Fluorinated Materials for Energy Conversion

Fluorinated Materials for Energy Conversion

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Fluorinated Materials for Energy Conversion Edited by

Tsuyoshi Nakajima Department of Applied Chemistry Aichi Institute of Technology Toyota-shi, Japan and

Henri Groult Pierre and Marie Curie University CNRS Paris, France

2005

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Preface Since the discovery of fluorine by Henri Moissan at the end of the 19th century, fluorine chemistry has developed many applications in energy conversion, medicine, biology, agriculture, telecommunication and so on. However, fluorine chemistry is not widely spread probably because of the difficulty in the fluorination techniques and handling of fluorinating reagents and fluorides. Elemental fluorine is a typical fluorinating gas with high reactivity arising from its small dissociation energy. Efficient production of elemental fluorine by molten salt electrolysis is still one of the important research subjects in fluorine chemistry notably because of industrial applications in nuclear energy field. A new and promising aspect in fluorine chemistry is the applications of fluorination reactions and various fluorides to energy conversion materials for lithium batteries, fuel cells, solar cells etc. Many examples regarding the introduction of fluorine into lithium battery materials, that is, fluorination of carbonaceous anodes and oxide cathodes, synthesis of new fluorine containing electrolytes, fluorination of organic solvents and so on, were recently reported and revealed the importance of fluorine chemistry in this field. It was also shown that the fluorinated materials had important roles in fuel cells and solar cells. This summarizes the recent advances on these topics. All authors are specialists actively working in fluorine chemistry, electrochemistry, polymer chemistry and solid state chemistry. We hope that the book offers new aspects of fluorine chemistry to readers in the various fields. Tsuyoshi Nakajima and Henri Groult

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Contributors Takeshi Abe Department of Energy and Hydrocarbon Chemistry, Kyoto University Japan

Guy Campet Institut de Chimie de la Matière Condensée de Bordeaux (ICMCBCNRS) France

Fannie Alloin Laboratoire d'Electrochimie et de Physico-Chimie des Matériaux et des Interfaces, INPG-CNRS UMR 5631 France

Vittorio Canevari IMEM-CNR Institute Italy

Bruno Améduri Laboratoire de Chimie Macromoléculaire, ENSCM-CNRS UMR 5076 France François Blanchard Faculté des Sciences, Université F. Rabelais France Alessio Bosio Department of Physics, University of Parma Italy Gérard Bosser Faculté des Sciences, Université F. Rabelais France Hubert Cachet Laboratoire des Interfaces et Systèmes Electrochimiques (UPR15-CNRS)–Université Pierre et Marie Curie France Magali Caillon-Caravanier Faculté des Sciences, Université F. Rabelais France

Bernard Carré Faculté des Sciences, Université F. Rabelais France Alexandre Chagnes Faculté des Sciences, Université F. Rabelais France Stephen E. Creager Depeartment of Chemistry, Clemson University U.S.A. Darryl D. DesMarteau Depeartment of Chemistry, Clemson University U.S.A. Marc Dubois Laboratoire des Matériaux Inorganiques, Université Blaise Pascal de Clermont-Ferrand France Olt E. Geiculescu Depeartment of Chemistry, Clemson University U.S.A. Henri Groult Laboratoire LI2C- CNRS UMR 7612, Université Pierre et Marie Curie France

viii

Contributors

Katia Guérin Laboratoire des Matériaux Inorganiques, Université Blaise Pascal de Clermont-Ferrand France

Kazuhiko Matsumoto Department of Fundamental Energy Science, Kyoto University Japan

Rika Hagiwara Depeartment of Fundamental Energy Science, Kyoto University Japan

Yoshiaki Matsuo Department of Materials Science and Chemistry, University of Hyogo Japan

André Hamwi Laboratoire des Matériaux Inorganiques, Université Blaise Pascal de Clermont-Ferrand France

Bengt-Erik Mellander Physics Engineering Physics, Chalmers University of Technology Sweden

Bernard Jousseaume Laboratoire de Chimie Organique et Organométallique, Université Bordeaux I France Kiyoshi Kanamura Department Applied Chemistry, Tokyo Metropolitan University Japan HanSu Kim Samsung Advanced Institute of Technology (SAIT) South Korea Chai-Won Kwon Samsung Corning Precision Glass South Korea Frédéric Lantelme Laboratoire LI2C- CNRS UMR 7612, Université Pierre et Marie Curie France Daniel Lemordant Faculté des Sciences, Université F. Rabelais France

Bénédicte Montigny Faculté des Sciences, Université F. Rabelais France Régine Naejus Faculté des Sciences, Université F. Rabelais France Tsuyoshi Nakajima Department of Applied Chemistry, Aichi Institute of Technology Japan Benjamin G. Nolan Department of Chemistry, Colorado State University U.S.A. Madeleine Odgaard IRD Fuel Cells A/S Denmark Zempachi Ogumi Department of Energy and Hydrocarbon Chemistry, Kyoto University Japan

Contributors

Yoshimi Ohzawa Department of Applied Chemistry, Aichi Institute of Technology Japan

Hidekazu Touhara Department of Chemistry, Shinshu University Japan

Nicola Romeo Department of Physics, University of Parma Italy

Thierry Toupance Laboratoire de Chimie Organique et Organométallique, Université Bordeaux I South Korea

Jean-Yves Sanchez Laboratoire d'Electrochimie et de Physico-Chimie des Matériaux et des Interfaces, INPG-CNRS UMR 5631 France Yukio Sasaki Department of Nanochemistry, Tokyo Polytechnic University Japan Johanna Saunier EA 401 Faculté de pharmacie France Christian Simon Laboratoire LI2C- CNRS UMR 7612, Université Pierre et Marie Curie France Renaud Souzy Laboratoire de Chimie Macromoléculaire, ENSCM-CNRS UMR 5076 France Steven H. Strauss Department of Chemistry, Colorado State University U.S.A. Masayuki Takashima Department Material Science and Engineering, Fukui University Japan

Shoichi Tsujioka Chemical Research Center, Central Glass Co., Ltd. Japan Pierre Turq Laboratoire LI2C- CNRS UMR 7612, Université Pierre et Marie Curie France Jan Uhlir Nuclear Research Institute Rez plc Czech Republic Jun-ichi Yamaki Institute of Material and Chemical Engineering, Kyushu University Japan Rachid Yazami Laboratoire d'Electrochimie et de Physico-Chimie des Matériaux et des Interfaces, INPG-CNRS UMR 5631 France Susumu Yonezawa Department of Material Science and Technology, Fukui University Japan Bin Zhu Chemical Engineering Technology/Chemical Reaction Engineering, Royal Institute of Technology (KTH) Sweden

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TABLE OF CONTENTS Preface Contributors Chapter 1: Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF H. Groult, C. Simon, A. Mantoux, F. Lantelme, and P. Turq

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1

Chapter 2: Applications of fluorinated carbon materials to primary and secondary lithium batteries T. Nakajima

31

Chapter 3: Synthesis and electrochemical properties of new carbon anodes prepared by chemical vapor infiltration Y. Ohzawa

61

Chapter 4: Electrochemical properties of fluorinated carbon nanotubes H. Touhara

89

Chapter 5: Fluorine-doped tin oxide electrodes for lithium batteries C.W. Kwon, H. Kim, T. Toupance, B. Jousseaume, and G. Campet

103

Chapter 6: Synthesis of fluorinated cathodes and fluoride electrolytes for lithium-ion batteries S. Yonezawa and M. Takashima

125

Chapter 7: Physicochemical properties of fluorine-containing electrolytes for lithium batteries D. Lemordant, F. Blanchard, G. Bosser, M. Caillon-Caravanier, B. Carré, A. Chagnes, B. Montigny and R. Naejus 137 Chapter 8: Fluorinated anions and electrode/electrolyte stability in lithium batteries R. Yazami and A. Martinent

173

Chapter 9: Electrochemical properties of lithium electrolytes based on bis(polyfluorodiolato)borate and tetrakis(polyfluoroalkoxy) aluminate superweak anions B.G. Nolan, S. Tsujioka, and S.H. Strauss

195

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Table of contents

Chapter 10: Fluorinated electrolytes based on lithium salts of strong Brønsted acids O.E. Geiculescu, S.E. Creager, and D.D. DesMarteau

223

Chapter 11: Electrolytes for lithium batteries K. Kanamura

253

Chapter 12: Thermally stable fluoro-organic solvents for lithium ion battery Jun-ichi Yamaki

267

Chapter 13: Physical and electrochemical properties and application to lithium batteries of fluorinated organic solvents Y. Sasaki

285

Chapter 14: PVdF-based polymers for lithium batteries J.-Y. Sanchez, F. Alloin, and J. Saunier

305

Chapter 15: Lithium-ion-conductive polymer electrolytes exhibit a high lithium-ion transference number with the incorporation of fluorine atoms T. Abe and Z. Ogumi

335

Chapter 16: Room-temperature molten salts as new electrolytes R. Hagiwara and K. Matsumoto

349

Chapter 17: Fluorine-intercalated graphite for lithium batteries A. Hamwi, K. Guérin, and M. Dubois

369

Chapter 18: Battery application of graphite intercalation compounds Y. Matsuo

397

Chapter 19: Fluoride-based electrolytes and their applications for intermediate temperature ceramic fuel cells B. Zhu and B.-E. Mellander

419

Chapter 20: The use of Nafion® as electrolyte in fuel cells M. Odgaard

439

Chapter 21: Functional fluoropolymers for fuel cell membranes R. Souzy and B. Ameduri

469

Chapter 22: Films and powders of fluorine-doped tin dioxide H. Cachet

513

Table of contents

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Chapter 23: Doped transparent conducting oxides suitable for the fabrication of high efficiency thin film solar cells A. Bosio, N. Romeo, and V. Canevari

535

Chapter 24: Fluoride technologies application within the Molten-Salt Reactors fuel cycle J. Uhlir

549

Subject Index

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Fluorinated Materials for Energy Conversion T Nakajima and H Groult (Editors) © 2005 Elsevier Ltd. All rights reserved.

1

Chapter 1

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF H. Groult, C. Simon, A. Mantoux, F. Lantelme, and P. Turq Laboratoire LI2C, CNRS UMR 7612, Université Pierre & Marie Curie- Case courrier 51, 4 place Jussieu, 75252 Paris cedex 05, France 1. PROPERTIES AND INDUSTRIAL USES OF FLUORINE GAS Fluorine gas is a yellowish, poisonous and highly corrosive gas which reacts with practically all organic and inorganic substances [1–6]; it reacts with all elements except helium, neon and argon to form ionic or covalent fluorides. Until World War II, there was no commercial production of elemental fluorine. However, due to the development of the atomic bomb and nuclear energy applications, the production of large amounts of elemental fluorine became necessary. Now, F2 gas can be considered as a necessary intermediate in uranium isotopic enrichment: separation of the isotopes of natural uranium is carried out by a diffusion process involving gaseous UF6. Uranium tetrafluoride (UF4) is first produced by the reduction of the oxide H2 or NH3 and fluorination by HF at 400–600°C according to UO3  H2 → UO2  H2O

(1)

UO2  4HF → UF4  2H2O

(2)

UF6 is then prepared by the reaction of fluorine gas and UF4 at a high temperature ( 1000°C) according to UF4  F2 → UF6

(3)

Uranium enrichment performed by gaseous diffusion or ultracentrifugation consists of increasing the content of natural uranium’s in isotope 235 to levels up to 3–5%. About 90% of the nuclear reactors currently in operation use this type of enriched uranium.

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In the coming decades, energy consumption is expected to rise significantly, due to dramatic increases in world population, coupled with the industrialisation of developing countries, notably in Asia and Latin America. Since the beginning of the 20th century, the emission of greenhouse gases has risen drastically because of the use of fossil fuels in industry and transportation. This emission is expected to increase considerably in the near future. Thus, to respect the climatic and environmental equilibrium, intense research programmes are being devoted to the development of renewable energies (solar, wind, etc.); however, the latter have, at the present time, a limited potential. Therefore, nuclear power, which could be considered as an environmentally safe form of energy since it does not produce harmful gases contributing to the greenhouse effect, should play an important role in the production not only of electricity, but also of other energy sources (hydrogen, etc.). For example, it generates 110 times less CO2 than natural gas and close to 240 times less CO2 than coal for electricity production. In the European Union, nuclear energy accounts for 35% of electricity production, thereby avoiding the emission of 300 million tonnes of CO2. In 2001, the installed capacity of all types of nuclear power plants in the world amounted to 358,000 MW, about a quarter in the United States (over 100,000 MW) and about 17% in France (almost 60,000 MW). Thirty-two reactors are under construction worldwide: 22 in Asia and 10 in central and eastern Europe. Fluorine gas is not only devoted to the synthesis of UF6, but is also widely used for the preparation of various fluorinated compounds involved in different industrial processes: WF6 for depositing tungsten on insulating or conducting substrates by CVD, NF3 for etching semiconductors, graphite fluorides (CFx, 0.5  x  1.24) for use as cathodes in primary lithium batteries and as lubricating agents, SF6 as insulating gas in electric devices, ClF3 to clean semiconductor fabrication vessels in the computer chip industry, CoF3 as solid fluorine carriers and F2–N2 mixture to strengthen the surface properties of plastics (impermeability, chemical resistance, barrier effect, etc.) or to control the fluorination of molecules in organic chemistry. Therefore, it seems to be of prime importance to study in detail the fluorine evolution reaction (FER) in order to optimise the process and to satisfy the increasing industrial requirement for this gas. The purpose of this review paper is to provide a brief overview of both experimental and theoretical aspects of the FER on carbon anode in KF–2HF. The process has been analysed of by correlating results deduced from electrochemical tests in KF–2HF, ex situ surface characterisations, notably by AFM and STM, and numerical calculations. Owing to molecular dynamics simulations of molten KF–2HF, the constituents of KF–nHF melt have been identified, depending on the temperature and the HF content in KF–nHF. The origin of the strong adhesion of fluorine bubbles on the surface is also discussed. Finally, the particular shape of fluorine bubbles generated on horizontal carbon anodes in KF–2HF

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

3

is studied from a theoretical point of view taking into account capillary forces between the electrode surface and the gas–liquid interface. 2. ANALYSIS OF THE PREPARATION PROCESS 2.1. Generalities

In 1886, the French scientist Henri Moissan prepared, for the first time, fluorine by electrolysis of anhydrous hydrogen fluoride containing a small amount of potassium fluoride in an electrochemical cell with platinum–iridium electrodes. KF was used to render HF conducting. The description of industrial cells is widely reported in the literature [1,2,7]; briefly, the cells operate at 6 kA and contain molten KF–2HF (40.8 wt% HF) with about 24 plate carbon anodes and steel or iron cathodes. Carbon anodes are used to avoid dissolution occurring with most metals in parallel with the evolution of fluorine; in addition, graphite must be avoided since exfoliation takes place due to co-intercalation of ionic species and fluorine gas between the lamellar graphene sheets. Monel skirts are also required to separate the hydrogen and fluorine gases formed at the cathode and the anode, respectively, and to avoid their explosive recombination. The global reaction involves HF decomposition: 2HF(liq) → F2 (g)  H2 (g)

(4)

The two corresponding half-cell reactions are supposed to involve the HF2 electrochemical species: HF2 →

1  2

F2  HF  e

(5)

at the anode, and 2 HF  e →

1  2

H2  HF2

(6)

at the cathode. First, molecular dynamic simulation of KF–nHF was investigated depending on the HF ratio and the temperature of the melt. 2.2. Molecular dynamics model for KF–nHF electrolytes

Within the framework of improvement of fluorine generation process, the model developed for KF–nHF electrolytes was intended to provide not only structural properties of the liquid (e.g. thermodynamical data, speciation, etc.) but also dynamical properties, especially transport coefficients such as self-diffusion coefficients, electrical conductivity and viscosity. This would make it possible, in principle, to address most of the previously cited problems on the microscopic scale.

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Among the available simulation techniques, ab initio molecular dynamics (AIMD) was the only one previously used to study KF–2HF [8]. While it is accurate, it needs very few hypotheses, and is computationally extremely intensive, making it unsuitable for the long simulations needed for transport properties computation. Lighter techniques such as Monte-Carlo simulations or integral equations based methods were also rejected because they were unable to predict transport properties. The only theoretical method left to investigate KF–HF electrolytes was therefore classical molecular dynamics (MD) [9]. This statistical mechanics-based simulation method is fast and provides all the desired results as soon as the model used therein is adequately defined. Transport properties have been examined with the herein presented iono-molecular model [10], but we will focus only on the structural features of the liquid. Molecular dynamics requires the description of the interaction forces between the liquid constituents. In pure molecular liquids, for example HF, this means defining the forces applied to each molecule by their surrounding molecular neighbours. In high-temperature molten salts, such as KF, this means defining the forces acting on each fluorine and potassium ion. But in KF–2HF, the microscopic nature of the constituents is still unknown: in electrochemistry, HF2 is often postulated, but there are no hints on how much HF are turned in that ionic form, how much in “neutral” HF. Oligomers are known to form in pure HF [11]. Polyfluorides, both centred F(HF)n and chained HnFn1 , have also been revealed in several condensed phases [12–15]. In fact, the question is whether KF–2HF is a molecular liquid or an ionic liquid. Is KF–2HF closer to pure HF or to molten KF? The point of view adopted is crucial for acidity definition. Up to now, the only attempt was based on Brönsted acidity [16], but why not try with Lewis fluoroacidity pF? This only depends on the nature of the species on the microscopic scale, which is in general known from direct observation. The paucity of available physical-chemistry data was an important obstacle to the definition of the model: to our knowledge, there are still no data on infrared, Raman spectroscopies, neutron scattering, and NMR spectroscopy (numerous corresponding data exist for the solid, but were not available for the liquid case). Therefore, the nature of the components was unknown, and a fortiori the forces acting between them. The only direct information about the liquid structure was given by AIMD [8], showing essentially centred polyfluorides and transitionally chained ones. We therefore decided to build an iono-molecular model to retrieve these F(HF)n, and their proportions, as a function of n. We denoted our model as iono-molecular since it describes, on the one hand HF molecules and K and F ions on the other. For alkali halides, the models developed by Fumi and Tosi [17] are indeed well tested and used to describe accurately the solid phases, pure molten salts, their mixtures and even aqueous solutions of the salts. For KF, we used the interactions given in Table 1.

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

5

The centred polyfluorides essentially consist of a fluorine ion solvated by HF molecules. In these F(HF)n complexes, the fluoride–hydrogen distance dHFranges from 1.35 to 1.7 Å depending on n, and is very similar to an ordinary H-bond. For pure HF, several models have been developed. Only one of them explicitly describes the H-bond between HF molecules (H-bond is implicit in the others and results from Coulomb or dipole interactions): the HF3 model derived by Klein and McDonald [18] (see Table 2). We therefore decided to use this model for our HF molecules, and to apply the H-bonding Morse potential (VHF) between HF molecules and fluoride. In addition to the attractive coulombic interaction between positively charged H and fluoride anions, this was intended to yield the right dHF shorter than the H-bond in pure HF (1.8 Å). The system was simulated at different temperatures ranging from 330 to 410 K (56.85 to 136.85°C). The initial simulation cell consisted of 384 atoms (64 times KF–2HF units) randomly arranged to get a density extrapolated, for each simulation temperature, from experimental data [19]. The coulombic forces were computed with an ordinary Ewald algorithm, the HF molecules were handled Table 1 Interaction potential for K and F ions Parameter







cij

1.25

1.00

0.75

σij (Å)

2.926

2.642

2.358

Cij (10-79 Jm6)

24.3

19.5

18.6

Dij(10-99 Jm8)

24.0

21.0

22.0

Note: Analytical form is Vij  ZiZje2\rbcij exp[B(σijr)] Cijr6Dijr8 with b  3.38 × 1019 J and B  2.96 × 106 m1.

Table 2 Interaction potential for HF molecules Pair

Potential

H-F

2(exp(10.6(r1.6))exp(5.6(r1.6)))

H-H

600exp(3.34r)

F-F

2  105exp(4.25r)VDISP

Note: Distances are in Å and energies in kcal mol1 with V  f(r)(220\r6  400\r8  4500\r10), where f(r)  exp((4.7/r1)2) if r 4.7 and f(r)  1 if r 4.7. DISP

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with a constraints algorithm, and the Verlet algorithm was used with a time step of 1.0 fs [10]. Simulation cells were equilibrated in 20,000 equilibration steps (i.e. 20 ps) to yield stationary averages for temperature, potential energy, and constant radial distribution functions. Simulations were then conducted in the micro-canonical ensemble. Visual examination of the cell after equilibration reveals the formation of polyfluorides (Fig. 1). This is a spontaneous formation of the complexes, since the initial configuration was randomly disordered. To investigate how many complexes F(HF)n of each n were formed during simulations, we had to choose a criterion to on which to base a decision on whether an HF molecule is bonded to a fluoride ion or not. This criterion is naturally derived from the radial distribution functions (RDF). In Fig. 2, the RDF for the hydrogen–fluoride pairs is plotted as a function of distance r. The first neighbour peak at 1.54 Å is perfectly in the range expected, clearly shorter than H-bonds in pure HF. Furthermore, the RDF decreases down to zero beyond this peak, between r  2.1 and 2.6 Å. This shows clearly that HF molecules, where the H atom is closer than 2.1 Å to a fluoride ion (or any cut-off distance up to 2.6 Å), are bonded to it. This allows one fluoride ions to determine how many molecules are bonded to each fluoride, and

Fig. 1. Instantaneous configuration of 2HF–KF liquid at 366 K. Grey spheres represent fluorine atoms (both molecular fluorine and fluoride), white spheres hydrogen atoms and black spheres potassium ions. Radii are set to the standard van der Waals radii.

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

7

Fig. 2. Radial distribution g(r) function for the H–F (fluorine ions) pair, in the liquid 2HF–KF at 366 K. The narrow peak at 1.54 Å results from the very strong bonding between fluorine and H (belonging to an HF molecule). g(r) decreases down to zero in the range 2.1–2.6 Å: closer pairs are bonded, more distant pairs are non-bonded.

to determine how many polyfluorides of each n exist. The results are reported in the next section. Following the above procedure, we obtained, at 366 K, for a simulation length of 2.6 ns, the average number of complexes F(HF)n for each n. The data obtained are displayed in Fig. 2, together with results from previous AIMD studies [8]. The results are very similar: both theoretical techniques reveal F(HF)n complexes up to n  4; the more common complex is F(HF)2 and not the simplest [FHF] . Fig. 3 shows only time-averaged results, but the complex populations undergo fluctuations during the simulation. It has been demonstrated that this is due to exchanges of HF molecules between complexes. This rapid exchange (on the simulation timescale) explains why the final observed populations do not depend on the starting random configuration: the system really reaches a dynamical chemical equilibrium. Fig. 4 illustrates this process: [F(HF)n…HF]   F(HF)m  F(HF)n  [FH…F(HF)m]  for each pair n,m.

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Fig. 3. Centred polyfluoride F(HF)n populations, depending on n. Black bars: results from our classical molecular dynamics simulations at 366 K. White bars: results from ab initio molecular dynamics by von Rosenvinge et al. Features of both distributions are the same.

Fig. 4. Exchange of HF molecules between two polyfluoride ions. One central fluorine is denoted by FA and the other by FB. The graphs show the distance between the transferred H atom (and therefore HF molecule) and the two centres, as a function of time.

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

9

To go beyond these results, other simulations were performed at higher temperatures up to 410 K and showed insignificant differences in complex populations. The absence of cell-size effect was also checked with simulation cell containing up to 1926 atoms (for details see Ref. [20]). Another factor potentially influencing the population of complexes is liquid composition, which will be discussed in the next section. Two other compositions of the liquid. KF–1.8HF and KF–2.4HF, were simulated using the same model from the RDF analysis, it appeared that the structure of these liquids is very similar to the one obtained for KF–2HF. It was therefore possible to apply the same procedure to determine populations of the different polyfluorides. Results are presented in Fig. 5. For the KF–1.8HF mixture, [FHF]  and F(HF)2 proportions are almost equal (but F(HF)2 is still predominant) and proportions of heavier polyfluorides are smaller than in KF–2HF. For the 2.4KF–HF mixture, F(HF)2 is also predominant but competes with F(HF)3 instead of [FHF] . In general, the relative proportions of heavier polyfluorides increase with increasing proportions of HF. This result seems quite intuitive but: ●

●

●

At the time the model was built such an effect was never observed and not even suggested. It was directly observed only later, in very different media such as tBu4NF/HF in solution and in cold freon mixtures [12]. In more similar media, such as (CH3)4NF/mHF, this behaviour is only suggested by more recent experiments.

Fig. 5. Centred polyfluoride F(HF)n populations, depending on n, for two different compositions. Black bars: 1.8 HF/KF; white bars 2.4 HF/KF. Distribution is shifted from low weight (less solvated fluorine) to higher weight (more solvated fluorine) while HF proportion is increased in the liquid.

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These results finally demonstrate the flexibility of the model and its ability to predict consistent and relevant structural results. To conclude, in spite of converging recent results about similar liquids, there is still no evidence for the existence of the polyfluorides in KF–2HF. We therefore tried to use NMR to address this problem, but HF exchange is too fast to have enough resolution on the spectra with current methods. Both theoretical and experimental work on similar systems with various cations is in progress to confirm the validity of the model and to get a more in depth understanding of the structure of these electrolytes. 2.3. Surface characterisations of carbon anodes fluorinated in KF–2HF by STM and AFM

The fluorine evolution process is characterised by a high current efficiency approaching 0.95 and also by a poor energy efficiency of about 0.3 [3–7, 21–23]. As a consequence, a large quantity of heat must be removed by cooling to maintain a constant temperature in the cell. The thermodynamic potential of HF decomposition is ca. 2.9 V [21,24], and not 1.8 V as reported in Ref. [25]. In fact, the total voltage in industrial cells is composed of five parts: the reversible decomposition voltage (2.9 V), the ohmic drop in the electrolyte, the ohmic drop in the electrodes, the cathode overvoltage, and the anode overvoltage. The high ohmic drop value is mainly due to the fact that a distance of several centimetres between anodes and cathodes is necessary to avoid explosive recombination of hydrogen and fluorine, in contrast to the NaOH-Cl2 preparation process in which the distance between anodes and cathodes is only a few millimetres. The high anode overvoltage (2.5 V) is commonly ascribed to the formation of a solid carbon–fluorine layer on carbon anodes during fluorine production [1–7, 21–23]. The inhibition of the FER is partly explained by the low surface energy of the film, which repels the electrolyte from the electrode. The contact angle is in the range of 120–160° [3]: fluorine bubbles have a lenticular form and are strongly adherent to the surface of the carbon anodes. This induces a significant decrease in the electroactive surface area of the electrode. Qualitative evidence of the formation of a passivating layer on the carbon anodes is given by cyclic voltammetry studies [7,26,27] and X-ray photoelectron spectroscopy (XPS) [3,5,6,27–30]. Indeed, the first voltammogram performed in KF–2HF with a new carbon anode exhibits an anodic passivation peak (Fig. 6) between 2.5 and 3.0 V vs. Pt–H2, which corresponds to the formation of a solid carbon–fluorine film (denoted C–F henceforth) on the anode surface. Many authors have concluded that, after reaching a high potential value in KF–2HF, the C–F film is composed of insulating graphite fluorides (denoted CFx) and it was assumed that the electron transfer occurs by tunnel effect through the passive CFx [31], the latter acting as an inhibiting barrier for the electron transfer during the FER. It leads to very low values of transfer coefficient, in agreement with a mechanism

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

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Fig. 6. I–E curves (v  0.4 V s1) obtained in KF–2HF with a graphite electrode.

involving electron tunnelling through a passive film. For such a mechanism, the probability of electron transfer depends on the thickness and the height of the potential barrier. In fact, XPS investigations performed with carbon anodes fluorinated in KF–2HF have shown that the position of the F1s and C1s peaks indicates the presence of ionic and semi-ionic C–F bonds [3,5]. Impedance measurements performed under “dry conditions”, i.e.without electrolyte (with a carbon/C–F/mercury structure) and in the presence of an aqueous solution containing a redox couple have revealed that the C–F film can be considered as an electronic conductor and thus cannot constitute a high-energy barrier for the electron transfer in FER. STM measurements on HOPG samples fluorinated at 6 V in KF–2HF [6,32] have confirmed these conclusions in revealing the presence of conducting GIC compounds, usually denoted CxF, with ionic and semiionic C–F bonds. In fact, two kinds of images (Fig. 7) were obtained with this technique: (i) In the first image, the same hexagonal symmetry as in pure HOPG (Fig. 7a) was observed, with a periodicity of 0.244 nm (Fig. 7b). Only half of the carbon atoms of a graphene layer exhibits high electronic density, due to the nonequivalence of the atomic sites resulting from the ABAB stacking of graphene layers. (ii) In the second image, all of the carbon atoms of the hexagonal rings were observed; this is due to the presence of fluorinated intercalants between two graphene sheets, which induce an increase in the distance between these two layers; as a consequence, the non-equivalence of the atomic sites resulting

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Fig. 7. Three-diamentional scanning tunnelling microscopy images. (a) Pure HOPG showing no overlap; z range, 15 nA/div. (b) Fluorinated HOPG (6 V) showing an overlap of electronic densities; z range: 5 nA/div. Experimental conditions : bias voltage 20 mV; constant height mode; Pt/Ir tip. (Images are reprinted from H. Groult et al., Electrochim. Acta, 44 (1999) 2793–2803.)

from the ABAB stacking of graphene layers is lost and all the carbon atoms of graphene layers are clearly visible. The spacing between two neighbouring atoms deduced from the corrugation amplitudes, is about 0.154 nm. In contrast two classical STM images observed in the case of pure HOPG, an overlap of the electronic densities between two neighbouring atoms was observed in the case of HOPG samples fluorinated in KF–2HF. The fluorination of the surface, coupled with the intercalation of fluorinated species between two graphene layers, induces the formation of carbon–fluorine bonds that modify the electronic density of each carbon atom. Therefore, the overlap of the electronic densities is due to two neighbouring F atoms. A schematic in-plane structural model corresponding to the image observed by STM in Fig. 7b is given in Ref. [32], showing the centred hexagonal lattice commensurate with the graphite lattice. STM observations on fluorinated HOPG have also revealed that in many parts of the electrode, no current can be detected even for very high bias value, i.e., no image can be recorded. This was attributed to the local presence of insulating compounds such as graphite fluorides (CFx). Finally, the C–F solid film generated at the surface of the carbon anode during the electrolysis of molten KF–2HF was supposed to be composed of conducting compounds belonging to the graphite intercalation compounds (GICs) family in which the C–F bonds are ionic and semi-ionic and insulating CFx with covalent C–F bonds. In spite of the presence of graphite fluorides, the fluorine evolution mechanism does not obey a mechanism involving electron tunnelling through a passive film as reported previously [31]: the electrons can be easily transferred from the electrolyte to the electrode. Nevertheless, no evidence was reported that proved effectively the presence of CFx in the C–F layer. This is why AFM measurements in contact

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

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mode were performed with HOPG samples fluorinated at 6 V in KF–2HF. In addition to the AFM characterisations, local electrical measurements were performed in ambient air with an original laboratory-made device derived from an AFM; this apparatus has been developed by Houzé [33–35] and allows one to cover nine decades of tip/sample resistance value (from 100 Ω to 100 GΩ). The fluorinated HOPG samples previously studied by STM were analysed on 5 μm  5 μm areas (Fig. 8). The electrical and topographical images are represented in Fig. 8a and b, respectively; the corresponding distribution of the electrical resistance measured is given in Fig. 8c. As shown in Fig. 8c, the resistance distribution is centred on 3  104 and 6  1010 Ω average values; for the former, most of the contact resistance values are measured between 8  103 and 2  105 Ω, associated with the red colours on the electrical images (Fig. 8b); for the resistance distribution centred on 8  1010 Ω, most of the contact resistance values are measured between 5  1010 and 2  1011 Ω, associated with the purple colours on the electrical images (Fig. 8b). Notice that the histogram also

(a)

(b)

(c)

Fig. 8. AFM investigations on HOPG fluorinated in KF-2HF at 6 V: (a) topographical image; (b) electrical image; (c) distribution of the measured resistance deduced from (b).

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reveals a continuous variation of the local resistance with two small shoulders around 107 and 4  108 Ω. These images clearly show that the surface of fluorinated HOPG samples does not appear to conduct uniformly. Also note that the presence of less conducting zones does not correspond to transitions between the two graphene sheets. As reported in the literature, graphite fluorides and chemically prepared graphite intercalation compounds [36–43] present a very large difference in terms of electrical resistivity. For instance, graphite fluoride with a composition of CF0.465 exhibits an electrical resistivity of about 107 Ω cm (about 104 Ω cm for the starting graphite material), whereas this value can reach about 105 Ω cm in the case of fluorine-GICs [36]. Based on this fact, we interpret the differences of resistivity observed on AFM images with HOPG samples fluorinated in KF–2HF (Fig. 8c) in the presence of GICs (with a resistance distribution centred on 3  104 Ω), graphite fluorides (with a resistance distribution centred on 6  1010 Ω), and intermediate compounds for which the composition varies from CFx to GICs and which give rise to intermediate colours (Fig. 8b) from red to purple. Therefore, these AFM measurements coupled with local electrical determinations confirm our previous assumptions deduced from STM measurements about the heterogeneity of composition of the C–F layer on carbon anodes fluorinated during electrolysis in molten KF–2HF. 2.4. Origin of the strong adhesion of F2 bubbles on the carbon surface

The CFx compounds in the C–F film a strongly influence the wettability of the electrode by KF–2HF and the kinetics rates of the FER. For example, we have shown [6] that in the case of carbon anode chemically fluorinated at high temperature under fluorine gas atmosphere prior to its introduction into the electrochemical cell, leading to a high amount of graphite fluorides on the surface, the fluorine evolution reaction is completely inhibited in KF–2HF. Here, we compare fluorine bubble evolution on carbon (Fig. 9a) and nickel (Fig. 9b) ring

Fig. 9. Evolution of fluorine in KF–2HF on (a) horizontal carbon and (b) nickel electrodes facing the top of the electrochemical cell (E  4 V vs. Cu/CuF2).

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

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disk electrodes; in both cases, only the upper part of the surface was electroactive, the electrodes being embedded into an insulating Teflon holder. As shown in Fig. 9a, the large fluorine bubble formed on the carbon anode has a lenticular form and is strongly adherent to the surface; almost the entire surface is covered by the gas and the contact angle is about 150°. A parallel drawn can be made with the electrolytic preparation of aluminium: when the alumina content of the cryolite melt decreases, a concomitant increase in the contact angle is seen. For a low amount of alumina, a rapid change in the wetting properties occurs and the contact angle jumps to around 180°. This results in a full coverage of the electrode by the gas phase followed by a dramatic increase in the overvoltage and the appearance of sparks around the electrode. This phenomenon, called “anode effect,” is due to the presence of solid fluorocarbon CFx compounds at the surface of the carbon electrode; the solid–liquid interfacial energy is very low and the liquid cryolite does not wet the anode any more. As similar explanation can be given in terms of wettability during the electrolysis of molten KF–2HF with carbon anodes, but no “anode effect” is seen. In contrast, in the case of a nickel electrode used for F2 evolution in KF–2HF (Fig. 9b), small, spherical fluorine bubbles with a zero contact angle form at the centre of the electrode; nevertheless, an important modification occurs on the lateral part of the electrode at the nickel/Teflon interface. Indeed, in this area, due to the presence of a perfluorinated carbon compound such as Teflon, the fluorine bubble that forms on nickel spreads over the adjacent coating (Teflon) and produces large and non-symmetric fluorine bubbles. In other words, the juxtaposition of two compounds with very different interfacial properties drastically modifies the shape of the fluorine bubble and its formation. In the case of fluorinated carbon anodes, STM and AFM measurements have revealed the presence of GIC and CFx; therefore, an analogy can be made with the phenomenon observed at the nickel/Teflon interface. The fluorine bubbles should form easily in the part of the electrode covered by GICs, but since these zones are located near the zones covered with CFx compounds, the fluorine bubbles spread over the surface covered by CFx and give rise to a large and strongly attached fluorine bubble. 2.5. Origin of the CFx compounds in the C–F surface film

Graphite fluorides used in primary lithium batteries or as lubricants are usually prepared by the chemical fluorination of carbon at high temperatures ( 350°C) using F2 gas. Depending on the operating conditions, the x value in CFx can vary from ~ 0.5 to ~ 1.3. As reported above, their electrical conductivity drastically decreases with increasing x values. In the case of the electrolysis of KF–2HF, CFx can also be generated from the electrofluorination of graphite oxides initially present on carbon surface. Indeed, these oxides are known to be easily fluorinated at low temperatures because of their high instability. For increasing potentials applied to the carbon electrodes in KF–2HF, oxygen might

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be easily exchanged with fluorine, to give rise to large amounts of insulating graphite fluoride [44,45]. For example, graphite oxide compounds such as CzO(OH)y could lead to the formation of CFx according to the reaction CzO(OH)y  (2  xz  3y) F → zCFx  (1  y) F2O  yHF  (2  xz  3y)e

(7)

C–O bonds could result to the presence of water in the electrolyte. In contrast, in the classical schematic representation of the carbon surface (Fig. 10a), several kinds of C–O bonds are present at the edges of graphene sheets and can make bridges between two stacks of graphene sheets. F atoms can also be easily exchanged with O atoms to form CF, CF2 and CF3 groups with covalent C–F bonds, as illustrated in Fig. 10b. In this case, very large insulating areas can be formed, explaining why very large insulating domains are observed by AFM with HOPG samples fluorinated in KF–2HF (Fig. 8c). Physicochemical characterisations of starting carbon materials have been made to determine if the model used for the surface representation (Fig. 10a) and our assumptions given above for the formation of large insulating domains are valid. The XRD pattern of the carbon used as anode is presented in Fig. 11. The bump generally observed around 20° due to the presence of amorphous phases is

Fig. 10. Schematic representation of carbon surface (a) before and (b) after electrochemical fluorination in KF–2HF.

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

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(002)

40 counts

(100) et (101)

0

20

40

(004)

60

80

100

120

140

2θ / deg.

Fig. 11. XRD pattern of carbon material.

not evidenced in the XRD pattern; in addition, the (002) diffraction line belonging to graphite is clearly visible, indicating a preferential orientation along the c-axis. Additionally, three minor contributions are pointed out due to the (100), (101) and (004) diffraction lines of graphite, but with a low intensity for each one. As a consequence, one may conclude that the carbon used for the preparation is similar to graphite but with a lower crystallinity. The distance, d002 between two graphene sheets, was determined to be from 0.34 nm the Bragg relation taking into account the position of the (002) diffraction line: the value very slightly higher than that usually observed for pure graphite (0.3354 nm). The crystallite size Lc along the c-axis was calculated from the broadness of the (002) diffraction line using Scherrer’s equation: 0.89λ1 Lc   B cos θ

(8)

where λ1 is the wavelength of the Kα1Co beam, θ the Bragg angle and B the angular full-width at half-maximum (FWHM) of the (002) diffraction line. It was found that Lc was close to 40 nm. Similar information has been provided by transmission electron microscopy analysis; as shown in Fig. 12, the carbon material is mainly composed of graphitised phases without clear orientation. Finally, Raman spectroscopy analysis was performed; this technique provides information about functional groups or chemical bonds in molecules. In a Raman spectrum, each line has a characteristic polarisation, and therefore, polarisation data provide information about the molecular structure. Graphitic carbons are strong Raman scatterers in spite of their intense optical absorption.

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Fig. 12. TEM image of carbon material.

Because of the weak interlayer bonding, disorder along the c-axis can occur in the graphite crystal, while at the same time, the strong interlayer C–C bonding maintains a high degree of order within the individual carbon sheets. Therefore, Raman spectra of carbon materials are usually characterised by a pair of bands called G- and D-bands. The G-band is assigned to the E2g2 carbon–carbon stretching mode, whereas the D-band is due to an A1g vibration mode in the disordered region of carbon materials or edge plane of powder carbon [46,47]. The ratio, R, of intensity of D-band, ID, to that of G-band, IG (R  ID/IG) depends on the structure of the carbon and indicates the degree of disordering of the surface of the carbon materials. In the presence of amorphous phases, an additional peak should be observed at around 1530 cm1. Fig. 13 shows the Raman spectrum of the starting carbon material. The G- and D-bands are observed at around 1600 and 1355 cm1. The R value obtained from our experimental spectra is close to 1.7. This value is very large compared with that usually observed for other kinds of carbonaceous materials: 0.07 for pyrolitic graphite, 0.20 for polycrystalline graphite and 0.76 for coke-type carbon. It indicates a high disordering at the surface of our carbon materials. It is also important to note that the experimental curve can be fitted by considering only the two contributions due to the G- and D-bands; in other words, it is not necessary to take into account an additional contribution at around 1530 cm1 due to the presence of amorphous phases to obtain a very good fitting of the experimental spectra. These conclusions are in good agreement with the results deduced from XRD analysis (no evidence of amorphous phases).

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

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Fig. 13. Raman spectra of carbon electrode.

To conclude, XRD and TEM analyses have pointed out the presence of graphitised phases without a clear orientation; these phases are necessary for the formation of GICs during the electrolysis of molten KF–2HF. However, Raman spectroscopy has shown that the carbon materials used for fluorine production are characterised by a high surface disordering, which means that according to the classical representation of the carbon surface described in Fig. 10a, many C–O bonds are initially present on the surface prior to electrolysis. These C–O bonds lead to the formation of insulating CFx compounds during electrolysis. The latter are not wetted by the melt and the electrode surface area involved in the FER is drastically limited. Some possibilities are available to limit the formation of CFx [5,7,48–54]. Typical examples are described briefly in the next section. 2.6. Improvements in the fluorine evolution process

For limiting the influence of CFx and improving the kinetics of the FER, surface treatments can be proposed. We present here one specific example that consists in performing an electrochemical activation of the carbon anode in KF–2HF at very high potential (40 V) during a short time period (1 min). The aim of this treatment is to burn vigorously the C–O groups and thereby avoid the formation of large amounts of CFx. The I–E curves recorded in KF–2HF (v  0.2 V s1) with a crude and an activated carbon electrode are shown in Fig. 14: if one considers a current density of 12 A dm2, the anodic overvoltage is decreased to 0.4 V in the case of activated carbon. One must notice that activation has two effects on the surface behaviour: first, the decrease in the amount of CFx on the surface leads to an increase in the electroactive surface area. Second, the activation of

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480

2

420

j (mA/cm²)

360 300

1

240 180 120 60 0 3.5

4.0

4.5

5.0

5.5

6.0

E (V) vs Cu/CuF2

Fig. 14. I–E curves (v  0.4 V s1) obtained in KF–2HF. (1) carbon after electrochemical passivation in KF–2HF; (2) carbon after electrochemical activation in KF–2HF at 40 V during 1 min.

the carbon anode in KF–2HF induces a smoothing and cleaning of the surface as evidenced recently by SEM [55]. Consequently, the porosity of the surface is modified and the influence of pores on the kinetics rate is decreased. Indeed, as described in a previous paper [55,56], fluorine is produced on the horizontal carbon surface and in the pores. In the latter, the walls stop the lateral growth of the bubble and the phenomenon that occurs is comparable to capillary rise with convex meniscus. The volume of the gaseous cap increases along the z-axis concomitantly with the decrease in the electroactive surface area. When the pore is filled with fluorine, the gas bubble spreads on the horizontal surface all around the pore; the same phenomenon occurs in the nearest pores and recombination of the gaseous bubbles is observed until a total coverage of the electrode is reached. Activation at high potential makes it possible to limit the influence of pores on the kinetics rate of the FER. The positive effect of activation procedures on the kinetics rate of the FER is clearly evidenced by faradaic impedance measurements performed in KF–2HF [57] at 4.2 V with crude and activated carbon anodes (Fig. 15): as shown in this figure that presents the impedance diagrams obtained in the Nyquist representation, the charge transfer resistance (⬇ diameter of the semi-circle) measured at medium frequency in the case of an activated carbon electrode (R2) is about nine times lower than that observed in the case of a crude carbon electrode (R1), indicating a faster kinetics in the case of activated carbon electrodes. Activation of laboratory-scale electrodes is a good, realisable procedure; however, such a procedure cannot be achieved on a large scale in industrial cells because: (i) the procedure is risky, since sparks are observed, and produces a high

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

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- Im(Z) (ohm cm2 )

60 50 40 30

1

20 10

2

0 0

20 R2

40

60

Re (Z) (ohm

cm2)

80

100 R1

Fig. 15. Impedance diagram in the Nyquist representation obtained in KF–2HF with carbon after electrochemical (1) passivation in KF–2HF; (2) activation in KF–2HF at 40 V for 1 min; E  4.2 V vs. Cu/CuF2.

quantity of heat that should be eliminated, and (ii) a special electrolyser is needed to perform such a pre-treatment of carbon anodes. 2.7. Interfacial properties and gas bubble formation

Generally, bubble evolution obeys a classic mechanism: nucleation, growth, coalescence, detachment and rise of bubbles, and is also related to their size and adherence to the electrode surface [58–60]. Practically, these processes occur concurrently, so, it is virtually impossible to separate them experimentally. As shown above, the interfacial properties also play an important role in the present electrochemical reaction. The size and adherence of bubbles depend on the properties of the liquid–gas interface. For many gas evolution reactions, the contact angle is nearly zero and the weakly adherent bubbles have a spherical shape. The electrode surface is not strongly modified in the presence of bubbles and most of the works deal with the influence of the bubble on the fluid motion in the vicinity of the electrode [61]. As shown in Fig. 9a, the fluorine bubble generated on a horizontal carbon surface electrode facing the top of the electrochemical cell has a particular shape; the fluorine gas coverage does not hinder the current flow even if a complete coating of the electrode by a fluorine gaseous film is observed and the volume of the gas bubble continues to increase. To explain this phenomenon and also to understand exactly the origin of the high anodic overvoltage that characterises this process, the influence of the mass transfer on the kinetics rate has been studied recently by impedance measurements using a rotating disk electrode [55]. The interpretation of the impedance diagrams obtained in the Nyquist representation allows us to propose a new model for the representation of the electrode/electrolyte interface as illustrated in Fig. 16, including the presence of an intermediate layer sandwiched between the

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Fig. 16. Schematic representation of the carbon/KF–2HF interface. Cross section of an horizontal carbon electrode facing the top of the electrochemical cell. A “fluidized” layer in which liquid KF–2HF and fluorine gas co-exist is sandwiched between the C–F film and F2 bubble. A gradient of concentration of fluorine in the solid C–F layer up to the surface of the carbon electrode is observed.

C–F surface layer and the gaseous phase. The current passes through this conductive layer at the periphery of the bubble where the gas layer is very thin. It should be pointed out that this hypothesis presents some similarity to the explanation of Brandon and Kelsall [62] in the case of the bubble departure radii of H2, Cl2, and O2 evolution using microelectrodes. They have proposed that a thin liquid film of electrolyte separates the gas and solid phases. Jennings et al. [63] have mentioned the existence of a mixed phase at the electrolyte/electrode interface composed of electrolyte and gas. Nevertheless, compared with fluorine bubbles, the shape of these three gas bubbles is completely different and their detachment easier. In the case of fluorine evolution, the intermediate conducting layer is supposed to be composed of a mixed phase comprising liquid KF–2HF and fluorine. Owing to this model, we are also able to propose an explanation for the origin of the high anodic overvoltage: it is due to the C–F film and to the intermediate conducting layer composed of a mixture of KF–2HF and nascent fluorine, giving rise to ηC–F (activation overvoltage for the FER) and ηfluid (ohmic drop in the conducting layer), respectively; ηT  ηC–F  ηfluid. For instance, it has been shown that, for a resistivity of the fluidised layer ρ ⬇ 10 Ω cm, a temperature of the melt θ  95°C, and a radius of electrode r0  0.8 cm, ηfluid is close to 1 V, comparable with ηC–F ⬇ 1.5 V. Therefore, the contribution of the “fluidised” layer must be taken into account for a good understanding of the fluorine evolution process.

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

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Let us now consider the mechanism of bubble growth and detachment to understand the origin of the shape of the F2 bubble formed on the horizontal surface facing upward (Fig. 9a). Visual examination [55,56] shows that a few small gas bubbles remain at the electrode surface just after the detachment of a fluorine bubble. These bubbles very rapidly coalesce, and a unique bubble grows and covers the electrode surface. Then, the bubble swells. A bulge appears in its centre and gives rise to a spherical bubble, which detaches through the electrolyte. A quantitative analysis is carried out considering that the current arising from the uncovered part of the electrode is calculated from the current density. The current arising from the conducting layer beneath the bubble is calculated as described in a previous paper [55]. The areas of the covered and uncovered surfaces depend on the gas volume and on the shape of the bubble. The amount of gas is readily calculated from the electrolysis current. At first, we have assumed that the curvature radius, R, at the gas–liquid interface is constant. Consequently, the bubble shape is a spherical cap. In situ observations have shown that the bubble is not exactly a spherical cap: the periphery of the bubble is flattened and the adherent bubble has the shape of a flying saucer (Fig. 9a). Therefore, to obtain a more realistic bubble shape, the model has been improved by considering that such a bubble shape is due to the presence of capillary forces between the electrode surface and the gas–liquid interface [64]. The increment of pressure, pint, due to the interface curvature, R, obeys the following equation [65]:

γ GL pint  ppub pext  2  R

(9)

where pbub is the pressure inside the bubble, pext the external pressure and γGL the gas–liquid surface tension. A phenomenological approach is used taking into account a simplified assumption: the capillary pressure at the gas–liquid interface is supposed to be proportional to the reverse of distance, hcap (Fig. 17): K pcap   hcap

(10)

where hcap  h  δ. The thickness, δ, of the conducting layer has been estimated from in situ observations [66] coupled with the exploitation of impedance diagrams [55]; δ  0.3 mm. K is the proportionality constant, which is homogenous to surface tension. In order to obtain an accurate description of the phenomenon, the hydrostatic pressure, phyd, was also introduced into the model phyd  hhyd dg

(11)

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Fig. 17. Scheme for the pressure balance at the gas–liquid interface.

where d is the density of the electrolyte and g the acceleration due to gravity. The distance, hhyd, to the electrolyte surface is determined from the distance, hsur, between the surface of the conductive layer and electrolyte surface: hhyd  hsur  h. The external pressure is pext  patm  phyd

(12)

where patm is the atmospheric pressure. The pressure inside the bubble obeys the equation pbub  patm  phyd  pcap  pint

(13)

In the present situation, R varies all along the gas–liquid interface. At a distance close to the electrode, the term pcap is large and the value of pint can be negative, which corresponds to a concave curve. In this case, according to Eq. (9), R is negative. At time t the volume of the bubble obeys nF2 RT V  pbub

(14)

with nF being the amount of gas generated by the electrolysis (in mol). 2 However, the gas bubble no longer has the shape of a spherical cap. To solve the problem, a step-by-step process was used. The interface profile was divided into small distance intervals, Δs. The coordinates of a point i1 were deduced from the coordinates xi and hi of the point i. At point i, the curvature radius, Ri, was determined from the internal pressure, pint (Eq. (13)).

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

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As an example, the profile for a bubble containing 1.8 μmol of fluorine is shown in Fig. 18a. The following parameters were used: patm  1 atm, hsur  1 cm, d  1.98 g cm3, g  981 cm s2, γGL  750 dyn cm1, K  740 dyn cm1 and pbub  1.0086 atm. Then, the bubble growth during electrolysis was studied using a stepby-step procedure. The time was divided into small time intervals, Δt, and the current at time t was deduced from the current arising from the covered and uncovered areas of the electrodes [64]. However, when the radius of the covered area, rcov, tends to the electrode radius, rD, the current still passes in the form of a very thin layer at the periphery of the bubble. The additional resistance, Radd, depends on the thickness of the layer and is assumed to be proportional to tan θ1. The angle θ1 is the value of θi in the external ring and is equal to the contact angle. In the model [64], the following empirical equation is used:

λρ Radd   tan θ1 2πr

(15)

where λ is a proportionality constant. Since the electrode surface is fully covered, the fluorine gas generated by the electrolysis induces a swelling of the bubble. The pressure inside the bubble and the contact angle increase, so that the calculated volume of the bubble fits the experimental volume deduced from the amount of gas. The bubble growth leads to the formation of a spherical excrescence, which soon detaches from the electrode surface under the action of the hydrostatic pressure. During that last phase, due to the formation of the evolving bubble, the pressure remains nearly constant. It results in a current plateau just before the bubble detachment.

1

d

h / cm

c 0.5

b a

0 0.3

0.6

0.9

1.2

x / cm

Fig. 18. Calculated profile for bubbles of various volumes: (a) 55 mm3 , (b) 220 mm3, (c) 460 mm3, (d) 700 mm3.

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The various parameters introduced in the model are adjusted in order to obtain simulated curves, which fit the experimental curves. The shape of the fluorine bubble at various stages of electrolysis is shown in Fig. 18. In contrast to the flat bubble obtained at the beginning of the growth (Fig. 18a), the bubble height just before the detachment (Fig. 18d) is now greater than the radius of the contact disk (bubble electrode). 3. CONCLUSIONS The uses of fluorine gas in various industrial fields confer an attractive aspect to this compound. In this work, a brief overview of the FER in molten KF–2HF is given in considering both a theoretical and experimental approach. First, the simulation of KF–nHF using molecular dynamics was presented to identify the species present in the electrolyte depending on the HF/KF ratio and the temperature of the melt. It has been shown that, under our experimental conditions (composition: KF–2HF; θ  95°C), the major species are presum-ably F(HF)2. Nevertheless, direct experimental evidence to confirm the validity of the model and to provide an deeper understanding of the structure of this electrolyte is welcome. Further comparisons with similar systems (such as EMIMF-2.3HF) both on the theoretical and experimental levels are in progress. Then, AFM investigations coupled with local electrical measurements in ambient air on HOPG fluorinated in KF–2HF allowed us to validate previous assumptions on the presence of both conducting GICs and insulating CFx on the carbon surface. Owing to these observations, coupled with physicochemical characterisations by XRD, TEM and Raman spectroscopy, the origin of the strong adhesion of F2 bubble on carbon surface was elucidated. Graphitised phases randomly oriented and C–O bonds are present at their edges; these C–O bonds are easily changed during electrolysis of molten KF–2HF to form insulating and non-wetted CFx compounds (CF, CF2 and CF3 groups). One example for improving the kinetics of the fluorine evolution process is presented; it consists of the activation at very high potential in KF–2HF, the aim is to limit the generation of CFx to enhance the wettability of the electrode, thereby increasing the electroactive surface area. Finally, the particular shape of the fluorine bubble evolving on a horizontal electrode surface upward of the electrochemical cell was discussed; in that frame, an original approach was made in considering the characteristic features of the process: (i) strong adherence of fluorine gas at the electrode surface, (ii) existence of an electrolytic current even when the electrode is fully covered with the gas film, and (iii) flat bubble with a nearly null contact angle. The experimental observation pointed to the existence of a thin conducting layer beneath the gas bubble. The variable

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF

27

curvature radius of the bubble is the consequence of a skin effect due to the influence of capillary forces between the electrode surface and the gas–liquid interface. ACKNOWLEDGEMENTS The authors thank Pr. D. Devilliers, Drs. S. Durand-Vidal, F. Nicolas, B. Morel, J.-P. Caire, C. Belhomme, F. Houzé, R. Baddour-Hadjean, F. Warmon and MM. A. Colisson, M. Combel, and M. Vogler for helpful and fruitful discussions and assistance in the experimental work. We also acknowledge the Comurhex-Cogema Company (Pierrelatte, France) for their joint support of this research project. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

[14] [15] [16] [17] [18] [19]

A.J. Rudge, “Production of elemental fluorine by electrolysis” in Industrial Electrochemical Processes, A.T. Kuhn (Ed.), Amsterdam, 1971, pp. 1–69, Chap. 1. P.T. Hough, W.V. Childs, and T. Fuchigami (Eds.), The ECS Proceeding Series, Pennington, 1997, PV 97-15, p. 113. N. Watanabe, T. Nakajima, and H. Touhara, Graphite Fluorides, Vol. 8, Elsevier, Amsterdam, 1988, pp. 1–22, Chap. 1. T. Nakajima, Fluorine-Carbon, and Fluoride-Carbon Materials, T. Nakajima (Ed.), M. Dekker, New York, 1995, pp. 1–31, Chap. 1. H. Groult, D. Devilliers, and M. Vogler, Current Topics in Electrochemistry, Research Trends, Vol. 4, Poojapura, Trivandrum, India, 1997, pp. 23–39. H. Groult, J. Fluorine Chem., 119 (2003) 173. D. Devilliers, M. Chemla, and T. Nakajima (Ed.), Fluorine-Carbon and Fluoride-Carbon Materials, M. Dekker, New York, 1995, pp. 283–331,Chap. 8. T. von Rosenvinge, M. Parrinello, and M.L. Klein, J. Chem. Phys., 107(19) (1997) 8012. M.P. Allen and D.J. Tildesley, Computer Simulation of Liquids, Oxford University Press, Oxford, 1987. C. Simon, T. Cartailler, and P. Turq, Phys.-Chem. Chem.-Phys., 3 (2001) 3119. D. Deraman, J.C. Dore, J.G. Powles, J.H. Holloway, and P. Chieux, Mol. Phys., 55(6) (1985) 1351. J.D. Forrester, M.E. Senko, A. Zalkin, and D.H. Templeton, Acta Crystallogr., 16 (1963) 58. I.G. Shenderovich, S.N. Smirnov, G.S. Denisov, V.A. Gindin, N.S. Golubev, A. Dunger, R. Reibke, S. Kirpekar, O.L. Malinka, and H.-H. Limbach, Ber. Bunsenges. Phys. Chem., 102 (1998) 422. R. Hagiwara, T. Hirashige, T. Tsuda, and Y. Ito, J. Electrochem. Soc., 149(1) (2002) D1. A. Tasaka, Y. Shodai, S. Kohara, and M. Inaba, Proceedings of Third French–Japanese Seminar on Fluorine in Inorganic Chemistry and Electrochemistry, Paris, 2003, p. 9. H. Dumont, S.Y. Qian, and B.E. Conway, J. Appl. Electrochem., 27 (1997) 267. F.G. Fumi and M.P. Tosi, J. Phys. Chem. Solids, 25 (1964) 31. M.L. Klein and I.R. McDonald, J. Chem. Phys., 71(1) (1979) 298. A.I. Semerikova and A.F. Alabyshev, Russian J. Phys. Chem., 35 (1961) 2791.

28 [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58]

H. Groult et al. C. Simon, T. Cartailler, and P. Turq, J. Chem. Phys., 117(8) (2002) 3772. O.R. Brown, Electrochim. Acta, 25 (1980) 367. H. Imoto, T. Nakajima, and N. Watanabe, Bull. Chem. Soc. Jpn., 48 (1975) 1633. L. Bai and B.E. Conway, J. Appl. Electrochem., 18 (1988) 839. D. Devilliers, F. Lantelme, and M. Chemla, J. Chim. Phys., 76 (1979) 428. H. Wendt and G. Kreysa, Electrochemical Engineering, Science and Technology in Chemical and Other Industries, Springer, Berlin, 1999, pp. 290–369. Chap. 11. D.M. Novak and P.T. Hough, J. Electroanal. Chem., 144 (1983) 121. O.R. Brown, B.M. Ikeda, and M.J. Wilmott, Electrochim. Acta, 32 (1987) 1163. H. Groult, D. Devilliers, M. Vogler, C. Hinnen, P. Marcus, and F. Nicolas, Electrochim. Acta, 38 (1993) 2413. L. Bai and B.E. Conway, J. Appl. Electrochem., 20 (1990) 916. P. Cadman, J.D. Scott, and J.M. Thomas, Carbon, 15 (1977) 75. M. Chemla and D. Devilliers, J. Electrochem. Soc., 136 (1989) 87. H. Groult, D. Devilliers, S. Durand-Vidal, F. Nicolas, and M. Combel, Electrochim. Acta, 44 (1999) 2793. F. Houzé, R. Meyer, O. Schneegans, and L. Boyer, Appl. Phys. Lett., 6(13) (1996) 1975. J.P. Kleider, C. Longeaud, R. Brüggemann, and F. Houzé, Thin Solid Films, 383 (2001) 57. S. Guessab, L. Boyer, F. Houzé, S. Noël, and O. Schneegans, Synth. Metals, 118 (2001) 121. N. Watanabe, T. Nakajima, and H. Touhara, Graphite Fluorides, Vol. 8, Elsevier, Amsterdam, 1988, pp. 240–261, Chap. 8. T. Mallouk and N. Bartlett, J. Chem. Soc. Chem. Commun., (3) (1983) 103. R. Hagiwara, M. Lerner, and N. Bartlett, J. Chem. Soc. Chem. Commun., (9) (1989) 573. Y. Sato, T. Kume, R. Hagiwara, and Y. Ito, Carbon, 41 (2003) 351. A. Hamwi, M. Daoud, and J.C. Cousseins, Synth. Metals, 30 (1989) 23. R. Yazami, P. Hany, P. Masset, and A. Hamwi, Mol. Cryst. Liq. Cryst., 310 (1998) 397. T. Nakajima, Y. Matsuo, B. Cemva, and A. Jesih, Carbon, 34 (1996) 1595. A. Tressaud, F. Moguet, S. Flandrois, M. Chambon, G. Guimon, G. Nanse, E. Papirer, V. Gupta, and O.P. Bahl, J. Phys. Chem. Solids, 57(6–8) (1996) 745. T. Nakajima and N. Watanabe, Graphite Fluorides and Carbon-Fluorine Compounds, CRC Press, Boca Raton, 1991, pp. 155–171, Chap. 7. T. Nakajima and M. Touma, J. Fluorine Chem., 57 (1992) 83. F. Tunistra and J.L. Koenig, J. Chem. Phys., 53 (1970) 1126. D.S. Knight and W.B. White, J. Mater. Res., 4 (1989) 385. T. Nakajima, T. Ogawa, and N. Watanabe, J. Electrochem. Soc., 134 (1987) 8. D. Devilliers, B. Teisseyre, and M. Chemla, Electrochim. Acta, 35 (1990) 153. P.T. Hough and D.M. Novak-Antoniou, US Patent 4 602 985, 1986. N. Watanabe, M. Inoue, and S. Yoshizawa, J. Electrochem. Soc. Jpn., 31 (1963) 113. T. Tojo and T. Nakajima (Ed.), Fluorine-Carbon and Fluoride-Carbon Materials, M. Dekker, New York, 1995, pp. 333–354, Chap. 9. Asahi Glass Co., JP-Kokai Patent 58 81 981, 1983. O.R. Brown and M.J. Wilmott, European Patent 255 225, 1988. H. Groult, D. Devilliers, F. Lantelme, J.–P. Caire, F. Nicolas, and M. Combel, J. Electrochem. Soc., 149 (2002) E485. H. Groult and F. Lantelme, J. Electrochem. Soc., 148 (2001) E13. F. Nicolas, H. Groult, D. Devilliers, and M. Chemla, Electrochim. Acta, 41 (1996) 911. S. Lubetkin, Electrochim. Acta, 48 (2002) 357.

Experimental and theoretical aspects of the fluorine evolution reaction on carbon anodes in molten KF–2HF [59] [60] [61] [62] [63]

29

H. Vogt, Electrochim. Acta, 42 (1997) 2695. F. Lantelme, D. Diamanacos, and J. Chevalet, Electrochim. Acta, 23 (1978) 717. G. Kreysa and M. Kuhn, J. Appl. Electrochem., 15 (1985) 517. N.P. Brandon and G.H. Kelsall, J. Appl. Electrochem., 5 (1984) 475. D. Jennings, A.T. Kuhn, J. Stepanek, and R. Whitehead, Electrochim. Acta, 20 (1975) 903. [64] F. Lantelme and H. Groult, J. Electrochem. Soc., 151(5) (2004) D121. [65] J.T. Davies and E.K. Rideal, Interfacial Phenomena, Academic Press, San Diego, 1961. [65] H. Roustan, Ph.D. Thesis, ENSEEG-INPG, Saint Martin d’Hères, France, 1998.

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Fluorinated Materials for Energy Conversion T Nakajima and H Groult (Editors) © 2005 Elsevier Ltd. All rights reserved.

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Chapter 2

Applications of fluorinated carbon materials to primary and secondary lithium batteries Tsuyoshi Nakajima Department of Applied Chemistry, Aichi Institute of Technology, Yakusa-cho, Toyota-shi 470-0392, Japan E-mail: [email protected] 1. INTRODUCTION Reaction of carbon materials with elemental fluorine yields two kinds of intercalation compounds: graphite fluorides, (CF)n and (C2F)n, and fluorine – graphite intercalation compound, Cx F [1–4]. Graphite fluorides with puckered (sp3) graphene layers to which fluorine atoms are covalently bonded are synthesized by the fluorination of various carbon materials at high temperatures of 300 to 600°C. For example, (CF)n is prepared from petroleum coke at 300–600°C and from natural or synthetic graphite at ca. 600°C. (C2F)n is obtained from high crystalline graphite in a limited temperature range of 350–400°C. Fluorination of a graphite between 400°C and ca. 550°C yields a mixture of (CF)n and (C2F)n. In a lowtemperature range less than ca. 100°C, fluorine – graphite intercalation compound, Cx F is synthesized in the presence of Lewis acid(s) such as HF. Cx F has planar (sp2) graphene layers with ionic or semi-ionic (semi-covalent) C–F bond. Synthesis of Cx F is usually performed under a fluorine atmosphere in the presence of Lewis acid(s) or in anhydrous liquid HF (aHF) with elemental fluorine. In most cases, Cx F is prepared at room temperature. (C2F)n and (CF)n are black and graywhitish in color, respectively, and both are electric insulators because of their C–F covalent bonds. The black color of (C2F)n may be due to a trace amount of Cx F type sp2 carbon. On the other hand, Cx F is black and an electric conductor because intercalated fluorine atoms are mobile at stage 2 or higher stages and somewhat mobile even at stage 1 in which semi-ionic (semi-covalent) C–F bond exists. Graphite fluoride, (CF)n, was used as the cathode material of the primary lithium battery, as solid lubricant, as water repellent and so on. Among them, the most important application is the use as a cathode material in primary lithium battery with metallic lithium anode and organic solvents [1,2]. Li/(CF)n primary

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Tsuyoshi Nakajima

battery has been practically used for many years. It has a high discharge voltage between 2 and 3 V, high discharge capacity (⬃800 mAh/g), high-energy density, high safety and long shelf life. Cx F is also a new candidate as cathode for primary lithium battery, having a higher discharge potential than that of (CF)n because fluorine atom semi-ionically bonded to graphene layer has a higher activity than covalently bonded one [2–10]. A disadvantage of Cx F is that its discharge capacity is less than that of (CF)n due to the lower fluorine content of Cx F. Cx F with the highest fluorine content (x ⬇ 2) is usually prepared in aHF with elemental fluorine [2,3,11–15] or using elemental fluorine in the presence of gaseous HF and IF5 [2–4,7–10]. Cx F with a higher fluorine content than C2F was recently synthesized by the fluorination of graphite using high oxidation state transition metal complex fluorides and elemental fluorine under pressure in aHF [16–18]. Recent research interest is mainly on the materials for secondary (rechargeable) lithium batteries, which use carbonaceous anodes, transition metal oxide cathodes and organic electrolyte solutions. Application of fluorination techniques and various fluorides to secondary lithium batteries is an interesting research subject. Polyvinylidenefluoride (PVDF) and LiPF6 are currently used as a binder and electrolyte for practical lithium ion secondary batteries, respectively. Imide and methide salts containing CF2/CF3 groups are new candidates as electrolytes, because of their high thermal and electrochemical stability although they cause corrosion of aluminum cathode current collector [19,20]. Light fluorination is one of the effective methods of surface modification for transition metal oxide cathodes [21] and carbonaceous anodes [22–27]. Surface treatment of lithium cobalt oxide by elemental fluorine increased the capacity and improved the cycleability [21]. Surface fluorination of natural graphite samples with different particle sizes increased the capacities without any decrease in the first coulombic efficiencies [22–25]. In case of petroleum cokes, first coulombic efficiencies of graphitized petroleum cokes were improved [26,27]. Corrosion of aluminum cathode current collector occurs in the solvents containing fluoro-organic electrolytes as mentioned above. The corrosion of aluminum is inhibited by fluorination [19,28]. The present chapter deals with recent results on the synthesis, structures and electrochemical behavior of highly fluorinated graphite as a cathode of primary lithium battery and those of surface-fluorinated graphites and petroleum cokes as anodes of secondary lithium battery. 2. INFLUENCE OF COINTERCALATED HF ON THE DISCHARGE BEHAVIOR OF HIGHLY FLUORINATED GRAPHITE AS A CATHODE OF PRIMARY LITHIUM BATTERY Fluorine – graphite intercalation compound, Cx F is usually synthesized by several different methods at room temperature in the presence of Lewis acid(s) [2–4]. Typical synthetic methods are a gas/solid reaction using graphite and

Applications of fluorinated carbon materials to primary and secondary lithium batteries

33

elemental fluorine in the presence of a small amount of gaseous HF [2–4,29] or in the presence of gaseous HF and IF5 [2–4,7–10], and a reaction of graphite with elemental fluorine in aHF [2–4,11–15]. The gas/solid reaction using graphite and elemental fluorine gives stage 1 and higher stage compounds with composition of ⬃C3F. The reaction of graphite with elemental fluorine in aHF or with HF and IF5 yields mainly stage 1 compounds with the higher fluorine contents of C4F–C2F. The reaction of graphite with high oxidation state transition metal complex fluoride in aHF also provides stage 1 Cx F with by-products being insoluble in aHF [30]. Stage 1 compounds with higher fluorine contents than C2F are synthesized by the reaction of graphite with high oxidation transition metal complex fluoride and elemental fluorine under pressure in aHF at room temperature [16–18]. In the present section, the synthesis, structure and discharge behavior of highly fluorinated stage 1 Cx F compounds are described [16–18]. 2.1. Synthesis and structure of highly fluorinated graphite, Cx F

When Cx F is synthesized in aHF, some amount of HF is cointercalated into graphite because the fluorination reaction proceeds via CxHF2, which is formed at the beginning of intercalation reaction of fluorine into graphite in aHF (Eq. (1), step 1). Mobile HF2  easily diffuses into graphite, and HF is gradually desolvated along with the formation of semi-ionic C–F bond at stage 1 (Eq. (1), step 2). Cointercalated HF thus remains in graphene layers after the formation of semi-ionic C–F bond. HF remaining in graphene layers is removed from Cx F by pumping. However, the complete removal of HF is usually difficult (Eq. (2)). Stage 1 Cx F sometimes contains stage 2 and 3 phases as minor components, where C–F bond is nearly ionic and the main intercalated species are HF2 [2–4,11–15]: xC  1/2F2  yHF → CxHF2(HF)(y1) → Cx F(HF)y

(1)

Cx F(HF)y → Cx F(HF)z  (yz)HF (by pumping)

(2)

Highly fluorinated graphite, Cx F, was synthesized by the reaction of graphite with high oxidation state transition metal complex fluoride (K2PdF6, K2MnF6, K2NiF6 or KAgF4) and elemental fluorine under pressure ((3.9–11.8)  105 Pa) in aHF at room temperature [16–18]. The raw products contained by-products such as PdF2/PdF3, MnF2/MnF3, NiF2 or AgF2 insoluble in aHF. PdF2/PdF3 or MnF2/MnF3 were removed from Cx F by adding KF, fresh HF and elemental fluorine to the raw product and exposing the mixture to UV light for 2 days and washing away as K2MF6 (M, Pd or Mn) soluble in aHF with an excess of KF in the form of yellow solution [18]. Solid by-product, NiF2 or AgF2, was separated from Cx F by adding aHF and AsF5 to the raw product and washing off as M(AsF6)2 (M: Ni or Ag), soluble in aHF in the form of yellow solution [16].

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Tsuyoshi Nakajima

Composition and X-ray diffraction data of Cx F samples are summarized in Table 1 [16–18]. The C/F ratio was in the range of 3.7–1.2, decreasing with increasing fluorine pressure from 3.9  105 to 11.8  105 Pa, and it was usually the lowest by the use of KAgF4 as a fluorinating agent. Hydrogen (H) was Table 1 X-ray diffraction data and composition of Cx F samples prepared by high oxidation state complex fluorides and elemental fluorine in anhydrous HF at room temperature Sample

Composition

F2

Fluoride

5

X-ray diffraction data a

(x10 Pa)

Stage

Ic (nm)

a0 (nm)

1

C3.7F(HF)1.2

5.9

K2PdF6

1

0.629

0.245

2

C2.6F(HF)0.68

5.9

K2PdF6

1 (2)

0.582 0.925

0.246

3

C2.7F(HF)0.41

7.9

K2MnF6

1

0.599,0.571

0.245

4

C2.9F(HF)0.65

7.9

K2MnF6

1

0.597

0.246

5

C2.3F(HF)0.31

3.9

K2NiF6

1 2 3

0.634 0.929 1.236

0.245

1 (2) (3)

0.678 0.927 1.245

0.245

1 (2) (3)

0.644 0.933 1.231

0.246

1 (2) (3)

0.673 0.921 1.226

0.245

6

7

8

C1.7F(HF)0.12

C1.9F(HF)0.58

C1.5F

7.9

7.9

7.9

K2NiF6

K2NiF6

K2NiF6

9

C1.6F

11.8

K2NiF6

1

0.647

0.247

10

C1.6F

11.8

K2NiF6

1

0.680

0.247

11

C1.5F(HF)0.35

11.8

K2NiF6

1 (2) (3)

0.639 0.942 1.240

0.247

12

C1.2F

11.8

KAgF4

1

0.631

0.247

13

C1.3F

11.8

KAgF4

1

0.626

0.247

14

C1.4F

11.8

KAgF4

1

0.628

0.248

a

( ) represents minor phase.

Applications of fluorinated carbon materials to primary and secondary lithium batteries

35

detected in some samples by elemental analysis, mainly existing as HF in them. As shown by IR spectra later, all the samples contained small amounts of HF. Fluorination of graphite by elemental fluorine in aHF usually yields Cx F with a high fluorine content. The maximum C/F ratio is approximately 2 [11–15]. The use of high oxidation state transition metal complex fluoride and elemental fluorine under pressure provides highly fluorinated graphite with C/F ratio less than 2 although H was detected in some samples by elemental analysis. Among Cx F samples in Table 1, those prepared using KAgF4 (samples 12–14) had the highest fluorine contents. Their composition would be in the range of 1 x 2, except traces of HF cointercalated in them. All the Cx F samples were composed of stage 1, or stage 1 with stage 2 and 3 phases as minor components, as given in Table 1. The repeat distances along the c-axis (Ic) of the stage 1 phases were in the range 0.57–0.68 nm. Most of the Ic values were larger than 0.60 nm, as given in Table 1. The smallest Ic value of stage 1 Cx F was reported to be 0.47 nm for C6F [12], which means that semi-ionically bonded fluorine atoms form a single intercalated layer between two graphene layers due to the low in-plane density of fluorine. With increasing in-plane density of fluorine, the single fluorine-intercalated layer gradually changes to the double rows where intercalated fluorine atoms are in contact with each other between two graphene layers. The Ic of stage 1 thus increases to a value higher than 0.6 nm with increasing fluorine content. Cointercalation of HF into graphite also contributes to the increase in the Ic values. The lattice parameters along the a-axis (a0) were in the range of 0.245–0.248 nm. The a0 values were 0.245 nm for the Cx F samples containing stage 2 and 3 phases and stage 1 compounds with relatively low fluorine contents. Pure stage 1 compounds (samples 9, 10, and 12–14) and C1.1F (sample 11) prepared under high fluorine pressure (11.8  105 Pa) had greater a0 values of 0.247–0.248 nm than that of graphite (0.246 nm). The lattice parameter a0 is directly correlated with the carbon – carbon bond length, i.e. it is proportional to C–C bond length. When the carbon – fluorine bonding of Cx F is ionic or nearly ionic (usually stage 2 or higher stage), the C–C bond length is slightly shorter than that of graphite itself due to the electron transfer from graphite to intercalated fluorine [3]. This is usually observed in a bond length of an acceptor-type graphite intercalation compound. However, the C–C bond is longer than 0.1421 nm of graphite lattice in a highly fluorinated stage 1 Cx F due to an increase in the covalent nature of C–F bond [3]. The formation of semi-ionic C–F bond at stage 1 causes the localization of electrons and may slightly change the sp2 nature of graphene layers. It was thought that stage 1 Cx F with a composition of ⬃C2F kept planar sp2 graphene layers. However, the increase in the C–C bond length and partial puckering of graphene layers may occur in a highly fluorinated phase in which the fluorine content is C2F or higher than C2F (C2F–C1F). The nature of C–F bonding of Cx F samples prepared using K2NiF6 and KAgF4 was evaluated by X-ray photoelectron spectroscopy (XPS) [16]. The

36

Tsuyoshi Nakajima

peaks for C1s and F1s electrons were observed at high binding energies, probably due to the charging effect caused by electron localization. For Cx F samples prepared using K2NiF6, C1s spectra showed two strong peaks at 287.7 and 290.2 eV, a medium peak at 291.8 eV and a weak one at 294.0 eV on average. The C1s peak at 287.7 eV corresponds to carbon atom unbound to fluorine. The peak at 290.2 eV is attributed to nearly semi-ionic C–F bond. Those observed at 291.8 and 294.0 eV are covalent C–F bond and –CF2 group. Corresponding to these C1s peaks, two F1s peaks were observed at 689.2 and 690.9 eV, indicating nearly semi-ionic and covalent C–F bonds, respectively. The Cx F samples prepared using KAgF4 had two strong C1s peaks at 287.7 and 290.2 eV and a strong F1s peak at 689.2 eV. In addition to this peak, a very weak F1s peak was present at 686.4 eV, indicating nearly ionic C–F bond. The lack of C1s peaks at 291.8 and 294.0 eV and F1s peak at 690.9 eV shows that fluorination degree is weaker in the Cx F prepared using KAgF4 than in the Cx F prepared with K2NiF6, which coincides with the fact that K2NiF6 is a stronger fluorinating agent than KAgF4. Highly fluorinated stage 1 compounds consist of several different phases as shown by IR spectra (Fig. 1) [18]. Table 2 summarizes the assignment of IR absorption peaks, C–C bond of graphene layers and possible Cx F phases [18]. The absorptions observed at 1084 cm1 and between 1123 and 1134 cm1 are assigned to the stretching vibration of semi-ionic C–F bond [13], probably arising from stage 1 C3F and C2F phases with sp2–sp3 graphene layers, respectively, in comparison with the composition in Table 1. The absorptions at 1225 and 1230 cm1 are due to C–F stretching vibration of covalent C–F bond with sp3 hybridized orbital, being observed for graphite fluoride, (CF)n [31,32]. A new absorption at 1196 cm1 may be assigned to the stretching vibration of nearly Table 2 IR absorption peaks (cm1) and assignments for Cx F samples Samplea

Assignment

4

7

9

11

14

1084

–

–

–

–

1123 –

Semi-ionic C–F

Graphene

Possible

layer

Cx F phase

sp2–sp3 2

C3F

1125

1126

1131

1134

Semi-ionic C–F

sp –sp

C2F

–

–

1196

1196

Nearly covalent C–F

sp2–sp3

C1F

1225

1225

1230

1230

1257

1250

1257

1256

1256

sp2

1524

1524

1525

1524

1523

sp2

1570

1575

1570

–

–

Sample number is the same as given in Table 1.

Covalent C–F

A2u

sp

3

1225

a

3

sp2

(CF)n

Applications of fluorinated carbon materials to primary and secondary lithium batteries

37

Fig. 1. IR absorption spectra of Cx F samples. 4, 7, 9, 11 and 14 in the figure: sample numbers in Table 1.

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covalent C–F bond in somewhat puckered graphene layers with the composition of C1F because this absorption has a larger vibration energy than that of C2F. It was recently reported that the absorption observed in the range 1250–1257 cm1 were due to C–C stretching in sp2 graphene layers of Cx F containing HF [14]. This absorption disappeared and a new absorption appeared at 1220 cm1 when HF was completely removed from Cx F [14]. It suggests that complete desolvation of HF from Cx F gives rise to the puckering of graphene layers by shortening the C–F bond length, i.e. the formation of graphite fluoride, (CF)n, with puckered sp3 graphene layers. The absorptions between 1250 and 1257 cm1 thus show that all the Cx F samples in Table 1 contain trace amounts of HF. In addition to above absorptions, two absorptions were observed in the range of 1570–1575 and 1523–1525 cm1. The absorptions at 1570 and 1575 cm1 are located at slightly lower wave numbers than those of graphite, 1580 cm1, being attributed to A2u mode indicating C–C stretching of graphene layers [33,34]. Since the absorptions have slightly weaker vibration energies than those of graphite itself, they may be derived from Cx F phase with relatively lower fluorine content such as C3F–C2F. The absorptions at 1523–1525 cm1 possess more weaker vibration energies corresponding to longer C–C bond length in sp2 graphene layers, probably arising from Cx F phase with a higher fluorine content, x ⬇ 2. The Cx F with higher fluorine content than C2F was prepared by the fluorination using high oxidation state transition metal complex fluorides and elemental fluorine under pressure in aHF. The IR absorption data indicate that the Cx F samples in Table 1 are composed of several different phases. Since graphite is polycrystal, the fluorination degree may not be uniform in crystallites constituting a graphite particle. If C3F or C2F phase with planar graphene layers exists in the same crystallite with graphite fluoride (CF)n having puckered graphene layers, such a structure may be unstable due to a high structural strain. Graphite fluoride (CF)n and stage 1 Cx F are both stable compounds even at high temperatures [1–4,35]. It is suggested therefore that CxF and (CF)n with planar and puckered graphene layers consist of different crystallites from each other, although some transient states from sp2 to sp3 structure may exist. The lattice parameter (a0)values in Table 1 were obtained by X-ray diffraction as average values for several different phases. 2.2. Influence of cointercalated HF on the discharge behavior of highly fluorinated graphite, Cx F

Discharge curves of Cx F samples were obtained in 1 mol/dm3 LiClO4–propylene carbonate (PC) solution at 25°C. Figs. 2–4 show the discharge curves obtained at a current density of 10 mA/g [16,18]. The discharge potential and capacity are governed by fluorine content, the nature of C–F bond and the amount of HF cointercalated in graphite. The discharge potential was at 3.1–3.2 V vs Li/Li at first and moved to a plateau at ca. 1.5 V as shown in Fig. 2. Since the electrochemical reduction of PC occurs at around 1 V, two plateaus observed at

Applications of fluorinated carbon materials to primary and secondary lithium batteries

39

3.1–3.2 and 1.5 V indicate the reduction of two different fluorine species. Sample 11 in Fig. 2 seems to have a larger amount of HF than other Cx F samples in Figs. 2–4 because it had the larger amount of H and stage 2 and 3 phases. Sample 11 had a long plateau at about 1.5 V, finally approaching 1 V. The same and short plateaus were also observed in samples 5 and 6 in Fig. 3. Cx F samples contain the semi-ionic, covalent and ionic C–F bonds as already mentioned. The ionic fluorine, HF2 , is in a more reduced state as anion than the semi-ionic and covalent fluorine atoms. It suggests that the equilibrium potential of F(semi-ionic and covalent)/F couple is higher than that of HF2 /F. Therefore, the discharge potential

Fig. 2. Discharge curves of Cx F samples, obtained at 10 mA/g in 1 mol/dm3 LiClO4-PC at 25°C. 9 and 11 in the figure: sample numbers in Table 1.

Fig. 3. Discharge curves of Cx F samples, obtained at 10 mA/g in 1 mol/dm3 LiClO4-PC at 25°C. 5, 6, 8 and 10 in the figure: sample numbers in Table 1.

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Tsuyoshi Nakajima

at 3.1–3.2 V can be assigned to the reduction of semi-ionic and covalent fluorine atoms and that at 1.5 V to the reduction of ionic fluorine, HF2 . The Cx F samples having H and stage 2 and 3 phases contained larger amounts of HF than others, and IR absorption spectra indicated that HF was present even in the samples in which no H was detected by elemental analysis. Samples 5, 6, 8 and 11 with small amounts of HF and stage 2 and 3 phases showed gradual decrease in the potentials with discharge. Stage 1 samples 9 and 10 exhibited the similar discharge curves. The discharge capacities of samples 5, 6 and 8–11 were in the range 260 to 580 mAh/g at a cut-off potential of 1.5 V vs. Li/Li. Samples 2, 3 and 4 also showed similar discharge curves with capacities of 420, 620 and 570 mAh/g, respectively. When K2NiF6, K2MnF6 and K2PdF6 were used as fluorinating agents, most of the Cx F samples contained H and/or stage 2 and 3 phases, which suggests that relatively larger amounts of HF were cointercalated in these samples, compared with those prepared using KAgF4 (samples 12–14). The a0 values of samples 1–11 in Table 1 were in the range 0.245–0.247 nm, and most of them were 0.245 or 0.246 nm, which also suggests that these samples contain ionic species, HF2 . On the other hand, samples 12, 13 and 14 gave flat discharge potentials at 3.2 V until 400–450 mAh/g at a current density of 10 mA/g and decreased to 1 V as shown in Fig. 4. They had higher fluorine contents without stage 2 and 3 phases and larger a0 values of 0.247–0.248 nm, which suggests that the amounts of HF and HF2  in samples 12–14 were less than those in samples 1–11. The influence of cointercalated HF was more clearly observed on the discharge at high current densities as shown in Fig. 5 [18]. The discharge capacity of Cx F containing a higher amount of HF was largely decreased at a high current density. The discharge capacity of sample 8 was significantly decreased from

Fig. 4. Discharge curves of Cx F samples, obtained at 10 mA/g in 1 mol/dm3 LiClO4-PC at 25°C. 12–14 in the figure: sample numbers in Table 1.

Applications of fluorinated carbon materials to primary and secondary lithium batteries

41

Fig. 5. Discharge curves of Cx F samples, obtained at 35 mA/g (sample 8) and 40 mA/g (sample 14) in 1 mol/dm3 LiClO4-PC at 25°C. 8 and 14 in the figure: sample numbers in Table 1.

460 mAh/g at a current density of 10 mA/g to 275 mAh/g by the discharge at 35 mA/g, while that of sample 14 was slightly reduced from 515 mAh/g at 10 mA/g to 500 mAh/g at 40 mA/g at the cut-off potential of 1.5 V. The discharge capacities of samples 8–10, obtained at a low current density of 10 mA/g and at a cut-off potential of 1.0 V, reached 93–96% of their theoretical values calculated from the composition. This shows that cointercalation of HF in Cx F causes the decrease in the discharge capacities at both low and high current densities (10 and 35–40 mA/g, respectively). The discharge potential and capacity are affected by several factors such as fluorine content, nature of C–F bond and the amount of cointercalated HF. Cx F with a high fluorine content generally shows a large discharge capacity. The Cx F samples in Table 1 possessed three kinds of C–F bonds, i.e. semi-ionic, covalent and nearly ionic C–F bonds with composition between C2F and C1F. Discharge reaction of graphite fluoride with covalent C–F bond proceeds with the formation of an intermediate discharge product composed

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Tsuyoshi Nakajima

of C, F, Li and solvent molecules, which is metastable, determining an open-circuit voltage (OCV) of Li/(CF)n cell [1,2,36,37]. The intermediate discharge product finally decomposes to carbon (C), LiF and solvent molecules (S). (CF)n  nLi·S  ne→ nC···F···Li···S → nC  nLiF  nS

(3)

The discharge reaction would be basically similar to that of graphite fluoride; however, the reaction rate of discharge changes depending on whether HF coexists or not: Cx F  Li·S  e → Cx···F···Li···S → x C  LiF  S (fast)

(4)

Cx F  HF  Li·S  e → Cx···F···HF···Li···S → x C  LiHF2  S (slow) (5) CxδHF2δ  Li·S  e → Cx···F···HF···Li···S → x C  LiHF2  S (slow) (6) The discharge capacities at the cut-off potential of 1.5 V vs. Li/Li strongly depended on the amounts of cointercalated HF and coexistence of stage 2 and 3 phases. The C–F bonding in stage 2 and 3 phases is nearly ionic, and the main intercalated fluorine species are mobile HF2δ. All the samples in Table 1 contained small amounts of HF as shown by IR absorption spectra even in case that H was not detected by elemental analysis. The cointercalation of HF in stage 1 phase also suggests the coexistence of a trace of HF2δ. The discharge capacity was larger in stage 1 Cx F samples containing smaller amount of HF without stage 2 and 3 phases as already shown. If HF does not coexist in Cx F, the intermediate discharge product, C x ···F···Li···S would smoothly decompose to carbon, LiF and solvent molecules because of a strong interaction between F and Li due to a large surface charge of a small fluoride ion (Eq. (4)). Therefore the discharge reaction may proceed well without potential decrease for Cx F containing a less amount of cointercalated HF. On the other hand, when HF is contained in Cx F, decomposition of the intermediate discharge product, C x···F···HF···Li···S, may be slow because of a weak interaction between HF2 and Li due to a small surface charge of a large HF2 anion (Eq. (5)). Slow decomposition of the intermediate discharge product would cause the gradual decrease in the discharge potential, leading to the reduction of the discharge capacity at the cut-off potential of 1.5 V. This is more clearly observed at a high current density. The discharge capacities of samples 8 and 10 were 460 and 423 mAh/g at a current density of 10 mA/g, decreasing to 275 and 175 mAh/g at 35 mA/g, respectively. Therefore, complete formation of stage 1 phase with semi-ionic or covalent C–F bond and thorough removal of HF from a prepared sample are needed to obtain a flat discharge potential and high capacity. For this purpose, KAgF4 seems the best fluorinating agent. Chemical diffusion coefficients of Li in the intermediate discharge product were obtained by the impedance measurements. For all samples, the diffusion

Applications of fluorinated carbon materials to primary and secondary lithium batteries

43

Table 3 Chemical diffusion coefficients of lithium ion F/C ratio

DLi (1012 cm2/s)

8

0.65

13

10

0.62

11

13

0.80

4.4

14

0.73

5.7

Samplea

a

Sample number is the same as given in Table 1.

coefficients were nearly constant irrespective of the discharge ratios. The data are given in Table 3 [18]. Samples 8 and 10 may have contained larger amounts of HF than samples 13 and 14, because sample 8 had stage 2 and 3 phases and discharge capacities of samples 8 and 10 at 35 mA/g (275 and 175 mAh/g, respectively) were much smaller than those of samples 13 and 14 at 40 mA/g (440 and 500 mAh/g, respectively), although the fluorine contents were somewhat higher in samples 13 and 14. Large HF2 anion would have a weaker interaction with Li cation than small F anion. Therefore HF2and Li would be mobile in the intermediate discharge product. This may be the reason why the larger diffusion coefficients were obtained for samples 8 and 10 than samples 13 and 14. 3. CHARGE/DISCHARGE BEHAVIOR OF SURFACE-FLUORINATED CARBON MATERIALS AS ANODES OF SECONDARY LITHIUM BATTERY Since electrochemical redox reactions occur at the surface of a solid electrode, surface structure is one of the decisive factors for determining the electrode performance. Surface modification is effective for improving the electrochemical characteristics of carbonaceous electrodes for lithium ion secondary battery. Electrode characteristics are governed by crystallinity, surface area, surface pore volume distribution, surface chemical species such as oxygen and so on. Several methods were applied to improve electrochemical behavior of carbon materials. They are surface oxidation [39–41], surface fluorination [22–27], thin metal coating [42], and carbon coating [43–47]. Light oxidation of carbon materials caused increase in their capacities by forming nanochannels at the surface while strong oxidation degraded surface structure, leading to increase in the irreversible capacity [39–41]. Carbon coating is an effective method for increasing the capacity and first coulombic efficiency [43–47]. In the present section, the effect of surface fluorination of natural graphite [22–25] and petroleum coke [26,27] on their electrochemical characteristics is described.

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3.1. Electrochemical behavior of surface-fluorinated natural graphite 3.1.1. Composition and surface structure change of natural graphite by fluorination

Natural graphite samples with average particle sizes of 7, 25 and 40 μm (abbreviated NG7, NG25 and NG40, respectively) were used as starting materials. Surface fluorination was performed between 150 and 500°C by elemental fluorine (purity: 99.4–99.7%) of 3  104 Pa for 2 min in a nickel reactor [22,23,25]. The same natural graphite samples were also subjected to plasma fluorination using CF4 [24,25]. Tables 4 and 5 show fluorine contents obtained by elemental analysis and surface fluorine concentration by XPS, respectively [22–25]. The fluorine contents in the samples fluorinated between 150 and 350°C were less than 1 at%, i.e. 0.2–0.6 at% while those fluorinated between 350 and 500°C had the larger values, i.e. 0.6–4.7 at%. The results coincide well with the fact that only the surface of graphite is fluorinated between 150 and 300°C [1–3]. Surface fluorine concentrations obtained by XPS showed the same trend, being in the range of 3.6–12.0 at% for the samples fluorinated between 150 and 350°C and in the range of 8.1–33.1 at% for those fluorinated between 350 and Table 4 Fluorine contents of surface-fluorinated graphite samples, obtained by elemental analysis Fluorination

Fluorine content (at%)

condition

NG7

NG25

NG40

Fluorinated by F2 150–300°C 350–500°C

0.5–0.6 0.6–2.2

0.3–0.4 0.6–4.7

0.2–0.4 1.7–4.2

Plasma-fluorinated

0

0.3

0.3

0 within detection limit ( 0.2 at%).

Table 5 Surface fluorine concentrations of fluorinated graphite samples, obtained by XPS Fluorination condition

Surface fluorine concentration (at%) NG7

NG25

NG40

Fluorinated by F2 150–300°C 350–500°C

6.0–10.4 11.3–14.6

4.5–6.9 8.1–28.9

3.6–12.0 14.3–33.1

Plasma-fluorinated

6.7–8.8

7.1–11.5

3.3

Applications of fluorinated carbon materials to primary and secondary lithium batteries

45

500°C. Fluorine content and surface fluorine concentration were both increased not only with increasing fluorination temperature but also with increasing particle size of graphite, i.e. with decreasing surface area of graphite. On the other hand, fluorine contents were lower in plasma fluorination than the fluorination by elemental fluorine, i.e. ⬃0.3 at%. Plasma-fluorinated samples exhibited rather constant surface fluorine concentrations, less than 10 at% in most cases. Small amounts of surface oxygen were detected by XPS, i.e. 1.0–3.7 at% for natural graphite samples fluorinated between 150 and 500°C, and 1.0–2.8 at% for plasma-fluorinated samples. Table 6 shows the BET surface areas, indicating the significant increase in the surface areas by the fluorination using elemental fluorine at 250 and 350°C and plasma fluorination using CF4 [23–25]. The increments in surface areas were ⬃77% and ⬃55% for the samples fluorinated by elemental fluorine and plasma treatment, respectively. The surface pore volume distribution was also changed by surface fluorination. Fluorination reduced the mesopores with diameters greater than 2–3 nm, and increased those with diameters of 1.5–2 and 2–3 nm. These surface structure changes may have been caused by carbon–carbon bond breaking due to the strong fluorination reactions. Fluorination of natural graphite powder accompanying carbon–carbon bond breaking also induces the increase in the surface structural disorder, which is detected by Raman spectroscopy. Carbon materials exhibit two Raman shifts at 1580 and 1360 cm1. The Raman shift observed at 1580 cm1 is based on the in-plane stretching vibration derived from graphitic structure (E2g2 mode, Gband), and that at 1360 cm1 indicates the A1g vibration mode due to the disordered structure and/or edge of carbon particles (D- band) [33,34]. Surface fluorination enhanced the D- band intensity in any case. The intensity ratio of two Raman shifts, R (ID/IG), shows the degree of surface disordering of carbon Table 6 Surface areas of natural graphite and surface-fluorinated samples, obtained by BET method Surface area (m2/g)

Fluorination condition

NG7

NG25

NG40

Original graphite

4.79

3.71

2.94

Fluorinated by F2 150°C 250°C 350°C

5.61 7.65 8.48

5.16 5.18 6.09

3.50 4.90 4.95

Plasma-fluorinated (60 min)

7.42

4.71

3.69

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Tsuyoshi Nakajima

materials. The R values calculated from the intensity ratios of two Raman shifts are given in Table 7, which indicate that R values were increased by surface fluorination, in particular, with increasing fluorination temperature and particle size of graphite powder [22–25]. Plasma fluorination also increased the R values; however, the values decreased with increasing duration of plasma treatment. This means that the surface-disordered parts are somewhat eliminated by the extended plasma treatment. Fig. 6 shows transmission electron microscopic images of graphite samples fluorinated by elemental fluorine [23,25]. The fluorinated layers exhibit disordered structures in contrast with the unreacted graphene layers. The thickness of fluorinated basal plane is about 3–4 nm, corresponding to 5–7 fluorine-intercalated layers [1–4]. The surface of edge plane is also disordered with a similar thickness. The nature of C–F bonding is evaluated by XPS. Graphite samples fluorinated by elemental fluorine showed C1s and F1s peaks, indicating C–F bond at 288.5 and 687.7 eV on average, respectively. These binding energies indicate that the C–F bonding of surface-fluorinated graphite samples is in an intermediate state between the semi-ionic and covalent bonds [22–25]. Weakly shifted C1s peaks were also observed at 291.0 eV indicating small amounts of covalent C–F bonds. For plasma-fluorinated samples, similar C1s and F1s peaks were observed at 288.1 and 687.6 eV on average, respectively, with weakly shifted C1s peaks at Table 7 R (ID /IG) values calculated from Raman shifts of surface-fluorinated graphite samples Fluorination

R value

condition

NG7

NG25

NG40

Original graphite

0.083

0.082

0.080

Fluorinated by F2 at 150°C 200°C 250°C 300°C 350°C 400°C 450°C 500°C

0.18 – 0.23 – – 0.25 – 0.29

0.15 – 0.19 0.18 – 0.40 0.70 0.56

0.13 0.14 0.16 0.23 0.45 0.74 0.55 0.73

Plasma-fluorinated for 60 min 100 min 140 min 180 min

0.20 0.14 0.13 0.13

0.15 – – –

0.14 – – –

Applications of fluorinated carbon materials to primary and secondary lithium batteries

47

Fig. 6. Transmission electron micrographs of surface-fluorinated graphite samples: (a) NG25 fluorinated by F2 at 250°C; (b) NG40 fluorinated by F2 at 300°C.

291.1 eV, indicating covalent C–F bonds. Thus the nature of C–F bonding is the same in graphite samples fluorinated by elemental fluorine and plasma treatment. 3.1.2. Charge/discharge characteristics of surface-fluorinated natural graphite

The profile of charge/discharge curves of surface-fluorinated samples is similar to that for non-fluorinated graphite as shown in Fig. 7, which shows the charge/discharge curves for original NG25 and that fluorinated at 250°C [23]. The charge/discharge characteristics were obtained at a current density of 60 mA/g between 0 and 3 V vs. Li/Li in 1 mol/dm3 LiClO4–ethylene carbonate (EC) / diethyl carbonate (DEC) (1:1 in volume) at 25°C [23,25]. The small plateau at 0.6–0.7 V in the first reduction curve is ascribed to the reduction of EC and subsequent formation of a thin surface film on graphite (solid electrolyte interface, SEI) [48]. The potential plateau indicating the decomposition of EC was no longer observed from the second cycle in any case. The surface-fluorinated NG25 exhibited higher capacity than original NG25 without any change in

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Tsuyoshi Nakajima

Fig. 7. Charge/discharge curves for (a) original NG25 and (b) NG25 fluorinated at 250 °C. —— : 1st cycle, ······· : 5th cycle, ----- : 10th cycle.

the profile of charge/discharge curves. NG7 and NG40 also showed the similar charge/discharge curves. The discharge capacities of surface-fluorinated NG7 samples were in the range 376–383 mAh/g at 60 mA/g, while original NG7 showed 360–363 mAh/g [22]. The discharge capacities of original and surface-fluorinated NG25 samples are shown in Fig. 8 as a function of cycle number [23]. NG25 had discharge capacities of 350–353 mAh/g smaller than those of NG7. As shown in Fig. 8, NG25 samples fluorinated between 150 and 400°C exhibited higher capacities than original NG25. Among them, the samples fluorinated between 200 and 300°C gave the highest capacities of 387–389 mAh/g, and those fluorinated at 150 and 350°C also provided high capacities of 382–384 mAh/g. The increments of the discharge capacities obtained at 10th cycle were ⬃10%, which is larger than ⬃5% for surface-fluorinated NG7. The results were similar in the case of NG40. The discharge capacities of original NG40 were 330–335 mAh/g smaller than those of NG7 and NG25. The NG40 samples fluorinated between 150 and 300°C showed high capacities of 374–377 mAh/g. The discharge capacities were

Applications of fluorinated carbon materials to primary and secondary lithium batteries

49

390

Capacity (mAh/g)

380

370

360

350

340 0

2

4

6 Cycle number

8

10

Fig. 8. Discharge capacities of original NG25 and surface-fluorinated NG25 samples as a function of cycle number. (䉬) NG25, (䊏)150°C, (䉱) 200°C, () 250°C, (䉭) 300°C, (䊉) 350°C, (䊊) 400°C, (ⵧ) 450°C, (䉫) 500°C.

gradually decreased with increase in the fluorination temperature. The increments of the discharge capacities were ⬃13%. Since the theoretical capacity of graphite is 372 mAh/g, corresponding to LiC6, many fluorinated samples had higher capacities than the theoretical value: NG7 fluorinated between 150 and 500°C, NG25 between 150 and 400°C and NG40 fluorinated 150 and 300°C. The first coulombic efficiencies of NG7, NG25 and NG40 were 79.7, 85.6 and 85.1%, respectively. The samples fluorinated between 150 and 300°C showed the same first coulombic efficiencies as those of original graphites. It means that the irreversible capacities induced by surface fluorine are negligible for the samples fluorinated in this temperature range. The surface fluorine was reduced by ⬃40% in the course of electrode preparation [22]. This may be one of the reasons why the first coulombic efficiencies were not decreased for the samples fluorinated between 150 and 300°C. The first coulombic efficiencies were decreased with increasing fluorination temperature from 350 to 500°C, i.e. with increasing fluorine content. Plasma-fluorinated graphite sample had the same potential profile as that of original graphite. Figs. 9 and 10 show the discharge capacities of plasmafluorinated NG7 and NG25 as a function of cycle number [24]. The discharge capacities of plasma-fluorinated samples were highly dependent on the duration of plasma fluorination (Fig. 9). The 30-min-fluorinated NG7 showed 370 mAh/g higher than that of original NG7. The 60-min-fluorinated sample gave the highest

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Tsuyoshi Nakajima

Capacity (mAh/g)

390

380

370

360

350

3

1

5

7

9

Cycle number

Fig. 9. Discharge capacities of original NG7 and plasma-fluorinated NG7 samples as a function of cycle number. (䉫) NG7, (䉭) 30-min fluorinated, (䊉) 60-min fluorinated, 100-min fluorinated, (䊊)140-min fluorinated, *180-min fluorinated.

Capacity (mAh/g)

400

380

360

340

1

3

5 7 Cycle number

9

Fig. 10. Discharge capacities of original NG25 and plasma-fluorinated NG25 samples as a function of cycle number. (䉬)NG25, (䊉)60-min fluorinated at room temperature, (䉱) 60-min fluorinated at 90°C.

capacities, 382 mAh/g, which is larger than not only that of NG7 but also the theoretical capacity of graphite, 372 mAh/g. When the duration of plasma fluorination was further extended to 100–180 min, the discharge capacities decreased to the values comparable with that of NG7. The observed capacities are closely related to R values of Raman shifts given in Table 7. The plasma-fluorinated NG7 samples having the larger R values exhibited higher capacities. The decrease in the R value with increase in the duration of fluorination probably arose from the elimination of surface-disordered layers by extended plasma treatment. With the increase in the particle size, the capacities of NG25 and NG40 were reduced

Applications of fluorinated carbon materials to primary and secondary lithium batteries

51

because the surface area decreases with increasing particle size. The effect of surface fluorination was more distinct as the particle size increases. The discharge capacities increased to 388 and 381 mAh/g for plasma-fluorinated NG25 and NG40, respectively. The increments in the discharge capacities by plasma fluorination were 5, 10 and 15% for NG7, NG25 and NG40, respectively. It was also found that first coulombic efficiencies were the same as those of original graphites, or slightly higher in some cases. Surface fluorination by elemental fluorine and plasma treatment both increased the discharge capacities of graphite samples to the higher value than theoretical capacity of graphite, 372 mAh/g. Surface fluorination increased the surface areas (Table 6) and mesopores with diameters of 1.5–2 and 2–3 nm. These surface structure changes would enhance the reaction kinetics and make possible the preservation of some excess lithium in surface mesopores. 3.2. Electrochemical behavior of surface-fluorinated petroleum coke 3.2.1. Composition and surface structure change of petroleum coke by fluorination

Starting materials were petroleum coke and those heat-treated at 1860, 2300 and 2800°C (abbreviated as PC, PC1860, PC2300 and PC2800C, respectively). The d spacings of (002) diffraction lines were 0.3450, 0.3385, 0.3366 and 0.3361 nm for PC, PC1860, PC2300 and PC2800, respectively. The d values were decreased with increasing heat-treatment (graphitization) temperature, indicating the increase in the crystallinity. However, all the d values were larger than that of natural graphite, 0.3354 nm. This means that the crystallinity of four petroleum coke samples is lower than that of natural graphite. Surface fluorination of petroleum coke was performed at 150, 200 and 300°C by 3  104 Pa elemental fluorine (purity: 99.4–99.7%) for 2 min, using a nickel reactor [26,27]. Table 8 shows composition obtained by elemental analysis and surface composition by XPS [26,27]. Fluorine contents were relatively higher in PC, in particular in PC fluorinated at 300°C. Smaller amounts of fluorine (0.3–0.6 at%) were detected for PC1860, PC2300 and PC2800 fluorinated at 300°C. Fluorine contents in the heat-treated petroleum cokes fluorinated at 150 and 200°C were in the detection limit ( 0.2 at%). Surface fluorine concentration had similar dependence on the heat-treatment temperature to that obtained by elemental analysis, decreasing from 50.2 at% to 5.2 at% with the increase in the graphitization temperature. Hydrogen (2.9 at% in Table 8) was detected in original PC by elemental analysis, which means that PC contained hydroxyl and carboxyl groups. PC and heat-treated petroleum cokes had the larger amounts of surface oxygen, 2.6–8.2 at% than natural graphite samples, 1.5–2 at%. The surface oxygen significantly decreased by heat – treatment from 8.2 to 2.6 at% and fluorination further reduced the surface oxygen to 0.9–2.5 at%. BET surface areas of petroleum cokes and surface-fluorinated samples are summarized in Table 9 [26,27]. The surface area of PC was large, decreasing to

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Tsuyoshi Nakajima

Table 8 Composition (at%) obtained by elemental analysis (upper column) and surface concentration (at%) by XPS (lower column) for surface-fluorinated petroleum cokes Fluorination temperature

PC

PC1860

C

F

O

C

F

Original 150°C 200°C 300°C

94.8 96.7 96.5 90.4

2.9 a 0.9 1.2 7.3

(2.3) (2.4) (2.3) (2.3)

99.4 99.8 99.8 99.2

– 0.0 0.0 0.6

Original 300°C

91.8 48.8

– 50.2

8.2 1.0

95.7 76.7

– 22.4

Petroleum coke PC2300 O

C

F

(0.6) 100.0 (0.2) 100.0 (0.2) 100.0 (0.2) 99.6 4.3 0.9

96.4 85.8

– 0.0 0.0 0.3 – 11.7

PC2800 O

C

F

O

(0.0) 100.0 – (0.0) (0.0) 99.8 0.0 (0.2) (0.0) 99.8 0.0 (0.2) (0.1) 99.5 0.4 (0.1) 3.6 2.5

97.4 93.2

– 5.2

2.6 1.6

0 for F within detection limit (CF2 covalent C-F

600°C

Fluorination post-treatment temperarure

550°C

450°C

400°C

350°C

250°C

150°C

R A W CF(LT)

1000

1100

1200 Wavenumber (cm−1)

1300

1400

Fig. 9. FT–IR spectra of room-temperature graphite fluoride post-treated at different fluorination temperatures in F2 gas. The spectrum of conventional CF(HT) is added for comparison.

3.3. Progressive fluorination limiting the conformational defects

EPR spectroscopy gives additional information on the conformational order of the post-treated materials and allows one to differentiate these samples to the conventional graphite fluoride CF(HT). Fig. 10 displays the EPR spectra of low (raw and re-fluorinated up to 600°C) and high-temperature fluorinated graphites. The origin of the main broad line (denoted as line A) was identified as carbon dangling bonds having a localized spin. Such spin carriers have been proposed for other fluorinated carbons obtained under F2 atmosphere : starting from natural graphite [24], amorphous carbon thin film [39,40] or nanosized graphite fluorides [41].

Fluorine-intercalated graphite for lithium batteries

Line Lin A

383

L Line B

CF( CF(LT)

X2

100°C

X2

Fluorination post-treatment temperature

150°C 200°C

X2

250°C

X2

300°C

X 1.5

350°C

X4

400°C

X4

450°C

X4

500°C

X4

550°C

X8

600°C

X 10

680°C X 10 CF(HT) X4

graphitized coke

CF(HT)

3300 3400 3500 3600 Magnetic field (G)

natural graphite Line C 3000

3200

3400

3600

3800

Magnetic field (G)

Fig. 10. EPR spectra of the CF(LT) post-treated in F2 gas at various temperatures; the spectra of two CF(HT) obtained by direct fluorination at 600°C of natural graphite and of graphitized coke are also displayed. EPR spectra were recorded at room temperature using a X Band Bruker EMX spectrometer operating at 9.653 GHz.

Irrespective of the sample, the g-factor, which is typical of free radicals and localized structural defects, is close to 2.003 0.002. The narrow signal (line B) was ascribed to the spin carriers formed in the fluorographite layers to

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accommodate the charge of IF6 in a classical intercalation model. The broad EPR line cannot be simulated by a pure Lorentzian or Gaussian profile. It results from an unresolved hyperfine structure of the dangling bond electrons interacting with the neighboring fluorine nuclei. As a matter of fact, the wings of this structure can be observed for TFPT 200°C (Fig. 10). Dangling bond centers could adopt different configurations of surrounding fluorine atoms; this disorder leads to an unresolved super hyperfine structure (SHFS). During the re-fluorination treatment, this disorder is significantly lowered because the fluorine content increases and the degree of crystallinity is still high as observed from XRD data. Then, the local environment of each dangling bond in the highly fluorinated carbon matrix becomes more and more organized. Therefore, for fluorination posttreatment temperatures higher than 400°C, the super hyperfine structure with the fluorine nuclei becomes more and more resolved as TFPT increases (Fig. 10) and the broad line splits into seven lines. As shown by Panich et al. [24], the hyperfine interaction between dangling bond electron and six neighboring fluorine 1 nuclei (nuclear spin I  2 , n is the number of 19F nuclei) results in the splitting of the EPR spectrum into seven lines, i.e. (2nI  1)  7. This environment of the residual dangling bonds can be described as follows: it could consist of three fluorine atoms linked to three carbons in the armchair carbon sheet containing the dangling bond and three others in the adjacent sheet (the simulation of the signal for TFPT  550°C leads to the hyperfine parameter A  45 2 G, a linewidth ΔΗpp  36 G 2 G and g  2.003 0.001). Contrary to the sample re-treated at 600°C, another type of spin carrier (defects with a different environment) is present only in CF(HT) as evidenced by an additional line which is present on the EPR spectrum of these materials, denoted as line C with ΔHpp  20 1 G. The relative intensity of this additional line in comparison with that of the dangling bonds is dependent on both the starting graphite material and its fluorination process. This is emphasized by the EPR spectra of CF(HT) obtained starting from natural graphite or graphitized coke. In both cases, the SHFS is present but the intensity of line C is higher for the natural graphite than for graphitized coke (Fig. 10). Line C suggests the presence of additional conformational defects in the armchair carbon sheets. The spin density decreases as a function of the fluorination post-treatment [27] because of the reaction of the dangling bonds with F2 during the post-treatment: this density in the raw CF(LT) is high (close to 1020 2  1019, spins g1 corresponding approximately to 1 spin carrier for 200 carbon atoms), whereas after a treatment of CF(LT) at 600°C in F2 gas, it drops to a value close to 1  1018 2  1017 spins g1. It should be noted that the spin density of CF(HT) (resulting from graphitized coke) is equal to 12  1018 2  1018 spins g1 indicating a greater proportion of conformational defects than in the CF(LT) post-treated at this particular temperature.

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The evaluation of EPR experiments with temperature is different for lines A and B: the behavior of the latter is similar to that of weakly fluorinated CF(LT) [42] and other acceptor GICs [43,44], since its linewidth ΔHpp increases with decrease in temperature [27]; dipole–dipole interaction (proposed by Di Vittorio et al. [42]) and spin–orbit relaxation mechanism (Davidov et al. [43]) are involved in the relaxation of these materials. On the contrary, the width of line A is temperature-independent in the range 110–300 K with spin carriers behaving as isolated spins. Motional narrowing due to hopping motion of the dangling bond electrons at low temperature and/or due to surrounding fluorine motion at high temperature was proposed by Yokomichi et al. [39,40] for fluorinated amorphous carbon films. This phenomenon is not efficient in posttreated CF(LT). In brief, during post-treatment in pure F2 atmosphere of room-temperature graphite fluoride, C–F bonds have been formed exhibiting a bonding character more and more covalent with an increase in post-treatment temperature; the nature of the C–F bonding changes progressively from semi-ionic to covalent. The planarity of the carbon sheets is maintained (sp2 carbon hybridization) for TFPT 450°C. When TFPT is increased between 400 and 500°C, the C–F bonds develop a hybrid structure (both semi-ionic and covalent C–F bonds coexist) as the carbon skeleton that consists of chair-type structure along with some planarity. At the highest post-treatment temperatures (550–600°C), each carbon atom is then covalently bonded to a fluorine atom thereby increasing the fluorine content. This occurs after the removal of iodine fluoride species. The chair-type structure then becomes more and more regular and the residual dangling bonds behave as isolated spins and have a regular environment. Since the fluorination occurs due to the presence of intercalated iodine species, the thermal post-treatment of CF(LT) in F2 avoids the formation of conformational defects, contrary to a direct reaction of F2 gas with graphite at 600°C leading to the formation of CF(HT). Because of their low mobility in the fluorocarbon interlayer space, the iodine fluoride species could hinder the diffusion of the F2 molecules, shield the dangling bonds, and strengthen the planar configuration of the carbon sheets. The role of the iodine species is well evidenced by the strong correlation between the evolution of the NMR lines of IFy and the fluorocarbon matrix (Fig. 5): the removal of the iodine species and the change of the C–F bond occurred simultaneously showing their strong interactions. The recent works of Sato et al. [45] show that the progressive fluorination is not possible by a post-treatment in 5.0 MPa (⬃ 50 atm) fluorine gas of fluorine-GIC because the material burned to form fluorocarbon gases in the temperature range 300–400°C. These authors [45] show that the direct conversion of stage 1 fluorine-GIC into CFx occurred only with 0.1 MPa (⬃ 1 atm) F2 gas in the same temperature range also forming graphite fluoride with few defects: the original structure of the raw material with semi-ionic C–F bonds and planar sp2 carbon

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sheets is maintained for post-treatment temperature lower than 400°C, whereas covalent C–F bonds and sp3 carbon atoms are formed at higher temperature. Sato et al. [45] suggest that the post-fluorination is facilitated by the rearrangement of originally intercalated fluorine atoms in the fluorine-GIC. Therefore, the progressive post-fluorination allows the formation of modified graphite fluorides with various physical properties and various electrochemical performances as discussed in the following section. 4. ELECTROCHEMICAL STUDIES OF FLUORINATED GRAPHITE USED AS CATHODIC MATERIALS IN LITHIUM BATTERIES The use of carbon fluorides as cathode materials in nonaqueous primary lithium battery started at the beginning of the 1970s, the electrolyte used being composed of a 1 mol L1 lithium salt (LiX, X  ClO4, PF6 or BF4) dissolved in aprotic solvents (usually employed: propylene carbonate (PC), dimethylsulfoxide (DMSO), γ -butyrolactone (BL), tetramethylene sulfone (TMS), dimethoxyethane (DME), etc.). In the case of fluorinated graphite CFx, the theoretical specific capacity Qth (mAh g1) is given by the following equation: Qth  (xF)/3.6M, where F is the Faraday constant and M the molar mass of CFx. When a discharge current is applied, a departure from the OCV is observed both due to the ohmic drop in the electrolyte and electrode overpotential; it primarily occurs with the charge transfer, ionic diffusion, and/or phase transformation. In practice, the electric energy Epr supplied by the battery when discharged is related to the measured closed-circuit voltage (CCV) and to the electrode utilization (faradic yield ρ F): Epr 

冕

ρF

CCV(z)F dz

0

where z is the cell reaction progress rate. 4.1. High-temperature graphite fluoride CF(HT)

Carbon fluorides CFx (with “x” close to unity) prepared at high temperatures (CF(HT)) have been studied actively as cathode materials in high-energy density lithium batteries [20,46,47]. It is expected that the OCV of the Li/CFx cell depends on the nature of the carbon–fluorine bonding. In fact, a strong covalent C–F bonding yields lower OCV than that measured in weaker C–F bonds found in purely ionic or in semi-ionic CFx materials (Fig. 2). This may relate to a higher CFx reduction overpotential with increase in C–F binding energy. Fig. 11 summarizes some average potentials and specific capacities of CFx prepared by direct fluorination of graphite at temperatures varying between 300 and 600°C, and used as cathodic materials in a lithium cell composed of a liquid electrolyte.

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2.7 600

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Fig. 11. Average potential (䊏) and specific capacity () (calculated with cutoff voltage of 1.5 V) as a function of CFx synthesis temperature for lithium cells operated at room temperature with 1 mol L1 LiClO4–PC electrolyte for a current density of 0.5 mA cm2.

4.2. Low-temperature graphite fluoride CF(LT)

CF(LT) exhibits ionic [48] or semi-ionic [49] C–F bonds, in which the sp2 hybridization of the original graphite is maintained. The graphite fluorination level x strongly depends on the chemical nature of the catalyst fluorides. Fig. 12 displays some typical average potential and specific capacities of CF(LT) synthesized at room temperature using various catalysts. For instance, a high fluorination yield (i.e. x  0.8) was achieved in the presence of a IF5 and HF mixture [17,49–51]. When compared with conventional high-temperature CF(HT), these particular CF(LT) yield a similar discharge capacity of about 600 Ah kg1 but differs by having a higher discharge voltage (⬇ 2.9 V) (⬇ 2.0 V for CF(HT) under the same discharge conditions). This difference results in an energy density increase of 30%. Moreover, CF(LT) displays a higher rate capability of up to 5C, while CF(HT) hardly sustains a 2C rate [51]. The C rate corresponds to the recovering of the nominal capacity of the cell in 1 h. 4.3. Re-fluorinated CF(LT): an improvement in the electrochemical performance

When re-fluorination of CF(LT) is performed in the 200–400°C temperature range, the fluorine content x increases to reach a value in the range 0.9–1 [50]. However, re-fluorinated compounds contain minute amounts of IFy catalysts. The latter are highly undesirable as they adversely alter the cell’s shelf life (self-discharge) and increase its internal impedance. In some special applications such as in implantable medical devices, a battery should last about 15 years at human body temperature of 37°C. Therefore the self-discharge rate should be kept very low.

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Fig. 12. Average potential (䊏) and specific capacity () (calculated with cutoff voltage of 1.5 V) as a function of x in CFx for lithium cells operated at room temperature with 1 mol L1 LiClO4–PC electrolyte for a current density of 0.5 mA cm2.

Recently, Guérin et al. have reported an increase in the electrochemical performance of the CF(LT) compounds by a re-treatment under fluorine atmosphere at different temperatures of these post-treated CF(LT) [52]. Li/LiClO4-PC 1 mol L1/CFx cells were studied by galvanostatic measurements of 10 A kg1. Fig. 13 shows the galvanostatic discharges obtained with the fluorination posttreated CF(LT) series. Galvanostatic discharge under various current densities was carried out at room temperature. The discharge capacities detailed here have been obtained for a cell cutoff at 2 V. Fig. 14 shows the fluorination post-treatment temperature dependence of the average discharge voltage E and the discharge capacity that reached at 2 V. At the early stage of discharge (Fig. 13), the voltage drops abruptly for a relatively short time and then re-increases steadily to the main voltage plateau. The initial voltage drop, usually called the “delay effect”, is characteristic of Li/CFx cells and is ascribed to the low electrical conductivity of the active cathode. As the conductivity is improved with the carbon formation (resulting from CFx reduction), the voltage increases due to enhanced discharge reaction kinetics. The relative voltage drop increases with the post-treatment temperature but is still lower than that in CF(HT) even for CF(LT) post-treated under fluorine at the highest temperature. This suggests a lower resistivity of all the post-treated samples when compared with CF(HT). For fluorination post-treatment temperatures between 100 and 250°C, both average discharge voltage and discharge capacity remain constant (around 3.0 V and 570 Ah kg1, respectively). Again, the high discharge voltage results from the semi-ionic character of the C–F bond. The OCV varies with the voltage

Fluorination post-treatment temperatures

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Fig. 13. Galvanostatic discharge curves of [Li/LiClO4–PC 1 mol L1/CF(LT) post-treated] cell at 10 A kg1 for different fluorination post-treatment temperatures.

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Fig. 14. Average potential (䊏) and specific capacity () (calculated with cutoff voltage of 2 V) as a function of the fluorination post-treatment temperature of CF(LT) for lithium cells operated at room temperature with 1 mol L1 LiClO4–PC electrolyte for a current density of 0.5 mA cm2.

plateau value but may result from a mixed potential of the post-treated CF(LT) and more active fluorine species. The energy density shown in Fig. 15 increases with the capacity and is already, for these fluorination post-treatment temperatures, 70% higher than that of CF(HT), whose energy density value is about

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Fig. 15. Specific capacity () (calculated with cutoff voltage of 2 V) and energy density (䊉) as a function of average discharge potential.

1000 Wh kg1. A discharge capacity of about 570 Ah kg1 is typical for low-temperature graphite fluorides [12]. For a theoretical CF1 composition, assuming all C–F sites are electrochemically active, a capacity close to 850 Ah kg1 can be found. This low discharge yield cannot be explained by inactive CF2 and CF3 groups, typically expected in CF(HT) [53] but by the fluorination level. In fact, CF2 and CF3 groups were not detected by FT–IR and 19F–NMR experiments on all the post-treated fluorinated graphite, except for a fluorination post-treatment at 600°C [27]. Moreover, despite a general composition of CF1 for post-treated CF(LT), some fluorine atoms arise from residual IFy species that contribute to the active mass whereas they are not electrochemically active. For a re-fluorination temperature between 300 and 550°C, the OCV and the average discharge voltage decrease gradually with increase in re-fluorination temperature (Fig. 14). This could be mainly explained by the C–F bond change toward a more covalent character. As shown in Fig. 15, the specific discharge capacity and energy density go through a maximum of 900 Ah kg1 and 2270 Wh kg1, respectively. This maximum falls at around 2.53 V average voltage that corresponds to CF(LT) post-treated at 550°C. The increase in the capacity correlates with the departure of iodine fluoride species and the concomitant increase in the fluorine content with re-fluorination treatment [21]. The energy density is particularly high for the hybrid graphite fluoride compounds (re-fluorination temperature between 400 and 500°C) and the two carbon hybridization states (i.e. sp2 and sp3) seem to favor good electrochemical performance. For a fluorination post-treatment temperature equal to 600°C, the average discharge voltage and the discharge capacity are lower than for CF(LT) post-treated at 550°C (Fig. 14). The increase in the inactive CF2 and CF3 groups

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identified by FT–IR and 19F–NMR measurements [27] could explain the lower capacity as in covalent CF(HT). The fact that despite a similar fluorination temperature (600°C), post-treated CF(LT) yields twice as high as energy density as CF(HT) is noteworthy (i.e. about 2000 vs. 1000 Wh kg1, respectively.) It should be noted that the discharge voltage varies monotonically from 3.15 to 2.3 V with re-fluorination temperature while keeping a high discharge capacity. Indeed, the discharge voltage can be correlated to C–F bond nature, which is a function of the fluorination post-treatment temperature and not of F/C ratio as is generally the case for low-temperature graphite fluorides [46]. The coexistence of sp2- and sp3-hybridized carbons with varying sp2/sp3 ratios that were discussed in the previous section, may explain such voltage evolution in the posttreated CF(LT) cathode materials. As the sp2-type carbons tend to be more electrochemically active and the sp3-type yield higher capacity, the post-treated CF(LT) materials offer a wide range of (voltage, capacity) pairs suitable for specific battery-operating requirements. Tuning of the fluorination post-treatment temperature is the key parameter in achieving such a specific material. 4.4. Reversibility of the electrochemical processes

The nature of the C–F bond in carbon fluorides synthesized at high temperature is covalent and causes a strong lithium fixation on fluorinated sites. Consecutively, during the first discharge, LiF is formed irreversibly [46]. Therefore, covalent carbon fluorides cannot be used in secondary lithium batteries. The physical and chemical studies of compounds, synthesized at room temperature and then re-fluorinated, have demonstrated the semi-ionic nature of the C–F bond for specific post-fluorination temperature and the lithium reversibility has been investigated again. Indeed, Yazami et al. [49] undertook a comparative study of the electrochemical behavior of the carbon fluorides, prepared by direct fluorination at high-temperature CF(HT) and by fluorination at low-temperature CF(LT) using catalysts, in a lithium/solid polymer electrolyte in order to determine the reversibility of the electrode reaction. Cyclic voltammetry with decreasing scan rate (from 1000 to 1 mV min1) was applied at the same temperature (80°C) to two cells, one with CF(HT) and the other with CF(LT). This clearly emphasizes that, whereas in the case of CF(HT), the voltammograms do not exhibit reversible behavior, when CF(LT) is used, associated reduction and oxidation peaks were present, which denote reversible behavior. Proceeding on the reversibility of lithium intercalation into graphite fluorides, all compounds (CF(LT)) post-treated under F2 at different temperatures were studied by cyclic voltammetry in Li/LiClO4-PC 1 mol L1/CFx cells between 2.0 and 4.5 V with a sweeping rate of 6 mV min1. As exemplified in Fig. 16 by CF(LT) post-treated at 200°C, a reduction peak of lithium associated with an oxidation one, centered at 3.0 and 3.2 V, respectively, is present for the fourth cycle as in the first

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ones (Fig. 16a). The same behavior was observed in the case of CF(LT) post-treated at 400°C (Fig. 16b), whereas no reduction peak is noticed for a fluorination posttreatment temperature of 500°C even during the first cycle (Fig. 16c). The current quantities involved in the reversible process are low and they decrease when the

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E (V) vs. Li+/Li

Fig. 16. Fourth cyclic voltammograms of [Li/LiClO4–PC 1 mol L1/CF(LT) post-treated] at TFPT  200°C (a), TFPT  400°C (b) and TFPT  500°C (c) (sweeping rate 6 mV min1).

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fluorination post-treatment temperature increases. More generally, a reduction peak associated with an oxidation peak appears for all CF(LT) post-treated at temperatures below 450°C. Such behavior can be correlated with the fact that for such posttreatment temperatures the planarity of the graphene sheets is maintained. For temperatures above 450°C, the planarity of the graphene sheets is partly broken and LiF is inevitably formed in the working electrode consuming all the fluorine atoms upon cycling. In this case, the sample behaves as the conventional CF(HT) [49], exhibiting a voltammogram without a well-defined peak, in particular in the oxidation wave (Fig. 16c). The reduction process occurs mainly at potentials lower than 2.5 V. These results on the electrochemical behavior of CF(LT) confirm the possible reversibility of the intercalation of lithium ions. This opens a new investigation field for fluorinated graphite. 5. CONCLUSION: TO DESIGN MATERIALS WITH AN ADJUSTED CAPACITY – VOLTAGE COUPLE Although lithium batteries using CFx are still the focus of intensive work [54], more attention has been paid during these last years on fluorinated graphite prepared at room temperature. The post-treatment of low-temperature graphite fluoride in fluorine gas in the range 150–680°C result in new derivatives of high-energy density associated with average discharge voltage ranging between 3.15 and 2.35 V and specific capacity between 600 and 900 Ah kg1. The dual nature of the C–F bond of post-treated CF(LT) and its evolution as a function of the fluorination posttreatment temperature, which are evidenced by a combination of characterizations (19F and 13C–NMR, FT–IR, XRD, EPR), make it possible to design materials with a (capacity, voltage) couple adjusted to particular requirements and lead to outstanding electrochemical performance as cathodes in primary lithium batteries. Such materials can also be suitable in secondary lithium batteries. ACKNOWLEGMENTS The authors wish to thank Pr F. Masin (Université Libre de Bruxelles, Belgium), Dr R. Yazami (INPG, LEPMI, St. Martin D’Hères, France), Dr. J. Giraudet, Dr. Z. Fawal (Université Libanaise, Faculté des Sciences III, Tripoli, Liban), Pr. P. Hoggan, and Dr. J.P. Pinheiro for their cooperation and fruitful discussion in this work. REFERENCES [1] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 83rd edn., CRC Press, Boca Raton, FL, 2002. [2] S.S. Chen, A.S. Rodgers, J. Chao, R.C. Wilhoit, and B.J. Zwolinski, J. Phys. Chem. Ref. Data, 4 (1975) 441.

394 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

A. Hamwi et al. D. Peters, J. Chem. Phys., 38 (1963) 561. S.L. Di Vittorio, M.S. Dresselhaus, and G. Dresselhaus, J. Mater. Res., 8 (1993) 1578. N. Watanabe, R. Hagiwara, and T. Nakajima, J. Electrochem. Soc., 131 (1984) 1980. A. Morita, N. Eda, T. Ijima, and H. Ogawa, Power Sources, Vol. 9, J. Thompson (Ed.), Academic Press, New York, 1983, p. 435. A. Hamwi, C. Latouche, V. Marchand, J. Dupuis, and R. Benoit, J. Phys. Chem. Solids, 57 (1996) 991. D. Claves, J. Giraudet, A. Hamwi, and R. Benoit, J. Phys. Chem. B, 105 (2001) 1739. I. Palchan, D. Davidov, and H. Selig, J. Chem. Soc., Chem. Commun., 12 (1983) 657–658. T. Mallouk and N. Bartlett, J. Chem. Soc. Chem. Commun., (1983) 103–105. R. Hagiwara, M. Lerner, and N. Bartlett, J. Chem. Soc. Chem. Commun., (1989) 573. A. Hamwi, J. Phys. Chem. Solids, 57 (1996) 677. A. Hamwi, M. Daoud, J.C. Cousseins, and R. Yazami, J. Power Sources, 27 (1989) 81. R. Yazami and P. Touzain, Solid State Ionics, 9 (1983) 489. M.J. Root, R. Dumas, R. Yazami, and A. Hamwi, J. Electrochem. Soc., 148 (2001) A339. T. Nakajima, In: Fluorine–Carbon and Flouride–Carbon Materials: Chemistry, Physics, and Applications, T. Nakajima (Ed.); Marcel Dekker, New York, 1995, p 11. A. Hamwi, M.Daoud, and J.C. Cousseins, Synth. Met., 26 (1988) 89. A.M. Panich, Synth. Metals, 100 (1999) 169. H. Touhara and F. Okino, Carbon, 38 (2000) 241. T. Nakajima and N. Watanabe, Graphites Fluorides and Carbon–fluorine Compounds, T. Nakajima (Ed.), CRC Press, Boca Raton, FL, 1991, p. 84. K. Guérin, J.P. Pinheiro, M. Dubois, Z. Fawal, F. Masin, R. Yazami, and A. Hamwi, Chem. Mat., 16 (2004) 1786. A.M. Panich, T. Nakajima, H.M. Vieth, A.F. Privalov, and S.D. Goren, J. Phys.: Condens. Matter, 10 (1998) 7633. A.M. Panich, T. Nakajima, and S.D. Goren, Chem. Phys. Lett., 271 (1997) 381. A.M. Panich, A.I. Shames, and T. Nakajima, J. Phys. Chem. Solids, 62 (2001) 959. T.R. Krawietz and J.F. Haw, Chem. Commun., 19 (1998) 2151. A. Hamwi, M. Daoud, D. Djurado, J.C. Cousseins, Z. Fawal, and J. Dupuis, Synth. Metals, 44 (1991) 75. M. Dubois, K. Guérin, J.P. Pinheiro, Z. Fawal, F. Masin, and A. Hamwi, Carbon, 42 (2004) 1931–1940. H. Selig, W.A. Sunder, M.J. Vasile, F.A. Stevie, P.K. Gallagher, and L.B. Ebert, J. Fluorine Chem., 12 (1978) 397. R.K. Heenan and R. Robiette, J. Mol. Struct., 55 (1979) 191. K.O. Christe and W.W. Wilson, Inorg. Chem., 28 (1989) 3275. N. Bartlett, S. Beaten, L.W. Reeves, and E.J. Wells, Can. J. Chem., 42 (1964) 2531. C.A. Wilkie, G.-Y. Lin, and D.T. Haworth, J. Solid State Chem., 30 (1979) 197. A.M. Panich and T. Nakajima, Mol. Cryst. Liq. Cryst., 340 (2000) 77. J. Giraudet, M. Dubois, K. Guérin, J.P. Pinheiro, A. Hamwi, W.E.E. Stone, P. Pirotte, and F. Masin, accepted in J. Solid State Chem. H.A Resing, J. Milliken, D.D. Domingues, and L.E. Iton, Proc. 17th Biennal Conf., Carbon, Lexington, Kentucky University, 1985. T. Mallouk, B.L. Hawkins, M.P. Conrad, K. Zilm, G.E. Maciel, and N. Bartlett, Phil. Trans. R. Soc. Lond. A, 314 (1985) 179. E.W. Hagaman, D.K. Murray, and G.D.Del Cul, Energy & Fuel, 12 (1998) 399. S. Mouras, A. Hamwi, D. Djurado, and J.C. Cousseins, Rev. Chimie Minérale, 24 (1988) 572. H. Yokomichi, T. Hayashi, T. Amano, and A. Masuda, J. Non-Cryst. Solids, 227 (1998) 641.

Fluorine-intercalated graphite for lithium batteries

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[40] H. Yokomichi and K. Morigaki, J. Non-Cryst. Solids, 266 (2000) 797. [41] K. Takai, H. Sato, T. Enoki, N. Yoshida, F. Okino, H. Touhara, and M. Endo, Mol. Cryst. Liq. Cryst., 340 (2000) 289. [42] S.L. Di Vittorio, T. Enoki, M.S. Dresselhaus, G. Dresselhaus, M. Endo, and T. Nakajima, Phys. Rev. B: Cond. Mater. Mater. Phys., 46 (1992) 12723. [43] R. Davidov, O. Milo, I. Palchan, and H. Selig, Synth. Metals, 8 (1983) 83. [44] M. Murata and H. Suematsu, J. Phys. Soc. Jpn., 51 (1982) 1337. [45] Y. Sato, S. Shiraishi, Z. Mazej, R. Hagiwara, and Y. Ito, Carbon, 41 (2003) 1971. [46] R. Yazami, Chemistry, Physics and Applications of Fluorine–Graphite and Fluoride–Carbon Compounds, T. Nakajima (Ed.), Marcel Dekker, New York, 1995, pp. 251–281. [47] N. Watanabe, Solid State Ionics, 1 (1980) 87. [48] H. Touhara, Y. Goto, N. Watanabe, K. Imaeda, T. Enoki, H. Inokuchi, and Y. Mizutani, Synth. Metals, 23 (1988) 461. [49] R. Yazami and A. Hamwi, Solid State Ionics, 28 (1988)1756. [50] A. Hamwi and R. Yazami, Flourine-containing carbonaceous substances, their preparation, and use as battery electrodes, WO Patent, 97/41061 (1997). [51] A. Hamwi, M. Daoud, and J.C. Cousseins, Synth. Metals, 30 (1989) 23. [52] K. Guérin, R. Yazami, and A. Hamwi, Electrochem. Solid State Lett., 7(6) (2004) A159. [53] Y. Kita, N. Watanabe, and Y. Fujii, J. Am. Chem. Soc., 101 (1979) 3832. [54] A.G. Ritchie, C.O. Giwa, P.G. Bowles, J. Burgess, E. Eweka, and A. Gilmour, J. Power Sources, 96 (2001) 180.

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Fluorinated Materials for Energy Conversion T Nakajima and H Groult (Editors) © 2005 Elsevier Ltd. All rights reserved.

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Chapter 18

Battery application of graphite intercalation compounds Yoshiaki Matsuo Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, 2167, Shosha, Himeji Hyogo, 671-2201, Japan 1. INTRODUCTION Graphite intercalation compounds are very useful for battery applications because of the high electrical conductivity and their two-dimensional layered structure, which enables further intercalation of foreign ions such as lithium ions. Therefore, they have been used for the electrode materials of batteries such as lithium primary battery [1–6], lithium-ion battery [7,8], alkaline battery [9,10] and thermo cells [11–14]. Graphite intercalation compounds are classified into three categories from the viewpoint of chemical bonding between graphite and intercalated species as shown in Fig. 1. In donor and acceptor type intercalation compounds, graphite is positively and negatively charged, respectively, and it is bound to intercalated species via ionic bonding. The intercalated species include alkaline metal, alkaline earth metal, transition metal, etc. to form donor-type intercalation compounds. Among them, it is well known that lithium-intercalated graphite has been used as an anode of lithium-ion battery. Halogens, acids, oxides, etc. form acceptor-type intercalation compounds and some of them have been tested as cathode-active materials of lithium primary battery and electrode material of alkaline cell. On the other hand, fluorine and oxygen with high electronegativities are known to form covalent-or semicovalent-type graphite intercalation compounds when they are allowed to react with graphite under appropriate conditions. They are poly(carbon monofluoride), (CF)n, poly(dicarbon monofluoride), (C2F)n, and graphite oxide (abbreviated as GO or it is sometimes called graphitic acid). In these compounds, the planarity of carbon sheet of graphite is completely lost and the carbon layer is thought to consist of cyclohexane-like zig-zag carbon array. These materials are insulators; however, in the presence of conducting additives they are used as cathode-active materials of lithium primary battery [1,5].

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Yoshiaki Matsuo

Acceptor type Acids Metal oxide Metal halides etc

Thermo cell

Donor type Alkaline metals Alkaline earth metals etc

Graphite (Residual carbon)

Δ Surface modification

Δ

Alkaline cell

Semi-covalent type fluorine (CxF, x