Electrochemical Characteristics of Graphite Intercalation Compounds with Fluorine-Lithium Cells
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NOBUATSU WATANABE, HIDEKAZU TOUHARA AND TSUYOSHI NAKAJIMA Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606, JAPAN
ABSTRACT The discharge reaction mechanism for two types of graphite fluorides, (CF)n and (C2F)n cathodes of lithium cells utilizing a 1 M LiC10 4 -propylene carbonate (PC) system were studied by means of x-ray diffraction, ESCA, NMR and transmission electron microscopy. It has been revealed that the discharge reaction proceeds through the formation of an intermediate solvated ternary compound of (C-F-Li),PC or (C 2 ' F.Li).PC, and then they decompose to graphite with low crystallinity and LiF. The decomposition process is not involved in the electrochemical reaction and this discharge reaction infers the open-circuit voltage of cells. New graphite intercalation compounds, CxF(MFn)y(MFn=AIF3, MgF 2 ) were prepared under a fluorine atmosphere of 1 atm in the temperature range between 20 and 4000C. These compounds are highly stable even in moist air and have high electrical conductivity. The discharge potentials of CxF(MFn)y-lithium cells are 2.5-2.8 V at current densities 40-400[±A, which are higher than those of (CF)n or (C 2 F)n-lithium cells below 300pA. INTRODUCTION Increasing interest has been shown in graphite intercalation compounds with fluorine or fluorides because of their important characteristics. Among them, graphite fluoride, (CF)n and (C 2 F)n which have layered structures with covalent bonds between carbon and fluorine atoms, are excellent cathode materials for lithium batteries with high and stable discharge potential, high energy density and long shelf life [1]; (CF)n-Li batteries have now been used on a large scale. The open-circuit voltage (OCV) for the (CF)n-Li cell is around 3.2 V, which is appreciably less than the value of the electromotive force (EMF) 4.57 V, calculated from free energies of formation of CF 1 . 0 0 and LiF [2]. The difference in the value between OCV and EMF has been interpreted to result from the formation of an intermediate nonstoichiometric ternary phase during discharge of graphite fluoride cathodes [3]. However, the species formed during discharge in a real cell are still ambiguous and a deeper understanding for the discharge reaction has been required to develop further improved batteries. In the present paper, compounds formed during the discharge of graphite fluoride cathodes were investigated and a new discharge reaction will be proposed. We also report the preparation of ternary intercalation compounds, CxF(MFn)y(MFn=AlF 3 , MgF 2 ) which are nonhygroscopic and stable. The electrochemical behavior of these compounds as a cathode material will be shown. DISCHARGE REACTION IN (CF)n AND (C 2 F)n-LITHIUM CELLS Experimental Graphite fluoride,
Mat.
Res. Soc.
Symp. Proc.
(CF)n and (C2F)n were prepared by direct fluorination of
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20 (1983) @Elsevier Science Publishing Co.,
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248 Reaction conditions are flaky natural graphite from Madagascar (50-604m). given elsewhere [1]. A new cell assembly was developed to
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