Two types of graphite fluorides, (CF)n and (C2F)n, and discharge characteristics and mechanisms of electrodes of (CF)n and (C2F)n in lithium batteries

Abstract During studies of the relationship between the composition and the structure of graphite fluoride, it has been confirmed that a new chemical compound, poly-dicarbon monofluoride, represented by the formula (C 2 F) n is formed under relatively mild reaction conditions. Its color changes from black to white. Accordingly, the color of graphite fluoride does not depend on the composition but on the reaction and treatment temperatures. Furthermore, it is noted that (C 2 F) n having properties comparable to those of (CF) n can be obtained in extremely high fields, up to 100% as opposed to (CF) n which is obtained in very poor yield. In the newly proposed structure of (C 2 F) n a diamond-like structure is partially formed, that is, C-C bonds in the direction perpendicular to the layers forming a packing structure of (C 2 F) n . The electrochemical behavior of two lithium batteries having respectively a (CF) n electrode and a (C 2 F) n electrode were studied in 1 M LiClO 4 -propylene carbonate solution by potentiometric and galvanostatic polarization and the potential sweep method. The initial open circuit voltage (OCV) of the battery with the (CF) n electrode was 3.36−2.96 V and with the (C 2 F) n electrode 3.55−2. 9 5 V, both of which values are much lower than the theoretical value, 4.5 V. The overpotential of the (C 2 F) n electrode was approximately 0.4 V lower than that of the (CF) n electrode. The mechanisms of discharge of the two kinds of graphites, (CF) n and (C 2 F) n , in lithium batteries were studied using X-ray diffraction, ESCA spectra and DTA. The interlayer spacing of (CF) n expanded with the progress of discharge while that of (C 2 F) n is almost unchanged. Some new peaks were observed in the ESCA spectra of F 1s , Li 1s and C 1s , and the DTA diagrams of both the (CF) n and (C 2 F) n electrodes after discharge and they are different from those of LiF and of the (CF) n and (C 2 F) n electrodes before discharge. From these results, it is assumed that the cell reaction of a Li(CF) n or (C 2 F) n battery is the formation not of LiF, but of a nonstoichiometric ternary intercalation compound CLi x F y or C 2 Li x F y , where x and y are each approximately unity or somewhat smaller.