LITHIUM STORAGE BEHAVIOR FOR VARIOUS KINDS OF CARBON ANODES IN LI IONSECONDARY BATTERY

A lithium secondary battery using carbon as a negative electrode has b een developed. Further improvements to increase the cell capacity are expected by modifying the structure of the carbonaceous materials. In the present paper, various kinds of carbon fibers are studied in order to cover a wide range of crystallinity and poly-p-phenylene (PPP)-bas ed carbon is also chosen as one of the highly disordered carbons becau se of its Li discharge capacity as a negative electrode in the battery . The discharge capacity of a carbon electrode depends on the crystall ite thickness Lc(002), in which the minimum of the discharge capacity occurs at an intermediate size. This dependency implies a different Li storage mechanism, as also suggested by their voltammograms. A Li int ercalation reaction to form stage compounds takes place in graphite fi bers with large crystallite thickness, while the doping of Li ions occ urs in carbon fibers with small Lc(002) crystallites. In carbon fibers with intermediate crystallite thickness (around 100 Angstrom), both i ntercalation and doping processes occur incompletely, thereby reducing the discharge capacity. Moreover, it is indicated from the voltammogr am that the Li doping process in extremely disordered PPP-based carbon , exhibiting a discharge capacity of 680 mAh g(-1) which is two times greater than that of graphite and consisting of very small crystallite s in the c-dimension, is very different even from that in carbon fiber s with small crystallite thickness.