Characterization of CoS_x (x=2.1-4.5) as cathode materials for rechargeable lithium batteries

CoS_2.1, CoS_2.5, and CoS_4.5 were synthesized by direct reaction in aqueous solution between stoichiometric amounts of lithium polysulfide and cobalt salts. The first material was found to be an n-type semiconductor while the other two were p-type semiconductors. The 20 degrees C resistivities were 16, 30, and 34 Omega-cm, respectively. The theoretical energy density of the Li/CoS_x couples was in the range of 1253-1634 Wh/kg. A spirally wound 2/3 A size cell was used as a test vehicle for cathode evaluation. The open-circuit voltage (OCV) of the cell after stabilization was 2.13 V. At room temperature and at a discharge rate of 50-150 mA, the initial discharge capacity was typically 0.6-1.1 Ah. This is equivalent to a practical energy density of 127-165 Wh/kg. However, after a few cycles, cell capacity declined and stabilized at 0.3-0.4 Ah. The maximum number of cycles to cell failure was 50. The charge-discharge reaction mechanism and the capacity decline mechanism are discussed.