Abstract
ABSTRACT
A composite of copper chevrel phase (Cu2Mo6S7.8) and sulfur was investigated as positive electrode materials for all-solid-sate lithium-sulfur batteries with an inorganic solid electrolyte (amorphous Li3PS4). The composites of Cu2Mo6S7.8 and sulfur in various mole ratios (Cu2Mo6S7.8 : S = 1 : 1 to 1 : 20) showed high specific capacities in the range of 200 to 480 mAhg−1, which was calculated on the base of the total weights of the composites. Those capacities were two to three times larger than that of the Cu2Mo6S7.8. The reaction mechanisms of the composite as cathode materials were investigated during the discharge process. First of all, only the mixing of Cu2Mo6S7.8 with sulfur caused a structural change of the chevrel phase (type 1) into type 1’ phase and a formation of copper sulfide (CuS). In the initial stage of the discharge process, lithium ions were electrochemically inserted into the type 1’ phase and CuS. The Li+ insertion into the CuS formed LixCuS phase. The newly formed LixCuS continuously reacted with elemental sulfur and changed into CuS and Li2S. This reaction was not electrochemical reaction, however, the generated CuS would be able to electrochemically react with farther lithium ions. This process caused the large electrochemical capacity of the composites. An all-solid-state battery with the composite of Cu2Mo6S7.8 and sulfur (1 : 7 in mole ratio) showed high specific capacity more than 250 mAhg−1 and the capacity was retained even after 30 discharge-charge cycles at 25 ˚C.
