Electrochemical properties of stoichiometric CuS coated on carbon fiber paper and Cu foil current collectors as cathode material for lithium batteries

We prepared stoichiometric CuS by spray pyrolysis at 400 degrees C, followed by low temperature heat treatment at 150 degrees C for 1 h to conduct electrochemical reaction mechanism studies. The cathode electrode coated on carbon fiber paper (CFP) and Cu foil current collectors were analyzed by ex situ XRD after cycling at 0.02C and 1.2-3.0 V with Li/Li+. Stoichiometric CuS coated on a CFP current collector suffered from the 1st cycle irreversibility and rapid capacity fading after the 5th cycle to 120 mA h g(-1), owing to the formation of a low crystallinity CuS after the 1st charge, and the dissolution of lithium polysulfides (Li2Sx, 2 < x <= 8) in the electrolyte. On the other hand, stoichiometric CuS coated on a Cu foil current collector showed improved capacity of 460 mA h g(-1) after the 5th cycle. The excess Cu cations on the Cu foil introduced to CuxS led to the formation of Cu1.96S instead of CuS in the fully charged state, which enhanced the capacity retention and the efficient Li2Sx utilization. However, the gradual Li2Sx dissolution in the electrolyte and migration to the lithium metal side after 15 cycles at 0.1C still caused cell deterioration. To trap Li2Sx, we inserted a CFP interlayer between the stoichiometric CuS electrode coated on a Cu foil current collector and separator. The modified cell showed a stable capacity of 440 mA h g(-1) for 200 cycles at 1C and excellent rate capability with a capacity of 340 mA h g(-1) at 10C.