Copper diffusion related phase change and voltage decay in CuS cathode

Copper sulfide(CuS)is a promising cathode for lithium-ion batteries(LIBs)due to its impeccable theoretical energy density(~1015 Wh·kg^(−1) and 4743 Wh·L^(−1)).However,it suffers from voltage decay leaded energy density loss and low energy efficiency,which hinders its application.In this work,with combined ex-situ/in-situ X-ray diffraction(XRD)and electrochemical analysis,we explore detailed degradation mechanisms.For the voltage decay,it is attributed to a spontaneous reaction between CuS cathode and copper current collector(Cu CC).This reaction leads to energy density loss and active materials degradation(CuS→Cu_(1.81)S).As for energy efficiency,CuS undergoes a series of phase transformations.The main phase transition processes are CuS→α-LiCuS→Li_(2−x)Cu_(x)S+Cu→Li_(2)S+Cu for discharge;Li_(2)S+Cu→Li_(2−x)Cu_(x)S→β-LiCuS→CuS for charge.Here,α-LiCuS,β-LiCuS,and Li_(2−x)CuxS are newly identified phases.These phase changes are driven by topotactic-reaction-related copper diffusion and rearrangement.This work demonstrates the significance of transition-metal diffusion in the intermediates formation and phase change in conversion-type materials.