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_1.81S). As for energy efficiency, CuS undergoes a series of phase transformations. The main phase transition processes are CuS → α-LiCuS → Li_2−xCu_xS + Cu → Li₂S + Cu for discharge; Li₂S + Cu → Li_2−xCu_xS → β-LiCuS → CuS for charge. Here, α-LiCuS, β-LiCuS, and Li_2−xCu_xS 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.