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-situlin-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 Cu x S 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.