Origins of initial deactivation of sodium in metal sulfides and its activation via presodiation

Metal sulfides are promising candidates for sodium (Na)-ion battery anodes due to their moderate theoretical capacity and cost-effectiveness. However, their application in practical full cells has been hindered by their low coulombic efficiency and poor electrochemical reactivity during the initial cycles. Here, we reveal the origins of the inhibited initial electrochemical performance in the Na-ion battery full cells employing a copper sulfide (CuS) anode, which exhibits excellent electrochemical performance. The initial sodiation/desodiation process in CuS generates a Na⁺ non-conductive intermediate phase which confines Na⁺ within its structure. This leads to the deactivation of the Na⁺, and the full cell ultimately fails within the early cycles. To address this issue, we employ a presodiation strategy that effectively mitigates the initial capacity loss by promoting the disintegration of bulk CuS into nanograins of intermediate phases and facilitating its conversion reaction to activate the confined Na. Our findings provide invaluable insights toward the development of metal sulfide-based Na-ion batteries with high power density and long cycle life.