Unraveling the Voltage Failure Mechanism in Metal Sulfide Anodes for Sodium Storage and Improving Their Long Cycle Life by Sulfur‐Doped Carbon Protection

Abstract Metal sulfides are emerging as a promising anode material for sodium‐ion batteries with high reversible capacities and fast reaction kinetics, but achieving long‐cycling‐life remains a great challenge. Here, taking cobalt sulfide as an example, its electrochemical sodium‐ion storage failure phenomenon is first reported, which indicates that the battery cannot reach the cut‐off voltage during charging. Detailed analyses demonstrate that such failure may originate from the dissolution and escape of polysulfide intermediates, further reacting with the released copper‐ions from the current collector and inducing the occurrence of the shuttle effect. Based on the explored failure mechanism, a sulfur‐doped carbon matrix with polar carbon sulfur bonds, which can firmly immobilize the dissolved polysulfides, is deliberately introduced into the Co 1− x S active particles (Co 1− x S/s‐C) to improve their cycle stability. Consequently, the cycle life of the Co 1− x S/s‐C anode for sodium‐ion storage is extended from the original 125 to present 2000 cycles, even at high‐rate current densities. Moreover, utilizing the carbon current collector instead of traditional copper can effectively delay the occurrence of the failure phenomenon. The present work promotes better fundamental understanding of the structural evolution of metal sulfide anodes during cycles, and the solution strategy can be extended to apply in other metal sulfides (ZnS, NiS).