Interface Modulation of Metal Sulfide Anodes for Long‐Cycle‐Life Sodium‐Ion Batteries

Abstract Although studies of transition metal sulfides (TMS) as anode materials for sodium‐ion batteries are extensively reported, the short cycle life is still a thorny problem that impedes their practical application. In this work, a new capacity fading mechanism of the TMS electrodes is demonstrated; that is, the parasitic reaction between electrolyte anions (i.e., ClO 4 − ) and metal sulfides yields non‐conductive and unstable solid‐electrolyte interphase (SEI) and meanwhile, corrosively turns metal sulfides into less‐active oxides. This knowledge guides the development of an electrochemical strategy to manipulate the anion decomposition and construct a stable interface that prevents extensive parasitic reactions. It is shown that introducing sodium nitrate to the electrolyte radically changes the Na + solvation structure by populating nitrate ions in the first solvation sheath, generating a stable and conductive SEI layer containing both Na 3 N and NaF. The optimized interface enables an iron sulfide anode to stably cycle for over 2000 cycles with negligible capacity loss, and a similar enhancement in cycle performance is demonstrated on a number of other metal sulfides. This work discloses metal sulfides’ cycling failure mechanism from a unique perspective and highlights the critical importance of manipulating the interface chemistry in sodium‐ion batteries.