Cu triggered phase transitions in Fe7S8@NS-C anode: A neglected factor affecting the electrochemical performance of sodium storage

Abstract

Iron sulfide (Fe7S8) with high theoretical capacity and abundant natural resource is an attractive anode candidate in sodium-ion batteries (SIBs). However, its abnormal capacity variation in ether-based electrolytes is inexplicable. Herein, N,S co-doped carbon-coated Fe7S8 (Fe7S8@NS-C) is constructed. As an anode for SIBs, the Na-storage capacity of Fe7S8@NS-C increases significantly from 503.9 to 565.8 mAh/g at 1 A/g after 120 cycles. Comprehensive characterizations demonstrate this capacity growth not only originates from the reduced FeSx particle size, but also from the Cu-triggered phase transition to form Cu5FeS4 and CuSx. Subsequently, the unstable Cu5FeS4 is converted to CuSx and FeSx, and the resultant capacity maintains relatively stable. Remarkably, Cu2O and Cu0 gradually appear as the cycle number increases, which may result from the side reactions of Cu+ with some electrolyte components. The constant Cu-related phase transition accelerates the corrosion of Cu foil, which weakens the adhesion of active materials on it and induces a capacity attenuation after ∼ 550 cycles. This work is helpful to understand the capacity variation of Fe7S8 anode during sodiation/desodiation processes in ether-based systems, which is of great significance for the design and application of metal sulfide materials in electrochemical energy storage.