Iron Oxide–Iron Sulfide Hybrid Nanosheets as High-Performance Conversion-Type Anodes for Sodium-Ion Batteries

Commercialization of Na-ion batteries is hindered by the shortage of abundant and environmentally benign electrode materials with high electrochemical performance. Most of the high-capacity alloying- and conversion-type anodes face rapid capacity loss during prolonged cycling. Herein, we report superior Na-ion storage performance of iron oxide–iron sulfide hybrid nanosheet anodes. Composite anodes containing Fe2O3–FeS and Fe3O4–FeS hybrid nanosheets demonstrated high specific capacities of 487 and 364 mA h g–1, respectively, at a 0.1C rate. These electrodes also exhibited excellent cycling performance, maintaining 330 mA h g–1 after 50 galvanostatic cycles at a 1C rate with ∼100% coulombic efficiency. Mechanistic investigations revealed a high degree of pseudocapacitive-type Na-ion storage (up to ∼65%) in these iron oxide–iron sulfide hybrid nanosheet anodes. Spectroscopic studies confirmed the complete disappearance of the starting oxide and sulfide structures. 57Fe Mössbauer spectroscopy confirmed Na-ion storage through the conversion reaction of iron oxide–iron sulfide hybrid anodes. Excellent Na-ion storing performance in these hybrid anodes compared with that of previously investigated iron sulfide- and iron oxide-based electrodes is accredited to the enhanced pseudocapacitive Na-ion diffusion caused by the two-dimensional microstructure, high surface area, and crystal mismatch between the iron oxide–iron sulfide nanograins of the hierarchical nanosheets.