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 similar to 100% coulombic efficiency. Mechanistic investigations revealed a high degree of pseudocapacitive-type Na-ion storage (up to similar to 65%) in these iron oxide-iron sulfide hybrid nanosheet anodes. Spectroscopic studies confirmed the complete disappearance of the starting oxide and sulfide structures. Fe-57 Mossbauer 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..
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