Transformation of Two-Dimensional Iron Sulfide Nanosheets from FeS2 to FeS as High-Rate Anodes for Pseudocapacitive Sodium Storage

Exploration of advanced anodes is significant for fabricating high-performance sodium ion batteries (SIBs). Two-dimensional (2D) transition-metal sulfide nanosheets have attracted great interest owing to their fascinating electrochemical performance. In this work, pyrite FeS2 nanosheets (FeS2 NSs) and their conversion to hexagonal FeS nanosheets with carbon coating (FeS/C NS) are designed as robust and high-rate anodes of SIBs. Transformation from the FeS2 NS to FeS/C NS by one-step annealing is investigated. Benefitting from the surface carbon coating and 2D morphology with large lateral size and thin thickness, the FeS/C NS possesses enhanced electron conductivity and ion diffusion coefficients, has a short ion diffusion distance and abundant active sites, and exhibits accelerated reaction kinetics. Consequently, the FeS/C NS shows significantly improved cycling stability and rate capability relative to the FeS2 and FeS counterparts. The FeS/C NS anode maintains a stable reversible capacity of 388 mA h g–1 after 300 cycles at 1 A g–1. Impressive capacities of 438 and 391 mA h g–1 can be yielded, respectively, at high current densities of 5 and 10 A g–1, thereby exhibiting superior high rate capability.