Cobalt‐Doped FeS 2 Nanospheres with Complete Solid Solubility as a High‐Performance Anode Material for Sodium‐Ion Batteries

Considering that the high capacity, long‐term cycle life, and high‐rate capability of anode materials for sodium‐ion batteries (SIBs) is a bottleneck currently, a series of Co‐doped FeS 2 solid solutions with different Co contents were prepared by a facile solvothermal method, and for the first time their Na‐storage properties were investigated. The optimized Co 0.5 Fe 0.5 S 2 (Fe0.5) has discharge capacities of 0.220 Ah g −1 after 5000 cycles at 2 A g −1 and 0.172 Ah g −1 even at 20 A g −1 with compatible ether‐based electrolyte in a voltage window of 0.8–2.9 V. The Fe0.5 sample transforms to layered Na x Co 0.5 Fe 0.5 S 2 by initial activation, and the layered structure is maintained during following cycles. The redox reactions of Na x Co 0.5 Fe 0.5 S 2 are dominated by pseudocapacitive behavior, leading to fast Na + insertion/extraction and durable cycle life. A Na 3 V 2 (PO 4 ) 3 /Fe0.5 full cell was assembled, delivering an initial capacity of 0.340 Ah g −1 .