Promoting Highly Reversible Sodium Storage of Iron Sulfide Hollow Polyhedrons via Cobalt Incorporation and Graphene Wrapping

Sodium ion batteries (SIBs) have drawn significant attention owing to their low cost and inherent safety. However, the absence of suitable anode materials with high rate capability and long cycling stability is the major challenge for the practical application of SIBs. Herein, an efficient anode material consisting of uniform hollow iron sulfide polyhedrons with cobalt doping and graphene wrapping (named as CoFeS@rGO) is developed for high-rate and long-life SIBs. The graphene-encapsulated hollow composite assures fast and continuous electron transportation, high Na⁺ ion accessibility, and strong structural integrity, showing an extremely small volume expansion of only 14.9% upon sodiation and negligible volume contraction during the desodiation. The CoFeS@rGO electrode exhibits high specific capacity (661.9 mAh g⁻¹ at 100 mA g⁻¹), excellent rate capability (449.4 mAh g⁻¹ at 5000 mA g⁻¹), and long cycle life (84.8% capacity retention after 1500 cycles at 1000 mA g⁻¹). In situ X-ray diffraction and selected-area electron diffraction patterns show that this novel CoFeS@rGO electrode is based on a reversible conversion reaction. More importantly, when coupled with a Na₃V₂(PO₄)₃/C cathode, the sodium ion full battery delivers a superexcellent rate capability (496.8 mAh g⁻¹ at 2000 mA g⁻¹) and ≈96.5% capacity retention over 200 cycles at 500 mA g⁻¹ in the 1.0–3.5 V window. This work indicates that the rationally designed anode material is highly applicable for the next generation SIBs with high-rate capability and long-term cyclability.