Hollow Microsphere Structure and Spin‐Polarized Surface Capacitance Endow Ultrafine Fe7S8 Nanocrystals with Excellent Fast‐Charging Capability in Wide‐Temperature‐Range Lithium‐Ion Batteries

Fe7S8 as a conversion‐type anode shows high capacity in lithium‐ion batteries (LIBs). Nevertheless, the sluggish ion transport rate, low electron conduction behavior, and large volume change upon cycling limit its applications in fast‐charging wide‐temperature‐range LIBs. Here, a simple hydrothermal and subsequent solid‐phase high‐pressure sulfidation route is proposed to synthesize a hollow Fe7S8/N‐doped C microsphere structure. The hollow space is enveloped by the spheres’ shell consisting of ultrafine Fe7S8 nanocrystals (≈8 nm) embedded into N‐doped C matrix, which enhances ion transport and electrical conduction, and accommodates the volume expansion of Fe7S8. Remarkably, in situ magnetometry reveals that spin‐polarized surface capacitance occurs during the stage of conversion reaction, in which the formed Fe and Li2S act as electrons and ions acceptor, respectively, to construct space charge zone at their interfaces, thus enhancing lithium transport and storage. Accordingly, the hollow microspheres show high gravimetric energy density and outstanding fast‐charging capability along with excellent cycling stability in Ah‐level pouch cells operating from ‐40 to 60 °C. For the first time, this work confirms the effectiveness of spin‐polarized surface capacitance effect on enhancing ion storage and transport in fast‐charging wide‐temperature‐range LIBs.