Hexaindium Heptasulfide/Nitrogen and Sulfur Co‐Doped Carbon Hollow Microspindles with Ultrahigh‐Rate Sodium Storage through Stable Conversion and Alloying Reactions

Abstract Group IIIA–VA metal sulfides (GMSs) have attracted increasing attention because of their unique Na‐storage mechanisms through combined conversion and alloying reactions, thus delivering large theoretical capacities and low working potentials. However, Na⁺ diffusion within GMSs anodes leads to severe volume change, generally representing a fundamental limitation to rate capability and cycling stability. Here, monodispersed In₆S₇/nitrogen and sulfur co‐doped carbon hollow microspindles (In₆S₇/NSC HMS) are produced by morphology‐preserved thermal sulfurization of spindle‐like and porous indium‐based metal organic frameworks. The resulting In₆S₇/NSC HMS anode exhibits theoretical‐value‐close specific capacity (546.2 mAh g⁻¹ at 0.1 A g⁻¹), ultrahigh rate capability (267.5 mAh g⁻¹ at 30.0 A g⁻¹), high initial coulombic efficiency (≈93.5%), and ≈92.6% capacity retention after 4000 cycles. This kinetically favored In₆S₇/NSC HMS anode fills up the kinetics gap with a capacitive porous carbon cathode, enabling a sodium‐ion capacitor to deliver an ultrahigh energy density of 136.3 Wh kg⁻¹ and a maximum power density of 47.5 kW kg⁻¹. The in situ/ex situ analytical techniques and theoretical calculation both show that the robust and fast Na⁺ charge storage of In₆S₇/NSC HMS arises from the multi‐electron redox mechanism, buffered volume expansion, negligible morphological change, and surface‐controlled solid‐state Na⁺ transport.