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

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 In 6 S 7 /nitrogen and sulfur co-doped carbon hollow microspindles (In 6 S 7 /NSC HMS) are produced by morphology-preserved thermal sulfurization of spindle-like and porous indium-based metal organic frameworks. The resulting In 6 S 7 /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 In 6 S 7 /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 In 6 S 7 /NSC HMS arises from the multi-electron redox mechanism, buffered volume expansion, negligible morphological change, and surface-controlled solid-state Na + transport.