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 In₆ S₇ /nitrogen and sulfur co-doped carbon hollow microspindles (In₆ S₇ /NSC HMS) are produced by morphology-preserved thermal sulfurization of spindle-like and porous indium-based metal organic frameworks. The resulting In₆ S₇ /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 In₆ S₇ /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 In₆ S₇ /NSC HMS arises from the multi-electron redox mechanism, buffered volume expansion, negligible morphological change, and surface-controlled solid-state Na⁺ transport.