Hydrogel-assisted in-situ encapsulation of transition metal sulfides: An effective strategy to significantly boost their electrochemical performance as an anode for sodium-ion batteries

Abstract

Transition metal sulfides (TMSs) have emerged as one of the most auspicious anode materials for sodium-ion batteries (SIBs), owing to their superior redox reversibility and high theoretical specific capacity. Nevertheless, substantial volume fluctuations during redox reactions often lead to structural degradation, resulting in rapid capacity fading and poor rate capability. Herein, we proposed an innovative strategy that employed the Chinese snack ice-powder derived hydrogel to fabricate a hard-carbon(HC) encapsulated Co₉S₈ composite (Co₉S₈@HC). Microstructural characterizations revealed the formation of C-S covalent bonds at the carbon/cobalt-sulfide interface, which upgraded the interfacial contact from a conventional "surface contact" to a "point-to-surface chemical rivet" configuration. Electrochemical evaluations demonstrated that the sample carbonized at 800 °C (Co₉S₈@HC-800) delivered an outstanding sodium storage capacity of 598.41 mAh g⁻¹ at 100 mA g⁻¹. After 300 cycles, the electrode still delivered a capacity of 578.19 mAh g⁻¹, corresponding to a capacity retention of 96.61 %, and exhibited superior rate capability (426.70 mAh g⁻¹ at 5000 mA g⁻¹). These remarkable properties are ascribed to the synergistic effect between the carbon shell and cobalt sulfide, which not only improves electronic conductivity but also effectively accommodates volume changes during repeated cycling, rendering Co₉S₈@HC-800 a highly promising advanced anode candidate for SIBs.