Surface-Sulfurized Bi/Co Fluffy Hollow Microspheres as High-Performance Electrode Materials for Hybrid Asymmetric Supercapacitors

Transition metal sulfides (TMS) are promising for supercapacitors due to superior theoretical capacity and redox activity compared to metal oxides yet suffer from structural instability and poor conductivity. To overcome these limitations, this study achieved molecular-level homogenization through a glycerol-mediated solvothermal process followed by in situ sulfidation to form a unique fluffy spherical Bi/Co@S heterostructure. This hierarchical design─featuring a porous hollow core and a uniformly sulfurized surface─synergistically improves charge transfer, ion diffusion, and mechanical stability. The resulting Bi/Co@S electrode delivers a high specific capacity of 1514.8 C g–1 at 1 A g–1, excellent rate performance (72% capacity retention at 20 A g–1), and exceptional cycling stability (80.9% capacity retention rate over 10,000 cycles). A hybrid supercapacitor assembled with activated carbon achieves an energy density of 47.67 Wh kg–1 at 800 W kg–1, outperforming most previously reported bismuth-based materials. This work offers a novel strategy for designing stable, high-performance TMS electrodes.