Encapsulation of Bi 2 S 3 Within Multifunctional Sulfurized Polyacrylonitrile Matrix Enables High‐Performance Wide‐Temperature Sodium/Potassium‐Ion Batteries Through Solid–Solid Conversion

ABSTRACT With high theoretical capacity and low cost, bismuth sulfide (Bi 2 S 3 ) demonstrates considerable potential as an anode for sodium/potassium‐ion batteries (SIBs/PIBs). Unfortunately, significant volume variation, low electrical conductivity, and dissolution of polysulfides drastically weaken their intrinsic merits. Herein, we fabricate a Bi 2 S 3 ‐SPAN composite by confining Bi 2 S 3 nanoparticles within a sulfurized polyacrylonitrile (SPAN) nanofiber matrix. In this architecture, the confinement effect of SPAN inhibits the aggregation of Bi 2 S 3 nanoparticles and alleviates the volume expansion during charge and discharge. Critically, the “solid–solid” conversion mechanism significantly suppresses the “dissolution–shuttle” effect of polysulfides. These merits ensure that Bi 2 S 3 ‐SPAN anode presents prominent sodium/potassium storage performance across a broad temperature range (−15°C to –50°C), demonstrating its environmental adaptability. Especially, in SIBs, Bi 2 S 3 ‐SPAN anode harvests capacities of 259 mAh g −1 (90.9% retention) over 770 cycles and 537 mAh g −1 (85.2% retention) over 230 cycles at 0.5 A g −1 under −15°C and 50°C, respectively. The full cells also achieve excellent electrochemical performances. Notably, Bi 2 S 3 ‐SPAN‐450||NVP pouch cell delivers a stable capacity of 183 mAh after 300 cycles at 0.2 A g −1 . This work is expected to offer inspiration for relieving volume expansion and the “dissolution–shuttle” effect of polysulfides in metal sulfide electrode materials for advanced SIBs/PIBs.