<i>In Situ</i> Transmission Electron Microscopy Visualization of Crystallographic Reversibility in 2D Bismuthene Anodes Enabling Ultrastable Potassium-Ion Battery Cycling

Alloy-type anode materials have attracted considerable attention in advanced rechargeable battery systems for exceptional theoretical capacities, yet their practical implementation has been hindered by structural degradation during repeated ion insertion/extraction. Here, utilizing <i>in situ</i> transmission electron microscopy, we demonstrate that few-layer bismuthene nanosheets exhibit excellent structural stability during potassium storage processes. Specifically, few-layer bismuthene nanosheets undergo reversible single-crystal structural evolution upon depotassiation, which originates from atomically coherent interfaces between the alloyed K₃Bi phase and regenerated Bi domains during potassium extraction, enabling lattice-structure inheritance and facilitating continuous epitaxial growth of the two-dimensional (2D) bismuthene framework. Particularly, such crystallographic reversibility shows strong size dependence, preferentially occurring in nanostructured few-layer bismuthene. This nanoconfinement effect also triggers a distinct phase transition pathway (Bi ↔ KBi₂ ↔ KBi ↔ K₅Bi₄ ↔ K₃Bi) that diverges from bulk material behavior. Electrochemical evaluations reveal exceptional cycling stability, with few-layer bismuthene electrodes delivering high reversible capacities of 352 and 327 mAh g^-1 after 1200 (2 A g^-1) and 2500 (5 A g^-1) cycles, respectively, while maintaining 82.1% retention under 20 A g^-1 over 3100 cycles. These findings not only elucidate the critical role of nanoscale dimensions in alloying-type anode design but provide a paradigm for developing durable 2D materials-based energy storage systems.