In situ atomic-scale tracking of unusual interface reaction circulation and phase reversibility in (de)potassiated alloy-typed anode

Alloy-typed anode materials, endowed innately with high theoretical specific capacity, hold great promise as an alternative to intercalation-typed counterparts for alkali-ion batteries. Despite tremendous efforts devoted to addressing drastic volume change and severe pulverization issues of such anodes, the underlying mechanisms involving dynamic phase evolutions and reaction kinetics have not yet been fully comprehended. Herein, taking antimony (Sb) anode as a representative paradigm, its microscopic operating mechanisms down to the atomic scale during live (de)potassiation cycling are systematically unraveled using in situ transmission electron microscopy. Highly reversible phase transformations at single-particle level, that are Sb ↔ KSb2 ↔ KSb ↔ K5Sb4 ↔ K3Sb, were revealed during cycling. Meanwhile, multiple phase interfaces associated with different reaction kinetics coexisted and this phenomenon was properly elucidated in the context of density functional theory calculations. Impressively, previously unexplored unidirectional circulation of reaction interfaces within individual Sb particle is confirmed for both potassiation and depotassiation. Based on the empirical results, the surface diffusion-mediated potassiation-depotassiation pathways at single-particle level are suggested. This work affords new insights into energy storage mechanisms of Sb anode and valuable guidance for targeted optimization of alloy-typed anodes (not limited to Sb) toward advanced potassium-ion batteries.