Na x P-Dominated Cycling Degradation Mechanisms in 3-Ah Phosphorus-Based Sodium-Ion Batteries

Phosphorus-based sodium-ion batteries (SIBs) are promising candidates for next-generation energy storage due to their high theoretical capacity and elemental abundance. However, their long-term degradation behavior remains poorly understood, especially at practical cell capacities. In this work, we systematically investigate a high-capacity 21700-type 3 Ah phosphorus-based SIB using 700-cycle electrochemical evaluation, advanced microstructural analysis, and specific surface area measurements. Strikingly, phosphorus-based anodes exhibit more severe capacity loss in a full cell than a half cell. Furthermore, the formation of (Na x P, x = 1–3) spherical particles on the anode surface is observed for the first time under extended cycling, revealing a key failure pathway. This study provides direct, cycle-scale evidence of the dominant degradation route in practical phosphorus-based SIBs, offering crucial insights for targeted anode material engineering and lifetime enhancement strategies.