Pinpointing Chemomechanical Origins of Na Cathode Degradation

The performance and longevity of sodium-ion batteries are heavily influenced by cathode degradation, particularly under high-voltage cycling. Despite ongoing research, the interplay between chemical and mechanical processes remains unclear. Here, we investigated the degradation mechanisms of an O3-NaLi_1/9Ni_2/9Fe_2/9Mn_4/9O₂ (NLNFM) cathode material using synchrotron-based nanoresolution chemical imaging. Oxygen loss at high voltages was identified as the primary trigger, causing unwanted phase transformations, disrupting sodium intercalation, and leading to capacity fade. Fluorine incorporation during cycling also induced stress and particle cracking, accelerating degradation. By analyzing particles of different sizes, we revealed distinct degradation pathways: small particles experience severe side reactions during early cycling due to their high specific surface area, while large particles develop progressive structural damage during extended cycling from intraparticle heterogeneity and stress. These findings highlight the particle-size-dependent nature of cathode degradation and inform strategies such as particle size optimization, doping, micromorphology design, and stress-tolerant structures to mitigate capacity fade in sodium-ion batteries.