Visualization of Thermal‐Induced Degradation Pathways of High‐Ni Cathode: a Comparative Study in Solid Chloride and Liquid Electrolytes

Abstract Nickel‐rich‐layered oxide cathodes are promising candidates for enhancing the energy density of lithium‐ion batteries. Higher energy density leads to severe oxygen release, poor thermal stability, and safety risks, as exothermic side reactions induce complex structural and chemical transformations at elevated temperatures. Herein, in‐situ heating scanning transmission X‐ray microscopy (STXM)‐ptychography to directly investigate the thermal degradation pathways of charged LiNi 0.8 Co 0.1 Mn 0.1 O 2 in both solid chloride and liquid electrolytes is employed. A key finding is the opposite spatial degradation behavior: in solid electrolytes, oxygen loss and Ni reduction occur from the core to surface, while in liquid electrolytes, the degradation proceeds from surface to core. These observations are closely linked to the local structural disorder around Ni atoms, as oxygen loss directly weakens the Ni─O bonding environment, promoting the reduction of Ni and accelerating lattice instability at high temperatures. Additionally, the extent of degradation is found to correlate with particle size, with solid electrolytes effectively stabilizing smaller particles. These results reveal strong spatial heterogeneity in thermal degradation and highlight the critical role of electrolyte chemistry in dictating thermal stability. Our study provides new insights into the structural and chemical evolution of Ni‐rich cathodes under thermal stress, offering valuable guidance for the design of safer, high‐performance lithium‐ion batteries.