Elucidating Thermal Decomposition Kinetic Mechanism of Charged Layered Oxide Cathode for Sodium‐Ion Batteries

Abstract The safety of the P2‐type layered transition metal oxides (P2‐Na_xTMO₂), a promising cathode material for sodium‐ion batteries (SIBs), is a prerequisite for grid‐scale energy storage systems. However, previous thermal runaway studies mainly focused on morphological changes resulting from gas production detection and thermogravimetric analysis, while the structural transition and chemical reactions underlying these processes are still unclear. Herein, a comprehensive methodology to unveil an interplay mechanism among phase structures, interfacial microcrack, and thermal stability of the charged P2‐Na_0.8Ni_0.33Mn_0.67O₂ (NNMO) and the P2‐Na_0.8Ni_0.21Li_0.12Mn_0.67O₂ (NNMO‐Li) at elevated temperatures is established. Combining a series of crystallographic and thermodynamic characterization techniques, the specific chemical reactions occurring in the NNMO materials during thermal runaway are clarified first and solidly proved that Li doping effectively hinders the dissolution of transition metal ions, reduces oxygen release, and enhances thermal stability at elevated temperatures. Importantly, based on Arrhenius and nonisothermal kinetic equations, the kinetic triplet model is successfully constructed to in‐depth elucidate the thermal decomposition reaction mechanism of P2‐Na_xTMO₂, demonstrating that such thermodynamic assessment provides a new perspective for building high‐safety SIBs.