candidates for next-generation LIBs, due to their high reversible capacities. [2] However, NRLOs suffer from rapid capacity fading and thermal instability, especially at high loading level (thick electrode) and elevated temperature, seriously restricting their large-scale application. [3] To overcome these problems, a fundamental understanding of degradation mechanism for the NRLOs electrodes is critical especially for thick electrodes under elevated operating temperature.Generally, the severe degradation of thick cathode mainly results from the following three factors: i) the limited electronic and Li + transport through the thick NRLOs electrode (e.g., longer pathway for charge transfer) [1a,3b,4] ii) the accelerated chemical/electrochemical side reactions between NRLOs and electrolyte at high temperature. [1a,3d,4c,d,5] iii) the overutilization of active materials on the surface and non-uniform potential distribution during high-temperature cycling. [4c,5a,6] Although previous studies put forward valuable insights into the degradation of NRLOs, the detailed degradation mechanism based on multiscale perspectives of ions, crystal structures, particles, electrodes and full-cells have not been thoroughlyThe thermal instability is a major problem in high-energy nickel-rich layered cathode materials for large-scale battery application. Due to the scarce investigation of thick electrodes at the practical full-cell level, the understanding of thermal failure mechanism is still insufficient. Herein, an intrinsic origin of thermal instability in fully charged industrial pouch cells during hightemperature storage is discovered. Through the investigation from crystals to particles, and from electrodes to cells, it is shown that serious top-down heterogeneous degradation occurs along the depth direction of the thick electrode, including phase transition, cationic disordering, intergranular/ intragranular cracks, and side reactions. Such degradation originates from the abundant oxygen vacancies and reduced catalytic Ni 2+ at cathode surface, causing microstructural defects and directly leading to the thermal instability. Nonmagnetic elements doping and surface modification are suggested to be effective in mitigating the thermal instability through modulating cationic disordering.
