Revealing the origin of high-thermal-stability of single-crystal Ni-rich cathodes toward higher-safety batteries

The poor thermal stability of Ni-rich cathode materials, resulting in thermal runaway of the battery, is a major safety threat to the development of lithium-ion batteries. However, the thermal degradation mechanism that determines thermal stability, especially for the promising single-crystal (SC) Ni-rich cathode material, has not been elucidated. More importantly, this is indeed a fundamental issue. Herein, via a series of in-situ/ex-situ probing technologies, the thermal degradation of SC Ni-rich material is elaborately diagnosed from surface to bulk phase and compared with polycrystalline (PC) Ni-rich material. A comprehensive oxygen release kinetic model including oxygen diffusion distance, mechanical stress and temperature is presented. This model reveals that the SC Ni-rich material exhibits a stable depth-dependent gradient oxygen release kinetics, while the PC Ni-rich material exhibits an accelerated oxygen release kinetics by grain boundaries, which reveals the origin of the high-thermal-stability of SC Ni-rich cathodes. This work highlights the importance of suppressing oxygen release kinetics (e.g., increase oxygen diffusion distance, increase mechanical stress) to improve thermal stability, facilitating the development of safer lithium-ion batteries based on Ni-rich cathodes.