The effect of high-temperature storage on the reaction heterogeneity of Ni-rich layered cathode materials

Ni-rich cathode materials with high energy and power densities have been used in Li-ion batteries for next-generation electric vehicles. However, Ni-rich cathode materials undergo severe structural degradation during operation in high-temperature environments, leading to critical performance degradation. Herein, we investigate the effect of high-temperature conditions on the crystal and electronic structure of Ni-rich cathode materials during the electrochemical reaction using synchrotron-based X-ray techniques. The bulk crystal structure of the Ni-rich layered material is maintained during high-temperature storage, but changes in the surface structure and particle integrity are observed. The conspicuous growth of an insulating NiO-like rock-salt phase on the particle surface results in an asymmetrical capacity loss in the form of charge fading, that is, capacity loss during charging. In addition to this surface degradation, particle fracture impedes the transport of Li-ions and electrons, producing several particle regions with reduced electrochemical activity. As a result, the reaction heterogeneity within the cathode is significantly intensified during the electrochemical reaction, aggravating the electrochemical performance. This study elucidates the origin of the performance degradation of Ni-rich cathode materials in high-temperature environments, which can offer valuable guidance for developing advanced Li-ion batteries with thermal durability.