Degradation Mechanism of Ni-Rich Cathode Materials: Focusing on Particle Interior

In the development of Li-ion batteries for electric vehicles (EVs), Ni-rich layered oxides are anticipated to be promising cathode materials. However, the rapid capacity fading originating from microcracks has prevented practical applications of Ni-rich cathodes. Herein, we systematically perform post-mortem analyses of Li­[Ni_xCo_yMn_1‑x‑y]­O₂ (x = 0.8 and 0.9) cathodes after long-term cycling, focusing on the particle interior. The results demonstrate that microcracks and the resultant degradation of the secondary particle interior by exposure to the deleterious electrolyte are dominant factors in the deterioration of Ni-rich cathodes. Moreover, cathode degradation significantly decreases the ionic and electrical conductivities, leading to the partial electrochemical insulation inside the cathode particles. This insulation contributes to the kinetic loss of capacity at high C-rates and induces structural inhomogeneity in the cathode. A comprehensive understanding of the degradation mechanism of Ni-rich cathodes suggests guidelines for developing Ni-rich cathode materials that are appropriate for application in EVs.