Initial Coulombic Efficiency as a Descriptor of Structural Evolution in Layered Cathode Materials

Maximizing energy density in Lithium-ion batteries requires careful attention to the initial coulombic efficiency (ICE) of the cathode. Even among layered metal oxides, the ICE of ternary cathodes (NCM) and lithium cobalt oxide (LCO) shows significant differences. Nevertheless, the fundamental causes of ICE loss remain poorly understood, particularly in distinguishing between kinetic and structural contributions. In this study, the mechanisms behind ICE loss in layered cathodes are systematically investigated. By introducing apparent and real irreversible capacities and their relationship with charging cut-off voltage and ICE, the roles of lithium-ion diffusion and structural degradation is differentiate, positioning ICE as a physical descriptor that correlates with phase transitions and structural evolution. For LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), below 4.1 V, the irreversible capacity loss originates almost entirely from lithium diffusion kinetics at the end of discharge, which can be recovered through constant-voltage discharge and shows a certain correlation with the reversible phase transition. Above 4.1 V, real irreversible capacity arises due to irreversible phase transitions and lattice distortion. These results establish a direct link between ICE and structural changes, positioning ICE as a key diagnostic tool for probing phase transitions in layered cathodes and offering insights for the design of next-generation, high-efficiency materials.