In situ characterization of crystal phase evolution of the LiNi_0.6Co_0.2Mn_0.2O₂ cathode at different current densities

With the rapid development of EVs, LiNi0.6Co0.2Mn0.2O2 (NCM622) ternary materials are widely used in lithium-ion batteries due to their high energy density and cycling performance. However, during cycles at different current densities, NCM622 materials undergo structural changes and bulging, leading to electrolyte decomposition and gas generation. Hence, studying the phase transition of NCM materials at different current densities is crucial. In this study, in situ X-ray diffraction is employed to investigate the phase transition and lattice parameter variations of NCM622 materials during the first two charge-discharge cycles at 0.5-10C. Three crystalline forms, namely hexagonal phases (H1, H2) and a solid solution (S) phase, are observed with gradual reduction in Li+ content. With increasing current densities, the H2 phase rapidly appears while the H1 phase slowly disappears during charging. This indicates a higher deintercalation of Li+, benefiting the increase in specific capacity. During discharging, the homogeneous S phase facilitates lithium-ion migration, but the change in lattice anisotropy decreases with higher current densities due to increased intercalation and stronger lattice interaction forces. At high current densities, incomplete Li+ migration results in relatively lower specific capacity, while the smaller lattice strain in the S phase may enhance cycling stability. Initiating the first two cycles at low current density alters the phase transition mechanism but induces lattice strain, potentially causing particle cracking, secondary reactions, and reduced cell longevity. While a dense surface electrolyte film forms at a low rate, the material exhibits better cycling stability with higher specific capacity during the initial 120 cycles compared to that at high current density.