Performance Degradation Mechanism of High-Nickel Cathode Depending on Discharge Rates and Charge Voltages during Long-Term Cycling

This study investigates the degradation mechanisms of high-nickel (Ni) layered oxide (LiNi_0.83Co_0.11Mn_0.06O₂) under varying discharge C-rates at a high cut-off voltage (4.3 V) during long-term cycling. Contradictory to conventional knowledge, a low discharge rate (0.1C) results in worse cycle performance than a high discharge rate (1C) at a high cut-off voltage. In-depth transmission electron microscopy analysis reveals that at a high C-rate discharge condition, more Ni ions are reduced from +3 to +2, yet the layered structure is maintained. In contrast, at a low C-rate, more Ni ions retain their +3 valence but the phase transition to the periodically ordered spinel occurs at some portion. The prolonged dwell time at high voltage forces Ni ions in Li layers to be locally ordered, and this phase transition more critically affects the cycling. Therefore, this study underscores that setting a proper cut-off voltage can be more significant to the cycle performance than the discharge C-rate.