Mechanism of Degradation of Capacity and Charge/Discharge Voltages of High-Ni Cathode During Fast Long-Term Cycling Without Voltage Margin

The authors reveal the mechanisms of degradation of capacity, charge voltage, and discharge voltage of commercially-available high-nickel cathode material when it is cycled without a voltage margin by two different charge protocols: constant-current charging and constant-current, constant-voltage charging. With repeated constant-current charging, the cathode material changes to a non-periodic cation-mixed state, which causes a relatively low voltage degradation, whereas during constant-current, constant-voltage charging, the cathode material changes from a layered structure to a periodic cation-mixed spinel-like phase, with consequent severe voltage decay. This decay results from a reduction in the equilibrium electrode potential and an increase of overpotential which are aggravated in a periodic cation-mixed state. The findings provide insights into the use of excess Li without charge-voltage margin in high-Ni cathode materials.