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. Constant voltage step at high voltage (4.3 V) entails periodic cation‐mixed states in high‐nickel cathode materials which leads to severe voltage decay. Thus, constant voltage step at high voltage should be avoided for high‐nickel cathode materials. More fundamentally, structural stability of high‐nickel cathode materials should be improved to alleviate lattice distortion induced by the phase transition between two hexagonal phases.