Understanding of the Irreversible Phase Transition and Zr-Doped Modification Strategy for a Nickel-Rich Cathode under a High Voltage

Increasing the nickel content and broadening the voltage window are important means for LiNixCoyMn1–x–yO2 layered cathodes with low cost and high energy density, but these nickel-rich cathodes often suffer from structural instability and unsatisfactory cyclic performance. The systematic and detailed degradation mechanism especially under a high voltage is still unclear, which hinders the further development of nickel-rich cathodes. Our results show that due to the migration of high valence nickel ions to lithium sites, especially upon the deep removal of Li+ ions, the nickel-rich cathode undergoes an irreversible phase transformation from a layered structure to a spinel or even rock-salt phase. Such irreversible phase transitions within a wide voltage window would cause insufficient lithium utilization and voltage decay, finally deteriorating the electrochemical performance of nickel-rich cathodes. In a narrow voltage range of 3.0–4.3 V, the capacity retention of the Ni-rich cathode is 93.4%, and the voltage fading is only 40 mV after 250 cycles. However, the cathode only exhibits a capacity retention of 77.4% with a significant voltage decay over 180 mV, as the voltage range further extends to 3.0–4.6 V. Furthermore, various characterizations and electrochemical performances demonstrate that the strengthened metal–oxygen bonds in the transition layer can produce stable structures and suppress phase transitions, thereby displaying superior electrochemical performance in the widened voltage window. As a result, the cycling retention of a Zr-doped cathode reaches 84.5%, and the voltage decay is only 50 mV after 250 cycles at 3.0–4.6 V, which exhibits excellent long-term cycle performance. These insights provide guidance for understanding the electrochemical mechanism and the design of high-voltage cathode materials.