Recovery of High-Voltage Oxygen Redox Activity by Eliminating Residual Oxygen Dimers

Abstract

Oxygen-redox layered oxides are promising electrode materials with the potential for superior energy density; however, their real-world employment is hindered by severe electrochemical irreversibility. The fundamental origin of this irreversibility has been difficult to pinpoint due to the complex evolution of both the structure and redox centers during cycling. Herein, we reveal that oxygen redox irreversibility is governed by the formation and persistence of oxygen dimer states and, for the first time, demonstrate the reversible recovery of the intrinsic high-voltage oxygen redox plateau (∼4.5 V vs Li/Li+), which is typically lost after initial charge. Our in-depth tracking of oxygen dimer behavior establishes a direct correlation between the recoverability of the high-voltage plateau and the residual concentration of oxygen dimers in the electrode material. Furthermore, we identify sluggish dimer dissociation kinetics as the root cause of the irreversible loss of high-voltage oxygen redox activity. More importantly, we demonstrate that these kinetically trapped oxygen dimers can be eliminated by facilitating electron transfer from transition metal to oxygen dimers via redox reshuffling at a moderately elevated temperature, thereby fully restoring the high-voltage oxygen redox activity. These findings clarify the role of oxygen dimer kinetics in redox irreversibility and provide valuable insights toward achieving voltage-hysteresis-free anionic redox in oxygen redox electrodes.