Stabilizing the oxygen lattice and reversible oxygen redox in Na-deficient cathode oxides

Abstract

To overcome the limitation of capacity from traditional oxide-based intercalation cathodes, oxygen redox reaction garners intense interest in Li-rich/Na-rich cathodes for rechargeable batteries. However, cycle degradation and voltage drop caused by the irreversible structural changes and O2 release upon electrochemical cycling hinder their commercial applications. Herein, a Na-deficient layered compound Na0.67Mn0.72Zn0.28O2 is employed to study the relationship between the oxygen redox reaction and the (O2)n− stabilization mechanism by using a combined experimental and computational approach. Our results indicate that Na0.67Mn0.72Zn0.28O2 has a long plateau of approximately 4.1 V with a total capacity of 157 mAh g−1, majorly stemming from the oxygen redox reactions of O2−/(O2)n−. The O-2p nonbonding states originating from the weak Zn–O interaction and the shortened O–O bonds induced by the absence of Na atoms in the alkali layers catalyze the formation of peroxo-like O–O dimer between the Mn–Mn atoms. The honeycomb TM layer of the Na-deficient materials remains stable with a small O–O bond length evolution from 2.55 Å to 2.48 Å during the oxygen redox process, which stabilizes the lattice oxygen in electrochemical cycles. This finding is important for further understanding the mechanism underlying the oxygen redox chemistry of high energy Na-deficient cathode materials.