Native Vacancy Enhanced Oxygen Redox Reversibility and Structural Robustness

Cathode materials with high energy density, long cycle life, and low cost are of top priority for energy storage systems. The Li-rich transition metal (TM) oxides achieve high specific capacities by redox reactions of both the TM and oxygen ions. However, the poor reversible redox reaction of the anions results in severe fading of the cycling performance. Herein, the vacancy-containing Na_4/7[Mn_6/7(◻_Mn)_1/7]O₂ (◻_Mn for vacancies in the MnO slab) is presented as a novel cathode material for Na-ion batteries. The presence of native vacancies endows this material with attractive properties including high structural flexibility and stability upon Na-ion extraction and insertion and high reversibility of oxygen redox reaction. Synchrotron X-ray absorption near edge structure and X-ray photoelectron spectroscopy studies demonstrate that the charge compensation is dominated by the oxygen redox reaction and Mn³⁺/Mn⁴⁺ redox reaction separately. In situ synchrotron X-ray diffraction exhibits its zero-strain feature during the cycling. Density functional theory calculations further deepen the understanding of the charge compensation by oxygen and manganese redox reactions and the immobility of the Mn ions in the material. These findings provide new ideas on searching for and designing materials with high capacity and high structural stability for novel energy storage systems.