Highly Reversible Oxygen-Redox Chemistry at 4.1 V in Na_{4/7− x}[□_1/7Mn_6/7]O₂ (□: Mn Vacancy)

Increasing the energy density of rechargeable batteries is of paramount importance toward achieving a sustainable society. The present limitation of the energy density is owing to the small capacity of cathode materials, in which the (de)intercalation of ions is charge-compensated by transition-metal redox reactions. Although additional oxygen-redox reactions of oxide cathodes have been recognized as an effective way to overcome this capacity limit, irreversible structural changes that occur during charge/discharge cause voltage drops and cycle degradation. Here, a highly reversible oxygen-redox capacity of Na₂Mn₃O₇ that possesses inherent Mn vacancies in a layered structure is found. The cross validation of theoretical predictions and experimental observations demonstrates that the nonbonding 2p orbitals of oxygens neighboring the Mn vacancies contribute to the oxygen-redox capacity without making the Mn−O bond labile, highlighting the critical role of transition-metal vacancies for the design of reversible oxygen-redox cathodes.