Highly Reversible Oxygen‐Redox Chemistry at 4.1 V in Na 4/7− x [□ 1/7 Mn 6/7 ]O 2 (□: 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 Na 2 Mn 3 O 7 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.