Research Featured on Inside Cover of Energy & Environmental Science

Increasing the energy density of battery materials is a crucial part of the global energy transition. Employing oxygen (anionic) redox in positive electrodes of lithium- and sodium-ion batteries has the potential to significantly improve their energy density by providing high voltage capacity beyond that of most transition metal redox couples. However, anionic redox typically drives rehybridization of highly oxidized oxide-anion species to form short O–O dimers, leading to structural disorder, voltage hysteresis, and voltage fade, negating much of the benefit. In this work, we report unambiguous experimental and computational spectroscopic confirmation of a redox mechanism that prevents such rehybridization and enables exceptionally low-voltage hysteresis (only 40 meV) in Na_2−xMn₃O₇. We demonstrate that large kinetic barrier for cation migration and coulombic interactions between oxidized oxide-anions and the interlayer Na vacancies disfavor rehybridization and stabilizes hole polarons on oxygen (O⁻) at 4.2 V vs. Na/Na⁺. The underlying principles and findings in our work can be applied to other materials with in-plane transition metal-vacancy ordering. This finding could lead to new compositions and structures with improved stability in a highly oxidized state, which has applications even beyond energy storage in fuel cells, catalysis, electrochromics, memory devices and more.