Redox Engineering of Fe‐Rich Disordered Rock‐Salt Li‐Ion Cathode Materials

Abstract The pursuit of high‐performance and cost‐effective Li‐ion batteries emphasizes the need for cathode materials composed of abundant elements, such as Fe. Disordered rock‐salt (DRX) cathode materials, known for their high compositional flexibility, offer a unique opportunity in this regard. However, Fe‐rich DRX (Fe‐DRX) cathodes, potentially the most cost‐effective among all DRXs, have seen limited research interest due to their comparatively restrained performance. This limitation stems from the inaccessibility of the Fe 3+ /Fe 4+ redox in the DRX structure, prompting the need for redox engineering to enable Fe‐DRXs with readily utilizable redox mechanisms. In this work, utilizing both experiments and theoretical study, reversible Fe 2+ /Fe 3+ redox in an Fe 2+ ‐based DRX cathode is demonstrated. This design minimizes the reliance on O redox, resulting in a high capacity (≈290 mAh g −1 ) and energy density (≈700 Wh kg −1 ), as opposed to an Fe 3+ ‐based DRX operating on the limited Fe 3+ /Fe 4+ redox and extensive O redox upon cycling. Overall, the study introduces a novel approach to redox engineering to develop low‐cost, high‐performing Fe‐rich cathode materials.