Design Rules for High-Valent Redox in Intercalation Electrodes

High-valent redox, where over-oxidation of oxygen or transition metals (TMs) drives extensive charge sharing through the formation of short covalent bonds, has historically been avoided in intercalation electrodes because of its association with structural disorder and electrochemical irreversibility. Here, we present a perspective on the origin of these undesirable behaviors and materials design criteria to mitigate them. Drawing parallels between oxygen redox and high-valent TM redox (e.g., CrIII/VI and VIII/V), we reveal that the defect formation energy landscape is the primary factor controlling the electrochemical reversibility of high-valent redox, as it determines which defects form to accommodate the short covalent bonds as well as the nature of those covalent bonding arrangements. By tuning the defect formation energy landscape, researchers can control the nature of the oxidized species while minimizing structural disorder. These concepts reveal a wide range of previously avoided redox mechanisms as promising candidates for high density energy storage.