Pushing the limit of 3d transition metal-based layered oxides that use both cation and anion redox for energy storage

Intercalation chemistry has dominated electrochemical energy storage for decades, and storage capacity worldwide has now reached the terawatt-hour level. State-of-the-art intercalation cathodes for Li-ion batteries operate within the limits of transition metal cation electrochemistry, but the discovery of anion-redox processes in recent decades suggests rich opportunities for substantially increasing stored energy densities. The diversity of compounds that exhibit anion redox in the solid state has inspired the exploration of new materials for next-generation cathodes. In this Review, we outline the mechanisms proposed to contribute to anion redox and the accompanying kinetic pathways that can occur in layered transition metal oxides. We discuss the crucial role of structural changes at both the atomic and mesoscopic scales with an emphasis on their impact on electrochemical performance. We emphasize the need for an integrated approach to studying the evolution of both the bulk structure and electrode–electrolyte interphase by combining characterization with computation. Building on the fundamental understanding of electrochemical reaction mechanisms, we discuss engineering strategies such as composition design, surface protection and structural control to achieve stable anion redox for next-generation energy storage devices. Layered oxide compounds with anion redox are among the most promising positive electrode materials for next-generation Li-ion batteries. In this Review, we discuss the thermodynamics and kinetics of the proposed redox mechanisms, and the implications of these mechanisms for designing engineering strategies to achieve stable anion redox.