As the demand for rechargeable lithium-ion batteries (LIBs) with higher energy density increases, the interest in lithium-rich oxide (LRO) with extraordinarily high capacities is surging. The capacity of LRO cathodes exceeds that of conventional layered oxides. This has been attributed to the redox contribution from both cations and anions, either sequentially or simultaneously. However, LROs with notable anion redox suffer from capacity loss and voltage decay during cycling. Therefore, a fundamental understanding of their electrochemical behaviors and related structural evolution is a prerequisite for the successful development of high-capacity LRO cathodes with anion redox activity. However, there is still controversy over their electrochemical behavior and principles of operation. In addition, complicated redox mechanisms and the lack of sufficient analytical tools render the basic study difficult. In this review, we aim to introduce theoretical insights into the anion redox mechanism and in situ analytical instruments that can be used to prove the mechanism and behavior of cathodes with anion redox activity. We summarized the anion redox phenomenon, suggested mechanisms, and discussed the history of development for anion redox in cathode materials of LIBs. Finally, we review the recent progress in identification of reaction mechanisms in LROs and validation of engineering strategies to improve cathode performance based on anion redox through various analytical tools, particularly, in situ characterization techniques. Because unexpected phenomena may occur during cycling, it is crucial to study the kinetic properties of materials in situ under operating conditions, especially for this newly investigated anion redox phenomenon. This review provides a comprehensive perspective on the future direction of studies on materials with anion redox activity.

The synergistic combination of oxygen vacancies, dual OER/ORR catalysts, and the complementary catalyst with oxygen-mediating capability in the Co3O4/Ru–CeO2/Li2O2−x successfully activated reversible anion redox reaction in lithia-based cathodes.

Cathode materials commonly experience volumetric changes that can reduce the cycle life of lithium-based rechargeable batteries. To improve stability in performance, materials must be designed to be structurally invariant throughout electrochemical cycling. Zero-strain cathode materials refer to those cathode materials that undergo negligible or zero volumetric changes during cell cycling. These can provide various benefits, including a high battery operating voltage, high capacity, and long-term stability. In this review, we summarize the problems of conventional cathode active materials originating from volumetric changes with the origin of strains and discuss the zero-strain behavior of the cathode. Recent advancements in the validation of engineering strategies to enhance cathode performance based on zero-strain behavior and identification of reaction mechanisms in zero-strain cathodes are highlighted. Further, analytical methods are introduced that can be used to demonstrate the strain behavior of cathodes with suppressed volumetric changes. Finally, a comprehensive outlook on the future direction of research on materials with zero-strain behavior is provided.

Chemical compatibility of various polymeric binders and solvents with lithia is investigated for improved cycling stability of lithia-based cathodes.