Anionic Redox in Rechargeable Batteries: Mechanism, Materials, and Characterization

Compared with conventional positive electrode materials in Li-ion batteries, Li-rich materials have a huge advantage of large specific capacities of >300 mAh g(-1). Anionic redox mechanism is proposed to explain the over-capacity, which means anions can participate in the redox process for charge compensation. The concept enriches the range and design considerations of high-energy-density positive electrode materials for both Li-ion and Na-ion batteries, which therefore arouses extensive attention. This review summarizes the progress of anionic redox in rechargeable batteries in recent years and discusses the fundamental mechanism that triggers anionic redox. Moreover, the state-of-the-art materials involving anionic redox are illustrated, accompanied by the challenges for practical applications. Furthermore, the common techniques for monitoring anionic redox are reviewed and compared for an advisable choice in future studies. Finally, the consideration and discussion for designing stable positive electrodes based on cationic and anionic redox are presented. The perspective is highlighted and this review provides a basic understanding of anionic redox in rechargeable batteries and paves the way to develop high-capacity positive electrodes for high-energy battery systems.