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

Abstract

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.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

“…Although, as mentioned above, numerous studies have been conducted on ARR on peroxides with a decreased O–O length, specific chemical oxygen bonding and alkali-rich systems, the basic mechanisms of O-redox are still controversial and unconfirmed. 345–347 Therefore, it is deemed advantageous and highly significant to precisely observe the involvement of oxygen anions in charge compensation, together with the resulting structural response to these anionic activities and the related electrochemical behaviour via sophisticated and targeted characterization measurements. 348,349 This will aid in the development of layered oxide cathodes with higher reversible capacity.…”

Facile Morphology Modulation to Enhance Stability and Fast Kinetics of Anionic Redox for Li-Rich Layered Oxides Cathodes

“…This alteration affects the covalency of the TM-O bond, mitigating oxygen release and enhancing the electrochemical stability of materials. 10,11 While these modification strategies partially address LLOs' issues, the complex treatment processes and introduction of extra inert materials either elevate costs or decrease total energy density, limiting their potential commercial application for high energy density cathodes. Instead of relying solely on subsequent material modifications, researchers should focus on the initial design of LLOs, particularly their composition, morphology, and crystal structure, to enhance their electrochemical properties.…”

“…Bulk structure modification strategies primarily refer to elemental doping, which regulates local electronic and crystal structures. This alteration affects the covalency of the TM-O bond, mitigating oxygen release and enhancing the electrochemical stability of materials. , While these modification strategies partially address LLOs’ issues, the complex treatment processes and introduction of extra inert materials either elevate costs or decrease total energy density, limiting their potential commercial application for high energy density cathodes.…”

Facilitating an Ultrastable O3-Type Cathode for 4.5 V Sodium-Ion Batteries via a Dual-Reductive Coupling Mechanism

“…However, although inducing oxygen redox contributes to an elevated specific capacity, the overoxidation of O would result in the irreversible oxygen loss as well as transition metal (TM) migration, − leading to the considerable structure degradation with a rapid capacity and voltage decay. To relieve the issues, TM doping, F doping, surface coating, etc.…”