Review on anionic redox for high-capacity lithium- and sodium-ion batteries

Rechargeable batteries, especially lithium-ion batteries, are now widely used as power sources for portable electronics and electric vehicles, but materials innovations are still needed to satisfy the increasing demand for larger energy density. Recently, lithium- and sodium-rich electrode materials, A2MO3-family layered compounds (A = Li, Na; M = Mn4+, Ru4+ etc.), have been extensively studied as potential high-capacity electrode materials for a cumulative cationic and anionic redox activity. Negatively charged oxide ions can potentially donate electrons to compensate the absence of oxidable transition metals as a redox center to further increase the reversible capacity. Understanding and controlling the state-of-the-art anionic redox processes is pivotal for the design of advanced energy materials, highlighted in rechargeable batteries. Hence, experimental and theoretical approaches have been consecutively developed to study the diverting processes, states and structures involved. In this review, we attempt to present a literature overview and provide insight into the reaction mechanism with respect to the anionic redox processes, proposing some opinions as target oriented. Hope that through this discussion, the search for anionic redox electrode materials with high-capacity rechargeable batteries can get further development, realizing practical applications as soon as possible.