Exploring the anionic redox chemistry in cathode materials for high-energy-density sodium-ion batteries

Improving the energy and power densities of sodium-ion batteries is a prime challenge to establish this energy storage technology to be on par with state-of-the-art lithium-ion batteries. The energy density of the sodium-ion batteries is limited due to the lower redox potential of their electrode materials compared to that of the corresponding Li analogues; however, it can be overcome by triggering the anionic redox. Although anionic redox has received significant research interest, a clear understanding of the underlying mechanism for delivery of high capacity by utilizing anionic redox is still lacking. Formidable challenges associated with the utilization of anionic redox such as rapid material degradation, voltage fade, and oxygen release hinder its practical applications. Given the great potential of anionic redox chemistry for developing high-energy batteries, in this mini-review, the recent mechanistic understanding, electrode material degradation pathways including oxygen release, and strategies to trigger anionic redox are discussed. An overview of the existing potential and future research directions of sodium-ion batteries involving anionic reaction is provided at the end.