Insights into the high voltage layered oxide cathode materials in sodium-ion batteries: Structural evolution and anion redox

Abstract The phase evolution and anion redox behaviours under electrochemical cycling of sodium transition metal layered oxides have both been intensively studied. In this review, we focus on the relationship between them at high voltage states-of-charge, in the context of growing evidence that anion redox begins at an earlier state of charge than previously thought, and that the final structural change can behave in either a biphasic or a pseudo-solid solution manner, depending on the transition metal element. By systematically reviewing major breakthroughs in experimental materials discovery, characterization techniques and theoretical understanding, we tentatively propose a new way to think about the phase transition dynamics and a path to improving the theory of anion redox.