The rich diversity of mechanisms that can explain “anion redox” in Li-ion and Na-ion cathode materials

The ability to harness redox processes beyond traditional transition metal-based charge compensation phenomena in Li-ion and Na-ion cathodes will enable significant increases in the energy density of electrochemical energy storage technologies. Anion redox remains poorly understood and is accompanied by irreversibilities and degradation mechanisms that lead to hysteresis, voltage fade and limited cycle life. While most electrode chemistries that exhibit anion redox achieve their excess capacity through structure altering redox (SAR) mechanisms that cause electrode degradation, several electrode chemistries exhibit reversible anion redox with minimal polarization. This suggests that the chemistry and crystal structure of the host plays a crucial role in enabling different redox mechanisms that have been interpreted as anion redox. First-principles statistical mechanics methods are an invaluable tool to help elucidate the true nature of anion redox, but they must be placed within a multi-scale framework to explain anomalous capacities in Li-ion and Na-ion batteries as well as other electrochemical properties that are peculiar to “anion-redox” cathodes.