Decoupling the effect of vacancies and electropositive cations on the anionic redox processes in Na based P2-type layered oxides

The activation of anionic redox couple is recognized as one of the best way to increase the energy density of positive electrode materials in both Li and Na-ion batteries. However, for such hope, to materialize a better understanding of the parameters governing the activation, reversibility and efficiency of the anionic redox in NaMO2 layered compounds is still sorely needed. Herein, we report a new P2–Na0.63[□0.036Mg0.143Mn0.820]O2 compound that combines vacancies and Mg doping as well-known sources for anionic redox activation and benchmark its electrochemical performances against P2–Na0.72[Mg0.31Mn0.69]O2. We found that vacancies and Mg doping trigger independently anionic redox processes that differ in terms of redox voltage and reversibility. The one associated to vacancies occurs at the lowest potential and is irreversible. Moreover, we evidenced by monitoring the structural evolution of the pristine phase during cycling the benefice of anionic processes in ensuring the stabilization of P2-type structure at high voltage over a wide range in Na content. These findings highlight the importance of the anionic redox process origin (e.g. vacancies vs. highly electropositive cations) in governing the material electrochemical properties, while providing a new way to efficiently stabilize, without capacity loss, the P2-type structure through the charge process in non A-rich compounds.