Layered P2-Nax[Mg, Ni, Mn]O2 Cathodes: What Are the Atomistic Effects of Cation Substitution?

Abstract

Layered sodium transition metal oxides such as Nax[Ni, Mn]O2 with P2 oxygen stacking structure are potential cathode materials for sodium-ion batteries. The main challenge is that they tend to show many plateaux in the charge-discharge profile, indicating numerous phase transitions resulted from sodium orderings, which has adverse effects on cyclability. Recently it has been reported that the substitution of non-redox active cations, such as Mg2+, Li+ and Zn2+, for transition metal ions results in smooth charge-discharge profile and improved rate performance. In this work, the P2-Nax[Mg, Ni, Mn]O2 system is investigated by both ab-initio and interatomic potential molecular dynamics simulations in order to understand the role of Mg2+ substitution. Na-ion diffusivities and migration barriers are simulated, and the atomistic effects of Mg2+ are discussed.