Layered sodium transition metal oxides such as Na_x[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-Na_x[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.