Impact of Mg and Ti doping in O3 type NaNi1/2Mn1/2O2 on reversibility and phase transition during electrochemical Na intercalation

Abstract

O3 type layered sodium nickel manganese oxide, NaNi_1/2Mn_1/2O₂, which is isostructural with α-NaFeO₂, has attracted attention as a promising positive electrode material for sodium-ion batteries owing to its large reversible capacity of ca. 200 mA h g⁻¹. To improve the cycle stability for practical use, O3 type NaNi_1/2Mn_1/2O₂ materials with Mg or/and Ti substitution are synthesized. The materials with Mg or Ti substitution exhibit better capacity capability, and Mg and Ti co-substituted material demonstrates even better capacity capability, with an initial discharge capacity of 200 mA h g⁻¹ without any capacity loss due to substitution. Substitution of Mg²⁺ and Ti⁴⁺, which are larger ions than Ni²⁺ or Mn⁴⁺, results in a larger in-plane lattice of the O3 type structure, in contrast to the shrinkage during charging, and this has the potential to delay the phase transition during charging. In contrast to the non-substituted NaNi_1/2Mn_1/2O₂, the Mg and Ti co-substituted material demonstrates more continuous phase transitions and lattice parameter changes, and no significant shrinkage of the interslab spacing in the layered structure, as evidenced by ex situ and operando X-ray diffraction. The coexistence of Mg and Ti enhances not only the reversibility of the structural change but also the structural stability at the surface, resulting in the excellent sodium battery performance.