High voltage Mg-doped Na_0.67Ni_0.3-xMg_xMn_0.7O₂ (x = 0.05, 0.1) Na-ion cathodes with enhanced stability and rate capability

Magnesium substituted P2-structure Na_0.67Ni_0.3Mn_0.7O₂ materials have been prepared by a facile solid-state method and investigated as cathodes in sodium-ion batteries. The Mg-doped materials described here were characterized by X-ray diffraction (XRD), ²³Na solid-state nuclear magnetic resonance (SS-NMR), and scanning electron microscopy (SEM). The electrochemical performance of the samples was tested in half cells vs Na metal at room temperature. The Mg-doped materials operate at a high average voltage of ca. 3.3 V vs Na/Na⁺ delivering specific capacities of ∼120 mAh g^-1, which remain stable up to 50 cycles. Mg doping suppresses the well-known P2-O2 phase transition observed in the undoped composition by stabilizing the reversible OP4 phase during charging (during Na removal). GITT measurements showed that the Na-ion mobility is improved by 2 orders of magnitude with respect to the parent P2-Na_0.67Ni_0.3Mn_0.7O₂ material. The fast Na-ion mobility may be the cause of the enhanced rate performance.