High voltage structural evolution and enhanced Na-ion diffusion in P2-Na_2/3Ni_{1/3-: X}Mg_xMn_2/3O₂ (0 ≤ x ≤ 0.2) cathodes from diffraction, electrochemical and ab initio studies

We have presented a detailed investigation of the effects of Mg substitution on the structure, electrochemical performance and Na-ion diffusion in high voltage P2-type Na_2/3Ni_1/3-xMg_xMn_2/3O₂ (0 < x < 0.2) cathode materials for Na-ion batteries. Structural analysis using neutron diffraction showed that Mg²⁺ substitutes at Ni²⁺ sites from ordered [(Ni²⁺/Mn⁴⁺)O₆] honeycomb units along the ab-plane, leading to an AB-type structure that can be indexed using the P6₃ space group. Within the sodium layers, high Mg-substitution levels (i.e. x = 0.2) caused a disruption in the typical Na zig-zag ordering observed in the undoped material, leading to a more disordered Na distribution in the layers. Load curves of the x = 0.1 and 0.2 materials show smooth electrochemistry, indicative of a solid-solution process. Furthermore, DFT calculations showed an increase in Na-ion diffusivity for the Mg-substituted samples. Enhanced cycling stability was also observed in these materials; structural analysis using high-resolution in operando synchrotron X-ray diffraction show that such an improved electrochemical performance is caused by the suppression of the O2 phase and switch to the formation of an OP4 phase. Ab initio studies support our experimental evidence showing that the OP4 phase (cf. O2) is the most thermodynamically stable phase at high voltages for Mg-substituted compounds. Finally, we have provided evidence using diffraction for the x = 1/2 and x = 1/3 intermediate Na⁺-vacancy ordered phases in P2-Na_2/3Ni_1/3Mn_2/3O₂.