Understanding doping effects on P2 NaxMn1−yMyO2 (M=Li, Mg, Al, Ti, V, Cr, Fe, Co, Ni) cathode materials for Na-ion batteries

The P2-layered oxide ${\mathrm{NaMnO}}_{2}$ is known as a cheap and high-capacity material for secondary batteries, but has been limited in application due to the large Jahn-Teller lattice distortion. Through the highly accurate hybrid functional method (HSE06), we computationally evaluate the doping influences on the lattice distortions, stabilities, electronic structures, redox potentials, and diffusion mechanisms. Our calculations indicate that dopants not only reduce the lattice distortion degree, especially for Li, Mg, Ti, and V cases, but also increase the stability of the structure, implying the dopants would alleviate the Jahn-Teller lattice distortion. At low Na concentrations, the Li dopant preferably diffuses out of the ${\mathrm{MnO}}_{2}$ layer, but hardly moves to the Na layer, suggesting that P2-layered oxides can prevent the dopant's migrations during the Na extractions. At full Na concentrations, all the considered dopants, except for Ti and V, have a small effect on the redox potential. The effect on the diffusion mechanism is described through the diffusion of a Na ion--polaron complex near the dopant's environments. Fe, Mg, Ti, and Cr dopants can hinder the Na ion-polaron complex diffusion with significantly higher activation energies, respectively, while the Al dopant almost remains the activation energy as well as the perfect structure. However, Li, V, Co, and Ni dopants benefit from such complex diffusion with much lower activation energy so the ion diffusivities increase significantly. It is found that the doping influence on the activation energy for Na ion diffusion is associated with the $M$--O bond change and charges of the neighboring dopants.