Computational Study of NaVOPO4 Polymorphs as Cathode Materials for Na-Ion Batteries: Diffusion, Electronic Properties, and Cation-Doping Behavior

Rechargeable sodium-ion batteries have gained considerable interest as potential alternatives to lithium-ion batteries, owing to their low cost and the wide abundance of sodium. Phosphate compounds are promising materials for sodium-ion batteries because of their high structural stability. Vanadium phosphates have shown high energy densities as cathode materials, but their Na-ion transport and cation-doping properties are not as yet fully understood. Here, we have combined density functional theory calculations and molecular dynamics techniques to study the diffusion, electronic properties, and cation doping of the α-, β-, and αI-NaVOPO4 polymorphs. The calculated Na-ion activation energies of these compounds (0.3–0.5 eV) are typical for Na-based cathode materials and the simulations predict Na-ion diffusion coefficients of 10–11–10–12 cm2 s–1. The cell voltage trends show an operating range of 3.1–3.3 V vs Na/Na+, with the partial substitution of vanadium by other metals (Al3+, Co2+, Fe3+, Mn4+, Ni2+, or Ti4+) increasing the cell voltage by up to 0.2–1.0 V vs Na/Na+. Our study provides new quantitative insights into the electrochemical behavior of a potentially important class of phosphate cathode materials for sodium-ion batteries.