First-Principles Insight into the Impact of Oxygen Substitution in Na3V2(PO4)2F3 Cathodes on the Structural Evolution, Redox Mechanism, and Na-Ion Migration

Sodium vanadium phosphate fluoride (Na3V2(PO4)2F3, NVPF) has emerged as a promising NASICON-type cathode material for sodium-ion batteries due to its 3D Na-ion diffusion channels, high voltage, and high theoretical capacity. However, issues with Na-ion diffusion kinetics and electrical conductivity have limited its electrochemical performance. In this work, first-principles calculations were employed to systematically investigate the structural evolution, average voltage, magnetism, and electronic structure of Pnnm NVPF and partially the O-substituted product Na3V2(PO4)2F2O. Compared to disordered P42/mnm, ordered Pnnm NVPF exhibited lower desodiation voltages and Na+ migration barriers, resulting in improved electrochemical properties. Additionally, Na3V2(PO4)2F2O enabled extract more Na+ near the electrolyte stability limit, enhancing capacity and energy density. However, lattice contraction from O substitution also increased Na+ diffusion barriers in Na3V2(PO4)2F2O. Distinct redox mechanisms were revealed for the two materials, offering vital information for optimizing NASICON cathodes.