Abstract

Sodium (Na) vanadium (V) fluorophosphate Na_xV₂(PO₄)₂F₃ (NVPF) is a highly attractive intercalation electrode material due to its high operation voltage, large capacity, and long cycle life. However, several practical issues limit the full utilization of NVPF’s energy density: (1) the high voltage plateau associated with extracting the “third” Na ion in the reaction N₁VPF → VPF (∼4.9 V vs Na/Na⁺) appears above the electrochemical stability window (ESW) of most practical electrolytes (∼4.5 V vs Na/Na⁺); and (2) a sudden drop in Na-ion diffusivity is observed near composition Na₁V₂(PO₄)₂F₃. Therefore, it is important to investigate the potential substitution of V by other transition metals (TMs) in NVPF derivatives, which can practically access the extraction of the third Na-ion. In this work, we investigate the partial substitution of V with molybdenum (Mo), niobium (Nb), or tungsten (W) in NVPF to improve its energy density. Using first-principles calculations, we examine the structural and electrochemical behaviors of Na_xV_2-yMo_y(PO₄)₂F₃, Na_xV_2-yNb_y(PO₄)₂F₃, and Na_xW₂(PO₄)₂F₃ across the whole Na composition region of 0 ≤ x ≤ 4, and at various transition metal (TM) substitution levels, namely, y = 0.5, 1.0, 1.5, and 2.0 for Mo, and y = 1.0 and 2.0 for Nb. We found that partial substitution of 50% V by Mo in NVPF reduces the voltage plateau for extracting the third Na ion by 0.6 volts, which enables further Na extraction from Na₁VMo(PO₄)₂F₃ and increases the theoretical gravimetric capacity from ∼128 to ∼174 mAh/g. Analysis of the migration barriers for Na-ions in Na₃VMo(PO₄)₂F₃ unveils improved kinetic properties over NVPF. The proposed Na₁VMo(PO₄)₂F₃ material provides an optimal gravimetric energy density of ∼577.3 Wh/kg vs ∼507 Wh/kg for the pristine NVPF, which amounts to an increase of ∼13.9%.