Unlocking the Inaccessible Energy Density of Sodium Vanadium Fluorophosphate Electrode Materials by Transition Metal Mixing

Sodium (Na) vanadium (V) fluorophosphate NaxV2(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 NaxV2–yMoy(PO4)2F3, NaxV2–yNby(PO4)2F3, and NaxW2(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 NaxVMo(PO4)2F3 unveils improved kinetic properties over NVPF. The proposed NaxVMo(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%.