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Sodium (Na) vanadium (V) fluorophosphate $\mathrm{Na_xV_2(PO_4)_2F_3}$ (NVPF)is a highly attractive intercalation electrode material due to its highoperation voltage, large capacity, and long cycle life. However, several issueslimit the full utilization of NVPF's energy density: 1) the high voltageplateau associated with extracting the "third" Na ion in the reaction N$_1$VPF$\rightarrow$ VPF (~4.9 V vs Na/Na$^+$) appears above the electrochemicalstability window of most practical electrolytes (~4.5 V); 2) a sudden drop inNa-ion diffusivity is observed near composition $\mathrm{Na_1V_2(PO_4)_2F_3}$.Therefore, it is important to investigate the potential substitution of V byother transition metals in NVPF derivatives, which can access the extraction ofthe third Na-ion. In this work, we investigate the partial substitution of Vwith molybdenum (Mo), niobium (Nb), or tungsten (W) in NVPF to improve itsenergy density. We examine the structural and electrochemical behaviors of$\mathrm{Na_xV_{2-y}Mo_y(PO_4)_2F_3}$, $\mathrm{Na_xV_{2-y}Nb_y(PO_4)_2F_3}$,and $\mathrm{Na_xW_{2}(PO_4)_2F_3}$ across the whole Na composition region of 0$\leq$ x $\leq$ 4, and at various transition metal substitution levels, namely,y=0.5, 1.0, 1.5, 2.0 for Mo, and y=1.0, 2.0 for Nb. We find that partialsubstitution of 50% V by Mo in NVPF reduces the voltage plateau for extractingthe third Na ion by 0.6 Volts, which enables further Na extraction from$\mathrm{Na_1Mo_{2}(PO_4)_2F_3}$ and increases the theoretical gravimetriccapacity from ~128 to ~174 mAh/g. Analysis of the migration barriers forNa-ions in $\mathrm{Na_xVMo(PO_4)_2F_3}$ unveils improved kinetic propertiesover NVPF. The proposed $\mathrm{Na_xVMo(PO_4)_2F_3}$ material provides anoptimal gravimetric energy density of ~577.3 Wh/kg versus ~507 Wh/kg for thepristine NVPF, which amounts to an increase of ~13.9%.

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