Theoretical and Experimental Study of Vanadium-Based Fluorophosphate Cathodes for Rechargeable Batteries

Abstract

A single-phase crystalline Na3V2O2(PO4)2F material has been prepared by the solvothermal method. Partial ion exchange between Na and Li was then used to form Na3–xLixV2O2(PO4)2F. The two materials were studied as positive cathodes by physical characterization, electrochemical measurements, and simulation. With density functional theory calculations, four stable phases of NaxV2O2(PO4)2F were identified at the Na concentrations of x = 0, 1, 2, 3. The transitions between these phases give rise to three values of the Na chemical potential and three voltage plateaus for Na intercalation. The lower two voltages, corresponding to removal of the first two Na per formula unit, agree well with the corresponding experimental electrochemical measurements. Removal of the third Na, however, is not observed experimentally, because it is outside of the (4.8 V) stability window of the electrolyte. This observation is consistent with our calculations that show that the last Na will only be removed at 5.3 V, owing to the stability of the V–O bonding state and a strong Coulomb attraction between the Na and the anions. Computational modifications of the material were considered to activate the third Na with an oxidation energy in the electrolyte stability window, including swapping the anions from O and F to less-electronegative Cl and Br. The most promising material, Na3V2Cl2(PO4)2F, is found to be stable and a good candidate as a Na cathode because all three Na ions can be reversibly removed without significant reduction in the cell potential or energy density of the material. Finally, we show that Li can partially replace Na and that these Li intercalate into the material with a higher rate owing to a lower diffusion barrier as compared to Na.