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. Computationa