Theoretical Description, Synthesis, and Structural Characterization of β-Na0.33V2O5 and Its Fluorinated Derivative β-Na0.33V2O4.67F0.33: Influence of Oxygen Substitution by Fluorine on the Electrochemical Properties

Abstract

The structure of β-Na0.33V2O4.67F0.33 has been investigated by both theoretical and experimental methods. It exhibits the same structure as that of the parent bronze β-Na0.33V2O5. The partial substitution of oxygen by fluorine has little effect on the average structure and cell parameters, but the sodium environment changes significantly. Using DFT calculations, we determined the most stable positions of fluorine atoms in the unit cell. It was found that the partial replacement of oxide by fluoride takes mainly place in the coordination sphere of Na producing a shortening of the Na-anion bond lengths. We also analyzed the electronic properties based on density of states and Bader charge distribution. The crystallochemical situation of sodium ions in β-Na0.33V2O4.67F0.33 oxyfluoride, detected by both experimental and computational methods, affects its mobility with respect to the parent oxide. The higher ionicity in the Na coordination sphere of β-Na0.33V2O4.67F0.33 is related to a sodium ion diffusion coefficient, DNa+, that is 1 order of magnitude lower (1.24 × 10-13 cm2 s-1) than in the case of β-Na0.33V2O5 (1.13 × 10-12 cm2 s-1). Electrochemical sodium insertion/deinsertion properties of the oxyfluoride have been also investigated and are compared to the oxide. Insertion/deinsertion equilibrium potential for the same formal oxidation state of vanadium increases due to fluorination (for instance reduction of V+4.3 occurs at 1.5 V in the oxide and at 1.75 V in the oxyfluoride). However, the capacity of Na0.33V2O4.67F0.33 at constant current is lower than in the case of β-Na0.33V2O5 due to a less adequate morphology, a lower DNa+, and a lower oxidation state of vanadium owing to the aliovalent O/F substitution.