Crystal structure and electrochemical properties vs. Na + of the sodium fluorophosphate Na 1.5VOPO 4F 0.5

Abstract

Pure Na 1.5VOPO 4F 0.5 was prepared by tuning the synthesis conditions previously reported by Barker et al. Using FTIR, Rietveld analysis and atomic absorption measurements, the stoichiometry and structure were unambiguously determined. The refined structure shows the same framework as the one ascribed to “NaVPO 4F” but clearly underline the presence of two different sodium sites (8h and 8j), one fluorine site (2a) and one octahedral V 4+ site [VO 5F]. We further examined the Na + insertion mechanism of this phase whose signature was similar to the one of “NaVPO 4F” and “Na 3V 2(PO 4) 2F 3”. Namely two voltage plateaux are found at 3.6 and 4.0 V vs. Na +/Na and are characteristics of two bi-phasic transitions. However the overall reversible capacity does not exceed 0.56 Na ion per formula unit, and is furthermore hindered by a high voltage phenomenon, most likely linked to electrolyte degradation. Finally AC impedance measurements carried out on a dense pellet showed a RT ionic conductivity of 1.8 × 10 −7 S / cm with an activation energy of 0.43 eV.