Figure 3: Synthesis and electrochemistry of Na4V2(PO4)2F3 by Na reductive ball milling.
From: Insertion compounds and composites made by ball milling for advanced sodium-ion batteries
Rietveld refinements of pristine Na3V2(PO4)2F3 (a), a mixture of Na3V2(PO4)2F3 and Na4V2(PO4)2F3 (b) and pure Na4V2(PO4)2F3 (c). The red crosses, black continuous line and bottom grey line represent the observed, calculated and difference patterns, respectively. Vertical blue tick bars mark the Bragg reflections arising from the Amam space group. Patterns are given in Q-space for allowing a direct comparison. Structure of Na3V2(PO4)2F3 (d,f) and of Na4V2(PO4)2F3 (e,g) as deduced from the refinement of the synchrotron patterns. The Na environment of Na3V2(PO4)2F3 and Na4V2(PO4)2F3 is respectively highlighted in (f,g). VO4F2 octahedral and PO4 tetrahedral are coloured in blue and grey, respectively. Na and F atoms are shown as orange and green balls, respectively. Vacancies on the Na1 and Na2 sites on Na3V2(PO4)2F3 are coloured in white. The electrochemical behaviour of composite ‘Na3.5V2(PO4)2F3’ (that is, with equal amounts of Na3V2(PO4)2F3 and Na4V2(PO4)2F3) in Na-half cell is shown in h.
