Mechanochemically assisted solid-state synthesis of sodium vanadium fluorophosphates

The present study describes the effect of the NaF/VPO4 ratio (3/2 or 1/1) and the conditions of the mechanochemically assisted solid-state synthesis (slow cooling or quenching) on the composition, structure, morphology, and electrochemistry of the final products in the Na-V-P-O-F system. It has been shown that the cooling rate doesn't affect much the phase composition and morphology of the final Na3V2(PO4)(2)F-3 product, formed in the NaF/VPO4 = 3/2 reagent mixture, which achieves the reversible capacity of 113 mAh g(-1) at C/10 cycling rate. On the contrary, the metastable crystalline NaVPO4F phase with the tavorite-like structure (S.G. C2/c) was formed via the solid-state synthesis using quenching the reagent mixture with the NaF/VPO4 = 1/1 ratio along with two other phases: Na3V2(PO4)(2)F-3 and Na2.7V4P4O17 (OH). The reversible capacity of such a multiphase product was about 61 mAh g(-1) mostly due to the contribution of Na3V2(PO4)(2)F-3, while tavorite-like NaVPO4F and Na2.7V4P4O17(OH) did not exhibit any noticeable electrochemical activity.