Enhanced electrochemical performance of vanadyl (IV) Na3(VO)2(PO4)2F by ex-situ carbon coating

A vanadyl (IV) Na3(VO)2(PO4)2F compound has been successfully synthesized by a novel single-step hydrothermal method. ESR measurements have confirmed the existence of a V+ 4 phase. This material has been carbon coated via an impregnation method followed by either a long thermal treatment (composite A) or a flash thermal treatment (composite B). X-Ray diffraction patterns showed that composite B exhibits the presence of a pure Na3(VO)2(PO4)2F phase while a mixture of Na3(VO)2(PO4)2F and Na3V2(PO4)3 appears in A. The carbon content was 2.65 and 4.5 wt. % for A and B, respectively. The Na3(VO)2(PO4)2F material shows in all cases two voltage plateaux at 3.6 and 4.0 V vs. Na/Na+. Composite B shows better electrochemical performance than uncoated Na3(VO)2(PO4)2F especially in terms of rate capabilities, by achieving 102 mAh/g at C/20 and 68 mAh/g at 1C.