Study on sodium storage properties of manganese-doped sodium vanadium phosphate cathode materials

Abstract

Na+ superionic conductor (NASICON)-structured Na4VMn(PO4)3 (NVMP) possesses stable cycling performance at 2.5–3.8 V by replacing V with lower cost Mn but suffers rapid capacity decay when further widening the voltage to 2.5–4.2 V, owing to a less stable V4+/V5+ redox couple. Herein, to stabilize the V4+/V5+ couple and improve the reversibility, a series of carbon-coated NVMP (NVMP@C) with different V/Mn ratios are compared, among which, Na3.25V1.75Mn0.25(PO4)3@C delivers an additional reversible V4+/V5+ capacity of 7 mAh g−1 at the voltage of 3.9–4.2 V. Based on this, to further activate the capacity of V4+/V5+, Al-doped Na3.25V1.75−yMn0.25Aly(PO4)3 structures are synthesized. When 0.5 Al is doped, 15 mAh g−1 capacity corresponding to V4+/V5+ can be released. In addition, it is found that the activation of V4+/V5+ is not conducive to electrochemical reversibility and C-rate performance, owing to the shrinkage of the whole framework structure with the oxidization of V4+ to V5+.