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

Abstract

Abstract Na + superionic conductor (NASICON)‐structured Na 4 VMn(PO 4 ) 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 V 4+ /V 5+ redox couple. Herein, to stabilize the V 4+ /V 5+ couple and improve the reversibility, a series of carbon‐coated NVMP (NVMP@C) with different V/Mn ratios are compared, among which, Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C delivers an additional reversible V 4+ /V 5+ capacity of 7 mAh g − 1 at the voltage of 3.9–4.2 V. Based on this, to further activate the capacity of V 4+ /V 5+ , Al‐doped Na 3.25 V 1.75− y Mn 0.25 Al y (PO 4 ) 3 structures are synthesized. When 0.5 Al is doped, 15 mAh g − 1 capacity corresponding to V 4+ /V 5+ can be released. In addition, it is found that the activation of V 4+ /V 5+ is not conducive to electrochemical reversibility and C‐rate performance, owing to the shrinkage of the whole framework structure with the oxidization of V 4+ to V 5+ .