Electrochemical Performance of High-Valence Mo and Low-Valence Mn Doped- Na V (PO ) @C Cathode for Sodium-Ion Batteries.

The sodium superionic conductor (NASICON)-Na V (PO ) (NVP) is an attractive cathode for sodium-ion batteries, which is still confronted with limited rate performance due to its low electronic conductivity. In this paper, a chemical strategy is adopted to partially replace V of the NVP framework by low-valence Mn and high-valence Mo substitution. The crystal structure, sodium-ion diffusion coefficient and electrochemical performance of Mn-Mo-doped [Na V Mo Mn(PO ) @C] cathode were investigated. X-ray diffraction confirmed the NASICON-type structure and XPS analysis confirmed the oxidation state of Mn and Mo in doped NVP cathode. The Na ion diffusion processes were inferred from Cyclic Voltammetry (CV), Galvanostatic intermittent titration technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) measurement, which clearly show rapid Na-ion diffusion in NASICON-type cathode materials. The Mn-Mo-substituted NVP shows smoother charge-discharge profiles, improved rate performance (64.80 mAh/g at 1 C rate), better energy density (308.61 mWh/g) and superior Na-ion kinetics than that of unsubstituted NVP@C cathode. Their enhanced performance is attributed to large interstitial volume mainly created by high valence Mo and enhanced capacity is attributed to the low valence Mn doping. These results demonstrate that Mn-Mo-doped NVP cathode is strongly promising cathode material for sodium-ion batteries.