Rationally designed NASICON-type Na3.75V1.25Mn0.75(PO4)3 cathode towards long life-span sodium ion batteries

Owning to non-toxic and high working potential, Mn-substituted NASICON-type Na3+xMnxV2-x(PO4)3 cathodes have promising application in sodium ion batteries (SIBs). However, the poor cycling stability limits their further practical applications. Herein, a succession of carbon coated Na3.75Mn0.75V1.25(PO4)3 cathodes are constructed via a sol–gel combining with heating treatment. The effects of carbon content on the structure, electrochemical properties and sodium ion diffusion kinetics are investigated. The optimal composition, Na3.75Mn0.75V1.25(PO4)3/2VitC, displays an outstanding specific capacity and long-cycling stability. It releases a discharge capacity of 105.6 mAh g-1 with a high initial Coulombic efficiency of 97.5% at 0.2C. It can also deliver a high capacity of 92.6 mAh g-1 with a capacity retention of 88.3% at 5C over 2000 cycling, and the reversible capacity can achieve to 79.1 mAh g-1 with 78% capacity retention after 4000 cycles at high rate of 10C. Ex-situ X-ray diffraction and X-ray photoelectron spectroscopy reveal a biphasic reaction process and high structural reversibility during sodium ion insertion/extraction. The good multiplicity performances, excellent long cycle stability and low cost of the optimized Na3.75Mn0.75V1.25(PO4)3 composite indicate that it is a promising cathode material for SIBs.