Stable cycling performance of lituium-doping Na3-xLixV2(PO4)3/C (0 ≤ x ≤ 0.4) cathode materials by Na-site manipulation strategy

Na3V2(PO4)(3) (NVP) has become a hot material in the research field of sodium-ion batteries (SIBs) as a result of its unique structural advantages. Unfortunately, the poor electronic conductivity and ion diffusion ability fundamentally limit the practical development of NVP. Considering the above defects, a series of Li+ doped Na3-xLixV2(PO4)(3)/C cathode materials for SIBs are synthesized through hydrothermal assisted sol-gel method. The cell volume steadily decline with the raise of Li+ doping amount, which is consistent with the usual doping effect. Moreover, with the increase of lithium doping amount, Li+ preferentially occupies Na-(2) site and then occupies Na-(1) site. Through the first principles calculation, it is clear that the introduction of Li+ can significantly reinforce the conductivity of the material system and diminish the electron localization phenomenon. The analysis of (1 1 6) crystal plane by TEM shows that Li doping will reduce the interplanar spacing. The change of V valence during the charge-discharge process of Na2.8Li0.2V2(PO4)(3)/C is consistent with that of NVP by V element XANES characterization. There is no conventional phase transition phenomenon in the ex-situ XRD high-voltage charging state. The above facts indicate that lithium doping obtains a stable solid solution NVP. The electrochemical test results show that Na2.8Li0.2V2(PO4)(3)/C can provide an amazing reversible capacity of 116.9 mAh/g (theoretical capacity of 117 mAh/g) and capacity retention rate of 99.82 % after 500 cycles. GITT results show that Li+ can significantly increase the Na+ diffusion coefficient of the material. Such excellent electrochemical performance is attributed to the fact that the doping of Li+ activates the activity of Na+ in Na-(2) site and improves the reaction efficiency.