Stable cycling performance of lituium-doping Na3−xLixV2(PO4)3/C (0 ≤ x ≤ 0.4) cathode materials by Na-site manipulation strategy
Na 3 V 2 (PO 4 ) 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 Na 3−x Li x V 2 (PO 4 ) 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 Na 2.8 Li 0.2 V 2 (PO 4 ) 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 Na 2.8 Li 0.2 V 2 (PO 4 ) 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.
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