Evaluation of the electrochemical behavior and structural reversibility of Li3−xNaxV2(PO4)3/C (0≤x≤3) as anode materials for Na-ion batteries

A series of Li3−xNaxV2(PO4)3/C (0≤x≤3) materials are successfully prepared by a simple solid-state reaction method and used for the first time as anode materials for Na-ion batteries. Powder X-ray diffraction (XRD) results show that the phase structures of Li3−xNaxV2(PO4)3/C evolve along with the change of Li/Na atomic ratio (0≤x≤3). With increasing x in Li3−xNaxV2(PO4)3/C from 0.0 to 3.0, the main phase in as-prepared sample transforms from monoclinic Li3V2(PO4)3 to rhombohedral Li3V2(PO4)3, and finally to rhombohedral Na3V2(PO4)3, which results in different sodium storage behavior and performance between Li3−xNaxV2(PO4)3/C (0≤x≤3) materials. Electrochemical results show that Li3−xNaxV2(PO4)3/C (x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0) can deliver the initial charge capacities of 21.1, 35.9, 33.8, 41.7, 43.3, 43.9 and 47.7 mAh g−1 at a current density of 10 mA g−1, respectively. After 45 cycles, the reversible capacities can be kept at 16.9, 45.1, 32.6, 44.6, 43.7, 37.8 and 27.3 mAh g−1 for Li3V2(PO4)3/C, Li2.5Na0.5V2(PO4)3/C, Li2NaV2(PO4)3/C, Li1.5Na1.5V2(PO4)3/C, LiNa2V2(PO4)3/C, Li0.5Na2.5V2(PO4)3/C and Na3V2(PO4)3/C, respectively. Furthermore, the structural reversibility of Li3−xNaxV2(PO4)3/C (x=1.0, 2.0, and 3.0) is also observed by in-situ XRD observation during sodiation/de-sodiation process. All these observed evidences indicate that only some of Li3−xNaxV2(PO4)3/C (0≤x≤3) can be used as possible sodium storage materials.