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 Li 3− x Na x V 2 (PO 4 ) 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 Li 3− x Na x V 2 (PO 4 ) 3 /C evolve along with the change of Li/Na atomic ratio (0≤x≤3). With increasing x in Li 3− x Na x V 2 (PO 4 ) 3 /C from 0.0 to 3.0, the main phase in as-prepared sample transforms from monoclinic Li 3 V 2 (PO 4 ) 3 to rhombohedral Li 3 V 2 (PO 4 ) 3 , and finally to rhombohedral Na 3 V 2 (PO 4 ) 3 , which results in different sodium storage behavior and performance between Li 3− x Na x V 2 (PO 4 ) 3 /C (0≤ x ≤3) materials. Electrochemical results show that Li 3− x Na x V 2 (PO 4 ) 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 Li 3 V 2 (PO 4 ) 3 /C, Li 2.5 Na 0.5 V 2 (PO 4 ) 3 /C, Li 2 NaV 2 (PO 4 ) 3 /C, Li 1.5 Na 1.5 V 2 (PO 4 ) 3 /C, LiNa 2 V 2 (PO 4 ) 3 /C, Li 0.5 Na 2.5 V 2 (PO 4 ) 3 /C and Na 3 V 2 (PO 4 ) 3 /C, respectively. Furthermore, the structural reversibility of Li 3− x Na x V 2 (PO 4 ) 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 Li 3− x Na x V 2 (PO 4 ) 3 /C (0≤ x ≤3) can be used as possible sodium storage materials.