Controllable synthesis of high-rate and long cycle-life Na₃V₂(PO₄)₃ for sodium-ion batteries

Structural and morphological control is an effective approach for improvement of electrochemical performance in rechargeable batteries. In this paper, three different morphological Na₃V₂(PO₄)₃ (irregular shaped, the porous sponge-like and plate like) were successfully prepared through controlling the amount of oxalic acid by a simple two-step reduction method. It is found that the amount of oxalic acid has vital impacts on the morphology of Na₃V₂(PO₄)₃; moreover, the morphological evolution and formation mechanism are proposed based on the reactions of different amount of oxalic acid occurring in the two-step reduction process. The excellent electrochemical performances of the porous sponge-like Na₃V₂(PO₄)₃ are attributed to the unique morphology. The initial capacity of the porous sponge-like Na₃V₂(PO₄)₃ is 101.77 mAh g⁻¹ at 30 C; after 700 cycles, it remains as high as 89.28 mAh g⁻¹ with only 12% capacity loss. When the current density increases to 50 C and 70 C, the capacity retentions of 81% after 600 cycles, and 92.5% after 500 cycles are achieved, respectively.