Synthesis of alluaudite-type Na2VFe2(PO4)3/C and its electrochemical performance as cathode material for sodium-ion battery

A new alluaudite-type Na2VFe2(PO4)(3) with carbon composite has been prepared for the first time via a simple sol-gel method using citric acid as an assistant agent and carbon source. The crystal structure and morphology of Na2VFe2(PO4)(3)/C composite are well characterized by X-ray diffraction, scan electron microscope, and energy-dispersive X-ray analysis. The experimental results show that a three-dimensional [VFe2(PO3)(3)](2-) framework in this alluaudite structure provides two types of tunnels to enable the sodium-ion transporting which contributes to the electrochemical performance. When evaluated as a cathode for sodium-ion batteries, Na2VFe2(PO4)(3)/C composite delivers the first specific charge capacity of 66.7 mAh g(-1) and discharge capacity of 65.8 mAh g(-1) at 5 mA g(-1) current density. After 100 cycles, the charge capacity (similar to 52.2 mAh g(-1)at 5 mA g(-1)) shows a good capacity retention (similar to 78%) compared to the first cycle. Cyclic voltammetric profiles confirm the activated multi-electron reactions including the Fe2+/Fe3+, V2+/V3+, and V3+/V4+ redox couples. Furthermore, the diffusion coefficient of sodium-ion (D (Na) (+)) in this structure is also calculated based on the result of electrochemical impedance spectroscopy.