Effects of nitrogen and sulfur atom regulation on electrochemical properties of Na3V2(PO4)2F3 cathode material for Na-ion batteries

The cathode material Na3V2(PO4)(2)F-3 of sodium-ion battery is well-known for its large number of ion migration channels and high working voltage. However, the electrochemical performance of Na3V2(PO4)(2)F-3 is not very outstanding. Thus, in the present study, Na3V2(PO4)(2)F-3 cathode materials were successfully synthesized by using the sol-gel method and mechanical milling method to enhance the electrochemical performance. The physicochemical properties of synthesized Na3V2(PO4)(2)F-3 were investigated by using X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transition electron microscopy. X-ray diffraction spectroscopy indicates that the doping of nitrogen and sulfur did not alter the crystal form of Na3V2(PO4)(2)F-3. Transition electron microscopy image shows that Na3V2(PO4)(2)F-3 has a thin carbon layer, and x-ray photoelectron spectroscopy illustrates the successful doping of nitrogen and sulfur into the carbon layer. The cyclic voltammetry curves show that the nitrogen and sulfur co-doped Na3V2(PO4)(2)F-3 samples have good reversibility and low polarization. Materials with 15% thiourea has a high discharge specific capacity (126.9 mA h g(-1) at 0.2 C) at the first cycle and excellent cycle stability (126.3 mA h g(-1) after 100 cycles, a capacity retention of 99.5%) among the synthesized cathode materials. In the present study, the electrochemical performance of the Na3V2(PO4)(2)F-3 cathode material was enhanced by regulation of co-doping of nitrogen and sulfur atoms.