Mechanism and enhanced performance of low-dose low-valence molybdenum-doped Na3V2(PO4)2F2O cathodes for sodium batteries

Sodium vanadium oxyfluorophosphate Na3V2(PO4)2F2O is an attractive cathode material for sodium ion batteries due to its high crystalline stability, high specific capacity and high discharge potential. Currently, the poor electronic conductivity and low diffusion rate of Na+ severely impede its development and application. In this work, low-dose Mo2+ doping is proposed through first-principle computation to enhance the structure and performance of Na3V2(PO4)2F2O. It is found that low-dose of Mo2+ doping can reduce the band gap and energy barrier for Na + diffusion in Na3V2(PO4)2F2O. However, heavy doping leads to serious Jahn-Teller distortion of the crystal structure. Therefore, low-dose Mo2+ doping, ranging from x = 0 to x = 0.06 in Na3V2-xMox(PO4)2F2O, is designed and experimentally carried out. The best performance with 118.5 mA h g−1 at 0.1C and 60.6 mA h g−1 at 20C is obtained in Na3V1.96Mo0.04(PO4)2F2O when x = 0.04 in comparison with other doping amounts. The specific discharge capacity at 0.5C decreases gradually from 105.4 to 77.9 mA h g−1 after 400 cycles, indicating a capacity retention of 73.9 %. These results demonstrate that low-dose Mo2+ doping is an effective strategy to enhance the electrochemical performance of Na3V2(PO4)2F2O, making it a promising cathode material for sodium batteries.