Co-doped Na2FePO4F fluorophosphates as a promising cathode material for rechargeable sodium-ion batteries

Due to the high discharge voltage and a favorable theoretical capacity, Na2FePO4F have been attracted much attention as the viable cathode materials for sodium-ion storage. However, the low intrinsic electronic conductivity of Na2FePO4F suppresses its practical applications. Herein, ion doping strategy is employed to improve the electrochemical performance of Na2FePO4F as the cathode of sodium ion batteries. We first used density functional theory (DFT) calculation to screen the optimum dopants by evaluating the structural, electronic and electrochemical properties of metal-doped Na2FePO4F. Our calculation results indicate that the Co-doped Na2FePO4F is the most promising candidate for practical applications. To further prove the validity of metal doping, Na2Fe0.94Co0.06PO4F/C was synthesized by sol-gel method and showed superior electrochemical performance compared with the pristine material. The specific capacity of the Na2Fe0.94Co0.06PO4F/C is 99.93 mAh g−1 at 0.2 C and the discharge capacity retention is 62.1% after 400 cycles at 1 C.