Synthesis of carbon coated Na2FePO4F as cathode materials for high-performance sodium ion batteries

In this paper, nano-sized carbon coated Na2FePO4F (Na2FePO4F/C) samples were successfully prepared via low-temperature solvothermal route and followed by carbon coating process. The effect of pH values of the precursor solutions on the morphology and structure of Na2FePO4F samples, and the electrochemical properties of the synthesized Na2FePO4F/C samples were systematically investigated. It was found that the morphology and structure of the Na2FePO4F samples were not only controlled by the pH values of the precursor solutions during the solvothermal route, but also changed in the subsequent carbon coating process, which finally influenced both the electronic conductivities and sodium ion diffusion ability. The solvothermal products exhibited spherical morphology consist of rod-like units at pH = 10.5. The spherical particles changed to irregular polyhedrons morphology with a large specific surface area of 126.2 m2 g−1 after carbon coating process. The sodium ion diffusion coefficient of the Na2FePO4F-10.5/C sample (synthesized at pH = 10.5) was tested to be as high as 2.836 × 10−11 cm2 s−1. The Na2FePO4F-10.5/C sample showed initial discharge capacity of 114.3 mAh g−1 at 0.1 C and high capacity retention of 93.3% after 100 cycles due to the great improvements of both electronic conductivities and sodium ion diffusion ability.