Na2FePO4F/C composite cathode with excellent ions migration property for high-performance sodium-ion batteries

For sodium-ion batteries, polyanionic cathode materials have been mainly studied due to their advantages of high safety, low-cost and environmental friendliness. However, some shortcomings of low intrinsic conductivity and slow reaction kinetics limit its further application. In our study, a uniform and dense amorphous carbon layer is coated onto the surface of fluorophosphate (Na2FePO4F/C) via high-energy ball milling and heat-treatment process. According to the results of SEM and TEM, it is seen that the surface of the active particles would be composited with a carbon layer. When the heat treatment temperature is 650 °C, the ratio of ID/IG of Raman is the largest, indicating that the highest disorder degree of the carbon layer. The obtained Na2FePO4F/C-650 has the largest initial discharge specific capacity (120 mAh·g−1 at 0.1 C), and the specific capacity retention rate is 76.54% at 10 C for 1000 cycles. Furthermore, the sodium-ion diffusion coefficient of Na2FePO4F/C-650 measured using AC impedance is up to 4.91 × 10−12 cm2·s−1. It could be resulted from the fact that amorphous carbon layer can absorb more sodium ions and is beneficial to ion diffusion kinetics according to the DFT calculation. This study has proposed a new idea to modify cathode material and facilitated the commercial application of Na2FePO4F.