Synthesis of Na2FePO4F/C as cathode materials for sodium ion batteries with different solvents

Na2FePO4F is a promising cathode material for sodium ion batteries because of its high capacity, good cyclic stability, and small volume expansion rate. In this paper, Na2FePO4F samples with different microstructures were synthesized by hydrothermal reaction with three different solvents and carbon coating process. The structures of the synthesized materials were characterized by X-ray diffraction, thermogravimetric analysis, scanning/transmission electron microscopy, and their electrochemical properties were tested. Based on the test results, a reasonable explanation is given for the reason leading to the morphology difference of the samples. The NFPF-EG sample synthesized with 30% ethylene glycol and 70% deionized water as the mixed solvent show the best electrochemical performance (112.3 mAh g(-1) at the rate of 0.1 C and 93.2% capacity retention rate after 200 cycles at 1 C). The results of cyclic voltammetry and electrochemical impedance spectrum suggest that NFPF-EG sample has small irreversible polarization and high conductivity, which is attributed to its uniform microscopic morphology and small particle size to shorten the transport path of Na+ and electrons.