Abstract: In the article, the electrolytes of mixed solvents for the hard carbon negative of sodium-ion batteries were designed, using sodium hexafluorophosphate (NaPF6) as the sodium salt and adding propylene carbonate (PC) and dimethyl carbonate (DMC) in an ethylene carbonate (EC) based solvent. The effect and mechanism of different solvent synergistic combinations on the electrochemical performance of hard carbon negative were explored. The sodium solvation structure and the diffusion coefficient of sodium ions in the electrolyte were analyzed by Molecular dynamics simulation (MD). The electrochemical performance of hard carbon negative electrode in EC based electrolyte was compared and analyzed through conductivity tests, charge-discharge tests, cycling performance tests, and rate capability tests. The kinetic behavior of sodium ions storage was also analyzed by cyclic voltammetry (CV), alvanostatic intermittent titration (GITT), and electrochemical impedance spectroscopy (EIS). The surface morphology and composition of hard carbon anodes before and after cycling were made by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), and so on. As a result, the synergistic effect of DMC and PC can significantly improve the ionic conductivity of the electrolyte and diffusion coefficient of sodium ions in electrolyte, optimize the solvation structure, reduce organic by-products and increase anion coordination, promote the generation of inorganic substances in the SEI film, effectively reduce the SEI film impedance and charge transfer impedance, and improve the diffusion coefficient of sodium ions in HC. Therefore, the hard carbon anode demonstrates superior rate capability and cycling performance in the 1 M NaPF6-EC/PC/DMC electrolyte at 50 mA g-1, 362mAh g-1 initial capacity, and the capacity decays to 353.4 mAh g-1 with a capacity retention rate of 97.6% after 50 cycles. Thereby, EC/PC/DMC mixed solvents are one of the most promising solvents for the HC anode of sodium ion batteries

Key words: Sodium ion battery；Electrolyte；Solvent, Electrochemical performance；Electrochemical behavior