High-Performance Sodium Secondary Batteries Using Synergistic Effect of Amorphous SiP2/C Anode and Ionic Liquid Electrolyte

Abstract

A silicon diphosphide-carbon composite (SiP2/C) was investigated as a negative electrode material for sodium secondary batteries with the Na[FSA]–[C3C1pyrr][FSA] (FSA− = bis(fluorosulfonyl)amide anion and C3C1pyrr+ = N-methyl-N-propylpyrrolidinium cation) ionic liquid electrolyte. Two amorphous silicon diphosphide materials, SiP2/C (80:20) and SiP2/C (70:30) (80:20 and 70:30 refer to the SiP2:C weight ratio), were prepared by a facile two-step high energy ball-milling process. SiP2/C (80:20) and SiP2/C (70:30) delivered high discharge capacities of 883 and 791 mAh g−1, respectively, at 100 mA g−1 in the first cycle at 90 °C, with the latter showing better cyclability. Comparison of the performance of SiP2/C (70:30) in the ionic liquid and organic electrolytes at 25 °C indicated the advantage of the ionic liquid electrolyte in terms of higher discharge capacity and improved cyclability. Electrochemical impedance spectroscopy revealed that the interfacial resistance decreased with cycling in the ionic liquid electrolyte at 25 °C but significantly increased at 90 °C. Ex situ X-ray diffraction revealed that the product remains amorphous even after charging and discharging in SiP2/C (70:30). This study demonstrated the importance of ionic liquids and phosphide based materials as high performance enablers for sodium secondary batteries.