Stable cross-linked gel terpolymer electrolyte containing methyl phosphonate for sodium ion batteries

Abstract

The novel bifunctional methyl phosphonate which combines the good polymerization ability of acrylate, the adjustable flexibility of ethylenedioxy group and the flame retardant properties of phosphonate is designed and synthesized. The corresponding ternary gel polymer electrolyte is prepared by radical cross-linking copolymerization. The gel terpolymer electrolyte has good thermal stability with the onset decomposition temperature of 250 °C, excellent electrochemical stability of up to 5.0 V (vs. Na+/Na) and high ionic conductivity of 5.13 mS cm−1. Moreover, the gel polymer electrolyte can enhance interfacial adhesion between the membrane and the electrodes. The Na3V2(PO4)3/Na cell using phosphorus containing gel terpolymer electrolyte shows ultra-long cycling performance and better cycle stability and rate performance than the corresponding conventional liquid electrolyte, phosphorus containing gel bipolymer electrolyte and other crosslinker based cells. Meanwhile, the glass fiber separator filled with gel polymer electrolyte maintains better integrity than liquid electrolyte after 1000 cycles, which means the gel polymer electrolyte has good mechanical property and compatibility. The gel polymer electrolyte based sodium-ion battery also exhibits superior cycle stability compared to the battery with liquid electrolyte when running at high temperature of 60 °C. The strong interaction between methyl phosphonate and sodium ion is studied by FT-IR, 31P NMR and HRMS, and demonstrated by theoretical calculation.