Effects of Comonomers on the Performance of Stable Phosphonate-Based Gel Terpolymer Electrolytes for Sodium-Ion Batteries with Ultralong Cycling Stability

Gel polymer electrolyte (GPE) is one of the most promising alternatives to solve the bottlenecks of nonaqueous liquid electrolytes such as decomposition, safety hazards, and growth of dendrites. In this work, three novel methyl phosphonate-based crosslinking gel terpolymer electrolytes with different comonomers are designed and prepared by in situ radical polymerization. The gel polymer electrolytes have excellent thermal stability, wide electrochemical windows (≥4.9 V), and high ionic conductivities (±3 mS cm–1), and may be used as less-flammable electrolytes for sodium-ion batteries. 31P NMR spectra, Arrhenius plot, and density functional theory (DFT) calculations confirm that multifunctional phosphonate structural units promote the dissociation of NaClO4 and help to transport the sodium ions freely in the polymer framework. X-ray photoelectron spectroscopy (XPS) results show that the gel polymer electrolytes have the capability of inhibiting liquid electrolyte decomposition and the formation of the stable solid electrolyte interphase (SEI) film. The Na3V2(PO4)3/GPE/Na cells exhibit better ultralong cycling stability and enhanced temperature performance than those of liquid cells. Strikingly, GPE1 has the best comprehensive electrochemical performance, especially the rate performance and long-term cycling stability with a capacity retention ratio of 82.6% after 3500 cycles, which indicates that different comonomers have obvious effects on the performance. Therefore, the full cell of SnS2/GPE1/Na3V2(PO4)3 is evaluated and delivers good cycling stability of 500 cycles, holding a great prospect for the design and production of phosphorus-containing electrolytes for safer sodium-ion batteries.