Spencer A. Langevin, Courtney McHale, Tanner Hamann, Ann Choi, M. Zhang, Yayuan Liu, Jesse S. Ko
A key advancement in emerging sodium-ion batteries is the integration of gel polymer electrolytes, which offer improved performance, scalability, and safety. Here, we report a series of sodium-ion–conducting gel polymer electrolytes synthesized via acrylate-based polymerization incorporating a phosphate linker to enhance both electrochemical stability and thermal runaway resistance. Full cells incorporating gel polymer electrolytes derived from triacrylate polymerization with phosphate linkers deliver an average discharge voltage of 2.7 V (projected specific energy of ~160 Wh kg⁻¹) and maintain >95% up to 100 cycles and >80% up to 500 cycles at C/5. In addition, the phosphate linker substantially improves safety, reducing the self-extinguishing time (flammability index: 14.83 s g⁻¹) and effectively preventing thermal runaway under severe overcharge conditions. A key advancement in emerging sodium-ion batteries is the integration of gel polymer electrolytes, which offer improved performance, scalability, and safety. Here, the authors synthesize sodium-ion–conducting gel polymer electrolytes with phosphate linkers, achieving high electrochemical stability and cycle retention, as well as improved safety by significantly reducing flammability and preventing thermal runaway.