Gel polymer electrolytes (GPEs) hold great promise for next-generation Li-ion batteries as they combine the high ionic conductivity and facile transport properties of liquid electrolytes with the improved safety and mechanical properties of solid polymer electrolytes. The selection of appropriate non-flammable solvents as well as polymer matrices with good electrochemical stability and robustness is a critical step in the design of efficient and safe GPEs. Here, we show that integration of an easy-to-make deep eutectic solvent (DES) based on N-methylacetamide and lithium bis(trifluoromethanesulfonyl)imide, in a versatile acrylate polymer host comprising 2-hydroxyethyl acrylate (HEA) and polyethylene glycol methyl ether acrylate (MPEGA), results in various GPE formulations with improved electrochemical stability compared to the liquid DES alone. At optimized HEA to MPEGA ratios, eutectic GPEs achieve ionic conductivities exceeding 0.4 mS cm⁻¹ and 1.2 mS cm⁻¹ at 20 °C and 40 °C respectively. When tested in Li₄Ti₅O₁₂/LiMn₂O₄ full cells, these GPEs outperform the liquid DES at 0.45C rate in terms of cycling stability. More impressively, the eutectic GPEs exhibit substantially improved non-flammability as well as remarkable robustness and stability to water compared to the liquid DES. The self-extinguishing time of the eutectic GPEs when exposed to a flame is suppressed by over a factor of seven. Regarding water stability, while the cycle life of full cells with a liquid DES is dramatically impacted by the presence of water, eutectic GPEs are significantly more tolerant, enabling more stable cycling and even the assembly of cells in fully ambient laboratory conditions. These findings provide useful insights into the design of novel, safe, robust GPEs based on environmentally friendly solvents.

We report UV-photopolymerized “water-in-bisalt” electrolytes that expand the electrochemical stability window, improving “water-in-bisalt” cycle life in lithium titanate full cells.