Polymer-Assisted Deep Supercooling of Lithium Salts Enables Solvent-Free Liquid Electrolytes with Near Single-Ion Conduction

The rate capability of lithium-ion batteries is constrained by the ionic conductivity and Li+ transference number (tLi) of the electrolyte. Conventional liquid electrolytes with low tLi suffer from concentration polarization during high-rate operation, underscoring the need for systems with higher tLi. Solid-state electrolytes intrinsically offer tLi = 1 but face interfacial and processing challenges. Here, we report a solvent-free liquid electrolyte with near-single-ion conduction, realized through polymer-assisted supercooling of lithium salts. A trace amount of polymer suppresses crystallization, stabilizing the salts in a deeply supercooled liquid state at room temperature while preserving near-unity tLi—the first demonstration of near-single-ion conduction in a liquid electrolyte under ambient conditions. This electrolyte enables high-rate operation in Li/LiCoO2 cells by eliminating concentration polarization and demonstrates applicability to high-energy-density lithium batteries through binder-free, thick electrodes exploiting their intrinsic adhesive properties. These findings establish polymer-assisted supercooled salts as a platform for non-flammable, single-ion conducting liquid electrolytes for next-generation batteries.