Ion-Solvent Interplay in Concentrated Electrolytes Enables Subzero Temperature Li-Ion Battery Operations

Despite the essential role of ethylene carbonate (EC) in solid electrolyte interphase (SEI) formation, the high Li⁺ desolvation barrier and melting point (36 °C) of EC impede lithium-ion battery operation at low temperatures and induce sluggish Li⁺ reaction kinetics. Here, we demonstrate an EC-free high salt concentration electrolyte (HSCE) composed of lithium bis(fluorosulfonyl)imide salt and tetrahydrofuran solvent with enhanced subzero temperature operation originating from unusually rapid low-temperature Li⁺ transport. Experimental and theoretical characterizations reveal the dominance of intra-aggregate ion transport in the HSCE that enables efficient low-temperature transport by increasing the exchange rate of solvating counterions relative to that of solvent molecules. This electrolyte also produces a <5 nm thick anion-derived LiF-rich SEI layer with excellent graphite electrode compatibility and electrochemical performance at subzero temperature in half-cells. Full cells based on LiNi_0.6Co_0.2Mn_0.2O₂||graphite with tailored HSCE electrolytes outperform state-of-the-art cells comprising conventional EC electrolytes during charge-discharge operation at an extreme temperature of -40 °C. These results demonstrate the opportunities for creating intrinsically robust low-temperature Li⁺ technology.