Lithium‐Ion Conduction Through Frozen Phase of Organic Electrolytes for Lithium Batteries

Overview

Frozen organic electrolytes composed of immobilized solvent molecules can function as molecular-solid Li+ conductors. Ethylene carbonate (EC) in a dilute LiTFSI solution (EC 0.2T, 0.2 m LiTFSI in EC) was predicted and demonstrated to form Li+-conductive channels in its crystalline frozen phase. Ionic transport occurs predominantly by Li+ hopping through a solid matrix of frozen solvent molecules, producing conductivities and transference numbers approaching those of liquid electrolytes while enabling a solvent-derived, Li2O-rich solid electrolyte interphase (SEI) on lithium metal.

Methods and approach

Crystallographic and computational analysis was used to identify channel-like motifs in frozen EC structures potentially permissive of Li+ migration. Experimental preparation involved controlled freezing of 0.2 m LiTFSI in EC to obtain the room-temperature ice-phase electrolyte (EC 0.2T). Ionic conductivity and Li+ transference number were quantified using electrochemical impedance spectroscopy, DC polarization, and transference measurements. Li metal cell performance was evaluated in lithium iron phosphate (LFP)||Li configurations to assess capacity retention and cycle life. Post-mortem interfacial characterization employed surface-sensitive techniques to determine SEI composition and morphology.

Results

EC 0.2T in its frozen state exhibited an ionic conductivity of approximately 0.64 mS cm−1 and a Li+ transference number of roughly 0.8, consistent with a hopping-dominated transport mechanism through immobilized solvent matrices. LFP||Li cells using frozen EC 0.2T delivered capacities comparable to liquid-electrolyte baselines while demonstrating substantially extended cycle life. Interfacial analysis revealed a solvent-derived SEI enriched in Li2O, correlated with improved lithium metal stability and reduced parasitic reactions during cycling.

Implications

The findings counter the prevailing assumption that frozen organic electrolytes are ionically inert, establishing a class of molecular-solid electrolytes that combine high Li+ transference with appreciable ionic conductivity via solid-state hopping. High Li+ selectivity and Li2O-rich SEI formation suggest a pathway to improved lithium metal cycling stability without reliance on polymeric or ceramic solid electrolytes. Further work is required to delineate the temperature-dependent transport regime, long-term chemical stability, scaling to higher concentrations and different solvent/salt chemistries, and the mechanistic origin of SEI formation under frozen-solvent conditions.