Ion transport and structural properties of highly concentrated organogel electrolytes based on Tetra-arm PEG networks

Abstract

We report a new class of poly(ethylene glycol) (PEG)-based gel electrolytes formed via salting-in polymer dissolution in highly concentrated LiFSA/AN electrolytes (FSA: bis(fluorosulfonyl)amide; AN: acetonitrile). We demonstrated that PEG exhibits a unique redissolution behavior at high Li salt concentrations, which enabled gelation of Tetra-arm PEG (TetraPEG) prepolymers in the highly concentrated electrolytes. Structural analysis using high-energy X-ray total scattering (HEXTS) combined with all-atom molecular dynamics (MD) simulations revealed that PEG chains coordinate with Li⁺ ions, which are indirectly stabilized through interactions with AN and FSA⁻: i.e., the PEG-coordinated Li⁺ ions act as solvation sites on the polymer chains, providing a molecular origin for the specific salting-in PEG dissolution observed in this system. The resulting TetraPEG gel electrolytes formed mechanically robust and elastic networks, and their mechanical properties were shown to be tunable by polymer concentration. Furthermore, the Li⁺ transference number in TetraPEG networks at the electrode interface exceeds that in the bulk, indicating enhanced interfacial Li-ion transport. Interfacial MD simulations under polarization revealed that FSA⁻ anions are excluded from the negatively charged electrode surface, while neutral molecules such as PEG and AN become enriched. This restructuring facilitates Li⁺ mobility and improves electrode reaction (Li⁺ insertion reaction) kinetics at the interface.