Conformational Flexibility of Solvent Molecules Enables Li-Ion Hopping in Highly Concentrated Electrolytes

We report the structural characteristics of a highly concentrated electrolyte composed of methyl pyruvate (MP) solvent and lithium bis(fluorosulfonyl)amide (LiFSA) salt, which exhibits specific ion conduction based on the lithium-ion (Li-ion) hopping/exchange mechanism. Infrared (IR) spectroscopy and density functional theory (DFT) calculations revealed that the Li salt concentration (c_Li) affects the Li-ion coordination structure in the LiFSA/MP electrolyte. At low c_Li (≤2.5 M), Li ions were solvated with two MP molecules in both cis- and trans-forms acting as bidentate ligands. As c_Li increased above 2.5 M, ionic aggregates were formed. High-energy X-ray total scattering (HEXTS) and all-atom molecular dynamics (MD) simulations demonstrated a specific solution structure (MP- and FSA^--bridged Li-ion ordered structure) in the electrolyte at high concentrations. Furthermore, DFT calculations were performed on the Li⁺-MP complex to investigate the potential energy surface of the internal conformational changing of the coordinated MP. The results show a change from cis-MP to trans-MP in the Li-ion solvation complex, as well as in the bulk solution, which is similar to the anion flexibility of FSA^- (cis-FSA/trans-FSA). Such molecular flexibilities play a key role in specific Li-ion conduction: in Li-ion ordered complexes, the MP and FSA^- bridged via bidentate coordination undergo a conformational change to unstable monodentate coordination with no chelating effect. This partial structural relaxation leads to ligand exchange and hopping of Li ions to other coordination sites, resulting in specific Li-ion conduction based on the ion hopping/exchange mechanism.