Molecular Flexibility-Controlled Ion Solvation and Electrode Reaction Kinetics in Sulfite-Based Lithium-Ion Battery...

The effects of solvent molecular flexibility on Li⁺ solvation and graphite electrode kinetics were investigated in lithium-ion battery (LIB) electrolytes using dimethyl sulfite (DMS), a linear sulfite solvent with high molecular flexibility. Raman spectroscopy revealed that, in the dilute solutions, Li⁺ ions are solvated by three DMS molecules to form Li(DMS)₃⁺ complexes, whereas the corresponding cyclic sulfite solvent, ethylene sulfite (ES), forms conventional four-coordinate Li(ES)₄⁺ complexes. Density functional theory (DFT) calculations revealed that the Li(DMS)₃⁺ complex consists of two monodentate DMS molecules and one bidentate DMS molecule, the latter adopting the trans-trans (TT) conformer, which is thermodynamically unfavorable in the bulk phase, but becomes stabilized within the Li⁺ solvation shell due to the strong electrostatic field of the Li⁺ ion. As a result, the Li(DMS)₃⁺ complex is energetically less stable than the conventional four-coordinate Li(ES)₄⁺ complex. In the highly concentrated region, Li⁺ ions formed similar ionically ordered structures interconnected through bis(fluorosulfonyl)amide (FSA) anions in both DMS and ES electrolytes. In the electrode reaction, the dilute DMS electrolyte exhibited an exceptionally low activation energy for Li⁺ insertion at the graphite electrode, attributed to the easier desolvation of weakly coordinated Li⁺ species. However, its reductive instability led to DMS-derived SEI films with poor stability and rapid capacity degradation. In contrast, the highly concentrated DMS electrolyte produced stable FSA-derived solid-electrolyte interphase (SEI) films and improved cycling stability, albeit with higher activation energy due to dominant Li⁺-anion interactions. These results provide molecular-level insights into how solvent flexibility governs Li⁺ coordination structures and electrode reaction kinetics in LIB electrolytes.