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

## TL;DR

This paper explores how the flexibility of sulfite solvents affects lithium ion solvation and battery performance, revealing how molecular structure influences battery efficiency and stability.

## Contribution

The study provides molecular-level insights into how solvent flexibility controls Li+ coordination and electrode reaction kinetics in sulfite-based electrolytes.

## Key findings

- DMS forms three-coordinate Li+(DMS)3 complexes, while ES forms four-coordinate Li+(ES)4 complexes.
- Dilute DMS electrolyte has low activation energy for Li+ insertion but poor stability due to reductive instability.
- Concentrated DMS electrolyte forms stable FSA-derived SEI films with improved cycling stability despite higher activation energy.

## Abstract

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.