Anion Packing and Li⁺ Coordination Topology in Ether-Functionalized Imidazolium Ionic-Liquid Electrolytes: FSI versus TFSI
Year: 2026
Type: article
Abstract: We ask how anion identity relates to L i + coordination and the transport penalty of lithium-salt addition in ether-functionalized [MOEmim] ionic liquids. [ MOEmim ] [ FSI ] , [ MOEmim ] [ TFSI ] and their matched Li-salt electrolytes ( x ( LiX ) = 0.167 ) were synthesized and studied by FTIR, rheometry, impedance spectroscopy, 25 ns polarizable molecular dynamics (APPLE&P) with atom-resolved coordination and Onsager transport analysis, and COSMO-RS. LiFSI more than doubles the viscosity of [ MOEmim ] [ FSI ] (21.3 → 52.4 mPa·s at 25°C), whereas LiTFSI raises that of [ MOEmim ] [ TFSI ] by only about 10%, though the FSI electrolyte remains the more conductive. Binding strength does not run in the direction required: COSMO-RS finds L i + coordination more favorable to TFS I − at every cluster size, by 11–29 kJ mol⁻¹. The difference lies in coordination topology. L i + holds a near-invariant oxygen coordination of 4.78–4.96, and the number of partners sharing it tracks anion size: 3.9 distinct anions alongside FS I − against 3.2 alongside TFS I − , so denticity follows arithmetically: 1.24 against 1.56. FSI aggregates are 1.7–1.8 × larger though non-percolating, and ether-oxygen coordination is a minority motif. The Li⁺ coordination topology and the transport penalty of salt addition therefore track anion packing, while ion correlations limit the fraction of the total ion motion that produces net charge transport. Because FSI⁻ and TFSI⁻ differ in more than volume, this identifies an operative packing explanation rather than isolating volume as an independent variable.
Source: Energy storage materials
Institution: Concordia University
Language: English
Subfield: Physical and Theoretical Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $5,400
OpenAlex ID: W7225314522