Understanding Lithium‐ion Transport in Sulfolane‐ and Tetraglyme‐Based Electrolytes Using Very Low‐Frequency Impedance Spectroscopy
Abstract
With the increasing interest in highly concentrated electrolyte systems, correct determination of the cation transference number is important. Pulsed‐field gradient NMR technique, which measures self‐diffusion coefficients, is often applied on liquid electrolytes because of the wide accessibility and simple sample preparation. However, since the assumptions of this technique, that is, complete salt dissociation, all ions participating in motion, and all of them moving independently, no longer hold true in concentrated solutions, the transference numbers, thus obtained are often over‐estimated. In the present work, impedance spectroscopy at a frequency range of 1 MHz to 0.1 mHz was used to examine the concentration effect on lithium‐ion transference number under anion‐blocking conditions for two electrolytes: lithium bis(fluorosulfonyl)imide (LiFSI) in sulfolane (SL) and lithium bis(trifluorosulfonyl)imide (LiTFSI) in tetraglyme (G4). The of the former was almost an order of magnitude higher than that of the latter. It also appeared to increase with increasing concentration while the latter followed an opposite trend. The faster Li+ transport in the SL system is attributed to the formation of a liquid structure consisting of extended chains/bridges of SL molecules and the anions, which facilitate a cation‐hopping/ligand‐exchanged‐typed diffusion mechanism by partially decoupling the cations from the anions and solvent molecules. The G4 system, in contrast, is dominated by the formation of long‐lived, stable [Li(G4)]+ solvation cages that results in a sluggish Li+ transport. The difference between the two transport mechanisms is discussed via comparison of the bulk ionic conductivity, viscosity, ion self‐diffusion coefficients, and the Onsager transport coefficients.
Document Details
- Document Type
- Pub Defense Publication
- Publication Date
- Mar 30, 2022
- Source ID
- 10.1002/eem2.12302
Entities
People
- Glenn R. Pastel
- Janet S Ho
- Kang Xu
- Lin Ma
- Marshall A Schroeder
- Michael S Ding
- Oleg A. Borodin
Organizations
- United States Army Research Laboratory