Ion Dynamics in Ionic‐Liquid‐Based Li‐Ion Electrolytes Investigated by Neutron Scattering and Dielectric Spectroscopy

Abstract

A detailed understanding of the diffusion mechanisms of ions in pure and doped ionic liquids remains an important aspect in the design of new ionic-liquid electrolytes for energy storage. To gain more insight into the widely used imidazolium-based ionic liquids, the relationship between viscosity, ionic conductivity, diffusion coefficients, and reorientational dynamics in the ionic liquid 3-methyl-1-methylimidazolium bis(trifluoromethanesulfonyl)imide (DMIM-TFSI) with and without lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) was examined. The diffusion coefficients for the DMIM cation and the role of ion aggregates were investigated by using the quasielastic neutron scattering (QENS) and neutron spin echo techniques. Two diffusion mechanisms are observed for the DMIM cation with and without Li-TFSI, that is, translational and local. The data additionally suggest that Li ion transport along with ion aggregates, known as the vehicle mechanism, may play a significant role in the ion diffusion process. These dielectric-spectroscopy investigations in a broad temperature and frequency range reveal a typical α-β-relaxation scenario. The α relaxation mirrors the glassy freezing of the dipolar ions, and the β relaxation exhibits the signatures of a Johari-Goldstein relaxation. In contrast to the translational mode detected by neutron scattering, arising from the decoupled faster motion of the DMIM ions, the α relaxation is well coupled to the dc charge transport, that is, the average translational motion of all three ion species in the material. The local diffusion process detected by QENS is only weakly dependent on temperature and viscosity and can be ascribed to the typical fast dynamics of glass-forming liquids.

Ionic transport in highly concentrated lithium bis(fluorosulfonyl)amide electrolytes with keto ester solvents: structural implications for ion hopping conduction in liquid electrolytes

“…In the equimolar complexes of Li[TFSA] and G3 or G4 (so-called solvate ionic liquids), D Li is found to be identical to D sol , indicating that long-lived Li complex ions are formed due to the strongly chelating properties of G3 and G4. 32 Nevertheless, in the glyme-Li salt solvate ionic liquids, the Li ion transport can generally be interpreted as being via the physical diffusion mechanism that premises the translational motion of the solvated ions in a similar manner to that for ionic liquids 34 as well as dilute electrolyte solutions. 29 In contrast to these examples, the diffusion behavior observed for the highly concentrated Li[FSA]/keto ester systems is more akin to our recently published observations for SL-based concentrated electrolytes.…”

Insight into the Solid Electrolyte Interphase Formation in Bis(fluorosulfonyl)Imide Based Ionic Liquid Electrolytes

“…Ionic liquids (ILs) with favorable properties such as low vapor pressure, low flammability, wide electrochemical stability windows, and high ionic conductivities have been extensively investigated as electrolytes for electric energy storage applications, mainly with lithium‐ion batteries (LIBs) and sodium‐ion batteries (SIBs). [ 1–6 ] The successful operation of LIBs and SIBs, particularly their long term cycling stability, depends on the quality of solid electrolyte interphases (SEIs) formed on the surfaces of both anode and cathode electrodes (or cathode electrolyte interphase (CEI), in the case of the cathode). Both the SEI and CEI are ionically conductive but electronically insulative and mainly formed during the initial battery formation cycles.…”

Structure and dynamics of highly concentrated LiTFSI/acetonitrile electrolytes

“…For ionic liquids QENS has revealed a complex picture of the dynamics with several different processes contributing to the local dynamics. [27][28][29][30][31][32] However, to the best of our knowledge no studies have been reported on the local dynamics in highly concentrated electrolytes using QENS.…”