Enhanced low-temperature ionic conductivity via different Li⁺ solvated clusters in organic solvent/ionic liquid mixed electrolytes

Abstract

We investigate Li coordination in mixed electrolytes based on ionic liquids (ILs) and organic solvents and its relation with the macroscopic properties such as phase behaviour and ionic conductivity. Using Raman spectroscopy we determine the solvation shell around Li in mixtures formed by the IL N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, the organic solvents ethylene carbonate and dimethyl carbonate (EC : DMC 1 : 1), and the salt LiTFSI. We find that the organic solvent molecules preferentially solvate Li as long as there are enough of them. Our results are consistent with a model where Li(EC)(DMC) and Li(EC)(DMC) are the main complexes formed by the organic solvent molecules and where TFSI mainly participates in Li(TFSI) clusters. As the amount of organic solvent is increased, the number of TFSI around Li rapidly decreases showing a higher affinity of the organic solvents to solvate Li. The changes in the local configurations are also reflected in the ionic conductivity and the phase behaviour. The formation of larger clusters leads to a decrease in the conductivity, whereas the presence of several different clusters at intermediate compositions effectively hinders crystallization at low temperatures. The result is an enhanced low-temperature ionic conductivity in comparison with the pure IL or organic solvent electrolytes.

Understanding transport mechanisms in ionic liquid/carbonate solvent electrolyte blends

“…1. Despite these deviations, our Raman results are similar to previously reported data 39,69 rendering the error margins of the obtained coordination numbers (see Fig. 1) rather negligible.…”

“…70 Note that the Raman peak at B740 cm À1 constitutes the most suitable peak for analyzing Li + -TFSI À interactions. 39,66,[70][71][72] No shift is observed for ''free'' TFSI À (fully solvated by weakly interacting Pyr 14 + cations) while coordinated TFSI À (ion pairs or aggregates)…”

“…Macroscopic properties such as thermal properties of Pyr 14 TFSI/EC/DMC blends 38 are commonly considered, while often neglecting the molecular details or ion speciation that govern the achievable ion transport properties of such blends, except for recent work. 39 However, when aiming at a comprehensive understanding of probable ion transport processes in Pyr 14 TFSI/EC/DMC blends in the presence of lithium salts, it is important to consider macroscopic properties of electrolytes including viscosities, self-diffusion coefficients, molar conductivities, apparent activation energies or transport numbers and the degree of ion dissociation, respectively. Note that the anion of the selected conducting salt typically corresponds to the anion of ILs.…”

Stabilizing lithium metal anode by octaphenyl polyoxyethylene-lithium complexation

“…13e, f) and viscosity ( Supplementary Fig. 9), hindering the ion transport and the stable SEI formation [52][53][54][55][56] . Due to the combination of these factors, the electrolytes with an OP-10 additive at a concentration of 5% were determined to be the most promising conditions for facilitating ion transport and reducing dendrite formation, and therefore were used for all following tests.…”

Enhancement of Functional Properties of Liquid Electrolytes for Lithium‐Ion Batteries by Addition of Pyrrolidinium‐Based Ionic Liquids with Long Alkyl‐Chains

“…These bands are shifted respectively at 904 and 933 cm À 1 when solvent molecules coordinate lithium ions. [42,58] In the case of electrolytes without ionic liquids, the three bands at 893, 904 and 916 cm À 1 are sharp. On the contrary in all IL-added electrolytes the intensity of the bands in the 850-950 cm À 1 region is reduced and all peaks are broader, especially in the band at 893 cm À 1 .…”