ORALS

Certain molten solvates of Li salts can be regarded as solvate ionic liquids [1, 2]. A typical example is equimolar mixtures of glymes (G3: triglyme and G4: tetraglyme) and Li[TFSA]([TFSA]=[NTf₂]) ([Li(glyme)][TFSA]). The amount of free glyme estimated by Raman spectroscopy and MD simulation was found to be a few percent in [Li(glyme)][TFSA]^-, and thereby could be regarded as solvate ionic liquids. The activity of Li⁺ in the glyme-Li salt mixtures was also evaluated by measuring the electrode potential of Li/Li⁺ as a function of concentration. At a higher concentration of Li salt, the amount of free glyme diminished in the solvate ionic liquids, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes, the solvation of Li⁺ by the glyme forms stable and discrete solvate ions ([Li(glyme)]⁺) in the solvate ionic liquids. This anomalous Li⁺ solvation had a great impact on the electrolyte properties and electrode reactions [1]. The electrochemical oxidation of glyme in [Li(glyme)][TFSA] is greatly enhanced due to the donation of lone pairs of ether oxygen atoms to the Li⁺ cation, resulting in the HOMO energy level lowering of a glyme molecule. This anomalous Li⁺ solvation induces interesting transport properties when interfacial electrochemical reactions proceed, which is not transport of solvated ions based on Stokes' law but a ligand (solvent)-exchange transport. Another intriguing aspect of the solvate ionic liquids is unusual solubility, which leads to the stable operation of the Li-S battery due to very low solubility of the discharge products (Li₂S_x). Li⁺-intercalated graphite was electrochemically formed in [Li(G3)₁][TFSA]. In contrast, the cointercalation of G3 and Li+ (intercalation of solvate [Li(G3)₁]⁺ cation) into graphite occurred in [Li(G3)_x][TFSA] electrolytes containing excess G3 (x > 1). In the solvate ionic liquid, the activity of the free solvent is very low, which would make the solvate ion unstable and the desolvation possible at the interface. Very recently, we demonstrate that Li⁺ hopping conduction, which cannot be explained by conventional Stokes' law, emerges in highly concentrated molten solvate electrolytes composed of LiBF₄ and sulfolane (SL) [3]. In the concentrated electrolytes with molar ratios of SL/LiBF₄ < 3, Li⁺ diffused faster than SL and BF₄^-, and thus the evolution of Li⁺ hopping conduction was found.

References:
[1] M. Watanabe, K. Dokko, K. Ueno, M.L. Thomas, From Ionic Liquids to Solvate Ionic Liquids: Challenges and Opportunities for Next Generation Battery Electrolytes, Bull. Chem. Soc. Jpn., 91 (2018) 1660-1682.
[2] M. Watanabe, M.L. Thomas, S. Zhang, K. Ueno, T. Yasuda, K. Dokko, Application of Ionic Liquids to Energy Storage and Conversion Materials and Devices, Chem. Rev., 117 (2017) 7190-7239.
[3] K. Dokko, D. Watanabe, Y. Ugata, M.L. Thomas, S. Tsuzuki, W. Shinoda, K. Hashimoto, K. Ueno, Y. Umebayashi, M. Watanabe, Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolyte, J. Phys. Chem. B, 122 (2018) 10736-10745.