Development of New Borate-Based Lithium Ionic Liquid for Next Generation Lithium-Ion Battery

Abstract

Solvate ionic liquids (SILs), which consist of a solvate ion and its counter ion, are expected as the electrolyte for the new generation Lithium-ion batteries [1]. Li-glymes SIL is an equimolar mixture of lithium salts and glyme of an oligoether such as triglyme and tetraglyme (Gn: CH3O-(CH2CH2O) n -CH3, n = 3 and 4). Li-glymes SIL shows favorable liquid properties like ordinary aprotic ionic liquids; negligible vapor pressure, practically non-inflammability, ionic conductivity and electrochemical stability, and so on. Watanabe et al. [2] have proposed that the specific Li+ ion hopping conduction occurs in the Li-glymes SIL. In Lithium-ion batteries with Li-glymes SIL, the desolvated Li+ ions are intercalated into graphite electrode, however, the co-intercalation of Li+ ion and glyme into the electrode occurs in the battery using the excess glyme electrolyte solution [3]. This suggests that the solvent activity affects the electrode reaction. In the present study, we prepared lithium ionic liquids with oligoether chains (Figure 1) and measured those physical and electrochemical properties.Infrared spectroscopic (IR) measurements were carried out to identify the prepared lithium ionic liquids. An intense peak, which cannot be observed in the IR spectra of the raw materials, appears in approximately 1000 cm-1. This peak is attributable to B-O vibration mode, suggesting that the lithium ionic liquids are synthesized. We here define the new lithium ionic liquids as LiB(Gn)4 (n = 3 or 4). The ionic conductivities of LiB(G3)4 and Li(B4)4 are 0.20 and 0.37 mS cm-1, respectively. In addition, Li+ ion transport number is approximately 0.5 for both LiB(G3)4 and Li(B4)4., which is higher than that of electrolyte solution used in a commercial lithium-ion battery. This suggests that Li+ ion hopping conduction may occur via the exchange among glymes which are side chain of anion.