Electrochemical Behavior of Li4Ti5O12 in Ionic Liquid/Water Bilayer Electrolyte

Abstract

Introduction Lithium-ion batteries (LIBs) have been used in portable electronic devices, and recently expected to be used in large-scale energy facilities for electric vehicle or large energy storage systems. Flammable organic solvents were used in LIBs and it threaten the safety of LIBs. Therefore, nonflammable aqueous electrolytes have been investigated[1]. However, its narrow potential windows restrict the use of negative electrodes, which makes it difficult to obtain high voltage. In this study, we utilized stable liquid-liquid interface between water and hydrophobic ionic liquids in order to enhance the negative potential window. The electrochemical behavior of Li4Ti5O12(LTO) thin-film electrode, which cannot be used in aqueous solution, was investigated. Experimental LTO thin-film electrodes were prepared on a Pt substrate by a sol-gel method. LTO thin-film electrode was used as a working electrode. The bilayer electrolyte was formed on the working electrode: the upper layer was 1.0 mol dm −3 LiNO3 aqueous solution, and the lower layer was butyltrimethylammonium bis(trifluoromethylsulfonyl)amide (BTMATFSA) containing 1.0 mol kg −1 of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA). The counter electrode was platinum wire and the reference electrode was Ag/AgCl electrode (sat. KCl), and these electrodes were placed in the aqueous solution layer. Cyclic voltammetry was performed. Hereafter, all potentials are referred to as Ag/AgCl. Results Figure 1(a) shows the cyclic voltammograms of LTO thin-film electrode in bilayer electrolyte. At volume ratio of ionic liquid:aqueous solution = 3:4, clear redox peaks around ca. −1.33 V were observed. This reaction can be assigned to lithium-ion insertion/extraction into/from LTO. In contrast, Figure 1(b) shows that only large reduction current was observed and the redox peaks of LTO were not detected at volume ratio = 1:29. At this ratio, ionic liquid was miscible with aqueous solution. Thus, the reduction current below 0.9 V can be assigned to the hydrogen evolution reaction from water. These results reveal that bilayer electrolyte with clear interface prevent the hydrogen evolution reaction and enable lithium-ion to insert/extract into/from LTO. Reference [1] W. Li, J. R. Dahn, and D. S. Wainwright, Science, 264, 1115 (1994). Acknowledgment This work was partially supported by CREST, JST. Figure 1