Investigation of Physicochemical Properties of Carbonate-Based Highly Concentrated Electrolytes with Different Anions

Introduction Recently, stable operation of renewable energy has been required for the realization of a low-carbon society. Rechargeable batteries expected as high-performance energy saving system. As one of the improvement methods of battery performance, "highly concentrated electrolytes" is attracting attention as new electrolyte systems. In the case of a conventional concentration electrolytes (Li salt concentration ≒ 1 mol L-1), there is free solvent which is not coordinated with Li cation. On the other hand, highly concentrated electrolytes (Li salt concentration> 3 mol L-1) has almost no free solvent. Highly concentrated electrolytes have been reported to show unique properties and good battery performances because of specific coordination structures [1]. For example, highly concentrated electrolytes have good thermal and electrochemical stability. From the viewpoint of energy density, increase of operating voltage is effective. Also, higher thermal stability contributes to improvement of safety. Therefore, we focused on ethylene carbonate (EC)-based highly concentrated electrolyte. In this study, we prepared EC-based liquid electrolytes using several Li salt at the concentration range of 5.6~14 mol kg-1. Prepared electrolytes were investigated the effect of anion species on physicochemical properties based on Walden’s law. Experimental Electrolyte samples were prepared by mixing LiN(SO2CF3)2 (LiTFSA) or LiN(SO2F)2 (LiFSA) with ethylene carbonate (EC) in various compositions in an argon-filled glove box. Prepared electrolyte was defined as xEC + LiTFSA and xEC + LiFSA, respectively ( e.g. x = 2 means EC : Li salt = 2 : 1 by mol). Temperature dependences for viscosity for prepared samples were measured by stavbinger-type viscosity measurement system. Temperature dependences for ionic conductivity were also measured by AC impedance method. Result and discussion Fig.1 shows temperature dependence of viscosity of prepared electrolyte. In all compositions, prepared electrolytes showed higher viscosity than 1 mol kg-1 (x=11.4). Viscosity of prepared electrolyte increased with Li salt concentration, and xEC + LiTFSA had higher viscosity than xEC + LiFSA owing to differences of their anion ionic radius. Fig.2 shows temperature dependence of ionic conductivity (σ) of prepared electrolytes. σ increased in inverse of viscosity, and xEC + LiFSA indicated higher ionic conductivity than xEC + LiTFSA in all Li salt composition. Fig.3 shows Walden plots of prepared electrolytes at 313.15 K. xEC + LiTFSA follows Walden's law, and plots are lower than ideal line in all Li salt composition. On the other hand, Walden's plots of xEC + LiFSA were close to the ideal line in the high Li salt concentration range of x=1.25 and 0.8. Therefore, the xEC + LiFSA should be changed solvation structure and ionic transport mechanism in the high Li salt concentration region. Reference [1] K.Yoshida et al., J. Am. Chem. Soc., 133,13121 (2011). Figure 1