Physicochemical Properties and Electrochemical Stabilities of Highly Concentrated Electrolytes

Abstract

Introduction 5 V class cathode active materials have been investigated intensively to increase the energy density of lithium batteries. However, the high electrode potential promotes the decomposition and side reactions of electrolytes, active material, binder, and conductive agent of the composite cathode, leading to low coulombic efficiency and high over potentials1,2). One approach to overcome this problem is to enhance the oxidative stability of the electrolyte. Our group has reported that the equimolar mixture of lithium bis (trifluoromethanesulfonyl)amide (Li[TFSA]) and triglyme (G3) or tetraglyme (G4) with high lithium salt concentration (≈3 mol dm−3) shows the properties similar to room temperature ionic liquids such as high thermal stabilities and low vapor pressure. In addition, the oxidative stability was enhanced by increasing the lithium salt concentration in Li[TFSA]/G3 and Li[TFSA]/G4 electrolytes owing to the lowering of HOMO energy level of glymes coordinating to the lithium cation3). The enhancement of oxidative stability in the highly concentrated electrolytes can be expected in other solvents. In this work, the physicochemical properties and electrochemical stabilities of highly concentrated electrolytes based on sulfone solvent will be mainly reported. Experimental Electrolytes were prepared by mixing LiBF4 and sulfolane (SL) in specific molar ratio under Ar atmosphere. The interaction of lithium cation and SL was observed by Raman spectroscopy using RMP-330, JASCO. The oxidative stabilities of electrolytes were investigated by linear sweep voltammetry (LSV) with three electrodes cell consisting of Pt disk working electrode, Pt coil counter electrode, and reference electrode of Li metal in 1 M Li[TFSA]/G3 with liquid junction. The scanning rate was 1 mV s−1 and the operation temperature was 30 °C. Result and Discussion Fig. 1 shows the concentration dependence of Raman spectra for LiBF4/SL in the range of 650-700 cm−1, corresponding to the C-S-C stretching vibration of SL4). The band gradually shifted to higher wave number with the increase of lithium salt. This suggests the population of uncoordinated (free) SL is decreasing with increase of lithium salt concentration. The LSV results for the mixtures of LiBF4:SL=1:7 (1.4 mol dm−3) and 1:1.35 (5.8 mol dm−3) are shown in Fig. 2. In the case of 1.4 mol dm−3 LiBF4/SL, oxidative current were observed at around 4.3 V (vs. Li/Li+). On the other hand, the highly concentrated electrolyte, 5.8 mol dm−3 LiBF4/SL, was stable up to 5.0 V (vs. Li/Li+). Apparently, the oxidative stability was enhanced by the increase of lithium salt concentration. The enhancement of oxidative stability of the SL molecule was attributed to the strong interaction between SL and Li+ ion. The lone pairs of oxygen atoms of SL are attracted to Li+ ion, and the electrons of the lone pairs become hard to be extracted. In the 5.8 mol dm−3 LiBF4/SL electrolyte, almost all the SL molecules coordinate to Li+ ions and uncoordinated SL scarcely exists, resulting in the extremely high oxidative stability of the electrolyte. Other physicochemical properties such as viscosity, ionic conductivity, and self-diffusion coefficients of components of electrolyte solutions will be also reported. Acknowledgment This research was supported by Japan Science and Technology Agency (JST) - Advanced Low Carbon Technology Research and Development Program - Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) of Japan.