Solvate Ionic Liquids and Their Application to Lithium Batteries: Glyme-Lithium Bis(fluorosulfonyl)amide Equimolar Compl
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Solvate Ionic Liquids and Their Application to Lithium Batteries: Glyme-Lithium Bis(fluorosulfonyl)amide Equimolar Complexes Kazuki Yoshida, Mizuho Tsuchiya, Naoki Tachikawa, Kaoru Dokko, and Masayoshi Watanabe Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ABSTRACT The physicochemical properties of glyme-Li[FSA] (FSA: bis(fluorosulfonyl)amide) equimolar complexes were investigated. The self-diffusion coefficients of glymes and Li+ as determined by pulsed-field gradient spin-echo nuclear magnetic resonance spectroscopy in equimolar complexes were almost identical, suggesting that all of the glyme molecules coordinated with Li+. Electrochemical characterization revealed that the oxidative stability of glyme molecules was enhanced by complexing with Li+. Using [Li(glyme)1][FSA] electrolytes and a LiFePO4 cathode, a lithium secondary battery could be stably operated for more than 100 cycles at room temperature. INTRODUCTION Lithium ion batteries (LIBs) with high energies and power densities have become essential for modern society as power sources for portable electronic devices.1 Conventional electrolytes used in LIBs are composed of mixed organic solvents (cyclic carbonate and linear carbonate) and LiPF6. Among them, linear carbonate solvents are extremely flammable and have flash points below room temperature. Thus, LIBs have thermal instability problems at elevated temperatures. To achieve a high degree of safety for LIBs, the development of thermally stable electrolytes is crucial. Room-temperature ionic liquids (RTILs), which consist entirely of cations and anions, have attracted much attention owing to their unique properties such as low volatility, high thermal stability, high ionic conductivity, wide potential window, and high chemical stability. RTILs have been extensively studied as electrolytes for LIBs.2 To replace the flammable organic solvents of LIBs, onium cation-based aprotic RTILs such as quaternary ammonium and imidazolium RTILs have been adopted as solvents for dissolving Li salts. However, the viscosities of RTILs increase with increasing Li salt concentration. Moreover, concentration polarization takes place in RTILs during the charge-discharge cycle of LIBs because there are two cationic species, Li+ and the onium cation, in the electrolyte. The Li+ transference number is generally very low (less than 0.13) and the maximum current density based on Li+ migration in RTILs does not meet the requirements for practical use in LIBs at room temperature. To achieve a high transference number for Li+ in RTILs, we prepared lithium ionic liquids consisting of Li+ and borate having electron-withdrawing groups, which reduces the anionic basicity, as well as Li-coordinating ether ligands to dissociate Li+ from the anionic centers.4,5 However, the ionic conductivity of the lithium ionic liquid at room temperature was low because of its high viscosity and low degree of dissociation. Recently, we reported that glyme-Li[TFSA] (TFSA: b
