The equimolar mixtures of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and glyme (CH₃-O-(CH₂-CH₂-O)_n-CH₃) are classified as solvate ionic liquids. They have been investigated for the electrolytes of electrochemical devices because of low vapor pressure, high electrochemical stability, and the acceptable ionic conductivity [1]. In particular, they are suitable electrolytes for sulfur cathode batteries because of the low solubility of polysulfides, Li₂S_x[2].

In order to apply the LiTFSA-glyme solvate ionic liquids to the electrolytes for practical rechargeable lithium batteries, the improvement of rate performance is one of the significant issues because the diffusion of Li species is generally slow due to the high viscosity of solvate ionic liquids. We have already reported that the local viscosity near the interface between an electrode and the solvate ionic liquids drastically changes during deposition and dissolution of Li on the electrode using the electrochemical quartz crystal microbalance (EQCM), which can monitor not only the change of mass of the electrode but also the change of the viscosity and density near the electrode [3]. It is expected that an increase in the local viscosity with increase in the local concentration of Li species particularly affect the rate performance of the batteries using the solvate ionic liquids. In the present study, the effects of temperature on the change of local viscosity near the electrode during dissolution of Li were investigated in LiTFSA-tetraglyme (G4) solvate ionic liquids.

The change of product of local viscosity and density (ηρ) was monitored during dissolution of Li deposited on a Ni-coated quartz crystal electrode in the LiTFSA-G4 solvate ionic liquids in the temperature range from 293 to 308 K with different current densities. An increase in the ηρ was observed during dissolution of Li, reflecting a descending concentration distribution of Li species was produced by diffusion of Li species from the electrode surface to the bulk of the solvate ionic liquid. The variation of the ηρ before and after dissolution for –0.25 mA cm^–2 was larger than that for –0.15 mA cm^–2 at a constant temperature, indicating the variation is a function of the concentration gradient depending on the current density. When the current density is constant, the variation of the ηρ decreased with elevating temperature, probably due to a decrease in the local concentration of Li species at the electrode surface by a decline in the viscosity. The limiting current density for dissolution of Li also increased with elevating temperature in a coin-type cell. Moreover, the performance of a LiFePO₄ cathode, such as polarization and capacity, was remarkably improved especially during extraction of Li⁺ from the LiFePO₄cathode at elevated temperature. These results indicated operation at higher temperatures is effective in improving the rate performance of the batteries using solvate ionic liquids, while most of conventional rechargeable batteries have to be cooled in order to avoid deterioration of their performance.

This work was partially supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency (JST).

[1] T. Tamura et al., Chem. Lett., 39, 753 (2010).

[2] K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013).

[3] N. Serizawa et al., J. Electrochem. Soc., 160, A1529 (2013).