Relationship Between the Rate Performance of Rechargeable Lithium-Sulfur Batteries and the Local Viscosity Change at the Interface Between the Electrode and Li[N(CF3SO2)2]-Glyme Solvate Ionic Liquid

Sulfur is one of the attractive candidates as a positive electrode material because of the high theoretical specific capacity and the low cost. However, the dissolution of the intermediate lithium polysulfide, Li₂S_x, into the organic electrolyte leads to the rapid capacity fading and the low coulomb efficiency. The equimolar mixtures of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and glyme (CH₃-O-(CH₂-CH₂-O)_n-CH₃) are liquid at ambient temperature and are classified as a solvate ionic liquids because of the low vapor pressure, the wide electrochemical potential window, and the acceptable ionic conductivity[1]. Moreover, the solubility of Li₂S_x is known to be low due to the low donor ability of the LiTFSA-glyme solvate ionic liquid, resulting in enabling the more than 400-cycle stable charge-discharge operation of sulfur/carbon composite electrode in 50:50 mol% LiTFSA-triglyme (n=3) [2].

     While the viscosity of the LiTFSA-glyme solvate ionic liquid is generally high, the addition of such non-flammable diluent as 1,1,2,2–tetrafluoroethyl 2,2,3,3–tetrafluoropropyl ether (HFE), which does not affect the complex cation structure, decreased the viscosity, leading to the improvement of the rate performance of the cell due to the increase in the diffusivity of Li⁺ in the electrolyte[2]. In particular, the charging reaction of Li-S battery (Li⁺extraction from the lithiated sulfur electrode), could be improved by adding HFE.

     The electrochemical quartz crystal microbalance (EQCM) is often used for the in-situ monitoring of the electrode mass. Moreover the change in the viscosity (η) and density (ρ) of the electrolyte near the electrode can be estimated using an impedance technique EQCM. We have already reported that the drastic changes in the local viscosity of 50:50 mol% LiTFSA-triglyme were observed during deposition and dissolution of lithium metal using an EQCM [3]. The change could be explained by the concentration distribution of Li⁺ in the diffusion layer and the transient change in the dissolved state of Li⁺. In this study, the EQCM measurements were conducted in 50:50 mol% LiTFSA-tetraglyme (n=4) solvate ionic liquid with and without HFE to compare the local physical property of the electrolyte during deposition and dissolution of lithium metal.

     The resonance resistance of a quartz crystal electrode is known to reflect the local viscosity and density of the contacting electrolyte on the electrode. The resistance decreased during deposition of Li and largely increased during dissolution of Li, and decayed toward the initial value after the interrupt of the current. The magnitude of the change in the resistance became small with addition of HFE. This indicated the local change in ηρ value due to the concentration distribution of Li⁺was suppressed in the presence of HFE.

     This work was partially supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency (JST) and Keio Leading-edge Laboratory of Science and Technology (KLL).

[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).