Highly concentrated electrolytes that behaves like ionic liquids, such as an equimolar mixture of certain Li salt, e.g. Li[TFSA] (lithium bis(trifluoromethanesulfonyl)amide: LiN(SO₂CF₃)₂) and glymes (Gn: (CH₃O(CH₂CH₂O)_nCH₃)), are attracting much attention recently. Li⁺ ion forms complex cation with glyme in 1:1 ratio when the chain length of glyme are n = 3 or 4 (G3 or G4), and nearly all glymes in the equimolar mixture coordinate to Li⁺. Therefore the mixture is composed of solvate cation [Li(glyme)]⁺ and counter anion [TFSA]⁻ exhibiting low volatility, low flammability, high thermal stability, and high electrochemical stability. Such mixture is classified as solvate ionic liquids (SILs).^1,2) The glyme-Li salt SILs can be applied to various Li secondary batteries as a thermally stable electrolyte.^2,3) Recently, the electrolyte dissolving Li[FSA] (lithium bis(fluorosulfonyl)amide: LiN(SO₂F)₂) at high concentration has getting attention for exhibiting unique properties that are not observed in conventional concentration electrolytes, such as stable Li⁺ intercalation/deintercalation into/from graphite layers,⁽⁴⁾ and stable Li metal deposition/dissolution at high current density. ⁽⁵⁾ In this work, the mixture of Li[FSA] and G3 are prepared in wide concentration range and their physicochemical properties including ionic conductivity, viscosity, self-diffusion coefficient and Raman spectra, and electrochemical properties including oxidative stability and Li metal deposition/dissolution will be discussed.

Figure 1 shows the concentration dependency of self-diffusion coefficient ratio between (a) Li⁺ and G3, and (b) Li⁺ and [FSA]⁻ expressed as D_G3/D_Li and D_FSA/D_Li respectively. The D_G3/D_Li decreased along with the increase of Li[FSA] concentration and reached unity at around 4 mol dm⁻³ which is equimolar composition. This indicates the formation of complex cation [Li(G3)]⁺, and Li⁺ and G3 diffuse together in the equimolar mixture. However, the D_G3/D_Li became lower than unity in the mixture of Li[FSA] concentration higher than 4 mol dm⁻³, indicating that Li⁺ diffuses faster than G3. In contrast, D_FSA/D_Li was constant around 1 at concentration lower than 4 mol dm⁻³ and sharply increased at higher concentration. These results suggest that the charge transport mechanism in the mixture of concertation higher than 4 mol dm⁻³ is different from that in the mixture of lower concertation. The [FSA]⁻ coordinates to Li⁺ when excess Li[FSA] is added to G3, and forms complex anion Li[FSA]_y^(y−1)−. This may induce another charge transport mechanism different from the simple vehicle mechanism. It is considered that the ligand exchange of Li⁺ takes a significant role during the charge transport in the mixture higher than 4 mol dm⁻³. The Raman spectra and electrochemical properties of glyme-Li salt SILs will also be reported.

Reference: (1) T. Mandai et al., Phys. Chem. Chem. Phys., 16, 8761 (2014). (2) K. Yoshida et al., J. Am. Chem. Soc., 113, 13121 (2011). (3) K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013). (4) Y. Yamada et al., J. Am. Chem. Soc., 136, 5039 (2014). (5) J. Qian et al., Nat. Commun., 6, 6362 (2015).