Lithium-sulfur battery has been attracted attention as a next-generation battery of large capacity. Lithium-sulfur battery has reversible theoretical capacity of 1,672mA h g^-1. This is 10 times the theoretical capacity compared with conventional positive electrode materials such as LiCoO₂ using Li-ion battery. One serious problem of lithium sulfur battery is dissolution of lithium polysulfide (Li₂S_x) as reaction intermediate into the electrolyte solution during charge/discharge reaction. This causes degradation of the charge/discharge cycle characteristics and coulombic efficiency of the battery. In order to solve this problem, “solvate ionic liquid (SIL)” is proposed as new electrolyte, because dissolution of Li₂S_x can be suppressed owing to weak Lewis acidity/basicity.

For example, significant improvement for stability of electrolyte solution has been reported by high-salt concentration (ether series^[1]/acetonitrile^[2]) owing to strong interaction between Li salt and solvent molecule. However, high-concentration electrolyte exhibits large viscosity. And the electrolyte systems has risks of rate performance for high rate charge/discharge operations. Therefore, in order to obtain low viscosity electrolyte, we proposed to add low viscosity dilute solvent into electrolyte^[3]. Most important demands for dilute solvent are,

1. Improvement of rate performances for batteries by low viscosity of electrolyte solution
2. Stabilization of solvated structure between Li salt and solvent molecule with/without dilute solvent. In this study, we investigated physicochemical effects for understanding compatibility of two effective approaches of super-concentrated Li salt electrolyte and non-interactive dilute solvent.

In this study SIL sample, [Li(G4)_0.8]TFSA (G4: CH₃-O-(C₂H₄O)₄-CH₃, TFSA:N(SO₂CF₃)₂) was prepared in Ar-filled glovebox. Molar ratio of [Li(G4)_0.8]TFSA was G4:LiTFSA=0.8:1. In addition We added given amount of 1,1,2,2, - tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (HFE) dilute solvent into SIL, and measured temperature dependences of viscosity and density of prepared sample.

Fig.1(a) showed the composition dependences of the viscosity from 10 to 80℃ (h) for SIL/HFE mixtures. We confirmed that diluting HFE decreased viscosity [Li(G4)_0.8]TFSA.

Fig.1(b) showed the temperature dependences of the density from 10 to 80℃ (ρ) for SIL/HFE mixtures. ρ values of HFE were larger than that of SIL ones owing to the difference of fluorine density between SIL and HFE. At this time, we assumed SIL as one molecule, excess densities (E_ρ) were expressed as following,

E_ρ=ρ-(xρ_SIL+(1-x)ρ_HFE)

Where x, ρ_SIL and ρ_HFE are mole fraction of SIL, ρ of neat SIL and HFE, respectively.

Fig.1(c) shows mole fraction dependences of E_r for SIL-HFE mixtures. E_r always shows a negative value in the temperature range below 30℃. Therefore, by mixing SIL and HFE, and suggested that the density decreasing. This result correlated with spectroscopic method that coordination structure of SIL and G4 doesn’t change significant in dilute HFE^[4].

In this presentation, relationships between lithium-sulfur battery performance (rate properties) and composition of [Li(G4)_0.8]TFSA/HFE will be reported, and also will be discussed optimum dilution amount of HFE.

References:

[1]K. Yoshida et al, J. Am, Chem. Soc. 2011, 133, 13121.

[2]Y. Yamada et al, J. Am. Chem. Soc. 2014, 136, 5039.

[3]K. Dokko et al, J. Electrochem. Soc. 2013, 160, A1304.

[4]S. Saito et al, J. Phys. Chem. B 2016, 120, 3378.