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, which is 10 times value compared with conventional positive electrode materials such as LiCoO₂ using Li-ion battery. Also, sulfur is obtained as by-products of petroleum, and low-cost batteries can be produced. 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. SIL is consisted of weak Lewis base/acid ion, which is coordinated Li cation with solvent, such as complex cations. SIL has physicochemical properties similar to conventional ionic liquid, which melting point lower than room temperature, thermal stability in wide temperature range and negligible vapor pressure. 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. When SIL electrolyte applied to Li-S batteries, high coulombic efficiency and long cycle performance are reported^[3]. However, if SIL was actually used as electrolyte, 1: 1 molar ratio of SIL was locally destroyed in vicinity of the electrode during charge/discharge reaction.Free glyme exists into electrolyte solution with electrochemical process, which is not temporarily coordinated between lithium cation and oxygen of glyme. As a result, Li₂S_x can be dissolved into free glyme, easily. In order to suppress of free glyme formation, we propose to suppress of Li₂S_x dissolution and improve performance of Li-S battery by using excess Li salt concentration of SIL. However, high-concentration electrolyte exhibits large viscosity, and 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^[4]. Most important demands for dilute solvent are,

1. Improvement of rate performances for batteries by low viscosity of electrolyte solution
2. Stabilization of solvate structure between Li salt and solvent molecule with/without dilute solvent.

Experiments

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 high Li salt concentrated SIL sample, [Li_1.25(G4)₁]TFSA (G4: CH3-O-(C₂H₄O)₄-CH₃, TFSA:N(SO₂CF₃)₂) was prepared in Ar-filled glovebox. Molar ratio of [Li_1.25(G4)₁]TFSA was G4:LiTFSA=1:1.25. In addition, we added given predetermined amount of 1,1,2,2, - tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (HFE) dilute solvent into SIL, and measured temperature dependence of viscosity and density for prepared sample.

Results & Discussion

Fig.1(a) showed the composition dependence of the viscosity from 10 to 80℃ (r) for SIL/HFE mixtures. We confirmed that diluting HFE decreased viscosity [Li_1.25(G4)₁]TFSA. When diluting HFE and SIL to about 1: 1, the viscosity decreased to about 1/10. HFE is very effective as a diluting solvent to lowering the viscosity of SIL. Fig.1(b) showed the composition dependence of the density from 10 to 80℃ (r) for SIL/HFE mixtures. r values of HFE were larger than that of SIL ones owing to the difference of fluorine density between SIL and HFE. In mixing SIL and HFE, the density didn’t take intermediate value and was mainly lower. To analyze this reason, we assumed SIL as one molecule, excess densities (E_ρ) were expressed as following,

E_ρ=ρ-(x_ρSIL+(1-x)_ρHFE)

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 dependence 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. Mixture of SIL and HFE indicates possibility of expansion / repulsion etc. The mixture of SIL-HFE was inferred to be a liquid state like phase separation significant interaction. This result correlated with spectroscopic method that coordination structure of SIL and G4 doesn’t change significant in dilute HFE^[5]. In this presentation, relationships between lithium-sulfur battery performance (rate properties) and composition of [Li_1.25(G4)₁]TFSA/HFE will be reported, and precise transport properties will be discussed.

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] S. Seki et al, Electrochemistry 2017, 85, 680.

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

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