Solvate Ionic Liquids for Lithium–Sulfur Batteries

Introduction

Glyme, which has a chemical structure of CH₃–O–(CH₂–CH₂–O)_n–CH₃, can coordinate Li⁺ ions forming a 1 : 1 complex cation of [Li(glyme)]⁺ when n is either 3 or 4. The coordination number of Li⁺ is typically 4–5, thus, n = 3 or 4 is suitable for the equimolar complexation. Consequently, the equimolar mixture of glyme and certain Li salt with weakly Lewis basic anion can be regarded as a room temperature ionic liquid consisting of [Li(glyme)]⁺ complex cations and the anions. ^[1]-[3] Herein, we present the solvate ionic liquids as excellent electrolyte candidates for Li-S batteries; these electrolytes greatly suppress the dissolution of lithium polysulfides. ^{[4] ,[5]}

Experiments

Electrolytes were prepared by mixing tetraglyme (G4) with lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) in an Ar-filled glove box. Sulfur was mixed with Ketjen black (KB, specific surface area: 1270 m² g^–1), and polyvinyl alcohol (PVA) for cathode preparation. The mass ratio of S/KB/PVA in the composite cathode was 60:30:10.

Results and discussion

Figure 1 shows charge–discharge stability of Li–S cells with [Li(G4)_x][TFSA] electrolyte measured at low current densities. The initial discharge capacity of each Li–S cell was in the range 800–1000 mAh g^–1, which corresponds to 50–60% of the theoretical capacity of elemental S (1672 mAh g^–1). The coulombic efficiency of the cell with [Li(G4)₄] [TFSA] rapidly decreased with each charge–discharge cycle. The lower coulombic efficiency is attributed to the occurrence of the redox shuttle mechanism of lithium polysulfides (Li₂S_m) within the cell during the charge and discharge processes. In contrast, the cell with [Li(G4)₁][TFSA] can be cycled more than 50 times while maintaining a coulombic efficiency above 98%. This high efficiency can be attributed to the low solubility of Li₂S_m in [Li(G4)₁][TFSA], indicating that the solid state Li₂S_m remains in the composite cathode and undergoes a reversible redox reaction. ^[5]

Acknowledgement

This study was supported in part by ALCA of Japan Science and Technology Agency (JST).

References

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

[2] K. Yoshida et al. J. Am. Chem. Soc. 133 (2011) 13121.

[3] K. Ueno et al., J. Phys. Chem. B 116 (2012) 11323.

[4] N. Tachikawa et al., Chem. Commun., 47 (2011) 8157.

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