Rechargeable lithium-ion batteries (LIBs) are necessary for our life to use as a mobile phone and so on. Lithium-sulfur (Li-S) battery is expected for next generation rechargeable battery owing to have high capacity (1,645 mAh/g) compared with conventional Li-ion batteries. Cycle performances are one of the index of battery. There is strongly related with a battery lifetime. The key issues of Li-S battery for cycle performances are the dissolution of sulfur species, the positive electrode active material, as Li₂S_x (lithium polysulfide). If we can suppress the dissolution of Li₂S_x, the battery life should be extended. Solvate ionic liquid (SIL) is mixture of 1:1 complex from low-molecular weight ether and Li salt, which have high thermal/electrochemical stabilities owing to strong interaction of between ether oxygen and Li cation. Also SIL electrolyte can suppress the dissolution of Li₂S_x. Recently, high Li salt concentration more than conventional SIL into electrolyte is important for high performance LIBs and Li-S batteries not only the high stability but also low Lewis basicity of electrolytes for low solubility of impurity with charge/discharge. Fig. 1 shows cycle performance of LiNi_1/3Mn_1/3Co_1/3O₂ | [Li(G3)_x]TFSA | Li cell. Excess Li salts achieved high cycle performances and stable charge-discharge operations [1]. Fig. 2 shows Charge-discharge profiles with C/18(Li-S | [Li(G3)₁]TFSA | Li cell). Over 1,000 mAh/g capacity at 1st charge-discharge, and 600 mAh/g capacity at 600 cycles were observed. We consider to importance for composition ratio glyme and LiTFSA for lithium-sulfur battery performance [2].However, quantitative analysis for dissolution of Li₂S_x into various SIL ([Li(G3)_x] [TFSA]) has not investigated. In this study, to clarify relationship between composition ratio and dissolution amount of Li₂S_x, saturated solubility of Li₂S_x was measured by electrochemical and UV-vis spectra.

All experiments were examined in a glove box with Ar atmosphere. SILs, [Li(G3)_1.25]TFSA, [Li(G3)_1.11]TFSA, [Li(G3)]TFSA, [Li(G3)_0.9]TFSA and [Li(G3)_0.8]TFSA (composition ratio of glyme (G3,tryglyme) and LiTFSA is 10:8, 10:9, 10:10, 10:9 and 10:8), were prepared. Mixture of S₈ and Li₂S (S₈:Li₂S=7:8, we defined as Li₂S₈) were prepared. SILs of each concentration ratio prepared with Li₂S₈ was stirred to a saturated state with a hot stirrer at 50 ℃. It was oxidized to S₈ by holding it at 3.0V for 12h using an electrolytic cell. The electrolytic and non-electrolytic SILs were each diluted 40 times (molar ratio) with HFE, and the absorbance was measured by UV-vis spectrometer. Lithium-sulfur batteries using electrolytes of each composition were prepared and charge and discharge cycle tests were carried out.

Fig. 3 shows appearances of five LiTFSA concentration SILs with saturated Li₂S₈. This shows high concentration lithium salt electrolyte suppresses dissolution of Li₂S_x into each SILs. Fig. 4 shows UV-vis spectra of non- or electrolytic SILs. Color strength (dark to yellow) were decreased with Li salt concentration into SILs, and considered the solubility of Li₂S_x was controlled with the Li salt concentrations. We confirmed that improvement in cycle performances were observed in the lithium-sulfur batteries using various lithium salt concentration SILs. In the presentation, we will report to results of UV-vis spectra and correlation of between dissolution amount of Li₂S_x and battery performances.

References:

[1] Seki et. al, RSC Adv., 6, 33049-33047 (2016).

[2] Seki et. al, Electrochem., 85, 680-682 (2017).