Effect of Electrolyte Composition on Performance and Stability of Lithium-Sulfur Batteries

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Rechargeable lithium-ion batteries are necessary for our life to use as a mobile phone, electric vehicle and so on. However, there is a limit to the capacity of lithium-ion batteries. Therefore it is necessary to develop batteries which has large capacity. 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. However, Li-S battery has serious problem that lithium polysulfide (Li2S x ) which is an intermediate product dissolves into electrolyte. If we can suppress the dissolution of Li2S x , the battery life should be extended. To suppress the dissolution of Li2S x , we use solvate ionic liquid (SIL) electrolytes. 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. SIL electrolytes have low solubility of Li2S x owing to their low Lewis acidity / basicity and suppress dissolution of Li2S x into electrolyte. Recently, it was found that high Li salt concentration more than 1:1 SIL electrolyte is effective for high performance lithium-ion batteries and Li-S batteries. Fig. 1 shows cycle performance of LiNi1/3Mn1/3Co1/3O2 | [Li(G3) x ]TFSA | Li cell. Excess Li salts achieved high cycle performances and stable charge-discharge operations. Li salt excess SIL electrolyte contributes to extension of cycle life for Li-ion batteries. According to these previous researches, it is shows that local composition changes of SIL electrolytes of glyme and Li salt near the electrode during the charge and discharge process. Li2S x should be dissolved in the released free glyme. Therefore, we considered that Li salt excess SIL electrolyte can suppress the formation of temporary free glyme. Therefore, the purpose of this study is to clarify the influence of the composition of SIL electrolyte on the solubility of Li2S x and the performance of Li-S batteries. All preparations and measurements were carried out in an inert atmosphere Ar-filled glove box and a sealed cell. 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 triglyme (G3) and LiTFSA is 10:8, 10:9, 10:10, 10:9 and 10:8), were prepared, respectively. Viscosity, density and thermal stability of SIL electrolyte samples were measured. Then, dissolution tests of Li2S x were carried out. Lithium polysulfide is an unstable substance, so S8 and Li2S were mixed with S8 and Li2S=7:8, we defined as Li2S8 were prepared. Prepared SIL electrolyte and Li2S8 were mixed for saturation. SILs were each diluted 50 times (molar ratio) with SIL electrolyte, and the absorbance was measured by UV-vis spectrometer. Fig. 2 shows appearances of five SIL electrolytes with saturated Li2S8. It was visually observed that the amount of free glyme decreased in the Li excess SIL electrolyte and the dissolution of Li2S x into SIL electrolyte was suppressed with LiTFSA concentration. From this result, improvement of Li-S battery performances were expected by using Li salt excess SIL electrolytes. Charge and discharge tests were carried out using [S|[Li(G3) x ]TFSA|Li] cells, and cycle performances were shown (Fig. 3) . All initial capacities were more than 800 mAh/g. The figure is the normalized cycle characteristics as deterioration rate with the initial capacity set to 100%. At 25 cycles [Li(G3) x ]TFSA x=0.8 (Li salt excess SIL electrolyte) showed a sufficient capacity retention about 10% higher than x=1 (equimolar). From the above results, the Li salt excess SIL electrolyte can conclude as effective electrolyte design for Li-S batteries. In the presentation, we will comprehensively report the thermal and transport properties of each SIL and the results of UV-vis measurement results. Figure 1