A Rational Balance Design of Ionic Liquid Based Electrolyte for Advanced Li-S Batteries

Lithium-sulfur (Li-S) battery has attracted worldwide attentions by virtue of high theoretical energy density, abundant material source and good environmental friendliness. However, the dissolution and shuttle of the intermediate polysulfides always result in low sulfur utilization, bad cycle stability and severe self-discharge issue. It is well known that room temperature ionic liquids (ILs) can reduce polysulfide dissolution owing to weak donor ability and delay polysulfide migration due to high viscosity. But high viscosity is a double-edged sword, which can also hinder the transport of Li ion, casing low capacity deliver and weak rate capability. Various low-viscous ethers (DME, DOL, TEGDME) has been attempted to cooperate with the IL, but most of them have a relatively high donor number and hence polysulfide dissolution is reversely promoted unavoidably. Here, we put forward the introduction of linear fluorinated ethers (LFEs) in the IL-based electrolyte as co-solvent. By adding moderate content of LFEs into the IL, the increased capacity and improved rate capability are demonstrated. On the one hand, LFEs own relatively lower viscosity compared to ILs and hence improve the ion conduction in the electrolyte. On the other hand, they serve to minimize polysulfide dissolution due to low solvation ability. Besides, these F-contained solvents stabilize SEI on Li anode as well. However, the binary system consisting of IL and LFE leads to low active material utilization instead, despite high coulombic efficiency (CE, close to 100%) and stable cycle is obtained. In other words, excessive restriction to polysulfide dissolution is not necessary for the improvement of cell performance. In contrast, the ternary system consisting of IL, LFE and ordinary ether (such as DOL) contributes to more reversible capacity, although the CE value (~90%) is a bit lower than those binary or pure IL cases (Fig.1 as an example). Therefore, it should be emphasized that a rational balance design between the polysulfide dissolution and shuttle suppression is more significant for constructing advanced Li-S batteries. Fig. 1 (a) Discharge capacities and (b) coulombic efficiencies of Li-S batteries in various electrolytes during cycling Reference: Lu, K. Zhang, Y. Yuan, et al, Electrochim. Acta, 2015, 161, 55-62 Lu, Y. Yuan, K. Zhang, et al, J. Electrochem. Soc., 2015, 162, A1460-A1465 Lu, Z. Chen, H. Du, et al, Ionics, DOI: 10.1007/s11581-018-2814-x Acknowledgment: This work was supported by the Natural Science Foundation of China (No. 51604221, 51372197 and 51574288), the Key Innovation Team of Shaanxi Province (2014KCT-04). Figure 1