Design of Sparingly Solvating Electrolytes for Li-S Batteries

Li-S batteries have received much attention as next-generation secondary batteries because cathode active material, S 8 , has a very high theoretical capacity (1672 mAh g -1 ) and is abundant in natural resources. One of the issues hindering the Li-S batteries from practical use is the dissolution of lithium polysulfide (Li 2 S m ), which are reaction intermediates of S 8 cathode. The dissolution of Li 2 S m causes poor cycle life and low coulombic efficiency of discharge/charge of a Li-S cell. To address these problems, electrolytes exhibiting weak solvating ability against Li 2 S m , called “sparingly solvating electrolytes”, have been developed. 1 In this study, we evaluated the solubility of Li 2 S m and S 8 in various organic electrolytes and investigated relationship between parameters of the electrolytes and the solubility of sulfur-based active materials. Based on the solubility data, we will demonstrate a rational design of sparingly solvating electrolytes and its application to Li-S batteries. 1M Li[TFSA] electrolytes were prepared using various organic solvents, and then Li 2 S 8 or S 8 was saturated in each electrolyte solution. The saturation solubility of Li 2 S 8 and S 8 was determined by UV-Vis spectroscopy. The donor number (DN) of the electrolytes was estimated using 23 Na NMR. 2 Among the electrolytes capable of suppressing the dissolution of Li 2 S 8 and S 8 , 1 M Li [TFSA] / n -butyl methyl ether ( n -BME) and the n -BME-based electrolyte with optimized salt concentration were applied to a Li-S batteries. The electrodes were prepared by using S 8 , Ketjen Black (KB), and CMC binder. 2032-type coin cells were assembled with a mixed composite electrode and a Li metal foil anode, and tested in the voltage range of 1.0-3.0 V at 30 ° C. We found a relatively good correlation between the Li 2 S 8 solubility and DN of the electrolytes. When the DN becomes lower than 14, the solubility of Li 2 S 8 remarkably decreases. However, some electrolytes showed different Li 2 S 8 solubilities even with the same DN. When classifying these solvents into three groups: monodentate ether, bidentate ether, and ester, we found relatively good correlation between the dielectric constant and the Li 2 S 8 solubility. We concluded that Li 2 S 8 solubility is dominated mainly by the DN of the electrolyte.When solvents have similar molecular structures and similar DN, however, the solubility was higher for the electrolytes with higher dielectric constant. In addition, although the solubility of S 8 is generally much lower than that of Li 2 S 8 , the solubility of S 8 was found to be high if the solvent has the solubility parameter of ~10 (cal cm −3 ) 1/2 . It is considered that the solubility of S 8 follows a common principle of solubility of neutral compounds, since the solubility parameter (10.2) of solute S 8 agrees with the solubility parameter of CS 2 showing the highest solubility of S 8 . The sparingly solvating electrolytes for Li-S batteries can be designed by using the DN of electrolytes and the solubility parameter of solvent as guideline. As a result of charge / discharge tests, we found that 1 M Li [TFSA] / n -BME was applicable to Li-S batteries. We will also discuss the battery characteristics with the optimized electrolyte. References [1] K. Dokko et al. , J. Electrochem. Soc., 160 , A1304 (2013). [2] M. Schmeisser et al ., Chem. Eur. J. 18 , 10969 (2012). Figure 1