Design of Sparingly Solvating Electrolytes for Li-S Batteries
Authors: Haga Shohei , Yanagi Masato , Seita Taisho , Dokko Kaoru , Watanabe Masayoshi , Ueno Kazuhide 
Published: December 2020

Li-S batteries have received much attention as next-generation secondary batteries because cathode active material, S8, 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 (Li2S
m
), which are reaction intermediates of S8 cathode. The dissolution of Li2S
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 Li2S
m
, called “sparingly solvating electrolytes”, have been developed.1 In this study, we evaluated the solubility of Li2S
m
and S8 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 Li2S8 or S8 was saturated in each electrolyte solution. The saturation solubility of Li2S8 and S8 was determined by UV-Vis spectroscopy. The donor number (DN) of the electrolytes was estimated using 23Na NMR.2 Among the electrolytes capable of suppressing the dissolution of Li2S8 and S8, 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 S8, 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 Li2S8 solubility and DN of the electrolytes. When the DN becomes lower than 14, the solubility of Li2S8 remarkably decreases. However, some electrolytes showed different Li2S8 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 Li2S8 solubility. We concluded that Li2S8 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 S8 is generally much lower than that of Li2S8, the solubility of S8 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 S8 follows a common principle of solubility of neutral compounds, since the solubility parameter (10.2) of solute S8 agrees with the solubility parameter of CS2 showing the highest solubility of S8. 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