Development and Mechanism Investigation of Novel Fluorinated Ethers As Secondary Solvent in Electrolyte for Lithium-Sulfur Batteries

Abstract

Sulfur (S) has been identified as one of the most promising cathode material due to its high theoretical specific capacity (1675 mAh/g), good environment compatibility, and low cost. These features make lithium-sulfur battery a potential candidate for the next-generation battery, especially in electric vehicles and portable devices. However, the application of lithium-sulfur (Li-S) battery is still impeded by some limitations, including the low electrical and ionic conductivity of the elemental sulfur and the discharge product (Li2S), and the shuttle effect due to the presence the dissolved polysulfide (PS) species in electrolyte. 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) has been identified as an effective secondary solvent in electrolyte for lithium sulfur batteries. In this study, fluorinated ethers (FEs) other than TTE were attempted in 1,3-dioxolane (DOL) based electrolyte, and comparison was made over cell performance, viscosity/conductivity of electrolyte, and aggregation states of lithium PS species. 1,1,2,2-Tetrafluoroethyl 2,2,2-trifluoroethyl Ether (HFE), ethyl 1,1,2,2-tetrafluoroethyl ether (ETE), and 1,1,2,3,3,3-hexafluoropropyl ethyl ether (HEE) were identified as promising alternatives to TTE to accommodate wider range of applications of Li-S batteries. However, amphiphilic FEs, like 1,1,1,2,2,3,3,4,4-Nonafluoro-6-(pentyloxy)hexane (NHPE) and 1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluoro-8-(pentyloxy)octane (TDPE) were not effective at all. Depending on formula weight, viscosity and conductivity, cell performance of Li-S batteries containing electrolytes with different FEs also varied. And FEs with coulombic efficiency higher than that of TTE were discovered. In DOL/FE based electrolyte, the aggregation states of lithium PS species were investigated via small-angle X-ray scattering (SAXS), and revealed a variation in dimension of assemblies of lithium PS species in the presence of different FEs. The difference of cell performance can therefore be concluded as the variation in transport behavior of lithium PS species in electrolyte. In addition, this would offer some insights into the failure mechanisms of Li-S batteries, and promote further development of electrolyte for Li-S batteries. Figure 1