Effect of the Solvate Electrolyte on Performance and Reaction Mechanism of Lithium-Sulfur Batteries
Chun-Yao Wang1, Tzu-Yun Lin1, Meng-Ping Xiong2, Ju-Ming Tsai2, Heng-Liang Wu2*
1Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan
2Center for Condensed Matter Science, National Taiwan University, Taipei, Taiwan
* Presenter:Heng-Liang Wu, email:hengliangwu@ntu.edu.tw
Solvate electrolytes have been proposed to suppress the polysulfide dissolution and enhance the capacity retention of lithium-sulfur (Li-S) batteries.[1-3] In this study, we report on our use of in situ spectroscopy including Raman and X-ray spectroscopy (X-ray diffraction and X-ray absorption spectroscopy) to investigate sulfur reaction mechanism and the interaction between polysulfide and electrolyte. Raman spectroscopy and cyclic voltammetry obtained from sulfur-carbon cathodes in the conventional ether-based electrolyte show that long chain polysulfides (S82-) are formed in the first reduction process at ~2.4 V vs Li/Li+ and short chain polysulfides such as S42-, S4-, S3․- and S2O42- are observed with continued discharge at ~2.3 V vs Li/Li+ in the second reduction process.[4] The elemental sulfur changes from orthorhombic phase to monoclinic phase in the conventional ether-based electrolyte during the first cycle. In acetonitrile-based solvate electrolyte, in situ spectroscopy results show that short chain polysulfides are formed at the early stage of discharge process and the formation of soluble long chain polysulfides is suppressed. Elemental sulfur with orthorhombic phase is reformed in acetonitrile-based solvate electrolyte during cycling. These results suggest that solvate electrolyte changes the sulfur reaction mechanism. The effect of solvate electrolyte on Li plating/stripping process was also studied. We next propose different solvate electrolytes with low polysulfide solubility and high stability toward Li metal to enhance the capacity retention of Li-S batteries.

[1] M. Cuisinier et al, Unique behaviour of nonsolvents for polysulphides in lithium–sulphur batteries. Energy Environ. Sci. 2014, 7, 2697.
[2] L. Cheng et al, Sparingly Solvating Electrolytes for High Energy Density Lithium–Sulfur Batteries. ACS Energy Lett. 2016, 1, 503−509.
[3] Q. Pang et al, Tuning the electrolyte network structure to invoke quasi-solid state sulfur conversion and suppress lithium dendrite formation in Li–S batteries. Nature Energy 2018, 3 (9), 783-791.
[4] H.-L. Wu et al, In Situ Raman Spectroscopy of Sulfur Speciation in Lithium–Sulfur Batteries. ACS Appl. Mater. Interfaces 2015, 7, 1709.

Keywords: lithium-sulfur batteries, polysulfide dissolution, sulfur reaction mechanism, solvate electrolytes