Probing the Role of Solvent and Salt in Lithium Sulfur Redox Reactions
Authors: Zou Qingli , Lu Yi-Chun 
Published: February 2020

Lithium sulfur (Li-S) battery has the potential to provide a 2 to 3 times higher specific energy than conventional lithium ion batteries.1 The development of Li-S batteries is, however, limited by the dissolution, diffusion and side reactions of soluble polysulfides in the electrolyte.1-3 Fundamental understanding of solvent’s influence on the stability of polysulfides and their activities in Li-S redox reactions is crucial for enabling rational design of electrolyte for Li-S batteries. We compare reported Li-S reaction pathways in high-DN solvents (e.g. DMSO, DMA)4-7 and low-DN solvents (DOL:DME8 / TEGDME9). Interestingly, major reactions proposed in high-DN solvents (e.g. DMSO, DMA)4-7 were also proposed in low-DN solvents (e.g. DOL:DME or TEGDME) in various studies,8-10including the electrochemical and chemical reactions of the trisulfur radical. In view of these proposed reaction pathways in the literature, it is unclear what the major difference is in the Li-S reaction mechanism/pathways in high- and low-DN solvents that contribute to the distinct electrochemical behaviors found in high- and low-DN solvents. Therefore, it is critical to understand and differentiate the major differences in Li-S reaction pathways in DMSO and DOL:DME in order to understand origins responsible for different Li-S behaviors.
We apply operando UV-Vis spectroscopy4, 9, 11, 12 to investigate the Li-S redox reactions in the commonly used low-DN solvent DOL: DME (v:v=1:1) and a classic high-DN solvent DMSO as model electrolytes.13 We revealed that Li-S redox reactions in DMSO undergo series of electrochemical and chemical reactions involving S8
2-, S6
2-, S4
2- and S3
•- where S3
•- is the most stable and dominant reaction intermediates.13 In DOL: DME, the dominant reaction intermediates of the Li-S redox reactions is found to be S4
2-.14 To investigate the stability of various polysulfides in DMSO and DOL: DME, we perform UV-Vis studies on a series of chemically-synthesized polysulfides14 in both solvents. Results indicate that (1) S3
•- is strongly stabilized in DMSO and can be formed facilely from S8
2-, S6
2-, and S4
2-; (2) S4
2- is strongly stabilized in DOL: DME and can be formed facilely from S8
2-, S6
2-, and S4
2-. These observations support that S3
•- and S4
2- is the main reaction intermediates of the Li-S redox reactions in DMSO and DOL: DME, respectively. Detailed solvent-dependent Li-S reaction pathways, the stability of S3
•- and S4
2-in the solvents and the implications in the Li-S batteries will be discussed.

Acknowledgement

This work is supported by a grant from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region (HKSAR), China, under Theme-based Research Scheme through Project No. T23-407/13-N, and by two RGC projects, No. CUHK24200414 and No. CUHK14200615.

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