Lithium-sulfur (Li/S) batteries promise significant specific energy improvements over traditional lithium-ion technology resulting from the high theoretical capacity of both the sulfur cathode and the lithium anode. However they are constrained by low voltage, safety issues due to the lithium anode, charge retention, and cycle life to name a few issues. A variety of these drawbacks are frequently attributed to the so-called “polysulfide shuttle” where partial discharge products from the cathode (polysulfide species such as Li₂S₆, Li₂S₄, etc.) dissolve in the electrolyte and migrate to the anode where they wreak havoc.^1–4 Many approaches have been developed to mitigate this issue, however few attempts have been made to measure the amount of polysulfide in the electrolyte, particularly during cycling in a representative cell. Here we describe an electrochemical in situ method for detecting and quantifying polysulfide species in the electrolyte of experimental Li/S cells during operation. This method has been applied to experimental Li/S and lithium sulfur-molybdenum disulfide (Li/S-MoS₂) cells developed previously in our laboratory.⁵ High concentration electrolytes and ceramic coated separators were also investigated using this method as they have also been shown to influence the performance (cycle life in particular) of Li/S cells.⁶ The method utilizes an auxiliary electrode inserted into the electrolyte for a quantitative determination of the redox behavior of the polysulfides. Several interesting trends were observed using this method, including that Li/S-MoS₂ cathodes, ceramic coated separators and concentrated electrolytes all display lower concentration of polysulfide compared to the baseline Li/S cells. The quantity of polysulfide species changes during discharge, increasing dramatically during the second discharge plateau. This technique can thus be used to determine the efficacy of new modifications (for example to the cathode or separator) designed to improve sulfur sequestration within or near the cathode

Acknowledgement:

This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The authors acknowledge the funding support of Army (CERDEC).

© 2017 California Institute of Technology. Government sponsorship acknowledged.

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