In-Situ Raman Spectroscopy of Lithium-Sulfur Cells
Authors: Langrud Strauss , Boateng Frimpong , Brow Ryan , Santhanagopalan Shriram , Xing Weibing 
Published: July 2022

With the increasing demand for higher capacity and energy density, new batteries are being developed to meet these demands. Among the most promising chemistries, Lithium-Sulfur (Li-S) is one of the most popular. This is because of its excellent theoretical energy density (2600 Wh/kg, active materials) [1]. There are challenges that prevent Li-S cells from being commercialized, including adverse impact on cell performance due to lithium polysulfide (PS) formation/dissolution, volume changes during cycling and lithium dendrite formation. The issue that retains the focus of many researchers is the formation and shuttling of PSs as it directly impacts the practicality of the cell in terms of longevity and efficiency. There have been various approaches to mitigate PS shuttling including the use of specially designed cathode materials. One such material is sulfur-polyacrylonitrile (S-PAN) where PAN acts as a backbone [1, 2, 3]. The structure of this material supports the full reduction of elemental sulfur to Li2S upon discharging. This full reduction prevents the formation of intermediate PSs and loss of sulfur through PS shuttling, thus improving the sulfur utilization of S-PAN cathodes. The hypothesis is that there are little to no intermediate PSs present in S-PAN cathodes. To prove this, we used Raman spectroscopy to compare the quantitative results between the S-PAN electrode and conventional sulfur electrode. Specifically, with Raman spectroscopy, the features of the spectra that we are comparing are the Raman peaks of intermediate PSs that may be present in the samples [4, 5]. If the hypothesis is correct, the intensities of the Raman peaks for intermediate PSs for S-PAN should be less pronounced than conventional sulfur. In other words, the S-PAN electrode should exhibit significantly fewer intermediate polysulfides than the conventional sulfur electrode. The test is conducted by discharging the Raman cell to a set of incremental potentials where Raman spectra are taken at each potential. This will allow for the comparison of the polysulfide compositions and concentrations at different potential levels.
We will present the effect of S-PAN on the charge-discharge cycle and compare it to a conventional Li-S cell. We will also utilize Raman Spectroscopy and in-situ Raman cells to characterize and compare PS species in both S-PAN and conventional S based cells.

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Acknowledgement

This work was supported in part by the U.S. Department of Energy, Office of Science, Office Workforce Development for Teachers and Scientists (WDTS) under the Science Community College Internship (CCI) Program as well as the Office of Energy Efficiency and Renewable Energy, Energy Storage Graduate Internship Program at the National Renewable Energy Laboratory.