Lithium sulfur (Li-S) batteries are a promising option for high energy density applications, such as electric vehicles and grid storage. In addition to a high theoretical specific energy, the Li-S system also boasts a lower cost and the use of relatively nontoxic, abundant materials compared to current lithium ion batteries. However, soluble, mobile polysulfide intermediates in the electrolyte lead to a polysulfide shuttling mechanism resulting in damage to the anode, loss of active material, and diminished cyclability. A better understanding of the polysulfide speciation as a function of depth of discharge is warranted to identify means to mitigate the polysulfide shuttle.

In response, polysulfide speciation is examined, under relevant conditions, in an operational coin cell, at varying states of charge and position, using in situ confocal Raman microscopy. Specifically, a spectroscopic 2032 coin cell was designed for use in in situ experiments while maintaining a representative environment for discharge. The coin cell consisted of a carbon-sulfur cathode, lithium metal anode, and 1:1 1,2-dimethoxyethane to 1,3-dioxolane with 1M LiTFSI and 0.25M LiNO₃electrolyte. To aid in comparison, polysulfide standards were synthesized and investigated using electrospray ionization-mass spectroscopy (ESI-MS). Raman and ESI-MS were utilized to map polysulfide speciation and relative abundances along the discharge curve. The electrolyte within the cell was probed as a function of position within the cell to determine the spatial distribution of differing polysulfide species. Challenges associated with performing Raman spectroscopy within this environment, to differentiate the varying polysulfide species in solution, were also investigated.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.