Lithium-sulfur (Li-S) batteries are a “beyond Li-ion” technology that provides high capacity (1675 mAh g^-1) while also being low cost, earth-abundant, and lightweight. Li-S batteries have a unique chemistry that achieves high capacity via chemical transformation rather than Li intercalation as in Li-ion. Elemental sulfur S₈ is reduced through a series of soluble Li polysulfides (Li₂S_x, 2 ≤ x ≤ 8) to a final solid discharge product Li₂S, and the process is reversed upon charging. However, Li-S suffers from unrealized theoretical capacity and rapid capacity fade due to loss processes that are not well-understood. Batteries are spatially and chemically heterogeneous. Deciphering the chemical states of sulfur and the distribution of dissolved polysulfides in the anode, cathode, and especially the electrolyte during cycling is critical for realizing the potential of Li-S. We present operando characterization using sulfur K-edge X-ray absorption spectromicroscopy performed at the Stanford Synchrotron Radiation Lightsource (SSRL, BL 14-3). Synchrotron radiation provides the high energy resolution and micron-scale beam size needed to distinguish complicated polysulfide spectral features in the tender X-ray regime. From this information, we can observe a spatially and chemically varied system to gain insight into how sulfur speciation and spatial distribution contribute to capacity fade.

Typically, operando X-ray characterization is conducted perpendicular to battery electrodes, providing an average of the chemistry throughout the entire stack. The cross-sectional pouch cell geometry presented here provides a spatially resolved picture of sulfur diffusion between the cathode and anode, which cannot be achieved by typical characterization methods. Cells consisting of a sulfur/carbon cathode, Li anode, and 1 M LiClO₄ + 0.5 LiNO₃ in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME) were discharged, and maps were collected at energies sensitive to the sulfur and Li polysulfide species. Tuning the incident X-ray energy allows for targeted observations of species such as polysulfides, sulfates, elemental sulfur, and lithium sulfide during cycling.

A novel electrochemical cell with well-defined electrode geometry, electrolyte volume, and current collection has been designed at SSRL to map the battery cross-section in fluorescence mode. This cell will mitigate the challenges of using pouch cells for X-ray experiments such as water and oxygen contamination and sample-to-sample variation. It will also provide more robust, longer-term cycling that better replicates the offline electrochemistry of a typical coin or pouch cell. Because the new cell will increase reproducibility, we can alter electrode architecture and electrolyte composition to directly compare polysulfide and sulfur distributions, illuminating the direct influence that these parameters have on battery chemistry. Using this new design, changes in sulfur and polysulfide chemistry are systematically correlated to structure and performance.