Species in Lithium-Sulfur Batteries Using Spatially Resolved Operando X-ray Absorption Spectroscopy and X-ray Fluorescence Mapping

The lithium-sulfur (Li-S) battery chemistry has attracted great interest in the last decade because of its outstanding theoretical gravimetric energy density compared to the state-of-the-art lithium-ion battery technology. However, practically achieved energy density is still far below the theoretical value, even in small laboratory-scale batteries. The problems seen in laboratory-scale batteries will inevitably increase during scale-up to large application-format cells, as the electrolyte to active material (AM) ratio will need to be reduced in these cells to achieve high gravimetric energy density on cell-level basis. Our study shows the unique possibility of X-ray fluorescence (XRF) mapping to visualize the spatial distribution of the AM inside operating Li-S batteries in all cell components [working electrode (WE), separator, and counter electrode (CE)]. Through a combination of operando XRF mapping and X-ray absorption spectroscopy, we show that unless self-discharge is efficiently prevented, the AM can completely dissolve and distribute throughout the cell stack within a time frame of 2 h, causing poor capacity retention. Using a polysulfide diffusion barrier between the WE and the CE, we successfully suppress these processes and thereby establish a tool for examining the sealed cathode electrode compartment, enabling sophisticated studies for future optimization of the WE processes.