Lithium-sulfur (Li-S) cells have the potential to provide high gravimetric density energy storage, but currently suffer from rapid capacity degradation due to redistribution of the electrochemically active material during cycling. Research to address this challenge typically aims to synthesise cathodes with homogeneously dispersed sulfur particles, seeking uniform current density and discharge product deposition to enable reversible utilisation of the electrochemically active interface. However, electrochemical1 and tomographic2 characterisation suggest that a diffusion-limited mechanism leads to agglomeration of sulfur during charging. In this work, we develop a metric to quantitatively compare the degree of homogeneity of sulfur distribution and relate this to electrochemical performance.
Laboratory and synchrotron X-ray computed tomography (XCT) were used to characterise the particle size distribution and spatial distribution of sulfur in Li-S in pristine, post-mortem, and in-situ samples. The homogeneity of sulfur is assessed as a ratio of the separation between similarly sized particles to the quantity of interstitial particles. Tracking the ‘trajectory’ of the metric for individual particles throughout a time series of in-situ tomograms quantitatively captures the preferential utilisation of smaller sulfur particles during discharge3. Additionally, the changes in the population-level homogeneity metric in uncycled and fully charged electrodes show the formation of sulfur during charging at localised sites which are readily available for oxidation of soluble polysulfide species, consistent with the diffusion-limited process. The methodology is extended using the local porosity to infer the distribution of low-density carbon in the tomography data, and therefore compare the electrical connectivity of sulfur/ carbon electrodes produced via different synthesis pathways4. By using this metric to quantify changes in the sulfur homogeneity as a function of state of charge beyond initial redistribution in the first cycle, we aim to apply this to determine the influence of experimental synthesis methods on electrochemical performance and identify parameters to prioritise in future optimisation.

1 T. Zhang et al., J. Electrochem. Soc., 165(1), A6001-A6004 (2018)
2 C. Tan et al., ACS Appl. Energy Mater., 1(9), 5090-5100 (2018)
3 D. Di Lecce et al., J. Power Sources, 472, 228424 (2020)
4 S. Yari et al., Adv. Energy Mater., 2402163 (2024).