Advancements in Optical Microscopy Techniques for Investigating and Quantifying Lithium-Sulfur Battery Degradation
Authors: Owen Rhodri E , Robinson James 
Published: December 2024

Lithium-sulfur (Li-S) batteries have emerged as promising candidates for next-generation energy storage systems due to their high theoretical energy density and the low cost of their component materials. However, their practical implementation faces significant challenges related to their complex electrochemical processes, such as polysulfide dissolution, shuttle effect, and electrode degradation. Understanding these phenomena at the microscale is critical for the advancement of Li-S battery technology. Optical microscopy techniques offer unique advantages in this regard, providing real-time, non-destructive imaging with high spatial resolution.

One of the primary challenges in Li-S battery research is tracking the dynamic behaviour of sulfur species during cycling and the tendency for the lithium anode to degrade during the stripping and plating processes. Optical microscopy techniques, enable direct visualisation of the changes occurring in the cathode, anode and electrolyte. By combining these techniques with advanced image analysis techniques, we are able to monitor morphological changes in the lithium anode, investigate the distribution of polysulfide intermediates in the electrolyte and monitor the sulfur cathode. Quantifying these insights provides invaluable insights for the design of sulfur-based electrodes and electrolytes with enhanced stability and performance.

The nature of optical microscopy facilitates high speed _in situ_ and _operando_ studies of Li-S batteries under realistic electrochemical conditions, offering spatially resolved information on polysulfide diffusion kinetics and highlights the importance of electrolyte composition and separator design in mitigating shuttle effects. Optical microscopy plays a crucial role in characterising the morphology and microstructure of electrode materials in Li-S batteries, which is particularly important for the study the degradation of the lithium anode where the formation of dendritic and ‘dead’ lithium contributes significantly towards Li-S battery degradation.

Using advanced photogrammetry techniques in combination with traditional optical microscopy technique allows us to begin to study and quantify the three-dimensional nature of these dendritic structures. Currently expensive and time-consuming techniques such as X-ray computed tomography are the main tools used for understanding these properties, however, beyond these time and cost limitations they are relatively poorly suited to the study of low-density materials such as the lithium dendrites present in lithium-sulfur systems. Here, for the first time we demonstrate the cost-effective use of micro-photogrammetry for studying such structures.

Our studies highlight and quantify the processes occurring providing a deeper understanding of how lithium-sulfur cells operate and degrade, providing quantification of the lithium fraction, the two and three-dimensional properties of the formed dendrites as well as providing means for tracking the concentrations and nature of the polysulfide species present in the electrolyte.

In summary, optical microscopy techniques represent powerful tools for unravelling the intricate mechanisms underlying Li-S batteries. From real-time imaging of lithium-sulflur cell to morphological characterisation of electrode materials, these techniques offer unprecedented insights into battery performance and degradation mechanisms. By harnessing the capabilities of optical microscopy, we can accelerate the development of high-energy-density, long-lasting Li-S battery systems, paving the way for sustainable energy storage solutions in the future.

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