Visualization of Dissolution-Precipitation Processes in Lithium-Sulfur Batteries

In this work, light is shed on the dissolution and precipitation processes S8 and Li2S, and their role in the utilization of active material in Li-S batteries. Combining operando X-ray Tomographic Microscopy and optical image analysis, in real-time; sulfur conversion/dissolution in the cathode, the diffusion of polysulfides in the bulk electrolyte, and the redeposition of the product of the electrochemical reaction, Li2S, on the cathode are followed. Using a custom-designed capillary cell, positioning the entire cathode volume within the field of view, the conversion of elemental sulfur to soluble polysulfides during discharge is quantitatively followed. The results show the full utilization of elemental sulfur in the cathode in the initial stage of discharge, with all solid sulfur converted to soluble polysulfide species. Optical image analysis shows a rapid diffusion of polysulfides as they migrate from the cathode to the bulk electrolyte at the start of discharge and back to the cathode in the later stages of discharge, with the formation and precipitation of Li2S. The results point to the redeposition of Li2S on all available surfaces in the cathode forming a continuous insulating layer, leaving polysulfide species remaining in the electrolyte, and this is the process limiting the cell's specific capacity.

Automated summary

This study sheds light on the dissolution and precipitation processes of S8 and Li2S, and their role in the utilization of active material in Li-S batteries. By combining operando X-ray Tomographic Microscopy and optical image analysis, the researchers were able to track the sulfur conversion/dissolution in the cathode, the diffusion of polysulfides in the electrolyte, and the redeposition of Li2S on the cathode in real-time. The results provide insights into the important material conversion processes in Li-S batteries and can contribute to the optimization of battery performance.