Microstructural Evolution Revealed By Operando X-Ray Tomographic Microscopy in Next Generation Batteries

Sulfur conversion chemistry boasts a high theoretical specific capacity of 1672 mAh/gs and promises to deliver a high-energy density battery when combined with a lithium metal anode (3862 mAh/gLi). The use of such chemistries has the additional benefits of utilizing abundant and low cost materials in the cathode, i.e. carbon and sulfur.[1] Despite clear advantages, lithium-sulfur (Li-S) batteries still face a low practical energy density, poor rate performance, and limited cycling life. These issues are a result of the complex conversion chemistry of the cathode, and the stripping/plating mechanism of the Li-metal anode. These processes include the dissolution and diffusion of polysulfide species through the cell’s electrolyte from the cathode resulting in capacity fade, in addition to the growth of dendritic structures from the anode causing the formation of ‘dead lithium’ and short circuiting of the battery. A plethora of strategies have been developed to address the issues posed by the soluble polysulfide species, from using additives to prevent polysulfide shuttling, to encapsulation of sulfur to prevent its dissolution. While to address the growth of Li-dendrites, electrolyte formulations and the use of interlayer materials have been investigated. When developing these strategies, a detailed understanding of the conversion chemistry and Li-metal plating/stripping mechanisms during cycling is required. This can be obtained by e.g. the use of Raman spectroscopy to probe chemical changes to electrolytes and observe polysulfide speciation,[2] or using phase-field modelling to investigate factors that influence the growth mechanisms of Li-metal during plating.[3] In addition, sulfur dissolution and Li-metal dendrite growth induce microstructural changes that need to be probed using techniques with an appropriate resolution and field-of-view, e.g. synchrotron x-ray tomographic microscopy (XTM), which is capable of micrometre resolution, a near millimetre wide field of view, and measurement times of less than 60 seconds. This allows synchrotron XTM to continuously probe microstructural changes during battery operation, giving quantitative insights into the complex electrochemical mechanisms of the sulfur cathode and Li-metal anode. In this contribution we address the conversion processes of sulfur and its relation in limiting the battery’s practical specific capacity by using a capillary cell type battery, placing the entire cathode within the field of view of the XTM measurement. This enables quantification of the sulfur phase and correlative analysis with simultaneously acquired electrochemical data.[4] We demonstrate the full dissolution of elemental sulfur, with further conversion of sulfur species occurring immediately, and that an efficient diffusion of dissolved polysulfide species through and from the cathode is crucial to achieve a high specific capacity of Li-S cells in practice. Furthermore, we find that in the final step of cell discharge, a uniform and porous Li2S layer is formed on the cathode surface, effectively limiting access to the carbon surface and preventing further polysulfide conversion. We also demonstrate XTM as a unique technique to follow the growth of Li-metal microstructures in real time showing the difference in growth mechanisms when using different electrolyte compositions and under different cycling conditions. We observed change in the growth mechanisms of Li-metal. from homogenous mossy growth of Li-metal, to island dendritic growth and the formation of dead Li, to the growth of a globular phase, showing a changes in the fundamental process of Li-metal growth. [1] M. Agostini, J.Y. Hwang, H.M. Kim, P. Bruni, S. Brutti, F. Croce, A. Matic, Y.K. Sun, Adv. Energy Mater. 8 (2018) 1–7. [2] M. Sadd, M. Agostini, S. Xiong, A. Matic, ChemPhysChem 23 (2022). [3] Y. Liu, X. Xu, M. Sadd, O.O. Kapitanova, V.A. Krivchenko, J. Ban, J. Wang, X. Jiao, Z. Song, J. Song, S. Xiong, A. Matic, Adv. Sci. 2003301 (2021) 1–11. [4] M. Sadd, S. De Angelis, S. Colding‐Jørgensen, D. Blanchard, R.E. Johnsen, S. Sanna, E. Borisova, A. Matic, J.R. Bowen, Adv. Energy Mater. 2103126 (2022) 2103126.