3D and 4D Characterisation of Lithium-Ion Battery Electrode Microstructures using X-ray Tomography

There is a direct link between electrode microstructure and their performance in lithium-ion batteries (LIBs); however, this relationship remains poorly understood. By utilising tomographic X-ray imaging techniques, it is possible to characterise LIB electrode microstructure in three dimensions. Moreover, extending these imaging techniques to explore changes in these materials gives rise to the notion of “four-dimensional” (4D) tomography to study microstructural evolution with time. This work focused on characterising, both qualitatively and quantitatively, the three-dimensional (3D) microstructure of LIB electrode materials at multiple length and time scales with the aid of laboratory and synchrotron X-ray sources. The suitability and reliability of direct 3D microstructural analysis for quantifying LIB electrodes was demonstrated by comparing it with stereological methods, which are shown to introduce bias when applied to inhomogeneous 3D microstructures. Silicon (Si) and metallic lithium (Li) are highly energy-dense electrode materials and promising candidates for use in LIBs; however, they experience significant microstructural degradation upon electrochemical cycling. Using a custom-built, X-ray transparent in-situ electrochemical cell, 4D characterisation of the microstructural evolution and degradation within the aforementioned electrode materials was performed both in-situ and in-operando. Phase transformation, fracture formation and propagation within individual Si particles was visualized and tracked in 3D during the course of a half-cell discharge. At a higher X-ray imaging resolution, microstructural evolution in Si microparticles as a result of repeated cycling was captured and quantified in 3D. Visualisation of formation and growth of pits and mossy lithium deposits along metallic Li electrode surfaces was also presented. Finally, an X-ray contrast-enhancement approach for imaging lowly attenuating electrode materials such as graphite was also demonstrated. This work has demonstrated X-ray tomography as a diagnostic tool for providing valuable insight into electrode microstructure which can aid the efficient design of these electrode materials in future generation LIB systems