Characterization and Chemical Mapping of High Capacity Intermetallic Li-Ion Anodes Using X-Ray Microscopy Techniques

Abstract

A combined electrochemical and x-ray imaging study has been performed for a high capacity Cu6Sn5 alloy anode material for Li-ion batteries. The electrodes investigated include Cu6Sn5 pellets and composite electrodes containing alloy, carbon additive, and a polymer binder. X-ray microtomography (µCT), segmentation, and analysis was performed on anodes subject to lithiation and cycling. The observation of affected and unaffected samples from lithiated, cycled, and pristine electrodes were compared. Significant structural changes were observed in pellet electrodes subjected to full lithiation (0 V vs. Li/Li+). Cycled composite electrodes show microstructural changes that correspond with capacity fade. These changes include decreases in both particle size, indicating pulverization of the active material, and reduction in pore/carbon/binder regions, suggesting impacts on effective transport properties. The µCT results show distinct absorption patterns indicating varied bulk chemical composition, but detailed chemical composition is difficult to discern. To address this drawback x-ray absorption near edge structure (XANES) imaging was performed, obtaining absorption spectra for Cu, Cu6Sn5, and Li2CuSn. Using these spectra, an analysis was performed on 2D images from samples of cycled electrodes to map the chemical composition in the phases that contained Cu. The capabilities to distinguish the different materials on mixed samples shows that microstructure and composition changes resulting from lithiation and delithiation in Cu6Sn5, can be observed with x-ray imaging methods. In operando XANES imaging of Cu6Sn5 lithiation in composite electrodes has been performed. The in operando testing methodology will be shown and explained; results show particle motion and disintegration during cycling, the analyzed particles will be characterized using absorption analysis, in order to accurately discern the evolution of the composite during cycling. These features appear mostly on delithiation and show that, in addition to microstructural change at the particle scale, the networks supporting mass and charge transport may evolve during cycling. This multiscale microstructural evolution is expected to have significant impact on cycling performance and capacity fade.