X-ray Diffraction Tomography for Investigating Li-Ion Batteries Under Operating Conditions

Li-ion batteries (LiBs) have become the preferred energy storage device choice for a variety of applications ranging from portable devices to (hybrid) electric vehicles, due to their relatively large energy density and life-cycle when compared to other competitive technologies. LiBs are though complex multi-component material systems and their performance directly depends on numerous parameters such as electro-chemical stability of the cell components and uniform current and temperature distributions for the desired operating conditions (voltage/current density), all of which are interlinked. A non-uniform current/charge distribution can have a negative impact on the performance of LIBs as it can lead to decreased capacity and power output, local temperature gradients and even to overcharge/overdischarge. Understanding how the local chemistry of the various system components is related to spatial gradients in the state of charge (SoC) and indeed capacity fade, is essential in order to optimise cell performance and design LiB materials with enhanced properties. However, investigating the evolving chemistry and the Li distribution in real-time is crucial to avoid relaxation problems coming from <i>ex situ </i>cell/material characterisation measurements.<br/>Over the past decade, several studies focusing on <i>in situ</i>/ <i>operando</i> investigation of commercial LiBs, but most have been performed with conventional X-ray absorption-contrast computed tomography which provides limited/no chemical information in the reconstructed images [1]. In contrast, combining scattering or spectroscopic techniques with tomography enables the extraction of local chemical and physical information within the interiors of intact materials and devices.<br/>In this work, X-ray diffraction computed tomography was employed to study the state-of-charge induced chemical heterogeneities in commercial cylindrical LiBs in a spatially-resolved manner. More precisely, the (de)lithation processes under operating conditions and the effect of cycling rate on the heterogeneities within both electrodes were investigated. In both these studies [2,3], we were able to identify regions of uneven Li distribution which is bound to have an effect on the current and temperature distribution, potentially leading to capacity loss and/or cell failure. Finally, the obtained results demonstrated the importance of this chemical imaging technique as an invaluable diagnostic tool for real-word commercial batteries. This technique can applied for a variety of different experiments related to battery degradation mechanisms, such as <i>in situ/operando</i> abusing testing to capture internal temperature gradients through the crystallographic behaviour of both electrodes.<br/>[1] Pietsch, Wood, Annual Review of Materials Research, 47, 451, 2017.<br/>[2] Vamvakeros et al., Small Methods, 5, 2100512, 2021.<br/>[3] Matras et al., Journal of Power Sources 539, 231589, 2022.