High-Resolution Kinetic Studies of Battery Degradation Using Laboratory X-Ray Microscopy

Advances in the design of new materials for efficient and durable energy storage and conversion systems are critical to current and future energy technologies. Performance and lifetime of batteries strongly depend on the 3D morphology of hierarchically structured materials and on morphology changes during operation. Morphological changes, i.e., particle fracture and surface deterioration, are among the most prominent sources of electrode degradation and eventual irreversible capacity loss. Advanced characterization techniques are needed to gain a deep understanding of the degradation mechanisms that affect performance and limit the lifetime of energy storage and conversion systems.<br/><br/>The potential of X-ray microscopy (TXM) and nano-XCT studies as a non-destructive method for characterizing kinetic processes in 3D-structured systems has been demonstrated mainly at synchrotron radiation beamlines. In this talk, we are presenting a customized solution for a laboratory nano-XCT tool with an integrated operando cell, and we are demonstrating its applicability for the study of kinetic processes in electrode materials, that cause degradation of batteries at the nanoscale, using a photon energy of 8 keV (Cu-Ka radiation). For laboratory nano-XCT tools, the selection of the material for the windows is particularly important because of the lower brilliance of the X-ray sources compared to synchrotron radiation sources. Ideally, the windows for an electrochemical<i> operando</i> cell must be transparent for photons in the selected energy range, it has to be rigid, nonpermeable to gases and moisture and electrically conductive. Glassy carbon, widely used in a variety of electrochemical and electrocatalytic applications, fulfils these requirements. It acts simultaneously as a current collector and a window. Glassy carbon is suitable for applications with most electrode compositions for Li- and Na-ion batteries. However, the thin glassy carbon windows are extremely fragile and require proper structural support from the cell enclosure.<br/><br/>A special custom-built electrochemical cell with glassy carbon windows/current collectors was designed and integrated into a TXM tool. The design of the electrochemical cell allows X-rays to pass through the sample at a wide range of angles, which allows limited-angle X-ray tomography to image the 3D morphology of the porous material and the evolution of defects like microcracks during battery cycling.<br/><br/>We demonstrate the capability of laboratory nano-XCT for the non-destructive imaging of microcracks in particles of Na_0.9Fe_0.45Ti_1.55O₄ (NFTO) cathode material in Li-ion batteries [1]. Many particles of the as-prepared cathode material show already initial microcracks. These microcracks start to grow at the first charge/discharge cycle. Further charging/discharging results in division of large particles into smaller ones. Chemo-mechanical stress caused by the supposed electrochemical substitution of Na by Li ions might accelerate the processes of microcrack formation and propagation. A notable observation is that the probability for formation and growth of microcracks is higher for large particles with low density than for the more dense and finer particles. A possible reason for the higher fracture susceptibility of large particles (especially at higher current densities) might be the state-of-charge (SOC) heterogeneity. The impact of the morphology on the degradation of battery materials, particularly the size- and density-dependence of the fracture behavior of the particles, is revealed based on a semi-quantitative analysis of the formation and propagation of microcracks in particles.<br/><br/>This study opens new perspectives for the nondestructive characterization of novel electrode materials for batteries. In-situ and operando X-ray microscopy allows kinetic studies of morphology changes and defect evolution during battery cycling with about 100nm resolution.<br/><br/>[1] V. Shapovalov, K. Kutukova, et al., Crystals (2022) 12, 3.