In-Operando Imaging of Electrodeposition and Dendrites Using Nanoscale X-Ray Computed Tomography

Abstract

The widespread implementation of lithium-ion batteries in systems ranging from portable electronics to electric vehicles has generated a remarkable demand for batteries with improved durability and performance. In order to satisfy said demand, the issue of dendrite formation must be addressed; dendrite growths inside batteries have been observed to create an internal short circuit that results in catastrophic failure.1 Consequently, dendrite propagation not only poses a significant safety hazard to existing lithium-ion batteries, but also is detrimental to the development of next-generation lithium based energy storage technologies such as lithium-air batteries. A better understanding of dendrite nucleation and growth behavior through direct visualization is crucial to developing effective dendrite mitigation techniques. Prior studies observed these phenomena using electron microscopy2 and micro-scale resolution X-ray computed tomography (micro-CT).3,4 These approaches present distinct challenges, including the vacuum environment of electron microscopes2,5 and insufficient resolution of micro-CT to image nucleation events.3 An interesting alternative is the use of the nano-scale resolution X-ray CT (nano-CT) that allows for three-dimensional observations within opaque materials at high resolution (50 nm) with any electrolyte with controlled environmental conditions. In this study, an in-operando cell for nano-CT imaging was designed and fabricated, in which the working electrode is prepared on a metal wire placed within electrolyte contained by a sealed capillary. This work addresses the particular challenges of preparing in-operando electrochemical cells for lab-scale nano-CT instruments, which include sealing, limited cell size, and the beam path length through the sample. Additional considerations include the development of high temporal resolution 4D (3D plus time) reconstruction algorithms to address the longer exposure times of lab-scale instruments. Preliminary experiments demonstrating these capabilities include the imaging of dendritic copper electrodeposition using copper wires and aqueous copper sulfate electrolyte. Figure 1 shows the images of the in-operando copper dendrite growth acquired using the nano-CT with a 65 mm field of view and an exposure time of 10 s. The results in Figure 1f show that following the initial cell transient, the cell current increases with the corresponding increased surface area of the dendrites.