Lithium Plating Detection in Extremely Fast-Charged Lithium-Ion Batteries Using Simultaneous Neutron and X-Ray Imaging

Abstract

Lithium-ion batteries (LIBs) have profoundly advanced the development of electric vehicles (EVs). However, one of the remaining bottlenecks in their widespread deployment is the long charging time required for commercial LIBs. Today, there is a global push towards extreme fast charging (XFC) to reduce charging times to 10-15 minutes.1 But existing LIBs cannot achieve this goal without significantly reducing battery performance, mainly due to the loss of active Li. The lost Li either becomes “dead” from being electronically disconnected after plating on the anode or “inactive” because of the irreversible reaction of Li with the electrolyte to form a solid electrolyte interphase. Since lithium plating is identified as one of the major degradation mechanisms from XFC in LIBs,2 understanding its origin and characteristics is essential to developing designs to enable fast-charged batteries. Several studies have characterized Li plating using techniques such as cryo-electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray micro-computed tomography (CT) among others.3 While valuable, these characterization techniques have limitations4 in terms of their sample preparation, spatial resolution, and imaging contrast required to distinguish between materials of similar atomic number (Z). In addition, due to the complex electrochemical reaction mechanisms of Li metal plating, multiple methods for detecting Li metal and understanding the mechanistic details of Li plating are needed.Here, we are using simultaneous neutron and X-ray-based dual-mode micro-computed tomography, a non-destructive imaging modality, to investigate the characteristics of Li plating during XFC in LIBs. Since X-rays and neutrons are sensitive to the electron and nuclear density of the material, respectively, dual-mode X-ray and neutron CT allows the easy separation of the different anode components in a LIB such as Li, graphite, and pores. Higher energy X-rays are needed to get through the metallic components in batteries such as copper and aluminum. However, they do not have sufficient contrast to separate lithium from graphite. Thus, neutrons are used due to their high sensitivity to lighter elements such as lithium, especially 6Li, and carbon.We have performed multi-modal imaging experiments at the Neutron and X-ray Tomography (NeXT)5 system located on the BT-2 beamline at the National Institute of Standards and Technology Center for Neutron Research. We characterized pristine and cycled graphite anode strips containing plated lithium at different regions. For cycled anode strips, we disassembled the battery pouch cells and imaged the graphite anodes cycled under XFC conditions for 450 cycles at fully discharged condition. These strips were used to get sample sizes small enough to achieve the highest possible spatial resolution. The spatial resolution achieved by this technique was ~10-15 μm, thus providing sufficient resolution to pinpoint the location of Li plating within the thickness of the anode. For image processing, we employed bivariate histogram segmentation that uses information from both X-rays and neutrons to segment different anode components. With these dual-imaging datasets, we are now investigating whether a link exists between the spatial heterogeneity of Li metal plating and the local microstructure of the battery anode including porosity, tortuosity, and thickness. We want to investigate why and where Li plating occurs on the battery anode. Addressing these fundamental questions will inform improved battery designs, graphite anode architectures, and charging protocols that will reduce Li plating during XFC in LIBs. Ultimately, these improvements based on our research findings will help make charging an EV similar to filling up at a gas station.