Investigation of Li Deposition Behavior in Graphite Anodes of Lithium-Ion Batteries Using Operando Computed Tomography

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Lithium-ion batteries are now widely applied in daily life due to their advantages such as excellent cycle characteristics and high energy density. In recent years, demands on lithium-ion batteries have also increased for the development of electric vehicles. Rapid charging of lithium-ion batteries is an essential element for the widespread use of electric vehicles and other vehicles. However, during rapid charging, polarization within the electrodes causes a reaction distribution, which may result in lithium filling only part of the graphite and further deposition of lithium metal. The deposition of lithium metal has a negative impact on battery capacity and safety. Understanding the reaction distribution and lithium metal deposition during charging is therefore very important to enable rapid charging. In this study, the three-dimensional structure within the graphite anode during charging is measured using operando X-ray computed tomography (CT) and the reaction distribution and lithium deposition within the electrode are discussed from the results. Laminated cells with graphite composite electrode as working electrode and lithium metal as counter electrode and 1 mol/L LiPF6/EC:MEC = 3:7 electrolyte were prepared as measurement samples. operando CT measurements were performed at SPring-8 BL20XU using an X-ray energy of 30 keV. Transmission images were acquired using a transmission X-ray microscope with a zone plate and the Zernike phase contrast method with a phase ring. For CT measurements, the sample was rotated 0-180°, during which 1800 transmission images were acquired. The transmission images were acquired using a transmission X-ray microscope with a zone plate and the Zernike phase contrast method with a phase ring. The combination of these two techniques allows structures in the battery to be measured with high sensitivity and spatial resolution. The batteries were charged (lithium insertion into the graphite) at room temperature and CT measurements were taken every 10% depth of charge (State of charge, SOC). Figure 1 shows cross-sectional images of graphite composite electrode at different SOCs measured by operando CT. The Zernike phase-contrast CT enabled the measurement of CT images that clearly distinguish between graphite, electrolyte, and lithium metal. Compared to the electrolyte, lithium appears darker in the field of view due to lower X-ray absorption. The process of lithium dendrite deposition in the battery during the late charging phase could be captured. Lithium metal deposition was observed on the separator side of the graphite composite anode at SOC 90%, although no lithium deposition was observed in the graphite composite anode up to SOC 80%. When the SOC reached 100%, even more significant lithium deposition was observed on the separator side. The results showed that the Li dendrites generated were growing with charging. In the presentation of the day, a more detailed analysis of the reaction distribution within the graphite composite observed by operando X-ray CT and the deposition of lithium during overcharging will be presented. Figure 1