Visualizing 3D Morphologies and Spatial Heterogeneities of Li after Fast-Charging via In-situ Neutron Tomography

Mechanistic understanding of Li plating is imperative to developing fast-charging batteries for sustainable electric vehicles. Here, we report in-situ 3D characterization of the morphological behavior and spatial heterogeneities of Li plating using neutron micro-computed tomography with a spatial resolution of 10-15 microns. Two batteries are imaged in charged and discharged states after: (1) 4 cycles of 1C and (2) 6 cycles of 6C charging. Neutron images show that Li plates at and around the edge of graphite, indicating that the graphite edge and the areas around the edge are the most susceptible to Li plating. However, certain areas of the cells form dead Li whereas others form active Li showing 3D spatial heterogeneities, which are discussed at four regions: (1) near Cu current collector (CC), (2) middle of graphite electrode, (3) graphite-separator interface, and (4) in separator. Specifically, Li near the Cu CC remains active whereas Li near the interface and in the separator becomes dead at both 1C and 6C. Additionally, a distinct 3D Li morphology is revealed at 1C vs. 6C. Particularly, tip-like Li deposits are observed mostly at 6C that become dead following battery discharging, suggesting a correlation between higher XFC-charging rate/cycling number and the increased formation of tip-like Li deposits.