Using advanced imaging approaches to characterize degradation modes during fast-charging of lithium-ion batteries
Keywords: Engineering.
Chemical Engineering.

Long charging times of lithium-ion batteries (LIBs) have impeded the widespread deployment of electric vehicles (EVs). Currently available EVs cannot charge at rates like those of refueling at a gas station. Thus, the U.S. Department of Energy and the U.S. Advanced Battery Consortium has identified extreme fast charging (XFC) to reduce LIB charging times to under 10-15 minutes as one of their main goals to meet their national goal of around 50 % plug-in EVs in U.S. by 2030. However, existing LIBs cannot achieve these XFC goals without significant capacity fade over cycling. This is mainly attributed to the irreversible loss of lithium (Li) after plating on the graphite electrode. While numerous methods have detected Li plating, they lack three-dimensional (3D) non-invasive quantification of plated Li on graphite electrodes in full cells during battery cycling. Advanced imaging approaches using neutrons and X-rays are promising characterization tools for non-destructive 3D visualization of battery materials. Here, neutrons are particularly sensitive to detecting Li on graphite due to the large difference in their total neutron cross-sections (Li:C ≈ 72:6 barns). First, I demonstrated the viability of simultaneous neutron- and X-ray based tomography (NeXT) as a non-destructive imaging platform for ex-situ 3D visualization of graphite electrode degradation following extreme fast charging (XFC). In addition, I underscored the benefits of the simultaneous nature of NeXT by combining the neutron and X-ray data from the same sample location for material identification and segmentation of one pristine and two XFC-cycled graphite electrodes (9C charge for 450 cycles). Finally, my ex-situ results and methodology development paved the way for the design of NeXT-friendly LIB geometries for operando and/or in-situ three-dimensional (3D) visualization of electrode degradation during XFC. Second, I outlined the design and characterization of a neutron-friendly LIB (NFB) coin-cell for visualization of plated Li at the separator-electrolyte-electrode interface during XFC. Using database-derived total neutron attenuation cross-sections, neutron path lengths, and the expected neutron transmission from the battery materials of interest, I designed our NFB. My electrochemical characterization results showed that the NFB can undergo XFC at 6C. My neutron radiography results exhibited excellent neutron transmission, enabling visualization of the graphite-separator-electrolyte interface. Third, I investigated the 3D morphological behavior and spatial heterogeneities of plated, dead, and active Li on thick graphite anodes following XFC using the designed NFB. My results revealed changes in plated, dead, and active Li morphologies from isolated deposits at 1C to mossier and denser outgrowths covering the entire circumference of the graphite anode at 6C. Here, dead Li denotes Li that has become electronically disconnected from the rest of the graphite anode, and hence cannot find a pathway back to the cathode during battery discharge. My data also demonstrated different 3D morphological growth mechanisms at 1C vs 6C, indicating associations between the (1) dendritic Li growth and higher XFC-charging rate; (2) root-growing mossy Li and a relatively slower XFC-charging rate. My 3D visualizations exhibited spatial heterogeneities of plated, dead, and active Li, thus revealing that certain areas around the graphite anode cultivated more dead as compared to the active Li. Overall, this thesis encompasses neutron and X-ray μ-CT methodology development, and electrochemical engineering to discover fundamental mechanisms of Li plating behavior during XFC in-situ in full cell LIBs in 3D non-destructively. My contributions to imaging fast-charging LIBs, will help design improved batteries for EVs and other applications such as grid-scale energy storage.