ABSTRACTHIGH-RESOLUTION CHARACTERIZATION OF SOLID-LIQUID INTERFACES IN ENERGY STORAGE MATERIALS USING MICROSCOPY: FROM SOLID LITHIUM METAL TO LIQUID SODIUM-POTASSIUM ANODES Hyeongjun Koh Eric A. Stach Eric Detsi

Battery technologies are key to reducing greenhouse gases. They not only enable the usage of renewable energy but also facilitate transition to electric vehicles in transportation sectors, thus lowering the emission of greenhouse gases. The utilization of different battery anodes can offer different advantages and disadvantages. Lithium metal anodes are ideal for the next generation batteries in automobile applications due to their high energy density. However, challenges including formation of dendrites and solid electrolyte interphases (SEI) can impact the long-term stability and thus their reliability. On the other hand, liquid metal batteries, such as liquid Na-K anodes, could potentially be used in large grid storage systems because they have fast kinetics, low cost, and potentially long cycle life. Despite these advantages. The development of liquid metal anodes has not been fully explored, making the practical feasibility of Na-K anodes uncertain.
In this thesis, we aimed to use cryogenic electron and ion beam microscopy to better understand and address these challenges. We first developed a technique to create electron-transparent battery samples for subsequent characterization by cryogenic transmission electron microscopy (cryo-TEM). This method is well-suited for understanding phenomena and challenges occurring to metal batteries as it enables high-resolution characterization with minimum artifacts. Using this approach, we find short-range ordering in the SEI of Li-metal batteries by using cryogenic four-dimensional scanning transmission electron microscopy (4D-STEM).  We also surmise that the structural ordering in SEIs plays a critical role in suppression of Li dendrites, increasing battery performance. Although previous studies have demonstrated that SEIs are a mixture of inorganic precipitates and organic matrix, our data suggests that the true morphology of SEIs is largely amorphous, indicating that those analyses may be strongly impacted by electron beam damage. Therefore, we tackled the first challenge by providing a platform to elucidate the structures of SEIs that impact battery performance. We also address the second challenge by understanding the behavior of liquid Na-K anodes with Na-ion electrolytes. Although it is difficult to characterize liquid metals due to their fluidic properties and high surface tension, cryogenic focused ion beam/scanning electron microscopy (cryo-FIB/SEM) combined with various characterizations clearly revealed dissolution of K, which makes the anodes not viable with Na-ion electrolytes. In our conclusion, we suggest future directions for SEI analysis that can provide additional insights into battery performance and demonstrate a cross-sectioning method for standard coin cells using cryo-FIB/SEM to further characterize liquid metal anodes.