(130f) Recent Developments in Operando Ultrasonic Characterization to Investigate Lithium Metal Cell Dynamics

We are in the midst of rapid global manufacturing scale-up of lithium-ion batteries for electric vehicles. This has led to a significant focus placed on developing higher energy density rechargeable batteries, such as the use of metallic lithium anodes. The bulk of academic battery research has focused on electrolyte design and fundamental materials chemistry underlying lithium metal anodes. As improved electrolytes are discovered and lithium metal is sufficiently protected, it becomes increasingly important to characterize cell-level behavior. This includes understanding of stack pressure and operating temperature effects, characterizing spatial heterogeneities of larger format lithium metal cells, and other systems-level phenomena. Here, I will present recent developments in an operando ultrasonic technique for quantifying lithium metal mechanics and imaging cell degradation, such as gas formation and lithium microstructural development.

First, I will discuss the advantages of ultrasound as a non-destructive and operando means of analysis, which is beneficial for closed-form electrochemical cells. I will describe a quantitative stiffness metric which can be estimated from acoustic wave signal processing. Using this technique, I present several examples of probing battery degradation, including the detection of gas formation, lithium metal deposition, and interfacial contact loss. Specifically, I will describe recent work in collaboration with General Motors and Mercedes-Benz Research and Development North America, where I apply operando ultrasonic analysis to develop a mechanistic understanding of lithium metal cells from initial cell formation to long-term degradation. In this study, high concentration and fluorinated ether electrolytes are shown to enhance both chemical stability of the interphases as well as mechanical stability of the electrodes. Lithium metal cell chemistry and morphology are shown to be a direct function of both stack pressure and temperature.