In-Operando Li Metal Morphological Analysis via Acoustic Impedance Spectroscopy

Non-destructive in-operando techniques provide unique insights into electrochemical processes. In this work, we apply a recently introduced operando acoustic technique, acoustic impedance spectroscopy (AIS), to Li metal batteries. AIS measures the acoustic resonances of a battery, which directly probes the mechanical properties of the system. During cycling, these resonances shift in frequency due to fluctuations in the mechanical properties of the Li metal caused by porous plating and stripping. Furthermore, we show that these mechanical changes evolve over extended cycling due to a transition from laterally homogeneous Li growth to laterally inhomogeneous Li growth. Finally, we introduce a quantification of this evolution known as a “symmetry score” and demonstrate the repeatability of this measurement over various current densities and cell chemistries. A decrease in the symmetry score accompanies the degradation of lithium metal morphology and stability. This value, just like the commonly used coulombic efficiency, can be used to characterize the health of a lithium metal battery.