Evolving contact mechanics and microstructure formation dynamics of the lithium metal-Li7La3Zr2O12 interface

The dynamic behavior of the interface between the lithium metal electrode and a solid-state electrolyte plays a critical role in all-solid-state battery performance. The evolution of this interface throughout cycling involves multiscale mechanical and chemical heterogeneity at the micro- and nano-scale. These features are dependent on operating conditions such as current density and stack pressure. Here we report the coupling of operando acoustic transmission measurements with nuclear magnetic resonance spectroscopy and magnetic resonance imaging to correlate changes in interfacial mechanics (such as contact loss and crack formation) with the growth of lithium microstructures during cell cycling. Together, the techniques reveal the chemo-mechanical behavior that governs lithium metal and Li7La3Zr2O12 interfacial dynamics at various stack pressure regimes and with voltage polarization.

between 1000 to 2000 m/s). Our recent work by Chang et al. is a useful resource for understanding how the acoustic waveform signals are processed in order to obtain quantitative information such as material stiffness and sound speed. 5 In the context of this work, the total amplitude as calculated and plotted in each of the main figures is the integral of the waveform, therefore capturing any change in the waveform intensity. The time-of-flight (ToF) is the time it takes for the sound wave to propagate through the medium, and is related to the thickness and sound speed by the following equation: where L is thickness, E is modulus, and is density For a cell stack which contains materials each with different bulk moduli and density, an averaged approach is generally taken (e.g., in geophysics, a Backus average takes the harmonic average of each material to estimate the overall property of the heterogeneously layered structure). 5,6 In fluids, the modulus is the bulk modulus, which is the volume change of the fluid under an applied hydrostatic pressure. In solids, waves propagate both longitudinally (primary, compressional wave) and transversely (secondary, shear wave), and therefore, the modulus is a function of both the bulk modulus and shear modulus. Generally, shear waves arrive much later than longitudinal waves, and the primary wave that is measured is the longitudinal wave. When the wave strikes an interface, it will typically be reflected and transmitted simultaneously. The degree of reflection vs transmission is related to the acoustic impedance of the material, and this effect along every interface ultimately results in the measured transmitted wave amplitude.
For operando acoustic studies, waveforms are continuously pulsed through the medium, with each waveform recorded on an oscilloscope. Individual waveforms can be superimposed on a heatmap to show the evolution over time. Likewise, total amplitudes of each waveform can be plotted to determine the change in total amplitude as a function of cell cycling (such as the current study).
Supplementary Figure 1. Plot of transmitted wave amplitude vs stack pressure, from the finitedifference solution of the 1D acoustic wave equation with a dispersion term to account for interfacial contact loss (adapted from reference [ 7 ] for solid-solid interfaces). This model shows that a decrease in stack pressure at any wave frequency results in a loss of transmitted amplitude due to formation of a rough interface (which impacts the dispersion term).
In the current study, there are no bulk phase changes in the electrodes or the electrolyte (as opposed to graphite intercalation or lithium metal oxide cathode phase changes in lithium-ion batteries). Therefore, the primary contributor to acoustic amplitude attenuation is interfacial roughening due void formation at the stripping interface. As shown in the SEM images (e.g. Supplementary Figure  3), the plated interface, while increasingly non-uniform, is fully intact after the polarization tests. The voids on the stripping electrode side will cause the wave to attenuate due to a lower transmission efficiency of a solid/gas interface, and therefore the degree of interfacial contact loss can be correlated with amplitude attenuation. While a full model to quantify this effect is out of scope of the current study, we show a relation between stack pressure and amplitude attenuation modeled using a finite difference scheme to solve the 1D acoustic wave equation with a dispersion term (see Supplementary Figure 1). This serves to demonstrate the potential quantitative nature of acoustic characterization for detecting the degree of contact loss at buried interfaces. The model shows consistency with the data in the study, which utilizes a 2.5 MHz central frequency. An increase in stack pressure improves interfacial contact and increases the predicted transmitted amplitude. For example, a 2 MHz wave at a stack pressure of 2 MPa will transmit amplitude at roughly 80% of that at 5 MHz.

Explanation of bulk magnetic susceptibility (BMS) effects.
Lithium metal exhibits a unique chemical shift depending on its orientation in the external magnetic field. This phenomenon arises due to bulk magnetic susceptibility (BMS) effects from Pauli paramagnetism in lithium metal. For example, a strip of lithium metal oriented perpendicular to the external magnetic field gives a 7 Li shift of 242 ppm. In contrast, if the lithium strip is oriented parallel to the field, the shift for lithium metal is at 272 ppm. This orientation-dependent 7 Li shift for lithium metal provides a sensitive readout of sample orientation during the experiment, as well as a convenient way to probe lithium microstructural growth, which is oriented perpendicular to the lithium metal electrode.
1.3 Discussion of chemical analysis at the lithium metal -LLZO interface. 7 Li NMR spectra at the lithium -LLZO interface in CSI experiments provide information regarding the chemical species at that interface. Shifts to lower frequency as compared to LLZO (which appears at 2. Supplementary Figure 11. Cross-sectional SEM images, showing the plated lithium foil on top, LLZO in the middle, and stripped lithium foil on the bottom. The plated interface is intact, whereas the stripped interface shows delamination, due to loss of contact from void formation. This was a lithium -LLZO -lithium cell galvanostatically polarized at 0.5 mA/cm 2 to 5 V inside the NMR probe.