Evolution of the Electrochemically Active Interface Between a Li-Metal Anode and a Single-Crystal LLZO under Mechanical Loads

Batteries with solid electrolytes promise increased energy density and improved safety by pairing a high-energy density anode and non-flammable, solid-state electrolyte ($\mathrm{Li_{6}La_{3}ZrTaO_{12}}$). The bottleneck in the development of solid-state batteries remains the Li$\mid$LLZO interface, a dominant source of degradation and capacity loss due to the development of voids, and impurity segregation during discharge. This study presents direct, in situ, and quantitative observations of the electrochemically active area during anodic dissolution of Li metal at initial current densities between 0.007--3.8 mA/cm$^{2}$ and loads up to $\sim$7 MPa. The transparency of the single-crystal LLZO and the reflectivity of the Li-metal under optical microscopy enable these experiments. The void morphology and size vary with the initial current density: for the same charge, low rates lead to fewer, but larger voids and vice versa. Stripping at high current densities generates a greater density of voids, and thus, an accelerated loss of the electrochemically active area. Applying a compressive load delays void growth, but can only mitigate loss of contact at the lowest rate. This study provides novel experimental evidence of the impact of stripping current and load on Li dissolution and provides information for interface engineering, numerical simulations, and charge-discharge protocols for solid-state batteries.