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-capacity anode with a non-flammable, solid-state electrolyte (Li 6.5 La3 Zr 1.5 Ta 0.5 O12 , LLZO). The bottleneck in the development of solid-state batteries remains the anode-electrolyte interface (Li|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 Li-metal stripping at initial current densities between 0.007 and 3.8 mA/cm 2 and loads up to ∼7 MPa. The transparency of the single-crystal LLZO and the reflectivity of the Li-metal under visible-light 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 compared to higher rates. Very slow stripping rates lead to angular and faceted voids. Stripping at high current densities generates a greater density of voids, and thus, an accelerated loss of the electrochemically active area. ... mehrApplying a compressive load delays void growth, but can only mitigate loss of contact at the lowest rate. This study presents experimental evidence demonstrating the impact of current density and load on Li electrochemical stripping and provides information for interface engineering, numerical simulations, and charge-discharge protocols for solid-state batteries.