Operando Visualization of Void Formation at Lithium/Solid Electrolyte Interfaces

Solid state batteries (SSBs) are a promising alternative to state-of-the-art Li-ion batteries, owing to their improved safety and high energy density. However, Li-metal SSBs experience Li dendrite and void formation at the Li/solid electrolyte (SE) interface at high current densities and cycle numbers, leading to capacity loss and catastrophic cell failure [1]. There is a need to understand the origin and growth mechanisms of these degradation modes; however voids are difficult to study because they exist at the buried SE/Li interface [2, 3]. To overcome this limitation, in this study, we used a transparent Al-doped Li7La3Zr2O12 (LLZO) SE to facilitate real-time visualization of void formation and growth through operando video microscopy synchronized with electrochemical cycling. Al-doped LLZO SE was fabricated by hot pressing and the transparency of 1mm pellets was examined using both ultraviolet visible spectroscopy and optical microscopy. Features as small as 2 μm in resolution could be optically resolved through these transparent LLZO pellets. In-plane Li metal symmetrical cells were fabricated on LLZO pellets and assembled in a custom-built visualization cell platform for o perando video microscopy tests during electrochemical cycling. Using this platform, void formation was observed at the Li/SE interface during stripping. Voids <10 μm in size were observed, which increased over time in both number and size. Image segmentation was performed on the operando optical images to analyze the coupled electrochemical-morphological effects on void formation and growth. This allowed for quantification of the void size, shape, and position throughout cycling. The void morphology was further confirmed by cross-sectional scanning electron microscopy (SEM). This work further enhances our understanding of how, when, and whyvoid formation occurs and presents a powerful operando method to study phenomena with as small as a 2μm resolution at buried interfaces in SSBs.