All-solid-state batteries are a promising technology for future high-energy-density energy storage because they replace flammable liquid electrolytes with solid materials and may enable the use of lithium metal anodes. Lithium metal is attractive because it can store far more charge than conventional graphite anodes, offering a pathway to batteries with greater energy density. However, lithium metal is also highly reactive, and its interface with sulfide solid electrolytes is often unstable. This instability can produce unwanted interphase layers, increase resistance, promote uneven lithium deposition, and ultimately limit battery lifetime. This project addresses the fundamental scientific challenge of stabilizing lithium metal anodes in all-solid-state batteries. The research combines electrochemical testing, materials analysis, and advanced characterizations to understand how lithium metal interacts with solid electrolytes and interlayer during battery operation. A central goal is to identify the chemical, structural, and mechanical factors that control interphase formation and degradation at the lithium metal/solid electrolyte interface. During this period, the project will focus on completing ongoing experiments, analyzing and integrating data, and publishing the principal scientific findings. Interphase characterization, including cryogenic electron microscopy, will be strengthened through SLAC capabilities and expertise. These measurements are especially important because lithium metal interfaces are highly sensitive and can be altered by conventional sample handling or imaging conditions. The expected outcome is a clearer mechanistic understanding of lithium stability in all-solid-state batteries and a set of design principles for improving solid-state battery interfaces. The results will support the broader development of safer, higher-energy, longer-lasting batteries for high energy density sulfide all solid state batteries.