Li-ion batteries are essential for clean transportation and sustainable development but face energy density limitations (~250 Wh/kg). To make electric vehicles a viable alternative to fossil fuel cars, batteries must reach ~400 Wh/kg. Solid-state Li-ion batteries, using high-voltage cathodes, Li metal anodes, and solid electrolytes with high ionic conductivity, can achieve up to ~500 Wh/kg. However, stable interfaces between the solid electrolyte and electrodes remain a major challenge. In particular, the cathode-to-electrolyte interface degrades over time due to mechanical and chemical instability. Volume changes in cathode particles during charge/discharge lead to contact loss and performance decline. To address this, five European research groups and the industrial partner are developing interface engineering solutions. Cathodes with low chemical expansion will be blended into composite structures within the solid electrolyte. Mechanofusion, ceramic processing, and physical vapor deposition will be used to optimize interface stability. Advanced microscopy and spectroscopy techniques will analyze structural and mechanical changes, while electrochemical testing and finite element simulations will refine the designs. This project aims to push Europe to the forefront of next-generation battery development in line with decarbonization goals.