All-solid Li batteries offer improved safety and power density yet suffer from drastic capacity fade, a result of electrolyte decomposition on electrode surfaces. We will introduce/optimize an interfacial protective layer that provides Li+ transport pathways and prevents direct exposure of solid electrolyte to extreme potentials. Our unique, in-situ approach allows real-time reaction monitoring during operation. Combined with the battery performance tests, the insights gained will facilitate rational protective layer design. Interlayer materials of interest include: 1) Li+ solvates exhibiting excellent stability at operating temperature, (10 – 50 C) and 2) LiAlS thin films prepared via atomic layer deposition.