All solid-state batteries (ASSBs) have the potential to deliver higher energy densities, wider operating temperature range and improved safety compared with today’s liquid electrolyte-based batteries. However, they still face challenges that limit their commercialization resulting from thick solid-state electrolytes (SSEs), and poor processability due to their unstable chemistries. Existing studies on solvent-binder based casting methods to reduce SSE thickness and improve processability still lack crucial fundamental understanding of the behavior and selection criteria for different SSE-binder-solvent systems. In this work, we attempt to isolate various SSE-binder-solvent system behaviors and study their chemical and electrochemical compatibilities using both bulk and surface sensitive characterization tools. With these findings, we introduce a novel binder-solvent combination that significantly reduces SSE thickness (~50 µm) and improves processability of ASSBs while maintaining high ionic conductivity (0.7 mS cm^-1). The resulting organic/inorganic composite SSE was found to be stable against lithium metal by plating and stripping over 2000 hours at 0.11 mA cm^-2 and 1.1 mAh cm^-2. This study suggests the importance of understanding fundamental SSE-binder-solvent interactions and provides a potential design strategy for scalable production of ASSBs.