Electrode/electrolyte interface was studied for all solid-state batteries using inorganic solid electrolyte with the crystalline thio-LISICON and glassy Li–Si–P–S–O systems. The formation of the interfacial phase depends on the electrolyte. The thio-LISICON (Li3.25Ge0.25P0.75S4) and the Li–Al negative electrode provided the best electrode/electrolyte interface for fast charge–discharge characteristics, while the SEI phase formed at the Li–Al/Li3PO4–Li2S–SiS2 glass boundary caused high interfacial resistance. The formation of the SEI phase is general behavior at the electrode/electrolyte interface of solid-state batteries, and the fast electrochemical reaction is attained as a result of optimization of the electrode/electrolyte combination.