Thin film batteries with amorphous sulfide-based solid electrolytes were investigated. The interfacial resistance of the battery has been drastically reduced by inserting a very thin LiNbO3 layer between the LiCoO2 cathode and solid electrolyte. The cycle performance of the battery is affected by the composition of the Li2S–P2S5 solid electrolyte. From XPS analysis, it is suggested that the electrochemical stability of the solid electrolyte changes as the composition of the solid electrolyte changes. Thin film batteries have shown both high capacity and good cycle performance with a solid electrolyte of suitable composition and silicon thin layer at the anode interface. Finally, a capacity of 400μAhcm−2 at 500 cycles has been achieved with a 9.1μm-thick cathode.