The decrease in the resistance at solid electrolyte and electrode interfaces has become a critical issue in the development of solid-state lithium batteries. To understand the ionic transport properties across solid electrolyte and electrode interfaces, thin-film batteries consisting of flat epitaxial films are of considerable importance. Herein, we report the operation of bottom-current-collector-free thin-film batteries using LiNi0.8Co0.2O2 epitaxial thin films. As the bottom current collector is not inserted between the substrates and LiNi0.8Co0.2O2, the surface of LiNi0.8Co0.2O2 is very flat, with a surface roughness average of approximately 0.44 nm. This study proposes a very flat electrode surface suitable for the investigation of interfacial phenomena at solid electrolyte and electrode interfaces.