We report on an all-solid-state battery that employs a closo-type complex hydride solid electrolyte and a LiCoO2 cathode. Interfacial modification between the solid electrolyte and cathode with a LiNbO3 buffer layer enables reversible charge-discharge cycling with a cell voltage of 3.9 V (vs. Li+/Li) at room temperature. Electrochemical analyses clarify that the given modification effectively suppresses side reactions at the cathode/solid electrolyte interface. The interfacial resistance is lowered by ca. 10 times with a 5 nm thick LiNbO3 buffer layer compared to that without a buffer layer, so that a discharge capacity of 109 mAh g−1 is achieved. These results suggest that interfacial modification can be a viable approach to the development of high-voltage all-solid-state batteries using closo-type complex hydride solid electrolytes and oxide cathodes.