LiNbO3-coated LiNi0.5Mn1.5O4 powders were synthesized by a sol-gel method, and their intercalation property as a cathode material was investigated using all-solid-state batteries with Li10GeP2S12 solid electrolyte and In-Li metal anode. The LiNbO3 coating delivered reversible lithium intercalation of LiNi0.5Mn1.5O4 through an electrochemical interface with the Li10GeP2S12. Oxygen-deficient LiNi0.5Mn1.5O4-delta with a higher electronic conductivity than LiNi0.5Mn1.5O4 improved the intercalation activity. An all-solid-state battery consisting of 3 wt%-LiNbO3-coated LiNi0.5Mn1.5O4-delta/Li10GeP2S12/In-Li exhibited a discharge capacity of 80 mAh g(-1) at the first cycle with an average discharge voltage of 4.1 V (vs. In-Li), which demonstrates the possibility of 5 V class all-solid-state batteries with a high voltage spinel cathode. (C) 2016 Published by Elsevier B.V.