The ever-increasing interest in sustainable mobility is driving the development of innovative batteries with increased energy densities relative to currently commercialized lithium-ion batteries. All-solid-state batteries using 5 V-class positive electrodes are one of those batteries due to their larger volumetric energy density and their superior durability. However, their power density tends to be limited by the large charge transfer resistance at their electrolyte/5 V-electrode interfaces; one explanation for this is the development of significant Li+ deficient layers at the interface. Here we propose a new interlayer material that would effectively resolve the Li+ deficient layers. The partially-crystallized Li56Nb22Ta22 oxide was identified using the molecular beam epitaxy (MBE) based high-throughput physical vapor deposition (HT-PVD) approach. Its higher ionic conductivity of 4.2 mu S cm(-1) and higher permittivity of 165 when measured at 254 kHz, relative to those of conventional LiNbO3 interlayer (1.8 mu S cm(-1) and 95, respectively) will be effective for fast charge transfer reactions at the electrolyte/cathode interfaces in 5 V-class all-solid-state batteries. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.