A lithium stable, lithium-ion conducting solid electrolyte, Li1.4Al0.4Ge0.2Ti1.4(PO4)(3) (LAGTP), was developed by protecting the surface with a polyethylene oxide (PEO) based polymer electrolyte. A surface of tape-cast LAGTP film with epoxy resin was coated with a PEO8Li(FSO3)(2)N-tetraethylene glycol dimethyl ether (G4)-10 wt% BaTiO3 composite polymer electrolyte. The electrical conductivity of the LAGTP film with a 4 wt% polymer electrolyte was 1.06 x 10(-4) S.cm(-1) at 25 degrees C and 5.92 x 10(-4) S.cm(-1) at 60 degrees C. A cell consisting of Li/4 wt% (PEO-G4-10 wt% BaTiO3)-coated LAGTP/4 M Li(CF3SO2)(2)N in dimethoxyethane/Li exhibited a steady cell resistance of 600 Omega . cm(2) at 25 degrees C for 4 months. The stable and low interface resistance confirms that the thin PEO-based layer succeeded in preventing the reduction from Ti4+/Ge4+ to Ti3+/Ge3+ in LAGTP by lithium. No lithium dendrite formation was observed at the interface of the surface-coated LAGTP and lithium at 1.0 mA . cm(-2) and 25 degrees C for 40 h lithium deposition. (C) 2016 The Electrochemical Society. All rights reserved.