A Li1.4Al0.4Ge0.2Ti1.4 (PO4)(3) (LAGTP) film was prepared by a tape-casting method using a fine LAGTP powder prepared by the sol-gel method. A LAGTP composite film formed with approximately 2 wt% epoxy resin was water-impermeable and had high mechanical strength. The highest lithium ion conductivity of 5.26 x 10(-4) S cm(-1) at 25 degrees C was obtained for the LAGTP-epoxy resin composite film. The LAGTP-epoxy resin composite film was stable in an aqueous solution with a high content of lithium ions. The three-point bending strength of the 75 mu m thick LAGTP-epoxy resin composite film was 125 N mm(-2). The water-impermeable composite film with excellent mechanical properties is an attractive candidate for the water-stable lithium electrode in rechargeable aqueous lithium-air batteries. (C) 2015 The Electrochemical Society. All rights reserved.