Solvent-free composite poly(ethylene oxide) (PEO)-ceramic fiber or mat electrolytes with high ionic conductivity and good interfacial stability have been developed using high-ionic-conductivity fibers and mats. The conducting ceramic fibers can penetrate the cross section of the electrolyte film to provide long-range lithium-ion transfer channels, thus producing composite electrolytes with high conductivity. In this work, a maximum room-temperature conductivity of S cm−1 was achieved for 20 wt % fiber in a PEO- mixture containing 12.5 wt % in PEO. The maximum transference number obtained was 0.7. The ceramic fibers in this composite electrolyte are coated by a very thin PEO layer, which is sufficient to provide good interfacial stability with lithium-ion and lithium-metal anodes. © 2004 The Electrochemical Society. All rights reserved.