A solid composite electrolyte membrane for use in a lithium battery is provided which exhibits a conductivity ranging from about 10−4 S cm−1 to about 10−3 S cm−1 at ambient temperature. The membrane is formed by providing a glass or glass-ceramic powder formed from a mixture of lithium carbonate, alumina, titanium dioxide, and ammonium dihydrogen phosphate. The powder is mixed with a conditioning agent and at least one solvent, followed by the addition of a binder and one or more plasticizers. The resulting slurry is cast into a tape which is then subjected to a binder burn-off and sintering process to form the membrane. The resulting membrane may be a glass-ceramic composite having a porosity ranging from 0 to 50%, or the membrane may be further infiltrated with a polymer to form a water-impermeable polymeric-ceramic composite membrane.