Amorphous oxide and oxynitride lithium electrolyte thin films were synthesized by r.f. magnetron sputtering of lithium silicates and lithium phosphates in Ar, Ar+O x, Ar+N" or N 2 . The composition, structure, and electrical properties of the films were characterized using ion and electron beam, X-ray, optical, photoelectron, and a .c. impedance techniques . For the lithium phosphosilicate films, lithium ion conductivities as high as 1 .4 X 10 -`S/ cm at 25 Β°C were observed, but none of these films selected for extended testing were stable in contact with lithium . On the other hand, a new thin-film lithium phosphorus oxynitride electrolyte, synthesized by sputtering Li,PO, in pure N r, was found to have a conductivity of 2X10 -6 S/cm at 25 Β°C and excellent long-term stability in contact with lithium. Thin-films cells consisting of a 1 Β΅m thick amorphous V 2 0 5 cathode, a 1 Β΅m thick oxynitride electrolyte film, and a 5 Β΅m thick lithium anode were cycled between 3 .7 and 1 .5 V using discharge rates of up to 100 Β΅A/cm 2 and charge rates of up to 20 Β΅A/cm'' . The open-circuit voltage of 3 .6 to 3 .7 V of fully-charged cells remained virtually unchanged after months of storage .