Fabrication and characterization of amorphous lithium electrolyte thin films and rechargeable thin-film batteries

Abstract

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 2, Ar + N 2, 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 × 10 −6 S/cm at 25 °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 3PO 4 in pure N 2, was found to have a conductivity of 2 × 10 -6 S/cm at 25 °C and excellent long-term stability in contact with lithium. Thin-films cells consisting of a 1 μm thick amorphous V 2O 5 cathode, a 1 μm thick oxynitride electrolyte film, and a 5 μm thick lithium anode were cycled between 3.7 and 1.5 V using discharge rates of up to 100 μA/cm 2 and charge rates of up to 20 μA/cm 2. The open-circuit voltage of 3.6 to 3.7 V of fully-charged cells remained virtually unchanged after months of storage.