CRADA Final Report: Properties of Vacuum Deposited Thin Films of Lithium Phosphorous Oxynitride (Lipon) with an Expanded Composition Range

摘要

Thin films of an amorphous, solid-state, lithium electrolyte, referred to as ''Lipon'', were first synthesized and characterized at ORNL in 1991. This material is typically prepared by magnetron sputtering in a nitrogen plasma, which allows nitrogen atoms to substitute for part of the oxygen ions of Li{sub 3}PO{sub 4}. Lipon is the key component in the successful fabrication of ORNL's rechargeable thin film microbatteries. Cymbet and several other US Companies have licensed this technology for commercialization. Optimizing the properties of the Lipon material, particularly the lithium ion conductivity, is extremely important, yet only a limited range of compositions had been explored prior to this program. The goal of this CRADA was to develop new methods to prepare Lipon over an extended composition range and to determine if the film properties might be significantly improved beyond those previously reported by incorporating a larger N component into the film. Cymbet and ORNL investigated different deposition processes for the Lipon thin films. Cymbet's advanced deposition process not only achieved a higher deposition rate, but also permitted independent control the O and N flux to the surface of the growing film. ORNL experimented with several modified sputtering techniques and found that by using sectored sputter targets, composed of Li{sub 3}PO{sub 4} and Li{sub 3}N ceramic disks, thin Lipon films could be produced over an expanded composition range. The resulting Lipon films were characterized by electrical impedance, infrared spectroscopy, and several complementary analytical techniques to determine the composition. When additional N plus Li are incorporated into the Lipon film, the lithium conductivity was generally degraded. However, the addition of N accompanied by a slight loss of Li gave an increase in the conductivity. Although the improvement in the conductivity was only very modest and was a disappointing conclusion of this study, forcing a higher N content in the Lipon may alleviate some of the run-to-run variations in the Lipon quality that have been problematical for years.