Lithium Phosphorous Sulfurous Oxynitride (LiPSON) as a New Solid Electrolyte Material for Application in Thin-Film Batteries and Electrochromic Devices

Compared to lithium ion batteries of current generation which contain liquid or gel-electrolytes with aprotic solvents, all-solid-state batteries promise long lifetime, high volumetric energy density and inherent safety. Current battery research therefore focuses on the search for stable and incombustible inorganic solid electrolytes with high lithium ion conductivity, negligible electronic conductivity and chemical stability in contact with the electrodes – particularly towards the metallic Li anode. For the application in thin-film batteries glassy lithium phosphorous oxynitride electrolyte (LiPON) has become the state-of-the-art material due to its superior chemical stability. Commercial cells comprising LiPON are already available. Being an insulator with a high optical bandgap, LiPON may be also well suited as electrolyte in electrochromic thin film devices.

However, LiPON has a relatively poor ionic conductivity (< 1 x 10^-6 S cm^-1) and needs to be applied as a very thin film in order to obtain sufficient current densities in a cell rendering pin-hole free large-area deposition a major technological challenge. New materials with higher conductivities would be highly desirable.

Here, we report for the first time on the successful deposition of lithium phosphorous sulfurous oxynitride (LiPSON) thin films by RF magnetron sputtering from a mixed Li₃PO₄ and Li₂SO₄ target in reactive nitrogen atmosphere. The electrical properties (ionic and electronic partial conductivities) of the films have been characterized by electrochemical impedance spectroscopy (EIS) and DC polarization techniques. Ionic conductivities as high as 6 x 10^-6 S cm^-1  (E_a= 0.47 eV) are achieved. Moreover, we succeeded also in preparing pure LiPON films with extraordinary high ionic conductivities and high deposition rates. Our detailed analysis of the films includes the characterization of compositions by energy-dispersive X-ray spectroscopy (EDX) and by inductively coupled plasma optical emission spectrometry (ICP-OES) as well as of chemical properties by photoelectron spectroscopy (XPS). In addition, we characterized the optical properties of the films by UV-VIS spectroscopy.

Finally, we will present first results of the electrical properties of cathode/LiPSON half-cells.