Electrical and Optical Properties of Lithium Phosphorous Oxynitride (LiPON) Electrolyte Thin Films with High Nitrogen Content Prepared By RF Sputtering

Lithium phosphorus oxynitride (LiPON) is one of the promising electrolyte material for the thin-film batteries due to its superior electrochemical stability, moderate ionic conductivity and low electronic conductivity. In addition, LiPON has a wide optical bandgap (>3 eV) which makes it transparent for UV-Vis light and suitable for using in electrochromic thin film devices.

The typical value of ionic conductivity of LiPON is in the range of 0.2 to 3.3×10^-6 S/cm[1]. Here we report the deposition of amorphous LiPON thin films by RF sputtering from a commercial Li₃PO₄ target in nigtrogen atmosphere with the ionic conductivities at room temperature as high as 6.5×10^-6 S/cm with an activation energy of 0.55 eV. The average growth rate is 14 nm per minute. The electronic conductivity is found to be 2.2×10^-12 S/cm. The film composition is determined to be Li_3.13PO_1.69N_1.39by inductively coupled plasma-optical emission spectrometry (ICP-OES) and Rutherford backscattering spectrometry (RBS), which demonstrates our LiPON sample contains relatively high nitrogen content and low oxygen content. The local structure is probed by XPS and Raman spectrometry. The incorporation of nitrogen in the matrix is confirmed by both methods and the nitrogen is present in two different chemical environments corresponding to  and P-N=P.

The optical properties of LiPON are studied by variable-angle spectroscopic ellipsometry (SE) and transmission measurements. The LiPON film is found to have a bandgap of 3 eV and is highly transparent for UV-VIS light. The optical constants include refractive index n, extinction coefficient k and absorption coefficient α in the energy range of 0.58~5.5 eV are derived. The Urbach tail is also observed in LiPON thin films when photon energy is lower than the bandgap. For its application in electrochromic device, the WO₃/LiPON half-cells are prepared and the electrochromic properties will be discussed.

[1] J.F. Ribeiro, R. Sousa, J.P. Carmo, L.M. Gonçalves, M.F. Silva, M.M. Silva, J.H. Correia, Thin Solid Films 522 (2012) 85-89.