Ultrathin lithium phosphorus oxynitride glass (LiPON) films with thicknesses down to 15 nm, deposited by reactive sputtering in nitrogen plasma, were found to be electronically insulating. Such ultrathin electrolyte layers could lead to high power outputs and increased battery energy densities. The effects of stoichiometry, film thickness, and substrate material on the ionic conductivity were investigated. As the amount of nitrogen in the layers increased, the activation energy of the ionic conductivity decreased from 0.63 to 0.53 eV, leading to a maximum conductivity of 1 × 10(-6) S/cm. No dependence of the ionic conductivity on the film thickness or substrate material could be established. A detailed analysis of the equivalent circuit model used to fit the impedance data is provided. Polarization measurements were performed to determine the electronic leakage in these ultrathin films. A 15-nm LiPON layer on a TiN substrate showed electronically insulating properties with electronic resistivity values around 10(15) Ω·cm. To our knowledge, this is the thinnest RF-sputtered LiPON layer shown to be electronically insulating while retaining good ionic conductivity.
Keywords: Li-ion rechargeable battery; LiPON; RF sputtering; impedance spectroscopy; thin film.