Characterization of RF Sputter-deposited Sodium Phosphorous Oxynitride Thin Films as a Solid-state Sodium-ion Conductor

We demonstrated the thin film deposition of sodium phosphorous oxynitride (NaPON) via RF magnetron sputtering of $Na_3PO_4$, as a solid-state Na-ion conductor similar to lithium phosphorous oxynitride (LiPON), which is a commonly used solid electrolyte. The deposited NaPON thin film was characterized by scanning electron microscopy, X-ray diffractometry, and electrochemical impedance spectroscopy, to investigate the feasibility of the solid-state electrolyte in several different cell configurations. The key properties of a solidstate electrolyte, i.e., ionic conductivity and activation energy, were estimated from the complex non-linear least square fitting of the measured impedance spectra at various temperatures in the range of $27-90^{\circ}C$. The ionic conductivity of the NaPON film was measured to be $8.73{\times}10^{-6}S\;cm^{-1}$ at $27^{\circ}C$, which was comparable to that of the LiPON film. The activation energy was estimated to be 0.164 eV, which was lower than that of the LiPON film (0.672 eV). The obtained values encourage the use of a NaPON thin film in the future as a reasonable solid-state electrolyte.