Unveiling the structural and electrochemical effects of Al2O3 incorporation within LiPON electrolyte thin films by atomic layer deposition

Abstract

Thin film solid-state electrolytes (SSEs) have emerged as a key component in the development of microenergy storage devices, offering improved safety, stability and compatibility with advanced electrode materials. Among these electrolytes, lithium phosphorus oxynitride (LiPON) stands out as a relevant SSE candidate for microsupercapacitor applications. As with most ionic conductors, a margin for improvement always exists to enhance their overall performance. Within this scope, efforts to optimize LiPON through doping have been extensively explored, primarily using Physical Vapour Deposition (PVD) to provide films with relatively high thicknesses (500 nm–1 µm) and superior ionic conductivity. In this study, the use of Atomic Layer Deposition (ALD) for doping LiPON thin films (<50 nm) was explored for the first time, by inserting aluminum oxide (Al₂O₃) as a network former. Through the ALD supercycle approach, Al₂O₃ doped-LiPON thin films were deposited at 330 °C, using lithium hexamethyldisilazide (LiHMDS) and diethyl phosphoramidate (DEPA), while Al₂O₃ traces were injected during the film growth via trimethylaluminum (TMA) and water (H₂O) pulses. The resulting amorphous films were depth profiled by Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and demonstrated a uniform distribution of aluminium throughout the film's thickness for different doping levels. The incorporation of Al₂O₃ is intended to enable additional Li⁺ transport pathways through modified bridging configurations, particularly in oxygen and nitrogen environments. The Fourier transform infrared spectroscopy (FTIR) analysis indicated a prevalence of Bridging Oxygen (BO) and divalent nitrogen (N_d) units upon doping. These findings were further supported by X-ray photoelectron spectroscopy (XPS), underlining an increase in the ratios of Bridging Oxygen to Non-Bridging Oxygen (BO/NBO) and divalent nitrogen to trivalent nitrogen (N_d/N_t) along with higher lithium and lower carbon concentrations. The obtained structural modifications were accompanied by a stimulated Li⁺ ionic transport and a reduced activation energy, while maintaining a good insulating property and an electrochemical stability over a wide voltage window (up to 6 V).