Highly Conformal Solid Electrolyte for Li-Ion Batteries By Atomic Layer Deposition

Abstract

Lithium phosphorus oxynitride (LiP x O y N z , commonly known as LiPON) is a state-of-the-art solid electrolyte material with suitably low resistivity and high ionic conductivity for all-solid-state thin-film microbatteries [1,2]. Thin and pinhole-free layers of LiPON can be fabricated using atomic layer deposition (ALD) [3]. Uniquely among thin-film fabrication methods, ALD is a technique that works for coating even challenging 3D interfaces [4,5]. In this work, LiPON ALD processes were optimized for two different lithium precursors at reasonable temperatures (~300 °C) and precursor pulse/purge lengths (< 5 s), and LiPON films were deposited onto PillarHall® lateral high-aspect-ratio (LHAR) test structures to investigate the conformality of the material. Comparative testing was performed between the two lithium precursors.The results indicated that more conformal LiPON films can be deposited using the appropriate precursors – in this case, lithium tert-butoxide (LiO t Bu) yielded greater penetration depths than lithium hexamethyldisilazide (Li-HMDS). The improved conformality observed when using LiO t Bu indicates that this precursor is a better choice for LiPON coatings on high-aspect-ratio structures. The ALD technique allowed for the deposition of highly conformal LiPON films, ~50–115 µm deep into 100–500 nm gaps. This opens possibilities for the use of LiPON as a solid electrolyte in batteries with high-surface-area electrodes. High surface areas are important not only for unlocking higher power densities (faster charging and discharging) in batteries, but also for enabling the use of thin-film technology in fabricating microbatteries with meaningful energy storage capacities.