ATOMIC LAYER DEPOSITION OF ALKALI PHOSPHORUS OXYNITRIDE ELECTROLYTES FOR BEYOND-LITHIUM NANOSCALE BATTERIES

Abstract

Lithium-ion batteries dominate portable energy storage systems today due to their light weight and high performance. However, with the continuing demand for battery capacity projected to outstrip the supply of lithium, alternative energy storage systems based on the more abundant Na and K alkali metals are attractive from both a resource perspective and their similar charge storage mechanism. Beyond limited lithium resources, there remains significant opportunity for innovation to improve battery architecture and thus performance. Nanostructured solid-state batteries (SSBs) are poised to meet the demands of next-generation energy storage technologies, with atomic layer deposition (ALD) being a powerful tool enabling high-performance nanostructured SSBs that offer competitive performance with their liquid-based counterparts. This dissertation has two main objectives: First, the development of the first reported ALD solid-state Na+ and K+ conductors are presented. Second, by leveraging the work on developing new solid- state Na+ ion conductors, a proof-of-principle nanoscale Na-SSB is fabricated and tested.ALD processes are developed for the Na and K based analogues of the well-known solid- state electrolyte (SSE) lithium phosphorus oxynitride (LiPON). In this case; NaPON and KPON. A comprehensive comparison of the structure, electrochemical, and processing parameters between the APON (A = Li, Na, K) family of materials is presented. The structure of NaPON closely resembles that of ALD LiPON, both possessing a N/P of 1, classifying them as alkali polyphosphazenes. Interestingly, KPON exhibits similar ALD process parameters to NaPON and LiPON, but the resulting film composition is quite different, showing little nitrogen incorporation and more closely resembling a phosphate glass. NaPON is determined to be a promising SSE with an ionic conductivity of 1.0 ́ 10-7 S/cm at 25 °C and a wide electrochemical stability window of 0-6.0V vs. Na/Na+. The electrochemical stability and performance of NaPON as a SSE is tested in liquid-based and all solid-state battery configurations comprised of a V2O5 cathode and Na metal anode. Electrochemical analysis suggests intermixing of the NaPON/V2O5 layers during the ALD NaPON deposition, and further reaction during the Na metal evaporation step. The reaction during the ALD NaPON deposition on V2O5 is determined to be two-fold: (1) reduction of V2O5 to VO2 and (2) Na+ insertion into VO2 to form NaxVO2. The Na metal evaporation process is found to exacerbate this reactivity, resulting in the formation of irreversible interphases leading to poor SSB performance. Despite the relatively poor performance, this work represents the first report of a nanoscale Na-SSB and showcases cryo- TEM as a powerful characterization technique to further the understanding of nanoscale SSBs. Looking forward, the intermixing during the ALD NaPON deposition does not impact the cycling of the NaxVO2 electrode in liquid-based cells, with NaPON-coated electrodes outperforming unsodiated V2O5 electrodes. This may be advantageous for the fabrication of SSBs, as the SSE deposition simultaneously could pre-sodiate a stable cathode material, excluding the need for ex-situ sodiation in liquid solutions or depositing a pre-sodiated electrode material. Strategies to pair this NaxVO2/NaPON cathode/electrolyte with a stable anode are discussed, with a focus on the ultimate realization of a high-performance Na-SSB. This work highlights the high reactivity of Na compared to Li based battery chemistries, not only necessitating the need for interfacial coatings in Na SSBs, but also the extreme caution required during fabrication of Na-SSBs or liquid sodium- ion batteries.