SYNTHESIS AND CHARACTERIZATION OF OXIDE THIN FILMS FOR ION TRANSPORT

Abstract

Metal oxides are a diverse category of materials that exhibit many material properties and applications. They can range from insulators to superconductors and have uses varying from electronic semiconductor devices to solar cells and even biomedical applications. Thin films, specifically, attract interest due to their unique properties compared to the bulk alternatives. Their small nano to micro scale size provides a large surface to volume ratio, which can affect defect density and alter the optical, electrical, or mechanical properties. The ease of fabrication and the tunability of thin films through techniques such as strain engineering or doping enables them to be widely utilized to characterize material properties and interfaces. Of these metal oxides, perovskite rare-earth nickelates exhibit intriguing electrical and optical properties, such as metal-to-insulator transitions. LaNiO3 is the only exception where the metallic phase is robust at all temperatures. Electrolyte gating can be an efficient method to manipulate the electronic behavior of LaNiO¬3 and induce an insulating phase. This work utilized ionic liquid gating with electric double-layer transistor devices to control the electronic properties of LaNiO3. The electrolyte gating leads to an insulating phase transition with an increased film resistivity by over six orders of magnitude. The electrolyte gating behaviors are found to be dependent on not only gating voltage and duration, but also the atmospheric environment. X-ray photoelectron spectroscopy (XPS) analysis revealed that ionic liquid gating promotes the removal of oxygen from the crystal to form oxygen vacancies, which also affects the Ni valence state. The insulating phase transition was attributed to enhanced electron correlation as well as an opening of the charge transfer gap due to the reduced overlap between Ni and O bands. The filling of carriers is controlled by the gate voltage. These results suggest that electrolyte gating devices can be useful for manipulating electron-electron correlation, furthering materials research in realizing exotic physics in correlated systems. Li-ion batteries are widely used in electronics, appliances, electric vehicles, and energy storage systems. The transport of the Li ions in Li-ion batteries can be affected by many factors, such as crystal structure or the formation of interfaces between the electrodes and the electrolyte used. Investigation of delithiation in a LiCoO2 cathode and the effects of Li removal on the structure demonstrated that charge compensation occurs via the formation of Co4+ and hole generation near O atoms. One drawback of conventional Li ion batteries is the flammability of the liquid electrolyte solvents. Li7La3Zr2O12 (LLZO) solid electrolytes have been considered an alternative to these liquid electrolytes. However, the reduced ionic conductivity of LLZO needs to be improved to fully realize their use in Li ion batteries. A hybrid electrolyte composed of LLZO with small amounts of liquid electrolyte can potentially resolve this issue by increasing Li conductivity between the LLZO and electrodes. This results in the formation of a solid-electrolyte interface, which can adversely affect the performance of the overall battery. To fully understand how this interface forms and how it impacts the performance, Ta-doped LLZO/Li2O multilayer films were exposed to a liquid electrolyte solution containing LiPF6. XPS analysis showed that carbon contaminants such as LiCO2 were reduced upon electrolyte exposure. Fluorinated metals such as LiF, LaF3, and ZrF4 were detected at depths up to 10 nm, suggesting that LLZO interacts more strongly with HF than other decomposition products of the electrolyte. Overcoming the formation of fluorinated metals is critical to realizing hybrid electrolytes that use the SE LLZO in combination with a LE containing LiPF6 without being inhibited by solid-electrolyte formation.