UC San Diego

Lithium-ion batteries have been growing rapidly for the past few decades, finding applications in portable devices such as cell phones, and laptop computers. Recently, the surging demand for electric vehicles and grid storage necessitates the development of safer batteries that offer higher performance, longer life, and lower cost. Spinel-type LiNi0.5Mn1.5O4 (LNMO) stands out as a promising cathode material for 5 V-class Li-ion batteries enabling high energy density and low material costs. However, common carbonate-based liquid electrolytes suffer from oxidative decomposition under high voltage, resulting in continuous cell degradation. In contrast, certain solid-state electrolytes have a wide electrochemical stability range and can withstand the required oxidative potential. In this study, we test a thin-film battery comprising an LNMO cathode with a solid lithium phosphorus oxynitride (LiPON) electrolyte and characterized their interface before and after cycling. Utilizing Li metal as the anode, this system can deliver stable performance for 600 cycles with an average Coulombic efficiency exceeding 99%. Neutron depth profiling reveals a slight overlithiated layer at the interface prior to cycling, a result that is consistent with the excess charge capacity measured during the first cycle. Additionally, cryogenic electron microscopy indicates intimate contact between LNMO and LiPON without any noticeable structure or chemical composition evolution after extended cycling, underscoring the superior stability of LiPON against a high voltage cathode. These findings accelerate the commercialization of a high voltage cell with solid or liquid electrolytes.In addition to thin film batteries, the thin film platform of battery materials offers another potential application in the form of a resistive switching device for future computing. With conventional silicon-based memory devices approaching their quantum mechanical limits, it is imperative to seek new functional materials to replace them. Therefore, in this thesis, solid-state electrolyte lithium lanthanum titanate (LLTO) is explored as a resistive switching device, and its material properties are investigated through micro and macroscopic characterization. Electronic conductivity changes that provoke the LLTO switching are investigated experimentally and computationally through the comparison of LLTO with different oxygen compositions, under the hypothesis that oxygen vacancy plays a role in altering local LLTO conductivity. Based on those data, switching mechanisms are proposed as filament growth created by local conductivity change through the simplified model.