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Understanding the Surface and Interface Properties of Electrode Materials in Alkali-ion Batteries : A Combination of Experimental and Computational Studies

Abstract

In order to realize the commercialization of alkali-ion batteries (AIB) in electric vehicles and smart grid systems, further improvements are demanded in multiple aspects, especially the energy/power density, safety and cycle life. Due to the complexity of the system, besides each separate component, surface and interface are critical in understanding and optimizing the system. In this dissertation, systematic studies are performed on the nanosizing effect, the surface phase transformation and surface coating of Li-intercalation cathode materials by a combination of first principles and experimental studies. In the first part, the surface spin transition of LiCoO₂ (LCO) was found based on first principles calculations. Different sizes of LCO were synthesized and the spin transition on the surface was confirmed by other characterization techniques. The OER/ORR activities profoundly increased at the spin-transited surfaces. The study shed lights on the electronic properties tuning in nanosizing process, which would affect functional properties. In the second part, the structural and chemical evolutions during electrochemical process of the Li-excess compound was probed by a combination of high-end characterization techniques especially STEM/EELS. A second phase generation together with oxygen vacancy formation and microstrain formation was found at the first cycle plateau region. A novel oxygen vacancy assisted transition metal migration mechanism was proposed using first principles calculations. These findings lead to an increased understanding of the performance fading mechanism of Li-excess family compounds, providing new insights in optimization their rate performances. In the third part, the surface coating of LLTO has been investigated for an improved electrochemical performance of the NCA combining computation with other electrochemical tests. The good ionic conductor LLTO increases the system's ion conductivity thus the electrochemical performance. Similar investigations have also been carried out in AlF₃ coatings. These studies help the understanding of coating effect of electrode materials. In the fourth part, a detailed STEM/EELS study on the P2 type Na cathode material was carried out

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