| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 102 === The recent changes across transportation, communication, health, and now wearable technology represent a significant paradigm shift in which electronic devices are ubiquitous to all facets of daily human life. Although considerable progress have been made towards the different technologies, the power sources of these devices, mainly rechargeable lithium ion batteries, have seen a slower progression in advancement due to several factors that include a lack of commercially available high energy-dense materials. This limitation, among others, proves to be the bottleneck across all technology. The lithium-rich high-capacity cathode material Li[NixLi1/3-2x/3Mn2/3-x/3]O2 are of high interest as the next generation of cathode materials due to it’s rechargeable capacity and energy density about twice that of current commercial cathode materials. Elucidation of multi-level surface and bulk reactions within the lithium-rich materials and their connection within the entire battery system would give deeper understanding to the materials electrochemistry.
This dissertation describes the development and utilization of advanced characterization techniques to understand the surface and bulk mechanisms of the lithium-rich high-capacity cathode materials Li[NixLi1/3-2x/3Mn2/3-x/3]O2. This work identified several surface phenomenons including the changes from the surface layered structure to a lithium transport-inhibiting spinel structure with increased number of electrochemical cycling and surface reactions. Surface studies on the lithium-excess/graphite full-cell battery identified the evolution of Li2O during charging and its effect on the overall battery environment. Using in situ surface-enhanced Raman spectroscopy via SiO2-encapsulated Au nanoparticles, Investigation of the first electrochemical cycling shows that Li2O will form during the oxygen-activation plateau and subsequently consumed towards the end of the plateau. The reaction of Li2O leads to LiOH formation on the graphite anode and changes in the battery environment that promote Li2CO3 precipitation onto the Li-excess cathode. During the oxygen activation plateau, activated oxygen species are formed that will hybridize with Mn causing the layered-to-spinel transformation and subsequent decrease in overall hybridization causing instability. The presents of Ni alleviates the subsequent decrease in hybridization and shows milder layered-to-spinel transformation due to the preferred hybridization of Ni with the activated oxygen species while maintaining Mn as a spectator metal. This result underlines the strong relationship between Ni and Mn in stabilizing the lithium-rich material. To mitigate the dilapidating surface formations and surface reconstruction, atomic-layer-deposited TiO2 coating on Li-excess and graphite electrodes were employed as a protection layer. Electrochemical studies showed improvements on the rate capability if TiO2 coating was applied to the cathode side and a decrease if applied to the anode side. While initial stability was found during cycling, TiO2-coating on a single electrode generally lead to lower stability compared to non-coated electrodes when cycled up to 100 cycles at 55 °C versus. TiO2 coating on both electrodes showed to be the most stable under the same condition due to protection from surface reactions on both electrodes. The multifaceted analysis of bulk and surface phenomenon has led to an increased understanding of the lithium-rich high-capacity cathode materials.
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