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Influence of Synthetic Conditions on the Polyol Process for High Power High Energy Materials: Structural and Electrochemical Properties

  • Author(s): Yoon, Hyojung
  • Advisor(s): Meng, Ying S
  • et al.
Abstract

With the rapid depletion of conventional energy sources based on fossil fuels and increasing energy demand, sustainable and renewable energy technologies, such as solar, wind, hydroelectric power, have been drawn worldwide attention in energy-based economics. Because of their uncontrollable and intermittent nature, large-scale high- power electrical energy storage (EES) system is required to increase reliability of renewable energy resources. Among various energy storage systems, lithium-ion batteries (LiBs) are considered as the most important rechargeable energy storage system. However current LiB technology is still inadequate to compete against the conventional fossil fuels and implement in the large-scale applications: Key questions are cost, energy density, cycling life, and safety issues. In the first part of this dissertation, rare-earth-free permanent magnets (PMs) are intensively studied. Our innovative approach, using polyol process, developed nanostructured cobalt carbide (NCC) consisting of an assembly of Co2C and Co3C nanoparticles. The resulting material exhibits an energy product of greater than 15.84 MGOe at room temperature before compaction, which is confirmed through vibrating sample magnetometer (VSM) and first order reversal curves (FORCs). An understanding of the formation mechanism, using in situ time-resolved x-ray absorption spectroscopy (XAS), and the correlation between phase contributions to the properties are described in detail. Our success in the development of rare-earth-free PMs may provide a cost- effective and sustainable platform for clean technology applications. Although the performances of LiBs have been drastically improved, the limit in terms of the energy density and material stability still needs to be resolved. In the second part, high-voltage cathode materials, spinel LiNi0.5Mn1.5O4 (LNMO) and olivine LiCoPO4, are investigated to understand the influence of synthetic conditions of polyol synthesis on their electrochemical properties and structural stabilities upon cycling. High- angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray photoelectron spectroscopy (XPS) allow us to determine the evolution of phase transformation accompanied with the migration of transition metal (TM) ions at the surface and electrode/electrolyte interfaces.

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