Browsing by Subject "phase change materials"

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    Structure-Property Relationships Of Nanostructured Materials For Electrochemical And Thermal Energy Storage Applications
    (2020-05) Tran, Nam
    Advance materials with nanostructure provide unique features to design a safe and efficient energy conversion and storage systems. In this thesis, the synthesis, characterization, and optimization of novel materials for electrical energy storage and thermal energy storage are explored. The first part of the thesis focuses on novel cathode materials, Li8ZrO6, for lithium-ion batteries. A synthesis method using pyrolysis of inorganic and organic precursors was utilized to prepare a nanostructured Li8ZrO6/C composite. The composite contained micron-sized particles of active material Li8ZrO6 in intimate contact of conductive carbon phase. The grain size of Li8ZrO6 was further reduced to below 50 nm using ball-milling approach. The composite can be used to make an electrode directly without any additional conductive phase. A new battery- testing program was developed to electrochemically and reversibly remove/re-insert up to 3 Li per formula unit of Li8ZrO6. A specific capacity of 221 mAh/g (corresponding to removal of 2 Li/f.u.) and 331 mAh/g (3 Li/f.u.) with 100% Coulombic efficiency were maintained for 140 cycles and 15 cycles, respectively. The structural change at different states of charge was predicted using quantum mechanical calculations and experimentally supported by XRD, XPS, and PDF data. Lithium intercalation (up to 2.5 Li/f.u.) of Li8ZrO6 followed a reversible path and lattice oxygen atoms were involved in the redox reaction during the charge/discharge processes. The effects of transition metals doping on bulk Li8ZrO6 were also investigated. The second part of the thesis describes the one-pot synthesis and properties of phase change material composites consisting of metal nanoparticles embedded in a mesoporous carbon network. The melting temperature, particle size, and the amount of energy stored/released was controlled by varying the loading of metals in the composite. The melting temperature of Bi metal nanoparticles in the composite can be tuned to 33 °C below the melting point of bulk metal. The specific enthalpies and the associated phase change temperatures of the metal nanoparticles were maintained over multiple melting/recrystallization cycles. The mesoporous carbon network prevented nanoparticles aggregation during and after the phase change and acted as a container to offset volume expansion of the metal during the melting process. The composites are stable in a sealed container for at least 11 months.