Microwave-assisted synthesis and characterization of inorganic materials for energy applications

Harrison, Katharine Lee
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Lithium-ion batteries play a crucial role in portable electronics, but require further innovation for electric vehicle and grid storage applications. To meet this demand, significant emphasis has been placed on developing safe, inexpensive, high energy density cathode materials. LiFePO₄ is a candidate cathode material for electric vehicle and grid storage applications. Vanadium-doped LiFePO₄ cathodes of the form [chemical formula] (0 ≤ x ≤ 0.25) were synthesized here by a facile, low-temperature microwave-assisted solvothermal (MW-ST) method. Such an approach offers manufacturing-energy and cost savings compared to conventional synthesis. Additionally, although [chemical formula] has been synthesized previously by conventional methods, it is shown here that the MW-ST method allows much higher doping levels than can be achieved at conventional temperatures, indicating that metastable phases can be isolated through the low-temperature microwave-assisted synthesis. LiFePO₄ suffers from poor ionic conductivity, but this limitation can be minimized by microwave-assisted synthesis through a tuning of the particle size, allowing for decreased Li⁺ diffusion paths. LiVOPO₄ is another polyanion material with higher energy density than LiFePO₄, but similar ionic conductivity limitations. It has not been previously synthesized by MW-ST. Thus, a MW-ST method was developed here to prepare LiVOPO₄. By varying reaction conditions, three polymorphic modifications of LiVOPO₄ were accessed and the electrochemical performance was optimized. LiVOPO₄ can be further discharged to Li₂VOPO₄, which has been suggested in the literature, but the structural transformation that accompanies this process has not been detailed. To this end, the delithiation process was studied by ex situ XRD measurements to better understand how the second lithium is accommodated. Finally, MW-ST has also been exploited to grow thin films of anatase TiO₂ phase on indium tin oxide (ITO)-coated glass substrates. The microwave field is selectively absorbed by the conductive ITO layer on the glass substrates, leading to ohmic heating. The resulting heated ITO layer acts as a favorable site for nucleation and growth. TiO₂ thin films have widespread applications in the energy and electronics sectors. Such selective microwave-assisted ohmic heating of solid materials within a growth solution represents a promising new avenue for microwave synthesis, which has been minimally explored in the literature.