Doctor of Philosophy

dissertationElectric vehicles using on-board electricity as a power source have been commercialized for application in a small part of the automobiles market. For wide substitution of the current gasoline-powered vehicles, a lot of effort should be placed on improving the performance of lithium-ion batteries (LIBs), which are the dominant power sources of recent groups of electric vehicles. In this work, we studied several promising cathode and solid-state electrolyte materials for realization of high-capacity, high-safety LIBs. Poly-anionic LiFePO4 and Li2FeP2O7 have been considered very promising cathode materials for LIBs. They have large specific capacities, high thermal and chemical stability, and low cost. However, both of them have the same problem of low ionic and electronic conductivities. In order to speed up the kinetics in the LIBs, these poly-anionic materials were synthesized by developing a simple and high-throughput solution-based technique. The sort of chelating agent and the amount of carbon atoms in the starting solution were varied and the optimal parameters were found for LiFePO4 and Li2FeP2O7, respectively. The safety issue is another important factor for electric vehicles; it is not ensured by current LIBs using organic liquid electrolytes, which are flammable and volatile, prone to leak and decompose at high temperatures. Therefore, recent research has been focused on developing solid-state electrolytes. In this work, high-quality garnet-type iv Li7La3Zr2O12 (LLZO) electrolytes were synthesized using a solution-based technique. The ionic conductivity of cubic LLZO was revealed to be 1.67×10-4 S/cm. A proto-type cell comprised of LLZO electrolyte, LiCoO2 cathode and lithium metal anode was assembled. The cell possessed a gravimetric discharge capacity of 3.4 mAh/g. This value is quite low compared to conventional cells, mainly due to its large interfacial resistance. For improving the interfacial contact, LLZO was fabricated into thin films by pulsed laser deposition technique. The films deposited at room temperature had amorphous structure, and exhibited a lithium-ion conductivity of 3.35×10-7 S/cm. The effects of annealing on the properties of the films were investigated. Films annealed properly were found to have an enhanced lithium-ion conductivity value of 7.36×10-7 S/cm. Moreover, the as-deposited thin films were found to be electrochemically stable against lithium metal

Garnet-type Li7La3Zr2O12 electrolyte prepared by a solution-based technique for lithium ion battery

pre-printHigh quality garnet-type Li7La3Zr2O12 solid electrolyte was synthesized using a solution-based technique. The electrolyte pellets were sintered at 900 oC, resulting in tetragonal phase, which then transformed to cubic phase after annealing at 1230 oC. The ionic conductivity of both phases was studied and revealed to be 3.67x10-7 S/cm and 1.67×10-4 S/cm, respectively. A prototype cell comprising of Li7La3Zr2O12 electrolyte, LiCoO2 cathode and lithium metal anode was assembled. The cell made with the cubic phase electrolyte exhibited superior performance than the one made with the tetragonal phase electrolyte. The former cell possessed a very promising gravimetric discharge capacity of 3.4 mAh/g, which is the highest value obtained among similar setups

Characterization of Li7La3Zr2O 12 thin films prepared by pulsed laser deposition

pre-printA pulsed laser deposition system was employed to fabricate thin films of Li7La3Zr2O12 solid electrolyte. The deposition process was carried out at room-temperature, resulting in amorphous films. These as-deposited films had a large optical band gap of 5.13 eV, and exhibited a lithium-ion conductivity of 3.35×10-7 S/cm. The films were then annealed, and the effect of annealing on the optical and electrical properties of the films was examined. After annealing at 1000 °C, the films were found to be cubic with a narrower band gap of 3.64 eV. In addition, these annealed films showed an inferior ionic conductivity than the as-deposited ones

Fabrication and characterization of Li7La3Zr 2O12 thin films for lithium ion battery

pre-printThin films of Li7La3Zr2O12 were deposited on SrTiO3 (100) and Sapphire (0001) substrates at room-temperature using a pulsed-laser-deposition technique. Detailed structural, compositional, optical, and electrochemical characterizations of the films were performed. The films deposited at room-temperature had amorphous structure, and exhibited a lithium-ion conductivity of 3.35 × 10−7 S/cm. The effects of thermal annealing and pulsed laser annealing on the properties of the films were investigated. Pulsed laser annealed films were found to have a superior lithium-ion conductivity value of 7.36 × 10−7 S/cm. Moreover, the Li7La3Zr2O12 films were found to be electrochemically stable against lithium metal

Modification of high potential, high capacity Li2FeP 2O7 cathode material for lithium ion batteries

pre-printLi2FeP2O7 is a newly developed polyanionic cathode material for high performance lithium ion batteries. It is considered very attractive due to its large specific capacity, good thermal and chemical stability, and environmental benignity. However, the application of Li2FeP2O7 is limited by its low ionic and electronic conductivities. To overcome the above problem, a solution-based technique was successfully developed to synthesize Li2FeP2O7 powders with very fine and uniform particle size (< 1 μm), achieving much faster kinetics. The obtained Li2FeP2O7 powders were tested in lithium ion batteries by measurements of cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge cycling. We found that the modified Li2FeP2O7 cathode could maintain a relatively high capacity even at fast discharge rates

Broadband squeezed light field by magnetostriction in an opto-magnomechanical

We present a novel mechanism for generating a wide bandwidth squeezed optical output field in an opto-magnomechanical system. In this system, the magnon (mechanical) mode in the yttrium-iron-garnet crystal is coupled to the microwave field (optical field) through magnetic dipole (radiation pressure) interaction. The magnetostrictive force induced by the yttrium-iron-garnet crystal causes a mechanical displacement and creates a quadrature squeezed magnon mode. Eventually, this quadrature squeezed mechanical mode is transferred to the output optical field through state-swap interaction. Our results demonstrate the optimal parameter range for obtaining a stable squeezed optical output field with a wide bandwidth. Moreover, the squeezed light field exhibits strong robustness to environmental temperature. The new scheme we propose has potential applications in quantum precision measurements, quantum wireless networks, quantum radar, etc

Multi-channel quantum noise suppression and phase-sensitive modulation in a hybrid optical resonant cavity system

Quantum noise suppression and phase-sensitive modulation of continuously variable in vacuum and squeezed fields in a hybrid resonant cavity system are investigated theoretically. Multiple dark windows similar to electromagnetic induction transparency (EIT) are observed in quantum noise fluctuation curve. The effects of pumping light on both suppression of quantum noise and control the widths of dark windows are carefully analyzed, and the saturation point of pumping light for nonlinear crystal conversion is obtained. We find that the noise suppression effect is strongly sensitive to the pumping light power. The degree of noise suppression can be up to 13.9 dB when the pumping light power is 6.5 Beta_th. Moreover, a phase-sensitive modulation scheme is demonstrated, which well fills the gap that multi-channel quantum noise suppression is difficult to realize at the quadrature amplitude of squeezed field. Our result is meaningful for various applications in precise measurement physics, quantum information processing and quantum communications of system-on-a-chip