Andrew N. Jansen

Development of physico-chemical models for electrochemical impedance spectroscopy

Abstract Development of physico-chemical or process models for impedance spectra requires consideration of the physics and chemistry associated with the specific system under investigation. In order to be useful for interpretation of experimental data, the model must also account for the chracteristics of the measurement. The procedure for development of physico-chemical models is reviewed, and a method is presented that can be used to identify measurement characteristics such as error structure and degree of stationarity.

Numerical study of the influence of reactor design on MOCVD with a comparison to experimental data

Abstract Comparisons were made between experimental data and a two-dimensional model for the MOCVD of GaAs from trimethylgallium and arsine in two horizontal air cooled reactor geometries. A unique feature of this work was that comparison was made, not only on the wafer, but over the entire deposition regime. Excellent agreement was achieved for growth at low system pressures. Experimental deposition profiles under atmospheric pressure were much less uniform than those predicted by the model. This difference could not be attributed to a pressure dependence of heterogeneous reactions. Inclusion of thermal diffusion had little effect on the uniformity of the calculated deposition profile but decreased the magnitude of the growth rate by up to 12%. Through model calculations and experiments, it was determined that a channel with a tilted upper wall and a horizontal susceptor has the same growth rate profile as does a standard horizontal upper wall channel with a tilted susceptor. Substrate rotation was predicted to cause growth rate uniformity within less than 4.3% in horizontal channel flow and 3.8% in a channel with tilted walls.

Olivine electrode engineering impact on the electrochemical performance of lithium-ion batteries

High energy and power density lithium iron phosphate was studied for hybrid electric vehicle applications. This work addresses the effects of porosity in a composite electrode using a four-point probe resistivity analyzer, galvanostatic cycling, and electrochemical impedance spectroscopy (EIS). The four-point probe result indicates that the porosity of composite electrode affects the electronic conductivity significantly. This effect is also observed from the cells pulse current discharge performance. Compared to the direct current (dc) methods used, the EIS data are more sensitive to electrode porosity, especially for electrodes with low porosity values.

Thermally Stimulated Deep‐Level Impedance Spectroscopy Application to an n‐GaAs Schottky Diode

Impedance spectroscopy with temperature as a parameter is shown to be sensitive to deep-level states in an n-GaAs Schottky diode. A mathematical model is developed which accounts for the influence of electronic transitions involving deep-level states on the impedance response of the device. Regression of the model to the data is shown to require a weighting strategy that accounts for the variance of the measurements. The parameters obtained are compared to those obtained by deep-level transient spectroscopy and those reported in the literature. Good agreement was seen for the number, concentration, and activation energies of deep-level states, but the reaction cross sections obtained differed from those obtained by deep-level transient spectroscopy by several orders of magnitude. This discrepancy is attributed to the need to refine the Drocess model to account for the influence of recombination pathways on the leakage current observed at low frequencies:

Identification of Deep-Level States in Electronic Materials by Optically Stimulated Deep Level Impedance Spectroscopy

Optically stimulated deep level impedance spectroscopy (OS-DLZS) is suggested for analysis of electronic transitions involving deep-level states in semiconductors with large bandgaps. The technique is based on interpretation of both the real and imaginary components of the impedance response measured over a continuous range of electrical frequencies under sub-bandgap illumination. The application of OS-DLZS is illustrated for a ZnS:Mn electroluminesce nt panel and for ZnO varistors. The lower frequency range of the impedance spectrum is shown to be more sensitive to electronic transitions caused by monochromatic sub-bandgap illumination than is the capacitance measured at high frequency.

Comparative costs of flexible package cells and rigid cells for lithium-ionhybrid electric vehicle batteries.

We conducted a design study to compare the manufacturing costs at a level of 100,000 hybrid vehicle batteries per year for flexible package (Flex) cells and for rigid aluminum container (Rigid) cells. Initially, the Rigid cells were considered to have welded closures and to be deep-drawn containers of about the same shape as the Flex cells. As the study progressed, the method of fabricating and sealing the Rigid cells was expanded to include lower cost options including double seaming and other mechanically fastened closures with polymer sealants. Both types of batteries were designed with positive electrodes containing Li(Ni{sub 1/3}Co{sub 1/3}Mn{sub 1/3})O{sub 2} and graphite negative electrodes. The use of a different combination of lithium-ion electrodes would have little effect on the difference in costs for the two types of cells. We found that 20-Ah cells could be designed with excellent performance and heat rejection capabilities for either type of cell. Many parts in the design of the Flex cells are identical or nearly identical to those of the Rigid Cell, so for these features there would be no difference in the cost of manufacturing the two types of batteries. We judged the performance, size and weight of the batteries to be sufficiently similar that the batteries would have the same value for their application. Some of the design features of the Flex cells were markedly different than those of the deep-drawn and welded Rigid cells and would result in significant cost savings. Fabrication and processing steps for which the Flex cells appear to have a cost advantage over these Rigid cells are (1) container fabrication and sealing, (2) terminal fabrication and sealing, and (3) intercell connections. The costs of providing cooling channels adjacent to the cells and for module and battery hardware appear to favor Rigid cell batteries slightly. Overall, Flex cell batteries appear to have an advantage of about

Overcharge Effect on Morphology and Structure of Carbon Electrodes for Lithium-Ion Batteries

1.20-

Aging characteristics of high-power lithium-ion cells with LiNiCoAlO and LiTiO electrodes

3.70 per cell for a 25-kW Battery of 20 cells or about

Capacity Fading Mechanism and Improvement of Cycling Stability of the SiO Anode for Lithium-Ion Batteries

24 to

Enabling Fast Charging: A Technology Gap Assessment

74 per battery. Container experts assisted with this study, including a paid consultant and personnel at container manufacturing companies. Some of the companies are considering entering the business of manufacturing containers for hybrid vehicle battery manufacturers. For this reason they provided valuable guidance on overall approaches to reducing the costs of the cell containers. They have retained the description of some specific designs and procedures for future possible work with battery manufacturers, with whom they are now in contact. Through the guidance of these experts, we determined that a new type of container could be manufactured that would have the best features of performance and low cost of both the Rigid and Flex containers. For instance, the aluminum layer in a tri-layer sheet can be sufficiently thick to form a rigid container that can be fabricated in two halves, much like a Flex container, and mechanically joined at the edges for strength. In addition to the mechanical joint, this container can be sealed at the edges, much like a Flex container, by means of an inner polymer liner that can be heat-sealed or ultrasonically welded. The terminals can be flat strips of metal sealed into the top of the container as part of the edge sealing of the container, as for the Flex cell. Ridges can be stamped into one side of the container to provide cooling channels and the exterior layer of the container stock can be coated with a thin, electrically insulating, polymer layer. We expect this type of container will provide excellent sealing and durability and be less expensive than either the Flex or the Rigid container, which the study initially considered. A major cost for the original Rigid container is the welding required for sealing the container. However, the welding of the current collector tabs to the terminal piece may be even more complex and costly than welding the container. It is important, therefore, to develop an inexpensive procedure for attachment of the foils to the terminal pieces. A lower-cost procedure, such as ultrasonic welding or mechanical clipping, might replace laser welding of the foils to the terminal pieces. A conclusion from our discussions with the container experts is that the manufacturing rate required for the containers for hybrid vehicle batteries is fairly low, and thus favors procedures requiring low tooling costs and little development effort. These conditions favor flexible packaging, heat sealing, shallow stamping, double seaming and ultrasonic welding. It works against deep drawing and untested procedures for welding and joining.