Lipka, Stephen M.

An automated procedure for integrated cycling and electrochemical impedance spectroscopy (EIS) testing of nickel sinter individualized pressure vessel electrodes for secondary nickel/hydrogen batteries was developed. Nickel electrodes from three major U.S. manufacturers were cycled under various conditions. The condition of the electrodes was monitored using both EIS and traditional electrochemical methods. In order to establish relationships between the status of the electrodes and the acquired impedance spectra, various cycling and electrode parameters were analyzed and compared with the EIS data. Nonlinear least squares (NLS) regression was used for analysis of the impedance data. An equivalent circuit was developed which produced good correlation with the impedance data at all states-of-charge and discharge rates. Problems with the experimental procedure which limit the validity of EIS testing were discussed.

Experiments were conducted to determine the stress corrosion cracking (SCC) susceptibility of various corrosion-resistant alloys which included: 17-4 PH, INCONEL 718 and A286. These alloys were studied for different aging (heat) treatments. Slow strain rate tests (extension rate = 4.7 x 10^-5 mm/s) were performed on each alloy in four different environments; including air and natural seawater acidified with reagent grade hydrochloric acid to a pH of 0.1, 1 and 3. During the experiments, the load versus time and the open circuit potential were monitored. Various parameters such as time-to-failure, energy-to-failure, maximum or failure stress and reduction-in-area were calculated in order to determine SCC susceptibility. Fractography using SEM was conducted to confirm whether any SCC occurred and, if so, to identify its mode (intergranular or transgranular). Limited potentiodynamic studies were also completed to evaluate the passive behavior of these alloys. The results are discussed in terms of the SCC susceptibility and the nature of the cracking. An attempt was also made to correlate alloy microstructure, slow strain rate test parameters and passivation behavior with SCC susceptibility.

Results are reported on a method to synthesize multi-wall carbon Nanotubes (MWNTs) using a liquid injection chemical vapor deposition (CVD) method centered around ferrocene as the catalyst precursor. These materials were specifically targeted as active materials for electrochemical double layer capacitors (EDLCs). A parameteric study was developed to optimize material synthesis based on growth parameters, characterize physical growth properties, and evaluate the performance of the MWNTs as electrodes in electrochemical capacitors. Physical data includes scanning electron micrographs (SEM), transmission electron micrographs (TEM), and x-ray powder diffraction (XRD). Electrochemical performance data is given based on cyclic voltammetry (CV) and impedance spectroscopy (EIS).

High energy density PAN-based carbon fiber anode materials for lithium-ion type batteries were developed. Commercially available organic precursors were thermally converted to carbons. The effects of precursor material, carbonization temperature, heating ramp rate, soak time and gaseous atmosphere during the thermal treatment on the electrochemical performance of the carbon fibers were studied. In order to evaluate the electrochemical performance of the carbon fibers, test cells were assemble using the carbon materials prepared in the laboratory and intercalation/deintercalation experiments were performed. The results indicated that the highest reversible capacity and lowest irreversible capacity loss was obtained for carbon fibers carbonized at 1100C at fast ramp rate of 26C/min. X-ray diffraction experiments revealed a relation between the capacity and the irreversible capacity loss on first cycle, and the size of the crystallites Lc. A phenomenological explanation for this behavior was developed. Using electrochemical impedance spectroscopy the diffusion coefficient of Li in the tested carbon fibers was calculated. In addition, the influence of electrolyte composition (solvent and salt) on the reversible and irreversible capacities as well as on the intercalation/deintercalation potential profile was investigated. An electrolyte containing 1M LiPF6 in EC:DEC:DMC (40:30:30 v/o) proved to be most suitable for these carbon fiber materials improving significantly their electrochemical performance. Finally, coin cells were assembled containing the carbon fiber material prepared in the laboratory. They were tested for reversible and irreversible capacity. The coin cells proved that the synthesized carbon anode materials possess high energy density and could be used in commercial applications.