Access Type

Dissertation/Thesis

Date of Award

January 2023

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Xin Wu

Second Advisor

Kwo Young

Abstract

Nowadays, multiphase transition metal-based AB2 metal hydride alloys and the rare-earth metal-based AB5 alloys (currently used in consumer NiMH batteries) are used as negative electrodes in NiMH batteries. It is of great interest to explore the possibility of significantly improving NiMH battery performance to compete with Li-ion batteries. In this area, the effect of dopant elements from Group-IA, -IIA, -IIIA non-transition metal family has not been thoroughly studied. In this study, lithium, magnesium, and boron (a metalloid considered metal in this study) were chosen as modifiers in the C14-phase predominant AB2 alloy by different dopant concentrations and synthesis methods, doping effects on phase composition, microstructure, hydrogen storage in gaseous and electrochemical phases, and magnetic properties were extensively investigated.The samples with different dopant amounts were processed through melting/casting. The produced constituents were analyzed with an inductively-coupled plasma optical emission spectrometer, the structures were characterized with a scanning electron microscope with an energy dispersive spectroscopy and an x-ray diffractometer, the gaseous phases storage performances were analyzed with a pressure-concentration-temperature system, the electrochemical phase storage performances were analyzed with a battery tester, and the magnetic properties for the surface structure corrosion in the electrochemical environment were measured with a vibration sample magnetometer. The results with non-transition metal dopants are compared with that without dopants. Lithium-doped alloys were processed through three synthesizing methods: melting/casting, mechanical alloying treated at different process duration, and solid-state diffusion/sintering treated at different temperatures. Solid-state diffusion synthesis (heat-treated at temperatures ranging from room to 850°C) successfully kept Li in alloys, with a higher participation rate obtained at ≤700°C. Structural analysis of the unit cell expansion indicated that Li entered the main hydrogen storage phase at lower temperatures but evaporated at higher sintering temperatures. While Li negatively affects electrochemical capacities and activation easiness, its surface structure improves the high-rate discharge performance. Magnesium has a high volatility during the melting/casting process leading to a reduced concentration from its designed stoichiometric concentration in the final ingot. A new Mg-rich cubic phase was formed, benefiting hydrogen storage capacity, high-rate dischargeability, and electrochemical activation; however, it was detrimental to the charge-transfer ability at room temperature and –40°C. Correlations of Mg-doped alloys showed that Ni-content was detrimental to the storage capacities, while Ti-content was unfavorable to the high-rate dischargeability but helpful to the charge-transfer ability. Boron element expanded unit cell volume by entering A-site in pairs (dumbbell model), helping the gaseous phase hydrogen storage capacity and lowering the equilibrium pressure. A new V3B2 phase was found in a higher B-concentration alloy that improved high-rate dischargeability, surface exchange current, and charge transfer ability at both room and low temperatures. However, the V3B2 phase didn’t contribute to the hydrogen storage capacities in either the gaseous phase or the electrochemical environment. In summary, the results show that Li and Mg benefit high-rate performance but lower storage capacity. B dopant helps the gaseous phase storage performance but severely degrades the electrochemical phase performance. Higher concentrations of these non-transition metals will worsen the overall performance. With these key findings, this research makes the 20 years of study of the doping effects on the electrochemical performance in NiMH batteries more complete. Further research should include the matrix effects of these modifiers and the applications of these modifiers to meet various requirements of specific products.

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