Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Prof. Andy (Xueliang) Sun

Abstract

Alkali metal-O2 batteries, i.e. Li- and Na-O2, are considered as the next generation of energy saving technologies with potential application in electric transportation. The high theoretical energy density in these cells is related to the use of high energy alkali metals as negative and oxygen as the positive electrode materials. The performance of alkali metal-O2 cells is highly dependent on the positive electrode material, where oxygen reduction and evolution reactions take place. Besides, the primary products of oxygen reduction reaction in these cells are typically metal oxides, which are insoluble in nonaqueous electrolytes, resulting in accumulation on the porous air electrode surface. Accordingly, an ideal air electrode should have a porous structure with appropriate pore volume and pore size distribution in addition to necessary characteristics such as conductivity, chemical stability, high surface area, and low cost. Several air electrode architectures are designed and developed in the studies presented in this thesis to improve the cyclability and performance of alkali metal-O2 cells.

In the first part of experimental results, the correlation between the surface area and pore size of the air electrode materials with the electrochemical behavior of Na-O2 cell was studied in detail. Series of specific air electrode materials with different surface area and porosity were synthesized using a heat-treatment procedure under various corrosive atmospheres using non-precious carbon black as starting materials. Then, the correlation between discharge capacity, surface area and porosity of the cathode materials was studied. The results indicate that the discharge capacity in Na-O2 cells is linearly correlated with surface area while morphology of the solid discharge product strongly depends on specific surface area and pore size. In addition, studying the kinetics of electrochemical reactions in Na-O2 cells revealed that different sodium oxides (peroxide and superoxide) species are produced during the discharge cycle of the cell.

To further improve the cyclability of Na-O2 cells, a binder-free three dimensional (3D) air electrode was designed and synthesized in the second part of this thesis. The air electrode was composed of vertically grown nitrogen doped carbon nanotubes on carbon paper. The electrochemical tests demonstrated that 3D architecture of the air electrode results in increased discharge capacity by optimizing the utilized area of the electrode material. Moreover, synchrotron-based X-ray absorption spectroscopy was employed to study the failure mechanism of the cell. The results revealed that formation of parasitic carbonate-based side products on the carbon surface increases the charging overpotential of the cell and results in battery failure.

Based on the results from previous parts, a combination of carbonaceous hierarchical 3D structured design with a mesoporous Mn3O4/Pd bifunctional catalyst was employed to prepare the air electrode in the next part. The catalyst layer in this design serves as a protective layer against the oxidation of the carbon surface by highly oxidative environment of the cell and also decreases the charging overpotential. Electrochemical studies showed a stable cycling performance as well as a synergistic catalytic effect in both Na- and Li-O2 cells. Spectroscopic investigations revealed a dynamic electron exchange between Pd and Mn3O4 during the discharge and charge cycles of the cell which is responsible for the synergetic effect. In addition, analysis of the discharge products in both Na- and Li-O2 cells demonstrated that the oxygen-bonding properties of the electrode surface may increase the oxygen-rich phase of the products by stabilizing the superoxide intermediate and hence reduce the charging overpotential of the cell.

In the final part of this study, the influence of oxygen-containing functional groups on the morphology and composition of discharge products in alkali metal-O2 cell is studied. The results suggest that functional groups on the carbon surface play a significant role in choosing either the surface-mediated or the solution-mediate mechanisms for formation of the discharge products. The results presented in this study help to better understand the electrochemical mechanism governing on alkali metal-O2 cells and contribute to improve the performance of the cells.

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