Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Sun, Xueliang

Abstract

Na-O2 batteries are considered as the promising candidates for electric vehicles due to their ultrahigh theoretical energy densities. However, state-of-the-art Na-O2 batteries suffer from serious challenges including 1) pore clogging and insufficient O2 transportation within the air electrode; 2) degradation of air electrode, 3) Na dendrite growth; and 4) Na corrosion induced by O2/O2- crossover. This thesis, therefore, focuses on rational design of cell configurations to address these problems and understanding the insight mechanisms.

3D printing of “O2 breathable” air electrodes for Na-O2 batteries were first developed. The unique air electrode structure features non-competitive pathways for O2, electrons, and Na+, leading to high-capacity and long-life Na-O2 batteries.

Except for air electrode, it is found that the Na anode also plays crucial roles in determining the Na-O2 cell performance. To prevent the Na dendrite growth and Na degradation induced by O2/O2- crossover, novel Na-O2 batteries using carbon paper protected Na anode was first designed. Besides, hybrid solid-state Na-O2 batteries based on solid-state electrolytes and protected Na anodes were successfully developed.

Furthermore, the cycling stability of Na@alucone anode was investigated. It is revealed that the chemical stability of Na protective layer against O2- is critical to the cycling stability of Na anode, and a universal approach was proposed to achieve high-performance Na-O2 batteries.

A flame-resistant, dendrite-free and O2/O2-- impermeable composite electrolyte was fabricated, both safety and electrochemical performance of Na-O2 batteries can be enhanced significantly.

In summary, the discoveries in this thesis provide important guidance to achieve high-performance Na-O2 batteries.

Summary for Lay Audience

With the depletion of fossil fuels and increasing environmental concerns, development of green and sustainable energy-storage systems has never been more urgent. Rechargeable Na-O2 batteries have aroused substantial attention due to their high theoretical energy densities, high energy efficiency, and abundance of sodium. However, state-of-the-art Na-O2 batteries suffer from key challenges that hinder their practical application, such as 1) pore clogging and insufficient O2 transportation within the air electrode, 2) degradation of air electrode, 3) Na dendrite growth and resultant cell short circuiting, and 4) Na corrosion induced by O2/O2- crossover. The thesis mainly focuses on addressing the challenges of Na-O2 batteries via rational design of cell configurations, as well as clarifying the underlying degradation mechanism of Na-O2 cell performance.

Freestanding “O2 breathable” air electrodes for Na-O2 batteries were first fabricated using 3D printing technique. The designed air electrode has a stacked mesh structure with macroscale open pores and conductive filaments composed of reduced graphene oxide sheets. The unique air electrode structure features separated pathways for O2, electrons, and electrolyte (Na+), leading to high-capacity and long-life Na-O2 batteries.

Except for air electrode, it is found that the Na anode also plays important roles in determining the Na-O2 cell performance. Novel Na-O2 batteries using electrically connected carbon paper and Na metal as protected Na anode were constructed, in order to improve the Na-O2 cell performance. To address the issues of Na dendrite growth and O2/O2- crossover comprehensively, hybrid solid-state Na-O2 batteries based on solid-state electrolytes and protected Na anodes were successfully developed.

Furthermore, by investigating the electrochemical behavior of alucone protected Na (Na@alucone) anode in Na-O2 batteries, it is revealed that the chemical instability of the Na protective layer against superoxide radical can be fatal to the Na-O2 cell performance. More importantly, a universal strategy was proposed to eliminate superoxide crossover for efficient Na dendrite suppressing film.

A flame-resistant, dendrite-free and O2/O2-- impermeable composite electrolyte was developed to construct quasi-solid-state Na-O2 battery. Benefiting from the merits of composite electrolyte, both safety and electrochemical performance of Na-O2 batteries can be improved significantly.

In summary, the strategies developed in this thesis provide important guidance to achieve high-performance superoxide-based Na-O2 batteries.

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