PhD Oral Exam - Zhao Yang, Chemical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Driven by the increasing demand for advanced energy storage technologies, it is critical to develop alternatives to traditional lithium-ion batteries (LIBs), which suffer from limitations in theoretical capacity, cost, performance, and safety. Among various candidates, lithium-sulfur (Li-S) batteries have attracted considerable interest because of their exceptional theoretical energy density (≈2600 Wh kg-1), the low cost, and the inherent environmental friendliness of sulfur as an active material. However, despite these advantages, the practical development of Li-S batteries is constrained by the intricate multi-interface challenges inside the sulfur cathode, resulting in sluggish sulfur redox kinetics, severe volume fluctuations during the S8/Li2S conversion, polysulfide dissolution in liquid-state systems, poor interfacial contact and detrimental solid-state electrolyte decomposition related to solid-state systems. Therefore, a comprehensive understanding of interfacial processes and the elaborate design of stable interfacial structures are fundamental to advancing Li-S battery technology.

This thesis begins with a comprehensive review of recent progress on the interfacial engineering of sulfur cathodes in Li-S batteries from liquid-state to solid-state systems. Next, three interfacial stabilized strategies of sulfur cathodes are designed to promote the electrochemical performance of Li-S batteries. Advanced characterization techniques were utilized, including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, X-ray adsorption spectroscopy (XAS), high-resolution transmission electron microscopy (HRTEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS).

In liquid system, a multifunctional self-healing poly(hindered urea) (PHU) polymer coating is developed to suppress polysulfide shuttling and accommodate large volume changes. The dynamic hindered-urea network provides self-healing property, chemical anchoring and stress-relaxation capability, enabling improved sulfur utilization and durable cycling, with high areal capacities maintained under practical sulfur loadings.

Following this, a dual-interface-dominant architecture that is realized through carbon host nanostructure engineering was presented in all-solid-state Li-S batteries (ASSLSBs). By stabilizing the sulfide solid electrolyte, mitigating the buildup of insulating by-products, accelerating sulfur redox kinetics, and enhancing sulfur utilization, ASSLSBs achieve a high initial capacity of 1111 mAh g-1 at 0.2 C with a sulfur loading of 5 mg cm-2, and exhibit long-term cycling stability, retaining 1234 mAh g-1 (93.3%) over 100 cycles at 0.1 C rate.

Finally, a novel PHU-engineered self-healing sulfur cathode is further demonstrated for ASSLSBs. The elastic, self-healing interfacial layer preserves structural integrity, inhibits solid electrolyte decomposition, and stabilizes long-term charge-transport pathways, enabling high reversible capacities and extended cycling stability of ASSLSBs.

Collectively, this thesis establishes interfacial engineering strategies that address the mechanochemical and electrochemical challenges of sulfur cathodes, offering guidance for the practical realization of high-energy Li-S batteries in both liquid and solid-state configurations.