PhD Oral Exam - Qihang Yu, Mechanical 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

All-solid-state batteries (ASSBs) have attracted significant attention in recent years due to their enhanced safety, high energy density, and long-term stability compared to conventional liquid electrolyte-based systems. Meanwhile, organic batteries, which utilize redox-active organic compounds as electrode materials, offer additional advantages such as structural tunability, sustainability, and the potential for low-cost, large-scale production. However, their practical application has long been limited by the dissolution of organic materials in liquid electrolytes, leading to rapid capacity fading and poor cycle life. By replacing liquid electrolytes with solid-state electrolytes, this dissolution issue can be fundamentally mitigated, enabling more stable electrochemical performance of organic electrodes. In recent years, the development of all-solid-state lithium organic batteries (ASSLOBs) combining organic active materials with inorganic solid-state electrolytes has emerged as a highly promising direction. These systems not only retain the environmental and structural advantages of organic batteries but also benefit from the mechanical robustness and electrochemical stability of solid electrolytes.

This thesis begins with an in-depth review of the development of lithium-ion batteries, with particular attention to the evolution of electrode materials and electrolyte systems. The first section traces the development in the field, highlighting key progress in both cathode and anode materials, as well as innovations in liquid electrolyte formulation and optimization. The discussion then turns to organic electrode materials, which have attracted growing interest due to their structural tunability, environmental compatibility, and potential for low-cost production. However, their practical deployment remains limited by issues such as poor cycle life and dissolution in liquid electrolytes. To address these challenges, the latter part of the thesis explores the development of solid-state electrolytes, which not only offer improved safety and thermal stability but also effectively suppress the dissolution of organic active materials. Recent progress in integrating inorganic solid-state electrolytes with organic electrodes is examined, revealing a promising pathway that leverages the strengths of both components.

First, compared to oxide solid electrolytes and sulfide solid electrolytes, halide solid electrolytes have attracted increasing attention due to their wider electrochemical stability windows. The first work investigates the compatibility between the organic cathode material indigo and halide-based solid electrolytes. The results demonstrate that, for organic positive electrode materials, the selection of solid-state electrolytes cannot rely solely on the electrochemical stability window of electrolytes. Instead, the interfacial compatibility and specific interactions between the electrolyte and the electrode material must also be carefully considered to ensure stable electrochemical performance.

Second, a bifunctional indigo natural dye was investigated that serves as both an active material and a solid molecular catalyst in sulfide-based ASSBs, addressing these compatibility challenges. Contrary to the prevailing view that chemical reactions between OEMs and sulfide SEs are detrimental, our study reveals that controlled reactions between indigo and Li6PS5Cl (LPSC) SE catalyze their synergistic redox process after optimizing electrode microstructures. This strategy enables a high reversible capacity of 583 mAh g-1 (LPSC contribution: 379 mAh g-1) at 0.1 C, a high areal capacity of 3.84 mAh cm-2, and excellent cycling stability at room temperature. These findings highlight the potential of bifunctional OEMs in sulfide-based ASSBs to overcome the key challenges of OEMs in practical applications.

Subsequently, this work underscores the critical importance of chemical compatibility in achieving optimal battery performance, demonstrating that carefully regulated interfacial reactions between organic electrode materials and sulfide solid electrolytes can catalyze reversible S²⁻ anionic redox processes. This catalytic mechanism enables a high electrode-level energy density of 476.6 Wh kg-1 and stable cycling over 2,000 cycles at room temperature in all-solid-state organic batteries. Mechanistic investigations into the OEM-catalyzed S²⁻ redox chemistry reveal two essential criteria for effective catalysis: (1) the redox potential of the OEM must be appropriately aligned to oxidize S²⁻ species without inducing over-oxidation, and (2) low cation–S²⁻ bond covalency is necessary to localize electron density on sulfur atoms, thereby facilitating reversible redox activity. These insights address key limitations associated with OEMs in all ASSBs and offer a strategic framework for designing next-generation sustainable ASSBs with enhanced energy density and long-term stability.

This thesis investigates the development of all-solid-state lithium organic batteries by exploring the interplay between organic electrode materials and solid-state electrolytes. It highlights the critical role of chemical and interfacial compatibility—beyond electrochemical stability windows—in achieving high energy density and long-term cycling stability.