PhD Oral Exam - Ximeng Zhang, 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

Lithium-ion batteries (LIBs) underpin today’s mobile and stationary storage, but cathodes based on transition-metal oxides face material-supply, cost, and sustainability constraints. Organic cathode materials (OCMs), composed of earth-abundant elements, offer low-temperature processing, structural diversity, and molecule-level tunability. Most prior progress has focused on carbonyl systems; by comparison, nitrogen-containing organics (azo, nitroso, and nitro aromatics) remain less explored even though a variation of their functional groups allows tuning of redox potential, kinetics, and intermolecular interactions. This thesis targets that gap by examining nitrogen-rich small molecules as LIB cathodes, identifying the main limits to performance, and developing practical routes to stabilize cycling.

The work begins with a critical review of working principles of LIBs including the development of electrodes and electrolytes. As promising cathode candidates, OCMs are reviewed with emphasis on their major classes and charge-storage mechanisms; we also consolidate strategies, including spanning molecular design, electrode and separator modification, and electrolyte engineering, that improve performance and advance OCMs toward practical use. Throughout the experimental chapters, electrochemical testing (cyclic voltammetry, galvanostatic charge–discharge, impedance spectroscopy) is coupled with structural ex situ characterization (Fourier-transform infrared (FTIR), Raman, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS)) to track functional-group evolution and to connect spectral changes to voltage profiles, polarization, and Coulombic efficiency. Three types of nitrogen-based organic materials have been subsequently studied and evaluated for their electrochemical performances as OCMs for LIBs.

Firstly, commercially available azo dye materials were surveyed as OCMs, with methyl orange (MO) as a low-cost lead candidate. Ex situ XRD, Raman, FTIR, and XPS confirm reversible lithiation/delithiation centered on the –N=N– group. To curb dissolution and enhance conductivity, MO was confined within porous conductive carbon (BP2000). The MO@BP2000 composite showed improved cyclic stability and rate response, delivering an initial 150 mAh g–1 and retaining 121 mAh g–1 after 50 cycles at 30 mA g–1.

Secondly, nitroso aromatics were probed to clarify their charge-storage pathways. Five representatives, including nitrosobenzene, 2-nitrosotoluene, 4-nitrosobenzoic acid and its lithium salt, and polymeric 1,4-dinitrosobenzene (DNSB), were examined. Ex situ FTIR/Raman/XPS revealed a common electrochemical reaction route: nitroso units dimerize and evolve toward azo-type linkages; small molecules convert rapidly and irreversibly, whereas DNSB proceeds gradually via azoxy intermediates with growing conjugation and slower kinetics. Encapsulation of DNSB in high-surface-area carbons (BP2000, Ketjenblack (KB)) improves utilization and interface stability. The DNSB@KB composite reaches a discharge capacity of 383 mAh g–1 initially and retains 102 mAh g–1 after 800 cycles at 150 mA g–1; impedance analysis indicates reduced charge-transfer resistance and more stable interfaces.

Thirdly, two stabilization strategies were evaluated for mitigating dissolution and improving the cycling durability for a promising cathode material, p-dinitrobenzene (p-DNB): (i) replacing the conventional liquid electrolyte with a poly(ethylene oxide)–based solid polymer electrolyte (PEO–LiTFSI) and (ii) applying poly(vinyl alcohol) (PVA) surface coatings on slurry-cast electrodes. The PEO–LiTFSI electrolyte enables acceptable capacity only at elevated temperature owing to sufficient ionic conductivity, but it fails to suppress thermally driven dissolution. In contrast, PVA surface coatings effectively mitigate dissolution in liquid electrolyte and enhance cycling stability at room temperature. With an optimized coating, p-DNB delivers 178 mAh g–1 after 150 cycles at 0.2 C. Electrochemical impedance spectroscopy reveals that excessively thick coatings increase interfacial resistance, highlighting the trade-off between interfacial protection and ion transport.

Overall, this thesis establishes a comprehensive understanding of how organic cathodes, with a particular focus on nitrogen-containing systems (azo, nitroso, and nitro aromatics), can be advanced toward practical application. It delineates the principal limitations that still hinder their deployment, including dissolution, intrinsically low conductivity, interfacial instability, and limited volumetric energy. At the same time, it demonstrates that strategies such as salt formation, controlled polymerization, carbon confinement, protective surface coatings, and electrolyte optimization can improve their electrochemical performances.