Yoojin Ahn

Advisor: Dr. Meilin Liu

will defend a doctoral thesis entitled,

Development of niobium-based oxide anodes with the Wadsley-Roth shear structure for fast-charging and durable lithium-ion batteries 

On

Monday, April 13 at 12:30 p.m. (EDT)

East Architecture Building, Room 107 

and

 Virtually via MS Teams 

https://teams.microsoft.com/meet/28474003007812?p=7HeY3ekJg08ccpmbda

Meeting ID: 284 740 030 078 12

Passcode: xz924qh3

Committee

Dr. Meilin Liu –  School of Materials Science and Engineering (advisor)

Dr. Matthew McDowell – School of Materials Science and Engineering

Dr. Hamid Garmestani – School of Materials Science and Engineering 

Dr. Preet Singh – School of Materials Science and Engineering 

Dr. Angus Wilkinson – School of Chemistry and Biochemistry

Abstract

    Lithium-ion batteries are among the most promising power sources for portable electronics, robotics, and electric vehicles; however, fast-charging capacity and volumetric energy density remain constrained by conventional anode materials such as graphite and silicon. Graphite is prone to lithium plating at high charging rates and offers limited volumetric capacity, whereas silicon suffers from large volume change, leading to structural instability and safety concerns. These limitations highlight the need for new anode materials capable of sustaining rapid cycling while maintaining structural stability. Among the candidates, niobium-based oxides with Wadsley-Roth shear frameworks have emerged as promising fast-charging anodes due to their block-structured pathways and robust cycling performance.

    This dissertation investigates strategies to optimize and characterize niobium-based oxide anodes, while elucidating the mechanisms governing fast and durable lithium storage. The first objective is to achieve high-capacity, stable Nb2O5 anodes through defect engineering, specifically by embedding the metastable M-Nb2O5 (tetragonal) domains within the stable H-Nb2O5 (monoclinic) matrix. Because M-Nb2O5 typically forms transiently during the phase transition from T-Nb2O5 (orthorhombic) to H-Nb2O5, controlling H/M phase coexistence introduces additional edge-sharing between metal-oxygen octahedra MO6 blocks, facilitates continuous Li-adsorption pathways, and mitigates structural strain, collectively enhancing rate capability and cycling stability.

    The second objective is to tune the Wadsley-Roth shear structure via entropy engineering by incorporating multiple cations to increase configurational entropy. Properly selected multi-cation compositions stabilize 3×4 octahedra block networks, suppress unfavorable metal-oxygen tetrahedra linkages, facilitate smooth lithium-ion transport under high-rate cycling, and mitigate structural changes during cycling. The synergistic effects of multi-cation mixing improve the intrinsic properties of niobium-based oxides, and their individual roles are analyzed to reveal the impact of entropy tuning on energy storage materials.

    Collectively, these two strategies–mixed-phase defect engineering and entropy tuning–are designed to enable niobium-based oxide anodes that combine fast-charging capability with long-term structural integrity, while elucidating the crystallographic and electrochemical principles underlying the performance enhancement. Their electrochemical and structural evolutions are systematically investigated using advanced characterization techniques, including operando measurements. Furthermore, the developed niobium-based oxide materials were demonstrated in practical configurations, such as high-mass loading full cells, flexible batteries, and all-solid-state batteries. Finally, the general methodology presented in this dissertation offers a transferable framework for enhancing the intrinsic properties of materials across a broad range of functional applications.