Lu, Yinan; (2025) Development of High-performance Vanadium Oxide Cathodes for Rechargeable Metal-ion Batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Vanadium pentoxide (V2O5) deems as promising cathode candidate for rechargeable metal-ion batteries owing to a layered structure, high chemical and thermal stability, electrochemical safety, low cost, and ease of preparation. As a cost-effective option for cathode materials in both lithium-ion batteries (LIBs) and zinc-ion batteries (ZIBs), V2O5 has attracted great attention due to its high theoretical specific capacity values. However, several challenges impede its practical use, including insufficient practical capacity, sluggish ion diffusion, low intrinsic conductivity, and limited cycling stability. To address these limitations, this thesis has developed strategies including hydrogenation, doping, and defect engineering to improve the electrochemical and optical performance of V2O5-based cathode materials. Through an in- depth examination of the physicochemical characteristics and a detailed study of the charge storage mechanisms, the thesis emphasizes the importance of V2O5 modifications in enhancing battery performance. The findings offer valuable insights for the future development of high- performance, rechargeable metal-ion batteries. This research consists of three projects: enhancing the optical and electrochemical properties of hydrogenated V2O5 for photo- accelerated lithium-ion batteries, designing pre-doped V2O5-based materials for high- performance zinc-ion batteries, and developing cation-doped V2O5 electrodes with engineered oxygen vacancies for lithium-ion batteries. The specific aims and outcomes are summarized below. A detailed summary of the objectives and key results of these three research areas is provided below. (1) Photovoltaic cells integrated with batteries face challenges such as additional ohmic resistance and inefficiencies caused by mismatches in energy transfer among their components. To address these limitations, researchers are exploring the development of photo-accelerated batteries that combine energy harvesting and storage into a single device. This approach could reduce expenses, boost energy conversion efficiency and develop lightweight devices. This work introduces a novel strategy for designing photocathodes using hydrogenated vanadium pentoxide (H:V2O5) nanofibers, which enhances optical activity, electronic conductivity, and ion diffusion rates within photo-accelerated Li-ion batteries. Compared to pristine V2O5, the H:V2O5 showed better electrochemical performance, with a 43% and 41% increase in specific capacity under dark and illuminated 2 conditions, respectively, at a current density of 2000 mA g-1 . Characterization techniques, like energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV-Visible (UV-Vis) spectroscopy, and photoluminescence (PL), revealed that the enhanced photocurrent generation was linked to defect mid-gap states, associated with oxygen-related defects and hydroxyl groups, which increased optical absorption and photoactivity. Moreover, density functional theory (DFT) simulations further confirmed that the presence of oxygen vacancies significantly reduces the energy barrier for Li-ion diffusion. This research holds great promise for the development of high-performance photocathodes in future energy storage applications. (2) Rechargeable ZIBs are gaining significant attention because of their affordability, abundant raw materials, and eco-friendly nature. The use of aqueous electrolytes in ZIBs also offers a substantial safety advantage over the flammable organic electrolytes commonly used in LIBs. Despite these benefits, the development of high-performance cathode materials for ZIBs remains a critical challenge. This work applies cation doping approach to synthesize and evaluate four distinct materials: pristine vanadium oxide (V2O5), sodium-doped vanadium oxide (Na-V2O5), potassium-doped vanadium oxide (K-V2O5), and ammonium- doped vanadium oxide (NH4-V2O5). Transmission electron microscopy (TEM) and X-ray diffraction (XRD) identify the interplanar distance of pristine V2O5 expanded upon cation insertion. Among all samples, the NH4-V2O5 electrode achieved the highest specific capacity, reaching 310.8 mAh g-1 at a current density of 100 mA g-1, along with superior cycling stability. This enhanced performance is attributed to the highest V4+/V5+ redox couple ratio, the pillar effect introduced by ammonium cations, and the increased interlayer spacing. These findings provide important insights for the design and development of advanced cathode materials in the future. (3) Different cations have been explored individually for LIBs, the mechanisms behind cation doping remain complex and not fully elucidated, posing challenges in selecting the most effective cations. In this research, hydrothermal synthesis was employed to create three distinct types of cation-doped V2O5 nanofibers, which were characterized using scanning electron microscopy (SEM) and TEM. The introduction of cations resulted in several beneficial modifications: expanded interlayer spacing, increased concentration of oxygen vacancies, and an elevated V4+/V5+ ratio. These structural and chemical enhancements3 collectively facilitated enhanced Li+ ion diffusion, improved capacity, stable cycling performance, and reduced charge transfer resistance. Among the synthesized materials, NAVO demonstrated outstanding electrochemical performance, achieving discharge capacities of 249.1 and 122.0 mAh g-1 at current densities of 100 and 2000 mA g-1 , respectively. Furthermore, it demonstrated impressive cycling stability, with an initial specific capacity of 222.1 mAh g-1 at 1000 mA g-1, retaining 150.5 mAh g-1 after 30 cycles, corresponding to over 67% capacity retention. This study sheds light on the impact of cation doping on the electrochemical performance in LIBs and paves the way toward the development of lithium-ion batteries with improved energy density, power performance, and cycle life.

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