Said, Samia; (2024) Understanding the cycling and degradation of alloy anodes for Li and Na-ion batteries with in-operando microscopy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The unique properties of ultra-thin two-dimensional (2D) and one-dimensional (1D) materials extracted from Van der Waals (vdW) layered structures have catalyzed a revolution in material science. These materials offer exceptional electrical/ionic conductivity, high specific surface areas (SSA), significantly high theoretical capacities, and therefore have garnered particular attention in energy storage applications. Among these, black phosphorus (BP) and its nanomaterials have gained traction for their extraordinary structural and electrochemical properties, becoming focal points of research in lithium-ion and sodium-ion batteries (LIB/NIB). This thesis reviews recent literature on the synthesis, properties, and application of ultra-thin BP-based electrode materials, highlighting their high theoretical capacity, mechanical strength, and good ion conductivity. However, challenges such as low electronic conductivity and significant volume changes during charge/discharge cycles restrict their practical applicability. Nanosizing BP and/or integrating it with stable carbonaceous materials like graphene, reduced graphene oxide (rGO), or carbon nanotubes (CNTs) show promise in improving electronic conductivity and mitigating volume expansion. Yet, beyond this basic understanding, there is limited fundamental insight into how these structures result in improved electrochemical performance. Additionally, the performance of BP-based anodes can vary due to differences in preparation methods. This significantly hinders the optimisation and commercialisation prospects of BP-based anodes. Therefore, understanding the interplay between preparation techniques, morphology, composite structure, and energy storage capabilities of BP-based electrodes are essential for advancing their practical applications. Furthermore, the impact of electrolytes and binders on the energy storage performance of BP-based electrodes remains inadequately explored. This thesis investigates the morphological, mechanical, and chemical changes in BP-based anode materials during electrochemical cycling, utilizing advanced characterization techniques such as operando electrochemical atomic force microscopy (EC-AFM), corroborated with ex-situ Raman spectroscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and x-ray photoelectron spectroscopy (XPS). Operando EC-AFM reveals the formation and evolution of the solid-electrolyte interphase (SEI) layer and severe material degradation mechanisms during intercalation/alloying even for nano-sized and hybridized BP, emphasizing the need for more effective stabilization strategies. The research also focuses on phosphorene nanoribbons (PNRs), showing the very first visual evidence of the sodiation mechanism of PNRs. This work demonstrated their improved stability and mechanical robustness compared to BP, withstanding alloying-induced volume expansions more effectively. As far as we are aware, this is the first direct evidence that confirms the predicted superior electrochemical performance of PNRs. The detailed understanding gained from this study marks a milestone in PNR research but also lays a foundation for harnessing the unique properties of these nanoribbons into practical applications. Overall, this thesis provides valuable insights into the dynamic electrochemical behaviour of BP-based anode materials, offering guidance for optimizing electrode design and improving battery performance. The results underscore the necessity for continued research to bridge the gap between laboratory conditions and practical applicability, advancing the development of next-generation energy storage devices.

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