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Structural Engineering of Advanced Cathode Materials for Aqueous Zinc-ion Batteries

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Universitat de Barcelona. Facultat de Física

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[eng] Aqueous zinc ion batteries (AZIBs) have garnered significant research attention due to their remarkably high-volume energy density, reaching up to 5,851 mAh mL-1. This surpasses the capabilities of state-of-the-art lithium-ion batteries (LIBs), making AZIBs a promising candidate for advanced energy storage technology. Additionally, the natural abundance, low cost, and non-toxic nature of zinc offer economic advantages and environmental sustainability, particularly beneficial for large-scale applications. One notable advantage of AZIBs is their ability to be fabricated in an air atmospheric environment, thanks to the air stability of the AZIBs system. This characteristic significantly simplifies the fabrication process, further enhancing the attractiveness of AZIBs for widespread adoption. However, the practical implementation of AZIBs still suffers from several intractable technical challenges, such as limited energy density and inadequate cycle life, which seriously hinder this technology from yielding practically viable energy density and cyclability.
Selecting appropriate cathode materials and implementing rational structural design engineering can effectively overcome the aforementioned challenges. In Chapter 1, I summarize the state of the art on advanced cathode materials for AZIBs and particularly detail structural engineering strategies to achieve high energy density and extended cycle life. 
In Chapter 2, I detail my work on the design and engineering of K+ pre-intercalated MnO2 nanorods (K-MnO2-NR) as an efficient cathode to overcome the limitations of AZIBs. The K-MnO2-NR is synthesized by a facile one-step chemical method with a size of less than 10 nm. Their unique structure provides a large surface area, abundant active sites for ion storage, and a short diffusion path for ion transport. The intercalation of K+ also improves the conductivity of the electrode and stabilizes the tunnel structure. Consequently, this K-MnO2-NR configuration delivers a high capacity of 285 mAh g-1 at 0.1 A g-1, while retaining 222 mAh g-1 at 2 A g-1. Kinetic reaction analysis reveals that even under high charging/discharging rates, ion diffusion-controlled capacity plays a crucial role, which is beneficial for achieving high capacity under such conditions. Assembled pouch cells with K-MnO2-NR also exhibit promising application prospects. This work has been accepted for publication in the journal Ceramics International and it is already available online (https://doi.org/10.1016/j.ceramint.2024.04.324).
However, the capacity of the enhanced MnO2 still falls short of expectations, hampering its practical application. The primary reason for this limitation is that the prepared crystalline MnO2 possess few defects, resulting in a reduced ion storage capacity. Hence, there arises a necessity to devise a novel defect engineering methodology to address this issue and obtain materials with high-density active sites, thereby enhancing their performance.
In Chapter 3, to further improve MnO2-based cathodes, I introduce a method to obtain manganese oxide materials with high-density active sites through the in situ phase transformation of MnSe, thereby regulating the defect structure. I detail my work on the structural engineering of reduced graphene oxide (rGO)-coated MnSe nanoparticles (MnSe@rGO) as a cathode material for AZIBs. The introduction of rGO provides a surface-confining effect against morphological evolution, thus preventing structural failure of the electrode. Furthermore, the intrinsically high electronic conductivity of rGO facilitates the MnSe phase transition, enabling the utilization of its full capacity potential. The optimized MnSe@rGO-3 cathode demonstrates a significant specific capacity of 290 mAh g-1 at 0.1C and retains a specific capacity of 178 mAh g-1 even at 5C. Through quantitative electrochemical analyses, first-principles calculations, and in situ characterization, the enhanced capacitive zinc-ion storage behavior and phase transformation mechanism of MnSe@rGO cathode materials are elucidated. Moreover, the mechanical stability of rGO ensures the successful electrohydrodynamic (EHD) jet printing of flexible ZIBs into a flexible integrated functional system. As an illustration, a flexible touch-controlled light-emitting diode (LED) array system incorporating as-fabricated MnSe@rGO-3-based ZIBs is developed. This approach showcases effective performance in both flat and bent configurations, offering the added advantages of enhanced safety and environmental sustainability. This work was published in ACS Nano in 2023 (https://doi.org/10.1021/acsnano.3c00672).
Despite the significant strides made in enhancing the specific capacity of Mn-based cathode materials through defect engineering, the persisting limitations associated with manganese dissolution and moderate cycle life continue to raise concerns. These issues indeed cast doubt on their viability for high-energy-density applications, particularly in application fields like wearables.
In Chapter 4, to increase the energy density of AZIBs, I explain my work on the development of a new cathode material based on a layered metal chalcogenide (LMC), bismuth telluride (Bi2Te3) nanodisks, coated with polypyrrole (PPy) as cathode material for aqueous ZIBs, and then explore its storage mechanism. In situ X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS) measurements, and density functional theory (DFT) calculations are employed to elucidate that the energy storage mechanism of Bi2Te3 is the insertion/extraction of protons rather than Zn ions within the (0 0 6) interlayers, coupled with the formation/deposition of Zn4SO4(OH)6·5H2O on the electrode surface. The PPy coating enhances the ionic conductivity of the LMC while preventing surface oxidation. Consequently, the Bi2Te3@PPy cathode exhibits remarkable rate performance and long-term cycling stability with ultra-long lifespans of over 5,000 cycles. They also present outstanding stability even under bending. This work was published in Advanced Materials in 2023 (https://doi.org/10.1002/adma.202305128).
Finally, the main conclusions of this thesis, including a comparison chart of the three cathode materials developed in the thesis, and some perspectives for future work are presented.

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[spa] Las baterías de iones de zinc en electrolito acuoso (AZIBs) han atraído notable atención por su excelente densidad volumétrica de energía, alcanzando hasta 5,851 mAh mL-1, superando a las baterías de iones de litio (LIB). Además, el zinc es abundante, económico y no tóxico, lo que beneficia aplicaciones a gran escala. Las AZIBs pueden fabricarse en un ambiente atmosférico, simplificando significativamente el proceso de fabricación. Sin embargo, enfrentan desafíos técnicos como densidad de energía limitada y vida útil corta.
En el Capítulo 1, se revisa el estado del arte sobre materiales catódicos avanzados para AZIBs, y se detallan estrategias para lograr alta densidad de energía y ciclo de vida extendido. En el Capítulo 2, se presenta el diseño e ingeniería de nanobarras de MnO2 preintercaladas con K+ (K-MnO2-NR) como cátodos. Este material, sintetizado mediante un método electroquímico sencillo, ofrece una alta capacidad de 285 mAh g 1 a 0.1 A g-1 y retiene 222 mAh g-1 a 2 A g-1. La intercalación de K+ mejora la conductividad y estabiliza la estructura, proporcionando una gran superficie y sitios activos para el almacenamiento de iones. Este trabajo se ha publicado en International Ceramics.
En el Capítulo 3, se introduce un método para mejorar aún más el cátodo a base de MnO2 mediante la transformación de fase de MnSe, creando materiales con alta densidad de sitios activos. Se diseñaron nanopartículas de MnSe recubiertas con óxido de grafeno reducido (rGO) (MnSe@rGO). El recubrimiento de rGO mejora la conductividad y estabiliza la estructura, evitando fallos estructurales. El cátodo MnSe@rGO-3 demuestra una capacidad específica de 290 mAh g-1 a 0.1 C y retiene 178 mAh g-1 a 5C. Este trabajo fue publicado en ACS Nano.
En el Capítulo 4, se explora un nuevo material catódico basado en nanodiscos de telururo de bismuto (Bi2Te3) recubiertos con polipirrol (PPy) para ZIBs acuosas. Mediante análisis XRD in situ, mediciones XPS y cálculos DFT, se dilucida que el mecanismo de almacenamiento de Bi2Te3 implica la inserción/extracción de protones y la formación de Zn4SO4(OH)6·5H2O. El recubrimiento de PPy mejora la conductividad iónica y previene la oxidación. El cátodo Bi2Te3@PPy exhibe excelente rendimiento y estabilidad a largo plazo, con una vida útil de más de 5,000 ciclos, incluso bajo flexión. Este trabajo fue publicado en Materiales Avanzados.
A pesar de estos avances, persisten desafíos como la disolución del manganeso y la vida útil limitada, cuestionando su viabilidad para aplicaciones de alta densidad de energía. La tesis concluye con una comparación de los tres cátodos desarrollados y ofrece perspectivas para futuros trabajos.

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Structural Engineering of Advanced Cathode Materials for Aqueous Zinc-ion Batteries