Macroporous metal oxide battery electrode performance analysis and operando spectroscopy

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Grant, Alex
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University College Cork
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Vast consumption of fossil fuels has catapulted greenhouse gas emissions which have contributed to increasing the global average temperature at a frightening rate. Thankfully, sun, wind and tides have massive potential to provide energy in an environmentally friendly way. However, maintaining these energy sources at the global scale required to realise the transition from fossil fuels to renewable energy is difficult, given that their supply is often location and weather dependent. Fortunately, there is a solution, and energy storage is the key to a green energy future. Currently, battery systems are the most suitable option. These systems can be implemented in large-scale grid storage facilities, electric vehicles (EVs), portable electronics like laptops and mobile phones, right down to fingernail-sized microbatteries for medical devices and hearing aids. Despite decades of research, the demand for increased functionality has grown at such a rate that the development of appropriate technology has proven difficult. Additionally, the materials typically used in production of commercial batteries such as cobalt, graphite and Li are coupled with concerns related to ethics, CO2 emissions, and availability. This thesis delves into three realms, all related to the development and optimisation of battery electrode materials, and efficient approaches to monitor their deterioration. The first realm is optical analysis of opal photonic crystals (PhCs) which are used as templates for battery electrodes. PhCs are a particular type of nanostructure which demonstrate iridescent colours when exposed to light. The observed colours originate from a phenomenon known as the photonic band gap (PBG), the band of frequencies which are reflected from the crystal. The PBG is dependent on the structure and refractive index contrast of the material. Consequently, the PBG can be controlled by changing the structural parameters of the crystal. Opals are a type of PhC which are used as templates for inverse opals (IOs), which have shown promise as battery electrodes due to their high surface area to volume ratio, thin pore walls and interconnectivity, which removes the need for conductive additives and binders. The most fascinating aspect of their potential is the correlation between their structural properties and their optical response since the primary route of battery performance deterioration is through the structural degradation of the battery electrode materials. Opals of different thicknesses can be constructed while the reflected colours remain unchanged once the periodicity of the crystal is maintained. However, the intensities of the reflected frequencies are thickness dependent. The optical response of these materials can therefore be used to determine crystal thickness, and vice versa. Optimising the electrochemical performance of IO (IO) battery electrodes requires formation of an excellent opal template which can be directed and evaluated by this non-destructive optical spectroscopy approach. With a method established for perfecting opal quality established, the second realm explores the electrochemical performance of sustainable battery systems, in the form of SnO2-based electrode materials in lithium-ion (Li-ion)and sodium-ion (Na-ion) chemistries. Li-ion is the most widely used commercial battery chemistry. As Li reserves are depleted, alternatives must be considered. Na stands as a practically infinite resource, extracted easily from sea salt. Na-ion batteries operate under similar principles to Li-ion, but optimisation of appropriate electrode materials has proved challenging. Graphite stands as the gold standard anode material for Li-ion, but its capacity is dramatically reduced in Na-ion systems. Furthermore, the carbon emissions from the processing of natural graphite and production of synthetic graphite are worrying. SnO2 is one of the thirty most abundant elements on earth and stands as a promising replacement for graphite in Na-ion batteries. However, massive volume expansion prevents its widespread adoption. Routes to overcome this volume expansion involve the design of state-of-the-art nanostructures, the route taken in this thesis using the IO architecture. While this electrochemical exploration provides an explanation of the underlying chemistry governing the rate behaviour and stability of the IO electrodes, the structural changes which arise in the electrode during cycling prompt deeper investigations. Furthermore, the rapid nature of the associated electrochemical processes prompt real-time measurements, which are addressed next in the thesis in the form of operando Raman spectroscopy. The third realm is the application of operando methods to battery performance analysis, an approach which litters the literature in recent years. Traditionally, analysis of battery electrodes and the electrolyte were performed ex-situ. The battery was disassembled, the materials were removed and analysed. Ex-situ analysis is useful prior to cycling and for postmortem analysis. A more effective analytical approach is the use of in-situ techniques. These involve stopping the battery at a certain voltage, disassembly, and analysing the electrodes and electrolyte in that state. This approach is useful for determining what changes occur to the battery at different states of charge. However, these materials are often sensitive to the environment, and the in-situ results are not representative of the state of the material prior to disassembly. Furthermore, electrochemical reactions can occur over nanoseconds or even less, while the structural changes can be short-lived. As a result, in-situ analysis can fail to capture many of the changes which occur during cycling. Operando techniques involve analysis of battery components during electrochemical cycling. Disassembly is not required, and structural changes can be tracked continuously, capturing any metastable and ultrashort phases. Operando Raman spectroscopy can be used to monitor structural changes to the electrode at the atomic scale, along with evolution of the electrolyte non-destructively. This thesis has been written to showcase the benefits of combining optical, structural, and electrochemical analysis, moving from ex-situ to operando techniques, and optimising performance of sustainable battery technologies with a view to transitioning from Li-ion to Na-ion chemistry with environmentally friendly electrode materials to develop the first PhC, lab-based operando battery diagnostics system.
Macroporous , Inverse opal , Electrochemistry , Nanomaterials , Optics , Photonic crystals , Spectroscopy , Operando , Li-ion batteries , Na-ion batteries , Batteries
Grant, A. 2023. Macroporous metal oxide battery electrode performance analysis and operando spectroscopy. PhD Thesis, University College Cork.
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