Colour-coded batteries: electro-photonic materials circuitry for advanced electrochemical energy storage

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Lonergan, Alex
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University College Cork
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The motivation behind this thesis was to develop an optical diagnostic system to assess the performance of lithium-ion battery electrodes. Ideally, this design would accommodate operando analysis; allowing for real-time assessment the of electrode performance during battery operation. Photonic crystals are explored throughout this thesis as a means to develop this system. Photonic crystal materials are ordered, periodic, dielectric structures which can influence the propagation of light. The ordered arrangement of material in these structures creates a photonic bandgap (complete light inhibition) or photonic stopband (partial light inhibition) depending on the refractive index contrast between the constituent materials. The photonic bandgap associated with photonic crystal materials is often compared to the electronic bandgap in semiconductor materials. In semiconductors with an electronic bandgap, the placement of atoms or molecules in the crystal structure creates a periodic potential with allowed electron energy levels for the semiconductor. The principle behind the photonic bandgap in photonic crystals is similar; the arrangement of material creates a periodic dielectric function which can act to inhibit light propagation over a wavelength range. The presence of this photonic bandgap/stopband, alongside its inherent wavelength sensitivity towards the refractive indices of the constituent materials and structural periodicity, is a standout feature of photonic crystals with a range of applications stemming from this optical phenomenon. Photonic crystal structures can be naturally occurring and are responsible for the vibrant colours seen in some opal gemstones, peacock feathers and butterfly wings; the wavelengths reflected by the photonic bandgaps in these ordered structures bring about the structural colour. From a research perspective in manufacturing these materials, some of the simplest photonic crystal designs arise from colloidal crystal templates, such as artificial opals and inverse opals. The periodicity and refractive index contrast is dictated by the order and properties of the template of colloidal particles; this creates a tuneable photonic bandgap for the structure. The possibility of a controllable and selective optical response has created applications for photonic crystals as refractive index sensors, photocatalysts, solar cells and optical waveguides. The wide assortment of materials candidates, porosity and ordered structural template are attractive material features, even outside of optical applications. Electrodes for lithium-ion batteries frequently adopt inverse opal designs to exploit the nanoscale dimensions of the structure. The ambition of this thesis was to create a combination of these applications for lithium-ion battery electrodes. Could a battery design be realised that would incorporate the benefits of a porous, nanoscale structure for the electrode while simultaneously allowing the signature wavelength position of the photonic bandgap to act as an operando material diagnostic? To answer this question, inverse opal structures are characterised in a variety of different operating conditions over the course of this thesis. We primarily focus on TiO2 inverse opals designs due to the numerous studies on the electrochemical and optical properties of the structure available for comparison. In particular, the optics of the inverse opal photonic crystal structure are studied extensively; starting with simple characterisation of the standalone structure before moving on to more complex characterisation in solvent-filled environments or metal particle decorated structures. All of the work presented is in pursuit of establishing the foundation for the optical diagnostic system for the electrode; the optics of the inverse opal needed to be thoroughly understood prior to testing in a battery cell. Ultimately, a proof-of-concept for the operando optical battery diagnostic system is shown for the first time as a culmination of the work in this thesis. The data shows promising results, signalling that operando monitoring of the photonic stopband for TiO2 electrodes can be used to detect changes in the electrode via non-invasive, real-time optical spectroscopy. In principle, this technique, showcased here for TiO2 inverse opals, is versatile and broadly applicable to a wide range of electrode materials, creating exciting opportunities for future material characterisation. Chapter 1 of this thesis contains a literature review which places emphasis on the range of optical applications for photonic crystal materials. Various designs of photonic crystals are utilised in a wide range of research fields. A broad scope of material arrangements is covered in the discussion, including 1D, 2D and 3D photonic crystal designs. This chapter reviews a gamut of photonic crystal materials and their signature optical response in relation to their implementation as refractive index sensors, photocatalysts, solar cells and optical waveguides. Through a consideration of the role of photonic crystal materials across a variety of disciplines, this chapter highlights and provides an understanding of the critical importance of the photonic response of these structures in each application. The concept of structural colouration and the benefits of an ordered, structured template are also explored here. Chapter 2 details the experimental theory and procedure for the various techniques used over the course of the thesis. This chapter is detailed, particularly in the background theory for each sample preparation or analytical technique. The intent of this chapter was to provide a thorough introduction to the key techniques used in the research, targeting those new and unfamiliar to the processes. A broad introduction to the operating principles of each technique is given, alongside a more focussed perspective on the role of these techniques in photonic crystal research. Finally, the general procedure and operating conditions are provided for the work carried out here. Chapter 3 is a results chapter which investigates the optical response of photonic crystal materials in air. The transmission spectra of polystyrene opal templates and TiO2 inverse opals are studied on conductive glass substrates. The optical behaviour of the photonic stopbands for these materials are examined for differing periodicities and angles of incidence using the Bragg-Snell relation. The opal templates conform excellently to the behaviour predicted from this relation. The TiO2 inverse opals deviate significantly from the expected response. A reduced interplanar spacing for the inverse opal is put forward to explain this discrepancy. Chapter 4 contains a comprehensive study of TiO2 and SnO2 inverse opals in solvent-filled environments. The photonic stopbands of these materials are studied with the air voids of the inverse opal filled with various solvents of different refractive indices. The behaviour of the photonic stopbands is compared for the two inverse opal materials. The red-shift in the stopband position is used to calculate a fill factor of crystalline material present in each inverse opal structure, with the SnO2 structure displaying a relatively higher percentage of material compared to the TiO2. Transmission spectra are also recorded with a varying angle of incidence while immersed in a solvent using a custom experimental set-up. This marks the first reported instance of this type of measurement. Chapter 5 explores the optical behaviour of metallo-dielectric photonic crystals (photonic crystals with metal particles included in the structure). The transmission spectra of polystyrene opal and TiO2 inverse opal samples are examined before and after metallisation of the surfaces of the structures with copper, nickel or gold particles. In particular, the photonic stopbands of the materials are studied. A blue-shift in the photonic stopband position is observed for every material with a layer of metal particles incorporated into the structure. The amount, placement and type of metal particle are shown to be related to the reported blue-shift. Chapter 6 presents the results obtained for the operando measurements of the photonic stopband for TiO2 inverse opal electrodes in a lithium-ion battery cell. The results of this chapter represent a new technique which has not been reported on before. A custom flooded-cell design and metal-coated glass electrode are used to record measurements. Through operando monitoring of the electrode several interesting optical effects are discovered. There is an irreversible decrease in transmission linked to the first discharge only, possibly related to the SEI layer. The photonic stopband breaks down and reforms over the course of the discharge/charge process at voltages linked to lithium insertion/removal. A gradual red-shift of the stopband position occurs over time, indicating long-term material changes.
Photonic crystals , Optical transmission spectroscopy , Inverse opal optics , Sol-gel film preparation , Optical thin films , Electrochemical techniques , Operando analysis techniques , Lithium-ion battery materials , Nanostructured materials , Photonic bandgap
Lonergan, A. T. 2021. Colour-coded batteries: electro-photonic materials circuitry for advanced electrochemical energy storage. PhD Thesis, University College Cork.