Li-ion batteries have redefined expectations for consumer electronics and are the key technology in enabling the electrification of road transport. Their contribution to decarbonisation only stands to increase in the coming decades as demand for Li-ion batteries continues to soar. However, as production continues to accelerate, maximising device-level level sustainability over the lifetime of cells, modules, and battery packs is of increasing importance. Currently, there is a broad suite of techniques available in laboratory settings for the analysis of Li-ion battery performance and degradation during operation. Studying batteries using these techniques has led to the design of safer, longer-lasting batteries over the past few decades. However, most of these techniques use large and/or expensive instrumentation or require the design of bespoke cells and are therefore not applicable as methods for monitoring commercial battery systems in the field. Currently, only a small collection of non-disruptive diagnostic techniques are available for these applications. In addition, these techniques generally measure broad metrics, which are extrapolated using statistical methods to estimate and predict cell performance, often resulting in systematic underutilisation of battery systems.

Optical techniques offer a potential solution to this problem owing to their low-cost, facile integration as part of commercial battery systems and their potential to monitor a range of mechanical, thermal, and chemical parameters through the application of optical fibres. By providing specific and detailed data in real-time, optical techniques coupled with a smart battery management system have the potential to improve battery lifetimes, reduced safety risks, and facilitate more advanced characterisation ahead of end-of-life protocols (e.g., re-use, recycling, etc).

In this thesis, we discuss the development of a novel optical sensing platform for monitoring [Li+] in battery electrolytes. The first chapter introduces key concepts around the basic operation of Li-ion batteries and the degradation processes that lead to loss of performance during their operation. Following this, the range of in situ and in operando techniques that have facilitated our current understanding of processes occurring inside Li-ion batteries are discussed as well as the state-of-the-art in the application of optical techniques. This chapter ends with a summary of the thesis and its objectives and serves as an introduction to the context in which this work was originally formulated.

The second chapter begins with an introduction to the field of optically active organic molecules, ionophores, and their convergence to form highly selective and sensitive optical chemosensors. Subsequently, the design, synthesis, and characterisation of two novel Li+-selective fluorescent chemosensors based on a naphthalene diimide core are described. Although both displayed poor solubility in salt-solvent systems reflective of the conditions in a Li-ion battery electrolyte, they were deemed to be a promising starting point for the development of a solid-supported chemosensor-based optical sensing platform for our applications.

Chapter III starts with a summary of the fabrication and operation principles of planar and nanoparticle optodes as well as their application as part of optical fibre sensors. Following this is a discussion of potential strategies for the immobilisation of a chemosensor onto a solid support for applications as an optode sensor. Subsequently, the chapter describes the optimisation of experimental parameters towards the development of a fabrication procedure for planar optodes functionalised with one of the novel chemosensors synthesised in Chapter II. The final planar optodes used a silica-based substrate loaded with a mesoporous surface film, which is functionalised with a modified derivative of the chemosensor. Characterisation of the planar optodes demonstrated their promising properties for monitoring [Li+] selectively in concentration ranges and solution conditions relevant to Li-ion battery electrolytes. This chapter also summarises attempts to apply the same fabrication procedure for the functionalisation of optical fibres to form ‘dip probes.’ However, these were largely unsuccessful.

To further characterise the properties of the optode sensor in emission mode, the planar optodes were integrated as part of microfluidic devices. This provided precise control over liquid samples on the optode surface and enabled full characterisation of their Li+-sensing capabilities in emission mode. Chapter IV provides a summary of this data, which showed the planar optodes to possess the necessary sensitivity and selectivity for application as an emission mode concentration sensor in battery electrolytes. In addition, the platform was shown to demonstrate strong cyclability, a rapid time response, and a level of spatial resolution that suggested the optodes could be applied for dynamic chemical imaging using fluorescence microscopy.

Chapter V is the final chapter and describes the application of the optode-based microfluidic devices for two proof-of-concept experiments. Firstly, ex-situ [Li+] measurements were taken using the devices on a pristine electrolyte and electrolyte extracted from a Li-ion pouch cell after cycling. These results provided a quantitative indication of bulk Li+ depletion in the electrolyte during cycling with a comparable level of accuracy to ICP-OES measurements run in parallel. Secondly, simple experimental conditions were designed to establish the ability of the optodes to track the evolution of Li+ concentration gradients in solution with time. These experiments yielded the first recorded example of spatial Li+ tracking using a solid-supported optical sensor to our knowledge. Modelling of the diffusion data showed that the experimental results could provide a comparative estimation of the Li+ self-diffusion coefficients for different solvent systems.