Abstract
Traditional lithium-ion batteries (LIBs) possess several issues regarding their safety, performance, and sustainability owing to their use of liquid electrolytes based upon flammable organic solvents. Consequently, there is growing interest in replacing the liquid component with a solid-state counterpart as a strategy to mitigate these flaws. Solid polymer electrolytes (SPEs) have gained considerable attention as alternative electrolyte materials for LIBs due to their low cost of manufacture, greater mechanical integrity, and solvent-free nature. The gold-standard material for SPE applications is poly(ethylene oxide) (PEO) due to its high ionic conductivity and ability to solvate lithium ions. However, PEO-based SPEs possess several shortcomings which hinder their practical application including low ambient temperature ionic conductivity, poor mechanical strength at elevated temperatures, and low lithium-ion transference numbers. Furthermore, there are concerns regarding the sustainability of PEO as it is currently fossil fuel derived. Therefore, it is necessary to design alternative host materials which can replace PEO to advance the field of SPEs for the next generation of rechargeable LIBs.The objective of the work that will be presented in this thesis is to develop high performance polymer electrolyte materials derived from renewable feedstocks. Sugar-based polymers have been identified as a promising platform for the development of SPEs due to their high oxygen content to promote coordination and dissolution of lithium salts. The structure and properties can be easily varied to explore a wide chemical space (e.g Tg, crystallinity, pendant functional groups, crosslinked networks) and tailor the properties of the materials towards the application. Finally, sugar-based polymers are non-toxic and can exhibit (bio)degradability which could facilitate the recycling of battery technologies and recovery of precious elements involved in their manufacture.
Chapter 2 of this thesis describes the derivatisation of a sugar-based oxazolidine-2-thione (OZT) as a novel strategy to overcome the functionalisation limitations of oxygenated renewable polymers. This simple methodology involves exploiting the reactivity of nucleophilic sulphur centre of the OZT group with alkyl bromides to introduce pendant functional groups either pre- or post polymerisation. In total, seven novel bio-derived monomers with different pendant functionalities were synthesised starting from D-xylose. Photo-initiated “thiol-ene” co-polymerisation of these monomers with dithiols yielded functional poly(ester-thioethers) with a broad spectrum of properties. Lastly, from a single non-functionalised OZT polymer, divergent post-polymerisation modification approaches were possible to functionalise the materials with different pendant groups.
Chapter 3 explores the application of the co-polymers described in Chapter 2 as solid polymer electrolytes for lithium-ion batteries. By utilising an in-situ “thiol-ene” co-polymerisation with a small quantity of a trifunctional thiol, the polymers could be crosslinked to prepare thin, transparent, flexible SPE films. Thermal analysis found the materials to be entirely amorphous and suitable thermal stability up to ~200 °C. Electrochemical characterisation revealed a maximum ionic conductivity of 2.6 x 10–5 S cm–1 at 80 °C which could be improved to 5.3 x 10–5 S cm–1 by incorporation of flexible pendant groups on to the OZT sugar core. Finally, these materials exhibited hydrolytic degradability and the potential to recover their mechanical properties after stress is applied.
Chapter 4 investigates the use of sugar-derived polyethers synthesised from a xylose-oxetane platform as polymer electrolyte. It was quickly found that these polymers were unsuitable for electrolyte applications, without modification, due to their high Tg, semi-crystallinity, and poor mechanical integrity. Therefore, alternative strategies were employed to prepare materials from these polymers. Firstly, co-polymerisation with a poly(ethylene glycol) (PEG) macroinitiator was investigated to prepare ABA-type triblock block co-polymers which imparted film forming properties and greater mechanical integrity. The resulting polymer electrolyte prepared from this material exhibited a high ionic conductivity of 1.1 x 10–4 S cm–1 at 80 °C and good electrochemical stability up to 4.8 V vs. Li/Li+. The second strategy involved post-polymerisation modification of the polyethers by deprotection and crosslinking with difunctional boronic acids to yield gel polymer electrolytes (GPEs). Electrochemical characterisation revealed that the gels displayed excellent room temperature ionic conductivity of 3.7 10–3 S cm–1 and remarkably high lithium-ion transference numbers (t+) = 0.88–0.92 owing to the presence of anion-trapping boron moieties in their structure.
Finally, Chapter 5 details efforts towards the preparation of cyclic OZT monomers
for ring-opening polymerisation (ROP). The primary aim of this work was to prepare an OZT-oxetane derivative which could then be employed in co-polymerisations with a variety of co-monomers however, synthesis of the monomer proved challenging and could only be isolated in diminished yields. Preliminary studies into the homopolymerisation and co-polymerisation with cyclic anhydrides were explored demonstrating the potential to use this platform to prepare functional polyether and polyester materials. As such, it is hoped that this work will provide a basis for future research projects.
| Date of Award | 2 Oct 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Antoine Buchard (Supervisor) & Frank Marken (Supervisor) |