Abstract
dc:description.abstractThe transition to a zero-carbon society will require a new generation of energy storage technology with high energy density. Central to this are novel electrode materials that possess high specific capacities (mAh g−1) and long cycling lifetimes. Modern lithium-ion batteries utilise graphite as an anode, being a cheap, safe and stable material. Sodium-ion batteries, a promising alternative to their lithium counterpart, utilise hard-carbon as an anode, a disordered matrix of graphene nano-sheets, due to the inability of sodium to bind effectively with pure graphite. However, the theoretical specific capacity of carbonaceous materials (372 mAh g−1 and 300 mAh g−1 for Li in graphite and Na in hard carbon, respectively) is low compared to other materials such as phosphorus (2596 mAh g−1 ) or silicon (3579 mAh g−1). Materials such as these come with their own caveats. High volume expansions, chemical instability, cost, or conductivity problems are a few potential issues encountered with Si or P anodes. Alternatively, chemical enhancement of graphite is considered an effective way of modifying the electrochemical ion-storage properties whilst retaining a high level of stability. Heteroatom substitution of carbon in a graphite lattice has been shown to produce high-capacity anode materials suitable for Li- and Na-ion batteries. This thesis develops an understanding of the structure and function of element-doped graphite within Li- and Na-ion batteries. Turbostratic doped graphitic materials were produced through pyrolysis of organic material. In Chapter 2, investigations into nitrogen-doping are made. Alterations to the synthetic procedure in combination with thorough structural and compositional analysis helps in understanding what factors make nitrogen-doped graphite effective as an anode. Notably, it was found that pyrolysis of organic precursors produced dense, solid spheres, previously thought to be hollow. Use of X-ray photoelectron spectroscopy, combined with ion-etching, revealed how the nitrogen dopant environment varied with increasing depth, and across annealing parameters. The effect of these environments on electrochemical performance could then be assessed. Chapter 3 focuses on boron as a dopant. The electron deficiency of boron compared to carbon is predicted to aid electron transfer, and subsequently facilitate intercalation of Li+ or Na+. Literature on boron-doped graphite largely considers systematic changes to the boron quantity present or focuses solely on applications. However, there is some debate about whether different precursors affect the final structure and performance of the graphite. In this chapter, a particular focus is on investigating the ability of different precursors to produce substitutionally-incorporated B-doped graphite. 11B solid-state nuclear magnetic resonance spectroscopy (SSNMR) combined with structural analysis are used to identify the phases boron is present in and how this is related to performance in Li- and Na-ion batteries. Relating the boron environments present to the voltage at which battery capacity is observed allows for the interpretation of how boron doping affects and facilitates the intercalation of Li+ and Na+ ions. In Chapter 4, an in-depth study of the function of phosphorus-doped graphite is presented. Ex-situ 31P SSNMR spectroscopic studies of doped-graphites, partially cycled to different voltages in Li- and Na-ion cells, led to proposed mechanisms of lithiation or sodiation. Furthermore, alternations to the synthetic procedure allowed reliable encapsulation of white phosphorus between graphene layers, enabling effective, reversible cycling of red phosphorus without degradation.
Degree
thesis:*- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2023
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Clark, Cassius
- Advisors dc:contributor.advisor
-
- Wright, Dominic
- Grey, Clare
Subjects
dc:subject × 15Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.96537
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/349478