Abstract
Lithium-ion batteries are required to meet increased future energy storage demands, but current technologies use flammable liquid electrolytes that raise significant safety concerns. One proposed method of alleviating these concerns is to replace these flammable liquid electrolytes with solid electrolytes. The use of solid electrolytes, however, causes other issues. In particular, solid electrolytes often possess low bulk ionic conductivities and high resistances at inhomogeneities, such as grain boundaries. The development of solid electrolytes could be accelerated by an improved understanding of the fundamental factors and phenomena that influence their ionic transport properties at these inhomogeneities. In this thesis, computational techniques are used to model solid electrolytes. From these models we gain insight into space-charge regions at inhomogeneities in solid electrolytes and the influence of dopant configuration on transport properties.In chapter three, a Poisson-Boltzmann model is used to investigate space-charge formation at three grain boundaries in doped Li3OCl. These calculations suggest space-charge regions will contribute to grain boundary resistance in this lithium-ion solid electrolyte and that predicted space-charge properties can be significantly influenced by features of the grain boundary core.
Poisson-Boltzmann models are the conventional method of modelling space-charge regions in solid electrolytes. This model, however, uses a mean-field treatment of electrostatic interactions that is only accurate for weak particle interaction strengths. In chapter four, to assess how grain boundary space-charge formation changes with electrostatic interaction strength, we perform kinetic Monte Carlo simulations of a simple model system that is analogous to the classical one-component plasma. These simulations reveal that, upon surpassing a critical interaction threshold, space-charge densities, n(z), become damped oscillatory. We compare this emergent phenomenon with similar interfacial behaviour previously observed in fluids.
In fluids, the long-range interfacial behaviour is known to be determined by bulk properties that are characterised by the bulk radial distribution function, g(r).In particular, n(z) and g(r) are thought to share the same long-range behaviour. In chapter five, we use Monte Carlo simulations to further investigate our model system. These simulations indicate that, in our model system, a similar relationship exists between n(z) and g(r), and that this relationship exists regardless of the type of inhomogeneity.
Finally, in chapter six, to investigate the influence of aliovalent dopant configuration on lithium ion transport, we perform molecular dynamics simulations of doped Li3OCl. We show that dopant configuration can have a small impact on measured transport properties. We also propose a simple conceptual model to explain why and how dopant configuration alters the potential energy surface of a solid electrolyte which, in turn, alters ionic transport behaviour.
| Date of Award | 21 Feb 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Benjamin Morgan (Supervisor) & Alison Walker (Supervisor) |