Energy storage technologies, in particular second-generation rechargeable batteries, are instrumental in addressing the current global demand for clean and sustainable energy. The progress of portable electronics, electric and hybrid-electric vehicles and large-scale grid storage depends on the available technology of rechargeable batteries. Innovation requires a fundamental atomicscale understanding of the properties of the constituent battery materials. Critical to a battery's performance is the electronic and ionic conductivity of the material. Poor ion transport leads to poor rate capability, practical capacity and cyclability. The current research into particle diffusion is predominantly based on single-particle potential energy calculations. Such an approach neglects not only the entropic contribution to diffusion, but the contributions of the collective dynamics which are present in a many particle configuration Using two novel, state-of-the-art enhanced sampling techniques, the 'Shooter' method' and metadynamics, particle diffusion within a many particle system is analysed. The 'Shooter' method is able to connect single particle translocation events into a general particle diffusion mechanism and elucidate diffusion pathways that are otherwise disregarded by single-particle potential energy calculations. This is achieved under consideration of all degrees of freedom, explicitly allowing for local structure changes and lattice dynamics. The application of metadynamics simulations to battery materials is shown for the first time. Using novel collective variables to distinguish between nondiffusive and diffusive regimes, the free energy barrier for diffusion is calculated, therefore considering the entropic contribution. The free energy surface is also reconstructed, highlighting the complex nature of particle diffusion with the olivine phosphates. Using the two novel approaches, the short-time evolution of disorder within the system can be followed and allows for the characterisation of highly complex and correlated translocation events. Additionally,rare events such as twodimensional diffusion and the formation of antisite defects are observed on nominal simulation timeframes. In summary, several novel computational approaches to analysing particle diffusion within battery materials have been developed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:768037 |
| Date | January 2018 |
| Creators | Flack, Timothy |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/118525/ |
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