Providing cleaner sources of energy will require significant improvements to the solid-state materials available for energy storage and conversion technologies. Rechargeable lithium and sodium batteries are generally regarded as the best available candidates for future energy storage applications, particularly with regard to implementation within hybrid or fully electric vehicles, due to their high energy density. However, production of the next generation of rechargeable batteries will require significant improvements in the materials available for the cathode, anode and electrolyte. Modern computer modelling techniques enable valuable insights into the fundamental defect, ion transport and voltage properties of battery materials at the atomic level. Polyanionic framework materials are being investigated as alternative cathodes to LiCoO2 in Li-ion batteries largely due to their greater stability, cost and environmental benefits. In this thesis, four types of polyanion materials are examined using computational techniques. Firstly, the pyrophosphate material, Li2FeP2O7 is investigated, which has the highest voltage (3.5 V) for an iron-based phosphate cathode. In this pyrophosphate material the anti-site defect in which the Li+ and Fe2+ cations exchange positions is the intrinsic defect type found with the lowest energy. Lithium ion diffusion will follow non-linear, curved paths in the b-axis and c-axis directions, which show low migration energies. Hence, in contrast to 1D diffusion in LiFePO4, fast Li+ transport in Li2FeP2O7 is predicted to be through a 2D network in the bc-plane, which is important for good rate capability and for the function of particles without nano-sizing. Favourable doping is found for Na+ on the Li+ site, and isovalent dopants (e.g., Mn2+, Co2+, Cu2+) on the Fe2+ site; the latter could be used in attempts to increase the Fe2+/Fe3+ redox potential towards 4V. Secondly, the relative abundance and low cost associated with Na-ion batteries now make them an attractive alternative for large-scale grid storage. Therefore, defect chemistry and ion migration results are presented for the sodium-based pyrophosphate framework, Na2MP2O7 (where M = Fe, Mn). Formation energies for Na/M ion exchange are found to be higher than Li/Fe exchange, which has been related to the larger size of the Na ion compared to the Li ion. Low activation energies are found for long-range diffusion in all crystallographic directions in Na2MP2O7 suggesting three-dimensional (3D) Na-diffusion. Thirdly, the search for high voltage cathodes for lithium-ion batteries has led to recent interest in the Li2Fe(SO4)2 material which has a voltage of 3.83 V vs lithium, the highest recorded for a fluorine-free iron-based compound. Ion conduction paths through the Li2M(SO4)2 (M = Fe, Mn, Co) marinite family of cathode materials, show low activation energies for lithium migration along the a-axis channels giving rise to long-range 1D diffusion, supported by molecular dynamics (MD) simulations. Density functional theory (DFT) simulations were used to reproduce the observed high voltage of Li2Fe(SO4)2 and to make predictions of the voltages of both Li2Mn(SO4)2 and Li2Co(SO4)2, and also examine local structural distortions on lithium extraction. Finally, the layered and tavorite polymorphs of LiFeSO4OH have recently attracted interest as sustainable cathode materials offering low temperature synthesis routes. Using DFT techniques the experimental voltage and structural parameters are accurately reproduced for the tavorite polymorph. An important result for the layered structure, is that similar accuracy in both cell voltage and structure can only be obtained if a van der Waals functional is included in the DFT methodology to account for the inter-layer binding.
|Date of Award||2 Jun 2014|
|Supervisor||Saiful Islam (Supervisor) & Paul Raithby (Supervisor)|