The importance of energy storage worldwide is increasing with the use of renewable energy sources and electric vehicles. With the intermittent nature of wind and solar power, large-scale grid storage is an extremely important progression needed to reduce the use of fossil fuels. For this to become a reality, rechargeable batteries beyond existing Li-ion technologies need consideration. The development of such batteries requires improvement of understanding their component materials. Modern computer modelling techniques enable valuable insights into the fundamental defect, ion transport and voltage properties of battery materials at the atomic level. Atomistic simulation and ab initio density functional theory (DFT) techniques have been used to study a number of potential cathode materials for Na-ion and Mg batteries. Firstly, the olivine and maricite forms of NaFePO4 are considered in terms of their defect formation energies and Na ion diffusion. The atomistic study indicates that anti-site disorder is the most favourable type of intrinsic defect. The activation energies for Na-ion migration in the olivine and maricite materials are 0.4 eV and 1.6 – 1.8 eV respectively. Moreover, molecular dynamics (MD) studies reveal that there is only substantial Na-ion diffusion in the olivine structure, with diffusion coefficients (DNa) at 300 K of 7 x 10−13 cm2s−1 for maricite and 4 x 10−9 cm2s−1 for olivine NaFePO4. The presence of anti-site defects is shown to decrease Na+ diffusion within the olivine structure, which is of relevance to its rate behaviour. Secondly, the effect of lattice strain on ion transport and defect formation in olivine-type LiFePO4 and NaFePO4 is investigated as a means to enhance their ion conduction properties. It is predicted that lattice strain can have a remarkable effect on the rate performance of olivine cathode materials, with a major increase in ionic conductivity and decrease in blocking defectsat room temperature. Thirdly, DFT techniques have been used to examinesurface and grain boundary formation in P2-NaCoO2. The coordination lossexperienced by ions present at surfaces is found to influence the resultingsurface energy. Layered oxide cathode materials were further investigated byconsidering the effect of Mg2+ doping on P2-Na2 [Ni1 Mn2 ]O2. Na vacancy 333formation energies decreased with 10% Mg2+ doping on the Ni site and an increase in Na diffusion was predicted with MD calculations. This positive effect on Na ion conductivity is caused by displacement of the Mg ions from the transition metal layer and the resulting change in electrostatic potential. Finally, Mg ion conduction, doping and voltage behaviour of MgFeSiO4 were studied. The Mg-ion migration activation energy is relatively low for an olivine-type silicate, and MD simulations predict a diffusion coefficient (DMg) of 10−9 cm2s−1, suggesting favourable electrode kinetics. Partial substitution of Fe by Co or Mn could increase the cell voltage from 2.3 V vs Mg/Mg2+ to 2.8 - 3.0 V.
|Date of Award||2 Jan 2018|
|Supervisor||Saiful Islam (Supervisor)|