Abstract
This thesis describes the application of solid state synthetic and characterisation techniques to the detailed analysis of potential solid electrolyte materials for lithium ion battery applications. The focus of this study is fast-ion conduction in lithium containing garnets and the understanding of the factors affecting the structure and properties of these materials. The phases studied during this project can be separated into two groups, the cubic garnet materials related to Li5Ln3Mv2O12 (Ln = La, Pr, Nd, Sm, Eu; Mv = Sb, Nb, Ta) and the tetragonal phases, related to Li7Ln3MIV2O12 (Ln = La, Nd; MIV = Zr, Sn). Synthetic studies indicated that a range of cubic garnet compositions are stable. All the cubic phases studied are found to show fast lithium ion conductivity. Doping on the Ln site to increase Li content was successful and accordingly improved the conductivity. Doping up to x = 1 was achieved in phases related to Li5+2AxLn3-xMv2O12 (A = Ca, Sr, Ba). Structural studies carried out using neutron diffraction analysis found a cubic garnet related structure in the space group Ia3d, consistent with previously published results on related phases. The structure contained Li in two partially occupied sites, an ideal tetrahedral site and a distorted “octahedral” site found to be tending towards a 4 coordinate geometry. Increasing Li content was accommodated on to the two existing sites without disruption to the rest of the structure, and increasing temperature caused some redistribution of Li between the sites. Tetragonal garnet related phases were unexpectedly produced when increasing the Li content to form Li7Ln3MIV2O12. Structural studies showed three Li sites, one tetrahedral and two distorted octahedra with the space group I41/acd, such cation ordering accounting for the very low conductivity observed for these phases. Thermal history effects on conductivity were observed in the vast majority of the phases studied. The main cause of these effects was thought to be atmospheric water, bringing about exchange of Li with H and the formation of Li containing species on the surface. The effects were variable and illustrated the sensitivity of these phases to external conditions.