Synthesis of new lithium ion conductors

Abstract

This thesis reports the synthesis and characterisation of a number of new Li-ion conductors with olivine and NASICON structure types. The new materials have been characterised by chemical analysis, diffraction and spectroscopy in order to establish relationships between the Li-ion conductivity and the crystal structure. The thesis starts describing Li-ion battery technology. The role of ceramic materials in battery development is highlighted and some of the state-of-the-art materials are introduced. The battery failure mechanism and fire risk are examined. Then defect chemistry and ionic conduction are presented to end with a literature review on families of Li-ion conductors indicating their conductivities and crystallographic features. The experimental techniques used throughout the thesis for the synthesis and characterisation of the new materials are also introduced. A large fraction of the work in the thesis focuses on the olivine-type structure of general composition LiMPO4 with M a 2+ cation. The main strategy developed in these chapters was the aliovalent substitution on M site in order to introduce Li-vacancies in the Li-ion diffusion pathways of the structure. A number of new modifications are reported, mainly based on the olivine material LiMgPO4. These series follow Vegard’s law up to a doping limit after which a plateau is reached. The impurity phases that appear in the plateau have been determined. In general, the synthesis of these new materials have needed higher temperatures than the parent phases, highlighting in this way the role of the entropy of mixing in this type of modification. Li-ion conductivity derived from AC impedance measurements are presented for some of the new samples. The rest of the work in the thesis is on the NASICON-type structure. A number of Li-ion analogues related to LiTaAl(PO4)3 have been investigated. The role of Li and P excess in the distortion of the unit cell has been studied with X-ray diffraction. For one of the related analogues, its phase diagram has been explored in a search for new Li-ion conductors with the NASICON-type structure. Densification methods to enable conductivity measurements have also been developed. Finally, Li-ion conductivity has been determined by AC impedance spectroscopy on a number of new phases. A summary chapter at the end of the thesis summarises the work and highlights the main conclusions and future directions.