PhD ThesisThree LISICON-based systems, Li3PO4-Li4GeO4, Li2MoO4-Li4GeO4 and Li2WO4-Li4GeO4, have been systematically investigated. Details of the phase behaviour, crystal structure and defect structure have been studied by powder X-ray and neutron diffraction and solid-state NMR, reverse Monte Carlo (RMC) modelling of neutron total scattering data and molecular dynamics simulations. Electrochemical impedance spectroscopy measurements have been used to characterise the electrical properties.
In the Li3PO4-Li4GeO4 system, a solid solution Li3+xGexPl-xO4, isostructural with the end member γ-Li3PO4 is found in the compositional range 0.00 ≤ x ≤ 0.90. Two main types of defect are identified and clustering of these defects is proposed. Conductivity measurements show the x = 0.75 composition exhibits the best total conductivity (σ250°C = ~ 1.8 x 10-2 S cm-1) with a low activation energy of 0.42 eV.
In the Li4-2xGe1-xMoxO4 system, compositions in the range 0.1 ≤ x ≤ 0.5, exhibit LISICON-type structures. Both the β and γ phase LISICON type polymorphs are observed in this system, the relative amounts of which vary with temperature. In this system, the highest conductivity (σ250°C = ~ 5.0 x 10-3 S cm-1) is obtained in the x = 0.2 composition with an activation energy of 0.67 eV.
In the Li2WO4-Li4GeO4 system, the solid solution only extends between 0.10 ≤ x ≤ 0.25. Low activation energy and high electric conductivity are observed throughout this system. The highest elevated temperature conductivity values are seen in the x = 0.15 composition (σ250°C = ~ 3.12 x 10-2 S cm-1). Molecular dynamics simulations suggest an order of magnitude higher conductivity values could be achieved through reduction of grain boundary resistances
