The aim of this thesis work has been focused on the development of Li-rich complex oxide materials and their advanced characterisation by a wide range of techniques for their application in Li batteries. To achieve this ultimate goal, it is necessary to consider the material design and discovery, the synthetic routes employed, and the characterisation of these materials to unpick the underpinning structure-property relations which govern functionality. Chapter 1 introduces the basic aspects of current Li-ion battery technologies and their limitations. This is followed by a description of the all-solid-state battery concept and an examination of solid electrolyte candidate materials. Lithium-rich garnet materials are described in the following section with the conductivity-crystal structure relationship detailed. The role of lithium-excess in complex oxides for battery applications is explored followed by a section introducing the novel concept of lithium-rich double perovskites and the Li6Hf2O7 system. Finally, a section reviewing the microwave and sol-gel synthetic pathways employed for battery materials will conclude this introductory chapter. The chemicals and synthetic approaches employed in this thesis to develop the materials under study are detailed in Chapter 2. The basics behind the characterisation techniques employed in this thesis, including powder X-ray diffraction (PXRD) and neutron powder diffraction (NPD) techniques for structural characterisation, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic cycling with potential limitation (GCPL) for electrochemical analyses, X-ray absorption spectroscopy (XAS) synchrotron-based techniques for local structure analyses and muon-spin relaxation (μ+SR) for local Li+ diffusion studies, among others, are also detailed. The first results chapter, Chapter 3, details the studies performed on Zn-, Ga-, Al-doped Li6BaLa2Ta2O12 (LBLTO) garnet materials as solid-state electrolytes. The achievement of shorter reaction times and temperatures compared to conventional solid-state chemistry methods is detailed. The role of the dopant in the structure is analysed by PXRD and XAS studies and its influence on the ionic conductivity of the materials is examined. For the undoped material, local Li+ diffusion analyses by μ+SR are also evaluated and discussed. Chapter 4 presents a novel microwave-assisted synthesis for Al- and Ga-doped Li7La3Zr2O12 (LLZO) garnets. The chapter discusses the stabilisation of the cubic phase of the LLZO garnet at lower temperatures and shorter reaction times. The structure of the material and dopant positions are analysed by PXRD, XAS and PND studies. The macro and micro ionic transport properties of the materials are examined by EIS and μ+SR and related to the macrostructure and dopant positions within the garnet structure. The preparation of the homologous Al-doped LLZO cubic garnet by sol-gel chemistry is explored in Chapter 5. The stabilisation of the highly conducting cubic phase even at lower temperatures is analysed by conventional PXRD, advanced in-situ NPD and Raman spectroscopy. The reasons behind the ionic transport behaviour of this sol-gel prepared material are analysed by EIS and local Li+ diffusion studied with μ+SR. Chapter 6 focuses on the synthesis and ionic conductivity studies of the novel Li-rich complex oxides In-and Y-doped Li6Hf2O7 as solid-state electrolytes for lithium-ion batteries. The analysis of this new family of materials and their crystallographic structures are presented. The transport properties and the role of the dopant is discussed, with the ionic conductivity and activation energy for macroscopic ionic conduction presented. In Chapter 7, a new family of Li-rich double perovskites as versatile novel materials for all-solid-state Li batteries is presented. The synthesis and structural characterisation of the Li1.5La1.5WO6 (LLWO) and Li1.5La1.5TeO6 (LLTeO) novel compounds by PXRD, NPD and XAS analyses is described. Investigation of Li1.5La1.5WO6 as a candidate negative insertion electrode was analysed by CV and GCPL experiments, as well as the macro and microscopic study of their transport properties by EIS and μ+SR techniques respectively. The chapter also includes the study and discussion of the redox stability and Li+ conduction properties of Li1.5La1.5TeO6 as a solid-state electrolyte and preliminary studies of a pseudo solid-state battery formed by these two novel Li-rich double perovskites. In Chapter 8, the homologous Na-rich double perovskite Na1.5La1.5TeO6 is presented. The crystal structure has been explored by PXRD, Raman spectroscopy and in-situ variable-temperature PXRD experiments. The transport properties have also been explored at the macroscopic and local level by EIS and μ+SR and its compatibility with Na metal electrodes analysed in symmetrical cells. To conclude, a summary of the main conclusions obtained from the work presented in this thesis, together with further lines of research to explore, are discussed in Chapter 9.