The Ba3MoNb1-xVxO8.5 family exhibits significant oxide ion conductivity, and is thus interesting in the potential use of solid oxide fuel cells as an electrolyte. Its structure contains palmierite-like layers consisting of metallic cations forming octahedral and tetrahedral polyhedra, through which the oxide ions can be conducted. Furthermore, the cationic vacancies present in the structure lead to complex stacking configurations beneficial to the conductivity. In this work the Ba3MoNb1-xVxO8.5 family, with x = 0.0, 0.1, 0.2, 0.3, 0.4, have been synthesised and analysed with the use of X-ray diffraction and subsequent refinement of their structures. The synthesis for each sample was performed successfully, and the first calcination step was further identified to bring a higher yield under higher temperature and longer time of the calcination. From the LeBail refinement the a lattice parameter was shown to correlate with Vegard's law, decreasing with an increase of substitution of Nb5+ with V5+, whilst the c parameter expands upon substitution of the smaller cation. This is hypothesised to be caused by repulsion, either coming from distortions caused by second-order Jahn Teller effect, or due to more tetrahedral coordination. Finally, the Rietveld refinement has given insight into the location and occupation of the metallic cations. V5+ has been incorporated into the solid solution, and sits on the same crystallographic sites as the other metallic cations, choosing to split off to either one of the two sites, thereby contradicting earlier literature. Furthermore, the vanadium doping also seems to move the overall concentration of Mo6+ and Nb5+ to one site in particular. These results have thereby given new insight into this family of solid electrolytes.

This study explores the potential of metal oxide fluorides as cathode materials for solid-state fluoride-ion batteries (FIBs), aiming to combine the stability of intercalation-based electrode materials with the high energy density of conversion-based materials. Through comprehensive experimental investigations using techniques such as electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), linear sweep voltammetry (LSV), and X-ray diffraction (XRD), the compatibility and electrochemical performance of transition metal oxides (Cu₂O, FeO, and Mn₂O₃) with CsPb_0.9K_0.1F_2.9 (PK10) solid electrolyte and Pb/PbF₂ composite anode are evaluated. Results indicate negligible room temperature capacity for Cu₂O, FeO, and Mn₂O₃, suggesting potential limitations related to the cathode fluorination reaction. Additionally, PK10 electrolyte displays slight instability at room temperature, indicating potential electrochemical activity. Symmetric cell testing using Pb/PbF₂ composite electrodes confirms the suitability of the Pb/PbF₂ composite as both counter and reference electrodes. Notably, Cu₂O full cells show enhanced specific capacity at elevated temperatures (60 °C), reaching 310.24 mAh/g during the first cycle, equivalent to 82.96% of the theoretical specific capacity. This considerable increase in capacity due to only a slightly higher temperature is attributed to reduced overpotential and enhanced fluoride ions diffusion rates. However, observation of capacity fade between cycles for the Cu₂O cell at 60 °C suggests irreversible reactions, necessitating further investigation. In conclusion, this study highlights the potential of metal oxide cathode materials in solid-state FIBs, emphasizing the importance of understanding electrolyte stability and cathode compatibility for battery performance enhancement.

The lack of a decent solid-state ionic conductor has hindered the large-scale application of solid-state batteries, which are considered to be the potential game changer for energy transition. The recently reported K doping CsPbF3 material system has shed light on this problem. This material possesses high ionic conductivity and a wide electrochemical stability window at the same time, making it a highly promising candidate for the next-generation fluoride ion solid-state battery. In order to have a clearer understanding of the structural information of this material and to find out what contributes to the outstanding properties it demonstrates, this thesis project uses Density functional theory(DFT) to calculate its ground state properties. Meanwhile, to better understand its local structure, the Nuclear magnetic resonance(NMR) parameters for this material are also calculated using DFT and analyzed in detail. Results generated from the calculations suggest that the coulombic interaction can be utilized to explain the structural deformation upon doping K into the CsPbF3 system. Additionally, the analysis of the optimized cell structure indicates a tendency for the material system to go through a cubic to tetragonal phase transition, which reproduces the trend observed experimentally and offers a potential explanation for the driving force behind it. Further investigation using Nudged elastic band calculations(NEB) also reveals a relatively low energy barrier for vacancies to diffuse in the crystal structure, which provides insight into the high ionic conductivity of this material. The findings manifested in this thesis project could potentially offer improvement directions for the K-doped CsPbF3 system and contribute to the development of other solid-state ionic conductors.

This study aims to assess the suitability of iron oxyfluoride (FeOF) as cathode material for fluoride-ion batteries based on the electrochemical performance and fluorination capability of ferrous oxide (FeO), as well as the defluorination of FeOF. Due to the pressing demand for electrochemical storage, alternatives to the widespread lithium-ion battery must be sought. One alternative can be the fluoride-ion battery (FIB). By trying to combine the stability of intercalation-based electrode materials and the high energy density of conversion-based materials, oxyfluorides might be the answer, especially if based on an abundant transition metal such as iron. In this report, the suitability of iron oxyfluoride as cathode material was evaluated. This was done by synthesising the electrode composites, evaluating their performance in a custom-made electrochemical cell and investigating the phase transitions of the researched materials. It was found that the ferrous oxide could not be fluorinated in an electrochemical environment and only reached a capacity of 0.75 mAh/g, which is equal to 0.2% of the theoretical capacity. It was also found that the iron oxyfluoride could not be electrochemically defluorinated. Therefore it is concluded that iron oxyfluoride is not suitable as cathode material for fluoride ion batteries.

In the coming energy transition, the availability of reliable and affordable energy storage will be of vital importance. Battery storage is a large factor in the energy storage sector, and current battery storage is dominated by lithium-based batteries. However, lately, alternatives to lithium have been given renewed attention, due to the insufficient abundance of lithium in the earth’s crust and the promising theoretical aspects of these other types of batteries. Fluoride-based batteries, for example, have high theoretical capacities and are theorized to be very suitable for application in solid-state battery technology. Fluoride-based batteries have only been produced on a lab-scale relatively recently, with the first reversible solid-state fluoride-ion battery produced in 2011, and the first room temperature reversible fluoride-ion battery produced in 2018, yet interest in this technology has drastically increased over the past few years. The current main issues fluoride- ion batteries are running into are its poor cyclability, its low current densities, and its inability to meet the theoretical capacities. In this report, multiple facets of fluoride-ion batteries have been analyzed in an effort to improve on these characteristics. In particular a focus has been placed on two commonly used materials in fluoride-ion batteries: electrolyte BaSnF4 and cathode material BiF3. The ionic conductivity dependence on pressure was deduced for each material, with BaSnF4 having its highest ionic conductivity at ∼280 MPa and BiF3 having its highest ionic conductivity at ∼680 MPa. To test the effect of oxides in BiF3, which form spontaneously when BiF3 is exposed to air or humidity, various Bi-O-F compounds were synthesized and had their ionic conductivity measured. It was found that each of the compounds that contained oxygen had a drastically lower (factor 1,000-10,000) ionic conductivity than pure BiF3. An attempt was also made to improve the ionic conductivity of BiF3 by doping that material with SnF2. BiF3 doped with SnF2 concentrations of 5-20% were synthesized, and had their ionic conductivity measured. It was found that the ionic conductivity was increased by dopant concentrations of 5% and 10%, with the material with 10% SnF2 having the highest ionic conductivity, while the materials with 15% and 20% SnF2 had a similar ionic conductivity to pure BiF3. Symmetric fluoride-ion batteries were also produced, with BiF3 as an electrode material and BaSnF4 as an electrolyte material. The produced batteries reached charge and discharge capacities with values of at most only 30% of the values reported in literature. The batteries also had poor capacity retention over multiple charge-discharge cycles. Batteries were also produced using BiF3 doped with 5% and 10% SnF2 as an electrode material. These batteries had higher initial capacities but had even poorer capacity retention over subsequent cycles. It was however also found that the critical current density for the batteries had increased as a result of doping with SnF2, allowing higher current densities to be applied to the batteries.