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Abstract: This thesis presents the results of my work on the sodium-ion battery cathode material NaNiO2, the parent system for all nickel-rich sodium-ion cathodes. This material class promises the highest theoretical capacities for sodium layered transition metal oxides, which are presently unrealised due to degradation induced irreversible capacity loss. This work advances understanding of structural evolution which occurs between discrete, stable, ordered phases during charging, solving complete atomistic structural models for the first time and answering the question of how structure is influenced by various potential ordering phenomena (charge, vacancy, and orbital). This establishes a framework for rational design of next-generation cathodes with enhanced performance via targeted transition metal ion doping to disrupt the observed ordering. Carefully controlled solid-state synthesis is carried out to produce phase-pure Oʹ3-NaNiO2, from which electrochemical synthesis of the desodiated phases (NaxNiO2, x = 2/3, 1/2, 2/5, 1/3) is possible. Novel approaches are developed to facilitate successful film-casting for coin-cell batteries, and for large-quantity powder cells to enable neutron powder diffraction on electrochemically derived samples. Dynamic structural stabilisation is reported in Pʹ3-Na2/3NiO2, using a suite of complementary characterisation techniques including combined Rietveld refinement of synchrotron X-ray and neutron diffraction data, magnetic susceptibility, and 23Na solid-state nuclear magnetic resonance spectroscopy. My experimental results are compared to my collaborator’s Ab Initio Molecular Dynamics simulations, which indicate multiple low-energy structures that are dynamically populated. In Na2/3NiO2, I observe a combination of competing effects which contribute to the resultant dynamic nature of the structure, including honeycomb ordering of mixed-valence nickel ions, orbital-ordering of Jahn-Teller distorted Ni3+, and zigzag Na+/vacancy ordering. I then solve structures for Pʹʹ3-Na1/2NiO2, Oʹʹ3-Na2/5NiO2, and Oʹʹʹ3-Na1/3NiO2, using ex situ synchrotron X-ray and neutron powder diffraction. Each phase contains unique Na+/vacancy orderings, which minimise intra-layer electrostatic repulsions between Na+ ions, and Nix+-charge orderings which minimise inter-layer repulsions by locating expanded NiO6 octahedra over vacancies. I present sequential Rietveld refinement of operando diffraction data, finding evidence for a transient Pʹʹʹ3-Na1/2 Oʹʹʹ3-Na1/3NiO2 transition. Finally, I develop methodology to facilitate operando and in situ neutron diffraction on sodium-ion batteries, using NaxNiO2 as a well characterised model system. Python-based code is developed for the tandem processing of diffraction/electrochemical data, which is freely available at GitHub and Zenodo. For this, I develop a novel approach for Rietveld refinement, wherein a significant component of the measured pattern is resultant from the complex sample environment background. My work opens up this technique to the industrially relevant transition-metal doped cathodes, which neutron diffraction is uniquely suited to, for the entire sodium-ion battery community.

Open Access

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Abstract: Ba2GdNbO6 has previously been reported to adopt either monoclinic, tetragonal, or cubic symmetry at room temperature. Using high-resolution synchrotron X-ray diffraction, neutron diffraction and neutron pair distribution function analysis we find that the compound adopts a tetragonal I4/m double-perovskite structure at room temperature (with a weak, temperature-independent second-order Jahn–Teller distortion in the NbO6 octahedra) and undergoes a phase transition to a monoclinic P21/n symmetry upon cooling to 2.4 K. Only upon heating above room temperature to T ≈ 450 K does Ba2GdNbO6 reversibly transition to a cubic Fm3̅m symmetry. Magnetic susceptibility measurements indicate predominant paramagnetic behavior down to 1.8 K, with minimal ferromagnetic short-range correlations (θ = 0.20(5) K) and a small exchange interaction (J1 = −0.0032(8) K). At 2 K and 9 T, the compound exhibits a maximum magnetic entropy change of −ΔSm = 15.75 J K–1 mol–1 and an adiabatic temperature change of ΔTad = 21 K, making it a promising candidate for low-temperature magnetocaloric applications. Heat capacity measurements confirm a rigid crystal lattice (TD = 267(3) K) and a corresponding small lattice entropy contribution in the low-temperature regime, highlighting the potential of Ba2GdNbO6 for effective cooling capability in magnetocaloric devices at cryogenic temperatures. This study elucidates the structural and magnetic characteristics of Ba2GdNbO6 and attests to its promise for low-temperature magnetocaloric refrigeration.

Open Access

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Abstract: NaNiO$_2$ is a Ni$^{3+}$-containing layered material consisting of alternating triangular networks of Ni and Na cations, separated by octahedrally-coordinated O anions. At ambient pressure, it features a collinear Jahn--Teller distortion below $T^\mathrm{JT}_\mathrm{onset}\approx480$\,K, which disappears in a broad first-order transition on heating to $T^\mathrm{JT}_\mathrm{end}\approx500$\,K, corresponding to the increase in symmetry from monoclinic to rhombohedral. It was previously studied by variable-pressure neutron diffraction [ACS Inorganic Chemistry 61.10 (2022): 4312-4321] and found to exhibit an increasing $T^\mathrm{JT}_\mathrm{onset}$ with pressure up to $\sim$5\,GPa. In this work, powdered NaNiO$_2$ was studied \textit{via} variable-pressure synchrotron x-ray diffraction up to pressures of $\sim$67\,GPa at 294\,K and 403\,K. Suppression of the collinear Jahn--Teller ordering is observed \textit{via} the emergence of a high-symmetry rhombohedral phase, with the onset pressure occurring at $\sim$18\,GPa at both studied temperatures. Further, a discontinuous decrease in unit cell volume is observed on transitioning from the monoclinic to the rhombohedral phase. These results taken together suggest that in the vicinity of the transition, application of pressure causes the Jahn--Teller transition temperature, $T^\mathrm{JT}_\mathrm{onset}$, to decrease rapidly. We conclude that the pressure-temperature phase diagram of the cooperative Jahn--Teller distortion in NaNiO$_2$ is dome-like.

Open Access

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Abstract: Metal–organic framework (MOF) composites are proposed as solutions to the mechanical instability of pure MOF materials. Here, we present a new compositional series of recently discovered MOF–crystalline inorganic glass composites. In this case, formed by the combination of a photocatalytic titanium MOF (MIL-125-NH2) and a phosphate-based glass (20%Na2O–10%Na2SO4–70%P2O5). This new family of composites has been synthesized and characterized using powder X-ray diffraction, thermal gravimetric analysis, differential scanning calorimetry, scanning electron microscopy, and X-ray total scattering. Through analysis of the pair distribution function extracted from X-ray total scattering data, the atom–atom interactions at the MOF–glass interface are described. Nitrogen and carbon dioxide isotherms demonstrate good surface area values despite the pelletization and mixing of the MOF with a dense inorganic glass. The catalytic activity of these materials was investigated in the photooxidation of amines to imines, showing the retention of the photocatalytic effectiveness of the parent pristine MOF.

Open Access

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Abstract: Metal-organic framework (MOF) composite materials containing ionic liquids (ILs) have been proposed for a range of potential applications, including gas separation, ion conduction, and hybrid glass formation. Here, an order transition in an IL@MOF composite is discovered using CuBTC (copper benzene-1,3,5-tricarboxylate) and [EMIM][TFSI] (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide). This transition – absent for the bare MOF or IL – provides an extended super-cooling range and latent heat at a capacity similar to that of soft paraffins, in the temperature range of ≈220 °C. Structural analysis and in situ monitoring indicate an electrostatic interaction between the IL molecules and the Cu paddle-wheels, leading to a decrease in pore symmetry at low temperature. These interactions are reversibly released above the transition temperature, which reflects in a volume expansion of the MOF-IL composite.