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The commercialisation of Li-ion batteries over the last decade has provided additional impetus for the improvement of existing energy storage technologies. Towards this, a major portion of the global efforts includes exploratory research aimed at the development of new material chemistries. Aligning with this theme, this Thesis explores the synthesis–structure–property relationships in Li- and Mn-rich layered oxides, a cost-effective high-capacity material system that shows promise as a positive electrode material for future Li-ion batteries. The compositional and crystallographic diversity of Li- and Mn-rich layered oxides make them particularly susceptible to synthesis-dependent variations and exacerbates structural characterisation. Therefore, understanding how synthetic variations influence their structural and electrochemical properties is a crucial step in realising their potential as positive electrode materials.

Even for simple compositions like Li₂MnO₃, dissimilar crystallographic ordering and particle morphologies are produced depending on whether a solid-state or sol-gel synthesis approach was implemented. Subsequently, due to the higher degree of structural disorder and larger surface area, the sol-gel sample exhibited higher initial electrochemical capacities. The structural features present in these compounds such as cation site-mixing and stacking faults, manifest over varying crystallographic regimes. Hence, complementary characterisation techniques that probe different structural length scales are necessary for an accurate structural characterisation of these compounds. This factor, together with their complex crystallography, have led to contradictory single- and multi-phase structure models being reported for complex Li- and Mn-rich layered oxides. By using a combination of diffraction, spectroscopic techniques and magnetic measurements it was discovered that Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ can exist in both single- and multi-phase structural forms if synthesised through sol-gel and solid-state methods, respectively. Further studies following the same theme revealed that when synthesised under common laboratory conditions these compounds are metastable. Here, the composition and synthesis play a critical role in the thermodynamic and kinetic factors affecting the resultant phase, domain structure and degree of cationic order. Finally, to encompass all the structural features contained in Li- and Mn-rich layered oxides, a supercell-based structure model for Li- and Mn-rich layered oxides, using Li_1.2Mn_0.6Ni_0.2O₂ as an example, is presented. Summing all the work together from the thesis, a critical evaluation of commonly used characterisation techniques is also provided as a guideline for future research in this field.

The transition towards a society with full electromobility depends heavily on the battery systems, which raises concerns about the environmental friendliness and sustainability of the current products on the market. Magnetite (Fe₃O₄) from the conversion type electrode family is one of the promising candidates in the search for a sustainable battery technology owing to its high theoretical storage capacity, high abundance, and low toxicity. However, the material suffers severely from capacity degradation in its practical use and hence requires better understanding and adjustments to reach its full potential.

Developing nanostructured electrode materials is one of the strategies to enhance its storage capacity, stability, and charging rate. Therefore, this thesis starts with synthesis and chemical lithiation of cubic (ferrimagnetic) Fe₃O₄ nanoparticles that are used as a model system to establish relationships between structural and magnetic properties upon lithiation. The thesis then explores the relationship between particle size, composition, crystal structure, and electrochemical performance of Fe₃O₄ electrodes via multi-operando techniques during cycles of lithiation and delithiation reactions. Magnetometry, known for its sensitivity to the chemical, compositional, and agglomeration state of the materials was exploited to measure the magnetic signal of the electrodes under operando conditions as a complement to operando SAXS and WAXS measurements. The results from the operando studies indicated that during electrochemi calcycling; LiFeO₂, FeO and metallic iron (Fe) are produced as intermediate compounds, but their stability regions differ greatly when using nanoparticles or bulk materials and also when compared against ex-situ analyzed specimens. In addition, commercial micron- and nanosized (paramagnetic) Co₃O₄ particles were employed to study evolution of structural and magnetic properties over cycles to shed light on the size-dependent reaction kinetics. The obtained results revealed that nanosizing leads to improved electrochemical performance, variations in surface reaction kinetics, and differences in aging mechanisms. The magnetic measurements were crucial in determining surface capacitance reactions that involve gel-like polymeric layer growth and degradation during Li removal and uptake.

Lastly, the magnetic properties of layered NMC-based cathode materials were studied. The differences in their magnetic properties provided important information on the transition metal ordering depending on the choice of synthesis method that is used. Magnetization measurements were used in combination with diffraction data to choose an appropriate structure model to describe actual atomic arrangement in each material. Consequently, the findings in this thesis suggest that (operando) magnetometry can be employed as a complementary tool to elucidate structural details of battery electrodes, potentially revealing insights beyond the detection limits of volume-averaged X-ray scattering techniques.