Synthesis–Structure–Property Relationships in Li- and Mn-rich Layered Oxides

Abstract: 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 Li1.2Mn0.54Ni0.13Co0.13O2 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 Li1.2Mn0.6Ni0.2O2 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.