Dottorato in PROCESSI CHIMICI PER L'INDUSTRIA E PER
L'AMBIENTE

Titolo della tesi: New positive electrode materials for next generation lithium-ion batteries

During PhD, the primary focus of the research revolved around the study of lithium-rich layered oxide materials. The specific objective was to delve into Co-free materials, a pursuit driven by the pressing socio-economic demands of our time. This investigation was conducted through a multifaceted approach that harnessed various scientific techniques to gain comprehensive insights into the challenges faced by these materials and to unravel their intricate structural, electronic, morphological, and electrochemical properties. As the research unfolded, a significant part of the effort was dedicated to devising strategies to enhance these materials. This process involved exploring the concept of lattice doping to rectify the issues commonly encountered with Co-free materials. In essence, the research aimed to provide explanations and solutions that could mitigate the challenges and limitations associated with lithium-rich layered oxide materials. This thesis consists of eight self-contained chapters, in each of which a different topic related to these materials is addressed, and it can be divide in two part. During the first part, the analysis and study of materials was completely based on first-principles calculations, in order to highlight the main structural and electronic peculiarities of these materials. The first provides the state of the art of lithium-ion batteries, the cathode materials used, developments and improvement strategies. The second explores, using first-principles calculations, the structural and electronic properties of the single-component materials that make up LRLO materials. This approach served us to compartmentalise the structural complexity that plagues these materials, and to validate our computational approach for subsequent, more complex studies. The third explores the impact of cobalt removal from the LRLO trigonal lattice on the structural, thermodynamic, and electronic properties of this material by means of density functional theory calculations. Cobalt removal was modelled by simultaneous substitution with Mn/Ni, thus leading to p-doping in the lattice. The fourth discusses the formation of antisite Li/Ni defects in the layered lattice of the LRLO compound using first-principles calculations. Our aim is to shed light on the native disorder in LRLO and how the synthesis protocol can promote the formation of antisite defects. The second part consists of analyses based on experimental techniques coupled with computational ones, supporting the identified experimental evidence. In the fifth, the results obtained from two different syntheses of new Co-free LRLO materials, namely Li1.25Ni0.125Mn0.625O2 (np-LRLO) with low nickel content, are discussed. Here, the crystal structure ambiguity of these materials is highlighted and analysed in detail, and a new prototype structure for Rietveld refinement of XRD characterisation is proposed. In addition, post-mortem characterisations will be discussed. The sixth, for the same np-LRLO material, uses multi-technique analysis to study the formation cycles that are key to the structural and electronic rearrangement of these materials, which then allows them to be activated from an electrochemical point of view. Some of these techniques are ICP, ISE, Operand XRD, Raman ex-situ, OEMS, Dilatometry, XAS and DFT. The seventh aims to verify the redox activity and the evolution of the electronic and local structure beyond the training cycles, after long cathode cycles. In addition, the intensity of the first shell peak in the Fourier Transform (FT) was evaluated as a function of temperature to characterise the local bond strength and structural disorder at different cycles. Finally, the eighth objective is to understand the role of cationic doping of these materials.