Reaction Mechanisms in Advanced Materials for Li and Na-ion Batteries Studied by X-ray Absorption Spectroscopy and Related Techniques

The research work presented in this thesis regarded the structural study of different materials employed as battery electrodes (for both Li and Na ion cells) by means of X-ray Absorption Spectroscopy (XAS), X-ray Photoelectron Spectroscopy (XPS) and RamanSpectroscopy. The main purpose of the thesis was to provide better insight, at a microscopic and atomic level, of all the mechanisms related to the insertion/de-insertion of lithium or sodium ions in the electrode structure at the electrolyte interface and the bulk. In particular we have tackled three main open problems concerning: i) the evolution of the local structure in zinc-ferrite conversion-alloying materials used in lithium-ion batteries, ii) the study of formation and evolution of the solid electrolyte interphase (SEI) in carbon-based anodes again used for lithium-ion cells, iii) the relationship between local structure distortion and electrochemical performances in a class of cathode materials for sodium-ion batteries. By using X-ray absorption spectroscopy, we have shown that in the very early stages of Li+ insertion (until 0.3Li+ per formula unit) carbon-coated zinc-ferrite nanoparticles anode retain the spinel structure while at higher level of Li uptake (> 0.3 Li+ per formula unit), Zn atoms migrate to vacant crystallographic sites. In this initial stage, Fe is found to be gradually reduced from Fe3+ to Fe2+ upon lithium insertion and remains in the original octahedral sites. Our EXAFS study indicates an increase in structural disorder upon lithiation. Lithiation proceeds with a continuous reduction of the Zn and Fe until those species are fully metallized in the form of nano-sized particles. Finally, we could provide direct proof of the reversible lithium-zinc alloying mechanism occurring in the very final stage of the lithiation. The evolution and stability of the SEI were studied using an arsenic-containing compound as electrolyte. Arsenic acts as local probe for SEI formation for XAS and XPS, giving an insight into the oxidation state and structure of the SEI. Both XAS and XPS revealed the presence of arsenic with oxidation state 3+ and 5+, possibly in the form of arsenic oxides (As2O5, As2O3) and arsenic-fluorine compounds (AsF3, AsF6–, LixAsF3-x). Moreover, XPS revealed the presence of As0 (not detected by XAS) that could be present, in a small quantity, only on the outer layer of the SEI. The organic fraction of the SEI has been also studied with XPS, showing the presence of different lithium alkyls species and carbonates as a result of the degradation of the electrolyte organic solvents. Those species may contain lithium atoms contributing to the total capacity of the cell, in agreement with recent results. In-situ microRaman experiments, specifically developed during this thesis, were attempted showing the modifications of the graphitic host structure during lithium insertion in the material. Finally, the structure of Mn-based layered oxides for sodium-ion cathodes, doped with Ti and Fe, was studied by X-ray Absorption Spectroscopy. We have verified that the oxidation states of Mn and Ti in P2-Na2/3Mn0.8Fe0.2-xTixO2 are in agreement with the expected theoretical values. Our structural XAS refinement, compared with the results of DFT calculations and XRD data, confirmed experimentally the Jahn-Teller induced distortion of the structure for all the materials under consideration. A slight decrease of the local structural disorder is observed in the material where both Fe and Ti are present with equal proportions. Most of the results presented in this thesis have been published in international journals, and the reader is referred to the published papers for further details.