Lithium manganese oxide (LiMn2O4) spinel surfaces and their interaction with the electrolyte content

Thesis (M.Sc. (Physics)) -- University of Limpopo, 2020.

This dissertation presents the results of the ab-initio based computational studies of spinel lithium manganese oxide (LiMn2O4) bulk, surfaces, and the adsorption of an organic electrolyte, ethylene carbonate. The spinel LiMn2O4 is one of the most promising cathode materials for Lithium-ion batteries because of its affordability, nontoxicity, and improved safety compared to commercially used LiCoO2. However, it also suffers from the irreversible capacity due to the electrolyte-cathode interactions which lead to manganese (Mn) dissolution. Using the spin-polarized density functional theory calculations with on site Coulomb interactions and long-range dispersion corrections [DFT+U−D3−(BJ)], we investigated the bulk properties, surface stability and surface reactivity towards the ethylene carbonate (EC) during charge/discharge processes. Firstly, we explored the structural, electronic, and vibrational bulk properties of the spinel LiMn2O4. It was found that the bulk structure is a stable face-centred cubic structure with a bandgap of 0.041 eV and pseudo-gap at the Fermi level indicating electronic stability. Calculated elastic constants show that the structure is mechanically stable since they obey the mechanical stability criteria. The plotted phonon curves show no imaginary vibrations, indicating vibrational stability. To study the charge/discharge surfaces, we modelled the fully lithiated and the partially delithiated slabs and studied their stability. For the fully lithiated slabs, Li-terminated (001) surface was found to be the most stable facet, which agrees with the reported experimental and theoretical data. However, upon surface delithiation, the surface energies increase, and eventually (111) surface becomes the most stable slab as shown by the reduction of the plane in the particle morphologies. Finally, we explored the surface reactivity towards the ethylene carbonate during charge/discharge processes. The ethylene carbonate adsorption on the fully lithiated and partly delithiated facets turn to enhance the stability of (111) surface. Besides the strong interaction with the (111) surfaces, a negligible charge transfer was calculated, and it was attributed by a large charge rearrangement that takes place within the surfactant upon adsorption. The wavenumbers of the C=O stretching showed a red shifting concerning the isolated EC molecule