Abstract:
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
