Thesis (Ph.D. (Physics)) -- University of Limpopo, 2019 / LiMn2O4 spinel (LMO) is a promising cathode material for secondary lithium-ion
batteries which, despite its high average voltage of lithium intercalation, suffers
crystal symmetry lowering due to the Jahn-Teller active six-fold Mn3+ cations.
Although Ni has been proposed as a suitable substitutional dopant to improve the
energy density of LiMn2O4 and enhance the average lithium intercalation voltage,
the thermodynamics of Ni incorporation and its effect on the electrochemical
properties of this spinel are not fully understood.
Firstly, structural, electronic and mechanical properties of spinel LiMn2O4 and
LiNixMn2-xO4 have been calculated out using density functional theory employing the
pseudo-potential plane-wave approach within the generalised gradient
approximation, together with Virtual Cluster Approximation. The structural
properties included equilibrium lattice parameters; electronic properties cover both
total and partial density of states and mechanical properties investigated elastic
properties of all systems. Secondly, the pressure variation of several properties was
investigated, from 0 GPa to 50 GPa. Nickel concentration was changed and the
systems LiNi0.25Mn1.75O4, LiNi0.5Mn1.5O4 LiNi0.75Mn1.25O4 and LiNi0.875Mn1.125O4 were
studied. Calculated lattice parameters for LiMn2O4 and LiNi0.5Mn1.5O4 systems are
consistent with the available experimental and literature results. The average
Mn(Ni)-O bond length for all systems was found to be 1.9 Å. The bond lengths
decreased with an increase in nickel content, except for LiNi0.75Mn1.25O4, which gave
the same results as LiNi0.25Mn1.75O4. Generally, analysis of electronic properties
predicted the nature of bonding for both pure and doped systems with partial density
of states showing the contribution of each metal in our systems. All systems are
shown to be metallic as it has been previously observed for pure spinel LiMn2O4,
and mechanical properties, as deduced from elastic properties, depicted their
stabilities.
Furthermore, the cluster expansion formalism was used to investigate the nickel
doped LiMn2O4 phase stabilities. The method determines stable multi-component
crystal structures and ranks metastable structures by the enthalpy of formation while iv
maintaining the predictive power and accuracy of first-principles density functional
methods. The ground-state phase diagram with occupancy of Mn 0.81 and Ni 0.31
generated various structures with different concentrations and symmetries. The
findings predict that all nickel doped LMO structures on the ground state line are
most likely stable. Relevant structures (Li4Ni8O16, Li12MnNi17O48, Li4Mn6Ni2O16,
Li4Mn7NiO16 and Li4Mn8O16) were selected on the basis of how well they weighed
the cross-validation (CV) score of 1.1 meV, which is a statistical way of describing
how good the cluster expansion is at predicting the energy of each stable structure.
Although the structures have different symmetries and space groups they were
further investigated by calculating the mechanical and vibrational properties, where
the elastic constants and phonon vibrations indicated that the structures are stable
in accordance with stability conditions of mechanical properties and phonon
dispersions.
Lastly, a computer program that identifies different site occupancy configurations for
any structure with arbitrary supercell size, space group or composition was
employed to investigate voltage profiles for LiNixMn2-xO4. The density functional
theory calculations, with a Hubbard Hamiltonian (DFT+U), was used to study the
thermodynamics of mixing for Li(Mn1-xNix)2O4 solid solution. The results suggested
that LiMn1.5Ni0.5O4 is the most stable composition from room temperature up to at
least 1000K, which is in excellent agreement with experiments. It was also found
that the configurational entropy is much lower than the maximum entropy at 1000K,
indicating that higher temperatures are required to reach a fully disordered solid
solution. The maximum average lithium intercalation voltage of 4.8 eV was
calculated for the LiMn1.5Ni0.5O4 composition which correlates very well with the
experimental value. The temperature has a negligible effect on the Li intercalation
voltage of the most stable composition. The approach presented here shows that
moderate Ni doping of the LiMn2O4 leads to a substantial change in the average
voltage of lithium intercalation, suggesting an attractive route for tuning the cathode
properties of this spinel. / National Research Foundation (NRF)
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ul/oai:ulspace.ul.ac.za:10386/3348 |
| Date | January 2019 |
| Creators | Malatji, Kemeridge Tumelo |
| Contributors | Maphanga, R. R., Ngoepe, P. E. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | xv, 176 [2] leaves |
| Relation |
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