Phuti E. Ngoepe

Lithium and oxygen adsorption at the β-MnO2 (110) surface

The adsorption and co-adsorption of lithium and oxygen at the surface of rutile-like manganese dioxide (β-MnO2), which are important in the context of Li–air batteries, are investigated using density functional theory. In the absence of lithium, the most stable surface of β-MnO2, the (110), adsorbs oxygen in the form of peroxo groups bridging between two manganese cations. Conversely, in the absence of excess oxygen, lithium atoms adsorb on the (110) surface at two different sites, which are both tri-coordinated to surface oxygen anions, and the adsorption always involves the transfer of one electron from the adatom to one of the five-coordinated manganese cations at the surface, creating (formally) Li+ and Mn3+ species. The co-adsorption of lithium and oxygen leads to the formation of a surface oxide, involving the dissociation of the O2 molecule, where the O adatoms saturate the coordination of surface Mn cations and also bind to the Li adatoms. This process is energetically more favourable than the formation of gas-phase lithium peroxide (Li2O2) monomers, but less favourable than the formation of Li2O2 bulk. These results suggest that the presence of β-MnO2 in the cathode of a non-aqueous Li–O2 battery lowers the energy for the initial reduction of oxygen during cell discharge.

Calculated cell discharge curve for lithium batteries with a V2O5 cathode

Electronic structure techniques now enable accurate estimates of cell voltages of solid state batteries employing intercalation cathodes: we calculate the variation of cell potential with the degree of discharge—a crucial factor determining the applicability and utility of cathode materials—for the LixV2O5 cathode system.

Briullouin scattering in superionic compounds : NdF3 and CaF2 (ReF3)

Abstract Using Brillouin scattering techniques, the acoustic mode frequencies associated with the set of elastic constants in NdF3 have been measured as a function of temperature from 300 to 1325 K. Significant deviations from linear behaviour for certain elastic constants were observed above about 1050 K and attributed to a diffuse phase transition into a superionic state. Similar studies on a series of compounds CaF2 (10 mold% ReF3) where Re represents La, Nd, Tb and Tm demonstrate a marked reduction in the superionic transition temperature, Tc, compared with pure CaF2. In the doped crystals significant differences in Tc and the subsequent development of disorder are dismissed in relation to the presence of extrinsic defect clusters.

Generating structural distributions of atomistic models of Li2O nanoparticles using simulated crystallisation

Simulated crystallisation has been used to predict that Li2O nanoparticles comprise octahedral morphologies bounded by {111} and truncated by {100} with inverse fluorite crystal structure. We observe that by changing the temperature of the (simulated) crystallisation, changes in the microstructure can be realised, such a strategy facilitates the generation of full atomistic models with microstructural distributions similar to the structural diversity observed synthetically.

Periodic modeling of zeolite Ti-LTA

We have proposed a combination of density functional theory calculations and interatomic potential-based simulations to study the structural, electronic, and mechanical properties of pure-silica zeolite Linde Type A (LTA), as well as two titanium-doped compositions. The energetics of the titanium distribution within the zeolite framework suggest that the inclusion of a second titanium atom with configurations Ti-(Si)0-Ti, Ti-(Si)1-Ti, and Ti-(Si)2-Ti is more energetically favorable than the mono-substitution. Infra-red spectra have been simulated for the pure-silica LTA, the single titanium substitution, and the configurations Ti-(Si)0-Ti and Ti-(Si)2-Ti, comparing against experimental benchmarks where available. The energetics of the direct dissociation of water on these Lewis acid sites indicate that this process is only favored when two titanium atoms form a two-membered ring (2MR) sharing two hydroxy groups, Ti-(OH)2-Ti, which suggests that the presence of water may tune the distribution of titanium atoms within the framework of zeolite LTA. The electronic analysis indicates charge transfer from H2O to the Lewis acid site and hybridization of their electronic states.

‘Breathing-crystals’ the origin of electrochemical activity of mesoporous Li–MnO2

Akin to Le Chataliers principle, we show that a mesoporous material can mitigate the effect of stress by expanding or contracting elastically into the pore space; we simulate this ‘breathing-crystal’ phenomenon using MD simulation. In particular, our simulations reveal that mesoporous Li–MnO2 is electrochemically active because the stress, associated with charge cycling, does not influence the structure or dimensions of the (unlithiated) 1 × 1 tunnels in which the lithium ions intercalate and reside. Conversely, the parent bulk material suffers structural collapse and blockage of the 1 × 1 tunnels under stress. The mechanism associated with Li deintercalation is presented together with the activation energy barriers, which are calculated to be 0.4 eV – irrespective of whether the mesoporous host is unstrained or under considerable (1.6 GPa) tensile or compressive stress.

Computer Modelling of Elastic Properties of LaF3 Using Free Energy Minimisation Techniques

Abstract An effective use of computer simultion methods in the study of solids depends on the derivation of reliable potential models. Hence, the empirically derived interionic potentials of LaF3 were tested by free energy minimisation techniques, within the quasi-harmonic approximation. This was achieved by calculating the temperature variation of elastic constants derived directly from the shell model potentials. These changes agree reasonably well with those calculated from an experimental lattice expansion, below 800 K. However, the departure from linearity and experimental results occur in the range 800–1100 K, which is below the transition temperature to the state of high ionic conductivity. This shows how far the fitted shell model potentials can reliably predict the elastic behaviour of LaF3.

First Principle Study of Ti50Al50 Alloys

The study on the Ti-based materials and its application has been the interest of many research industries. These alloys are known to have an ordered B2 phase at high temperatures and transform to a stable low B19 martensitic phase. First principle approach has been used to study L10, B32, B2 and B19 Ti50Al50 alloys and the results compared well with the available experimental data. The equilibrium lattice constants are in good agreement with the experimental values (within 3% agreement). Furthermore, the elastic constants of these alloys are calculated, and revealed stability for L10 and B19 structures, while B2 and B32 gave C′<0 (condition of instability).

Advances in Ti-Based Systems as High Temperature Shape Memory Alloys

In this study, we investigate the effect of ternary addition on the structural, mechanical properties and temperature dependence of Ti-based as potential shape memory alloys using molecular dynamics approach. We found that binary Ti-Pt alloys exhibit shape memory properties and display possible martensitic transformation from B2 to B19 phases. Partial addition with Zr, Co, Pd, Ir showed preferential ternary high temperature shape memory alloys formation of 6.25 at. % X composition (Ti-Pt-X). We found that the equilibrium lattice constants are in better agreement with the available experimental values. The heats of formation and elastic properties reveal possible composition and phases at temperature above 900 K with good shape memory properties. Their structures were confirmed using the X-ray diffraction patterns at different temperatures.

Amorphisation and recrystallisation study of lithium intercalation into TiO2 nano-architecture.

Titanium dioxide is playing an increasingly significant role in easing environmental and energy concerns. Its rich variety of polymorphic crystal structures has facilitated a wide range of applications such as photo-catalysis, photo-splitting of water, photoelectrochromic devices, insulators in metal oxide, semiconductors devices, dye sensitized solar cells (DSSCs) (energy conversions), rechargeable lithium batteries (electrochemical storage). The complex structural aspects in nano TiO 2 , are elucidated by microscopic visualization and quantification of the microstructure for electrode materials, since cell performance and various aging mechanisms depend strongly on the appearance and changes in the microstructure. Recent studies on MnO 2 have demonstrated that amorphisation and recrystallisation simulation method can adequately generate various nanostructures, for Li-ion battery compounds. The method was also previously employed to produce nano-TiO 2 . In the current study, the approach is used to study lithiated nanoporous structure for TiO 2 which have been extensively studied experimentally, as mentioned above. Molecular graphic images showing microstructural features, including voids and channels have accommodated lithium’s during lithiation and delithiation. Preliminary lithiation of TiO 2 will be considered.