Structural, Electronic, Mechanical, and Thermodynamic Properties of Na Deintercalation from Olivine NaMnPO4: First-Principles Study.

The impact of Na atom deintercalation on olivine NaMnPO4 was investigated in a first-principle study for prospective use as cathode materials in Na-ion batteries. Within the generalized gradient approximation functional with Hubbard (U) correction, we used the plane-wave pseudopotential approach. The calculated equilibrium lattice constants are within 5% of the experimental data. The difference in equilibrium cell volumes for all deintercalated phases was only 6%, showing that NaMPO4 is structurally more stable. The predicted voltage window was found to be between 3.997 and 3.848 V. The Na1MnPO4 and MnPO4 structures are likely to be semiconductors, but the Na0.75MnPO4, Na0.5MnPO4, and Na0.25MnPO4 structures are likely to be metallic. Furthermore, all independent elastic constants for NaxMPO4 structures were shown to meet the mechanical stability requirement of the orthorhombic lattice system.

Electronic, Structural, and Optical Properties of Mono-Doped and Co-Doped (210) TiO2 Brookite Surfaces for Application in Dye-Sensitized Solar Cells-A First Principles Study.

Titanium dioxide (TiO2) polymorphs have recently gained a lot of attention in dye-sensitized solar cells (DSSCs). The brookite polymorph, among other TiO2 polymorphs, is now becoming the focus of research in DSSC applications, despite the difficulties in obtaining it as a pure phase experimentally. The current theoretical study used different nonmetals (C, S and N) and (C-S, C-N and S-N) as dopants and co-dopants, respectively, to investigate the effects of mono-doping and co-doping on the electronic, structural, and optical structure properties of (210) TiO2 brookite surfaces, which is the most exposed surface of brookite. The results show that due to the narrowing of the band gap and the presence of impurity levels in the band gap, all mono-doped and co-doped TiO2 brookite (210) surfaces exhibit some redshift. In particular, the C-doped, and C-N co-doped TiO2 brookite (210) surfaces exhibit better absorption in the visible region of the electromagnetic spectrum in comparison to the pure, S-doped, N-doped, C-S co-doped and N-S co-doped TiO2 brookite (210) surfaces.

Optical and Electrochemical Applications of Li-Doped NiO Nanostructures Synthesized via Facile Microwave Technique.

Nanostructured NiO and Li-ion doped NiO have been synthesized via a facile microwave technique and simulated using the first principle method. The effects of microwaves on the morphology of the nanostructures have been studied by Field Emission Spectroscopy. X-ray diffraction studies confirm the nanosize of the particles and favoured orientations along the (111), (200) and (220) planes revealing the cubic structure. The optical band gap decreases from 3.3 eV (pure NiO) to 3.17 eV (NiO doped with 1% Li). Further, computational simulations have been performed to understand the optical behaviour of the synthesized nanoparticles. The optical properties of the doped materials exhibit violet, blue and green emissions, as evaluated using photoluminescence (PL) spectroscopy. In the presence of Li-ions, NiO nanoparticles exhibit enhanced electrical capacities and better cyclability. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results show that with 1% Li as dopant, there is a marked improvement in the reversibility and the conductance value of NiO. The results are encouraging as the synthesized nanoparticles stand a better chance of being used as an active material for electrochromic, electro-optic and supercapacitor applications.

Amorphization and recrystallization study of lithium insertion into manganese dioxide.

Various polymorphs of MnO(2) are widely used as electrode materials in Li/MnO(2) batteries. Electrolytic manganese dioxide (EMD) is the most electrochemically active form of MnO(2) and is very difficult to characterize. Their structural details are still largely unknown owing to the poor quality of X-ray diffraction (XRD) patterns obtained from most MnO(2) samples. Simulated amorphisation and crystallization technique was used to derive microstructural models for Li-MnO(2) which included most microstructural details that one would expect to find in the real material. Specifically, pyrolusite-MnO(2), comprising about 25,000 atoms, was amorphised (strain-induced) under molecular dynamics (MD) and different concentrations of lithium ions were inserted. Each system was then crystallized under MD simulation. The resulting models conformed to the pyrolusite polymorph, with microstructural features including: extensive micro-twinning and more general grain-boundaries, stacking faults, dislocations and isolated point defects and defect clusters. Molecular graphical images, showing the atom positions for the microstructural features together with simulated XRD patterns they give rise to, are presented and compared with measured XRD. The calculated XRD are in accord with experiment thus validating the structural models.