Showing papers by "Penghao Xiao published in 2012"
TL;DR: A generalized solid-state nudged elastic band (G-SSNEB) method is presented for determining reaction pathways of solid-solid transformations involving both atomic and unit-cell degrees of freedom and it is demonstrated that the method is robust for mechanisms dominated either by atomic motion or by unit- cell deformation.
Abstract: A generalized solid-state nudged elastic band (G-SSNEB) method is presented for determining reaction pathways of solid–solid transformations involving both atomic and unit-cell degrees of freedom. We combine atomic and cell degrees of freedom into a unified description of the crystal structure so that calculated reaction paths are insensitive to the choice of periodic cell. For the rock-salt to wurtzite transition in CdSe, we demonstrate that the method is robust for mechanisms dominated either by atomic motion or by unit-cell deformation; notably, the lowest-energy transition mechanism found by our G-SSNEB changes with cell size from a concerted transformation of the cell coordinates in small cells to a nucleation event in large cells. The method is efficient and can be applied to systems in which the force and stress tensor are calculated using density functional theory.
687 citations
TL;DR: In this article, a density functional theory (DFT+U) electronic structure calculation of Li2MnO3-based lithium-rich layered cathodes was performed and it was shown that substitution of Mn with other transition metals (Ti and Co) significantly affects the amount of oxygen loss and capacity during the first charge/discharge cycle.
Abstract: Oxygen loss can lead to high-capacity Li2MnO3-based lithium-rich layered cathodes. Substitution of Mn with other transition metals (Ti and Co) significantly affects the amount of oxygen loss and capacity during the first charge/discharge cycle. An explanation of these results is provided with density functional theory (DFT+U) electronic structure calculations. Oxygen is found to bind more strongly to Ti and more weakly to Co. The influence of the substitution is attributed to changes of the band gap. Ti lifts the nonbonding band and increases the band gap of the compound, thus raising the energy required to redistribute the electrons released upon oxygen loss. Co lowers the nonbonding band and facilitates oxygen loss.
100 citations
TL;DR: In this article, the lithium insertion behavior of nanoparticle (3D) and nanosheet (2D) architectures of TiO2(B) is quite different, as observed by differential capacity plots derived from galvanostatic charging/discharge experiments.
Abstract: The lithium insertion behavior of nanoparticle (3-D) and nanosheet (2-D) architectures of TiO2(B) is quite different, as observed by differential capacity plots derived from galvanostatic charging/discharge experiments. DFT+U calculations show unique lithiation mechanisms for the different nanoarchitectures. For TiO2(B) nanoparticles, A2 sites near equatorial TiO6 octahedra are filled first, followed by A1 sites near axial TiO6 octahedra. No open-channel C site filling is observed in the voltage range studied. Conversely, TiO2(B) nanosheets incrementally fill C sites, followed by A2 and A1. DFT+U calculations suggest that the different lithiation mechanisms are related to the elongated geometry of the nanosheet along the a-axis that reduces Li+–Li+ interactions between C and A2 sites. The calculated lithiation potentials and degree of filling agree qualitatively with the experimentally observed differential capacity plots.
85 citations
TL;DR: Li et al. as mentioned in this paper demonstrate that the undercoordinated Fe2+/Fe3+ redox couple at the surface gives a high barrier for charge transfer, but it can be stabilized by nitrogen or sulfur adsorption.
Abstract: Despite the great achievement in understanding the materials properties and powder engineering of LiFePO4, the chemical bonding at the surface has been almost ignored. Herein, we demonstrate that the undercoordinated Fe2+/Fe3+ redox couple at the surface gives a high barrier for charge transfer, but it can be stabilized by nitrogen or sulfur adsorption. The surface modification improves greatly the charge transfer kinetics and the charge/discharge performance of a LiFePO4 cathode. Density functional theory (DFT) calculation estimates the origin of the improvement in terms of an electronic and ionic contribution based on a surface model probed by time of flight secondary ion mass spectrometry (TOF-SIMS); the calculation agrees well with an experimental rate-constant analysis.
63 citations
TL;DR: A new reaction mechanism between graphite and diamond with nucleation characteristics that has a lower activation energy than the concerted mechanism is found, resolving a conflict between theory and experiment.
Abstract: Phase transitions between carbon allotropes are calculated using the generalized solid-state nudged elastic band method. We find a new reaction mechanism between graphite and diamond with nucleation characteristics that has a lower activation energy than the concerted mechanism. The calculated barrier from graphite to hexagonal diamond is lower than to cubic diamond, resolving a conflict between theory and experiment. Transitions are calculated to three structures of cold compressed graphite: bct C4, M, and Z-carbon, which are accessible at the experimentally relevant pressures near 17 GPa.
32 citations
TL;DR: In this article, Nitrogen and sulfur adsorption/chemisorption on LiFePO4 particles is characterized by DFT calculations and time of flight secondary ion mass spectrometry.
Abstract: Nitrogen and sulfur adsorption/chemisorption on LiFePO4 particles is characterized by DFT calculations and time of flight secondary ion mass spectrometry.
7 citations