Computational structure analysis of multicomponent oxides

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references.First principles density functional theory (DFT) energy calculations combined with the cluster expansion and Monte Carlo techniques are used to understand the cation ordering patterns of multicomponent oxides. Specifically, the lithium ion battery cation material LiNi0.5Mn0.5O2 and the thermoelectric material P2-NaxCoO2 (0.5 =/< x =/< 1) are investigated in the course of this research. It is found that at low temperature the thermodynamically stable state of LiNi0.5Mn0.5O2 has almost no Li/Ni disorder between the Li-rich and transition metal-rich (TM) layer, making it most suitable for battery applications. Heating the material above ~600°C causes an irreversible transformation, which yields a phase with 10~12% Li/Ni disorder and partial disorder of cations in the TM layer. Phase diagrams for the NaxCoO2 system were derived from the results of calculations making use of both the Generalized Gradient Approximation (GGA) to DFT and GGA with Hubbard U correction (GGA+U). This enabled us to study how hole localization, or delocalization, on Co affects the ground states and order-disorder transition temperatures of the system. Comparison of ground states, c lattice parameter and Na1/Na2 ratio with experimental observations suggest that results from the GGA, in which the holes are delocalized, matches the experimental results better for 0.5 =/< x =/< 0.8. We also present several methodological improvements to the cluster expansions. An approach to limit phase space and methods to deal with multicomponent charge balance constrained open systems while including both weak, long-range electrostatic interactions and strong, short-range interactions in a single cluster expansion.by Yoyo Hinuma.Ph.D

Comparison of Matlantis and VASP bulk formation and surface energies in metal hydrides, carbides, nitrides, oxides, and sulfides

Generic neural network potentials without forcing users to train potentials could result in significantly acceleration of total energy calculations. Takamoto et al. [Nat. Commun. (2022), 13, 2991] developed such a deep neural network potential (NNP) and made it available in their Matlantis package. We compared the Matlantis bulk formation, surface, and surface O vacancy formation energies of metal hydrides, carbides, nitrides, oxides, and sulfides with our previously calculated VASP values obtained from first-principles with the PBEsol(+U) functional. Matlantis bulk formation energies were consistently ~0.1 eV/atom larger and the surface energies were typically ~10 meV/{\AA}^2 smaller than the VASP counterpart. Surface O vacancy formation energies were generally underestimated within ~0.8 eV. These results suggest that Matlantis energies could serve as a relatively good descriptor of the VASP bulk formation and surface energies

Configuration entropy effect on temperature coefficient of redox potential of P2-Na x CoO2

The temperature coefficient (α) of redox potential (V) is a significant material parameter that coverts thermal energy into electric energy. In this paper, we determined α of P2-type Na x CoO2 against the Na+ concentration (x). The solid component (α solid) of α scatters from −0.18 to 0.64 mV K−1. The phase separation model cannot reproduce the x-dependence even qualitatively. We interpreted the unexpected disagreement in terms of the residual configuration entropy in the pseudo-disordered region between the Na+ ordered phases

Nihon University CubeSat Program

The CubeSat program is now proceeding in Japan and U.S. academic institutions and radio groups. CubeSat is a 10cm cubed, 1kg weighted micro-satellite. The first launch of CubeSats is scheduled in May 2002. The second launch will be realized in autumn 2002 or later. Subsequent launch is also planned. Nihon University is going to join the second launch. Our program consists of two phases. At the first phase, we are developing a CubeSat for the second launch opportunity. The purpose of the first phase is that the students learn the whole process of the micro satellite development and operation. At the second phase, we intend to challenge some engineering mission. We have been studying on a mission of deploying an inflatable structure model. In this paper we show the latest status of the first phase of our program , and the plan for the second phase

Lithium Diffusion in Graphitic Carbon

Graphitic carbon is currently considered the state-of-the-art material for the negative electrode in lithium-ion cells, mainly due to its high reversibility and low operating potential. However, carbon anodes exhibit mediocre charge/discharge rate performance, which contributes to severe transport-induced surface-structural damage upon prolonged cycling, and limits the lifetime of the cell. Lithium bulk diffusion in graphitic carbon is not yet completely understood, partly due to the complexity of measuring bulk transport properties in finite-sized, non-isotropic particles. To solve this problem for graphite, we use the Devanathan-Stachurski electrochemical methodology combined with ab-initio computations to deconvolute, and quantify the mechanism of lithium-ion diffusion in highly oriented pyrolytic graphite (HOPG). The results reveal inherent high lithium-ion diffusivity in the direction parallel to the graphene plane (ca. 10^-7 - 10^-6 cm2 s-1), as compared to sluggish lithium-ion transport along grain boundaries (ca. 10^-11 cm^2 s^-1), indicating the possibility of rational design of carbonaceous materials and composite electrodes with very high rate capability.Comment: 9 pages, 3 figure