First Principles Design of Efficient and Durable Energy Storage and Convergence Devices using Hybrid Interface Materials

Abstract

We utilize first principles density functional theory (DFT) calculations to investigate electronic and interfacial structures, thermodynamic and kinetic properties of hybrid interface materials. Hybrid interfaces are composed of heterogeneous materials creating unique properties unavailable with pure substances, such as enhanced mechanical, electrical, magnetic, and optical properties. To design the hybrid interface materials with enhance properties atomic level understanding of the structures are the first step. This thesis utilizes density functional theory calculations based on quantum mechanics to accomplish the goals, and additionally applies ab-initio and statistical thermodynamic formalism and molecular dynamics simulations. With the methodologies emerging materials for energy convergence and storage devices are computationally predicted and designed to acquire best performance of the systems. Specifically the thesis focused on two types of hybrid interface materials: thin Film coating to stabilize active electrode of Li-ion batteries and Dispersion of uniform nanoparticles to create multi functionality (mechanical and electric properties). In the first part of the thesis, we study how to enhance thermal stability of cathodes with high Ni composition in Li-ion battery application. It is found that several atomic layers of amorphous Al2O3 is very useful for enhancing the durability, which originates from strong attractive interaction between LiNiO2 (012) and Al2O3 deposits interface via highly ionic chemical bonding of Al and O. If combined with the experimental atomic layer deposition (ALD) technique the results can guide developing high performance electrode materials in Li-ion batteries. Secondly, the thesis demonstrates that catalytic activities towards oxygen reduction and evolution reactions (ORR and OER) in a Li–O2 battery can be substantially improved via hybrid materials with doped graphene and metallic supports. The activities are accurately predicted by thermodynamic free energy diagrams, by which the rate-determining step dominantly controlling overpotentials is identified. Thirdly, DFT calculations are applied to band gap engineering of zirconia (ZrO2), which has a wide band gap (~ 5 eV). It is shown that the width of the band gap is sensitive function of the concentration of oxygen vacancies. By controlled magnesiothermic reduction white ZrO2 transforms into oxygen deficient black ZrO2-x, which has substantially lower band gap (~ 1.5 eV). Combined with experimental fabrication and measurement it is nicely shown that for the first time, the black ZrO2-x interact with solar light with dramatically enhanced absorbance and significant activity H2 production with excellent stability. The forth part of this thesis reports that DFT calculations can accurately predict mechanical properties of fabricated single crystal Ni2Si nanowire (NW). The material is experimentally synthesized using simple three-step processes: casting a ternary Cu-Ni-Si alloy, nucleate and growth of Ni2Si NWs as embedded in the alloy matrix via designing discontinuous precipitation (DP) of Ni2Si nanoparticles and thermal aging, and finally chemical etching to decouple the Ni2Si NWs from the alloy matrix. DFT calculations of strain-stress curve for the Ni2Si NW is validated by direct application of uniaxial tensile tests to the sample and high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX). Lastly, DFT calculations and ab-initio thermodynamics are applied to develop completely new hybrid interface materials by Cu-based alloys to obtain high-mechanical properties (strength, ductility) and electrical conductivity simultaneously. It was not so successful in previous studies since the tow properties are mutually exclusive as conventionally speculated. It is, however, demonstrated that two contradictory material properties can be improved simultaneously if the interfacial energies between Cu matrix and uniform structures of Al2O3 nanoparticle are carefully controlled. The key underlying mechanism is elucidated as uniform dispersion of γ-Al2O3 nanoparticles over Cu matrix by adding small amounts of Ti solutes. Amazingly, Ti dramatically drives phase transformations of γ-Al2O3 particles from irregular shapes to homogeneous spherical morphologies via regulation of interfacial energies, leading to substantial enhancement of the mechanical property of Cu matrix without degrading its electric conductivity. We propose that the systematic methodologies pursued in this thesis significantly contributes to opening innovative ways to design various kinds of hybrid interface materials leading to improving performances of energy devices, advancing nanotechnology and commercializaing emerging materials into industries.