(Lecture, Jan 16) Topics on energy and materials science
January 15,2018 23:37:36 readCount:96
Time: Tuesday, Jan 16, 2018
Venue: Room 207, Building B5, University Town Campus
Topic 1: Surface and Interface Design for Energy Storage
Speaker: Sun Xueliang
Time: 9:15-10:00
Topic 2: Beyond electrostatic effects at oxide hetero-interfaces: Electrochemical phase change, strong electric fields, and elastic strain
Speaker: Bilge Yildiz
Time: 10:00-10:45
Transition metal oxide hetero-interfaces are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity and magnetism. These interfaces are the source for local heterogeneities in composition, atomic structure and electronic structure. Classically, defect redistribution is quantified at the continuum level by concurrent solution of Poisson’s equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. It is possible to inform this level of modeling with first principles calculations of band off-sets, and defect formation and segregation energies at thermodynamically relevant conditions. This approach had numerous successful implementations, including the quantification of charge transport properties at surfaces and grain boundaries. In this talk, I will discuss three phenomena that also need to be considered in a broader framework of defect structures and distributions at oxide hetero-interfaces. 1) Presence of strong electric fields that can cause polarization of defective systems and affect the defect abundance and structure. We have assessed this effect on neutral oxygen vacancies in simple binary oxides from first principles calculations. 2) Phase change under the effect of local electrostatic potential because of a change in the electrochemical potential of oxygen. We have assessed the ability to trigger phase change electrochemically in two classes of oxides, SrCoOx and VOx, and have quantified the phases and the corresponding distinctly different electronic properties by combining in operando x-ray diffraction and x-ray photoelectron and absorption spectroscopy. The results have implications both for oxide hetero-interfaces and for oxide electronic devices that aim to control properties electrically. 3) Elastic strain, that affect the stability and mobility of defects. In this recent work, we have focused on the stability of electronic defects, specifically the electron polarons versus free electrons SrTiO3, as a function of temperature and hydrostatic stress, by combining first principles calculations and quasi harmonic approximation. Our results demonstrate that it is possible to control the type of electronic defect, and so the transport properties, by means of electro-chemo-mechanics.
Topic 3: Tailoring the Semiconductor-Catalyst Interface for More Efficient Solar Energy Hydrogen Evolution
Speaker: Francesco Ciucci
Time: 11:00-11:45
Solid-state electrolytes with fast lithium conduction are the core of the all-solid-state Li battery technology. By substituting the organic electrolyte with a piece of non-flammable ceramic material, we can achieve a better safety, a higher specific capacity and a higher energy density. To date, the major bottleneck for this technology is the slow bulk diffusion of solid-state electrolyte and the interfacial incompatibility between the electrolyte and electrodes. To resolve these issues, several families of fast ionic conductors have been developed. The understanding of Li diffusion in these materials is essential to the development of novel family fast ionic conductors. In this sense, atomistic modeling provides us with a unique tool to obtain comprehensive information on the atom motion, which is difficult to access with experimental techniques. In this work, we use density functional theory (DFT) calculations as well as large scale classical molecular dynamics (MD) simulations to simulate the Li diffusion in a novel family of superionic conductor, lithium-rich anti-perovskites and provide an understanding of the Li diffusion behavior.
Lithium-rich anti-perovskites (LiRAPs) are a promising family of solid electrolytes, which exhibit ionic conductivities above 10−3 S cm−1 at room temperature, among the highest reportedvalues to date. We investigate the defect chemistry and the associated lithium transport in Li3OCl, a prototypical LiRAP, using DFT calculations and classical MD simulations. We studied three types of charge neutral defect pairs, namely the LiCl Schottky pair, the Li2O Schottky pair, and the Li interstitial with a substitutional defect of O on the Cl site. Among them the LiCl Schottky pair has the lowest binding energy and is the most energetically favorable for diffusion as computed by DFT. This is confirmed by classical MD simulations, where the computed Li ion diffusion coefficients for LiCl Schottky systems are significantly higher than those for the other two defects considered and the activation energy in LiCl deficient Li3OCl is comparable to experimental values. The high conductivities and low activation energies of LiCl Schottky systems are explained by the low energy pathways of Li between the Cl vacancies. We propose that Li vacancy hopping is the main diffusion mechanism in highly conductive Li3OCl.
Topic 4: Computational accelerated design of materials and interfaces for all-solid-state li-ion batteries
Speaker: Yifei Mo
Time: 15:40-16:25
All-solid-state Li-ion battery based on solid electrolytes is a promising next-generation battery technology with high energy density, intrinsic safety, long-life cyclability, and high-rate charging/discharging. However, multiple challenges, such as low ionic conductivity of solid electrolytes and poor interfacial compatibility at the solid electrolyte-electrode interfaces, are impeding the development of this novel technology. To resolve these materials challenges, we will design new materials and interfaces through an accelerated approach guided by computation, in contrast to a conventional trial-and-error approach. We will leverage an array of computation techniques to provide unique materials insights into the fundamental materials limitations and to establish general design principles of materials for overcoming such challenges. In the first part of the presentation, we will use first-principles atomistic modeling to reveal the origin of ultra-fast diffusion in lithium super-ionic conductors, which uniquely exhibit several orders of magnitude higher ionic conductivity than most solids. Materials design principles for fast ion conductors will be established based on the newly gained understanding, and such design principles will be demonstrated in our computation-guided discovery and design of new super-ionic conductors. In addition, we will present our first-principles database approach in investigating the compatibility of heterogeneous interfaces between electrolyte and electrodes, which are difficult to access in experiments. Key limiting factors at the solid electrolyte-electrode interfaces will be identified, and corresponding interfacial design using new materials will be proposed from computation guidance to address these interfacial limitations. The demonstrated computation capabilities represent a transferable model in designing new materials and interfaces for emerging technologies.

Announced by School of Environment and Energy