Modelling correlated electron-ion diffusion in nano-scale TiO2: beyond periodic model and density functional theory

当前研究的重大挑战之一的主要焦点是减少社会对使用化石燃料的依赖，从而降低二氧化碳排放水平。这一挑战的一个主要战略组成部分是用太阳能电池、氢燃料电池和电池等替代品取代作为汽车和航空航天应用主要燃料来源的液体燃料（汽油、柴油和煤油）。特别是，几种使用锂电池的汽车型号已经批量生产。然而，电动汽车占据的市场利基仍然很小，因为电池充电时间长，能量容量小，老化。例如，在伦敦居民中很受欢迎的G-Wiz充电时间为8小时，最大行驶里程约为50英里，而一款新的（2008年生产）全电动跑车Tesla Roadster需要长达17小时的充电时间才能行驶240英里。需要对锂电池特性进行非增量改进，以大幅增加电动汽车的比例，从而大大降低噪音、污染和CO2排放水平，这反过来将具有有益的经济、环境和健康影响。使用纳米结构电极材料，特别是纳米结构TiO 2基化合物，成为提高锂电池性能特征的一种有前途的策略。该研究项目旨在通过对锂离子迁移与电子跳跃耦合的原子尺度机制和动力学的量子力学建模，促进新电极材料的开发。我们将调查的Li+ -电子相互作用的影响，通过比较它们的迁移动力学单独和作为一对。为了研究纳米结构化的影响，我们将比较Li+和e-物种在体中和晶界附近的迁移动力学。理解Li+/e-耦合的效果和界面结构的效果将为我们提供用于控制和优化能量容量和充电速度的各种协议。这些协议中的一些可以包括选择TiO 2纳米结构的特征尺寸和控制电子注入。尽管是一个普遍的问题，相关的离子-电子迁移在氧化物还没有得到妥善解决的量子力学水平。这是因为广泛使用的基于密度泛函理论（DFT）的从头计算方法严重低估了绝缘体和半导体的带隙，这导致对捕获电子的性质的定性描述不正确。此外，这些方法采用周期性边界条件模型，这不适用于模拟复杂的非周期性结构。在本提案中，我们将突破这些限制，并应用专门为不规则表面和界面中的缺陷建模而设计的嵌入式聚类方法。作为该提案的一部分，我们将开发一致的长程静电和短程嵌入电位的新形式。这将使我们能够应用精确的量子化学方法进行电子结构计算，而不会受到传统DFT技术的缺点。这些嵌入电位可用于与TiO 2块体，表面，界面和纳米结构相关的许多电子现象的其他计算研究。为了促进它们的广泛应用，我们将与量子化学计算计算机软件包的开发人员合作。

A principal focus of one of the current research grand challenges is on reducing society's dependence on the use of fossil fuels and thus decreasing the CO2 emission levels. A major strategic component of this challenge is replacing liquid fuels (petrol, diesel, and kerosene), as main fuel sources for automotive and aerospace applications, with alternatives such as solar cells, hydrogen fuel cells, and electric batteries. In particular, several models of automobiles operating on Li batteries are already mass-produced. However, the market niche taken up by the electric cars remains small due to long battery charging times, small energy capacity, and aging. For example, the G-Wiz, which is popular among inner London residents, has a charging time of 8 hours and maximum range of ~50 miles, while a new (2008 production) all-electric sports car the Tesla Roadster needs up to 17 hours of charging time for a 240 mile journey. Non-incremental improvements of Li battery characteristics are needed to dramatically increase the fraction of electric automobiles and, thus, considerably reduce noise, pollution, and CO2 emission levels, which, in turn, will have beneficial economical, environmental, and health implications. The use of nano-structured electrode materials, in particular, nano-structured TiO2 based compounds, emerged as a promising strategy for increasing performance characteristics of Li batteries. This research project aims to facilitate the development of new electrode materials through quantum-mechanical modelling of the atomic-scale mechanisms and kinetics of Li ion migration coupled with electron hopping. We will investigate the effects of the Li+ - electron interaction by comparing the kinetics of their migration individually and as a pair. To investigate the effects of nano-structuring, we will compare the migration kinetics of the Li+ and e- species in the bulk and in the vicinity of the grain boundary. Understanding the effect of the Li+/e- coupling and that of the interface structure would provide us with a variety of protocols for controlling and optimising the energy capacity and charging speed. Some of these protocols may include selecting characteristic size of TiO2 nano-structures and controlling electron injection. Despite being a generic problem, correlated ion-electron migration in oxides has not been properly addressed on the quantum mechanical level. This is because widely used ab initio methods based on the density functional theory (DFT) severely underestimate the band gap of insulators and semiconductors, which results in a qualitatively incorrect description of the properties of trapped electrons. In addition, these methods employ a periodic boundary conditions model, which is not applicable to modelling complex non-periodic structures. In this proposal, we will break through these limitations and apply an embedded cluster method specifically designed for modelling defects in irregular surfaces and interfaces. As a part of this proposal, we will develop new forms of consistent long-range electrostatic and short-range embedding potentials. This will allow us to apply accurate quantum-chemical methods for electronic structure calculations, which do not suffer from drawbacks of conventional DFT techniques. These embedding potentials can be used in other computational studies of numerous electronic phenomena associated with TiO2 bulk, surfaces, interfaces, and nano-structures. To facilitate their wider availability, we will collaborate with developers of computer packages for quantum-chemical calculations.