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

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.

Here we list beneficiaries of the proposed research, summarise its relevance to the beneficiaries, and identify potential for impact arising from this work. 1) Academic community (see also Academic Beneficiaries section), public, government, and commercial organisations involved in the research and development will benefit from New knowledge: fundamental understanding of the coupled ion-electron diffusion mechanisms in the TiO2 bulk and along grain boundaries. This will impact understanding of generic interfacial electronic processes in TiO2 and other materials and stimulate the design of new ion-electron conductors. New tools: researchers involved in the computational modelling can use the methodology and computer codes, which we propose to develop, in their work. Impact: expanding the field of studied systems, further development of the computational techniques either complementary or alternative to the standard periodic model and widely used methods based on the density functional theory. New expertise: we will promote and will be open to share our experience in the modelling complex processes and design of the embedding potentials. Impact: formation of interdisciplinary collaborative networks centred on one or several topics including interfacial processes in TiO2, electronic processes in oxide semiconductors, complex diffusion mechanisms, and development of libraries of the embedding potentials for a wider user community and broader applications. 2) Developers and manufactures of novel Li batteries will benefit from the specific knowledge of the mechanisms and timescales of Li ion migration in the bulk and along grain boundaries, which will impact the choice of optimal conditions for the materials processing used in the manufacturing and design of the higher power-density and more reliable batteries with shorter charging times. 3) Developers of the NWChem and MOLCAS computer codes (our project partners) will benefit from increased functionality of their respective codes. Being a spinoff company, MOLCAS can increase their revenues through higher circulation and, thus, stimulate further development. Advances in the NWChem code will increase international visibility of the EMSL User Facility (DoE) and will further promote formation of international collaborative links. Impact: enhanced computational capabilities made available to a broad community of users. 4) The PDRA working on the project will acquire new skills and attain experience important to secure long-term employment potential. Impact: contribution to the development of skilled workforce. 5) Society in general will benefit indirectly through a broader use of the improved Li batteries in automobile industry and households, reducing dependence on the use of fossil fuels, and decreasing unnecessary power loss. Impact: decreased levels of the heat and CO2 emissions, lower noise and pollution levels. We plan to maximise potential impacts by: 1) Establishing and maintaining collaborations with developers of widely used computational packages for materials modelling and distributing our methodology through these codes both in the UK and overseas. 2) Collaborating with experimentalists and materials modellers working on ionic transport for energy applications and, in particular, engaging with researchers working on applications of TiO2 in energy-conversion devices and photo-catalysis. 3) Communicating the results of our work via scientific publications, conference presentations, seminars, and discussion meetings. 4) Communicating our achievements to a broader audience, including undergraduate students, through UCL-based webpage.