Abstract
Lithium-ion batteries have found an important place in our daily life, used in portable electrical devices e.g. mobile phones and laptops. Despite their success they are still open for improvement, particularly if their applications should be extended to hybrid (HEV) and electrical vehicles (EV). The batteries currently available on the market show problems with capacity (energy density), cell-potential, charge/discharge rates and lifetime. In (most) commercial Li-ion batteries on the market today, the technology in both the anode and cathode materials are based on classical Li-intercalation processes, where Li-ions are extracted or inserted from an open host structure. The disadvantages with these materials are their failure to often incorporate more than one Li-ion per transition metal (TM) ion, resulting in low capacity (energy density).The introduction of nano-materials in the field of Li-ion batteries has re-opened the investigations on simple transition metal oxides, sulphides, and nitrides as attractive new anode materials. Experiments show that the reactions of these TM compounds with Li are different from the classical Li-intercalation processes. Instead the transition metals are reduced in the presence of Li-ions to form metal nano-particles of 2 to 8 nm in size, dispersed in a Li2X (X=O, S or N) matrix. Owing to the fact that the Li/TM ratio associated with these reactions are larger than one, these materials show high energy storage; in some materials above 1000 mAh/g, which is about three times as high as in commercial graphite anodes. However, the capacity associated with conversion reactions often decrease rapidly after the first charge/discharge cycle. Hence, to improve the properties of these materials we need to characterise these reactions in detail on an atomic level.One powerful tool, to gain atomic information is the combination of first-principles and classical inter-atomic potential simulations. Here we propose to use such an arrangement to determine the stability and reactivity of TM-oxide nano-particles (2-8 nm) investigated as potential anode materials in conversion reactions. The theories we will apply are well established in the field of battery modelling and material science, but they have rarely or never been applied to conversion reactions of nano-particles before. The calculations will, therefore, partly be guided by collaboration with experimental groups studying the corresponding systems. For this purpose we need to study a system that is well documented, and that we have experience synthesising and characterising here in Kent. The material should, of course, also have the potential as an anode material in Li-ion batteries. Fe3O4 satisfies the requested criterions. Further, Fe3O4 is highly available (low cost) and non-toxic. The PI has expertise in modelling iron-bearing materials, using both first principles and classical inter-atomic potential techniques. We will establish the energetically most stable Fe3O4 nano-particle structures as a function of Li-ion concentration and particle size, allowing us to also calculate the cell voltage of the material. The rate and power capability of the material can be understood by determining the Li-ion reactivity and band gap of the particles, respectively.
Planned Impact
Identifying fossil fuels as one of (or) the main source to global warming, has accelerated our search for alternative energy sources, gaining much attention in media with impacts on governmental decisions. Despite the general awareness much research is still needed into this field and the development of anode materials for lithium ion batteries is one contribution. In a long-term perspective, reducing the CO2 footprint would have a direct impact on the environment and human health. Re-chargeable lithium ion (Li-ion) batteries have in a short time become commercially highly successful in portable electronic applications such as laptops and mobile phones. However, they also have applications in micro-batteries for use in eg. hearing aids. The Global Industry Analyst, Inc. produced a report in October 2008, predicting an increase of unit sales by >8%, corresponding to an increase in value sales of > 7%. Only in Europe the LiMnO2-based batteries are expected to reach 1.19 billion units, worth ~$310 million, by 2012. Despite their success they need further research for applications in high-energy storage applications, such as electrical vehicles (EV) and hybrid electrical vehicles (HEV). The reason is that the materials used in commercial Li-ion batteries today fail to fulfil the requirement in capacity (energy storage). However, research presented in the last five years has demonstrated promising results towards applications for EVs and HEVs. As an example Transport for London is currently running a test case, in which Li-ion batteries are used in HEV buses, and if improved charge/discharge rates could be obtained Li-ion batteries would be desirable in devices used by the military. We have existing links to industries within the UK and EU via the Alistore-ERI network, and the Enterprise office at the University of Kent. These contacts may encourage future industrial funding via, for example, KTP, SEEDA and EPSRC/CASE studentships. Intellectual property (IP) that may arise from the project will be handled by the Enterprise and research offices at the University of Kent. Immediate beneficiaries of the research will be academic groups working on energy storage materials, but also experimental and theoretical groups in the field of nano-technology. The computational models employed in the current proposal might be employed to study materials for other technological applications, and we confirm that results of interest to a wider audience will be submitted to accessible high impact journals. The PI has a satisfying publication record. Highlights of the results will also be presented at key meetings, and published at a web-page on Energy and Environmental Research in Kent maintained by the PI. The project involves up and coming technology with wide implications. Hence, the work will benefit from contacts with existing collaborations, both experimental and theoretical groups, but it would also serve to establish new contacts, particularly with academic groups, but also industries. As a result further applications for PhD and Post-doctoral research assistant (PDRA) positions, will be submitted to, for example, UK research councils, but also EU-funding will be sought after via collaborations within the Alistore-ERI network. Training of new expertise in the field of Li-ion batteries has important future benefits. The host institution organise successful outreach activities, and one possible route to get contacts with a more general public is to present highlights of results achieved in the project in connection with these activities. In addition, the work is suitable for Caf Scientific , a seminar series on popular science organised by the University of Kent (UKC). Dissemination of relevant results to media and the general public will be assisted by the Kent information office, while possible intellectual property (IP) arising from the project will be dealt with by the Enterprise and research offices at UKC.