3rd TYC Energy Workshop: Materials for Electrochemical Energy Storage
Research on electrochemical energy storage is gaining great importance in recent years. The increasing drive from carbon-based to renewable energy generation, such as wind and solar, has posed the question of how to best store the electricity generated in order to overcome the problem of intermittency inherent to many of the renewable technologies. Another long-standing problem is the development of improved batteries with longer lifetime, shorter charging cycles and higher power densities/capacitance for applications in mobile technologies including cell phones and laptops, battery powered biomedical implants, and automobiles. Moreover, the question of availability and cost of materials used in commercial batteries today comes under greater scrutiny, especially so in light of a proposed development of large-scale energy-storage facilities for power grid applications. There is clearly a need to improve and optimise existing materials and to design entirely new battery materials with obvious benefits to society and economy.
Computational electrochemistry and battery materials research plays a significant role in addressing these issues. Due to the rapid increase of computational capabilities and the development of massively parallel computer codes, it is now possible to study realistic models of electrode-electrolyte interfaces from first principles. Such simulations have the potential to unravel the atomistic structure of electrochemical interfaces, and, in combination with experimental methods such as NMR and IR spectroscopies, the nature of the active species and the mechanism of charge/discharge cycles. However, in order to fully exploit their potential, first principles methods should be used/interfaced within a multi-scale computational approaches that capture relevant time and length scales of the problem. Very promising advances have also recently been made in the ab initio and high throughput structure prediction of functional materials, including battery materials. Developments in the latter area are particularly important for providing advanced tools that can efficiently narrow down the shear number of compositions and stoichiometries of possible new materials for cathodes and anodes. Relevant tools and approaches are being developed in adjacent fields of oxide fuel cells, microelectronics and electro-catalysis.
To bridge gaps between different communities and develop multi-scale strategies for future developments, we propose to organise a focused workshop for 2014, that will showcase the very best of research in the area of computational electrochemistry and battery materials. The workshop will provide a forum for the exchange of best practice and discussion of future directions of this growing field of computer simulation. We will bring together both computational scientists and experimentalists working on electrochemical energy storage on the small scale (Li-ion and Li-air batteries), on the large scale (flow batteries) and for high power density applications (supercapacitors). In addition, there will be a session on fundamental aspects of electrochemical interfaces and their simulation by ab initio and classical atomistic methods.
This event will be the third edition in a series of workshops established by the London Thomas-Young-Centre (TYC), on the timely topic of energy materials. The TYC, of which the organisers are members, is a London-wide interdisciplinary community of research groups working to address challenges of society and industry through the theory and simulation of materials. With about 80 participating groups, and ambitious programmes of events, the TYC is a supportive community for research students and young researchers and a source of new collaborations for visiting scientists. Our vision is to establish the TYC energy workshop as a fixture in the diaries of computational physicists and chemists that share our interest in energy related problems. To achieve this goal, we have created an international scientific advisory committee, comprised of 12 eminent experts in this field and leaders in computational or experimental materials research. The topic of the workshop and the list of the invited speakers attached to this proposal are based on the suggestions made by the committee.
In previous years two successful focused workshops on energy materials have been organised by the TYC. The first one in September 2010 on photo-induced energy conversion, hydrogen storage
and fuel cells, www.thomasyoungcentre.org/events/archive/279/energy-materials-electro-and-photo- chemical-interfaces/
and the second one in June 2012 (co-sponsored by Psi-k and CECAM),
on the theory and simulation of charge transfer in energy materials. Each of these events attracted both eminent scientists and young researchers, and more than 100 participants. The research presented and discussed was published in two themed issues Blumberger et al. Phys. Chem. Chem. Phys. 13, pp 7602 (2011) and Baletto et al. Phys. Chem. Chem. Phys 15, pp 4475 (2013) providing a lasting legacy of these events. Comments on the previous workshops captured by our questionaires have indicated overwhelming support of the scientific content and format of the workshop.
Here we request financial support from CECAM, which will be vital for the organisation of the 3rd workshop in this series. We plan to have 4 dedicated half-day sessions.
11 positive responses from leading computational and experimental researchers in the field have been received already and a few confirmations are still outstanding. There will also be the opportunity for younger researchers to
present their work in form of a contributed talk (about 8 slots) or poster presentation. Contributed talks will be selected depending on the quality of the abstracts submitted.
The current state of research on the energy-storage technologies, including Li-and Na-ion, Li-air and Na-S batteries, has been reviewed in a recent special issue of Advanced Functional Materials. While Li-ion batteries made a tremendous progress in the past decade and enabled a wide range of consumer electronics technologies, their application for the automobile market is far more challenging. (In particular, the long-term stability of the high-energy cathodes and high-capacity anodes need to be improved.) There is increasing interest in alternative technologies, such as Li-S and Li-air batteries which have theoretical specific energies of 2600 and 12000 Wh/kg, respectively (in comparison to the current goal of 250 Wh/kg for Li-ion batteries), but suffer from poor reversibility and poor stability of the electrode materials. Improving the charging rates, stepping up safety of operation and reducing the cost are some of the areas in which further progress is required in order to significantly reduce the dependence on the fossil fuels. Density functional theory and atomistic simulations were successfully used to model, for example, composite electrode materials and Li-oxygen discharge products . Calculations on the stability and phase transformation of the cathodes upon lithiation and de-lithiation [3,4] have revealed the effect of oxygen deficiency on the electrochemical stability of cathodes . It is now recognised that modelling of complex phenomena, such as memory effects  and correlated electron-ion transport in nano-structured materials requires true multiscale simulation approaches .
Electrochemical capacitors (ECs) or supercapacitors, including double-layer, pseudo-capacitors and hybrid capacitors, are nowadays emerging as energy storage devices thanks to their high power densities and long lifetime. They are characterised by a charge/discharge rate that is much faster than the one for batteries, of the order of few seconds, making them highly desirable in a number of applications ranging from portable electronics to hybrid vehicles. The state-of-the-art of ECs has been recently reviewed [8-10]. In order to become commercially available, the performance of supercapacitors should be improved, in particular their energy density, which should become more similar to the one Li-batteries, and their capacitance. For instance, higher capacitance could be achieved by increasing the specific surface area of the electrodes in double-layer supercapacitors based on nanostructured carbon-based electrodes or by nanostructuring redox active metal oxides in pseudo-capacitors . The design of the next generation of high-capacitance materials will benefit from computational studies that can unravel charge transfer mechanisms and the behaviour of ions confined in subnanometer pores as highlighted recently .
Significant progress has been made in recent years on the modelling of electrochemical interfaces using a number of different approaches. For instance, effective Hamiltonians describing the electrolyte-electrode interface
could be parametrized using electronic structure calculations[13-15], cluster models were successfully devised[16-17] and tools from computational surface science were extended and applied to electrocatalysis at the metal-water interface[18-20]. On a most rigorous level, although computationally highly demanding, all atom DFT-based molecular dynamics simulations approaches were developed for the calculation of absolute electrode potentials.[21-24] A central question in the field is how to align the electronic energy levels of the electrode and electrolyte when they are brought into contact. Two such approaches have been developed, the workfunction method and, very recently, the hydrogen insertion method of Sprik and co-workers. First applications of the latter method to a rutile-water interface have shown that the results are rather sensitive on the functional used, indicating that the improved performance of hybrid and range-separated functionals over GGAs for the description of band gaps carries over to the description of the energy levels at the electrochemical interface. Important issues to discuss are how these methods can be further developed (i) to gain atomistic insight into the kinetics of redox reactions at the electrode (ii) to help understand the atomistic details of the charge/discharge cycles of batteries and supercapacitors.
Francesca Baletto (King''s College, London, UK) - Organiser
Jochen Blumberger (University College London) - Organiser
Alexander L. Shluger (University College London) - Organiser
Peter Sushko (Pacific Northwest National Laboratory) - Organiser