- Mathieu Salanne
Sorbonne University, France
- Marie-Liesse DOUBLET
Institut Charles GERHARDT, France
Introduction and motivation
How much energy can a device store? How fast can it be charged? These two questions are at the heart of the research on electrochemical energy storage (EES). Two main families of devices coexist: supercapacitors which accumulate the charge at the surface of the electrodes through ion adsorption [1,2], and batteries in which the storage mechanism is based on redox reactions occurring in the bulk electrodes . Li-ion batteries have a high specific energy, keeping cellular phones, laptop and even cars working throughout several hours. For rapid power delivery and recharging, i.e. for high specific power applications, supercapacitors are then used.
Due to the recent advances in the field of materials science, the range of applications of EES devices has tremendously increased over the past two decades. The development of systems with improved performances and lower costs, as well as their large-scale production are now considered as vital issues for many countries. This can be seen from the recent creation of networks or institutes that gather academics and industrials, both at the national and European levels [4-6].
Most of the recent breakthroughs have however implied complex materials, often at the nanoscale. It is thus necessary to control the chemistry at the molecular level in all the active components of the devices, i.e. the two electrodes, the electrolytes. The various interfaces also have to be characterized and understood which implies considering potential dependent mechanistic approaches. Over the year, atomistic and molecular simulations have therefore appeared as one of the main keys to success in designing tomorrow’s high-energy and high-power EES devices, in complement with in situ and/or in operando spectroscopy techniques [7,8]. This is now well established in academic laboratories, which are now routinely building consortiums with synthesis, electrochemical and spectroscopic characterizations, together with modeling for developing new materials. However, this habit does not seem to be adopted yet by the industrial companies in the field. The objective of this workshop is therefore to bring together some of the worldwide experts in the field of EES simulations (and in particular the researchers who are developing the corresponding simulation tools) with the interested industrial partners. We hope that such a workshop could help bridging the gap between needs and supply, which would put simulation at the centre of the future industrial developments of EES devices.
The state-of-the-art can be considered at two levels: 1/ Simulation methods which are routinely used to simulate EES devices. 2/ Initiatives which are currently undertaken to bring simulation tools and/or results within the reach of non-specialist users.
From the methodological point of view, many different methods are used or developed depending on the nature of the material, the targeted properties and the necessary time/length scales.
-The workhorse for studying the redox activity of bulk electrode materials is standard Density Functional Theory (DFT) since it is necessary to have access to the electronic structure.
-For electrolytes, determining the transport properties involves the use of molecular dynamics. Depending on the availability of correct force fields, classical or DFT-based MD are generally used .
-Then further statistics or larger systems are generally studied by using lattice-based methods, such as kinetic Monte Carlo or Lattice Boltzmann.
Generally, standard DFT or MD packages can be used to study bulk materials. However in the case of interfaces, additional difficulties need to be overcome so that several groups are developing specific methodologies and/or simulation packages [9-11].
Despite the large growth in the simulation communities (especially DFT and MD) over the past decades, using these tools often requires lots of efforts for experimentalists and/or engineers in the industry. For this reason, several groups are currently developing user-friendly interfaces, either in specific programs or directly accessible from website . For efficiency reasons, it is necessary to develop high-throughput frameworks and to link these tools with accurate databases [13,14]. This implies that a common language is established between the communities of theorists and experimentalists, in order to build appropriate databases that will be helpful for material designers.
Finally, we should mention that several research groups are developing tools that aim to simulate systems at much larger scales [15,16]. The objective is to provide a direct link with experiments, by directly computing macroscale properties similar to the ones obtained in electrochemistry experiments. Such multi-scale methods, most often based on the Butler-Volmer equation, are typically top-down approaches that aim to account for the material or electrolyte specificity in an effective manner through appropriate parameterizations. Huge efforts are being devoted to the development of bottom-up approaches, with however major issues due to transferability between different scales.
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