From the methodological point of view, in a first step we have focused on the development of an approach in which a finite field is applied to an electrochemical cell made of a single electrode in contact with an electrolyte. It has two interesting features: i) it allows to switch from 2D to 3D periodic boundary conditions, which will accelerate the simulations. ii) the method is readily extended to perform finite electric displacement simulations, that is to simulate an electrochemical system under open circuit conditions. Another important methodological advance is the introduction of a mass-zero constrained molecular dynamics algorithm, which has the advantage to be symplectic and time reversible. Finally, we have developed the theoretical framework of a semiclassical Thomas-Fermi model to tune the metallicity of electrodes in molecular simulations. By systematically varyiing the Thomas-Fermi length, we have shown that all the interfacial properties are modified by screening within the metal: the capacitance decreases significantly and both the structure and dynamics of the adsorbed electrolyte are affected. In a last step, we have implemented an algorithm allowing to account for the polarization of the electrolyte together with the polarization of the electrode, as well as the possibility to fix the electrochemical potential of the atoms rather than the electrode potential, allowing to simulate multi-component materials.
In parallel, we have also simulated practical systems for energy storage and conversion applications. A large amount of work focused on the physico-chemical properties of the family of water-in-salts electrolytes. Firstly, we have studied the transport and interfacial properties of the most common water-in-salt, namely the one based on the LiTFSI salt. We have shown that the nanostructuration of the liquid impacts the diffusivities of all the species and the ionic correlations inside the melts. This work was further extended to the study of a large variety of water-in-salt anions. Our simulations have also shown an intriguing feature, namely the polymerization of lithium ions inside the liquid, which can have some important consequencies on the interfacial properties as well. We have also performed many simulations aiming at the interpretation of a series of experimental results: On the one hand, we have shown the occurence of aqueous biphasic systems when mixing water-in-salts with simpler aqueous electrolytes as well as the physical processes at the origin of their formation. On the other hand we have provided further understanding on the formation of a stable solid electrolyte interphase in water-in-salt-based Li-ion batteries by establishing a competitive salt precipitation/dissolution during the reduction of water at the interface. The mechanism of the anion decomposition was studied in detail through the use of ab initio molecular dynamics simulations. As an extension to this work, we have started a study on the effect of confining water in organic electrolytes (instead of ionic ones as in water-in-salts). We have shown that these systems provide very important information on the influence of the chemical speciation of water over its interfacial reactivity, with potential impact in the field of electrocatalysis.
The second series of systems that we targeted, biredox ionic liquids, needed more developments. In particular, there was no force field available to simulate them so we started by performing an (electronic) Density Functional Theory study of their electrochemical properties. The obtained data was then used to develop polarizable force fields in order to perform classical molecular dynamics simulations. We could thus study in detail the structure of the solvation shell of anthraquinone and TEMPO redox-active species in an acetronitrile solvent as well as in a pure ionic liquid. When simulating the pure biredox ionic liquid, we observed some peculiar nanostructural organization, with the formation of nanodomains enriched with the redox-active groups, which can strongly impact the performance of the supercapacitors and explain why higher than expected energy densities are obtained.
