Periodic Reporting for period 3 - AMPERE (Accounting for Metallicity, Polarization of the Electrolyte, and Redox reactions in computational Electrochemistry)
Berichtszeitraum: 2021-04-01 bis 2022-09-30
This is due to the difficulty of accounting for complex effects arising from (i) the degree of metallicity of the electrode (i.e. from semimetals to perfect conductors), (ii) the mutual polarization occurring at the electrode/electrolyte interface and (iii) the redox reactivity through explicit electron transfers. Current understanding therefore relies on standard theories that derive from an inaccurate molecular-scale picture.
My objective is to fill this gap by introducing a whole set of new methods for simulating electrochemical systems. They will be provided to the computational electrochemistry community as a cutting-edge MD software adapted to supercomputers. First applications will aim at the discovery of new electrolytes for energy storage. Here I will focus on:
(1) ‘‘water-in-salts’’ to understand why these revolutionary liquids enable much higher voltage than conventional solutions
(2) redox reactions inside a nanoporous electrode to support the development of future capacitive energy storage devices.
These selected applications are timely and rely on collaborations with leading experimental partners. The results are expected to shed an unprecedented light on the importance of polarization effects on the structure and the reactivity of electrode/electrolyte interfaces, establishing MD as a prominent tool for solving complex electrochemistry problems.
In parallel, we have also started to simulate practical systems. We mostly focused on the water-in-salts electrolytes. In a first step we have studied the transport and interfacial properties of the most common water-in-salt, namely the one based on the LiTFSI salt (Li et al., J. Phys. Chem. C, 122, 23917, 2018 & J. Phys. Chem. B, 123, 10514, 2019). We have shown that the nanostructuration of the liquid impacts the diffusivities of all the species and the ionic correlations inside the melts. Then we have used our simulations to interpret 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 (Dubouis et al., ACS Central Sci., 5, 640, 2019). 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 (Bouchal et al., Angew. Chem., Int. Ed., 59, 15913, 2020). 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) (Dubouis et al., J. Phys. Chem. Lett., 9, 6683, 2018). 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 (Dubouis et al., Nature Catalysis, 3, 656, 2020).
The second series of systems that we target, 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 (Reeves et al., Phys. Chem. Chem. Phys., 22, 10561, 2020). The obtained data are now being used to develop polarizable force fields; and we expect to have finished this step by the end of 2020 in order to concentrate on the simulation of redox properties of these systems using classical molecular dynamics.
It is worth noting that another lead was followed, which was not anticipated upon writing the project. In order to develop a multi-scale approach, we have started to perform simulations in which the molecular dynamics step are replaced by molecular DFT calculations, both to determine capacitive properties (Jeanmairet et al., J. Chem. Phys., 151, 124111, 2019) and electron transfer reactions (Jeanmairet et al., Chem. Sci., 10, 2130, 2019). This approach allows a much more efficient sampling since it consists in computing the minimum of free energy of the system. The gains in computer time could be huge, but many difficulties have to be overcome for simulating complex systems such as water-in-salts or biredox ionic liquids.
Once these developments are finished, it will be possible to study much more deeply biredox ionic liquids, in order to understand their charging mechanism in supercapacitor applications. In particular our objective is to study the interplay between diffusion and electron transfer kinetics, and how it impacts the power performance of the devices.