| Summary: | There are critical problems hinder the further application of the batteries. One of these problems is related to the efficiency of the charging/discharging process. A simple idea has been proposed to solve this problem in different ways. This idea is proposed in the Prof. White's group: by shorten the distance between two electrodes to the nanometer scale, the electrolyte cannot completely screen the electric field originated from the electrodes, the unscreened electric field can provide driven force to the migration of ions in the electrolyte, and such effect is named as the "Coulomb Transport effect". Computational simulations are performed to validate this idea and explore the mechanisms from the atomistic view point.
Firstly, the electrolyte consisting 2591 water molecules, one excess proton and one chloride anion sandwiched by two plane electrodes are simulated under a range of different applied voltages. It is shown in the simulation that migration rate of the excess proton is correlated to the applied voltages. Secondly, another system consisting of Mg[TFSI]2 in MeCN solution is setup and simulated under applied voltages using the polarizable electrode model. It has been proposed for long time that the Mg2+ tends to form clusters in electrolyte, which may largely decrease the efficiency of batteries in two ways: 1) it can decrease the effective charge of the Mg 2+, e.g., [MgTFSI]+ only possesses +1 charge; 2) migration of larger ion cluster can be slower compared with transportation of small clusters. As it is expected, the simulation results support this idea. Thirdly, we proposed to add supporting charges into the electrolyte. The simulations of the electrolyte consisting of [IrCl6]2-/3- redox couple has shown this trend that by adding different concentrations of NaNO 3 as supporting charges, the migration of the ions can be less affected by the Coulomb Transport effect.
We have also explored the hydrated proton solvation and diffusion near the electrode surface using the same model. The proton transportation process is related to the complex collective re-orientational motions of the surrounding water molecules, and the slow dynamics of adsorbed water molecules may affect the mechanism near the electrode. Through simulation, we have proposed a new proton transport mechanism that the proton may hop between the adsorbed water layer and the second water layer to avoid being trapped by the slow dynamics of highly ordered water molecules adsorbed onto the electrode surface.
Besides all of the atomistic simulations described above, we have also tried to improve the simulation efficiency by constructing coarse-grained models in two ways. The first way is to construct the solvent free model which can reproduce the structural, dynamics, and thermodynamics properties of the atomistic models. The second model is constructed onto the center of charge of molecules, which can improve the simulation efficiency without scarifying the important information of the screening effect of the electrolyte molecules.
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