Transport Properties of Li-TFSI Water-in-Salt Electrolytes

Abstract

Water-in-salts are a new family of electrolytes that may allow the development of aqueous Li-ion batteries. They have a structure which is reminiscent of the one of ionic 1 liquids, and they are characterized by a large concentration of ionic species. In this work we study their transport properties and how they evolve with concentration by using molecular dynamics simulations. We first focus on the choice of the force field. By comparing the simulated viscosities and self diffusion coefficients with experimental measurements, we select a set of parameters that reproduces well the transport properties. We then use the selected force field to study in detail the variations of the self and collective diffusivities of all the species as well as the transport number of the lithium ion. We show that correlation between ions and water play an important role over the whole concentration range. In the water-in-salt regime, the anions form a percolating network which reduces the cation-anion correlations and leads to rather large values for the transport number compared to other standard electrolytes.

Computational Screening of the Physical Properties of Water‐in‐Salt Electrolytes**

“…Figure 3c shows the values obtained for H 2 O molecules, lithium cations and the various anions. In agreement with previous works, [12,42] it can be seen that water molecules are the most mobile species, followed by lithium cations and anions. This corresponds to an opposite behaviour with respect to the dynamics of the ions in typical ionic liquid electrolytes, where anions generally diffuse faster than cations [12,39,47] …”

“…The simulations predict a LiTFSI‐H 2 O viscosity in good agreement with experimental data (≈33 cP instead of 22 cP, [42] ) while they underestimate the ionic conductivity by a factor 2, [6,15,42] which corresponds to typical error for the prediction of transport properties in electrolytes by non‐polarizable molecular dynamics [43] . Although the use of a different parameterization for the partial charges and Lennard‐Jones parameters of the anion could improve the situation, [42] the present parameters were chosen in order to keep consistency between all the different anions studied and to compare them without introducing any bias.…”

Computational Screening of the Physical Properties of Water-in-Salt Electrolytes

“…36 Figure 3c shows the values obtained for H 2 O molecules, lithium cations and the various anions. In agreement with previous works, 12,32 it can be seen that water molecules are the most mobile species, followed by lithium cations and anions. This corresponds to an opposite behaviour with respect to the dynamics of the ions in typical ionic liquid electrolytes, where anions generally diffuse faster than cations.…”

“…WiS. 32 In order to reach proper density, each system was firstly equilibrated at 298.15 K and 1 bar for 4 ns in the NpT ensemble by using Nosé-Hoover thermostat and barostat [48][49][50] with relaxation times of 10 and 500 femtoseconds, respectively. Then we performed a second equilibration of 60 ns within an NVT ensemble, followed by a production run of around 90 ns (with a time step dt = 1 fs) that was used to obtain structural and dynamic information about the systems.…”

Understanding the electrochemistry of “water-in-salt” electrolytes: basal plane highly ordered pyrolytic graphite as a model system

“…The less solvated uorinated anions form a percolating network that reduces the cation-anion pairs which enhances the transport number of the cations, compared to traditional dilute electrolytes. 15 The structure of such water-in-salt systems is best described by the presence of highly solvated cations with 3D percolating channels for fast cation transport; and a less solvated aggregates of an anionic network with slow relaxation time that immobilises the anion movement. 16 There have been some reports, e.g.…”