Water-in-Salt LiTFSI Aqueous Electrolytes. 1. Liquid Structure from Combined Molecular Dynamics Simulation and Experimental Studies

Abstract

The concept of water-in-salt electrolytes was introduced recently, and these systems have been successfully applied to yield extended operation voltage and hence significantly improved energy density in aqueous Li-ion batteries. In the present work, results of X-ray scattering and Fourier-transform infrared spectra measurements over a wide range of temperatures and salt concentrations are reported for the LiTFSI (lithium bis­(trifluoro­methane sulfonyl)­imide)-based water-in-salt electrolyte. Classical molecular dynamics simulations are validated against the experiments and used to gain additional information about the electrolyte structure. Based on our analyses, a new model for the liquid structure is proposed. Specifically, we demonstrate that at the highest LiTFSI concentration of 20 m the water network is disrupted, and the majority of water molecules exist in the form of isolated monomers, clusters, or small aggregates with chain-like configurations. On the other hand, TFSI– anions are connected to each other and form a network. This description is fundamentally different from those proposed in earlier studies of this system.

Water or Anion? Uncovering the Zn2+ Solvation Environment in Mixed Zn(TFSI)2 and LiTFSI Water-in-Salt Electrolytes

“…The intensity of this peak shrinks with increasing LiTFSI concentration and becomes unnoticeable in 10 and 20 m LiTFSI solutions. This behavior is consistent with our previous observation for the LiTFSI aqueous electrolytes . This peak is due to the correlation among TFSI – anions that are fully solvated by water molecules .…”

The Nanostructure of Water-in-Salt Electrolytes Revisited: Effect of the Anion Size

“…Thus, TFSI anions could form long-range clusters almost exclusively via hydrophobic interactions, as discussed by Borodin et al . and Zhan g et al . In turn, the Tf anion network is formed by intercalation of hydrophobic interactions between CF 3 groups and Coulombic interactions between anion–Li + , consistent with a lower dissociation degree, probably with more than one or two anions per Li + ion.…”

“…On one hand, TFSI anions have hydrophobic CF 3 groups on both ends, while Tf anions have SO 3 charged/ hydrophilic and CF 3 hydrophobic regions. Thus, TFSI anions could form long-range clusters almost exclusively via hydrophobic interactions, as discussed by Borodin et al 13 and Zhang et al 20 In turn, the Tf anion network is formed by intercalation of hydrophobic interactions between CF 3 groups and Coulombic interactions between anion−Li + , consistent with a lower dissociation degree, probably with more than one or two anions per Li + ion. The different nature of the extended clustering could explain the different trends on the parameters and be related to the percolation threshold.…”

“…Despite the increasing interest in WiS electrolytes, only a couple of very recent experimental results confirm the structural heterogeneity rather than the dynamical one and, to the best of our knowledge, the few systematic studies available are mostly focused on LiTFSI 20,21 or rely on crude approximations, assuming spherical water clusters. 22 The purpose of this work is to shed some light on the morphological characteristics of this nanostructure and try to determine the driving parameters that affect it.…”

Ion Transport in the EMITFSI/PVDF System at Different Temperatures: A Molecular Dynamics Simulation

“…In addition, we calculate the relevant coordination number ( CN ) of ion pairs by numerically integrating the PCF, which is expressed as 55 , 56 where ρ is the average number density and g ( r ) is the PCF, where the cutoff radius ( r c ) is usually chosen to be the first local minimum of the corresponding PCF, that is, the first solvation shell.…”