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

Y Zhang and NHC Lewis and J Mars and G Wan and NJ Weadock and CJ Takacs and MR Lukatskaya and HG Steinruck and MF Toney and A Tokmakoff and EJ Maginn, JOURNAL OF PHYSICAL CHEMISTRY B, 125, 4501-4513 (2021).

DOI: 10.1021/acs.jpcb.1c02189

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.

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