Structure and Transport of “Water-in-Salt” Electrolytes from Molecular Dynamics Simulations

Abstract

Currently used lithium ion batteries for portable electronics utilize flammable and often toxic non-aqueous electrolytes in order to achieve high energy densities. They also require a low humidity manufacturing environment resulting in an increased cost. Aqueous electrolytes have recently emerged as potential intrinsically nonflammable alternatives after their electrochemical stability window was expanded beyond 3.0 V by employing a new class of “Water-in-Salt” electrolytes. In such super-concentrated electrolyte, the decomposition of salt anion occurs preferentially on the anode before hydrogen evolution takes place, creating a kinetic protection against electrochemical decomposition via a dense solid electrolyte interphase (SEI). In this presentation, results from classical molecular dynamics (MD) simulations using a polarizable APPLE&P force field are analyzed in order to examine in detail the ion transport mechanism in bis(trifluoromethane sulfonyl)imide (LiTFSI-water) “Water-in-Salt” electrolytes (WiSE) for safe, green and low cost aqueous lithium ion batteries. They are complemented by Born Oppenheimer MD simulations of smaller systems that yield similar structural features. Simulations revealed an unusually low activation energy and fast ion transport for highly concentrated solutions even at low temperatures that is quite different from the dramatic increase of the activation energy for conductivity found in traditional battery electrolytes. A high conductivity and lithium transference number in WiSE is attributed to the formation of fast ion transporting pathways that are connected to the unexpected structure of WiSE electrolytes, which was confirmed by small angle neutron scattering experiments (SANS). The ability of MD simulations to describe dynamics of ion and solvent in WiSE electrolytes was further validated via pfg-NMR and conductivity measurements, while IR spectroscopy measurements provide a comprehensive picture of the salt electrolyte aggregation that is coupled with ion transport. The connection between the double layer structure of WiSE electrolytes and its electrochemical stability will be briefly discussed.