Structure and ion transport in super-concentrated water-in-salt electrolytes: Insights from molecular dynamics simulations

Water-in-salt electrolyte (WISE) systems hold promise as materials for high performance, more reliable and affordable, energy storage devices including batteries and supercapacitors. However, the structure and ion dynamics in these systems is significantly different from those of aqueous electrolytes and are poorly understood. Here, we present molecular dynamics investigation of water-in-salt electrolyte materials having pure as well as eutectic mixtures of Li-NTF2, and Li-MM3411 salts at several concentrations. We find the existence of small water clusters such as dimers and trimers even at extremely high salt concentrations, but not pockets of bulk water molecules. We find that the water molecules are bound to the cation as well as anion at extremely high salt concentrations. It is found that Li ions display a variety of solvation environments in super concentrated electrolyte solutions depending on the salt and its concentration in water. Our simulation results suggests that ion transport follows a solvent-assisted Li-ion diffusion mechanism whereby Li ion diffuses along with the water molecules, with the Li-ions hopping from one anion site to the other. The cation transport number in WISE systems can be regulated by adding small amount of another salt; for example, the transference of WISE materials Li-NTF2 salt can be improved by adding small amount Li-MM3411 salt. Further, we show that there exists an optimal salt concentration in water, at which high apparent transference number for Li+ as well as high ionic conductivity can be achieved. These observations will provide a guidance for the design of new electrolyte systems and interphases with consequences for accelerating the development of batteries and supercapacitors with enhanced energy density and cycling stability.