Ionic conductance and viscous drag in water-in-salt electrolytes for lithium and sodium ion batteries and supercapacitors

• MD simulations of water-in-salt electrolytes (WiSE) at various temperatures. • Calculations of Conductivity, shear viscosity and the associated activation energy. • Dynamics of NaTFSI is more than LiTFSI. • LiTFSI possessed a more activation energy barrier than NaTFSI. • Connection of ionic conductivity and viscosity to the lifetime of ion-cage formation. The electrochemical window of water can expanded to ∼3 V using water-in-salt electrolytes (WiSE), which make use of high concentrated aqueous solution of lithium and sodium salts as electrolytes for batteries as well as supercapacitors. In order to understand the dynamics present in these solutions and the effect of temperature on conductance, we performed classical molecular dynamics simulations of WiSE systems: ∼10 m aqueous lithium and ∼8 m sodium bis(trifluorosulfonimide)(LiTFSI and NaTFSI). We find the ionic correlation is weaker in NaTFSI electrolyte and it has more ionic conductivity. However, both aqueous LiTFSI and NaTFSI exhibit much higher ion transport than non-aqueous electrolytes. The viscous drag experienced by NaTFSI is much lesser than that by that of LiTFSI. From the calculations of shear viscosity and the associated activation energy of these electrolyte systems, we find that LiTFSI possess a more substantial energy barrier. The opposite ions stay paired for a shorter period at higher temperatures and contribute to conductivity through their fast motion. The ionic conductivity and viscosity are connected to the lifetime of ion-cage formation, which facilitates the overall dynamics of the electrolyte. We conclude that these WiSE systems are impressive candidates that may produce a paradigm shift in the arena of electrolytes for energy storage devices.