Correlations between ionic conductivity and co-solvent modulated water structure and dynamics in aqueous Zn-ion battery electrolytes

The addition of a co-solvent with a higher donor number than water in aqueous zinc ion (Zn2+) battery (AZIB) electrolyte systems is a promising approach to prevent Zn-dendrite formation and metal anode corrosion. We have investigated here how a co-solvent in an aqueous mixture affects both the structure and dynamics of water molecules that are engaged in solvating Zn2+ ions and whether the solution ionic conductivity constructs any correlation to the altered structure and dynamics of these solvating water molecules. For this purpose, we have considered an experimental AZIB system with tetramethylurea (TMU) as a co-solvent at several compositions. Molecular dynamics simulations have been performed to extract ionic conductivity using the Onsager transport coefficients, tetrahedral H-bond network, intermittent H-bond lifetimes, and residence times of water molecules surrounding Zn2+ ions. We have found that the water-water H-bond network and orientational structure are significantly perturbed due to the presence of Zn2+. Moreover, both the residence time and H-bond lifetime of the solvating water molecules increase in the presence of TMU. The average number of water-water H-bonds is found to be positively correlated to the simulated conductivity, while H-bond lifetimes and water residence times depict anti-correlation. These data suggest that water-water H-bonds surrounding Zn2+ ions critically determine the Zn2+ transport, whereas the increase in water-water H-bond lifetimes and water residence times reinforces the idea of vehicular transport for Zn2+ ions. This microscopic structural and dynamical information explains the role of TMU in this AZIB system and provides a microscopic explanation for the observed mundane viscosity dependence of ionic conductivity.