The Local Structure in Zn-Cl Electrolytes and The Potential for Operando Study of The Interfacial Electrolyte Structure in Zn Ion Batteries using Valence-To-Core X-Ray Emission Spectroscopy

The determination and modeling of the local structure around ions in electrolytes is a classic problem in physical chemisry that also has contemporary impact spanning electrical energy storage, geochemistry, and ionic liquids (broadly defined). This problem is greatly enriched by the occurrence of cation-anion correlations, such as in contact ion pairing, giving at its extreme the 'water in salt electrolyte' (WiSE) regime that poses extreme theoretical challenges while also showing improved energy density in some batteries. Here, we report several studies of ion pairing in the aqueous Zn-Cl solutions as a function of concentration, anion activity, nanoconfinement, and temperature. To this end, we demonstrate that laboratory-based valence-to-core x-ray emission spectroscopy (VTC-XES) gives an improved method to determine the local symmetry and average composition of the Zn(II) first shell coordination. Starting with bulk solutions at ambient conditions, analysis of VTC-XES gives a robust characterization of the first coordination shell without the analytical complications of extended x-ray absorption fine structure (EXAFS), for which the changing second-shell population at high concentrations generally requires independent theoretical modeling by advanced molecular dynamics simulation. Next, we find that nanoconfinement by nanoporous carbon electrode material increases the mean ion pairing and also, in agreement with prior WiSE results, increases the electrochemical window for Zn-ion batteries. Finally, we return to the bulk system to study the temperature dependence of ion pairing, giving new insight into the geochemical problem of Zn transport in brines while also clearly demonstrating the limitations of models based on extrapolation of solubility and thermodynamic information from the dilute regime.<br/>These results illustrate the power of VTC-XES to investigate electrolyte structure, even when measured in the laboratory rather than at synchrotron x-ray light sources. The future extension to non-aqueous electrolytes would have even more direct relevance for beyond-Li-ion batteries. In addition, the work here lays important groundwork for future operando studies of the near-electrode interfacial structure of Zn-ion and other metal-ion electrolytes via synchrotron-based measurements. Such studies would immediately interrogate the connection between electrolyte structure and the operating voltage window for Zn-ion and other metal-ion battery systems.