Evolution of Local Coordination of Zn Ions in Aqueous Electrolytes: Chemical, Thermal and Confinement Effects

Abstract

The accelerating global transition towards renewable energy sources necessitates innovative and sustainable energy storage solutions to effectively manage the intermittent nature of renewable power generation. A key facet of this transition is the adoption of grid-scale energy storage technologies capable of accommodating the variability in renewable energy production. Aqueous batteries, distinguished by their water-based electrolytes, have emerged as promising candidates for grid storage due to their inherent safety, cost-effectiveness, and environmental compatibility. However, the limited electrochemical voltage window (EVW) of aqueous electrolytes, attributed to hydrogen and oxygen evolution reactions at the electrodes, presents a significant hurdle for achieving energy-dense aqueous batteries. To that end, controlling the solvation structure of active ions in the electrolyte, for example by increasing the salt concentration of the electrolyte, has been shown to be an effective way of expanding the EVW of aqueous electrolytes. Consequently, the ability to detect and study the solvation structure of active ions in aqueous electrolytes from dilute to concentrated state is central to further research and development on such electrolyte systems. This research focuses on Zn-halide solutions for aqueous Zn-ion battery (ZIB) electrolytes. The study demonstrates that valence-to-core X-ray emission spectroscopy (VTC-XES) serves as a valuable tool for investigating the local structure of Zn2+ ions in aqueous solutions from dilute to extreme concentrations, quantifying coordination numbers without relying on thermodynamic formation constants. The developed methods contribute to the fundamental understanding of the solvation behavior of Zn2+ ions in aqueous solutions under varying conditions. The investigation reveals that, under ambient conditions, higher ion activity leads to increased desolvation and contact ion pairing. This behavior is further observed with elevated solution temperatures. Additionally, aqueous ZnCl2 electrolytes under nanoconfinement exhibit enhanced contact ion pairing, correlating with their increased EVW under nanoconfinement. These findings establish VTC-XES as a robust tool for studying local structures in aqueous Zn-halide solutions in general and contribute to the fundamental understanding of the physical chemistry of these specific systems. Hence, first, this work establishes a new methodology for studying ion complexation in aqueous electrolytes. Second, it gives new inquiry into the connection between ion pairing and the electrochemical voltage window, especially for nanoconfined conditions immediately relevant for metal ion batteries. Third, the observed temperature dependence of ion pairing in the system under study directly challenges theoretical approaches used to predict ion complexation in geophysical brines under environmentally-relevant conditions, and also has relevance for higher-temperature operation of ZIB.