Tailoring local chemical environments in aqueous electrolytes for efficient energy storage: From dilute to concentrated

This thesis systematically explores electrolyte engineering strategies to enhance the efficiency and energy density of aqueous batteries, focusing on zinc-ion batteries (AZIBs) and the effects of electrolyte concentration on key electroreduction reactions, particularly hydrogen evolution (HER) and oxygen reduction (ORR). First part of this thesis investigates the impact of electrolyte concentration, ranging from dilute to Water-in-Salt (WIS) regimes, on Zn speciation, plating/stripping efficiency, and cycling stability. While WIS electrolytes are often assumed optimal for Zn plating/stripping, our findings reveal that they may result in lower Coulombic efficiency (CE) and slower kinetics. We demonstrate that coordinating anions like acetate can enable a WIS-like Zn coordination environment even in relatively dilute conditions, promoting prolonged cycling with an electrolyte that offers higher conductivity and lower viscosity compared to traditional WIS formulations, thus reducing overpotentials and higher rates of Zn plating/stripping. Additionally, this work introduces a method to quantify Zn²⁺ transference number in multicomponent systems, uncovering that higher Zn²⁺ transference correlates with improved cycling performance and suppresses dendrite formation, revealing that co-salts intended to boost conductivity can hinder Zn migration, impacting overall performance. The thesis also investigates HER suppression and ORR kinetics across different concentrations. Increased concentrations reduce HER activity due to changes in water activity, hydrogen bonding, and local pH, with each factor quantified for its impact on HER kinetics. For ORR, WIS electrolytes possess a decrease oxygen solubility due to modified hydrogen bonding while preserving favorable ORR kinetics, allowing selective control of this reaction. This comprehensive approach to electrolyte engineering—linking structure-performance correlations, transport properties, kinetics, and mechanisms—significantly advances the stability and performance of AZIBs and related aqueous energy systems.