Lithium-ion batteries are widely implemented as energy storage devices due to their high energy density and low cost. They enabled modern portable electronics and electric ve-hicles, and are key to manage the integration of intermittent renewable electricity sources, such as solar and wind, into the power grid and decentralized microgrids. The use of flam-mable organic electrolytes however raises safety concerns, particularly for large-scale sta-tionary batteries. Furthermore, supply chain risks and the use of critical raw materials are a major challenge for the ongoing battery revolution. Aqueous electrolytes offer promising properties, as they are inherently non-flammable and potentially more cost effective. Combined with electrode materials based on abundant raw materials, aqueous batteries are among the most promising candidates to replace cur-rent battery chemistries relying on critical cobalt and flammable organic electrolytes. How-ever, due to the narrow electrochemical stability window of water (practically ca. 1.5 V), aqueous batteries have not yet met the energy density to be serious competitors to current lithium-ion battery technology. The goal of this thesis was to develop aqueous electrolytes with enhanced electrochem-ical stability, thus enabling new high-voltage cell chemistries. Exploring aqueous electrolytes with high salt concentrations, so-called water-in-salt electrolytes, we significantly increased the electrochemical stability and developed highly stable 2 V class aqueous sodium-ion bat-teries. Operating near the solubility limit of the electrolyte salt, crystallization turned out to be a major challenge for water-in-salt electrolytes. In this thesis, we demonstrated that add-ing highly asymmetric anions into the solution can effectively suppress crystallization, allow-ing stable operation of saturated electrolytes at temperatures as low as -10 °C. We further examined the impact of anion asymmetry on the local solution structure and dynamics using molecular dynamics simulations, relating the enhanced supercooling to sustained rotational motion of the asymmetric anion. Further, we demonstrate that the anion has a crucial impact on solution structure and cell performance. We establish that high salt concentration alone does not guarantee high-voltage stability and that due to the limited solubility of most sodium salts new approaches are needed. We provide guidelines for future work and the evaluation of experimental data in order to spur progress in the field of aqueous batteries.