135 pages ; Development of safe, rechargeable, and high energy density batteries is crucial for transitioning from fossil fuels to renewable energy resources which are available intermittently. Zinc metal batteries are a promising candidate for this owing to their low cost, high theoretical energy density and environmental friendliness. However, one of the major challenges to their development is unstable metal deposition which leads to formation of ramified, low-density, fragile structures called dendrites. During cell charging, these dendrites can cause rapid loss of storage capacity by breaking away from the electrode mass to form electrically disconnected or “dead” metal. Over time, they can also grow through the separators causing an internal short circuit, leading to poor reversibility and premature failure of the metal anodes. To overcome these challenges, polymers have been widely used as electrolyte additives in batteries to stabilize electrodeposition. In this work, we look at the effects of polymers in two scenarios - a fundamental study of hydrodynamic instability, and secondly in a more application-oriented study on zinc iodide systems.In the first part, we investigate the effect of salt concentration and polymer additive on electroconvection which is a hydrodynamic instability occurring at high voltages. We perform electrochemical measurements and direct flow visualization, which reveals that electroconvection is delayed and suppressed at all voltages in the presence of oligomers. Our experiments also indicate the importance of considering the interfacial effects of PEG adsorption in addition to the bulk effects of oligomer additives. To corroborate our experimental results, we also present a perturbation analysis study, which reveals that the underlying stability mechanism involves the formation of an oligomer layer at the interface, which in response to perturbation is believed to exert an opposing body force on the surrounding fluid to preserve the layer structure and in so doing suppresses ...