Elucidating the Influence of Electrolyte Additives on Iron Electroplating Performance Towards Application in Iron-Based Batteries

All-iron redox flow batteries are a promising alternative for grid-scale energy storage; however, their efficiency and lifetime are hampered by the poorly understood plating process, the limited reversibility of the negative half-cell, and the presence of competitive reactions. In this work, we propose a methodology to deconvolute the nucleation parameters of iron via a suite of electrochemical techniques, spectroscopy, and analytical models, coupled with microscopic and crystallographic techniques. We perform a systematic analysis with iron-based electrolytes to deconvolute the simultaneous plating and hydrogen evolution reactions, and investigate an array of additives to tune electroplating descriptors. We find that all additives studied are able to regulate the plating process and find that highly stable iron-complexes based on buffers, such as iron-borates or -citrates deliver greater battery performance in symmetric configuration, with nearly 50% higher cyclability than the baseline electrolyte. These additives show superior selectivity, with improvements in faradaic efficiencies from 60% to ~90% due to the balanced effects of enhanced nucleation and side reaction suppression. Herewith, the aim of this study is to bridge the knowledge gap between the role of additives, kinetics and efficiency of the electrodeposition reaction, and their interplay defining battery cycling performance.