Synergistic Design of High Steric Hindrance and Negatively Charged Anolyte Enables 6000‐Cycle Stability for Alkaline All‐Iron Flow Batteries

ABSTRACT The alkaline all‐iron flow battery (AIFB) adopting Fe complexes in both half‐cells is an essential pathway to large‐scale energy storage with inherent merits of long discharge duration. However, inferior electrochemical reversibility and ligand crossover hinder the long cycling stability of the AIFB. Herein, we delicately design the Fe complex anolyte with large‐space steric hindrance and a negatively charged protective layer, which significantly boosts the long‐term stability of the AIFB. While the coordination of Fe 3+ with polydentate multi‐ligands abundant in hydroxyl and sulfonic acid groups renders Fe complex a high steric hindrance, the negatively charged interface of Fe complex also effectively prevents OH − attack and active species crossover by virtue of electrostatic repulsion, thereby synergistically achieving high electrochemical stability and low membrane permeation. Based on the design guidelines, the anolyte design process starts with 12 organic ligands as building blocks, followed by constructing 11 distinct Fe complexes with tailored structures. After multiple rounds of screening, the AIFB adopting the [Fe(HPF)BHS] 4− anolyte exhibits a record‐breaking ultra‐long cycling stability over 6000 cycles at 80 mA cm − 2 . This work provides deep insights into efficient anolyte design and offers a universal Fe complex design strategy, which is beneficial to promoting the application of high‐performance iron‐based flow batteries.