Shaohao Xiao, Jinan Li, Jie Zhang*, Zhiyong Fu*, Zhenxing Liang
Aqueous all-iron redox flow batteries (RFBs) represent a promising technology for large-scale and long-term energy storage due to their extremely low cost and inherent safety. However, these systems still suffer severe capacity decay arising from the dissociation of iron coordination complexes and iron precipitation during prolonged, high-concentration operation. Herein, an ethylenediamine tetramethylenephosphonic (EDTMP) acid-coordinated Fe ion [Fe(EDTMP)], that reaches 1.5 M solubility under mild conditions (pH ≈ 8), is designed as anolyte to overcome the challenges for neutral all-iron redox flow batteries (RFBs). The Fe(III) complex with a robust six-coordination structure undergoes stable reversible redox cycles with a potential at −0.4 V vs. Ag/AgCl. It develops an all-soluble all-iron RFB with high concentration, high capacity, and long-term cyclability. While operated with a 1.3 M Fe(EDTMP) electrolyte, the battery delivers a capacity of 31.67 Ah L−1 and retains 97% of its initial capacity after 1000 cycles (equivalent to 815 h of operation).