A Universal Additive Design Strategy to Modulate Solvation Structure and Hydrogen Bond Network toward Highly Reversible Fe Anode for Low-Temperature All-Iron Flow Batteries

Aqueous all-iron redox flow batteries (RFBs) are promising competitors for next-generation grid-scale energy storage applications. However, the high-performance operation of all-iron RFBs in a wider temperature range is greatly hindered by inferior iron plating/stripping reaction and low solid-liquid transition temperature at Fe anode. Herein, a universal electrolyte additive design strategy for all-iron RFBs is reported, which realizes a highly reversible and dendrite-free Fe anode at low temperatures. Quantum chemistry calculations first screen several organic molecules with oxygen-containing functional groups and identify N,N-Dimethylacetmide (DMAc) as a potential candidate with low cost, high solubility, and strong interactions with Fe2+ and H2O. Combined experimental characterizations and theoretical calculations subsequently demonstrate that adding DMAc into the FeCl2 solution effectively reshapes the primary solvation shell of Fe2+ via the Fe2+-O (DMAc) bond and breaks hydrogen-bonding network of water through intensified H-bond interaction between DMAc and H2O, thereby affording the Fe anode with enhanced Fe/Fe2+ reversibility and lower freezing point. Consequently, the assembled all-iron RFB achieves an excellent combination of high power density (25 mW cm-2), long charge-discharge cycling stability (95.59% capacity retention in 103 h), and preeminent battery efficiency at -20 degrees C (95% coulombic efficiency), which promise a future for wider temperature range operation of all-iron RFBs. A universal additive design strategy is proposed for all-iron flow batteries at low-temperature operation. By reshaping the solvation structure and breaking hydrogen-bonding network of water, N,N-Dimethylacetmide (DMAc) proves to realize both enhanced Fe/Fe2+ reversibility and lower freezing point, which enables the all-iron flow cell an excellent combination of high power density, long cycling stability and preeminent battery efficiency at -20 degrees C.image