A Nonaqueous Eutectic Electrolyte for Rechargeable Iron Batteries

Iron metal has attracted great interest as an anode material for the development of aqueous rechargeable batteries due to its huge abundance in the earth’s crust, offering significantly lower cost per cell. However, the intractable side reactions at the negative iron anode and parasitic hydrogen evolution in aqueous media hamper the technology being unattainable for practical evaluations. Herein, we demonstrate a nonaqueous, eutectic electrolyte based on triethylamine hydrochloride ((Et₃NH)Cl) and FeCl₃ as an efficient electrolyte for the application of rechargeable iron batteries (RIBs). The eutectic formation is achieved by dual intermolecular interactions, namely, Lewis acid-base interactions and hydrogen bonding, resulting in hybrid organic-inorganic active ionic complex species. The optimized eutectic electrolyte offers appreciably high ionic conductivity (∼1.2 mS cm^-1) at room temperature, high plating and stripping efficiency (∼100%), a long cycling stability of ≅400 h in a symmetric iron cell, and a wide operating potential window (∼2.5 V on Mo or carbon-coat Al). Differential scanning calorimetry (DSC) reveals that the electrolyte renders the liquid phase at −11 °C. A hydrothermally synthesized V₂O₅ nanowire cathode paired against an iron anode in the optimized eutectic electrolyte renders a good capacity of ∼140 mAh g^-1 at a current density of 10 mA g^-1. The charge storage mechanism of the cell is thoroughly investigated by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction on the galvanostatically cycled electrodes. The addition of AlCl₃ extends the electrolyte stability window to 3.2 V on SS, resulting in enhanced cell performance that maintains stability for >100 cycles. This work introduces a eutectic electrolyte class that can enable safe RIBs at low-cost and a wide operating potential window.