An Air‐Operated, High‐Performance Fe‐Ion Secondary Battery Using Acidic Electrolyte

Fe²⁺ have emerged as the ideal charge carriers to construct aqueous batteries as one of the most competitive candidates for next-generation low-cost and safe energy storage. Unfortunately, the fast oxidation of Fe²⁺ into Fe³⁺ at ambient conditions inevitably requires the assembly process of the cells in an oxygen-free glovebox. Up to date, direct air assembly of aqueous Fe-ion battery remains very desirable yet highly challenge. Here oxidation of Fe²⁺ is found at ambient condition and is completely inhibited in an acidic electrolyte. A proton/O₂ competitive mechanism in the acidic electrolyte is revealed with reduced coordinated O₂ in the Fe²⁺ solvated shell for this unexpected finding. Based on this surprise, for the first time, air-operated assembly of iron-ion batteries is realized. Meanwhile, it is found that the acidic environment induces the in situ growth of active α-FeOOH derivate on the VOPO₄·2H₂O surface. Strikingly, the acidic electrolyte enables an air-operated Fe-ion battery with a high specific capacity of 192 mAh g^-1 and ultrastable cycling stability over 1300 cycles at 0.1 A g^-1. This work makes a break through on the air-assembly of Fe-ion battery without oxygen-free glovebox. It also reveals previously unknown proton/O₂ competitive mechanisms in the Fe²⁺ solvated shell and cathode surface chemistry for aqueous Fe²⁺ storage.