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.