Two‐electron redox chemistry enables potassium‐free copper hexacyanoferrate as high‐capacity cathode for aqueous Mg‐ion battery

Prussian blue analogs (PBAs) are potential contestants for aqueous Mg‐ion batteries (AMIBs) on account of their high discharge voltage and three‐dimensional open frameworks. However, the low capacity arising from single reaction site severely restricts PBAs' practical applications in high‐energy‐density AMIBs. Here, an organic acid co‐coordination combined with etching method is reported to fabricate defect‐rich potassium‐free copper hexacyanoferrate with structural water on carbon nanotube fiber (D‐CuHCF@CNTF). Benefiting from the high‐valence‐state reactive sites, arrayed structure and defect effect, the well‐designed D‐CuHCF@CNTF exhibits an extraordinary reversible capacity of 146.6 mAh g−1 with two‐electron reaction, nearly close to its theoretical capacity. It is interesting to unlock the reaction mechanism of the Fe2+/Fe3+ and Cu+/Cu2+ redox couples via x‐ray photoelectron spectroscopy. Furthermore, density functional theory calculations reveal that Fe and Cu in potassium‐free D‐CuHCF participate in charge transfer during the Mg2+ insertion/extraction process. As a proof‐of‐concept demonstration, a rocking‐chair fiber‐shaped AMIBs was constructed via coupling with the NaTi2(PO4)3/CNTF anode, achieving high energy density and impressive mechanical flexibility. This work provides new possibilities to develop potassium‐free PBAs with dual‐active sites as high‐capacity cathodes for wearable AMIBs. Freestanding defect‐rich potassium‐free copper hexacyanoferrate (D‐CuHCF@CNTF) with two‐electron redox chemistry was successfully prepared by organic acid co‐coordination and etching strategy. Benefiting from the high‐valence‐state reactive sites, arrayed structure, and defect effect, the resulting D‐CuHCF@CNTF exhibits an extraordinary reversible capacity of 146.6 mAh g−1. Fiber‐shaped aqueous Mg‐ion batteries assembled with D‐CuHCF@CNTF cathodes and NaTi2(PO4)3@CNTF anodes hold promise for wearable energy‐storage devices.