Cobalt-Doping Unlocks Copper Self-Healing Chemistry in Copper Hexacyanoferrate for Long-Cycling Aqueous Mg-Ion Storage

Fiber-shaped aqueous Mg-based batteries (FAMBs) represent an emerging platform for wearable electronics, yet their development is hindered by the lack of high-performance cathode materials. Copper hexacyanoferrate (CuHCF), a promising Prussian blue analogue, suffers from low capacity, rapid capacity decay due to limited active sites, Jahn–Teller distortion, and irreversible Cu dissolution. Herein, we demonstrate that cobalt doping enables effective modulation of the CuHCF. It induces a reversible Cu dissolution/self-healing behavior, suppresses Jahn–Teller distortion to maintain structural integrity, and increases the availability of active sites, thereby significantly enhancing the Mg2+ storage performance. The resulting Co-doped CuHCF nanoblocks on carbon nanotube fiber (CoCuHCF@CNTF) deliver a high specific capacity of 153.2 mAh g–1, exceptional cycling stability (90% retention after 30,000 cycles), and remarkable rate capability. The combination of this cathode and NaTi2(PO4)3@CNTF anode yields a flexible FAMB with a high energy density of 89 mWh cm–3, capable of powering a mobile phone. This work establishes a robust design principle for developing high-performance cathode materials through multifunctional cation doping, providing new insights into Mg2+-dominated storage behavior in advanced aqueous multivalent-ion energy-storage systems.