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.