Controllable N-Doped CuCo₂ O₄ @C Film as a Self-Supported Anode for Ultrastable Sodium-Ion Batteries

Rational synthesis of flexible electrodes is crucial to rapid growth of functional materials for energy-storage systems. Herein, a controllable fabrication is reported for the self-supported structure of CuCo₂ O₄ nanodots (≈3 nm) delicately inserted into N-doped carbon nanofibers (named as 3-CCO@C); this composite is first used as binder-free anode for sodium-ion batteries (SIBs). Benefiting from the synergetic effect of ultrasmall CuCo₂ O₄ nanoparticles and a tailored N-doped carbon matrix, the 3-CCO@C composite exhibits high cycling stability (capacity of 314 mA h g^-1 at 1000 mA g^-1 after 1000 cycles) and high rate capability (296 mA h g^-1 , even at 5000 mA g^-1 ). Significantly, the Na storage mechanism is systematically explored, demonstrating that the irreversible reaction of CuCo₂ O₄ , which decomposes to Cu and Co, happens in the first discharge process, and then a reversible reaction between metallic Cu/Co and CuO/Co₃ O₄ occurrs during the following cycles. This result is conducive to a mechanistic study of highly promising bimetallic-oxide anodes for rechargeable SIBs.