Fabrication of an MOF-derived heteroatom-doped Co/CoO/carbon hybrid with superior sodium storage performance for sodium-ion batteries

Metal-organic frameworks (MOFs) have gained significant attention as precursors for the fabrication of porous hybrid materials due to their highly controllable composition, structure and pore size. However, at present, MOF-derived materials have rarely been investigated as anode materials for sodium-ion batteries. In this work, we report the fabrication of a Ni-doped Co/CoO/N-doped carbon (NC) hybrid using bimetallic Ni-Co-ZIF as the starting precursor. The resulting Ni-doped Co/CoO/NC hybrid is highly microporous with a high specific surface area of 552 m² g^-1. When employed as an anode material for sodium-ion batteries, the Ni-doped Co/CoO/NC hybrid exhibited both good rate performance with a high discharge capacity of 218 mA h g^-1 at a high current density of 500 mA g^-1 and good cycling stability, as a high discharge capacity of 218.7 mA h g^-1 can be retained after 100 cycles at 500 mA g^-1, corresponding to a high capacity retention of 87.5%. The excellent electrochemical performance of the Ni-doped Co/CoO/NC hybrid for SIBs may be attributed to the synergistic effects of various factors, including: (i) the presence of a carbon matrix which provides protection against aggregation and pulverization during sodiation/desodiation; (ii) the highly microporous nature along with the presence of a few mesopores which facilitates better insertion/de-insertion of Na⁺ ions; (iii) the Ni-doping which introduces defect sites into the atomic structure of CoO via partial substitution, thus enhancing the conductivity of the cobalt oxide (CoO) component and hence, the overall hybrid material, and (iv) the N-doping which promotes a faster migration speed of sodium ions (Na⁺) across the carbon layer by creating defect sites, thereby improving the conductivity of the carbon frameworks in the hybrid material.