Structure and electrochemical properties of hierarchically porous carbon nanomaterials derived from hybrid ZIF-8/ZIF-67 bi-MOF coated cyclomatrix poly (organophosphazene) nanospheres
Hybrid bi-ZIF nanocrystals consisting of ZIF-8/ZIF-67 were synthesised in the presence of cyclomatrix poly(organophosphazene) (POP) nanospheres and formed POP/bi-ZIF core@shell nanospheres. The POP/bi-ZIF showed excellent thermal stability up to 478°C, with well-preserved core@shell structures during carbonization at 850°C. The resultant core@shell carbon nanospheres exhibited hierarchically mesoporous structures. The porous carbon core was derived from the carbonised covalent inorganic-organic polyphosphazene framework, containing in situ doped heteroatoms such as N, P, S and O; the shell structure was derived from the bi-ZIF containing up to 40% of Zn and Co elements. The bi-ZIF derived carbon shell showed a BET surface area of 1347.76 m² g-1 and a Langmuir surface area of 1882.71 m² g-1, and the total BET surface area of the core@shell structure reached 1025.00 m² g-1. When applied as an anode material in lithium ion batteries, the core@shell carbon structure reached a charge capacity of 595 mA h g-1 with a discharge capacity of 546 mA h g-1, and remained reversible charge/discharge capacity at 400 mA h g-1 after 140 cycles, which is higher than the theoretical capacity of graphite anode. A good cycling stability with 83% capacity retention in the C-rate tests was achieved. This work provides a facile and scalable method to produce mesoporous carbon nanostructures with in situ doped metal elements and heteroatoms, which benefits the high rate electrochemical properties of lithium ion batteries.