The influence on flame retardant epoxy composites by a bird's nest-like structure of Co-based isomers evolved from zeolitic imidazolate framework-67

•The controlled synthesis of three different nanostructured Co-based isomers, namely hollow nanocages, multi-yolk@shell nanostructures and single yolk@shell nanostructures, was achieved by simple solvothermal method.•A novel multi-yolk@shell nanostructure (m-CBC@LDH) was constructed by restricting several cobalt basic carbonate (CBC) nanoparticles in the nanocage assembled by CoCo-LDH.•For m-CBC@LDH, the improved flame retardant efficiency is supported by the rapid char formation mechanism of CoCo-LDH exposing high active sites and the enhanced charring mechanism of CBC nanoparticles.•An inspiration for the design of complex nanostructures and the influence on the comprehensive performance of composites.