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Regulating the composition and microstructure of nanomaterials is a major challenge in the development of high-performance materials for electromagnetic wave (EMW) absorption. This paper introduces a new lightweight EMW-absorbing material Co@N/C with a yolk–shell structure and compositions of ferromagnetic metal and carbon materials. The minimum reflection loss (RL min) of the as-designed Co@N/C nanospheres with a filler loading of 25 wt% reaches −67.68 dB and the effective absorption bandwidth of a 1.9 mm thickness is 5.3 GHz. The exceptional absorption performance of the EMW absorption is primarily achieved through the synergistic effect between the Co–carbonaceous material and the yolk–shell structure, which optimizes the impedance matching. The polarization loss, conductivity loss, magnetic loss, and multiple scattering mainly contribute to the excellent EMW-absorption ability of the materials. The proposed method for regulating the structure and morphology of Co@N/C can efficiently construct high-performance EMW absorbers. [Display omitted] • Yolk-Shell structure nanocomposite with excellent electromagnetic wave absorption properties was synthesized. • An excellent reflection loss (RL) of −67.68 dB at 15.88 GHz with an ultra-thin thickness (1.7 mm). • Effective absorption bandwidth (<−10 dB) up to 5.3 GHz under a single thickness (1.9 mm). [ABSTRACT FROM AUTHOR]