Yolk–shell structured Co-C/Void/Co₉S₈ composites with a tunable cavity for ultrabroadband and efficient low-frequency microwave absorption

A yolk–shell structured Co-C/Void/Co₉S₈ ternary composite composed of a Co nanoparticle-embedded porous carbon core and Co₉S₈ shell was synthesized by the sulfidation of a Co-based zeolitic imidazolate framework and subsequent pyrolysis. The composition and interior cavity of the Co-C/Void/Co₉S₈ composite could be precisely modulated by controlling the sulfidation reaction. Due to the abundant heterointerfaces, well-controlled cavity, and magnetic–dielectric synergistic effects, the Co-C/Void/Co₉S₈ exhibited excellent and tunable microwave-absorbing properties. The optimized Co-C/Void/Co₉S₈, having a loading of 25 wt.% and thickness only 2.2 mm, displayed an ultrabroad absorption bandwidth of 8.2 GHz at high frequencies. Moreover, the composite could achieve an extremely high reflection loss of–54.02 dB at low frequencies by adjusting its loading to 30 wt.%. This study provides a new insight into promising lightweight microwave-absorbing materials with ultrabroad absorption bandwidths and strong low-frequency absorption.