Multifunctional cellulose nanofibers (CNF)-based composites have great promise for applications in advanced electronics. In this work, an onionskin-cell-like biomimetic structure was developed within the CNF matrix based on the volume repulsion mechanism, which can construct dual-continuous conductive networks of nanosilver flowers (AgNFs) and multi-walled carbon nanotubes (CNTs). The resulting composite films with 4.18 vol% AgNF and 20 vol% CNTs exhibit a through-plane thermal conductivity (TC) of 3.13 W/mK and a remarkable thermal conductivity enhancement coefficient (TCE) of 821 %. This exceptional performance is attributed to the high interconnectivity (25.9 %) of the Ag–Ag pathways within the primary thermally conductive channel. The superior thermal behavior of these composite films with such bionic structure has also been demonstrated through infrared thermography analysis. Temperature distributions of the CNF-based films with different structures is simulated by transient finite element method to investigate the reinforcement mechanism of such onionskin-cell-like bionic structure. Meanwhile, the composite film (150 μm) with such a dual-channel structure can achieve an impressive electromagnetic interference shielding effectiveness (EMI SE) up to 43.7 dB due to the reflection of electromagnetic waves through the densely interconnected conductive network, which means a shielding efficiency exceeding 99.99 %. Building on this foundation, these films were introduced into a Janus composite as the conductive A-layer, which present multifunctional properties, including an out-of-plane TC of 2.86 W/mK, electrical insulation (1.32 × 109 Ω cm), and a specific shielding effectiveness (SSE) of 233 dB/mm. Overall, this research contributes a novel structural design concept to meet the multifunctional requirements of electronic device applications.
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