A symmetric gradient structure enables robust CNF/FeCo/LM composite films with excellent electromagnetic interference shielding and electrical insulation

The exceptional performance of electromagnetic interference (EMI) shielding materials often stems from their high conductivity. However, EMI shielding materials with high electrical conductivity pose a risk of current leakage, which is an issue that cannot be ignored. Herein, a unique electrical insulation cellulose nanofiber/ferro cobalt/liquid metal (CNF/FeCo/LM) composite film with a symmetric gradient structure was fabricated using the vacuum-assisted filtration and cold compression techniques. In the CNF/FeCo/LM composite film, CNF/FeCo serves as the electromagnetic wave (EMW) absorption layer and electrical insulation protective layer, CNF/FeCo/LM functions as the EMW transmission layer, and the LM acts as the middle EMI shielding enhancement layer. The prepared symmetric gradient CNF/FeCo/LM composite film exhibits a satisfactory EMI shielding effectiveness of 39.32 dB at a thickness of only 0.17 mm, with 20 wt% of LM and 20 wt% FeCo. This performance is attributed to the symmetric gradient composite film “absorption-weak reflection-strong reflection-reabsorption” mechanism. Furthermore, this composite film also exhibits an excellent in-plane thermal conductivity of 4.85 W m−1 K−1 and outstanding volume resistance of 5.58 × 1011 Ω cm. The CNF/FeCo/LM composite film achieves a tensile strength of 56.28 MPa, an elongation at break of 21.47%, and a toughness of 7.2 MJ m−3 due to the interaction between CNF and the LM, along with a zigzag fracture path that develops within the interlayer, which facilitates stress transfer and energy absorption. This work offers a viable approach to designing high EMI shielding materials with electrical insulation, which is crucial for promoting the use of CNF matrix composites in electronic packaging.