Amplified Heterogeneous Interface for Modulating High‐Frequency Polarization Response in Confined Space

Abstract Heterogeneous interface engineering is a promising strategy for high‐efficiency EM wave absorption. However, Ostwald ripening induced by high temperatures imposes inhibitory constraints on phase growth, thereby limiting the effects of heterogeneous interface amplification. In this study, heterogeneous interface modulation, atomic site design, and elemental competition strategies are employed to induce multiphase fission and atomic reconstruction processes, enabling the fabrication of (Co _z Ni _1−z ) _x Se _y @C composites with tailored interfacial microstructure and crystallographic phase composition. The confined carbon encapsulation structure is beneficial for promoting the in situ growth of selenides, which successfully induces an ultrahigh density heterostructure system (CoSe ₂ , CoSe, NiSe ₂ , and NiSe) at the nanoscale, breaking through the limits of heterointerface amplification in conventional binary element systems. This unique broadband dielectric evolution mechanism enables precise modulation of multistage quantum‐confined polarization resonance, resulting in a gradient leap forward of the dielectric loss tangent (tanδ _ɛ ) with increasing frequency in the high‐frequency regime, which demonstrates intense polarization relaxation response. The (Co _z Ni _1−z ) _x Se _y @C microspheres with ultrathin and strong attenuation properties achieve a minimum reflection loss (RL _min ) of −51.5 dB at a thickness of only 1.5 mm. A synergistic multiple‐loss model is designed to amplify heterogeneous interfaces and modulate high‐frequency polarization response, providing inspiration for fabricating advanced EM wave absorbers.