Study on Broadband and High-Performance Microwave-Absorbing Spinel NiCo₂O₄ Regulated by Fe Doping

Spinel NiCo₂O₄ has emerged as a promising microwave absorption material due to its unique crystal structure and abundant defect sites. Nevertheless, its low intrinsic electrical conductivity leads to insufficient conductive loss and unsatisfactory high-frequency impedance matching, severely limiting the simultaneous realization of strong electromagnetic attenuation and broad absorption bandwidth. Fe³⁺ doping is an effective modification strategy for NiCo₂O₄ by virtue of its matched ionic radius and dual modulation capability for dielectric and magnetic properties. Herein, pristine and Fe-doped NiCo₂O₄ absorbers with different doping contents (4%, 6%, 8%) were fabricated via a hydrothermal-calcination route, and the correlation between Fe doping concentration, microstructure, electronic structure, electromagnetic properties, and microwave absorption performance was systematically investigated. Benefiting from moderate 6% Fe doping, the optimized F6 sample exhibits a refined porous nano-agglomerate structure, which provides abundant heterogeneous interfaces and pore channels for electromagnetic wave scattering and attenuation. The introduced oxygen vacancies and balanced Ni²⁺/Ni³⁺, Co²⁺/Co³⁺, and Fe²⁺/Fe³⁺ mixed-valence states effectively strengthen interfacial and dipole polarization, while the optimized electrical conductivity and magnetic properties synergistically boost conductive and magnetic losses. Owing to the dual-loss synergism and superior impedance matching (58% proportion of Δ < 0.4), the F6 sample achieves an excellent minimum reflection loss of -62.7 dB at 2.2 mm and a wide effective absorption bandwidth of 4.6 GHz. This work clarifies the intrinsic structure-performance mechanism of Fe-doped NiCo₂O₄, providing a reliable and feasible strategy for the design and preparation of high-performance spinel-type microwave-absorbing materials.