Defect engineering enables the efficient management of electromagnetic parameters and the enhancement of electromagnetic wave (EMW) absorption. In this study, (Fe_1−xMn_x)₂AlB₂ transition metal boride (MAB) phases with a layered structure were prepared via Joule heating-driven ultrafast synthesis, and their EMW absorption properties were investigated. The experimental results demonstrate that the incorporation of Mn atoms at the M site can effectively modulate the impedance matching and EMW absorption properties of the material through the introduction of defects and lattice distortions. Notably, (Fe_0.3Mn_0.7)₂AlB₂ exhibits a reflection loss as high as −47.8 dB at 12.24 GHz, with a maximum effective absorption bandwidth of 4.16 GHz (10.24–14.40 GHz) at an ultrasmall thickness of 1.5 mm. This study provides a promising avenue for the development of excellent microwave-absorbing materials, which are essential for meeting the evolving requirements of advanced electronics. Additionally, this work offers a paradigm for enhancing other properties of MAB phases through defect engineering.

High-entropy oxides (HEOs) have received significant attention because of their tunable mechanical properties and wide range of functional applications. However, the conventional method used for sintering HEOs requires prolonged processing time, which results in excessive grain growth, thereby compromising their performance. Here, an ultrafast high-temperature sintering (UHS) strategy was adopted, and rock-salt composite (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O was selected as model materials. Experimental parameters were tuned to illustrate the influence of applied current and soaking time on the densification process and resulting grain size. Additionally, the electrochemical performance of UHS-synthesized microparticles as anode materials in lithium-ion batteries was investigated. The results show that the ultrafast heating rate results in fine grains with a diameter of ~6–8 μm and density of 95%, which are much smaller and similar to those obtained using the conventional sintering method (25 μm and 96%). Moreover, the high surface area and reactivity of the microparticles, as well as their sluggish diffusion effect and structural stability, contribute to outstanding performance with high capacity (336 mA·h/g at 1 A/g) and ultralong cyclability (1000 cycles). This novel technique offers valuable insights into the densification process of HEOs and other materials and can thus broaden their application range.