Electric Double‐Layer Structured Grain Boundaries in Medium‐Entropy Perovskite Enable Robust Electromagnetic Interference Shielding after 1200 °C Oxidation

High-temperature electromagnetic interference (EMI) shielding materials are essential for aerospace applications, but achieving both efficiency and stability above 1000 °C remains challenging. To address this challenge, it is proposed to develop electric double-layer (EDL) structured grain boundaries (GBs) within oxide ceramics by point defect segregation at GBs. As a proof of concept, the highly deficient medium-entropy (ME) perovskite is selected with the nominal composition (SrBaLa)_1/3TiO₃, which contains a high concentration of Ti and O vacancies. After sintering, the Ti and O vacancies segregate at GBs, forming a back-to-back EDL structure analogous to that in EDL supercapacitors. As a result, the sintered (SrBaLa)_1/3TiO₃ bulk exhibits significantly enhanced dipole polarization than its powders, leading to higher complex permittivity than most oxide ceramics. Its EMI shielding effectiveness exceeds 32 dB in the X-band and remains stable even after annealing at 1200 °C in air. In addition, the (SrBaLa)_1/3TiO₃ ceramic also possesses the highest flexural strength (217.2 MPa) and hardness (11.6 GPa) among the perovskite oxides due to the vacancy clustering to both GBs and grains, which validates the protocol to fabricate mechanically and thermally robust EMI shielding materials for aerospace applications.