Ultrahigh Energy Storage in Relaxor Ferroelectric Ceramics with Core–Shell Grains

Abstract The achievement of record‐high energy storage performance in relaxor‐ferroelectric bulk ceramics represents a major advancement in the field of dielectric capacitors. Nonetheless, a trade‐off between breakdown strength and polarization has typically limited the optimization of overall performance. Here, guided by a rational composition design, K 0.5 Na 0.5 NbO 3 ‐based bulk ceramics are fabricated with grain core–shell structures and polymorphic nanodomains, leading to a synergistic enhancement of breakdown strength and polarization. This results in an unprecedented recoverable energy density of ≈20.4 J·cm −3 and an energy efficiency of ≈90% at an electric field of ≈1020 kV·cm −1 . Additionally, the ceramics exhibits excellent charge–discharge performance, including a high discharge energy density of ≈6.0 J·cm −3 and an ultrashort discharge time of ≈42 ns at 500 kV·cm −1 , along with superior reliability and stability. These advancements are expected to provide valuable insights for the exploration and utilization of advanced dielectric materials.