Outstanding Energy Storage Performance in High‐Hardness (Bi0.5K0.5)TiO3‐Based Lead‐Free Relaxors via Multi‐Scale Synergistic Design

Abstract Lead‐free dielectric ceramics with ultrahigh energy storage performance are the best potential stocks used in next‐generation advanced pulse power capacitors. Here, an ultrahigh recoverable energy storage density W rec of ≈7.57 J cm −3 and a large efficiency η of ≈81.4% are first realized in (Bi 0.5 K 0.5 )TiO 3 (BKT)‐based relaxor ferroelectric ceramics with an ultrahigh Vickers hardness H v ≈ 8.63 Gpa by adding BaTiO 3 and NaNbO 3 in order to synergistically design the domain and microstructure in multiscale, leading to the existence of ultrasmall polar nanoregions, ultrafine grain size, compact grain boundaries, dense microstructure, and large band gap E g simultaneously. Encouragingly, an excellent energy storage temperature stability ( W rec ≈ 4.31 ± 0.25 J cm −3 , η ≈ 86 ± 5%, 20–200 °C), frequency stability ( W rec ≈ 5.14 ± 0.12 J cm −3 , η ≈ 81.3 ± 1.2%, 5–100 Hz), and excellent charge/discharge performance (power density P D ≈ 103.2 MW cm −3 , discharge energy density W D ≈ 2.4 J cm −3 , discharge rate t 0.9 ≈ 130 ns) are also achieved in BKT‐based ceramics. The results demonstrate that BKT‐based ceramics can be very competitive lead‐free relaxors for energy storage capacitors in pulsed power devices.