| Summary: | Dielectric materials are highly desired for pulsed power capacitors due to their ultra-fast charge-discharge rate and excellent fatigue behavior. Nevertheless, the low energy storage density caused by the low breakdown strength has been the main challenge for practical applications. Herein, we report the electric energy storage properties of (1 − <i>x</i>) Bi<sub>0.5</sub>(Na<sub>0.8</sub>K<sub>0.2</sub>)<sub>0.5</sub>TiO<sub>3</sub>-<i>x</i>Bi<sub>0.2</sub>Sr<sub>0.7</sub>TiO<sub>3</sub> (BNKT-BST; <i>x</i> = 0.15–0.50) relaxor ferroelectric ceramics that are enhanced via a domain engineering method. A rhombohedral-tetragonal phase, the formation of highly dynamic PNRs, and a dense microstructure are confirmed from XRD, Raman vibrational spectra, and microscopic investigations. The relative dielectric permittivity (2664 at 1 kHz) and loss factor (0.058) were gradually improved with BST (<i>x</i> = 0.45). The incorporation of BST into BNKT can disturb the long-range ferroelectric order, lowering the dielectric maximum temperature <i>T<sub>m</sub></i> and inducing the formation of highly dynamic polar nano-regions. In addition, the <i>T<sub>m</sub></i> shifts toward a high temperature with frequency and a diffuse phase transition, indicating relaxor ferroelectric characteristics of BNKT-BST ceramics, which is confirmed by the modified Curie-Weiss law. The rhombohedral-tetragonal phase, fine grain size, and lowered <i>T<sub>m</sub></i> with relaxor properties synergistically contribute to a high <i>P<sub>max</sub></i> and low <i>P<sub>r</sub></i>, improving the breakdown strength with BST and resulting in a high recoverable energy density <i>W<sub>rec</sub></i> of 0.81 J/cm<sup>3</sup> and a high energy efficiency <i>η</i> of 86.95% at 90 kV/cm for <i>x</i> = 0.45.
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