Synergistic Relaxor Engineering and Microstructural Densification Enable Ultrahigh Energy Storage in BNT‐Based Lead‐Free Ceramics

ABSTRACT Lead‐free dielectric ceramics with simultaneously high recoverable energy‐storage density and efficiency are highly desirable for pulsed‐power capacitors, yet their performance is fundamentally limited by the trade‐off among polarization strength, hysteresis loss, and breakdown strength. Here, we report a multiscale synergistic strategy in (0.94− x )(Bi 0.5 Na 0.5 )TiO 3 ‐0.06KNbO 3 ‐ x SrTiO 3 [(0.94− x )BNT‐0.06KN‐ x ST] ceramics by coupling relaxor regulation with microstructural densification. KN incorporation induces R 3 c / P 4 bm local phase coexistence in the BNT matrix, weakening long‐range ferroelectric order and initiating a transition toward a relaxor state. Further ST addition enhances local structural heterogeneity, random fields, and polar nanoregions, leading to highly reversible polarization with suppressed hysteresis. In parallel, tape casting produces a dense and homogeneous microstructure, effectively mitigating defect‐induced field concentration and enhancing breakdown strength. The optimized x = 0.32 composition exhibits a high breakdown strength of 804 kV cm −1 and delivers an ultrahigh recoverable energy density of 10.1 J cm −3 with an efficiency of 97.6% at 820 kV cm −1 . Frequency‐ and temperature‐dependent measurements at 450 kV cm − 1 further demonstrate robust energy‐storage behavior under the tested field condition. This work establishes a disorder‐engineered relaxor strategy combined with microstructural densification to reconcile polarization reversibility and electric‐field endurance in lead‐free dielectric ceramics.