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

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_0.5Na_0.5)TiO₃-0.06KNbO₃-xSrTiO₃ [(0.94−x)BNT-0.06KN-xST] ceramics by coupling relaxor regulation with microstructural densification. KN incorporation induces R3c/P4bm 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.