Enhanced energy storage performance of lead-free sodium strontium niobate-based tungsten bronze ceramics via relaxor regulation and grain engineering

Balancing a large recoverable energy density ( W rec ) with superior efficiency ( η ) is a key prerequisite for implementing lead-free tungsten bronze-based dielectric ceramics in high-power capacitive energy-storage systems. Herein, a heterogeneous relaxor-relaxor solid-solution design strategy is adopted to synergistically optimize the polarization response and breakdown endurance of the ceramics through relaxor regulation and grain-size engineering. Introducing the multicationic perovskite-type relaxor phase (Bi 0.2 K 0.2 Ba 0.2 Sr 0.2 Ca 0.2 )TiO 3 (BKBSCT) into the Sr 2 NaNb 3.5 Ta 1.5 O 15 (SNNT) matrix induces local BO 6 octahedral distortion and promotes the emergence of field-responsive nanoscale polar regions (PNRs). Meanwhile, BKBSCT-driven grain refinement generates abundant grain boundaries that impede charge transport and suppress breakdown-channel propagation, thereby improving the dielectric breakdown field ( E b ). The optimized SNNT-0.05BKBSCT composition exhibits W rec = 7.50 J/cm 3 and η = 84.59% at 760 kV/cm, while retaining robust stability against temperature, cycling, and frequency variations. These findings establish heterogeneous relaxor-relaxor solid-solution engineering as a rational design strategy for reconciling low-loss polarization response with high breakdown endurance in lead-free tungsten bronze-based dielectric ceramics.