Synergistic Domain and Defect Engineering Enables Giant Electrostrain With High Piezoelectric Sensitivity in Lead-Free Ferroelectrics

Simultaneously achieving high piezoelectric sensitivity and large electric-field-induced strain remains a fundamental challenge for lead-free piezoelectric ceramics, as these responses are typically limited by competing polarization stability and mobility. Here, we present a general electromechanical design paradigm that overcomes this trade-off through the cooperative regulation of hierarchical domain structures and defect-mediated internal bias fields in (K_0.5Na_0.5)NbO₃ (KNN)-based ferroelectrics. Using 0.96K_0.48Na_0.52NbO₃-0.04Bi_0.5Li_0.5HfO₃ as a model system, partial substitution of Nb⁵⁺ with Sb⁵⁺ serves as a multifunctional tuning parameter to simultaneously modulate phase constitution, domain architecture, and defect chemistry. At an optimal Sb⁵⁺ concentration, hierarchical multivariant domain configurations are stabilized, while aligned defect dipoles associated with coupled A-site and oxygen vacancies introduce controlled energetic asymmetry into the polarization landscape. This asymmetric yet dynamically flexible polarization state enables low-barrier polarization rotation and reversible domain switching under electric fields. As a result, the optimized ceramics exhibit a high piezoelectric coefficient of approximately 350 pC N⁻¹ together with an ultrahigh electrostrain of about 1.14%, placing them among the top-performing lead-free piezoelectric systems reported to date. More broadly, this work provides a transferable strategy for designing next-generation lead-free piezoelectrics with concurrently enhanced piezoelectric and electrostrain responses.