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

ABSTRACT 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 _0.5 Na _0.5 )NbO ₃ (KNN)‐based ferroelectrics. Using 0.96K _0.48 Na _0.52 NbO ₃ ‐0.04Bi _0.5 Li _0.5 HfO ₃ as a model system, partial substitution of Nb ⁵⁺ with Sb ⁵⁺ serves as a multifunctional tuning parameter to simultaneously modulate phase constitution, domain architecture, and defect chemistry. At an optimal Sb ⁵⁺ 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.