Freestanding Relaxor Ferroelectric Single‐Crystalline Thin Films Enable Flexible Piezoelectric Energy Harvester With Giant Power Density

ABSTRACT Piezoelectric‐based flexible nanogenerators have appealed tremendous attention due to their significant application potential in wearable and implantable devices. However, materials with high piezoelectricity commonly used in these nanogenerators are typically rigid oxides, which often suffer from low strain tolerance due to their intrinsic brittleness. Here, we report the development of flexible, high‐performance freestanding relaxor ferroelectric 0.68Pb(Mg _1/3 Nb _2/3 )O ₃ ‐0.32PbTiO ₃ single‐crystal nanoscale membranes, achieved through crystallographic orientation control and a sacrificial‐layer‐based wet etching process. The flexible nanogenerator fabricated from freestanding (111)‐oriented membranes exhibit outstanding electromechanical performance, with a converse piezoelectric coefficient as high as 1350 pm V ⁻¹ . Under an applied strain of ∼2.4%, the nanogenerator devices generate an open‐circuit voltage of ∼20 V and a short‐circuit current of ∼72.6 nA, corresponding to a giant power density of ∼147 mW cm ^{− 3} , while maintaining stable operation over >30000 mechanical deformation cycles. Combined synchrotron‐based X‐ray reciprocal‐space mapping, atomically resolved electron microscopy, and comprehensive electrical characterizations reveal that substrate release induces crystallographic symmetry lowering and suppresses clamping, thereby enhancing polarization rotation and domain‐wall mobility. This work establishes a general strategy for integrating high‐performance relaxor oxide thin films into flexible platforms, advancing next‐generation energy‐harvesting and flexible electronic technologies.