Lead zirconate titanate (PZT)‐based piezoelectric ceramics are important functional materials for various electromechanical applications, including sensors, actuators, and transducers. High piezoelectric coefficient and mechanical quality factor are essential for the resonant piezoelectric application. However, since these properties are often inversely proportional, simultaneously high performances are hard to achieve, consequently, a wide range of applications are strongly restricted. In the present study, exceptionally well‐balanced performances are achieved in PZT‐based ceramics via innovative defect engineering, which involves multi‐scale coordination among defect dipole, domain‐wall density, and grain boundary. These materials are superior to many state‐of‐the‐art commercial counterparts, which can potentially satisfy high‐end requirements for advanced electromechanical applications, such as energy harvesting, structural health monitoring, robotic sensors, and actuator.

Dielectric capacitors have aroused extensive attentions as energy storage devices in the past few decades on account of their extremely fast charging-discharging process and consequent high power density, great voltage endurance, and low cost, 1,2 which gives them a great advantage in the application of pulsed power and power electronics such as hybrid electric vehicles, radar transmitters, frequency inverters, pacemakers, and electric weapon systems. [3][4][5] However, low-energy storage density and efficiency are currently the most challenging problems of dielectric capacitors.As is well-known, high breakdown strength E b and dielectric constant ε r , large maximum polarization P max , small remnant polarization P r and coercive field E c are crucial to obtain high energy storage density and high efficiency simultaneously. [5][6][7] Dielectric polymers usually possess ultrahigh breakdown strength (≥5000 kV cm −1 ) but low dielectric constants (≤12), which limits their energy storage density. [8][9][10][11][12][13] In addition, relatively low maximum operating temperature

Since the extrinsic contribution is the key to electric-field-induced strain in ferroelectrics, engineering the interaction between defect and domain-wall motion has been an effective approach for enhancing the strain performance. While acceptor doping has been frequently employed in the lead-free (K, Na)NbO3 (KNN) system, the individual influence of intrinsic defects is still ill-understood. In this work, pure KNN ceramics with various concentrations of intrinsic defects were prepared by hot-pressing at different temperatures. Meanwhile, the microstructure, electrical properties, and defect chemistry were systematically investigated. An enhanced normalized strain d33* of 320 pm/V with good temperature stability was obtained in the KNN sample hot-pressed at 1000 °C, which is two times larger than that of reported normally sintered KNN. Besides, an asymmetric bipolar strain was found accompanied by the presence of offset polarization. The phenomena can be explained by a qualitative model involving unswitchable domains and intrinsic defects. The present study could enable further understanding of defect engineering and demonstrate a possible manipulation of intrinsic defects to enhance the strain performance of KNN-based piezoceramics.

Flexible piezoelectric thin films are raising interest in energy harvesting and wearable electronics, although their direct fabrication is challenging in the selection of substrates and thermal processing. In this work, we developed direct fabrication of flexible lead-free (K, Na)NbO 3 (KNN)-based piezoelectric films on commercially available metallic foils by sol−gel processing. Stainless steel and platinum foils are selected as flexible substrates because of their good thermal stability, robust flexibility, and cost-efficiency. The sol−gel-processed KNN-based thin films on both of the metallic foils show good flexibility, with the bending radii reaching ±3 mm. The flexible thin films grown on stainless steel and platinum foils present high breakdown electric fields that reach 1760 and 2530 kV/cm, respectively, resulting from the fine-grained dense structure, limited leakage current density, and suppressed mobility of charged carriers. Improved effective piezoelectric coefficient d 33, eff * (75.4 pm/V) with a slight decrease after bending was obtained in the flexible thin films on Pt when compared to their rigid counterparts. The flexible lead-free piezoelectric thin films with combined high breakdown electric fields and piezoelectric and energy storage properties may pave the way for integrating KNN-based multifunctional thin films into flexible electronics.

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