Grain size engineered lead-free ceramics with both large energy storage density and ultrahigh mechanical properties

Lead-free dielectric ceramics with both a high recoverable energy storage density (Wrec) and excellent mechanical performance are highly desirable for practical applications in next-generation advanced pulsed power capacitors (APPCs). However, lead-free dielectric ceramics exhibit low Wrec owing to small breakdown strength (Eb) and poor mechanical properties because of their large pore size and low relative density, which restrict devices miniaturization and operation in severe environments. Here, we propose a new strategy, namely, grain size engineering, to develop K0.5Na0.5NbO3 (KNN)-based ceramics with both an extremely high Wrec and large mechanical properties. Interestingly, a large Wrec of 2 J cm−3 was achieved in 0.9K0.5Na0.5NbO3-0.1BiFeO3 (0.9KNN-0.1BF) ceramics at 206 kV cm−1, which is superior to other lead-free dielectric ceramics under moderate electric fields (<220 kV cm−1). Encouragingly, the hardness (H) of 0.9KNN-0.1BF ceramics reached 2.45 GPa, which is approximately 5 times higher than that of pure KNN ceramics (0.45 GPa). Moreover, the bending strength (σf), elastic modulus (E) and compression strength (σc) of 0.9KNN-0.1BF ceramics are also enhanced by ~200% over those of pure KNN ceramics. Compared with other lead-free dielectric ceramics, grain size engineered 0.9KNN-0.1BF is the first high-performance ceramic material with both an exceptionally large Wrec and ultrahigh mechanical properties, which can accelerate the practical use of APPCs. Most importantly, the findings in this work will not only provide a guideline for developing other lead-free dielectric ceramics with both extremely high energy storage properties and superior mechanical performance but also open a window to the mechanical properties of dielectric ceramics for APPCs.