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 (W _rec ) 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 W _rec owing to small breakdown strength (E _b ) 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 K _0.5 Na _0.5 NbO ₃ (KNN)-based ceramics with both an extremely high W _rec and large mechanical properties. Interestingly, a large W _rec of 2 J cm ⁻³ was achieved in 0.9K _0.5 Na _0.5 NbO ₃ -0.1BiFeO ₃ (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 (σ _f ), elastic modulus (E) and compression strength (σ _c ) 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 W _rec 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.