Significantly Enhanced Energy Storage Density and Efficiency at Low Electric Fields in Lead-Free Bi_0.5Na_0.25K_0.25TiO₃-K_{0 .5}Na_0.5NbO₃ Piezoceramics

Bi-Na-K-TiO3 and K-Na-NbO3 lead-free piezoceramicshave been widely used in next-generationadvanced pulsed-power capacitors owing to their environmental friendlinessand exceptional electromechanical and thermal behavior. However, theenormous challenge of obtaining ultrahigh recoverable energy storagedensity "W (rec)" correspondingto ultrahigh efficiency "& eta;" has persisted andhas become a fundamental barrier inhibiting the development of lead-freepiezoceramics in cutting-edge energy storage applications. To raisethe "W (rec)" and "& eta;"substantially, we proposed a strategy to prepare a composite of lead-freebulk piezoceramics. To demonstrate the effectiveness of this approach,frequency- and temperature-dependent composites of (1-x)Bi0.5Na0.25K0.25TiO3-(x)K0.5Na0.5NbO3 (BNKT-KNN) ceramics were used as a representative in thiswork. (1-x)BNKT-(x)KNN piezoceramicswith sub-nanometer grains (approximately 150 nm) were prepared usinga solid-state reaction route followed by two-step sintering. The resultantceramics had a dense structure with minimal pores, exhibiting pseudo-cubicsymmetry and strong relaxor characteristics. The frequency- and temperature-dependentdielectric and ferroelectric properties, along with their relaxorbehavior and energy storage properties, have been investigated. Thelarge "W (rec)" & SIM; 40mJ/cm(3) at 10 Hz and "& eta;" & SIM; 63%at 100 Hz accomplished by applying a shallow external electric field(35 kV/cm) for 75BNKT-25KNN ceramics is comparable to other reportedBi-Na-K-TiO3- and K-Na-NbO3-based lead-free bulk ceramics. These outcomes demonstratethat the (1-x)BNKT-(x)KNNceramics are preferred materials for advanced pulsed-power capacitors.This study paves the way to design a novel class of piezoceramic materialswith high-energy storage applications to fulfill the stringent criteriaof modern energy storage applications.