Significantly Enhanced Energy Storage Density and Efficiency at Low Electric Fields in Lead-Free Bi<sub>0.5</sub>Na<sub>0.25</sub>K<sub>0.25</sub>TiO<sub>3</sub>–K<sub>0.5</sub>Na<sub>0.5</sub>NbO<sub>3</sub> Piezoceramics
Dhanranjan Kumar, S.K. Rout
Abstract
Bi–Na–K–TiO 3 and K–Na–NbO 3 lead-free piezoceramics have been widely used in next-generation advanced pulsed-power capacitors owing to their environmental friendliness and exceptional electromechanical and thermal behavior. However, the enormous challenge of obtaining ultrahigh recoverable energy storage density “ W rec ” corresponding to ultrahigh efficiency “η” has persisted and has become a fundamental barrier inhibiting the development of lead-free piezoceramics in cutting-edge energy storage applications. To raise the “ W rec ” and “η” substantially, we proposed a strategy to prepare a composite of lead-free bulk piezoceramics. To demonstrate the effectiveness of this approach, frequency- and temperature-dependent composites of (1– x )Bi 0.5 Na 0.25 K 0.25 TiO 3 -( x )K 0.5 Na 0.5 NbO 3 (BNKT-KNN) ceramics were used as a representative in this work. (1– x )BNKT-( x )KNN piezoceramics with sub-nanometer grains (approximately 150 nm) were prepared using a solid-state reaction route followed by two-step sintering. The resultant ceramics had a dense structure with minimal pores, exhibiting pseudo-cubic symmetry and strong relaxor characteristics. The frequency- and temperature-dependent dielectric and ferroelectric properties, along with their relaxor behavior and energy storage properties, have been investigated. The large “ W rec ” ∼ 40 mJ/cm 3 at 10 Hz and “η” ∼ 63% at 100 Hz accomplished by applying a shallow external electric field (35 kV/cm) for 75BNKT-25KNN ceramics is comparable to other reported Bi–Na–K–TiO 3 - and K–Na–NbO 3 -based lead-free bulk ceramics. These outcomes demonstrate that the (1– x )BNKT-( x )KNN ceramics are preferred materials for advanced pulsed-power capacitors. This study paves the way to design a novel class of piezoceramic materials with high-energy storage applications to fulfill the stringent criteria of modern energy storage applications.