Boosting energy-storage performance in lead-free ceramics via polyphase engineering in the superparaelectric state

Dielectric energy storage devices are important components of high-power and pulsed electronic systems. High recoverable energy density (Wrec) and high efficiency (ƞ) are critical parameters for such applications. In this work we propose a strategy of polyphase engineering in the superparaelectric (SPE) state to achieve high-performance energy storage. Through careful engineering of the proportions of rhombic (R) phase and tetragonal (T) phase by a linear dielectric additive, CaTi0.8Hf0.2O3 (CTH), in 0.94Na0.5Bi0.5TiO3-0.06BaTiO3-based ceramics, the SPE state can be shifted to ambient temperature when R/T ≤ 0.16. Thanks to the features of isolated polar nano-domains (PNRs) and the fine P-E hysteresis loop of the SPE state, an ultrahigh Wrec of 8.91 J/cm3 and high ƞ of 78.4% are achieved in our 0.75(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-0.25CaTi0.8Hf0.2O3 sample. The sample was also characterized by excellent temperature and frequency stability, achieving ultra-high power density (178 MW/cm3) and ultra-fast transient discharge time (40 ns). Our work proves that polyphase engineering in the SPE state is a powerful approach to the design of dielectric energy storage materials with high performance.