Core-shell structure and domain engineering in Bi0.5Na0.5TiO3-based ceramics with enhanced dielectric and energy storage performance

Core–shell structured Bi0.5Na0.5TiO3KTaO3 + x% (in mass) Li2CO3 ceramics were fabricated in this study. Increasing x from 0 to 2 leads to the decrease of sintering temperature from 1 175 °C to 1 020 °C. The limited diffusion of Ta5+ results in chemical heterogeneities and core–shell microstructures. The Ta5+-depleted cores show the nanodomains (∼10 nm), while the Ta5+-rich shells display the polar nanoregions (1–2 nm). From x = 0 to 1, the appearance of cores with nanodomains contributes to the increase of dielectric constant and maximum polarization, while the further addition of Li2CO3 suppresses the dielectric and polarization responses due to the reduced grain sizes and polarization coupling. The enhanced dielectric relaxation and existence of core-shell microstructure with different polarization levels help to optimize the dielectric temperature stability. The x = 2 ceramics exhibit a stable high dielectric constant ∼1 400 over a wide temperature range of 20–520 °C. More encouragingly, the ultrafine grain size and core–shell microstructure in the x = 2 ceramics greatly benefit the improvement of breakdown strength. Combined with the delayed polarization saturation and high ergodicity, a high recoverable energy density of ∼5.07 J/cm3 is obtained under 44 kV/mm, with a high efficiency of ∼85.17%.