Lead-free medium-entropy (Na0.47(1-x)Bi0.47(1-x)Ba0.06(1-x)Sr0.7xNd0.2x)TiO3 relaxor ceramics with robust energy-storage performance

High-entropy engineering is becoming a new strategy in dielectric energy storage capacitors. However, high-entropy strategy alone is insufficient to produce good energy storage performance, as the polarization in-tensity is greatly reduced by the highly disordered state. We, herein, propose an "entropy engineering + poly-phase" strategy and highlight that materials with medium entropy values, no necessity of high entropy values, can yield good energy storage performance under condition that polyphases are introduced in the materials. To illustrate the effectiveness of this strategy, the medium-entropy ceramics (Na0.47(1-x)Bi0.47(1-x)Ba0.06 (1-x)Sr0.7xNd0.2x)TiO3 with x = 0.1, 0.2, 0.3, 0.4, and 0.5 were designed by doping ions with different ionic radii and valence states at the A-site. Investigations on the energy storage performance reveal that the sample of x = 0.4, with the maximum entropy value of 1.39R, has the largest recyclable energy storage density Wrec = 8.71 J/ cm3 with a high efficiency eta = 88.5%. The sample also shows excellent thermal stability of dielectric constant (Delta epsilon '/epsilon ' 25oC <=+/- 15%) over a temperature range of -61-144 degrees C. The excellent performance is due to the intro-duction of multiphase in the medium entropy ceramics, which effectively refines the grain and domain sizes, leading to a high polarization intensity (44.23 mu C/cm2) and excellent energy storage performance. This work shows that "entropy engineering + polyphase" is an effective strategy for excellent energy storage performance and paves a way of developing new energy storage materials by entropy engineering.