Dielectric energy-storage ceramics have the advantages of high power density and fast charge and discharge rates, and are considered to be excellent candidate materials for pulsed power-storage capacitors. At present, the application of dielectric energy-storage ceramics is hindered by their low energy density and the fact that most of them contain elemental lead. Therefore, lead-free dielectric energy-storage ceramics with high energy storage density have become a research hot spot. In this paper, we first present the requirements that dielectric energy-storage capacitors impose on the properties of ceramic materials. We then review our previous research work combined with research progress into bismuth (Bi)-based lead-free energy-storage ceramics including Bi _0.5 Na _0.5 TiO ₃ (BNT), BiFeO ₃ , and Bi _0.2 Sr _0.7 TiO ₃ , in which the composition design ideas and related energy-storage characteristics of BNT-based lead-free energy-storage ceramics are emphasized. At the same time, we highlight the problems faced by Bi-based lead-free energy-storage ceramics and some strategies for addressing them. Finally, we examine the future prospects of research into Bi-based lead-free energy-storage ceramics.

Compared to batteries and electrochemical capacitors, dielectric capacitors are widely studied because of their huge advantages in terms of charging/discharging speed and power density. In this work, high-entropy (Bi_0.2Na_0.2Sr_0.2Ba_0.2Ca_0.2)TiO₃ lead-free relaxor-ferroelectric ceramics were prepared by both conventional sintering (CS) and spark plasma sintering (SPS). The results showed that the ceramic prepared by SPS obtained a high energy storage density of 6.66 J/cm³ and a satisfied energy storage efficiency of 77.2% under an electric field of 430 kV/cm. This is directly related to the high density, fine grains, number of oxygen vacancies, and composition uniformity of the SPS samples. This study provides a new path for the preparation of high-entropy dielectric energy storage ceramics with high energy storage properties.

Electronic states of ions in lead-free perovskite-type dielectric oxides have been investigated with a first-principle cluster calculation. For this calculation a double-perovskite cluster model based upon the simple cubic ABO₃ was used; A and B are both the cations, and O is the oxygen anion. Systematic variations of ionic species for A and B, and lengths of the model cube edge were given to the model. Results of charge transfers of the ions show that their magnitudes depend on the edge length; the lager length leads to the higher transfer magnitude. This tendency implies spatial tolerance of the ions to the clusters, and are expected to correlate with electric polarizability and dipole reversibility of this kind of oxides. The density of states and the overlap population indicate that the higher cation valence causes the higher covalency of the anions. Considering all results together provides us an idea to obtain lead-free high-performance ferroelectrics, as high as the lead-based solid solutions.

The effective ionic charges of lead-free perovskite dielectric complex compounds were investigated with molecular orbital calculation. The base model was a double perovskite cluster that consisted of octahedral oxygen cages with a transition metal ion of titanium, niobium, or zirconium located at each of their centers, and alkali and/or alkaline earth metal ions located at the body center, corners, edge centers, or face centers of the cluster. The results showed significant covalent bonds between the transition metals and the oxygens, and the alkali metals, especially sodium and oxygen. On the other hand, the alkaline earth metals have weak covalency. Calculation was also performed with the replacement of some of the oxygens with chlorine or fluorine; such replacement enhances the covalency of the transition metals. These trends provide good guidelines for the design properties of lead-free perovskite piezoelectrics based on ubiquitous sodium use.