Dielectric capacitors have been recognized as promising devices for advanced pulse power systems due to their high-power density and fast charge-discharge rates. The dielectrics must simultaneously achieve large energy storage density and high efficiency to support the rapid development of dielectric capacitors. Among the various dielectric ceramics investigated so far, the Bi_0.5Na_0.5TiO₃ (BNT)-based lead-free relaxor ferroelectric has recently become increasingly attractive for dielectric energy storage owing to its high spontaneous polarization and temperature corresponding to the peak of maximum permittivity (T_m) in dielectric constant spectroscopy. Extensive efforts have been devoted to developing high-performance BNT-based ceramics in extreme conditions, and significant progress has been made. To meet the application demands of energy storage devices across diverse electric fields, it is imperative to understand the fundamental principles of energy storage and devise targeted optimization strategies for BNT-based ceramics. This review provides an overview of energy storage theory and essential determinants governing capacitive performance of dielectric materials, encompassing polarization response, breakdown characteristics, relaxation behavior, and dielectric properties. Furthermore, we elucidate tailored multiscale design strategies to optimize the energy storage capability of BNT-based ceramics across various electric field (E-field) regions:  low E-field (< 300 kV/cm), moderate E-field (300 - 500 kV/cm), and high E-field (˃ 500 kV/cm). We further present the developmental progress and future outlook of BNT-based ceramics for advanced electrostatic capacitor applications.

Nickel–cobalt layered double hydroxides (NiCo-LDHs) are promising electrode materials for hybrid supercapacitors (HSCs) due to their high theoretical charge storage capacity and excellent reversibility. However, their practical application is limited by low electrical conductivity and a tendency to agglomerate, which suppress their electrochemical performance. To address these challenges, NiCo-LDH nanosheets (NiCo-LDH NSs)/carbon microtubes derived from poplar catkins (CMT-PC) composite electrode material is synthesized via a hydrothermal method. This composite integrates NiCo-LDH NSs as a coating and CMT-PC with a high specific surface area as the framework. In a three-electrode system, the NiCo-LDH NSs/CMT-PC electrode demonstrated a specific capacity of 228.4 mAh·g⁻¹ (1644.5 F·g⁻¹, 822.2 C·g⁻¹) at a current density of 1 A·g⁻¹, and maintained a specific capacity of 101.7 mAh·g⁻¹ (732.2 F·g⁻¹, 366.1 C·g⁻¹) even at 30 A·g⁻¹. After 5000 cycles, the material exhibited excellent stability, retaining 96.5% of its capacity, with a decrease from 193.6 to 186.9 mAh·g⁻¹. To explore its practical application in HSCs, we assembled NiCo-LDH NSs/CMT-PC//activated carbon (AC) HSCs, using the NiCo-LDH NSs/CMT-PC as the positive electrode and AC as the negative electrode. The assembled device exhibited a specific capacity of 88.3 mAh·g⁻¹ at 1 A·g⁻¹ and an energy density of 72.2 Wh·kg⁻¹ at a power density of 508.6 W·kg⁻¹. Impressively, after 9000 cycles at 3 A·g⁻¹, the specific capacity increased from 64.2 to 66.5 mAh·g⁻¹, demonstrating exceptional cycling stability and suitability for practical applications.

AgNbO₃-based antiferroelectric ceramics can be used to prepare dielectric ceramic materials with energy storage performance. However, their efficiency is much lower than that of relaxors, which is one of the biggest obstacles for their applications. To overcome this problem, AgNbO₃ ceramics co-doped with Eu³⁺ and Ta⁵⁺ at the A- and B-sites were prepared in this work. The Ag_0.97Eu_0.01Nb_0.85Ta_0.15O₃ sample has a W_r of 6.9 J/cm³ and an η of 74.6%. The ultrahigh energy storage density and efficiency of Ag_0.97Eu_0.01Nb_0.85Ta_0.15O₃ has been ascribed to the synergistic effect of the increase in the breakdown electric field, the enhancement of antiferroelectric stability, the construction of multiphase coexistence, and the modification of the domain structure morphology. The Ag_0.97Eu_0.01Nb_0.85Ta_0.15O₃ ceramic is expected to be one of the options for preparing dielectric capacitors.

Heterovalent doped (K_0.48-0.07xNa_0.52-0.43xBi_0.5x)(Nb_0.95-0.95xSb_0.05-0.05xZr_x)O₃ ceramics were fabricated using conventional solid-state reaction. Then, the phase structures, dielectric, ferroelectric, and electric-strain properties were investigated. The compositions were tuned to be located at polymorphic phase boundary with increasing heterovalent Bi³⁺ and Zr⁴⁺ doping levels. A large strain of 0.19% was obtained at relatively low electric fields of 30 kV/cm in the composition of x = 0.04. The normalized large-signal d₃₃* values were approximately 633 pm/V under a low driving electric field of 30 kV/cm, which were comparable or larger than the values reported for other lead-free families. The large strains obtained can be attributed to the formation of nanodomains and high-density domain walls, which were confirmed by the observations of domain morphology using transmission electron microscopy (TEM) technique. Excellent temperature stability of the strain properties of the x = 0.04 sample could be ascribed to the sluggish behaviour for the local structural heterogeneity in heterovalent-ion doped KNN ceramic. Theoretical simulations revealed that the Zr⁴⁺ produce the local stress at the BO₆ octahedra and Bi³⁺ could yield off-centering of AO₁₂ ployhedron due to the nature of Bi 6s lone pair electrons, which induced lattice expansion and local distortions in the sample. The local displacements are strongly anisotropic in heterovalent-ion doped system. It is believed that random local fields exist in these compositions owing to the eixstence of charge distribution. Such heterovalent doping of Bi³⁺ and Zr⁴⁺ could destory simultaneously the orthorhombic symmetry and the short-range ferroelecctric order, leading to the formation of complex nanodomains and local structral hetergenenity. Heterovalent doping may, therefore, offer a new avenve to design novel K_0.5Na_0.5NbO₃ (KNN) -based materials for their mutifunctional applications.