Sm doped [(Na_0.5Bi_0.5)_0.93Ba_0.07]_1-xSm_xTiO₃ (BNBST) lead-free dielectric energy storage ceramics were prepared by solid-phase reaction sintering method. Effects of the Sm doping content on phase structure, microstructure, ferroelectric, dielectric, energy storage properties and d.c., and a.c. conductance of BNBST ceramics were systematically investigated. Results indicate that the as-fabricated BNBST ceramics exhibit single-phase perovskite structure, and Sm dopants get into the A-site lattice of (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃ matrix. Dense and uniform grains are obtained by grain growth inhibition of Sm dopants with average grain size within 2 μm. The remanent polarization and coercivity of BNBST ceramics sharply decrease after introducing Sm dopants, and the double hysteresis loops are observed with a little decrease of saturation polarization. The energy storage density and efficiency increase firstly and then decrease with increasing Sm doping content, the energy storage density reaches a maximum value of 0.70 J/cm³ at x=0.02 and 70 kV/cm electric field, with corresponding efficiency of 40%. The BNBST ceramics show an obvious relaxation ferroelectric characteristic and its dielectric constant peaks of T_m decrease and planarize with increasing Sm doping content. The electric insulativity of BNBST ceramics has strong temperature dependence, and the excellent electric insulativity can be kept when ambient temperature is below 300℃.

Compared to polymers and their nanocomposites, dielectric ceramics are considered as promising candidates for the pulsed-power devices because of their excellent temperature stability and good anti-fatigue characteristic. Nevertheless, relatively low energy storage density is the main disadvantage for dielectric ceramics, which does not meet the requirement of miniaturization for pulsed-power devices. Therefore, how to improve the energy storage density of dielectric ceramics has become one of hot topics on the research of functional ceramics in recent years. In this paper, the basic principle of the capacitor for electric energy storage was introduced firstly and then the research advances of BaTiO₃-based, BiFeO₃-based, (K_0.5Na_0.5)NbO₃-based lead-free relaxor ceramics and (Bi_0.5Na_0.5)TiO₃-based, and AgNbO₃-based lead-free anti-ferroelectric ceramics were reviewed based on our group’s research, in which the composition design strategies of different material systems were especially summarized. Finally, the opportunities and challenges of lead-free nonlinear energy-storage ceramics were analyzed, and the coping strategies as well as the future development direction were also proposed.

In this study, BaTiO₃ nanofibers were synthesized by a two-step hydrothermal method and subsequently incorporated into poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix to prepare nanocomposites for energy storage application. The crystalline phase, morphology and microstructure of BaTiO₃ nanofibers were observed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy, respectively. The dielectric properties and energy storage performance of the nanocomposites were characterized by dielectric and ferroelectric analyzer. The BaTiO₃ nanofibers with tetragonal phase structure exhibited high aspect ratios, good dispersibility and compatibility in polymer matrix. The effects of volume fraction of BaTiO₃ nanofibers on the dielectric constant, breakdown strength and discharged energy density of the nanocomposites were investigated systematically. The dielectric constant of the BaTiO₃-P(VDF-HFP) nanocomposites remarkably improved with the increase of BaTiO₃ nanofiber contents at the same frequency. At 1 kHz, the maximum dielectric constant of the composite with 20vol% BaTiO₃ nanofibers is up to 30.69. The composite with 5vol% BaTiO₃ nanofibers achieves the maximum energy storage density (4.89 J/cm³) and discharged energy density (2.58 J/cm³) at 240 kV/mm.

(1-x)(Bi_0.5Na_0.5)_0.935Ba_0.065TiO₃-xBiScO₃ (BNBT-xBS) lead-free ceramics were fabricated by conventional ceramic sintering process and modified by BiScO₃. Effects of BiScO₃ content on microstructure, energy storage, field-induced strain and dielectric properties of BNBT-xBS ceramics were investigated. The results indicated that the structure of BNBT-xBS ceramics without impurity phase transferred from the co-existence phase of rhombohedral and tetragonal phase to pseudo-cubic phase. Average grain size of BNBT-xBS ceramics grew slightly with increment of doping content. The long-range ferroelectric order of BNBT-xBS ceramics was destroyed by BiScO₃, which resulted in weak polarization. Meanwhile, the phase transition of BNBT-xBS ceramics was observed from a typical ferroelectric phase to relaxor phase. BiScO₃ dopants improved energy storage and strain performance of ceramics as well, whose maximum energy storage density and high strain were 0.46 J/cm³ and 0.25% at 70 kV/cm. The dielectric constant decreased with doping content increasing. Relaxor ferroelectric characteristics were also verified by temperature-dependence dielectric spectra. The resistance of BNBT-xBS ceramics illustrated a negative temperature coefficient and excellent electrical insulativity below 450℃.

High-k composites have been actively pursued in the past few years for potential applications in embedded capacitors and energy-storage devices. In this study, Ag@TiO₂ core@shell particles were synthesized by a hydrolysis from titanate alkoxides at room temperature. Composites filled with the particle fillers were characterized for I/V, dielectric and energy-storage characteristics. Mechanisms of influences of Ag@TiO₂ fillers on dielectric properties of composites were investigated. Scanning electron microscopy and energy dispersive spectra exhibit that the synthesized Ag@TiO₂ particles have spherical and fully-coated core@shell structures. X-ray diffraction pattern confirms the phase of Ag and TiO₂ in the particles. The polydimethylsiloxane composites filled with Ag@TiO₂ fillers exhibit a small leakage current of 10 ^-8A/cm ², a high dielectric permittivity of 108, and a very low dielectric loss of 0.2%, and a large energy storage density of 8.58×10 ^-3J/cm ³. Theoretical model containing effective medium theory (EMT) and Maxwell theory were used to compare with experimental results, and interfacial polarizations were proposed to enhance the permittivities of the composites. The composites filled with Ag@TiO₂ fillers show potential applications in the embedded capacitors.