High-Performance Dielectric Ceramic Films for Energy Storage Capacitors: Progress and Outlook

Based on the concept of band bending at metal/semiconductor interfaces as an energy filter for electrons, we present a theory for the enhancement of the thermoelectric properties of semiconductor materials with metallic nanoinclusions. We show that the Seebeck coefficient can be significantly increased due to a strongly energy-dependent electronic scattering time. By including phonon scattering, we find that the enhancement of $ZT$ due to electron scattering is important for high doping, while at low doping it is primarily due to a decrease in the phonon thermal conductivity.

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Theory of enhancement of thermoelectric properties of materials with nanoinclusions.

Materials exhibiting high energy/power density are currently needed to meet the growing demand of portable electronics, electric vehicles and large-scale energy storage devices. The highest energy densities are achieved for fuel cells, batteries, and supercapacitors, but conventional dielectric capacitors are receiving increased attention for pulsed power applications due to their high power density and their fast charge-discharge speed. The key to high energy density in dielectric capacitors is a large maximum but small remanent (zero in the case of linear dielectrics) polarization and a high electric breakdown strength. Polymer dielectric capacitors offer high power/energy density for applications at room temperature, but above 100 °C they are unreliable and suffer from dielectric breakdown. For high-temperature applications, therefore, dielectric ceramics are the only feasible alternative. Lead-based ceramics such as La-doped lead zirconate titanate exhibit good energy storage properties, but their toxicity raises concern over their use in consumer applications, where capacitors are exclusively lead free. Lead-free compositions with superior power density are thus required. In this paper, we introduce the fundamental principles of energy storage in dielectrics. We discuss key factors to improve energy storage properties such as the control of local structure, phase assemblage, dielectric layer thickness, microstructure, conductivity, and electrical homogeneity through the choice of base systems, dopants, and alloying additions, followed by a comprehensive review of the state-of-the-art. Finally, we comment on the future requirements for new materials in high power/energy density capacitor applications.

Electroceramics for High-Energy Density Capacitors: Current Status and Future Perspectives

High-performance lead-free piezoelectric materials are in great demand for next-generation electronic devices to meet the requirement of environmentally sustainable society. Here, ultrahigh piezoelectric properties with piezoelectric coefficients (d33 ≈700 pC N-1 , d33 * ≈980 pm V-1 ) and planar electromechanical coupling factor (kp ≈76%) are achieved in highly textured (K,Na)NbO3 (KNN)-based ceramics. The excellent piezoelectric properties can be explained by the strong anisotropic feature, optimized engineered domain configuration in the textured ceramics, and facilitated polarization rotation induced by the intermediate phase. In addition, the nanodomain structures with decreased domain wall energy and increased domain wall mobility also contribute to the ultrahigh piezoelectric properties. This work not only demonstrates the tremendous potential of KNN-based ceramics to replace lead-based piezoelectrics but also provides a good strategy to design high-performance piezoelectrics by controlling appropriate phase and crystallographic orientation.

Ultrahigh Piezoelectric Properties in Textured (K,Na)NbO3-Based Lead-Free Ceramics

Ultrahigh discharge energy density (Wdis = 10.5 J cm−3) and efficiency (η = 87%) have been obtained in doped BiFeO3–BaTiO3 ceramic multilayers by achieving an electrically rather than chemically homogeneous microstructure. Back scattered scanning and transmission electron microscopy combined with energy dispersive X-ray spectroscopy mapping of (0.7 − x)BiFeO3–0.3BaTiO3–xNd(Zn0.5Zr0.5)O3 (0.05 ≤ x ≤ 0.10) ceramics revealed a core–shell grain structure which switched from a bright to dark contrast as x increased. Compositions with x = 0.08 were at the point of cross over between these two manifestations of core–shell contrast. Dielectric measurements together with the absence of macrodomains in diffraction contrast TEM images suggested that compositions with x = 0.08 exhibited relaxor behaviour within both the core and shell regions. Impedance spectroscopy demonstrated that, despite being chemical dissimilar, the grains were electrically homogeneous and insulating with little evidence of conductive cores. Multilayers of x = 0.08 had enhanced breakdown strength, EBDS > 700 kV cm−1 and a slim hysteresis loop which resulted in large Wdis and high η which were temperature stable to <15% from 25 to 150 °C.

Ultrahigh energy storage density lead-free multilayers by controlled electrical homogeneity

Currently, bismuth-based perovskite-type ceramics are considered as promising species in energy storage applications because of easy phase- and micro/macro-structure modulations and high performances. In this work, the composition dependent phase structure and ferroelectric properties of novel Bi0.5K0.5TiO3–Ba(Mg1/3Nb2/3)O3 (BKT–BMN) ceramics are studied. Relaxor properties are gradually enhanced with increasing BMN content. The BKT–0.15BMN composition is selected to further explore energy storage performance via the hot-pressing (HP) technique. The results show that the recoverable energy storage density (WR = 3.14 J cm−3) for the HP sample is more than two times larger than that of the conventional sintering (CS) one. The outstanding WR also exhibits super stability against temperature and frequency variations. Besides, the energy storage efficiency (η) is up to 83.7% for the HP sample. In particular, the stored energy can be released in a very short time of ∼0.12 μs at room temperature, indicating a fast discharging speed for the HP sample. The enhanced performance is due to the decrease of grain size and the increase of grain boundaries, the mechanism underlying the microstructure effect on the breakdown strength (BDS) value is disclosed by numerical simulations (COMSOL). This work not only enriches the bismuth-based ceramic systems in pulsed power applications, but also deepens the understanding of the intrinsic mechanism of a refined microstructure that boosts energy storage performance.

Fine-grain induced outstanding energy storage performance in novel Bi0.5K0.5TiO3–Ba(Mg1/3Nb2/3)O3 ceramics via a hot-pressing strategy

Energy storage performance of Na0.5Bi0.5TiO3-SrTiO3 lead-free relaxors modified by AgNb0.85Ta0.15O3

High-performance (Na0.5Bi0.5)TiO3 (NBT) lead-free incipient ceramics are promising piezoactuator materials, but high driving fields to deliver the large strain limits their practical applications. Herein, we report a giant piezoelectric strain (d33*) of 810 pm V−1 at a low driving field of 4 kV mm−1 in a novel ternary (0.94 − x)(Na0.5Bi0.5)TiO3–0.06Ba(Zr0.05Ti0.95)O3−x(Sr0.8Bi0.1□0.1)TiO2.95 solid solution with x = 0.05 (SBT5). The field of SBT5 critical composition is notably reduced compared with other NBT-based ceramics, while its strain properties are maintained at a high level along with an excellent thermal stability. The dopant induces a randomly distributed local polarization field, which breaks the symmetry of Landau potential curves, boosts the ferroelectric instability and favors a more disordered relaxor structure. The giant strain in the critical composition SBT5 is due to a field-induced reversible relaxor-ferroelectric phase transformation, while the reduced driving field results from two synergistic effects: remanent quasi-ferroelectric order as the seed for the growth of polar domains helps the system skip the nucleation process; the local defects further facilitate the growth of ferroelectric domains. The composition, temperature and electric-field dependence of structural evolutions were systematically elucidated from the micro- and macroscopic view. This study opens up a feasible and effective way for achieving giant electrostrain in lead-free actuator materials.

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Giant electrostrain accompanying structural evolution in lead-free NBT-based piezoceramics

In this study, the Bi-nonstoichiometric 099Bix(Na08K02)05TiO3-001SrTiO3 (BNKST) ceramics with x = 05–0535 mol (Bi50-Bi535) were prepared by a conventional solid-state reaction method The effects of Bi excess on structural transition and ferroelectric stability of BNKST ceramics were systematically investigated by the Raman spectra, dielectric analyses and electromechanical measurements The introduction of excess Bi3+ could significantly break the long-range ferroelectric order and favor the presence of relaxor phase, then the ferroelectric-relaxor transition temperature (TFR) can be effectively tuned to around room temperature by Bi nonstoichiometry, giving rise to an enhanced room-temperature strain property The positive strain Spos and dynamic piezoelectric constant d33* of Bi525 critical composition reach 033% and 440 pm/V, respectively at 6 kV/mm The high recoverable strain of Bi525 sample can be attributed to the electric-field-induced reversible relaxor-ferroelectric phase transition The present work may be helpful for further understanding and designing high-performance NBT-based lead-free ceramics for piezoelectric actuator applications

Feng Li

Papers

Fine-grain induced outstanding energy storage performance in novel Bi0.5K0.5TiO3–Ba(Mg1/3Nb2/3)O3 ceramics via a hot-pressing strategy

Energy storage performance of Na0.5Bi0.5TiO3-SrTiO3 lead-free relaxors modified by AgNb0.85Ta0.15O3

Giant electrostrain accompanying structural evolution in lead-free NBT-based piezoceramics

Tuning the ferroelectric-relaxor transition temperature in NBT-based lead-free ceramics by Bi nonstoichiometry

A Study on the Relationship Between Grain Size and Electrical Properties in (K,Na)NbO3-Based Lead-Free Piezoelectric Ceramics