Enhancement of energy storage performance in lead-free barium titanate-based relaxor ferroelectrics through a synergistic two-step strategy design
Introduction
Dielectric capacitors, which possess the advantages of rapid charge − discharge speed and ultrahigh power density, have been utilized extensively in advanced pulsed power capacitor systems (APPCSs) [1], [2], [3]. Nevertheless, the low energy storage density (Wrec), small capacity storage efficiency (η) and poor thermal stability hinder their general applications in APPCSs. Coupled with the trend of miniaturization and integration of capacitors, improvements to their comprehensive performance have become more urgent. Their properties are usually determined based on their polarization–electric field (P–E) hysteresis loops and the following formulas [4]:
where Pm denotes the maximum polarization and Pr denotes remanent polarization. As estimated, high Wrec and η values demand a high breakdown strength (BDS, Eb) and a large difference (ΔP) between Pm and Pr. Among the five commonly available dielectric types, linear dielectrics (LDs) have ultrahigh η; however, their low Pm limits the acquisition of high Wrec [5], [6], [7]. While normal ferroelectrics (FEs) have a large Pm, their high Pr and Wloss limit E, thus yielding low Wrec [8], [9]. Antiferroelectrics (AFEs) have an ideal large Pm and low Pr, whereas the FE-AFE phase transition occurs under a high electric field accompanied by internal stress concentration and failure, which cause poor fatigue resistance [10], [11], [12]. Relaxor antiferroelectrics (RAFEs) combine the advantages of the mentioned AFEs and the latter relaxor ferroelectrics (RFEs) together, thus realizing a high Wrec and a high η under a high electric field [13]. Meanwhile, RFEs with small Pr, large Pm and low Wloss can undertake high Eb, enabling efficient implementation of a large Wrec. Moreover, the permittivity of RFEs is generally thermally insensitive within a given temperature range owing to the diffusion phase transition, which promotes the temperature stability of RFEs in energy-storage applications.
Hence, eco-friendly lead-free RFEs are considered as promising candidates for use in energy-storage capacitors. BaTiO3 (BT)-based RFEs account for a significant portion of candidate RFEs [14], [15]. Although the derived Ba1−xSrxTiO3 (BST) matrix can improve some characteristics of BT, some deficiencies remain to be solved: (1) BST possesses a relatively high Pm of approximately 27 μC/cm2, mainly owing to the long-range ferroelectric ordered Ti-O coupling at the B-site; however, this ordering is likely to be broken by the new coupling of B-site dopants, thus reducing Pm and Wrec. (2) BST is a normal FE material with regular FE domains accompanied by a large Pr and coercive field (Ec), leading to a high Wloss and low η. (3) While the Curie temperature (TC) of BT can be easily moved to approximately 66 °C by increasing the Sr2+ ion concentration, the TC peak remains high and sharp, which is not conducive to good energy storage performance (ESP) over a wide temperature range. Because an RFE state can be induced by substituting ions into a normal FE material at sufficiently high concentrations, Bi-containing compounds (BiMeO3, where Me denotes different cations) are typically used to dope BT to activate the FE-to-RFE phase transition and optimize the ESP [16]. For instance, Wang et al. synthetized BT-Bi(Mg2/3Nb1/3)O3 samples and obtained a Wrec approximately 1.13 J/cm3 [17]. Jiang et al. achieved an enhanced Wrec of 1.25 J/cm3 with a high η of 95% at 185 kV/cm in BT-Bi(Mg0.5Zr0.5)O3 systems [18]. Hu et al. recorded an ultrahigh Wrec of 4.49 J/cm3 with an η of 93% in BT-Bi(Mg1/2Ti1/2)O3 ceramics at room temperature via polarization mismatch [19].
Herein, we present a synergistic two-step strategy for enhancing the comprehensive ESP of Ba0.8Sr0.2TiO3-based RFEs, achieving outstanding Wrec, η, and thermal stability. As depicted in the left half of Fig. 1, the high Pm and Pr, as well as low Eb, are maintained in BST, in which the ferroelectric-to-paraelectric phase transition occurs at roughly 66 °C. In the first step, we chose Bi-containing Bi(Zn2/3Nb1/3)O3 (BZN) to broaden the Tm peak and disturb the long-range FE order of BST simultaneously, resulting in typical relaxor behaviors and the conversion from macrodomains to polar nanoregions (PNRs), which improves Pm, reduces Pr, and broadens the temperature range of stable permittivity. (1) For the A-site doping of BZN, Bi3+ has an ionic radius (117 pm) comparable to that of Ba2+ (135 pm) and a valence electron configuration (6s26p0) similar to that of Pb2+, where the Bi 6p and O 2p orbitals can be hybridized to yield a larger Pm [20]. (2) The insertion of B-site composite ions can improve the symmetry of eccentric crystals, disrupt the long-range FE order, and create PNRs. The emergence of PNRs facilitates domain switching after removing the electric field, which suppresses energy loss during charge and discharge, thus creating a negligible Pr. (3) The substitution of BZN reduces the Tm of BST to 16 °C and widens the TCC2 °C value that characterizes the temperature stability of permittivity in the range of − 44.9 °C to 88.1 °C, indicating that a corresponding stable polarization occurs in this temperature platform; the dominant paraelectric state above Tm is accompanied by an ultrahigh η. Accordingly, the large Pm, small Pr, and wide TCC2 °C contribute to an intermediate Wrec, and high-temperature stability is attained in BST–BZN ceramics [see Fig. 1(b)]. However, the Eb obtained from this one-step process is not significantly improved, and the overall improvement of the ESP is limited. As shown in the right half of Fig. 1, by reducing the pore concentration, Eb can be efficiently increased. Hence, the second step is to optimize the preparation process using viscous polymer processing (VPP), a simple ceramic production approach that is reasonably inexpensive with minimal equipment requirements [18], [21], [22]. After sintering, ceramics with dense structures and ultralow porosity can be produced by continuously rolling and diluting ceramic micelles to improve the green density. This significantly increases Eb and ameliorates Wrec simultaneously, realizing a satisfactory ESP in BST–BZNVPP ceramics [see Fig. 1(c)].
In this work, BST-based ceramics exhibit significant improvements in energy storage density, efficiency, and temperature stability through our synergistic design strategy including the introduction of BZN in the first step and the use of VPP technology in the second step. Accordingly, we report an exceptional room-temperature ESP (Wrec = 5.16 J/cm3, η = 82.3%) in BZN9VPP ceramics. The Wrec fluctuates within 3% (Wrec = 3.6 J/cm3) and the η varies within 2% (η = 92.8%) from 20 to 120 °C, indicating good temperature stability of the ESP. These results demonstrate the efficacy of our synergistic design strategy and the competitiveness of BZN9VPP ceramics in pulsed power capacitor applications.
Section snippets
Fabrications of samples
The conventional solid-state process was adapted for the synthesis of (1 − x)(Ba0.8Sr0.2)TiO3-x Bi(Zn2/3Nb1/3)O3 (BZNx, x = 0, 0.05, 0.09, 0.12, 0.15, 0.20) ceramic samples. Thereafter, 0.91(Ba0.8Sr0.2)TiO3–0.09Bi(Zn2/3Nb1/3)O3 ceramics were optimized by the VPP technique and denoted as BZN9VPP ceramics. High-purity (AR-grade) SrCO3, BaCO3, TiO2, Bi2O3, ZnO, and Nb2O5 as raw materials were weighed in stoichiometric amounts depending on the above nominal formulas. The raw powders were ground in
Results and discussion
Fig. 2(a) depicts the XRD patterns of the BZNx ceramic powders. All specimens have single perovskite structures with no heterophases, implying that Bi3+, Zn2+, and Nb5+ are completely diffused into the main BST lattice. Fig. 2(b) reveals that as the BZN content increases, the diffraction peaks shift to lower angles, indicating increased cell volume, because Zn2+ (74 pm) and Nb5+ (68 pm) have larger radii than Ti4+ (60.5 pm). Fig. 2(c) and (d) show the Rietveld refinement of the XRD data, error
Conclusions
In summary, the BZN9VPP ceramics synthesized using the synergistic design strategy exhibited improved ESP. In the first step of BZN doping, the temperature range of stability and enhanced relaxor behavior can be attributed to the broadening of the permittivity peak and the generation of highly dynamic PNRs. In the second step of the VPP process, the increased Eb benefits from the relatively small grain size and increased band gap. Consequently, the designed BZN9VPP ceramic acquires the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 52172127, 51772239 and 51761145024) and the Fundamental Research Funds for the Central Universities (XJTU). The SEM work was done at International Center for Dielectric Research (ICDR), Xi'an Jiaotong University, Xi'an, China. The authors also thank Shiyanjia Lab (www.shiyanjia.com) for the TEM analysis.
References (73)
- et al.
Effect of HfO2 addition as intergranular grains on the energy storage behavior of Ca0.6Sr0.4TiO3 ceramics
J. Eur. Ceram. Soc.
(2016) - et al.
Enhanced energy storage density and its variation tendency in CaZrxTi1-xO3 ceramics
J. Alloys Compd.
(2016) - et al.
Dielectric and energy storage properties of BaTiO3-Bi(Mg1/2Ti1/2)O3 ceramic: Influence of glass addition and biasing electric field
Ceram. Int.
(2017) - et al.
Grain size engineered lead-free ceramics with both large energy storage density and ultrahigh mechanical properties
Nano Energy
(2019) - et al.
Giant energy density and high efficiency achieved in silver niobate-based lead-free antiferroelectric ceramic capacitors via domain engineering
Energy Storage Mater.
(2021) - et al.
Mechanism of enhanced energy storage density in AgNbO3-based lead-free antiferroelectrics
Nano Energy
(2021) - et al.
Lead-free BaTiO3-Bi0.5Na0.5TiO3-Na0.73Bi0.09NbO3 relaxor ferroelectric ceramics for high energy storage
J. Eur. Ceram. Soc.
(2017) - et al.
Perovskite lead-free dielectrics for energy storage applications
Prog. Mater. Sci.
(2019) - et al.
Achieve ultrahigh energy storage performance in BaTiO3-Bi(Mg1/2Ti1/2)O3 relaxor ferroelectric ceramics via nano-scale polarization mismatch and reconstruction
Nano Energy
(2020) - et al.
Ultrahigh dielectric breakdown strength and excellent energy storage performance in lead-free barium titanate-based relaxor ferroelectric ceramics via a combined strategy of composition modification, viscous polymer processing, and liquid-phase sintering,
Chem. Eng. J.
(2020)