Enhanced dielectric breakdown strength and energy storage density in lead-free relaxor ferroelectric ceramics prepared using transition liquid phase sintering

Abstract

Lead-free relaxor ferroelectric ceramics have been widely explored for high power energy storage applications because of their high polarization saturation and low remnant polarization. However, lead-free relaxor ceramics with the bulk form exhibit low recoverable energy storage density (W_rec < 2 J cm⁻³) owing to low dielectric breakdown strength (DBS <200 kV cm⁻¹). Here we use a strategy (the transition liquid phase sintering) to decrease the porosity and increase DBS of lead-free relaxor ferroelectric ceramics. This is achieved by introducing ZnO into 0.8(K_0.5Na_0.5)NbO₃–0.2Sr(Sc_0.5Nb_0.5)O₃ (0.8KNN–0.2SSN) ceramics. A dense microstructure (a low porosity) and submicron sized grains were found for 0.8KNN–0.2SSN–x mol%ZnO ceramics, which is responsible for a large DBS (400 kV cm⁻¹). Both a large W_rec (2.6 J cm⁻³) and high energy storage efficiency (73.2%) were achieved for 0.8KNN–0.2SSN–0.5 mol%ZnO ceramics. The W_rec of 2.6 J cm⁻³ exceeds all the other reported results of lead-free bulk ceramics. The 0.8KNN–0.2SSN–0.5%ZnO ceramics are believed to be an attractive material for high power energy storage applications.

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Introduction

Solid-state dielectric capacitors are widely used in hybrid electric vehicles and advanced pulsed power systems, because dielectric capacitors usually possess higher power density than other energy storage devices (fuel cells, Li-ion batteries, and super-capacitors) due to their very fast charge/discharge speed (millisecond or microsecond).^1–10 Currently, there are two major types of conventional dielectric materials for capacitor technologies: ceramics and polymers.^11–22 Polymer dielectric materials usually possess high energy storage density owing to their high dielectric breakdown strength (DBS). However, these polymer dielectric materials must be operated within a limited temperature range (<100 °C),^11,23,24 which limit their applications in some high temperature fields such as hybrid and electric vehicles, aerospace power electronics, and underground oil and gas exploration.²³ In contrast, dielectric ceramics exhibit superior mechanical and thermal properties, and are thus considered the potential best candidate for the high-power energy storage applications. Unfortunately, dielectric ceramics exhibit low the recoverable energy storage density (W_rec < 2 J cm⁻³) owing to low DBS (<200 kV cm⁻¹).^25–31 Therefore, a number of studies on electrical energy storage ceramics had mainly been focused on searching new dielectric ceramics with high W_rec.^25–30 On the other hand, in recent years, some countries have required all new electronic products to be lead-free due to the toxicity of lead.^32–36 Therefore, there is an increasing demand of research for developing lead-free dielectric ceramics with high W_rec for the environmental protection and human health reasons.

At present, extensive studies on lead-free electrical energy storage ceramics are focused on (Bi_0.5Na_0.5)TiO₃ (BNT) and BaTiO₃ (BT) based relaxor ferroelectric ceramics because of their relatively high W_rec and simple preparing conditions.^17–24 Chu et al. reported that the 10 mol% Bi(Mg_1/2Ti_1/2)O₃ modified (Na_0.5Bi_0.5)_0.92Ba_0.08TiO₃ ceramics showed a W_rec of 2 J cm⁻³ at 135 kV cm^{−1 37} Xu et al. found that the 10 mol% NaNbO₃ modified (Na_0.5Bi_0.5)_0.92Ba_0.08TiO₃ ceramics showed a W_rec of 0.71 J cm⁻³ at 70 kV cm^{−1 38} Luo et al. reported that 0.90(Bi_0.5Na_0.5)TiO₃–0.10KNbO₃ ceramics showed the W_rec of 1.17 J cm⁻³ at 104 kV cm^{−1 27} Gao et al. found that the 0.89(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃–0.05(K_0.5Na_0.5)NbO₃ bulk ceramics have the W_rec of 0.59 J cm⁻³ under an electric field of 56 kV cm^{−1 28} Wang et al. obtained the 0.90BaTiO₃–0.05Bi(Mg_2/3Nb_1/3)O₃ ceramics with the W_rec of 1.13 J cm⁻³ at 150 kV cm^{−1 39} Shen et al. reported a W_rec of 0.71 J cm⁻³ in 0.91BaTiO₃–0.09BiYbO₃ ceramics at 243 kV cm^{−1 25} As above mentioned, the W_rec of lead-free bulk ceramics is smaller (<2 J cm⁻³) than that of thin film (about 15 J cm⁻³) owing to the relatively low DBS (<200 kV cm⁻¹). Therefore, it is reasonable to expect that the W_rec could be significantly enhanced with increasing the DBS of lead-free bulk ceramics.

In general, the DBS of ceramics depends on several internal factors (porosity, grain size and introduction of second phase) and external parameters (sample thickness, sample area, and electrode configuration). Among these factors, the porosity appears to be a dominant factors in affecting the DBS.^40,41 Zhang et al. confirmed that the lack of control over the microstructure (a high porosity) of BaTiO₃ ceramics resulted in poor DBS.⁴¹ Recently, our group found that the DBS of 0.8(K_0.5Na_0.5)NbO₃–0.2Sr(Sc_0.5Nb_0.5)O₃(0.8KNN–0.2SSN) ceramics increased to 295 kV cm⁻¹ because the compositions drive the grain size of KNN-based ceramics to submicron scale, which lead to a high W_rec of 2.02 J cm⁻³.⁴² Although 0.8KNN–0.2SSN ceramics exhibited high DBS owing to the fine and submicron sized grains, there are still many pores at grain boundaries. Now, a question arises: if we can decrease or eliminate the porosity, can we further increase DBS of 0.8KNN–0.2SSN ceramics?

It is well known that the liquid phase sintering technology is an effective approach to prepare ceramics with little or no porosity.^43,44 However, the second phase formation could easily be induced owing to the addition of sintering aids, such as glass and oxides with low melting point, which could lead to the deterioration of energy storage properties.^43,44 Therefore, an ideal sintering aids should have the following characters: first, the liquid phase can be formed at the initial stage during the sintering process, then, the liquid phase wets and covers the surface of grains, and the densification is promoted by liquid phase sintering, and in the final stage of the sintering, most of the liquid phase reabsorbed into the grains. This process can be called the transition liquid phase sintering. Fig. 1 shows a schematic diagram of the transition liquid phase sintering.

Fig. 1 Schematic diagram of transition liquid phase sintering.

ZnO as a sintering additive has been successfully utilized to promote the densification of (K_0.5Na_0.5)NbO₃ (KNN) based ceramics owing to the presence of the liquid phase during the sintering process.^45–48 For example, Kang et al. reported that the addition of ZnO can decrease the sintering temperature and assisted the densification of KNN ceramics.⁴⁵ Marcos et al. found that the addition of ZnO helps to increase the density of KNN-based ceramics and avoids the deliquescence.⁴⁶ Pan et al. also confirmed that the addition of ZnO can enhance the sinterability of KNN-based ceramics and optimize the microstructure and electric properties.⁴⁷ Therefore, in this study, 0.8(K_0.5Na_0.5)NbO₃–0.2Sr(Sc_0.5Nb_0.5)O₃–x mol%ZnO(0.8KNN–0.2SSN–x%ZnO) ceramics were prepared using pressure-less solid-state sintering method to see if ZnO is suitable for decreasing the porosity and increasing the DBS of 0.8KNN–0.2SSN ceramics through transition liquid phase sintering. The goal of our work is to develop a dielectric ceramics with as a high W_rec as possible through increasing the DBS of the candidate ceramics materials.

Here we report our recent advance on 0.8KNN–0.2SSN–x%ZnO ceramics prepared using pressure-less solid-state sintering method. Large DBS (400 kV cm⁻¹) was achieved for 0.8KNN–0.2SSN–x%ZnO ceramics through transition liquid phase sintering. High W_rec of 2.6 J cm⁻³ was obtained for 0.8KNN–0.2SSN–0.5%ZnO ceramics. The W_rec of our sample is superior to all the other reported results of lead-free bulk ceramics. The results demonstrate that the 0.8KNN–0.2SSN–0.5%ZnO ceramics were promising lead-free candidate materials for high power energy storage applications.

Experimental

0.8(K_0.5Na_0.5)NbO₃–0.2Sr(Sc_0.5Nb_0.5)O₃–x mol%ZnO (0.8KNN–0.2SSN–x%ZnO, x = 0, 0.5, 1.0, 1.5 and 2.0) ceramics in this study were fabricated by conventional solid-state reactions. Reagent-grade oxides and carbonate powders of K₂CO₃, Na₂CO₃, Sc₂O₃, SrCO₃, Nb₂O₅ and ZnO were used as starting raw materials. Before being weighed, these powders were first separately dried in an oven at 150 °C for 5 h to avoid the absorption of moisture by potassium carbonate. They were milled for 24 h using planetary milling with 2–5 mm diameter zirconia ball media and alcohol. After ball milling, slurry was dried at 120 °C prior to calcination. Calcination was performed in an alumina crucible that was heated at 950 °C for 5 h. The 0.8KNN–0.2SSN and ZnO powders were weighted according to the formula and re-milled for 24 h. After drying the powder, these powders were mixed with 5 wt% solution of PVA as a binder. The samples were pressed into cuboid of 15 mm × 15 mm × 1 mm under 300 MPa. After burning off PVA, the samples were sintered at 1000–1200 °C for 5 h soaking period in air in covered alumina crucibles. In order to suppress the high volatilization of alkali metal oxides at a high temperature during sintering, a double crucible method was used in this study.⁴⁹ For electrical measurements, the sintered samples were polished to achieve parallel and smooth faces with a thickness of 0.2–0.5 mm prior to the application of silver paste. In order to prevent the samples edge breakdown, the silver paste with 6 mm × 6 mm was screen printed by mesh screen mask of 200 lines per inch on the both surfaces of the samples as electrode. A silver electrode was obtained by firing at 820 °C for 20 min.

The bulk density of the sintered samples was measured using the Archimedes method. The phase structures of the sintered ceramics were examined using an X-ray diffractometer (Philips X-PertPro Diffracto meter, Almelo, The Netherlands). The microstructure of the ceramic samples was observed using scanning electron microscopy (SEM, model JSM-6360, JEOL, Tokyo, Japan). The grain size distributions were calculating using a free software called SPIP. The average grain sizes were calculated according to the grain size distribution. In order to investigate the DBS and energy storage properties of this materials, the samples with 0.2 mm in thickness were immersed in silicone oil to avoid surface flashover. For each composition, at least three samples were measured to obtain average maximum electric field strength. All measurements were conducted in two-electrode setup with the bottom electrode connected through a copper plate underneath the sample. In order to obtain the polarization–electric field (P–E) hysteresis loops, all samples were measured using a ferroelectric test system (TF Analyzer 2000; aixACCT, Aachen, Germany) under a triangular waveform of different amplitudes at the room temperature and 1 Hz.

Results and discussion

Fig. 2 shows X-ray diffraction (XRD) patterns of 0.8KNN–0.2SSN–x%ZnO ceramics at room temperature. As can be seen from these patterns, a single perovskite phase was formed for the specimens with x < 1.5, however, the peaks for the secondary phase were observed for the samples with x ≥ 2.0. Therefore, it can be concluded that a very small amount of Zn ions was incorporated into the host lattice of the perovskite structure. However, when a large amount of ZnO was added into the samples, most of the Zn ions cannot enter into the matrix of the 0.8KNN–0.2SSN ceramics, and formed the second phase. In this compositions, some Zn²⁺ could enter the lattice of ABO₃ structure, which do not meant to substitute Nb⁵⁺ and Sc³⁺. On the other hand, Nb⁵⁺or Sc³⁺ replaced by some Zn²⁺ also site between the lattice of ABO₃ structure.

Fig. 2 XRD patterns of the 0.8KNN–0.2SSN–xZnO ceramics.

Fig. 3 shows the density variation of 0.8KNN–0.2SSN–x%ZnO ceramics as a function of the ZnO content for samples sintered at the optimum sintering temperature. When increasing ZnO content, the bulk density increases from 4.59 g cm⁻³ for samples without ZnO to 4.63 g cm⁻³ for 1.0 mol% and 1.5 mol% of ZnO, then decrease to 4.56 g cm⁻³ for 2.0 mol% of ZnO. The percent of theoretical density of all samples exceeds 98%. The increase in bulk density at low values of x (x < 1.5) may be attributed to the densification of 0.8KNN–0.2SSN–x%ZnO ceramics through transition liquid phase sintering. The decrease in bulk density observed at high values of x (x > 2.0 mol%) probably due to the formation of the secondary phase.

Fig. 3 Density of 0.8KNN–0.2SSN–xZnO as a function of the ZnO content for the ceramics sintered at the optimum sintering temperature.

Fig. 4 shows SEM micrographs obtained for the surface of 0.8KNN–0.2SSN–x%ZnO samples sintered at the optimum sintering temperature. All the specimens exhibited the dense microstructure (a low porosity), which is beneficial to improve the DBS of ceramic samples. A small amount of the liquid phase was already observed in the 2.0 mol% ZnO added specimen, as indicated by the red arrows in Fig. 4(d). Generally, the square grains are unfavourable for the densification, however, in this study, the grain size of samples is submicron, and the pore size is nanometre scale. So the samples with the submicron grains show high densification. In order to identify the influences of ZnO content on their grain size distribution, Fig. 5 exhibits the statistics on the grain size distributions of 0.8KNN–0.2SSN–x%ZnO ceramics. The average grain sizes were calculated according to the grain size distribution. The average grain size of 0.8KNN–0.2SSN–x%ZnO ceramics slightly increases with the addition of ZnO owing to the transition liquid phase sintering. The average grain size of 0.8KNN–0.2SSN–x%ZnO ceramics are about 0.45 μm, 0.53 μm, 0.57 μm, and 0.60 μm for x = 0.5, 1.0, 1.5 and 2.0, respectively.

Fig. 4 SEM images of the 0.8KNN–0.2SSN–x%ZnO ceramics with (a) x = 0.5; (b) x = 1.0; (c) x = 01.5; (d) x = 2.0.

Fig. 5 Grain size distribution of the 0.8KNN–0.2SSN–x%ZnO ceramics.

Fig. 6 shows the transparency of the 0.8KNN–0.2SSN–x%ZnO (x = 0 and 0.5) ceramics polished to a thickness of 0.3 mm. The letters can be clearly read through the samples, suggesting that 0.8KNN–0.2SSN–xZnO (x = 0 and 0.005) ceramics have good transparencies in the visible spectra. Similar phenomena have been found in other KNN-based ceramics.^33,34 As can be seen, 0.8KNN–0.2SSN ceramics show achromatization, while 0.8KNN–0.2SSN–0.5%ZnO ceramics show a specific brown. Generally speaking, ferroelectric ceramics are opaque because of the scatter and reflection of light in various sites, including the grains and the grain boundaries, as well as the pores within the grains and the grain boundaries.^50,51 Some theoretical and experimental results revealed that the scatter of the grains and the grain boundaries can be reduced as soon as possible, when the grain size becomes smaller than the wavelength of light.^52,53 Therefore, it can be concluded that the high transmittance of the 0.8KNN–0.2SSN–xZnO (x = 0 and 0.005) ceramics can be attributed to submicron-sized grains in this study. On the other hand, the high transparency of the 0.8KNN–0.2SSN–0.5%ZnO ceramics also further confirms that the ceramics possess the dense microstructure (a low porosity).

Fig. 6 Transparency of the 0.8KNN–0.2SSN–x%ZnO (x = 0 and 0.5) ceramics polished to a thickness of 0.3 mm (a) x = 0 (b) x = 0.5.

A serious problem for KNN-based ceramics is highly hygroscopic when the ceramic samples were exposed to humidity, which limits the practice applications of KNN-based ceramics.⁵⁴ Park et al. confirmed that the ZnO modified KNN ceramics showed no deliquescence when they are exposed to water owing to the dense microstructure.⁴⁸ In order to assess whether 0.8KNN–0.2SSN–x%ZnO ceramics easily deliquesce, the samples were immersed under water for 3 months. Fig. 7 shows the specimens of the 0.8KNN–0.2SSN–x%ZnO ceramics immersed under water for 3 months. The results confirmed that all ceramic samples showed no deliquescence. The reason should be attributed to high densification in 0.8KNN–0.2SSN–x%ZnO ceramics.

Fig. 7 Photograph of 0.8KNN–0.2SSN–x%ZnO ceramics immersed under water for 3 months. (a) x = 0.5; (b) x = 1.0; (c) x = 1.5; (d) x = 2.0.

The energy storage behavior of the 0.8KNN–0.2SSN–x%ZnO ceramics with different ZnO content was investigated by the P–E hysteresis loops. Fig. 8(a) shows the P–E loops of 0.8KNN–0.2SSN–x%ZnO ceramics at selected applied electric fields (200 kV cm⁻¹). It can be seen from Fig. 8(a) that the P–E hysteresis loops became slimmer with increasing the addition of ZnO. This result indicates that the ferroelectric properties of 0.8KNN–0.2SSN–xZnO ceramics are gradually decreased with the increase of ZnO content, namely, P_s and P_r decrease substantially with the increase of ZnO content. For example, the P_s and P_r value are 16.2, 13.1, 12.5, 11.4, 10 μC cm⁻² and 2.0, 1.4, 1.3, 1.2, 1.1 μC cm⁻² for 0.8KNN–0.2SSN–x%ZnO ceramics with x = 0, 0.5, 1.0, 1.5, and 2.0, respectively. This phenomenon can be explained as follow: the ionic radius of Zn²⁺ (0.074 nm, CN = 6) is close to that of B-site ionic (Nb⁵⁺: 0.064 nm, CN = 6; Sc³⁺: 0.075 nm, CN = 6). Based on ionic size and Pauling's rules, Zn²⁺ is introduced in the B site of the ABO₃ perovskite structure. In this case, Zn²⁺ could replace the Nb⁵⁺or Sc³⁺ in octahedral sites, gives rise to the oxygen vacancies in anion array for keeping the charge balancing. The oxygen vacancies reduce the volume of the cell and are believed to play a major role in the ‘hardening” effect, which is so called acceptor substitution.

Fig. 8 (a) P–E hysteresis loops at selected applied electric field for 0.8KNN–0.2SSN–xZnO ceramics; (b) P_s and P_r of 0.8KNN–0.2SSN–x%ZnO ceramics; (c) P–E hysteresis loops at their critical breakdown strength for 0.8KNN–0.2SSN–x%ZnO ceramics.

For energy storage applications, good electric field endurance is one of the crucial factors to consider. The P–E hysteresis loops at their critical breakdown strength for all the samples were shown in Fig. 8(c), which were measured at 1 Hz until the samples undergone a break down. It can be seen clearly from Fig. 8(c) that DBS value shows a maximum 400 kV cm⁻¹ in 0.8KNN–0.2SSN–x%ZnO ceramics. It is worth mentioning that DBS values of 400 kV cm⁻¹ are much higher than other lead-free bulk ceramics. The high DBS of samples were directly attributed to their submicron size grains and the dense microstructure (a low porosity).

For relaxor ferroelectric ceramics, the electric energy storage density W can be estimated from their P–E hysteresis loops by using the following equation:⁵⁵

where E is the electric field, and P is the polarization. When the electric field increases from zero to the maximum E_max, the polarization increases to the maximum P_max, namely, P_s and electric energy is stored in relaxor ferroelectric ceramics. During the discharge process, the recoverable energy storage density W_rec is then released from E_max to zero, the W_rec as represented by the shaded area in Fig. 9(a). W_rec could be calculated according to the equation below:⁵⁵where P_r is the remanent polarization. Consequently, part of the stored energy is unrecovered because of the hysteresis loss, which is called energy loss density W_loss. In practical application, apart from higher W_rec values, larger high energy storage efficiency η is also always desired. The energy storage efficiency η is calculated as the following formula:⁵⁵

Fig. 9 (a) Unipolar P–E hysteresis loops of 0.8KNN–0.2SSN–xZnO ceramics at room temperature; (b) the calculated energy storage density W and the recoverable energy storage density W_rec from (a); (c) the calculated energy loss density W_loss and the energy storage efficiency η.

In order to calculate the energy storage properties, the unipolar P–E hysteresis loops of 0.8KNN–0.2SSN–xZnO the ceramics were examined. Fig. 9(a) shows the unipolar P–E hysteresis loops of the 0.8KNN–0.2SSN–x%ZnO ceramics at room temperature. It can be seen from Fig. 9(b) and (c) that W, W_rec and W_loss decrease with the increasing of the content of ZnO. The largest W and W_rec (3.55 J cm⁻³ and 2.60 J cm⁻³) were obtained at 0.80KNN–0.20SSN–0.5%ZnO ceramics under a high electric field (400 kV cm⁻¹). In this study, the W and W_rec values are considerably higher than previously reported results of other lead-free ceramics, indicating that the ceramics used in this study are promising candidates for high power energy storage applications. Except for the high W_rec, the high η is also desirable for the practical applications. As seen from Fig. 9(c), the η slightly decreases with the increment of ZnO content from 73.2% for 0.8KNN–0.2SSN–0.5%ZnO ceramics to 72.1% for 0.8KNN–0.2SSN–2%ZnO ceramics. Fig. 10 shows W_rec and DBS of some lead-free ceramics.^{25–31,42,56–63} It can be seen Fig. 10 that W_rec of 2.60 J cm⁻³ excessed that of other lead-free bulk ceramics in the literature. At the same time, it can be found that the large W_rec of 0.80KNN–0.20SSN–0.5%ZnO ceramics are directly attributed to high DBS (400 kV cm⁻¹).

Fig. 10 W_rec and DBS of some lead-free ceramics.

To evaluate the potential application of 0.80KNN–0.20SSN–0.5%ZnO ceramics in energy storage, the dependence of energy-storage properties on applied field was investigated. Fig. 11(a) presents the variations of unipolar P–E hysteresis loops of 0.80KNN–0.20SSN–0.5%ZnO ceramics at room temperature under different electric fields. The largest polarization increases from 4.8 μC cm⁻² to 16.7 μC cm⁻² with electric fields increasing from 100 kV cm⁻¹ to 400 kV cm⁻¹, which suggests that a much higher energy density can be achieved by improving dielectric strength of the ceramics.

Fig. 11 (a) Unipolar P–E hysteresis loops of 0.80KNN–0.20SSN–0.5%ZnO ceramics at room temperature under different electric fields. (b) The calculated energy storage density W and the recoverable energy storage density W_rec; (c) the calculated energy loss density W_loss and the energy storage efficiency η.

Fig. 11(b) and (c) show energy storage properties of 0.80KNN–0.20SSN–0.5%ZnO ceramics under different electric fields at room temperature. W and W_rec increase from 0.25 J cm⁻³ and 0.22 J cm⁻³ to 3.55 J cm⁻³ and 2.60 J cm⁻³ as electric field increase from 100 kV cm⁻¹ to 400 kV cm⁻¹. The results indicate that higher electric field is favorable for energy-storage properties. Similar to the trend of W and W_rec, W_loss increases with an increase of the applied electric field and reaches 0.95 J cm⁻³ at 400 kV cm⁻¹. On the contrary, it can be seen from Fig. 11(c) that η decreases with the increment of applied electric field from 88% for 100 kV cm⁻¹ to 73.2% for 400 kV cm⁻¹ due to the higher W_loss at high electric fields, and the value for all samples is relatively high (73.2%) at measured electric fields.

Generally, the ideal energy storage ceramics should combine a higher energy density with a low energy loss simultaneously. However, in this study, W_rec increase from 0.25 J cm⁻³ and 0.22 J cm⁻³ to 3.55 J cm⁻³ and 2.60 J cm⁻³ as electric field increase from 100 kV cm⁻¹ to 400 kV cm⁻¹, and η decreases with the increment of applied electric field from 88% for 100 kV cm⁻¹ to 73.2% for 400 kV cm⁻¹. Though η decreases with the increment of applied electric field, the value of 73.2% is relatively high and should satisfy the requirements for practical applications.

Conclusion

In conclusion, this work demonstrated the development of a new lead-free relaxor ferroelectric ceramic for high power energy storage applications. 0.8KNN–0.2SSN–x%ZnO solid solution has been synthesized using pressure-less solid-state sintering. The dense microstructure (a low porosity) and submicron sized grains were found for 0.8KNN–0.2SSN–x%ZnO ceramics, which is responsible for large BDS (400 kV cm⁻¹). Both the large W_rec (2.6 J cm⁻³) and high η (73.2%) were obtained at 0.8KNN–0.2SSN–0.5%ZnO ceramics under an electric field of 400 kV cm⁻¹. We believe that the material system is a promising candidate for high power energy storage applications. In addition, these results also confirm that the KNN-based relaxor ferroelectrics can be regarded as an alternative direction for the development of high power energy storage applications.

Acknowledgements

Authors gratefully acknowledge the supports of the National Science Foundation of China (NSFC No. 51502109) and China Postdoctoral Science Foundation Funded Project (2014T71007).

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