Introduction

In recent years, dielectric capacitors have played a critical role in advanced electronic power systems and energy storage devices, owing to their rapid charge-discharge characteristics and remarkable power density1,2,3,4,5. Polymer-based dielectric capacitors are highly attractive to researchers because of their high Eb, low mass, stable structure, and good flexibility. However, low energy storage density compared with batteries and super capacitors limits their broad use in the energy storage device market. To improve their discharged energy density (Udis), it is essential to improve the breakdown strength (Eb) and dielectric constant (εr) of their dielectric components according to the follow equation6,7:

$${U}_{{{{\rm{dis}}}}}=\frac{1}{2}{\varepsilon }_{0}{\varepsilon }_{{{{\rm{r}}}}}{{E}_{{{{\rm{b}}}}}}^{2}$$
(1)

where ε0 is the permittivity of vacuum (8.85 × 10−12 F m−1)2. In this context, filling 2D dielectric materials including boron nitride nanosheets (BNNS)8, Ca2Nb3O109, Mxene10 and montmorillonite (MMT)11 into the polymer matrix is a valid method to enhance the tortuosity of conductive pathways at large electric fields, thereby improving the Eb of the composites. For instance, Li et al.8 reported that adding 12 vol% BNNS to poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymers greatly improved the Eb from 4000 to 6500 kV cm−1. Bao et al.9,12 obtained a significantly enhanced Eb of 7920 kV cm−1 in PVDF-based nanocomposite capacitors by adding negatively charged Ca2Nb3O10 nanosheets. Unfortunately, the εr of commonly used 2D materials, such as BNNS (~3–4)8, is relatively low, which is not helpful for improving the εr of composites, resulting in a relatively low Udis. Moreover, the widely used PVDF-based composites possess a low energy storage efficiency (η) of ~60%, which means that much energy is lost and converted into heat, being harmful for long-term operation. Therefore, developing dielectric energy storage materials with simultaneously high Udis and η remains a challenge.

After comprehensive consideration, a type of bismuth layer-structured ferroelectric material (Na0.5Bi4.5Ti4O15, NBT) with simultaneously high εr and low dielectric loss (tan δ) has attracted our attention13,14,15,16,17, and more importantly, the charge distribution within 2D layers of Na0.5Bi4.5Ti4O15 is more uniform than that in other bismuth layer-structured materials (e.g., Bi4Ti3O12 and SrBi2Ta2O9), facilitating the achievement of large Eb18,19. Uniform and flat micro-sized NBT sheets prepared via the molten salt method have been proved to be a potential 2D functional material20. Interestingly, by optimizing the molten salt method process, a specific, dimensionally homogeneous 2D NBT ferroelectric with a size of ~1–2 μm can be achieved, which could be suitable as a filler with the large faces aligned perpendicular to the direction of the applied electric field, thus forming a barrier to block the passage of electrons to increase the Eb21. Finite element simulations (Supplementary Fig. 1) demonstrate the advantages of the NBT sheets (1–2 μm) in increasing the Eb of polyetherimides (PEI) polymers relative to BNNS nanosheets (~100 nm) and NBT sheets with a size of ~20 μm. Owing to small size, BNNS are prone to irregular motion due to Brownian motion, leading to a chaotic distribution in the polymer matrix22. In contrast, the NBT sheets with sizes of 1–2 μm and ~20 μm can be aligned with the biggest faces parallel to each other in the matrix. Consequently, the BNNS exhibit the slight blocking effect on electrical tree evolution, and the NBT sheets with the large size (~20 μm) are directly penetrated by the electrical tree. Excitingly, the 2D NBT sheets with the size of 1–2 μm effectively serve barriers that impede the direct penetration of electrical trees, making it a very promising candidate for improving Eb.

In addition, multilayer composites, consisting of high Eb and large-εr layers have demonstrated the ability to integrate the merits of each functional layer effectively. The interface effect, including the Maxwell–Wagner–Sillars (MWS) effect, appears in heterogeneous systems and is favored when the electrical characteristics (e.g., εr, electron affinity, and band gap) of the constituent phases significantly vary23,24,25,26,27,28. Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) is a potential candidate for capacitor applications due to its high εr (~9)29. However, P(VDF-HFP)-based composites usually exhibit low η (<60%) when applied in high electric fields. In contrast, linear dielectric PEI exhibits a high Eb (~5000 kV cm−1), an extremely low residual polarization (Dr) and a high η (>90%), but a low εr (~3)30,31,32. Therefore, constructing multilayer structure may be a valid method to enhance the capacitive performance.

Herein, we propose a synergistic enhancement strategy to obtain large energy density and efficiency in the NBT-PEI/NBT-P(VDF-HFP) bilayer-structured composite film through incorporation of 2D bismuth layer-structured NBT ferroelectric micro-sheets, as shown in Fig. 1. Interestingly, by combining the electric barrier effect of 2D NBT sheets and the interface effect of the bilayer structure (Fig. 1a, d), an ultra-high Udis of 25.0 J cm−3, accompanied by a large η of 81.2% is achieved at an ultra-high Eb of 8283 kV cm−1, which far exceeds those of most reported energy storage materials. In addition, the composite film has excellent actual charge-discharge capacity, exhibiting an extremely fast discharge time of 46.6 ns and an ultra-high-power density of 62.2 MV cm−3, making it a potential material for utilization in high-power devices.

Fig. 1: Schematic diagram of the designed bilayer-structured NBT-PEI/NBT-P(VDF-HFP) composites.
figure 1

a Simulation of the real-time evolving distributions of the electric field in NBT-polymer composites. b Schematic diagram of bilayer-structured composites. c Schematic diagram of the trap energy level caused by the addition of the NBT sheets into composites. The trap energy level is calculated using Φe = EANBT −  EAP, where EANBT and EAP are electron affinities of the NBT sheets and the polymer, respectively. d Schematic diagram of the interface effect occurring in heterogeneous systems of PEI and P(VDF-HFP).

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Results and discussion

NBT sheets and bilayer-structured composite films quality

Figure 2a indicates that the X-ray diffraction (XRD) pattern of the NBT sheets matches that of the standard Na0.5Bi4.5Ti4O15 (PDF#74-1316), which proves that these sheets have a bismuth layered structure. Figure 2b, c presents scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) mapping images of a 2D NBT sheet. Owing to its bismuth layered structure, it has a platelet shape with the size around 1–2 μm and the Bi, Na, Ti, and O constituents are uniformly distributed overall. The thickness of an NBT sheet is approximately 257 nm, as shown in Fig. 2d. Its high-aspect-ratio platelike shape is beneficial for parallel alignment and uniform dispersion in the polymer matrix, which can be observed from the surface SEM images of the bilayer-structured NBT-PEI/NBT-P(VDF-HFP) composite film (Fig. 2e) and the NBT-PEI and NBT-P(VDF-HFP) composite films with various NBT filler contents (Supplementary Figs. 2 and 3). Obviously, the NBT sheets are uniformly distributed in the polymer matrix without aggregation at low filler contents (<0.75 wt.%), which is helpful for forming a barrier to prevent electrons from passing through when applied in a high electric field. However, the aggregation appears when the filler content exceeds 0.75 wt.%, which can easily lead to local electric field distortion, decreasing Eb of the composite films. Figure 2f shows a macroscopic digital photograph of the NBT-PEI/NBT-P(VDF-HFP) bilayer composite film. The film demonstrates good flexibility, curves in its natural state and is ideal for deformation as conditions change. Figure 2g shows the focused ion beam (FIB)-SEM image of the cross-section of the NBT-PEI/NBT-P(VDF-HFP) composite film. There is a clear demarcation between layers, and the composite films have an overall thickness of 7.7 μm. In addition, in order to distinguish the composites of the different layers, EDS mapping images of the PVDF-specific element F and the PEI-specific element O are shown in Fig. 2h, i. F and O are uniformly dispersed in the top and bottom layers, respectively, suggesting the good homogeneity of the bilayer-structured composite films.

Fig. 2: Characterization of NBT sheets and composite films.
figure 2

a XRD pattern and phase structure of 2D NBT sheets. b SEM image and c EDS mapping images of Bi, Na, Ti, and O in an NBT sheet. d Atomic force microscopy (AFM) image of a single NBT sheet. e Local surface SEM image and f Macroscopic digital photograph of the bilayer-structured NBT-PEI/NBT-P(VDF-HFP) composite film. g FIB-SEM image, and EDS mapping images of h F and i O in the cross-section of NBT-PEI/NBT-P(VDF-HFP).

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Energy storage performance optimization of composite films

The dependence of the εr of the 2D NBT sheets on the frequency at room temperature shows that the sheets have a high εr of 119.0 at 1 kHz (Supplementary Fig. 4). The dielectric and capacitive performances of the NBT-PEI and NBT-P(VDF-HFP) composites with various contents of NBT fillers (Supplementary Figs 5 and 6 and Supplementary Tables 1 and 2) show that the εr values of the NBT-PEI and NBT-P(VDF-HFP) composites notably increase with increasing content of the NBT filler because of its higher εr relative to those of PEI and P(VDF-HFP) while the tan δ values of the composites are low. Additionally, the εr of the NBT-P(VDF-HFP) composite films reduces with increasing frequency because the polar groups in P(VDF-HFP) cannot keep up with the movement at high frequencies. When the NBT filler content is 0.75 wt.%, the NBT-PEI composite film displays the largest Eb of 7484 kV cm−1 and the largest Dmax-Dr value of 4.53 μC cm−2, resulting in a maximum Udis of 16.7 J cm−3, accompanied by a high η of 92.3%. Compared with that of pristine PEI (Udis = 8.2 J cm−3), the Udis of the NBT-PEI composite film is enhanced by 104%. Moreover, for the NBT-P(VDF-HFP) composite film, at a 0.75 wt.% NBT sheet filler content, it presents the largest Eb of 6084 kV cm−1 and the largest Dmax-Dr value of 6.30 μC cm−2, possessing a maximum Udis of 15.5 J cm−3, accompanied by a η of 62.4%. Compared with that of pristine P(VDF-HFP) (Udis = 9.8 J cm-3), the Udis of the NBT-P(VDF-HFP) composite film is improved by 58%. The resistivities and hopping conduction models of NBT-PEI and NBT-P(VDF-HFP) composite films (Supplementary Figs. 7 and 8 and Supplementary Note 1) show that the resistivities of the NBT-PEI and NBT-P(VDF-HFP) slowly reduce with enhancing electric field compared to those of pristine PEI and P(VDF-HFP), and the charge hopping distances are reduced from 0.46 nm for PEI to 0.32 nm for NBT-PEI and from 0.48 nm for P(VDF-HFP) to 0.40 nm for NBT-P(VDF-HFP). These indicate that the additions of the 2D NBT sheets are of important value for causing the increase of the Eb in the NBT-PEI and NBT-P(VDF-HFP) composite films.

After the aforementioned optimization of the NBT filler content in the NBT-PEI and NBT-P(VDF-HFP) composite films, the bilayer-structured NBT-PEI/NBT- P(VDF-HFP) composite films are further designed and fabricated with three different structures: 3-7, 5-5 and 7-3 type composites, according to the thickness ratio of the NBT-PEI and NBT-P(VDF-HFP) layers. The optimized dielectric and capacitive properties are achieved in the 7-3 type composite film with an ultra-high Eb of 8283 kV cm−1 (Supplementary Fig. 9, Supplementary Table 3 and Supplementary Note 2). Finite element simulations are used to study the electric potential and electric field distributions of the three different types of bilayer-structured composites at 6000 kV cm−1 (Supplementary Fig. 10). The distributions of the electric potential and electric field tend to become more uniformly as the NBT-PEI layer content increases, which is beneficial for avoiding localized areas of high potential and reducing the risk of localized dielectric breakdown. Figure 3 shows the summarized dielectric and capacitive performance parameters of above optimized composites. As shown in Fig. 3a, the NBT-PEI/NBT-P(VDF-HFP) composite film has a medium εr of 5.0 and a low tan δ of 0.02 at 1 kHz. In addition, the εr decreases little at high frequencies, which is attributed to the introduction of PEI with a rigid molecular structure. Figure 3b shows the Eb of different kinds of composites obtained via the following Weibull distribution functions33:

$${X}_{i}={{\mathrm{ln}}}({E}_{i})$$
(2)
$${Y}_{i}={{\mathrm{ln}}}(-{{\mathrm{ln}}}(1-i/(n+1)))$$
(3)
Fig. 3: Dielectric and energy storage performance.
figure 3

a Frequency dependence of the εr and tan δ. b Weibull distribution plots, c D-E loops and d Udis, Utot and η values of different types of composites. e Capacitive performances of previous reported advanced composites.

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In this context, n and i represent the total number of samples and the index of each individual sample, respectively, whereas Ei denotes the breakdown electric field for each sample. The slope of the fitted linear relationship between Xi and Yi, known as the shape parameter (β), serves as an indicator of the reliability of the breakdown electric field (Eb) measurements. A higher β corresponds to greater reliability of the experimental data. The Eb is determined as the intersection of the linear fit with Yi = 0. Notably, the minimum β for the NBT-PEI composite film is 16.4, underscoring the high quality and consistency of these composite films. The Eb massively increases from 4986 kV cm−1 for pristine P(VDF-HFP) to 6084 kV cm−1 for NBT-P(VDF-HFP) composite film when the 2D NBT sheets are introduced. Moreover, the Eb further enhances to 8283 kV cm−1 by designing the bilayer structure. Compared with those of pristine P(VDF-HFP), the D-E loops of the NBT-PEI/NBT-P(VDF-HFP) composite film are slimmer owing to the NBT-PEI layer with low Dr, as shown in Fig. 3c. Therefore, the NBT-PEI/NBT-P(VDF-HFP) composite film exhibits the highest Udis of 25.0 J cm−3, accompanied by a high η of 81.2%, as plotted in Fig. 3d. To demonstrate the merits of this composite for dielectric capacitor applications, we summarize capacitive performances of the previously reported advanced composites8,29,34,35,36,37,38,39,40,41,42,43,44,45,46 in Fig. 3e (Supplementary Table 4). As observed, it is challenging to achieve large Ue and η in a kind of composite. For example, the BaTiO3-PVDF/PEI has a relatively high η of 84.5%, but the Udis of 8.0 J cm−3 is low36. The BaTiO3/PVDF has a high Udis of 19.1 J cm−3, but the η of 68.6% is low37. Interestingly, in this study, by introducing 2D bismuth layer-structured NBT micro-sheets and designing the unique bilayer structure, a large Udis of 25.0 J cm−3 and a high η of 81.2% are simultaneously achieved in the NBT-PEI/NBT-P(VDF-HFP) composite film.

Interface effect

The substantial enhancement in Eb can be partially attributed to the interface effect, which typically occurs in heterogeneous systems and is particularly pronounced if the constituent phases exhibit obviously different properties (e.g., εr, electron affinity, and band gap)47. To elucidate the interface effect between the NBT micro-sheets and the surrounding polymers, energy band diagrams at the PEI/NBT and P(VDF-HFP)/NBT interfaces (Fig. 4a, b) were constructed based on the ultraviolet photoelectron spectroscopy (Supplementary Figs. 11–13 and Supplementary Notes 3 and 4) and UV-Vis spectroscopy (Supplementary Figs. 14 and 15) results. As plotted in Fig. 4a, b, the NBT micro-sheets possess much higher electron affinity (EANBT, e.g., 6.2 eV) than both PEI (EA1, e.g., 2.8 eV) and P(VDF-HFP) (EA2, e.g., 2.4 eV), which causes the formation of deep trap energy levels (Φe1 = EANBT - EA1, e.g., 3.4 eV, and Φe2 = EANBT-EA2, e.g., 3.8 eV) to capture injected and excited electrons through strong electrostatic attraction. As a result, Eb is effectively enhanced. Furthermore, to further study the interface effect between NBT-PEI and NBT-P(VDF-HFP), the space charge development in the NBT-PEI/NBT-P(VDF-HFP) composite film were measured via the pulsed-electro- acoustic (PEA) test (Fig. 4c, d). By integrating the interface space charge peak, the obtained interface charge density at 300 kV cm−1 is 1.13 × 10−3 C m−3 (Fig. 4d and Supplementary Note 5), which is much higher than the MWS theoretical value (3.29 × 10-5 C m−2, Supplementary Note 5). As we know, MWS effect is only part of the interface effect. The higher interface charge density may suggest that a large amount of deep traps form in the interface region of the bilayer-structured composite film, which also leads to a large accumulation of space charges47. Such unique structure of the interface is believed to play an important role in enhancing the Eb. In addition, the resistivities and hopping conduction models of the NBT-PEI, NBT-P(VDF-HFP) and NBT-PEI/NBT-P(VDF-HFP) composite films (Fig. 4e,f) show that the resistivity of NBT-PEI/NBT-P(VDF-HFP) decrease slowly with increasing the electric field, and the charge hopping distance is decreased from 0.40 nm for NBT-P(VDF-HFP) composite film and 0.32 nm for NBT-PEI composite film to 0.30 nm for NBT-PEI/NBT-P(VDF-HFP) composite film. All above these results demonstrate that the bilayer-structured NBT-PEI/NBT-P(VDF-HFP) composite film has significantly enhanced electrical breakdown resistance owing to the interface effect.

Fig. 4: Interface effect.
figure 4

a Energy band diagrams at the NBT/PEI interfaces and b NBT/P(VDF-HFP) interfaces, where EA1, EA2, and EANBT respectively represent electron affinities of PEI, P(VDF-HFP), and NBT sheets, Evac and Ec are vacuum level and conduction band. c, d Space charge density development under 300 kV cm−1 in the 7-3 type composite film as functions of the position and time. e Resistivity and f leakage current density of the NBT-P(VDF-HFP), NBT-PEI, and NBT-PEI/NBT- P(VDF-HFP) composite films at various electric fields.

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Finite element simulation of the real-time evolution of the electric potential and electric field distribution

To further investigate the enhancement mechanism for Eb, the real-time distribution evolving of the electric potential and electric field in the pristine PEI, NBT-PEI, and NBT-PEI/NBT-P(VDF-HFP) composite films were simulated at 7000 kV cm−1 via COMSOL Multiphysics and MATLAB (Fig. 5, Supplementary, Fig. 16 and Supplementary Note 6). As shown in Fig. 5a, because pristine PEI has a monolithic structure, it has no difference in the overall electric field distribution, which leads to the preferential growth pathway of electric dendrites along the direction of the applied electric field. Consequently, the electrical trees show few branches and little energy loss during the evolution process, which leads to quick growth of electrical trees to the top boundary. In contrast, as shown in Fig. 5b, the introduction of 2D NBT sheets changes the distributions of the electric field in the NBT-PEI composite film. To be specific, a localized high electric field region is formed around the 2D NBT sheets, and the differences of the electric field between the tips of the electrical trees and the NBT is small. Therefore, the electrical trees tend to produce more branches toward the surrounding low electric field regions. Consequently, the tortuous growth path of electrical trees and the appearance of many electrical tree branches lead to a large energy loss during their evolution process. Furthermore, as shown in Fig. 5c, the electric field distribution is dramatically changed via designing the bilayer structure with a clear demarcation formed at the interface. When electrical trees pass through the interface, the appearance of much lower electric field regions near such interface makes the electrical trees prone to produce more branches. Therefore, more energy loss occurs during the evolution process. All these effects slow down the development of electrical trees at the upper boundary, leading to a substantial enhancement in the Eb.

Fig. 5: Simulation of the real-time evolution of the electric field distribution.
figure 5

a Pristine PEI, b NBT-PEI composite film and c NBT-PEI/NBT-P(VDF-HFP) composite film.

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Charge-discharge capacity

Compared with energy storage properties, the practical charge-discharge characteristics for dielectric capacitors are also critical for the application purpose. Hence, the underdamped and overdamped discharge properties of the NBT-PEI/NBT-(PVDF-HFP) composite film are tested through a discharge resistance, inductance, and capacitance load (RLC) circuit48,49,50. Underdamped pulsed discharge current, the maximum current (Imax) and current density (CD = Imax/S, where S is the electrode area) of the NBT-PEI/NBT-(PVDF-HFP) composite film at various electric fields are shown in Figure 6a, b. As presented, the composite film possesses an Imax of 51.5 A and an ultra-high CD of 102.5 A cm−2 at 3000 kV cm−1. Figure 6c, d shows the discharge energy density (Wd) at different electric fields, which is calculated by the following equation48,49,50:

$${W}_{d}=\frac{R\int I{(t)}^{2}{dt}}{V}$$
(4)

Where V, I, and R are the sample volume, the discharge current acquired by overdamped charge-discharge tests, and the load resistor (R = 300 Ω), respectively. Typically, the parameter t0.9, which means the time required for releasing 90% of the Wd, can characterize the discharged rate of a material. The NBT-PEI/NBT-(PVDF-HFP) composite film presents a rapid discharge rate of 46.6 ns, a large Wd of 2.9 J cm−3 and an ultra-high power density (PD, PD = Wd/t0.9) of 62.2 MW cm−3 at 3000 kV cm−1. Figure 6e displays the Wd of NBT-PEI/NBT-(PVDF-HFP) composite film at 3000 kV cm−1 when the cycle number is increased from 1 to 103. As shown, Wd varies in the range of 2.74 to 3.01 J cm−3, indicating good fatigue resistance. Encouragingly, as illustrated in Fig. 6f, compared with previously reported advanced composites34,35,39,40,51,52,53,54, our bilayer-structured composite film has both a much higher power density and a faster discharge rate, demonstrating very promising application potential in high pulsed power devices.

Fig. 6: Charge-discharge capacity.
figure 6

a Underdamped pulsed discharge current. b Maximum current and current density under different electric fields. c Discharge energy density acquired by the overdamped charge-discharge test. d Maximum current and discharge energy density under various electric fields. e Discharge energy density at different cycle numbers and 3000 kV cm−1. f Charge-discharge performances of previously reported advanced composites.

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In summary, we have successfully prepared an attractive NBT-PEI/NBT-P(VDF-HFP) composite film by integrating 2D ferroelectric NBT micro-sheets and bilayer structure, which can significantly increase the Eb of the composite films. Specifically, adding 0.75 wt.% NBT filler increases the Eb from 5467 kV cm−1 for pristine PEI to 7484 kV cm−1 for the NBT-PEI composite film. The designed bilayer structure further proves positive effect in boosting the Eb of composite films. The large interface charge density of 1.13 × 10−2 C m−2 obtained from the PEA test indicates that the further improvement in the Eb is due to the interface effect. Moreover, optimizing the thickness ratio of NBT-PEI to NBT-P(VDF-HFP), the NBT-PEI/NBT-P(VDF-HFP) bilayer-structured composite film exhibits an ultra-high Eb of 8283 kV cm−1, resulting in an ultra-high Udis of 25.0 J cm−3 and large η of 81.2%. Besides, the composite film possesses an ultra-high power density of 62.2 MV cm−3 and a fast discharge rate of 46.6 ns. Compared with previously reported advanced composites, both high energy storage density and efficiency are realized in our bilayer-structured composite film. This study provides a novel strategy for developing high-performance polymer-based energy storage materials.

Methods

Preparation of 2D bismuth layer-structured NBT sheets

Na0.5Bi4.5Ti4O15 sheets were prepared via a molten salt growth method. Na2CO3 (99.5%, Aladdin, Shanghai, China), Bi2O3(≥99.9%, Aladdin), and TiO2 (≥99.0%, Aladdin) were used as raw materials, NaCl (≥99.5%, Aladdin, Shanghai, China) was used as molten salt. All materials were weighed based on the weight ratio of raw material:molten salt = 1:15 and mixed via ball-milling in ethanol for 24 h. The mixtures were calcined at 850 °C for 1 h to fabricate fine-grain matrix, and the excess impurities were washed away. Finally, 2D NBT sheets with the size of approximately 1–2 µm were obtained after drying.

Fabrication of NBT-P(VDF-HFP) and NBT-PEI composites

NBT-PEI composites were fabricated by a casting method, PEI (PolyK, USA) pellets were introduced into N-methyl pyrrolidone (NMP) (Macklin, 99.5%) solvent by stirring for 4 h to obtain transparent PEI solution. At the same time, the 2D NBT sheets were also distributed in NMP solvent by ultrasonically stirring for 20 min. Then, the above two solutions were mixed together and stirred for 12 h at room temperature to achieve uniform dispersion of NBT sheets. The solution was then cast onto a glass substrate by casting machine and dried at 70 °C for 12 h in vacuum oven to volatilize the NMP. The NBT-PEI composites with a thickness of approximately 7–10 μm were stripped from the glass substrate, and covered with Au electrodes with the diameter of 2 mm. Similar to the fabrication of NBT-PEI composites, the NBT-P(VDF-HFP) composites were successfully obtained and covered with Au electrodes with a diameter of 2 mm for the measurement of electrical performances.

Fabrication of bilayer-structured NBT-PEI/NBT-P(VDF-HFP) composite films

The NBT-PEI/NBT-P(VDF-HFP) composite film was fabricated by layer-by-layer casting from the prepared NBT-PEI and NBT-P(VDF-HFP) solutions. First, the NBT-PEI solution was cast on a glass substrate and dried at 70 °C for 12 h in vacuum as the bottom film. The NBT-P(VDF-HFP) solution was subsequently cast on the NBT-PEI film at 70 °C for 12 h in vacuum as the top film, because the PEI matrix is a thermosetting polymer and not dissolvable in the NBT-P(VDF-HFP) solution. Finally, three different types of NBT-PEI/NBT-P(VDF-HFP) composite films with different thickness ratios of the two layers, i.e., 0.7NBT-PEI/0.3NBT-P(VDF-HFP) (7-3 type), 0.5NBT-PEI/0.5NBT-P(VDF-HFP) (5-5 type) and 0.3NBT-PEI-0.7NBT-P(VDF-HFP) (3-7 type) composite films, were successfully prepared.

Characterization

X-ray diffraction (XRD, D/max 2400, Rigaku, Tokyo, Japan) was used to determine the phase structure. The microstructure and chemical composition were determined via field-emission scanning electron microscopy (FE-SEM, Merlin Compact, Germany) equipped with energy-dispersive spectroscopy (EDS). Cross-sectional images were measured via focused ion beam scanning electron microscopy (FIB-SEM, ZEISS Crossbeam 540, Helios G4 PFIB Hxe, Helios 600i). The thicknesses of the 2D sheets were measured via atomic force microscopy (AFM, Dimension Fastscan, Bruker, Germany). The dependences of the dielectric constant and loss on the frequency were obtained with a TH 2827 A precision LCR meter (Tonghui Electronic Co., Ltd., China). The Eb was achieved by the NJC5010 withstand voltage tester (Gogo Instruments Technology, China). The D-E loops, leakage currents, and resistivities at different electric fields were acquired with a Precision LC II ferroelectric testing system (Radiant Technologies Inc., USA). The energy levels were tested by ultraviolet photoelectron spectroscopy (UPS, Thermo Fisher Scientific Nexsa G2, USA). The band gaps were determined via UV-Vis spectroscopy (Hitachi UH4150, Japan). The space charge distributions were tested by the pulsed-electro-acoustic (PEA, HY-PEA-DPT01, China) method. Discharge resistance, inductance, and capacitance (RLC) load circuit (CFD-003, Gogo Instruments Technology, China) were used to analyze the charge-discharge performance.