1 Introduction

Electrostatic capacitors possess the ultra-high power density and fast charge–discharge rate that electrochemical capacitors and batteries cannot match, which enable them to have broad application prospects in modern electrical and electronic systems, such as hybrid electric vehicles (HEVs), grid-connected photovoltaic systems, wind power generation, and underground oil and gas exploration [1,2,3]. However, high temperature, high current, and high power are the working conditions faced by capacitors in actual application scenarios [4, 5]. For example, the operating temperature of capacitors in HEVs is 140 °C, and even higher than 250 °C in underground oil and gas exploration. However, the current state-of-the-art polymer-based dielectric biaxially oriented polypropylene (BOPP) has a maximum operating temperature of no more than 105 °C [6], which requires an additional cooling system to ensure the effective operation of BOPP, resulting in reduced energy efficiency and higher cost and weight [7]. Therefore, there are urgent needs to develop polymer dielectric films that can adapt to extreme environments.

To realize stable application of capacitors at high temperatures, it is inevitable to select polymers with high glass transition temperature (Tg). At present, the polymer dielectric materials with high Tg include polyimide (PI, Tg ≈ 360 °C), polyetherimide (PEI, Tg ≈ 217 °C), polyetheretherketone (PEEK, Tg ≈ 143 °C), polycarbonate (PC, Tg ≈ 150 °C), fluoropolyester (FPE, Tg ≈ 330 °C), and polyamide-imide (PAI, Tg > 300 °C) [8]. Although the Tg of these polymers are relatively high, their energy density and efficiency deteriorate seriously contrast with that at room temperature (RT) due to rapidly increased conduction loss. For instance, at 150 °C and 250 MV/m, the energy density and efficiency of PEEK is 0.5 J/cm3 and 30%. This is because the barrier height at the interface between electrode and dielectric material is reduced due to the dual effects of high temperature and mirror force, so that electrons are more easily injected from electrode [9, 10]. In addition, most of the molecular chain structures of high-temperature resistant polymers have a large number of benzene rings, resulting in a lower band gap [11,12,13] (PEI of 3.2 eV contrast with PP of 8.8 eV), which makes electrons easier to excite and aggravates the increase in leakage current [14]. Moreover, the Joule heating effect caused by the conduction loss accelerates the breakdown process under high temperature and high electric field, which seriously affects the long-term stability of polymer dielectric.

Recent years, researchers have proposed some methods to suppress leakage current. For instance, Wang found that fillers with wide band gap have obvious impact on energy storage performance and Eb of dielectric nanocomposites [15, 16]. Li utilized the strong electrostatic attraction of molecular semiconductors to immobilize free electrons and impede electric charge injection and transport in dielectric polymers [17]. In addition, inorganic materials of wide band gap such as silicon dioxide (SiO2) and boron nitride have been deposited on dielectric films via chemical vapor deposition (CVD) to increase the height of electrode/dielectric interface barrier [18, 19]. Thereby, the injection of electrons is suppressed. However, large-scale production through either uniformly dispersed fillers or CVD is an unsolved problem at present. Therefore, there is an urgent need for a scalable method to improve the energy storage performance of polymer dielectric materials at elevated temperature.

BNNS with wide band gap of 5.9 eV has been shown to have significant effect on improving Eb and charge–discharge efficiency of polymer-based dielectric nanocomposites owing to its high intrinsic Eb and insulation [20,21,22,23,24,25,26,27,28]. These advantages make it an ideal carrier scattering center. PI is a very common high-temperature resistant polymer material, and its precursor PAA has good solubility which means it has excellent film-forming properties. Mature preparation process and low price make PI have the potential of scalable application in the field of high-temperature dielectric energy storage. However, PI possess poor high-temperature dielectric energy storage performance caused by severe conduction loss. To solve this problem, dielectric films for high-temperature application combined PI and boron nitride interlayer are prepared by typical solution casting which exhibits the advantages of low energy consumption, scalability, and simple preparation process compared with CVD. The experiment results indicate that a small amount of boron nitride interlayer can increase the Eb and suppress leakage current, leading to significant increase in energy storage density at high temperature. Method reported in this paper provides a scalable and effective approach for the preparation of high-temperature polymer dielectric materials.

2 Experiment

2.1 Materials

The commercially available polyamic acid (PAA) solution was supplied by RTP (USA). N, N-dimethylacetamide (DMAc, water≦30 ppm (by K.F.), 99.8%, Safe-Dry with molecular sieve) was purchased from Adamas. Hexagonal boron nitride (h-BN) powders (2–3 µm, > 99%) were purchased from Qinhuangdao Eno High-Tech Material Development Co., Ltd (China). Isopropanol (IPA, 99.8%) was purchased from Adamas. All above materials are used directly without further purification.

2.2 Preparation of BNNS

BNNS were fabricated via liquid-phase exfoliation method. A total of 3 g h-BN powders were dispersed in 100 ml IPA and 100 ml deionized water. After 5 min sonication bath dispersion, the mixture was placed into cell grinder with ice bath for 480 min. The obtained dispersions were centrifuged at 4000 rpm for 20 min and then collected the supernatant. Afterward, BNNS was obtained via filtering and oven drying. Finally, the dried BNNS was grinded for subsequent use.

2.3 Preparation of sandwich structured composite films

The fabrication process of sandwich-structured films is illustrated in Fig. 1. A total of 60 mg BNNS powders were dispersed in 20 ml IPA and then ultrasonic dispersed for 240 min to obtain stable dispersions. Then, PAA solution with a certain mass fraction was casted onto a clean glass substrate through a doctor blade served as the first layer. The glass substrates were placed into vacuum oven for 1 day to remove solvent. After the first layer of PAA was dry, BNNS dispersions were casted onto PAA film repeatedly in the same way and dried thoroughly and the solid content and dispersions volume of BNNS can be precisely controlled by pipetting gun. To be specific, using a pipette to suck up 250 μL of boron nitride dispersions, adjusting the height of the doctor blade so that the boron nitride dispersions can just cover the entire PPA base film, putting it in a 70 °C blast oven for 5 min to dry, and then taking it out and carry out the above operations until the total volume of the scraping dispersions is the desired total volume. Last, the composites films were put into vacuum oven and further imidized. The curing conditions are 100 °C, 150 °C, 200 °C, 230 °C, and 260 °C for 1 h each. Afterwards, composite films were peeled off and dried for 1 day. Six sandwich structure films were donated as PBP2.25, PBP4.5, PBP6.75, PBP9, PBP11.25, and PBP13.5, and the number represented the mass of boron nitride interlayer (the unit of mass is milligram). The thickness of PBP films is 10–15 µm.

Fig. 1
figure 1

Schematic diagram of PBP films preparation process

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2.4 Characterization

The morphology of PI and PBP films were characterized with scanning electron microscopy (SEM, JEOL JSM5900LV), and the surface morphology and thickness of exfoliated BNNS was measured by atomic force microscope (AFM). X-ray diffraction (XRD, X’Pert pro MPD, Philips Company) was used to characterize the h-BN crystal structure before and after exfoliation with scanning range of 10–90°. Fourier transform infrared spectroscopy (FT-IR) was used to characterize the chemical structure of PI and PBP films. The dielectric constant and dielectric loss were measured by Novocontrol concept 50 in the frequency range of 1–106 Hz at RT, 80 °C, 150 °C, and 200 °C. The breakdown strength (Eb) of the PBP were measured by breakdown voltage tester (SG-255G, DC voltage) with test voltage range of 0–20 kV and test temperature of RT, 80 °C and 150 °C. Hysteresis loop tester (Premier II-100 V, American Radiant Company) was used to characterize polarization of materials under alternating electric field and different test temperature. Differential scanning calorimeter (DSC, PerkinElmer Instruments) is used to confirm the PI curing is complete under a nitrogen atmosphere and at a heating/cooling rate of 10 °C min−1. TGA was conducted on a thermogravimetric analyzer (TG 209F1 Iris, Netzsch, Germany) under dry air.

3 Results and discussion

The morphology of BNNS after exfoliation is examined using AFM as shown in Fig. S1. BNNS exposes obvious difference in diameter and its thickness is about 20 nm, which indicate BNNS was successfully exfoliated. Although the diameter of BNNS was in the relatively large range of 100–500 nm, the smaller BNNS can embed between the large BNNS to make boron nitride interlayer more compact. As show in Figs. 2b and S2a–e, it is noted that BNNS were forced to arrange in parallel by shearing effect of doctor blade and overlap with each other. Different from integral and dense boron nitride film fabricated via CVD, boron nitride interlayer of different mass content shows rough and porous morphology compared to smooth and flat PI surface. This distinguishing feature may help the upper and lower layers of PI film bond well since PAA can be connected through holes. To verify whether the crystal structure of h-BN has changed after ultrasonic exfoliation, XRD was carried out, and the result is shown in Fig. 2e, it can be observed that the distinctive peak of BNNS was located at 26.7° the same as h-BN, which suggest the crystal structure is well preserved even though going through 480-min ultrasound exfoliation. Boron nitride interlayer was fabricated via solution casting and drying which means that good uniformity and stability of BNNS dispersions in IPA are important to the quality of boron nitride interlayer. Otherwise, the boron nitride interlayer deposited by volatilizing solvent is likely to form visible unevenness and granular agglomerates. Therefore, BNNS dispersion was free settled for a week and its optical image is shown in Fig. S3. It can be seen that there is no obvious difference compared with the BNNS dispersions just after ultrasonic treatment indicating rather good stability.

Fig. 2
figure 2

a–b SEM images of surface of PI, PBP2.25, respectively. c–d Cross-section SEM images of PI, PBP2.25, respectively (all scale bars are 10 µm). e Statistical chart of BN interlayer thickness. f XRD results for h-BN and BNNS

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The PBP films were fabricated by three individual solution casting process, in order to confirm whether the sandwich structure is formed; the cross-section of PBP films were characterized with SEM as shown in Figs. 2c–d and S2f–j. Each PBP films expose a clear three-layer structure and boron nitride interlayer is located at the middle of PBP and the thickness statistic chart of PBP films is shown in Fig. 2f. It can be confirmed that the thickness of boron nitride interlayer increases linearly with increasing mass content of BNNS at low contents, whereas the thickness of boron nitride interlayer of PBP13.5 is nearly 2.5 µm, far beyond the expected value of 1.2 µm (extrapolated value calculated from the curve, as shown in Fig. 2f). The reasons may be that different from in situ synthesis of inorganic films by CVD and other methods, boron nitride interlayer is deposited on PAA films by solution volatilization, which means that there must be voids between BNNS. With the increase of boron nitride content, the void volume will increase and undesirable accumulation will occur along the vertical direction; this makes the thickness of boron nitride interlayer significantly different at high content. Owing to the good fluidity of PAA solution, when the third layer of PAA covers the boron nitride interlayer, PAA penetrated into voids, causing the greatly increase of thickness of the boron nitride interlayer. The conjecture is further confirmed by Fig. S4. It is observed that compared to the rather flat surface of PBP2.25, PBP13.5 possess rougher and more porous surface which can be seen at the locations indicated by the marks.

Incomplete curing would greatly reduce the Eb and meanwhile increasing the dielectric loss. To verify that PI based films is fully cured, DSC was performed on the cured PI and the result is shown in Fig. S5a. It is confirmed that the curing conditions used are sufficient for PI to cure completely. Further evidence is shown in Fig. S5b, the absorption band at 3252 cm−1 and 1640 cm−1 result from the N–H and C = O stretching of PAA and their disappearance indicates that PAA is completely reacted. As shown in Fig. S5c, peaks at 1775 cm−1, 1720 cm−1, 1341 cm−1, and 723 cm−1 represent the C = O asymmetric stretching, C = O symmetric stretching, C = N stretching, and C = O bending of PI. The results of FT-IR indicate that PI and PBP films are cured completely.

To reveal the influence of boron nitride interlayer on dielectric performance of PI, the dielectric constant and loss as a function of frequency and temperature were studied. As shown in Figs. 3a–b and S6a, b, PBP films exhibit a slight increase in dielectric constant compared with pristine PI film (from 3.5 to 3.8), attributable to similar dielectric constant of PI (εr = 3.5) and BNNS (εr ≈ 4). In addition, interfacial polarization caused by the interface between PI and boron nitride interlayer can also cause an increase in dielectric constant [29,30,31,32,33,34,35,36]. Unlike ceramics and conductive fillers, BNNS does not significantly increase the dielectric loss of the composite films due to its intrinsic electrical inertness. As a result, the dielectric loss of each PBP films is under 0.01. The dielectric stability of dielectric films at different temperatures is of great significance for practical applications, owing to the actual working environment temperature may be variable. In order to verify the temperature stability of dielectric properties of PBP films, continuous tests from 15 to 200 °C were conducted. As shown in Figs. 3c–d and S6c, d, the dielectric constant of PI and PBP films shows independent characteristic with temperature and the variation of dielectric constant is no more than 1.75% at 10 Hz, 102 Hz, 103 Hz, and 104 Hz as shown in Fig. 3e. The variation of dielectric constant of films shows a downward trend as frequency rises. According to the polarization mechanism, several polarizations such as orientational and interface become relaxed as frequency rises which mean they no longer contribute to the increase of dielectric constant [29, 37]. Actually, these two polarizations have a strong dependence on temperature which could lead to the variation in dielectric constant at different temperatures.

Fig. 3
figure 3

a–b Dielectric constant and loss of PI, PBP films in the frequency range of 1 to 106 Hz. c–d dielectric constant and loss of PI, PBP films in the temperature range of 15 to 200 °C. e The variation in dielectric constant at 15 and 200 °C. f The Eb of PI and PBP films at RT, 80 °C and 150 °C

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The dielectric constant can only reflect the polarization of dielectric materials, but the Eb can determine the maximum electric field that materials can load [38]. Without considering energy loss, the greater electric field that materials can load, the greater energy density would be. Therefore, the Eb of PI and PBP films were characterized base on the two-parameter Weibull distribution, which can be described as followed equation:

$$\mathrm{P}(\mathrm{E})=1-\mathrm{exp}\left[-{(\frac{\mathrm{E}}{{\mathrm{E}}_{\mathrm{b}}})}^{\upbeta }\right]$$
(1)

where P(E) is the cumulative probability of electric failure, E is experimental breakdown strength, Eb is the characteristic breakdown strength of cumulative failure probability of 63.2%, and β is the shape parameter which represent the dielectric reliability of materials. The Eb of PI and PBP films is summarized in Fig. 3f. Apparently, the Eb of PBP films greatly increases with the mass content of BNNS from 2.25 mg to 11.25 mg and further addition of BNNS lead to substantial reduction in improvement of Eb. At RT, the highest Eb of 489 kV/mm is achieved with BNNS mass content of 4.5 mg which is 54.2% higher than pristine PI (Eb = 317 kV/mm), and even at the highest mass content of 13.5 mg, the Eb is also increased by 18% compared with PI. Noteworthiness, as shown in Fig. S7, the β value of PBP films is lower than pristine PI, representing the dielectric properties of PI films are more reliable than PBP films. Similarly, the β and Eb of PI and PBP films show the same trend at 80 °C. The reason is supposed to be the macroscopic heterogeneity of boron nitride interlayer. At elevated temperature of 150 °C, the β value of PI decreases from 18.7 to 12.3, suggesting high-temperature condition does have negative impact on the properties of PI films. In contrast, there was no significant decrease in β value of PBP films which proves that boron nitride interlayer has an inhibitory effect on the growth of breakdown phase [39,40,41]. It is worth noted that the Eb of PBP films show a significant improvement comparing with PI even it has detectable decrease at high temperature. The peak value of 442.3 kV/mm achieves at the BNNS mass content of 6.75 mg, 51% improvement compared to PI. Regardless of the temperature, the Eb of PBP13.5 shows a relatively low enhancement effect. This is due to the high intrinsic Eb of boron nitride, which inhibits the growth of the breakdown phase and improves the Eb. However, when the boron nitride content is high, cavities are easily generated in the boron nitride interlayer. The increase of this kind of defect may make the upper layer PAA solution unable to fully infiltrate when the boron nitride interlayer is very thick which leads to reduction of the Eb. Under the dual effects of these two effects on the increase and decrease of the Eb, the increase of the Eb of PBP13.5 is not very obvious compared with other PBPs.

The actual working conditions of capacitor films generally are alternating high voltage. However, the dielectric constant and loss can reveal the polarization of dielectric materials, but cannot truly reflect the energy storage performance of dielectric films at high temperatures, because the test voltage applied is only 1 to 2 V. Thus, to further uncover the energy storage performance of PI and PBP films, D-E loops were studied. As shown in Fig. S8, a narrow enclosing area of loop indicates the low energy loss and high Dmax suggesting high energy storage density. Owing to the intrinsic linear dielectric properties of PI and BNNS, as shown in Figs. 4a and S9, it is obviously found that the PI and PBP films expose more than 90% efficiency at the electric field up to 475 kV/mm at RT. Due to the enhancement in Eb caused by boron nitride interlayer, PBP11.25 can reach 3.69 J/cm3 at 450 kV/mm comparing with the pristine PI of 1.55 J/cm3 at 300 kV/mm. When temperature rises to 80 °C, as shown in Fig. S10, the D-E loops of PI film become larger with increasing electric field which means conduction loss becomes obvious gradually. The efficiency of PBP films maintain excellent level even at 375 kV/mm shown in Fig. 4b. Among PBPs, PBP2.25 films deliver the energy density of 3.36 J/cm3 and 93% efficiency versus 0.92 J/cm3 and 79% of pristine PI.

Fig. 4
figure 4

a–d Discharge energy density and efficiency of PI and PBP films at RT, 80 °C, 150 °C, and 200 °C. e Discharge energy density and charge–discharge efficiency of PI and PBP at 150 °C and 200 MV/mm. f At the electric field of 200MV/m, the charge–discharge efficiency of PI, BOPP, and PBP at RT, 80 °C, 150 °C, and 200 °C

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In the absence of external cooling system, high-temperature dielectric films are required to have stable energy storage performance at above 140 °C. In order to meet this requirement, D-E loop of PI and PBP films at 150 °C and 200 °C were studied and shown in Figs. S11 and S12. By integrating the curve in Fig. S11, it is found that the conduction loss of PI film increases rapidly with increasing electric field, which indicates that the number of free electrons injected by the electrode and self-excited inside materials increases sharply under high temperature and high electric field. Surprisingly, PBP9 films can still maintain the efficiency of 90% at 375 kV/mm as shown in Fig. 4c, suggesting the boron nitride interlayer does play the role of electron scattering center and barrier. Overall, the improving effect of boron nitride interlayer on energy storage performance is most remarkable at the intermediate mass content and the reasons should be the thickness and compactness of boron nitride interlayer. At low mass content, owing to the pores, the boron nitride interlayer is not thick enough to completely block the electrons accelerated by electric field and partial electrons can still pass through the boron nitride interlayer by pores. At high mass content, the number of electrons conducted along the interface increases due to increased number of defects. In spite of the possibilities mentioned above, compared with PI films, PBP films still exhibit obvious effect of restraining conduction loss, which is attributed to the intrinsic characteristics of wide band gap and high insulation of BNNS. As a result, PBP9 possess an energy density of 2.58 J/cm3 and efficiency of 90%, much higher than pristine PI of 0.75 J/cm3 and 65% at 150 °C. At high temperature, the Eb of dielectric materials is also a considerable factor that restricts the increase in energy storage density, because the Eb directly determines the maximum loaded alternating electric field of dielectric materials, which in turn affects the maximum electric displacement. PBP films exhibit higher Eb than pristine PI because of the impediment to the growth of breakdown phase of boron nitride interlayer.

PI possess excellent high-temperature resistance performance. According to the type of monomer diamine, PI synthesized by aromatic diamine has a decomposition temperature of near 600 °C which means PI has the possibility of application at higher temperatures [42, 43]. According to the TGA result shown in Fig. S13, the decomposition temperature of PI and PBP films is around 585 °C, suggesting excellent thermal stability. As shown in Fig. 4d, obviously, energy storage properties decline sharply at 200 °C. However, the PBP9 films can maintain at around 80% efficiency and 1.81 J/cm3 versus PI of 0.58 J/cm3 and 58%. In order to explore the performances of PI and PBP films in practical application scenarios, the working conditions of capacitors in hybrid electric vehicles of 150 °C and 200 kV/mm were chosen for comparison. As shown in Fig. 4e, all PBP films have an efficiency of over 90% and the energy density of PBP films is twice as much as PI films. Figure 4f compares the charge–discharge efficiency of PI films, PBP films and BOPP as a function of temperature measured at 200 MV/m. Obviously, the PBP films exhibit better dielectric properties than PI and BOPP. At 150 °C, PBP films have a charge–discharge efficiency of over 90%. Even at 200 °C, PBP films still maintain an efficiency of over 80% while that of PI films drop to below 70%.

Since the polarization of PBP films has not been significantly improved, the enhancement in high-temperature energy storage performance of materials is mainly due to the improvement in charge–discharge efficiency and the main reasons for the reduction of efficiency of materials is determined by conduction loss. The conduction loss is reflected in the form of leakage current density. Thus, in order to further illustrate the restraining effect of boron nitride interlayer on leakage current, leakage current density was characterized and the results at 150 °C are shown in Fig. 5a. While the leakage current density increases with electric field, the value of leakage current density of PBP films is consistently lower than that of PI and the difference reaches the maximum at 200 kV/mm. Summarizing the leakage current density of PI and PBP films at 200 kV/mm in a chart (Fig. 5b), it is obvious that the leakage current density of PBP films achieves a substantial reduction contrast with pristine PI. The lowest leakage current density of 2.16 × 10−8 A/cm2 is achieved at BNNS mass content of 9 mg, nearly 1/35 of PI, and even the PBP13.5 shows leakage current density 1/7 of PI. These results prove that boron nitride interlayer has clear effect on restraining leakage current.

Fig. 5
figure 5

a Leakage current density of PI and PBP films at 150 °C. b Leakage current density of PI and PBP films at 150 °C and 200 kV/mm

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To further demonstrate the superiority of the boron nitride interlayer over traditional polymer composites on restraining leakage current, finite element simulation of PBP and randomly parallel distributed PI/BNNS composite were conducted by COMSOL. The models are shown in Fig. 6a, and the simulation results are shown in Fig. 6b–c. In PBP, it can be seen that the current density in most of the regions are at relatively low level contrast with PI/BNNS of almost half of the regions are at a rather high level. Interestingly, as shown in Fig. S14, the current density of upper surface of PBP shows much lower than PI/BNNS composite; this may indicate that the number of electrons injected into the upper surface is less than that of PI/BNNS. In addition, it is obvious that high current density areas are blocked by the boron nitride interlayer, while the high current density region connects the upper and lower surfaces in the PI/BNNS model. This suggests that the boron nitride interlayer acts as an effective electron scattering center and barrier to hinder the transportation of free electrons, and leading to enhancement in energy storage efficiency.

Fig. 6
figure 6

a Schematic diagram of computational simulation models of PBP films (left) and randomly parallel distributed PI/BNNS composite films (right). b–c Simulation results of three-dimensional current density (A/m2) distribution of above two models (from left to right are XYZ-view, XY-view, XZ-view). Loading electric field is 200 kV/mm and lower surface grounding

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In order to figure out the advantage of boron nitride interlayer, Fig. 7 lists the energy storage performances of dielectric polymer composites at 150 °C and 200 °C in recent years [44,45,46,47,48,49,50,51,52]. It can be seen that PBP films in this work show classy energy storage performance than most of the high-temperature polymer composites reported recently. Especially in Fig. 7a, the energy density at the level of efficiency above 90% (or the maximum value of efficiency available for capacitors), PBP9 can reach 2.58 J/cm3 which is larger than other PI composites. For example, PI/BaTiO3, PI/BZT-BCT, and PI/BN (PI coated with BN through physical vapor deposition) have energy density of 2.1 J/cm3, 0.493 J/cm3, and 1.83 J/cm3, respectively. At higher temperature of 200 °C, Fig. 7b summarizes up-to-date works which studied energy storage performance at 200 °C. Although the energy storage performance of PBP9 is not the top at 200 °C, energy density of 1.82 J/cm3 and efficiency of 78% can also reach the classy level among previous works. In addition, the energy storage performance of PBP films was compared with that of high-temperature resistant polymers in literature and the results are shown in Fig. S15. It is obviously found that the energy storage performances of PBP9 outperform PC, PEI, FPE, and PEEK, showing great application potential. Therefore, embedding boron nitride interlayer into polymer matrix is a feasible and effective method.

Fig. 7
figure 7

Comparison charts of discharge energy density and charge–discharge efficiency of dielectric materials at a 150 °C and b 200 °C. BZT-BCT: 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3. BN boron nitride, BTNP barium titanate nanoparticles, c-DPAES crosslinked poly(arylene ether sulfone)s, BT barium titanate, c-BCB crosslinked divinyltetramethyldisiloxane­bis(benzocyclobutene), BCB benzocyclobutene, BTNFs barium titanate nanofibers, Al2O3-NPLs alumina nanoplates, A-P-A-P-A Al2O3-PI-Al2O3-PI-Al2O3

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4 Conclusions

In this work, sandwich structure composite films were prepared by embedding boron nitride interlayer into PI matrix via simple layer-by-layer casting method. The BNNS directly overlap with each other on PI surface, forming a compact layer of boron nitride located at the middle of PI film. Compared with the traditional solution mixing and casting, the method used in this work can achieve obvious improvement in energy storage performance with rather low loadings of BNNS. It was experimentally confirmed that the boron nitride interlayer serves as an electron scattering center and barrier that suppress the leakage current and hinder the propagation of breakdown phase. Such result was further confirmed by computational simulation. Consequently, the Eb and charge–discharge efficiency were significantly improved due to the boron nitride interlayer. As a result, Eb of PBP9 (442.3 kV/mm) exhibit 41% higher than pristine PI at 150 °C and greatly improved energy storage density of 2.58 J/cm3 versus 0.75 J/cm3 of PI. This work provides a feasible and scalable strategy for designing high performance dielectric polymer composites used at elevated temperatures.