Excellent Energy Storage Properties with High-Temperature Stability

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Excellent energy storage properties with high temperature stability in sandwich-structured polyimide-based composite films Qingguo Chi, Zhiyou Gao, Tiandong Zhang, Changhai Zhang, Yue Zhang, Qingguo Chen, Xuan Wang, and Qingquan Lei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04370 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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ACS Sustainable Chemistry & Engineering

Excellent energy storage properties with high temperature stability in sandwichstructured polyimide-based composite films Qingguo Chi, †,‡,§, Zhiyou Gao, †,‡, Tiandong Zhang, †,‡,Changhai Zhang,,†, Yue Zhang, †,‡,

Qingguo Chen,,†,‡, Xuan Wang, †,‡, Qingquan Lei, †

†Key

Laboratory of Engineering Dielectrics and Its Application, Ministry of Education,

Harbin University of Science and Technology, Harbin 150080, PR China ‡School

of Electrical and Electronic Engineering, Harbin University of Science and

Technology, Harbin 150080, PR China §State

Key Laboratory of Electronic Thin Films and Integrated Devices, University of

Electronic Science and Technology of China, Chengdu, 610054, PR China Corresponding

Author: E-mail: [email protected], [email protected].

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT In recent years, the polymer-based dielectric capacitors have been attracted much more attention due to the advantages of excellent flexibility, light weight and high power density. However, most studies focus on energy storage performances of polymer-based dielectrics at room temperature, and there is relatively less investigations in polymerbased dielectrics working under the high temperature condition, which is much closer to the practical applications. Besides, dielectric capacitors operating in high temperature environment require an excellent temperature stability of structure and performance. In this paper, the high-temperature resistant polyimide (PI) is selected as the matrix material, and 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) nanofibers is used as the filling phase. By analyzing the energy storage behaviors of BZT-BCT/PI composites at different temperatures, it can be found that when the doping content of BZT-BCT nanofibers is more than 1 vol%, the dielectric strength of the composites drops sharply when temperature increases from 25 °C to 150 °C, resulting in serious deterioration of energy storage properties. Based on this, a composite film with a sandwich structure has been designed, where BZT-BCT/PI with different volume fractions is intermediate layer and hexagonal boron nitride (h-BN) with good thermal and insulating properties is introduced in the top and bottom layers with the content of 5 vol%. Consequently, the results have shown that the energy storage properties of the constructed sandwiched dielectric composite films exhibit excellent temperature stability. The maximum field strength of the composite film with the BZT-BCT content of 1 vol% in the intermediate layer is 360 kV/mm and 350 kV/mm under the temperature of 25 °C and 150 °C, and the storage


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density is 2.3 J/cm3 and 1.83 J/cm3 respectively. KEYWORDS: polyimide, dielectric, energy storage density, high temperature



INTRODUCTION

Capacitors are increasingly used as energy storage devices for its excellent energy storage capabilities in modern society. Compared to the supercapacitors and batteries, dielectric capacitors offer unparalleled advantages of fast charge and discharge, which release stored energy in a very short period of time (in microseconds). Recently, emerging products related to renewable energy, such as hybrid electric vehicles (HEV), gridconnected photovoltaic power generation and wind turbine generators, have generated huge demand for dielectric capacitors.1,2 Furthermore, some dielectric capacitors need to operate at high temperatures in practical applications. For instance, in HEV, aerospace power systems, underground oil and gas exploration, advanced propulsion systems, and high-temperature electronic equipment, capacitors with great power, high current and stable dielectric properties at high temperatures are often required.3-5 Therefore, it is essential to improve the durability and reliability of dielectrics to obtain a great energy density and excellent stability under temperature fluctuations. In addition, it is also necessary to develop polymer-based dielectrics that can withstand various thermal stresses, chemical corrosion resistance, good flexibility, easy processing, and low cost. The polymer-based dielectric capacitors with excellent energy storage properties are the focus of current researches due to its light weight and easy processing, leading to the advanced, integrated and flexible energy storage devices. Furthermore, the dielectric selfhealing mechanism and high breakdown strength of the polymer reinforce the reliability



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of capacitor.6-8 However, most polymer-based capacitors, such as biaxially oriented polypropylene (BOPP), a major dielectric of current mainstream capacitors for inverters, can only operate at temperatures below 105 °C due to significant changes in dielectric strength and mechanical properties at elevated temperatures.9 In a hybrid car, the engine temperature can exceed 140 °C. A mismatch between ambient temperature (140 °C) and BOPPs maximum operating temperature (~105 °C) requires active cooling systems or all electronics and circuits to be redesigned and remanufactured. At present, the manufacturer's research and development strategy, in addition to the engine radiator, also introduces additional cooling system set at 65 °C in order to stabilize the operation of the inverter. This auxiliary cooling circuit introduces additional weight, volume and complexity in the design of the power system, which are detrimental to the manufacturing cost and performance of the HEV.10-12 Therefore, it is particularly important to find a dielectric capacitor with excellent high-temperature energy storage characteristics. Pan et al. found that PEEK has a storage density of 2.37 J/cm3 at 140 °C, but its efficiency is less than 35% due to the heat enhancement of charge carrier conduction at high temperature.13 Sun et al. found that although the dielectric constant and electrical displacement of low volume fraction barium titanate/polyimide increased, the thermal runaway caused by Joule heat accumulation under high electric field greatly reduced the energy storage density and breakdown strength of the composites.14 Li et al. greatly improved

the

dielectric

strength

and

thermal

conductivity

of

divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) by introducing boron nitride, which made the composites exhibit good energy storage characteristics at high


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temperatures.15 At the same time, by adding a high dielectric barium titanate/BCB composite film in the middle, the complementary properties of the multi-components of the space structure are integrated, and the permittivity and discharge energy density are improved while maintaining low loss and high discharge efficiency at high temperatures.16 In this paper, polyimide (PI) with good electrical insulation properties and excellent structural temperature stability is selected as the matrix material which has high breakdown strength and working temperature (350 °C).17-19 A high dielectric constant 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3

(BZT-BCT)

ceramic

(εr~3200)

at

the

morphotropic phase boundary (MPB) is used as the filling phase, and the large aspect ratio BZT-BCT nanofibers have been prepared by electrospinning to increase the electrical displacement of the composites.20-23 Moreover, the sandwich structure has been designed to reduce the high temperature loss and improve the dielectric strength of the composites, more importantly, the hexagonal boron nitride (h-BN) with good electrical insulation properties and great thermal conductivity are chosen as the filling phase for constructing h-BN/PI top layer and bottom layer and the BZT-BCT/PI acts as intermediate layer. The results indicate that the sandwich-structured composites exhibit an excellent high-temperature energy storage properties compared with single-layer BZTBCT/PI composite films.



EXPERIMENT SECTION Materials: Pyromellitic dianhydride (PMDA, ≥98.5%), 4,4'-diaminodiphenyl ether

(ODA, ≥98%), ethanol (≥99.7%), N,N-dimethylformamide (DMF, ≥99%), acetic acid



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(≥99.5%), acetylacetone (≥99%), Ca(OH)2 (≥95%), Ba(OH)2∙8 H2O (≥98%),and titanium tetrabutyl acid (≥98%) were obtained from Sinopharm Chemical Reagent Co., Ltd.,

China.

Zirconium

acetylacetonate

was

bought

from

Alfa

Aesar,

Polyvinylpyrrolidone (PVP, Mw=1300000), hexagonal boron nitride (h-BN, ≥99.9%) were purchased from Aladdin Industrial Corporation.

Preparation of PI Composite Films: A certain amount of BZT-BCT (h-BN) was dispersed in 33 mL of DMF. The details on preparation of BZT-BCT nanofibers have been reported in previous work.20 After ultrasonic stirring for 5 min, 2.00 g of ODA was added and dissolved by stirring for 0.5 h. 2.22 g of PMDA was added to the above solution in portions under stirring to finally obtain a BZT-BCT (h-BN)/PAA precursor with a certain viscosity, and continuously stirred for another 12 h. The BZT-BCT (h-BN)/PAA precursor was subjected to vacuum treatment to remove bubbles in the precursor. The precursor is scraped on a clean and flat glass plate. The sandwich-structured composite films were prepared by layer-by-layer solution casting and it needs to be dried at 80 °C for 0.5 h before next scraping. Finally, the wet sandwich-structured films were placed into an oven for imidization: 1 hour at 80 °C, 0.5 h at 160 °C, 0.5 h at 200 °C, 0.5 h at 240 °C, 0.5 h at 280 °C, 0.5 h at 320 °C, and 1 h at 350 °C. After natural cooling, the glass plate was placed in distilled water to obtain the dense films. For convenience, the single-layer and sandwich-structured composite films with different volume fractions BZT-BCT are expressed by 1BZT-BCT/PI, 3BZT-BCT/PI, 5BZT-BCT/PI, 7BZT-BCT/PI and 1BZT-BCT/PI-S, 3BZT-BCT/PI-S, 5BZT-BCT/PI-S, 7BZT-BCT/PI-S respectively.


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Characterization: The Emperean X-ray diffractometer (XRD) in the Netherlands was used to phase analysis of composite films. Cu target was irradiated by Kα, and the operating current was 40 mA with 40 kV working voltage. The composite films was subjected to microstructure observation by Hitachi SU8020 scanning electron microscope (SEM). Dielectric properties were tested by a Novocontrol broadband dielectric and impedance spectrometer. Before the test, the aluminum electrodes on both side of the samples were fabricated by using a ZHD-400 high vacuum resistance evaporation coating machine, the diameter of the electrodes was of 2.5 cm. The measured frequency range was 101 to 106 Hz, and the temperature range was 0 to 150 °C. The D-E characteristic curve and current density of the composite at 10 Hz was measured by Radiant Premier II. In order to better analyze the influence of temperature on the energy storage properties of polyimide-based composite films, four temperature points were selected for testing: room temperature (25 °C), 50 °C, 100 °C, 150 °C.



RESULTS AND DISCUSSION

Morphology of the Composites. Figure 1 shows comparison of XRD images of BZTBCT/PI and BZT-BCT/PI-S composite films with different BZT-BCT doping content. The broad peak appearing at about 2θ=18° corresponds to the characteristic diffraction peak of the amorphous PI. The characteristic diffraction peaks appeared at 21.96°, 31.32°, 38.64°, 45.02°, 55.97° and 65.63° in figure 1a and figure 1b correspond to (100), (110), (111), (200), (211), (220) crystal faces of BZT-BCT, respectively. With the addition of BZT-BCT nanofibers, the intensity of the diffraction peak of PI is significantly reduced, while the diffraction peak of BZT-BCT is getting stronger. This is because the increase



of BZT-BCT nanofibers destroyed the ordered molecular structure of the PI matrix polymer and decreased the packing density of the molecular chains.24 The characteristic peak appeared at 26.75° in figure 1b and figure S1 is the (002) crystal face of h-BN. In addition, it can be clearly seen from the XRD patterns that the diffraction peak intensity of BZT-BCT in the sandwich-structured BZT-BCT/PI-S is slightly smaller than that of single layer BZT-BCT/PI composite films, which is mainly caused by the smaller relative content of BZT-BCT in sandwich-structured composite films compared with single layer BZT-BCT/PI composite films.

Figure 1 XRD patterns of (a) single layer BZT-BCT/PI composite films (b) sandwich-structured BZTBCT/PI-S composite films.


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Figure 2 The cross-sectional SEM images of sandwich-structured BZT-BCT/PI-S composite films with different doping content (a) 1BZT-BCT/PI-S, (b) 3BZT-BCT/PI-S, (c) 5BZT-BCT/PI-S, (d) 7BZT-BCT/PI-S.

The cross-sectional SEM images of sandwich-structured composite films at different volume fractions are shown in figure 2. It illustrated that the total thickness of the sandwich-structured composite films are about 16 or 17 μm, and the thickness of each layer is substantially same. No obvious agglomeration of h-BN fillers in the PI matrix in both top and bottom layer, which indicates there is good compatibility between h-BN and PI. Compared with BZT-BCT in the middle layer of 1BZT-BCT/PI-S, 3BZT-BCT/PI-S, there are caves starting to appear in the middle of 5BZT-BCT/PI-S composite film, leading to greatly reduced dielectric strength of the composite at high temperatures.

Dielectric Properties of the Composites. Dielectric properties are important parameter for characterizing energy storage performance. Figure 3a shows the dielectric properties of single layer BZT-BCT/PI composite films with different content of BZT-



BCT fillers at room temperature. The dielectric constant of the composites rises as the volume fraction of BZT-BCT nanofibers increases and keeps good frequency stability. The dielectric constant of 7BZT-BCT/PI at 10 Hz is 5.99, which is 1.64 times than that of PI. Compared with the single layer BZT-BCT/PI composite films, the dielectric constant of BZT-BCT/PI-S composite films, as shown in figure 3b, is not improved obviously with increased BZT-BCT doping content. This phenomenon can be explained as: firstly, the relative volume fraction of BZT-BCT nanofibers in the sandwichstructured composite films is lower compared with single layer BZT-BCT/PI composite films; secondly, the dielectric constant of outer layers in the sandwich-structured composite films is lower due to the small dielectric constant of h-BN and PI, resulting in the slightly increase of average dielectric constant of the sandwich-structured BZTBCT/PI-S composite films. The dielectric properties of h-BN/PI composite films the volume fractions of 2.5%, 5%, 7.5% and 10% are given in figure S2. The dielectric constant of h-BN/PI composite films increases first and then decreases with the increase of the filled h-BN content. When the filled h-BN is low (≤5 vol%), the h-BN fillers uniformly distributes in PI matrix, so that the increased dielectric constant of composite films is attributed to increased interfacial polarization and excellent dispersion.25 However, the obvious aggregation can be found in the composites filled with 7.5 vol% h-BN and 10 vol% h-BN, leading to the decreased dielectric constant. The dielectric loss tanδ of BZT-BCT/PI-S composite films show relative lower values (≤0.01) than that of BZT-BCT/PI single layer composite films, especially measured at high frequency.


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Figure 3 Dielectric constant and loss of (a) single layer BZT-BCT/PI composite films (b) sandwichstructured BZT-BCT/PI-S composite films, and dielectric constant versus temperature changes of (c) 5BZT-BCT/PI (d) 5BZT-BCT-S compare with PI.

It is well known that dielectric loss mainly included conductance loss and polarization loss.26 At low frequencies, the electric field changes slowly, and the polarization of the dielectric material is basically synchronized with the change of the external electric field. Therefore, the conductance loss is dominant due to low polarization loss occurring. Because the polymer has good insulating properties, as shown in figure 3a, the dielectric loss of single-layer composite films below 104 Hz is very small. The sandwich-structured composite films form an interface polarization due to the discontinuous dielectric constant between layers, resulting in an increasing loss below 104 Hz as seen in figure 3b. The polarization of the dielectrics at high frequencies cannot be synchronized with the change of the applied electric field.27 The dipoles composed of positive and negative charges will tend to array along the direction of the electric field, and gradually start to



rotate to generate polarization loss, so both of the conductance loss and the polarization loss are exist and give rise to the increased dielectric loss above 104 Hz. The dielectric loss of the sandwich-structured composite films at high frequency is significantly reduced compared with the single layer composite films, and the overall loss is less than 0.015. The dielectric loss of 5 vol% h-BN/PI composite film is less than 0.01 at high frequency, as can be seen in figure S2, which is much smaller than that of the composite films of BZT-BCT/PI. Conductive loss plays an important role on the dielectric loss, so the low dielectric loss of h-BN/PI composites at high frequency may be attributed to its excellent insulativity.28-30 The reduced dielectric loss of BZT-BCT/PI-S composite films may be caused by the increased resistivity after introducing h-BN/PI as outer layer. The smaller dielectric loss is beneficial to reduce the energy loss and leakage current of the composite films. To further elaborate the dielectric properties of the composites, the represented dielectric constant versus temperature changes of single-layer and sandwich-structured composite films with 5 vol% BZT-BCT compared with pure PI are shown in figure 3c and figure 3d, respectively. The dielectric constant of the composite films decreases with temperature increasing. The internal charge of PI moves faster at high temperatures, so the distance between positive and negative charge centers decreases, resulting in a decrease in the electric-dipole moment. The downward trend of pure PI began to increase at 80 °C, while the decreased trend of the composite film appears at around 50 °C. This is probably because the incorporation of the filler phase changes the molecular chain structure of the polymer's original sequence and reduces the energy required for molecular


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chain motion.31

Energy Storage of the Composites. In general, the energy storage density of a capacitor can be calculated using an integral formula: U e   EdD

(1)

While the energy storage density can be computed by formula for linear dielectrics. Ue 

1  0 r Eb 2 2

(2)

Where Ue is the energy storage density; 0 is the vacuum dielectric constant, and its value is about 8.8510-12 F/m; r is the relative dielectric constant; Eb is the breakdown strength.9, 13, 14, 32 As a linear dielectric materials, theoretically, the energy storage density of polyimide can be calculated by equation (2). However, because temperature and electric field have an obvious effect on the dielectric properties and polarization behaviors of PI composite materials, so the energy storage integral formula is used to calculate the energy storage density in this study. Figure S3 shows that the efficiency of pure PI decreases with increasing electric field and varies significantly with temperature. The calculated energy storage characteristics of BZT-BCT/PI composite films at different temperatures are displayed in figure 4. For the sake of revealing the effect of the temperature on the energy storage properties more clearly, the maximum field strength (Emax), the comparison of storage density (Ut/U25, Ut is defined as the energy storage density at t temperature, here, t=25, 50, 100, 150) and energy storage efficiency (ηt/η25, t=25, 50, 100, 150) are given in figure 5. BZT-BCT has limited surface energy because of its large aspect ratio, making it difficult to agglomerate, so it randomly disperse in the



polyimide matrix at lower volume fraction. Nanofibers in the films tend to along an inplane direction in coating process, which can restrict the concentration of electric field in vertical direction.33-35 So 1BZT-BCT/PI composite film show high maximum field strength. When the contents of BZT-BCT nanofibers is larger than 1 vol%, the maximum field strength decreases, which may be due to the more defect structure are employed in the polymer matrix.20,21 Although the maximum field strength of 1BZT-BCT/PI shows an excellent temperature stability from 25 °C to 150 °C, reaching 360 kV/mm, the energy storage density varies obviously with temperature. For instance, the energy storage density of 1BZT-BCT/PI is 3.12 J/cm3 at 50 °C, which is 37% higher than that of the film at 25 °C. But for 150 °C, the storage density of 1BZT-BCT/PI is 1.3 J/cm3, which is 57% of that at 25 °C. The decreased energy storage density of 1BZT-BCT/PI may be attributed to the decreased polarization and increased dielectric loss at 150 °C. Figure 8a shows that the electrical displacement of 1BZT-BCT/PI is severely affected by temperature. It tends to increase first and then decrease with increasing temperature, and is reduced severely at 150 °C. As the temperature increases, the activity of polymer molecular chain increases, so the polarization at high field increases.36 When the temperature rises to a certain value, the polarization inside the composite films tends to be saturated. Then the depolarization occurs as the temperature continues to increase, so the inherent dipole moment of the polymer itself decreases.


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Figure 4 Discharge energy density and efficiency of single layer PI composite films (a) 1BZT-BCT/PI, (b) 3BZT-BCT/PI, (c) 5BZT-BCT/PI, (d) 7BZT-BCT/PI.

Figure 5 Maximum field strength, energy density and efficiency comparison chart of single layer PI composite films at different temperature (a) 1BZT-BCT/PI, (b) 3BZT-BCT/PI, (c) 5BZT-BCT/PI, (d) 7BZT-BCT/PI.

When the doping content is greater than 1 vol%, its maximum field strength gradually



decreases with the increased temperature. The Emax of 3BZT-BCT/PI decreases 70 kV/mm from 25 °C to 150 °C. However, its energy density is little higher than 1BZTBCT/PI at 25 °C and 150 °C, which can be attributed to the higher electric displacement. This decline is especially serious at 5 vol% and 7 vol%, resulting in a severe decrease in storage density. At 150 °C, the maximum field strength of the 7BZT-BCT/PI composite film is only 150 kV/mm, which is nearly half of the value of it at 25 °C, and the storage density of 7BZT-BCT/PI is only 0.48 J/cm3, which is just 33% of that at 25 °C. It was reported that thermal runaway determines the maximum operation field of dielectric polymers, so it becomes a constraint factor for high-field capacitive energy storage at high temperatures.15, 16 Thermal loss at higher temperature may be the key factor that leads to the lower applied maximum electric field and higher energy loss. The thermal conductivity of polyimide is less than 0.2 W∙(m∙K)-1,37, 38 and BZT-BCT also has poor thermal conductivity, which is not conducive to the thermal diffusion of composite films. The heat accumulation of the composite films makes the internal electrons more active, and the leakage current increases, which greatly reduces the breakdown strength of the composite films.13,

14

In addition, as the volume fraction of BZT-BCT increases, the

efficiency of the composite films decreases obviously. At 150 °C, the efficiency of 1BZTBCT/PI composite film at the maximum field strength is 0.56, which is 78% of that at 25 °C; the efficiency of 3BZT-BCT/PI composite film is only 0.36, but 5BZT-BCT/PI and 7BZT-BCT/PI composite films exhibit relatively high efficiency of 0.51 and 0.61 due to their lower maximum field strength.


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Figure 6 Discharge energy density and efficiency of sandwich-structured PI composite films (a) 1BZT-BCT/PI-S, (b) 3BZT-BCT/PI-S, (c) 5BZT-BCT/PI-S, (d) 7BZT-BCT/PI-S.

Figure 7 Maximum field strength, energy density and efficiency comparison chart of sandwichstructured PI composite films at different temperature (a) 1BZT-BCT/PI-S, (b) 3BZT-BCT/PI-S, (c) 5BZT-BCT/PI-S, (d) 7BZT-BCT/PI-S.

Sandwich-structured BZT-BCT/PI-S composite films have been designed to improve



the energy storage properties and temperature stability. The energy storage density of hBN/PI composite films with different content of h-BN at 25 °C and 150 °C are given in figure S4. At 25 °C, 2.5 vol% h-BN/PI can endure the highest applied field strength and possess excellent energy storage properties. However, the decreased energy storage density and efficiency for 2.5 vol% h-BN/PI composites can be observed at 150 °C, which may be caused by the accumulated joule heat. Although the applied electric field for 5 vol% h-BN/PI composites is slightly lower than that of 2.5 vol% h-BN/PI, the energy storage density and efficiency is higher, the higher content of h-BN is beneficial to increase the heat dissipated ability and decrease the energy loss.13-16 With the further increase of h-BN filled content, the applied electric field for the composites sharply reduce, which may be caused by the structural defects or voids make at higher filling content20.21. Under the consideration of the electrical properties of h-BN/PI composite with different content measured at 25 °C and 150 °C, the 5 vol% h-BN/PI is chosen for constructing the sandwich-structured composite films. The energy storage properties of the sandwich-structured composite films and the effect of temperature on it and the Emax, Ut/U25, ηt/η25 are displayed in figure 6 and figure 7, respectively. The maximum field strength of sandwich-structured composites decreases slightly as the temperature increased to 150 °C. Notably, the energy storage density of sandwich-structured composite are also significantly improved, and shows a good temperature stability compared with single layer composite films. Although the 1BZT-BCT/PI-S composite film has a lower electrical displacement at 25 °C than a single layer film, the composite film exhibit relatively stable energy storage characteristics over


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the entire temperature range because of a better electrical displacement at a higher temperature. At high temperatures, 5 vol% h-BN/PI composite films on both sides provide relatively stable polarization behaviors. Besides, the uneven distribution of the electric field resulting from the dielectric difference between the layers lead to a interfacial polarization, which can alleviate the influence of the depolarization of the composite films at high temperatures.39 These all can contribute to exhibit stable polarization for sandwich-structured composite films at elevated temperatures. The storage density of the 1BZT-BCT/PI-S film at 150 °C is 1.83 J/cm3, which is 80% of that at 25 °C and 41% higher than that of 1BZT-BCT/PI. At the same time, the composite film still has a high efficiency of 0.67. The field strength and the energy storage density decreases slowly at high temperature with the volume fraction increases. When the doping content is less than 7%, the Emax and U of sandwich-structured composite films at 150 °C is 350 kV/mm, 340 kV/mm, 330 kV/mm and 1.83 J/cm3, 1.81 J/cm3, 1.69 J/cm3, respectively. The maximum field strength of the 7BZT-BCT/PI-S composite film at 150 °C is still 280 kV/mm, meanwhile the storage density is 1.46 J/cm3, which is 72% of that at 25 °C, and the efficiency is 0.6. The interfaces between the h-BN/PI layer and BZT-BCT/PI layer protect the complete breakdown and obstruct the development of conductive paths, which lead to the enhanced breakdown strength and reduced leakage current. Meanwhile, the excellent high thermal conductivity and insulation of h-BN in both top and bottom layers improves the thermal conductivity of the composite films, which is effectively suppressed their thermal runaway under high temperature conditions and the leakage current is greatly reduced.15,

16

So the composite films exhibits good



dielectric strength and energy storage characteristics at high temperatures. There is a comparison of energy storage properties for some representative polymer-based composite films at 150 °C in table S1.

Figure 8 D-E curve of (a) 1BZT-BCT/PI and (b) 1BZT-BCT/PI-S at different temperature.

Leakage Current Density of the Composites. The current densities vs electric field of the single layer and sandwich-structured composite films at selected temperature (25 °C and 150 °C) are shown in figure 9. The current density gradually increases with the increased electric field and exhibits a nonlinear relationship for all of samples, while the current density at 150 °C is significantly greater than 25 °C. The loss at hightemperature are induced by the Schottky charge injection (thermionic emission) and the Poole-Frenkel (P-F) emission, both of which are contributed to the thermally excited charge carriers.15, 16 Schottky emission refers to the fact that electrons existed in metal electrodes can obtain enough thermal activation energy, which pass through the electrode/dielectric interface and inject into the dielectric material. As the temperature and electric field increase, the height and width of the energy barrier are reduced, and the charge transfer rate is increased. P-F emission represents electrothermal excitation of electrons emitted from a trap into a dielectric conduction band. Since the introduction of


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the filler phase increases defects and space charge, the current density becomes larger as the volume fraction of BZT-BCT increases.14, 40 The current densities of BZT-BCT/PI-S composite films are obviously smaller than that of BZT-BCT/PI composite films, especially at lower electric field, which may attributed to the enhancement of heat dissipation ability induced by the filling h-BN in the outer layers. Moreover, the presence of h-BN increases the barrier height of the composite and promotes the formation of deeper traps, effectively preventing the injected charges from electrodes and inhibiting the development of thermally activated charge carriers at high temperatures, which greatly reduces the conduction loss of the composite.15, 16

Figure 9 Current density of single layer PI composite films at (a) 25 °C (b) 150 °C and sandwichstructured composite films at (c) 25 °C (d) 150 °C under different electric fields.



CONCLUSIONS

In this paper, the high temperature performance of the polyimide-based composite



films has been improved by the constructed sandwich-structured composite films. The 5 vol% h-BN/PI composite film are selected as the outer layers for its excellent energy storage property at high temperature. The outer layers filled with h-BN remain the dielectric strength and restrain the leakage current at high temperature, leading to the excellent energy storage properties and high temperature stability of the sandwichstructured composites. The sandwich structure can prevent completely breakdown of composite films at high field and inhibit the formation of the conductive path. At the same time, h-BN with high thermal conductivity may improve the heat dissipation capacity of BZT-BCT/PI-S composite films, which greatly reduces the leakage current of the dielectric composite at high temperature and improves its breakdown strength. Based on the high field strength and stable electric displacement at high temperature, the storage density at 150 °C of all sandwich-structured composite films in this study can still remain above 72% compared with that under 25 °C, and a relative high efficiency is obtained simultaneously. For the 1BZT-BCT/PI-S, the discharge energy storage density and efficiency are 1.83 J/cm3 and 0.67 at 150 °C, which is 80% and 68% compared with that under 25 °C respectively. The results in this study provide an effective method to improve the energy storage property of dielectric capacitors application at high temperatures.



ASSOCIATED CONTENT

Supporting Information The Supporting Information includes the microstructure and energy storage property for h-BN/PI composite films with different h-BN content.


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

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]; [email protected]. ORCID Changhai Zhang: 0000-0003-4441-1963 Notes The authors declare no competing financial interest. 

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of the National Science Foundation of China (61640019), the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201601), Science Funds for the Young Innovative Talents of HUST (201102).



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Table of Content

Sandwich-structured polyimide-based composite films exhibit excellent energy storage properties and maximum field strength with high temperature stability.


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Figure 1 XRD patterns of (a) single layer BZT-BCT/PI composite films (b) sandwich-structured BZT-BCT/PI-S composite films 56x22mm (300 x 300 DPI)



Figure 2 The cross-sectional SEM images of sandwich-structured BZT-BCT/PI-S composite films with different doping content (a) 1BZT-BCT/PI-S, (b) 3BZT-BCT/PI-S, (c) 5BZT-BCT/PI-S, (d) 7BZT-BCT/PI-S. 174x125mm (300 x 300 DPI)


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Figure 3 Dielectric constant and loss of (a) single layer BZT-BCT/PI composite films (b) sandwich-structured BZT-BCT/PI-S composite films, and dielectric constant versus temperature changes of (c) 5BZT-BCT/PI (d) 5BZT-BCT-S compare with PI. 149x106mm (300 x 300 DPI)



Figure 4 Discharge energy density and efficiency of single layer PI composite films (a) 1BZT-BCT/PI, (b) 3BZT-BCT/PI, (c) 5BZT-BCT/PI, (d) 7BZT-BCT/PI 146x104mm (300 x 300 DPI)


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Figure 5 Maximum field strength, energy density and efficiency comparison chart of single layer PI composite films at different temperature (a) 1BZT-BCT/PI, (b) 3BZT-BCT/PI, (c) 5BZT-BCT/PI, (d) 7BZTBCT/PI. 153x106mm (300 x 300 DPI)



Figure 6 Discharge energy density and efficiency of sandwich-structured PI composite films (a) 1BZTBCT/PI-S, (b) 3BZT-BCT/PI-S, (c) 5BZT-BCT/PI-S, (d) 7BZT-BCT/PI-S 145x106mm (300 x 300 DPI)


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Figure 7 Maximum field strength, energy density and efficiency comparison chart of sandwich-structured PI composite films at different temperature (a) 1BZT-BCT/PI-S, (b) 3BZT-BCT/PI-S, (c) 5BZT-BCT/PI-S, (d) 7BZT-BCT/PI-S 106x77mm (300 x 300 DPI)



Figure 8 D-E curve of (a) 1BZT-BCT/PI and (b) 1BZT-BCT/PI-S at different temperature 53x20mm (300 x 300 DPI)


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Figure 9 Current density of single layer PI composite films at (a) 25°C (b) 150°C and sandwich-structured composite films at (c) 25°C (d) 150°C under different electric fields 139x101mm (300 x 300 DPI)


Excellent Energy Storage Properties with High-Temperature Stability

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