Sandwich-structured polymers with electrospun boron nitrides layers as high-temperature energy storage dielectrics

Journal Pre-proofs Sandwich-Structured Polymers with Electrospun Boron Nitrides Layers as High-Temperature Energy Storage Dielectrics Guang Liu, Tiandong Zhang, Yu Feng, Yongquan Zhang, Changhai Zhang, Yue Zhang, Xubin Wang, Qingguo Chi, Qingguo Chen, Qingquan Lei PII: DOI: Reference:

S1385-8947(20)30434-4 https://doi.org/10.1016/j.cej.2020.124443 CEJ 124443

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

8 November 2019 15 January 2020 12 February 2020

Please cite this article as: G. Liu, T. Zhang, Y. Feng, Y. Zhang, C. Zhang, Y. Zhang, X. Wang, Q. Chi, Q. Chen, Q. Lei, Sandwich-Structured Polymers with Electrospun Boron Nitrides Layers as High-Temperature Energy Storage Dielectrics, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124443

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Sandwich-Structured Polymers with Electrospun Boron Nitrides Layers as High-Temperature Energy Storage Dielectrics

Guang Liua,b, Tiandong Zhanga,b, Yu Feng*a,b, Yongquan Zhanga,b, Changhai Zhanga,b, Yue Zhanga,b, Xubin Wanga,b, Qingguo Chi*a,b, Qingguo Chena,b, Qingquan Leia,b

a

Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150080, P. R. China b

School of Electrical and Electronic Engineering, Harbin University of Science and Technology, Harbin 150080, P. R. China

*Corresponding author E-mail: [email protected] (Y. Feng) E-mail: [email protected] (Q. G. Chi)

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Abstract Most dielectric polymers have unsatisfactory energy storage characteristics at high temperature. In this work, hexagonal boron nitride (h-BN) layers with different thickness playing roles in thermal conduction and carrier blocking are designed and transferred on both sides of polycarbonate (PC) films. They existing in the aggregation status of h-BN plates are fabricated via electrospinning technology. It is found that the thermal conductivity of the composite films is improved due to the existence of h-BN layers. The composite film with layer thickness of 1 μm (BN-1) has the low leakage current density, high breakdown strength and excellent high-temperature energy storage characteristics. The energy storage density of BN-1 is 5.52 J/cm3 under 500 MV/m electric field at 100℃, which is 15.10% higher than that of pure PC. At the same time, the density and efficiency stability of BN-1 are superior to pure PC in the temperature range of RT-100℃. Finally, the operation status of composite capacitor in practical applications is simulated by using the dynamic data concerning thermal conductivity and leakage current of composite films at different temperatures. The outstanding operation status in composite capacitor indicates that a promising, scalable and affordable route to high-temperature dielectric is provided.

Keywords: polycarbonate films; h-BN; thermal conductivity; high-temperature; energy storage characteristics

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1. Introduction Electric energy plays an increasingly important role in human industrial production and daily life, and reusable energy storage devices (such as secondary ion batteries, photosensitive cells, etc.) have also being widely studied and applied.[1-3] Although rechargeable ion batteries (such as lithium ion batteries, magnesium ion batteries, zinc ion batteries, etc.) have higher charge and discharge capacities, its low charge-discharge rate greatly limits the application in some specific places.[4-5] Dielectrics capacitors have attracted considerable attention due to their superior power density (approximately million walt) and fast charge-discharge capability (approximately microseconds). They are usually used as electrical energy storage devices in advanced electronic and power systems.[6-10] Generally ceramic dielectrics have super high dielectric constants and excellent thermal stability, however, polymer dielectric capacitors are wise choice for new flexible devices because of their light weight, mechanical flexibility, scalability, especially their higher breakdown strength compared to ceramic dielectrics.[11-15] For polymer dielectrics, PVDF and its composites have been extensively studied for their large dielectric constant and high energy storage density. However, its large loss leads to a low charge-discharge efficiency, which is undesirably causing a serious waste of energy. In addition, dielectric capacitors have a high operating ambient temperature, while PVDF has poorer temperature tolerance, which makes it a long way to achieve industrial applications. In order to reduce the temperature of dielectric capacitors and obtain better energy storage characteristics, the added secondary cooling system is the main way, but its unsatisfactory to increase the weight and the volume of the integrated power system, and the utilization of energy is also unsatisfactory reduced.[16-19] In order to meet the requirements of industrial applications, polymer dielectrics with better temperature tolerance such as (Polyetherimide PEI), Polyimide (PI) and Benzocyclobutene (BCB) have received extensive attention and research. Although these dielectrics possess better temperature tolerance, it is well known that 3

in many applications, dielectric capacitors operating at ambient temperatures above 80°C, and various temperature and field-dependent conduction mechanisms at high temperatures and high fields can cause a sharp increase in electrical conductivity, for example, charge injection at the electrode/dielectric interface.[20-23] The increased electrical conductivity also causes the leakage current to increase, resulting in a large amount of Joule heating, eventually leading to thermal runaway of the dielectric polymer.[24-25] Recently, computer modeling was conducted on metal/polyethylene interfaces. The calculation also found an important role of the metal/polyethylene interface for charge injection and charge transport mechanisms. The calculations also found an important role for charge injection and charge transport mechanisms.[26-28] In addition, the poor thermal conductivity of the polymer film also causes excessive accumulation of heating in the polymer dielectric during application, which in turn increases the leakage current and increases the electrical conductivity of the polymer dielectric. This creates a vicious cycle that eventually leads to premature breakdown of the polymer dielectric. Therefore, in addition to finding materials with better temperature tolerance, certain measures must be taken to improve the heat dissipation ability of the polymer dielectric and to inhibit the injection of charges from the electrode/dielectric interface. Hexagonal boron nitride (h-BN), has a large band gap of 5.97 eV, high insulation and excellent heat dissipation performance.[29-32] Related studies have found that filling h-BN into the interior of the polymer can improve the heat dissipation performance of the polymer dielectric, avoid thermal runaway of the polymer film, and ultimately obtain improved high temperature energy storage characteristics.[23] It was also found that the h-BN on the surface of polymer film can effectively inhibit carrier injection from the electrode/dielectric interface, thereby reducing the electrical conductivity. Finally, improved high temperature energy storage characteristics were obtained.[7]

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Although a large amount of work has been done on high-temperature dielectrics, the related research is mainly to prepare composite films by Chemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD) method, and such methods are always accompanied with strict operating environment and high cost, which are harmful for scalable production. Therefore, finding a method that is environmentally friendly, inexpensive, and scalableproduced should be the most noteworthy at present. Polycarbonate (PC) is a widely used engineering thermoplastic with high impact strength, high processability, light weight and better thermal stability. It is easy to process and harmless to the human body. On the basic of performance of PC, it has been predicted as a potential dielectric capacitor in moderate temperatures (RT-120℃).[33-35] Besides, as one of the effective ways to prepare nanomaterials, electrospinning technology has received extensive attention for its advantages of easy operation, low cost, wide applicability and controllable process.[36-39] In this paper, surface functionalized sandwich-structure dielectric films (h-BN/PC/h-BN) have been successfully prepared by electrospinning technology and hot-pressing, as shown in Fig. 1a. This method is not only simple to prepare, but also allows the film to maintain good mechanical properties (as Fig. S8). The h-BN layer on the surface of the film can make heating to diffuse along the surface, which effectively reduces heating concentration of the film. In addition, the injection of carriers from metal electrode into dielectric films is critically dependent on the interfacial barrier height. h-BN has a large band gap, the introduction of hBN layer builds a potential barrier due to their differences in bandgap and electron affinity. So the h-BN layer can increase the barrier between the electrode and the dielectric film, and effectively inhibit the charge injection from the electrode/dielectric interface, thus reducing the electric conductivity and leakage current density of the film, and the decrease of leakage current density will reduce the generation of Joule heating, thereby reducing the accumulation of heating[7]. Its working principle is shown in Fig. 1b. As expected, It was found that the thermal conductivity of the sandwich-structure composite film was significantly improved 5

due to the presence of the h-BN thermally conductive layer. The sandwich-structure composite film with h-BN thickness of 1 μm (BN-1) at 100℃ has the maximum thermal conductivity, low leakage current density, high applied electric field, and excellent hightemperature energy storage characteristics. The discharge energy density (energy storage density) of BN-1 composite film is 5.52 J/cm3 under an electric field of 500 MV/m at 100℃, which is 15.10% higher than that of pure PC. At the same time, it also has superior chargedischarge efficiency (87.25%). At the same time, the density and efficiency stability of BN-1 are superior to pure PC in the temperature range of RT-100℃. In order to investigate the temperature distribution of the sandwich composite films in practical applications, the thermal field of dielectric capacitor wound by sandwich-structure composite films is simulated by using the dynamic data concerning thermal conductivity and leakage current of composite films at different temperatures. The experimental and simulation results simultaneously confirm the superiority of composite dielectric film in the Lab and application. It should be noted that the above sandwich-structure composite films can also be industrially produced. In industrial production, the film is generally produced by extrusion, as shown in Fig. 1c, in order to facilitate industrial production, only the spray coating and hot pressing device are added (as the inside part of the dotted line in the figure), thus, a surface-functionalized sandwich structure of h-BN/PC/h-BN film can be obtained. The present work provides a new method for processing surface-functionalized films. The outstanding operation status in composites indicates that a promising, scalable and affordable route to high-temperature dielectric is provided.

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Fig. 1. Flow chart of preparation of sandwich structure film (a); working principle diagram of surface h-BN functional layer(b); predicted flow chart of industrial production (c).

2. Experimental 2.1. Materials The materials and chemical reagents used in the experiment are: PC (PolyK Technologies, LLC), h-BN (Shanghai Macklin Biochemical Co.,Ltd.), Tetrahydrofuran (Shanghai Aladdin Biochemical Technology Co., Ltd.), Anhydrous ethanol (Tianli Chemical Reagent Co.,Ltd.). The above materials and chemical reagents are directly used in the experiment without further processing and purification. 2.2. Preparation of PC film 7

Firstly, PC particles were weighed and dissolved in 10 mL tetrahydrofuran solution. The homogeneous solution was obtained by continuous stirring with a magnetic stirrer for 12 hours. The obtained solution was then casted onto a clean glass plate and the films were dried at 60℃ for 12 h, finally, the pure PC films were obtained. 2.3. Preparation of h-BN/PC/h-BN composite film In order to prepare h-BN/PC/h-BN sandwich composite films, the h-BN film was firstly prepared by electrospinning technology, and then transferred to both sides of the PC film by hot pressing, thus the surface functionalized composite films were obtained. The specific preparation process is shown in Fig. 1a: firstly, a certain amount of h-BN powder was weighed and dispersed in 10 mL anhydrous ethanol, and uniform h-BN dispersion solution was obtained by magnetic stirring and ultrasonic treatment. Then, the hBN dispersion solution was sprayed onto aluminium foil by electrospinning technology. During the preparation process, the voltage was control as 13 KV and 10 KV, respectively. The h-BN films with different thickness of 1 μm, 2 μm, 3 μm, 4 μm and 5 μm, were obtained by adjusting the volume of injection. Finally, the h-BN film was transferred to both sides of the PC film by hot pressing. respectively. The composite films with different h-BN thicknesses were recorded as BN-1, BN-2, BN-3, BN-4 and BN-5, respectively. 2.4. Structure characterizations X-ray diffraction (XRD) patterns of the sandwich-structure films were all tested by the EMPYREAN X-ray diffractometer, which adopted copper target as the radiation source, and the operating voltage and current are 40 kV and 40 mA. The cross-sectional structure morphologies of the sandwich-structure films were observed by the HITACHI S-3400N scanning electron microscope (SEM). Fourier transform infrared spectroscopy (FT-IR) of the sandwich-structure films were conducted on a NEXUS 670 FT-IR spectrometer in the range of 4000 to 500 cm-1. 8

2.5. Thermal and Electrical performance measurements The in-plane and through-plane thermal conductivity of the sandwich-structure films were tested using the German NETZSCH 114 laser thermal conductivity tester. The dielectric performance of the sandwich-structure films were characterized by a broadband dielectric spectrometer (Novocontrol GmbH, Germany) from frequency 100 to 107 Hz. The D-E loops and current density were characterized by a Precision LC ferroelectric test system (Radiant Technologies, USA).

3. Results and discussion 3.1. Structural characterization To observe the morphology of sandwich structure films, the cross sections of these films were characterized by scanning electron microscopy (SEM). Fig. 2a-f are the cross-sectional SEM images of PC, BN-1, BN-2, BN-3, BN-4 and BN-5, respectively. The corresponding EDS and element mapping spectra are shown at the right side of respective image.

Fig. 2. Cross-sectional SEM image of several composite films and their EDX mapping and energy spectra: PC (a); BN-1 (b); BN-2 (c); BN-3 (d); BN-4 (e); BN-5 (f). 9

As can be seen that the thickness of the PC layer in all films is about 10-13 μm, the cross sections of PC layer is relatively flat and compact, and there are no obvious defects such as holes, indicating that the prepared PC film is excellent in quality. In addition, the thickness of h-BN layer is consistent with the designed thickness (1-5 μm in turn). It can be seen from elements mapping and EDS that there are no B and N elements in pure PC films. The distribution of B and N elements in h-BN/PC/h-BN sandwich-structure composite films is uniform. With the increase of h-BN thickness, the diffraction peaks of B and N elements in the corresponding EDS energy spectrum are gradually enhanced, which illustrates that the hBN content in the composite films is gradually increasing. The phase structures of the films were analyzed by XRD, as shown in Fig. 3a. There is a wide peak in the vicinity of 2θ about 18°, which corresponds to the characteristic peak of PC amorphous diffraction.[40-42] There are two characteristic diffraction peaks of the composite films at 2θ of 26.75°and 54.94° correspond to the (002) and (004) crystal planes of h-BN, respectively, which proves the existence of h-BN.[43-45] The presence of h-BN has no effect on the diffraction peak of PC film. Moreover, there are no other characteristic peaks except for the characteristic diffraction peaks of h-BN and PC in the composite films, indicating that no impurities existed, and there is only physical mixing between the PC and h-BN.

Fig. 3. XRD patterns (a) and FT-IR diagrams (b) of several composite films. 10

In addition, the existence of h-BN was also proved by Fourier transform infrared spectroscopy (FT-IR), as shown in Fig. 3b. The characteristic absorption peaks of h-BN at 799 cm-1 and 1499 cm-1 wavelength represent the stretching vibration of B-N bond and the bending vibration of N-B-N bond, respectively.[46-48] The characteristic absorption peak at a wavelength of 3419 cm-1 represents the stretching and bending vibration of the O-H bond.[4950]

By comparison, it can be found that the characteristic absorption peaks of PC and h-BN

exist simultaneously in composite films, and the existence of h-BN in the composite film is also confirmed. 3.2. Electrical and Thermal Performance As the most important electrical property of dielectric capacitors, energy storage density has an important impact on its practical application. The energy storage density (J/cm3) of dielectric capacitors under a specific electric field can generally be calculated by Equation (1).[51-52] U e   EdD

(1)

where E and D are the applied electric filed and the electric displacement, respectively. In Equation (1) D   0 r E . Where  0 is the vacuum dielectric constant, and  r is the relative dielectric constant.[53-54] Therefore, the relative dielectric constant of dielectric materials has an important influence on its energy storage density. Firstly, the dielectric properties of the prepared dielectric films were tested. At the same temperature, the dielectric constant of the material gradually decreases as the frequency increases (Fig. S1). This is because almost all of polarizations can catch up with AC electrical field at lower frequencies, the dielectric constant is higher. The relaxation polarization cannot keep up with the frequency change at high frequencies, the polarization is lowered, so the dielectric constant is lowered.[30,55] The composite film is sufficiently polarized at low frequencies, and the rotation of the dipole can keep up with the frequency change at low 11

frequencies, so that electrical conductivity loss rather than polarization loss plays a dominant role. It can be seen that the dielectric loss of both BN-1 and BN-2 composites is kept at a low level close to or even lower than the pure PC, and much lower than that of other composite materials. When the thickness of h-BN layer is more than 3 μm, the dielectric loss increases rapidly. This is because when the thickness of h-BN layer is thicker, the weak bonding force between h-BN and h-BN increases defects such as voids, and then the charge can inject into the dielectric, and the electrical conductivity increases, resulting in a significant increase in dielectric loss of the composite film.[56]

Fig. 4. Dielectric constant and dielectric loss of composite films at different temperatures: RT (a); 50℃ (b); 80℃ (c); 100℃ (d); 120℃ (e); 150℃ (f). As shown in Fig. 4, the dielectric constant of each material at 10 Hz is significantly higher than that at 103 Hz. And at the same temperature, as the thickness of the h-BN layer 12

increases, the dielectric constant of the composite film gradually increases. There are two main reasons for this increasing trend, and the first factor is that the dielectric constant of hBN (4 to 5) is slightly higher than that of PC (about 3). Therefore, the h-BN on the surface of PC film can increase the dielectric constant of the composite films. Secondly, when placing hBN on the surface of the PC film, a phase interface is formed between h-BN and PC film. Besides as the thickness of h-BN increases, the interface between h-BN and h-BN increases, resulting in a larger relaxation polarization which will increases the dielectric constant.[57-58] In order to gain a deeper understanding of the polarization behavior of the films prepared, the temperature dependence of dielectric properties of them were investigated. Fig. 5a-f show the temperature dependence of dielectric constant and loss of several dielectric films prepared at specific frequencies. As can be seen from the diagram, with the increase of frequency, the dielectric constant peak and loss peak of dielectric films shift significantly to the high temperature region, which is caused by typical thermal activation relaxation polarization.[59-60] By comparison, it can be found that the dielectric constant of the composite films with h-BN on its surface shows a more significant increase with temperature increasing, indicating greater relaxed polarization. In addition, there is a distinct peak in the dielectric constant and dielectric loss at 141°C, which is mainly due to the transition of the glass state to rubbery state of PC at this temperature.[61] In order to confirm this phenomenon, and study the effect of hBN on the thermal properties of the film, we performed Differential Scanning Calorimeter (DSC) tests and Thermogravimetric Analysis (TGA) on the materials. The results are shown in Fig. S2. It can be seen from the chart that as the temperature increases, the dielectric constant and dielectric loss of the sandwich composite film show the same change rule as pure PC. And the change is most intense around 141°C, which is also related to the glass transition temperature of the material itself. It can also be seen that as the thickness of the h-BN increases, the dielectric constant peak and the dielectric loss peak caused by typical thermal activation relaxation polarization become less obvious. 13

Fig. 5. Temperature-dependent changes of the dielectric constant and the dielectric loss for composite films: PC (a); BN-1 (b); BN-2 (c); BN-3 (d); BN-4 (e); BN-5 (f). In addition to the dielectric properties, leakage current density also has an important influence on the energy storage properties of dielectric films. It is a direct reflection of conductivity characteristics in a dielectric under high electric field. The relationships between current density of films and electric field at a specific temperature are shown in Fig. 6. The samples show very low leakage current at low electric field, but a sharp increase in leakage current at high electric field. The leakage current density of several films at room temperature is relatively low, because both PC film and h-BN have good insulation properties. As the temperature increases, molecular thermal motion is intense, and charge is more easily injected into the dielectric to reduce its breakdown strength. In addition, as the charge is injected, a large amount of joule heating is generated, and eventually leads to thermal runaway of films, 14

which will affect their electrical performance.[19] Charge can obtain enough thermal activation energy in metal electrodes at high temperature and high field, which makes it easier to inject into dielectric films through the electrode/dielectric interface. With the increase of temperature and electric field, the energy barrier decrease, and the transfer rate of charges increases. Therefore, the current density increases significantly with the increase of temperature and electric field.[16,62] At high temperatures, general electrical conductivity loss predominate, so reducing the current density of the dielectric films under high temperature conditions can effectively reduce the electrical conductivity loss and prevent thermal runaway of dielectric films. The hBN with large band gap on the surface of film can increase the barrier between the electrode and the dielectric film, effectively inhibit the injection of charges, reduce the leakage current density, and greatly reduce the electrical conductivity loss of the composites.[7] Moreover, because h-BN has good heat dissipation performance, the existence of h-BN on the surface of PC film can help to improve the heat dissipation of composite film, and can effectively prevent the thermal runaway of the film. It can be seen that the leakage current density of composite film is obviously less than that of pure PC film when the thickness of h-BN is 1 μm, and then with the increase of h-BN thickness, the current density of composite film increases gradually, even higher than that of pure PC film. This is mainly due to the fact that as the thickness of h-BN increases, the h-BN layer is less dense and more defects are generated, so that the charge is more easily injected into the film, so the leakage current density increases rapidly and even higher than the pure PC film.[63] At 100℃, the current density of the BN-1 composite film is as low as 1.25E-8 A/cm2 at an electric field strength of 100 MV/m, which is lower than the pure PC of 3.85E-8 A/cm2 and other films prepared. This is not only because h-BN can inhibit charge injection, but also has an excellent thermal conductivity. The superior thermal conductivity of h-BN allows heating to be conducted along the in-plane direction of

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the film, reducing the temperature of film surfaces, increasing the energy barrier, thereby reducing the possibility of charge injection.

Fig. 6. Current density of composite films at different temperatures: RT (a); 50℃ (b); 80℃ (c); 100℃ (d); 120℃ (e); 150℃ (f). In order to prove the effect of the h-BN layer on the thermal conductivity of the film, we calculated the thermal conductivity of several films at different temperatures according to Equation (2), and the results are shown in Table1.[64-66]  T    T  CT   T 

(2)

In the formula above,  T  refers to the thermal conductivity of the dielectric at a specific temperature,  T  the thermal diffusion coefficient of the dielectric at a specific temperature, C T  the specific heat of the dielectric at a specific temperature, and  T  the 16

density of the dielectric at a specific temperature. The BN-1 has the best through-plane thermal conductivity, its through-plane thermal conductivity can reach 0.308 W/m·K at 100°C, while the through-plane thermal conductivity of pure PC is only 0.285 W/m·K (Table1 and Fig. S3). According to the data listed in Table 1, the contour plot of throughplane thermal conductivity as a function of temperature and thickness of BN is shown in Figure 7a. BN-1 possesses the excellent thermal conductivity in the wide temperature range, especially from 80°C to 120°C. Table 1. Through-plane and In-plane thermal conductivities of composite films In-plane (Through-plane) thermal conductivity (W/m·K) Materials 25℃

50℃

80℃

100℃

120℃

150℃

PC

0.558 (0.245)

0.598 (0.248)

0.728 (0.266)

0.835 (0.285)

0.738 (0.268)

0.623 (0.256)

BN-1

1.912 (0.260)

2.091 (0.248) )

2.180 (0.271)

2.366 (0.308) )

2.151 (0.288) ))

1.973 (0.281)

BN-2

1.648 (0.217)

1.891 (0.223)

2.022 (0.235)

2.124 (0.242)

1.947 (0.244)

1.867 (0.238)

BN-3

1.332 (0.201)

1.633 (0.211) ))

1.774 (0.217)

1.886 (0.230)

1.626 (0.217)

1.593 (0.230)

BN-4

1.258 (0.165)

1.351 (0.170) )

1.555 (0.194)

1.742 (0.222)

1.596 (0.206)

1.541 (0.198)

BN-5

1.147 (0.159)

1.279 (0.165) )

1.455 (0.189)

1.512 (0.209)

1.405 (0.195)

1.382 (0.198)

Fig. 7. Through-plane and In-plane thermal conductivities of composite films. The contour plot of through-plane thermal conductivity as a function of temperature and thickness of BN is shown in Fig. 7b. The trend of in-plane thermal conductivity is similar to the through-plane one. The thermal conductivity for in-plane direction of BN-1 at 100°C is 17

the highest of 2.366 W/m·K which is higher than that of pure PC, while the in-plane thermal conductivity of composite film with h-BN of 3 μm is only 1.886 W/m·K (Table 1 and Fig. S3). It should be noted that the interface between h-BN and h-BN increases with the thickness of h-BN, and this will generate a lot of interfacial thermal resistance, which hinders the conduction of heat.[67-68] Therefore, when the thickness of h-BN is greater than 1 μm, the thermal conductivity of the composite decreases significantly, even lower than that of pure PC. The above analysis can show that the thermal conductivity of BN-1 is significantly higher than that of other films, especially its ultra-high in-plane thermal conductivity. Excellent inplane thermal conductivity facilitates heating transfer along the in-plane direction of the dielectric film, which effectively reduces the temperature of the film, thereby preventing thermal runaway of the film at high temperature, and is beneficial to obtain better high temperature energy storage characteristics. The D-E hysteresis loops of pure PC and composites at various temperatures are shown in Fig. S4. According to the illustration in Fig. S5, the energy storage density and efficiency of pure PC and composites at respective breakdown strength are calculated and shown in Fig. S6 along with Table 2. At the same temperature and electric field, the energy storage density of the dielectric film increases with the increase of the thickness of h-BN. This is due to the higher dielectric constant of the sandwich-structure composite films. However, with the increase of the thickness of h-BN, the maximum electric field strength of the sandwichstructure films gradually decreases. This is because the interlayer defects of h-BN gradually increase and the current density gradually increases with the increase of the thickness of h-BN (as shown in Fig. 6), which makes the film more vulnerable to breakdown and makes the dielectric film unable to obtain higher energy storage density. As the temperature gradually increases, the energy storage density of the dielectric film at the same electric field strength gradually increases, and when the temperature exceeds 100℃, the energy storage density decreases rapidly. When the temperature exceeds 100℃, the dielectric film begin to turn to 18

high-elastic state at about 120℃, which has an important effect on its dielectric constant (as Fig. 5), so that the energy storage density decreases rapidly. As the temperature increases, the current density increases, and various losses (especially the electric conductivity loss) gradually increase, so that the breakdown field strength of the dielectric film gradually decreases, and the efficiency also gradually decreases. Just like the results in Fig. 6 and Fig. 7, the most outstanding energy storage performance is discovered in BN-1. As can be seen from Table 2, the maximum energy density of BN-1 can reach 5.52 J/cm3, which is 15.10% higher than that of pure PC. At the same time, BN-1 also has a higher charge-discharge efficiency (87.25%). Table 2 The maximum energy storage density and corresponding efficiency of composite films. Maximum Energy Storage Tensity (J/cm3) and Efficiency (%) Materials RT

50℃

80℃

100℃

120℃

150℃

PC

4.39 (94.38)

4.63 (88.67)

4.81 (88.51)

4.80 (90.16)

2.94 (86.77)

2.93 (68.75)

BN-1

4.96 (90.66)

5.11 (89.02)

5.30 (87.67)

5.52 (87.25)

3.64 (87.14)

3.39 (78.59)

BN-2

5.01 (80.82)

5.19 (85.96)

5.13 (91.81)

4.91 (87.31)

4.18 (84.08)

3.10 (76.75)

BN-3

4.37 (83.93)

4.86 (92.94)

5.50 (89.98)

5.31 (79.67)

4.49 (80.70)

1.82 (78.59)

BN-4

3.49 (77.01)

5.41 (91.35)

2.37 (96.97)

3.61 (91.47)

2.44 (88.10)

2.08 (83.79)

BN-5

2.76 (80.33)

5.33 (91.3)

5.15 (88.99)

5.35 (81.52)

3.26 (80.61)

2.66 (85.94)

The present experimental (electrical and thermal) results are in accordance with our expectation and design route. The h-BN has a large band gap and high thermal conductivity, which can effectively inhibit charge injection form the interface of electrodes/dielectric, and can effectively reduce the electric conductivity loss (this is the main factor causing film failure at high temperature and high field), which makes the composite film has ideal electric field strength and charge-discharge efficiency. In particular, BN-1 has the largest thermal conductivity at 100℃ and can effectively conduct heating along the surface of the film, which can effectively reduce the internal temperature of the film, minimize the leakage current

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density and reduce the electric conductivity effectively. Therefore, at 100℃, BN-1 has the best energy storage characteristics. In addition to the energy storage characteristics of dielectric films, the stability of energy storage density of dielectric film with the changes of temperature also has an important significant on their practical application. Fig. 8a-b show the rate of change of the maximum energy storage density and charge-discharge efficiency of the prepared dielectric films at different temperatures relative to room temperature. From Fig. 8a, it can be seen that the energy storage densities of PC, BN-1 and BN-2 have better temperature stability, and within 100°C their relative rates of change to room temperature are kept within 20%. In addition, the charge-discharge efficiency of BN-1 has the best temperature stability, its relative rates of change to room temperature are kept within 10%, as shown in Fig. 8b.

Fig. 8. Growth rate of storage density and efficiency of composite film with temperature: PC (a); BN-1 (b); BN-2 (c); BN-3 (d); BN-4 (e); BN-5 (f). 20

In order to further understand the influence of high thermal conductivity and low electrical conductivity induced by h-BN layer on the temperature distribution of dielectric films in practical applications, we simulated the steady temperature distribution of film capacitors by using COMSOL Multiphysics through the dynamic data concerning thermal conductivity and leakage current of composite films at different temperatures. As shown in Fig. 9a, the capacitor is composed of the composite films on which an aluminum electrode is deposited. The aluminum electrode layer and films has a thickness of 40 nm and 15 μm respectively. The height of dielectric film equals to 40 mm, and the direction corresponds to the axial direction of the cylindrical capacitor. It is assumed that the film size along the diameter of the cylindrical capacitor is sufficiently long to be finally wound into a cylindrical capacitor having a diameter of D = 40 mm. Finally, the steady-state temperature distribution of the six dielectric film capacitors under the applied electric field of 100 MV/m and ambient temperature of 80℃ is calculated by heat conduction equation (see supplementary information for details). The results are shown in Fig. 9b-g. Capacitors consisting of pure PC film have a maximum internal temperature of 94℃, which is 14℃ higher than the ambient temperature. The maximum temperatures of capacitors composed of BN-1 and BN-2 are 85℃ and 91℃, respectively. When the ambient temperature is 50℃ the internal temperature of the capacitor wound pure PC film is about 66℃, while the internal temperature of the capacitor wound by BN-1 is only less than 60℃ (see Fig. S7). That is because they have excellent higher thermal conductivity, especially the superior in-plane thermal conductivity, which can lead the heating diffuse along the in-plane direction, thus decreasing the internal temperature of capacitors. In addition, both of them have lower conductivity (as shown in Table S2), so they produce lower Joule heating, especially BN-1 has the lowest heat generation power. The internal temperature of the capacitors supported by BN-3, BN-4 and BN-5 is higher than 100℃, which is due to the high Joule heating generated and their low thermal conductivity. It can be seen from Fig. 9 that the overall temperature of the capacitor made of BN-1 is the 21

lowest, which is not only due to its higher thermal conductivity, especially its high in-plane thermal conductivity is more benefical to dissipate heating along the in-plane direction, which is very effective in avoiding heat concentration inside the capacitor and avoiding thermal runaway of the capacitor. The simulations results show the obvious superiority of BN-1 as capacitor materials compared with other samples.

Fig. 9. A schematic representation of a real capacitor with a height of H = 40 mm and a diameter of D = 40 mm made by winding h-BN/PC/h-BN nanocomposite films (a). Internal temperature distribution when the ambient temperature is 80℃, operating at an applied electric field of 100 MV/m in different capacitors made by composite films of PC (b); BN-1 (c); BN-2 (d); BN-3 (e); BN-4 (f); BN-5 (g).

4. Conclusion In this work, a method that is environmentally friendly, simple to operate and easy to realize industrial production is proposed to prepare a surface functionalized polymer dielectric. Mainly using the electrospinning technology and hot pressing method, the h-BN layer with large band gap and high thermal conductivity was successfully transferred to both sides of the PC film, and the surface functionalization of the h-BN/PC/h-BN sandwich structure films were obtained, which was used in high temperature energy storage dielectrics. The composite film not only has a high surface barrier, but also has a higher thermal conductivity because of 22

the existence of h-BN, which allows heating to be conducted faster in the in-plane direction, effectively lowering the temperature of the film and inhibiting thermal runaway of the film. It is beneficial to obtain the ideal high temperature energy storage characteristics. The experimental results show that when the thickness of the h-BN functional layer on the surface of the film is 1 μm, the dielectric film not only has lower dielectric loss, lowest conductivity, but also the highest thermal conductivity. This not only reduces the heating generation of the capacitor during operation, but also effectively conducts heating along the surface of the film, avoids heat concentration of the capacitor, and finally obtains excellent high-temperature energy storage characteristics. At 100℃, the dielectric film has a storage density of 5.52 J/cm3 under an electric field strength of 500 MV/m at 100℃ and a charge-discharge efficiency of up to 87.25%. This method is easy to operate and has extensive applicability. It can not only transfer h-BN to both sides of the polymer film, but also is applicable to other two-dimensional materials to produce various surface functionalized polymer films. The outstanding operation status in composites indicates that a promising, scalable and affordable route to high-temperature dielectric is provided, and is of great significance for the development and production of dielectric capacitors.

Acknowledgements This research was funded by National Natural Science Foundation of China (No. 51977050, 51807041 and 51807042), Natural Science Foundation of Heilongjiang Province of China (No. QC2018067 and TD2019E002), China Postdoctoral Science Foundation (No. 2018M640302), Heilongjiang Postdoctoral Financial Assistance (No. LBH-Z18098), and Fundamental Research Fundation for Universities of Heilongjiang Province (No. LGYC2018TD001 and LGYC2018JC019).

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Appendix A. Supporting information Supporting information available: Experimental details and results of any supplementary information available should be included here, which associated with this article can be found in the online version at http://dx.doi.org/xxxxxxxxxx.

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Highlights  The h-BN/PC/h-BN film is prepared by a simple and environmentally friendly method.  The h-BN layer can inhibit charge injection, and facilitates heat dissipation.  The composite film obtains excellent high-temperature energy storage performance. 32



Efficiency of 87.25% and density of 5.52 J/cm3 are obtained at 100℃.

Declaration of Interest Statement No conflict of interest exists in the submission of “Sandwich-Structured Polymers with Electrospun Boron Nitrides Layers: A Scalable and Affordable Route to High-Temperature Energy Storage Dielectrics”, and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

Q. G. Chi Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150080, P.R. China E-mail: [email protected] Tel./Fax: +86 451 86391681

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