Ultrahigh discharge efficiency and excellent energy density in oriented core-shell nanofiber-polyetherimide composites

Journal Pre-proof Ultrahigh discharge efficiency and excellent energy density in oriented core-shell nanofiber-polyetherimide composites Feng Yu, Zhou Yinhua, Zhang Tiandong, Zhang Changhai, Zhang Yongquan, Zhang Yue, Chen Qingguo, Chi Qingguo PII:

S2405-8297(19)31012-8

DOI:

https://doi.org/10.1016/j.ensm.2019.10.016

Reference:

ENSM 959

To appear in:

Energy Storage Materials

Received Date: 31 July 2019 Revised Date:

12 October 2019

Accepted Date: 12 October 2019

Please cite this article as: F. Yu, Z. Yinhua, Z. Tiandong, Z. Changhai, Z. Yongquan, Z. Yue, C. Qingguo, C. Qingguo, Ultrahigh discharge efficiency and excellent energy density in oriented coreshell nanofiber-polyetherimide composites, Energy Storage Materials, https://doi.org/10.1016/ j.ensm.2019.10.016. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Ultrahigh discharge efficiency and excellent energy density in oriented core-shell nanofiber-polyetherimide composites

Feng Yu,a, b Zhou Yinhua,a, b Zhang Tiandong,a, b Zhang Changhai,a, b* Zhang Yongquan,a, b Zhang Yue,a, b Chen Qingguo,a, b Chi Qingguo,a, 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: [email protected] **Corresponding author: [email protected]

Graphical Abstract A new energy-storage dielectric system—polyetherimide composite with ultrahigh efficiency of 90% and outstanding density of 11.3 J/cm3 is developed.

Ultrahigh discharge efficiency and excellent energy density in oriented core-shell nanofiber-polyetherimide composites

Abstract A new energy storage dielectric system-polyetherimide (PEI) based composite was developed and investigated intensively. The system possesses the advantages of excellent energy storage efficiency along with energy storage density. A comprehensive energy storage performance with ultrahigh efficiency of 90% and outstanding density of 11.3 J/cm3 is simultaneously achieved in PEI/BaTiO3 nanofiber coated by thick SiO2 layer (PEI/BT@T-SiO2) composites. The optimized energy storage efficiency is ascribed to the lower intrinsic dielectric loss of PEI matrix, insulted SiO2 shell outside the BaTiO3 nanofibers, and the oriented arrangement (parallel to the composite surface) of above “core-shell” structured fillers. In addition, three types of “core-shell” structured fillers ((BaSr)TiO3@SiO2, BT@SiO2 and BT@T-SiO2) were fabricated. The effects of dielectric constant for “core” and thickness for “shell” in the “core-shell” structured fillers on the dielectric and energy storage performance of composites were also investigated. The combination of experimental and simulated results indicates that larger dielectric constant of “core” doesn’t mean the high polarization it can provide, and thickness of “shell” can influence the electric field distribution on “core”. This work not only provides a new choice for matrix selection, but also points out a design route for the “core-shell” structured fillers used in the polymer based dielectric composites.

Keywords: PEI; nanofiber; core-shell; energy storage efficiency; energy storage density 1

1. Introduction With the continuous consumption of non-renewable energy materials and the emergence of new clean energy materials, the much higher requirements are placed on the storage and conversion of electrical energy.[1] Compared with the electrochemical energy storage systems (Li-ion batteries, electrochemical supercapacitors, and fuel cells), the dielectric capacitors possess the intrinsic advantage of high power energy density and fast charge/discharge speed, which has been widely applied in pulsed power systems, such as hybrid electric vehicles, telecommunications equipment, and medical devices.[2,

3]

There are two main kinds of dielectrics for the

capacitors-inorganic and organic. Inorganic dielectrics always have high dielectric constant and low breakdown strength.[4-6] In contrast, organic dielectrics generally have excellent breakdown strength as well as inferior dielectric constant.[7] Organic dielectrics, namely polymer dielectrics have attracted more attention owing to their high breakdown strength, flexibility and lightweight.[8-10] Taking the commercialized electrostatic capacitor as an example, the state-of-the-art dielectrics are biaxially oriented polypropylene (BOPP) films with a low energy density (Ue) of 2 J/cm3.[11] For example, 35% and 23% of volume and weight in DC bus capacitors are occupied by power inverters of electrical vehicles. It is urgent to develop polymer dielectric materials with high energy density hence to miniaturize the size and weight of electrostatic capacitors. As one kind of pivotal dielectric materials, ferroelectric polymers, especially polyvinylidene fluoride (PVDF) and its copolymers, have been subjected to an upsurge of research.[12-14] Recently, 2

PVDF-based polymers have become one of the most frequently used dielectric polymer matrices for the fabrication of high-energy-density capacitors.[15, 16] Although the dielectric constant (≥10) of PVDF is higher than that (3-5) of other common dielectric polymers, the acceptable energy storage density cannot be achieved in the ௉

pure PVDF-based polymer based on the formula ܷ௘ = ‫׬‬௉ ೘ ‫ܧ‬dܲ.[17, 18] Optimizing the ೝ

dielectric constant of polymer by means of inorganic filling becomes a widespread approach. Obviously, the insulated inorganic fillers (for example ABO3 perovskite oxides) are superior to conductive ones (including metal, carbon based materials) because conductive ones also bring the high dielectric loss and conductivity into the composites.[19-21] In addition to the conduction types of inorganic filler, researchers are mainly concerned with the influence of shape factor (0-dimension, 1-dimension or 2-dimension) for filler on the energy storage performances of obtained composites.[22-24] The spherical fillers (0-dimension) are the first research object, but the energy storage enhancement of composites is limited and always achieved at high filler concentration.[25, 26] 1-dimension (1-D) filler is one of the most potential fillers in present, because the orientation configuration of 1-D filler is conducive to the composite with excellent polarization and breakdown strength.[27-30] Furthermore, various 1-D nanofibers and corresponding polymer based composites with highly oriented 1-D nanofibers can be fabricated via several approaches.[31-35] The investigations on the composites with 2-dimension (2-D) filler have been explored, but a few 2-D fillers can be chosen, for example NaNbO3 sheet and BN sheet.[36, 37] At the same time of experimental development, advances in simulation analysis also 3

offer researchers more information for designing the composites structures with a series of different dimensional fillers and distribution configurations of fillers.[17, 38] However, the lower energy storage efficiency (<80%) has limited the widespread application and industrial development for PVDF based composites. The efficiency of 80% means that energy of 2 J will be wasted when 10 J is stored in a circle of charge-discharge process. Obviously, higher energy storage density can’t make up for the lower energy storage efficiency. The comprehensive energy storage performance including high density and efficiency rather than solely high density is the real aim for researchers.[39, 40] Because the intrinsic dielectric loss of pure PVDF is not excellent (~0.02 at 100 Hz), the value for the PVDF based composites is seriously higher. This disadvantage cannot be easily optimized by filler filling or hybrid method, which is confirmed by experimental and theoretical results.[41] Discovery and development of a new energy storage system with both high density and efficiency are urgently needed. In the present study, four aspects in a composite: the matrix, the filler, the configuration of 1-D filler and the organic-inorganic interface are considered and designed. For the matrix, the PEI is chosen because of the high breakdown strength and low dielectric loss.[42] The Ba0.65Sr0.35TiO3 (BST for short) and BaTiO3 (BT for short) nanofibers fabricated by electrospinning method are chosen as fillers.[43] The dielectric constant of BST ( ߝ୆ୗ୘ ~12000 − 14000 ) is higher than that of BT (ߝ୆୘ ~1500 − 2500 ) at room temperature.[44] In order to improve the interface insulation and reduce the interfacial polarization between matrix and filler, the SiO2 layer is considered to be the interface layer.[43, 45] It should be noted that the shell SiO2 4

possesses an outstanding insulation property (energy band gap is ~9 eV, Eb is 800-1000 kV/mm) and a remarkable thermal conductivity than most of polymers (~2.5 W/(m·K)).[46] Furthermore, three kinds (BST@SiO2, BT@SiO2 and BT@T-SiO2) of “core-shell” structured fillers have been synthesized via sol-gel method, then incorporated into the PEI matrix (T-SiO2 means thicker SiO2 layer). The PEI based composites with oriented 1-D fibers were fabricated via high speed electrospinning method as reported in our previous works.[43] The 1-D fillers are distributed in the direction parallel to composite surface (perpendicular to applied electric field), which may hinder the formation of conductive path along the direction of electric field. Finally, it is worth highlighting that this work mainly focuses on the following two points. On the one hand, the influence of dielectric constant of core part in the “core-shell” structured fillers on the energy storage of composites is the research object. On the other hand, the effect of thickness of shell part on the dielectric and energy storage performance of composites is another key point. The aim behind above investigation is to obtain a kind of dielectrics with high energy density and efficiency.

2. Experimental section 2.1 Materials Polyetherimide

(PEI)

was

provided

by

PolyK

Technologies.

N-methylpyrrolidone (NMP), tetraethoxysilane (TEOS, C8H20O4Si) were produced in Tianjin Bailunsi Biotechnology Co., Ltd. Polyvinylpyrrolidone (PVP, Mw=1,300,000)

5

and strontium acetate (C4H6O4Sr) were obtained from Aladdin. Ammonia solution (25%), barium hydroxide octahydrate (Ba(OH)2·8H2O) were provided by Tianjin Tianli Chemical Reagent Co., Ltd. Acetic acid, acetylacetone, absolute ethanol and tetrabutyltitanate (Ti(OCH2CH2CH2CH3)4) were produced in Sinopharm Chemical Reagent Co., Ltd. Cetyltrimethyl ammonium bromide (CTAB, C19H42BrN) was purchased in China Hui Shi Biochemical Reagent Co., Ltd. 2.2. Preparation of nanofiber, core-shell structured nanofiber and composites The BT and BST nanofibers (NFs) were prepared via the sol-gel and electrospinning process. The process scheme for the nanofibers is described in Supporting Information in detail and shown in Figure S1. The BT NFs and BST NFs were coated by a nano-layer of amorphous silica phase via chemical reaction of a modified sol-gel method. The process scheme for the “core-shell” structured nanofibers (BST@SiO2, BT@SiO2, and BT@T-SiO2) is described in Supporting Information and shown in Figure S2. The aligned PIE/BT@SiO2 and PEI/BST@SiO2 composites loaded with different nanofibers contents were fabricated by high speed electrospinning technology and hot-pressing method as shown in Scheme 1.[31, 47] A certain amount of SiO2 coated nanofibers were taken and dispersed in NMP solution by sonication for 5 minutes. Then, the PEI particles were dissolved in the above dispersion in proportion, and the magnetic stirrer temperature was set to 50 °C, and stirring was continued for 12 hours to obtain a uniform and stable suspension. The obtained mixed liquid was placed in a vacuum oven to take a vacuum, and the bubbles were discharged to form a spinning precursor. The spinning precursor was placed in a 6

syringe, and the directional spinning receiver has a higher rotational speed of approximately 2000 r/min. After drying the electrospun film, the film was hot-pressed at a temperature of 180 °C and a pressure of 15 MPa for 15 minutes to obtain a series of PEI composites. The hot-pressure temperature is determined via DSC data as shown in Figure S3. 2.3. Characterization The X-ray diffraction (XRD) analysis was carried out on the PANalytical Empyrean. Scanning electron microscopy (SEM) of PEI-based composites were obtained on Hitachi SU8020. Fourier transform infrared spectroscopy (FTIR, JASCO 6100) was used to investigate the molecular structure of the fillers and corresponding composites. Transmission electron microscope (TEM) was carried out on FEI Tecnai 2-12. The Al electrodes (25 mm and 3 mm in diameter) were evaporated on two sides of PEI-based composites for the following electric measurements. Dielectric properties were collected at the frequency range of 0.01 Hz to 1 MHz at room temperature on the Novocontrol GmbH Alpha-A. PEI-based composites were subjected to the electric breakdown strength tests via YDZ-560. Radiant Premier II Ferroelectric Test System was used to measure the current-voltage (I-V) behavior and the electric displacement-electric field (P-E) hysteresis loops of PEI-based composites at frequency of 10 Hz.

7

Scheme 1. Preparation scheme for PEI composites with oriented nanofibers by means of high speed electrospinning technology and hot-pressing method

3. Results and discussion In order to directly observe the microstructures of nanofibers, the SEM and TEM images are used to evaluate the size and shape of nanofibers. Figure 1(a1) and (b1) present the SEM images of the BT nanofibers and BST nanofibers, respectively. (For a further comparison, the SEM images for the BT and BST nanofibers before sintering process can be referred in Figure S4.) A statistical distribution concerning aspect ratio of inorganic fibers is shown in Figure S5. According to Figure S5, the average aspect ratios of both BT (Figure 1 (a1)) and BST (Figure 1 (b1)) nanofibers are in the range of 5-6. The average aspect ratio of BT nanofibers (5.46) is slightly larger than that of BST nanofibers (5.14). The Ba, Ti, and O element mappings in Figure 1(a1) are shown in Figure 1(a2), (a3), and (a4). According to elemental mappings, the chemical elements of Ba, Ti, and O are homogeneously distributed in 8

the BT nanofibers. For the BST nanofiber, the Ba, Sr, and O element mappings in Figure 1(b1) are shown in Figure 1(b2), (b3), and (b4), which also confirms the homogeneous distribution of chemical elements. Figure 1(c) and (d) show the TEM images of BT@SiO2 and BT@T-SiO2, respectively. Obviously, the SiO2 coating in BT@SiO2 is thinner than that in BT@T-SiO2. The process for tuning the thickness of SiO2 coating is described in Supporting Information. The thickness distribution of SiO2 coating in BT@SiO2 and BT@T-SiO2 is calculated and shown in Figure S6. The thickness of SiO2 coating in BT@SiO2 is in the range of 12-19 nm. The average thickness is about 15 nm. The thickness range and average value of SiO2 coating in BT@T-SiO2 are 16-24 nm and 21 nm, respectively. The SEM and TEM images indicate that the different “core” (BT and BST) and various “shell” (SiO2 and T-SiO2) in the core-shell fillers have been achieved successfully.

9

Figure 1. SEM images of (a1) BT and (b1) BST nanofibers. Element distributions of (a2) Ba, (a3) Ti, (a4) O in Figure 1(a1). Element distributions of (b2) Ba, (b3) Sr, (b4) O in Figure 1(b1). TEM images of (c) BT@SiO2 and (d) BT@T-SiO2 The XRD and FTIR were employed to investigate the phase and molecular structure of the inorganic nanofibers along with coating layer. As illustrated in Figure 2(a), the (100), (110), (111), (200), (211), (220), (310), and (222) planes of the BT and BST are well indexed without other impurity phases, which indicate the existence of pseudocubic perovskite structure (JCPDS, 81-2203). Figure 2(a) inset shows the enlarged image of XRD pattern in the 2θ range of 30-33°. The right shift for this

10

diffraction peak corresponding to the (111) plane is found. It should be noted that all the diffraction peaks in BST shift toward right as compared with those in BT, which is due to incorporation of Sr ion in BT crystal. The introduction of Sr will decreases the lattice parameters along with the Curie Temperature of BT crystal.[48] According to the FTIR spectra as shown in Figure 2(b), the infrared characteristic absorption peaks of BT and BST nanofibers in the inorganic fillers were revealed near ~588 cm-1.[43] Comparing to the uncoated BT and BST, the coated nanofibers show a characteristic peak of O-Si stretching vibration near ~786 cm-1, and characteristic absorption peak of Si-O-Si stretching vibration near 1081 cm-1.[49] The peaks appeared at ~3459 cm-1 and ~1646 cm-1 for BST@SiO2, BT@SiO2, and BT@T-SiO2 belong to the -OH stretching vibration and bending vibration, respectively.[50] It was reported that the hydroxyl group can promote a strong interaction between the inorganic fillers and the polymer matrix.[22, 51] Other additional absorption peaks, such as ~2925 and ~2850 cm-1, may be induced by the secondary intermediates which formed in the hydrolysis condensation reaction process.[43]

Figure 2. (a) XRD pattern of BT and BST, inset is the enlarged image of XRD pattern in the 2θ of 30-33°. (b) FTIR spectra of BST, BT, BST@SiO2, BT@SiO2 and 11

BT@T-SiO2 In order to confirm the orientation of nanofibers in the composites, the cross-section SEM images of three composites are shown in Figure 3(a), (b), and (c). As shown in Figure 3(a1), the BST@SiO2 nanofibers tend to align the parallel orientation in the PEI/1 vol% BST@SiO2 composite. The orientation direction of nanofibers is marked by lines. The cross-sectional structure is flat, no obvious agglomeration and defect can be observed. The element distributions of Ba, Sr, Ti, Si, O and N in Figure 3(a1) are shown in Figure 3(a2)-(a7). The chemical elements of Ba, Sr, Ti and Si are mainly distributed in the BST@SiO2 nanofibers. Meanwhile, the element of O is distributed in both nanofiber and matrix. Clearly, the element of N is distributed only in PEI matrix. The element-mapping clearly indicates the distribution and orientation of the nanofibers in the matrix. For the composites with BT@SiO2 and BT@T-SiO2 nanofibers, the similar oriented distribution of nanofibers can be found in Figure 3(b) and (c). The element-mapping of Ba and Ti on the cross-section of PEI/BT@SiO2 and PEI/BT@T-SiO2 composites can also help us to confirm the orientation of nanofibers (see Figure S7). The high speed electrospinning method used in present work is confirmed again to be a successful approach to obtain oriented nanofibers.[43] As illustrated in Figure 3(d) of XRD patterns for pure PEI and composites with three fillers (BST@SiO2, BT@SiO2, and BT@T-SiO2), the characteristic diffraction peaks of inorganic fillers and PEI matrix are indexed. For example, the characteristic diffraction plane of (110), (111), (200), (210), (211) for BT/BST nanofibers is at 2θ~31.5°, ~38.7°, ~45.1°, ~51, ~55.9°, respectively. 12

According to the above XRD pattern analysis, the inorganic fillers of BT and BST nanofibers are with perovskite structure in the composites, and no other impurity phase is contained. For the PEI, there is only a diffuse scattering peak located at about 20°. For the composites with 2 vol% nanofibers, the peak belonging to PEI is also found at about 20°. However, the diffuse scattering peaks in the composites with 4 vol% nanofibers shift toward high-angle direction, which indicates that more nanofibers will make polymer molecule varied. Most possibly, nanofibers reduce the spacing between molecules according to the Bragg’s equation: 2݀sinߠ = nߣ, where d, θ, and λ is interplanar distance, scattering angle and X-ray wavelength, respectively. As shown in Figure 3(e) of FTIR spectra for three kinds of composites, the characteristic absorption peaks of PEI are indexed near 750, 1375, 1720 and 1780 cm-1. The peaks located at 750 and 1375 cm-1 correspond to the C-N bending and C-N stretching.[52] The peaks found at 1720 and 1780 cm-1 belong to the imide carbonyl asymmetrical symmetrical stretching in PEI matrix.[53, 54]

13

Figure 3. Cross-section SEM images of (a1) PEI/1% BST@SiO2, (b) PEI/1% BT@SiO2 and (c) PEI/1% BT@T-SiO2. Element distributions of (a2) Ba, (a3) Sr, (a4) Ti, (a5) Si, (a6) O and (a7) N in Figure 3(a1). XRD pattern (d) and FTIR spectra (e) of PEI/BST@SiO2, PEI/BT@SiO2, and PEI/BT@T-SiO2 composites For the energy storage performance of certain a dielectric, dielectric properties

14

including dielectric constant, dielectric loss, AC conductivity, breakdown strength, leakage current are critical factors. The dependency of the dielectric constant, dielectric loss and AC conductivity of PEI/BST@SiO2, PEI/BT@SiO2, and PEI/BT@T-SiO2 composites on the frequency are shown in Figure S8. The dielectric constant, dielectric loss and AC conductivity of composites at lower frequency increase with the concentration of fillers no matter in which kind of composites.[55] As the frequency increasing, dielectric constant of all the composites decreases, because different types of polarizations cannot keep up with the variation of the AC frequency and will gradually vanish one by one.[50] In order to highlight the enhancement of dielectric properties in composites, the dependency of the dielectric constant, dielectric loss and AC conductivity of PEI/BST@SiO2, PEI/BT@SiO2, and PEI/BT@T-SiO2 composites on the filler concentration at given frequencies are shown in Figure 4. For the Figure 4(a), the frequency of 100 Hz is chosen. Except for the PEI/BT@T-SiO2, PEI/BST@SiO2 and PEI/BT@SiO2 composites possess a continuously increased tendency of dielectric constant. It can be acceptable because the dielectric constant of BST and BT is far greater than those of matrix and interface SiO2 layer (ߝୗ୧୓మ ~4). The dielectric constant of PEI/BT@T-SiO2 decreases firstly, then increases with the filler concentration, which is reported in some previous works.[56] Actually, the reduced dielectric constant is always reported to exist in the SiO2 filled polymers, such as PEI, PI and so on.[57-60] In this work, the shell material in the core-shell fillers is SiO2 which is believed to responsible for the reduced dielectric constant.[60] Firstly, the dielectric constant of SiO2 itself is relatively small (ε=3-4).[61] 15

In this work, the thickness of SiO2 shell in the BT@T-SiO2 nanofiber is largest among the three fillers (BST@SiO2, BT@SiO2 and BT@T-SiO2). The relatively more SiO2 concentration in the BT@T-SiO2 nanofibers is beneficial to remaining lower dielectric constant in this kind of composite. Secondly, there are certain influences from SiO2 surface on the molecule chain of PEI. The FTIR results in Figure 2(b) indicate that – OH bond exists on the surface of SiO2 shell. The –OH bond is always considered to provide an “interaction” for the connection between inorganic and molecule chain.[62] This “interaction” possibly constrains/restricts the mobility of molecule, especially side groups.[63, 64] It is always called “chain-anchoring effects”. This anchoring effect is also confirmed by some theoretical calculations.[65, 66] Commonly, side groups are more favorable to rotation, which would entail a significant increase in the polarizability along with dielectric constant. When the mobility of side groups is restricted, the polarizability of matrix molecule chain will be expected to decrease. The Debye equation can explain the reduction of dielectric constants due to the decreased polarizability as follows.[67, 68] ఌೝ ିଵ ఌೝ

ே

ఓమ

= ଷఌ (ߙ௘ + ߙௗ + ଷ௞்) ାଶ బ

(1)

where εr is the relative dielectric constant of the samples, ε0 represents the dielectric constant of vacuum, N is the number density of dipoles in the system, αe and αd are the electronic and distortion polarization in the molecule, µ is the orientation polarizability, k is Boltzmann’s constant, and T is the temperature in K. The µ polarizability and dielectric constant of polymer matrix are determined by the ability of orientable dipoles to orient under an applied electric field.[69] That means that if 16

molecule chain orientation movement of polar groups in the BT@T-SiO2/PEI composites is restricted by the BT@T-SiO2 nanofibers, dielectric constant will decrease. According to the Equation 1, there are two efficient design routes for researchers to obtain the polymer composites with low dielectric constant.[70] One is reducing the number of dipoles (decreased N). The another one is decreasing the polarizability of polymer. Finally, the effect of nanofibers on the molecule chain dimension/length of polymer cannot be ignored. The molecule chain dimension/length of polymer is commonly reduced because of filler with larger size.[71] According to the recent density functional theory calculation, the polarizability and dielectric constant of polymer will be reduced due to the decreased molecule chain length.[72] In our opinion, the second factor (restricted molecule mobility) is the dominant reason for the reduced dielectric constant. It should be noted that the reduced dielectric constant is also found in the polyimide/SiO2 composites recently, and the restricted molecule mobility is also regarded as origin of this phenomenon.[67]

17

Figure 4. Dielectric constant, dielectric loss and AC conductivity of PEI/BT@T-SiO2, PEI/BST@SiO2 and PEI/BT@SiO2 composites with various filler concentrations at 100 Hz (a, b, c) and 1 Hz (d, e, f) Among the three kinds of composites with same filler concentration, the dielectric constant of PEI/BST@SiO2 is largest, and PEI/BT@T-SiO2 possesses the lowest value. The results indicate that the dielectric constant of core part in the “core-shell” structured filler system brings a positive effect on the dielectric constant of composites (comparing PEI/BST@SiO2 with PEI/BT@SiO2). When the PEI/BT@SiO2 is in comparison with the PEI/BT@T-SiO2, the thickness of shell part in the “core-shell” structured filler is also confirmed to influence the dielectric constant of composites. For the dielectric loss and AC conductivity of three 18

composites, the tendency with filler concentration is almost identical—rising. Similar to the dielectric constant, PEI/BST@SiO2 composites have the largest dielectric loss along with AC conductivity, and the lowest values are observed in PEI/BT@T-SiO2 composite. In addition, the dielectric properties of three composites with various filler concentrations at 1 Hz are shown in Figure 4(b). The overall tendency of dielectric properties illustrated in Figure 4(b) is consistent with that in Figure 4(a). In order to investigate the dielectric properties further, the leakage current behavior of these composites has been measured and shown in Figure 5(a1), (b1) and (c1). For the PEI/BST@SiO2, the leakage current of composites with 0, 0.5, 1, 2, 3, and 4 vol% fillers is 2.17×10-6, 4.19×10-6, 4.70×10-6, 7.11×10-6, 1.09×10-5, and 3.21×10-5 A/cm2 under the applied field of 100 kV/mm. The leakage current of composites increases with the filler concentrations. The leakage current of PEI/BT@SiO2 composites with 0.5, 1, 2, 3, and 4 vol% fillers is 3.05×10-6, 5.07×10-6, 6.40×10-6, 6.58×10-6, and 7.42×10-6 A/cm2, respectively. Similar to PEI/BST@SiO2, there is a continuously increasing tendency for the leakage current in PEI/BT@SiO2. The increasing rate for the leakage current with filler concentration is lower compared with that in PEI/BST@SiO2. Originally, there is electric field distortion between matrix and filler existing because of the dielectric properties difference between them. In such case, the larger the dielectric difference between them is, the more serious the electric field distortion is. The dielectric constant of BST is larger than that of BT, so there are more evident electric field distortion existing between PEI matrix and BST filler, which is confirmed by following simulation analysis in Figure 8. The serious 19

electric field distortion around the interface will lead to the higher leakage current in the interface. For the PEI/BT@T-SiO2 composites, the leakage current of composites with 0.5 and 1 vol% filler is even lower than that of pure PEI. The values are 1.07×10-6 and 9.57×10-7 A/cm2, respectively. The leakage current of rest composites with 2, 3, and 4 vol% is larger than that of pure PEI, but lower than those of the composites with same filler concentration of BST@SiO2 and BT@SiO2. It should be noted that the lowest leakage current is achieved in the PEI/BT@T-SiO2 composite with 1 vol% filler. The two-parameter Weibull distribution is used to accumulate the characteristic breakdown strength based on the cumulative breakdown probability, which is described and fitted by Equation 2: ln൫− ln൫1 − ܲ(‫)ܧ‬൯൯ = ߚ ln(‫ )ܧ‬− β ln(‫ܧ‬௕ )

(2)

Where P(E) is the cumulative breakdown probability, E is the tested breakdown strength, β is the shape parameter representing the degree of dispersion of the tested breakdown strength values, Eb is the breakdown strength at P=63.2%. According to Figure 5(a2), the Eb value of the PEI/BST@T-SiO2 composites is 325, 342, 309, 303 and 255 kV/mm, at the nanofiber content of 0.5, 1, 2, 3 and 4 vol%, respectively. These values are lower than Eb (449 kV/mm) of pure PEI. Thus, the Eb value decreases gradually, as the content of BST@SiO2 in the composite increases. There are similar variation tendency for the Eb in PEI/BT@ SiO2 and PEI/BT@T-SiO2. There is no composite possessing more excellent Eb than pure PEI. Because the space and the distance between the nanofibers become much smaller, and even the 20

agglomeration of the inorganic filler appears in composite, as the content of hybrid nanofibers increase. Therefore, it is easier for the charge carriers to transport between fillers, and the Eb is reduced.

Figure 5. Leakage current and breakdown strength of (a) PEI/BST@SiO2, (b) PEI/BT@SiO2 and (c) PEI/BT@T-SiO2 composites After the comprehensive analysis on dielectric properties of composite, the polarization behavior of three kinds of composites (PEI/BST@SiO2, PEI/BT@SiO2 and PEI/BT@T-SiO2) are further studied, as illustrated in Figure S9, S10, and S11. According to the shape of hysteresis loops in all composites, the linear dielectric character of PEI based composites can be proved. In addition, the larger energy storage efficiency can be expected and predicted in this kind of composites. The maximum electric field strength as shown in the P-E curves decreases with the increase of the nanofiber content, indicating that the electric field strength is mainly

21

subjected to the inorganic nanofiber content. According to the P-E curves, the energy storage parameters of the composite can be calculated by Equation 3 and Equation 4, respectively. These formulas are described as follows: [31, 47, 73] ௉

ܷ௘ = ‫׬‬௉ ೘ ‫ܧ‬dܲ ೝ

ߟ=

௎೐ ು೘ ‫׬‬బ ாୢ௉

(3) (4)

Where Ue is the discharged energy density, Pm is the maximum polarization, Pr is the residual polarization, E is the applied electric field, and η is the charge-discharge efficiency. The discharged energy density and charge-discharge efficiency of three composites are plotted in Figure 6. Figure 6(a), (b) and (c) correspond to PEI/BST@SiO2, PEI/BT@SiO2 and PEI/BT@T-SiO2 composites, respectively. As illustrated in Figure 6(a), for the PEI/BST@SiO2 composites, the Ue reaches ~7.3, ~9.2, ~7.7, ~6.8, ~7.6 and ~3.5 J/cm3 with η of ~76.9%, ~74.6%, ~89.5%, ~88.7%, 80.4% and ~82.3% at electric field of ~420, ~410, ~360, ~310, ~320 and ~230 kV/mm, when the concentration is 0, 0.5, 1, 2, 3 and 4 vol%, respectively. Hence, as the filler concentration increases, the Ue of composite is obviously increased at the same applied electric field. The largest energy storage density (9.2 J/cm3) is obtained at filler concentration of 0.5 vol%, while the corresponding efficiency is only 74.6%. For the PEI/BT@SiO2 composites, the Ue reaches ~7.7, ~7.0, ~7.3, ~6.6 and ~4.2 J/cm3 with η of ~89.3%, ~86.9%, ~87.4%, 84.8% and ~83.4% at electric field of ~400, ~400, ~370, ~330 and ~270 kV/mm, when the concentration is 0.5, 1, 2, 3 and 4 vol%, respectively. It should be noted that the η of all the composites is greater than 80%, which indicates the low energy loss during the charge-discharge 22

process. For the PEI/BT@T-SiO2 composites, the Ue reaches ~7.8, ~11.3, ~13.7, ~2.6 and ~3.4 J/cm3 with η of ~91.8%, ~90.0%, ~81.6%, 93.7% and ~85% at electric field of ~390, ~470, ~500, ~240 and ~300 kV/mm, when the concentration is 0.5, 1, 2, 3 and 4 vol%, respectively. In the present study, breakdown strength from P-E loops in some composites is superior to those obtained from DC breakdown measurement, which may be attributed to the difference of electrode area. The Al electrode diameter (2.5 cm) on both sides of samples in the DC breakdown measurement is larger than that (0.3 cm) in the P-E loop measurements. The higher breakdown strength is always achieved in the dielectric film with smaller electrode area.[74, 75] The composites with 1 and 1.5 vol% fillers attract more attention because former possesses overall energy storage performance (Ue≥11 J/cm3 and η≥90%) and later has largest energy storage density (Ue=13.7 J/cm3) among all the tested samples. The enhanced Ue achieved in the composite above is always attributed the larger dielectric constant and polarization of core part (BST or BT) in “core-shell” structured fillers. The excellent energy storage efficiency is generally obtained in the PEI/BT@T-SiO2 composites can be ascribed to following reasons. Firstly, the oriented arrangement of nanofibers (parallel to the composite surface) in the composites is achieved by means of high speed electrospinning technology. This kind of configuration will be beneficial to suppress the formation of conductive pathway along the direction of electric field in total composite. Secondly, lower intrinsic dielectric loss of polymer matrix is a prerequisite to obtaining the lower energy loss in the corresponding composites. The dielectric loss of pure PEI is far lower than those of ferroelectric polymers, for example PVDF and 23

its copolymers. Thirdly, smaller dielectric constant difference between nanofiber and matrix will reduce the magnitude of local electric field distortion around the interface region. Compared with BST, BT as the core part of nanofibers will bring weak electric field distortion, and thus the lower energy loss. This point can be confirmed by contrasting PEI/BST@SiO2 with PEI/BT@SiO2 composites. Finally, the insulated SiO2 shell in the “core-shell” structured fillers alleviates the carrier concentration at the interface. The thicker the SiO2 shell is, the more obvious the effect of suppressing carries charge is. This point can be verified by comparing the PEI/BT@SiO2 and PEI/BT@T-SiO2 composites. However, the relatively large concentration of filler deteriorates the Eb of composite, so the excessive content of inorganic filler is not conducive to improve the energy storage performance.

24

Figure 6. Energy storage density and efficiency of (a) PEI/BST@SiO2, (b) PEI/BT@SiO2 and (c) PEI/BT@T-SiO2 composites On the whole, Figure 7 demonstrates the comparison for discharged energy density (Ue), breakdown strength (Eb), dielectric constant (ε), charge-discharge efficiency (η), dielectric loss (tan δ) and conductivity (σ) of six composites to evaluate the comprehensive performance for composites. The final result indicates that the composite with an excellent comprehensive energy storage performance appears in composite with a large area of the enclosed pattern. Evidently, the energy storage 25

performance of PEI/BT@T-SiO2 with 1 vol% filler is better than that of PEI/BST@SiO2, due to the higher Ue and lower tan δ as well as σ. It also is superior to PEI/BT@SiO2. Furthermore, the energy storage performance of PEI/BT@T-SiO2 with 2 vol% filler is upper than that of PEI/BST@SiO2 and PEI/BT@SiO2 owing to the higher Ue and Eb. Finally, the effect of BT@T-SiO2 on promoting energy storage performance is most obvious. Both the Ue and η enhancement can be achieved in this kind of composites simultaneously.

Figure 7. Comparison of comprehensive energy storage performance for six composites (PEI/BST@SiO2 1vol%, PEI/BST@SiO2 2vol%, PEI/BT@SiO2 1vol%, PEI/BT@SiO2 2vol%, PEI/BT@T-SiO2 1vol%, and PEI/BT@T-SiO2 2vol%)

Based on the above results and analyses, the finite element simulation for electric field distribution in three composites at the same filler content (about 1.3 vol%) has 26

been carried out by the COMSOL Multiphysics. The detailed information concerning the finite element simulation has been descripted in Supporting Information. The finite element simulation for applied electric field strength (300 kV/mm) in PEI/BST@TiO2, PEI/BT@TiO2, and PEI/BT@T-SiO2 composites is show in Figure 8, which can intuitively show the distribution states of electric field strength in the PEI matrix, SiO2 interface and BST (or BT) nanofibers of composites. The three-dimensional model is on the basis of the composites with oriented nanofibers as shown in Figure 8. The legend with the unit of kV/mm represents the redistributed electric field strength in composite. Meanwhile, Figure 8(a2), (b2) and (c2) are the results of cross-section finite element simulation for three composites. Furthermore, Figure 8(a3), (b3) and (c3) are partial enlarged views of the simulation results of Figure 8(a2), (b2) and (c2). The BST and BT with large dielectric constant have the lower redistributed electric field, and the redistributed electric field on the SiO2 shell is higher, and the redistributed electric field for the polymer matrix is middle-level. The SiO2 shell itself has a great resistance to applied electric field, so the composite can withstand a higher applied electric field. As illustrated in Figure 8(a3), the redistributed electric field on the BST core and SiO2 shell is about 0.17 kV/mm and 1.16×103 kV/mm in the PEI/BST@SiO2 composite. However, the values in the PEI/BT@SiO2 are 1.21 kV/mm and 1.14×103 kV/mm, and in the PEI/BT@T-SiO2 are 1.22 kV/mm and 0.88×103 kV/mm. By comparing Figure 8(a3) with (b3), the conclusions can be revealed that exorbitant dielectric constant of core part will lead to lower redistributed electric field on itself, but higher distorted electric field on the 27

shell layer. In such case, two results will be led to. One is insufficient polarization of ferroelectric BST nanofibers under lower electric field. Assuming that the P-E hysteresis loop of BST is similar to that of BT, their loops are illustrated as shown in Figure 8(d). When the redistributed electric field (0.17 kV/mm) obtained from simulated result is applied on the BST, the corresponding polarization of BST is approximately located at A point. PA is the polarization of fiber when 0.17 kV/mm is applied on fiber. When the redistributed electric field 1.21 kV/mm and 1.22 kV/mm are applied on the BT, the corresponding polarization of BT is approximately located at B Point and C point, respectively. PB and PC are the polarizations of fiber when 1.21 kV/mm and 1.22 kV/mm are applied on fiber. Obviously, PB is greater than PA. Although the dielectric constant of BST is larger than that of BT, the simulated results indicate that the BT than BST can provide greater polarization under the real energy storage circumstance because of the larger redistributed electric field on BT. The another one is increased possibility that breakdown appears firstly in the SiO2 shell. The significantly distorted electric field is concentrated on the SiO2 shell, which will doubtlessly promote the charge carrier transportation in it. This point can be used to explain the inferior insulating properties (higher dielectric loss, AC conductivity and leakage current) in the PEI/BST@SiO2.

28

Figure 8. Finite element simulation of electric field distribution for three kinds of composites: (a) PEI/BST@SiO2, (b) PEI/BT@SiO2, and (c) PEI/BT@T-SiO2. (d) Polarization status of core part in the core-shell fillers

By comparing Figure 8(b3) with (c3), the conclusions can be revealed that thicker SiO2 shell will not only relieve the electric field concentration on itself, but also enhanced the electric field on the core part. Former result will reduce the probability that breakdown occurs firstly in the SiO2 shell. Later will give rise to the 29

sufficient polarization of ferroelectric core part as shown in Figure 8(d) since value of PB is greater than that of PA. It should be mentioned that the final steady-state in the composite is without the consideration of a variable dielectric constant of BST and BT (due to the ferroelectricity of BST and BT) under a changing applied electric field. However, the tendency can be used to design the microstructure of composites (including matrix, interface and filler). The experimental and simulated results suggest following two points: larger dielectric constant of core part doesn’t mean the high polarization it can offer, and thickness of shell part in “core-shell” structured fillers can affect the electric field distribution on core part. For comparison, the energy storage density and efficiency of representative polymer-based dielectrics (including PVDF based composites and all organic polymer) published recently are given in Figure 9. The samples located below the yellow diagonal line always exhibit outstanding Ue, and the ones located above the yellow diagonal line display excellent η. The expected performance of composite should be present in the direction of yellow arrow. The PEI/BT@T-SiO2 composites in this work show a combination of high energy density and high efficiency, which seems to have an advantage over the reported polymer dielectrics.

4. Conclusion Three kinds of PEI based composites with BST@SiO2, BT@SiO2 and BT@T-SiO2 fillers were fabricated via high speed electrospinning technology and hot-pressing method. In the aspects of comprehensive energy storage performance (density and efficiency), PEI/BT@T-SiO2 composites are superior to PEI/BST@SiO2 30

and PEI/BT@SiO2 composites. The most comprehensive energy storage performance is discovered in the PEI/BT@T-SiO2 with 1 vol% filler in which η is 90% and Ue is 11.3 J/cm3. The highest Ue of 13.7 J/cm3 is achieved in the PEI/BT@T-SiO2 with 2 vol% filler. In addition to the experiments, the finite element analysis is used to investigate the electric field distribution in three kinds of composites with various “core-shell” structured fillers. On the basis of experimental and simulated results, two points can be suggested that larger dielectric constant of “core” in core-shell structured fillers doesn’t equal to high polarization, and thickness of “shell” in core-shell structured fillers can obviously impact the electric field distribution on “core” part. This work develops a new energy storage dielectric system-PEI for the matrix chosen, and offers a design route for the “core-shell” structured filler which can enhance the energy storage density of composite without sacrificing the efficiency.

Figure 9. Comparison of previous published data and our work (1#,[7] 2#,[19] 3#,[9] 4#,[18] 5#,[28] 6#,[29] 7#,[30] 8#,[26] 9#,[10] 10#,[32] 11#,[34] 12#,[76]

31

13#,[12] 14#,[46] 15#,[77] 16#,[9] 17#,[78] 18#,[34] 19#,[35] and 20#[79])

Acknowledgement This research was funded by National Natural Science Foundation of China (No. 51807041, 51977050 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).

Conflict of interest The authors declare no conflict of interest.

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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Figure Captions Scheme 1. Preparation scheme for PEI composites with oriented nanofibers by means of high speed electrospinning technology and hot-pressing method Figure 1. SEM images of (a1) BT and (b1) BST nanofibers. Element distributions of (a2) Ba, (a3) Ti, (a4) O in Figure 1(a1). Element distributions of (b2) Ba, (b3) Sr, (b4) O in Figure 1(b1). TEM images of (c) BT@SiO2 and (d) BT@T-SiO2 Figure 2. (a) XRD pattern of BT and BST, inset is the enlarged image of XRD pattern in the 2θ of 30-33°. (b) FTIR spectra of BST, BT, BST@SiO2, BT@SiO2 and BT@T-SiO2 Figure 3. Cross-section SEM images of (a1) PEI/1% BST@SiO2, (b)PEI/1% BT@SiO2 and (c) PEI/1% BT@T-SiO2. Element distributions of (a2) Ba, (a3) Sr, (a4) Ti, (a5) Si, (a6) O and (a7) N in Figure 3(a1). XRD pattern (d) and FTIR spectra (e) of PEI/BST@SiO2, PEI/BT@SiO2, and PEI/BT@T-SiO2 composites Figure 4. Dielectric constant, dielectric loss and AC conductivity of PEI/BT@T-SiO2, PEI/BST@SiO2 and PEI/BT@SiO2 composites with various filler concentrations at 100 Hz (a, b, c) and 1 Hz (d, e, f) Figure 5. Leakage current and breakdown strength of (a) PEI/BST@SiO2, (b) PEI/BT@SiO2 and (c) PEI/BT@T-SiO2 composites Figure 6. Energy storage density and efficiency of (a) PEI/BST@SiO2, (b) PEI/BT@SiO2 and (c) PEI/BT@T-SiO2 composites Figure 7. Comparison of comprehensive energy storage performance for six composites (PEI/BST@SiO2 1vol%, PEI/BST@SiO2 2vol%, PEI/BT@SiO2 1vol%, PEI/BT@SiO2 2vol%, PEI/BT@T-SiO2 1vol%, and PEI/BT@T-SiO2 2vol%) Figure 8. Finite element simulation of electric field distribution for three kinds of composites: (a) PEI/BST@SiO2, (b) PEI/BT@SiO2, and (c) PEI/BT@T-SiO2. (d) Polarization status of core part in the core-shell fillers Figure 9. Comparison of previous published data and our work 38

Highlights


A new energy-storage dielectric system—polyetherimide (PEI) composite is developed.




The orientation of core-shell structured nanofibers is achieved in the composites.




Efficiency of 90% and density of 11.3 J/cm3 are simultaneously obtained.


Core and shell structures in nanofibers will impact the performance of composites.

Conflict of Interest Form No conflict of interest exists in the submission of “Ultrahigh discharge efficiency and excellent energy density in oriented core-shell nanofiber-polyetherimide composites”, 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