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Multilayered ferroelectric polymer films incorporating low-dielectric-constant components for concurrent enhancement of energy density and charge–discharge efficiency
Author’s Accepted Manuscript Multilayered Ferroelectric Polymer Films Incorporating Low-Dielectric-Constant Components for Concurrent Enhancement of Energy Density and Charge–Discharge Efficiency Jie Chen, Yifei Wang, Qibin Yuan, Xinwei Xu, Yujuan Niu, Qing Wang, Hong Wang www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(18)30745-6 https://doi.org/10.1016/j.nanoen.2018.10.028 NANOEN3106
To appear in: Nano Energy Received date: 26 July 2018 Revised date: 27 September 2018 Accepted date: 13 October 2018 Cite this article as: Jie Chen, Yifei Wang, Qibin Yuan, Xinwei Xu, Yujuan Niu, Qing Wang and Hong Wang, Multilayered Ferroelectric Polymer Films Incorporating Low-Dielectric-Constant Components for Concurrent Enhancement of Energy Density and Charge–Discharge Efficiency, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.10.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Multilayered Ferroelectric Polymer Films Incorporating Low-Dielectric-Constant Components for Concurrent Enhancement of Energy Density and Charge–Discharge Efficiency
Jie Chena, Yifei Wanga, Qibin Yuana, Xinwei Xua, Yujuan Niub, Qing Wangc, Hong Wanga,b,* a
State Key Laboratory for Mechanical Behavior of Materials & School of Electronic and
Information Engineering, Xi’an Jiaotong University, Xi’an, 710049, China b
Department of Materials Science and Engineering, Southern University of Science and
Technology, Shen Zhen 518055, China c
Department of Materials Science and Engineering, The Pennsylvania State University,
University Park, Pennsylvania, 16802, USA
*
[email protected]
ABSTRACT Ferroelectric polymers are the materials of choice for capacitive energy storage owing to their highest dielectric constants (K) and the best energy densities among the current dielectric polymers. Herein, different from the conventional approaches based on the incorporation of high-K fillers into the single-layer films to enhance the capacitive performance, a low-K polymer, i.e. PMMA with a K value of 3–4, is selected as an example and introduced into the layered configurations of the ferroelectric polymer. Both improvements in the energy density (Ue)
and charge–discharge efficiency (η) over those of the pristine polymer have been achieved via the establishment of multiple interlaminar interfaces and modulation of component ratios. The influence of film configuration on the capacitive performance has been systematically studied. The trilayered all-polymer film with optimized component ratio is capable of operating with a charge–discharge efficiency as high as 84% and concurrently delivering an energy density up to 20.3 J cm−3, surpassing the capacitive performance of the currently available polymer dielectrics that present the upper limits of Ue of ~20 J cm-3 and η of ~80%. Along with excellent stability of dielectric and mechanical properties of the polymer films, this work suggests great potential of the multicomponent ferroelectric polymers with layered architecture for electrical energy storage applications.
Graphical Abstract
Keywords: ferroelectric polymers; multilayer films; dielectrics; electrical energy storage; power density
1. Introduction Electrostatic capacitors with polymer dielectrics as storage media of electric charges are critical components in advanced electronics and electrical power systems, such as hybrid electric vehicles, medical defibrillators and power regulations in smart grid, due to their capability of delivering the highest power density (on the order of million watt per cubic centimeter), low cost, lightweight, and scalability [1-4]. Yet, most dielectric polymers possess relatively low energy densities (Ue) owing to their comparatively low dielectric constants (i.e. K ≤ 4 at 1 kHz) and small electric displacement (D), which seriously hinder their applications in next-generation power systems [5-8]. For instance, biaxially oriented polypropylene (BOPP) films, the state-ofthe-art commercially available dielectric polymer, only has a K of ~2.2 and a D of <1.2 μC cm−2; these limit the maximum Ue of ~2.5 J cm−3 obtained at an electric field of 500 MV m-1, which is at least an order of magnitude lower than their electrochemical counterparts such as batteries, electrochemical capacitors, with Ue of ~20 J cm−3. Exceptionally, the ferroelectric polymers represented by poly(vinylidene fluoride) (PVDF) and its co- and ter-polymers possess highly polar C–F bonds and spontaneous orientation of dipoles in the crystalline phases, resulting in higher K values of 10~12 at 1 kHz and higher D of 9~10 μC cm−2 than other classes of dielectric polymers [9-14]. Consequently, the ferroelectric polymers could potentially endow an order of magnitude enhancement in Ue (e.g. ~10 J cm−3). The most popular route to further raise Ue of the ferroelectric polymers is the introduction of high-K inorganic ceramic nanofillers, such as titanium oxide (TO) [12, 13], barium titanate (BT) [14-18], barium strontium titanate (BST) [19, 20], into the polymer matrix to form polymer nanocomposites. However, PVDF exhibits the bulk conductivity (i.e. ~10-9 S m-1) that is at least
eight orders of magnitude higher than that of benchmark BOPP (i.e. ~10-17 S m-1) [17,18], which significantly reduces its charge–discharge efficiency (<70%) at high electric fields (>300 MV m1
) [19-26]. Besides, the drawback of the polymer nanocomposite approach is that the utilization
of inorganic fillers with high K always confers to even higher bulk conductivity, giving rise to an abrupt rise in conductive loss of the resulting polymer nanocomposites and sharply decreased the charge–discharge efficiency under high electric fields, especially at high filler concentrations (e.g. ~50%). For instance, when Ue is enhanced to over 20 J cm−3, which is the state-of-the-art value of discharged energy density, the charge–discharge efficiency is reduced to less than 60% [24]. From the practical application viewpoint, low charge–discharge efficiency denotes the presence of large conduction loss, the dominant energy loss mechanism at high fields, which causes Ohmic (Joule) heating to induce thermal runaway and degrade the lifetime of devices during continuous operations [21-22]. Therefore, it is important that not only the energy density but also the charge–discharge efficiency should be evaluated for dielectric materials. In addition to the capacitive energy-storage capability, the stability of the performance and the structural integrity over charging-discharging cycling and mechanical bending tests are other crucial criteria in the evaluation of energy storage devices, e.g. wound capacitors, during the continuous operation [1]. To reduce the conduction loss and improve the charge–discharge efficiency of PVDF, a variety of low-K polymers, such as polyamide (PA) [27], polyimide (PI) [28], polypropylene (PP) [29,30], polystyrene (PS) [31], and poly(methyl methacrylate) (PMMA) [32-34], have been added into PVDF to form polymer blends. In particular, PMMA is fully compatible with PVDF, resulting in the blends with much reduced conduction loss and improved charge–discharge efficiency under the high electric fields [35]. Nonetheless, in order to achieve large enhancement
in the charge–discharge efficiency (i.e. >80%), it is required to add high loadings of PMMA (e.g. >40 vol.%), which in turn substantially deteriorates the K, and consequently, Ue of the resulting blends. For example, the highest Ue achieved under a charge–discharge efficiency of 80% is only 10.5 J cm−3 for the PVDF-based polymer blends [35]. Therefore, it is urgently needed to develop completely new approaches in order to achieve high energy densities along with outstanding charge–discharge efficiencies simultaneously via utilizing complementary features of highly insulating PMMA and highly polar ferroelectric polymer. Recently, the topologically-structured composites including the insulating layer to avoid early breakdown under high electric fields due to the blocking effect of the interfaces between the layers have been used in the field of electrical insulation [36-38]. Compared with the layered films with the same composition, the conventional blend films always exhibit shorter dielectric lifetime and lower dielectric strength [39]. In the blend films, the interfacial areas are random relative to the direction of the applied electric field, which could be susceptible to dielectric breakdown [40]. By contrast, with the interfacial areas that are all perpendicular to the direction of the applied electric field in the layered films, the insulating layers are effective in deflecting the hot electron propagation and impeding the breakdown across the film thickness, leading to higher breakdown strength of the layered films [41]. Herein, we describe the development of the multilayered dielectric polymer films consisting of highly insulating PMMA and ferroelectric copolymer poly(vinylidene fluoride-cohexafluoropropene) P(VDF-HFP) via simple solution-casting process, which are capable of balancing the contradicted parameters including dielectric loss, energy density, charge–discharge efficiency, power density and mechanical reliability to exhibit the capacitive performance exceeding those reported for the currently available polymer dielectrics. It is found that,
compared with the single layer blend film with an energy density of 17.5 J cm −3 and a chargedischarge efficiency of 72% measured at 440 MV/m, the trilayered architecture film with intimate multiple interfaces has been proven to be effective in reduction of high-field conduction loss and leakage current over more than on order of magnitude, thus giving rise to a much improved discharged efficiency of 84%) and higher energy density of 20.3 J cm −3. Superior energy storage properties are achieved via synergic integrations of the multiple components in the layered structures and deliberately adjusting of the interfaces and the relative ratios of the constituent layers. Furthermore, excellent cycling stability and mechanical reliability are demonstrated over the rigorous cycling tests in the trilayered all-polymer films. Therefore, it is anticipated that our work will open a new pathway to the flexible all-polymer dielectrics with impressive capacitive energy storage capability and great reliability. 2. Experimental methods 2.1. Fabrication of the trilayered all-polymer films Compared to PVDF, P(VDF-HFP) shows lower dielectric loss and higher breakdown strength and is selected as the high-K component of the layered structures. The influence of different molecular weights on the dielectric and energy storage properties of P(VDF-HFP) films have been studied. Considering the comprehensive dielectric and energy storage performance (Fig. S1 and Fig. 3), the intermediate-molecular-weight P(VDF-HFP) film (Mw=530,000) are used in this study. For the fabrication of trilayered all-polymer films, 1g P(VDF-HFP) (Arkema, molecular weight of 530,000) and 2g PMMA were respectively dissolved in 10mL N, Ndimethylformamide (DMF, Letai Co., China) by stirring for 4 h to form a homogeneous and stable suspension. Subsequently, the P(VDF-HFP) suspension was cast onto the glass plate (20cm×20cm) by a laboratory casting equipment (MSK-AFA-L800, Hefei Ke Jing Materials
Technology Co., Ltd.) at a casting rate of 25mm/s and dried as an outer layer film after evaporating solvent in vacuum at 80 ℃ for 1h. The inner layer of the PMMA suspension was then casted onto the outer layer at a casting rate of 25mm/s. Finally, the P(VDF-HFP) suspension was cast onto the PMMA layer as the outer layer at a casting rate of 25mm/s. The trilayered allpolymer films were dried at 60 °C for 10 h in vacuum for complete evaporation of solvents. The dried films were further heated under vacuum at 200 °C for 5 min and then immediately quenched in ice water to obtain the trilayerd all-polymer films. The final quenched films were dried at 100 °C overnight to complete remove residual water and peeled off the glass substrates. Quenching in ice-water resulted in the decrease of crystallinity from 29.98% to 16.53% (Fig. S2). The thickness of each layer in the trilayered films can be tuned by controlling the solution concentration and scraper height. The volume fractions of P(VDF-HFP) to PMMA were 100: 0, 90: 10, 70: 30, 50: 50, 30: 70, 10: 90 and 0: 100. Meanwhile, the single layer film of P(VDFHFP)/PMMA blends and bilayered films were also prepared with the same volume ratio for comparison. The thickness of all dielectric films, including blends and bilayered films, was carefully controlled to around 20 μm (Fig. S4). The cross-sections of the films were obtained by freeze fracture in liquid nitrogen and the morphologies were observed by scanning electron microscopy (SEM, Quanta 250FEG, FEI, Ltd.). 2.2. Characterization The crystal structures were characterized by an X-ray diffraction meter (XRD, Bruker D8Advance) with the X-ray wavelength of 1.542 Å (Cu Kα radiation, 40 kV, and 100 mA), the 2θ diffraction angle from 10° to 60°, the rate of 15°/min, and the step of 0.02°. All data were analyzed using Jade software with a Gaussian-Lorentz superposition fitting function. For the electrical measurements, gold electrodes of a typical thickness of 100 nm and diameter of 2 mm
were sputtered on both sides of the polymer films by the auto fine coater (JFC-1600, JEOL, LTD.). Dielectric properties were measured using an impedance analyzer (4990A, Agilent Technologies, Inc.) in a frequency range of 1 kHz to 50 MHz. The DC electric resistivities of the films were collected using a High-Voltage Test System (PolyK Technologies, State College, PA, USA). The electric displacement-electric field loops and leakage current of the film samples were measured by a ferroelectric polarization tester (Radiant Technologies, Inc.) with a limited current of 0.19 mA at room temperature and 10 Hz. The fast discharge tests were performed using a PK-CPR1502 test system (PolyK Technologies). Trek 10/10B high voltage amplifier was used to charge the capacitor sample soaked in silicone dielectric fluid and the typical charging time was ∼1 s. Then the charged capacitor was discharged to a high voltage noninductive resistor through a high-speed metal-oxide-semiconductor field-effect transistor switch. The resistance of the load resistor (RL) was selected as 100 kΩ to match the capacitance of the sample and achieve certain discharge RC time constant. The charge–discharge cycle was controlled by a LabVIEW program. The load resistor, 𝑅𝐿, and the applied field were 100 kΩ and 150 MV/m, respectively. At least 20 samples were measured for each test. A computer aided homemade bending test system was used for bending tests (Fig. S9). 3. Results and Discussion 3.1. Preparation and Morphology Characterization We fabricated a series of the trilayered all-polymer films using the layer-by-layer solution casting method. Solution casting technique is straightforward, versatile, and cost-effective when compared to the multistep extrusion blowing and zone-heated uniaxially stretching process [42]. Fig. 1(a) shows a schematic illustration of the fabrication of the trilayered polymer films. P(VDF-HFP) and PMMA were dissolved into DMF separately to prepare the respective
solutions. Then, the solutions were poured onto a smooth glass substrate in sequence to obtain uniform trilayer films, which was placed into a vacuum oven to accelerate the evaporation of solvent. Fig. 1(c) shows a photograph of an as-prepared freestanding trilayered polymer film. The trilayer configuration with P(VDF-HFP) as the outer layer and PMMA as the center layer has been characterized by cross-section scanning electron microscopy (SEM), as shown in Fig. 1(b). High-magnification cross-section SEM shown in Fig. 1(d)–(f) suggests the intimate interfaces between adjacent layers without noticeable defects at a scale of 0.1 mm. This is apparently attributed to the chemical compatibility between amorphous PMMA and semicrystalline P(VDF-HFP) [6]. The thickness of the component layer has been further confirmed by high-magnification cross-sectional SEM. The total thickness of all the samples is fixed to 20±1 μm by simply controlling the solution concentration and the scraper height. The thickness of the middle layer increases gradually from 2.4 μm to 5.9 μm and 10.5 μm. The ratio of the thickness of the middle layer to the total thickness was calculated to determine the volume fraction of PMMA. The resulting films are thus termed as 10 vol.%, 30 vol.%, 50 vol.%, 70 vol.% and 90 vol.% according to the volume fraction of PMMA, respectively. 3.2. Dielectric and Electrical Insulation Characteristics Since the polarization and relaxation of electrical charges and dipoles of dielectrics are dependent on time, dielectric constant and dielectric loss are generally variable with respect to frequency of electric field [24]. The dielectric properties of the trilayer-structured all-polymer films with varied volume fractions of PMMA are presented in Fig. 2(a) and (b). As revealed in the dielectric spectra in the frequency range from 1 kHz to 20 MHz, all the trilayered all-polymer films display the distinct weak-field dielectric properties of neat P(VDF-HFP). The K value of the trilayered all-polymer films decreases monotonically with the increase of PMMA volume
fraction because of a lower K (i.e., 3–4) of PMMA relative to that of P(VDF-HFP) (i.e. ~10 at 1 kHz). Concurrently, the dielectric loss peak attributable to the dielectric relaxation in the ferroelectric polymer is observed at about 10 MHz, which is gradually weakened with increasing the PMMA volume fraction [19]. As a result, a much lower dielectric loss (~0.03) has been achieved in the 90 vol.% film when compared to that of pure P(VDF-HFP) film (~0.1 at 100 kHz). It is worth pointing out that low dielectric loss reduces the chances of self-heating and thermal runaway of dielectrics, which is crucial to the films for capacitor applications [24]. In addition to weak-field (0.025 MV/m) dielectric loss, the direct current (DC) electrical resistivity at varied electric fields is of importance in the judgement of the electrical insulation characteristics of dielectrics [1]. Consistent with the trends of weak-field dielectric loss, as presented in Fig. 2(c), it is found that the trilayered all-polymer films with different volume fractions of PMMA show the direct current (DC) electrical resistivity that is one order of magnitude higher than that of pristine P(VDF-HFP) films, e.g., 2.02 × 1012 for the 10 vol.% film and 6.9 × 1012 for the 90 vol.% film vs. 2.6 × 1011 Ω m for P(VDF-HFP) films masured at an applied field of 50 MV m−1. DC electrical resistivities of the layered films with different volume fractions of PMMA are at least one order of magnitude higher than that of pristine P(VDF-HFP) films, e.g., 3.02 × 1011 Ω m of the 10 vol.% film and 4.2 × 1012 Ω m of the 90 vol.% film vs. 8.4 × 109 Ω m of P(VDF-HFP) films measured at an applied field of 150 MV/m. As a result, much depressed dielectric loss and greatly improved electrical insulation characteristic can be achieved by tuning the thickness of the component layers. 3.3. Capacitive Energy-storage Capability The electrical displacement–electrical field (D–E) loops of the trilayer films are measured with a 10 Hz unipolar triangle signal (Fig. 3). Widening (or opening) of D–E loops depicts a
deviation from the linear behavior of the electrical displacement versus the applied field and corresponds to the energy loss (space-charge, conduction, etc.) that is indicated by the enclosed area between the charge and discharge curves. It is evident that the introduction of PMMA into P(VDF-HFP) drastically narrows the D–E loops, indicative of reduced high-field energy loss with thicker layers of PMMA. Fig. S6(b) summarizes the energy loss of the trilayered allpolymer films with varied PMMA volume fractions along with pristine P(VDF-HFP) film measured at varied applied electric fields. For example, at an applied field of 440 MV m−1, the pristine P(VDF-HFP) exhibits an energy loss of 10.4 J cm−3, whereas the losses of the trilayered films inserted with 10 vol.% and 30 vol.% PMMA are only 5.3 and 3.9 J cm−3, respectively. This can be attributed to improved electrical resistivity as a result of the presence of PMMA in the layered structures. The capacitive energy-storage capability of the films was evaluated from the D–E loops by a modified Sawyer-Tower circuit, where the stored energy density (U) is derived from the D–E loop by integration of the area between the charge curve and the ordinate, and the discharged energy density (Ue) is determined by the area between the discharge curve and the ordinate (Fig. S5). We found that the improvement in electrical resistivity and corresponding decrease in energy loss results in substantial enhancements in discharged energy density and charge– discharge efficiency (η= Ue/U) of the trilayered all-polymer films simultaneously [46]. Fig. 4(a) compares the charge–discharge efficiencies of the trilayered all-polymer films with that of P(VDF-HFP). As expected, much higher charge-discharge efficiencies are delivered in all the trilayered films compared to P(VDF-HFP). For instance, all-polymer trilayered films exhibit higher charge–discharge efficiencies over 85% in comparison to 65% of pure P(VDF-HFP) at 300 MV m−1. Even at 440 MV m−1, the layered polymer films still retain an efficiency of 80%,
while the efficiency drops to 60% for P(VDF-HFP). It is well known that low charge–discharge efficiency generates vast waste heat and inevitably reduces the service lifetime of film capacitors. The discharged energy densities are summarized in Fig. 4(b), in which marked improvements are manifest with the trilayered all-polymer films with 10 vol.% ~ 70 vol.% PMMA. For instance, at 440 MV m−1, the discharged energy densities of 10 vol.% and 30 vol.% films are 20.7 and 20.3 J cm−3, respectively, corresponding to an ~30% enhancement over P(VDF-HFP) (16 J cm−3), with their charge–discharge efficiencies >80%. We further compare the discharged energy density and the charge–discharge efficiency of the trilayered all-polymer films with those of the representative dielectric polymers and nanocomposites, are presented in Fig. 5. Evidently, the trilayered all-polymer film with the optimized component ratio (30 vol.% PMMA) exhibits a Ue ~20.3 J cm−3 and a η ~84%, surpassing the overall capacitive performance of the currently available polymer dielectrics which have the upper limits of Ue of ~20 J cm-3 and η of ~80%. Specifically, a very high η of 91% with a Ue of 16.2 J cm−3 has been achieved in the trilayered film with 70 vol.% PMMA at 400 MV m-1, indicating an enormous ~700% enhancement of energy density over the commercial benchmark BOPP (~2 J cm−3), while carries a η comparable to BOPP (96%) [9]. In addition, some researchers have deposited ferroelectric thin films on commercial flexible polymeric Kapton films via hydrothermal process that exhibits high capacitance densities and acceptable dielectric loss [50]. However, layer-by-layer solution casting technique is straightforward, time saving and cost-effective compared to the deposition polycrystalline thin films on commercial flexible polymeric films. The large improvements of the capacitive energy storage performance in comparison to neat ferroelectric polymer could be attributed to the synergetic optimization of slightly decreased maximum displacement and sharply suppressed remnant displacement by tailoring the
component ratios. As plotted in Fig. 4(c) and (d), at 100 MV m-1, the maximum displacement decreases mildly from ∼2.4 μC cm-2 of P(VDF-HFP) to ∼2.1 μC cm-2 of 70 vol.%, while the remnant displacement substantially decreases from ∼0.27 μC cm-2 to ∼0.04 μC cm-2 of 70 vol.%. Especially, this trend still exists even at an applied electric field of 400 MV m-1. During the charging process, the maximum displacement offers a high charging energy density under the applied voltages, whereas the large reduction in remnant displacement during discharging confers low loss and thus gives rise to large enhancements in both discharged energy densities and the charge-discharge efficiency. To in-depth rationalize the observed excellent energy storage performance of the trilayered all-polymer films, different film configurations including bilayered and single-layered blends films with the same volume fraction of PMMA (i.e. 30 vol.%) were prepared by solution casting for the purpose of comparison. As corroborated in the high-magnification cross-section SEM images of Fig. 5 (a)-(c), the total thickness of bilayered and single-layered blend film is equal to that of the trilayered film (20±1 μm) via precisely controlling the height of the scraper. As exemplified in Fig. S8(a) and (b), it is evident that the film configurations have a marginal effect on the weak-field dielectric constants of the films. On the other hand, the dielectric loss of the layered configurations films is substantially lower than that of single-layered blends. More striking results are observed by comparing their D-E loops, which become slimmer with the increase of the interface number (Fig. S9), indicative of reduced high-field energy loss with more layered interfaces, as shown in Fig. S10(b). To further investigate the energy loss mechanisms in different configurations, the conductive loss is deduced from the D–E loops with the assumption that the measured displacement at zero field (DE=0) comes mainly from the leakage current and plays a dominant role in the high-field dielectric loss of dielectric films [36]. Fig. S10(c)
represents the conductive loss of the films with different configurations at varied electric fields. Obviously, the tri-layered architecture film shows the lowest conductive loss (< 7%) in comparison to other architecture films. We further compare the discharged energy density and the charge–discharge efficiency of different film configurations as a function of applied electric field (Fig. 5(d) and (e)). As expected, the trilayered all-polymer film possess the highest discharged energy density of 20.3 J cm−3 and the best charge–discharge efficiency of 84% in comparison to the single-layer blend (17.5 J cm−3 ＆ 72%) and the bilayered film (17.5 J cm−3 ＆ 77%) measured at 440 MV m−1. The trilayered all-polymer films exhibits much higher discharged energy density and charge– discharge efficiency than those of the representative single-layer blend polymers, as presented in Fig. S7. Higher energy storage performance in the trilayered all-polymer film than those of other configuration films could be attributed to the decreased remnant displacement at various electric fields (Fig. 5(f)), while maintaining a similar maximum displacement (Fig. S10(a)). For instance, at 440 MV m−1, the trilayered all-polymer films possess the lowest remnant displacement of ~0.71 μC cm−2 in comparison to ∼1.16 μC cm−2 of the bilayered film and ∼1.35 μC cm−2 of the single-layered blend. The important role of interfaces in suppression of remnant displacement has also been verified in the DC electric resistivity (Fig. S10(d)) and the leakage current-electric field (I-V) measurements (Fig. 5(g)). For instance, the leakage current density (5.45 × 10-7 A/cm2) of the trilayered film is much lower than those of the single-layered blend (3.42 × 10-6 A/cm2) and the bilayered film (1.68 × 10-6 A/cm2) as well as P(VDF-HFP) (7.38 × 10-6 A/cm2) at 200 MV/m as summarized in Table S1. The highest electric resistivity and the lowest leakage current density of the trilayered all-polymer film are endowed when compared to the other film configurations with fewer interfaces. The results indicate that the establishment of multiple
interlaminar interfaces can efficiently inhibit conductance loss, thus simultaneously improving discharge energy densities and the corresponding charge-discharge efficiencies. 3.4. Fast-discharge Capability and Mechanical Reliability In addition to the capacitive energy-storage capability, the fast-discharge capability (power density) is another crucial criterion in the evaluation of the performance of energy storage devices. To demonstrate the ultra-fast discharge rate of the trilayered all-polymer films and their high power densities, a typical fast-discharge experiment was carried out on both the trilayered film and BOPP at an identical RC time constant (Fig. S11), in which samples are charged at 150 MV m-1 followed by discharging across a 100 kΩ resistor. The experimental discharge time, τ0.9, is defined as the time to discharge 90% of the energy into the load (Fig. S12(a). As expected, a superior power density of 0.13 MW cm-3 with a faster discharge time τ0.9 of 11.2 μs has been obtained in the trilayered all-polymer film, which is more than six times that of BOPP, i.e. 0.02 MW cm-3 with a τ0.9 of 17.2 μs, in addition to superior energy densities (1.53 J cm−3 versus 0.34 J cm−3). Moreover, the cyclic fast discharge experiments have been carried out at an applied field of 150 MV m−1 to examine the stability of the trilayered all-polymer films (Fig. S13). Remarkably, as shown in Fig. S12(b), no sign of degradation in the discharged energy density has been detected in the trilayered all-polymer films over a straight 50 000 cycles of charge– discharge (i.e., a 55 h continuous operation), which is of great importance for the long term operation of electronic devices. To further confirm the stability of the layered structure, rigorous mechanical bending tests (Fig. S14(a)) have been conducted on the trilayered all-polymer films at a radius of curvature (ROC) of 5 mm, as presented in Fig. 7(a). Significantly, noticeable degradation in the discharged
energy density and the charge–discharge efficiency has not been detected in the trilayered allpolymer films over consecutive 20,000 bending cycles (Fig. 7(b) and S14(b)). We have verified the structural integrity after rigorous bending cycles. As shown in cross-section SEM (Fig. S14(c)), no detectable cracks or flaws in the interfaces of adjacent layers have been found. Moreover, we have studied the chemical stability before and after cycling and the effect of moisture on the polymer. The diffraction peaks at 18.4° and 20.2° are assigned to α (020) and α (021), respectively; and the diffraction peaks at 26.8° is ascribed to γ (022) [47-48]. As shown in the XRD patterns (Fig. S15), no shift in the diffraction peaks has been found in the trilayered allpolymer films after cycling and exposure to moisture due to the hydrophobicity of the polymer. These results suggest good chemical stability of the layered polymer films. The excellent dielectric and mechanical stability and reliability might be attributed to the following factors: (i) the all-organic architecture is intrinsic mechanically and electrical stable, which benefits the long-time cycling stability; (ii) high efficiency means less waste heat, better reliability and longer cycling lifetime; and (iii) a strong interfacial force between the adjacent PMMA/P(VDF-HFP) layers. 4. Conclusions In summary, as a result of synergic contributions of the presence of the multiple interfaces in the layered structures and control of the relative thickness of the component layer, much depressed dielectric loss and greatly improved electrical insulation have been achieved in the trilayered polymer films. Consequently, the layered polymer films exhibit remarkably enhanced capacitive energy storage performance, exceeding those reported for the current polymer dielectrics with the upper limits of Ue of ~20 J cm-3 and η of ~80%. The uniqueness of the layered structures in the realization of concurrent improvements in energy density and the
charge-discharge efficiency, which typically contradict to each other, has been clearly demonstrated in the comparison with other film configurations such as single layer and bilayered films. In addition, the trilayered all-polymer films possess excellent cycling stability and mechanical reliability as shown in consecutive and rigorous cycling tests. Along with the simplicity and scalability of the solution-based processing, this work opens a facile route to boost the performance of ferroelectric polymers for electrical energy storage devices. Acknowledgements The work was supported by National 973 projects of China (No. 2015CB654603) and the National Natural Science Foundation of China (No. 61471290, 61631166004). Notes The authors declare no conflict of interest. Appendix A. Supporting Information Supplementary data associated with this article can be found in the online version at …
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Jie Chen is currently a Ph.D. candidate in electronic science and technology in Xi’an Jiaotong University. He received the B.S. and M.S. in materials science and Engineering from the Xi’an University of Architecture and Technology and Guilin University of Technology in 2012 and 2015, respectively. His research interest is on the polymer dielectrics with high energy storage density.
Yifei Wang is a Ph.D. candidate of electronic science and technology at Xi’an Jiaotong University. He received his B.S. degree in electronic science and technology from Xi’an Jiaotong University, Xi’an, China, in 2013. His research focuses on polymer nanocomposites for dielectric energy storage applications.
Qibin Yuan received the B.S. degree in materials physics and the M. E. degree in materials engineering from the Shaanxi University of Science and Technology, China in 2009 and 2013, respectively. He is currently a doctoral candidate in electronic science and technology in Xi’an Jiaotong University Xi’an, China. His research interest is focused on high energy storage density ceramics and devices.
Xinwei Xu is a Ph.D. candidate of electronic science and technology at Xi’an Jiaotong University. He received his B.S. degree in electronic science and technology from Xi’an Jiaotong University, Xi’an, China, in 2015. His research focuses on high-temperature dielectric energy storage applications.
Yujuan Niu is a Research Associate Professor of Academy for Advanced Interdisciplinary Studies, South University of Science and Technology of China. She received the Ph.D. degree in Electronic Science and Technology from Xi’an Jiaotong University, and then as a postdoctoral worked at Xi’an Jiaotong University, Xi’an, China. Her research interests are focused on polymer matrix composites materials for energy storage and surface modification for composites.
Qing Wang is Professor of Materials Science and Engineering at The Pennsylvania State University, University Park, PA, USA. He received his Ph.D. in 2000 at University of Chicago. Prior to joining the faculty at Penn State in 2002, he was a postdoctoral fellow at Cornell University. His research programs are centered on using chemical and material engineering approaches towards the development of novel functional polymers and polymer nanocomposites with unique dielectric, electronic and transport properties for applications in energy harvesting and storage. Hong Wang is a chair professor of Southern University of Science and Technology, Shenzhen, China. Her main research interests include dielectric materials, ceramic-polymer composites, and dielectric measurements for applications in passive integration and electronic devices. She has authored and co-authored more than 220 peer-reviewed papers and 2 book chapters. She holds 27 Chinese patents and 1 U.S. patent and has presented over 40 invited talks in international academic conferences. She is a senior member of IEEE, the chair of the Executive committee of the Asian Electroceramic Association (AECA), and a member of IEEE UFFC society’s Ferroelectric committee
Fig. 1. (a) Schematic illustration of the trilayered all-polymer films. (b) SEM image of cross section of the trilayered all-polymer film with 30 vol.% PMMA. (c) Optical image of the freestanding trilayered all-polymer film with 30 vol.% PMMA. (d)-(f) High-magnification SEM image of cross section of the trilayered all-polymer films with 10 vol.%, 30 vol.%, 50 vol.% PMMA films, respectively.
Fig. 2. Frequency-dependent (a) dielectric constant and (b) dielectric loss of the trilayered allpolymer films with different PMMA volume fractions. Insets show the (a) dielectric constant and (b) dielectric loss of the trilayered all-polymer films at 100 kHz as a function of PMMA content. (c) DC electric resistivity of the trilayered all-polymer films with different volume fractions of PMMA at varied electric fields.
Fig. 3. D-E loops at varied electric fields of (a) pristine P(VDF-HFP) and (b-g) trilayered allpolymer films with different PMMA volume fractions.
Fig. 4. (a) Charge–discharge efficiency, b) discharged energy density, (c) maximum displacement and (d) remnant displacement of pristine P(VDF-HFP) and the trilayered allpolymer films with different PMMA volume fractions as a function of the applied field.
Fig. 5. Comparison of the discharged energy density and the charge-discharge efficiency of the trilayered all-polymer films with 30 vol.% and 70 vol.% PMMA films with other representative dielectric polymers.
Fig. 6. Cross-section SEM images of (a) single-layered blend film, (b) bilayered film, and (c) trilayered film. (d) Discharged energy density, (e) the charge–discharge efficiency, (f) the remnant displacement and (g) leakage current density of P(VDF-HFP) and different configuration polymer films as a function of electric field.
Fig. 7. (a) The schematic and digital photographs of the bending test and (b) energy storage capability of the trilayered all-polymer films with 30 vol.% PMMA before and after 20000 cycles of bending.
Highlights:
1.
Multilayered ferroelectric polymer films with excellent comprehensive energy storage performance have been achieved via layer by layer solution-casting process.
2.
The obtained excellent cycling stability and mechanical reliability is of great importance for the long term operation of flexible electronic devices.
3.
The uniqueness of the layered structures in the realization of comprehensive energy storage performance has been demonstrated in the comparison with other film configurations such as single layer and bilayered films.
PII: DOI: Reference:
S2211-2855(18)30745-6 https://doi.org/10.1016/j.nanoen.2018.10.028 NANOEN3106
To appear in: Nano Energy Received date: 26 July 2018 Revised date: 27 September 2018 Accepted date: 13 October 2018 Cite this article as: Jie Chen, Yifei Wang, Qibin Yuan, Xinwei Xu, Yujuan Niu, Qing Wang and Hong Wang, Multilayered Ferroelectric Polymer Films Incorporating Low-Dielectric-Constant Components for Concurrent Enhancement of Energy Density and Charge–Discharge Efficiency, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.10.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Multilayered Ferroelectric Polymer Films Incorporating Low-Dielectric-Constant Components for Concurrent Enhancement of Energy Density and Charge–Discharge Efficiency
Jie Chena, Yifei Wanga, Qibin Yuana, Xinwei Xua, Yujuan Niub, Qing Wangc, Hong Wanga,b,* a
State Key Laboratory for Mechanical Behavior of Materials & School of Electronic and
Information Engineering, Xi’an Jiaotong University, Xi’an, 710049, China b
Department of Materials Science and Engineering, Southern University of Science and
Technology, Shen Zhen 518055, China c
Department of Materials Science and Engineering, The Pennsylvania State University,
University Park, Pennsylvania, 16802, USA
*
[email protected]
ABSTRACT Ferroelectric polymers are the materials of choice for capacitive energy storage owing to their highest dielectric constants (K) and the best energy densities among the current dielectric polymers. Herein, different from the conventional approaches based on the incorporation of high-K fillers into the single-layer films to enhance the capacitive performance, a low-K polymer, i.e. PMMA with a K value of 3–4, is selected as an example and introduced into the layered configurations of the ferroelectric polymer. Both improvements in the energy density (Ue)
and charge–discharge efficiency (η) over those of the pristine polymer have been achieved via the establishment of multiple interlaminar interfaces and modulation of component ratios. The influence of film configuration on the capacitive performance has been systematically studied. The trilayered all-polymer film with optimized component ratio is capable of operating with a charge–discharge efficiency as high as 84% and concurrently delivering an energy density up to 20.3 J cm−3, surpassing the capacitive performance of the currently available polymer dielectrics that present the upper limits of Ue of ~20 J cm-3 and η of ~80%. Along with excellent stability of dielectric and mechanical properties of the polymer films, this work suggests great potential of the multicomponent ferroelectric polymers with layered architecture for electrical energy storage applications.
Graphical Abstract
Keywords: ferroelectric polymers; multilayer films; dielectrics; electrical energy storage; power density
1. Introduction Electrostatic capacitors with polymer dielectrics as storage media of electric charges are critical components in advanced electronics and electrical power systems, such as hybrid electric vehicles, medical defibrillators and power regulations in smart grid, due to their capability of delivering the highest power density (on the order of million watt per cubic centimeter), low cost, lightweight, and scalability [1-4]. Yet, most dielectric polymers possess relatively low energy densities (Ue) owing to their comparatively low dielectric constants (i.e. K ≤ 4 at 1 kHz) and small electric displacement (D), which seriously hinder their applications in next-generation power systems [5-8]. For instance, biaxially oriented polypropylene (BOPP) films, the state-ofthe-art commercially available dielectric polymer, only has a K of ~2.2 and a D of <1.2 μC cm−2; these limit the maximum Ue of ~2.5 J cm−3 obtained at an electric field of 500 MV m-1, which is at least an order of magnitude lower than their electrochemical counterparts such as batteries, electrochemical capacitors, with Ue of ~20 J cm−3. Exceptionally, the ferroelectric polymers represented by poly(vinylidene fluoride) (PVDF) and its co- and ter-polymers possess highly polar C–F bonds and spontaneous orientation of dipoles in the crystalline phases, resulting in higher K values of 10~12 at 1 kHz and higher D of 9~10 μC cm−2 than other classes of dielectric polymers [9-14]. Consequently, the ferroelectric polymers could potentially endow an order of magnitude enhancement in Ue (e.g. ~10 J cm−3). The most popular route to further raise Ue of the ferroelectric polymers is the introduction of high-K inorganic ceramic nanofillers, such as titanium oxide (TO) [12, 13], barium titanate (BT) [14-18], barium strontium titanate (BST) [19, 20], into the polymer matrix to form polymer nanocomposites. However, PVDF exhibits the bulk conductivity (i.e. ~10-9 S m-1) that is at least
eight orders of magnitude higher than that of benchmark BOPP (i.e. ~10-17 S m-1) [17,18], which significantly reduces its charge–discharge efficiency (<70%) at high electric fields (>300 MV m1
) [19-26]. Besides, the drawback of the polymer nanocomposite approach is that the utilization
of inorganic fillers with high K always confers to even higher bulk conductivity, giving rise to an abrupt rise in conductive loss of the resulting polymer nanocomposites and sharply decreased the charge–discharge efficiency under high electric fields, especially at high filler concentrations (e.g. ~50%). For instance, when Ue is enhanced to over 20 J cm−3, which is the state-of-the-art value of discharged energy density, the charge–discharge efficiency is reduced to less than 60% [24]. From the practical application viewpoint, low charge–discharge efficiency denotes the presence of large conduction loss, the dominant energy loss mechanism at high fields, which causes Ohmic (Joule) heating to induce thermal runaway and degrade the lifetime of devices during continuous operations [21-22]. Therefore, it is important that not only the energy density but also the charge–discharge efficiency should be evaluated for dielectric materials. In addition to the capacitive energy-storage capability, the stability of the performance and the structural integrity over charging-discharging cycling and mechanical bending tests are other crucial criteria in the evaluation of energy storage devices, e.g. wound capacitors, during the continuous operation [1]. To reduce the conduction loss and improve the charge–discharge efficiency of PVDF, a variety of low-K polymers, such as polyamide (PA) [27], polyimide (PI) [28], polypropylene (PP) [29,30], polystyrene (PS) [31], and poly(methyl methacrylate) (PMMA) [32-34], have been added into PVDF to form polymer blends. In particular, PMMA is fully compatible with PVDF, resulting in the blends with much reduced conduction loss and improved charge–discharge efficiency under the high electric fields [35]. Nonetheless, in order to achieve large enhancement
in the charge–discharge efficiency (i.e. >80%), it is required to add high loadings of PMMA (e.g. >40 vol.%), which in turn substantially deteriorates the K, and consequently, Ue of the resulting blends. For example, the highest Ue achieved under a charge–discharge efficiency of 80% is only 10.5 J cm−3 for the PVDF-based polymer blends [35]. Therefore, it is urgently needed to develop completely new approaches in order to achieve high energy densities along with outstanding charge–discharge efficiencies simultaneously via utilizing complementary features of highly insulating PMMA and highly polar ferroelectric polymer. Recently, the topologically-structured composites including the insulating layer to avoid early breakdown under high electric fields due to the blocking effect of the interfaces between the layers have been used in the field of electrical insulation [36-38]. Compared with the layered films with the same composition, the conventional blend films always exhibit shorter dielectric lifetime and lower dielectric strength [39]. In the blend films, the interfacial areas are random relative to the direction of the applied electric field, which could be susceptible to dielectric breakdown [40]. By contrast, with the interfacial areas that are all perpendicular to the direction of the applied electric field in the layered films, the insulating layers are effective in deflecting the hot electron propagation and impeding the breakdown across the film thickness, leading to higher breakdown strength of the layered films [41]. Herein, we describe the development of the multilayered dielectric polymer films consisting of highly insulating PMMA and ferroelectric copolymer poly(vinylidene fluoride-cohexafluoropropene) P(VDF-HFP) via simple solution-casting process, which are capable of balancing the contradicted parameters including dielectric loss, energy density, charge–discharge efficiency, power density and mechanical reliability to exhibit the capacitive performance exceeding those reported for the currently available polymer dielectrics. It is found that,
compared with the single layer blend film with an energy density of 17.5 J cm −3 and a chargedischarge efficiency of 72% measured at 440 MV/m, the trilayered architecture film with intimate multiple interfaces has been proven to be effective in reduction of high-field conduction loss and leakage current over more than on order of magnitude, thus giving rise to a much improved discharged efficiency of 84%) and higher energy density of 20.3 J cm −3. Superior energy storage properties are achieved via synergic integrations of the multiple components in the layered structures and deliberately adjusting of the interfaces and the relative ratios of the constituent layers. Furthermore, excellent cycling stability and mechanical reliability are demonstrated over the rigorous cycling tests in the trilayered all-polymer films. Therefore, it is anticipated that our work will open a new pathway to the flexible all-polymer dielectrics with impressive capacitive energy storage capability and great reliability. 2. Experimental methods 2.1. Fabrication of the trilayered all-polymer films Compared to PVDF, P(VDF-HFP) shows lower dielectric loss and higher breakdown strength and is selected as the high-K component of the layered structures. The influence of different molecular weights on the dielectric and energy storage properties of P(VDF-HFP) films have been studied. Considering the comprehensive dielectric and energy storage performance (Fig. S1 and Fig. 3), the intermediate-molecular-weight P(VDF-HFP) film (Mw=530,000) are used in this study. For the fabrication of trilayered all-polymer films, 1g P(VDF-HFP) (Arkema, molecular weight of 530,000) and 2g PMMA were respectively dissolved in 10mL N, Ndimethylformamide (DMF, Letai Co., China) by stirring for 4 h to form a homogeneous and stable suspension. Subsequently, the P(VDF-HFP) suspension was cast onto the glass plate (20cm×20cm) by a laboratory casting equipment (MSK-AFA-L800, Hefei Ke Jing Materials
Technology Co., Ltd.) at a casting rate of 25mm/s and dried as an outer layer film after evaporating solvent in vacuum at 80 ℃ for 1h. The inner layer of the PMMA suspension was then casted onto the outer layer at a casting rate of 25mm/s. Finally, the P(VDF-HFP) suspension was cast onto the PMMA layer as the outer layer at a casting rate of 25mm/s. The trilayered allpolymer films were dried at 60 °C for 10 h in vacuum for complete evaporation of solvents. The dried films were further heated under vacuum at 200 °C for 5 min and then immediately quenched in ice water to obtain the trilayerd all-polymer films. The final quenched films were dried at 100 °C overnight to complete remove residual water and peeled off the glass substrates. Quenching in ice-water resulted in the decrease of crystallinity from 29.98% to 16.53% (Fig. S2). The thickness of each layer in the trilayered films can be tuned by controlling the solution concentration and scraper height. The volume fractions of P(VDF-HFP) to PMMA were 100: 0, 90: 10, 70: 30, 50: 50, 30: 70, 10: 90 and 0: 100. Meanwhile, the single layer film of P(VDFHFP)/PMMA blends and bilayered films were also prepared with the same volume ratio for comparison. The thickness of all dielectric films, including blends and bilayered films, was carefully controlled to around 20 μm (Fig. S4). The cross-sections of the films were obtained by freeze fracture in liquid nitrogen and the morphologies were observed by scanning electron microscopy (SEM, Quanta 250FEG, FEI, Ltd.). 2.2. Characterization The crystal structures were characterized by an X-ray diffraction meter (XRD, Bruker D8Advance) with the X-ray wavelength of 1.542 Å (Cu Kα radiation, 40 kV, and 100 mA), the 2θ diffraction angle from 10° to 60°, the rate of 15°/min, and the step of 0.02°. All data were analyzed using Jade software with a Gaussian-Lorentz superposition fitting function. For the electrical measurements, gold electrodes of a typical thickness of 100 nm and diameter of 2 mm
were sputtered on both sides of the polymer films by the auto fine coater (JFC-1600, JEOL, LTD.). Dielectric properties were measured using an impedance analyzer (4990A, Agilent Technologies, Inc.) in a frequency range of 1 kHz to 50 MHz. The DC electric resistivities of the films were collected using a High-Voltage Test System (PolyK Technologies, State College, PA, USA). The electric displacement-electric field loops and leakage current of the film samples were measured by a ferroelectric polarization tester (Radiant Technologies, Inc.) with a limited current of 0.19 mA at room temperature and 10 Hz. The fast discharge tests were performed using a PK-CPR1502 test system (PolyK Technologies). Trek 10/10B high voltage amplifier was used to charge the capacitor sample soaked in silicone dielectric fluid and the typical charging time was ∼1 s. Then the charged capacitor was discharged to a high voltage noninductive resistor through a high-speed metal-oxide-semiconductor field-effect transistor switch. The resistance of the load resistor (RL) was selected as 100 kΩ to match the capacitance of the sample and achieve certain discharge RC time constant. The charge–discharge cycle was controlled by a LabVIEW program. The load resistor, 𝑅𝐿, and the applied field were 100 kΩ and 150 MV/m, respectively. At least 20 samples were measured for each test. A computer aided homemade bending test system was used for bending tests (Fig. S9). 3. Results and Discussion 3.1. Preparation and Morphology Characterization We fabricated a series of the trilayered all-polymer films using the layer-by-layer solution casting method. Solution casting technique is straightforward, versatile, and cost-effective when compared to the multistep extrusion blowing and zone-heated uniaxially stretching process [42]. Fig. 1(a) shows a schematic illustration of the fabrication of the trilayered polymer films. P(VDF-HFP) and PMMA were dissolved into DMF separately to prepare the respective
solutions. Then, the solutions were poured onto a smooth glass substrate in sequence to obtain uniform trilayer films, which was placed into a vacuum oven to accelerate the evaporation of solvent. Fig. 1(c) shows a photograph of an as-prepared freestanding trilayered polymer film. The trilayer configuration with P(VDF-HFP) as the outer layer and PMMA as the center layer has been characterized by cross-section scanning electron microscopy (SEM), as shown in Fig. 1(b). High-magnification cross-section SEM shown in Fig. 1(d)–(f) suggests the intimate interfaces between adjacent layers without noticeable defects at a scale of 0.1 mm. This is apparently attributed to the chemical compatibility between amorphous PMMA and semicrystalline P(VDF-HFP) [6]. The thickness of the component layer has been further confirmed by high-magnification cross-sectional SEM. The total thickness of all the samples is fixed to 20±1 μm by simply controlling the solution concentration and the scraper height. The thickness of the middle layer increases gradually from 2.4 μm to 5.9 μm and 10.5 μm. The ratio of the thickness of the middle layer to the total thickness was calculated to determine the volume fraction of PMMA. The resulting films are thus termed as 10 vol.%, 30 vol.%, 50 vol.%, 70 vol.% and 90 vol.% according to the volume fraction of PMMA, respectively. 3.2. Dielectric and Electrical Insulation Characteristics Since the polarization and relaxation of electrical charges and dipoles of dielectrics are dependent on time, dielectric constant and dielectric loss are generally variable with respect to frequency of electric field [24]. The dielectric properties of the trilayer-structured all-polymer films with varied volume fractions of PMMA are presented in Fig. 2(a) and (b). As revealed in the dielectric spectra in the frequency range from 1 kHz to 20 MHz, all the trilayered all-polymer films display the distinct weak-field dielectric properties of neat P(VDF-HFP). The K value of the trilayered all-polymer films decreases monotonically with the increase of PMMA volume
fraction because of a lower K (i.e., 3–4) of PMMA relative to that of P(VDF-HFP) (i.e. ~10 at 1 kHz). Concurrently, the dielectric loss peak attributable to the dielectric relaxation in the ferroelectric polymer is observed at about 10 MHz, which is gradually weakened with increasing the PMMA volume fraction [19]. As a result, a much lower dielectric loss (~0.03) has been achieved in the 90 vol.% film when compared to that of pure P(VDF-HFP) film (~0.1 at 100 kHz). It is worth pointing out that low dielectric loss reduces the chances of self-heating and thermal runaway of dielectrics, which is crucial to the films for capacitor applications [24]. In addition to weak-field (0.025 MV/m) dielectric loss, the direct current (DC) electrical resistivity at varied electric fields is of importance in the judgement of the electrical insulation characteristics of dielectrics [1]. Consistent with the trends of weak-field dielectric loss, as presented in Fig. 2(c), it is found that the trilayered all-polymer films with different volume fractions of PMMA show the direct current (DC) electrical resistivity that is one order of magnitude higher than that of pristine P(VDF-HFP) films, e.g., 2.02 × 1012 for the 10 vol.% film and 6.9 × 1012 for the 90 vol.% film vs. 2.6 × 1011 Ω m for P(VDF-HFP) films masured at an applied field of 50 MV m−1. DC electrical resistivities of the layered films with different volume fractions of PMMA are at least one order of magnitude higher than that of pristine P(VDF-HFP) films, e.g., 3.02 × 1011 Ω m of the 10 vol.% film and 4.2 × 1012 Ω m of the 90 vol.% film vs. 8.4 × 109 Ω m of P(VDF-HFP) films measured at an applied field of 150 MV/m. As a result, much depressed dielectric loss and greatly improved electrical insulation characteristic can be achieved by tuning the thickness of the component layers. 3.3. Capacitive Energy-storage Capability The electrical displacement–electrical field (D–E) loops of the trilayer films are measured with a 10 Hz unipolar triangle signal (Fig. 3). Widening (or opening) of D–E loops depicts a
deviation from the linear behavior of the electrical displacement versus the applied field and corresponds to the energy loss (space-charge, conduction, etc.) that is indicated by the enclosed area between the charge and discharge curves. It is evident that the introduction of PMMA into P(VDF-HFP) drastically narrows the D–E loops, indicative of reduced high-field energy loss with thicker layers of PMMA. Fig. S6(b) summarizes the energy loss of the trilayered allpolymer films with varied PMMA volume fractions along with pristine P(VDF-HFP) film measured at varied applied electric fields. For example, at an applied field of 440 MV m−1, the pristine P(VDF-HFP) exhibits an energy loss of 10.4 J cm−3, whereas the losses of the trilayered films inserted with 10 vol.% and 30 vol.% PMMA are only 5.3 and 3.9 J cm−3, respectively. This can be attributed to improved electrical resistivity as a result of the presence of PMMA in the layered structures. The capacitive energy-storage capability of the films was evaluated from the D–E loops by a modified Sawyer-Tower circuit, where the stored energy density (U) is derived from the D–E loop by integration of the area between the charge curve and the ordinate, and the discharged energy density (Ue) is determined by the area between the discharge curve and the ordinate (Fig. S5). We found that the improvement in electrical resistivity and corresponding decrease in energy loss results in substantial enhancements in discharged energy density and charge– discharge efficiency (η= Ue/U) of the trilayered all-polymer films simultaneously [46]. Fig. 4(a) compares the charge–discharge efficiencies of the trilayered all-polymer films with that of P(VDF-HFP). As expected, much higher charge-discharge efficiencies are delivered in all the trilayered films compared to P(VDF-HFP). For instance, all-polymer trilayered films exhibit higher charge–discharge efficiencies over 85% in comparison to 65% of pure P(VDF-HFP) at 300 MV m−1. Even at 440 MV m−1, the layered polymer films still retain an efficiency of 80%,
while the efficiency drops to 60% for P(VDF-HFP). It is well known that low charge–discharge efficiency generates vast waste heat and inevitably reduces the service lifetime of film capacitors. The discharged energy densities are summarized in Fig. 4(b), in which marked improvements are manifest with the trilayered all-polymer films with 10 vol.% ~ 70 vol.% PMMA. For instance, at 440 MV m−1, the discharged energy densities of 10 vol.% and 30 vol.% films are 20.7 and 20.3 J cm−3, respectively, corresponding to an ~30% enhancement over P(VDF-HFP) (16 J cm−3), with their charge–discharge efficiencies >80%. We further compare the discharged energy density and the charge–discharge efficiency of the trilayered all-polymer films with those of the representative dielectric polymers and nanocomposites, are presented in Fig. 5. Evidently, the trilayered all-polymer film with the optimized component ratio (30 vol.% PMMA) exhibits a Ue ~20.3 J cm−3 and a η ~84%, surpassing the overall capacitive performance of the currently available polymer dielectrics which have the upper limits of Ue of ~20 J cm-3 and η of ~80%. Specifically, a very high η of 91% with a Ue of 16.2 J cm−3 has been achieved in the trilayered film with 70 vol.% PMMA at 400 MV m-1, indicating an enormous ~700% enhancement of energy density over the commercial benchmark BOPP (~2 J cm−3), while carries a η comparable to BOPP (96%) [9]. In addition, some researchers have deposited ferroelectric thin films on commercial flexible polymeric Kapton films via hydrothermal process that exhibits high capacitance densities and acceptable dielectric loss [50]. However, layer-by-layer solution casting technique is straightforward, time saving and cost-effective compared to the deposition polycrystalline thin films on commercial flexible polymeric films. The large improvements of the capacitive energy storage performance in comparison to neat ferroelectric polymer could be attributed to the synergetic optimization of slightly decreased maximum displacement and sharply suppressed remnant displacement by tailoring the
component ratios. As plotted in Fig. 4(c) and (d), at 100 MV m-1, the maximum displacement decreases mildly from ∼2.4 μC cm-2 of P(VDF-HFP) to ∼2.1 μC cm-2 of 70 vol.%, while the remnant displacement substantially decreases from ∼0.27 μC cm-2 to ∼0.04 μC cm-2 of 70 vol.%. Especially, this trend still exists even at an applied electric field of 400 MV m-1. During the charging process, the maximum displacement offers a high charging energy density under the applied voltages, whereas the large reduction in remnant displacement during discharging confers low loss and thus gives rise to large enhancements in both discharged energy densities and the charge-discharge efficiency. To in-depth rationalize the observed excellent energy storage performance of the trilayered all-polymer films, different film configurations including bilayered and single-layered blends films with the same volume fraction of PMMA (i.e. 30 vol.%) were prepared by solution casting for the purpose of comparison. As corroborated in the high-magnification cross-section SEM images of Fig. 5 (a)-(c), the total thickness of bilayered and single-layered blend film is equal to that of the trilayered film (20±1 μm) via precisely controlling the height of the scraper. As exemplified in Fig. S8(a) and (b), it is evident that the film configurations have a marginal effect on the weak-field dielectric constants of the films. On the other hand, the dielectric loss of the layered configurations films is substantially lower than that of single-layered blends. More striking results are observed by comparing their D-E loops, which become slimmer with the increase of the interface number (Fig. S9), indicative of reduced high-field energy loss with more layered interfaces, as shown in Fig. S10(b). To further investigate the energy loss mechanisms in different configurations, the conductive loss is deduced from the D–E loops with the assumption that the measured displacement at zero field (DE=0) comes mainly from the leakage current and plays a dominant role in the high-field dielectric loss of dielectric films [36]. Fig. S10(c)
represents the conductive loss of the films with different configurations at varied electric fields. Obviously, the tri-layered architecture film shows the lowest conductive loss (< 7%) in comparison to other architecture films. We further compare the discharged energy density and the charge–discharge efficiency of different film configurations as a function of applied electric field (Fig. 5(d) and (e)). As expected, the trilayered all-polymer film possess the highest discharged energy density of 20.3 J cm−3 and the best charge–discharge efficiency of 84% in comparison to the single-layer blend (17.5 J cm−3 ＆ 72%) and the bilayered film (17.5 J cm−3 ＆ 77%) measured at 440 MV m−1. The trilayered all-polymer films exhibits much higher discharged energy density and charge– discharge efficiency than those of the representative single-layer blend polymers, as presented in Fig. S7. Higher energy storage performance in the trilayered all-polymer film than those of other configuration films could be attributed to the decreased remnant displacement at various electric fields (Fig. 5(f)), while maintaining a similar maximum displacement (Fig. S10(a)). For instance, at 440 MV m−1, the trilayered all-polymer films possess the lowest remnant displacement of ~0.71 μC cm−2 in comparison to ∼1.16 μC cm−2 of the bilayered film and ∼1.35 μC cm−2 of the single-layered blend. The important role of interfaces in suppression of remnant displacement has also been verified in the DC electric resistivity (Fig. S10(d)) and the leakage current-electric field (I-V) measurements (Fig. 5(g)). For instance, the leakage current density (5.45 × 10-7 A/cm2) of the trilayered film is much lower than those of the single-layered blend (3.42 × 10-6 A/cm2) and the bilayered film (1.68 × 10-6 A/cm2) as well as P(VDF-HFP) (7.38 × 10-6 A/cm2) at 200 MV/m as summarized in Table S1. The highest electric resistivity and the lowest leakage current density of the trilayered all-polymer film are endowed when compared to the other film configurations with fewer interfaces. The results indicate that the establishment of multiple
interlaminar interfaces can efficiently inhibit conductance loss, thus simultaneously improving discharge energy densities and the corresponding charge-discharge efficiencies. 3.4. Fast-discharge Capability and Mechanical Reliability In addition to the capacitive energy-storage capability, the fast-discharge capability (power density) is another crucial criterion in the evaluation of the performance of energy storage devices. To demonstrate the ultra-fast discharge rate of the trilayered all-polymer films and their high power densities, a typical fast-discharge experiment was carried out on both the trilayered film and BOPP at an identical RC time constant (Fig. S11), in which samples are charged at 150 MV m-1 followed by discharging across a 100 kΩ resistor. The experimental discharge time, τ0.9, is defined as the time to discharge 90% of the energy into the load (Fig. S12(a). As expected, a superior power density of 0.13 MW cm-3 with a faster discharge time τ0.9 of 11.2 μs has been obtained in the trilayered all-polymer film, which is more than six times that of BOPP, i.e. 0.02 MW cm-3 with a τ0.9 of 17.2 μs, in addition to superior energy densities (1.53 J cm−3 versus 0.34 J cm−3). Moreover, the cyclic fast discharge experiments have been carried out at an applied field of 150 MV m−1 to examine the stability of the trilayered all-polymer films (Fig. S13). Remarkably, as shown in Fig. S12(b), no sign of degradation in the discharged energy density has been detected in the trilayered all-polymer films over a straight 50 000 cycles of charge– discharge (i.e., a 55 h continuous operation), which is of great importance for the long term operation of electronic devices. To further confirm the stability of the layered structure, rigorous mechanical bending tests (Fig. S14(a)) have been conducted on the trilayered all-polymer films at a radius of curvature (ROC) of 5 mm, as presented in Fig. 7(a). Significantly, noticeable degradation in the discharged
energy density and the charge–discharge efficiency has not been detected in the trilayered allpolymer films over consecutive 20,000 bending cycles (Fig. 7(b) and S14(b)). We have verified the structural integrity after rigorous bending cycles. As shown in cross-section SEM (Fig. S14(c)), no detectable cracks or flaws in the interfaces of adjacent layers have been found. Moreover, we have studied the chemical stability before and after cycling and the effect of moisture on the polymer. The diffraction peaks at 18.4° and 20.2° are assigned to α (020) and α (021), respectively; and the diffraction peaks at 26.8° is ascribed to γ (022) [47-48]. As shown in the XRD patterns (Fig. S15), no shift in the diffraction peaks has been found in the trilayered allpolymer films after cycling and exposure to moisture due to the hydrophobicity of the polymer. These results suggest good chemical stability of the layered polymer films. The excellent dielectric and mechanical stability and reliability might be attributed to the following factors: (i) the all-organic architecture is intrinsic mechanically and electrical stable, which benefits the long-time cycling stability; (ii) high efficiency means less waste heat, better reliability and longer cycling lifetime; and (iii) a strong interfacial force between the adjacent PMMA/P(VDF-HFP) layers. 4. Conclusions In summary, as a result of synergic contributions of the presence of the multiple interfaces in the layered structures and control of the relative thickness of the component layer, much depressed dielectric loss and greatly improved electrical insulation have been achieved in the trilayered polymer films. Consequently, the layered polymer films exhibit remarkably enhanced capacitive energy storage performance, exceeding those reported for the current polymer dielectrics with the upper limits of Ue of ~20 J cm-3 and η of ~80%. The uniqueness of the layered structures in the realization of concurrent improvements in energy density and the
charge-discharge efficiency, which typically contradict to each other, has been clearly demonstrated in the comparison with other film configurations such as single layer and bilayered films. In addition, the trilayered all-polymer films possess excellent cycling stability and mechanical reliability as shown in consecutive and rigorous cycling tests. Along with the simplicity and scalability of the solution-based processing, this work opens a facile route to boost the performance of ferroelectric polymers for electrical energy storage devices. Acknowledgements The work was supported by National 973 projects of China (No. 2015CB654603) and the National Natural Science Foundation of China (No. 61471290, 61631166004). Notes The authors declare no conflict of interest. Appendix A. Supporting Information Supplementary data associated with this article can be found in the online version at …
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Jie Chen is currently a Ph.D. candidate in electronic science and technology in Xi’an Jiaotong University. He received the B.S. and M.S. in materials science and Engineering from the Xi’an University of Architecture and Technology and Guilin University of Technology in 2012 and 2015, respectively. His research interest is on the polymer dielectrics with high energy storage density.
Yifei Wang is a Ph.D. candidate of electronic science and technology at Xi’an Jiaotong University. He received his B.S. degree in electronic science and technology from Xi’an Jiaotong University, Xi’an, China, in 2013. His research focuses on polymer nanocomposites for dielectric energy storage applications.
Qibin Yuan received the B.S. degree in materials physics and the M. E. degree in materials engineering from the Shaanxi University of Science and Technology, China in 2009 and 2013, respectively. He is currently a doctoral candidate in electronic science and technology in Xi’an Jiaotong University Xi’an, China. His research interest is focused on high energy storage density ceramics and devices.
Xinwei Xu is a Ph.D. candidate of electronic science and technology at Xi’an Jiaotong University. He received his B.S. degree in electronic science and technology from Xi’an Jiaotong University, Xi’an, China, in 2015. His research focuses on high-temperature dielectric energy storage applications.
Yujuan Niu is a Research Associate Professor of Academy for Advanced Interdisciplinary Studies, South University of Science and Technology of China. She received the Ph.D. degree in Electronic Science and Technology from Xi’an Jiaotong University, and then as a postdoctoral worked at Xi’an Jiaotong University, Xi’an, China. Her research interests are focused on polymer matrix composites materials for energy storage and surface modification for composites.
Qing Wang is Professor of Materials Science and Engineering at The Pennsylvania State University, University Park, PA, USA. He received his Ph.D. in 2000 at University of Chicago. Prior to joining the faculty at Penn State in 2002, he was a postdoctoral fellow at Cornell University. His research programs are centered on using chemical and material engineering approaches towards the development of novel functional polymers and polymer nanocomposites with unique dielectric, electronic and transport properties for applications in energy harvesting and storage. Hong Wang is a chair professor of Southern University of Science and Technology, Shenzhen, China. Her main research interests include dielectric materials, ceramic-polymer composites, and dielectric measurements for applications in passive integration and electronic devices. She has authored and co-authored more than 220 peer-reviewed papers and 2 book chapters. She holds 27 Chinese patents and 1 U.S. patent and has presented over 40 invited talks in international academic conferences. She is a senior member of IEEE, the chair of the Executive committee of the Asian Electroceramic Association (AECA), and a member of IEEE UFFC society’s Ferroelectric committee
Fig. 1. (a) Schematic illustration of the trilayered all-polymer films. (b) SEM image of cross section of the trilayered all-polymer film with 30 vol.% PMMA. (c) Optical image of the freestanding trilayered all-polymer film with 30 vol.% PMMA. (d)-(f) High-magnification SEM image of cross section of the trilayered all-polymer films with 10 vol.%, 30 vol.%, 50 vol.% PMMA films, respectively.
Fig. 2. Frequency-dependent (a) dielectric constant and (b) dielectric loss of the trilayered allpolymer films with different PMMA volume fractions. Insets show the (a) dielectric constant and (b) dielectric loss of the trilayered all-polymer films at 100 kHz as a function of PMMA content. (c) DC electric resistivity of the trilayered all-polymer films with different volume fractions of PMMA at varied electric fields.
Fig. 3. D-E loops at varied electric fields of (a) pristine P(VDF-HFP) and (b-g) trilayered allpolymer films with different PMMA volume fractions.
Fig. 4. (a) Charge–discharge efficiency, b) discharged energy density, (c) maximum displacement and (d) remnant displacement of pristine P(VDF-HFP) and the trilayered allpolymer films with different PMMA volume fractions as a function of the applied field.
Fig. 5. Comparison of the discharged energy density and the charge-discharge efficiency of the trilayered all-polymer films with 30 vol.% and 70 vol.% PMMA films with other representative dielectric polymers.
Fig. 6. Cross-section SEM images of (a) single-layered blend film, (b) bilayered film, and (c) trilayered film. (d) Discharged energy density, (e) the charge–discharge efficiency, (f) the remnant displacement and (g) leakage current density of P(VDF-HFP) and different configuration polymer films as a function of electric field.
Fig. 7. (a) The schematic and digital photographs of the bending test and (b) energy storage capability of the trilayered all-polymer films with 30 vol.% PMMA before and after 20000 cycles of bending.
Highlights:
1.
Multilayered ferroelectric polymer films with excellent comprehensive energy storage performance have been achieved via layer by layer solution-casting process.
2.
The obtained excellent cycling stability and mechanical reliability is of great importance for the long term operation of flexible electronic devices.
3.
The uniqueness of the layered structures in the realization of comprehensive energy storage performance has been demonstrated in the comparison with other film configurations such as single layer and bilayered films.