BaTiO3/MWNTs/Polyvinylidene Fluoride Ternary Dielectric Composites with Excellent Dielectric Property, High Breakdown Strength, and High-Energy Storage Density.

To improve the dielectric performance of polyvinylidene fluoride (PVDF), BaTiO3/MWNTs/PVDF ternary composites were prepared by the solution casting method. The percolation threshold (fraction of MWNTs) has dropped greatly below 0.4 vol %, with the enhancement of dielectric constant and breakdown field. For the BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composite, the dielectric constant is 59, the loss is below 0.055, and the maximum operating electric field is 324 MV/m, so the discharged energy density can be of up to 10.3 J/cm3 with the efficiency of above 77.2%. The reason of improvement was revealed by the scanning electron microscope images and the X-ray diffraction data. It is found that uniform distribution of filler in the composites and the increase of the β phase of polymers result in the enhancement of polarization and improvement of dielectric constant of PVDF. The third-phase spherical inorganic particles prevent the formation of conductive networks and improve the uniformity of local electric field, so the breakdown strength of composites can be enhanced greatly. Here, this paper provides a method to get the composites with high energy storage density for supercapacitors.

Introduction

Dielectric capacitors with high discharged energy densities and power densities are highly desired and have received tremendous attention with the rapid development of compact, low-cost electronic, and electrical power systems.[1,2] Dielectric materials as capacitors possess the abilities to obtain high discharged energy densities,[3,4] which also exhibits high charge–discharge speeds (milliseconds or microseconds) and almost infinite cycle life. In general, the discharged energy storage density (Wrev) of dielectric materials can be described by Wrev = ∫E dD, where E represents the electric field and D represents the electric displacement that positively relate to the dielectric constant (ε).[5,6] It can be seen from this equation that large dielectric constant and high breakdown field strength are required for high energy storage densities. Although inorganic materials have large dielectric constants and displacement polarization, the low electric breakdown strength limits their further applications.[7] Polymer-based capacitors have become a hot spot in the capacitor industry because of the superior performance, such as high breakdown strength, high energy density, high power density, low loss, light weight, and self-healing capability.[8−10] However, the dielectric constant of polymeric materials is generally low, leading to a lower energy density. For example, biaxial-oriented polypropylene has an excellent breakdown strength of up to 640 MV/m, but their too small dielectric constant (∼2) leads to the unsatisfactory energy storage density (∼1.2 J/cm3).[11,12] Therefore, a lot of works have been focused on how to increase the dielectric constant of polymer materials. At present, two main methods have been widely used to improve dielectric constant by adding conductive particles (Ni, Zn, and Fe3O4)[13,14] and high dielectric ceramic particles (BaTiO3 and PbZrTi1–O3).[15−17] For the high-dielectric constant ceramic particle and polymer composites, the increase of dielectric constant is limited even at higher content. For example, the maximum dielectric constant of these polymer-based composites can only reach 100 at the volume fraction of 50%. Moreover, these composites at high fractions also have very serious limitations, such as sharp declined flexibility and mechanical properties and increased weight and density, restricting their further applications in the electronic material field. Therefore, more and more researchers concentrate on tailoring the dielectric performance of polymer-based composites and maintaining their excellent flexibility simultaneously,[18] because flexible polymer nanocomposites have become one of the best candidates for various field applications.[11,19] For the percolative composites based on the conductive particles and polymers, the dielectric constant increases rapidly when the content of the filled particles is close to the percolation threshold. It overcomes the aforementioned shortcomings of ceramic and polymer composites and enables a large increase in dielectric constant at lower volume fractions. Based on the percolation theory, the dielectric constant would rapidly increase when the system changed from insulators to conductors near the percolation threshold, with the sharp increase of dielectric loss at the same time.[20,21] Generally speaking, the percolation threshold is about 15–20% and the filling of conductive particles also destroys the flexibility and mechanical properties of the polymers.[17] Moreover, the incompatibility between filler particles and polymer matrixes also decreases the breakdown strength and greatly reduces the ability of charge storing. Therefore, in order to meet the requirements of electromechanical applications, it is necessary to reduce the percolation threshold of conductive particles. Thus, how to achieve the percolation transition at a low filling content is a key issue.

It has been found that an effective conductive network can be formed at a low content when low-dimensional nanoconductive particles are used as the fillers in the dielectric composites.[22] It can greatly reduce the percolation threshold, and the flexibility and easy processing characteristics of polymers can be maintained. It is reported that zero-dimension zinc oxide (ZnO), one-dimensional carbon nanotube (CNT), and two-dimensional graphene nanosheet (GN) have been added into the polymer to achieve percolation transition at a lower fraction (ZnO-10 wt %,[23] CNT-1.6 vol %[20] and GN-2.5 wt %[24]), with great enhancement of dielectric constant. However, the defects induced by the filled particles greatly reduced the breakdown strengths of these composites. For example, the breakdown strength of the multi-walled carbon nanotubes/polyvinylidene fluoride (MWNTs/PVDF) composites drops below 100 MV/m.[25] Meanwhile, the agglomeration and uniform distribution of filled nanoparticles in the polymer also leads to the declined breakdown strength.

In this paper, in order to improve the electric breakdown strength of MWNTs/PVDF composites, a certain amount of third-phase spherical inorganic particles (BaTiO3) were added to prevent the formation of conductive networks and improve the uniformity of local electric field strength. It was found that the breakdown strength of BaTiO3/MWNTs/PVDF ternary composites could be enhanced more than threefold to 324 MV/m. Meanwhile, the dielectric constant was also improved greatly with the synergetic effect of high dielectric particles and percolation effect. Therefore, a material with high dielectric constant, high breakdown strength, and high energy storage density can be obtained, which helps to open another method for supercapacitors.

Results and Discussion

Microstructure of BaTiO3, MWNTs, and Composites

Figure displays the scanning electron microscopy (SEM) images of MWNT nanoparticles, BaTiO3 nanoparticles, and the freeze-fractured BaTiO3/MWNTs/PVDF ternary composites. It can be observed from Figure a,b that MWNTs are dispersed well after ultrasonic cell disruption, and the average size of monodispersed BaTiO3 nanoparticles is about 20 nm. As shown in Figure c,d, the MWNT and BaTiO3 nanoparticles are uniformly dispersed in the PVDF matrix, indicating that a homogeneous BaTiO3/MWNTs/PVDF composite can be obtained in this work.

Figure 1

SEM images of (a) MWNT nanoparticles dispersed in DMF after ultrasonic treatment, illustration for unprocessed MWNTs and (b) BaTiO3 nanoparticles. (c) SEM image of the fractured surfaces of BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites. (d) Enlarged view of the square area of the SEM image.

Figure shows the X-ray diffraction (XRD) spectrum of PVDF and BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) at room temperature. For the PVDF film, the peaks at 2θ = 18.4° and 20.8° are the characteristic diffraction of (020) of the nonpolar phase (α phase) and (110)/(200) of the polar phase (β phase), respectively.

Figure 2

XRD spectrum of PVDF and BaTiO3/MWNTs/PVDF composites.

Furthermore, the diffraction peaks at 2θ = 21.9°, 31.3°, 38.6°, 44.9°, 50.6°, and 55.9° can be indexed to (100), (110), (111), (200), (210), and (211) planes of BaTiO3 according to the standard PDF (no. #31-0174), respectively. It is found that the relative ratio of α and β phases decreases greatly with the increase of fraction of BaTiO3, as shown in Figure S1. The increase of β phase and the decrease of α phase induce the enhancement of polarization of PVDF. In addition, the diffraction peaks of MWNTs cannot be observed in the XRD pattern because of the small content and amorphous behavior of MWNTs.[5,26]

Dielectric Properties of the BaTiO3/MWNTs/PVDF

The dielectric constant (ε) and dielectric loss (tan δ) of BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) ternary composites at room temperature are presented in Figure . It has been found that the MWNTs/PVDF binary composites exhibit the typical percolative behaviors with the percolation threshold of 0.4 vol % (shown in Figure S2). To control the dielectric loss at a relative low level, here the volume of MWNTs is fixed at 0.35 vol %, where the value of tan δ is below 0.05. The ε in Figure a shows gradual decrease with frequency, whereas the tan δ in Figure b exhibits no change at low frequency and a broad peak at 106 Hz. The Maxwell–Wagner–Sillars interfacial polarization and space charge polarization could not catch up the switching fields, resulting in the decrease of ε and the increase of tan δ at higher frequency. To reveal the influence of BaTiO3 nanoparticles on the dielectric properties, dependence of ε and tan δ of both BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) and BaTiO3/PVDF (x/100 – x) composites on the fraction of BaTiO3 is presented in Figure c. Obviously, the ε increases with increasing of the BaTiO3 volume fraction in these two types of composites. However, the enhancement of dielectric constant of the BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) is better than that of BaTiO3/PVDF (x/100 – x) at the same volume fraction of BaTiO3. Furthermore, the superiority is becoming distinct with the increase of the volume fraction.

Figure 3

Dependence of (a) dielectric constant and (b) dielectric loss of PVDF and BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites on the frequency. (c) Dependence of dielectric constant and loss of the BaTiO3/PVDF (x/100 – x) and BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites at 100 Hz.

The ε of BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) can be improved up to 60 when the fraction of BaTiO3 is 11.5 vol %. It is 6, 2.4, and 2.7 times larger than that of PVDF, MWNTs/PVDF, and BaTiO3/PVDF, respectively. Here, the enhancement of dielectric constant in the BaTiO3/MWNTs/PVDF composites might be caused by the following reasons: (1) the hydroxyl groups on the surface of self-synthesized BaTiO3 can act as the surfactant and promote the compatibility between the filled particles and PVDF matrix. (2) BaTiO3 themselves are the high ε particles which can significantly improve the dielectric properties of the composites. (3) The increase of the β phase of PVDF (discussed above) caused by the introduction of BaTiO3 also helps to improve the dielectric constant of composites. At the same time, the dielectric loss of BaTiO3/MWNTs/PVDF composites shows no obvious change and is successfully kept at a low level. For instance, the dielectric loss is below 0.075 at 100 Hz when the fraction of BaTiO3 is 16.5 vol %, which implies small leakage current and low energy loss of the composites.[27]

Energy Storage Behavior of the BaTiO3/MWNTs/PVDF

The measured polarization versus electric field (P–E) hysteresis loops at different electric fields are shown in Figure a. All loops show linear response at low field and exhibit nonlinear ferroelectric hysteresis at high field, which is the characteristic of ferroelectrics. Although the breakdown strength decreases gradually with the increasing filler, the corresponding polarization of BaTiO3/MWNTs/PVDF composites increases. The polarization reaches the maximum (7.6 μC/cm2) when the fraction of BaTiO3 is 11.5 vol %, which is 2 times higher than that of BaTiO3/PVDF binary composites (shown in Figure S3), indicating that the MWNTs have great effects on the polarization of BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites. Meanwhile, as shown in Figure S4a, the P–E hysteresis loops of MWNTs/PVDF exhibit a large leakage behavior and show very low breakdown field because of the formation of the conduction path of MWNTs. Considering all results above, the filled MWNTs and BaTiO3 play a dominant role in the polarization behavior of BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites.

Figure 4

(a) P–E hysteresis loops; (b) the Pmax – Pr values; (c) Weibull distribution of breakdown strength; and (d) leakage current of PVDF and BaTiO3/MWNTs/PVDF composites.

It has been generally accepted that Pmax – Pr values and electric breakdown strength are the two key factors to affect the energy storage performance of composites. Along this line, the Pmax – Pr values versus the electric field for PVDF and BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites are presented in the Figure b. It can be observed that Pmax – Pr value increases with the increasing BaTiO3 content which should be beneficial in achieving excellent energy storage density.[6,28] The breakdown electric fields of ferroelectrics are found to follow Weibull distributions and the average dielectric strength can be determined from the variation between different tests[29]where X and Y are two parameters in the Weibull distribution function, E is the breakdown field in each test, n is the number of tests, and i is the serial number. The breakdown fields increase in the order of E1 ≤ E2 ≤ ... ≤ E ≤ ... ≤ E. Through the linear fitting of Y(X) curves, the Weibull modulus γ and average dielectric strength Eb were obtained where γ is the slope and Eb is X at Y = 0. The Weibull plots of the breakdown field for BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites at room temperature are illustrated in Figure c. It can be found that the breakdown strength decreases significantly with the increase of BaTiO3 content. The defects and voids induced by the filled BaTiO3, lead to the heterogeneous distribution of electric field.[29] By the way, although the breakdown strength of BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites decreases with the increase of BaTiO3 content, the breakdown strength of the BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composite can be of upto 324 MV/m, which is greatly higher than that of the 0.35MWNTs/PVDF composite (Figure S4, 30 MV/m). Here the barrier characteristic of BaTiO3 might be realized because of the steric effect, which avoids the premature agglomeration of fillers and leads to improvement of breakdown strength.[30,31]

Additionally, the leakage current, which is sensitive to the defects and grain boundaries, has great effect on the electric breakdown strength. Therefore, it is necessary to investigate the effect of BaTiO3 content on the leakage current behaviors of nanocomposites for optimizing the electric breakdown strength. As shown in Figure d, the leakage current densities of PVDF and BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites are in the range of 10–7 to 10–5 A/cm2 and follow a reasonable increasing trend with the rise of applied electric field.

Although the leakage current densities of BaTiO3/MWNTs/PVDF composites gradually increase with the increasing BaTiO3 content caused by the defects and space charges, it remains much lower than that of MWNTs/PVDF composites (Figure S4b) because of the existence of conduction barrier induced by BaTiO3.[6,32] The energy densities are calculated from the P–E loops and shown in Figure a. It can be obviously found that BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites exhibit significantly higher energy density than that of the PVDF film. The maximum energy density obtained in the film with 11.5 vol % BaTiO3 nanoparticles is 10.3 J/cm3 at 324 MV/m, which is two times larger than that of the PVDF film (5.3 J/cm3 at 411 MV/m). As discussed above, the energy density is determined by the Pmax – Pr and electric breakdown values, therefore, the enhancement of the energy density of BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites is because of the following aspects:

Figure 5

(a) Storage energy density and (b) efficiency of PVDF and BaTiO3/MWNTs/PVDF composites.

The introduction of surface-modified BaTiO3 not only increases the polarization (dielectric constant) of BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) composites, but also provides a conduction barrier for the breakdown path. In this case, their polarization increases while breakdown strength decreases with the increasing BaTiO3 content. When the content of BaTiO3 reaches 11.5 vol %, the synergetic optimization effect of polarization and breakdown strength leads to the highest energy density of 10.3 J/cm3.

A large amount of electric charge can be enriched under the electric field at the interfaces between MWNTs and PVDF also improve the polarization of BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites.[33,34]

The increase of polar phase (β phase) could also help to increase the polarization of BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites. For practical applications of dielectric capacitors, the high field efficiency (η) is another key factor to characterize the energy storage performance of dielectric materials. As shown in Figure b, the efficiency decreases with the increasing BaTiO3 content because of the fatted hysteresis loops. It can be seen that the efficiency of BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites is 77.2%, indicating these composites have great potential in applications of energy storage.[5]

Figure a is the P–E hysteresis loops of BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites at 245 MV/m (75% of the maximum breakdown field strength) with different temperatures. It can be seen that the hysteresis of the BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composite gets thicker and wider with the increase of temperature. The corresponding energy storage performances of BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites at different temperatures are shown in Figure b. When the temperature reaches 80 °C, the energy density and efficiency remain about 7.5 J/cm3 and 70%, respectively. It indicates that the BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites has a wide range of temperature applications.

Figure 6

(a) P–E hysteresis loops and (b) efficiency of BaTiO3/MWNTs/PVDF (11.5/0.35/88.15) composites at 245 MV/m with different temperatures.

Conclusions

In summary, BaTiO3/MWNTs/PVDF ternary composites with excellent dielectric and energy storage performance were prepared. The dispersion of MWNTs in the composites can be greatly improved by cell disruption, leading to the reduction of percolation threshold. Additionally, the second filler (BaTiO3) not only further improves the dispersion of MWNTs, but also increases the dielectric performance without consuming the electric field strength, and thus an enhanced energy storage density can be obtained.

Experimental Details

Materials

MWNTs—the average diameter and length of MWNTs are about 20 nm and 2–5 μm, respectively.[26] Triethylene glycol (TEG), N,N-dimethylformamide (DMF) and Ba(OH)2·8H2O (AR 98%, Aladdin Industrial Corporation, China), Ti(OC4H9)4 (AR 99.0%, Aladdin Industrial Corporation, China), and PVDF (3F Corporation, Shanghai, China). All chemicals were of analytical grade and directly used without further purification.

Preparation of BaTiO3 Nanoparticles

Highly stable BaTiO3 nanoparticles of about 20 nm were synthesized via controlling hydrolysis of Ti(OC4H9)4 in alcohols. Ba(OH)2·8H2O (1.8 M), Ti(OC4H9)4 (1.5 M), and polyvinyl pyrrolidone (PVP, K30) (34.8 mg/mL) were added to the TEG solvents. The mixture was then gradually heated to 160 °C with stirring by a magnetic agitator for 2 h. After cooling, deionized water was employed to remove the attached TEG on the BaTiO3 surface and precipitate the sols. Subsequently, the as-prepared BaTiO3 nanoparticles were washed about six times with ethanol and deionized water through a high-speed centrifugation and then dried in 80 °C for 12 h. Additionally, the surface of BaTiO3 synthesized via this method already has −OH groups, and thus further functionalization process is not required.[9]

Preparation of MWNTs/PVDF and BaTiO3/MWNTs/PVDF Composites

In order to decrease the agglomeration of MWNTs and reduce the percolation threshold of MWNTs in MWNTs/PVDF, MWNTs were first treated by the cell disruption technology. In this study, the content of MWNTs was fixed at 0.35 vol % (detail discussion can be found in Figure S2). After this, BaTiO3/MWNTs/PVDF ternary composites with different BaTiO3 contents BaTiO3/MWNTs/PVDF (x/0.35/99.65 – x) were prepared by solution casting through the following process.

First, the powders of PVDF were fully dissolved in DMF solution, followed by the dispersion of MWNTs and BaTiO3 powders. Then, the mixed solution was thoroughly blended under an ultrasonication for 0.5 h using a cell crusher, and then blended for 24 h by magnetic stirring. Subsequently, the solution was slowly dropped onto conductive glasses, and then the samples were transferred into a vacuum oven and heated at 200 °C for 0.5 h. By controlling the concentration and volume of the solution, the film thickness can be controlled at 10–15 μm. The final volume of BaTiO3 in the BaTiO3/MWNTs/PVDF ternary composites varied from 3 to 16.5 vol %. For comparison, the 0.35vol%MWNTs/PVDF and BaTiO3/PVDF (x/100 – x) binary composites were also prepared.

Characterization

The morphologies of the MWNTs, BaTiO3 particles, and BaTiO3/MWNTs/PVDF ternary composites were investigated through a scanning electron microscope (SU-70, Hitachi). The phase structures of samples were performed on an X-ray diffractometer (D8 ADVANCE, Bruker). Before the electric properties measurement, top gold electrodes with a size of 0.016 cm2 were deposited by a 108-Auto sputter coater (Cressington Scientific Instruments, UK). The dielectric performances of samples were measured by the Agilent precision impedance analyzer 4294A at room temperature. The P–E hysteresis loops were obtained on a Premier II ferroelectric material test system (Radiant Technologies, USA).