Improving the Energy Density and Efficiency of the Linear Polymer PMMA with a Double-Bond Fluoropolymer at Elevated Temperatures.

A variety of applications can be found for high-temperature film capacitors, including energy storage components and pulsed power sources. In this work, in order to increase the energy density (U e), poly(vinylidene fluoride-chlorotrifluoroethylene-double bond) (P-DB) is introduced into poly(methyl methacrylate) (PMMA) to manufacture composite films by a solution casting process. In the case of the pure PMMA film, there is significant improvement in the polarization (P max) and breakdown field (E b) of the composite film. These improvements can effectively increase the U e of the composite film at room temperature and the elevated temperature. The results show that at an elevated temperature of 90 °C and at 350 MV/m, the U e of 40 vol % P-DB reaches 8.7 J/cm3, and the efficiency (η) of 77% is also considerable. Compared with biaxially oriented polypropylene (2.0 J/cm3), the proposed film exhibits 4 times enhancement in the energy storage density, meaning that it can be an energy storage capacitor with huge potential at high temperatures.

Introduction

For the purpose of boosting the progress of the energy storage and conversion technology, the electronics industry advancements call for lighter, more flexible, and much more higher-performance energy storage materials.[1−3] Because of their ability to charge and discharge quickly, capacitors are widely used in hybrid vehicles as the basic component of the power system.[4,5] Researchers have made great progress in the study of dielectric polymers and ceramic polymer composites in recent years.[6−8] The key parameter for the performance of dielectric materials is the energy storage density (Ue), which can be expressed by Ue = ∫E dP. According to the formula, the Ue of dielectric materials is related to the electric strength (E) and polarization (P), and P is proportional to the dielectric constant (εr).[9] In the application research of film capacitors, some higher εr (>10) ferroelectric polymers were selected, such as poly(vinylidene difluoride) (PVDF)-based copolymers and ternary polymers. For example, the energy density of melt-stretching P(VDF-CTFE) has exceeded 25 J/cm3.[10,11]

PVDF and copolymer-based composites have addressed the demand for energy storage at room temperature.[12−16] However, in advanced electronic equipment, such as automotive and aerospace power systems, the performance of polymer materials is dramatically diminished as the working temperature rises.[17−19] This is due to a significant increase in electrical conduction caused by various temperature and field-related conduction mechanisms, such as the injection of charge at the electrode/dielectric interface.[20,21] In the hybrid and electric vehicles, for example, the working temperature of power inverters is around 70–90 °C. Special thermal management must be included in integrated power systems to use dielectric polymers at high temperatures. At the same time, this introduces new issues, such as an increased weight, increased energy consumption, and a significant reduction in efficiency, particularly in certain applications, which unavoidably require a large amount of energy.[22] At present, many high-performance linear polymers are used as candidates for high-temperature dielectric capacitors, such as polyimide (PI),[22] polyetherimide (PEI),[22] polyester,[23] polycarbonate (PC),[23] polymethyl methacrylate (PMMA),[11] polyetherketoneketone,[24] polyether ether ketone,[18] polystyrene,[25] polypropylene (BOPP),[18] aromatic polyurea,[23] aromatic polythiourea (ArPTU),[26] poly(tetrafluoroethylene-hexafluoropropylene) (PVDF-HFP),[18] and functionalized poly(4-methyl-1-pentene).[27,28]

However, even though liner polymers have attracted a lot of attention due to their excellent high-temperature resistance properties, their Ue is not sufficient due to their low dielectric constant. For example, biaxially oriented polypropylene (BOPP) with low efficiency (η) (0.02%) and breakdown electric field (Eb) (720 MV/m) is widely used in film capacitors. Unfortunately, its Ue is only 2 J/cm3 owing primarily to the low εr. In the previous research, the PVDF polymer (contained with the double-bond polymer) possesses high Eb and εr.[29−31] In order to achieve the energy density, linear PMMA blended with the PVDF polymer had been studied because PMMA was completely compatible with PVDF based on polymers[31−36] such as P(VDF-HFP)/PMMA, PVDF/PMMA, P(VDF-TrFE-CFE)/PMMA, and tri-layer PVDF/PMMA/P(VDF-HFP) composites. However, they were particularly concerned with room-temperature dielectric performance, with only a few cases involving high-temperature energy storage performance. As a result, achieving both a high energy density and a high efficiency in a single polymer material at high temperatures is not easy. The design and fabrication of all-organic composites is becoming increasingly important to meet the stricter requirements of both scientific and technological progress.[37]

In this research, PMMA was chosen as the matrix due to its high efficiency, high temperature resistance, and high glass-transition temperature. The PVDF with double-bond polymer poly(vinylidene fluoride-chlorotrifluoroethylene-double bonds) (defined as P-DB)[38] with Eb and εr was introduced into linear PMMA to prepare composite films by a simple solution cast method. Under high electric fields, the composite’s conductive loss considerably decreased, and its charge–discharge efficiency improved.[33] Then, the energy storage reliability was investigated at different temperatures and under a cycling test up to 105 cycles. The results show that the composite polymer can obtain the highest energy density of 8.7 J/cm3 and maintain a comparable efficiency at 90 °C. The enhanced energy storage capability indicates that it can be an excellent high-energy storage capacitor candidate at enhanced temperatures.

Results and Discussion

Figure a shows the scanning electron microscopy (SEM) image of 40 vol % P-DB composite. There are no pores or cracks in the composite film, demonstrating the polymer film’s homogeneity and uniformity. The macroscopic morphology shown in Figure b displays that the composite film is transparent. The crystalline information of P-DB and PMMA/P-DB films was characterized with differential scanning calorimetry (DSC). PMMA has no obvious characteristic peaks because PMMA is amorphous. Unlike PMMA, semi-crystalline P-DB has an obvious melting peak in the DSC results. The area and height of these curves increase with the addition of additives, as shown in Figure c, indicating that crystallization in the polymer increases. Simultaneously, with the addition of P-DB, the polymer’s peak gradually shifts to the right, indicating that the higher the crystallinity, the larger the crystal size, because of the addition of more VDF (vinylidene fluoride). Most importantly, the presence of crystals in the composite has a significant influence on εr.

Figure 1

(a) SEM images of 40 vol % P-DB composite films. (b) Digital image of the 40 vol % P-DB composite films. (c) DSC curves of P-DB, PMMA, and PMMA/P-DB composite films with different contents.

Figure presents the results of dielectric performance at room temperature. Increasing the amount of P-DB in the composite results in a higher εr, for example, the εr of PVDF is 10.8 and the εr of PMMA is 5.3 at 1 kHz. The permittivity of 70 vol % P-DB at the same frequency is 9.6, about 1.8 times that of pure PMMA (Figure a). This is primarily due to the fact that P-DB has a much higher polarity than PMMA, making it more prone to orientation polarization. Dipole moment excitation characteristics at different electric fields and frequencies are defined as dielectric loss (tan δ).[37,38] According to Figure b, the tan δ of pure P-DB and composite materials decreases first and then increases with increasing frequency because the electric field change time at higher frequencies is shorter than the establishment time of space charge polarization and dipole orientation polarization. The difference is that pure PMMA has a downward trend in the frequency range because although the PMMA molecule has a certain polarity, the polarity of the side chain is relatively small, and there is no relaxation peak in the high frequency. At 1 kHz, the tan δ of the composite is similar to that of the pure polymer, and tan δ is less than 0.05. The AC conductivities of P-DB, PMMA, and P-DB/PMMA are shown in Figure c. The low conductivity at room temperature indicates that the leakage current will be very small when the material is used in a capacitor. The dielectric parameters at 1 kHz of different samples are summarized in Figure d.

Figure 2

(a) Dielectric permittivity, (b) dielectric loss, and (c) AC conductivity as a function of frequency at room temperature for P-DB, PMMA, and PMMA/P-DB composites. (d) Summary of dielectric parameters at 1 kHz of different samples.

The dielectric performances at different frequencies (1k, 10k, and 100k Hz) as a function of temperature are depicted in Figures and S1. At 1k, 10k, and 100k Hz frequencies, the εr and tan δ of the pure polymer and the composite film are found to be nearly constant. As expected, the dielectric properties of P-DB are strongly dependent on the temperature (Figure e,f), while the dielectric properties of PMMA are almost independent of the temperature (Figure a,b). The data also show that the εr and tan δ of 40 vol % PMMA/P-DB composite do not change significantly as the temperature increases from 30 to 90 °C at 1 kHz (Figure c,d). Stable dielectric properties provide the foundation for high-temperature applications.

Figure 3

Temperature dependence of dielectric permittivity of (a) PMMA, (c) 40 vol % PMMA/P-DB composite, and (e) P-DB and dielectric loss of (b) PMMA, (d) 40 vol % PMMA/P-DB composite, and (f) P-DB.

To represent the breakdown properties of the composites straightforward, we use the double-parameter Weibull statistics to evaluate the breakdown strength. The formula is as follows:where F(x) is defined as the probability of cumulative electrical failure on the sample and E is related to the electric breakdown strength.[4]Figure shows the Weibull plots of all films at room temperature and 90 °C. The Eb of composites increases with the content of P-DB at room temperature (Figure a) due to the fact that P-DB possesses a high Eb (435 MV/m). Compared with pure PMMA (365 MV/m), the Eb of 70 vol % P-DB reaches 420 MV/m. The Weibull Eb of all samples decreases at elevated temperature, as presented in Figure b. Like the trend at room temperature, the Eb increases with an increase in P-DB. The difference is that compared with room temperature, the Eb of the 90 °C film decreases. For example, at room temperature, the breakdown field strengths of P-DB and 40 vol % P-DB are 430 and 400 MV/m, respectively, while at 90 °C, the Eb of P-DB and the composite drop to 384 and 360 MV/m, respectively. The decrease in Eb at high temperatures, as shown in other polymers such as aromatic polyurea, is caused by the increased loss or conductivity associated with thermal breakdown.[39] In addition, as shown in Figure S2, it can be found that the β of the composite films decreases more slowly than that of pure P-DB in the same temperature range. For example, at room temperature, the β of P-DB and 40 vol % P-DB are 18 and 15, respectively. However, at 90 °C, the numbers are 12 and 9, respectively. The results indicate that the dispersion of the composite films has a lower dependence of temperature, which is beneficial to the application of the film at 90 °C.

Figure 4

Weibull plots of pristine P-DB, PMMA, and PMMA/P-DB composites with a series of different PMMA contents at (a) room temperature and (b) 90 °C.

Figure S3 presents the typical P–E loops of composite films and pure polymer films under different electric fields at 10 Hz. Both the high polarization (Pmax) and high Eb are conducive to the application of energy storage capacitors.[8,39,40] Compared with the pure PMMA film, the Pmax of composite films is larger, which is mainly due to the higher Pmax of P-DB. Unfortunately, under the same electric field, the higher the content of P-DB, the higher the residual polarization (Pr) of the composite films, which is related to the decrease of the efficiency of energy storage. As a result, we expect to discover a suitable component in the experiment so that the film can attain a high energy density and efficiency. The major factors for high-pulse capacitance applications are polymers’ high-energy storage (Udischarged) and high efficiency (η). The P–E loop, Udischarged, and η of the composites are presented in Figure . Figure a depicts the variation of the P–E loops of the P-DB/PMMA composite films when the applied voltage is less than 300 MV/m. The electrical displacement of the composite films declines with the rise of PMMA, but the P–E loop becomes significantly thinner. Both Pmax and Pr of the composites improve with the addition of PVDF. Among them, 70 vol % P-DB composite has the largest Pmax, reaching 5.67 μC/cm2. Compared with the 1.85 μC/cm2 of pure PMMA, the Pmax polarization increased by about 3 times. As shown in Figure b, compared with PMMA, the more the P-DB content in the mixture, the higher its energy density is. Under a 100 MV/m electric field, the difference in the discharge energy density between pure and composite films is negligible. However, it clearly increases when the electric field exceeds 300 MV/m, as illustrated in Figure b. For example, at 300 MV/m, the Udischarged increases from 2.6 J/cm3 of PMMA to 6.8 J/cm3 of 70 vol % P-DB composites, which is 2.6 times that of pure PMMA. However, on the other hand, as more P-DB is added to the composites, the η continuously decreases, as observed from Figure c. For instance, at 300 MV/m, the efficiency decreases from 92% of pure PMMA films to 89% of 60 vol % PMMA/P-DB composites and 67% of pure P-DB films. At room temperature, when the testing electric field is increased to 400 MV/m, the efficiency of the composite films with 40 vol % P-DB is still above 85%, and the Udischarged can reach 9.3 J/cm3.

Figure 5

Pristine P-DB, PMMA, and PMMA/P-DB composite films at room temperature: (a) P–E loop under 300 MV/m, (b) discharged energy density, and (c) discharging efficiency with the electric field.

Typical P–E loops of P-DB, composite films, and PMMA at 90 °C are also given in Figure S4. Compared with the hysteresis loops at room temperature, the P–E loops of the films all widen to varying degrees. Meanwhile, as the P-DB content grows, the polarization degree of the composite films steadily increases, mirroring the pattern at ambient temperature. It is worth noting that when compared with P-DB, PMMA retains obvious linear characteristics at high temperatures, making it suitable for the application of films at high temperatures. Figure a depicts a comparison of the polarization changes of composite materials at 90 °C and at an electric field of 300 MV/m. Both Pmax and Pr of the composite material increase with the increase of the P-DB content. When the content of P-DB exceeds 50 vol %, the Pr of the composite increases significantly, which indicates that P-DB has a great influence on the loss of the composite at high temperatures, which may lead to the decrease of the film efficiency.

Figure 6

Pristine P-DB, PMMA, and composite films at 90 °C (a) P–E loop under 300 MV/m, (b) discharged energy density, and (c) discharging efficiency with the electric field. (d) The discharged energy density (Udischarged) vs E of 40 vol % PMMA/P-DB composite and selected polymers at elevated temperatures: ArPU (90 °C), BOPP (70 °C), c-BCB (150 °C), PC (150 °C), PEI (150 °C), PI (150 °C), PMMA (70 °C), PPEK (130 °C), P(TFE-HTP) (70 °C), P(VDF-TrFE-CTFE) (60 °C), P(VDF-TrFE-CTFE)-g-PMMA (60 °C), and 40 vol % PMMA/P-DB (90 °C).

The energy density of the composite material is improved not only at room temperature but also at 90 °C. The difference is that the energy density at room temperature is improved by the P-DB component, but at high temperatures, the energy density is obviously reflected. For example, at room temperature, the more the P-DB is added, the higher the energy storage of the composite will be, but at 90 °C, under the same electric field, the value of the 70 vol % composite material is lower than that of the 40 vol % composite films as shown in Figure b. The efficiency graph is shown in Figure c. Specifically, under the same electric field, the P-DB efficiency decreased from 63% at room temperature to 35% (350 MV/m) at 90 °C, while 40 vol % composite can still reach 77% (350 MV/m). In summary, even at 90 °C, 40 vol % P-DB shows the highest energy density of 8.7 J/cm3 and an efficiency of 77% under 350 MV/m. Figure d and Table S1 show the Udischarged and η of selected polymers at elevated temperatures for comparision.[10,11,18,22,40,41] It can be observed that the Udischarged of 40 vol % P-DB composite is larger than those of other polymers, and of critical importance are the high energy density of 8.7 J/cm3 and η of 77%, which have been obtained at an elevated temperature of 90 °C at the same time.

Therefore, we select the 40 vol % P-DB composite film for a further study of thermal stability. Thermal stability, discharge duration, and fatigue are most important parameters that affect the performance of a capacitor in high-temperature energy storage applications. The related parameters of the three kinds of films PMMA, P-DB, and 40 vol % P-DB composites from room temperature to 90 °C in 300 MV/m are given in Figure , and the related P–E loops are shown in Figure S5. As shown in Figure S5a, with the increase of the temperature, the hysteresis loop becomes wider obviously, the polarization value of P-DB increases, and Pr becomes higher, which indicate that the efficiency of P-DB decreases at high temperatures. The efficiency of pristine P-DB films decreases from 65% at 30 °C to 37% at 90 °C from Figure b. However, the P–E loops of PMMA exhibit good linearity at different temperatures. Even at 90 °C, the efficiency is up to 90%. It is interesting that the 40 vol % P-DB composite also shows high thermal stability where the Pmax slightly increases and Pr maintains the same values over the temperature range, as presented in Figure S5a. As a result, the Ue of 40 vol % P-DB is 6.5 J/cm3 (300 MV/m), which is 2 times that of PMMA (Figure a), while the efficiency is above 80% from 30 to 90 °C. Besides, compared to P-DB (5.0 J/cm3), the discharged energy density of 40 vol % films corresponds to ∼30% enhancement.

Figure 7

(a) Discharged energy density and (b) efficiency as a function of temperature at 300 MV/m for P-DB, PMMA, and 40 vol % P-DB composite.

Fast discharge performance is also critical to the application of energy storage capacitors. It is intended to characterize the discharge speed and discharge energy of a high-speed capacitor discharge circuit while maintaining the same RC time constant. Under a 200 MV/m electric field, 40 vol % P-DB composite films at different temperatures were tested.[10,35] In order to characterize the discharge performance of the capacitor, we define the time corresponding to 90% of the maximum discharge energy in the figure as the discharge time.[10,41] The experimental results are given in Figure . It is discovered that the composite film releases the stored energy at a rate of microseconds, which is equal to the rate of release of BOPP (23 μs). Furthermore, the discharge speed of composite films is not adversely affected by the operating temperature to a significant extent. At room temperature and 90 °C, the discharge times were 14.9 and 15.5 μs, respectively. According to the testing results, the composite film has a steady discharge energy density as well as a fast discharge speed during the discharge process. When it comes to the design of pulse power capacitors at high temperatures, all of these advantages are essential.

Figure 8

Discharged energy density as a function of time of 40 vol % PMMA/P-DB. All samples are charged in an electric field of 200 MV/m.

In the charge and discharge cycle tests, high fatigue durability is very important for the long-term stable use of capacitors. 40 vol % P-DB composite films are investigated as a function of the charge/discharge cycles up to 5 × 105 cycles at room temperature and 105 cycles at 90 °C under 200 MV/m (∼60% of the Eb), and the Ue and η of composite film are given in Figure . In pulse power applications, if the loss of Ue reaches 5%, we believe that the capacitor has failed.[24] At room temperature, the results show that the Ue increase is 2.6%, and the efficiency is beyond 90%, as observed from Figure a. Outstandingly, at 90 °C, the composite film displays excellent stability with merely 2.4% variation of Ue, and the efficiency is 85% (Figure b). The P–E loop of the charge/discharge cycling process is shown in Figure S6. It can be observed that the loops become slightly wider at high temperatures compared with those at room temperature. These striking properties provide the possibility of stable use of 40 vol % P-DB composite films in high-temperature atmospheres.

Figure 9

Discharged energy density and efficiency of 40 vol % P-DB film as a function of the cycling number under 200 MV/m: (a) at room temperature with a cycling number of 5 × 105 and (b) at 90 °C with a cycling number of 1 × 105.

Conclusions

In this work, a P-DB/PMMA composite has been proposed and studied at room temperature and elevated temperatures. Compared with pristine PMMA and P-DB, 40 vol % P-DB composite exhibits better energy density performance at high temperature. A discharged energy density of 9.3 J/cm3 and an efficiency of 86% are achieved at room temperature. Even at 90 °C, 40 vol % P-DB composite possesses a high energy density (8.7 J/cm3) at 350 MV/m, which is 4 times that of BOPP (2 J/cm3). Specifically, it presents high efficiency and excellent cycling stability at high temperature. In addition, the discharge times of 40 vol % P-DB composite are measured to be 14.9 and 15.5 μs at room temperature and elevated temperature, respectively. This type of polymer composite therefore has strong application potential for dielectric energy storage at high temperatures due to its combination of high breakdown strength, outstanding energy storage density, and quick discharge speed.

Experimental Section

The P-DB matrix was synthesized according to a previous report,[35,36] whose detailed information is described in Supporting Information Section 1. First, 3 g of P-DB and 5 g of PMMA were each weighed and dissolved in 100 mL of dimethyl formamide solvent separately and stirred for 5 h to obtain uniform and stable solutions. Then, the two solutions were mixed according to different volume fractions. The components of the five composite membranes are 20, 40, 50, 60, and 70% volume fractions of PMMA. In the following description, the composite material is expressed as the volume ratio of P-DB, for example, 40 vol % P-DB, to represent the composite material with a P-DB volume fraction of 40%. 1.5 mL of the mixed solution was uniformly cast on the glass substrates at 70 °C for solvent evaporation. Then, the dried film on the glass plate was placed in a vacuum drying oven at 200 °C. After heating for 1 h, it was immediately placed in ice water to quench. Finally, films were released from the glass plate to obtain all composites (defined as PMMA/P-DB composites). Both sides of polymer films were sputtered Au electrodes for electrical characterization. The details of materials and characterization are given in the Supporting Information.