Enhanced Discharged Efficiency and High Energy Density at Elevated Temperature in Polymer Dielectric via Manipulating Relaxation Behavior

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Oct 2020Enhanced Discharged Efficiency and High Energy Density at Elevated Temperature in Polymer Dielectric via Manipulating Relaxation Behavior Yu Zhang, Zheng Liu, Lixue Zhu, Jie Liu, Yunhe Zhang and Zhenhua Jiang Yu Zhang Engineering Research Center of Super Engineering Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 , Zheng Liu Engineering Research Center of Super Engineering Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 , Lixue Zhu Engineering Research Center of Super Engineering Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 , Jie Liu Engineering Research Center of Super Engineering Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 , Yunhe Zhang *Corresponding author: E-mail Address: [email protected] Engineering Research Center of Super Engineering Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 and Zhenhua Jiang Engineering Research Center of Super Engineering Plastics, Ministry of Education, College of Chemistry, Jilin University, Changchun 130012 https://doi.org/10.31635/ccschem.020.201900111 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Polymer dielectrics with excellent dielectric properties and energy storage performance under elevated temperature are urgently needed in electrical power systems. Polyetherimide (PEI), which is supposed to be the most promising candidate among polymer dielectric materials, has a limitation for high-temperature dielectric applications especially at high electric field owing to secondary relaxation. Herein, a series of self-cross-linkable oligomers are prepared by combining the advantages of PEI and phenylethynyl groups. Cross-linked polymer dielectric films with enhanced voltage resistance and low leakage current density are obtained by cross-linking under oxygen. The occurrence of β-relaxation is diminished by cross-linking, which makes polymer dielectrics exhibit desirable dielectric stability over a broad temperature and frequency range. Particularly, the dielectric loss of c-10%PEPA-PEI at 1000 Hz is 0.0037 and 0.0043 at room temperature and 150 °C, respectively. Simultaneously, the polymer dielectric maintains a still low dielectric loss (<0.010) between 102 and 106 Hz. Furthermore, c-10%PEPA-PEI possesses excellent high-temperature energy storage performance owing to much interchain reaction originating from proper chain length, and exhibits ultrahigh charge–discharge efficiency (>95.0%) and improved energy density (3.6 J/cm3) at 150 °C. The authors believe that these cross-linked films showing excellent dielectric performance have a promising future in high-temperature applications. Download figure Download PowerPoint Dielectric capacitors with dielectric materials as media for electric charges storage play a crucial role in the modern electronics industry and advanced energy storage applications.1–3 The emergence of applications such as electric vehicles, wind turbine generators, and pulsed power systems promotes the requirements for dielectric materials for next-generation microelectronics and power systems.4–6 In comparison with ceramic dielectric materials, polymer dielectric materials are more suitable for energy storage capacitor applications owing to their unique features, including high electric breakdown strength, low dielectric loss, excellent flexibility, and easy processing.7,8 Therefore, more and more researchers are focused on polymer dielectric materials.9–11 However, most polymer dielectric materials are limited to relatively low operating temperatures, and fail to meet the requirements for the further development of numerous applications such as hybrid electric vehicles and aerospace power conditioning, in which the devices always have to work at elevated temperatures.12 For example, commercialized biaxially oriented polypropylene (BOPP), as the state-of-the-art capacitor film, exhibits immense breakdown strength at room temperature and is used in power inverters, which are the most important electronic components of hybrid electric vehicles. Nevertheless, BOPP can be operated only below 105 °C, and the charge-discharge efficiency and breakdown strength fall steeply with the rise of temperature under high applied electric fields.13 Therefore, it is urgent to develop high-temperature polymer dielectric materials with good thermal stability, dielectric properties, and energy storage properties in order to meet the demand for high-temperature applications. Polyetherimide (PEI) is a widely used engineering polymer with low cost and high yield, due to its favorable heat resistance and processing properties, excellent mechanical properties, and chemical and dimensional stability.14 Meanwhile, it has a low dissipation factor and outstanding energy storage properties; it has been reported that the charge–discharge efficiency of PEI can reach more than 90.0% and the energy density exceeds 0.5 J/cm3 at 150 °C and 200 MV/m.15 Therefore, it is supposed to be the most promising candidate among polymer dielectric materials for high-temperature energy storage applications. However, the linear structure of PEI, containing carbonyl groups, aromatic imide rings, and ether linkages, has local segment chain and functional groups movement along with the increase of temperature, which will cause secondary relaxation,16 where the localized rotational fluctuations of the dipole vectors of carbonyl groups cause β-relaxation.17 The relaxation is detrimental to the dielectric properties of PEI,18 especially in the case of high temperature and high electric field, which makes the leakage current of the material increase and the energy storage density decrease. Under an alternating electric field, the conduction loss of the material will increase dramatically and the charge–discharge efficiency will be greatly reduced. Cross-linking is an effective strategy for improving material temperature resistance as well as enhancing the breakdown strength and charge–discharge efficiency.19–21 Khanchaitit et al.21 presented a thermal cross-linking approach to address the critical issue of energy in ferroelectric polymers under high electric fields, and the ferroelectric polymer exhibited significantly reduced dielectric loss and greatly improved breakdown strength. In addition, Li et al.22 prepared composites by introducing cross-linkable divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) and boron nitride nanosheets and greatly improved the dielectric properties at high temperature by constructing a cross-linking network; the charge–discharge efficiency of c-BCB exceeds 75.0% at 150 °C and 250 MV/m. Moreover, a new class of all-polymer prepared by cross-linking of melt-processable fluoropolymers was reported, and the cross-linked polymers exhibited large discharged energy densities and great charge–discharge efficiencies along with excellent breakdown strength at elevated temperatures.23 Inspired by these previous studies, we introduce reactive groups into molecular chains as cross-linking points to reduce the occurrence of relaxation and the motion of the molecular chain, and then cure them by heating to obtain high-performance dielectrics. The phenylethynyl group has the desirable comprehensive performance and provides the following advantages.24–27 First, the degree of cross-linking can be controlled by adjusting the length of the molecular chain. Second, the cross-linking reaction without cross-linking agent is a self-cross-link, which means that the process has no volatile matter to escape and the generation of voids is avoidable. This property is essential for the preparation of high-quality films. Furthermore, the thermal and mechanical properties of the polymer can be greatly improved due to the formation of the space network structure after cross-linking.28,29 In recent years, the introduction of phenylethynyl groups into polyimide and other high-performance resins has become more extensive.30,31 Herein, combining the advantages of PEI and phenylethynyl groups, self-cross-linkable 4-phenylethynylphthalic anhydride (PEPA)-terminated ether imide oligomers derived from 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride (BPADA) and 4,4′-oxydianiline (4,4′-ODA) were prepared, as shown in Figure 1a. Specifically, the oligomer with an excess of 10% amino groups is called 10%PEPA-PEI. It has an average molecular weight (Mn) of 28,070 g/mol and polydispersity of 1.60 ( Supporting Information Table S1). Afterward, the cross-linked film named c-10%PEPA-PEI was prepared via the solution casting and heat-cross-linking under oxygen and 320 °C (Figure 1b). Visible light images of 10%PEPA-PEI films obviously display the cracks (Figure 1c), which may be due to the low molecular weight and poor mechanical properties of oligomers. Significantly, the films after self-cross-linking under oxygen exhibit good flexibility (Figure 1d). Furthermore, the effect of cross-links on the film surface roughness is minimal. The atomic force microscopy (AFM) images show little differences before and after cross-linking as a result of the unique cross-linking characteristics of phenylethynyl group (no volatile matter escapes), where the surface roughnesses are 0.331 and 0.345 nm, respectively (Figures 1e and 1f). In consequence, this general approach is conducive to the industrial production of high-quality films owing to its simple fabrication process. Figure 1 | (a) Synthetic route of the phenylethynyl-terminated PEI oligomers. (b) Schematic of preparation process of the uncured and cured films. The digital photos of (c) 10%PEPA-PEI and (d) c-10%PEPA-PEI films cured under O2-320 °C-2 h. The AFM images of (e) 10%PEPA-PEI and (f) c-10%PEPA-PEI. Download figure Download PowerPoint Alternatively, the atmosphere and temperature in the cross-linking process will affect the properties of films. The frequency- and temperature-dependent dielectric constant and dissipation factor of the films cross-linked under different conditions are shown in Figures 2a and 2b. Specifically, the dielectric constants of cross-linked films are between 2.9 and 3.2 at room temperature and 1000 Hz. At 150 °C and 1000 Hz, the dielectric constants are between 2.9 and 3.1, indicating that the cross-linked films possess excellent dielectric stability. Meanwhile, the dissipation factor of cross-linked films decreases with the increase of cross-linking temperature, whether under oxygen or in vacuum, and maintains a low value (<0.010). In addition, relaxation is more effectively suppressed in films cured under oxygen than in vacuum, which indicated that the molecular chain segments of the films cross-linked under oxygen are more restricted than those cured in vacuum at high temperature. More interestingly, the discharged energy density (Ue) and charge–discharge efficiency (η) of films exhibit the regularity that η increases while Ue decrease with the improvement of cross-linking temperature in both oxygen and vacuum conditions, which can be seen in Figures 2c and 2d. Meanwhile, the breakdown strength and η of films cross-linked under oxygen, especially under high electric field, are more desirable than that in vacuum. It is clearly observed that the films cross-linked under oxygen and 320 °C possess outstanding η. Moreover, the discharged energy density of this film is relatively low, owing to the smaller dielectric constant derived from the restriction on the movement of molecular chain segments. However, its voltage resistance and charge–discharge efficiency are more excellent than the others, which is of great significance to the development of high-performance polymer dielectric films. Furthermore, Figures 2e and 2f provide evidence that the films cross-linked under oxygen have low leakage current density and volume conductivity, which greatly improves the dielectric performance of the films. The distinction of dielectric performance may be probably attributed to the different structure caused by cross-linked mechanism ( Supporting Information Figures S2 and S3 and Table S2).26,27 Meanwhile, the curing process under oxygen is more convenient and easier than curing in vacuum, so it is a green and efficient approach. In summary, a film with favorable dielectric and energy storage performance can be obtained when cross-linked under oxygen at 320 °C. Figure 2 | (a) Dielectric constant and dissipation factor of dielectrics with different curing process at room temperature as a function of frequency. (b) Dielectric constant and dissipation factor of dielectrics with different curing process at 1000 Hz as a function of temperature. Discharged energy density and charge–discharge efficiency of dielectrics at (c) room temperature and (d) 150 °C as a function of electric fields. (e) Current density of the dielectrics as a function of electric fields at room temperature. (f) Volume conductivity of dielectrics at room temperature and 200 MV/m. Download figure Download PowerPoint As far as we know, the charge–discharge efficiency of PEI will be greatly reduced when electric field exceeds 400 MV/m at high temperature, which may be owing to the increased dielectric loss caused by β-relaxation, though PEI is considered as the most promising candidate among polymer dielectric materials. Consequently, secondary relaxation is detrimental to the films in high-temperature dielectric applications. It has been previously reported that PEI exhibits β-relaxation at about 100 °C, which is also proved in our study ( Supporting Information Figure S4). The β-relaxation obviously appears at 100 °C and 100 Hz, and the relaxation peak moves toward higher temperature with the increase of frequency. When the frequency reaches 100 kHz, the relaxation peak reaches about 140 °C. It is well acknowledged that cross-linking can limit the movement of molecular chains. To maximally limit β-relaxation caused by rotational fluctuations of carbonyl group dipoles, and simultaneously to maintain excellent dielectric and energy storage properties, films of different cross-linking degree were prepared at 320 °C and 2 h under oxygen. First, phenylethynyl-terminated oligomers with an excess of 5% and 20% amino groups were prepared, which are called 5%PEPA-PEI and 20%PEPA-PEI (Figure 1a). These oligomers show Mn of 48,630 and 18,850 g/mol, and polydispersities of 1.63 and 1.49, respectively ( Supporting Information Table S1). Then the cross-linked films, named c-5%PEPA-PEI and c-20%PEPA-PEI, were parpared via the heat-cross-linking (Figure 1b). The cured films display excellent thermostability and satisfactory mechanical properties ( Supporting Information Figures S5–S7), which is extremely significant for desirable dielectric films. As shown in Figure 3, with an increasing degree of cross-linking, the dielectric loss of c-10%PEPA-PEI is much lower than that of c-5%PEPA-PEI, possibly due to the restrictions on the movement of molecular chains by interchain cross-links. Remarkably, the β-relaxation peak gradually moves toward higher temperature, which increases the dielectric performance of the films. Nevertheless, the dielectric constant and dielectric loss of c-20%PEPA-PEI are slightly greater than that of c-10%PEPA-PEI. Owing to the short molecular chain length, 20%PEPA-PEI may be take place much intrachain reaction. This may induce a slight increase in the interchain spacing, thereby reducing polarization barriers of dipoles in the polymer chains along the applied electric field and generating a much higher dielectric constant.10 At the same time, the spacing may produce defects which increase the dielectric loss. Figure 3 | (a) Dielectric constant and (b) dissipation factor of pristine PEI and cured films at different degrees of cross-linking at room temperature as a function of frequency. (c) Dielectric constant and (d) dissipation factor of pristine PEI and cured films at different degrees of cross-linking at 1000 Hz as a function of temperature. Download figure Download PowerPoint X-ray diffraction data of cured films at different degree of cross-linking were carried out to probe structural changes (Figure 4). The broad X-ray peak for the cured films arises from interchain segment scattering in the amorphous state. The broad X-ray peak of the c-5%PEPA-PEI, c-10%PEPA-PEI, and c-20%PEPA-PEI is at ca. 2θ = 16.0°, 17.0°, and 16.4°, respectively. The X-ray diffraction data indicate that the interchain spacing of c-10%PEPA-PEI is less than that of c-5%PEPA-PEI. The decrease of interchain spacing may restrict the movement of the molecular chains and also contribute to the limitation of β-relaxation. Meanwhile, the interchain spacing for c-20%PEPA-PEI is greater than that of c-10%PEPA-PEI. The expanded interchain spacing of c-20%PEPA-PEI is mainly induced by the intrachain reaction, which facilitates dipole polarization and and leads to a higher dielectric constant than that of c-10%PEPA-PEI. However, the expanded interchain spacing may cause defects and thus damage dielectric properties. The molecular chains structural changes are consistent with the dielectric properties varieties of cross-linked films. To summarize: by introducing cross-linkable phenylacetylene groups, the relaxation of PEI has successfully been reduced and c-10%PEPA-PEI possesses excellent dielectric properties owing to the optimal network structure. Figure 4 | (a) X-ray diffraction data of cured films at different degrees of cross-linking. Background subtracted data of (b) c-5%PEPA-PEI, (c) c-10%PEPA-PEI, and (d) c-20%PEPA-PEI. Download figure Download PowerPoint Energy storage properties for the films at different degrees of cross-linking were carried out to study the high-temperature dielectric performance. As shown in Figure 5a, the P–E loops of cured membranes under an electric field of 300 MV/m at 150 °C, especially for c-10%PEPA-PEI, are much slimmer than for PEI. The P-E loops also reflect that the cured films have higher charge–discharge efficiency and discharged energy density than PEI (Figures 5b and 5c). For example, the discharged energy density of c-10%PEPA-PEI is up to 3.6 J/cm3 at 500 MV/m and 150 °C, and its charge–discharge efficiency is 96.5%. However, the breakdown strength and charge–discharge efficiency of c-5%PEPA-PEI and c-20%PEPA-PEI are reduced compared with c-10%PEPA-PEI. It may be because the molecular chain length of 5%PEPA-PEI is slightly longer, so that the interchain reaction is incomplete. Meanwhile, the interchain spacing coming from intrachain reaction of c-20%PEPA-PEI causes more defects, reducing the breakdown strength, which may account for mechanical property decreases. At the same time, the cross-linked films process excellent breakdown strength and charge–discharge efficiency at 200 °C ( Supporting Information Figure S8). The discharged energy density and charge–discharge efficiency of c-10%PEPA-PEI at 400 MV/m are 2.0 J/cm3 and 72.2%, respectively. In general, this study shows that the films have outstanding charge–discharge efficiency and discharged energy density (Figure 5d), which is the most excellent polymer dielectric, with outstanding high-temperature energy storage performance, compared with the reported dielectrics so far.32–36 Figure 5 | (a) Polarization of pristine PEI and cured films at different degrees of cross-linking at 150 °C as a function of electric fields. (b) Discharged energy density and (c) charge–discharge efficiency of the dielectrics at 150 °C as a function of electric fields. (d) Reported discharged energy density and charge–discharge efficiency of the polymer dielectrics under 150 °C. Download figure Download PowerPoint In conclusion, a series of polymer dielectric films with excellent dielectric properties have been prepared. The feedback of cross-linked films to dielectric properties under oxygen is more effective than that in vacuum. Meanwhile, cross-linking can limit the movement of molecular chains and thus reduce the occurrence of β-relaxation caused by the localized rotational fluctuations of carbonyl groups, which can effectively improve dielectric properties of films. In addition, the length of polymer chain has an impact on the dielectric properties of cross-linked films. Incomplete interchain reactions, as well as intrachain reactions can be detrimental to dielectric properties. Specifically, c-10%PEPA-PEI with appropriate chain length exhibits ideal features for optimal network structure, including significantly enhanced discharged density and remarkably improved charge–discharge efficiency. This study may lay a foundation for the development of high-temperature polymer dielectric materials in advanced energy storage applications. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. 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