**4.2 Surface modification**

Because of its simple method, the compounding of fillers and polymers to produce composite materials has become accepted. However, preparation methods, external conditions, and other complications can give rise to many structural defects and electric field concentrations between the two phases of the filler and the polymer matrix. Therefore, surface treatment of the filler using a coupling agent, or decorative insulating, or conductive particles has become a key area of research [27, 28].

Halloysite (Al2Si2O5(OH)42H2O) is an aluminosilicate clay, which has a unique tubular structure. It has a high ε<sup>r</sup> (6–8), but extremely low dielectric loss (10<sup>3</sup> ). Because there are moderate hydroxyl groups on the surface that can be chemically modified, and suitable surface modification can be performed, halloysite nanotubes (HNTs) may be an ideal filler for the preparation of dielectric polymer-based composites with high ε<sup>r</sup> and low dielectric loss characteristics. Zhu et al. [29] used KH550 (3-aminopropyltriethoxysilane) and polyaniline (PANI) to modify the surface of HNT, and prepared HNT/PI, KH550 modified HNT/PI and PANI-HNT/PI nanocomposite membranes. Among these, at 100 Hz, the PANI-HNT/PI films attained a maximum ε<sup>r</sup> of 17.3, while the dielectric loss was only 0.2. Notably, the prepared composite has high breakdown strength (>110.4 kV mm<sup>1</sup> ), and a maximum discharge energy density of 0.93 J cm<sup>3</sup> ; these properties could still be maintained at temperatures ≤300°C [29]. Wang et al. [30] prepared a nanocomposite with high thermal conductivity by introducing amidefunctionalized MWCNT [MWCNT@p-phenylenediamine (PPD)] into a PEI matrix, as shown in **Figure 9**. Compared with unmodified MWCNT, MWCNT@PPD could participate in the in situ polymerization of PEI to form covalent bonds in the matrix, thereby improving the dispersibility of the filler. This method solved the disadvantages of the traditional CNT acid treatment that can destroy their conjugate structure and greatly affect the aspect ratio. The results showed that the thermal conductivity of nanocomposites containing 4.0 wt% MWCNT@ PPD ≤0.43 W m<sup>1</sup> K<sup>1</sup> [30].

**Figure 9.**

*Schematic for the preparation of multi-walled carbon nanotubes@azide polyacrylic acid (MWCNT@PPD) [30].*

### *High-Temperature Polyimide Dielectric Materials for Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.92260*

Yang et al. [27] investigated the dielectric properties of PI incorporating CCTO/ Ag nanoparticles (CCTO@Ag). The use of Ag coating to modify the surface of CCTO nanoparticles increased the conductivity of the intermediate layer, thereby enhancing the space charge polarization and Maxwell-Wagner-Sillars effect, improving the electric field distortion. The results showed that when the content of CCTO@Ag was 3 vol%, the ε<sup>r</sup> of PI/CCTO@Ag composites was significantly increased to 103, which was about 30 times the ε<sup>r</sup> of pure PI. At the same time, the dielectric loss was very low at 0.018 [27]. Wang et al. [31] used 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) and TH-615 acrylic-acrylate-amide copolymers to modify BT nanoparticles, and then prepared a BT/PI composite film by in situ polymerization. The results showed that this surface modification method could improve the dispersion uniformity of filler particles in the matrix and improve the interfacial compatibility between the two phases. At 103 Hz, a BT/PI film modified with 8% PBTCA had a ε<sup>r</sup> of 23.5, a dielectric loss of 0.00942, a breakdown strength of 80 MV m<sup>1</sup> , and a *Ue* of 0.67 J cm<sup>3</sup> . At the same frequency, the composite modified with 6% TH-615 had a ε<sup>r</sup> of 20.3, a dielectric loss of 0.00571, a breakdown strength of 73 MV m<sup>1</sup> , and a Ue of 0.68 J cm<sup>3</sup> [31]. Due to its unique chemical structure, GO shows great potential in the field of capacitors. The graphene oxide sheet has many hydroxyl groups and epoxy groups on the surface while carboxyl groups are mainly located on the edges. However, most studies involving graphenebased composites have used only marginal carboxyl groups, so the polymer chains were attached to the edges only. Fang et al. [32] made full use of oxygen functional groups to prepare PPD-carboxyl-functionalized graphene oxide (CFGO)/PI composites. The polymer chain was fixed on the base surface, and the graphene oxide sheet was effectively separated. Thermogravimetric analysis (TGA) tests showed that PPD-CFGO/PI composites had good thermal stability below 500°C. When the content of PPD-CFGO was 4 wt%, the ε<sup>r</sup> increased to 36.9, which was 12.5 times higher than that of pure PI polymer (≈3.0), the dielectric loss was only 0.0075, and the breakdown strength remained at a high level [32].

However, the covalent functionalization method adopted by Fang et al. [32] reduced the conductivity of graphene by destroying the π-π conjugate structure of graphene. In order to overcome this shortcoming, Feng et al. decorated the surface of rGO with a solid π-π stack by insulating reduced polyaniline (R-PANI) to introduce a space effect and effectively prevent the irreversible agglomeration of rGO. At 1 kHz, the highest ε<sup>r</sup> (25.84) was observed in nanocomposite films containing 20 wt% rGO@R-PANI, and the dielectric loss was 0.11. The ε<sup>r</sup> and dielectric loss of rGO/PI nanocomposite films were 8.23 and 56.4, respectively. Furthermore, the 5 wt% weight loss temperature for 20 wt% rGO@ R-PANI/PI nanocomposite film was 480°C, indicating that the nanocomposite film has great potential in the field of high-temperature dielectric materials [5]. Yue et al. [5] introduced reduced barium titanate (rBT), sintered in a reducing atmosphere (95N2/5H2), to PI without using any modifier or surfactant ingredients in the matrix. Surface defects of rBT and interface interactions between two phases caused by the reducing atmosphere lead to an increase in ε<sup>r</sup> and Ue. Compared with pure PI, the rBT/PI composite with 30 wt% rBT exhibited the following characteristics: The ε<sup>r</sup> at 1000 kHz was ≤31.6 (pure PI = 4.1), the material maintained a low dielectric loss (0.031), the Ue of 9.7 J cm<sup>3</sup> at 2628 kV cm<sup>1</sup> represented an increase of >400% (for pure PI Ue = 1.9 J cm<sup>3</sup> at 3251 kV cm<sup>1</sup> ) [5].

## **4.3 Core-shell structure**

Recently, much work has focused on introducing an intermediate layer or an insulating shell on the surface of the filler to prevent them from directly connecting to each other. Fillers in composite materials can increase electrical conductivity and cause excessive polarization interfaces. Researchers are also attempting to introduce intermediate layers or oxide shells between fillers to reduce dielectric loss. Studies have also shown that the core-shell structure can achieve a high εr, low dielectric loss, and high energy density [28, 33].

Liu et al. [34] synthesized a sandwich-shaped core-shell SiO2@GO hybrid to prepare a novel SiO2@GO/PI flexible composite film using in situ polymerization. The dense SiO2 layer grafted onto the GO surface can effectively suppress leakage current. The results showed that at 40 Hz, the ε<sup>r</sup> of the composite material containing 20 wt% SiO2@GO was as high as 73, which was 21 times that of pure PI (3.0), and the dielectric loss was only 0.39. In order to improve interfacial compatibility, two coupling agents, 3-aminopropyl triethoxysilane and 3-glycidoxypropyltrimethoxysilane (GPTS), were used to modify the surface of SiO2@GO: At 40 Hz, the ε<sup>r</sup> of the GPTS-SiO2@GO/PI composite increased to 79 and the loss decreased to 0.25. This significant improvement in the dielectric properties was due to the improved dispersibility of the filler following GPTS modification. Wang et al. [28] prepared a core-shell structure of BT@SiO2 nanofibers by electrospinning, and successfully prepared a nanocomposite membrane composed of core-shell BT@SiO2 nanofibers and PI. Because SiO2 has very low dielectric loss (0.00002) and moderate εr, using a thin layer of SiO2 to isolate PI from BT nanofibers can alleviate the local field concentration. The latter is caused by the large difference in ε<sup>r</sup> between the concentrations of the two phases, thereby enhancing the breakdown strength of the PI nanocomposite film. Compared with pure PI, the composite film filled with 3 vol% BT@SiO2 nanofibers had a maximum Ue of 2.31 J cm<sup>3</sup> at 346 kV mm<sup>1</sup> (pure PI *Ue* = 1.42 J cm<sup>3</sup> at 308 kV mm<sup>1</sup> ). TGA also showed that below 500°C, BT@SiO2/PI nanocomposite films had good thermal stability [28]. Wang et al. [35] prepared a core-shell AgNW/PI composite film with high ε<sup>r</sup> and low loss (see **Figure 10**). The insulating shell could protect the silver cores from being directly connected to each other, so that when the ε<sup>r</sup> of the composite film reached its maximum value (126), the dielectric loss remained at a low level [35].

Weng et al. [33] synthesized a novel core-shell of Ag@Al2O3 nanoparticles as conductive fillers and doped them into PI to prepare Ag@Al2O3/PI composite films. The composite film containing 10% by weight of Ag@Al2O3 had a ε<sup>r</sup> of 21, which was seven times higher than that of pure PI (3.1). This increase in ε<sup>r</sup> may be due to the high electrical conductivity of the Ag@Al2O3 filler, which caused interfacial polarization inside the composite in the applied electric field. Hence, when the mass fraction of Ag@Al2O3 was increased to 30%, the maximum value of ε<sup>r</sup> was 124 [33].

### **4.4 Multilayer structure**

Most polymer nanocomposites are expected to achieve high energy density by combining the high breakdown strength of the polymer matrix with the high ε<sup>r</sup> of the filler. In fact, when the filler is introduced into the polymer matrix, the breakdown strength often decreases, especially when the volume fraction of the composite filler is high, which does not improve the energy density of the nanocomposite. Therefore, there is a need to expand nanocomposites into multilayer structures to compensate for the reduced breakdown strength [36].

Chen et al. [36] designed a three-layer PI composite membrane by combining KTa0.5Nb0.5O3 (KTN) nanoparticles with PI. Pure PI (with high breakdown field strength) was used as the middle layer with KTN/PI nanocomposite as the two outer layers to improve the energy storage performance of the entire composite film. The results showed that the maximum discharge energy density of the triple-layer composite film (t-KPI) was 3.0 J cm<sup>3</sup> at 300 kV mm<sup>1</sup> , which was much larger

*High-Temperature Polyimide Dielectric Materials for Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.92260*

**Figure 10.**

*(a) TEM image of core-shell structured AgNW. (b) TEM image of an individual AgNW. (c) SEM image of core-shell structured AgNW. (d) SEM image of AgNW/PI hybrid film [35].*

than the maximum discharge energy density of the equivalent single-layer composite film (1.5 J cm<sup>3</sup> , at 210 kV mm<sup>1</sup> ); at a high electric field of 300 kV mm<sup>1</sup> , the t-KPI composite film could still maintain 88% charge and discharge efficiency [36]. Amin Azizi et al. [37] prepared large-scale high-quality hexagonal boron nitride (h-BN) films using vapor deposition technology (CVD) and transferred them to PEI films to synthesize h-BN/PEI/h-BN composite film. As shown in **Figure 11**, this composite film exhibits excellent charge-discharge efficiency and dielectric stability at high temperatures. At 100°C, the discharge energy density of h-BN-coated PEI reached 2.93 J cm<sup>3</sup> , and its charge-discharge efficiency was >90%. As the operating temperature increased, its advantages become more obvious. At 200°C, the energy density of h-BN-coated PEI film was 1.19 J cm<sup>3</sup> . Rapid cyclic discharge experiments were performed at 150°C and 200 MV m<sup>1</sup> to test the stability of h-BN/PEI/h-BN composite films under electric fields and high temperature. The results demonstrated that the h-BN/PEI/h-BN film coated with 19 layers of h-BN did not show any reduction in discharge energy density and charge-discharge efficiency over 55,000 charge-discharge cycles [37].

Chen et al. [38] prepared an amino-modified CNT/PI (NH2-MWCNT/PI) flexible composite film with a three-layer structure in which a high-dielectric NH2- MWCNT was inserted between pure PI layers (serving as the bottom and top layers) of the complex. Since the conductive paths of the insulating layer could be effectively isolated, the three-layer composite film showed high ε<sup>r</sup> and low dielectric loss. It is worth noting that at 1 kHz, when the NH2-MWCNT content of the intermediate layer was 10 wt%, the multilayer composite film (P-10-P) gave the highest ε<sup>r</sup> of 31.3, while the dielectric loss was 0.0016. In addition, the maximum energy density of the composite membrane containing 5 wt% NH2-MWCNT in the intermediate layer (P-5-P) was as high as 1.95 J cm<sup>3</sup> , which is more than 50% higher than that of pure PI (1.41 J cm<sup>3</sup> ). The maximum energy density of the composite film P-10-P also remained at 1.31 J cm<sup>3</sup> [38]. Among the various films,

**Figure 11.**

*Charge-discharge efficiency of the dielectrics as a function of temperatures measured at an applied field of (a) 200, (b) 300, and (c) 400 MVm<sup>1</sup> . (d) Discharged energy density achieved at above 90% charge-discharge efficiency at varied temperatures [37].*

h-BN/PI composite film filled with 5 vol% h-BN as the outer layer could improve the heat dissipation ability of the three-layer composite material, thereby maintaining the dielectric strength and suppressing leakage current at high temperatures. Hence, this sandwich structure composite material had excellent energy storage properties and high temperature stability. At 25 and 150°C, the maximum field strengths of the composite film with a Zr and Ca modified BT (BZT-BCT) content of 1 vol% in the intermediate layer were 360 and 350 kV mm<sup>1</sup> respectively, while the storage densities were 2.3 and 1.83 J cm<sup>3</sup> , respectively [39]. Zhou et al. [40] proposed a method for preparing high-performance polymer dielectrics at high temperatures (designed roll-to-roll plasma enhanced CVD), which was easily adapted to large-scale production of various surface-functionalized polymer films. In this experiment, they uniformly deposited wide-band gap SiO2 on the dielectric polymer film at ambient temperature and atmospheric pressure, and their productivity was comparable to that of melt extrusion. The results showed that the introduced SiO2 layer increased the potential barrier at the electrode/dielectric interface, resulting in a significant decrease in conductivity. Therefore, compared with the pure polymer (see **Figure 12**), the SiO2-coated film exhibited good hightemperature capacitance performance and had a higher energy storage efficiency (η) value. For example, at 150°C, when η > 90%, the maximum Ue values of PEI-SiO2, PEN-SiO2, PI-SiO2, PC-SiO2, and FPE-SiO2 composite films were 2.12, 1.75, 1.24, 1.79, and 2.06 J cm<sup>3</sup> , which were respectively 236, 672, 510, 1279, and 644% greater than the corresponding pure films. At 100°C, when η > 90%, Ue for PEI-SiO2 was 3.0 J cm<sup>3</sup> [40].

*High-Temperature Polyimide Dielectric Materials for Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.92260*

### **Figure 12.**

*(a) Charge-discharge efficiency and discharged energy density of BOPP and BOPP-SiO2 films with 180 nm coating layer on each side of the polymer measured at 120°C. (b) Charge-discharge efficiency of the various dielectric films before and after coating measured at 150°C. (c) Maximum discharged energy density of the various dielectric films before and after coating achieved at above 90% charge-discharge efficiency measured at 150°C. (d) Discharged energy density obtained from cyclic fast discharge tests of pristine BOPP and BOPP-SiO2 films [40].*
