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

nanocrystals to the PMDA-1,3-bis(4-aminophenoxy)benzene (BAPB) PI system to generate nanocomposites. Compared with the ε<sup>r</sup> (2.8) of pure PMDA-BAPB PI, the ε<sup>r</sup> of composites containing 18 vol% BST increased to 6.2; below 1 MHz, the dielectric loss of composite materials with different contents of BST was less than 0.04. At an addition level of 10 vol% BST, the breakdown strength of PMDA-BAPB/BST nanocomposites increased, to reach a maximum value 296 V μm�<sup>1</sup> , while the energy density of the composite was twice that of pure PMDA-BAPB PI. The observed relative increases in ε<sup>r</sup> and breakdown strength together with the reduction in dielectric loss for the nanocomposite with 10 vol% BST are desirable characteristics for practical applications [21]. Wang et al. [22] prepared PI-based composites with good dielectric properties using CaCu3Ti4O12 (CCTO) and Zr-modified CaCu3Ti3.95Zr0.05O12 (CCTZO) particles as fillers. The results showed that at a filling content of 40 vol%, the ε<sup>r</sup> of the CCTZO/PI composite film could reach a value of 70 at 10 Hz, and this was higher than that of the CCTO/PI composite film under the same conditions; at 150°C, the ε<sup>r</sup> of the CCTZO/PI composite material reached ≈260 [22].

By changing the design of the inorganic filler, interface problems between the filler and the polymer can be improved, such as poor flake/fiber morphology. The nanosheets can increase the breakdown strength of composites because they provide a uniform insulating center and a curved path for the electrons. Boron nitride nanosheets (BNNSs) have a layered structure like graphene and are wide band gap (6 eV) insulators. Unlike traditional dielectric materials (high-ε<sup>r</sup> ceramics and conductive fillers), polymer/BNNS nanocomposites may provide higher breakdown field strengths. Wan et al. [23] prepared three-phase composites of BNN, BT-fibers, and PI (BNNS@BT-fiber/PI) using in situ polymerization. The combination of BNNS and BT fibers can facilitate the dispersion of BNNS nanosheets in BT fibers, thereby improving energy storage performance. When the content of BNNS@BTfiber was 20% by weight, the ε<sup>r</sup> of the composite material was 47.57 at room temperature and 43.03 at 200°C at 100 kHz, demonstrating a reasonable thermal stability. At a BNNS@BT-fiber content of 1 wt%, the maximum Ue of the composite at 3438 kV cm�<sup>1</sup> was 7.1 J cm�<sup>3</sup> , that is, about three times that of pure PI [23].

In order to achieve better dispersion and alignment of the filler in the PI matrix, Gu et al. [24] prepared micron boron nitride (mBN)/PI composites by in situ polymerization and electrostatic spinning technology. At 30 wt% mBN, the mBN/PI composite material exhibited a high ε<sup>r</sup> (3.77) and low dielectric loss (0.007); the material also showed good thermal stability (λ = 0.696 W m�<sup>1</sup> K�<sup>1</sup> ), a high temperature index (279°C), and Tg was 240°C [24]. Cheng et al. [25] considered that molybdenum disulfide (MoS2) had an appreciable band gap and excellent heat resistance, and prepared MoS2/PI nanocomposite films. Compared with the pure PI film, the ε<sup>r</sup> of the composite film was significantly increased, while the dielectric loss remained relatively low. At a filler content of 1 vol%, the breakdown field strength reached 395 MV m�<sup>1</sup> , while *Ue* increased to ≈3.35 J cm�<sup>3</sup> . Furthermore, at 395 MV m�<sup>1</sup> , the charge and discharge efficiency could still be maintained above 80% [25]. Alumina (Al2O3) filler has good insulation performance, high thermal conductivity, and is relatively inexpensive. Therefore, it can be added to the polymer matrix as a filler to improve thermal performance. Choi et al. [26] used 6FDA, 4,4<sup>0</sup> -methylenedianiline, and bis(3-aminopropyl)-terminated polydimethylsiloxane to prepare PI films with different siloxane content. Since PI-3, PI-4 and PI-5 films were independent and flexible, PI/Al2O3 composite films were prepared at different concentrations of Al2O3 using these three PIs. The results showed that the thermal conductivity of the composite film increased with increasing Al2O3 content. The composite film containing 75% by weight of Al2O3 was flexible. The composite film containing 80 wt% Al2O3 showed improved thermal conductivity (>1.3 W m�<sup>1</sup> K�<sup>1</sup> ). Compared with traditional polysiloxane/Al2O3 composite materials, PI/Al2O3 composite films demonstrated improved thermal properties [26].
