Enhancement thermal stability of polyetherimide-based nanocomposites for applications in energy storage

Highlights

• SrTiO₃ nanoparticles/polyetherimide nanocomposite film are designed and prepared by grafting method.

• The composite film obtains excellent high-temperature energy storage performance.

• An excellent high-temperature discharged energy density of 6.6 J/cm³ at 100 °C is achieved.

Abstract

Improving thermal stability of high-performance polymer-based nanocomposite films for electrical energy storage is essential to meet ever-increasing demands for the electrical industry, especially at harsh environment applications. Here, the polyetherimide (PEI)-based composites films are prepared via grafting method in the presence of SrTiO₃ (ST) nanofillers to substantially improved capacitive performances at elevated temperature. The composites films with the optimized filler compositions show a high discharged energy density of 6.76 J/cm³ under 600 MV/m at room temperature. Meantime, excellent high-temperature discharged energy density of 6.6 J/cm³ at 100 °C for the composites films is also achieved, which is superior to most of the previously reported. The simulations further demonstrate that the ST nanoparticles embedded into the composites films could effectively improve heat dissipation in comparison with pristine PEI matrix, resulting in enhancement high-temperature energy storage capabilities. This work gives considerable promise for high energy density polymer-based nanocomposite films capacitors under harsh environments.

Introduction

Electrostatic capacitors as storage media of electric charges have created a broad spectrum of applications for them in microelectronics and electric power systems, due to their capability of delivering high power density and extremely discharge time among the electrical energy devices [[1], [2], [3], [4], [5]]. The demand for high-temperature electrostatic capacitors arises from numerous emerging applications such as electric vehicles, wind generators, solar converters, aerospace power conditioning, and down hole oil and gas explorations, in which the power systems and electronic devices have to operate at elevated temperatures [4,[6], [7], [8], [9]]. The biaxially oriented polypropylene (BOPP), the material of the state-of-the-art commercial polymer film capacitors, possesses high breakdown strength (E_b) and very low dielectric loss (~0.0002) [10]. However, the discharge energy density (U_d) of BOPP (<2 J/cm³) is limited by its relatively low dielectric constant (2.2). Moreover, the working temperature of BOPP is limited by 120 °C, owing to low U_d to only 0.27 J/cm³ [11].

To address these imperative needs, a variety of well-established engineering polymers, such as polyetherimide (PEI), polyimide (PI), polyphenylenesulphide (PPS), polyarylene ether nitrile (PEN), crosslinked divinyltetramethyldisiloxanebis (benzocyclobutene) (c-BCB), and fluorene polyester (FPE), have been exploited as the matrix for application in high-temperature composites materials [7,[12], [13], [14], [15], [16], [17], [18], [19]]. As these engineering polymers have high tensile strength, excellent mechanical properties, high glass transition temperature (T_g) and superior thermal stability [7,15,20]. When the temperature is close to T_g, the polymer chains begin to move, losing dimensional and mechanical stability and thus causing very sharp fluctuations in dielectric constant and dielectric loss factor [21]. For example, Wang and Li et al. reported that the boron nitride nanosheets (BNNSs) and Al₂O₃ nanostructures incorporated into c-BCB achieved excellent high temperatures energy storage performances [4,6,22]. In order to restrict polymer chains moves at the high temperature, the poly(aryl ether sulfone) (DPAES) on the surface of functionalized boron nitride nanosheets (BN-BCB) could effectively improve the dielectric and energy storage performances of polymer/ceramic composites at high temperatures [21]. However, not only is c-BCB a very expensive material, but it also requires high temperature curing at 250 °C in an inert gas environment to complete the film preparation processes, which further increases cost and complexity.

Polyimide (PI) is a thermoset polymer synthesized from dianhydride and diamine (or diisocyanate) monomers through a condensation reaction followed by chemical imidization [7,19,23]. PI possesses the inherent advantages including exceptional resistance to heat and chemicals and decent mechanical strength. These advantages are originates from the imide structure and aromatic structure, results in very high T_g [23]. In addition, the PI have excellent insulating properties, low dissipation factor, high breakdown strength, and high volume resistivity, in which makes a good candidate for high-temperature film capacitor applications. Most of researches have demonstrated that the high dielectric constant inorganic nanofillers incorporated into the PI matrix could significantly enhance energy storage capability at room temperature and high temperature [[24], [25], [26], [27], [28], [29], [30], [31]].

Poly(ether imide) (PEI), a modified version of PI, is a class of recyclable thermoplastic amorphous polymers with good solubility and processability. Therefore, the PEI is used as polymer matrix. In particular, dispersing inorganic fillers evenly in PEI matrix is a key factor that affects the properties of composites films, and the surface modification of inorganic particles can improve the compatibility between the inorganic phase and the organic phase [32,33]. In this work, the SrTiO₃/polyetherimide (ST/PEI)-based composites films are prepared via grafting method. The paraelectric phase ST with relatively high dielectric constant and low hysteresis are conductive to enhancing energy storage density as comparison of the other ferroelectric and antiferroelectric phase fillers. The dielectric constant and breakdown strength of composites films could be improved effectively through loading ST nanofillers, resulting in enhancement energy storage density at room temperature (RT). Especially, the composites film with 5 vol% ST NPs exhibits high discharged energy density of 6.76 J/cm³ under 600 MV/m at RT. In addition, the finite element simulations and experimental results indicate that the high-temperature energy storage capability of composites films could be improved. Moreover, a superb high-temperature discharged energy density of 6.6 J/cm³ at 100 °C for the composites film with 5 vol% ST NPs is also attained. The results indicate that this work might provide a simple and effective design approach for development high-temperature polymer dielectric composites.