Polymer dielectrics play a crucial role in contemporary electronics because of their flexibility, low cost, high operating voltage, corrosion resistance, self-healing, etc., which are widely used in oil and gas exploration, and electric vehicles at operating temperatures (>105 ℃). However, polymers necessitate the maintenance of a substantial volume during usage due to their low dielectric constant, thereby resulting in a diminished energy storage density. Furthermore, polymers demonstrate a relatively low thermal conductivity, and are susceptible to failure when exposed to high-temperature conditions. To function effectively in high-temperature conditions exceeding 105 ℃, an extra refrigeration system must be incorporated into an electronic power system. This undoubtedly results in an increase in both energy consumption and overall weight of the system. Consequently, developing a new generation of polymer-based film capacitors with superior high-temperature energy storage capabilities becomes a necessity. In this paper, we utilized a multidimensional synergistic concept to comprehensively improve the high-temperature energy storage performance of PI-based polymer dielectrics via combining the electrical and thermal advantages of inorganic materials and polyimide (PI) in different dimensions.

(BNNS-TiO₂-BNNS)/PI sandwich-structured composite films (denoted as BTB/PI film) were prepared by a casting method based on a concept of multidimensional synergy. Titanium dioxide (TiO₂) with a high dielectric constant was prepared into a one-dimensional nanofiber structure by an electrostatic spinning method with PI as an interlayer of BTB/PI film. Electrically insulating and thermally conductive boron nitride was prepared into a two-dimensional nanosheet structure by a liquid phase exfoliation method with PI as an insulating and thermally conductive outer layer of the BTB/PI film. The frequency-/temperature-dependence of the films were examined by a model LRS-003 high and low temperature cooling and heating system and a model E4980 precision LCR meter. The PE loops for the films were determined by a linked test rig consisting of a model ZJ-6A quasi-static d33 tester, a model MODEL 610C Trek HV amplifier and a model BWT-001 variable temperature test rig at 25 ℃ and 200 ℃, respectively.

The (BNNS-TiO₂-BNNS)/PI sandwich composite films with the thicknesses of 22-25 μm prepared are dense and uniform, without obvious holes and obvious delamination, having superior dielectric properties, breakdown strength and high-temperature energy storage properties. The results by frequency-/temperature-dependent tests indicate that the dielectric constant of BTB/PI films increases with the increase of BNNS content. Compared with the dielectric constant of pure PI film at 1kHz (i.e., 3.12), BTB/PI-3, BTB/PI-5, 2BTB/PI-7 and BTB/PI-10 improve to 4.20, 4.37, 4.67 and 4.92, respectively. The dielectric loss of the composite film at 10³-10⁶ Hz and 25-200 ℃ is <0.025. From the breakdown strength of the material, the carriers inside the composite film have greater energy and concentration at high temperatures, making the PI chain more vulnerable to attack and breakage. The E_b of pure PI film thus decreases from 380 MV/m to 337 MV/m and the β reduces from 14.60 to 8.04 when the temperature increases from room temperature to 200 ℃. However, the E_b and β of BTB/PI-5 and BTB/PI-7 are higher than those of the pure PI films at the tested temperature. At an optimal BNNS volume ratio, BTB/PI-7 achieves the maximum E_b and β at 25, 100, 150 ℃, and 200 ℃. This can be attributed to the incorporation of wide-band, high-thermal-conductivity BNNS, preventing the composite films from experiencing breakdown caused by electrical or thermal runaway. At 25, 100, 150 ℃ and 200 ℃, the energy storage performance of BTB/PI film is higher than that of pure PI film. At an optimum BNNS volume ratio, BTB/PI-7 film achieves the maximum energy density and maintains a stable high efficiency in the tested temperature range. The energy density and efficiency of BTB/PI-7 film are 4.61 J/cm³ and 88.75% (at 465 MV/m) at 25 ℃, 3.95 J/cm³ and 88.08% (at 450 MV/m) at 100 ℃, 3.65 J/cm³ and 88.53% (at 440 MV/m) at 150 ℃, 3.04 J/cm³ and 88.00% (at 380 MV/m) at 200 ℃.

The polymer-based composite films were obtained to enhance the high temperature resistant energy storage performance based on the multidimensional synergistic design. The BTB/PI film exhibited stable frequency/temperature-dependence with a dielectric loss of lower than 0.025 at 10³-10⁶ Hz and 25-200 ℃. At 200 ℃, BTB/PI-7 achieved a high energy density of 3.04 J/cm³. Moreover, the energy efficiency of BTB/PI-7 was higher than 88% at 25, 100, 150 ℃ and 200 ℃. It was demonstrated that different materials with advantages (i.e., high polarization, great breakdown strength, and high temperature resistance) were obtained via incorporating various dimensional fillers into the polymer and implementing rational structural design. Consequently, the high temperature-resistant energy storage performance of PI-based composite films could be significantly enhanced.