Abstract：Faced with the application requirements of various electromagnetic devices and new energy vehicle inverters, film capacitors that combine easy processability with high charge/discharge density and efficiency are receiving more attention. However, the commonly used commercial BOPP films on the market cannot be used in high temperature applications due to their own structural limitations, and the charge/discharge energy density and efficiency are also low, so the development of high energy storage density film capacitors for high temperature applications is the future development trend. In order to solve the problem of poor energy storage performance due to the low breakdown field strength (Eb) of pure polymer energy storage films, composite structural design of broadband inorganic materials with polymers is an effective way. In this work, in order to reduce the effects of poor compatibility between inorganic and polymer surfaces due to excessive differences in physicochemical properties, resulting in electric field distortion, polyetherimide (PEI) was chosen as the matrix, BNNSs with wide forbidden bands and high thermal conductivity were used as fillers, a potential barrier layer doped with BNNSs was constructed on both sides of the pure PEI film by electrostatic spinning, and sandwich structured polymer-based composite films were designed and prepared. By increasing the overall barrier height of the composite film and suppressing the carrier injection and internal breakdown path development at the electrode at high temperature, the Eb of the composite film is finally enhanced and the conductivity loss is reduced. Moreover, by comparing the doping of different particle sizes of BNNSs, it was found that the filler size plays a role in regulating the microstructure and macroscopic properties in the structure design, and eventually the smaller size of 200 nm BNNSs obtained higher energy storage density and charge/discharge efficiency, and the energy storage density and charge/discharge efficiency of the composite film filled with 5% BNNSs reached 5.65 J/cm3 and 96% efficiency at room temperature; even at 150℃, the energy storage density of 2.52 J/cm3 and 95% charge/discharge efficiency can be achieved. In addition, the breakdown mechanism of polymer-based composite films is complex, and the physical process of breakdown cannot be captured experimentally. Therefore, this work simulated the breakdown path evolution of polymer-based composite films filled with BNNSs of different particle sizes using a stochastic breakdown model and found the following conclusions: (1) The potential barrier layer on both sides of the pure PEI effectively reduces the carrier injection at the electrode/dielectric and its transport inside the dielectric, especially the sandwich structure, which has a significant inhibitory effect during the development of the breakdown path, while the inorganic material doped in the form of nanosheets also provides this resistance. (2) The composite films doped with smaller sized BNNSs have higher breakdown strength, attributed to the higher number of smaller sized BNNSs at the same volume fraction, providing a higher chance of hindrance. (3) Excessive filler volume fraction will lead to local agglomeration in the composite film, resulting in distortion of the local electric field and severe degradation of Eb. In this work, the composite film with 5% BNNSs exhibited the optimal energy storage performance.
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2024, Vol. 39 
