With the continuous development of new power systems, polymer film capacitors have been widely used in the fields of high-voltage direct current transmission, power electronics and new energy vehicles due to their excellent charging and discharging efficiency, excellent energy storage density and high operational reliability. However, the conductivity loss at high temperatures and high fields (more than 150°C) is the key to the performance improvement. The results show that doping wide bandgap inorganic fillers in polymer polymers is an effective solution, but the composite of organic matrix and inorganic fillers is often accompanied by problems such as interface compatibility, dielectric mismatch, and local electric field distortion, which will damage the structure and properties of the materials in serious cases. Based on this, the research group of Fang Cheng of North China Electric Power University proposed an interface fluorination strategy to effectively solve the above pain points. In this study, PEI composite films with hydroxylated BNNSs (BNNS-OH) and fluorinated BNNSs (BNNS-F) fillers were prepared, and the mechanism of fluorination strategy in BNNS regulating the band structure was clarified by analyzing the changes of the band structure of BNNS-OH and BNNS-F, and the heterojunction structure evolved from type I in BNNS-OH/PEI to type II in BNNS-F/PEI, and realized the dual capture of electrons and holes. In addition, interfacial fluorination significantly improves the breakdown strength of the composite film, making it exhibit a discharge density of 5.73 J/cm3 and an ultra-high efficiency of 91.22% at 575 kV/mm and 150°C. The related results were published in the Chemical Engineering Journal with the title of "Interface engineering of polymer composite films for high-temperature capacitive energy storage". The first author and corresponding author of the paper are Associate Professor Yu Xiang and Associate Professor Fan Sidi from the School of Electrical and Electronic Engineering, North China Electric Power University, respectively, and doctoral students Yang Rui and Zhang Wenqi have made important contributions to simulation.
Outline of Research
Fig.1 BNNS packing modification process First, the BNNS was thinned to nanometer thickness by ball mill stripping, and then the BNNS was hydroxylated to improve the grafting efficiency of the fluorinated coupling agent. Subsequently, BNNS-F was added to the PEI solution, and the BNNS-F/PEI composite film was prepared by solution casting method, and the PEI and BNNS-OH/PEI films were prepared as controls.
Fig.2 The microscopic morphology and physicochemical properties of the composite film were observed by SEM, and the introduction of BNNS filler did not damage the matrix structure, and the composite film showed a complete and dense state. Compared with BNNS-OH/PEI, interfacial fluorination resulted in a stronger intermolecular interaction between BNNS-F and PEI, which effectively inhibited the motility of the PEI molecular chain. As a result, the Tg of the BNNS-F/PEI composite film has been increased, allowing it to withstand higher operating temperatures and exhibit higher energy storage densities under extreme conditions.
Fig.3 The dielectric strength and band arrangement structure interface fluorination strategy effectively improved the breakdown field strength of the composite film, and the dielectric strength of BNNS-F/PEI film was increased from 456 kV/mm to 626 kV/mm of pure PEI, and the breakdown field strength of 589 kV/mm was maintained even at 150 °C. Fitting the J-E curve by the jump conduction equation, BNNS-F/PEI exhibits the lowest jump distance, implying an increase in trap density in the dielectric film, which plays an important role in hindering charge transport and suppressing leakage current. The band structure changes of BNNS-OH and BNNS-F were analyzed by UV-Vis and UPS measurements, which explained the internal mechanism of their performance improvement, and the type I heterojunction structure between PEI and BNNS-OH can temporarily inhibit charge transport by facilitating the accumulation of electrons at the interface through the energy barrier. In contrast, the type II heterojunction structure formed between PEI and BNNS-F constructs an electron trap at the interface, which realizes the capture and binding of electrons at the interface. At the same time, the accumulation of holes at the interface further hinders carrier transport, thus achieving dual inhibition of electrons and holes.
Fig.4 The mechanical strength of the composites and the phase field breakdown development simulated pure PEI film of the pure PEI film is 2.61 GPa, which increases to 2.81 GPa after the addition of BNNS-F filler. This is due to the stronger interfacial interaction and compatibility between BNNS-F and PEI, which exhibits greater resistance to mechanical deformation under the action of high electric fields, resulting in higher breakdown strength. The influence mechanism of interfacial fluorination on the breakdown development process was explored by the phase field method, and considering the differences in the properties of the interfacial region of the fillers, the core-shell structures of BNNS-OH and BNNS-F were designed during the model construction, and the interfacial properties before and after fluorination were represented by assigning different parameters to the shells. In the original PEI model, the breakdown path propagates rapidly with partial discharge, but in the BNS-OH/PEI model, the presence of BNNS packing changes the transmission path of the electric tree branch, making it more inclined to be closer to the BNNS filler, and then evolves into an electric tree branching state, which weakens the electric tree propagation rate and local electric field distortion. In the BNNS-F/PEI model, the fluorinated interface has a higher breakdown strength than the -OH layer, so the branched electric tree growing along the interface is further inhibited, which effectively hinders the development of the breakdown path. ConclusionIn this study, BNNS-F/PEI high-temperature energy storage films with high discharge density and energy storage efficiency were prepared based on the interfacial fluorination strategy, and the composite films exhibited a Ue of 5.73 J/cm3 and an ultra-high discharge efficiency of 91.22% at 150°C. The core advantages of interfacial fluorination are as follows: First, it promotes the interfacial interaction between the polymer matrix and the nanofiller, and provides a gain for the breakdown field strength through mechanical enhancement. Secondly, the dielectric constant is slightly reduced due to the low dipole moment of -F. However, the resulting large increase in breakdown field strength still contributes to a higher discharge density. In addition, the interfacial fluorination realizes the transformation of the band structure arrangement of BNNS and PEI, and compared with the interfacial electron accumulation formed by the traditional I-type structure, the II-type band structure realizes the dual capture of electrons and holes, and highly suppresses the leakage current, thereby significantly improving the energy storage efficiency. In this study, an interface engineering strategy based on fluorination effect was proposed, which effectively solved the interface problem caused by filler doping, and provided theoretical guidance for the preparation and application of polymer-based high-temperature energy storage films.
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Original link:
https://doi.org/10.1016/j.cej.2024.154056 Source: Frontiers of Polymer Science