Influences of Modified Montmorillonite on the High-Temperature Energy Storage Performance of Polyetherimide Dielectric Composites

Polyetherimide with its high glass transition temperature (i.e., 217 ℃) and melting point (i.e., 247 ℃) is considered as a promising high-temperature dielectric material. However, the π–π conjugated structure of its benzene rings leads to a sharp increase in conductivity loss at high temperatures and electric fields, thus reducing efficiency and energy storage density. Montmorillonite possesses a large bandgap and good insulation properties. Incorporating montmorillonite nanosheets into the polymer matrix is an effective approach to enhance the carrier transition barrier and dissipate carriers in the in-plane direction, thereby suppressing conduction current and improving the high-temperature energy storage performance of the polymer. However, the significant differences between the flexible matrix and the rigid inorganic filler pose some challenges for the compatibility at the organic-inorganic interface. In this paper, polyethylenimine was modified onto the surface of montmorillonite nanosheets to improve their interface compatibility with polyetherimide, allowing for uniform dispersion within the organic matrix. In addition, the impact of modified montmorillonite on the dielectric constant, breakdown strength and energy storage performance of polyetherimide composites was also investigated.

The modified montmorillonite was prepared via cation exchanging with polyethylenimine, and MMT/PEI composites were prepared by a tape casting method. PEI particles were firstly dissolved in NMP solvent under heating and stirring at 50 ℃ for 6 h. Once the PEI particles were completely dissolved, modified montmorillonite with a target content were firstly dispersed in the solvent under ultrasonication for 0.5 h. The resulting suspension was then cast onto a clean glass plate with a scraper and dried in a vacuum oven at 80 ℃ for 12 h. Afterwards, the material was dried at 200 ℃ for 4 h to completely remove the NMP solvent. The glass substrate was then placed in deionized water and left to stand for 10 min, allowing for the composites film to detach from the glass substrate, resulting in a composite film with a thickness of approximately 10 μm.

The relative dielectric constant of the composites exhibits only a slight decrease in the frequency range of 10² Hz to 10⁶ Hz, indicating a good stability. The dielectric constant of the composites at 1000 Hz increases from 3.25 for pure PEI to 3.54, while the dielectric loss remains below 0.02 in the entire frequency range as the amount of montmorillonite increases. Moreover, the dielectric constant of the composites increases from 3.13 to 3.32 with the addition of montmorillonite at 150 ℃ and 1000 Hz. The dielectric loss remains at a low level. Thus, the dielectric breakdown strength of pure PEI at 150 ℃ is 429 MV/m. After introducing modified montmorillonite, the breakdown strength of the composites initially increases and then decreases. At a doping modified montmorillonite amount of 0.1%, the breakdown strength increases to 487.2 MV/m, and with a doping modified montmorillonite amount of 0.2%, it further increases to 507.5 MV/m. However, the breakdown strength decreases to 459.5 MV/m when the doping modified montmorillonite amount continues to increase. The discharge energy density of pure PEI at 150 ℃ and at 450 MV/m is 2.56 J/cm³, with an efficiency of only 72.5%. After introducing the modified montmorillonite nanosheets, the breakdown strength and dielectric constant of the composites both are enhanced, leading to an improvement in charge storage capability. At 0.1% of modified montmorillonite doping amount, the maximum discharge energy density increases to 3.17 J/cm³ at 150 ℃ and 500 MV/m, although the efficiency remains low at 77.8%. When the doping modified montmorillonite amount increases to 0.2%, the maximum discharge energy density reaches 3.15 J/cm³ at 150 ℃ and 450 MV/m, with the efficiency improving to 90.1%, thus meeting the standard operational requirements for film capacitors. At 150 ℃ and 500 MV/m, the discharge energy density further increases to 3.54 J/cm³, while the efficiency stabilizes at 86.7%. However, when the doping modified montmorillonite amount increases to 0.3%, the maximum discharge energy density and efficiency both decline. This indicates that an appropriate amount of montmorillonite nanosheets can effectively enhance the energy storage properties of PEI-based composites. The excessive doping can lead to a negative effect on the energy storage performance of composites, likely due to aggregation or suboptimal dispersion of the nanosheets.

Polyethyleneimine molecules were effectively loaded onto the surface and interlayers of montmorillonite nanosheets through cation exchange. This process improved the dispersibility of the montmorillonite nanosheets in solvents and their compatibility with the polyetherimide matrix, ensuring the structural uniformity of the composite materials. After doping with modified montmorillonite, the energy storage performance of the composites exhibited the optimum performance observed at a doping modified montmorillonite amount of 0.2%. At this doping amount, the dielectric constant was 3.45 at 1000 Hz and 150 ℃, while the breakdown strength reached 507.5 MV/m. At an electric field strength of 450 MV/m, the charge-discharge efficiency achieved 90.1%, with a discharge energy density of 3.15 J/cm³; at 500 MV/m, the charge-discharge efficiency was 86.7%, and the maximum discharge energy density reached 3.54 J/cm³. The results of this work indicated that doping with montmorillonite nanosheets could be an effective strategy for enhancing the high-temperature energy storage performance of polyetherimide, paving a way for the development of high-strength, high-performance high-temperature dielectric polymers.