Film capacitors have the advantages of ultra-high charge and discharge rates, high voltage resistance, low cost, and light weight, and play an important role in the modern electronic and electrical field. Commercial biaxially oriented polypropylene film (BOPP), as the most commonly used flexible energy storage material, has excellent energy storage efficiency at room temperature, but when the ambient temperature is higher than 100 ° C, its electrical performance under high electric field And the energy storage efficiency is significantly reduced. In terms of energy dissipation mechanism, for linear dielectrics, the leakage current generated under high electric field is an important way of energy loss. To this end, some studies have introduced some highly insulating inorganic particles into the polymer matrix to reduce the leakage current density of the composite material and improve its breakdown and energy storage performance. However, the introduction of inorganic particles usually brings about defects such as agglomeration and excessive surface energy. Based on this, research work on the surface modification of inorganic particles has continuously emerged in recent years. Most researchers mainly use siloxane coupling agents or dopamine-based interface modifiers to reduce the surface energy of inorganic particles to improve their dispersion performance, but relying solely on these small molecule interface modifiers to improve their dispersibility, often With little success.
Research results
Recently, the research group of Professor He Jinliang and Associate Professor Li Qi of Tsinghua University has made important progress in the field of polymer-based high heat-resistant energy storage dielectrics. In this study, the author first reacted magnesium oxide (MgO) nanoparticles with a siloxane coupling agent to achieve amination of the nanoparticles. The aminated nanoparticles are reacted with polypropylene grafted maleic anhydride (PP-g-MAH) to form a PP-g-MAH / MgO composite. PP-g-MAH / MgO composite and PP are blended to form PP / PP-g-MAH / MgO composite. PP-g-MAH acts as a "bridge" between polymer PP and inorganic MgO particles, not only by providing a deep well to suppress leakage current, but also by polar elements to increase its dielectric constant. In particular, the energy storage density of PP-MAH-MgO nanocomposites at 120 oC is 1.66 J / cm3, and the energy storage density at η> 90% is even better than that of pure PP at 80 ℃ (ie 1.39 J / cm3) 19% higher. This result shows that through interfacial modification, the working temperature of PP nanocomposite films can be increased to 120 ℃. The work was published in Energy Storage Materials, a top academic journal of international energy, with the title "Interface-modulated nanocomposites based on polypropylene for high-temperature energy storage".
Research ideas and discussion of specific research results
MgO nanoparticles are aminated under the treatment of siloxane coupling agent (APTES). The amino groups in the aminated MgO nanoparticles are condensed with the ester groups on PP-g-MAH. The two The coupling is carried out to obtain the MgO / PP-g-MAH conjugate with PP-g-MAH as the shell, where the shell thickness is 10 nm. Blending the above conjugate with polypropylene, due to the extremely high compatibility between PP-g-MAH and the PP matrix, the MgO nanoparticles can be evenly dispersed in the PP matrix.
When designing a composite system with high energy storage performance at high temperatures, first of all, the surface modifier must have a high degree of compatibility with the polymer matrix and a high thermal stability to avoid the formation of defects at the interface. Second, nanoparticles should not have a high dielectric constant and a small electric field distortion, because the low dielectric constant PP leakage current is easily increased by the distortion of its electric field under high temperature conditions. Third, deep wells should be introduced through surface functionalization to suppress conduction losses.
The author chose PP-g-MAH as the interface modifier for inorganic particles, because the strong polar anhydride groups can enhance the dielectric response of the composite. The dielectric constant of PP / PP-g-MAH / MgO composites formed by PP-g-MAH modified MgO increased from 2.22 (pure PP) to 2.47. This is mainly due to the polar groups attached to the surface of the MgO coating make the nanoparticles more uniformly dispersed in the PP matrix and form a larger interface polarization area. As the temperature increases, the dielectric constant of PP / PP-g-MAH / MgO composite material can maintain a relatively stable state. In contrast, composite materials grafted with siloxane coupling agent only on the surface of MgO particles have poor dielectric thermal stability. At the same time, the nanocomposite materials containing PP-g-MAH / MgO nanoparticles show a loss factor that is comparable to or even lower than that of pure PP at high temperatures and low frequencies. This means that the conductive loss of PP / PP-g-MAH / MgO nanocomposites is suppressed, which shows that the interface modifier PP-g-MAH plays an important role in the dielectric thermal stability of the composite.
In order to obtain direct evidence of the existence of deep traps in the modified region of the interface, a Kelvin probe force microscope (KPFM) with nanometer spatial resolution was used to detect the surface potential attenuation in the interface. The author first obtained the morphology of the frozen ultra-thin sliced film sample to locate the position of the nanoparticles. Then choose to pass through the center of the nanoparticle to perform charging and ISPD measurements. Obviously, the surface charge decay form of PP-MAH-MgO / PP nanocomposites is different from that of untreated MgO / PP nanocomposites, especially the interface charge trap at the interface is much slower than un-MgO / PP It shows that the interface charge trap in PP-MAH-MgO / PP is much deeper than that in untreated MgO / PP.
In order to characterize its energy storage performance at high temperatures, the measurement temperature was set to 120 oC (its temperature exceeds the maximum operating temperature of commercial BOPP films (ie 105 oC)). As expected, when the electric field exceeds 198 MV / m, the η of the original PP drops below 90%, and drops to 62% at 400 MV / m, which is attributed to the nanoparticle / polymer interface modification inhibiting it Conductivity. Under high electric field and high temperature, PP-MAH-MgO nanocomposites show the highest energy storage density and energy storage efficiency. It is worth noting that at 120 oC (ie 1.66 J / cm3), the energy storage density of PP-MAH-MgO nanocomposites at η> 90% is even better than that of pure PP at 80 oC (ie 1.39 J / cm3). 19% higher. This result shows that through interfacial modification, the working temperature of PP nanocomposite films can be increased to 120 oC.
In addition, during the continuous 50000 charge-discharge cycles, no signs of decreased energy storage density and energy storage efficiency were observed in PP-MAH-MgO / PP nanocomposites, and good charge-discharge cycle characteristics may be attributed to nanocomposites Increase in resistivity.
Research summary
This work uses interface adjustment to improve the high-temperature energy storage performance of polymer-based nanocomposites. SEM and TEM confirmed that PP-MAH-MgO nanoparticles have good compatibility and dispersion in PP matrix. PP / PP-MAH-MgO nanocomposites have stable dielectric properties and improve their breakdown strength at high temperatures. In addition, PP / PP-MAH-MgO nanocomposites have significantly improved charge-discharge efficiency, energy storage density and cycle stability.
References: Interface-modulated nanocomposites based on polypropylene for high-temperature energy storage. Energy Storage Materials, 28 (2020) 255–263. DOI: 10.1016 / j.ensm.2020.03.017.
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