High energy density and discharge efficiency polypropylene nanocomposites for potential high-power capacitor

Film capacitor, one typical type of electrostatic capacitors, exhibits its unique advantages in the high-power energy storage devices operating at a high electric field due to the high electrical breakdown strength ( E b ) of the polymeric films . However, the development of film capacitor towards high energy storage density is severely hindered by the low dielectric constant ( ε ) and low charge-discharge efficiency ( η ) of the polymeric films. The film of polypropylene (PP), the most used polymeric film with a market share of 50%, owns a high η due to its low inherent hysteresis loss . Yet the low ε (2.2 ​at 10 3  ​Hz) impedes the increase of its energy storage density (1–2 ​J/cm 3 ). Here we demonstrate that the discharged energy density ( U e ) of PP film could be largely increased from 1.40 ​J/cm 3 of pure PP film to 3.86 ​J/cm 3 of PP nanocomposite film by incorporating a small loading of core-shell structured [email protected] 3 ( [email protected] ) nanoparticles (2.27 ​vol%) into PP matrix. The obtained U e of 3.86 ​J/cm 3 , to the best of our knowledge, is the highest reported value for the PP based films using commercial PP resin. Meanwhile, the η merely undergoes a slight decrease from 99.5% to 94.1%. Similarly, the obtained η of 94.1%, to the best of our knowledge, is also the highest value for the polymeric films reported in the previous works. The significant increase of U e (175.7%) and negligible decrease of η (5.4%) are mainly attributed to the increases of ε from 2.2 to 3.7 and E b from 361 ​MV/m to 448 ​MV/m. Compared with the films of raw BT/PP nanocomposites, the U e exhibits a significant increase of 365.1% from 0.83 ​J/cm 3 to 3.86 ​J/cm 3 and the η also displays an increase of 7.4% from 87.6% to 94.1%. The significant improvement of energy storage performance achieved in the film of [email protected] /PP nanocomposite suggests that the organic PMMA shell plays an important role in the amelioration of compatibility of BT nanoparticles and PP matrix and the alleviation of local electric field concentration. Moreover, the PMMA shell could also provide a robust scaffold to hinder the early breakdown failure of nanocomposites due to its high E b (425 ​MV/m). Thus, high ε , low loss and high E b are simultaneously achieved. Specifically, the high hot stretch ratio (1:4) of nanocomposite film indicates the strong feasibility of industrialized film processing. Apart from the experimental analysis, theoretical analysis using the simulation of finite element is also carried out to figure out the influence of organic PMMA shell on the dielectric performance of PP nanocomposites films. These findings enable the development of film capacitor using non-polar PP based film towards high energy storage density.