Gradient Ethylene-Styrene Copolymer Nanocomposites Towards High Energy Storage Capability

Polymer film capacitors are extensively utilized in solar grid-connected power systems, new energy vehicles, and aircraft. However, due to the low storage energy density of commercial polymer dielectrics, it is difficult for commercial film capacitors to be miniaturized. We introduced a soluble semi-crystalline gradient ethylene-styrene copolymer (PESt) system with controllable microphase interfaces. The distance of each microphase interface is controlled within 3–7 nm, thus generating abundant microphase interfaces with large-scale modulated dielectric performances. Compared with typical energy storage dielectric polymers, the ethylene-styrene gradient copolymer with a styrene content of 44% (PESt44) exhibits significantly enhanced energy storage density of 7.26 J/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at 700 MV/m, with charge-discharge efficiency > 95% at 20°C, versus 2 J/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> of Biaxially-oriented Polypropylene (BOPP) at similar film thickness of 10μm. Here, in this work, to further increase storage energy density, the composites of PESt44 and alumina (Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>) nanoparticles are prepared by solution-mixing. When the mass fraction of Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> reaches 1wt%, the composite exhibits a high discharged energy density of about 8.43 J/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at the applied fields up to 800 MV/m, as well as high charge-discharge efficiency (90.63%). In addition, the composite owns a low dielectric loss (10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup>~10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>@20°C), which guarantees long-term stable operation of the film capacitors. However, PESt44 doped with 5wt% Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> only exhibits 3.30 J/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at the applied fields up to 500 MV/m, owning to overlapping interface regions and reducing dispersion. Thus, nano-doping strategy is only helpful with low nanoparticles content in the PESt44. This work contributes to the development of polymeric composite for film capacitors towards high energy density, which is essential for their industrialization in high-power electrical systems.