Improved Working Temperature and Capacitive Energy Density of Biaxially Oriented Polypropylene Films with Alumina Coating Layers

High-temperature dielectric energy-storage properties are crucial for polymer-based capacitors for harsh environment applications. However, biaxially oriented polypropylene (BOPP), a state-of-the-art commercial capacitor dielectric, can work only below 105 °C. Here, we present a versatile method to enhance its working temperature by depositing alumina (Al 2 O 3 ) layers onto BOPP films via magnetron sputtering. Compared with a pure BOPP film, the sandwiched Al 2 O 3 /BOPP/Al 2 O 3 structure shows a higher dielectric constant, a lower electrical conduction loss, stronger mechanical properties, higher thermal conductivity, and especially increased working temperature. As a result, the composite film delivers a high discharged energy density of 0.45 J/cm 3 under 200 MV/m (the actual operating electric field in hybrid electric vehicles) at 125 °C. The discharged energy density and energy-storage efficiency (∼97.7%) are highly stable over 5000 cycles at 125 °C. This work provides an effective route to develop high-temperature polymer-based capacitors.