"Polymers are ideal for energy storage for transportation due to their light weight, scalability and high dielectric strength," said research leader Qing Wang, professor of materials science and engineering. "However the existing commercial polymer used in hybrid and electric vehicles, called BOPP, cannot stand up to the high operating temperatures without considerable additional cooling equipment. This adds to the weight and expense of the vehicles."
As explained by researchers, in normal two-dimensional polymer films, increasing the dielectric constant, the strength of the electric field, is in conflict with stability and charge-discharge efficiency. The stronger the field, the more likely a material is to leak energy in the form of heat. Penn State researchers originally attacked this problem by mixing different materials while trying to balance competing properties in a two-dimensional form. While this increased the energy capacity, they found that the film broke down at high temperatures when electrons escaped the electrodes and were injected into the polymer, which caused an electric current to form.
That’s when they came up with the sandwich-like structure. The outer layers, composed of boron nitride nanosheets in a polymer matrix, are excellent insulators, while the central layer is a high dielectric constant material called barium titanate.
A comparison of BOPP, or biaxial-oriented polypropylene, and the sandwich structure nanocomposite, termed SSN-x, in which the x refers to the percentage of barium titanate nanocomposites in the central layer, shows that at 150° C, SSN-x has essentially the same charge-discharge energy as BOPP at its typical operating temperature of 70° C.
However SSN-x has several times the energy density of BOPP, which makes SSN-x highly preferable for electric vehicle and aerospace applications as an energy storage device due to the ability to significantly reduce the size and weight of the electronics while improving system performance and stability. The elimination of bulky and expensive cooling equipment required for BOPP is an additional bonus, researchers said.
The next step will involve processability studies to determine if the material can be produced economically on a large scale.