Enhancing the Dielectric Breakdown Strength and Energy Density of Solid-State Polymeric Capacitors by Chain End Manipulations

The need for high power density, flexible, pulsed power, and lightweight energy storage devices in advanced applications necessitates the use of polymer film-based dielectric capacitors. The maximum energy density of a dielectric capacitor is limited by the breakdown of the dielectric material at high electric fields, otherwise known as dielectric breakdown strength. Thus, for enhancing the energy density of the polymeric dielectric capacitors for advanced applications, the physics governing the dielectric breakdown of polymers needs to be understood and utilized in devising novel strategies to enhance the breakdown strength and energy density of polymeric dielectric capacitors. Theoretically, it has been shown that chain ends contribute adversely to the electrical breakdown of polymer dielectrics at high electric fields due to the free volume associated with them, resulting in low energy density in polymer capacitors. In this work, we experimentally demonstrate the role of chain ends indirectly and directly in the dielectric breakdown by using block copolymers (BCP) and cyclic polymer films respectively. The BCPs show an enhanced breakdown strength due to chain end segregation, resulting in energy densities of 5 J/cm³, which is higher than the industry standard of 1-2 J/cm³. Given the role of the chain ends is explored in the BCP films indirectly, we synthesized the cyclic polymers with high purity to explore the adverse effect of chain ends in the dielectric breakdown directly. The cyclic polymer films demonstrate ~50% enhancement in dielectric breakdown strength and as a result, the capacitive energy density in the cyclic polymer films increases by ~80% as compared to their linear counterparts. Interestingly, the cyclic polymer films show a higher refractive index and packing density as compared to their linear counterparts, which might be attributed to the elimination of chain end-based free volume. These novel insights into correlating the polymer structure and topology with electric properties and energy density will help design next-generation polymeric energy storage materials and devices.