Synergistic Modulation of Free Volume and Band Structure Assist the High Energy Storage Performance of Polymer Dielectrics

Abstract Metallized film capacitors are widely used for their high‐power density, high breakdown strength and prominent machining performance. However, achieving high energy density and high discharge efficiency simultaneously in dielectric polymers remains a significant challenge. To address this issue, the novel random copolymer P(MMA‐VN) is synthesized via the copolymerization of 2‐vinyl naphthalene (2‐VN) and methyl methacrylate (MMA). This approach takes advantage of the free volume adjustment capability of rigid conjugated naphthalene rings as side groups, as well as their energy band modulation effect, to enhance energy storage performance. Experimental results reveal that the incorporation of naphthalene rings increases the free volume of the methyl side groups, thereby improving polarization and suppressing relaxation losses. Furthermore, the modification of the energy levels in naphthalene introduces trap energy levels, which enhance carrier trapping ability and effectively reduce leakage loss and the probability of thermal and electrical breakdown. As a result, the P(MMA‐VN) with 5 mol% VN unit exhibits outstanding performance, including an ultrahigh energy density (19.3 J cm ^{− 3} ) and remarkable discharging efficiency (>89%) at 800 MV m ⁻¹ . This study proposed a novel strategy to reconcile conflict between high energy density and high efficiency in dielectric polymers by regulating free volume and energy level structures.