Liquid metal interface mechanochemistry disentangles energy density and biaxial stretchability tradeoff in composite capacitor film

Dielectric polymer composites for film capacitors have advanced significantly in recent decades, yet their practical implementation in industrial-scale, thin-film processing faces challenges, particularly due to limited biaxial stretchability. Here, we introduce a mechanochemical solution that applies liquid metal onto rigid dielectric fillers (e.g. boron nitride), dramatically transforming polymer-filler interface characteristics. This approach significantly reduces modulus mismatch and stress concentration at the interface region, enabling polypropylene composites to achieve biaxial stretching ratio up to 450 × 450%. Furthermore, liquid metal integration enhances boron nitride's dielectric polarization while maintaining inherent insulation, producing high-dielectric-constant, low-loss films. These films, only microns thick yet quasi square meters in area, achieve a 55% increase in energy density over commercial biaxially-oriented polypropylene (from 2.9 to 4.5 J cm