A class of multiferroic, three-phase particulate composites of Tb–Dy–Fe alloy, lead–zirconate–titanate (PZT), and polymer are investigated, in which a small volume fraction $f$ of Tb–Dy–Fe alloy particles are dispersed in a PZT/polymer mixture. The measured dielectric, piezoelectric, and magnetoelectric properties demonstrate that a percolation transition occurs at $\text{f∼0.12}$ in the composites. When $f$ is low (e.g., $\text{f<0.07),}$ the composites exhibit piezoelectric and increasing magnetoelectric response. In the critical $f$ range of $\text{0.07<f<0.12,}$ such piezo- and magnetoelectric responses sharply drop, and equal zero at the percolation threshold, above which the composite becomes a conductor and a magnetostrictive composite only.

Flexible polymer dielectrics for capacitive energy storage that can function well at elevated temperatures are increasingly in demand for continuously advancing and miniaturizing electrical devices. However, traditional high-resistance polymer dielectrics composed of aromatic backbones have a compromised band gap (E_g) and hence suffer from low breakdown strength and a huge loss at high temperatures. Here, based on the density functional theory (DFT) calculations, rigid and non-coplanar alicyclic segments are introduced into the polyimide backbone to overcome the incompatibility of a high glass transition temperature (T_g) and large E_g. Thanks to the large optical E_g (∼4.6 eV) and high T_g (∼277 °C), the all-alicyclic polyimide at 200 °C delivers a maximum discharge energy density (U_e) of 5.01 J cm⁻³ with a charge–discharge efficiency (η) of 78.1% at 600 MV m⁻¹, and a record U_e of 2.55 J cm⁻³ at η = 90%, which is 10-fold larger than that of the state-of-art commercial polyetherimides (PEIs). In addition, compared with aromatic polyimides, the all-alicyclic polyimide possesses a better self-clearing characteristic due to a smaller ratio of carbon to hydrogen and oxygen, which facilitates its long-term reliability in practical applications.