Having established that proper modulation of LM-BN and filler content balances εr, Eb, and dielectric loss, we selected the optimal BOPP/15-LM-BN with 2.5 vol% filler content to assess capacitive energy storage performance above 90% discharge efficiency (ƞ) (Fig. 5a and Supplementary Fig. 23–24). The discharge energy density (Ud) of BOPP/LM-BN (4.5 J cm− 3 at 550 MV/m) significantly surpasses that of commercial BOPP film (2.9 J cm− 3 at 550 MV/m) and BOPP/BN film (2.9 J cm− 3 at 500 MV/m). Additionally, BOPP/LM-BN exhibits a high ƞ of 90% at 550 MV/m. Durability tests show BOPP/LM-BN sustains 10,000 charge-discharge cycles at 400 MV/m with minimal degradation (Fig. 5b), indicating long-term reliability. A fast discharge speed is a critical performance to assess the dielectric capacitors versus other electrical energy storage devices such as electrochemistry batteries. The discharge time τ0.9 was defined as the discharged time at the 90% energy density from the discharge tests. Under 200 MV/m, representative discharge profiles from a load resistor RL of 100 Ω for BOPP, BOPP/BN, and BOPP/LM-BN films show a close τ0.9 of 18.8, 25.4, and 20.0 ns, respectively (Fig. 5c), indicating the well-maintained pulse discharge advantage of BOPP/LM-BN films. Corresponding fast Ud is increased from 0.31 J cm− 3 of commercial BOPP to 0.42 J cm− 3 of BOPP/LM-BN film, suggesting the greatly improved power density of BOPP/LM-BN.
In Fig. 5d and Supplementary Table. S1, we compare our BOPP/LM-BN dielectrics with currently representative capacitor films, including solution-casted polymers, biaxially-stretched composites, and surface-coated films at above 90% ƞ and 550 MV/m1, 4, 12, 17, 28, 37–41. While novel polymer strategies achieve high Ue values through elaborate molecular design, such as polyimide derivatives and ladderphane copolymer, they are limited by complex preparation and are not amenable to large-area, high-quality biaxial stretching. Surface-coated films maintain biaxial stretchability but offer limited permittivity and Ue enhancement. Composite films typically suffer from reduced stretchability; only with very low filler content (< 1 vol%) is stretchability preserved, but with negligible Ue improvement. Our BOPP/LM-BN, however, achieves a desirable balance in high Ue and biaxial stretching ratios, making it a promising candidate in the applications of miniaturized film capacitors.
The application potential of BOPP/LM-BN in multilayer foil biaxial-orientation composite film capacitors (MFBOCFC) is demonstrated with large-area (9 × 9 cm) prototypes fabricated through stacking, pressing, leading, and sealing (Supplementary Fig. 25–26). These prototypes can be encapsulated similarly to commercial film capacitors through methods like winding encapsulation or stacked encapsulation (Fig. 5e), with dielectric film layer exhibiting a uniform thickness of around 10 µm while Al electrode foil is around 6 µm. The capacitance and dissipation factors of the BOPP, BOPP/BN and BOPP/LM-BN capacitor with the same layer numbers as a function of frequency are shown in Fig. 5f. Capacitance measurements show BOPP/LM-BN capacitors outperforming BOPP and BOPP/BN capacitors in capacitance (85 nf vs. 72 nf and 65 nf, respectively) at 100 Hz, indicating potential for significant volume reduction by 31% in capacitor design. Additionally, BOPP/LM-BN capacitors exhibit excellent capacitance stability under bending (Fig. 5g), highlighting their flexibility and suitability for diverse flexible applications. Meanwhile, the BOPP/LM-BN has a much higher thermal conductivity (4.1 W m − 1 K− 1) than that of BOPP/BN (2.6 W m− 1 K− 1) which can improve the high-temperature performance of devices (Supplementary Fig. 27) 42 . The raw materials cost of our composite capacitor film (around $6.0/kg) is much lower than other BOPP alternating materials ($11.0/kg to 350.0/kg) (Supplementary Table. S2).
In summary, our study presents a novel approach to fabricate composite capacitor films with a soft and functional interface, successfully resolving the trade-off between enhanced energy density and high biaxial stretchability. By incorporating a soft LM liquid, we effectively modulate the modulus mismatch between the soft PP matrix and the hard BN filler. This modulation reduces stress concentration and blunts crack formation, thus enabling a higher biaxial stretching ratio. The LM-BN heterostructure exhibits wide band gaps and electron traps, yet simultaneously enhances dipole polarization, which positively impacts the permittivity and breakdown strength of the composite. These unique characteristics lead to a spectrum of exceptional capacitive energy storage properties, including εr, Eb, Ud, ƞ, cyclic stability, thermal conductivity, and device capacitance, which pave the way for scaled-up fabrication of cost-effective composite dielectric films and the development of more compact capacitive energy storage devices. Moreover, we believe that this soft and functional interface approach sets a promising foundation for designing polymer-filler interface, holding potential for broad application in improving both biaxial stretchability and functionality of composite materials across various fields, including packaging, barrier films, permeable films, battery films, and optical films.