Significant improvements in energy density and efficiency of CQDs/PVDF-based dielectric nanocomposite via stretching effect

With the advancement towards lightweight, integrated, and intelligent capacitors, there is an urgent need to develop flexible dielectric materials with high power density and high energy storage performance. Poly(vinylidene fluoride) (PVDF) based nanocomposite has garnered widespread attention due to its high polarization and ease of processing. However, the inverse relationship between the dielectric constant and breakdown strength constrains its potential for achieving high energy density . Herein, a two-pronged approach effectively combining the incorporation of organic carbon quantum dots (CQDs) nanofiller and PVDF polymer crystallization behavior modulation is utilized to boost the energy density of nanocomposites. Particularly, CQDs increase the crystallinity and decrease the leakage current density of the nanocomposites effectively, resulting in enhanced breakdown strength. Moreover, through optimizing process parameters of stretching temperatures and rate during uniaxial stretching, significantly higher crystallinity , polar phase transition, increased chain orientation, and smoother surface are achieved to construct a denser and more stable microstructure that effectively hinders the formation of electrical breakdown pathways, thereby enhancing the electric polarization and breakdown strength of the nanocomposites. As a consequence, the ultrahigh discharge energy density of 30.8 J/cm 3 with an efficiency of 74.1 % was yielded at 944.2 kV/mm, representing improvements of 124.8 % and 42.8 %, respectively, compared with pure PVDF (13.7 J/cm 3 , 51.9 %). These facile uniaxial stretching films present excellent energy storage performance as those obtained by complicated processes, which shows great potential for practical large-scale applications. • CQDs increase the crystallinity and reduce leakage current density, which is key to achieving superior dielectric properties. • Optimizing stretching improves crystallinity, polar phase transitions, chain orientation of PVDF-based polymers. • The nanocomposite achieves 30.8 J/cm 3 and 74.1% at 939 kV/mm, representing huge improvement compared to pure PVDF. • The uniaxial stretching process is a simple and scalable method, making it promising for practical large-scale applications. • This work reveals microstructure-property links and enables facile, scalable energy storage.