Crystal phase transition dependence of the energy storage performance of poly(vinylidene fluoride) and poly(vinylidene fluoride-hexafluoropropene) copolymers

Stretching Aligned Hydrogen Bonding Network to Evoke Mechanically Robust and High-Energy-Density P(VDF-HFP) Dielectric Film Capacitors

Polymer-based dielectric film capacitors are essential energy storage components in electronic and power systems due to their ultrahigh power density and ultra-fast charge storage/release capability. Nonetheless, their relatively low energy density does not fully meet the requirements of power electronics and pulsed power systems. Herein, a scalable composite dielectric film based on a ferroelectric polymer with edge hydroxylated boron nitride nanosheets (BNNS-OH) is fabricated via the construction of a hydrogen bonding network and stretching orientation strategy. The presence of hydroxyl groups on boron nitride aids in forming a robust hydrogen bonding network within the ferroelectric polymer, leading to a significant increase in Young's modulus and superior dielectric performance. Furthermore, the stretching process aligns the BNNS-OH and the hydrogen bonding network along the drawing direction via covalent and hydrogen bonding interaction, resulting in a remarkable tensile strength (109 MPa), breakdown strength (688 MV m-1), and energy density (28.2 J cm-3), outperforming mostrepresentative polymer-based dielectric films. In combining the advantages of a simple preparation process, extraordinary energy storage performance, and low-cost raw materials, this strategy is viable for large-scale production of polymer-based dielectric films with high mechanical and dielectric performance and opens a new path for the development of next-generation energy storage applications.

Introduction of Nanoscale Si3N4 to Improve the Dielectric Thermal Stability of a Si3N4/P(VDF-HFP) Composite Film

In order to improve the dielectric thermal stability of polyvinylidene fluoride (PVDF)-based film, nano silicon nitride (Si3N4) was introduced, and hence the energy storage performance was improved. The introduction of nano Si3N4 fillers will induce a phase transition of P(VDF-HFP) from polar β to nonpolar α, which leads to the improved energy storage property. As such, the discharging energy density of Si3N4/P(VDF-HFP) composite films increased with the amount of doped Si3N4. After incorporating 10wt% Si3N4 in Si3N4/P(VDF-HFP) films, the discharging density increased to 1.2 J/cm3 under a relatively low electric field of 100 MV/m. Compared with a pure P(VDF-HFP) film, both the discharging energy density and thermal dielectric relaxor temperature of Si3N4/P(VDF-HFP) increased. The working temperature increased from 80 °C to 120 °C, which is significant for ensuring its adaptability in high-temperature energy storage areas. Thus, this result indicates that Si3N4 is a key filler that can improve the thermal stability of PVDF-based energy storage polymer films and may provide a reference for high-temperature capacitor materials.

High Breakdown Strength and Energy Density in Multilayer-Structured Ferroelectric Composite

All-organic dielectric composites are drawing increased attention owing to their high operating voltage, low loss, and superior processability. However, polymers usually possess a relatively lower dielectric constant than most the other dielectrics, which seriously suppresses the improvement of their energy density. In this work, multilayer-structured composites with excellent dielectric and energy storage properties are prepared by the stacking method, and the effect of layer numbers on the performance of the composites is studied. High-κ polymers such as poly(vinylidenefluoride) (PVDF) and poly(vinylidenefluoride-ter-trifluoroethylene-ter-chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)) are used to prepare the composites with different layers. It is found that the dielectric constant is up to 14.45 at 1 kHz, which is increased with the volume fraction of the P(VDF-TrFE-CTFE) layer and layer number of the composites. Due to the increased dielectric constant, an ultrahigh discharge energy density of 18.12 J/cm³ is achieved at the electric field of 620 kV/mm. This study exhibits an effective routine to prepare flexible high-performance dielectric materials.

Negatively Charged Nanosheets Significantly Enhance the Energy-Storage Capability of Polymer-Based Nanocomposites

Polymer-based dielectric materials play a key role in advanced electronic devices and electric power systems. Although extensive research has been devoted to improve their energy-storage performances, it is a great challenge to increase the breakdown strength of polymer nanocomposites in terms of achieving high energy density and good reliability under high voltages. Here, a general strategy is proposed to significantly improve their breakdown strength and energy storage by adding negatively charged Ca₂ Nb₃ O₁₀ nanosheets. A dramatically enhanced breakdown strength (792 MV m^-1 ) and the highest energy density (36.2 J cm^-3 ) among all flexible polymer-based dielectrics are observed in poly(vinylidene fluoride)-based nanocomposite capacitors. The strategy generalizability is verified by the similar substantial enhancements of breakdown strength and energy density in polystyrene-based nanocomposites. Phase-field simulations demonstrate that the further enhanced breakdown strength is ascribed to the local electric field, produced by the negatively charged Ca₂ Nb₃ O₁₀ nanosheets sandwiched with the positively charged polyethyleneimine, which suppresses the secondary impact-ionized electrons and blocks the breakdown path in nanocomposites. The results demonstrate a new horizon of high-energy-density flexible capacitors.

High field antiferroelectric-like dielectric of poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene)-graft-poly(styrene-methyl methacrylate) for high pulse capacitors with high energy density and low loss

The insulating performance of PSt segments in MMA offers a strategy for the synthesis of low loss dielectrics.

Improved battery performance contributed by the optimized phase ratio of β and α of PVDF

Inorganic salts (LiCl) were induced to improve the ratio of β and α of PVDF by the solution method. The vibrational spectra of PVDF polymorphic polymers were obtained by Fourier transform infrared spectroscopy (FTIR), and the results showed that the ratio of β and α of pristine PVDF was elevated from 43.66% to 53.27%. A small amount of LiCl grains was detected to be decorated on the surface of LiCl-involved electrodes by the SEM and EDS tests. The rate capability of the modified samples was evaluated when charge-discharged at 5C. The capacity of the 1/10LiCl@PVDF samples remained at a high level of 71.64% when charge-discharged at 5C, which was much higher than the value of 54.66% for pristine samples. The results of the CV and EIS tests revealed that the electrochemical polarization increasing rate and charge transfer resistance of the 1/10LiCl@PVDF samples were smaller than those of the pristine PVDF samples.

Inorganic salts (LiCl) were induced to improve the ratio of β and α of PVDF by the solution method.