Enhancing electrical energy storage capability of dielectric polymer nanocomposites via the room temperature Coulomb blockade effect of ultra-small platinum nanoparticles

Synchronously enhanced breakdown strength and energy storage ability of cellulose acetate flexible films via introducing ultra-low content of carbonized polymer dots

Developing green and environmentally friendly biomass materials for energy storage and application is of great significance to sustainable development. Novel composite films containing cellulose acetate (CA) and carbonized polymer dots (CPDs) are reported herein. The CPDs have strong hydrogen bonding interactions with CA matrix, in which CPDs act as the physical crosslinking points and enhance the entanglement density of the matrix. And the composite films demonstrate a significant enhancement in breakdown strength (Eb), reaching up to 520.58 MV/m with the addition of 0.1 wt% CPDs (1.62 times higher than 321.94 MV/m of pure CA). Furthermore, the discharging energy density (Ud) achieves 2.55 J/cm3 at 450 MV/m, which is 1.36 times higher than that of the pure CA film (1.87 J/cm3 at 400 MV/m) and simultaneously, the energy efficiency (η) is maintained at 73.3 %. The Coulomb-blockade effect induced by the ultra-low content of CPDs effectively inhibiting carrier migration, and the enhanced entanglement density of the matrix improving mechanical properties and reducing polarization loss, mainly contribute to the enhanced dielectric performances. Furthermore, CPDs also improve the mechanical properties of the composite films apparently. This work provides some references for the fabrication of the next generation of environmentally friendly dielectric composite films.

Increased Deep Trap Density in Interfacial Engineered Nanocomposite Revealed by Sequential Kelvin Probe Force Microscopy for High Dielectric Energy Storage

Nanocomposites combining inorganic nanoparticles with high dielectric constant and polymers with high breakdown strength are promising for the high energy density storage of electricity, and carrier traps can significantly affect the dielectric breakdown process. Nevertheless, there still lacks direct experimental evidence on how nanoparticles affect the trap characteristics of nanocomposites, especially in a spatially resolved manner. Here, a technique is developed to image the trap distribution based on sequential Kelvin probe force microscopy (KPFM) in combination with the isothermal surface potential decay (ISPD) technique, wherein both shallow and deep trap densities and the corresponding energy levels can be mapped with nanoscale resolution. The technique is first validated using the widely-used commercial biaxially oriented polypropylene, yielding consistent results with macroscopic ISPD. The technique is then applied to investigate polyvinylidene fluoride-based nanocomposites filled with barium titanate nanoparticles, revealing higher deep trap density around surface-modified nanoparticles, which correlates well with its increased breakdown strength. This technique thus provides a powerful spatially resolved tool for understanding the microscopic mechanism of dielectric breakdown of nanocomposites.

Achieving Significantly Boosted Dielectric Energy Density of Polymer Film via Introducing a Bumpy Gold/Polymethylsilsesquioxane Granular Blocking Layer

Polymer dielectrics are the key materials for pulsed energy storage systems, but their low energy densities greatly restrict the applications in integrated electronic devices. Herein, a unique bumpy granular interlayer consisting of gold nanoparticles (Au NPs) and polymethyksesquioxane (PMSQ) microspheres is introduced into a poly(vinylidene fluoride) (PVDF) film, forming trilayered PVDF-Au/PMSQ-PVDF films. Interestingly, the Au/PMSQ interlayer arouses a dielectric enhancement of 47% and an ultrahigh breakdown strength of 704 MV m-1, which reaches 153% of pure PVDF. It is revealed that the greatly enhanced breakdown strength originated from the Coulomb-blockade effect of Au NPs and the excellent insulating properties of PMSQ microspheres with a special molecular-scale organic-inorganic hybrid structure. Benefiting from the concurrently enhanced dielectric and breakdown performances, an outstanding energy density of 22.42 J cm-3 with an efficiency of 67.1%, which reaches 249% of that of the pure PVDF, is achieved. It is further confirmed that this design strategy is also applicable to linear dielectric polymer polyethyleneimine. The composites exhibit an energy density of 8.91 J cm-3 with a high efficiency of ≈95%. This work offers a novel and efficient strategy for concurrently enhancing the dielectric and breakdown performances of polymers toward pulsed power applications.

Improved Energy Density at High Temperatures of FPE Dielectrics by Extreme Low Loading of CQDs

Electrostatic capacitors, with the advantages of high-power density, fast charging-discharging, and outstanding cyclic stability, have become important energy storage devices for modern power electronics. However, the insulation performance of the dielectrics in capacitors will significantly deteriorate under the conditions of high temperatures and electric fields, resulting in limited capacitive performance. In this paper, we report a method to improve the high-temperature energy storage performance of a polymer dielectric for capacitors by incorporating an extremely low loading of 0.5 wt% carbon quantum dots (CQDs) into a fluorene polyester (FPE) polymer. CQDs possess a high electron affinity energy, enabling them to capture migrating carriers and exhibit a unique Coulomb-blocking effect to scatter electrons, thereby restricting electron migration. As a result, the breakdown strength and energy storage properties of the CQD/FPE nanocomposites are significantly enhanced. For instance, the energy density of 0.5 wt% CQD/FPE nanocomposites at room temperature, with an efficiency (η) exceeding 90%, reached 9.6 J/cm3. At the discharge energy density of 0.5 wt%, the CQD/FPE nanocomposites remained at 4.53 J/cm3 with an efficiency (η) exceeding 90% at 150 °C, which surpasses lots of reported results.

Influence of Ferroelectric Filler Size and Clustering on the Electrical Properties of (Ag-BaTiO3)-PVDF Sub-Percolative Hybrid Composites

The paper presents a study concerning the role of ferroelectric filler size and clustering in the dielectric properties of 20%BaTiO₃-80%PVDF and of 20% (2%Ag-98%BaTiO₃)-PVDF hybrid nanocomposites. By finite element calculations, it was shown that using fillers with ε > 10³ does not provide a permittivity rise in the composites and the effective dielectric constant tends to saturate to specific values determined by the filler size and agglomeration degree. Irrespective of the ferroelectric filler sizes, the addition of metallic ultrafine nanoparticles (Ag) results in permittivity intensification and the effect is even stronger if the metallic nanoparticles are connected to a higher degree with the ferroelectric particles' surfaces. When using coarse ferroelectric fillers, the probability of clustering is higher, thus favoring the permittivity increase by field concentration in small regions close to the interfaces separating dissimilar materials. The modeling results were validated by an experimental dielectric analysis performed in a series of PVDF-based thick films with the same amount of BaTiO₃ fillers or with Ag-BaTiO₃ hybrid fillers. Similar trends as predicted by simulations were found experimentally but with slightly higher permittivity values which were assigned to the modifications of the polymer phase composition due to the presence of nanofillers and the local sample inhomogeneity (the presence of clustering, in particular for coarse BaTiO₃ grains), which create regions with enhanced local fields.

Dielectric Characterization of Core-Shell Structured Poly(vinylidene fluoride)-grafted-BaTiO3 Nanocomposites

Dielectric properties of poly(vinylidene fluoride)-grafted-BaTiO₃ (PVDF-g-BT) core-shell structured nanocomposites obtained from Reversible Addition Fragmentation chain Transfer (RAFT) polymerization of VDF were investigated by Broadband Dielectric Spectroscopy (BDS). The dielectric constant increased along with the BT content, about +50% by addition of 15 vol% of BT, which was around 40% more than expected from predictions using the usual dielectric modeling methods for composite materials, to be ascribed to the effect of the interfacial core-shell structure. The known dielectric relaxations for PVDF were observed for the neat polymer as well as for its nanocomposites, not affected by the presence of nanoparticles. A relaxation process at higher temperatures was found, due to interfacial polarization at the amorphous-crystalline interface, due to the high crystallinity of materials produced by RAFT. Isochronal BDS spectra were exploited to detect the primary relaxation of the amorphous fraction. Thermal analysis demonstrated a very broad endotherm at temperatures much lower than the usual melting peaks, possibly due to the ungrafted fraction of the polymer that is more easily removable by repeated washing of the pristine material with acetone.

High Conduction Band Inorganic Layers for Distinct Enhancement of Electrical Energy Storage in Polymer Nanocomposites

Dielectric polymer nanocomposites are considered as one of the most promising candidates for high-power-density electrical energy storage applications. Inorganic nanofillers with high insulation property are frequently introduced into fluoropolymer to improve its breakdown strength and energy storage capability. Normally, inorganic nanofillers are thought to introducing traps into polymer matrix to suppress leakage current. However, how these nanofillers effect the leakage current is still unclear. Meanwhile, high dopant (> 5 vol%) is prerequisite for distinctly improved energy storage performance, which severely deteriorates the processing and mechanical property of polymer nanocomposites, hence brings high technical complication and cost. Herein, boron nitride nanosheet (BNNS) layers are utilized for substantially improving the electrical energy storage capability of polyvinylidene fluoride (PVDF) nanocomposite. Results reveal that the high conduction band minimum of BNNS produces energy barrier at the interface of adjacent layers, preventing the electron in PVDF from passing through inorganic layers, leading to suppressed leakage current and superior breakdown strength. Accompanied by improved Young's modulus (from 1.2 GPa of PVDF to 1.6 GPa of nanocomposite), significantly boosted discharged energy density (14.3 J cm-3) and charge-discharge efficiency (75%) are realized in multilayered nanocomposites, which are 340 and 300% of PVDF (4.2 J cm-3, 25%). More importantly, thus remarkably boosted energy storage performance is accomplished by marginal BNNS. This work offers a new paradigm for developing dielectric nanocomposites with advanced energy storage performance.

Enhanced Energy Storage Performance of PVDF-Based Composites Using BN@PDA Sheets and Titania Nanosheets

With the rapid development of modern electrical and electronic applications, the demand for high-performance film capacitors is becoming increasingly urgent. The energy density of a capacitor is dependent on permittivity and breakdown strength. However, the development of polymer-based composites with both high permittivity (εr) and breakdown strength (Eb) remains a huge challenge. In this work, a strategy of doping synergistic dual-fillers with complementary functionalities into polymer is demonstrated, by which high εr and Eb are obtained simultaneously. Small-sized titania nanosheets (STNSs) with high εr and high-insulating boron nitride sheets coated with polydopamine on the surface (BN@PDA) were introduced into poly(vinylidene fluoride) (PVDF) to prepare a ternary composite. Remarkably, a PVDF-based composite with 1 wt% BN@PDA and 0.5 wt% STNSs (1 wt% PVDF/BN@PDA-STNSs) shows an excellent energy storage performance, including a high εr of ~13.9 at 1 Hz, a superior Eb of ~440 kV/mm, and a high discharged energy density Ue of ~12.1 J/cm3. Moreover, the simulation results confirm that BN@PDA sheets improve breakdown strength and STNSs boost polarization, which is consistent with the experimental results. This contribution provides a new design paradigm for energy storage dielectrics.

Increasing Permittivity and Mechanical Harvesting Response of PVDF-Based Flexible Composites by Using Ag Nanoparticles onto BaTiO3 Nanofillers

The role of Ag addition on the structural, dielectric, and mechanical harvesting response of 20%(xAg - (1 - x)BaTiO3) - 80%PVDF (x = 0, 2, 5, 7 and 27 vol.%) flexible composites is investigated. The inorganic fillers were realized by precipitating fine (~3 nm) silver nanoparticles onto BaTiO3 nanoparticles (~60 nm average size). The hybrid admixtures with a total filling factor of 20 vol.% were embedded into the PVDF matrix. The presence of filler enhances the amount of β-PVDF polar phase and the BaTiO3 filler induces an increase of the permittivity from 11 to 18 (1 kHz) in the flexible composites. The addition of increasing amounts of Ag is further beneficial for permittivity increase; with the maximum amount (x = 27 vol.%), permittivity is three times larger than in pure PVDF (εr ~ 33 at 1 kHz) with a similar level of tangent losses. This result is due to the local field enhancement in the regions close to the filler-PVDF interfaces which are additionally intensified by the presence of silver nanoparticles. The metallic addition is also beneficial for the mechanical harvesting ability of such composites: the amplitude of the maximum piezoelectric-triboelectric combined output collected in open circuit conditions increases from 0.2 V/cm2 (PVDF) to 30 V/cm2 for x = 27 vol.% Ag in a capacitive configuration. The role of ferroelectric and metallic nanoparticles on the increasing mechanical-electric conversion response is also been explained.

Analysis of the charging kinetics in silver nanoparticles-silica nanocomposite dielectrics at different temperatures

Dielectric nanocomposite materials are now involved in a large panel of electrical engineering applications ranging from micro-/nano-electronics to power devices. The performances of all these systems are critically dependent on the evolution of the electrical properties of the dielectric parts, especially under temperature increase. In this study we investigate the impact of a single plane of silver nanoparticles (AgNPs), embedded near the surface of a thin silica (SiO2) layer, on the electric field distribution, the charge injection and the charge dynamic processes for different AgNPs-based nanocomposites and various temperatures in the range 25°C–110°C. The electrical charges are injected locally by using an Atomic Force Microscopy (AFM) tip and the related surface potential profile is probed by Kelvin Probe Force Microscopy (KPFM). To get deeper in the understanding of the physical phenomena, the electric field distribution in the AgNPs-based nanocomposites is computed by using a Finite Element Modeling (FEM). The results show a strong electrostatic coupling between the AFM tip and the AgNPs, as well as between the AgNPs when the AgNPs-plane is embedded in the vicinity of the SiO2-layer surface. At low temperature (25°C) the presence of an AgNPs-plane close to the surface, i.e., at a distance of 7 nm, limits the amount of injected charges. Besides, the AgNPs retain the injected charges and prevent from charge lateral spreading after injection. When the temperature is relatively high (110°C) the amount of injected charges is increased in the nanocomposites compared to low temperatures. Moreover, the speed of lateral charge spreading is increased for the AgNPs-based nanocomposites. All these findings imply that the lateral charge transport in the nanocomposite structures is favored by the closely situated AgNPs because of the strong electrostatic coupling between them, additionally activated by the temperature increase.

Enhanced Energy Storage Performance of Polymer/Ceramic/Metal Composites by Increase of Thermal Conductivity and Coulomb-Blockade Effect

Polymer-based dielectrics have attracted considerable attention for a wide range of applications as energy storage devices with high power. However, high loss from low thermal conductivity (K) and leaky current may limit their practical utilization greatly. To overcome these issues, two-dimensional hexagonal boron nitride (h-BN) modified with polydopamine (PDA) and metal palladium nanoparticles (h-BN@PDA@Pd NPs) are introduced into a poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) copolymer matrix. The PDA coating improves the compatibility between the ceramic h-BN filler and the polymer matrix. Contrary to the general idea, the metallic Pd NPs enhance the breakdown strength of the polymer nanocomposites through the Coulomb-blockade effect. The nanocomposite film filled with 6 vol % h-BN@PDA@Pd NPs exhibits significantly improved recoverable energy density (U_rec) of 58.6 J cm^-3, which is increasedby 496% compared to pure P(VDF-HFP) film, maintaining an efficiency of 65%, even under a high voltage of 500 MV m^-1. The in-plane thermal conductivity of the nanocomposites was improved from 0.21 to 1.02 W m^-1 K^-1 with increasing ceramic h-BN content. This study suggests that a dielectric polymer with surface-engineered ceramic h-BN fillers through a Coulomb-blockade effect of metal Pd NPs might be a promising strategy for high energy storage devices.

MgAl LDH nanosheets loaded with Ni nanoparticles: a multifunctional filler for improving the energy storage performance of PVDF-based nanocomposites

The dielectric constant and breakdown strength of the PVDF-based nanocomposites can be increased simultaneously by adding multifunctional Ni–MgAl LDH nanosheets, thus the nanocomposites can exhibit excellent energy storage performance.

Interface design for high energy density polymer nanocomposites

This review provides a detailed overview on the latest developments in the design and control of the interface in polymer based composite dielectrics for energy storage applications. The methods employed for interface design in composite systems are described for a variety of filler types and morphologies, along with novel approaches employed to build hierarchical interfaces for multi-scale control of properties. Efforts to achieve a close control of interfacial properties and geometry are then described, which includes the creation of either flexible or rigid polymer interfaces, the use of liquid crystals and developing ceramic and carbon-based interfaces with tailored electrical properties. The impact of the variety of interface structures on composite polarization and energy storage capability are described, along with an overview of existing models to understand the polarization mechanisms and quantitatively assess the potential benefits of different structures for energy storage. The applications and properties of such interface-controlled materials are then explored, along with an overview of existing challenges and practical limitations. Finally, a summary and future perspectives are provided to highlight future directions of research in this growing and important area.

Dielectric properties of poly(arylene ether nitrile ketone) copolymers

A series of poly(arylene ether nitrile ketone) (PENK) random copolymers are successfully synthesized by the nucleophilic aromatic substitution polymerization of 2,6-dichlorobenzonitrile, 4,4′-difluorobenzophenone with various bisphenol monomers (4,4′-biphenol, bisphenol A, phenolphthalein, and hydroquinone). Compared with poly(arylene ether ketone), the PENK copolymers possess better solubility in polar solvents such as N-methyl-2-pyrrolidone, N, N-dimethylacetamide, N, N-dimethylformamide, dimethylsulfoxide, and so on. Because of the different molecular structures, the PENK copolymers exhibit thermal properties with their glass transition temperature ( T g) in the range of 171–237°C and 5% weight loss temperature ( T d) ranging from 409°C to 554°C. Moreover, all polymers with an intrinsic viscosity of approximately 1 dL/g show excellent film-forming properties and outstanding mechanical strength higher than 85 MPa. The temperature dependence of the dielectric constant and dielectric loss of all derived copolymers is stable before their T g. The breakdown strength of the hydroquinone-derived PENK copolymer is as high as 253 kV/mm, resulting in an energy storage density of 1.00 J/cm3. These solvent processable PENK copolymers are potential dielectric candidates for high temperature applications.

Largely enhanced energy storage capability of a polymer nanocomposite utilizing a core-satellite strategy

The development of new generation dielectric materials toward capacitive energy storage has been driven by the rise of high-power applications such as electric vehicles, aircraft, and pulsed power systems. Here we demonstrate remarkable improvements in the energy density and charge-discharge efficiency of poly(vinylidene fluoride) (PVDF) upon the incorporation of core-satellite structures, namely NaNbO3(NN)@polydopamine (PDA)@Ag nanowires. As compared to the NN NWs/PVDF and NN@PDA NWs/PVDF nanocomposites, the NN@PDA@Ag NWs/PVDF nanocomposites exhibit greatly enhanced energy density and significantly suppressed energy loss. As a result, the NN@PDA@Ag NWs/PVDF nanocomposite films with optimized filler content exhibit an excellent discharge energy density of 16.04 J cm-3 at 485 MV m-1, and maintain a high discharge efficiency of 62.8%. Moreover, the corresponding nanocomposite films exhibit a superior power density of 2.1 MW cm-3 and ultra-fast discharge speed of 153 ns. Ultimately, the excellent dielectric and capacitive properties of the polymer nanocomposites could pave the way for widespread applications in modern electronics and power modules.