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With the increased demand for electrical devices, there is an urgent requirement to explore advanced materials for capacitors. Composite is an effective way to improve performance through materials and structure design. Herein, an asymmetric three-layer structure with gradient dielectric constant through the solution casting method is reported. This unique design of construction consists of a buffer layer pristine poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) embedded between high insulation linear materials polyurea (PUA) and high polarization non-linear materials P(VDF-HFP)/BT nanocomposite. The experimental and simulation results demonstrate that the linear layer ensures high insulation of the composite and relatively high efficiency, while the nanocomposite layer acts as a high dielectric layer to enhance polarization. Typically, the buffer layer could effectively avoid overconcentration of the electric field on the PUA layer, which results in high breakdown strength of the trilayer composite. As a result, an ultrahigh breakdown strength of 5970 kV/cm and excellent energy storage density of 8.2 J/cm3 can be obtained, which were 45% and 2.15 times higher than that of pristine P(VDF-HFP). This asymmetric structure strategy provides a successful case in exploring high-performance energy storage materials.

High energy storage density with low-energy loss polymer films are essential for high-performance electric devices. To avoid the high-energy loss of utilizing nonlinear polymer materials, a sandwich nanostructure comprising a linear polymer poly(methyl methacrylate) (PMMA) matrix embedded with a high dielectric constant BaTiO3 (BT) interlayer and poly(vinylidene fluoride) (PVDF) binder was constructed using a solution casting strategy. This structural design takes advantage of each component in the composite. The good dispersion of BT particles in the binder, which was incorporated between PMMA, enabled a high dielectric constant and fewer defects. Additionally, the excellent film formation ability of the PVDF binder guarantees the uniform thickness and stable structure of the BT mid-layer, and good miscibility between PVDF and PMMA enhanced the interaction between each layer. Interestingly, since the dielectric constant of PVDF was between BT fillers and PMMA, a dielectric gradient distribution mitigated the local electric field concentration, as proven by the simulation results. Consequently, a low-loss linear PMMA composite film exhibited satisfying breakdown strength and excellent discharged energy density, which were 25% and 460% higher than those of pristine PMMA, respectively.

Modified MXene (Ti3C2Tx) is attractive as a flexible electrode for wearable energy storage devices. In this work, a convenient and effective method was proposed to change the conventional 2D boron carbon nitride (BCN) nanosheets into three-dimensional 3D BCN microspheres that were obtained by tube furnace drying under N2 flow and annealing. Then, the MXene/BCN microflowers were applied to all-solid-state flexible microsupercapacitors (MSCs) as a high-performance electrode material. It was found that the areal capacitance can reach up to 89 mF cm−2 for a single MSC under 0.5 mA cm−2. Furthermore, the MSCs can achieve remarkable mechanical flexibility such that the capacitance will not be evidently decreased even after bending by up to 180°. In addition, 90.1% capacity retention was obtained even after 10 000 cycles and the highest energy density and power density reached 0.0124 mW h cm−2 (volumetric energy density of approximately 17.7 mW h cm−3) and 3.1 mW cm−2 (volumetric power density of approximately 4.5 W cm−3). These results demonstrate the synthesis of MXene/BCN composite materials with excellent power density and large scalability and can provide distinctive insights into high-performance flexible device storage systems.

Nanocomposite polymer materials are commonly used in energy storage devices on account of the excellent dielectric performance. However, there is a long-standing contradiction between dielectric constant and breakdown strength of nanocomposite. In this study, polyurea (PUA) is designed to in situ modify BaTiO3 (BT) nanoparticles. Based on the excellent dispersity, favorable compatibility and outstanding insulating performance of BT@PUA particles, the P(VDF-CTFE)/BT@PUA nanocomposite exhibits enhanced breakdown strength and reliability. In addition, the introduction of BT@PUA enhanced the dielectric constant to 10.72 and 35% larger than that of pure P(VDF-CTFE). As a result, the energy density of BT@PUA composite reaches 8.94 J/cm3, which is 1.65 times than P(VDF-CTFE) (5.41 J/cm3) and 4.5 times than the commercial biaxially oriented polypropylenes. Meanwhile, the quantified simulation illustrates the necessity of extremely high breakdown strength of the shell. Experiment results and simulation reveal that coating PUA is a novel and effective way to enhance energy storage performance for polymer nanocomposites with a promising broad application prospect.

The application prospects of dielectric materials such as polyvinylidene fluoride polymers in the field of energy storage have stimulated extensive research, and the performances are mainly affected by crystallization. However, most of them are focused on the influences of single-layer crystalline rather than macroscopic structures. In this study, a unique sandwich structure is proposed by combining layer-by-layer casting and multi-step thermal processing methods to control the macroscopic crystallization in each layer. Four heating temperatures were first set, and it was found that as the drying temperature increases, the films present different crystallization, which results in different dielectric performances. According to this feature, a sandwich structure was proposed for modification of macroscopic crystallization and the influences on energy storage performance were studied. After macroscopic modification of the crystallization, a low loss [Formula: see text], high energy density (7.12[Formula: see text]J/cc) with the highest charge-discharging efficiency was obtained, which presents the advantages of this methods, and the related mechanisms were discussed.

The improvement of energy density is desired for dielectric materials in capacitors according to the developing trend of electric devices. In this article, poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-CTFE)) and polyurea (PUA) are blended in different ratios to obtain high dielectric performance composite, and the dielectric polymer composite films are fabricated and investigated. The composite films combine the advantages of both contents, showing high dielectric constant of P(VDF-CTFE) and high energy storage efficiency of PUA. The results demonstrate that the composite film contains PUA with a volume fraction of 10% exhibits the best performance, achieving a breakdown strength as high as 5020 kV/cm, which is higher than that of P(VDF-CTFE)/polythiourea (PTU) composite ever reported. Moreover, the breakdown strength and energy storage efficiency are 1.35 times and 1.65 times higher than pure P(VDF-CTFE), respectively. The article provides a new high-performance dielectric material fabricated through a simple and effective blending method, and it has a great potential in electric energy storage capacitors.

Scalable and cost-effective fabrication of three-dimensional (3D) boron carbon nitride (BCN) microspheres was first demonstrated by hydrothermal and annealing methods. In particular, the specific surface area of 3D-BCN-4 reached 1390.12 m2 g-1 and had a high hierarchical pore structure. An all-printed solid-state flexible microsupercapacitor (MSC) based on 3D-BCN-4 microspheres as an electrode material was fabricated for the first time by a screen printing method, which also provided efficacious properties. The single MSC areal capacitance reached 41.6 mF cm-2. Furthermore, the remarkable mechanical flexibility was also achieved for the device with evidence that no obvious capacitance loss occurred even upon bending to 180°, and the device had a 93.3% capacitance retention after 1000 cycles. In addition, the maximum energy density reached 0.00832 mW h cm-2, and the highest power density was 2 mW cm-2. These results show the synthesis of 3D-BCN by a facile and effective method with excellent electrochemical performance, which should provide a promising direction to wearable energy storage devices.

High performance dielectric polymer materials are a key point for energy storage capacitors, especially film capacitors. In this paper, a sandwich-structured polymer film is constructed to achieve high energy density and high efficiency. High dielectric materials of poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) doped with barium titanate (BaTiO3) are used as the outer layer to achieve a high dielectric constant, and a boron nitride nanosheet (BNNS) layer is inserted between P(VDF-HFP)/BaTiO3 to obtain a high breakdown field strength of composite films. The results indicate that when the doping amount of the BaTiO3 nanoparticles reaches 10 wt% and the mass fraction of the BNNS layer is 0.75 wt%, a significant improvement of energy storage performance is obtained. The energy storage density of the P(VDF-HFP)/BaTiO3/BNNSs composite film can reach 8.37 J cm−3, which is higher than 6.65 J cm−3 of the pure P(VDF-HFP) film. Compared with the P(VDF-HFP) film doped with BaTiO3, significant improvement of the breakdown field strength (about 148.5%) is achieved and the energy storage density increases 235% accordingly, resulting from the inserted BNNSs layer blocking the growth of electrical branches and suppressing leakage current. This novel sandwich-structured film shows promising future applications for high performance dielectric capacitors.

The properties of solution-cast poly(vinylidene fluoride) (PVDF) films for energy storage are affected greatly by solvents used. In this letter, surface micrographs, crystalline phases, and energy storage properties of PVDF films crystallized from different solvents were studied. Under an electric field of 1000 kV/cm, the charge-discharge efficiency of the PVDF film crystallized from N,N-dimethylformamide (DMF) (with low vapor pressure) is 2.6 times higher than that of the PVDF film crystallized from N-methyl-2-pyrrolidone (NMP) (with high vapor pressure). Meanwhile, the resistivity of the former is up to 6.6 times higher than that of the latter.

All-solid-state flexible microsupercapacitors have been intensely investigated in order to meet the rapidly growing demands for portable microelectronic devices. Herein, we demonstrate a facile, readily scalable and cost-effective laser induction process for preparing reduced graphene oxide/multi-walled carbon nanotube composite, which can be used as the interdigital electrodes in microsupercapacitors. The obtained composite exhibits high volumetric capacitance about 49.35 F cm−3, which is nearly 5 times higher than that of the pristine reduced graphene oxide film in aqueous 1.0 M H2SO4 solution (measured at a current density of 5 A cm−3 in a three-electrode testing). Additionally, an all-solid-state flexible microsupercapacitor employing these composite electrodes with PVA/H3PO4 gel electrolyte delivers high volumetric energy density of 6.47 mWh cm−3 at 10 mW cm−3 under the current density of 20 mA cm−3 as well as achieve excellent cycling stability retaining 88.6% of its initial value and outstanding coulombic efficiency after 10,000 cycles. Furthermore, the microsupercapacitors array connected in series/parallel can be easily adjusted to achieve the demands in practical applications. Therefore, this work brings a promising new candidate of prepare technologies for all-solid-state flexible microsupercapacitors as miniaturized power sources used in the portable and wearable electronics.

Flexible supercapacitors that maintain electrochemical performance under deformation have attracted much attention for the potential application in the flexible electronics market. A compressible and flexible free-standing electrodes sponge and all-solid-state symmetric supercapacitors based on as-prepared electrodes are presented. The carbon nanotubes (CNTs) framework is synthesized by chemical vapor deposition (CVD) method, and then composited with poly (3,4-ethylenedioxythiophene) PEDOT by the electrodeposition. This CNTs/PEDOT sponge electrode shows highest mass-specific capacitance of 147 Fg-1 at 0.5 A g−1, tuned by the PEDOT mass loading, and exhibits good cyclic stability with the evidence that more than 95% of capacitance is remained after 3000 cycles. Furthermore, the symmetric supercapacitor shows the highest energy density of 12.6 Wh kg−1 under the power density of 1 kW kg−1 and highest power density of 10.2 kW kg−1 with energy density of 8 Wh kg−1, which exhibits both high energy density and power density. The electrochemical performance of composite electrode also indicates that the operate voltage of device could be extend to 1.4 V by the n-doping and p-doping process in different potential of PEDOT component. This flexible supercapacitor maintains stable electrochemical performance working on different bending condition, which shows promising prospect for wearable energy storage applications.

To obtain high-performance electrode on fabric substrate for flexible supercapacitors (SCs), a new composite electrode incorporating the conducting polymer poly-3,4-ethylenedioxythiophene (PEDOT) within the nanocomposite film of the reduced graphene oxide/carbon nanotubes (rGO/CNTs) has been proposed and fabricated successfully. By repeating spaying process and using vapor phase polymerization (VPP) method, an rGO/CNTs/PEDOT nanocomposite film is developed on the flexible non-woven fabric substrate and the composite electrode with hierarchical structure is obtained. The unique interconnected structure ensures that the flexible fabric electrode has a specific capacitance high up to 164 F/g at current density 0.1 A/g and an equivalent-series resistance (R ESR ) lower than 3.53 Ω. Moreover, the composite electrode exhibits good stability with capacitance retentions over 93% even it is bent 1000 times. Devices based on this composite electrode are constructed and reversibly cycle at a voltage of 1 V, which provide a volume energy density of 22.8 mWh/g and retain 98% capacitance of the initial value under a 90° bending angle. This work suggests that our new composite electrode is a promising candidate for the high-performance energy-storage devices, and flexible and wearable electronic components.

Here we demonstrate a facile electrochemical polymerization and laser induction process to fabricate poly (3,4-ethylenedioxythiophene)/reduced graphene oxide (PEDOT/rGO) nanocomposite, which can be used as supercapacitor electrode material. Firstly, a PEDOT film is deposited on ITO substrate using an electrochemical polymerization method and a graphene oxide (GO) film is successively deposited on as-prepared PEDOT film through a spin-coating method. Then, by using a laser-writing method, the GO film is transformed into the rGO and a PEDOT/rGO nanocomposite is obtained. The resulting nanocomposite shows high areal capacitance about 43.75 mF/cm 2 , which is nearly 3 times higher than that of the PEDOT film at a current density of 0.2 mA/cm 2 . The PEDOT/rGO nanocomposite exhibits excellent cyclic stability, which can retain 83.6% of its initial capacitance after 1000 charge-discharge cycles. Furthermore, this nanocomposite can be deposited on varied substrates as electrode materials, which shows promising application to prepare high performance energy storage materials.

The molecular weight shows great influence on dielectric and energy storage performance of poly(vinylidene fluoride) (PVDF) films and related devices. In this letter, the influences of molecular weight on the dielectric and energy storage properties of PVDF films were studied. It has been found that, under a 1000 kV/cm electric field, the low-molecular-weight PVDF film presents a much higher energy storage efficiency as high as 80.10%, nearly three times as much as the high-molecular-weight PVDF film. Moreover, the low-molecular-weight PVDF film also shows high resistivity, representing an order of magnitude improvement over the high-molecular-weight PVDF film, which is more desirable and promising for high performance pulse discharge capacitor application.

Construct dielectric films with high energy density and efficiency are the key factor to fabricate high-performance dielectric film capacitors. In this paper, an all organic composite film was constructed based on high dielectric polymer and linear dielectric polymer. After the optimized polycondensation reaction of a linear dielectric polymer aromatic polythiourea (ArPTU), the proper molecular weight ArPTU was obtained, which was introduced into poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) terpolymer for a composite dielectrics. The results indicate that the addition of ArPTU molecules reduces the dielectric loss and improves the breakdown field strength of the PVDF-TrFE-CFE effectively. For the PVDF-TrFE-CFE/ArPTU (90/10) composite film, the maximum energy density about 22.06 J/cm3 at 407.57 MV/m was achieved, and high discharge efficiency about 72% was presented. This composite material can be casted on flexible substrate easily, and PVDF-TrFE-CFE/ArPTU organic composite films having high energy density, high breakdown field strength, low dielectric loss, and higher discharge efficiency are obtained. This is an unreported exploration about high energy density organic dielectric films based on PVDF-TrFE-CFE matrix and linear polymer dielectrics, and the findings of this research can provide a simple and scalable method for producing flexible high energy density materials for energy storage devices.

A novel electrode material incorporating renewable biomass-derived juglone biomolecules with commercial activated carbon (AC) granules has been through simple ultrasonic dispersion and dissolution–recrystallization and was found to exhibit good electrochemical performance. The juglone biomolecules are prepared by an ultrasound-assisted extraction method from abandoned walnut peel, which decreases pollution and increases economic efficiency. Through the dissolution–recrystallization process with AC, a hierarchical structure with nanosized juglone particles was obtained, and the AC particles worked as scaffolding to strengthen the slight biomolecules, thus expanding the active sites and effectively reducing the dissolution of the active materials. The pseudocapacitance fading mechanism was investigated by ex situ FTIR measurement and the porous structure ensures that the composite electrode has an enhanced specific capacitance of 248 F g−1 compared to 172.8 and 62.5 F g−1 for the respective AC and juglone samples. Besides, the excellent cyclic stability (retained 75% after 3000 charge–discharge cycles) was demonstrated. The highest area-specific capacitance of the composites was 1300 mF cm−2. An asymmetric supercapacitor based on this composite electrode was assembled with an AC electrode as the counter electrode and exhibited good cyclic performance at a voltage of 1.2 V (retained 77% after 3000 charge–discharge cycles), which provides a high energy density of 12 W h kg−1 at a power density of 0.18 kW kg−1 and a high power density of 2 kW kg−1 at an energy density of 9 W h kg−1. This work explores the application of biomolecule-based composites in energy storage devices and provides a potential strategy for constructing environmentally friendly electrodes.

The three-dimensional (3D) porous nanostructures have shown attractive promise for flexible microsupercapacitors due to their merits of more exposed electrochemical active sites, higher ion diffusion coefficient, and lower charge-transfer resistance. Herein, a highly opened 3D network of reduced graphene oxide/poly(3,4-ethylenedioxythiophene) (rGO/PEDOT) was constructed through the laser-assisted treatment and in situ vapor phase polymerization methods, which can be employed with gel electrolyte to prepare flexible microsupercapacitors, without conductive additives, polymer binder, separator, or complex processing. These porous open network structures endow the obtained microsupercapacitors with a maximum specific capacitance (35.12 F cm−3 at 80 mA cm−3), the corresponding energy density up to 4.876 mWh cm−3, remarkable cycling stability (with only about 9.8% loss after 4000 cycles), and excellent coulombic efficiency, which are comparable with most previous reported rGO-based microsupercapacitors. Additionally, the microsupercapacitors connected in series/parallel have been conveniently fabricated, followed by being integrated with solar cells as efficient energy harvesting and storage systems. Moreover, the working voltage or energy density of microsupercapacitors array can be easily tailored according to the practical requirements and this work provides a promising approach to prepare high-performance flexible micro-energy device applied in the wearable electronics accordingly.

Polymer materials are preferred to show a high advantage for electrostatic capacitors energy storage and pulsed power applications. In this work, a linear material, aromatic polyurea, has been successfully synthesized by stepwise polycondensation, then films with different isocyanate index of 1.00, 1.03, 1.06 and 1.09 were prepared. The Young’s modulus increases with the increase of isocyanate index, which means that film with higher isocyanate index presents better mechanical property. The dielectric permittivity of samples with isocyanate index of 1.03, 1.06 and 1.09 are 3.72, 3.63, 3.79 respectively, more than 1.5 times high than Biaxially Oriented Polypropylene (BOPP). Film with isocyanate index of 1.06 presents highest energy storage efficiency because of the complete reaction and lower reaction monomer residual, which can be proved by FTIR and the dielectric loss. At 3000 kV/cm electric filed, sample with isocyanate index of 1.06 still shows a very high energy storage efficiency of 89%. Considering the comprehensive property, polyurea film with isocyanate index of 1.06 is promising for high performance pulse capacitors application.

The crystal structure shows great influence on dielectric and energy storage performance of poly(vinylidene fluoride) (PVDF) films. In this paper, a γ-phase PVDF film was fabricated by crystallization of PVDF from a DMF solution through a solution-casting method and a quenching treatment. The influences of crystalline phases on the dielectric and energy storage properties of the films were studied. It has been found that, compared with common α- and β-phase, the obtained γ-phase PVDF film presents much higher relative permittivity of about 9.8 in a 1 kHz electric field. The ferroelectric hysteresis loop investigation indicates that the γ-phase PVDF film exhibits much slimmer polarization–electric field hysteresis loops than α- and β-phase PVDF films and a high energy efficiency more than 72 % is obtained. This γ-phase PVDF film also shows low leakage current at high working voltage, which is more desirable and promising for high performance pulse discharge capacitor application.

All-organic composites are widely used in energy storage application due to the high breakdown strength performance, but the improvement of energy storage was limited by the relatively low dielectric constant. Therefore, to satisfy the high demands of dielectric materials, energy storage properties of polymer composites should be further enhanced. In this article, poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-CTFE)) and polyurea (PUA), which are known as high dielectric ferroelectric material and linearly high energy storage efficiency material respectively, are composited through double layer (DL) casting method for the first time. The properties of DL structured composite film is contrasted with solution blending structure especially in energy storage efficiency, and the results demonstrate that DL structure design can make great use of advantages of two materials and also can avoid the influence of phase separation between P(VDF-CTFE) and PUA efficiently. Moreover, high breakdown strength (6180 kV/cm) and high energy storage efficiency (77%) of DL composites can be realized simultaneously by incorporating PUA as an insulating layer, and the mechanism is discussed in detail. This work provides an effective route to improve the energy storage properties of polymer dielectric materials and shows great application potential.

The application prospects of dielectric materials such as poly(vinylidene fluoride) (PVDF) based polymers in the field of energy storage have stimulated extensive research. In this study, the energy storage characteristics of poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) films crystallized from several solvents with different vapor pressure were investigated. The charge-discharge efficiency of the P (VDF-HFP) film crystallized from N, N-dimethylformamide (DMF) demonstrates an efficiency of 85% under an electric field of 1000 kV cm−1, which is 1.42 times of the film crystallized from dimethyl sulfoxide (DMSO). Meanwhile, the insulation resistivity of the film crystallized from DMF is also higher than other films, and the energy storage characteristics are optimal in this study. This work provides guidance for the preparation of PVDF-based polymer films using the solution method.

We report chemical in situ deposition of conducting polymer poly (3,4-ethylenedioxythiophene) (PEDOT) on reduced graphene oxide (rGO) nanosheets through a simple hydrothermal polymerization method. The functional groups on graphene oxide (GO) were directly employed as an oxidant to trigger the polymerization of 3,4-ethylenedioxythiophene (EDOT), and the GO nanosheets were reduced into rGO accordingly in an aqueous environment. Well anchoring of ultrathin PEDOT on rGO through this oxidant-free method was confirmed by UV-Vis spectrum, FT-IR spectrum, SEM, and TEM analysis. The obvious enhancement of conductivity was observed after the covering of PEDOT on rGO, and this composite showed high conductivity about 88.5 S/cm. The electrochemical performance results revealed that rGO/PEDOT composite electrode exhibits high specific capacitance about 202.7 F/g. The good synergetic effect between PEDOT and rGO also makes sure highly stable reversibility of composite electrode during charging/discharging process, and more than 90% initial capacitance retains after 9000 times cycles. In addition, the electrode based on rGO/PEDOT deposited on the cotton fabric shows excellent flexible ability with the evidence that 98% of the initial capacitance of electrode maintained after three thousands of free bending, which shows promising energy storage performance for flexible devices. .

Here we report the novel design of Co9S8 nanoparticles anchoring onto three dimensional (3D) porous sulfur-doping reduced graphene oxide network (Co9S8@S-rGO) by the facile hydrothermal reaction and heat annealing process, during which the abundant pores on the S-rGO framework and Co9S8 nanoparticles can be obtained simultaneously. This may be attributed to the Co9S8 nanoparticles as the catalyst, which can contribute to the transformation of the S atoms in rGO into S-relative gases (such as H2S or SO2), resulting in the pores-rich nanostructures. Furthermore, the holey network S-rGO matrix effectively facilitate electron/ion transport, and the Co9S8 nanoparticles intercalating in S-rGO framework can drastically reduce the agglomeration of S-rGO nanosheets and provide abundant pseudocapacitance reaction sites simultaneously. The results demonstrate that the Co9S8@S-rGO-800 electrode delivers a higher gravimetric capacitance of 348.45 F g−1 and an outstanding cycling stability of 92.6% after 5000 charge/discharge cycles at 1 A g−1. As expected, an asymmetric supercapacitor is assembled and exhibits a high energy density of 39.51 Wh kg−1 at a power density of 260 W kg−1 (23.13 Wh kg−1 at 8410 W kg−1), as well as the excellent cycling stability with 90.6% of the initial specific capacitance after 5000 cycles.

Untreated and annealed PVDF films in same crystalline phase were fabricated via different processes, and the effects of annealing treatment on the dielectric properties of PVDF films were studied. It has been found that the annealing treatment eliminates the micro pores in the PVDF films. As a result, the annealing treatment shows to exert strong influence on the relative permittivity and leakage current of PVDF films. Compared with common untreated PVDF film, the annealed PVDF film presents a higher relative permittivity about 8.7 in a 1 kHz electric field and a lower leakage current around 3.04 μA under a 1000 kV/cm electric field. However, the annealing treatment has little effect on the dissipation factor.