Charge-discharge Efficiency Research Articles

N- and F-Co-Doped Carbon Quantum Dots Coated on a Ni Foam Substrate as Current Collector for Highly Stable Li-Air Batteries

Because of their high theoretical density, Li-air batteries (LABs) are expected to be critical components of future energy storage devices. However, the commercial use of LABs is limited by their low charge-discharge efficiency, high overpotential, and low cyclic stability. Therefore, to improve the cyclic stability of LABs, we manufactured N- and F-co-doped carbon quantum dots (NF-CQDs), thinly coated them on the surface of a Ni foam current collector via ultrasonic spray coating, and used the as-fabricated NF-CQD/Ni as the current collector of an LAB cathode. The NF-CQD/Ni-based cathode maintained remarkable cycling stability over 42 cycles (with a limited capacity was 1000 mAh/g) and exhibited a superior specific discharge capacity of approximately 5121.91 mAh/g (at 100 mA/g). NF-CQD/Ni-based cathode exhibits superior electrochemical properties, including its high cycling stability and high discharge capacity, primarily contributed by the NF-CQD coating layer, which prevented the oxidation of the Ni foam substrate and increased the electrical conductivity and electrocatalytic activity of the cathode. Therefore, NF-CQD/Ni can serve as an excellent current collector for LAB cathodes.

Hydrogen Bonding of Aramid Boosts High-Temperature Capacitive Properties of Polyetherimide Blends

Polyetherimide (PEI) is the state-of-the-art commercial high-temperature polymer dielectric with excellent thermal and chemical stability and relatively high high-temperature capacitive energy storage properties. The rotation of the dipoles in the PEI chains brings the β-relaxation which seriously increases the leakage current and decreases the charge-discharge efficiency. In this work, hydrogen bonds have been introduced to limit the dipole rotation of PEI by blending aramids [1,4-poly(ether fluoromethyl naphthalene amide), PNFA] into the PEI matrix. By introducing 10 wt % PNFA, the β-relaxation of the blend has been significantly reduced which could be observed from the dielectric spectrum. To explore the mechanism of limited β-relaxation, we analyze the hydrogen bonds in the blend films by infrared spectroscopy and found that the maximum content of hydrogen-bonded carbonyl formed between PNFA and PEI chains was 14.3% when the content of PNFA was 30 wt %. The content of hydrogen bonds formed between PNFA and PEI was positively correlated with the energy storage performance of the blends. The maximum discharged energy density with an efficiency above 90% of the blend film with 30 wt % PNFA reaches 4.1 J cm-3 at 150 °C, which is about 350% higher than that of pristine PEI. This work shows that composing hydrogen bonds by the blending method could be a viable strategy for enhancing the high-temperature energy storage performance of polymer dielectrics, which could be achieved by large-scale preparation and has feasible industrial production prospects.

Crosslinked PAES-based sandwich-structured polymer nanocomposites with covalently strengthened interface towards high-temperature capacitive energy storage

As a key material for capacitors, high-performance polymer dielectric films still exhibit the problem of low energy storage density, which cannot meet the requirements of high-power and high-temperature conditions. Here, a sandwich-structured composite film with inner layer doped TiO2-BCB (titanium dioxide – benzocyclobutene) and outer layer doped BN-BCB (boron nitride – benzocyclobutene) nanoparticles was prepared, which exhibits reduced dielectric loss, leakage current and enhanced dielectric constant (ε) and breakdown strength (Eb). Diels-Alder reaction and radical polymerization reactions are utilized to form the entire cross-linked sandwich structure. Moreover, the thermal-induced crosslinked structure replaces the traditional multilayer structure established by Van der Waals forces, thus improving the dielectric and mechanical qualities of the film. Specifically, the sandwich structure composites achieved a large discharge energy density (4.0 J/cm3) and high charge-discharge efficiency (84%) at 150 °C and 450 MV/m, and the dielectric dissipation factor is less than 0.005. These results demonstrated that designing a cross-linked network in multilayer nanocomposite structure is a feasible method to improve the high-temperature energy storage performance of these films.

All-organic sandwich-structured BOPP/PVDF/BOPP dielectric films with significantly improved energy density and charge–discharge efficiency

Biaxially orientated polypropylene (BOPP) is widely used in film capacitors for its easy process, high breakdown strength and competitive cost. However, it is still a real challenge to further improve its energy storage density. Herein, we combined BOPP with polyvinylidene fluoride (PVDF), which has the best electroactive properties among conventional dielectric polymers, into an all-organic sandwich-structured composite to improve the energy storage performance. In the configuration, commercial BOPP films served as outer layers and the binder chlorinated polypropylene (CPP) was blended with PVDF as the middle layer to strengthen the interfacial adhesion. When PVDF and CPP were blended at a weight ratio of 1:1, an enhanced discharge energy density (Ue) of 5.07 J/cm3 was obtained, with charge–discharge efficiency (η) of 82.5 %, much higher than that of BOPP (2.71 J/cm3 with η of 87.3 %). Furthermore, CPP was grafted with four different methacrylates. With the polymer-grafted CPP in the middle layer, the Ue and η of the sandwich-structured composites were further improved. For example, with methyl methacrylate as grafting monomer, the Ue and η of the sandwich-structured film were up to 5.89 J/cm3 and 80.8 %, respectively, while with nonafluorohexyl methacrylate as grafting monomer, the corresponding value of Ue and η were 5.70 J/cm3 and 88.1 %, respectively. This work demonstrated a facile strategy to combine the advantages of BOPP and PVDF by a sandwich structure to raise the energy density significantly and keep the high efficiency simultaneously.

Enhanced energy storage performance of polyetherimide-based sandwich-structured dielectric films in elevated temperature range via using cyanoethyl cellulose layer

In order to satisfy the application of electronic device in the harsh environment, polymer-based dielectrics should have high discharge energy density (Ud) and high-temperature resistance synchronously. Herein, a series of sandwich-structured polymer films consisting of cyanoethyl cellulose (CEC) and polyetherimide (PEI) were fabricated by the solution casting method. The PEI-based sandwich-structured dielectric films showed high dielectric constant because of highly polarized CEC layer and the macroscopical interfacial polarization. In addition, the polymer films displayed the improved breakdown strength owning to the insulating PEI, barrier effect of the interface, and electric field redistribution. As a result, the maximal Ud of a positive sandwich-structured P–5C–P film with outer PEI layer and central CEC layer reached 6.8 J/cm3 (151% of pure PEI) along with charge-discharge efficiency of 80%. More importantly, at 100 °C and 150 °C, a high Ud of 5.5 J/cm3 and 3.0 J/cm3 were achieved in the P–5C–P film due to both high glass transition temperature (Tg) of PEI and CEC, which was 1.72 times and 1.36 times than that of pristine PEI, respectively. This work provides a new method for the preparation of high-performance sandwich-structured polymer dielectrics via selecting highly polarized central polymer layer with high Tg.

Improved energy storage performance in flexible (PbLa)ZrO3 thin films via nanocrystalline engineering

Flexible film capacitors with high energy storage density (Wrec) and charge–discharge efficiency (η) are a cutting-edge research topic in the current field of energy storage.

Design of heterogeneous sandwich-structured dielectric composites with excellent charge–discharge efficiency and energy storage density

With the rapid development of new energy fields, polymer-based film capacitors have received extensive attention in the field of dielectric capacitors due to their high power density.

Flexible cellulose/PVDF composite films with improved breakdown strength and energy density for dielectric capacitors

• Cellulose as biomass material was used for energy-storage dielectrics. • Regenerated cellulose/polyvinylidene fluoride dielectric composite films (RC-PVDF) were prepared by immersion method. • The energy storage density of RC-PVDF reached 8.3 J/cm 3 under the electric field of 410 MV/m, which was 144% and 176% higher than that of RC and PVDF. • The introduction of PVDF reduced the water absorption and improved the dielectric properties of the RC-PVDF films. Environment-friendly dielectric materials with high charge-discharge efficiency and energy density are urgently needed in the field of energy storage. Cellulose has the advantages of renewability, non-pollution, wide source, cheapness and so on. However, cellulose is easy to absorb water which causes the decrease of energy storage density in high humidity environment. In this work, polyvinylidene fluoride (PVDF) was used to reduce the hydrophilic of cellulose. Regenerated cellulose (RC)/PVDF dielectric composite films (RC-PVDF) were prepared by immersion method. When the concentration of PVDF solution was 3 wt%, the composite had the best comprehensive performance. The energy storage density of RC-PVDF reached 8.3 J/cm 3 under the electric field of 410 MV/m, which was 144% and 176% higher than that of RC and PVDF, respectively. This work expands the application of cellulose in humid environment and provides a good choice to develop high-performance biopolymer-based dielectric materials.

Design and characterization of molecular, crystal and interfacial structures of PVDF-based dielectric nanocomposites for electric energy storage.

PVDF-based polymers with polar covalent bonds are next-generation dielectric materials for electric energy storage applications. Several types of PVDF-based polymers, such as homopolymers, copolymers, terpolymers and tetrapolymers, were synthesized by radical addition reactions, controlled radical polymerizations, chemical modifications or reduction with the monomers of vinylidene fluoride (VDF), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), hexafluoropropylene (HFP) and chlorotrifluoroethylene (CTFE). Owing to rich molecular structures and complicated crystal structures, PVDF-based dielectric polymers can show versatile dielectric polarization properties, including normal ferroelectrics, relaxor ferroelectrics, anti-ferroelectrics and linear dielectrics, which are beneficial for designing polymer films with high capacity and high charge-discharge efficiency for capacitor applications. Furthermore, to satisfy the requirements of practical high-capacity capacitors, the polymer nanocomposite method is another promising strategy to achieve high-capacitance dielectric materials by the addition of high-dielectric ceramic nanoparticles, moderate-dielectric nanoparticles (MgO, and Al2O3), high-insulation nanosheets (BN), etc. It is concluded with the current problems and future perspectives of interfacial engineering, such as core-shell strategies and hierarchical interfaces in polymer-based composite dielectrics for high-energy-density capacitor applications. In addition, an in-depth understanding of the roles of interfaces on the dielectric properties of nanocomposites can be achieved by indirect analysis techniques (theoretical simulation) and direct analysis techniques (scanning probe microscopy). Our systematic discussions on molecular, crystal and interfacial structures provide guidance for designing fluoropolymer-based nanocomposites for high-performance capacitor applications.

Enhanced High-Temperature Capacitive Performance of a Bilayer-Structured Composite Film Employing a Charge Blocking Layer

The great development potential of polymer dielectric capacitors in harsh environments urgently requires enhancing capacitive performance at high temperatures. However, the exponentially increased conduction loss at high temperature and high field results in a drastic drop in energy density and charge-discharge efficiency. Here, a bilayer-structured polyimide (PI) composite film containing a wide-band gap inorganic layer as a charge blocking layer is designed. The inorganic layer improves the charge trapping ability and regulates the charge mobility at the electrode/dielectric interface. The charge injection mechanism in the interface-optimized PI/boron nitride nanosheet (BNNS) composite films is investigated by finite element simulation, and the effect of the BNNS layer on high temperature conduction is further understood. An appropriate thickness of the charge blocking layer establishes an effective energy barrier. Therefore, the composite films exhibit significantly suppressed conduction loss and excellent capacitive performance at a high temperature. A high energy density of 4.37 J cm-3 with efficiency of 92% is obtained at 200 °C and 500 MV m-1, which is superior to reported high-temperature dielectric polymers and their composite films. This work provides a promising approach to improve the energy storage performance of polymer materials at high temperatures.

Enhancement of Energy Storage Performances in PMMA/PVDF Nanocomposites with Low Dielectric Bismuth Carbonate Nanosheets

Both high discharge energy density (Ud) and charge–discharge efficiency (η) for nanocomposites are crucial to the development and application of modern electronic devices. However, the large dielectric contrast and ferroelectric phase greatly limit the improvement of Ud and η of poly(vinylidene fluoride) (PVDF)-based nanocomposites. Herein, Bi2O2CO3 (simplified as BOC) nanosheets with low dielectric constant and wide bandgaps and linear poly(methyl methacrylate) (PMMA) were filled into PVDF by solution casting to fabricate x vol % BOC/PAPF (PAPF is short for PMMA/PVDF) nanocomposites. The contributions of BOC nanosheets and linear PMMA, especially for the BOC nanosheet, have been systematically studied. Both experimental and simulation results show that BOC nanosheet and PMMA could improve the breakdown strength and reduce the dielectric loss and leakage current of nanocomposites simultaneously, realizing Ud and η improvement at the same time. As a result, the 1.0 vol % BOC/PAPF nanocomposite shows the highest energy storage performance with Ud = 13.68 J/cm3 and η = 85.72%, which is much superior to PVDF. Therefore, this study might provide a proof-of-concept for constructing nanocomposites with excellent Ud and η concurrently.

Enhanced breakdown strength and electrostatic energy density of polymer nanocomposite films realized by heterostructure ZnO-ZnS nanoparticles

We demonstrate that introduction of heterostructure nanoparticles into a polymer matrix is an effective strategy to substantially enhance dielectric breakdown strength (Eb) and thus a high electrostatic energy storage density (Ue) can be obtained, which is highly desired in modern electronic and electrical systems for energy storage and conversion. This is realized through the special “electrical rectification” effect of heterostructure nanoparticles on the charge transport, which stems from capturing and confining charge carriers in the nanocomposites by potential well and potential barrier formed at the heterojunction of ZnO-ZnS nanoparticles. The leakage current of the ZnO-ZnS/polyetherimide nanocomposites (ZnO-ZnS/PEI) is suppressed by the introduced heterojunction, which is accompanied with the simultaneous increase of dielectric displacement and charge–discharge efficiency, resulting in significant enhancement of Ue. Notably, the 1 wt% ZnO-ZnS/PEI nanocomposite films possess high discharged energy density at room temperature, i.e. 6.9 J cm−3 at 650 MV m−1, and even at 150 °C, the Ue still remains 3.6 J cm−3 at 500 MV m−1. Outstanding fatigue resistance over 50,000 charge–discharge cycles at 200 MV m−1 and 150 °C demonstrates high temperature cyclic stability of the heterostructure in confining charge carriers. This work provides a novel and scalable strategy to obtain polymer-based dielectrics with superior energy storage performance.

Enhancement of high temperature capacitive performance of sulfone-containing polyimide by suppressing carrier transport

With the increasing demand for safe operation in harsh environments, polymer dielectric capacitors with high energy density (Ue) and charge-discharge efficiency (η) operating at high temperatures are urgently needed. In this work, the sulfone group-containing polyimide (SO2PI) film with optimal imidization degree is prepared by regulating the kinetics of imidization reaction. The functional groups obtained by partial imidization enable polyimide to achieve excellent energy storage performance, and it is found that the presence of sulfone groups is beneficial for the further improvement of η. The optimal SO2PI film achieves a Ue of 2.40 J cm−3 at 400 MV m−1 and 150 °C, while it maintains a high η of 92%. Moreover, its preparation process without any additional modification steps matches commercial production equipment, indicating a simple and effective strategy for fabrication of a high-performance dielectric film. Thus, this work exhibits great potential in the field of high temperature energy storage.

Biodegradable Poly(l-Lactic Acid) Films with Excellent Cycle Stability and High Dielectric Energy Storage Performance

Polymer-based film capacitors play key roles in numerous applications, such as converter/inverter systems in hybrid electric vehicles (HEVs), smart grids, and pulsed power sources. However, nearly all actively studied dielectric polymers are nondegradable. In this work, we prepare flexible biodegradable poly(l-lactic acid) (PLLA) films via a simple solvent casting method and achieve enhanced dielectric performances by polymer crystallization. The recoverable energy density (Urec) with charge–discharge efficiency (η) of 90% was improved from ∼2.9 J/cm3 for amorphous PLLA to ∼5.7 J/cm3 for the crystallized PLLA film at room temperature. Under 200 MV/m at 85 °C (the operation conditions of commercial biaxially oriented polypropylene-based capacitors in HEVs), Urec of 0.82 J/cm3 with η of 95% is achieved in the crystallized PLLA film, which is much higher than that of BOPP (below 0.5 J/cm3). In particular, the remarkable cyclic stability of the crystallized PLLA film is demonstrated by charge–discharge tests for 20 000 cycles at both room temperature and 85 °C under 200 MV/m. Moreover, the low C/(H+O) atom ratio helps metalized PLLA films exhibit a valuable self-healing ability after breakdown. With excellent recoverable energy density, high efficiency, good cyclic reliability, low-cost preparation method, self-healing ability, and eco-friendliness, the crystallized biodegradable PLLA film provides an eco-friendly and high-performance candidate to develop high-energy-storage capacitors.

Ultra-low dielectric loss and high thermal stability achieved by hierarchical microcapacitor structure in nanocomposites via surface topological modulation

The urgent need for miniaturized high-reliable electronic devices requires polymer film capacitors with stable dielectric properties under extreme environments. The traditional surface coating structure can barely boost the performance of energy storage dielectric materials, which are confronted with a high dielectric loss and low breakdown strength. Herein, reduced polyaniline (R-PANI) grows in the form of vertically oriented nanorods on the surface of porous carbon embedded with ultrafine BaTiO3, resulting in a hierarchical microcapacitor structure. The surface topology modulation of R-PANI dramatically increases the aspect ratio of the nanoparticles perpendicular to the external electric field, thus suppressing the leakage current and restraining the subsequent dielectric loss of the nanocomposite. The nanocomposite filled with 8 wt% C@BT@R-PANI exhibits a breakdown strength of 316.50 MV/m, a high dielectric constant of 66.8, and an ultra-low dielectric loss of 0.00663 at 1 kHz. Under a moderate electric field of 150 MV/m, the energy density increases to 2.75 J/cm3 and the charge-discharge efficiency (η) reaches 80.09%. In addition, the nanocomposite exhibits an excellent dielectric-thermal stability in the range of 30–150 °C, making it a potential candidate for high-temperature dielectric materials.

High energy density and superior charge/discharge efficiency polymer dielectrics enabled by rationally designed dipolar polarization

Although many dielectric polymers exhibit high energy storage density (Ue) with enhanced dipolar polarization at room temperature, the substantially increased electric conduction loss at high applied electric fields and high temperatures remains a great challenge. Here, we report a strategy that high contents of medium-polar ester group and end-group (St) modification are introduced into a biodegradable polymer polylactic acid (PLA) to synergistically reduce the loss and enhance Ue and charge-discharge efficiency (η). The resultant St-modified PLA polymer (PLA-St) exhibits an Ue of 6.5 J/cm3 with an ultra-high η (95.4%), far outperforming the best reported dielectric polymers. It is worth noting that the modified molecular structures can generate deep trap centers and restrict the local dipole motions in the polymer, which are responsible for the reduction of conduction loss and improvements in high-temperature capacitive performance. In addition, the PLA-St polymer shows intrinsically excellent self-healing ability and cyclic stability surviving over 500 000 charge-discharge cycles. This work offers an efficient route to next-generation eco-friendly dielectric polymers with high energy density, low loss, and long-term stability.

Suppressing the Loss of Polymer-Based Dielectrics for High Power Energy Storage.

Polymer-based dielectrics have received intensive interest from academic community in the field of high-power energy storage owing to their superior flexibility and fast charge-discharge ability. Recently, how to suppress the loss of polymer-based dielectrics has been increasingly recognized as a critical point to attain a high charge-discharge efficiency in the film capacitors. Some achievements are made in analyzing the source of loss and suppressing loss via Edison's trial and error method. In this review, the significance of suppressing loss in polymer-based dielectrics is firstly emphasized. Then, different sources of loss are discussed carefully and an in-depth analysis of the related measurements is presented. Next, recent research results in suppressing loss are summarized and discussed in detail according to different strategies. Finally, the challenges and opportunities in the loss suppression research for the rational design of high-efficiency polymer-based dielectrics are proposed.

Greatly enhanced energy density in polymer nanocomposites coated with high-k perovskite nanosheets

Polymer dielectric materials with high energy storage density are vital component in advanced electronic devices. However, the increase of conduction loss at relatively high electric field will lead to film failure and hinder the exploration of high energy density in the dielectric polymer. Herein, a general strategy is proposed to effectively enhance the breakdown strength and energy density by coating wide bandgap Ca2Nb3O10 nanosheets on the surface of the polyvinylidene fluoride film using Langmuir-Blodgett and spray coating technologies. The coating layers of Ca2Nb3O10 nanosheets exhibit high insulation feature, and therefore, suppress the leakage current density of the nanocomposite film, resulting in remarkably enhanced breakdown strength of 710 MV/m and discharged energy density of 25.1 J/cm3, with a satisfactory charge–discharge efficiency of 78%. Our work may provide a scalable, and yet effective route to tailor the energy storage density of polyvinylidene fluoride-based polymers.

Tailoring Poly(Styrene-co-maleic anhydride) Networks for All-Polymer Dielectrics Exhibiting Ultrahigh Energy Density and Charge-Discharge Efficiency at Elevated Temperatures.

Polymer film capacitors have been widely used in electronics and electrical power systems due to their advantages of high power densities, fast charge-discharge speed, and great stability. However, the exponential increase of electrical conduction with temperature and applied electric field substantially degrades the capacitive performance of dielectric polymers at elevated temperatures. Here, the first example of controlling the energy level of charge traps in all-organic crosslinked polymers by tailoring molecular structures that significantly inhibit high-field high-temperature conduction loss, which largely differs from current approaches based on the introduction of inorganic fillers, is reported. The polymer network with optimized crosslinking structures exhibits an ultrahigh discharged energy density of 7.02 J cm-3 with charge/discharge efficiencies of >90% at 150°C, far outperforming current dielectric polymers and composites. The charge-trapping effects in different crosslinked structures, as the origins of the marked improvements in the high-temperature capacitive performance, are comprehensively investigated experimentally and confirmed computationally. Moreover, excellent cyclability and self-healing features are demonstrated in the polymer film capacitors. This work offers a promising pathway of molecular structure design to scalable high-energy-density polymer dielectrics capable of operating under harsh environments.

Improved energy storage performance of polyimide nanocomposites by constructing the meso- and macroscopic interfaces

Polyimide (PI) is a promising material for the dielectric capacitor due to its excellent electrical insulation and mechanical properties and so on. However, the low dielectric constant limits the enhancement of energy storage density and its further practical application. Here, we construct the sandwich-structured PI/BaTiO3 nanocomposites by in-situ synthesis and solution casting techniques. The energy density of the sandwich-structured nanocomposites reaches the maximum value of 7.44 J/cm3, contributed by the synergistically enhanced breakdown strength and dielectric constant (486.34 kV/mm and 7.24), while keeping a good charge-discharge efficiency. Combined with the three-dimensional simulation results, it can be seen that the organic/inorganic interfacial polarization in polarized layer and the interlayer polarization can significantly increase the dielectric constant. Meanwhile, the sandwich structure can redistribute the electric field and mitigate the electric field concentration in the polarized layer. The outer insulation layer can block the high current and heat breakdown paths caused in the middle layer. Consequently, the breakdown strength and dielectric constant are improved synergistically, leading to an enhancement in the energy density. The work provides a new paradigm to explore polyimide nanocomposites with enhanced energy density based on the combination of experiments and simulations.