High Energy Density Capacitors Research Articles

Bilayer structured PVDF-based composites via integrating BaTiO3 nanowire arrays and BN nanosheets for high energy density capacitors

Polymer based dielectric nanocomposites with high energy density are highly desirable to satisfy the lightweight and integration of electrostatic capacitors in electronic and electrical equipment. Recently, it is proved that polymer incorporated with aligned nanowire array represents an effective strategy for significantly improving the permittivity due to largely enhanced space charge and orientation polarization. Nevertheless, it is still a great challenge to overcome the contradictory relationship between permittivity and electric breakdown strength. In this work, a unique spatial design in bilayer structured PVDF based nanocomposites are proposed by incorporating BaTiO3 nanowire array with high-permittivity and BN nanosheets with wide-band gap, respectively. The bottom BaTiO3/PVDF layer and upper BN/PVDF layer are constructed for the synergy ascension of permittivity and breakdown strength by the optimization of BN nanosheet content. As a consequence, the dielectric permittivity of BaTiO3/BN-PVDF nanocomposite reaches 35.3 at 1 kHz, which is higher than that of 33.9 for BaTiO3/PVDF nanocomposite and 9.1 for pristine PVDF film, respectively. Also, the significantly increased electric displacement, e.g., 12.1 μC cm−2, enhanced Eb, e.g., 500.3 kV mm−1 and suppressed leakage current density are obtained in BaTiO3/BN-PVDF nanocomposites. The highest discharged energy density reaches 16.1 J cm−3, which shows a great improvement comparing to that of commercial biaxially oriented polypropylene (BOPP) film. This work demonstrates a unique design of spatial structure integrating various anisotropic nanofillers in polymer-based nanocomposite, whose outstanding energy storage performance makes it as a promising candidate for high-performance electrostatic capacitors application.

Temperature resistant amorphous polyimides with high intrinsic permittivity for electronic applications

High–dielectric–constant (high–k) polymers are highly desirable for high energy density capacitors and low–voltage organic thin-film transistors (OTFTs). However, conventional high–k polymers suffer from low temperature resistance or crystalline state, hampering their applications in electronic devices in general environments. Herein, by accurate monomer design and mild polycondensation, we synthesize a new polyimide (STP–PI) featuring amorphous state, intrinsic high–k, very high thermal stability, and self-standing properties. Having polar side groups and rigid twisty backbones, the STP–PI exhibits both high and stable permittivity (5.2 ∼ 6.0 from 1 MHz to 100 Hz) and excellent thermal stability (Td5% > 450 °C and Tg > 300 °C), superior to its counterpart without the polar side group. When applied for energy storage, STP–PI delivers a discharged energy density of 3.68 J cm−3 with discharge efficiency of 84% at 350 MV m−1 even at 200 °C, which makes a record among the state–of–the–art performance of instinct polymer dielectrics. When applied in low-voltage OTFTs as the dielectric layers, the devices show a low threshold voltage of −0.30 V and mobility value of 4.03 cm2 V−1 s−1. The design strategy paves a way of making high–k polyimides for high temperature energy storage and electronic applications.

Tailored Doubly Confined Mos2 Bilayer with Rich S-Mo-S Sites to Achieve High-Energy Density and Ultra-Stable Potassium-Ion Capacitor

Tailored Doubly Confined Mos2 Bilayer with Rich S-Mo-S Sites to Achieve High-Energy Density and Ultra-Stable Potassium-Ion Capacitor

Anti-polar state in BiFeO3/NdFeO3 superlattices

Antiferroelectrics are promising materials for high energy density capacitors, and the search for environmentally friendly and efficient systems is actively pursued. An elegant strategy to create and design new (anti)ferroic system relies on the use of nanoscale superlattices. We report here the use of such a strategy and the fabrication of nanoscale BiFeO3/NdFeO3 superlattices and in depth characterization using high-resolution x-ray diffraction and transmission electron microscopy. The structural analysis at the atomic scale demonstrates that such superlattices host anti-polar ordering most likely described by an antiferroelectric-like Pbnm symmetry. Temperature dependence of the anti-polar state and structural transition further hint that the stability of the anti-polar state is controlled by the BiFeO3 layer thickness within the stacking and, in a more moderate way, by interlayer strain. Discovery of such a polar arrangement in superlattices and the possible generalization to the whole rare-earth family pave the way to new platforms for energy storage applications as well as nano-electronic devices.

Scalable Polyimide-Poly(Amic Acid) Copolymer Based Nanocomposites for High-Temperature Capacitive Energy Storage.

The developments of next-generation electric power systems and electronics demand for high temperature (≈150°C), high energy density, high efficiency, scalable, and low-cost polymer-based dielectric capacitors are still scarce. Here, the nanocomposites based on polyimide-poly(amic acid) copolymers with a very low amount of boron nitride nanosheets are designed and synthesized. Under the actual working condition in hybrid electric vehicles of 200 MV m-1 and 150°C, a high energy density of 1.38 J cm-3 with an efficiency higher than 96% is achieved. This is about 2.5 times higher than the room temperature energy density (≈0.39 J cm-3 under 200 MV m-1 ) of the commercially used biaxially oriented polypropylene, the benchmark of dielectric polymer. Especially, the energy density and efficiency at 150°C show no sign of degradation after 20000 cycles of charge-discharge test and 35 days' high-temperature endurance test. This research provides an effective and low-cost strategy to develop high-temperature polymer-based capacitors.

Lead-based antiferroelectrics revisited for high energy density capacitors and large strain actuators

Lead zirconate stannate titanate (PZST) ceramics with the generic composition of Pb1-0.5xNbx[(ZrySn1-y)1-zTiz]1-xO3 with x representing Nb content (x = 0 or 0.02), y representing Zr:Sn ratio (y = 0.57 or 0.60), and z representing Ti content (z = 0.06 or 0.08), and lead lanthanum zirconate titanate (PLZT) ceramics with the general composition of Pb1-xLax(ZryTi1−y)1−x/4O3 with x = 0.050 − 0.120 were synthesized using solid state reaction method and sintering. The energy density and field induced strain behavior of these compositions were investigated for capacitor and actuator applications, respectively. The focus of the study was to evaluate and discuss the change in the polar state of these ceramics from normal ferroelectric to antiferroelectric to relaxor antiferroelectric to linear dielectric with compositional modification. The highest releasable energy density (1.053 J/cm3) was obtained from the antiferroelectric Pb[(Zr0.60Sn0.40)0.94Ti0.06]O3 composition among the PZST compositions that were investigated, but the efficiencies were generally low (max. 60%) due to the large hysteresis that was observed in these compositions. The highest field induced strain level of 0.26% was also measured in the same composition. In the case of PLZT compositions, the highest releasable energy density values (0.313 J/cm3) with much higher efficiencies (76%) and highest field induced strain levels (0.23%) were obtained from the PLZT compositions with relaxor antiferroelectric character.

Poly(lactic acid)‐Based Film with Excellent Thermal Stability for High Energy Density Capacitor Applications

AbstractDielectric capacitors play a key role in high power electronics, electric vehicles, and medical devices. Due to the biodegradability, high thermal stability, and ease of processing, poly(lactic acid)‐based (PLA) films are widely used in applications of tissue engineering, actuators, and sensors. In this investigation, PLA films are prepared using a solution casting method for high energy density capacitor applications. It is found that the stretched poly(d‐lactic acid) (PDLA) film exhibits an energy density and charge discharge efficiency of 18.5 J cm−3 and 95%, respectively, at room temperature. Furthermore, after a heating time of 30 min at 100 °C, it is found that the energy density and chargedischarge efficiency of the stretched PDLA films are maintained at 11.2 J cm−3 and 76%, respectively. In comparison, the energy density and efficiency of the stretched PDLA film are 1.03‐fold and 53% higher, respectively, than those of the stretched poly(vinylidene fluoride‐hexafluoropropylene) film at 100 °C with the same heat treatment. Compared with a high temperature polyimide material, the energy density of the stretched PDLA film is 1.98‐fold higher at 100 °C. The PLA‐based material shows great promise for application in high energy density capacitors operating at a high temperature (100 °C).

Lead-free AgNbO3/poly(vinylidene fluoride‐hexafluoropropylene) antiferroelectric nanocomposite for high energy density capacitor applications

In the past few decades, ceramic/polymer nanocomposites have been extensively studied due to their relatively large dielectric constant, high electric energy density, and large charge–discharge efficiency. Antiferroelectric (AFE) materials possess a low remnant polarization (P r) and relatively high saturation polarization (P s), which can induce higher storage energy density than relaxor ferroelectric materials. In this investigation, AgNbO3-based AFE ceramic nanoparticles were grafted with a layer of poly(methyl methacrylate) (PMMA) to form AgNbO3–g-PMMA nanoparticles and then the obtained nanoparticles were embedded into a poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) matrix to form an AgNbO3–g-PMMA/PVDF-HFP (AN–g-PMMA/HFP) nanocomposite film. After characterization, it was found that the dielectric constant of the AN–g-PMMA/HFP-5 (5% AgNbO3) nanocomposite film was 12.3, which was 43% higher than that of the pristine PVDF-HFP film. The AN–g-PMMA/HFP-5 nanocomposite film exhibited the largest breakdown field of 500 MV m−1 and the highest energy density of 13 J cm−3, which was increased by 32.5% (from 9.82 to 13 J cm−3), as compared with that of the pristine PVDF-HFP film. The AN–g-PMMA/HFP-5 film exhibited a charge–discharge efficiency of 69.5% at 500 MV m−1, which is 9.5% higher than that of the pristine PVDF-HFP film. This AgNbO3-based nanocomposite film shows great promise for AFE nanocomposite for high energy density capacitor applications.

High electric energy and power density achieved in [formula omitted] antiferroelectric ceramics by La dopants

In order to improve the energy storage and discharge performance of ceramic dielectric capacitors, antiferroelectric materials have drawn wide attention. In this work, a comprehensive investigation on Pb1−xLax[(Zr0.55Sn0.45)0.92Ti0.08]1−x∕4O3 (x = 0%, 2%, 4%, 6%) antiferroelectric ceramics are fabricated by a rolling process. With the addition of lanthanum(La), the average grain size and the electric-field-induced strain are suppressed. However, the energy conversion efficiency, particularly the efficiency of −60 °C, is enhanced with increasing La content. In addition, the released energy density calculated by the “quasi-static” method(Wre) and “dynamic” method(Wdis) were investigated comparatively. As the content of La increase, the discrepancy betweenWre andWdis gradually decreased. Specifically, the composition with 4%La doped has the best comprehensive performance, with the highest recoverable energy density of 5.61 J/cm3 and ultrahigh efficiency of 90% at 410 kV/cm. Moreover, a discharge energy density of 3.6 J/cm3 can be obtained at 222 kV/cm and 90% can be released in 0.68μs. By directly short experiment, a current density of 1599A/cm2 and power density of 177 MW∕cm3 were obtained. These results indicated that PLZST 4/55/45/8 is promising for high electric energy and power density capacitors.

Electroceramics for High-Energy Density Capacitors: Current Status and Future Perspectives.

Materials exhibiting high energy/power density are currently needed to meet the growing demand of portable electronics, electric vehicles and large-scale energy storage devices. The highest energy densities are achieved for fuel cells, batteries, and supercapacitors, but conventional dielectric capacitors are receiving increased attention for pulsed power applications due to their high power density and their fast charge–discharge speed. The key to high energy density in dielectric capacitors is a large maximum but small remanent (zero in the case of linear dielectrics) polarization and a high electric breakdown strength. Polymer dielectric capacitors offer high power/energy density for applications at room temperature, but above 100 °C they are unreliable and suffer from dielectric breakdown. For high-temperature applications, therefore, dielectric ceramics are the only feasible alternative. Lead-based ceramics such as La-doped lead zirconate titanate exhibit good energy storage properties, but their toxicity raises concern over their use in consumer applications, where capacitors are exclusively lead free. Lead-free compositions with superior power density are thus required. In this paper, we introduce the fundamental principles of energy storage in dielectrics. We discuss key factors to improve energy storage properties such as the control of local structure, phase assemblage, dielectric layer thickness, microstructure, conductivity, and electrical homogeneity through the choice of base systems, dopants, and alloying additions, followed by a comprehensive review of the state-of-the-art. Finally, we comment on the future requirements for new materials in high power/energy density capacitor applications.

High-temperature resistant polypropylene films enhanced by atomic layer deposition

Last two decades has witnessed various technical programs towards the development of new dielectric films for high temperature and high energy density capacitor technology. Yet, the organic polypropylene film still holds its top position for film capacitors required for electric and electronic power applications such as power grid, hybrid electric vehicles, oil and gas exploration, and aviation. However, its low temperature stability determined by its structural nature limits its further adoption for more rising markets. This work thus developed a creative method (ultrathin coating of inorganic compound using atomic layer deposition) to increase the high temperature stability and the dielectric strength of such a strategically important polymer film from below 105 °C to above 140 °C. Several techniques also confirmed the effective minimization of the dimensional change and crystallinity loss at higher temperatures, i.e., Thermomechanical analysis, Dynamic mechanical analysis, Dynamic Scanning calorimetry, x-ray diffraction and electron microscopy techniques. This breakthrough discovery adds a huge value to the commercial PP films and relevant capacitor industry and will extend the operation of high-performance PP film capacitors to various high standard applications.

Superior discharged energy density in polyetherimide composites enabled by ultra-low ZnO@BN core-shell fillers

The primary strategy to increase dielectric constant and energy storage density is adding a high fraction of fillers with high dielectric constant to polymer matrix. Yet, it has not been very successful without encountering processing difficulties, lowered breakdown strength, and mechanical inflexibility. Herein, we introduce an unconventional approach using ultra-low fraction of zinc oxide core and boron nitride shell (ZnO@BN) filler with very high thermal conductivity dispersed in polyetherimide (PEI) for the same purpose. For the 0.1 vol% core-shelled fillers, the PEI composite film exhibits a superior dischargeable energy density of 10.8 J/cm3 and a high efficiency of 92.1% at an electric field of 500 kV/mm. The polarization responses under high electric fields and simulation reveal the association with the increased interfacial polarization of the thermally conductive core-shelled structure. This strategy lays out a new route to further investigate of high energy density composite and capacitor technologies.

Electronic and optical properties of Y-doped $$\hbox {BaBiO}_{3}$$

We investigate the nature of bond disproportionation in $$\hbox {BaBi}_{0.9}\hbox {Y}_{0.1}\hbox {O}_{3}$$ (BYBO) and its manifestation in the valence band and the optical absorption spectra. Our room-temperature (RT) structural results show that both the parent and the doped (BYBO) compounds stabilise in monoclinic structure but with a change in space group from I2/m to P21/n on Y doping. The Raman scattering studies suggest decrement in the breathing mode distortion of $$\hbox {BiO}_{6}$$ octahedra. The valence band spectra show an increase in the gap and close to the Fermi edge, we observe significant modifications in the fine structures on Y doping. These were studied using band structure calculations under TB-mBJ. The results show that the electronic states of the Y ions play significant role in driving the electronic structure. We also observe the importance of O 2p holes and also transfer of electrons from O 2p to Bi1 6s, Bi2 6s, and Y states to bring about bond disproportionation in the doped compound. The intensity variations and the features in the optical absorption spectra were understood based on the E vs k point calculations performed in the irreducible part of the Brillouin zone. The behaviour of the gap on doping is also in line with the calculations and optical absorption studies. The behaviour of the optical absorption spectra suggests possible use in solar cell and optical sensor applications. A composite of the doped compound with $$\hbox {BaTiO}_{3}$$ may find applications in the field of high-energy density capacitor. We believe that our results will be helpful in understanding the role of the electronic states of the dopants in superconductivity, especially in comparison with the monoclinic phase exhibited in the superconducting phase of $$\hbox {BaPb}_{0.75}\hbox {Bi}_{0.25}\hbox {O}_{3}$$ .

Strained Polymer Thermal Conductivity Enhancement Counteracted by Additional Off-Axis Strain

Thin-film (5–20 μm) polymer dielectrics are a critical component in high energy density capacitors. Understanding thermal transport in these materials is critical to addressing thermally enabled di...

Structure design boosts concomitant enhancement of permittivity, breakdown strength, discharged energy density and efficiency in all‐organic dielectrics

Polymer-based nanocomposites with excellent flexibility and intrinsic high breakdown strength are promising candidates for high energy density capacitors compared to ceramics counterparts. However, their energy density is relatively low due to the trade-off between permittivity and breakdown strength. In this work, the authors proposed a ferroconcrete-like structure for all-organic nanocomposites via combinatorial electrospinning and hot-pressing method. In this structure, polymethyl methacrylate (PMMA) serves as matrix while poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) serves as reinforcement phase. This novel structure is highly effective in breaking the paradox of improved discharged energy density with decreased efficiency, as evidenced by the concurrently improved discharged energy density (∼12.15 J/cm3 compared to 8.82 J/cm3 of the matrix) and efficiency (∼81.7% compared to 76.8% of the matrix). Compared to conventional blending composite films, samples with ferroconcrete-like structure exhibit higher permittivity, breakdown strength, discharged energy density and efficiency. The superior energy storage performance is attributed to large aspect ratio P(VDF-HFP) fibres distributed perpendicularly to the external field, which brings about the extra enhancement of permittivity. Besides, mechanical properties are improved and restriction on carrier motion is facilitated, leading to enhanced breakdown strength and suppressed conduction. This work provides a new way to design dielectric composite for high energy density and efficiency applications.

Evolution of Relaxor Behavior in Multiferroic Pb(Fe2/3W1/3)O3-BiFeO3 Solid Solution of Complex Perovskite Structure

Mutiferroic materials like bismuth ferrite BiFeO3 have attracted much interest in the last decade due to their promising potential for such applications as spintronics and magnetoelectric data storage devices. On the other hand, relaxor ferroelectrics have been intensively studied for their complex structures with quenched disorder and polar nanoregions which play an important role in their outstanding piezoelectric performance. Much less studied are the single-phase multiferroics that exhibit ferroelectric and/or magnetic relaxor behavior and the correlation between their structure and intricate magneto-electric interactions. In this work, we investigate the evolution of the structure and relaxor behavior in the solid solution between the complex perovskite multirelaxor Pb(Fe2/3W1/3)O3 [PFW] and canonical multiferroic BiFeO3 [BFO], (1-x)PFW-xBFO (with a solubility limit of x = 0.30). The temperature dependences of the dielectric permittivity and loss tangent measured in the frequency range from 100 Hz to 1 MHz indicate characteristic relaxor ferroelectric properties for compositions of x ≤ 0.15, with a frequency-dependent dielectric permittivity peak and its temperature, Tm, satisfying the Vogel-Fulcher law. Detailed studies of the evolution of the relaxor behavior with composition reveal that Tm decreases firstly with a small amount (x = 0.05) of BFO substitution and then increases with further increase of BFO concentration. The degree of relaxor character, as defined by ΔTm [Tm (1 MHz) - Tm (100 Hz)], increases monotonously with increasing BFO content, signifying an enhancement of relaxor behavior with BFO substitution, which is confirmed by the Lorenz-type quadratic variation of the static permittivity. A temperature - composition phase diagram is constructed in terms of the characteristic Burns temperature (TB) and freezing temperature (Tf), which delimits a paraelectric state (PE) above TB, a non-ergotic relaxor state (NR) below Tf, and an ergotic relaxor state (ER) in between. The observed enhancement of relaxor behavior is explained by an increase in the number and size distribution of polar nanoregions in the ER phase, resulting from increased compositional and charge disorders as a result of BFO substitution. The evolution of relaxor behavior and its microscopic mechanisms studied in this work are insightful for a better understanding the multirelaxor properties in multiferroics. Moreover, further substitution of BFO (x ≥ 0.2) flattens the permittivity curves and leads to a temperature-stable variation of high dielectric constant (≈ 103) in a wide temperature range, making the PFW-BFO solid solution attractive for such applications as high energy density capacitors.

Effect of defect dipoles on the colossal dielectric behaviors of TiO2-x ceramics

Considerable efforts have been made to explore high-dielectric constant ceramics for their promising applications as high-energy density capacitors and fine metal-oxide-semiconductor field effect transistors. Here, we investigated the colossal dielectric behaviors of TiO2-x ceramics influenced by defect dipoles introduced by oxygen vacancies and Ti3+. As temperature decreases from room temperature, three thermally activated relaxations were observed. A room temperature relaxation (R3) was attributed to Maxwell-Wagner relaxation due to the electrode/sample contact. A low-temperature Maxwell-Wagner relaxation (R1) featuring nearly constant loss behavior was associated with frozen carriers. The middle-temperature relaxation (R2) was verified to be the polaron relaxation composed of two segment contributions: polaron hopping and polaron reorienting. The defect dipoles gradually changed from a disordered state into an ordered state in external electric field when temperature falls and reaches a frozen-in state below 217 K. The ordered phase will turn into disordered again when temperature increases to 217 K due to thermal perturbation corresponding to a phase transition. These results indicate that the colossal dielectric constant properties of TiO2 system are attributed to defect dipole clusters. This work exhibits a way to identify the dipole effect. Moreover, we found that the dielectric properties of TiO2-x ceramics hardly depend on temperature and frequency. This implies that TiO2-x ceramics is a kind of promising colossal dielectric materials which can work at various temperatures.

Revealing Anomalous High Capacitance of Individual Porous Nanoparticles Employing Nano-Impact Electrochemistry

As a class of promising energy storage devices, supercapacitors have attracted great attention, regarding the deployment of high-power output in renewable energy technologies. The electrification of transportation and the dramatic increase of using portable electronic devices, require significant improvement in this field. Electric double-layer capacitors (EDLCs) are an important class of supercapacitors, in which the electrochemical double layer formed at the electrode/electrolyte interface is used to store energy. However, despite their high power density and remarkable cycle life, the prevalent application of EDLCs is limited because of their still insufficient energy density. Therefore, increasing the capacitance of EDLC electrodes is of great interest. One emerging approach for enhancing energy density has been employment of porous structures as novel electrodes for EDLCs [1].From a theoretical point of view, a conventional EDLC electrode can be considered as a parallel-plate capacitor that delivers a capacitance (C) according to C = ε A / d, where ε is the permittivity of the electrolyte, A is the electrode surface area, and d is the thickness of EDL. Consequently, increasing A of the EDLC electrodes using meso- or nanostructured materials is a classic approach toward realizing large specific capacitance and high energy density. Alternatively, it was experimentally proved that confinement effect on d in microporous EDLCs (pore size < 1 nm) leads to an anomalous increase in the specific capacitance as the pore width decreases. These findings suggest non-classical behaviors of EDLs emerged in sub nanometer pores. However, except for a few theoretical simulation results [2], there has been very limited research carried out on the application of the confinement effect in two-dimensional materials as an additional strategy toward high-energy density capacitors [3]. Herein, we propose single particle electrochemistry to study capacitive properties of individual porous nanoparticles, as an ideal candidate for fabrication of EDLCs.Employing “nano-impact” electrochemistry, an enhanced current is measured, which is caused by randomly colliding nanoparticles with a microelectrode [4,5]. In capacitive nano-impacts, the colliding particle causes the perturbation of a portion of the EDL with the subsequent charging-discharging processes which give rise to transient current-time events. This can be related to the electronic properties/capacitance of the particles [6]. Potentially this method will provide a powerful tool to study the properties of individual nanoparticles without any ensemble effects and overlaying effects of neighboring particles. In the present study, capacitance of individual porous platinum nanoparticles is investigated, by using PtNP impacts at platinum microelectrode in aqueous solution. These experiments are exploited to extract physico-chemical information about the structural effects on the energy density of EDLCs. The gained insights will help to improve our fundamental understanding of the EDL formation and will, thus, provide guidance towards the development of new and improved EDLCs.

Lead-free relaxor thin films with huge energy density and low loss for high temperature applications

We report record energy storage density (>80 J cm−3) in Pb-free relaxor ferroelectrics based on Mn-doped BiFeO3–BaTiO3 thin films. Rapid interval deposition was used to impose layer-by-layer growth improving crystallinity and lowering unwanted defects concentration. The growth and Mn doping produced an order of magnitude lower leakage, with strongly reduced dielectric loss (from room temperature to >300 °C, and 100 Hz to 1 MHz), e.g. by a factor of 5 at 225 °C and 25 kHz. At room temperature (RT), the dielectric breakdown strength increased by a factor of 1.5 to >3000 kV cm−1 while the dielectric constant remained flat, at ~1000 from RT to 350 °C. The films perform better than competing materials (e.g. PZT and SrTiO3-based) while being Pb-free and while operating up to 350 °C, which SrTiO3-based systems do not. Our work gives considerable promise for high energy and power density capacitors for harsh environments.

Nitrogen-doped carbon nanotubes as an anode for a highly robust potassium-ion hybrid capacitor.

Potassium ion hybrid capacitors (KIHCs) have drawn growing interest owing to their outstanding energy density, power density and excellent cycling stability. However, the large ionic radius of potassium triggers a huge volume change during continuous K+ insertion/extraction processes, restricting the development of KIHCs. Here, we report N-doped carbon nanotubes (NCNTs) for high-performance K+ storage. The NCNTs possess a hierarchical structure and N functional groups and not only offer sufficient space to relieve the volume expansion, but also provide highly efficient channels to transport electrons and ions. As a result, the NCNTs anode presents a high specific capacity and an excellent cycling stability with an average decay rate of 0.0238% per cycle (the lowest value among the reported carbon-based anodes for K-ions batteries) during 3600 continuous cycles. A potassium ion hybrid capacitor (KIHC) was also designed with the NCNT anode and a commercial active carbon cathode and achieved both a high energy/power density (117.1 W h kg-1/1713.4 W kg-1) and a long cycle life (2000 cycles at 1 A g-1). Moreover, the in situ Raman and ex situ element mapping characterization demonstrate the outstanding electrochemical reversibility of the NCNTs. This work provides a superior strategy to design low-cost anode materials with excellent K+ storage electrochemistry.