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Although manganese dioxide (MnO2) holds great promise as a cathode material for aqueous Zn-ion batteries (ZIBs), its practical application is still impeded by its low tapping density, sluggish electron transfer and rapid capacity fading during cycling. Herein, a novel 3D high-density MXene–MnO2 composite cathode material has been developed via a gas-phase spray drying strategy in which MnO2 nanoparticles are encapsulated in the crumpled and rippled MXene nanosheets, effectively constructing a robust and conductive 3D microflower-like architecture that is beneficial for rapid ion/electron transfer and high structural stability. These 3D MXene@MnO2 microflowers were used as a ZIB cathode and exhibited a large reversible specific capacity (∼301.2 mA h g−1), remarkable rate capability and outstanding cycling stability over 2000 cycles. When the mass loading was increased to 8.0 mg cm2, a high specific capacity of ∼287.6 mA h g−1 with satisfactory rate and cycling performances was still achieved in the corresponding aqueous ZIB. Furthermore, the phase evolution and electrochemical mechanism during the charge/discharge process were elucidated in depth using an in situ Raman investigation. Based on this, a flexible aqueous ZIB was assembled as a proof of concept and achieved reliable electrochemical behavior under various deformation states, revealing its potential for application in portable/wearable electronics. [ABSTRACT FROM AUTHOR]

Sodium-ion capacitors are a new type of energy storage devices that possess the respective advantages of both sodium-ion batteries and electrochemical capacitors. However, the low energy density of sodium-ion capacitors is the main reason restricting their real applications. The key point is to match the kinetic behaviors of the anode and cathode. In this study, similar pore microstructures for both anode and cathode are constructed by using bacterial cellulose as a precursor; the products exhibit identical hierarchical micropore–mesopore networks and consistent pore distributions of 1–3 nm. These similar pore microstructures can effectively improve the behaviors of charge transport and ion diffusion. Benefiting from the merits of the advanced structures and good performance of the anode and cathode materials, a constructed sodium-ion capacitor delivers a high energy density of 124 W h kg−1 at a power density of 210 W kg−1 and a power density of 15 500 W kg−1 at an energy density of 22 W h kg−1 within the potential range of 0–4.2 V; meanwhile, an outstanding cycle performance of 2000 cycles was obtained, with a capacity retention of 99.6%. This study provides a simple and promising method to match the capacity and kinetic behavior of the anode and cathode to build high-performance sodium-ion capacitors. [ABSTRACT FROM AUTHOR]

Although potassium-ion storage technology has started to receive significant attention, its development is still in its infancy; thus, exploration of suitable electrode materials is critical at this stage. Herein, onion-like carbon (OLC) was obtained from the combustion soot as the candle burnt and used as a promising anode material for potassium-ion storage. This OLC delivered high reversible capacity and exhibited good rate performance and cycling stability. Taking advantage of these features, a potassium-ion hybrid capacitor (PIHC) with OLC as the battery-type anode and activated carbon as the capacitor-type cathode was constructed. The constructed device displayed the high energy density of 142 W h kg−1, the ultrahigh power output of 21 kW kg−1, and a long cycling life (83% capacity retention after 6000 cycles). Thus, this device may simultaneously cater to the requirements of high power and high energy densities in a wide range of applications. [ABSTRACT FROM AUTHOR]

Supercapacitors using ionic liquids (ILs) as electrolytes have triggered great interest due to their much higher energy density when compared to aqueous supercapacitors. Although manganese oxides have obvious capacitive contribution in ILs and thus can be used as electrode materials for IL-based supercapacitors, they suffer from low specific capacitance in ILs. Here Mn3O4 nanodots loaded on nitrogen-doped graphene sheets (denoted as Mn3O4 NDs@NG) are prepared through a facile one-pot solvothermal method with the presence of octylamine as the surfactant. Octylamine plays an important role in obtaining quantum-sized Mn3O4 NDs and controlling their dispersion degree on the surface of NG. With an optimal loading mass of Mn3O4 NDs, the corresponding Mn3O4 NDs@NG material is able to achieve a high specific capacitance of 158.9 F g−1 in a given IL and shows excellent rate capability. On this basis, a symmetric supercapacitor is assembled based on such a Mn3O4 NDs@NG, which delivers a high energy density of 90.7 W h kg−1 in the IL electrolyte. Furthermore, an asymmetric supercapacitor is also built by using such a Mn3O4 NDs@NG and activated carbon as the negative and positive electrode, respectively. This asymmetric device shows a higher energy density of 124.4 W h kg−1 compared to the symmetric one, and it still can deliver 55.8 W h kg−1 at a large power density of 29.9 kW kg−1. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs) have demonstrated great promise as a new platform for the synthesis of porous electrode materials for energy storage. Research effort on MOFs and MOF derived nanostructures has focused mainly on tuning the chemical composition at the molecular level and developing highly porous frameworks in which enhancing the capacity and reducing the transport path of ions are favorable. Here we report an approach using the MOF (polyhedral ZIF-8) as a novel precursor to synthesize two electrode materials with different energy-storage mechanisms: the capacitor-like porous carbon polyhedra and the battery-like MoS2–ZIF composite. The porous carbon polyhedra have a continuous 3D porous network with an extremely high surface area of 3680.6 m2 g−1 and a well-controlled pore size distribution, and the MoS2–ZIF composite shows a three-dimensional (3D) nanostructure with an open framework. Furthermore, a novel hybrid supercapacitor is fabricated by employing these two 3D nanostructured MOF-derived electrode materials, which shows the best properties among the current hybrid supercapacitors with respect to energy, power and cycling life. The presented strategy for the controlled design and synthesis of 3D MOF-derived nanostructures provides prospects in developing high-performance active materials in advanced energy storage devices. [ABSTRACT FROM AUTHOR]

Mesoporous Ni-doped MnCo2O4 hollow nanotubes (denoted as MCNO-HNTs) are successfully prepared through simple single-nozzle electrospinning combined with thermal treatment. MCNO-HNTs obviously exhibit a hollow structure and are assembled by a lot of small nanoparticles. When used as an anode material for sodium-ion batteries (SIBs), this electrode exhibits remarkable capacity retention of 81% at 1 A g−1 even after 11 000 cycles. The outstanding electrochemical performance can be attributed to the unique hollow mesoporous structure that alleviates stress caused by large volume changes, suppresses the agglomeration of the pulverized nanoparticles, and facilitates the transfer of electrons and electrolyte ions during prolonged cycling. Furthermore, the pseudocapacitive behavior of this material also effectively improves the electrochemical reaction kinetics. Therefore, due to the simple single-nozzle electrospinning technique and high electrochemical performance, mesoporous MCNO-HNTs have great potential as an anode material for rechargeable SIBs. [ABSTRACT FROM AUTHOR]

Lithium-ion hybrid capacitors (LIHCs) are receiving intense interest because they can combine the distinctive advantages of Li-ion batteries and supercapacitors. Their main limitations, however, are slow anode kinetics and poor cycle life when compared to supercapacitors. Here we demonstrate for the first time that conductive porous niobium nitride (p-NbN) with a cubic crystal structure, one-step prepared from commercial powdery Nb2O5, is an ideal anode material having pseudocapacitive characteristics (high rate lithium ion storage and excellent long-term cycling stability) for LIHCs. As a consequence, a novel LIHC is fabricated using p-NbN as the anode and activated carbon as the cathode. This device with a wide potential window of 4.0 V exhibits a high energy density of 149 W h kg−1 and a high power density of 45 kW kg−1 as well as a superior capacity retention of 95% after 15 000 cycles at 1.0 A g−1. In view of the excellent electrochemical characteristics, the simple manufacturing of p-NbN as well as the high-density nature of the NbN material, our results are of great importance for future development of long-life LIHCs with high gravimetric/volumetric performances. [ABSTRACT FROM AUTHOR]

Assembling asymmetric supercapacitors (SCs) combined with ionic liquid (IL) electrolytes is a very efficient strategy to enhance the energy density of SCs. However, the poor cycle stability of pseudocapacitive metal oxides in ILs seriously affects the performance of this class of asymmetric SCs. Improving the structural stability of metal oxides during the charge/discharge process is one of the greatest challenges at present. Herein, RuO2 nano-dots/reduced graphene oxide (RGO) composites are firstly prepared, and an IL-based asymmetric SC is built using the component-optimized composite (20 wt% RuO2/RGO) as the cathode and activated polyaniline-derived carbon nanorods (denoted as APDC) as the anode. It exhibits a high energy density of 108 W h kg−1, but shows poor cycling stability. In order to solve this problem, an ultrathin carbon layer originating from glucose is employed to encapsulate RuO2 nano-dots anchoring on RGO, forming a core/shell structure of RuO2@C. With the protection of the carbon shell, the as-made RuO2@C/RGO//APDC asymmetric SC exhibits superior long-term stability with 98.5% capacitance retention after 100 000 cycles in the IL electrolyte, as well as a high energy density of 103 W h kg−1 with a potential window of 3.8 V. Furthermore, this protection mechanism of the carbon layer is analyzed by electrochemical quartz crystal microbalance experiments. [ABSTRACT FROM AUTHOR]

Although flexible all-solid-state supercapacitors (f-SSCs) have been receiving much attention as promising flexible energy storage devices, most of them cannot operate at high temperatures due to the volatility or flammability of currently used aqueous and organic electrolytes. Here, we report an ionic liquid (IL) gel-based asymmetric supercapacitor having excellent heat-resistant performance and flexibility. To this end, low-cost γ-FeOOH is firstly electrodeposited on carbon cloth, and its pseudocapacitive behavior in a typical IL is investigated through an electrochemical quartz crystal microbalance (EQCM) for the first time. The results show that the pseudocapacitance mainly originates from a diffusion-controlled insertion process of the cations. By taking advantage of the prominent pseudocapacitance of γ-FeOOH, as well as excellent characteristics of IL gel electrolytes (thermostability, non-flammability, chemical inertness and wide potential), an advanced high-temperature f-SSC is fabricated by using γ-FeOOH as the anode and porous N-doped activated carbon as the cathode. The f-SSC exhibits outstanding electrochemical performance at elevated temperatures, and can achieve a maximum volumetric energy density of 1.44 mW h cm−3 (based on the whole device volume) at 200 °C. Moreover, it is able to maintain a stable energy-storage ability during the bending process even at 180 °C, providing the highest reported temperature for flexibility tests in f-SSCs to date. [ABSTRACT FROM AUTHOR]

Rutile TiO2 embedded in carbon submicron-tablets (TiO2/C) with a “blueberry muffin” morphology was fabricated via a two-step pyrolysis from a metal–organic framework precursor. Such a unique structure of the TiO2/C submicron-tablets provides the ideal anode characteristics (high reversible capacity, superior rate capability and excellent long-term cycling stability) for fast rechargeable lithium ion batteries. [ABSTRACT FROM AUTHOR]

To study the formation mechanism and influencing factors of graphene quantum dots (GQDs), GQDs with different average sizes were prepared using a modified hydrothermal method with hydrogen peroxide (H2O2) as an etching agent and ammonia as an assistant. It is found that size-controlled GQDs were prepared by adjusting the amount of ammonia and porous reduced graphene oxide (PRGO) debris can be synthesized by reducing the hydrothermal reaction time. Structural changes of final products were mainly attributed to the changes in the etching ability of the hydroxyl radical (OHṖ) against the reduction ability of the hydroxyl group (OH−) in different alkaline environments regulated by ammonia. Furthermore, we studied the electrochemical properties of GQDs and PRGO. The results showed that the specific capacitance of all samples increases linearly with the size and the smallest GQDs can work at the highest scan rate of as high as 5000 V s−1 with an ultra-fast power response (τ0 = 63.3 μs). Thus, these findings elucidate the formation mechanism of GQDs and demonstrate that GQDs are applicable in microelectronic devices with high power response requirements. [ABSTRACT FROM AUTHOR]

Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions, as observed with transition metal oxides in alkaline aqueous electrolytes. Here, we demonstrate that pseudocapacitive behavior of Fe2O3 can occur in a 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid (IL), and it is closely related to the chemical state variation between Fe3+ and Fe2+ on the surface of a Fe2O3 electrode during the charging/discharging process. By taking advantage of such pseudocapacitance, we prepared a promising electrode material, i.e., graphene nanosheet-supported Fe2O3 nanoparticles (denoted as Fe2O3@GNS), and then built high-performance asymmetric supercapacitors (ASs) using Fe2O3@GNS as the battery-type electrode material, commercial activated carbon (AC)/or activated polyaniline-derived carbon nanorods (denoted as APDC) as the capacitor-type electrode material, and EMIMBF4 IL as the electrolyte. The as-made ASs are able to work reversibly in a full operation voltage region of 0-4 V and exhibit very high energy density. Especially, the AS of Fe2O3@GNS//APDC achieves an extremely high energy density of 177 W h kg-1 and shows a superior combination of high energy and power density (the energy density still remains 62.4 W h kg-1 even at a high power density of 8 kW kg-1). [ABSTRACT FROM AUTHOR]

A series of hybrid electrochemical capacitors were fabricated by using the flower-like cobalt hydroxide (Co(OH)2) and urchin-like vanadium nitride (VN) as the positive and negative electrode materials, respectively. Both Co(OH)2 and VN electrode materials showed excellent electrochemical performance due to their unique structure and fast reversible Faradic reaction characteristics. With different operation voltage window (OVW) and negative/positive mass ratios, the impact on capacitance performance of the hybrid supercapacitor was investigated thoroughly, which demonstrated that both mass ratio and OVW played an important role in their capacitance performance. Furthermore, theoretical modeling was performed and the simulation result was found to be in agreement with the experimental result for the influence of the negative/positive mass ratio on capacitance performance of the hybrid supercapacitor. When an optimized negative/positive mass ratio was located, the Co(OH)2//VN hybrid supercapacitor could be cycled reversibly in the high-voltage region of 0-1.6 V and delivered a high energy density of 22 W h kg_1. Even at a large power density of 15.9 kW kg-1 the hybrid supercapacitor still possessed a desirable specific energy density of 9 W h kg_1. Such an impressive hybrid supercapacitor was expected to be a highly promising candidate for application in high-performance energy storage systems. [ABSTRACT FROM AUTHOR]

Rutile TiO2 embedded in carbon submicron-tablets (TiO2/C) with a "blueberry muffin" morphology was fabricated via a two-step pyrolysis from a metal-organic framework precursor. Such a unique structure of the TiO2/C submicron-tablets provides the ideal anode characteristics (high reversible capacity, superior rate capability and excellent long-term cycling stability) for fast rechargeable lithium ion batteries.