Amorphous Heterostructure Derived from Divalent Manganese Borate for Ultrastable and Ultrafast Aqueous Zinc Ion Storage.

Aqueous zinc-manganese (Zn-Mn) batteries have promising potential in large-scale energy storage applications since they are highly safe, environment-friendly, and low-cost. However, the practicality of Mn-based materials is plagued by their structural collapse and uncertain energy storage mechanism upon cycling. Herein, this work designs an amorphous manganese borate (a-MnBOx ) material via disordered coordination to alleviate the above issues and improve the electrochemical performance of Zn-Mn batteries. The unique physicochemical characteristic of a-MnBOx enables the inner a-MnBOx to serve as a robust framework in the initial energy storage process. Additionally, the amorphous manganese dioxide, amorphous Znx MnO(OH)2 , and Zn4 SO4 (OH)6 ·4H2 O active components form on the surface of a-MnBOx during the charge/discharge process. The detailed in situ/ex situ characterization demonstrates that the heterostructure of the inner a-MnBOx and surface multicomponent phases endows two energy storage modes (Zn2+ /H+ intercalation/deintercalation process and reversible conversion mechanism between the Znx MnO(OH)2 and Zn4 SO4 (OH)6 ·4H2 O) phases). Therefore, the obtained Zn//a-MnBOx battery exhibits a high specific capacity of 360.4 mAh g-1 , a high energy density of 484.2 Wh kg-1 , and impressive cycling stability (97.0% capacity retention after 10 000 cycles). This finding on a-MnBOx with a dual-energy storage mechanism provides new opportunities for developing high-performance aqueous Zn-Mn batteries.

A Pseudocapacitor Diode Based on Ion-Selective Surface Redox Effect.

Supercapacitor diode (CAPode) is a novel device that integrates ion diode functionality into a conventional electrical double-layer capacitor and is expected to have great applications in emerging fields such as signal propagation, microcircuit rectification, logic operations, and neuromorphology. Here, a brand new pseudocapacitor diode is reported that has both high charge storage (50.2 C g-1 at 20 mV s-1 ) and high rectification (the rectification ratio of 0.79 at 200 mV s-1 ) properties, which is realized by the ion-selective surface redox reaction of spinel ZnCo2 O4 in aqueous alkaline electrolyte. Furthermore, an application of the integrated device is demonstrated in the logic gate of circuit system to realize the logic operations of "AND" and "OR". This work not only expands the types of CAPodes, but also provides a train of thought for constructing high-performance capacitive ionic diodes.

Low-Temperature Synthesis of Amorphous FePO4@rGO Composites for Cost-Effective Sodium-Ion Batteries.

The dramatic growth of the sodium-ion battery market evokes a high demand for high-performance cathodes. In this work, a nanosized amorphous FePO4@rGO composite is developed using coprecipitation combined with low-temperature hydrothermal synthesis, which registered a surface area of 179.43 m2 g-1. The composites maintain three-dimensional mesoporous morphology with a pore size in the range of 3-4 nm. Uniform distribution of amorphous FePO4 allows a reversible capacity of 175.4 mA h g-1 at 50 mA g-1 while maintaining a stable cycle life of 500 cycles at 200 mA g-1. The amorphous FePO4@rGO, obtained by energy-efficient synthesis, significantly improved the rate performance compared to the crystalline material prepared at high temperatures. Cyclic voltammetry tests reveal that the fast reaction kinetics can be attributed to the pseudocapacitive behavior of the electrode. In addition, we demonstrated the promise of FePO4@rGO cathodes for low-temperature sodium-ion batteries.

Construction of Supercapacitor-Based Ionic Diodes with Adjustable Bias Directions by Using Poly(ionic liquid) Electrolytes.

The newly emerging supercapacitor-diode (CAPode), integrating the characteristics of a diode into an electrical-double-layer capacitor, can be employed to extend conventional supercapacitors to new technological applications and may play a crucial role in grid stabilization, signal propagation, and logic operations. However, the reported CAPodes have only been able to realize charge storage in the positive-bias direction. Here, bias-direction-adjustable CAPodes realized by using a polycation-based ionic liquid (IL) or a polyanion-based IL as electrolyte in an asymmetric carbon-based supercapacitor architecture are proposed. The resulting CAPodes exhibit charge-storage function at only the positive- or negative-bias direction with a high rectification ratio (≈80% for rectification ratio II, RRII ) and an outstanding cycling life (4500 cycles), representing a crucial breakthrough for designing high-performance capacitive ionic diodes.

Enhanced field emission performance of MXene-TiO2 composite films.

A facile method to produce an MXene-TiO2 composite is demonstrated for enhanced field emission display applications. The field emission performance of two-dimensional free-standing and linear-shaped field emitters has been systematically investigated and enhanced electron emission behaviors (e.g. emission current, stability and emission patterns) are achieved by compositing MXene and TiO2 nanowires. The relationship between the emission current density, electric field and anode-cathode gap distance is studied and the emitters, especially the cross-section of the composite film, show good performance. The emission current from the cross-section of the composite film can reach 289 mA cm-2, which is the best result of the state of the art compared to single MXene and TiO2 nanowires. We have also reported a triboelectric nanogenerator powered by free-standing MXene-TiO2 composite emitters, implying the feasibility of the self-powering field emission devices and possibly enlarging the applications of cold emitters in various fields.

Ti3C2 (MXene) based field electron emitters.

As a novel class of two-dimensional materials, MXene has provoked tremendous progress for various applications in functional devices. Here, we pioneer a preliminary understanding on the field emission behavior of MXene for the first time. Ti3C2 paper is fabricated by using facile filtration method, and multiple vertical sheets appear on the surface of MXene paper with high electrical conductivity (2.93 × 105 S m-1) and low work function (3.77 eV). The field electron emission performance and electric field distribution on MXene emitters are measured and simulated under planar and standing conditions. Both emitter conditions exhibit stable, uniform electron emission pattern, and the standing emitter achieves high emission current density of 59 mA cm-2 under 7.5 V µm-1. This work demonstrates the feasibility of MXene as cold electron source, establishing a preliminary foundation for its applications in field emission-based devices.

Hybrid Aqueous/Nonaqueous Water-in-Bisalt Electrolyte Enables Safe Dual Ion Batteries.

Dual ion batteries (DIBs) have recently attracted ever-increasing attention owing to the potential advantages of low material cost and good environmental friendliness. However, the potential safety hazards, cost, and environmental concerns mainly resulted from the commonly used nonaqueous organic solvents severely hinder the practical application of DIBs. Herein, a hybrid aqueous/nonaqueous water-in-bisalt electrolyte with both broad electrochemical stability window and excellent safety performance is developed. The lithium-based DIB assembled using KS6 graphite and niobium pentoxide as the active materials in the cathode and anode exhibits good comprehensive performance including capacity, cycling stability, rate performance, and medium discharge voltage. Initial capacities of ≈47.6 and 29.6 mAh g-1 retention after 300 cycles can be delivered with a medium discharge voltage of around 2.2 V in the voltage window of 0-3.2 V at the current density of 200 mA g-1 . Good rate performance for the battery can be indicated by 29.7 mAh g-1 discharge capacity retention at 400 mA g-1 . It is noteworthy that the coulombic efficiency of the battery can reach as high as 93.9%, which is comparable to that of the corresponding DIBs using nonaqueous organic electrolytes.

Revealing the Impact of Oxygen Dissolved in Electrolytes on Aqueous Zinc-Ion Batteries.

Aqueous zinc-ion batteries (ZIBs) are promising low-cost and high-safety energy storage devices. However, their capacity decay especially at the initial cyclic stage is a serious issue. Herein, we reveal that the dissolved oxygen in aqueous electrolyte has significant impact on the electrochemistry of Zn anode and ZIBs. After removing oxygen, the symmetrical set-up of Zn/Zn is capable of reversible plating/stripping with a 20-fold lifetime enhancement compared with that in oxygen enrichment condition. Taking aqueous Zn-MnO2 battery as an example, although the presence of oxygen can contribute an extra capacity over 20% at the initial cycles due to the electrocatalytic activity of MnO2 with oxygen, the corrosion of Zn anode can be eliminated in the oxygen-free circumstance and thus offering a better reversible energy storage system. The impact of the dissolved oxygen on the cycling stability also exists in other ZIBs using vanadium-based compounds, Birnessite and Prussian blue analog cathodes.

Porous g-C3N4 and MXene Dual-Confined FeOOH Quantum Dots for Superior Energy Storage in an Ionic Liquid.

Owing to their unique nanosize effect and surface effect, pseudocapacitive quantum dots (QDs) hold considerable potential for high-efficiency supercapacitors (SCs). However, their pseudocapacitive behavior is exploited in aqueous electrolytes with narrow potential windows, thereby leading to a low energy density of the SCs. Here, a film electrode based on dual-confined FeOOH QDs (FQDs) with superior pseudocapacitive behavior in a high-voltage ionic liquid (IL) electrolyte is put forward. In such a film electrode, FQDs are steadily dual-confined in a 2D heterogeneous nanospace supported by graphite carbon nitride (g-C3N4) and Ti-MXene (Ti3C2). Probing of potential-driven ion accumulation elucidates that strong adsorption occurs between the IL cation and the electrode surface with abundant active sites, providing sufficient redox reaction of FQDs in the film electrode. Furthermore, porous g-C3N4 and conductive Ti3C2 act as ion-accessible channels and charge-transfer pathways, respectively, endowing the FQDs-based film electrode with favorable electrochemical kinetics in the IL electrolyte. A high-voltage flexible SC (FSC) based on an ionogel electrolyte is fabricated, exhibiting a high energy density (77.12 mWh cm-3), a high power density, a remarkable rate capability, and long-term durability. Such an FSC can also be charged by harvesting sustainable energy and can effectively power various wearable and portable electronics.

"Water in salt/ionic liquid" electrolyte for 2.8 V aqueous lithium-ion capacitor.

Development of high-voltage electrolytes with non-flammability is significantly important for future energy storage devices. Aqueous electrolytes are inherently non-flammable, easy to handle, and their electrochemical stability windows (ESWs) can be considerably expanded by increasing electrolyte concentrations. However, further breakthroughs of their ESWs encounter bottlenecks because of the limited salt solubility, leading to that most of the high-energy anode materials can hardly function reversibly in aqueous electrolytes. Here, by introducing a non-flammable ionic liquid as co-solvent in a lithium salt/water system, we develop a "water in salt/ionic liquid" (WiSIL) electrolyte with extremely low water content. In such WiSIL electrolyte, commercial niobium pentoxide (Nb2O5) material can operate at a low potential (-1.6 V versus Ag/AgCl) and contribute its full capacity. Consequently, the resultant Nb2O5-based aqueous lithium-ion capacitor is able to operate at a high voltage of 2.8 V along with long cycling stability over 3000 cycles, and displays comparable energy and power performance (51.9 Wh kg-1 at 0.37 kW kg-1 and 16.4 Wh kg-1 at 4.9 kW kg-1) to those using non-aqueous electrolytes but with improved safety performance and manufacturing efficiency.

CeOx-Decorated Hierarchical NiCo2S4 Hollow Nanotubes Arrays for Enhanced Oxygen Evolution Reaction Electrocatalysis.

Developing hollow self-supported nanotube arrays with hierarchical microporous and abundant multiactive sites shows great promise for oxygen evolution reaction (OER) electrocatalysis. Herein, a facile and low-cost strategy of NiCo2S4 hollow nanotubes arrays decorated with CeOx nanoparticles (NPs) assembled on a flexible support carbon cloth (CC) for enhanced OER performance is reported. The obtained hierarchical nanoarrays CeOx/NiCo2S4/CC exhibited excellent activity toward OER with an overpotential (270 mV) at 10 mA cm-2, relatively weak Tafel slope, and distinguished durability. CeOx/NiCo2S4/CC nanoarrays not only provide fast electronic transmission and well-defined connection to the substrate but also defective sites and electron transfer by the introduction of CeOx NPs. This new strategy was offered to construct low-cost and effectively hierarchical structural electrocatalysts containing rare-earth species.

Recent Advances of Cellulose-Based Materials and Their Promising Application in Sodium-Ion Batteries and Capacitors.

Cellulose as the most abundant natural biopolymer on earth has shown promising potential because of its excellent physical, mechanical, and biocompatible properties, which are very important for sustainable energy storage systems (ESSs). In this review, a comprehensive summary of the applications involving all kinds and forms of cellulose in the advanced Na-related ESSs, including sodium ion batteries (SIBs) and sodium ion capacitors (SICs), is presented. For cellulose, the impact of various structures and surface chemical properties on the electrochemical performance is focused on. In particular, the latest developments in cellulose-based binders and separators are highlighted. In addition, an in-depth understanding of the structure and performance of electrode materials and the storage mechanism of a hard carbon anode derived from cellulose for SIBs is provided. Further, the manufacturing of full-cellulose-based SICs assembled by all parts of devices including hard carbon anodes, active carbon cathodes, binders, and separator based on cellulose or cellulose derivatives is reviewed. Finally, the prospects of cellulose-based energy storage systems on several issues that need further exploration are presented.

A Dual Carbon-Based Potassium Dual Ion Battery with Robust Comprehensive Performance.

Dual carbon-based potassium dual ion batteries (K-DCBs) have recently attracted ever-increasing attention owing to the potential advantages of high performance-to-cost ratio, good safety, and environmental friendliness. However, the reported K-DCBs still cannot simultaneously meet the requirements of high capacity, long cycling stability, and low cost, which are necessary for practical applications. In this study, a K-DCB with good comprehensive performance including capacity, cycling stability, medium discharge voltage, and energy density is developed by introducing the optimal cathode and anode materials, i.e., KS6 and natural graphite, respectively. An initial capacity of ≈54.6 mAh g-1 and 92.5% capacity retention after 400 cycles can be delivered in a wide voltage window of 2.4-5.4 V at the current density of 100 mA g-1 . A high medium discharge voltage around 4.2 V and an energy density up to 158.3 Wh kg-1 are meanwhile delivered by the K-DCB. In addition, the working mechanism of the devices is understood in detail. It is believed that valuable contributions to the electrochemical performance improvement of the related devices toward practical applications can be provided by this study.

Opening Magnesium Storage Capability of Two-Dimensional MXene by Intercalation of Cationic Surfactant.

Two-dimensional (2D) Ti3C2 MXene has attracted great attention in electrochemical energy storage devices (supercapacitors and lithium-ion and sodium-ion batteries) due to its excellent electrical conductivity as well as high volumetric capacity. Nevertheless, a previous study showed that multivalent Mg2+ ions cannot reversibly insert into MXene, resulting in a negligible capacity. Here, we demonstrate a simple strategy to achieve high magnesium storage capability for Ti3C2 MXene by preintercalating a cationic surfactant, cetyltrimethylammonium bromide (CTAB). Density functional theory simulations verify that intercalated CTA+ cations reduce the diffusion barrier of Mg2+ on the MXene surface, resulting in the significant improvement of the reversible insertion/deinsertion of Mg2+ ions between MXene layers. Consequently, the MXene electrode exhibits a desirable volumetric specific capacity of 300 mAh cm-3 at 50 mA g-1 as well as outstanding rate performance. This work endows MXene material with an application in electrochemical energy storage and, simultaneously, introduces magnesium battery materials as a member.

Realizing the Embedded Growth of Large Li2O2 Aggregations by Matching Different Metal Oxides for High-Capacity and High-Rate Lithium Oxygen Batteries.

Large Li2O2 aggregations can produce high-capacity of lithium oxygen (Li-O2) batteries, but the larger ones usually lead to less-efficient contact between Li2O2 and electrode materials. Herein, a hierarchical cathode architecture based on different discharge characteristics of α-MnO2 and Co3O4 is constructed, which can enable the embedded growth of large Li2O2 aggregations to solve this problem. Through experimental observations and first-principle calculations, it is found that α-MnO2 nanorod tends to form uniform Li2O2 particles due to its preferential Li+ adsorption and similar LiO2 adsorption energies of different crystal faces, whereas Co3O4 nanosheet tends to simultaneously generate Li2O2 film and Li2O2 nanosheets due to its preferential O2 adsorption and different LiO2 adsorption energies of varied crystal faces. Thus, the composite cathode architecture in which Co3O4 nanosheets are grown on α-MnO2 nanorods can exhibit extraordinary synergetic effects, i.e., α-MnO2 nanorods provide the initial nucleation sites for Li2O2 deposition while Co3O4 nanosheets provide dissolved LiO2 to promote the subsequent growth of Li2O2. Consequently, the composite cathode achieves the embedded growth of large Li2O2 aggregations and thus exhibits significantly improved specific capacity, rate capability, and cyclic stability compared with the single metal oxide electrode.

Silica-grafted ionic liquids for revealing the respective charging behaviors of cations and anions in supercapacitors.

Supercapacitors based on activated carbon electrodes and ionic liquids as electrolytes are capable of storing charge through the electrosorption of ions on porous carbons and represent important energy storage devices with high power delivery/uptake. Various computational and instrumental methods have been developed to understand the ion storage behavior, however, techniques that can probe various cations and anions of ionic liquids separately remain lacking. Here, we report an approach to monitoring cations and anions independently by using silica nanoparticle-grafted ionic liquids, in which ions attaching to silica nanoparticle cannot access activated carbon pores upon charging, whereas free counter-ions can. Aided by this strategy, conventional electrochemical characterizations allow the direct measurement of the respective capacitance contributions and acting potential windows of different ions. Moreover, coupled with electrochemical quartz crystal microbalance, this method can provide unprecedented insight into the underlying electrochemistry.

2-Methylimidazole-Derived Ni-Co Layered Double Hydroxide Nanosheets as High Rate Capability and High Energy Density Storage Material in Hybrid Supercapacitors.

A new method based on one-step solvothermal reaction is demonstrated to synthesize ultrathin Ni-Co layered double hydroxide (LDH) nanosheets, which grow directly on a flexible carbon fiber cloth (NiCo-LDH/CFC). Through using 2-methylimidazole as complex and methanol as solvent, the as-prepared NiCo-LDH/CFC shows a (003) facet preferential growth and an expanded interlayer spacing structure, resulting in a unique 3D porous nanostructure with a thickness of nanosheets of around 5-7 nm that shows high energy storage performance. By controlling the ratio of Ni/Co = 4:1 in the precursor solution, the electrode shows a specific capacitance of 2762.7 F g-1 (1243.2 C g-1) at a current density of 1 A g-1. Nevertheless, the optimal composition is obtained with Ni/Co = 1:1, which produces a specific capacitance of 2242.9 F g-1 (1009.3 C g-1) at 1 A g-1 and shows an excellent rate capability with 61% of the original capacitance being retained at a current density of 60 A g-1. The hybrid supercapacitor (HSC) based on the NiCo-LDH/CFC exhibits a maximum energy density of 59.2 Wh kg-1 and power densities of 34 kW kg-1, respectively. Long-term stability test shows that 82% of the original capacitance of the HSC remains after 5000 cycles. Importantly, the electrochemical performance of the solid-state flexible supercapacitors based on the prepared NiCo-LDH/CFC electrode showed a negligible change when the device was bent up to 180°. The performance of synthesized NiCo-LDH/CFC indicates the great potential of the material for delivering both high energy density and high power density in energy storage devices.

An Asymmetric Supercapacitor with Both Ultra-High Gravimetric and Volumetric Energy Density Based on 3D Ni(OH)2/MnO2@Carbon Nanotube and Activated Polyaniline-Derived Carbon.

Development of a supercapacitor device with both high gravimetric and volumetric energy density is one of the most important requirements for their practical application in energy storage/conversion systems. Currently, improvement of the gravimetric/volumetric energy density of a supercapacitor is restricted by the insufficient utilization of positive materials at high loading density and the inferior capacitive behavior of negative electrodes. To solve these problems, we elaborately designed and prepared a 3D core-shell structured Ni(OH)2/MnO2@carbon nanotube (CNT) composite via a facile solvothermal process by using the thermal chemical vapor deposition grown-CNTs as support. Owing to the superiorities of core-shell architecture in improving the service efficiency of pseudocapacitive materials at high loading density, the prepared Ni(OH)2/MnO2@CNT electrode demonstrated a high capacitance value of 2648 F g-1 (1 A g-1) at a high loading density of 6.52 mg cm-2. Coupled with high-performance activated polyaniline-derived carbon (APDC, 400 F g-1 at 1 A g-1), the assembled Ni(OH)2/MnO2@CNT//APDC asymmetric device delivered both high gravimetric and volumetric energy density (126.4 Wh kg-1 and 10.9 mWh cm-3, respectively), together with superb rate performance and cycling lifetime. Moreover, we demonstrate an effective approach for building a high-performance supercapacitor with high gravimetric/volumetric energy density.

Electrospinning Synthesis of Mesoporous MnCoNiOx@Double-Carbon Nanofibers for Sodium-Ion Battery Anodes with Pseudocapacitive Behavior and Long Cycle Life.

In this work, MnCoNiOx (denoted as MCNO) nanocrystals (with a size of less than 30 nm) finely encapsulated in double-carbon (DC, including reduced graphene oxide and amorphous carbon derived by polymer) composite nanofibers (MCNO@DC) were successfully synthesized via an electrospinning method followed by a sintering treatment. The as-obtained MCNO@DC nanofibers present superior sodium storage performance and retain an especially high specific capacity of 230 mAh g-1 with a large capacity retention of about 96% at 0.1 A g-1 after 500 cycles and a specific capacity of 107 mAh g-1 with capacity retention of about 89% at 1 A g-1 after 6500 cycles. The outstanding cycle characteristic is mainly due to the tiny MCNO nanoparticles, which shorten the ion migration distance, and the three-dimensional DC framework, which remarkably promotes the electronic transfer and efficiently limits the volume expansion during the progress of insertion and extraction of Na+ ions. Moreover, nitrogen doped in carbon is able to improve the electrochemical capability as well. Finally, kinetic analysis of the redox reactions is used to verify the pseudocapacitive mechanism in charge storage and the feasibility of using MCNO@DC composite nanofibers as an anode for sodium-ion batteries with the above-mentioned behavior.

Morphology Engineering of Co3O4 Nanoarrays as Free-Standing Catalysts for Lithium-Oxygen Batteries.

The effective shape-controlled synthesis of Co3O4 nanoarrays on nickel foam substrates has been achieved through a simple hydrothermal strategy. When they served as the binder- and conductive-agent-free porous cathodes for nonaqueous Li-O2 batteries, they sufficiently reflect the favorable catalytic characteristic of Co3O4 and alleviate the problems of serious pore blocking and surface passivation caused by insoluble and insulating discharge products. In particular, Co3O4 rectangular nanosheets exhibit superior electrocatalytic performance comparing with Co3O4 nanowires and hexagonal nanosheets, leading to higher specific capacity and better cycling stability over 54 cycles at 100 mA g(-1), which relate to their good pore structure, large specific surface area, and highly active {112} exposed plane, effectively promoting the mass transport and reversible formation and decomposition of discharge products in the cathode. These comparisons further indicate the morphology effect of nanostructured Co3O4 on their performances as free-standing catalysts for Li-O2 batteries, which also have been proved through the further analysis of discharge products on different shapes of Co3O4 nanoarrays electrodes.