Two‐Dimensional Cobalt‐/Nickel‐Based Oxide Nanosheets for High‐Performance Sodium and Lithium Storage

Two‐dimensional (2D) nanomaterials are one of the most promising types of candidates for energy‐storage applications due to confined thicknesses and high surface areas, which would play an essential role in enhanced reaction kinetics. Herein, a universal process that can be extended for scale up is developed to synthesise ultrathin cobalt‐/nickel‐based hydroxides and oxides. The sodium and lithium storage capabilities of Co₃O₄ nanosheets are evaluated in detail. For sodium storage, the Co₃O₄ nanosheets exhibit excellent rate capability (e.g., 179 mA h g^?1 at 7.0 A g^?1 and 150 mA h g^?1 at 10.0 A g^?1) and promising cycling performance (404 mA h g^?1 after 100 cycles at 0.1 A g^?1). Meanwhile, very impressive lithium storage performance is also achieved, which is maintained at 1029 mA h g^?1 after 100 cycles at 0.2 A g^?1. NiO and NiCo₂O₄ nanosheets are also successfully prepared through the same synthetic approach, and both deliver very encouraging lithium storage performances. In addition to rechargeable batteries, 2D cobalt‐/nickel‐based hydroxides and oxides are also anticipated to have great potential applications in supercapacitors, electrocatalysis and other energy‐storage‐/‐conversion‐related fields.     

A Microwave Synthesis of Mesoporous NiCo2O4 Nanosheets as Electrode Materials for Lithium‐Ion Batteries and Supercapacitors

A facile microwave method was employed to synthesize NiCo₂O₄ nanosheets as electrode materials for lithium‐ion batteries and supercapacitors. The structure and morphology of the materials were characterized by X‐ray diffraction, field‐emission scanning electron microscopy, transmission electron microscopy and Brunauer–Emmett–Teller methods. Owing to the porous nanosheet structure, the NiCo₂O₄ electrodes exhibited a high reversible capacity of 891 mA h g^?1 at a current density of 100 mA g^?1, good rate capability and stable cycling performance. When used as electrode materials for supercapacitors, NiCo₂O₄ nanosheets demonstrated a specific capacitance of 400 F g^?1 at a current density of 20 A g^?1 and superior cycling stability over 5000 cycles. The excellent electrochemical performance could be ascribed to the thin porous structure of the nanosheets, which provides a high specific surface area to increase the electrode–electrolyte contact area and facilitate rapid ion transport.     

Core–Shell Carbon‐Coated CuO Nanocomposites: A Highly Stable Electrode Material for Supercapacitors and Lithium‐Ion Batteries

Herein we present a simple method for fabricating core–shell mesostructured CuO@C nanocomposites by utilizing humic acid (HA) as a biomass carbon source. The electrochemical performances of CuO@C nanocomposites were evaluated as an electrode material for supercapacitors and lithium‐ion batteries. CuO@C exhibits an excellent capacitance of 207.2 F g^?1 at a current density of 1 A g^?1 within a potential window of 0–0.46 V in 6 M KOH solution. Significantly, CuO electrode materials achieve remarkable capacitance retentions of approximately 205.8 F g^?1 after 1000 cycles of charge/discharge testing. The CuO@C was further applied as an anode material for lithium‐ion batteries, and a high initial capacity of 1143.7 mA h g^?1 was achieved at a current density of 0.1 C. This work provides a facile and general approach to synthesize carbon‐based materials for application in large‐scale energy‐storage systems.     

Mesoporous MnCo2O4 with a Flake‐Like Structure as Advanced Electrode Materials for Lithium‐Ion Batteries and Supercapacitors

A mesoporous flake‐like manganese‐cobalt composite oxide (MnCo₂O₄) is synthesized successfully through the hydrothermal method. The crystalline phase and morphology of the materials are characterized by X‐ray diffraction, field‐emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller methods. The flake‐like MnCo₂O₄ is evaluated as the anode material for lithium‐ion batteries. Owing to its mesoporous nature, it exhibits a high reversible capacity of 1066 mA h g^?1, good rate capability, and superior cycling stability. As an electrode material for supercapacitors, the flake‐like MnCo₂O₄ also demonstrates a high supercapacitance of 1487 F g^?1 at a current density of 1 A g^?1, and an exceptional cycling performance over 2000 charge/discharge cycles.     

Mesoporous Nickel Oxide Nanowires: Hydrothermal Synthesis,Characterisation and Applications for Lithium‐Ion Batteries and Supercapacitors with Superior Performance

Mesoporous nickel oxide nanowires were synthesized by a hydrothermal reaction and subsequent annealing at 400 °C. The porous one‐dimensional nanostructures were analysed by field‐emission SEM, high‐resolution TEM and N₂ adsorption/desorption isotherm measurements. When applied as the anode material in lithium‐ion batteries, the as‐prepared mesoporous nickel oxide nanowires demonstrated outstanding electrochemical performance with high lithium storage capacity, satisfactory cyclability and an excellent rate capacity. They also exhibited a high specific capacitance of 348 F g^?1 as electrodes in supercapacitors.     

Three‐Dimensional MoS2 Hierarchical Nanoarchitectures Anchored into a Carbon Layer as Graphene Analogues with Improved Lithium Ion Storage Performance

Much attention has recently been focused on the synthesis and application of graphene analogues of layered nanomaterials owing to their better electrochemical performance than the bulk counterparts. We synthesized graphene analogue of 3D MoS₂ hierarchical nanoarchitectures through a facile hydrothermal route. The graphene‐like MoS₂ nanosheets are uniformly dispersed in an amorphous carbon matrix produced in situ by hydrothermal carbonization. The interlaminar distance between the MoS₂ nanosheets is about 1.38 nm, which is far larger than that of bulk MoS₂ (0.62 nm). Such a layered architecture is especially beneficial for the intercalation and deintercalation of Li⁺. When tested as a lithium‐storage anode material, the graphene‐like MoS₂ hierarchical nanoarchitectures exhibit high specific capacity, superior rate capability, and enhanced cycling performance. This material shows a high reversible capacity of 813.5 mAh g^?1 at a current density of 1000 mA g^?1 after 100 cycles and a specific capacity as high as 600 mAh g^?1 could be retained even at a current density of 4000 mA g^?1. The results further demonstrate that constructing 3D graphene‐like hierarchical nanoarchitectures can effectively improve the electrochemical performance of electrode materials.     

Sucrose assisted hydrothermal synthesis of SnO2/graphene nanocomposites with improved lithium storage properties

SnO₂/graphene nanocomposites are synthesized by a new hydrothermal treatment strategy under the assistance of sucrose. From the images of the scanning electron microscope (SEM) and transmission electron microscope (TEM), it can be observed that SnO₂ nanoparticles with the size of 4~5 nm uniformly distribute on the graphene nanosheets. The result demonstrates that sucrose can effectively prevent graphene nanosheets from restacking during hydrothermal treatment and subsequently treatment. The charging/discharging test result indicates that the SnO₂/graphene nanocomposites exhibit high specific capacity and excellent cycleability. The first reversible specific capacity is 729 mAh.g^?1 at the current density of 50 mA.g^?1, and remains 646 mAh.g^?1 after 30 cycles at the current density of 100 mA.g^?1, 30 cycles at the current density of 200 mA.g^?1, 30 cycles at the current density of 400 mA.g^?1, 30 cycles at the current density of 800 mA.g^?1, and 30 cycles at the current density of 50 mA.g^?1.     

Rational Design of Ni Nanoparticles on N‐Rich Ultrathin Carbon Nanosheets for High‐Performance Supercapacitor Materials: Embedded‐ Versus Anchored‐Type Dispersion

Highly dispersed Ni nanoparticles (NPs) and abundant functional N‐species were integrated into ultrathin carbon nanosheets by using a facile and economical sol–gel route. Embedded‐ and anchored‐type configurations were achieved for the dispersion of Ni NPs in/on N‐rich carbon nanosheets. The anchored‐type composite exhibited outstanding pseudocapacitance of 2200 F g^?1 at 5 A g^?1 with unusual rate capability and extraordinary cyclic stability over 20 000 cycles with little capacitance decay. Aqueous asymmetric supercapacitors fabricated with this composite cathode demonstrated a high energy density of 51.3 Wh kg^?1 at a relatively large power density of 421.6 W kg^?1, along with outstanding cyclic stability. This approach opens an attractive direction for enhancing the electrochemical performances of metal‐based supercapacitors and can be generalized to design high‐performance energy‐storage devices.     

Reduced Graphene Oxide‐Supported TiO2 Fiber Bundles with Mesostructures as Anode Materials for Lithium‐Ion Batteries

Although the synthesis of mesoporous materials is well established, the preparation of TiO₂ fiber bundles with mesostructures, highly crystalline walls, and good thermal stability on the RGO nanosheets remains a challenge. Herein, a low‐cost and environmentally friendly hydrothermal route for the synthesis of RGO nanosheet‐supported anatase TiO₂ fiber bundles with dense mesostructures is used. These mesostructured TiO₂‐RGO materials are used for investigation of Li‐ion insertion properties, which show a reversible capacity of 235 mA h g^?1 at 200 mA g^?1 and 150 mA h g^?1 at 1000 mA g^?1 after 1000 cycles. The higher specific surface area of the new mesostructures and high conductive substrate (RGO nanosheets) result in excellent lithium storage performance, high‐rate performance, and strong cycling stability of the TiO₂‐RGO composites.     

Supercritical Carbon Dioxide Assisted Deposition of Fe3O4 Nanoparticles on Hierarchical Porous Carbon and Their Lithium‐Storage Performance

A composite of highly dispersed Fe₃O₄ nanoparticles (NPs) anchored in three‐dimensional hierarchical porous carbon networks (Fe₃O₄/3DHPC) as an anode material for lithium‐ion batteries (LIBs) was prepared by means of a deposition technique assisted by a supercritical carbon dioxide (scCO₂)‐expanded ethanol solution. The as‐synthesized Fe₃O₄/3DHPC composite exhibits a bimodal porous 3D architecture with mutually connected 3.7 nm mesopores defined in the macroporous wall on which a layer of small and uniform Fe₃O₄ NPs was closely coated. As an anode material for LIBs, the Fe₃O₄/3DHPC composite with 79 wt % Fe₃O₄ (Fe₃O₄/3DHPC‐79) delivered a high reversible capacity of 1462 mA h g^?1 after 100 cycles at a current density of 100 mA g^?1, and maintained good high‐rate performance (728, 507, and 239 mA h g^?1 at 1, 2, and 5 C, respectively). Moreover, it showed excellent long‐term cycling performance at high current densities, 1 and 2 A g^?1. The enhanced lithium‐storage behavior can be attributed to the synergistic effect of the porous support and the homogeneous Fe₃O₄ NPs. More importantly, this straightforward, highly efficient, and green synthetic route will definitely enrich the methodologies for the fabrication of carbon‐based transition‐metal oxide composites, and provide great potential materials for additional applications in supercapacitors, sensors, and catalyses.     

Three‐Dimensional NiMoO4 Nanosheets Supported on a Carbon Fibers@Pre‐Treated Ni Foam (CF@PNF) Substrate as Advanced Electrodes for Asymmetric Supercapacitors

Herein, we report a nanoarchitectured nickel molybdate/carbon fibers@pre‐treated Ni foam (NiMoO₄/CF@PNF) electrode for supercapacitors. The synthesis of NiMoO₄/CF@PNF mainly consists of a direct chemical vapor deposition (CVD) growth of dense carbon fibers (CFs) onto pre‐treated Ni foam (PNF) as the substrate, followed by in situ growth of NiMoO₄ nanosheets (NSs) on the CF@PNF substrate by means of a hydrothermal process. The NiMoO₄/CF@PNF electrode exhibits a high areal capacitance (5.14 F cm^?2 at 4 mA cm^?2) and excellent cycling stability (97 % capacitance retention after 2000 cycles at 10 mA cm^?2). Furthermore, we have successfully assembled NiMoO₄ NSs//activated carbon (AC) asymmetric supercapacitors, which can achieve an energy density of 45.6 Wh kg^?1 at 674 W kg^?1, and excellent stability with 93 % capacitance retention after 2000 cycles at 5 mA cm^?2. These superior properties hold great promise for energy‐storage applications.     

Self‐Assembling Synthesis of Free‐standing Nanoporous Graphene–Transition‐Metal Oxide Flexible Electrodes for High‐Performance Lithium‐Ion Batteries and Supercapacitors

The synthesis of nanoporous graphene by a convenient carbon nanofiber assisted self‐assembly approach is reported. Porous structures with large pore volumes, high surface areas, and well‐controlled pore sizes were achieved by employing spherical silica as hard templates with different diameters. Through a general wet‐immersion method, transition‐metal oxide (Fe₃O₄, Co₃O₄, NiO) nanocrystals can be easily loaded into nanoporous graphene papers to form three‐dimensional flexible nanoarchitectures. When directly applied as electrodes in lithium‐ion batteries and supercapacitors, the materials exhibited superior electrochemical performances, including an ultra‐high specific capacity, an extended long cycle life, and a high rate capability. In particular, nanoporous Fe₃O₄–graphene composites can deliver a reversible specific capacity of 1427.5 mAh g^?1 at a high current density of 1000 mA g^?1 as anode materials in lithium‐ion batteries. Furthermore, nanoporous Co₃O₄–graphene composites achieved a high supercapacitance of 424.2 F g^?1. This work demonstrated that the as‐developed freestanding nanoporous graphene papers could have significant potential for energy storage and conversion applications.     

Superior Lithium‐Ion Storage Properties of Mesoporous CuO–Reduced Graphene Oxide Composite Powder Prepared by a Two‐Step Spray‐Drying Process

Mesoporous CuO–reduced graphene oxide (rGO) composite powders were prepared by using a two‐step spray‐drying process. In the first step, hollow CuO powders were prepared from a spray solution of copper nitrate trihydrate with citric acid and were wet milled to obtain a colloidal spray solution. In the second step, spray drying of the colloidal solution that contained dispersed GO nanosheets produced mesoporous CuO–rGO composite powders with particle sizes of several microns. Thermal reduction of GO nanosheets to rGO nanosheets occurred during post‐treatment at 300 °C. Initial discharge capacities of the hollow CuO, bare CuO aggregate, and CuO–rGO composite powders at a current density of 2 A g^?1 were 838, 1145, and 1238 mA h g^?1, respectively. Their discharge capacities after 200 cycles were 259, 380, and 676 mA h g^?1, respectively, and their corresponding capacity retentions measured from the second cycle were 67, 48, and 76 %, respectively. The mesoporous CuO–rGO composite powders have high structural stability and high conductivity because of the rGO nanosheets, and display good cycling and rate performances.     

Enhancement of Lithium Storage Performance of Carbon Microflowers by Achieving a High Surface Area

High‐surface‐area, nitrogen‐doped carbon microflowers (A‐NCFs‐4) assembled from porous nanosheets are prepared in a three‐step process: soft‐templating self‐assembly, thermal decomposition, and KOH activation. The hydrazine hydrate used in our experiment serves not only as a structure‐directing agent, but also as a nitrogen source. The resultant A‐NCFs‐4 has a hierarchical porous structure and its specific surface area is as high as 2309 m² g^?1. When used as anode, it exhibits a reversible capacity as high as 807 mAh g^?1 at 300 mA g^?1 after 100 cycles, and an excellent rate capability of 200 mAh g^?1 at a high current density of 8 A g^?1. Compared with unactivated counterpart, A‐NCFs‐4 exhibits a significantly improved lithium storage capacity and rate capability; this can be attributed to its unique structural characteristics and high surface area. The hierarchical micro‐/mesopore structure, high surface area, and nitrogen doping of A‐NCFs‐4 could guarantee fast mass transport for lithium species, enhance the A‐NCFs‐4/electrolyte contact area, shorten the lithium‐ion diffusion length, and accommodate strain induced by volume changes during the electrochemical reaction. The results indicate that the as‐prepared A‐NCFs‐4 could be a promising candidate as a high‐performance anode for lithium‐ion batteries.     

Ultrathin Titanate Nanosheets/Graphene Films Derived from Confined Transformation for Excellent Na/K Ion Storage

Confined transformation of assembled two‐dimensional MXene (titanium carbide) and reduced graphene oxide (rGO) nanosheets was employed to prepare the free‐standing films of the integrated ultrathin sodium titanate (NTO)/potassium titanate (KTO) nanosheets sandwiched between graphene layers. The ultrathin Ti‐based nanosheets reduce the diffusion distance while rGO layers enhance conductivity. Incorporation of graphene into the titanate films produced efficient binder‐free anodes for ion storage. The resulting flexible NTO/rGO and KTO/rGO electrodes exhibited excellent rate performances and long cycling stability characterized by reversible capacities of 72 mA h g^?1 at 5 A g^?1 after 10000 cycles and 75 mA h g^?1 after 700 cycles at 2 A g^?1 for sodium and potassium ion batteries, respectively. These results demonstrate the superiority of the unique sandwich‐type electrodes.     

Sulfur‐Impregnated Core–Shell Hierarchical Porous Carbon for Lithium–Sulfur Batteries

Core–shell hierarchical porous carbon spheres (HPCs) were synthesized by a facile hydrothermal method and used as host to incorporate sulfur. The microstructure, morphology, and specific surface areas of the resultant samples have been systematically characterized. The results indicate that most of sulfur is well dispersed over the core area of HPCs after the impregnation of sulfur. Meanwhile, the shell of HPCs with void pores is serving as a retard against the dissolution of lithium polysulfides. This structure can enhance the transport of electron and lithium ions as well as alleviate the stress caused by volume change during the charge–discharge process. The as‐prepared HPC‐sulfur (HPC‐S) composite with 65.3 wt % sulfur delivers a high specific capacity of 1397.9 mA h g^?1 at a current density of 335 mA g^?1 (0.2 C) as a cathode material for lithium–sulfur (Li‐S) batteries, and the discharge capacity of the electrode could still reach 753.2 mA h g^?1 at 6700 mA g^?1 (4 C). Moreover, the composite electrode exhibited an excellent cycling capacity of 830.5 mA h g^?1 after 200 cycles.     

Boron‐Doped,Carbon‐Coated SnO2/Graphene Nanosheets for Enhanced Lithium Storage

Heteroatom doping is an effective method to adjust the electrochemical behavior of carbonaceous materials. In this work, boron‐doped, carbon‐coated SnO₂/graphene hybrids (BCTGs) were fabricated by hydrothermal carbonization of sucrose in the presence of SnO₂/graphene nanosheets and phenylboronic acid or boric acid as dopant source and subsequent thermal treatment. Owing to their unique 2D core–shell architecture and B‐doped carbon shells, BCTGs have enhanced conductivity and extra active sites for lithium storage. With phenylboronic acid as B source, the resulting hybrid shows outstanding electrochemical performance as the anode in lithium‐ion batteries with a highly stable capacity of 1165 mA h g^?1 at 0.1 A g^?1 after 360 cycles and an excellent rate capability of 600 mA h g^?1 at 3.2 A g^?1, and thus outperforms most of the previously reported SnO₂‐based anode materials.     

A New Anode for Lithium‐Ion Batteries Based on Single‐Walled Carbon Nanotubes and Graphene: Improved Performance through a Binary Network Design

Carbon nanomaterials, especially graphene and carbon nanotubes, are considered to be favorable alternatives to graphite‐based anodes in lithium‐ion batteries, owing to their high specific surface area, electrical conductivity, and excellent mechanical flexibility. However, the limited number of storage sites for lithium ions within the sp²‐carbon hexahedrons leads to the low storage capacity. Thus, rational structure design is essential for the preparation of high‐performance carbon‐based anode materials. Herein, we employed flexible single‐walled carbon nanotubes (SWCNTs) with ultrahigh electrical conductivity as a wrapper for 3D graphene foam (GF) by using a facile dip‐coating process to form a binary network structure. This structure, which offered high electrical conductivity, enlarged the electrode/electrolyte contact area, shortened the electron‐/ion‐transport pathways, and allowed for efficient utilization of the active material, which led to improved electrochemical performance. When used as an anode in lithium‐ion batteries, the SWCNT‐GF electrode delivered a specific capacity of 953 mA h g^?1 at a current density of 0.1 A g^?1 and a high reversible capacity of 606 mA h g^?1 after 1000 cycles, with a capacity retention of 90 % over 1000 cycles at 1 A g^?1 and 189 mA h g^?1 after 2200 cycles at 5 A g^?1.     

Facile Synthesis of Nitrogen‐Containing Mesoporous Carbon for High‐Performance Energy Storage Applications

Porous carbon with high specific surface area (SSA), a reasonable pore size distribution, and modified surface chemistry is highly desirable for application in energy storage devices. Herein, we report the synthesis of nitrogen‐containing mesoporous carbon with high SSA (1390 m² g^?1), a suitable pore size distribution (1.5–8.1 nm), and a nitrogen content of 4.7 wt % through a facile one‐step self‐assembly process. Owing to its unique physical characteristics and nitrogen doping, this material demonstrates great promise for application in both supercapacitors and encapsulating sulfur as a superior cathode material for lithium–sulfur batteries. When deployed as a supercapacitor electrode, it exhibited a high specific capacitance of 238.4 F g^?1 at 1 A g^?1 and an excellent rate capability (180 F g^?1, 10 A g^?1). Furthermore, when an NMC/S electrode was evaluated as the cathode material for lithium–sulfur batteries, it showed a high initial discharge capacity of 1143.6 mA h g^?1 at 837.5 mA g^?1 and an extraordinary cycling stability with 70.3 % capacity retention after 100 cycles.     

Compact Coupled Graphene and Porous Polyaryltriazine‐Derived Frameworks as High Performance Cathodes for Lithium‐Ion Batteries

It is highly desirable to develop electroactive organic materials and their derivatives as green alternatives of cathodes for sustainable and cost‐effective lithium‐ion batteries (LIBs) in energy storage fields. Herein, compact two‐dimensional coupled graphene and porous polyaryltriazine‐derived frameworks with tailormade pore structures are fabricated by using various molecular building blocks under ionothermal conditions. The porous nanosheets display nanoscale thickness, high specific surface area, and strong coupling of electroactive polyaryltriazine‐derived frameworks with graphene. All these features make it possible to efficiently depress the dissolution of redox moieties in electrolytes and to boost the electrical conductivity of whole electrode. When employed as a cathode in LIBs, the two‐dimensional porous nanosheets exhibit outstanding cycle stability of 395 mAh g^?1 at 5 A g^?1 for more than 5100 cycles and excellent rate capability of 135 mAh g^?1 at a high current density of 15 A g^?1.