Facile Preparation and Lithium Storage Properties of TiO2@Graphene Composite Electrodes with Low Carbon Content

Over the past decade, TiO₂/graphene composites as electrodes for lithium ion batteries have attracted a great deal of attention for reasons of safety and environmental friendliness. However, most of the TiO₂/graphene electrodes have large graphene content (9–40 %), which is bound to increase the cost of the battery. Logically, reducing the amount of graphene is a necessary part to achieve a green battery. The synthesis of TiO₂ nanosheets under solvothermal conditions without additives is now demonstrated. Through mechanical mixing TiO₂ nanosheets with different amount of reduced graphene (rGO), a series of TiO₂@graphene composites was prepared with low graphene content (rGO content 1, 2, 3, and 5 wt %). When these composites were evaluated as anodes for lithium ion batteries, it was found that TiO₂+3 wt % rGO manifested excellent cycling stability and a high specific capacity (243.7 mAh g^?1 at 1 C; 1 C=167.5 mA g^?1), and demonstrated superior high‐rate discharge/charge capability at 20 C.     

A Robust Solid Electrolyte Interphase Layer Augments the Ion Storage Capacity of Bimetallic‐Sulfide‐Containing Potassium‐Ion Batteries

Metal–organic framework‐derived NiCo_2.5S₄ microrods wrapped in reduced graphene oxide (NCS@RGO) were synthesized for potassium‐ion storage. Upon coordination with organic potassium salts, NCS@RGO exhibits an ultrahigh initial reversible specific capacity (602 mAh g^?1 at 50 mA g^?1) and ultralong cycle life (a reversible specific capacity of 495 mAh g^?1 at 200 mA g^?1 after 1 900 cycles over 314 days). Furthermore, the battery demonstrates a high initial Coulombic efficiency of 78 %, outperforming most sulfides reported previously. Advanced ex situ characterization techniques, including atomic force microscopy, were used for evaluation and the results indicate that the organic potassium salt‐containing electrolyte helps to form thin and robust solid electrolyte interphase layers, which reduce the formation of byproducts during the potassiation–depotassiation process and enhance the mechanical stability of electrodes. The excellent conductivity of the RGO in the composites, and the robust interface between the electrodes and electrolytes, imbue the electrode with useful properties; including, ultrafast potassium‐ion storage with a reversible specific capacity of 402 mAh g^?1 even at 2 A g^?1.     

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.     

Hierarchical Nanotubes Constructed by Co9S8/MoS2 Ultrathin Nanosheets Wrapped with Reduced Graphene Oxide for Advanced Lithium Storage

Nanostructured hybrid metal sulfides have attracted intensive attention due to their fascinating properties that are unattainable by the single‐phased counterpart. Herein, we report an efficient approach to construct cobalt sulfide/molybdenum disulfide (Co₉S₈/MoS₂) wrapped with reduced graphene oxide (rGO). The unique structures constructed by ultrathin nanosheets and synergetic effects benefitting from bimetallic sulfides provide improved lithium ions reaction kinetics, and they retain good structural integrity. Interestingly, the conductive rGO can facilitate electron transfer, increase the electronic conductivity and accommodate the strain during cycling. When evaluated as anode materials for lithium‐ion batteries (LIBs), the resultant reduced graphene oxide‐coated cobalt sulfide/molybdenum disulfide (Co₉S₈/MoS₂@rGO) nanotubes deliver high specific capacities of 1140, 948, 897, 852, 820, 798 and 784 mAh g^?1 at the various discharging current densities of 0.2, 0.5, 1, 2, 3, 4 and 5 A g^?1, respectively. In addition, they can maintain an excellent cycle stability with a discharge capacity of 807 mAh g^?1 at 0.2 A g^?1 after 70 cycles, 787 mAh g^?1 at 1 A g^?1 after 180 cycles and 541 mAh g^?1 at 2 A g^?1 after 200 cycles. The proposed method may offer fundamental understanding for the rational design of other hybrid functional composites with high Li‐storage properties.     

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.     

Sodium‐Ion Storage Properties of FeS–Reduced Graphene Oxide Composite Powder with a Crumpled Structure

The sodium‐ion storage properties of FeS–reduced graphene oxide (rGO) and Fe₃O₄‐rGO composite powders with crumpled structures have been studied. The Fe₃O₄‐rGO composite powder, prepared by one‐pot spray pyrolysis, could be transformed to an FeS‐rGO composite powder through a simple sulfidation treatment. The mean size of the Fe₃O₄ nanocrystals in the Fe₃O₄‐rGO composite powder was 4.4 nm. After sulfidation, FeS nanocrystals of size several hundred nanometers were confined within the crumpled structure of the rGO matrix. The initial discharge capacities of the FeS‐rGO and Fe₃O₄‐rGO composite powders were 740 and 442 mA h g^?1, and their initial charge capacities were 530 and 165 mA h g^?1, respectively. The discharge capacities of the FeS‐rGO and Fe₃O₄‐rGO composite powders at the 50th cycle were 547 and 150 mA h g^?1, respectively. The FeS‐rGO composite powder showed superior sodium‐ion storage performance compared to the Fe₃O₄‐rGO composite powder.     

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.     

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.     

Ultrathin Hexagonal Hybrid Nanosheets Synthesized by Graphene Oxide‐Assisted Exfoliation of β‐Co(OH)2 Mesocrystals

In the present study, we report the synthesis of a high‐quality, single‐crystal hexagonal β‐Co(OH)₂ nanosheet, exhibiting a thickness down to ten atomic layers and an aspect ratio exceeding 900, by using graphene oxide (GO) as an exfoliant of β‐Co(OH)₂ nanoflowers. Unlike conventional approaches using ionic precursors in which morphological control is realized by structure‐directing molecules, the β‐Co(OH)₂ flower‐like superstructures were first grown by a nanoparticle‐mediated crystallization process, which results in large 3D superstructure consisting of ultrathin nanosheets interspaced by polydimethoxyaniline (PDMA). Thereafter, β‐Co(OH)₂ nanoflowers were chemically exfoliated by surface‐active GO under hydrothermal conditions into unilamellar single‐crystal nanosheets. In this reaction, GO acts as a two‐dimensional (2D) amphiphile to facilitate the exfoliation process through tailored interactions between organic and inorganic molecules. Meanwhile, the on‐site conjugation of GO and Co(OH)₂ promotes the thermodynamic stability of freestanding ultrathin nanosheets and restrains further growth through Oswald ripening. The unique 2D structure combined with functionalities of the hybrid ultrathin Co(OH)₂ nanosheets on rGO resulted in a remarkably enhanced lithium‐ion storage performance as anode materials, maintaining a reversible capacity of 860 mA h g^?1 for as many as 30 cycles. Since mesocrystals are ubiquitous and rich in morphological diversity, the strategy of the GO‐assisted exfoliation of mesocrystals developed here provides an opportunity for the synthesis of new functional nanostructures that could bear importance in clean renewable energy, catalysis, photoelectronics, and photonics.     

Coated/Sandwiched rGO/CoSx Composites Derived from Metal–Organic Frameworks/GO as Advanced Anode Materials for Lithium‐Ion Batteries

Rational composite materials made from transition metal sulfides and reduced graphene oxide (rGO) are highly desirable for designing high‐performance lithium‐ion batteries (LIBs). Here, rGO‐coated or sandwiched CoS_x composites are fabricated through facile thermal sulfurization of metal–organic framework/GO precursors. By scrupulously changing the proportion of Co²⁺ and organic ligands and the solvent of the reaction system, we can tune the forms of GO as either a coating or a supporting layer. Upon testing as anode materials for LIBs, the as‐prepared CoS_x‐rGO‐CoS_x and rGO@CoS_x composites demonstrate brilliant electrochemical performances such as high initial specific capacities of 1248 and 1320 mA h g^?1, respectively, at a current density of 100 mA g^?1, and stable cycling abilities of 670 and 613 mA h g^?1, respectively, after 100 charge/discharge cycles, as well as superior rate capabilities. The excellent electrical conductivity and porous structure of the CoS_x/rGO composites can promote Li⁺ transfer and mitigate internal stress during the charge/discharge process, thus significantly improving the electrochemical performance of electrode materials.     

Enhancement of Sodium Ion Battery Performance Enabled by Oxygen Vacancies

The utilization of oxygen vacancies (OVs) in sodium ion batteries (SIBs) is expected to enhance performance, but as yet it has rarely been reported. Taking the MoO_3?x nanosheet anode as an example, for the first time we demonstrate the benefits of OVs on SIB performance. Moreover, the benefits at deep‐discharge conditions can be further promoted by an ultrathin Al₂O₃ coating. A series of measurements show that the OVs increase the electric conductivity and Na‐ion diffusion coefficient, and the promotion from ultrathin coating lies in the effective reduction of cycling‐induced solid‐electrolyte interphase. The coated nanosheets exhibited high reversible capacity and great rate capability with the capacities of 283.9 (50 mA g^?1) and 179.3 mAh g^?1 (1 A g^?1) after 100 cycles. This work may not only arouse future attention on OVs for sodium energy storage, but also open up new possibilities for designing strategies to utilize defects in other energy storage systems.     

Nanocomposites of Sulfonic Polyaniline Nanoarrays on Graphene Nanosheets with an Improved Supercapacitor Performance

A new nanocomposite, poly(aniline‐co‐diphenylamine‐4‐sulfonic acid)/graphene (PANISP/rGO), was prepared by means of an in situ oxidation copolymerization of aniline (ANI) with diphenylamine‐4‐sulfonic acid (SP) in the presence of graphene oxide, followed by the chemical reduction of graphene oxide using hydrazine hydrate as a reductant. The morphology and structure of PANISP/rGO were characterized by field‐emission (FE) SEM, TEM, X‐ray photoelectron spectroscopy (XPS), Raman, FTIR, and UV/Vis spectra. The electrochemical performance was evaluated by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. The PANISP/rGO nanocomposite showed a nanosized structure, with sulfonic polyaniline nanoarrays coated homogeneously on the surface of graphene nanosheets. This special structure of the nanocomposite also facilitates the enhancement of the electrochemical performance of the electrodes. The PANISP/rGO nanocomposite exhibits a specific supercapacitance up to 1170 F g^?1 at the current density of 0.5 A g^?1. The as‐prepared electrodes show excellent supercapacitive performance because of the synergistic effects between graphene and the sulfonic polyaniline copolymer chains.     

SnS2 Nanoplatelet@Graphene Nanocomposites as High‐Capacity Anode Materials for Sodium‐Ion Batteries

Na‐ion batteries have been attracting intensive investigations as a possible alternative to Li‐ion batteries. Herein, we report the synthesis of SnS₂ nanoplatelet@graphene nanocomposites by using a morphology‐controlled hydrothermal method. The as‐prepared SnS₂/graphene nanocomposites present a unique two‐dimensional platelet‐on‐sheet nanoarchitecture, which has been identified by scanning and transmission electron microscopy. When applied as the anode material for Na‐ion batteries, the SnS₂/graphene nanosheets achieved a high reversible specific sodium‐ion storage capacity of 725 mA h g^?1, stable cyclability, and an enhanced high‐rate capability. The improved electrochemical performance for reversible sodium‐ion storage could be ascribed to the synergistic effects of the SnS₂ nanoplatelet/graphene nanosheets as an integrated hybrid nanoarchitecture, in which the graphene nanosheets provide electronic conductivity and cushion for the active SnS₂ nanoplatelets during Na‐ion insertion and extraction processes.     

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.     

TiO2–B Nanosheets/Anatase Nanocrystals Co‐Anchored on Nanoporous Graphene: In Situ Reduction–Hydrolysis Synthesis and Their Superior Rate Performance as an Anode Material

A unique hybrid, TiO₂–B nanosheets/anatase nanocrystals co‐anchored on nanoporous graphene sheets, can be synthesized by a facile microwave‐induced in situ reduction–hydrolysis route. The as‐formed nanohybrid has a hierarchically porous structure, involving both mesopores of approximately 4 nm and meso‐/macropores of 30–60 nm in the graphene sheets, and a large surface area. Importantly, electrodes composed of the nanohybrid exhibit superior rate capability (160 mA h g^?1 at ca. 36 C; 154 mA h g^?1 at ca. 72 C) and excellent cyclability. The synergistic effects of conductive graphene with numerous nanopores and the pseudocapacitive effect of ultrafine TiO₂–B nanosheets and anatase nanocrystals endow the hybrid a superior rate capability.     

Highly Ordered Mesoporous Few‐Layer Graphene Frameworks Enabled by Fe3O4 Nanocrystal Superlattices

While great progress has been achieved in the synthesis of ordered mesoporous carbons in the past decade, it still remains a challenge to prepare highly graphitic frameworks with ordered mesoporosity and high surface area. Reported herein is a simple synthetic methodology, based on the conversion of self‐assembled superlattices of Fe₃O₄ nanocrystals, to fabricate highly ordered mesoporous graphene frameworks (MGFs) with ultrathin pore walls consisting of three to six stacking graphene layers. The MGFs possess face‐centered‐cubic symmetry with interconnected mesoporosity, tunable pore width, and high surface area. Because of their unique architectures and superior structural durability, the MGFs exhibit excellent cycling stability and rate performance when used as anode materials for lithium‐ion batteries, thus retaining a specific capacity of 520 mAh g^?1 at a current density of 300 mA g^?1 after 400 cycles.     

Synthesis of 2D/2D Structured Mesoporous Co3O4 Nanosheet/N‐Doped Reduced Graphene Oxide Composites as a Highly Stable Negative Electrode for Lithium Battery Applications

Mesoporous Co₃O₄ nanosheets (Co₃O₄‐NS) and nitrogen‐doped reduced graphene oxide (N‐rGO) are synthesized by a facile hydrothermal approach, and the N‐rGO/Co₃O₄‐NS composite is formulated through an infiltration procedure. Eventually, the obtained composites are subjected to various characterization techniques, such as XRD, Raman spectroscopy, surface area analysis, X‐ray photoelectron spectroscopy (XPS), and TEM. The lithium‐storage properties of N‐rGO/Co₃O₄‐NS composites are evaluated in a half‐cell assembly to ascertain their suitability as a negative electrode for lithium‐ion battery applications. The 2D/2D nanostructured mesoporous N‐rGO/Co₃O₄‐NS composite delivered a reversible capacity of about 1305 and 1501 mAh g^?1 at a current density of 80 mA g^?1 for the 1st and 50th cycles, respectively. Furthermore, excellent cyclability, rate capability, and capacity retention characteristics are noted for the N‐rGO/Co₃O₄‐NS composite. This improved performance is mainly related to the existence of mesoporosity and a sheet‐like 2D hierarchical morphology, which translates into extra space for lithium storage and a reduced electron pathway. Also, the presence of N‐rGO and carbon shells in Co₃O₄‐NS should not be excluded from such exceptional performance, which serves as a reliable conductive channel for electrons and act as synergistically to accommodate volume expansion upon redox reactions. Ex‐situ TEM, impedance spectroscopy, and XPS, are also conducted to corroborate the significance of the 2D morphology towards sustained lithium storage.     

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.     

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.     

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.