High capacity Li storage in sulfur and nitrogen dual-doped graphene networks

Three-dimensional (3D) networks composing of S and N dual-doped graphene (SNG) were synthesized by a chemical vapor deposition approach using MgSO₄-containing whiskers as templates and S source and NH₃ as N source. Energy dispersive spectrometer mapping and X-ray photoelectron spectroscopy coupled with Raman analysis have revealed that S and N atoms with concentrations of 5.2 and 1.8 atom%, respectively, have been substitutionally incorporated into the graphene networks via covalent bonds. The SNG, as an anode material for lithium ion batteries (LIBs), exhibits extremely high capacity (3525 mAh/g at the current density of 50 mA/g) and superior rate capability (870 mAh/g at 1000 mA/g) with excellent cycling stability (remaining a reversible capacity of 400 mAh/g at 10 A/g after 2500 cycles). The enhanced conductivity, the 3D porous network with many disorders and the intrinsically high Li storage capacity of S and N-doped carbon segments have led to the excellent electrode performance of the SNG networks. The effects of binder content and calendaring pressure on the electrode performance have been investigated. The full LIB with SNG as anode and LiCoO₂ as cathode can afford a high reversible capability (164 mAh/g at 0.2 C) and good cycling stability.     

Exceptional electrochemical performance of nitrogen-doped porous carbon for lithium storage

Crumpled nitrogen-doped porous carbon sheets are successfully fabricated via chemical activation of polypyrrole-functionalized graphene sheets with KOH (APGs). The obtained APGs with nitrogen doping, high surface area, porous and crumpled structure exhibit exceptional electrochemical performances as the electrode material for LIBs, including a superhigh reversible specific capacity of 1516.2 mAh g⁻¹, excellent cycling stability over 10,000 cycles, and good rate capability (133.2 mAh g⁻¹ even at a very high current density of 40 A g⁻¹). The chemical activation synthesis strategy might open new avenues for the design of high-performance carbon-based anode materials.     

High-capacity graphene oxide/graphite/carbon nanotube composites for use in Li-ion battery anodes

A composite of graphene oxide sheets, carbon nanotubes (CNTs), and commercial graphite particles was prepared. The composite’s use as a high-capacity and binder-free anode material for Li-ion batteries was examined. Results showed that this novel composite had a very high reversible Li-storage capacity of 1172.5 mA h g⁻¹ at 0.5C (1C = 372 mA g⁻¹), which is thrice that of commercial graphite anode. The composite also exceeded the theoretical sum of capacities of the three ingredients. More importantly, its reversible capacity below 0.25 V can reach up to 600 mA h g⁻¹. In summary, the graphene oxide/graphite/CNT composite had higher reversible capacity, better cycling performance, and similar rate capability compared with the graphene oxide/graphite composite.     

Preparation of graphene supported porous Si@C ternary composites and their electrochemical performance as high capacity anode materials for Li-ion batteries

Graphene supported porous Si@C ternary composites had been synthesized by various routes and their structural, morphological and electrochemical properties were investigated. Porous Si spheres coated with carbon layer and supported by graphene have been designed to form a 3D carbon conductive network. Used as anode materials for lithium ion batteries, graphene supported porous Si@C ternary composites demonstrate excellent electrochemical performance and cycling stability. The first discharge capacity is 2184.7 mA h/g at a high current density of 300 mA/g. After 50 cycles, the reversible capacity is 652.4 mA h/g at a current density of 300 mA/g and the coulomb efficiency reaches at 98.7%. Due to their excellent electrochemical properties, graphene supported porous Si@C ternary composites can be a kind of promising anode materials for lithium ion batteries.     

Electrospinning combined with hydrothermal synthesis and lithium storage properties of ZnFe2O4-graphene composite nanofibers

ZnFe₂O₄-graphene composite nanofibers were prepared through electrospinning technique, then with graphene oxide by the facile solvothermal method to get the final products for the first time. The obtained ZnFe₂O₄ nanofibers composed of numerous same size nanoparticles wrapped by graphene sheets to form a unique nanostructure. When the ZnFe₂O₄-graphene electrode was evaluated as anode for lithium-ion batteries, good rate capability and long-term cycling stability could be achieved. The ZnFe₂O₄-graphene electrode exhibited a first discharge capacity of 2166 mAh g⁻¹ cycling at 0.05 C, remained an average reversible capacity of 1000 mAh g⁻¹ after 50 cycles, and kept the high rate capacities of 899, 822, 760 and 711 mAh g⁻¹ at the current rates of 0.5, 1, 2 and 5 C, respectively. The excellent electrochemical performance could be ascribed to the following reasons: the large electrochemical active surface area provided by the composite nanofibers; the graphene sheets decreased the internal resistance of the lithium-ion batteries, which resulted from the electrical conductivity of the graphene sheets; the graphene sheets as conductive network could effectively restrain the agglomeration of ZnFe₂O₄ nanopaiticals.     

Hollow ZnSnO3 cubes@carbon/reduced graphene oxide ternary composite as anode of lithium ion batteries with enhanced electrochemical performance

The ternary composite, carbon coated hollow ZnSnO₃ (ZS@C) cubes encapsulated in reduced graphene oxide sheets (ZS@C/rGO), was synthesized via low-temperature coprecipitation and colloid electrostatic self-assembly. The uniform carbon-coating layer not only plays a role in buffering the volume change of ZnSnO₃ cubes in the charging/discharging processes, but also forms three-dimensional network with the cooperation of graphene to maintain the structural integrity and improve the electrical conductivity. The results show that the reduced graphene oxide sheets encapsulated ZS@C microcubes with a typical core-shell structure of ~700 nm in size exhibit an improved electrochemical performance compared with bare ZS@C microcubes. The ZS@C/rGO electrode delivered an initial discharge capacity of 1984 mA h g⁻¹ at a current density of 0.1 A g⁻¹ and maintained a capacity of 1040 mA h g⁻¹ after 45 cycles. High specific capacity and superior cycle stability indicate that the ZS@C/rGO composite has a great potential for the application of lithium-ion anode material.     

Silver nanorods attached to graphene sheets as anode materials for lithium-ion batteries

We employed a homogenizing method to disperse silver nanorods onto graphene nanosheets (GNs) in liquid phase, forming a three-dimensional Ag/GN framework. The Ag/GN hybrid material was used to investigate Li-ion insertion/extraction properties. A microwave-assisted polyol approach was adopted to synthesize highly crystalline Ag nanorods. The as-prepared Ag/GN hybrid anode shows improved Li-storage capacity, excellent rate capability, and good cycling stability, as compared with the pure GN anode. The hybrid anode offers a reversible capacity of 1015 mA h g⁻¹ at 0.1 C and a high capacity retention of 64.1% based on the capacity ratio at 5 C–0.1C rates. The enhancement of anode performance can be attributed to an increased electrode conductivity and interlayer distance due to the insertion of metallic Ag nanorods onto a percolated GN network, thus preventing the re-stacking of graphene sheets.     

Graphenothermal reduction synthesis of MnO/RGO composite with excellent anodic behaviour in lithium ion batteries

Mn(II) oxide/graphene oxide (MnO/RGO) composites were synthesized by an easy and cost-effective graphenothermal reduction method. The surface morphology, structure, chemical composition and electrochemical behaviour of the resulting composites were investigated in detail. The MnO/RGO composite exhibited a high surface area (115.7 m²/g), which led to the high discharge capacity, enhanced cycling stability, and outstanding rate capability as anode in Li-ion batteries (LIBs). The MnO/RGO composite exhibited an higher initial discharge capacity of 1607 mA h/g at a current density of 100 mA/g and maintained 94% of its reversible capacity over 100 consecutive cycles. Furthermore, MnO/RGO composite could preserve a significantly higher capacity of 847 mA h/g for 150 cycles even at a high current density of 250 mA/g. The excellent electrochemical properties result from the existence of highly conductive RGO and a short transportation span for both Li-ions and electrons. The developed MnO/RGO composite materials hold highly promising prospects in LIBs.     

Ultrafine SnO2 nanoparticles as a high performance anode material for lithium ion battery

In this paper, the ultrafine tin oxides (SnO₂) nanoparticles are fabricated by a facile microwave hydrothermal method with the mean size of only 14 nm. Phase compositions and microstructures of the as-prepared nanoparticles have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the ultrafine SnO₂ nanoparticles are obtained to be the pure rutile-structural phase with the good dispersibility. Galvanostatic cycling and cyclic voltammetry results indicate that the first discharge capacity of the ultrafine SnO₂ electrode is 1196.63  mAh g⁻¹, and the reversible capacity could retain 272.63 mAh g⁻¹ at 100 mA g⁻¹ after 50 cycles for lithium ion batteries (LIBs). The excellent electrochemical performance of the SnO₂ anode for LIBs is attributed to its ultrafine nanostructure for providing active sites during lithium insertion/extraction processes. Pulverization and agglomeration of the active materials are effectively reduced by the microwave hydrothermal method.     

Preparation of high-surface-area carbon nanoparticle/graphene composites

A method has been developed for synthesizing high-surface-area carbon nanoparticle/graphene composites. Functionalized carbon nanoparticles were anchored to the graphene planes and function as spacers to prevent the restacking of graphene sheets during drying. The composite has a layered structure in which functionalized carbon nanoparticles are sandwiched between graphene stacks. This layering leads to a porous structure with a specific surface area as high as 1256 m²/g. Such a structure provides easy access to both sides of the graphene for either gas or liquid species and allows their fast transfer. A specific capacitance as high as 324.6 F/g at a current density of 0.3 A/g was achieved using the composites in a supercapacitor.     

Reduced graphene oxide anchored with δ-MnO2 nanoscrolls as anode materials for enhanced Li-ion storage

We developed a one-pot in situ synthesis procedure to form nanocomposite of reduced graphene oxide (RGO) sheets anchored with 1D δ-MnO₂ nanoscrolls for Li-ion batteries. The as-prepared products were characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The electrochemical performance of the δ-MnO₂ nanoscrolls/RGO composite was measured by galvanostatic charge/discharge cycling and electrochemical impedance spectroscopy. The results show that the δ-MnO₂ nanoscrolls/RGO composite displays superior Li-ion battery performance with large reversible capacity and high rate capability. The first discharge and charge capacities are 1520 and 810 mAh g⁻¹, respectively. After 50 cycles, the reversible discharge capacity is still maintained at 528 mAh g⁻¹ at the current density of 100 mAh g⁻¹. The excellent electrochemical performance is attributed to the unique nanostructure of the δ-MnO₂ nanoscrolls/RGO composite, the high capacity of MnO₂ and superior electrical conductivity of RGO.     

Control of number of graphene layers using ultrasound in supercritical CO2 and their application in lithium-ion batteries

This study reports a novel strategy using ultrasound in supercritical CO₂ for exfoliating graphite directly into single and few-layer graphene sheets. The mutually complementary characterizations of the as-exfoliated samples via atomic force microscopy, transmission electron microscopy and Raman spectroscopy indicate that the ultrasonic power greatly affects the number of layers and the lateral size of the graphene. Single-layer graphene with a lateral size of 50–100 nm and two-layer graphene with a lateral size of 0.5–10 μm are obtained using an ultrasonic power of 300 and 120 W, respectively. As-exfoliated graphene sheets heighten the electrochemical performance of LiFePO₄ cathode materials, demonstrating graphene's remarkable electrical conductivity. The specific capacity of the LiFePO₄/graphene composite cathode achieves 160 mAh/g and displays stable cycling for more than 15 cycles. This technique will enable cost-effective mass production of graphene sheets with good quality, and the as-exfoliated graphene will find wide applications, including lithium-ion batteries.     

The synthesis of a hybrid graphene–nickel/manganese mixed oxide and its performance in lithium-ion batteries

Mixing of aqueous suspensions of delaminated NiMn layered double hydroxide (LDH) and graphene oxide leads to the instantaneous precipitation of a hybrid material that after calcination under inert atmosphere at 450 °C leads to Ni₆MnO₈ nanoparticles deposited on larger reconstituted graphene sheets. This material exhibits electrical conductivity similar to graphite, superparamagnetism and can be used as an anode for Li-ion batteries. A maximum capacity value of 1030 mA h g^?1 was found during the first discharge, and capacity values higher than 400 mA h g^?1 were still achieved after 10 cycles. The methodology used here should allow the preparation of a large variety of hybrid graphene-metal oxide materials starting from other LDHs in which the properties derived from both constituents coexist.     

High quality graphene sheets from graphene oxide by hot-pressing

We report a simple and effective route to convert graphene oxide sheets to good quality graphene sheets using hot pressing. The reduced graphene oxide sheets obtained from graphene oxide by low temperature thermal exfoliation are annealed at 1500 °C and 40 MPa uniaxial pressures for 5 min in vacuum. No appreciable oxygen content was observed from X-ray photoelectron spectroscopy and no D peak was detected in the Raman spectrum. The graphene sheets produced had a much higher electron mobility (1000 cm² V⁻¹ S⁻¹) than other chemically modified graphenes.     

Porous Fe2O3 nanorods anchored on nitrogen-doped graphenes and ultrathin Al2O3 coating by atomic layer deposition for long-lived lithium ion battery anode

Porous iron oxide (Fe₂O₃) nanorods anchored on nitrogen-doped graphene sheets (NGr) were synthesized by a one-step hydrothermal route. After a simple microwave treatment, the iron oxide and graphene composite (NGr-I-M) exhibits excellent electrochemical performances as an anode for lithium ion battery (LIB). A high reversible capacity of 1016 mAh g⁻¹ can be reached at 0.1 A g⁻¹. When NGr-I-M electrode was further coated by 2 ALD cycles of ultrathin Al₂O₃ film, the first cycle Coulombic efficiency (CE), rate performance and cycling stability of the coated electrode can be greatly improved. A stable capacity of 508 mAh g⁻¹ can be achieved at 2 A g⁻¹ for 200 cycles, and an impressive capacity of 249 mAh g⁻¹ at 20 A g⁻¹ can be maintained without capacity fading for 2000 cycles. The excellent electrochemical performance can be attributed to the synergy of porous iron oxide structures, nitrogen-doped graphene framework, and ultrathin Al₂O₃ film coating. These results highlight the importance of a rational design of electrode materials improving ionic and electron transports, and potential of using ALD ultrathin coatings to mitigate capacity fading for ultrafast and long-life battery electrodes.     

A binary solvent system for improved liquid phase exfoliation of pristine graphene materials

Up to now, the cost-effective mass production of pristine graphene sheets remains a significant challenge. We report a binary solvent system of dimethylformamide and n-butylalcohol that facilitates the high concentration production of unoxidized graphene through liquid-exfoliation assisted with sonication. A stable dispersion with concentration as high as 6.5 mg/ml, was received via this method. The conductivity of a film made from the graphene sheets had a conductivity of 27,000 S/m. The supercapacitor fabricated by graphene exhibits a giant capacitance of 600 F/g and an outstanding long-term stability.     

Dopamine derived nitrogen-doped carbon sheets as anode materials for high-performance sodium ion batteries

Designed as an anode material for sodium ion batteries, nitrogen-doped carbon sheets (NCSs) were successfully synthesized using graphene and dopamine as template and carbon precursor, respectively. The NCSs demonstrate high reversible capacity and excellent rate performance, delivering a high reversible capacity of 382 mAh g⁻¹ at 50 mA g⁻¹ after 55 cycles. Even up to 10 A g⁻¹, a rate capacity of 75 mAh g⁻¹ can be obtained. Furthermore, NCSs also have remarkable cycling stability with specific capacity of 165 mAh g⁻¹ after 600 cycles (under 200 mA g⁻¹). The excellent performance of NCSs can be ascribed to the nitrogen-doped two-dimension sheet structure.     

Graphene nanosheets based on controlled exfoliation process for enhanced lithium storage in lithium-ion battery

A facile and rapid approach was used for the fabrication of chemically derived graphene nanosheets based on the reduction of graphite oxide (GO) in tube furnace assembly at different temperatures. The morphologies, microstructures, specific surface areas and other features of GO and graphene nanosheets were characterized. Structure characterization indicates that the platelet thickness of graphene nanosheets obtained at 300 °C was 1.62 nm, which corresponds to an approximately 5 layers stacking of the monoatomic graphene nanosheets. Electrochemical performances of the as-prepared graphene nanosheets were performed, the result of which could prove the above observation that graphene nanosheets (5 layers) obtained at 300 °C actually displayed the most remarkable electrochemical performances: the first discharge and charge capacities of graphene nanosheets were as high as 2137 mAh/g and 994 mAh/g, respectively, and after 100 cycles graphene nanosheets still possessed a high capacity of 478 mAh/g.     

Aqueous slurry of S-doped carbon nanotubes as conductive additive for lithium ion batteries

S-doped carbon nanotubes (SCNTs) obtained by a post treatment approach are used as conductive additive for LiFePO₄ (LFP) cathodes in Lithium ion batteries (LIBs). The SCNTs exhibit higher specific surface area, higher conductivity and better hydrophily as compared to the pristine CNTs because of S doping. Thus the SCNTs can be stably dispersed in water, forming an aqueous conductive slurry. The LFP cathode using the aqueous SCNTs slurry as conductive additive exhibits excellent electrochemical performances in terms of capacity (143 mA h g⁻¹ at 2 C), rate capability and cycling stability (99.6% of initial capacity after 200 cycles) due to the uniform dispersibility of SCNTs in the bulk of electrodes forming a continuous conductive network. The full cell configuration with graphite as anode, affords a high reversible capability (150 mA h g⁻¹ at 0.2 C), good cycling stability (capacity retention of 87.6% at 2 C), ultrahigh energy density of 163.7 W h kg⁻¹ and power density of 296.8 W kg⁻¹. Our results provide an easy approach to prepare high performance LIB cathodes using water as solvent, thus leading to lower cost and more secure for the electrode production.     

Iodine doped graphene as anode material for lithium ion battery

I-doped graphene is synthesized by a facile heat treatment method and used as anode material for lithium ion battery. The doped graphene exhibits high reversible capacity (1690 mAh g⁻¹ at 100 mA g⁻¹), good cyclability (retaining 92.6% reversible capacity after 200 cycles) and excellent rate performance compared with undoped graphene. The superior electrochemical performance of the I-doped graphene is explained by the change of graphene lattice, defects and positive charge density introduced by the doping of I atoms.