Ordered mesoporous carbon–TiO2 materials for improved electrochemical performance of lithium ion battery

A series of ordered mesoporous carbon–TiO₂ (OMCT) materials with various weight percentages of TiO₂ (50–75 wt%) were synthesized by evaporation-induced self-assembly and in-situ crystallization at various calcination temperatures (600–1200 °C) to evaluate the Li-ion storage performance. The OMCT has ordered 2D hexagonal mesoporous structures and the TiO₂ nanocrystals with different phases are embedded into the frameworks of carbonaceous matrix. The reversible capacity of OMCT is highly dependent on the phase and content of TiO₂, and the anatase TiO₂ is a superior crystalline phase to rutile and TiN for Li-ion insertion. The OMCT₆₅ which contains 35 wt% carbon and 65 wt% TiO₂ shows a high capacity of 500 mAh g^?1 at 0.1C after 80 cycles. In addition, OMCT₆₅ exhibits a good cyclability and rate capability. The reversible capacity remains at 98 mAh g^?1 at a high rate of 5C, and then recoveries to 520 mAh g^?1 at 0.1C after 105 cycles. The excellent reversible capacity and rate capability of OMCT₆₅ are attributed to the embedment of well-dispersed anatase TiO₂ nanocrystals into the specific porous structure of OMCT, which can not only facilitate the fast Li-ion charge transport but can also strengthen the carbon–TiO₂ co-constructing channels for lithiated reactions.     

Hydrothermal synthesis of a monoclinic VO2 nanotube–graphene hybrid for use as cathode material in lithium ion batteries

The hydrothermal reaction of a mixture of a colloidal dispersion of graphite oxide and ammonium vanadate yielded a hybrid made of graphene and a nanotubular metastable monoclinic polymorph of VO₂, known as VO₂(B). The formation of VO₂(B) nanotubes is accompanied by the reduction of graphite oxide. Initially the partially scrolled graphite oxide layers act as templates for the crystallization of VO₂(B) in the tubular morphology. This is followed by the reduction of graphite oxide to graphene resulting in a hybrid in which VO₂(B) nanotubes are dispersed in graphene. Electron microscopic studies of the hybrid reveal that the VO₂(B) nanotubes are wrapped by and trapped between graphene sheets. The hybrid shows potential to be a high capacity cathode material for lithium ion batteries. It exhibits a high capacity (～450 mAh/g) and cycling stability. The high capacity of the hybrid is attributed to the interaction between the graphene sheets and the VO₂(B) tubes which improves the charge-transfer. The graphene matrix prevents the aggregation of the VO₂(B) nanotubes leading to high cycling stability.     

Combustion synthesis of graphene oxide–TiO2 hybrid materials for photodegradation of methyl orange

Graphene oxide (GO)–TiO₂ hybrid materials with enhanced photocatalytic properties were synthesized by a one-step combustion method using urea and titanyl nitrate as the fuel and oxidizer, respectively. During the synthesis procedure, the precursors containing GO, fuel, and oxidizer were maintained at different combustion temperatures (300–450 °C) for 10 min to ignite the combustion reaction. The effects of combustion temperatures on the weight loss, chemical status and photocatalytic properties were studied by thermogravimetry and differential scanning calorimetry, X-ray photoelectron spectroscopy, Raman, and photoluminescence. GO in the GO–TiO₂ hybrids were not oxidized, but thermally reduced by decomposition of partial oxygen-containing groups. Meantime, the nitrogen doping of GO was achieved. Compared to the neat TiO₂ obtained at same condition, GO–TiO₂ hybrid obtained at 350 °C exhibited enhanced photodegradation performance, which is attributed to the effective photo-generated electron transferring from TiO₂ to partially reduced GO, which confirmed by the photoluminescence quenching of TiO₂.     

Chemical-free synthesis of graphene–carbon nanotube hybrid materials for reversible lithium storage in lithium-ion batteries

Graphene–carbon nanotube hybrid materials were successfully prepared through the π–π interaction without using any chemical reagent. We found that the ratio between carbon nanotube and graphene had critical influences on the state in aqueous solution and morphology of hybrid materials. Field emission scanning electron microscope and transmission electron microscope analysis confirmed that graphene nanosheets wrap around individual carbon nanotubes and form a homogeneous three-dimensional hybrid nanostructure. When applied as an anode material in lithium ion batteries, graphene–carbon nanotube hybrid materials demonstrated a high reversible lithium storage capacity, a high Coulombic efficiency and an excellent cyclability.     

Core–shell CNT–Ni–Si nanowires as a high performance anode material for lithium ion batteries

Core–shell carbon nanotube (CNT)–Si heterogeneous nanowires have been identified as one of the most promising candidates for future anode materials in lithium ion batteries. However, stress in these nanostructures, is the long-existing bottleneck, rendering severe fading of the capacities and even failure of the batteries. We prove that the interfaces between CNT cores and Si shells play a critical role in the stress engineering. With rationally engineered interfaces, our core–shell nanowires with CNT–Ni–Si structure are able to offer excellent capacity retention and rate performance. Introduction of the rough Ni interfacial layer and utilization of the CNT cores lead to reinforced stability of the structure, well accommodated stress and enhanced charge transfer, which are responsible for the improved performance. This core–shell CNT–Ni–Si nanostructure provides a simple but effective pathway towards realization of long lifetime and high performance lithium ion batteries.     

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.     

Preparation and electrochemical performance of SiCN–CNTs composite anode material for lithium ion batteries

The composite of silicon carbonitride (SiCN) and carbon nanotubes (CNTs) was synthesized by sintering the mixture of polysilylethylenediamine-derived amorphous SiCN and multi-walled CNTs at a temperature of 1,000 °C for 1 h in argon. The as-prepared SiCN–CNTs material, which was used as anode active substance in a lithium ion battery, showed excellent electrochemical performance. Charge–discharge tests showed the SiCN–CNTs anode provided a high initial specific discharge capacity of 1176.6 mA h g⁻¹ and a steady specific discharge capacity of 450–400 mA h g⁻¹ after 30 charge–discharge cycles at 0.2 mA cm⁻². Both of the abovementioned values are higher than that of pure polymer-derived SiCN, CNTs, and commercial graphite at the same charge–discharge condition. It was deduced that the CNTs in the composite not only improved the electronic conductivity and offered channels and sites for the immigrating and intercalating of Li⁺ but also stabilized the structure of the composite.     

“Concrete” inspired construction of a silicon/carbon hybrid electrode for high performance lithium ion battery

Graphene and other carbon materials have been combined with various silicon (Si) nanostructures to accommodate the volume change of Si and enhance their electrical conductivity. However, for most of the formed hybrids, their low initial Coulombic efficiency (CE), fragile structures and poor stability cannot meet the practical application of battery. In this work, inspired by the structure and composition of reinforced concrete, a Si nanoparticles embedded in porous carbon/graphene (Si-C/G) electrode is fabricated through directly calcining a Si-polyacrylonitrile/graphene oxide precursor on a current collector. In this concrete-like structure, amorphous carbon, the carbonization product of polyacrylonitrile, acts as the “cement” and binds all components together. The flexible graphene network effectively enhances the strength, flexibility and conductivity of the electrode, as does the reinforcing rod framework in concrete. This carbon/graphene scaffold can accommodate the volume expansion of Si and isolate Si from electrolyte. Such Si-C/G electrode with small surface area and compact structure achieves a high initial CE of 78% and a reversible capacity of 1711 mAh g⁻¹, as well as outstanding rate and cycling performances.     

Template synthesis of SnO2/α-Fe2O3 nanotube array for 3D lithium ion battery anode with large areal capacity

Electrodes with three-dimensional (3D) nanostructure are expected to improve the energy and power densities per footprint area of lithium ion microbatteries. Herein, we report a large-scale synthesis of a SnO(2)/α-Fe(2)O(3) composite nanotube array on a stainless steel substrate via a ZnO nanowire array as an in situ sacrificial template without using any strong acid or alkali. Importantly, both SnO(2) and α-Fe(2)O(3) contribute to the lithium storage, and the hybridization of SnO(2) and α-Fe(2)O(3) into an integrated nanotube structure provides them with an elegant synergistic effect when participating in electrochemical reactions. Large areal capacities and good rate capability are demonstrated for such a composite nanotube array. Particularly noteworthy is that the areal capacities (e.g. 1.289 mAh cm(-2) at a current rate of 0.1 mA cm(-2)) are much larger than those of many previous thin-film/3D microbattery electrodes. Our work suggests the possibility of further improving the areal capacity/energy density of 3D microelectrodes by designing ordered hybrid nanostructure arrays.     

Composites of V2O3–ordered mesoporous carbon as anode materials for lithium-ion batteries

The composites of V₂O₃–ordered mesoporous carbon (V₂O₃–OMC) were synthesized and used as anode materials for Li-ion intercalation. These materials exhibited large reversible capacity, high rate performance and excellent cycling stability. For instance, a reversible capacity of V₂O₃–OMC composites was 536 mA h g⁻¹ after 180 cycles at a current density of 0.1 A g⁻¹. The high electrochemical performance of the V₂O₃–OMC composites is attributed to the anchoring of nanoparticles on mesoporous carbon for improving the electrochemical active of V₂O₃ particles for energy storage applications in high performance lithium-ion batteries.     

Nanocrystalline carbon–TiO2 hybrid hollow spheres as possible electrodes for solar cells

Nanocrystalline C-doped TiO₂ hybrid hollow spheres were prepared by solvothermal synthesis by controlling calcinations of mixtures of furfural, chitosan or saccharose with titanium isopropoxide. The origin of the carbon influences the texture, the crystalline framework, and the optical and photoelectrochemistry properties of the TiO₂. The results indicate that the carbon–TiO₂ hybrid hollow spheres may be used as TiO₂-based film electrodes for use in solar cells.     

Ultra-thin carbon nanosheets coated with SnO2–NbC nanoparticles as high-performance anode materials for lithium-ion batteries

A SnO₂–NbC–C ternary composite was constructed using hydrothermal and ball milling methods. For this special design, the SnO₂–NbC nanoparticles were uniformly coated with ultra-thin carbon nanosheets. It should be noted that the NbC nanoparticles can accommodate the volume variation of SnO₂ particles and prevent Sn agglomeration, resulting in good electrical contact between particles and low charge transfer resistance. After 100 cycles of 0.2 Ag^-1, the SnO₂–NbC–C nanocomposite exhibited a high capacity of 956.0 mAhg^-1, and a rate capacity of 955.2 mAhg^-1, and after 500 cycles of 5.0 Ag^-1, it achieved a capacity of 704.1 mAhg^-1. In addition, the SnO₂–NbC–C composite remained stable after 200 and 700 cycles without agglomeration.     

Experimentally derived axial stress–strain relations for two-dimensional materials such as monolayer graphene

A methodology is presented here for deriving true experimental axial stress–strain curves in both tension and compression for monolayer graphene through the shift of the 2D Raman peak (Δω) that is present in all graphitic materials. The principle behind this approach is the observation that the shift of the 2D wavenumber as a function of strain for different types of PAN-based fibres is a linear function of their Young’s moduli and, hence, the corresponding value of Δω over axial stress is, in fact, a constant. By moving across the length scales we show that this value is also valid at the nanoscale as it corresponds to the in-plane breathing mode of graphene that is present in both PAN-based fibres and monolayer graphene. Hence, the Δω values can be easily converted to values of σ in the linear elastic region without the aid of modelling or the need to resort to cumbersome experimental procedures for obtaining the axial force transmitted to the material and the cross-sectional area of the two-dimensional membrane.     

One-pot synthesis of core–shell-structured tin oxide–carbon composite powders by spray pyrolysis for use as anode materials in Li-ion batteries

Core–shell-structured tin oxide–carbon composite powders with mixed SnO₂ and SnO tetragonal crystals are prepared by one-pot spray pyrolysis from a spray solution with tin oxalate and polyvinylpyrrolidone (PVP). The aggregate, made up of SnO_x nanocrystals (several tens of nanometers), is uniformly coated with an amorphous carbon layer. The initial discharge capacities of the bare SnO₂ and SnO_x–carbon composite powders at a current density of 1 A g⁻¹ are 1473 and 1667 mA h g⁻¹, respectively; their discharge capacities after 500 cycles are 78 and 1033 mA h g⁻¹, respectively. The SnO_x–carbon composite powders maintain their spherical morphology even after 500 cycles. On the other hand, the bare SnO₂ powder breaks into several pieces after cycling. The structural stability of the SnO_x–carbon composite powders results in a low charge transfer resistance and high lithium ion diffusion rate even after 500 cycles at a high current density of 2 A g⁻¹. Therefore, the SnO_x–carbon composite powders have superior electrochemical properties compared with those of the bare SnO₂ powders with a fine size.     

Biotemplate synthesis of mesoporous α-Fe2O3 hierarchical structure with assisted pseudocapacitive as an anode for long-life lithium ion batteries

The oriented biotemplate synthesis of nanostructured metal oxides as anode materials for lithium-ion batteries (LIBs) has recently attracted widespread attentions. Herein, mesoporous α-Fe₂O₃ hierarchical tubes (named as Fe-400) were successfully prepared by facile iron salt impregnation and air calcination at 400 °C using waste poplar branch as biotemplate. The hierarchical structure of Fe-400 is constructed from cross-linked small nanoparticles (~29 nm), which then results in large specific surface area (37.7 m² g^?1) and uniform mesoporous size distribution (12.2 nm). As anode material for LIBs, Fe-400 displays reversible capacity of 880.7 mA h g^?1 after long-cycle of 800^th at 1 A g^?1, indicating that this material has high capacity retention and good long-cycle stability. The prominent electrochemical properties are mainly ascribed to the large specific surface area, unique homogeneous mesopores, and the assisted pseudocapacitive behaviors of Fe-400. In view of the low-cost, environment-friendly and easily large-scale synthesis of Fe-400 electrode material, the present biotemplate strategy can present useful reference for the synthesis of other transition metal oxide-based anode materials for LIBs.     

Improved activity of a graphene–TiO2 hybrid electrode in an electrochemical supercapacitor

We present a simple and fast approach for the synthesis of a graphene–TiO₂ hybrid nanostructure using a microwave-assisted technique. The microstructure, composition, and morphology were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. The electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. Structural analysis revealed a homogeneous distribution of nanosized TiO₂ particles on graphene nanosheets. The material exhibited a high specific capacitance of 165 F g⁻¹ at a scan rate of 5 mV s⁻¹ in 1 M Na₂SO₄ electrolyte solution. Theenhanced supercapacitance property of these materials could be ascribed to the increased conductivity of TiO₂ and better utilization of graphene. Moreover, the material exhibited long-term cycle stability, retaining ∼90% specific capacitance after 5000 cycles, which suggests that it has potential as an electrode material for high-performance electrochemical supercapacitors.     

Sol–gel derived hybrid materials as heterogeneous catalysts for the epoxidation of olefins

CH₃ReO₃ has been heterogenized inside the porous system of hybrid silica matrixes via the sol–gel method using 1,4-bis(triethoxysilyl)benzene as a co-condensation agent and 4-((3-triethoxysilyl)propylamino)pyridine hydrochloride as a hydrolysable ligand. The resulting solids are stable and recyclable epoxidation catalysts.     

Status and prospects in sulfur–carbon composites as cathode materials for rechargeable lithium–sulfur batteries

Sulfur stands as a very promising cathode candidate for the next-generation rechargeable batteries due to its high energy density, natural abundance, low cost and environmental friendliness. However, the application of lithium–sulfur batteries suffers from low sulfur utilization and poor cycle life of the sulfur cathode. The problems are mainly ascribed to the electrically insulating nature of sulfur and the discharge products, and to the dissolution of the reaction intermediates of polysulfides. Among various approaches, fabricating sulfur–carbon composite cathodes with sulfur embedded within conductive carbon frameworks has been proven promising. Carbon materials, including nanoporous carbon, carbon nanotubes, graphene nanosheets and some other forms, have excellent conductivity, robust chemistry, good mechanical stability, and great abundance. By constraining sulfur within carbon frameworks, the conductivity of the sulfur electrode can be greatly enhanced, and the dissoluble loss of intermediate sulfur species in the liquid electrolyte can also be restrained due to the sorption properties of carbon, leading to a much improved electrochemical performance. This review summarizes the progresses in the sulfur–carbon composite cathodes for lithium–sulfur batteries in recent years, and introduces the roles and the effectiveness of various carbon structures on the electrochemical properties.