Carbon-coated silicon/graphite oxide composites as anode materials for highly stable lithium-ion batteries.

Silicon (Si) has been widely investigated as an anode material for lithium-ion batteries (LIBs) due to its high theoretical capacity. However, the huge volume expansion and low electrical conductivity limit its practical application to some extent. Here, we prepared silicon/reduced graphene oxide/amorphous carbon (Si/G/C) anode materials for lithium-ion batteries using a facile synergistic cladding layer. The protective effect of different carbon layers was explored and it was found that ternary composites have excellent electrochemical properties. In this work, the surface of Si was first modified using ammonia, and the positively charged Si was tightly anchored to the graphene sheet layer. In contrast, amorphous carbon was used as a reinforcing coating for further coating to synergistically build up the cladding layer of Si NPs with graphene oxide. The ternary composite (Si/G/C) material greatly ensures the structural integrity of the composites and shows excellent cycling as well as rate performance compared to Si/reduced graphene oxide and Si/carbon composites. For the Si/G/C composite, at a current density of 1 A g-1, it can be stably cycled over 267 times with 70% capacity retention (only 0.0711% capacity reduction per cycle).

Using Sandwiched Silicon/Reduced Graphene Oxide Composites with Dual Hybridization for Their Stable Lithium Storage Properties.

Using silicon/reduced graphene oxide (Si/rGO) composites as lithium-ion battery (LIB) anodes can effectively buffer the volumetric expansion and shrinkage of Si. Herein, we designed and prepared Si/rGO-b with a sandwiched structure, formed by a duple combination of ammonia-modified silicon (m-Si) nanoparticles (NP) with graphene oxide (GO). In the first composite process of m-Si and GO, a core-shell structure of primal Si/rGO-b (p-Si/rGO-b) was formed. The amino groups on the m-Si surface can not only hybridize with the GO surface to fix the Si particles, but also form covalent chemical bonds with the remaining carboxyl groups of rGO to enhance the stability of the composite. During the electrochemical reaction, the oxygen on the m-Si surface reacts with lithium ions (Li+) to form Li2O, which is a component of the solid-electrolyte interphase (SEI) and is beneficial to buffering the volume expansion of Si. Then, the p-Si/rGO-b recombines with GO again to finally form a sandwiched structure of Si/rGO-b. Covalent chemical bonds are formed between the rGO layers to tightly fix the p-Si/rGO-b, and the conductive network formed by the reintroduced rGO improves the conductivity of the Si/rGO-b composite. When used as an electrode, the Si/rGO-b composite exhibits excellent cycling performance (operated stably for more than 800 cycles at a high-capacity retention rate of 82.4%) and a superior rate capability (300 mA h/g at 5 A/g). After cycling, tiny cracks formed in some areas of the electrode surface, with an expansion rate of only 27.4%. The duple combination of rGO and the unique sandwiched structure presented here demonstrate great effectiveness in improving the electrochemical performance of alloy-type anodes.

A New Structure with Localized sp2 Bonding for Fivefold Twinning in Diamond.

Changes in atomic bonding configuration in carbon from sp3 to sp2 are known to exist in certain structural defects in diamond, such as twin boundaries, grain boundaries, and dislocations, which have a significant impact on many properties of diamond. In this work, the atomic structure of fivefold twinning in detonation synthesized ultra-dispersed diamonds is investigated using a combination of techniques, including spherical aberration-corrected high-resolution electron microscopy (HREM), HREM image simulations, and molecular mechanics (MM) calculations. The experimental HREM images reveal clearly that the fivefold twinning in diamond has two distinct structures. In addition to the concentric fivefold twins, where the core structure is the intersection of five {111} twinning boundaries, a new extended core structure with co-hybridization of bonding is identified and analyzed in fivefold twinning. The atomic structure forming these fivefold twinning boundaries and their respective core structures is proposed to involve both the tetrahedral sp3 and planar graphitic sp2 bonding configurations, in which a co-hybridized planar hexagon of carbon serves as a fundamental structural unit. The presence of this sp2 -bonded planar unit of hexagonal carbon rings in general grain boundaries is also discussed.

Co(OH)2 nanosheet-decorated graphene-CNT composite for supercapacitors of high energy density.

A composite of graphene and carbon nanotubes has been synthesized and characterized for application as supercapacitor electrodes. By coating the nanostructured active material of Co(OH)2 onto one electrode, the asymmetric supercapacitor has exhibited a high specific capacitance of 310 F g-1, energy density of 172 Wh kg-1 and maximum power density of 198 kW kg-1 in ionic liquid electrolyte EMI-TFSI.

Facile synthesis of morphology-controlled hybrid structure of ZnCo2O4 nanosheets and nanowires for high-performance asymmetric supercapacitors.

Controllable synthesis of electrode materials with desirable morphology and size is of significant importance and challenging for high-performance supercapacitors. Herein, we propose an efficient hydrothermal approach to controllable synthesis of hierarchical porous three-dimensional (3D) ZnCo2O4 composite films directly on Ni foam substrates. The composite films consisted of two-dimensional (2D) nanosheets array anchored with one-dimensional (1D) nanowires. The morphologies of ZnCo2O4 arrays can be easily controlled by adjusting the concentration of NH4F. The effect of NH4F in the formation of these 3D hierarchical porous ZnCo2O4 nanosheets@nanowires films is systematically investigated based on the NH4F-independent experiments. This unique 3D hierarchical structure can help enlarge the electroactive surface area, accelerate the ion and electron transfer, and accommodate structural strain. The as-prepared hierarchical porous ZnCo2O4 nanosheets@nanowires films exhibited inspiring electrochemical performance with high specific capacitance of 1289.6 and 743.2 F g-1 at the current density of 1 and 30 A g-1, respectively, and a remarkable long cycle stability with 86.8% capacity retention after 10 000 cycles at the current density of 1 A g-1. Furthermore, the assembled asymmetric supercapacitor using the as-prepared ZnCo2O4 nanosheets@nanowires films as the positive electrode and active carbon as negative electrode delivered a high energy density of 39.7 W h kg-1 at a power density of 400 W kg-1. Our results show that these unique hierarchical porous 3D ZnCo2O4 nanosheets@nanowires films are promising candidates as high-performance electrodes for energy storage applications.

Tuning oxygen-containing functional groups of graphene for supercapacitors with high stability.

To investigate the relationship between the oxygen-containing functional groups of graphene and the stability of supercapacitors, reduced graphene oxide (rGO) containing different oxygenic functional groups was prepared by varying the reduction time of GO using hydrazine as the reducing agent. TEM, XRD, Raman, and XPS characterizations revealed that, as the reduction time increased, the sp2 structure in the rGO sheet was restored and the obtained rGO had good crystallinity accompanied by removal of the oxygenic functional groups. The analysis of the content of the different functional groups also suggested that the reduction rate of the oxygenic functional group was C-O > C[double bond, length as m-dash]O > O-C[double bond, length as m-dash]O. The supercapacitive performance of rGO showed that the oxygenic functional groups contributed to some pseudocapacitance and resulted in a larger specific capacitance. At the same time, however, it is also accompanied by poorer rate performance and durability, which will be improved by removing the oxygenic functional groups by extending the reduction time. With an optimized reaction condition of a reduction time of 24 h, the obtained rGO exhibited excellent stability in floating tests at 3.0 V and 45 °C for 60 days. These findings pave the way for the development of high quality graphene materials for cost-effective and practical graphene supercapacitors.

Achieving High-Energy-Density Graphene/Single-Walled Carbon Nanotube Lithium-Ion Capacitors from Organic-Based Electrolytes.

Developing electrode materials with high voltage and high specific capacity has always been an important strategy for increasing the energy density of lithium-ion capacitors (LICs). However, organic-based electrolytes with lithium salts limit their potential for application in LICs to voltages below 3.8 V in terms of polarization reactions. In this work, we introduce Li[N(C2F5SO2)2] (lithium Bis (pentafluoroethanesulfonyl)imide or LiBETI), an electrolyte with high conductivity and superior electrochemical and mechanical stability, to construct a three-electrode LIC system. After graphite anode pre-lithiation, the anode potential was stabilized in the three-electrode LIC system, and a stable solid electrolyte interface (SEI) film formed on the anode surface as expected. Meanwhile, the LIC device using LiBETI as the electrolyte, and a self-synthesized graphene/single-walled carbon nanotube (SWCNT) composite as the cathode, showed a high voltage window, allowing the LIC to achieve an operating voltage of 4.5 V. As a result, the LIC device has a high energy density of up to 182 Wh kg-1 and a 2678 W kg-1 power density at 4.5 V. At a current density of 2 A g-1, the capacity retention rate is 72.7% after 10,000 cycles.

Carbon composite microelectrodes fabricated by electrophoretic deposition.

Single-walled carbon nanotubes were deposited on one end of the etched carbon fiber electrodes by an electrophoretic method. The carbon nanotube bundles formed a dense network on the carbon fiber surface. The electrochemical properties of the composite carbon electrodes were studied in the buffered neutral solutions. The results in cyclic voltammetry's characteristic indicate that the electrons on the electrodes transfer very fast. Furthermore, the redox reactions of dopamine (DA) on the composite electrodes show good sensitivity. When the DA concentration was 0.02 mM, the peak current in differential pulse technique reached 1.33 microA after performing the background subtraction. In addition, the simultaneous detection of DA and ascorbic acid (AA) showed that the interference effect was not observed. It was suggested that the carbon composite microelectrodes have potential applications as electrochemical sensors inside a single cell.

Effects of low work-function lanthanum oxides on stable electron field emissions from nanoscale emitters.

Nanoscale electron field emitters are known to produce more stable electron emissions than conventional emitters. This has been attributed to size effects; nanoscale emitters can operate with a small emission current and a low extraction voltage, which reduces the bombardment of residual gas ions on the emitter tip. However, our experiments discovered that nanoscale LaB6 emitters had extremely stable emissions, suggesting that chemical effects are present in addition to size effects. This suggests that during operations, a material other than LaB6 may be deposited on the surface of the tip to enhance the stability of emissions. Therefore, we searched for possible materials theoretically within the La-B-O ternary system and found that lanthanum oxides (LaO) and oxygen-deficient La2O3 (La2O3-x ) had good electrical conductivity and a low work function comparable to that of LaB6. These lanthanum oxides are chemically less reactive to residual gases than LaB6. Thus, if they are present on the LaB6 surface, they could stabilize electron emissions without diminishing the emission performance. These findings suggest that lanthanum oxides could be used for electron field emitters.

Facile preparation of flexible binder-free graphene electrodes for high-performance supercapacitors.

A facile two-step strategy to prepare flexible graphene electrodes has been developed for supercapacitors using thermal reduction of graphene oxide (GO) and thermally reduced graphene oxide (TRGO) composite films. The tunable porous structure of the GO/TRGO film provided channels to release the high pressure generated by CO2 gas. The graphene electrode obtained from reduced-GO/TRGO (1 : 1 in mass ratio) film showed great flexibility and high film density (0.52 g cm-3). Using the EMI-BF4 electrolyte with a working voltage of 3.7 V, the as-fabricated free-standing reduced-GO/TRGO (1 : 1) film achieved a great gravimetric capacitance of 180 F g-1 (delivering a gravimetric energy density of 85.6 W h kg-1), a volumetric capacitance of 94 F cm-3 (delivering a volumetric energy density of 44.7 W h L-1), and a 92% retention after 10 000 charge/discharge cycles. In addition, the solid state flexible supercapacitor with the free-standing reduced-GO/TRGO (1 : 1) film as the electrodes and the EMI-BF4/poly (vinylidene fluoride hexafluopropylene) (PVDF-HFP) gel as the electrolyte also demonstrated a high gravimetric capacitance of 146 F g-1 with excellent mechanical flexibility, bending stability, and electrochemical stability. The strategy developed in this study provides great potentials for the synthesis of flexible graphene electrodes for supercapacitors.

Direct measurement of the friction between and shear moduli of shells of carbon nanotubes.

We report measurements of the shear modulus of each shell and the friction between the two shells of double-shell carbon nanotubes in single nanotube-based nanoelectromechanical devices operated in a transmission electron microscope. In situ nanobeam electron diffraction is applied to obtain the chiral indices of each shell of the nanotube and it allows us to establish a quantitative correlation between the atomic structure and properties of the nanotube under investigation.

Rapid and precise scanning helium ion microscope milling of solid-state nanopores for biomolecule detection.

We report the formation of solid-state nanopores using a scanning helium ion microscope. The fabrication process offers the advantage of high sample throughput along with fine control over nanopore dimensions, producing single pores with diameters below 4 nm. Electronic noise associated with ion transport through the resultant pores is found to be comparable with levels measured on devices made with the established technique of transmission electron microscope milling. We demonstrate the utility of our nanopores for biomolecular analysis by measuring the passage of double-strand DNA.

Graphene and carbon nanotube composite electrodes for supercapacitors with ultra-high energy density.

We describe a graphene and single-walled carbon nanotube (SWCNT) composite film prepared by a blending process for use as electrodes in high energy density supercapacitors. Specific capacitances of 290.6 F g(-1) and 201.0 F g(-1) have been obtained for a single electrode in aqueous and organic electrolytes, respectively, using a more practical two-electrode testing system. In the organic electrolyte the energy density reached 62.8 Wh kg(-1) and the power density reached 58.5 kW kg(-1). The addition of single-walled carbon nanotubes raised the energy density by 23% and power density by 31% more than the graphene electrodes. The graphene/CNT electrodes exhibited an ultra-high energy density of 155.6 Wh kg(-1) in ionic liquid at room temperature. In addition, the specific capacitance increased by 29% after 1000 cycles in ionic liquid, indicating their excellent cyclicity. The SWCNTs acted as a conductive additive, spacer, and binder in the graphene/CNT supercapacitors. This work suggests that our graphene/CNT supercapacitors can be comparable to NiMH batteries in performance and are promising for applications in hybrid vehicles and electric vehicles.

Nanostructured LaB6 field emitter with lowest apical work function.

LaB(6) nanowires are ideal for applications as an electrical field-induced ion and electron point source due to their miniature dimensions, low work function, as well as excellent electrical, thermal, and mechanical properties. We present here a reliable method to fabricate and assemble single LaB(6) nanowire-based field emitters of different crystal orientations. The atomic arrangement, emission brightness from each crystal plane, and field emission stability have been characterized using field ion microscopy (FIM) and field emission microscopy (FEM). It is found that the 001 oriented LaB(6) nanowire emitter has the highest field emission symmetry while the 012 oriented LaB(6) nanowire has the lowest apical work function. The field emission stability from the single LaB(6) nanowire emitter is significantly better than either the LaB(6) needle-type emitter or W cold field emitters.

Roles of water in the formation and preparation of graphene oxide.

The functional groups and physical properties of graphene oxide (GO) are found to be sensitive to and can be controlled by the water content in the reactions when GO samples are prepared at different concentrations of sulfuric acid using a modified Hummers method. GO prepared with 93% sulfuric acid (H2SO4) showed fewer structural defects, less π-π conjugation, and larger interlayer spacing than GO prepared with 99% H2SO4. The intensity ratio of the D-band to the G-band of the Raman spectrum is 0.89 ± 0.01 and 1.02 ± 0.01, corresponding to average interlayer spacing of 0.91 nm and 0.86 nm, respectively. The yield and carbon to oxygen ratio of the GO sheets prepared from different concentrations of H2SO4 are nearly identical. More importantly, compared with GO synthesized with 99% H2SO4, GO prepared with 93% H2SO4 contains more carbon-oxygen single bonds, such as epoxy groups and hydroxyl groups, but fewer carbonyl groups.

Correction: Roles of water in the formation and preparation of graphene oxide.

[This corrects the article DOI: 10.1039/D0RA10026A.].

A stable LaB6 nanoneedle field-emission point electron source.

A material with a low work function exhibiting field-emission of electrons has long been sought as an ideal point electron source to generate a coherent electron beam with high brightness, long service life, low energy spread, and especially stable emission current. The quality and performance of the electron source are now becoming limiting factors for further improving the spatial resolution and analytical capabilities of the electron microscope. While tungsten (W) is still the only material of choice as a practically usable field emission filament since it was identified more than six decades ago, its electron optical performance remains unsatisfactory, especially the poor emission stability (>5% per hour), rapid current decay (20% in 10 hours), and relatively large energy spread (0.4 eV), even in an extremely high vacuum (10-9 Pa). Herein, we report a LaB6 nanoneedle structure having a sharpened tip apex with a radius of curvature of about 10 nm that is fabricated and finished using a focused ion beam (FIB) and show that it can produce a field emission electron beam meeting the application criteria with a high reduced brightness (1010 A m-2 sr-1 V-1), small energy spread (0.2 eV), and especially high emission stability (<1% fluctuation in 16 hours without decay). It can now be used practically as a next-generation field-emission point electron source.

Effect of porous structural properties on lithium-ion and sodium-ion storage: illustrated by the example of a micro-mesoporous graphene1-x (MoS2) x anode.

In this work, we synthesized micro-mesoporous graphene1-x (MoS2) x with different compositional ratios via co-reduction of graphite oxide and exfoliated MoS2 platelets. We systematically studied the performance of the micro-mesoporous graphene1-x (MoS2) x as anodes in lithium-ion batteries and sodium-ion batteries. The results show that the specific surface areas of the composites decrease with introducing MoS2. The irreversible capacitance, which is related to the formation of solid electrolyte interphases, also decreases. Besides specific surface area, we found that micropores can benefit the lithiation and sodiation. We demonstrated that a specific charge capacity of 1319.02 mA h g-1 can be achieved at the 50th cycle for the graphene½(MoS2)½ anode in lithium-ion batteries. Possible relationships between such a high specific capacity and the micro-mesoporous structure of the graphene1-x (MoS2) x anode are discussed. This work may shed light on a general strategy for the structural design of electrode materials in lithium-ion batteries and sodium-ion batteries.

Stable field-emission from a CeB6 nanoneedle point electron source.

A single CeB6 nanoneedle structure has been fabricated using a focused ion beam (FIB) and its field emission characteristics have been evaluated. A converged electron beam has been obtained, attributed to its sharpened tip with a radius of curvature of about 10 nm. Combined with its low work function, the required electric field is as low as 1.6 V nm-1 to generate a field emission current of 50 nA. The most outstanding feature of the CeB6 nanoneedle emitter is its excellent current stability that enabled continuous emission for 16 hours with a fluctuation of 1.6% and without deterioration even in a vacuum of 10-7 Pa. The stable field-emission is attributed to the nanometric tip radius that led to reduction in gas adsorption and desorption. In addition, the downward dipolar structure on the emission surface is also beneficial for making the surface inert. These performance factors make CeB6 a practical field-emission point electron source for microscopy applications.

Effects of surfactants on spinning carbon nanotube fibers by an electrophoretic method.

Thin fibers were spun from a colloidal solution of single-walled carbon nanotubes (SWNTs) using an electrophoretic method. Sodium dodecylbenzenesulfonate (NaDDBS) was chosen as a surfactant and showed good performance owing to its special chemical structure. The highest spinning velocity reached 0.5 mm s-1. The resulting SWNT fibers had a tensile strength of 400 MPa and a conductivity of 355 S cm-1. Their mechanical and electrical properties were markedly improved after adding NaDDBS as the dispersant in water.