Wanci Shen

Nitrogen-enriched electrospun porous carbon nanofiber networks as high-performance free-standing electrode materials

Nitrogen-enriched porous carbon nanofiber networks (NPCNFs) were successfully prepared by using low-cost melamine and polyacrylonitrile as precursors via electrospinning followed by carbonization and NH3 treatments. The NPCNFs exhibited inter-connected nanofibrous morphology with a large specific surface area, well-developed microporous structure, relatively high-level nitrogen doping and great amount of pyridinic nitrogen. As free-standing new anode materials in lithium-ion batteries (LIBs), the NPCNFs showed ultrahigh capacity, good cycle performance and superior rate capability with a reversible capacity of as high as 1323 mA h g−1 at a current density of 50 mA g−1. These attractive characteristics make the NPCNFs materials very promising anode candidates for high-performance LIBs and, as free-standing electrode materials to be used in other energy conversion and storage devices.

Facile synthesis of nitrogen-doped carbon nanosheets with hierarchical porosity for high performance supercapacitors and lithium–sulfur batteries

Magnesium citrate and potassium citrate are two commonly used food additives in our daily life. Herein, we prepared nitrogen-doped hierarchical porous carbon nanosheets (N-HPCNSs) through direct pyrolysis of their mixtures and subsequent NH3 treatment. The as-prepared N-HPCNS shows hierarchical porosity (specific surface area of 1735 m2 g−1 and pore volume of 1.71 cm3 g−1), and a moderate nitrogen doping of 1.7%. Moreover, it can be effectively applied in various energy storage/conversion systems. When used as supercapacitor electrodes, it shows a high specific capacitance of 128 F g−1 in organic electrolytes and retains 45% of the original capacitance even at an ultrahigh current density of 100 A g−1. It can also serve as an effective sulfur carrier in lithium–sulfur batteries. The N-HPCNS/sulfur cathode shows high discharge capacities of 1209 mA h g−1 at 0.2C and 493 mA h g−1 even at 4C. Over 500 charge/discharge cycles at 1C, it still retains a high discharge capacity of 486 mA h g−1 with an ultralow capacity loss of 0.051% per cycle and a high average coulombic efficiency of 99.4%.

A comparative investigation on multiwalled carbon nanotubes and carbon black as conducting additive in LiNi0.7Co0.3O2

A comparative investigation was carried out on multiwalled carbon nanotubes (MWCNTs) and carbon black as a conducting additive in LiNi 0.7 Co 0.3 O 2 for lithium ion batteries. All LiNi 0.7 Co 0.3 O 2 particles were connected together by well-crystallized MWCNTs to form a conductive network wiring. The discharge capacities of the composite cathode were improved to be 223 mAh/g with 89.9% efficiency at the C/10 rate and 214 mAh/g at the 1C rate in the initial cycle when carbon black was replaced by MWCNTs. The results proved that MWCNTs wrapping was an effective way to improve electron conducting and reversible capacity with high cycle efficiency of cathode materials.

Electrospinning Preparation of Nanosilicon/Disordered Carbon Composite as Anode Materials in Li-Ion Battery

Electrospinning is employed to prepare Si/C composite anode materials. By electrospinning of Si-poly(vinyl alcohol) and by subsequent pyrolysis under argon flow, the nanosilicon particles coated by disordered carbon are uniformly and tightly embedded in the irregular porous network of a disordered carbon matrix, which leads to the gradual release of large capacity from the pure Si component in the composite anode. After 50 cycles, the reversible discharge capacity of the Si/C composite can reach as high as 892.5 mAh/g.

New origin of spirals and new growth process of carbon whiskers

Abstract A new kind of carbon whisker, different from others, has been found by means of high resolution electron microscopy and field emission scanning electron microscopy. Carbon layers are almost perpendicular to the whisker axis and a spiral structure is formed around it. Structure analysis indicates the growth process consists of two stages; firstly, carbon layers stacked in turbostratic manner, and then gradually graphitized. Accordingly, the final structure of whiskers is perfect graphitized texture. The spirals are not similar to the so-called ‘growth spirals’ formed by the screw dislocation mechanism, and a more reasonable growth mechanism of whiskers is suggested in accordance with the analysis.

A new method synthesizing the encapsulated ZrC with graphitic layers

Abstract A new method has been found to synthesize the filled carbon particles, which is different from other methods for synthesizing the encapsulated carbides. It involves high energy milling the carbon with ZrO2 ball and succedent heat treatment in 1800°C. In addition, a particle with sunken surface is observed while surface of others are convex.

Silicon-Encapsulated hollow carbon nanofiber networks as binder-free anodes for lithium ion battery

Silicon-encapsulated hollow carbon nanofiber networks with ample space around the Si nanoparticles (hollow Si/C composites) were successfully synthesized by dip-coating phenolic resin onto the surface of electrospun Si/PVA nanofibers along with the subsequent solidification and carbonization. More importantly, the structure and Si content of hollow Si/C composite nanofibers can be effectively tuned by merely varying the concentration of dip solution. As-synthesized hollow Si/C composites show excellent electrochemical performance when they are used as binder-free anodes for Li-ion batteries (LIBs). In particular, when the concentration of resol/ethanol solution is 3.0%, the product exhibits a large capacity of 841mAh g-1 in the first cycle, prominent cycling stability, and good rate capability. The discharge capacity retention of it was ∼90%, with 745mAh g-1 after 50 cycles. The results demonstrate that the hollow Si/C composites are very promising as alternative anode candidates for high-performance LIBs.

Nitrogen-enriched hierarchical porous carbon with enhanced performance in supercapacitors and lithium–sulfur batteries

It is quite desirable but challenging to prepare highly active materials for various energy storage applications at low cost. Here, an efficient strategy to produce nitrogen-enriched hierarchical porous carbon (N-HPC) is reported by facile pyrolysis of magnesium citrate and subsequent NH3 treatment. As-prepared N-HPC presents a developed hierarchical micro- and trimodal meso-porosity with a high specific surface area of 1290 m2 g−1 and pore volume of 3.04 cm3 g−1. It also shows an abundant nitrogen doping of 3.6%. When used for electrochemical electrodes in supercapacitors and lithium–sulfur (Li–S) batteries, significantly enhanced performances have been obtained compared with commercially available activated carbon. In supercapacitor testing, the N-HPC electrode shows a specific capacitance of 101 F g−1 in a nonaqueous electrolyte. And the capacitance retains 67% even at a 200-fold charge/discharge rate. Moreover, its performance in Li–S batteries is more outstanding. It enables a very high sulfur loading (76.2% by weight) and the resulting N-HPC/S cathode shows high discharge capacities of 1153 mA h g−1sulfur (or 702 mA h g−1electrode) at 0.2C and 671 mA h g−1 even at 4C. And it still remains 600 mA h g−1 over 300 charge/discharge cycles at 1C with an average coulombic efficiency of 99.0%.

Whiskers with apex angle 135° growing by a disclination growth mechanism

A type of carbon whiskers was synthesized at 2100°C. The field emission scanning electron microscopy (FESEM) shows that there are spirals in the whiskers. Transmission electron microscopy (TEM) shows that the carbon layers of the whiskers are almost perpendicular to the whisker axis. The apex angle of the whisker with the conical shape is about 135°, which is in agreement with one of the apex angles predicted by Double and Hellawell, was not observed till now. It is inferred that the growth mechanism should be disclination one. In addition, the initiation of the disclination might be related to the zirconium carbides (ZrC).

A kind of carbon whiskers in new structure and morphology

Carbon whiskers with new structure and morphology were observed when heating the milled graphite. Transmission electron microscopy and high resolution electron microscopy (HREM) show that carbon layers are almost perpendicular to the growth axes of carbon whiskers. Field emission scanning electron microscopy (FESEM) indicates that there are spirals appearing on the surface of the whiskers. The structure analysis shows that the growth mechanism of carbon whiskers is related to the trace amount of ZrC in the heated samples.