The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Nitrogen-doped graphene can be used for lithium storage, but the nitrogen content is often quite low. Here, the authors synthesize graphene analogous with high nitrogen content using a zeolitic imidazolate framework, which show exceptional battery performances.

/pdf/high-lithium-anodic-performance-of-highly-nitrogen-doped-2f1npjwt34.pdf

High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework

In this Letter, we reported on the preparation and Li-ion battery anode application of ultrasmall Sn nanoparticles (∼5 nm) embedded in nitrogen-doped porous carbon network (denoted as 5-Sn/C). Pyrolysis of Sn(Salen) at 650 °C under Ar atmosphere was carried out to prepare N-doped porous 5-Sn/C with the BET specific surface area of 286.3 m2 g–1. The 5-Sn/C showed an initial discharge capacity of 1014 mAh g–1 and a capacity retention of 722 mAh g–1 after 200 cycles at the current density of 0.2 A g–1. Furthermore, a reversible capacity of ∼480 mAh g–1 was obtained at much higher current density of 5 A g–1. The remarkable electrochemical performance of 5-Sn/C was attributed to the effective combination of ultrasmall Sn nanoparticles, uniform distribution, and porous carbon network structure, which simultaneously solved the major problems of pulverization, loss of electrical contact, and particle aggregation facing Sn anode.

Ultrasmall Sn Nanoparticles Embedded in Nitrogen-Doped Porous Carbon As High-Performance Anode for Lithium-Ion Batteries

Heteroatom modification represents one of the largest studied areas of research related to nanostructured carbon materials, with integrated applications stretching from energy production and storage to sustainability and medical uses. While a wide variety of dopants (boron, phosphorus, iodine, fluorine, etc.) have been studied, doping carbon structures with nitrogen ad-atoms has arguably experienced the greatest progress and brought the most attention over the last several years. Research in this field has conclusively demonstrated that nitrogen doping is an effective way to tailor the properties of carbon and tune the material for various applications of interest. This review provides a comprehensive overview of advances in the last half decade on state-of-the-art carbon modification with nitrogen heteroatoms. Improvements in well-established fabrication/modification processes are discussed as well as novel strategies. Additionally, recent theoretical and experimental findings related to the benefits and effects of nitrogen modification for specific applications in the energy and environmental fields are reviewed.

Recent progress on nitrogen/carbon structures designed for use in energy and sustainability applications

Graphitic carbon nitride (g-C3N4) behaving as a layered feature with graphite was indexed as a high-content nitrogen-doping carbon material, attracting increasing attention for application in energy storage devices. However, poor conductivity and resulting serious irreversible capacity loss were pronounced for g-C3N4 material due to its high nitrogen content. In this work, magnesiothermic denitriding technology is demonstrated to reduce the nitrogen content of g-C3N4 (especially graphitic nitrogen) for enhanced lithium storage properties as lithium ion battery anodes. The obtained nitrogen-deficient g-C3N4 (ND-g-C3N4) exhibits a thinner and more porous structure composed of an abundance of relatively low nitrogen doping wrinkled graphene nanosheets. A highly reversible lithium storage capacity of 2753 mAh/g was obtained after the 300th cycle with an enhanced cycling stability and rate capability. The presented nitrogen-deficient g-C3N4 with outstanding electrochemical performances may unambiguously promot...

Nitrogen-Deficient Graphitic Carbon Nitride with Enhanced Performance for Lithium Ion Battery Anodes

In this article, the structure and morphology of the carbon anode materials with different dimensions have been characterized through SEM and TEM. The performances of electrochemical intercalation and deintercalation of lithium-ions have been studied. The results show that graphene as the two dimensional nanomaterials possess more advantages of microstructure and better Li-ions intercalation performances than carbon nanotubes (CNTs) and graphite. The superior abilities of Li-ions intercalation and deintercalation are attributed to increasing lithium storage space, decreasing Li diffusion distance, and higher specific surface area for Li-ions.

Carbon Nanomaterials with Different Dimensions for Anode of Li-Ion Batteries

The graphene sheets were obtained using rapid thermal expansion in nitrogen atmosphere from graphene oxide sheets prepared from graphite powder through improved Hummers’ method. The structure and morphology of the graphene sheets as anode materials with different dimensions were characterized through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscope (SEM). The electrochemical performances were evaluated in coin-type cells versus metallic lithium. The results showed that graphene as the two dimensional nanomaterials possessed more advantages in microstructure and better Li-ions intercalation performances than the traditional materials.

Synthesis and Characterization of Graphene Sheets as an Anode Material for Lithium-Ion Batteries

Lithium storage performance of carbon nanotubes with different nitrogen contents as anodes in lithium ions batteries

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Carbon Nanomaterials with Different Dimensions for Anode of Li-Ion Batteries

Synthesis and Characterization of Graphene Sheets as an Anode Material for Lithium-Ion Batteries

Lithium storage performance of carbon nanotubes with different nitrogen contents as anodes in lithium ions batteries