No abstract

Graphene has been widely used to dramatically improve the capacity, rate capability, and cycling performance of nearly any electrode material for batteries. However, the binding between graphene and these electrode materials has not been clearly elucidated. Here we report oxygen bridges between graphene with oxygen functional groups and NiO from analysis by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy and confirm the conformation of oxygen bridges by the first-principles calculations. We found that NiO nanosheets (NiO NSs) are bonded strongly to graphene through oxygen bridges. The oxygen bridges mainly originate from the pinning of hydroxyl/epoxy groups from graphene on the Ni atoms of NiO NSs. The calculated adsorption energies (1.37 and 1.84 eV for graphene with hydroxyl and epoxy) of a Ni adatom on oxygenated graphene by binding with oxygen are comparable with that on graphene (1.26 eV). However, the calculated diffusion barriers of the Ni adatom on the oxygenated graphene surface (2.23 and 1.69 eV for graphene with hydroxyl and epoxy) are much larger than that on the graphene (0.19 eV). Therefore, the NiO NS is anchored strongly on the graphene through a C-O-Ni bridge, which allows a high reversible capacity and excellent rate performance. The easy binding/difficult dissociating characteristic of Ni adatoms on the oxygenated graphene facilitates fast electron hopping from graphene to NiO and thus the reversible lithiation and delithiation of NiO. We believe that the understanding of this oxygen bridge between graphene and NiO will lead to the development of other high-performance electrode materials.

Although the rechargeable lithium–sulfur battery is an advanced energy storage system, its practical implementation has been impeded by many issues, in particular the shuttle effect causing rapid capacity fade and low Coulombic efficiency. Herein, we report a conductive porous vanadium nitride nanoribbon/graphene composite accommodating the catholyte as the cathode of a lithium–sulfur battery. The vanadium nitride/graphene composite provides strong anchoring for polysulfides and fast polysulfide conversion. The anchoring effect of vanadium nitride is confirmed by experimental and theoretical results. Owing to the high conductivity of vanadium nitride, the composite cathode exhibits lower polarization and faster redox reaction kinetics than a reduced graphene oxide cathode, showing good rate and cycling performances. The initial capacity reaches 1,471 mAh g−1 and the capacity after 100 cycles is 1,252 mAh g−1 at 0.2 C, a loss of only 15%, offering a potential for use in high energy lithium–sulfur batteries.

A unique sandwich structure is designed with pure sulfur between two graphene membranes, which are continuously produced over a large area, as a very simple but effective approach for the fabrication of Li–S batteries with ultrafast charge/discharge rates and long lifetimes.

Amorphous carbon nitride (ACN) with a bandgap of 1.90 eV shows an order of magnitude higher photocatalytic activity in hydrogen evolution under visible light than partially crystalline graphitic carbon nitride with a bandgap of 2.82 eV. ACN is photocatalytically active under visible light at a wavelength beyond 600 nm.