Oxygen bridges between NiO nanosheets and graphene for improvement of lithium storage

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.