Formation of Silicon/Graphene Heterostructures through Co-Gas-Phase Synthesis

The introduction of 2D materials as an active or enabling component is of significant interest in the field of electrochemical energy storage because it provides properties such as mechanical stability, large surface area and at the same time low material requirements. Furthermore, graphene-like 2D structures show strong electrical conductivity and electrochemical stability. While each individual 2D material has unique advantages, the deliberate assembly of heterostructures made up of different flakes in configurations such as 2D/2D or particle-flake (0D/2D) emerges as an important advancement. Such finely built heterostructures not only improve functioning but also have potential to push the performance of energy storage devices to levels that were not previously possible. A simple, competitive and scalable strategy to produce heterostructures by combining 0D nanoparticles and 2D graphene in the gas phase is reported in this research work. Gas-phase synthesis of graphene is known for more than a decade which is now further developed to give production rates of few hundred mg/h of few-layer graphene (FLG). By combining two independent gas phase reactors, this few-layer graphene is controllably mixed with other functional materials in the gas phase to obtain heterostructures without the use of structure directing agents. The reactor system was involving a microwave plasma and a hot wall reactor. Silicon nanoparticles were synthesized in the hot wall reactor, meanwhile graphene was produced in the MW plasma reactor. Both reactors were connected in such a way that the nanoparticles and graphene get in contact with each other directly in the gas phase after their inception. In this way the self-assembled heterostructures of graphene and nanoparticles are produced with the production rate reaching almost 800 mg/h. The materials were characterized with TEM, Raman spectroscopy, XPS and XRD to investigate the morphology, composition and structure. TEM images indicate silicon is present as almost spherical nanoparticles that form small aggregates. These particles are interconnected by graphene. EDX mapping shows silicon nanoparticles having graphene surrounding them. It is present between the aggregates of nanoparticles as well as connecting them to each other. Raman as well as XRD spectra confirmed the presence of silicon and graphene in heterostructures with extraordinary purity. The presence of silicon in crystalline as well as amorphous form was also confirmed from Raman spectra. It will be shown that the successful formation of 0D/2D silicon/graphene heterostructures used as anode material for lithium-ion batteries has illustrated some significant improvements. Compared to pure silicon, their combination results in excellent long-term stability of the anode while also increasing its Coulombic efficiency. Thus, the formation of these heterostructures in the gas phase offers the possibility to exploit the potential of silicon to significantly increase the anode storage capacity.