(a) A low-magnification tunneling electron microscope (TEM) image of Gr–Si nanoparticle (NP). (b) A higher-magnification image for the same Gr–Si NP from the white box in a. (Insets) The line profiles from the two red boxes indicate that the interlayer spacing between graphene layers is ~3.4 Å, in good agreement with that of typical graphene layers based on van der Waals interaction. (c) A high-magnification TEM image visualizing the origins (red arrows) from which individual graphene layers grow. (d) A schematic illustration showing the sliding process of the graphene coating layers that can buffer the volume expansion of Si.
Silicon is receiving considerable attention as an active material for next- generation lithium-ion battery anodes because of its gravimetric capacity--a measure in mAh/g of the total charge capacity stored by the cell or battery, per gram of the battery's weight.
Unfortunately, the large volume change of silicon during charge–discharge cycles has in the past weakened its competitiveness. Recently, however, a research group from Samsung reported in the publication Nature Communications that using direct graphene growth over silicon nanoparticles without silicon carbide formation resulted in a near doubling of run-time by expanding energy density-- the amount of stored power in a given area -- to 1.8 times that of current batteries.
The graphene layers anchored onto the silicon surface accommodate the volume expansion of silicon via a sliding process between adjacent graphene layers. When paired with a commercial lithium cobalt oxide cathode, the silicon carbide-free graphene coating allows the full cell to reach volumetric energy densities of 972 and 700 Wh l−1 at first and 200th cycle, respectively, 1.8 and 1.5 times higher than those of current commercial lithium-ion batteries.
This observation suggests that a two-dimensional layered structure of graphene and its silicon carbide-free integration with silicon can serve as a prototype toward advancing silicon anodes to commercially viable technology.