Building on previous knowledge acquired through research on thin-film batteries, three-dimensional (3D) porous macroscopic particles consisting of curved two-dimensional (2D) nano-structures of Si may bring unique advantages for Si anode technology. Prior work on thin Si films showed that during Li insertion, large-area Si films mostly accommodate the volume changes via variation in thickness. Therefore, the changes in the external surface area can fundamentally be minimized and thus, formation of a stable, solid electrolyte interphase (SEI) should be easier to achieve. In contrast, Si nanoparticles expand uniformly in all dimensions and thus, their outer surface area (where SEI forms) changes dramatically during insertion/extraction of Si. The low elasticity of the SEI makes it difficult to achieve the long-term stability under cycling load. Further, thin Si films have lower surface area (for the same mass), in comparison to Si nanoparticles, and better potential for achieving low irreversible capacity losses on the first and subsequent cycles.
Thin Si films coated on porous 3D particles composed of curved 2D graphene sheets have been synthesized utilizing techniques that allow for tunable properties. Since graphene exhibits specific surface area up to 100 times higher than carbon black or graphite, the deposition of the same mass of Si on graphene is much faster in comparison — a factor which is important for practical applications. In addition, the distance between graphene layers is tunable and variation in the thickness of the deposited Si film is feasible. Both of these characteristics allow for optimization of the energy and power characteristics. Thicker films will allow higher capacity, but slower rate capabilities. Thinner films will allow more rapid charging, or higher power performance.
In this innovation, uniform deposition of Si and C layers on high-surface-area graphene produced granules with specific surface area (SSA) of ≈ 5 m2g–1. The over 100 times reduction in SSA of the initial graphene material is important for high Coulombic efficiencies on the first and subsequent cycles. Here, the low surface area of the composite resulted in an average Coulombic efficiency in excess of 99%. The anode composed of the nanocomposite particles exhibited specific capacity in excess of 2,000 mAhg–1 at the current density of 140 mAg–1 and excellent stability for 150 cycles, significantly exceeding the theoretical capacity of graphite and graphene. While only a Si-containing composite was demonstrated, the synthesis techniques utilized are applicable for other high-capacity materials that can be conformally coated on graphene. Similarly, while only curved graphene was used as a substrate for Si deposition, other curved, thin, 2D substrates (which do not react with Si precursors) may be used for conformal coating by Si or other high-capacity materials.
This work was performed by Gleb Yushin, Kara Evanoff, and Alexander Magasinski of Georgia Institute of Technology for Glenn Research Center.
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