Spherical Carbon with Unique Architectures and Properties

V.G. Pol, K.C. Lau, L.A. Curtiss, J.G. Wen, D.J. Miller, and M.M. Thackeray, Argonne National Laboratory

Carbon atoms can be arranged in different ways to yield various structure types, such as graphene (and graphite), diamond, and fullerenes, e.g., hollow nanotubes and spherical ‘buckyballs.’ The properties of carbon are controlled at the nanometer length scale, so there is great potential to tailor their structures during synthesis. Argonne researchers at the Energy Frontier Research Center, the Center for Electrical Energy Storage: Tailored Interfaces, have used a high temperature, high pressure autogenic process to fabricate dense and mono-dispersed carbon spheres (2–4 µm in diameter) (Fig. 1a). The process involves the decomposition of hydrocarbon precursors, e.g., mesitylene (C₉H₁₂) or plastic waste (polyethylene), above the critical temperature for decomposition, e.g., 700°C. Spheres synthesized at this temperature have smooth surfaces and are comprised of turbostratically disordered carbon (inset, Fig. 1a).

The graphitic character of the carbon spheres can be increased by heating the spheres to 2800°C in an inert atmosphere. Even though the spheres remain intact (Fig. 1b), there is evidence of sintering and a significant roughening of the surface, characterized by curved, dome-like graphitic sheets (inset, Fig. 1b). Classical molecular dynamics methods were used to simulate the changes in crystal structure and bonding of the spheres when heated from 273 to 2730°C. At 2730°C, the carbon spheres were predicted to be defective and comprised of a mixture of sp² graphene fragments with sp³ linkages (Fig. 1c).

These carbon spheres have technological implications. They have been investigated as an intercalation anode for lithium-ion batteries; if heated between 2400 and 2800°C, the carbon spheres remain intact on repeated lithiation and delithiation, yielding a stable electrochemical capacity of approximately 250 mAh/g. These materials also have interesting tribological properties.

Three images, the first two of which (labeled 'a' and 'b') are black and white SEM and TEM images of spherical carbon particles, further described in the caption, and the third of which (labeled 'c') shows a gray and red diagram of carbon nanoparticles with simulated defects that are described in the caption.

Fig. 1. SEM and TEM (inset) images of spherical carbon particles: a) prepared at 700°C, and b) after heating at 2800°C; c) Simulated defects in carbon nanoparticles (T = 3000 K) at edges, surfaces, and within the bulk.

Reference: V.G. Pol and M.M. Thackeray, “Spherical Carbon Particles and Carbon Nanotubes Prepared by Autogenic Reactions: Evaluation as Anodes in Lithium Electrochemical Cells,” Energy & Environmental Science, 2011, 4, 1904–1912.

July 2012