Page 302 - Handbook of Battery Materials

272 10 Carbons

of graphite, and there are regions in the carbon particles which appear to be
amorphous. These observations are typical for carbon blacks which are heat-treated
at graphitizing temperatures. The particle size restricts the motion of the layer
planes and the stresses that result inhibit the formation of a highly graphitized
structure that is similar to that of pure graphite.
A terminology to identify carbons that are graphitizable or those that are
nongraphitizable by heat treatment has been adopted. Hard carbons are those
carbons that are nongraphitizable and are mechanically hard – hence the name. In
contrast, soft carbons are mechanically soft and can be graphitized. Hard carbons
are obtained by carbonizing precursors such as thermosetting polymers (e.g.,
phenol-formaldehyde resins), furfuryl alcohol, divinylbenzene-styrene copolymer,
cellulose, charcoal, and coconut shells. These carbons are usually formed by
solid-state transformation during the carbonization steps. One explanation for
the inability of hard carbons to form a graphitic structure by heat treatment
3
is the presence of strong sp crosslinking bonds which impede movement and
reorientation of the carbon atoms to form the ordered layer structure of graphite.
Soft carbons are formed by carbonizing precursors such as petroleum coke, oil,
and coal-tar pitch. In these materials the formation of carbon proceeds through
an intermediate liquid-like phase (referred to as a mesophase), which facilitates the
three-dimensional ordering that is necessary to create a graphite-like structure.
Besides the discussion by Kinoshita [1], an extensive review that describes the
formation of carbonaceous materials and their physical properties is presented by
researchers from Japan [7].
Natural graphite is classified as flake, vein, or microcrystalline (amorphous),
depending on the crystallite size and particle shape. Major sources of natural
graphite are found in Mexico, China, and Brazil. Flake graphite is anisotropic
and has a crystallinity similar to that of single-crystal graphite. One problem with
many sources of natural graphite is their ash content (e.g., Fe, Si), which can be
as high as 25%. Much of this ash can be removed by leaching in concentrated
acid or exposure to halogen gases. Synthetic or artificial graphite is produced by
heat treatment of a precursor carbon such as petroleum coke to temperatures in
the region of 2800 C or higher. Solid graphite structures for bipolar separators or
◦
electrode substrates for batteries are obtained by extrusion or molding of blended
mixtures of petroleum coke and a binder of coal-tar pitch which are heat-treated to
graphitization temperatures [8].
A variety of amorphous carbons such as carbon black, active carbon, and glassy
carbon is available. With the exception of glassy carbon, these amorphous carbons
generally have high surface area, high porosity, and small particle size. Carbon
−1
2
blacks, for example, are available with surface areas that are >1000 m g , particle
size <50 nm, and density much less than the theoretical value for graphite (2.25 g
−3
cm ). In addition, the morphology of carbon blacks may resemble individual
spheres of about 250 nm diameter (i.e., thermal blacks) or a cluster of fused carbon
particles of <50 nm diameter (i.e., furnace blacks). The morphology of carbon
black particles has been the subject of much discussion [9–12]. Active carbons are
typically granular carbons which are produced by carbonizing materials such as