Page 466 - Handbook of Battery Materials

15.2 Graphitic and Nongraphitic Carbons 439

less common ABC (rhombohedral graphite). Due to the small amount of required
energy for transformation of AB into ABC stacking (and vice versa), perfectly
stacked graphite crystals are not readily available. For instance, typically about 5%
of the graphene layers in natural graphite are arranged rhombohedrally. Therefore,
the term ‘graphite’ is often used regardless of stacking order.
The actual structure of practical carbonaceous materials deviates more or less
from the ideal graphite structure. Moreover, carbonaceous materials consisting
of aggregates of graphite crystallites are called graphites as well. For instance, the
terms ‘natural graphite’, ‘artificial’or‘synthetic graphite’, and ‘pyrolytic graphite’are
commonly used, although the materials are polycrystalline [66]. The crystallites may
vary considerably in size, ranging from the order of nanometers to micrometers. In
some carbons, the aggregates are large and relatively free of defects, for example,
in highly oriented pyrolytic graphite (HOPG). Furthermore, texture effects can be
observed as the crystallites may be differently oriented to each other. In addition
to essentially graphitic crystallites, carbons may also include crystallites containing
carbon layers (or packages of stacked carbon layers) with significant, randomly
distributed misfits and misorientation angles of the stacked segments to each
other (turbostratic orientation or turbostratic disorder [67]). The latter disorder can
be identified from a nonuniform, and on average increased, interlayer spacing
compared with graphite [66, 68].
When the disorder in the structure becomes more dominant among the crystal-
lites, the carbonaceous material can no longer be considered graphitic but must be
regarded as a nongraphitic carbon. For carbon samples that contain both charac-
teristic graphitic and nongraphitic structure units, the classification into graphitic
and nongraphitic types can be somewhat arbitrary and in many cases is only made
for the sake of convenience.
In the case of nongraphitic (disordered) carbons, most of the carbon atoms are
arranged in a planar hexagonal network, too. Though layered structure segments are
probable, there is actually no far-reaching crystallographic order in the c-direction.
The structure of these carbons is characterized by amorphous areas embedding
and partially crosslinking more graphitic (layered) structure segments [69–71]
(Figure 15.4). The number and the size of the areas vary, and depend both on
the precursor material and on the manufacturing process, for example, on the
manufacturing temperature and pressure. Using a simple model [19, 42] the
complex X-ray diffraction (XRD) patterns of nongraphitic carbons can be correlated
with the probability of finding unorganized (randomly oriented and amorphous)
and organized (layered) areas. As a result the lithium storage capacity of a specific
nongraphitic carbon material can be predicted approximately.
Most nongraphitic carbons are prepared by pyrolysis of organic polymer or
◦
hydrocarbon precursors at temperatures below ∼1500 C. Further heat treatment
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of most nongraphitic carbons at temperatures from ∼1500 to ∼3000 Cmakes it
possible to distinguish between two different types of carbons.
Graphitizing carbons develop the graphite structure continuously during the
heating process. The carbon layers are mobile enough to form graphite-like
crystallites as crosslinking between the layers is weak. Nongraphitizing carbons