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15.2 Graphitic and Nongraphitic Carbons 453

when nongraphitic carbons with specific charges of up to ∼500 Ah kg −1 were
synthesized [200]. When lithium is stored in the carbon bulk, one can suppose that
a higher specific charge (in ampere-hour per kilogram) requires a correspondingly
higher volume (i.e., lower density) of the carbonaceous matrix to accommodate
the lithium. As a consequence, the charge densities (in ampere-hour per liter)
of the high-specific-charge carbons should be comparable with those of graphite.
Several models have been suggested to explain the high specific charge of these
lithiated carbons. This may be because the variety of precursor materials and
of manufacturing processes leads to carbonaceous host materials with various
structures and compositions.
Yazami et al. [41, 207–211] proposed the formation of nondendritic metallic
(Figure 15.11a) lithium multilayers on external graphene sheets and surfaces.
Peled et al. [127, 128] suggested that the extra charge gained by mild oxidation of
graphite is attributed to the accommodation of lithium at the prismatic surfaces
(Figure 15.3) between two adjacent crystallites and in the vicinity of defects. Sato
et al. [212, 213] suggested that lithium molecules occupy nearest-neighbor sites in
intercalated carbons. Yata’s and Yamabe’s groups discussed the possibility of the
formation of LiC 2 in carbons with a high interlayer spacing of ∼0.400 nm (graphite:
0.335 nm) for their ‘polyacenic semiconductor’ (Figure 15.11b) [214–219]. In Refs
[220–225] it is assumed that carbons with a small particle size can store a
considerable amount of lithium on graphite edges and surfaces in addition to
the lithium located between the graphene layers (Figure 15.11c). The existence of
different ‘Li storage sites’ (Figure 15.11d) was discussed in Refs [226–228], too.
Others [40, 229–232] proposed that additional lithium can be accommodated in
◦
nanocavities which are present in the carbon at temperatures below ∼800 C [233]
(Figure 15.11e). Kureha Chemical Industry Co. proposes a cluster-like storage of
lithium in pores where the electrolyte solvent cannot enter (Figure 15.11f ). This
carbon, so-called ‘Carbotron P’ (Morimoto, S. Kureha Chemical Industry Co., Ltd.,
personal communication) or ‘pseudo isotropic carbon (PIC)’ [234], was used in the
second generation of Sony’s lithium-ion cell [235].
The probabilities of the models have been discusses rather controversially [50, 51,
236–238]. There are various attempts by several researchers (in particular, Dahn’s
group [50, 51, 239–244]) to interpret the behavior of the high-specific-charge carbons
systematically. Many graphitizing (soft) and nongraphitizing (hard) carbons which
were prepared below approximately 800–900 C and which show very high specific
◦
charges, exhibit a hysteresis [50, 51, 215, 241, 242]: the lithium intercalation
+
occurs close to 0 V vs Li/L whereas the lithium de-intercalation occurs at much
more positive potentials (Figure 15.12b). The potential hysteresis seems to be
proportional to the hydrogen content in the carbon. The ratio of hydrogen to carbon
in the material is high (i) when a substantial amount of hydrogen is present during
manufacture, either because hydrogen is already incorporated in the precursor
material and/or because manufacture takes place under an H 2 atmosphere and
(ii) when the manufacturing temperature has been so low that hydrogen has not
yet been removed. It has been suggested that lithium is bound near H-terminated
edge carbon atoms, which induces a partial bond change at the carbon from