Page 476 - Handbook of Battery Materials

15.2 Graphitic and Nongraphitic Carbons 449

(a)
graphene layers

(b)

(c)

Donor solvent
Decomposed solvent
Li
Film component
Figure 15.9 Schematic model of the film-formation mech-
anism on/in graphite: (a) the situation before reaction, (b)
formation of ternary lithiated graphite Li x (solv) y C n ,and (c)
film formation due to decomposition of Li x (solv) y . Prepared
with data from Ref. [152].

investigations indicate that the film-formation process on graphite can be even
more complex [107, 160–163].
In analogy to unsolvated intercalation reactions, solvent co-intercalation takes
place via the prismatic surfaces only; not only the total external surface area but
also the ratio between the prismatic and basal surfaces affect the irreversible charge
loss [117]. This is in good agreement with the observations of Imanishi et al.
[164, 165], who found that the tendency for PC co-intercalation into graphitized
carbon fibers depends on the fiber texture. Carbon fibers in which the graphite
packages are concentrically arranged expose a smaller amount of prismatic surfaces
to the electrolyte, that is, they are less sensitive to solvent co-intercalation than
fibers with a radial texture.
To take into account the effect of the thickness of the respective graphite flake on
the formation of Li x (solv) y C, several simple models have been suggested recently
[117]. The intercalation of all kinds of species into graphite generally requires
energy to expand the gaps between the graphene layers held together by van der