Page 463 - Handbook of Battery Materials

436 15 Lithiated Carbons

of metallic lithium may be the most reactive (although due to kinetic effects
they are not necessarily so), and thus they can cause safety problems as well as
handling difficulties. This will be discussed further in the next section of this
chapter.

15.1.1
Why Lithiated Carbons?

Among the mentioned lithium insertion materials above, lithiated carbons (Li x C n )
are considered to be the most promising at present. Carbonaceous materials exhibit
higher lithium storage capacities and more negative redox potentials versus the
cathode than polymers, metal oxides, or chalcogenides. Furthermore, they show
long-term cycling performance superior to Li alloys due to their better dimensional
stability. In addition, most carbons suitable as anodes for lithium-ion cells are
cheap and abundant compared with the other materials.
Though considerable safety improvements were the major driving force for
the introduction of lithiated carbons into rechargeable lithium cells, it has to
be kept in mind that the lithium activity of lithium-rich carbons is similar to
+
that of metallic lithium. Thus the redox potential vs Li/Li is quite close to 0 V
(Figure 15.2) and the reactivity is high. Additionally, the particle size of Li x C n in
practical electrodes is only in the order of 10 µm, that is, the reactive surface area
is large. Moreover, ex situ investigations after cycling have shown that cycling of
graphite electrodes increases the specific surface area of Li x C n by a factor of 5
[36]. Recent differential scanning calorimetry studies on polymer-bonded lithiated
◦
carbons reveal that the SEI films degrade at temperatures of approx. 120–140 C,
then undergo a reaction with the electrolyte and the binder material at temperatures
◦
above 200 C. The degradation reactions are proportional to the surface area of
the carbon [37], and furthermore can be expected to depend on the SEI films
formed, that is, the electrolytes used. The tendency of the SEI film to peel off
the carbon anode is assumed to be suppressed (the adherence between carbon
and SEI is supposed to be improved) by proper surface pre-treatment of the
carbon [38].
However, thereactionrateofLi x C n depends on the lithium concentration at
the surface of the carbon particles, which is limited by the rather slow transport
1)
kinetics of lithium from the bulk to the surface [17–19]. As the melting point of
metallic lithium is low (∼180 C) there is some risk of melting of lithium under
◦
abuse conditions such as short-circuiting, followed by a sudden breakdown of the
SEI and a violent reaction of liquid lithium with the other cell components. In
contrast, there is no melting of lithiated carbons.

1) Lithiated carbons are mostly multiphase causes experimental problems because the
systems. Hence, the determination of propagation of a reaction front has to be
chemical diffusion coefficients for Li + considered.