Page 473 - Handbook of Battery Materials

446 15 Lithiated Carbons

emphasized that chemically prepared lithiated carbons that are exposed to the
electrolyte behave similarly to metallic lithium (and not similarly to lithiated
carbons which have been prepared electrochemically, e.g., Figure 15.7) regarding
film formation.
Finally, regarding long-term cycling of metallic lithium and of lithiated carbon,
respectively, and its influence on the SEI, the surface of a metallic lithium electrode
is periodically renewed during cycling, causing irreversible formation of a ‘new’ SEI
in each cycle. Unless the lithium electrode becomes electrochemically inactive due
to passivation, this process can be repeated until the lithium and/or the electrolyte
are completely consumed. In contrast, the surface of the lithiated carbon electrode
is passivated by the SEI throughout the cycling process.
Since film formationonLi x C 6 is associated with the irreversible consumption of
material (lithium and electrolyte), the corresponding charge loss is frequently called
‘irreversible specific charge’or‘irreversible capacity.’ Reversible lithium intercalation,
on the other hand, is called ‘reversible specific charge’or ‘reversible capacity.’ The losses
have to be minimized because the losses of charge and of lithium are detrimental to
the specific energy of the whole cell and, moreover, increase the material expenses
because of the necessary excess of costly cathode material, which is the lithium
source in a lithium-ion cell after cell assembly.
The extent of the irreversible charge losses due to film formation depends
to a first approximation on the surface area of the lithiated carbon which is
wetted by the electrolyte [36, 65, 117–121]. Electrode manufacturing parameters
influencing the pore size distribution within the electrode [36, 118, 121, 122] and
the coverage of the individual particles by a binder [121, 123] have an additional
influence on the carbon electrode surface exposed to the electrolyte. These and
other technical aspects which are important in this respect are reviewed in recent
papers [2, 6].
Besides the irreversible charge loss caused by electrolyte decomposition, several
authors claim that the following reactions are also responsible for (additional)
irreversible charge losses:
1) irreversible reduction of impurities such as H 2 Oor O 2 on the carbon surface,
2) reduction of ‘surface complexes’ such as ‘surface oxides’ at the prismatic
surfaces of carbon, and
3) irreversible lithium incorporation into the carbon matrix (‘formation of
residue compounds’ [124–126], e.g., by irreversible reduction of ‘internal
surface groups’ at prismatic surfaces of domain boundaries in polycrystalline
carbons).

In order to improve the electrochemical performance with respect to lower
irreversible capacity losses, several attempts have been made to modify the car-
bon surface. Here the work of Peled’s [38, 127–129] and Takamura’s groups
[130–135] deserves mention. A more detailed discussion can be found in Part III,
Chapter 17.