Page 475 - Handbook of Battery Materials

448 15 Lithiated Carbons

formation are more positive than those for the formation of the corresponding com-
+
pound Li x C 6 .Atthisstage of Li intercalation the coulombic interaction between
+
the lithium guest layer (Li ) and the balancing negative charge distributed over the
−
graphene layers (C ) is weak, and space to accommodate large solvent molecules is
n
still available [24, 26, 144, 145]. As a further result of the low coulombic interactions,
the mobility of the intercalants is high, and therefore the guest distribution among
the interlayer spaces is incommensurable or random. Bycontrast,the lithium guests
in the binary phase LiC 6 form a commensurable structure, that is, they are organized
according to the honeycomb ‘raster’ of the graphene layers [90]. The solvated GICs
are thermodynamically unstable with respect to the reduction of the co-intercalated
solvent molecules [136]. The kinetically controlled reduction depends on the type
of co-intercalated solvent. It is slow for, for example, dimethyl sulfoxide, where
even staging of solvated GICs can be observed [138, 146], but very much faster,
for example, for PC [63, 137, 147–151], where the electrochemical intercalation
+
followed by fast decomposition of the intercalated Li (solv) y can be misunderstood
+
as simple electrolyte decomposition. Anyway, the reduction of Li (solv) y inside
the graphite is associated with an increase of irreversible losses of charge and of
material.
Effects resulting from solvent co-intercalation, mechanical destruction, and
higher irreversible specific charge losses seriously complicate the operation of
graphitic anode materials. Since ternary lithiated carbons are thermodynamically
favored at low lithium concentrations in the graphite, that is, at the beginning of
intercalation, kinetic measures have to be applied to prevent, or at least suppress,
solvent co-intercalation. This can be accomplished by using electrolyte components
that form effectively protecting SEI films on the external graphite surfaces in the
very early stages of the first reduction, at potentials which are positive relative to the
potentials of a significant formation of Li x (solv) y C n .The first [65],and for a long
time the only, effective solvent in this respect seemed to be ethylene carbonate (EC)
[136, 152, 153]. Since the viscosity of electrolytes based on pure EC is rather high,
mixtures of EC with low viscosity solvents such as dimethyl carbonate (DMC) and
diethyl carbonate (DEC) are widely used [154–159]. Several papers which report on
investigations and applications of these electrolyte blends are compiled in recent
reviews [2, 6].
Although the formation of binary Li–GICs prevails in EC-based electrolytes,
many investigations indicate a mechanism of film formation in which solvated
lithiated graphites also participate. Film formation in the first cycle during the
first reduction of the host material due to electrolyte decomposition is not a
simple surface reaction but a rather complex three-dimensional process, taking
place basically at a potential of ∼1to ∼0.8 V vs Li/Li + (Figure 15.7a). In-situ
dilatometric [152], scanning tunneling microscopy (STM) [160, 161] and Raman
[106] methods indicate a (fairly large) expansion of the graphite host corresponding
to the (intermediate) formation of Li x (solv) y C n at those potentials. The reduction
of Li x (solv) y on parts of the internal surfaces between the graphene layers results in
an ‘extra’ film, which penetrates into the bulk of the graphite host (Figure 15.9).
Correspondingly ‘extra’ irreversible charge losses are observed [152]. Several other