Page 477 - Handbook of Battery Materials

450 15 Lithiated Carbons

Waals forces. Firstly, the ‘expansion energy’ (related to the threshold intercalation
potential) depends on the mechanical flexibility of those graphene layers that are
deformed by the intercalation process. The average ‘expansion energy’ increases
with the thickness of the graphite flake, or more precisely with the number of
adjacent graphene layers on both sides of a particular gap. Therefore, intercalation
typically starts close to the basal planes of the flake, in the gaps adjacent to
the end basal plane. Then the intercalation progresses toward internal layer
gaps.
Secondly, the ‘expansion energy’ increases with the size of the guest species.
Intercalation of large solvated lithium ions into the outer van der Waals gaps
produces a considerable deformation (bending) of the outer graphene layers.
Further intercalation into the internal gaps increases the bending angles of the
outer graphene layers. However, when the outer graphene layers cannot be bent
any more, the intercalation of solvated lithium into the internal van der Waals
gaps is hindered. In conclusion, the graphite particle thickness effect should be
particularly regarded for the intercalation of solvated lithium ions (Figure 15.10
and [117, 166]). This means, furthermore, that it should be possible to diminish
some expansion due to solvent co-intercalation by sufficient external pressure on
the electrode, for example, by close packing of the electrodes in the cell.
Thirdly, strong solvent co-intercalation, in particular into internal van der Waals
gaps, can only be expected for kinetically stable ternary compounds Li x (solv) y C n .For
example, comparison of DMC and DEC with dimethoxyethane (DME) shows that
the kinetic stability of Li x (DME) y C n can be considered to be much higher than that
of Li x (DMC) y C n and Li x (DEC) y C n (and of course Li x (EC) y C n ) [166]. With EC/DME,
solvent co-intercalation proceeds on a macroscopic scale, that is, the external van der
Waals gaps and some internal ones can participate in the solvent co-intercalation
reaction. When DMC or DEC is used as co-solvent, solvent co-intercalation can be
expected to take place at the more external gaps only. Instrumentation such as STM
with which it is possible to investigate the edge of a basal plane surface can still
detect a local expansion [160, 161], whereas instrumentation providing information
on a macroscopic scale, such as dilatometry [152] or XRD, cannot.
Numerous research activities have focused on the improvement of the protec-
tive films and the suppression of solvent co-intercalation. Beside EC, significant
improvements have been achieved with other film-forming electrolyte components
such as CO 2 [153, 166–174], N 2 O [167, 174], SO 2 [152, 166, 174–176], S 2− [167, 174,
x
177, 178], ethyl propyl carbonate [179], ethyl methyl carbonate [180, 181] and other
asymmetric alkyl methyl carbonates [182], vinyl PC [183], ethylene sulfite [184],
S,S-dialkyl dithiocarbonates [185], vinylene carbonate [186], and chloroethylene
carbonate [187–191] (which evolves CO 2 during reduction [192]). In many cases
the suppression of solvent co-intercalation is due to the fact that the electrolyte
components form effective SEI films already at potentials which are positive rela-
tive to the potentials of solvent co-intercalation. An excess of DMC or DEC in the
electrolyte inhibits PC co-intercalation into graphite, too [180].
+
Furthermore, the molecular size of the Li -solvating solvents may affect the
tendency for solvent co-intercalation. Crown ethers [19, 149–151, 193, 194] and