Page 479 - Handbook of Battery Materials

452 15 Lithiated Carbons

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other bulky electrolyte additives [193] are assumed to coordinate Li ions in solu-
tion in such a way that solvent co-intercalation is suppressed. The electrochemical
formation of binary lithiated graphites Li x C 6 was also reported for the reduction of
graphite in electrolytes containing high-molecular-mass polymers as solvent. The
claimed lithium intercalation, however, proceeds in a potential region where usually
solvated lithiated graphites appear [41, 43, 195, 196] (for comparison, see Ref. [197]).
Graphitic anodes which have been ‘pre-filmed’ in an electrolyte ‘A’ containing
effective film-forming components before they are used in a different electrolyte
‘B’ with less effective film-forming properties show lower irreversible charge losses
and/or a decreased tendency to solvent co-intercalation [152, 198, 199]. However,
sufficient insolubility of the pre-formed films in electrolyte ‘B’ is required to
ascertain long-term operation of the anode.

15.2.4
Lithiated Nongraphitic Carbons

The use of nongraphitic (disordered) carbons as anode materials in lithium-ion
cells is highly attractive for two reasons:
1) The crosslinking between the graphene layers (or packages of graphene layers)
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by sp -hybridized carbon atoms (Figure 15.4) mechanically suppresses the
formation of solvated lithiated graphites, Li x (solv ) y C n , [19, 26, 65, 152]. As a
result the gap between the layers cannot expand very much, and thus there
is not enough space for the solvent to co-intercalate. Moreover, these carbons
have the advantage that they can operate in EC-free electrolytes. Consequently,
the first practically applicable lithiated carbon anodes [200, 201] were based on
these nongraphitic carbons and not on graphitic materials. Furthermore, the
use of composite carbonaceous materials comprising a ‘core’ of graphite and a
protective ‘shell’ of nongraphitic carbon is an alternative to inhibit the solvent
co-intercalation reaction in graphite [202–205].
2) In comparison with graphite, nongraphitic carbons can provide additional
sites for lithium accommodation. As a result, they show a higher capability of
reversible lithium storage than graphites, that is, stoichiometries of x > 1in
Li x C 6 are possible.
The latter, so-called ‘high specific charge’ or ‘high capacity,’ carbons have
received considerable attention in recent research and development. Usually they
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are synthesized at rather low temperatures, ranging from ∼500 to ∼1000 C, and
can exhibit reversible specific charges from ∼400 to ∼2000 Ah kg −1 (x =∼1.2
to ∼5in Li x C 6 ), depending on the heat treatment, the organic precursor, and
the electrolyte [206]. 3) Such materials have been known since the late 1980s,

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3) Carbons chemically pre-lithiated before the specific charges greater than 400 Ah kg ,
production of the electrode can exhibit too [206].