Page 526 - Handbook of Battery Materials

16.3 SEI Formation on Carbonaceous Electrodes 499

molecules that form large and stable solvated lithium cations will have a smaller
tendency for co-intercalation into the graphite.
In conclusion, it seems that solvents appropriate for lithium-ion batteries em-
0
ploying a graphite anode must have high solvation energy, high E , and high
i 0 for reduction in order to slow the co-intercalation of the solvated ion, and to
enhance the formation of the SEI at the most positive potential (far from the Li/Li +
potential).
Another phenomenon which has practical importance, especially for large pris-
matic cells, is gas production during the first charge [95, 96]. There are two
sources of gas: (i) the reduction of the electrolyte with the formation of methane
or ethane, CO, and hydrogen and (ii) replacement of gases adsorbed on the carbon
by SEI-building materials during the formation of the SEI. Gases such as H 2 ,O 2 ,
CO 2 ,N 2 , and small organic molecules are released from the micropores of the
carbon. The second source of gases is very significant if the carbons being used are
disordered, but less so if the carbons are graphite-like.
It was concluded [97, 98] that, on long cycling of the lithium-ion battery, the
passivating layer on the carbon anode becomes thicker and more resistive, and is
responsible, in part, for capacity loss.
16.3.3
Parameters Affecting Q IR

It has been shown [8, 72, 75] that Q IR is consumed mainly in the building of the
SEI. However, for a variety of carbons and graphites, Q IR may have other sources
[6, 72] (Equation 16.6):

Q IR = Q SEI + Q SP + Q U + Q T (16.6)
where Q SEI is the capacity needed for the formation of the SEI, Q U is the unused
capacity under specified experimental conditions (it is usable at low rates and high
potentials), Q SP is the capacity associated with the formation of soluble reduction
products [6, 72], and Q T is the capacity associated with the trapping of lithium
inside the structure of the carbon, generally as a result of irreversible reaction with
heteroatoms present on the inner surface of closed pores [68].
Q IR depends on the electrolyte type (solvent and salts), the impurity level of
the carbon and the electrolyte, the real surface area of the carbon including
inner pores which the electrolyte can enter, the surface morphology, and the
chemical composition of the carbon. It typically decreases in the order: powders >
microbeads > fibers. Impurities such as acids and alcohols, water, or heavy metals
may contaminate the SEI, causing side-reactions [1, 2] such as hydrogen evolution
and electrolyte reduction; this results in larger Q IR values [99, 100].
Since in many publications the impurity level in the carbons and electrolytes is
not specified, it is difficult to correlate Q IR reliably with the type of solvent or salt.
However, it seems that in cases where the electrolyte reduction products are Li 2 CO 3
and LiF (as in the cases when EC, CO 2 , and fluorinated anions are used), Q IR is
lowest. We believe that controlled reduction-induced formation of nonconducting