Nauka /Interperiodica

 
x
CoO
2
. After a 30-day
storage at room temperature, the residual capacity of
these batteries amounted to 97% of the nominal value.
At 
55°ë
, the self-discharge accelerated by 4–5 times.
According to [63], the effect of the self-discharge on
the degradation of the capacity of LIB they tested is not
great, for the self-discharge is of a reversible character.
When analyzing the self-discharge of LIB, one has
however to take into account not so much the reversible
component (insertion of lithium into the positive elec-
trode) as the irreversible component (oxidation of elec-
trolyte at its surface) [64–66].
During oxidation of an organic electrolyte with a
solvent (El) at the surface of an electrode there occurs
the reaction El 
e + El
+
[66] with the subsequent
spending of liberated electrons in an electrochemical
process with the participation of the material of the pos-
itive electrode (metal oxide M
é
 
z
) and lithium ions
(MO
 
z
+ 
 
y
e
+ 
 
y
Li
+
Li
 
y
MO
 
z
). As a result, there
occurs intercalation of lithium into the positive elec-
trode, i.e. the decrease in the degree of its chargeness
without the imposition of an external current.
As the self-discharge rate of LIB is limited chiefly
by the solvent oxidation rate, the solvent stability plays
the most important role in the preservation of LIB.
Pistoia 
 
et al.
[66] examined the self-discharge of
three major metal oxide cathodes (Li
 
x
Mn
2
O
4
, 
Li
 
x
CoO
2
,
Li
 
x
NiO
2
) in electrolyte systems based on LiPF
6
, 
LiBF
4
,
and LiClO
4
. They established that, even in a state of
moderate oxidation (at stoichiometry of lithiated oxides
Li
0.5
Mn
2
O
4
, 
Li
0.5
ëÓ
O
2
,
and Li
0.4
NiO
2
), these materials
were prone to self-discharge. In all cases, during the
self-discharge, there occurs the solvent oxidation on the
positive electrode, albeit differently in different sys-
tems. In some cases there was noticed the clogging of
pores of the positive electrode by the oxidation prod-
ucts, which led to a discernible increase in the electrode
impedance and a decrease in the charge–discharge rate,
which is enough to bring about an irreversible self-dis-
charge of LIB with a decrease in the gross capacity. The
authors of [66] proposed alternative self-discharge
mechanisms. One of these involves the electrolyte
decomposition on the electrode; another, a spontaneous
insertion of lithium into the positive electrode; yet
another, the dissolution of the positive-electrode mate-
rial. The dissolution on the positive electrode decreased
twofold upon replacing the carbon negative electrode
with platinum; this allowed the authors of [66] to allege
that a perceptible role in the self-discharge of the posi-
tive electrode is played by lithium ions that form at the
negative electrode.
THE OVERCHARGE OF LIB
The overcharge of LIB of all types leads to some
undesirable processes, which result in irreversible deg-
radation of LIB and a decrease in the capacity and
energy density.
As a result of overcharge of the negative carbon
electrode there occurs the deposition of metallic lith-
ium on it. The main reason for the said process is, as a
rule, an exceedingly great excess of lithium in LIB at
the expense of unbalanced initial ratio between weights
of positive and negative electrodes. When such an
unbalanced LIB is being charged, the potential of the
positive electrode fails to reach its normal state and
remains far more negative. As the control parameter for
the polarity change and for the passing to a regime of
discharge is the potential difference between positive
and negative electrodes, and this difference of poten-
tials in the case of electrodes unbalanced with respect
to lithium is below nominal, the charging process still
continues after the negative electrode reached a state
saturated with respect to lithium. As a result this there

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RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41
No. 1
2005
DEGRADATION OF LITHIUM-ION BATTERIES
5
occurs overcharge of the negative carbon electrode and
the deposition of metallic lithium on it. Another reason
for overcharge of the negative electrode is a forced
charging, which leads in some cases to overpolarization
of the negative electrode [67].
The lithium that was deposited on the negative car-
bon electrode rapidly reacts with the solvent to form on
the surface of the negative electrode a film covered by
a film of salt (Li
2
CO
3
and LiF [68]) and other products.
The film blocks the pores in the carbon electrode and
diminishes the magnitude of its working surface area,
which leads to a decrease in the activity of the negative
electrode and to the capacity degradation.
The number of electrochemical and chemical reac-
tions that proceed on an overcharged positive electrode
is quite large. The reactions depend on particular con-
ditions: the electrode material, the electrolyte composi-
tion, the temperature, and so on. An overcharge may
lead to the capacity loss because of the formation of an
inert material, for example, Co
3
O
4
in the case of a cath-
ode of lithium cobaltite [69]; LiNi
2
O
4
, in the case of a
cathode of Li