Nauka /Interperiodica

 
et al.
performed a comprehensive analy-
sis of the degradation of the capacity of LIB with dif-
ferent depths of discharge (monitored from the LIB
voltage), which had been stored for a long time at 
15–
60°ë
. A year’s storage at 60 and 
30°ë
led to a capacity
loss of 15–20 and 7–8%. No unambiguous dependence
of the degradation on the depth of discharge was dis-
covered.
Investigating the change in the discharge character-
istics of LIB with a positive electrode of LiCoO
2
with
cycling, Choi and Lim [57] established that the cycle
life of LIB is defined not so much by the conditions of
discharge (in particular, by the temperature) as by the
discharge mode. Exceeding a cut-off charging voltage

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RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41
No. 1
2005
DEGRADATION OF LITHIUM-ION BATTERIES
3
or charging LIB at 4.2 V for a very long time tells on the
workability of LIB utterly negatively. The use of a
forced-charging mode (full charging within <1 h) is
also undesirable. According to [57], the major factor
responsible for the degradation of LIB is the electrolyte
electrooxidation at the surface of the positive electrode.
The magnitude of the impedance may serve as a cer-
tain indicator of the workability of LIB on the whole
and that of individual electrodes. By analyzing the
value of the impedance of a system and its constituents,
one can judge, to a certain degree of probability, on
both the changes that occur in the system under some
actions or others and the possible reasons for these
changes.
Wu 
 
et al.
[58], by using LIB the size 18650 with a
reference electrode built in the battery, managed to
measure the impedances of the positive and negative
electrodes separately. They established that the imped-
ance of the positive electrode, which initially was twice
the impedance of the negative electrode, grew after 100
charge–discharge cycles by almost two times, whereas
the latter remained practically invariant.
In [59], the impedance of LIB the size 18650S
(Sony) was periodically determined during a prolonged
(up to 800 cycles) cycling; besides, there was measured
the impedance of individual electrodes, (negative, of
graphite; positive, of lithium cobaltite) extracted out of
the battery after 10 and 800 charge–discharge cycles.
The obtained results on the whole confirmed the results
obtained by Wu 
 
et al.
[58]: after the 10th cycle, the
impedance of the positive electrode exceeded the
impedance of the negative electrode by three times, and
after the 800th cycle, by five times. The ohmic resis-
tance of the electrolyte underwent no change whatso-
ever, while the discharge capacity of LIB decreased by
30%. The change in the impedance of electrodes was
attributed in [59] to a change in the structure and com-
position of a solid-electrolyte surface layer during
cycling. Using electron probe microanalysis, it was
established that the concentration of oxygen and fluo-
rine in a surface layer of electrodes increased consider-
ably, which was taken to be the reason for the increase
in the interfacial resistance.
According to the data of Fellner 
 
et al.
[60], who
investigated characteristics of LIB the size 18650 with
the positive electrode based on a mixture of LiNiO
2
and
LiCoO
2
, the discharge capacity of batteries (one-hour
regime of discharge), when cycled, decreased linearly
with the number of cycles. A similar dependence links
the final discharge voltage and the number of cycles at
a depth of discharge equal to 40%. The authors of [60]
established that the impedance of LIB as a whole
increases linearly with the number of cycles and that
the rate of the change in the impedance at 
10°ë
is not
as great as that at 
30°ë
.
Jungst 
 
et al.
[43] studied the decrease in the capacity
of LIB the size 18650 with the positive electrode of
LiNi
0.8
Co
0.15
Al
0.05
O
2
and the negative electrode of
graphite MAG-10, which were intended for powering
an electric vehicle. The batteries with the chargeness
degrees of 60, 80, and 100% were stored at 25–55
°
C
and their impedance was periodically measured. The
LIB impedance steadily increased with the storage
duration.
The authors of [61, 62] analyzed variations in the
impedance of commercial cylindrical LIB the size
18650S (Sony) and prismatic batteries UF653467
(Sanyo) during their cycling. The character of varia-
tions in the impedance spectra demonstrated that the
resistance of interphase boundaries of positive and neg-
ative electrodes increase during the cycling. The elec-
tron microscopy and x-ray diffraction studies of elec-
trodes, performed in parallel with the measurements of
the impedance spectra after the cycling, showed that the
decrease in the capacity of LIB during their cycling
could have been caused by the disordering of the lay-
ered crystalline structure of LiCoO
2
, resulting in deac-
tivation of a fraction of lithium ions in the cathode, and
by the emergence, in the LiCoO
2