Nauka /Interperiodica

 
z
) in a molecule of
Li
1
+
 
x
Mn
2
–
 
x
O
4
–
 
z
. With the aim of decreasing the speed
of the decrease in capacity it was suggested to decrease,
if possible, the volume of the electrolyte and the mois-
ture content in it, the more so when employing LiPF
6
as
the electrolyte salt. An increased stability is exhibited
by doped and undoped lithium–manganese spinels that
are synthesized at relatively not high temperatures
(below 
700°ë
). Manev and Faulkner [103] have pat-
ented a multistage process for annealing a lithium–
manganese–cobalt spinel, which consists of that the
process is performed successively at temperatures of
400–500, 500–600, and 
600–700°ë
.
Lithium cobaltite LiCoO
2
, which possesses a far
higher stability than LiMn
2
O
4
does, is nevertheless
also subjected to chemical dissolution, which leads in
particular to the formation of Co
2+
. The Co
2+
ions can
then undergo discharge at the surface of the negative
electrode. The authors of [104] reported on a direct
link between the amount of metallic cobalt at the sur-
face of the negative carbon electrode and the decrease
in the capacity of LIB that had been charged to a volt-
age in excess of 4.2 V. The value of the decrease in the
capacity of LIB with a positive electrode based on lith-
ium cobaltite depends on the technology of the prepa-
ration of the latter, in the first place, on the character of
thermal treatment during the synthesis of the active
material.
THE PROBLEM OF STABILITY 
OF THE NEGATIVE CARBON ELECTRODE 
OF LIB
To the minds of many researchers, the major “perpe-
trator” responsible for the worsening of electrochemi-
cal characteristics of LIB is the negative carbon elec-
trode. This point of view is adhered to, in particular, by
Broussely 
 
et al.
[8], who assert that the major reason for
the decrease in the capacity of LIB is hidden in the oxi-
dation of lithium on the negative electrode, as the solid-
electrolyte film on this electrode is incapable of com-
pletely blocking the interaction between lithium inter-
calated into carbon and the solvent. To tell the truth,
however, authors believe that, with time, the solid-elec-
trolyte film on the surface of graphite turns denser, and
the reaction process that leads to the delithiation of a
battery slows down.
Zheng 
 
et al.
[105] established that the magnitude of
the activation energy for the process of the decrease
in 
the capacity of the negative carbon electrode
(39.8 kJ mol
–1
) is close to the value of the activation
energy for the degradation processes of LIB as a whole.
On the grounds of this they claim that the degradation
of the negative carbon electrode is a primate in the pro-
cess of the overall worsening of electrochemical char-
acteristics of a battery.
Kida 
 
et al.
[106], when comparing the results of the
cycling of LIB with a positive electrode prepared from

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RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41
No. 1
2005
DEGRADATION OF LITHIUM-ION BATTERIES
7
lithium cobaltite and negative electrodes of various car-
bonaceous materials, such as coke, natural graphite,
and coke–graphite mixtures (“hybrid” carbon), showed
that the last system is characterized by a far smaller
decrease in the capacity during their cycling than the
first two systems (with a high enough overall capacity).
The reason for the increase in the stability of character-
istics of LIB, in the opinion of authors, is the voltage
decrease during discharge, as a result of which the side
reactions between graphite and electrolyte (1 M LiPF
6
in the EC–DMC and EC–DEC mixtures) are sup-
pressed.
Based on the results published in [106] and bearing
in mind that, by employing an alternative negative elec-
trode (in particular, an electrode prepared from
Li
4/3
Ti
5/3
O
4
[107]), it is sometimes possible to achieve
a high selectivity of LIB during cycling, it was con-
cluded [108] that the main reason for the decrease in the
capacity of LIB is the spurious processes occurring at
the negative carbon electrode.
At the same time, one must not consider the stability
of characteristics of a negative carbon electrode in iso-
lation from the processes that proceed on the positive
electrode. Wang