Nauka /Interperiodica

 
y
< 0.3
NiO
2
[69]; and a compound of di- or
tetravalent Mn in the case of a cathode of LiMn
2
O
4
[70].
The high potentials (sometimes in excess of 4.5 V),
which are realized in conditions of overcharge on posi-
tive electrodes, may lead to exothermic reactions of
oxidation of organic solvents with the formation of gas-
eous and insoluble solid products, in particular, Li
2
CO
3
,
which block the electrode pores [71]. The combination
most stable against the oxidation happens to be a mix-
ture of EC with dimethyl carbonate (DMC) [72].
Leising et al. [15] were analyzing the behavior, in
conditions of overcharge, of model cells of three types,
specifically, graphite/LiCoO
2
, 
Li
/LiCoO
2
, and
Li/graphite. Comparing the heat evolution in the said
cells, the said authors established that the main source
of the heat evolution in LIB is the reactions that proceed
on the positive electrode (in this particular case, based
on lithium cobaltite).
With the aim of lowering the danger of the decom-
position of the electrolyte as a result of its oxidation, it
was suggested to add various additives into the compo-
sition of the electrolyte, for example, LiI [73], which
performed the role of “internal chemical shuttles” that
provided for the occurrence, in LIB, of parallel pro-
cesses in the cases where the voltage across LIB
exceeds a certain level.
With LiI, a process occurs in the following manner.
At an anodic potential that is sufficiently high and
smaller than the potential of the fully charged positive
electrode, the I
–
ions are subjected to a two-step oxida-
tion (
3I
–
+ 2Â, 2
3I
2
+ 2Â) to iodine,
which reacts with metallic lithium and undergoes
regeneration to LiI.
The major way to prevent LIB from overcharging is
to ensure a balance with regard to lithium between the
positive and negative electrodes. In a number of pat-
I
3
–
I
3
–
ents, it is suggested to realize the solution of this prob-
lem at the expense of regulation of the balance between
nominal capacity of relevant electrodes [74], their
weights [75], or geometrical size (width, thickness) [76].
Besides these measures, in order to prevent LIB
from overcharging, the latter are of late supplied with
microchips that automatically block the supply of the
charging current onto the battery at the instant of reach-
ing a critical voltage on it.
STRUCTURAL AND CHEMICAL CHANGES 
IN THE MATERIAL OF THE POSITIVE 
ELECTRODE OF LIB
In the process of exploitation (cycling and storage)
of LIB, the material of the positive electrode may expe-
rience various changes in its bulk and on its surface,
which unavoidably leads to the worsening of character-
istics of the battery. The most substantial changes occur
on electrodes of lithium–manganese spinels.
During the normal cycling at room temperature, the
two-phased structure LiMn
2
O
4
, which is relatively
unstable, converts into a stable single-phased one, with
the loss of Mn
3+
and the formation of MnO
2
[70, 77,
78]. Upon the intercalation of lithium, the latter trans-
forms into inactive LiMnO
2
with a layered structure.
Following overcharge of the positive electrode
based on a lithium–manganese spinel up to potentials
of below 3.5 V there reveals itself the so-called distor-
tion of a crystalline structure after Jahn–Teller [79–81],
which in turn leads to the dissolution of the spinel and
a slow degradation of capacity during cycling.
The process of distortion after Jahn–Teller may be
substantially slowed down by increasing the lithium
excess (i.e. by increasing the average state of the oxida-
tion degree of manganese) in the initial spinel [82]. A
high stability during the cycling is exhibited, in partic-
ular, by a spinel of the composition Li
1.06
Mn
1.94
O
4
[83].
The same result may be achieved by doping LiMn
2
O
4
with other cations, in particular, with Ni, Co, and Cr
[84–90]. The authors of [91] offered a technique for
increasing the efficiency of operation of LIB at the
expense of utilizing positive electrodes of a lithium–
manganese spinel doped with lithium and cobalt simul-
taneously.
The investigations of the electrochemical behavior
of electrodes prepared from lithium–manganese spinels
undoped and doped by various metals
(LiM
0.05
Mn
1.95
O
4
, where M is Li, B, Al, Co, Ni), per-
formed by voltammetric, galvanostatic, and x-ray dif-
fraction methods in [92], showed that the major reason
for the degradation of the said materials during a
charge–discharge process is the changes in the volume
of a unit cell of their crystal lattice, which are repeated
during the cycling process. Inserting metals, which
diminish relative alteration of crystallographic parame-
ter a of a unit cell of a spinel during lithiation, into
spinel allows one to considerably stabilize the crystal

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6
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41 No. 1 2005
KANEVSKII, DUBASOVA
lattice and sharply diminish the speed of the decrease in
the capacity of electrodes. The chemical dissolution of
a spinel during cycling plays a secondary role.
Certain phase alterations were also discovered in the
material of positive electrodes prepared based on lith-
ium cobaltite. Wang et al. [93], who investigated the
change in the structure of the said materials during the
cycling of LIB in the interval of voltages extending
from 2.5 to 4.35 V by a transmission electron micros-
copy method, established the appearance, during the
cycling, of multiple defects in the crystal lattice of lith-
ium cobaltite. According to the opinion of these
authors, these defects are precisely the factor that is
responsible for the degradation of the capacity of LIB
in the case of overcharge and prolonged cycling.
On the whole, phase alterations of electrode materi-
als, which are observed when cycling electrodes based
on LiCoO
2
and LiNiO
2
, are usually not accompanied by
such a noticeable degradation of capacity as that exhib-
ited by electrodes prepared from lithium–manganese
spinels [50].
Apart from the phase alterations occurring in the
material of the positive electrode, the latter can undergo
dissolution. The principal factors that exert an influence
on the occurrence of this process are defects of the crys-
talline structure of the active material, exceedingly high
electrode potentials, and the presence of carbon in the
composite positive electrodes [50].
The dissolution of the lithium–manganese spinel
LiMn
2
O
4
, which proceeds through the mechanism of
disproportionation (
2Mn
3+
Mn
4+
+ Mn
2+
), is
accompanied by the accumulation of ions Mn
2+
in the
electrolyte. These ions can discharge on the surface of
the negative electrode and block its pores [94–97]. The
process of dissolution considerably accelerates with
increasing temperature (in excess of 
80°ë) [98], the
more so when solutions based on LiPF
6
that are dried
not well enough are used in the role of electrolyte.
Hydrolysis of LiPF
6
leads to the formation of acid prod-
ucts, in the first place, HF (LiPF
6
+ H
2
O
POF
3
+
LiF + 2HF) [99], which enter a reaction with the spinel
[100]. In order to prevent the occurrence of this pro-
cess, it was recommended, along with thorough dehy-
dration of the electrolyte, to insert 1,1,1,3,3,3-hexame-
thyldisilazan 
(CH
3
)
3
SiNSi
(ëç
3
)
3
into the electrolyte
[98]. Apart from that, in order to diminish the washing
of manganese out a lithium–manganese spinel, it was
suggested to insert oxides of lanthanum, strontium,
neodymium, and samarium into the composition of the
spinel [101].
The authors of [102], having convincingly proved
that the major reason for the decrease in the capacity of
LIB with positive electrodes manufactured based on a
lithium–manganese spinel is the slow dissolution of the
spinel in a weakly acidic electrolyte, which proceeds
via a disproportionation reaction, established that the
degree of degradation of spinel electrodes correlates
with the lack of oxygen (indicator