Nauka /Interperiodica

m
COO
]
n
, decompo-
sition of LiPF
6
, and dissolution of the active material of
the positive electrode. After a 40-day storage, the
impedance of the negative carbon electrode increased
by 8, 28, and 35% and the reversible capacity decreased
by 5, 12, and 18% at storage temperatures of 50, 65, and
75°ë.
It proved possible to considerably improve the
capacity characteristics of LIB and their stability dur-
ing exploitation at elevated temperatures by adding VC
in the electrolyte [131, 171]. Using the chronopotenti-
ometry, electrochemical quartz microbalance, imped-
ance, infrared spectroscopy, and x-ray diffraction anal-
ysis methods, Aurbach et al. [115] established that the
presence of VC reduces the irreversible capacity of
lithiated carbon electrodes at elevated temperatures,
increases their stability during cycling, and diminishes
impedance. The authors of [115] linked the said effects
with the capability of VC to form, during electroreduc-
tion, dense elastic polymer films on the surface of the
carbon negative electrode. The said films strongly
adhere to the carbon surface and provide for the passi-
vation of this surface, which is better than that in their
absence. The oligomer films that form on the surface of
positive electrodes in the presence of VC hamper direct
interaction of the material of the said electrodes with
products of decomposition and hydrolysis of LiPF
6
.
The degradation of LIB and the decrease in the their
capacity after storage at elevated temperatures (up to
70°ë) may be caused not only by destructive processes
on negative electrodes but also by quite a number of
processes occurring on positive electrodes.
Aurbach et al. [130] showed that, during prolonged
storage and/or cycling at elevated temperatures in an
electrolyte containing LiPF
6
, positive electrodes based
on lithium cobaltite undergo noticeable degradation.
An analysis of an electrolyte solution testifies to the
presence of a considerable amount of Co
2+
, which may
discharge and undergo deposition on the counterelec-
trode surface, worsening the counterelectrode charac-
teristics. However, the major reason for the degradation
of a battery, in the opinion of the authors of [130] is not
so much the change in the composition of the active
mass of the positive electrode as the processes occur-
ring on its surface. The latter are caused by the forma-
tion on a cobaltite electrode of a film of LiF, which
forms as a result of interaction of lithium cobaltite with
traces of hydrofluoric acid. The said film is responsible
for the increase in the impedance of the positive elec-
trode, which is connected with the difficultness of
migration of lithium ions through a solid-electrolyte
film on the electrode surface.
Amine et al. [16] were studying the conservation of
the capacity and stability of LIB the size 18650 of the
system graphite–LiNi
0.8
Co
0.2
O
2
with electrolyte LiPF
6
in an equimolar mixture of EC and DEC, which were
stored at temperatures ranging from 40 to 
70°ë. The
observed degradation of LIB (in the first place, the
decrease in the discharge capacity) was accompanied
by a considerable increase in the impedance of the pos-
itive electrode, which was connected, in opinion of the
authors of [16], with an excessive increase in the thick-
ness of the solid-electrolyte film on the electrode. The
correctness of this assertion is confirmed by the results
obtained in [172, 173], whose authors used a variety of
methods to investigate the properties and composition
of surface films on the positive electrode based on
LiNi
0.8
Co
0.2
O
2
after a sufficiently long storage at ele-
vated temperatures in an electrolyte based on lithium
hexafluorophosphate. It was established that after stor-
age the solid-electrolyte film contained a large amount
of LiF; in so doing, the weight of the film after storing
LIB at 
70°ë was greater than that after storage at 50°ë
by 10%. With the aim of increasing stability, the
authors of [172, 173] think it necessary to replace lith-
ium hexafluorophosphate by some other salt.
Dokko et al. [174] used an ingenious microvoltam-
metric method of investigation to investigate the elec-
trochemical stability of individual particles of various
cathodic materials (LiMn
2
O
4
, 
LiNi
0.85
Co
0.15
O
2
,
Li
1.10
Cr
0.048
Mn
1.852
O
4
, LiëÓé
2
) during cycling in per-
chlorate, tetrafluoborate, and hexafluorophosphate
electrolytes based on a two-component solvent (PC–
EC) at temperatures of 25 and 
50°ë. It was established
that lithium–manganese spinel possesses good stability
in such electrolytes as LiClO
4
/PC–EC and LiBF
4
/PC–
EC at both room temperature and an elevated tempera-
ture (after 50 charge–discharge cycles the stability
exceeds 95%) but undergoes degradation when heated
in the Li
êF
6
/PC–EC (the more so in an insufficiently
desiccated electrolyte). Doping the lithium–manganese
spinel with chromium allows one to achieve a suffi-
ciently high stability in a lithium hexafluorophosphate
electrolyte. The stability of lithium cobaltite cycled in
the potential region extending from 3.6 to 4.3 V in the
LiClO
4
/PC–EC heated to 50
°C is unsatisfactory; but
narrowing the cycling range to 3.6–4.0 V leads to a sub-
stantial bettering of characteristics. Roughly speaking,
the behavior of LiNi
0.85
Co
0.15
O
2
is identical to that of
lithium cobaltite.
During storage at elevated temperatures and when
discharged by high currents, when the temperature
inside LIB may exceed 
80°ë, there can occur a phase

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12
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41 No. 1 2005
KANEVSKII, DUBASOVA
transformation of cobaltate and a disproportionation of
Li
0.5
ëÓO
2
to Li
ëÓO
2
and 
ëÓ
3
O
4
[175].
When stored in conditions of an elevated tempera-
ture, a lithium–manganese spinel also may be subjected
to disproportionation (
2Mn
3+
Mn
4+
+ Mn
2+
) [80, 94].
In the presence of hydrofluoric acid, which forms as a
result of hydrolysis of LiPF
6
, at an elevated temperature
the spinel undergoes dissolution via the reaction
2LiMn
2
O
4
+ 4H
+
3
λ-MnO
2
+ Mn
2+
+ 2Li
+
+ 2H
2
O
[109, 169]. Water generated in this reaction accelerates
the electrode degradation and the capacity sharply
decreases.
The problem of stability of positive electrodes may
be solved by replacing LiPF
6
by other salts, for exam-
ple, by LiClO
4
, LiBF
4
, or LiAsF
6
, which form no acid
components as a result of hydrolysis.
A very perspective electrolyte salt for LIB is thought
to be lithium bis(perfluoroethylsulfonyl)imide
LiN
(C
2
F
5
SO
2
)
2
(LiBETI), which possesses a very high
thermal stability and is insensitive to the action of water
[41, 52, 99]. An electrolyte on its basis possesses a high
stability up to 
60°ë and provides for stable characteris-
tics of LIB even in the presence of moderate humidifi-
cation.
It is the opinion of the authors of [52] that a compos-
ite electrolyte based on LiF and a complexing agent,
specifically, tris(hexafluorophenyl)boran 
(ë
6
F
6
)
3
B,
possesses characteristics that are optimum for LIB.
This electrolyte is tolerant with respect to water and
thermally stable. It ensures a high electroconductance
of solutions in a broad temperature interval [176]. The
efficiency of the cycling of electrochemical cell
Li
/LiMn
2
O
4
with the composite electrolyte LiF
–
(ë
6
F
6
)
3
B in an equimolar mixture of EC and DMC in a
three-hour discharge in the interval of 10–50 cycles at
55
°C not only did not decrease but even increased
somewhat and amounted to nearly 97% (the efficiency
of similar cycling in an electrolyte based on LiPF
6
within the same time period dropped from 97 to 40%).
Authors explain the reason for the high stability both by
a high thermal stability of components LiF and
(ë
6
F
6
)
3
B of the electrolyte and by the fact that acid
products that bring about the degradation of the mate-
rial of the positive electrode fail to form even in the case
of decomposition of the electrolyte.
The authors of [177–179] reported about a new syn-
thesized salt, namely, lithium bis(oxalate)borate
LiB
(C
2
O
4
)
2
. The chelated borate anion of this salt has a
unique structure containing no hydrogen or labile fluo-
rine. According to the data presented in [176], an elec-
trolyte based on the said salt is not subjected to any
transformations on the negative and positive electrodes
and provides for stable discharge characteristics of LIB
in conditions of elevated temperatures; besides, it pos-
sesses passivating properties with respect to an alumi-
num current lead and does not activate its corrosion.
After 80 charge–discharge cycles, in a LIB with an
electrolyte based on LiPF
6
, at a temperature of 
50°ë
,
the capacity of LIB amounted to 0.56 mA h cm
–2
, and
in a LIB with an electrolyte based on LiB
(
C
2
O
4
)
2
it was
equal to 1 mA h cm
–2
at 60
°
C and 0.97 mA h cm
–2
at
70°ë
.
With the aim of developing an optimum electrolyte
that would ensure a satisfactory workability of LIB in a
broad interval of temperatures extending from –30 to
60°ë
, performed were capacity tests of LIB [20] with
different electrolytes based on three salts, specifically,
LiBF
4
and LiB
(
C
2
O
4
)
2
, which ensure a high and suffi-
ciently stable capacity at an elevated temperature, and
LiPF
6
, which possesses no such quality. It was estab-
lished that a multicomponent electrolyte 1 M
LiB
(
C
2
O
4
)
2
in a 1 : 1 : 1 mixture of PC, EC, and EMC
made it possible to realize sufficiently stable operation
of LIB in a wide temperature interval.
CONCLUSIONS
Analyzing the above literature data devoted to
investigations of stability of energetic characteristics of
LIB, one can draw the following conclusions.
The reason for the decrease in capacity during the
cycling of LIB is various chemical and electrochemical
reactions that proceed on electrodes as well as in the
bulk electrolyte. The major processes that are responsi-
ble for the worsening of characteristics of LIB can be as
follows: (1) the overcharge, which leads to the oxida-
tion of electrolyte on the positive electrode and the dep-
osition of metallic lithium on the negative electrode, (2)
the electroreduction of electrolyte components (both
the solvent and the salt) and various impurities on the
negative electrode, (3) self-discharge of electrodes, and
(4) the dissolution and phase alterations of the active
material of the positive electrode.
Some factors or others may prevail in each particu-
lar case; these factors determine parameters of stability
of the system. In so doing, due to its complexity and
multifactoredness, it is very difficult to