Nauka /Interperiodica

 
n
, where 
 
n
= 3–14 [152, 153], or
organosiliconboron compounds with the general formula
where R
1
–
R
9
are atoms of nitrogen and halogens and
alkyl groups [154].
One of the ways to improve the stability of electro-
lytes is to add additives that would improve the stability
of electrolytes to oxidation. The additives suggested for
this purpose include triphenylmethane, tetraphenyl-
methane, and the like [155], as well as solvents that
belong with the group of thiocarbonates or thioethers,
in which the oxygen of the ether group is replaced by
sulfur [156].
Introducing salicyl alcohol or salicylaldehyde into
an electrolyte assists the formation of a stable film on
the positive electrode. The film preserves the electrode
from direct contact with the electrolyte, which reduces
the probability of oxidation of the latter [157]. The
same aim is pursued by adding into the electrolyte addi-
tives with the general formula
where R stands for a nitrogen-containing group and M
denotes an alkali metal or a hydrogen atom [158]. The
R
8
–Si–O–B–O–Si–R
5
,
–
R
7
–R
9
–
R
6
–R
4
–O
–
R
1
–Si–R
3
–R
2
MS N SM,
N N
–
–
=
=
–
–
–
=
–
R
film that forms on the positive electrode upon inserting
the said compound possesses a low ohmic resistance.
The electrolyte decomposition as a result of elec-
troreduction on the negative electrode leads as a rule to
a decrease in the capacity and cycle life of LIB as a con-
sequence of the irreversible decrease in the content of
salt and solvent. Besides, the said phenomena may
serve as a reason for the failure of batteries caused by
of a seal failure resulting from gas evolution and an
increase in the internal pressure.
On the whole, the process of electroreduction of var-
ious carbonate electrolytes on the negative carbon elec-
trode obeys the same regularities as on metallic lithium,
due to the closeness of potentials of lithium and com-
pletely lithiated carbon [159]. In either case the elec-
troreduction of the electrolyte leads to the formation of
a carbonate film of a solid electrolyte at the electrode
surface and the evolution of gaseous products (propy-
lene, ethylene).
Various electrolyte salts also may be subjected to
electroreduction, converting into difficultly soluble
products, specifically, into LiF in the electrolytes based
on LiPF
6
, 
LiAsF
6
,
and LiBF
4
, and into LiCl in the elec-
trolytes based on lithium perchlorate [159–161]. These
products, when deposited on the surface of electrodes,
may either stabilize the passivating film on the surface
or, vice versa, give rise to a layer with an increased
ohmic resistance, which would block the surface, thus
unfavorably affecting the impedance of LIB.
CORROSION OF MATERIALS 
OF CURRENT LEADS IN LIB
The most important parameters that are discussed
when considering the question of materials for current
leads for LIB is the formation of a passivating film on a
surface, adhesion to the electrode material, and either
localized or overall corrosion, which determine in some
cases the internal impedance of LIB.
The investigations of the corrosion behavior of met-
als that are usually used for current leads in LIB (cop-
per, aluminum) in electrolytes based on PC (PC–DEC)
and EC (EC–DMC), which were performed in [162]
with the aid of a complex of electrochemical and phys-
icochemical methods, established that aluminum is
prone to localized corrosion and copper, to corrosion
cracking. The pitting corrosion of aluminum, which
serves as the current lead for the positive electrode,
occurs at high positive potentials, which are typical for
the completion of the charging of LIB with positive
electrodes based on a lithium–manganese spinel and
lithium cobaltite. It was reported that, during the first
few hundreds of cycles, the PC–DEC electrolyte is less
aggressive than EC–DMC; increasing the potential of
the charging leads to a decrease in the corrosion resis-
tance, and adding a small amount of water into electro-
lyte facilitates an increase in the resistance, with the
overall picture of corrosion being independent of the

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RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41
No. 1
2005
KANEVSKII, DUBASOVA
cycling duration. The character of corrosion of copper,
which is usually employed for manufacturing current
leads of the negative electrode, is typical for the behav-
ior of metals at negative potentials. In so doing, a very
important role is played by metallurgical prehistory of
the material of the current lead, in particular, the solid-
ification regime, which is responsible for the character
of the size of grains.
An investigation of the corrosion behavior of alumi-
num and copper current leads for positive and negative
electrodes of LIB showed [163] that the passivating
film on aluminum current leads turns thicker during
cycling, increasing the internal resistance. The cracking
of copper current leads is revealed very distinctly in the
case of negative electrodes of LIB kept in a charged
state (the electrode potential close to the lithium poten-
tial). The cracking is especially pronounced in the cases
where the leads are made of copper solidified during
metallurgical conversion.
With the aim of raising the corrosion resistance of
aluminum current leads and protecting them from
excessive passivation it was recommended to deploy a
conversion chromate coating on them [163].
A certain problem from the viewpoint of corrosion
resistance of an aluminum current lead emerges in the
case of utilization of electrolytes based on lithium
imide, which possesses enhanced corrosion activity
with regard to aluminum [164, 165]. An effective
method to eliminate this in the addition of additives of
LiBF
4
into the electrolyte [50].
EFFECT OF TEMPERATURE 
ON THE CAPACITY CHARACTERISTICS OF LIB
Exploitation of LIB at low temperatures (down to
−
40
°
C) as a rule leads to irreversible decrease in the
capacity of LIB. According to the data published in [1],
whose authors conducted comparative investigations of
characteristics of commercial LIB produced by various
manufacturers, the decrease in the capacity of LIB at
low temperatures as compared with the nominal capac-
ity amounted to 17–35% at 
–20°ë
; 43–76%, at 
–30°ë
;
and 78–100%, at 
−
40°ë
. In the opinion of the authors
of [166–168], such a degradation of LIB is impossible
to explain by either a decrease in the ohmic resistance
inside the positive electrode or a change in the proper-
ties of the solid-electrolyte film on its surface. The
decrease in the capacity of LIB at low temperatures is
usually attributed in the first place to the deposition of
metallic lithium on the surface of negative electrodes
during a charging process and to complications that
arise with the transport of lithium ions in the bulk of the
electrode due to the decrease in the rate of their solid-
phase diffusion in the carbon material [168].
An analysis of the change in the potentials of posi-
tive and negative electrodes of LIB with a reference
electrode built in it showed [166] that the potential of
the negative electrode of LIB fully discharged at a low
temperature (
–40°ë) happens to be more positive than
the reversible potential of lithium by approximately
300 mV. This testifies to that a considerable amount of
lithium undergoes no deintercalation from the electrode
during the discharge process.
An increase in the activation resistance of the pro-
cess of discharge of lithium ions and the concentration
polarization by lithium ions at low temperatures makes
the electrode potential noticeably more negative, which
eventually leads to electrodeposition of metallic lith-
ium on the carbon surface. The electrolyte reduction on
the freshly formed surface of metallic lithium is respon-
sible for the change in the structure (an increase in the
density and thickness) of the solid-electrolyte film on
the negative electrode, which is accompanied by an
increase in its ohmic resistance, which in turn gives rise
to an additional increase in the polarization. Such
changes in the surface films lead to a gradual irrevers-
ible decrease in the capacity [166]. While the discharge
capacity changed during cycling to the 15th cycle fol-
lowing a relatively small decrease in the temperature
(to 
–10°ë) by no more than 5%, the capacity drop at
−20°ë amounted to 35%. Returning to room tempera-
ture did not lead to restoration of the initial state of the
battery: the discharge capacity amounted to a mere
85% of the initial value.
The stability of negative electrodes is also nega-
tively affected by the LIB storage at temperatures in
excess of 40
°C. The capacity decrease of fully charged
carbon electrodes during storage is always smaller than
that of fully discharged electrodes [105]. The reason for
this phenomenon is thought to be the destruction of the
surface film of a solid electrolyte, which leads to con-
tinuous delithiation of the bulk of the electrode with a
subsequent interaction of lithium atoms with the elec-
trolyte, i.e. to irreversible self-discharge of LIB.
To evaluate the contribution made by the negative
electrode to the self-discharge of LIB in a three-elec-
trode cell with a lithium counterelectrode and a lithium
reference electrode, Yazami et al. [169] investigated the
decrease in the capacity of a carbon electrode subjected
to a tenfold cycling and then stored at 
70°ë. For the
electrolyte they employed 1 M LiPF
6
in an EC–DMC
mixture. Based on the impedance investigations (the
electrode impedance perceptibly increased after stor-
age) and charge–discharge characteristics (the elec-
trode capacity dropped by 1.5–2.5 times as a function
of the storage duration), the authors of [169] make the
conclusion that, during storage at an elevated tempera-
ture, lithium that was intercalated into carbon diffuses
out of the space between graphene planes towards their
external surfaces, thus making it easier for the chemical
reactions between lithium and electrolyte and its impu-
rities to occur. The deposition of the products of these
reactions at the electrode surface is responsible for the
impedance increase, which in turn compromises the
discharge characteristics.

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RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41 No. 1 2005
DEGRADATION OF LITHIUM-ION BATTERIES
11
The change in the structure and surface properties of
the negative carbon electrode during the storage of LIB
at elevated temperatures was studied in [170] with the
aid of x-ray diffraction analysis, scanning electron
microscopy, and Fourier transform infrared spectros-
copy. The positive electrodes of LIB were manufac-
tured from LiMn
1.7
Al
0.3
O
4
doped with aluminum and
the electrolyte was 1 M LiPF
6
in a mixture of EC, DEC,
and EMC. The surface of the freshly prepared electrode
contained no noticeably pronounced film, whereas after
storage at 
50–75°ë the surface was covered by a thick,
nonuniform in thickness (from 40 to 200 nm), multilay-
ered solid-electrolyte film, which contained products of
polymerization of EC 
[(CH
2
CH
2
O
)