Nauka /Interperiodica

 
p
-tolylsul-
fide, and diphenylsulfide [129].
The authors of [24, 130] investigated the effect of
the storage duration, temperature (45–80
°
C), and elec-
trolyte additives on the characteristics of industrial LIB
containing negative electrodes prepared from mesocar-
bon microbeads mixed with mesocarbon fibers. The
positive electrodes were fabricated of Li
 
x
CoO
2
and the
electrolyte was a 1 M LiPF
6
solution in an EC–EMC
mixture containing such additives as VC and a lithium
organoboron complex (Li-OBC). They demonstrated
that the electrochemical behavior of the negative car-
bon electrodes depends on the stability of the surface
solid-electrolyte film, whose structure alters during the
storage at a constant potential due to partial dissolution,
and on the secondary reactions occurring in the initially
formed film. Adding VC and Li-OBC into the electro-
lyte slightly stabilizes the solid-electrolyte film.
Besides, Li-OBC hampers the deposition of metallic
cobalt on the carbon surface. The cobalt is presumed
[104] to emerge during the discharge of ions Co
2+
,
which are the product of dissolution of Li
 
x
CoO
2
that
forms during the cycling and prolonged storage of LIB.
Adding dimethyl pyrocarbonate and lithium disali-
cylateborate into the electrolyte exerts salubrious effect
on both the negative graphite electrode and the positive
electrode prepared from lithium cobaltite or lithium–
manganese spinel [131]. Adding lithium disalicylateb-
orate into the electrolyte makes the impedance of both
negative and positive electrodes decrease in the temper-
ature interval extending from 25 to 60
°
C, suppresses
reactions leading to the formation of 
, 
PF
5
, and HF,
and raises rates of electrode reactions. Adding dimethyl
pyrocarbonate into electrolytes based on LiPF
6
at an
elevated temperature (80
°
C) leads to the formation of a
stable passivating film at the surface of the negative
graphite electrode.
A strong film of a solid electrolyte can be created on
the negative carbon electrode by using electrolytes fab-
ricated of a lithium salt with an organic anion selected
from derivatives of phthalimide or phthalimidin [132]
and from imides or metides, for example,
LiN
(
CF
3
SO
2
)
2
, 
LiC
(CF
3
SO
2
)
3
, and LiN(C
2
F
5
SO
2
)
2
[133]. Other efficient electrolytes are made of a mixture
of two salts, including lithium tetrafluoborate [134].
Still others are prepared from a mixture of salts of two
groups, one of which includes LiPF
6
, LiBF
4
, LiAsF
6
,
and LiSbF
6
, while the other comprises derivatives of
lithium hexafluorophosphate or lithium tetrafluoborate
of the type LiPF
a
(C
b
F
2b + 1
)
6 – a
, where a
= 1–5 and b 
≥ 1,
or of the type LiPF
c
(C
d
F
2d + 1
SO
2
)
6 – c
, where c
= 1–5 and
d
≥ 1 [135].
DECOMPOSITION OF ELECTROLYTE IN LIB
A standard modern electrolyte for LIB is manufac-
tured chiefly based on lithium hexafluorophosphate
LiPF
6
. Using the latter involves some problems con-
nected with the fact that the salt is relatively unstable:
when solid, it decomposes at 
30°ë [136], and when dis-
solved, at 
80–85°ë [137]. Far better stability is inherent
in lithium perchlorate LiClO
4
, lithium triphlate
LiCF
3
SO
3
, lithium tris(trifluoromethylsulfonyl)imide
LiN
(CF
3
SO
2
)
3
[138], and lithium tris(trifluoromethyl-
sulfonyl)metid LiC
(CF
3
SO
2
)
3
[139, 140]. A very high
thermal stability is exhibited by recently synthesized
lithium bis[salicylate(2-)]borate and its derivatives
[141], whose decomposition temperatures in air exceed
260°ë.
In moistened electrolytes, lithium hexafluorophos-
phate readily undergoes hydrolysis with the formation
of acidic products (POF
3
, HF); it is at equilibrium with
PF
5
(LiPF
6
LiF + PF
5
) [137], which actively reacts
with the solvent. In particular, EC reacts with PF
5
and
undergoes polymerization with the evolution of carbon
dioxide [142]. Hydrofluoric acid easily enters reactions
with the material of the positive electrode, especially
with lithium–manganese spinel [143].
To hamper or completely suppress the hydrolysis of
lithium hexafluorophosphate and restrict the possibili-
ties of deleterious influence the acid components exert
on the electrode materials and electrolyte, it was sug-
gested to limit the overall content of hydrofluoric acid
and water in the electrolyte, reducing it to 1% [144].
According to [145], the decrease in the water content
may be achieved at the expense of regulation of techno-
logical process of the manufacturing of electrodes and
LIB as a whole. In so doing, provisions are made for the
PF
6
–

---

RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41
No. 1
2005
DEGRADATION OF LITHIUM-ION BATTERIES
9
performance of the process of the manufacturing of
electrode masses and electrodes and the assembling of
LIB in conditions of a low relative humidity of an air
atmosphere (<30%) as well as a preliminary drying of
electrodes at temperatures exceeding 150
°
C. To reach
the same aim, it is also proposed to add a sorbent in the
electrolyte (specifically, zeolite) with a specific surface
area in excess of 1000 m
2
g
–1
[146]. Efficient desiccants
are organophosphorus compounds. Hydrofluoric acid
and water can efficiently be absorbed by the organosil-
icon and organotin compounds [147] and by derivatives
of oxalic and carbonic acids [148]. For water absor-
bents one can also use such organosilicon additives as
vinyltrichlorosilane, trimethylsilane, and hexamethyld-
isilazan [98, 149]. The suppression of the harmful influ-
ence of acid components of the electrolyte may be
achieved by means of inserting into it substances of
alkaline nature, specifically, pyridine and its derivatives
[150] or amines NR
1
R
2
R
3
, where R
1
, 
R
2
,
and R
3
are
hydrogen atoms or organic radicals [151].
Some patents recommend to contain the hydrolytic
decomposition of lithium hexafluorophosphate at the
expense of inserting into the electrolyte derivatives of
phosphazine 
(
PNR
2
)