Nauka /Interperiodica

 
et al.
[109] showed that the acid impu-
rities that are generated at the positive electrode may
destroy the solid-electrolyte layer on the negative elec-
trode, which leads, as was demonstrated in [8], to
amplified deintercalation of lithium out of intercalate
LiC
6
.
Aurbach 
 
et al.
have performed fundamental investi-
gation of processes of the decrease in the characteristics
of LIB and carbon electrodes. According to [110], the
reasons for the decrease in the capacity of negative
graphite electrodes are the friability of graphite parti-
cles, which is caused by relatively weak links between
the graphene planes, which leads to stratification (exfo-
liation) of graphite, and the occurrence of aggressive
surface reactions on carbon in conditions of a surface
passivated not sufficiently enough, especially in the
cases where gas evolution occurs during a reaction,
which facilitates the loosening and disintegration of the
active mass. Anyway, even in the case of a relatively
stable and reversible anodic process, a prolonged oper-
ation on the negative carbon electrode is accompanied
by a discernible increase in its impedance. The reason
for this phenomenon is that a certain change in the
graphite volume caused by the lithium intercalation
leads to destruction of the surface film, which then
regenerates and its thickness increases.
Analyzing processes that occur on the surface of
electrodes in LIB, factors that lead to the worsening of
electrochemical characteristics of LIB, and ways to
improve the workability of LIB, Aurbach points out in
[23] that graphite electrodes, when cycled in most elec-
trolytes containing ethers and esters, certain unramified
alkyl carbonates, and propylene carbonate (PC), gradu-
ally loose their capability to reversible insertion of lith-
ium. The primary explanation of this phenomenon
involves insufficient passivation of electrode accompa-
nied by co-intercalation with Li
+
of solvent molecules
and exfoliation of graphene planes, as well as deactiva-
tion caused by the reduction of solvent molecules at the
crystal faces between the graphene planes, which leads to
the blocking of the transport of lithium ions in graphite.
The chief factor in the mechanism of degradation of
graphite electrodes in all electrolytes during prolonged
cycling, especially at elevated temperatures, is the
deactivation of graphite particles by surface films,
which gain thickness at the expense of co-intercalation
of the solvent and isolate the graphite particles from the
bulk electrode [111]. This is accompanied by a substan-
tial increase in the electrode impedance caused by the
decrease in the area of active surface of the electrode
and by the increase in the ohmic resistance of the film.
The principal path leading to a more stable operation of
negative electrodes is to improve physicomechanical
properties of the surface film, in the first place, to make
it more elastic and better adhere to the surface.
From the viewpoint of ensuring stable characteris-
tics of LIB with graphite anodes, the electrolyte sys-
tems based on LiAsF
6
and LiClO
4
are the most accept-
able (and accessible), as was established in [23] by
means of comprehensive investigation of the behavior
of negative and positive electrodes in various electro-
lytes, specifically, in solutions of LiPF
6
, 
LiAsF
6
,
LiClO
4
, and LiSO
3
CF
3
in traditional alkyl carbonate
solvents, i.e. in EC mixed with DMC, DEC, and ethyl-
methyl carbonate (EMC). Such electrolyte systems
contain no acid particles of the HF type. The film of the
composition 
(ëç
2
éëé
2
Li
)
2
, which forms on the sur-
face of graphite electrodes in such electrolytes as a
result of EC reduction, is stable, has a low impedance,
and exhibits excellent passivating and protecting prop-
erties. However, neither of the above salts is applicable
for practical use in LIB: one is ecologically hazardous
and the other is unsuitable for elevated temperatures.
An alternative acceptable from any viewpoint hap-
pened to be a new electrolyte system based on lithium
perfluoroalkyl phosphate LiPF
3
(
CF
2
CF
3
)
3
. Such an
electrolyte contains no acid components, its thermal
stability is higher than that of electrolytes based on
LiPF
6
, and it ensures long cycle life of the negative
graphite electrode and the positive electrode prepared
from LiMn
2
O
4
. The mechanism governing the behavior
of electrodes in an electrolyte based on
LiPF
3
(CF
2
CF
3
)
3
, which was determined with the aid of
impedance, differs from the mechanism governing the
behavior of electrodes in an electrolyte based on
hexafluorophosphate. In the former case, surface reac-
tions on the electrodes involve solvent molecules, while
in the latter case, the 
anions and HF. The thermal
stability of electrolytes based on LiPF
3
(
CF
2
CF
3
)
3
is
also marginally better than that of hexafluorophosphate
electrolytes [39]. The only factor restricting wide appli-
cation of the said electrolyte is its relatively high cost.
PF
6
–

---

8
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 41
No. 1
2005
KANEVSKII, DUBASOVA
The optimization of the electrolyte composition is
not the only way to substantially improve the cycling
characteristics of LIB. Another way is to insert various
additives into the electrolyte, in particular, vinylene
carbonate (VC) [112–114]. The latter forms on the sur-
face of electrodes a stable polymeric solid-electrolyte
film, which hinders electroreduction of the salt anion
with the formation of a layer of lithium fluoride at the
surface of the negative carbon electrode [115].
An efficient additive into an electrolyte based on PC
is ethylene sulfite [116, 117]. In the presence of this sul-
fur-containing analogue of EC, the region of reduction
of the solvent on the negative carbon electrode shifts in
the direction of negative potentials by more than 0.8 V
[118]. As a result, electrolytes that are used in LIB may
be based on PC, which is in some cases more preferable
than other electrolytes.
In addition to these additives, proposed were divi-
nylethylene carbonate [119]; dibenzyl carbonate [120];
derivatives of vinylethylene carbonate [121], phenyl
carbonates [122], 
γ
-halogenated cyclic ethers [123],
thiols, thioethers, and thioacetates [124]; phosphoric
anhydride [125], boron fluoride, and fluoboric acid and
their complexes [126]; phthalimide and phthalimidin
[127]; and organic additives containing boron and oxy-
gen [128]. To enhance the efficiency of action of indi-
vidual additives, it was suggested to add into an electro-
lyte simultaneously four to six different substances,
specifically, PC, VC, 1,3-propanesulfone, di-