Electrochimica Acta 45 (1999) 67±86
www.elsevier.nl/locate/electacta
On the correlation between surface chemistry and
performance of graphite negative electrodes for Li ion
batteries
D. Aurbach a,*, B. Markovsky a, I. Weissman a, E. Levi a, Y. Ein-Eli b
a
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
b
Electric Fuel Ltd., P.O. Box 641, Beit Shemesh 9900, Israel
Received 3 February 1999; received in revised form 13 April 1999
Abstract
This paper discusses some important aspects of the correlation between surface chemistry, 3D structure, and the
electrochemical behavior of lithiated graphite electrodes. By reviewing results obtained with di€erent electrolyte
solutions (e.g. ethylene carbonate-based solutions, propylene carbonate solutions, and ether-based systems), we
describe the stabilization and capacity fading mechanisms of graphite electrodes. One of the failure mechanisms
occurs at potentials >0.5 V Li/Li+, and relates to an increase in the electrode's impedance due to improper
passivation and a simultaneous change in the electrode's morphology, probably due to gas formation. At low
potentials (depending on the electrolyte solution involved), phenomena such as exfoliation and amorphization of the
graphite electrodes can be observed. Stabilization mechanisms are also discussed. In general, surface stabilization of
the graphite is essential for obtaining reversible lithiation and a long electrode cycle life. The latter usually relates to
precipitation of highly compact and insoluble surface species, which adhere well, and irreversibly, to the active
surface. Hence, the choice of appropriate electrolyte solutions in terms of solvents, salts and additives is very critical
for the use of graphite anodes in Li batteries. The major analytical tools for this study included FTIR and
impedance spectroscopies, XPS, and in situ and ex situ XRD in conjunction with standard electrochemical
techniques. # 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Lithium electrodes; Lithiated graphite; Li metal and Li ion batteries; Surface chemistry; FTIR; XPS; EDAX; In situ
XRD; EIS
1. Introduction
Li ion batteries are becoming increasingly important
in the world market of energy storage and conversion
devices. In parallel, this ®eld attracts increasingly more
prominent research groups in material, surface, and
* Corresponding author. Tel.: +972-3-531-8317; fax: +9703-535-1250.
E-mail address: [email protected] (D. Aurbach)
electrochemical science, because these systems are
highly complex and thus, their basic study is very challenging. Reviewing the highly proli®c literature in this
area shows clearly that the most popular anode materials for these batteries are carbons of graphitic structure [1,2].
Historically speaking, graphite was the ®rst type of
carbon with which reversible lithiation was explored. It
was found to be a good basis for the anodic reaction
in rechargeable Li batteries [3,4]. As is well known,
lithiation of graphite is an intercalation process in
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 1 9 4 - 2
68
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
Fig. 1. Typical voltammetric behavior of graphite electrodes at a very slow scan rate, n=4 mV/s in EC-DMC 1:3/LiAsF6 solution.
(Graphite ¯akes, KS-6, Timcal). Ultrathin, thin and thick electrodes correspond to submicronic (0.16 mg/cm2, see preparation procedure in Refs. [5,9]), 10 microns (3.3 mg/cm2), and 140 mm (12 mg/cm2), respectively. The current for the thin and ultrathin electrodes was normalized with that of the thick electrodes according to their mass ratios in order to have the 3 CV on the same scale.
The electrode area was 2.4 cm2 for the thin and the ultrathin, and 10.2 cm2 for the thick electrode.
which lithium is inserted between graphene planes. The
process involves phase transition between intercalation
stages according to the following set of equations:
tence of two phases (as veri®ed by in situ XRD
measurements [6,7]), and to the plateaus which characterize galvanostatic lithiation of these electrodes [1±7].
ÿÿ
*
C6 ‡ 0:083Li‡ ‡ 0:083eÿ )
ÿ
ÿLi0:083 C6 Diluted stage I, Lix C6 , xR0:083
…1†
ÿÿ
*
Diluted stage I Li0:083 C6 ‡ 0:083eÿ ‡ 0:083Li‡ )
ÿ
ÿLi0:166 C6 stage IV
…2†
ÿÿÿ
*
Stage VI Li0:166 C6 ‡ 0:056eÿ ‡ 0:056Li‡ )
ÿLi0:222 C6 Stage III
…3†
ÿÿ
*
Stage III Li0:222 C6 ‡ 0:278eÿ ‡ 0:278Li‡ ÿ
)
ÿLi0:5 C6 Stage II
…4†
ÿ
*
Stage II Li0:5 C6 ‡ 0:5eÿ ‡ 0:5 Li‡ ÿ
)
ÿ
ÿLiC6 Stage I
…5†
This nature of the lithiation process of graphite in
several types of nonaqueous Li salt solutions leads to
the voltammetric behavior of graphite electrodes
obtained at very low scan rates, shown in Fig. 1 (taken
from Ref. [5]). The various stages and the relevant
equations are marked in this ®gure. The four sets of
peaks corresponding to Eqs. (2)±(5) relate to coexis-
Practical graphite electrodes are usually composed of
micronic particles bound with polymers such as polyvinylidene ¯uoride (PVDF) to a metallic current collector. The structure thus formed is porous, and allows
the solution to penetrate among the particles and interact with them. As the particles in these electrodes are
more oriented (e.g. the basal planes of the graphite
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
particles are parallel to the current collector) and thinner, the peaks of their voltammograms (see Fig. 1) are
sharper, the hysteresis between the intercalation±deintercalation sets of peaks is smaller, and the speci®c
charge capacity may approach 372 mA h/g (LiC6),
which is the theoretical value. In such cases, these
composite electrodes may be considered as being an
array of microelectrodes (the graphite particles), which
react in parallel with lithium ions from the solution
phase [5,7±9].
However, from the early stages of the study of these
electrodes it became clear that obtaining the above optimal behavior depends strongly on the solution used,
due to the unique surface chemistry developed on the
electrodes as a function of the various solution components. It was suggested that exfoliation of the graphite particles due to cointercalation of Li ions and
solvent molecules destroys the active mass during Li
intercalation into graphite in a large variety of solution
compositions [1±4]. Hence, the necessary condition for
stabilization of graphite electrodes against such
phenomena is the formation of thin, passivating surface ®lms which allow only Li ion migration through
them, leaving out the solution species. In order to
obtain such a stabilization, these surface layers have to
be formed at suciently high potentials above Li intercalation potentials [10,11]. Indeed, during recent years
we have seen intensive work done in two parallel
areas, namely, the search for new solvents, salts, and
additives in which graphites behave reversibly due to
the unique surface chemistry developed [12±21], and a
rigorous study of the surface chemistry developed on
graphite in a variety of electrolyte solutions [10,11,22±
46]. Attempts were made to identify the composition
of the various surface species formed by FTIR spectroscopy
[10,11,22,27,29,32,34,37±39,42,43,45,46]).
However, there are also successful attempts to use
other surface sensitive techniques such as in situ
Raman spectroscopy[31,33], XPS[36], and ellipsometry[40]. In situ morphological studies of these electrodes by AFM should also be mentioned[30,35,38].
There are also reports on the application of TEM for
the study of exfoliated graphite particles after failure
of the electrode in solutions based on propylene carbonate[28].
These studies show that there are only a few cases in
which graphite electrodes behave reversibly during prolonged cycling in single solvent solutions. These
include asymmetric alkyl carbonates[47] (e.g., methyl
ethyl carbonate[25], methyl propyl carbonate[26]),
chloropropylene carbonate[48] and chloroethylene carbonate [13]. There is also a report on the reversible
behavior of graphite electrodes in 1±3 dioxolane/
LiAsF6 solutions[49]. However, since the solutions
used in this study contained no stabilizer, it is quite
possible that the stability of this system was obtained
69
due to the presence of oligomers of 1±3 dioxolane. It
is well known that in the absence of basic additives,
this solution is not stable, and the solvent tends to
polymerize via cationic mechanisms due to the unavoidable presence of trace Lewis acids in solutions[50].
A remarkable stabilization of graphite electrodes in
a large variety of solutions is obtained by the use of
EC as a cosolvent[23,24,51±53]. The reason for this is
discussed later in this paper.
The sensitivity of graphite electrodes to the composition of the electrolyte solutions, and the fact that in a
large variety of solutions the electrode speci®c charge
capacity deteriorates upon prolonged charge±discharge
cycling, led to an intensive search for other types of
carbons. In general, as graphitic carbons are less
ordered than normal graphite, and their structure
includes turbostratic disorder, so they are more stable
upon cycling because their exfoliation does not occur
as easily as it does in natural or synthetic graphite
¯akes [54,55].
Intensive e€orts were made in recent years to ®nd
alternative carbon anodes for Li ion batteries.
However, although this subject is beyond the scope of
the present paper, we should mention the continuous
attempts to develop hard, highly disordered carbons in
which a high capacity of lithium insertion can be
obtained[56±65] up to a stoichiometry of LiC2 [56,57].
It should be noted that the ®rst charging process of
these carbons (Li insertion) is accompanied by huge,
irreversible charge loss (several hundreds of mA/g).
Other types of carbon which are currently studied are
the soft, graphitizable carbons, which include coke,
mesophase pitch based carbons, and ®bers at di€erent
degrees of graphitization[66±75]. A common denominator in all the soft carbons is that the gain in stability
(due to partially disordered structure) is achieved at
the expense of capacity, which is usually less than that
obtained with graphite.
In spite of the above drawbacks of graphite electrodes in terms of stability and cycle life, and in parallel
to the search for and development of alternative carbons, extensive work is currently being carried out
with graphite electrodes in the following directions:
1. The electroanalytical behavior of graphite electrodes
as a model host material for lithium intercalation is
being studied extensively. Points of interest are
intercalation mechanisms[5±9,76±82], stage structures[83±91], electrode impedance,[9,92±94] and Liion solid state di€usion[9,92,95].
2. The performance of graphite anodes as a function
of their structure[96±100], morphology [101,102],
surface treatments and operation conditions is currently being explored. [103±108]
3. Grinding (e.g. by a ball mill) [109,110], addition of
deposits such as silver [111], chemical passivation
70
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
[112], expansion of the graphite structure by means
of acid treatment [113] or intercalation of large ions
such as K+ (e.g., using KC8 as the starting material) [114] were described as routes for improving
the performance of carbon anodes whose precursor
was graphite.
The above review demonstrates how vital the study
of graphite electrodes is in connection with R&D of Li
ion batteries, and shows that a great deal of work has
been devoted to these systems. Nevertheless, there are
still unresolved questions that relate mostly to stabilization and capacity fading mechanisms of these electrodes. The present paper combines previous and new
results in an attempt to improve our understanding of
how lithiated graphite electrodes are stabilized in particular solutions, and when and how they fail in other
solutions. The tools for this study included surface sensitive FTIR spectroscopy, XPS, in situ and ex situ
XRD, SEM, chronopotentiometry, slow scan rate cyclic voltammetry, and impedance spectroscopy.
2. Experimental
We used composite graphite electrodes, which were
comprised of graphite ¯akes (Timcal AG, Switzerland,
Timrex Ð synthetic graphites KS-6, KS-25, KS-44, or
natural graphite from Superior Graphite Inc.) bound
with PVDF (5±10% by weight) onto copper current
collectors. Their preparation and study by standard
electrochemical techniques (e.g., chronopotentiometry)
have already been described[5±11,22±27].
Surface studies of graphite electrodes by FTIR spectroscopy were described in Refs. [23±26]. In brief,
graphite powder from electrodes before and after electrochemical processes was analyzed using di€use re¯ectance or transmittance modes. In the last mode, the
carbon particles were pelletized with KBr. The performance of EIS measurements of these electrodes was
described in Refs. [9,27,92]. Our in situ and ex situ
XRD measurements of graphite electrodes are reported
in Refs. [6,7,27]. Our XPS measurements were
described in Ref. [24].
We used alkyl carbonate solvents of the highest
purity from Merck Inc., Tomiyama Inc., and
Mitsubishi Inc. LiPF6 and LiC(SO2CF3)3 solutions in
EC-DMC were obtained from Merck Inc. (1 and 0.75
M, respectively). LiAsF6 was obtained from FMC Inc.
The water content in the solutions was usually around
20 ppm, and the HF content in the LiPF6 solutions
was a few tens of ppm (<100 ppm)[6]. All the electrochemical measurements were carried out under highly
pure argon atmosphere at room temperature
(258C2 0.5) in glove boxes or in hermetically sealed
cells. All the preparations for the spectroscopic studies
Fig. 2. (a) FTIR spectra measured from synthetic graphite
particles taken from an electrode that was lithiated and
delithiated in EC/LiAsF6 1 M solution (transmittance mode)
(b) Same as for (a), the solution was EC-DEC 3:1/LiAsF6 1
M under 6 atm of CO2. (c) The FTIR spectrum of the EC
cathodic electrolysis product (in THF(C4H9)4NClO4 0.2 M
solution),
isolated
as
a
Li
salt
(identi®ed
as
(CH2OCO2Li)2[115]).
were done under highly pure argon atmosphere in
glove boxes.
3. Results and discussion
3.1. On the surface chemistry of highly reversible
lithiated graphite electrodes
It is well known that EC can be considered as a
magic cosolvent with which lithiated graphite anodes
behave highly reversibly in a large variety of electrolyte
solutions. Even in cases where a reversible Li intercalation cannot be obtained at all, such as in single solvent
solutions based on DEC, DMC or ethers (e.g. THF,
2Me-THF, glymes), the addition of EC to these solutions leads to highly reversible behavior of lithiated
graphite anodes[23,24,51±53]. This fact is especially
interesting, because the lithiation of graphite cannot be
obtained at all in PC solutions (PC as a single solvent),
although the only di€erence between EC and PC molecules is the methyl group in the last molecule. Hence,
the obvious question is what is unique in EC that
leads to the highly reversible behavior of lithiated
graphite in its solutions? As explained in the introduction section, the answer lies in the surface chemistry of
the graphite electrodes in the presence of EC.
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
71
Fig. 3. FTIR spectra measured from synthetic graphite particles taken from electrodes lithiated±delithiated in 3 EC-DMC 1:1 Li
salt solutions, as indicated (LiAsF6 1 M, LiPF6 1 M, and LiC(SO2CF3)3 0.75 M) di€use re¯ectance mode.
Figure 2 compares FTIR spectra measured from
graphite particles after one lithiation±delithiation cycle
in EC/LiAsF6 1 M solution (2a), EC-DEC 3:1/LiAsF6
1 M solution under 6 atmospheres of CO2 (2b), and
spectrum of (CH2OCO2Li)2 (2c). The last reference
compound is the major electrolysis (reduction) product
of EC in THF/(C4H9)4NClO4 solution (with CH2CH2
as the coproduct), crystallized as a Li salt [115]. Partial
peak assignments appear in the ®gure. In spite of the
diculties in obtaining high-resolution spectra from
the surface species, on graphite particles, both spectra
2a and 2b are very similar to spectra 2c. As already
shown, CO2 reacts on noble metals and graphite electrodes at low potentials in the presence of Li salt to
form Li2CO3 (and probably CO as the coproduct)
[10,11,22]. Hence, graphite electrodes which are
lithiated in a large variety of solutions based on ethers,
esters and open-chain alkyl carbonates (e.g. DMC,
DEC) under CO2 atmosphere, are usually covered by
surface ®lms in which Li2CO3 is a major constituent.
However, spectrum 2b, which relates to an experiment conducted under CO2 atmosphere (6 atm),
shows no pronounced peaks of Li2CO3. The spectrum
is dominated by the peaks of the EC reduction product. Hence, one can conclude that EC is a highly
reactive solvent and therefore, when it is a major component in an electrolyte solution, its reduction domi-
nates the lithiated graphite surface chemistry and
suppresses other reactions such as those of CO2 on the
active electrode's surface. Fig. 3 compares FTIR spectra measured from graphite particles taken from electrodes that were cycled in EC-DMC (1:1) solutions of
LiAsF6, LiPF6 and LiC(SO2CF3)2 (as indicated). The
three spectra di€er from each other in their general
shape, relative height of the peaks, and also in some
regions, especially in the high wavenumber region
(>2500 cmÿ1). However, all of them contain, as dominant absorptions, the typical peaks of the EC reduction products, one of which is probably
(CH2OCO2Li)2, as demonstrated in the comparison of
the spectra in Fig. 2.
The nCH peaks in the 2700 cmÿ1ÿ2820 cmÿ1 region
appearing in part of the spectra prove that the surface
®lms thus formed also contain alkoxy species (e.g.
CH3OLi), as indeed expected for these systems[23±26].
The peaks in the 3000±3700 cmÿ1 region appearing in
the spectra related to the LiPF6 solutions are typical of
hydroxy groups [116]. This can be attributed to the
fact that LiPF6 solutions are always contaminated by
HF. Hence, reaction of HF with (CH2OCO2Li)2 may
form surface species such as CH2(OCO2H)
CH2OCO2Li (and LiF as a coproduct) to which the
nOH peaks in Fig. 3 may belong. Surface element
analysis of these electrodes by EDAX clearly showed
72
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
Fig. 4. XPS carbon 1s peaks obtained from a lithium electrode and a graphite electrode (as indicated). The dashed lines
are spectra measured after 30 s of sputtering (argon ions).
Both electrodes were treated in EC-DMC/LiAsF6 1 M solutions. The Li electrode was freshly prepared in this solution,
followed by storage for a few hours. The graphite electrode
was lithiated±delithiated (one cycle) before the measurement.
that all three salts are also reduced on the graphite surfaces. The EDAX spectra of graphite electrodes treated
in LiAsF6, LiPF6 and LiC(SO2CF3)3 solutions contained As, P and S peaks, respectively, in addition to
the expected C, O and F (PVDF) peaks. Hence, the
FTIR spectra of Fig. 3 are obviously complicated by
absorption of groups such as LixAsFy (major peak
around 700 cmÿ1) [117], LixPFy and LiPFyOz (major
peaks around 850 and 1000 cmÿ1) [118], or LixSOyCFz
(peaks around 1000, 1100, 1200 and 1300 cmÿ1) [117],
which are the expected reduction products of LiAsF6,
LiPF6 and LiC(SO2CF3)3, respectively [117,118]. The
obvious presence of such groups in the surface ®lms
on the graphite particles may be one of the reasons for
the observed di€erences among the spectra of Fig. 3.
However, comparison between these spectra and spectra of reduction products of the salts used [117,118]
leads to the conclusion that none of these spectra of
Fig. 3 contain pronounced peaks of salt reduction products.
Hence, our conclusion from these studies is that no
matter what salt is used, the surface ®lms covering the
graphite electrodes in EC-DMC 1:1 solutions are
dominated by the EC reduction products. In addition
to (CH2OCO2Li)2 and CH2CH2, reduction of EC on
graphite may form polymers, as suggested by several
authors[42,119]. However, none of the spectral tools
that we used so far (FTIR, XPS, EDAX) could verify
this possibility. XPS studies of both Li and lithiated
graphite electrodes treated in the above solutions contributed another piece of information that could not
be reached at by FTIR spectroscopy, namely, the presence of species with Li±C bonds in the surface ®lms
(C1S peaks of binding energies below 283 eV). This is
demonstrated in Fig. 4, which compares carbon spectra
of Li and Li-graphite electrodes after being treated in
EC-DMC/LiAsF6 solutions. The lithium electrode was
freshly prepared in solution, stored for 3 h and
measured as already described [120]. The graphite electrode was lithiated±delithiated before the measurement.
The peak assignments appearing in this ®gure are
based on Refs. [120,121, 130], and the previous knowledge from the FTIR spectroscopic studies [10,11,22±
27]. A possible EC reduction product which contains a
Li±C bond is LiCH2CH2OCO2Li.
Figure 5 shows a general reaction scheme of the
possible EC reactions on graphite surfaces based on
the above studies. Note that the major EC reduction
product that we analyzed (CH2OCO2Li)2 may be
formed either by disproportionation of the anion radical (formed by a single electron transfer to EC), or by
a two-electron transfer which form CO.3, which further
attacks nucleophilically another molecule of EC (as
already discussed)[27]. The coproduct in the above
reaction pattern CH2CH2 can either be liberated as
ethylene gas, or polymerize on the carbon surface.
This scheme also suggests a possible formation of
species such as LiCH2CH2OCO2Li, as evident from
the XPS studies. It should be emphasized that PC reduction patterns may be quite similar to those of EC.
However, as already discussed [10,22,27,115], its major
reduction product contains a methyl group (see Fig. 5)
which may detrimentally in¯uence its sedimentation in
passivating surface ®lms [10,22,27].
In comparing EC and PC, the methyl group of PC
may have another important e€ect. It slows down the
kinetics of PC reduction on active surfaces as compared with EC. We studied the reactions of EC and
PC with Li/Hg amalgam, from which it was clear that
EC is much more reactive than PC in reduction processes (which form a mixture of ROCO2Li and
Li2CO3). The formation of e€ectively passivating surface ®lms may also be connected to the fast kinetics of
the reduction of solvent molecules to form insoluble Li
salts (which is faster than detrimental processes such
as the coinsertion of solution species into the graphite).
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
73
Fig. 5. The various reduction patterns of EC on graphite and the relevant product distribution. The chemical structure of the
expected major PC reduction product is also shown for a comparison.
Another important discovery is the superiority of
asymmetric alkyl methyl carbonate as a basis for single
solvent solutions in which lithiated graphite anodes
behave irreversibly[47]. We explored the surface chemistry of lithiated graphite in solutions based on DMC,
MEC and MPC in an attempt to understand the di€erence in the behavior of graphite electrodes in DMC
(poor) and the other methyl alkyl carbonates.
Figure 6 compares an FTIR spectrum measured
from a graphite electrode after being cycled in a MPC/
LiAsF6 solution (6a) and three reference spectra.
Spectra 6b and 6c were obtained from Li electrodes
freshly prepared in this solution and in DMC/CH3OH
0.1 M, respectively, and were then measured as
already described. [122] Spectrum 6d belongs to a
thin ®lm of CH3OLi on a re¯ective Li surface, prepared and measured as already described [123]. The
peak assignment appears near these spectra. In a
previous study, we synthesized CH3CH2CH2OLi and
CH3CH2CH2OCO2Li, which are possible products of
MPC reduction, and measured their IR spectra [124].
The above comparison and related analysis show
that on lithium the major surface species in MPC are
CH3OLi and CH3OCO2Li, while on graphite it seems
that CH3OCO2Li and Li2CO3 are important surface
species. It is assumed that PrOLi and PrOCO2Li are
also formed on the active surfaces. However, the actual
major, stable components in the surface ®lms are the
methoxy derivative and Li2CO3. The latter compound
can be formed either by further reduction of ROCO2Li
in the presence of Li ions, or by reaction of ROCO2Li
with trace water to form Li2CO3, ROH and CO2
[118,124].
Hence, we concluded that the stability of the graphite electrodes in solvents such as MEC or MPC relates
to a selective deposition of Li2CO3 and CH3OCO2Li,
which together form highly passivating ®lms. We
assume that this selectivity is achieved by a relatively
higher solubility of ROCO2Li or ROLi species in
which R is a bulkier group than CH3, compared with
CH3OLi and CH3OCO2Li. Another possibility that
should be mentioned is a nucleophilic reaction between
CH3OLi, when formed, and solvent molecules according to the following equation:
74
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
Fig. 6. (a) FTIR spectra of graphite particles taken from an electrode which was lithiated±delithiated (one cycle) in MPC/1 M
LiAsF6 solution (di€use re¯ectance mode). (b) FTIR spectra measured ex situ (external re¯ectance mode) from a lithium electrode
freshly prepared in MPC/1 M LiAsF6 solution and stored in it for 3 h (see experimental details in Ref. [26]). (c) Same as (b), Li
electrode, DMC/CH3OH 0.1 M solution. (d) A reference FTIR spectrum of a thin ®lm of CH3OLi on a re¯ective Li surface (external re¯ectance mode, experimental details in Ref. [123]).
CH3 OLi ‡ ROCO2 CH3 4 CH3 OR ‡ CH3 OCO2 Li
…6†
It should be noted that CH3OLi can react nucleophilically with these molecules to also produce ROLi and
ROCO2Li. However, due to their expected higher solubility than CH3OCO2Li, they do not participate in the
surface ®lm formation, as is evident from the spectral
studies. We attribute the advantage of solvents such as
MEC or MPC over DMC for lithiated graphite
anodes, to the high concentration of Li2CO3 in the surface ®lms formed on graphite in the ROCO2CH3 solvents, as is evident from the spectral studies. This is
due to the fact that the di€erence in solubility between
Li2CO3 (formed by a direct two-electron reduction
process or by reactions of ROCO2Li with trace water)
and the organic Li salts (formed by solvent reduction)
is expected to be higher in ROCO2CH3 than in
CH3OCO2CH3 (DMC) because the latter solvent is
more polar.
The last subject in this review which can provide an
important clue to stabilization mechanisms of graphite
electrodes relates to the e€ect of additives. We describe
herein the e€ect of SO2 and CO2 gases as additives to
solutions in which lithiation of graphite is not a reversible process.
Figure 7, reproduced from our previous paper [125],
shows typical chronopotentiograms of graphite electrodes and ®rst cycle voltammograms of platinum electrodes in LiAsF6/DMC and LiAsF6/DMC/SO2 solutions,
as indicated. This ®gure demonstrates the typical deterioration of lithiated graphite electrodes in DMC/
LiAsF6 solutions during the ®rst few cycles [11,25].
However, the addition of even a low percentage of
SO2 (1%), changes the behavior considerably. The relevant chronopotentiogram in Fig. 7 re¯ects the reduction of SO2 at potentials between 2.5±3 V (Li/Li+).
This reaction indeed involves a pronounced irreversible
charge capacity loss in the ®rst lithiation process.
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
75
denced by the strong peaks at 900 and 1000±1100
cmÿ1 that dominate spectra 8b,c. Spectrum 8d demonstrates clearly that in the presence of CO2, the graphite
electrode surface chemistry becomes dominated by
Li2CO3 formation according to the following general
equation [115].
2CO2 ‡ 2Li ‡ 2eÿ 4 Li2 CO3 ‡ CO
Fig. 7. Chronopotentiograms of graphite electrodes in DMC/
LiAsF6 1 M solution containing 10% (by weight), SO2 (thick
line), and in SO2 free DMC/LiAsF6 1 M solution (thin line)
C/15 rate. In the insert, ®rst cycle voltammograms of Pt electrodes in DMC/LiAsF6 1 M, 1% by weight SO2 (thick solid
line), and in SO2 free solutions are shown (20 mV/s). For
both experiments, lithium foils served as counter and reference electrodes.
However, it is followed by completed lithiation of the
carbon, and further highly reversible behavior of the
lithiated graphite electrode. The voltammograms
shown in this ®gure indicate that some reduction process around 2.5 V (Li/Li+) occurs on Pt in the
LiAsF6/DMC/SO2 solution and leads to electrode passivation (low cathodic currents of potentials below 2 V
compared with the SO2 free solution). Similar behavior
was obtained with CO2-containing solutions, but with
one di€erence: CO2 is reduced on noble metal or carbon electrodes at much lower potentials than SO2
(<1.2 V vs. Li/Li+)[22,126]. The e€ect of these two
additives on the surface chemistry of graphite electrodes is demonstrated in Fig. 8, which shows FTIR spectra obtained from graphite electrodes treated in SO2/
DMC and CO2/g-butyrolactone (BL) solutions (1 M
LiAsF6). Both solvents are reduced on graphite electrodes at potentials below 1.2 V (see Fig. 7 as an example
for DMC solutions). After being cycled in SO2-free
DMC solutions, the FTIR spectra of graphite electrodes are somewhat similar to the spectrum of Fig. 6c
(which belongs to a combination of CH3OLi and
CH3OCO2Li, as discussed above).
Spectrum 8e is the typical FTIR spectrum of graphite electrode in BL solutions, and re¯ects the formation
of the g-alkoxy cyclic b-keto ester anion which is the
major BL reduction product on lithium and noble
metals at low potentials [127,131,132]. In the presence
of SO2, the surface chemistry of graphite becomes
dominated by the formation of Li2S and LixSOy compounds (e.g. Li2SO3, Li2S2O4, Li2S2O5) [125], as evi-
…7†
In situ studies by FTIR spectroscopy of CO2 reduction on noble metals in Li salt solutions provided
some evidence for the formation of CO as a coproduct
to Li2CO3 [128]. The above results converge to the
conclusion that stabilization of lithiated graphite electrodes needs the formation of highly compact and passivating surface ®lms at potentials as high as possible.
In order to obtain such ®lms, the surface species
should be as small as possible, and should contain centers through which they can adhere well to the graphite
surface. Formation of surface polymeric species is not
essential for stabilization of these electrodes.
The list of good passivating agents includes
(CH2OCO2Li)2, Li2CO3, LixSOy, Li2O, CH3OLi and
CH3OCO2Li. All of these species have a common denominator: They are highly compact and polar, and
we can attribute their passivating properties to their
possible good adhesion to the graphite surfaces and/or
their cohesion due to polarity (ionic structure).
In Fig. 5 we suggest some possible conformations of
these surface species (demonstrated for the carbonates
as an example) on the graphite surface, in which good
adhesion can be obtained between them and the active
surface due to attractive interactions. We can speculate
on the existence of possible electrostatic interactions
between the negatively charged graphite and partially
positively charged carbons and lithium ions of the surface species. Li ions adsorbed to the carbon surface
may also play a role in adhering surface species to the
graphite via their interaction with the partially negatively charged oxygen of the above surface species (as
shown schematically in Fig. 5).
3.2. Impedance spectroscopy of lithiated graphite
electrodes
Impedance spectroscopy may serve as an excellent
tool for in situ characterization of the properties of
insertion electrodes in general, and lithiated graphite
electrodes in particular. The impedance characteristics
of pristine electrodes, as well as impedance features of
electrodes during cycling, provide very useful information on the stabilization and failure mechanisms of
Li-graphite electrodes. In this section we show some
examples of impedance characteristics of reversible
lithiated graphite electrodes as an introduction to the
next section, which deals with the failure mechanisms
76
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
Fig. 8. FTIR spectra measured from graphite particles taken from electrodes treated in various LiAsF6 1 M solutions containing
additives (transmittance mode). (a) Electrode was immersed in DMC/LiAsF6/SO2 (10% by weight) solution and was held at open
circuit for a few hours before the measurement. (b) Same as (a), electrode was polarized to 2.5 V (Li/Li+) and equilibrated at this
potential. (c) Same as (b), electrode was polarized and equilibrated at 0.6 V (Li/Li+). (d) The graphite electrode was cycled once
(lithiation±delithiation) in g-butyrolactone/1 M LiAsF6 solution under 6 atm of CO2. (e) Same as (d), argon atmosphere.
of these electrodes. The Li intercalation into graphite
is a serial multistep process in which Li ions have to
®rst migrate through the surface ®lms that cover the
electrodes, after which the insertion into the carbon is
accompanied by charge transfer at the ®lm-carbon
interface, followed by solid state di€usion of lithium
into the graphite. Finally, lithium accumulates within
the bulk via phase transition between the various intercalation stages. We made an intensive study of the EIS
of lithiated graphite electrodes in a variety of con®gurations and solution compositions[5,7,8,9,23,24,27,92]
and found that the impedance spectra of these electrodes clearly re¯ect the serial, multistep nature of the Li
insertion±deinsertion processes. A reasonable separation of time constants is obtained. This is demonstrated in Fig. 9, which relates to a thin (submicronic,
see preparation procedure in Ref. [5]) composite graphite electrode in an EC-DMC/LiAsF6 1 M solution (in
which these electrodes are highly reversible and stable).
The big semicircle (high-medium frequencies) which
can be modeled by a `Voigt'-type analog (as indicated),
belongs to Li migration within the surface ®lms
coupled with ®lm capacitance (in the order of a few
mF/cm2 or less). At lower frequencies, the spectra
include features which are usually semicircular in
shape and probably relate to some kind of charge
transfer (potential-dependent, as indicated), coupled
with high capacitance (in the order of several mF/
cm2).
Charge transfer resistance can be related to three
di€erent processes:
1. Li-ion transfer at the solution±surface ®lm interface,
2. Li-ion transfer at the surface ®lm±graphite interface,
3. Interparticle electron transfer.
The high pseudo capacitance coupled with this
charge transfer resistance can be related either to the
high surface area, the outer part of the surface ®lms,
which should be porous (as is the case of lithium electrodes) [129], or to adsorption phenomena at the surface ®lm±graphite interface. As the graphite electrode
is thicker and the particles are less oriented, these low
frequency features are more developed (see further
results). This may indicate that these spectral features
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
77
gration through the surface ®lms is supposed to be
potential-independent. This means that the big semicircle shown in the spectra of Fig. 10 also re¯ects
some kind of interfacial charge transfer in addition
to Li+-ion migration through the surface ®lms.
Fig. 9. Typical impedance spectrum measured from an ultrathin electrode (same as that of Fig. 1) at equilibrium potentials, as indicated. The electrode was lithiated±delithiated
several times before these measurements in order to stabilize
the surface ®lms. EC-DMC 1:3/LiAsF6 1 M solution. The
various elements in the spectra, marked as equivalent circuit
analogs, are speci®ed.
indeed relate to interparticle electron transfer and to
the degree of porosity of the composite electrode.
At the lower frequencies, the impedance spectra contain a potential-dependent Warburg element that
relates to the potential-dependent solid state di€usion
of lithium into graphite. Finally, at the very low frequencies, the Z0 vs. Z' plot becomes very steep, and in
fact, re¯ects the intercalation capacitance (Cint 1 ÿ1/
oZ0, o=2 pf 4 0)). It is important to note that at
o 4 0, 1/oZ0 as a function of potential correlates very
well with the slow scan-rate CV shown in Fig. 1. The
thinner the electrode and the more oriented the graphite particles which compose it, the better the correlation, and the closer the values of the Cint(V)
calculated from the CV and the EIS.
Figure 10 shows impedance spectra measured at a
few selected potentials from a 10 micron thick electrode composed of natural graphite particles, in ECDMC 1:3/LiAsF6 1 M solution. These spectra also
show a clear separation of time constants and, in fact,
contain the same elements as the spectrum of Fig. 9.
However, there are two di€erences that should be
noted.
1. At the very low frequencies, Z0 vs. Z ' is much less
steep than for submicronic thick electrodes.
Consequently, only a qualitative correlation exists
between Cint calculated from the SSCV and that calculated from Z0 at o 4 0.
2. The large high-medium frequency semicircle is also
potential-dependent for thick electrodes. It expands
(reversibly) at low potentials, as shown in the insert
appearing in Fig. 10. The resistance for Li+ ion mi-
Figure 11 presents a typical e€ect of prolonged
cycling on the electrode impedance. Spectra measured
from a thick (140 mm) electrode comprised of synthetic
graphite ¯akes whose average width was around 25
mm, in an EC-DMC/LiAsF6 solution after 3 and 75
charge±discharge cycles, are compared. As demonstrated in this ®gure, in spite of the initially stable and
highly passivating surface ®lms formed on Li-graphite
electrodes in this solution, upon cycling their impedance increases considerably. As proven by in situ and
ex situ XRD, as well as by SEM observation and
chronopotentiometric studies, in these solutions Ligraphite electrodes can be cycled hundreds of cycles
and still remain integrated, retaining their capacity
upon prolonged cycling. Hence, Fig. 11 mostly re¯ects
changes in the surface structure of these electrodes
which increase their impedance. We assume that upon
prolonged cycling there are phenomena such as expansion and contraction of the graphite particles' volume,
as well as some degree of microexfoliation of the
graphite particles on their surface, which leads to local
breakdown in the electrode's passivation (on a microscopic level). This allows the continuous reduction of
solution species. While this process occurs on a very
low scale, it thickens the surface ®lms and consequently, the electrode's impedance increases, particularly in the time constants that relate to Li+-ion
migration through the surface ®lms, whose increasing
thickness upon cycling makes them more resistive.
The last point dealt with in this section relates to
solvent e€ects on the stabilization of Li graphite electrodes, as monitored by impedance spectroscopy.
Lithiated graphite electrodes were cycled (a few galvanostatic charge±discharge cycles), and then stored
periodically at a low (Li intercalation) potential. The
electrodes were measured during these experiments by
EIS after di€erent periods of storage at the low potential (close to the storage potential).
Figures 12 and 13 show typical Nyquist plots
measured at 90 mV (Li/Li+) after di€erent periods of
storage time at this potential, from similar Li-graphite
electrodes in EC-DMC 1:1 and EC-DMC 1:5 LiAsF6 1
M solutions, respectively. As clearly shown in these
®gures, at a high concentration of EC the electrode's
impedance is higher, but highly stable upon prolonged
storage. The spectra of Fig. 12 are very similar to the
spectra of lithium electrodes in these solutions [129].
This means that the dominant features in the electrode's impedance relate to the surface ®lms. In contrast,
at a lower concentration of EC (Fig. 13), the surface
78
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
Fig. 10. A family of impedance spectra measured at di€erent equilibrium potentials, as indicated, from graphite electrodes comprised of natural graphite particles, in EC-DMC 1:3/LiAsF6 1 M solution. The electrode's area was 10.8 cm2 (1.5 mg total active
mass). The electrode was cycled (a few lithiation±delithiation cycles) before these measurements in order to assure formation of
stable surface ®lms. The insert shows the dependence of R1 ± the electrode's interfacial resistance on the potential (calculated from
the diameter of the high frequency semicircles in the Nyquist plots).
®lms are not the dominant factor that determines the
electrode's impedance, and the behavior is not stable,
i.e., the electrode's impedance, especially in the features
which relate to the surface ®lms, increases upon storage. Hence, we can conclude that the high EC concentration and the prolonged storage of the graphite
electrode at low potential, probably leads to the formation of surface ®lms in which the EC reduction pro-
duct is the major, if not the only, constituent. The
expected uniformity of the surface ®lms in this case
probably leads to their high stability (Their relatively
high resistance may be an intrinsic feature of a highly
uniform surface ®lm, with only a small concentration
of defects). In contrast, using solutions of a low EC
concentration, DMC reduction also contributes to the
electrode's surface chemistry and hence, the compo-
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
Fig. 11. Impedance spectra measured from a thick (140 mm,
active mass was 12 mg/cm2) graphite electrode (Timcal KS-25
synthetic graphite ¯akes) in EC-DMC 1:1/LiAsF6 1 M solution, at 110 mV vs. Li/Li+. (a) After 3 galvanostatic cycles
(white circles). (b) After 75 galvanostatic cycles at C/7 rate
(black circles).
sition of the surface ®lms is much less homogeneous.
This may obviously lead to a much less resistive ®lm
(compared with a homogeneous one), due to the possible large number of defects. However, for the same
reason, it is also a better electronic conductor than a
homogeneous ®lm. Thereby, stabilization takes much
longer, due to possible continuous low-scale reduction
of solution species via electron tunneling through such
surface ®lms.
79
Fig. 13. Same as Fig. 12. EC-DMC 1:5/LiAsF6 1 M solution.
(a)±(d) Ð steps 1, 2, 3, 4, EIS measurements after 10, 275,
375 and 425 h rest periods.
3.3. Failure mechanisms of lithiated graphite electrodes
An obvious failure mechanism of graphite anodes
that was discussed in the literature [1±4] is the exfoliation of the graphene planes due to cointercalation of
solution species, together with the Li ions, in the
absence of appropriate passivation of the electrodes.
This is demonstrated in Fig. 14, which relates to
graphite electrodes in LiClO4 0.5 M/diglyme
Fig. 12. Impedance spectra measured from graphite electrodes (140 mm thick, area of 1 cm2, 12 mg active mass per cm2) at 90 mV
(Li/Li+) after di€erent periods of storage at this potential in EC-DMC 1:1/LiAsF6 1 M solution. The experiment included repeated
consecutive steps. In each step the electrode passed a few intercalation±deintercalation cycles, then rested at 90 mV (Li/Li+) for as
many hours as indicated, followed by an EIS measurement. (a) First step, EIS measured after 100 h. of rest at 90 mV (Li/Li+). (b)
Second step, EIS measured after 200 h rest period. (c) Third step, EIS measured after 620 h rest period.
80
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
Fig. 14. XRD patterns measured ex situ from graphite electrodes (140 mm thick) comprised of Timcal KS-25 particles treated in
CH3±OCH2CH2±OCH2CH2±OCH2 (glyme)/LiClO4 0.5 M solution after several steps of galvanostatic polarization. The ®gure also
shows a typical chronopotentiogram of these electrodes in the glyme solutions with the points (a±c) after which the electrodes were
measured, marked on the V vs. capacity (mA h/g) curve (corresponding to the letters near the XRD patterns).
(CH3OCH2CH2OCH2CH2OCH3) solution. The ®gure
shows XRD patterns of graphite electrodes measured
ex situ after di€erent periods of lithiation in galvanostatic mode. The ®gure also presents a typical chronopotentiogram of a graphite electrode which is
polarized cathodically in this solution. The various
points in which the process was terminated for proceeding with the XRD measurements are also marked
on the V vs. capacity curve. The XRD patterns clearly
indicate progressive destruction of the active mass as
the process continues. The graphite peaks become
smaller and a new amorphous phase appears (broad
peak centers at 2y=208). When the electrode reaches
potentials close to 0 V (Li/Li+) in this solution, the
active mass becomes carbon dust, and the electrode
disintegrates completely. However, there are other failure mechanisms as well, as demonstrated in Fig. 15
which relates to a similar experiment (as for Fig. 14)
of a graphite electrode in a PC/LiAsF6 solution. This
®gure shows a typical chronopotentiogram of a graphite electrode polarized galvanostatically from OCV to
low potentials, and several XRD patterns measured ex
situ from graphite electrodes after di€erent periods of
galvanostatic polarization. The relevant states of the
electrodes measured in terms of end potential are
marked on the V(t) curve in Fig. 15.
Both the XRD patterns and the electrochemical studies show no indication that graphite can be at all
lithiated in this solution (no visible shifts in the major
graphite XRD peaks). In fact, further polarization
leads to lithium deposition rather than to Li±C intercalation. It is very signi®cant that in spite of the above,
the XRD pattern shows that the active mass remains
pure graphite, although it cannot intercalate lithium.
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
81
Fig. 15. Same as Fig. 14. XRD patterns (ex situ) and chronopotentiogram of graphite electrodes in a PC/LiAsF6 1 M solution.
The letters near the V vs. capacity curve correspond to the 4 XRD patterns (a±d). The peak at 2y=248 left of the large 002 peak
of the graphite comes from the same plane due to the radiation of CuKb (which in¯uences the XRD patterns when the peaks are
very intense).
Hence, it is clear that in PC solutions we may have a
failure mechanism which di€ers from that relevant to
the glyme solutions discussed above. In the present
Fig. 16. Typical impedance spectra of graphite electrodes
(Timcal KS-6 particles, 10 mm thick, 15 cm2, 13 mg/cm2),
measured at 0.3 V (Li/Li+) after being polarized to this potential (and equilibration) in 1 M LiAsF6 solutions of ECDMC and PC, as indicated.
case, the electrode fails not because of a completed
exfoliation and destruction of the active mass, but
rather due to some kind of electrical blocking.
Figure 16 compares impedance spectra measured
from graphite electrodes polarized to 0.3 V (Li/Li+)
and then equilibrated in EC-DMC and PC solutions of
LiAsF6 1 M. The impedance spectra in this ®gure
indeed re¯ect a remarkable di€erence between the
Fig. 17. A schematic presentation of sections in pristine and
processed graphite electrodes comprised of synthetic graphite
¯akes. Concluded from SEM observation of electrodes after
being polarized to low potentials in PC solutions [27].
82
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
Fig. 18. A scheme of a proposed failure mechanism of graphite electrodes in PC solutions. The solvent molecules are reduced to
ROCO2Li species and propylene gas. Due to the lack of passivation, the reduction process is intense, solvent molecules can also
percolate, and when inserted into the graphite, can be reduced there. Gas bubbles form local pressure which cracks the graphite
particles. This enables percolation of solution species into the cracks, their reduction and electrical isolation of active mass.
behavior of graphite electrodes in the two classes of
electrolyte solutions (in terms of reversible lithiation).
The impedance spectrum of graphite in DMC-EC solution is typical of an electrode covered by compact,
passivating surface ®lms and thereby, the very low frequency impedance is almost purely capacitive. In contrast, the impedance of the graphite electrode in the
PC solution is typical of a highly resistive porous electrode which is not properly passivated.
SEM observations of graphite electrodes after being
polarized in PC solutions[27] are illustrated in Fig. 17.
Originally, the pristine electrodes are composed of
highly oriented graphite ¯akes, as shown in this ®gure.
Polarization to low potentials leads to visible changes
in the particles' orientation, although the electrode
remains integrated. Hence, these observations correlate
well with the studies by XRD (Fig. 15) which also
show that upon cathodic polarization of these electro-
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
des in a PC solution to 0 V (Li/Li+), the active mass
remains pure graphite, although Li intercalation does
not occur. Hence, the failure mechanism in this case
relates to some kind of electrical blocking of the active
mass, with a considerable increase of its impedance,
and not to an exfoliation of the graphite structure (as
in the ®rst case dealt with above). Of particular interest
is the fact that this failure mechanism occurs in PC
and not at all in EC solutions, in spite of the similarity
in the chemical structure of the two solvent molecules.
It should be noted that in many cases, polarization of
these electrodes to 0 V (Li/Li+) also leads to their
destruction. However, the particles collected after
destruction of these electrodes also show XRD patterns of nearly pure graphite. Thus, in contrast to the
former case where destruction of the active mass is via
exfoliation, in the present case, when disintegration of
the electrode occurs, it relates to mechanical fractures
of the active mass.
We wish to propose the failure mechanism that is
schematically illustrated in Fig. 18. As discussed in section a above, the methyl group of the PC reduction
product CH3CH(OCO2Li)CH2OCO2Li [115], prevents
the precipitation of this product in compact surface
layers, as for the case of the EC reduction product
(CH2OCO2Li)2. Thereby, no ecient passivation of the
graphite electrode can be obtained in PC solutions.
The reduction of PC also forms propylene gas as a
coproduct. Due to this lack of passivation, massive PC
reduction forms gas bubbles. We can assume that in
the absence of good passivation, PC molecules are also
inserted into the graphite and are reduced there. The
pressure of the propylene gas thus formed between the
graphite planes can crack part of the graphite particles,
leading to an increase in the electrode's surface area,
thus making it more porous. This further interferes
with the possibility to obtain stable and passivating
surface ®lms. The change in the particle orientation
due to cracking also allows solution species to penetrate between graphite particles, and thus isolate them
electrically from the bulk. It seems that this mechanism
in which pressure formation within the graphite particles and particle cracking are involved, explains the
results obtained by XRD, SEM, impedance spectroscopy and the electrochemical measurements
described above. It should be emphasized that the
behavior of Li±C electrodes in di€erent solutions
depends not only on the speci®c surface chemistry
developed in each solution, but also on the type of carbon used. Graphite ¯akes are the most sensitive carbonaceous material to the solution composition in
electrochemical lithiation processes. This is because of
the layered structure of graphite, and the relatively
weak forces that hold the graphene planes together.
Hence, it is relatively easy to crack graphite particles
and, mechanically and chemically, to separate gra-
83
phene planes of graphite (e.g. by intercalation of toolarge species). As the carbon is less ordered, so the
lithiation is less sensitive to the solution composition.
This is because disorder adds rigidity to the carbon
structure. Hence, while lithium can not be inserted
reversibly into graphite ¯akes in PC solutions, as
shown above, reversible lithiation in PC solution can
be obtained with hard carbons, petroleum coke, and
even with graphite ®bers. While the large scale destruction mechanisms described above are much less relevant to carbons other than graphite, they can be good
models for small scale processes which occur during
prolonged cycling of any kind of Li±C electrode, and
lead to some capacity fading.
4. Conclusion
The use of graphite ¯akes as the active mass in
anodes for Li ion batteries requires a choice of electrolyte solutions in which a surface stabilization of the
graphite particles is obtained, because the forces which
hold the graphene planes together are relatively weak.
Thereby, an obvious failure mechanism of lithiated
graphite electrodes relates to a cointercalation of solvent molecules between the graphene planes together
with Li-ions. This forces a splitting between graphene
planes, and thus the exfoliation of the graphite particles into dust. Strong evidence for such a failure
mechanism was obtained for graphite electrodes processed in ethereal solutions such as diglyme (CH3O±
CH2CH2±OCH2CH2OCH3). There is, however,
another failure mechanism in which the graphite electrode remains integrated and the active mass retains its
graphite structure. However, the electrodes develop a
high impedance, and the active mass seems to be
blocked for any possible Li insertion.
There is evidence that the active mass in these cases
becomes deactivated due to the formation of thick surface ®lms, which leads to electrical isolation of the
graphite particles. Such a failure mechanism was found
in PC solutions. The formation of thick surface ®lms
and the electrical isolation of the graphite particles in
these cases are attributed to the chemical structure of
the PC reduction product (which contains a methyl
group) which does not allow formation of compact
surface ®lms, and to a massive gas formation (propylene) whose bubbles may form local pressure that may
crack the graphite particles. This allows the solution
species to percolate inside the cracks, and be reduced
within the cracks. Thus, a great part of the active mass
becomes electrically insulated, while the basic graphitic
structure is retained.
Stabilization of graphite electrodes is achieved when
highly reactive solution components are reduced at suf®ciently high potentials (above those of Li insertion) to
84
D. Aurbach et al. / Electrochimica Acta 45 (1999) 67±86
species which can adhere to the graphite surface and
form highly compact surface ®lms. Such surface species
include (CH2OCO2Li)2 (EC reduction product),
CH3OLi and CH3OCO2Li (CH3OCO2R, where R are
alkyl, reduction products), Li2CO3 (reduction product
of CO2), Li2S, Li2O and LixSOy compounds (reduction
products of SO2). When these species are formed at
suciently high potentials, they precipitate as passivating surface ®lms which prevent a massive reduction of
solution species (which may also lead to detrimental
gas formation) and cointercalation of solution species
into the graphite. The uniformity of these protective
surface ®lms in terms of chemical composition is also
an important factor in these passivation properties.
Therefore, enhanced stabilization of graphite electrodes
is obtained in solvents in which the precipitation of
surface species is selective, i.e., not all the possible reduction products of the solution components are
involved in the precipitation of the surface ®lms (e.g.,
as in the case of CH3OCO2R-type solvents).
Acknowledgements
This work was partially supported by the NEDO
Organization, Japan.
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