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
baDepartment of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel bElectric 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 dierent 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
electrochemical science, because these systems are highly complex and thus, their basic study is very chal-lenging. Reviewing the highly proli®c literature in this area shows clearly that the most popular anode ma-terials for these batteries are carbons of graphitic struc-ture [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
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Fig. 1. Typical voltammetric behavior of graphite electrodes at a very slow scan rate,n=4mV/s in EC-DMC 1:3/LiAsF6solution.
(Graphite ¯akes, KS-6, Timcal). Ultrathin, thin and thick electrodes correspond to submicronic (0.16 mg/cm2, see preparation
pro-cedure in Refs. [5,9]), 10 microns (3.3 mg/cm2), and 140mm (12 mg/cm2), respectively. The current for the thin and ultrathin
elec-trodes was normalized with that of the thick elecelec-trodes according to their mass ratios in order to have the 3 CV on the same scale. The electrode area was 2.4 cm2for the thin and the ultrathin, and10.2 cm2for 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:
C60:083Li0:083eÿÿÿ)ÿÿ*Li0
:083C6 Diluted stage I, LixC6,xR0:083 1
Diluted stage I Li0:083C60:083eÿ0:083Liÿÿ)ÿÿ*Li0:166C6 stage IV 2
Stage VI Li0:166C60:056eÿ0:056Liÿÿ)ÿÿ*Li0:222C6 Stage III 3
Stage III Li0:222C60:278eÿ0:278Liÿÿ)ÿÿ*Li0:5C6 Stage II 4
Stage II Li0:5C60:5eÿ0:5 Liÿÿ)ÿÿ*LiC6 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-tence of two phases (as veri®ed by in situ XRD measurements [6,7]), and to the plateaus which charac-terize galvanostatic lithiation of these electrodes [1±7].
particles are parallel to the current collector) and thin-ner, the peaks of their voltammograms (see Fig. 1) are sharper, the hysteresis between the intercalation±dein-tercalation 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 op-timal behavior depends strongly on the solution used, due to the unique surface chemistry developed on the electrodes as a function of the various solution com-ponents. It was suggested that exfoliation of the graph-ite 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 sur-face ®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 suciently high potentials above Li inter-calation 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 spec-troscopy [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 ellipsome-try[40]. In situ morphological studies of these electro-des 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 car-bonate[28].
These studies show that there are only a few cases in which graphite electrodes behave reversibly during pro-longed 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 car-bonate [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
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 una-voidable 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 compo-sition 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 eorts 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 dierent degrees of graphitization[66±75]. A common denomi-nator 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 electro-des in terms of stability and cycle life, and in parallel to the search for and development of alternative car-bons, 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 struc-tures[83±91], electrode impedance,[9,92±94] and Li-ion solid state diusLi-ion[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 cur-rently being explored. [103±108]
[112], expansion of the graphite structure by means of acid treatment [113] or intercalation of large ions such as K+ (e.g., using KC
8 as the starting
ma-terial) [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 stabiliz-ation and capacity fading mechanisms of these electro-des. The present paper combines previous and new results in an attempt to improve our understanding of how lithiated graphite electrodes are stabilized in par-ticular solutions, and when and how they fail in other solutions. The tools for this study included surface sen-sitive FTIR spectroscopy, XPS, in situ and ex situ XRD, SEM, chronopotentiometry, slow scan rate cyc-lic 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 spec-troscopy were described in Refs. [23±26]. In brief, graphite powder from electrodes before and after elec-trochemical processes was analyzed using diuse re¯ec-tance or transmitre¯ec-tance modes. In the last mode, the carbon particles were pelletized with KBr. The per-formance 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). LiAsF6was 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 electro-chemical measurements were carried out under highly pure argon atmosphere at room temperature (258C20.5) in glove boxes or in hermetically sealed cells. All the preparations for the spectroscopic studies
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 intercala-tion 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 sol-utions 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 dierence between EC and PC mol-ecules 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 introduc-tion secintroduc-tion, the answer lies in the surface chemistry of the graphite electrodes in the presence of EC.
Fig. 2. (a) FTIR spectra measured from synthetic graphite particles taken from an electrode that was lithiated and delithiated in EC/LiAsF61 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
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 diculties 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
elec-trodes 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 Li2CO3is amajorconstituent.
However, spectrum 2b, which relates to an exper-iment conducted under CO2 atmosphere (6 atm),
shows no pronounced peaks of Li2CO3. The spectrum
is dominated by the peaks of the EC reduction pro-duct. Hence, one can conclude that EC is a highly reactive solvent and therefore, when it is a major com-ponent in an electrolyte solution, its reduction
domi-nates the lithiated graphite surface chemistry and suppresses other reactions such as those of CO2on the
active electrode's surface. Fig. 3 compares FTIR spec-tra measured from graphite particles taken from elec-trodes that were cycled in EC-DMC (1:1) solutions of LiAsF6, LiPF6 and LiC(SO2CF3)2 (as indicated). The
three spectra dier 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
domi-nant absorptions, the typical peaks of the EC re-duction products, one of which is probably (CH2OCO2Li)2, as demonstrated in the comparison of
the spectra in Fig. 2.
ThenCHpeaks 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 LiPF6solutions 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
that all three salts are also reduced on the graphite sur-faces. The EDAX spectra of graphite electrodes treated in LiAsF6, LiPF6 and LiC(SO2CF3)3 solutions
con-tained 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], Li
xPFy and LiPFyOz (major
peaks around 850 and 1000 cmÿ1) [118], or Li
xSOyCFz
(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 dierences among the spectra of Fig. 3. However, comparison between these spectra and spec-tra 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 pro-ducts.
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 con-tributed another piece of information that could not be reached at by FTIR spectroscopy, namely, the pre-sence 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/LiAsF6solutions. The lithium electrode was
freshly prepared in solution, stored for 3 h and measured as already described [120]. The graphite elec-trode was lithiated±delithiated before the measurement. The peak assignments appearing in this ®gure are based on Refs. [120,121, 130], and the previous knowl-edge 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 rad-ical (formed by a single electron transfer to EC), or by a two-electron transfer which form CO3., 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 re-duction 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 eect. It slows down the kinetics of PC reduction on active surfaces as com-pared 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 pro-cesses (which form a mixture of ROCO2Li and
Li2CO3). The formation of eectively passivating
sur-face ®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 graph-ite).
Fig. 4. XPS carbon 1s peaks obtained from a lithium elec-trode and a graphite elecelec-trode (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
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 chem-istry of lithiated graphite in solutions based on DMC, MEC and MPC in an attempt to understand the dier-ence 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,
pre-pared 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 graph-ite 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
accord-ing to the followaccord-ing equation:
CH3OLiROCO2CH34CH3ORCH3OCO2Li 6
It should be noted that CH3OLi can react
nucleophi-lically with these molecules to also produce ROLi and ROCO2Li. However, due to their expected higher
solu-bility 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 Li2CO3in the
sur-face ®lms formed on graphite in the ROCO2CH3
sol-vents, as is evident from the spectral studies. This is due to the fact that the dierence 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 eect of additives. We describe herein the eect of SO2and CO2gases as additives to
solutions in which lithiation of graphite is not a revers-ible process.
Figure 7, reproduced from our previous paper [125], shows typical chronopotentiograms of graphite des and ®rst cycle voltammograms of platinum electro-des in LiAsF6/DMC and LiAsF6/DMC/SO2 solutions,
as indicated. This ®gure demonstrates the typical de-terioration 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
rel-evant chronopotentiogram in Fig. 7 re¯ects the re-duction of SO2at potentials between 2.5±3 V (Li/Li+).
This reaction indeed involves a pronounced irreversible charge capacity loss in the ®rst lithiation process. Fig. 6. (a) FTIR spectra of graphite particles taken from an electrode which was lithiated±delithiated (one cycle) in MPC/1 M LiAsF6solution (diuse re¯ectance mode). (b) FTIR spectra measured ex situ (external re¯ectance mode) from a lithium electrode
freshly prepared in MPC/1 M LiAsF6solution 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
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 pro-cess around 2.5 V (Li/Li+) occurs on Pt in the
LiAsF6/DMC/SO2solution and leads to electrode
pas-sivation (low cathodic currents of potentials below 2 V compared with the SO2free solution). Similar behavior
was obtained with CO2-containing solutions, but with
one dierence: CO2is reduced on noble metal or
car-bon electrodes at much lower potentials than SO2
(<1.2 V vs. Li/Li+)[22,126]. The eect of these two
additives on the surface chemistry of graphite electro-des is demonstrated in Fig. 8, which shows FTIR spec-tra obtained from graphite electrodes treated in SO2/
DMC and CO2/g-butyrolactone (BL) solutions (1 M
LiAsF6). Both solvents are reduced on graphite
electro-des 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 electro-des 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 graph-ite electrode in BL solutions, and re¯ects the formation of theg-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
com-pounds (e.g. Li2SO3, Li2S2O4, Li2S2O5) [125], as
evi-denced by the strong peaks at 900 and 1000±1100 cmÿ1that dominate spectra 8b,c. Spectrum 8d
demon-strates clearly that in the presence of CO2,the graphite
electrode surface chemistry becomes dominated by Li2CO3 formation according to the following general
equation [115].
2CO22Li2eÿ4Li2CO3CO 7
In situ studies by FTIR spectroscopy of CO2
re-duction 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 elec-trodes needs the formation of highly compact and pas-sivating 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 cen-ters 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
de-nominator: 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 sur-face species. Li ions adsorbed to the carbon sursur-face may also play a role in adhering surface species to the graphite via their interaction with the partially nega-tively 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 infor-mation 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 Fig. 7. Chronopotentiograms of graphite electrodes in DMC/
LiAsF61 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 elec-trodes in DMC/LiAsF6 1 M, 1% by weight SO2 (thick solid
line), and in SO2 free solutions are shown (20 mV/s). For
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 diusion of lithium into the graphite. Finally, lithium accumulates within the bulk via phase transition between the various inter-calation stages. We made an intensive study of the EIS of lithiated graphite electrodes in a variety of con®gur-ations and solution compositions[5,7,8,9,23,24,27,92] and found that the impedance spectra of these electro-des clearly re¯ect the serial, multistep nature of the Li insertion±deinsertion processes. A reasonable separ-ation of time constants is obtained. This is demon-strated in Fig. 9, which relates to a thin (submicronic, see preparation procedure in Ref. [5]) composite graph-ite 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 dierent 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 elec-trodes) [129], or to adsorption phenomena at the sur-face ®lm±graphite intersur-face. 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 Fig. 8. FTIR spectra measured from graphite particles taken from electrodes treated in various LiAsF61 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
indeed relate to interparticle electron transfer and to the degree of porosity of the composite electrode.
At the lower frequencies, the impedance spectra con-tain a potential-dependent Warburg element that relates to the potential-dependent solid state diusion of lithium into graphite. Finally, at the very low fre-quencies, theZ0vs.Z'plot becomes very steep, and in fact, re¯ects the intercalation capacitance (Cint1ÿ1/
oZ0, o=2 pf40)). It is important to note that at o40, 1/oZ0as 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 graph-ite particles which compose it, the better the corre-lation, 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 elec-trode composed of natural graphite particles, in EC-DMC 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 dierences 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 betweenCintcalculated from the SSCV and that
cal-culated fromZ0ato40.
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-gration through the surface ®lms is supposed to be potential-independent. This means that the big semi-circle 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.
Figure 11 presents a typical eect of prolonged cycling on the electrode impedance. Spectra measured from a thick (140mm) 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 demon-strated 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 impe-dance increases considerably. As proven by in situ and ex situ XRD, as well as by SEM observation and chronopotentiometric studies, in these solutions Li-graphite 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 expan-sion 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 micro-scopic level). This allows the continuous reduction of solution species. While this process occurs on a very low scale, it thickens the surface ®lms and conse-quently, the electrode's impedance increases, particu-larly 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 eects on the stabilization of Li graphite elec-trodes, as monitored by impedance spectroscopy. Lithiated graphite electrodes were cycled (a few galva-nostatic charge±discharge cycles), and then stored periodically at a low (Li intercalation) potential. The electrodes were measured during these experiments by EIS after dierent periods of storage at the low poten-tial (close to the storage potenpoten-tial).
Figures 12 and 13 show typical Nyquist plots measured at 90 mV (Li/Li+) after dierent periods of
storage time at this potential, from similar Li-graphite electrodes in EC-DMC 1:1 and EC-DMC 1:5 LiAsF61
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 electro-de's impedance relate to the surface ®lms. In contrast, at a lower concentration of EC (Fig. 13), the surface Fig. 9. Typical impedance spectrum measured from an
ultra-thin electrode (same as that of Fig. 1) at equilibrium poten-tials, 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
®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 sto-rage. Hence, we can conclude that the high EC concen-tration and the prolonged storage of the graphite electrode at low potential, probably leads to the for-mation 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-Fig. 10. A family of impedance spectra measured at dierent equilibrium potentials, as indicated, from graphite electrodes com-prised of natural graphite particles, in EC-DMC 1:3/LiAsF61 M solution. The electrode's area was10.8 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
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 poss-ible 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.
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 exfolia-tion of the graphene planes due to cointercalaexfolia-tion 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. 11. Impedance spectra measured from a thick (140mm, active mass was 12 mg/cm2) graphite electrode (Timcal KS-25
synthetic graphite ¯akes) in EC-DMC 1:1/LiAsF6 1 M
sol-ution, at 110 mV vs. Li/Li+. (a) After 3 galvanostatic cycles
(white circles). (b) After 75 galvanostatic cycles at C/7 rate (black circles).
Fig. 13. Same as Fig. 12. EC-DMC 1:5/LiAsF61 M solution.
(a)±(d) Ð steps 1, 2, 3, 4, EIS measurements after 10, 275, 375 and 425 h rest periods.
Fig. 12. Impedance spectra measured from graphite electrodes (140mm thick, area of 1 cm2, 12 mg active mass per cm2) at 90 mV
(Li/Li+) after dierent periods of storage at this potential in EC-DMC 1:1/LiAsF
61 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)
(CH3OCH2CH2OCH2CH2OCH3) solution. The ®gure
shows XRD patterns of graphite electrodes measured ex situ after dierent periods of lithiation in galvano-static mode. The ®gure also presents a typical chrono-potentiogram of a graphite electrode which is polarized cathodically in this solution. The various points in which the process was terminated for pro-ceeding with the XRD measurements are also marked on theVvs. 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 fail-ure 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 graph-ite electrode polarized galvanostatically from OCV to low potentials, and several XRD patterns measured ex situ from graphite electrodes after dierent 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 stu-dies 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 interca-lation. 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. Fig. 14. XRD patterns measured ex situ from graphite electrodes (140mm thick) comprised of Timcal KS-25 particles treated in CH3±OCH2CH2±OCH2CH2±OCH2(glyme)/LiClO40.5 M solution after several steps of galvanostatic polarization. The ®gure also
Hence, it is clear that in PC solutions we may have a failure mechanism which diers from that relevant to the glyme solutions discussed above. In the present
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 dierence between the Fig. 15. Same as Fig. 14. XRD patterns (ex situ) and chronopotentiogram of graphite electrodes in a PC/LiAsF61 M solution.
The letters near theVvs. capacity curve correspond to the 4 XRD patterns (a±d). The peak at 2y=248left 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).
Fig. 16. Typical impedance spectra of graphite electrodes (Timcal KS-6 particles, 10mm thick,15 cm2, 13 mg/cm2),
measured at 0.3 V (Li/Li+) after being polarized to this
po-tential (and equilibration) in 1 M LiAsF6 solutions of
EC-DMC and PC, as indicated.
behavior of graphite electrodes in the two classes of electrolyte solutions (in terms of reversible lithiation). The impedance spectrum of graphite in DMC-EC sol-ution is typical of an electrode covered by compact, passivating surface ®lms and thereby, the very low fre-quency impedance is almost purely capacitive. In con-trast, the impedance of the graphite electrode in the PC solution is typical of a highly resistive porous elec-trode 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-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
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 notat allin 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 pat-terns 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 sec-tion a above, the methyl group of the PC reducsec-tion 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 ecient 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 pene-trate between graphite particles, and thus isolate them electrically from the bulk. It seems that this mechanism in which pressure formation within the graphite par-ticles and particle cracking are involved, explains the results obtained by XRD, SEM, impedance spec-troscopy and the electrochemical measurements described above. It should be emphasized that the behavior of Li±C electrodes in dierent solutions depends not only on the speci®c surface chemistry developed in each solution, but also on the type of bon used. Graphite ¯akes are the most sensitive car-bonaceous 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-phene planes of graphite (e.g. by intercalation of too-large 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 destruc-tion mechanisms described above are much less rel-evant 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 electro-lyte 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 sol-vent molecules between the graphene planes together with Li-ions. This forces a splitting between graphene planes, and thus the exfoliation of the graphite par-ticles into dust. Strong evidence for such a failure mechanism was obtained for graphite electrodes pro-cessed in ethereal solutions such as diglyme (CH3O±
CH2CH2±OCH2CH2OCH3). There is, however,
another failure mechanism in which the graphite elec-trode 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 sur-face ®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 (propy-lene) 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.
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 LixSOycompounds (reduction
products of SO2). When these species are formed at
suciently high potentials, they precipitate as passivat-ing 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 re-duction 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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