Calorimetric investigation of the reactivity of the passivation film on lithiated graphite at elevated temperatures

Electrochimica Acta 49 (2004) 581–589

Calorimetric investigation of the reactivity of the passivation film on lithiated graphite at elevated temperatures M. Holzapfel1 , F. Alloin∗ , R. Yazami Laboratoire d’Electrochimie et de Physicochimie des Matériaux et des Interfaces (LEPMI), Institut National Polytechnique de Grenoble (INPG), 1130, rue de la piscine, BP 75, 38402 St. Martin d’Hères Cedex, France Received 4 July 2003; received in revised form 9 September 2003; accepted 9 September 2003

Abstract Thermal storage of lithiated graphite electrodes has been performed between 40 and 90 ◦ C for 8 h to 3 weeks. The results were compared for two separators: Celgard 2402 and a microporous PVdF membrane. The effects of storage on the capacity losses have been discussed with respect to the passivation film on the graphite electrodes in contact with the electrolyte solution EC:DMC:DEC (2:2:1)–1 M LiPF6 . The capacity loss shows a thermally activated character, which has been related to transformations of the passivation film at moderate temperatures. At higher temperatures, reaction of the intercalated lithium takes place, controlled by Li+ -ion diffusion. DSC measurements were performed on passivated and lithiated graphite electrodes. Two peaks could be distinguished. An effect of the elevated temperature storage on the intensity and onset temperature of the first peak in DSC is evidenced. This peak could be attributed to the transformation of the passivation film. The second peak is due to the diffusion of lithium ions and the subsequent reaction with the liquid electrolyte. The effect of washing the electrode with DMC was thoroughly investigated. Our results allowed to attribute the transformation of the passivation film upon DSC analysis to a reaction taking place in the presence of LiPF6 . © 2003 Elsevier Ltd. All rights reserved. Keywords: Graphite; Differential scanning calorimetry; Passivation film; Reactivity; Rechargeable lithium batteries

1. Introduction Galvanostatic and voltammetric cycling using twoelectrode button-type coin cells, a carbon one as working electrode and a lithium electrode as counter and reference electrode, are most common techniques to determine the reversible capacity and cycling behaviour of graphite [1–8]. A complete lithium-ion cell, with graphite as negative electrode and lithium metal oxide as positive electrode does not allow to separate the limitations resulting from transformations of each of the two electrodes. On the other hand, in half-cells, kinetics are limited only by the working electrode, the lithium counter electrode is much faster and provides a stable potential and a re∗ Corresponding author: Tel.: +33-4-76-82-65-60; fax: +33-4-76-82-66-70. E-mail addresses: [email protected] (M. Holzapfel), [email protected] (F. Alloin). 1 Present address: Paul-Scherrer-Institut, 5232 Villigen Switzerland. Tel.: +41-56-310-2116; fax: +41-56-310-4415.

PSI,

0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.09.012

serve of lithium. This design permits, hence, to study the electrochemical behaviour of each electrode material separately. In most studies the charge/discharge following the storage begins with a relithiation (discharge). This does not permit to separate a new irreversible capacity, due to the possible reformation of the passivation film, from the capacity loss due to delithiation. The present work deals with the determination of the effects of storage at elevated temperatures on graphite which has only rarely been presented hitherto [9–13], more precisely in an electrolyte system based on a microporous PVdF membrane designed for a full lithium metal oxide–graphite battery. The use of microporous PVdF membranes has gained much interest, as they show a very good compromise between electrochemical and thermal stability, sufficient interaction with the liquid electrolyte, easy fabrication, low cost, mechanical stability and some adherence on the active material [14–16]. The graphite electrode is studied by both, galvanostatic cycling and DSC measurements.

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The passivation film is expected to transform or disrupt upon storage at elevated temperatures (and during DSC analysis) and the reactions occurring have been proposed to be the transformation of lithium ethylene dicarbonate to lithium carbonate [11,12,17,18]. We propose a new mechanism for the transformation of the passivation film which is in accordance with our experimental results.

2. Experimental 2.1. Electrode The graphite electrode used in this study was provided by Saft-Alcatel and consists of a mixture of mesocarbon microbeads (MCMB) and a graphitised carbon, further called MC. MC it is made of 96% carbon material and 4% binder, cast on copper foil. The electrodes were obtained ready for use, dried over night in vacuum at 60 ◦ C and handled in an argon filled glove box (<1 ppm O2 and H2 O). 2.2. Electrolyte “Battery grade” solvents EC, DMC and DEC (<30 ppm H2 O) from Merck (Darmstadt) were dried further over activated molecular sieve (3 Å, Roth, Karlsruhe) before use (<5 ppm H2 O). The LiPF6 “battery grade”, from Merck (Darmstadt), was used as received, to make 1 M solutions. A 100 ␮m thick microporous PVdF-membrane was used as separator. It was prepared as follows: 3.4 g of PVdF (301F, Atofina) are dissolved in 16.6 g acetone at 60 ◦ C in a hermetically closed vessel. The solution is then cast in a thickness of approximately 500 ␮m on an aluminium substrate. It was then put into a bath made of pure ethanol for 45 s and dried, first at room temperature, then at 60 ◦ C over night. 2.3. Experimental cell Electrochemical cycling has been performed in button-type coin cells (CR 2430). The graphite electrode was cut as a 16 mm diameter disk of about 100 ␮m thickness. The lithium counter electrode consisted of a disc of 20 mm in diameter. It was pre-passivated by thionyl chloride in order to operate in DEC-containing electrolytes. Stainless steel discs were used as spacers and current collectors in the coin cells.

cling after storage was performed in a newly prepared coin cell beginning with delithiation. 2.5. DSC measurements The DSC measurements were performed on a TA Instruments 2920 CE. The samples were placed in aluminium crucibles (volume: 100 ␮l). Approximately 40 ␮l of electrolyte are added and the crucibles are hermetically sealed with a PTFE O-ring. The heating rate was 5 K/min to a maximum temperature of 210 ◦ C, if not mentioned otherwise. The crucibles began to leak at higher temperatures because of the high internal vapour pressure. The electrodes were folded before putting them into the DSC pans, so that the copper current collector prevented a direct contact between the aluminium of the pans and the electrode active material, which otherwise could lead to alloying reactions. The reference crucible contained the current collector (without the active material) and electrolyte.

3. Results and discussion 3.1. Cycling and storage experiments The electrolyte used consisted of a ternary solvent mixture EC:DMC:DEC (2:2:1) containing 1 M LiPF6 (all proportions are given in volumes), further called the ternary electrolyte. This electrolyte is of interest because of its excellent behaviour in rechargeable lithium-ion batteries [19]. 3.2. Influence of DEC 3.2.1. Experimental observations During the first charge/discharge experiments with half-cells, problems in reproducibility occurred and strong variations in the rechargeable capacity were observed. Eventually higher charge (delithiation) than discharge (lithiation) capacity were obtained. The reason for this behaviour was determined, by lithium plating-stripping experiments, to be the DEC content of the electrolyte. These experiments are further described in [20]. Already Aurbach et al. [21,22] indicated the poor electrochemical behaviour of lithium metal electrodes for electrolytes containing less than 50% of EC, and also the presence of DEC has been indicated to be problematic [23]. The lithium counter electrodes were, hence, pre-passivated by thionyl chloride vapours prior to use, in order to operate in this electrolyte.

2.4. Galvanostatic cycling 3.3. Influence of storage on the capacity loss The cells were cycled on a MacPile battery cycler (Bio Logic, Claix, France) in galvanostatic cycling mode, at C/10 rate, between 1.5 and −5 mV versus Li/Li+ . Storage experiments have been performed for the electrodes in their lithiated state at different temperatures, between 40 and 90 ◦ C and at different storage times, between 8 h and 3 weeks. Cy-

In Fig. 1, an example of high temperature storage experiments is given for the graphite electrodes using a microporous PVdF separator. The difference between the first discharge and the first charge is more important than on the subsequent cycles. This difference is of irreversible nature

M. Holzapfel et al. / Electrochimica Acta 49 (2004) 581–589

28

MC-graphite (PVdF separator) storage at 60˚C 1 week

300

24

MC : 60 C

20 dQ/%

capacity/mAh g-1

350

583

dQ

16

rev

12 8

250

4 dQ

0 200

0

2

4

0

6 8 10 12 14 16 cycle number

28

Fig. 1. Typical result of the elevated temperature storage of the MC graphite. Electrolyte: EC:DMC:DEC (2:2:1) containing 1 M LiPF6 , separator: microporous PVdF. Passivated lithium is used for the counter electrode.

3.4. Differential scanning calorimetry (DSC) study MC-electrodes have been cycled and stored as described earlier. After storage, different samples (of about 18 mg of active material) have been recovered from the coin cell in the glove box and put into the DSC pans. A typical DSC trace is shown in Fig. 3, and compared to a sample in which only the ternary electrolyte in presence of a non-cycled electrode has been measured. The result obtained on the

10 15 20 storage time/days

25

MC : 1 week

24 dQ/%

20 dQ

16

rev

12 8 4

dQ

irrev

0 (b)

30 40 50 60 70 80 90 100 T/˚C

Fig. 2. Capacity losses for graphite. (a) MC electrode—different storage times for T = 60 ◦ C; (b) MC electrode—1-week storage at different temperatures.

cycled electrode consists of two exothermic peaks: one with onset at 100–105 ◦ C and passes through a maximum at 125–130 ◦ C. A second peak is present too, the maximum of which is not observed in our measurements, because they had to be stopped at 210 ◦ C due to beginning solvent leakage at higher temperatures. The presence of two peaks in DSC analysis was reported in literature [24–29] and has been explained as follows: (i) the first signal is due to a rupture of the passivation film, (ii) the second signal corresponds to

600

-1

heat flow/mW g of graphite

and is commonly attributed to the formation of a passivation film on graphite. The capacity associated with the formation of the passivation layer, is of 25 mAh/g, which corresponds to 6.8% of the total capacity. The cycle capacity is not stable but declines slowly. This behaviour may result from contact losses upon cycling. The difference between charge and discharge capacity, however, is quite small and shows the good reversibility of the lithiation/delithiation process. The jump in cycle 7 is due to the reversible capacity loss upon storage. It can be explained with the fact that cycling after storage begins with a complete delithiation, where the reversible loss leads to less capacity. The difference between the last discharge capacity before storage and the first charge capacity after storage divided by the capacity before storage represents the relative capacity loss dQrev . The results are presented as follows: the capacity loss is given as a function of either storage time at a given temperature or temperature at a given time of storage. We did not find a significant difference in the cycling behaviour of the graphite electrode in presence of either microporous PVdF or Celgard. In the following examples microporous PVdF will be used. Fig. 2a and b shows some characteristic results for the capacity losses upon storage. For 1 week storage, it reveals to be important only for temperatures of 60 ◦ C or higher, and for storage of 1 week or longer. As would be predicted, the capacity losses increased with temperature and storage time.

5

(a)

irrev

lithiated graphite non-cycled graphite

500 400 300 200 100 0 0

50

100 150 T/˚C

200

250

Fig. 3. Typical DSC curves for the reaction of a lithiated and non-cycled MC graphite with the ternary electrolyte EC:DMC:DEC (2:2:1)–1 M LiPF6 .

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the reaction of the intercalated lithium, which becomes accessible due to the partial dissolution of the passivation film, with the electrolyte by diffusion processes [17,24,30,31]. Under rupture of the passivation film one understands the disappearance of its limiting character towards the process of Li+ -ion diffusion. This may be a mechanical process, due to constraints at high temperatures or related to chemical reactions of the compounds present in the film. Lithiated graphite could also react with the polymer binder present in the composite electrode [25]. The capacity loss is mainly due to the consumption of the intercalated lithium by the action of the liquid electrolyte. This loss is partly recovered upon the next lithium intercalation at room temperature as the lithium counter electrode constitutes a large lithium reserve. Published results show that in carbonate based electrolytes the onset temperature primarily depends on the electrolyte salt used and less on the chosen solvent mixture [25,29–33]. 3.5. Influence of the amount of intercalated lithium on the DSC response The lithium content x in Lix C6 has an important effect on its thermal signature as was reported in [17] for EC :DEC (2:1)–1 M LiPF6 by ARC and in [31] for EC:DMC (1:1)–1 M LiBF4 by DSC. The latter study shows the presence of two peaks in which the first, attributed to the rupture of the passivation film, remains invariant with x, whereas the second increases in intensity and shifts to lower temperatures for higher x values. We undertook an investigation on the influence of the lithium content in MC graphite using EC:DMC:DEC (2:2:1)–1 M LiPF6 and four x values : 0 (lithiated − redelithiated), 0.2, 0.5 and 1, the results are shown in Fig. 4. The curves are quite similar for all the lithium containing compounds. No temperature shift is observed (i) on the first signal, with an onset temperature of approximately 105 ◦ C, for the different lithium contents and (ii) on the beginning

-1

heat flow/mW g of graphite

350 300

Li C 0.2

250

Li C

200

Li C

150

0.5

1.0

Li C 0.0

6

6

6

6

100 50 0 50

100

150

200

T/˚C Fig. 4. Influence of the intercalation level in the graphite material on the DSC results.

of the second peak; no significant difference in the intensity of the signal is observed. For the delithiated graphite (x = 0), however, only the first signal is observed, which corresponds to −32 J/g, a value consistent with the literature values [28,31,32]. In this last sample the second signal does not appear, therefore its attribution to the reaction of the intercalated lithium with the electrolyte seems justified. As the two signals often overlap, the energy released during the initial reaction is taken as equal to the area under the curve prolonged by extrapolation of the experimental data. The generated heat for the first peak is about −100 J/g for all the lithiated samples. This means that the transformation process of the passivation film is not influenced by x for x > 0.2, but that already some reaction of intercalated lithium accompanies the transformation of the film. The reactivity of the intercalated lithium with electrolyte is not influenced, in the range of the explored temperatures, by the quantity of lithium. The total energy in any of these experiments does not exceed −200 J/g, which is less than 10% of the total amount of energy liberated at the reaction of a lithiated graphite with electrolyte (cf. [34]), so in any case we are in a situation where lithium is in excess. 3.6. Washing with DMC An extinction of the heat flow signals associated to the transformation of the passivation film and the reaction of the electrolyte has been observed by Du Pasquier et al. [24] upon washing of lithiated graphite electrodes with DMC. The authors related this to the dissolution of the passivation film upon washing and the subsequent reaction of the intercalated lithium with DMC. Our experiments on the formation of the passivation film on lithiated HOPG in different electrolytes [34], however, show that DMC does form a protective passivation film and does not fully delithiate lithiated graphite. In Fig. 5, we show the DSC results obtained with different samples (MC-electrodes in their lithiated state) cycled in presence of EC:DMC:DEC (2:2:1)–1 M LiPF6 , washed with DMC and dried. The samples were studied either in their dry state or after addition of DMC or EC:DMC:DEC (2:2:1)–1 M LiPF6 . The results are compared to an unwashed electrode in presence of EC:DMC:DEC (2:2:1)–1 M LiPF6 . The dry sample exhibits nearly no exothermic reaction. Therefore, the absence of electrolyte does not permit the oxidation of the intercalated lithium, but also the signal of the transformation of the passivation film, the first peak, is absent. In presence of DMC the first peak is absent, the second peak, however, reappears, with relatively smaller intensity. In presence of the ternary electrolyte both peaks reappear, with slightly lower intensity as in the case of the unwashed electrode (analysed with ternary electrolyte). This shows that the intercalated lithium can not have fully reacted with DMC during the washing of the electrode, since intercalated lithium is present. The results could, however, be explained if one supposes a transformation of the passivation film

M. Holzapfel et al. / Electrochimica Acta 49 (2004) 581–589

1200

6

300 250 200 150

1000 800

lithiated non-lithiated

600 400 200 0

100 50

50

100

(a)

150

200

T/˚C

100

150 T/˚C

200

250

Fig. 5. Effect of DMC:DSC analyses of samples that have been washed with DMC prior to the measurement and then analysed under the conditions given in the figure. The DSC curve of an unwashed sample is given for comparison.

during the washing with DMC, with the formation of, mainly, lithium methylate [34]. This compound does not undergo any transformation upon heating and thus no signature in the temperature region 100–130 ◦ C is expected. This new film could be more stable upon heating which would inhibit the reaction with liquid electrolyte and shift the second peak to higher temperatures. 3.7. The effect of LiPF6 In order to validate this assumption, additional experiments have been undertaken. The electrodes were cycled in DMC–1 M LiPF6 and afterwards analysed by DSC in their lithiated and delithiated states. The results are shown in Fig. 6a. As for the samples analysed in presence of ternary electrolyte, the DSC curves of the lithiated samples show two signals in the same temperature range, which means that they can be attributed to the same two effects: the first one to the transformation of the passivation film, slightly shifted in its onset temperature by approximately 10 ◦ C (95 ◦ C instead of 105 ◦ C), and the second one to the reaction of the intercalated lithium with DMC. For the delithiated samples the second signal is absent. The assumption that washing causes a transformation of the film to a methylate-based material which does not show any exothermic transformation on heating is, therefore, not correct. Further experiments have been undertaken in order to deal with these results. Fig. 6b and c show the DSC curves of different electrodes, in their lithiated and delithiated states, respectively, that have been cycled in DMC–1 M LiPF6 , washed with DMC and measured in the presence of either DMC or DMC–1 M LiPF6 . In the absence of LiPF6 a complete disappearance of the first signal is observed in both,

560 480

DMC DMC-LiPF

6

400 320 240 160 80 0 50

100

(b)

150 T (˚C)

200

300

-1

50

heat flow/mW g-1 of graphite

0

heat flow/mW g of graphite

-1

heat flow/mW g of graphite

350

heat flow/mW g-1 of graphite

dry DMC EC:DMC:DEC (2:2:1) - 1M LiPF effet d'un lavage au DMC 6 unwashed + EC:DMC:DEC (2:2:1) - 1M LiPF

585

250

6

200 150 100 50 0 -50 -100 50

(c)

DMC DMC-LiPF

100

150

200

T/˚C

Fig. 6. Influence of LiPF6 : DSC analyses of samples that have been cycled in DMC–1 M LiPF6 . (a) The samples have been analysed in their lithiated or delithiated state. (b) The lithiated sample (x = 1) has been cycled in DMC–1 M LiPF6 , washed with DMC and analysed either with DMC or with DMC–1 M LiPF6 . (c) The delithiated sample (x = 0) has been cycled in DMC–1 M LiPF6 , washed with DMC and analysed either with DMC or with DMC–1 M LiPF6 .

lithiated and delithiated electrodes, whereas with LiPF6 this first peak is present. LiPF6 seems to play a major role in the appearance of the first transformation. The second peak is also modified. In absence of LiPF6 the heat flow is lower. This could be explained by a higher stability of the film versus a non-saline solvent, the reaction between intercalated lithium and the electrolyte occurs, but at higher temperatures. The film transformation facilitates the diffusion

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250

temperature [11,12,17,18]. According to these studies, the principal reaction is the transformation of the semicarbonates (lithium alkylcarbonates and alkylenedicarbonates) into lithium carbonate. The main reaction proposed in the literature is the following:

200

(CH2 OCO2 Li)2 → Li2 CO3 + CO2 + C2 H4 + 21 O2

150

the formation of oxygen, however, is somehow improbable in a strongly reducing environment. Dahn and co-workers [17,18] proposed a reaction with the formation of lithium carbonate and ethylene:

EC:DMC:DEC (2:2:1) EC:DMC:DEC (2:2:1) - 1M LiPF

6

-1

heat flow/mW g of graphite

350 300

100 50 0 50

2Li + (CH2 OCO2 Li)2 → 2Li2 CO3 + C2 H4 100

150

200

250

T/˚C Fig. 7. DSC measurement of lithiated graphite (x = 1), cycled in EC:DMC:DEC (2:2:1)–1 M LiPF6 , washed with EC:DMC:DEC (2:2:1) and analysed with either EC:DMC:DEC (2:2:1) or EC:DMC:DEC (2:2:1)–1 M LiPF6 .

of molecules and therefore promotes the reactivity of the intercalated lithium with the electrolyte. Hence, its disappearance in absence of LiPF6 must influence notably the second signal in DSC. When no modification of the passivation film has taken place, a higher temperature would be required to achieve a fast diffusion of the reactants. Moreover it was observed, in the study on the reactivity of lithiated HOPG [34,35], that the energy produced upon reduction with or without participation of LiPF6 is roughly the same; so a modification of the energy released can not be associated to the reactions taking place. In order to verify the conclusions drawn for DMC we made the same experiment, for lithiated graphite, in presence of the ternary electrolyte. In this case the electrodes were cycled in the ternary electrolyte, then washed in their lithiated state with EC:DMC:DEC (2:2:1) in order to completely remove LiPF6 . DSC measurements were performed in presence of ternary electrolyte either with or without LiPF6 (Fig. 7). As in the case of DMC a strong influence of LiPF6 is found: in absence of LiPF6 the transformation at 110–150 ◦ C is not observed and the reaction of the intercalated lithium has a lower yield than in the case of LiPF6 -containing electrolyte. This is probably due to an increased stabilisation of the passivation film without LiPF6 . As a LiPF6 -containing electrolyte with pristine graphite does not show any DSC signal, it can be concluded that the reaction of LiPF6 with the passivation film accounts for the first signal in DSC. In a lithiated sample, the signal amplitude is increased as the reaction of some intercalated lithium with the electrolyte starts, but the reaction can happen even with totally delithiated graphite. 3.8. A possible explanation Numerous studies have been published which discuss the transformation of the passivation film on graphite at elevated

(1)

(2)

the main difference to the former reaction is the participation of lithium, this reaction is therefore an active reduction of the lithium alkylcarbonate. In the study of the reactivity of lithiated HOPG powder with different electrolytes [34], we have calculated the reaction energy of such reactions starting from the formation enthalpies of the starting and end products. We obtain, by using the data mentioned there, the following values for these two reactions: reaction (1) : +320 kJ(+2 kJ/g of (CH2 OCO2 Li)2 reacted) reaction (2) : −1400 kJ(−8.7 kJ/g of (CH2 OCO2 Li)2 reacted). It is obvious that the first reaction is unfavourable and can not account for the first DSC signal. The second reaction can not describe the first signal which is also observed in the case of delithiated graphite. Another difference from the literature is that LiPF6 does not seem to play any role in these reactions, which contrasts with our results. Therefore, LiPF6 should be considered in the reaction mechanism. LiPF6 is always slightly dissociated into LiF and PF5 . This reaction is an equilibrium which is shifted to PF5 formation at higher temperatures. Thus, larger amounts of PF5 can be formed at high temperature, in particular because the LiF formed is precipitated due to its insolubility in the electrolyte mixture. PF5 is a strong Lewis acid and we can speculate that it may react with nucleophilic compounds, especially when they are not sterically hindered. Among the species present in the electrolyte + passivation film system, lithium alkylates ROLi are the most reactive and can therefore react preferentially with PF5 under formation of a dative bonding between the oxygen of the alkylate anion and the phosphorous of PF5 (Fig. 8) and formation of a ROPF5 − anion (P(V)). The consumption of PF5 shifts the equilibrium, so that more PF5 is produced. The compound formed is likely to be soluble in the electrolyte and so more nucleophilic compounds can be dissolved. A part of the lithium alkylene carbonates and other products discussed in [32,36] can also react with PF5 , and probably be dissolved. As this reaction should be exothermic (due to dissolution of

M. Holzapfel et al. / Electrochimica Acta 49 (2004) 581–589

F F P F

+

F F

LiF

R O

-1

+

400 heat flow/mW g of graphite

Li+ + PF6-

Li+ F F P

R O F

587

F F

300 200 100 0 50

Li+

150

200

Fig. 9. Elevated temperature storage of lithiated graphite, investigated by DSC analysis. The samples were precycled in EC:DMC:DEC (2:2:1)–1 M LiPF6 , stored 1 week and analysed with EC:DMC:DEC (2:2:1)–1 M LiPF6 .

3.9. Storage experiments followed by DSC We examined the influence of the storage of lithiated graphite at elevated temperature by DSC analysis. Fig. 9 shows the results obtained after 1-week storage at different temperatures. The amounts of energy liberated in these experiments are summarised in Fig. 10, the reference being an electrode that has not been stored but measured just after lithiation. The energies given for the first peak are obtained by extrapolation. A decrease in released energy for the first peak is observed upon storage. This decrease becomes important at 60 ◦ C after 1 week of storage and could even be measured at 40 ◦ C in the case of 3 weeks storage. A good agreement is obtained between the storage temperatures or times for which a loss of capacity is observed and those relating to a reduction in energy, associated with the passivation film transformation, measured by DSC. There is also the effect of the still remaining low water content (less than 5 ppm) and/or of other impurities that may act already at lower temperatures and 120

-1

products with small crystallisation energies but high solvation energies)—an exothermic signature in the DSC could be expected, which would explain the first peak observed. This process results in a weakening of the passivation film as it becomes less homogeneous and more porous. In the case of delithiated graphite the reaction is stopped when all the accessible alkylcarbonate is complexed, the reaction considered produces a weak exothermic signal at 100–120 ◦ C. In the case of lithiated graphite, the passivation film becomes porous, which allows the reaction between the electrolyte and the intercalated lithium with reformation of alkylcarbonate. This formation of alkylcarbonate allows the continued reaction with PF5 and the released energy is thus more significant. Moreover, should the passivation film become sufficiently porous, the reaction between electrolyte and lithium would become kinetically more favourable. This reaction is associated to the second DSC signal which often can not be well separated from the first signal. We want to note that this mechanism is, at present, speculative and that this study needs to be completed for other electrolyte salts. As a matter of fact, an influence of the salt on the onset temperature of the first peak has been acknowledged in literature [25,26,28,30]. The reported temperature shift could be attributed to the film composition, which is influenced by the nature of the salt or to the reaction between the electrode surface and the electrolyte salt during the DSC analyses. However, for salts such as LiAsF6 and LiBF4 , which decompose at higher temperatures to form AsF5 and BF3 Lewis acids, reactions of the same type as the ones discussed could be expected, whereas salts as LiTFSI, which do not form Lewis acids on heating should not be expected to undergo such reactions. Indeed, in their DSC measurements on deintercalated graphite electrodes, Andersson et al. showed that for LiCF3 SO3 and LiTFSI, such peaks do not appear in the temperature range where they occur for LiBF4 and LiPF6 , but only about 50 ◦ C higher and to smaller extent [30].

100 T/ C

- Q/J g of graphite

Fig. 8. Possible reaction mechanism for the transformation of the passivation film at elevated temperatures.

40 C 60 C 70 C 80 C 90 C

100

Energy first peak

80

1 week

60

3 weeks ref

40 20 0 20 30 40 50 60 70 80 90 100 T/˚C

Fig. 10. Energies liberated during the DSC analysis of the stored samples. The values shown correspond to the energies measured for the first peak, attributed to the transformation of the passivation film.

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400

5 K/min 1.5 K/min 0.5 K/min

300

-1

heat flow/W g of graphite

500

200 100 0 50

100

150

200

250

T ( C) Fig. 11. Influence of the heating rate on the DSC curves of lithiated graphites. In any case EC:DMC:DEC (2:2:1)–1 M LiPF6 -electrolyte was used.

over the storage period. This may partly reduce the released energy and can explain the small difference in the released energies for the reference and the 40 ◦ C sample for which an effect of the temperature should be very small. An increasing storage temperature does also shifts the onset temperatures of the first peak to lower values; the separation between the two effects becomes more important. A partial transformation of the passivation film according to the reaction discussed above can be supposed to happen upon storage. If the samples stored are partially transformed, the transformation that happens during the DSC measurement becomes less significant and thus the DSC signal for this first peak diminishes. In order to explain the reduction in generated heat of the first peak, with storage temperature and time, we measured the effect of the scanning rate on the onset temperature of the film transformation. In Fig. 11, we show the influence of the heating rate on the DSC signal of lithiated graphite electrode with ternary electrolyte. The amplitudes of the different signals have been corrected for the heating rate, so that the signals are directly comparable. A difference in amplitude of the DSC signal persists which may be due to the better separation of the two exothermic signals at lower heating rate. The onset temperature for the first peak shifts to lower values : it is at 110 ◦ C for 5 K/min, 102–103 ◦ C for 1.5 K/min and 95 ◦ C for 0.5 K/min. Extrapolating these results suggests that for steady state conditions a slow transformation begins as low as 60 ◦ C, in agreement with the electrochemical experiments. The microporous membrane used (Celgard or PVdF) has no significant influence on the modification of the thermal behaviour of stored samples. The same decrease of the first DSC signal was observed with microporous PVdF and microporous polyolefine. The polymeric membrane seems to have no influence of the transformation of the passivation film. The amplitude of the second reaction depends on the storage conditions and, as expected, higher storage tempera-

tures either diminish the signal of the second peak or shift it towards higher temperatures. A diminution of the signal is not very likely: results for different lithium intercalation levels (x) did not show any difference in the second peak even for x = 0.2. Such a low intercalation level is not attained under these conditions, so it is more reasonable to suppose that the peak must be shifted towards higher temperatures. This would be consistent with an important modification of the passivation film during storage which would result in reduced mobility of the electrolyte species slowing down the reaction with the intercalated lithium, which then is shifted to higher temperatures. The total energy is relatively small: −300 J/g, compared to −2600 J/g determined for the complete reaction with DEC [34] and to the values given in literature : −1600 J/g [24,33] (EC:DMC (1:1)–1 M LiPF6 ), −1300 J/g [25] (PC:EC:DMC (1:1:3)–1 M LiPF6 ) and −700 J/g [29] (EC:DMC:DEC (4:3:3)–1 M LiPF6 ). This difference is due to the fact that the second peak is cut only in its ascent when the DSC experiment has to be stopped at 210 ◦ C. We performed an isotherm at 210 ◦ C for 10 h in order to allow the reaction to terminate. The values obtained for three different samples are not much higher: −630 and −800 J/g for the reference sample; −1040 J/g for a sample stored for 3 weeks at 40 ◦ C and −460 and −630 J/g for a sample stored for 1 week at 90 ◦ C even if it seems that the energy diminishes for the higher storage temperature. Anyway, the values remain below the ones for the total reaction, so obviously, even after 10 h of reaction; a part of the intercalated lithium is not accessible for the reaction with electrolyte.

4. Conclusion The effect of elevated temperature storage on the passivation film on graphite electrodes with EC:DMC:DEC (2:2:1)–1 M LiPF6 electrolyte is examined by electrochemical cycling and DSC. The cycling experiments show that the capacity loss is a function of both temperature and time of storage. The microporous PVdF membrane as separator has been shown to provide comparable results on electrochemical cycling and storage at elevated temperatures as polyolefinic Celgard separator. Good performance is obtained with this new microporous separator. Our study confirms the interpretation generally given in the literature, namely the attribution of the first peak to the transformation of the passivation film and the attribution of the second one to the reaction of intercalated lithium with the liquid electrolyte. An effect of storage at elevated temperatures on the intensity and onset temperature of the first peak is evidenced, which reflects the alteration of the passivation film upon storage. The effect of washing the electrode with DMC, reported to deintercalate the lithiated graphite, and the role of different electrolytes with or without LiPF6 on the DSC signals were investigated and allowed us to attribute the transformation of the passivation film upon storage to a

M. Holzapfel et al. / Electrochimica Acta 49 (2004) 581–589

reaction taking place only in the presence of LiPF6 salt. We propose, as mechanism for this transformation, the decomposition of LiPF6 at higher temperatures, with formation of the Lewis acid PF5 which reacts and dissolves partially the nucleophilic species present in the passivation film.

[15] [16] [17]

Acknowledgements

[18] [19]

The authors are indebted to Richard Michel (LEPMI, Grenoble) for its help in the values for the energies measured by DSC, Pr. Jean-Yves Sanchez (LEPMI, Grenoble) for the many scientific discussions and would like to thank SAFT-Alcatel, ATOFINA and the French ministry of industry for the financial support within of the “reactifs”-programm.

References

[20]

[21] [22] [23] [24] [25]

[1] M. Broussely, J. Power Sources 81–82 (1999) 140. [2] M. Broussely, P. Biensan, B. Simon, Electrochim. Acta 45 (1999) 3. [3] Y. Ein-Eli, B. Markovsky, D. Aurbach, Y. Carmeli, H. Yamin, S. Luski, Electrochim. Acta 39 (1994) 2559. [4] B. Simon, S. Flandrois, K. Guérin, A. Fevrier-Bouvier, J. Teulat, P. Biensan, J. Power Sources 81–82 (1999) 312. [5] Z.X. Shu, R.S. McMillan, J.J. Murray, J. Electrochem. Soc. 140 (1993) 922. [6] H. Shi, J. Power Sources 75 (1998) 64. [7] R.S. Rubino, H. Gan, E.S. Takeuchi, J. Electrochem. Soc. 148 (2001) A1029. [8] T.D. Tran, J.H. Feikert, R.W. Pekala, K. Kinoshita, J. Appl. Electrochem. 26 (1996) 1161. [9] A. Martinent, Ph.D. thesis, Institut National Polytechnique de Grenoble (2001). [10] R. Yazami, Y. Reynier, Electrochim. Acta 47 (2002) 1217. [11] T. Zheng, A.S. Gozdz, G.G. Amatucci, J. Electrochem. Soc. 146 (1999) 4014. [12] A.M. Andersson, K. Edström, J. Electrochem. Soc. 148 (2001) A1100. [13] C. Sauvage, Diploma, Institut National Polytechnique de Grenoble, 1999. [14] A.S. Gozdz, J.-M. Tarascon, C. Schmutz, P.C. Warren, F.K. Shokoohi, in: Proceedings of the Rechargeable Lithium and Lithium-ion Bat-

[26] [27] [28] [29] [30] [31] [32] [33] [34]

[35]

[36]

589

teries Symposium, paper 117, 186th ECS Meeting, Miami, Florida, 1994. A. DuPasquier, P.C. Warren, D. Culver, A.S. Gozdz, G.G. Amatucci, J.-M. Tarascon, Solid State Ionics 135 (2000) 249. F. Bodin, X. Andrieu, C. Jehoulet, I.I. Olsen, J. Power Sources 81–82 (1999) 804. M.N. Richard, J.R. Dahn, J. Electrochem. Soc. 146 (1999) 2068. D.D. MacNeil, D. Larcher, J.R. Dahn, J. Electrochem. Soc. 146 (1999) 3596. J. Saunier, F. Alloin, J.-Y. Sanchez, G. Caillon, J. Power Sources 119 (2003) 454. M. Holzapfel, F. Alloin, R. Yazami, Reactivity of the Passivation Film on Lithiated Graphite: a DSC study, in Proceedings of the 203rd Electro-chemical Society Meeting in Paris (poster presentation), France, 27 April–2 May 2003. To be published in the Proceedings, vol. 1. D. Aurbach, B. Markosky, M.D. Levi, E. Levi, A. Schechter, M. Moshkovich, Y. Cohen, J. Power Sources 81–82 (1999) 95. D. Aurbach, J. Power Sources 89 (2000) 206. G. Zhuang, Y. Chen, P.R. Ross, Langmuir 15 (1999) 1470. A. Du Pasquier, F. Disma, T. Bowmer, A.S. Gozdz, G. Amatucci, J.-M. Tarascon, J. Electrochem. Soc. 145 (1998) 472. P. Biensan, B. Simon, J.P. Pérès, A. de Guibert, M. Broussely, J.M. Bodet, F. Perton, J. Power Sources 81–82 (1999) 906. K. Edström, A.M. Andersson, A. Bishop, L. Fransson, J. Lindgren, A. Hussénius, J. Power Sources 97 (2001) 87. A.M. Andersson, Ph.D. thesis, Uppsala University, 2001. L. Fransson, T. Eriksson, K. Edström, T. Gustafsson, J.O. Thomas, J. Power Sources 101 (2001) 1. H. Maleki, G. Deng, A. Anani, J. Howard, J. Electrochem. Soc. 146 (1999) 3224. A.M. Andersson, M. Herstedt, A.G. Bishop, K. Edström, Electrochim. Acta 47 (2001) 1885. A.M. Andersson, K. Edström, J.O. Thomas, J. Power Sources 81–82 (1999) 8. Z. Zhang, D. Fouchard, J.R. Rea, J. Power Sources 70 (1998) 16. P. Novak, F. Joho, M. Lanz, B. Rykart, J.-C. Panitz, D. Alliata, R. Kötz, O. Haas, J. Power Sources 97 (2001) 39. M. Holzapfel, F. Alloin, R. Yazami, J. Phys. Chem. B 106 (2002) 13165; M. Holzapfel, Ph.D. thesis, Institut National Polytechnique de Grenoble, 2002. M. Holzapfel, F. Alloin, R. Yazami, Poster presentation, in: Proceedings of the 11. IMLB, abstract # 246, Monterey, California, 2002. Y. Wang, S. Nakamura, M. Ue, P.B. Balbuena, J. Am. Chem. Soc. 123 (2001) 11708.