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Sodium reactivity with carbons
0022~3697(95)00366-5
Pergamon
SODIUM L. JONCOURT,? tLaboratoire
REACTIVITY
M. MERMOUX,?
J t’hys. Chem Solids Vol 57. Nos 6-8. pp. 877-882. 1996 Copyright 8 1596 Elwier Science Ltd Printed in Great Britain. All rights reserved 0022.3697196 515.00 + 0.00
WITH CARBONS
PH. TOUZAIN,? and B. ALLARDS
L. BONNETAIN,?
D. DUMASS
Science des Surfaces et Materiaux Carbon&, ERS CNRS no 101, ENSEEG-INPG, BP 75 Domaine Universitaire, F-38402, Saint-Martin d’Htres Cedex, France SLRE, Carbone Savoie, 10, rue de l’lndustrie BP 16, F-6963 1, Vknissieux Cedex, France (Received 28 May 1995; accepted 3 1 May 1995)
Abstract-We
report in this paper some results on the reactivity of different carbon materials, and in particular a set of pitch-coke samples heat treated between 800 and 24Oo”C,with sodium. An experimental cell has been designed, which allows carbon to react with sodium vapor at relative pressure close to 0.9 up to 800°C. The different carbon-sodium samples were examined using X-ray diffraction. The sodium uptake has been related both to intercalation and adsorption or capillary condensation. It is shown that the amount of intercalated sodium cannot be related to one of the structural parameters deduced from the X-ray diffraction patterns or from the Raman spectra of the pristine samples. We then discuss the uncertainty of our results as a function of the crystallite size (L, and L,) of the carbon samples, assuming an in-plane ratio C : Na of 8. Finally, some preliminary results concerning the electrochemical intercalation of sodium in pitch-coke samples heat-treated at different temperatures in a solid state sodium-carbon battery using a polymer electrolyte will be presented in order to assess the amount of intercalated sodium. Keywords: A. inorganic compounds, B. chemical synthesis, C. X-ray diffraction, D. microstructure.
1. INTRODUCTION Different carbon materials, pitch cokes or petroleum cokes, anthracitic carbons, etc., are widely used as cathode blocks in aluminum electrolysis cells, and there is a general agreement that penetration of sodium atoms into the carbon lattice is responsible for the swelling and subsequent destruction of the electrodes [l]. The more graphitic materials tend to have superior characteristics in terms of decreased electrical resistance and increased resistance to sodium penetration, but their abrasion resistance is lower. This is the reason why carbons heat treated to moderate temperatures (11OO- 1600°C) are generally used as cathodic materials for aluminum electrolysis. It is well known that sodium intercalation in graphite is very different from the intercalation of the other alkali metals since only high stage compounds (8th to 6th stage) have been observed, but not in a reproducible way. The interaction of sodium in graphitic carbons has been studied by several authors [2-61, and it has been shown that different phenomena may occur. Regarding the graphitizing carbons (pitchcoke, petroleum-coke, etc.), intercalation is actually observed, and the lower the heat treatment temperature (HTT) of the coke, the greater the uptake. This has been interpreted in terms of the position of the Fermi level: the lower the Fermi level, the greater the intercalation. However, the amount of intercalated
sodium, as determined from X-ray diffraction, cannot be considered as being related to chemical analyses. A sodium excess is sometimes evidenced and can be attributed to capillary condensation in the porosity of the material or adsorption. This has mostly been observed for non-graphitizing carbons which are characterized by a highly porous microtexture. We presented recently [7] some results on the intercalation of sodium in different carbon samples, especially in anthracitic carbons. It was shown that the amount of intercalated sodium was HTT-dependent for all the different samples examined. Moreover, we noticed that the interpretation of the results, the discrimination between the intercalated and the condensed or adsorbed amount of sodium in particular, was strongly based upon the X-ray diffraction spectra. We present in this paper some results about the behavior of pitch-coke samples heat treated between 800 and 2400°C. Intercalation was carried out both chemically using sodium vapor at 700°C and electrochemically using a polymer electrolyte at 80°C. The electrochemical method was used in order to control continuously the amount of intercalated sodium. Some of these experiments have been conducted in order to obtain a hypothetical correlation between structural parameters and the ability of carbon to intercalate sodium as was done for the lithium 877
L. JONCOURT et al.
878
HTT [“Cl
intercalation in carbon 1891, to control the uptake of sodium, and to observe the staging process.
700
1100
1500
1900
2300
2. EXPERIMENTAL
Pitch-coke samples were provide by the Carbone Savoie. They were heat treated at various temperatures from 800°C up to 2400°C in an induction graphite furnace under a nitrogen flow at atmospheric pressure. The residence time at the maximum temperature was 2 h. No pretreatment of the samples to remove contaminants and heteroatoms (oxygen in particular) was undertaken. This set of samples was characterized by using both X-ray diffraction and Raman spectroscopy. X-ray diffraction measurements were carried out on a Phillips diffractometer using CuK, (0.15418 nm) radiation. Patterns were analyzed according to known procedures [lo, 111, the average interlayer spacing (doo2),the crystallite sizes along the a axis (L,) and along the c axis (L,,), the structural strain eC along the c axis were determined for each sample. Micro-Raman spectra were recorded on a multichannel Dilor XY spectrometer using the 5 14.5 nm radiation of an argon ion laser, and analyzed according to known procedures [12, 131. Density measurements were carried out with a standard helium pycnometer, and surface area measurements were carried out using the standard BET method with nitrogen. Sodium intercalation was first carried out using sodium vapor according to a procedure described in Ref. [ 141.Since sodium may react with Pyrex glass, the preparations of the sodium-carbon compounds were carried out in a reactor made of refractory steel. The sodium relative pressure was about 0.9. in most cases, the carbon temperature was kept constant at 700°C and the reaction time was 5 h. About 2 g of carbon were introduced in the reactor for each run. The grain size of the carbon samples was between 74 and 104pm. As the reaction products are extremely sensitive to air, for the X-ray diffraction analysis, the samples were protected from air exposure using a container equipped with a beryllium window. The total sodium content of the intercalated samples was determined by means of a boiling-water extraction and a subsequent titration of the resulting alkaline solution. Results were compared to those obtained for sodium intercalation in graphite (natural from Madagascar) and to those in a pyrocarbon sample obtained at 2100°C. Electrochemical intercalation was performed in Na/ C cells using polymer electrolyte to prevent intercalation of solvated sodium. Polymer electrolytes of composition P(E0)t2NaCF3S0, [P(EO) = commercial 5 M polyethylene oxide] and composite cathodes
50 , 60
0 x
70 80 -
Fig. 1. Variation in the sodium uptake in pitch coke samples as a function of HTT. A: Total uptake determined from the elemental analysis; 0: uptake of chemically intercalated sodium determined from the X-ray diffraction patterns; x: uptake of electrochemically intercalated sodium determined from coulometric measurements.
containing the carbon of interest (grain size lower than 25 pm), P(E0) and NaCFsSOs were used. Intercalation was carried out using slow scan voltammetry with a Mac Pile System [I 51. Voltage steps of 20 mV/ 6 h were systematically used. The intercalation rate was estimated from coulometric measurements assuming no parasitic side reactions. A cutoff voltage of 0.02V vs. Na+/Na was used to prevent the interference of sodium plating which could occur at about 0 V in these sodium cells.
3. RESULTS AND DISCUSSION
Firstly, the chemical reaction results are presented. In order to test our experimental procedure, sodium intercalation in pure graphite was first studied. No intercalation was observed using the experimental conditions described in the Experimental. Next, sodium was intercalated in the pyrocarbon sample. Both chemical analysis and the X-ray diffraction pattern (Fig. 2e) were consistent with a stage 3 compound. The in-plane ratio deduced from the elemental analysis was close to NaCs and the increase of interlayer spacing was 0.120 nm. These results were in full agreement with those found in the literature 161. The sodium uptake for the pitch-coke, as a function of the HTT is plotted in Fig. 1. Some of these results were presented in Ref. [7]. The sodium uptake was found to be maximum for a 800°C HTT, after which it decreased, and then became insignificant above 2200°C. These results are in agreement with those found in the references [2,5,6] for similar graphitizing carbons. Approximate compositions between NaCs
Sodium reactivity with carbons
18 20 22
18 20 22 24 26 28 30 32 34 28
879
24
30 32 34
(b)
18 20 22 24 26 28 30 32 34
18 20 22 24 26
20
28
30 32 34
28
Cd)
CC)
20
28
20
(a)
10
26
30
2o
40
50
60
Fig. 2. Examples of X-ray diffraction patterns before and after the chemical intercalation of sodium. (a) Pitch-coke HTT = 1900°C; (b) pitch-coke H?T = 1500°C; (c) pitch-coke HTT = 1100°C; (d) pitch-coke HTT = 800°C; (e) pyrocarbon sample.
and NaCt2 were obtained for the lowest HTT, while compositions of NaCm and below were obtained for the highest HTT. Some of the X-ray diffraction patterns of the intercalated samples are presented in Fig. 2. For the lowest HTT, a single (OO/) reflection line was observed while for the highest HTT a series of (001) reflection appeared. For most of the X-ray diffraction patterns, the position of the lines fall
between those that would be obtained for two successive stage values. So, this observation of a single set of (001) lines suggests a random mixture of two successive stages in most of the samples examined. Thus, assuming non-integral stage values and an in-plane ratio C/Na of 8 whatever the HTT was, the amount of intercalated sodium could be deduced from the position of the (001) reflections [6] (plotted in Fig. 1).
880
L. JONCOURT et al.
-9 E ;
0.06 0.05
0.00
’ 700
1150
1600
2050
2500
HIT(T)
Fig. 3. Porous volume estimated from helium density measurements (*) or estimated from the excess amount of sodium (0) as a function of HTT.
Comparing these values with the chemical analysis, an excess amount of sodium was always evidenced. It is also known that other phenomena than intercalation may occur when carbon samples are exposed to a sodium vapor. Sodium adsorption and capillary condensation have been proved in the work of Metrot et al. [2, 51. Moreover, sodium diffraction lines were sometimes seen in the X-ray diffraction patterns of some of the samples (see for example Fig. 2c and d). The porous volume of the samples has been roughly determined from density measurements. Assuming that sodium fills all the porous volume, we obtained the results presented in Fig. 3. The results are also in satisfactory agreement, except for the lowest HTT. These experiments and the previously published ones have shown that the amount of intercalated sodium strongly depends on the HTT of the carbon samples. It is known that the structure and the microtexture of carbons is also HTT-dependent. So, it is tempting to try to correlate the amount of intercalated sodium to the structural parameters deduced from the structural analysis of the pristine carbon using X-ray diffraction or Raman spectroscopy. However, such a correlation was impossible to obtain. For example, chemical analysis and X-ray diffraction results indicate that the amount of intercalated sodium was similar for the pyrocarbon sample and for a pitchcoke sample heat-treated at 1100°C (formation of a stage 3 compound), while the structural analysis showed that the dmZ,L, and I!,~values were completely different for both samples. Thus, sodium intercalation appears not to be related to these “geometric” parameters. This seems to confirm Mering’s hypothesis which correlates the uptake of sodium to the Fermi level in the carbon samples. This explains in particular why on the contrary
to pure graphite, boronated graphite intercalates sodium, the presence of boron into the carbon lattice inducing a lowering of the Fermi level. Sodium intercalation in disordered carbons seems to be related to the presence of acceptors defects within the carbon planes, their nature and/or concentration being HTT dependent. This observation rules out graphitization models based on the assumption of the invariance of the elementary layer supposed to be identical to that of graphite. According to Maire and Mering [lo], these defects may be interstitial carbon atoms. It must also be pointed out that the formation of regularly stacked stages in particular for high stage numbers may not be very clearly observed in such poorly organized carbons. For example, it was not possible to observe X-ray diffraction lines corresponding to sodium, either in a superstructure or in nonepitaxic compounds, as is found, for example, in the potassium-carbon compounds. All the results presented above and all the literature data are based on indexing of the X-ray diffraction patterns assuming a constant in-plane ratio. It is clear that this assumption may be justified for the highest HTT values (1800°C and above) for which the La values are higher than ca 1Onm. On the contrary, for the lowest HTT, the L, values are low for such disordered carbons and are basically comprised between 2 and 5 nm for HTT lower than 16OO”C,and this hypothesis may be unrealistic. Indeed, in the case of very small crystallite sizes, the expected in-plane C/Na ratio could be greater than 8 because the regularity of the in-plane superlattice is disturbed at the edge of a graphite plane. Following Fujimoto et al. [ 16- 181,we have undertaken calculations to determine a more appropriate in-plane density for the case of perfect small crystallites for which the diameter is L, and the thickness is L,. The detail of such calculations, carried out for the case of lithium intercalation in disordered carbons, may be found in Ref. [ 181. Summarizing, it is supposed that each graphite plane develops isotropically to the a and b axes: the first elemental unit is the benzene ring, the second is the coronene one, and so on. It can be shown that the amount of intercalated sodium for a hypothetical first stage compound varies as a function of L, and L, as described by the following equation: $8~
(L, + &GL,12 Wf
-&d
x (r, +
ho2)
LC
The result of the calculation for a crystallite whose L, value is 2nm, L, value is 1.5nm and doo2 value is 0.344nm (these value have been deduced from the X-ray diffraction pattern of the 800°C coke sample) shows that NaC,2,5 is a more realistic limit composition for a hypothetical first stage compound.
881
Sodium reactivity with carbons
C
..
ISOO’ 220d.. lP”O . . . . .. . . . ... .. . ... . . ..
0
0
0. I
0.2
0.3 x
0.4
.,2lio. ..I.......... 0.5
0.6
I5
80”
Fig. 4. V(x) curves for the different Na/pitch-coke cells. The inset is a detail of the V(x) curve for the coke heat-treated at 2200°C.
24
27
30
Fig. 5. Diffraction profiles in the (002) region during the intercalation in the coke sample heat-treated at 800°C for different values of x from NaC,.
of our different current vs. time curves has confirmed this result. Finally, for the pitch-coke heat-treated at 22OO”C,the voltage/composition curve tends to flatten (see the inset in Fig. 4) while no evidence of plateaus indicative of two phases coexistence was observed for the other HTT. So, these measurements give no evidence for the formation of regular staged phases for the cokes heat-treated below 2200°C. At the present time, no in situ X-ray diffraction measurements are available to confirm this observation. As a preliminary X-ray investigation, a series of spectra has been recorded for the chemical intercalation in a coke sample heat-treated at 800°C for various reaction times. The set of spectra over the (002) region is presented in Fig. 5. It can be seen that the (002) line smoothly shifts, while the half width of the line remains approximately constant with the exception of the first spectra for which the line results from carbon and intercalated carbon. This behavior confirms that regular staging does not occur during the intercalation in such disordered phases. For this particular case, the final interlayer spacing is consistent with a second stage compound while chemical analyses are more consistent with a first stage compound. In other words, the lattice expansion may not be consistent with the 0.12nm diameter of intercalated sodium. This behavior has been observed by Boehm et al. during the study of intercalation of sodium in carbon blacks. Similar investigations for other HTTs are currently under progress. Finally, the results of this study are also in qualitative agreement with some results of lithium intercalation in carbon. For example, Dahn et al. 1211have shown that staging is not observed for carbon heattreated below 1900-2200°C. This can be understood remembering that the in-plane crystallite diameter is examination
Thus, it seems necessary to follow more precisely the reaction of sodium into carbon. Electrochemical intercalation measurements have been carried out for this purpose. The results are presented in Fig. 4 for five different HTT samples. There are systematic differences between these curves. First, it can be seen that the voltage intercalation threshold is higher as HTT is lower. Sodium intercalation begins near 1.8 V/Na for coke heat-treated at 8Oo”C, while it begins at I .4 V/Na for coke heat-treated at 2200°C. Thus, sodium intercalation is easier for the lowest HTT samples. It may be noted that true reductive intercalation of lithium in graphite occurs below 0.4 V/Li. Thus, the corresponding process for sodium is expected to occur only below 0.1 V/Na, the standard reduction potentials for Li/Li+ and Na/Na+ are -3.0 and -2.7 V, respectively. From this observation, most of the sodium intercalation in graphite is expected to occur below the sodium plating potential and thus cannot be observed. Second, the amount of intercalated sodium is HTT-dependent. The total amount of intercalated sodium is also plotted in Fig. 1. The comparison of the amount of chemically or electrochemically intercalated sodium is in good agreement, with the exception of the lower HTT for which the amount of chemically intercalated sodium is higher. This may be also explained remembering that the in-plane ratio NaCs assumption is an overestimation for such disordered carbons. Only a few results of electrochemical sodium intercalation in carbon are available in the literature. Nevertheless, our results are in qualitative agreement with those given by Doeff et al. [19,20]. Moreover, these authors indicated the need to study sodium intercalation in low granulometry powders, indicating a rather low diffusion coefficient of sodium in carbon. The
21 28
0.7
from Na,C,
18
882
L. JONCOURT
et al. REFERENCES
lower than ca IOnm for such carbon samples, preventing the clustering of the intercalated sodium atoms. 4. CONCLUSIONS
2. 4.
We have reported results about the chemical and the electrochemical intercalation of sodium in disordered carbons. Both chemical and electrochemical measurements have shown that the amount of intercalated sodium depends on the HTT of the carbon. The results were in good agreement with those previously reported and the amount of sodium electrochemically intercalated has been found to correspond to that chemically intercalated. No direct correlation between the amount of intercalated sodium and the structural parameters describing the in-plane or the out-of-plane ordering has been obtained. This seems to confirm Mering’s hypothesis which correlates the uptake of sodium to the Fermi level in the carbon samples. Furthermore, the shape of the V(x) curves tends to indicate that staging does not occur in a regular way as it would be obtained for graphite for HTT values lower than 2200°C. Preliminary sequential X-ray measurements have confirmed this observation. These results may be compared to those known for the lithium intercalation in similar carbon samples. The effect of a small in-plane crystal size on the maximum amount of sodium considering an octal structure has been analyzed and it was shown that the calculated amount of sodium may be quite different from NaCs for an hypothetical first stage compound. In situ X-ray measurements in an electrochemical sodium cell are now planned, in order to obtain a more detailed scheme of the intercalation process in these coke samples.
20.
Acknowledgement-The authors are grateful to Y. Chabre (Laboratoire de SpectromktriePhysique, Grenoble) for very
21.
fruitful discussions and advice.
6.
8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
Dewing E. W., Trans. Melall. Sot. A.I.M.E. 227, 1328 (1963). Metrot A., Ph. D. Thesis, Nancy (1969). Berger D., Third cycle Thesis, Nancy (1971). Marsh H., Murdie N., Edwards I. A. S. and Boehm H. P., Chemistry and Physics of Carbon (Edited by P. L. Walker and P. T. Thrower), Vol. 20, p. 213. Dekker, New York (1987). Berger D., Carton B., Metrot A. and Herold A., Chemistry and Physics of Carbon (Edited by P. L. Walker and P. T. Thrower), Vol. 12, p. 1. Dekker, New York (1968). Robert M. C., Oberlin M. and Mering J., Chemistry and Physics of Carbon (Edited by P. L. Walker and P. T. Thrower), Vol. 10, p. 142. Dekker, New York (1970). Sechet C., Sarmeo D., Mermoux M., Touzain Ph., Bonnetain L., Allard B., Dumas D. and Paulus R., Mol. Cryst. Liq. Cryst. 245, 153 (1994). Dahn J. R., Sleigh A. K., Shi H., Reimeirs J. R., Zhong Q, and Way B. M., Electrochim. Acta 38, 1179 (1993). Tatsumi K., Iwashita N., Sakaebe H., Shioyama H., Higuchi S., Mabuchi A. and Fujimoto H., J. Elecfrothem. Sot. 142, 716 (1995). Maire J. and Mering J., Chemistry and Physics of Carbon (Edited bv P. L. Walker and P. T. Thrower).,_ Vol. 6. b. 125. Ddkker, New York (1970). Iwashita N. and Inagaki M., Carbon 13, 1107 (1993). Tuinstra F. and Koenig J. L., J. Chem. Phys. 53, 1126 (1970). Rouzaud J. N., Oberlin A. and Beny Bassez C., Thin Solid Films 105, 75 (1983). Sechet C., Thesis, Grenoble (1993). Chabre Y., NATO ASZSeries B305, 181 (1993). Fujimoto H., Tokumitsu K., Mabuchi A., Kasuh T. and Shiraishi M., Ext. Abstrs. Carbon ‘94, Grenade, 3-8 July, p. 134 (1994). Fujimoto H., Tokumitsu K., Mabuchi A., Kasuh T. and Shiraishi M., Carbon 32, 1249 (1994). Fujimoto H., Tokumitsu K., Mabuchi A., Kasuh T. and Akuzawa N., Ext. Abstrs. Carbon ‘94, Grenade, 3-8 July, p. 604 (1994). Doeff M. C., Ma Y., Visco S. J. and De Jonghe L. C., J. Elecrrochem. Sot., 140, 169 (1993). Doeff M. C., Ma Y ., Visco S. J. and De Jonghe L. C., The Elecrrochem. Sot. Ext. Abstrs., Vol. 93- 1, abstract 1895, p. 2583, Honolulu, HI, Meeting, 16-21 May (1993). Dahn J. R., Fong R. and Spoon M. J., Phys. Rev. 842, 6424 (1993).
Pergamon
SODIUM L. JONCOURT,? tLaboratoire
REACTIVITY
M. MERMOUX,?
J t’hys. Chem Solids Vol 57. Nos 6-8. pp. 877-882. 1996 Copyright 8 1596 Elwier Science Ltd Printed in Great Britain. All rights reserved 0022.3697196 515.00 + 0.00
WITH CARBONS
PH. TOUZAIN,? and B. ALLARDS
L. BONNETAIN,?
D. DUMASS
Science des Surfaces et Materiaux Carbon&, ERS CNRS no 101, ENSEEG-INPG, BP 75 Domaine Universitaire, F-38402, Saint-Martin d’Htres Cedex, France SLRE, Carbone Savoie, 10, rue de l’lndustrie BP 16, F-6963 1, Vknissieux Cedex, France (Received 28 May 1995; accepted 3 1 May 1995)
Abstract-We
report in this paper some results on the reactivity of different carbon materials, and in particular a set of pitch-coke samples heat treated between 800 and 24Oo”C,with sodium. An experimental cell has been designed, which allows carbon to react with sodium vapor at relative pressure close to 0.9 up to 800°C. The different carbon-sodium samples were examined using X-ray diffraction. The sodium uptake has been related both to intercalation and adsorption or capillary condensation. It is shown that the amount of intercalated sodium cannot be related to one of the structural parameters deduced from the X-ray diffraction patterns or from the Raman spectra of the pristine samples. We then discuss the uncertainty of our results as a function of the crystallite size (L, and L,) of the carbon samples, assuming an in-plane ratio C : Na of 8. Finally, some preliminary results concerning the electrochemical intercalation of sodium in pitch-coke samples heat-treated at different temperatures in a solid state sodium-carbon battery using a polymer electrolyte will be presented in order to assess the amount of intercalated sodium. Keywords: A. inorganic compounds, B. chemical synthesis, C. X-ray diffraction, D. microstructure.
1. INTRODUCTION Different carbon materials, pitch cokes or petroleum cokes, anthracitic carbons, etc., are widely used as cathode blocks in aluminum electrolysis cells, and there is a general agreement that penetration of sodium atoms into the carbon lattice is responsible for the swelling and subsequent destruction of the electrodes [l]. The more graphitic materials tend to have superior characteristics in terms of decreased electrical resistance and increased resistance to sodium penetration, but their abrasion resistance is lower. This is the reason why carbons heat treated to moderate temperatures (11OO- 1600°C) are generally used as cathodic materials for aluminum electrolysis. It is well known that sodium intercalation in graphite is very different from the intercalation of the other alkali metals since only high stage compounds (8th to 6th stage) have been observed, but not in a reproducible way. The interaction of sodium in graphitic carbons has been studied by several authors [2-61, and it has been shown that different phenomena may occur. Regarding the graphitizing carbons (pitchcoke, petroleum-coke, etc.), intercalation is actually observed, and the lower the heat treatment temperature (HTT) of the coke, the greater the uptake. This has been interpreted in terms of the position of the Fermi level: the lower the Fermi level, the greater the intercalation. However, the amount of intercalated
sodium, as determined from X-ray diffraction, cannot be considered as being related to chemical analyses. A sodium excess is sometimes evidenced and can be attributed to capillary condensation in the porosity of the material or adsorption. This has mostly been observed for non-graphitizing carbons which are characterized by a highly porous microtexture. We presented recently [7] some results on the intercalation of sodium in different carbon samples, especially in anthracitic carbons. It was shown that the amount of intercalated sodium was HTT-dependent for all the different samples examined. Moreover, we noticed that the interpretation of the results, the discrimination between the intercalated and the condensed or adsorbed amount of sodium in particular, was strongly based upon the X-ray diffraction spectra. We present in this paper some results about the behavior of pitch-coke samples heat treated between 800 and 2400°C. Intercalation was carried out both chemically using sodium vapor at 700°C and electrochemically using a polymer electrolyte at 80°C. The electrochemical method was used in order to control continuously the amount of intercalated sodium. Some of these experiments have been conducted in order to obtain a hypothetical correlation between structural parameters and the ability of carbon to intercalate sodium as was done for the lithium 877
L. JONCOURT et al.
878
HTT [“Cl
intercalation in carbon 1891, to control the uptake of sodium, and to observe the staging process.
700
1100
1500
1900
2300
2. EXPERIMENTAL
Pitch-coke samples were provide by the Carbone Savoie. They were heat treated at various temperatures from 800°C up to 2400°C in an induction graphite furnace under a nitrogen flow at atmospheric pressure. The residence time at the maximum temperature was 2 h. No pretreatment of the samples to remove contaminants and heteroatoms (oxygen in particular) was undertaken. This set of samples was characterized by using both X-ray diffraction and Raman spectroscopy. X-ray diffraction measurements were carried out on a Phillips diffractometer using CuK, (0.15418 nm) radiation. Patterns were analyzed according to known procedures [lo, 111, the average interlayer spacing (doo2),the crystallite sizes along the a axis (L,) and along the c axis (L,,), the structural strain eC along the c axis were determined for each sample. Micro-Raman spectra were recorded on a multichannel Dilor XY spectrometer using the 5 14.5 nm radiation of an argon ion laser, and analyzed according to known procedures [12, 131. Density measurements were carried out with a standard helium pycnometer, and surface area measurements were carried out using the standard BET method with nitrogen. Sodium intercalation was first carried out using sodium vapor according to a procedure described in Ref. [ 141.Since sodium may react with Pyrex glass, the preparations of the sodium-carbon compounds were carried out in a reactor made of refractory steel. The sodium relative pressure was about 0.9. in most cases, the carbon temperature was kept constant at 700°C and the reaction time was 5 h. About 2 g of carbon were introduced in the reactor for each run. The grain size of the carbon samples was between 74 and 104pm. As the reaction products are extremely sensitive to air, for the X-ray diffraction analysis, the samples were protected from air exposure using a container equipped with a beryllium window. The total sodium content of the intercalated samples was determined by means of a boiling-water extraction and a subsequent titration of the resulting alkaline solution. Results were compared to those obtained for sodium intercalation in graphite (natural from Madagascar) and to those in a pyrocarbon sample obtained at 2100°C. Electrochemical intercalation was performed in Na/ C cells using polymer electrolyte to prevent intercalation of solvated sodium. Polymer electrolytes of composition P(E0)t2NaCF3S0, [P(EO) = commercial 5 M polyethylene oxide] and composite cathodes
50 , 60
0 x
70 80 -
Fig. 1. Variation in the sodium uptake in pitch coke samples as a function of HTT. A: Total uptake determined from the elemental analysis; 0: uptake of chemically intercalated sodium determined from the X-ray diffraction patterns; x: uptake of electrochemically intercalated sodium determined from coulometric measurements.
containing the carbon of interest (grain size lower than 25 pm), P(E0) and NaCFsSOs were used. Intercalation was carried out using slow scan voltammetry with a Mac Pile System [I 51. Voltage steps of 20 mV/ 6 h were systematically used. The intercalation rate was estimated from coulometric measurements assuming no parasitic side reactions. A cutoff voltage of 0.02V vs. Na+/Na was used to prevent the interference of sodium plating which could occur at about 0 V in these sodium cells.
3. RESULTS AND DISCUSSION
Firstly, the chemical reaction results are presented. In order to test our experimental procedure, sodium intercalation in pure graphite was first studied. No intercalation was observed using the experimental conditions described in the Experimental. Next, sodium was intercalated in the pyrocarbon sample. Both chemical analysis and the X-ray diffraction pattern (Fig. 2e) were consistent with a stage 3 compound. The in-plane ratio deduced from the elemental analysis was close to NaCs and the increase of interlayer spacing was 0.120 nm. These results were in full agreement with those found in the literature 161. The sodium uptake for the pitch-coke, as a function of the HTT is plotted in Fig. 1. Some of these results were presented in Ref. [7]. The sodium uptake was found to be maximum for a 800°C HTT, after which it decreased, and then became insignificant above 2200°C. These results are in agreement with those found in the references [2,5,6] for similar graphitizing carbons. Approximate compositions between NaCs
Sodium reactivity with carbons
18 20 22
18 20 22 24 26 28 30 32 34 28
879
24
30 32 34
(b)
18 20 22 24 26 28 30 32 34
18 20 22 24 26
20
28
30 32 34
28
Cd)
CC)
20
28
20
(a)
10
26
30
2o
40
50
60
Fig. 2. Examples of X-ray diffraction patterns before and after the chemical intercalation of sodium. (a) Pitch-coke HTT = 1900°C; (b) pitch-coke H?T = 1500°C; (c) pitch-coke HTT = 1100°C; (d) pitch-coke HTT = 800°C; (e) pyrocarbon sample.
and NaCt2 were obtained for the lowest HTT, while compositions of NaCm and below were obtained for the highest HTT. Some of the X-ray diffraction patterns of the intercalated samples are presented in Fig. 2. For the lowest HTT, a single (OO/) reflection line was observed while for the highest HTT a series of (001) reflection appeared. For most of the X-ray diffraction patterns, the position of the lines fall
between those that would be obtained for two successive stage values. So, this observation of a single set of (001) lines suggests a random mixture of two successive stages in most of the samples examined. Thus, assuming non-integral stage values and an in-plane ratio C/Na of 8 whatever the HTT was, the amount of intercalated sodium could be deduced from the position of the (001) reflections [6] (plotted in Fig. 1).
880
L. JONCOURT et al.
-9 E ;
0.06 0.05
0.00
’ 700
1150
1600
2050
2500
HIT(T)
Fig. 3. Porous volume estimated from helium density measurements (*) or estimated from the excess amount of sodium (0) as a function of HTT.
Comparing these values with the chemical analysis, an excess amount of sodium was always evidenced. It is also known that other phenomena than intercalation may occur when carbon samples are exposed to a sodium vapor. Sodium adsorption and capillary condensation have been proved in the work of Metrot et al. [2, 51. Moreover, sodium diffraction lines were sometimes seen in the X-ray diffraction patterns of some of the samples (see for example Fig. 2c and d). The porous volume of the samples has been roughly determined from density measurements. Assuming that sodium fills all the porous volume, we obtained the results presented in Fig. 3. The results are also in satisfactory agreement, except for the lowest HTT. These experiments and the previously published ones have shown that the amount of intercalated sodium strongly depends on the HTT of the carbon samples. It is known that the structure and the microtexture of carbons is also HTT-dependent. So, it is tempting to try to correlate the amount of intercalated sodium to the structural parameters deduced from the structural analysis of the pristine carbon using X-ray diffraction or Raman spectroscopy. However, such a correlation was impossible to obtain. For example, chemical analysis and X-ray diffraction results indicate that the amount of intercalated sodium was similar for the pyrocarbon sample and for a pitchcoke sample heat-treated at 1100°C (formation of a stage 3 compound), while the structural analysis showed that the dmZ,L, and I!,~values were completely different for both samples. Thus, sodium intercalation appears not to be related to these “geometric” parameters. This seems to confirm Mering’s hypothesis which correlates the uptake of sodium to the Fermi level in the carbon samples. This explains in particular why on the contrary
to pure graphite, boronated graphite intercalates sodium, the presence of boron into the carbon lattice inducing a lowering of the Fermi level. Sodium intercalation in disordered carbons seems to be related to the presence of acceptors defects within the carbon planes, their nature and/or concentration being HTT dependent. This observation rules out graphitization models based on the assumption of the invariance of the elementary layer supposed to be identical to that of graphite. According to Maire and Mering [lo], these defects may be interstitial carbon atoms. It must also be pointed out that the formation of regularly stacked stages in particular for high stage numbers may not be very clearly observed in such poorly organized carbons. For example, it was not possible to observe X-ray diffraction lines corresponding to sodium, either in a superstructure or in nonepitaxic compounds, as is found, for example, in the potassium-carbon compounds. All the results presented above and all the literature data are based on indexing of the X-ray diffraction patterns assuming a constant in-plane ratio. It is clear that this assumption may be justified for the highest HTT values (1800°C and above) for which the La values are higher than ca 1Onm. On the contrary, for the lowest HTT, the L, values are low for such disordered carbons and are basically comprised between 2 and 5 nm for HTT lower than 16OO”C,and this hypothesis may be unrealistic. Indeed, in the case of very small crystallite sizes, the expected in-plane C/Na ratio could be greater than 8 because the regularity of the in-plane superlattice is disturbed at the edge of a graphite plane. Following Fujimoto et al. [ 16- 181,we have undertaken calculations to determine a more appropriate in-plane density for the case of perfect small crystallites for which the diameter is L, and the thickness is L,. The detail of such calculations, carried out for the case of lithium intercalation in disordered carbons, may be found in Ref. [ 181. Summarizing, it is supposed that each graphite plane develops isotropically to the a and b axes: the first elemental unit is the benzene ring, the second is the coronene one, and so on. It can be shown that the amount of intercalated sodium for a hypothetical first stage compound varies as a function of L, and L, as described by the following equation: $8~
(L, + &GL,12 Wf
-&d
x (r, +
ho2)
LC
The result of the calculation for a crystallite whose L, value is 2nm, L, value is 1.5nm and doo2 value is 0.344nm (these value have been deduced from the X-ray diffraction pattern of the 800°C coke sample) shows that NaC,2,5 is a more realistic limit composition for a hypothetical first stage compound.
881
Sodium reactivity with carbons
C
..
ISOO’ 220d.. lP”O . . . . .. . . . ... .. . ... . . ..
0
0
0. I
0.2
0.3 x
0.4
.,2lio. ..I.......... 0.5
0.6
I5
80”
Fig. 4. V(x) curves for the different Na/pitch-coke cells. The inset is a detail of the V(x) curve for the coke heat-treated at 2200°C.
24
27
30
Fig. 5. Diffraction profiles in the (002) region during the intercalation in the coke sample heat-treated at 800°C for different values of x from NaC,.
of our different current vs. time curves has confirmed this result. Finally, for the pitch-coke heat-treated at 22OO”C,the voltage/composition curve tends to flatten (see the inset in Fig. 4) while no evidence of plateaus indicative of two phases coexistence was observed for the other HTT. So, these measurements give no evidence for the formation of regular staged phases for the cokes heat-treated below 2200°C. At the present time, no in situ X-ray diffraction measurements are available to confirm this observation. As a preliminary X-ray investigation, a series of spectra has been recorded for the chemical intercalation in a coke sample heat-treated at 800°C for various reaction times. The set of spectra over the (002) region is presented in Fig. 5. It can be seen that the (002) line smoothly shifts, while the half width of the line remains approximately constant with the exception of the first spectra for which the line results from carbon and intercalated carbon. This behavior confirms that regular staging does not occur during the intercalation in such disordered phases. For this particular case, the final interlayer spacing is consistent with a second stage compound while chemical analyses are more consistent with a first stage compound. In other words, the lattice expansion may not be consistent with the 0.12nm diameter of intercalated sodium. This behavior has been observed by Boehm et al. during the study of intercalation of sodium in carbon blacks. Similar investigations for other HTTs are currently under progress. Finally, the results of this study are also in qualitative agreement with some results of lithium intercalation in carbon. For example, Dahn et al. 1211have shown that staging is not observed for carbon heattreated below 1900-2200°C. This can be understood remembering that the in-plane crystallite diameter is examination
Thus, it seems necessary to follow more precisely the reaction of sodium into carbon. Electrochemical intercalation measurements have been carried out for this purpose. The results are presented in Fig. 4 for five different HTT samples. There are systematic differences between these curves. First, it can be seen that the voltage intercalation threshold is higher as HTT is lower. Sodium intercalation begins near 1.8 V/Na for coke heat-treated at 8Oo”C, while it begins at I .4 V/Na for coke heat-treated at 2200°C. Thus, sodium intercalation is easier for the lowest HTT samples. It may be noted that true reductive intercalation of lithium in graphite occurs below 0.4 V/Li. Thus, the corresponding process for sodium is expected to occur only below 0.1 V/Na, the standard reduction potentials for Li/Li+ and Na/Na+ are -3.0 and -2.7 V, respectively. From this observation, most of the sodium intercalation in graphite is expected to occur below the sodium plating potential and thus cannot be observed. Second, the amount of intercalated sodium is HTT-dependent. The total amount of intercalated sodium is also plotted in Fig. 1. The comparison of the amount of chemically or electrochemically intercalated sodium is in good agreement, with the exception of the lower HTT for which the amount of chemically intercalated sodium is higher. This may be also explained remembering that the in-plane ratio NaCs assumption is an overestimation for such disordered carbons. Only a few results of electrochemical sodium intercalation in carbon are available in the literature. Nevertheless, our results are in qualitative agreement with those given by Doeff et al. [19,20]. Moreover, these authors indicated the need to study sodium intercalation in low granulometry powders, indicating a rather low diffusion coefficient of sodium in carbon. The
21 28
0.7
from Na,C,
18
882
L. JONCOURT
et al. REFERENCES
lower than ca IOnm for such carbon samples, preventing the clustering of the intercalated sodium atoms. 4. CONCLUSIONS
2. 4.
We have reported results about the chemical and the electrochemical intercalation of sodium in disordered carbons. Both chemical and electrochemical measurements have shown that the amount of intercalated sodium depends on the HTT of the carbon. The results were in good agreement with those previously reported and the amount of sodium electrochemically intercalated has been found to correspond to that chemically intercalated. No direct correlation between the amount of intercalated sodium and the structural parameters describing the in-plane or the out-of-plane ordering has been obtained. This seems to confirm Mering’s hypothesis which correlates the uptake of sodium to the Fermi level in the carbon samples. Furthermore, the shape of the V(x) curves tends to indicate that staging does not occur in a regular way as it would be obtained for graphite for HTT values lower than 2200°C. Preliminary sequential X-ray measurements have confirmed this observation. These results may be compared to those known for the lithium intercalation in similar carbon samples. The effect of a small in-plane crystal size on the maximum amount of sodium considering an octal structure has been analyzed and it was shown that the calculated amount of sodium may be quite different from NaCs for an hypothetical first stage compound. In situ X-ray measurements in an electrochemical sodium cell are now planned, in order to obtain a more detailed scheme of the intercalation process in these coke samples.
20.
Acknowledgement-The authors are grateful to Y. Chabre (Laboratoire de SpectromktriePhysique, Grenoble) for very
21.
fruitful discussions and advice.
6.
8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
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