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Surface-fluorinated graphite anode materials for Li-ion batteries
Journal of Fluorine Chemistry 126 (2005) 1111–1116 www.elsevier.com/locate/fluor
Surface-fluorinated graphite anode materials for Li-ion batteries Henri Groult a,*, Tsuyoshi Nakajima b, Laurent Perrigaud a, Yoshimi Ohzawa b, Hitoshi Yashiro c, Shinichi Komaba c, Naoaki Kumagai c a
Pierre and Marie Curie University, LI2C Laboratory, UMR CNRS 7612, 4 Place Jussieu, 75252 Paris Cedex 05, France b Aichi Institute of Technology, Department of Applied Chemistry, Yakusa-cho, Toyota-shi 470-0392, Japan c Department of Frontier Materials and Functional Engineering, Iwate University, Morioka, Iwate 020-8551, Japan Received 8 March 2005; received in revised form 17 March 2005; accepted 18 March 2005 Available online 24 May 2005 Dedicated to Professor Hebert W. Roesky on the occasion of his 70th birthday.
Abstract Surface modification of graphite powder has been performed by chemical fluorination using elemental fluorine at 200 8C and 300 8C. This process leads to an increase of the BET surface area due to partial C–C bond breaking. Surface analyses performed by secondary ions mass spectrometry have shown that the H + O content at the surface of graphite is significantly decreased by this fluorination treatment. Fluorinated graphite powders have been tested as negative electrodes in Li-ion battery, chronopotentiometry measurements have shown that the fluorinated graphite exhibits better electrochemical performances than raw graphite powder notably due to an increase of the surface area which allows the storage of a higher amount of lithium into the host lattice. In addition, impedance measurements performed in a delithiated state have shown a significant decrease of the total cell resistance, i.e. a decrease of both the charge transfer resistance and the resistance related to the solid electrolyte interface (SEI) layer. # 2005 Elsevier B.V. All rights reserved. Keywords: Graphite; Lithium; Fluorine; Li-ion battery; Anode; Surface modification
1. Introduction Rechargeable lithium-ion batteries are widely used as power sources in electrical equipments. These batteries are commercially available and are composed of LiCoO2, LiNiO2 or LiMn2O4 as positive electrode [1–8], and lithiated graphite instead of metallic lithium as negative electrode [9– 18]. The use of lithiated graphite allows to avoid growth of metallic Li dendrite usually observed upon charge– discharge cycles with a metallic lithium anode. Owing to the lamellar structure of carbon materials, Li cations can be easily inserted and deinserted between two carbon sheets. The maximum lithium content is one lithium guest atom per six carbon host atoms, giving rise to a composition of LiC6 and the theoretical specific capacity of 372 mAh/g. * Corresponding author. Tel.: +33 1 4427 3534; fax: +33 1 4427 3856. E-mail address: [email protected] (H. Groult). 0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2005.03.014
However, the electrochemical lithium insertion/deinsertion process is also characterised by an irreversible reaction which occurs during the initial charge procedure, i.e. during the first insertion of lithium cations into the host structure, which results in the formation of a passivating layer called solid electrolyte interface (SEI). Through this surface film, lithium-ion is desolvated and intercalated into graphite without any degradation of the host lattice structure. Surface composition of a starting graphite material has obviously a great influence on the electrochemical performances of graphite anode, notably on the value of irreversible capacity since the electrochemical reactions occur at the surface of the electrode. For example, the surface oxygen species are well known to have a significant effect on the reaction kinetics and capacity values. Heating of carbon fibers up to 1000 8C under vacuum is a suitable technique to enhance the electrochemical performances of graphite anode by reducing the influence of the oxygen
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species. In order to modify more easily the influence of the SEI film formed during the first cycle on the kinetic rate and on the irreversible capacity loss, several additives dissolved into an electrolyte solution have been also successfully found to be effective: HF, CO2, and vinylene carbonate [19–22]. These additives partially suppress the initial irreversibility and improve cycle life of the batteries. Recently, surface fluorination by elemental fluorine and radiofrequency plasma fluorination using CF4 has been proposed to improve the performances of graphite powders [23]. The effect of fluorination is the reduction of surface oxygen and increase of the surface disorder. In the present study, the electrochemical performances of natural graphite powder and surface-fluorinated samples have been investigated as anode materials of Li-ion battery.
2. Results and discussion 2.1. Physico-chemical characterisations It is well known that fluorination of graphite [24–27] at T > 350 8C leads to the formation of graphite fluorides, (CF)n and (C2F)n, having covalent C–F bonds. In the lower temperature region of 100–300 8C, only the surface of graphite is fluorinated. In the present case, the fluorine contents in the fluorinated samples is less than 1 at.%: 0.4 at.% for Sample 2 (T = 200 8C) and 0.7 at.% for Sample 3 (300 8C). The surface areas of fluorinated graphite samples were measured by BET method and compared to that of raw graphite powder (4.79 m2/g). As shown in Fig. 1, the surface area increases with increasing fluorination temperature due to partial C–C bond breaking. The X-ray diffraction patterns (not shown here) revealed the presence of well-defined peak around 2u = 26.58 due to the (0 0 2) diffraction line of graphite. No difference was observed between the raw graphite powder and the fluorinated samples, i.e. the XRD patterns do not reveal the presence of the peaks related to fluorocarbons. It indicates that the fluorination treatment does not concern the bulk of the graphite particles but mainly their surface region.
The interlayer distance, d0 0 2, was deduced from the relation of Bragg. In the case of raw graphite, the d0 0 2 is 0.3354 nm. After fluorination, the d0 0 2 values slightly increased with increasing temperature: 0.3359 nm for Sample 2 and 0.3367 nm for Sample 3. Surface characterisations by time of flight secondary ions mass spectrometry (ToF-SIMS) were done. Compared to XPS, this technique provides chemical and molecular information on carbon sample surface in a lower depth analysis. This analysis consists in bombardment of samples with a pulsed primary ion beam with a dose <1013 at./cm2. This low dose minimises surface damage and erosion, allowing ‘‘static’’ analysis of molecular ions emitted from the outermost layer (1.5 nm). This technique offers also the other advantage to provide a very low detection limit (<10 ppm) and a high lateral resolution (0.2 mm). Thus, ToF-SIMS allows to analyse the molecular structure of the outermost mono-layers of a material. By contrast, the depth analysed by XPS is usually in the order of magnitude of several nanometers. However, a quantitative exploitation of the spectra is somewhat difficult because the intensity corresponding to one element is a function of the ionisation yield, which depends also on the molecular environment. Therefore, only the ratios of the intensities related to hydrogen and oxygen to that of carbon, r1 ¼ I1 Hþ16 O =I12C , of the intensities related to fluorine to that of carbon, r2 ¼ I19F =I12C , and of the intensities related to the sum of C–F groups (BBC–F, >CF2, and CF3) to that of carbon, r3 ¼ IP CFi ði¼13Þ =I12C , have been compared for the raw and fluorinated graphite samples. Results deduced from the exploitation of the secondary ions mass spectrometry (SIMS) spectra are summarised in Table 1. Static SIMS negative spectra obtained for Samples 1–3 are presented in Fig. 2. As shown in Table 1, the oxygen and hydrogen content at the surface of the graphite powder was decreased when the surface of the graphite powder was fluorinated by F2. Moreover, the H + O content on the surface decreased significantly with increasing temperature: for instance it was found that r1 = 0.42 for Sample 1 (raw graphite) and r1 = 0.24 for Sample 3 (T = 300 8C). Finally, the amount of fluorine content at the surface was obviously higher for highest fluorination temperature. Surface disordering was also investigated by Raman spectroscopy. The Raman spectra of carbon materials are usually characterised by a pair of bands at around 1580 cm1 and 1360 cm1, called G- and D-bands, Table 1 Results of SIMS measurements: ratio of intensity related to hydrogen and oxygen to that of carbon, r1 ¼ I1Hþ16O =I12C , of the intensities related to fluorine to that of carbon, r2 ¼ I19F =I12C , of the intensities related to the sum of C–F groups (BBC–F, >CF2, and CF3) to that of carbon, r3 ¼ ISCFiði¼1to3Þ =I12C .
Fig. 1. Surface area of original and fluorinated samples obtained by BET method.
Samples
r1
r2
r3
1 2 3
0.42 0.34 0.24
– 6.1 8.3
– 0.06 0.11
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Fig. 2. Static SIMS negative spectrum in m/z range of Samples 2 (a) and 3 (b).
respectively. The G-band is assigned to the E2g2 carbon– carbon stretching mode whereas the D-band is due to an A1g vibration mode in the disordered region of carbon materials or edge plane of powdery carbon. The ratio, R, of intensity of D-band, ID, to that of G-band, IG, (R = ID/IG) varies, depending on the structure of the carbon and indicates the degree of disordering of the carbon surface. The intensities of the G- and D-bands were determined by drawing a base line along the scattering background and by measuring the intensity above the base line. Fig. 3a shows the Raman spectra of Sample 1. Several spectra were recorded at different points of the surface. As shown in this figure, the intensity of the D-band can be rather null, giving rise to R-values comprised between 0 and 0.09. It means that the surface of the starting materials is not homogeneous. After fluorination, same investigations were performed at different points of the surface. In this case, and whatever the temperature of the fluorination treatment, no difference was observed and the Raman spectra were similar to each other: the fluorination treatment induces an homogenization of the surface. The spectra obtained for Samples 2 and 3 are given in Fig. 3b. The G- and D-bands are observed at around 1580 cm1 and 1345 cm1. The highest D-band intensity was observed for Sample 3 giving rise to the highest R-values: 0.11 for Sample 2 and 0.29 for Sample 3. The fluorination slightly increases the surface disordering and this increase is much pronounced for higher fluorination temperature in agreement with previous results [23].
Important bulk and surface structure changes have been pointed out notably by Raman spectroscopy and XRD. It should have a significant influence on their electrochemical performances when they are used as anodes in Li-ion batteries. It will be discussed in the following paragraph. 2.2. Electrochemical performances Raw and fluorinated graphite powders were tested as negative electrode in lithium-ion battery. A typical charge/ discharge curve obtained in the case of Sample 1 is presented in Fig. 4. The formation of the SEI layer gives rise to a shoulder at around 0.6 V versus Li/Li+. The insertion of lithium-ions occurs in the potential range 0.3–0 V versus Li/ Li+. The charge–discharge voltage profile in galvanostatic mode exhibits three distinct plateaux appearing approximately at around 0.21 V, 0.11 V, and 0.06–0.08 V versus Li/ Li+ due to a staging mechanism [12–18]. These potentials have been assigned to two-phases coexistence regions caused by the stage transformations between dilute stage 1 $ stage 4, stage 2L $ stage 2 and stage 2 $ stage 1 transitions, respectively, of lithium–graphite intercalation compound. This staging phenomenon is due to repulsive interactions between the lithium-inserted layers. An increment of the charge capacities was observed after fluorination of graphite powder whatever the fluorination temperature: the capacity values observed for Samples 2 and 3 were about 380 mAh/g; this value is slightly higher than
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Fig. 5. Impedance spectra in the Nyquist representation of Samples 1 and 3 after delithiation in 1 M LiClO4-EC:DEC (1:1).
Fig. 3. Raman spectra of (a) Sample 1 recorded at different points of the surface and (b) Samples 2 and 3.
the theoretical one (372 mAh/g) and larger than the experimental one (350 mAh/g) usually obtained with non-modified graphite powders. As shown above (Fig. 1), the fluorination of graphite leads to increase in the surface disorder with increase of mesopores: as a consequence, a larger amount of lithium can be stored in fluorinated graphite. In addition, the increase of the BET surface area did not have a significant influence on the first coulombic efficiency since the latter was rather the same whatever the sample: 80% for Sample 1 to 78% for Sample 2.
Fig. 4. Typical first charge/discharge curves in galvanostatic mode obtained with Sample 1 in 1 M LiClO4-EC:DEC (1:1).
To complete results deduced from chronopotentiometry, impedance measurements were performed. Each impedance spectra were recorded in the Nyquist representation at the end of the delithiation (i.e. after reoxidation) to obtain the resistance of the SEI layer. As an example, the Nysquist diagrams obtained for Samples 1 and 3 after delithiation are presented in Fig. 5. The electrochemical impedance response of the system was analysed using the equivalent circuit given in Fig. 6 [11,16,28]. In this circuit, Rb is the bulk resistance of the cell (electrolyte, separator, and electrodes), RSEI and CSEI the resistance and capacitance of the SEI layer formed at the surface of graphite during the first discharge step, respectively, Rct the charge transfer resistance, Cdl the double layer capacitance, and W is the Warburg impedance. In very high frequency region, the semi-circle in the Nyquist diagram (Fig. 5) is related to the contribution of the SEI layer. In the middle frequency range, the second semi-circle (Fig. 5) is related to charge transfer; finally, in low frequency region, the Warburg is considered. For the latter, a straight line showing a 458 angle against the real axis is usually obtained. Nevertheless, as mentioned above, the impedance spectra presented in Fig. 5 were obtained after complete extraction of lithium from the graphite lattice; it explains why a straight line with a slope of 908 is observed at low frequency instead of 458. As shown in Fig. 5, both the resistances related to the SEI and to the charge transfer were lower for the fluorinated sample: RSEI 15 V and Rct 19 V for Sample 1 and RSEI 5.5 V and Rct 8 V for Sample 3. It means that the charge transfer at the fluorinated graphite interface is faster than that observed in the case of pure graphite. The decrease of the resistance of the SEI could be related to the decrease of the H + O content on the surface deduced from SIMS measurements with increasing fluorination temperature: it was found that r1 = 0.42 for Sample 1 (raw graphite) and r1 = 0.24 for Sample 3 (T = 300 8C). The reduction of the resistance of the SEI could result from a strong modification of the composition of the layer and/or a decrease of its thickness. Consequently, the total cell resistance is drastically decreased by using surface-fluorinated graphite powders.
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two and three times lower for fluorinated graphite samples than those observed for raw graphite.
4. Experimental 4.1. Fluorination of carbon powders Fig. 6. Equivalent circuit used for the fitting of the impedance spectra.
The increase of the reversible capacities observed from the analysis of chronopotentiograms is here clearly understood. Finally, if we compare the variation of the charge transfer resistances using the surface areas deduced from BET method with the mass of active materials, we obtain: 0.95 V m2 for Sample 1 and 0.69 V m2 for Sample 3. It means that (i) for these two samples, the BET surface area cannot be regarded as the effective electrode surface area for the charge transfer reaction and (ii) the charge transfer reaction takes place on a larger surface in the case of fluorinated samples.
3. Conclusion The effect of surface modification of natural graphite powder by chemical fluorination using elemental F2 was studied. XRD measurements have revealed a slight increase of the d0 0 2 interlayer spacing with increasing fluorination temperature. In the same way, the surface area deduced from BET method increased with increasing fluorination temperature due to partial C–C bond breaking. Surface analyses performed by secondary ions mass spectrometry have shown that the ratio of the intensities related to hydrogen and oxygen to that of carbon, r1 ¼ I1 Hþ16 O =I12C was decreased in the case of fluorinated compounds; this ratio is also dependent on the of the fluorination temperature. Raman analyses have shown that the surface disordering increased with increasing temperature of the fluorination. Electrochemical tests performed by impedancemetry and chronopotentiometry have shown that the electrochemical performances of fluorinated graphite powders are better than those of raw graphite powder. Indeed, due to an increase of the surface area in the case of fluorinated samples, a larger amount of lithium can be stored in the case of fluorinated graphite, i.e. larger charge capacity values were observed for fluorinated samples. Nevertheless, irrespective to the increase in the BET surface area, the first coulombic efficiency was slightly higher in the case of fluorinated samples. Finally, impedance measurements performed in a delithiated state have revealed a decrease of the total cell resistance, i.e. a decrease of both the charge transfer resistance and the resistance related to the SEI layer: Rct and RSEI were about
Raw material was natural graphite powder samples (denoted Sample 1) with average particles diameter of 7 mm. Surface fluorination was performed at 200 8C and 300 8C by elemental fluorine (purity: 99.4–99.7%) of 3 104 Pa for 2 min in a nickel reactor. Hereafter, the different samples numbered Samples 1–3 correspond to raw graphite, graphite fluorinated at 200 8C, and graphite fluorinated at 300 8C, respectively. 4.2. Physico-chemical characterisations The compositions of surface-fluorinated samples were determined by elemental analysis of carbon and fluorine. The structural characterisation of the samples was performed by X-ray diffraction using a Siemens D5000 X´˚ ray diffractometer with a Cu Ka radiation (l1 = 1.54178 A ) and a nickel filter. The presence of H and O contents on the surface of the graphite powder was examined by secondary ions mass spectrometry using a TFS Surface Analyser operated at 109 Torr, equipped with a liquid Ga-ion source and pulse electron flooding. During the analysis, the targets were bombarded by the 10 keV Ga+ beams with pulsed primary ion current varying from 0.3 to 0.5 pA. The total collection time was 300 s and rastered over a 12 mm 12 mm area. The Raman spectra were recorded at room temperature using a micro-Raman system with a Dilor XY spectrometer equipped with a charge coupled device (CCD) detector. An argon-ion laser (514.5 nm) was employed as the excitation source. A 50X objective was used to focus the laser light on sample surface to a spot of 5 mm2 and the laser power was kept to 1 mW to avoid any degradation of the film. Finally, the spectra were measured in backscattering geometry. 4.3. Electrochemical studies The electrochemical lithium insertion/deinsertion reaction was studied in 1 M LiClO4-EC:DEC (1:1) solution at room temperature in a glove box (water content <10 ppm) under argon atmosphere. The counter and reference electrodes were metallic lithium foils. The galvanostatic charge–discharge was performed using a potentiostat/ galvanostat (VMP Bio-Logic) in the potential range: 0.02–2.5 V in 1 M LiClO4-EC:DEC (1:1) at a current density of 60 mA/g. Impedance measurements were performed using an Autolab frequency analyser between 106 Hz and 103 Hz.
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Acknowledgements The authors are grateful to Drs. A. Khan-Harari (ENSCP, Paris-France) and R. Baddour-Hadjean (LADIR CNRS, Thiais-France) for the XRD and Raman experiments, respectively.
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Surface-fluorinated graphite anode materials for Li-ion batteries Henri Groult a,*, Tsuyoshi Nakajima b, Laurent Perrigaud a, Yoshimi Ohzawa b, Hitoshi Yashiro c, Shinichi Komaba c, Naoaki Kumagai c a
Pierre and Marie Curie University, LI2C Laboratory, UMR CNRS 7612, 4 Place Jussieu, 75252 Paris Cedex 05, France b Aichi Institute of Technology, Department of Applied Chemistry, Yakusa-cho, Toyota-shi 470-0392, Japan c Department of Frontier Materials and Functional Engineering, Iwate University, Morioka, Iwate 020-8551, Japan Received 8 March 2005; received in revised form 17 March 2005; accepted 18 March 2005 Available online 24 May 2005 Dedicated to Professor Hebert W. Roesky on the occasion of his 70th birthday.
Abstract Surface modification of graphite powder has been performed by chemical fluorination using elemental fluorine at 200 8C and 300 8C. This process leads to an increase of the BET surface area due to partial C–C bond breaking. Surface analyses performed by secondary ions mass spectrometry have shown that the H + O content at the surface of graphite is significantly decreased by this fluorination treatment. Fluorinated graphite powders have been tested as negative electrodes in Li-ion battery, chronopotentiometry measurements have shown that the fluorinated graphite exhibits better electrochemical performances than raw graphite powder notably due to an increase of the surface area which allows the storage of a higher amount of lithium into the host lattice. In addition, impedance measurements performed in a delithiated state have shown a significant decrease of the total cell resistance, i.e. a decrease of both the charge transfer resistance and the resistance related to the solid electrolyte interface (SEI) layer. # 2005 Elsevier B.V. All rights reserved. Keywords: Graphite; Lithium; Fluorine; Li-ion battery; Anode; Surface modification
1. Introduction Rechargeable lithium-ion batteries are widely used as power sources in electrical equipments. These batteries are commercially available and are composed of LiCoO2, LiNiO2 or LiMn2O4 as positive electrode [1–8], and lithiated graphite instead of metallic lithium as negative electrode [9– 18]. The use of lithiated graphite allows to avoid growth of metallic Li dendrite usually observed upon charge– discharge cycles with a metallic lithium anode. Owing to the lamellar structure of carbon materials, Li cations can be easily inserted and deinserted between two carbon sheets. The maximum lithium content is one lithium guest atom per six carbon host atoms, giving rise to a composition of LiC6 and the theoretical specific capacity of 372 mAh/g. * Corresponding author. Tel.: +33 1 4427 3534; fax: +33 1 4427 3856. E-mail address: [email protected] (H. Groult). 0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2005.03.014
However, the electrochemical lithium insertion/deinsertion process is also characterised by an irreversible reaction which occurs during the initial charge procedure, i.e. during the first insertion of lithium cations into the host structure, which results in the formation of a passivating layer called solid electrolyte interface (SEI). Through this surface film, lithium-ion is desolvated and intercalated into graphite without any degradation of the host lattice structure. Surface composition of a starting graphite material has obviously a great influence on the electrochemical performances of graphite anode, notably on the value of irreversible capacity since the electrochemical reactions occur at the surface of the electrode. For example, the surface oxygen species are well known to have a significant effect on the reaction kinetics and capacity values. Heating of carbon fibers up to 1000 8C under vacuum is a suitable technique to enhance the electrochemical performances of graphite anode by reducing the influence of the oxygen
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species. In order to modify more easily the influence of the SEI film formed during the first cycle on the kinetic rate and on the irreversible capacity loss, several additives dissolved into an electrolyte solution have been also successfully found to be effective: HF, CO2, and vinylene carbonate [19–22]. These additives partially suppress the initial irreversibility and improve cycle life of the batteries. Recently, surface fluorination by elemental fluorine and radiofrequency plasma fluorination using CF4 has been proposed to improve the performances of graphite powders [23]. The effect of fluorination is the reduction of surface oxygen and increase of the surface disorder. In the present study, the electrochemical performances of natural graphite powder and surface-fluorinated samples have been investigated as anode materials of Li-ion battery.
2. Results and discussion 2.1. Physico-chemical characterisations It is well known that fluorination of graphite [24–27] at T > 350 8C leads to the formation of graphite fluorides, (CF)n and (C2F)n, having covalent C–F bonds. In the lower temperature region of 100–300 8C, only the surface of graphite is fluorinated. In the present case, the fluorine contents in the fluorinated samples is less than 1 at.%: 0.4 at.% for Sample 2 (T = 200 8C) and 0.7 at.% for Sample 3 (300 8C). The surface areas of fluorinated graphite samples were measured by BET method and compared to that of raw graphite powder (4.79 m2/g). As shown in Fig. 1, the surface area increases with increasing fluorination temperature due to partial C–C bond breaking. The X-ray diffraction patterns (not shown here) revealed the presence of well-defined peak around 2u = 26.58 due to the (0 0 2) diffraction line of graphite. No difference was observed between the raw graphite powder and the fluorinated samples, i.e. the XRD patterns do not reveal the presence of the peaks related to fluorocarbons. It indicates that the fluorination treatment does not concern the bulk of the graphite particles but mainly their surface region.
The interlayer distance, d0 0 2, was deduced from the relation of Bragg. In the case of raw graphite, the d0 0 2 is 0.3354 nm. After fluorination, the d0 0 2 values slightly increased with increasing temperature: 0.3359 nm for Sample 2 and 0.3367 nm for Sample 3. Surface characterisations by time of flight secondary ions mass spectrometry (ToF-SIMS) were done. Compared to XPS, this technique provides chemical and molecular information on carbon sample surface in a lower depth analysis. This analysis consists in bombardment of samples with a pulsed primary ion beam with a dose <1013 at./cm2. This low dose minimises surface damage and erosion, allowing ‘‘static’’ analysis of molecular ions emitted from the outermost layer (1.5 nm). This technique offers also the other advantage to provide a very low detection limit (<10 ppm) and a high lateral resolution (0.2 mm). Thus, ToF-SIMS allows to analyse the molecular structure of the outermost mono-layers of a material. By contrast, the depth analysed by XPS is usually in the order of magnitude of several nanometers. However, a quantitative exploitation of the spectra is somewhat difficult because the intensity corresponding to one element is a function of the ionisation yield, which depends also on the molecular environment. Therefore, only the ratios of the intensities related to hydrogen and oxygen to that of carbon, r1 ¼ I1 Hþ16 O =I12C , of the intensities related to fluorine to that of carbon, r2 ¼ I19F =I12C , and of the intensities related to the sum of C–F groups (BBC–F, >CF2, and CF3) to that of carbon, r3 ¼ IP CFi ði¼13Þ =I12C , have been compared for the raw and fluorinated graphite samples. Results deduced from the exploitation of the secondary ions mass spectrometry (SIMS) spectra are summarised in Table 1. Static SIMS negative spectra obtained for Samples 1–3 are presented in Fig. 2. As shown in Table 1, the oxygen and hydrogen content at the surface of the graphite powder was decreased when the surface of the graphite powder was fluorinated by F2. Moreover, the H + O content on the surface decreased significantly with increasing temperature: for instance it was found that r1 = 0.42 for Sample 1 (raw graphite) and r1 = 0.24 for Sample 3 (T = 300 8C). Finally, the amount of fluorine content at the surface was obviously higher for highest fluorination temperature. Surface disordering was also investigated by Raman spectroscopy. The Raman spectra of carbon materials are usually characterised by a pair of bands at around 1580 cm1 and 1360 cm1, called G- and D-bands, Table 1 Results of SIMS measurements: ratio of intensity related to hydrogen and oxygen to that of carbon, r1 ¼ I1Hþ16O =I12C , of the intensities related to fluorine to that of carbon, r2 ¼ I19F =I12C , of the intensities related to the sum of C–F groups (BBC–F, >CF2, and CF3) to that of carbon, r3 ¼ ISCFiði¼1to3Þ =I12C .
Fig. 1. Surface area of original and fluorinated samples obtained by BET method.
Samples
r1
r2
r3
1 2 3
0.42 0.34 0.24
– 6.1 8.3
– 0.06 0.11
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Fig. 2. Static SIMS negative spectrum in m/z range of Samples 2 (a) and 3 (b).
respectively. The G-band is assigned to the E2g2 carbon– carbon stretching mode whereas the D-band is due to an A1g vibration mode in the disordered region of carbon materials or edge plane of powdery carbon. The ratio, R, of intensity of D-band, ID, to that of G-band, IG, (R = ID/IG) varies, depending on the structure of the carbon and indicates the degree of disordering of the carbon surface. The intensities of the G- and D-bands were determined by drawing a base line along the scattering background and by measuring the intensity above the base line. Fig. 3a shows the Raman spectra of Sample 1. Several spectra were recorded at different points of the surface. As shown in this figure, the intensity of the D-band can be rather null, giving rise to R-values comprised between 0 and 0.09. It means that the surface of the starting materials is not homogeneous. After fluorination, same investigations were performed at different points of the surface. In this case, and whatever the temperature of the fluorination treatment, no difference was observed and the Raman spectra were similar to each other: the fluorination treatment induces an homogenization of the surface. The spectra obtained for Samples 2 and 3 are given in Fig. 3b. The G- and D-bands are observed at around 1580 cm1 and 1345 cm1. The highest D-band intensity was observed for Sample 3 giving rise to the highest R-values: 0.11 for Sample 2 and 0.29 for Sample 3. The fluorination slightly increases the surface disordering and this increase is much pronounced for higher fluorination temperature in agreement with previous results [23].
Important bulk and surface structure changes have been pointed out notably by Raman spectroscopy and XRD. It should have a significant influence on their electrochemical performances when they are used as anodes in Li-ion batteries. It will be discussed in the following paragraph. 2.2. Electrochemical performances Raw and fluorinated graphite powders were tested as negative electrode in lithium-ion battery. A typical charge/ discharge curve obtained in the case of Sample 1 is presented in Fig. 4. The formation of the SEI layer gives rise to a shoulder at around 0.6 V versus Li/Li+. The insertion of lithium-ions occurs in the potential range 0.3–0 V versus Li/ Li+. The charge–discharge voltage profile in galvanostatic mode exhibits three distinct plateaux appearing approximately at around 0.21 V, 0.11 V, and 0.06–0.08 V versus Li/ Li+ due to a staging mechanism [12–18]. These potentials have been assigned to two-phases coexistence regions caused by the stage transformations between dilute stage 1 $ stage 4, stage 2L $ stage 2 and stage 2 $ stage 1 transitions, respectively, of lithium–graphite intercalation compound. This staging phenomenon is due to repulsive interactions between the lithium-inserted layers. An increment of the charge capacities was observed after fluorination of graphite powder whatever the fluorination temperature: the capacity values observed for Samples 2 and 3 were about 380 mAh/g; this value is slightly higher than
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Fig. 5. Impedance spectra in the Nyquist representation of Samples 1 and 3 after delithiation in 1 M LiClO4-EC:DEC (1:1).
Fig. 3. Raman spectra of (a) Sample 1 recorded at different points of the surface and (b) Samples 2 and 3.
the theoretical one (372 mAh/g) and larger than the experimental one (350 mAh/g) usually obtained with non-modified graphite powders. As shown above (Fig. 1), the fluorination of graphite leads to increase in the surface disorder with increase of mesopores: as a consequence, a larger amount of lithium can be stored in fluorinated graphite. In addition, the increase of the BET surface area did not have a significant influence on the first coulombic efficiency since the latter was rather the same whatever the sample: 80% for Sample 1 to 78% for Sample 2.
Fig. 4. Typical first charge/discharge curves in galvanostatic mode obtained with Sample 1 in 1 M LiClO4-EC:DEC (1:1).
To complete results deduced from chronopotentiometry, impedance measurements were performed. Each impedance spectra were recorded in the Nyquist representation at the end of the delithiation (i.e. after reoxidation) to obtain the resistance of the SEI layer. As an example, the Nysquist diagrams obtained for Samples 1 and 3 after delithiation are presented in Fig. 5. The electrochemical impedance response of the system was analysed using the equivalent circuit given in Fig. 6 [11,16,28]. In this circuit, Rb is the bulk resistance of the cell (electrolyte, separator, and electrodes), RSEI and CSEI the resistance and capacitance of the SEI layer formed at the surface of graphite during the first discharge step, respectively, Rct the charge transfer resistance, Cdl the double layer capacitance, and W is the Warburg impedance. In very high frequency region, the semi-circle in the Nyquist diagram (Fig. 5) is related to the contribution of the SEI layer. In the middle frequency range, the second semi-circle (Fig. 5) is related to charge transfer; finally, in low frequency region, the Warburg is considered. For the latter, a straight line showing a 458 angle against the real axis is usually obtained. Nevertheless, as mentioned above, the impedance spectra presented in Fig. 5 were obtained after complete extraction of lithium from the graphite lattice; it explains why a straight line with a slope of 908 is observed at low frequency instead of 458. As shown in Fig. 5, both the resistances related to the SEI and to the charge transfer were lower for the fluorinated sample: RSEI 15 V and Rct 19 V for Sample 1 and RSEI 5.5 V and Rct 8 V for Sample 3. It means that the charge transfer at the fluorinated graphite interface is faster than that observed in the case of pure graphite. The decrease of the resistance of the SEI could be related to the decrease of the H + O content on the surface deduced from SIMS measurements with increasing fluorination temperature: it was found that r1 = 0.42 for Sample 1 (raw graphite) and r1 = 0.24 for Sample 3 (T = 300 8C). The reduction of the resistance of the SEI could result from a strong modification of the composition of the layer and/or a decrease of its thickness. Consequently, the total cell resistance is drastically decreased by using surface-fluorinated graphite powders.
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two and three times lower for fluorinated graphite samples than those observed for raw graphite.
4. Experimental 4.1. Fluorination of carbon powders Fig. 6. Equivalent circuit used for the fitting of the impedance spectra.
The increase of the reversible capacities observed from the analysis of chronopotentiograms is here clearly understood. Finally, if we compare the variation of the charge transfer resistances using the surface areas deduced from BET method with the mass of active materials, we obtain: 0.95 V m2 for Sample 1 and 0.69 V m2 for Sample 3. It means that (i) for these two samples, the BET surface area cannot be regarded as the effective electrode surface area for the charge transfer reaction and (ii) the charge transfer reaction takes place on a larger surface in the case of fluorinated samples.
3. Conclusion The effect of surface modification of natural graphite powder by chemical fluorination using elemental F2 was studied. XRD measurements have revealed a slight increase of the d0 0 2 interlayer spacing with increasing fluorination temperature. In the same way, the surface area deduced from BET method increased with increasing fluorination temperature due to partial C–C bond breaking. Surface analyses performed by secondary ions mass spectrometry have shown that the ratio of the intensities related to hydrogen and oxygen to that of carbon, r1 ¼ I1 Hþ16 O =I12C was decreased in the case of fluorinated compounds; this ratio is also dependent on the of the fluorination temperature. Raman analyses have shown that the surface disordering increased with increasing temperature of the fluorination. Electrochemical tests performed by impedancemetry and chronopotentiometry have shown that the electrochemical performances of fluorinated graphite powders are better than those of raw graphite powder. Indeed, due to an increase of the surface area in the case of fluorinated samples, a larger amount of lithium can be stored in the case of fluorinated graphite, i.e. larger charge capacity values were observed for fluorinated samples. Nevertheless, irrespective to the increase in the BET surface area, the first coulombic efficiency was slightly higher in the case of fluorinated samples. Finally, impedance measurements performed in a delithiated state have revealed a decrease of the total cell resistance, i.e. a decrease of both the charge transfer resistance and the resistance related to the SEI layer: Rct and RSEI were about
Raw material was natural graphite powder samples (denoted Sample 1) with average particles diameter of 7 mm. Surface fluorination was performed at 200 8C and 300 8C by elemental fluorine (purity: 99.4–99.7%) of 3 104 Pa for 2 min in a nickel reactor. Hereafter, the different samples numbered Samples 1–3 correspond to raw graphite, graphite fluorinated at 200 8C, and graphite fluorinated at 300 8C, respectively. 4.2. Physico-chemical characterisations The compositions of surface-fluorinated samples were determined by elemental analysis of carbon and fluorine. The structural characterisation of the samples was performed by X-ray diffraction using a Siemens D5000 X´˚ ray diffractometer with a Cu Ka radiation (l1 = 1.54178 A ) and a nickel filter. The presence of H and O contents on the surface of the graphite powder was examined by secondary ions mass spectrometry using a TFS Surface Analyser operated at 109 Torr, equipped with a liquid Ga-ion source and pulse electron flooding. During the analysis, the targets were bombarded by the 10 keV Ga+ beams with pulsed primary ion current varying from 0.3 to 0.5 pA. The total collection time was 300 s and rastered over a 12 mm 12 mm area. The Raman spectra were recorded at room temperature using a micro-Raman system with a Dilor XY spectrometer equipped with a charge coupled device (CCD) detector. An argon-ion laser (514.5 nm) was employed as the excitation source. A 50X objective was used to focus the laser light on sample surface to a spot of 5 mm2 and the laser power was kept to 1 mW to avoid any degradation of the film. Finally, the spectra were measured in backscattering geometry. 4.3. Electrochemical studies The electrochemical lithium insertion/deinsertion reaction was studied in 1 M LiClO4-EC:DEC (1:1) solution at room temperature in a glove box (water content <10 ppm) under argon atmosphere. The counter and reference electrodes were metallic lithium foils. The galvanostatic charge–discharge was performed using a potentiostat/ galvanostat (VMP Bio-Logic) in the potential range: 0.02–2.5 V in 1 M LiClO4-EC:DEC (1:1) at a current density of 60 mA/g. Impedance measurements were performed using an Autolab frequency analyser between 106 Hz and 103 Hz.
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Acknowledgements The authors are grateful to Drs. A. Khan-Harari (ENSCP, Paris-France) and R. Baddour-Hadjean (LADIR CNRS, Thiais-France) for the XRD and Raman experiments, respectively.
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