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The feasibility of a pyrrolidinium-based ionic liquid solvent for non-graphitic carbon electrodes
Electrochemistry Communications 13 (2011) 1256–1259
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
The feasibility of a pyrrolidinium-based ionic liquid solvent for non-graphitic carbon electrodes Junyoung Mun a, Taeeun Yim a, Sunhyung Jurng a, Jang-Hoon Park b, Sang-Young Lee b, Ji Heon Ryu a, Young Gyu Kim a, Seung M. Oh a,⁎ a b
Department of Chemical and Biological Engineering and WCU Program of C2E2, Seoul National University, Seoul 151-744, Republic of Korea Department of Chemical Engineering, Kangwon National University, Chuncheon, 446-599, Republic of Korea
a r t i c l e
i n f o
Article history: Received 20 July 2011 Received in revised form 18 August 2011 Accepted 19 August 2011 Available online 26 August 2011 Keywords: Lithium-ion batteries Room-temperature ionic liquids Non-graphitic carbons Soft carbons Hard carbons
a b s t r a c t The feasibility of a pyrrolidinium-based room-temperature ionic liquid (RTIL) as the solvent for lithium-ion batteries is tested by analyzing its intercalation behavior and thermal stability. The RTIL-cations are intercalated into a graphitic carbon and a part of them are irreversibly trapped inside the graphene layers. These trapped cations block Li + intercalation to give only a marginal capacity. In contrast, such a cation insertion/trapping is absent in two non-graphitic carbons; hard carbon and soft carbon. A stable cycle performance with a Li + insertion capacity of about 200 mAh g− 1 is attained. The absence of RTIL-cation insertion is evidenced by the cyclic voltammograms and Raman spectra. A calorimetric study reveals that this RTIL has a higher thermal stability and less reactivity with lithiated carbons as compared with the carbonate-based solvent. The use of this RTIL solvent for the non-graphitic carbons seems to be feasible. © 2011 Elsevier B.V. All rights reserved.
1. Introduction With an expansion of the market for lithium-ion batteries (LIBs), the safety of LIBs has been an increasing focus of attention. The accidents known as thermal runaway are frequently triggered by some abusing events such as overcharging, internal short or local overheating. Once the cell heating is triggered, additional heats are evolved by the decomposition of cell constituents, which are electrochemically/thermally unstable and combustible. Along this line, the room-temperature ionic liquids (RTILs) have been projected as the replacement of the conventional carbonate-based solvents indebted to their low vapor pressure and nonflammability [1–11]. At present, graphite is the most popular negative electrode for LIBs but still carries a weak point for safety concerns. It is known that thermal runaway is triggered by decomposition of solid electrolyte interphase, and exothermic reactions between lithiated graphite and electrolyte solutions [12–14]. A variety of RTILs have been tested as the replacement of carbonate solvents, from which a high thermal/ electrochemical stability of RTILs has been reported [1–5,9,10]. The previous reports [3,6], however, complain that some RTIL cations such as pyrrolidinium and piperidinium ions are intercalated into graphite to degrade the negative electrodes. The prime concern in this work is to see if non-graphite carbons (hard carbon and soft carbon) are degraded by the insertion of
⁎ Corresponding author. Tel.: + 82 2 880 7074; fax: + 82 2 872 5755. E-mail address: [email protected] (S.M. Oh). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.08.030
RTIL-cation. To this end, a pyrrolidinium-based RTIL was tested as the solvent. The second concern is the thermal stability and reactivity of this RTIL solvent against the exothermic reactions with the lithiated non-graphite carbons. To this end, a calorimetric study is made with the RTIL and carbonate solvent to compare the onset temperature for exothermic reactions and heat evolution. 2. Experimental N-methyl-N-propylpyrrolidinium-bis(trifluromethanesulfonyl)imide (PMPyr-TFSI) was prepared according to the previous report [15]. The electrolyte was prepared by dissolving 1.0 M Li-TFSI (3M) into PMPyr-TFSI solvent. Two different non-graphitic carbons were tested as the negative electrode; hard carbon (Carbotron P, Kureha Chemical) and soft carbon (heat-treated green mesocarbon microbead (MCMB), Osaka Gas Co.). For comparison, the same measurements were made with a graphitic carbon (graphitized MCMB-10-28, Osaka Gas Co.). To prepare the composite electrodes, a mixture of the carbon powder, polyvinylidenefluoride (Solvay), and Super P (8:1:1, weight ratio) was dispersed in N-methyl-2-pyrrolidone. The resulting slurry was coated on a piece of copper foil, and the resulting electrode plates were dried in a vacuum oven at 120 °C for 12 h. Galvanostatic discharge–charge cycling was made with a coin-type cell (2032), in which a piece of lithium foil (Cyprus Co.) and glass filter (Advanter, GA-55, thickness: 0.21 mm, pore size: 0.6 μm) were loaded as the counter electrode and separator, respectively. Cyclic voltammograms were obtained in a three-electrode configuration, in which a silver wire was used as the pseudo-reference electrode and Pt plate as the
J. Mun et al. / Electrochemistry Communications 13 (2011) 1256–1259
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Fig. 1. The galvanostatic discharge (lithiation) and charge (de-lithiation) voltage profiles obtained with the Li/carbon cells (a–c), and the specific capacity with cycling (d). In (d), the open and filled symbols represent the lithiation and de-lithiation capacity, respectively. Current density = 25 mA g− 1. Voltage cut-off = 2.0–0.01 V (vs. Li/Li+).
counter electrode. The pseudo-reference electrode was calibrated by using the ferrocene/ferrocenium (Fc/Fc+) couple. Raman spectra were obtained in a backscattering mode using a Raman spectrometer HR800
(Jobin Yvon Horiba) with Ar laser (632.8 nm). For the differential scanning calorimetry (DSC, Dupont Q2000), the carbon electrodes were lithiated down to 0.01 V (vs. Li/Li+) in both RTIL-based and carbonate electrolyte, and collected after removing the current collector. The DSC measurement was made without any further treatments of the electrode samples. In this report, the lithiation was expressed as discharging, whereas the de-lithiation charging on the basis of the standard lithium-ion cell configuration. 3. Results and discussion
Fig. 2. Cyclic voltammograms obtained in the neat PMPyr-TFSI electrolyte. Scan rate = 10 mV s− 1.
Fig. 1 shows the discharge/charge voltage profiles of three carbon electrodes. In the first cycle, the graphite electrode shows the Li+ intercalation/de-intercalation behavior at 0.0–0.3 V (vs. Li/Li+). In addition, the plateau-like voltage profiles appear at 0.7–0.3 V in discharge and at 0.9–1.1 V upon charge, suggesting that additional redox reaction is involved. Four features are noted on this additional redox reaction. First, the reduction reaction (at 0.7–0.3 V) takes place before Li+ intercalation (at 0.0–0.3 V), whereas the oxidation (at 0.9–1.1 V) after Li+ de-intercalation. The same observation was made in the previous works [3,6,16], which reported that several RTIL-cations such as imidazolium and pyrrolidinium are intercalated into graphene layers before Li+ intercalation and removed after Li+ de-intercalation. Hence, the plateau-like feature at 0.7–0.3 V can be ascribed to the RTIL-cation (PMPyr + in this work) intercalation and that at 0.9–1.1 V to the cation de-intercalation. Second, the capacity associated with the cation de-intercalation is much smaller than that for the cation intercalation, implying that some of the intercalated cations are trapped inside the graphene layers. Third, the RTIL-cation intercalation/de-intercalation becomes less significant from the second cycle as evidenced by the shortened voltage plateaus at both 0.7–0.3 V and 0.9–1.1 V regions. In the 5th cycle, this cation-relevant redox reaction totally disappears to give only a discharge/charge capacity delivered by Li+ intercalation/de-intercalation. The resulting Li+ intercalation/de-intercalation capacity is presented in Fig. 1d. Only a marginal capacity is observed
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J. Mun et al. / Electrochemistry Communications 13 (2011) 1256–1259
Fig. 3. Raman spectra of the fresh (the upper panel) and potential-swept (the bottom) carbon samples. The latter samples were prepared by a potential sweep down to − 2.5 V (vs. Fc/Fc+) in the neat PMPyr-TFSI electrolyte (Fig. 2).
from the 5th cycle. Surely, the trapped cations hinder Li+ accommodation. Fourth, the graphite electrode is passivated as evidenced by the evolution of irreversible capacity that is associated with electrolyte decomposition and solid electrolyte interphase (SEI) formation at 0.7– 0.3 V. The RTIL-cation insertion/de-insertion is not occurring with the nongraphitic carbons as evidenced by the absence of voltage plateaus (Fig. 1b and c). Only the Li+ insertion/de-insertion takes place with the sloping discharge/charge voltage profiles at 1.0–0.01 V. The 1st coulombic efficiency is low (hard carbon; 55% and soft carbon; 69%) due to the irreversible capacity that is caused by SEI formation (passivation) and Li+ trapping in some defect sites [17,18]. From the second cycle, however, both electrodes show a coulombic efficiency of N90% (hard carbon; 91.9% and soft carbon; 92.8% in the second cycle) indebted to the negligible irreversible reactions. A stable cycle performance with a capacity of ca. 200 mA h g− 1 is attained for both electrodes thereafter. Unambiguously, Li+ insertion/de-insertion is successfully carried out in two non-graphitic carbons without interference by the trapped RTIL cations. A separate measurement made in the carbonate electrolyte (1.0 M LiTFSI/EC:DEC) illustrates that the capacity values are slightly larger (hard carbon; 254 mA h g− 1 and soft carbon; 228 mA h g− 1 in the 20th cycle) than those obtained in the RTIL, which seems to be due to a higher ionic conductivity for the carbonate-based electrolytes.
To ascertain the presence/absence of RTIL-cation insertion, cyclic voltammograms were recorded in the neat PMPyr-TFSI solvent without lithium salt. Three carbon electrodes exhibit a reduction current at b−2.0 V (vs. Fc/Fc +) in the first negative scan (Fig. 2), which may come from the RTIL-cation insertion and the reductive decomposition of PMPyr-TFSI. In the positive scan, however, only the graphite electrode shows an oxidation current at −2.3 V. This oxidation current has been ascribed to the RTIL-cation de-intercalation in the previous works [3,6]. The evolution of RTIL-cation de-intercalation ensures the RTIL-cation intercalation into the graphite electrode. The nongraphitic carbon electrodes show only the reduction current, reflecting that the SEI formation by RTIL decomposition takes place without cation insertion. Fig. 3 displays the Raman G-band spectra, where the E2g symmetry vibration of carbons appears [6,19]. In the fresh state, the graphite gives a G-band at ca. 1585 cm − 1, which is the in-plane vibration of the graphene layers. Upon a negative potential sweep, this G-band moves to ca. 1602 cm − 1. The development of this new G-band has been ascribed to the intercalation of RTIL-cations into the graphite structure [6]. The other carbons show their own Gbands at 1580–1600 cm − 1 in their fresh state, but no blue-shift is observed even after the negative potential sweep. Obviously, the cation (PMPyr +) is not inserted into the non-graphitic carbons. The second concern in this work is to see if the used RTIL is thermally/ electrochemically more stable than the conventional carbonate-based solvents. The DSC data in Fig. 4 illustrates that the exothermic peaks appear at 150–230 °C when the lithiated non-graphitic carbons are contacted with the carbonate solvent (EC/DEC). With the RTIL, however, this exothermic peak develops at the higher temperatures (N250 °C). The total heat generation at 150–350 °C is smaller for the RTIL: hard carbon; 399.4 J g− 1 in PMPyr-TFSI and 695.6 J g− 1 in EC:DEC, and for soft carbon; 272.5 J g− 1 in PMPyr-TFSI and 517.4 J g− 1 in EC:DEC. The exothermic peaks in this temperature range are known to evolve by the reaction between the lithiated carbons and solvents [12]. The higher onset temperature and the less significant heat evolution for the RTIL illustrate that the RTIL is less reactive with the lithiated carbons. Namely, the RTIL is electrochemically and thermally more stable than the carbonate solvents.
4. Conclusion
Fig. 4. DSC data obtained with the lithiated non-graphitic carbon electrodes. Heating rate = 10 °C min− 1. The total heat generation from 150 to 350 °C is presented in the inset. The carbonate solvent is the mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol. ratio).
The insertion behavior of pyrrolidinium cation is compared for the graphitic and non-graphitic carbon electrodes. Contrary to the graphitic carbon, the non-graphitic ones do not show any RTIL-cation insertion/ trapping, such that the desired Li+ insertion is not hindered. A good cycle performance is attained. The RTIL shows a superior behavior to the carbonate solvent with respect to the electrochemical/thermal stability and heat evolution.
J. Mun et al. / Electrochemistry Communications 13 (2011) 1256–1259
Acknowledgement This work was supported by the WCU program through the National Research Foundation of Korea funded by the MEST (R31-10013 and NRF-2010-C1AAA001-2010-0029065). References [1] H. Sakaebe, H. Matsumoto, Electrochemistry Communications 5 (2003) 594–598. [2] E. Markevich, V. Baranchugov, D. Aurbach, Electrochemistry Communications 8 (2006) 1331–1334. [3] Z. Honghe, J. Kai, T. Abe, Z. Ogumi, Carbon 44 (2006) 203–210. [4] M. Egashira, M. Tanaka-Nakagawa, I. Watanabe, S. Okada, J.I. Yamaki, Journal of Power Sources 160 (2006) 1387–1390. [5] V. Baranchugov, E. Markevich, E. Pollak, G. Salitra, D. Aurbach, Electrochemistry Communications 9 (2007) 796–800. [6] V. Baranchugov, E. Markevich, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, Journal of the Electrochemical Society 155 (2008) A217–A227. [7] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nature Materials 8 (2009) 621–629.
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[8] A. Lewandowski, A. Swiderska-Mocek, Journal of Power Sources 194 (2009) 607–609. [9] J. Mun, Y.S. Jung, T. Yim, H.Y. Lee, H.-J. Kim, Y.G. Kim, S.M. Oh, Journal of Power Sources 194 (2009) 1068–1074. [10] J. Jin, H.H. Li, J.P. Wei, X.K. Bian, Z. Zhou, J. Yan, Electrochemistry Communications 11 (2009) 1500–1503. [11] J. Mun, S. Kim, T. Yim, J.H. Ryu, Y.G. Kim, S.M. Oh, Journal of the Electrochemical Society 157 (2010) A136–A141. [12] N.-S. Choi, I.A. Profatilova, S.-S. Kim, E.-H. Song, Thermochimica Acta 480 (2008) 10–14. [13] T. Doi, L. Zhao, M. Zhou, S. Okada, J. Yamaki, Journal of Power Sources 185 (2008) 1380–1385. [14] J.W. Jiang, J.R. Dahn, Electrochimica Acta 49 (2004) 4599–4604. [15] T. Yim, H.Y. Lee, H.J. Kim, J. Mun, S. Kim, S.M. Oh, Y.G. Kim, Bulletin of Korean Chemical Society 28 (2007) 1567. [16] T.E. Sutto, T.T. Duncan, T.C. Wong, Electrochimica Acta 54 (2009) 5648–5655. [17] K. Naoi, N. Ogihara, Y. Igarashi, A. Kamakura, Y. Kusachi, K. Utsugi, Journal of the Electrochemical Society 152 (2005) A1047–A1053. [18] H. Li, Z.X. Wang, L.Q. Chen, X.J. Huang, Advanced Materials 21 (2009) 4593–4607. [19] A.C. Ferrari, J. Robertson, Physical Review B 61 (2000) 14095–14107.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
The feasibility of a pyrrolidinium-based ionic liquid solvent for non-graphitic carbon electrodes Junyoung Mun a, Taeeun Yim a, Sunhyung Jurng a, Jang-Hoon Park b, Sang-Young Lee b, Ji Heon Ryu a, Young Gyu Kim a, Seung M. Oh a,⁎ a b
Department of Chemical and Biological Engineering and WCU Program of C2E2, Seoul National University, Seoul 151-744, Republic of Korea Department of Chemical Engineering, Kangwon National University, Chuncheon, 446-599, Republic of Korea
a r t i c l e
i n f o
Article history: Received 20 July 2011 Received in revised form 18 August 2011 Accepted 19 August 2011 Available online 26 August 2011 Keywords: Lithium-ion batteries Room-temperature ionic liquids Non-graphitic carbons Soft carbons Hard carbons
a b s t r a c t The feasibility of a pyrrolidinium-based room-temperature ionic liquid (RTIL) as the solvent for lithium-ion batteries is tested by analyzing its intercalation behavior and thermal stability. The RTIL-cations are intercalated into a graphitic carbon and a part of them are irreversibly trapped inside the graphene layers. These trapped cations block Li + intercalation to give only a marginal capacity. In contrast, such a cation insertion/trapping is absent in two non-graphitic carbons; hard carbon and soft carbon. A stable cycle performance with a Li + insertion capacity of about 200 mAh g− 1 is attained. The absence of RTIL-cation insertion is evidenced by the cyclic voltammograms and Raman spectra. A calorimetric study reveals that this RTIL has a higher thermal stability and less reactivity with lithiated carbons as compared with the carbonate-based solvent. The use of this RTIL solvent for the non-graphitic carbons seems to be feasible. © 2011 Elsevier B.V. All rights reserved.
1. Introduction With an expansion of the market for lithium-ion batteries (LIBs), the safety of LIBs has been an increasing focus of attention. The accidents known as thermal runaway are frequently triggered by some abusing events such as overcharging, internal short or local overheating. Once the cell heating is triggered, additional heats are evolved by the decomposition of cell constituents, which are electrochemically/thermally unstable and combustible. Along this line, the room-temperature ionic liquids (RTILs) have been projected as the replacement of the conventional carbonate-based solvents indebted to their low vapor pressure and nonflammability [1–11]. At present, graphite is the most popular negative electrode for LIBs but still carries a weak point for safety concerns. It is known that thermal runaway is triggered by decomposition of solid electrolyte interphase, and exothermic reactions between lithiated graphite and electrolyte solutions [12–14]. A variety of RTILs have been tested as the replacement of carbonate solvents, from which a high thermal/ electrochemical stability of RTILs has been reported [1–5,9,10]. The previous reports [3,6], however, complain that some RTIL cations such as pyrrolidinium and piperidinium ions are intercalated into graphite to degrade the negative electrodes. The prime concern in this work is to see if non-graphite carbons (hard carbon and soft carbon) are degraded by the insertion of
⁎ Corresponding author. Tel.: + 82 2 880 7074; fax: + 82 2 872 5755. E-mail address: [email protected] (S.M. Oh). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.08.030
RTIL-cation. To this end, a pyrrolidinium-based RTIL was tested as the solvent. The second concern is the thermal stability and reactivity of this RTIL solvent against the exothermic reactions with the lithiated non-graphite carbons. To this end, a calorimetric study is made with the RTIL and carbonate solvent to compare the onset temperature for exothermic reactions and heat evolution. 2. Experimental N-methyl-N-propylpyrrolidinium-bis(trifluromethanesulfonyl)imide (PMPyr-TFSI) was prepared according to the previous report [15]. The electrolyte was prepared by dissolving 1.0 M Li-TFSI (3M) into PMPyr-TFSI solvent. Two different non-graphitic carbons were tested as the negative electrode; hard carbon (Carbotron P, Kureha Chemical) and soft carbon (heat-treated green mesocarbon microbead (MCMB), Osaka Gas Co.). For comparison, the same measurements were made with a graphitic carbon (graphitized MCMB-10-28, Osaka Gas Co.). To prepare the composite electrodes, a mixture of the carbon powder, polyvinylidenefluoride (Solvay), and Super P (8:1:1, weight ratio) was dispersed in N-methyl-2-pyrrolidone. The resulting slurry was coated on a piece of copper foil, and the resulting electrode plates were dried in a vacuum oven at 120 °C for 12 h. Galvanostatic discharge–charge cycling was made with a coin-type cell (2032), in which a piece of lithium foil (Cyprus Co.) and glass filter (Advanter, GA-55, thickness: 0.21 mm, pore size: 0.6 μm) were loaded as the counter electrode and separator, respectively. Cyclic voltammograms were obtained in a three-electrode configuration, in which a silver wire was used as the pseudo-reference electrode and Pt plate as the
J. Mun et al. / Electrochemistry Communications 13 (2011) 1256–1259
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Fig. 1. The galvanostatic discharge (lithiation) and charge (de-lithiation) voltage profiles obtained with the Li/carbon cells (a–c), and the specific capacity with cycling (d). In (d), the open and filled symbols represent the lithiation and de-lithiation capacity, respectively. Current density = 25 mA g− 1. Voltage cut-off = 2.0–0.01 V (vs. Li/Li+).
counter electrode. The pseudo-reference electrode was calibrated by using the ferrocene/ferrocenium (Fc/Fc+) couple. Raman spectra were obtained in a backscattering mode using a Raman spectrometer HR800
(Jobin Yvon Horiba) with Ar laser (632.8 nm). For the differential scanning calorimetry (DSC, Dupont Q2000), the carbon electrodes were lithiated down to 0.01 V (vs. Li/Li+) in both RTIL-based and carbonate electrolyte, and collected after removing the current collector. The DSC measurement was made without any further treatments of the electrode samples. In this report, the lithiation was expressed as discharging, whereas the de-lithiation charging on the basis of the standard lithium-ion cell configuration. 3. Results and discussion
Fig. 2. Cyclic voltammograms obtained in the neat PMPyr-TFSI electrolyte. Scan rate = 10 mV s− 1.
Fig. 1 shows the discharge/charge voltage profiles of three carbon electrodes. In the first cycle, the graphite electrode shows the Li+ intercalation/de-intercalation behavior at 0.0–0.3 V (vs. Li/Li+). In addition, the plateau-like voltage profiles appear at 0.7–0.3 V in discharge and at 0.9–1.1 V upon charge, suggesting that additional redox reaction is involved. Four features are noted on this additional redox reaction. First, the reduction reaction (at 0.7–0.3 V) takes place before Li+ intercalation (at 0.0–0.3 V), whereas the oxidation (at 0.9–1.1 V) after Li+ de-intercalation. The same observation was made in the previous works [3,6,16], which reported that several RTIL-cations such as imidazolium and pyrrolidinium are intercalated into graphene layers before Li+ intercalation and removed after Li+ de-intercalation. Hence, the plateau-like feature at 0.7–0.3 V can be ascribed to the RTIL-cation (PMPyr + in this work) intercalation and that at 0.9–1.1 V to the cation de-intercalation. Second, the capacity associated with the cation de-intercalation is much smaller than that for the cation intercalation, implying that some of the intercalated cations are trapped inside the graphene layers. Third, the RTIL-cation intercalation/de-intercalation becomes less significant from the second cycle as evidenced by the shortened voltage plateaus at both 0.7–0.3 V and 0.9–1.1 V regions. In the 5th cycle, this cation-relevant redox reaction totally disappears to give only a discharge/charge capacity delivered by Li+ intercalation/de-intercalation. The resulting Li+ intercalation/de-intercalation capacity is presented in Fig. 1d. Only a marginal capacity is observed
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Fig. 3. Raman spectra of the fresh (the upper panel) and potential-swept (the bottom) carbon samples. The latter samples were prepared by a potential sweep down to − 2.5 V (vs. Fc/Fc+) in the neat PMPyr-TFSI electrolyte (Fig. 2).
from the 5th cycle. Surely, the trapped cations hinder Li+ accommodation. Fourth, the graphite electrode is passivated as evidenced by the evolution of irreversible capacity that is associated with electrolyte decomposition and solid electrolyte interphase (SEI) formation at 0.7– 0.3 V. The RTIL-cation insertion/de-insertion is not occurring with the nongraphitic carbons as evidenced by the absence of voltage plateaus (Fig. 1b and c). Only the Li+ insertion/de-insertion takes place with the sloping discharge/charge voltage profiles at 1.0–0.01 V. The 1st coulombic efficiency is low (hard carbon; 55% and soft carbon; 69%) due to the irreversible capacity that is caused by SEI formation (passivation) and Li+ trapping in some defect sites [17,18]. From the second cycle, however, both electrodes show a coulombic efficiency of N90% (hard carbon; 91.9% and soft carbon; 92.8% in the second cycle) indebted to the negligible irreversible reactions. A stable cycle performance with a capacity of ca. 200 mA h g− 1 is attained for both electrodes thereafter. Unambiguously, Li+ insertion/de-insertion is successfully carried out in two non-graphitic carbons without interference by the trapped RTIL cations. A separate measurement made in the carbonate electrolyte (1.0 M LiTFSI/EC:DEC) illustrates that the capacity values are slightly larger (hard carbon; 254 mA h g− 1 and soft carbon; 228 mA h g− 1 in the 20th cycle) than those obtained in the RTIL, which seems to be due to a higher ionic conductivity for the carbonate-based electrolytes.
To ascertain the presence/absence of RTIL-cation insertion, cyclic voltammograms were recorded in the neat PMPyr-TFSI solvent without lithium salt. Three carbon electrodes exhibit a reduction current at b−2.0 V (vs. Fc/Fc +) in the first negative scan (Fig. 2), which may come from the RTIL-cation insertion and the reductive decomposition of PMPyr-TFSI. In the positive scan, however, only the graphite electrode shows an oxidation current at −2.3 V. This oxidation current has been ascribed to the RTIL-cation de-intercalation in the previous works [3,6]. The evolution of RTIL-cation de-intercalation ensures the RTIL-cation intercalation into the graphite electrode. The nongraphitic carbon electrodes show only the reduction current, reflecting that the SEI formation by RTIL decomposition takes place without cation insertion. Fig. 3 displays the Raman G-band spectra, where the E2g symmetry vibration of carbons appears [6,19]. In the fresh state, the graphite gives a G-band at ca. 1585 cm − 1, which is the in-plane vibration of the graphene layers. Upon a negative potential sweep, this G-band moves to ca. 1602 cm − 1. The development of this new G-band has been ascribed to the intercalation of RTIL-cations into the graphite structure [6]. The other carbons show their own Gbands at 1580–1600 cm − 1 in their fresh state, but no blue-shift is observed even after the negative potential sweep. Obviously, the cation (PMPyr +) is not inserted into the non-graphitic carbons. The second concern in this work is to see if the used RTIL is thermally/ electrochemically more stable than the conventional carbonate-based solvents. The DSC data in Fig. 4 illustrates that the exothermic peaks appear at 150–230 °C when the lithiated non-graphitic carbons are contacted with the carbonate solvent (EC/DEC). With the RTIL, however, this exothermic peak develops at the higher temperatures (N250 °C). The total heat generation at 150–350 °C is smaller for the RTIL: hard carbon; 399.4 J g− 1 in PMPyr-TFSI and 695.6 J g− 1 in EC:DEC, and for soft carbon; 272.5 J g− 1 in PMPyr-TFSI and 517.4 J g− 1 in EC:DEC. The exothermic peaks in this temperature range are known to evolve by the reaction between the lithiated carbons and solvents [12]. The higher onset temperature and the less significant heat evolution for the RTIL illustrate that the RTIL is less reactive with the lithiated carbons. Namely, the RTIL is electrochemically and thermally more stable than the carbonate solvents.
4. Conclusion
Fig. 4. DSC data obtained with the lithiated non-graphitic carbon electrodes. Heating rate = 10 °C min− 1. The total heat generation from 150 to 350 °C is presented in the inset. The carbonate solvent is the mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol. ratio).
The insertion behavior of pyrrolidinium cation is compared for the graphitic and non-graphitic carbon electrodes. Contrary to the graphitic carbon, the non-graphitic ones do not show any RTIL-cation insertion/ trapping, such that the desired Li+ insertion is not hindered. A good cycle performance is attained. The RTIL shows a superior behavior to the carbonate solvent with respect to the electrochemical/thermal stability and heat evolution.
J. Mun et al. / Electrochemistry Communications 13 (2011) 1256–1259
Acknowledgement This work was supported by the WCU program through the National Research Foundation of Korea funded by the MEST (R31-10013 and NRF-2010-C1AAA001-2010-0029065). References [1] H. Sakaebe, H. Matsumoto, Electrochemistry Communications 5 (2003) 594–598. [2] E. Markevich, V. Baranchugov, D. Aurbach, Electrochemistry Communications 8 (2006) 1331–1334. [3] Z. Honghe, J. Kai, T. Abe, Z. Ogumi, Carbon 44 (2006) 203–210. [4] M. Egashira, M. Tanaka-Nakagawa, I. Watanabe, S. Okada, J.I. Yamaki, Journal of Power Sources 160 (2006) 1387–1390. [5] V. Baranchugov, E. Markevich, E. Pollak, G. Salitra, D. Aurbach, Electrochemistry Communications 9 (2007) 796–800. [6] V. Baranchugov, E. Markevich, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, Journal of the Electrochemical Society 155 (2008) A217–A227. [7] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nature Materials 8 (2009) 621–629.
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