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Cite this: Chem. Commun., 2013, 49, 11194 Received 31st August 2013, Accepted 8th October 2013
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A superconcentrated ether electrolyte for fast-charging Li-ion batteries† Yuki Yamada,ab Makoto Yaegashi,a Takeshi Abebc and Atsuo Yamada*ab
DOI: 10.1039/c3cc46665e www.rsc.org/chemcomm
We have found ultrafast Li+ intercalation into graphite in a superconcentrated ether electrolyte, even exceeding that in a currently used commercial electrolyte. This discovery is an important breakthrough toward fast-charging Li-ion batteries far beyond present technologies.
Fast-charging Li-ion batteries are urgently required for electric vehicles to gain in popularity over gasoline cars in terms of refueling (charging) time. In the most popular battery chemistry of a graphite anode and a lithium transition metal oxide cathode, the charging rate largely depends on Li+ intercalation kinetics at the graphite anode. This is because the reaction potential of graphite is quite close to that of dangerous Li metal, and the overpotential at the graphite anode must primarily be reduced to ensure a high level of both safety and charging rate. Electrolyte composition is an important factor that dominates the kinetics of electrochemical Li+ intercalation. As a first prerequisite, ionic conductivity (and Li+ transference number) must be high to facilitate rapid Li+ transport to the electrode surface. Second, it is well known that the kinetics of interfacial charge transfer (i.e., activation barrier) depends on electrolyte solvents.1 For the graphite anode, however, there is no other option of organic electrolytes but ethylene carbonate (EC) based solutions2 or a part of ionic liquids,3 because other organic electrolytes do not allow for highly reversible Li+ intercalation into graphite under normal conditions.4 Due to the limitation of electrolyte compositions, there has long been no remarkable progress in the kinetics of the graphite anode reaction. Recently, we have found that the graphite anode reversibly works without EC solvent in Li salt superconcentrated organic electrolytes.5 Although their rate performance was low due to high viscosity at that time, this a
Department of Chemical System Engineering, The University of Tokyo, Tokyo 113-8656, Japan. E-mail: [email protected]; Fax: +81 3 5841 7488; Tel: +81 3 5841 7295 b Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyoto 615-8246, Japan c Department of Energy and Hydrocarbon Chemistry, Kyoto University, Kyoto 615-8510, Japan † Electronic supplementary information (ESI) available: Experimental details, Raman band assignments, charge–discharge curves, cycling tests, and LiFePO4 charge–discharge properties. See DOI: 10.1039/c3cc46665e
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salt-concentrating strategy, expanding the graphite anode reaction for a wide variety of organic solvents, allows us to design a new high-rate organic electrolyte without EC. Ethers are attractive electrolyte solvents with strong Lewis basicity (i.e., high donor number), high chemical stability, and low prices. Ethers can easily dissolve Li salts through strong coordination to Li+ by the lone pair of oxygen atoms. Among various ether solvents, 1,2-dimethoxyethane (DME) is one of the best electrolyte solvents in terms of rate performance; due to the low viscosity and strong bidentate coordination to Li+, Li salt–DME solutions exhibit remarkably high ionic conductivity.6 Unfortunately, the graphite anode does not work reversibly in ether solutions,4d but our salt-concentrating strategy ensures the reversibility of the graphite anode, listing the DME solutions in the candidates of a high-rate electrolyte for Li-ion batteries. Herein we report ultrafast Li+ intercalation into graphite in a DME solution containing a superhigh concentration of LiN(SO2F)2 (lithium bis(fluorosulfonyl)amide, LiFSA), which is even much faster than that in a currently used commercial EC-based electrolyte. We chose a LiFSA salt because it is characterized by weak interaction between Li+ cations and FSA anions,7 presumably providing low viscosity and high ionic conductivity even at high LiFSA concentrations. Actually, FSA-based room-temperature ionic liquids work as low-viscosity superior-rate electrolytes for Li-metal batteries.8 However, the Li+ transference number in ionic liquids is essentially low (e.g., 0.1–0.2)8d due to the presence of reaction-irrelevant organic cations. In this regard, our LiFSA-superconcentrated ether solution is expected to exceed the FSA-based ionic liquids in rate performance because it contains only Li+ as a cation. A salt-concentrated solution was prepared by adding LiFSA salt to DME solvent with mild stirring and heating in a pure Ar atmosphere. The resultant superconcentrated 3.6 mol dm 3 LiFSA–DME mixture is a transparent, colorless liquid at room temperature (Fig. 1a). The 3.6 mol dm 3 concentration corresponds to the LiFSA : DME molar ratio of ca. 1 : 2, which is the criterion for salt concentration required for reversible Li+ intercalation into graphite in our previous reports on other combinations of Li salts and solvent.5 As shown in Fig. 1b, we confirmed reversible Li+ intercalation reaction at a natural graphite electrode in the present 3.6 mol dm 3 LiFSA–DME electrolyte, while the reaction was totally irreversible in a conventional This journal is
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Table 1
Fig. 1 (a) Image of 3.6 mol dm 3 LiFSA–DME solution. (b) Cyclic voltammograms of a natural graphite electrode in 1.0 and 3.6 mol dm 3 LiFSA–DME electrolytes.
Fig. 2
Raman spectra of LiFSA–DME solutions at various salt concentrations.
dilute LiFSA–DME solution due to the co-intercalation of Li+ and solvent to destroy the layered structure of graphite.4d In a charge–discharge test (Fig. S1 and S2, ESI†), the reversible capacity was retained at ca. 320 mA h g 1 (theoretical value: 372 mA h g 1) even after 50 cycles with almost 100% coulombic efficiency, which is equal to the performance in a commercial EC-based electrolyte. These results validate our salt-concentrating strategy of expanding the graphite anode reaction for a wide variety of organic solvents other than EC. To characterize the solution structures, Raman spectra were obtained in a pure Ar atmosphere (Fig. 2). The Raman spectra were completely different for conventional dilute 1.0 mol dm 3 and superconcentrated 3.6 mol dm 3 solutions, suggesting that the 3.6 mol dm 3 LiFSA–DME solution presented in this work is totally a new material with unique structural characteristics. The Raman bands at 700–780 cm 1 arise from the vibration of FSA anions; the bands at 720 cm 1, 732 cm 1, and 746 cm 1 are from a free FSA anion, a contact-ion pair (CIP, a FSA anion interacting with one Li+), and an aggregate (AGG, a FSA anion interacting with two or more Li+), respectively (Table S1, ESI†), referring to the assignment on bis(trifluoromethanesulfonyl)amide (TFSA).9 In 1.0 mol dm 3 solution, the majority of FSA exists as a free anion, due to the high dissociative character of the LiFSA salt and the strong Li+ coordinating ability of DME solvent. In 3.6 mol dm 3 solution, however, there are no free FSA anions and all of the FSA anions exist as CIPs or AGGs interacting with Li+. Several Raman bands at 800–900 cm 1 arise from the vibration of DME solvent; the two bands at 823 cm 1 and 851 cm 1 are from free DME solvents with several conformers, while the band at 879 cm 1 is from Li+-solvating DME solvents (Table S1, ESI†).10 In 1.0 mol dm 3 solution, there are both free and solvating DME solvents, due to the excess amount of DME beyond its maximum threefold coordination to Li+.11 On the other hand, This journal is
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Physical properties of LiFSA–DME solutions at 30 1C
Concentration (mol dm 3)
Molar ratio (DME/LiFSA)
Viscosity (mPa s)
Conductivity (mS cm 1)
1.0 3.6
8.8 1.9
1.2 25.1
17 7.2
there are no bands for free DME solvents in 3.6 mol dm 3 solution, indicating that all DME solvents coordinate to Li+. The average solvation number is 1.9 which is equal to the molar ratio of LiFSA : DME. The series of Raman analyses suggest that in the 3.6 mol dm 3 concentrated solution, all FSA anions and DME solvents interact with Li+ to form a polymeric fluid network of Li+ and FSA . This structure is unique to such superhigh salt concentrations and this is totally a new material different from conventional organic solutions or ionic liquids. Basic physicochemical properties of LiFSA–DME solutions are presented in Table 1. Even at a superhigh concentration of 3.6 mol dm 3, the ionic conductivity was retained at 7.2 mS cm 1 despite increased viscosity (25.1 mPa s). This ionic conductivity is almost comparable to that of commercial EC-based solutions (B10 mS cm 1) with much lower viscosity (B5 mPa s).12 The remarkably high ionic conductivity arises primarily from a large amount of charge carriers (i.e., cations and anions); the volumetric molar concentration of cations and anions is even larger than in some room-temperature ionic liquids (e.g., 3.3 mol dm 3 in N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME)-TFSA), due to smaller cations and anions with only a few solvents. It is this situation that led us to focus on the LiFSA-superconcentrated ether solution as a high-rate electrolyte for Li-ion batteries. Having established the unique structural and electrochemical features of the superconcentrated solution, the kinetics of Li+ intercalation into graphite was studied with a natural graphite/Li metal half cell. Fig. 3a and b show Li+ intercalation voltage curves of graphite at various C-rates in 3.6 mol dm 3 LiFSA–DME and commercial 1.0 mol dm 3 LiPF6/EC : DMC (dimethyl carbonate) (1 : 1 by vol.) electrolytes, respectively. In both cases, the voltage curve shifted downward with increasing C-rates. This phenomenon is referred to as polarization, and the polarization width is a good indicator of electrochemical reaction kinetics in batteries; smaller polarization indicates a faster electrochemical reaction. Upon comparing the voltage curves of the two electrolytes, it is clear that the polarization is much smaller in 3.6 mol dm 3 LiFSA–DME, indicating that electrochemical Li+ intercalation into graphite is outstandingly fast in the new electrolyte, even exceeding that in a currently used commercial EC-based electrolyte. The difference in the reaction kinetics becomes remarkable in Li+ intercalation capacity at high C-rates. Fig. 3c shows reversible capacity of a natural graphite/Li metal half cell at various C-rates, where charge and discharge were conducted at the same C-rate without using a constant-voltage mode. At all C-rates, the natural graphite electrode exhibited higher capacity in 3.6 mol dm 3 LiFSA– DME than in a commercial EC-based electrolyte, and the gap became much larger at higher C-rates. This is due to considerably small polarization in 3.6 mol dm 3 LiFSA–DME, which allows for massive Li+ intercalation before the voltage reaches 0 V cut-off at which dangerous Li metal deposition occurs. There are several possible factors accounting for the ultrafast Li+ intercalation reaction (i.e., small polarization) in the Chem. Commun., 2013, 49, 11194--11196
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Fig. 3 Discharge curves of a natural graphite/Li half cell with (a) 3.6 mol dm 3 LiFSA–DME and (b) commercial 1.0 mol dm 3 LiPF6/EC : DMC (1 : 1 by vol.) electrolytes at various C-rates (C/20, C/5, C/2, 1C, and 2C) at 25 1C. (c) Reversible capacity of a natural graphite/Li half cell with the two electrolytes at various C-rates and 25 1C. Charge and discharge were conducted at the same C-rate without using a constant-voltage mode at both ends of charge and discharge, and the charge (Li+ de-intercalation) capacity was plotted.
salt-superconcentrated system. In general, polarization consists of several overpotentials arising from (i) ohmic resistance of ion/electron conduction (resistance polarization), (ii) activation of a reaction (activation polarization), and (iii) diffusion of reactive species (concentration polarization). First, the resistance polarization should be small in the present case, because the bulk ionic conductivity of 3.6 mol dm 3 LiFSA–DME is quite high (7.2 mS cm 1) despite its high viscosity. However, since it is still slightly lower than that of an EC-based solution (B10 mS cm 1),12 the other two factors (i.e., activation and concentration polarization) should also contribute largely to the fast Li+ intercalation reaction. We postulate that (a) Li+ de-solvation (or ‘‘de-complexation’’ of CIPs or AGGs in this case) kinetics, (b) an excellent surface film facilitating Li+ intercalation, (c) high Li+ concentration at the interface with a peculiar electric double-layer structure, and/or (d) the high Li+ transference number should be the possible reasons for the ultrafast Li+ intercalation. Detailed discussions will be presented in a full paper with evidential experimental results. To ensure the practical validity of our concept, we next studied the charging (Li+ de-intercalation) rate of a LiFePO4 cathode in the superconcentrated electrolyte (Fig. S3 and S4, ESI†). Although the improvement in Li+ de-intercalation kinetics is not remarkable, an important point is that the charging rate capability at the cathode is much higher than that at a graphite anode with comparable capacity per unit area. This indicates that the overall charging rate of Li-ion batteries is dominated by Li+ intercalation kinetics at the graphite anode. Hence, the ultrafast Li+ intercalation into graphite, enabled by our new electrolyte, will undoubtedly make a considerable contribution to development of fast-charging Li-ion batteries beyond the present technologies. For example, the reversible capacity of a graphite anode at the 2C rate was ca. 250 mA h g 1 in 3.6 mol dm 3 LiFSA–DME (Fig. 3c), which corresponds to almost 70% charge for only 30 minutes without dangerous lithium plating on graphite. A full-cell test is underway and the results will be reported elsewhere. In summary, we have discovered outstanding reaction kinetics of a graphite anode in a LiFSA-superconcentrated DME electrolyte. This organic electrolyte is totally a new material with unique structural and electrochemical features and is fully different from
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a conventional low-concentration solution. The kinetics of Li+ intercalation even far exceeds that in a currently used commercial EC-based electrolyte. The discovery of ultrafast Li+ intercalation into graphite, enabled by a new electrolyte material, is an important breakthrough toward superfast-charging Li-ion batteries with expanding application to automobiles. This work was supported by the Cabinet Office, the Government of Japan and the Funding Program for World-Leading Innovative R&D on Science & Technology.
Notes and references 1 (a) T. Abe, F. Sagane, M. Ohtsuka, Y. Iriyama and Z. Ogumi, J. Electrochem. Soc., 2005, 152, A2151; (b) K. Xu, Y. Lam, S. S. Zhang, T. R. Jow and T. B. Curtis, J. Phys. Chem. C, 2007, 111, 7411; (c) Y. Yamada, Y. Iriyama, T. Abe and Z. Ogumi, Langmuir, 2009, 25, 12766. 2 H. Ikeda, S. Narukawa and H. Nakajima, Jp. Pat., 1769661, 1981. 3 M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko and M. Kono, J. Power Sources, 2006, 162, 658. 4 (a) A. N. Dey and B. P. Sullivan, J. Electrochem. Soc., 1970, 117, 222; (b) J. O. Besenhard, Carbon, 1976, 14, 111; (c) D. Aurbach, M. D. Levi, E. Levi, A. Schechter and E. Granot, J. Power Sources, 1997, 68, 91; (d) T. Abe, N. Kawabata, Y. Mizutani, M. Inaba and Z. Ogumi, J. Electrochem. Soc., 2003, 150, A257. 5 (a) S.-K. Jeong, M. Inaba, Y. Iriyama, T. Abe and Z. Ogumi, Electrochem. Solid-State Lett., 2003, 6, A13; (b) Y. Yamada, Y. Takazawa, K. Miyazaki and T. Abe, J. Phys. Chem. C, 2010, 114, 11680. 6 J. T. Dudley, D. P. Wilkinson, G. Thomas, R. LeVae, S. Woo, H. Blom, C. Horvath, M. W. Juzkow, B. Denis, P. Juric, P. Aghakian and J. R. Dahn, J. Power Sources, 1991, 35, 59. 7 S. Tsuzuki, W. Shinoda, S. Seki, Y. Umebayashi, K. Yoshida, K. Dokko and M. Watanabe, ChemPhysChem, 2013, 14, 1993. 8 (a) H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko and M. Kono, J. Power Sources, 2006, 160, 1308; (b) Y. Matsui, S. Kawaguchi, T. Sugimoto, M. Kikuta, T. Higashizaki, M. Kono, M. Yagmagata and M. Ishikawa, Electrochemistry, 2012, 80, 808; (c) M. Yamagata, Y. Matsui, T. Sugimoto, M. Kikuta, T. Higashizaki, M. Kono and M. Ishikawa, J. Power Sources, 2013, 227, 60; (d) H. Yoon, P. C. Howlett, A. S. Best, M. Forsyth and D. R. MacFarlane, J. Electrochem. Soc., 2013, 160, A1629. 9 D. M. Seo, O. Borodin, S.-D. Han, P. D. Boyle and W. A. Henderson, J. Electrochem. Soc., 2012, 159, A1489. 10 (a) H. Yoshida and H. Matsuura, J. Phys. Chem. A, 1998, 102, 2691; (b) B. L. Papke, M. A. Ratner and D. F. Shriver, J. Electrochem. Soc., 1982, 129, 1434. 11 W. A. Henderson, J. Phys. Chem. B, 2006, 110, 13177. 12 (a) M. S. Ding, K. Xu, S. S. Zhang, K. Amine, G. L. Henriksen and T. R. Jow, J. Electrochem. Soc., 2001, 148, A1196; (b) B. Klassen, R. Aroca, M. Nazri and G. A. Nazri, J. Phys. Chem. B, 1998, 102, 4795.
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Cite this: Chem. Commun., 2013, 49, 11194 Received 31st August 2013, Accepted 8th October 2013
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A superconcentrated ether electrolyte for fast-charging Li-ion batteries† Yuki Yamada,ab Makoto Yaegashi,a Takeshi Abebc and Atsuo Yamada*ab
DOI: 10.1039/c3cc46665e www.rsc.org/chemcomm
We have found ultrafast Li+ intercalation into graphite in a superconcentrated ether electrolyte, even exceeding that in a currently used commercial electrolyte. This discovery is an important breakthrough toward fast-charging Li-ion batteries far beyond present technologies.
Fast-charging Li-ion batteries are urgently required for electric vehicles to gain in popularity over gasoline cars in terms of refueling (charging) time. In the most popular battery chemistry of a graphite anode and a lithium transition metal oxide cathode, the charging rate largely depends on Li+ intercalation kinetics at the graphite anode. This is because the reaction potential of graphite is quite close to that of dangerous Li metal, and the overpotential at the graphite anode must primarily be reduced to ensure a high level of both safety and charging rate. Electrolyte composition is an important factor that dominates the kinetics of electrochemical Li+ intercalation. As a first prerequisite, ionic conductivity (and Li+ transference number) must be high to facilitate rapid Li+ transport to the electrode surface. Second, it is well known that the kinetics of interfacial charge transfer (i.e., activation barrier) depends on electrolyte solvents.1 For the graphite anode, however, there is no other option of organic electrolytes but ethylene carbonate (EC) based solutions2 or a part of ionic liquids,3 because other organic electrolytes do not allow for highly reversible Li+ intercalation into graphite under normal conditions.4 Due to the limitation of electrolyte compositions, there has long been no remarkable progress in the kinetics of the graphite anode reaction. Recently, we have found that the graphite anode reversibly works without EC solvent in Li salt superconcentrated organic electrolytes.5 Although their rate performance was low due to high viscosity at that time, this a
Department of Chemical System Engineering, The University of Tokyo, Tokyo 113-8656, Japan. E-mail: [email protected]; Fax: +81 3 5841 7488; Tel: +81 3 5841 7295 b Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyoto 615-8246, Japan c Department of Energy and Hydrocarbon Chemistry, Kyoto University, Kyoto 615-8510, Japan † Electronic supplementary information (ESI) available: Experimental details, Raman band assignments, charge–discharge curves, cycling tests, and LiFePO4 charge–discharge properties. See DOI: 10.1039/c3cc46665e
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salt-concentrating strategy, expanding the graphite anode reaction for a wide variety of organic solvents, allows us to design a new high-rate organic electrolyte without EC. Ethers are attractive electrolyte solvents with strong Lewis basicity (i.e., high donor number), high chemical stability, and low prices. Ethers can easily dissolve Li salts through strong coordination to Li+ by the lone pair of oxygen atoms. Among various ether solvents, 1,2-dimethoxyethane (DME) is one of the best electrolyte solvents in terms of rate performance; due to the low viscosity and strong bidentate coordination to Li+, Li salt–DME solutions exhibit remarkably high ionic conductivity.6 Unfortunately, the graphite anode does not work reversibly in ether solutions,4d but our salt-concentrating strategy ensures the reversibility of the graphite anode, listing the DME solutions in the candidates of a high-rate electrolyte for Li-ion batteries. Herein we report ultrafast Li+ intercalation into graphite in a DME solution containing a superhigh concentration of LiN(SO2F)2 (lithium bis(fluorosulfonyl)amide, LiFSA), which is even much faster than that in a currently used commercial EC-based electrolyte. We chose a LiFSA salt because it is characterized by weak interaction between Li+ cations and FSA anions,7 presumably providing low viscosity and high ionic conductivity even at high LiFSA concentrations. Actually, FSA-based room-temperature ionic liquids work as low-viscosity superior-rate electrolytes for Li-metal batteries.8 However, the Li+ transference number in ionic liquids is essentially low (e.g., 0.1–0.2)8d due to the presence of reaction-irrelevant organic cations. In this regard, our LiFSA-superconcentrated ether solution is expected to exceed the FSA-based ionic liquids in rate performance because it contains only Li+ as a cation. A salt-concentrated solution was prepared by adding LiFSA salt to DME solvent with mild stirring and heating in a pure Ar atmosphere. The resultant superconcentrated 3.6 mol dm 3 LiFSA–DME mixture is a transparent, colorless liquid at room temperature (Fig. 1a). The 3.6 mol dm 3 concentration corresponds to the LiFSA : DME molar ratio of ca. 1 : 2, which is the criterion for salt concentration required for reversible Li+ intercalation into graphite in our previous reports on other combinations of Li salts and solvent.5 As shown in Fig. 1b, we confirmed reversible Li+ intercalation reaction at a natural graphite electrode in the present 3.6 mol dm 3 LiFSA–DME electrolyte, while the reaction was totally irreversible in a conventional This journal is
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Table 1
Fig. 1 (a) Image of 3.6 mol dm 3 LiFSA–DME solution. (b) Cyclic voltammograms of a natural graphite electrode in 1.0 and 3.6 mol dm 3 LiFSA–DME electrolytes.
Fig. 2
Raman spectra of LiFSA–DME solutions at various salt concentrations.
dilute LiFSA–DME solution due to the co-intercalation of Li+ and solvent to destroy the layered structure of graphite.4d In a charge–discharge test (Fig. S1 and S2, ESI†), the reversible capacity was retained at ca. 320 mA h g 1 (theoretical value: 372 mA h g 1) even after 50 cycles with almost 100% coulombic efficiency, which is equal to the performance in a commercial EC-based electrolyte. These results validate our salt-concentrating strategy of expanding the graphite anode reaction for a wide variety of organic solvents other than EC. To characterize the solution structures, Raman spectra were obtained in a pure Ar atmosphere (Fig. 2). The Raman spectra were completely different for conventional dilute 1.0 mol dm 3 and superconcentrated 3.6 mol dm 3 solutions, suggesting that the 3.6 mol dm 3 LiFSA–DME solution presented in this work is totally a new material with unique structural characteristics. The Raman bands at 700–780 cm 1 arise from the vibration of FSA anions; the bands at 720 cm 1, 732 cm 1, and 746 cm 1 are from a free FSA anion, a contact-ion pair (CIP, a FSA anion interacting with one Li+), and an aggregate (AGG, a FSA anion interacting with two or more Li+), respectively (Table S1, ESI†), referring to the assignment on bis(trifluoromethanesulfonyl)amide (TFSA).9 In 1.0 mol dm 3 solution, the majority of FSA exists as a free anion, due to the high dissociative character of the LiFSA salt and the strong Li+ coordinating ability of DME solvent. In 3.6 mol dm 3 solution, however, there are no free FSA anions and all of the FSA anions exist as CIPs or AGGs interacting with Li+. Several Raman bands at 800–900 cm 1 arise from the vibration of DME solvent; the two bands at 823 cm 1 and 851 cm 1 are from free DME solvents with several conformers, while the band at 879 cm 1 is from Li+-solvating DME solvents (Table S1, ESI†).10 In 1.0 mol dm 3 solution, there are both free and solvating DME solvents, due to the excess amount of DME beyond its maximum threefold coordination to Li+.11 On the other hand, This journal is
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Physical properties of LiFSA–DME solutions at 30 1C
Concentration (mol dm 3)
Molar ratio (DME/LiFSA)
Viscosity (mPa s)
Conductivity (mS cm 1)
1.0 3.6
8.8 1.9
1.2 25.1
17 7.2
there are no bands for free DME solvents in 3.6 mol dm 3 solution, indicating that all DME solvents coordinate to Li+. The average solvation number is 1.9 which is equal to the molar ratio of LiFSA : DME. The series of Raman analyses suggest that in the 3.6 mol dm 3 concentrated solution, all FSA anions and DME solvents interact with Li+ to form a polymeric fluid network of Li+ and FSA . This structure is unique to such superhigh salt concentrations and this is totally a new material different from conventional organic solutions or ionic liquids. Basic physicochemical properties of LiFSA–DME solutions are presented in Table 1. Even at a superhigh concentration of 3.6 mol dm 3, the ionic conductivity was retained at 7.2 mS cm 1 despite increased viscosity (25.1 mPa s). This ionic conductivity is almost comparable to that of commercial EC-based solutions (B10 mS cm 1) with much lower viscosity (B5 mPa s).12 The remarkably high ionic conductivity arises primarily from a large amount of charge carriers (i.e., cations and anions); the volumetric molar concentration of cations and anions is even larger than in some room-temperature ionic liquids (e.g., 3.3 mol dm 3 in N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME)-TFSA), due to smaller cations and anions with only a few solvents. It is this situation that led us to focus on the LiFSA-superconcentrated ether solution as a high-rate electrolyte for Li-ion batteries. Having established the unique structural and electrochemical features of the superconcentrated solution, the kinetics of Li+ intercalation into graphite was studied with a natural graphite/Li metal half cell. Fig. 3a and b show Li+ intercalation voltage curves of graphite at various C-rates in 3.6 mol dm 3 LiFSA–DME and commercial 1.0 mol dm 3 LiPF6/EC : DMC (dimethyl carbonate) (1 : 1 by vol.) electrolytes, respectively. In both cases, the voltage curve shifted downward with increasing C-rates. This phenomenon is referred to as polarization, and the polarization width is a good indicator of electrochemical reaction kinetics in batteries; smaller polarization indicates a faster electrochemical reaction. Upon comparing the voltage curves of the two electrolytes, it is clear that the polarization is much smaller in 3.6 mol dm 3 LiFSA–DME, indicating that electrochemical Li+ intercalation into graphite is outstandingly fast in the new electrolyte, even exceeding that in a currently used commercial EC-based electrolyte. The difference in the reaction kinetics becomes remarkable in Li+ intercalation capacity at high C-rates. Fig. 3c shows reversible capacity of a natural graphite/Li metal half cell at various C-rates, where charge and discharge were conducted at the same C-rate without using a constant-voltage mode. At all C-rates, the natural graphite electrode exhibited higher capacity in 3.6 mol dm 3 LiFSA– DME than in a commercial EC-based electrolyte, and the gap became much larger at higher C-rates. This is due to considerably small polarization in 3.6 mol dm 3 LiFSA–DME, which allows for massive Li+ intercalation before the voltage reaches 0 V cut-off at which dangerous Li metal deposition occurs. There are several possible factors accounting for the ultrafast Li+ intercalation reaction (i.e., small polarization) in the Chem. Commun., 2013, 49, 11194--11196
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Fig. 3 Discharge curves of a natural graphite/Li half cell with (a) 3.6 mol dm 3 LiFSA–DME and (b) commercial 1.0 mol dm 3 LiPF6/EC : DMC (1 : 1 by vol.) electrolytes at various C-rates (C/20, C/5, C/2, 1C, and 2C) at 25 1C. (c) Reversible capacity of a natural graphite/Li half cell with the two electrolytes at various C-rates and 25 1C. Charge and discharge were conducted at the same C-rate without using a constant-voltage mode at both ends of charge and discharge, and the charge (Li+ de-intercalation) capacity was plotted.
salt-superconcentrated system. In general, polarization consists of several overpotentials arising from (i) ohmic resistance of ion/electron conduction (resistance polarization), (ii) activation of a reaction (activation polarization), and (iii) diffusion of reactive species (concentration polarization). First, the resistance polarization should be small in the present case, because the bulk ionic conductivity of 3.6 mol dm 3 LiFSA–DME is quite high (7.2 mS cm 1) despite its high viscosity. However, since it is still slightly lower than that of an EC-based solution (B10 mS cm 1),12 the other two factors (i.e., activation and concentration polarization) should also contribute largely to the fast Li+ intercalation reaction. We postulate that (a) Li+ de-solvation (or ‘‘de-complexation’’ of CIPs or AGGs in this case) kinetics, (b) an excellent surface film facilitating Li+ intercalation, (c) high Li+ concentration at the interface with a peculiar electric double-layer structure, and/or (d) the high Li+ transference number should be the possible reasons for the ultrafast Li+ intercalation. Detailed discussions will be presented in a full paper with evidential experimental results. To ensure the practical validity of our concept, we next studied the charging (Li+ de-intercalation) rate of a LiFePO4 cathode in the superconcentrated electrolyte (Fig. S3 and S4, ESI†). Although the improvement in Li+ de-intercalation kinetics is not remarkable, an important point is that the charging rate capability at the cathode is much higher than that at a graphite anode with comparable capacity per unit area. This indicates that the overall charging rate of Li-ion batteries is dominated by Li+ intercalation kinetics at the graphite anode. Hence, the ultrafast Li+ intercalation into graphite, enabled by our new electrolyte, will undoubtedly make a considerable contribution to development of fast-charging Li-ion batteries beyond the present technologies. For example, the reversible capacity of a graphite anode at the 2C rate was ca. 250 mA h g 1 in 3.6 mol dm 3 LiFSA–DME (Fig. 3c), which corresponds to almost 70% charge for only 30 minutes without dangerous lithium plating on graphite. A full-cell test is underway and the results will be reported elsewhere. In summary, we have discovered outstanding reaction kinetics of a graphite anode in a LiFSA-superconcentrated DME electrolyte. This organic electrolyte is totally a new material with unique structural and electrochemical features and is fully different from
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a conventional low-concentration solution. The kinetics of Li+ intercalation even far exceeds that in a currently used commercial EC-based electrolyte. The discovery of ultrafast Li+ intercalation into graphite, enabled by a new electrolyte material, is an important breakthrough toward superfast-charging Li-ion batteries with expanding application to automobiles. This work was supported by the Cabinet Office, the Government of Japan and the Funding Program for World-Leading Innovative R&D on Science & Technology.
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