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Properties of LiMn2O4 cathode in electrolyte based on N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide
Electrochimica Acta 55 (2010) 1990–1994
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Properties of LiMn2 O4 cathode in electrolyte based on N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ´ Andrzej Lewandowski ∗ , Agnieszka Swiderska-Mocek, Ilona Acznik Faculty of Chemical Technology, Pozna´ n University of Technology, Plac Marii Sklodowskiej-Curie, PL-60 965 Pozna´ n, Poland
a r t i c l e
i n f o
Article history: Received 6 August 2009 Received in revised form 2 November 2009 Accepted 6 November 2009 Available online 11 November 2009 Keywords: Ionic liquid Lithium-ion battery LiMn2 O4 cathode
a b s t r a c t LiMn2 O4 was examined as a cathode material for lithium-ion batteries, working together with a room temperature ionic liquid electrolyte, obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf2 ) in liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide (MePrPipNTf2 ), with the formation of a liquid LiNTf2 –MePrPipNTf2 system. The Li/LiMn2 O4 cell was tested by galvanostatic charging/discharging and by impedance spectroscopy. The LiMn2 O4 cathode showed good cyclability and Coulombic efficiency in the presence of 10 wt.% of vinylene carbonate (VC) as an additive to the ionic liquid. The flash point of the LiNTf2 –MePrPipNTf2 –VC(10%) electrolyte was estimated to be above 300 ◦ C. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction A lithium salt (LiPF6 ) solution in a mixture of cyclic carbonates usually serves as the electrolyte for lithium-ion batteries. Instead of a salt solution in volatile solvents, non-volatile molten salts may be applied as electrolytes. Salts liquid at room temperature, usually called room temperature ionic liquids (RTILs), or simply ionic liquids (ILs), have been proposed as potential electrolytes for electrochemical capacitors and lithium-ion batteries [1,2]. Most of low-melting-point salts are quaternary ammonium salts, showing negligible vapour pressure and broad electrochemical stability [1]. Dissolution of a solid lithium LiX salt in a liquid AX salt (RTIL) leads to a new LiX–AX ionic liquid containing a lithium cation. During the last decade there has been increasing interest in such LiX–AX ionic liquids as electrolytes for lithium and lithium-ion batteries, due to their non-volatility and hence, non-flammability. LiMn2 O4 is one of the most frequently studied cathode materials for the use in Li-ion batteries. It was also examined in RTILs based on N,N,N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (LiNTf2 –Me3 HexNNTf2 ) [3]. Electrochemical behavior of the LiMn2 O4 cathode in the ionic liquid was strongly influenced by the temperature. Optimal charging/discharging capacity was obtained at a temperature of ca. 30 ◦ C. The quaternary ammonium room temperature ionic liquid (Me3 HexNNTf2 ) containing 20 vol.% of chloroethylene carbonate facilitated the cycling of the spinel cathode with satisfactory capac-
∗ Corresponding author. E-mail address: [email protected] (A. Lewandowski). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.11.020
ity and reversibility [4]. A number of ionic liquids, including those based on the N-methyl-N-propylpiperidinium [MePrPip+ ] and Nbutyl-N-methylpyrrolidinium [BuMePyr+ ] cations as well as the bis(trifluoromethanesulfonyl)imide anion [NTf2 − ] were studied as potential electrolytes for Li-ion cells with a LiMn1.5 Ni0.5 O4 mixed spinel cathode [5]. The LiMn1.5 Ni0.5 O4 spinel is a high voltage cathode in both LiNTf2 –MePrPipNTf2 and LiNTf2 –BuMePyrNTf2 electrolytes; the Li/IL/LiMn1.5 Ni0.5 O4 cell was charged up to 4.8 V. The LiNTf2 –MePrPipNTf2 room temperature ionic liquid was studied as an electrolyte compatible both with the LiCoO2 cathode [6–9] and such anode materials as amorphous Si, graphite, hard carbon and sulfur–carbon composite [10–13]. The general aim of the present paper was to study the LiMn2 O4 cathode in the LiNTf2 –MePrPipNTf2 electrolyte. 2. Experimental 2.1. Materials LiMn2 O4 powder (Aldrich), graphite KS-15 (G) (Lonza), poly(vinylidene fluoride) (PVDF, Fluka), vinylene carbonate (VC, Aldrich), lithium foil (0.75 mm thick, Aldrich) and lithium bis(trifluoromethanesulfonyl)imide (LiNTf2 , Fluka) were used as received. N-Methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (MePrPipNTf2 ) was prepared according to the literature [11], by reacting N-methylpiperidine (Aldrich) with propylbromide (Aldrich) followed by metathesis with lithium bis(trifluoromethanesulfonyl)imide. The LiNTf2 –MePrPipNTf2 ionic liquid containing Li+ cation was obtained by dissolution of solid LiNTf2 in liquid MePrPipNTf2 (0.4 M solution of LiNTf2
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in MePrPipNTf2 ). Positive electrodes were prepared by casting a slurry of LiMn2 O4 , graphite (G) and PVDF (at a ratio of 85:5:10) in N-methyl-2-pyrrolidone (NMP, Fluka) on a golden current collector (diameter 12 mm). The layer of the cathode was formed by evaporation of the solvent (NMP) at 120 ◦ C in vacuum. 2.2. Measurements Cycling efficiency of the cathode was measured in a two compartment cell against a counter electrode consisting of the lithium foil, separated by the glass micro-fibre GF/A separator (Whatmann), placed in an adopted 0.5 Swagelok® connecting tube. The LiMn2 O4 /LiNTf2 –MePrPipNTf2 /Li and LiMn2 O4 /LiNTf2 –MePrPipNTf2 –VC(10%)/Li cells were assembled in a dry argon atmosphere in a glove box. The cycling measurements were done with the use of the ATLAS 0461 MBI multichannel electrochemical system (Atlas-Solich, Poland) at different current rates (C/10–C/2). Constant current charging/discharging cycles were conducted between 3.2 and 4.3 V versus the lithium-metal reference. Interface resistance at the electrode/electrolyte interface was measured using an ac impedance analyzer (Atlas-Solich System, Poland). Flash point of electrolytes (LiNTf2 –MePrPipNTf2 and LiNTf2 –MePrPipNTf2 –VC(10%)) was measured with the use of an open-cup home-made apparatus, based on the Cleveland instrument, with a 1.5 ml cup. The cup was heated electrically through a sand bath, and temperature of the electrolyte was measured with the M-3850 Metex (Korea) digital thermometer. The apparatus was scaled with a number of compounds of known flash points. 3. Results and discussion 3.1. LiNTf2 –MePrPipNTf2 –VC(10%) electrolyte composition The solubility of the solid LiNTf2 salt in the liquid MePrPipNTf2 salt was ca. 0.4 M at room temperature. The density of the resulting ternary ionic liquid was ca. 1.576 g/cm3 [11]. At higher LiNTf2 concentrations a crystalline solid phase formation was observed. The lithium-ion content in the LiNTf2 –MePrPipNTf2 ionic liquid is lower in comparison to a corresponding system, obtained recently by dissolution of the solid LiNTf2 salt in the liquid MePrPyrNTf2 salt, which resulted in a ternary LiNTf2 –MePrPyrNTf2 ionic liquid [14]. 3.2. Galvanostatic charging/discharging Fig. 1a shows the charging/discharging curve for the LiMn2 O4 /0.4 M LiNTf2 in MePrPipNTf2 /Li cell (without any additive to the ionic liquid electrolyte). The capacity of the charging (qch ) and discharging (qdis ) processes at the first cycle was 107 and 94 mAh/g, respectively. As the result, the initial Coulombic efficiency at the first cycle was ca. 87%. The capacity of the electrode, both qch and qdis , gradually decreased during the cycling (Fig. 2). However, the Coulombic efficiency, proportional to the qdis /qch ratio, increased during the cycling to reach almost 100% after the fourth cycle (Fig. 3). Fig. 1b illustrates charging/discharging curves for the spinel electrode working with the electrolyte containing 10 wt.% of VC as an additive. The content of VC is relatively high and the resulting electrolyte may be called a solvent-in-salt solution. After the first cycle, the capacity of the spinel was ca. 125 mAh/g. The electrode showed ca. 100% of its initial discharge capacity after 25th cycles, at charge/discharge current of 12 mA/g (C/10). The addition of VC (10 wt.%) considerably improved both the charging and discharging capacities, in comparison to the ionic liquid without any additive (discharge capacity and the Coulombic efficiency for the spinel electrode working together with the neat ionic liquid, as well as with additive are shown in
Fig. 1. Galvanostatic charging/discharging of (a) the LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 |Li and (b) the LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li cells. Spinel mass in the cathode: (a) 2.5 mg and (b) 2.6 mg. Current: 12 mA/g (C/10).
Figs. 2 and 3). The improvement is probably due to the ability of the additive to SEI (Solid Electrolyte Interface) formation on the lithium surface. The discharge capacity of the LiMn2 O4 /0.4 M LiNTf2 in MePrPipNTf2 + 10%VC/Li cell depends on the current rates (Fig. 4). The highest capacity (ca. 120 mAh/g) was obtained at the lowest C/10 rate. The cell could work relatively efficiently, over many cycles, at higher rates. The charging/discharging rate C/5 led to the electrode capacity of ca. 110 mAh/g. However, at C/3
Fig. 2. Discharge capacity of the spinel cathode.
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Fig. 5. Impedance plots for the symmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 |Li cell kept under open circuit conditions as a function of time; (a) 0–96 h and (b) 264–624 h.
Fig. 3. Coulombic efficiency of the spinel cathode.
and C/2 rate, the discharge capacity dropped to 97 and 72 mAh/g, respectively. 3.3. Impedance spectroscopy (SEI formation) The gradual decrease of the cathode capacity in the LiMn2 O4 /0.4 M LiNTf2 in MePrPipNTf2 /Li cell may be due to (i) problems at the LiMn2 O4 /electrolyte interface and (ii) inefficient work of the lithium counter electrode. Low potential anodes, such as lithium-metal, are very reactive in contact with electrolytes. Use of electrolyte additives is an effective method of improving the cycle-life of lithium-metal or lithiated-graphite anodes. Additive enables the formation of the SEI on the surface of the anode. The electrochemical reaction of the electrolyte additive (for example lithium carbonate, salicyloborate, vinylene carbonate etc.) at electrode material may result in the formation of a coating and modification of the electrode surface. Metallic lithium is not a part of lithium-ion batteries (it has been replaced by graphite) however, in laboratory tests of a cathode or anode, lithium-metal is commonly used as a counter electrode. The Li/electrolyte/LiMn2 O4 cells tested in this study contained small amounts of the cathode material, at the level of 2–6 mg. The counter electrode was lithium
Fig. 4. Discharge capacity of the LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li cell at various discharging rates.
foil of ca. 0.75 mm in thickness and surface area of ca. 1 cm2 . The mass and capacity of the lithium-metal counter electrode was much higher in comparison to the tested LiMn2 O4 electrode. Consequently, after gradual degradation of lithium during the cell cycling, its capacity was still much higher than that characteristic of the tested one. The possible passivation of both the lithium counter electrode and the LiMn2 O4 cathode were investigated by impedance spectroscopy. Fig. 5 illustrates the impedance of the symmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 |Li cell as a function of time under open circuit conditions. The initial impedance of ca. 102 increases with time, to reach a value of ca. 104 after 1 month of storage. This indicates corrosion of metallic lithium in contact with the electrolyte with the formation of a highly resistive coating. In the presence of 10 wt.% of VC the impedance increase is almost two orders of magnitude lower (Fig. 6). During galvanostatic charging/discharging of the Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC |Li cell the protective coating of low resistance is efficiently formed which is reflected in impedance plots (Fig. 7). Fig. 8 shows evolution of the impedance of the symmetrical, fully charged (delithiated LiMn2 O4 ) (a) LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 |LiMn2 O4 and (b) LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|LiMn2 O4 systems, recorded just after their assembling and as a function of time. It can be seen that the impedance of the charged cathode change with time, indicating that the cathode may also be covered with SEI. This is not unexpected, as cathodes may form SEI of different structure in comparison to that formed at anodes [15]. The electrochemical process of the lithium intercalation into the cathode material includes the necessary step of Li+ migration through the SEI layer
Fig. 6. Impedance plots for the symmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li cell kept under open circuit conditions as a function of time.
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Fig. 7. Impedance plots for the symmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li system recorded after galvanostatic charging/discharging with current of 1 mA.
[15]. However, impedance plots suggest that the LiMn2 O4 cathode does not react with the electrolyte under study with the formation of a resistive film. Figs. 9 and 10 show impedance evolution in unsymmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 |LiMn2 O4 and Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|LiMn2 O4 systems, respectively. In this case a dramatic difference can be seen between the cell without the SEI forming VC (Fig. 9) and that containing VC (Fig. 10). In the absence of VC in the electrolyte, the evolution of cell impedance from ca. 500 (at low frequencies) to 100 k after 33 days was observed. This indicates strong passivation of the Li/IL interface. However, the system containing 10 wt.% of VC does not show this effect (Fig. 10), which suggests formation of a coating on the lithium-metal surface, protecting it from the further corrosion. Fig. 11 shows the impedance plot for the Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|LiMn2 O4 systems recorded after the 25th charging/discharging cycle. The plot consists of two semicircles and was analyzed according to an equivalent circuit consisting of the electrolyte bulk resistance, Rs , in series with two sub-circuits, both represented by resistance R parallel to capacity C. Determined
Fig. 9. Impedance plots for the unsymmetrical LiMn2 O4 |0.4 M LiNTf2 MePrPipNTf2 |Li cell as a function of time.
in
Fig. 10. Impedance plot for the unsymmetrical LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li cell, kept under open circuit conditions, as a function of time.
Fig. 8. Impedance plots for the symmetrical cells (a) LiMn2 O4 |0.4 M in MePrPipNTf2 |LiMn2 O4 and (b) LiMn2 O4 |0.4 M LiNTf2 in LiNTf2 MePrPipNTf2 + 10%VC|LiMn2 O4 , kept under open circuit conditions, as a function of time. Cathodes (LiMn2 O4 ) were in the charged (deintercalated) state.
Fig. 11. Impedance plot for the unsymmetrical LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li system recorded after 25th charge/discharge cycles.
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equivalent series resistance Rs and charge transfer resistance Rct at the cathode (the second semicircle) were 34 and 252 , respectively. The LiMn2 O4 cathode does not require the presence of the SEI forming additive, while the anodes, both metallic lithium (laboratory test cells) and lithiated graphite (the lithium-ion battery) work properly only in the presence of the SEI forming additive [11,14]. The Li//LiCoO2 system with a room temperature ionic liquid as an electrolyte (RTIL based on N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide) was studied by impedance spectroscopy [16]. The impedance plots consisted of two semicircles, which were assigned to the anode/electrolyte interface (the high frequency semicircle) and to the cathode/electrolyte interface (the low frequency semicircle). The LiMn2 O4 and LiCoO2 cathodes were investigated in another ionic liquid (RTIL based on N,N,N-trimethyl-Nhexylammonium bis(trifluoromethanesulfonyl)imide) [3,17]. The Li/LiMO2 impedance plots also consisted of two semicircles. Lithium-ion transfer through the electrode/electrolyte interface was assumed to be the rate controlling step of the lithium intercalation into the LiMn2 O4 electrode. Measurements with the LiMn2 O4 cathode were performed at different temperatures and the charge transfer resistance values, Rct , obtained from the impedance spectra deconvolution, were plotted as log Rct = f(T−1 ), to give the activation energy for the charge transfer process (0.57 eV) [3]. It has been concluded that the high cathode/electrolyte resistance and the large charge transfer resistance were probably associated with the poor wettability of the electrode. 3.4. Electrolyte flammability In lithium-ion cells containing classical electrolytes the solvent may start to evaporate and consequently, the vapour may ignite. Therefore, for safety reasons, it is of practical significance to look for new non-volatile electrolytes. Many ionic liquids, due to the strong ion–ion interactions, are characterised by negligible vapour pressure at room temperature. On the other hand, the SEI forming additives, such as VC, are volatile organic compounds and it is necessary to estimate the flash point of such mixtures. The flash point of the LiNTf2 –MePrPipNTf2 –VC(10%) electrolyte was estimated to be above 300 ◦ C. Lithium concentration in the ionic liquid is at the level of ca. 0.4 M, while the VC concentration is only 0.27 M, which suggests that the entire amount of volatile VC may be bound to solvation shells of Li+ , MePrPip+ or NTf2 − ions. The effect of a high salt concentration in a molecular solvent on reduced flammability of the electrolyte may be found in the available literature. The following battery: C/LiPF6 –MePrPipNTf2 (50 wt.%)-EC-DMC-EMC/LiCoO2
was examined from the point of view of its performance and the electrolyte non-flammability [13]. Flammability was examined by soaking a glass filter with the electrolyte and exposing it to a flame for 10 s. High contents of the MePrPipNTf2 salt (at least 40 wt.%) resulted in the non-flammability of the system. This suggests that the dissolution of a salt, liquid or solid at room temperature, may lead to a kind of a solvent-in-salt solution, of a decreased vapour pressure of molecular components and hence, practically to nonflammability of the system. 4. Conclusions 1. The LiMn2 O4 cathode shows good cyclability and Coulombic efficiency working with the LiNTf2 –MePrPipNTf2 room temperature ionic liquid as an electrolyte, in the presence of 10 wt.% of vinylene carbonate as an additive to the ionic liquid. 2. The flash point of the LiNTf2 –MePrPipNTf2 –VC(10%) electrolyte was estimated to be above 300 ◦ C. Acknowledgement The support of the DS31-178/09 grant is gratefully acknowledged. References [1] A. Weber, G.E. Blomgren, in: W. van Schalkwijk, B. Scrosati (Eds.), Advances in Lithium Ion Batteries, Kluwer, 2002, p. 185. [2] A. Fernicola, B. Scrosati, H. Ohno, Ionics 12 (2006) 95. [3] H. Zheng, H. Zhang, Y. Fu, T. Abe, Z. Ogumi, J. Phys. Chem. B 109 (2005) 13676. [4] H. Zheng, B. Li, Y. Fu, T. Abe, Z. Ogumi, Electrochim. Acta 52 (2006) 1556. [5] V. Borgel, E. Markevich, D. Aurbach, G. Semrau, M. Schmidt, J. Power Sources 189 (2009) 331. [6] H. Sakaebe, H. Matsumoto, Electrochem. Commun. 5 (2003) 594. [7] H. Sakaebe, H. Matsumoto, K. Tatsumi, J. Power Sources 146 (2005) 693. [8] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources 160 (2006) 1308. [9] J. Xu, J. Yang, Y. NuLi, J. Wang, Z. Zhang, J. Power Sources 160 (2006) 621. [10] L.X. Yuan, J.K. Feng, X.P. Ai, Y.L. Cao, S.L. Chen, H.X. Yang, Electrochem. Commun. 8 (2006) 610. [11] A. Lewandowski, A. Swiderska-Mocek, J. Power Sources 171 (2007) 938. [12] V. Baranchugov, E. Markevich, E. Pollak, G. Salitra, D. Aurbach, Electrochem. Commun. 9 (2007) 796. [13] H. Nakagawa, Y. Fujino, S. Kozono, Y. Katayama, T. Nukuda, H. Sakaebe, H. Matsumoto, K. Tatsumi, J. Power Sources 174 (2007) 1021. [14] A. Lewandowski, A. Swiderska-Mocek, J. Power Sources 194 (2009) 502. [15] D. Aurbach, in: W. van Schalkwijk, B. Scrosati (Eds.), Advances in Lithium Ion Batteries, Kluwer, 2002, p. 7. [16] S. Seki, Y. Kobayashi, H. Miyashiro, Y. Ohno, Y. Mita, A. Usami, N. Terada, M. Watanabe, Electrochem. Solid State 8 (2005) A577. [17] H. Zheng, J. Qin, Y. Zhao, T. Abe, Z. Ogumi, Solid State Ionics 176 (2005) 2219.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Properties of LiMn2 O4 cathode in electrolyte based on N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ´ Andrzej Lewandowski ∗ , Agnieszka Swiderska-Mocek, Ilona Acznik Faculty of Chemical Technology, Pozna´ n University of Technology, Plac Marii Sklodowskiej-Curie, PL-60 965 Pozna´ n, Poland
a r t i c l e
i n f o
Article history: Received 6 August 2009 Received in revised form 2 November 2009 Accepted 6 November 2009 Available online 11 November 2009 Keywords: Ionic liquid Lithium-ion battery LiMn2 O4 cathode
a b s t r a c t LiMn2 O4 was examined as a cathode material for lithium-ion batteries, working together with a room temperature ionic liquid electrolyte, obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf2 ) in liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide (MePrPipNTf2 ), with the formation of a liquid LiNTf2 –MePrPipNTf2 system. The Li/LiMn2 O4 cell was tested by galvanostatic charging/discharging and by impedance spectroscopy. The LiMn2 O4 cathode showed good cyclability and Coulombic efficiency in the presence of 10 wt.% of vinylene carbonate (VC) as an additive to the ionic liquid. The flash point of the LiNTf2 –MePrPipNTf2 –VC(10%) electrolyte was estimated to be above 300 ◦ C. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction A lithium salt (LiPF6 ) solution in a mixture of cyclic carbonates usually serves as the electrolyte for lithium-ion batteries. Instead of a salt solution in volatile solvents, non-volatile molten salts may be applied as electrolytes. Salts liquid at room temperature, usually called room temperature ionic liquids (RTILs), or simply ionic liquids (ILs), have been proposed as potential electrolytes for electrochemical capacitors and lithium-ion batteries [1,2]. Most of low-melting-point salts are quaternary ammonium salts, showing negligible vapour pressure and broad electrochemical stability [1]. Dissolution of a solid lithium LiX salt in a liquid AX salt (RTIL) leads to a new LiX–AX ionic liquid containing a lithium cation. During the last decade there has been increasing interest in such LiX–AX ionic liquids as electrolytes for lithium and lithium-ion batteries, due to their non-volatility and hence, non-flammability. LiMn2 O4 is one of the most frequently studied cathode materials for the use in Li-ion batteries. It was also examined in RTILs based on N,N,N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (LiNTf2 –Me3 HexNNTf2 ) [3]. Electrochemical behavior of the LiMn2 O4 cathode in the ionic liquid was strongly influenced by the temperature. Optimal charging/discharging capacity was obtained at a temperature of ca. 30 ◦ C. The quaternary ammonium room temperature ionic liquid (Me3 HexNNTf2 ) containing 20 vol.% of chloroethylene carbonate facilitated the cycling of the spinel cathode with satisfactory capac-
∗ Corresponding author. E-mail address: [email protected] (A. Lewandowski). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.11.020
ity and reversibility [4]. A number of ionic liquids, including those based on the N-methyl-N-propylpiperidinium [MePrPip+ ] and Nbutyl-N-methylpyrrolidinium [BuMePyr+ ] cations as well as the bis(trifluoromethanesulfonyl)imide anion [NTf2 − ] were studied as potential electrolytes for Li-ion cells with a LiMn1.5 Ni0.5 O4 mixed spinel cathode [5]. The LiMn1.5 Ni0.5 O4 spinel is a high voltage cathode in both LiNTf2 –MePrPipNTf2 and LiNTf2 –BuMePyrNTf2 electrolytes; the Li/IL/LiMn1.5 Ni0.5 O4 cell was charged up to 4.8 V. The LiNTf2 –MePrPipNTf2 room temperature ionic liquid was studied as an electrolyte compatible both with the LiCoO2 cathode [6–9] and such anode materials as amorphous Si, graphite, hard carbon and sulfur–carbon composite [10–13]. The general aim of the present paper was to study the LiMn2 O4 cathode in the LiNTf2 –MePrPipNTf2 electrolyte. 2. Experimental 2.1. Materials LiMn2 O4 powder (Aldrich), graphite KS-15 (G) (Lonza), poly(vinylidene fluoride) (PVDF, Fluka), vinylene carbonate (VC, Aldrich), lithium foil (0.75 mm thick, Aldrich) and lithium bis(trifluoromethanesulfonyl)imide (LiNTf2 , Fluka) were used as received. N-Methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (MePrPipNTf2 ) was prepared according to the literature [11], by reacting N-methylpiperidine (Aldrich) with propylbromide (Aldrich) followed by metathesis with lithium bis(trifluoromethanesulfonyl)imide. The LiNTf2 –MePrPipNTf2 ionic liquid containing Li+ cation was obtained by dissolution of solid LiNTf2 in liquid MePrPipNTf2 (0.4 M solution of LiNTf2
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in MePrPipNTf2 ). Positive electrodes were prepared by casting a slurry of LiMn2 O4 , graphite (G) and PVDF (at a ratio of 85:5:10) in N-methyl-2-pyrrolidone (NMP, Fluka) on a golden current collector (diameter 12 mm). The layer of the cathode was formed by evaporation of the solvent (NMP) at 120 ◦ C in vacuum. 2.2. Measurements Cycling efficiency of the cathode was measured in a two compartment cell against a counter electrode consisting of the lithium foil, separated by the glass micro-fibre GF/A separator (Whatmann), placed in an adopted 0.5 Swagelok® connecting tube. The LiMn2 O4 /LiNTf2 –MePrPipNTf2 /Li and LiMn2 O4 /LiNTf2 –MePrPipNTf2 –VC(10%)/Li cells were assembled in a dry argon atmosphere in a glove box. The cycling measurements were done with the use of the ATLAS 0461 MBI multichannel electrochemical system (Atlas-Solich, Poland) at different current rates (C/10–C/2). Constant current charging/discharging cycles were conducted between 3.2 and 4.3 V versus the lithium-metal reference. Interface resistance at the electrode/electrolyte interface was measured using an ac impedance analyzer (Atlas-Solich System, Poland). Flash point of electrolytes (LiNTf2 –MePrPipNTf2 and LiNTf2 –MePrPipNTf2 –VC(10%)) was measured with the use of an open-cup home-made apparatus, based on the Cleveland instrument, with a 1.5 ml cup. The cup was heated electrically through a sand bath, and temperature of the electrolyte was measured with the M-3850 Metex (Korea) digital thermometer. The apparatus was scaled with a number of compounds of known flash points. 3. Results and discussion 3.1. LiNTf2 –MePrPipNTf2 –VC(10%) electrolyte composition The solubility of the solid LiNTf2 salt in the liquid MePrPipNTf2 salt was ca. 0.4 M at room temperature. The density of the resulting ternary ionic liquid was ca. 1.576 g/cm3 [11]. At higher LiNTf2 concentrations a crystalline solid phase formation was observed. The lithium-ion content in the LiNTf2 –MePrPipNTf2 ionic liquid is lower in comparison to a corresponding system, obtained recently by dissolution of the solid LiNTf2 salt in the liquid MePrPyrNTf2 salt, which resulted in a ternary LiNTf2 –MePrPyrNTf2 ionic liquid [14]. 3.2. Galvanostatic charging/discharging Fig. 1a shows the charging/discharging curve for the LiMn2 O4 /0.4 M LiNTf2 in MePrPipNTf2 /Li cell (without any additive to the ionic liquid electrolyte). The capacity of the charging (qch ) and discharging (qdis ) processes at the first cycle was 107 and 94 mAh/g, respectively. As the result, the initial Coulombic efficiency at the first cycle was ca. 87%. The capacity of the electrode, both qch and qdis , gradually decreased during the cycling (Fig. 2). However, the Coulombic efficiency, proportional to the qdis /qch ratio, increased during the cycling to reach almost 100% after the fourth cycle (Fig. 3). Fig. 1b illustrates charging/discharging curves for the spinel electrode working with the electrolyte containing 10 wt.% of VC as an additive. The content of VC is relatively high and the resulting electrolyte may be called a solvent-in-salt solution. After the first cycle, the capacity of the spinel was ca. 125 mAh/g. The electrode showed ca. 100% of its initial discharge capacity after 25th cycles, at charge/discharge current of 12 mA/g (C/10). The addition of VC (10 wt.%) considerably improved both the charging and discharging capacities, in comparison to the ionic liquid without any additive (discharge capacity and the Coulombic efficiency for the spinel electrode working together with the neat ionic liquid, as well as with additive are shown in
Fig. 1. Galvanostatic charging/discharging of (a) the LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 |Li and (b) the LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li cells. Spinel mass in the cathode: (a) 2.5 mg and (b) 2.6 mg. Current: 12 mA/g (C/10).
Figs. 2 and 3). The improvement is probably due to the ability of the additive to SEI (Solid Electrolyte Interface) formation on the lithium surface. The discharge capacity of the LiMn2 O4 /0.4 M LiNTf2 in MePrPipNTf2 + 10%VC/Li cell depends on the current rates (Fig. 4). The highest capacity (ca. 120 mAh/g) was obtained at the lowest C/10 rate. The cell could work relatively efficiently, over many cycles, at higher rates. The charging/discharging rate C/5 led to the electrode capacity of ca. 110 mAh/g. However, at C/3
Fig. 2. Discharge capacity of the spinel cathode.
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Fig. 5. Impedance plots for the symmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 |Li cell kept under open circuit conditions as a function of time; (a) 0–96 h and (b) 264–624 h.
Fig. 3. Coulombic efficiency of the spinel cathode.
and C/2 rate, the discharge capacity dropped to 97 and 72 mAh/g, respectively. 3.3. Impedance spectroscopy (SEI formation) The gradual decrease of the cathode capacity in the LiMn2 O4 /0.4 M LiNTf2 in MePrPipNTf2 /Li cell may be due to (i) problems at the LiMn2 O4 /electrolyte interface and (ii) inefficient work of the lithium counter electrode. Low potential anodes, such as lithium-metal, are very reactive in contact with electrolytes. Use of electrolyte additives is an effective method of improving the cycle-life of lithium-metal or lithiated-graphite anodes. Additive enables the formation of the SEI on the surface of the anode. The electrochemical reaction of the electrolyte additive (for example lithium carbonate, salicyloborate, vinylene carbonate etc.) at electrode material may result in the formation of a coating and modification of the electrode surface. Metallic lithium is not a part of lithium-ion batteries (it has been replaced by graphite) however, in laboratory tests of a cathode or anode, lithium-metal is commonly used as a counter electrode. The Li/electrolyte/LiMn2 O4 cells tested in this study contained small amounts of the cathode material, at the level of 2–6 mg. The counter electrode was lithium
Fig. 4. Discharge capacity of the LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li cell at various discharging rates.
foil of ca. 0.75 mm in thickness and surface area of ca. 1 cm2 . The mass and capacity of the lithium-metal counter electrode was much higher in comparison to the tested LiMn2 O4 electrode. Consequently, after gradual degradation of lithium during the cell cycling, its capacity was still much higher than that characteristic of the tested one. The possible passivation of both the lithium counter electrode and the LiMn2 O4 cathode were investigated by impedance spectroscopy. Fig. 5 illustrates the impedance of the symmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 |Li cell as a function of time under open circuit conditions. The initial impedance of ca. 102 increases with time, to reach a value of ca. 104 after 1 month of storage. This indicates corrosion of metallic lithium in contact with the electrolyte with the formation of a highly resistive coating. In the presence of 10 wt.% of VC the impedance increase is almost two orders of magnitude lower (Fig. 6). During galvanostatic charging/discharging of the Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC |Li cell the protective coating of low resistance is efficiently formed which is reflected in impedance plots (Fig. 7). Fig. 8 shows evolution of the impedance of the symmetrical, fully charged (delithiated LiMn2 O4 ) (a) LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 |LiMn2 O4 and (b) LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|LiMn2 O4 systems, recorded just after their assembling and as a function of time. It can be seen that the impedance of the charged cathode change with time, indicating that the cathode may also be covered with SEI. This is not unexpected, as cathodes may form SEI of different structure in comparison to that formed at anodes [15]. The electrochemical process of the lithium intercalation into the cathode material includes the necessary step of Li+ migration through the SEI layer
Fig. 6. Impedance plots for the symmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li cell kept under open circuit conditions as a function of time.
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Fig. 7. Impedance plots for the symmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li system recorded after galvanostatic charging/discharging with current of 1 mA.
[15]. However, impedance plots suggest that the LiMn2 O4 cathode does not react with the electrolyte under study with the formation of a resistive film. Figs. 9 and 10 show impedance evolution in unsymmetrical Li|0.4 M LiNTf2 in MePrPipNTf2 |LiMn2 O4 and Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|LiMn2 O4 systems, respectively. In this case a dramatic difference can be seen between the cell without the SEI forming VC (Fig. 9) and that containing VC (Fig. 10). In the absence of VC in the electrolyte, the evolution of cell impedance from ca. 500 (at low frequencies) to 100 k after 33 days was observed. This indicates strong passivation of the Li/IL interface. However, the system containing 10 wt.% of VC does not show this effect (Fig. 10), which suggests formation of a coating on the lithium-metal surface, protecting it from the further corrosion. Fig. 11 shows the impedance plot for the Li|0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|LiMn2 O4 systems recorded after the 25th charging/discharging cycle. The plot consists of two semicircles and was analyzed according to an equivalent circuit consisting of the electrolyte bulk resistance, Rs , in series with two sub-circuits, both represented by resistance R parallel to capacity C. Determined
Fig. 9. Impedance plots for the unsymmetrical LiMn2 O4 |0.4 M LiNTf2 MePrPipNTf2 |Li cell as a function of time.
in
Fig. 10. Impedance plot for the unsymmetrical LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li cell, kept under open circuit conditions, as a function of time.
Fig. 8. Impedance plots for the symmetrical cells (a) LiMn2 O4 |0.4 M in MePrPipNTf2 |LiMn2 O4 and (b) LiMn2 O4 |0.4 M LiNTf2 in LiNTf2 MePrPipNTf2 + 10%VC|LiMn2 O4 , kept under open circuit conditions, as a function of time. Cathodes (LiMn2 O4 ) were in the charged (deintercalated) state.
Fig. 11. Impedance plot for the unsymmetrical LiMn2 O4 |0.4 M LiNTf2 in MePrPipNTf2 + 10%VC|Li system recorded after 25th charge/discharge cycles.
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equivalent series resistance Rs and charge transfer resistance Rct at the cathode (the second semicircle) were 34 and 252 , respectively. The LiMn2 O4 cathode does not require the presence of the SEI forming additive, while the anodes, both metallic lithium (laboratory test cells) and lithiated graphite (the lithium-ion battery) work properly only in the presence of the SEI forming additive [11,14]. The Li//LiCoO2 system with a room temperature ionic liquid as an electrolyte (RTIL based on N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide) was studied by impedance spectroscopy [16]. The impedance plots consisted of two semicircles, which were assigned to the anode/electrolyte interface (the high frequency semicircle) and to the cathode/electrolyte interface (the low frequency semicircle). The LiMn2 O4 and LiCoO2 cathodes were investigated in another ionic liquid (RTIL based on N,N,N-trimethyl-Nhexylammonium bis(trifluoromethanesulfonyl)imide) [3,17]. The Li/LiMO2 impedance plots also consisted of two semicircles. Lithium-ion transfer through the electrode/electrolyte interface was assumed to be the rate controlling step of the lithium intercalation into the LiMn2 O4 electrode. Measurements with the LiMn2 O4 cathode were performed at different temperatures and the charge transfer resistance values, Rct , obtained from the impedance spectra deconvolution, were plotted as log Rct = f(T−1 ), to give the activation energy for the charge transfer process (0.57 eV) [3]. It has been concluded that the high cathode/electrolyte resistance and the large charge transfer resistance were probably associated with the poor wettability of the electrode. 3.4. Electrolyte flammability In lithium-ion cells containing classical electrolytes the solvent may start to evaporate and consequently, the vapour may ignite. Therefore, for safety reasons, it is of practical significance to look for new non-volatile electrolytes. Many ionic liquids, due to the strong ion–ion interactions, are characterised by negligible vapour pressure at room temperature. On the other hand, the SEI forming additives, such as VC, are volatile organic compounds and it is necessary to estimate the flash point of such mixtures. The flash point of the LiNTf2 –MePrPipNTf2 –VC(10%) electrolyte was estimated to be above 300 ◦ C. Lithium concentration in the ionic liquid is at the level of ca. 0.4 M, while the VC concentration is only 0.27 M, which suggests that the entire amount of volatile VC may be bound to solvation shells of Li+ , MePrPip+ or NTf2 − ions. The effect of a high salt concentration in a molecular solvent on reduced flammability of the electrolyte may be found in the available literature. The following battery: C/LiPF6 –MePrPipNTf2 (50 wt.%)-EC-DMC-EMC/LiCoO2
was examined from the point of view of its performance and the electrolyte non-flammability [13]. Flammability was examined by soaking a glass filter with the electrolyte and exposing it to a flame for 10 s. High contents of the MePrPipNTf2 salt (at least 40 wt.%) resulted in the non-flammability of the system. This suggests that the dissolution of a salt, liquid or solid at room temperature, may lead to a kind of a solvent-in-salt solution, of a decreased vapour pressure of molecular components and hence, practically to nonflammability of the system. 4. Conclusions 1. The LiMn2 O4 cathode shows good cyclability and Coulombic efficiency working with the LiNTf2 –MePrPipNTf2 room temperature ionic liquid as an electrolyte, in the presence of 10 wt.% of vinylene carbonate as an additive to the ionic liquid. 2. The flash point of the LiNTf2 –MePrPipNTf2 –VC(10%) electrolyte was estimated to be above 300 ◦ C. Acknowledgement The support of the DS31-178/09 grant is gratefully acknowledged. References [1] A. Weber, G.E. Blomgren, in: W. van Schalkwijk, B. Scrosati (Eds.), Advances in Lithium Ion Batteries, Kluwer, 2002, p. 185. [2] A. Fernicola, B. Scrosati, H. Ohno, Ionics 12 (2006) 95. [3] H. Zheng, H. Zhang, Y. Fu, T. Abe, Z. Ogumi, J. Phys. Chem. B 109 (2005) 13676. [4] H. Zheng, B. Li, Y. Fu, T. Abe, Z. Ogumi, Electrochim. Acta 52 (2006) 1556. [5] V. Borgel, E. Markevich, D. Aurbach, G. Semrau, M. Schmidt, J. Power Sources 189 (2009) 331. [6] H. Sakaebe, H. Matsumoto, Electrochem. Commun. 5 (2003) 594. [7] H. Sakaebe, H. Matsumoto, K. Tatsumi, J. Power Sources 146 (2005) 693. [8] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources 160 (2006) 1308. [9] J. Xu, J. Yang, Y. NuLi, J. Wang, Z. Zhang, J. Power Sources 160 (2006) 621. [10] L.X. Yuan, J.K. Feng, X.P. Ai, Y.L. Cao, S.L. Chen, H.X. Yang, Electrochem. Commun. 8 (2006) 610. [11] A. Lewandowski, A. Swiderska-Mocek, J. Power Sources 171 (2007) 938. [12] V. Baranchugov, E. Markevich, E. Pollak, G. Salitra, D. Aurbach, Electrochem. Commun. 9 (2007) 796. [13] H. Nakagawa, Y. Fujino, S. Kozono, Y. Katayama, T. Nukuda, H. Sakaebe, H. Matsumoto, K. Tatsumi, J. Power Sources 174 (2007) 1021. [14] A. Lewandowski, A. Swiderska-Mocek, J. Power Sources 194 (2009) 502. [15] D. Aurbach, in: W. van Schalkwijk, B. Scrosati (Eds.), Advances in Lithium Ion Batteries, Kluwer, 2002, p. 7. [16] S. Seki, Y. Kobayashi, H. Miyashiro, Y. Ohno, Y. Mita, A. Usami, N. Terada, M. Watanabe, Electrochem. Solid State 8 (2005) A577. [17] H. Zheng, J. Qin, Y. Zhao, T. Abe, Z. Ogumi, Solid State Ionics 176 (2005) 2219.