Functionalized ionic liquids based on quaternary ammonium cations with three or four ether groups as new electrolytes for lithium battery

Electrochimica Acta 56 (2011) 4663–4671

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Functionalized ionic liquids based on quaternary ammonium cations with three or four ether groups as new electrolytes for lithium battery Shaohua Fang a,b , Yide Jin a , Li Yang a,b,∗ , Shin-ichi Hirano b , Kazuhiro Tachibana c , Shingo Katayama d a

School of Chemistry and Chemical Technology, Shanghai Jiaotong University, Shanghai 200240, China Hirano Institute for Materials Innovation, Shanghai Jiaotong University, Shanghai 200240, China Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University, Yamagata 992-8510, Japan d Shoei Chemical Inc., Tokyo 198-0025, Japan b c

a r t i c l e

i n f o

Article history: Received 22 November 2010 Received in revised form 27 February 2011 Accepted 28 February 2011 Available online 4 March 2011 Keywords: Lithium battery Ionic liquid Electrolyte Functionalized cation

a b s t r a c t New functionalized ILs based on quaternary ammonium cations with three or four ether groups and TFSI− anion were synthesized and characterized. Physical and electrochemical properties, including melting point, thermal stability, viscosity, conductivity and electrochemical stability were investigated for these ILs. Five ILs with lower viscosity in these ILs were applied in lithium battery as new electrolytes. Behavior of lithium redox and charge–discharge characteristics of lithium battery were investigated for these IL electrolytes with 0.6 mol kg−1 LiTFSI. Lithium plating and striping on Ni electrode could be observed in these IL electrolytes. Li/LiFePO4 cells using these IL electrolytes without additives had good capacity and cycle property at the current rate of 0.1 C, and the N(2o1)3 (2o2)TFSI and N2(2o1)3 TFSI electrolytes owned better rate property. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction During the past decade, ionic liquids (ILs) have attracted great interest of researchers due to their extraordinary properties, including nonvolatility, nonflammability, good thermal stability, good electrochemical stability and high ionic conductivity [1,2]. Basing on these properties, ILs have showed potential as safe electrolytes for high-energy-density lithium battery system, which uses lithium metal anode with high theoretical capacity (more than 3860 mAh g−1 ) [3–6]. In different kinds of ILs which have been used as electrolytes in lithium battery, quaternary ammonium and cyclic quaternary ammonium ILs have been investigated intensively, because of better electrochemical stability (low cathodic limiting potential) [3,4,7–20]. At present, functionalized IL is a very noticeable topic in the field of IL research. Introducing different functional groups into cations, can markedly change the physicochemical properties of ILs, and provide more choices for applications of ILs [21,22]. Compared with other functional groups, one short ether group can help to reduce the viscosities and melting points of ILs, and not result in obvious degradation of electrochemical stability of ILs [23–28]. Many

∗ Corresponding author at: School of Chemistry and Chemical Technology, Shanghai Jiaotong University, No. 800, Dongchuan Road, Shanghai 200240, China. Tel.: +86 21 54748917; fax: +86 21 54741297. E-mail address: [email protected] (L. Yang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.02.107

researchers have paid more attention to the functionalized ILs with ether group, and one ether group has been introduced into imidazolium cations [29–34], quaternary ammonium cations [23–27,35], piperidinium cations [26,28], pyrrolidinium caions [28], morpholinium cations [28], oxazolidinium cations [28], guanidinium cations [36], sulfonium cations [37] and quaternary phosphonium cations [27]. And some ILs with one ether group have been selected as new electrolytes for different electrochemical devices. N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) and triethyl(2methoxyethyl) phosphonium bis(trifluoromethanesulfonyl)imide (P222(2o1)-TFSI) electrolytes have been reported to own good cycle performance, when they are used in lithium battery at low rate without additives [38–42]. Electric double layer capacitor using DEME-BF4 electrolyte exhibits excellent cycle performance even at temperature over 100 ◦ C [43]. 3-[2-(2-methoxyethoxy)ethyl]1-methyl imidazolium iodide (MEOII) have been applied in dye-sensitized solar cells [44]. In contrast to the ILs with one ether group, researches on the ILs with two or more ether groups are quite rare. A series of novel imidazolium ILs based on cations with two identical ether groups have been developed, but the viscosities of these ILs are higher than 80 mPa s at 25 ◦ C due to long chains of ether groups [45]. Kärnä and co-workers have synthesized several quaternary ammonium ILs based on cations with two identical ether groups (2-ethoxyethyl group or 4-methoxybenzyl group), and the thermal properties of these ILs have been investigated [46].

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O

O

O

O N O

R1

N

TFSI O

TFSI Abbreviation

Abbreviation

N2(2o1)3TFSI R1=CH2CH3 N3(2o1)3TFSI R1=CH2CH2CH3 N4(2o1)3TFSI R1=CH2CH2CH2CH3 R1=CH2CH2CH2CH2CH3 N5(2o1)3TFSI N(2o1)4TFSI R1=CH2CH2OCH3 R1=CH2CH2OCH2CH3 N(2o1)3(2o2)TFSI N(2o1)3(3o1)TFSI R1=CH2CH2CH2OCH3

R2

R2=CH2CH3 R2=CH2CH2CH3

N2(2o1)2(2o2)TFSI N3(2o1)2(2o2)TFSI

N4(2o1)2(2o2)TFSI R2=CH2CH2CH2CH3 R2=CH2CH2CH2CH2CH3 N5(2o1)2(2o2)TFSI N(2o1)2(2o2)2TFSI R2=CH2CH2OCH2CH3 R2=CH2CH2CH2OCH3 N(2o1)2(2o2)(3o1)TFSI

Fig. 1. Structures of functionalized quaternary ammonium ILs with three or four ether groups.

In order to find more ILs with low viscosity and good electrochemical stability for the applications in lithium battery as electrolytes, we tried to synthesize new ILs based on quaternary ammonium cations with three or four ether groups. When our research work was in progress, six quaternary ammonium ILs with three or four identical ether groups (2-methoxyethyl group) in cations had been reported recently, and their properties have been investigated [47]. In this paper, more quaternary ammonium ILs with three or four ether groups were prepared by a facile method, and different ether groups (2-methoxyethyl group, 2-ethoxyethyl group or 3-methoxypropyl) were introduced into quaternary ammonium cations. The structures of these ILs are shown in Fig. 1. Except N2(2o1)3 TFSI and N(2o1)4 TFSI, the other eleven ILs were reported for the first time. We investigated melting point, thermal stability, viscosity, conductivity and electrochemical window of these ILs. Five ILs with lower viscosity were applied in lithium battery as new electrolytes. Behavior of lithium redox on Ni electrode was investigated for these IL electrolytes with 0.6 mol kg−1 LiTFSI, and charge–discharge characteristics of Li/LiFePO4 cells were examined. 2. Experimental 2.1. Reagents and materials Commercially available reagents were purchased from Sinopharm Chemical Reagent Corporation Ltd., or Alfa Aesar. Lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) was kindly provided by Morita Chemical Industries Corporation Ltd. These reagents were analytical grade and used as received. 2.2. Preparation of tertiary amines containing three ether groups 2.2.1. N,N,N-tri-(2-methoxyethyl)amine (N(2o1)3 ) Bis(2-methoxyethyl)amine (50.0 g, 375 mmol) reacted with chloroethyl methyl ether (18.0 g, 190 mmol) at 140 ◦ C for 48 h in a 100 mL autoclave. The solid salt formed was filtered off, and the residue was distilled under reduced pressure using a 25 cm vigreuxcolumne. The product was collected at 102–103 ◦ C (boiling point) when the pressure was about 2 Pa. Colorless liquid; 1 H NMR: ı (ppm) 3.45–3.41 (t, 6H), 3.30 (s, 9H), 2.75–2.72 (t, 6H). 2.2.2. N,N-di-(2-methoxyethyl)-N-2-ethoxyethylamine (N(2o1)2 (2o2)) Bis(2-methoxyethyl)amine (48.0 g, 360 mmol) reacted with chloroethyl ethyl ether (19.7 g, 181 mmol) at 145 ◦ C for 48 h in a 100 mL autoclave. The solid salt formed was filtered off, and the

residue was distilled under reduced pressure using a 25 cm vigreuxcolumne. The product was collected at 125–126 ◦ C (boiling point) when the pressure was about 2 Pa. Colorless liquid; 1 H NMR: ı (ppm) 3.50–3.42 (m, 8H), 3.31 (s, 6H), 2.77–2.73 (t, 6H), 1.18–1.14 (t, 3H). 2.3. Preparation of ILs based on quaternary ammonium cations with three or four ether groups 2.3.1. N-ethyl-N,N,N-tri-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (N2(2o1)3 TFSI) N(2o1)3 (9.1 g, 48 mmol) reacted with bromoethane (11.2 g, 104 mmol) at 60 ◦ C for 24 h in a 50 mL autoclave with methanol (10 mL) as solvent. The produced bromide was acquired after washing with ether. The bromide was recrystallized twice from acetone and THF, and dried under high vacuum at 60 ◦ C. The bromide and LiTFSI was dissolved in deionized water and mixed for 24 h at ambient temperature. The crude IL was dissolved with dichloromethane, and washed with deionized water until no residual halide anions in the deionized water used to rinse the IL could be detected by AgNO3 . The dichloromethane was removed by rotating evaporation. The product was dried under high vacuum for more than 24 h at 105 ◦ C. Colorless liquid; 1 H NMR: ı (ppm) 3.75–3.73 (m, 6H), 3.62–3.60 (t, 6H), 3.57–3.52 (m, 2H), 3.34 (s, 9H), 1.35–1.32 (t, 3H); 13 C NMR: ı (ppm) 124.86–115.37, 65.91, 59.66, 59.17, 59.13, 57.27, 8.14. 2.3.2. N-propyl-N,N,N-tri-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (N3(2o1)3 TFSI) N(2o1)3 (10.0 g, 52 mmol) reacted with 1-bromopropane (10.0 g, 81 mmol) at 80 ◦ C for 24 h in a 50 mL autoclave with methanol (10 mL) as solvent. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.73–3.71 (m, 6H), 3.62-3.59 (t, 6H), 3.36–3.30 (m, 11H), 1.77–1.67 (m, 2H), 0.96–0.92 (t, 3H); 13 C NMR: ı (ppm) 124.87–115.32, 65.86, 63.19, 60.10, 59.09, 15.94, 10.32. 2.3.3. N-butyl-N,N,N-tri-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (N4(2o1)3 TFSI) A mixture of N(2o1)3 (10.0 g, 52 mmol), 1-bromobutane (14.0 g, 102 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.74–3.72 (m, 6H), 3.63–3.61 (t, 6H), 3.40–3.32 (m, 11H), 1.71–1.64 (m, 2H), 1.39–1.30 (m, 2H), 0.98–0.94 (t, 3H); 13 C NMR: ı (ppm) 124.95–115.29, 65.90, 61.82, 60.09, 59.16, 24.14, 19.53, 13.49.

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2.3.4. N-amyl-N,N,N-tri-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (N5(2o1)3 TFSI) A mixture of N(2o1)3 (10.0 g, 52 mmol), 1-bromopentane (15.0 g, 99 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.73–3.72 (m, 6H), 3.64–3.62 (t, 6H), 3.39–3.34 (m, 11H), 1.73–1.66 (m, 2H), 1.38–1.24 (m, 4H), 0.93–0.89 (t, 3H); 13 C NMR: ı (ppm) 124.89–115.27, 65.90, 61.92, 60.01, 59.05, 28.24, 22.04, 21.90, 13.81. 2.3.5. N,N,N,N-quart-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (N (2o1)4 TFSI) A mixture of N(2o1)3 (10.0 g, 52 mmol), bromoethyl methyl ether (7.2 g, 52 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.76–3.74 (m, 8H), 3.71–3.69 (t, 8H), 3.33 (s, 12H); 13 C NMR: ı (ppm) 124.88–115.29, 66.02, 61.13, 59.01. 2.3.6. N-2-ethoxyethyl-N,N,N-tri-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (N(2o1)3 (2o2)TFSI) A mixture of N(2o1)3 (10.0 g, 52 mmol), 2-bromoethyl ethyl ether (8.0 g, 52 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.78–3.68 (m, 16H), 3.51–3.46 (t, 2H), 3.33 (s, 9H), 1.18–1.15 (t, 3H); 13 C NMR: ı (ppm) 124.95–115.30, 67.06, 66.05, 63.92, 61.18, 59.11, 14.86. 2.3.7. N-3-methoxypropylN,N,N-tri-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (N(2o1)3 (3o1)TFSI) A mixture of N(2o1)3 (10.0 g, 52 mmol), 3-bromopropyl methyl ether (8.0 g, 52 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.65–3.67 (m, 6H), 3.58–3.56 (t, 6H), 3.44–3.40 (m, 2H), 3.34–3.32 (t, 2H), 3.24 (s, 9H), 3.20 (s, 3H), 1.93–1.86 (m, 2H); 13 C NMR: ı (ppm) 124.82–115.45, 68.73, 65.85, 60.24, 59.38, 59.03, 58.63, 22.77. 2.3.8. N-ethyl-N,N-di-(2-methoxyethyl)N-2-ethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N2(2o1)2 (2o2)TFSI) N(2o1)2 (2o2) (10.0 g, 49 mmol) reacted with bromoethane (11.0 g, 100 mmol) at 65 ◦ C for 48 h in a 50 mL autoclave with methanol (10 mL) as solvent. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.77-3.74 (m, 6H), 3.63–3.60 (t, 6H), 3.57–3.46 (m, 4H), 3.34 (s, 6H), 1.36–1.32 (t, 3H), 1.18–1.15 (t, 3H); 13 C NMR: ı (ppm) 124.82–115.22, 67.00, 66.85, 63.74, 59.56, 59.06, 57.18, 14.74, 8.01.

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2.3.10. N-butyl-N,N-di-(2-methoxyethyl)N-2-ethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N4(2o1)2 (2o2)TFSI) A mixture of N(2o1)2 (2o2) (10.0 g, 49 mmol), 1-bromobutane (14.0 g, 102 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The produced bromide was acquired after washing with ether. The bromide was recrystallized twice from ethyl acetate, and dried under high vacuum at 60 ◦ C. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.76–3.72 (m, 6H), 3.63–3.60 (m, 6H), 3.51–3.46 (m, 2H), 3.40–3.36 (m, 2H), 3.33 (s, 6H), 1.72–1.64 (m, 2H), 1.38–1.29 (m, 2H), 1.18–1.14 (t, 3H), 0.97–0.93 (t, 3H). 2.3.11. N-amyl-N,N-di-(2-methoxyethyl)N-2-ethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N5(2o1)2 (2o2)TFSI) A mixture of N(2o1)2 (2o2) (10.0 g, 49 mmol), 1-bromopentane (15.0 g, 99 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.77–3.72 (m, 6H), 3.63–3.60 (m, 6H), 3.51–3.46 (m, 2H), 3.39–3.31 (m, 8H), 1.74–1.66 (m, 2H), 1.38–1.25 (m, 4H), 1.18–1.15 (t, 3H), 0.92–0.89 (t, 3H); 13 C NMR: ı (ppm) 124.93–115.29, 67.01, 65.87, 63.81, 61.98, 60.03, 59.93, 58.96, 28.19, 22.05, 21.88, 14.86, 13.77. 2.3.12. N,N-di-(2-methoxyethyl)N,N-di-(2-ethoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (N(2o1)2 (2o2)2 TFSI) A mixture of N(2o1)2 (2o2) (10.0 g, 49 mmol), 2-bromoethyl ethyl ether (9.0 g, 59 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.75–3.71 (m, 8H), 3.69–3.66 (m, 8H), 3.48–3.43 (m, 4H), 3.31 (s, 6H), 1.16–1.12 (t, 6H); 13 C NMR: ı (ppm) 124.99–115.34, 66.97, 65.97, 63.90, 61.11, 59.03, 14.78. 2.3.13. N-3-methoxypropyl-N,N-di-(2-methoxyethyl)-N-2ethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N(2o1)2 (2o2)(3o1)TFSI) A mixture of N(2o1)2 (2o2) (10.0 g, 49 mmol), 3-bromopropyl methyl ether (9.0 g, 59 mmol) and methanol (10 mL) in a 250 mL flask was refluxed at 85 ◦ C for more than 72 h under an N2 atmosphere. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.74–3.71 (m, 6H), 3.62–3.59 (m, 6H), 3.51–3.44 (m, 4H), 3.40–3.38 (t, 2H), 3.31 (s, 6H), 3.28 (s, 3H), 1.96–1.91 (m, 2H), 1.16–1.12 (t, 3H); 13 C NMR: ı (ppm) 124.85–115.26, 68.71, 67.12, 65.90, 63.79, 60.26, 59.48, 59.11, 58.76, 22.94, 14.86. 2.4. Measurement The structure of synthesized IL was confirmed by 1 H NMR and 13 C NMR (Avance III 400), and chloroform-d was used as the solvent.

2.3.9. N-propyl-N,N-di-(2-methoxyethyl)N-2-ethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N3(2o1)2 (2o2)TFSI) N(2o1)2 (2o2) (10.0 g, 49 mmol) reacted with 1-bromopropane (13.0 g, 106 mmol) at 80 ◦ C for 72 h in a 50 mL autoclave with methanol (10 mL) as solvent. The other procedures were identical with N2(2o1)3 TFSI. Colorless liquid; 1 H NMR: ı (ppm) 3.77–2.72 (m, 6H), 3.64–3.62 (t, 6H), 3.52–3.46 (m, 2H), 3.36–3.32 (m, 8H), 1.77–1.68 (m, 2H), 1.19–1.15 (t, 3H), 0.97–0.93 (t, 3H); 13 C NMR: ı (ppm) 124.77–115.26, 67.05, 65.87, 63.79, 63.18, 60.10, 60.00, 59.09, 15.88, 14.78, 10.14.

The water content of the dried IL was detected by a moisture titrator (Metrohm 73KF coulometer) basing on Karl–Fischer method, and the value was less than 50 ppm. Calorimetric measurement of IL was performed by using a differential scanning calorimeter (DSC, Perkin-Elmer Pyris 1) in the temperature range −60 ◦ C to a predetermined temperature. Each sample with an average weight of 4–6 mg was sealed in aluminum pan in a dry chamber, and then heated and cooled at scan rate of 10 ◦ C min−1 . The thermal data were collected during heating in the second heating-cooling scan. The thermal stability was measured with TGA (Perkin-Elmer, 7 series thermal analysis system). Each

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sample with an average weight of 4–6 mg was placed in the platinum pan, and heated at 10 ◦ C min−1 from room temperature to 600 ◦ C under nitrogen. The viscosity value was measured by using viscometer (DV-III ULTRA, Brookfield Engineering Laboratories, Inc.). The density was determined by measuring the weight of prepared IL (1.0 mL) in a dry chamber at 25 ◦ C. The ionic conductivity was measured by using DDS-11A conductivity meter in a dry chamber. The electrochemical stability was investigated by linear sweep voltammogram measurement, which was performed by using CHI 660D electrochemical working station in an argon-filled UNILAB glove box ([O2 ] < 10 ppm, [H2 O] < 10 ppm). Glassy carbon (3 mm diameter) was used as the working electrode. Platinum wire and silver wire were used as the counter and reference electrodes respectively. The glassy carbon electrode was polished with alumina paste (d = 0.1 ␮m). And the polished electrode was washed with deionized water and dried under vacuum. 0.6 mol kg−1 of LiTFSI was added to the dried IL to prepare IL electrolyte, and this procedure was carried out in the glove box. The plating and stripping behaviors of lithium in the IL electrolytes were examined by using cyclic voltammogram (CV) method in the glove box. The nickel disk (2 mm diameter) was used as the working electrode. Platinum wire and silver wire were used as the counter and reference electrodes respectively. The Ni electrode was polished with alumina paste (d = 0.1 ␮m). And the polished electrode was washed with deionized water and dried under vacuum. The cyclic voltammogram was performed by CHI 660D electrochemistry workstation at room temperature (25 ◦ C). Coin cell was used to evaluate the performances of IL electrolyte for the application in lithium battery. Lithium metal was used as anode. And cathode was fabricated by spreading the mixture of LiFePO4 , acetylene black and PVDF (initially dissolved in N-methyl2-pyrrolidone) with a weight ratio of 8:1:1 onto Al current collector (battery use). Loading of active material was about ca. 1.5 mg cm−2 and this thinner electrode was directly used without pressing. The separator was glass filter made of borosilicate glass (GF/A from Whattman). Cell construction was carried out in the glove box, and all the components of cell were dried under vacuum before placed into the glove box. The cells were sealed and then stayed at room temperature for 4 h before performance test. The cell performances were examined by the galvanostatic charge–discharge (C–D) cycling test using a CT2001A cell test instrument (LAND Electronic Co., Ltd.) at room temperature (25 ◦ C). Current rate was determined by using the nominal capacity of 170 mAh g−1 for Li/LiFePO4 cell. Charge included two processes: (1) constant current at a rate (cut-off voltage of 4.0 V) (2) constant voltage at 4.0 V (1 h), and discharge had one process: constant current (cut-off voltage of 2.0 V).

Tm

Tc

(e)

Tm

endo

Tc

(d)

Tm Tc

(c)

Tm

Tc

(b)

exo (a) -60

-50

-40

-30

-20

-10

0

10

20

30

40

o

Temperature/ C Fig. 2. DSC curves of (a) N2(2o1)3 TFSI, (b) N3(2o1)3 TFSI, (c) N(2o1)4 TFSI, (d) N (2o1)3 (2o2)TFSI and (e) N(2o1)2 (2o2)2 TFSI.

of freedom of cation and anion [26,28,48,49]. Introducing one short ether group into imidazolium [34], quaternary and cyclic quaternary ammonium [25,28], quaternary phosphonium [27] and guanidinium cations [36], has been proved to be helpful to reduce the lattice energy of ILs and result in low melting points of ILs, due to weakening electrostatic interaction between the cation and anion (which resulted from the electron donation action of ether group), reducing symmetry of cations, and high flexibility of ether group. When three or four ether groups were incorporated into quaternary ammonium cations, some ILs with low melting points could also be acquired. As shown in Table 1, the melting points of nine ILs were lower than −60 ◦ C, and all the thirteen ILs were liquid at room temperature. The cation of N(2o1)4 TFSI owned good symmetry, and its melting point was higher. It was interesting that N3(2o1)3 TFSI, N(2o1)3 (2o2)TFSI and N(2o1)2 (2o2)2 TFSI also had high melting points, though their cations owned low symmetry. The thermal stabilities of the prepared ILs were examined by variable-temperature TGA experiments. Like the five ILs, which were shown in Fig. 3 as examples, all the thirteen ILs had onestage decomposition behavior. According to Table 1, the thermal decomposition temperatures (Td ) of the ILs with three ether groups were slightly higher the ILs with four ether groups except N(2o1)3 (3o1)TFSI and N(2o1)2 (2o2)(3o1)TFSI, and the thermal stabilities of N(2o1)3 (3o1)TFSI and N(2o1)2 (2o2)(3o1)TFSI were close to the ILs with three ether groups. Compared with the published

100 90 80

3.1. Properties of these ILs The phase transitions of these ILs were investigated by differential scanning calorimetry (DSC), and the DSC curves of five ILs are shown in Fig. 2 as examples. N3(2o1)3 TFSI, N(2o1)4 TFSI, N(2o1)3 (2o2)TFSI and N(2o1)2 (2o2)2 TFSI owned one crystallization transition (Tc ) before melting transition (Tm ). Like N2(2o1)3 TFSI (Fig. 2a), the other eight ILs did not show any phase transition behaviors until −60 ◦ C, which was the inferior temperature limit of DSC measurement. As organic molecular compound, the melting point of IL was determined by the strength of its crystal lattice, which could be controlled by three main factors: symmetries of cation and anion, interactions between cation and anion, and conformational degrees

Weight loss/%

3. Results and discussion

70

N2(2o1)3TFSI

60

N3(2o1)3TFSI

50

N4(2o1)3TFSI N(2o1)4TFSI

40

N(2o1)3(2o2)TFSI

30 20 10 0

0

100

200

300

400

500

600

o

Temperature/ C Fig. 3. TGA traces of N2(2o1)3 TFSI, N3(2o1)3 TFSI, N4(2o1)3 TFSI, N(2o1)4 TFSI and N (2o1)3 (2o2)TFSI.

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Table 1 Physical and thermal properties of these ILs. ILs

Mwa (g mol−1 )

N2(2o1)3 TFSI N3(2o1)3 TFSI N4(2o1)3 TFSI N5(2o1)3 TFSI N(2o1)4 TFSI N(2o1)3 (2o2)TFSI N(2o1)3 (3o1)TFSI N2(2o1)2 (2o2)TFSI N3(2o1)2 (2o2)TFSI N4(2o1)2 (2o2)TFSI N5(2o1)2 (2o2)TFSI N(2o1)2 (2o2)2 TFSI N(2o1)2 (2o2)(3o1)TFSI

500.48 514.50 528.53 542.56 530.50 544.53 544.53 514.50 528.53 542.56 556.58 558.55 558.55

c d e f g h

<−60 11.6 <−60 <−60 16.6 18.4 <−60 <−60 <−60 <−60 <−60 2.9 <−60

dc (g cm−3 )

Cd (mol dm−3 )

e (mPa s)

 f (mS cm−1 )

g (S cm2 mol−1 )

Td h (◦ C)

1.36 1.31 1.31 1.31 1.36 1.31 1.32 1.36 1.30 1.29 1.24 1.33 1.31

2.72 2.55 2.48 2.41 2.56 2.41 2.42 2.64 2.46 2.38 2.23 2.38 2.35

85 101 116 127 104 90 117 76 93 107 110 87 111

1.38 1.08 0.88 0.77 1.04 1.10 1.00 1.13 1.20 0.84 0.79 1.10 0.89

0.51 0.42 0.35 0.32 0.41 0.46 0.41 0.43 0.49 0.35 0.35 0.46 0.38

331.5 325.7 323.2 331.7 315.7 312.2 324.0 327.0 328.7 329.5 326.3 315.2 329.2

Molecular weight. Melting point. Density at 25 ◦ C. Concentration at 25 ◦ C. Viscosity at 25 ◦ C. Conductivity at 25 ◦ C. Molar conductivity at 25 ◦ C. Decomposition temperature of 10% weight loss.

data [25,47], the Td of these quaternary ammonium ILs with three or four ether groups were lower than the quaternary ammonium ILs with one ether gourp or two ether gourps. In other words, the Td decreased with the increasing of the number of ether group in cations of quaternary ammonium ILs. Furthermore, some research results have shown that the variable-temperature TGA experiment might overestimate the thermal decomposition temperature of IL or IL electrolyte [50,51], and the more detailed behaviors of thermal decomposition can be investigated for these new functionalized ILs by the isothermal TGA experiment in the future. Replacing one alkyl group in the cations by one flexible ether group with the similar size and formula weigh, could cause the reducing of viscosity, because of weakening electrostatic interaction between the cation and anion which resulted from the electron donation action of ether group [25–28,36,52]. When three or four ether groups were introduced into cations of quaternary ammonium ILs, Van der Waals interactions between cations and anions increased due to the obvious increasing of cation sizes, and it should counteract the electron donation action of three or four ether groups. Though the viscosities of the ILs with three or four ether groups were higher than some quaternary ammonium ILs based on small cations with or without one group [23,25,27,47], the viscosities of five ILs were still lower than 100 mPa s at 25 ◦ C, and the viscosity of N2(2o1)2 (2o2)TFSI was 76 mPa s at 25 ◦ C. In order to discuss viscosity property distinctly, these ILs were classified to NR(2o1)3 series (N2(2o1)3 TFSI, N3(2o1)3 TFSI, N4(2o1)3 TFSI, N5(2o1)3 TFSI, N(2o1)4 TFSI, N(2o1)3 (2o2)TFSI and N(2o1)3 (3o1)TFSI) and NR(2o1)2 (2o2) series (N2(2o1)2 (2o2)TFSI, N3(2o1)2 (2o2)TFSI, N4(2o1)2 (2o2)TFSI, N5(2o1)2 (2o2)TFSI, N(2o1)3 (2o2)TFSI, N(2o1)2 (2o2)2 TFSI and N(2o1)2 (2o2)(3o1)TFSI). Fig. 4 shows the changing of viscosity with alkyl or ether group in cations at room temperature for the NR(2o1)3 series and NR(2o1)2 (2o2) series. It could be found that the viscosity increased with the increasing of the alkyl group size. If the alkyl group (butyl group or amyl group) was replaced by the ether group with similar size (2-methoxyethyl group or 2-ethoxyethyl group), the viscosities of ILs could reduce obviously, and the viscosities of the ILs with 2-ethoxyethyl group were lower than the ILs with 2-methoxyethyl group. The viscosities of the ILs with 3-methoxypropyl group were very close to the ILs with amyl group. Furthermore, the viscosities of the ILs in the NR(2o1)3 series were higher than the ILs in the NR(2o1)2 (2o2) series with the same alkyl or ether group in cations, although the former had smaller cation sizes.

The conductivity of IL had been regarded as an important property for application of IL as electrolyte in different electrochemical devices, and it could be mainly governed by the viscosity, density and formula weight of IL [28,53]. The low viscosity IL usually displayed high conductivity, and substituting one alkyl group in the cation by one flexible ether group with the similar size and formula weight, could also improve the conductivity of IL [25,27,28,34,36,52]. As the viscosity property, the conductivities of these ILs with three or four ether groups were lower than some quaternary ammonium ILs based on small cations with or without one group, because of stronger Van der Waals interactions between cations and anions after introducing three or four ether groups into cations. The temperature dependence of viscosity was investigated for N2(2o1)3 TFSI, N3(2o1)3 TFSI, N4(2o1)3 TFSI, N(2o1)4 TFSI and

160 140 120

η/mPa s

a b

Tm b (◦ C)

100 80 60

NR(2o1)3 Series NR(2o1)2(2o2) Series

40 20 0

C2

C3

C4

C2OC

C5

C2OC2 C3OC

Alkyl or ether group in cations Fig. 4. Changing of viscosity with alkyl or ether group in cations of these functionalized ILs at room temperature. (C2 = CH2 CH3 , C3 = CH2 CH2 CH3 , C4 = CH2 CH2 CH2 CH3 , C5 = CH2 CH2 CH2 CH2 CH3 , C2OC = CH2 CH2 OCH3 , C2OC2 = CH2 CH2 OCH2 CH3 and C3OC = CH2 CH2 CH2 OCH3 .).

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S. Fang et al. / Electrochimica Acta 56 (2011) 4663–4671

5.0

3

4.5

2

4.0 -2

1

j/mA cm

ln(η/mPa s)

a

3.5

N2(2o1)3TFSI N3(2o1)3TFSI

3.0

-1

N4(2o1)3TFSI 2.5

N2(2o1)3TFSI

N(2o1)4TFSI 6.0

6.5

7.0

7.5

8.0

8.5

9.0

N3(2o1)3TFSI

-2

N(2o1)3(2o2)TFSI 2.0

0

9.5 10.0 10.5

-3 -4

-1

1000/(T-T0)/K

-3

-2

-1

0

1

2

3

+

E vs (Ag/Ag )/V

b

2.5

Fig. 6. Linear sweep voltammograms of N2(2o1)3 TFSI and N3(2o1)3 TFSI at 25 ◦ C. Working electrode: glassy carbon; counter electrode: platinum wire; reference electrode: silver wire; scan rate: 10 mV s−1 .

-1

ln(σ/ms cm )

2.0 1.5 1.0

N2(2o1)3TFSI N3(2o1)3TFSI

0.5

N4(2o1)3TFSI N(2o1)4TFSI

0.0

N(2o1)3(2o2)TFSI 7

8

9

10

11

12

-1

1000/(T-T0)/K

Fig. 5. VTF plots of (a) viscosity and (b) conductivity for five functionalized ILs.

N(2o1)3 (2o2)TFSI over the temperature range 25–80 ◦ C, and the  = 0 exp



B T − T0



(1)

VTF plots of viscosity according to Eq. (1) [54,55] were shown in Fig. 5(a). The temperature dependence of viscosity suited the VTF model very well. Similarly, the temperature dependence of conductivity was also investigated for the five ILs over the temperature range 25–80 ◦ C, and the VTF plots of conductivity according to Eq. (2) [54] were shown in Fig. 5(b)  = 0 exp

 −B  T − T0

(2)

Like the viscosity, the temperature dependence of conductivity suited the VTF model very well. 3.2. Electrochemical windows of these ILs and lithium redox in five IL electrolytes If ILs owned wide electrochemical windows, they would have a promise to be used as new electrolytes in high-energy devices, including lithium battery and electrochemical capacitor. The electrochemical stabilities of the thirteen ILs were investigated by linear sweep voltammogram (LSV). The LSV curves of two ILs at 25 ◦ C were shown as examples in Fig. 6. The cathodic limiting potentials of N2(2o1)3 TFSI and N2(2o1)3 TFSI were about −2.6 V and −2.8 V versus Ag/Ag+ , and their anodic limiting potentials were about +1.8 V versus Ag/Ag+ . So their electrochemical windows were about 4.4 V and 4.6 V, respectively. All the results of LSV measurements were listed in Table 2. The cathodic limiting potentials,

anodic limiting potentials and electrochemical windows of these ILs did not show distinct relationships with their cation structures, and the values changed in narrow range. N4(2o1)3 TFSI had the widest electrochemical window (4.8 V) in these ILs. As already reported, introducing one ether group into the cations of quaternary ammonium and cyclic quaternary ammonium ILs could reduce the electrochemical stability [25,28]. It could also be found that the electrochemical windows of these ILs with three or four ether groups were lower than the quaternary ammonium ILs without ether group [56]. Although the electrochemical stabilities of these ILs with three or four ether groups were not as good as the quaternary ammonium ILs without ether group, their electrochemical windows were still higher than 4.0 V, and the values of five ILs were higher than 4.5 V. So these new functionalized ILs could belong to the ILs with good electrochemical stability. Five ILs with lower viscosity (N2(2o1)2 (2o2)TFSI, N3(2o1)2 (2o2)TFSI, N(2o1)2 (2o2)2 TFSI, N(2o1)3 (2o2)TFSI and N2(2o1)3 TFSI), were chosen to dissolve 0.6 mol kg−1 of LiTFSI as the IL electrolytes for lithium battery. The lithium redox in these IL electrolytes was examined by cyclic voltammogram (CV) method at 25 ◦ C. The CV curves are shown in Fig. 7(a)–(e), and the plating and stripping of lithium on Ni electrode can be clearly observed. In the first cycle for the N2(2o1)2 (2o2)TFSI electrolyte, the plating of lithium was at about −2.83 V versus Ag/Ag+ , and the anodic peak at about −2.30 V versus Ag/Ag+ in the returning scan corresponded to the stripping of lithium. The lithium redox in this electrolyte might be caused by the generation of a certain surface film (SEI) on the Ni electrode. The peak currents of the lithium redox decreased slightly with the cycle number, and it suggested that the SEI film changed so that the lithium redox was restrained. The cathodic peak at about −2.34 V versus Ag/Ag+ could be found in the first cycle. This cathodic peak might be assigned to the electrochemical reduction of the electrolyte, and it could be presumed that this reduction might generate the SEI film on Ni electrode. In the second and third cycles the current of this peak decreased, and it could mean that SEI film generating in the first cycle also restrained the reduction of the electrolyte. Some differences of lithium redox behaviors on the Ni electrode were found in Fig. 7(a)–(e) for the five IL electrolytes. The peak currents of the lithium redox were different, and how the anodic peaks of lithium changed with the cycle number was also different. For example, for the N(2o1)3 (2o2)TFSI electrolyte the peak currents of the lithium redox in the first cycle were smallest in the five IL electrolytes, and the peak currents of the lithium redox in the second

S. Fang et al. / Electrochimica Acta 56 (2011) 4663–4671

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Table 2 Electrochemical windows of these ILs. ILs

Cathodic limiting potential E versus (Ag/Ag+ )/V

Anodic limiting potential E versus (Ag/Ag+ )/V

Electrochemical window (V)

N2 (2o1)3 TFSI N3 (2o1)3 TFSI N4 (2o1)3 TFSI N5(2o1)3 TFSI N (2o1)4 TFSI N(2o1)3 (2o2)TFSI N(2o1)3 (3o1)TFSI N2(2o1)2 (2o2)TFSI N3(2o1)2 (2o2)TFSI N4(2o1)2 (2o2)TFSI N5(2o1)2 (2o2)TFSI N(2o1)2 (2o2)2 TFSI N(2o1)2 (2o2)(3o1)TFSI

−2.6 −2.8 −2.8 −2.6 −2.4 −2.6 −2.5 −2.6 −2.5 −2.7 −2.6 −2.8 −2.5

+1.8 +1.8 +2.0 +1.7 +2.3 +2.0 +1.8 +1.9 +1.7 +1.7 +1.7 +1.8 +1.9

4.4 4.6 4.8 4.3 4.7 4.6 4.3 4.5 4.2 4.4 4.3 4.6 4.4

Working electrode: glassy carbon; counter electrode: platinum wire; reference electrode: silver wire; scan rate: 10 mV s−1 ; cut-off current density: 0.1 mA cm−2 .

a

2.0 1.5

Cycle 1 Cycle 2 Cycle 3

j/mA cm-2

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

-3.0

-2.5

-2.0

-1.5

-1.0

+

-0.5

E vs (Ag/Ag )/V

b

c

1.5

-2

0.5

j/mA cm

-2

0.5 0.0

0.0

-0.5

-0.5

-1.0

-1.0

-1.5

-3.0

-2.5

-2.0

-1.5

-1.0

Cycle 1 Cycle 2 Cycle 3

1.0

Cycle 1 Cycle 2 Cycle 3

1.0

j/mA cm

1.5

-1.5

-0.5

-3.0

e

0.8

0.2

j/mA cm

-2

-1.0

0.0 -0.2

0.0 -0.2 -0.4

-0.6

-0.6 -2.5

-2.0

-1.5 +

E vs (Ag/Ag )/V

-1.0

-0.5

Cycle 1 Cycle 2 Cycle 3

0.2

-0.4

-3.0

-0.5

0.8

0.4 -2

Cycle 1 Cycle 2 Cycle 3

0.4

j/mA cm

-1.5

0.6

0.6

-0.8

-2.0

E vs (Ag/Ag )/V

E vs (Ag/Ag )/V

d

-2.5

+

+

-0.8

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

+

E vs (Ag/Ag )/V

Fig. 7. Cyclic voltammograms for these IL electrolytes with 0.6 mol kg−1 LiTFSI at 25 ◦ C (−3.2 V to −0.2 V versus Ag/Ag+ ): (a) N2(2o1)2 (2o2)TFSI electrolyte; (b) N3(2o1)2 (2o2)TFSI electrolyte; (c) N(2o1)2 (2o2)2 TFSI electrolyte; (d) N(2o1)3 (2o2)TFSI electrolyte; (e) N2(2o1)3 TFSI electrolyte. Working electrode: Ni; counter electrode: platinum wire; reference electrode: silver wire; scan rate: 10 mV s−1 .

S. Fang et al. / Electrochimica Acta 56 (2011) 4663–4671

160

120

140

110

Coulombic efficiency/%

Discharge capacity/mAh g

-1

4670

120 100 N2(2o1)2(2o2)TFSI electrolyte

80

N3(2o1)2(2o2)TFSI electrolyte N(2o1)2(2o2)2TFSI electrolyte

60

N(2o1)3(2o2)TFSI electrolyte

20

90 80

N2(2o1)2(2o2)TFSI electrolyte

70

N3(2o1)2(2o2)TFSI electrolyte

40

0

10

20

30

40

50

60

70

N(2o1)2(2o2)2TFSI electrolyte

60

N(2o1)3(2o2)TFSI electrolyte N2(2o1)3TFSI electrolyte

50

N2(2o1)3TFSI electrolyte

40

100

80

90

100

0

10

20

30

70

80

90

100

1.0

0.8

0.6

0.4

0.2

N2(2o1)2(2o2)TFSI electrolyte N3(2o1)2(2o2)TFSI electrolyte N(2o1)2(2o2)2TFSI electrolyte N(2o1)3(2o2)TFSI electrolyte N2(2o1)3TFSI electrolyte

3.3. Charge–discharge characteristics of Li/LiFePO4 cells

0.0 0.0

0.5

1.0

1.5

Discharge rate/C Fig. 10. Rate properties of Li/LiFePO4 cells using the IL electrolytes. Charge current rate is 0.1 C, and discharge capacity is normalized on the basis of discharge capacity at 0.1 C rate, which is the value after the cycle performance of cell reaches stability.

the rate of 0.1 C, and the discharge capacity at the rate of 1.5 C was about 61 mAh g−1 , which retained 45% of the capacity at the rate of 0.1 C. Basing on the normalized capacity at the discharge rates of 1.0 C and 1.5 C, the rate property of the N(2o1)2 (2o2)2 TFSI elec4.0

3.5

Cell potential/V

The C–D characteristics of Li/LiFePO4 cells using IL electrolytes without additives were examined at 0.1 C rate. Fig. 8 shows the discharge capacity during cycling of Li/LiFePO4 cells using the five IL electrolytes. The initial discharge capacity of the cell using the N2(2o1)2 (2o2)TFSI electrolyte was about 99 mAh g−1 , and the discharge capacity increased gradually with the cycle number, which could result from the improved wettability of the IL electrolyte to the LiFePO4 cathode during the C–D processes. The discharge capacity was stable after 30 cycles, and the value retained about 134 mAh g−1 until the 100th cycle. The changing of discharge capacity with the cycle number for the other four IL electrolytes was similar to the N2(2o1)2 (2o2)TFSI electrolyte. The discharge capacity of the N(2o1)2 (2o2)2 TFSI electrolyte after reaching stability (about 136 mAh g−1 ) was close to the N2(2o1)2 (2o2)TFSI electrolyte, and higher than the N3(2o1)2 (2o2)TFSI, N(2o1)3 (2o2)TFSI and N2(2o1)3 TFSI electrolytes (about 130, 126 and 127 mAh g−1 , respectively). Fig. 9 shows the cycle number dependence of coulombic efficiencies of Li/LiFePO4 cells at 0.1 C rate. The coulombic efficiencies of the five IL electrolytes were higher than 96% after the initial several cycles, and the coulombic efficiencies of the N3(2o1)2 (2o2)TFSI, N(2o1)3 (2o2)TFSI and N2(2o1)3 TFSI electrolytes were a little higher than the N2(2o1)2 (2o2)TFSI and N(2o1)2 (2o2)2 TFSI electrolytes after the 30th C–D cycle. The rate properties of Li/LiFePO4 cells are shown in Fig. 10, and the discharge capacity is normalized on the basis of discharge capacity at 0.1 C rate, which is the value after the cycle performance of cell reach stability. The discharge capacity decreased with the increasing of the discharge rate. As shown in Fig. 11, the discharge capacity for the N2(2o1)2 (2o2)TFSI electrolyte at the discharge rate of 1.0 C was about 84 mAh g−1 , which retained 63% of the capacity at

60

Fig. 9. The cycle number dependences of coulombic efficiency of Li/LiFePO4 cells using the IL electrolytes. Charge–discharge current rate is 0.1 C.

Normalized discharge capacity

and third cycles almost disappeared, which indicated that the SEI film on Ni electrode restrained the lithium redox intensively. Furthermore, one cathodic peak in the range from −2.0 V to −1.25 V versus Ag/Ag+ or one cathodic peak in the range from −0.75 V to −1.5 V versus Ag/Ag+ could be found in the first cycle for the five IL electrolyte, which might be caused by the reactions of the trace water or oxygen in IL electrolyte on the Ni electrode, and these peaks disappeared in the second and third cycles due to the SEI film forming in the first cycle. This kind of cathodic peak caused by the trace impurities, could also be found in the reported experimental results of CV for some other IL electrolytes [4,7,26,57].

50

Cycle number

Cycle number Fig. 8. Discharge capacity during cycling of Li/LiFePO4 cells using the IL electrolytes. Charge–discharge current rate is 0.1 C.

40

3.0

2.5

2.0

1.5C 0

20

40

60

1.0C 80

Capacity/mAh g

0.5C 0.2C 0.1C 100

120

140

-1

Fig. 11. Discharge curves of Li/LiFePO4 cells using N2(2o1)2 (2o2)TFSI electrolyte. Charge current rate is 0.1 C, and discharge current rate is indicated in this figure.

S. Fang et al. / Electrochimica Acta 56 (2011) 4663–4671

trolyte was unideal, and the rate properties of the N(2o1)3 (2o2)TFSI and N2(2o1)3 TFSI electrolytes were better in the five IL electrolytes. For the differences of viscosity among the five ILs were not obvious, it was possible that the rate property was affected primarily by some other factors besides the viscosity of IL, such as the interfacial characteristics at both the LiFePO4 cathode/electrolyte and lithium metal anode/electrolyte interfaces. 4. Conclusions New functionalized ILs based on quaternary ammonium cations with three or four ether groups were synthesized by a facile method. Physical and electrochemical properties, including melting point, thermal stability, viscosity, conductivity and electrochemical window, were investigated for these ILs. All these ILs were liquid at room temperature, and the viscosity of N2(2o1)2 (2o2)TFSI was 76 mPa s at 25 ◦ C. Good electrochemical stabilities of these ILs permitted them to become potential electrolytes used in electrochemical devices. Behavior of lithium redox on Ni electrode was investigated for five IL electrolytes with 0.6 mol kg−1 LiTFSI, and the lithium plating and striping on Ni electrode could be observed in these IL electrolytes without additives due to the forming of SEI film. Li/LiFePO4 cells using the five IL electrolytes without additive showed good discharge capacity and stable cycle property at 0.1 C current rate, and the N(2o1)3 (2o2)TFSI and N2(2o1)3 TFSI electrolytes owned better rate property. Acknowledgements The authors thank the research center of analysis and measurement of Shanghai JiaoTong University for the help in the NMR characterization. This work was financially supported by the National Key Project of China for Basic Research under Grant No. 2006CB202600, the National High Technology Research and Development Program of China under Grant No. 2007AA03Z222, and Shoei Chemical Inc. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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