Article pubs.acs.org/IECR
Functionalized Ionic Liquids Based on Trialkylimidazolium Cations with Alkoxymethyl Group at the N‑1 Position: Synthesis, Characterization, and Application as Electrolytes for a Lithium Ion Battery Yide Jin,† Shaohua Fang,*,† Jianhao Zhang,† Zhengxi Zhang,† Kaiqiang Yu,† Jianzhi Song,† Li Yang,*,†,‡ and Shin-ichi Hirano‡ †
School of Chemistry and Chemical Technology, Shanghai Jiaotong University, Shanghai 200240, China Hirano Institute for Materials Innovation, Shanghai Jiaotong University, Shanghai 200240, China
‡
S Supporting Information *
ABSTRACT: Twenty-four new functionalized ionic liquids (ILs) based on trialkylimidazolium cations with the alkoxymethyl group at the N-1 position were synthesized and characterized. Physicochemical properties of these ILs, such as melting point, thermal stability, density, viscosity, conductivity, and electrochemical stability, were studied systematically. Twenty-one ILs appeared in the liquid state at room temperature, and 14 ILs showed a melting point lower than −60 °C. Introduction of the alkoxymethyl group at the N-1 position of trialkylimidazolium cations could not be more beneficial than that of the alkoxyethyl group for reducing viscosity. Li/LiFePO4 cells employing three trialkylimidazolium ILs with methoxymethyl-group-based electrolytes showed good discharge capacity and cycle stability at a current rate of 0.1 C. This is the first report of ILs with the alkoxymethyl group used as electrolytes without an additive for a lithium ion battery.
1. INTRODUCTION Molten salts with melting points close to or lower than ambient temperature are defined as ionic liquids (ILs). The unique properties of ILs, such as low vapor pressure, nonflammability, good thermal stability, great chemical and electrochemical stability, and high ionic conductivity, have draw great attention from researchers during the past decade.1−3 ILs have been used as new media for organic reactions,4−7 industrial and biological catalysis,8−10 and separation processes.11−13 In addition, ILs are widely applied as a “novel solvent” in different electrochemical devices, including lithium ion batteries,14−18 electrochemical capacitors,19,20 fuel cells,21,22 and dye-sensitized solar cells.23−25 In various kinds of ILs, a lot of emphasis has been placed on 1,3-dialkylimidazolium ILs as a result of easy synthesis, low viscosity, and high conductivity.26−29 However, low cathodic stability of 1,3-dialkylimidazolium ILs (about 1 V versus Li/Li+) resulting from acidic proton at the C-2 position has restricted their applications as electrolytes in lithium ion batteries.30−32 When the acidic proton is replaced by alkyl group at the C-2 position, 1,2,3-trialkylimidazolium ILs can be obtained with improvement of electrochemical stability, which are more appropriate electrolytes for lithium ion batteries.33−35 Nowadays, ether-functionalization is a prominent topic in the IL research field. Unlike some groups with electron-withdrawing action, such as nitrile and ester groups, introducing ether groups with electron-donating action into IL cations can reduce melting point and viscosity without resulting in apparent degradation of electrochemical stability.30,36−39 Up to now, almost all kinds of IL cations with one ether group have been reported,40−47 and researchers also have an interest in ILs with two or more ether groups. For instance, Pernak et al. have first © 2014 American Chemical Society
reported several symmetrical 1,3-dialkoxymethylimidazolium ILs with viscosities higher than 80 cP at room temperature.48 Two identical 2-ethoxyethyl or 4-methoxybenzyl groups are introduced into quaternary ammonium cations, and thermal properties of these IL are investigated.49 Han et al. have also reported some quaternary ammonium ILs with two, three, or four identical 2-methoxyethyl groups and studied physicochemical properties of these ILs.50 Our group has synthesized guanidinium and cyclic quaternary ammonium ILs with two identical or different ether groups and quaternary ammonium ILs with three or four different ether groups.51−53 When the reported physicochemical properties of etherfunctionalized ILs are analyzed, it can be found that the structure of the ether group affects physicochemical properties, especially thermal stability and viscosity. Generally, alkoxymethyl (1OR) groups or alkoxyethyl (2OR) groups are selected and linked to nitrogen or phosphonium atom of the cation. Introduction of ether groups in IL cations is negative for thermal stability, and the 1OR group can reduce thermal stability more obviously than the 2OR group.43,47,48,54 The effect of ether groups on viscosity is complicated. For quaternary ammonium, phosphonium, pyrrolidinium, and piperidinium ILs, it is certain that the 1OR group is more beneficial than the 2OR group for reducing viscosity.42,43,47 For instance, the viscosity of P-methoxymethyl-P,P,P-triethylphosphonium bis(trifluoromethylsulfonyl)imide (P222(1O1)Received: Revised: Accepted: Published: 2860
July 16, 2013 November 24, 2013 January 24, 2014 January 24, 2014 dx.doi.org/10.1021/ie4022804 | Ind. Eng. Chem. Res. 2014, 53, 2860−2871
Industrial & Engineering Chemistry Research
Article
Figure 1. Structures of the functionalized ILs based on trialkylimidazolium cations with alkoxymethyl group at the N-1 position.
TFSA, 35 cP) is lower than that of P-ethoxymethyl-P,P,Ptriethylphosphonium TFSA (P222(2O1)-TFSA, 44 cP) at 25 °C, and the viscosity of N-methoxymethyl-N,N,N-triethylammonium TFSA (N222(1O1)-TFSA, 69 cP) is lower than that of N-ethoxymethyl-N,N,N-triethylammonium TFSA (N222(2O1)-TFSA, 85 cP) at 25 °C.43 However, for 1,3dialkylimidazolium ILs, according to published data,45,55 the effect of the 1OR and 2OR groups on viscosity seems to be contradictory. Zhou et al. have reported 1,3-dialkylimidazolium ILs with 1OR group had higher viscosities than ILs with 2OR group; for example, the viscosity of 1-methoxymethyl-3methylimidazolium perfluoromethyltrifluoroborate (IM1(1O1)-CF3BF3, 55 cP) is higher than that of 1-ethoxymethyl3-methylimidazolium perfluoromethyltrifluoroborate (IM1(2O1)-CF3BF3, 43 cP) at 25 °C.45 But Fei et al. have reported 1,3 dialkylimidazolium ILs with 1OR group had viscosities lower than that with 2OR group; for example, the viscosities of 1-methoxymethyl-3-methylimidazolium TFSA (IM1(1O1)TFSA, 49.9 cP) and 1-ethoxymethyl-3-methylimidazolium TFSA (IM1(1O2)-TFSA, 42.0 cP) are lower than that of 1methoxyethyl-3-methylimidazolium TFSA (IM1(2O1)-TFSA, 58.5 cP) at 21 °C.55 Thus, it is necessary and meaningful to further identify the effect of 1OR and 2OR groups on viscosity of imidazolium ILs.
According to published data of electrochemical window for ether-functionalized phosphonium, quaternary ammonium, pyrrolidinium, and piperidinium ILs,42,43,47,56 it can be found that electrochemical stability of ILs with the 1OR group are close to homologous ILs with the 2OR group. To date, various ILs with one or more 2OR groups can be used as electrolytes in lithium ion battery, such as N,N-diethyl-N-methyl-N-methoxyethylammonium TFSA (DEME-TFSA, N122(2O1)-TFSA), P,P,P-triethyl-P-methoxyethylphophonium TFSA (P222(2O1)-TFSA), N,N,N,N-tetramethyl-N-methyl-N-methoxyethylguanidinium TFSA (1g1(2O1)-TFSA), N,N-dimethoxyethylpyrrolidinium TFSA (Py(2O1)(2O1)-TFSA) and N,Ndimethoxyethylpiperidinium TFSA (PP(2O1)(2O1)TFSA).43,51,52,57 Nevertheless, no IL with the 1OR group has been reported as an electrolyte for lithium ion batteries, though its electrochemical stability indicates the possibility of application. Recently, our group focused on ether-functionalized trialkylimidazolium ILs, and has reported two series of trialkylimidazolium ILs with the 2OR group at the N-1 position or the 1OR group at the C-2 position.58,59 In this paper, we continued to prepare 24 new ILs based on trialkylimidazolium cations with the 1OR group at the N-1 position. The structures and abbreviations of these ILs are illustrated in Figure 1. Arabic numbers denote alkyl groups, and 2861
dx.doi.org/10.1021/ie4022804 | Ind. Eng. Chem. Res. 2014, 53, 2860−2871
Industrial & Engineering Chemistry Research
Article
Table 1. Physical and Thermal Properties of Functionalized ILs Based on Trialkylimidazolium Cations with Alkoxymethyl Group at the N-1 Position series
ionic liquids
MWa (g mol−1)
Tmb (°C)
dc (g cm−3, ±5%)
ηd (mPa s, ±5%)
σe (mS cm−1, ±5%)
Tdf (°C)
IM(1O1)1R
IM(1O1)11-TFSA IM(1O1)12-TFSA IM(1O1)13-TFSA IM(1O1)14-TFSA IM(1O1)1(2O1)-TFSA IM(1O1)1(2O2)-TFSA IM(1O1)21-TFSA IM(1O1)22-TFSA IM(1O1)23-TFSA IM(1O1)24-TFSA IM(1O1)2(2O1)-TFSA IM(1O1)2(2O2)-TFSA IM(1O2)11-TFSA IM(1O2)12-TFSA IM(1O2)13-TFSA IM(1O2)14-TFSA IM(1O2)1(2O1)-TFSA IM(1O2)1(2O2)-TFSA IM(1O2)21-TFSA IM(1O2)22-TFSA IM(1O2)23-TFSA IM(1O2)24-TFSA IM(1O2)2(2O1)-TFSA IM(1O2)2(2O2)-TFSA
421.4 435.4 449.4 463.4 465.4 479.4 435.4 449.4 463.4 477.4 479.4 493.4 435.4 449.4 463.4 477.4 479.4 493.4 449.4 463.4 477.4 491.4 493.4 507.4
22 36 21
