Synthesis and electrochemical properties of new dicationic ionic liquids

Accepted Manuscript Synthesis and electrochemical properties of new dicationic ionic liquids

Xinyi Mei, Zheng Yue, Qiang Ma, Hamza Dunya, Braja K. Mandal PII: DOI: Reference:

S0167-7322(18)33684-5 doi:10.1016/j.molliq.2018.10.085 MOLLIQ 9830

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

18 July 2018 28 September 2018 16 October 2018

Please cite this article as: Xinyi Mei, Zheng Yue, Qiang Ma, Hamza Dunya, Braja K. Mandal , Synthesis and electrochemical properties of new dicationic ionic liquids. Molliq (2018), doi:10.1016/j.molliq.2018.10.085

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ACCEPTED MANUSCRIPT Synthesis and Electrochemical Properties of New Dicationic Ionic Liquids

Xinyi Mei, Zheng Yue, Qiang Ma, Hamza Dunya, Braja K. Mandal*

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Department of Chemistry, Illinois Institute of Technology, Chicago, IL 60616, United States. *

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[email protected] (B.K. Mandal).

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Corresponding author. Tel.: +1 312 5673446; fax: +1 312 5673494. E-mail address:

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Keywords: Ionic liquids; Dicationic ionic liquids; Rechargeable lithium batteries; Liquid

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electrolytes; Ionic conductivity; Electrochemical stability.

ACCEPTED MANUSCRIPT ABSTRACT Three classes of dicationic ionic liquids (DILs) were synthesized, in which the cationic species [Nmethyl pyrrolidinium (MPy), N-butyl imidazolium (BIm) or diethylsulfide (DES)] were separated with

oligoether

moieties.

The

counter

anions

are

either

dicyanamide

(DCA)

or

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C-NMR analysis. The DILs were investigated for their

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identified by FT-IR， 1H-NMR and

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bis(trifluoromethane sulfonyl)imide (TFSI). The molecular structures of these compounds were

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potential as electrolyte solvents. Thermal behaviors were also studied by thermogravimetric

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analysis (TGA) and differential scanning calorimetry (DSC) showing excellent thermal stability up to 150 °C for all DILs. Their physicochemical properties were comprehensively characterized

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regarding their densities, melting points, viscosities, and miscibility with polar and nonpolar solvents. To compensate high viscosity limitation of DILs, two low viscosity ionic liquids

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(thinners), [MImEt] [DCA] and [MImEt] [TFSI] were used. To further improve the lithium ion

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conduction ability of the electrolyte systems, a lithium salt (LiTFSI) was added. Then several

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ternary electrolyte systems, LiTFSI in DILs/thinner (the weight ratio of DILs/thinner was fixed equal to 1/1) were developed, and their ionic conductivity and electrochemical stability were

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analyzed.

ACCEPTED MANUSCRIPT 1. Introduction Over the last several years ionic liquids (ILs) have been widely promoted as more reliable class of “green solvents” compared with organic carbonates, attributing to their unique properties, such as thermal stability [1], high electrical conductivity [2], high polarity [3], and negligible vapor

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pressures [4], coupled with a wide liquid range. ILs are a class of designable organic compounds

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which display two characteristic features: first, they consist only of ions which are poorly

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coordinated. Second, they are mostly liquids below 100℃ [5]. If the ILs are liquid at room temperature, they are called room-temperature ionic liquids (RTILs) [6]. Through combining

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organic cations with suitable anions allows tailoring the appropriate physical, chemical and

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biological properties for ILs, while some even possess unexpected functions resulting from synergetic collaboration between the two components. Hence, the attractive flexibility or

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‘tunability’ in the design have driven phenomenal interest in ILs synthesis [7]. The unique structure

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and performance of ILs as a platform not only offers additional breaks to modify these ionic materials’ physical properties (e.g. melting point, density, polarity, viscosity, hydrophobicity/

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hydrophilicity, solubility) for specific applications, but also offers other gorgeous chemical

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features, such as fundamental ionic conductivity, tremendous thermal, chemical, and electrochemical stability [8].

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By either a physical combination of ILs or chemical modification (covalent functionalization or ion-exchange metathesis process of the ionic constituents, specific functional groups can be easily incorporated into the ILs skeleton), various IL-containing composite materials and functional ILs have been effective realized and applied in several areas. Notable efforts with ILs have also been made to design safe and environmental friendly solvents [9], because conventional organic solvents are often toxic, flammable and volatile. Compared with the volatile organic

ACCEPTED MANUSCRIPT compounds, ILs display excellent dissolution performance for organic, inorganic, and polymer materials, and their immeasurable vapor pressure [10] and non-flammability properties provide them the ability to avoid atmospheric pollution, because there would be no loss of solvent through evaporation. The thermal decomposition temperatures higher than 300 °C could enhance their

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recycling efficiency [11]. Accordingly, ILs can be applied to replace traditional volatile organic

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reactions [12], extraction, catalysis, and separation processes [8].

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solvents for a host of practices linked to green chemistry and clean technology, such as organic

ILs possess a wide range of viscosities ranging from 20 and 40,000 cp vs. viscosities of typical

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organic solvents which range from 0.2 and 100 cp. ILs exhibit excellent electrochemical stability

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in a wide electrochemical window of 2~ 5 V [13]. Most ILs are also nonflammable. Thus, ILs would be excellent candidates for prospective applications as electrolytes for lithium secondary

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batteries and other energy-related applications, such as fuel cells, supercapacitors [14], and dye-

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sensitized solar cells [15–17].

Dicationic ionic liquids (DILs) consist of an anion and a doubly charged cation, which is

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composed of two singly charged cations as head groups linked by a variable length of alkyl or

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oligo ethylene glycol (OEG) chain as a rigid or flexible spacer. Thus, the DILs can offer the opportunity to investigate (a) the influence of cation and anion variation, and (b) the influence of

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the chain length. Recently, most researchers have focused on mono cationic-type ILs, although the number of DILs described in the literature is increasing [15]. Anderson et al. presented the synthesis and characterization of a variety of DILs containing hydrocarbon spacers [17]. Kubisa and Biedron investigated DILs obtained by functionalization of OEG with triphenylphosphine [18]. Ohno and co-workers have shown that dicationic OEG-based molten salts bearing imidazolium cations, which displayed good ion conducting properties [19]. Dicationic and

ACCEPTED MANUSCRIPT polycationic ILs linked by alkyl chains, published by Armstrong and coworkers [17], displayed extraordinarily higher chemical and thermal stability than their mono-cationic analogs [20]. ILs should have the capability to dissolve more lithium salts in order to have a higher Li+ conductivity, if ILs are intended to be applied in Li-ion batteries. Thus, by incorporating an OEG segment into

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the DILs structure, the conductivity could be enhanced primarily by improving cation transport

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through the OEG segment, although the viscosity of DILs did not decrease [21] due to the weak

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interaction of ethylene oxide segment compared to the alkyl chain [22]. However, for DILs, the relationship studies between their structure and physicochemical characteristics and molecular

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structures are, as yet, still relatively rare [23]. Therefore, it is worthwhile to explore other new

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DILs structures to gain further understanding and extend the applications of DILs as electrolyte components.

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In this paper, we report a strategic design and synthesis of TFSI & DCA-based DILs to

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address the polysulfide shuttle (PSS) effect in lithium-sulfur (Li-S) battery electrolytes. The solvent-in-salt (SIS) electrolytes displayed outstanding suppression of lithium polysulfide (Li2Sn)

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dissolution via the use of heavily saturated electrolyte solutions; however, the common electrolyte

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solvents (1:1 DME/DOL) used are extremely flammable [24]. By contrast, TFSI-based DILs have attracted great attentions due to their non-flammable and thermal stability, although, in general,

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TFSI-based ILs cannot dissolve adequate lithium salt (typically <0.5M) to form a relatively high concentration. We hope to improve the solubility of lithium salt, through the inserting different length of EO chains into the two cationic moieties. Different from TFSI-based DILs, by using DCA as the counter anion for the DILs, namely DCA-based DILs have much lower viscosity and tend to display much better ionic conductivity [25]. DCA anion is normally unusable for Li-ion

ACCEPTED MANUSCRIPT battery applications (4V electrochemical window), but can perhaps be used for Li-S batteries, as a result of the much lower operating voltage (~ 3V) [26]. On the other hand, imidazolium cation-based DILs are promising candidates to replace volatile solvents, due to their chemical and thermal stability, non-volatility, high ionic

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conductivity, large electrochemical window and good solvent behavior [27]. Pyrrolidinium-based

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ILs show a better compatibility toward the lithium anode [28]. Lots of publications have been

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devoted to imidazolium-based, and pyrrolidinium-based DILs and the other DILs, such as sulfonium-based DILs have seldom been discussed. Sulfonium cation-based ILs could show

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higher cell efficiencies, because of lower charge transfer resistance [29, 30]. Additionally, the EO

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units in the cation of the ILs effectively provide relatively higher ion mobility, which may contribute to high ionic conductivity. The TFSI and DCA anions can facilitate stable

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charge/discharge properties of the Li-S cells.

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2. Experimental 2.1 Materials.

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Diethylene glycol (99%), triethylene glycol (99%), tetraethylene glycol (99%), pentaethylene

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glycol (98%), 1,4-dioxane (99.8%), thionyl chloride (99%), pyridine (99.8%), thiourea (99%), 1methylpyrrolidine (98%), 1-butyl imidazolium (95%), diethyl sulfide (98%), potassium hydroxide

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(90%), 4-toluenesulfonyl chloride (99%), lithium bis(trifluoromethyl sulfonyl) imide (99.5%), sodium dicynamide (96%), Dowex® 22 were purchased from Sigma-Aldrich Chemical Co., (USA). All solvents were supplied by the Fisher Scientific Co., (USA). 2.2 General Procedures for the Preparation of MPy-based DILs. As shown in Scheme 1, the synthesis of MPy-based DILs involves three steps:

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ACCEPTED MANUSCRIPT

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Scheme 1. Synthetic Routes of MPy-based DILs 2.2.1 Synthesis of DILs [DiMPy1O] [TFSI].

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1,1'-(oxy bis (ethane-2, 1-diyl)) bis (1-methylpyrrolidin-1-ium) bis ((trifluoromethyl)

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sulfonyl) amide: [DiMPy1O] [TFSI]. Bis (chloroethyl) ether (1b, 4.06 g, 28.4 mmol) and 1methylpyrrolidine (5.13 g, 60.2 mmol) were dissolved in 10 mL dry acetonitrile and refluxed for

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4h under stirring at 80 ̊C. The mixture was cooled down to room temperature and stirred overnight. After the solvent had been removed by vacuum evaporation, the residue was washed three times with 15 ml ethyl ether. The remaining volatiles were removed under high vacuum to obtain the desired compound 1, 1'-(oxy bis (ethane-2,1-diyl)) bis(1-methylpyrrolidin-1-ium) dichloride (1d, yield: 8.05 g, 90.5%) as a brown liquid. Halide anion exchange of DILs with [TFSI]- and [DCA]- anions was performed according to the procedure reported in the literature [31]. 1d (1.85 g, 5.9 mmol) with a slight excess of LiTFSI

ACCEPTED MANUSCRIPT (3.55 g, 12.4 mmol) were dissolved in 8 ml deionized water. The solution was stirred overnight at room temperature, extracted the solution with 30 ml dichloromethane. The obtained organic layer was washed three times with saturated brine (10 ml), dried over anhydrous magnesium sulfate and active carbon. The residue was purified by column chromatography on alumina and concentrated

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under reduced pressure to give the final product [DiMPy1O] [TFSI] (yield: 3.32 g, 70.2%) as a

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brown liquid.

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IR: 3563 cm-1 (w), 2973 cm-1, 2901 (C-H, s), 2386.94 cm-1, 1931.99-1797.99 cm-1, 1633.0 cm-1 1476.73 cm-1, (S=O, s), 1351.76 cm-1 (C-N, w), 1186.75 cm-1 (C-O, s), 1055.42 cm-1(C-F, s),

NMR (300 MHz, CDCl3): δ (ppm) 3.96 (t, J=3.51 Hz, 4H), 3.85–3.51 (m, 12H), 3.04 (s,

6H), 2.19 (m, 8H).

13C

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1H

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999.41-876.99 cm-1, 789.95-740.46 cm-1, 645.75-571.02 cm-1.

NMR (300 MHz, CDCl3): δ (ppm) 119.8 (q, J=159 Hz), 69.33, 64.41,

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63.98, 61.81, 47.93, 20.84. Anal. calcd. for C18H30F12N4O9S4 (%): C, 26.93; H, 3.77; N, 6.98; S,

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15.98. Found: C, 26.83; H, 3.58; N, 6.83; S, 15.91. 2.2.2 Synthesis of DILs [DiMPy2O] [TFSI].

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1,1'-((ethane-1,2-diylbis(oxy)) bis (ethane-2,1-diyl)) bis (1-methylpyrrolidin-1-ium) bis

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((trifluoromethyl)sulfonyl) amide: [DiMPy2O] [TFSI]. An analogous procedure was used to prepare 2d starting from 2b (2.99 g, 16 mmol), 1-methylpyrrolidine (2.86 g, 33.6 mmol), to give

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the desired compound as a brown liquid (2d, yield: 5.43 g, 95.1%). The corresponding IL was prepared analogously from 2d (4.08 g, 11.4 mmol) and LiTFSI (6.49 g, 22.6 mmol) to give the final product as a brown liquid [DiMPy2O] [TFSI] (yield: 6.49 g, 67.1%).

ACCEPTED MANUSCRIPT IR: 3563 cm-1 (w), 2964 cm-1, 2902.84 (C-H, s), 2379.03 cm-1, 1931.76-1798.58 cm-1 (w), 1634.43 cm-1, 1463.43 cm-1, (S=O, s), 1352.48 cm-1 (C-N, w), 1193.83 cm-1 (C-O, s), 1054.56 cm1

(C-F, s), 953.83-877.41 cm-1, 789.69-739.92 cm-1. 1H

NMR (300 MHz, CDCl3): δ (ppm) 3.84 (t, J=3.51 Hz, 4H), 3.73–3.50 (m, 16H), 3.03 (s,

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6H), 2.17 (m, 8H). 13C NMR (300 MHz, CDCl3): δ (ppm) 119.8 (q, J=159 Hz), 70.2, 65.4, 65.0,

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63.5, 48.6, 21.1. Anal. calcd. for C20H34F12N4O10S4 (%): C, 28.37; H, 4.05; N, 6.62; S, 15.15.

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Found: C, 28.22; H, 4.00; N, 6.46; S, 15.03. 2.2.3 Synthesis of DILs [DiMPy3O] [TFSI].

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1,1'-(((oxybis(ethane-2,1-diyl)) bis(oxy)) bis(ethane-2,1-diyl)) bis(1-methylpyrrolidin-1-ium)

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bis((trifluoromethyl)sulfonyl) amide: [DiMPy3O] [TFSI]. An analogous procedure was used to prepare 3d starting 3b (4.01 g, 17.3 mmol), 1-methylpyrrolidine (3.14 g, 36.9 mmol), to give the

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desired compound as a dark brown liquid (3d, yield: 6.58 g, 94.5%).

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The corresponding IL was prepared analogously from 3d (2.01 g, 5.0 mmol) and LiTFSI (3.02 g, 10.5 mmol) to give the final product as a dark brown liquid [DiMPy3O] [TFSI], (yield:

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3.17 g, 71%).

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IR: 3563 cm-1 (w), 2973 cm-1, 2877.73 (C-H, s), 2387.01 cm-1, 1931.61-1797.99 cm-1 (w), 1673.96 cm-1, 1462.84 cm-1, (S=O, s), 1352.42 cm-1 (C-N, w), 1193.55 cm-1 (C-O, s), 1056.58 cm(C-F, s), 998.26-877.85 cm-1, 789.24-741.40 cm-1. 1H

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NMR (300 MHz, CDCl3): δ (ppm) 3.87 (t, J=3.51 Hz, 4H), 3.75–3.54 (m, 20H), 3.08(s,

6H), 2.21 (m, 8H). 13C NMR (300 MHz, CDCl3): δ (ppm) 119.8 (q, J=159 Hz), 70.63,70.16, 65.50, 64.90, 48.69, 21.2. Anal. calcd. for C22H38F12N4O11S4 (%): C, 29.66; H, 4.30; N, 6.29; S, 14.40. Found: C, 29.50; H, 4.18; N, 6.22; S, 14.29. 2.2.4 Synthesis of DILs [DiMPy4O] [TFSI].

ACCEPTED MANUSCRIPT 1,1'-(3,6,9,12-tetraoxatetradecane-1,14-diyl)

bis

(1-methylpyrrolidin-1-ium)

bis

((trifluoromethyl)sulfonyl) amide: [DiMPy4O] [TFSI]. An analogous procedure was used to prepare 4d starting 4b (3.02 g, 11 mmol), 1-methylpyrrolidine (2.00 g, 23.5 mmol), to give the desired compound as dark brown liquid (4d, yield: 4.02 g, 82.4%).

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The corresponding IL was prepared analogously from 4d (2.66 g, 6.0 mmol) and LiTFSI

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(3.58 g, 12.5 mmol) to give the final product as a dark brown liquid [DiMPy4O] [TFSI] (yield:

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3.97 g, 70.9%).

IR: 3542 cm-1 (w), 2880.44 (C-H, s), 2387.01 cm-1, 1932.03-1723.60 cm-1 (w), 1673.96 cm, 1462.71 cm-1, (S=O, s), 1352.47 cm-1 (C-N, w), 1193.89 cm-1 (C-O, s), 1056.94 cm-1 (C-F, s),

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1H

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998.59-839.90 cm-1, 789.16-740.05 cm-1.

NMR (300 MHz, CDCl3): δ (ppm) 3.87 (t, J=3.51 Hz, 4H), 3.73–3.52 (m, 24H), 3.11 (s,

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6H), 2.20 (m, 8H). 13C NMR (300 MHz, CDCl3): δ (ppm) 119.8 (q, J=159 Hz), 70.3, 70.0, 65.50,

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64.90, 48.7, 21.2. Anal. calcd. for C24H42F12N4O12S4 (%): C, 30.83; H, 4.53; N, 5.99; S, 13.72. Found: C, 30.71; H, 4.44; N, 5.82; S, 13.66.

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2.2.5 Synthesis of DILs [DiMPy1O] [DCA].

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Low viscous DCA-based RTILs have great potential as a co-solvent for lithium rechargeable battery electrolytes. However, a few research papers have been published in this area, probably

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because of the difficulty of the synthesis of DCA-based RTILs. The solubility of sodium dicyanamide (NaN(CN)2) is very low in most of the organic solvents; that leads to the difficulty in the metathesis reaction. As shown in Scheme 2, the most commonly used method is the use of silver salt as the reaction intermediate [24]. It should be noted that Tanner et al. reported a 24 h metathesis reaction usingNaN(CN)2 in acetone [32].

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Scheme 2. NaN(CN)2 metathesis via the silver salt intermediate We have developed a concise metathesis procedure, depending on the solubility difference of

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NaCl and NaN(CN)2 in methanol. At 25 ̊C the solubility of NaCl in methanol is 14.9 g/L [33]. We measured the solubility of NaN(CN)2 at room temperature by gradually adding the salt into 30 ml

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methanol, and the result is approximately 37 g/L. Therefore, the metathesis reaction can be carried

easy to separate from the system.

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out directly with NaN(CN)2 in methanol, because of the formation of NaCl precipitation, which is

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However, similar reactions are not available for other anions including TsO-, I- or Br-. Thus,

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we try to exchange the TsO- or Br- ions into Cl- ion by an ion-exchange resin (Dowex® 22, chloride form), which is a styrene-divinylbenzene cross-linked polymer beads with plenty of pending

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dimethyl ethanol benzyl ammonium chloride groups. The procedure follows the reported work

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that produces IL by ion-exchange resin [34]. Passing the solution of our ILs in methanol or MeCN through a column filled with the ion-exchange resin, the OTs, I or Br ions will be exchanged with chloride and absorbed in the resin, while Cl will flow out with the cations. Dowex® 22 (Chloride form) could exchange with anions at the efficiency of 0.6 mmol/ml. NaN(CN)2 (3.71 g, 41.7 mmol) was dissolved in 100 ml methanol and added to 1d (6.21 g, 19.8 mmol) dissolved in 20ml methanol. The mixture was stirred overnight at room temperature, removed methanol by vacuum distillation until some precipitation came out. Then the solid was

ACCEPTED MANUSCRIPT filtered and 15 ml acetone was added to the filtrate. Removed the solvent by vacuum distillation until some precipitation came out again. Repeat the foregoing procedures for several times until no precipitation came out. Then the filtrate was concentrated under reduced pressure to give the final product [DiMPy1O] [DCA] (yield: 5.22 g, 70.3%) as dark brown liquid.

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IR: 3020.64-2891.08 (C-H, s), 2367.93 cm-1, 2235.04-2133.44 cm-1(CN, m), 1659.68 -

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1461.34 cm-1, 1365.82 cm-1 (C-N, w), 1135.15 cm-1 (C-O, s), 1076.29-987.63 cm-1, 935.08-878.26

1H

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cm-1, 817.78-665.65 cm-1.

NMR (300 MHz, D2O): δ (ppm) 4.11 (t, J=3.51 Hz, 4H), 3.98–3.71 (m, 12H), 3.23 (s,

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6H), 2.34 (m, 8H). 13C NMR (300 MHz, D2O): δ (ppm) 119.9, 70.9, 65.3, 63.5, 48.7, 43.8, 21.4.

2.2.6 Synthesis of DILs [DiMPy2O] [DCA].

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Anal. calcd. for C18H30N8O (%): C, 57.73; H, 8.07; N, 29.92. Found: C, 57.62; H, 8.01; N, 29.81.

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The corresponding IL was prepared analogously from 2d (1.35, 3.8 mmol) and NaN(CN)2

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(1.00 g, 11.2mmol) to give the desired compound as a dark brown liquid [DiMPy2O] [DCA] (yield: 1.14 g, 72%).

cm-1. 1H

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, 1311.80 cm-1 (C-N, w), 1122.03 cm-1 (C-O, s), 1049.5-998.57 cm-1, 935.26-905.96 cm-1, 814.49

NMR (300 MHz, D2O): δ (ppm) 3.95 (t, J=3.51 Hz, 4H), 3.72–3.55 (m, 16H), 3.08 (s,

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IR: 3023.71-2898.27 (C-H, s), 2365 cm-1, 2238.72-2137.4 cm-1(CN, m), 1644.48-1471.8 cm-

6H), 2.20 (m, 8H). 13C NMR (300 MHz, D2O): δ (ppm) 119.9, 70.8, 69.6, 64.88, 62.9, 48.6, 43.5, 21.1. Anal. calcd. for C20H34N8O2 (%): C, 57.39; H, 8.19; N, 26.77. Found: C, 57.24; H, 8.11; N, 26.64. 2.2.7 Synthesis of DILs [DiMPy3O] [DCA].

ACCEPTED MANUSCRIPT The corresponding IL was prepared analogously from 3d (4.87, 12.1 mmol) and NaN(CN)2 (2.28 g, 25.6 mmol) to give the desired compound as a dark brown liquid [DiMPy3O] [DCA] (yield: 4.24 g, 75.7%). IR: 3020.64-2874.89 (C-H, s), 2367 cm-1, 2235.49-2135.21 cm-1(CN, m), 1708.16 -1462.47

NMR (300 MHz, D2O): δ (ppm) 3.90 (t, J=3.51 Hz, 4H), 3.75–3.51 (m, 20H), 3.04 (s,

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1H

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cm-1, 1352.87 cm-1 (C-N, w), 1116.4 cm-1 (C-O, s), 998.02-905.64 cm-1, 833.73-663.06 cm-1.

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6H), 2.15 (m, 8H). 13C NMR (300 MHz, D2O): δ (ppm) 119.9, 70.9, 69.6, 64.7, 62.9, 48.5, 43.3, 30.3, 21.1. Anal. calcd. forC22H38N8O3 (%): C, 57.12; H, 8.28; N, 24.22. Found: C, 57.01; H, 8.20;

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N, 24.11.

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2.2.8 Synthesis of DILs [DiMpy4O] [DCA].

The corresponding IL was prepared analogously from 4d (2.32 g, 5.2 mmol) and NaN(CN)2

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(0.98 g, 11.0 mmol) to give the desired compound as a dark brown liquid [DiMPy4O] [DCA]

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(yield: 1.98 g, 74.9%).

IR: 3024.97-2902.28 (C-H, s), 2370.32 cm-1, 2239.22-2138.81 cm-1(CN, m), 1707.50-

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1462.18 cm-1, 1312.43 cm-1 (C-N, w), 1111.11 cm-1 (C-O, s), 998.41-906.78 cm-1, 833.92-665.65

1H

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cm-1.

NMR (300 MHz, D2O): δ (ppm) 3.93 (t, J=3.51 Hz, 4H), 3.72–3.55 (m, 24H), 3.07 (s,

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6H), 2.180 (m, 8H). 13C NMR (300 MHz, D2O): δ (ppm) 119.9, 71.8, 69.6, 64.8, 63.0, 48.5, 43.4, 30.4, 21.2. Anal. calcd. for C24H42N8O4 (%): C, 56.90; H, 8.36; N, 22.12. Found: C, 56.78; H, 8.31; N, 22.01. 2.3 General Procedures for the Preparation of BIm-based DILs. As shown in Scheme 3, the synthesis of BIm-based DILs involves three steps.

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Scheme 3. Synthetic Routes of BIm-based DILs 2.3.1 Synthesis of DILs [DiBIm1O] [TFSI].

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3, 3’-(oxy bis (ethane-2, 1-diyl)) bis (1-butyl-1H-imidazol-3-ium) bis ((trifluoromethyl)

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sulfonyl) amide: [DiBIm1O] [TFSI]. Bis (chloroethyl) ether (1b, 4.09 g, 28.6 mmol) and 1-

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butylimidazolium (7.48 g, 60.2 mmol) were dissolved in 10 ml dry acetonitrile and refluxed 2h under stirring. The mixture was cooled down to room temperature and stirred overnight. After the solvent had been removed by vacuum evaporation, the residue was washed three times with 15 ml ethyl ether. The remaining volatiles were removed by high vacuum to give the desired compound 3,3'-(oxy bis(ethane-2,1-diyl))bis(1-butyl-1H-imidazol-3-ium) dichloride (1e, yield: 7.85 g, 87.5%) as a brown liquid.

ACCEPTED MANUSCRIPT 1e (2.44 g, 7.8 mmol) with LiTFSI (4.69 g, 16.3 mmol) were dissolved in 15 ml deionized water. The solution was stirred overnight at room temperature, and extracted the solution with 30 ml dichloromethane. The obtained organic layer was washed three times with saturated brine 10 ml, dried over anhydrous magnesium sulfate and active carbon. The residue was purified by

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product [DiBIm1O] [TFSI] (yield: 4.97 g, 79.6%) as a brown liquid.

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column chromatography on alumina and concentrated under reduced pressure to give the final

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IR: 3563 cm-1 (w), 3152.68-3117.53 cm-1 (=C-H, s), 2967.35 -2880.04 cm-1 (C-H, s), 2389.19 cm-1, 1932.66-1712.46 cm-1 (w), 1650.43 cm-1 (C=N, m), 1566.03-1464.08 cm-1, (C=C, m),

US

1351.77 cm-1 (C-N, w), 1191.84 cm-1 (C-O, s), 1056.74 cm-1(C-F, s), 949.51-845.29 cm-1, 789.84-

1H

AN

570.39 cm-1.

NMR (300 MHz, D-acetone): δ (ppm) 9.18 (s, 2H), 7.85 (d, J=1.2 Hz, 2H), 7.72 (d, J=1.2

M

Hz, 2H), 4.55 (t, J=4.5 Hz, 4H), 4.34 (t, J=7.2 Hz, 4H), 3.97 (t, J=6.9 Hz, 4H), 1.90 (m, 4H), 1.37

ED

(m, 4H), 0.92 (t, J=7.2 Hz, 6H). 13C NMR (300 MHz, D-acetone): δ (ppm) 135.5, 123.2, 122.2, 119.8 (q, J=159 Hz), 68.7, 49.5, 49.4, 31.8, 19.3, 12.7. Anal. calcd. for C22H32F12N6O9S4 (%): C,

PT

30.00; H, 3.66; N, 9.54; S, 14.56. Found: C, 29.84; H, 3.54; N, 9.47; S, 14.43.

CE

2.3.2 Synthesis of DILs [DiBIm2O] [TFSI]. 3,3'-((ethane-1,2-diylbis(oxy)) bis (ethane-2,1-diyl)) bis (1-butyl-1H-imidazol-3-ium) bis

AC

((trifluoromethyl)sulfonyl) amide: [DiBIm2O] [TFSI]. An analogous procedure was used to prepare 2e starting 2b (2.93 g, 15.6 mmol), 1-butylimidazolium (4.09 g, 32.9 mmol), to give the desired compound as a brown liquid (2e, yield: 5.01 g, 89.7%). The corresponding IL was prepared analogously from 2e (1.97 g, 5.5 mmol) and LiTFSI (3.32 g, 11.5 mmol) to give the final product as a brown liquid [DiBIm2O] [TFSI] (yield: 3.74 g, 80.3%).

ACCEPTED MANUSCRIPT IR: 3563 cm-1 (w), 3152.04-3116.51 cm-1 (=C-H, s), 2966.46 -2879.14 cm-1 (C-H, s), 2389.30 cm-1, 1932.15-1712.48 cm-1 (w), 1650.43 cm-1 (C=N, m), 1565.96-1463.53 cm-1, (C=C, m), 1352.18 cm-1 (C-N, w), 1191.82 cm-1 (C-O, s), 1056.64 cm-1(C-F, s), 929.50-834.69 cm-1, 789.62559.98 cm-1. NMR (300 MHz, CDCl3): δ (ppm) 8.7 (s, 2H), 7.46 (d, J=1.2 Hz, 2H), 7.4 (d, J=1.2 Hz,

T

1H

IP

2H), 4.34 (t, J=4.5 Hz, 4H), 4.17 (t, 6.9 Hz, 4H), 3.79 (t, J=5.1 Hz, 4H), 3.48 (t, J=3.9 Hz, 4H),

CR

1.83 (m, 4H), 1.2 (m, 4H), 0.91 (t, 6H). 13C NMR (300 MHz, CDCl3): δ (ppm) 135.5, 123.2, 122.2, 119.8 (q, J=159 Hz), 70.2, 68.6, 50.8, 49.8, 31.8, 19.2, 13.0. Anal. calcd. for C24H36F12N6O10S4

US

(%): C, 31.17; H, 3.92; N, 9.09; S, 13.87. Found: C, 31.05; H, 3.88; N, 9.01; S, 13.75.

AN

2.3.3 Synthesis of DILs [DiBIm3O] [TFSI].

3,3'-(((oxybis(ethane-2,1-diyl)) bis (oxy)) bis (ethane-2,1-diyl)) bis (1-butyl-1H-imidazol-3-

M

ium) bis((trifluoromethyl) sulfonyl) amide: [DiBIm3O] [TFSI]. An analogous procedure was

ED

used to prepare 3e starting 3b (3.78 g, 16.4 mmol), 1-butylimidazolium (4.28 g, 34.4 mmol), to give the desired compound as a brown liquid (3e, yield: 5.86 g, 89.2%).

PT

The corresponding IL was prepared analogously from 3e (2.74 g, 6.8 mmol) and LiTFSI (4.11

CE

g, 14.3 mmol) to give the final product as a brown liquid [DiBIm3O] [TFSI] (yield: 4.78g, 78.7%). IR: 3563 cm-1 (w), 3151.15-3115.88 cm-1 (=C-H, s), 2965.73 -2878.65 cm-1 (C-H, s), 2387.36

AC

cm-1, 1929.38-1714.20 cm-1 (w), 1650.43 cm-1 (C=N, m), 1565.61-1464.09 cm-1, (C=C, m), 1351.86 cm-1 (C-N, w), 1191.84 cm-1 (C-O, s), 1057.15 cm-1(C-F, s), 936.01-838.87 cm-1, 789.4570.62 cm-1. 1H

NMR (300 MHz, D-acetone): δ (ppm) 9.04 (s, 2H), 7.74 (d, J=1.2 Hz, 2H), 7.72 (d, J=1.2

Hz, 2H), 4.50 (t, J=4.2 Hz, 4H), 4.35 (t, J=6.9 Hz, 4H), 3.91 (t, J=4.2 Hz, 4H), 3.65-3.20 (t, J=3.9 Hz, 8H), 1.90 (m, 4H), 1.37 (m, 4H), 0.92 (t, J=7.2 Hz, 6H). 13C NMR (300 MHz, D-acetone): δ

ACCEPTED MANUSCRIPT (ppm) 135.5, 123.2, 122.2, 119.8 (q, J=159 Hz), 70.1, 68.5, 49.6, 49.4, 31.8, 19.1, 12.8. Anal. calcd. for C26H40F12N6O11S4 (%): C, 32.23; H, 4.16; N, 8.67; S, 13.24. Found: C, 32.21; H, 4.04; N, 8.55; S, 13.11. 2.3.4 Synthesis of DILs [DiBIm4O] [TFSI]. bis

(1-butyl-1H-imidazol-3-ium)

bis

T

3,3'-(3,6,9,12-tetraoxatetradecane-1,14-diyl)

IP

((trifluoromethyl)sulfonyl) amide: [DiBIm4O] [TFSI]. An analogous procedure was used to

CR

prepare 4e starting 4b (2.95 g, 10.7 mmol), 1-butylimidazolium (2.83 g, 22.8 mmol), to give the desired compound as a brown liquid (4e, yield: 4.28 g, 89.6%).

US

The corresponding IL was prepared analogously from 4e (1.93 g, 4.3 mmol) and LiTFSI (2.61

AN

g, 9.1 mmol) to give the final product as a brown liquid [DiBIm4O] [TFSI] (yield: 3.23 g, 79.8%). IR: 3542.72 cm-1 (w), 3149.39-3114.79 cm-1 (=C-H, s), 2963.48 -2877.37 cm-1 (C-H, s),

M

2389.19 cm-1, 1926.6-1846.11 cm-1 (w), 1650.43 cm-1 (C=N, m), 1565.6-1463.43 cm-1, (C=C, m),

ED

1352.27 cm-1 (C-N, w), 1191.84 cm-1 (C-O, s), 1057.7 cm-1(C-F, s), 939.86-840.24 cm-1, 789.03570.54 cm-1.

NMR (300 MHz, D-acetone): δ (ppm) 9.05 (s, 2H), 7.75 (d, J=1.2 Hz, 2H), 7.73 (d, J=1.2

PT

1H

CE

Hz, 2H) 4.49 (t, J=4.5 Hz, 4H), 4.35 (t, J=7.2 Hz, 4H), 3.88 (t, J=4.8 Hz, 4H), 3.71-3.37 (t, J=1.8 Hz, 12H), 1.90 (m, 4H), 1.37 (m, 4H), 0.92 (t, J=7.2 Hz, 6H). 13C NMR (300 MHz, D-acetone): δ

AC

(ppm) 135.5, 123.2, 122.2, 119.8 (q, J=159 Hz), 70.2, 68.4, 49.6, 49.4, 31.8, 19.1, 12.8. Anal. calcd. for C28H44F12N6O12S4 (%): C, 33.20; H, 4.38; N, 8.30; S, 12.66. Found: C, 33.04; H, 4.25; N, 8.22; S, 12.52. 2.3.5 Synthesis of DILs [DiBIm1O] [DCA]. NaN(CN)2 (1.36 g, 15.3 mmol) was dissolved in 40 ml methanol, and added to 1e (2.28 g, 7.3 mmol) dissolved in 5ml methanol. The mixture was stirred overnight at room temperature,

ACCEPTED MANUSCRIPT removed methanol by vacuum distillation until some precipitation came out. Then the precipitation was filtered, and 15 ml acetone was added into the filtrate. The solvent was removed by vacuum distillation until some precipitation came out again. Repeat the foregoing procedures for several times until no precipitation came out. Then the filtrate was concentrated under reduced pressure to

T

give the final product [DiBIm1O] [DCA] (yield: 2.03 g, 74.7%) as a dark brown liquid.

(CN, m), 1633.47 cm-1 (C=N, m), 1565.21-1463.45 cm-1, (C=C, m), 1316.78 cm-1 (C-N, w),

1165.31-1129.01 cm-1 (C-O, s), 907.02-752.42 cm-1.

NMR (300 MHz, D2O): δ (ppm) 8.74 (s, 2H), 7.46 (d, J=1.2 Hz, 2H), 7.42 (d, J=1.2 Hz,

US

1H

CR

1

IP

IR: 3144.59-3106.59 cm-1 (=C-H, s), 2962.44 -2875.54 cm-1 (C-H, s), 2240.53-2138.22 cm-

AN

2H), 4.34 (t, J=4.5 Hz, 4H), 4.15 (t, J=7.2 Hz, 4H), 3.83 (t, J=4.8 Hz, 4H), 1.8 (m, 4H), 1.23 (m, 4H), 0.86 (t, J=7.2 Hz, 6H). 13C NMR (300 MHz, D2O): δ (ppm) 135.6, 122.8, 122.3, 119.7, 68.6,

M

49.5, 31.4, 30.3, 18.9, 12.8. Anal. calcd. for C22H32N10O (%): C, 58.39; H, 7.13; N, 30.95. Found:

ED

C, 58.24; H, 7.04; N, 30.81.

2.3.6 Synthesis of DILs [DiBIm2O] [DCA].

PT

The corresponding IL was prepared analogously from 2e (2.98 g, 8.3 mmol) and NaN(CN)2

CE

(1.56 g, 17.5 mmol) to give the desired compound as a dark brown liquid [DiBIm2O] [DCA] (yield: 2.61 g, 74.8%).

1

AC

IR: 3143.53-3105.28 cm-1 (=C-H, s), 2962.23 -2875.06 cm-1 (C-H, s), 2236.83-2135.58 cm(CN, m), 1644.07 cm-1 (C=N, m), 1565.05-1464.30 cm-1, (C=C, m), 1313.82 cm-1 (C-N, w),

1165.47-1116.54 cm-1 (C-O, s), 906.6-752.89 cm-1. 1H

NMR (300 MHz, D2O): δ (ppm) 8.74 (s, 2H), 7.48 (d, J=1.2 Hz, 2H), 7.47 (d, J=1.2 Hz,

2H), 4.33 (t, J=4.5 Hz, 4H), 4.15 (t, J=6.9 Hz, 4H), 3.82 (t, J=5.1 Hz, 4H), 3.60 (t, 4H), 1.8 (m, 4H), 1.24 (m, 4H), 0.85 (t, J=7.2 Hz, 6H).

13C

NMR (300 MHz, D2O): δ (ppm) 135.7, 122.7,

ACCEPTED MANUSCRIPT 122.3, 119.7, 69.7, 68.5, 49.5, 31.4, 30.3, 18.9, 12.8. Anal. calcd. for C24H36N10O2 (%): C, 58.05; H, 7.31; N, 28.20. Found: C, 58.01; H, 7.21; N, 28.11. 2.3.7 Synthesis of DILs [DiBIm3O] [DCA]. The corresponding IL was prepared analogously from 3e (2.12 g, 5.3 mmol) and NaN(CN)2

T

(0.99 g, 11.1 mmol) to give the desired compound as a dark brown liquid [DiBIm3O] [DCA]

IP

(yield: 1.85 g, 75.8%).

1

CR

IR: 3143.50-3105.37 cm-1 (=C-H, s), 2961.67 -2874.75 cm-1 (C-H, s), 2237.71-2136.15 cm(CN, m), 1644.40 cm-1 (C=N, m), 1564.91-1463.74 cm-1, (C=C, m), 1314.05 cm-1 (C-N, w),

NMR (300 MHz, D2O): δ (ppm) 8.74 (s, 2H), 7.51 (d, J=1.2 Hz, 2H), 7.48 (d, J=1.2 Hz,

AN

1H

US

1165.26-1116.69 cm-1 (C-O, s), 907.06-752.89 cm-1.

2H), 4.36 (t, J=4.2 Hz, 4H), 4.17 (t, J=6.9 Hz, 4H), 3.85 (t, J=4.2 Hz, 4H), 3.59-3.64 (t, J=3.9 Hz,

M

8H), 1.8 (m, 4H), 1.23 (m, 4H), 0.86 (t, J=7.2 Hz, 6H). 13C NMR (300 MHz, D2O): δ (ppm) 135.7,

ED

122.8, 122.3, 119.7, 69.8, 69.6, 68.5, 49.5, 31.4, 30.3, 19.0, 12.9. Anal. calcd. for C26H40N10O3 (%): C, 57.76; H, 7.46; N, 25.91. Found: C, 57.62; H, 7.31; N, 25.82.

PT

2.3.8 Synthesis of DILs [DiBIm4O] [DCA].

CE

The corresponding IL was prepared analogously from 4e (2.35 g, 5.3 mmol) and NaN(CN)2 (0.99 g, 11.1 mmol) to give the desired compound as a dark brown liquid [DiBIm4O] [DCA]

AC

(yield: 2.04 g, 76.2%).

IR: 3143.97-3107.37 cm-1 (=C-H, s), 2961.01 -2875.06 cm-1 (C-H, s), 2239.71-2138.17 cm1

(CN, m), 1633.23 cm-1 (C=N, m), 1565.11-1464.06 cm-1, (C=C, m), 1315.39 cm-1 (C-N, w),

1114.62 cm-1 (C-O, s), 945.74-752.33 cm-1. 1H

NMR (300 MHz, D2O): δ (ppm) 8.74 (s, 2H), 7.48 (d, J=1.2 Hz, 2H), 7.42 (d, J=1.2 Hz,

2H), 4.3 (t, J=4.5 Hz, 4H), 4.11 (t, J=7.2 Hz, 4H), 3.8 (t, J=4.8 Hz, 4H), 3.56-3.61 (t, J=1.8

ACCEPTED MANUSCRIPT Hz,12H), 1.8 (m, 4H), 1.23 (m, 4H), 0.86 (t, J=7.2 Hz, 6H). 13C NMR (300 MHz, D2O): δ (ppm) 135.7, 122.7, 122.2, 119.8, 71.7, 70.8, 69.7, 68.5, 49.4, 31.8, 30.3, 19.0, 12.7. Anal. calcd. for C28H44N10O4 (%): C, 57.52; H, 7.58; N, 23.95. Found: C, 57.43; H, 7.44; N, 23.84. 2.4 General Procedures for the Preparation of DES-based DILs.

T

As shown in Scheme 4, the synthesis of DES-based DILs along with TFSI anions involves

AC

CE

PT

ED

M

AN

US

CR

IP

three steps, and DCA anions reactions involve three steps.

Scheme 4. Synthetic Routes of DES-based DILs. 2.4.1 Synthesis of DILs [DiDES1O] [TFSI].

ACCEPTED MANUSCRIPT (oxy bis (ethane-2,1-diyl)) bis (diethyl sulfonium) bis ((trifluoromethyl) sulfonyl) amide: [DiDES1O] [TFSI]. Diethylene glycol 1a (4.01 g, 37.8 mmol), and 4-methylbenzenesulfonyl chloride (TsCl) (15.08 g, 79.4 mmol) were dissolved in 50 ml DCM. KOH (6.05 g, 108 mmol) was added slowly into the solution with ice-water bath and vigorous stirring. After stirring for

T

overnight at room temperature, 50 ml DCM was added to dilute the mixture, and then it was

IP

washed with 50 ml water one time and 20 ml brine two times. The organic layer was separated and

CR

dried with anhydrous MgSO4. Then the solvent was removed to get the desired product, (oxy bis (ethane-2,1-diyl)) bis(4-methylbenzenesulfonate): (1f, Yield: 14.56 g, 93%), which was colorless

US

viscous liquid.

AN

1f (10.15 g, 24.5 mmol) and diethyl sulfide (8.88 g, 98.4mmol) were mixed in a neat flask and refluxed two days. The remains were washed with 15 mL ethyl ether three times and dried

M

under high vacuum to obtain a pale yellow viscous liquid product (oxy bis (ethane-2, 1-diyl)) bis

ED

(diethyl sulfonium) 4-methylbenzenesulfonate (1g, yield: 11.16 g, 76.6%). 1g (4.54 g, 7.6 mmol) and LiTFSI (4.8 g, 16.7 mmol) were dissolved in 10 ml deionized

PT

water. The solution was reflux overnight at 80 ℃, and extracted the solution with 30 ml

CE

dichloromethane. The obtained organic layer was washed three times with saturated brine (10 ml), dried over anhydrous magnesium sulfate. The residue was purified by column chromatography on

AC

alumina and concentrated under reduced pressure to give the final product [DiDES1O] [TFSI] (Yield: 4.43g, 71.5%) as a pale liquid. IR: 2987.41 -2885.08 cm-1 (C-H, s), 1934.93-1790.59 cm-1 (w), 1456.03-1351.84 cm-1 (C-H, m), 1189.75 cm-1 (C-O, s), 1056.67-977.13 cm-1(C-F, s), 789.49-740.25 cm-1, 654-570.4 cm-1. 1H

NMR (300 MHz, CDCl3): δ (ppm) 3.95 (t, J=4.2 Hz, 4H), 3.45 (t, J=2.4 Hz, 4H), 3.23 (m,

8H), 1.46 (t, J=7.2 Hz, 12H). 13C NMR (300 MHz, CDCl3): δ (ppm) 119.8 (q, J=159 Hz), 65.2,

ACCEPTED MANUSCRIPT 33.8, 32.8, 8.5. Anal. calcd. for C16H28F12N2O9S6 (%): C, 23.64; H, 3.47; N, 3.45; S, 23.67. Found: C, 23.50; H, 3.35; N, 3.33; S, 23.54. 2.4.2 Synthesis of DILs [DiDES2O] [TFSI]. ((ethane-1,2-diylbis(oxy)) bis (ethane-2,1-diyl)) bis (diethylsulfonium) bis ((trifluoromethyl)

T

sulfonyl) amide: [DiDES2O] [TFSI]. An analogous procedure was used to prepare 2f (8.03 g,

CR

IP

86.8%) starting with 2a (3.03 g, 20.2 mmol), KOH (4.52 g, 80.6 mmol) and TsCl （8.42 g, 44.3mmol）. Then 2f (6.03 g, 13.1 mmol) was taken to react with DES (4.75 g, 52.7 mmol) to

US

give the desired compound as a yellow liquid (2g, yield: 6.85 g, 81.6%).

AN

The corresponding IL was prepared analogously from 2g (3.57 g, 5.6 mmol) and LiTFSI (3.56 g, 12.4 mmol) to give the final product as a yellow liquid [DiDES2O] [TFSI] (yield: 3.64 g, 76%).

M

IR: 2987.54 -2881.83 cm-1 (C-H, s), 1456.03-1351.84 cm-1 (C-H, m), 1189.75 cm-1 (C-O, s),

1H

ED

1056.67-977.13 cm-1(C-F, s), 789.49-740.25 cm-1, 654-570.39 cm-1. NMR (300 MHz, CDCl3): δ (ppm) 3.93 (t, J=5.1 Hz, 4H), 3.59 (t, J=4.2 Hz, 4H), 3.46 (t,

PT

J=2.4 Hz, 4H), 3.29 (m, 8H), 1.44 (t, J=7.2 Hz, 12H). 13C NMR (300 MHz, CDCl3): δ (ppm) 119.8

CE

(q, J=159 Hz), 70.3, 65.1, 34.2, 32.7, 8.5. Anal. calcd. for C18H32F12N2O10S6 (%): C, 25.23; H, 3.76; N, 3.27; S, 22.45. Found: C, 25.11; H, 3.61; N, 3.20; S, 22.33.

AC

2.4.3 Synthesis of DILs [DiDES3O] [TFSI]. (((oxybis(ethane-2,1-diyl)) bis (oxy)) bis (ethane-2,1-diyl)) bis (diethylsulfonium) bis ((trifluoromethyl) sulfonyl) amide: [DiDES3O] [TFSI]. An analogous procedure was used to prepare 3f (10.59 g, 89.2%) starting 3a (4.59 g, 23.6 mmol), KOH (5.3 g, 94.5 mmol) and TsCl (9.87 g, 52 mmol). Then 3f (9.59 g, 19.1 mmol) was taken to react with DES (6.88 g, 76.3 mmol) to give the desired compound as a yellow liquid (3g, yield: 9.87 g, 75.7%).

ACCEPTED MANUSCRIPT The corresponding IL was prepared analogously from 3g (3.05 g, 4.5 mmol) and LiTFSI (2.78 g, 9.7 mmol) to give the final product as a yellow liquid [DiDES3O] [TFSI] (yield: 3.11 g, 69.8%). IR: 2992.44 -2879.84 cm-1 (C-H, s), 1934.93-1793.37 cm-1 (w), 1455.73-1351.99 cm-1 (C-H,

NMR (300 MHz, CDCl3): δ (ppm) 3.94 (t, J=5.1 Hz, 4H), 3.7-3.5 (t, J=4.2 Hz, 8H), 3.46

IP

1H

T

m), 1190.57 cm-1 (C-O, s), 1057.11-976.06 cm-1(C-F, s), 789.49-740.25 cm-1, 663.24-555.13cm-1.

CR

(t, J=2.4 Hz, 4H), 3.23 (m, 8H), 1.46 (t, J=7.2 Hz, 12H). 13C NMR (300 MHz, CDCl3): δ (ppm) 119.8 (q, J=159 Hz), 70.5, 70.1, 65.3, 34.2, 32.8, 8.6. Anal. calcd. for C20H36F12N2O11S6 (%): C,

US

26.66; H, 4.03; N, 3.11; S, 21.36. Found: C, 26.55; H, 4.00; N, 3.02; S, 21.28.

AN

2.4.4 Synthesis of DILs [DiDES4O] [TFSI].

(3, 6, 9, 12-tetraoxatetradecane-1, 14-diyl) bis (diethylsulfonium) bis ((trifluoromethyl)

M

sulfonyl) amide: [DiDES4O] [TFSI]. An analogous procedure was used to prepare 4f (10.11 g,

ED

82.8%) starting 4a (4.87 g, 20.4 mmol), KOH (4.59 g, 81.8 mmol) and TsCl （8.54 g, 45 mmol）.

PT

Then 4f (9.18 g, 16.8 mmol) was taken to react with DES (6.1 g, 67.6 mmol) to give the desired compound as a dark yellow liquid (4g, yield: 10.11 g, 82.8%).

CE

The corresponding IL was prepared analogously from 4g (2.58 g, 3.6 mmol) and LiTFSI (2.22

75.7%).

AC

g, 7.7 mmol) to give the final product as a dark yellow liquid [DiDES4O] [TFSI] (yield: 2.54 g,

IR: 2946.74 -2875.87 cm-1 (C-H, s), 1931.78-1798.10 cm-1 (w), 1456.54-1351.42 cm-1 (C-H, m), 1189.75 cm-1 (C-O, s), 1056.67-977.13 cm-1(C-F, s), 789.49-740.25 cm-1, 654-570.39 cm-1. 1H

NMR (300 MHz, CDCl3): δ (ppm) 3.92 (t, J=2.1 Hz, 4H), 3.7-3.51 (t, J=5.1 Hz, 12H),

3.43 (t, J=5.7 Hz, 4H), 3.18 (m, 8H), 1.36 (t, J=6.6 Hz, 12H).

13C

NMR (300 MHz, CDCl3): δ

(ppm) 119.8 (q, J=159 Hz), 71.1, 70.4, 69.5, 65.1, 34.2, 32.8, 8.5. Anal. calcd. for

ACCEPTED MANUSCRIPT C22H40F12N2O12S6 (%): C, 27.96; H, 4.27; N, 2.96; S, 20.36. Found: C, 27.81; H, 4.19; N, 2.85; S, 20.28. 2.4.5 Synthesis of DILs [DiDES1O] [DCA]. (oxy bis (ethane-2,1-diyl)) bis (diethyl sulfonium) dicyanamide: [DiDES1O] [DCA].

T

Dowex® 22 (Chloride form) resin was washed three times with methanol and dried in the oven

IP

under 60 ̊C to remove all moisture. 20 ml dried Dowex® 22 was contained in a column and wetted

CR

with 15 mL methanol, 1g (6.42 g, 10.8 mmol) was dissolved in 15 mol methanol. This solution was passed through the resin column with dropping rate controlled to ~1 drop/second. 60mL

US

methanol was added to wash the column. Then all methanol solution was combined and the solvent

AN

was removed. The desired product (oxy bis (ethane-2, 1-diyl)) bis (diethyl sulfonium) dichloride 1h (Yield: 2.97g, 85.1%) was obtained as brown viscous liquid.

M

NaN(CN)2 (1.39g, 15.6 mmol) was totally dissolved in 40ml methanol, 1h (2.37 g, 7.3 mmol)

ED

with another 5ml methanol was added together. The mixture was stirred overnight at room temperature, removed methanol by vacuum distillation until some precipitation came out. Then

PT

the precipitation was filtered, and 15 ml acetone was added to the filtrate. The solvent was removed

CE

by vacuum distillation until some precipitation came out again. Repeat the preceding procedures for several times until no precipitation came out. Then the filtrate was concentrated under reduced

AC

pressure to give the final product [DiDES1O] [DCA] (yield: 2.1 g, 74.3%) as a yellow liquid. IR: 2987.41 -2877.08 cm-1 (C-H, s), 2236.96-2135.93 cm-1(CN, m), 1650.71 cm-1 (C=N, m), 1565.11-1422.63 cm-1, (C=C, m), 1310.94 cm-1 (C-N, w), 1112.76 cm-1 (C-O, s), 976.20-790.97 cm-1, 665.31 cm-1. 1H

NMR (300 MHz, CDCl3): δ (ppm) 3.97 (t, J=4.2 Hz, 4H), 3.64 (t, J=2.4 Hz, 4H), 3.4-3.18

(m, 8H), 1.46 (t, J=7.2 Hz, 12H). 13C NMR (300 MHz, CDCl3): δ (ppm) 119.9, 65.2, 33.8, 32.8,

ACCEPTED MANUSCRIPT 8.5. Anal. calcd. for C16H28N6OS2 (%): C, 49.97; H, 7.34; N, 21.85; S, 16.68. Found: C, 49.84; H, 7.28; N, 21.72; S, 16.55. 2.4.6 Synthesis of DILs [DiDES2O] [DCA]. ((ethane-1,2-diylbis(oxy))

bis(ethane-2,1-diyl))

bis(diethylsulfonium)

dicyanamide:

T

[DiDES2O] [DCA]. An analogous procedure was used to prepare 2h starting with 2g (3.09 g, 4.8

IP

mmol), to give the desired compound as a yellow liquid (2h, yield: 1.55 g, 87.2%).

CR

The corresponding IL was prepared analogously from 2h (1.15 g, 3.1 mmol) and NaN(CN)2 (0.36 g, 4.1 mmol) to give the final product as a yellow liquid [DiDES2O] [DCA] (yield: 1.01 g,

US

75.7%).

AN

IR: 2979.21-2877.82 cm-1 (C-H, s), 2238.92-2137.95 cm-1(CN, m), 1650.89 cm-1 (C=N, m), 1581.58-1455.26 cm-1, (C=C, m), 1313.77 cm-1 (C-N, w), 1119.62 cm-1 (C-O, s), 1034.44-790.97

NMR (300 MHz, CDCl3): δ (ppm) 3.93 (t, J=5.1 Hz, 4H), 3.59 (t, J=4.2 Hz, 4H), 3.46 (t,

ED

1H

M

cm-1, 665.31 cm-1.

J=2.4 Hz, 4H), 3.29-3.19 (m, 8H), 1.36 (t, J=7.2 Hz, 12H). 13C NMR (300 MHz, CDCl3): δ (ppm)

PT

119.9, 69.9, 64.7, 33.5, 32.0, 8.0. Anal. calcd. for C18H32N6O2S2 (%): C, 50.44; H, 7.53; N, 19.61;

CE

S, 14.96. Found: C, 50.36; H, 7.51; N, 19.49; S, 14.82. 2.4.7 Synthesis of DILs [DiDES3O] [DCA].

AC

(((oxy bis (ethane-2,1-diyl)) bis (oxy)) bis (ethane-2,1-diyl)) bis (diethyl sulfonium) dicyanamide: [DiDES3O] [DCA]. An analogous procedure was used to prepare 3h starting with 3g (6.72 g, 9.8 mmol), to give the desired compound as a yellow liquid (3h, yield: 3.38 g, 83.4%). The corresponding IL was prepared analogously from 3h (2.48 g, 6.0 mmol) and NaN(CN)2 (1.13 g, 12.6 mmol) to give the final product as a yellow liquid [DiDES3O] [DCA] (yield: 2.16 g, 75.8%);

ACCEPTED MANUSCRIPT IR: 3029.41-2877.08 cm-1 (C-H, s), 2248.57-2139.07 cm-1(CN, m), 1650.61 cm-1 (C=N, m), 1581.72-1397.34 cm-1, (C=C, m), 1318.19 cm-1 (C-N, w), 1121.96 cm-1 (C-O, s), 1033.73-790.11 cm-1, 665.37 cm-1. 1H

NMR (300 MHz, CDCl3): δ (ppm) 3.93 (t, J=5.1 Hz, 4H), 3.8-3.53 (t, J=4.2 Hz, 8H), 3.45

T

(t, J=2.4 Hz, 4H), 3.3-3.23 (m, 8H), 1.39 (t, J=7.2 Hz, 12H). 13C NMR (300 MHz, CDCl3): δ (ppm)

CR

17.78; S, 13.57. Found: C, 50.68; H, 7.62; N, 17.66; S, 13.48.

IP

119.9, 70.0, 69.6, 64.7, 38.6, 33.7, 8.1. Anal. calcd. for C20H36N6O3S2 (%): C, 50.82; H, 7.68; N,

2.4.8 Synthesis of DILs [DiDES4O] [DCA].

US

(3, 6, 9, 12-tetraoxatetradecane-1, 14-diyl) bis (diethylsulfonium) dicyanamide: [DiDES4O]

AN

[DCA]. An analogous procedure was used to start with 4g (7.43 g, 10.2 mmol), to give the desired compound as a yellow liquid (4h, yield: 3.81 g, 81.8%).

M

The corresponding IL was prepared analogously from 4h (2.81 g, 6.2 mmol) and NaN(CN)2

ED

(1.15 g, 13 mmol) to give the final product as a yellow liquid [DiDES4O] [DCA] (yield: 2.32 g, 72.6%).

PT

IR: 2987.41 -2877.08 cm-1 (C-H, s), 2236.96-2135.93 cm-1(CN, m), 1650.71 cm-1 (C=N, m),

cm-1, 665.31 cm-1.

NMR (300 MHz, D2O): δ (ppm) 3.95 (t, J=2.1 Hz, 4H), 3.8-3.53 (t, J=5.1 Hz, 12H), 3.45

AC

1H

CE

1565.11-1422.63 cm-1, (C=C, m), 1310.94 cm-1 (C-N, w), 1112.76 cm-1 (C-O, s), 976.20-790.97

(t, J=5.7 Hz, 4H), 3.3-3.18 (m, 8H), 1.6 (t, J=6.6 Hz, 12H). 13C NMR (300 MHz, D2O): δ (ppm) 120, 69.9, 69.7, 69.4, 64.7, 33.6, 32.1, 8.1. Anal. calcd. for C22H40N6O4S2 (%): C, 51.14; H, 7.80; N, 16.26; S, 12.41. Found: C, 51.22; H, 7.68; N, 16.21; S, 12.30.

2.5 Characterization

ACCEPTED MANUSCRIPT The flammability test was performed by igniting our samples and recording the selfextinguishing time (SET) of them. Solubility of our products was tested in various polar and nonpolar organic solvents. The electrolytic mixtures were prepared by mixing (in the proper mole fractions) LiTFSI with DILs/[MImEt][DCA], which were continuously stirred until to obtain

T

homogeneous samples (e.g., full dissolution of LiTFSI in ILs) [45]. The ternary mixtures, 0.5 M

IP

LiTFSI in DILs/[MImEt] [DCA] (the DILs: [MImEt] [DCA] weight ratio was fixed equal to 1:1),

CR

were prepared as electrolytes. Ionic conductivity of the electrolytes was characterized in a controlled environment dry room from 25 °C to 70 °C using a two-electrode cell and a Gamry

US

potentiostat. The conductivity values were then calibrated to a standard 0.1 mol kg−1 KCl aqueous

AN

solution with conductivity at 25.0 °C of 0.0128 S cm−1 [21].

M

3. Results and Discussion

ED

In order to make the best choice of the electrolyte composition for use in practical lithium rechargeable batteries, we should take into account the flammability, ionic conductivity, and

PT

viscosity of the electrolytes [35]. Diethylene glycol, triethylene glycol, tetraethylene glycol,

CE

pentaethylene glycol with different length of EO chains were chosen as the ideal compounds, because of their structural similarity and tunability (i.e., the ability to alter, control, and tailor their

AC

structure as desired) are contributed to understanding better the structural change that corresponds to a particular physical property. They were modified by converting the end groups to an appropriate leaving group, such as chloride. The subsequent quaternization reaction with MPy (or BIm) at elevated temperature (reflux) led to the corresponding DILs with Cl- anions, whereas TFSI (or DCA) obtained via further anion exchange. However, applying the same procedure for the preparation of DES-based DILs did not result in products of a good yield. In order to obtain the

ACCEPTED MANUSCRIPT product in high yields, we changed the strategy by replacing of the dichloride precursor with the more reactive di-OTs compound and extending the reaction time up to 72 h, we got the desired products finally. Applying the presented synthesis route, the series of DILs with various cationic head groups

T

and containing different anionic counterparts could be obtained with high yields. Furthermore,

IP

using this method, we have the ability to control and tune the properties of DILs to get the desirable

CR

solvation properties and thermal stabilities [36].

Three series of DILs with EO spacer lengths from 1 to 4 have been synthesized and some of

US

their properties, such as density, viscosity, and appearance at room temperature have been

AN

investigated [36]. All the physicochemical properties of the DILs are listed in TABLE I. Compounds [DiMPy1O][DCA] and [DiMPy1O][TFSI] with one EO chain spacer length, were

ED

remained in the liquid state.

M

solidified at room temperature, whereas other compounds with spacer length (EO chains≥2),

PT

The density is the most often measured and reported physical property of ILs because nearly every application requires knowledge of the density. Shirota et al [37] reported the liquid density

CE

of a series of methyl imidazolium DILs, in which the two cations are linked by -(CH2)- chain of

AC

different length (Scheme 5), and associated with various anion, as shown in TABLE I:

CR

IP

T

ACCEPTED MANUSCRIPT

US

Scheme 5. Structure and abbreviation of the reported DILs.

AN

TABLE I. Liquid density & Viscosity of the reported DILs [37] (at 297K). Density (g cm−3)

M

Cations

1.546

BF4−

TFSI−

PFSI−

1.639

-

738.9

2601

1.615

-

649.5

2012

BF4−

[C8(MIm)2]2+

1.500

-

-

662.9

AC

CE

[C6(MIm)2]2+

1.570

PT

[C5(MIm)2]2+

PFSI−

ED

TFSI−

Viscosity (cP)

[C9(MIm)2]2+

1.479

1.553

1.255

678.8

1935

8222

[C10(MIm)2]2+

1.462

1.536

1.240

720.6

1951

12310

[C12(MIm)2]2+

1.428

1.503

1.209

842.4

1958

15540

TABLE II

contains density values of our samples. In general, DILs are denser than either

organic solvents and water (TABLE I). The density of the DILs decreases when longer -(CH2)- is

ACCEPTED MANUSCRIPT linking the two cations, possibly a longer alkane chain prevents the compact stacking of the cations. and it increases when anions with larger molecular weight are paired with the dication, so that for a given cation, the density order is PFSI− > TFSI− > BF4−. Similar to this report, our DILs containing TFSI anions have a higher density than those containing DCA anions. For instance,

T

[DiMPy1O][TFSI] (1.56 g cm−3 at 25°C) has higher density than [DiMPy1O][DCA] (1.19 g cm−3

IP

at 25 °C), [DiBIm1O][TFSI] (1.51 g cm−3 at 25°C) has higher density than [DiBIm1O][DCA]

CR

(1.21 g cm−1 at 25°C), [DiDES1O][TFSI] (1.60 g cm−1 at 25°C) has higher density than [DiDES1O][DCA] (1.25 g cm−1 at 25°C). Since none of the anions have a long chain, it is possible

US

that they all can occupy favorably, relatively close-approach positions around the cation, resulting

AN

in the high densities [38]. However, the density of our DILs trends to increase when the polyether chain between the two cations becomes longer, which is opposite to the trends of the reported

M

DILs. For example, the density of [DiMPy1O][TFSI] = 1.56 g cm−3, [DiMPy2O][TFSI] = 1.57 g

ED

cm−3, [DiMPy3O][TFSI] = 1.59 g cm−3, and [DiMPy4O][TFSI] = 1.61 g cm−3. We believe this can be explained by that the strong electronegativity of the O atoms in the polyether chains leads

PT

to more compact stacking of the ions, caused a different effect to the density.

CE

On the other hand, MPy-based DILs have a larger density than those of BIm-based DILs with the same EO chains, for example, [DiMPy1O][TFSI] (1.56 g cm−1 at 25°C) has a higher density

AC

than [DiBIm1O][TFSI] (1.51 g cm−1 at 25°C). With the increasing of EO chains, the density of the corresponding MPy-based DILs and BIm-based DILs are increasing. Moreover, the replacement of the cationic head groups by DES group decreases the fluid density, for example, [DiDES2O][TFSI] (1.57 g cm−1 at 25°C) has a lower density than [DiDES1O][TFSI] (1.60 g cm−1 at 25°C) [25].

ACCEPTED MANUSCRIPT The viscosity of an IL is a very important parameter in electrochemical studies owing to that it provides an engineering point of view as it plays a major role in stirring, mixing and pumping operations; in addition, it also affects the mass transport rate in electrolyte system. Nevertheless, the viscosity is much less investigated than the density, and only limited data are available in the

T

literature. DILs are generally viscous liquids with viscosities ranging typically from 10 to 500 mPa

IP

s at ambient temperature [39]. As shown in TABLE II, the influence of temperature on viscosity is

CR

much more significant than on density: a strong decrease is observed with increasing temperature making DILs easier to apply at super ambient conditions, due to the rise in the fluidity of the

US

solution by increasing the kinetic energy of molecule [40]. The bridging EO chains have an

AN

irregular influence on their viscosity. The viscosities of ILs are determined essentially by van der Waals interactions, hydrogen bonding, and freedom of molecular rotation. EO chain lengthening

M

makes the ILs more viscous, due to increased van der Waals interactions [41], the high viscosity

ED

of DILs seems to limit charge transport in the electrolyte.

PT

TABLE II. Physicochemical properties of the DILs Ionic conductivity

Molecular

Density

Appearance (Before Neat (mS cm-1)

weight

CE

Compounds

Viscosity

-3 a

(g cm )

(cp)

a

decolorization)

AC

(g mol-1)

25°C

70°C

[MImEt] [DCA]

177.21

1.10

14.82

Pale yellow liquid

26.3

62.3

[MImEt] [TFSI]

391.31

1.47

25

Colorless liquid

10.62

27.1

374.48

1.19

>2000

Dark brown crystal

0.44

8.12

418.54

1.21

64.31

Brown liquid

2.3

12.95

[DiMPy1O] [DCA] [DiMPy2O] [DCA]

ACCEPTED MANUSCRIPT

462.59

1.22

60.44

Dark brown liquid

2.5

12.93

506.64

1.22

49.49

Dark brown liquid

1.45

9.47

802.69

1.56

>2000

Brown crystal

1.23

7.39

846.75

1.57

377.5

Brown liquid

1.4

6.01

890.8

1.59

279

CR

1.01

4.9

934.85

1.61

189.6

Dark brown liquid

0.97

4.54

452.26

1.21

429.2

Dark brown liquid

2.31

14.7

496.61

1.21

169

Dark brown liquid

1.53

10.16

1.21

134.9

Dark brown liquid

1.70

10.11

CE

[DiMPy3O]

584.71

1.22

75.33

Dark brown liquid

1.36

11.31

880.77

1.51

411.4

Brown liquid

0.65

4.56

924.82

1.53

364

Brown liquid

0.79

5.26

968.87

1.54

332.8

Brown liquid

0.77

4.74

1012.92

1.57

298.6

Brown liquid

0.72

4.75

[DCA] [DiMPy4O] [DCA] [DiMPy1O]

T

[TFSI]

IP

[DiMPy2O] [TFSI] [DiMPy3O]

[DiMPy4O]

M

[DCA]

[DiBIm3O]

PT

540.66 [DCA] [DiBIm4O] [DCA]

AC

[DiBIm1O]

ED

[DiBIm2O] [DCA]

AN

[TFSI] [DiBIm1O]

Dark brown liquid

US

[TFSI]

[TFSI]

[DiBIm2O] [TFSI] [DiBIm3O] [TFSI] [DiBIm4O] [TFSI]

ACCEPTED MANUSCRIPT [DiDES1O] 384.56

1.25

368.5

Yellow liquid

1.87

13.1

428.62

1.24

361.1

Yellow liquid

0.67

5.39

472.67

1.24

142.9

Yellow liquid

1.43

11.74

516.72

1.23

85.22

Yellow liquid

3.41

16.8

812.77

1.60

366.1

2.47

10.28

856.83

1.57

235.7

Yellow liquid

0.89

4.48

900.88

1.54

128.1

Yellow liquid

1.27

7.54

944.93

1.53

110.4

Dark yellow liquid

2.4

11.55

[DCA] [DiDES2O] [DCA] [DiDES3O]

T

[DCA]

IP

[DiDES4O]

[DiDES1O]

[DiDES2O]

M

[TFSI]

ED

[DiDES4O] [TFSI] Measured at 25 °C

CE

3.1 Flammability Test.

PT

a

AN

[TFSI] [DiDES3O]

Pale yellow liquid

US

[TFSI]

CR

[DCA]

A battery might be placed in more hazardous conditions, such as in the presence of fire or

AC

sparks. In order to investigate the flammable behavior of the mixed electrolytes containing DIL and [MImEt][DCA], we have carried out a flammability test called self-extinguishing time (SET) [42]. Each electrolyte was tested three times: the burner was switched on above the sample for 5 s and then switched off. The ignition time after flame setting and the self-extinguish time after removing the burner flame were measured as indices of the non-flammability [43] and the time it took for the flame to extinguish was normalized against liquid mass to give the SET in sg-1 [44].

ACCEPTED MANUSCRIPT The electrolyte was judged to be nonflammable if the electrolyte never ignited during the testing, or if the ignition of electrolyte ceased when the flame was removed [35]. It is apparent from Figure 1 that all the three series of electrolytes with various of DILs are certified to be nonflammable, since they cannot be ignited, meaning their SET is all measured to

T

be 0 s. We also confirmed that neat DILs were completely non-flammable in nature, and contribute

AC

CE

PT

ED

M

AN

US

CR

IP

to the high safety of the electrolyte system.

ACCEPTED MANUSCRIPT Figure 1. Development of the flames for different DILs based electrolytes during SET test directly at the burner was turned on (at start) and was turned off (after 1s) respectively. 3.2 Solubility Test. The synthesized DILs were tested for their solubility behavior in polar and non-polar solvents

IP

T

(shown in TABLE III). DILs derived from [DCA] anions exhibited good solubility in polar solvents

CR

such as dimethyl sulfoxide (DMSO), acetonitrile (MeCN), methanol (MeOH), water (H2O), acetone and tetrahydrofuran(THF). Simultaneously, all of them were partially miscible in

US

dichloromethane (DCM) and ethyl acetate (EtOAc), and non-soluble in non-polar solvents such as hexane, diethylether (Et2O). The solubility behavior of another set of DILs derived from [TFSI]

AN

anion displayed different solubility properties, they are well soluble in DCM and EtOAc, but not

M

soluble in H2O [3], possibly because of the highly hydrophobic -CF3 groups in [TFSI]. So that, for DILs cation replacing the anions with [DCA] increases the solubility of the IL in water while

ED

replacement with the [TFSI] ion dramatically decreases the water solubility.

CE

Compounds

PT

TABLE III. Comparison of solubility in various organic solvents

AC

DMSO

Miscibility

MeCN

MeOH

H2O

Et2O

DCM

EtOAc

Acetone

THF

Hexane

[MImEt][DCA]

m

m

m

m

i

p

p

m

m

i

[MImEt][TFSI]

m

m

m

i

i

m

m

m

m

i

m

m

m

m

i

p

p

m

m

i

m

m

m

m

i

p

p

m

m

i

[DiMPy1O] [DCA] [DiMPy2O] [DCA]

ACCEPTED MANUSCRIPT

m

m

m

m

i

p

p

m

m

i

m

m

m

m

i

p

p

m

m

i

m

m

m

i

i

m

m

m

m

i

m

m

m

i

i

m

m

m

i

m

m

m

i

i

m

CR

m

m

i

m

m

m

i

m

US

m

m

m

i

m

m

m

m

i

p

p

m

m

i

m

m

m

m

i

p

p

m

m

i

m

m

m

m

i

p

p

m

m

i

m

m

m

i

p

p

m

m

i

AC

[DiMPy3O]

m

m

m

i

i

m

m

m

m

i

m

m

m

i

i

m

m

m

m

i

m

m

m

i

i

m

m

m

m

i

m

m

m

i

i

m

m

m

m

i

[DCA] [DiMPy4O] [DCA] [DiMPy1O]

[DiMPy2O] [TFSI] [DiMPy3O] [TFSI] [DiMPy4O]

[DCA]

[DCA] [DiBIm4O]

[DiBIm1O]

CE

m

[DCA]

PT

[DiBIm3O]

ED

[DiBIm2O]

M

[DiBIm1O] [DCA]

i

AN

[TFSI]

m

IP

T

[TFSI]

m

[TFSI]

[DiBIm2O] [TFSI] [DiBIm3O] [TFSI] [DiBIm4O] [TFSI]

ACCEPTED MANUSCRIPT

m

m

m

m

i

p

p

m

m

i

m

m

m

m

i

p

p

m

m

i

m

m

m

m

i

p

p

m

m

i

m

m

m

m

i

p

m

m

i

m

m

m

i

i

m

CR

m

m

i

m

m

m

i

US

[DiDES1O]

m

m

m

m

i

m

m

m

i

m

m

m

m

i

m

m

m

i

m

m

m

m

i

[DCA] [DiDES2O] [DCA] [DiDES3O]

[DiDES4O] [DCA] [DiDES1O] [TFSI] [DiDES2O]

i

AN

[TFSI] [DiDES3O]

i

p

ED

[DiDES4O]

i

M

[TFSI]

m

IP

T

[DCA]

[TFSI]

PT

i, immiscible; m, miscible; p, partly miscible

CE

3.3 Ionic Conductivity test

The as-prepared electrolyte formulations were listed in TABLE IV. The ionic conductivity data

AC

of neat DILs were compared in Figure 2, 4, 6, and the ionic conductivity data of DILs based electrolytes were paralleled in Figure 3, 5, 7. TABLE IV. Formulations of the electrolytes with 0.5M LiTFSI in DILs/[MImEt][DCA]

Ionic conductivity Components weight (g) Formulation

(mS cm-1)

DILs abbreviation DILs

[MImEt][DCA]

LiTFSI

25 °C

70°C

[DiMPy1O][DCA]

0.1002

0.1008

0.0246

6.2

22.3

E2a

[DiMPy2O][DCA]

0.1

0.1

0.0242

9.1

25.3

E3a

[DiMPy3O][DCA]

0.1015

0.0987

0.0242

5.2

18.54

E4a

[DiMPy4O][DCA]

0.0995

0.1002

0.0240

3.68

15.19

E5a

[DiMPy1O][TFSI]

0.1103

0.1105

0.0238

8.1

26.6

E6a

[DiMPy2O][TFSI]

0.0985

0.1

9.74

28.5

E7a

[DiMPy3O][TFSI]

0.0998

0.102

0.0216

6.95

22.1

E8a

[DiMPy4O][TFSI]

0.1022

0.1002

0.0215

4.68

16.37

E1b

[DiBIm1O][DCA]

0.1022

0.0245

5.23

21.4

E2b

[DiBIm2O][DCA]

0.1021

0.0998

0.0244

5.01

18.71

E3b

[DiBIm3O][DCA]

0.1012

0.1001

0.0243

4.93

17.99

E4b

[DiBIm4O][DCA]

0.102

0.1002

0.0244

4.77

16.98

E5b

[DiBIm1O][TFSI]

0.1001

0.1002

0.0219

5.19

18.51

E6b

[DiBIm2O][TFSI]

0.1048

0.105

0.0228

6.85

20.9

E7b

[DiBIm3O][TFSI]

0.1032

0.1025

0.0223

6.11

18.9

E8b

[DiBIm4O][TFSI]

0.1024

0.1006

0.0218

8.35

20.8

E1c

[DiDES1O][DCA]

0.101

0.1008

0.0241

5.08

19.45

[DiDES2O][DCA]

0.1017

0.1001

0.0241

5.05

17.55

E3c

[DiDES3O][DCA]

0.1007

0.1

0.0240

4.49

17.73

E4c

[DiDES4O][DCA]

0.1017

0.1001

0.0242

8.19

22.7

E5c

[DiDES1O][TFSI]

0.1176

0.116

0.0249

6.31

19.99

E6c

[DiDES2O][TFSI]

0.102

0.0998

0.0216

6.13

19.22

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0.1002

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0.1017

0.1006

0.0219

6.84

20.8

E8c

[DiDES4O][TFSI]

0.1012

0.1003

0.0219

6.78

24.3

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Figure 2. Ionic Conductivity versus Temperature of neat MPy-based DILs

Figure 3. Ionic Conductivity versus Temperature of MPy-based electrolytes

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Figure 4. Ionic Conductivity versus Temperature of neat BIm-based DILs

Figure 5. Ionic Conductivity versus Temperature of MIm-based electrolytes

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Figure 6. Ionic Conductivity versus Temperature of neat DES-based DILs

Figure 7. Ionic Conductivity versus Temperature of DES-based electrolytes In TABLE V, the ternary mixtures 0.7 M LiTFSI in [DiMPy2O][DCA]/[MImEt] [DCA] (E9a) along with [DiMPy2O][TFSI] (E10a) were prepared, and in TABLE VI, another ternary mixtures 0.5

ACCEPTED MANUSCRIPT M LiTFSI in [DiMPy2O][DCA]/[MImEt] [TFSI] (E11a), along with [DiMPy2O][TFSI] (E12a) were prepared as parallel groups. And their ionic conductivity data were listed in Figure 8. TABLE V. Formulations of the electrolytes with 0.5M and 0.7M LiTFSI in [MImEt][DCA]/DILs

Ionic conductivity Components weight (g) (mS/cm) DILs

[MImEt][DCA]

E2a

[DiMPy2O][DCA]

0.1

0.1

E6a

[DiMPy2O][TFSI]

0.0985

0.1

E9a

[DiMPy2O][DCA]

0.1009

0.1018

E10a

[DiMPy2O][TFSI]

0.1033

0.104

70 °C

0.0242

9.1

25.3

0.0213

9.74

28.5

0.0343

5.79

20

0.0312

7.44

22.5

LiTFSI

25 °C

Notes

0.5M LiTFSI

0.7M LiTFSI

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Compounds

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TABLE VI. Formulations of the electrolytes with 0.5M LiTFSI in DILs/[MImEt][TFSI]

Ionic conductivity

Components weight (g)

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(mS/cm)

Notes

DILs

[MImEt][TFSI]

LiTFSI

25 °C

70 °C

E11a

[DiMPy2O][DCA]

0.1001

0.1006

0.0217

2.4

11.25

0.5M

E12a

[DiMPy2O][TFSI]

0.1139

0.114

0.0215

2.8

10.32

LiTFSI

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Compounds

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Figure 8. Ionic Conductivity versus Temperature of [DiMPy2O]-based electrolytes with

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different components.

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Many factors can affect their conductivity, such as viscosity, density, ion size, anionic charge delocalization, aggregations and ionic motions. It is apparent that the ionic conductivity increases

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with increasing temperature for all the investigated neat DILs and DILs based electrolytes. The

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decrease in viscosity [45] and faster migration of carrier ions and easier motion in the electrolyte system at higher temperatures are possibly responsible for this phenomenon [46].

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The neat DILs showed poor ionic conductivity, because of very high viscosity. As we are known that the conductivity of ILs would decrease significantly upon mixing with a lithium salt [45]. So, the lithium-ion batteries fabricated only using neat DILs as the electrolyte may exhibit more disappointing ionic conductivity, which can be ascribed to the following two reasons: First, the decreasing ionic conductivity of the electrolytes caused by the increased viscosity at low temperature; and second, the largely decreased lithium-ion diffusion rate across electrolyte system at low temperature. To overcome this limitation of DILs, they must be blended with other low

ACCEPTED MANUSCRIPT viscosity and high stability solvents, then the ternary electrolyte system, 0.5 M LiTFSI in DILs/[MImEt] [DCA] (the DILs: [MImEt] [DCA] weight ratio was fixed equal to 1:1), were developed. [MImEt][DCA] with low viscosity properties (14.82 cp) acts as a thinner solvent in this electrolyte system. The addition of [MImEt][DCA] is able to enhance the mobility of those

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dissociated ions by decreasing the viscosity of the mixture, and hence, increase the conductivity

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of the whole system, as compared with the single solvent electrolyte [47]. Meanwhile, DILs serve

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not only as nonflammable and non-volatile plasticizers but also as ionic carriers, because DILs with different length of EO chains are good solvents for lithium salts (LiTFSI) forming

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homogeneous liquid solutions. So, comparing with neat DILs (Figures 2, 4 and 6), the series of

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DILs based electrolytes (Figures 3, 5 and 7) showed much higher conductivity. Figures 3, 5 and 7, E6a exhibits the highest ionic conductivity over the whole temperature

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range studied, with a conductivity of 9.74 mS cm-1 at 25°C (see TABLE IV). [DiMPy2O][TFSI] has

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two EO chains, and one methyl group on each pyrrolidine ring, the closely related [DiMPy2O][DCA] in E2a also with two EO chains but with different anions [DCA] shows the

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second highest ionic conductivity (TABLE IV). The ether groups were effective in lowering the

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crystallinity of DILs and also in improving the ionic conductivity. Thus, ionic conductivity depends on the length of EO chains in the cations and on the concentration of the salt applied [48].

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Figure 8 suggests that the concentration of charge carriers of these DILs based electrolytes predominate over their segmental mobility in governing the conductivities. By comparing E2a and E9a (TABLE V), with increasing the concentration of LiTFSI in the composite electrolyte from 0.5M up to 0.7M, the ionic conductivities decrease from 9.1 mS cm-1 to 5.79 mS cm-1 at 25°C. The similar trend also happens between E6a and E10. This behavior in conductivity is a general feature of DILs based electrolytes, and can be explained as a trade-off between increasing number of

ACCEPTED MANUSCRIPT charge carriers and reduced chain mobility upon salt addition [49]. While [MImEt][TFSI] instead of [MImEt][DCA] as a thinner solvent, the ionic conductivity of E11a is only 2.4 mS cm -1 and E12a is only 2.8 mS cm-1 at 25 °C (see TABLE VI). 3.4 Cyclic Voltammetry.

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Figure 9 shows the cyclic voltammetry of best four DILs based electrolytes. As is shown, all

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of the four electrolytes exhibited pretty similar high voltage stability. E2a shows the

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electrochemical stability in the potential range of -2.5~5.0 V; E6a exhibits in the range of -2~3.8 V, E8b performs in the range of -2.5~4.5 V and E4c displays in the range of -1.8~4.5 V.

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E8b has the widest electrochemical window when the anodic potential is swept to the very

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positive region, an oxidation current arises at of +4.5 V, suggesting the oxidative decomposition of the ILs system. This onset potential (+4.5 V) can be regarded as a “cathodic limit” of the E8b,

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which defines the highest potential for the DILs-based electrolyte to be stable at. Obviously, this

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potential value of the cathodic limit is far positive than the highest charging potential (~3 V) of the sulfur cathode and therefore it is expected that not only the E8b electrolyte can be used for

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rechargeable sulfur cathodes, but also all the other three electrolyte system. When the potential is

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scanned to the negative potential region, the CV curve shows a featureless background line until 2.5 V, indicating the chemical and electrochemical stability of the ILs system against lithium. With

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the presence of lithium ions in the electrolyte, the cathodic peak appeared at 0.5 V show the electrochemical deposition of Li ions and the anodic peak appeared at -2.5 V indicates successive anodic oxidation of metallic lithium [50]. The appearance of a pair of oxidation-reduction peaks of lithium suggests that lithium can be very well charged and discharged in the E8b electrolyte system without any electrochemical hindrance. Except for these features, there are no any other current bands observed in the CV curve, showing a noble electrochemical stability in the potential

ACCEPTED MANUSCRIPT range of -2.5~4.5 V and a sufficient potential window of the 0.5 M LiTFSI in [DiBIm4O][TFSI] /[MImEt][DCA] (E8b) for Li/S cells [51]. The stability of the DILs based electrolytes to achieve high voltages against lithium is confirmed, which ensures their capability for further use in high

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potential electrolyte materials.

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Figure 9. CV curves for best four DILs based electrolytes

4. Conclusion

In conclusion, three series of DILs based on MPy, BIm and DES have been successfully synthesized and characterized. All prepared DILs displayed good thermal stability (up to 200 °C) and non-flammability. Moreover, the ternary electrolytes of 0.5 M LiTFSI in DILs combination with [MImEt][DCA] at 1:1 weight ratios have been prepared, and their properties have been

ACCEPTED MANUSCRIPT investigated. The DILs based electrolytes showed wide electrochemical stability and acceptable ion conductivity. Such impressive results, especially excellent electrochemical performance at low–medium temperatures, make the DILs-based electrolytes prepared in this work promising

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electrolytes to make safe lithium rechargeable batteries.

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ACCEPTED MANUSCRIPT Highlights 

Three classes of new dicationic ionic liquids (DILs) have been synthesized and characterized. The density, miscibility, viscosity, and neat ionic conductivity are reported for each.

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Additionally, several electrolyte systems containing DILs and LiTFSI were tested for

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flammability and ionic conductivity.

All DILs exhibited thermal stability up to 150 °C and non-flammability.

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These DIL-based electrolyte systems were determined to be electrochemically suitable

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for use in lithium-sulfur batteries.