IONIC LIQUID AND PLASTIC CRYSTAL

Abstract

The present invention provides an ionic liquid or plastic crystal comprising an anion and a cation, the anion comprising [C(SO.sub.2F).sub.3].sup.−, and the cation comprising at least one member selected from the group consisting of 1-ethyl-3-methylimidazolium ([EMI].sup.+), N,N-diethyl-N-methyl-(2-methoxyethyl)ammonium ([DEME].sup.+), N-methyl-N-propylpyrrolidinium ([Py.sub.13].sup.+), N-methyl-N-propylpiperidinium ([PP.sub.13].sup.+), tetramethylammonium ([N.sub.1111].sup.+), tetraethylammonium ([N.sub.2222].sup.+), trimethylhexylammonium ([N.sub.6111].sup.+), triethylhexylammonium ([N.sub.6222].sup.+), N-methyl-ethylpyrrolidinium ([Py.sub.12].sup.+), 1-butyl-3-methylimidazolium ([C.sub.4mim].sup.+), and 1-hexyl-3-methylimidazolium ([C.sub.6mim].sup.+).

Claims

1. An ionic liquid or plastic crystal comprising an anion and a cation, the anion comprising [C(SO.sub.2F).sub.3].sup.−, and the cation comprising at least one member selected from the group consisting of 1-ethyl-3-methylimidazolium ([EMI].sup.+), N,N-diethyl-N-methyl-(2-methoxyethyl)ammonium ([DEME].sup.+), N-methyl-N-propylpyrrolidinium ([Py.sub.13].sup.+), N-methyl-N-propylpiperidinium ([PP.sub.13].sup.+), tetramethylammonium ([N.sub.1111].sup.+), tetraethylammonium ([N.sub.2222].sup.+), trimethylhexylammonium ([N.sub.6111].sup.+), triethylhexylammonium ([N.sub.6222].sup.+), N-methyl-N-ethylpyrrolidinium ([Py.sub.12].sup.+), 1-butyl-3-methylimidazolium ([C.sub.4mim].sup.+), and 1-hexyl-3-methylimidazolium ([C.sub.6mim].sup.+).

2. The ionic liquid or plastic crystal according to claim 1, wherein the cation comprises at least one member selected from the group consisting of 1-ethyl-3 -methylimidazolium ([EMI].sup.+), N,N-diethyl-N-methyl-(2-methoxyethyl)ammonium ([DEME].sup.+), N-methyl-N-propylpyrrolidinium ([Py.sub.13].sup.+), and N-methyl-N-propylpiperidinium ([PP.sub.13].sup.+).

3. The ionic liquid or plastic crystal according to claim 1, which is an ionic liquid, wherein the cation comprises [EMI].sup.+.

4. A non-aqueous electrolyte for lithium secondary batteries, the electrolyte comprising an anion and a cation, the anion comprising [C(SO.sub.2F).sub.3].sup.−, and the cation comprising at least one member selected from the group consisting of 1-ethyl-3-methylimidazolium ([EMI].sup.+), N,N-diethyl-N-methyl-(2-methoxyethyl)ammonium ([DEME].sup.+), N-methyl-N-propylpyrrolidinium ([Py.sub.13].sup.+), N-methyl-N-propylpiperidinium ([PP.sub.13].sup.+), tetramethylammonium ([N.sub.1111].sup.+), tetraethylammonium ([N.sub.2222].sup.+), trimethylhexylammonium ([N.sub.6111].sup.+), triethylhexylammonium ([N.sub.6222].sup.+, N-methyl-N-ethylpyrrolidinium ([Py.sub.12].sup.+), 1-butyl-3-methylimidazolium ([C.sub.4mim].sup.+), and 1-hexyl-3-methylimidazolium ([C.sub.6mim].sup.+).

5. The non-aqueous electrolyte for lithium secondary batteries according to claim 4, wherein the cation comprises at least one member selected from the group consisting of 1-ethyl-3-methylimidazolium ([EMI].sup.+), N,N-diethyl-N-methyl-(2-methoxyethyl)ammonium ([DEME].sup.+), N-methyl-N-propylpyrrolidinium ([Py.sub.13].sup.+), and N-methyl-N-propylpiperidinium ([PP.sub.13].sup.+).

6. The non-aqueous electrolyte for lithium secondary batteries according to claim 4, wherein the cation comprises [EMI].sup.+.

7. The non-aqueous electrolyte for lithium secondary batteries according to claim 4, further comprising Li[C(SO.sub.2F).sub.3].

8. A lithium secondary battery comprising the non-aqueous electrolyte of claim 4.

9. The lithium secondary battery according to claim 8, comprising a Li metal negative electrode as a negative electrode.

10. The ionic liquid or plastic crystal according to claim 2, which is an ionic liquid, wherein the cation comprises [EMI].sup.+.

11. The non-aqueous electrolyte for lithium secondary batteries according to claim 5, wherein the cation comprises [EMI].sup.+.

12. The non-aqueous electrolyte for lithium secondary batteries according to claim 5, further comprising Li[C(SO.sub.2F).sub.3].

13. The non-aqueous electrolyte for lithium secondary batteries according to claim 6, further comprising Li[C(SO.sub.2F).sub.3].

14. A lithium secondary battery comprising the non-aqueous electrolyte of claim 5.

15. A lithium secondary battery comprising the non-aqueous electrolyte of claim 6.

16. A lithium secondary battery comprising the non-aqueous electrolyte of claim 7.

17. The lithium secondary battery according to claim 14, comprising a Li metal negative electrode as a negative electrode.

18. The lithium secondary battery according to claim 15, comprising a Li metal negative electrode as a negative electrode.

19. The lithium secondary battery according to claim 16, comprising a Li metal negative electrode as a negative electrode.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0030] FIG. 1: Cyclic voltammogram of Li[f.sub.3C]-containing [EMI] [f.sub.3C] on a platinum electrode.

[0031] FIG. 2: Cyclic voltammogram of Li[f.sub.3C]-containing [EMI] [f.sub.3C] on a nickel electrode.

[0032] FIG. 3: Electrochemical AC impedance spectra of Li metal symmetrical cells.

[0033] FIG. 4: 1C Charge-discharge cyclic performance of LiCoO.sub.2/Li cells, in terms of [EMI] [f.sub.3C] and [EMI] [Tf.sub.2N].

[0034] Upper graph: Cycle dependency of discharge capacity.

[0035] Lower graph: Charging and discharging coulomb efficiency.

[0036] FIG. 5: AC impedance plots of charged LiCoO.sub.2/Li cells.

[0037] a) [EMI] [f.sub.3C]

[0038] b) [EMI] [Tf.sub.2N]

[0039] FIG. 6: Thermogravimetric changes in a nitrogen atmosphere.

[0040] FIG. 7: DSC measurement results of [EMI] [f.sub.3C] and [EMI] [Tf.sub.3C].

[0041] FIG. 8: DSC measurement results of [DEME] [f.sub.3C] and [DEME] [Tf.sub.3C].

[0042] FIG. 9: DSC measurement results of [Py.sub.13 ] [f.sub.3C], [PP.sub.13] [f.sub.3C], [Py.sub.13] [Tf.sub.3C], and [PP.sub.13] [Tf.sub.3C].

DESCRIPTION OF EMBODIMENTS

[0043] The anion of the ionic liquid or plastic crystal of the present invention comprises [f.sub.3C].sup.−, and the cation comprises at least one member selected from [EMI].sup.+, [DEME].sup.+, [Py.sub.13].sup.+, [PP_].sup.+, [N.sub.1111].sup.+, [N.sub.2222].sup.+, [N.sub.6111].sup.+, [N.sub.6222].sup.+, [Py.sub.12].sup.+, [C.sub.4mim].sup.+, and [C.sub.6mim].sup.+. The ionic liquid is obtained when the cation is [EMI].sup.+, [N.sub.6111].sup.+, [N.sub.62222].sup.+, [C.sub.4mim].sup.+, and [C.sub.6mim].sup.+, and the plastic crystal is obtained when the cation is any of [DEME].sup.+, [Py.sub.13].sup.+, [PP.sub.13].sup.+, [N.sub.1111].sup.+, [N.sub.2222].sup.+, and [Py.sub.12].sup.+.

[0044] The anion may consist only of [f.sub.3C].sup.−, or a combination of [f.sub.3C].sup.− and other anions. Examples of other anions include FSI.sup.− ([(FSO.sub.2).sub.2N].sup.−, TFSI.sup.− ([(CF.sub.3SO.sub.2).sub.2N].sup.−), FTA.sup.− ([(FSO.sub.2) (CF.sub.3SO.sub.2)N].sup.+), Tf.sub.3C.sup.− ([(CF.sub.3SO.sub.2).sub.3C].sup.−), and the like. The content of the [f.sub.3C] anion is preferably 50 mol % or more, preferably 70 mol % or more, more preferably 80 mol % or more, even more preferably 90 mol % or more, particularly preferably 95 mol % or more, and most preferably 100 mol %, of the total anion.

[0045] The cation is more preferably [EMI].sup.+, [DEME].sup.+, [Py.sub.13].sup.+, and [PP.sub.13].sup.+, and still more preferably [EMI].sup.+. It is also possible to combine [EMI].sup.+ with at least one member selected from [DEME].sup.+, [Py.sub.13].sup.+, [PP.sub.13].sup.+, [N.sub.1111].sup.+, [N.sub.2222].sup.+, [N.sub.6111].sup.+, [N.sub.6222].sup.+, [Py.sub.12].sup.+, [C.sub.4mim].sup.+, and [C.sub.6mim].sup.+. In one preferable embodiment, the content of [EMI].sup.+ in the total cation is 30 mol % or more, preferably 50 mol %, or more, more preferably 70 mol % or more, still more preferably 90 mol % or more, particularly preferably 95 mol % or more, and most preferably 100 mol %.

[0046] The following are the abbreviations and structural formulas of cations and anions used in the present specification.

##STR00001##

[0047] In this specification, an ionic liquid refers to a substance that is liquid at room temperature (melting point: 35° C. or lower), and a plastic crystal undergoes, at a temperature lower than the above melting point, a solid-solid phase transition accompanied by a great calorimetric change that is equivalent to a calorific value obtained when a general solid or liquid is subjected to melting, and achieves, as a result, a melting entropy of about 20 J K.sup.−1 mol.sup.−1 or less. To be identified as the plastic crystal of the present invention, one or both of the following requirements must be satisfied, i.e., undergoing solid-solid phase transition at a temperature lower than the melting point, and achieving a melting entropy of about 20 J K.sup.−1 mol.sup.−1 or less (J. Timmermans, J. Phys. and. Chem. Solids, 18(1), (1961)). A plastic crystal, which is completely different from general crystalline materials, is characterized as being a flexible, sticky solid.

[0048] More specifically, the ionic liquid is [EMI] [f.sub.3C], [N.sub.6111] [f.sub.3C], [N.sub.6222] [f.sub.3C], [C.sub.4mim] [f.sub.3C], and [C.sub.6mim] [f.sub.3C], and the plastic crystal is [DEME] [f.sub.3C], [Py.sub.13] [f.sub.3C], [PP.sub.13] [f.sub.3C], [N.sub.1111] [f.sub.3C], [N.sub.2222] [f.sub.3C], and [Py.sub.12] [f.sub.3C].

[0049] An alkali metal salt comprising [f.sub.3C].sup.− is a known substance and may be produced by a known production method. The following is a preferable production scheme.

##STR00002##

[0050] In the formula, M represents an alkali metal, and preferably Na, K, or Li; Z represents [EMI].sup.+, [DEME].sup.+, [Py.sub.13].sup.+, [PP.sub.13].sup.+, [N.sub.1111].sup.+, [N.sub.2222].sup.+, [N.sub.6111].sup.+, [N.sub.6222].sup.+, [Py.sub.12].sup.+, [C.sub.4mim].sup.+, or [C.sub.6mim].sup.+.

[0051] Compound (1), which is a starting material, is sulfated with oleum to yield Compound (2), which is reacted with SF.sub.4 to obtain Compound (3). Compound (3) is reacted with a base such as alkali metal carbonate or alkali metal hydrogen carbonate (e.g., sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, lithium carbonate, or lithium hydrogen carbonate) to form a salt (4) of an alkali metal (M), which is then reacted with [EMI].sup.+, [DEME].sup.+, [Py.sub.13].sup.+, [PP.sub.13].sup.+, [N.sub.1111].sup.+, [N.sub.2222].sup.+, [N.sub.6111].sup.+, [N.sub.6222].sup.+, [Py.sub.12].sup.+, [C.sub.4mim].sup.+, or [C.sub.6mim].sup.+ to perform cation exchange. In this manner, a target ionic liquid or plastic crystal of the present invention is obtained.

[0052] The lithium second battery of the present invention has active material layers on collectors, and the ionic liquid or plastic crystal of the present invention is used as the electrolyte. One electrode is separated by a separator.

[0053] As a collector usable as a positive electrode, it is possible to use metal sheet, such as aluminum, stainless steel, nickel, and titanium, and the like. In addition, it is preferable to use aluminum and stainless steel whose surface is coated with carbon, nickel, titanium, or silver, and alloys obtained by incorporating carbon, nickel, titanium, or silver into the surface of the aluminum or stainless steel.

[0054] As a collector used as a negative electrode, it is preferable to use copper, stainless steel, nickel, and titanium. The collector is usually used in the form of a film or a sheet; however, a porous body, a foam, or the like may also be used. The thickness of the collector is not particularly limited, and is preferably 1 to 500 μm. The surface of the collector is preferably provided with irregularities by a surface treatment.

[0055] Examples of positive electrode active materials include, but are not limited to, Li.sub.0.3MnO.sub.2, Li.sub.4Mn.sub.5O.sub.12, V.sub.2O.sub.5, LiCoO.sub.2, LiMn.sub.2O.sub.4, LiNiO.sub.2, LiFePO.sub.4, LiCO.sub.1/3Ni.sub.1/3Mn.sub.1/3O.sub.2, Li.sub.1.2(Fe.sub.0.5Mn.sub.0.5).sub.0.8O.sub.2, Li.sub.1.2(Fe.sub.0.4Mn.sub.0.4Ti.sub.0.2).sub.0.8O.sub.2, Li.sub.1+x(Ni.sub.0.5Mn.sub.0.5).sub.1−xO.sub.2, LiNi.sub.0.5Mn.sub.1.5O.sub.4, Li.sub.2MnO.sub.3, Li.sub.0.76Mn.sub.0.51Ti.sub.0.49O.sub.2, LiNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2, Fe.sub.2O.sub.3, LiCoPO.sub.4, LiMnPO.sub.4, Li.sub.2MPO.sub.4F (M=Fe, Mn), LiMn.sub.0.875Fe.sub.0.125PO.sub.4, Li.sub.2FeSiO.sub.4, Li.sub.2−xMSi.sub.1−xP.sub.xO.sub.4 (M=Fe, Mn), LiMBO.sub.3 (M=Fe, Mn), FeF.sub.3, Li.sub.3FeF.sub.6, Li.sub.2TiF.sub.6, Li.sub.2FeS.sub.2, TiS.sub.2, MoS.sub.2, FeS, and the like (however, x is within the range of 0 to 1).

[0056] The negative electrode active material is not particularly limited, and known negative electrode active materials may be used. Examples of negative electrode active materials preferably used in the non-aqueous electrolyte lithium secondary battery of the present invention include carbon materials, as well as metal oxides, metal nitrides, and the like that are capable of incorporating lithium ions. Examples of carbon materials include natural graphite, artificial graphite, pyrolytic carbons, cokes, meso-carbon microbeads, carbon fibers, active carbons, pitch-coated graphite, and the like. Example of metal oxides capable of incorporating lithium ions include metal compounds that contain tin or silicon, such as tin oxide and silicon oxide. Examples of metal nitrides include Li.sub.2.6Co.sub.0.4N and the like. Examples also include a mixture comprising graphite, a tin alloy, and a binding agent; a silicon thin film; and a lithium foil.

[0057] The active material layers contain the positive electrode active material or negative electrode active material described above, and preferably further contain a conducting agent and a binding agent. A filler and a lithium salt may also be incorporated as additional materials. Examples of a conducting agent include natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, metal powders, metal fibers, polyphenylene derivatives, and the like. Examples of a binding agent include water-soluble polymers such as carboxymethyl cellulose, cellulose, diacetyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, sodium alginate, polyacrylic acid, sodium polyacrylate, polyvinyl phenol, polyvinyl methyl ether, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylonitrile, polyacrylamide, polyhydroxy(meth)acrylate, and styrene-maleic acid copolymer; emulsions (latexes), such as polyvinyl chloride, polytetrafluoroethylene, polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM) sulfonated EPDM, polyvinyl acetal resin, (meth)acrylic acid ester-containing (meth)acrylic acid ester copolymers, such as methyl methacrylate and 2-ethylhexyl acrylate, vinyl ester-containing polyvinyl ester copolymers, such as (meth)acrylic acid ester-acrylonitrile copolymer and vinyl acetate, styrene-butadiene copolymers, acrylonitrile-butadiene copolymers, polybutadiene, neoprene rubber, fluororubber, polyethylene oxide, polyester-polyurethane resin, polyether-polyurethane resin, polycarbonate-polyurethane resin, polyester resin, phenol resin, and epoxy resin; and the like.

[0058] The filler is not particularly limited, as long as it is a fibrous material that does not undergo a chemical change in the non-aqueous electrolyte secondary battery of the present invention. Usually, olefin-based polymers, such as polypropylene and polyethylene, and fibers, such as glass and carbon, are used. The amount of the filler added is not particularly limited, and is preferably 0 to 30 mass %.

[0059] Examples of lithium salts include those that exhibit thermal resistance at a temperature within an intermediate temperature range, such as LiBF.sub.4, LiCF.sub.3SO.sub.3, LiC(CF.sub.3SO.sub.2).sub.3, LiC(FSO.sub.2).sub.3, LiN(CF.sub.3SO.sub.2).sub.2, LiN(FSO.sub.2).sub.2, LiN(CF.sub.3CF.sub.2SO.sub.2).sub.2, LiN(CF.sub.3SO.sub.2) (C.sub.2F.sub.5SO.sub.2), LiN(CF.sub.3SO.sub.2) (C.sub.4F.sub.9SO.sub.2), LiN(CF.sub.3CF.sub.2CO).sub.2, and LiBOB. These salts may be used alone, or in a combination of two or more. A preferable lithium salt is LiC(FSO.sub.2).sub.3.

[0060] In a preferable embodiment, the lithium secondary battery of the present invention may have a structure of a laminate cell itself. It is preferable that the separator has excellent capability of being impregnated with an ionic liquid, and has excellent thermal resistance. Specific examples of the separator include a silica nanoparticle-containing polyolefin separator and an inorganic glass filter.

[0061] Preferable examples of the positive electrode active material of a battery include LiFePO.sub.4, which has excellent thermal resistance.

[0062] It is assumed that batteries that are operable at room temperature may be operable as is at high temperatures. In fact, however, batteries that use a previously known organic solvent electrolyte solution cannot be operated at high temperatures, and must be sufficiently air-cooled. Further, the formation of an air pathway reduces the space filling rate; thus, even if it is possible to achieve a great capacity per unit cell, the efficiency of the whole system is reduced. The non-aqueous electrolyte for lithium secondary batteries of the present invention, which has excellent thermal resistance and excellent durability, contributes to the development of a battery with excellent thermal resistance, and is preferable, in particular, for use in large-scale stationary storage batteries because the efficiency greatly improves.

[0063] The lithium secondary battery of the present invention is applicable to any shape, such as a sheet, a square, and a cylinder. The shape of the electrode may also be optimally selected according to the shape of the non-aqueous electrolyte secondary battery.

[0064] The positive electrode active material layer and the negative electrode active material layer are provided on a collector. The positive electrode active material layer and negative electrode active material layer may be provided on one surface or both surfaces of a collector. It is more preferable to use an electrode having positive and negative electrode active material layers on both surfaces. There is no particular limitation to the size of the negative-electrode plate relative to the size of the positive-electrode plate. The positive electrode plate area is preferably 0.9 to 1.1, and particularly preferably 0.95 to 1.0, based on the positive electrode plate area being 1. The electrode is obtained by applying an active material-containing coating liquid to the surface of a collector, followed by drying and further pressing to form an active material layer.

[0065] As the coating liquid, for example, a slurry coating liquid may be used, optionally comprising a conducting agent mentioned above, a binding agent mentioned above, and a dispersion medium, such as N-methyl-2-pyrroIidone (NMP), water, and toluene.

[0066] Examples of the application method include reverse roll coating, direct roll coating, blade coating, knife coating, extrusion coating, curtain coating, gravure coating, bar coating, dip coating, and squeeze coating. Of these, blade coating, knife coating, and extrusion coating are preferable. The application is preferably performed at a rate of 0.1 to 100 m/min. The application method may be selected from the above in view of the solution properties and drying properties of the coating liquid, and in this way, it is possible to obtain an excellent surface state of the coating layer. The application of the coating liquid may be performed sequentially with respect to one surface at a time or both surfaces simultaneously.

[0067] The electrolyte solution of a lithium secondary battery may be obtained, for example, by dissolving or mixing an alkali metal salt (supporting electrolyte) that is believed to exhibit desired thermal resistance with an ionic liquid or plastic crystal that contains an anion, such as [f.sub.3C].sup.−, which shows excellent thermal resistance. These can be uniformly mixed by heating to the melting point or higher of a mixed salt that is obtained by mixing with a salt given as an example in the present invention. Examples of anions of the supporting electrolyte include [BF.sub.4].sup.−, [(FSO.sub.2).sub.2N].sup.−, [(FSO.sub.2) (CF.sub.3SO.sub.2)N].sup.−, [(CF.sub.3SO.sub.2).sub.2N].sup.−, [(CF.sub.3CF.sub.2SO.sub.2).sub.2]N.sup.−, [(C.sub.2H.sub.4O.sub.2).sub.2B].sup.−, [(CF.sub.3SO.sub.2).sub.3C].sup.−, [CF.sub.3SO.sub.3].sup.−, and the like. Examples further include [f.sub.3C].sup.−, i.e., the anion of the electrolyte of the present invention, and the like. As the anion of the supporting electrolyte, it is preferable to use [f.sub.3C].sup.−, which is the same anion used in the electrolyte of the present invention. As the cation of the supporting electrolyte, if is preferable to use Li.sup.+.

[0068] In a preferable embodiment, the lithium secondary battery of the present invention has a laminate structure in which the positive electrode active material layer described above is provided on one surface of a separator, and the negative electrode active material layer is provided on the other surface of the separator. Further, a collector is provided on the active material layer surfaces opposite to the separator. The separator layer is impregnated with an electrolyte mixture that comprises a supporting electrolyte and the electrolyte of the present invention, the electrolyte mixture being in a dissolution state by heating if necessary.

[0069] The laminate structure is not limited to a simple, single layer laminate, and may be, for example, a multilayer laminate structure that comprises a plurality of these laminate structures, a structure that comprises a combination of laminates in which layers are formed on both surfaces of a collector, and a structure obtained by winding these formations.

[0070] A non-portable (stationary) lithium secondary battery has a multilayer laminate structure. However, the electrolyte of the present invention, which is highly stable at a temperature within an intermediate temperature range, can reduce or eliminate the number of fans for cooling or air pathways for cooling, making it possible to increase the storage capacity per unit volume in a state of an assembled battery in which single cells axe assembled.

EXAMPLES

[0071] The present invention is described below in more detail with reference to Examples and Comparative Examples.

[0072] In Examples and Comparative Examples, commercially available products were used for cation components, i.e., [EMI].sup.+, ([C.sub.2mim].sup.+, [DEME].sup.+, [Py.sub.12].sup.+, [Py.sub.13].sup.+, [Py.sub.14].sup.+, [PP.sub.13].sup.+, [PP.sub.14].sup.+, [N.sub.1111].sup.+, [N.sub.2222].sup.+, [N.sub.6111].sup.+, [N.sub.6222].sup.+, [C.sub.1mim].sup.+, [C.sub.4mim].sup.+, and [C.sub.6mim].sup.+; and for anion components, i.e., [Tf.sub.2N].sup.− (=[(CF.sub.3SO.sub.2).sub.2N].sup.+), [f.sub.2N].sup.− (=[(FSO.sub.2).sub.2N].sup.−), and [Tf.sub.3C].sup.− (=[(CF.sub.3SO.sub.2).sub.3C].sup.−).

[0073] The thermal gravimetric analysis (TGA) was performed using a Seiko Instruments TG/DTA 6200 in a nitrogen stream at a scan rate of 10° C. per minute.

[0074] The differential scanning calorimetry (DSC) was performed using a Perkin Elmer Pyris 1 at a scan rate of 10° C. per minute.

[0075] The thermal resistance was evaluated based on the thermal-decomposition temperature measured by TGA. The state of a liquid, a solid, or a plastic crystal was determined in view of the presence or absence of phase transition temperature, its temperature, and calorimetric change, as measured by DSC.

Production Example 1

Synthesis of Anion (f.SUB.3.C)

[0076] Compound (2) (58.4 mg), which is a commercially available product obtained from Alcatraz Chemicals (Gujarat, India), was reacted with 194 mg of SF.sub.4 to obtain 51.6 mg of compound (3). Then, 92.4 mg of compound (3) was reacted with an excess amount of potassium carbonate to form 93.9 mg of potassium salt (4) comprising f.sub.3C anion.

Example 1

[0077] Potassium salt (4) comprising f.sub.3C anion obtained id Production Example 1 was reacted with an equimolar bromide of [EMI].sup.+, [DEME].sup.+, [Py.sub.13].sup.+, or [PP.sub.13].sup.+ to perform cation exchange. In this manner, a target ionic liquid ([EMI] [f.sub.3C]) of the present invention and target plastic crystals ([DEME] [f.sub.3C], [Py.sub.13] [f.sub.3C], and [PP.sub.13] [f.sub.3C]) of the present invention were obtained.

[0078] The following shows the physical property values of the obtained ionic liquid and plastic crystals. Table 1 shows the melting point, glass transition temperature, and solid-solid phase transition temperature.

1) [EMI] [f.sub.3C]
.sup.1H-NMR (CD.sub.3CN, 300 MHz): δ=1.45 (t, J=7.2 Hz, 3H), 3.81 (s, 3H), 4.16 (q, J=7.2 Hz, 2H), 7.32 (s, 1H), 7.37 (s, 1H), 8.39 (s, 1H): .sup.19F-NMR (CD.sub.3CN, 283 MHz): δ=71.5 (s, 3F).
Elemental analysis values (theoretical values): H 2.99% (2.98%); C 22.71% (22.58%); N 7.58% (7.52%); F 15.36% (15.31%). Ionic conductivity at 25° C.: 6.2 mS cm.sup.−1. Viscosity at 25° C.: 39 mPa.Math.s. Density at 25° C.: 1.55 g mL.sup.−1. Melting entropy: 40 J K.sup.−1 mol.sup.−1. Thermal-decomposition temperature (at the time of 10% reduction): 246° C.
2) [DEME] [f.sub.3C]
.sup.1-NMR (CD.sub.3CN, 300 MHz): δ=1.26 (t of t, J=7.2 Hz and 1.9 Hz, 6H), 2.92 (s, 3H), 3.27-3.37 (complex, 9H), 3.71 (m, 2H): .sup.19F-NMR (CD.sub.3CN, 283 MHz): δ=71.6 (s, 3F).
Elemental analysis values (theoretical values): H 4.81% (4.95%); C 26.41% (26.53%); N 3.44% (3.44%); F 14.03% (13.99%). Melting entropy: 6.1 J K.sup.−1 mol.sup.−1. Thermal-decomposition temperature (at the time of 10% reduction): 325° C.
3) [Py.sub.13] [f.sub.3C]
1H-NMR (CD.sub.3CN, 300 MHz): δ=0.96 (t, J=7.2 Hz, 3H), 1.75 (m, 2H), 2.15 (m, 4H), 2.93 (s, 3H), 3.17 (m, 2H), 3.39 (m, 4H); .sup.19F-NMR (CD.sub.3CN, 283 MHz): δ=71.5 (s, 3F).
Elemental analysis values (theoretical values): H 4.63% (4.66%); C 27.67% (27.76%); N 3.4% (3.60%); F 14.88% (14.64%). Melting entropy: 9.3 J K.sup.−1 mol.sup.−1. Thermal-decomposition temperature (at the time of 10% reduction): 343° C.
4) [PP.sub.13] [f.sub.3C]
.sup.1H-NMR (CD.sub.3CN, 300 MHz): δ=0.963 (t, J=7.2 Hz, 3H), 1.59-1.88 (m, 8H), 2.92 (s, 3H), 3.14-3.24 (complex, 6H); .sup.19F-NMR (CD.sub.3CN, 283 MHz): δ=71.5 (s, 3F).
Elemental analysis values (theoretical values): H 4.97% (5.00%); C 29.59% (29.77%); N 3.47% (3.47%; F 14.22% (14.13%). Melting entropy: 8.6 J K.sup.−1 mol.sup.−1. Thermal-decomposition temperature (at the time of 10% reduction): 373° C.

Example 2

[0079] Potassium salt (4) comprising f.sub.3C anion obtained in Production Example 1 was reacted with an equimolar bromide of [N.sub.6111].sup.+, [N.sub.6222].sup.+, [N.sub.1111].sup.+, [N.sub.2222].sup.+, [Py.sub.12].sup.+, [C.sub.4mim].sup.+, and [C.sub.6mim].sup.+ to perform cation exchange. In this manner, target ionic liquids of the present invention [N.sub.6111] [f.sub.3C], [N.sub.6222].sup.+ [f.sub.3C], [N.sub.1111] [f.sub.3C], [N.sub.22222] [f.sub.3C], [PP.sub.14] [f.sub.3C], [Py.sub.12] [f.sub.3C], [Py.sub.14] [f.sub.3C], [C.sub.1mim] [f.sub.3C], [C.sub.4mim] [f.sub.3C], and [C.sub.6mim] [f.sub.3C] were obtained. The following shows the physical property values of the obtained ionic liquids or plastic crystals. Table 1 shows the melting point, glass transition temperature, and solid-solid phase transition temperature.

1) [N.sub.1111] [f.sub.3C]
.sup.1H NMR (DMSO-d6, 300 MHz): δ=3.06 (s, 12H); .sup.19F NMR (DMSO-d6, 283 MHz): δ=71.9 (s, 3F).
Thermal-decomposition temperature (at the time of 10% reduction): 348° C.
2) [N.sub.2222] [f.sub.3C]
.sup.1H NMR (CDCl.sub.3, 300 MHz): δ=1.35 (t, J=7.2 Hz, 4×3H), 3.23 (q, J=7.2 Hz, 4×2H); .sup.19F NMR (CDCl.sub.3, 283 MHz): δ=71.2 (s, 3F).
Thermal-decomposition temperature (at the time of 10% reduction): 317° C.
.sup.1H NMR (CDCl.sub.3, 300 MHz): δ=0.91 (t, J=7.2 Hz, 3H), 1.15-1.47 (complex, 3×2H), 1.76 (m, 2H), 3.13 (s, 3×3H), 3.26 (m, 2H); .sup.19F NMR (CDCl.sub.3, 283 MHz): δ=71.5 (s, 3F).
Thermal-decomposition temperature (at the time of 10% reduction): 346° C.
4) [N.sub.6222] [f.sub.3C]
.sup.1H NMR (CDCl.sub.3, 300 MHz): δ=0.91 (t, J=7.2 Hz, 3H), 1.23-1.41 (complex, 3×3H, 3×2H), 1.64 (m, 2H), 3.21 (m, 2H), 3.26 (q, J=7.2 Hz, 3×2H); .sup.19F NMR (CDCl.sub.3, 283 MHz): δ=71.2 (s, 3F).
Thermal-decomposition temperature (at the time of 10% reduction): 340° C.
5) [Py.sub.12] [f.sub.3C]
.sup.1H NMR (DMSO-d6, 300 MHz): δ=1.24 (t, J= 7.2 Hz, 3H), 2.04 (br, 2×2H) , 2.92 (s, 3H), 3.23-3.50 (complex, 3×2H); .sup.19F NMR (DMSO-d6, 283 MHz): δ=71.5 (s, 3F).
Thermal-decomposition temperature (at the time of 10% reduction): 284° C.
6) [C.sub.4mim] [f.sub.3C]
.sup.1H NMR (DMSO-d6, 300 MHz): δ=0.87 (t, J=7.2 Hz, 3H), 1.21 (m, 2H), 1.73 (m, 2H), 3.81 (s, 3H), 4.13 (t, J=7.2 Hz, 2H), 7.67 (t, J=1.7 Hz, 1H), 7.73 (t, J=1.7 Hz, 1H), 9.07 (s, 1H); .sup.19F NMR (DMSO-d6, 283 MHz): δ=71.8 (s, 3F).
Thermal-decomposition temperature (at the time of 10% reduction): 320° C.
7) [C.sub.6mim] [f.sub.3C]
.sup.1H NMR (DMSO-d6, 300 MHz): δ=0.83 (t, J=7.2 Hz, 3H), 1.15-1.32 (complex, 3×2H), 1.76 (m, 2H), 3.81 (s, 3H), 4.12 (t, J=7.2 Hz, 2H), 7.66 (t, J=1.7 Hz, 1H), 7.73 (t, J=1.7 Hz, 1H), 9.07 (s, 1H); .sup.19F NMR (DMSO-d6, 283 MHz): δ=71.9 (s, 3F).
Thermal-decomposition temperature (at the time of 10% reduction): 327° C.

Comparative Example 1

[0080] Potassium salt (4) comprising f.sub.3C anion obtained in Production Example 1 was reacted with an equimolar bromide of [PP.sub.14].sup.30, [Py.sub.14].sup.+, and [C.sub.1mim].sup.+ to perform salt exchange. In this manner, [PP.sub.14] [f.sub.3C], [Py.sub.14] [f.sub.3C], and [C.sub.1mim] [f.sub.3C] were obtained. The following shows the physical property values of the obtained ionic liquids or plastic crystals. Table 1 shows the melting point, glass transition temperature, and solid-solid phase transition temperature.

1) [PP.sub.14] [f.sub.3C]
.sup.1H NMR (DMSO-d6, 300 MHz): δ=0.91 (t, J=7.2 Hz, 3H), 1.29 (m, 2H), 1.49 (m, 2H), 1.61 (m, 2H), 1.74 (br, 2×2H), 2.94 (s, 3H), 3.16-3.42 (br, 3×2H); .sup.19F NMR (DMSO-d6, 283 MHz): δ=71.9 (s, 3F).
Thermal-decomposition temperature (at the time of 10% reduction): 349° C.
2) [Py.sub.14] [f.sub.3C]

[0081] .sup.1H NMR (DMSO-d6, 300 MHz): δ=0.90 (t, J=7.2 Hz, 3H), 1.27 (m, 2H), 1.64 (m, 2H), 2.05 (br, 2×2H), 2.94 (s, 3H), 3.30 (t, J=7.2 Hz, 2H), 3.40 (m, 2×2H); .sup.19F (DMSO-d6, 283 MHz): δ=71.9 (s, 3F).

Thermal-decomposition temperature (at the time of 10% reduction): 351° C.
3) [C.sub.1mim] [f.sub.3C]

[0082] .sup.1H NMR (DMSO-d6, 300 MHz): δ=3.81 (s, 2×3H), 7.64 (s, 1H), 7.65 (s, 1H), 8.99 (s, 1H); .sup.19F NMR (DMSO-d6, 283 MHz): δ=71.9 (s, 3F).

Thermal-decomposition temperature (at the time of 10% reduction): 333° C.

Comparative Example 2

[0083] A commercially available potassium salt comprising Tf.sub.3C anion was reacted with an equimolar bromide of [EMI].sup.+, [N.sub.1111].sup.+, [N.sub.6111].sup.+, [N.sub.6222].sup.+, [PP.sub.14].sup.+, [Py.sub.12].sup.+, [Py.sub.14].sup.+, [C.sub.1mim].sup.+, [C.sub.4mim].sup.+, and [C.sub.6mim].sup.+ to perform salt exchange. In this manner, [EMI] [Tf.sub.3C], [N.sub.1111] [Tf.sub.3C], [N.sub.6111] [Tf.sub.3C], [N.sub.6222] [Tf.sub.3C], [PP.sub.14] [Tf.sub.3C], [Py.sub.12] [Tf.sub.3C], [Py.sub.14] [Tf.sub.3C], [C.sub.1mim] [Tf.sub.3C], [C.sub.4mim] [Tf.sub.3C], and [C.sub.6mim] [Tf.sub.3C] were obtained. Table 2 shows the melting point and Tg of the obtained Tf.sub.3C salts.

Comparative Example 3

[0084] A commercially available potassium salt comprising f.sub.2N anion was reacted with an equimolar bromide of EMI.sup.+ to perform cation exchange. In this manner, [EMI] [f.sub.2N] was obtained. The stability with respect to the Li metal was evaluated in Test Example 1(2).

Comparative Example 4

[0085] A commercially available potassium salt comprising Tf.sub.2N anion was reacted with an equimolar bromide of EMI.sup.+ to perform salt exchange. In this manner, [EMI] [Tf.sub.2N] was obtained. An evaluation was performed in Test Example 1(2) and 1(3).

TABLE-US-00001 TABLE 1 Phase transition temperature and the state at 35° C. f.sub.sC.sup.− State T.sub.m .sup.1)(° C.) T.sub.g .sup.2)(° C.) T.sub.s-s .sup.3)(° C.) C.sub.1mim.sup.+ S .sup.4) 61 none .sup.5) none C.sub.2mim (EMI.sup.+) L .sup.6) 22 none none C.sub.4mim.sup.+ L 33 none none C.sub.6mim.sup.+ L none −85 none Py.sub.12.sup.+ PC .sup.7) decomp .sup.8) none −63 Py.sub.13.sup.+ PC 177 none 24 Py.sub.14.sup.+ S 85 none none PP.sub.13.sup.+ PC 146 none 9.1 PP.sub.14.sup.+ S 36 −78 none N.sub.1111.sup.+ PC decomp none −23 N.sub.2222.sup.+ PC decomp none −1 DEME.sup.+ PC 69 none −29 N.sub.6111.sup.+ L 29 None none N.sub.6222.sup.+ L none −85 none .sup.1) Melting point; .sup.2) glass transition temperature; .sup.3) solid-solid phase transition temperature (only of those that are significantly exothermic (equal to or more than several kJ/mol, which is achieved by a general solid); .sup.4) solid; .sup.5) not observed; .sup.6) liquid; .sup.7) plastic crystal; .sup.8) underwent thermal decomposition before melting.

TABLE-US-00002 TABLE 2 Phase transition temperature and the state at 35° C. Tf.sub.3C.sup.− State T.sub.m .sup.1) T.sub.g .sup.2) T.sub.s-s .sup.3) C.sub.1mim.sup.+ S .sup.4) 41 none .sup.5) none C.sub.2mim (EMI.sup.+) L .sup.6) 29 −72 none C.sub.4mim.sup.+ L none −70 none C.sub.6mim.sup.+ L none −79 none Py.sub.12.sup.+ S 81 None none Py.sub.13.sup.+ S 71 −59 none Py.sub.14.sup.+ S 56 None none PP.sub.13.sup.+ S 54 −51 none PP.sub.14.sup.+ S 37 −55 none N.sub.1111.sup.+ S 137 None none DEME.sup.+ L none −77 none N.sub.6111.sup.+ S 53 −48 none N.sub.6222.sup.+ L none −55 none .sup.1) Melting point; .sup.2) glass transition temperature; .sup.3) solid-solid phase transition temperature (only of those that are significantly exothermic (equal to or more than several kJ/mol, which is achieved by a general solid); .sup.4) solid; .sup.5) not observed; .sup.6) liquid.

[0086] In Tables 1 and 2, “PC” represents a plastic crystal, “L” represents a liquid, and “S” represents a solid. Also, “decomp” means that it reached the decomposition temperature before melting was observed.

[0087] With reference to Table 1, the obtained products were identified as plastic crystals (PCs) when they satisfied the following two points, i.e., the plastic crystal-specific solid-solid phase transition (T.sub.S-S) accompanied by a considerable amount of exothermic heat to melting and endothermic heat to coagulation at low temperature was observed, and it was sticky at room temperature.

Test Example 1

(1) Electrochemical Stability

[0088] A cyclic voltammogram was measured in [EMI] [f.sub.3C] containing 0.37 mol kg.sup.−1 of Li[f.sub.3C] on a platinum electrode (FIG. 1). Further, a cyclic voltammogram was measured in [EMI] [f.sub.3C] containing 0.37 mol kg.sup.−1 of Li[f.sub.3C] on a nickel electrode (FIG. 2).

[0089] It was reveled that the coexistence of Li salt allowed the f.sub.3anion-containing EMI ionic liquids to exhibit high oxidation stability (high enough to be applied to a positive-electrode material with a voltage of about 4.5 V)) and high reduction stability (high enough to enable lithium metal deposition/remelting).

(2) Stability with Respect to Li Metal

[0090] The electrochemical AC impedance spectra (25° C., 500 kHz-0.1 Hz, amplitude: ±10 mV) were measured for Li-metal symmetrical cells containing the [EMI] [f.sub.3C] obtained in Example 1, the [EMI] [Tf.sub.3C] obtained in Comparative Example 2, the [EMI] [f.sub.2N] obtained in Comparative Example 3, or the [EMI] [Tf.sub.2N] obtained in Comparative Example 4 (FIG. 3).

[0091] The charge transfer reaction rate constant at the Li metal interface is proportional to the reciprocal of the width of a semicircular arc (interfacial change-transfer resistance) observed in the AC impedance measurement. Thus, an anion that forms a circular arc as small as possible is preferable when Li metal is used as a negative electrode. With reference to this point, a smaller reaction rate on the Li negative electrode is achieved in the following order: a system comprising f.sub.2N.sup.−, which was 20Ω after cell preparation (smallest); a system comprising Tf.sub.2N.sup.− (80Ω); a system comprising f.sub.3C.sup.− (130Ω); and a system comprising Tf.sub.3C.sup.− (1500Ω). Li metal is an active metal, forms a solid-electrolyte interface (commonly SEI) through, for example, reduction decomposition caused when in contact with an electrolyte solution, and achieves an effect of inhibiting the decomposition of electrolyte solution. FIG. 3 also shows dotted lines representing the results obtained almost 1.5 year after the preparation of Li metal symmetrical cells, by which a significant increase in the size of circular arcs are confirmed. The system comprising f.sub.3C.sup.− of the present invention achieved only a 2.5-fold increase while the other Comparative Examples achieved a 22-fold, 45-fold, or 170-fold increase. This suggests that the solid-electrolyte interface formed by the system comprising f.sub.3C.sup.− of the present invention was more excellent than those formed by the other systems, achieving a stable Li metal-ionic liquid interface. Further, in view of the results of FIG. 3, even if the [EMI] [f.sub.2N] was stabile over many cycles, the [EMI] [f.sub.2N] resulted in a high increase in the resistance at the Li electrode side. Considering this, it is assumed that f.sub.3C.sup.− is more excellent in terms of long-terra storage characteristics of a battery.

[0092] (3) Novel Ionic Liquid LiCoO.sub.2/Li Cycle Test

[0093] FIG. 4 shows the results of the measurement of 1C charge-discharge cyclic performance of LiCoO.sub.2/Li cells using the [EMI] [f.sub.3C] (Example 1) and the [EMI] [Tf.sub.2N] (Comparative Example 4). Upper graph: cycle dependency of discharge capacity. Lower graph: charging and discharging coulomb efficiency. The [EMI] [Tf.sub.3C] to which Li[Tf.sub.3C] was incorporated showed high inter facial charge-transfer resistance as described in (2), and was thus never active as a battery because of a high internal resistance due to its excessively high viscosity. In contrast, the [EMI] [f.sub.3C] of the present invention was active in an excellent manner and had much more excellent cycle stability than the previously known [EMI] [Tf.sub.2N]. The coulomb efficiency was about 1, which clarifies that the decomposition of the electrolyte solution, deterioration of the electrode, and the like were suppressed.

[0094] When [EMI] [f.sub.3C] was used, up to 500 cycles were stably achieved with a coulomb efficiency of about 100%. This clarifies the achievement of the stability at the Li interface as shown in FIG. 3, as well as oxidation stability high enough to be applied to a 4 V class positive electrode.

[0095] Further, the AC impedance plots in terms of charged LiCoO.sub.2/Li cells were assessed using the [EMI] [f.sub.3C] (Example 1), and the [EMI] [Tf.sub.2N] (Comparative Example 4). FIG. 5 shows the results.

[0096] The electrode interfacial charge-transfer resistance is determined based on the width of the circular arc at the time of charge; the width of [EMI] [f.sub.3C] was much smaller than that of the previously known [EMI] [Tf.sub.2N] salt, and almost no change was observed even after 500 charge/discharge cycles. This suggests that the interface between the positive electrode and electrolyte solution was stably constructed.

[0097] The analysis was performed for the [EMI] [Tf.sub.3C] (Comparative Example 2) as well; however, an operation was unable to be performed from the first time due to too high internal resistance. In comparison with the results of the Tf.sub.2N system, it is clear that the only replacement of the CF.sub.3SO.sub.2 group of Tf.sub.3C with FSO.sub.2 group reduced the viscosity and the interfacial charge-transfer resistance, which, as a result, reduced the internal resistance. This not only greatly improved the performance but also greatly improved the stability of the electrolyte solution at the positive-electrode interface.

(4) Thermogravimetric Change

[0098] The thermogravimetric change of the [EMI] [f.sub.3C], [DEME] [f.sub.3C], [Py.sub.13] [f.sub.3C], and [PP.sub.13] [f.sub.3C] was observed in a nitrogen atmosphere. FIG. 6 shows the results. The results clarified that all of the salts had thermal resistance (about 250° C.).

(5) DSC

[0099] FIG. 7 shows the DSC measurement results of the [EMI] [f.sub.3C] (Example 1) and the [EMI] [Tf.sub.3C] (Comparative Example 2: [Tf.sub.3C]=[(CF.sub.3SO.sub.2).sub.3C] (at the time of temperature elevation; temperature elevation rate: 10° C. per minute).

[0100] The results of FIG. 7 clarify that the [f.sub.3C] anion of the present invention, when forming an ionic liquid with a typical EMI cation, gives a salt having a lower melting point.

[0101] Further, FIG. 8 shows the DSC measurement results of the [DEME] [f.sub.3C] (Example 1) and the [DEME] [Tf.sub.3C] (Comparative Example 2: [Tf.sub.3C]=[(CF.sub.3SO.sub.2).sub.3C]) (at the time of temperature elevation; temperature elevation rate; 10° C. per minute).

[0102] The results of FIGS. 7 and 8 clarify that the [f.sub.3C] anion of the present invention gives a salt having a melting point much higher than room temperature with the DEME cation, which most easily forms an ionic liquid next to EMI. However, its melting entropy is as significantly low as 6.1 J K.sup.−1 mol.sup.−1, which indicates that this salt is a plastic crystal that is expected to be used as a unique solid electrolyte. Further, the [DEME] [f.sub.3C] had a much lower glass transition temperature, and presumably achieves very high ion mobility in the plastic crystal. Therefore, it is clear that the [f.sub.3C] anion of the present invention is an anion that easily forms a plastic crystal.

[0103] Further, FIG. 9 shows the DSC measurement results of the [Py.sub.13] [f.sub.3C] and [PP.sub.13] [f.sub.3C] (Example 1), and the [Py.sub.13] [Tf.sub.3C] and [PP.sub.13] [Tf.sub.3C] (Comparative Example 1: [Tf.sub.3C]=[CF.sub.3SO.sub.2).sub.3C]) (at the time of temperature elevation; temperature elevation rate: 10° C. per minute).

[0104] With Py.sub.13 and PP.sub.13 cations as well, the f.sub.3C salts had a melting point that is about 100° C. higher than that of the Tf.sub.3C salts, and a melting entropy that is greatly lower than the definition of the plastic crystal of Timmermans (ΔS.sub.m<20 J K.sup.−1 mol.sup.−1, J. Timmermans, J. Phys. Chem. Solids, 18 (1), (1961), and references therein). This indicates that both of these f.sub.3C salts were plastic crystals. For use as an electrolyte, plastic crystals that have a higher melting point, which is an upper-limit temperature that represents a plastic crystal phase, may be used in a wider temperature range as a solid electrolyte. The salt including f.sub.3C anion exhibits a broad endotherm (equivalent to the temperature at which the salt becomes a plastic crystal) at around room temperature. This indicates that the f.sub.3C anion is an excellent anion that provides a plastic crystal phase within a wide temperature range, i.e., room temperature or higher, which is important for practical use.