JP4239537B2 - Room temperature molten salt electrolyte and electrochemical device using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a room temperature molten salt electrolyte excellent in reduction resistance. The present invention also relates to an electrochemical device using the same.
[0002]
[Prior art]
In recent years, as power sources for electronic devices, power storage power sources, electric vehicle power sources, etc., which have been improved in performance and size, electric power such as various non-aqueous batteries and electric double layer capacitors that can obtain high energy density. Chemical devices are attracting attention.
[0003]
In general, non-aqueous batteries and electric double layer capacitors include ionic compounds composed of lithium cations that are solid at room temperature in organic solvents that are not liquid ionic compounds, such as propylene carbonate and γ-butyrolactone. Lithium salts) and liquid nonaqueous electrolytes (nonaqueous electrolytes) in which ionic compounds (ammonium salts) composed of aliphatic ammonium cations are dissolved are frequently used.
[0004]
On the other hand, it has been proposed to use a room temperature molten salt electrolyte that is liquid at room temperature for electrochemical devices such as the non-aqueous electrolyte battery and the electric double layer capacitor. Such a room temperature molten salt electrolyte does not require the use of the flammable organic solvent, and has extremely high flame retardancy, so that when used as an electrolyte for an electrochemical device, an electrochemical device having high safety can be obtained. Is possible.
[0005]
As the electrolytes for electrochemical devices, conventionally proposed room temperature molten salt electrolytes include ionic compounds having quaternary ammonium cations having aromatic rings such as imidazolium ions and pyridinium ions, and ions having lithium cations. (For example, refer to Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4).
[0006]
However, the conventional room temperature molten salt electrolyte has the following problems. That is, a conventional room temperature molten salt electrolyte using a quaternary ammonium cation having an aromatic ring such as an imidazolium ion or a pyridinium ion has a lower reduction potential than a general non-aqueous electrolyte. . On the other hand, in an electrochemical device such as a battery, for example, the negative electrode operating potential of a non-aqueous electrolyte battery using lithium ions as carrier ions is generally equivalent to the potential of metallic lithium (−3.045 V vs. NHE), which is very basic. For this reason, when a conventional room temperature molten salt electrolyte is used, the quaternary ammonium cation and the negative electrode active material itself are reduced and decomposed, and the electrical characteristics such as cycle performance and charge / discharge efficiency performance deteriorate. there were.
[0007]
Moreover, since the room temperature molten salt electrolyte described in Patent Document 1 contains an aluminum halide, when applied to an electrochemical device due to the corrosiveness of aluminum halide ions (for example, AlCl ₄ ⁻ ), There was a problem of degrading the performance. Furthermore, aluminum halides are generally very reactive and difficult to handle.
[0008]
In contrast, Patent Document 5 uses an ionic compound (lithium salt) having a lithium cation of 0.5 mol / l or more in combination to reduce the reduction potential and eliminate the need for an aluminum halide that is difficult to handle. A room temperature molten salt electrolyte has been proposed.
[0009]
However, particularly when applying to an electrochemical device using a non-aqueous electrode involving lithium ions, a room temperature molten salt electrolyte having a lower reduction potential has been demanded.
[0010]
Patent Document 6 describes a technique for improving the flame retardancy of a non-aqueous electrolyte by adding an ionic compound having an imidazolium cation to the non-aqueous electrolyte.
[0011]
[Patent Document 1]
JP-A-4-349365 [Patent Document 2]
Japanese Patent Laid-Open No. 10-92467 [Patent Document 3]
Japanese Patent Laid-Open No. 11-86905 [Patent Document 4]
Japanese Patent Laid-Open No. 11-260400 [Patent Document 5]
JP 2002-110230 A [Patent Document 6]
Japanese Patent No. 3060107 [Non-Patent Document 1]
Study Group of Molten Salt / Thermal Technology “Basics of Molten Salt / Thermal Technology”, Agne Technology Center, 1993, 313p (ISBN 4-900041-24-6)
[0012]
[Problems to be solved by the invention]
This invention is made | formed in view of the said problem, and it aims at providing the normal temperature molten salt type electrolyte excellent in reduction resistance. Moreover, it aims at providing the electrochemical device excellent in the electrical property.
[0013]
[0014]
[0015]
[Means for Solving the Problems]
According to the present invention, as described in claim 1 , the room temperature molten salt electrolyte containing an imidazolium cation having the structure of (Chemical Formula 5) further includes a pyrrolium cation having the structure of (Chemical Formula 1) . Is a room temperature molten salt electrolyte.
[Chemical formula 5]
However,
R1 and R3 are alkyl groups having 1 to 6 carbon atoms,
R2, R4, and R5 are a hydrogen atom or an alkyl group having 1 to 6 carbon atoms,
It is.
[0016]
[Chemical 6]
[0017]
However,
R1 is an alkyl group having 1 to 6 carbon atoms,
R2 to R8 are a hydrogen atom or an alkyl group having 1 to 6 carbon atoms,
It is.
[0018]
Further, according to the present invention, as described in claim 2 , the room temperature molten salt electrolyte containing an imidazolium cation having the structure of (Chemical Formula 5) further includes a pyrrolium cation having the structure of (Chemical Formula 2). There is characterized in Rukoto.
[0019]
[Chemical 7]
[0020]
However,
R1 and R2 are alkyl groups having 1 to 6 carbon atoms,
R3 to R6 are a hydrogen atom or an alkyl group having 1 to 6 carbon atoms,
It is.
[0021]
[0022]
[0023]
[0024]
[0025]
[0026]
[0027]
According to the above configuration, the reduction resistance of a room temperature molten salt electrolyte containing an imidazolium cation having the structure of (Chemical Formula 5) can be improved .
[0028]
[0029]
[0030]
[0031]
[0032]
[0033]
[0034]
[0035]
[0036]
[0037]
[0038]
[0039]
[0040]
[0041]
[0042]
[0043]
Further, according to the present invention, as described in claim 3 , the room temperature molten salt electrolyte is composed of an anion consisting of only a nonmetallic element.
[0044]
According to such a configuration, the room temperature molten salt electrolyte is formed using an anion consisting of only a nonmetallic element, and thus does not contain aluminum halide ions. It is possible to further add features such as no deterioration in battery performance and difficulty in handling.
[0045]
Moreover, this invention is characterized by the said normal temperature molten salt type electrolyte containing lithium ion, as described in Claim 4 .
[0046]
According to such a configuration, it is excellent in reduction resistance in a wide potential region and high lithium ion mobility can be obtained, so that it is particularly applied to an electrochemical device using lithium ions as carrier ions, such as a lithium battery. Then, an electrochemical device having excellent electrical characteristics can be provided.
[0047]
Here, the lithium ion content is preferably in the range of 0.1 to 3 mol / l. When the content of the lithium salt is 0.1 mol / l or more, the resistance as an electrolyte does not increase too much, and the charge / discharge efficiency of the battery can be kept high. Moreover, by setting the content of the lithium salt to 3 mol / l or less, the melting point of the room temperature molten salt electrolyte is kept low, and the liquid state is easily maintained at room temperature. From the above viewpoint, the lithium salt content in the non-aqueous electrolyte is preferably in the range of 0.5 to 2 mol / l.
[0048]
Moreover, this invention is an electrochemical device using the said normal temperature molten salt type electrolyte, as described in Claim 5 .
[0049]
According to such a configuration, since the room temperature molten salt electrolyte excellent in reduction resistance according to the present invention is used for the electrolyte of the electrochemical device, the electrochemical device such as a battery or an electric double layer capacitor is electrically used. It can be excellent in characteristics.
[0050]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to these descriptions.
[0051]
The ionic compound having a cation represented by ( Chemical Formula 1) , (Chemical Formula 2), or (Chemical Formula 5) has a high flame retardancy, a relatively low melting point, and a room temperature molten salt that exhibits a liquid at room temperature. It is a constitutive compound. Therefore, when a room temperature molten salt electrolyte is used, not only high ionic conductivity can be obtained, but also flame retardancy can be imparted.
[0052]
Examples of the pyrrolium cation having the structure of (Chemical Formula 1) include 1,2-dimethylpyrrolium ion, 1-ethyl-2-methylpyrrolium ion, 1-propyl-2-methylpyrrolium ion, 1-butyl-2. -Methyl pyrrolinium ion etc. are mentioned, However, It is not limited to these.
[0053]
Examples of the pyrrolium cation having the structure of (Chemical Formula 2) include 1,1-dimethylpyrrolium ion, 1-ethyl-1-methylpyrrolium ion, 1-methyl-1-propylpyrrolium ion, 1-butyl-1-methylpyrrole. Um ion etc. are mentioned, but it is not limited to these.
[0054]
[0055]
[0056]
As the imidazolium cation having the structure of (Chemical Formula 5), as the dialkylimidazolium ion, 1,3-dimethylimidazolium ion, 1-ethyl-3-methylimidazolium ion, 1,3-diethylimidazolium ion, Examples of the trialkylimidazolium ion include 1-butyl-3-methylimidazolium ion, 1,2,3-trimethylimidazolium ion, 1,2-dimethyl-3-ethylimidazolium ion, 1,2-dimethyl. Examples include, but are not limited to, -3-propylimidazolium ion, 1-butyl-2, 3-dimethylimidazolium ion, and the like.
[0057]
[0058]
The room temperature molten salt electrolyte of the present invention is preferably composed of an anion consisting only of a nonmetallic element. Examples of the anion consisting of the nonmetallic element include BF ₄ ⁻ , PF ₆ ⁻ , CF ₃ SO ₃ ⁻ , N (CF ₃ SO ₂ ) ₂ ⁻ , N (C ₂ F ₅ SO ₂ ) ₂ ⁻ , N (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) ⁻ , C (CF ₃ SO ₂ ) ₃ ^— and C (C ₂ F ₅ SO ₂ ) ₃ ^— are preferably selected. By selecting the anion in this way, it becomes easy to form a room temperature molten salt electrolyte having a low melting point, so that a room temperature molten salt electrolyte having a high ionic conductivity can be surely obtained. Two or more of these anions may be mixed.
[0059]
From this point of view, it is preferable to select an ionic compound having an anion as described above for adjusting the room temperature molten salt electrolyte of the present invention. Two or more of these anions may be mixed.
[0060]
The normal temperature molten salt referred to in the present application refers to an ionic compound that is at least partially liquid at normal temperature. The normal temperature is a temperature range in which the electrochemical device is assumed to normally operate, and the upper limit is about 100 ° C., in some cases about 60 ° C., and the lower limit is about −50 ° C., and in some cases about −20 ° C. . On the other hand, an inorganic molten salt such as Li ₂ CO ₃ —Na ₂ CO ₃ —K ₂ CO ₃ used for various electrodepositions as described in Non-Patent Document 1 has a melting point of 300 ° C. or higher. However, it does not exhibit a liquid form within a temperature range in which an electrochemical device is normally assumed to operate, and is not included in the room temperature molten salt referred to in the present application.
[0061]
Moreover, the room temperature molten salt electrolyte referred to in the present application is an electrolyte made of the room temperature molten salt. The room temperature molten salt electrolyte of the present application is composed of only an ionic compound, as shown in the examples described later, and is ionic such as propylene carbonate and γ-butyrolactone used in general lithium battery electrolytes. Organic solvents that are not compounds can be excluded. By not containing an organic solvent that is not an ionic compound, higher flame retardancy and higher thermal stability can be enjoyed. Therefore, since it consists only of a flame-retardant ionic compound, when this is used as an electrolyte, it becomes possible to obtain the electrochemical device which has high safety | security. Furthermore, since it does not contain a volatile organic solvent, it offers a high degree of freedom in selecting the material of the exterior body. For example, an electrochemical device using a metal resin composite film represented by an aluminum laminate film is provided. It becomes extremely easy to do. Furthermore, in the manufacturing process of electrochemical devices, when a conventional non-aqueous electrolyte is used, an electrolyte injection process is arranged immediately before the sealing process close to the final process in order to avoid volatilization of the electrolyte. Although consideration was required, by not including the organic solvent, there is an advantage that the consideration is not necessary.
[0062]
In addition, you may use together the organic solvent which is not an ionic compound to such an extent that the said advantages, such as the heat stability, a flame retardance, and nonvolatility, are not impaired. Examples of organic solvents that are not ionic compounds include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, diethyl carbonate Chain carbonates such as ethyl methyl carbonate; chain esters such as methyl formate, methyl acetate, methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, Ethers such as 1,4-dibutoxyethane and methyldiglyme; Nitriles such as acetonitrile and benzonitrile; Dioxolane or derivatives thereof; Ethylene sulfide, sulfolane, sultone or the like Examples thereof include a single derivative or a mixture of two or more thereof, but are not limited thereto.
[0063]
By the way, the room temperature molten salt electrolyte in the present invention may be used by solidifying it into a gel by complexing a polymer. Here, examples of the polymer include, for example, polyethylene oxide, polypropylene oxide, polyacrylonitrile, polymethyl methacrylate, polyvinylidene fluoride, various acrylic monomers, methacrylic monomers, acrylamide monomers, allyl monomers, and styrene monomers. Examples include, but are not limited to, polymers. These may be used alone or in combination of two or more.
[0064]
Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these descriptions.
[0065]
( Reference Example 1)
The pin Lori pyridinium an ionic compound having a cation 1-butyl-1-methyl-pyrrolidinium bis (trifluoromethanesulfonyl) imide, by mixing one mole of _{LiN (CF 3 SO 2) 2} , room temperature molten A salt electrolyte was obtained.
[0066]
(Example 2)
To 1-ethyl-1-methylpyrrolium tetrafluoroborate, which is an ionic compound having a pyrrolium cation having the structure of (Chemical Formula 2), 2 mol of 1-ethyl-3-methylimidazolium tetrafluoroborate and A normal temperature molten salt electrolyte was obtained by mixing 1 mol of LiBF ₄ .
[0067]
(Example 3)
1,1-dimethylpyrrolium hexafluorophosphate, which is an ionic compound having a pyrrolium cation having the structure of (Chemical Formula 2), and 2 mol of 1-butyl-3-methylimidazolium hexafluorophosphate and 1 A normal temperature molten salt electrolyte was obtained by mixing molar LiPF ₆ .
[0068]
( Reference Example 4)
An ionic compound having a pin Ratho cation in 1,2-dimethyl-pyrazolium bis (perfluoroalkyl ethane sulfonyl) imide, mixed with 3 moles of 1-ethyl-3-methylimidazolium bis (perfluoroalkyl ethane sulfonyl) imide Thus, a room temperature molten salt electrolyte was obtained.
[0069]
(Example 5)
2 mol 1-ethyl-3-methylimidazolium tetrafluoroborate which is an ionic compound having an imidazolium cation having the structure of (Chemical Formula 5) has an ionicity having a pyrrolium cation having the structure of (Chemical Formula 1). A room temperature molten salt electrolyte was obtained by mixing 2 mol of 1-ethylpyrrolium tetrafluoroborate as a compound and 1 mol of LiBF ₄ .
[0070]
( Reference Example 6)
2 mol of 1-ethyl-1-methylpyrrolium bis (perfluoroethylsulfonyl) imide, which is an ionic compound having a pyrrolium cation having the structure of (Chemical Formula 2), and 2 mol of trimethylhexylammonium bis (perfluoroethylsulfonyl) ) Imide and 1 mol of LiN (CF ₃ SO ₂ ) ₂ were mixed to obtain a room temperature molten salt electrolyte.
[0071]
( Reference Example 7)
2 moles of 1,2-dimethyl-pyrazolium bis (trifluoromethanesulfonyl) imide is an ionic compound having a pin Ratho cation, 2 moles of 1-butyl is an ionic compound having a pin Lori pyridinium cation - A normal temperature molten salt electrolyte was obtained by mixing 1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide and 1 mol of LiN (CF ₃ SO ₂ ) ₂ .
[0072]
( Reference Example 8)
2 moles of 1-butyl pyridinium tetrafluoroborate as an ionic compound having a pin lysine cation, and 2 moles of 1-ethyl-1-methyl piperidinium tetrafluoroborate, 1 mole of LiN (CF ₃ SO ₂ ) ₂ was mixed to obtain a room temperature molten salt electrolyte.
[0073]
(Comparative Example 1)
1 mol of LiBF ₄ was mixed with 1-ethyl-3-methylimidazolium tetrafluoroborate to obtain a room temperature molten salt electrolyte.
[0074]
(Comparative Example 2)
A liquid non-aqueous electrolyte (non-aqueous electrolyte) was obtained by mixing 1 mol of LiBF ₄ with propylene carbonate.
[0075]
(Reduction resistance test)
Using the room temperature molten salt electrolytes obtained in Examples 2, 3, 5, Reference Examples 1, 4, 6-8 and Comparative Example 1, a reduction resistance test was performed. The test cell was a three-terminal cell, and platinum was used for the working electrode. The scanning potential range was from natural electrode potential to −2 V (vs. Li ^/ Li ⁺ ) in the reduction direction. The potential scanning speed was 5 mV / sec. Table 1 shows the results. Here, when measurement was performed using each electrolyte, a reduction peak was observed at a potential lower than 0 V and lower than 0 V observed in the room temperature molten salt electrolyte obtained in Comparative Example 1. The result was indicated by a circle, and the reduction peak not observed at a potential nobler than 0 V was indicated by a mark.
[0076]
As a result , all of the room temperature molten salt electrolytes of Examples 2, 3, 5 and Reference Examples 1, 4, 6-8 have better reduction resistance than the room temperature molten salt electrolyte of Comparative Example 1. I understood.
[0077]
(Battery performance test)
A lithium battery was produced as an electrochemical device using the electrolytes obtained in the above Examples and Comparative Examples. A cross-sectional view of the produced lithium battery is shown in FIG. The lithium battery according to the present invention includes a pole group 4 including a positive electrode 1, a negative electrode 2, and a separator 3, an electrolyte, and a metal resin composite film 5 as an exterior material. The positive electrode 1 is formed by applying a positive electrode mixture 11 on a positive electrode current collector 12. The negative electrode 2 is formed by applying a negative electrode mixture 21 on a negative electrode current collector 22. The electrolyte is impregnated in the pole group 4. The metal resin composite film 5 covers the pole group 4 and is sealed on all four sides by heat welding.
[0078]
Next, a method for manufacturing the lithium battery having the above configuration will be described. The positive electrode 1 was obtained as follows. First, LiCoO ₂ and acetylene black, which is a conductive agent, are mixed, and an N-methyl-2-pyrrolidone solution of polyvinylidene fluoride is further mixed as a binder, and this mixture is used as a positive electrode current collector 12 made of an aluminum foil. After coating on one side, it was dried and pressed so that the thickness of the positive electrode mixture 11 was 0.1 mm. The positive electrode 1 was obtained by the above process. The negative electrode 2 was obtained as follows. First, Li _4/3 Ti _5/3 O _{4 as} a negative electrode active material and Ketjen black as a conductive agent are mixed, and further, an N-methyl-2-pyrrolidone solution of polyvinylidene fluoride is mixed as a binder. The mixture was applied to one side of a negative electrode current collector 22 made of copper foil, dried, and pressed so that the thickness of the negative electrode mixture 21 was 0.1 mm. The negative electrode 2 was obtained by the above process. As the separator 3, a polyethylene microporous film was used. The pole group 4 was configured by facing the positive electrode mixture 11 and the negative electrode mixture 21, placing the separator 3 therebetween, and laminating the positive electrode 1, the separator 3, and the negative electrode 2 in this order. Next, the electrode group 4 was impregnated with the electrolyte by immersing the electrode group 4 in the electrolyte. Furthermore, the pole group 4 was covered with the metal resin composite film 5, and the four sides were sealed by heat welding.
[0079]
As the electrolyte, the examples 2, 3 and 5, Reference Examples 1 and 4, to prepare a lithium battery using 6, 7, 8 and Comparative Examples 1 and 2 electrolyte according to each. This respectively present invention batteries 2,3,5, and Reference Example batteries 1,4, 6, 7, 8 and comparative batteries 1 and 2. Since the electrolyte according to Reference Example 4 did not contain lithium ions, no lithium battery was produced.
[0080]
(Initial discharge capacity test)
An initial discharge capacity test was performed on the above-described battery of the present invention , the reference battery, and the comparative battery. The test temperature was 20 ° C. The charging was a constant current charging with a current of 1 mA and a final voltage of 2.7 V. The discharge was a constant current discharge with a current of 1 mA and a final voltage of 1.2V. The obtained battery capacity was defined as the initial discharge capacity. The design capacities of the present invention battery , the reference example battery, and the comparative battery are all the same.
[0081]
(Temperature cycle test)
A temperature cycle test was performed on the above-described battery of the present invention , the reference battery, and the comparative battery. A battery whose initial capacity was confirmed under the same conditions as in the initial discharge capacity test described above was charged under the conditions described above, then stored at 100 ° C. for 3 hours and then stored at room temperature for 21 hours, and repeated for 30 days. The discharge capacity after storage was measured under the conditions described above to determine the self-discharge rate. The results of the initial discharge capacity test and the temperature cycle test are also shown in Table 1. The self-discharge rate was calculated by (Equation 1).
[0082]
[Formula 1]
[0083]
[Table 1]
[0084]
As is apparent from Table 1, the battery of the present invention and the reference example battery had an initial discharge capacity of about 100% of the design capacity for each battery, and therefore the charge / discharge efficiency was also about 100%. I understood. About the self-discharge rate by a temperature cycle test, it was superior to the comparative battery 2 and showed performance comparable to that of the comparative battery 1.
[0085]
In addition, although the lithium battery was demonstrated to the example as a use of the normal temperature molten salt type electrolyte illustrated above, it can utilize suitably also as electrolytes for other electrochemical devices, such as an electric double layer capacitor.
[0086]
Since the room temperature molten salt electrolyte of the present invention is excellent in reduction resistance as described above, the battery of the present invention using the electrolyte has excellent electrical characteristics such as repeated charge / discharge cycle performance.
[0087]
【The invention's effect】
As described above, according to the present invention, a room temperature molten salt electrolyte excellent in reduction resistance can be provided, and an electrochemical device using the electrolyte can be excellent in electrical characteristics.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a lithium battery according to an example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Positive electrode 11 Positive electrode mixture 12 Positive electrode collector 2 Negative electrode 21 Negative electrode mixture 22 Negative electrode collector 3 Separator 4 Electrode group 5 Metal resin composite film

Claims (5)

A room temperature molten salt electrolyte, wherein the room temperature molten salt electrolyte containing an imidazolium cation having the structure of (Chemical Formula 5) further contains a pyrrolium cation having the structure of (Chemical Formula 1).
However,
R1 and R3 are alkyl groups having 1 to 6 carbon atoms,
R2, R4, and R5 are a hydrogen atom or an alkyl group having 1 to 6 carbon atoms,
It is.
However,
R1 is an alkyl group having 1 to 6 carbon atoms,
R2 to R8 are a hydrogen atom or an alkyl group having 1 to 6 carbon atoms,
It is. A room temperature molten salt electrolyte, wherein the room temperature molten salt electrolyte containing an imidazolium cation having the structure of (Chemical Formula 5) further contains a pyrrolium cation having the structure of (Chemical Formula 2).
However,
R1 and R3 are alkyl groups having 1 to 6 carbon atoms,
R2, R4, and R5 are a hydrogen atom or an alkyl group having 1 to 6 carbon atoms,
It is.
However,
R1 and R2 are alkyl groups having 1 to 6 carbon atoms,
R3 to R6 are a hydrogen atom or an alkyl group having 1 to 6 carbon atoms,
It is. The room temperature molten salt electrolyte according to claim 1 or 2 , wherein the room temperature molten salt electrolyte is composed of an anion consisting of only a nonmetallic element. Room temperature molten salt electrolyte is a room temperature molten salt electrolyte according to any one of claims 1 to 3, are Nde contains lithium ions. The electrochemical device using the normal temperature molten salt type electrolyte in any one of Claims 1-4 .