JP4706267B2 - Multi-substituted imidazolium molten salt and its use - Google Patents

Description

  The present invention relates to a molten salt having an anion and a cation having a predetermined structure and a method for producing the same. Such a molten salt is useful, for example, as a medium constituting an electrolyte of various power storage elements.

As a cation component capable of constituting a salt that exhibits a liquid state in a temperature range near room temperature (hereinafter also referred to as “room temperature molten salt”), it may be referred to as 1-ethyl-3-methylimidazolium cation (hereinafter referred to as “EMI”). 1,3-dialkylimidazolium cations such as.) Are known. Representative examples of room temperature molten salt having this kind of cation, EMI and bis (trifluoromethanesulfonyl) imide anion ^{_{[- N (SO 2 CF 3}} ) 2, hereinafter also referred to as "TFSI". And a salt thereof. Such a room temperature molten salt can be used, for example, as a component constituting an electrolyte of a non-aqueous battery (such as a lithium ion secondary battery). For example, Patent Document 1 described below describes a nonaqueous electrolyte battery including a nonaqueous electrolyte containing a room temperature molten salt having an imidazolium cation. The following patent documents 2 and 3 are mentioned as another prior art document regarding the electrolyte using an imidazolium salt.
JP 2002-324579 A Japanese Patent Laid-Open No. 11-306858 Japanese Patent Laid-Open No. 11-306859

By the way, generally, the reduction potential of a room temperature molten salt having an imidazolium cation is relatively high. Therefore, it is beneficial if the reduction potential of the salt having an imidazolium cation can be lowered while maintaining the above property of being liquid in the temperature range near room temperature.
An object of the present invention is to provide a salt that exhibits a liquid state in a temperature range near room temperature and has a low reduction potential, and a method for producing the same. Another object of the present invention is to provide an electrolyte medium mainly composed of such a salt. Still another object is to provide an electrolyte containing such a medium and a storage element (battery, capacitor, etc.) provided with the electrolyte.

  The present inventor has found that by selecting and combining specific anions with respect to the polysubstituted imidazolium cation, a salt having a low reduction potential and a low melting point can be constituted by these cations and anions.

ここで、上記式中のＲ¹¹およびＲ¹³は、それぞれ独立に、炭素数１〜４のアルキル基から選択され得る。また、Ｒ¹²，Ｒ¹⁴およびＲ¹⁵のうち一つは炭素数１〜４のアルキル基であり得る。他の二つは、それぞれ独立に、水素原子および炭素数１〜４のアルキル基から選択され得る。
また、上記方法は、下記式（２）で表されるアニオンを選択する工程を含み得る。

ここで、上記式中のＲｆ¹およびＲｆ²は、それぞれ独立に、エーテル結合を含むパーフルオロアルキル基（これを「パーフルオロアルキルエーテル基」という。）およびエーテル結合を含まないパーフルオロアルキル基から選択され得る。
上記方法は、上記式（１）で表されるカチオンと、上記式（２）で表されるアニオンとの塩を生じさせる工程を含み得る。 One invention disclosed herein relates to a method for producing a molten salt that exhibits a liquid state in a temperature range (normal temperature range) near normal temperature. The method may include a step of selecting a cation represented by the following formula (1).

Here, R ¹¹ and R ¹³ in the above formula can be independently selected from alkyl groups having 1 to 4 carbon atoms. One of R ¹² , R ¹⁴ and R ¹⁵ may be an alkyl group having 1 to 4 carbon atoms. The other two may be independently selected from a hydrogen atom and an alkyl group having 1 to 4 carbon atoms.
Moreover, the said method may include the process of selecting the anion represented by following formula (2).

Here, Rf ¹ and Rf ² in the above formula are each independently a perfluoroalkyl group containing an ether bond (hereinafter referred to as “perfluoroalkyl ether group”) and a perfluoroalkyl group not containing an ether bond. Can be selected.
The said method may include the process of producing the salt of the cation represented by the said Formula (1), and the anion represented by the said Formula (2).

  An imidazolium cation in which an alkyl group is introduced at at least one of the 2, 4, and 5 positions of the imidazole ring is an imidazolium cation having no such substituent (alkyl group) (for example, a 1,3-dialkylimidazolium). Compared to (cation), an imidazolium salt having a lower reduction potential can be formed. The reduction potential of the imidazolium salt tends to decrease as the number of substituents of the imidazole ring increases. On the other hand, as the number of alkyl groups in the imidazole ring increases, the melting point of the imidazolium salt (related to the lower limit of the temperature range in which the salt can be maintained in a liquid state) tends to increase. For this reason, it is difficult to realize a molten salt in which both a low reduction potential and a low melting point are highly compatible only by selecting a cation. According to the invention disclosed herein, for the polysubstituted imidazolium cation represented by the above formula (1), a specific anion represented by the above formula (2) is selected and combined with the cation. A molten salt having a low reduction potential and a low melting point can be realized.

In one preferred embodiment of the invention disclosed herein, R ¹² in the formula (1) is an alkyl group having 1 to 4 carbon atoms. Further, at least one of R ¹⁴ and R ¹⁵ may be selected from an alkyl group having 1 to 4 carbon atoms. The rest can be selected from a hydrogen atom and an alkyl group having 1 to 4 carbon atoms. Therefore, one of R ¹⁴ and R ¹⁵ may be an alkyl group having 1 to 4 carbon atoms and the other may be a hydrogen atom, and both R ¹⁴ and R ¹⁵ are each independently an alkyl group having 1 to 4 carbon atoms. It may be. For example, any of R ^{11 to} R ¹⁵ can be independently a cation having a structure of an alkyl group having 1 to 4 carbon atoms.
In another preferred embodiment of the invention disclosed herein, Rf ¹ in the formula (2) may be a C 1-5 perfluoroalkyl group or a perfluoroalkyl ether group. Further, Rf ² in the formula (2) may be a perfluoroalkyl group or a perfluoroalkyl ether group having 1 to 3 carbon atoms. Rf ² is preferably a perfluoroalkyl group.

In any of the methods disclosed herein, the step of generating the salt is represented by the first raw material which is a salt of a cation and an anion represented by the above formula (1) and the above formula (2). It can carry out suitably using the 2nd raw material which is a salt of an anion and an alkali metal cation. Typically, the salt of the cation represented by the formula (1) and the anion represented by the formula (2) can be synthesized by mixing the first material and the second material. As a suitable example of the first raw material used in this step, a cation represented by the above formula (1) and the following formula (3):
^- OSO ₂ Rf ^{3 (3)}
(Rf ³ may be a perfluoroalkyl group with or without an ether linkage);
A salt with an anion represented by can be exemplified. As another preferred example, a salt of a cation represented by the above formula (1) and an anion represented by X ⁻ can be exemplified. Here, X is a halogen atom. Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Among them, preferred are a chlorine atom, a bromine atom and an iodine atom, and particularly preferred are a chlorine atom and a bromine atom from the economical viewpoint.

One invention disclosed herein relates to a medium mainly composed of a room temperature molten salt of a cation represented by the above formula (1) and an anion represented by the above formula (2). For example, it is possible to provide a medium (preferably an electrolyte medium) that is a liquid in a normal temperature range and is mainly composed of a molten salt produced by any of the methods described above. Such a liquid medium can be useful as an electrolyte used for an electrochemical device such as a power storage element or a component thereof. For example, the medium can be used as a solvent for dissolving the supporting electrolyte.
In one preferred embodiment of the medium disclosed herein, the medium comprises: (a) a reduction potential for metallic lithium of 1 V or less; and (b) a melting point of about 10 ° C. or less. At least one of the conditions can be satisfied. In a more preferred embodiment, the medium can satisfy both conditions (a). And (b). Since such a medium has a low reduction potential and a low melting point, it is suitable as a constituent component for an electrolyte constituting a power storage element such as a lithium ion secondary battery.

In addition, according to the present invention, there can be provided an electrolyte that exhibits a liquid state in a normal temperature range and includes any of the above-described media. The electrolyte can contain a compound (supporting electrolyte) that can supply inorganic cations (for example, alkali metal ions) to the electrolyte. An electrolyte having a composition in which such a compound (for example, an alkali metal salt) is dissolved in the medium may be used. In one preferred embodiment, the electrolyte includes a lithium salt as a supporting electrolyte. For example, the electrolyte may have a composition in which a lithium salt is dissolved in the medium.
In one preferred embodiment of the electrolyte disclosed herein, the electrolyte has (a) a reduction potential with respect to metallic lithium of 0.5 V or less, and (b) a melting point of about 10 ° C. or less. May satisfy at least one of the conditions. In a more preferred embodiment, the medium can satisfy both conditions (a). And (b). Since such an electrolyte has a low reduction potential and a low melting point, it is suitable as a component of a power storage element such as a lithium ion secondary battery.

  Furthermore, according to the present invention, a power storage device including any one of the above-described electrolytes is provided. Here, the “storage element” refers to various electrochemical batteries (including primary batteries and secondary batteries. Examples include lithium ion batteries and nickel metal hydride batteries) and capacitors (for example, electric double layer capacitors). It is a concept that includes both of them. According to the present invention, a lithium battery (typically a lithium ion secondary battery) including any one of the above-described electrolytes is provided. In a preferred embodiment, the battery includes an electrolyte having a composition in which a lithium salt is dissolved in any of the media described above.

ここで、上記式中のＲ¹¹〜Ｒ¹⁵は、それぞれ独立に、炭素数１〜４のアルキル基から選択され得る。Ｒｆ¹は、炭素数１〜５のパーフルオロアルキル基であり得る。Ｒｆ²は、炭素数１〜３のパーフルオロアルキル基であり得る。このような塩は、還元電位の低さと融点の低さとが高度に両立したものとなり得る。したがって、電解質（例えば、リチウムイオン二次電池等の蓄電素子用の電解質）またはその構成成分として有用な材料となり得る。 According to one invention disclosed herein, there is provided a molten salt that is liquid at room temperature and is represented by the following formula (4).

Here, R ^{11 to} R ¹⁵ in the above formula may be independently selected from alkyl groups having 1 to 4 carbon atoms. Rf ¹ may be a C 1-5 perfluoroalkyl group. Rf ² may be a C 1-3 perfluoroalkyl group. Such a salt can be a highly compatible low reduction potential and low melting point. Therefore, it can be a material useful as an electrolyte (for example, an electrolyte for a storage element such as a lithium ion secondary battery) or a component thereof.

  Hereinafter, preferred embodiments of the present invention will be described in detail. It should be noted that technical matters other than the contents particularly mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters for those skilled in the art based on the prior art. The present invention can be carried out based on the technical contents disclosed in the present specification and the common general technical knowledge in the field.

<Cation component>
The molten salt disclosed herein may be a salt having a cation having a structure including an imidazole ring having at least three alkyl groups (substituents). In a typical embodiment of such a cation, two nitrogen atoms constituting the imidazole ring each have an alkyl group, and at least one (preferably two or more) of the three carbon atoms constituting the imidazole ring are alkyl groups. Have Those alkyl groups (substituents) may be open chain or may form a ring structure. As used herein, “open chain” is a concept including both an open chain having no branch (straight chain) and an open chain having a branch (branched chain) unless otherwise specified. An open chain (particularly linear) alkyl group is preferred. Two or more substituents bonded to the imidazole ring may be bonded to each other to form a ring structure.
In a preferred embodiment, any of the three or four or more substituents of the imidazole ring are each independently an alkyl group having 1 to 4 (more preferably 1 to 3) carbon atoms. An open-chain alkyl group is preferable, and a linear alkyl group is more preferable. Specific examples of such alkyl _{groups, -CH 3, -CH 2 CH 3} , -CH 2 CH 2 CH 3, -CH 2 CH 2 CH 2 CH 3, -CH (CH 3) CH 3, -CH (CH _{3) CH 2 CH 3, -CH} 2 CH (CH 3) CH 3, and the like _{-C (CH 3) 2 CH 3} . Of these, —CH ₃ , —CH ₂ CH ₃ , and —CH ₂ CH ₂ CH ₃ are particularly preferred. Such an alkyl group has a structure in which the 1st and 3rd position nitrogen atoms of the imidazole ring are bonded to at least one (preferably 2 or more) carbon atoms of the 2nd, 4th and 5th positions. Cations are preferred.

  Preferred examples of the cation constituting the molten salt disclosed herein include 1,2,3-trimethylimidazolium ion, 1,2,3-triethylimidazolium ion, and 1-ethyl-2,3-dimethylimidazolium. Ions, 1,2-dimethyl-3-propylimidazolium ions, trialkylimidazolium ions such as 2-ethyl-1,3-dimethylimidazolium ions; 1,2,3,4-tetramethylimidazolium ions, Tetraalkylimidazolium ions such as 1,2,3,4-tetraethylimidazolium ion, 2-ethyl-1,3,4-trimethylimidazolium ion; 1,2,3,4,5-pentamethylimidazolium Pentaalkyl such as ions, 1-ethyl-2,3,4,5-tetraalkylimidazolium ions Mi Dazo potassium ions; and the like. A cation having 3 to 10 carbon atoms in total of three or more alkyl groups (substituents) bonded to the imidazole ring is preferable, and a cation having 4 to 8 carbon atoms is more preferable.

<About the anion component>
Molten salt disclosed ^herein, - N may be (CorF ¹⁾ salt having an anion component represented by (SO ^₂ Rf _2). This anion component has a structure in which a sulfonyl group (—SO ₂ —) and a carbonyl group (—CO—) are bonded to one nitrogen atom (N). A perfluoroalkyl group or a perfluoroalkyl ether group (Rf ¹ ) is bonded to the carbonyl group. In addition, a perfluoroalkyl group or a perfluoroalkyl ether group (Rf ² ) is bonded to the sulfonyl group. As described above, in the anion component, both the sulfonyl group and the carbonyl group bonded to the nitrogen atom have a perfluoro group (a group in which all the hydrogen atoms of the organic group are replaced with fluorine atoms). In one typical embodiment, both the sulfonyl group and the carbonyl group have a perfluoroalkyl group. In another exemplary embodiment, the sulfonyl group has a perfluoroalkyl group, and the carbonyl group has a perfluoroalkyl ether group.

When the group (Rf ¹ ) bonded to the carbonyl group is a perfluoroalkyl group, the perfluoroalkyl group may be an open chain or may form a ring structure. An open chain (particularly linear) perfluoroalkyl group is preferred. The total number (carbon number) of carbon atoms contained in Rf ¹ may be ¹ to 10, for example. The carbon number is preferably 1 to 5, and more preferably 1 to 3. One preferred example of Rf ¹ is a linear or branched (more preferably linear) perfluoroalkyl group that satisfies the above carbon number. Specific examples of such perfluoroalkyl groups include —CF ₃ , —CF ₂ CF ₃ , —CF ₂ CF ₂ CF ₃ , —CF ₂ (CF ₂ ) ₂ CF ₃ , and —CF ₂ (CF _{2 4} CF ₃ is mentioned. Of these, —CF ₃ , —CF ₂ CF ₃ , and —CF ₂ CF ₂ CF ₃ are preferable.

When Rf ¹ is a perfluoroalkyl ether group, the perfluoroalkyl ether group may be an open chain or may form a ring structure. An open chain (particularly linear) perfluoroalkyl ether group is preferred. The total number (carbon number) of carbon atoms contained in Rf ¹ may be 2 to 16, for example. The carbon number is preferably 2 to 10, and more preferably 2 to 5. The number of ether bonds contained in Rf ¹ can be, for example, 1-5. The number of ether bonds is preferably 1 to 3. Preferable examples of Rf ¹ include linear or branched perfluoroalkyl ether groups satisfying the above carbon number and the number of ether bonds.

The Rf ¹ is represented by the following formula (5):
^{-Rf 4 -O (-Rf 5 -O)} n -Rf 6 (5);
The perfluoroalkyl ether group represented by these may be sufficient. Here, Rf ⁴ and Rf ⁵ may each independently be a linear or branched perfluoroalkylene group having 1 to 3 carbon atoms. Rf ⁶ may be a linear or branched perfluoroalkyl group having 1 to 5 carbon atoms. n may be a number from 0 to 4 (preferably 0 to 2). When a plurality of Rf ⁵ are contained in the above formula (5), these Rf ⁵ may be the same or different.

In one preferred embodiment of the molten salt disclosed herein, Rf ¹ is, for example, —CF ₂ OCF ₃ , —CF ₂ OCF ₂ CF ₃ , —CF ₂ OCF ₂ CF ₂ CF ₃ , —CF ₂ CF _2. OCF ₃ , —CF ₂ CF ₂ OCF ₂ CF ₃ , —CF ₂ CF ₂ CF ₂ OCF ₃ , —CF ₂ OCF ₂ CF ₂ OCF ₃ , —CF ₂ CF ₂ OCF ₂ CF ₂ OCF ₃ , —CF (CF ₃ ) OCF ₃ , —CF (CF ₃ ) OCF ₂ CF ₃ , —CF (CF ₃ ) OCF ₂ CF ₂ CF ₃ , and —C (CF ₃ ) ₂ OCF ₃ It is a fluoroalkyl ether group. Of these, preferred Rf ¹ is —CF ₂ OCF ₃ , —CF ₂ OCF ₂ CF ₃ , —CF ₂ CF ₂ OCF ₃ , —CF ₂ OCF ₂ CF ₂ OCF ₃ , and —CF (CF ₃ ) OCF ₂ CF ₂ CF ₃ is mentioned.

On the other hand, the group (Rf ² ) bonded to the sulfonyl group may be a perfluoroalkyl group or a perfluoroalkyl ether group similar to Rf ¹ . The total number (carbon number) of carbon atoms contained in Rf ² may be 1 to 10, for example. The carbon number is preferably 1 to 5, and more preferably 1 to 3. An open chain (particularly straight chain) perfluoroalkyl group satisfying such carbon number is preferred. Specific examples of such perfluoroalkyl groups include —CF ₃ , —CF ₂ CF ₃ , —CF ₂ CF ₂ CF ₃ , —CF ₂ (CF ₂ ) ₂ CF ₃ , and —CF ₂ (CF _{2 4} CF ₃ is mentioned. Of these, —CF ₃ , —CF ₂ CF ₃ , and —CF ₂ CF ₂ CF ₃ are preferable. It is particularly preferred that Rf ² is —CF ₃ .

  Any of the above-described salts of a cation and an anion (typically, a salt of a cation represented by the above formula (1) and an anion represented by the above formula (2)) is at least in the normal temperature range. A part of the salt may be a liquid salt (sometimes referred to as a room temperature molten salt, a room temperature molten salt, an ionic liquid, or the like). Here, the “room temperature range” is, for example, an upper limit of about 80 ° C. (typically about 60 ° C., and in some cases about 40 ° C.), and a lower limit of about −20 ° C. (typically about 0 ° C., In some cases, it refers to a temperature range of approximately 20 ° C. A molten salt in which at least a part of the temperature range is liquid (molten state) is preferable. Further, at least a part (preferably all) is in a liquid state (molten state) in a temperature range of at least about 20 to 40 ° C. (more preferably about 0 to 60 ° C., more preferably about −20 to + 80 ° C.). A molten salt capable of maintaining the temperature is preferred.

  The anion constituting the molten salt disclosed herein includes (1) a sulfonyl group and a carbonyl group bonded to one nitrogen atom, (2) a perfluoroalkyl group or a perfluoroalkyl ether to the sulfonyl group. A group (preferably a perfluoroalkyl group) is bonded, (3). A perfluoroalkyl group or a perfluoroalkyl ether group is bonded to the carbonyl group. A salt having such an anion can have a lower melting point than a salt having an anion that does not satisfy one or more of the above characteristics (1) to (3). Therefore, an anion having the above characteristics (1) to (3) is combined with an (polysubstituted) imidazolium cation in which an alkyl group is introduced at at least one of positions 2, 4, and 5 of the imidazole ring. The resulting molten salt can be a molten salt in which a low reduction potential and a low melting point are highly compatible. Such a molten salt can have a lower viscosity than a salt of an anion that does not satisfy one or more of the above characteristics (1) to (3) and a polysubstituted imidazolium cation. . Moreover, it can become a thing with higher ion conductivity. Further, it can exhibit practically preferable ion conductivity even at a lower temperature range. According to the molten salt provided by the present invention, one or more of these effects can be realized.

<Use as media>
A medium mainly composed of a molten salt (imidazolium salt) disclosed herein and showing a liquid state in a normal temperature range is preferably used for various applications as an ion conductive material, a non-flammable solvent, a non-volatile solvent, and the like. Can be done. For example, it can be useful as an electrolyte (for example, an electrolyte for a non-aqueous battery) or a component thereof. The medium may contain only one kind of the above-mentioned imidazolium salt, or may contain two or more kinds of molten salts. Moreover, the medium disclosed here can further contain components other than the molten salt mentioned above. For example, an ionic compound (salt) having a cation other than the imidazolium cation can be contained. Salts (room temperature molten salts) that are liquid in the temperature range near room temperature are preferred. Moreover, the medium disclosed here can contain a general solvent. Non-aqueous solvents (typically organic solvents) are preferred. A medium exhibiting a liquid state at about 20 ° C. (preferably about 10 ° C., more preferably about 0 ° C., further preferably about −20 ° C.) is preferred. In addition, a medium capable of maintaining a liquid state in a temperature range of at least about 20 to 40 ° C. (more preferably about 0 to 60 ° C., more preferably about −20 to + 80 ° C.) is preferable.
As a preferred example of the medium disclosed herein, a medium substantially composed of one or more of the imidazolium salts described above can be mentioned. Another preferred example is a medium that is substantially composed of one or more of the imidazolium salts described above and another ion-binding compound. In this case, the ratio of the imidazolium salt to the entire medium is preferably about 50% by mass or more, and more preferably about 70% by mass or more.
In one preferred embodiment of the medium disclosed herein, the medium is in a liquid state in the normal temperature range with substantially no components (for example, an organic solvent) other than the above-described molten salt. Alternatively, the molten salt and the other ion-binding compound are not substantially contained (that is, in a state containing substantially only the ion-binding compound (salt)), and are in a liquid state in the temperature range.

  Any of the media disclosed herein has (a) a reduction potential relative to metallic lithium of about 1 V or less (preferably about 0.8 V or less), and (b) a melting point of about 10 ° C. or less (preferably Of about 0 ° C. or less, more preferably about −20 ° C. or less). In a preferred embodiment, the medium can satisfy both conditions (a) and (b). Since such a medium has a low reduction potential and a low melting point, it is suitable as a medium for an electrolyte constituting a power storage element such as a lithium ion secondary battery. For example, it can be suitably used as a medium (electrolyte medium) for dissolving a supporting electrolyte in an electrolyte for an electricity storage element that is liquid in a normal temperature range. At least one of (a) and (b) above (more preferably) in a state that does not substantially contain a component other than the imidazolium salt described above (or a component other than the imidazolium salt and other room temperature molten salt). Is preferably a medium that can satisfy both conditions.

<Electrolyte and battery including the electrolyte>
[Electrolytes]
An electrolyte containing any one of the above-described media and showing a liquid state in a normal temperature range can be used as, for example, an electrolyte of a battery that is charged and discharged by a cation traveling between a positive electrode and a negative electrode. For example, it is suitable as an electrolyte constituting a battery (typically a lithium ion secondary battery) in which the cation is lithium ion. In addition to any of the imidazolium salts described above, the electrolyte used in such a battery can contain a compound (lithium source) that can supply lithium ions to the electrolyte. Examples of such a compound (also referred to as a supporting electrolyte) include LiPF ₆ , LiBF ₄ , LiPF ₅ (CF ₃ ), LiPF ₄ (CF ₃ ) ₂ , LiPF ₄ (CF ₂ CF ₃ ) ₂ , LiBF ₃ (CF ₃ ), LiBF ₂ (CF ₃ ) ₂ , LiBF (CF ₃ ) ₃ , LiBF ₃ (C ₂ F ₅ ), LiAsF ₆ , LiClO ₄ , LiSCN, LiOCOCF ₃ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN Lithium salts such as (SO ₂ C ₂ F ₅ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiOCOC ₆ F ₅ , LiN (SO ₂ CF ₃ ) ₂ (sometimes expressed as “LiTFSI”). You can choose.

The electrolyte disclosed herein has the ^{formula: - N (CORf 1) (} SO 2 Rf 2); anion salts of lithium ion represented by may contain. That is, the electrolyte disclosed here may have a composition containing a lithium salt (supporting electrolyte) represented by the following formula: LiN (CORf ¹ ) (SO ₂ Rf ² ); For example, each of Rf ¹ and Rf ² is a perfluoroalkyl group having 1 to 4 carbon atoms, such as LiN (COCF ₃ ) (SO ₂ CF ₃ ), LiN (COCF ₂ CF ₃ ) (SO ₂ CF ₃ ), etc. The lithium salt which is can be contained. Further, LiN (COCF ₂ OCF ₃ ) (SO ₂ CF ₃ ), LiN (COCF ₂ OCF ₂ CF ₃ ) (SO ₂ CF ₃ ), LiN (COCF ₂ CF ₂ OCF ₃ ) (SO ₂ CF ₃ ), LiN ( Rf ¹ having 2 to 5 carbon atoms such as COCF ₂ OCF ₂ CF ₂ OCF ₃ ) (SO ₂ CF ₃ ), LiN (COCF (CF ₃ ) OCF ₂ CF ₂ CF ₃ ) (SO ₂ CF ₃ ), etc. A lithium salt which is a perfluoroalkyl ether group and Rf ² is a perfluoroalkyl group having 1 to 4 carbon atoms can be contained. The anion component constituting the lithium salt and the anion component constituting the molten salt may be the same or different. For example, it can be set as the electrolyte containing the salt of the same anion component and lithium ion as the anion component which comprises molten salt (imidazolium salt). Such an electrolyte is preferable from the viewpoint of, for example, recoverability after use.

  The concentration of the supporting electrolyte is not particularly limited. For example, it can be set as the composition which contains about 0.1-20 mol support electrolyte (lithium salt) per liter (L) of electrolyte. Usually, the content of the supporting electrolyte is suitably in the range of 0.3 to 15 mol / L, and preferably in the range of 0.5 to 10 mol / L. In a temperature range of at least 10 ° C. or higher (preferably 0 ° C. or higher), the concentration is preferably set to a level at which the supporting electrolyte can be stably dissolved (precipitation is not observed).

The electrolyte can contain a general solvent (typically an organic solvent). Solvents preferably used include carbonates such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate, diethyl carbonate (DEC), and ethyl methyl carbonate; cyclic esters such as γ-butyrolactone; methyl acetate and acetic acid Esters such as ethyl, methyl formate and ethyl formate; cyclic ethers such as tetrahydrofuran and 1,3-dioxolane; ethers such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether and diethylene glycol dimethyl ether; Examples thereof include aprotic solvents used for the electrolyte solution of the secondary battery. Moreover, 3,3,4,4-tetrafluorotetrahydrofuran, trifluoropropylene carbonate, CF ₃ COOCH ₃ , C ₂ F ₅ COOCH ₃ , C ₃ F ₇ COOCH ₃ , CF ₃ COOCF ₂ CF ₂ H, CF ₃ COOCH ₂ CF ₃ , CF ₃ COOCH ₂ CF ₂ CF ₂ H, C ₃ F ₇ OCH ₃ , C ₄ F ₉ OCH ₃ , CF ₃ CFHCF ₂ OCH ₂ CF ₃ , C ₃ F ₇ OCF (CF ₃ ) COOCH ₃ , F [ CF (CF ₃ ) CF ₂ O] ₃ CF (CF ₃ ) COOCH _3, F [CF (CF ₃ ) CF ₂ O] ₃ CF (CF ₃ ) COOCH ₃ or the like containing at least one fluorine atom in the molecule Solvent (fluorine organic solvent): trimethyl phosphate, triethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, tripropyl phosphate Phosphate esters containing or not containing fluorine atoms such as phosphate, tributyl phosphate, tri (trifluoromethyl) phosphate, tri (trifluoroethyl) phosphate, tri (perfluoroethyl) phosphate; be able to. These solvents are preferred because they are nonflammable or flame retardant. One or more selected from the above-described solvents and the like can be contained in the electrolyte.
Although not particularly limited, in an electrolyte containing such a solvent, the content ratio of the solvent can be made equal to or less than the content ratio of the imidazolium salt. That is, the content of the solvent can be 50 parts by mass or less (preferably 25 parts by mass or less, more preferably 10 parts by mass or less) with respect to 50 parts by mass of the imidazolium salt constituting the electrolyte. Thus, the electrolyte which has the said imidazolium salt as a main component is a preferable example of the electrolyte disclosed here.

  The electrolyte disclosed herein can have a composition that does not substantially contain a solvent (typically, a composition that consists essentially of an ion-binding compound (salt)). For example, it may be a composition substantially composed of one or more of the imidazolium salts and a supporting electrolyte (for example, a lithium salt). You may further contain the normal temperature molten salt of the said imidazolium salt. A liquid electrolyte having such a composition and having a normal temperature range (for example, about 20 ° C. to 40 ° C., preferably about 0 ° C. to 60 ° C., more preferably about −20 ° C. to + 80 ° C.) is preferable. An electrolyte that can maintain a liquid state even at about −30 ° C. is particularly preferable. A lithium ion secondary battery provided with such an electrolyte can function appropriately in the normal temperature range, for example. In particular, it can be a lithium ion secondary battery exhibiting improved characteristics (for example, ion conductivity) in a temperature range of about 20 ° C. or lower (and further 0 ° C. or lower).

  Also, any of the electrolytes disclosed herein have (a) a reduction potential with respect to metallic lithium of about 0.5 V or less (preferably about 0.3 V or less), and (b) a melting point of about 10 It is possible to satisfy at least one of the following conditions: 1 ° C. or less (preferably about 0 ° C. or less, more preferably about −20 ° C. or less). In a preferred embodiment, the electrolyte can satisfy both the conditions (a) and (b). Since such an electrolyte has a low reduction potential and a low melting point, it is suitable as an electrolyte constituting a power storage element such as a lithium ion secondary battery. For example, at least one of (a) and (b) above (more than the above-mentioned imidazolium salt (or the imidazolium salt and other room temperature molten salts) and a component other than the supporting electrolyte). An electrolyte capable of satisfying both conditions is preferred.

[Other battery components]
A battery including the electrolyte as described above may include a positive electrode having an active material capable of reversibly inserting and extracting lithium (for example, insertion and extraction). As such an active material for a positive electrode, various lithium composite oxides containing lithium and a transition metal as constituent elements can be preferably used. Examples of such complex oxides include Li—Ni oxides, Li—Mn oxides, Li—Co oxides, and the like. Here, “Li—Ni-containing oxide” means an oxide having Li and Ni as constituent elements, and at least one other metal element other than Li and Ni (that is, a transition metal element other than Li and Ni) And / or an oxide containing a typical metal element). The metal element is a kind selected from the group consisting of Co, Al, Mn, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. Or it can be two or more. The same applies to the Li—Mn-containing oxide and the Li—Co-containing oxide.

  The positive electrode of the battery disclosed herein may have a configuration in which the positive electrode active material as described above is held on a conductive member. As the conductive member (current collector), a rod-like body, a plate-like body, a foil-like body, a net-like body or the like mainly composed of a metal such as aluminum (Al), nickel (Ni), titanium (Ti), or the like is used. be able to. Alternatively, carbon paper or the like may be used. For example, it can be set as the structure which provided the layer (active material content layer) containing an active material in the surface of a sheet-like electroconductive member (for example, Al foil). In addition to the active material, the active material-containing layer can contain one or more materials used for a general positive electrode as necessary. Examples of such materials include conductive materials and binders. As the conductive material, one or more selected from carbon materials such as carbon black (acetylene black and the like), conductive metal powders such as nickel powder, and the like can be used. As the binder, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), styrene butadiene block copolymer (SBR), carboxymethyl cellulose ( One or more selected from CMC) and the like can be used.

The battery disclosed herein can include a negative electrode having an active material capable of reversibly inserting and extracting lithium ions (for example, insertion and extraction). As the negative electrode active material, a carbon material having an amorphous structure and / or a graphite structure can be used. For example, natural graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, soft carbon, etc., one or more active materials used for the negative electrode of general lithium ion secondary batteries Can be preferably used. Alternatively, oxide, chalcogenide, or the like can be used. For example, lithium titanate (for example, Li ₄ Ti ₆ O ₁₂ ) can be preferably used as the negative electrode active material. Such an active material can be made into a negative electrode having a configuration in which a conductive member made of metal or the like is held together with a binder or the like as necessary. As the conductive member (current collector), a rod-like body, a plate-like body, a foil-like body, a net-like body mainly composed of a metal such as copper (Cu), aluminum (Al), nickel (Ni), titanium (Ti), etc. Etc. can be used. Alternatively, carbon paper or the like may be used. As the binder, the same as the positive electrode can be used. For example, it can be set as the structure which provided the layer (active material content layer) containing an active material in the surface of a sheet-like electroconductive member (for example, Cu foil).

The battery disclosed here can be configured such that the electrolyte is disposed between the positive electrode and the negative electrode. In addition, a separator may be disposed between the positive electrode and the negative electrode, and an electrolyte may be infiltrated into the separator. As the separator, for example, a porous film made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP) can be used. Moreover, you may use the woven fabric or nonwoven fabric which consists of a polypropylene, a polyethylene terephthalate (PET), methylcellulose (MC) etc.
Examples of the electrolyte disclosed herein include polyethylene oxide (PEO), ethylene oxide-propylene oxide copolymer (EO-PO), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVdF), and polyvinylidene fluoride-hexafluoro. It can be used as a solid electrolyte by molding (film formation) together with a support such as propylene copolymer (PVdF-HFP).

  The technique disclosed herein, i.e., in the cation, the number of substituents of the imidazole ring is increased to improve the performance of forming an imidazolium salt having a low reduction potential, and in the anion, the feature (1). The technology of selecting an anion satisfying (3) and combining these cations and anions to form a molten salt that achieves both a low reduction potential and a low melting point is the substitution of the imidazole ring. It can also be applied to imidazolium salts in which some or all of the groups are substituents other than alkyl groups. Such a substituent is, for example, a substituent capable of forming an imidazolium cation capable of forming an imidazolium salt having a lower reduction potential compared to a 1,3-dialkylimidazolium cation (typically EMI). (Typically, it is an organic group. For example, it may be an organic group containing an atom other than a hydrogen atom and a carbon atom.). By applying the present invention to such an imidazolium salt, for example, a room temperature molten salt having a low reduction potential and a low melting point, a medium containing such a molten salt, an electrolyte containing the medium, and a battery comprising the electrolyte ( Lithium ion secondary batteries, etc.) may be provided.

  Examples of the present invention will be described below, but the present invention is not intended to be limited to the specific examples.

<Synthesis of imidazolium salt>
[Synthesis Example 1]
Under the reaction conditions shown in Table 1 below, polysubstituted (3-5 substituted) imidazolium salts of asymmetric amides in which the sulfonyl group and the carbonyl group each have a perfluoroalkyl group were synthesized. That is, 10 mmol of 1,2,3,4,5-pentamethylimidazolium trifluoromethanesulfonate (Compound 1; first raw material) and 5 mL of water were placed in a reaction vessel. A solution prepared by dissolving 10.5 mmol of sodium salt of N- (trifluoromethanesulfonyl) trifluoroacetamide (compound 2; second raw material) in 5 mL of water was added to this solution, and the mixture was vigorously stirred at room temperature for 10 minutes. This was left still, the upper layer part was removed, and the solvent was distilled off from the lower layer part. The remaining liquid was washed repeatedly with water. Then, it was dried at 110 ° C. for 3 hours under reduced pressure by a vacuum pump. In this way, 1,2,3,4-tetramethylimidazolium N- (trifluoromethanesulfonyl) trifluoroacetamide (product), which was the target product, was obtained. The melting point of the imidazolium salt was not more than room temperature (about 20 ° C. here).

[Synthesis Examples 2 to 6]
Using each imidazolium salt (compound 1) and amide metal salt (compound 2) shown in Table 1 and Table 2 below, the reaction and workup (solvent distillation) were performed in the same manner as in Synthesis Example 1 with the solvents and reaction conditions shown in the same table. The product shown in the same table was obtained by carrying out (drying and drying). The product yields are also shown in the table. The melting points of these imidazolium salts were all below normal temperature (about 20 ° C. here).

Elemental analysis of each product obtained in Synthesis Examples 1 to 6 was performed, and it was confirmed that the calculated value and the analyzed value were almost the same. In addition, each product was dissolved in acetonitrile-d ₃ and a ¹⁹ F-NMR spectrum was measured using C ₆ F ₆ as a standard. Further, each product was dissolved in acetonitrile-d ₃ and a ¹ H-NMR spectrum was measured. The data obtained is shown in Table 3.

[Synthesis Example 7]
Under the reaction conditions shown in Table 4 below, a polysubstituted (3-5 substituted) imidazolium salt of an asymmetric amide having a perfluoroalkyl ether group was synthesized. That is, 15 mmol of 1,2,3-trimethylimidazolium trifluoromethanesulfonate (Compound 1; first raw material) and 15 mL of water were placed in a reaction vessel. While stirring this, sodium salt of N- (trifluoromethanesulfonyl) -2- (trifluoromethoxy) difluoroacetamide [NaN (COCF ₂ OCF ₂ ) (SO ₂ CF ₃ )] (Compound 2; second raw material) 15 A solution prepared by dissolving .8 mmol in 15 mL of water was added, and the mixture was stirred at room temperature for 10 minutes. Thereafter, the lower oil layer was separated and repeatedly washed with water. The resulting oil was dried at 110 ° C. for 3 hours under reduced pressure using a vacuum pump. Thus, 10.35 mmol of 1,2,3-trimethylimidazolium N- (trifluoromethanesulfonyl) -2- (trifluoromethoxy) difluoroacetamide was obtained. The yield was 69%. The melting point of the imidazolium salt was not more than room temperature (about 20 ° C. here).
The amide sodium salt (compound 2) used in this synthesis example is, for example, sodium trifluoromethanesulfonamide (NaNHSO ₂ CF ₃ ) and 2- (trifluoromethoxy) difluoroacetic anhydride [(CF ₃ OCF ₂ CO ) ₂ O)] in a suitable solvent (such as diethyl ether). The acid anhydride is available, for example, from Exfluor Research Corp. (Texas, USA).

[Synthesis Examples 8 to 15]
Using each imidazolium salt (compound 1) and amide sodium salt (compound 2) shown in Tables 4 to 6 below, the reaction and post-treatment (distilling off the solvent) were carried out in the same manner as in Synthesis Example 7 using the solvents and reaction conditions shown in the same table. And drying), each product shown in the same table was obtained. The yield of each product is also shown in the table. The melting points of these imidazolium salts were all below normal temperature (about 20 ° C. here).
The compound 2 used in Synthesis Example 11 is, for example, (CF ₃ CF ₂ CF ₂ CF) instead of (CF ₃ OCF ₂ CO) ₂ O in the synthesis method of NaN (COCF ₂ OCF ₂ ) (SO ₂ CF ₃ ) described above. It can be synthesized by using OCF ₂ CO) ₂ O. The compound 2 used in Synthesis Example 12, for example, in place of _{(CF 3 OCF 2 CO) 2} O in the synthesis method of the above _{NaN (COCF 2 OCF 2) (} SO 2 CF 3) (CF 3 OCF 2 It can be synthesized by using (CF ₂ CO) ₂ O. These acid anhydrides are all available from, for example, Exfluor Research Corp. (Texas, USA).

Elemental analysis of each product obtained in Synthesis Examples 7 to 15 was performed, and it was confirmed that the calculated values and the analytical values were almost the same. For these compositions, ¹⁹ F-NMR spectrum and ¹ H-NMR spectrum were measured by the above method. The obtained data is shown in Table 7.

<Measurement of ionic conductivity>
The ionic conductivity of several salts obtained by the synthesis examples described above was measured. The measurement is performed using the model name “Model 3100” manufactured by YSI, and the argon atmosphere using the product name “3417” manufactured by YSI as the probe, the product name “MI-915” or “MI-905” manufactured by MicroElectrodes. Went under.

About each salt (samples 1-5) of the structure shown in Table 8, the ionic conductivity in 20 degreeC of measurement temperature was measured by the said method, and the measurement result shown in the same table was obtained. These salts are salts synthesized by the respective synthesis examples shown in parentheses in the table.
Further, as shown in Table 9 below, a salt of a polysubstituted imidazolium cation and a bis (trifluoromethanesulfonyl) imide anion (sample 6), and a salt of the cation and trifluoromethanesulfonate (sample 7) The ionic conductivity was measured by the above method, and the measurement results shown in the same table were obtained. In addition, the ionic conductivity of the salt of the polysubstituted imidazolium cation and tetrafluoroborate (samples 8 and 9) is quoted from the 43rd Annual Meeting of Battery Discussion (1B09), pp. 256-257 (2002). It is shown in the same table.
The salts of Sample 6 and Sample 7 were solid at the measurement temperature (20 ° C.) described above. Thus, a sample that is solid at the measurement temperature has substantially no ionic conductivity compared to a liquid sample. Therefore, for these samples 6 and 7, the ionic conductivity is indicated as “˜0”.
As can be seen from Tables 8 and 9, the salts of Samples 1 to 5 obtained by the synthesis examples described above clearly had better ionic conductivity at 20 ° C. than the salts of Samples 6 to 9. Therefore, the battery using the electrolyte or the salt of Samples 1 to 5 as its constituent component can be more excellent in battery performance. Moreover, it can function appropriately in a wider temperature range (for example, a room temperature or lower temperature range).

<Measurement of potential width>
The electrochemical stability of each of the samples 10 to 13 shown in Table 10 was evaluated. Here, Samples 10, 12 and 13 are imidazolium salts obtained by Synthesis Examples 3, 4 and 2, respectively. Sample 11 is a composition prepared by dissolving lithium bis (trifluoromethanesulfonyl) imide as a supporting electrolyte in the imidazolium salt obtained in Synthesis Example 3 at a concentration of about 9.3 mol%. This composition was liquid at 20 ° C.
The electrochemical stability was evaluated as follows. That is, a three-electrode electrochemical cell was constructed using each sample as an electrolyte, a glassy carbon electrode as a working electrode, lithium metal as a counter electrode, and lithium metal as a reference electrode. The thus produced cell was subjected to linear sweep voltammetry measurement at room temperature (about 23 ° C.) at a sweep rate of 10 mV / sec. From the graph of temperature vs. conductivity obtained by the measurement, the reduction potential (vs. lithium metal) of each sample and the oxidation potential were read. Furthermore, the width of the potential window (potential width) was calculated from the difference between the oxidation potential and the reduction potential. The results are shown in Table 10.
As can be seen from Table 10, the imidazolium salts (samples 10, 12 and 13) obtained in Synthesis Examples 2 to 4 all showed a low reduction potential of 0.6 to 0.7 V (that is, 1 V or less). It was. In addition, all of these imidazolium salts exhibited a high oxidation potential of about 5.4 to 5.8 V (that is, 5 V or more). Since the reduction potential is low and the oxidation potential is high as described above, all of these salts have a wide potential width of 4.5 V or more.
Moreover, the composition (sample 11) in which LiTFSI was dissolved in the imidazolium salt synthesized in Synthesis Example 3 had a low reduction potential of about 0.1 V (ie 0.5 V or less) and about 5.2 V (ie 5 V or more). ) Showed a high oxidation potential. As a result, this composition had a wide potential width of 5 V or more.

Furthermore, each composition shown in Table 11 was prepared, and their electrochemical stability was evaluated in the same manner as described above. Sample 14 shown in Table 11 is a composition obtained by dissolving lithium salt (supporting electrolyte) of asymmetric amide having a perfluoroalkyl ether group at a concentration of 12 mol% in the imidazolium salt obtained in Synthesis Example 10. Sample 15 is a composition in which LiTFSI is dissolved in the imidazolium salt at a concentration of 11 mol%. Sample 16 is a composition in which lithium 2,2,2-trifluoro-N- (trifluoromethanesulfonyl) acetamide is dissolved in the imidazolium salt at a concentration of 7 mol%. All of these compositions were liquid at 20 ° C.
The supporting electrolyte (lithium salt) used for sample 14 is, for example, lithium trifluoromethanesulfonamide (LiNHSO ₂ CF ₃ ) and (CF ₃ OCF ₂ CO) ₂ O in an appropriate solvent (such as diethyl ether). It can be synthesized by reacting.
As can be seen from Table 11, the compositions of Samples 14 to 16 all showed a low reduction potential of −0.2 to 0.1 V (that is, 0.5 V or less, and further 0.3 V or less). All of these compositions exhibited high oxidation potentials of 5.2 to 5.6 V (that is, 5 V or more). For this reason, all of these compositions have a wide potential width of 5 V or more. In particular, Sample 14 exhibited a particularly low reduction potential (−0.2 V) and a particularly high oxidation potential (5.6 V), and thus had a particularly wide potential width (5.8 V).

<Measurement of ionic conductivity and potential width>
For samples 17 to 19 shown in Table 12, the ionic conductivity at a measurement temperature of 25 ° C. was measured by the above method. Moreover, the reduction potential of these samples was measured by the above method. The measurement results are shown in Table 12 together with the melting points of the respective samples.
As can be seen from Table 12, Sample 18 (EMI-TFSI) with a small number of substituents on the imidazole ring had a high reduction potential of 1.4 V with respect to lithium metal. Further, it polysubstituted and imidazolium cation ^- sample 17 salts with BF ₄ has a melting point significantly increased despite the reduction potential was decreased as compared with sample 18. On the other hand, Sample 17, which is a salt of a polysubstituted imidazolium cation and an anion represented by the above formula (2), was able to achieve both a low reduction potential and a low melting point. Also, this sample 17 clearly showed better ionic conductivity than the sample 19.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
In addition, the technical elements described in the present specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology illustrated in the present specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

Claims (12)

A method for producing a molten salt that is liquid at room temperature,
Following formula (1):

(R ¹¹ and R ¹³ are each independently an alkyl group having 1 to 4 carbon atoms. One of R ¹² , R ¹⁴ and R ¹⁵ is an alkyl group having 1 to 4 carbon atoms, and the rest is hydrogen. Selected from atoms and alkyl groups of 1 to 4 carbon atoms);
A step of selecting a cation represented by:
Following formula (2):

(Rf ¹ and Rf ² are each independently a perfluoroalkyl group containing or not containing an ether bond);
A step of selecting an anion represented by:
Forming a salt of the cation and the anion;
A process for producing a molten salt comprising R ¹² in the formula (1) is an alkyl group having 1 to 4 carbon atoms, at least one of R ¹⁴ and R ¹⁵ is selected from an alkyl group having 1 to 4 carbon atoms, and the remainder is a hydrogen atom and a carbon number. The method of claim 1, wherein the method is selected from 1 to 4 alkyl groups. The method according to claim 1, wherein R ¹² , R ¹⁴ and R ¹⁵ in the formula (1) are each independently selected from alkyl groups having 1 to 4 carbon atoms. 4. The method according to claim ¹ , wherein Rf ¹ in the formula (2) is a C 1-5 perfluoroalkyl group containing or not containing an ether bond. Rf ² in the formula (2) is a perfluoroalkyl group having 1 to 3 carbon atoms, The method according to any one of claims 1 to 3.   In the step of generating the salt, a first raw material that is a salt of a cation and an anion represented by the formula (1), and a salt of an anion and an alkali metal cation represented by the formula (2). The method according to any one of claims 1 to 5, wherein the two raw materials are mixed. The anion constituting the first raw material is
Following formula (3):
^- OSO ₂ Rf ^{3 (3)}
(Rf ³ is a perfluoroalkyl group with or without an ether bond);
An anion represented by
The method according to claim 6, which is an anion represented by X ⁻ (X is a halogen atom). It is a medium that shows liquid in a normal temperature range,
An electrolyte medium mainly composed of a molten salt produced by the method according to claim 1. The following conditions:
The reduction potential for metallic lithium is 1 V or less; and
A melting point of 10 ° C. or lower;
The medium of claim 8, wherein: An electrolyte that is liquid at room temperature,
An electrolyte comprising the medium according to claim 8 or 9 and a lithium salt as a supporting electrolyte. The following conditions:
The reduction potential for metallic lithium is 0.5 V or less; and
A melting point of 10 ° C. or lower;
The electrolyte according to claim 10, wherein   A lithium ion secondary battery comprising the electrolyte according to claim 10 or 11.