JP6038463B2 - Non-aqueous electrolyte and power storage device using the same - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte and an electricity storage device using the same.

  Various studies have been made on various power storage devices such as secondary batteries having a charging and discharging mechanism, electrolytic capacitors, and electric double layer capacitors for the purpose of ensuring safety and improving cycle characteristics and energy density.

Patent Documents 1 and 2 propose to improve battery characteristics from the surface of a non-aqueous electrolyte. Patent Document 1 discloses a non-aqueous electrolyte that hardly causes expansion of a battery package or hydrolysis of an electrolyte even when exposed to high temperatures. Discloses a non-aqueous electrolyte solution composed of lithium bis (fluorosulfonyl) imide and a lactone. Patent Document 2 discloses LiN (CF ₃ SO ₂ ) _{2 and the} like, LiPF ₆ , LiBF ₄ , LiCl as non-aqueous electrolytes that can improve the storage stability and cycle characteristics of the battery.
A non-aqueous electrolyte solution in which an O ₄ or LiAsF ₆ electrolyte is dissolved in a non-aqueous solvent is disclosed. However, in these patent documents, the properties of the non-aqueous electrolyte under a low-temperature environment (for example, 0 ° C. or lower) and the use under a low-temperature environment such as a battery equipped with the non-aqueous electrolyte are examined. It has not been.

JP 2004-165151 A JP-A-8-335465

  The present invention has been made paying attention to the above-described circumstances, and an object thereof is to provide a nonaqueous electrolytic solution in which a decrease in ionic conductivity in a low temperature environment is suppressed, and a power storage device using the same. There is.

The non-aqueous electrolytic solution of the present invention that has achieved the above-mentioned object is obtained by using an electrolyte (1), a general formula (2); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ (X, X ′ are fluorine An atom, an alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms, wherein at least one of X and X ′ is a fluorine atom) and a carbonate-based solvent. The total molar concentration of the electrolyte (1) and the compound represented by the general formula (2) is 1.2 mol / L or more, and the molar concentration of the compound represented by the general formula (2) is 0.2 mol. / L or more (non-aqueous electrolyte (I)).

The electrolyte (1) is preferably lithium hexafluorophosphate (LiPF ₆ ).

  The freezing point of the nonaqueous electrolytic solution (I) is the same molar concentration as the total molar concentration (mol / L) of the electrolyte (1) and the compound represented by the general formula (2) contained in the nonaqueous electrolytic solution. It is preferable that it is lower than the freezing point of the non-aqueous electrolyte containing the electrolyte (1). Further, the viscosity of the non-aqueous electrolyte (I) is the same as the total molar concentration (mol / L) of the electrolyte (1) and the compound represented by the general formula (2) contained in the non-aqueous electrolyte. Those having a viscosity lower than that of the nonaqueous electrolytic solution containing the electrolyte (1) having a molar concentration (mol / L) are also preferable.

  The non-aqueous electrolyte (I) preferably further contains a cyclic carbonate having a carbon-carbon double bond. In the nonaqueous electrolytic solution (I) of the present invention, it is desirable that the compound represented by the general formula (2) is lithium bis (fluorosulfonyl) imide.

  The present invention also includes a nonaqueous electrolytic solution (nonaqueous electrolytic solution (II)) containing the compound represented by the general formula (2) and a solvent having a freezing point of −30 ° C. or higher.

  The nonaqueous electrolytic solution (II) of the present invention preferably contains an electrolyte different from the compound represented by the general formula (2). The solvent preferably has an ethylene carbonate content of 40% by mass or more in 100% by mass of the solvent having a freezing point of −30 ° C. or higher. Furthermore, the aspect in which nonaqueous electrolyte (II) contains the compound represented by General formula (2) 0.1 mol / L or more, the aspect containing the cyclic carbonate which has a carbon-carbon double bond, In addition, general formula ( The embodiment in which the compound represented by 2) is lithium bis (fluorosulfonyl) imide is a preferred embodiment of the non-aqueous electrolyte (II).

  Furthermore, the electrical storage device using the said non-aqueous electrolyte (I) or non-aqueous electrolyte (II) is also contained in this invention.

  The non-aqueous electrolytic solution of the present invention maintains a solution state even in a low temperature environment, and suppresses an increase in viscosity and a decrease in ionic conductivity. Therefore, it is considered that the electricity storage device using the non-aqueous electrolyte of the present invention can operate even in a low temperature environment.

FIG. 1 is a graph showing the results of Experimental Example 4. FIG. 2 is a graph showing the results of Experimental Example 6.

The non-aqueous electrolyte of the present invention is
(I) Electrolyte (1) and general formula (2); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ (X and X ′ are fluorine atoms, alkyl groups having 1 to 6 carbon atoms, or carbon numbers) 1 to 6 fluoroalkyl groups, and at least one of X and X ′ is a fluorine atom (hereinafter sometimes referred to as compound (2)), a carbonate solvent, and an electrolyte. The total molar concentration of the compound represented by (1) and the general formula (2) is 1.2 mol / L or more, and the molar concentration of the compound represented by the general formula (2) is 0.2 mol / L. Or above (hereinafter referred to as non-aqueous electrolyte (I)), or
(II) including the above compound (2) and a solvent having a freezing point of −30 ° C. or higher (hereinafter referred to as non-aqueous electrolyte (II)),
However, it has characteristics.

  The present inventors have repeatedly studied to develop a power storage device with higher performance. Surprisingly, (I) In addition to the electrolyte (1), the compound contains the compound (2), and the electrolyte ( The nonaqueous electrolytic solution containing 1) and the compound (2) at a specific concentration can maintain the solution state even in a low temperature environment, and the increase in the viscosity of the nonaqueous electrolytic solution is suppressed. Is higher than a non-aqueous electrolyte containing no compound (2), and (II) non-aqueous electrolysis having a freezing point lower than the freezing point of the solvent alone by mixing the compound (2) and the solvent. As a result, the present invention was completed. The present invention will be described below.

<Non-aqueous electrolyte (I)>
[Electrolyte (1)]
The electrolyte (1) is a substance that dissociates into ions in the non-aqueous electrolyte (I) of the present invention and functions as a charge carrier. Since the present invention has a gist in that the compound (2) is used in addition to the electrolyte (1), the electrolyte (1) is not particularly limited, and is used as an electrolyte in electrolytes of various power storage devices. Any conventionally known electrolyte can be used. The electrolyte (1) is preferably one that has a large dissociation constant in the electrolyte and generates an anion that is difficult to solvate with the solvent described below. Specifically, LiPF ₆ , L
One or more compounds selected from the group consisting of iClO ₄ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiCl and LiI are used as the electrolyte (1) according to the present invention. Among these, LiPF ₆ and LiBF ₄ are preferable, and LiPF ₆ is more preferable. The electrolyte (1) may be used alone or in combination of two or more.

  The concentration of the electrolyte (1) is preferably 0.05 mol / L or more and 2.5 mol / L or less. More preferably, it is 0.1 mol / L-2.0 mol / L, More preferably, it is 0.15 mol / L-1.8 mol / L.

[Compound (2)]
The nonaqueous electrolytic solution (I) of the present invention contains a compound (2) represented by the general formula (2); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ . Similar to the electrolyte (1), the compound (2) also dissociates into an anion and a cation in the electrolytic solution, so that it can also serve as a charge carrier supply source. Therefore, it is conceivable that only the compound (2) is used instead of the electrolyte (1). However, the present inventors have used the compound (2) at a specific concentration in addition to the electrolyte (1), thereby increasing the charge. In addition to its function as a carrier, it has been found that the freezing point of the nonaqueous electrolytic solution can be lowered and the viscosity increase of the nonaqueous electrolytic solution can be suppressed.

  In the general formula (2), X and X ′ represent a fluorine atom, an alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms, and at least one of X and X ′ is a fluorine atom. . The alkyl group having 1 to 6 carbon atoms is preferably a linear or branched alkyl group, and examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. It is done. Examples of the fluoroalkyl group having 1 to 6 carbon atoms include those in which some or all of the hydrogen atoms of the alkyl group are substituted with fluorine atoms, such as a fluoromethyl group, a difluoromethyl group, and a trifluoromethyl group. , Fluoroethyl group, difluoroethyl group, trifluoroethyl group, pentafluoroethyl group and the like. Among the alkyl groups or fluoroalkyl groups, a fluorine atom, a trifluoromethyl group, and a pentafluoroethyl group are preferable. Preferred compounds (2) include lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (methylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) ) Imide, lithium (fluorosulfonyl) (ethylsulfonyl) imide, more preferably lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethyl) Sulfonyl) imide, more preferably lithium bis (fluorosulfonyl) imide.

  As compound (2), a commercially available product may be used, or a compound synthesized by a conventionally known method may be used.

  The concentration of the compound (2) in the nonaqueous electrolytic solution (I) of the present invention is preferably 0.2 mol / L or more and 2.5 mol / L or less. More preferably, it is 0.2 mol / L-2.0 mol / L, More preferably, it is 0.2 mol / L-1.8 mol / L.

  The total concentration of the electrolyte contained in the nonaqueous electrolytic solution (I) of the present invention (the total of the electrolyte (1) and the compound (2) is preferably 1.2 mol / L or more and 2.5 mol / L or less. More preferably, it is 1.2 mol / L to 2.0 mol / L, and further preferably 1.2 mol / L to 1.8 mol / L. When the total electrolytic mass is 2.5 mol / L or more, the electrolyte solution If the total electrolytic mass is less than 1.2 mol / L, the amount of lithium ions is small, so the movement of lithium ions may be suppressed and the function of the battery may not be sufficiently exhibited. The capacity may become low, and the battery performance may be low.

  Note that when the total of the electrolyte (1) and the compound represented by the general formula (2) is 100 mol%, the compound (2) is 50 mol% or less (not including 0 mol%) (that is, the electrolyte (1 ) Is 50 mol% or more). The amount of compound (2) is preferably 5 mol% or more, more preferably 8 mol% or more, and even more preferably 10 mol% or more. If the blending amount of LiFSI is too small, it may be difficult to use in a low temperature environment.

[solvent]
In preparing the non-aqueous electrolyte (I) of the present invention, various non-aqueous solvents conventionally used in non-aqueous electrolytes can be used as the solvent for dissolving the electrolyte. For example, saturated cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate; tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, Ethers such as 1,1-dimethoxyethane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane; γ-butyrolactone, γ-valerolactone, α-methyl-γ-butyrolactone Lactones such as: chain carboxylic acid esters such as methyl propionate and methyl butyrate can be used. Among these non-aqueous solvents, carbonate-based solvents such as saturated cyclic carbonates and chain carbonates can be preferably used because they are hardly decomposed and stable when a voltage is applied.

  In addition, the said non-aqueous solvent may be used independently and may mix and use 2 or more types. In the case of using two or more kinds of non-aqueous solvents, those containing saturated cyclic carbonates as essential solvents are preferred, and in this case, the content of cyclic carbonates in the mixed solvent is preferably 5% by mass or more. . More preferably, it is 10 mass% or more, More preferably, it is 20 mass% or more, It is preferable that it is 95 mass% or less, More preferably, it is 90 mass% or less, More preferably, it is 80 mass% or less. As the saturated cyclic carbonate, ethylene carbonate is preferred.

[Additive]
The non-aqueous electrolyte (I) of the present invention may contain an additive. Examples of the additive include a cyclic carbonate having a carbon-carbon double bond (C═C). If these additives are contained in the non-aqueous electrolyte, when the non-aqueous electrolyte is used in an electricity storage device, a film is formed on the surface of the electrode, causing deterioration of the electrode, further decomposition of the electrolyte, etc. Can be suppressed. Among the cyclic carbonates having a carbon-carbon double bond, vinylene carbonate (VC), vinyl ethylene carbonate (VEC), methyl vinylene carbonate (MVC), and ethyl vinylene carbonate (EVC) are preferable. More preferred are vinylene carbonate and vinyl ethylene carbonate, and even more preferred is vinylene carbonate.

  The concentration of the additive in the nonaqueous electrolytic solution (I) of the present invention is preferably 0.1% by mass to 10% by mass. More preferably, it is 0.1 mass%-5 mass%.

[Freezing point, viscosity]
The non-aqueous electrolyte (I) of the present invention is
(1) An electrolyte having a freezing point of the nonaqueous electrolytic solution (I) equal to the total molar concentration (mol / L) of the electrolyte (1) and the compound (2) contained in the nonaqueous electrolytic solution (I) What is lower than the freezing point of the non-aqueous electrolyte (I ′) containing (1), and / or
(2) The viscosity of the nonaqueous electrolytic solution (I) is the same molar concentration (mol / mol) as the total molar concentration (mol / L) of the electrolyte (1) and the compound (2) contained in the nonaqueous electrolytic solution (I). / L) that is lower than the viscosity of the non-aqueous electrolyte (I ′) containing the electrolyte (1),
Is preferred.

  The non-aqueous electrolyte (I) of the present invention contains the electrolyte (1) and the compound represented by the compound (2) in the electrolyte, thereby containing only the electrolyte (1) at the same molar concentration. It has a feature that the freezing point and the viscosity are lowered as compared with the electrolyte. The detailed reason why the freezing point and the viscosity are thus lowered is unknown, but by making the compound (2) an essential component of the non-aqueous electrolyte (I) due to the high affinity with the solvent of the compound (2). It is considered that the viscosity and freezing point of the nonaqueous electrolytic solution are lowered.

  If the freezing point of the nonaqueous electrolytic solution is low, the nonaqueous electrolytic solution can be maintained in a solution state even in a low temperature environment. Therefore, the use of the nonaqueous electrolytic solution (I) of the present invention for an electricity storage device can suppress a reduction in the performance of the electricity storage device due to a decrease in temperature. In addition, the freezing point of the nonaqueous electrolytic solution (I) of the present invention is not particularly limited, but is preferably, for example, −20 ° C. or lower, more preferably −25 ° C. or lower.

  The difference between the freezing point of the nonaqueous electrolytic solution (I) and the freezing point of the nonaqueous electrolytic solution (I ′) is preferably 2 ° C. or higher, more preferably 3 ° C. or higher, and further preferably 5 ° C. or higher. is there. Further, the upper limit of the difference between the freezing point of the nonaqueous electrolytic solution (I) and the freezing point of the nonaqueous electrolytic solution (I ′) may be about 50 ° C.

  On the other hand, the viscosity of the nonaqueous electrolyte affects the conductivity of ions in the electrolyte. That is, when the viscosity is too high, the movement of ions stagnate, so that sufficient ionic conductivity tends to be difficult to obtain. Further, the viscosity tends to increase as the temperature decreases. However, the temperature of the nonaqueous electrolyte solution (I) of the present invention in which the compound (2) is used in addition to the electrolyte (1) is decreased. Even if it is a case, the raise of a viscosity is suppressed compared with the case where a compound (2) is not used together, and it is thought that it is suitable for the use in a low-temperature environment.

  In the present invention, the freezing point is the temperature of the first peak confirmed in the DSC curve during temperature drop when DSC measurement is performed under a predetermined condition using a differential scanning calorimeter, while the viscosity is Is a value measured using a cone plate viscometer according to the measuring method defined in JISK7117-2. These measurement methods will be described in detail in Examples.

<Non-aqueous electrolyte (II)>
The nonaqueous electrolytic solution (II) of the present invention has a gist in that it contains the compound (2) and a solvent having a freezing point of −30 ° C. or higher. As a result of repeated investigations on non-aqueous electrolytes having excellent low-temperature characteristics, the present inventors have found that the affinity between the compound (2) and a specific solvent having a freezing point of −30 ° C. or higher is high, and that the mixture is It was found that the freezing point is lower than the freezing point of the solvent alone.

[Compound (2)]
As the compound (2), the same compound as the non-aqueous electrolyte (I) can be used. The concentration of the compound (2) in the non-aqueous electrolyte (II) is preferably 0.05 mol / L or more and the saturation concentration or less. More preferably, they are 0.1 mol / L or more and 2.5 mol / L or less, More preferably, they are 0.1 mol / L or more and 2 mol / L or less.

[Solvent with freezing point of -30 ° C or higher]
As the solvent of the non-aqueous electrolyte (II), one type of solvent may be used alone, or two or more types of solvents may be used as long as the freezing point in a state not containing a solute such as an electrolyte is −30 ° C. or higher. You may mix and use. When two or more solvents are mixed, two or more solvents having a freezing point of −30 ° C. or higher may be mixed, a solvent having a freezing point of −30 ° C. or higher, and a solvent having a freezing point of less than −30 ° C. One or more of these may be mixed and used.

  As the solvent for the nonaqueous electrolytic solution (II), the nonaqueous solvent exemplified as the solvent for the nonaqueous electrolytic solution (I) can be used. Among the above non-aqueous solvents, as the solvent having a freezing point of −30 ° C. or higher, saturated cyclic carbonates such as ethylene carbonate and 2,3-butylene carbonate; chain carbonates such as dimethyl carbonate; 1,4-dioxane and the like Ethers. On the other hand, as a solvent having a freezing point of less than −30 ° C., saturated cyclic carbonates such as propylene carbonate and chloroethylene carbonate; chain carbonates such as ethyl methyl carbonate and diethyl carbonate; tetrahydrofuran, 2-methyltetrahydrofuran, 1,1- Ethers such as dimethoxyethane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane; lactones such as γ-butyrolactone, γ-valerolactone, α-methyl-γ-butyrolactone Chain carboxylic acid esters such as methyl propionate and methyl butyrate;

  Among the above non-aqueous solvents, carbonates that are solid at normal temperature (25 ° C.), or mixed solvents that include one or more of the above-mentioned non-aqueous solvents and carbonates that are solid at normal temperature and have a freezing point of −30 ° C. or higher are Suitable as a solvent for the aqueous electrolyte (II). The inventors of the present invention have a particularly high affinity for the above compound (2) with carbonates, and even if both the compound (2) and the carbonates used as a solvent are in a solid state, they are mixed with each other when mixed. It was found that it melted and became a liquid mixture.

  Among the non-aqueous solvents, carbonate-based solvents are widely used as solvents for electrolyte solutions. However, carbonates that are solid at room temperature, such as ethylene carbonate, have a high melting point and low fluidity, and thus have a problem that they are difficult to use alone or in a high content as an electrolyte. For example, it has been used for electrolyte solutions as a mixed solvent with chain carbonates such as ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) and saturated cyclic carbonates such as propylene carbonate. However, as the present inventors have found, normal temperature solid carbonates such as ethylene carbonate and compound (2) (solid at normal temperature) have high affinity, and a mixture in a liquid state is obtained by mixing them. It is done. Therefore, when the compound (2) is an essential electrolyte, carbonates that are solid at room temperature can be used alone or in a high content as a solvent for a non-aqueous electrolyte. As a result, even when the electrolytic solution is used in a high-temperature environment, it is possible to prevent the occurrence of problems such as the evaporation of the low boiling point solvent and the expansion of the battery package due to decomposition, and to provide a safer power storage device. . Further, when the compound (2) is an essential electrolyte, the nonaqueous electrolyte (II) has a lower freezing point than the solvent alone, and further, the electrolyte using another electrolyte other than the compound (2). Therefore, it is considered to be suitably used for use in a low temperature environment.

  Examples of the solvent that is solid at room temperature include ethylene carbonate and 2,3-butylene carbonate. Among these, ethylene carbonate is preferable. The amount of the solvent that is solid at room temperature is preferably 30% by volume or more based on the total amount of the solvent. More preferably, it is 40 volume% or more, More preferably, it is 45 volume% or more, More preferably, it is 50 volume% or more, More preferably, it is 60 volume% or more.

  When a solvent that is solid at room temperature is used in combination with another non-aqueous solvent (for example, a solvent in a liquid state at room temperature), the amount of the other non-aqueous solvent used is 70% by volume or less based on the total amount of the solvent. More preferably, it is 60 volume% or less, More preferably, it is 55 volume% or less, More preferably, it is 50 volume% or less, More preferably, it is 40 volume% or less.

[Others]
The non-aqueous electrolyte (II) may contain other additives and other electrolytes. As an additive, the additive of said non-aqueous electrolyte (I) and a conventionally well-known additive are mentioned. On the other hand, as the other electrolyte, any conventionally known electrolyte such as the electrolyte (1) of the non-aqueous electrolyte (I) can be used. The additive concentration in the non-aqueous electrolyte (II) is preferably 0.1% by mass or more and 10% by mass or less (more preferably 0.1% by mass or more and 5% by mass or less). On the other hand, the concentration of the other electrolyte is 0 mol / L or more and 3 mol / L or less (more preferably 0 mol / L or more and 2.5 mol / L or less, more preferably 0 mol / L or more and 2 mol / L or less). Is recommended. The electrolyte concentration (total amount of the compound (2) and other electrolytes) in the non-aqueous electrolyte (II) is preferably 0.1 mol / L or more. More preferably, they are 0.1 mol / L-2.5 mol / L, More preferably, they are 0.5 mol / L-2 mol / L.

[Power storage device]
The non-aqueous electrolytes (I) and (II) of the present invention are suitable for use in a low temperature environment. Therefore, the non-aqueous electrolyte of the present invention is not limited to batteries having charging and discharging mechanisms such as primary batteries, lithium (ion) secondary batteries, and fuel cells, but also various storage devices such as electrolytic capacitors, electric double layer capacitors, and solar cells. Is preferably used. In addition, the structure of various electrical storage devices is not specifically limited, The electrolyte solution of this invention is applicable to a well-known electrical storage device.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental Example 1 Freezing Point Measurement Experimental Example 1-1
As an electrolyte (1), 1.52 g (1 mol / L) of lithium hexafluorophosphate (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd., LBG grade) was weighed into a 10 mL volumetric flask, and ethylene carbonate (EC) and ethyl methyl carbonate ( EMC) (both manufactured by Kishida Chemical Co., Ltd., LBG grade) was made up with a mixed solvent having a volume ratio of 1/1 ((EC / EMC)) to prepare an electrolytic solution. The sample was placed in an aluminum pan and subjected to differential scanning calorimetry (DSC) using a differential scanning calorimeter (DSC 6220, manufactured by Seiko Instruments Inc.) Measurement conditions were 2 ° C./min. After the temperature was lowered and reached the respective set temperature (−80 ° C., 50 ° C.), the temperature was held for 3 minutes at the same set temperature. Click measured .3 cycle was freezing point, the 2 cycles after adopted as data, the average value was determined. Table 1 shows the results.

Experimental Example 1-2
Electrolyte (1) lithium hexafluorophosphate as (LiPF _6, Kishida Chemical Co., Ltd., LBG grade) 1.37 g (0.9 mol / L), lithium bis (fluorosulfonyl) as the compound (2) imide (LiFSI, stock The freezing point was measured in the same manner as in Experimental Example 1-1 except that 0.19 g (0.1 mol / L) manufactured by Nippon Shokubai Co., Ltd. was used.

Experimental Example 1-3
Lithium hexafluorophosphate (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd., LBG grade) 1.14 g (0.75 mol / L) as electrolyte (1), lithium bis (fluorosulfonyl) imide (LiFSI, stock) as compound (2) The freezing point was measured in the same manner as in Example 1-1, except that 0.47 g (0.25 mol / L) manufactured by Nippon Shokubai Co., Ltd. was used.

Experimental Example 1-4
Electrolyte (1) lithium hexafluorophosphate as (LiPF _6, Kishida Chemical Co., Ltd., LBG grade) 0.76 g (0.5 mol / L), lithium bis (fluorosulfonyl) as the compound (2) imide (LiFSI, stock The freezing point was measured in the same manner as in Experimental Example 1-1 except that 0.94 g (0.5 mol / L) manufactured by Nippon Shokubai Co., Ltd. was used. The results are shown in Table 1. For comparison, the result of DSC measurement using only a mixed solvent of ethylene carbonate and ethyl methyl carbonate (EC / EMC = 1/1, volume ratio) as a sample solution is shown as Reference Example 1.

Experimental Examples 1-5 to 1-7
Except for changing the electrolyte concentration as shown in Table 1, the freezing point was measured in the same manner as in Example 1-1. The results are shown in Table 1.

In Table 1, LiPF ₆ represents lithium hexafluorophosphate, and LiFSI represents lithium bis (fluorosulfonyl) imide. In the lower part of each cell, the amount of the electrolyte (1) or compound (2) used to prepare 10 ml of the electrolytic solution is shown in parentheses (the same applies to the following tables).

Experimental Examples 1-8 to 1-10
Examples 1-1, 1-2 and 1 except that a mixed solvent having a volume ratio of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) of 3/7 (EC / EMC) was used as the solvent. The freezing point was measured in the same manner as -4. The results are shown in Table 2. For comparison, the result of DSC measurement using only a mixed solvent of ethylene carbonate and ethyl methyl carbonate (EC / EMC = 3/7, volume ratio) as a sample solution is shown as Reference Example 2.

Experimental Examples 1-1-1 to 1-13
The freezing point was measured in the same manner as in Experimental Example 1-1 except that the electrolyte concentration was changed as shown in Table 2. The results are shown in Table 2.

  In Table 2, “ND” means that no exothermic peak was observed in the DSC curve up to −80 ° C. (the same applies to Table 3).

Experimental Examples 1-14 to 1-15
As a solvent, the volume ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and propylene carbonate (PC) is 4.5 / 4.5 / 1 (EC / EMC / PC, each solvent is manufactured by Kishida Chemical Co., Ltd.) , LBG grade) and the electrolyte concentration was changed as shown in Table 3, a sample solution was prepared in the same manner as in Experimental Example 1-1, and the freezing point was measured. For comparison, the result of measuring the freezing point using only ethylene carbonate, ethyl methyl carbonate and propylene carbonate (EC / EMC / PC = 4.5 / 4.5 / 1, volume ratio) as a sample solution is referred to as Reference Example 3. Show.

  From Tables 1 to 3, it was confirmed that the freezing point of the nonaqueous electrolytic solution was lowered by using the compound (2) (LiFSI) in addition to the electrolyte (1). From this result, it is considered that by using the nonaqueous electrolytic solution of the present invention, a battery in which the battery performance is hardly deteriorated even in a low temperature environment can be obtained.

Experimental Example 2 Measurement of Viscosity Experimental Examples 2-1 to 2-6
Ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (EC / EMC = 3/7 (volume ratio), manufactured by Kishida Chemical Co., Ltd., LBG grade) was used as the solvent, and the electrolyte concentration was changed as shown in Table 4. Except that, a sample solution was prepared in the same manner as in Experimental Example 1-1. The viscosity of the sample solution was measured according to JISK7117-2 using a cone plate viscometer (Cone: CP10) manufactured by BROOKFIELD. The measurement was performed by putting about 2 ml of the sample solution into the sample cup and adjusting the rotation speed so that the rotation speed was gradually increased and the torque was 50 to 80%. Table 4 shows the results of measuring the viscosity of the sample solution at 50 ° C to -30 ° C.

  From Table 4, in Experimental Example 2-1, in which the electrolyte (1) is used alone, it can be confirmed that the viscosity increases as the temperature decreases, but in addition to the electrolyte (1), the compound (2) (LiFSI) It can be seen that even when the mass fraction of the electrolyte in the sample solution is increased, the increase in viscosity in a low-temperature environment can be suppressed.

Experimental Examples 2-7 to 2-9
Table 5 shows that ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (EC / EMC = 1/1 (volume ratio), manufactured by Kishida Chemical Co., Ltd., LBG grade) was used as the solvent. A sample solution was prepared in the same manner as in Experimental Example 1 except for the change. Similarly to Experimental Example 2-1, etc., the viscosity of the sample solution at 50 ° C. to −25 ° C. was measured using a cone plate viscometer (Cone: CP10) manufactured by BROOKFIELD. The results are shown in Table 5.

Experimental Examples 2-10 to 2-12
As a solvent, ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / propylene carbonate (PC) (EC / EMC / PC = 4.5 / 4.5 / 1 (volume ratio), each solvent is manufactured by Kishida Chemical Co., Ltd. A sample solution was prepared and the freezing point was measured in the same manner as in Experimental Example 1 except that a mixed solvent of LBG grade) was used and the electrolyte concentration was changed as shown in Table 6.

  From Tables 4-6, even if a solvent composition changes, it is the fall of temperature by using compound (2) (LiFSI) in addition to electrolyte (1) similarly to the case of Experimental example 2-1, 2-2. It can be seen that the increase in the viscosity due to can be suppressed.

Experimental Example 3 Measurement of Ionic Conductivity Experimental Examples 3-1 to 3-8
Using a mixed solvent in which the volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (both manufactured by Kishida Chemical Co., Ltd., LBG grade) is 3/7 (EC / EMC), the electrolyte (1) and A sample solution (non-aqueous electrolyte) was prepared in the same manner as in Experimental Example 1 except that the concentration of the compound (2) was changed as shown in Table 7. Next, the ion conductivity of the sample solution was measured using “IMPEDANCE ANALYZER SI 1260” manufactured by Solartron. The results are shown in Table 7.

Experimental Examples 3-9 to 3-11
A mixed solvent in which the volume ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and propylene carbonate (PC) is 4.5 / 4.5 / 1 (each solvent is LBG grade, manufactured by Kishida Chemical Co., Ltd.). A sample solution was prepared and the freezing point was measured in the same manner as in Experimental Example 1 except that the sample solution was changed and the electrolyte concentration was changed as shown in Table 8. The results are shown in Table 8.

  From Tables 7 and 8, even if the solvent composition changes, the total electrolyte is the same as the electrolytic solution using the electrolyte (1) alone, and the electrolytic solution using the compound (2) (LiFSI) in addition to the electrolyte (1) Was found to exhibit higher conductivity at any temperature.

As described above, from the results of Experimental Examples 1 to 3, by replacing a part of LiPF ₆ which is the electrolyte (1) of the conventional lithium ion secondary battery with the compound (2) (LiFSI), the characteristics of the electrolyte solution at low temperature It was confirmed that Therefore, the results of the above experimental examples suggest that the battery characteristics in a low temperature region are improved by using the nonaqueous electrolytic solution of the present invention.

Experimental Example 4 Charge / Discharge Cycle Test Experimental Examples 4-1 to 4-5
[Preparation of electrolyte]
The electrolyte (1), the compound (2), and the additive were mixed so that the concentrations shown in Table 9 were ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3/7 (EC / EMC, both manufactured by Kishida Chemical Co., Ltd., LBG). Grade) mixed solvent to prepare an electrolytic solution.

[Production of laminate cell and charge / discharge before test]
One commercial positive electrode sheet (active material: lithium cobaltate, 3 mAh / cm ² ) having an area of the positive electrode active material layer of 12 cm ² (3 × 4 cm) and an area of the negative electrode active material layer of 13.44 cm ² (3. 2 × 4.2 cm) of a commercially available negative electrode sheet (active material: graphite, 3.2 mAh / cm ² ) was laminated so as to face each other, and one polyolefin separator was sandwiched therebetween. The positive and negative electrode sheets were sandwiched between two aluminum laminate films, and the aluminum laminate film was filled with an electrolytic solution and sealed in a vacuum state.

  Using a charge / discharge test apparatus (ACD-01, manufactured by Asuka Electronics Co., Ltd.), charge / discharge was performed once at 30 ° C. at a charge / discharge rate of 0.2 C (constant current mode) and 3.5 V to 4.2 V. Thereafter, the laminate cell was opened and then sealed again in a vacuum state. Under the same conditions, charging / discharging was repeated 5 times to complete a laminated lithium battery.

[Charge / discharge cycle test]
About the obtained laminate type lithium battery, charge / discharge rate 0.2C (constant current mode), 3.5V to 4.2V at 30 ° C. using a charge / discharge test device (ACD-01, manufactured by Asuka Electronics Co., Ltd.). In each charge and discharge, a cycle test (210 to 240 times) was performed with a charge / discharge pause time of 10 minutes. The results are shown in Table 9 and FIG.

LiPF ₆ : lithium hexafluorophosphate (manufactured by Kishida Chemical Co., Ltd., LBG grade)
LiFSI: Lithium bis (fluorosulfonyl) imide (manufactured by Nippon Shokubai Co., Ltd.)
LiTFSI: Lithium bis (trifluoromethanesulfonyl) imide (manufactured by Kishida Chemical Co., Ltd., LBG grade)
VC: Vinylene carbonate (manufactured by Kishida Chemical Co., Ltd., LBG grade)

  From Table 9, in addition to electrolyte (1), by using LiFSI (Experimental example 4-2) as a compound (2), compared with the case where this is not used (Experimental example 4-1, 4-4). It can be seen that the cycle characteristics can be improved. It can also be seen that the use of an additive (VC) in addition to LiFSI can further improve the cycle characteristics (Experimental Example 4-3, FIG. 1). In addition, since only the electrolyte (1) is contained in the electrolyte solution according to Experimental Example 4-5, the results of Experimental Example 2 are inferior in low-temperature characteristics as compared with Experimental Examples 4-2 and 4-3 including LiFSI. It is considered a thing. From the above results, it can be expected that the laminate type lithium batteries of Experimental Examples 4-2 and 4-3 using the nonaqueous electrolytic solution according to the present invention show good battery characteristics even at low temperatures.

Experimental Example 5 Freezing point measurement, solubility evaluation Experimental Examples 5-1 to 5-11
Using ethylene carbonate (EC) alone or a mixed solvent having a volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) of 1/1 (EC / EMC) as a solvent, LiPF ₆ and LiFSI The solidification point was measured and the solubility was evaluated in the same manner as in Experimental Example 1 except that the concentrations were changed as shown in Tables 10 and 11. The solubility was evaluated by visual observation as to whether the mixture was in a liquid state or whether a solid component remained. The results are shown in Table 10 and Table 11.

  In Tables 10 and 11, “insoluble” indicates that a solid component remained in the mixture.

The solubility of LiPF ₆ and LiFSI (both solid at room temperature) in ethylene carbonate (EC), which is solid at normal temperature (25 ° C.), has a freezing point of 36 ° C., and what is LiPF ₆ and ethylene carbonate? Even though they were mixed, they remained solid and did not become liquid (Experimental Example 5-1), whereas LiFSI was compatible with ethylene carbonate to be in a liquid state, and the freezing point was −9.9 ° C. Turned out to be. Further, when LiPF ₆ and LiFSI were mixed in a mixed solvent having a volume ratio of ethylene carbonate and ethyl methyl carbonate of 1/1, the freezing point of the mixture containing LiFSI decreased to −55.5 ° C. ( Experimental Example 5-5). The freezing point of Experimental Example 5-5 is compared with the case of containing only the mixed solvent (Experimental Example 5-3, freezing point 6.2 ° C.) and the mixed solvent and LiPF ₆ (Experimental Example 5-4, freezing point −21.1 ° C.). Thus, it is much lower than 30 ° C. Furthermore, the same tendency was observed when the content of ethylene carbonate was increased to 75% by volume (Experimental Examples 5-9 to 5-11).

  From this result, it is understood that the compound (2) represented by LiFSI has high affinity with cyclic carbonates such as ethylene carbonate, and a mixture (electrolyte solution) having a low freezing point can be obtained by mixing the two. . Therefore, it can be seen that the nonaqueous electrolytic solution of the present invention containing the compound (2) and a solvent having a freezing point of −30 ° C. or higher is hard to solidify even in a low temperature environment and has fluidity.

Experimental Example 6 Low-temperature characteristic test [Production of laminate cell and charge / discharge before test]
The electrolytic solution was prepared by dissolving the electrolyte in a mixed solvent having a volume ratio of ethylene carbonate and ethyl methyl carbonate of 3/7 so that the concentrations shown in Table 12 were obtained.
Preparation and charging / discharging before the test of the laminate cell were carried out using a positive electrode sheet (active material: LiNi _1/3 Mn _1/3 Co _1/3 , ² mAh / cm ² ) and a negative electrode sheet (active material: artificial graphite, 2.2 mAh / A laminated lithium battery was completed in the same manner as in Experimental Example 4 except that cm ² ) was used.

[Low temperature characteristics test]
About the obtained laminate type lithium battery, using a charge / discharge test apparatus (ACD-01, manufactured by Asuka Electronics Co., Ltd.), charging is performed up to 4.2 V at 25 ° C. and a charging rate of 0.2 C (constant current mode). Based on the discharge capacity when discharged to 3 V at a discharge rate of 0.2 C in an environment of 25 ° C. The charge at the time of measuring the characteristics at a low temperature is similarly performed at 25 ° C., and is discharged at −30 ° C. at a discharge rate of 0.2 C, 0.5 C, 1.0 C, and 2.0 C to 3.0 V. Characteristics were measured. The results are shown in Table 12 and FIG. In Table 12, the discharge capacity retention rate means the ratio of the discharge capacity at each temperature of the discharge capacity to the discharge capacity (100%) at 25 ° C.

  From Table 12 and FIG. 2, the electrolytic solution of Experimental Example 6-2 containing a predetermined amount of the compound (2) is discharged in a low temperature environment of −30 ° C. as compared with the electrolytic solution of Experimental Example 6-1 that does not contain the compound (2). It can be seen that the decrease in capacity is suppressed. From this result, it can be seen that the electrolytic solution of the present invention is also suitably used for devices used in a low temperature environment.

Claims (6)

(XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ (X, X ′ represents a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms. And at least one of X and X ′ is a fluorine atom), and a non-aqueous electrolyte containing a solvent having a freezing point of −30 ° C. or higher,
Containing 0.6 mol / L or more of the compound represented by the general formula (2), and
A nonaqueous electrolytic solution, wherein ethylene carbonate is 40% by volume or more in 100% by volume of a solvent having a freezing point of −30 ° C. or higher. The nonaqueous electrolytic solution according to claim 1 , comprising an electrolyte different from the compound represented by the general formula (2). The nonaqueous electrolytic solution according to claim 1 or 2 , comprising 2 mol / L or less of the compound represented by the general formula (2). The nonaqueous electrolytic solution according to any one of claims 1 to 3 , comprising a cyclic carbonate having a carbon-carbon double bond. The nonaqueous electrolytic solution according to any one of claims 1 to 4 , wherein the compound represented by the general formula (2) is lithium bis (fluorosulfonyl) imide. Electricity storage device using the nonaqueous electrolytic solution according to any one of claims 1 to 5.