JP4903983B2 - Non-aqueous electrolyte and secondary battery using the same - Google Patents

Description

  The present invention relates to an electrolyte for a non-aqueous electrolyte secondary battery and a secondary battery using the same, and in particular, an electrolyte for a lithium ion battery with improved conductivity and flame retardancy of the electrolyte for a lithium ion battery, and The present invention relates to a lithium ion battery using this.

  A non-aqueous electrolyte secondary battery is generally configured as shown in FIG. 1. In this configuration, a positive electrode 1 capable of inserting or extracting lithium is placed inside a positive electrode can 2 via an aluminum foil 3. On the other hand, a negative electrode 4 capable of inserting or extracting lithium is accommodated in a sealing plate 5. The positive electrode can 2 and the sealing plate 5 are caulked and sealed with a gasket 7 in a state in which the positive electrode 1 and the negative electrode 4 sandwich the separator 6 made of a microporous film in which an electrolytic solution is impregnated. (For example, refer to Patent Document 1).

  As a solvent for a lithium ion battery, which is one of non-aqueous electrolyte secondary batteries, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and the like have been used. Among these, propylene carbonate has a high dielectric constant, excellent solubility of lithium salts, etc., and high electrical conductivity at low temperatures, and therefore has excellent performance as a main solvent for an electrolytic solution. However, when graphite or graphitized carbon is used as the negative electrode material, there is a problem that propylene carbonate is decomposed on the surface of the carbon material, so that ethylene carbonate is used as the high dielectric constant solvent. However, ethylene carbonate has a higher freezing point than propylene carbonate and remarkably lowers lithium ion conductivity in a low-temperature environment. Therefore, ethylene carbonate is used by mixing with a low-viscosity solvent such as dimethyl carbonate.

  However, with such a conventional electrolyte, the ionic conductivity can only be improved to the order of several mS / cm. In order to further improve the ionic conductivity, it has been reported that it is effective to reduce the viscosity of the electrolyte by increasing the low-viscosity solvent instead of the high dielectric constant solvent such as ethylene carbonate and propylene carbonate. (For example, refer nonpatent literature 1.). However, since the low-viscosity solvent has high volatility, it has a problem that its safety is lowered as the amount added is increased.

  In view of such a situation, a good room temperature molten salt has been developed (see, for example, Patent Document 2). This room temperature molten salt is chemically stable as a liquid at room temperature, and since water can be easily removed out of the system, in a lithium secondary battery using an electrolytic solution containing this molten salt, It has been reported that it has the effect of being excellent in performance, durability and stability.

Japanese Patent Laid-Open No. 11-121022 JP-A-11-297355 Boris Ravdel et al., Abstract of the 204th “The Electrochemichal Society, Inc.” Meeting 273, 2003

  However, simply adding the above-mentioned room temperature molten salt to the electrolyte of the non-aqueous electrolyte secondary battery does not provide a good improvement in ionic conductivity, and is used for a non-aqueous electrolyte secondary battery. There is a demand for an optimum content as an electrolytic solution. In the present invention, the room temperature molten salt that stably exists as a liquid at room temperature is referred to as an ionic liquid.

  Accordingly, the present invention provides a novel non-aqueous electrolyte for a non-aqueous electrolyte secondary battery that has improved safety and further improved ionic conductivity, and a non-aqueous electrolyte secondary battery using the same. The purpose is that.

The non-aqueous electrolyte for a non-aqueous electrolyte secondary battery of the present invention comprises an alkali salt , a non-aqueous solvent that is a mixture of a cyclic ester and a chain ester , a dimethylethylbutylammonium cation and a bistrifluoromethylsulfonylimide anion. The ionic liquid is composed of an ionic liquid, and the content of the ionic liquid with respect to the total weight of the non-aqueous solvent and the ionic liquid is 35 to 50% by weight .

  The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode including an active material capable of occluding and releasing alkali ions, a negative electrode including an active material capable of occluding and releasing alkali ions, and the non-aqueous electrolysis of the present invention. It is characterized by comprising a non-aqueous electrolyte for a liquid secondary battery.

  According to the present invention, by containing an ionic liquid in the nonaqueous electrolytic solution, the diffusion constant of alkali ions and the transport number are improved, and the ionic conductivity is improved. The content of is less than 80% by weight based on the total weight of the non-aqueous solvent and the ionic liquid, thereby avoiding a decrease in the diffusion constant of alkali ions due to an increase in viscosity.

  This improvement in ionic conductivity is presumably due to the following principle. In the case of a conventional non-aqueous solvent-only electrolyte, solvent molecules with dipoles around alkali ions are oriented in an energetically stable direction, and attractive forces are generated between the ions and dipole molecules. Work. Since alkali ions have a small ion radius, the surface area is small, that is, the charge density per unit area is large and the dipole moment is large. Therefore, since the energy required for alkali ions to move to other sites is large, the ionic conductivity is low. On the other hand, in the nonaqueous electrolytic solution of the present invention, by containing the ionic liquid, the ionic liquid and the alkali salt are mixed and act on the alkali salt that has not been dissociated, and the ionic liquid and Monovalent ions are formed. Therefore, the ion radius can be increased, that is, the surface area can be increased, and the dipole moment can be reduced.

  In addition, the ionic liquid in the present invention does not generate gas due to volatilization, unlike conventional low-viscosity solvents, so that the non-aqueous electrolyte can be maintained in flame retardancy, resulting in safety. The effect that it is excellent also is acquired.

  The non-aqueous electrolyte for a non-aqueous electrolyte secondary battery of the present invention is composed of an alkali salt, a non-aqueous solvent, and a specific ionic liquid. The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode including an active material capable of occluding and releasing alkali ions, a negative electrode including an active material capable of occluding and releasing alkali ions, and the non-aqueous electrolysis of the present invention. It is comprised from the non-aqueous electrolyte for liquid secondary batteries. Specifically, in a dry atmosphere, the positive electrode and the negative electrode are accommodated in a positive electrode can and a sealing plate, the positive electrode and the negative electrode are laminated so as to sandwich the separator, and the laminate is sealed. The positive electrode can and the sealing plate can be caulked and sealed through a gasket to produce the nonaqueous electrolyte secondary battery of the present invention. Hereinafter, these components will be described.

1. Nonaqueous Electrolytic Solution (1) Alkali Salt The alkaline salt in the nonaqueous electrolytic solution of the present invention is a solute and is preferably a lithium salt. Specifically, inorganic lithium salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , or LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ Fluorine-containing organic lithium salts such as SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) _{3 and the} like can be mentioned. Two or more kinds of these alkali salts may be used as a mixture, but in the present invention, it is a more preferable embodiment that the alkali salt is at least one of LiPF ₆ and LiBF ₄ .

  Further, the molar concentration of the solute alkali salt in the electrolytic solution is preferably 0 M or more and 3 M or less, and more preferably 0.5 M. When deviating from this range, the electrical conductivity of the non-aqueous electrolyte is lowered, and the battery performance is degraded.

(2) Non-aqueous solvent The non-aqueous solvent in the present invention includes cyclic esters such as ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone, which are high dielectric constant solvents, and diethyl carbonate and ethyl methyl carbonate, which are low-boiling solvents. And chain esters such as dimethyl carbonate and methyl ethyl carbonate. These non-aqueous solvents may be used as a mixture of two or more. In the present invention, it is a more preferable embodiment that the non-aqueous solvent is a mixture of a cyclic ester and a chain ester.

(3) Ionic liquid
In the non-aqueous electrolyte for a non-aqueous electrolyte secondary battery of the present invention, the greatest feature is that the ionic liquid represented by the following chemical formula 1 is contained in a specific amount. Here, R ₁ to ₄ represent an alkyl group , the cation portion is specifically a dimethylethylbutylammonium cation, and X ⁻ represents bistrifluoromethylsulfonylimide (TFSI-) shown in the following chemical formula 2. Yes.

  In addition, the content of the ionic liquid relative to the total weight of the non-aqueous solvent and the ionic liquid in the present invention is essential to be more than 0% by weight and less than 80% by weight, preferably more than 5% by weight and 60% by weight. More preferably, it is more than 10% by weight and less than 50% by weight, and more preferably more than 35% by weight and less than 50% by weight. If this ionic liquid is not contained, the diffusion constant and transport number of alkali ions are not improved, and the ionic conductivity cannot be improved. On the other hand, when the content of the ionic liquid is 80% by weight or more, the viscosity of the non-aqueous solvent increases, the diffusion constant of alkali ions decreases, and as a result, the ionic conductivity also decreases.

2. Positive electrode In the non-aqueous electrolyte secondary battery of the present invention, the positive electrode active material is kneaded with an electronic conductor such as acetylene black and a binder such as polyvinylidene fluoride, and this is made into a slurry, It can be produced by applying it on the body, drying it and molding it into any shape. Examples of the positive electrode active material include materials capable of inserting and extracting lithium, such as transition metal oxides, transition metal sulfides, composite oxides of lithium and transition metals, conductive polymer materials, carbon fiber materials, or mixtures thereof. As a material for the positive electrode current collector, aluminum, titanium, tantalum, alloys thereof, or the like can be used.

3. Negative electrode For the negative electrode in the non-aqueous electrolyte secondary battery of the present invention, as with the positive electrode, the negative electrode active material was kneaded with an electronic conductor such as acetylene black and a binder such as polyvinylidene fluoride, and this was made into a slurry. Then, it can apply | coat and dry on a positive electrode electrical power collector, and can shape | mold and shape in arbitrary shapes. Examples of the negative electrode active material include metallic lithium, lithium-containing alloy, or lithium ion doped, undopeable carbon material, lithium ion doped, detachable metal oxide and other materials capable of occluding and releasing lithium. As the material of the negative electrode current collector, metals such as copper, nickel, and stainless steel are used, and among these, a copper foil is preferable from the viewpoint of easy processing into a thin film and cost.

4). Other constituent members The material of the positive electrode can and the sealing plate in the present invention is an outer can for accommodating the positive electrode, the negative electrode, and the like, and stainless steel or aluminum is preferably used as the material. The separator is a member impregnated with the non-aqueous electrolyte of the present invention and sandwiched between the positive electrode and the negative electrode, and a porous sheet or nonwoven fabric made of a polyolefin such as polyethylene or polypropylene can be used. .

  Moreover, as another aspect of the present invention, an electric device or a transport device using the non-aqueous electrolyte secondary battery is included.

Hereinafter, the effects of the present invention will be described in detail by way of specific examples.
1. Preparation of Nonaqueous Electrolyte for Nonaqueous Electrolyte Secondary Battery < Comparative Example 1 >
As shown in Table 1, a mixed solvent of trimethylhexylammonium bistrifluoromethylsulfonylimide, which is an ionic liquid, and a 1: 3 mixed solution of ethylene carbonate and ethyl methyl carbonate in a weight ratio of 35:65 is used. LiBF ₄ was dissolved in 0.5 M to prepare a non-aqueous electrolyte for a non-aqueous electrolyte secondary battery of Comparative Example 1 of the present invention.

<Example 1 >
As shown in Table 1, a mixed solvent in which dimethylethylbutylammonium bistrifluoromethylsulfonylimide, which is an ionic liquid, and a 1: 3 mixed solution of ethylene carbonate and ethyl methyl carbonate was 35:65 by weight ratio. In addition, LiPF ₆ was dissolved in 0.5 M to prepare a nonaqueous electrolyte solution for a nonaqueous electrolyte secondary battery of Example 1 of the present invention.

< Comparative example 2 >
As shown in Table 1, a mixed solvent in which a 1: 3 mixed solution of dimethylethylbutylammonium bistrifluoromethylsulfonylimide, which is an ionic liquid, and ethylene carbonate and ethylmethyl carbonate, is 10:90 by weight ratio. In addition, LiBF ₄ was dissolved in 0.5 M to prepare a non-aqueous electrolyte for a non-aqueous electrolyte secondary battery of Comparative Example 2 of the present invention.

<Example 2 >
As shown in Table 1, a mixed solvent in which dimethylethylbutylammonium bistrifluoromethylsulfonylimide, which is an ionic liquid, and a 1: 3 mixed solution of ethylene carbonate and ethyl methyl carbonate was 35:65 by weight ratio. Then, LiBF ₄ was dissolved in 0.5 M to prepare a non-aqueous electrolyte for a non-aqueous electrolyte secondary battery of Example 2 of the present invention.

<Example 3 >
As shown in Table 1, a mixed solvent in which a 1: 3 mixture of dimethylethylbutylammonium bistrifluoromethylsulfonylimide, which is an ionic liquid, and ethylene carbonate and ethylmethyl carbonate, was 50:50 by weight. In addition, LiBF ₄ was dissolved in 0.5M to prepare a non-aqueous electrolyte for a non-aqueous electrolyte secondary battery of Example 3 of the present invention.

< Comparative Example 3 >
As shown in Table 1, a mixed solvent in which a dimethylethylbutylammonium bistrifluoromethylsulfonylimide, which is an ionic liquid, and a 1: 3 mixed solution of ethylene carbonate and ethyl methyl carbonate were mixed at a weight ratio of 75:25. In addition, LiBF ₄ was dissolved in 0.5 M to prepare a nonaqueous electrolyte solution for a nonaqueous electrolyte secondary battery of Comparative Example 3 of the present invention.

<Comparative Example 4 >
As shown in Table 1, LiBF ₄ was dissolved in 1M in a 1: 3 mixture of ethylene carbonate and ethyl methyl carbonate, and non-aqueous electrolysis for a non-aqueous electrolyte secondary battery of Comparative Example 4 for the present invention. A liquid was prepared.

<Comparative Example 5 >
As shown in Table 1, LiPF ₆ was dissolved in 1.25 M in a 1: 3 mixed solution of ethylene carbonate and ethyl methyl carbonate, and the nonaqueous electrolyte secondary battery of Comparative Example 5 for the present invention was used. A water electrolyte was prepared.

<Comparative Example 6 >
As shown in Table 1, LiBF ₄ was dissolved in 1M in dimethylethylbutylammonium bistrifluoromethylsulfonylimide, which is an ionic liquid, and nonaqueous electrolysis for a nonaqueous electrolyte secondary battery according to Comparative Example 6 for the present invention. A liquid was prepared.

2. Ionic conductivity evaluation
About the nonaqueous electrolyte for nonaqueous electrolyte secondary batteries of Example 1 and Comparative Examples 1 and 4 to 6 produced as described above, conductivity meter (trade name: CM-60, manufactured by TOA) Was used to measure the ionic conductivity at −30 ° C. to 25 ° C., and the temperature characteristics were examined. The results are shown in FIG. Moreover, about the nonaqueous electrolyte for nonaqueous electrolyte secondary batteries of Examples 2 and 3 and Comparative Examples 2 to 4 and 6 produced as described above, a conductivity meter (trade name: CM-60, Using TOA), the ionic conductivity at 25 ° C. was measured, and the ionic liquid concentration dependency was examined. The results are shown in FIG.

As is apparent from FIG. 2, the temperature characteristics are Comparative Examples 4 and 5 which are conventional non-aqueous electrolytes not containing an ionic liquid, and Comparative Example 6 where the non-aqueous solvent is only an ionic liquid. As compared with Comparative Example 1 , it was shown that Example 1 which is the present invention has improved ion conductivity at room temperature. As is clear from FIG. 3, the dependency on the ionic liquid concentration is shown in Comparative Example 4 that does not contain an ionic liquid, Comparative Example 6 in which the ionic liquid concentration is 100% by weight , and the ionic liquid concentration. Compared to Comparative Example 2 which is 10% by weight and Comparative Example 3 which is 70% by weight, Example 2 in which the ionic liquid concentration is 35 % by weight and Example 3 in which 50% by weight are superior in ionic conductivity. It was shown that

3. Application to Nonaqueous Electrolyte Secondary Battery For the positive electrode, a mixture of LiCoO ₂ that is a positive electrode active material, acetylene black, and polyvinylidene fluoride in a weight ratio of 89: 5: 6 was used as N-methyl-2. -Dispersed with pyrrolidone into a slurry form was uniformly applied onto a positive electrode current collector aluminum foil having a thickness of 20 μm, dried, and then punched into a predetermined shape. On the other hand, for the negative electrode, a mixture of mesocarbon microbeads, which are negative electrode active materials, acetylene black, and polyvinylidene fluoride in a weight ratio of 89: 5: 6 is dispersed in N-methyl-2-pyrrolidone and slurried. The obtained product was uniformly applied onto a negative electrode current collector 18 μm thick copper foil, dried, and then punched into a predetermined shape.

  Next, in a dry argon atmosphere, the positive electrode was accommodated inside a stainless steel positive electrode can via an aluminum foil, while the negative electrode was accommodated via a copper foil in a stainless steel sealing plate. Next, these positive electrode and negative electrode are laminated so as to sandwich a separator made of a polypropylene microporous film impregnated with the nonaqueous electrolyte solution for a nonaqueous electrolyte secondary battery of each of the above examples. The positive electrode can and the sealing plate were caulked and sealed through a gasket so as to enclose the laminate, and a coin-type non-aqueous electrolyte secondary battery as shown in FIG. 1 was produced.

  The non-aqueous electrolyte secondary battery produced as described above is superior to the conventional non-aqueous electrolyte secondary battery because the ionic conductivity of the electrolyte is improved and increased as described above. It was shown that the battery characteristics were obtained.

It is the schematic diagram which showed the cross section of the button type non-aqueous electrolyte secondary battery. It is the diagram which showed the ionic conductivity with respect to the temperature of an Example and a comparative example. It is the diagram which showed the ionic conductivity with respect to the density | concentration of the ionic liquid of an Example and a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Positive electrode, 2 ... Positive electrode can, 3 ... Aluminum foil, 4 ... Negative electrode, 5 ... Sealing plate, 6 ... Separator,
7 ... gasket.

Claims (6)

Alkali salt,
A non-aqueous solvent that is a mixture of a cyclic ester and a chain ester ;
An ionic liquid composed of dimethylethylbutylammonium cation and bistrifluoromethylsulfonylimide anion ,
The non-aqueous electrolyte for a non-aqueous electrolyte secondary battery, wherein the content of the ionic liquid with respect to the total weight of the non-aqueous solvent and the ionic liquid is 35 to 50% by weight . The non-aqueous electrolyte for a non-aqueous electrolyte secondary battery according to claim 1 , wherein the alkali salt is a lithium salt. The lithium salt, a non-aqueous electrolyte The non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized in that at least one of LiPF ₆ and LiBF _4. A positive electrode containing an active material capable of occluding and releasing alkali ions;
A negative electrode containing an active material capable of occluding and releasing alkali ions;
Non-aqueous electrolyte secondary battery, characterized by comprising a non-aqueous electrolyte according to any one of claims 1 to 3. An electric device using the nonaqueous electrolyte secondary battery according to claim 4 . Transportation equipment using the non-aqueous electrolyte secondary battery according to claim 4 .

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