WO2016117399A1 - Lithium-ion secondary battery - Google Patents

Abstract

　The purpose of the present invention is to provide a lithium-ion secondary battery electrolytic solution wherein, even at low temperatures of -30°C or lower, the viscosity of the electrolytic solution does not easily increase, thus providing a high output, very safe lithium-ion battery.　A lithium-ion secondary battery electrolytic solution, said solution comprising ethylene carbonate and a chain carbonate. The lithium-ion secondary battery electrolytic solution has a dicarboxylic acid diester represented by formula 1 in the range of 10 to 15 vol% of the lithium-ion secondary battery electrolytic solution.

Description

Lithium ion secondary battery

The present invention relates to an electrolyte solution for a lithium ion secondary battery and a lithium ion secondary battery using the same.

In recent years, lithium batteries with high energy density have been developed as power sources for electric vehicles and hybrid vehicles. For in-vehicle use, it is desirable to use at a low temperature around −30 ° C.

By the way, a lithium ion battery is charged and discharged by moving lithium ions in an electrolytic solution provided between the positive and negative electrodes of the battery. The electrolyte is composed of a lithium salt for supplying lithium ions and an organic solvent, which is an organic compound. Depending on the type of organic solvent, the viscosity of the electrolyte increases or the solvent coagulates at a low temperature around -30 ° C Such a phenomenon occurs. The increase in the viscosity of the electrolytic solution and the solidification of the solvent cause a decrease in the mobility of lithium ions in the liquid, resulting in a decrease in battery performance.

Patent Document 1 discloses a technique relating to an electrolytic solution having ethylene carbonate as a cyclic carbonate and further having ethyl methyl carbonate and dimethyl carbonate as chain carbonates in order to improve the low temperature characteristics of the electrolytic solution.

Patent Document 2 discloses a technique of using a dicarboxylic acid diester as an auxiliary agent for an electrolytic solution in order to improve capacity maintenance characteristics and cycle characteristics after high-temperature storage.

JP 2009-123671 A JP2011-138759A

As in Patent Document 1, low temperature characteristics can be improved by using ethyl methyl carbonate as the cyclic carbonate. However, when further low temperature characteristics are required, it is effective to increase the ratio of chain carbonates such as ethyl methyl carbonate and dimethyl carbonate having a low melting point. For example, it may be difficult to satisfy a flash point of 21 ° C. or higher and lower than 70 ° C. of the second petroleum defined by the Japanese Fire Service Act. Therefore, there is room for studying compounds other than the conventional cyclic carbonate and chain carbonate.

When a small amount of dicarboxylic acid diester is used as an electrolyte additive as in Patent Document 2, capacity retention characteristics and cycle characteristics after high-temperature storage are improved, but charge / discharge performance at low temperatures such as −30 ° C. or lower is improved. The effect is low.

In the present invention, there is provided an electrolyte for a lithium ion secondary battery that can realize a lithium ion battery with high output and high safety that hardly increases the viscosity of the electrolyte even at a low temperature of −30 ° C. or lower. Aimed.

In an electrolyte for a lithium ion secondary battery having ethylene carbonate and a chain carbonate, the electrolyte for a lithium ion secondary battery uses a dicarboxylic acid diester represented by Formula 1 as an electrolyte for a lithium ion secondary battery. On the other hand, an electrolyte solution for lithium ion secondary batteries having a range of 10 to 15 vol% and having ethylene carbonate of 23.0 vol% or more with respect to the electrolyte solution for lithium ion secondary batteries.

[Chemical 1]

(In Formula 1, R ₁ and R ₃ are alkyl groups having 1 or 2 carbon atoms, and R ₂ is an alkylene group having 1 or 2 carbon atoms. R ₁ and R ₃ may be the same or different. Good.)
Preferably, in the electrolyte for a lithium ion secondary battery, the ratio of the dicarboxylic acid diester represented by Formula 1 to the ethylene carbonate (EC) (dicarboxylic acid diester / EC) is 0.7 or less. Electrolyte.

According to the present invention, there is provided an electrolytic solution for a lithium ion secondary battery capable of realizing a lithium ion battery with high output and high safety that hardly raises the viscosity of the electrolytic solution even at a low temperature of −30 ° C. can do.

External perspective view of prismatic secondary battery Exploded perspective view of wound electrode group

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various modifications by those skilled in the art are within the scope of the technical idea disclosed in this specification. Changes and modifications are possible.

FIG. 1 is an example of an external perspective view of a battery according to an embodiment of the present invention.
The square secondary battery C <b> 1 includes a battery can 1 and a lid (battery lid) 6. A wound group 3 is accommodated in the battery can 1, and an opening of the battery can 1 is sealed by a battery lid 6. The battery lid 6 has a substantially rectangular flat plate shape, and is welded so as to close the upper opening of the battery can 1 to seal the battery can 1. The battery lid 6 is provided with a positive external terminal 8A and a negative external terminal 8B. The wound group 3 shown in FIG. 2 is charged via the positive external terminal 8A and the negative external terminal 8B, and power is supplied to the external load. The battery cover 6 is integrally provided with a gas discharge valve 10, and when the pressure in the battery container rises, the gas discharge valve 10 opens to discharge gas from the inside, and the pressure in the battery container is reduced. Thereby, the safety of the square secondary battery C1 is ensured. Moreover, the liquid injection stopper 11 provided in the battery lid 6 closes the liquid injection port for injecting the electrolytic solution. After injecting the electrolyte into the battery can 1, an injection stopper 11 is joined to the battery lid 6 by laser welding to seal the injection port, and the rectangular secondary battery C <b> 1 is sealed.

FIG. 2 is an exploded perspective view showing a state where a part of the wound electrode group is developed.

The square secondary battery C <b> 1 has a wound group 3 in the battery can 1.
The winding group 3 is configured by winding the negative electrode 32 and the positive electrode 31 in a flat shape with a separator 33 therebetween. In the winding group 3, the outermost electrode is the negative electrode 32, and the separator 33 is wound outside thereof. The separator 33 has a role of insulating between the positive electrode 31 and the negative electrode 32.

The portion of the negative electrode 32 to which the negative electrode mixture 32b is applied is larger in the width direction than the portion of the positive electrode 31 to which the positive electrode mixture 31b is applied. It is comprised so that it may be pinched | interposed into the part to which the mixture 32b was apply | coated. The positive foil exposed portion 31c and the negative foil exposed portion 32c are bundled at a plane portion and connected by welding or the like. The separator 33 is wider than the portion where the negative electrode mixture 32b is applied in the width direction, but is wound to a position where the metal foil surface at the end is exposed at the positive electrode foil exposed portion 31c and the negative electrode foil exposed portion 32c. Therefore, it does not hinder the welding when bundled.

The positive electrode 31 has a positive electrode active material mixture on both surfaces of a positive electrode foil that is a positive electrode current collector, and a positive electrode foil in which the positive electrode active material mixture is not applied to one end in the width direction of the positive electrode foil An exposed portion 31c is provided.

The negative electrode 32 has a negative electrode active material mixture on both sides of a negative electrode foil that is a negative electrode current collector, and the negative electrode foil in which the negative electrode active material mixture is not applied to the other end in the width direction of the positive electrode foil An exposed portion 32c is provided. The positive foil exposed portion 31c and the negative foil exposed portion 32c are regions where the metal surface of the electrode foil is exposed, and are wound so as to be disposed at one side and the other side in the winding axis direction.

The positive electrode foil exposed portion 31c of the wound group 3 is electrically connected to a positive external terminal 8A provided on the battery lid 6 via a positive current collector plate. Further, the negative electrode foil exposed portion 32c of the wound group 3 is electrically connected to a negative external terminal 8B provided on the battery lid 6 via a negative current collector (current collector terminal). As a result, power is supplied from the wound group 3 to the external load via the positive current collector and the negative current collector, and external generated power is supplied to the wound group 3 via the positive current collector and the negative current collector. And charged.
<Negative electrode>
For the negative electrode, artificial graphite powder as a negative electrode active material and polyvinylidene fluoride (PVdF) as a binder are mixed at a weight ratio of 93: 7, for example, and a dispersion solvent N-methylpyrrolidone (NMP) is added thereto. The slurry of the kneaded negative electrode mixture can be applied to both surfaces of a rolled copper foil having a thickness of 10 μm, which is a negative electrode current collector, and then dried, pressed, and cut.

Other examples of the negative electrode active material include graphitized materials obtained from natural graphite, petroleum coke, coal pitch coke, etc., heat-treated at a high temperature of 2500 ° C. or higher, mesophase carbon or amorphous carbon, carbon fiber, lithium A metal to be alloyed or a material having a metal supported on the surface of carbon particles is used. For example, a metal or alloy selected from lithium, silver, aluminum, tin, silicon, indium, gallium, and magnesium. Further, the metal or an oxide of the metal can be used as a negative electrode active material. Furthermore, lithium titanate can also be used. In the present invention, the selection of the negative electrode active material is important for obtaining the effects of the present application. As the negative electrode active material, natural graphite, artificial graphite, amorphous carbon, or lithium titanate is preferably used.
<Positive electrode>
For the positive electrode, a lithium-containing composite oxide powder that is a positive electrode active material, scaly graphite that is a conductive material, and polyvinylidene fluoride (PVdF) that is a binder are mixed in a weight ratio of 85: 10: 5, for example. The slurry of N-methylpyrrolidone (NMP), a dispersion solvent added thereto and kneaded, is applied to both surfaces of the positive electrode current collector aluminum foil, and then dried, pressed and cut. Can do.

Other examples of the positive electrode active material are LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ . In addition, LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , Li ₄ Mn ₅ O ₁₂ , LiMn _2−x MxO ₂ (however, at least selected from the group consisting of M = Co, Ni, Fe, Cr, Zn, Ti) 1 type, x = 0.01 to 0.2), Li ₂ Mn ₃ MO ₈ (however, M = at least one selected from the group consisting of Fe, Co, Ni, Cu, Zn), Li _1-x A _x Mn ₂ O ₄ (however, at least one selected from the group consisting of Mg, B, Al, Fe, Co, Ni, Cr, Zn, Ca, x = 0.01 to 0.1), LiNi _1-x M _x O ₂ (however, at least one selected from the group consisting of M = Co, Fe, and Ga, x = 0.01 to 0.2), LiFeO ₂ , Fe ₂ (SO ₄ ) ₃ , LiCo _1-x M _x O ₂ (however, M = Ni, Fe, when less is selected from the group consisting of Mn One, x = 0.01 ~ 0.2), LiNi 1-x M x O 2 ( however, M = Mn, Fe, Co , Al, Ga, Ca, at least one selected from the group consisting of Mg, x = 0.01 to 0.2), Fe (MoO ₄ ) ₃ , FeF ₃ , LiFePO ₄ , LiMnPO _{4 and the} like can be listed. One or more positive electrode active materials may be mixed and used. The particle size of the positive electrode active material is usually defined so as to be equal to or less than the thickness of the mixture layer formed from the positive electrode active material, the conductive agent, and the binder. When there are coarse particles having a size equal to or greater than the thickness of the mixture layer in the positive electrode active material powder, the coarse particles can be removed in advance by sieving classification or wind classification to produce particles having a thickness of the mixture layer thickness or less. preferable.
<Binder>
As the binder in the positive electrode mixture and the negative electrode mixture, for example, PVDF can be used. In addition, polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, butyl rubber, nitrile rubber, styrene / butadiene rubber, polysulfide rubber, nitrocellulose, cyanoethyl cellulose, various latexes, acrylonitrile, vinyl fluoride, vinylidene fluoride, fluorine Polymers such as propylene fluoride and chloroprene fluoride, and mixtures thereof can also be used.
<Electrolyte>
An electrolytic solution is injected into the battery can 1. The electrolytic solution has, as a solvent, ethylene carbonate, a chain carbonate such as dimethyl carbonate and ethyl methyl carbonate, and a dicarboxylic acid diester represented by Formula 1.

[Chemical 1]

In Formula 1, R ₁ and R ₃ are alkyl groups having 1 or 2 carbon atoms, and R ₂ is an alkylene group having 1 or 2 carbon atoms. R ₁ and R ₃ may be the same or different.

The electrolytic solution has ethylene carbonate which is a cyclic carbonate. Ethylene carbonate has high ionic conductivity and can improve the output characteristics of the battery. Moreover, it is a component with a high flash point of 143 ° C. and high safety. However, since the melting point is as high as 38 ° C., phenomena such as an increase in the viscosity of the electrolyte and solidification of the solvent may occur at low temperatures.

Therefore, as other components, a chain carbonate such as ethyl methyl carbonate or diethyl carbonate is generally mixed as a solvent for the electrolytic solution. Chain carbonates, for example, have a melting point of -15 ° C. for ethyl methyl carbonate and 5 ° C. for dimethyl carbonate, which is lower than that of ethylene carbonate. Can be suppressed. In addition, for example, propylene carbonate or diethyl carbonate can be used as the chain carbonate.

Thus, to improve the output performance and safety of lithium batteries, it is effective to increase the ratio of ethylene carbonate with high ionic conductivity and flash point, but as the temperature decreases, the electrolyte solution solidifies. The charge / discharge performance of the battery is reduced. On the other hand, in order to improve low-temperature charge / discharge performance, it is effective to increase the ratio of ethyl methyl carbonate and dimethyl carbonate having a low melting point. However, the flash point of ethyl methyl carbonate is 23 ° C, and the flash point of dimethyl carbonate is 15 ° C. By increasing the ratio of these chain carbonates, the flash point of the second petroleum defined by the Fire Service Law is 21 ° C or higher, 70 ° C. It may be difficult to satisfy less than.

The present invention has a dicarboxylic acid diester represented by Formula 1 in the electrolytic solution. As the dicarboxylic acid diester represented by Formula 1, for example, dimethyl malonate, diethyl malonate, diethyl succinate, and dimethyl succinate can be used. Of these, malonic acid diester has a melting point of −62 ° C. and a flash point of 189 ° C., diethyl malonate has a melting point of −50 ° C. and a flash point of 199 ° C., diethyl succinate has a melting point of −20 ° C. and a flash point of 198 ° C. is there. From the viewpoint of improving the low temperature characteristics, dimethyl malonate having a lower melting point is effective, and from the viewpoint of contribution to safety, diethyl malonate is effective.

Thus, the physical properties of the dicarboxylic acid diesters have a melting point lower than a low temperature of −30 ° C. and a flash point sufficiently higher than room temperature. Therefore, it is possible to improve the low temperature characteristics of the electrolytic solution while ensuring the safety of the electrolytic solution.

The concentration of the dicarboxylic acid diester represented by Formula 1 with respect to the electrolytic solution is preferably in the range of 10 to 15 vol%. When it is 10 vol% or less, the concentration of the dicarboxylic acid diester is low, and the effect of preventing a decrease in ionic conductivity at low temperatures is low. In order to prevent a decrease in ionic conductivity at low temperatures, it is important to use this substance as a solvent. For this reason, it is preferable to contain at least 10 vol% or more. When the dicarboxylic acid diester concentration represented by Formula 1 with respect to the electrolytic solution is 15 vol% or more, the ionic conductivity at normal temperature may be lowered. The ionic conductivity at room temperature is higher in the above-described chain carbonates such as ethylene carbonate and cyclic carbonates such as ethyl methyl carbonate and diethyl carbonate than the dicarboxylic acid diester represented by Formula 1. For this reason, it is considered that the concentration of chain carbonates such as ethylene carbonate and cyclic carbonates such as ethyl methyl carbonate and diethyl carbonate due to ionic conduction at room temperature is relatively lowered.

From the viewpoint of ensuring the output characteristics of the battery at normal temperature, that is, from the viewpoint of ensuring ionic conductivity, the concentration of ethylene carbonate with respect to the entire electrolyte is preferably 23.0 vol% or more. When it is less than 23 vol%, the electrolyte has a low ionic conductivity. Further, in the electrolytic solution having ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate and the dicarboxylic acid diester represented by the formula 1 as a solvent, the amount of ethylene carbonate is preferably 40% or less, and preferably 29% or less. Ethylene carbonate has a high ionic conductivity, but when used in a large amount, the viscosity is high and the ionic conductivity tends to decrease. Further, by controlling the amount of ethylene carbonate within this range, the ratio of chain carbonate and dicarboxylic acid diester represented by Formula 1 can be increased, and as a result, low temperature characteristics at −30 ° C. can be secured.

In addition, the volume ratio (dicarboxylic acid diester / ethylene carbonate) of ethylene carbonate and the dicarboxylic acid diester represented by Formula 1 is preferably 0.7 or less. The reason is not clear, but it seems to be related to the difference in the number and mechanism of lithium ions coordinated with each compound. The polar functional group to which the lithium ion necessary for the solvent used in the electrolytic solution is coordinated has an ester group, but due to the difference in chemical structure from the carbonate group that is the polar functional group of the carbonate-based solvent, Coordination power is different. Furthermore, the number of polar functional groups in the carbonate solvent selected in the present invention is one in the molecule, but in the case of dimethyl malonate belonging to the dicarboxylic acid diester compound, there are two polar functional groups in the molecule. . For this reason, when coordinated to lithium ions, one molecule may coordinate at two locations, and the structure coordinated to lithium ions is expected to be different from carbonate-based solvents. Is done. This is considered to be a factor that determines the optimum range of the mixing ratio of the carbonate solvent and dimethyl malonate.

The total amount of chain carbonate with respect to the entire electrolytic solution is preferably in the range of 60 to 77 vol%, particularly in the range of 60 to 66 vol%. The chain carbonate preferably has 60 vol% or more because it has a lower melting point than ethylene carbonate and is excellent in low temperature characteristics. In the case of 60 vol% or less, the ionic conductivity at low temperatures tends to decrease. Moreover, when it is 77 vol% or more, the flash point of the electrolytic solution tends to be high. Moreover, since the amount of ethylene carbonate is relatively reduced, the ionic conductivity tends to be low.

Any of the chain carbonates may be used alone, or a plurality of chain carbonates may be mixed and used. When using a plurality of chain carbonates, it is preferable to use ethylene carbonate and dimethyl carbonate.

When ethylene carbonate (EMC) and dimethyl carbonate (DMC) are used, the ratio (EMC / DMC) is preferably in the range of 0.6 to 1.8. It has also been reported in Patent Document 1 (for example, FIG. 2 etc.) that the low temperature characteristics are improved by adjusting the ratio of EMC and DMC. In particular, in the case of a system of four solvents having ethylene carbonate, EMC, DMC, and a dicarboxylic acid diester represented by the formula 1, it is preferably in the range of 0.6 to 1.8. Among chain carbonates, DMC has low viscosity and high ionic conductivity, but has a high melting point. Therefore, in the conventional ethylene carbonate and chain carbonate system, in addition to DME as a chain carbonate, Low temperature characteristics are ensured by using EMC. However, when a dicarboxylic acid diester having high low temperature characteristics is used for the electrolyte, a large amount of DMC can be used.

The electrolytic solution preferably contains various lithium salts as a lithium ion source. The lithium _{salt, LiPF 6, LiBF 4, LiClO} 4, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, or imide salts such as lithium represented by lithium trifluoromethane sulfonimide, many types of lithium A salt can be used. An electrolytic solution obtained by dissolving these salts in the above solvent can be used as an electrolytic solution for a battery. As the electrolyte salt, LiPF ₆ , LiBF ₄ , and LiClO ₄ are particularly preferable.

The concentration of the lithium salt is related to the ratio to the number of polar functional groups coordinated to lithium ions in the solvent molecule. As the lithium salt concentration increases, the number of lithium ions in the electrolytic solution increases. At the same time, the viscosity of the electrolytic solution also increases, thereby inhibiting the movement of ions. Therefore, the lithium salt concentration is preferably in the range of 0.7 to 1.5 mol / liter with respect to the electrolytic solution. A concentration of 0.9 to 1.2 mol / liter is particularly desirable. A plurality of lithium salts may be used. In addition to the above solvent components, additives can be appropriately added to the electrolytic solution. Examples of the additive include components that form a film on the electrode, such as vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propene sultone, ethylene sulfate, or a derivative thereof.

Moreover, various overcharge inhibitors, flame retardants, and oxidation inhibitors can be added.

Hereinafter, each embodiment of the present invention will be described.

As the solvent and volume ratio of the non-aqueous electrolyte, ethylene carbonate (EC): 24.7 vol%, ethyl methyl carbonate (EMC): 32.7 vol%, dimethyl carbonate (DMC): 32.7 vol% and dicarboxylic acid diester A certain amount of dimethyl malonate was mixed at 10.0 vol%, and then the nonaqueous electrolytic solution was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0 mol / liter.
(Melting point measurement)
Differential scanning calorimetry (DSC) measurement was performed under conditions of a temperature drop rate of 5 ° C./min and a temperature range of room temperature to −80 ° C. The melting point obtained from the measurement was -79 ° C. The solvent composition and melting point results are shown in Table 1.
(Ion conductivity evaluation)
The ionic conductivity at room temperature was evaluated. 5 mL of the sample solution was put into a glass bottle, and after inserting the electrode of a digital conductivity meter (manufactured by Toa Denpa, CM-60V) into the sample solution, the measurement was started after being held in a thermostatic bath for 1 hour 30 minutes. The measurement temperature was 25 ° C.

As the solvent and volume ratio of the non-aqueous electrolyte, ethylene carbonate (EC): 28.0 vol%, ethyl methyl carbonate (EMC): 31.0 vol%, dimethyl carbonate (DMC): 31.0 vol%, and dicarboxylic acid diester After mixing at a certain dimethyl malonate: 10.0 vol%, the non-aqueous electrolyte was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

As in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. The melting point obtained from the measurement was -61 ° C. The solvent composition and melting point results are shown in Table 1.

As the solvent and volume ratio of the non-aqueous electrolyte, ethylene carbonate (EC): 27.0 vol%, ethyl methyl carbonate (EMC): 36.0 vol%, dimethyl carbonate (DMC): 27.0 vol% and dicarboxylic acid diester After mixing at a certain dimethyl malonate: 10.0 vol%, the non-aqueous electrolyte was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

As in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. The melting point obtained from the measurement was −63 ° C. The solvent composition and melting point results are shown in Table 1.

Next, a fourth embodiment will be described below. As the solvent and volume ratio of the nonaqueous electrolyte, ethylene carbonate (EC): 29.0 vol%, ethyl methyl carbonate (EMC): 39.0 vol%, dimethyl carbonate (DMC): 22.0 vol% and dicarboxylic acid diester After mixing at a certain dimethyl malonate: 10.0 vol%, the non-aqueous electrolyte was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

As in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. From the measurement, a result corresponding to the solidification of the electrolytic solution was not obtained, and the melting point was −80 ° C. or lower. The solvent composition and melting point results are shown in Table 1. The notation “<−80” in the table indicates that the melting point was below the measurement limit of −80 ° C. in this experimental system.

Next, a fifth embodiment will be described below. As the solvent and volume ratio of the non-aqueous electrolyte, ethylene carbonate (EC): 23.0 vol%, ethyl methyl carbonate (EMC): 31.0 vol%, dimethyl carbonate (DMC): 31.0 vol%, and dicarboxylic acid diester After mixing at a certain dimethyl malonate: 15.0 vol%, the non-aqueous electrolyte was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

As in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. The melting point obtained from the measurement was -64 ° C. The solvent composition and melting point results are shown in Table 1.

Next, a sixth embodiment will be described below. As the solvent and volume ratio of the non-aqueous electrolyte, ethylene carbonate (EC): 28.0 vol%, ethyl methyl carbonate (EMC): 28.5 vol%, dimethyl carbonate (DMC): 28.5 vol%, and dicarboxylic acid diester After mixing at a certain dimethyl malonate: 15.0 vol%, the non-aqueous electrolyte was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

As in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. The melting point obtained from the measurement was -66 ° C. The solvent composition and melting point results are shown in Table 1.

Next, a seventh embodiment will be described below. As the solvent and volume ratio of the non-aqueous electrolyte, ethylene carbonate (EC): 25.0 vol%, ethyl methyl carbonate (EMC): 22.5 vol%, dimethyl carbonate (DMC): 37.5 vol% and dicarboxylic acid diester After mixing at a certain dimethyl malonate: 15.0 vol%, the non-aqueous electrolyte was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

As in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. From the measurement, a result corresponding to the solidification of the electrolytic solution was not obtained, and the melting point was −80 ° C. or lower. The solvent composition and melting point results are shown in Table 1.

Example 1 An electrolytic solution was prepared in the same manner as in Example 1 except that dimethyl malonate was replaced with diethyl malonate (MDM).

As in Example 1, the melting point obtained by differential scanning calorimetry (DSC) measurement was −75 ° C.

Example 1 An electrolytic solution was prepared in the same manner as in Example 1 except that dimethyl malonate was replaced with diethyl succinate (SDM).

The melting point obtained by carrying out differential scanning calorimetry (DSC) measurement in the same manner as in Example 1 was −65 ° C.
(Comparative Example 1)
Next, the first comparative example will be described below. After mixing the solvent of the non-aqueous electrolyte and its volume ratio, ethylene carbonate (EC): 30.0 vol%, ethyl methyl carbonate (EMC): 30.0 vol%, dimethyl carbonate (DMC): 40.0 vol%, The nonaqueous electrolytic solution was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

In the same manner as in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. The melting point obtained from the measurement was -58 ° C. The solvent composition and melting point results are shown in Table 1.
(Comparative Example 2)
Next, a second comparative embodiment will be described below. After mixing the solvent of the non-aqueous electrolyte and its volume ratio, ethylene carbonate (EC): 35.0 vol%, ethyl methyl carbonate (EMC): 20.0 vol%, dimethyl carbonate (DMC): 40.0 vol%, The nonaqueous electrolytic solution was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

In the same manner as in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. The melting point obtained from the measurement was -52 ° C. The solvent composition and melting point results are shown in Table 1.
(Comparative Example 3)
Next, a third comparative embodiment will be described below. After mixing the solvent of the non-aqueous electrolyte and its volume ratio, ethylene carbonate (EC): 30.0 vol%, ethyl methyl carbonate (EMC): 25.0 vol%, dimethyl carbonate (DMC): 45.0 vol%, The nonaqueous electrolytic solution was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

In the same manner as in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. The melting point obtained from the measurement was −51 ° C. The solvent composition and melting point results are shown in Table 1.
(Comparative Example 4)
Next, a fourth comparative embodiment will be described below. As the solvent and the volume ratio of the non-aqueous electrolyte, ethylene carbonate (EC): 13.0 vol%, ethyl methyl carbonate (EMC): 36.0 vol%, dimethyl carbonate (DMC): 36.0 vol% and dicarboxylic acid diester After mixing at a certain dimethyl malonate: 15.0 vol%, the non-aqueous electrolyte was adjusted so that the concentration of lithium hexafluorophosphate (LiPF6) was 1.0M.

In the same manner as in Example 1, differential scanning calorimetry (DSC) measurement and ion conductivity measurement were performed. The melting point obtained from the measurement was below -80 ° C. However, since the amount of ethylene carbonate was as low as 13.0%, the ionic conductivity at room temperature was low.
According to a comparison between Examples 1 to 9 and Comparative Examples 1 to 3, by using dicarboxylic acid diester as the electrolyte of 10 to 15 vol%, the melting point can be reduced to −58 ° C. or lower, and the electrolysis has high characteristics at low temperatures. It was confirmed that the liquid could be realized. It was also confirmed that the ionic conductivity at room temperature was high.

Comparative Example 4 is an example in which the amount of ethylene carbonate is 13 vol% with respect to the electrolytic solution. Since it has 15% of dicarboxylic acid diester, it can be seen that the electrolyte has a low melting point and excellent low-temperature characteristics. On the other hand, since the EC concentration is 13 vol%, which is low with respect to the electrolyte, and the ratio of dicarboxylic acid diester / EC is high, the ionic conductivity is low.

From the above results, it was confirmed that by using dicarboxylic acid diester in the 10-15 vol% electrolyte, the melting point could be −58 ° C. or lower, and an electrolyte having high characteristics at low temperatures could be realized. Further, from the viewpoint of securing the output characteristics of the battery at room temperature, the concentration of EC with respect to the electrolytic solution is preferably 23.0 vol% or more, and the ratio of dicarboxylic acid diester / EC is about 0.7 vol%. It was confirmed that the following is preferable.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various designs can be made without departing from the spirit of the present invention described in the claims. It can be changed. For example, the above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to one having all the configurations described. Further, a part of the configuration of an embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of an embodiment. Furthermore, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

C1 Rectangular secondary battery 1 Battery can 3 Winding group 6 Battery lid 8A Positive electrode external terminal 8B Negative electrode external terminal 10 Gas discharge valve 11 Injection plug 32 Negative electrode 32b Negative electrode mixture 32c Negative electrode foil exposed portion 31 Positive electrode 31b Positive electrode mixture 31c Positive foil exposed portion 33 Separator

Claims (9)

1. In an electrolyte for a lithium ion secondary battery having ethylene carbonate and chain carbonate,
   The lithium ion secondary battery electrolyte has a dicarboxylic acid diester represented by Formula 1 in a range of 10 to 15 vol% with respect to the lithium ion secondary battery electrolyte;
   The electrolyte solution for lithium ion secondary batteries which has 23.0 vol% or more of said ethylene carbonate with respect to the electrolyte solution for said lithium ion secondary batteries.
   [Chemical 1]
   

   

   (In Formula 1, R ₁ and R ₃ are alkyl groups having 1 or 2 carbon atoms, and R ₂ is an alkylene group having 1 or 2 carbon atoms. R ₁ and R ₃ may be the same or different. Good.)
2. In claim 1,
   For the lithium ion secondary battery, the ratio (dicarboxylic acid diester / EC) of the dicarboxylic acid diester represented by the formula 1 to the ethylene carbonate (EC) is 0.7 or less in the electrolyte for the lithium ion secondary battery. Electrolytic solution.
3. In claim 2,
   The chain carbonate is an electrolyte solution for a lithium ion secondary battery, which is at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and propylene carbonate.
4. In claim 3,
   The electrolyte for a lithium ion secondary battery is an electrolyte for a lithium ion secondary battery having dimethyl carbonate and ethyl methyl carbonate as the chain carbonate.
5. In claim 4,
   The dicarboxylic acid diester represented by the formula 1 is an electrolyte solution for a lithium ion secondary battery, which is any one of dimethyl malonate, diethyl malonate, and diethyl succinate.
6. In claim 5,
   The electrolyte for a lithium ion secondary battery, wherein the concentration of the chain carbonate with respect to the electrolyte for a lithium ion secondary battery is in the range of 60.0 to 77.0 vol%.
7. In claim 6,
   The electrolyte solution for a lithium ion secondary battery, wherein a volume ratio (EMC / DMC) of the dimethyl carbonate (DMC) to the ethyl methyl carbonate (EMC) is in a range of 0.6 to 1.8.
8. In claim 7,
   The lithium ion secondary battery electrolyte has a lithium salt,
   The lithium salt is an electrolyte solution for a lithium ion secondary battery, which is at least one of LiPF ₆ , LiBF ₄ , and LiClO ₄ .
9. In a lithium ion secondary battery having a positive electrode, a negative electrode, and an electrolyte,
   The said electrolyte solution is a lithium ion secondary battery which is the electrolyte solution for lithium ion secondary batteries in any one of Claim 1 thru | or 8.

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