JP4959465B2 - Non-aqueous electrolyte for lithium secondary batteries with excellent high-temperature storage characteristics - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte for lithium secondary batteries having excellent high-temperature storage characteristics. More specifically, the present invention relates to a lithium salt, an organic solvent, and a non-ionic electrolyte containing a specific imidazolium salt as an additive for improving high-temperature storage characteristics. The present invention relates to an aqueous electrolyte. In particular, the non-aqueous electrolyte solution according to the present invention can exhibit excellent high-temperature storage characteristics in secondary batteries for vehicles that should be operated at high temperatures, such as electric vehicles and hybrid electric vehicles.

  As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources has increased rapidly. Among secondary batteries, lithium secondary batteries showing high energy density and discharge voltage are widely used. It has been.

  A lithium secondary battery has a porous separation membrane interposed between a positive electrode and a negative electrode, each of which is coated with an active material on a current collector. It is constituted by impregnating a non-aqueous electrolyte containing a salt. Mainly, the positive electrode active material is made of lithium cobalt-based oxide, lithium manganese-based oxide, lithium nickel-based oxide, lithium composite oxide, or the like, and the negative electrode oxide is made of carbon-based material.

However, in a lithium secondary battery, of lithium transition metal oxide and the electrolyte as a cathode active material is accelerated at high temperatures, to produce a by-product of increasing the resistance of the positive electrode, the shelf life at high temperatures There is a problem that it drops rapidly.

  Recently, secondary batteries are also attracting attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been proposed to solve air pollution caused by existing gasoline vehicles and diesel vehicles using fossil fuels. Therefore, the amount of use is expected to increase further, so that the problems with respect to high-temperature storage characteristics are greatly highlighted.

  Therefore, there is an urgent need to develop a lithium-ion battery that exhibits excellent cycle characteristics even during high-temperature storage and has no performance degradation so that it can be used appropriately as a next-generation power source for electric vehicles, hybrid electric vehicles, and the like. .

  In connection with this, as described later, the present invention presents a non-aqueous electrolytic solution containing a specific imidazolium-based inorganic salt as an additive for improving high-temperature storage characteristics.

  A part of the technique using an imidazolium-based compound as an electrolyte is already known. For example, Japanese Patent Application Publication No. 2003-229333 discloses a technique using an imidazole compound as a main cation supply source mainly in an electrolytic capacitor or an electrolytic solution for an electric double layer capacitor. Japanese Patent Application Publication No. 2002-260966 discloses an electrolytic solution for an electric double layer capacitor, an organic solvent such as propylene carbonate and sulfolane, and 1,3-dimethyl-2-fluoroimidazo as a main cation source. A technique using a fluorine-containing imidazolium salt such as lithium tetrafluoroborate is disclosed.

  However, the above application relates to an electrolytic solution generally used for an electrolytic capacitor, an electric double layer capacitor, or the like having a configuration different from that of a lithium secondary battery, and an imidazolium salt is used as a main cation supply source of the electrolytic solution. Therefore, it has a concept different from the present invention in which a specific imidazolium-based inorganic salt is used as an additive for the purpose of improving high-temperature storage characteristics.

On the other hand, Japanese Patent Application Publication No. 2002-110231 discloses a technique for adding an imidazolium salt to a non-aqueous electrolyte for a secondary battery containing a lithium salt in order to improve safety and ensure a sufficient level of battery performance. is doing. In the above application, 1,3-dimethylimidazolium ion, 1-ethyl-3-methylimidazolium ion, 1-methyl-3-ethylimidazolium ion, 1-butyl-3 are used as the cation component of the imidazolium salt. -Dialkylimidazolium ions such as methylimidazolium ion, 1,2,3-trimethylimidazolium ion, 1,2-dimethyl-3-ethylimidazolium ion, 1,2-dimethyl-3-propylimidazolium ion, Examples include trialkylimidazolium ions such as 1-butyl-2,3-dimethylimidazolium ion, and the anion components include BF ₄ , PF ₆ , ClO ₄ , CF ₃ SO ₃ , and N (CF ₃ SO _{2) 2, N (C 2} F 5 SO 2) 2, N (CF 3 SO 2) (C 4 F 9 _{SO 2), C (CF 3} SO 2) 3, C (C 2 F 5 SO 2) and ₃ and the like are exemplified. That is, the above application suggests that an imidazolium salt is used as an additive for improving physical properties in a non-aqueous electrolyte for a lithium secondary battery.

  However, the above application only illustrates the possibility of using various imidazolium salts and does not describe the properties of these inorganic salts. That is, the detailed description of the above application exemplifies various kinds of substances described above, but the examples show experimental results of 1-ethyl-3-methylimidazolium tetrafluoroborate only. ing.

  As a result of extensive research and various experiments, the inventors of the present application have found an imidazolium salt containing a cation having a specific alkyl substituent structure defined by the following chemical formula 1 among many imidazolium salts. When added to the electrolytic solution, it was confirmed that the high temperature storage characteristics were much better than when other imidazolium salts were added, and the present invention was completed.

上記の式で、Ｘは、ハロゲン、ＣｌＯ_４、Ｂ_１０Ｃｌ_１０、ＰＦ_６、ＣＦ_３ＳＯ_３、ＣＦ_３ＣＯ_２、ＡｓＦ_６、ＳｂＦ_６、ＡｌＣｌ_４、ＣＨ_３ＳＯ_３、ＣＦ_３ＳＯ_３、Ｃ_２Ｆ_５ＳＯ_２、（ＣＦ_３ＳＯ_２）（Ｃ_４Ｆ_９ＳＯ_２）、ＣＦ_３ＳＯ_２及び低級脂肪族カルボン酸からなる群から選択される。 A non-aqueous electrolyte solution for a lithium secondary battery according to the present invention is a non-aqueous electrolyte solution for a lithium secondary battery excellent in high-temperature storage characteristics containing a lithium salt and an organic solvent, and is represented by the following chemical formula 1. A salt (imidazolium salt) containing ethyl-2,3-dimethylimidazolium ion as a cation is contained.

In the above formula, X is halogen, ClO ₄ , B ₁₀ Cl ₁₀ , PF ₆ , CF ₃ SO ₃ , CF ₃ CO ₂ , AsF ₆ , SbF ₆ , AlCl ₄ , CH ₃ SO ₃ , CF ₃ SO ₃ , It is selected from the group consisting of C ₂ F ₅ SO ₂ , (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), CF ₃ SO ₂ and lower aliphatic carboxylic acids.

  Therefore, the electrolytic solution according to the present invention exhibits remarkably improved high-temperature storage characteristics by adding an imidazolium salt containing a trialkylimidazolium ion of a specific substituent as a cation component to the electrolytic solution.

  The imidazolium salt of Chemical Formula 1 is often dissociated in the electrolyte, and the dissociated cations and anions react with the positive electrode and the negative electrode of the lithium secondary battery, respectively. In the imidazolium cation, two methyl groups and one ethyl group substituted at specific positions of the imidazole ring structure act as electron donors, helping to form a protective film on the electrode surface, Presumed to suppress decomposition. That is, it is presumed that the imidazolium salt according to the present invention assists the firm formation of a SEI (Solid Electrolyte Interface) film that provides a protective function for the electrode active material under the initial charge / discharge conditions. Accordingly, the high temperature storage characteristics of the battery are greatly improved by preventing the increase in internal resistance of the battery and the heat generation phenomenon induced by the decomposition reaction of the electrolyte due to the partial destruction of the SEI film at a high temperature.

  According to the experiments by the inventors of the present application, the remarkable improvement in the high-temperature storage characteristics as described above is a salt containing an imidazolium cation having a predetermined lower alkyl group substituted at a specific position, as shown in the chemical formula. It was confirmed that That is, as confirmed from the following experimental examples, the high-temperature storage characteristics are improved by 15% or more compared to imidazolium salts in which only one or two of the substituents are substituted with alkyl groups. A significant increase in effect can be obtained that is more than expected.

As an anion component, BF ₄ specified in the example of Japanese Patent Application Publication No. 2002-110231 described above is excluded from X. According to experiments by the inventors of the present application, an electrolytic solution containing an imidazolium salt containing BF ₄ as an anion component cannot exhibit a desired degree of high-temperature storage characteristics.

In particular, a preferred example of X is hexafluorophosphate (PF ₆ ).

Imidazolium salts of Formula 1, are preferably 0.5 to 5 wt% based on the total mass of the electrolytic solution. In other words, if the amount of imidazolium salt added is too small, it is difficult to obtain the effect of the addition. Conversely, if the amount of imidazolium salt added is too large, problems such as increase in the viscosity of the electrolyte are induced. This is not preferable.

The lithium salt constituting the non-aqueous electrolyte of the present invention is a substance that is well dissolved in an organic solvent. For example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, chloroborane lithium, lower aliphatic lithium carboxylate, lithium 4-phenylborate, imide, etc. are used. Of these, LiPF ₆ is preferably used.

  Examples of the organic solvent that is a main component constituting the electrolytic solution include N-methyl-2-pyrrolidinone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate ( DEC), ethyl methyl carbonate (EMC), gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, Nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propiyl Emissions carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, but an aprotic organic solvent such as ethyl propionate, but is not limited thereto.

  As the organic solvent, polypropylene carbonate (PC) having a high degree of dissociation of lithium salt or a mixed solvent of PC and linear carbonate is used.

  In the non-aqueous electrolyte solution of the present invention, in order to further prevent aging and / or deterioration of the negative electrode due to decomposition of the electrolyte solution, an ester compound having a cyclic molecular structure and a C═C unsaturated bond in the ring (for example, A small amount of vinylene carbonate) can be added.

  In addition, in order to improve the charge / discharge characteristics and flame retardancy, the electrolyte includes, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene. Derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride and the like are added. In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride is further included to impart incombustibility, and carbon dioxide gas is further included to improve high-temperature storage characteristics.

  The present invention provides a lithium secondary battery in which the non-aqueous electrolyte is added to an electrode assembly of a positive electrode, a negative electrode, and a separation membrane.

  The positive electrode is manufactured, for example, by applying a mixture of a positive electrode active material, a conductive material, and a binder on a positive electrode current collector and then drying, and if necessary, a filler may be further added to the mixture. is there.

  Generally, the positive electrode current collector is manufactured to a thickness of 3 to 500 μm. The positive electrode current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the battery, for example, stainless steel, aluminum, nickel, titanium, plastic carbon, Alternatively, an aluminum or stainless steel surface treated with carbon, nickel, titanium, silver or the like is used. In addition, the current collector can also form fine irregularities on its surface to enhance the binding force of the positive electrode active material, and can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics. Forms are possible.

The positive active material is mainly lithium cobalt oxide (LiCoO _2), lithium manganese oxide, or layered compounds such as lithium nickel oxide (LiNiO _2), substituted in one or more of the transition metal Compounds; lithium copper oxide (Li ₂ CuO ₂ ); vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O ₇ ; chemical formula LiNi _1-x M _x A Ni-site type lithium nickel oxide represented by O ₂ (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3); Chemical formula LiMn _2−x M _x O ₂ (where M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1).

  In one preferred example, as the positive electrode active material, a lithium manganese-based metal oxide having safety and lower price than lithium cobalt-based oxide or lithium nickel-based oxide can be used as a main component.

Preferred examples of the lithium manganese metal oxide include chemical formula Li _{1 + x} Mn _2-x O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO _{2, and the} like. lithium manganese oxide; (where, M = Co, Ni, Fe , Cr, with Zn or Ta, is x = 0.01 to 0.1) formula _{LiMn 2-x} M _{x O 4} or _Li 2 Mn ₃ MO ₈ (where M = Fe, Co, Ni, Cu or Zn); and LiMn ₂ O in which a part of Li is substituted with an alkaline earth metal ion in the chemical formula ₄ etc. are mentioned.

Usually, the conductive material is added based on the total mass of a mixture containing a positive electrode active material 50 mass%. The conductive material is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; carbon black, acetylene black Carbon black such as ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powder such as carbon fluoride, aluminum and nickel powder; zinc oxide And conductive whiskers such as potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

The binder is a component that assists the binding between the active material and the conductive material and the binding to the current collector, and is usually added in an amount of 1 to 50% by mass based on the total mass of the mixture including the positive electrode active material. Examples of the binder include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer. (EPDM), sulfonated EPDM, styrene butylene rubber, fluororubber, and various copolymers.

  The filler is selectively used as a component that suppresses the expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material that does not induce a chemical change in the battery. For example, polyethylene, polypropylene And a fibrous substance such as glass fiber and carbon fiber.

  The negative electrode is manufactured by applying a negative electrode material on a negative electrode current collector and drying it, and further includes the above-described components as necessary.

  Generally, the negative electrode current collector is manufactured with a thickness of 3 to 500 μm. The negative electrode current collector is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, plastic carbon Alternatively, a copper or stainless steel surface treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy is used. In addition, the negative electrode current collector, like the positive electrode current collector, can also form fine irregularities on the surface to enhance the binding force of the negative electrode active material, film, sheet, foil, net, porous body, It is used in various forms such as foam and nonwoven fabric.

Examples of the negative electrode active material include carbon such as non-graphitizable carbon and graphite-based carbon; Li _x Fe ₂ O ₃ (0 ≦ x ≦ 1), Li _x WO ₂ (0 ≦ x ≦ 1), Sn _x Me _1-x Me ′ _y O _z (Me: Mn, Fe, Pb, Ge; Me ′: Al, B, P, Si, Group 1, Group 2, Group 3 element of the periodic table, halogen; 0 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8), etc .; lithium metal; lithium alloy; silicon alloy; tin alloy; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , Bi ₂ O _5, etc. And; conductive polymers such as polyacetylene; and Li—Co—Ni-based materials.

  In one preferred example, a non-graphitic carbon material such as non-graphitizable carbon is used as a main component as the negative electrode active material. Therefore, by using a negative electrode active material in which a graphite-based carbon material is mixed at less than 50%, it is possible to use propylene carbonate (PC) having a relatively low viscosity at a low temperature while having a high dielectric constant as an electrolyte solvent. enable.

  The separation membrane is interposed between the positive electrode and the negative electrode, and an insulating thin film having high ion permeability and mechanical strength is used. Generally, the pore diameter of the separation membrane is 0.01 to 10 μm and the thickness is 5 to 300 μm. As the separation membrane, for example, a sheet or a nonwoven fabric made of an olefin polymer such as polypropylene having chemical resistance and hydrophobicity and glass fiber or polyethylene is used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separation membrane.

  The secondary battery according to the present invention is manufactured in various forms. For example, the electrode assembly is manufactured in a jelly-roll type, a stack type, etc., and the battery is in the form of a cylindrical can, a rectangular can, or a battery case of a laminate sheet including a metal layer and a resin layer. is there. Since this is widely known in the art, a detailed description thereof will be omitted.

  The lithium secondary battery according to the present invention is preferably used as a high-power, large-capacity battery or a battery pack unit battery, and particularly for vehicles such as electric vehicles and hybrid electric vehicles that require excellent high-temperature storage characteristics. It is preferable to use it as a power source.

  The non-aqueous electrolyte for a lithium secondary battery according to the present invention includes a specific imidazolium salt, so that the high-temperature storage characteristics can be greatly improved. Therefore, a secondary battery that should operate at a high temperature such as an electric vehicle and a hybrid electric vehicle. Widely and effectively applied to.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, the category of this invention is not limited by this Example.

[Example 1]
Ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) 4: 3: in the non-aqueous electrolyte solvent were mixed in mass ratio of 3, a lithium salt is dissolved 1M-LiPF _6, overall quality amounts to produce a 1.5 mass% of vinylene carbonate (VC) and 3 mass% of 1-ethyl-2,3-dimethyl-imidazolium hexafluorophosphate was added electrolyte on the basis. A positive electrode containing lithium manganese oxide as an active material and a negative electrode containing hard carbon as an active material were laminated together with a separation membrane, and the electrolyte solution was added to form a laminate type lithium ion battery.

[Comparative Example 1]
A battery was constructed by producing an electrolytic solution by the same method as in Example 1 except that 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate was not added.

[Comparative Example 2]
Electrolytic solution in the same manner as in Example 1 except that 1-ethyl-3-methylimidazolium hexafluorophosphate was added instead of 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate To produce a battery.

[Comparative Example 3]
In the same manner as in Example 1 except that 1-ethyl-2,3-dimethylimidazolium tetrafluoroborate was added instead of 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate. An electrolyte solution was manufactured to constitute a battery.

[Comparative Example 4]
Instead of 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate, 1,2,3-trimethylimidazolium tetrafluoroborate was added.

[Experimental Example 1]
High temperature characteristic experiments were performed on the batteries manufactured in Example 1 and Comparative Examples 1 to 4 described above. In the high temperature characteristic experiment, after the battery was stored at 65 ° C. for 5 weeks, the increase in resistance was measured at room temperature, and the data for the measurement result is shown in FIG. As shown in FIG. 1, when the resistance increase rate after 2 weeks of storage is compared, the battery of Example 1 has a resistance of about 60% as compared with the battery of Comparative Example 1 in which no imidazolium salt is added to the electrolyte. Compared to the batteries of Comparative Examples 2 to 4 to which other imidazolium salts were added, it was greatly reduced by 70% or more. The resistance increase rate thus reduced corresponds to an apparent value that deviates from the range that can be predicted by the addition of a conventional imidazolium salt. Furthermore, the resistance increase rate of the battery of Example 1 according to the present invention increased to 12 weeks after storage and showed an increase rate of about 7%, and then stabilized without increasing any longer. It exhibits stable characteristics even when stored at high temperatures.

  Therefore, it can be seen from the results of the above experimental examples that the high-temperature storage characteristics can be greatly improved when the electrolytic solution for a lithium secondary battery to which the imidazolium compound according to the present invention is added is used.

  Those having ordinary knowledge in the technical field to which the present invention pertains can be applied and modified in various ways within the scope of the present invention based on the above contents.

It is a graph which shows the resistance increase rate with respect to the battery of Example 1 and Comparative Examples 1-4 by the high temperature storage characteristic experiment of Experimental example 1. FIG.

Claims (9)

A non-aqueous electrolyte for a lithium secondary battery containing a lithium salt and an organic solvent,
An electrolytic solution comprising a salt containing 1-ethyl-2,3-dimethylimidazolium ion represented by the following chemical formula (1) as a cation.

[In the above formula, X is hexafluorophosphate (PF ₆ ) . ] The imidazolium salts of formula 1, wherein the total mass of the electrolytic solution on the basis, characterized in that it is 0.5 to 5 mass%, the electrolytic solution according to claim 1. The electrolyte solution, characterized by using a mixed solvent of profiles propylene carbonate (PC), or PC and a linear carbonate as a solvent, the electrolytic solution of claim 1.   The lithium secondary battery comprised by including the non-aqueous electrolyte solution as described in any one of Claims 1-3 in the electrode assembly of a positive electrode, a separation membrane, and a negative electrode structure.   The lithium secondary battery according to claim 4, wherein the battery includes a lithium manganese metal oxide as a positive electrode active material. Lithium manganese oxide in which the lithium manganese-based metal oxide has the chemical formula Li _{1 + X} Mn _{2 -X} O ₄ (wherein X is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ ,
Chemical formula LiMn _{2 -X} M _X O ₄ (wherein M is Co, Ni, Fe, Cr, Zn or Ta and X is 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (formula Wherein M is Fe, Co, Ni, Cu or Zn), or
The lithium secondary battery according to claim 5, wherein LiMn ₂ O _{4 in} which a part of Li in the chemical formula is substituted with an alkaline earth metal ion.     The lithium secondary battery according to claim 4, wherein the battery includes a non-graphite carbon material as a negative electrode active material.     The lithium secondary battery according to claim 4, wherein the battery is used as a high-power / high-capacity battery or a battery pack unit battery.     The lithium secondary battery according to claim 4, wherein the battery is used as a power source of an electric vehicle or a hybrid electric vehicle.

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