KR20070082927A - Non-aqueous electrolyte lithium secondary battery with excellent high-temperature storage properties and low-temperature output properties - Google Patents

Abstract

A non-aqueous lithium electrolyte is provided to improve low-temperature output properties while maintaining high-temperature storage and cycle characteristics by adding an acetate-based electrolyte and a LiTFSI salt to the conventional electrolyte. A non-aqueous lithium electrolyte comprises a lithium salt and a non-aqueous electrolyte for dissociation and dispersion of the lithium salt. The non-aqueous electrolyte contains an ester-based electrolyte. The lithium salt contains a LiTFSI salt. The non-aqueous electrolyte is a mixture of cyclic carbonates and linear carbonates. The ester-based electrolyte is one, two or more selected from the group consisting of methyl formate, methyl acetate, ethyl acetate, isopropyl acetate, isoamyl acetate, methyl propionate, ethyl propionate, methyl butylate, and ethyl butylate.

Description

Non-aqueous Electrolyte Lithium Secondary Battery with Excellent High-Temperature Storage Properties and Low-Temperature Output Properties}

The present invention relates to a non-aqueous lithium electrolyte having excellent high temperature storage characteristics and low temperature output characteristics, and a lithium secondary battery including the same. More particularly, an ester-based electrolyte is mixed with an existing carbonate electrolyte in an appropriate ratio, and conventional lithium By mixing LiTFSI salt with salt at a predetermined ratio to form a nonaqueous electrolyte, it is possible to improve low temperature output characteristics while maintaining high temperature storage and cycle characteristics in secondary batteries that must be operated at high temperature as well as low temperature, such as electric vehicles and hybrid electric vehicles. It relates to a non-aqueous lithium electrolyte and a lithium secondary battery comprising the same.

As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing, and lithium secondary batteries showing high energy density and discharge voltage are commercially used in such secondary batteries.

In recent years, hybrid electric vehicles have been developed to improve fuel efficiency and reduce emissions of internal combustion engine vehicles. The high power battery, which is an auxiliary power source, must have high power density and excellent durability such as cycle and high temperature storage. In addition, the low temperature output must also be high to start the car at low temperatures.

In general, a nonaqueous electrolyte of a lithium secondary battery is used by dissolving LiPF ₆ lithium salt in a carbonate solvent (especially a mixture of linear carbonate and cyclic carbonate). Mainly used as the cyclic carbonate electrolyte is ethylene carbonate, propylene carbonate and the like. Ethylene carbonate is widely used due to its excellent dielectric constant, but it has a disadvantage in that it has a low melting point due to its high melting point, which exists in a solid state at room temperature.In the case of propylene carbonate, its low melting point is superior to that of ethylene carbonate, When using a crystalline carbon-based negative electrode has the disadvantage that the electrode is unstable due to the reactivity with the negative electrode. Therefore, in the case of the conventional nonaqueous electrolyte, in order to improve the low temperature characteristics, a mixture of linear carbonate electrolytes having good low temperature characteristics was used.

In addition, an electrolyte consisting of a mixture of a linear carbonate, a cyclic carbonate, an alkyl acetate, and a lithium salt has been proposed as an electrolyte improving method for improving low temperature characteristics (Japanese Patent Laid-Open Nos. 1997-171825, 1996-195221, Korean Patent Publication No. 2000-0040642, Korean Patent Registration No. 322449). These methods have improved low temperature properties by the addition of the alkyl acetate, but there was room for improvement in storage performance and cycle characteristics at high temperature since no technical supplement was prepared to prevent the high temperature property deterioration.

Therefore, there is an urgent need to develop a lithium ion battery that maintains cycle and high temperature storage performance and improves low temperature output to be suitable as a next-generation power source for battery vehicles and hybrid electric vehicles.

Accordingly, an object of the present invention is to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

The inventors of the present application, after continuing in-depth research and various experiments, mixed the ester-based electrolyte, especially the acetate-based electrolyte in a certain ratio in the electrolyte composition in order to improve the low-temperature characteristics of the existing electrolyte, and to reduce the high temperature characteristics of concern It has been found that, by the addition of a predetermined amount of LiTFSI salt to prevent the composition of the electrolyte, it is surprisingly possible to improve the low temperature output while maintaining the high temperature storage and cycle characteristics. The present invention has been completed based on such an unpredictable discovery.

Accordingly, the non-aqueous lithium electrolyte having excellent high temperature storage characteristics and low temperature output characteristics according to the present invention is a non-aqueous lithium electrolyte composed of a lithium salt and a non-aqueous electrolyte for dissociation and dispersion of the lithium salt. It provides a non-aqueous lithium electrolyte that is mixed and the LiTFSI salt is mixed with the lithium salt.

As the non-aqueous electrolyte, an organic solvent usually used as a non-aqueous electrolyte can be used. Among them, a cyclic carbonate, a linear carbonate or a mixture thereof is preferable, and a mixture of a cyclic carbonate and a linear carbonate is particularly preferable. Examples of the cyclic carbonates include propylene carbonate, ethylene carbonate, butylene carbonate, and the like, and linear carbonates include, for example, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and the like. no.

The ester electrolyte has a lower freezing point than the linear carbonate electrolyte and has excellent viscosity and dielectric constant at low temperatures. In the present invention, such an ester electrolyte is mixed with the existing electrolyte composition to lower battery resistance at low temperatures and improve output characteristics.

Examples of the ester electrolyte, methyl formate, methyl acetate, ethyl acetate, isopropyl acetate, isoamyl acetate, methyl propiotate (methyl propionate), ethyl propionate, methyl butylate, ethyl butylate and the like, but are not limited thereto. These may be used alone or in combination of two or more. Especially, acetate type electrolyte solutions, such as methyl acetate, ethyl acetate, and isopropyl acetate, are preferable, and ethyl acetate and methyl acetate are especially preferable.

The ester electrolyte may be added in 20 to 40% by volume based on the total volume of the nonaqueous electrolyte. In this case, when the ester-based electrolyte is added in excess of the above range, the high temperature performance may drop, and if too little is added, the desired low temperature output performance may not be expected.

However, since a low molecular weight ester electrolyte has a low freezing point, the composition of the electrolyte solution containing acetate as described above is somewhat inferior in high temperature characteristics. LiTFSI salt, which is good in high temperature performance due to low generation, is mixed with the existing lithium salt in an appropriate ratio.

The LiTFSI salt is represented by the formula LiN (CF ₃ SO ₂ ) ₂ , and may be added in an amount of 15 to 30 wt% based on the total lithium salt. When the LiTFSI salt is used in excess, the low temperature performance may be reduced due to the increase in viscosity. On the contrary, when the LiTFSI salt is used too much, it is not preferable because it is difficult to expect to improve the high temperature characteristic due to the addition.

The lithium salt is a good material to be dissolved in the non-aqueous electrolyte, 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 carboxylic acid lithium, lithium tetraphenyl borate, and and imide, among LiPF ₆ is more desirable.

In some cases, the non-aqueous lithium electrolyte of the present invention includes pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme for the purpose of improving charge / discharge characteristics, flame retardancy, and the like. glyme), hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol , Aluminum trichloride, or the like may be added. In some cases, in order to impart nonflammability, a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride may be further included.

The secondary battery including the non-aqueous electrolyte according to the present invention can be applied to a high-output large-capacity battery used in electric vehicles, hybrid battery vehicles, etc., which must operate at low temperature as well as high temperature.

The present invention also provides a lithium secondary battery comprising the non-aqueous lithium electrolyte in the positive electrode, the negative electrode and the separator.

The positive electrode is prepared by, for example, applying a mixture of a positive electrode active material, a conductive agent, and a binder onto a positive electrode current collector, followed by drying, and optionally, a filler may be further added to the mixture.

The positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as 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 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 and the like; Ni-site type lithium nickel oxide represented by the formula LiNi _1-x M _x O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 to 0.3; Formula LiMn _2-x M _x O ₂ (wherein M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (wherein M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ and the like, but are not limited to these.

The positive electrode current collector is generally made to a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The conductive agent is typically added in an amount of 1 to 50 wt% based on the total weight of the mixture including the positive electrode active material. Such a conductive agent is not particularly limited as long as it has conductivity without causing chemical change to the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component that assists in bonding the active material and the conductive agent to the current collector, and is generally added in an amount of 1 to 50 wt% based on the total weight of the mixture including the positive electrode active material. Examples of such binders 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, fluorine rubber, various copolymers, and the like.

The filler is optionally used as a component for inhibiting expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material without causing chemical change in the battery. Examples of the filler include olefinic polymers such as polyethylene and polypropylene; Fibrous materials, such as glass fiber and carbon fiber, are used.

The negative electrode is manufactured by applying and drying a negative electrode material on the negative electrode current collector, and if necessary, the components as described above may be further included.

The negative electrode current collector is generally made to a thickness of 3 to 500 ㎛. Such a negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver and the like on the surface, aluminum-cadmium alloy and the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The negative electrode material may be, for example, carbon such as hardly graphitized carbon or graphite 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' Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen, 0 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); Lithium metal; Lithium alloys; Silicon-based alloys; Tin-based alloys; 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 , and metal oxides such as Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

Especially, amorphous carbon type negative electrode active materials, such as hardly graphitized carbon, etc. are especially preferable. This is because the ester electrolyte may decompose due to its excellent reactivity in the crystalline carbon-based negative electrode, but does not occur in the amorphous carbon-based negative electrode.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

Hereinafter, the present invention will be described in detail by way of examples, but the scope of the present invention is not limited thereto.

Example 1

Ethylene carbonate, methylethyl carbonate and ethyl acetate were mixed in a volume ratio of 3: 4: 3, and then non-aqueous electrolytes were prepared by dissolving respective lithium salts to 0.8 M LiPF ₆ and 0.2 M LiTFSI. A positive electrode containing lithium manganese oxide and a negative electrode containing carbon were laminated together with a separator and the nonaqueous electrolyte was added to constitute a laminate type lithium ion battery.

Example 2

The electrolyte was prepared in the same composition and method as in Example 1 to construct a battery, but the lithium salt composition was changed to 0.6 M LiPF ₆ and 0.4 M LiTFSI.

Example 3

An electrolyte was prepared in the same composition and method as in Example 1 to construct a battery, but methyl acetate was used instead of ethyl acetate.

Example 4

In the same composition and method as in Example 1 to prepare an electrolyte solution, but using a methyl acetate instead of ethyl acetate, the composition of the lithium salt was changed to 0.4 M LiPF ₆ and 0.6 M LiTFSI.

Comparative Example 1

Ethylene carbonate and methylethyl carbonate were mixed in a volume ratio of 3: 7, and then a lithium salt was dissolved to obtain 1.0 M LiPF ₆ to prepare a nonaqueous electrolyte. Then, a battery was constructed in the same manner as in Example 1.

Comparative Example 2

An electrolyte was prepared in the same composition and method as in Comparative Example 1 to form a battery, but the lithium salt composition was changed to 0.8 M LiPF ₆ and 0.2 M LiTFSI.

 Comparative Example 3

An electrolyte was prepared in the same composition and method as in Example 1 to construct a battery, but the lithium salt composition was changed to 1.0 M LiPF ₆ .

 Experimental Example 1

Low temperature characteristics were performed on the batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 3. In the low temperature characteristic experiment, a constant current of 10 A was applied at −30 ^° C. for 10 seconds to measure battery internal resistance and output, and the results are shown in Table 1 below.

As shown in Table 1, the batteries of Examples 1 to 3 and Comparative Example 3 to which the ester electrolyte was added have a lower low temperature resistance and a lower temperature output than the batteries of Comparative Examples 1 and 2 without using the ester electrolyte. You can see the excellent.

In addition, although the low-temperature output of the battery of Examples 1 to 3 was slightly lower than that of the battery of Comparative Example 3 due to the addition of LiTFSI salt, it was confirmed that the level was not significant.

Experimental Example 2

In addition, the high temperature characteristics of the batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 3 were performed. The high temperature patent experiment measured the capacity and resistance at room temperature after storing the cell at 65 ° C. for 10 weeks, and comparing the measured result with the value before storage is shown in Table 2 below.

As shown in Table 2, the batteries of Examples 1 to 3 and Comparative Examples 1 and 2 show similar values of capacity reduction rate and resistance increase rate, but the battery of Comparative Example 3 has a large decrease in capacity and a large resistance. It can be seen that the increase.

Through the results of Experimental Example 1 and Experimental Example 2, using the non-aqueous electrolyte according to the present invention, it is possible to improve the low-temperature performance while maintaining the overall high-temperature performance by improving the high-temperature performance when using the ester-based electrolyte It can be seen.

Those skilled in the art to which the present invention pertains will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

As described above, the non-aqueous lithium electrolyte excellent in the high temperature storage characteristics and the low temperature output characteristics according to the present invention and the lithium secondary battery including the same have been lowered in the high temperature characteristics by adding a certain amount of an ester electrolyte in order to improve the conventional low temperature characteristics. Unlike the above, by mixing the ester-based electrolyte solution at the proper ratio and mixing the LiTFSI salt at the predetermined ratio to prepare the electrolyte solution, it is possible to improve the low-temperature output characteristics while maintaining the high temperature storage characteristics and cycle characteristics of electric vehicles and hybrid electric vehicles As described above, the present invention can be widely applied to secondary batteries that need to operate at low temperatures as well as high temperatures.

Claims (11)

A non-aqueous lithium electrolyte comprising a lithium salt and a nonaqueous electrolyte for dissociation and dispersion of the lithium salt, wherein the non-aqueous electrolyte is mixed with an ester electrolyte and a LiTFSI salt is mixed with the lithium salt. Electrolyte. The non-aqueous lithium electrolyte according to claim 1, wherein the nonaqueous electrolyte is a mixture of a cyclic carbonate and a linear carbonate. The non-aqueous lithium electrolyte of claim 1, wherein the lithium salt is LiPF ₆ . According to claim 1, wherein the ester electrolyte, methyl formate, methyl acetate, ethyl acetate, isopropyl acetate, isoamyl acetate, methyl propiotate, ethyl propionate, methyl butyrate and ethyl butyrate Non-aqueous lithium electrolyte, characterized in that one or two or more selected from. The non-aqueous lithium electrolyte according to claim 1, wherein the ester electrolyte is an acetate electrolyte. 6. The non-aqueous lithium electrolyte according to claim 5, wherein the acetate electrolyte is ethyl acetate and / or methyl acetate. The non-aqueous lithium electrolyte of claim 1, wherein the ester electrolyte is added in an amount of 20 to 40% by volume based on the total volume of the nonaqueous electrolyte. The non-aqueous lithium electrolyte of claim 1, wherein the LiTFSI salt is added in an amount of 15 to 30 wt% based on the total lithium salt. A lithium secondary battery comprising a non-aqueous lithium electrolyte according to any one of claims 1 to 8. 10. The lithium secondary battery of claim 9, wherein the negative electrode of the secondary battery is made of an amorphous carbon-based negative electrode active material. The lithium secondary battery according to claim 9, wherein the secondary battery is a high output large capacity battery for an electric vehicle or a hybrid battery vehicle.