JP5282346B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery and an electrolyte used therefor. More specifically, the present invention relates to a non-aqueous electrolyte secondary battery that has little decomposition of the electrolyte, high charge / discharge efficiency, and excellent storage characteristics and cycle characteristics at high temperatures, and an electrolyte used therefor.

With the recent reduction in weight and size of electrical products, development of lithium secondary batteries having a high energy density is in progress. In addition, with the expansion of the application field of lithium secondary batteries, improvement of battery characteristics is also demanded.
As a solvent used for the electrolyte of the non-aqueous electrolyte secondary battery, ethylene carbonate having a high dielectric constant is frequently used. However, the freezing point of ethylene carbonate is as high as 36.4 ° C., and it is solid at room temperature. Therefore, a low-viscosity solvent such as ethyl methyl carbonate or diethyl carbonate is mixed as an auxiliary solvent in the electrolytic solution using ethylene carbonate. However, a low-viscosity solvent generally has a low boiling point and a low dielectric constant. Therefore, when mixed in large quantities, the performance of the electrolytic solution decreases due to a decrease in the degree of dissociation of the lithium salt, or a salt precipitates due to evaporation of the solvent. There is a problem in terms of safety, such as the flash point is lowered. On the other hand, if only a small amount is mixed, problems of electrical conductivity and viscosity at low temperatures remain.

By the way, in non-aqueous electrolyte secondary batteries using carbonaceous materials such as coke, artificial graphite, and natural graphite for the negative electrode, the formation of dendrite is suppressed because lithium does not exist in the metallic state, and excellent battery life and safety are achieved. It is known to show.
However, when a carbonaceous material having a high degree of crystallinity such as graphite is used for the negative electrode, the non-aqueous solvent is decomposed or the carbonaceous material is peeled off, which may increase the irreversible capacity. In particular, when propylene carbonate is used for the non-aqueous solvent and graphite material is used for the negative electrode, there is a problem that the propylene carbonate is vigorously decomposed on the surface of the graphite and the battery characteristics are deteriorated.

  In order to suppress such decomposition reaction, many studies have been made to contain various compounds in the electrolytic solution. For example, chloroethylene carbonate (H. Katayama, J. Arai, H. Akahoshi, J. Power Sources 1999, 81-82, 705-708.), Fluoroethylene carbonate (R. McMillan, H. Slegr, ZX Sho, W. Wang, J. Power Sources 1999, 81-82, 20-26.), Ethylene sulfite (GH Wrodnigg, JO Besenhard, M. Winter, J. Electrochem. Soc. 1999 , 146, 470.), vinylene carbonate (J. Barker, F. Gao, US Patent No. 5,712,059 (1998), Y. Naruse, S. Fujita, A. Omaru, US Patent No. 5,714,281 (1998)) Ethylene carbonate derivatives and analogs have been studied. These compounds are usually considered to be reduced at a high electrode potential during initial charging to form a film on the surface of the electrode.

Problems to be solved by the invention

[Problems to be solved by the invention]
However, the effects of the additives described above are insufficient, and further improvements are desired. The present invention decomposition of the electrolytic solution is small, the charge-discharge efficiency is an object of the invention to provide a high torquecontrol aqueous electrolyte secondary battery.

Means for solving the problem

[Means for Solving the Problems]
As a result of intensive investigations in view of such circumstances, the present inventors have found that the electrolyte solution of the non-aqueous electrolyte secondary battery contains a nitrile compound having a carbon-carbon unsaturated bond, thereby reducing the decomposition of the electrolyte solution. It found that it is possible to discharge efficiently obtain a high torquecontrol aqueous electrolyte secondary battery, thereby completing the present invention.

  That is, the gist of the present invention is an electrolyte solution in a non-aqueous electrolyte secondary battery including a negative electrode and a positive electrode capable of inserting and extracting lithium, and an electrolyte solution in which a lithium salt is dissolved in a non-aqueous solvent. Contains a nitrile compound having a carbon-carbon unsaturated bond, and a non-aqueous electrolyte solution used in the non-aqueous electrolyte solution.

BEST MODE FOR CARRYING OUT THE INVENTION The nitrile compound having a carbon-carbon unsaturated bond used in the present invention is any compound as long as it has at least one carbon-carbon unsaturated bond and cyano group in the molecule. It may be. Such compounds include acrylonitrile, methacrylonitrile, crotononitrile, 3-methylcrotononitrile, 2-methyl-2-butenenitrile, 2-pentenenitrile, 3,7-dimethyl-2,6-octadiene. Nitrile, fumaronitrile, 2-methyleneglutaronitrile, cinnamonitrile, 4-methoxycinnamonitrile, 3-methoxyacrylonitrile, 3-ethoxyacrylonitrile, (1-ethoxyethylidene) malononitrile, 1-cyanovinyl acetate, 2-acetoxy- 3- butenenitrile, ethyl-2-cyanoacrylate, ethyl-2-cyano-3-methyl-2-butenoate, ethyl-2-cyano-2-pentenoate, diethyl dicyanofumarate, ethyl-2-cyano-3-ethoxy Acrylate Methyl -α- cyano cinnamate, ethyl-2-cyano-3-phenyl-2-butenoate, ethyl 2-cyano-3,3-diphenylacrylate, 2-furonitrile, 2-cyanoethyl acrylate, ethyl-2-cyano - 3-dimethylamino acrylate etc. are mentioned. The nitrile compound having a carbon-carbon unsaturated bond is preferably contained in the electrolytic solution so as to be 0.001 to 10% by weight, particularly 0.01 to 3% by weight. As the non-aqueous solvent used in the present invention, alkylene carbonate having 2 to 4 carbon atoms of alkylene group, dialkyl carbonate having 1 to 4 carbon atoms of alkyl group, cyclic ether, chain ether, cyclic ester, chain ester, What is necessary is just to select suitably from what can be used as a solvent of nonaqueous electrolyte solution, such as a sulfur-containing organic solvent and a phosphorus-containing organic solvent, and these solvents may be mixed and used. Examples of the alkylene carbonate having 2 to 4 carbon atoms in the alkylene group include ethylene carbonate, propylene carbonate, and butylene carbonate. Of these, ethylene carbonate and propylene carbonate are preferred. Examples of the dialkyl carbonate having 1 to 4 carbon atoms in the alkyl group include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, and ethyl-n-propyl carbonate. . Of these, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are preferred.

Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran.
Examples of chain ethers include dimethoxyethane and dimethoxymethane.
Examples of the cyclic ester include γ-butyrolactone and γ-valerolactone.

Examples of the chain ester include methyl acetate, methyl propionate, and ethyl propionate.
Examples of the sulfur-containing organic solvent include sulfolane and diethyl sulfone.
Examples of the phosphorus-containing organic solvent include trimethyl phosphate and triethyl phosphate.

  Among these nonaqueous solvents, alkylene carbonates having 2 to 4 carbon atoms in the alkylene group and dialkyl carbonates having 1 to 4 carbon atoms in the alkyl group are preferable. Particularly preferred is an alkylene carbonate having 2 to 4 carbon atoms in the alkylene group and a dialkyl carbonate having 1 to 4 carbon atoms in the alkyl group, each containing 20% by weight or more, and these are 70% by weight of the total. It is a mixed solvent occupying the above.

In view of high temperature stability, the non-aqueous solvent preferably contains an organic solvent having a relative dielectric constant of 25 or more in a proportion of 60% by weight or more, particularly 85% by weight or more. When this ratio is low, when a low-viscosity solvent having a low boiling point is used in combination, the battery internal pressure increases during high-temperature storage, and the battery is likely to be deformed or leaked.
Examples of the organic solvent having a relative dielectric constant of 25 or more include ethylene carbonate, propylene carbonate, γ-butyrolactone, γ-valerolactone, and the like. These may be used alone or in combination of two or more.

The electrolytic solution may further contain auxiliary agents such as a film forming agent, an overcharge preventing agent, a dehydrating agent, and a deoxidizing agent.
For example, as a film forming agent, carbonates such as vinylene carbonate and vinyl ethylene carbonate; sulfites such as ethylene sulfite; sulfonic acid esters such as propane sultone; carboxylic acid anhydrides such as succinic anhydride, maleic anhydride, and phthalic anhydride Consisting of nitrogen-containing heterocyclic compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methylsuccinimide When a compound selected from the group is contained in an amount of 0.01 to 3% by weight in the electrolytic solution, capacity maintenance characteristics and cycle characteristics are improved.

  Examples of the overcharge inhibitor include benzene derivatives described in JP-A-8-203560, JP-A-7-302614, JP-A-9-50822, JP-A-8-273700, JP-A-9-17447, and the like; No. 9-106835, No. 9-171840, No. 10-32258, No. 7-302614, No. 7-302614, No. 11-162512, and Nos. 2939469 and 2963898. Biphenyl and derivatives thereof; pyrrole derivatives described in JP-A Nos. 9-45369 and 10-32258; and JP-A Nos. 7-320778 and 7-302614 Aromatic compounds such as aniline derivatives; ether compounds described in Japanese Patent No. 2983205; It can be exemplified compounds described in 001-15158 JP.

Furthermore, in order to improve the wettability with the separator and the electrode material, a surfactant may be added to the electrolytic solution so as to be 0.01 to 2% by weight.
A lithium salt is used as a solute of the electrolytic solution used in the present invention. As the lithium salt, any known lithium salt can be used as the solute of the non-aqueous electrolyte solution.
1) Inorganic lithium _{salts: LiPF 6, LiAsF 6, LiBF} 4, LiTaF 6, LiAlF 4, LiAlF 6, LiSiF inorganic fluoride salts ₆ and the like, perhalogenate 2) organic lithium salts such as LiClO _4: LiCF ₃ SO organic sulfonic acid salts such as _{3, LiN (CF 3 SO 2} ) 2, LiN (C 2 F 5 SO 2) 2, LiN (CF 3 SO 2) (C 4 F 9 SO 2) perfluoroalkylsulfonic acid such as Imido salts, perfluoroalkylsulfonic acid methides such as LiC (CF ₃ SO ₂ ) ₃ , inorganic fluorides such as LiPF ₃ (C ₂ F ₅ ) ₃ , LiBF ₂ (CF ₃ ) ₂ , LiBF ₃ (CF ₃ ) salt by substituting a part of fluorine salt with perfluoroalkyl _{group, LiB (CF 3 COO) 4} , LiB (OCOCF 2 COO) 2, LiB (OCOC 2 F 4 COO) 2, lithium etc. Tetrakis (perfluoro-carboxylate) borate salts. These may be used as a mixture.

Among these, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₃ ) are considered in terms of solubility, ion dissociation degree, and electrical conductivity characteristics. ₂ ) (C ₄ F ₉ SO ₂ ), LiPF ₃ (CF ₃ ) ₃ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiBF ₂ (C ₂ F ₅ ) ₂ LiB (OCOCF ₂ COO) ₂ are preferred, and LiPF ₆ LiBF ₄ is more preferable. In particular, when the non-aqueous solvent contains 60% by weight or more of γ-butyrolactone, it is preferable that LiBF ₄ accounts for 50% by weight or more of the entire lithium salt.

The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 3 mol / liter. If the concentration is too low, the electrical conductivity of the electrolytic solution becomes insufficient due to an absolute lack of concentration. Conversely, if the concentration is too high, the electrical conductivity decreases due to an increase in viscosity, and precipitation at low temperatures is likely to occur.
Materials for the negative electrode constituting the battery of the present invention include carbonaceous materials such as pyrolysates of organic matter, artificial graphite, natural graphite, and mixtures thereof under various conditions; tin oxide, antimony tin oxide, silicon monoxide Metal oxides such as vanadium oxide; lithium metal; alloys with lithium such as aluminum, silicon, tin, antimony, lead, arsenic, zinc, bismuth, copper, cadmium, silver, gold, platinum, palladium, magnesium, sodium, potassium A metal that can be made into an alloy (including an intermetallic compound); a metal that can be alloyed with lithium and a composite alloy compound of lithium and an alloy that contains the metal; lithium metal nitride such as lithium cobalt nitride; Can be mentioned. Note that the above materials may be mixed and used.

  Of these, artificial graphite, purified natural graphite produced by subjecting easily graphitizable pitch obtained from various raw materials to high-temperature heat treatment, and graphite materials obtained by subjecting these graphite to surface treatment with various pitches are preferable. As such a graphite material, the d value (interlayer distance) of the lattice plane (002 plane) obtained by X-ray diffraction by the Gakushin method is 0.335 to 0.34 nm, particularly 0.335 to 0.337 nm. Those are preferred. The ash content is preferably 1% by weight or less, more preferably 0.5% by weight or less, and particularly preferably 0.1% by weight or less. The crystallite size (Lc) determined by X-ray diffraction by the Gakushin method is preferably 30 nm or more, more preferably 50 nm or more, and still more preferably 100 nm or more.

Further, the median diameter of the carbonaceous material by the laser diffraction / scattering method is preferably 1 to 100 μm, more preferably 3 to 50 μm, further preferably 5 to 40 μm, and particularly preferably 7 to 30 μm.
BET specific surface area is preferably 0.3~25.0m ² / g, more preferably 0.5 to 20.0 m ² / g, still more preferably 0.7~15.0m ² / g, 0.8 ˜10.0 m ² / g is particularly preferred.

Carbonaceous material, when Raman spectrum analysis using argon ion laser light, a peak in the range of 1570~1620cm ^-1 PA peak in the range of (peak intensity IA) and 1300~1400cm ^-1 PB (peak intensity IB) the intensity ratio R = IB / IA is from 0.01 to 1.0, especially 0.1 to 0.7 are preferred, the half width of the peak in the range of 1570~1620cm ^-1, 26cm ^-1 or less, particularly 25cm ^-1 or less is preferable.

The alloy is preferably a metal alloy selected from the group consisting of tin, antimony, silver, copper and gold, and tin-antimony alloy, tin-silver alloy, copper-antimony alloy, gold-antimony alloy are used. Particularly preferred.
The metal or alloy used for the negative electrode may be one type or a mixture of two or more types. The average particle diameter is preferably 1 to 1000 nm, more preferably 10 to 500 nm, and still more preferably 30 to 400 nm. If the average particle size is too large, capacity deterioration due to repeated charge / discharge cycles may increase and the usefulness as an electrode may be impaired. Conversely, if it is too small, the surface area increases and the safety of the battery decreases. Further, the particle size distribution is preferably within these ranges.

  Using these negative electrode materials, the negative electrode can be produced by a conventional method. For example, a negative electrode material can be produced by adding a binder, a thickener, a conductive material, a solvent, etc. to a negative electrode material as necessary to form a slurry, which is applied to a substrate of a current collector and dried. it can. Further, the negative electrode material can be roll-formed as it is to obtain a sheet electrode, or a pellet electrode can be obtained by compression molding.

Any binder can be used as long as it is a material that is stable with respect to the solvent and electrolyte used in the electrode production. Specific examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene / butadiene rubber, isoprene rubber, and butadiene rubber.
Examples of the thickener include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein.

Examples of the conductive material include metal materials such as copper and nickel, and carbonaceous materials such as graphite and carbon black.
Examples of the material for the current collector for the negative electrode include metals such as copper, nickel, and stainless steel. Among these, copper foil is preferable from the viewpoint of easy processing into a thin film and cost.
Examples of the positive electrode material include materials capable of inserting and extracting lithium, such as lithium transition metal composite oxide materials such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide.

A positive electrode can be manufactured according to the manufacturing method of said negative electrode. That is, if necessary, a binder, a conductive material, a solvent, and the like are added to the positive electrode material and mixed, and then applied to a current collector substrate to form a sheet electrode, or press molding to form a pellet electrode can do.
Examples of the material for the positive electrode current collector include metals such as aluminum, titanium, and tantalum, and alloys thereof, and among these, aluminum or an alloy thereof is preferable in terms of energy density.

The separator used in the battery of the present invention is only required to be stable with respect to the electrolytic solution and excellent in liquid retention, and a porous sheet or nonwoven fabric made of polyolefin such as polyethylene and polypropylene is used. preferable.
The battery according to the present invention can be manufactured by a conventional method using the above-described negative electrode, positive electrode, and non-aqueous electrolyte.

  The battery can have any commonly used shape. Examples include a cylinder type in which a sheet electrode and a separator are spiral, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, and a coin type in which a pellet electrode and a separator are stacked.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to these examples unless it exceeds the gist.
Example 1
5 parts by weight of polyvinylidene fluoride was mixed with 95 parts by weight of artificial graphite powder (TIMREX KS6), and dispersed with N-methyl-2-pyrrolidone to form a slurry. This was uniformly coated on a stainless steel mesh, dried and pressed to obtain a negative electrode.

Under a dry argon atmosphere, 1 part by weight of acrylonitrile was added to 99 parts by weight of propylene carbonate, and LiN (CF ₃ SO ₂ ) ₂ was dissolved at 1 mol / liter to obtain an electrolyte.
A glass cell is filled with the electrolyte solution, the negative electrode is used as a working electrode, lithium metal is used as a counter electrode and a reference electrode, an electrochemical cell is produced, and cyclic voltammetry is performed at a room temperature potential scanning speed of 0.05 mV / sec. It was measured.

(Comparative Example 1)
An electrochemical cell was prepared and cyclic voltammetry was measured in the same manner as in Example 1 except that an electrolytic solution in which LiN (CF ₃ SO ₂ ) ₂ was dissolved in propylene carbonate so as to be 1 mol / liter was used. did.
(Example 2)
An electrochemical cell was prepared in the same manner as in Example 1 except that an electrolyte prepared by adding 1 part by weight of methacrylonitrile was used instead of acrylonitrile in Example 1, and cyclic voltammetry was measured.

The result of Example 1 is shown in FIG. 1, the result of Comparative Example 1 is shown in FIG. 2, and the result of Example 2 is shown in FIG.
In the case of Comparative Example 1, only a large reduction current associated with the decomposition of the electrolytic solution is observed in the vicinity of about 0.8 V, and no current associated with the insertion and extraction of lithium is observed.
In the case of Examples 1 and 2, a large reduction current due to the occlusion of lithium was observed from about 0.2 V, and no current accompanying the decomposition of the electrolyte solution of about 0.8 V was observed. An oxidation current due to the release of lithium is also observed, and it can be seen that the insertion and release of lithium proceed smoothly.

FIG. 1 shows a first potential-current curve obtained by cyclic voltammetry measurement in Example 1. FIG. 2 shows a first potential-current curve obtained by cyclic voltammetry measurement in Comparative Example 1. FIG. 3 shows a first potential-current curve obtained by cyclic voltammetry measurement in Example 2.

Claims (5)

Lithium can be occluded / released, a negative electrode containing a graphite material having a d-value of 0.335 to 0.34 nm on a lattice plane (002 plane) determined by X-ray diffraction by the Gakushin method, and lithium An electrolyte used in a non-aqueous electrolyte secondary battery including a positive electrode that can be occluded / released and an electrolyte in which a lithium salt is dissolved in a non-aqueous solvent, the acrylonitrile, methacrylonitrile, crotono Nitrile, 3-methylcrotononitrile, 2-methyl-2-butenenitrile, 2-pentenenitrile, 3,7-dimethyl-2,6-octadienenitrile, fumaronitrile, 2-methyleneglutaronitrile, cinnamonitrile, 4-methoxycinnamonitrile, 3-methoxyacrylonitrile, 3-ethoxyacrylonitrile, (1-ethoxyethylidene) malononito Le, 1-cyano-vinyl acetate, 2-acetoxy-3-butenenitrile, ethyl-2-cyanoacrylate, ethyl-2-cyano-3-methyl-2-butenoate, ethyl 2-cyano-2-pentenoate, diethyl dicyano Fumarate, ethyl-2-cyano-3-ethoxyacrylate, methyl-α-cyanocinnamate, ethyl-2-cyano-3-phenyl-2-butenoate, ethyl-2-cyano-3,3-diphenylacrylate , 2 -Containing 0.01 to 3 wt% of a nitrile compound having at least one carbon-carbon unsaturated bond selected from the group consisting of furonitrile, 2-cyanoethyl acrylate , and ethyl-2-cyano-3-dimethylaminoacrylate. An electrolyte for a non-aqueous electrolyte secondary battery.
The non-aqueous solvent comprises 20% by weight or more of alkylene carbonate having 2 to 4 carbon atoms in the alkylene group and dialkyl carbonate having 1 to 4 carbon atoms in the alkyl group, and these carbonates are 70% by weight of the total. The electrolyte solution for a non-aqueous electrolyte secondary battery according to claim 1, wherein the electrolyte solution occupies% or more.
2. The electrolyte solution for a non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous solvent contains 60% by weight or more of an organic solvent having a relative dielectric constant of 25 or more.
4. The non-aqueous system according to claim 3 , wherein the organic solvent having a relative dielectric constant of 25 or more is selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, and γ-valerolactone. Electrolyte for electrolyte secondary battery.
Nonaqueous electrolyte secondary battery characterized by comprising a non-aqueous electrolyte secondary battery electrolyte according to any one of claims 1 to 4.

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