JPH11126601A - Negative electrode for lithium ion secondary battery and battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve coulomb efficiency in an initial discharge cycle by previously forming a limited permeability layer on a negative electrode. SOLUTION: A limited permeability film is made of a material which is insoluble in an electrolyte and can be molded into an electrode surface layer, and for example, polyacrylonitrile or the like is available. A fraction value of the limited permeability film layer ranges between a maximum molecular weight of a solute moving to the solvent side and a minimum molecular weight of a solute which does not pass through the film, and the desirable fraction value ranges form 8 to 100. If the faction molecular weight is lowered further, lithium ions can hardly pass the film, so that ability as a battery is lost. On the other hand, if the fraction molecular weight is increased further, a solvent actuating an inconvenient reaction is brought into contact with carbon serving as an active material, and a disadvantageous condition is caused. The thickness of the film is not limited specially, however, the thinner film is better from the standpoint of an ion permeating speed, and the thickness ranges about 5-100 μm. If the film is excessively thin, film strength is lowered, while volume of the electrode is increased and a charging energy density is lowered if the film is too thick.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode for a lithium ion secondary battery having high performance, especially high reversible capacity and low irreversible capacity.

[0002]

2. Description of the Related Art Lithium secondary batteries have been expected as next-generation secondary batteries because of their high charging energy density. However, safety issues have not been solved, and lithium of the negative electrode has been occluded with lithium ions.・ Replacement of lithium ion rechargeable batteries, which have been replaced with charcoal to be released, metals that form alloys by reversibly absorbing lithium ions, and metal compounds that form composite compounds, rapidly increase the usage. Is being done. However, they are inferior in terms of charging energy density as compared with lithium secondary batteries, and an increase in capacity is desired. One of the reasons why the energy density, particularly the discharge capacity is not sufficient, is that in the first charge / discharge cycle, a decomposition reaction of the electrolyte solution occurs on the surface of the negative electrode active material, and lithium ions are consumed to passivate the passive electrode (SEI).
lyte interface). Such a reaction easily occurs when carbon, particularly graphite, is used as the active material. This reaction still occurs, albeit much less, after the second cycle. As a result, the charge / discharge efficiency does not reach 100%, and the capacity gradually decreases with repeated cycles. Since SEI penetrates between graphite layers,
It is believed that the formation not only consumes lithium ions, but also consumes the lithium-containing portion between the graphite layers, thereby lowering the performance of the battery. It is also considered that the diffusion rate of lithium ions into graphite is reduced. The present invention has been made in such a situation.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a battery having a high coulomb efficiency in an initial charge / discharge cycle. Another object of the present application is to provide a battery having a large discharge capacity. Still another object of the present invention is to provide a battery having excellent cycle characteristics.

[0004]

Means for Solving the Problems A first invention of the present application is an electrode having a current collector and an active negative electrode material containing carbon applied to the current collector as constituent elements, the surface of which is provided. The negative electrode for a lithium ion secondary battery, wherein a restricted permeable membrane layer is formed in advance, and the second invention is characterized in that the limited permeable membrane layer has a cut-off molecular weight of 8 to 100. The negative electrode for a secondary battery, the third invention is the negative electrode for a lithium ion secondary battery according to claim 1, wherein the molecular weight cutoff of the restricted permeable membrane layer is 10 to 80, and the fourth invention is a current collector And an active negative electrode material containing carbon covering the surface thereof, and a negative electrode having a restricted permeable membrane layer formed on the surface thereof in advance, an electrolyte solution, and a positive electrode. In a fifth invention, the solvent of the electrolyte solution is carbonic acid. A lithium ion secondary battery according to claim 4, wherein the ethers, the sixth invention is a lithium ion secondary battery according to claim 5, wherein fractionation molecular weight of between 8 and 100 of the semi-permeable membrane layer. This suppresses a decrease in Coulomb efficiency in the first charge / discharge cycle, increases the discharge capacity, and improves cycle characteristics.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION There is no particular limitation on carbon used as a negative electrode active material. Natural graphite, artificial graphite, expanded graphite,
Polyacrylonitrile-based or petroleum-pitch-based carbon fibers or graphitized products thereof, mesocarbon microbeads (MCMB), vapor-grown carbon fibers or graphitized products thereof, carbon nanotubes, activated carbon, amorphous carbon, and other various carbons can be used. From the viewpoint of the energy density of the battery, a battery mainly composed of a graphite structure is preferred, and when used at a large current, a battery which is ground or cut to some extent is preferred. These negative electrode active materials are mixed with a binder solution by a conventional method and applied to a current collector. There is no particular limitation on the current collector. Metal foil, metal mesh, punched metal, sintered metal filament, and other forms can be used. The material is not particularly limited as long as it has good conductivity,
Copper is most preferably used as the negative electrode because copper itself is ionized and hardly eluted. The coated negative electrode is preferably cut in accordance with the size of a target battery, and after a lead wire is attached, a restricted permeable membrane layer is formed.

[0006] The material of the restricted permeable membrane is not particularly limited as long as it is insoluble in the common electrolytic solution and can be molded as an electrode surface layer. Vinyl polymers such as polyacrylonitrile and polymethyl methacrylate, polyolefin derivatives such as polyethylene, polypropylene and polyviridene fluoride, cellulose derivatives such as cellulose acetate and nitrocellulose, and condensation polymers such as polyethylene terephthalate and polyamide A polymer, a thermosetting resin such as a phenol resin or an epoxy resin, an elastic polymer such as silicone rubber or polybutadiene, or the like can be freely selected within a range that does not swell or dissolve in an electrolytic solution used for a battery. In a preferred embodiment, the material used is a cation exchange resin.

[0007] The material of the restricted permeation membrane must have a membrane-forming ability, but for that purpose, it must have a high molecular weight, be a linear polymer, and expand well without agglomeration of molecules in a solvent. That is, it is necessary to use a good solvent as a solvent. As a binder for the active material, polyvinylidene fluoride is generally used with N-methyl-2-vinylpyrrolidone as a solvent. Polyvinylidene fluoride used has a very high molecular weight but poor solubility, so the film forming ability is low, and even if a film can be formed by choosing the conditions, the microcrystal gap is too large and the purpose of the present application is It does not function as a restricted permeable membrane.

[0008] As a molding method, a polymer solution is applied to the electrode, or the electrode is dipped in the solution, then dipped in a non-solvent and solidified, a wet method is used, and instead of dipping in a non-solvent, a dry method is used, Examples of the method include a method of coating a polymer and a reaction method of immersing in a monomer and then heating or immersing in a polymerization catalyst solution. When using a solvent, especially in the case of a dry method, the molecular weight of the solvent is often larger than the required fractionation value, in which case the required fractionation value cannot be obtained or the solvent cannot be completely removed. Therefore, it is also preferable to replace and remove the solvent with a low molecular liquid during drying. When the electrode is immersed in a solution or a molten material, the electrode is immersed under reduced pressure or vacuum in order to minimize the space generated between the graphite and the restricted permeation membrane, and is returned to a line pressure, and then pulled up from the liquid. It is also preferred.

In the wet method, in order to control the fractionation value, there are a polymer concentration, a coagulation temperature, a coagulation liquid composition, drying conditions and the like. Since the fractionation value used in the present invention is smaller than that of a normal film, it is necessary to make the aggregate of the molecules or the crystals as fine as possible. On the other hand, in order to increase the permeation speed of lithium ions, the layer is densified. Coordination of the two issues of not being done is necessary. When using a good solvent as the solvent as much as possible and using a solidifying liquid with high coagulability,
The pores on the layer surface can be reduced and the inner layer can be roughened. Further, when the molten polymer is applied or immersed in the molten polymer, only the cooling rate is controlled, and rapid cooling with a cooled non-solvent is preferred. After completion of the molding, the pore size can be reduced by repeating drying and swelling with the swelling liquid. In this way, a restricted permeable membrane layer having pores smaller than the molecules of the solvent or the swelling liquid can be formed.

The hole referred to here is not necessarily a hole having a circular cross section that opens from the surface toward the inner surface.
Usually refers to the gap between microcrystals or microassemblies,
The rough part of the membrane molecule twists and folds between the surface and the inner surface, and the size varies variously. Some of them are interrupted on the way, but at least a part penetrates both surfaces. The fraction value is almost determined by its largest gap. It is preferable that the restricted permeable membrane is formed by wrapping the entire negative electrode in a liquid-tight manner.

A method for evaluating the fraction value of the formed restricted permeation membrane will be described. Here, the fraction value refers to the maximum value that passes through the membrane. Here, it is expressed by molecular weight. The same film as the limited permeation film formed on the surface of the coated negative electrode is formed on a glass plate or mercury. The formation conditions are exactly the same as those for the negative electrode. The membrane is peeled off from the glass plate or mercury and set, for example, on a dialysis machine. A solution in which a plurality of solutes having different molecular weights is dissolved is placed on one side of the membrane, and a solvent is placed on the other side. If necessary, the solution side may be pressurized in the dialysis device, or an electric field may be applied when the solute is an electrolyte to accelerate the permeation of the solute through the membrane. After some time, the concentration ratio of each solute on both sides of the membrane is analyzed.

The fraction value exists between the maximum molecular weight of the solute that has moved to the solvent side and the minimum molecular weight that has not passed. In the present invention, the fractionation value of the restricted permeable membrane layer is preferably 8
-100, more preferably 10-80. When the molecular weight cut off is further reduced, the passage of lithium ions becomes difficult, and the ability as a battery is lost. Cannot be reached sufficiently. It is preferable to form the membrane so that the permeability of a solute having a molecular weight equal to or less than the fraction value is as high as possible. For that purpose, it is preferable to form the inside of the film roughly. When the fractionation value differs in the thickness direction of the film, the fractionation value of the portion having the smallest fractionation value becomes the fractionation value of the film. Usually, the fractional value of the membrane surface is the smallest, which determines the fractional value of the membrane.

The thickness of the restricted permeable membrane layer is not particularly limited, but is preferably small from the viewpoint of ion transmission speed. Since there are some irregularities on the surface of the negative electrode before the film is formed, the film thickness is hard to be strictly determined, but is about 5 to 100 microns. If it is too thin, the membrane strength will be low. If it is too thick, the electrode will be bulky, and the charging energy density per volume and weight of the battery will be low. The negative electrode on which the restricted permeable membrane layer is formed is incorporated into a battery by a conventional method.

As long as the solvent of the electrolytic solution does not dissolve or invade the restricted permeation membrane, a known solvent can be used as it is. Specifically, ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole, dimethoxyethane, acetonitrile, propionitrile, butyronitrile, valeronitrile, nitriles such as benzonitrile, γ-butyrolactone, etc. Amides, ketones such as 4-methyl-2-pentanone, propylene carbonate, ethylene carbonate, vinylene carbonate, diethyl carbonate,
Carbonate esters such as dimethyl carbonate and methyl-ethyl-carbonate, and singly and a mixture of two or more such as dimethylsulfoxide, dimethylformamide, and sulfolane can be used without the above-mentioned problems. For example, dimethylsulfoxide and dimethylformamide cannot be used when polyacrylonitrile is used as the blind permeable membrane. According to the invention of the present application, when a graphite material is used for the negative electrode, a decomposition reaction of the solvent is very liable to occur, and carbonates, especially propylene carbonate, which have been considered unsuitable for use alone, can be used favorably.

Known electrolytes can also be used without limitation. For example, LiClO ₄ , LiBF
_{4, LiAsF 6, LiPF 6,} LiCF 3 CO 2, L
iCF ₃ SO _{3 and the} like can be mentioned as a part thereof. As the positive electrode, those known as a negative electrode of a lithium secondary battery and a positive electrode of a lithium ion secondary battery can be used as they are. LiCoO ₂ , LiMnO as part of examples of active materials
₂ , LiMn ₂ O ₄ , LiNiO ₂ , and the like. These can be dispersed in a binder solvent together with a conductive agent and a binder, and applied to a current collector such as an aluminum foil to form a positive electrode. Known separators can be widely used.

[0016]

EXAMPLE 30 g of polyvinylidene fluoride was dissolved in 420 ml of N-methyl-2-pyrrolidone, 270 g of natural graphite was added, and the mixture was sufficiently dispersed with an ultrasonic disperser. The obtained dispersion was applied to a copper foil having a thickness of 10 μm. The coating amount after drying was 1.56 g / cm ² . The obtained material was cut into a predetermined size, and the lead wire was welded. on the other hand,
Polyacrylonitrile with a molecular weight of about 60,000
It was dissolved in dimethyl sulfoxide by weight. Here, the coated negative electrode was immersed, pulled up, immersed in a large amount of water at 40 ° C., and solidified. This was repeated three times with drying at 100 ° C. and immersion in water to form a restricted permeable membrane layer.

A limited permeation film was similarly formed on a glass plate instead of the negative electrode. The final drying was performed by peeling the film from the glass plate. A dialyzer was assembled that formed a dialysis chamber on both sides of the membrane that was peeled off from the glass plate and dried. Each dialysis chamber was provided with a platinum electrode. Then, one of the dialysis chambers contains 0.1 mL of CaCl ₂ and ZnCl ₂ respectively.
An aqueous solution dissolved at a concentration of 1 mol / liter was charged, pure water was charged into the other, a voltage of +1 V (with respect to the solution side) was applied to the pure water side, and after 10 hours, the sample on the pure water side was subjected to atomic absorption spectrometry. When the calcium ion concentration and the zinc ion concentration were measured by analysis, 0.012 mol / liter and 0
Mol / l. That is, it became clear that the molecular weight cutoff exists between the calcium ion atomic weight of 40 and the zinc ion molecular weight of 65.4 in the restricted permeable membrane formed on the negative electrode.

The above-mentioned negative electrode is used. A positive electrode is made by coating LiCoO ₂ as a positive electrode active material, acetylene black as a conductive agent, polyvinylidene fluoride as a binder on an aluminum foil as a current collector, and polypropylene as a separator. The porous sheet was used as an electrolyte at 1.5 mol / mol.
A cylindrical battery was assembled using 1 liter of LiPF ₆ propylene carbonate solution. LiCoO used for positive electrode
_The weight ratio of No. ₂ to the natural graphite used for the negative electrode was 3.
It was set to be 0. The battery was charged and discharged repeatedly at a voltage of 2.5 to 4.1 V at a current of 3.7 mA / g with respect to graphite. The initial charge / discharge efficiency ratio (discharge amount / charge amount) was 99.5%, and the discharge capacity after 1000 cycles was 320.3 mAhr / g per natural graphite.

[0019]

Comparative Example 1 Example 1 except that no restricted permeation film was formed.
When a battery manufactured in exactly the same way was subjected to the same test,
The initial charge / discharge efficiency was 70.2%, and the discharge capacity after 1000 cycles was 105.3 mAhr / g per natural graphite.

[0020]

Embodiment 3 The cut and lead-welded negative electrode prepared in Embodiment 1 was used. Polyacrylonitrile having a molecular weight of about 100,000 was dissolved in dimethyl sulfoxide at a concentration of 3%, and the negative electrode was immersed. Then put it at -3 ° C
Immersed in an aqueous solution of dimethyl sulfoxide having a concentration of 50,
Next, it was immersed in a 20% aqueous solution of dimethyl sulfoxide at −3 ° C., and was thoroughly washed with water. Thereafter, it was sufficiently dried at 80 ° C.

On the other hand, a film was formed in exactly the same manner except that the negative electrode was replaced with a glass plate. Drying was performed after peeling the film from the glass plate. The obtained membrane was assembled in the same dialysis apparatus as in Example 1. In one dialysis chamber, water in which 0.05 mol / l of SnCl ₂ was dissolved was charged with pure water, and after applying a voltage of 1 V, the pure water side sample was subjected to atomic absorption analysis 10 hours later. The calcium ion concentration was 0.0015 mol / l. That is, it is understood that the molecular weight cut-off is larger than 118.7 of tin ion. The same battery as in Example 1 was assembled using the negative electrode having the above-described restricted permeation film formed on the surface. Further, the same charge / discharge test as in Example 1 was performed. The initial charge / discharge efficiency was 86.5%, and the discharge capacity after 1000 cycles was 202.6 mAhr / g per graphite weight.

[0022]

According to the present invention, since the diffusion of the solvent to the carbon surface is restricted, the decomposition reaction of the solvent is suppressed, so that the initial charge / discharge efficiency is high and the discharge capacity after repeated charge / discharge is reduced. A high lithium ion secondary battery can be obtained.
Further, the amount of gas generated by decomposition of the electrolytic solution is small, and the pressure in the battery is also small, so that the safety in various accidents is increased. Further, even if activated carbon having a low charge capacity is used as an active material because its surface is occupied by a solvent or solvated lithium ions, a battery with a high charge capacity can be obtained. Furthermore, if the fractionation value of the limiting transmission membrane can be reduced and the cutoff can be sharpened,
It also has the advantage that contact between water and the negative electrode can be prevented, and that much care is not required for dehydration in the electrolytic solution.

Claims (6)

[Claims] 1. An electrode comprising a current collector and a carbon-containing active negative electrode material applied to the current collector as constituent elements, wherein a restricted permeable membrane layer is previously formed on the surface of the electrode. Characteristic negative electrode for lithium ion secondary batteries. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the molecular weight cutoff of the restricted permeable membrane layer is 8 to 100. 3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the molecular weight cutoff of the restricted permeation membrane is 10 to 80. 4. A negative electrode comprising a current collector and an active negative electrode material containing carbon covering the surface thereof, a negative electrode having a restricted permeable membrane layer previously formed on the surface thereof, an electrolyte solution, and a positive electrode. A lithium ion secondary battery characterized by the above-mentioned. 5. The lithium ion secondary battery according to claim 4, wherein the solvent of the electrolyte solution is a carbonate. 6. The lithium ion secondary battery according to claim 5, wherein the molecular weight cutoff of the restricted permeation membrane is 8 to 100.