JPH11126608A - Active material for secondary battery negative electrode and secondary battery - Google Patents

Abstract

(57) [Summary] [Object] To provide a metal ion secondary battery having high charge / discharge efficiency and excellent cycle characteristics. [Constitution] In a metal ion secondary battery, the surface of carbon used as a negative electrode active material is previously covered with a restricted permeable membrane, and a battery is formed using the negative electrode using the same. The molecular weight cut-off of the restricted permeation membrane is 8 to 80, more preferably 10 to 80.
15 is preferred.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode active material for a secondary battery having high performance, particularly high reversible capacity and small 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, the safety problem has not been solved, and the lithium of the negative electrode has been charged with lithium ions. Lithium-ion rechargeable batteries replaced with carbon that emits, metals that form alloys by reversibly absorbing lithium ions, and metal compounds that form composite compounds are being replaced and their usage is rapidly expanding. .
However, they are inferior in energy density as compared with lithium secondary batteries, and an increase in capacity is desired.

[0003] One of the reasons that the energy density, particularly the discharge capacity is not sufficient, is that in the first charge / discharge cycle, a decomposition reaction of the electrolytic solution occurs on the surface of the negative electrode active material, consuming lithium ions and causing a passivation film (SEI; solid). select
It is said to form a role 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 layers of graphite, its formation is considered to consume not only lithium ions but also the lithium-containing portion of graphite, 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.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a metal ion secondary battery having high coulomb efficiency in the first charge / discharge cycle, particularly a lithium ion secondary battery. 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.

[0005]

Means for Solving the Problems The first invention of the present application is a negative electrode active material for a secondary battery made of carbon coated on the surface with a restricted permeable membrane, and the second invention is that the secondary battery is a lithium ion secondary material. 3. The active material for a negative electrode of a secondary battery according to claim 1, wherein the active material is a battery.
The active material for a negative electrode of a secondary battery according to claim 2, wherein the molecular weight cutoff of the restricted permeation membrane is 10 to 15.
The second invention is the secondary battery anode according to the first aspect, wherein the carbon is graphite. The sixth invention is the graphite forming an intercalation compound. 7. The negative electrode active material for a secondary battery according to claim 5, wherein the seventh invention comprises a positive electrode, a negative electrode having carbon as a negative electrode active material whose surface is coated with a restricted permeable membrane, and an electrolytic solution. An eighth invention is the secondary battery according to claim 7, wherein the electrolyte is a lithium ion battery containing a lithium salt, and a ninth invention is a secondary battery in which the range of the fractionation value of the restricted permeable membrane is limited. The secondary battery according to claim 8, which has a molecular weight of 10 to 80, and the tenth invention is the secondary battery according to claim 8, wherein the molecular weight cut-off of the supermembrane is 10 to 15, and the eleventh invention. 11. The secondary battery according to claim 10, wherein the solvent of the electrolyte is water.

As a result, a decrease in the initial charge / discharge efficiency is suppressed, the discharge capacity is increased, and not only cycle characteristics are improved, but also safety is improved. If the molecular weight cutoff is between lithium and water, the use of water as a solvent is also possible in lithium ion secondary batteries, and when water is used,
Compared to the case of using an organic solvent that is flammable by itself,
The type of electrolyte can be selected from a wide range of lithium salts, and the safety against needle sticks, overcharge, overdischarge, crushing, burner heating, impact, and the like can be increased.

[0007]

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 their graphitized products, mesocarbon microbeads (MCMB), vapor-grown carbon fibers or their graphitized products, carbon nanotubes, activated carbon, amorphous carbon, and other various carbons can be used. . From the viewpoint of the charging energy density of the battery, a battery mainly composed of a graphite structure capable of forming an intercalation compound is preferable, and when used at a large current, a material that has been ground or cut to some extent is preferable.
However, the presence of the fine powder indicates that when a restricted permeation membrane layer is formed, a carbon ratio is reduced when a restricted permeation membrane is formed with respect to its high specific surface area, and the battery ratio per negative electrode weight and per negative electrode volume is reduced. It is not preferable because the capacity is reduced. Those having an average particle size or fiber diameter of 0.5 μm or more and a specific surface area of 10 m ² / g or less are preferred. On the surface of these carbon powders or grains, a restricted permeable membrane layer is previously formed and coated. Preferably, the surface is completely covered by the layer.

[0008] The material of the restricted permeable membrane is not particularly limited as long as it is insoluble in the common electrolyte 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. Regardless of the material, a molecular weight and a linear form that can form a film on the carbon surface are required. The choice of solvent also greatly affects the ability to form a film. A so-called good solvent, which can take a form in which polymer molecules are sufficiently elongated in a solution, is selected according to the polymer used. Polyvinylidene fluoride commonly used as a binder for an electrode active material is N-methyl-
When 2-pyrrolidone is used as a solvent, the film-forming ability is poor, and only a bridge is formed between active material particles. Even if the membrane can be formed by selecting the conditions, the molecular weight cut-off is very large and does not substantially act as the restricted permeation membrane intended in the present application.

As a method for forming the restricted permeable membrane layer, a wet method in which carbon is immersed in a membrane material solution and then pulled up, and then immersed in a non-solvent while being stirred to solidify, or agitated instead of immersed in the non-solvent A dry method of drying while immersing in a film-forming monomer and then heating with stirring or immersing in a polymerization catalyst solution while stirring, and mixing with a melt of a film material and throwing it into a high-speed stirred cold non-solvent For example, a melting method. When a solvent is used, particularly in the case of a dry method, the molecular weight of the solvent is often larger than the required fractionation value, and the required fractionation value may not be obtained or may not be completely removed. It is also preferable to perform displacement removal with a low molecular liquid before or during the process.

When carbon is immersed in a solution or monomer of the membrane material, the carbon is immersed under reduced pressure or vacuum to reduce the space between the surface of the carbon particles and the restricted permeation membrane as much as possible, and then returned to normal pressure. It is also preferred to pull up from the solution or monomer afterwards. 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 fractional molecular weight which is preferable in the present invention is smaller than the value of a normal membrane, it is necessary to make the aggregate or crystal of the membrane material molecules as small as possible. In order to increase the size, it is necessary to adjust the two issues of not densifying the layer. When a good solvent is used as much as possible and a coagulating liquid having a high coagulability is used, the pores on the surface of the layer can be reduced and the inner layer can be roughened. After the solidification is completed, the pore diameter can be reduced by repeating drying and swelling with the swelling liquid.
Thus, a restricted permeable membrane layer having pores smaller than molecules of the solvent or the swelling liquid can be formed.

The hole mentioned here does not necessarily mean a hole having a circular cross section opening 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 determined by its largest part. It is preferable that the restricted permeable membrane is formed by wrapping the whole of the powder or the grains. If the viscosity of the solution, melt, or the mixture of monomer and carbon is high and there is stickiness to the current collector of the electrode, apply it in that state, then coagulate, cool, polymerize, etc. A restricted permeable membrane layer can also be formed.

A method for evaluating the fractional value of the formed restricted permeation membrane will be described. Here, the molecular weight cutoff refers to the molecular weight of the largest molecule or ion passing through the membrane. The same membrane as the restricted permeation membrane formed on the surface of the coated carbon is formed on a glass plate or mercury. The formation conditions are exactly the same as those for forming on the carbon surface. The formed film is peeled off from the glass plate or mercury, and is set in, for example, a dialysis device.
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 or ion on both sides of the membrane is analyzed. The fractionation value is between the maximum molecular weight of the solute or ion transferred 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 depends on the metal ion used,
Preferably it is 10-80. The molecular weight cutoff is preferably larger than the molecular weight of the metal that transports electrons, and smaller than the molecular weight of the solvent that hinders battery characteristics. When the electrolyte contains water, the molecular weight cut-off must be 15 or less.

It is preferable to form the membrane so that the permeability of the molecular weight below the fraction value is as high as possible. 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 the 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 0.1 to 5 microns.

[0014] Carbon as a negative electrode active material having a restricted permeable membrane layer formed on its surface is 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, but copper foil is usually used as a negative electrode because its ions do not elute from the material. The negative electrode thus formed is assembled into a battery by a conventional method.

As long as the solvent for the electrolyte used in the battery is not dissolved or attacked by 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, 4-methyl-2
-Ketones such as pentanone, propylene carbonate, ethylene carbonate, vinylene carbonate, diethyl carbonate, dimethyl carbonate, methyl-
Carbonic esters such as ethyl-carbonate, and dimethylsulfoxide, dimethylformamide, sulfolane and the like can be used alone or as a mixture of two or more as long as the above-mentioned problems are not caused. For example, when polyacrylonitrile is used as the restricted permeation membrane, it is natural that dimethyl sulfoxide and dimethylformamide cannot be used. 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.

Further, the fractionation value of the restricted permeation membrane is 10 to 15
However, in the case where water does not pass through, and the molecular weight is smaller than the molecular weight of water such as lithium ion and easily passes through the restricted permeable membrane, water can be used for a part or all of the solvent. In that case, if the presence of water causes a problem in the positive electrode, it is possible to form a restricted permeation film in the positive electrode as well.

Known electrolytes can also be used without limitation. For example, in the case of a lithium ion secondary battery, LiClO ₄ , LiBF ₄ , LiAsF ₆ , L
iPF ₆ , LiSbF ₆ , LiAlCI ₄ , LiSC
N, LiCF ₃ SO _{3 and the} like can be mentioned as a part thereof. Although salts of other metals can be used, when water is used as a solvent, lithium halides and other lithium salts other than those described above can be widely used as lithium salts. The metal on the cation side is not limited to lithium as long as it is reversibly absorbed by carbon and exhibits a negative potential, and is reversibly absorbed by the positive electrode and exhibits a positive potential.

The active material for the positive electrode can reversibly absorb and release metal ions, and various compounds can be used as long as the standard electrode potential in the state of absorbing metal ions is positive. In the case of a lithium secondary battery, those known as a positive electrode of a lithium secondary battery and a positive electrode of a lithium ion secondary battery are directly used, for example, LiCoO ₂ , LiMnO ₂ , Li
Mn ₂ O ₄ , LiNiO ₂ or the like can be used. When water is not used as the solvent, a composite oxide of a metal other than lithium may be used depending on the salt for the electrolytic solution to be used. As usual, a positive electrode is prepared by dispersing in a binder solvent together with a binder and a conductive agent, and coating the resultant on a current collector such as an aluminum foil. Known separators can be widely used.

[0019]

EXAMPLE 1 A solution of polyacrylonitrile having a molecular weight of about 40,000 and a dimethyl sulfoxide solution having a concentration of 15% by weight was prepared, and natural graphite having a particle size of about 3 μm was charged therein.
After sufficient stirring, the mixture was taken out, applied to a 10 μm thick copper foil used as a current collector for an electrode, and immersed in methanol at −10 ° C. After solidifying sufficiently, take out, wash with water,
Dry at 00 ° C, then immerse in 50 ° C water
After drying at 50 ° C, immersion in water at 50 ° C and drying at 100 ° C were alternately repeated three times to obtain a negative electrode. On the other hand, in a dish containing mercury,
A copper foil having a thickness of 10 μm with a hole having a diameter of 5 mm is floated, and a dimethyl sulfoxide solution of the above-mentioned polyacrylonitrile is allowed to flow over the copper foil so that the hole overflows from the hole.
A large amount of cold methanol was gently poured into the flask. The copper foil was removed from the mercury and treated as described above. This copper foil was assembled in a dialyzer. Electrodes were provided in the dialysis chamber on both sides of the foil. One dialysis chamber was charged with an aqueous solution having both a concentration of lithium chloride and potassium chloride of 3 mol / liter, and the other was charged with pure water, and a voltage of plus 1 V was applied to the solution side.
After 24 hours, a sample taken from the pure water side was subjected to atomic absorption spectroscopy. As a result, the presence of 0.061 mol / liter of lithium was confirmed, but the presence of potassium was not recognized.

The above-mentioned negative electrode was used. The positive electrode was coated with LiCoO ₂ as a positive electrode active material, acetylene black as a conductive agent, polyvinylidene fluoride as a binder, and 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.
The battery was assembled using 1 liter of LiPF ₆ propylene carbonate solution. LiCoO ₂ used for the positive electrode,
The weight ratio to natural graphite used for the negative electrode was set to be 3.0. The battery is operated with a current of 2.
Charging and discharging were repeated at a current of 0 mA / g at a voltage of 2.5 to 4.1 V. The initial charge / discharge efficiency (discharge amount / charge amount) was 99.1%, and the charge amount was 37% per graphite weight of the negative electrode.
The discharge amount at 0 mAhr / g and the 1000th charge / discharge cycle was 310 mAhr / g.

[0021]

Example 2 The same battery was assembled as in Example 1 except that the electrolytic solution was changed to a 2 mol / L aqueous solution of lithium chloride. Initial charge amount is 240 mAhr / g, discharge capacity is 2
It was 33 mAhr / g. The discharge capacity after repeating charge and discharge 1000 times was 192 mAhr / g per graphite weight.

[0022]

Comparative Example 1 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. Using this negative electrode, a battery was assembled in the same manner as in Example 1, and a similar test was performed. The first charge capacity is 373
mAhr / g, the discharge capacity was 292 mAhr / g, and the discharge capacity at the 1000th charge / discharge cycle was 132 m
Ahr / g.

[0023]

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 discharge capacity is high, the charge / discharge efficiency is high, and the cycle characteristics are also high. A good secondary battery can be provided. Further, when activated carbon or the like was used as the negative electrode active material, the metal ions contacted the carbon surface or penetrated into the internal pores while being solvated, so that a low charge capacity could not be obtained, but according to the present application, High capacity is obtained due to limited permeation of solvents and solvated ions. Further, in some cases, it is possible to use water as the electrolyte solvent. Even a lithium ion secondary battery, which is considered safer than a lithium secondary battery, may ignite in a needlestick test due to the organic solvent used in the electrolyte, but aqueous electrolytes are not flammable. No flammable gas is generated, heating from outside, temperature rise due to external short circuit, temperature rise due to internal short circuit due to needle stick or impact deformation, overcharge,
It is highly safe against overdischarge and the danger caused by such an accident can be prevented.

Claims (11)

[Claims] An active material for a negative electrode of a secondary battery, comprising carbon coated on the surface with a restricted permeable membrane. 2. The active material for a negative electrode of a secondary battery according to claim 1, wherein the secondary battery is a lithium ion secondary battery. 3. The active material for a negative electrode of a secondary battery according to claim 2, wherein the molecular weight cutoff of the restricted permeation membrane is from 8 to 80. 4. The active material for a negative electrode of a secondary battery according to claim 2, wherein the molecular weight cutoff of the restricted permeation membrane is 10 to 15. 5. The secondary battery negative electrode according to claim 1, wherein the carbon is graphite. 6. The graphite according to claim 5, wherein said graphite forms an intercalation compound.
The active material for a secondary battery negative electrode according to the above. 7. A secondary battery comprising a positive electrode, a negative electrode having carbon as a negative electrode active material, the surface of which is coated with a restricted permeable membrane, and an electrolytic solution. 8. The secondary battery according to claim 7, wherein the electrolyte is a lithium ion battery containing a lithium salt. 9. The secondary battery according to claim 8, wherein the molecular weight cutoff of the restricted permeation membrane is from 8 to 80. 10. The secondary battery according to claim 8, wherein the molecular weight cutoff of the restricted permeation membrane is 10 to 15. 11. The secondary battery according to claim 10, wherein the solvent of the electrolytic solution is water.