JPH11149942A - Manufacturing method of porous composite electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain increase in resistance of an electrode by immersing an electrode in which an active substance where a polymer solution consisting of a solvent and a polymer having compatibility to water carried on a surface as is main component integrally molded with an aluminum base substance in a water solution containing alcohol. SOLUTION: An electrode body is immersed in a polymer solution in which a polymer is dissolved in an organic solvent having compatibility with water. Thereby, the polymer solution uniformly permeates into a hole in an active substance layer of the electrode body. Next, when an electrode in which an active substance where the polymer solution is carried on a surface is integrally molded with the aluminum base substance is soaked in alcohol-added water, an organic solvent of the polymer solution is extracted in the water, the polymer is solidified in a state in which the thus solvent-removed part becomes porous, the porous polymer is uniformly formed at the hole in the active substance layer. With this method, a good electrode without increase in resistance can be fabricated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a porous composite electrode having a porous polymer formed on the surface.

[0002]

2. Description of the Related Art Lithium batteries and lithium-ion batteries (hereinafter referred to as lithium-based batteries) are different from lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries, etc., in which an aqueous solution is used as an electrolyte. Various safety elements such as a safety valve, a protection circuit, a PTC element, and the like are provided to further ensure safety in using the electrolyte. However, this raises a problem that the cost is increased, and in order to solve this problem, an attempt has been made to use a solid polymer electrolyte having less chemical reactivity instead of the organic electrolyte. The use of the solid polymer electrolyte is a useful technique that is effective in solving the above-mentioned problems and at the same time providing flexibility in battery shape, simplifying a manufacturing process, reducing manufacturing costs, and the like.

[0003] As a solid polymer electrolyte,
Many studies have been made on ion-conductive polymers composed of complexes of polyethers such as polyethylene oxide and polypropylene oxide with alkali metal salts.
However, with polyether, it is difficult to obtain high ionic conductivity while maintaining sufficient mechanical strength, and the conductivity is greatly affected by temperature, so that sufficient conductivity cannot be obtained at room temperature. Recently, comb-type polymers having polyethers in the side chains, copolymers of polyether chains and other monomers, polysiloxanes or polyphosphazenes having polyethers in the side chains, cross-linked polyethers, etc. Is being considered.

On the other hand, as another form of solid polymer electrolyte, studies have been made on a gel-like solid electrolyte obtained by impregnating a polymer with an electrolytic solution. In this gel-like solid electrolyte, as a polymer, polyacrylonitrile, polyvinylidene fluoride, polyvinyl chloride, polyvinyl sulfone, polyvinylpyrrolidinone, or the like is used. For the purpose of facilitating the impregnation of the electrolytic solution and improving the electrical conductivity, a copolymer of vinylidene fluoride and hexafluoropropylene is used after further measures such as using a copolymer. In addition, a solid polymer electrolyte obtained by impregnating a polymer film formed by drying a latex such as nitrile rubber, styrene butadiene rubber, polybutadiene, polyvinylpyrrolidone or the like with an electrolyte has been studied.

One of the methods of using such a solid polymer electrolyte is to form a solid polymer electrolyte on the surface of an electrode active material. By using this method, it is possible to improve the safety of the electrode. Become. in this case,
In order for ions to move smoothly in the battery, the active material particles and the solid polymer electrolyte need to be in good contact with each other as much as possible, and it is necessary to sufficiently fill the pores of the active material layer with the polymer. There is. In addition, even when a solid polymer electrolyte and a free electrolyte are used in combination, it is possible to fill the pores of the active material layer with as much polymer as possible in order to reduce the amount of the free electrolyte and improve safety. desirable.

However, in the solid polymer electrolyte, ions are less likely to move than in a conventional electrolyte in a liquid phase, and the above-described electrode has a problem that the resistance at the electrode portion is increased. Therefore, in order to solve such a problem, for example, as described in Japanese Patent Application Laid-Open No. 09-219197, the solid polymer electrolyte in the electrode hole is made porous, Electrodes that facilitate the movement of ions as a whole by efficient ion diffusion are being studied. According to this electrode, the amount of free electrolyte can be reduced, and the resistance at the electrode portion can be reduced.

[0007] As one method for producing a porous polymer, a polymer solution obtained by dissolving a polymer in a solvent is immersed in a solvent that is insoluble and compatible with the solvent. There is a solvent extraction method in which a solvent is extracted, and the portion from which the solvent has been removed becomes pores to solidify the polymer to obtain a porous polymer. According to this method, since a communication hole composed of spherical uniform holes can be formed, a solvent extraction method is suitable as a method for producing an electrode having a porous solid polymer electrolyte as described above.

[0008]

When an electrode having a porous solid polymer electrolyte is prepared using the above-mentioned solvent extraction method, the most inexpensive solvent that is insoluble in a polymer and compatible with a solvent is used. It is preferable to use a large amount of available water.

However, when aluminum is used as the electrode base material, the performance of the electrode is unexpectedly poor, and as a result of investigating the cause, the aluminum of the base reacts with water and an aluminum oxide film grows on the electrode surface. It was found that the resistance of the manufactured electrode itself increased. Since the increase in the resistance of the electrode is directly related to the increase in the internal resistance of the battery when assembled as a battery, it was an important issue to stop the increase in the resistance.

[0010]

The present inventor has conducted various studies to solve the above-mentioned problems, and as a result, has found that the addition of alcohol to water can suppress the increase in the resistance. The present invention has led to the present invention.

[0011] That is, the first invention of the present application provides an electrode formed by integrally forming an active material having a surface on which a polymer solution mainly composed of a solvent having compatibility with water and a polymer is formed with an aluminum base, with an alcohol. A method for producing a porous composite electrode, characterized by immersing the composite electrode in an aqueous solution containing When the electrode obtained by the present invention is used as an electrode for a secondary battery, the polymer preferably functions as a solid polymer electrolyte, and more preferably swells with an electrolytic solution to become a gel-like ionic conductive polymer. good.

The second invention of the present application is the method for producing a porous composite electrode according to the above-mentioned production method, wherein the active material is a general formula LiMO ₂ (M is a transition metal element). In this case, the effect of the present invention appears more remarkably.

The third invention of the present application is the method for producing a porous composite electrode according to the first or second invention, wherein the solvent of the polymer solution is NMP. Also in this case, the effect of the present invention appears more remarkably.

A fourth invention of the present application is the method for producing a porous composite electrode according to the first, second or third invention, wherein the alcohol is methanol. In this case, more preferably, the content of methanol in the aqueous solution is 1 to 70 wt%.

The sixth invention of the present application is characterized by a method for producing an electrode in which the active material carrying the polymer solution according to the above inventions on the surface is formed integrally with an aluminum substrate. A method for producing a porous composite electrode, comprising forming an electrode by integrally molding an aluminum substrate and immersing the same in the polymer solution.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The invention of the present application will be further described below using the positive electrode used in a lithium secondary battery as an embodiment.

As the positive electrode active material used in the lithium secondary battery, for example, a composition formula LixMO ₂ or Liy
M ₂ O ₄ (where M is a transition metal, 0 ≦ x ≦ 1, 0 ≦ y ≦
Complex oxides, oxides having tunnel-like vacancies, and metal chalcogenides having a layered structure represented by 2) can be used. Specific examples thereof include LiCoO ₂ , LiN
iO ₂ , LiMn ₂ O ₄ , Li ₂ Mn ₂ O ₄ , MnO ₂ , FeO ₂ , V ₂ O
₅ , V ₆ O ₁₃ , TiO ₂ , TiS _{2 and the} like.

The solid polymer electrolyte which fills the pores of the active material layer produced using these active materials has an elasticity capable of changing its shape following the volume expansion and contraction of the active material due to charge and discharge. Therefore, it is preferable to use a gel-like ion-conductive polymer obtained by swelling a polymer with an electrolytic solution. Specifically, as the polymer, for example, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, polyethylene oxide, polyether such as polypropylene oxide, polyacrylonitrile, polyvinylidene fluoride, polyvinylidene chloride, polymethyl methacrylate, polymethyl Acrylate, polyvinyl alcohol, polymethacrylonitrile,
Polyvinyl acetate, polyvinyl pyrrolidone, polyethylene imine, polybutadiene, polystyrene, polyisoprene, or a derivative thereof can be used alone or as a mixture. Further, a polymer obtained by copolymerizing various monomers constituting the polymer can also be used.

In order to produce an electrode in which an active material having a polymer solution supported on its surface is formed integrally with an aluminum substrate using the above active material and a polymer, for example, active material particles and a conductive material such as carbon are used. By applying a paste formed by kneading the binder and the binder to a thin aluminum plate and drying the paste, first, an electrode body in which the active material is integrally formed with the aluminum base is produced. Next, the electrode body is immersed in a polymer solution obtained by dissolving the polymer in an organic solvent compatible with water. As a result, the polymer solution uniformly penetrates into the holes in the active material layer of the electrode body.

Next, the electrode obtained by molding the active material having the surface of the polymer solution thus obtained and formed integrally with the aluminum substrate is immersed in water to which alcohol has been added. Extracted inside,
As a result, the polymer is solidified in a state where the portion from which the solvent has been removed becomes a pore, and a uniformly porous polymer is formed in the pores in the active material layer. Moreover, the holes are spherical and uniform,
Moreover, pores communicating from the polymer surface to the active material surface can be formed.

As the alcohol, various alcohols such as methanol and ethanol can be used, but methanol is preferably used, and more preferably 1 to 70 wt%.

When the electrode thus obtained is immersed in an electrolytic solution for a lithium secondary battery, the electrolytic solution swells the polymer, and the polymer becomes a gel-like ionic conductive polymer. Thus, a porous composite positive electrode for a lithium secondary battery comprising the substance is completed.

By the above method, a good electrode can be obtained without increasing the resistance. One of the reasons that such an effect can be obtained is that in the drying step of the positive electrode plate after the solvent extraction of the polymer solution, the presence of alcohol having a low boiling point allows water molecules to be quickly removed from the electrode, thereby increasing the temperature. The progress of the formation of the aluminum film below is suppressed.

The increase in resistance when conventional water is used is more likely to occur when an active material represented by the general formula LiMO ₂ (M is a transition metal element) is used. This tends to appear when an active material that contains an alkaline component and elutes the alkaline component in water is used. In addition, it easily appears when an organic solvent that elutes an alkaline component in water is used as the organic solvent of the polymer solution. Therefore, when such a material is used, the present invention exerts its effect remarkably, and suppresses an increase in the resistance of the electrode.

[0025]

EXAMPLES (Example 1) A positive electrode plate of a battery using an aluminum current collector will be described below as a preferred example of the present invention.

First, a mixture of 70% by weight of lithium cobalt oxide, 6% by weight of acetylene black, and 15% by weight of n-methyl-pyrrolidone (NMP) in which 9% by weight of polyvinylidene fluoride (PVdF) was dissolved was used.
m, a length of 480 mm and a thickness of 20 μm were applied to an aluminum foil, dried at 90 ° C. to evaporate NMP.
The above operation was performed on both surfaces of the aluminum foil to prepare an electrode body. Thereafter, a polymer solution in which 4 wt% of PVdF was dissolved in NMP was supported on the electrode body. After this,
The electrode body was immersed in water containing 1 to 70 wt% of methanol to perform solvent extraction. After removing this electrode,
Drying was performed at 130 ° C. Thereafter, pressing was performed to obtain a positive electrode (A). The thickness of the positive electrode after pressing was 170 μm. The total weight of the active material and the conductive agent filled per unit area was 23 mg / cm ² . As a comparative example,
Water was used as a solvent for extraction of NMP, and the subsequent treatment was the same as the above-described positive electrode (A), and an electrode (B) was prepared. FIG. 1 shows the results of measuring the surface resistance of these porous polymer electrolyte composite positive electrodes. According to the measurement result, NMP
It can be seen that the positive electrode (A) of the present invention, from which NMP was extracted using water containing methanol, and the positive electrode (A) had lower surface resistance than the positive electrode (B) according to the conventional treatment in which the extraction was performed.
From FIG. 1, when methanol is used, preferably 25 to
It can be seen that 70 wt%, more preferably around 50 wt% is good.

Similarly, when ethanol is used instead of methanol, the amount is preferably 60 to 80 wt%, more preferably about 70 wt%.

Next, a battery manufactured using the positive electrode (A) of Example 1 will be described.

The negative electrode was manufactured as follows. Graphite 81wt%, PVDF 9wt%, NMP10wt
% On a copper foil with a thickness of 14 μm,
Drying at 150 ° C. evaporated the NMP. After performing the above operation on both sides of the copper foil, press,
A negative electrode was used. The thickness of the negative electrode after pressing was 190 μm.

A 26 .mu.m thick polypropylene microporous separator (trade name: DURAGA
C) and wound up, height 47.0 mm, width 22.2 m
m, inserted into a stainless steel case with a thickness of 6.4 mm,
A prismatic battery was assembled. Thereafter, 2.5 g of a mixed (volume 1: 1) electrolyte solution of ethylene carbonate containing 1 M LiPF ₆ and diethyl carbonate was added.
Performs time aging treatment, with a nominal capacity of 400 mAh,
The battery (C) of the present invention was manufactured. As a comparative example, a battery (D) was manufactured through the same steps using the positive electrode (B). FIG. 2 shows the results of the internal resistance measurement test after aging. According to this result, the internal resistance of the battery (C) of the present invention in which NMP was extracted with water containing methanol was lower than that of the battery (D) having a conventionally known positive electrode in which NMP was extracted with water. Recognize.

Next, the discharge performance of the battery will be described.

The above-mentioned battery (C) of the present invention and conventionally known battery (D) were subjected to a current of 1 CA at room temperature.
The battery was charged to 1 V, subsequently charged at a constant voltage of 4.1 V for 2 hours, and then discharged to 2.5 V at a current of 1 CA. FIG. 3 shows the results of this discharge test. These results show that the battery (C) according to the present invention exhibited better discharge performance than the conventionally known battery (D) using water.

From these results, it can be said that the battery manufactured according to the present invention solves the problem of the conventional battery that has been extracted using water, and exhibits excellent performance.

In the present invention, the porous ion-conductive organic polymer electrolyte was prepared on the positive electrode plate by a wet method using water containing methanol. As a result, the internal resistance of the battery was lower than that of the conventional method using water, and the discharge was reduced. A battery with good performance can be manufactured.

In the above embodiment, graphite is used as a compound as a negative electrode material.
alloys of lithium with l, Si, Pb, Sn, Zn, Cd and the like, transition metal composite oxides such as LiFe ₂ O ₃ and WO ₂ , W
O _2, MoO transition metal oxides such as _2, graphite, carbonaceous materials such as carbon, Li ₅ (Li ₃ N) lithium nitride such or metal lithium foil, or may be a mixture thereof.

Further, in the above embodiment, a polypropylene microporous separator is used, but the present invention is not limited to this. For example, a polyethylene microporous membrane or a polymer constituting the organic polymer electrolyte may be used. A configured separator may be used.

Further, as the electrolytic solution, a mixed (volume 1: 1) electrolytic solution of ethylene carbonate containing 1M LiPF ₆ and diethyl carbonate is used. However, the present invention is not limited to this. For example, ethylene carbonate, propylene carbonate , Dimethyl carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide,
Dimethylacetamide, 1,2-dimethoxyethane,
1,2-diethoxyethane, tetrahydrofuran, 2-
A polar solvent such as methyl tetrahydrofuran, dioxolan, methyl acetate, or a mixture thereof may be used.

Further, in the above embodiment, LiPF ₆ is used as a salt to be contained in the electrolytic solution. In addition, for example, LiBF ₄ , LiAsF ₆ , LiClO ₄ ,
SCN, LiI, LiCF ₃ SO ₃ , LiCl, LiB
r, a lithium salt such as LiCF ₃ CO ₂ or a mixture thereof may be used.

In the above embodiment, methanol is used as the alcohol, but ethanol, isopropyl alcohol, etc. may be used.

Further, in the above embodiment, LiC was used as the compound capable of occluding and releasing the alkali metal as the positive electrode material.
Although oO ₂ was used, it is not limited to this.
In addition, as the inorganic compound, for example, a composition formula Li
xMO ₂ or LiyM ₂ O ₄ (where M is a transition metal, 0 ≦ x
.Ltoreq.1, 0.ltoreq.y.ltoreq.2), a composite oxide, an oxide having tunnel-like vacancies, and a metal chalcogenide having a layered structure can be used. As a specific example, LiC
oO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , Li ₂ Mn ₂ O ₄ , MnO ₂ ,
Examples include FeO ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , and TiS ₂ .

Further, in the above embodiment, polyvinylidene fluoride is used as the polymer in the organic polymer electrolyte. However, the polymer is not limited to polyvinylidene fluoride. For example, polyvinyl chloride, polyacrylonitrile, polyethylene oxide, polypropylene oxide Such as polyether, polyacrylonitrile, polyvinylidene fluoride, polyvinylidene chloride, polymethyl methacrylate,
Polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinyl pyrrolidone, polyethylene imine, polybutadiene,
Polystyrene, polyisoprene, or derivatives thereof may be used alone or as a mixture. Also,
A polymer obtained by copolymerizing various monomers constituting the organic polymer may be used.

[0042]

According to the present invention, the resistance of the electrode can be suppressed by reducing the corrosion of aluminum on the electrode substrate. When the electrode according to the manufacturing method of the present invention is used for a battery, the internal resistance is reduced. As a result, the performance of the battery can be improved.

[Brief description of the drawings]

FIG. 3 is a characteristic diagram showing a relationship between a methanol concentration and an increase in surface resistance.

FIG. 5 is a characteristic diagram showing a relationship between a methanol concentration and an internal resistance of a battery before and after aging.

FIG. 5 is a characteristic diagram showing a discharge curve after 4.1 V charging of a battery (C) using an electrode according to the present invention and a battery (D) using a conventional electrode.

[Procedure amendment]

[Submission date] February 24, 1998

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Change

[Correction contents]

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a relationship between a methanol concentration and an increase in surface resistance.

FIG. 2 is a characteristic diagram showing a relationship between a methanol concentration and a battery internal resistance before and after aging.

FIG. 3 is a characteristic diagram showing a discharge curve after 4.1 V charging of a battery (C) using an electrode according to the present invention and a battery (D) using a conventional electrode.

Claims (6)

[Claims] 1. An electrode, in which an active material having a polymer solution mainly composed of a solvent having compatibility with water and a polymer as a main component, formed on a surface of an aluminum substrate is immersed in an aqueous solution containing alcohol. A method for producing a porous composite electrode, comprising: 2. The method for producing a porous composite electrode according to claim 1, wherein the active material is a general formula LiMO ₂ (M is a transition metal element). 3. The method for producing a porous composite electrode according to claim 1, wherein the solvent is NMP. 4. The method for producing a porous composite electrode according to claim 1, wherein the alcohol is methanol. 5. The method according to claim 1, wherein the content of methanol in the aqueous solution is 1-7.
The method for producing a porous composite electrode according to claim 4, wherein the content is 0 wt%. 6. After an active material and an aluminum substrate are integrally formed, the electrode is immersed in the polymer solution to form an electrode in which the active material having the polymer solution supported on the surface is integrally formed with the aluminum substrate. The method for producing a porous composite electrode according to any one of claims 1, 2, 3, 4, and 5, wherein the method is performed.