JP6741641B2 - Imide polymer solution - Google Patents

Description

The present invention relates to an imide-based polymer solution having excellent safety, high capacity, and good charge/discharge cycle characteristics, which can be used for producing an electrode for a lithium secondary battery.

In a lithium secondary battery, scratches or irregularities on the surface of the electrode may cause the electric insulation of the separator in contact with the electrode to be destroyed. As a result, an internal electrical short circuit may occur.

In order to prevent this internal short circuit, it has been proposed to provide a protective layer made of an insulating porous film on the electrode surface. The porous film serving as the protective layer is formed of a water-soluble polymer (cellulose derivative, polyacrylic acid derivative, polyvinyl alcohol derivative, etc.), a fluorine resin, a rubber resin, etc. , Patent Documents 1 to 4 propose a porous membrane having pores formed by mixing a large amount of fine particles such as zirconia.

As another method for forming the protective layer, after forming a coating film for forming the protective layer on the electrode surface, before drying it, dip it in a coagulation bath containing a poor solvent to cause phase separation of the coating film. A method for obtaining a porous protective layer has also been proposed (Patent Documents 5 and 6).

On the other hand, in a lithium secondary battery using a high-capacity active material such as silicon, a spirally wound electrode body in which a positive electrode and a negative electrode are spirally wound via a separator is used as a prismatic (square tubular) outer can. It is general that the battery is constructed by loading the inside of the outer casing of the laminated film or the laminated film. In that case, the capacity may decrease with repeated charging/discharging, or the thickness may greatly increase due to swelling of the battery. In order to improve such a problem, a large amount of fine particles such as silicon dioxide and alumina are blended on the outer surface of the active material layer of the electrode (negative electrode) to form an imide polymer such as polyimide having pores formed therein. A method has been proposed in which a porous layer is provided to mitigate the volume change and deformation of the electrode (Patent Document 7).

International Publication 1997/008763 Japanese Patent No. 5071056 Japanese Patent No. 5262323 Japanese Patent No. 5370356 Japanese Patent No. 3371839 Japanese Patent No. 3593345 JP, 2011-233349, A

The electrode provided with the porous layer on the surface as described above, because the adhesion between the active material layer and the porous layer is low, the effect of preventing a short circuit is not always sufficient, and to ensure the safety of the battery. There are points to be improved from the viewpoint. The ion permeability of the porous protective layer is also not sufficient. Such an electrode does not sufficiently relax stress due to a change in the volume of the active material, and therefore the cycle characteristics of the electrode are not necessarily sufficiently improved. Further, in the electrode obtained by the method of causing phase separation using a coagulation bath containing a poor solvent such as water or alcohol, the entire active material layer is in contact with the coagulation bath, so that the poor solvent has the original characteristics of the active material layer. May be damaged. Further, with this method, since a waste liquid containing a poor solvent is generated from the coagulation bath, there is a problem as a manufacturing method from the viewpoint of environmental compatibility.

Therefore, the present invention is to solve the above problems, by improving the adhesiveness between the porous layer and the active material layer, lithium lithium having excellent safety, high discharge capacity and good cycle characteristics. An object is to provide an electrode for a secondary battery and a method for manufacturing the same.

The present inventors have achieved the above-mentioned problems by using as a electrode a laminate in which an ion-permeable porous layer formed of an imide polymer having a specific porosity is provided on the outer surface of an electrode active material layer. The inventors have found that they can be solved and have completed the present invention.

The present invention has the following gist.

1) A imide polymer solution, a good solvent in which the solvent is against the imide polymer, a mixed solvent of a poor solvent is higher boiling than the good solvent, poor solvent, ether solvents And the compounding amount of the ether solvent is 40 to 90% by mass with respect to the total amount of the solvent, and the imide polymer solution for forming an ion-permeable porous layer on the surface of the electrode active material layer. ..

2) On the surface of the electrode active material layer, the imide polymer solution of 1) is applied to form a coating film, and thereafter, when the solvent in the coating film is removed, a poor solvent remaining in the coating film. laminated integrally acting together with the phases separated within a coating film allowed to form an ion-permeable porous layer consisting of an imide-based polymer by using, and the electrode active material layer and the ion-permeable porous layer of A method for manufacturing an electrode for a lithium secondary battery, comprising:

Since the electrode for a lithium secondary battery of the present invention does not need to contain a large amount of fine particles such as alumina and silicon dioxide particles in order to form the pores of the ion-permeable porous membrane, It is possible to improve the cushioning property and ensure good adhesion between the porous layer and the active material layer. Therefore, it can be suitably used as an electrode for a lithium secondary battery having excellent safety, high discharge capacity and good cycle characteristics. Moreover, in the manufacturing method of the present invention, the electrode of the present invention can be easily manufactured by a simple process.

FIG. 4 is a cross-sectional view of an electrode in which an imide porous layer is laminated and integrated on the outer surface of a positive electrode active material layer. It is an expanded sectional view of the electrode of FIG. It is a figure which shows the external appearance of what peeled the positive electrode active material layer from the imide porous layer. FIG. 4 is an enlarged view of a portion where the positive electrode active material layer in FIG. 3 is almost peeled off. FIG. 4 is an enlarged view of a portion where the positive electrode active material layer in FIG. 3 remains.

The lithium secondary battery electrode of the present invention is formed by laminating and integrating an ion-permeable porous layer formed of an imide-based polymer and having a porosity of 30 to 90% by volume on the outer surface of the electrode active material layer. It was formed. The lithium secondary battery electrode is an electrode constituting a lithium ion secondary battery, and is a positive electrode in which a positive electrode active material layer is bonded to a positive electrode current collector, or a negative electrode active material layer is bonded to a negative electrode current collector. The negative electrode is called. The electrode active material layer is a general term for the positive electrode active material layer and the negative electrode active material layer.

As the current collector, a metal foil such as a copper foil, a stainless foil, a nickel foil, or an aluminum foil can be used. Aluminum foil is preferably used for the positive electrode and copper foil is preferably used for the negative electrode. The thickness of these metal foils is preferably 5 to 50 μm, more preferably 9 to 18 μm. The surface of these metal foils may be subjected to a roughening treatment or a rustproofing treatment for improving the adhesiveness with the active material layer.

The positive electrode active material layer is a layer obtained by binding the positive electrode active material particles with a resin binder. The material used as the positive electrode active material particles is preferably a material that can store and store lithium ions, and examples thereof include those commonly used as the positive electrode active material of a lithium secondary battery. For example, oxides (LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4, etc.), complex oxides (LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , Li(LiaNixMnyCoz) O _2, etc.), phosphoric acid Examples of the active material particles include iron-based (LiFePO ₄ , Li ₂ FePO ₄ F, etc.) and polymer compound-based (polyaniline, polythiophene, etc.). Among these, LiCoO ₂ , LiNiO ₂ , and LiFePO ₄ are preferable. In order to reduce the internal resistance of the positive electrode active material layer, conductive particles such as carbon (graphite, carbon black, etc.) particles and metal (silver, copper, nickel, etc.) particles are blended in an amount of about 1 to 30% by mass. May be.

The negative electrode active material layer is a layer obtained by binding the negative electrode active material particles with a resin binder. The material used as the negative electrode active material particles is preferably a material that can store and store lithium ions, and examples thereof include materials commonly used as a negative electrode active material for lithium secondary batteries. For example, graphite, amorphous carbon, silicon-based, tin-based active material particles and the like can be mentioned. Of these, graphite particles and silicon-based particles are preferable. Examples of the silicon-based particles include particles of silicon simple substance, silicon alloy, silicon/silicon dioxide composite, and the like. Among these silicon-based particles, particles of simple silicon (hereinafter sometimes abbreviated as “silicon particles”) are preferable. Single silicon means crystalline or amorphous silicon having a purity of 95% by mass or more. In order to reduce the internal resistance of the negative electrode active material layer, conductive particles such as carbon (graphite, carbon black, etc.) particles and metal (silver, copper, nickel, etc.) particles are blended in an amount of about 1 to 30% by mass. May be.

The particle diameter of the active material particles and the conductive particles is preferably 50 μm or less, and more preferably 10 μm or less for both the positive electrode and the negative electrode. On the contrary, if the particle diameter is too small, the binding with the resin binder becomes difficult, so that the particle diameter is usually 0.1 μm or more, preferably 0.5 μm or more.

The porosity of the electrode active material layer is preferably 5 to 50% by volume, and more preferably 10 to 40% by volume for both the positive electrode and the negative electrode.

The thickness of the electrode active material layer is usually about 20 to 200 μm.

Examples of the above-mentioned resin binder for binding the active material particles include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and styrene-butadiene copolymer. Examples thereof include rubber, polytetrafluoroethylene, polypropylene, polyethylene, imide polymer and the like. Among these, polyvinylidene fluoride, styrene/butadiene copolymer rubber, and imide polymer are preferable.

In the electrode of the present invention, an ion-permeable imide porous layer is laminated and integrated on the outer surface of the electrode active material layer.

The imide-based polymer forming the imide porous layer is a polymer having an imide bond in its main chain or a precursor thereof. Typical examples of the polymer having an imide bond in the main chain include polyimide, polyamideimide, polyesterimide and the like. However, it is not limited to these.

Among the imide-based polymers, for example, polyimide or polyamide-imide can be preferably used. As the polyimide, a polyamic acid type polyimide using polyamic acid as its precursor (applied to a polyimide that becomes insoluble in a solvent when the polyimide is used) or a soluble polyimide (soluble in a solvent as a polyimide) can be used. Among these imide polymers, aromatic polyimides and aromatic polyamideimides having excellent mechanical properties and heat resistance are preferable from the viewpoint of ensuring excellent safety and good cycle characteristics of the lithium secondary battery electrode. The aromatic polyimide or the aromatic polyamideimide may be thermoplastic or non-thermoplastic. Among them, aromatic polyimide or aromatic polyamideimide having a glass transition temperature of 200° C. or higher can be preferably used.

It is essential that the porosity of the imide porous layer in the present invention is 30 to 90% by volume. It is preferably 40 to 80% by volume, and more preferably 45 to 80% by volume. By setting the porosity in this manner, good mechanical properties and good cushioning properties for stress relaxation due to volume change of the active material are simultaneously secured. Therefore, it is possible to obtain an electrode having excellent safety and good cycle characteristics. The porosity of the imide porous layer is a value calculated from the apparent density of the imide porous layer and the true density (specific gravity) of the imide polymer forming the imide porous layer. Specifically, the porosity (volume %) is calculated by the following formula when the apparent density of the imide porous layer is A (g/cm ³ ) and the true density of the imide-based polymer is B (g/cm ³ ). To be done.
Porosity (% by volume) = 100-A* (100/B)

The imide porous layer in the present invention is preferably firmly bonded to the active material layer. That is, from the viewpoint of improving the safety of the battery, the adhesive strength between the electrode active material layer and the imide porous layer is preferably higher than the strength of the electrode active material layer. Whether the adhesive strength is higher than the strength of the electrode active material layer is determined by whether cohesive failure or interfacial peeling occurs at the interface when the electrode active material layer is peeled from the imide porous layer. You can When cohesive failure occurs, it is determined that the strength of the adhesive interface is higher than the strength of the electrode active material layer. When a fragment of the active material layer adheres to a part of the surface of the imide porous layer after peeling (adhesive surface to the electrode active material layer), it is determined to be cohesive failure. Conventionally, such an electrode that undergoes cohesive failure is not known, and in the electrode of the present invention, such a high adhesive force greatly contributes to the improvement of the safety of the battery.

The average pore diameter of the imide porous layer in the present invention is preferably 0.1 to 10 μm, more preferably 0.5 to 5 μm. By setting the average pore diameter in this way, good ion permeability is secured. Whether the ion permeability is good or not can be determined from the permeation time of the solvent when the solvent for the electrolytic solution constituting the battery is dropped on the electrode surface. The details of the determination method will be described later. In the electrode of the present invention, this penetration time is preferably 300 seconds or less, more preferably 150 seconds or less.

The thickness of the imide porous layer in the present invention is preferably 1 to 100 μm, more preferably 10 to 50 μm.

The imide porous layer in the present invention may be either insulating or conductive. When the imide porous layer is insulative, it is convenient because this layer also has a function as a separator that prevents electrical contact between the positive electrode and the negative electrode of the lithium secondary battery. When the imide porous layer is made conductive, for example, conductive particles such as carbon (graphite, carbon black, etc.) particles and metal (silver, copper, nickel, etc.) particles, etc. It may be added to the layer. From the viewpoint of ensuring the cushioning property and adhesiveness of the imide porous layer, the content of these conductive particles is preferably 20% by mass or less.

Next, a method for manufacturing the electrode for a lithium secondary battery of the present invention will be described.

For example, the electrode for a lithium secondary battery of the present invention can be manufactured by the following process.

(1) A dispersion containing the above-mentioned binder, active material particles, and a solvent (hereinafter, may be abbreviated as “active material dispersion”) is applied to the surface of a metal foil, which is a current collector, and dried. An electrode active material layer is formed on the metal foil.

(2) Next, on the surface of the electrode active material layer, a coating liquid containing an imide polymer capable of forming an imide porous layer by phase separation and a solvent (hereinafter, abbreviated as "imide coating liquid"). Yes) is applied.

(3) After that, by removing the solvent in the coating liquid, phase separation is caused in the imide porous layer to form pores in the imide porous layer, and at the same time, the electrode active material layer and the imide porous layer are formed. Are laminated and integrated.

Upon drying for forming the electrode active material layer, the residual solvent content in the active material layer is preferably 0.5 to 50% by mass. By doing so, the strength of the adhesive interface between the electrode active material layer and the imide porous layer can be improved.

In order to form an imide porous layer by phase separation using an imide polymer, for example, a poor solvent induced phase separation method can be preferably used. The poor solvent-induced phase separation method refers to a method of inducing phase separation and expressing a porous structure by utilizing the action of a solvent that becomes a poor solvent for a solute in a coating liquid.

As the poor solvent-induced phase separation method, a dry phase separation method is preferable from the viewpoint of simplicity of manufacturing process and environmental compatibility. The dry phase separation method, when drying and solidifying a coating film of an imide-based coating liquid composed of a mixed solvent of a good solvent and a poor solvent having different boiling points, by utilizing the action of the poor solvent remaining in the coating film A method of causing phase separation.

The imide-based coating liquid used in the dry phase separation method is a good solvent that dissolves the imide-based polymer, which is a solute, when the above-mentioned polyamic acid, soluble polyimide, and solution-polymerized polyamideimide are manufactured by solution polymerization. This can be easily obtained by using a mixed solvent having a boiling point higher than that of the good solvent and having a solute mixed with a poor solvent. A good solvent means a solvent having a solubility of 1% by mass or more in an imide-based polymer at 25°C, and a poor solvent means a solvent having a solubility of less than 1% by mass in an imide-based polymer at 25°C.
The boiling point difference between the good solvent and the poor solvent is preferably 5° C. or higher, more preferably 20° C. or higher, even more preferably 50° C. or higher.

An amide-based solvent is preferably used as the good solvent. Examples of the amide solvent include N-methyl-2-pyrrolidone (NMP boiling point: 202°C), N,N-dimethylformamide (DMF boiling point: 153°C), N,N-dimethylacetamide (DMAc boiling point: 166°C). Is mentioned. These may be used alone or in combination of two or more.

An ether solvent is preferably used as the poor solvent. Examples of the ether solvent include diethylene glycol dimethyl ether (boiling point: 162° C.), triethylene glycol dimethyl ether (boiling point: 216° C.), tetraethylene glycol dimethyl ether (boiling point: 275° C.), diethylene glycol (boiling point: 244° C.), triethylene glycol (Boiling point: 287° C.) and the like. These may be used alone or in combination of two or more. The compounding amount of the poor solvent is preferably 40 to 90% by mass, and more preferably 60 to 80% by mass with respect to the total amount of the solvent. With such a solvent composition, strong adhesion between the imide porous layer and the active material layer as described above can be obtained.

Examples of the imide-based coating liquid are the product name "U-imide varnish BP" (polyamic acid type polyimide varnish), the product name "U-imide varnish SP" (soluble polyimide varnish), which are commercially available from Unitika Ltd. for forming a porosity The name "U imide varnish IP" (polyamide imide varnish) and the like can be mentioned.

The imide-based coating solution composed of a polyamic acid solution or a soluble polyimide solution used in the dry phase separation method may be a commercially available product described above, but the raw materials tetracarboxylic dianhydride and diamine are mixed in approximately equimolar amounts. However, a polyamic acid solution or a soluble polyimide solution obtained by polymerizing it in the mixed solvent described above is also preferably used. Further, after a polymerization reaction only in a good solvent to obtain a solution, a method of adding a poor solvent to it, or after a polymerization reaction only in a poor solvent to obtain a suspension, a method of adding a good solvent to this Then, an imide-based coating liquid can be obtained.

Examples of the tetracarboxylic acid dianhydride include pyromellitic acid, 3,3′,4,4′-biphenyltetracarboxylic acid, 3,3′,4,4′-benzophenonetetracarboxylic acid, 3,3′, 4,4'-diphenylsulfone tetracarboxylic acid, 3,3',4,4'-diphenyl ether tetracarboxylic acid, 2,3,3',4'-benzophenone tetracarboxylic acid, 2,3,6,7-naphthalene Tetracarboxylic acid, 1,4,5,7-naphthalene tetracarboxylic acid, 1,2,5,6-naphthalene tetracarboxylic acid, 3,3′,4,4′-diphenylmethane tetracarboxylic acid, 2,2-bis (3,4-dicarboxyphenyl)propane, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane, 3,4,9,10-tetracarboxyperylene, 2,2-bis[4-( A dianhydride such as 3,4-dicarboxyphenoxy)phenyl]propane or 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]hexafluoropropane is used. These may be used alone or in combination of two or more. Among these, pyromellitic acid and 3,3′,4,4′-biphenyltetracarboxylic acid are preferable.

Examples of the diamine include p-phenylenediamine, m-phenylenediamine, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane and 3,3'-dimethyl-4,4. ′-Diaminodiphenylmethane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 1,2-bis(anilino)ethane, diaminodiphenyl sulfone, diaminobenzanilide, diaminobenzoate, diaminodiphenyl sulfide, 2, 2-bis(p-aminophenyl)propane, 2,2-bis(p-aminophenyl)hexafluoropropane, 1,5-diaminonaphthalene, diaminotoluene, diaminobenzotrifluoride, 1,4-bis(p-amino) Phenoxy)benzene, 4,4'-bis(p-aminophenoxy)biphenyl, diaminoanthraquinone, 4,4'-bis(3-aminophenoxyphenyl)diphenyl sulfone, 1,3-bis(anilino)hexafluoropropane, 1 1,4-bis(anilino)octafluorobutane, 1,5-bis(anilino)decafluoropentane, and 1,7-bis(anilino)tetradecafluoroheptane are used. These may be used alone or in combination of two or more. Among these, p-phenylenediamine, 4,4'-diaminodiphenyl ether and 2,2-bis[4-(4-aminophenoxy)phenyl]propane are preferable.

The solid content concentration of the polyamic acid in the polyimide precursor solution is preferably 1 to 50% by mass, more preferably 5 to 25% by mass. The polyamic acid contained in the polyimide precursor solution may be partially imidized. The viscosity of the polyimide precursor solution at 30° C. is preferably 1 to 150 Pa·s, more preferably 5 to 100 Pa·s.

The imide-based coating liquid consisting of the polyamide-imide solution used in the dry phase separation method may be a commercially available product as described above, but the raw materials trimellitic anhydride and diisocyanate are blended in approximately equimolar amounts, and A solution obtained by carrying out a polymerization reaction in the mixed solvent is also preferably used. Further, after a polymerization reaction only in a good solvent to obtain a solution, a method of adding a poor solvent to it, or after a polymerization reaction only in a poor solvent to obtain a suspension, a method of adding a good solvent to this It is also possible to obtain an imide-based coating liquid composed of a polyamide-imide solution.

As the trimellitic acid anhydride, those partially substituted with pyromellitic acid anhydride, benzophenonetetracarboxylic acid anhydride, or biphenyltetracarboxylic acid anhydride may be used.

Examples of the diisocyanate include m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 4,4′-diphenyl ether diisocyanate, diphenyl sulfone-4,4′-diisocyanate, diphenyl-4,4′-diisocyanate, o-Tolidine diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, xylylene diisocyanate and naphthalene diisocyanate are used. These may be used alone or in combination of two or more. Among these, 4,4'-diphenylmethane diisocyanate is preferable.

The solid content concentration of the polyamideimide in the polyamideimide solution is preferably 1 to 50% by mass, more preferably 10 to 30% by mass.

The viscosity at 30° C. of the polyamide-imide solution is preferably 1 to 150 Pa·s, more preferably 5 to 100 Pa·s.

If necessary, known additives such as various surfactants and organic silane coupling agents may be added to the imide-based coating liquid within a range that does not impair the effects of the present invention. If necessary, a polymer other than the imide-based polymer may be added to the imide-based coating liquid within a range that does not impair the effects of the present invention.

The imide-based coating liquid is applied to the surface of the electrode active material layer, dried at 100 to 150° C., and then heat-treated at 250 to 350° C. if necessary, so that the porosity is 30 to 90% by volume. It is possible to simultaneously form the porous layer and integrate the electrode active material layer and the imide porous layer. At this time, the porosity can be adjusted to 30 to 90% by volume by selecting the type and blending amount of the solvent (good solvent and poor solvent) in the imide-based coating liquid. The porosity can also be adjusted by selecting the drying conditions.

If necessary, it is preferable that the surface of the obtained imide porous layer is subjected to physical polishing treatment such as sand blast treatment or scratch blast treatment, or chemical etching treatment. As a result, the surface area of the imide porous layer is increased and the porosity is increased, so that good ion permeability of the imide porous layer can be secured.

When applying the active material dispersion or the imide-based coating liquid, a roll-to-roll continuous coating method or a sheet-like coating method can be employed, and any method is applicable. As the coating device, a die coater, a multilayer die coater, a gravure coater, a comma coater, a reverse roll coater, a doctor blade coater, or the like can be used.

As described above, the electrode of the present invention can be easily manufactured by a simple process.

Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to the examples.

The electrode active material layers (for the positive electrode and the negative electrode) formed on the current collector used in the following Examples and Comparative Examples were obtained as follows.

(Cathode active material layer)
86 parts by mass of LiFePO ₄ particles (average particle size 0.5 μm) that is a positive electrode active material, 8 parts by mass of carbon black (acetylene black) that is a conductive additive, and 6 parts by mass of polyvinylidene fluoride that is a binder resin are used as a solvent. Was uniformly dispersed in N-methylpyrrolidone to obtain a positive electrode active material dispersion. This dispersion was applied to a positive electrode current collector having a thickness of 15 μm of aluminum foil, and the obtained coating film was dried at 130° C. for 10 minutes and then hot pressed to obtain a positive electrode active material layer having a thickness of 50 μm.

(Negative electrode active material layer)
Silicon particles (average particle size 0.7 μm) that are negative electrode active materials, graphite particles (average particle size 0.7 μm) that are conductive agents, and polyamic acid solution that is a binder resin (manufactured by Unitika Ltd., trade name “U-imide varnish”). CR" and a solid content concentration of 18% by mass) were uniformly dispersed in N-methylpyrrolidone (NMP) to obtain a negative electrode active material dispersion having a solid content concentration of 25% by mass. The mass ratio of the silicon particles, the graphite particles and the polyamic acid solution was 70:10:20. This dispersion was applied to a copper foil having a thickness of 18 μm, which is a negative electrode current collector, and the obtained coating film was dried at 120° C. for 10 minutes to obtain a negative electrode active material layer having a thickness of 40 μm. 22% by mass of NMP remained in this active material layer.

The characteristics and the like of the electrodes obtained in the following examples and comparative examples were evaluated by the following methods.

(1) Ion permeability 5 μL of a mixed solvent of ethylene carbonate, ethylmethyl carbonate and dimethyl carbonate (volume ratio 1:1:1) set at 30° C. is dropped onto the surface of the electrode, which completely penetrates. This was visually observed to measure the permeation time, and the permeation time was used to evaluate the ion permeability.

(2) Adhesiveness The electrode active material layer was forcibly peeled by hand in the opposite direction of 180 degrees from the laminated and integrated product of the electrode active material layer and the imide porous layer. At that time, the quality of the adhesiveness was determined by whether or not the fragments of the electrode active material layer adhered to a part of the surface (adhesion surface with the electrode active material) of the imide porous layer after peeling. That is, when the fragments are attached, peeling is unlikely to occur at the interface between the electrode active material layer and the imide porous layer, and they are cohesively destroyed, so the adhesiveness between the electrode active material layer and the imide porous layer is " It was determined to be “good”. Further, when the fragments were not attached, peeling occurred at the interface, and therefore the adhesiveness was determined to be “poor”.

<Example 1>
Approximately equimolar amounts of trimellitic anhydride (TMA) and 4,4'-diphenylmethane diisocyanate (DMI), 30 parts by mass of N-methylpyrrolidone (NMP) as a good solvent, and tetraethylene glycol dimethyl ether 70 as a poor solvent. The reaction was carried out in a mixed solvent with parts by mass to obtain a uniform polyamideimide solution (P-1) having a solid content concentration of 15% by mass. This solution was applied to the outer surface of the positive electrode active material layer described above, dried at 130° C. for 10 minutes, and then the surface was subjected to polishing treatment to form an imide porous layer having a thickness of 23 μm on the outer surface of the positive electrode active material layer. An electrode (positive electrode) "C-1" that was laminated and integrated was obtained. Table 1 shows the evaluation results of the obtained electrodes.

The SEM images of the cross section of the positive electrode “C-1” are shown in FIGS. In FIG. 1, three layers are shown above and below. The lowermost layer is a positive electrode current collector, the intermediate layer is a positive electrode active material layer, and the uppermost layer is an imide porous layer. FIG. 2 shows the interface between the positive electrode active material layer and the imide porous layer and its vicinity. From these figures, it is understood that the average pore diameter of the imide porous layer is about 3 μm.

3 to 5 show SEM images of the surface of the imide porous layer on the side in contact with the active material layer when the active material layer of the positive electrode "C-1" was forcibly peeled by hand in the opposite direction by 180 degrees. .. From FIG. 3, it can be seen that, after the peeling, the portion where the active material layer is almost peeled off and the portion where the fragment of the active material layer remains coexist. FIG. 4 shows an enlarged SEM image of the part with the number “1” in FIG. 3 (where the active material layer is almost peeled off). From this SEM image, it can be seen that a large number of pores are present on the surface of the imide porous layer at the interface. FIG. 5 shows an enlarged SEM image of the portion of the number “2” in FIG. 3 (where the fragments of the active material layer remain). From this SEM image, it can be seen that many pores are present in the active material layer at the interface. It is considered that the pores present at the interface shown in FIGS. 4 and 5 contribute to the favorable ion permeability of the positive electrode “C-1”.

<Example 2>
Approximately equimolar amounts of trimellitic anhydride and 4,4′-diphenylmethane diisocyanate are reacted in a mixed solvent of 25 parts by mass of NMP and 75 parts by mass of tetraethylene glycol dimethyl ether to give a solid concentration of 10% by mass. A uniform polyamide-imide solution (P-2) was obtained. This solution was applied to the outer surface of the positive electrode active material layer described above, dried at 130° C. for 10 minutes, and then the surface was subjected to polishing treatment to form an imide porous layer having a thickness of 20 μm on the outer surface of the positive electrode active material layer. An electrode (positive electrode) "C-2" that was integrally laminated was obtained. Table 1 shows the evaluation results of the obtained electrodes.

<Example 3>
Approximately equimolar amounts of trimellitic anhydride and 4,4'-diphenylmethane diisocyanate are reacted in a mixed solvent of 35 parts by mass of NMP and 65 parts by mass of tetraethylene glycol dimethyl ether to give a solid concentration of 17% by mass. A uniform polyamide-imide solution (P-3) was obtained. This solution was applied to the outer surface of the positive electrode active material layer described above, dried at 130° C. for 10 minutes, and then the surface was subjected to polishing treatment to form an imide porous layer having a thickness of 25 μm on the outer surface of the positive electrode active material layer. An electrode (positive electrode) “C-3” integrated in a laminated form was obtained. Table 1 shows the evaluation results of the obtained electrodes.

<Comparative Example 1>
Approximately equimolar amounts of trimellitic anhydride and 4,4′-diphenylmethane diisocyanate are reacted in a mixed solvent of 65 parts by mass of NMP and 35 parts by mass of tetraethylene glycol dimethyl ether to give a solid concentration of 17% by mass. A uniform polyamide-imide solution (P-4) was obtained. This solution was applied to the outer surface of the positive electrode active material layer described above, dried at 130° C. for 10 minutes, and then the surface was subjected to polishing treatment to form an imide porous layer having a thickness of 25 μm on the outer surface of the positive electrode active material layer. An electrode (positive electrode) "C-4" that was laminated and integrated was obtained. Table 1 shows the evaluation results of the obtained electrodes.

<Comparative example 2>
Approximately equimolar amounts of trimellitic anhydride and 4,4'-diphenylmethane diisocyanate were reacted in NMP to obtain a uniform polyamideimide solution (P-5) having a solid content concentration of 19% by mass. This solution was applied to the outer surface of the positive electrode active material layer described above, dried at 130° C. for 10 minutes, and then the surface was subjected to polishing treatment to form an imide porous layer having a thickness of 25 μm on the outer surface of the positive electrode active material layer. An electrode (positive electrode) "C-5" which was laminated and integrated was obtained. Table 1 shows the evaluation results of the obtained electrodes.

<Comparative example 3>
Alumina particles having an average particle size of 0.5 μm were uniformly mixed and dispersed in the polyamideimide solution (P-5) obtained in Comparative Example 2 to obtain an alumina filler dispersion (P-6) having a solid content concentration of 25% by mass. It was The mass ratio of polyamideimide and alumina particles was 5:95 (polyamideimide:alumina particles). This dispersion was applied to the outer surface of the positive electrode active material layer described above and dried at 130° C. for 10 minutes, whereby a 25 μm-thick imide porous layer was laminated and integrated on the outer surface of the positive electrode active material layer. An electrode (positive electrode) "C-6" was obtained. Table 1 shows the evaluation results of the obtained electrodes.

<Example 4>
Approximately equimolar amounts of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA) were used as N,N-dimethylacetamide ( 20 parts by mass of DMAc) and 80 parts by mass of tetraethylene glycol dimethyl ether as a poor solvent were reacted to obtain a uniform polyamic acid solution (P-7) having a solid content concentration of 15% by mass. This solution is applied to the outer surface of the above-mentioned negative electrode active material layer, dried at 130° C. for 10 minutes, heat-treated at 300° C. for 120 minutes to convert polyamic acid into polyimide, and then the surface is polished to obtain a thickness. To obtain an electrode (negative electrode) “A-1” in which an imide porous layer having a thickness of 23 μm was laminated and integrated on the outer surface of the negative electrode active material layer. Table 1 shows the evaluation results of the obtained electrodes.

Next, the battery characteristics of this negative electrode "A-1" were evaluated. Specifically, this negative electrode was punched out into a circular shape having a diameter of 14 mm, a separator made of a polypropylene porous film and a lithium foil were sequentially laminated on the imide porous surface side, and this was placed in a stainless coin-shaped outer container. Stowed. An electrolytic solution (solvent: mixed solvent in which ethylene carbonate, ethylmethyl carbonate, and dimethyl carbonate were mixed at a volume ratio of 1:1:1, electrolyte: 1MLiPF ₆ ) was injected into the outer container, and the outer container was filled. A polypropylene cap was used to cover and fix a 0.2 mm thick stainless steel cap, and the battery can was sealed, and a cell with a diameter of 20 mm and a thickness of about 3.2 mm for evaluating discharge capacity and cycle characteristics was used. Obtained. Using the obtained cell, a charging/discharging cycle of charging at 30° C. to a constant current of 0.05 C to 2 V and discharging to a constant current of 0.05 C to 0.02 V was performed. As a result, the initial discharge capacity of the negative electrode "A-1" was 2200 [mAh/g-active material layer], and the discharge capacity after 10 cycles was 2050 [mAh/g-active material layer], which was a high initial discharge capacity and good. Good cycle characteristics were confirmed.

<Example 5>
Almost equimolar amounts of 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 4,4′-oxydianiline in a mixed solvent of 30 parts by mass of DMAc and 70 parts by mass of triethylene glycol dimethyl ether. To obtain a uniform polyamic acid solution (P-8) having a solid content concentration of 15% by mass. This solution is applied to the outer surface of the above-mentioned negative electrode active material layer, dried at 130° C. for 10 minutes, heat-treated at 300° C. for 120 minutes to convert polyamic acid into polyimide, and then the surface is polished to obtain a thickness. To obtain an electrode (negative electrode) “A-2” in which an imide porous layer having a thickness of 23 μm was laminated and integrated on the outer surface of the negative electrode active material layer. Table 1 shows the evaluation results of the obtained electrodes.

<Example 6>
A commercially available polyimide precursor varnish for forming a porous film containing a polyamic acid obtained by reacting pyromellitic dianhydride and 4,4′-oxydianiline (“Uimide varnish BP” manufactured by Unitika Ltd.: P -9) is applied to the outer surface of the above-mentioned negative electrode active material layer, dried at 130° C. for 10 minutes, heat-treated at 300° C. for 120 minutes to convert the polyamic acid into polyimide, and then the surface is subjected to polishing treatment, An electrode (negative electrode) “A-3” was obtained in which a 25 μm-thick imide porous layer was laminated and integrated on the outer surface of the negative electrode active material layer. Table 1 shows the evaluation results of the obtained electrodes.

<Comparative example 4>
Almost equimolar amounts of 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 4,4′-oxydianiline in a mixed solvent of 70 parts by mass of DMAc and 30 parts by mass of tetraethylene glycol dimethyl ether. To obtain a uniform polyamic acid solution (P-10) having a solid content concentration of 15% by mass. This solution is applied to the outer surface of the above-mentioned negative electrode active material layer, dried at 130° C. for 10 minutes, heat-treated at 300° C. for 120 minutes to convert polyamic acid into polyimide, and then the surface is polished to obtain a thickness. To obtain an electrode (negative electrode) “A-4” in which an imide porous layer having a thickness of 20 μm was laminated and integrated on the outer surface of the negative electrode active material layer. Table 1 shows the evaluation results of the obtained electrodes.

<Comparative Example 5>
Approximately equimolar amounts of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 4,4'-oxydianiline are reacted in DMAc to give a solid concentration of 15% by mass. A uniform polyamic acid solution (P-11) was obtained. This solution is applied to the outer surface of the above-mentioned negative electrode active material layer, dried at 130° C. for 10 minutes, heat-treated at 300° C. for 120 minutes to convert polyamic acid into polyimide, and then the surface is polished to obtain a thickness. To obtain an electrode (negative electrode) “A-5” in which an imide porous layer having a thickness of 18 μm was laminated and integrated on the outer surface of the negative electrode active material layer. Table 1 shows the evaluation results of the obtained electrodes.

<Comparative example 6>
Alumina particles having an average particle diameter of 0.5 μm were uniformly mixed and dispersed in the polyamic acid solution (P-10) obtained in Comparative Example 4 to obtain an alumina filler dispersion (P-12) having a solid content concentration of 25% by mass. Got The mass ratio of polyamideimide to alumina particles was 5:95 (polyamideimide:alumina particles). This dispersion was applied to the outer surface of the above-mentioned negative electrode active material layer, dried at 130° C. for 10 minutes, and heat-treated at 300° C. for 120 minutes to convert polyamic acid into polyimide, and to form a 25 μm thick imide porous film. An electrode (negative electrode) “A-6” in which the layers were laminated and integrated on the outer surface of the negative electrode active material layer was obtained. Table 1 shows the evaluation results of the obtained electrodes.

<Comparative Example 7>
Almost equimolar amounts of 3,3′,4,4′-biphenyltetracarboxylic dianhydride and 4,4′-oxydianiline in a mixed solvent of 30 parts by mass of NMP and 70 parts by mass of γ-butyrolactone. The reaction was carried out to obtain a uniform polyamic acid solution (P-13) having a solid content concentration of 15% by mass. This solution is applied to the outer surface of the negative electrode active material layer, dried at 130° C. for 10 minutes, heat-treated at 300° C. for 120 minutes to convert the polyamic acid into polyimide, and then the surface is polished to obtain a thickness. To obtain an electrode (negative electrode) “A-7” in which an imide porous layer having a thickness of 20 μm was laminated and integrated on the outer surface of the negative electrode active material layer. Table 1 shows the evaluation results of the obtained electrodes.

<Comparative Example 8>
Approximately equimolar amounts of 3,3',4,4'-biphenyltetracarboxylic dianhydride and 4,4'-oxydianiline were reacted in diethylene glycol dimethyl ether to give a solid content of 15% by mass. An attempt was made to obtain a uniform polyamic acid solution (P-14). However, it was not possible to obtain a uniform solution.

As described above, as shown in Examples and Comparative Examples, the lithium secondary battery electrode of the present invention is an amide solvent as a good solvent for the imide polymer, an ether solvent having a higher boiling point than the amide solvent as a poor solvent. Since the dry phase separation method using is used, good ion permeability can be secured. Further, since the lithium secondary battery electrode of the present invention does not need to be mixed with a large amount of fine particles such as alumina and silicon dioxide particles in order to form the pores of the ion permeable porous membrane, the ion permeable porous electrode Good adhesion between the layer and the active material layer can be ensured. Therefore, it can be suitably used as an electrode for a lithium secondary battery having excellent safety, high discharge capacity and good cycle characteristics. Further, according to the manufacturing method of the present invention, the electrode can be easily manufactured by a simple process having high environmental compatibility.

Claims (2)

A imide polymer solution, a good solvent in which the solvent is against the imide polymer, a mixed solvent of a poor solvent is higher boiling than the good solvent, the poor solvent is an ether solvent An imide-based polymer solution for forming an ion-permeable porous layer on the surface of the electrode active material layer, characterized in that the blending amount of the ether-based solvent is 40 to 90 mass% with respect to the total amount of the solvent. The surface of the electrode active material layer is coated with the imide-based polymer solution according to claim 1 to form a coating film, and thereafter, when the solvent in the coating film is removed, a poor residue remaining in the coating film. Using the action of a solvent to cause phase separation in the coating film to form an ion-permeable porous layer made of an imide polymer, and stacking the electrode active material layer and the ion-permeable porous layer A method for manufacturing an electrode for a lithium secondary battery, characterized by being integrated.