JP6403576B2 - Method for producing electrode for lithium secondary battery - Google Patents

Description

  The present invention relates to an electrode for a lithium secondary battery having excellent safety, high capacity and good charge / discharge cycle characteristics, and a method for producing the same.

  In a lithium secondary battery, the electrical insulation of the separator in contact with the electrode may be destroyed due to scratches or irregularities on the electrode surface. As a result, an electrical internal 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.), fluorine-based resin, rubber-based resin, etc., and alumina, silicon dioxide, etc. In addition, porous membranes in which pores are formed by blending a large amount of fine particles such as zirconia have been proposed (Patent Documents 1 to 4).

  As another method for forming a protective layer, after a coating film for forming a protective layer is formed on the electrode surface, it is immersed in a coagulation bath containing a poor solvent before drying to cause phase separation of the coating film. A method of 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 wound electrode body in which a positive electrode and a negative electrode are wound in a spiral shape via a separator is used as a rectangular (square tube) outer can. In general, the battery is configured by being loaded inside a laminate film outer package. In that case, the capacity may decrease with repeated charging and discharging, or the thickness may increase greatly due to battery swelling. In order to improve such a problem, an imide polymer such as polyimide having pores formed by mixing a large amount of fine particles such as silicon dioxide and alumina on the outer surface of the active material layer of the electrode (negative electrode). A method has been proposed in which a porous layer is provided to mitigate electrode volume changes and deformation (Patent Document 7).
Patent Document 1: International Publication No. 1997/008763 Patent Document 2: Japanese Patent No. 5071056
Patent Literature 3: Japanese Patent No. 5262323 Patent Literature 4: Patent No. 5370356 Patent Literature 5: Patent No. 3371839 Patent Literature 6: Patent No. 3593345 Patent Literature 7: Japanese Patent Laid-Open No. 2011-233349

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

  Accordingly, the present invention solves the above-described problems, and improves the adhesion between the porous layer and the active material layer, thereby improving the safety of the lithium secondary battery having high discharge capacity and good cycle characteristics. It aims at providing the electrode for secondary batteries, and its manufacturing method.

  The present inventors have solved the above problem by using, as an electrode, a laminate in which an ion-permeable porous layer formed of an imide-based polymer having a specific porosity is provided on the outer surface of the electrode active material layer. As a result, the present invention has been completed.

  The present invention has the following objects.

1) A method for producing an electrode for a lithium secondary battery, wherein a dispersion containing a binder, active material fine particles, and a solvent is applied to the surface of a metal foil that is a current collector, and dried. Then, an electrode active material layer is formed on the surface of the electrode active material layer, and then a solvent obtained by mixing an imide polymer, a good solvent amide solvent and a poor solvent ether solvent on the surface of the electrode active material layer. A coating liquid is formed by applying a coating liquid containing the coating liquid, and then, when the solvent in the coating film is removed, phase separation is caused in the coating film by utilizing the action of the poor solvent remaining in the coating film. When the electrode active material layer and the imide polymer porous layer are laminated and integrated , the ion permeability of the imide polymer porous layer evaluated by the following method is 150 seconds or less, and the electrode active material layer adhesion strength between the material layer and the imide-based polymer porous is higher than the strength of the electrode active material layer Method for producing a lithium secondary battery electrode, wherein the door.
<Ion permeability evaluation method>
5 μL of a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (volume ratio 1: 1: 1) set at 30 ° C. is dropped on the surface of the imide polymer porous layer, and this completely penetrates. This is visually observed and the permeation time is measured, and this permeation time is evaluated as ion permeability.

2) d ether-based solvent, a manufacturing method of the electrode for a lithium secondary battery is 1), characterized in that triethylene glycol dimethyl ether and / or tetraethylene glycol dimethyl ether.
3) The method for producing an electrode for a lithium secondary battery according to 1) or 2), wherein the compounding amount of the poor solvent is 40 to 90% by mass with respect to the total amount of the solvent.

  The electrode for the lithium secondary battery of the present invention does not require a large amount of fine particles such as alumina and silicon dioxide particles to form pores of the ion permeable porous membrane. The cushioning property can be improved, and good adhesion between the porous layer and the active material layer can be ensured. Accordingly, it can be suitably used as an electrode for a lithium secondary battery that is excellent in safety and has a high discharge capacity and good cycle characteristics. In the manufacturing method of the present invention, the electrode of the present invention can be easily manufactured by a simple process.

It is sectional drawing of the electrode by which the imide porous layer was laminated | stacked and integrated on the outer surface of the 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 a positive electrode active material layer in FIG. 3 is almost peeled off. FIG. 4 is an enlarged view of a portion where a positive electrode active material layer in FIG. 3 remains.

  In the lithium secondary battery electrode of the present invention, an ion-permeable porous layer formed of an imide-based polymer and having a porosity of 30 to 90% by volume is laminated and integrated on the outer surface of the electrode active material layer. It is formed. An electrode for a lithium secondary battery is an electrode constituting a lithium ion secondary battery, and 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. Said negative electrode. An electrode active material layer is a general term for a positive electrode active material layer and a negative electrode active material layer.

  As the current collector, a metal foil such as a copper foil, a stainless steel foil, a nickel foil, or an aluminum foil can be used. Aluminum foil is preferably used for the positive electrode, and copper foil is 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 an antirust treatment for improving the adhesiveness with the active material layer.

The positive electrode active material layer is a layer obtained by binding positive electrode active material particles with a resin binder. The material used as the positive electrode active material particles is preferably a material capable of occluding and storing lithium ions, and examples thereof include materials generally used as a positive electrode active material for lithium secondary batteries. For example, oxide type (LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ etc.), complex oxide type (LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , Li (LiaNixMnyCoz) O ₂ etc.), phosphoric acid Active material particles such as iron-based (LiFePO ₄ , Li ₂ FePO ₄ F, etc.) and polymer compound-based (polyaniline, polythiophene, etc.) can be mentioned. 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. It may be.

  The negative electrode active material layer is a layer obtained by binding negative electrode active material particles with a resin binder. The material used as the negative electrode active material particles is preferably a material capable of occluding and storing lithium ions, and examples thereof include materials generally used as a negative electrode active material for lithium secondary batteries. Examples thereof include active material particles such as graphite, amorphous carbon, silicon-based, and tin-based materials. Among these, graphite particles and silicon-based particles are preferable. Examples of the silicon-based particles include particles of silicon alone, a silicon alloy, a silicon / silicon dioxide composite, and the like. Among these silicon-based particles, particles of silicon alone (hereinafter sometimes abbreviated as “silicon particles”) are preferable. Silicon simple substance 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. It may be.

  The particle diameter of the active material particles and the conductive particles is preferably 50 μm or less for both the positive electrode and the negative electrode, and more preferably 10 μm or less. On the contrary, if the particle diameter is too small, it becomes difficult to bind with the resin binder.

  As for the porosity of an electrode active material layer, 5-50 volume% is preferable in both a positive electrode and a negative electrode, and 10-40 volume% is more preferable.

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

  Examples of the resin binder for binding the active material particles include, for example, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and styrene / butadiene copolymer. Examples thereof include rubber, polytetrafluoroethylene, polypropylene, polyethylene, and an imide polymer. 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 the main chain or a precursor thereof. Typical examples of the polymer having an imide bond in the main chain include polyimide, polyamideimide, and polyesterimide. However, it is not limited to these.

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

The porosity of the imide porous layer in the present invention is essential to be 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 way, good mechanical properties and good cushioning properties for stress relaxation accompanying the volume change of the active material can be ensured at the same time. For this reason, 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 constituting the imide porous layer. Specifically, the porosity (% by 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 polymer is B (g / cm ³ ). Is done.
Porosity (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 interface debonding occurs at the interface when the electrode active material layer is peeled from the porous imide layer. Can do. 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. It is determined as cohesive failure when a fragment of the active material layer adheres to a part of the surface of the imide porous layer after peeling (adhesion surface with the electrode active material layer). Conventionally, such an electrode that causes cohesive failure is not known, and in the electrode of the present invention, such a high adhesive force greatly contributes to an improvement in battery safety.

  The average pore diameter of the imide porous layer in the present invention is preferably 0.1 to 10 μm, and more preferably 0.5 to 5 μm. By setting the average pore diameter in this way, good ion permeability is ensured. The quality of the ion permeability can be determined from the permeation time of the solvent when the solvent for the electrolyte solution constituting the battery is dropped on the electrode surface. Details of the determination method will be described later. In the electrode of the present invention, the permeation time is preferably 300 seconds or shorter, and more preferably 150 seconds or shorter.

  1-100 micrometers is preferable and, as for the thickness of the imide porous layer in this invention, 10-50 micrometers is more preferable.

  The imide porous layer in the present invention may be either insulating or conductive. When the imide porous layer is insulative, it is advantageous because this layer also functions 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 are used in an amount of about 5 to 50% by mass. What is necessary is just to mix | blend with a layer. From the viewpoint of ensuring cushioning properties and adhesiveness of the imide porous layer, the blending amount of these conductive particles is preferably 20% by mass or less.

  Next, the manufacturing method of the electrode for lithium secondary batteries of this invention is demonstrated.

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

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

  (2) Next, a coating liquid containing an imide polymer and a solvent that forms an imide porous layer by phase separation on the surface of the electrode active material layer (hereinafter abbreviated as “imide coating liquid”). Apply).

  (3) Thereafter, the solvent in the coating liquid is removed to cause phase separation in the imide porous layer to form pores in the imide porous layer, and the electrode active material layer, the imide porous layer, Are laminated and integrated.

  In the drying for forming the electrode active material layer, the residual solvent content in the active material layer is preferably set to 0.5 to 50% by mass. By doing in this way, the intensity | strength of the adhesion interface of an electrode active material layer and an 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 a phase separation to develop a porous structure by utilizing the action of a solvent that is a poor solvent for a solute in a coating liquid.

  As the poor solvent-induced phase separation method, a dry phase separation method is preferred from the viewpoint of simplicity of the production process and environmental compatibility. The dry phase separation method utilizes the action of the poor solvent remaining in the coating film when the coating film of the imide-based coating liquid composed of a mixed solvent of a good solvent and a poor solvent having different boiling points is dried and solidified. A method for 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 that is a solute when the above-described polyamic acid, soluble polyimide, polyamideimide and the like are produced by solution polymerization in a solvent, It can be easily obtained by using a mixed solvent having a higher boiling point than this good solvent and a solute mixed with a solvent that becomes a poor solvent. A good solvent refers to a solvent having a solubility in an imide polymer of 1% by mass or more at 25 ° C., and a poor solvent refers to a solvent having a solubility in an imide polymer of less than 1% by mass at 25 ° C.
The difference in boiling point between the good solvent and the poor solvent is preferably 5 ° C. or higher, more preferably 20 ° C. or higher, and further preferably 50 ° C. or higher.

  As the good solvent, an amide solvent is preferably used. 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.

  As the poor solvent, an ether solvent is preferably used. Examples of ether solvents 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. And a solvent such as (boiling point: 287 ° C.). These may be used alone or in combination of two or more. The blending 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. By setting it as such a solvent composition, the firm adhesion | attachment with an above imide porous layer and an active material layer is obtained.

  Examples of imide-based coating liquids include the product name “Uimide varnish BP” (polyamic acid type polyimide varnish), a product name “Uimide varnish SP” (soluble polyimide varnish), and products sold by Unitika Ltd. for porous formation. The name “Uimide varnish IP” (polyamideimide varnish) and the like can be mentioned.

  The imide-based coating solution composed of a polyamic acid solution, a soluble polyimide solution, etc. used in the dry phase separation method may use the above-mentioned commercially available products, but contains tetracarboxylic dianhydride and diamine as raw materials in approximately equimolar amounts. In addition, a polyamic acid solution or a soluble polyimide solution obtained by polymerization reaction in the mixed solvent described above is also preferably used. In addition, after a polymerization reaction only in a good solvent to obtain a solution, a method of adding a poor solvent thereto, or after a polymerization reaction only in a poor solvent to obtain a suspension, a method of adding a good solvent thereto. Thus, an imide-based coating liquid can also be obtained.

  Examples of the tetracarboxylic 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-naphthalenetetracarboxylic acid, 1,2,5,6-naphthalenetetracarboxylic acid, 3,3 ', 4,4'-diphenylmethanetetracarboxylic acid, 2,2-bis (3,4-dicarboxyphenyl) propane, 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane, 3,4,9,10-tetracarboxype Dianhydrides such as len, 2,2-bis [4- (3,4-dicarboxyphenoxy) phenyl] propane, 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, diaminodiphenylsulfone, 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'-bis (p-aminophenoxy) biphenyl, diaminoanthraquinone, 4,4'-bis (3-aminophenoxyphenyl) diphenyl sulfone, 1,3-bis (anilino) hexafluoropropane, 1,4-bis ( Anilino) octafluorobutane, 1,5-bis (anilino) decafluoropentane, 1,7-bis (anilino) tetradecafluoroheptane is 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.

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

  The imide-based coating liquid composed of the polyamide-imide solution used in the dry phase separation method may be a commercially available product as described above, but the raw material trimellitic anhydride and diisocyanate are blended in approximately equimolar amounts, A solution obtained by polymerization reaction in the mixed solvent is also preferably used. In addition, after a polymerization reaction only in a good solvent to obtain a solution, a method of adding a poor solvent thereto, or after a polymerization reaction only in a poor solvent to obtain a suspension, a method of adding a good solvent thereto. Thus, an imide-based coating liquid composed of a polyamideimide solution can also be obtained.

  As trimellitic acid anhydride, a part of which is substituted with pyromellitic acid anhydride, benzophenone tetracarboxylic acid anhydride, or biphenyl tetracarboxylic 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, diphenylsulfone-4,4′-diisocyanate, diphenyl-4,4′-diisocyanate, o-Tolidine diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, xylylene diisocyanate, naphthalene diisocyanate are used. These may be used alone or in combination of two or more. Among these, 4,4'-diphenylmethane diisocyanate is preferable.

  1-50 mass% is preferable and, as for the solid content density | concentration of the polyamide-imide in a polyamide-imide solution, 10-30 mass% is more preferable.

  The polyamideimide solution has a viscosity at 30 ° C. of preferably 1 to 150 Pa · s, and 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 as long as the effects of the present invention are not impaired. Moreover, you may add other polymers other than an imide type polymer to the imide-type coating liquid in the range which does not impair the effect of this invention as needed.

  An imide-based coating liquid is applied to the surface of the electrode active material layer, dried at 100 to 150 ° C., and then subjected to heat treatment at 250 to 350 ° C. as necessary, whereby the porosity of the imide is 30 to 90% by volume. The formation of the porous layer and the integration of the electrode active material layer and the imide porous layer can be performed simultaneously. 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, the surface of the obtained imide porous layer is preferably subjected to a physical polishing treatment such as a sand blast treatment or a scratch blast treatment, or a chemical etching treatment. As a result, the surface area of the imide porous layer is increased and the porosity is also increased, so that good ion permeability of the imide porous layer can be ensured.

  When applying the active material dispersion or the imide-based coating liquid, a method of continuous application by roll-to-roll or a method of coating by sheet can be adopted, and any method may be used. 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 positive electrode and negative electrode) formed on the current collector used in the following Examples and Comparative Examples were obtained as follows.

(Positive electrode active material layer)
86 parts by mass of LiFePO ₄ particles (average particle size 0.5 μm) as a positive electrode active material, 8 parts by mass of carbon black (acetylene black) as a conductive additive, and 6 parts by mass of polyvinylidene fluoride as a binder resin Were uniformly dispersed in N-methylpyrrolidone as a positive electrode active material dispersion. This dispersion was applied to an aluminum foil having a thickness of 15 μm as a positive electrode current collector, 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 as the negative electrode active material (average particle size 0.7 μm), graphite particles as the conductive additive (average particle size 0.7 μm), and polyamic acid solution as the binder resin (trade name “Uimide varnish, manufactured by Unitika Ltd.”) 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 silicon particles, graphite particles, and polyamic acid solution was 70:10:20. This dispersion was applied to a copper foil having a thickness of 18 μm as 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. In this active material layer, 22% by mass of NMP remained.

  The characteristics 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, ethyl methyl carbonate and dimethyl carbonate (volume ratio 1: 1: 1) set at 30 ° C. was dropped on the electrode surface, and it was visually observed that this completely penetrated. The penetration time was measured, and the ion permeability was evaluated based on the penetration time.

(2) Adhesiveness The electrode active material layer was forcibly peeled by hand in the opposite direction by 180 degrees from the laminated integrated product of the electrode active material layer and the imide porous layer. At that time, whether or not the adhesiveness was good was determined based on whether or not a fragment of the electrode active material layer was attached to a part of the surface of the imide porous layer after peeling (the adhesive surface with the electrode active material). 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 the cohesive failure is caused. Therefore, the adhesion between the electrode active material layer and the imide porous layer is “ It was determined as “good”. In addition, when the fragments were not attached, peeling at the interface occurred, so the adhesiveness was determined as “poor”.

<Example 1>
About equimolar 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 It was made to react in the mixed solvent with a mass part, and the uniform polyamideimide solution (P-1) whose solid content concentration is 15 mass% was obtained. This solution is 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 is polished so that an imide porous layer having a thickness of 23 μm is formed on the outer surface of the positive electrode active material layer. A laminated and integrated electrode (positive electrode) “C-1” was obtained. The evaluation results of the obtained electrode are shown in Table 1.

  The SEM image of the cross section of positive electrode "C-1" is shown to FIGS. FIG. 1 shows three layers on the top and bottom. 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 the vicinity thereof. From these figures, it can be seen 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 off by 180 degrees in the opposite direction by hand. . From FIG. 3, it can be seen that after peeling, the portion where the active material layer is almost peeled off and the portion where the fragments of the active material layer remain coexist. FIG. 4 shows an enlarged SEM image of a portion indicated by the numeral “1” in FIG. 3 (a portion where the active material layer is almost peeled). From this SEM image, it can be seen that many pores exist on the surface of the imide porous layer at the interface. FIG. 5 shows an enlarged SEM image of the portion “2” in FIG. 3 (where the fragments of the active material layer remain). From this SEM image, it can be seen that many pores exist in the active material layer at the interface. It is considered that the pores existing at the interface shown in FIGS. 4 and 5 contribute to the good ion permeability of the positive electrode “C-1”.

<Example 2>
An approximately equimolar 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, and the solid content concentration is 10% by mass. A uniform polyamideimide solution (P-2) was obtained. This solution is 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 is polished so that an imide porous layer having a thickness of 20 μm is formed on the outer surface of the positive electrode active material layer. A laminated and integrated electrode (positive electrode) “C-2” was obtained. The evaluation results of the obtained electrode are shown in Table 1.

< Reference Example 1 >
A substantially equimolar 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, and the solid content concentration is 17% by mass. A uniform polyamideimide solution (P-3) was obtained. This solution is 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 is polished so that an imide porous layer having a thickness of 25 μm is formed on the outer surface of the positive electrode active material layer. A laminated and integrated electrode (positive electrode) “C-3” was obtained. The evaluation results of the obtained electrode are shown in Table 1.

<Comparative Example 1>
A substantially equimolar 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, and the solid content concentration is 17% by mass. A uniform polyamideimide solution (P-4) was obtained. This solution is 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 is polished so that an imide porous layer having a thickness of 25 μm is formed on the outer surface of the positive electrode active material layer. A laminated and integrated electrode (positive electrode) “C-4” was obtained. The evaluation results of the obtained electrode are shown in Table 1.

<Comparative Example 2>
Approximately equimolar trimellitic anhydride and 4,4'-diphenylmethane diisocyanate were reacted in NMP to obtain a uniform polyamideimide solution (P-5) having a solid content of 19% by mass. This solution is 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 is polished so that an imide porous layer having a thickness of 25 μm is formed on the outer surface of the positive electrode active material layer. A laminated integrated electrode (positive electrode) “C-5” was obtained. The evaluation results of the obtained electrode are shown in Table 1.

<Comparative Example 3>
In the polyamideimide solution (P-5) obtained in Comparative Example 2, alumina particles having an average particle size of 0.5 μm are uniformly mixed and dispersed to obtain an alumina filler dispersion (P-6) having a solid content concentration of 25 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 and dried at 130 ° C. for 10 minutes, whereby an imide porous layer having a thickness of 25 μm was laminated and integrated on the outer surface of the positive electrode active material layer. Electrode (positive electrode) “C-6” was obtained. The evaluation results of the obtained electrode are shown in Table 1.

<Example 4>
Substantially equimolar 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA) were mixed with N, N-dimethylacetamide as a good solvent ( DMAc) Reaction was performed in a mixed solvent of 20 parts by mass of tetraethylene glycol dimethyl ether as a poor solvent 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 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 to polyimide, and then the surface is polished to obtain a thickness. Obtained 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. The evaluation results of the obtained electrode are shown in Table 1.

Next, the battery characteristics of this negative electrode “A-1” were evaluated. Specifically, this negative electrode is punched into a circle having a diameter of 14 mm, and a separator made of a polypropylene porous film and a lithium foil are sequentially laminated on the porous surface of the imide, and this is laminated in a stainless steel coin-type outer container. Stowed. An electrolytic solution (solvent: a mixed solvent in which ethylene carbonate, ethylmethyl carbonate, and dimethyl carbonate are mixed at a volume ratio of 1: 1: 1, electrolyte: 1 M LiPF ₆ ) is poured into the outer container, and the outer container is filled with the electrolyte. A 0.2 mm-thick stainless steel cap is placed and fixed through a polypropylene packing, and the battery can is sealed, and a cell for evaluating discharge capacity and cycle characteristics having a diameter of 20 mm and a thickness of about 3.2 mm is obtained. Obtained. Using the obtained cell, a charge / discharge cycle was performed at 30 ° C. with a constant current of 0.05 C to 2 V and a discharge with a constant current of 0.05 C to 0.02 V. 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]. Cycle characteristics were confirmed.

<Example 5>
About equimolar 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride and 4,4′-oxydianiline were mixed 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 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 to polyimide, and then the surface is polished to obtain a thickness. Obtained 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. The evaluation results of the obtained electrode are shown in Table 1.

<Example 6>
A commercially available polyimide precursor varnish for forming a porous film containing 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 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 to polyimide, and then polishing the surface. An electrode (negative electrode) “A-3” in which an imide porous layer having a thickness of 25 μm was laminated and integrated on the outer surface of the negative electrode active material layer was obtained. The evaluation results of the obtained electrode are shown in Table 1.

<Comparative example 4>
About equimolar 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride and 4,4′-oxydianiline were mixed 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 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 to polyimide, and then the surface is polished to obtain a thickness. Obtained 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. The evaluation results of the obtained electrode are shown in Table 1.

<Comparative Example 5>
An approximately equimolar amount of 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride and 4,4′-oxydianiline are reacted in DMAc, and the solid content concentration is 15% by mass. A uniform polyamic acid solution (P-11) was obtained. 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 to polyimide, and then the surface is polished to obtain a thickness. Obtained 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. The evaluation results of the obtained electrode are shown in Table 1.

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

<Comparative Example 7>
About equimolar 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride and 4,4′-oxydianiline are mixed in a mixed solvent of 30 parts by mass of NMP and 70 parts by mass of γ-butyrolactone. The reaction was performed 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 to polyimide, and then the surface is polished to obtain a thickness. Obtained an electrode (negative electrode) “A-7” in which a 20 μm imide porous layer was laminated and integrated on the outer surface of the negative electrode active material layer. The evaluation results of the obtained electrode are shown in Table 1.

<Comparative Example 8>
An approximately equimolar amount of 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride and 4,4′-oxydianiline were reacted in diethylene glycol dimethyl ether to obtain a solid concentration of 15% by mass. An attempt was made to obtain a uniform polyamic acid solution (P-14). However, a uniform solution could not be obtained.

As described above in Examples and Comparative Examples, the electrode for the lithium secondary battery of the present invention is an amide solvent as a good solvent for the imide polymer, and 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 ensured. In addition, 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 pores of the ion permeable porous membrane, the ion permeable porous Good adhesion between the layer and the active material layer can be ensured. Accordingly, it can be suitably used as an electrode for a lithium secondary battery that is excellent in safety and has a high discharge capacity and good cycle characteristics. Moreover, according to the manufacturing method of the present invention, an electrode can be easily manufactured by a simple process with high environmental compatibility.

Claims (3)

A method for producing an electrode for a lithium secondary battery, wherein a dispersion containing a binder, active material fine particles, and a solvent is applied to a surface of a metal foil as a current collector and dried to form an electrode on the metal foil An active material layer is formed, and then the surface of the electrode active material layer is coated with an imide polymer and a solvent obtained by mixing an amide solvent that is a good solvent and an ether solvent that is a poor solvent. A liquid is applied to form a coating film, and then, when the solvent in the coating film is removed, phase separation is caused in the coating film by utilizing the action of the poor solvent remaining in the coating film , When the electrode active material layer and the imide polymer porous layer are laminated and integrated , the ion permeability of the imide polymer porous layer evaluated by the following method is 150 seconds or less, and the electrode active material layer adhesion strength between the imide polymer porous and is higher than the strength of the electrode active material layer Method for producing a lithium secondary battery electrode, wherein.
<Ion permeability evaluation method>
5 μL of a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (volume ratio 1: 1: 1) set at 30 ° C. is dropped on the surface of the imide polymer porous layer, and this completely penetrates. This is visually observed and the permeation time is measured, and this permeation time is evaluated as ion permeability.   The method for producing an electrode for a lithium secondary battery according to claim 1, wherein the ether solvent is triethylene glycol dimethyl ether and / or tetraethylene glycol dimethyl ether.   3. The method for producing an electrode for a lithium secondary battery according to claim 1, wherein the compounding amount of the poor solvent is 40 to 90% by mass with respect to the total amount of the solvent.