CN104508870A - Negative electrode for lithium secondary battery - Google Patents

Abstract

The present invention is a negative electrode for a lithium secondary battery provided with an active material layer containing a particulate silicon-based active material and a binder, wherein the content of the silicon-based active material in the active material layer is more than 50% by mass, and the 20 th discharge capacity when the battery cell is repeatedly charged and discharged 20 times is 1500 mAh/g-silicon-based active material or more under the configuration and charge and discharge conditions of the battery cell shown below. Constitution of the cell > cell: bipolar pouch cell, counter electrode: metallic lithium, electrolyte: LiPF₆A mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate dissolved at a concentration of 1mol/L (volume ratio 1: 1: 1), < charge and discharge conditions > measurement temperature: 30 ℃, voltage range: 0.01-2V, charging current and discharging current: 500 mA/g-silicon-based active material.

Description

Negative electrode for lithium secondary battery

technical field

The present invention relates to a negative electrode for a lithium secondary battery using silicon-based particles as an active material.

Background technique

Conventionally, negative electrodes of lithium ion secondary batteries have been used in which an active material layer containing a particulate carbon-based active material such as graphite powder and an insulating binder is formed on the surface of a foil-shaped current collector such as copper foil. As the binder, polyvinylidene fluoride, polytetrafluoroethylene, or the like is used.

However, the discharge capacity of a negative electrode using a carbon-based active material is at most about 350 mAh/g, so an active material with a higher capacity is required. Therefore, a negative electrode using a particulate silicon-based active material as a next-generation active material instead of a carbon-based active material has been proposed. It is known that silicon exhibits a discharge capacity several times higher than that of graphite through an alloying reaction with lithium.

However, since the silicon-based active material has a large volume change associated with charge and discharge, the silicon-based active material is pulverized or detached from the current collector as charge and discharge are repeated. Therefore, there is a problem that the current collection property and the amount of active material of the negative electrode decrease, and the discharge capacity of the negative electrode decreases significantly. As a method of improving the degradation of the cycle characteristics caused by the volume change during repeated charging and discharging, Patent Document 1 proposes to bond silicon particles with an average particle size of 1 to 10 micrometers by using polyimide with excellent mechanical properties. The formed active material is provided on the surface of a current collector made of a specific copper foil to make a negative electrode. In Patent Document 2, it is proposed to use a composite composed of silicon and carbon as an active material, and use the active material as Polyimide bonded negative electrode. In addition, Non-Patent Documents 1 and 2 propose negative electrodes using polyamideimide and polyacrylic acid as binders for silicon particles, respectively.

Furthermore, Patent Document 3 proposes laminating a conductive adhesive layer in which a high-concentration binder is blended on the surface of the current collector, and providing an adhesive layer containing polyimide or the like on the outer surface of the conductive adhesive layer. The silicon negative electrode of the silicon active material layer of the binder.

prior art literature

patent documents

Patent Document 1: Specification of Patent No. 4471836

Patent Document 2: International Publication No. 2011/056847

Patent Document 3: Specification of Patent No. 4212392

non-patent literature

Non-Patent Document 1: Journal of Power Sources 177(2008) 590-594

Non-Patent Document 2: ACS Appl. Mater. Interfaces, 2010, 2(11), 3004-3010

Contents of the invention

However, even with the negative electrodes described in the aforementioned prior art documents, it is difficult to sufficiently suppress the decrease in discharge capacity accompanying repeated charge and discharge, and a negative electrode that maintains a high discharge capacity even after repeated charge and discharge is required.

Therefore, in order to solve the above-mentioned problems, an object of the present invention is to provide a negative electrode for a lithium ion secondary battery capable of maintaining a high discharge capacity even after repeated charge and discharge when a silicon-based active material is used.

The inventors of the present invention have made intensive studies in order to solve the above-mentioned problems, and as a result, have completed the present invention. That is, the gist of the present invention is as follows.

(1) A negative electrode for a lithium secondary battery, characterized in that it is a negative electrode for a lithium secondary battery provided with an active material layer containing a particulate silicon-based active material and a binder, and the silicon-based active material in the active material layer The active material content is more than 50% by mass, and the 20th discharge capacity is 1500mAh/g-silicon-based active material or more when repeated charging and discharging 20 times under the battery cell configuration and charge-discharge conditions shown below.

＜Composition of the battery unit＞

Battery: Bipolar pouch cell

Counter electrode: lithium metal

Electrolyte: LiPF ₆ is a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate dissolved at a concentration of 1mol/L (volume ratio 1:1:1)

＜Charge and discharge conditions＞

Measuring temperature: 30°C

Voltage range: 0.01 ~ 2V

Charge current and discharge current: 500mA/g-silicon-based active material

(2) The negative electrode for a lithium secondary battery according to (1), wherein the active material layer has a porosity of 15 to 40% by volume and an electrolytic solution absorption rate of 300 seconds or less.

(3) The negative electrode for a lithium secondary battery as described in (1) or (2), wherein the laminate in which the active material layer is laminated on the outer surface of the conductive adhesive layer formed on the sheet-shaped current collector constitute.

(4) The negative electrode for a lithium secondary battery according to any one of (1) to (3), wherein the active material is a particle composed of silicon alone.

(5) The negative electrode for a lithium secondary battery according to any one of (1) to (4), wherein the silicon-based active material has an average particle diameter of less than 1 μm.

Since the negative electrode of the present invention maintains a high discharge capacity even after repeated charge and discharge, it can be suitably used as a negative electrode for lithium secondary batteries.

Description of drawings

FIG. 1 is a graph showing charge and discharge curves when using the negative electrode of Example 1 of the present invention.

Detailed ways

The negative electrode for a lithium secondary battery of the present invention is provided with an active material layer containing a particulate silicon-based active material and a binder, and the content of the silicon-based active material in the active material layer is greater than 50% by mass. In addition, it has the characteristic that the discharge capacity at the 20th time when charging and discharging is repeated 20 times is 1500mAh/g-silicon-based active material or more under the battery cell configuration and charge-discharge conditions shown below.

＜Composition of the battery unit＞

Battery: Bipolar pouch cell

Counter electrode: lithium metal

Electrolyte: LiPF ₆ is a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate dissolved at a concentration of 1mol/L (volume ratio 1:1:1)

＜Charge and discharge conditions＞

Measuring temperature: 30°C

Voltage range: 0.01 ~ 2V

Charge current and discharge current: 500mA/g-silicon-based active material

"mAh/g-silicon-based active material" shown here as a unit of discharge capacity refers to the electric quantity (mAh ) are converted into values per gram of silicon-based active material.

The above-mentioned discharge capacity is more preferably 1700 mAh/g or more, still more preferably 2000 mAh/g or more. In this way, a negative electrode for a lithium secondary battery with excellent cycle characteristics and high discharge capacity can be obtained.

The discharge capacity was measured using a known pouch-type (laminated sheet-type) battery cell. Unlike coin-type battery cells, this battery cell is a battery cell that uses a flexible aluminum laminate film (a laminated film of resin film and aluminum foil) as an exterior material, and is measured in a state where no pressure is applied to the electrodes during charging and discharging. discharge capacity of the battery cell. This pouch-shaped battery cell can be produced, for example, as follows.

The obtained sheet-shaped negative electrode was cut into a rectangle of 10 mm×40 mm, leaving an active material area of 10 mm×10 mm, and covered with a welded film. As a counter electrode, a lithium plate with a thickness of 1 mm was cut into a rectangle of 30 mm×40 mm, and a nickel lead wire (5 mm×50 mm) with a thickness of 0.5 mm was folded in half and crimped. Only the negative electrode was placed in a bag-shaped separator (30 mm×20 mm), and then faced to the counter electrode to obtain an electrode group. As the spacer, a rectangular polypropylene resin porous film (thickness: 25 μm) was used. This electrode group was covered with a set of two rectangular aluminum laminated films (50 mm×40 mm), and after the three sides were sealed, 1 mL of electrolyte solution was injected into the bag-shaped aluminum laminated film. As the electrolytic solution, an electrolytic solution obtained by dissolving LiPF ₆ at a concentration of 1 mol/L in a mixed solvent in which EC, DEC, and EMC were mixed at a volume ratio of 1:1:1 was used. Thereafter, the remaining one side was sealed, and the inside of the bag-shaped aluminum laminated film was sealed. In addition, when sealing the inside of the bag-shaped aluminum laminate film, the negative electrode and one end of the nickel lead were extended to the outside to serve as terminals. In this way, a test battery cell was obtained. All these operations were carried out in an operation box under an argon atmosphere.

The content of silicon-based active material particles (hereinafter sometimes abbreviated as "silicon-based particles") in the active material layer needs to be more than 50% by mass, preferably more than 60% by mass. If it is 50% by mass or less, even if the silicon-based active material as the active material has a high discharge capacity per gram, the discharge capacity per gram of the active material layer decreases, making it difficult to obtain a negative electrode with a high discharge capacity.

Examples of the aforementioned silicon-based particles include particles such as silicon monomer, silicon alloy, and silicon-silica composite, and the shape may be any shape such as indeterminate shape, spherical shape, or fibrous shape. Among these silicon-based particles, silicon-only particles (hereinafter sometimes abbreviated as "silicon particles") have the highest discharge capacity, and thus can be preferably used. Here, silicon alone refers to crystalline or amorphous silicon having a purity of 95% by mass or higher.

The average particle diameter of the silicon-based particles is preferably 5 μm or less, more preferably less than 1 μm. The smaller the average particle diameter is, the larger the surface area of the particles is, so a high discharge capacity can be obtained. Here, the average particle diameter refers to, for example, a volume-based average particle diameter measured by a laser diffraction particle size distribution analyzer. This average particle size can also be confirmed from an SEM image of the surface of a negative electrode obtained using the above-mentioned silicon-based particles.

By mixing a binder with the silicon-based particles, the silicon-based particles are bonded to each other to form a film-like active material layer. The type of binder to be used is not limited, but it is preferable to use a polyimide-based polymer having excellent mechanical properties and excellent adhesion to silicon-based particles. Here, the polyimide-based polymer refers to a polymer having an imide bond in the main chain. Specific examples include polyimide, polyamideimide, polyesterimide, etc. Polyimide-based polymers are not limited to them, as long as the main chain has an imide bond Use any polymer. These resins are usually used alone, or two or more of them may be used in combination.

Among these polyimide-based polymers, it is preferable to use polyimides that are particularly excellent in mechanical properties, and among polyimides, it is more preferable to use aromatic polyimides. Here, the aromatic polyimide means a polyimide having a structure represented by the following general formula (1).

[chemical 1]

In the general formula (1), R1 is a tetravalent aromatic residue, and R2 is a divalent aromatic residue.

The aromatic polyimide may be thermoplastic or non-thermoplastic. As the polyimide, a precursor type polyimide obtained by thermally curing a polyimide precursor such as polyamic acid dissolved in a solvent, and a solvent-soluble polyimide can be used. Body polyimide. In addition, the detail of the precursor type polyimide using a polyamic acid is mentioned later.

A commercial item can also be used as said polyimide-type polymer. For example, polyamic acid type such as "U IMIDE AR", "U IMIDE AH", "U IMIDE CR", "U IMIDE CH" (all made by UNITIKA Co., Ltd.) or U VARNISH A (manufactured by Ube Industries, Ltd.) can be used. Varnish, solvent-soluble polyimide varnish made by dissolving "RIKACOAT SN-20" (manufactured by Shinnippon Chemical Co., Ltd.) or "MATRIMID5218" (manufactured by Huntsman Co., Ltd.) in a solvent, VYLOMAX HR-11NN (manufactured by Toyobo Co., Ltd.) and other polyamideimide varnishes.

From the viewpoint of discharge capacity and cycle characteristics, the content of the polyimide-based polymer in the active material layer is preferably 5 to 30% by mass, more preferably 15 to 25% by mass. By setting in this manner, the porosity of the active material layer described later can be set within a preferable range, and a negative electrode for a lithium secondary battery with excellent cycle characteristics and high discharge capacity can be obtained.

The porosity of the active material layer is preferably 15 to 40% by volume, more preferably 25 to 35% by volume. By setting the porosity in this way, the stress on the active material layer due to the volume change accompanying the charge and discharge of the silicon active material can be absorbed by the pores, so that no cracks are generated in the active material layer during charge and discharge. to obtain good cycle characteristics. Therefore, when the porosity is out of this range, the intended high discharge capacity after repeated charging and discharging may not be obtained.

The porosity of the above-mentioned active material layer is a value calculated from the apparent density of the active material layer, the true density (specific gravity) and the compounding amount of each material (silicon-based particles, binder, conductive particles, etc.) constituting the active material layer, It varies according to the compounding amount and particle size of each material. Specifically, silicon-based particles (true density A g/cm ³ ) are compounded by X mass %, binders (true density B g/cm ³ ) are compounded by Y mass %, conductive particles (true density C g/cm ³ ) The porosity (volume %) when the apparent density of the active material layer compounded Z mass % is Dg/cm ³ was calculated by the following calculation formula. Here, the true density of each material is obtained by measuring based on JIS Z8807.

Porosity (volume%)=100-D(X/A+Y/B+Z/C)

In the present invention, in order to obtain a high discharge capacity after charge-discharge cycles, it is preferable to set the porosity to 15 to 40% by volume, and to set the electrolyte absorption rate of the active material layer to 300 seconds or less. Here, the electrolytic solution absorption rate is preferably 200 seconds or less, more preferably 100 seconds or less. By setting the absorption rate of the electrolyte solution to 300 seconds or less, the electrolyte solution can be more effectively in contact with the surface of the silicon-based particles as the active material during repeated charge and discharge, so that a high discharge capacity. Therefore, when the electrolytic solution absorption rate is out of this range, the intended high discharge capacity after repeated charging and discharging may not be obtained. In addition, the details of the electrolytic solution absorption rate will be described later.

Here, the electrolytic solution absorption rate can be measured by the following method. That is, 5 μL of an electrolyte solution at 20°C mixed with ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) at a volume ratio of 1:1:1 was added dropwise to the active material layer. s surface. Then, after the electrolytic solution is added dropwise to the surface of the active material layer, the electrolytic solution dropped is completely absorbed into the layer from the surface of the active material layer, and the time until the liquid drop disappears on the surface of the active material layer is visually measured. time. The measured time was defined as the electrolytic solution absorption rate.

In the present invention, the thickness of the active material layer is optional, and can be set to a thickness of about 10 to 300 μm.

In the present invention, in order to reduce the internal resistance of the active material layer, it is preferable to contain conductive particles in the active material layer. As the electroconductive particles, for example, particulate carbon materials and metal materials can be used. As the carbon material, graphite and carbon black are preferable, and graphite is more preferable. As the metal material, for example, silver, copper, nickel can be used. The particle size of these carbon particles and metal particles preferably has an average particle size of 5 μm or less.

The content of the conductive particles in the active material layer is preferably 1 to 30% by mass, more preferably 5 to 25% by mass.

The negative electrode of the present invention is preferably a laminate in which a conductive adhesive layer is provided between the active material layer and the sheet-shaped current collector. Thereby, stress generated at the interface between the active material layer and the current collector due to expansion and contraction of the active material layer during charge and discharge can be relaxed, and peeling of the active material layer from the current collector can be suppressed.

The above-mentioned conductive adhesive layer is a layer in which conductive particles are mixed with a binder. The kind of binder used here is not limited, for example, the above-mentioned polyimide-based polymer can be preferably used. Among the polyimide-based polymers, it is particularly preferable to use polyamide-imide and solvent-soluble polyimide that are excellent in adhesion properties to current collectors such as copper foil. Here, polyamide-imide refers to a polyamide-imide having a structure represented by the following general formula (2).

[Chem 2]

In the general formula (2), R3 is a trivalent aromatic residue, and R4 is a divalent aromatic residue. The binder in the conductive adhesive layer may be the same as or different from the binder in the active material layer.

As the electroconductive particle used for the said electroconductive contact bonding layer, a particulate carbon material and a metal material can be used, for example. As the carbon material, graphite and carbon black are preferable, and graphite is more preferable.

As the metal material, for example, silver, copper, nickel can be used. The particle size of these carbon particles and metal particles preferably has an average particle size of 5 μm or less. These electroconductive particles may be the same kind as the electroconductive particle mix|blended with the said active material layer, and may differ.

Preferably, the content of the binder in the above-mentioned conductive adhesive layer is less than 30% by mass, that is, the content of the conductive particles is 70% by mass or more, more preferably the content of the binder in the above-mentioned conductive adhesive layer is less than 20% by mass. , That is, the content of the electroconductive particles is 80% by mass or more.

If the content of the binder is 30% by mass or more, that is, the conductive adhesive layer with the content of the conductive particles less than 70% by mass, it is difficult to make the porosity of the active material layer 15 to 40% by volume and to absorb the electrolyte solution. The speed is below 300 seconds. For example, regarding the negative electrode in which the polyimide content in the conductive adhesive layer described in Example 1 of Patent Document 3 is 80% by mass, in studies by the inventors of the present invention, electrolysis of the active material layer The liquid absorption speed is 1000 seconds or more. The reason for this is not clear, but it has been confirmed that regardless of the type of binder in the adhesive layer, as the content of the binder increases, the absorption rate of the electrolyte solution in the active material layer becomes slower, so it is considered that the conductive adhesive layer The composition of has no effect on the electrolyte absorption rate of the active material layer.

In addition, when the amount of the binder in the conductive adhesive layer is relatively small, by bonding conductive particles such as graphite, which has almost no volume expansion and contraction, with the binder in the same content as the active material layer, In addition, sufficient strength as a conductive adhesive layer can be obtained by using a binder having high adhesiveness to a current collector such as copper foil.

The thickness of the conductive adhesive layer is preferably 1 to 15 μm, more preferably 2 to 5 μm, from the viewpoint of conductivity and adhesiveness between the current collector and the active material layer.

As the current collector, for example, metal foils such as copper foil, stainless steel foil, and nickel foil can be used, and copper foil such as electrolytic copper foil and rolled copper foil is preferably used. The thickness of the metal foil is preferably 5 to 50 μm, more preferably 9 to 18 μm. In order to improve the adhesiveness of the metal foil and the conductive adhesive layer, the surface of the metal foil may be roughened or rust-proofed.

The negative electrode for a lithium secondary battery of the present invention can be easily produced by, for example, the following steps.

First step: Graphite particles are blended into a polyamide-imide solution to obtain a graphite dispersion (coating material for forming a conductive adhesive layer).

2nd process: After apply|coating a graphite dispersion on a copper foil, it dries and obtains a conductive coating film.

3rd process: Silicon particle and graphite particle are mixed with polyimide precursor solution, and silicon dispersion (coating material for active material layer formation) is obtained.

Step 4: After coating the silicon dispersion on the conductive coating film, it is dried to obtain a silicon-containing coating film.

Step 5: By heat-treating the silicon-containing coating film, the polyimide precursor is thermally cured and converted into polyimide.

By the method described above, a negative electrode in which a current collector, a conductive adhesive layer containing graphite particles and polyamide-imide, and an active material layer containing silicon particles, graphite particles, and polyimide are sequentially laminated can be easily produced. .

The drying temperature in the second step (conductive coating film forming step) is preferably 200°C or lower, more preferably 150°C or lower. From the viewpoint of drying efficiency, the drying temperature in the second step (conductive coating film forming step) is preferably 100° C. or higher.

In the second step, it is preferable to dry the conductive coating film until about 5 to 30% by mass of the solvent used for the graphite dispersion remains in the conductive coating film. This remaining solvent contributes to the expression of good adhesive strength between the conductive adhesive layer and the active material layer.

The drying temperature in the fourth step (active material coating film forming step) is preferably 200°C or lower, more preferably 150°C or lower. From the viewpoint of drying efficiency, the drying temperature in the fourth step (active material coating film forming step) is preferably 100° C. or higher.

The heat treatment temperature in the fifth step is preferably 250 to 500° C. from the viewpoint that the polyimide precursor in the silicon-containing coating film can be sufficiently converted into polyimide without causing thermal damage to the negative electrode. The heat treatment is preferably performed under an inert gas atmosphere such as nitrogen, but may also be performed in air or under vacuum. In addition, after the heat treatment, heat and pressure treatment may be performed as necessary.

The application of each dispersion in the second step and the fourth step may be performed only once, or divided into multiple times.

As the method of applying the dispersion of the conductive material to the current collector and the method of applying the dispersion of the active material to the conductive coating film, the method of continuously applying the roll-to-roll method, the method of coating in the form of a sheet, and the method of applying the active material dispersion to the conductive coating film can be used Either of the cloth methods. As a coating device, for example, a die coater, a multilayer die coater, a gravure coater, a comma coater, a reverse roll coater, or a knife coater can be used.

As the polyamide-imide solution, commercially available ones as described above can be used, but it is preferable to use a solution obtained by mixing trimellitic anhydride and diisocyanate as raw materials in approximately equimolar amounts and polymerizing them in a solvent.

As trimellitic anhydride, a compound obtained by substituting a part thereof with pyromellitic anhydride, benzophenone tetracarboxylic anhydride, or biphenyltetracarboxylic anhydride can also be used.

As the diisocyanate, for example, m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-diphenyl ether diisocyanate, diphenylsulfone-4,4' -diisocyanate, diphenyl-4,4'-diisocyanate, o-toluene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, xylene diisocyanate, naphthalene diisocyanate. These may be used individually or in combination of 2 or more types. Among these, 4,4'-diphenylmethane diisocyanate is preferable.

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

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

As the polyimide precursor solution, commercially available ones as mentioned above can be used, and polyamic acid obtained by mixing tetracarboxylic dianhydride and diamine as raw materials with approximately equimolarity and polymerizing them in a solvent is also preferably used.

As the tetracarboxylic dianhydride, for example, pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, 3,3',4,4'-benzophenone tetracarboxylic 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-(3,4-dicarboxyphenoxy)phenyl]propane, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]hexafluoropropane, etc. Dianhydride. These may be used individually or in combination of 2 or more types. Among these, pyromellitic acid and 3,3',4,4'-biphenyltetracarboxylic acid are preferable.

As diamines, for example, p-phenylenediamine, m-phenylenediamine, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, Phenylmethane, 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, diaminotrifluorobenzene, 1,4-bis(p-aminophenoxy)benzene, 4,4 '-bis(p-aminophenoxy)biphenyl, diaminoanthraquinone, 4,4'-bis(3-aminophenoxyphenyl)diphenylsulfone, 1,3-bis(anilino)hexafluoropropane , 1,4-bis(anilino)octafluorobutane, 1,5-bis(anilino)decafluoropentane, 1,7-bis(anilino)tetrafluoroheptane. These may be used individually or in combination of 2 or more types. Among these, p-phenylenediamine, 4,4'-diaminodiphenyl ether, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane are preferable.

As solid content concentration of the polyamic acid in a polyimide precursor solution, 1-50 mass % is preferable, and 5-25 mass % is more preferable. The polyamic acid contained in a polyimide precursor solution may also be partially imidized.

The viscosity in 30 degreeC of a polyimide precursor solution becomes like this. Preferably it is 1-150 Pa*s, More preferably, it is 10-100 Pa*s.

The solvent used for the polyamide-imide solution and the polyimide precursor solution is not particularly limited as long as it can dissolve polyamide-imide and polyamic acid, but an amide-based solvent is preferably used. Examples of the amide-based solvent include N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc). These may be used individually or in combination of 2 or more types.

Known additives such as various surfactants and organosilane coupling agents can be added to the polyamide-imide solution and the polyimide precursor solution as needed within a range that does not impair the effects of the present invention. In addition, other polymers other than polyamideimide and polyimide precursor can also be added to the polyamideimide solution and polyimide precursor solution as needed within the range that does not impair the effect of the present invention. things.

Example

Hereinafter, examples of the present invention will be described in detail, but the present invention is not limited to these examples.

Measurement and evaluation of various characteristic values in Examples and Comparative Examples are as follows.

(1) Porosity and electrolyte absorption rate of the active material layer

The porosity and electrolyte solution absorption rate of the active material layer were measured by the methods described above.

(II) Discharge characteristics of silicon-based active material layer

First, using the obtained sheet-shaped negative electrode, a bipolar pouch-shaped battery cell (laminated battery cell) was fabricated as a test battery cell for measuring the discharge capacity of the negative electrode by the following method.

The obtained sheet-shaped negative electrode was cut into a rectangle of 10 mm×40 mm, leaving an active material area of 10 mm×10 mm, and covered with a welded film. As a counter electrode, a lithium plate with a thickness of 1 mm was cut into a rectangle of 30 mm×40 mm, and a nickel lead wire (5 mm×50 mm) with a thickness of 0.5 mm was folded in half and crimped. Only the negative electrode was placed in a bag-shaped separator (30 mm×20 mm), and then faced to the counter electrode to obtain an electrode group. As the spacer, a rectangular polypropylene resin porous film (thickness: 25 μm) was used. This electrode group was covered with a set of two rectangular aluminum laminated films (50 mm×40 mm), and after the three sides were sealed, 1 mL of electrolyte solution was injected into the bag-shaped aluminum laminated film. As the electrolytic solution, an electrolytic solution obtained by dissolving LiPF ₆ at a concentration of 1 mol/L in a mixed solvent in which EC, DEC, and EMC were mixed at a volume ratio of 1:1:1 was used. Thereafter, the remaining one side was sealed, and the inside of the bag-shaped aluminum laminated film was sealed. In addition, when sealing the inside of the bag-shaped aluminum laminate film, the negative electrode and one end of the nickel lead were extended to the outside to serve as terminals. In this way, a test battery cell was obtained. All these operations were carried out in an operation box under an argon atmosphere.

Next, using the obtained test battery cell, charge and discharge were repeated according to the above-mentioned <charge and discharge conditions>, and the discharge capacity at the 20th cycle was obtained.

The preparation methods of the binder solution, the silicon dispersion for forming an active material layer, and the graphite dispersion for forming a conductive adhesive layer used in Examples and Comparative Examples are as follows.

[Preparation of Polyimide Precursor Solution]

Reaction of approximately equimolar 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 4,4'-oxydianiline (ODA) in NMP to obtain a solid component as polyimide A homogeneous polyamic acid solution (P-1) having a concentration of 20% by mass and a viscosity of 25 Pa·s at 30°C.

[Preparation of polyamideimide solution]

Approximately equimolar trimellitic anhydride (TMA) and 4,4'-diphenylmethane diisocyanate (DMI) were reacted in NMP to obtain a uniform solid content concentration of 18% by mass and a viscosity of 15 Pa·s at 30°C. Polyamideimide solution (P-2).

[Preparation of silicon dispersion for active material layer formation]

To the solution (P-1) obtained above, silicon particles (purity: 99% by mass) with an average particle diameter of 0.7 μm and graphite particles with an average particle diameter of 3 μm were added in the composition shown in Table 1 to uniformly disperse After stirring in the same way, NMP was added. In this way, a silicon dispersion having a composition shown in Table 1 (solid content concentration about 25% by mass) was obtained.

[Table 1]

<Composition of Silicon Dispersion for Active Material Layer Formation>

[Preparation of Conductive Particle Dispersion for Forming Conductive Adhesive Layer]

In the above-obtained solution (P-1) or solution (P-2), add graphite particles or carbon black (Ketjen black) with an average particle diameter of 3 μm in the composition shown in Table 2, in a uniformly dispersed manner After stirring, NMP was added. Thus, the electroconductive particle dispersion a1-a10 (solid content concentration about 30 mass %) which has a composition shown in Table 2 was obtained.

[Table 2]

<Composition of Conductive Particle Dispersion>

[table 3]

<Negative Electrode Characteristics>

<Example 1>

The conductive particle dispersion a1 was uniformly coated on one side surface of an electrolytic copper foil (manufactured by Furukawa Electric Co., Ltd., F2-WS) with a thickness of 18 μm in sheets using a bar coater, and dried at 130° C. for 10 minutes. A conductive coating film was obtained. The coating amount of the graphite dispersion was prepared so that the thickness of the obtained conductive adhesive layer would be 3 to 4 μm. Next, the silicon dispersion A1 was uniformly coated in sheets using a bar coater on the surface of the conductive coating film, and dried at 130° C. for 10 minutes to obtain an active material coating film. The coating amount of the silicon dispersion was prepared so that the thickness of the obtained active material layer was 40 to 50 μm. In this manner, a laminate in which the electrolytic copper foil, the conductive coating film, and the active material coating film were sequentially laminated was obtained. Next, the obtained laminate was heated from 100° C. to 350° C. over 2 hours under a nitrogen atmosphere, and then heat-treated at 350° C. for 1 hour. This heat treatment converts the polyamic acid in the active material coating film into polyimide. In this manner, the negative electrode A1a1 in which the electrolytic copper foil, the conductive adhesive layer, and the active material layer were sequentially laminated was obtained. Table 3 shows the properties of this negative electrode.

In addition, the charge-discharge curves of the first, fifth, tenth and twentieth cycles during the charge-discharge cycle of the negative electrode are shown in FIG. 1 . It can be seen from Figure 1 that a high discharge capacity can be obtained only for the first time, and in the repeated charge and discharge from the second to the 20th time, although the charge and discharge curve fluctuates, the decrease in the discharge capacity of the negative electrode due to repetition is slight of.

<Examples 2 to 5>

Negative electrode A1a2-A1a5 was obtained by the method similar to Example 1 except having changed electroconductive particle dispersion a1 into electroconductive particle dispersion a2-a5. Table 3 shows the properties of this negative electrode. In these negative electrodes, the charge-discharge curves also showed the same tendency as negative electrode A1a1.

<Examples 6-7>

Negative electrodes A2a1 to A3a1 were obtained in the same manner as in Example 1 except that silicon dispersion A1 was changed to silicon dispersions A2 to A3. Table 3 shows the properties of this negative electrode. In these negative electrodes, the charge-discharge curves also showed the same tendency as negative electrode A1a1.

<Comparative examples 1 to 5>

Negative electrode A1a6-A1a10 was obtained by the method similar to Example 1 except having changed electroconductive particle dispersion a1 into electroconductive particle dispersion a6-a10. Table 3 shows the properties of this negative electrode. In these negative electrodes, the discharge capacity significantly decreases with repeated charge and discharge, and the discharge capacity at the 20th cycle is as low as less than 1000 mAh/g-silicon-based active material.

＜Comparative example 6＞

Negative electrode A4a1 was obtained by the same method as Example 1 except having changed silicon dispersion A1 into silicon dispersion A4. Table 3 shows the properties of this negative electrode. In this negative electrode, the 20th discharge capacity showed a value of 1500mAh/g-silicon-based active material, but the content of the silicon-based active material in the active material layer was as low as 45% by mass, so the discharge capacity as a negative electrode was low.

As described above, the negative electrode of the example of the present invention has a high discharge capacity and excellent charge-discharge cycle characteristics, so it can be suitably used as a negative electrode for a lithium secondary battery.

Claims (5)

1. A negative electrode for a lithium secondary battery, characterized in that it is provided with a negative electrode for a lithium secondary battery that contains an active material layer containing a particulate silicon-based active material and a binding agent, The content of the silicon-based active material in the active material layer is greater than 50% by mass, Under the battery cell configuration and charge-discharge conditions shown below, the discharge capacity at the 20th time when charge-discharge is repeated 20 times is 1500mAh/g-silicon-based active material or more, ＜Composition of the battery unit＞ Battery: Bipolar pouch cell, Counter electrode: metal lithium, Electrolyte: LiPF ₆ is a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate dissolved at a concentration of 1mol/L (volume ratio 1:1:1), ＜Charge and discharge conditions＞ Measuring temperature: 30°C, Voltage range: 0.01 ~ 2V, Charge current and discharge current: 500mA/g-silicon-based active material. 2 . The negative electrode for lithium secondary batteries according to claim 1 , wherein the active material layer has a porosity of 15 to 40% by volume and an electrolyte absorption rate of 300 seconds or less. 3 . 3. The negative electrode for a lithium secondary battery according to claim 1 or 2, comprising a laminate in which an active material layer is laminated on an outer surface of a conductive adhesive layer formed on a current collector. 4. The negative electrode for a lithium secondary battery according to any one of claims 1 to 3, wherein the active material is a particle composed of silicon alone. 5 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the silicon-based active material has an average particle diameter of less than 1 μm.