JP2024041052A - Negative electrode composition for lithium ion batteries and method for producing negative electrodes for lithium ion batteries - Google Patents

Abstract

An object of the present invention is to provide a negative electrode composition for a lithium ion battery that can produce a negative electrode with high electrolyte permeability and excellent charge/discharge efficiency (Coulombic efficiency). [Solution] A negative electrode composition for a lithium ion battery containing a binder resin, a conductive aid, and a negative electrode active material, wherein the binder resin has a liquid absorption rate of 10 when immersed in an electrolytic solution at 50°C for 3 days. -40%, and has an ionic conductivity of 1 x 10-6 to 1 x 10-8 S/cm when immersed in the electrolytic solution at 50°C for 3 days. [Selection diagram] None

Description

The present invention relates to a negative electrode composition for lithium ion batteries and a method for producing a negative electrode for lithium ion batteries.

Lithium-ion batteries have the characteristics of high voltage and high energy density, so they are widely used in the field of mobile information devices and have established themselves as the standard battery for mobile devices such as mobile phones and notebook computers. There is. Its applications are expanding, and in addition to conventional applications, applications such as hybrid vehicles and electric vehicles are being considered, and some have already been put into practical use. In order to further popularize these batteries, higher capacity and higher output are required for lithium ion batteries, and various technologies are being applied.

One way to improve the capacity of secondary batteries is to improve electrode density. More capacity can be obtained by densely packing the active material. However, when the density of the electrode is increased, it becomes difficult for the electrolyte to penetrate into the electrode, resulting in a problem that a smaller capacity than the theoretical value can be obtained and the output characteristics deteriorate.

In order to solve such problems, Patent Document 1 discloses a technique in which electrolyte permeability can be improved by providing grooves on the electrode surface. Patent Document 2 discloses a technique for improving the permeability of an electrolytic solution by modifying the particle size and shape of an active material. Further, Patent Document 3 discloses a technique for improving the permeability of an electrolytic solution by adjusting electrode density.

Japanese Patent Application Publication No. 2008-27633 Japanese Patent Application Publication No. 2012-151088 JP2020-053282A

However, the method disclosed in Patent Document 1 involves a step of pressing with a roller with unevenness in order to form grooves on the electrode surface, and there is a problem in that new equipment needs to be introduced.
Moreover, although a certain improvement in permeability was observed in the methods of Patent Documents 2 and 3, the effect was not sufficient.

The present invention solves the above problems, and provides an electrode composition for lithium ion batteries, particularly a negative electrode composition, which can produce an electrode with high electrolyte permeability and excellent charge/discharge efficiency (Coulombic efficiency). The purpose is to

The present inventors have arrived at the present invention as a result of intensive studies.
The present invention is a negative electrode composition for a lithium ion battery comprising a binder resin, a conductive aid, and a negative electrode active material, wherein the binder resin has a liquid absorption rate of 10 when immersed in an electrolytic solution at 50°C for 3 days. 40%, and the ionic conductivity when immersed in the electrolyte at 50°C for 3 days is 1×10 ⁻⁶ to 1×10 ⁻⁸ S/cm, and the electrolyte is ethylene carbonate (EC ), an electrolytic solution in which LiPF ₆ is dissolved as an electrolyte to a concentration of 1 mol/L in a mixed solvent in which diethyl carbonate (DEC) is mixed at a volume ratio of EC:DEC=3:7,
The liquid absorption rate is determined by the following formula: Liquid absorption rate (%) = [(Weight of binder resin after immersion in electrolyte solution - Weight of binder resin before immersion in electrolyte solution) / Weight of binder resin before immersion in electrolyte solution] x 100
is required,
A negative electrode composition for a lithium ion battery, wherein the ionic conductivity is an ionic conductivity at 25° C. determined by an AC impedance method, and the binder resin, conductive aid, negative electrode active material, and aqueous solvent. a mixing step of preparing a slurry containing lithium, a coating step of applying the slurry to a current collector, and a drying step of drying the slurry after the coating step to form a negative electrode active material layer on the current collector. The present invention relates to a method for manufacturing a negative electrode for an ion battery.

According to the present invention, it is possible to provide a negative electrode composition for a lithium ion battery that can produce a negative electrode that has high electrolyte permeability and excellent charge/discharge efficiency (Coulombic efficiency).

[Negative electrode composition for lithium ion batteries]
The negative electrode composition for lithium ion batteries of the present invention is a negative electrode composition for lithium ion batteries containing a binder resin, a conductive additive, and a negative electrode active material.
The binder resin has a liquid absorption rate of 10 to 40% when immersed in an electrolytic solution at 50° C. for 3 days.
The liquid absorption rate when immersed in the electrolytic solution is determined by the following formula by measuring the weight of the binder resin before and after immersion in the electrolytic solution.
Liquid absorption rate (%) = [(Weight of binder resin after immersion in electrolyte solution - Weight of binder resin before immersion in electrolyte solution) / Weight of binder resin before immersion in electrolyte solution] x 100

The electrolytic solution for determining the liquid absorption rate was a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of EC:DEC=3:7, and 1 mol/L of LiPF ₆ as an electrolyte. An electrolyte solution is used that has been dissolved to the desired concentration.

Samples for the liquid absorption test are prepared as follows.
A binder resin solution with a resin concentration of 30% by weight (N'-dimethylformamide (hereinafter referred to as DMF) was used as the solvent) was poured into a PP tray (14 cm x 10 cm) to a height of about 3 mm, and dried at 70°C in a fair air. A resin sheet was prepared by drying in a vacuum dryer for 3 hours and then in a vacuum dryer (>-90 kPa) for 3 hours.
This resin sheet was cut into 1.5 cm square pieces, and the thickness of the sample was measured using a handy film thickness meter (manufactured by Mitutoyo) and confirmed to be about 1 mm, and then used for testing.

Immersion in the electrolytic solution when determining the liquid absorption rate is performed at 50° C. for 3 days. The resin sheet was immersed in the electrolytic solution by placing it in a screw tube (No. 7, capacity 50 mL), and the resin sheet was completely immersed in the electrolytic solution. By immersing at 50° C. for 3 days, the binder resin becomes saturated with liquid absorption. Note that the saturated liquid absorption state refers to a state in which the weight of the binder resin does not increase even if it is further immersed in the electrolytic solution.
Note that the electrolytic solution used when manufacturing a lithium ion battery using the binder resin of the present invention is not limited to the above electrolytic solution, and other electrolytic solutions may be used.

If the liquid absorption rate is less than 10%, the electrolytic solution will not easily penetrate into the binder resin, resulting in low lithium ion conductivity and the battery may not exhibit its full performance as a lithium ion battery. When the liquid absorption rate exceeds 40%, the binder resin expands excessively in the electrode, which tends to destroy the electrode structure, and the conductive path may be cut, leading to accelerated deterioration of the lithium ion battery.
The liquid absorption rate is preferably 20% or more, more preferably 30% or more.

The liquid absorption rate of the binder resin when immersed in an electrolytic solution at 50° C. for 3 days can be adjusted by the SP value of the binder resin. By increasing the SP value of the binder resin (reducing the SP value difference with the electrolyte), the electrolyte absorption rate of the binder resin can be increased.
The SP value of the binder resin is preferably 10.0 to 15.0, more preferably 12.5 to 14.0.
Note that the SP value (solubility parameter) [unit: (cal/cm ³ ) ^1/2 ] in the present invention is determined by the Fedors method (Polymer Engineering and Science, February, 1974, Vol. 14, No. 2 P. 147-154) ), using the values (heat of vaporization and molar volume at 25 °C of atoms or functional groups) described on page 152 (Table.5), and the value calculated by the method described in formula (28) on page 153. be.

The binder resin has an ionic conductivity of 1×10 ⁻⁶ to 1×10 ⁻⁸ S/cm when immersed in the electrolytic solution at 50° C. for 3 days.
The ionic conductivity is determined by measuring the conductivity of the binder resin at 25° C. by an AC impedance method when the binder resin is immersed in the electrolytic solution at 50° C. for 3 days.

The electrolytic solution for determining the ionic conductivity is a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of EC:DEC=3:7, and 1 mol/L of LiPF ₆ as an electrolyte. An electrolyte solution is used that has been dissolved to the desired concentration.

In the present application, the lithium ion conductivity of the binder resin is determined by measuring with an AC impedance method using a modular charge/discharge measurement system [Toyo Technica Co., Ltd.].
Preparation of the sample for lithium ion conductivity measurement and immersion in the electrolytic solution are performed in the same manner as for the sample for the liquid absorption test.
AC impedance measurement is performed at 25° C. at a frequency of 2.0×10 ⁴ Hz to 1.0×10 ⁻¹ Hz, voltage swing width is 1000 mA, and real number impedance component R (Ω) is determined. The ion conductivity σ (S/cm) of the resin sheet is determined from the impedance component R (Ω), the thickness d (cm) of the resin sheet, and the contact area A (cm ² ) between the electrode and the resin sheet.
Ion conductivity σ (S/cm) = d/(R x A)

If the ionic conductivity of the binder resin when immersed in the electrolytic solution at 50° C. for 3 days is less than 1×10 ⁻⁶ S/cm, the binder resin will swell too much with the electrolytic solution and will not adhere to the current collector. This may deteriorate the cycle characteristics. Furthermore, if the ionic conductivity of the binder resin exceeds 1×10 ⁻⁸ S/cm, the movement of lithium ions between the active material and the electrolyte will be hindered, and the performance as a lithium ion battery will not be fully demonstrated. There is.
The ionic conductivity is preferably 1×10 ⁻⁷ to 9×10 ⁻⁷ S/cm, more preferably 2×10 ⁻⁷ to 5×10 ⁻⁷ S/cm.

The binder resin is prepared by adding LiPF ₆ as an electrolyte to a mixed solvent in which ethylene carbonate (EC) and polycarbonate (PC) are mixed in a volume ratio of EC:PC=1:1 to the electrolyte at a concentration of 1 mol/L. Even when using an electrolytic solution dissolved in a liquid, the ionic conductivity in a saturated liquid absorption state is preferably 1×10 ⁻⁶ to 1×10 ⁻⁸ S/cm.
Further, it is more preferable that the binder resin has an ionic conductivity of 1×10 ⁻⁶ to 1×10 ⁻⁸ S/cm even when not impregnated with an electrolyte.

The binder resin was heated at 50° C. for 3 days, and 1 mol/L of LiPF ₆ was added as an electrolyte to the electrolytic solution (a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of EC:DEC=3:7). The ionic conductivity when immersed in an electrolytic solution dissolved to a concentration of L can be adjusted by the SP value of the binder resin. By increasing the SP value of the binder resin (reducing the difference in SP value from the electrolyte), the electrolyte absorption rate increases and the ionic conductivity can be increased.
The SP value of the binder resin is preferably 10.0 to 15.0, more preferably 12.5 to 14.0.

It is preferable that the binder resin has a contact angle of the electrolytic solution with respect to the binder resin of less than 30°. When the contact angle of the electrolytic solution to the binder resin is less than 30°, the electrolytic solution will have good permeability into the binder resin, and lithium ion conductivity will be high, resulting in improved battery performance. The contact angle of the electrolytic solution with the binder resin is more preferably 10 to 25°.
The contact angle of the electrolytic solution with respect to the binder resin can be adjusted by the SP value of the binder resin. By increasing the SP value of the binder resin (reducing the SP value difference with the electrolyte), the contact angle of the electrolyte with the binder resin can be decreased.

The contact angle of the electrolytic solution with respect to the binder resin is measured using a dynamic contact angle measuring device "DMo-701" manufactured by Kyowa Interface Science Co., Ltd.
In the present application, the contact angle of the electrolytic solution with respect to the binder resin is measured as follows.
After dissolving 3.0 g of binder resin in 30 g of DMF, it was applied onto a horizontal glass plate and air-dried for half a day at room temperature. Next, it was allowed to stand for 3 hours at -90 to -100 kMPa in a vacuum dryer heated to 160°C, and then cooled to 25°C to prepare a measurement sample. Next, using a dynamic contact angle measuring device "DMo-701" manufactured by Kyowa Interface Science Co., Ltd. as a measurement device, the measurement was performed at 25°C 30 seconds after a 2.5 μL electrolyte droplet landed on the sample. The value was measured as the contact angle (°).

When the binder resin is insoluble in DMF, the contact angle of the electrolytic solution with respect to the binder resin is determined in the same manner as above except that 3.0 g of the binder resin is dissolved in 30 g of isopropanol and the vacuum drying temperature is changed to 80°C. (°) was measured.

It is preferable that the binder resin has a contact angle of water with respect to the binder resin of less than 40°. When the contact angle of water with respect to the binder resin is less than 40°, the binder resin and water will be compatible with each other, and it will be easier to prepare a uniform aqueous slurry especially when preparing a negative electrode. The contact angle of water with respect to the binder resin is more preferably 10 to 25°.
The contact angle of water with respect to the binder resin can be adjusted by the weight average molecular weight and/or SP value of the binder resin. If the weight average molecular weight of the binder resin is decreased, the contact angle of water with respect to the binder resin tends to decrease, and by increasing the SP value of the binder resin (reducing the difference in SP value with water), the binder resin The contact angle of water to the resin can be reduced.

The contact angle of water with respect to the binder resin is measured using a dynamic contact angle measuring device "DMo-701" manufactured by Kyowa Interface Science Co., Ltd.
In the present application, the contact angle of water with respect to the binder resin is measured as follows.
Ion-exchanged water was added to the binder resin to adjust the solid content concentration to 5% by weight. The binder resin aqueous solution after adjusting the solid content concentration was applied onto a copper foil and dried in an oven at 50° C. for 10 minutes to produce a binder film with a thickness of 100 μm. 1 μL of ion-exchanged water was dropped onto the obtained binder film, and the water droplet on the binder film was photographed 1 minute after the droplet landed under conditions of a temperature of 25°C and a humidity of 50%, and the contact angle was measured. It was measured by the θ/2 method. The operation from dropping distilled water to measuring the contact angle was performed a total of five times at different locations on the binder film, and the average value of the five measured values was taken as the contact angle of water with respect to the binder resin.

When the binder resin is insoluble in ion-exchanged water, the contact angle (°) of water with respect to the binder resin was measured in the same manner as above except that the ion-exchanged water used to prepare the binder resin aqueous solution was changed to isopropanol. .

The weight average molecular weight of the binder resin is preferably 95,000 to 200,000. When the weight average molecular weight of the binder resin is 95,000 to 200,000, the resulting electrode has excellent strength. The weight average molecular weight of the binder resin is more preferably 120,000 to 140,000.

The weight average molecular weight of the binder resin can be determined by GPC (gel permeation chromatography) measurement under the following conditions. The sample polymer was dissolved in a solvent such as orthodichlorobenzene, N'-dimethylformamide (DMF), or tetrahydrofuran (THF) to prepare a 0.25% by weight solution, and the undissolved portion was removed using a 1 μm diameter tube. The solution filtered through a PTFE filter is used as a sample solution.
Equipment: Alliance GPC V2000 (manufactured by Waters)
Solvent: orthodichlorobenzene, DMF, THF
Standard material: polystyrene Sample concentration: 3mg/ml
Column stationary phase: PLgel 10um, MIXED-B 2 in series (manufactured by Polymer Laboratories)
Column temperature: 135℃

The binder resin contains the monomer (a1) in an amount of 90% by weight or more based on the total weight of the constituent monomers of the binder resin, and the monomer (a1) contains acrylic acid. , methyl acrylate, methacrylic acid, and methyl methacrylate. With the above composition, the negative electrode active material and the conductive additive are compatible with each other, and the electrode strength is improved.

The negative electrode composition for lithium ion batteries of the present invention contains a conductive additive. There are no particular limitations on the conductive aid as long as it is a conductive material.
Examples of conductive aids include metals [aluminum, stainless steel (SUS), silver, gold, copper, titanium, etc.], carbon [graphite (flake graphite (UP)), carbon black (acetylene black, Ketjen black, furnace black, channel black, thermal lamp black, etc.), carbon nanofibers (CNF), etc.], and mixtures thereof.
These conductive aids may be used alone or in combination of two or more. Also, alloys or metal oxides of these may be used. From the viewpoint of electrical stability, acetylene black is preferred.

The negative electrode composition for a lithium ion battery of the present invention includes a negative electrode active material.
As the negative electrode active material, carbon-based materials [graphite, non-graphitizable carbon (hard carbon), amorphous carbon, fired resin bodies (for example, carbonized products by firing phenol resin and furan resin, etc.), coke, etc. (e.g. pitch coke, needle coke, petroleum coke, etc.) and carbon fibers], silicon-based materials [silicon, silicon oxide (SiOx), silicon-carbon composites (carbon particles whose surfaces are coated with silicon and/or silicon carbide) silicon particles or silicon oxide particles whose surfaces are coated with carbon and/or silicon carbide, silicon carbide, etc.) and silicon alloys (silicon-aluminum alloys, silicon-lithium alloys, silicon-nickel alloys, silicon-iron alloys) , silicon-titanium alloy, silicon-manganese alloy, silicon-copper alloy, silicon-tin alloy, etc.), conductive polymers (e.g., polyacetylene and polypyrrole, etc.), metals (tin, aluminum, zirconium, titanium, etc.), metals Examples include oxides (titanium oxide and lithium titanium oxide, etc.), metal alloys (for example, lithium-tin alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.), and mixtures of these with carbon-based materials. It will be done.
From the viewpoint of battery capacity, graphite or hard carbon is preferable as the negative electrode active material.

The weight proportion of the binder resin in the negative electrode composition for lithium ion batteries of the present invention is 1 to 5% by weight based on the weight of the negative electrode composition for lithium ion batteries from the viewpoint of binding performance and electrical properties. It is preferable that there be.
Further, from the viewpoint of battery performance, the weight proportion of the negative electrode active material in the negative electrode composition for lithium ion batteries is preferably 90 to 98% by weight based on the weight of the negative electrode composition for lithium ion batteries.

[Method for manufacturing negative electrode for lithium ion battery]
The present invention includes a mixing step of preparing a slurry containing the binder resin, a conductive aid, a negative electrode active material, and an aqueous solvent, a coating step of applying the slurry to a current collector, and drying the slurry after the coating step. and a drying step of forming a negative electrode active material layer on the current collector.

(Mixing process)
The present invention includes a mixing step of preparing a slurry containing the binder resin, conductive aid, negative electrode active material, and an aqueous solvent.
In the mixing step, a slurry containing the binder resin, conductive support agent, negative electrode active material, and an aqueous solvent is prepared. The method for mixing the binder resin, conductive aid, and negative electrode active material with the aqueous solvent is not particularly limited, and any known method can be used.
Furthermore, there is no restriction on the order of mixing, and the binder resin, conductive aid, negative electrode active material, and aqueous solvent may be mixed in any order.

As the aqueous solvent, any liquid that has water as an essential component can be used without any restriction, such as water, an aqueous solution of an organic solvent, an aqueous solution of a surfactant, an aqueous solution of a water-soluble polymer, a mixture of two or more of these, etc., which will be described later. can be used.

(Coating process)
The present invention includes a coating step of applying the slurry obtained in the mixing step to a current collector.
The coating method is not particularly limited, and a known coating device such as a bar coater can be used.

Examples of the material constituting the current collector include metal materials such as copper, aluminum, titanium, stainless steel, nickel, and alloys thereof, as well as fired carbon, conductive polymer materials, conductive glass, and the like.
The shape of the current collector is not particularly limited, and may be a sheet-like current collector made of the above-mentioned material or a deposited layer made of fine particles made of the above-mentioned material.
The thickness of the current collector is not particularly limited, but is preferably 50 to 500 μm.

(drying process)
The present invention includes a drying step of drying the slurry after the coating step to form a negative electrode active material layer on the current collector.
It is preferable to remove the aqueous solvent contained in the slurry in the drying step. As a method for removing the aqueous solvent, drying using an oven or a vacuum oven is preferable. Examples of the atmosphere for removing the aqueous solvent include air, an inert gas, and a vacuum state. Further, the temperature at which the aqueous solvent is removed is preferably 60 to 250°C.

The following items are disclosed in this specification.

This disclosure (1) is
A negative electrode composition for a lithium ion battery comprising a binder resin, a conductive aid, and a negative electrode active material, wherein the binder resin has a liquid absorption rate of 10 to 40% when immersed in an electrolytic solution at 50°C for 3 days. The ionic conductivity when immersed in the electrolyte at 50° C. for 3 days is 1×10 ⁻⁶ to 1×10 ⁻⁸ S/cm, and the electrolyte is ethylene carbonate (EC), diethyl carbonate, etc. It is an electrolytic solution in which LiPF ₆ is dissolved as an electrolyte to a concentration of 1 mol/L in a mixed solvent in which (DEC) is mixed at a volume ratio of EC:DEC=3:7,
The liquid absorption rate is determined by the following formula: Liquid absorption rate (%) = [(Weight of binder resin after immersion in electrolyte solution - Weight of binder resin before immersion in electrolyte solution) / Weight of binder resin before immersion in electrolyte solution] x 100
is required,
The negative electrode composition for a lithium ion battery is characterized in that the ionic conductivity is an ionic conductivity at 25° C. determined by an AC impedance method.

The present disclosure (2) is the negative electrode composition for a lithium ion battery according to the present disclosure (1), wherein a contact angle of the electrolytic solution with the binder resin is less than 30°.

The present disclosure (3) is the negative electrode composition for a lithium ion battery according to the present disclosure (1) or (2), wherein the binder resin has a weight average molecular weight of 95,000 to 200,000.

The present disclosure (4) provides that the weight ratio of the binder resin in the negative electrode composition for lithium ion batteries is 1 to 5% by weight based on the weight of the negative electrode composition for lithium ion batteries, and The lithium ion according to any one of the present disclosure (1) to (3), wherein the weight ratio of the negative electrode active material in the negative electrode composition is 90 to 98% by weight based on the weight of the negative electrode composition for lithium ion batteries. This is a negative electrode composition for batteries.

The present disclosure (5) includes a mixing step of preparing a slurry containing the binder resin, a conductive aid, a negative electrode active material, and an aqueous solvent, a coating step of applying the slurry to a current collector, and a step of applying the slurry to a current collector, and after the coating step. The method of manufacturing a negative electrode for a lithium ion battery includes a drying step of drying a slurry to form a negative electrode active material layer on the current collector.

EXAMPLES Next, the present invention will be specifically explained with reference to examples, but the present invention is not limited to the examples unless it departs from the gist of the present invention.

(Production Example 1: Production of binder resin (A-1))
150 parts by weight of DMF was charged into a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube, and the temperature was raised to 75°C. Next, a monomer composition containing 90 parts by weight of acrylic acid, 10 parts by weight of methacrylic acid and 50 parts by weight of DMF, 0.5 parts by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) and 2, An initiator solution prepared by dissolving 0.5 parts by weight of 2'-azobis(2-methylbutyronitrile) in 30 parts of DMF was continuously added to a four-necked flask over a period of 2 hours using a dropping funnel under stirring while blowing nitrogen. Radical polymerization was carried out by adding the mixture dropwise. After the dropwise addition was completed, the reaction was continued at 75°C for 3 hours. Next, the temperature was raised to 80° C. and the reaction was continued for 3 hours to obtain a binder resin solution with a resin concentration of 30% by weight. This binder resin solution was dried under reduced pressure at 160° C. and 0.01 MP for 3 hours, and DMF was distilled off to obtain a binder resin (A-1) (Mw: 70,000).

(Production Example 2: Production of binder resin (A-2))
150 parts by weight of DMF was charged into a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube, and the temperature was raised to 75°C. Next, a monomer composition containing 90 parts by weight of acrylic acid, 10 parts by weight of methacrylic acid, and 50 parts by weight of DMF, and 0.2 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) and 2,2 parts by weight were added. An initiator solution prepared by dissolving 0.5 parts by weight of '-azobis(2-methylbutyronitrile) in 30 parts by weight of DMF was continuously added to a four-necked flask over a period of 2 hours using a dropping funnel under stirring while blowing nitrogen into the 4-necked flask. Radical polymerization was carried out by adding the mixture dropwise. After the dropwise addition was completed, the reaction was continued at 75°C for 3 hours. Next, the temperature was raised to 80° C. and the reaction was continued for 3 hours to obtain a binder resin solution with a resin concentration of 30% by weight. This binder resin solution was dried under reduced pressure at 160° C. and 0.01 MP for 3 hours, and DMF was distilled off to obtain a binder resin (A-2) (Mw: 120,000).

(Production Example 3: Production of binder resin (A-3))
150 parts by weight of DMF was charged into a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube, and the temperature was raised to 75°C. Next, a monomer composition containing 90 parts by weight of acrylic acid, 10 parts by weight of methacrylic acid and 50 parts by weight of DMF, 0.2 parts by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) and 2, An initiator solution prepared by dissolving 0.3 parts by weight of 2'-azobis(2-methylbutyronitrile) in 30 parts by weight of DMF was added to a four-necked flask while blowing nitrogen through the dropping funnel for 2 hours under stirring. Radical polymerization was carried out by continuous dropwise addition. After the dropwise addition was completed, the reaction was continued at 75°C for 3 hours. Next, the temperature was raised to 80° C. and the reaction was continued for 3 hours to obtain a binder resin solution with a resin concentration of 30% by weight. This binder resin solution was dried under reduced pressure at 160° C. and 0.01 MP for 3 hours, and DMF was distilled off to obtain a binder resin (A-3) (Mw: 250,000).

(Production Examples 4 to 7 and Comparative Production Examples 3 and 4: Production of binder resins (A-4) to (A-7) and (AX-4) and (AX-5))
Binder resins (A-4) to (A-7), (AX-4), and (AX-5) were prepared in the same manner as in Production Example 1, except that the type of monomer and the weight parts charged were changed as shown in Table 1. was manufactured.
As the binder resin (AX-1), styrene-butadiene rubber (hereinafter referred to as SBR) [Nippon Zeon Co., Ltd., trademark BM-400B, solid content 40% by weight, Tg -5°C] was used at 100°C and 0.01MP. The sample was dried under reduced pressure for 3 hours before use.

(Production Example 8: Production of binder resin (A-8))
A four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube was charged with 300 parts by weight of ion-exchanged water, and the temperature was raised to 70°C. Next, a monomer composition containing 90 parts by weight of acrylic acid, 10 parts by weight of methyl methacrylate, and 50 parts by weight of ion-exchanged water, and 2.5 parts by weight of 2,2'-azobis(2-methylpropionamidine) dihydrochloride were added. Radical polymerization was carried out by continuously dropping an initiator solution in which parts by weight were dissolved in 50 parts by weight of ion-exchanged water using a dropping funnel over a period of 3 hours while stirring and blowing nitrogen into a four-necked flask. After the dropwise addition was completed, the reaction was continued at 80°C for 2 hours. A binder resin solution with a resin concentration of 20% by weight was obtained. This binder resin solution was dried under reduced pressure at 100° C. and 0.01 MP for 1 hour, and the ion exchange water was distilled off to obtain a binder resin (A-8) (Mw: 130,000).

(Production Example 9: Production of binder resin (A-9))
A four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube was charged with 300 parts by weight of ion-exchanged water, and the temperature was raised to 70°C. Next, a monomer composition containing 90 parts by weight of acrylic acid, 10 parts by weight of methyl methacrylate, and 50 parts by weight of ion-exchanged water, and 2.0 parts by weight of 2,2'-azobis(2-methylpropionamidine) dihydrochloride were added. Radical polymerization was carried out by continuously dropping an initiator solution in which parts by weight were dissolved in 50 parts by weight of ion-exchanged water using a dropping funnel over a period of 3 hours while stirring and blowing nitrogen into a four-necked flask. After the dropwise addition was completed, the reaction was continued at 80°C for 2 hours. A binder resin solution with a resin concentration of 20% by weight was obtained. This binder resin solution was dried under reduced pressure at 100° C. and 0.01 MP for 1 hour, and the ion-exchanged water was distilled off to obtain a binder resin (A-9) (Mw: 180,000).

(Production Example 10: Production of binder resin (A-10))
A four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube was charged with 300 parts by weight of ion-exchanged water, and the temperature was raised to 70°C. Next, a monomer composition containing 90 parts by weight of acrylic acid, 10 parts by weight of methyl methacrylate, and 50 parts by weight of ion-exchanged water, and 1.5 parts by weight of 2,2'-azobis(2-methylpropionamidine) dihydrochloride were added. Radical polymerization was carried out by continuously dropping an initiator solution in which parts by weight were dissolved in 50 parts by weight of ion-exchanged water using a dropping funnel over a period of 3 hours while stirring and blowing nitrogen into a four-necked flask. After the dropwise addition was completed, the reaction was continued at 80°C for 2 hours. A binder resin solution with a resin concentration of 20% by weight was obtained. This binder resin solution was dried under reduced pressure at 100° C. and 0.01 MP for 1 hour, and the ion exchange water was distilled off to obtain a binder resin (A-10) (Mw: 240,000).

(Production Example 11: Production of Binder Resin (A-11))
A four-neck flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel and a nitrogen gas inlet tube was charged with 300 parts by weight of ion-exchanged water and heated to 70°C. Next, a monomer composition containing 50 parts by weight of acrylic acid, 50 parts by weight of hydroxyethyl acrylate and 50 parts by weight of ion-exchanged water, and an initiator solution containing 2.5 parts by weight of 2,2'-azobis(2-methylpropionamidine)dihydrochloride dissolved in 50 parts by weight of ion-exchanged water were continuously dropped into the four-neck flask with nitrogen for 3 hours under stirring to carry out radical polymerization. After the dropwise addition was completed, the reaction was continued for 2 hours at 80°C. A binder resin solution with a resin concentration of 20% by weight was obtained. This binder resin solution was dried under reduced pressure at 100°C and 0.01MP for 1 hour, and the ion-exchanged water was distilled off to obtain a binder resin (A-11) (Mw: 150,000).

(Comparative production example 1: Production of binder resin (AX-2))
150 parts by weight of isopropanol was charged into a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube, and the temperature was raised to 70°C. Next, a monomer containing 65 parts by weight of 2-ethylhexyl methacrylate, 30 parts by weight of 2-ethylhexyl acrylate, 4.6 parts by weight of acrylic acid, 0.4 parts by weight of 1,6-hexanediol dimethacrylate, and 50 parts by weight of isopropanol was prepared. The composition, 0.2 parts by weight of 2,2'-azobis(2,4-dimethylvaleronitrile) and 0.3 parts by weight of 2,2'-azobis(2-methylbutyronitrile) in 30 parts by weight of isopropanol. Radical polymerization was carried out by continuously dropping the dissolved initiator solution into a four-necked flask over a period of 2 hours using a dropping funnel while stirring and blowing nitrogen into the flask. After the dropwise addition was completed, the reaction was continued at 70°C for 3 hours. Next, the temperature was raised to 80° C. and the reaction was continued for 3 hours to obtain a binder resin solution with a resin concentration of 30% by weight. This binder resin solution was dried under reduced pressure at 100° C. and 0.01 MP for 3 hours, and isopropanol was distilled off to obtain a binder resin (AX-2) (Mw: 150,000).

(Comparative production example 2: Production of binder resin (AX-3))
150 parts by weight of isopropanol was charged into a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube, and the temperature was raised to 70°C. Next, a monomer composition containing 65 parts by weight of 2-ethylhexyl methacrylate, 30 parts by weight of 2-ethylhexyl acrylate, 5 parts by weight of acrylic acid and 50 parts by weight of isopropanol, and 2,2'-azobis(2,4-dimethyl Nitrogen was blown into a four-necked flask with an initiator solution prepared by dissolving 0.2 parts by weight of valeronitrile and 0.3 parts by weight of 2,2'-azobis(2-methylbutyronitrile) in 30 parts by weight of isopropanol. While stirring, the mixture was continuously added dropwise from a dropping funnel over 2 hours to carry out radical polymerization. After the dropwise addition was completed, the reaction was continued at 70°C for 3 hours. Next, the temperature was raised to 80° C. and the reaction was continued for 3 hours to obtain a binder resin solution with a resin concentration of 30% by weight. This binder resin solution was dried under reduced pressure at 100° C. and 0.01 MP for 3 hours, and isopropanol was distilled off to obtain a binder resin (AX-2) (Mw: 120,000).

(Examples 1 to 11, Comparative Examples 1, 4, 5: Preparation of negative electrode composition for lithium ion battery and preparation of negative electrode)
A resin solution was obtained by dissolving the binder resin in ion-exchanged water at a concentration of 10% by weight.
94 parts by weight of graphite, 3 parts by weight of acetylene black (AB) [DENKA BLACK Li-100], 1.5 parts by weight of sodium carboxymethylcellulose, 1.5 parts by weight of the above resin solution, and 100 parts by weight of ion-exchanged water were prepared. A slurry composition for a negative electrode was prepared by stirring with a foaming Rentaro at 2000 rpm for 1 minute. This negative electrode slurry composition was applied to a 30 μm thick copper foil using an applicator so that the film thickness after drying would be about 100 μm, and dried at 100° C. for 2 hours to obtain an electrode sheet. This electrode sheet was punched out to a diameter of 16 mm to produce a negative electrode for a lithium ion battery.

(Comparative Examples 2 and 3: Preparation of negative electrode composition for lithium ion battery and preparation of negative electrode)
A negative electrode for a lithium ion battery was produced in the same manner as in Example 1, except that a resin solution was obtained by dissolving the binder resin in isopropanol at a concentration of 10% by weight.

<Evaluation of electrolyte permeability (electrolyte permeation time: injection speed)>
5 μL of the electrolytic solution was dropped onto the produced negative electrode for electrolyte permeability evaluation, and the time (seconds) until the electrolytic solution soaked into the electrode was measured. The shorter the time until penetration, the better the permeability.
When an electrolytic solution is dripped onto the negative electrode for evaluating electrolyte permeability, the color of the part where the electrolytic solution is dripped becomes darker. Since the color returns to its original color when the electrolyte permeates into the electrode, the time required for the negative electrode for electrolyte permeability evaluation to return to its original color was measured.
The measurement results are shown in Table 2.
The negative electrode for electrolyte permeability evaluation was obtained by pressing the prepared negative electrode for a lithium ion battery using a pressure press to form a predetermined void (15%). The electrolytic solution used is one in which LiPF ₆ is dissolved at a ratio of 1 mol/L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 3:7).

(Preparation of battery for performance evaluation)
The produced negative electrode for a lithium ion battery was placed at both ends of a 2032 type coin cell together with a positive electrode for a lithium ion battery punched out to a diameter of 15 mm. As the current collector on the positive electrode side, aluminum foil with a thickness of 20 μm was used. A separator (Celguard 3501) was inserted between the electrodes to produce a battery cell for evaluation.
The electrolytic solution was injected into the evaluation battery cells and sealed, thereby producing evaluation batteries according to Examples and Comparative Examples.

<Battery evaluation>
At room temperature, using a charge/discharge measuring device [HJ0501SM8A] [manufactured by Hokuto Denko Co., Ltd.], the battery was charged at CC-CV (cutoff current 0.01C) at 0.1C to 4.2C, and then rested for 1 hour. Afterwards, discharge was performed at 0.1C to 2.5V. The charging capacity at this time was defined as the initial charging capacity X0, and the discharging capacity was defined as the initial capacity Y0. Thereafter, charging and discharging were repeated to obtain a discharge capacity Y1 at the 50th cycle.
The initial coulombic efficiency was calculated using the following formula, and the results are shown in Table 2.
Initial coulombic efficiency (%) = Initial discharge capacity (Y0) / Initial charge capacity (X0) x 100

The cycle characteristics were measured as follows: 50-cycle discharge capacity retention rate (%)=Y1/Y0×100. The results are shown in Table 2.

<Rate test evaluation>
At room temperature, using a charge/discharge measuring device [HJ0501SM8A] [manufactured by Hokuto Denko Co., Ltd.], the battery was charged by CC-CV (cutoff current 0.005C) at 0.05C to 4.2V, and then rested for 1 hour. Afterwards, discharge was performed at 0.05C to 2.5V. After resting for 1 hour, it was charged by CC-CV (cutoff current 0.01C) at 0.1C to 4.2V, and after resting for 1 hour, it was discharged to 2.5V at 0.1C. After that, tests were conducted under the same conditions at a charging rate of 0.1C, but increasing the C rate for each cycle with a discharge rate of 0.2C, 0.33C, 0.5C, 1C, 2C, 3C, and 5C. . At this time, let the discharge capacity at 0.1C be Z(0.1C), and the discharge capacities obtained at 1C, 3C, and 5C be Z(1C), Z(3C), and Z(5C), respectively, and use the following formula. The discharge rates (%) of 1C, 3C, and 5C were calculated using the formula. The results are shown in Table 2.
Discharge rate (1C) = Z (1C) / Z (0.1C) x 100 (%)
Discharge rate (3C) = Z (3C) / Z (0.1C) x 100 (%)
Discharge rate (5C) = Z (5C) / Z (0.1C) x 100 (%)

From Table 2, it can be seen that the electrodes of each Example had a shorter electrolyte permeation time and higher initial Coulombic efficiency than the electrodes of each Comparative Example.

The lithium ion battery obtained from the negative electrode composition for a lithium ion battery of the present invention is particularly useful as a lithium ion battery used for mobile phones, personal computers, hybrid vehicles, and electric vehicles.

Claims (5)

A negative electrode composition for a lithium ion battery comprising a binder resin, a conductive aid, and a negative electrode active material,
The binder resin has a liquid absorption rate of 10 to 40% when immersed in the electrolytic solution at 50°C for 3 days, and an ionic conductivity of 1 x 10 when immersed in the electrolytic solution at 50°C for 3 days. ^-6 to 1×10 ^-8 S/cm,
The electrolyte was prepared by dissolving LiPF ₆ as an electrolyte at a concentration of 1 mol/L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of EC:DEC=3:7. It is an electrolyte,
The liquid absorption rate is determined by the following formula: Liquid absorption rate (%) = [(Weight of binder resin after immersion in electrolyte solution - Weight of binder resin before immersion in electrolyte solution) / Weight of binder resin before immersion in electrolyte solution] x 100
is required,
A negative electrode composition for a lithium ion battery, wherein the ionic conductivity is an ionic conductivity at 25° C. determined by an AC impedance method. The negative electrode composition for a lithium ion battery according to claim 1, wherein a contact angle of the electrolytic solution with the binder resin is less than 30°. The negative electrode composition for a lithium ion battery according to claim 1, wherein the binder resin has a weight average molecular weight of 95,000 to 200,000. The weight proportion of the binder resin in the negative electrode composition for lithium ion batteries is 1 to 5% by weight based on the weight of the negative electrode composition for lithium ion batteries,
The negative electrode composition for lithium ion batteries according to claim 1, wherein the weight ratio of the negative electrode active material in the negative electrode composition for lithium ion batteries is 90 to 98% by weight based on the weight of the negative electrode composition for lithium ion batteries. thing. A mixing step of preparing a slurry containing the binder resin, conductive aid, and negative electrode active material, and an aqueous solvent, a coating step of applying the slurry to the current collector, and drying the slurry after the coating step to prepare the collector. A method for producing a negative electrode for a lithium ion battery, including a drying step of forming a negative electrode active material layer on an electric body.