TW202147673A - Negative electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery using same capable of improving cycle properties and rate properties while suppressing thixotropic changes in the negative electrode coating material - Google Patents

Abstract

Provided is a negative electrode for a nonaqueous electrolyte secondary battery, which is capable of improving cycle properties and rate properties while suppressing thixotropic changes in the negative electrode coating material to suppress dry cracking. The negative electrode for a nonaqueous electrolyte secondary battery according to one embodiment comprises a graphite-based negative electrode active substance, a silicon-based negative electrode active substance, a carboxymethyl cellulose and/or an alkali metal salt thereof, and a methane sulfonic acid formaldehyde condensate and/or an alkali metal salt thereof.

Description

Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery using the same

Embodiments of the present invention relate to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

Non-aqueous electrolyte secondary batteries, including lithium ion secondary batteries (LiB), are widely used as power storage devices due to their light weight, high voltage, and large capacity. Graphite-based materials are mainly used in the negative electrode of non-aqueous electrolyte secondary batteries, but the use of silicon-based negative electrode active materials has been proposed for the purpose of further increasing the capacity, and it is also known to use graphite-based negative electrode active materials and silicon-based negative electrode active materials in combination. . For example, as a method of using silicon oxide as a silicon-based negative electrode active material in a negative electrode, Patent Document 1 discloses a method of coating the surface of silicon oxide particles with a carbon layer.

On the other hand, in the production of the negative electrode coating material when producing the negative electrode, there are polyacrylic acid, polyacrylamide, polyvinyl alcohol, polyethylene glycol, and the like as the dispersing agent used in the water-based formulation. For example, Patent Document 2 discloses that polyacrylic acid and polyethylene glycol are used in the case where graphite is used as the negative electrode active material. In addition, Patent Document 3 and Patent Document 4 disclose that, in the case where graphite is used as the negative electrode active material, polyacrylic acid and Styrene Butadiene Rubber (SBR) are used in combination as a binding agent. In addition, Patent Document 5 discloses the use of polyacrylic acid as a silicon-based negative electrode active material. [Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 2002-42806 [Patent Document 2] Japanese Patent Laid-Open No. 11-354125 [Patent Document 3] Japanese Patent No. 3062304 [Patent Document 4] Japanese Patent No. 4441935 [Patent Document 5] Japanese Patent No. 3703667

[The problem to be solved by the invention] The combination of the above-mentioned graphite-based negative electrode active material and silicon-based negative electrode active material has the advantage of realizing high capacity. reduce.

On the other hand, in the dispersion of highly hydrophobic active materials or conductive aids, those with structures with strong hydrophobic interactions are preferred, and Carboxy Methyl Cellulose (CMC) is suitable. And use a dispersant like polyacrylic acid. However, the viscosity of the coating for negative electrodes is affected by the properties of the dispersing agent. In particular, the combined use of carboxymethyl cellulose and acrylic tends to increase the thixotropy, and the shrinkage during drying is strong, causing drying cracks. In addition, there are Unexpected inconveniences such as the following: the fluctuation of thixotropy due to the amount of addition is large, and the fluctuation of the basis weight at the time of coating is large.

An object of an embodiment of the present invention is to provide a negative electrode for a non-aqueous electrolyte secondary battery which can suppress the change in thixotropy of the negative electrode coating material, suppress drying cracks, and can improve cycle characteristics and rate characteristics.

[Means of Solving Problems] The negative electrode for a non-aqueous electrolyte secondary battery according to the embodiment of the present invention includes a graphite-based negative electrode active material, a silicon-based negative electrode active material, carboxymethyl cellulose and/or an alkali metal salt thereof, and a naphthalenesulfonic acid formaldehyde condensate and/or its alkali metal salts.

In one embodiment, the negative electrode for a non-aqueous electrolyte secondary battery may contain 0.5 parts by mass or more and 3.0 parts by mass or less with respect to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material. The carboxymethyl cellulose and/or its alkali metal salt, and the naphthalenesulfonic acid formaldehyde condensate containing 0.5 times or more and 3 times or less the mass of the carboxymethyl cellulose and/or its alkali metal salt and/or its alkali metal salts.

In one embodiment, the negative electrode for a non-aqueous electrolyte secondary battery may further contain styrene butadiene rubber. In this case, the styrene butadiene rubber may be contained in an amount of 0.5 parts by mass or more and 3.0 parts by mass or less with respect to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material.

A non-aqueous electrolyte secondary battery according to an embodiment of the present invention includes the above-described negative electrode for a non-aqueous electrolyte secondary battery.

[Effect of invention] According to the embodiment of the present invention, drying cracks can be suppressed, and cycle characteristics and rate characteristics can be improved.

The negative electrode for a non-aqueous electrolyte secondary battery of the present embodiment includes (A) a graphite-based negative electrode active material, (B) a silicon-based negative electrode active material, (C) carboxymethyl cellulose and/or an alkali metal salt thereof, and ( D) Naphthalenesulfonic acid formaldehyde condensate and/or its alkali metal salt, and a graphite-based negative electrode active material and a silicon-based negative electrode active material are used as the negative electrode active material.

[(A) Graphite-based negative electrode active material] The graphite-based negative electrode active material can be used without particular limitation as long as it is graphite capable of electrochemically occluding and releasing lithium ions. For example, natural graphite, artificial graphite, a mixture of natural graphite and artificial graphite, natural graphite coated with artificial graphite, etc. may be mentioned, and two or more of these may be used in combination. The particle shape of the graphite-based negative electrode active material is not particularly limited, and may be spherical or scaly.

The average particle diameter (D50) of the graphite-based negative electrode active material is not particularly limited, and may be, for example, 1 μm to 50 μm, or 5 μm to 30 μm. Graphite-based negative electrode active material, the specific surface area is not particularly limited, for example, may be ^{0.5 m 2 / g ~ 10 m} 2 / g, may also be ^{1.0 m 2 /g~5.0 m 2 / g} .

In this specification, the average particle size and specific surface area of the negative electrode active material can be measured, for example, using a laser scattering particle size distribution analyzer (for example, "Mastersizer 2000" manufactured by Malvern Panalytical). ”), these negative electrode active materials were dispersed in a medium in which the negative electrode active material was not dissolved, and the measurement was performed.

[(B) Silicon-based negative electrode active material] The silicon-based negative electrode active material contains silicon (atoms) and can electrochemically occlude and release lithium ions. Examples of the silicon-based negative electrode active material include particles of simple silicon, particles of silicon compounds, and the like. Examples of the silicon compound include those commonly used as negative electrode active materials of lithium ion secondary batteries, and examples thereof include silicon oxides, silicon alloys, and the like. The silicon-based negative electrode active material is preferably one or two or more selected from the group consisting of silicon, silicon alloys, and silicon oxides represented by SiOx (wherein x represents 0.5≦x≦1.6), more preferably silicon oxides .

The average particle size of the silicon-based negative electrode active material is not particularly limited, and may be, for example, 0.1 μm to 30 μm, or 1 μm to 20 μm. The specific surface area of the silicon-based negative electrode active material is not particularly limited, and may be, for example, 0.5 m ² /g to 15 m ² /g, or 1.0 m ² /g to 10 m ² /g.

The ratio of the graphite-based negative electrode active material to the silicon-based negative electrode active material is not particularly limited, and the silicon-based negative electrode active material is preferably 2 to 30 mass %, more preferably 3 mass %, in the total amount of the two, 100 mass %. % to 20 mass %, more preferably 5 to 15 mass %.

[(C) Carboxymethylcellulose and/or its alkali metal salt] Carboxymethyl cellulose (hereinafter, sometimes referred to as CMC) and/or its alkali metal salts (hereinafter, sometimes referred to as CMC salts) have hydroxyl groups in the glucose residues constituting cellulose substituted with carboxymethyl ether groups The resulting structure may have a carboxyl group or the form of a metal carboxylate, or both may be used in combination. As an alkali metal salt, a sodium salt, a lithium salt, a potassium salt, etc. are mentioned.

In aqueous formulations, CMC and/or CMC salt function as a dispersant for negative electrode active materials or conductive aids, as a viscosity modifier (thickener) for negative electrode coatings, and also as a bond between negative electrode active materials and current collectors. The agent works.

The CMC and/or the CMC salt are not particularly limited, and for example, those having a degree of etherification of 0.4 to 1.5 can be preferably used. When the degree of etherification is 0.4 or more, the solubility with respect to water can be improved, and when it is 1.5 or less, the thickening effect with respect to the negative electrode coating material can be improved, and useless introduction of a substituted ether can be suppressed. The cost increase caused by the high degree of chemical CMC. The etherification degree is preferably 0.5 or more, more preferably 0.6 or more. The etherification degree is preferably 1.2 or less, more preferably 1.0 or less.

In addition, as CMC and/or CMC salt, those having a viscosity of 1 mass % aqueous solution (25°C, B-type viscometer) of 100 mPa·s to 10,000 mPa·s can be preferably used, and the viscosity of the obtained coating for negative electrodes can be obtained. The concentration of the aqueous solution to be used is appropriately adjusted so that it becomes an appropriate several thousand mPa·s (eg, 3000 mPa·s to 5000 mPa·s). Here, when the concentration of the aqueous solution of CMC and/or CMC salt in the negative electrode coating material is 0.5 mass % or more, the dispersibility of the active material can be improved, the generation of aggregates of the negative electrode active material can be suppressed, and the coating at the time of coating can be suppressed. uneven. Moreover, by being 3 mass % or less, the coating material for negative electrodes can be made into an appropriate viscosity, and the coating unevenness at the time of coating can be suppressed, and the rise of the resistance value of a battery can also be suppressed. The more preferable aqueous solution concentration in the coating material for negative electrodes is 0.6 mass % to 2.4 mass %, and 100 mPa·s to 8000 mPa·s is preferable as a 1 mass % aqueous solution viscosity usable at this concentration. Furthermore, by making the viscosity of the 1 mass % aqueous solution 100 mPa·s or more, the thickness of the electrode at the time of coating can be easily adjusted to the extent that the viscosity of the negative electrode coating material is increased to suppress uneven coating, and by being 8000 mPa·s or less, The viscosity increase of the coating material for negative electrodes can be suppressed to the extent which can apply|coat uniformly, and it can suppress uneven application|coating.

The content of CMC and/or CMC salt is not particularly limited, but is preferably 0.5 to 3.0 parts by mass relative to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material. By setting it as such a content, the dispersibility of a graphite-type negative electrode active material or a silicon-type negative electrode active material can be improved, and the viscosity of the coating material for negative electrodes can be raised, and the formability of a coating film can be improved. The content is preferably 0.7 parts by mass or more, and is preferably 2.5 parts by mass or less.

[(D) Naphthalenesulfonic acid formaldehyde condensate and/or its alkali metal salt] Naphthalenesulfonic acid formaldehyde condensate (hereinafter, sometimes referred to as NSF) and/or its alkali metal salt (hereinafter, sometimes referred to as NSF salt) is a condensate of naphthalenesulfonic acid and formaldehyde, or its alkali metal salt, or both. mixture of . As an alkali metal salt, a sodium salt, a lithium salt, a potassium salt, etc. are mentioned.

By containing NSF and/or NSF salt, the volume change accompanying charge and discharge of the silicon-based negative electrode active material can be suppressed, and the cycle characteristics can be improved. In addition, the rate characteristic can be improved. In addition, when combined with CMC and/or CMC salt, the increase in thixotropy can be suppressed, drying cracks caused by shrinkage during drying can be suppressed, and changes in basis weight during coating caused by changes in thixotropy can be suppressed. .

In addition, as for NSF, as long as the performance is not impaired, as a monomer, for example, and alkyl naphthalene (such as methyl naphthalene, ethyl naphthalene, butyl naphthalene, etc.), hydroxynaphthalene, naphthalene carboxylic acid, anthracene, It is produced by co-condensation of aromatic compounds that can be co-condensed with naphthalenesulfonic acid, such as phenol, cresol, or derivatives thereof.

The weight average molecular weight (Mw) of NSF and/or NSF salt is not particularly limited, but is preferably 300 to 3,000. When the weight average molecular weight is 300 or more, the dispersibility becomes favorable, and when the weight average molecular weight is 3000 or less, the deterioration of the productivity can be suppressed, and the cost increase can be suppressed. The weight average molecular weight is preferably 350 or more, more preferably 450 or more, and is preferably 2500 or less, more preferably 1000 or less. Here, the weight average molecular weight is measured by gel permeation chromatography (Gel Permeation Chromatography, GPC).

As NSF and/or NSF salt, a commercial item can be used, and what was prepared according to a well-known manufacturing method can also be used. For example, NSF is obtained by sulfonating naphthalene using a sulfonating agent such as concentrated sulfuric acid and fuming sulfuric acid, and then performing a condensation reaction using formaldehyde. The NSF salt can also be obtained by neutralizing the obtained NSF with an alkali agent, and the water-insoluble matter by-produced by neutralization can also be removed.

The content of NSF and/or NSF salt is preferably 0.5 to 3 times, more preferably 0.6 to 2.5 times by mass relative to the content of CMC and/or CMC salt.

[(E) Styrene butadiene rubber] The negative electrode for a non-aqueous electrolyte secondary battery of the present embodiment may further contain (E) styrene butadiene rubber (hereinafter, sometimes referred to as SBR). SBR acts as a binding agent, and by using it together with CMC and/or a CMC salt, excellent binding properties can be obtained in a combined system of a graphite-based negative electrode active material and a silicon-based negative electrode active material. In addition, if it is an SBR containing a rubber component, since flexibility can be imparted to the coating film, drying cracks when the thickness of the coating film is increased can be suppressed.

As SBR, for example, an oil droplet emulsion in water (SBR latex) or the like can be used, which is usually used as a binding agent for a graphite-based negative electrode active material.

When SBR is used, its content is not particularly limited, but is preferably 0.5 to 3.0 parts by mass relative to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material. The content is preferably 0.7 parts by mass or more, and is preferably 2.0 parts by mass or less.

[Other additives] In the negative electrode for a non-aqueous electrolyte secondary battery of the present embodiment, in addition to the components described above, for example, a conductive aid, a dispersant other than the above, a binding agent, and the like may be added as necessary.

Examples of the conductive aid include carbon blacks such as acetylene black and Ketjen Black, carbon nanotubes, carbon fibers, and the like, and any one or two or more of these can be used. The content of the conductive assistant is not particularly limited, for example, it may be 0.1 parts by mass to 20 parts by mass, or 0.5 parts by mass to 10 parts by mass relative to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material. parts by mass.

Examples of adhesives other than SBR include various resin emulsions such as polyurethane emulsions, polyvinyl acetate emulsions, and acrylic resin emulsions, which can be used together with or in place of SBR.

[Negative electrode for non-aqueous electrolyte secondary battery] A negative electrode for a non-aqueous electrolyte secondary battery according to an embodiment may include a current collector and a negative electrode active material layer formed on the current collector, and the negative electrode active material layer preferably contains the components (A) to (D) component, (A) component - (E) component are preferable.

The negative electrode active material layer can be formed by, for example, applying a negative electrode coating material on a current collector and drying it. In this case, the coating material for negative electrodes contains the components (A) to (D) described above, preferably the components (A) to (E). The coating material for negative electrodes is preferably a water-based formulation, that is, the coating material for negative electrodes contains water as a solvent. In one embodiment, the water-based coating material for negative electrodes is made by dissolving the (C) component and the (D) component in water, and mixing (A) the graphite-based negative electrode active material and (B) the silicon-based negative electrode active material with (E) as particles. ) SBR is dispersed in water together and forms a slurry or paste.

The current collector is not particularly limited as long as it is an electron conductor that does not adversely affect the constituted battery. For example, in addition to copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymers, conductive glass, Al-Cd alloys, etc., for the purpose of improving adhesion, conductivity, and oxidation resistance, It is also possible to use those obtained by treating the surface of copper or the like with carbon, nickel, titanium, silver, or the like. These current collectors may be obtained by oxidizing the surface. As for the shape of the current collector, in addition to a foil shape, a film shape, a sheet shape, a mesh shape, a punched or expanded shape, a shaped body such as a lath body, a porous body, and a foam body can be used. The thickness is not particularly limited, and for example, a thickness of 1 μm to 100 μm can be used.

The mass per unit area (weight per unit area) of the negative electrode active material layer formed on the current collector is not particularly limited, and may be, for example, 5 mg/cm ² to 30 mg/cm ² or 7 mg/cm ² ~20 mg/cm ² .

[Non-aqueous electrolyte secondary battery] The nonaqueous electrolyte secondary battery of the present embodiment includes the negative electrode as the negative electrode. Specifically, the non-aqueous electrolyte secondary battery includes a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte, and the negative electrode of the above-described embodiment is used as the negative electrode. As one embodiment, the non-aqueous electrolyte secondary battery may include a laminate in which a negative electrode and a positive electrode are alternately laminated with separators interposed therebetween, a container for accommodating the laminate, and a non-aqueous electrolyte injected into the container. electrolytes such as liquid. As the non-aqueous electrolyte, for example, a lithium ion secondary battery can be constituted by dissolving a lithium salt serving as a supporting electrolyte in an organic solvent, for example. [Example]

Hereinafter, the present invention will be described in more detail based on Examples and Comparative Examples, but the present invention is not limited thereto.

[Production of carboxymethylcellulose sodium salt] (Manufacturing example 1: CMC-1) 500 parts by mass of solvent (isopropyl alcohol/water = 80/20 (mass ratio)), 50 parts by mass of sodium hydroxide, and 100 parts by mass of fragmented cellulose were put into a biaxial kneader type reactor, and the mixture was heated to 35 The alkali cellulose reaction was carried out at °C for 60 minutes.

A mixture of 55 parts by mass of monochloroacetic acid adjusted to 25° C. and 27 parts by mass of a solvent (isopropyl alcohol/water = 80/20 (mass ratio)) was added dropwise to the obtained reaction solution over 60 minutes (dropwise added). The temperature in the solution was maintained at 30°C to 40°C), the temperature was raised to 80°C over 25 minutes, and the etherification reaction was further performed for 60 minutes.

Next, the obtained reaction solution was cooled to 60° C., adjusted to pH 9.5 using a 50% by mass acetic acid aqueous solution, and then the reaction solvent was vaporized and recovered using a decompression filter to obtain crude carboxymethylcellulose sodium salt (nonvolatile matter). 40% by mass).

Sodium hydroxide was added to the obtained crude carboxymethylcellulose sodium salt to adjust the pH to 9.5. Then, it wash|cleaned using the solvent (methanol/water=70/30 (mass ratio)) of 10 times the mass of crude carboxymethylcellulose sodium salt, and it dried at 105 degreeC for 45 minutes after that. The obtained dried product was pulverized with an impact mill (Pulverizer, manufactured by Hosokawa Micron Co., Ltd.), and classified with an 80-mesh standard sieve to obtain a carboxymethyl group Cellulose sodium salt (CMC-1).

The obtained carboxymethylcellulose sodium salt (CMC-1) had a degree of etherification of 0.71, and a 1 wt % aqueous solution viscosity (25°C) of 3450 mPa·s.

(Manufacturing example 2: CMC-2) 500 parts by mass of solvent (isopropyl alcohol/methanol/water = 70/10/20 (mass ratio)), 61 parts by mass of sodium hydroxide and 100 parts by mass of flaky cellulose were put into a biaxial kneader type reactor , the alkali cellulose reaction was carried out at 35°C for 60 minutes.

A mixture of 54 parts by mass of monochloroacetic acid adjusted to 25° C. and 26 parts by mass of a solvent (isopropyl alcohol/methanol/water=70/10/20 (mass ratio)) was added dropwise to the obtained reaction liquid over 60 minutes After that (the temperature during the dropwise addition was maintained at 30°C to 40°C), the temperature was raised to 80°C over 25 minutes, and the etherification reaction was further performed for 60 minutes.

Next, the obtained reaction solution was cooled to 60° C., adjusted to pH 9.0 using a 50% by mass acetic acid aqueous solution, and then 2 parts by mass of 20% hydrogen peroxide was added, and the reaction solvent was vaporized and recovered using a decompression filter to obtain Crude carboxymethyl cellulose sodium salt (40% by mass of non-volatile content).

Sodium hydroxide was added to the obtained crude carboxymethylcellulose sodium salt to adjust the pH to 9.5. Then, it wash|cleaned using the solvent (methanol/water=70/30 (mass ratio)) of 10 times the mass of crude carboxymethylcellulose sodium salt, and it dried at 105 degreeC for 45 minutes after that. The obtained dried product was pulverized with an impact mill (Pulverizer, manufactured by Hosokawa Micron Co., Ltd.), and classified with an 80-mesh standard sieve to obtain carboxymethyl Cellulose sodium salt (CMC-2).

The obtained carboxymethyl cellulose sodium salt (CMC-2) had a degree of etherification of 0.68, and a 1 wt % aqueous solution viscosity (25°C) of 178 mPa·s.

(Production Example 3: CMC-3) 500 parts by mass of a solvent (isopropyl alcohol/water = 80/20 (mass ratio)), 52 parts by mass of sodium hydroxide and 100 parts by mass of flaky cellulose were put into a biaxial kneader type reactor, and the mixture was heated to 30 The alkali cellulose reaction was carried out at °C for 60 minutes.

A mixture of 66 parts by mass of monochloroacetic acid adjusted to 25° C. and 32 parts by mass of a solvent (isopropyl alcohol/water = 80/20 (mass ratio)) was added dropwise to the obtained reaction solution over 60 minutes (dropwise added). The temperature in the solution was maintained at 30°C to 40°C), the temperature was raised to 80°C over 25 minutes, and the etherification reaction was further performed for 60 minutes.

Next, the obtained reaction solution was cooled to 60° C., adjusted to pH 9.5 using a 50% by mass acetic acid aqueous solution, and then the reaction solvent was vaporized and recovered using a decompression filter to obtain crude carboxymethylcellulose sodium salt (nonvolatile matter). 40% by mass).

Sodium hydroxide was added to the obtained crude carboxymethylcellulose sodium salt to adjust the pH to 9.5. Then, it wash|cleaned using the solvent (methanol/water=70/30 (mass ratio)) of 10 times the mass of crude carboxymethylcellulose sodium salt, and it dried at 105 degreeC for 45 minutes after that. The obtained dried product was pulverized with an impact mill (Pulverizer, manufactured by Hosokawa Micron Co., Ltd.), and classified with an 80-mesh standard sieve to obtain carboxymethyl Cellulose sodium salt (CMC-3).

The obtained carboxymethylcellulose sodium salt (CMC-3) had a degree of etherification of 0.85, and a 1 wt % aqueous solution viscosity (25°C) of 3620 mPa·s.

[Method for measuring physical properties of carboxymethylcellulose sodium salt] (degree of etherification) 0.6 g of carboxymethylcellulose or its salt was dried at 105°C for 4 hours. After accurately weighing the dry matter, wrap it in filter paper and incinerate it in a magnetic crucible. Transfer the ash to a 500 mL beaker, add 250 mL of water and 35 mL of 0.05 mol/L sulfuric acid aqueous solution, and boil for 30 minutes. After cooling, the excess acid was reverse-titrated with a 0.1 mol/L potassium hydroxide aqueous solution. Furthermore, phenolphthalein was used as an indicator. Using the measurement results, the degree of etherification was calculated by the following formula. Formula: (degree of etherification)=162×A/(10000-80A) A=(af-bf1)/weight of dry matter (g) A: The amount of 0.05 mol/L aqueous sulfuric acid solution consumed by the binding base in 1 g of the sample (mL) a: The usage amount of 0.05 mol/L sulfuric acid aqueous solution (mL) f: Titration rate of 0.05 mol/L sulfuric acid aqueous solution b: Titration amount of 0.1 mol/L potassium hydroxide aqueous solution (mL) f1: Titration rate of 0.1 mol/L potassium hydroxide aqueous solution

(1 mass % aqueous solution viscosity) Place carboxymethylcellulose or its salt (about 2.2 g) into a 300 mL conical flask with a stopper and weigh accurately. To this, an amount of water calculated by the calculation formula "sample (g)×(99-moisture content (mass %))" was added, and the mixture was left to stand for 12 hours, followed by mixing for 5 minutes. Using the obtained solution, according to Japanese Industrial Standards (JIS) Z8803, the viscosity at 25° C. was measured using a BM type viscometer (single cylinder type rotational viscometer). At this time, the rotor rotation speed was set to 60 rpm, and when the upper limit of the measurement was reached, the measurement was performed by sequentially changing to 30 rpm and 12 rpm.

[Production of Naphthalenesulfonic Acid Formaldehyde Condensate Sodium Salt] (Production Example 4: NSF-1) 250 parts by mass of concentrated sulfuric acid and 250 parts by mass of naphthalene were added to a 2 L four-neck flask equipped with a thermometer, a condenser, and a stirring bar. part, the temperature was raised to 160 °C and reacted for 1 hour. The product was cooled to 90° C., 80 parts by mass of water was added, and 120 parts by mass of 37% by mass formalin was added dropwise, and the condensation reaction was performed at 103° C. for 5 hours. After completion of the reaction, 180 parts by mass of water and 60 parts by mass of a 48 mass % aqueous sodium hydroxide solution were added, and the mixture was stirred at 80° C. for 30 minutes for neutralization. Next, in order to remove residual sulfuric acid as gypsum, 50 parts by mass of Ca(OH) _{2 was added} , and the mixture was further stirred for 1 hour. The gypsum was removed by filtration using a filter aid to obtain a 45% by mass aqueous solution of naphthalenesulfonic acid formaldehyde condensate sodium salt (NSF-1).

The weight average molecular weight of the obtained naphthalenesulfonic acid formaldehyde condensate sodium salt (NSF-1) was about 720.

(Production Example 5: NSF-2) In a 2 L four-necked flask equipped with a thermometer, a condenser, and a stirring bar, 220 parts by mass of 15% oleum was put, and 220 parts by mass of naphthalene was placed, and the temperature was raised to 160°C. React for 1 hour. The product was cooled to 90° C., 60 parts by mass of water was added, and 140 parts by mass of 37% by mass formalin was added dropwise, and the reaction was carried out at 103° C. for 5 hours. After completion of the reaction, 165 parts by mass of water and 150 parts by mass of 48% by mass NaOH were added, and the mixture was stirred at 80° C. for 30 minutes. Next, in order to remove residual sulfuric acid as gypsum, 50 parts by mass of Ca(OH) _{2 was added} , and the mixture was further stirred for 1 hour. The gypsum was removed by filtration using a filter aid to obtain a 40% by mass aqueous solution of naphthalenesulfonic acid formaldehyde condensate sodium salt (NSF-2).

The weight average molecular weight of the obtained naphthalenesulfonic acid formaldehyde condensate sodium salt (NSF-2) was about 2700.

[Measurement of Weight Average Molecular Weight of Naphthalenesulfonic Acid Formaldehyde Condensate Sodium Salt] It was measured by GPC (Gel Permeation Chromatography; Gel Permeation Chromatography). The measurement conditions are as follows. Device detector: UV detector UV-8000 Tosoh pump: CCPD type Tosoh column: TSK gel G3000SW + G4000SW + guard column (guard column) Tosoh integrator : SC-8010 manufactured by Tosoh Co., Ltd. Measurement method Mobile phase solvent: 40/60 [vol%] acetonitrile/0.005 mol sodium acetate aqueous solution Sample preparation: Dissolve about 66 mg of active ingredient in 10 mL of mobile phase solvent and filter After filtration, inject 10 μL Analysis conditions Flow rate [mL/min] = 0.85, measurement wavelength [nm] = 254, measurement pressure [kg/cm ² ] = 40 to 60, measurement time [min] = 60 Standard substance Sodium polystyrene sulfonate with known molecular weight

[Preparation of coating material for negative electrode] (Example 1: coating material 1) 86.4 g of graphite-based negative electrode active material A (spherical natural graphite, average particle size 18.6 μm, specific surface area 1.3 m ² /g) was mixed with a planetary mixer. , 9.6 g of silicon-based negative electrode active material A (silicon oxide, average particle size 7.0 μm, specific surface area 2.2 m ² /g), 1.0 g of carbon black (made by Timcal, Super-P), 1.0 g g of CMC-1 and 2.2 g of a 45 mass % aqueous solution of NSF-1 were mixed. To this, 97.8 g of water and 2 g of a 50 mass % aqueous dispersion of styrene butadiene rubber (SBR) were added and mixed to obtain a slurry-like coating material 1 having a solid content of 50 mass % of Example 1 .

(Example 2 to Example 10 and Comparative Example 1 and Comparative Example 2: Paint 2 to Paint 12) As shown in Table 1, except that the type and usage amount of each component were changed, the paints 2 to 10 of Examples 2 to 10, and the paints of Comparative Example 1 and Comparative Example 2 were prepared in the same manner as the paint 1. 11. Paint 12. In addition, the compounding quantity of each component in Table 1 is the mass part of solid content excluding water, and the quantity of water was adjusted for each coating material, and it was set as the slurry of 50 mass % of solid content.

The graphite-based negative electrode active material B, the silicon-based negative electrode active material B, and the polyacrylamide among the components in Table 1 are as follows. Graphite-based negative electrode active material B: artificial graphite, average particle size 10.6 μm, specific surface area 3.6 m ² /g Silicon-based negative electrode active material B: silicon oxide, average particle size 4.5 μm, specific surface area 5.5 m ² /g Polyethylene Acrylamide: Sharoll AM-253P (manufactured by Daiichi Industrial Pharmaceutical Co., Ltd.)

[Evaluation of Coating for Negative Electrodes] (Dispersed State) Evaluation was performed by visual observation according to the following evaluation criteria. ○: When the paint was applied so that the weight per unit area was 7 mg/cm ² without filtering the paint, it was visually confirmed that no agglomerates or streaks caused by the agglomerates were generated on the coated surface. Case ×: When the coating material was applied so that the weight per unit area was 7 mg/cm ² without filtering the paint, the occurrence of aggregates was visually confirmed, or the occurrence of streaks caused by the aggregates occurred on the coating surface.

(Paint viscosity/thixotropy) According to the JIS Z8803 standard, using a BM type viscometer (single-cylinder rotational viscometer), the viscosity at 25°C, i.e. paint viscosity (60 rpm) and paint viscosity (6 rpm) were measured at rotor revolutions of 60 rpm and 6 rpm. ). In addition, as thixotropy, TI value=paint viscosity (6 rpm)/paint viscosity (60 rpm) was calculated.

[Preparation of Negative Electrode] Using a coater, the coating material 1 of Example 1 was applied to electrolytic copper having a thickness of 10 μm as a current collector so that the basis weight of the negative electrode active material layer was 7 mg/cm ^{2 .} On the foil, after drying at 120° C., a roll pressing process was performed, whereby the negative electrode 1 of Example 1 ^{having an electrode density of 1.5 mg/cm 3 was obtained.} Similarly, using the paints 2 to 10 of Examples 2 to 10, and the paints 11 and 12 of Comparative Examples 1 and 2, negative electrodes 2 to 10 of Examples 2 to 10 and Comparative Examples were obtained, respectively. 1. The negative electrode 11 and the negative electrode 12 of the comparative example 2.

[Evaluation of the negative electrode] (electrode surface state 1) About the negative electrode 1 - the negative electrode 12 obtained as mentioned above, the state of the electrode surface was evaluated. In the evaluation, the presence or absence of spots, protrusions, and streaks was visually judged, and the evaluation was performed according to the following criteria. ○: No crack or crack ×: Cracks or cracks are present

(Electrode Surface State 2) Negative electrodes were produced using paints 1 to 12 in the same manner as in the above [Preparation of Negative Electrode], except that the coating was performed so that the basis weight of the negative electrode active material layer was 9 mg/cm ^{2 .} , and the state of the electrode surface was evaluated in the same manner as described in (Electrode Surface State 1).

(Measurement of peel strength) A double-sided tape with a width of 18 mm and a length of 120 mm was attached to the stainless steel plate, and the surfaces of the negative electrode 1 to the negative electrode 12 on the negative electrode active material layer side were attached to the double-sided tape. About the negative electrode 1 - the negative electrode 12, it was set as the state which overlapped with the double-sided tape except for the length of 30 mm of the one end side, and was 18 mm in width and 150 mm in length. A stainless steel plate was sandwiched between the clamps on the lower side of a tensile testing machine (manufactured by Shimadzu Corporation, Autograph AGS-X), the one end side of the negative electrode was folded upward, and the current collector on the one end side was (Copper foil) One end of the mending tape was attached to the part, and the other end of the mending tape was sandwiched by the clamp on the upper side and pulled upward at a speed of 50 mm/min. Peel strength when peeled in 180° direction.

[Preparation of Lithium Ion Secondary Battery] (Preparation of Positive _{Electrode) 92 parts by mass of LiNi 1/3} Mn _1/3 Co _1/3 O ₂ (LNMCO) and acetylene black (DENTCA) were mixed with a planetary mixer. Production, Li-400) 4 mass parts, polyvinylidene fluoride 4 mass parts, and N-methyl-2-pyrrolidone 60 mass parts were mixed, and the slurry with a solid content of 62.5 mass % was obtained. The positive electrode was obtained by applying this slurry on an aluminum foil using a coater so that LNMCO might be 18 mg/cm ² , drying at 140° C., and then performing a roll pressing process.

(Preparation of Battery) The negative electrode 1 of Example 1 obtained above was combined with the positive electrode, a polyolefin-based (PE/PP) separator as a separator was sandwiched between the electrodes and laminated, and the positive and negative electrodes were laminated. Ultrasonic welding of positive and negative terminals. The laminate was placed in an aluminum laminate packaging material, and the opening for liquid injection was left and heat-sealed to produce a pre-liquid-injected battery with a ^{positive electrode area of 18 cm 2} and a negative electrode area of 19.8 cm ^{2 .} _{Next, an electrolyte solution in which LiPF 6} (1.0 mol/L) was dissolved was poured into a solvent mixed with ethylene carbonate and diethyl carbonate (30/70 vol ratio), and the opening was heat-sealed to obtain Example 1. Lithium-ion secondary battery. Negative electrodes 2 to 10 of Examples 2 to 10, Comparative Example 1, and negative electrodes 11 and 12 of Comparative Example 2 were carried out in the same manner to obtain Examples 2 to 10, Comparative Example 1, and Comparative Example, respectively. 2 lithium-ion secondary batteries.

[Evaluation of battery performance] A performance test at 25° C. was performed on the produced lithium ion secondary battery. The test method is as follows.

(Discharge Rate Characteristics) As discharge rate characteristics, the 5 C capacity retention ratio, the 7 C capacity retention ratio, and the 10 C capacity retention ratio were obtained by the following methods. In an environment of 25°C, CC (constant current) charging is performed at a current density equivalent to 0.5 C until 4.2 V, switching to CV (constant voltage) charging at 4.2 V, and after charging for 3.5 hours, a current equivalent to 1 C is charged. The density was CC-discharged up to 2.7 V. Then, CC (constant current) charging was performed up to 4.2 V at a current density equivalent to 0.5 C, switched to CV (constant voltage) charging at 4.2 V, and after charging for 3.5 hours, CC discharge was performed at a current density equivalent to 5 C until 2.7V. Similarly, CC discharge was performed at a discharge rate of 7 C and 10 C, and the ratio of the discharge capacity at each discharge rate to the discharge capacity equivalent to 1 C (100%) was set as the capacity retention ratio (%).

(Charge-discharge cycle characteristics) As the charge-discharge cycle characteristics, the 200-cycle capacity retention rate was obtained by the following method. The following cycles were performed for 200 cycles: CC (constant current) charging at a current density equivalent to 1 C up to 4.2 V in an environment of 25°C, then switching to CV (constant voltage) charging at 4.2 V for 1.5 After 1 hour, CC discharge was performed at a current density equivalent to 1 C up to a cycle of 2.7 V, and the ratio of the 1 C discharge capacity after 200 cycles at this time to the initial 1 C discharge capacity was set to be maintained at 200 cycles. Rate(%).

[Table 1] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Comparative Example 1 Comparative Example 2 Recipe (mass parts) Graphite-based negative electrode active material A 86.4 86.4 86.4 84.6 84.6 84.6 86.4 86.4 - 86.4 86.4 86.4 Graphite-based negative electrode active material B - - - - - - - - 86.4 - - - Silicon-based negative electrode active material A 9.6 - 9.6 9.4 11.9 11.9 9.6 9.6 9.6 9.6 9.6 9.6 Silicon-based negative electrode active material B - 9.6 - - - - - - - - - - carbon black 1 1 1 1 1 1 1 1 1 1 1 1 CMC-1 1 1 1 1 1 1 - - 1.4 - 1 1 CMC-2 - - - - - - 1.5 - - - - - CMC-3 - - - - - - - 1 - 3 - - NSF-1 1 1 - 3 0.5 1 1 1 1 3 - - NSF-2 - - 1 - - - - - - - - - Polypropylene amide - - - - - - - - - - - 1 SBR 1 1 1 1 1 0.5 0.5 1 0.6 - 2 1 total 100 100 100 100 100 100 100 100 100 100 100 100 scattered state ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ × Paint Viscosity (60 rpm) 4990 5230 3720 5800 4670 5110 6220 4750 6410 4920 3330 4370 Paint Viscosity (6 rpm) 14500 15200 9670 15700 12540 13590 10500 12100 20000 11200 9320 20100 TI value (6 rpm/60 rpm) 2.9 2.9 2.6 2.7 2.7 2.7 1.7 2.5 3.1 2.3 2.8 4.6 Electrode surface state 1 (7 mg/cm ² ) ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ × Electrode surface state 2 (9 mg/cm ² ) ○ ○ ○ ○ ○ ○ ○ ○ ○ × ○ × Peel strength [N/cm] 0.070 0.068 0.065 0.065 0.101 0.048 0.145 0.080 0.134 0.404 0.230 0.113 200 cycles capacity retention rate [%] 79.7 77.1 75.1 82.7 77.2 77.2 79.7 77.2 78.2 88.0 68.1 72.8 5 C capacity retention rate [%] 94.7 92.1 89.4 90.5 92.3 93.2 93.9 92.3 92.1 79.0 89.1 89.4 7 C capacity retention rate [%] 87.3 87.5 82.2 84.0 88.1 88.2 87.4 88.1 87.7 72.0 75.5 75.2 10 C capacity retention rate [%] 70.1 69.9 60.6 64.4 70.4 73.4 72.3 70.4 68.4 59.0 50.3 47.3

The results are shown in Table 1. Compared with Comparative Example 1 in which CMC salt was prepared in the combined system of graphite-based negative electrode active material and silicon-based negative electrode active material, and in Comparative Example 2 in which polyacrylamide was further prepared, the increase in thixotropy was large, and when drying shrinkage and drying cracks were generated in the negative electrode. In addition, since the thixotropy was greatly changed with respect to Comparative Example 1, it was difficult to set coating conditions in order to obtain a predetermined basis weight.

On the other hand, in the case of Examples 1 to 10 in which the CMC salt and the NSF salt were mixed together in the combined use system of the graphite-based negative electrode active material and the silicon-based negative electrode active material, the cycle characteristics compared to Comparative Example 1 were similar to those of Comparative Example 1. Speed characteristics improved. Moreover, since the change of thixotropy was suppressed compared with the comparative example 1, drying cracks like the comparative example 2 did not generate|occur|produce, and setting of the coating conditions was also easy.

In addition, according to the comparison between Examples 1 to 9 and Example 10, by adding SBR together with CMC salt and NSF salt, drying cracks can be suppressed even when the basis weight of the negative electrode active material layer is increased. .

As mentioned above, although some embodiment of this invention was described, these embodiment is presented as an example, Comprising: It does not intend to limit the scope of the invention. These embodiments can be implemented in other various forms, and various omissions, substitutions, and changes can be made without departing from the gist of the invention. These embodiments, their omissions, substitutions, and changes are also included in the inventions described in the claims and their equivalents in the same way as they are included in the scope and gist of the invention.

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Claims (5)

A negative electrode for a non-aqueous electrolyte secondary battery, comprising a graphite-based negative electrode active material, a silicon-based negative electrode active material, carboxymethyl cellulose and/or its alkali metal salt, and naphthalenesulfonic acid formaldehyde condensate and/or its alkali metal salt . The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, which contains 0.5 parts by mass or more and 3.0 parts by mass relative to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material the following carboxymethylcellulose and/or its alkali metal salts, and The naphthalenesulfonic acid formaldehyde condensate and/or the alkali metal salt thereof is contained in a mass of 0.5 times or more and 3 times or less of the mass of the carboxymethyl cellulose and/or its alkali metal salt. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 or claim 2, further comprising styrene butadiene rubber. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 3, which is 0.5 parts by mass or more and 3.0 parts by mass relative to 100 parts by mass of the total amount of the graphite-based negative electrode active material and the silicon-based negative electrode active material The styrene butadiene rubber described below. A non-aqueous electrolyte secondary battery, comprising: the negative electrode for a non-aqueous electrolyte secondary battery according to any one of claim 1 to claim 4.