JP2003272619A - Slurry for forming negative electrode coating for nonaqueous secondary battery and adjustment method of slurry - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aqueous slurry for forming a negative electrode coating for a nonaqueous secondary battery, easy to handle, excellent in adhesiveness between a synthetic carbon material such as refractory graphilic carbon or mesophase carbon microbeads or a phosphorus-like or a phosphorus piece-like graphite material and a collector, with a low deterioration of discharge capacity, excellent in durability against charge/discharge cycle, with improve dispersion stability and high stability with time with corrosion prevented, and an adjustment method of the slurry. <P>SOLUTION: The slurry with basic structure of a solid content and water medium consisting of a carbon material active substance and a binder has a pH adjuster added to make the pH at 9 or higher. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an aqueous slurry for forming a negative electrode coating film suitable for a non-aqueous secondary battery and a method for preparing the slurry.

[0002]

2. Description of the Related Art As negative electrode active materials for lithium-ion secondary batteries, carbon materials such as mesophase carbon microbeads (MCMB) and non-graphitizable carbon are mainly used. Further, a fluorine-based resin typified by polyvinylidene fluoride (PVDF) resin is mainly used as the binder, and these resins are used as an organic solvent such as N-methyl-2-pyrrolidone (NMP) together with the negative electrode active material. By kneading and forming a slurry, a slurry for forming a negative electrode coating film of a lithium ion secondary battery is obtained. Lithium ion secondary batteries are widely used as rechargeable power sources for notebook computers, mobile phones, etc., but there is a demand for higher capacity and higher voltage batteries to further expand the range of use. In order to meet the demands for higher capacity and higher voltage of such a battery, it is essential to increase the capacity of the negative electrode material and enhance the potential stability, and it is necessary to consider using a graphite material as the negative electrode active material. It is being advanced. This is because the graphite material forms a theoretical lithium-graphite intercalation compound because the crystallinity is high, and a value close to the theoretical charge / discharge capacity of 372 mAh / g can be obtained, and the potential stability is also high. Because it is expensive.

However, since the crystal structure of the graphite material has a high cohesive force in the layer direction, pulverization produces a flaky or phosphorus-like flaky particle powder. Therefore, a binder made of a fluorine-based resin typified by PVDF, which has been mainly used in the past, has a low film-forming property, and sufficient adhesion between graphite particles and a copper foil as a current collector cannot be obtained. . As a result, even if graphite powder, which is advantageous in terms of crystal structure, is used as the negative electrode active material, the charge and discharge capacity is reduced due to its high electric resistance value, and the capacity is greatly reduced at large current. Further, there is a drawback that the capacity is greatly reduced in a charge / discharge cycle in which charge / discharge is repeated. Further, it is pointed out that the fluorine-based resin decomposes at a high temperature, and the released fluorine and lithium react violently as a safety problem. Further, when using a fluorine-based resin represented by PVDF as a binder, an organic solvent such as NMP is used as a solvent for slurrying, but in recent years consideration for the environment and safety of workers and From the viewpoint of price and the like, it is preferable to make the solvent for slurrying aqueous. It should be noted that, as a matter of course, even as an aqueous slurry, a negative electrode having the same or any performance improved as a coating film obtained from a conventional solvent-based slurry, such as reducing the adhesion of the obtained coating film and the expansion and contraction at the time of charging and discharging. Must be a coating. As a slurry or a negative electrode coating film for forming a negative electrode coating film of a non-aqueous secondary battery having a water-based emulsion resin, a water-based binder as a thickener, and a carbon material active material as a basic structure, for example, JP-A-4-342966. Discloses a slurry having a specified ratio of carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR). Further, JP-A-8-250122 discloses a negative electrode composed of a styrene-butadiene latex having a specified butadiene content and a carbon material active material.

[0004]

SUMMARY OF THE INVENTION The present invention provides the above-mentioned object, namely, to provide an aqueous slurry excellent in handling safety, and to obtain a synthetic carbon material such as non-graphitizable carbon or MCMB, or a phosphorus-like or phosphorus-containing material. Excellent adhesion between flake graphite material and current collector, less decrease in discharge capacity, excellent durability against charge / discharge cycles, improved dispersion stability and high stability over time with non-aqueous system. The present invention provides a slurry for forming a negative electrode coating film of a secondary battery and a method for preparing this slurry.

[0005]

In order to solve the above-mentioned problems, the slurry of the present invention is a non-aqueous two-component slurry whose solids are a carbonaceous material active material and a binder and whose medium is water. In a slurry for forming a negative electrode coating film of a secondary battery, a pH adjusting agent is contained in this slurry,
The feature is that H is 9 or more. The binder used in this slurry is composed of an aqueous dispersion emulsion resin and a water-soluble polymer, the content of the water-soluble polymer in the binder is in the range of 30 to 65% by mass, and the solid content in the slurry is The content of the binder in the content is in the range of 1 to 5% by mass, and this water-dispersed emulsion resin is a natural rubber (NR) latex or styrene
Butadiene rubber (SBR) latex, butadiene rubber (BR) latex, acrylonitrile / butadiene copolymer rubber (NBR) latex, methyl methacrylate / butadiene copolymer rubber (MBR) latex, styrene butadiene / styrene copolymer (SBS) latex And at least one selected from acrylic ester resin emulsions, and the water-soluble polymer has a viscosity of a 1% by mass aqueous solution at 25 ° C. of 200 to 2,500.
mPa · s of at least one selected from sodium alginate, potassium alginate, ammonium alginate, propylene glycol alginate, carboxymethyl cellulose and its sodium salt or ammonium salt, hydroxyethyl cellulose, polyethylene oxide, polyvinyl alcohol, casein, sodium polyacrylate. To do. Further, the carbonaceous material used in this slurry contains 50 mass% or more of the massive graphite particle group composed of phosphorus-like or flake-like natural graphite particles, and the massive graphite particle group is subjected to laser light diffraction. The cumulative 50% diameter (D50 diameter) in the method is 10 to 25 μm, the specific surface area in the nitrogen gas adsorption method is 2.5 to ⁵ m ² / g,
The apparent density in the static method is 0.45 g / cm ³ or more,
The apparent density in the tap method is 0.70 g / cm ³ or more. In addition, in adjusting the pH, the pH is adjusted in the range of 9 to 12 when ammonia water is used as the pH adjuster, and the pH is adjusted to 9 to 1 when lithium hydroxide is used.
It is also within the scope of the present invention to set the range to 0. Another feature of the present invention is to form a negative electrode coating film of a non-aqueous secondary battery in which a main constituent material is a carbon material active material, a binder and an aqueous medium. For forming a negative electrode coating film of a non-aqueous secondary battery, which comprises adjusting pH to 9 to 12 using ammonia water for pH adjustment or adjusting pH to 9 to 10 using lithium hydroxide The present invention provides a method for adjusting a slurry for use.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The first feature of the present invention is the formation of a negative electrode coating film for a non-aqueous secondary battery whose solid content is a carbonaceous material active material and a binder and whose medium is water. A slurry for negative electrode coating film having a pH of 9 or more. When the pH is less than 9, the graphite particles are not sufficiently dispersed, and therefore when the viscosity of the water-soluble polymer described below is low or when the addition amount is small, the particles are remarkably settled, and when the water-soluble polymer is stored for a long period of time, it may be decomposed or moldy. Occurrence occurs.

Ammonia water or lithium hydroxide can be used to adjust the pH.
The range is from -12. If the pH exceeds 12, there is a risk that the current collector of the electrode, copper, will be corroded, and that there is a peculiar odor of ammonia itself, which may lower the working environment for handling the slurry. Moreover, pH is set to 9-10 when using lithium hydroxide. If lithium hydroxide is added and the pH exceeds 10, the dispersion of the emulsion will be destroyed and the function as a paint will not be achieved.

Further, the blending amount of the binder in the solid content of the slurry in the present invention is in the range of 1 to 5% by mass, preferably 1.5 to 3.0% by mass. The content of the water-soluble polymer in the binder is in the range of 30 to 65% by mass, preferably 45 to 55% by mass. When the content of the water-soluble polymer in the binder is less than 30% by mass, the dispersibility of graphite particles is poor and it is difficult to obtain a uniform coating film, and the charge / discharge capacity is reduced. On the other hand, the content of water-soluble polymer is 6
If it exceeds 5% by mass, the flexibility of the coating film is low, cracks may occur in the coating film when wound, and similarly, the charge / discharge capacity, especially the capacity at large current, is reduced. Therefore, the water-soluble polymer in the binder is a material that not only contributes to dispersion as an organic component in the slurry but also affects the film-forming property of the formed negative electrode coating film and contributes to the discharge capacity,
It can be said that the water-dispersed emulsion resin is a material that contributes to the obtained coating film adhering to the substrate. The solid content of the slurry, that is, when the amount of the binder compounded in the total amount of the carbon material active material and the substantial solid content of the binder is less than 1% by mass, the obtained negative electrode coating film is easily peeled off from the substrate. As a result, the compounding amount is small. On the other hand, if the amount of the binder compounded exceeds 5% by mass, the charge / discharge capacity sharply decreases because the binder is not a material that absorbs lithium ions.

As the water-dispersed emulsion resin constituting the binder of the present invention, a natural rubber (NR) latex having a minimum film forming temperature of 10 ° C. or lower, a styrene-butadiene rubber (SBR) latex, a butadiene rubber (BR) latex, Acrylonitrile-butadiene copolymer rubber (N
BR) latex, methyl methacrylate / butadiene copolymer rubber (MBR) latex, styrene butadiene / styrene copolymer (SBS) latex, and acrylic ester resin emulsion, and at least one selected from these water-dispersed emulsion resins. Alternatively, a mixture can be used. As the water-soluble polymer, the viscosity of a 1% by mass aqueous solution at 25 ° C. is 200
~ 2,500 mPa · s of sodium alginate, potassium alginate, ammonium alginate, propylene glycol alginate, carboxymethyl cellulose and its sodium salt or ammonium salt,
Hydroxyethyl cellulose, polyethylene oxide,
It is one or more selected from polyvinyl alcohol, casein, and sodium polyacrylate. These water-soluble polymers can be used alone or as a mixture.

Forming a coating film with a water-dispersed emulsion resin component is to aggregate emulsion particles constituting the emulsion resin, and the critical temperature for this aggregation of particles is called the minimum film-forming temperature. This temperature is an important factor in handling emulsion-containing paint, and when the slurry is applied and dried at a temperature lower than this critical temperature to form a coating film, the emulsion particles only partially aggregate. In some cases, the coating film may be discontinuous, or it may simply become powder and cannot be formed into a coating film. Therefore, in order to form a coating film with a coating material containing an emulsion, it is necessary to handle at a minimum film forming temperature or higher. The upper limit of the minimum film-forming temperature of the water-dispersed emulsion resin used in the present invention is room temperature (25 ° C)
It is lower than 10 ° C. Therefore, when the slurry is applied to form a coating film, the coating material does not require any additional equipment such as special heating or constant temperature.

It should be noted that 2% of a 1% by mass aqueous solution of a water-soluble polymer.
The viscosity at 5 ° C. depends on the molecular weight of the water-soluble polymer, the bonding form of the polymer, and the like. Generally, the lower the viscosity value, the lower the molecular weight or the degree of polymerization. When the viscosity of the aqueous solution exceeds 2,500 mPa · s, the water-soluble polymer has a high degree of polymerization, the viscosity of the slurry increases, the solid content is low, and the slurry becomes difficult to handle.
On the other hand, when the viscosity of the aqueous solution is less than 200 mPa · s, the viscosity of the slurry also decreases and the sedimentation of the particles becomes remarkable. The preferred viscosity range is 300 to 1,500 mPa · s.

Further, the carbonaceous material active material applied to the slurry of the present invention contains 50 mass% or more of agglomerated graphite particles composed of phosphorous or flaky natural graphite particles, and the agglomerated graphite particles are contained. The group has a cumulative 50% diameter (D50 diameter) in the laser light diffraction method of 10 to 25 μm and a specific surface area in the nitrogen gas adsorption method of 2.5 to ⁵ m ² / g.
The apparent density in the static method is 0.45 g / cm ³ or more,
The apparent density in the tap method is 0.70 g / cm ³ or more.

If the value of D50 diameter (average particle diameter) is less than 10 μm, the particle diameter of the agglomerated graphite particles is too small, and the resulting negative electrode coating film has increased contact resistance between graphite particles and is formed. The conductivity of the product tends to deteriorate. Therefore, as the battery characteristics, the charge / discharge capacity and the charge / discharge load characteristics are deteriorated, and the charge / discharge efficiency accompanying the decomposition of the electrolytic solution is decreased. On the other hand, when the value of D50 diameter exceeds 25 μm, the particle diameter of the graphite particle group is too large, and it takes time for the lithium ion in the negative electrode coating film to diffuse inside and outside the graphite during charging / discharging. As the discharge load characteristics deteriorate,
The formed coating film may have poor smoothness, and lithium may be locally deposited during charging.

Further, there is a correlation with the value of the D50 diameter (average particle diameter), but the specific surface area by the nitrogen gas adsorption method is 2.
When it is less than 5 m ² / g, the value of the specific surface area of the graphite particle group is low, and the particle group becomes coarse. Therefore, as a negative electrode coating film, it takes time for the diffusion of lithium ions into and out of graphite during charging / discharging, the charging / discharging load characteristics are deteriorated, and the smoothness of the coating film formed is deteriorated. Lithium may be deposited. On the other hand, when the specific surface area by the nitrogen gas adsorption method exceeds 5 m ² / g, the graphite particles become a fine particle group, and the contact resistance between the graphite particles increases as a negative electrode coating film, and the conductivity of the coating film formed is increased. Deteriorates, charge and discharge capacity and charge and discharge load characteristics decrease, charge and discharge efficiency decreases with the decomposition of the electrolytic solution, aggregation tends to proceed to a particle group with a low bulk density, and the specific surface area is It is not preferable if it is larger than the value.

Further, the massive graphite particle group in the present invention
Apparent density by static method of 0.45g / cm^Threethat's all,
Apparent density by tap method is 0.70g / cm^ThreeAbove
is there. Apparent density by static method and appearance by tap method
The method for measuring the applied density is the pigment test method (JIS K51
01). The static method and the
The apparent density by tapping and tapping is the Hosokawa Micron
It was measured using the Udertester PT-R type.
It The apparent density is measured by the static method by passing it through a sieve mesh.
Put the sample in the receiver and the volume is 100 cm^ThreeBecame
It is evaluated by measuring the mass of mushrooms. On the other hand
The apparent density measurement method using the tap method
After tapping the receiver 180 times while putting it in the container,
Product 100cm^ThreeEvaluation by measuring the mass per hit
To do. Apparent density 0.45g / cm by static method^ThreeOh
And apparent density of 0.70 g / cm by tap method^Threeof
The value is the lower limit value of the graphite particle group applied to the present invention.
It To meet the demand for higher energy density of lithium-ion batteries
On the other hand, increasing the packing density of the active material, in other words
For example, high density coating is essential. For that, you can
It is necessary to form only thick coatings. The inventors
As a result of the investigation, the solid content of the slurry for forming the coating film
A good coating film can be formed when the content is 45% by mass or more.
Found. To achieve its solids content, stand still
Apparent density by the method is 0.45g / cm^ThreeTap
Apparent density by method is 0.70g / cm ^ThreeGreater than or equal to
I found it to be better. Also, less than these apparent densities
, The fluctuation of the film thickness during coating becomes large, resulting in sufficient adhesion strength.
The amount of binder needed to obtain high
There is a concern that this will cause a decrease in capacity.

[0016]

EXAMPLES The present invention will be described below in more detail with reference to examples, but the present invention is not limited to the examples. <Examples 1 to 5> (Preparation of slurry) Specific surface area 4.2 m ² / g, D50 particle size 19.3 μm, apparent density 0.55 g / by static method
cm ³ , apparent density by tap method 0.85 g / cm ³
After mixing the water-dispersed emulsion shown in Table 1 with a binder made of a water-soluble resin, ammonia water was added to the carbon material active material made of lumpy graphite particles of flaky natural graphite
Was adjusted to obtain a slurry sample.

[0017]

[Table 1]

(Coating) In a 25 ° C. environment, these slurries are applied to a rolled copper foil serving as a current collector with a gap 2
Apply using a doctor blade of 00μm, 120 ℃
After drying for 10 minutes, a negative electrode coating film of 1.5 g / cm ³ was formed by roll pressing. (Adhesiveness) A cellophane tape having a width of 18 mm was attached on the negative electrode coating film and pressure-bonded with a load of 2 kg, and then the load required to peel off the cellophane tape was measured with a push-pull gauge. Further, the peeled (destructed) state of the negative electrode coating film was observed. (Electrode characteristics) A negative electrode coating film was punched together with a copper foil with a punch to prepare an electrode. A coin-shaped model cell was prepared using metallic lithium as a counter electrode and 1M-LiPF ₆ / EC (ethylene carbonate) + DMC (dimethyl carbonate) as an electrolyte, and 0.01 V (at a current density of 0.5 mA / cm ² ). vs. Li ^/ Li ⁺ ) lithium was occluded (charged) in the negative electrode at a constant current to obtain the charge capacity. The initial discharge capacity was determined by discharging to 1.1 V (vs. Li / Li ⁺ ) at a constant current of 0.5 mA / cm ² . Furthermore, after charging at 0.5 mA / cm ² , 6 m
1.1 V (vs. Li / Li ⁺ ) at a current density of A / cm ^2.
Discharge capacity up to 0.5mA / c
The ratio with the capacity when discharged at m ² was obtained, and the discharge load characteristic (discharge rate) was evaluated. (Preservability of paste) The prepared paste sample was stored at 30 ° C, and the state of the paste after 1 month was observed.

The results of the above-mentioned various evaluations of the paste samples 01 to 05 are shown in Table 2 as Examples 1 to 5.

[Table 2] In each of the samples of Examples 1 to 5 within the scope of the present invention described in the table, the coatability, the strength of the obtained coating film, the electrode characteristics and the storage stability of the paste were all good.

<Comparative Examples 1 to 9> Samples 11 to 14 and 21 to 25 shown in Tables 3 and 5 were used as comparative samples and evaluated by the same measuring method as in the examples. The evaluation results are shown in Table 4 for Comparative Examples 1 to 4, and Table 6 for Comparative Examples 5 to 5.
Shown as 9.

[0021]

[Table 3]

[0022]

[Table 4] In Sample 11 (Comparative Example 1), the viscosity of the 1% by mass aqueous solution of the water-soluble polymer was outside the range of Claim 2, and the coating film was faint due to the low viscosity, and the particles separated and settled during storage. Was remarkable. In Sample 12 (Comparative Example 2), the viscosity of the 1% by mass aqueous solution of the water-soluble polymer was outside the scope of Claim 2, the solid content of the slurry was low, and bleeding occurred during coating. Sample 1
In Nos. 3 and 14 (Comparative Examples 3 and 4), the content of the water-soluble polymer in the binder was outside the range defined in claim 2, and Sample 13 had a faint coating film, and Sample 14 had high water solubility. Due to the large number of molecules, the discharge load characteristics deteriorated.

[0023]

[Table 5]

[0024]

[Table 6]

In Sample 21 (Comparative Example 5), the amount of the binder blended in the slurry solids was small, and the coating film was scratched and the adhesion strength was low. In Sample 22 (Comparative Example 6), the amount of the binder was large and the discharge load characteristics were deteriorated. In Sample 23 (Comparative Example 7), since the minimum film-forming temperature of the emulsion was high, sufficient film formation was not obtained and the adhesion of the coating film was weak, causing deterioration of discharge capacity and discharge load. Sample 24 (Comparative Example 8) exhibited good coatability and electrode characteristics, but during storage for a long period of time, particles and a dispersion medium separated in the slurry, causing decomposition. Sample 25 (Comparative Example 9) showed good coatability and electrode characteristics similar to Sample 24, but had a high pH, and when left in a coated state, a blue aqueous solution was observed and the copper foil was corroded. .

<Examples 6 to 8, Comparative Example 10> Specific surface area 3.7 m ² / g, D50 particle diameter 20.5 μm, apparent density 0.46 g / cm ³ by static method, apparent density 0.4 by tap method. After mixing the water-dispersed emulsion shown in Table 7 and the water-soluble resin with the lumpy graphite particles composed of 80 g / cm ³ of flake shaped natural graphite, lithium hydroxide was added to adjust the pH to obtain a slurry.

[0027]

[Table 7]

[0028]

[Table 8] Examples 6 to 8 (Sample 0) within the scope of the present invention described in the table
6 to 08), the coatability, the strength of the obtained coating film, the electrode characteristics and the storage stability of the paste were all good. However, in Comparative Example 10 of Sample 31, the pH exceeded the range of the present invention (Claim 8), the dispersed state of the emulsion was destroyed and the paint gelled, and evaluation of coating and charging / discharging could not be performed. It was

[0029]

According to the present invention, it is possible to obtain a slurry for a negative electrode of a non-aqueous secondary battery, which has good coating film strength and coating film density of the battery and is excellent in various electrode characteristics and long-term storage stability of the slurry.

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 5H029 AJ03 AJ05 AJ13 AK11 AL07                       AM01 DJ08 EJ05 EJ12 HJ01                       HJ05 HJ07 HJ08 HJ10 HJ14                 5H050 AA07 AA08 AA15 AA18 BA17                       CA17 CB08 DA03 DA11 EA11                       EA23 FA17 GA10 GA22 GA27                       HA01 HA05 HA07 HA08 HA10                       HA14

Claims (8)

[Claims] 1. A slurry for forming a negative electrode coating film of a non-aqueous secondary battery, the solid content of which is a carbon material active material and a binder, and the medium of which is water.
A slurry for forming a negative electrode coating film of a non-aqueous secondary battery, which contains an H adjuster and has a pH of 9 or more. 2. The binder is composed of a water-dispersed emulsion resin and a water-soluble polymer, the content of the water-soluble polymer in the binder is in the range of 30 to 65% by mass, and the solid content of the slurry. The content of the binder is 1 to 5% by mass, and the water-dispersed emulsion resin has a minimum film forming temperature of 10 ° C. or less. One or more selected from butadiene copolymer rubber latex, methyl methacrylate / butadiene copolymer rubber latex, styrene butadiene / styrene copolymer latex, and acrylic ester resin emulsion, wherein the water-soluble polymer is 1% by mass. Viscosity of the aqueous solution at 25 ° C is 200 to 2,500 mPa · s
Sodium alginate, potassium alginate, ammonium alginate, propylene glycol alginate, carboxymethyl cellulose and its sodium salt or ammonium salt, hydroxyethyl cellulose, polyethylene oxide, polyvinyl alcohol,
The slurry for forming a negative electrode coating film of a non-aqueous secondary battery according to claim 1, which is one or more selected from casein and sodium polyacrylate. 3. The carbonaceous material contains, as the carbonaceous material, 50% by mass or more of a lumpy graphite particle group composed of phosphorus-like or flake-like natural graphite particles, and the lumpy graphite particle group comprises:
The cumulative 50% diameter (D50 diameter) in the laser light diffraction method is 10 to 25 μm, the specific surface area in the nitrogen gas adsorption method is 2.5 to ⁵ m ² / g, and the apparent density in the stationary method is 0.4.
5 g / cm ³ or more, apparent density in tap method is 0.
The slurry for forming a negative electrode coating film of a non-aqueous secondary battery according to claim 1, which has a content of 70 g / cm ³ or more. 4. The pH adjusting agent is aqueous ammonia,
The slurry according to claim 1, wherein the pH of the slurry is in the range of 9-12. 5. The slurry according to claim 1, wherein the pH adjuster is lithium hydroxide and the pH of the slurry is in the range of 9 to 10. 6. A slurry for forming a negative electrode coating film of a non-aqueous secondary battery, the main constituents of which are a carbon material active material and a binder, and the medium of which is water, and a pH adjuster is added to the slurry. A method for adjusting a slurry for forming a negative electrode coating film of a non-aqueous secondary battery, which comprises adjusting the pH of the slurry to an alkaline region of 9 or more. 7. The slurry adjusting method according to claim 6, wherein ammonia water is used as the pH adjusting agent to adjust the pH to a range of 9 to 12. 8. The slurry adjusting method according to claim 6, wherein lithium hydroxide is used as the pH adjusting agent and the pH is adjusted within a range of 9 to 10.