JP2001015114A - Slurry for forming negative electrode film for nonaqueous electrolyte secondary battery and negative electrode film for nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slurry for forming a negative electrode film for a nonaqueous electrolyte secondary battery and a negative electrode film for a nonaqueous electrolyte secondary battery having high safety in handling, excellent adhesion between a synthetic carbon material or a graphite material and a current collecting body, suppressed decrease of discharge capacity, excellent durability against a charging and discharging cycle. SOLUTION: This slurry for forming a negative electrode film for a nonaqueous electrolyte secondary battery has a basic structure consisting of a carbon material, a binding agent and a water medium. A portion or the entirety of the binding agent uses one or more alginate salt selected from sodium alginate, potassium alginate, or ammonium alginate and/or propylene glycol alginate ester. A negative electrode film is formed by using it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slurry for forming a negative electrode film of a nonaqueous electrolyte secondary battery and a negative electrode film of the battery.

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries, for example, lithium ion secondary batteries, have been widely used as rechargeable power sources for notebook computers, mobile phones, and the like. And higher voltage are desired. The non-aqueous electrolyte secondary battery is composed of members such as a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. As a negative electrode material of a non-aqueous electrolyte secondary battery, a carbon material such as mesophase carbon microbeads (MCMB), non-graphitizable carbon, and natural graphite is dispersed in a medium in which a binder is dissolved, as a negative electrode active material. Into a slurry for forming a negative electrode coating film, and apply this slurry onto a rolled copper foil as a current collector.
What was dried and used as the negative electrode coating film is used. Conventionally, as a medium of the slurry for forming a negative electrode coating film, N-methyl-
An organic solvent represented by 2-pyrrolidone (NMP) is used. Further, as a binder to be combined with the NMP, a fluorine-based resin material represented by polyvinylidene fluoride (PVDF) is used.

[0003] In order to satisfy the above demand for higher battery capacity, it is important to increase the capacity of the negative electrode material, that is, how much lithium ions can be absorbed by the negative electrode and the lithium ions can be released. Becomes However, MCMB conventionally used as a negative electrode active material
Is insufficient in graphitization, the resulting discharge capacity is 300
It stays at about mAh / g. For this reason, batteries (negative electrode materials)
As a measure to achieve the demand for higher capacity, studies are underway to use graphite particles such as natural graphite having high crystallinity as a negative electrode active material. This is because these graphite particles can obtain a value close to the theoretical charge / discharge capacity of 372 mAh / g. When graphite particles are used as the negative electrode active material, it becomes suitable for increasing the voltage of a non-aqueous electrolyte secondary battery.

[0004]

However, since the crystal structure of the graphite material has a high bonding force in the layer direction, the graphite material becomes flake-like or flake-like flake-like particles by pulverization. For this reason, a binder made of a fluorine-based resin typified by PVDF, which is conventionally mainly used, has a low film-forming property, and does not provide sufficient adhesion between graphite particles and a copper foil as a current collector. Was. As a result, even if graphite powder which is advantageous in terms of crystal structure is applied as the negative electrode active material, the charge / discharge capacity is reduced due to the high electric resistance value, and the capacity decrease at the time of a large current is increased. Further, there is a disadvantage that the capacity is significantly reduced in a charge / discharge cycle in which charge / discharge is repeatedly performed. further,
It has been pointed out that fluorine-based resin decomposes at high temperature and violent reaction between released fluorine and lithium occurs as a safety problem.

In the case of using a fluororesin represented by PVDF as a binder, it is necessary to use an organic solvent such as NMP as a solvent for slurrying. It is preferable that the solvent for slurrying is made aqueous from the viewpoints of the environment, the safety of the worker, and the price. As such an aqueous slurry, Japanese Patent Application Laid-Open No. 6-310142 discloses a method of producing a graphite intercalation compound containing sulfuric acid by immersing graphite powder in a mixed solution of concentrated sulfuric acid and concentrated nitric acid. It is disclosed that a slurry (paste) for forming a negative electrode of a lithium ion secondary battery is formed by using a binder of an acrylic resin and an aqueous solution of carboxymethyl cellulose on artificial graphite powder having a volume expansion of 5 times or less the original volume. ing.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is excellent in handling safety. In addition to synthetic carbon materials such as non-graphitizable carbon and MCMB, phosphorus-like materials can be used. Alternatively, the slurry for forming a negative electrode coating film of a nonaqueous electrolyte secondary battery having excellent adhesion between a flaky graphite material and a current collector, a small decrease in discharge capacity, and excellent durability against charge / discharge cycles, and a non-aqueous electrolyte It is intended to provide a negative electrode coating film for an aqueous electrolyte secondary battery.

[0007]

In order to solve the above-mentioned problems, a slurry for forming a negative electrode coating film of a non-aqueous electrolyte secondary battery according to the present invention comprises a carbon material, a binder and an aqueous medium. In the slurry for forming a negative electrode coating film of a nonaqueous electrolyte secondary battery, a part or all of the binder used is an alginate selected from one or more of sodium alginate, potassium alginate and ammonium alginate, and / or
Or propylene glycol alginate, the amount of which is 0.1 to 15 parts by weight with respect to 100 parts by weight of the carbon material in the slurry. To provide. Another invention related to the present invention relates to a negative electrode coating film of a non-aqueous electrolyte secondary battery having a carbon material and a binder as basic components, wherein a part or all of the binder is alginate and / or Or propylene glycol alginate, the amount of which is determined by the carbon material 10 in the negative electrode coating.
It is intended to provide a negative electrode coating film of a non-aqueous electrolyte secondary battery in an amount of 0.1 to 15 parts by weight with respect to 0 parts by weight.

[0008] Even if the carbon material used as the negative electrode active material is a flaky graphite powder or a phosphorous graphite powder having a high bonding force in the layer direction, the negative electrode coating film has a high film-forming property and a good inter-graphite graphite powder. Also, a good adhesion state to the copper foil as the current collector can be obtained. In addition, the negative electrode is filled with a larger volume of active material in a fixed volume, so that a more miniaturized non-aqueous electrolyte secondary battery can be provided.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is characterized in that a slurry for forming a negative electrode coating film of a non-aqueous electrolyte secondary battery and a negative electrode coating film of a non-aqueous electrolyte secondary battery include a part or all of a binder. The point is that alginate and / or propylene glycol alginate was used. That is, the binder in the present invention is a simple substance of sodium alginate, a simple substance of potassium alginate, a simple substance of ammonium alginate, a simple substance of propylene glycol alginate, or a mixture of alginate or alginate and propylene glycol alginate. May be in any form. In addition, alginates and / or propylene glycol alginate may be used in combination with other aqueous binders such as acrylic resin emulsions and carboxymethyl cellulose. In the case of a slurry containing alginate and / or propylene glycol alginate, carbon particles in the slurry can be dispersed well. This is because the hydroxyl groups (—OH) scattered in the molecule have an affinity for the aqueous medium and the other hydrophobic groups have an affinity for the carbon material, and thus contribute to the dispersion of the carbon particles in the aqueous medium. It seems to be in.

In addition, the negative electrode coating film containing alginate and / or propylene glycol alginate has a coating obtained in comparison with the case where another water-based binder is added in the same amount with respect to the carbon material. It is excellent in the characteristics of the membrane, particularly in terms of increasing the capacity of the non-aqueous electrolyte secondary battery.
The reason is that alginate and / or propylene glycol alginate is a material having excellent ion conductivity as compared with other aqueous binders, for example, carboxymethylcellulose and acrylic resin, so that alginate and / or Alternatively, in the negative electrode coating film according to the present invention to which propylene glycol alginate has adhered, lithium ions easily enter and exit, which is considered to contribute to an increase in the capacity of the nonaqueous electrolyte secondary battery. In addition, alginate and / or propylene glycol alginate are typical materials used as synthetic pastes and thickening stabilizers for food additives. For this reason, in addition to the fact that the slurry medium is water-based, consideration for the environment and safety for workers can be further ensured.

The compounding amount of the alginate and / or propylene glycol alginate is determined according to the amount of the carbon material 1 in the slurry for forming the negative electrode coating film of the nonaqueous electrolyte secondary battery.
0.1 to 15 parts by weight based on 00 parts by weight.
If it is less than 0.1 part by weight based on 100 parts by weight of the carbon material,
It is not preferable because the adhesion between the graphite particles and the adhesion with the copper foil are reduced. On the other hand, when the amount exceeds 15 parts by weight, the carbon material surface is excessively coated, the conductivity is deteriorated, and the capacity is reduced.
It is not preferable because the ratio of the active material in the coating film decreases. In addition, when each of alginate and / or propylene glycol alginate alone or a mixture thereof is used alone as a binder, a preferable blending amount is 3 to 15 parts by weight based on 100 parts by weight of the carbon material. And a particularly preferred range is 4 to 10 parts by weight. On the other hand, when other aqueous binders such as acrylic emulsion and carboxymethylcellulose are used as binders in combination with each of alginate and / or propylene glycol alginate alone or a mixture thereof, alginate is used. The preferred blending amount of propylene glycol alginate is 0.1 parts by weight to 12 parts by weight with respect to 100 parts by weight of the carbon material.
Although it is in the range of parts by weight, the optimum amount is determined by the amount of the other aqueous binder used in combination.

[0012]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples, but the present invention is not limited to these Examples. <Example 1> (Preparation of slurry) Scaly graphite having an average particle diameter of 20 µm (manufactured by Hitachi Powdered Metals Co., Ltd., trade name: GP-820) 100
Table 1 shows the blending amounts of various alginate and / or propylene glycol alginate with respect to parts by weight.
And kneaded with a planetarium mixer using pure water as a solvent to prepare a slurry having a solid content of 33% by weight. The alginate and propylene glycol alginate used are as follows. Sodium alginate: manufactured by Kibun Food Chemifa Co., Ltd., trade name: Duck Algin NSPM, potassium alginate
-Kimitsu Chemical Industry Co., Ltd., trade name: Kimitsu Algin K, ammonium alginate-Kibun Food Chemifa Co., Ltd.
Trade name: Duck Ammon, propylene glycol alginate, manufactured by Kimitsu Chemical Industry Co., Ltd., trade name: Kimiloid HV. These slurries were applied on a rolled copper foil as a current collector using a doctor blade having a gap of 200 μm, dried at 120 ° C. for 10 minutes, and passed through a roll press to form a negative electrode coating having a coating density of 1.35 g / cc. did.

(Coating property) As described above, when the slurry is applied on the rolled copper foil, the coating film is blurred, the coating film surface is roughened,
The coating property was evaluated by observing the bleeding of the solvent and the fluidity. Further, the dispersibility of the particles in the slurry was also observed from the degree of separation and sedimentation of the particles when the slurry was allowed to stand for 12 hours. (Electrode characteristics) The negative electrode coating was punched out together with a copper foil with a punch to produce a negative electrode. Lithium metal was used as the counter electrode, and opposed via a separator of a polypropylene porous membrane (trade name: Celgard # 2400, manufactured by Hoechst Japan KK). LiPF6 / EC + DMC (manufactured by Toyama Pharmaceutical Co., Ltd.) was used as the electrolyte. A coin-type model cell using a trade name: LI-PASTE / PF1) was prepared, and lithium was inserted into the negative electrode at a constant current of 0.01 V (vs. Li / Li +) at a current density of 0.5 mA / cm2 ( Charge) to determine the charge capacity. The initial discharge capacity was determined by discharging the battery to 1.1 V (vs. Li / Li +) at a constant current of 0.5 mA / cm2. Table 1 shows the binder composition in the studied slurry and the evaluation results of the obtained negative electrode coating film.

[0014]

[Table 1]

As shown in Sample Nos. 11 to 16, when the content of alginate and / or propylene glycol alginate in graphite is in the range of 3 to 15 parts by weight,
The adhesion strength increases as the blending amount increases. Further, the adhesion between particles and between the copper foils is good, and the ion permeability allows the discharge capacity to be high and the discharge rate to show good values. On the other hand, as described in Comparative Example 1, when the blending amount of propylene glycol alginate was 2 parts by weight (sample number: Comparative 1), the adhesion strength was low and the coating film was peeled off from the interface with the copper foil. When the adhesion is low, the contact state between the particles and the copper foil is not good, so that the discharge capacity and the discharge rate are low. When ammonium alginate and sodium alginate were mixed and the blending amount exceeded 15 parts by weight (sample number: comparative 2),
The influence of the surface coating increased, and both the discharge capacity and the discharge rate decreased.

<Example 2> Similar to Example 1, flaky graphite having an average particle diameter of 20 μm (GP-820, manufactured by Hitachi Powdered Metals Co., Ltd.) was used as the negative electrode active material and bound. Acrylic styrene copolymer emulsion (high pressure gas industry)
A trade name: LI-401 manufactured by K.K.) was mixed with alginic acid and / or propylene glycol alginate. After dissolving sodium alginate using pure water as a solvent, adding graphite and mixing, 5 parts by weight of an acrylic styrene emulsion is added to the graphite and kneaded using a planetarium mixer to obtain a slurry having a solid content of 33% by weight. Was prepared. Evaluation of coating properties, adhesion, and electrode characteristics was performed in the same manner as in Example 1. Table 2 shows the evaluation results.
Shown in

[0017]

[Table 2]

When the amount of alginate and / or propylene glycol alginate in the graphite is 0.1 to 12 parts by weight as in Sample Nos. 21 to 26, the amount of the alginate is increased as in the case of Example 1. As the strength increases, the adhesion strength increases. In addition, adhesion between particles and between copper foils is good, and because of ion permeability, the discharge capacity is high and the discharge rate shows a good value. On the other hand, as described in Comparative Example 2, when no alginate and / or propylene glycol alginate was contained (Sample No .: Comparative 3), the adhesion strength was low and the coating film was peeled off from the interface with the copper foil. The dispersibility of the particles in the slurry was poor, and the particles settled after standing for 12 hours. In addition, blurring occurred during coating. Furthermore, the discharge capacity and discharge rate are low due to poor adhesion and poor contact between particles and the copper foil. When the blending amount of alginate and / or propylene glycol alginate exceeds 12 parts by weight (Sample No .: Comparative 4), the effect of the surface coating becomes large as in Example 1, and the discharge capacity and discharge rate decrease. .

[0019]

According to the present invention, there is provided a non-aqueous electrolyte secondary battery having excellent adhesion between a synthetic carbon material and a graphite material and a current collector, little decrease in discharge capacity, and excellent durability in a charge / discharge cycle. A slurry for forming a negative electrode coating film and a negative electrode coating film of the same battery using the same can be provided.

Claims (3)

[Claims] 1. A slurry for forming a negative electrode coating film of a non-aqueous electrolyte secondary battery comprising a carbon material, a binder and an aqueous medium as basic components, wherein sodium alginate is used as part or all of the binder. A slurry for forming a negative electrode coating film of a non-aqueous electrolyte secondary battery, wherein at least one alginate selected from potassium alginate and ammonium alginate and / or propylene glycol alginate is used. 2. The amount of alginate and / or propylene glycol alginate in the slurry for forming a negative electrode coating film is 0.1 to 15 parts by weight based on 100 parts by weight of the carbon material. The slurry for forming a negative electrode coating film of the nonaqueous electrolytic solution secondary battery according to 1. 3. A negative electrode coating film for a non-aqueous electrolyte secondary battery comprising a carbon material and a binder as a basic composition, wherein a part or all of the binder is sodium alginate;
At least one alginate and / or propylene glycol alginate selected from potassium alginate and ammonium alginate; the amount of the alginate and / or propylene glycol alginate is 0.1% by weight based on 100 parts by weight of the carbon material; A negative electrode coating film for a nonaqueous electrolyte secondary battery, wherein the coating amount is 1 part by weight to 15 parts by weight.