JP2017182887A - Conductive assistant of electrode for nonaqueous electrolyte secondary battery and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a conductive assistant to obtain a high-performance nonaqueous electrolyte secondary battery electrode, which can effectively and efficiently show an high conductivity; and a method for manufacturing the conductive assistant.SOLUTION: A conductive assistant of an electrode for a nonaqueous electrolyte secondary battery comprises carbon originating from cellulose nanofiber. The conductive assistant is preferably 5-60 m/g in BET specific surface area.SELECTED DRAWING: None

Description

  The present invention relates to a conductive additive for a nonaqueous electrolyte secondary battery electrode.

  A non-aqueous electrolyte secondary battery such as a lithium ion secondary battery includes an electrode, a separator, and an electrolyte. Such an electrode is obtained by dispersing an electrode active material and a conductive additive in a solvent containing a binder to form an electrode slurry, which is coated on a metal foil for an electrode current collector and dried to form a composite layer. It is manufactured by doing.

  Although the conductive additive used when manufacturing such an electrode contributes to the improvement of the electronic conductivity in the formed composite material layer, the addition of the conductive additive tends to decrease the lithium ion conductivity. As a result, it may be difficult to improve the performance of the secondary battery as a whole. Furthermore, as the amount of conductive additive added increases, the amount of active material responsible for battery capacity per unit area or unit volume of the electrode increases, and as a result, battery capacity may be reduced.

  Under these circumstances, in order to improve the performance of the obtained secondary battery, techniques using various conductive assistants have been developed. For example, Patent Document 1 discloses a slurry for forming a positive electrode mixture layer in which a styrene-containing binder resin and a carbon fiber having an aspect ratio of 2 to 1000 are used in combination, and the electrical resistance of the positive electrode mixture layer is reduced. Is possible. Patent Document 2 discloses a secondary battery having a positive electrode layer containing fibrous carbon such as carbon nanotubes (CNT) and spherical carbon such as acetylene black as a conductive additive, and an active material. A good conductive network is formed between them.

JP 2010-262664 A Japanese Patent Laid-Open No. 2006-9679

  However, in the technique described in Patent Document 1, the resin binder to be used is limited to the styrene-containing binder resin, and the degree of freedom in selecting the resin binder is low. Moreover, in the technique described in Patent Document 2, the carbon nanotube that can be used as the conductive auxiliary agent is also affected by the high cost, so that the versatility is low and it is difficult to apply to industrial use.

  Accordingly, an object of the present invention is to provide a nonaqueous electrolyte secondary battery conductive additive and a method for producing the same for obtaining a high performance electrode for a nonaqueous electrolyte secondary battery that can exhibit high conductivity. It is in.

  Therefore, the present inventors have made various studies, and it is possible to produce a nonaqueous electrolyte secondary battery that can produce good rate characteristics effectively and efficiently if it is a conductive additive using cellulose nanofiber (CNF). As a result, the present invention has been completed.

That is, this invention provides the conductive support agent of the electrode for nonaqueous electrolyte secondary batteries which consists of carbon derived from a cellulose nanofiber.
The present invention also includes a step (I) of obtaining a granulated product by spray drying a slurry containing cellulose nanofibers and water, and firing the obtained granulated product in a reducing atmosphere or an inert atmosphere. Process (II)
The manufacturing method of the conductive support agent of the electrode for nonaqueous electrolyte secondary batteries provided with this is provided.

  With the conductive aid of the present invention, when an electrode slurry is formed together with an electrode active material, a resin binder, and an electrolytic solution, the absorption amount of the solvent such as the electrolytic solution is effectively reduced, and high density and good battery characteristics are obtained. The electrode which has can be manufactured simply. Moreover, it is also possible to reduce the addition amount of a conductive support agent, and an electrode can be manufactured effectively and efficiently.

Hereinafter, the present invention will be described in detail.
The conductive auxiliary agent for the electrode for a nonaqueous electrolyte secondary battery of the present invention is composed of carbon derived from cellulose nanofibers, that is, formed by carbonizing cellulose nanofibers. Such cellulose nanofiber is a skeletal component that occupies about 50% of all plant cell walls, and is a lightweight high-strength fiber that can be obtained by defibrating plant fibers constituting such cell walls to nano size. The cellulose molecular chain constituting the cellulose nanofiber is formed with a periodic structure of carbon. Such cellulose nanofibers have a fiber diameter of 1 to 500 nm, a fiber length of 1 to 500 μm, and have good dispersibility in water. Therefore, a slurry in which cellulose nanofibers are uniformly dispersed can be obtained by mixing cellulose nanofibers and water to obtain a slurry and then stirring the slurry with an appropriate stirring force. The fiber length of the cellulose nanofiber to be used is preferably 1 to 470 μm, more preferably 1 to 450 μm, and further preferably 1 to 430 μm. For example, SERISH KY-100G (fiber length: 200 to 400 μm, Daicel) A commercially available product such as Finechem Co., Ltd. can be suitably used.

After the slurry containing cellulose nanofibers is spray-dried, the cellulose nanofibers are carbonized by firing in a reducing atmosphere or firing in an inert atmosphere, and the conductive additive of the present invention comprising such carbon can be obtained. Since the obtained conductive auxiliary agent has a small specific surface area, when the electrode slurry is formed using the electrode active material, the resin binder, and the electrolytic solution together with this, the amount of absorption of the solvent such as the electrolytic solution can be effectively reduced. In addition, an electrode having high density and good battery characteristics can be obtained. Specifically, the BET specific surface area of the conductive additive of the present invention is preferably 5 to 60 m ² / g, more preferably 5 to 55 m ² / g, and further preferably 5 to 50 m ² / g. is there.

  Although the conductive support agent of this invention exhibits the aggregate or the fiber body which the fragment of this aggregate gathered, it is preferable that it is a fiber body. When the conductive auxiliary agent of the present invention is a fibrous body, the fiber length is preferably 1 to 470 μm, more preferably 1 to 450 μm, and further preferably 1 to 430 μm. Moreover, when the conductive support agent of this invention is an aggregate, the average particle diameter becomes like this. Preferably it is 1-100 micrometers, More preferably, it is 1-80 micrometers, More preferably, it is 1-50 micrometers.

The composite layer of the electrode for a non-aqueous electrolyte secondary battery of the present invention is obtained using the above-described conductive auxiliary agent, and contains an electrode active material, a resin binder, and the conductive auxiliary agent of the present invention, and includes a non-aqueous electrolyte secondary electrode. The battery electrode is provided.
The electrode active material contained in the composite material layer may be either a positive electrode active material of a secondary battery or a negative electrode active material of a secondary battery, and is not particularly limited. Examples include those using LiFePO ₄ , Li ₂ MnSiO ₄ , and NaFePO ₄ oxides, and examples of the negative electrode active material include those using Li ₄ Ti ₅ O ₁₂ oxides.

The resin binder contained in the composite material layer is used for bonding the constituent materials of the electrodes. For example, in the positive electrode, in order to bond the oxide and the current collector used in the positive electrode active material, In the negative electrode, it is used for bonding the oxide used for the negative electrode active material and the current collector. More specifically, for example, the resin binder used for the positive electrode is polyvinylidene fluoride (PVDF), and the resin binder used for the negative electrode is carboxymethyl cellulose (CMC), polyvinylidene fluoride, styrene-butadiene rubber ( SBR).
In addition, when using polyvinylidene fluoride as a resin binder for both the positive electrode active material and the negative electrode active material, since polyvinylidene fluoride is not dissolved or dispersed in water, N-methyl-2-pyrrolidone (NMP), which is an organic solvent, is used in combination. There is a need to.

  Such a composite layer preferably contains 16 to 48 parts by mass of an electrode active material and 0.5 to 2 parts by mass of a resin binder, more preferably an electrode active material, with respect to 1 part by mass of the conductive additive of the present invention. 17 to 48 parts by mass and 0.5 to 1.5 parts by mass of a resin binder. Specifically, a slurry containing these conductive assistants, electrode active materials, and resin binders can be prepared and formed by a manufacturing method described later.

The method for producing a conductive additive for an electrode for a non-aqueous electrolyte secondary battery according to the present invention comprises a step (I) of obtaining a granulated product by spray-drying a slurry containing cellulose nanofibers and water, and the obtained structure Baking the granules in a reducing or inert atmosphere (II)
Is provided.

  Step (I) is a step of obtaining a granulated body by spray drying a slurry containing cellulose nanofibers and water. The content of cellulose nanofibers in the slurry is preferably 10 to 50 parts by mass, and more preferably 10 to 25 parts by mass with respect to 100 parts by mass of water.

  The slurry is preferably stirred, and from the viewpoint of satisfactorily dispersing cellulose nanofibers in the slurry, it is preferable to use a disperser (homogenizer) for the stirring. Examples of such a disperser include a disaggregator, a beater, a low-pressure homogenizer, a high-pressure homogenizer, a grinder, a cutter mill, a ball mill, a jet mill, a short-axis extruder, a twin-screw extruder, an ultrasonic stirrer, and a home juicer mixer. Can be mentioned. Among these, an ultrasonic stirrer is preferable from the viewpoint of the dispersion efficiency of cellulose nanofibers. The degree of dispersion of cellulose nanofibers in the slurry can be quantitatively evaluated by, for example, light transmittance using a UV / visible light spectroscope or viscosity using an E-type viscometer. By confirming that the degree is uniform, it is possible to simply evaluate. The time for stirring with the disperser is preferably 0.5 to 6 minutes, and more preferably 1 to 4 minutes.

Next, in step (I), a slurry containing cellulose nanofibers and water is spray-dried to obtain a granulated body. The particle size of the granulated material is a D ₅₀ value in the particle size distribution based on the laser diffraction / scattering method, preferably 1 to 20 μm, more preferably 2 to 15 μm. Here, the D ₅₀ value in the particle size distribution measurement is a value obtained by a volume-based particle size distribution based on the laser diffraction / scattering method, and the D ₅₀ value means a particle size (median diameter) at a cumulative 50%. . In addition, it is preferable to use a spray dryer for spray drying, and the particle size of the granulated body may be adjusted to a desired value by appropriately optimizing the operating conditions of the spray dryer.
The temperature for spray drying is preferably 100 to 320 ° C., more preferably 150 to 250 ° C., from the viewpoint of incorporating cellulose nanofibers as fiber bodies or aggregates in the obtained granulated body.

  Step (II) is a step of firing the granulated product obtained in step (I) in a reducing atmosphere or an inert atmosphere. Thereby, the conductive support agent which consists of carbon derived from a cellulose nanofiber is obtained. The firing conditions of the step (II) are a reducing atmosphere or an inert atmosphere, the firing temperature is preferably 700 to 1200 ° C., more preferably 800 to 1200 ° C., and the firing time is preferably 5 minutes to 3 The time is more preferably 5 minutes to 1.5 hours. Step (II) is a step of firing only the granulated body obtained in Step (I), and the firing conditions can be selected in consideration of the circumstances of the granulated body alone. Therefore, it is possible to obtain a carbonaceous conductive additive having high crystallinity and excellent conductivity.

  The manufacturing method of the conductive support agent of the electrode for nonaqueous electrolyte secondary batteries of this invention may be further equipped with the process (III) which grind | pulverizes the conductive support agent obtained at process (II). By passing through the step (III), the fibrous body or aggregate structure of the cellulose nanofibers that are maintained from before being carbonized to after being carbonized are destroyed, and the conductive assistant is made into a fiber or a sheet. be able to. Therefore, in this step (III), from the viewpoint of preventing the conductive auxiliary agent from being pulverized into particles due to excessive compressive force or the like, the container containing only the conductive auxiliary agent is transferred without introducing the medium. It is preferable to perform ball mill pulverization for several tens of seconds with a small amount of medium or a small amount of medium.

  A non-aqueous electrolyte secondary battery electrode is obtained by forming a composite layer by using an electrode active material and a resin binder together with a conductive aid for the electrode for a non-aqueous electrolyte secondary battery of the present invention. A secondary battery electrode can be obtained. Specifically, the compound material layer is preferably 16 to 48 parts by mass of the electrode active material and preferably 0.5 to 2 parts by mass of the resin binder with respect to 1 part by mass of the conductive additive of the present invention. More preferably, the electrode active material is mixed with 17 to 48 parts by mass, and the resin binder is mixed with 0.5 to 1.5 parts by mass. Next, an electrolytic solution is added and sufficiently kneaded to prepare an electrode slurry (electrode mixture), which is applied to a current collector made of an aluminum foil or the like having a thickness of 5 to 40 μm and dried. Thereafter, the current collector on which the obtained composite material layer is formed is punched into a predetermined shape and size, and then pressed to obtain an electrode. A non-aqueous electrolyte secondary battery is obtained by incorporating and accommodating the obtained electrode (positive electrode or / and negative electrode) and a separately prepared separator.

  The above electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent that is usually used for an electrolyte solution of a lithium ion secondary battery or a sodium ion secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones Nitriles, lactones, oxolane compounds and the like can be used.

The type of the supporting salt is not particularly limited, but in the case of a lithium ion secondary battery, an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , an organic material selected from LiC (SO ₃ CF ₃ ) ₂ and LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ) It is preferably at least one of a salt and a derivative of the organic salt. In the case of a sodium ion secondary battery, an inorganic salt selected from NaPF ₆ , Nb1F ₄ , NaClO ₄ and Na1sF ₆ , a derivative of the inorganic salt, NaSO ₃ CF ₃ , NaC (SO ₃ CF ₃ ) ₂ and NaN (SO ₃ CF ₃ ) ₂ , NaN (SO ₂ C ₂ F ₅ ) _2, and an organic salt selected from NaN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and at least one derivative of the organic salt It is preferable.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

[Production Example 1: Production of positive electrode active material (LiFePO ₄ / C)]
To a slurry water composed of 1272 g of LiOH.H ₂ O and 4000 mL of water, 1153 g of 85% phosphoric acid aqueous solution was dropped to obtain a slurry containing Li ₃ PO ₄ . Next, 2780 g of FeSO ₄ .7H ₂ O was added to the obtained slurry to obtain a mixed solution, and then the obtained mixed solution was subjected to a hydrothermal synthesis reaction to obtain LiFePO ₄ . After adding 1000 g of water and 100 g of glucose (anhydrous crystalline glucose, manufactured by Nippon Shokuhin Kako Co., Ltd.) to 1000 g of the obtained LiFePO ₄ and obtaining a composite by spray drying, the composite is subjected to firing in a reducing atmosphere. Then, LiFePO ₄ / C (carbon amount = 2.0% by mass) on which carbon derived from glucose was supported was obtained.

[Production Example 2: Production of positive electrode active material (Li ₂ MnSiO ₄ / C)]
6027 g of MnSO ₄ .5H ₂ O was added to slurry water composed of 2141 g of LiOH.H ₂ O, 10479 g of Na ₄ SiO ₄ .nH ₂ O and 15000 mL of water to obtain a mixed solution, and then the obtained mixed solution was washed with water. Li ₂ MnSiO ₄ was obtained by a thermal synthesis reaction. After 1000 g of water and 225 g of glucose were added to 1000 g of the obtained Li ₂ MnSiO ₄ and spray-dried to obtain a composite, the composite was subjected to firing in a reducing atmosphere, and Li ₂ on which glucose-derived carbon was supported. MnSiO ₄ / C (carbon amount = 4.5% by mass) was obtained.

[Production Example 3: Production of positive electrode active material (NaFePO ₄ / C)]
To a slurry water composed of 1200 g of NaOH and 18000 mL of water, 1154 g of a 85% phosphoric acid aqueous solution was dropped to obtain a slurry containing Na ₃ PO ₄ . Next, 2780 g of FeSO ₄ .7H ₂ O was added to the obtained slurry to obtain a mixed solution, and then the obtained mixed solution was subjected to a hydrothermal synthesis reaction to obtain NaFePO ₄ . After 1000 g of water and 100 g of glucose were added to 1000 g of the obtained NaFePO ₄ and spray-dried to obtain a composite, the composite was subjected to firing in a reducing atmosphere, and NaFePO ₄ / C on which glucose-derived carbon was supported. (Amount of carbon = 2.0 mass%) was obtained.

[Production Example 4: Production of negative electrode active material (Li ₄ Ti ₅ O ₁₂ / C)]
1597 g of anatase TiO _2, 1478 g of Li ₂ CO ₃ and 30 g of ethanol were mixed by a ball mill and fired in the air to obtain Li ₄ Ti ₅ O ₁₂ . After 1000 g of water and 100 g of glucose were added to 1000 g of the obtained Li ₄ Ti ₅ O ₁₂ and spray-dried to obtain a composite, the composite was subjected to firing in a reducing atmosphere to support glucose-derived carbon. Li ₄ Ti ₅ O ₁₂ / C (carbon amount = 2.0% by mass) was obtained.

[Example 1]
4500 mL of water and 500 g of cellulose nanofiber (Delcel Finechem Selish FD200L (fiber length: 300 to 500 μm), water content 20 mass%) were mixed to obtain slurry water. The resulting slurry water is spray-dried at 180 ° C. (spray dryer; MDL-050M, manufactured by Fujisaki Electric Co., Ltd.) while stirring with an ultrasonic stirrer (manufactured by IKA, T25), and aggregates of cellulose nanofibers (Average particle size 3 μm) was obtained. The obtained aggregate was fired at 1000 ° C. for 1 hour in an argon hydrogen atmosphere (hydrogen concentration 3%), and then 10 g of the fired product was subjected to ball milling using 50 g of zirconia balls having a diameter of 3 mm for 30 seconds, A conductive additive obtained by carbonizing cellulose nanofibers was obtained.

[Example 2]
Except for preparing slurry water by mixing 4000 mL of water and 1000 g of cellulose nanofiber (Cerish KY100G manufactured by Daicel Finechem (fiber length: 200 to 400 μm), water content: 10 mass%), the conductive additive was prepared in the same manner as in Example 1. Obtained.

[Comparative Example 1] (acetylene black)
Commercially available acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., granular grade) was used as a conductive aid.

[Comparative Example 2] (Ketjen Black)
A commercially available ketjen black (EC300J manufactured by Lion) was used as a conductive aid.

<< Measurement of BET specific surface area of conductive additive >>
Using a specific surface area measuring device (FlowSorbIII 2305 manufactured by Shimadzu Corporation), the BET specific surface area of the conductive assistants obtained in Examples and Comparative Examples was measured by a nitrogen adsorption method.
The results are shown in Table 1.

<< Evaluation of charge / discharge characteristics using secondary battery >>
Using the conductive assistants obtained in Examples 1 and 2 and Comparative Examples 1 and 2, positive electrodes and negative electrodes of lithium ion secondary batteries and sodium ion batteries were produced. Specifically, each positive electrode active material or negative electrode active material obtained in Production Examples 1 to 4, conductive assistants obtained in Examples 1 and 2 and Comparative Examples 1 and 2, and polyvinylidene fluoride in a weight ratio of 90: The mixture was mixed at a mixing ratio of 5: 5, and N-methyl-2-pyrrolidone was added thereto and sufficiently kneaded to prepare a positive electrode or negative electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. In the case of the negative electrode slurry, coating on the copper foil was performed. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode or a negative electrode.
Next, a coin-type lithium ion secondary battery or a sodium ion secondary battery was constructed using the positive electrode or the negative electrode. A lithium foil (in the case of a lithium ion battery) or a sodium foil (in the case of a sodium ion battery) punched to φ15 mm was used as the counter electrode. In the electrolyte solution, LiPF ₆ (in the case of a lithium ion secondary battery) or NaPF ₆ (in the case of a sodium ion secondary battery) is mixed with a mixed solvent in which ethylene carbonate and ethyl methyl carbonate are mixed at a volume ratio of 3: 7. Those dissolved at a concentration of 1 mol / L were used. As the separator, a known one such as a polymer porous film such as polypropylene was used. These battery components were assembled and housed in a conventional manner in an atmosphere with a dew point of −50 ° C. or lower to produce a coin-type lithium secondary battery (CR-2032).

Using the manufactured secondary battery, a charge / discharge test was performed according to the following method.
In the case of the positive electrode active material, the charging condition is a constant current constant voltage charging with a current density of 170 mA / g and a voltage of 4.5 V, the discharging condition is a current density of 170 mA / g, a final voltage of 2.0 V, and a current density of 1700 mA / g. The discharge capacity was determined for two types of constant current discharge with a final voltage of 2.0V.
In the case of the negative electrode active material, the charging conditions are a constant current and constant voltage charging with a current density of 175 mA / g and a voltage of 3.0 V, the discharging conditions are a current density of 175 mA / g, a final voltage of 1.0 V, and a current density of 1750 mA / g. The discharge capacity was determined for two types of constant current discharge with a final voltage of 2.0V.
The results are shown in Tables 2-5.
The capacity ratio (%) shown in each table is an index of the rate characteristic obtained from the following formula, and the larger the value, the faster the charge / discharge is possible.
Capacity ratio (%) = {Discharge capacity (mA / g) / current density 1700 mA / g /
Discharge capacity (mA / g) × 100} at a current density of 170 mA / g

  From the above results, the conductive assistants of the examples can be obtained because the BET specific surface area is small as compared with the conductive assistants of the comparative examples, so that the absorption amount of the solvent such as the electrolytic solution can be effectively reduced. It can be seen that the same or better performance can be exhibited in the battery.

Claims (8)

  A conductive additive for a nonaqueous electrolyte secondary battery electrode comprising carbon derived from cellulose nanofibers. The conductive auxiliary agent for an electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the BET specific surface area is 5 to 60 m ² / g.   The conductive additive for an electrode for a nonaqueous electrolyte secondary battery according to claim 1 or 2, which is a fiber body having a fiber length of 1 to 470 µm or an aggregate having an average particle diameter of 1 to 100 µm.   The electrode active material is 16 to 48 parts by mass, and the resin binder is 0.5 to 2 with respect to 1 part by mass of the conductive additive of the electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 3. The composite layer of the electrode for nonaqueous electrolyte secondary batteries containing a mass part. Step (I) for obtaining a granulated product by spray-drying a slurry containing cellulose nanofibers and water, and step (II) for firing the obtained granulated product in a reducing atmosphere or an inert atmosphere.
The manufacturing method of the conductive support agent of the electrode for nonaqueous electrolyte secondary batteries provided with this.   The content of cellulose nanofibers in the slurry used in step (I) is 10 to 50 parts by mass with respect to 100 parts by mass of water. Production of a conductive additive for an electrode for a nonaqueous electrolyte secondary battery according to claim 5. Method. The electrode for a non-aqueous electrolyte secondary battery according to claim 5 or 6, wherein the granule obtained in step (I) has a D ₅₀ value of 1 to 20 µm in a particle size distribution based on a laser diffraction / scattering method. A method for producing a conductive auxiliary.   The baking temperature in process (II) is 700-1200 degreeC, The manufacturing method of the conductive support agent of the electrode for nonaqueous electrolyte secondary batteries of any one of Claims 5-7.