JP2014170685A - Binder composition for secondary battery electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a binder for a secondary battery which achieves good adhesiveness between a current collector and an electrode layer and good dispersibility of the binder in the electrode layer, and allows the electrode layer to have good windability; and an application thereof.SOLUTION: A binder composition for a secondary battery electrode comprises carboxymethyl cellulose or a salt thereof, and polyacrylic acid or a salt thereof. A composition for an electrode layer of a secondary battery comprises the binder composition and an active material. A secondary battery electrode comprises an electrode layer formed from the composition for an electrode layer, and a current collector. A secondary battery comprises the electrode. A method for manufacturing a secondary battery comprises: the step of forming a laminate of a multilayer structure including a pair of electrodes, at least one of which consists of the secondary battery electrode described above, and a separator located between the paired electrodes and impregnated with a nonaqueous electrolyte, or the step of forming the laminate, followed by winding the laminate; and the step of putting the laminate in a container.

Description

  The present invention relates to a binder composition for a secondary battery electrode and use thereof.

  Secondary batteries such as small and light lithium-ion batteries are installed in a wide range of electronic devices and electric devices such as mobile phones, notebook computers, digital cameras, digital videos, and portable music players. Lithium ion batteries have been put into practical use as a power source for transportation such as cars called eco-cars, and have been vigorously studied as power storage devices for power leveling and smart grids.

  Each of the positive electrode and the negative electrode of a lithium ion battery has a structure in which a current collector is sandwiched between a pair of electrode layers containing an active material and a binder. The positive electrode is usually produced by applying and drying a mixture (solution) of an active material such as lithium cobaltate and a binder on both surfaces of a current collector such as an aluminum foil. The negative electrode is usually manufactured by applying and drying a mixture (solution) of an active material such as a carbon material and a binder on a current collector such as a copper foil.

  A lithium ion battery includes a positive electrode, a negative electrode, and a porous insulating film through which ions can move, and has a structure in which a positive electrode, an insulating film, and a negative electrode are stacked in layers. For example, a lithium-ion battery is manufactured by winding a multilayer body having a separator such as an insulating film between a positive electrode and a negative electrode so that each electrode has a Baumkuchen shape (concentric shape), and inserting the electrode into a container. Can be welded, electrolyte solution can be injected, and the container can be sealed to obtain a lithium ion battery. When the shape of the lithium ion battery is cylindrical, the electrode multilayer body is wound into a cylindrical shape in accordance with the shape of the can. When the shape of the lithium ion battery is square, the electrode multilayer body is wound into a flat shape in accordance with the shape of the can.

  As a binder constituting the electrode of a lithium ion battery, polyvinylidene fluoride, polytetrafluoroethylene, styrene butadiene latex (SBR), carboxymethyl cellulose or a salt thereof, polyacrylic acid or a salt thereof, and the like are disclosed (Patent Document 1). And Patent Document 2).

JP 2007-115671 A JP 2009-80971 A

  However, the electrode obtained using a conventional binder has insufficient adhesion between the electrode layer and the current collector. Further, the conventional binder has a problem that the dispersibility is insufficient and aggregates are generated.

  Further, as secondary batteries such as lithium ion batteries are widely used in various fields, improvement in the electric capacity of secondary batteries is required. One method for improving the electric capacity is to increase the thickness of the electrode layer of each electrode.

  However, when the electrode layer is made thick in the conventional lithium ion battery, the electrode layer is easily broken, and in particular, when the multilayer body is wound in a Baumkuchen shape, the problem that the electrode layer is broken occurs. In addition, the suitability (resistance to cracking) of the electrode layer when the multilayer body is wound or bent is referred to as winding property.

  Accordingly, the present invention provides a binder for a secondary battery electrode that has excellent adhesion between the current collector and the electrode layer, and dispersibility of the binder in the electrode layer, and the electrode layer has good winding properties, and uses thereof. The purpose is to provide.

The present invention provides the following [1] to [7].
[1] A binder composition for a secondary battery electrode, comprising carboxymethylcellulose or a salt thereof and polyacrylic acid or a salt thereof.
[2] The binder composition according to the above [1], wherein the degree of etherification of carboxymethylcellulose or a salt thereof is 0.5 to 1.2.
[3] The binder composition according to the above [1] or [2], wherein the degree of polymerization of polyacrylic acid or a salt thereof is 100,000 or less.
[4] A composition for an electrode layer of a secondary battery, comprising the binder composition according to any one of [1] to [3] and an active material.
[5] An electrode for a secondary battery comprising an electrode layer formed from the composition as described in [4] above and a current collector.
[6] A secondary battery comprising the electrode according to [5].
[7] A multilayer body including a pair of electrodes including at least one electrode according to [5] above and a separator formed between the electrodes and impregnated with a nonaqueous electrolyte is laminated or wound after lamination. A method for manufacturing a secondary battery, which is inserted into a container.

  ADVANTAGE OF THE INVENTION According to this invention, the binder composition for secondary battery electrodes which is excellent in the adhesiveness of a collector and an electrode layer, and the dispersibility of the binder in an electrode layer, and has favorable winding property, and it The composition for electrode layers using the electrode, the electrode, and the method for producing the secondary battery can be provided.

(Binder composition)
The binder composition for electrodes of the present invention contains carboxymethyl cellulose or a salt thereof and a mixture of polyacrylic acid or a salt thereof.

  Carboxymethyl cellulose or a salt thereof (hereinafter sometimes abbreviated as CMC) used in the present invention has a structure in which a hydroxyl group in a glucose residue constituting cellulose is substituted with a carboxymethyl ether group. Carboxymethylcellulose may be in the form of a salt. Examples of the carboxymethyl cellulose salt include metal salts such as carboxymethyl cellulose sodium salt.

  In the present invention, cellulose means a polysaccharide having a structure in which D-glucopyranose (also simply referred to as “glucose residue” or “anhydroglucose”) is linked by β, 1-4 bonds. Cellulose is generally classified into natural cellulose, regenerated cellulose, fine cellulose, microcrystalline cellulose excluding non-crystalline regions, etc. from the origin, production method and the like.

  Examples of natural cellulose include cellulose produced by microorganisms such as bleached or unbleached pulp, refined linter, and acetic acid bacteria. The raw material of bleached or unbleached pulp is not specifically limited, For example, wood, cotton, straw, bamboo, etc. are mentioned. The method for producing bleached or unbleached pulp is not particularly limited, and examples thereof include a mechanical method, a chemical method, or a method combining mechanical methods and chemical methods. Examples of bleached or unbleached pulp include mechanical pulp, chemical pulp, groundwood pulp, sulfite pulp, kraft pulp, and papermaking pulp. Examples of bleached or unbleached pulp also include dissolved pulp that is chemically refined and used as a main raw material for artificial fibers, cellophane, etc., mainly used after being dissolved in chemicals.

  Examples of the regenerated cellulose include regenerated cellulose obtained by dissolving cellulose in a solvent such as a copper ammonia solution, a cellulose xanthate solution, or a morpholine derivative and spinning it again.

  As the fine cellulose, there are fine cellulose and cellulosic materials obtained by depolymerizing cellulosic materials such as natural cellulose and regenerated cellulose by acid hydrolysis, alkali hydrolysis, enzymatic decomposition, explosion treatment, vibration ball mill treatment, etc. Examples thereof include fine cellulose obtained by mechanical treatment.

  In manufacturing the CMC used in the present invention, a known CMC manufacturing method can be applied. For example, CMC can be produced by preparing mercerized cellulose (alkali cellulose) by treating cellulose with a mercerizing agent (alkali), and then adding an etherifying agent to mercerized cellulose to cause an etherification reaction. .

  As the raw material cellulose, any cellulose can be used as long as it is the above-mentioned cellulose, but those having high cellulose purity are preferred, and dissolved pulp or linter is more preferred. By using these, CMC with high purity can be obtained.

  Examples of mercerizing agents include alkali metal hydroxide salts such as sodium hydroxide and potassium hydroxide. Examples of the etherifying agent include monochloroacetic acid and sodium monochloroacetate.

  In the general method for producing water-soluble carboxymethylcellulose, the molar ratio of mercerizing agent to etherifying agent (mercelizing agent / etherifying agent) is 2.00-2. 45 is common. The reason is that when it is 2.00 or more, the etherification reaction can be sufficiently performed, and it is possible to prevent the unreacted monochloroacetic acid from remaining and being wasted. By being 2.45 or less, it is economical because it is possible to prevent the side reaction by an excess mercerizing agent and monochloroacetic acid from proceeding to produce an alkali metal glycolate.

  In the present invention, the CMC may be a commercial product. As a commercial item, the brand name "Sunrose" by Nippon Paper Chemicals Co., Ltd. is mentioned, for example.

In the present invention, the degree of etherification of CMC refers to the proportion of groups substituted with carboxymethyl ether groups (—OCH ₂ COOH) in hydroxyl groups (—OH) in glucose residues constituting cellulose.

  The CMC used in the present invention preferably has a carboxymethyl group substitution degree per glucose residue (hereinafter abbreviated as CM-DS) in the range of 0.5 to 1.2. When CM-DS is 0.5 or more, solubility in water can be kept good, and generation of undissolved substances can be suppressed. Moreover, when CM-DS is 1.2 or less, an increase in the spinnability of the liquid can be suppressed and handling can be easily maintained. CM-DS can be calculated by the method shown in the embodiment. The B-type viscosity of the 1% CMC aqueous solution at 20 ° C. is preferably 100 to 15,000 mPa · s.

  One type of CMC may be contained in the binder composition of the present invention, or a combination of two or more types of CMCs having different degrees of etherification, CM-DS, B-type viscosity, molecular weight and the like may be used.

  The polyacrylic acid and polyacrylic acid salt (hereinafter abbreviated as PAA) used in the present invention preferably has a degree of polymerization of 100,000 or less, and more preferably 70,000 or less. When the degree of polymerization is 100,000 or less, the dispersibility of the active material is good and the winding property can be maintained. The lower limit of the degree of polymerization is usually 1,000 or more and preferably 2,500 or more. The PAA may be polymethacrylic acid or polymethacrylate.

  One type of PAA may be contained in the binder composition of the present invention, or a combination of two or more types of PAAs having different molecular weights may be used.

  In the binder composition of the present invention, the mass ratio of CMC to PAA is usually 8: 2 to 2: 8, preferably 6: 4 to 4: 6.

  The binder composition of the present invention may have a binder other than CMC and PAA as long as the desired effect is not impaired. Examples of the binder that can be used in combination include styrene butadiene rubber (SBR), nitrile butadiene rubber, methyl methacrylate butadiene rubber, chloroprene rubber, and carboxy-modified styrene butadiene rubber. Of these, SBR is preferable.

  When the binder composition of this invention contains SBR, it is preferable that the mass ratio of SBR is 0.5-10 with respect to the sum total of CMC and PAA, and it is more preferable that it is 1-3. One type of SBR may be used, or a combination of two or more types of SBR having different molecular weights, styrene / butadiene contents, and the like may be used.

The binder composition of the present invention can be used as a binder for an electrode of a secondary battery. Content in the electrode of the binder composition of this invention is 0.1-10 mass% normally with respect to the whole electrode, Preferably it is 1-6 mass%, More preferably, it is 1-2 mass%. .
In the present invention, the secondary battery may be either an aqueous electrolyte secondary battery or a non-aqueous electrolyte secondary battery, but is preferably a non-aqueous electrolyte secondary battery. Examples of non-aqueous electrolyte secondary batteries include alkaline secondary batteries and lithium ion batteries, with lithium secondary batteries being preferred. Examples of the lithium secondary battery include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, and a lithium ion polymer secondary battery, and a lithium ion secondary battery is preferable.

  The binder composition of this invention can comprise the electrode layer of a secondary battery with an active material. The present invention provides a composition for an electrode layer of a secondary battery containing a binder composition and an active material.

  The active material is a negative electrode active material when the electrode layer is a negative electrode layer, and is a positive electrode active material when the electrode layer is a positive electrode.

  As the negative electrode active material, graphite (natural graphite, artificial graphite, etc.), coke, carbon fiber and other graphite materials; elements capable of forming an alloy with lithium, for example, Al, Si, Sn, Ag, Bi, An element such as Mg, Zn, In, Ge, Pb, Ti; a compound containing an element capable of forming an alloy with lithium; an element capable of forming an alloy with lithium and the compound; and carbon and / or Examples thereof include a composite with the graphite material or a nitride containing lithium. Of these, graphite materials are preferred, and graphite is more preferred.

As the positive electrode active material, LiFePO ₄ , LiMe _x O _y (Me represents a transition metal containing at least one of Ni, Co, and Mn. _X and y represent arbitrary numbers) based positive electrode active material Is preferred.

  The content of the active material in the electrode layer is usually 90 to 99% by mass, preferably 91 to 99% by mass, and more preferably 92 to 99% by mass.

  The electrode for secondary batteries of the present invention contains the binder composition of the present invention, an electrode layer containing an active material, and a current collector. The electrode usually has a structure in which an electrode layer is in contact with at least one surface of the current collector.

  As the current collector, any electrical conductor that does not cause a fatal chemical change in the configured electrode or battery can be used. When the electrode is a negative electrode, a negative electrode current collector can be used, and when the electrode is a positive electrode, a positive electrode current collector can be used.

  Examples of the material for the current collector for the negative electrode include those obtained by attaching carbon, nickel, titanium, or silver to the surface of stainless steel, nickel, copper, titanium, carbon, copper, or stainless steel. Of these, copper or a copper alloy is preferred, but copper is more preferred.

  Examples of the material for the positive electrode current collector include metals such as aluminum and stainless steel, with aluminum being preferred.

  Examples of the shape of the current collector include nets, punched metal, foam metal, foil processed into a plate shape, and the like, and foil processed into a plate shape is preferable.

  It is preferable that the electrode further has a conductive material. When the electrode has the conductive material, battery characteristics can be improved. The conductive material is usually included in the electrode layer.

  Examples of the conductive material include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. The carbon material may be one type or a combination of two or more types. Carbon black is preferred as the conductive material.

  The manufacturing conditions of the electrode are not particularly limited, but the following manufacturing method can be given as an example. The component which comprises electrode layers, such as an active material, is added to the binder composition of this invention as aqueous solution, and it mixes, stirring. The obtained mixture (which may be in the form of liquid, paste, or slurry) is laminated on the current collector by methods such as blade coating, bar coating, and die coating, and then dried, heated, etc. It is manufactured by performing the process.

  Although the shape of the electrode for secondary batteries of this invention is not specifically limited, Usually, it is a sheet form. The thickness of the electrode layer when the electrode is in sheet form (the thickness of the mixture layer formed from the electrode composition excluding the current collector portion) depends on the composition of the electrode composition, production conditions, etc. Is usually 30 to 200 μm.

  The secondary battery of this invention is equipped with the electrode for secondary batteries of the said invention. The secondary battery of the present invention may include the secondary battery electrode of the present invention as a positive electrode or a negative electrode, and the other electrode may be an electrode other than the electrode of the present invention. The secondary battery of the present invention includes a separator positioned between the electrodes and a non-aqueous electrolyte.

  The secondary battery of the present invention comprises a multilayer body including a pair of electrodes including at least one electrode for a secondary battery of the present invention and a separator positioned between the electrodes, or is wound after stacking into a container. It may be manufactured by a method including inserting. The container after insertion is usually sealed. The non-aqueous electrolyte is usually injected into the container before sealing the container.

  Hereinafter, the embodiments of the present invention will be described by way of examples, but the present invention is not limited thereto.

[Example 1]
Graphite powder 98% by weight, carboxymethyl cellulose (CM-DS = 0.92, 1% aqueous solution B-type viscosity = 2000 mPa · s (20 ° C.)) 0.5% by weight, sodium polyacrylate (reagent: Wako Pure Chemical) Manufactured, polymerization degree 22000-70000) 0.5% by mass and styrene butadiene rubber (SBR) 1% by mass were mixed and stirred at a homomixer 5000 rpm for 30 minutes. After stirring, the obtained paint was applied onto a copper foil with a 200 μm applicator and dried at 105 ° C. for 12 hours to obtain an electrode.

<Measuring method of CM-DS>
About 2.0 g of the sample was precisely weighed and placed in a 300 mL conical flask with a stopper. A solution obtained by adding 100 mL of special grade concentrated nitric acid to 1000 mL of nitric acid methanol was added and shaken for 3 hours to convert carboxymethylcellulose salt (CMC) to H-CMC (carboxymethylcellulose). The absolute dry H-CMC was accurately weighed in an amount of 1.5 to 2.0 g and placed in an Erlenmeyer flask with a 300 mL stopper. H-CMC was moistened with 15 mL of 80% methanol, 100 mL of 0.1 N NaOH was added, and the mixture was shaken at room temperature for 3 hours. As an indicator, using phenolphthalein was back titrated excess NaOH with 0.1N-H ₂ SO _4. CM-DS was calculated by the following equation.

(formula)
A = [(100 × F−0.1N—H ₂ SO ₄ (mL) × F ′) × 0.1] / (absolute dry weight of H-CMC (g))
CM-DS = 0.162xA / (1-0.058xA)
A: 1N-NaOH amount required for neutralizing 1 g of H-CMC (mL)
F: Factor of 0.1N—H ₂ SO ₄ F ′: Factor of 0.1N—NaOH

<Evaluation of dispersion state>
The prepared electrode was visually observed, and the case where there was little aggregate and uniform electrode plate was obtained was marked with ○, and the case where there was a lot of aggregate was marked with ×.

<Measurement of adhesive strength>
The prepared electrode was stored at 23 ° C. and 50% relative humidity for 24 hours at constant temperature and humidity, and then the adhesive strength was measured under the conditions of 180 ° direction peeling and peeling rate of 100 mm / min.

<Evaluation of winding property>
The case where no cracks were observed when the obtained electrode plate was bent was marked with ◯, and the one where many cracks occurred was marked with ×.

[Comparative Example 1]
98% by mass of graphite powder, 1% by mass of carboxymethyl cellulose (CM-DS = 0.92, 1% aqueous solution B-type viscosity = 2000 mPa · s (20 ° C.)) and 1% by mass of styrene butadiene rubber (SBR) are mixed as a binder. The mixture was stirred at a homomixer of 5000 rpm for 30 minutes. After stirring, the obtained paint was applied onto a copper foil with a 200 μm applicator and dried at 105 ° C. for 12 hours to obtain an electrode plate.

[Comparative Example 2]
98% by mass of graphite powder, 1% by mass of sodium polyacrylate (polymerization degree 22000-70000) as a binder and 1% by mass of styrene butadiene rubber (SBR) were mixed and stirred at a homomixer at 5000 rpm for 30 minutes. After stirring, the obtained paint was applied onto a copper foil with a 200 μm applicator and dried at 105 ° C. for 12 hours to obtain an electrode plate.

  Table 1 shows the evaluation results of Examples and Comparative Examples.

Claims (7)

  The binder composition for secondary battery electrodes containing carboxymethylcellulose or its salt, and polyacrylic acid or its salt.   The binder composition according to claim 1, wherein the degree of etherification of carboxymethylcellulose or a salt thereof is 0.5 to 1.2.   The binder composition according to claim 1 or 2, wherein the degree of polymerization of polyacrylic acid or a salt thereof is 100,000 or less.   The composition for electrode layers of the secondary battery containing the binder composition and active material as described in any one of Claims 1-3.   The electrode for secondary batteries containing the electrode layer formed from the composition of Claim 4, and a collector.   A secondary battery comprising the electrode according to claim 5.   A multilayer body including a pair of electrodes including at least one electrode according to claim 5 and a separator formed between the electrodes and impregnated with a nonaqueous electrolyte is laminated or wound after lamination and inserted into the container. A method for manufacturing a secondary battery.