JP7690810B2 - Binder for non-aqueous electrolyte secondary battery electrodes, electrode composition for non-aqueous electrolyte secondary battery, and electrode for non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a binder for electrodes of non-aqueous electrolyte secondary batteries, an electrode composition for non-aqueous electrolyte secondary batteries, and an electrode for non-aqueous electrolyte secondary batteries.

In recent years, electronic devices, particularly portable devices such as mobile phones, PDAs (personal digital assistants), and notebook computers, have become smaller, lighter, thinner, and more powerful, and the use of portable devices is becoming more widespread. As the range of uses for such portable devices becomes more diverse, the batteries that drive them have become extremely important components. Among such batteries, nonaqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, which have high energy density and high capacity, are widely used.

Normally, non-aqueous electrolyte secondary batteries are manufactured as follows. That is, a negative electrode containing a negative electrode active material made of a carbon material capable of absorbing and releasing lithium ions, and a positive electrode containing a positive electrode active material made of a lithium-containing transition metal composite oxide (e.g., LiCoO2, LiNiO2, LiMn2O4, etc.) are formed in a sheet shape on the surface of a metal foil as a current collector substrate (current collector), respectively, to obtain a sheet-shaped positive electrode and a sheet-shaped negative electrode. Then, the sheet-shaped positive electrode and the sheet-shaped negative electrode are wound or stacked with a separator also formed in a sheet shape interposed therebetween, and are stored in a case. The sheet-shaped positive electrode and the sheet-shaped negative electrode have a structure including a metal foil as a current collector substrate (current collector) and a mixture layer containing an active material formed on the surface of the metal foil, and can be formed by applying a negative electrode active material slurry (or paste) or a positive electrode active material slurry (or paste) to the current collector and drying it.

The negative electrode active material slurry (paste) contains a binder in addition to the negative electrode active material made of a carbon material capable of absorbing and releasing lithium ions. Patent Document 1 discloses a binder for negative electrodes whose main component is styrene/butadiene latex (SBR).

According to Patent Document 1, carboxymethyl cellulose, a water-soluble thickener, is dissolved in water to prepare an aqueous solution, which is then mixed with SBR and the negative electrode active material to produce a slurry. The slurry is applied as a coating liquid onto a substrate and dried to form a sheet-shaped negative electrode.

On the other hand, in the manufacture of positive electrodes for non-aqueous electrolyte secondary batteries, organic solvents such as N-methyl-2-pyrrolidone (NMP) have traditionally been used as the solvent. However, in recent years, water has come to be used as the solvent in order to reduce the cost of handling it and to reduce the impact on the environment when it is discharged.

The positive electrode active material slurry (paste) contains a lithium-containing transition metal composite oxide (e.g., LiCoO2, LiNiO2, LiMn2O4, etc.) as a positive electrode active material, carbon as a conductive material, and a binder. Patent Document 2 describes a binder such as carboxymethylcellulose, which has a viscosity of 4000 mPa·s or more in a 1% aqueous solution. Patent Document 2 describes the preparation of an active material paste by adding carboxymethylcellulose to pure water together with a conductive material, polytetrafluoroethylene (PTFE), etc.

Japanese Patent Application Publication No. 5-74461 JP 2003-157847 A

However, both Patent Document 1 and Patent Document 2 had issues with the battery capacity retention rate after cycle testing.

The present invention aims to obtain a binder for electrodes of non-aqueous electrolyte secondary batteries that has an excellent battery capacity retention rate.

As a result of extensive efforts, the present inventors have found that the problems can be solved by the following [1] to [7].
That is, according to the present invention,
[1] A binder for electrodes of non-aqueous electrolyte secondary batteries, comprising at least carboxymethylcellulose having a degree of carboxymethyl substitution per anhydrous glucose unit of 0.5 to 1.2 and/or a salt thereof, and a molybdate.
[2] The binder for a non-aqueous electrolyte secondary battery electrode according to [1], wherein the molybdate is contained in an amount of 30 to 200% by weight relative to 100% by weight of a solid content of the carboxymethyl cellulose or a salt thereof.
[3] The binder for a nonaqueous electrolyte secondary battery electrode according to [1] or [2], wherein when 2 L of a 0.3 mass % aqueous solution of the carboxymethyl cellulose or a salt thereof having a dry mass B is prepared, the whole is filtered through a 250 mesh filter under reduced pressure conditions of -200 mmHg, and the dry mass A of the residue on the filter after filtration is measured, the ratio of the dry mass A to the dry mass B is less than 50 ppm.
[4] The electrode binder for a non-aqueous electrolyte secondary battery according to any one of [1] to [3], wherein the molybdate salt includes at least one selected from ammonium molybdate and lithium molybdate.
[5] An electrode composition for a non-aqueous electrolyte secondary battery, comprising the binder for a non-aqueous electrolyte secondary battery electrode according to any one of [1] to [4] and a silicon-based compound.
[6] An electrode for a non-aqueous electrolyte secondary battery, comprising the electrode composition for a non-aqueous electrolyte secondary battery according to [5].
[7] A non-aqueous electrolyte secondary battery using the electrode composition for a non-aqueous electrolyte secondary battery according to [5].

According to the present invention, it is possible to obtain a binder for electrodes of non-aqueous electrolyte secondary batteries that has an excellent battery capacity retention rate.

Hereinafter, the electrode binder for non-aqueous electrolyte secondary batteries (hereinafter sometimes referred to as "electrode binder") of the present invention will be described. The electrode binder of the present invention contains at least carboxymethylcellulose and/or its salt having a degree of carboxymethyl substitution per anhydrous glucose unit of 0.5 to 1.2, and a molybdate salt.

<Component A: Carboxymethylcellulose or its salt>
In the present invention, carboxymethylcellulose or its salt has a structure in which the hydroxyl group in the glucose residue constituting cellulose is replaced with a carboxymethyl ether group. Carboxymethylcellulose may be in the form of a salt. Examples of the salt of carboxymethylcellulose include metal salts such as sodium carboxymethylcellulose.

In the present invention, cellulose refers to a polysaccharide in which D-glucopyranose (also simply called "glucose residue" or "anhydroglucose") is linked together through β,1-4 bonds. Cellulose is generally classified into natural cellulose, regenerated cellulose, fine cellulose, microcrystalline cellulose (without amorphous regions) and the like, based on its origin and manufacturing method.

Examples of natural cellulose include bleached or unbleached pulp (bleached or unbleached wood pulp); linters, refined linters; and cellulose produced by microorganisms such as acetic acid bacteria. The raw materials for bleached or unbleached pulp are not particularly limited, and examples include wood, cotton, straw, and bamboo. The manufacturing method for bleached or unbleached pulp is also not particularly limited, and may be a mechanical method, a chemical method, or a method that combines the two in between. Examples of bleached or unbleached pulp classified by manufacturing method include mechanical pulp, chemical pulp, groundwood pulp, sulfite pulp, and kraft pulp. In addition to paper pulp, dissolving pulp may also be used. Dissolving pulp is chemically refined pulp, and is mainly used by dissolving it in chemicals, and is the main raw material for artificial fibers, cellophane, etc.

Examples of regenerated cellulose include cellulose dissolved in some solvent, such as cuprammonium solution, cellulose xanthate solution, or morpholine derivatives, and then spun again.

Examples of fine cellulose include those obtained by subjecting cellulosic materials, such as the above-mentioned natural cellulose and regenerated cellulose, to depolymerization treatment (e.g., acid hydrolysis, alkaline hydrolysis, enzymatic decomposition, explosive crushing treatment, vibrating ball mill treatment, etc.), and those obtained by mechanically treating the above-mentioned cellulosic materials.

It is important that the carboxymethyl cellulose or salt thereof of the present invention has a degree of carboxymethyl substitution per anhydrous glucose unit of 0.5 or more, and preferably 0.6 or more. If the degree of carboxymethyl substitution is less than 0.5, there is a risk that the solubility in water will be insufficient.

In the present invention, anhydroglucose units refer to the individual anhydroglucose (glucose residues) that make up cellulose. The degree of carboxymethyl substitution (also called the degree of etherification) refers to the proportion of hydroxyl groups (-OH) in the glucose residues that make up cellulose that have been substituted with carboxymethyl ether groups (-OCH2COOH). The degree of carboxymethyl substitution is sometimes abbreviated as DS or CM-DS.

The upper limit of the degree of carboxymethyl substitution per anhydrous glucose unit of carboxymethyl cellulose or a salt thereof is preferably 1.2 or less, more preferably 1.0 or less.

The degree of carboxymethyl substitution can be confirmed by measuring the amount of base, such as sodium hydroxide, required to neutralize the carboxymethyl cellulose in the sample. In this case, if the carboxymethyl ether group of carboxymethyl cellulose or its salt is in the form of a salt, it is converted to carboxymethyl cellulose before measurement. When making the measurement, a base, back titration using an acid, and an indicator such as phenolphthalein can be appropriately combined.

In the present invention, the viscosity of a 1% by weight aqueous solution of carboxymethyl cellulose or a salt thereof, measured at 25°C with a Brookfield viscometer (30 rpm), is preferably 1,000 to 20,000 mPa·s, more preferably 1,500 to 15,000 mPa·s, and even more preferably 2,000 to 10,000 mPa·s. With a viscosity in the above range, it is difficult to settle and an electrode slurry with good coatability can be produced, making it suitable for nonaqueous electrolyte secondary batteries.

In addition, when 2 liters of an aqueous solution of 0.3% by mass of carboxymethylcellulose or its salt is entirely filtered through a 250 mesh filter under reduced pressure conditions of -200 mmHg, the dry mass of the residue on the filter is taken as mass M, and the mass of carboxymethylcellulose or its salt dissolved in the aqueous solution is taken as mass m, it is preferable that the ratio of mass M to mass m is less than 50 ppm. If it is 50 ppm or more, when an electrode is formed using carboxymethylcellulose or its salt, the electrode may have appearance defects such as streaks or pinholes, which may reduce the quality of the battery. There is no particular lower limit for the ratio of mass M to mass m, and the lower the ratio, the better.

In the present invention, the method for producing carboxymethylcellulose or a salt thereof is not limited, and any known method for producing carboxymethylcellulose or a salt thereof can be applied. That is, the raw material cellulose is treated with a mercerizing agent (alkali) to prepare mercerized cellulose (alkali cellulose), and then an etherifying agent is added to carry out an etherification reaction, thereby producing the carboxymethylcellulose or a salt thereof in the present invention.

As the raw cellulose, any of the above-mentioned celluloses can be used without any particular restrictions, but cellulose with high purity is preferred, and dissolving pulp and linters are particularly preferred. By using these, it is possible to obtain carboxymethyl cellulose or a salt thereof with high purity.

As a mercerizing agent, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide can be used. As an etherifying agent, monochloroacetic acid, sodium monochloroacetate, etc. can be used.

In the case of a general method for producing water-soluble carboxymethylcellulose, the molar ratio of the mercerizing agent to the etherifying agent is generally set to 2.00 to 2.45 when monochloroacetic acid is used as the etherifying agent. This is because if it is less than 2.00, the etherification reaction may not proceed sufficiently, resulting in unreacted monochloroacetic acid remaining and causing waste, and if it exceeds 2.45, a side reaction between the excess mercerizing agent and monochloroacetic acid may proceed, resulting in the production of an alkali metal glycolate, which may be uneconomical.

In the present invention, commercially available carboxymethylcellulose or its salts may be used as is, or may be treated as necessary before use. An example of a commercially available product is "Sunrose" (sodium salt of carboxymethylcellulose) manufactured by Nippon Paper Industries Co., Ltd.

[Crushing process]
In the present invention, the carboxymethylcellulose or its salt may be the above-mentioned carboxymethylcellulose or its salt as it is, or may be a product that has been subjected to a pulverization treatment (a pulverized product). The pulverization treatment is usually a mechanical pulverization treatment performed by a machine. The pulverization method of carboxymethylcellulose or its salt includes both a dry pulverization method in which the cellulose is treated in a powder state and a wet pulverization method in which the cellulose is treated in a state of being dispersed or dissolved in a liquid. Either of these methods may be selected in the present invention.

When an aqueous solution of carboxymethylcellulose or a salt thereof is prepared, gel particles derived from carboxymethylcellulose or a salt thereof remain in the aqueous solution as undissolved matter. By mechanically dry- or wet-grinding carboxymethylcellulose or a salt thereof, the gel particles are made finer in the aqueous solution of the mechanically pulverized product of carboxymethylcellulose or a salt thereof. As a result, it is believed that when an electrode is formed using an aqueous solution of the mechanically pulverized product of carboxymethylcellulose or a salt thereof, it is possible to further suppress the occurrence of coarse undissolved matter that causes streaks, peeling, pinholes, etc. on the surface of the electrode.

The grinding devices that can be used for the mechanical grinding process in the present invention include the following dry grinders and wet grinders.

Examples of dry grinding machines include cutting mills, impact mills, airflow mills, and media mills. These can be used alone or in combination, and even in several stages using the same model, but airflow mills are preferred.

Examples of cutting type mills include Mesh Mill (manufactured by HORAI Co., Ltd.), Atoms (manufactured by Yamamoto Hyakuma Seisakusho Co., Ltd.), Knife Mill (manufactured by Parman Co., Ltd.), Granulator (manufactured by Herbolt Co., Ltd.), Rotary Cutter Mill (manufactured by Nara Machinery Co., Ltd.), etc.

Examples of impact mills include the Pulverizer (manufactured by Hosokawa Micron Corporation), Fine Impact Mill (manufactured by Hosokawa Micron Corporation), Super Micron Mill (manufactured by Hosokawa Micron Corporation), Sample Mill (manufactured by Seishin Corporation), Bantam Mill (manufactured by Seishin Corporation), Atomizer (manufactured by Seishin Corporation), Tornado Mill (Nikkiso Co., Ltd.), Turbo Mill (Turbo Kogyo Co., Ltd.), and Bevel Impactor (Aikawa Iron Works Co., Ltd.).

Examples of airflow mills include the CGS type jet mill (manufactured by Mitsui Mining Co., Ltd.), the jet mill (manufactured by Sansho Industry Co., Ltd.), the Ebara Jet Micronizer (manufactured by Ebara Corporation), the Selenium Mirror (manufactured by Masuko Sangyo Co., Ltd.), and the supersonic jet mill (manufactured by Nippon Pneumatic Mfg. Co., Ltd.).

Examples of media mills include vibrating ball mills.

Examples of wet grinding machines include the Masscolloider (Masuko Sangyo Co., Ltd.), the high-pressure homogenizer (Sanmaru Kikai Kogyo Co., Ltd.), and the media mill. Examples of media mills include the bead mill (Aimex Co., Ltd.).

<Molybdate>
It is important that the electrode binder of the present invention contains a molybdate. The molybdate in the present invention may be potassium molybdate, sodium molybdate, calcium molybdate, lithium molybdate, ammonium molybdate, etc. Among these, lithium molybdate and ammonium molybdate are preferred.

When such molybdate is used as an electrode binder together with carboxymethylcellulose, the hydroxyl groups in the molybdate form a crosslinked structure through hydrogen bonds with the carboxyl groups of the carboxymethylcellulose. This crosslinked structure contains more hydroxyl and carboxyl groups in the molecule than carboxymethylcellulose alone, so it is more reactive through hydrogen bonds with the anode material such as SiOx dispersed in the anode slurry. As a result, a stable oxide film can be formed on the surface of the anode material such as SiOx, which suppresses decomposition of the electrolyte and prevents the SEI (Solid Electrolyte Interface) film from becoming excessively thick on the electrode surface, which is presumably linked to improved battery characteristics.

In addition, when a lithium molybdate salt is used, the molybdate can exhibit neutrality or weak alkalinity when added to an electrode binder. If the molybdate salt is not in the form of a lithium salt, the electrode composition in the form of an aqueous solution becomes strongly acidic, which is undesirable because it reduces the viscosity of the electrode composition and causes uneven polarity distribution, resulting in unevenness when the electrode composition is applied to the current collector. Furthermore, since the molybdate salt is a lithium salt, there is less inclusion of impurities when using a lithium-based active material, making it easier to demonstrate electrical performance.

<Electrode Binder>
It is important that the binder for electrodes of non-aqueous electrolyte secondary batteries of the present invention contains at least carboxymethyl cellulose and/or its salt having a carboxymethyl substitution degree per anhydrous glucose unit of 0.5 to 1.2, and a molybdate. In such an electrode binder, the molybdate is preferably contained in the range of 30 to 200% by weight, more preferably 40 to 170% by weight, and even more preferably 50 to 150% by weight, relative to the carboxymethyl cellulose or its salt. It is presumed that when the blending ratio is within this range, the formation of the crosslinked structure between the carboxymethyl cellulose and the molybdate occurs more effectively.

The electrode binder of the present invention can also be in the form of an aqueous solution. There are no particular limitations on the conditions for producing such an aqueous solution. For example, the electrode binder can be added to water (e.g., distilled water, purified water, tap water, etc.) and dissolved by stirring, etc., as necessary. Alternatively, carboxymethylcellulose or a salt thereof can be dissolved in water, etc., and then a molybdate can be added and dissolved by stirring, etc. Similarly, molybdate can be dissolved in water, etc., and then carboxymethylcellulose or a salt thereof can be dissolved by stirring, etc.

The aqueous solution of such an electrode binder preferably has a pH in the range of 1 to 8, more preferably in the range of 2 to 8, even more preferably in the range of 3 to 8, and particularly preferably in the range of 6 to 8. If the pH of the aqueous solution is biased toward the acidic side, it becomes difficult for the aqueous solution to exhibit the viscosity expected of the aqueous solution. Therefore, if the pH is in the range of 6 to 8, a particularly balanced viscosity and solubility can be achieved, making it possible to obtain an aqueous solution with excellent coatability.

The aqueous solution of the electrode binder preferably has a viscosity of 5 mPa·s or more, more preferably 250 mPa·s or more, and preferably 15,000 mPa·s or less, more preferably 10,000 mPa·s or less, and even more preferably 8,000 mPa·s or less, as measured at 25°C with a Brookfield viscometer (30 rpm). When the aqueous solution of the electrode binder is in the above viscosity range, it can exhibit suitable thickening and binding properties when added to the electrode composition.

The electrode binder of the present invention can also contain other components constituting the electrode composition, provided that the effects of the present invention are not impaired. Examples of such additive components include inorganic salts that are cationized in an aqueous solution. The presence of cations reinforces the crosslinked structure of carboxymethylcellulose, thereby enabling the material to exhibit excellent electrical properties. As such cations, monovalent, divalent, trivalent, and other cations can be used as appropriate, and specifically, those containing Fe ions, etc. are preferred.

<Electrode composition>
The electrode binder of the present invention can constitute an electrode composition together with an active material of an electrode. The form of the electrode composition is not particularly limited, and it may be in the form of a slurry or a paste.

In the present invention, the content of carboxymethylcellulose or a salt thereof in the electrode composition is preferably 0.1 to 4.0% by mass based on the total amount of the electrode composition.

The electrode composition may contain various components depending on whether the electrode formed from the composition is a negative electrode or a positive electrode.

In the case of an electrode composition for a negative electrode, a negative electrode active material is usually included. Examples of the negative electrode active material include graphite materials such as graphite (natural graphite, artificial graphite), coke, and carbon fiber; elements capable of forming an alloy with lithium, i.e., elements such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, and Ti; compounds containing elements capable of forming an alloy with lithium; composites of elements and compounds capable of forming an alloy with lithium, carbon, and/or the graphite material; and nitrides containing lithium. Among these, graphite materials and/or silicon-based compounds are preferred, and it is more preferred to include graphite and/or silicon-based compounds, and it is preferable to include at least a silicon-based compound.

In the case of an electrode composition for a positive electrode, a positive electrode active material is usually included. As the positive electrode active material, a LiMexOy (Me means a transition metal including at least one of Ni, Co, and Mn. x and y mean any number) type positive electrode active material is preferable. The LiMexOy type positive electrode active material is not particularly limited, but LiMn2O4 type, LiCoO2 type, and LiNiO2 type positive electrode active materials are preferable. Examples of LiMn2O4 type, LiCoO2 type, and LiNiO2 type positive electrode active materials include compounds in which various metal elements are substituted with LiMnO2, LiMn2O4, LiCoO2, and LiNiO2 as the main skeleton. LiMn2O4-based, LiCoO2-based, and LiNiO2-based positive electrode active materials have excellent performance as positive electrode active materials, such as excellent diffusion performance of electrons and lithium ions, and therefore lithium ion secondary batteries with high charge/discharge efficiency and good cycle characteristics can be obtained. Of these, LiCoO2-based positive electrode active materials are preferred, with LiCoO2 being more preferred. On the other hand, it is preferable to use LiMn2O4-based positive electrode active materials because of their low material costs.

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

In the case of an electrode composition for a positive electrode, it is preferable that the electrode composition contains a conductive material. When the electrode composition contains a conductive material, the characteristics of the manufactured positive electrode are improved. In addition, the conductive material can ensure the electrical conductivity of the positive electrode. Examples of conductive materials include one or a mixture of two or more carbon substances such as carbon black, acetylene black, and graphite. Of these, carbon black is preferable.

The electrode composition may contain a binder other than the aqueous solution of carboxymethylcellulose or its salt. Examples of binders for the electrode composition for the negative electrode include synthetic rubber-based binders. As the synthetic rubber-based binder, one or more selected from the group consisting of styrene butadiene rubber (SBR), nitrile butadiene rubber, methyl methacrylate butadiene rubber, chloroprene rubber, carboxy-modified styrene butadiene rubber, and latexes of these synthetic rubbers can be used. Among these, styrene butadiene rubber (SBR) is preferred. Examples of binders for the electrode composition for the positive electrode include the synthetic rubber-based binders listed as the binders for the negative electrode, as well as polytetrafluoroethylene (PTFE), of which polytetrafluoroethylene (PTFE) is preferred.

The binder content in the electrode composition is usually 0.5 to 5 mass%, preferably 0.5 to 3 mass%, and more preferably 0.5 to 2 mass%.

There are no particular limitations on the manufacturing conditions for the electrode composition. For example, other components constituting the electrode composition are added to an aqueous solution of carboxymethylcellulose or its salt, and mixed while stirring as necessary.

The form of the electrode composition is not particularly limited. For example, it may be in any form, such as liquid, paste, or slurry.

The electrode composition is used to manufacture electrodes for non-aqueous electrolyte secondary batteries. Electrodes for non-aqueous electrolyte secondary batteries may be manufactured by laminating the electrode composition on a current collector substrate (current collector). Examples of lamination methods include blade coating, bar coating, and die coating, with blade coating being preferred. For example, in the case of blade coating, a method of casting the electrode composition on a current collector substrate using a coating device such as a doctor blade is exemplified. The lamination method is not limited to the above specific example, and a method of discharging the electrode composition from an extrusion type injector having a slot nozzle onto a current collector substrate wound around a backup roll and running is also exemplified. In blade coating, after casting, drying by heating (for example, 80 to 120°C, heating time, for example, 4 to 12 hours) or pressurization by roll pressing may be performed as necessary.

Any electrical conductor that does not cause a fatal chemical change in the constructed battery can be used as the current collecting substrate.

As the current collector substrate for the negative electrode active material, stainless steel, nickel, copper, titanium, carbon, copper, or the above-mentioned stainless steel surface treated with carbon, nickel, titanium or silver can be used. Of these, copper or a copper alloy is preferred, with copper being the most preferred.

Examples of materials for the current collector substrate for the positive electrode include metals such as aluminum and stainless steel, with aluminum being preferred. The shape of the current collector substrate can be a mesh, punched metal, foam metal, or foil processed into a plate shape, with foil processed into a plate shape being preferred.

The shape of the electrode for a non-aqueous electrolyte secondary battery formed from the electrode composition is not particularly limited, but is usually in the form of a sheet. The thickness of a sheet-shaped electrode plate (the thickness of the mixture layer formed from the electrode composition, excluding the current collecting substrate portion) is difficult to define uniquely because it is appropriately selected depending on the composition of the composition and manufacturing conditions, but is usually 30 to 150 μm.

The electrode formed from the composition is used as an electrode for a non-aqueous electrolyte secondary battery. That is, the present invention includes an electrode formed from the composition, and a non-aqueous electrolyte secondary battery can be manufactured using the electrode. The non-aqueous electrolyte secondary battery can have a structure in which positive electrodes and negative electrodes are alternately stacked with separators interposed therebetween and wound many times. The separator is usually impregnated with a non-aqueous electrolyte.
The negative electrode and/or positive electrode may be formed from the electrode composition described above. The nonaqueous electrolyte secondary battery uses carboxymethyl cellulose or a salt thereof, which has excellent solubility, and can omit a process such as filtration using a filter, so that it is excellent in productivity, and the initial irreversible capacity is significantly improved, and high battery characteristics can be exhibited.

The following describes the embodiments of the present invention using examples, but the present invention is not limited to these.

In the present examples and comparative examples, the indices of carboxymethylcellulose or a salt thereof are measured by the following methods.
<Method for measuring degree of carboxymethyl substitution (CM-DS)>
Approximately 2.0 g of the carboxymethylcellulose pulverized sample was weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of methanol (a solution of 100 mL of concentrated nitric acid added to 1000 mL of methanol) was added and shaken for 3 hours to convert the carboxymethylcellulose salt (CMC salt) to H-CMC (carboxymethylcellulose). 1.5 to 2.0 g of bone-dried H-CMC was weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. The 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. Excess NaOH was back-titrated with 0.1 N H2SO4 using phenolphthalein as an indicator. CM-DS was calculated by the following formula 1.
(Formula 1)
A = [(100 x F - (0.1 N H2SO4 (mL)) x F') x 0.1] / (bone dry weight of H-CMC (g))
Carboxymethyl substitution degree (CM-DS) = 0.162 x A / (1 - 0.058 x A)
A: The amount of 1N NaOH required to neutralize 1 g of H-CMC (mL)
F': Factor of 0.1N H2SO4 F: Factor of 0.1N NaOH

<Viscosity>
Carboxymethylated cellulose or a salt thereof was weighed into a 1000 mL glass beaker and dispersed in 900 mL of distilled water to prepare an aqueous dispersion with a solid content of 1% (w/v). The aqueous dispersion was stirred at 600 rpm for 3 hours at 25°C using a stirrer. Thereafter, the viscosity after 3 minutes was measured using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.) with a No. 1 rotor at a rotation speed of 30 rpm in accordance with the method of JIS-Z-8803.

<Measurement of the mass ratio of the mass of the filtration residue to the dry mass of carboxymethyl cellulose dissolved in an aqueous solution>
2 L of an aqueous solution of 0.3% by mass (mass % based on the dry mass of carboxymethyl cellulose or its salt) of carboxymethyl cellulose or its salt was prepared. 2 L of this aqueous solution was filtered through a 250 mesh filter (stainless steel, mesh size 63 μm) using a filter ("Separot", Kiriyama Seisakusho) under reduced pressure conditions of -200 mmHg. The residue remaining on the 250 mesh filter was dried by blowing air at a temperature of 105°C for 16 hours, and the mass of the dried residue was measured and expressed as a mass percentage (ppm) relative to the mass of carboxymethyl cellulose in the aqueous carboxymethyl cellulose solution.

The coin-type nonaqueous electrolyte secondary batteries obtained in the examples and comparative examples were evaluated as follows.

<Battery evaluation>
(Discharge capacity (charge/discharge rate test))
The charge and discharge rate test of the coin-type nonaqueous electrolyte secondary batteries obtained in the Examples and Comparative Examples was carried out using Nagano Corporation's BTS2004 in a thermostatic chamber at 25° C., with 52 cycles being performed, with one cycle being a charge and discharge cycle in that order. Note that the charging conditions were constant current constant voltage (CC-CV) (CC current 0.2 C, CV voltage 4.2 V, final current 0.02 C) in all cycles.
As a condition for the discharge treatment, the final voltage was set to 3.0 V. In the first cycle, the discharge treatment was performed at a constant current of 0.2 C, and the discharge capacity (mAh/g) after one cycle was measured.
Thereafter, up to the 52nd cycle, the constant current for the discharge treatment was set as follows, and the discharge capacity (mAh/g) was measured after each cycle of discharge.
(Constant current of discharge treatment in each cycle)
2 to 10 cycles: Discharge treatment at a constant current of 0.2C
Cycles 11 to 20: constant current 1C for discharge treatment
21 cycles: constant current of 0.2C for discharge treatment
Cycles 22 to 31: constant current 2C for discharge treatment
32 cycles: constant current of 0.2C for discharge treatment
Cycles 33 to 42: Discharge treatment at constant current of 3C
Cycles 43 to 52: Discharge treatment at a constant current of 0.2C

(Capacity maintenance rate)
The capacity retention rate was calculated from the discharge capacity (mAh/g) in each cycle test described above.
The capacity retention rate was calculated according to the formula: "Capacity retention rate = Discharge capacity after 1 cycle (mAh/g)/Discharge capacity after 52 cycles (mAh/g) x 100".

Example 1
<Preparation of electrode binder>
A twin-shaft kneader with the rotation speed adjusted to 100 rpm was charged with 2720 g of isopropyl alcohol, 170 g of sodium monochloroacetate, and 58 g of sodium hydroxide dissolved in 480 g of water, and 160 g of dry weight of linter pulp was charged when it was dried at 30 ° C for 60 minutes. After stirring and mixing for 90 minutes at 30 ° C to prepare mercerized cellulose, the temperature was raised to 70 ° C and a carboxymethylation reaction was carried out for 90 minutes. After the reaction was completed, the mixture was neutralized with acetic acid to about pH 7, deliquored, dried and pulverized to obtain a sodium salt of carboxymethyl cellulose (hereinafter sometimes referred to as "CMC1") having a carboxymethyl substitution degree of 0.70, a viscosity of a 1% by weight aqueous solution measured with a B-type viscometer at 25 ° C of 7,900 mPa · s, and a filtration residue of 48 ppm relative to the dry mass of carboxymethyl cellulose dissolved in the aqueous solution. The above-mentioned sodium carboxymethylcellulose salt and 150% by mass of hexaammonium heptamolybdate tetrahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) based on the sodium carboxymethylcellulose salt were weighed into a 300 mL glass beaker, and distilled water was added to prepare an aqueous dispersion so that the solid content concentration of carboxymethylcellulose was 2% (w/v). The aqueous dispersion was stirred at 500 rpm for 2 hours at 25°C using a stirrer to obtain electrode binder 1.

<Preparation of negative electrode plate>
As the negative electrode material, SiOx, acetylene black (manufactured by Stream Chemical Co.), electrode binder 1, and styrene butadiene rubber (SBR, manufactured by JSR Corporation, product number S2910(E)-12-Na) were mixed so that the solid weight ratio was 97:0.5:1.0:1.5, water was added so that the slurry concentration was 45.6 mass%, and the mixture was thoroughly stirred using Mazerustar (manufactured by Kurabo Industries, Ltd., KK-250S) to obtain slurry 1. This slurry was applied with an applicator to a copper foil (manufactured by Furukawa Electric Co., Ltd., NC-WS) of length 320 mm x width 170 mm x thickness 17 μm, and then air-dried for 30 minutes, followed by drying in a dryer at 60 ° C. for 30 minutes. Further, the plate was pressed using a small tabletop roll press (manufactured by Tester Sangyo Co., Ltd., SA-602) at 5 kN and a roll peripheral speed of 50 m/min to obtain a negative electrode plate 1 having a basis weight of 19.7 g/m2 and an effective discharge capacity of 2100 mAh/g.

<Preparation of coin-type non-aqueous electrolyte secondary battery>
The obtained negative electrode plate 1 and a LiCoO2 positive electrode plate (manufactured by Hosen Co., Ltd., basis weight 110.2 g/m2, effective discharge capacity 145 mAh/g) were punched out into a circle having a diameter of 16 mm, and the punched negative electrode plate and positive electrode plate were vacuum dried at 120° C. for 12 hours.

Similarly, a separator (manufactured by CS Tech, a 20 μm thick polypropylene separator) was punched out to a circle with a diameter of 17 mm, and vacuum dried at 60°C for 12 hours.

Then, the negative electrode plate 1 was placed in a stainless steel circular dish-shaped container with a diameter of 20.0 mm, and then a separator, a positive electrode plate, a spacer (diameter 15.5 mm, thickness 1 mm), and a stainless steel washer (manufactured by Hosen Co., Ltd.) were stacked in this order, and then 300 μL of electrolyte (1 mol/L LiPF6, ethylene carbonate and diethyl carbonate in a volume ratio of 1:1) was added to the circular dish-shaped container. A stainless steel cap was placed over this via a polypropylene packing, and the container was sealed with a coin battery crimping machine (Hosen Co., Ltd.), obtaining a coin-shaped nonaqueous electrolyte secondary battery 1.

Example 2
An electrode binder 2 was obtained in the same manner as in Example 1, except that lithium molybdate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used in place of hexaammonium heptamolybdate tetrahydrate at 50 mass% relative to the sodium carboxyl cellulose salt.

A slurry, a negative electrode plate, and a coin-type nonaqueous electrolyte secondary battery were prepared in the same manner as in Example 1, except that electrode binder 2 was used instead of electrode binder 1.

(Comparative Example 1)
Except for not adding the molybdate, an electrode binder 3 was obtained in the same manner as in Example 1. In addition, except for using the electrode binder 3 instead of the electrode binder 1, a slurry, a negative electrode plate, and a coin-type nonaqueous electrolyte secondary battery were produced in the same manner as in Example 1.

The measurement and evaluation results for the examples and comparative examples are shown in Table 1 below.

As shown in Table 1, a non-aqueous electrolyte secondary battery using a binder for electrodes of a non-aqueous electrolyte secondary battery containing at least carboxymethyl cellulose and/or its salt with a carboxymethyl substitution degree of 0.5 to 1.2 per anhydrous glucose unit and a molybdate salt had an excellent battery capacity retention rate.

Claims (7)

A binder for electrodes of non-aqueous electrolyte secondary batteries, comprising at least carboxymethyl cellulose and/or a salt thereof having a carboxymethyl substitution degree of 0.5 to 1.2 per anhydrous glucose unit , and a molybdate, the molybdate comprising at least one selected from ammonium molybdate and lithium molybdate . The binder for non-aqueous electrolyte secondary battery electrodes according to claim 1, wherein the molybdate is contained in an amount ranging from 30 to 200% by weight relative to 100% by weight of the solid content of the carboxymethyl cellulose or its salt. The electrode binder for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein when 2 liters of a 0.3 mass % aqueous solution of the carboxymethyl cellulose or its salt having a dry mass B is prepared, the entire solution is filtered through a 250 mesh filter under reduced pressure conditions of -200 mmHg, and the dry mass A of the residue on the filter after filtration is measured, the ratio of the dry mass A to the dry mass B is less than 50 ppm. 4. The binder for a nonaqueous electrolyte secondary battery electrode according to claim 1, wherein the carboxymethylated cellulose or the salt thereof has a viscosity of 1,000 to 20,000 mPa·s in a 1% by mass aqueous solution measured at 25° C. with a B-type viscometer (30 rpm) . An electrode composition for a non-aqueous electrolyte secondary battery, comprising the binder for a non-aqueous electrolyte secondary battery electrode according to any one of claims 1 to 4 and a silicon-based compound. An electrode for a non-aqueous electrolyte secondary battery using the electrode composition for a non-aqueous electrolyte secondary battery according to claim 5. A non-aqueous electrolyte secondary battery using the electrode composition for non-aqueous electrolyte secondary batteries described in claim 5.