JP7748408B2 - Carboxymethyl cellulose and/or its salt, electrode composition for non-aqueous electrolyte secondary battery, electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to carboxymethyl cellulose and/or a salt thereof used as an electrode binder for non-aqueous electrolyte secondary batteries, as well as to an electrode composition for non-aqueous electrolyte secondary batteries, electrodes for non-aqueous electrolyte secondary batteries, and non-aqueous electrolyte secondary batteries that use this carboxymethyl cellulose and/or a salt thereof.

In recent years, with the rapid spread of small mobile devices such as smartphones and tablets, as well as stationary storage batteries, there has been a growing demand for small, high-energy-density batteries to power them.

Generally, graphite-based materials are used for the negative electrodes of lithium ion secondary batteries, but the theoretical capacity of graphite-based materials is 372 mAh/g (LiC ₆ ), which is currently approaching its limit.

Furthermore, in order to improve the energy density of lithium-ion secondary batteries, it is necessary to select new materials. Therefore, materials made by alloying lithium with silicon, tin, and other materials that have the lowest potential and large specific capacity after carbon and lithium are attracting attention.

Among these materials, silicon can absorb up to 4.4 lithium atoms per 1 silicon atom, theoretically providing approximately 10 times the capacity of graphite-based carbon materials. However, because silicon particles expand in volume approximately three to four times when they absorb lithium, repeated charge and discharge cycles cause degradation and a decrease in capacity, creating a problem. Detailed analysis of this phenomenon has shown that when lithium is inserted into a silicon-containing active material, the volume expansion causes microcracks to form within the electrode. The electrolyte penetrates these microcracks, forming a new coating (SEI layer). This creates irreversible capacity, resulting in a decrease in battery capacity. This phenomenon manifests itself in changes in charge and discharge efficiency during cycling. The decrease in cycle efficiency, particularly during the early stages of cycling when volume change is significant, significantly impacts the battery's lifespan when combined with a positive electrode with high charge and discharge efficiency. Therefore, when using silicon-containing active materials, minimizing changes in electrode structure due to this volume expansion is a key challenge.

In light of these circumstances, Patent Document 1 aims to improve battery characteristics by using a binder containing three essential components: carboxymethyl cellulose or a metal salt thereof, polyacrylic acid or a metal salt thereof, and styrene-butadiene rubber or polyvinylidene fluoride. However, according to the examples and comparative examples in Patent Document 1, when only two components (carboxymethyl cellulose and styrene-butadiene rubber, or carboxymethyl cellulose and polyvinylidene fluoride) were used as the binder, the desired battery characteristics were not obtained.

JP 2015-198038 A

Patent Document 1 states that three components, including carboxymethyl cellulose or its metal salt, are essential as a binder, but does not specifically mention the physical properties of carboxymethyl cellulose or its metal salt. The inventors' research has revealed that these physical properties affect battery characteristics, particularly the electrical resistance of the resulting electrode layer.

That is, the object of the present invention is to provide carboxymethyl cellulose and/or a salt thereof for use as an electrode binder for non-aqueous electrolyte secondary batteries, which enables the formation of electrode layers with low electrical resistance; and to provide an electrode composition for non-aqueous electrolyte secondary batteries, an electrode for non-aqueous electrolyte secondary batteries, and a non-aqueous electrolyte secondary battery that use this carboxymethyl cellulose and/or a salt thereof.

After extensive research, the inventors discovered that the above problem can be solved by using a material with a specific degree of dispersion.

That is, according to the present invention,
(1) Carboxymethyl cellulose and/or a salt thereof used as an electrode binder for a non-aqueous electrolyte secondary battery, the degree of carboxymethyl substitution per anhydroglucose unit being 0.5 to 1.2, the degree of dispersion in methanol (BS) measured by Turbine Scan being 3 to 10%, and the ratio of particle diameter D90 to particle diameter D10 (particle diameter D90/particle diameter D10) being 7.5 to 11;
(2) The carboxymethyl cellulose and/or salt thereof according to (1), having a particle size D50 of 10 to 20 μm.
(3) The carboxymethyl cellulose and/or salt thereof according to (1) or (2), wherein the difference between the particle diameter D90 and the particle diameter D50 (particle diameter D90 - particle diameter D50) is 5 to 30 μm.
(4) The carboxymethyl cellulose and/or salt thereof according to (1) or (2), wherein the difference between the particle diameter D50 and the particle diameter D10 (particle diameter D50 - particle diameter D10) is 5 to 20 μm.
(5) The carboxymethyl cellulose and/or salt thereof according to (1) or (2), wherein the difference between the particle diameter D90 and the particle diameter D10 (particle diameter D90 - particle diameter D10) is 10 to 50 μm.
(6) The carboxymethyl cellulose and/or salt thereof according to (1) or (2), having a viscosity of 1% by mass aqueous solution measured at 25 ° C. with a Brookfield viscometer (30 rpm) of 1,000 to 20,000 mPa s.
(7) The carboxymethyl cellulose and/or salt thereof according to (1) or (2), wherein 2 liters of a 0.3% by mass aqueous solution of the carboxymethyl cellulose or salt thereof having a dry mass m is prepared, and the whole is filtered through a 250-mesh filter under a reduced pressure condition of -200 mmHg, and the dry mass M of the residue on the filter after filtration is measured. When the ratio of the dry mass M to the dry mass m is less than 200 ppm,
(8) An electrode composition for a non-aqueous electrolyte secondary battery, comprising the carboxymethyl cellulose and/or salt thereof according to (1) or (2) and a styrene-butadiene rubber having an average particle size of 50 nm to 300 nm.
(9) The electrode composition for a non-aqueous electrolyte secondary battery according to (7), wherein the styrene-butadiene rubber has a glass transition temperature of −50° C. to 50° C.
(10) A non-aqueous electrolyte secondary battery electrode composition containing the carboxymethyl cellulose and/or its salt according to (1) or (2).
(11) An electrode for a non-aqueous electrolyte secondary battery, using the electrode composition for a non-aqueous electrolyte secondary battery according to (8).
(12) A non-aqueous electrolyte secondary battery is provided using the electrode composition for a non-aqueous electrolyte secondary battery according to (8).

The present invention provides carboxymethyl cellulose and/or a salt thereof for use as an electrode binder for non-aqueous electrolyte secondary batteries, enabling the production of electrode layers with low electrical resistance. Also provided are an electrode composition for non-aqueous electrolyte secondary batteries, an electrode for non-aqueous electrolyte secondary batteries, and a non-aqueous electrolyte secondary battery that use this carboxymethyl cellulose and/or a salt thereof.

The carboxymethyl cellulose and/or salt thereof of the present invention is described below. The carboxymethyl cellulose and/or salt thereof of the present invention is used as an electrode binder for non-aqueous electrolyte secondary batteries, and has a carboxymethyl substitution degree per anhydroglucose unit of 0.5 to 1.2 and a dispersion index (BS) in methanol measured by turbine scanning of 3 to 10%.

<Carboxymethylcellulose and/or its salt>
The carboxymethyl cellulose and/or its salt (hereinafter sometimes abbreviated as CMC) contained in the active material layer of the present invention has a structure in which the hydroxyl groups in the glucose units constituting the cellulose are substituted with carboxymethyl ether groups. The carboxymethyl cellulose may be in the form of a salt. Examples of the salt of carboxymethyl cellulose include metal salts such as sodium carboxymethyl cellulose.

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

Examples of natural cellulose include bleached or unbleached pulp, 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 examples include mechanical methods, chemical methods, and methods that combine mechanical and chemical methods. Examples of bleached or unbleached pulp include mechanical pulp, chemical pulp, groundwood pulp, sulfite pulp, kraft pulp, and papermaking pulp. Another example of bleached or unbleached pulp is dissolving pulp, which is chemically refined and primarily dissolved in chemicals before use, and is the main raw material for artificial fibers, cellophane, and other products.

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

Examples of fine cellulose include fine cellulose obtained by depolymerizing cellulosic materials such as natural cellulose and regenerated cellulose through acid hydrolysis, alkaline hydrolysis, enzymatic decomposition, blasting, vibrating ball mill treatment, etc., and fine cellulose obtained by mechanically treating cellulosic materials.

When producing the CMC used in the present invention, known CMC production methods can be applied. For example, CMC can be produced by treating cellulose with a mercerizing agent (alkali) to prepare mercerized cellulose (alkali cellulose), and then adding an etherifying agent to the mercerized cellulose to cause an etherification reaction.

The cellulose used as the raw material can be any of the above-mentioned celluloses, but cellulose with high purity is preferred, and dissolving pulp or linter is more preferred. Using these allows for the production of highly pure CMC.

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

In a typical method for producing water-soluble carboxymethyl cellulose, the molar ratio of the mercerizing agent to the etherifying agent (mercerizing agent/etherifying agent) is generally 2.00 to 2.45 when monochloroacetic acid is used as the etherifying agent. The reason for this is that a ratio of 2.00 or more allows the etherification reaction to proceed sufficiently, preventing unreacted monochloroacetic acid from being wasted. A ratio of 2.45 or less prevents a side reaction between the excess mercerizing agent and monochloroacetic acid from proceeding and producing an alkali metal glycolate, which is economical.
In the present invention, the CMC may be a commercially available product, for example, a product name "Sunrose" manufactured by Nippon Paper Industries Co., Ltd.

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

(Degree of substitution of carboxymethyl group)
The CMC used in the present invention has a degree of substitution of carboxymethyl groups per anhydroglucose unit (hereinafter sometimes referred to as DS value) of 0.5 to 1.2. A DS value of 0.5 or more can maintain good solubility in water and suppress the generation of undissolved matter. Furthermore, a DS value of 1.2 or less can suppress an increase in spinnability of the liquid and maintain easy handling. Therefore, the DS value of the CMC of the present invention is 0.5 to 1.2, preferably 0.5 to 1.0, and more preferably 0.6 to 1.0.

The degree of substitution of carboxymethyl groups is measured as follows:
Approximately 2.0 g of sample is weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of a solution of 100 mL of concentrated nitric acid in 1000 mL of methanol is added, and the mixture is shaken for 3 hours to convert carboxymethyl cellulose salt (CMC) to H-CMC (hydrogen-form carboxymethyl cellulose). 1.5 to 2.0 g of the bone-dry H-CMC is weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. The H-CMC is moistened with 15 mL of 80% methanol, 100 mL of 0.1 N NaOH is added, and the mixture is shaken at room temperature for 3 hours. Using phenolphthalein as an indicator, excess NaOH is back-titrated with 0.1 N H ₂ SO ₄ , and the degree of carboxymethyl substitution (DS value) is calculated using the following formula:
A = [(100 × F' - 0.1N-H ₂ SO ₄ (mL) × F) × 0.1] / (bone dry mass of H-CMC (g))
Degree of carboxymethyl substitution=0.162×A/(1−0.058×A)
F': Factor of 0.1N-H ₂ SO ₄ F: Factor of 0.1N-NaOH

(viscosity)
Furthermore, the viscosity of a 1% by mass aqueous solution of the carboxymethyl cellulose or salt thereof of the present invention, measured at 25°C with a Brookfield viscometer (30 rpm), is preferably 1,000 to 20,000 mPa·s, more preferably 1,000 to 15,000 mPa·s, and even more preferably 1,000 to 10,000 mPa·s. If this viscosity is too high, problems arise such as the inability to mix well with the active material and conductive additive during slurry preparation, or poor fluidity when applying the slurry to a current collector, making it impossible to apply. If the viscosity is too low, the slurry may run off the current collector when applied to the current collector, making it difficult to apply, or may cause migration of the active material and binders such as SBR, resulting in high electrical resistance.

The viscosity was measured as follows:
Carboxymethyl cellulose or a salt thereof is weighed into a 1000 mL glass beaker and dispersed in 900 mL of distilled water to prepare an aqueous dispersion with a solids content of 1% (w/v). The aqueous dispersion is stirred at 600 rpm for 3 hours at 25°C using a stirrer. Thereafter, the viscosity after 3 minutes is measured at 30 rpm using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.) in accordance with the method of JIS-Z-8803.

(Dispersibility in methanol (BS))
The carboxymethyl cellulose and/or its salt of the present invention has a dispersion in methanol (BS) of 3 to 10%, preferably 5 to 10%, as measured by turbine scanning. When the dispersion in methanol (BS) is within this range, CMC exhibits good dispersibility when used as a binder or dispersant in a negative electrode composition, thereby improving the electrical resistance. On the other hand, if the dispersion in methanol (BS) is too high, the CMC will not be uniformly dispersed in the negative electrode composition, which may result in a deterioration in the electrical resistance.

Here, the degree of dispersion in methanol (BS) was obtained by measuring the backscattered light intensity (%) using a Turbiscan (TURBISCAN LAb, manufactured by Eiko Seiki Co., Ltd.). Here, "BS" refers to the backscattered light intensity (%). Specifically, the measurement method involves placing 0.075 g of carboxymethyl cellulose and 15 ml of methanol in a measurement container. The container is then stirred for 10 seconds using a vortex mixer, and the container is then quickly placed in the device to begin measurement. The backscattered light intensity at a height of 20 mm after 30 minutes was calculated as the degree of dispersion in methanol (BS).

(Angle of repose)
The angle of repose of the carboxymethyl cellulose and/or salt thereof of the present invention is preferably 42° or more, more preferably 45° or more. If the angle of repose is too small, the powder will flow out of the outlet of the feeder, making it impossible to dissolve a predetermined amount of carboxymethyl cellulose, and the function as a binder or dispersant in the negative electrode will be reduced, resulting in a problem of increased electrical resistance.
Here, the angle of repose was measured using a powder property tester (Powder Tester PT-X (manufactured by Hosokawa Micron Corporation)) with an angle measurement method set to "Peak Operation" using a sieve with a mesh size of 710 μm and a linearity of 450 μm. The moisture content of the carboxymethyl cellulose used in the measurement was adjusted to 6.0 to 9.0%.

(collapse angle)
The collapse angle of the carboxymethyl cellulose and/or its salt of the present invention is preferably 19° or more, more preferably 21° or more. If the collapse angle is too small, the powder will flow out of the outlet of the feeder without any effort, making it impossible to dissolve a predetermined amount of carboxymethyl cellulose, and the function as a binder or dispersant in the negative electrode will be reduced, resulting in a problem of increased electrical resistance.
Here, the collapse angle was measured using a powder tester (Powder Tester PT-X (manufactured by Hosokawa Micron Corporation)) with an angle measurement method set to "Peak Operation" using a sieve with a mesh size of 710 μm and a linearity of 450 μm. The moisture content of the carboxymethyl cellulose used in the measurement was adjusted to 6.0 to 9.0%.
(difference angle)
The value obtained by subtracting the value of the angle of collapse from the value of the angle of repose can be expressed as a difference angle.

(Amount of filtration residue)
Furthermore, the carboxymethyl cellulose and/or salt thereof of the present invention preferably has a filtration residue amount within a predetermined range. That is, when 2 L of a 0.3 mass % aqueous solution of the carboxymethyl cellulose or salt thereof having a dry mass m is prepared and completely filtered through a 250 mesh filter under reduced pressure conditions of −200 mmHg, and the dry mass M of the residue on the filter after filtration is measured, the ratio of the dry mass M to the dry mass m is preferably less than 200 ppm, and more preferably less than 50 ppm.
If this value is too large, clogging occurs easily when filtering the electrode slurry, and the amount of carboxymethyl cellulose in the filtered slurry becomes less than the specified amount, which reduces its function as a binder and dispersant, resulting in the problem of increased electrical resistance.

(Particle diameter)
The value obtained by subtracting the particle diameter D10 from the particle diameter D90 of the carboxymethyl cellulose and/or salt thereof of the present invention (particle diameter D90 - particle diameter D10) is preferably 10 to 50 μm, more preferably 10 to 30 μm, and even more preferably 20 to 30 μm. If this value is too large, the particles may undergo layer separation, resulting in an inhomogeneous solution and high electrical resistance.

Furthermore, the value obtained by subtracting the particle diameter D50 from the particle diameter D90 of the carboxymethyl cellulose and/or its salt of the present invention (particle diameter D90 - particle diameter D50) is preferably 5 to 30 μm, more preferably 10 to 30 μm, and even more preferably 10 to 20 μm. If this value is too large, the particles will separate into layers, resulting in an inhomogeneous solution and high electrical resistance.

Furthermore, the value obtained by subtracting the particle diameter D10 from the particle diameter D50 of the carboxymethyl cellulose and/or its salt of the present invention (particle diameter D50 - particle diameter D10) is preferably 5 to 20 μm, more preferably 5 to 15 μm, and even more preferably 8 to 12 μm. If this value is too large, the particles will separate into layers, which can cause the problem of not forming a homogeneous solution.

Here, D10 is the particle size that contains 10% of the particles when calculated from the minimum value in the particle size distribution based on the volume average particle size, D50 is the particle size that contains 50% of the particles when calculated from the minimum value, and is also known as the average particle size. Furthermore, D90 is the particle size that contains 90% of the particles when calculated from the minimum value. The particle size distribution based on the volume average particle size can be measured using a laser diffraction/scattering particle size distribution analyzer, for example, using methanol as a dispersant.

The particle diameter D10 of the carboxymethyl cellulose and/or its salt of the present invention is preferably 1 to 10 μm, the particle diameter D50 is preferably 10 to 20 μm, and the particle diameter D90 is preferably 20 to 40 μm.

[Crushing process]
In the present invention, carboxymethyl cellulose or a salt thereof may be finely pulverized. As a method for finely pulverizing carboxymethyl cellulose or a salt thereof, either a dry pulverization method in which the carboxymethyl cellulose or a salt thereof is treated in a powder state or a wet pulverization method in which the carboxymethyl cellulose or a salt thereof is treated in a dispersed or dissolved state in a liquid may be selected.

By mechanically dry- or wet-pulverizing carboxymethyl cellulose or its salts, the gel particles derived from carboxymethyl cellulose or its salts that exist as undissolved matter in aqueous solution are refined. As a result, it is believed that the formation of coarse undissolved matter that causes streak defects, peeling, pinholes, etc. on the surface of the negative electrode can be suppressed.

Examples of fine grinding devices that can be used in the present invention include the following:
Dry mills include cutting mills, impact mills, and airflow mills. These can be used alone or in combination, and the same type of mill can be used for multiple stages of processing.

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), Granulator (manufactured by Herbolt), and Rotary Cutter Mill (manufactured by Nara Machinery Works Co., Ltd.).

Examples of impact mills include the Pulperizer (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 a CGS-type jet mill (manufactured by Mitsui Mining Co., Ltd.), a jet mill (manufactured by Sansho Industry Co., Ltd.), an Ebara jet micronizer (manufactured by Ebara Corporation), a Selenium mirror (manufactured by Masuko Sangyo Co., Ltd.), and a supersonic jet mill (manufactured by Nippon Pneumatic Mfg. Co., Ltd.).
An example of a media mill is a vibrating ball mill.

Examples of wet mills include the Mass Colloider (manufactured by Masuko Sangyo Co., Ltd.), and examples of media mills include the Bead Mill (manufactured by Imex Co., Ltd.) and the High-Pressure Homogenizer (manufactured by Sanmaru Machinery Industry Co., Ltd.).

In the present invention, a step of classifying the pulverized carboxymethyl cellulose or a salt thereof based on particle size can be provided.
The classification step may be performed during or after the pulverization step. Any known method may be used for classification. Examples of dry classifiers include cyclone classifiers, DS separators, turbo classifiers, microseparators, and air separators. Examples of wet classifiers include liquid cyclone type, centrifugal settlers, and hydroseparators.

The CMC used in the present invention may be one type, or a combination of two or more types of CMC that differ in degree of etherification, DS value, viscosity, molecular weight, etc.

<Binder for non-aqueous electrolyte secondary battery>
The carboxymethyl cellulose and/or its salt of the present invention is used as an electrode binder for a non-aqueous electrolyte secondary battery. Typically, an aqueous solution containing carboxymethyl cellulose and/or its salt is used as an electrode binder for a non-aqueous electrolyte secondary battery.

The concentration of carboxymethyl cellulose or its salt in the aqueous solution of carboxymethyl cellulose and/or its salt is typically 0.1 to 10% by mass, preferably 0.2 to 4% by mass, and more preferably 0.5 to 2% by mass.

There are no particular restrictions on the conditions for producing an aqueous solution of carboxymethyl cellulose and/or its salt. For example, carboxymethyl cellulose and/or its salt can be added to water (e.g., distilled water, purified water, tap water, etc.) and stirred as necessary to dissolve the solution.

In addition to carboxymethyl cellulose and/or its salt, binders for non-aqueous electrolyte secondary batteries may also contain other binders. Examples of binders used in electrode compositions for negative electrodes include synthetic rubber binders. Examples of synthetic rubber binders that can be used include 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. Among these, styrene butadiene rubber (SBR) is preferred. Examples of binders used in electrode compositions for positive electrodes include the synthetic rubber binders listed above as binders for negative electrodes, as well as polytetrafluoroethylene (PTFE).

Here, the average particle size (D50) of the SBR is preferably 50 to 300 nm, and more preferably 50 to 200 nm. If the average particle size (D50) of the SBR is too large, the SBR will not adhere uniformly to the active material, reducing its binder function and resulting in high electrical resistance. If the average particle size (D50) of the SBR is too small, the SBR will cover the active material, resulting in high electrical resistance.

The glass transition temperature (Tg) of the SBR is preferably from -50°C to 50°C.
When the SBR is within the above range, it mixes well with the carboxymethyl cellulose and/or salt thereof of the present invention, and when used in a negative electrode layer, it has appropriate flexibility, so that the electrical resistance is unlikely to become high.

<Electrode composition for nonaqueous electrolyte secondary batteries>
The electrode composition for a non-aqueous electrolyte secondary battery of the present invention (hereinafter sometimes referred to as the "electrode composition")
) contains at least an electrode active material and the carboxymethyl cellulose and/or a salt thereof of the present invention as the binder for a non-aqueous electrolyte secondary battery.

In other words, the carboxymethyl cellulose and/or its salt of the present invention can serve as an electrode binder and, together with an electrode active material, constitute an electrode composition. In this case, the content of carboxymethyl cellulose and/or its salt in the electrode composition is preferably 0.1 to 4.0% by mass based on the total electrode composition.

When using the other binders listed above, the content of the binder for non-aqueous electrolyte secondary batteries in the electrode composition is preferably 1 to 10% by mass, more preferably 1 to 6% by mass, and even more preferably 1 to 2% by mass.

(electrode active material)
The electrode active material contained in the active material layer constituting the present invention is a negative electrode active material when the electrode for a nonaqueous electrolyte secondary battery is a negative electrode, and is a positive electrode active material when the electrode is a positive electrode.

Examples of negative electrode active materials include graphite materials such as graphite (natural graphite, artificial graphite, etc.), coke, and carbon fiber; elements capable of forming an alloy with lithium, such as silicon-based compounds, Al, 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 with carbon and/or the graphite materials, or nitrides containing lithium. Of these, graphite materials and silicon-based compounds are preferred, and silicon particles or silicon oxide particles are more preferred as graphite and silicon-based compounds.

In the present invention, silicon oxide is represented by SiO _x (0<x≦2). In the present invention, the active material layer is more preferably a composite of a silicon-based compound and a graphite material.

When the negative electrode active material is a composite of a graphite material and a silicon-based compound, the graphite material and the silicon-based compound are preferably mixed in a ratio of graphite material:silicon-based compound of 10:90 to 90:10, and more preferably 50:50 to 80:20.

As the positive electrode active material, LiFePO ₄ and LiMe _x O _y (Me represents a transition metal including at least one of Ni, Co, and Mn, and x and y represent any numbers) based positive electrode active materials are preferred.

The content of the electrode active material in the electrode layer is typically 90 to 99% by mass, preferably 91 to 99% by mass, more preferably 92 to 99% by mass, even more preferably 95 to 99% by mass, particularly preferably 96 to 99% by mass, and most preferably 98 to 99% by mass.

The electrode composition may also contain a conductive additive, if necessary. Examples of conductive additives include conductive carbons such as carbon black, acetylene black, and ketjen black. The content of the conductive additive in the electrode composition is typically 0.01 to 20% by mass, and preferably 0.1 to 10% by mass.

Furthermore, the solvent used in the electrode composition is preferably an aqueous solvent. There are no particular restrictions on the type of aqueous solvent, but water, a water-soluble organic solvent, or a mixture of these is preferred, with water being more preferred.

A water-soluble organic solvent is an organic solvent that dissolves in water. Examples include methanol, ethanol, 2-propanol, butanol, glycerin, acetone, methyl ethyl ketone, 1,4-dioxane, N-methyl-2-pyrrolidone, tetrahydrofuran (THF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide, dimethyl sulfoxide (DMSO), acetonitrile, methyl succinate triglycol diester, acetic acid, and combinations thereof.

When the above-mentioned mixed solvent is used as the aqueous solvent, the amount of water-soluble organic solvent in the mixed solvent is preferably 10% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more. There is no upper limit to this amount, but it is preferably 95% by mass or less, and more preferably 90% by mass or less. Furthermore, the aqueous solvent may contain a water-insoluble organic solvent, as long as it does not impair the effects of the invention.

The conditions for producing the electrode composition are not particularly limited. For example, other components constituting the electrode composition are added to an aqueous solution of carboxymethyl cellulose and/or a salt thereof, and the mixture is mixed with stirring as necessary.
The form of the electrode composition is not particularly limited, and may be, for example, a liquid, a paste, a slurry, or any other form.

<Electrode for non-aqueous electrolyte secondary battery>
An electrode layer can be formed on a current collector by applying the electrode composition to the current collector. Examples of application methods include blade coating, bar coating, and die coating, with blade coating being preferred. For example, blade coating can be performed by casting the electrode composition onto the current collector using a coating device such as a doctor blade. Furthermore, the lamination method is not limited to the specific example described above. Another example is a method in which the electrode composition is discharged from an extrusion-type injector having a slot nozzle onto a running current collector wound around a backup roll. In blade coating, after casting, the electrode layer can be obtained by further drying, such as by heating (at a temperature of, for example, 80 to 120°C for a heating time of, for example, 4 to 12 hours), or by applying pressure using a roll press, as needed.

The shape of the electrode for the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, but is typically sheet-like. The thickness of a sheet-like electrode plate (the thickness of the electrode layer formed from the electrode composition, excluding the current collector portion) is difficult to specify because it depends on the composition and manufacturing conditions, but is typically 30 to 150 μm.

(Current collector)
Any electrical conductor that does not cause a fatal chemical reaction in the constructed electrode or battery can be used as the current collector. If the electrode is a negative electrode, a negative electrode current collector can be used, and if the electrode is a positive electrode, a positive electrode current collector can be used.

Examples of materials for the negative electrode current collector include stainless steel, nickel, copper, titanium, carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium or silver, etc. Among these, copper or a copper alloy is preferred, and copper is more preferred.
Examples of materials for the positive electrode current collector include metals such as aluminum and stainless steel, with aluminum being preferred.
The current collector may be in the form of a mesh, punched metal, foam metal, or foil processed into a plate shape, with foil processed into a plate shape being preferred.
<Nonaqueous electrolyte secondary battery>
The electrode for a non-aqueous electrolyte secondary battery of the present invention is used as an electrode for a non-aqueous electrolyte secondary battery.

That is, the present invention also provides a non-aqueous electrolyte secondary battery. A non-aqueous electrolyte secondary battery can have a structure in which positive electrodes and negative electrodes are alternately stacked with separators interposed between them and wound multiple times. Furthermore, a non-aqueous electrolyte secondary battery can be obtained by placing the wound multiple times stacked positive electrode, separator, and negative electrode into a battery container, injecting non-aqueous electrolyte, and sealing the container.

The shape of the non-aqueous electrolyte secondary battery is not particularly limited, and can be cylindrical, prismatic, flat, coin, button, sheet, etc. The material of the battery container is also not particularly limited as long as it can prevent moisture from entering the battery, and examples include metals, aluminum laminates, etc.

The separator is typically impregnated with a non-aqueous electrolyte. Examples of separators that can be used include microporous membranes or nonwoven fabrics made of polyolefins such as polyethylene and polypropylene.

The non-aqueous electrolyte typically contains a lithium salt and a non-aqueous solvent. Examples of lithium salts include LiPF ₆ , LiAsF ₆ , LiBF ₄ , and LiClO ₄ . Examples of non-aqueous solvents include ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and methyl ethyl carbonate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination. The concentration of the lithium salt in the non-aqueous electrolyte is typically 0.5 to 2.5 mol/L.

Electrodes for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries manufactured using an electrode composition that uses the carboxymethyl cellulose and/or its salt of the present invention as an electrode binder for non-aqueous electrolyte secondary batteries have low resistance and excellent battery characteristics due to the small amount of undissolved gel.

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples.
<Measurement and evaluation methods>
In the examples and comparative examples, measurements and evaluations were carried out as follows.

(Measurement of Degree of Carboxymethyl Substitution (DS Value))
Approximately 2.0 g of sample was weighed and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of a solution of 100 mL of special-grade concentrated nitric acid in 1000 mL of methanol was added, and the mixture was shaken for 3 hours to convert carboxymethyl cellulose salt (CMC) to H-CMC (hydrogen-form carboxymethyl cellulose). 1.5 to 2.0 g of the bone-dried H-CMC was weighed 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. Using phenolphthalein as an indicator, excess NaOH was back-titrated with 0.1 N H ₂ SO ₄ , and the degree of carboxymethyl substitution (DS value) was calculated using the following formula:
A = [(100 × F' - 0.1N-H ₂ SO ₄ (mL) × F) × 0.1] / (bone dry mass of H-CMC (g))
Degree of carboxymethyl substitution=0.162×A/(1−0.058×A)
F': Factor of 0.1N-H ₂ SO ₄ F: Factor of 0.1N-NaOH

(Measurement of 1 mass% viscosity of CMC)
Carboxymethyl 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 solids 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.) at a rotation speed of 30 rpm in accordance with the method of JIS-Z-8803.

(Particle diameter)
The particle diameters D10, D50 and D90 of the carboxymethyl cellulose or salts thereof used in the examples and comparative examples were determined from the particle diameter distribution based on the volume average particle diameter.

Particle size distribution was measured using a laser diffraction/scattering particle size distribution analyzer (Mastersizer 2000E, Spectris, Inc.). For the measurement, the sample was dispersed in methanol and then ultrasonically treated for at least one minute.

(Dispersibility in methanol (BS))
The dispersibility in methanol (BS) of the carboxymethyl cellulose or its salt used in the examples and comparative examples was measured by measuring the backscattered light intensity (%) using a Turbiscan (Turbiscan LAb, manufactured by Eiko Seiki Co., Ltd.). Specifically, 0.075 g of carboxymethyl cellulose and 15 ml of methanol were placed in a measurement container. The mixture was then stirred for 10 seconds using a vortex mixer, and the container was immediately placed in the device and measurement was initiated. The transmittance after 30 minutes at a height of 20 mm was calculated as the dispersibility in methanol (BS).

(Angle of repose)
The angle of repose of the carboxymethyl cellulose or its salt used in the examples and comparative examples was measured using a powder tester (Powder Tester PT-X, manufactured by Hosokawa Micron Corporation). Specifically, the angle of repose was measured using a powder property tester (Powder Tester PT-X (manufactured by Hosokawa Micron Corporation)) with the angle measurement method set to "Peak Operation" using a sieve with a mesh size of 710 μm and a linearity of 450 μm. The moisture content of the carboxymethyl cellulose used in the measurements was adjusted to 6.0 to 9.0%.

(collapse angle)
The collapse angle of the carboxymethyl cellulose or salt thereof used in the examples and comparative examples was measured using a powder tester (Powder Tester PT-X, manufactured by Hosokawa Micron Corporation) under the same conditions as those for measuring the angle of repose described above.
(difference angle)
Using the angle of repose and angle of collapse measured above, the angle of difference was calculated by the formula: angle of repose - angle of collapse.

(impedance)
The electrode compositions obtained in the examples and comparative examples were applied to a current collector (copper foil (NC-WS, manufactured by Furukawa Electric Co., Ltd.) measuring 320 mm in length, 170 mm in width, and 17 μm in thickness), dried at room temperature for 30 minutes, and then dried at 60°C for 30 minutes. After drying, the resultant was pressed at 5.0 kN using a small tabletop roll press (SA-602, manufactured by Tester Sangyo Co., Ltd.) to obtain a negative electrode plate having a negative electrode active material layer on the current collector. The obtained negative electrode plate and a _LiCoO2 positive electrode plate (manufactured by Hosen Co., Ltd., basis weight: 227.1 g/ ^m2 , effective discharge capacity: 145 mAh/g) were each punched out to a circle with a diameter of 16 mm, and the punched negative electrode plate 1 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. The negative electrode plate was then placed in a 20.0 mm diameter stainless steel circular dish-shaped container. The separator, positive electrode plate, spacer (15.5 mm diameter, 1 mm thick), and stainless steel washer (manufactured by Hosen Co., Ltd.) were then stacked in this order. 300 μL of electrolyte (1 mol/L LiPF ₆ , ethylene carbonate and diethyl carbonate in a volume ratio of 1:1) was then added to the circular dish-shaped container. A stainless steel cap was placed over the container via a polypropylene gasket and sealed using a coin battery crimping machine (Hosen Co., Ltd.), yielding a coin-shaped nonaqueous electrolyte secondary battery. The resulting coin-type battery was subjected to one charge-discharge cycle in a thermostatic chamber at 25°C using a secondary battery charge-discharge tester (BTS2004, manufactured by Nagano Corporation). Impedance measurements were then performed using an impedance tester (VPS, manufactured by Toyo Corporation), and resistance values were calculated using ZView (manufactured by Scribner Associates). A smaller resistance value indicates a nonaqueous electrolyte battery with better performance when used as a battery.

(Measurement of the amount of filtration residue of carboxymethyl cellulose or its salt)
Two liters 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. This 2 liters of aqueous solution was filtered through a 250-mesh filter (stainless steel, 63 μm mesh) using a filter ("Separote", manufactured by Kiriyama Seisakusho) under a reduced pressure of -200 mmHg. The residue remaining on the 250-mesh filter was dried with air at a temperature of 105°C for 16 hours, and the mass of the dried residue was measured and expressed as a percentage by mass (ppm) relative to the mass of carboxymethyl cellulose in the aqueous carboxymethyl cellulose solution.
The results of the above measurements and evaluations are shown in Table 1.

<Preparation of electrode composition>
Example 1
<Production of CMC>
A twin-screw kneader with the rotation speed adjusted to 100 rpm was charged with 550 parts of isopropanol and 40 parts of sodium hydroxide dissolved in 80 parts of water, and 100 parts of linter pulp (dry weight after drying at 100°C for 60 minutes) was added. The mixture was stirred and mixed at 30°C for 90 minutes to prepare mercerized cellulose. While stirring, 50 parts of monochloroacetic acid was added, and after stirring for 30 minutes, the mixture was heated to 70°C and allowed to undergo a carboxymethylation reaction for 90 minutes. After completion of the reaction, the mixture was neutralized with acetic acid to a pH of approximately 7, deliquored, dried, and pulverized to obtain CMC1. The DS value of CMC1 was 0.92, and the 1% viscosity was 1650 mPa·s.
<Production of electrode composition>
As the negative electrode active material, 1.0 g of 98% by mass graphite powder and 1.0 g of 98% by mass SiO _x powder were used, 0.01 g of 98% by mass acetylene black was used as a conductive additive, 1.0 g of an aqueous dispersion of CMC1 (2% by mass) was used as a binder, 63 mg of 48% by mass styrene butadiene rubber (SBR), and 1.5 g of water were mixed with Mazerustar (manufactured by Kurabo Industries, Ltd., Mazerustar KK-250S) to prepare an electrode composition. The Tg of the SBR was 7 ° C. and the average particle size was 165 nm.

Example 2
<Production of CMC>
A twin-screw kneader with the rotation speed adjusted to 100 rpm was charged with 650 parts of isopropanol and 60 parts of sodium hydroxide dissolved in 100 parts of water, and 100 parts of linter pulp (dry weight after drying at 100°C for 60 minutes) was added. The mixture was stirred and mixed at 30°C for 90 minutes to prepare mercerized cellulose. Further, 70 parts of monochloroacetic acid was added with stirring, and after stirring for 30 minutes, the mixture was heated to 70°C and allowed to undergo a carboxymethylation reaction for 90 minutes. After completion of the reaction, the mixture was neutralized with acetic acid to a pH of approximately 7, deliquored, dried, and pulverized to obtain CMC2. The DS value of CMC2 was 0.84, and the 1% viscosity was 4580 mPa·s.
<Production of electrode composition>
An electrode composition was prepared in the same manner as in Example 1, except that the type of CMC was changed to CMC2 obtained above.

Example 3
<Production of CMC>
A twin-screw kneader with the rotation speed adjusted to 100 rpm was charged with 600 parts of isopropanol and 55 parts of sodium hydroxide dissolved in 100 parts of water, and 100 parts of linter pulp (dry weight after drying at 100°C for 60 minutes) was added. The mixture was stirred and mixed at 30°C for 90 minutes to prepare mercerized cellulose. Further, 65 parts of monochloroacetic acid was added with stirring, and after stirring for 30 minutes, the mixture was heated to 70°C and allowed to undergo a carboxymethylation reaction for 90 minutes. After completion of the reaction, the mixture was neutralized with acetic acid to a pH of approximately 7, deliquored, dried, and pulverized to obtain CMC3. The DS value of CMC3 was 0.70 and the 1% viscosity was 4900 mPa·s.
<Production of electrode composition>
An electrode composition was prepared in the same manner as in Example 1, except that the type of CMC was changed to CMC3 obtained above.

Example 4
<Production of CMC>
A twin-screw kneader with the rotation speed adjusted to 100 rpm was charged with 600 parts of isopropanol and 38 parts of sodium hydroxide dissolved in 80 parts of water, and 100 parts of linter pulp (dry weight after drying at 100°C for 60 minutes) was added. The mixture was stirred and mixed at 30°C for 90 minutes to prepare mercerized cellulose. While stirring, 46 parts of monochloroacetic acid was added, and after stirring for 30 minutes, the mixture was heated to 70°C and subjected to a carboxymethylation reaction for 90 minutes. After completion of the reaction, the mixture was neutralized with acetic acid to a pH of approximately 7, deliquored, dried, and pulverized to obtain CMC4. The DS value of CMC4 was 0.70 and the 1% viscosity was 8980 mPa·s.
<Production of electrode composition>
An electrode composition was prepared in the same manner as in Example 1, except that the type of CMC was changed to the CMC4 obtained above.

(Comparative Example 1)
<Production of electrode composition>
An electrode composition was prepared in the same manner as in Example 1, except that the type of CMC was changed to CMC5 having a DS value of 0.93 and a 1% viscosity of 3460 mPa·s.

(Comparative Example 2)
<Production of electrode composition>
An electrode composition was prepared in the same manner as in Example 1, except that the type of CMC was changed to CMC6 having a DS value of 0.69 and a 1% viscosity of 6,340 mPa·S.

As shown in Table 1, electrode layers obtained from electrode compositions using carboxymethyl cellulose and/or its salts, which are used as electrode binders for non-aqueous electrolyte secondary batteries, and which have a carboxymethyl substitution degree per anhydroglucose unit of 0.5 to 1.2 and a dispersion index (BS) in methanol measured by turbine scanning of 3 to 10%, have low resistance values and are shown to have good battery performance when used in non-aqueous electrolyte secondary batteries.

Claims (12)

Carboxymethyl cellulose and/or its salts are used as electrode binders for non-aqueous electrolyte secondary batteries, and have a degree of carboxymethyl substitution per anhydroglucose unit of 0.5 to 1.2, a dispersion index (BS) in methanol measured by turbine scanning of 3 to 10%, and a ratio of particle diameter D90 to particle diameter D10 (particle diameter D90/particle diameter D10) of 7.5 to 11. Carboxymethyl cellulose and/or its salt according to claim 1, having a particle diameter D50 of 10 to 20 μm. Carboxymethylcellulose and/or its salt according to claim 1 or 2, wherein the difference between particle diameter D90 and particle diameter D50 (particle diameter D90 - particle diameter D50) is 5 to 30 μm. Carboxymethyl cellulose and/or its salt according to claim 1 or 2, wherein the difference between particle diameter D50 and particle diameter D10 (particle diameter D50 - particle diameter D10) is 5 to 20 μm. Carboxymethyl cellulose and/or a salt thereof according to claim 1 or 2, wherein the difference between particle diameter D90 and particle diameter D10 (particle diameter D90 - particle diameter D10) is 10 to 50 μm. Carboxymethyl cellulose and/or a salt thereof according to claim 1 or 2, wherein the viscosity of a 1% by weight aqueous solution measured at 25°C using a Brookfield viscometer (30 rpm) is 1,000 to 20,000 mPa·s. Carboxymethyl cellulose and/or a salt thereof according to claim 1 or 2, wherein when 2 liters of a 0.3% by mass aqueous solution of carboxymethyl cellulose or a salt thereof having a dry mass m is prepared and the entire solution is filtered through a 250-mesh filter under reduced pressure conditions of -200 mmHg, and the dry mass M of the residue on the filter after filtration is measured, the ratio of the dry mass M to the dry mass m is less than 200 ppm. An electrode composition for a non-aqueous electrolyte secondary battery, comprising the carboxymethyl cellulose and/or its salt according to claim 1 or 2, and styrene-butadiene rubber having an average particle size of 50 nm to 300 nm. The electrode composition for a non-aqueous electrolyte secondary battery according to claim 7, wherein the glass transition temperature of the styrene-butadiene rubber is -50°C to 50°C. A non-aqueous electrolyte secondary battery electrode composition containing the carboxymethyl cellulose and/or its salt according to claim 1 or 2. An electrode for a non-aqueous electrolyte secondary battery using the electrode composition for a non-aqueous electrolyte secondary battery described in claim 8. A nonaqueous electrolyte secondary battery using the electrode composition for a nonaqueous electrolyte secondary battery described in claim 8.