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

Abstract

The present invention provides a binder for non-aqueous electrolyte secondary battery electrodes that exhibits excellent adhesion strength when used as an electrode and excellent battery characteristics when used as a battery. [Solution] The solution comprises carboxymethylcellulose or a salt thereof having a carboxymethyl substitution degree of 0.5 to 1.3 per anhydrous glucose unit, and an alkaline compound containing an alkali metal, wherein the viscosity of a 1% by mass aqueous solution measured with a B-type viscometer (30 rpm) at 25°C is 1,000 mPa·s or more and 12,000 mPa·s or less, and the pH at 25°C is 8.0 or more and less than 9.5.

Description

Technical Field

[0001] The present invention relates to a binder for non-aqueous electrolyte secondary battery electrodes, an electrode composition for non-aqueous electrolyte secondary batteries, an electrode for non-aqueous electrolyte secondary batteries, and a non-aqueous electrolyte secondary battery.

Background Art

[0002] In recent years, due to the rapid spread of small portable terminals represented by smartphones and tablets, etc., and stationary batteries, the demand for small and high-energy-density batteries that drive them has been increasing.

[0003] Generally, graphite-based materials are used for the negative electrodes of lithium-ion secondary batteries. However, the theoretical capacity of graphite-based materials is 372 mAh / g (LiC6), and currently, it is approaching its limit.

[0004] Furthermore, in order to improve the energy density of lithium-ion secondary batteries, the selection of new materials is necessary. Therefore, materials obtained by alloying silicon, tin, etc., which have a low potential and a large specific capacity after carbon and lithium, with lithium, have attracted attention.

[0005] Among these materials, silicon can absorb up to 4.4 lithium atoms per 1 silicon atom in molar ratio, theoretically achieving about 10 times the capacity of graphite-based carbon materials. However, when silicon particles absorb lithium, their volume expands to approximately 3 to 4 times its original size, leading to degradation and a decrease in capacity due to repeated charging and discharging. Detailed analysis of this phenomenon reveals that when lithium is inserted into a silicon-containing active material, the volume expansion causes microscopic cracks to form within the electrode. Electrolyte then penetrates these cracks, forming a new film (SEI layer). This irreversible loss of capacity occurs, resulting in a decrease in battery capacity. This phenomenon manifests as changes in charge-discharge efficiency during the cycle. In particular, the decrease in cycle efficiency in the early stages of the cycle, when volume changes are large, significantly impacts the battery's lifespan when combined with a positive electrode that has high charge-discharge efficiency. Therefore, minimizing the change in electrode structure due to this volume expansion is a crucial challenge when using silicon-containing active materials.

[0006] Given this situation, Patent Document 1 attempts to improve battery characteristics by using a binder (binding agent) that includes three essential components: carboxymethylcellulose or its metal salt, polyacrylic acid or its metal salt, and styrene-butadiene rubber or polyvinidene fluoride. However, according to the examples and comparative examples in Patent Document 1, when only two components (carboxymethylcellulose and styrene-butadiene rubber, or carboxymethylcellulose and polyvinidene fluoride) were used as the binder, the desired battery characteristics were not obtained.

[0007] Furthermore, Patent Document 2 states that the properties required of a polymer used as a binder include the ability to bond the active materials together, the ability to adhere the active materials to the current collector, abrasion resistance during the process of winding the electrodes, and adhesion such as powder shedding resistance, which prevents fine powder of the active materials from falling off the coated and dried composition film (hereinafter also referred to as the "active material layer") even when cut afterwards. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2015-198038 [Patent Document 2] International Publication No. 2016 / 039067 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The object of the present invention is to provide a binder for non-aqueous electrolyte secondary battery electrodes that has excellent adhesion strength when used in electrodes and exhibits excellent battery characteristics when used in batteries, and also to provide a non-aqueous electrolyte secondary battery electrode composition, a non-aqueous electrolyte secondary battery electrode, and a non-aqueous electrolyte secondary battery using this binder. [Means for solving the problem]

[0010] As a result of diligent research, the inventors have found that the above problem can be solved by using carboxymethylcellulose or a salt thereof under predetermined conditions and a predetermined compound.

[0011] In other words, according to the present invention, (1) A binder for non-aqueous electrolyte secondary battery electrodes comprising carboxymethylcellulose or a salt thereof having a carboxymethyl substitution degree of 0.5 to 1.3 per anhydrous glucose unit, and an alkaline compound containing an alkali metal, wherein the viscosity of a 1% by mass aqueous solution measured at 25°C with a B-type viscometer (30 rpm) is 1,000 mPa·s or more and 12,000 mPa·s or less, and the pH at 25°C is 8.0 or more and less than 9.5. (2) An electrode composition for a non-aqueous electrolyte secondary battery comprising the binder for non-aqueous electrolyte secondary battery electrodes described in (1), (3) An electrode composition for a non-aqueous electrolyte secondary battery as described in (2), wherein the electrode active material contains 5% by mass or more of a silicon-based compound. (4) An electrode for a non-aqueous electrolyte secondary battery using the electrode composition for a non-aqueous electrolyte secondary battery described in (3), (5) A non-aqueous electrolyte secondary battery using the electrode composition for non-aqueous electrolyte secondary batteries described in (3), It will be provided. [Effects of the Invention]

[0012] The present invention provides a binder for non-aqueous electrolyte secondary battery electrodes that has excellent adhesion strength when used in electrodes and exhibits excellent battery characteristics when used in batteries. Furthermore, the present invention provides an electrode composition for non-aqueous electrolyte secondary batteries, an electrode for non-aqueous electrolyte secondary batteries, and a non-aqueous electrolyte secondary battery using this binder for non-aqueous electrolyte secondary battery electrodes. [Modes for carrying out the invention]

[0013] The binder for non-aqueous electrolyte secondary battery electrodes of the present invention will be described below. The binder for non-aqueous electrolyte secondary battery electrodes of the present invention (hereinafter sometimes referred to as "electrode binder") comprises carboxymethylcellulose or a salt thereof having a carboxymethyl substitution degree of 0.5 to 1.3 per anhydrous glucose unit, and an alkaline compound containing an alkali metal. The viscosity of a 1% by mass aqueous solution measured with a B-type viscometer (30 rpm) at 25°C is 1,000 mPa·s or more and 12,000 mPa·s or less, and the pH at 25°C is 8.0 or more and less than 9.5.

[0014] <Carboxymethylcellulose or its salts> The carboxymethylcellulose or its salt (hereinafter sometimes abbreviated as CMC) included in the present invention has a structure in which the hydroxyl groups in the glucose units constituting cellulose are replaced with carboxymethyl ether groups. Carboxymethylcellulose may also be in the form of a salt. Examples of carboxymethylcellulose salts include metal salts such as carboxymethylcellulose sodium salt.

[0015] In the present invention, cellulose means a polysaccharide having a structure in which D-glucopyranose (also simply referred to as "glucose unit" or "anhydroglucose") is linked by β,1-4 bonds. Cellulose is generally classified into natural cellulose, regenerated cellulose, microcrystalline cellulose, microcrystalline cellulose excluding amorphous regions, etc. according to its origin, production method, etc.

[0016] Examples of natural cellulose include sun-dried or undried pulp, purified lint, cellulose produced by microorganisms such as acetic acid bacteria, etc. The raw material of sun-dried or undried pulp is not particularly limited, and examples include wood, cotton, straw, bamboo, etc. The production method of sun-dried or undried pulp is also not particularly limited, and mechanical methods, chemical methods, or methods combining mechanical and chemical methods are exemplified. Examples of sun-dried or undried pulp include mechanical pulp, chemical pulp, groundwood pulp, sulfite pulp, kraft pulp, and pulp for papermaking. Also, as sun-dried or undried pulp, there is also exemplified dissolving pulp which is chemically purified and mainly used by dissolving it in chemicals and which is the main raw material for artificial fibers, cellophane, etc.

[0017] Examples of regenerated cellulose include regenerated cellulose obtained by dissolving cellulose in a solvent such as a copper ammonia solution, a cellulose xanthate solution, a morpholine derivative, etc. and then spinning it again.

[0018] Examples of microcrystalline cellulose include microcrystalline cellulose obtained by depolymerizing cellulose-based materials such as natural cellulose and regenerated cellulose by acid hydrolysis, alkaline hydrolysis, enzymatic hydrolysis, blasting treatment, vibration ball mill treatment, etc., and microcrystalline cellulose obtained by mechanically treating cellulose-based materials.

[0019] In producing CMC used in the present invention, known production methods of CMC can be applied. For example, after treating cellulose with a mercerizing agent (alkali) to prepare mercerized cellulose (alkali cellulose), CMC can be produced by adding an etherifying agent to the mercerized cellulose and causing an etherification reaction.

[0020] As the raw material cellulose, any of the above-mentioned celluloses can be used without particular limitation, but those with high cellulose purity are preferred, and dissolving pulp or linter is more preferred. By using these, CMC with high purity can be obtained.

[0021] Examples of the mercerizing agent include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. Examples of the etherifying agent include monochloroacetic acid and sodium monochloroacetate.

[0022] In the production method of water-soluble general 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 is that when it is 2.00 or more, the etherification reaction can be sufficiently carried out, and it is possible to prevent unreacted monochloroacetic acid from remaining and being wasted. When it is 2.45 or less, it is possible to prevent side reactions by the excess mercerizing agent and monochloroacetic acid from proceeding to generate alkali metal glycolate, which is economical. In the present invention, CMC may be a commercially available product. Examples of commercially available products include the product named "Sun Rose" manufactured by Nippon Paper Industries Co., Ltd.

[0023] In the present invention, the degree of etherification of CMC indicates the ratio of the groups in the hydroxyl groups (-OH) in the glucose units constituting cellulose that are substituted with carboxymethyl ether groups (-OCH2COOH).

[0024] (Degree of carboxymethyl substitution) The CMC used in the present invention has a carboxymethyl substitution degree per anhydrous glucose unit (hereinafter sometimes referred to as the DS value) of 0.5 to 1.3. A DS value of 0.5 or higher ensures good solubility in water and suppresses the generation of undissolved substances. A DS value of 1.3 or lower suppresses an increase in the stringiness of the liquid and makes it easy to handle. Therefore, the DS value of the CMC in the present invention is 0.5 to 1.3, preferably 0.5 to 1.0, and more preferably 0.6 to 1.0.

[0025] The method for measuring the degree of carboxymethyl substitution is as follows: Accurately weigh approximately 2.0 g of the sample and place it in a 300 mL stoppered Erlenmeyer flask. Add 100 mL of a solution made by adding 100 mL of special grade concentrated nitric acid to 1000 mL of methanol, and shake for 3 hours to convert the carboxymethylcellulose salt (CMC) to H-CMC (hydrogen-type carboxymethylcellulose). Accurately weigh 1.5 to 2.0 g of the oven-dried H-CMC and place it in a 300 mL stoppered Erlenmeyer flask. Wet the H-CMC with 15 mL of 80% methanol, add 100 mL of 0.1 N-NaOH, and shake at room temperature for 3 hours. Using phenolphthalein as an indicator, back titrate the excess NaOH with 0.1 N-H2SO4 and calculate the degree of carboxymethyl substitution (DS value) using the following formula. A = [(100 × F' - 0.1N-H2SO4(mL) × F) × 0.1] / (Dry mass of H-CMC (g)) Degree of carboxymethyl substitution = 0.162 × A / (1 - 0.058 × A) F': Factor of N-H2SO4 F: Factor of 0.1N-NaOH

[0026] (Viscosity of CMC) The viscosity of a 1% by mass aqueous solution of carboxymethylcellulose or its salt, measured at 25°C with a Type B viscometer (30 rpm), is preferably 10 to 20,000 mPa·s, more preferably 500 to 10,000 mPa·s, and even more preferably 1,000 to 10,000 mPa·s.

[0027] The method for measuring viscosity is as follows: Measure carboxymethylcellulose or its salt into a 1,000 mL glass beaker and disperse it in 900 mL of distilled water to prepare an aqueous dispersion with a solid content of 1% (w / v). Stir the aqueous dispersion at 25°C using a stirrer at 600 rpm for 3 hours. Then, measure the viscosity after 3 minutes at a rotation speed of 30 rpm using a Type B viscometer (manufactured by Toki Sangyo Co., Ltd.) in accordance with the method of JIS-Z-8803.

[0028] <Alkaline compounds containing alkali metals> The binder for non-aqueous electrolyte secondary battery electrodes of the present invention contains an alkaline compound containing an alkali metal. Examples of alkaline compounds containing alkali metals include alkaline compounds containing sodium, such as sodium hydroxide, sodium carbonate, and sodium acetate; alkaline compounds containing potassium, such as potassium hydroxide, potassium carbonate, and potassium acetate; and alkaline compounds that can form complexes, such as lithium molybdate and calcium hydroxide. When alkaline compounds containing sodium, such as sodium hydroxide, sodium carbonate, and sodium acetate, or alkaline compounds containing potassium, such as potassium hydroxide, potassium carbonate, and potassium acetate, are used, the carboxyl groups in carboxymethylcellulose are neutralized and become electrically charged. This electron repulsion improves the dispersibility of carboxymethylcellulose in the binder, thereby improving adhesion strength and 3C discharge capacity. On the other hand, by using alkaline compounds that can form complexes such as lithium molybdate and calcium hydroxide, in addition to the electron repulsion mentioned above, the crosslinking effect of carboxymethylcellulose can suppress the expansion and contraction of the negative electrode active material during charging and discharging. Furthermore, the crosslinking effect broadens the molecular chains, making it easier for lithium ions to move, thus further improving the 3C discharge capacity.

[0029] The content of the alkali metal-containing alkaline compound in the binder for non-aqueous electrolyte secondary battery electrodes of the present invention is not particularly limited as it varies depending on the alkaline compound, but for example, it is preferably 0.01 parts by mass or more, more preferably 0.1 parts by mass or more, preferably 10 parts by mass or less, and more preferably 1 part by mass or less, per 100 parts by mass of CMC.

[0030] <Binding agent for non-aqueous electrolyte secondary battery electrodes> The method for producing the binder for non-aqueous electrolyte secondary battery electrodes of the present invention is not particularly limited, but can be obtained, for example, by dispersing or dissolving an alkaline compound containing CMC and an alkali metal in a solvent such as water.

[0031] (Viscosity of binders for non-aqueous electrolyte secondary battery electrodes) The viscosity of a 1% by mass aqueous solution of the binder for non-aqueous electrolyte secondary battery electrodes of the present invention, measured at 25°C with a B-type viscometer (30 rpm), is 1,000 to 12,000 mPa·s, preferably 2,000 to 12,000 mPa·s, and more preferably 3,000 to 12,000 mPa·s. If the viscosity is too high, there are problems such as inability to properly mix the active material and conductive additive during slurry preparation, and insufficient fluidity when coating the slurry onto the current collector, making coating impossible. If the viscosity is too low, the slurry flows off the current collector when applied, making coating impossible, and the active material and binders such as SBR migrate, leading to increased electrical resistance.

[0032] The method for measuring viscosity is as follows: A binder for non-aqueous electrolyte secondary battery electrodes is measured 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 is stirred at 600 rpm for 3 hours at 25°C using a stirrer. Then, the viscosity is measured after 3 minutes at a rotation speed of 30 rpm using a Type B viscometer (manufactured by Toki Sangyo Co., Ltd.) in accordance with the method of JIS-Z-8803.

[0033] (pH of binders for non-aqueous electrolyte secondary battery electrodes) The pH of the binder for non-aqueous electrolyte secondary battery electrodes of the present invention is 8.0 or higher, preferably 8.2 or higher, more preferably 8.5 or higher, and less than 9.5, preferably less than 9.3, and more preferably less than 9.0. The pH was measured at 25°C using a glass electrode pH meter.

[0034] The pH of the binder for non-aqueous electrolyte secondary battery electrodes of the present invention is within the above range, which causes electron repulsion between CMCs and improves the dispersibility of CMCs. Therefore, when used in a battery, it has excellent adhesion strength and excellent battery performance, especially at high rates. Furthermore, the binder for non-aqueous electrolyte secondary battery electrodes of the present invention also has good cellulase resistance and can maintain its viscosity during storage.

[0035] <Electrode composition for nonaqueous electrolyte secondary batteries> The electrode composition for non-aqueous electrolyte secondary batteries of the present invention (hereinafter sometimes referred to as "electrode composition") comprises at least an electrode active material and the above-mentioned binder for non-aqueous electrolyte secondary battery electrodes.

[0036] In other words, the electrode binder of the present invention can constitute an electrode composition together with an electrode active material. In this case, the content of carboxymethylcellulose or its salt in the electrode composition is preferably 0.1 to 4.0% by mass of the total electrode composition.

[0037] Furthermore, the electrode composition of the present invention may contain other binders in addition to the electrode binder of the present invention. When using the electrode composition for the negative electrode, examples of other binders include carboxymethylcellulose, carboxymethylcellulose salts (excluding lithium salts), and synthetic rubber binders. As synthetic rubber binders, 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. Of these, styrene-butadiene rubber (SBR) is preferred. Furthermore, when using the electrode composition for the positive electrode, examples of other binders include the synthetic rubber binders shown as binders used when using the electrode composition for the negative electrode, as well as polytetrafluoroethylene (PTFE).

[0038] When using the above-mentioned other binders, the total content of the electrode binder and the other binders 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, on a dry mass basis.

[0039] (electrode active material) The electrode active material contained in the composition for non-aqueous electrolyte secondary batteries is a negative electrode active material when the electrode for the non-aqueous electrolyte secondary battery is a negative electrode, and a positive electrode active material when it is a positive electrode.

[0040] Examples of negative electrode active materials include graphite (natural graphite, artificial graphite, etc.), coke, carbon fiber, and other graphite-based materials; elements capable of forming alloys 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 alloys with lithium; composites of elements capable of forming alloys with lithium and the aforementioned compounds with carbon and / or the graphite-based material, or nitrides containing lithium. Of these, graphite-based materials and silicon-based compounds (i.e., Si or compounds containing Si) are preferred, and graphite and silicon particles or silicon oxide particles as silicon-based compounds are more preferred.

[0041] In addition, when using a silicon-based compound as the electrode active material, it is preferable to contain 5% by mass or more of the silicon-based compound with respect to the entire electrode active material, and more preferably 10% by mass or more.

[0042] Note that the silicon oxide in the present invention is represented by SiO x (0 < x ≤ 2). Also, in the present invention, as the electrode layer, a composite of a silicon-based compound and a graphite material can also be used.

[0043] When the negative electrode active material is a composite of a graphite material and a silicon-based compound, the mixing ratio of the graphite material and the silicon-based compound is preferably graphite material:silicon-based compound = 10:90 to 90:10, and more preferably 50:50 to 80:20.

[0044] As the positive electrode active material, LiFePO4, LiMe x O y (Me means a transition metal containing at least one of Ni, Co, and Mn. x and y mean arbitrary numbers.) - based positive electrode active materials are preferred.

[0045] The content of the electrode active material in the electrode layer is usually 90 to 99% by mass, preferably 91 to 99% by mass, more preferably 92 to 99% by mass, still more preferably 95 to 99% by mass, particularly preferably 96 to 99% by weight, and most preferably 98 to 99% by mass.

[0046] In addition, the electrode composition may contain a conductive aid as necessary. Examples of the conductive aid include conductive carbons such as carbon black, acetylene black, and ketjen black. The content of the conductive aid in the electrode composition is usually 0.01 to 20% by mass, preferably 0.1 to 10% by mass.

[0047] Furthermore, an aqueous solvent is preferred as the solvent used in the electrode composition. The type of aqueous solvent is not particularly limited, but it is preferably water, a water-soluble organic solvent, or a mixture thereof, with water being more preferred.

[0048] Water-soluble organic solvents are organic solvents that dissolve 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 triglycol diester succinate, acetic acid, and combinations thereof.

[0049] When the above 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 also contain water-insoluble organic solvents as long as the effects of the invention are not impaired.

[0050] 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 a salt thereof and mixed while stirring as necessary.

[0051] Furthermore, the properties of the electrode composition are not particularly limited. For example, it may be liquid, paste, or slurry, and any of these may be used.

[0052] <Non-aqueous electrolyte secondary battery electrode> An electrode layer can be formed on a current collector by applying the above electrode composition onto the current collector. Examples of application 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 onto the current collector using a coating device such as a doctor blade is exemplified. Furthermore, the method of lamination is not limited to the above specific example, and a method of applying the electrode composition by discharging it from an extrusion-type injector having a slot nozzle onto a current collector that is wound around a backup roll and running is also exemplified. In blade coating, after casting, the electrode layer can be obtained by further drying as needed by heating (for example, at a temperature of 80 to 120°C, for a heating time of 4 to 12 hours) and pressurizing by a roll press or the like.

[0053] The shape of the electrode for the non-aqueous electrolyte secondary battery of the present invention is not particularly limited, but it is usually in the form of a sheet. In the case of a sheet-shaped electrode plate, the thickness (thickness of the electrode layer formed from the electrode composition, excluding the current collector portion) is difficult to specify as it depends on the composition of the composition and manufacturing conditions, but it is usually 30 to 150 μm.

[0054] (Current collector) Any electrical conductor that does not cause a fatal chemical change in the constituent electrodes or battery can be used as the current collector. A negative electrode current collector can be used when the electrode is negative, and a positive electrode current collector can be used when the electrode is positive.

[0055] Examples of materials for the negative electrode current collector include stainless steel, nickel, copper, titanium, carbon, copper, or stainless steel with carbon, nickel, titanium, or silver deposited on its surface. Of these, copper or copper alloys are preferred, and copper is more preferred.

[0056] Examples of materials for the positive electrode current collector include metals such as aluminum and stainless steel, with aluminum being preferred.

[0057] Examples of current collector shapes include mesh, punched metal, formed metal, and foil processed into a plate shape, with foil processed into a plate shape being preferred.

[0058] <Nonaqueous electrolyte secondary battery> The electrode for a non-aqueous electrolyte secondary battery of the present invention is used as an electrode in a non-aqueous electrolyte secondary battery. In other words, the present invention also provides a non-aqueous electrolyte secondary battery. A non-aqueous electrolyte secondary battery can have a structure in which positive and negative electrodes are alternately stacked with a separator in between and wound many times. Alternatively, a non-aqueous electrolyte secondary battery can be obtained by placing the stack of positive electrode, separator, and negative electrode wound many times into a battery container, injecting a non-aqueous electrolyte, and sealing the container.

[0059] The shape of the non-aqueous electrolyte secondary battery is not particularly limited, and cylindrical, rectangular, flat, coin-shaped, button-shaped, sheet-shaped, etc., can be used. Furthermore, the material of the battery container is not particularly limited as long as it can achieve the purpose of preventing moisture from entering the inside of the battery, and examples include metal, aluminum laminate, etc.

[0060] The separator is typically impregnated with a non-aqueous electrolyte. For example, a microporous membrane or nonwoven fabric made of polyolefin such as polyethylene or polypropylene can be used as the separator.

[0061] Non-aqueous electrolytes typically consist of a lithium salt and a non-aqueous solvent. Examples of lithium salts include LiPF6, LiAsF6, LiBF4, and LiClO4. Examples of non-aqueous solvents include ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and methyl ethyl carbonate. Non-aqueous solvents may be used individually or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte is typically 0.5 to 2.5 mol / L.

[0062] The electrode for a non-aqueous electrolyte secondary battery produced by the electrode composition using the binder for an electrode of the present invention has excellent adhesion strength, and a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery using this electrode has excellent battery characteristics such as an improved discharge capacity at a high rate.

Examples

[0063] Hereinafter, embodiments of the present invention will be described by way of examples, but the present invention is not limited thereto.

[0064] <Measurement methods and evaluation methods> In the present examples and comparative examples, measurement and evaluation of each index were carried out by the following methods. The measurement and evaluation results are shown in Table 1.

[0065] <Measurement method for carboxymethyl substitution degree (CM-DS) of CMC> The measurement method for the carboxymethyl substitution degree is as follows: Approximately 2.0 g of the sample was precisely weighed and placed in a 300 mL conical flask with a stopper. 100 mL of a solution obtained by adding 100 mL of special grade concentrated nitric acid to 1000 mL of methanol was added, and the mixture was shaken for 3 hours to convert the salt of carboxymethyl cellulose (CMC) to H-CMC (hydrogen type carboxymethyl cellulose). Approximately 1.5 - 2.0 g of the absolute dry H-CMC was precisely weighed and placed in a 300 mL conical flask with a stopper. The H-CMC was wetted with 15 mL of 80% methanol, 100 mL of 0.1N-NaOH was added, and the mixture was shaken at room temperature for 3 hours. Using phenolphthalein as an indicator, the excess NaOH was back-titrated with 0.1N-H2SO4, and the carboxymethyl substitution degree (DS value) was calculated by the following formula. A = [(100 × F’ - 0.1N-H2SO4 (mL) × F) × 0.1] / (absolute dry mass of H-CMC (g)) Carboxymethyl substitution degree = 0.162 × A / (1 - 0.058 × A) F’: Factor of 0.1N-H2SO4 F: Factor of 0.1N-NaOH

[0066] <1% viscosity of the binder for an electrode> The viscosity of the electrode binder was measured using a Type B viscometer (manufactured by Toki Sangyo Co., Ltd.) at a rotation speed of 30 rpm for 3 minutes with a concentration of 1% by mass of the electrode binder.

[0067] <pH of electrode binder> The pH (hydrogen ion concentration) of the electrode binder was measured at a temperature of 25°C using a glass electrode pH meter (HM-30P, manufactured by Toa DKK) with a concentration of 1% by mass of the electrode binder.

[0068] <Battery Evaluation Method> (Discharge capacity (charge / discharge rate test)) The charge-discharge rate tests of the coin-type non-aqueous electrolyte secondary batteries obtained in the examples and comparative examples were conducted using Nagano Corporation's BTS2004 in a constant temperature bath at 25°C. 52 cycles were performed, with each cycle consisting of charging followed by discharging. The charging conditions for all cycles were constant current constant voltage (CC-CV) (CC current 0.2C, CV voltage 4.2V, termination current 0.02C).

[0069] For the discharge process, the termination voltage was set to 3.0V. For the first cycle, a constant discharge current of 0.2C was applied, and the discharge capacity (mAh / g) after one cycle was measured.

[0070] For the subsequent 52 cycles, the constant current for the discharge process was set as described below, and the discharge capacity (mAh / g) was measured after each discharge cycle. The discharge capacity after 33 cycles was defined as the 3C discharge capacity.

[0071] The constant current for the discharge process in each cycle is as follows: 2-10 cycles: Constant current 0.2C for discharge treatment. 11-20 cycles: Constant current 1C for discharge treatment 21 cycles: Constant current 0.2C for discharge processing. Cycles 22-31: Constant current 2C for discharge processing. 32 cycles: Constant current 0.2C for discharge processing. Cycles 33-42: Constant current 3C for discharge treatment Cycles 43-52: Constant current 0.2C for discharge treatment.

[0072] <Evaluation of adhesion strength> The negative electrode plates obtained in the examples and comparative examples were cut with a cutter to a width of 250 mm. The coated surface of the negative electrode plate was attached to a wooden board (300 mm long x 200 mm wide x 3 mm thick) with double-sided tape (manufactured by Nitto Denko Corporation), and the force required to peel the copper foil from the coating layer was measured using a TENSILON RTC-1210A (manufactured by A&D Company, Limited), and the adhesion strength was determined.

[0073] <Viscosity retention evaluation> To 100 mL of a 1% by mass aqueous solution of the electrode binder obtained in the examples and comparative examples, 0.1 ppm of cellulase (manufactured by Yakult Pharmaceutical Co., Ltd.) was added. The viscosity (mP·s) immediately after cellulase addition was measured at 25°C at a rotation speed of 30 rpm for 3 minutes using a Type B viscometer (manufactured by Toki Sangyo Co., Ltd.) in accordance with the method of JIS-Z-8803. After the electrode binder aqueous solution was allowed to stand for 3 hours after cellulase addition, the viscosity was measured using a Type B viscometer under the same conditions as immediately after cellulase addition.

[0074] The viscosity retention rate (%) was calculated using the viscosity immediately after cellulase addition (mP·s) and the viscosity 3 hours after addition (mP·s), as described above, using the formula: "Viscosity 3 hours after cellulase addition (mP·s) / Viscosity immediately after cellulase addition (mP·s) × 100".

[0075] <Example 1> (Preparation of electrode binder CMC(a)) A sodium salt of carboxymethylcellulose with a 1% by mass viscosity of 7,900 mPa·s and a DS of 0.70 was dispersed in ultrapure water to a concentration of 1% by mass of sodium salt of carboxymethylcellulose. Sodium hydroxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the dispersion to adjust the pH to 8.0-9.0 to obtain an electrode binder CMC(a) with a viscosity of 10,800 mPa·s and a DS of 0.70.

[0076] (Fabrication of negative electrode plate (a)) As a negative electrode material, SiOx, acetylene black (manufactured by Stream Chemical), electrode binder CMC(a), and styrene-butadiene rubber (SBR, manufactured by JSR, product code TRAD104A) were mixed in a solid weight ratio of 97:0.5:1.0:1.5. Water was added to achieve a slurry concentration of 45.6% by mass, and the mixture was thoroughly stirred using a Mazelstar (manufactured by Kurabo Industries Ltd., KK-250S) to obtain slurry (a).

[0077] Slurry (a) was applied to a copper foil (manufactured by Furukawa Electric Co., Ltd., NC-WS) measuring 320 mm in length, 170 mm in width, and 17 μm in thickness using an applicator. After air-drying for 30 minutes, it was dried in a dryer at 60°C for 30 minutes. Furthermore, it was pressed using a small tabletop roll press (manufactured by Tester Sangyo Co., Ltd., SA-602) under conditions of 5 kN and a roll peripheral speed of 50 m / min, resulting in a basis weight of 36.9 g / m². 2 A negative electrode plate (a) with an effective discharge capacity of 2100 mAh / g was obtained.

[0078] (Fabrication of a coin-type non-aqueous electrolyte secondary battery (a)) The obtained negative electrode plate 1 and LiCoO2 positive electrode plate (manufactured by NEI, basis weight 110.2 g / m²) 2 The negative and positive electrodes, each with an effective discharge capacity of 145 mAh / g, were punched out into a circle with a diameter of 16 mm, and then vacuum-dried at 120°C for 12 hours.

[0079] Similarly, a separator (20μm thick polypropylene separator manufactured by CS Tech) was punched out to form a circle with a diameter of 17mm, and then vacuum-dried at 60°C for 12 hours.

[0080] Next, a negative electrode plate 1 was placed in a 20.0 mm diameter stainless steel circular dish-shaped container, followed by the stacking of a separator, positive electrode plate, spacer (15.5 mm in diameter, 1 mm thick), and stainless steel washer (manufactured by Hosen Co., Ltd.) in that order. Then, 300 μL of electrolyte (1 mol / L LiPF6, with a volume ratio of ethylene carbonate to diethyl carbonate of 1:1) was added to the circular dish-shaped container. A stainless steel cap was then placed over this container via a polypropylene packing, and it was sealed using a coin cell crimping machine (manufactured by Hosen Co., Ltd.) to obtain a coin-type non-aqueous electrolyte secondary battery (a).

[0081] <Example 2> (Fabrication of electrode binder CMC (b), negative electrode plate (b), and non-aqueous electrolyte secondary battery (b)) An electrode binder CMC(b) with a viscosity of 3,300 mPa·s and a DS of 0.70 was prepared in the same manner as in Example 1, except that a sodium salt of carboxymethylcellulose with a 1% by mass viscosity of 3,140 mPa·s and a DS of 0.70 was used. Subsequently, a negative electrode plate (b) and a coin-type non-aqueous electrolyte secondary battery (b) were prepared in the same manner as in Example 1, except that the electrode binder CMC(b) was used as the electrode binder.

[0082] <Example 3> (Fabrication of electrode binder CMC(c), negative electrode plate(c), and non-aqueous electrolyte secondary battery(c)) A sodium carboxymethylcellulose salt with a 1% by mass viscosity of 7,900 mPa·s and a DS of 0.70 was dispersed in ultrapure water to a concentration of 1% by mass. Lithium molybdate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to adjust the pH to 8.0-9.0 to obtain an electrode binder CMC(c) with a viscosity of 4,300 mPa·s and a DS of 0.70. Subsequently, a negative electrode plate (c) and a coin-type non-aqueous electrolyte secondary battery (c) were fabricated in the same manner as in Example 1, except that the electrode binder CMC(c) was used as the electrode binder.

[0083] <Comparative Example 1> (Fabrication of electrode binder CMC (d), negative electrode plate (d), and non-aqueous electrolyte secondary battery (d)) A sodium carboxymethylcellulose salt with a 1% by mass viscosity of 7,900 mPa·s and a DS of 0.70 was dispersed in ultrapure water to a concentration of 1% by mass of sodium carboxymethylcellulose, and this aqueous dispersant was used as the electrode binder CMC(d). Subsequently, a negative electrode plate (d) and a coin-type non-aqueous electrolyte secondary battery (d) were fabricated in the same manner as in Example 1, except that the electrode binder CMC(d) was used.

[0084] <Comparative Example 2> (Fabrication of electrode binder CMC(e), negative electrode plate(e), and non-aqueous electrolyte secondary battery(e)) A sodium carboxymethylcellulose salt with a 1% by mass viscosity of 4,900 mPa·s and a DS of 0.65 was dispersed in ultrapure water to a concentration of 1% by mass of sodium carboxymethylcellulose, and this aqueous dispersant was used as the electrode binder CMC(e). Subsequently, a negative electrode plate (e) and a coin-type non-aqueous electrolyte secondary battery (e) were fabricated in the same manner as in Example 1, except that the electrode binder CMC(e) was used.

[0085] <Comparative Example 3> (Fabrication of electrode binder CMC(f), negative electrode plate(f), and non-aqueous electrolyte secondary battery(f)) Except for adjusting the pH with lysine instead of sodium hydroxide, the procedure was the same as in Example 2 to obtain an electrode binder CMC(f) with a viscosity of 2,800 mPa·s and a DS of 0.70. Subsequently, a negative electrode plate (f) and a coin-type non-aqueous electrolyte secondary battery (f) were fabricated in the same manner as in Example 1, except that the electrode binder CMC(f) was used as the electrode binder.

[0086] [Table 1]

[0087] As shown in Table 1, a binder for non-aqueous electrolyte secondary battery electrodes containing carboxymethylcellulose or a salt thereof with a carboxymethyl substitution degree of 0.5 to 1.3 per anhydrous glucose unit, and an alkaline compound containing an alkali metal, wherein the viscosity of a 1% by mass aqueous solution measured at 25°C with a B-type viscometer (30 rpm) is 1,000 mPa·s or more and 12,000 mPa·s or less, and the pH at 25°C is 8.0 or more and less than 9.5, exhibits excellent viscosity retention, electrodes obtained using electrode compositions with this binder exhibit excellent adhesion strength, and non-aqueous electrolyte secondary batteries obtained from these electrode compositions exhibit excellent 3C discharge capacity.

Claims

1. A binder for non-aqueous electrolyte secondary battery electrodes, comprising carboxymethylcellulose or a salt thereof having a carboxymethyl substitution degree of 0.5 to 1.3 per anhydrous glucose unit, and an alkaline compound containing an alkali metal, wherein the viscosity of a 1% by mass aqueous solution measured at 25°C with a B-type viscometer (30 rpm) is 1,000 mPa·s or more and 12,000 mPa·s or less, and the pH at 25°C is 8.0 or more and less than 9.

5.

2. An electrode composition for a non-aqueous electrolyte secondary battery, comprising the binder for non-aqueous electrolyte secondary battery electrodes described in claim 1.

3. The electrode composition for a non-aqueous electrolyte secondary battery according to claim 2, wherein the electrode active material contains 5% by mass or more of a silicon-based compound.

4. An electrode for a non-aqueous electrolyte secondary battery, using the electrode composition for a non-aqueous electrolyte secondary battery described in claim 3.

5. A non-aqueous electrolyte secondary battery using the electrode composition for a non-aqueous electrolyte secondary battery described in claim 3.

Images