Carboxymethyl cellulose lithium salt, method for producing carboxymethyl cellulose lithium salt, carboxymethyl cellulose nanofiber lithium salt, electrode composition for non-aqueous electrolyte secondary battery, electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

To provide a carboxymethylcellulose lithium salt which gives a high-viscosity carboxymethylcellulose nanofiber dispersion capable of giving a nonaqueous electrolyte secondary battery excellent in battery characteristics such as initial capacity when used as a binder for an electrode of the nonaqueous electrolyte secondary battery.SOLUTION: A carboxymethyl cellulose lithium salt having a carboxymethyl substitution degree per anhydrous glucose unit of 0.2 or more and less than 0.5, wherein a viscosity of a 1% by mass aqueous dispersion of the carboxymethyl cellulose lithium salt measured at 25 °C with a B-type viscosimeter is 200mPa·s or less, and an amount of lithium ions contained in the carboxymethyl cellulose lithium salt 1g is 5000 to 20, 000ppm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carboxymethyl cellulose lithium salt, a method for producing the carboxymethyl cellulose lithium salt, a carboxymethyl cellulose nanofiber lithium salt, an electrode composition for a non-aqueous electrolyte secondary battery, an electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, the rapid spread of small portable devices such as smartphones and tablets, as well as stationary storage batteries, has led to an increasing demand for small, high-energy-density batteries to power them.

[0003] 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 (LiC6), which is currently approaching its limit.

[0004] 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, etc., which have the second lowest potential and large specific capacity after carbon and lithium, are attracting attention.

[0005] Among these materials, silicon can absorb up to 4.4 lithium atoms per 1 silicon atom in molar ratio, theoretically providing approximately 10 times the capacity of graphite-based carbon materials. However, silicon particles expand in volume approximately three to four times when they absorb lithium, leading to degradation and capacity loss with repeated charge-discharge cycles. Detailed analysis of this phenomenon has revealed that when lithium is inserted into silicon-containing active materials, the volume expansion causes microcracks to form within the electrode, allowing the electrolyte to penetrate these microcracks and form a new coating (SEI layer). This creates irreversible capacity loss, resulting in a decrease in battery capacity. This phenomenon manifests itself in changes in charge-discharge efficiency during cycling. The decrease in cycle efficiency, especially during the early stages of cycling when volume change is large, significantly impacts the battery's lifespan when combined with a positive electrode with high charge-discharge efficiency. Therefore, minimizing changes in electrode structure due to this volume expansion is a key challenge when using silicon-containing active materials.

[0006] In view 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 of Patent Document 1, when only two components (carboxymethyl cellulose and styrene-butadiene rubber, or carboxymethyl cellulose and polyvinylidene fluoride) were used as the binder, desired battery characteristics were not obtained.

[0007] Furthermore, since the ions involved in charging and discharging in lithium ion secondary batteries are lithium ions, and the presence of other metal ions may degrade battery characteristics, the use of carboxymethylcellulose lithium salt as a metal salt of carboxymethylcellulose has also been considered (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-198038 [Patent Document 2] Japanese Patent Publication No. 2023-8785 Summary of the Invention [Problem to be solved by the invention]

[0009] However, in the manufacturing method of Patent Document 2, sodium hydroxide is used, and therefore sodium ions derived from the manufacturing method remain in the finally obtained carboxymethyl cellulose lithium salt, and there are concerns about the influence on the battery characteristics when used in a lithium ion secondary battery.

[0010] Therefore, an object of the present invention is to provide a lithium carboxymethyl cellulose salt that, when used as an electrode binder for a nonaqueous electrolyte secondary battery, gives a high-viscosity carboxymethyl cellulose nanofiber dispersion that can yield a nonaqueous electrolyte secondary battery with excellent battery characteristics such as initial capacity.Another object of the present invention is to provide a method for producing the lithium carboxymethyl cellulose salt. [Means for solving the problem]

[0011] As a result of extensive research to achieve this object, the present inventors have found that the above problems can be solved by a specific procedure, and have completed the present invention.

[0012] The present invention provides the following: (1) A lithium carboxymethyl cellulose salt having a degree of carboxymethyl substitution per anhydroglucose unit of 0.2 or more and less than 0.5, wherein the viscosity of a 1% by mass aqueous dispersion of the lithium carboxymethyl cellulose salt measured at 25°C with a Brookfield viscometer is 200 mPa·s or less, and the amount of lithium ions contained in 1 g of the lithium carboxymethyl cellulose salt is 5,000 to 20,000 ppm. (2) The carboxymethylcellulose lithium salt according to (1), wherein the crystallinity of the cellulose type I of the carboxymethylcellulose lithium salt is 50% or more. (3) The carboxymethyl cellulose lithium salt according to (1), characterized in that the carboxymethyl cellulose lithium salt does not contain sodium ions. (4) A method for producing a lithium carboxymethyl cellulose salt having a degree of carboxymethyl substitution per anhydroglucose unit of 0.2 or more but less than 0.5, comprising: step (A): treating cellulose with a 12% by mass or less aqueous solution of lithium hydroxide to obtain mercerized cellulose; and step (B): reacting the mercerized cellulose with a carboxymethylating agent in a mixed solvent containing an organic solvent in an amount of 50 to 99% by mass based on the total amount of water and the organic solvent to obtain carboxymethyl cellulose, wherein the viscosity of a 1% by mass aqueous dispersion of the lithium carboxymethyl cellulose lithium salt measured at 25°C with a Brookfield viscometer is approximately 200 mPa s or less, and the amount of lithium ions contained in 1 g of the lithium carboxymethyl cellulose lithium salt is 5,000 to 20,000 ppm. (5) The method for producing a carboxymethyl cellulose lithium salt according to (4), wherein the cellulose I type crystallinity of the carboxymethyl cellulose lithium salt is 50% or more. (6) Carboxymethyl cellulose nanofiber lithium salt having a degree of carboxymethyl substitution per anhydroglucose unit of 0.2 or more and less than 0.5, wherein the viscosity of a 1 mass% aqueous dispersion of the carboxymethyl cellulose nanofiber lithium salt measured at 25°C with a Brookfield viscometer is approximately 10,000 to 100,000 mPa·s, and the amount of lithium ions contained in 1 g of the carboxymethyl cellulose nanofiber lithium salt is 5,000 to 20,000 ppm. (7) An electrode composition for a non-aqueous electrolyte secondary battery, comprising the carboxymethyl cellulose nanofiber lithium salt according to (6). (8) An electrode for a non-aqueous electrolyte secondary battery, which uses the electrode composition for a non-aqueous electrolyte secondary battery according to (7). (9) A non-aqueous electrolyte secondary battery using the electrode composition for a non-aqueous electrolyte secondary battery according to (7). [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a carboxymethyl cellulose lithium salt that, when used as an electrode binder for a nonaqueous electrolyte secondary battery, gives a high-viscosity carboxymethyl cellulose nanofiber dispersion that can yield a nonaqueous electrolyte secondary battery with excellent battery characteristics such as initial capacity. Furthermore, according to the present invention, it is possible to provide a method for producing the carboxymethyl cellulose lithium salt. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below. In the present invention, "to" includes the extreme values. That is, "X to Y" includes the values ​​X and Y at both ends.

[0015] <Carboxymethylcellulose lithium salt> The lithium carboxymethyl cellulose salt of the present invention, having a degree of carboxymethyl substitution per anhydroglucose unit of 0.2 or more and less than 0.5, has a viscosity of 200 mPa·s or less as measured at 25°C with a Brookfield viscometer as a 1% by mass aqueous dispersion of the lithium carboxymethyl cellulose salt, and the amount of lithium ions contained in 1 g of the lithium carboxymethyl cellulose salt is 5,000 to 20,000 ppm.

[0016] The carboxymethyl cellulose lithium salt (hereinafter sometimes abbreviated as "CMC-Li") of the present invention can be produced using cellulose as a raw material. Carboxymethyl cellulose is a cellulose in which carboxymethyl groups are ether-bonded to some of the hydroxyl groups of glucose residues, and in the present application, it takes the form of a lithium salt.

[0017] The carboxymethyl cellulose produced by the present invention preferably maintains at least a part of its fibrous shape when dispersed in water, i.e., when an aqueous dispersion of carboxymethyl cellulose is observed under an electron microscope, a fibrous substance can be observed, and when the carboxymethyl cellulose is measured by X-ray diffraction, a peak of cellulose type I crystals can be observed.

[0018] The CMC-Li of the present invention has a degree of carboxymethyl substitution (hereinafter sometimes referred to as the "DS value") per anhydroglucose unit of cellulose of 0.2 or more and less than 0.5. From the viewpoint of viscosity after defibration, the DS value is 0.2 or more. Furthermore, from the viewpoint of maintaining at least a portion of the fibrous shape even when dispersed in water, the DS value is less than 0.5, preferably 0.4 or less. Therefore, the DS value of the CMC-Li obtained by the present invention is 0.2 or more and less than 0.5, preferably 0.2 to 0.4, and more preferably 0.25 to 0.4. By introducing carboxymethyl groups into cellulose, the cellulose molecules electrically repel each other, making it possible to defibrate into nanofibers. However, if the degree of carboxymethyl substitution is less than 0.2, there is a risk that defibration will not be possible due to the small amount of electronic repulsion. If the degree of substitution is 0.5 or more, the cellulose will be more likely to dissolve in water, making it difficult to maintain the fibrous form and making it difficult to defibrate into nanofibers. There is also a risk that a high viscosity will not be obtained after defibration. The degree of carboxymethyl substitution can be adjusted by controlling the amount of the carboxymethylating agent to be reacted, the amount of the mercerizing agent, and the composition ratio of water to the organic solvent.

[0019] In the present invention, the term "anhydroglucose unit" refers to each anhydroglucose (glucose residue) constituting cellulose. The term "carboxymethyl substitution degree (also referred to as "etherification degree") refers to the proportion of hydroxyl groups in the glucose residues constituting cellulose that have been substituted with carboxymethyl ether groups (the number of carboxymethyl ether groups per glucose residue).

[0020] The degree of carboxymethyl substitution is measured as follows: Weigh out approximately 2.0 g of sample and place it in a 300 mL Erlenmeyer flask with a stopper. Add 100 mL of a solution of 1000 mL of nitric acid methanol and 100 mL of special-grade concentrated nitric acid, and shake for 3 hours to convert carboxymethyl cellulose salt (CMC) to H-CMC (hydrogen-form carboxymethyl cellulose). Weigh out 1.5 to 2.0 g of the bone-dry H-CMC and place it in a 300 mL Erlenmeyer flask with a stopper. Wet the H-CMC with 15 mL of 80% methanol, add 100 mL of 0.1 N NaOH, and shake for 3 hours at room temperature. Using phenolphthalein as an indicator, back-titrate the excess NaOH with 0.1 N H2SO4, and calculate the degree of carboxymethyl substitution (CM-DS) using the following formula: A = [(100 × F - 0.1N-H2SO4 (mL) × F') × 0.1] / (bone-dry mass of H-CMC (g)) Carboxymethyl substitution degree (CM-DS) = 0.162 × A / (1-0.058 × A) A: Amount of 1N NaOH (mL) required to neutralize 1 g of H-CMC F': Factor of 0.1N-H2SO4 F: Factor of 0.1N NaOH.

[0021] (Viscosity of CMC-Li) Furthermore, from the viewpoint of workability during defibration, the viscosity of a 1% by mass aqueous dispersion of the CMC-Li obtained in this manner, measured at 25°C with a Brookfield viscometer (30 rpm), is 200 mPa s or less, more preferably 1 to 100 mPa s, and even more preferably 1 to 50 mPa s. If this viscosity is too high, air entrapment is likely to occur during defibration, which places a strain on the equipment.

[0022] The viscosity was measured as follows: CMC-Li was weighed into a 1,000 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. The viscosity was then measured after 3 minutes at 30 rpm using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.) according to the method of JIS-Z-8803.

[0023] (Lithium ion content) The amount of lithium ions contained per gram of the carboxymethyl cellulose lithium salt of the present invention is 5,000 to 20,000 ppm, preferably 5,000 to 15,000 ppm, and more preferably 7,000 to 15,000 ppm, from the viewpoint of maintaining discharge capacity when used in a nonaqueous electrolyte secondary battery. If the amount of lithium ions is too low below the lower limit, the DS will be less than 0.2, which may result in low viscosity after defibration. If the amount of lithium ions is too high above the upper limit, the DS will exceed 0.50, which may result in easy dissolution in water, making it difficult to maintain the fibrous form and making defibration into nanofibers difficult. The amount of lithium ions can be adjusted by the amount of mercerizing agent used.

[0024] The carboxymethyl cellulose lithium salt produced by the present invention preferably has a sodium ion content per gram of 3,000 ppm or less, more preferably 2,000 ppm or less, and even more preferably 1,000 ppm or less, from the viewpoint of discharge capacity when used in a nonaqueous electrolyte secondary battery. It is particularly preferable that the carboxymethyl cellulose lithium salt does not contain sodium ions, from the viewpoint of discharge capacity when used in a nonaqueous electrolyte secondary battery.

[0025] (crystallinity) The degree of crystallinity of the cellulose in the CMC-Li of the present invention is preferably 50% or more, more preferably 60% or more, of crystalline type I, from the viewpoint of being able to develop high viscosity after defibration. The crystallinity of cellulose can be controlled by the concentration of the mercerizing agent, the treatment temperature, and the degree of carboxymethylation. Since high concentrations of alkali are used in mercerization and carboxymethylation, type I cellulose crystals are likely to be converted to type II, but the desired crystallinity can be maintained by adjusting the degree of modification, for example by adjusting the amount of alkali (mercerizing agent) used. There are no particular limitations on the upper limit of the crystallinity of type I cellulose. In reality, the upper limit is thought to be around 90%.

[0026] The method for measuring the crystallinity of cellulose type I of CMC-Li is as follows: The sample was placed in a glass cell and measured using an X-ray diffraction measurement device (LabX XRD-6000, Shimadzu Corporation). The crystallinity was calculated using the method of Segal et al., where the diffraction intensity at 2θ = 10° to 30° in the X-ray diffraction pattern was used as the baseline, and the crystallinity was calculated using the diffraction intensity of the 002 plane at 2θ = 22.6° and the diffraction intensity of the amorphous part at 2θ = 18.5° using the following formula.

[0027] Xc=(I002c-Ia) / I002c×100 Xc = Crystallinity of cellulose type I (%) I002c: 2θ=22.6°, diffraction intensity of the 002 plane Ia: 2θ=18.5°, diffraction intensity of the amorphous part.

[0028] <Method of manufacturing carboxymethyl cellulose lithium salt> The method of the present invention for producing a lithium carboxymethyl cellulose salt having a degree of carboxymethyl substitution per anhydroglucose unit of 0.2 or more but less than 0.5 comprises the steps of: (A) treating cellulose with a 12% by mass or less aqueous solution of lithium hydroxide to obtain mercerized cellulose; and (B) reacting the mercerized cellulose with a carboxymethylating agent in a mixed solvent containing 50 to 99% by mass of an organic solvent relative to the total mass of water and the organic solvent to obtain carboxymethyl cellulose.

[0029] (Process (A)) In step (A), cellulose is treated with a 12% by mass or less aqueous solution of lithium hydroxide to obtain mercerized cellulose. This step is a mercerization step, and in the present invention, lithium hydroxide is used as a mercerizing agent.

[0030] (cellulose) In the present invention, cellulose refers to a polysaccharide having a structure in which D-glucopyranose (also simply referred to as "glucose residue" or "anhydroglucose") units are linked together via β-1,4 bonds. Cellulose is generally classified into native cellulose, regenerated cellulose, fine cellulose, microcrystalline cellulose (which is cellulose obtained by removing the amorphous region), etc., based on its origin and production method. In the present invention, any of these celluloses can be used as a raw material for mercerized cellulose.

[0031] Examples of natural cellulose include bleached pulp or unbleached pulp (bleached wood pulp or unbleached wood pulp); linters, purified linters; and cellulose produced by microorganisms such as acetic acid bacteria. The raw materials for bleached pulp or unbleached pulp are not particularly limited, and examples include wood, cotton, straw, bamboo, hemp, jute, and kenaf. The method for producing bleached pulp or unbleached pulp is also not particularly limited, and may be a mechanical method, a chemical method, or a method that is an intermediate combination of the two. Examples of bleached or unbleached pulp classified by production method include mechanical pulp (thermomechanical pulp (TMP), groundwood pulp), chemical pulp (sulfite pulp such as softwood unbleached sulfite pulp (NUSP) and softwood bleached sulfite pulp (NBSP), and kraft pulp such as softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), and hardwood bleached kraft pulp (LBKP)). In addition to papermaking pulp, dissolving pulp may also be used. Dissolving pulp is chemically refined pulp that is primarily dissolved in chemicals before use and is the main raw material for artificial fibers, cellophane, etc.

[0032] Examples of regenerated cellulose include cellulose dissolved in a certain solvent such as a cuprammonium solution, a cellulose xanthate solution, or a morpholine derivative, and then spun again. Examples of fine cellulose include those obtained by depolymerizing cellulosic materials, including the above-mentioned natural cellulose and regenerated cellulose (e.g., acid hydrolysis, alkaline hydrolysis, enzymatic decomposition, crushing treatment, vibrating ball mill treatment, etc.), and those obtained by mechanically treating the above-mentioned cellulosic materials.

[0033] (mercerization) Mercerized cellulose (also called alkali cellulose) is obtained by adding a mercerizing agent (alkali) to the above-mentioned cellulose as a raw material. In the present invention, lithium hydroxide is used as the mercerizing agent, and water is used as the solvent in the mercerization reaction. A mixed solvent of an organic solvent and water is used in the subsequent carboxymethylation, making it possible to economically obtain CMC-Li having a desired degree of carboxymethyl substitution.

[0034] In order to completely dissolve lithium hydroxide (mercerizing agent) in the solvent and to carry out a uniform mercerization reaction, the solvent used in mercerization must be 100% by mass of water, but it may contain up to 49% by mass of an organic solvent other than water as long as it does not impair the effects of the present invention.

[0035] Examples of solvents other than water (used in combination with water) used in the mercerization process include organic solvents used as solvents in the subsequent carboxymethylation step. Examples include alcohols such as methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, and tertiary butanol; ketones such as acetone, diethyl ketone, and methyl ethyl ketone; and dioxane, diethyl ether, benzene, and dichloromethane. These solvents, or mixtures of two or more of these, can be added to water in an amount of 49% by mass or less and used as the solvent for mercerization, as long as the effects of the present invention are not impaired. The organic solvent content in the mercerization reaction is preferably 30% by mass or less, and more preferably 10% by mass or less. The lower limit of the organic solvent content is not limited and may be 0% by mass.

[0036] In the present invention, it is necessary to use lithium hydroxide as the mercerizing agent. The mercerizing agent can be added to the reactor as an aqueous solution of 12 mass % or less, preferably 1 to 12 mass %, more preferably 3 to 6 mass %. If the concentration of the lithium hydroxide aqueous solution is too high above the upper limit, lithium hydroxide (mercerizing agent) will precipitate, which may increase the amount of by-products.

[0037] The amount of mercerizing agent used is not particularly limited as long as it allows carboxymethyl cellulose to achieve the desired degree of carboxymethyl substitution, but it is preferable to use an amount that allows the production of carboxymethyl cellulose having a degree of carboxymethyl substitution of 0.2 or more and less than 0.5. In one embodiment, the amount of mercerizing agent used is preferably 0.1 mol or more and 10.0 mol or less, more preferably 0.5 mol or more and 5.0 mol or less, and even more preferably 0.6 mol or more and 2.5 mol or less, per 100 g (bone dry) of cellulose.

[0038] The amount of solvent (water) used during mercerization is not particularly limited as long as it allows stirring and mixing of the raw materials, but is preferably 1.5 to 20 times by mass, more preferably 2 to 10 times by mass, relative to the cellulose raw material.

[0039] The mercerization treatment is preferably carried out by mixing the raw material (cellulose) with a solution of a mercerizing agent (aqueous lithium hydroxide solution) and adjusting the temperature of the reactor to less than 50°C. By keeping the temperature below 50°C, the reaction is more likely to proceed uniformly, making it possible to produce carboxymethyl cellulose with the desired viscosity. Furthermore, side reactions can be suppressed, thereby improving the effective utilization rate of the carboxymethylating agent described below. The temperature of the reactor is preferably from 0°C to less than 50°C, more preferably from 10°C to less than 50°C, and more preferably from 10°C to 40°C.

[0040] The raw material (cellulose) is added to a reactor whose temperature has been adjusted as described above, and an aqueous solution of a mercerizing agent (lithium hydroxide) is added. Mercerization is carried out by stirring for 15 minutes to 8 hours, preferably 30 minutes to 7 hours, and more preferably 30 minutes to 3 hours. Mercerized cellulose (alkali cellulose) is thus obtained.

[0041] The pH during mercerization is preferably 9 or higher, which allows the mercerization reaction to proceed. The pH is more preferably 11 or higher, even more preferably 12 or higher, and may be 13 or higher. There is no particular upper limit to the pH.

[0042] Mercerization can be carried out using a reactor capable of mixing and stirring the above components while controlling the temperature, and various reactors conventionally used for mercerization reactions can be used. For example, a batch-type stirring device having two shafts for stirring and mixing the above components is preferred from the viewpoints of both uniform mixing and productivity.

[0043] (Process (B)) The production method of the present invention includes step (B): reacting the mercerized cellulose with a carboxymethylating agent in a mixed solvent containing 50 to 99 mass% of an organic solvent based on the total mass of water and the organic solvent to obtain carboxymethyl cellulose.

[0044] (Carboxymethylation) Carboxymethyl cellulose is obtained by adding a carboxymethylating agent (also called an etherifying agent) to mercerized cellulose. In the present invention, a mixed solvent of water and an organic solvent is used as the solvent in this carboxymethylation reaction. By using a 100% water solvent during mercerization and a mixed solvent of water and an organic solvent during carboxymethylation, it is possible to obtain carboxymethyl cellulose in which the carboxymethyl groups are uniformly substituted.

[0045] Examples of the carboxymethylating agent include monochloroacetic acid, sodium monochloroacetate, methyl monochloroacetate, ethyl monochloroacetate, and isopropyl monochloroacetate. Of these, monochloroacetic acid is more preferred from the viewpoints of easy availability of the raw material and the fact that it does not contain sodium ions.

[0046] The amount of the carboxymethylating agent used is not particularly limited as long as it achieves the desired degree of carboxymethyl substitution in carboxymethyl cellulose. However, it is preferable to use an amount that enables the production of carboxymethyl cellulose having a crystallinity of 50% or more and a degree of carboxymethyl substitution of 0.2 or more but less than 0.5, as described below. When the crystallinity of cellulose type I is 50% or more and the degree of carboxymethyl substitution is less than 0.5, the fibrous shape of carboxymethyl cellulose is maintained and it is easily defibrated into cellulose nanofibers. In one embodiment, the amount of the carboxymethylating agent used is preferably in the range of 0.5 to 1.5 mol per anhydroglucose unit of cellulose. The lower limit of this range is more preferably 0.6 mol or more, even more preferably 0.7 mol or more, and the upper limit is more preferably 1.3 mol or less, even more preferably 1.1 mol or less. The carboxymethylating agent can be added to the reactor as, for example, a 5 to 80 mass % aqueous solution, more preferably 30 to 60 mass % aqueous solution, or it can be added undissolved in powder form.

[0047] When monochloroacetic acid is used as the carboxymethylating agent, the molar ratio of the mercerizing agent (lithium hydroxide) to the carboxymethylating agent (mercerizing agent / carboxymethylating agent) is generally set to 0.9 to 2.45. This is because if the ratio is less than 0.9, the carboxymethylation reaction may be insufficient, resulting in unreacted monochloroacetic acid remaining and wasting, and if the ratio exceeds 2.45, a side reaction between the excess mercerizing agent and monochloroacetic acid may proceed, resulting in the production of an alkali metal glycolate, which may be uneconomical.

[0048] In the present invention, the effective utilization rate of the carboxymethylating agent is preferably 15% or higher. It is more preferably 20% or higher, even more preferably 25% or higher, and particularly preferably 30% or higher. The effective utilization rate of the carboxymethylating agent refers to the proportion of carboxymethyl groups introduced into cellulose among the carboxymethyl groups in the carboxymethylating agent. In the present invention, by using water as a solvent during mercerization and a mixed solvent of water and an organic solvent during carboxymethylation, it is possible to produce a lithium carboxymethylcellulose salt that can produce a cellulose nanofiber dispersion with a desired degree of carboxymethyl substitution with a high effective utilization rate of the carboxymethylating agent (i.e., economically, without significantly increasing the amount of carboxymethylating agent used). There are no particular limitations on the upper limit of the effective utilization rate of the carboxymethylating agent, but in practice, it is approximately 80%.

[0049] The effective utilization rate of the carboxymethylating agent is calculated as follows: Effective utilization rate = (DS × moles of cellulose) / moles of carboxymethylating agent DS: Degree of carboxymethyl substitution Number of moles of cellulose: Pulp mass (dry mass after drying at 100°C for 60 minutes) / 162 (162 is the molecular weight per glucose unit of cellulose).

[0050] The concentration of the cellulose raw material in the carboxymethylation reaction is not particularly limited, but is preferably 1 to 40% (w / v) from the viewpoint of effective utilization of the carboxymethylating agent.

[0051] Simultaneously with the addition of the carboxymethylating agent, or before or immediately after the addition of the carboxymethylating agent, an organic solvent or an aqueous solution of an organic solvent is appropriately added to the reactor to form a mixed solvent of water and an organic solvent. In the present invention, the carboxymethylation reaction is allowed to proceed in this mixed solvent of water and an organic solvent. The timing of the addition of the organic solvent is not particularly limited as long as it is between the end of the mercerization reaction and immediately after the addition of the carboxymethylating agent, but is preferably within 30 minutes before or after the addition of the carboxymethylating agent.

[0052] Examples of organic solvents include alcohols such as methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, and tertiary butanol; ketones such as acetone, diethyl ketone, and methyl ethyl ketone; and dioxane, diethyl ether, benzene, and dichloromethane. These can be used alone or in combination with water to form a solvent for carboxymethylation. Among these, monohydric alcohols having 1 to 4 carbon atoms are preferred, and monohydric alcohols having 1 to 3 carbon atoms are more preferred, due to their excellent compatibility with water.

[0053] The proportion of organic solvent in the mixed solvent during carboxymethylation is 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more, of the total of water and organic solvent. The higher the proportion of organic solvent, the more uniformly substituted the carboxymethyl groups are in the carboxymethyl cellulose. The upper limit of the proportion of organic solvent is 99% by mass or less. Considering the cost of the organic solvent to be added, the proportion is preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less, and even more preferably 80% by mass or less.

[0054] The present invention is characterized in that water is used as a solvent during mercerization and a mixed solvent containing 50 to 99% by mass of the organic solvent relative to the total mass of water and organic solvent is used during carboxymethylation. However, using water as a solvent during carboxymethylation also makes it difficult to obtain carboxymethyl cellulose having a degree of carboxymethyl substitution of 0.2 or more but less than 0.5 and a crystallinity of cellulose type I of 50% or more. Specifically, when attempting to achieve a degree of carboxymethyl substitution of 0.2 or more using water as a solvent during carboxymethylation, the crystallinity of cellulose type I tends to decrease to less than 50%. The method of the present invention makes it possible to produce carboxymethyl cellulose having a degree of carboxymethyl substitution of 0.2 or more but less than 0.5 and a crystallinity of cellulose type I of 50% or more. Furthermore, nonaqueous electrolyte secondary batteries produced using the resulting CMC-Li have the advantage of having a large initial capacity.

[0055] A mixed solvent of water and an organic solvent is prepared, and the carboxymethylating agent is added to the mercerized cellulose. The mixture is stirred for 15 minutes to 4 hours, preferably 15 minutes to 1 hour, while maintaining a constant temperature, preferably in the range of 10 to 40°C. Mixing of the mercerized cellulose-containing liquid with the carboxymethylating agent is preferably carried out in multiple batches or by dropwise addition to prevent the reaction mixture from becoming too hot. After adding the carboxymethylating agent and stirring for a certain period of time, the temperature is raised, if necessary, to 30 to 90°C, preferably 40 to 90°C, more preferably 60 to 80°C, and the etherification (carboxymethylation) reaction is carried out for 30 minutes to 10 hours, preferably 1 to 4 hours, to obtain the lithium salt of carboxymethyl cellulose (CMC-Li).

[0056] In the carboxymethylation, the reactor used in the mercerization may be used as it is, or a separate reactor may be used which is capable of mixing and stirring the above components while controlling the temperature.

[0057] After the reaction is complete, the remaining alkali metal salt may be neutralized with a mineral acid or an organic acid. If necessary, by-products such as inorganic salts and organic acid salts may be removed by washing with aqueous methanol, followed by drying, pulverization, and classification to obtain CMC-Li. Examples of equipment used in dry pulverization include impact mills such as hammer mills and pin mills, media mills such as ball mills and tower mills, and jet mills. Examples of equipment used in wet pulverization include homogenizers, mass colloiders, and pearl mills.

[0058] <Production of carboxymethyl cellulose nanofiber lithium salt> The carboxymethyl cellulose lithium salt obtained by the method of the present invention can be converted into cellulose nanofibers with nanoscale fiber diameters by defibrating it. The carboxymethyl cellulose nanofibers obtained by the method of the present invention (hereinafter sometimes abbreviated as "CM-CNF") do not contain sodium ions and have high viscosity in the state of a 1% by mass aqueous dispersion.

[0059] For defibration, a dispersion of the carboxymethyl cellulose lithium salt obtained by the above method is prepared. The dispersion medium is preferably water for ease of handling. The concentration of carboxymethyl cellulose is preferably 0.01 to 10% (w / v), taking into account the efficiency of defibration and dispersion.

[0060] The device used to defibrate carboxymethyl cellulose is not particularly limited, but devices such as high-speed rotation, colloid mill, high-pressure, roll mill, and ultrasonic devices can be used. It is preferable to apply a strong shear force to the carboxymethyl cellulose dispersion during defibration. In particular, for efficient defibration, it is preferable to apply a pressure of 50 MPa or more to the dispersion and to use a wet high-pressure or ultra-high-pressure homogenizer capable of applying a strong shear force. The pressure is more preferably 100 MPa or more, and even more preferably 140 MPa or more. Furthermore, prior to defibration and dispersion treatment using a high-pressure homogenizer, the dispersion may be pre-treated, if necessary, using a known mixing, stirring, emulsifying, or dispersing device such as a high-speed shear mixer.

[0061] A high-pressure homogenizer is a device that uses a pump to pressurize (high pressure) a fluid and eject it from an extremely fine gap in the flow path, thereby emulsifying, dispersing, breaking down, pulverizing, and ultra-fine-graining particles through the combined energy of collisions between particles and shear forces caused by pressure differences.

[0062] By defibrating the carboxymethyl cellulose, it is possible to obtain CM-CNF having an average fiber diameter of 3 to 500 nm and an aspect ratio of 50 or more. The average fiber diameter is preferably 3 to 150 nm, more preferably 3 to 20 nm, even more preferably 5 to 19 nm, and even more preferably 5 to 15 nm.

[0063] The average fiber diameter and average fiber length of carboxymethyl cellulose or carboxymethyl cellulose nanofibers can be measured by analyzing 200 randomly selected fibers using an atomic force microscope (AFM) if the diameter is 20 nm or less, or a field emission scanning electron microscope (FE-SEM) if the diameter is 20 nm or more, and calculating the average. The aspect ratio can be calculated using the following formula: Aspect ratio = average fiber length / average fiber diameter.

[0064] The degree of carboxymethyl substitution in carboxymethyl cellulose nanofibers is usually the same as the degree of carboxymethyl substitution in the carboxymethyl cellulose before being made into nanofibers, and the proportion of type I crystals in the carboxymethyl cellulose nanofibers is usually the same as that in the carboxymethyl cellulose before being made into nanofibers.

[0065] By defibrating the carboxymethyl cellulose lithium salt obtained by the method of the present invention into nanofibers, a dispersion of high-viscosity carboxymethyl cellulose nanofiber lithium salt (hereinafter sometimes abbreviated as "CM-CNF-Li") can be obtained. The viscosity of the CM-CNF thus obtained, for example, of an aqueous dispersion with a solids content of 1% (w / v) measured using a Brookfield viscometer (at 25°C and 6 rpm for 3 minutes) is preferably 10,000 to 100,000 mPa·s, more preferably 15,000 to 50,000 mPa·s, and even more preferably 20,000 to 40,000 mPa·s, from the viewpoint of achieving both adequate coating properties and adhesion strength when used as an electrode binder for nonaqueous electrolyte secondary batteries. By using the CMC-Li of the present invention, it is possible to produce cellulose nanofibers that are free of sodium ions and have high viscosity.

[0066] However, if the defibration strength is too high, nano-size advances but the viscosity decreases, which is undesirable for use as an electrode binder. To achieve the above preferred viscosity range, for example, a high-pressure homogenizer is used, and a pressure of preferably 50 to 250 MPa, more preferably 100 to 200 MPa, is applied to a 0.1 to 2.0 mass% aqueous dispersion, and the treatment is preferably 1 to 5 times, more preferably 1 to 3 times.

[0067] (Binder for non-aqueous electrolyte secondary batteries) The CMC-Li obtained by the production method of the present invention can be used as an electrode binder for non-aqueous electrolyte secondary batteries in the form of cellulose nanofibers obtained by subjecting an aqueous dispersion of this CMC-Li to a defibration treatment. Typically, an aqueous dispersion containing CM-CNF-Li is used as an electrode binder for non-aqueous electrolyte secondary batteries. The concentration of CM-CNF-Li in the aqueous dispersion of CM-CNF-Li is typically 0.1 to 10% by mass, preferably 0.2 to 4% by mass, and more preferably 0.5 to 2% by mass.

[0068] If the CMC-Li having a DS value of 0.2 or more and less than 0.5 obtained by the production method of the present invention is used as a binder for a nonaqueous electrolyte secondary battery without being defibrated, the viscosity may be too low, which may result in poor coatability and adhesive strength.In addition, since the crystallinity is high within the above DS value range, lumpy portions will remain, which may become resistance components when the battery is made, which is not preferable.

[0069] In the present invention, not only cellulose fine fibers obtained by subjecting CMC-Li to a strong defibration treatment and completely converting them into nano-sized particles, but also cellulose fine fibers obtained by subjecting CMC-Li to a weak defibration treatment and partially converting them into nano-sized particles are referred to as cellulose nanofibers.

[0070] There are no particular restrictions on the conditions for producing the aqueous dispersion of CM-CNF-Li. For example, it can be prepared by adding CM-CNF-Li to water (e.g., distilled water, purified water, tap water, etc.) and dispersing it by stirring as necessary.

[0071] In addition, binders for nonaqueous electrolyte secondary batteries may include other binders in addition to CM-CNF-Li. Examples of binders used in electrode compositions for negative electrodes include carboxymethyl cellulose, carboxymethyl cellulose salts (excluding lithium salts), and 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).

[0072] <Electrode composition for nonaqueous electrolyte secondary batteries> The electrode composition for a non-aqueous electrolyte secondary battery (hereinafter, sometimes referred to as "electrode composition") of the present invention uses the CM-CNF-Li obtained as described above as a binder for a non-aqueous electrolyte secondary battery. For example, it can be obtained by mixing at least CM-CNF-Li, an electrode active material, and an aqueous solvent.

[0073] That is, the above-mentioned CM-CNF-Li can be used as an electrode binder to constitute an electrode composition together with an electrode active material and an aqueous solvent. In this case, the content of CM-CNF-Li in the electrode composition is preferably 0.1 to 4.0 mass% in terms of solid content with respect to the total mass of the electrode composition.

[0074] When the other binders are used, the content of the binder for a non-aqueous electrolyte secondary battery in the electrode composition is preferably 1 to 10 mass %, more preferably 1 to 6 mass %, and even more preferably 1 to 2 mass %.

[0075] (electrode active material) The electrode active material contained in the electrode composition for a non-aqueous electrolyte secondary battery is a negative electrode active material when the electrode for a non-aqueous electrolyte secondary battery is a negative electrode, and is a positive electrode active material when the electrode is a positive electrode.

[0076] As the negative electrode active material, graphite materials such as graphite (natural graphite, artificial graphite, etc.), coke, and carbon fiber; elements capable of forming an alloy with lithium, that is, for example, elements such as silicon-based compounds, Al, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Ti, etc.; compounds containing elements capable of forming an alloy with lithium; composites of elements capable of forming an alloy with lithium and the above compounds with carbon and / or the graphite material, or nitrides containing lithium, etc. can be exemplified. Among these, graphite materials and silicon-based compounds are preferred, and silicon particles or silicon oxide particles as the graphite and silicon-based compounds are more preferred.

[0077] 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.

[0078] 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:silicon-based compound is preferably 10:90 to 90:10, and more preferably 50:50 to 80:20.

[0079] 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.

[0080] 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, further preferably 95 to 99% by mass, particularly preferably 96 to 99% by weight, and most preferably 98 to 99% by mass.

[0081] The electrode composition may also contain a conductive aid as needed. Examples of conductive aids 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, and preferably 0.1 to 10% by mass.

[0082] (aqueous solvent) The solvent used in the method for producing an electrode composition is an aqueous solvent. The type of aqueous solvent is not particularly limited, but is preferably water, a water-soluble organic solvent, or a mixed solvent thereof, and more preferably water.

[0083] The water-soluble organic solvent is an organic solvent that dissolves in water. Examples of water-soluble organic solvents 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.

[0084] When the mixed solvent is used as the aqueous solvent, the amount of the 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, 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.

[0085] The conditions for producing the electrode composition are not particularly limited. For example, other components constituting the electrode composition are added to an aqueous dispersion of CM-CNF-Li, and the mixture is mixed while 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.

[0086] <Nonaqueous electrolyte secondary battery electrode> 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 achieved by casting the electrode composition onto the current collector using a coating device such as a doctor blade. The lamination method is not limited to the specific example described above, and can also be achieved by discharging the electrode composition 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 (for example, at a temperature of 80 to 120°C, for example, for a heating time of 4 to 12 hours), or by applying pressure using a roll press, as needed.

[0087] The shape of the electrode for a non-aqueous electrolyte secondary battery of the present invention is not particularly limited, but is usually a sheet. The thickness of the sheet-shaped 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 production conditions of the composition, but is usually 30 to 150 μm.

[0088] (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.

[0089] 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.

[0090] <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 therebetween and wound multiple times. For example, a non-aqueous electrolyte secondary battery can be obtained by using the electrode for a non-aqueous electrolyte secondary battery of the present invention as a negative electrode, stacking this negative electrode, a separator, and a positive electrode, and winding the stack multiple times, placing the resulting stack in a battery container, injecting a non-aqueous electrolyte, and sealing the container.

[0091] The shape of the nonaqueous electrolyte secondary battery is not particularly limited, and may be a cylindrical shape, a prismatic shape, a flat shape, a coin shape, a button shape, a sheet shape, etc. The material of the battery container is not particularly limited as long as it can prevent moisture from entering the battery, and examples thereof include a laminate of metal, aluminum, etc.

[0092] The separator is usually impregnated with a non-aqueous electrolyte. The separator may be, for example, a microporous membrane or nonwoven fabric made of polyolefin such as polyethylene or polypropylene.

[0093] The non-aqueous electrolyte typically contains 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. 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.

[0094] The CMC-Li of the present invention has a very low viscosity of 200 mPa·s or less when dispersed as a 1% by mass aqueous dispersion, which reduces the burden on the defibration device when defibrating it. Furthermore, the cellulose nanofibers obtained by defibration have a high viscosity, so when used as an electrode binder for non-aqueous electrolyte secondary batteries, the electrode composition containing this has excellent coatability to the current collector and the resulting electrode layer has excellent adhesion strength to the current collector. Furthermore, the cellulose nanofibers obtained by defibrating the CMC-Li of the present invention do not contain sodium ions, so when used as an electrode binder for non-aqueous electrolyte secondary batteries, non-aqueous electrolyte secondary batteries with excellent initial capacity can be obtained. [Example]

[0095] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0096] <Measurement and evaluation methods> The measurement and evaluation of the various indices for the carboxymethylcellulose salts (lithium carboxymethylcellulose salt or sodium carboxymethylcellulose salt) obtained in the examples and comparative examples were carried out by the following methods.

[0097] <Lithium ion content> The lithium ion content was measured by placing 1 g of carboxymethyl cellulose salt in a crucible, heating it in an electric furnace at 500°C for 1 to 3 hours, and then cooling it to room temperature. After that, 1 mL of 6N hydrochloric acid was added, and the mixture was evaporated on a hot plate at 100°C until the solvent was completely removed. After cooling to room temperature, 1 mL of 6N hydrochloric acid was added again, and the crucible was heated on a hot plate at 50°C for approximately 10 minutes. After cooling to room temperature, ultrapure water was added, and the mixture was transferred to a 50 mL measuring flask, and further ultrapure water was added to bring the volume to 50 mL. The solution thus prepared was measured by ion chromatography.

[0098] <Crystallization degree of cellulose type I> The crystallinity of carboxymethyl cellulose (cellulose type I) was measured as follows: a sample was placed in a glass cell and measured using an X-ray diffraction analyzer (LabX XRD-6000, Shimadzu Corporation). The crystallinity was calculated using the method of Segal et al., using the diffraction intensity at 2θ = 10° to 30° in the X-ray diffraction pattern as the baseline, and was calculated using the diffraction intensity of the 002 plane at 2θ = 22.6° and the diffraction intensity of the amorphous part at 2θ = 18.5° using the following formula: If there is no peak in the amorphous portion at 2θ=18.5°, the crystallinity is taken to be 0%. Xc = (I002c - Ia) / I002c × 100 Xc = Crystallinity of cellulose type I (%) I002c: 2θ=22.6°, diffraction intensity of the 002 plane Ia: 2θ=18.5°, diffraction intensity of the amorphous part.

[0099] <Carboxymethyl Substitution Degree (CM-DS)> Approximately 2.0 g of ground carboxymethylcellulose salt sample was weighed and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of a solution of 1,000 mL of methanol and 100 mL of concentrated nitric acid was added and the mixture was shaken for 3 hours to convert carboxymethylcellulose salt (CMC salt) to H-CMC (carboxymethylcellulose). 1.5-2.0 g of 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. Excess NaOH was back-titrated with 0.1 N H2SO4 using phenolphthalein as an indicator. CM-DS was calculated using the following equation. (Formula 1) A = [(100 × F - (0.1N H2SO4 (mL)) × F') × 0.1] / (bone-dry mass of H-CMC (g)) Carboxymethyl substitution degree (CM-DS) = 0.162 × A / (1-0.058 × A) A: Amount of 1N NaOH (mL) required to neutralize 1g of H-CMC F': Factor of 0.1N H2SO4 F: Factor of 0.1N NaOH

[0100] <Viscosity before defibration> Carboxymethylcellulose salt was weighed into a 1,000 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. The viscosity was then measured after 3 minutes at 30 rpm using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.) according to the method of JIS-Z-8803.

[0101] <Viscosity after defibration> Carboxymethylcellulose salt was weighed into a 1,000 mL glass beaker and dispersed in 900 mL of distilled water to prepare an aqueous dispersion with a solids content of 1% (w / v). This aqueous dispersion was defibrated by treating it once using a high-pressure homogenizer at 25°C and 150 MPa. The viscosity of the defibrated aqueous dispersion was measured after 3 minutes at 6 rpm using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.) according to the method of JIS-Z-8803.

[0102] <Battery evaluation> (Discharge capacity (charge / discharge rate test)) The charge / discharge rate test for the coin-type nonaqueous electrolyte secondary batteries obtained in the Examples and Comparative Examples was carried out using Nagano Corporation's BTS2004 in a thermostatic chamber at 25°C, with the coin-type nonaqueous electrolyte secondary batteries undergoing one cycle of charge / discharge followed by a charge / discharge. The charge conditions were a constant current / constant voltage (CC-CV) method (CC current 0.2C, CV voltage 4.2V, end current 0.02C), and the discharge conditions were set to an end voltage of 3.0V. The discharge was carried out at a constant current of 0.2C, and the discharge capacity (mAh / g) after one cycle was measured.

[0103] <Adhesion strength evaluation> The negative electrode plates obtained in the examples and comparative examples were cut with a cutter so that the width became 25 mm, and the coated surface of the negative electrode plate and a wooden plate (300 mm in length × 200 mm in width × 3 mm in thickness) were pasted with double-sided tape (manufactured by Nitto Denko Corporation). The force when peeling the copper foil from the coating layer using TENSILON RTC-1210A (manufactured by A&D Company, Limited) was taken as the adhesion strength.

[0104] (Example 1) <Preparation of CMC-Li> To a 5 L capacity twin-screw kneader with the rotation speed adjusted to 100 rpm, a solution of 18 parts of lithium hydroxide dissolved in 434 parts of water was added, and 100 parts were charged based on the dry mass when broadleaf pulp (manufactured by Nippon Paper Industries Co., Ltd., LBKP) was dried at 100 °C for 60 minutes. It was stirred and mixed at 30 °C for 60 minutes to prepare mercerized cellulose. While further stirring, 70 parts of monochloroacetic acid was added, then 18 parts of lithium hydroxide was added and stirred for 30 minutes. 1407 parts of isopropanol (IPA) was added, stirred at 30 °C for 30 minutes, then heated to 70 °C over 30 minutes, and a carboxymethylation reaction was carried out at 70 °C for 60 minutes. After the reaction ended, it was neutralized, washed, de-liquored, dried, and pulverized to obtain a lithium salt of carboxymethylated cellulose (hereinafter sometimes referred to as "CMC-Li1") with a carboxymethyl substitution degree of 0.31, a viscosity (before defibrination) of a 1 mass% aqueous dispersion measured with a B-type viscometer at 25 °C of 5.2 mPa·s, and a crystallinity of cellulose I type of 75%. The measurement of the carboxymethyl substitution degree, the viscosity of the 1 mass% aqueous dispersion (before defibrination), and the crystallinity of cellulose I type was carried out as described above.

[0105] The obtained CMC-Li was made into a 1 mass% aqueous dispersion, and by treating it once (defibrinating) under the conditions of 25 °C and 150 MPa using a high-pressure homogenizer, an aqueous dispersion with a viscosity of 33,300 mPa·s was obtained.

[0106] <Preparation of negative electrode plate> SiOx as the negative electrode material, acetylene black (manufactured by Stream Chemical Co.) as a conductive additive, a 1% by mass aqueous dispersion of defibrated CMC-Li1 as a binder, and styrene butadiene rubber (SBR, manufactured by ENEOS Materials Corporation, product number TRD 104A) were mixed to a solids weight ratio of 97:0.5:1.0:1.5, and water was added to a slurry concentration of 45.6% by mass. The mixture was thoroughly stirred using a Mazerustar (manufactured by Kurabo Industries, Ltd., KK-250S) to obtain Slurry 1. This slurry was applied with an applicator to a copper foil (manufactured by Furukawa Electric Co., Ltd., NC-WS) measuring 320 mm in length, 170 mm in width, and 17 μm in thickness, and then air-dried for 30 minutes. The mixture was then dried in a dryer at 60 °C for 30 minutes. Furthermore, using a small tabletop roll press (Tester Sangyo Co., Ltd., SA-602), the fabric was pressed at 5 kN and a roll peripheral speed of 50 m / min, resulting in a weight of 19.7 g / m 2 A negative electrode plate with an effective discharge capacity of 2100 mAh / g was obtained.

[0107] <Fabrication of coin-type non-aqueous electrolyte secondary battery> The obtained negative electrode plate and a LiCoO2 positive electrode plate (manufactured by Hosensha, basis weight 110.2 g / m) were 2 The negative electrode plate and the positive electrode plate (effective discharge capacity 145 mAh / g) were punched out into a circle having a diameter of 16 mm, and the punched negative electrode plate and the positive electrode plate were dried in a vacuum at 120° C. for 12 hours.

[0108] Similarly, a separator (manufactured by CS Tech, polypropylene separator having a thickness of 20 μm) was punched out into a circle having a diameter of 17 mm, and vacuum dried at 60° C. for 12 hours.

[0109] The negative electrode plate was then placed in a stainless steel circular dish-shaped container with a diameter of 20.0 mm, followed by stacking a separator, a positive electrode plate, a spacer (diameter 15.5 mm, thickness 1 mm), and a stainless steel washer (manufactured by Hosen Co., Ltd.) in that order. 300 μL of electrolyte (1 mol / L LiPF6, 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 packing, and the container was sealed using a coin battery crimping machine (manufactured by Hosen Co., Ltd.) to obtain a coin-shaped nonaqueous electrolyte secondary battery.

[0110] (Comparative Example 1) A solution of 12 parts of lithium hydroxide in 434 parts of water was added to a 5-L twin-screw kneader adjusted to 100 rpm, and 100 parts of hardwood pulp (LBKP, manufactured by Nippon Paper Industries Co., Ltd.) (dry mass after drying at 100°C for 60 minutes) was added. The mixture was stirred and mixed at 30°C for 60 minutes to prepare mercerized cellulose. 47 parts of monochloroacetic acid was added with further stirring, followed by 12 parts of lithium hydroxide and stirring for 30 minutes. 1407 parts of isopropanol (IPA) was added, and the mixture was stirred at 30°C for 30 minutes. After stirring, the mixture was heated to 70°C over 30 minutes and subjected to a carboxymethylation reaction at 70°C for 60 minutes. After the reaction was completed, the mixture was neutralized, washed, drained, dried, and pulverized to obtain a lithium salt of carboxymethylated cellulose (CMC-Li2) having a degree of carboxymethyl substitution of 0.18, a viscosity of a 1% by mass aqueous dispersion measured at 25°C with a Brookfield viscometer of 1.9 mPa s, and a crystallinity of cellulose type I of 76%. The obtained CMC-Li2 was defibrated in the same manner as in Example 1 to obtain an aqueous dispersion with a viscosity of 11,100 mPa s. A coin-type nonaqueous electrolyte secondary battery was obtained in the same manner as in Example 1, except that the defibrated aqueous dispersion obtained in this manner was used.

[0111] (Comparative Example 2) To a 5 L capacity twin-screw kneader with the rotation speed adjusted to 100 rpm, a solution prepared by dissolving 20 parts of sodium hydroxide in 434 parts of water was added, and 100 parts were charged based on the dry mass when hardwood pulp (manufactured by Nippon Paper Industries Co., Ltd., LBKP) was dried at 100 °C for 60 minutes. It was stirred and mixed at 30 °C for 60 minutes to prepare mercerized cellulose. While further stirring, 47 parts of monochloroacetic acid were added, then 20 parts of sodium hydroxide were added and stirred for 30 minutes. 1407 parts of isopropanol (IPA) were added, stirred at 30 °C for 30 minutes, then heated to 70 °C over 30 minutes, and a carboxymethylation reaction was carried out at 70 °C for 60 minutes. After the reaction was completed, it was neutralized, washed, de-liquored, dried, and pulverized to obtain a sodium salt of carboxymethylated cellulose (CMC-Na1) with a carboxymethyl substitution degree of 0.32, a viscosity of 4.9 mPa·s for a 1 mass% aqueous dispersion measured with a B-type viscometer at 25 °C, and a crystallinity of cellulose I type of 72%. Using the obtained CMC-Na1, fibrillation was carried out in the same manner as in Example 1 to obtain an aqueous dispersion with a viscosity of 26,200 mPa·s. A coin-type non-aqueous electrolyte secondary battery was obtained in the same manner as in Example 1 except that the fibrillated aqueous dispersion thus obtained was used.

[0112] (Comparative Example 3) <Preparation of Sodium Salt of Carboxymethyl Cellulose> To a twin-screw kneader with the rotation speed adjusted to 100 rpm, 2720 g of isopropanol, 170 g of monochloroacetic acid Na, and a solution prepared by dissolving 58 g of sodium hydroxide in 480 g of water were added, and 160 g were charged based on the dry weight when lint pulp was dried at 30 °C for 60 minutes. It was stirred and mixed at 30 °C for 90 minutes to prepare mercerized cellulose, then heated to 70 °C and a carboxymethylation reaction was carried out for 90 minutes. After the reaction was completed, it was neutralized with acetic acid to a pH of about 7, de-liquored, dried, and pulverized to obtain a sodium salt of carboxymethyl cellulose (hereinafter sometimes referred to as CMC-Na2) with a carboxymethyl substitution degree of 0.70 and a viscosity of 7,900 mPa·s for a 1 mass% aqueous solution measured with a B-type viscometer at 25 °C.

[0113] <Preparation of H-Type Carboxymethyl Cellulose> 100 mL of a 10 vol% nitric acid methanol solution and 10 g of CMC-Na2 were added to a 200 mL beaker and stirred at room temperature for 2 hours. After the reaction was completed, filtration was performed. 200 mL of an 80 vol% methanol aqueous solution and the filtration residue were added to a 300 mL beaker, stirred at room temperature for 30 minutes, and the filtration operation was carried out once. Then, 100 mL of methanol and the filtration residue were added to a 200 mL beaker, stirred at room temperature for 30 minutes, filtered, and the filtration residue was dried at 105 °C for 2 hours to obtain H-type carboxymethyl cellulose.

[0114] <Preparation of CMC-Li> 50 mL of a 75 vol% methanol aqueous solution and 1.75 g of lithium hydroxide were added to a 200 mL beaker and stirred at room temperature for 1 hour to dissolve lithium hydroxide. 5 g of the H-type carboxymethyl cellulose obtained as described above was added to the solution and stirred at room temperature for 3 hours. After the reaction was completed, 80 wt% acetic acid was added until the pH of the reaction solution reached 7.0, and filtration was performed. Then, a 90 vol% methanol aqueous solution and the filtration residue were added to a 100 mL beaker, stirred at room temperature for 30 minutes, and the filtration operation was carried out twice. The filtration residue was dried at 80 °C overnight to obtain lithium carboxymethyl cellulose (CMC-Li3) with a carboxymethyl substitution degree of 0.70, a viscosity of 2,210 mPa·s of a 1 mass% aqueous solution measured with a B-type viscometer at 25 °C, and a crystallinity of 0% of cellulose I type. A coin-type non-aqueous electrolyte secondary battery was obtained in the same manner as in Example 1 except that the obtained CMC-Li3 was used. In addition, a water dispersion was obtained by fibrillating CMC-Li3 in the same manner as in Example 1. The viscosity of the aqueous solution obtained after fibrillation was 93 mPa·s.

[0115] (Comparative Example 4) 18 parts of lithium hydroxide was mixed with 434 parts of isopropanol, but lithium hydroxide hardly dissolved and remained solid. The solid lithium hydroxide had poor reactivity with pulp and could hardly mercerize the pulp, so the experiment was terminated. If the experiment had been continued, the DS value of the CMC-Li obtained could be expected to be less than 0.2.

[0116] Table 1 shows the measurement and evaluation results of the carboxymethylcellulose lithium salt or carboxymethylcellulose sodium salt and the nonaqueous electrolyte secondary batteries obtained in the examples and comparative examples.

[0117] [Table 1]

[0118] As shown in Table 1, a nonaqueous electrolyte secondary battery using, as a binder, carboxymethyl cellulose nanofibers obtained by dispersing the carboxymethyl cellulose lithium salt of Example 1, which has a degree of carboxymethyl substitution per anhydroglucose unit of 0.2 or more but less than 0.5, and in which a 1% by mass aqueous dispersion of the carboxymethyl cellulose lithium salt has a viscosity of 200 mPa·s or less as measured at 25°C using a B-type viscometer, and in which the amount of lithium ions contained per gram of the carboxymethyl cellulose lithium salt is 5,000 to 20,000 ppm, was found to have a higher discharge capacity after one cycle than the nonaqueous electrolyte secondary batteries obtained in Comparative Examples 1 and 2. This indicates that the initial capacity of the battery was large. Furthermore, a negative electrode plate using cellulose nanofibers prepared from the carboxymethyl cellulose lithium salt of the present invention exhibited excellent adhesion strength between the copper foil used as a current collector and the electrode layer. The carboxymethyl cellulose lithium salt of Comparative Example 3 lost crystallinity due to its high degree of carboxymethyl substitution, and therefore had a low viscosity after disintegration. In addition, in Comparative Example 3, the lithium carboxymethyl cellulose salt was used as a binder before defibration, but the adhesive strength when made into a negative electrode plate was inferior to that of Example 1. In Comparative Example 4, lithium hydroxide was hardly dissolved, and a uniform mercerization reaction did not proceed, so the desired lithium carboxymethyl cellulose salt was not obtained.

Claims

1. A lithium carboxymethyl cellulose salt having a degree of carboxymethyl substitution per anhydroglucose unit of 0.2 or more and less than 0.5, The viscosity of a 1% by mass aqueous dispersion of the carboxymethyl cellulose lithium salt measured at 25°C using a Brookfield viscometer is 200 mPa s or less, The amount of lithium ions contained in 1 g of the carboxymethyl cellulose lithium salt is 5,000 to 20,000 ppm.

2. The carboxymethyl cellulose lithium salt according to claim 1, wherein the crystallinity of the cellulose I type of the carboxymethyl cellulose lithium salt is 50% or more.

3. The carboxymethyl cellulose lithium salt according to claim 1, characterized in that the carboxymethyl cellulose lithium salt does not contain sodium ions.

4. A method for producing a lithium salt of carboxymethyl cellulose having a degree of carboxymethyl substitution per anhydroglucose unit of 0.2 or more but less than 0.5, comprising: Step (A): treating cellulose with a 12% by mass or less aqueous solution of lithium hydroxide to obtain mercerized cellulose; Step (B): reacting the mercerized cellulose with a carboxymethylating agent in a mixed solvent containing 50 to 99% by mass of an organic solvent based on the total mass of water and the organic solvent to obtain carboxymethyl cellulose, The viscosity of a 1% by mass aqueous dispersion of the carboxymethyl cellulose lithium salt measured at 25°C using a Brookfield viscometer is 200 mPa s or less, The method for producing carboxymethyl cellulose lithium salt, wherein the amount of lithium ions contained in 1 g of the carboxymethyl cellulose lithium salt is 5,000 to 20,000 ppm.

5. The method for producing carboxymethyl cellulose lithium salt according to claim 4, wherein the crystallinity of cellulose I type of the carboxymethyl cellulose lithium salt is 50% or more.

6. A carboxymethyl cellulose nanofiber lithium salt having a carboxymethyl substitution degree per anhydroglucose unit of 0.2 or more and less than 0.5, The viscosity of a 1% by mass aqueous dispersion of the carboxymethyl cellulose nanofiber lithium salt measured at 25°C using a Brookfield viscometer is approximately 10,000 to 100,000 mPa s, The carboxymethyl cellulose nanofiber lithium salt has a lithium ion content of 5,000 to 20,000 ppm per 1 g of the carboxymethyl cellulose nanofiber lithium salt.

7. An electrode composition for a non-aqueous electrolyte secondary battery, comprising the carboxymethyl cellulose nanofiber lithium salt according to claim 6.

8. An electrode for a non-aqueous electrolyte secondary battery, which uses the electrode composition for a non-aqueous electrolyte secondary battery according to claim 7.

9. A non-aqueous electrolyte secondary battery using the electrode composition for a non-aqueous electrolyte secondary battery according to claim 7.

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