Negative electrode slurry and method for manufacturing the same

Abstract

To provide a negative electrode slurry having a viscosity suitable for coating current collectors. [Solution] The negative electrode slurry for a non-aqueous electrolyte secondary battery contains a negative electrode active material, a thickening polysaccharide, an alcohol with 5 or fewer carbon atoms, and water. The alcohol content is 0.70 to 3.50% by weight of the negative electrode slurry.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode slurry and a method for producing the same. [Background technology]

[0002] It is known that the negative electrode of a secondary battery is manufactured by applying a negative electrode slurry to a current collector. However, bacteria may unintentionally become contaminated in the negative electrode slurry during manufacturing (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2013-114959 [Patent Document 2] Patent No. 7548010 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Bacteria present in the negative electrode slurry can multiply using the thickening polysaccharides contained in the slurry as a nutrient source, potentially causing a decrease in the viscosity and spoilage of the negative electrode slurry. A decrease in the viscosity of the negative electrode slurry reduces its coating properties on the current collector.

[0005] To suppress bacterial growth, it is conceivable to add preservatives, antibacterial agents, or disinfectants to the negative electrode slurry. However, when negative electrode slurry containing these agents is stored, its viscosity increases, which can reduce its applicability to current collectors.

[0006] This disclosure aims to provide a negative electrode slurry having a viscosity suitable for coating a current collector, and a method for manufacturing the same. [Means for solving the problem]

[0007] [1] A negative electrode slurry for a non-aqueous electrolyte secondary battery, It contains a negative electrode active material, a thickening polysaccharide, an alcohol with 5 or fewer carbon atoms, and water. The negative electrode slurry has an alcohol content of 0.70 to 3.50% by weight of the negative electrode slurry. [2] The number of colonies measured by the following procedure is 1.0 × 10 5 The negative electrode slurry described in [1], having a concentration of 1 / mL or less. (Procedure for measuring the number of colonies) The negative electrode slurry is diluted 10,000 times and the resulting solution is inoculated into a film culture medium. The medium is then left in a constant temperature bath at 32°C for 24 hours, and the number of colonies formed on the film culture medium is measured. [3] The thickening polysaccharide is a cellulose derivative, as described in [1] or [2]. [4] The negative electrode slurry according to [3], wherein the cellulose derivative is carboxymethylcellulose. [5] A negative electrode slurry according to any one of [1] to [4], further comprising a binder. [6] The negative electrode slurry according to [5], wherein the binder comprises at least one of styrene-butadiene rubber and polyacrylic acid. [7] The negative electrode slurry according to any one of [1] to [6], wherein the alcohol is ethanol. [8] A method for producing a negative electrode slurry for a non-aqueous electrolyte secondary battery, To obtain a kneaded product containing a negative electrode active material, a thickening polysaccharide, and water, The process includes adding an alcohol with 5 or fewer carbon atoms to the aforementioned kneaded mixture and stirring it, A method for producing a negative electrode slurry, wherein the alcohol content in the negative electrode slurry is 0.70 to 3.50% by weight of the negative electrode slurry. [9] Obtaining the aforementioned mixture involves kneading the negative electrode active material, the thickening polysaccharide, and water, and then further kneading the binder and water. The method for producing a negative electrode slurry according to [8], wherein the binder comprises at least one of styrene-butadiene rubber and polyacrylic acid.

[10] The number of colonies measured by the following procedure is 1.0 × 10 5 A method for producing a negative electrode slurry according to [8] or [9], wherein the number of particles / mL or less. (Procedure for measuring the number of colonies) The negative electrode slurry is diluted 10,000 times and the resulting solution is inoculated into a film culture medium. The medium is then left in a constant temperature bath at 32°C for 24 hours, and the number of colonies formed on the film culture medium is measured. [Effects of the Invention]

[0008] According to this disclosure, it is possible to provide a negative electrode slurry having a viscosity suitable for coating onto a current collector. [Brief explanation of the drawing]

[0009] [Figure 1] This is a flowchart illustrating the manufacturing process of the negative electrode slurry according to the embodiment. [Modes for carrying out the invention]

[0010] In this specification, numerical ranges such as "x~y" include upper and lower limits unless otherwise specified. That is, "x~y" represents a numerical range of "x or greater and y or less". A number arbitrarily selected from within the numerical range may be used as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a number within the numerical range with a number listed in another part of this specification, in a table, or in a figure.

[0011] <Negative electrode slurry> The negative electrode slurry of this embodiment (hereinafter also referred to as "this slurry (1)") can be used to form the negative electrode active material layer of the negative electrode of a non-aqueous electrolyte secondary battery (hereinafter also referred to as "secondary battery"). The negative electrode can have a current collector and a negative electrode active material layer formed on the current collector. The negative electrode active material layer can be formed by drying, compressing, etc. the slurry (1) applied on the current collector. Examples of the secondary battery include a lithium-ion secondary battery that performs charge and discharge by intercalating and deintercalating lithium ions.

[0012] This slurry (1) contains a negative electrode active material, a thickening polysaccharide, an alcohol having 5 or less carbon atoms (hereinafter also referred to as "lower alcohol"), and water. The content of the lower alcohol in this slurry (1) is a ratio based on weight with respect to this slurry (1), and is 0.70 to 3.50% by weight of this slurry (1).

[0013] It is considered that bacteria may be mixed into this slurry (1) during the production of this slurry (1). Examples of the production time of this slurry (1) include steps such as the preparation of this slurry (1), the transfer of this slurry (1) by piping performed after the preparation of this slurry (1), and / or the storage of this slurry (1) after the preparation of this slurry (1) until it is applied to the current collector. Since this slurry (1) contains 0.70% by weight or more of the lower alcohol, the growth of bacteria in this slurry (1) can be suppressed, so that it is possible to suppress a decrease in the viscosity of this slurry (1) during storage. Since the content of the lower alcohol in this slurry (1) is 3.50% by weight or less, it is possible to suppress an increase in the viscosity of this slurry (1) during storage. Thereby, this slurry (1) can have a viscosity suitable for coating when applying this slurry (1) to the current collector in the production of the negative electrode, so that it is possible to suppress a decrease in the coating property of this slurry (1).

[0014] The viscosity of the slurry (1) prepared by mixing the negative electrode active material, thickening polysaccharide, lower alcohol, and water is, for example, 8 to 20 Pa·s, but may also be 9 to 18 Pa·s, 10 to 16 Pa·s, or 11 to 18 Pa·s. In this specification, the viscosity of the negative electrode slurry is defined as the shear rate of 0.1 s in the flow curve obtained by measuring with a rheometer at 25°C. -1 This refers to the value at [location].

[0015] After preparing the slurry (1) and storing it at 25°C for 7 days, the viscosity of the slurry (1) (measured at 25°C) is, for example, 8 to 23 Pa·s, but may also be 10 to 20 Pa·s, 12 to 18 Pa·s, or 15 to 20 Pa·s.

[0016] The solid content of this slurry (1) is, for example, 40-60% by weight, but may also be 43-59% by weight, 47-58% by weight, or 51-57% by weight.

[0017] The negative electrode active material may include, for example, at least one of a carbon-based active material and a metallic active material, or it may include both a carbon-based active material and a metallic active material. Examples of carbon-based active materials include one or more selected from the group consisting of graphite such as natural graphite and artificial graphite, hard carbon, soft carbon, and amorphous coated graphite. Examples of metallic active materials include metallic elements such as elemental metals or metal oxides containing elements selected from the group consisting of silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), and germanium (Ge). The metallic active material is preferably a Si-containing active material containing silicon (Si). The Si-containing active material preferably contains one or more selected from the group consisting of Si, SiOx (x=0.5~1.5), and SiC, and more preferably contains SiC or is SiC. SiOx is preferably SiO.

[0018] The negative electrode active material preferably contains or is a carbon-based active material, more preferably contains or is graphite, and even more preferably contains or is natural graphite.

[0019] When the negative electrode active material includes a carbon-based active material, the content of the carbon-based active material is preferably 90% by mass or more, may be 92% by mass or more, may be 95% by mass or more, may be 90-99% by mass, may be 92-98% by mass, or may be 95-97% by mass, relative to the negative electrode active material.

[0020] Examples of thickening polysaccharides include cellulose derivatives, xanthan gum, gellan gum, guar gum, carrageenan, dextrin, pregelatinized starch, pectic acid, and alginic acid. Examples of cellulose derivatives include carboxymethylcellulose (hereinafter also referred to as "CMC"), methylcellulose, hydroxyethylcellulose, and hydroxypropylcellulose. Among the thickening polysaccharides, those that can take the form of a salt or an acid may be in the form of a salt or an acid. Examples of salts include alkali metal salts such as lithium and sodium, and ammonium salts. The thickening polysaccharide may contain one or more of the above-mentioned compounds.

[0021] The thickening polysaccharides preferably contain or are cellulose derivatives, and more preferably contain or are CMC. The CMC is preferably in the form of a salt, and more preferably an alkali metal salt, such as a sodium salt.

[0022] The amount of thickening polysaccharides in the slurry (1) is not particularly limited and can be selected according to the viscosity of the slurry (1). For example, it may be 0.1 to 2.0% by weight relative to the total amount of negative electrode active material, but may also be 0.2 to 1.5% by weight, 0.3 to 1.0% by weight, or 0.4 to 0.8% by weight. If the slurry (1) contains two or more types of thickening polysaccharides, the amount of thickening polysaccharides is the total amount of the two or more types of thickening polysaccharides.

[0023] Lower alcohols are added to this slurry (1) to inhibit bacterial growth. The lower alcohols are not particularly limited as long as they have 5 or fewer carbon atoms. The number of carbon atoms in the lower alcohol may be 4 or less, 3 or less, 2 or less, 2 to 5, 2 to 4, or 2 or 3. The lower alcohols may include one or more from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, n-butyl alcohol, isobutyl alcohol, sec-butyl alcohol, and tert-butyl alcohol. The lower alcohols preferably include or are ethanol.

[0024] The lower alcohol content in the slurry (1) may be 0.70 to 3.50% by weight, 0.80 to 3.20% by weight, 0.90 to 3.00% by weight, 0.95 to 2.50% by weight, 1.00 to 2.20% by weight, or 1.10 to 2.00% by weight, preferably 0.99 to 1.96% by weight, and more preferably 1.08 to 1.48% by weight. If the slurry (1) contains two or more lower alcohols, the lower alcohol content is the total amount of the two or more lower alcohols.

[0025] If the lower alcohol content is low, it becomes more difficult to suppress the growth of bacteria in the negative electrode slurry. If the lower alcohol content is too high, the viscosity of the negative electrode slurry tends to increase during storage. It is presumed that an increase in the lower alcohol content in the negative electrode slurry makes it easier for the dispersion state of thickening polysaccharides in the negative electrode slurry to change, thus increasing viscosity. If the lower alcohol content is too high, the increase in viscosity of the negative electrode slurry will cause the amount of slurry dispensed during application to be less than the required amount, thus reducing the productivity of the negative electrode.

[0026] The water is preferably pure water. Examples of pure water include distilled water, ion-exchanged water, water produced by a reverse osmosis membrane (RO water), purified water, and the like.

[0027] The number of colonies measured by the procedure shown in the following "Colony Counting Procedure" using this slurry (1) is preferably 1.0×10 5 colonies / mL or less, more preferably 5.0×10 4 colonies or less / mL, may be 4.0×10 4 colonies or less / mL, may be 2.0×10 4 colonies or less / mL, may be 1.0×10 4 colonies or less / mL, and more preferably less than 1.0×10 4 colonies / mL. (Colony Counting Procedure) Inoculate a dilution of this slurry (1) diluted 10,000 times into a film medium and measure the number of colonies formed on the film medium when left in a constant temperature bath at 32°C for 24 hours.

[0028] The colony counting procedure is as described above. More specifically, it can be measured according to the procedure described in the examples. The slurry (1) inoculated into the film medium is the slurry immediately before coating on the current collector. For example, when the slurry is transferred through pipes or stored after the preparation of this slurry (1), it is the slurry after these processes. When the number of colonies is within the above range, the growth of bacteria during the storage of this slurry (1) can be suppressed, and the decrease in the viscosity of this slurry (1) can be inhibited. A negative electrode slurry with a large number of colonies formed on the film medium is considered to be likely to have a decrease in viscosity during storage.

[0029] The types of bacteria contained in the colonies are not particularly limited, but examples include bacteria belonging to one or more genera selected from the group consisting of the genera Celluomonas, Chryseobacterium, and Bacillus. The bacteria contained in the colonies may include bacteria of the genus Celluomonas. Bacteria of the genus Celluomonas tend to cause a decrease in the viscosity of the negative electrode slurry. This slurry (1), in which the number of colonies containing bacteria of the genus Celluomonas falls within the above range, can suppress a decrease in viscosity during storage.

[0030] This slurry (1) may further contain, in addition to the negative electrode active material, thickening polysaccharide, lower alcohol, and water, at least one of a binder and a conductive material, or both.

[0031] The slurry (1) preferably contains a binder. In this specification, the binder refers to a binder other than thickening polysaccharides. Examples of binders include polyacrylic acid (hereinafter also referred to as "PAA"), styrene-butadiene rubber (hereinafter also referred to as "SBR"), polyvinyl alcohol, polyvinyl acetate, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer and ethylene-propylene-butadiene copolymer, acrylonitrile-butadiene rubber (NBR), polyacrylonitrile (PAN), polyimide, polyamide, and acrylic resins such as ethylene-acrylic acid copolymer. PAA may be in the form of an acid or a salt. Examples of salts include alkali metal salts such as lithium and sodium, and ammonium salts. The slurry (1) may contain one or more of the above binders.

[0032] The slurry (1) preferably contains at least one of SBR and PAA, and more preferably contains SBR.

[0033] The binder content in the slurry (1) is not particularly limited, but for example, it may be 0.5 to 2.5% by weight relative to the total amount of negative electrode active material, and may also be 0.7 to 2.0% by weight, or 1.0 to 1.5% by weight.

[0034] Examples of conductive materials include carbon materials such as fibrous carbon, carbon black (acetylene black, Ketjen black, etc.), coke, and activated carbon. Examples of fibrous carbon include carbon nanotubes (hereinafter also referred to as "CNTs"). CNTs may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes such as double-walled carbon nanotubes (DWCNTs). This slurry (1) may contain one or more of the above conductive materials.

[0035] <Method for manufacturing negative electrode slurry> Figure 1 is a flowchart illustrating the manufacturing process of the negative electrode slurry according to the embodiment. The method for manufacturing the negative electrode slurry (hereinafter also referred to as "this method") is a method for manufacturing a negative electrode slurry for secondary batteries (non-aqueous electrolyte secondary batteries). The negative electrode slurry manufactured by this method (hereinafter also referred to as "this slurry (2)") can be used to form the negative electrode active material layer of the negative electrode of a secondary battery. This slurry (2) may be this slurry (1) described above, or it may be a negative electrode slurry other than this slurry (1).

[0036] This manufacturing method includes obtaining a mixture containing a negative electrode active material, a thickening polysaccharide, and water (hereinafter also referred to as the "step for obtaining the mixture"), and adding an alcohol with 5 or fewer carbon atoms (lower alcohol) to the mixture and stirring it (hereinafter also referred to as the "stirring step"). The lower alcohol content in this slurry (2) is expressed as a weight-based percentage of this slurry (2), and is between 0.70 and 3.50% by weight of this slurry (2).

[0037] According to this manufacturing method, in the step of obtaining the kneaded product, a kneaded product containing the negative electrode active material, thickening polysaccharides, and water, but without lower alcohols, is obtained, and in the stirring step, lower alcohols are added to the kneaded product and stirred. As a result, in the step of obtaining the kneaded product, it is easier to obtain a kneaded product in which the negative electrode active material and thickening polysaccharides are uniformly dispersed, and thus it is easier to obtain the slurry (2) in which the negative electrode active material and thickening polysaccharides are well dispersed. On the other hand, if lower alcohols are added when kneading the negative electrode active material and thickening polysaccharides, it becomes difficult to obtain a kneaded product in which the negative electrode active material and thickening polysaccharides are uniformly dispersed, and the dispersion state of the negative electrode active material and thickening polysaccharides in the negative electrode slurry tends to be uneven.

[0038] In this manufacturing method, since the lower alcohol content in the slurry (2) is within the range described above, it is possible to suppress the decrease in viscosity of the negative electrode slurry due to bacterial growth and the increase in viscosity of the negative electrode slurry due to the presence of lower alcohol during storage of the slurry (2). As a result, a slurry (2) with good coating properties for current collectors can be obtained.

[0039] The kneaded mixture may contain, in addition to the negative electrode active material, thickening polysaccharides, and water, at least one of a binder and a conductive material.

[0040] The process for obtaining the compound may include a step of kneading the negative electrode active material, thickening polysaccharides, and water. This step is a step of kneading the negative electrode active material, thickening polysaccharides, and water without adding lower alcohols. The kneading step may be, for example, (a) A step of powder mixing the negative electrode active material and the thickening polysaccharide, Step (b) involves adding water to the mixture obtained in step (a) and kneading it, The process may also include step (c) of adding water to the kneaded mixture (b) obtained in step (b) to dilute it, and then adding a binder and kneading it.

[0041] The compound obtained in the step of obtaining the compound may be compound (b), or compound (c) obtained in step (c). The conductive material may be added in at least one of step (a) and step (b).

[0042] The mixing and stirring processes can be carried out using known mixing equipment. Examples of mixing equipment include agitators with rotating blades, fill mixers, media stirring mills, planetary mixers, and twin-shaft mixers.

[0043] In this manufacturing method, the negative electrode active material, thickening polysaccharide, lower alcohol, water, binder, and conductive material can each be those described in slurry (1).

[0044] The number of colonies measured using the slurry (2) in the procedure for measuring the number of colonies described above may be within the range of the number described for slurry (1). Keeping the number of colonies within this range helps to suppress bacterial growth and a decrease in the viscosity of slurry (2) during storage. [Examples]

[0045] The present disclosure will be further described below with reference to examples and comparative examples. [Comparative Example 1] Natural graphite was prepared as the negative electrode active material, sodium salt of CMC as the thickening polysaccharide, SBR as the binder, and pure water as the water. The negative electrode active material and the thickening polysaccharide were mixed in powder form to obtain a mixture. Water was added to the mixture and kneaded to obtain kneaded product (b). After diluting kneaded product (b) with water, the binder was added and stirred to obtain kneaded product (c). Kneaded product (c) was degassed and passed through a sieve to obtain a negative electrode slurry. The composition ratio of the components contained in the negative electrode slurry was negative electrode active material:thickening polysaccharide:binder = 98.3:0.5:1.2.

[0046] [Examples 1-4, Comparative Example 2] Ethanol was prepared as the lower alcohol. A mixture (c) was obtained using the procedure described in Comparative Example 1. The lower alcohol was added to the mixture (c), stirred, degassed, and passed through a sieve to obtain a negative electrode slurry. The amount of lower alcohol added was adjusted to the content shown in Table 1. The lower alcohol content shown in Table 1 indicates the lower alcohol content in the negative electrode slurry.

[0047] [Measurement of the number of colonies] Using pure water with a bacterial count of 10 CFU / mL or less, the negative electrode slurry obtained above was diluted 10,000 times to obtain a diluent. 1 mL of the diluent was inoculated into a simple film medium (3M Petrifilm AC plate (aerobic count)) and left in a 32°C incubator for 24 hours. The number of colonies formed on the simple film medium after standing was measured. The results are shown in Table 1.

[0048] In Comparative Example 1 and Example 1, bacteria of the genera Celluomonas and Chryseobacterium were detected in the simple film culture media inoculated with the negative electrode slurries. In Examples 2-4 and Comparative Example 2, the types of bacteria present in the simple film culture media inoculated with the negative electrode slurries could not be identified.

[0049] [Viscosity measurement of negative electrode slurry] (Measurement of viscosity during preparation) The viscosity of the negative electrode slurry obtained above at 25°C was measured using a rheometer. The flow curve obtained from the measurement was measured at a shear rate of 0.1 s. -1 The value obtained was defined as the viscosity during the preparation of the negative electrode slurry. The results are shown in the "Viscosity during preparation" column of Table 1.

[0050] (Measurement of viscosity after static storage) The negative electrode slurry prepared as described above was placed in a storage container and stored at 25°C for 7 days. The viscosity of the negative electrode slurry after storage was measured using the procedure described above for measuring viscosity during preparation. The results are shown in the column for viscosity after static storage in Table 1.

[0051] (Measurement of viscosity after stirring) The negative electrode slurry was stirred after being left to stand. The viscosity of the stirred negative electrode slurry was measured using the procedure described above for measuring viscosity during preparation. The results are shown in the "Viscosity after stirring" column of Table 1.

[0052] [Table 1]

[0053] As shown in Table 1, in Examples 1 to 4, bacterial growth was suppressed during storage of the negative electrode slurry, thereby preventing a decrease in the viscosity of the negative electrode slurry, and an increase in the viscosity of the negative electrode slurry was also suppressed.

Claims

1. A negative electrode slurry for a non-aqueous electrolyte secondary battery, It contains a negative electrode active material, a thickening polysaccharide, an alcohol with 5 or fewer carbon atoms, and water. The negative electrode slurry has an alcohol content of 0.70 to 3.50% by weight of the negative electrode slurry.

2. The number of colonies measured using the following procedure is 1.0 × 10⁻⁶. ５ The negative electrode slurry according to claim 1, wherein the number of particles / mL or less. (Procedure for measuring the number of colonies) The negative electrode slurry is diluted 10,000 times and the resulting solution is inoculated onto a film culture medium. The medium is then left in a constant temperature bath at 32°C for 24 hours, and the number of colonies formed on the film culture medium is measured.

3. The anode slurry according to claim 1 or 2, wherein the thickening polysaccharide comprises a cellulose derivative.

4. The negative electrode slurry according to claim 3, wherein the cellulose derivative is carboxymethylcellulose.

5. Furthermore, the negative electrode slurry according to claim 1 or 2, further comprising a binder.

6. The negative electrode slurry according to claim 5, wherein the binder comprises at least one of styrene-butadiene rubber and polyacrylic acid.

7. The negative electrode slurry according to claim 1 or 2, wherein the alcohol is ethanol.

8. A method for manufacturing a negative electrode slurry for a non-aqueous electrolyte secondary battery, To obtain a kneaded product containing a negative electrode active material, a thickening polysaccharide, and water, The process includes adding an alcohol with 5 or fewer carbon atoms to the aforementioned kneaded mixture and stirring it, A method for producing a negative electrode slurry, wherein the alcohol content in the negative electrode slurry is 0.70 to 3.50% by weight of the negative electrode slurry.

9. Obtaining the aforementioned mixture involves kneading the negative electrode active material, the thickening polysaccharide, and water, and then further kneading with the addition of a binder and water. The method for producing a negative electrode slurry according to claim 8, wherein the binder comprises at least one of styrene-butadiene rubber and polyacrylic acid.

10. The number of colonies measured using the following procedure is 1.0 × 10⁻⁶. ５ A method for producing a negative electrode slurry according to claim 8 or 9, wherein the number of particles / mL or less. (Procedure for measuring the number of colonies) The negative electrode slurry is diluted 10,000 times and the resulting solution is inoculated onto a film culture medium. The medium is then left in a constant temperature bath at 32°C for 24 hours, and the number of colonies formed on the film culture medium is measured.

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