Electroconductive aid dispersion composition for battery, positive electrode paste, positive electrode for secondary battery, secondary battery, and electroconductive aid dispersant

Abstract

Provided is an electroconductive aid dispersion composition for a battery, the composition containing an electroconductive aid, a cellulose ether, and N-methyl-2-pyrrolidone. The cellulose ether is such that a portion of the hydroxyl groups derived from cellulose are etherified using (i) one or more substituents selected from the group consisting of methyl groups, ethyl groups, hydroxyethyl groups, and hydroxypropyl groups, and another portion of the hydroxyl groups derived from cellulose are etherified using (ii) a substituent represented by general formula (1) (wherein, R represents an alkyl group having 10 to 22 carbon atoms). The viscosity of a 2 mass% aqueous solution of the cellulose ether at 20°C is 3 to 60,000 mPa·s. This electroconductive aid dispersion composition for a battery is configured such that an electroconductive aid having high cohesiveness, such as carbon nanotubes, can be contained at a relatively high concentration in a uniform and favorable dispersion state while reducing the addition amount of polymer dispersant.

Description

Conductive additive dispersion composition for batteries, paste for positive electrodes, positive electrodes and secondary batteries for secondary batteries, and conductive additive dispersant

[0001] The present invention relates to a conductive additive dispersion composition for batteries, a paste for positive electrodes, a positive electrode for secondary batteries and a secondary battery, and a conductive additive dispersant.

[0002] Among secondary batteries, lithium-ion batteries are particularly characterized by their high energy density. The market has expanded due to the rapid spread of mobile devices such as mobile phones and notebook computers, and their performance has improved dramatically as a result. In recent years, as part of efforts to realize a sustainable society, the electrification of automobiles and the improvement of the efficiency of power storage systems have progressed, and the market for lithium-ion batteries is expected to continue to expand.

[0003] The positive electrode of a lithium-ion secondary battery mainly consists of a positive electrode active material, a conductive additive, a binder, and a current collector. Since the capacity of a lithium-ion secondary battery is mainly determined by the amount of active material, it is preferable to reduce materials other than the active material, such as conductive additives, as much as possible in order to increase the capacity of the battery. In recent years, carbon-based conductive additives with high specific surface area, high structure, and high aspect ratio have been used to impart high conductivity with smaller amounts of additive.

[0004] Generally, carbon-based conductive additives have a high tendency to aggregate. Therefore, in recent years, a method has become commonly used in which conductive additives are pre-dispersed uniformly in a dispersion medium such as an organic solvent using a polymer dispersant to create a conductive additive dispersion, and then a positive electrode paste (also called positive electrode composite paste or positive electrode composite slurry) is obtained by mixing a positive electrode active material and a binder into this conductive additive dispersion. This paste prevents aggregation of the conductive additive and forms an electrode with uniform dispersion.

[0005] However, among carbon-based conductive additives, particularly those with high specific surface area, high structure, and high aspect ratio, such as carbon nanotubes, there was a problem in that aggregation and poor dispersion of the conductive additive tended to occur significantly in the aforementioned conductive additive dispersion. When the conductive additive aggregated due to poor dispersion in the conductive additive dispersion, the viscosity of the dispersion increased, resulting in poor coating properties of the positive electrode paste manufactured using it. This prevented the acquisition of a smooth composite coating film, ultimately leading to a shortened lifespan of the secondary battery due to uneven load distribution. Furthermore, when the conductive additive was not uniformly dispersed, it became necessary to unintentionally reduce the solid content concentration in the dispersion. As a result, the solid content concentration of the positive electrode paste coated on the current collector foil also decreased, leading to a decrease in the surface density of the electrode and a reduction in capacity. In addition, poor dispersion of the conductive additive increased the internal resistance of the electrode, which also contributed to a decrease in the battery's charge and discharge capacity.

[0006] One known method for uniformly dispersing such carbon nanotubes as a conductive additive is a conductive additive dispersion using polyvinylpyrrolidone, a nonionic polymer dispersant (Japanese Patent Publication No. 2005-154630 (Patent Document 1)).

[0007] Furthermore, as a nonionic polymer dispersant other than polyvinylpyrrolidone, a carbon nanotube dispersion using ethyl cellulose, which is derived from cellulose, a natural polymer with a low environmental impact, is known (International Publication 2017 / 188175 (Patent Document 2)).

[0008] Japanese Patent Publication No. 2005-154630, International Publication No. 2017 / 188175

[0009] Incidentally, the conductive additive dispersion used in the manufacture of positive electrode paste (positive electrode slurry) requires not only that the conductive additive be contained uniformly and in a good dispersion state, as mentioned above, but also that the amount of polymer dispersant added be kept to a minimum, and furthermore, that the concentration of the conductive additive in the conductive additive dispersion be high. This is for the following reasons. Specifically, polymer dispersants can inhibit the electronic conductivity within the electrode as an electrical resistance component when assembled into a secondary battery. In other words, if the amount of polymer dispersant added is large, the internal resistance of the secondary battery containing a positive electrode made using the conductive additive dispersion will increase, which may result in a decrease in the energy capacity of the secondary battery. Furthermore, if the concentration of the conductive additive in the conductive additive dispersion is low, the amount of dispersion medium and solvent (i.e., polymer dispersant and organic solvent) in the positive electrode paste manufactured using the conductive additive dispersion will relatively increase, which may decrease the solid content concentration of the positive electrode paste when coated onto the current collector. Furthermore, a decrease in the solid content concentration of the positive electrode paste could lead to an increase in the drying time required for the positive electrode and an increase in the amount of dispersion medium and solvent removed during drying, potentially causing solid components such as binders to migrate and disrupt their uniformity.

[0010] On the other hand, carbon-based conductive additives, which generally have high cohesiveness, tend to disperse more easily when diluted. Therefore, it has been difficult to prevent conductive additive dispersions, which are required to contain the conductive additive in a uniform and well-dispersed state, from becoming dispersions diluted with a large amount of dispersion medium.

[0011] In the dispersion described in Patent Document 1, 5,000 to 10,000 parts by mass of a polymer dispersant (polyvinylpyrrolidone) and 1,000 parts by mass of a surfactant are added per 100 parts by mass of carbon nanotubes, which are the conductive additive, to assist in the dispersion of the carbon nanotubes. Furthermore, the carbon nanotube content in the conductive additive dispersion is very low, at 0.01% by mass or less, which was undesirable for a conductive additive dispersion for batteries.

[0012] In the dispersion described in Patent Document 2, the amount of ethyl cellulose, a polymer dispersant, added is large, at 100 to 300 parts by mass per 100 parts by mass of carbon nanotubes, which are conductive additives. Furthermore, although the content of the conductive additive in the conductive additive dispersion in Patent Document 2 is small, at 0.01 to 0.1% by mass, it is stated that if this is increased to 2% by mass or more, the fluidity of the dispersion will be lost and uniform dispersion will be difficult.

[0013] The present invention has been made in view of the above circumstances, and aims to provide a conductive additive dispersion composition for batteries that enhances the dispersibility of highly cohesive conductive additives such as carbon nanotubes, a conductive additive dispersion composition for batteries that contains highly cohesive conductive additives such as carbon nanotubes at a relatively high concentration in a uniform and good dispersion state while reducing the amount of polymer dispersant used for dispersion, a positive electrode paste, a positive electrode for secondary batteries and a secondary battery using the conductive additive dispersion composition for batteries, and a conductive additive dispersant that enables the high concentration of highly cohesive conductive additives such as carbon nanotubes with the addition of a small amount in the conductive additive dispersion composition for batteries, particularly in N-methyl-2-pyrrolidone.

[0014] The inventors diligently studied to achieve the above objective and tried various dispersants from among numerous polymer materials to obtain one that could improve the dispersibility of carbon nanotubes in N-methyl-2-pyrrolidone with a small amount of addition while increasing the content of the conductive additive in the dispersion. In the process, they focused on cellulose derivatives and repeatedly experimented with the application of various cellulose derivatives. Surprisingly, the inventors found that when using a cellulose ether as a dispersant, in which some of the hydroxyl groups derived from cellulose are etherified with one or more groups selected from the group consisting of methyl, ethyl, hydroxyethyl, and hydroxypropyl groups, and other parts of the hydroxyl groups derived from cellulose are etherified with a specific structure of hydroxyalkyl group having a linear alkyl group with 10 to 22 carbon atoms, it is possible to obtain a dispersion in which carbon nanotubes are uniformly and well dispersed with a high content of conductive additive while keeping the amount of dispersant added low.

[0015] In other words, the present invention provides the following conductive additive dispersion composition for batteries, a paste for positive electrodes, a positive electrode for secondary batteries, a secondary battery, and a conductive additive dispersant. 1. A conductive additive dispersion composition for batteries containing a conductive additive, cellulose ether, and N-methyl-2-pyrrolidone, wherein the cellulose ether is etherified by (i) one or more substituents selected from the group consisting of methyl, ethyl, hydroxyethyl, and hydroxypropyl groups, and the other part of the cellulose hydroxyl groups is (ii) the following general formula (1) 1. A conductive additive dispersion composition for batteries, wherein the cellulose ether is etherified with a substituent represented by (wherein R represents an alkyl group having 10 to 22 carbon atoms), and the viscosity of a 2% by mass aqueous solution of the cellulose ether at 20°C is 3 to 60,000 mPa·s. 2. The conductive additive dispersion composition for batteries according to 1, wherein the substituent of (i) in the cellulose ether is a methyl group and a hydroxypropyl group. 3. The conductive additive dispersion composition for batteries according to 1 or 2, wherein the substituent of (ii) in the cellulose ether is an n-octadecyl group in the general formula (1) above. 4. The conductive additive dispersion composition for batteries according to any one of 1 to 3, wherein the viscosity of a 2% by mass aqueous solution of the cellulose ether at 20°C is 15 to 2,000 mPa·s. 5. The conductive additive dispersion composition for batteries according to any one of 1 to 3, wherein the viscosity of a 2% by mass aqueous solution of the cellulose ether at 20°C is 5,000 to 60,000 mPa·s. 6. 11. A conductive additive dispersion composition for batteries according to any one of 1 to 5, wherein the conductive additive is a carbon nanotube. 7. A conductive additive dispersion composition for batteries according to any one of 1 to 6, wherein the content of the conductive additive is 0.5 to 10% by mass. 8. A conductive additive dispersion composition for batteries according to any one of 1 to 7, wherein the amount of cellulose ether added is 5 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the conductive additive. 9. A positive electrode paste comprising the conductive additive dispersion composition for batteries according to any one of 1 to 8, a positive electrode active material, and a binder. 10. A positive electrode for a secondary battery comprising a current collector and a positive electrode composite layer which is a coated and dried film of the positive electrode paste according to 9 formed on the current collector. 11. A secondary battery comprising the positive electrode for a secondary battery according to 10, a negative electrode, an electrolyte, and a separator. 12. (i) A portion of the cellulose-derived hydroxyl groups is etherified with one or more substituents selected from the group consisting of methyl, ethyl, hydroxyethyl, and hydroxypropyl groups, and (ii) a portion of the cellulose-derived hydroxyl groups is (ii) the following general formula (1) A conductive additive dispersant comprising cellulose ether, which is etherified with a substituent represented by (wherein R represents an alkyl group having 10 to 22 carbon atoms) and has a viscosity of 3 to 60,000 mPa·s in a 2% by mass aqueous solution at 20°C. 13. The conductive additive dispersant according to 12, for the dispersion of carbon nanotubes in N-methyl-2-pyrrolidone.

[0016] According to the present invention, it is possible to provide a conductive additive dispersion composition containing conductive additives such as carbon nanotubes, which have conventionally been difficult to disperse, in a uniform and good dispersion state. Furthermore, according to the present invention, it is possible to provide a conductive additive dispersion composition containing conductive additives such as carbon nanotubes, which are difficult to disperse, at a relatively high concentration in a uniform and good dispersion state by adding a small amount of polymer dispersant. Moreover, by using the conductive additive dispersion composition for batteries of the present invention, it is expected that the defect rate in secondary battery manufacturing will be reduced and the energy capacity of secondary batteries will be increased.

[0017] [Conductive additive dispersion composition for batteries] The conductive additive dispersion composition for batteries according to the present invention will be described below. The conductive additive dispersion composition for batteries according to the present invention is a conductive additive dispersion composition for batteries containing a conductive additive, a cellulose ether and N-methyl-2-pyrrolidone, wherein the cellulose ether is a cellulose ether in which a portion of the hydroxyl groups derived from cellulose is etherified with (i) one or more substituents selected from the group consisting of methyl groups, ethyl groups, hydroxyethyl groups and hydroxypropyl groups, and the other portion of the hydroxyl groups derived from cellulose is etherified with (ii) a substituent represented by the following general formula (1), and the viscosity of a 2% by mass aqueous solution of the cellulose ether at 20°C is 3 to 60,000 mPa·s. (In the formula, R represents an alkyl group having 10 to 22 carbon atoms.)

[0018] (Conductive additives) Conductive additives are added to enhance the conductivity of electrodes formed by the conductive additive dispersion composition for the battery (specifically, the positive electrode in the lithium-ion secondary battery described later). Examples include acetylene black, furnace black, thermal black, Ketjen black, and carbon nanotubes. Among the conductive additives, those having a high specific surface area and / or high structure or aspect ratio are preferred because they can impart high conductivity to the electrodes with a small amount of additive. Among the aforementioned conductive additives, carbon nanotubes are particularly preferred because they have a high specific surface area and a high aspect ratio. The carbon nanotubes may be single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0019] The BET specific surface area (specific surface area measured by the BET method) of the aforementioned conductive additive is 130 to 1500 m, from the viewpoint of the dispersibility of the conductive additive in N-methyl-2-pyrrolidone and the conductivity imparted to the positive electrode for lithium-ion secondary batteries. 2 Preferably, it is 150 to 1200 m / g. 2 It is more preferable that the amount is / g, and 160 to 1100m 2 It is particularly preferable that the value be / g.

[0020] When carbon nanotubes are used as a conductive additive, a smaller average outer diameter of the carbon nanotubes is considered to result in a higher aspect ratio conductive additive. From the viewpoint of dispersibility in N-methyl-2-pyrrolidone and conductivity imparted to the positive electrode of lithium-ion secondary batteries, the average outer diameter of the carbon nanotubes is preferably 0.5 to 25 nm, and particularly preferably 1 to 15 nm. The average outer diameter of the carbon nanotubes can be measured by scanning electron microscope (SEM).

[0021] The content of the conductive additive is preferably 0.5 to 10% by mass, and more preferably 0.6 to 6% by mass, from the viewpoint of the fluidity and ease of handling of the conductive additive dispersion composition to be prepared (that is, preferably 0.5 to 10 parts by mass, and more preferably 0.6 to 6 parts by mass, per 100 parts by mass of the total amount of the conductive additive dispersion composition for the battery (preferably the sum of the conductive additive, cellulose ether, and N-methyl-2-pyrrolidone)).

[0022] Furthermore, two or more different types of conductive additives may be used in combination, such as a combination of carbon nanotubes and acetylene black. When used in combination, the amount of conductive additive added is preferably 0.4 to 15 parts by mass, more preferably 0.5 to 10 parts by mass, and even more preferably 0.6 to 6 parts by mass, per 100 parts by mass of the total amount of the conductive additive dispersion composition for the battery (preferably the total of the conductive additives, methylcellulose, and N-methyl-2-pyrrolidone, including the combined additives).

[0023] (Cellulose ether (conductive additive dispersant)) The cellulose ether used in the present invention is used as a dispersant to improve the dispersibility of conductive additives, and is a cellulose ether in which a portion of the hydroxyl groups derived from cellulose is etherified with (i) one or more substituents selected from the group consisting of methyl groups, ethyl groups, hydroxyethyl groups and hydroxypropyl groups (i.e., the hydrogen atoms of the hydroxyl groups are substituted with substituents of (i)), and the other portion of the hydroxyl groups derived from cellulose is etherified with (ii) substituents represented by the following general formula (1) (i.e., the hydrogen atoms of the hydroxyl groups are substituted with substituents of (ii)). In other words, this cellulose ether is a cellulose ether having a structure in which (i) one or more substituents selected from the group consisting of methyl, ethyl, hydroxyethyl, and hydroxypropyl groups are bonded via ether bonds in place of some of the hydroxyl groups derived from cellulose in the glucose ring constituting the cellulose ether (i.e., one or more groups selected from the group consisting of methoxy, ethoxy, hydroxyethoxy, and hydroxypropoxy groups are bonded), and (ii) substituents represented by the following general formula (1) are bonded via ether bonds in place of some of the other hydroxyl groups derived from cellulose (i.e., groups represented by the general formula (2) described later are bonded), and exhibits high dispersibility in N-methyl-2-pyrrolidone for conductive additives that have high cohesiveness and are very difficult to disperse, such as carbon nanotubes. (In the formula, R represents an alkyl group having 10 to 22 carbon atoms.)

[0024] As a result of our investigations, we found that cellulose ethers in which some of the hydroxyl groups derived from cellulose are etherified with the substituents in (i) above can exhibit solubility in N-methyl-2-pyrrolidone. Through this etherification (substitution), the hydroxyl groups derived from cellulose become methoxy groups when the substituent in (i) is a methyl group, ethoxy groups when it is an ethyl group, hydroxyethoxy groups when it is a hydroxyethyl group, and hydroxypropoxy groups when it is a hydroxypropyl group. Hereafter, the degree of substitution or the number of substituted moles when modified with the substituent in (i) will be described as the degree of substitution of methoxy groups, ethoxy groups, or the number of substituted moles of hydroxyethoxy groups, hydroxypropoxy groups, etc., as measured and calculated.

[0025] When (i) is modified with methyl or ethyl groups as substituents, the degree of substitution (DS) of the methoxy and ethoxy groups is preferably 1.2 to 2.2, more preferably 1.4 to 2.0, from the viewpoint of solubility in N-methyl-2-pyrrolidone. In this case, the degree of substitution (DS) refers to the average number of methoxy or ethoxy groups per anhydrous glucose unit. The degree of substitution (DS) of the methoxy and ethoxy groups in cellulose ether can be determined by converting the values ​​obtained by measuring according to the degree of substitution analysis method for methylcellulose and ethylcellulose in the 18th edition of the Japanese Pharmacopoeia.

[0026] When (i) is modified with a hydroxyethyl group or a hydroxypropyl group as a substituent, the number of substituted moles (MS) of hydroxyethoxy and hydroxypropoxy groups is preferably 0.1 to 3.7, more preferably 0.14 to 3.6, from the viewpoint of solubility in N-methyl-2-pyrrolidone. In this case, the number of substituted moles refers to the average number of substituted moles of hydroxyethoxy or hydroxypropoxy groups per mole of anhydrous glucose unit. The number of substituted moles (MS) of hydroxyethoxy and hydroxypropoxy groups in cellulose ether can be determined by converting the values ​​obtained by the degree of substitution analysis method for hydroxyethylcellulose and hypromellose (hydroxypropyl methylcellulose) in the 18th edition of the Japanese Pharmacopoeia.

[0027] The cellulose ether used in the present invention may contain two or more substituents from the four substituent groups shown in (i). When two or more substituents are included, preferred combinations include at least one methyl group and an ethyl group and at least one hydroxyethyl group and a hydroxypropyl group, and more preferred combinations are a methyl group and a hydroxyethyl group; a methyl group and a hydroxypropyl group; an ethyl group and a hydroxyethyl group; or an ethyl group and a hydroxypropyl group.

[0028] Furthermore, when (i) contains two or more substituents, in the preferred combination of substituents of (i), the sum of the degree of substitution (DS) of the methoxy group and ethoxy group and the number of substituted moles (MS) of the hydroxyethoxy group and hydroxypropoxy group is preferably 1.4 to 3.9, more preferably 1.5 to 3.6, and even more preferably 1.5 to 2.3, from the viewpoint of solubility in N-methyl-2-pyrrolidone.

[0029] The inventors, assuming that a cellulose ether in which some of the cellulose-derived hydroxyl groups described above are etherified by the substituent in (i) above is used as a dispersant, anticipated that using a dispersant with high affinity for conductive additives such as carbon nanotubes would improve the dispersibility of conductive additives such as carbon nanotubes in the conductive additive dispersion composition, and proceeded with their investigations. As a result, they found that an alkyl group of a predetermined length as a substituent replacing the cellulose-derived hydroxyl groups in the cellulose ether exhibits affinity for conductive additives such as carbon nanotubes, but also found that the alkyl group has low affinity for N-methyl-2-pyrrolidone. From further investigations, the inventors anticipated that if the substituent in the cellulose ether that replaces the cellulose-derived hydroxyl groups and is bonded to the glucose ring via an ether bond has a predetermined structure (i.e., the alkyl group and the ether bond are bonded via a linking group of a predetermined structure), as represented by the general formula (1), it is possible to compensate for the low affinity of R (alkyl group) for N-methyl-2-pyrrolidone and increase the affinity of the entire cellulose ether molecule for N-methyl-2-pyrrolidone. Based on these predictions, various studies were conducted, and it was found that cellulose ethers in which some of the hydroxyl groups derived from cellulose (these are other hydroxyl groups different from those modified by the substituent in (i) above) are etherified by the substituent in (ii) above, represented by the following general formula (1), exhibit high affinity for N-methyl-2-pyrrolidone as a whole molecule, while also showing affinity for conductive additives such as carbon nanotubes. (In the formula, R represents an alkyl group having 10 to 22 carbon atoms.)

[0030] In general formula (1), R represents an alkyl group having 10 to 22 carbon atoms, preferably an alkyl group having 12 to 20 carbon atoms, and more preferably an alkyl group having 14 to 18 carbon atoms. Examples of R include linear alkyl groups such as n-undecyl group, n-dodecyl group, n-tridecyl group, n-tetradecyl group, n-pentadecyl group, n-hexadecyl group, n-heptadecyl group, n-octadecyl group, n-nonadecyl group, n-icosyl group, n-henicosyl group, and n-docosyl group; and branched alkyl groups such as 16-methylheptadecyl group and 18-methylicosyl group. From the viewpoint of affinity with conductive additives, n-hexadecyl group, n-heptadecyl group, and n-octadecyl group are preferred.

[0031] When the cellulose-derived hydroxyl groups of the cellulose ether are etherified with the substituent represented by (ii) in the above general formula (1), the cellulose-derived hydroxyl groups are replaced by the substituent represented by the following general formula (2). Hereafter, the number of substituted moles when modified with the substituent of (ii) is shown as the number of substituted moles of the substituent represented by the following general formula (2), which has been measured and calculated. (In the formula, R represents an alkyl group having 10 to 22 carbon atoms.)

[0032] Here, R in general formula (2) is the same as R in general formula (1) above.

[0033] The number of substituted moles (MS) of the substituent represented by general formula (2) is preferably 0.0005 or more and less than 0.1, more preferably 0.001 or more and less than 0.05, even more preferably 0.0015 or more and less than 0.02, and particularly preferably 0.002 or more and less than 0.01. The number of substituted moles of the substituent represented by general formula (2) refers to the average number of substituted moles of the substituent represented by general formula (2) per mole of anhydrous glucose unit. In cellulose ether, the number of substituted moles (MS) of the substituent represented by general formula (2) is determined from the value obtained by the quantitative method (degree of substitution measurement method) for the stearyloxyhydroxypropoxyl group described in the section on hydrophobized hydroxypropylmethylcellulose in the 2017 Standards for Additives for Quasi-drugs when the number of carbon atoms in R of general formula (2) is 18, and is determined by converting the value obtained in accordance with the above quantitative method when the number of carbon atoms in R of general formula (2) is other than 18.

[0034] The viscosity of a 2% by mass aqueous solution of cellulose ether at 20°C is preferably 3 to 60,000 mPa·s, more preferably 15 to 40,000 mPa·s, and even more preferably 15 to 30,000 mPa·s, from the viewpoint of dispersibility of the conductive additive and conductivity of the electrode formed by the conductive additive dispersion composition for the battery. Furthermore, when the specific surface area of ​​the conductive additive to be dispersed is relatively large (for example, when the BET specific surface area of ​​the conductive additive is 500 m²) 2 In cases where the amount exceeds 1 / g or when the concentration of the conductive additive in the conductive additive dispersion composition for batteries is low (for example, when the content of the conductive additive in the conductive additive dispersion composition for batteries is 2% by mass or less), it is particularly preferable that the concentration is 15 to 2,000 mPa·s, and the specific surface area of ​​the conductive additive to be dispersed is relatively small (for example, the BET specific surface area of ​​the conductive additive is 500 m²). 2When the concentration of the conductive additive in the conductive additive dispersion composition for batteries is high (for example, when the content of the conductive additive in the conductive additive dispersion composition for batteries is 2% by mass or more), the viscosity is particularly preferably 5,000 to 60,000 mPa·s, especially preferably 5,000 to 40,000 mPa·s, and most preferably 7,000 to 30,000 mPa·s. Considering resistance to elution into the electrolyte solvent for batteries, the viscosity of a 2% by mass aqueous solution of cellulose ether at 20°C is preferably 5,000 mPa·s or more. The viscosity of a 2% by mass aqueous solution of cellulose ether at 20°C can be obtained by the viscosity measurement method described in the 18th edition of the Japanese Pharmacopoeia for hypromellose (hydroxypropyl methylcellulose) (the same applies hereinafter in the examples).

[0035] From the viewpoint of dispersibility of the conductive additive and conductivity of the electrodes formed by the conductive additive dispersion composition for the battery, the amount of cellulose ether added is preferably 5 to 100 parts by mass, more preferably 5 to 50 parts by mass, and even more preferably 10 to 40 parts by mass, per 100 parts by mass of the conductive additive in the conductive additive dispersion composition for the battery. Furthermore, from the viewpoint of dispersibility of the conductive additive and conductivity of the electrodes formed by the conductive additive dispersion composition for the battery, the cellulose ether content is preferably 0.1 to 5.0% by mass, more preferably 0.1 to 1.0% by mass (i.e., preferably 0.1 to 5.0 parts by mass, and more preferably 0.1 to 1.0 parts by mass, per 100 parts by mass of the total amount of the conductive additive dispersion composition for the battery (preferably the sum of carbon black, cellulose ether, and N-methyl-2-pyrrolidone)).

[0036] As long as the above cellulose ether can be obtained, its production method is not particularly limited. For example, a cellulose ether in which a part of the hydroxyl groups derived from cellulose is etherified with one or more substituents selected from the group consisting of (i) a methyl group, an ethyl group, a hydroxyethyl group, and a hydroxypropyl group (however, the other part of the hydroxyl groups derived from cellulose is (ii) not etherified with the substituent represented by the general formula (1). Hereinafter, also referred to as "raw material cellulose ether") is dispersed in a dispersion medium to obtain a dispersion liquid, and then a step of stirring and mixing the dispersion liquid and an aqueous alkali metal hydroxide solution, and further adding an etherifying agent to the stirred mixture to carry out an etherification reaction on the raw material cellulose ether to obtain a reaction product thereof, a step of neutralizing the reaction product with an acid to obtain a neutralized reaction product, a step of cooling the neutralized reaction product, a step of washing the cooled neutralized reaction product and recovering the solid content, and a step of drying and pulverizing the solid content to obtain a cellulose ether can be obtained by a production method of cellulose ether including these steps. When the viscosity of the cellulose ether obtained by the above production method is high, a step of depolymerizing the cellulose ether can be added to obtain a low-viscosity cellulose ether within the viscosity range defined in the present invention.

[0037] Hereinafter, an example of the above production method for obtaining the cellulose ether used as the conductive assistant dispersant in the present invention from the raw material cellulose ether will be described. First, the step of dispersing the raw material cellulose ether in a dispersion medium to obtain a dispersion liquid and then stirring and mixing the dispersion liquid and an aqueous alkali metal hydroxide solution will be described.

[0038] Examples of the raw material cellulose ether include methyl cellulose (hereinafter, also referred to as "MC"), ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose (hereinafter, also referred to as "HPMC"), hydroxyethyl methyl cellulose, and the like.

[0039] The raw material cellulose ether may be synthesized by a known method or a commercially available product may be used. For example, methyl cellulose and ethyl cellulose can be prepared by reacting cellulose pulp with an alkali to obtain alkali cellulose, then reacting the obtained alkali cellulose with a methylating agent or an ethylating agent to obtain a reaction product, and then subjecting the obtained reaction product to treatments such as washing, drying, and pulverization. Also, hydroxyethyl cellulose and hydroxypropyl cellulose can be prepared by reacting cellulose pulp with an alkali to obtain alkali cellulose, then reacting the obtained alkali cellulose with a hydroxyethylating agent or a hydroxypropylating agent to obtain a reaction product, and then subjecting the obtained reaction product to treatments such as washing, drying, and pulverization. Further, hydroxypropyl methyl cellulose can be prepared by reacting cellulose pulp with an alkali to obtain alkali cellulose, then reacting the obtained alkali cellulose with a methylating agent and a hydroxypropylating agent to obtain a reaction product, and then subjecting the obtained reaction product to treatments such as washing, drying, and pulverization. Furthermore, hydroxyethyl methyl cellulose can be prepared by reacting cellulose pulp with an alkali to obtain alkali cellulose, then reacting the obtained alkali cellulose with a methylating agent and a hydroxyethylating agent to obtain a reaction product, and then subjecting the obtained reaction product to treatments such as washing, drying, and pulverization.

[0040] The degree of substitution of the methoxy group and ethoxy group and the number of moles of substitution of the hydroxyethoxy group and hydroxypropoxy group of the raw material cellulose ether are the same as those of the methoxy group, ethoxy group, hydroxyethoxy group, and hydroxypropoxy group in the cellulose ether used as the conductive aid dispersant in the present invention.

[0041] The viscosity of a 2% by mass aqueous solution of the raw material cellulose ether at 20°C is preferably 4 to 100,000 mPa·s, more preferably 10 to 80,000 mPa·s, from the viewpoint of dispersibility of the conductive additive and conductivity of the electrode formed by the conductive additive dispersion composition for batteries. The viscosity of a 2% by mass aqueous solution of the raw material cellulose ether at 20°C can be obtained by the viscosity measurement method described in the 18th edition of the Japanese Pharmacopoeia for hypromellose (hydroxypropyl methylcellulose) (the same applies hereinafter in the examples).

[0042] The dispersion medium used to disperse the raw material cellulose ether is preferably one that does not dissolve the raw material cellulose ether and has good affinity with the alkali metal hydroxide solution added later. Examples include methanol, ethanol, n-propyl alcohol, sec-propyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, ethylene glycol, acetonitrile, and dimethylformamide. Two or more of the above may be used in combination as the dispersion medium, and water may be included in an amount that does not dissolve the raw material cellulose ether. The amount of dispersion medium used is preferably 100 to 50,000 parts by mass, and more preferably 300 to 10,000 parts by mass, per 100 parts by mass of raw material cellulose ether.

[0043] Examples of alkali metal hydroxide solutions include aqueous solutions of sodium hydroxide or potassium hydroxide.

[0044] From an economic and ease of handling standpoint, the concentration of alkali metal hydroxide in the alkali metal hydroxide solution is preferably 5 to 60% by mass.

[0045] The amount of alkali metal hydroxide solution used can be appropriately set depending on the number of substitution moles (MS) of the substituent represented by general formula (2) of the cellulose ether to be produced and the concentration of alkali metal hydroxide in the alkali metal hydroxide solution. Preferably, the amount is 0.005 to 2.0 parts by mass of alkali metal hydroxide per unit mass part (1.00 parts by mass) of raw material cellulose ether, and more preferably 0.01 to 1.5 parts by mass.

[0046] The stirring and mixing of the dispersion and the alkali metal hydroxide solution can be carried out using a reactor, such as a reactor with an internal stirrer. The atmosphere inside the reactor can be appropriately selected to adjust the viscosity of the cellulose ether. For example, a nitrogen atmosphere may be used to suppress excessive reduction of viscosity of the cellulose ether, or an air atmosphere may be used to promote reduction of viscosity to an appropriate viscosity. The stirring and mixing time is preferably 0.1 to 24 hours.

[0047] Next, we will describe the step of adding an etherifying agent to the stirred mixture and carrying out an etherification reaction of the raw material cellulose ether to obtain the reaction product. In this step, the etherifying agent is used to introduce a substituent represented by general formula (1) into the hydroxyl group portion derived from cellulose of the raw material cellulose ether.

[0048] Preferably, the etherifying agent is a glycidyl ether compound having an alkyl group with 10 to 22 carbon atoms corresponding to R in general formula (1). Examples include n-undecylglycidyl ether, n-dodecylglycidyl ether, n-tridecylglycidyl ether, n-tetradecylglycidyl ether, n-pentadecylglycidyl ether, n-hexadecylglycidyl ether, n-heptadecylglycidyl ether, n-octadecylglycidyl ether (also known as stearylglycidyl ether), n-nonadecylglycidyl ether, n-icosylglycidyl ether, n-henicosylglycidyl ether, n-docosylglycidyl ether, 16-methylheptadecylglycidyl ether, 18-methylicosylglycidyl ether, and the like.

[0049] The amount of etherifying agent added can be appropriately determined so that the number of substituted moles (MS) of substituents represented by general formula (2) in the cellulose ether to be produced falls within the range described above, but it is preferably 0.01 to 1.0 parts by mass per 1.0 part by mass of raw material cellulose ether.

[0050] The reaction temperature for the etherification reaction using the etherifying agent is preferably 30 to 100°C. The reaction time for the etherification reaction is preferably 1 to 10 hours.

[0051] Next, we will describe the step of neutralizing the reaction product with an acid to obtain a neutralization reaction product. Any acid that can be removed in the subsequent washing step can be used without issue, and examples include hydrochloric acid, nitric acid, acetic acid, citric acid, oxalic acid, and phosphoric acid. Here, neutralization in this step means adding an acid to the reaction product to obtain a neutralization reaction product with a pH of 5 to 9, and the neutralization step is terminated when it is confirmed that the pH of the neutralization reaction product has become 5 to 9. Since heat is generated during the neutralization step due to the heat of neutralization, the neutralization step may be carried out while cooling the reaction composition to be neutralized in an ice bath or the like.

[0052] Next, the step of cooling the neutralization reaction product will be described. The reaction solution containing the neutralization reaction product is cooled before washing the neutralization reaction product. The temperature of the neutralization reaction product after cooling is preferably 10 to 30°C from the viewpoint of preventing the volatilization of the solvent used in the subsequent washing step.

[0053] Next, we will describe the process of washing the neutralization reaction product after cooling and recovering the solid component. The neutralization reaction product after cooling is washed with a solvent that does not dissolve the generated cellulose ether.

[0054] Since the components to be removed by washing are hydrophilic and hydrophobic substances with different properties, it is advisable to wash using different types of solvents, such as hydrophobic solvents, amphiphilic solvents, and affinity solvents. For example, a washing method may involve washing with a hydrophobic solvent such as hexane or diethyl ether, followed by washing with an amphiphilic solvent such as acetone, acetonitrile, dimethylformamide, or isopropyl alcohol, and finally washing with a highly hydrophilic solvent such as methanol, ethanol, or hot water. During washing, the solids and washing solution are separated using commonly known methods such as filtration or decantation, and the solids are recovered.

[0055] Next, the process of drying and grinding the solid to obtain cellulose ether will be described. By drying and grinding the obtained solid, powdered cellulose ether is obtained. There are no restrictions on the equipment that can be used in the drying process, but examples include a forced-air dryer. Similarly, there are no restrictions on the equipment that can be used in the grinding process as long as it is capable of grinding cellulose ether, but examples include an impact grinder or a ball mill.

[0056] If the viscosity of the cellulose ether obtained in the above grinding step is high when it is a 2% by mass aqueous solution at 20°C, the viscosity can be adjusted to the desired level by adding a step of depolymerizing the cellulose ether. Depolymerization can be carried out according to conventional methods. For example, if an acid-based depolymerization method is used, a hydrogen halide such as hydrogen chloride can be used as the acid catalyst for depolymerization. The acid may also be used in the form of an aqueous solution. The concentration of hydrogen chloride in the hydrogen chloride aqueous solution is preferably 1 to 45% by mass.

[0057] The amount of acid used is preferably 0.04 to 1 part by mass per 100 parts by mass of cellulose ether before depolymerization. The reaction temperature for depolymerization with acid is preferably 40 to 85°C. The reaction time for depolymerization with acid is preferably 0.1 to 4 hours.

[0058] After the depolymerization with acid is complete, the acid can be removed by reducing the pressure in the reactor. Alternatively, if necessary, a weak alkali such as sodium bicarbonate or sodium carbonate may be added to neutralize the acid used in depolymerization. Through these steps, cellulose ether, to be used as a conductive additive dispersant in the present invention, is obtained.

[0059] (N-methyl-2-pyrrolidone) N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") is an organic solvent used in the manufacture of the positive electrode of lithium-ion secondary batteries, and is also included as a dispersion medium for the conductive additive in the conductive additive dispersion composition for batteries of the present invention.

[0060] There are no particular restrictions on the NMP content, but from the viewpoint of ease of handling of the conductive additive dispersion composition for batteries, 85 to 99.9% by mass is preferred, and 90 to 99.3% by mass is more preferred (that is, preferably 85 to 99.9 parts by mass, and more preferably 90 to 99.3 parts by mass, per 100 parts by mass of the total amount of the conductive additive dispersion composition for batteries (preferably the sum of the conductive additive, cellulose ether, and N-methyl-2-pyrrolidone)).

[0061] Furthermore, from the viewpoint of improving the affinity between each component (components of the conductive additive dispersion composition for the battery and the positive electrode paste), one or more other solvents may be used in combination. Examples of solvents other than N-methyl-2-pyrrolidone include water, N-ethyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylpropionamide, N,N-diethylacetamide, and 4-acetylmorpholine. In the present invention, it is preferable to use NMP alone as the solvent.

[0062] (Other Components) The conductive additive dispersion composition for batteries of the present invention may contain other components within the scope of the objectives of the present invention, or within the scope that does not impede the dispersibility of the conductive additive or its performance when formed into electrodes. Examples of other components include surfactants, defoamers, pH adjusters, and viscosity modifiers.

[0063] The method for producing the conductive additive dispersion composition of the present invention is not particularly limited as long as the above components are uniformly mixed and the conductive additive can be dispersed. For example, it can be prepared by a dispersion process in which the conductive additive, cellulose ether, NMP, etc. are dispersed using a dispersion apparatus. In this case, the conductive additive and cellulose ether, which are powder raw materials, may be mixed in advance and then dispersed in NMP, or the cellulose ether may be dissolved in a solvent such as NMP beforehand and then mixed with the conductive additive.

[0064] Dispersion devices used include homogenizers, homodispersers, planetary mixers, paint conditioners, bead mills, and thin-film swirling high-speed mixers.

[0065] In the dispersion process described above, from the viewpoint of further improving the dispersibility of the conductive additive, the dispersion process may be carried out in stages using multiple dispersion devices. For example, one method is to uniformly mix the powder raw materials (conductive additive, cellulose ether) and dispersion medium (NMP) using a homodisperser or planetary mixer, and then disperse the conductive additive using a bead mill or thin-film swirling high-speed mixer.

[0066] [Positive Electrode Paste] The positive electrode paste of the present invention comprises the above-described conductive additive dispersion composition for batteries, positive electrode active material, and binder, and can be used in the manufacture of a positive electrode for a lithium-ion secondary battery.

[0067] As for the positive electrode active material, there are no particular restrictions as long as it is a material used as the positive electrode of a lithium-ion secondary battery, but transition metal oxides containing lithium elements, or transition metals in which some of the transition metal elements contained in the transition metal oxide are substituted with other elements can be used. Specifically, lithium cobaltate, lithium nickelate, lithium manganeseate, ternary (NCM) active materials, and NCA active materials fall into this category. In addition, unlike the oxide-based active materials mentioned above, phosphate ions (PO4) can be used. 3- ) and silicate ions (SiO4 4- By introducing multivalent anions such as ), polyanionic cathode active materials that enhance the ionic properties of transition metals can also be used, specifically including lithium iron phosphate and lithium iron silicate.

[0068] The content of the positive electrode active material is 35 to 80% by mass, preferably 40 to 70% by mass (35 to 80 parts by mass, preferably 40 to 70 parts by mass, per 100 parts by mass of positive electrode paste). If the content of the positive electrode active material is less than 35% by mass, the energy density when assembled as a battery will be poor, and if it exceeds 80% by mass, the positive electrode paste will harden, which may make the paste coating process on the current collector difficult.

[0069] As a binder, fluorine-based polymer materials are preferred from the viewpoint of durability of the formed cathode composite layer, such as polyvinylidene fluoride, polyvinyl fluoride, and tetrafluoroethylene. Furthermore, from the viewpoint of the binding properties and coating properties of the cathode paste, the weight-average molecular weight of the polymer material used as a binder is preferably 200,000 to 2,000,000, and more preferably 600,000 to 1,500,000. The weight-average molecular weight of the polymer material used as a binder can be measured by known methods such as gel permeation chromatography.

[0070] The binder content is 0.3 to 10% by mass, preferably 0.5 to 6% by mass (i.e., 0.3 to 10 parts by mass, preferably 0.5 to 6 parts by mass, per 100 parts by mass of positive electrode paste). If the binder content is less than 0.3% by mass, the strength of the positive electrode composite layer will be insufficient, and peeling or cracking of the composite layer may occur. If it exceeds 10% by mass, the electrical resistance inside the electrode may increase.

[0071] The positive electrode paste of the present invention can be prepared by mixing the above-described conductive additive dispersion composition for batteries, the positive electrode active material, and the binder, and the dispersion apparatus shown in the above-described method for producing the conductive additive dispersion composition for batteries can be used as the mixing apparatus.

[0072] When manufacturing the positive electrode paste, a solvent may be added from the viewpoint of viscosity adjustment. As the solvent, the solvent (such as NMP) shown in the conductive additive dispersion composition for batteries of the present invention described above can be used. The amount of solvent (NMP) contained in the positive electrode paste at this time (i.e., the sum of the amount of NMP contained in the conductive additive dispersion composition for batteries and the amount of NMP added) is 10 to 70% by mass, and preferably 20 to 60% by mass.

[0073] Furthermore, there are no particular limitations on the procedure for adding materials during mixing. All materials may be added and mixed simultaneously, or the mixing process may be carried out in stages, such as adding the battery conductive additive dispersion composition after mixing the binder, positive electrode active material, and solvent. Regarding the binder, it is recommended to dissolve it in the solvent beforehand to prepare a solution and then add it, in order to prevent foreign matter such as undissolved studs from contaminating the positive electrode paste.

[0074] The positive electrode paste of the present invention is suitable for manufacturing positive electrodes for lithium-ion secondary batteries. Specifically, the positive electrode paste can be applied to a current collector to an arbitrary thickness to form a coating film, the coating film can be dried, and the resulting coated and dried film can be used as a positive electrode composite layer to manufacture a positive electrode for a lithium-ion secondary battery.

[0075] Here, the current collector can be a film foil made of a metal or alloy such as iron, stainless steel, copper, aluminum, or nickel, with aluminum being particularly preferred from the viewpoint of potential stability at the positive electrode. The current collector may be subjected to surface treatment such as carbon coating from the viewpoint of reducing interfacial resistance.

[0076] There are no particular restrictions on the equipment used for coating the positive electrode paste, but examples include knife coaters, comma coaters, die coaters, and gravure coaters. In addition, to improve the electrical resistance of the electrode and the bonding properties of the positive electrode composite layer, the positive electrode paste coated on the current collector or the coated and dried film of the positive electrode paste may be subjected to rolling treatment using a roll press or the like.

[0077] The coating thickness of the positive electrode paste is preferably 50 to 1000 μm, and more preferably 100 to 500 μm.

[0078] The coating film of the positive electrode paste is dried in a drying oven after coating. The drying temperature and time are preferably 50 to 180°C for 0.5 to 1200 minutes, more preferably 60 to 140°C for 1 to 600 minutes. Vacuum drying may be performed at this time to remove moisture and solvent components.

[0079] This provides a positive electrode for a lithium-ion secondary battery comprising a current collector and a positive electrode composite layer which is a coated and dried film of the positive electrode paste of the present invention. The thickness of the positive electrode composite layer is preferably 10 to 800 μm, and more preferably 30 to 400 μm.

[0080] [Lithium-ion secondary battery] The lithium-ion secondary battery of the present invention comprises a positive electrode for lithium-ion secondary batteries, a negative electrode, an electrolyte, and a separator, the positive electrode for lithium-ion secondary batteries containing the positive electrode composite layer of the present invention described above, and has a structure in which the electrolyte is impregnated in a sealed state with a separator sandwiched between the positive electrode and the negative electrode.

[0081] In this case, the negative electrode of the lithium-ion secondary battery has a negative electrode composite material layer on one or both sides of the negative electrode current collector.

[0082] Similar to the current collectors used in the positive electrodes of lithium-ion secondary batteries, metal or alloy film foils are used for the negative electrode current collector. However, copper foil and nickel foil are preferred from the viewpoint of potential stability in the negative electrode.

[0083] The negative electrode composite layer contains one or more negative electrode active materials capable of intercalating and deintercalating lithium ions, and may optionally contain a negative electrode binder, a negative electrode conductive additive, and a negative electrode dispersant.

[0084] There are no particular restrictions on the negative electrode active material as long as it is a material used as a negative electrode active material in lithium-ion secondary batteries, but specific examples include carbon-based negative electrode materials such as natural graphite and artificial graphite, oxide-based negative electrode materials such as lithium titanate, and Si-based negative electrode materials such as nanosilicon, silicon alloys, silicon monoxide, and silicon-carbon composites.

[0085] As a binder for the negative electrode, one or more of the following can be used: polymer materials, synthetic rubber, etc. Examples of polymer materials include polyvinylidene fluoride, polyimide, polyamide-imide, aramid, polyacrylic acid, lithium polyacrylate, sodium carboxymethylcellulose, etc. Examples of synthetic rubbers include styrene-butadiene rubber, fluorine-based rubber, ethylene propylenediene, etc.

[0086] As a conductive additive for the negative electrode, one or more carbon materials such as acetylene black, Ketjen black, graphite, carbon nanotubes, and carbon nanofibers can be used.

[0087] As a dispersant for the negative electrode, one or more of the following can be used: methylcellulose, hydroxypropylmethylcellulose, ethylcellulose, sodium carboxymethylcellulose, polyvinyl alcohol, polyvinylpyrrolidone, and polyurethane.

[0088] A separator electronically insulates the positive and negative electrodes of a lithium-ion secondary battery, preventing current short circuits caused by contact between the two electrodes, while allowing lithium ions to pass through by impregnating the electrolyte. This separator is formed from a porous membrane made of, for example, synthetic resin or ceramic, and may have a laminated structure in which two or more porous membranes are stacked. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, and polyethylene, and examples of ceramics include alumina.

[0089] The electrolyte is a composition that mediates ion conduction between the positive and negative electrodes. For example, the electrolyte is prepared by dissolving a lithium salt, such as lithium hexafluoride phosphate, in a non-aqueous solvent mixture of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate. Additives may also be added to the electrolyte to improve stability, safety, and reduce resistance.

[0090] As described above, the conductive additive dispersion composition for batteries of the present invention exhibits excellent dispersibility even when containing a relatively high concentration of a highly cohesive conductive additive. By using this conductive additive dispersion composition for batteries, the coating properties of the positive electrode paste are improved, reducing the defect rate in the manufacture of lithium-ion secondary batteries and lowering production costs. Furthermore, since the increase in the internal resistance of the manufactured electrodes is suppressed, it becomes possible to increase the energy capacity of the lithium-ion secondary battery.

[0091] In this invention, "good dispersibility of the conductive additive in the conductive additive dispersion composition for batteries" means that the conductive additive is uniformly dispersed in the conductive additive dispersion composition for batteries, has a smooth surface appearance, and exhibits good dispersibility evaluation and electrical resistance evaluation by rheology.

[0092] Here, the rheological evaluation of the dispersibility of the conductive additive dispersion composition for batteries is performed using a rotary rheometer. Specifically, a rotary rheometer (Anton Paar "MCR702") is used, a cone plate is used as the measuring fixture, the set temperature is set to 25°C, and the viscosity of the composition is measured while sweeping the shear rate in the range of 0.1 to 1000 / s. If the viscosity decreases with increasing shear rate across the entire shear rate range, the dispersibility of the conductive additive in the conductive additive dispersion composition for batteries is considered good. On the other hand, if any increase in viscosity is observed even in part during the process of increasing shear rate, the dispersibility of the conductive additive in the conductive additive dispersion composition for batteries is considered poor.

[0093] Furthermore, the electrical resistance evaluation of the conductive additive dispersion composition for batteries is performed by mixing the conductive additive dispersion composition for batteries with a binder solution used in the positive electrode paste to prepare an evaluation slurry, and then evaluating the DC resistance of the slurry and the volume resistivity of the coating obtained by coating the evaluation slurry onto a glass substrate, thereby evaluating the dispersibility of the conductive additive from the perspective of electrical resistance.Since electrical resistance mainly depends on the physical properties of the conductive additive used, an evaluation slurry is prepared to have a predetermined conductive additive concentration for each conductive additive used, and evaluation criteria for electrical resistance (DC resistance component of the evaluation slurry and volume resistivity of the coating) are established and evaluated for each conductive additive used by the following method. (Measurement of DC resistance component of evaluation slurry) An appropriate amount of evaluation slurry is poured into an electrode cell (HIOKI E.E. CORPORATION "SA9001"), and impedance measurement is performed using measurement software (HIOKI E.E. CORPORATION "SA2633") with an LCR meter (HIOKI E.E. CORPORATION "IM3536-01") equipped with a test fixture (HIOKI E.E. CORPORATION "SA9002") to measure the DC resistance component of the evaluation slurry. If the DC resistance is lower than the set evaluation standard value, it is evaluated as good, and if it is higher than that value, it is evaluated as poor. (Measurement of volume resistivity of coating film) The above evaluation slurry is placed on a smooth glass plate and coated with an applicator with a gap of 300 μm. A coating film is obtained by drying the coated glass plate in a dryer at 110°C for 30 minutes. The surface resistance of the obtained coating film is measured using a low resistivity meter (Loresta GX MCP-T710, manufactured by Nitto Seiko Analytech Co., Ltd.), and the volume resistivity (Ω・cm) at room temperature is calculated by dividing the surface resistance by the coating film thickness measured using a thickness gauge (MFC-101A, manufactured by Nikon Corporation). A volume resistivity lower than the set evaluation standard value is considered good, and a value higher than that is considered poor. The dispersibility of the conductive additive is considered good if both the evaluation results of the DC resistance component of the evaluation slurry and the volume resistivity of the coating film are good. In all other cases (i.e., if only one or both evaluation results of the DC resistance component of the evaluation slurry and the volume resistivity of the coating film are poor), the dispersibility of the conductive additive is considered poor.

[0094] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples. The viscosity of a 2% by mass aqueous solution of cellulose ether at 20°C was measured using an Ubbelohde viscometer as specified in JIS K2283-1993. The degree of substitution of methoxy groups (DS) was determined by converting the value obtained by measuring according to the degree of substitution analysis method for methylcellulose in the 18th edition of the Japanese Pharmacopoeia. The number of substituted moles of hydroxypropoxy groups (MS) was determined by converting the value obtained by measuring according to the degree of substitution analysis method for hypromellose (hydroxypropyl methylcellulose) in the 18th edition of the Japanese Pharmacopoeia. The number of substituted moles (MS) of substituents represented by the general formula (2) (where R in formula (2) is an octadecyl group) was determined according to the stearyloxyhydroxypropoxyl group substitution degree measurement method described in the section on hydrophobized hydroxypropyl methylcellulose in the 2017 Standards for Additives in Quasi-drugs. Furthermore, the amount of elution of the synthesized cellulose ether (dispersant) into the electrolyte solvent was measured according to the following procedure. (Measurement of elution amount of cellulose ether into electrolyte solvent) A 2% by mass aqueous solution of cellulose ether was poured into a stainless steel mold and heated and dried at 60°C to form a film. This was cut into strips measuring 1 cm x 2 cm to obtain test pieces of 40-60 mg. After recording the weight of the obtained test pieces, they were immersed in a battery research reagent (ethylene carbonate / dimethyl carbonate = 1:1 (V / V%) mixed solvent, manufactured by Kanto Chemical Co., Ltd.), which was the electrolyte solvent, and left to stand in a constant temperature bath at 40°C for 3 days. After that, the immersed film was washed on the surface with dimethyl carbonate and then dried in a dry heat oven at 100°C. The amount of elution into the electrolyte solvent was evaluated by dividing the difference between the weight of the film after drying and the weight of the film before immersion by the weight of the film before immersion. Specifically, the amount of elution into the electrolyte solvent was determined by the following formula. Elution amount (%) = (Weight of film before immersion (mg) - Weight of film after immersion (mg)) / Weight of film before immersion × 100

[0095] Furthermore, the average outer diameter of the carbon nanotubes was determined by scanning electron microscopy (SEM), and the BET specific surface area was measured using the BET method. The room temperature was 23°C.

[0096] <Preparation of Cellulose Ether> (Synthesis Example 1: Synthesis of CE-1) At room temperature, 400 g of hydroxypropyl methylcellulose (hereinafter referred to as HPMC, manufactured by Shin-Etsu Chemical Co., Ltd.), which has a viscosity of 10,000 mPa·s in a 2% by mass aqueous solution at 20°C, a degree of substitution (DS) of methoxy groups (MeO groups) of 1.9, and a number of moles of substituted hydroxypropoxy groups (HPO groups) of 0.25, and 4000 g of tert-butyl alcohol were added to a reactor and stirred to disperse the HPMC in the tert-butyl alcohol. Next, 200 g of a 6% by mass aqueous solution of sodium hydroxide was added to the reactor and stirred under a nitrogen atmosphere for 2 hours. 90 g of stearyl glycidyl ether was added while stirring and stirred at 50°C for 5 hours to obtain the reaction product. After the etherification reaction was complete, the reaction product was neutralized with acetic acid to obtain a neutralization reaction product with a pH of 7. The neutralization reaction product was then cooled to 10-20°C. The solids of the cooled neutralization reaction product were filtered off, washed with hexane, then with acetone, and further washed with hot water at 80°C to recover the solids. The washed solids were dried in a forced-air dryer to a moisture content of 2% by mass or less, and then pulverized using a Victory Mill impact pulverizer to obtain powdered cellulose ether (cellulose ether intermediate product). Next, a 14% by mass aqueous hydrochloric acid solution at room temperature was sprayed onto 100 parts by mass of the obtained cellulose ether intermediate product so that the hydrogen chloride content was 0.35 parts by mass. This was transferred to a glass reactor, and the cellulose ether sprayed with the hydrochloric acid solution was reacted for 120 minutes while the glass reactor, with a jacket temperature of 80°C, was rotated to depolymerize it. Subsequently, while maintaining the jacket temperature at 80°C, the reactor was left under a reduced pressure of 5000 Pa for 30 minutes to remove the hydrochloric acid from the glass reactor. Then, sodium bicarbonate equivalent to 1 / 2 mole of the added hydrogen chloride was added to neutralize the cellulose ether, thereby obtaining cellulose ether-1 (hereinafter also referred to as "CE-1").

[0097] (Synthesis Example 2: Synthesis of CE-2) The process was carried out in the same manner as in Synthesis Example 1 until an intermediate cellulose ether product was obtained. A 14% hydrochloric acid aqueous solution at room temperature was sprayed onto 100 parts by mass of the obtained cellulose ether intermediate product so that the hydrogen chloride content was 0.40 parts by mass. This was transferred to a glass reactor, and the cellulose ether to which the hydrochloric acid aqueous solution had been sprayed was reacted for 110 minutes while the glass reactor, with a jacket temperature of 80°C, was rotated to depolymerize it. After that, while maintaining the jacket temperature at 80°C, the reactor was left under a reduced pressure of 5000 Pa for 30 minutes to remove the hydrochloric acid in the glass reactor, and sodium bicarbonate equivalent to 1 / 2 mole of the added hydrogen chloride was added to neutralize the cellulose ether, thereby obtaining cellulose ether-2 (hereinafter also referred to as "CE-2").

[0098] (Synthesis Example 3: Synthesis of CE-3) The process was carried out in the same manner as in Synthesis Example 1 until an intermediate cellulose ether product was obtained. A 14% hydrochloric acid aqueous solution at room temperature was sprayed onto 100 parts by mass of the obtained cellulose ether intermediate product so that the hydrogen chloride content was 0.35 parts by mass. This was transferred to a glass reactor, and the cellulose ether sprayed with the hydrochloric acid aqueous solution was reacted for 90 minutes while the glass reactor, with a jacket temperature of 80°C, was rotated to depolymerize it. After that, while maintaining the jacket temperature at 80°C, the reactor was left under a reduced pressure of 5000 Pa for 30 minutes to remove the hydrochloric acid in the glass reactor, and sodium bicarbonate equivalent to 1 / 2 mole of the added hydrogen chloride was added to neutralize the cellulose ether, thereby obtaining cellulose ether-3 (hereinafter also referred to as "CE-3").

[0099] (Synthesis Example 4: Synthesis of CE-4) At room temperature, 400 g of HPMC (manufactured by Shin-Etsu Chemical Co., Ltd.), which has a viscosity of 4,000 mPa·s in a 2% by mass aqueous solution at 20°C, a degree of substitution (DS) of methoxy groups (MeO groups) of 1.8, and a number of moles of substituted hydroxypropoxy groups (HPO groups) of 0.15, and 4000 g of tert-butyl alcohol were added to a reactor and stirred to disperse the HPMC in the tert-butyl alcohol. Next, 100 g of a 6% by mass aqueous solution of sodium hydroxide was added to the reactor and stirred under a nitrogen atmosphere for 2 hours. Then, 30 g of stearyl glycidyl ether was added while stirring and stirred at 50°C for 5 hours. After the etherification reaction was complete, the reaction product was neutralized with acetic acid to obtain a pH 7 neutralization reaction product. The neutralization reaction product was then cooled to 10-20°C. The solids of the cooled neutralization reaction product were filtered off, washed with hexane, then with acetone, and further washed with hot water at 80°C to recover the solids. The washed solids were dried in a forced-air dryer to a moisture content of 2% by mass or less, and then pulverized using a Victory Mill impact pulverizer to obtain powdered cellulose ether (cellulose ether intermediate product). Next, a 14% by mass aqueous hydrochloric acid solution at room temperature was sprayed onto 100 parts by mass of the obtained cellulose ether intermediate product so that the hydrogen chloride content was 0.18 parts by mass. This was transferred to a glass reactor, and the cellulose ether sprayed with the hydrochloric acid solution was reacted for 90 minutes while the glass reactor, with a jacket temperature of 80°C, was rotated to depolymerize it. After that, while maintaining the jacket temperature at 80°C, the reactor was left under a reduced pressure of 5000 Pa for 30 minutes to remove the hydrochloric acid in the glass reactor, and sodium bicarbonate equivalent to 1 / 2 mole of the added hydrogen chloride was added to neutralize the cellulose ether, thereby obtaining cellulose ether-4 (hereinafter referred to as "CE-4").

[0100] (Synthesis Example 5: Synthesis of CE-5) At room temperature, 400 g of HPMC (manufactured by Shin-Etsu Chemical Co., Ltd.), which has a viscosity of 4,000 mPa·s in a 2% by mass aqueous solution at 20°C, a degree of substitution (DS) of methoxy groups (MeO groups) of 1.4, and a number of moles of substituted hydroxypropoxy groups (HPO groups) of 0.2, and 4000 g of tert-butyl alcohol were added to a reactor and stirred to disperse the HPMC in the tert-butyl alcohol. Next, 150 g of a 6% by mass aqueous solution of sodium hydroxide was added to the reactor and stirred under a nitrogen atmosphere for 2 hours. Then, 60 g of stearyl glycidyl ether was added while stirring and stirred at 50°C for 5 hours. After the etherification reaction was complete, the reaction product was neutralized with acetic acid to obtain a pH 7 neutralization reaction product. The neutralization reaction product was then cooled to 10-20°C. The solids of the cooled neutralization reaction product were filtered off, washed with hexane, then with acetone, and further washed with hot water at 80°C to recover the solids. The washed solids were dried in a forced-air dryer to a moisture content of 2% by mass or less, and then pulverized using a Victory Mill impact pulverizer to obtain powdered cellulose ether (cellulose ether intermediate product). Next, a 14% by mass aqueous hydrochloric acid solution at room temperature was sprayed onto 100 parts by mass of the obtained cellulose ether intermediate product so that the hydrogen chloride content was 0.20 parts by mass. This was transferred to a glass reactor, and the cellulose ether sprayed with the hydrochloric acid solution was reacted for 90 minutes while the glass reactor, with a jacket temperature of 80°C, was rotated to depolymerize it. After that, while maintaining the jacket temperature at 80°C, the reactor was left under a reduced pressure of 5000 Pa for 30 minutes to remove the hydrochloric acid in the glass reactor, and sodium bicarbonate equivalent to 1 / 2 mole of the added hydrogen chloride was added to neutralize the cellulose ether to obtain cellulose ether-5 (hereinafter referred to as "CE-5").

[0101] (Synthesis Example 6: Synthesis of CE-6) The synthesis was terminated when the cellulose ether intermediate product from Synthesis Example 1 was obtained. That is, the cellulose ether intermediate product from Synthesis Example 1 was used as cellulose ether-6 (hereinafter also referred to as "CE-6").

[0102] (Synthesis Example 7: Synthesis of CE-7) At room temperature, 120 g of HPMC (manufactured by Shin-Etsu Chemical Co., Ltd.), which has a viscosity of 50 mPa·s in a 2% by mass aqueous solution at 20°C, a degree of substitution (DS) of methoxy groups (MeO groups) of 1.9, and a number of moles (MS) of hydroxypropoxy groups (HPO groups) of 0.23, and 960 g of sec-propyl alcohol were added to a reactor and stirred to disperse the HPMC in the sec-propyl alcohol. Next, 9.9 g of a 49% by mass aqueous solution of sodium hydroxide was added to the reactor and stirred under air for 10 minutes. Then, 10 g of stearyl glycidyl ether was added while stirring and stirred at 80°C for 5 hours. After the etherification reaction was complete, the reaction product was neutralized with acetic acid to obtain a pH 7 neutralization reaction product. The neutralization reaction product was then cooled to 10-20°C. The solid component of the cooled neutralization reaction product was filtered off, and the obtained solid component was washed with sec-propyl alcohol, and then washed again with hot water at 95°C to recover the solid component. The washed solid component was dried in a forced-air dryer to a moisture content of 2% by mass or less, and then pulverized using a Victory Mill impact pulverizer to obtain powdered cellulose ether (cellulose ether intermediate process product). This was used as cellulose ether-7 (hereinafter also referred to as "CE-7").

[0103] (Synthesis Example 8: Synthesis of CE-8) At room temperature, 100 g of HPMC (manufactured by Shin-Etsu Chemical Co., Ltd.), which has a viscosity of 4,000 mPa·s in a 2% by mass aqueous solution at 20°C, a degree of substitution (DS) of methoxy groups (MeO groups) of 1.9, and a number of moles (MS) of hydroxypropoxy groups (HPO groups) of 0.24, and 150 g of sec-propyl alcohol were added to a reactor and stirred to disperse the HPMC in the sec-propyl alcohol. Next, 8.2 g of a 49% by mass aqueous solution of sodium hydroxide was added to the reactor and stirred under air for 10 minutes. Then, 2.8 g of stearyl glycidyl ether was added while stirring and stirred at 80°C for 5 hours. After the etherification reaction was complete, the reaction product was neutralized with acetic acid to obtain a pH 7 neutralization reaction product. The neutralization reaction product was then cooled to 10-20°C. The solids of the cooled neutralization reaction product were filtered off, washed with sec-propyl alcohol, and then washed again with hot water at 95°C to recover the solids. The washed solids were dried in a forced-air dryer to a moisture content of 2% by mass or less, and then pulverized using a Victory Mill impact pulverizer to obtain powdered cellulose ether (cellulose ether intermediate product). This was then used as cellulose ether-8 (hereinafter also referred to as "CE-8").

[0104] (Synthesis Example 9: Synthesis of CE-9) At room temperature, 60 g of methylcellulose (MC) (manufactured by Shin-Etsu Chemical Co., Ltd.), having a viscosity of 15 mPa·s in a 2% by mass aqueous solution at 20°C and a degree of methoxy group (MeO group) substitution (DS) of 1.7, and 480 g of sec-propyl alcohol were added to a reactor and stirred to disperse the MC in the sec-propyl alcohol. Next, 5.2 g of a 49% by mass aqueous solution of sodium hydroxide was added to the reactor and stirred under air for 10 minutes. Then, 6.4 g of stearyl glycidyl ether was added while stirring and stirred at 80°C for 5 hours. After the etherification reaction was complete, the reaction product was neutralized with acetic acid to obtain a pH 7 neutralization reaction product. The neutralization reaction product was then cooled to 10-20°C. The solid content of the cooled neutralization reaction product was filtered off, the obtained solid content was washed with sec-propyl alcohol, and then washed with hot water at 95°C to recover the solid content. The washed solids were dried to a moisture content of 2% by mass or less using a forced-air dryer, and then pulverized using a Victory Mill impact pulverizer to obtain powdered cellulose ether (cellulose ether intermediate process product). This was then used as cellulose ether-9 (hereinafter also referred to as "CE-9").

[0105] Table 1 shows the viscosity of a 2% by mass aqueous solution at 20°C, the degree of substitution (DS) and number of substituted moles (MS) of various substituents, and the amount of elution into the electrolyte solvent for the cellulose ethers obtained in Synthesis Examples 1 to 9. Of the groups present in the obtained cellulose ethers, the methoxy group (MeO group) and hydroxypropoxy group (HPO group) correspond to those modified with substituent (i), and the group represented by the following formula (2A) corresponds to those modified with substituent (ii).

[0106]

[0107] [Test Materials] The materials used in the battery conductive additive dispersion compositions in the examples and comparative examples are shown below. <Conductive Additive> ・Carbon nanotube A (hereinafter, "CNT-A"): Multiwall carbon nanotube, average outer diameter 10 nm, BET specific surface area 205 m² 2 / g • Carbon nanotube B (hereinafter, "CNT-B"): Multiwalled carbon nanotube, average outer diameter 6 nm, BET specific surface area 631 m²2 / g · Carbon nanotube C (hereinafter, "CNT-C"): Multi-walled carbon nanotube, average outer diameter 15 nm, BET specific surface area 165 m 2 / g · Carbon nanotube D (hereinafter, "CNT-D"): Single-walled carbon nanotube, average outer diameter 1.3 nm, BET specific surface area 950 m 2 / g

[0108] <Dispersant> · Cellulose ether-1 (CE-1) · Cellulose ether-2 (CE-2) · Cellulose ether-3 (CE-3) · Cellulose ether-4 (CE-4) · Cellulose ether-5 (CE-5) · Cellulose ether-6 (CE-6) · Cellulose ether-7 (CE-7) · Cellulose ether-8 (CE-8) · Cellulose ether-9 (CE-9) · Polyvinylpyrrolidone K30 (PVP): Manufactured by FUJIFILM Wako Pure Chemical Corporation

[0109] <Preparation of conductive auxiliary agent dispersion composition for battery> [Example 1] A conductive auxiliary agent dispersion composition for battery was prepared according to the following procedure. 0.88 g of carbon nanotube A ("CNT-A") as a conductive auxiliary agent and 0.18 g of cellulose ether-1 ("CE-1") (20 parts by mass with respect to 100 parts by mass of the conductive auxiliary agent) as a dispersant were mixed as powders. To this, 34.0 g of N-methyl-2-pyrrolidone (NMP) was added and pre-mixed for 5 minutes under the condition of a peripheral speed of 1.3 m / s in a wet bead mill dispersing device ("Easy Nano RMB II type" manufactured by Imex Co., Ltd.). Then, 133 g of zirconia beads with a diameter of 1.0 mm were added, and further dispersed for 90 minutes in a wet bead mill dispersing device under the condition of a peripheral speed of 10 m / s. The obtained dispersion was filtered through a stainless steel mesh (a filter for filtering particles with a diameter of 1.0 mm) to obtain a conductive auxiliary agent dispersion composition for battery.

[0110] [Example 2] In Example 1, cellulose ether-2 (CE-2) was used instead of CE-1, and the same procedure as in Example 1 was followed to obtain a conductive auxiliary agent dispersion composition for battery.

[0111] [Example 3] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 1, except that cellulose ether-3 (CE-3) was used instead of CE-1.

[0112] [Example 4] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 1, except that cellulose ether-4 (CE-4) was used instead of CE-1.

[0113] [Example 5] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 1, except that cellulose ether-5 (CE-5) was used instead of CE-1.

[0114] [Example 6] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 1, except that cellulose ether-7 (CE-7) was used instead of CE-1.

[0115] [Example 7] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 1, except that cellulose ether-8 (CE-8) was used instead of CE-1.

[0116] [Example 8] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 1, except that cellulose ether-9 (CE-9) was used instead of CE-1.

[0117] [Comparative Example 1] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 1, except that cellulose ether-6 (CE-6) was used instead of CE-1.

[0118] [Comparative Example 2] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 1, except that polyvinylpyrrolidone K30 (PVP) was used instead of CE-1.

[0119] [Example 9] 0.35 g of carbon nanotube B ("CNT-B") as a conductive additive and 0.14 g of cellulose ether-1 ("CE-1") as a dispersant (40 parts by mass per 100 parts by mass of conductive additive) were mixed in powder form. 34.5 g of N-methyl-2-pyrrolidone (NMP) was added to this mixture and pre-mixed for 5 minutes at a peripheral speed of 1.3 m / s in a wet bead mill dispersion apparatus ("Easy Nano RMB II" manufactured by AIMEX Co., Ltd.). Then, 158 g of zirconia beads with a diameter of 0.5 mm were added and dispersed for a further 90 minutes at a peripheral speed of 12 m / s in the wet bead mill dispersion apparatus. A conductive additive dispersion composition for batteries was obtained by filtering the zirconia beads from the resulting dispersion using a stainless steel mesh (a filter that filters out particles with a diameter of 0.5 mm).

[0120] [Comparative Example 3] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 9, except that polyvinylpyrrolidone K30 (PVP) was used instead of CE-1.

[0121] [Example 10] 1.93 g of carbon nanotube C ("CNT-C") as a conductive additive and 0.29 g of cellulose ether-1 ("CE-1") as a dispersant (15 parts by mass per 100 parts by mass of conductive additive) were powder-mixed. 32.8 g of N-methyl-2-pyrrolidone (NMP) was added to this mixture and pre-mixed for 5 minutes at a peripheral speed of 1.3 m / s in a wet bead mill dispersion apparatus ("Easy Nano RMB II" manufactured by AIMEX Co., Ltd.). Then, 133 g of zirconia beads with a diameter of 1.0 mm were added and dispersed for a further 90 minutes at a peripheral speed of 10 m / s in the wet bead mill dispersion apparatus. A conductive additive dispersion composition for batteries was obtained by filtering the zirconia beads from the resulting dispersion using a stainless steel mesh (a filter that filters out particles with a diameter of 1.0 mm).

[0122] [Comparative Example 4] An attempt was made to produce a battery dispersion composition in the same manner as in Example 10, except that polyvinylpyrrolidone K30 (PVP) was used instead of CE-1. However, after dispersion in a bead mill for 90 minutes, the contents of the dispersion apparatus showed that the solid composition and zirconia beads were firmly adhered together, making it impossible to filter out the composition from the zirconia beads, and thus no dispersion composition was obtained. For this reason, no further evaluation was performed.

[0123] [Example 11] 0.21 g of carbon nanotube D ("CNT-D") as a conductive additive and 0.08 g of cellulose ether-1 ("CE-1") as a dispersant (38 parts by mass per 100 parts by mass of conductive additive) were mixed in powder form. 34.7 g of N-methyl-2-pyrrolidone (NMP) was added to this mixture and pre-mixed for 5 minutes at a peripheral speed of 1.3 m / s in a wet bead mill dispersion apparatus ("Easy Nano RMB II" manufactured by AIMEX Co., Ltd.). Then, 158 g of zirconia beads with a diameter of 0.5 mm were added and dispersed for a further 90 minutes at a peripheral speed of 12 m / s in the wet bead mill dispersion apparatus. A conductive additive dispersion composition for batteries was obtained by filtering the zirconia beads from the resulting dispersion using a stainless steel mesh (a filter that filters out particles with a diameter of 0.5 mm).

[0124] [Example 12] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 11, except that 0.18 g of CNT-D, 0.18 g of CE-1 (100 parts by mass per 100 parts by mass of conductive additive), and 35.6 g of NMP were used.

[0125] [Comparative Example 5] A conductive additive dispersion composition for batteries was obtained in the same manner as in Example 11, except that polyvinylpyrrolidone K30 (PVP) was used instead of CE-1.

[0126] (Evaluation Method) <Dispersibility (Appearance)> 2 g of the obtained conductive additive dispersion composition for batteries was placed on a glass plate and its dispersibility (appearance) was evaluated visually. In the composition, if the conductive additive was uniformly dispersed and the surface was smooth, the dispersibility of the conductive additive was good (○). If no clear separation of the conductive additive and the dispersion medium was observed, but some of the conductive additive in the composition had aggregated, resulting in fine irregularities on the surface, the dispersibility of the conductive additive was partially poor (△). If the separation of the conductive additive and the dispersion medium was clearly confirmed (specifically, if the separation of NMP from the conductive additive was observed), the dispersibility of the conductive additive was poor (×). If the composition could not be obtained in the production of the above-mentioned dispersion composition for batteries, it was deemed impossible to evaluate (××). Furthermore, if it was determined that the dispersion composition for batteries could not be obtained and therefore impossible to evaluate, it was disqualified (-) because subsequent rheological dispersibility evaluation and electrical resistance evaluation could not be performed.

[0127] <Evaluation of Dispersibility by Rheology> The dispersibility of the obtained conductive additive dispersion composition for batteries was evaluated using a rotary rheometer. Specifically, a rotary rheometer (Anton Paar "MCR702") was used, a parallel plate (diameter 50 mm) was used as the measurement fixture, the set temperature was set to 25°C, and the viscosity of the conductive additive dispersion composition for batteries was measured with a measurement gap of 0.5 mm while sweeping the shear rate in the range of 0.1 to 1000 / s. If the conductive additive is well dispersed in the conductive additive dispersion composition for batteries, the viscosity of the conductive additive dispersion composition for batteries decreases with increasing shear rate. On the other hand, if the dispersibility of the conductive additive is poor, a temporary increase in viscosity is observed during the process of increasing shear rate. In the entire shear rate range described above, if the viscosity decreased with increasing shear rate, it was judged as good dispersibility of the conductive additive (○). If any increase in viscosity was observed during the process of increasing shear rate, it was judged as poor dispersibility of the conductive additive (×).

[0128] <Dispersibility (Electrical Resistance Evaluation)> The obtained conductive additive dispersion composition for batteries and the binder solution were mixed to prepare an evaluation slurry. The dispersibility of the conductive additive was evaluated from the perspective of electrical resistance by performing DC resistance evaluation in the form of the slurry and volume resistivity evaluation of the coating film obtained by coating the evaluation slurry onto a glass substrate. Since electrical resistance mainly depends on the physical properties of the conductive additive used, an evaluation slurry was prepared for each conductive additive used as described below, and evaluation criteria for electrical resistance were established and evaluated.

[0129] [Evaluation using CNT-A] 1.24 g of the conductive additive dispersion composition for batteries obtained in Examples 1 to 8 and Comparative Examples 1 and 2, and 15 g of an NMP solution of polyvinylidene fluoride (weight-average molecular weight: 630,000) (solution concentration: 8% by mass) were added to a stirring container and kneaded for 5 minutes at a rotation speed of 2000 rpm using a defoaming kneader (ARV-310, manufactured by Thinky Co., Ltd.) to obtain an evaluation slurry.

[0130] An appropriate amount of the obtained evaluation slurry was poured into an electrode cell (HIOKI E.E. CORPORATION "SA9001"), and impedance measurement was performed using measurement software (HIOKI E.E. CORPORATION "SA2633") with an LCR meter (HIOKI E.E. CORPORATION "IM3536-01") equipped with a test fixture (HIOKI E.E. CORPORATION "SA9002") to measure the DC resistance component of the evaluation slurry. When using the above CNT-A, a DC resistance lower than 3.0 MΩ was evaluated as good, and a DC resistance of 3.0 MΩ or higher was evaluated as poor.

[0131] After measuring the DC resistance of the evaluation slurry, the slurry was placed on a smooth glass plate and coated using an applicator with a 300 μm gap. The coated glass plate was dried in a drying oven at 110°C for 30 minutes to obtain a coating film. The surface resistance of the obtained coating film was measured using a low resistivity meter (Loresta GX MCP-T710, manufactured by Nitto Seikou Analytech Co., Ltd.), and the volume resistivity (Ω・cm) at room temperature was calculated by multiplying the coating film thickness (measured with a thickness gauge, MFC-101A, manufactured by Nikon Corporation) by the surface resistance. When using the above CNT-A, a volume resistivity lower than 60 Ω・cm was considered good, and a volume resistivity of 60 Ω・cm or higher was considered poor.

[0132] If both the DC resistance of the evaluated slurry and the volume resistivity of the coating film were measured, the dispersibility of the conductive additive was good (○); if only one of them was poor, the dispersibility of the conductive additive was partially poor (△); and if both were poor, the dispersibility of the conductive additive was poor (×).

[0133] [Evaluation using CNT-B] 1.84 g of the conductive additive dispersion composition for batteries obtained in Example 9 and Comparative Example 3 and 15 g of an NMP solution of polyvinylidene fluoride (weight-average molecular weight: 630,000) (solution concentration: 8% by mass) were added to a stirring container and kneaded for 5 minutes at a rotation speed of 2000 rpm using a defoaming kneader (ARV-310, manufactured by Thinky Co., Ltd.) to obtain an evaluation slurry.

[0134] The DC resistance of the evaluation slurry and the volume resistivity of the coating film were measured in the same manner as when using CNT-A. The evaluation criteria were as follows: for the DC resistance of the evaluation slurry, a value lower than 3.0 MΩ was considered good, and a value of 3.0 MΩ or higher was considered poor; for the volume resistivity of the coating film, a value lower than 25 Ω·cm was considered good, and a value of 25 Ω·cm or higher was considered poor. Similar to CNT-A, if all of the measurement results were good, the dispersibility of the conductive additive was evaluated as good (○); if only one was poor, the dispersibility of the conductive additive was partially poor (△); and if both were poor, the dispersibility of the conductive additive was poor (×).

[0135] [Evaluation using CNT-C] 0.91 g of the conductive additive dispersion composition for batteries obtained in Example 10 and 15 g of an NMP solution of polyvinylidene fluoride (weight-average molecular weight: 630,000) (solution concentration: 8% by mass) were added to a stirring container and kneaded for 5 minutes at a rotation speed of 2000 rpm using a defoaming kneader (ARV-310, manufactured by Thinky Co., Ltd.) to obtain an evaluation slurry.

[0136] The DC resistance of the evaluation slurry and the volume resistivity of the coating film were measured in the same manner as when using CNT-A. The evaluation criteria were as follows: for the DC resistance of the evaluation slurry, a value lower than 3.5 MΩ was considered good, and a value of 3.5 MΩ or higher was considered poor; for the volume resistivity of the coating film, a value lower than 50 Ω·cm was considered good, and a value of 50 Ω·cm or higher was considered poor. Similar to CNT-A, if all of the measurement results were good, the dispersibility of the conductive additive was evaluated as good (○); if only one was poor, the dispersibility of the conductive additive was partially poor (△); and if both were poor, the dispersibility of the conductive additive was poor (×).

[0137] [Evaluation using CNT-D] 0.60 g of the conductive additive dispersion composition for batteries obtained in Examples 11, 12 and Comparative Example 5, and 15 g of an NMP solution of polyvinylidene fluoride (weight-average molecular weight: 630,000) (solution concentration: 8% by mass) were added to a stirring container and kneaded for 5 minutes at a rotation speed of 2000 rpm using a defoaming kneader (ARV-310, manufactured by Thinky Co., Ltd.) to obtain an evaluation slurry.

[0138] The DC resistance of the evaluation slurry and the volume resistivity of the coating film were measured in the same manner as when using CNT-A. The evaluation criteria were as follows: for the DC resistance of the evaluation slurry, a value lower than 4.5 MΩ was considered good, and a value of 4.5 MΩ or higher was considered poor; for the volume resistivity of the coating film, a value lower than 30 Ω·cm was considered good, and a value of 30 Ω·cm or higher was considered poor. Similar to CNT-A, if all of the measurement results were good, the dispersibility of the conductive additive was evaluated as good (○); if only one was poor, the dispersibility of the conductive additive was partially poor (△); and if both were poor, the dispersibility of the conductive additive was poor (×). The results are shown in Table 2.

[0139]

[0140] As shown in Examples 1 to 12, a conductive additive dispersion composition for batteries using a cellulose ether satisfying the requirements of the present invention, i.e., a cellulose ether in which some of the hydroxyl groups derived from cellulose are etherified with one or more groups selected from the group consisting of methyl, ethyl, hydroxyethyl, and hydroxypropyl groups, and other parts of the hydroxyl groups derived from cellulose are etherified with a hydroxyalkyl group of a specific structure having a linear alkyl group with 10 to 22 carbon atoms, and having a predetermined viscosity, showed good dispersibility of the conductive additive in appearance evaluation, rheological evaluation, and electrical resistance evaluation. On the other hand, in Comparative Example 1, which used a cellulose ether in which some of the hydroxyl groups derived from cellulose are etherified with one or more groups selected from the group consisting of methyl, ethyl, hydroxyethyl, and hydroxypropyl groups, and other parts of the hydroxyl groups derived from cellulose are etherified with a hydroxyalkyl group of a specific structure having a linear alkyl group with 10 to 22 carbon atoms, but the viscosity of a 2% by mass aqueous solution at 20°C exceeds 60,000 mPa·s, some parts of the appearance evaluation and electrical resistance evaluation were poor, and the rheological evaluation was poor. Furthermore, the conductive additive dispersion compositions for batteries of Examples 1 to 8 showed significantly better results in terms of dispersibility of the conductive additive than the composition of Comparative Example 2, which used polyvinylpyrrolidone, a conventionally used dispersant. Moreover, when using carbon nanotube B (aspect ratio not shown), a conductive additive with a large specific surface area, Comparative Example 3, which used polyvinylpyrrolidone, showed poor results in all aspects of dispersibility of the conductive additive, including appearance evaluation, rheological evaluation, and electrical resistance evaluation. In contrast, Example 9, which used cellulose ether satisfying the requirements of the present invention, showed significantly superior dispersibility of the conductive additive. Additionally, when the content of the conductive additive was increased, no composition could be obtained in Comparative Example 4, which used polyvinylpyrrolidone, whereas Example 10, which used cellulose ether satisfying the requirements of the present invention, showed that a battery dispersion composition with excellent dispersibility of the conductive additive could be obtained.Even when using single-walled carbon nanotubes, which have a higher aggregation tendency than multi-walled carbon nanotubes and make it difficult to obtain a battery dispersion composition, Examples 11 and 12, which used cellulose ether satisfying the requirements of the present invention, showed good dispersibility of the conductive additive in appearance evaluation, rheological evaluation, and electrical resistance evaluation. In Comparative Example 5, the dispersion of the conductive additive by the dispersion device did not proceed well during the composition preparation stage, and when zirconia beads were filtered from the contents of the dispersion device after dispersion, aggregated coarse conductive additive was removed along with the zirconia beads, resulting in a decrease in the concentration of the conductive additive in the battery conductive additive dispersion composition from the target concentration (amount added), which is thought to have worsened the electrical resistance evaluation. In contrast, in Examples 11 and 12, which used cellulose ether satisfying the requirements of the present invention, the conductive additive was well dispersed during the composition preparation stage, and a battery dispersion composition with the target conductive additive concentration was obtained, which is thought to have resulted in good conductivity evaluation.

[0141] <Preparation of Positive Electrode Paste> A positive electrode paste was prepared using the battery conductive additive dispersion compositions obtained in Examples 1 to 12 described above, according to the following procedure. 6.2 g of the battery conductive additive dispersion composition from Example 1 and 9.7 g of an NMP solution of polyvinylidene fluoride (weight-average molecular weight: 1,110,000) (solution concentration: 7% by mass) were added to a stirring container and kneaded for 5 minutes at a rotation speed of 2000 rpm using a defoaming kneader (ARV-310, manufactured by Thinky Co., Ltd.). 25.0 g of ternary positive electrode active material (NCM622) and 2.3 g of N-methyl-2-pyrrolidone (NMP) were added to this mixture and kneaded for 5 minutes at a rotation speed of 2000 rpm using the defoaming kneader to obtain a positive electrode paste (solid content concentration 60% by mass). Positive electrode pastes were also obtained for the battery carbon dispersion compositions from Examples 2 to 12 by the same method as described above. All of the positive electrode pastes were uniformly dispersed and exhibited good coating properties.

[0142] <Fabrication of Lithium-Ion Secondary Battery Positive Electrode> The positive electrode of a lithium-ion secondary battery was fabricated using the positive electrode paste described above by the following procedure. The positive electrode paste was applied and dried onto a current collector made of aluminum foil (thickness: 20 μm, width: 14 mm) using an electrode coating and drying machine (Clean Technology Co., Ltd. "LiB-W140"). The coating and drying conditions were a coating thickness of 180 μm, a transport speed of 0.2 m / min, and a furnace temperature of 115-125°C in a drying oven (furnace length: 50 cm). Subsequently, the coated and dried film obtained from the above coating and drying process was rolled together with the current collector at a linear pressure of 200 kg / cm using a tabletop roll press machine (Tester Industry Co., Ltd. "SA-602") to obtain the lithium-ion secondary battery positive electrode. The lithium-ion secondary battery positive electrode was successfully fabricated using any of the positive electrode pastes described above.

[0143] Although the present invention has been described using the above embodiments, the present invention is not limited to these embodiments. Other embodiments, additions, and modifications can be made within the scope that a person skilled in the art can conceive, and any embodiment that achieves the effects of the present invention is included within the scope of the present invention.

Claims

1. A conductive additive dispersion composition for batteries containing a conductive additive, cellulose ether, and N-methyl-2-pyrrolidone, wherein the cellulose ether is etherified by (i) one or more substituents selected from the group consisting of methyl, ethyl, hydroxyethyl, and hydroxypropyl groups, and (ii) the following general formula (1) A conductive additive dispersion composition for batteries, comprising a cellulose ether etherified with a substituent represented by (wherein R represents an alkyl group having 10 to 22 carbon atoms), wherein the viscosity of a 2% by mass aqueous solution of the cellulose ether at 20°C is 3 to 60,000 mPa·s.

2. The conductive additive dispersion composition for batteries according to claim 1, wherein the substituent of (i) in the cellulose ether is a methyl group and a hydroxypropyl group.

3. The conductive additive dispersion composition for batteries according to claim 1, wherein the substituent of (ii) in the cellulose ether is an n-octadecyl group in the general formula (1).

4. The conductive additive dispersion composition for batteries according to claim 1, wherein the viscosity of a 2% by mass aqueous solution of the cellulose ether at 20°C is 15 to 2,000 mPa·s.

5. The conductive additive dispersion composition for batteries according to claim 1, wherein the viscosity of a 2% by mass aqueous solution of the cellulose ether at 20°C is 5,000 to 60,000 mPa·s.

6. The conductive additive dispersion composition for batteries according to claim 1, wherein the conductive additive is a carbon nanotube.

7. The conductive additive dispersion composition for batteries according to claim 1, wherein the content of the conductive additive is 0.5 to 10% by mass.

8. The conductive additive dispersion composition for batteries according to claim 1, wherein the amount of cellulose ether added is 5 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the conductive additive.

9. A positive electrode paste comprising a conductive additive dispersion composition for batteries, a positive electrode active material, and a binder, as described in any one of claims 1 to 8.

10. A positive electrode for a secondary battery comprising a current collector and a positive electrode composite layer which is a coated and dried film of the positive electrode paste described in claim 9, formed on the current collector.

11. A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator as described in claim 10.

12. A portion of the cellulose-derived hydroxyl groups is etherified with (i) one or more substituents selected from the group consisting of methyl, ethyl, hydroxyethyl, and hydroxypropyl groups, and the remaining portion of the cellulose-derived hydroxyl groups is (ii) the following general formula (1) A conductive additive dispersant comprising cellulose ether, which is etherified with substituents represented by (wherein R represents an alkyl group having 10 to 22 carbon atoms) and having a viscosity of 3 to 60,000 mPa·s in a 2% by mass aqueous solution at 20°C.

13. The conductive additive dispersant according to claim 12, for dispersing carbon nanotubes in N-methyl-2-pyrrolidone.

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