JP2017188203A - Method of evaluating stability of positive electrode material slurry for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of evaluating the stability of a slurry of a positive electrode material for a lithium ion secondary battery, capable of performing stable evaluation with good reproducibility by suppressing the effect of moisture to a minimum and improving sensitivity to alkali on a positive electrode active material surface.SOLUTION: In a method of evaluating the stability of a positive electrode material slurry for a lithium ion secondary battery, the stability of a positive electrode material slurry is evaluated by changes in slurry viscosity over time. The method includes: a slurry producing step S1 in which a positive electrode active material, a conductive assistant, and a binder are kneaded and dispersed into an organic solvent, thereby forming a positive electrode material slurry; and a viscosity measurement step S2 in which evaluation is performed by measuring changes in slurry viscosity over time of the positive electrode material slurry obtained in the slurry producing step. In the method, the positive electrode material slurry producing step and the viscosity measurement step are performed under an environment where a dew point is controlled at -30°C or below, and polyvinylidene fluoride with a functional group is used for the binder.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a stability evaluation method for a positive electrode material slurry for a lithium ion secondary battery, wherein the stability of the positive electrode material slurry is evaluated based on a change in slurry viscosity with time.

  Since lithium ion secondary batteries have high energy density, they are widely used in portable electronic devices such as mobile phones and notebook computers that are required to be smaller and lighter in recent years. Moreover, development as a clean energy source is actively performed for automobile applications, and high performance and low cost such as small size, light weight, high capacity, and high output are required.

  The positive electrode used in these lithium ion secondary batteries is mainly composed of a lithium-containing transition metal oxide such as lithium cobaltate having a diameter of several μm to several tens of μm, and a diameter of several nm to several hundreds for improving electron conductivity. It is composed of three conductive assistants such as carbon black having a structure structure at nm and a binder such as polyvinylidene fluoride (hereinafter also referred to as PVDF) for fixing them to a current collector with a desired thickness. These materials are generally not used as powders, and are handled by being slurried to an appropriate viscosity with an organic solvent such as N-methyl-2-pyrrolidone (NMP). The positive electrode material slurry produced by adding an organic solvent to the material constituting the positive electrode and kneading is precisely applied to an aluminum current collector with a thickness of several tens of μm using a precision coating device such as a die coater or a roll coater. After that, it becomes a positive electrode through drying, compression, and cutting processes.

  Currently, lithium cobaltate is mainly used as a positive electrode active material for lithium ion secondary batteries, which occupies most of the market, and its capacity is about 150 mAh / g. Among them, a lithium nickel-based positive electrode material capable of obtaining a high capacity of 200 mAh / g has been developed as a positive electrode material for hybrid vehicles and electric vehicle batteries.

  The lithium nickel-based positive electrode material is a very attractive material in terms of capacity, but has a higher water absorption and higher alkalinity than the lithium cobalt-based positive electrode material (the pH of water in which the positive electrode material is immersed is about 11 to 12). Becomes). When such a positive electrode material is kneaded with polyvinylidene fluoride (PVDF), which is generally used as a binder, to produce a positive electrode, there are problems such as rapid viscosity increase and gelation (slurry non-fluidization). Occur. This phenomenon occurs when the HF in PVDF is desorbed by the reaction with the alkali of the positive electrode active material, the C—H bond is cut, and the C═C bond is generated, and the crosslinking reaction proceeds at once from this double bond. It has been reported. In addition, lithium nickel cobalt manganese ternary positive electrode materials are also being studied for nickel-rich composition with the aim of increasing the capacity. However, while the capacity increases, the alkalinity also increases, Similarly, slurry stability problems such as gelation are occurring.

  Therefore, in order to rapidly develop a lithium nickel-based positive electrode material, not only evaluation of normal battery performance but also evaluation of slurry stability is a very important factor.

  Conventionally, the positive electrode material slurry stability evaluation for a lithium ion secondary battery has been evaluated by a method of monitoring a change in viscosity over time or by the presence or absence of visual gelation (non-fluidization of the slurry). Since the main cause of this increase in viscosity and gelation is due to the alkali in the positive electrode active material described above, it is preferable that the evaluation of the stability of the slurry is an evaluation that correlates with the strength of the properties. However, the positive electrode active material has a high water absorption rate, and the water absorption rate of N-methyl-2-pyrrolidone used as a solvent is infinite. In an environment where moisture exists, they absorb water, The alkali is extracted from the inside of the positive electrode active material and gelation is accelerated. In addition, when the moisture content in the slurry is further increased, PVDF is deposited even if there is no alkali, and the slurry becomes non-fluidized. From these facts, evaluation with high reproducibility could not be performed in the conventional environment where moisture management is not performed and when the moisture was attached to the positive electrode active material member.

  For example, Patent Document 1 describes that in a method for producing a positive electrode sheet for a battery, the temperature at the time of kneading, feeding, and storing positive electrode material paste is maintained at a low temperature, and the atmosphere is set to a low humidity atmosphere. ing.

  In addition, as another evaluation method, there is a method of measuring the pH by immersing the positive electrode active material in water, but since alkali is extracted from the inside of the positive electrode active material by moisture, when producing a slurry in an organic solvent It was difficult to obtain a correlation with the alkalinity of the surface of the positive electrode active material in contact with the PVDF in question.

Japanese Patent Laid-Open No. 11-120991

  As described above, in order to perform stability evaluation of the positive electrode slurry for a lithium ion secondary battery stably with good reproducibility, control of moisture is important, and less moisture is better. However, when the evaluation is performed with a minimum amount of moisture, the reactivity with alkali (also referred to as sensitivity) becomes too small, and the gelation of the evaluation sample does not occur, and there is a problem that the slurry stability cannot be evaluated. It was.

  Patent Document 1 describes that a positive electrode sheet for a battery is produced in a low-humidity atmosphere. However, specific conditions and a response to a decrease in sensitivity to alkali that occurs when moisture is minimized are described. It has not been.

  The present invention has been proposed in view of such circumstances, and can suppress the influence of moisture to the minimum, improve the sensitivity of the positive electrode active material surface to alkali, and perform stable evaluation with good reproducibility. Provided is a method for evaluating the slurry stability of a positive electrode material for a lithium ion secondary battery.

  As a result of examining the solution of the above problem in detail, the present inventors have conducted a method for minimizing the influence of moisture in order to stably evaluate the slurry stability of the positive electrode material for a lithium ion secondary battery with good reproducibility. In order to improve the sensitivity to the alkali on the surface of the positive electrode active material, it is effective to carry out the production of the positive electrode material slurry and the viscosity measurement in an environment in which the dew point is controlled to -30 ° C. or less. As a binder used in the above, the knowledge that it is effective to use a functionalized polyvinylidene fluoride was obtained, and the present invention was completed.

  That is, one aspect of the present invention is a method for evaluating the stability of a positive electrode material slurry for a lithium ion secondary battery, which evaluates the stability of the positive electrode material slurry by a change in slurry viscosity with time, and includes a positive electrode active material, a conductive additive, A slurry production process in which a binder is kneaded and dispersed in an organic solvent to form a positive electrode material slurry, and a viscosity measurement process in which evaluation is performed by measuring a change in slurry viscosity with time of the positive electrode material slurry obtained in the slurry production process. In addition, the positive electrode material slurry manufacturing step and the viscosity measuring step are performed in an environment where the dew point is controlled to -30 ° C. or lower, and the functionalized polyvinylidene fluoride is used as the binder.

  According to one aspect of the present invention, by performing the positive electrode material slurry manufacturing step and the viscosity measurement step in an environment in which the dew point is controlled to -30 ° C. or less, the influence of moisture is minimized, and the functional group is added to the polyfluorinated binder. By using vinylidene, the sensitivity to the alkali on the surface of the positive electrode active material can be improved.

  At this time, in one embodiment of the present invention, the positive electrode active material may be vacuum-dried at 120 ° C. for 3 hours or more.

  Moisture contained in the positive electrode active material can be removed by drying the positive electrode active material under vacuum under reduced pressure.

  At this time, the conductive additive may be carbon black that has been vacuum-dried at 120 ° C. for 3 hours or more.

  Similarly, it is preferable to remove moisture in advance by conducting vacuum drying under reduced pressure.

  In one embodiment of the present invention, the organic solvent may be N-methyl-2-pyrrolidone having a water content of 30 ppm or less.

  It is preferable to use an organic solvent having a low water content.

  According to one embodiment of the present invention, a positive electrode for a lithium ion secondary battery that can suppress the influence of moisture to a minimum, improve sensitivity to alkali on the surface of the positive electrode active material, and perform stable evaluation with high reproducibility. A method for evaluating the slurry stability of a material can be provided.

It is process drawing which shows the outline of the process of the slurry stability evaluation method of the positive electrode material for lithium ion secondary batteries which concerns on one embodiment of this invention. It is a figure which shows the change over time of the viscosity of the positive electrode material slurry in Examples 1-4. It is a figure which shows the change over time of the viscosity of the positive electrode material slurry in the comparative example 1. It is a figure which shows the change over time of the viscosity of the positive electrode material slurry in the comparative example 2.

Hereinafter, embodiments of the present invention will be described in detail in the following order with reference to the drawings. The present embodiment described below does not unduly limit the contents of the present invention described in the claims, and all the configurations described in the present embodiment are essential as means for solving the present invention. Not necessarily.
1. 1. Slurry stability evaluation method of positive electrode material for lithium ion secondary battery 1-1. Slurry manufacturing process 1-2. Viscosity measurement process

<1. Method for evaluating slurry stability of positive electrode material for lithium ion secondary battery>
In FIG. 1, the outline of the process of the slurry stability evaluation method of the positive electrode material for lithium ion secondary batteries which concerns on one embodiment of this invention is shown.

  One embodiment of the present invention is a method for evaluating the stability of a positive electrode material slurry for a lithium ion secondary battery by evaluating the stability of the positive electrode material slurry by a change in slurry viscosity with time, comprising a positive electrode active material, a conductive additive, and a binder And a viscosity measurement step S2 in which the slurry viscosity is measured by measuring a change in slurry viscosity with time of the positive electrode material slurry obtained in the slurry production step. It is characterized in that it is carried out in an environment in which the positive electrode material slurry production step and the viscosity measurement step are controlled at a dew point of −30 ° C. or less, and the functionalized polyvinylidene fluoride is used as the binder.

  Thus, by performing the positive electrode material slurry manufacturing step and the viscosity measurement step in an environment where the dew point is controlled to -30 ° C. or less, the influence of moisture is suppressed to a minimum, and the functionalized polyvinylidene fluoride is used as the binder. The sensitivity to the alkali on the surface of the positive electrode active material can be improved, and the stability of the positive electrode material slurry can be evaluated with good reproducibility.

  Hereafter, each process is demonstrated in order, respectively.

(1-1. Slurry manufacturing process)
In the slurry manufacturing step S1, a positive electrode active material, a conductive additive, and a binder are kneaded and dispersed in an organic solvent to obtain a positive electrode material slurry.

  The basic composition of the positive electrode slurry for a lithium ion secondary battery produced in the present invention is the same as that of a conventionally known battery, and is composed of a positive electrode active material, a conductive additive, and a binder. About those mixing ratios, it can be set as the mixing ratio matched with the mixing ratio of the battery which is a final product.

  As the positive electrode active material, a lithium-containing transition metal oxide can be used. Examples include lithium cobaltate, lithium manganate, lithium nickelate, lithium iron phosphate, lithium transition metal composite oxide (lithium oxide containing two or more selected from the group consisting of cobalt, nickel, and manganese). It is done. For the positive electrode active material selected from these lithium-containing transition metal oxides, the stability of the positive electrode material slurry is evaluated.

  Conductive aids include carbon-based nanoparticles or nanofibers such as acetylene black, vapor grown carbon fiber (VGCF (registered trademark) manufactured by Showa Denko KK), Ketjen Blank, artificial graphite, and natural graphite. These conductive additives can be used alone or in combination.

  As the binder, it is necessary to use a PVDF binder in which a functional group is added to the PVDF molecule. The type of the functional group is not particularly limited, but it is preferable that HF is eliminated from the PVDF due to alkali attack, water is generated, or crosslinking proceeds by an oxygen addition reaction.

  In order to perform stability evaluation of the positive electrode material slurry stably with good reproducibility, it is important to control the moisture, and it is better that the amount of moisture adhering is small. There was a problem in that it no longer reacts with alkali and the sensitivity to alkali becomes too small. In the present invention, in order to solve this problem, it is usually used as a binder for a negative electrode, and a functionalized polyvinylidene fluoride (PVDF) having high sensitivity to alkali is applied as a binder for a positive electrode material slurry. Thereby, even in the state where the influence of moisture is suppressed to a minimum, the sensitivity to the alkali on the surface of the positive electrode active material can be improved, and stable evaluation can be performed with good reproducibility.

  It is preferable to remove the moisture adhering to the positive electrode active material and the conductive additive, and it is preferable to dry under vacuum under reduced pressure at 120 to 240 ° C. for 3 hours or more.

  As the organic solvent, it is preferable to use a dehydrated grade (for example, moisture content of 30 ppm or less) or a battery grade (for example, moisture content of 100 ppm or less) N-methyl-2-pyrrolidone having a low moisture content. It is more preferable to use

  In the slurry production step S1, these positive electrode active material, conductive additive and binder are kneaded and dispersed in an organic solvent. In the slurry production step S1, these operations are performed in an environment having a dew point of −30 ° C. or higher, thereby minimizing moisture contamination.

  In preparing the positive electrode material slurry, a planetary mixer, a twin-screw kneader, a homogenizer, and a rotation / revolution mixer can be used. It is preferable to use a rotation / revolution mixer that can be kneaded in a sealed container.

(1-2. Viscosity measurement process)
In the viscosity measurement step S2, the evaluation is performed by measuring the change over time in the slurry viscosity of the positive electrode material slurry obtained in the slurry production step S1.

  Viscosity is measured using a viscometer in a dry room controlled at a dew point of −30 ° C., and a change with time in the slurry viscosity of the positive electrode material slurry is measured. Even during storage other than during measurement, the dew point is controlled to -30 ° C. That is, in the viscosity measurement step S2, the slurry viscosity measurement environment and the slurry storage environment are set to a dew point of −30 ° C. or lower.

  The viscosity of the positive electrode material slurry is preferably adjusted to 1,000 to 8,000 cps, and more preferably adjusted to 2,000 to 3,000 cps from the ease of determination of gelation. The determination of gelation can be made when the viscosity of the positive electrode material slurry shows a rising tendency from the viscosity at the time of slurry preparation. In the viscosity measuring step S2, gelation is judged based on whether the viscosity of the positive electrode material slurry is within a range of 20 to 30% from the viscosity at the time of slurry preparation, and the stability of the slurry is determined by checking the elapsed time until gelation. Sexual evaluation can be performed.

  As described above, according to the slurry stability evaluation method for a positive electrode material for a lithium ion secondary battery according to an embodiment of the present invention, the influence of moisture is minimized by controlling the dew point to -30 ° C. Providing a slurry stability evaluation method for positive electrode materials for lithium ion secondary batteries that improves the sensitivity to alkali on the surface of the positive electrode active material by using polyvinylidene fluoride with a functional group and enables stable evaluation with high reproducibility. can do.

  In addition, although the positive electrode material slurry manufactured with the slurry stability evaluation method of the positive electrode material for lithium ion secondary batteries according to one embodiment of the present invention is for evaluation, the positive electrode material for lithium ion secondary batteries is actually used. It may be used as That is, the above-described positive electrode active material, conductive additive and binder are kneaded and dispersed in an organic solvent to produce a positive electrode material slurry, and a positive electrode for a lithium ion secondary battery can be produced by a sheet method or a coating method. .

  EXAMPLES The present invention will be described in further detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

Example 1
[Moisture removal of positive electrode active material]
In Example 1, lithium nickelate was used as the positive electrode active material. The positive electrode active material was subjected to vacuum reduced pressure drying at 120 ° C. for 8 hours with a vacuum dryer to remove moisture.

[Moisture removal for conductive material]
Carbon black (Denka Black (registered trademark), model number HS100, manufactured by Denka Kogyo Co., Ltd.) was used as the conductive additive, and vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer.

[Cathode slurry production]
The positive electrode material slurry was prepared in a dry room controlled at a dew point of −30 ° C. or lower by using a stirrer so that the above-mentioned positive electrode active material, conductive additive, and binder removed from water had a solid mass ratio of 91: 6: 3. It knead | mixed using the Shinkey Co., Ltd. product made from Awatori Nertaro (trademark) (model number: ARE-310). As the binder, a PVDF solution with a functional group (manufactured by Kureha, NMP solution type model number # L9305) was used. Thereafter, an appropriate amount of dehydrated grade (moisture content of 30 ppm or less) NMP (manufactured by Kanto Chemical Co., Inc.) was added and kneaded using the same device to obtain a positive electrode material slurry having a desired viscosity.

[Viscosity measurement]
The positive electrode material slurry was measured at a shear rate of 1/50 s using a viscometer (manufactured by Brookfield, DV-II + PRO) in a dry room controlled at a dew point of −30 ° C. The viscosity of was measured. The initial viscosity was 2,071 cps. The slurry was placed in a polypropylene sealed container and stored in a dry room controlled at a dew point of −30 ° C. After that, as a result of continuously measuring the viscosity, the viscosity after 28 hours was 4,739 cps, and the viscosity of the lithium nickel-based positive electrode material having high alkalinity was greatly increased. FIG. 2 shows changes over time in the viscosity of the positive electrode material slurry in Example 1.

(Example 2)
[Moisture removal of positive electrode active material]
In Example 2, lithium cobalt oxide was used as the positive electrode active material. The positive electrode active material was subjected to vacuum reduced pressure drying at 120 ° C. for 8 hours with a vacuum dryer to remove moisture.

[Moisture removal for conductive material]
Carbon black (Denka Black (registered trademark), model number HS100, manufactured by Denka Kogyo Co., Ltd.) was used as the conductive additive, and vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer.

[Cathode slurry production]
The positive electrode material slurry was prepared under the same conditions as in Example 1.

[Viscosity measurement]
The viscosity of the positive electrode slurry obtained at a shear rate of 1/50 s was measured using a viscometer (manufactured by Brookfield, DV-II + PRO) in a dry room controlled at a dew point of −30 ° C. The initial viscosity was 1,651 cps. The slurry was placed in a polypropylene sealed container and stored in a dry room controlled at a dew point of −30 ° C. Thereafter, the viscosity was continuously measured. As a result, the viscosity after 28 hours was 1,706 cps, and no increase in viscosity was observed in the lithium cobalt-based positive electrode material. FIG. 2 shows changes in the viscosity of the positive electrode material slurry in Example 2 over time.

(Example 3)
[Moisture removal of positive electrode active material]
In Example 3, lithium transition metal composite oxide (nickel: cobalt: manganese ratio 1: 1: 1) was used as the positive electrode active material, and vacuum vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer.

[Moisture removal for conductive material]
Carbon black (Denka Black (registered trademark), model number HS100, manufactured by Denka Kogyo Co., Ltd.) was used as the conductive additive, and vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer.

[Cathode slurry production]
The positive electrode material slurry was prepared under the same conditions as in Example 1.

[Viscosity measurement]
The viscosity of the resulting positive electrode slurry was measured at a shear rate of 1/50 s using a viscometer (Brookfield, DV-II + PRO) in a dry room controlled at a dew point of -30 ° C. The initial viscosity was 1,563 cps. The slurry was placed in a polypropylene sealed container and stored in a dry room controlled at a dew point of −30 ° C. Thereafter, the viscosity was continuously measured, and as a result, the viscosity after 28 hours was 2,865 cps. FIG. 2 shows changes over time in the viscosity of the positive electrode material slurry in Example 3. The increase in viscosity of the lithium transition metal composite oxide (nickel: cobalt: manganese ratio 1: 1: 1) material is small compared to the lithium nickel positive electrode material of Example 1, but the lithium cobalt based positive electrode material of Example 2 It was a big result when compared.

Example 4
[Moisture removal of positive electrode active material]
In Example 4, a lithium transition metal composite oxide having a high nickel ratio (nickel: cobalt: manganese ratio 5: 1: 2) was used as the positive electrode active material, and vacuum vacuum drying at 120 ° C. for 8 hours in a vacuum dryer. went.

[Moisture removal for conductive material]
Carbon black (Denka Black (registered trademark), model number HS100, manufactured by Denka Kogyo Co., Ltd.) was used as the conductive additive, and vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer.

[Cathode slurry production]
The positive electrode slurry was prepared under the same conditions as in Example 1.

[Viscosity measurement]
The viscosity of the resulting positive electrode slurry was measured at a shear rate of 1/50 s using a viscometer (Brookfield, DV-II + PRO) in a dry room controlled at a dew point of -30 ° C. The initial viscosity was 1,912 cps. The slurry was placed in a polypropylene sealed container and stored in a dry room controlled at a dew point of −30 ° C. Thereafter, the viscosity was continuously measured, and as a result, the viscosity after 28 hours was 4,278 cps. The change over time of the viscosity of the positive electrode material slurry in Example 4 is shown in FIG. In the lithium transition metal composite oxide having a high nickel ratio (nickel: cobalt: manganese ratio 5: 1: 2), a large increase in viscosity was observed after the lithium nickel positive electrode material.

  From the above, in Examples 1 to 4 in which the stability evaluation of the positive electrode material slurry was performed in an environment in which moisture was suppressed to a minimum, a larger increase in viscosity was observed as the ratio of nickel having high alkalinity was higher. Therefore, by applying the slurry stability evaluation method for the positive electrode material for a lithium ion secondary battery according to an embodiment of the present invention, a quantitative evaluation correlated with the alkalinity of the positive electrode active material surface can be performed. I understand that.

(Comparative Example 1)
[Moisture removal of positive electrode active material]
In Comparative Example 1, the same lithium composite oxide as in Example 4 (nickel: cobalt: manganese ratio 5: 1: 2) was used as the positive electrode active material, and vacuum vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer. .

[Moisture removal for conductive material]
Carbon black (Denka Black (registered trademark), model number HS100, manufactured by Denka Kogyo Co., Ltd.) was used as the conductive additive, and vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer.

[Cathode slurry production]
The positive electrode slurry was prepared in a dry room controlled at a dew point of −30 ° C. or lower, and the agitator (Sinky Corp.) was used so that the positive electrode active material, the conductive additive and the binder were removed at a solid mass ratio of 91: 6: 3. Kneaded using Awatori Nertaro (registered trademark) (model number: ARE-310). A PVDF solution having no functional group (manufactured by Kureha, NMP solution type model number # L7208) was used as a binder. Thereafter, an appropriate amount of dehydrated grade (water content 30 ppm or less) NMP (manufactured by Kanto Chemical Co., Inc.) was added and kneaded using the same device to obtain a positive electrode slurry having a desired viscosity.

[Viscosity measurement]
The viscosity of the positive electrode slurry obtained at a shear rate of 1/50 s was measured using a viscometer (manufactured by Brookfield, DV-II + PRO) in a dry room controlled at a dew point of −30 ° C. The initial viscosity was 2,302 cps. The slurry was put in a polypropylene sealed container and stored in a dry room controlled at a dew point of -30 ° C. Thereafter, the viscosity was continuously measured, and as a result, the viscosity after 48 hours was 2,103 cps which was the same as the initial viscosity. The change over time of the viscosity of the positive electrode material slurry in Comparative Example 1 is shown in FIG. In Example 1, an increase in viscosity was observed in the lithium composite oxide (nickel: cobalt: manganese ratio 5: 1: 2), but it was prepared and stored using general PVDF having no functional group in a -30 ° C environment. As a result, no increase in viscosity of the slurry was observed, and the quantitative evaluation could not be correlated with the alkalinity of the surface of the positive electrode active material.

(Comparative Example 2)
[Moisture removal of positive electrode active material]
In Comparative Example 2, the same lithium cobalt oxide as in Example 2 was used as the positive electrode active material, and vacuum vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer.

[Moisture removal for conductive material]
Carbon black (Denka Black (registered trademark), model number HS100, manufactured by Denka Kogyo Co., Ltd.) was used as the conductive additive, and vacuum drying was performed at 120 ° C. for 8 hours in a vacuum dryer.

[Cathode slurry production]
The positive electrode slurry was prepared under the same conditions as in Example 1.

[Viscosity measurement]
The viscosity of the slurry obtained at a shear rate of 1/50 s was measured using a viscometer (manufactured by Brookfield, DV-II + PRO) in a dry room controlled at a dew point of −30 ° C. At that time, the initial viscosity was 2,183 cps. The slurry was stored in a condition where the humidity was controlled at 40-50% without covering the polypropylene container, and the viscosity was continuously measured. As a result, the viscosity increased to 6,234 cps after 26 hours. Thereafter, gelation of the slurry occurred after 27 hours. FIG. 4 shows changes in the viscosity of the positive electrode material slurry in comparison 2 over time. When the slurry is stored at a humidity of 40 to 50%, even if it is a cobalt-based material that hardly undergoes gelation, gelation occurs, and quantitative evaluation that can be correlated with the alkalinity of the surface of the positive electrode active material I didn't.

Claims (4)

A method for evaluating the stability of a positive electrode material slurry for a lithium ion secondary battery, wherein the stability of the positive electrode material slurry is evaluated by a change in slurry viscosity with time,
A slurry manufacturing process in which a positive electrode active material, a conductive additive and a binder are kneaded and dispersed in an organic solvent to form a positive electrode material slurry;
Having a viscosity measurement step of performing evaluation by measuring a change in slurry viscosity with time of the positive electrode material slurry obtained in the slurry production step;
A positive electrode material slurry for a lithium ion secondary battery, wherein the positive electrode material slurry production step and the viscosity measurement step are performed in an environment where the dew point is controlled to -30 ° C. or less, and a functionalized polyvinylidene fluoride is used as the binder. Stability evaluation method.   2. The method for evaluating the stability of a positive electrode material slurry for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material is vacuum-dried at 120 ° C. for 3 hours or more.   3. The method for evaluating the stability of a positive electrode material slurry for a lithium ion secondary battery according to claim 1, wherein the conductive additive is carbon black that is vacuum-dried at 120 ° C. for 3 hours or more.   4. The positive electrode material slurry for a lithium ion secondary battery according to claim 1, wherein the organic solvent is N-methyl-2-pyrrolidone having a water content of 30 ppm or less. 5. Stability evaluation method.