DE102022201385A1 - Method of evaluating the solution quality of a binder solution for secondary battery electrodes and manufacturing method of an electrode slurry for secondary batteries according to the evaluation method - Google Patents

Abstract

A method for evaluating a solution quality of a binder solution for a secondary battery electrode of the present invention comprises: measuring a turbidity value of a predetermined amount of a sample of the binder solution by transmission of light through the binder solution; and evaluating a solution quality of the binder solution based on a magnitude of the turbidity value.Furthermore, a method for producing an electrode slurry for a secondary battery comprises: measuring a turbidity value of a predetermined amount of a sample of the binder solution by transmitting light through the binder solution; evaluating a solution quality of the binder solution based on a quantity of the turbidity value; and preparing an electrode slurry by mixing an active material for an electrode and a conductive material with a binder solution in a solution quality having a turbidity value equal to or smaller than a predetermined value.

Description

[Technical Field]

The present invention relates to a method for evaluating the solution quality of a binder solution for a secondary battery electrode. More particularly, the present invention relates to a method for evaluating the solution quality of a binder solution for a secondary battery electrode, in which the solution quality can be quantitatively evaluated by measuring a turbidity value.

Further, the present invention relates to a method for producing an electrode slurry for a secondary battery, which is based on the solution quality evaluation method.

[Technical background of the invention]

With the development of technologies for mobile devices, automobiles and energy storage and the associated increasing demand, the demand for batteries as a source of energy has also increased rapidly. For this reason, many studies have been made on lithium secondary batteries having high energy density and high discharge voltage, and such lithium secondary batteries have been widely used.

An electrode used for a lithium secondary battery is formed by applying a mixture layer containing an active material onto an electrode substrate made of metal foil, and the mixture layer is coated onto an electrode substrate as an electrode slurry of a positive or negative electrode and then dried. The electrode slurry is prepared by mixing solids such as a positive/negative electrode active material and a conductive material with a binder solution and then drying the mixture.

The binder makes it possible to bind an active material to an active material or to bind an active material to an electrode substrate, thereby improving the adhesion of the electrode and adjusting the viscosity of the slurry. Since the binder is mixed with an active material, a conductive material, etc. in the form of a binder solution dissolved in a predetermined solvent, the solution quality of the binder affects the quality of an electrode slurry or an electrode. For example, if the solution quality of a binder such as CMC or PVDF is not good, various problems such as increase in slurry viscosity, surface defect of electrode coating, etc. may be caused. The solution quality of the binder depends on the degree of minimization of the insolubles in the binder solution.

Heretofore, the dissolution quality of a binder solution has been evaluated simply by observing the binder not completely dissolved in the solution with the naked eye, or after applying a binder solution of a certain thickness to an OHP film with a blade of a certain thickness by counting the number of the foreign matter on the film with the naked eye. However, with this method, it is difficult to assess the overall quality of the solution due to the small amount of sample. In addition, the measuring accuracy is not high since foreign substances can be introduced which cannot be distinguished from the undissolved substances. In addition, depending on the appraiser, there may be a significant error due to the appraisal with the naked eye.

In the case that the solution quality of the binder solution is not strictly evaluated, even if a binder solution obtained by dissolving a similar binder in a solvent is used, the quality characteristics such as the surface defect of an electrode or an electrode slurry for a secondary battery vary depending on the case solution quality significantly. As a result, it is not possible to ensure a stable quality of an electrode, and it becomes difficult to improve the surface defect of the electrode.

Accordingly, there is a need for a technology for improving the surface defect of an electrode by quantitatively evaluating the solution quality of a binder solution for manufacturing a secondary battery electrode.

[State of the art]

[patent documents]

Japanese Patent Publication No. 2013-053919 (March 21, 2013)

[Description of the invention]

[Technical problem]

The present invention provides a solution to at least some of the above problems. For example, one aspect of the present invention provides a method for evaluating the solution quality of a binder solution for a secondary battery electrode, in which the solution quality can be quantitatively evaluated by a turbidity value of the binder solution.

Another aspect of the present invention provides a method for producing an electrode slurry for a secondary battery using a binder solution whose solution quality has been evaluated based on the method for evaluating the solution quality of the binder solution.

[Technical solution]

A method of evaluating a solution quality of a binder solution for a secondary battery electrode to solve the above problems includes: measuring a turbidity value of a predetermined amount of a sample of the binder solution by transmitting light through the binder solution; and evaluating a solution quality of the binder solution based on a magnitude of the turbidity value.

Here, the turbidity value is a ratio between a scattering transmittance and a total transmittance of the binder solution when light is irradiated through the binder solution.

A binder which is a dissolved component of the binder solution can, for example, be at least one selected from the group consisting of a non-aqueous binder such as polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, Carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer (EPDM), sulfonated EPDM or polytetrafluoroethylene (PTFE), an aqueous binder such as acrylonitrile butadiene rubber , styrene butadiene rubber (SBR) or acrylic rubber and a polymer resin such as hydroxyethyl cellulose or carboxymethyl cellulose.

A solvent of the binder solution can be, for example, at least one selected from the group consisting of an organic solvent such as N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone or dimethylacetamide, and water.

In a concrete example, the binder solution may be a CMC solution obtained by dissolving a predetermined amount of CMC in water.

For example, the solution quality of the CMC solution can be quantitatively evaluated by respectively measuring the turbidity values of a plurality of CMC solutions prepared by varying at least one of mixing time, mixing temperature, mixer type and mixing blade type.

As another aspect of the present invention, a method for producing an electrode slurry for a secondary battery includes: measuring a turbidity value of a predetermined amount of a sample binder solution by transmitting light through the binder solution; evaluating a solution quality of the binder solution based on a quantity of the turbidity value; and preparing an electrode slurry by mixing an active material for an electrode and a conductive material with a binder solution in a solution quality having a turbidity value equal to or smaller than a predetermined value.

In a concrete example, the binder solution may be a CMC solution obtained by dissolving a predetermined amount of CMC in water.

In one example, the electrode slurry may be a negative electrode slurry.

In a specific example, the turbidity value of the CMC solution can be equal to or less than 2%. In a more preferred embodiment, the turbidity value of the CMC solution may be less than or equal to 1.9%, preferably less than or equal to 1.8%, more preferably less than or equal to 1.7%.

[Beneficial Effects]

According to the present invention, it is possible to predict the surface defect of an electrode for a secondary battery by preparing an electrode slurry using a binder solution whose solution quality has been quantitatively evaluated from a turbidity value.

Further, it is possible to improve the surface defect of an electrode for a secondary battery by preparing an electrode slurry using a binder solution in a solution quality having a turbidity value equal to or smaller than a predetermined value.

character list

• 1 Fig. 12 is a figure showing a form of a surface defect of an electrode for a secondary battery.

[Best embodiment of the invention]

In the following, the present invention will be described in detail with reference to the drawings. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms, and the inventor can properly define the concept of the terms to best describe his invention. The terms and words should be interpreted in meaning and concept in accordance with the technical idea of the present invention.

As used in the present application, terms such as "including" or "having" are intended to indicate that a feature, number, step, process, component, part, or combination thereof is described in the specification, and do not exclude anticipate the possibility of the presence or addition of one or more other features or numbers, steps, operations, components, parts or combinations thereof. Furthermore, when a part, such as a layer, film, surface, plate, etc., is said to be "overlying" another part, this does not just include the case where the part is "directly on top" of the other part lies, but also the case that another part is interposed therebetween. Likewise, when a part, such as a layer, film, region, disk, etc., is referred to as lying "under" another part, this includes not only the case where the part lies "directly under" the other part, but also the case that another part is interposed. Furthermore, arranged “on” in the present application can include both the case of an arrangement on the bottom and on the top.

The binder as a component of the electrode slurry for a secondary battery enables an active material to be bonded to each other or to fix an active material on the current collector, thereby improving the adhesion of the coated electrode. The binder is mixed with an active material for an electrode, a conductive material, etc. in the form of a binder solution obtained by dissolving binder solids in a predetermined solvent to form an electrode slurry. However, if the binder contained in the binder solution is not completely dissolved and undissolved substances remain in a fine form such as microgel, the part where the undissolved substances are located cannot play the original role of the binder. Such a portion appears as a surface defect of an electrode, e.g., a gray spot, a pinhole, a crater shape, etc., and if this defect is severe, it may become a region lacking active materials, thereby deteriorating the adhesion of the electrode .

1 Fig. 12 is a figure showing a shape of a surface defect of an electrode for a secondary battery. As in 1 1, surface defects in the form of gray spots are seen in the electrode for a secondary battery.

Therefore, the solution quality of a binder solution of an electrode for a secondary battery is an important factor for improving the surface defect of the electrode, but in the prior art, it has been difficult to accurately measure the solution quality. In the present invention, a method of evaluating the solution quality of a binder solution by the magnitude of a turbidity value by measuring the turbidity value of the binder solution was proposed to quantitatively evaluate the solution quality of the binder solution for improving the surface defect of an electrode before preparing an electrode slurry. In particular, since it was difficult to make an accurate quantitative evaluation of the solution quality of a binder solution by applying a binder solution with a blade on an overhead projector film and determining the number of foreign objects on the film with the naked eye, as is conventional in the art, a method for Proposed measurement of the turbidity value of a sample of the binder solution.

Haze is the phenomenon that due to the inherent nature of a material, rays create an opaque appearance in addition to reflection or absorption when light is transmitted through the interior of a transparent material.

The haze value can be expressed by Formula 1 as follows.

As shown in Formula 1 above, when light is transmitted through the binder solution to measure the turbidity value, the ratio between the scattered transmission and the total transmission of the binder solution becomes the turbidity value of the binder solution. Namely, the turbidity value of the binder solution relates to the degree of scattering of light in the transmission of light. When foreign matter or undissolved substance particles having a size of hundreds of nm or more are present, the larger the difference between the refractive indexes of these particles and the solvent and the larger the particles, the more the haze value increases. Therefore, the greater the number of undissolved substances (microgels) in the solution, the greater the turbidity value. In this context, measuring the turbidity value does not mean counting the number of microgels, but is a concept for measuring the intensity of light scattered from undissolved substances.

A method for evaluating the solution quality of a binder solution for a secondary battery electrode of the present invention comprises: measuring a turbidity value of a predetermined amount of a sample of the binder solution by transmission of light through the binder solution; and evaluating the solution quality of the binder solution based on the magnitude of the turbidity value.

Before measurement of the turbidity value, a binder is added to a predetermined solvent and then mixed so as to prepare a predetermined binder solution.

The binder, which is a dissolved component of the binder solution, can be at least one selected from the group consisting of a non-aqueous binder such as polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer (EPDM), sulfonated EPDM or polytetrafluoroethylene (PTFE), an aqueous binder such as acrylonitrile butadiene rubber, styrene butadiene rubber (SBR) or acrylic rubber and a polymer resin such as hydroxyethyl cellulose or carboxymethyl cellulose.

Further, a solvent of the binder solution may be at least one selected from the group consisting of an organic solvent such as N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone or dimethylacetamide, and water.

At this time, the solution quality of the binder solution changes depending on the mixing time, mixing temperature, type of mixing blade, etc., even if the same content of solute is added to the solvent. Also, the solution quality varies depending on the kind, size, etc. of undissolved matter contained in a solution. Therefore, the solution quality cannot be accurately evaluated solely by information about the concentration of the binder solution.

It is therefore necessary to quantitatively evaluate the solution quality of the CMC solution by measuring the turbidity values of a plurality of CMC solutions, each of which differs at least in terms of mixing time, mixing temperature, mixer type and mixing blade type. TK mixers, Homo Disper mixers, etc. can be selected as mixers. As the mixing blade, a pitched blade agitator, an anchor agitator, a helical agitator, etc. can be used. Here, one of them can be used, or two or more of them can be used together. To prepare a binder solution for a secondary battery electrode, a binder can be dissolved basically at 30 to 130°C.

The turbidity value can be measured with a turbidimeter. As the turbidity meter, for example, HM-150 made by Murakami Color Research Laboratory, etc. can be used. As will be described later, the haze value was measured in accordance with JIS 7136 in the present specification. A standard D65 light source can be used in the turbidimeter, which can also be used as the light source for irradiating the binder solution. Concretely, a predetermined amount (e.g. 3 to 4 ml) of the binder solution was taken as a sample for the turbidimeter, which was then placed in a quartz glass cuvette, and the quartz glass cuvette was set in the turbidimeter. Thereafter, light from a light source was irradiated to measure the turbidity value of the binder solution.

If the size of the turbidity value is measured, the solution quality of the binder solution can be evaluated quantitatively. Thus, the dissolution quality of the binder solution can be quantitatively evaluated by measuring the turbidity value even if it is a binder solution prepared by dissolving the same content of solutes.

As another aspect of the present invention, a method for producing an electrode slurry for a secondary battery includes: measuring a turbidity value of a predetermined amount of a sample binder solution by transmitting light through the binder solution; evaluating a solution quality of the binder solution based on a quantity of the turbidity value; and preparing an electrode slurry by mixing an active material for an electrode and a conductive material with a binder solution in a solution quality having a turbidity value equal to or smaller than a predetermined value. Concretely, the method for producing an electrode slurry for a secondary battery of the present invention comprises measuring a turbidity value of a binder solution and then quantitatively evaluating the solution quality of the binder solution as described above. In measuring the solution quality of the binder solution, a binder solution in a solution quality with a turbidity value equal to or smaller than a predetermined value is selected to improve the surface defect of an electrode or related properties of an electrode to be produced, and an active material for an electrode and a conductive material are mixed with the binder solution to thereby prepare an electrode slurry.

The electrode active material may include a predetermined positive electrode active material and a predetermined negative electrode active material. The positive electrode active material may be a lithium-containing oxide, and a lithium-containing transition metal oxide may be used as the lithium-containing oxide.

The lithium-containing transition metal oxide can, for example, be any oxide or a mixture of two or more oxides selected from the group consisting of Li _x CoO ₂ (0.5<x<1.3), Li _x NiO ₂ (0.5<x<1.3), Li _x MnO ₂ (0.5<x<1.3), Li _x Mn ₂ O ₄ (0.5<x<1.3), Li _x (Ni _a Co _b Mn _c )O ₂ (0.5<x<1.3, 0<a<1, 0<b <1, 0<c<1, a+b+c=1), Li _x Ni _1-y Co _y O ₂ (0.5<x<1.3, 0<y<1), Li _x Co _1-y Mn _y O ₂ (0.5<x<1.3, 0≤y<1), Li _x Ni _1-y Mn _y O ₂ (0.5<x<1.3, 0<y<1), Li _x (Ni _a Co _b Mn _c ) O ₄ (0.5<x<1.3, 0<a<2, 0<b<2, 0<c<2, a+b+c=2), Li _x Mn _2-z Ni _z O ₄ (0.5<x <1.3, 0<z<2), Li _x Mn _2-z Co _z O ₄ (0.5<x<1.3, 0<z<2), Li _x CoPO ₄ (0.5<x<1.3), and Li _x FePO ₄ (0.5<x<1.3). In addition, the lithium-containing transition metal oxide may be coated with a metal such as aluminum (Al) or a metal oxide. Furthermore, besides the lithium-containing transition metal oxide, one or more of sulfides, selenides and halides can be used.

The negative electrode active material may be a carbon material, lithium metal, silicon, or tin. When a carbon material is used as a negative electrode active material, both low crystalline and high crystalline carbon can be used. Typical examples of low crystalline carbon are soft carbon and hard carbon. Typical examples of highly crystalline carbon are natural graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesocarbon microbeads, mesophase pitch, and high-temperature calcined carbons such as petroleum coke or coal tar.

The conductive material is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery. Examples are graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, Ketjen black (Ketjen black), channel black (channel black), furnace black (furnace black), lamp black (lamp black) and summer black (summer black); conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives and the like.

For example, a CMC solution obtained by dissolving a certain CMC content in water as a solvent can be selected as a preferred binder solution to evaluate the solution quality by measuring the turbidity value. Most of the undissolved matter in the CMC solution ranges in size from 7 to 15 µm. Since the CMC solution contains undissolved substances, which enable the turbidity value to be measured, the solution quality can be assessed relatively accurately using the turbidity value.

Moreover, the electrode slurry of the present invention may be a negative electrode slurry.

Further, an electrode slurry for a positive electrode or a negative electrode can be prepared by mixing the active material for an electrode, the conductive material, etc. with the binder solution, the solution quality of which has been evaluated by measuring the turbidity value. It is possible to remarkably improve the surface defect of an electrode by preparing an electrode slurry using a binder solution having a turbidity value (solution quality) equal to or smaller than a predetermined value.

examples

Hereinafter, the present invention will be described in detail, but the present invention is not limited to these examples.

Comparative example 1

A CMC solution having a concentration of 1.5% was prepared by adding a certain amount of CMC to distilled water. The solution was stirred with a Homo Disper mixer for different mixing times.

The solution quality of the prepared CMC solution was evaluated by the conventional evaluation method. The result of this observation of the number of foreign matters on the film with the naked eye after coating the CMC solution in a thickness of 200 µm on a 5×5 cm OHP film with a squeegee is as follows. [Table 1] sample no #1 #2 #3 #4 #5 #6 #7 #8th Mixing Time (min) 60 80 40 140 100 120 160 180 number of microgels 21 19 23 1 3.5 1 0.5 0

As shown in Table 1, the number of foreign matters tended to decrease as the mixing time increased. Despite the different mixing times in samples no. 4 and no. 6, it was not possible to determine which sample had a better solution quality. In addition, since the solution quality is determined by the number of foreign matter, there have been situations where it was difficult to determine the solution quality to evaluate quantitatively when the number of foreign substances of the respective samples was too large or too small. In particular, in the case of Sample No. 8, since the number of foreign matter decreased to zero, it was difficult to judge how far the solution quality could be quantified.

example 1

A CMC solution having a concentration of 1.5% was prepared by mixing under the same conditions as in Comparative Example 1.

From the prepared solution, 4 ml was taken and placed in a quartz glass cuvette, which was then set in a turbidimeter to measure the turbidity value. A HM-150 made by Murakami Color Research Laboratory was used as a turbidity meter, and the turbidity value of the binder solution was measured in accordance with JIS 7136. [Table 2] sample no #1 #2 #3 #4 #5 #6 #7 #8th Mixing Time (min) 60 80 40 140 100 120 160 180 Haze (%) 2.44 2.35 2.48 1.96 2.10 2.02 1.82 1.7

As shown in Table 2, the turbidity value gradually decreases as the mixing time increases. This trend is comparable to that in Table 1. However, unlike Comparative Example 1, the haze values of Samples 4 and 6 were different values of 1.96 and 2.02, respectively. Thus, the solution quality could be clearly differentiated quantitatively. Since the quality of the solution is clearly indicated by the turbidity value, e.g.

example 2

In order to evaluate the surface defects of an electrode depending on the turbidity value (solution quality) of a CMC solution, CMC solutions having the same concentration (1.5%) and different turbidity values from Example 1 were selected.

Graphite as the negative electrode active material, carbon black as a conductive material and a CMC solution as a binder were prepared, and graphite and the conductive material were added to the CMC solution in which an amount of the solvent (water) was adjusted that the active material, the conductive material and the binder have a weight ratio of 97:1:2 and the solid content of the active material, the conductive material and the binder is 40%. This mixture was then mixed for 2 hours at 80 rpm by a TK mixer so as to prepare a negative electrode slurry. The prepared negative electrode slurry was applied to a surface of the copper current collector under specified coating conditions (coating width: 200 mm, coating speed: 5 m/min, loading amount: 300 mg/cm ² ), which was then dried at a temperature of 100°C and was then rolled so as to produce a negative electrode.

In this way, electrodes of the same composition were prepared using CMC solutions with different turbidity values. The number of surface defects was measured and the results are shown in Table 3 below. Each of the electrodes prepared was cut to a length of 50 cm, and the number of surface defects per 10 cm × 10 cm was counted with the naked eye. [Table 3] sample no #3 #6 #8th Haze (%) 2.48 2.02 1.7 Number of surface defects 534 324 107

As is apparent from Table 3, the number of surface defects decreases as the haze value becomes smaller. From this, the mixing time to improve the solution quality can be identified. In addition, it is possible to discriminate and predict a binder solution having a turbidity value or a solution quality for improving the surface defect of an electrode for a secondary battery. In Example 2, the number of surface defects decreased at a haze value of 2.02 versus a haze value of 2.48.

Further, when the haze value was 1.7, the number of surface defects decreased by more than 400. Thus, there was a significant difference in the surface defect properties depending on the haze value. As a result, using a solution having a turbidity value of the CMC solution of 2.0% or less can significantly reduce the number of surface defects of the negative electrode slurry.

From the above results, it can be seen that the solution quality is judged by the turbidity value of the binder solution, and an electrode slurry is prepared using a binder solution having a certain turbidity value or below, to thereby predict and control surface defects of an electrode.

In the above statements, the present invention has been described in more detail with reference to the drawings and examples. Therefore, the embodiments described in the specification and the configurations described in the drawings are only the preferred embodiments of the present invention and do not represent all the technical ideas of the present invention. It is understood that there are various equivalents and variations instead of them at the time of filing of the present application.

QUOTES INCLUDED IN DESCRIPTION

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Patent Literature Cited

• JP 2013053919 [0009]

Claims (10)

A method for evaluating a solution quality of a binder solution for a secondary battery electrode, the method comprising: measuring a turbidity value of a predetermined amount of a sample of the binder solution by transmission of light through the binder solution; and Evaluation of a solution quality of the binder solution based on the size of the turbidity value. procedure according to claim 1 , where the turbidity value is a ratio between a scattering transmittance and a total transmittance of the binder solution when light is radiated through the binder solution. procedure according to claim 1 wherein a binder which is a dissolved component of the binder solution is at least one selected from the group consisting of a non-aqueous binder comprising polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol , carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer (EPDM), sulfonated EPDM or polytetrafluoroethylene (PTFE), an aqueous binder comprising acrylonitrile butadiene rubber, styrene butadiene rubber (SBR) or acrylic rubber, and a polymer resin comprising hydroxyethyl cellulose or carboxymethyl cellulose. procedure according to claim 3 wherein the solvent of the binder solution is at least one selected from the group consisting of an organic solvent comprising N-methylpyrrolidone (NMP), dimethylformamide (DMF), acetone or dimethylacetamide, and water. procedure according to claim 1 , wherein the binder solution is a CMC solution obtained by dissolving a predetermined amount of CMC in water. procedure according to claim 5 wherein the solution quality of the CMC solution is quantitatively evaluated by respectively measuring the turbidity values of a plurality of CMC solutions prepared by varying at least one of mixing time, mixing temperature, mixer type and mixing blade type. A method for producing an electrode slurry for a secondary battery, the method comprising: measuring a turbidity value of a predetermined quantity of a sample binder solution by transmission of light through the binder solution; evaluating a solution quality of the binder solution based on a variable of the turbidity value; and Preparation of an electrode slurry by mixing an active material for an electrode and a conductive material with a binder solution in a solution quality having a turbidity value equal to or smaller than a predetermined value. procedure according to claim 7 , wherein the binder solution is a CMC solution obtained by dissolving a predetermined amount of CMC in water. procedure according to claim 7 wherein the electrode slurry is a negative electrode slurry. procedure according to claim 7 , where the turbidity value of the CMC solution is equal to or less than 2%.

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