JP6926942B2 - Manufacturing method of positive electrode - Google Patents

Description

The present invention relates to a method for producing a positive electrode.

Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are used in hybrid vehicles (HVs), plug-in hybrid vehicles (PHVs), electric vehicles (EVs), and the like.
The non-aqueous electrolyte secondary battery includes a pair of electrodes, a positive electrode and a negative electrode, a separator that insulates between them, and a non-aqueous electrolyte. As a structure of an electrode (positive electrode or negative electrode) for a non-aqueous electrolyte secondary battery, a structure including a current collector made of a metal foil or the like and an electrode active material layer formed on the current collector is known.

The positive electrode of a lithium ion secondary battery is formed by applying a paste-like electrode mixture for a positive electrode containing a positive electrode active material such as a lithium-containing composite oxide on a current collector such as an aluminum foil, drying it, and pressing it. Is manufactured.
_{Patent Document 1 describes lithium cobalt composite oxide (LiCoO 2} ), lithium nickel composite oxide (LiNiO ₂ ), and lithium nickel cobalt aluminum composite oxide (LiNi) as lithium-containing composite oxides used as positive electrode active materials. _{x Co y Al 1-x-} y O 2) (NCA) and the like are cited (paragraphs 0005,0039, etc.).

JP-A-2016-087776

Conventionally, a paste-like electrode mixture for a positive electrode generally contains a conductive agent such as carbon powder, a binder such as polyvinylidene fluoride (PVDF), and a dispersion medium such as N-methyl-2-pyrrolidone (NMP). It is produced by a method including a step of preparing a preliminary paste containing the positive electrode active material and not containing the positive electrode active material, and a step of adding and mixing the positive electrode active material to the preliminary paste.

When a lithium-containing composite oxide such as NCA, which is a positive electrode active material, comes into contact with moisture in the atmosphere, excess Li on the surface of the active material reacts with the moisture to generate lithium hydroxide (LiOH), which is a basic compound. May be done. The reaction formula at this time is represented by _{Li 2} O + H _{2 O → 2 LiOH.}
When polyvinylidene fluoride (PVDF), which is commonly used as a binder, comes into contact with the basic compound lithium hydroxide (LiOH), an unsaturated reaction occurs. Therefore, when a positive electrode active material stored in the atmosphere is used as a raw material, or when the prepared paste-shaped electrode mixture for positive electrode is stored in the air, the viscosity of the prepared electrode mixture for positive electrode increases. There is a risk of gelling. Since it is difficult to apply the gelled electrode mixture for the positive electrode onto the current collector, it is difficult to stably manufacture the positive electrode. Therefore, when the electrode mixture for the positive electrode is gelled, the quality of the positive electrode to be produced is not good, and it is difficult to stably produce a lithium ion secondary battery having excellent performance.

The present invention has been made in view of the above circumstances, and provides a method for producing a positive electrode capable of stably producing a high-quality positive electrode by suppressing gelation of a paste-like electrode mixture for a positive electrode. The purpose is.

The method for producing a positive electrode of the present invention
A step (A) of preparing a paste-like electrode mixture for a positive electrode containing a positive electrode active material containing a lithium nickel cobalt aluminum composite oxide, a conductive agent, polyvinylidene fluoride, and a dispersion medium.
The present invention relates to a method for producing a positive electrode, which comprises a step (B) of applying and drying the paste-like electrode mixture for a positive electrode on at least one surface of a current collector.
In the method for producing a positive electrode of the present invention, in the step (A), a preliminary paste containing the conductive agent, polyvinylidene fluoride, and the dispersion medium and not containing the positive electrode active material is prepared, and an acid is added to the preliminary paste. This is a step of adding the positive electrode active material after the addition.

According to the present invention, it is possible to provide a method for producing a positive electrode capable of stably producing a high-quality positive electrode by suppressing gelation of the electrode mixture for the positive electrode.

It is a schematic whole view which shows the structural example of the non-aqueous electrolyte secondary battery of one Embodiment which concerns on this invention. It is a schematic cross-sectional view of the electrode laminate in the non-aqueous electrolyte secondary battery of FIG. 1A. It is a schematic cross-sectional view of the electrode of one Embodiment which concerns on this invention. It is a flowchart which shows the preparation method of the electrode mixture for positive electrode in the comparative example 1. FIG. It is a graph which shows the relationship between the standing time (air exposure time) in the air, and the viscosity of the electrode mixture for a positive electrode obtained in Comparative Example 1. In Comparative Example 1, the positive electrode combination after 2 hours of air exposure to the initial resistance of the lithium ion secondary battery (B1-X) obtained by using the positive electrode mixture (P1-X) without air exposure. It is a graph which shows the ratio of the initial resistance of the lithium ion secondary battery (B1-Y) obtained by using the agent (P1-Y). It is a flowchart which shows the preparation method of the electrode mixture for positive electrode in Example 1. FIG. In Example 1, it is a graph which shows the relationship between the type / addition amount of an acid, and the basicity of a positive electrode mixture after exposure to the atmosphere for 2 hours. In Example 1, it is a graph which shows the relationship between the type of acid, the basicity of the electrode mixture for a positive electrode after exposure to the air for 2 hours, and the viscosity of the electrode mixture for a positive electrode after exposure to the air for 2 hours. FIG. 5 is a graph showing the relationship between the type of acid, the basicity of the electrode mixture for the positive electrode after exposure to the air for 2 hours, and the initial resistivity ratio of the obtained lithium ion secondary battery in Example 1.

The present invention relates to a method for manufacturing a positive electrode for a lithium ion secondary battery.

[Lithium-ion secondary battery]
First, the configuration of the lithium ion secondary battery of one embodiment according to the present invention will be described with reference to the drawings.
FIG. 1A is a schematic overall view of the lithium ion secondary battery of the present embodiment.
FIG. 1B is a schematic cross-sectional view of the electrode laminate.
FIG. 1C is a schematic cross-sectional view of an electrode according to an embodiment of the present invention. The electrode shown in this figure is a positive electrode or a negative electrode in a lithium ion secondary battery.

As shown in FIG. 1A, the lithium ion secondary battery 1 of the present embodiment contains an electrode laminate 20 and a non-aqueous electrolyte (reference numeral omitted) in an exterior body (battery container) 11. Two external terminals (plus terminal and minus terminal) 12 for external connection are provided on the outer surface of the exterior body 11.
As shown in FIG. 1B, the electrode laminate 20 is formed by laminating a pair of electrodes 21 via a separator 22 that insulates them. The pair of electrodes 21 are a positive electrode 21A and a negative electrode 21B.

As shown in FIG. 1C, the electrode 21 (positive electrode 21A or negative electrode 21B) is an electrode containing an electrode active material (positive electrode active material or negative electrode active material) on at least one surface of a current collector 110 made of a metal foil or the like. The active material layer 120 is formed. In the illustrated example, the electrode active material layer 120 is formed on one surface of the current collector 110.

(Positive electrode)
The positive electrode includes a current collector and a positive electrode active material layer containing a positive electrode active material formed on at least one surface of the current collector.
As the positive electrode current collector, aluminum foil or the like is preferably used.
In the present embodiment, the positive electrode active material layer is formed by using a paste-like positive electrode mixture containing a positive electrode active material, a conductive agent, a binder, and a dispersion medium, and if necessary, a thickener. It is a paste. Each component can be used alone or in combination of two or more.

As the positive electrode active _{material, LiCoO 2, LiMnO 2, LiMn} 2 O 4, LiNiO 2, LiNi x Co 1-x O 2, LiNi x Co y Mn 1-x-y O 2, and LiNi _x Co _y Al _{1- Examples thereof include lithium-containing composite oxides such as xy} O ₂ (lithium nickel cobalt aluminum composite oxide, NCA) (in the formula, 0 <x <1, 0 <y <1).
The present invention can be preferably applied when the positive electrode active material contains excess Li.
The present invention can, for example, the positive electrode active material _{LiNi x Co y Al 1-x} -y O 2 (the lithium-nickel-cobalt-aluminum composite oxide, NCA) if it contains, is preferably applied.
Examples of the binder include polyvinylidene fluoride (PVDF) and the like.
Examples of the conductive agent include carbon materials such as acetylene black (AB) and graphite.
Examples of the thickener include carboxymethyl cellulose (CMC) and the like.
As the dispersion medium, an organic solvent such as N-methyl-2-pyrrolidone (NMP) is preferable.
The various solid components described above may be used in the production of an electrode mixture in the form of a solution or a dispersion liquid containing an arbitrary solvent or dispersion medium at the raw material stage. In this case, the dispersion medium in the electrode mixture includes the solvent or the dispersion medium in the raw material.

(Negative electrode)
The negative electrode includes a current collector and a negative electrode active material layer containing a negative electrode active material formed on at least one surface of the current collector.
A copper foil or the like is preferably used as the negative electrode current collector.
The negative electrode active material layer can be formed by using a paste-like electrode mixture for a negative electrode containing a negative electrode active material, a binder and a dispersion medium, and optionally a conductive agent and / or a thickener. .. Each component can be used alone or in combination of two or more.

The negative electrode active material is not particularly limited, and a material having a lithium storage capacity of 2.0 V or less based on Li / Li + is preferably used. Examples of the negative electrode active material include carbon such as graphite, metallic lithium, lithium alloys, transition metal oxides / transition metal nitrides / transition metal sulfides capable of dope / removal of lithium ions, and these. Combinations and the like can be mentioned.
Examples of the binder include a styrene-butadiene copolymer (SBR) and the like.
Examples of the conductive agent include carbon materials such as acetylene black (AB) and graphite.
Examples of the thickener include carboxymethyl cellulose (CMC) and the like.
Examples of the dispersion medium include water and the like.
The various solid components described above may be used in the production of an electrode mixture in the form of a solution or a dispersion liquid containing an arbitrary solvent or dispersion medium at the raw material stage. In this case, the dispersion medium in the electrode mixture includes the solvent or the dispersion medium in the raw material.

(Non-aqueous electrolyte)
As the non-aqueous electrolyte, known ones can be used, and liquid, gel-like or solid non-aqueous electrolytes can be used.
For example, a non-resolved lithium-containing electrolyte is dissolved in a mixed solvent of a high dielectric constant carbonate solvent such as propylene carbonate or ethylene carbonate and a low-viscosity carbonate solvent such as diethyl carbonate, methyl ethyl carbonate, and dimethyl carbonate. A water electrolyzed solution is preferably used.
As the mixed solvent, for example, a mixed solvent of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) is preferably used.
Examples of the lithium-containing electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{(2 k + 1)} (integer of k = 1 to 8), LiPF _n {C _k F _{(2 k + 1). )} } _(6-n) (integer of n = 1 to 5, integer of k = 1 to 8) and the like, and combinations thereof.

(Separator)
The separator may be a film that electrically insulates the positive electrode and the negative electrode and allows lithium ions to pass through, and a porous polymer film is preferably used. As the separator, a porous film made of polyolefin such as a porous film made of PP (polypropylene), a porous film made of PE (polyethylene), or a laminated porous film of PP (polypropylene) -PE (polyethylene) is preferably used. ..

(Exterior body (battery container))
A known exterior body can be used. The type of the secondary battery includes a cylindrical type, a coin type, a square type, a laminated type (also referred to as a film type), and the like, and the exterior body can be selected according to a desired type.

[Manufacturing method of positive electrode]
The method for producing a positive electrode of the present invention
A step (A) of preparing a paste-like electrode mixture for a positive electrode containing a positive electrode active material containing a lithium nickel cobalt aluminum composite oxide (NCA), a conductive agent, polyvinylidene fluoride (PVDF), and a dispersion medium. ,
It has a step (B) of applying and drying a paste-like electrode mixture for a positive electrode on at least one surface of a current collector.

In the method for producing a positive electrode of the present invention, in step (A), a preliminary paste containing a conductive agent, polyvinylidene fluoride (PVDF), and a dispersion medium and not containing a positive electrode active material is prepared, and an acid is added to the preliminary paste. This is a step of adding the positive electrode active material. Please refer to the flowchart of the first embodiment shown in FIG.

As described in the section [Problems to be Solved by the Invention], conventionally, a paste-like electrode mixture for a positive electrode has generally been a conductive agent such as carbon powder and a binder such as polyvinylidene fluoride (PVDF). A method including a step of preparing a preliminary paste containing the paste and a dispersion medium such as N-methyl-2-pyrrolidone (NMP) and not containing the positive electrode active material, and a step of adding and mixing the positive electrode active material to the preliminary paste. Manufactured by. Please refer to the flowchart of Comparative Example 1 shown in FIG.
When a lithium-containing composite oxide such as NCA, which is a positive electrode active material, comes into contact with moisture in the atmosphere, excess Li on the surface of the active material reacts with the moisture to generate lithium hydroxide (LiOH), which is a basic compound. May be done. The reaction formula at this time is represented by _{Li 2} O + H _{2 O → 2 LiOH.}

When polyvinylidene fluoride (PVDF), which is commonly used as a binder, comes into contact with the basic compound lithium hydroxide (LiOH), an unsaturated reaction occurs. Therefore, when a positive electrode active material stored in the air is used as a raw material, or when the prepared paste-shaped electrode mixture for positive electrode is stored in the air, the viscosity of the prepared electrode mixture for positive electrode increases. There is a risk of gelling. Since it is difficult to apply the gelled electrode mixture for the positive electrode onto the current collector, it is difficult to stably manufacture the positive electrode. Therefore, when the electrode mixture for the positive electrode is gelled, the quality of the positive electrode to be produced is not good, and it is difficult to stably produce a lithium ion secondary battery having excellent performance.

In the method for producing a positive electrode of the present invention, a preliminary paste containing no positive electrode active material is prepared, an acid is added to the preliminary paste, and then the positive electrode active material is added. In this method, since the preliminary paste is pre-biased to the acidic side and then the positive electrode active material is added, the positive electrode active material of the raw material contains lithium hydroxide (LiOH) produced by the reaction with the moisture in the atmosphere. Even if it is, lithium hydroxide (LiOH), which is a basic compound contained in the raw material, can be neutralized with an acid. Further, by leaving the finally obtained paste-like electrode mixture for the positive electrode in the air, the positive electrode active material in the electrode mixture for the positive electrode reacts with the moisture in the atmosphere and is contained in the electrode mixture for the positive electrode. Even if lithium hydroxide (LiOH) is produced in the body, the produced basic compound lithium hydroxide (LiOH) can be neutralized with an acid.
Therefore, in the method for producing a positive electrode of the present invention, polyvinylidene fluoride (PVDF) used as a binder and lithium hydroxide (LiOH), which is a basic compound, are suppressed from basicization of the electrode mixture for the positive electrode. It is possible to suppress the reaction with and suppress the increase in viscosity and gelation due to this reaction. As a result, a high quality positive electrode can be stably produced. By using the positive electrode manufacturing method of the present invention, a high-performance lithium ion secondary battery can be stably manufactured.

In general, the degree of basicity can be indexed by pH. However, in the method for producing a positive electrode of the present invention, the pH of the positive electrode mixture cannot be accurately measured for the following reasons.
In order to suppress the formation of lithium hydroxide (LiOH), it is preferable that the electrode mixture for the positive electrode does not contain water, but the presence of OH ions and H ions is necessary for pH measurement. Further, it is generally difficult to measure the pH of a paste having a relatively high viscosity as it is. Therefore, it is necessary to add water to dilute the paste-like electrode mixture for the positive electrode, which does not contain water and has a relatively high viscosity, before measuring the pH. However, when water is added, new lithium hydroxide (LiOH) is generated, so that the pH changes, and the pH of the positive electrode mixture cannot be accurately measured.

In the method for producing a positive electrode of the present invention, it is preferable to use "basicity" as an index of basicity.
As shown in the section of [Example] below, the basicity of the positive electrode mixture after preparation and standing in the air for 2 hours (after exposure to the air for 2 hours) is preferably 7.7 or less. It is preferably 7.6 or less. In this case, the increase in viscosity after exposure to the atmosphere is suppressed, and gelation is effectively suppressed, which is preferable. The viscosity after exposure to the air for 2 hours is preferably 85,000 mPa · s or less, more preferably 80,000 mPa · s or less.
In the present specification, the basicity and viscosity shall be determined by the method described in the section [Examples] below.

The acid is not particularly limited, and examples thereof include benzoic acid and acetic anhydride. These can be used alone or in combination of two or more.
Since the increase in basicity and viscosity of the positive electrode mixture can be effectively suppressed, the amount of acid added to the positive electrode mixture is preferably 3% by mass or more, more preferably 4% by mass or more.
Since the acid is originally a component that is not necessary for the positive electrode active material layer, it is not preferable to add a large amount of the unnecessary component. The amount of the acid added to the positive electrode mixture is preferably 3 to 5% by mass, more preferably 4 to 5% by mass.
The solid content of the positive electrode mixture is preferably about 70 to 90%.

Step (B) can be carried out by a known method. The paste-like electrode mixture for the positive electrode obtained in the step (A) can be applied onto the current collector, dried, and pressed to form a positive electrode active material layer. The positive electrode active material layer can be formed on one side or both sides of the current collector.

As described above, according to the present invention, it is possible to provide a method for producing a positive electrode capable of stably producing a high-quality positive electrode by suppressing gelation of the electrode mixture for the positive electrode.

Hereinafter, examples and comparative examples according to the present invention will be described.
In Example 1 and Comparative Example 1, a lithium ion secondary battery was manufactured by changing the manufacturing method of the positive electrode. Common conditions were used except for the method for manufacturing the positive electrode.

[Evaluation items and evaluation methods]
(Basicity of electrode mixture for positive electrode)
The basicity of the electrode mixture for the positive electrode was calculated from the ionization degree of ions measured by a non-aqueous titration method using a Sorbotoload electrode manufactured by Metrohm Japan.

(Viscosity of electrode mixture for positive electrode)
The viscosity of the positive electrode mixture was measured using a rheometer.

(Initial resistance of lithium-ion secondary battery)
The obtained lithium ion secondary battery was charged and discharged for one cycle, and the initial resistance value was determined based on the results of measuring the current value and the voltage value at the time of discharging. The initial resistance value was calculated from the discharge capacity when discharged for 10 seconds under the conditions of discharge rates of 0.3C, 1C, 3C, and 5C.

[Ingredient for positive electrode]
As a cathode active material, lithium-nickel-cobalt-aluminum composite oxide _{(LiNi x Co y Al 1-} x-y O 2) ( wherein, x is 0.75 to 0.95, y is 0.05 to 0.25, For 1-xy, 0.01 to 0.1) (NCA) was prepared.
Acetylene black (AB) was prepared as a conductive agent.
Polyvinylidene fluoride (PVDF) was prepared as a binder.
N-Methyl-2-pyrrolidone (NMP) was prepared as a dispersion medium.
Benzoic acid and acetic anhydride were prepared as acids.

[Comparative Example 1]
(Preparation and evaluation of electrode mixture for positive electrode)
First, a preliminary paste containing a conductive agent (AB), a binder (PVDF), and a dispersion medium (NMP) was prepared by a known method. Next, the positive electrode active material (NCA) was added to the preliminary paste and mixed. As described above, a paste-like electrode mixture for positive electrodes having a solid content of 70% by mass was prepared. FIG. 2 shows a flowchart showing a method for preparing the electrode mixture for the positive electrode in Comparative Example 1.
The solid content ratio of the raw material in the electrode mixture for the positive electrode was as follows.
Active material (NCA) / Conductive agent (AB) / Binder (PVDF) (mass ratio) = 93/6/1

The electrode mixture for the positive electrode immediately after preparation had a viscosity of 3330 mPa · s and a basicity of 7.5. When the obtained positive electrode mixture was allowed to stand in the air, the viscosity of the positive electrode mixture increased with the passage of time. FIG. 3 shows the relationship between the standing time (air exposure time) of the positive electrode mixture in the air and the viscosity. As shown in FIG. 3, the viscosity increased with the lapse of the standing time in the air (air exposure time), and after standing for 2 hours, the viscosity of the positive electrode mixture increased to 13826 mPa · s and gelled. After standing for 2 hours, the basicity of the positive electrode mixture increased to 8.3.
When the lithium-containing composite oxide (NCA), which is a positive electrode active material, comes into contact with moisture in the atmosphere, excess Li on the surface of the active material reacts with the moisture to generate lithium hydroxide (LiOH), which is a basic compound. It is considered that the basicity was increased. Further, it is considered that when polyvinylidene fluoride (PVDF) comes into contact with lithium hydroxide (LiOH), which is a basic compound, an unsaturated reaction occurs, and the electrode mixture for the positive electrode becomes thicker and gels.
Hereinafter, the positive electrode mixture immediately after preparation (without exposure to the atmosphere) is referred to as "positive electrode mixture (P1-X)", and the positive electrode mixture after exposure to the air for 2 hours is referred to as "positive electrode mixture (P1-X)". Y) ".

(Manufacturing of positive electrode)
Immediately after preparation (without exposure to the air), a positive electrode mixture (P1-X) for a positive electrode is applied onto one surface of an aluminum foil, which is a current collector, by a known method, dried, and pressed. A positive electrode (E1-X) was manufactured.
A positive electrode (E1-Y) was produced in the same manner except that the positive electrode mixture (P1-Y) after exposure to the atmosphere for 2 hours was used as the positive electrode mixture.

(Negative electrode)
A paste containing graphite as a negative electrode active material, styrene-butadiene copolymer (SBR) latex as a binder, carboxymethyl cellulose (CMC) as a thickener, and ion-exchanged water as a dispersion medium. An electrode mixture for the negative electrode was prepared. A negative electrode mixture obtained was applied to one surface of a copper foil as a current collector by a known method, dried, and pressed to produce a negative electrode.

(Separator)
A commercially available separator made of a PP (polypropylene) porous film was prepared.

(Non-aqueous electrolyte)
A non-aqueous electrolyte solution was prepared by dissolving _{LiPF 6} which is a lithium salt as an electrolyte at a concentration of 1 mol / L using ethylene carbonate (EC) / diethyl carbonate (DEC) = 1/1 (volume ratio) as a solvent.

(Manufacturing of lithium-ion secondary batteries)
A positive electrode (E1-X) obtained by using an electrode mixture (P1-X) for a positive electrode immediately after preparation (without exposure to the atmosphere), the negative electrode, the separator, and a commercially available laminated exterior body (8 × 12 cm). A battery cell was manufactured by a known method. The non-aqueous electrolytic solution was injected into the obtained battery cell to produce a lithium ion secondary battery (B1-X).
A lithium ion secondary battery (B1-Y) is used in the same manner except that a positive electrode (E1-Y) obtained by using an electrode mixture (P1-Y) for a positive electrode after exposure to the air for 2 hours is used as the positive electrode. Manufactured.

(Evaluation of lithium-ion secondary battery)
The initial resistances of the obtained lithium ion secondary batteries (B1-X) and (B1-Y) were determined, respectively.
The initial resistance of the lithium ion secondary battery (B1-X) obtained by using the positive electrode mixture (P1-X) immediately after preparation (without exposure to the atmosphere) was set to a reference value of 100%. A lithium ion secondary battery (B1-Y) obtained by using a positive electrode mixture (P1-Y) for a positive electrode after exposure to the air for 2 hours with respect to the initial resistance of the reference lithium ion secondary battery (B1-X). ) The ratio of the initial resistance was calculated. The evaluation results are shown in FIG. As shown in FIG. 4, an increase in the initial resistance was observed in the lithium ion secondary battery (B1-Y) obtained by using the positive electrode mixture (P1-Y) for the positive electrode after exposure to the atmosphere for 2 hours. The initial resistivity ratio of the lithium ion secondary battery (B1-Y) was 130%.

[Example 1]
(Preparation and evaluation of electrode mixture for positive electrode)
A preliminary paste was prepared in the same manner as in Comparative Example 1. In Example 1, an acid was added to the preliminary paste under different conditions and mixed. After adding and mixing the acid, the positive electrode active material (NCA) was added and mixed in the same manner as in Comparative Example 1 to prepare a paste-like electrode mixture for the positive electrode having a solid content of 70% by mass. FIG. 5 shows a flowchart showing a method for preparing the electrode mixture for the positive electrode in Example 1.
The type of acid was two conditions, benzoic acid and acetic anhydride.
The amount of acid added to the positive electrode mixture was set to 1, 2, 3, 4, and 5% by mass under five conditions.

The electrode mixture for the positive electrode immediately after preparation had a viscosity of 3330 mPa · s and a basicity of 7.5. The obtained positive electrode mixture was allowed to stand in the air for 2 hours, and then the viscosity and basicity were measured.
Tables 1 and 6 show the relationship between the type and amount of acid added and the basicity of the positive electrode mixture after exposure to the atmosphere for 2 hours. In FIG. 6, as reference data, the basicity = 7.5 immediately after the preparation of the electrode mixture for the positive electrode (without atmospheric exposure) and the basicity = 8.3 after 2 hours of exposure to the atmosphere in Comparative Example 1 are shown by broken lines. be.
FIG. 7 shows the relationship between the type of acid, the basicity of the positive electrode mixture after 2 hours of air exposure, and the viscosity of the positive electrode mixture after 2 hours of air exposure. Since the viscosity of the positive electrode mixture for the positive electrode in Comparative Example 1 after exposure to the atmosphere for 2 hours was 13826 mPa · s, the estimated value of 140000 mPa · s is shown by a broken line in FIG. 7 as reference data. In FIG. 7, as reference data, the target value of viscosity of 85,000 mPa · s after exposure to the atmosphere for 2 hours is shown by a broken line.

As shown in Tables 1, 6 and 7, in Example 1 in which the acid was added to the positive electrode mixture, 2 hours compared to Comparative Example 1 in which the acid was not added to the positive electrode mixture. It was possible to suppress an increase in the basicity and viscosity of the positive electrode mixture after exposure to the air. Regardless of the type of acid, the larger the amount of acid added, the lower the basicity and viscosity of the positive electrode mixture after exposure to the atmosphere for 2 hours. The basicity was 7.7 or less, and the viscosity was 83975 Pa · s, which was 85000 mPa · s or less, which was the target value, and gelation did not occur.
By adding benzoic acid or acetic anhydride in an amount of 3% by mass or more or 4% by mass or more, the basicity of the electrode mixture for the positive electrode after exposure to the air for 2 hours is 7.7 or less, preferably 7.6 or less, and the viscosity is increased. It could be 85,000 mPa · s or less, preferably 80,000 mPa · s or less.

(Manufacturing of positive electrode, manufacturing and evaluation of lithium ion secondary battery)
A positive electrode and a lithium ion secondary battery were manufactured in the same manner as in Comparative Example 1 using a plurality of types of positive electrode mixture after exposure to the air for 2 hours obtained by changing the acid addition conditions. The initial resistance of the obtained lithium ion secondary battery was determined. Initial resistance when the initial resistance of the lithium ion secondary battery (B1-Y) obtained by using the positive electrode mixture (P1-Y) for the positive electrode after exposure to the air for 2 hours in Comparative Example 1 is set to a reference value of 100%. The ratio was calculated.

FIG. 8 shows the relationship between the type of acid, the basicity of the electrode mixture for the positive electrode after exposure to the air for 2 hours, and the initial resistivity ratio of the obtained lithium ion secondary battery. In all the lithium ion secondary batteries obtained in Example 1, the lithium ion secondary batteries (B1-) obtained by using the positive electrode mixture (P1-Y) for the positive electrode after exposure to the air for 2 hours in Comparative Example 1 A decrease in initial resistance was observed with respect to Y). In particular, when the basicity of the positive electrode mixture for the positive electrode after exposure to the air for 2 hours is 7.5 to 7.7, the electrode mixture for the positive electrode (P1-Y) after the exposure to the air for 2 hours is used in Comparative Example 1. The initial resistance of the obtained lithium ion secondary battery (B1-Y) could be reduced by 10% or more.

1 Non-aqueous electrolyte secondary battery 20 Electrode laminate 21 Electrode 21A Positive electrode 21B Negative electrode 22 Separator 110 Current collector 120 Electrode active material layer

Claims (1)

A step (A) of preparing a paste-like electrode mixture for a positive electrode containing a positive electrode active material containing a lithium nickel cobalt aluminum composite oxide, a conductive agent, polyvinylidene fluoride, and a dispersion medium.
A method for manufacturing a positive electrode, which comprises a step (B) of applying and drying the paste-like electrode mixture for a positive electrode on at least one surface of a current collector.
In the step (A), a preliminary paste containing the conductive agent, polyvinylidene fluoride, and the dispersion medium and not containing the positive electrode active material is prepared, and benzoic acid and / or acetic anhydride is added to the preliminary paste. , The step of adding the positive electrode active material .
A method for producing a positive electrode, wherein the amount of benzoic acid and / or acetic anhydride added to the positive electrode mixture is 3 to 5% by mass.

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