Method for producing positive electrode mixture slurry and method for producing positive electrode

Abstract

To provide a manufacturing method of a slurry for a positive electrode mixture capable of suppressing a gelatinous state of the slurry.SOLUTION: A slurry for a positive electrode mixture contains an active material represented by the following formula (I): Li(1+x)NiyMe(1-y)O2 [in the formula, it leads 0≤x≤0.1 and 0.8≤y<1 and Me contains two or more selected from among Mn, Co, Al and Ti] and a modified product of polyvinylidene fluoride of which the weight average molecular weight is 1,000,000 or more. A manufacturing method of the slurry includes the steps of: (1) supplying carbon dioxide into N-methyl-2-pyrolidone; and (S2) kneading a modified product of N-methyl-2-pyrolidone, the active material and polyvinylidene fluoride after the step (S1). A concentration of carbon dioxide of N-methyl-2-pyrolidone used in the step (S2) is equal to or more than 70 ppm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a positive electrode mixture slurry and a method for producing a positive electrode. [Background technology]

[0002] In non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, a composite oxide of lithium and a transition metal such as nickel is sometimes used as the positive electrode active material contained in the active material layer of the positive electrode plate. When forming the active material layer of the positive electrode plate, a slurry containing the composite oxide and a binder is prepared. It is known that LiO2 contained on the particle surfaces of the composite oxide reacts with water to form LiOH, making the slurry alkaline, causing the binder to become three-dimensional and gelling the slurry (e.g., Patent Documents 1 and 2).

[0003] Patent Document 1 discloses that in order to prevent gelation of the slurry, a composite oxide is brought into contact with carbon dioxide under dry conditions to neutralize LiOH. Patent Document 2 describes that the use of a composite oxide that has a pH within a specific range when dispersed in water prevents the pH of the slurry from becoming alkaline. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-3891 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-90917 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the method disclosed in Patent Document 1, not only does LiOH on the surface of the composite oxide react with carbon dioxide, but lithium (Li) present inside the composite oxide also reacts with carbon dioxide. Since the reaction produces a large amount of lithium carbonate (LiCO3) and water, battery cells fabricated using the slurry described in Patent Document 1 are prone to gas generation. The method disclosed in Patent Document 2 requires the selection of a composite oxide that, when dispersed in water, has a pH within a specific range.

[0006] An object of the present disclosure is to provide a novel method for producing a positive electrode mixture slurry that can prevent the slurry from becoming gelatinous. [Means for solving the problem]

[0007] [1] A method for producing a positive electrode mixture slurry, The slurry contains an active material represented by the following formula (I) and a modified polyvinylidene fluoride having a weight-average molecular weight of 1,000,000 or more, The manufacturing method includes: A step (S1) of supplying carbon dioxide into N-methyl-2-pyrrolidone; and a step (S2) of kneading N-methyl-2-pyrrolidone, the active material, and the modified polyvinylidene fluoride after the step (S1), The method for producing a slurry, wherein the carbon dioxide concentration of the N-methyl-2-pyrrolidone used in the step (S2) is 70 ppm or more. Li (1+x) Ni y Me (1-y) O2(I) [In the formula, 0≦x≦0.1 and 0.8≦y<1, and Me includes two or more elements selected from the group consisting of Mn, Co, Al, and Ti.] [2] The method for producing a slurry according to [1], wherein the carbon dioxide concentration of the N-methyl-2-pyrrolidone used in the step (S2) is 100 ppm or more and 300 ppm or less. [3] The method for producing a slurry according to [1] or [2], wherein the step (S2) is carried out under a temperature condition of 50°C or less. [4] The method for producing a slurry according to any one of [1] to [3], wherein the step (S2) is carried out in an atmosphere having a carbon dioxide concentration of 10% by volume or more. [5] The method for producing a slurry according to any one of [1] to [4], wherein the step (S1) is a step of bubbling carbon dioxide through N-methyl-2-pyrrolidone. [6] A method for producing a positive electrode mixture slurry, comprising: The slurry contains an active material represented by the following formula (I) and a modified polyvinylidene fluoride having a weight-average molecular weight of 1,000,000 or more, The production method includes a step (S3) of kneading N-methyl-2-pyrrolidone, the active material, and the modified polyvinylidene fluoride in an atmosphere having a carbon dioxide concentration of 10% by volume or more. Li (1+x) Ni y Me (1-y) O2(I) [In the formula, 0≦x≦0.1 and 0.8≦y<1, and Me includes two or more elements selected from the group consisting of Mn, Co, Al, and Ti.] [7] The method for producing a slurry according to [6], wherein the step (S3) is carried out under a temperature condition of 50°C or less. [8] A method for producing a positive electrode plate having a current collector and an active material layer formed on the current collector, comprising: [1] to [7], and a step of producing a slurry by the method for producing a positive electrode mixture slurry according to any one of [1] to [7]. and applying the slurry onto the current collector and drying the slurry. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to prevent the positive electrode mixture slurry from becoming gelatinous. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a flowchart showing a method for producing a positive electrode mixture slurry according to an embodiment. [Figure 2]10 is a flowchart showing a method for producing a positive electrode mixture slurry according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Embodiment 1] (Method for producing positive electrode mixture slurry (1)) 1 is a flowchart showing a method for producing a positive electrode mixture slurry according to an embodiment. The method for producing a positive electrode mixture slurry (hereinafter also referred to as "slurry (1)") according to this embodiment is a method for producing a positive electrode mixture slurry for a non-aqueous electrolyte secondary battery (hereinafter also referred to as "battery"). The slurry (1) can be used to form an active material layer of a positive electrode plate of the battery.

[0011] The slurry (1) contains an active material represented by formula (I) (hereinafter also referred to as "active material (I)") and a modified polyvinylidene fluoride (hereinafter also referred to as "PVDF modified material") having a weight-average molecular weight of 1,000,000 or more. Li (1+x) Ni y Me (1-y) O2(I) [In the formula, 0≦x≦0.1 and 0.8≦y<1, and Me includes two or more elements selected from the group consisting of Mn, Co, Al, and Ti.]

[0012] The method for producing the slurry (1) includes the steps of: A step (S1) of supplying carbon dioxide (hereinafter also referred to as "CO2") into N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP"); and a step (S2) of kneading NMP, the active material (I), and the PVDF modified material after the step (S1). The CO2 concentration of the N-methyl-2-pyrrolidone used in the step (S2) is 70 ppm or more.

[0013] The active material (I) can be used as the active material for the positive electrode plate to increase the capacity of the battery. LiO2 may be present as an impurity on the particle surface of the active material (I). When this LiO2 reacts with water present in the slurry, lithium hydroxide (LiOH) is formed, which tends to make the slurry alkaline. The PVDF modified material is prone to gelation in alkaline slurry. In the method for producing the slurry (1), CO2 is supplied to NMP in step (S1), and NMP with a CO2 concentration of 70 ppm or more, the active material (I), and the PVDF modified material are kneaded in step (S2). This allows LiOH to react with CO2 in the NMP to produce lithium carbonate (LiCO3) and water, making the slurry (1) less alkaline. As a result, gelation of the PVDF modified material is suppressed, and the slurry (1) is prevented from becoming gelatinous and becoming highly viscous.

[0014] As a method for neutralizing LiOH on the surface of the active material (I), instead of using NMP supplied with CO2, a method of placing the active material (I) before kneading in a CO2 atmosphere is also considered, as shown in the comparative example described below. However, this method makes CO2 more likely to react with lithium (Li) inside the active material (I), producing a relatively large amount of LiCO3 and water, which is thought to make the battery more likely to generate gas. In contrast, since slurry (1) is produced by kneading NMP containing CO2 with the active material (I), LiOH on the surface of the active material (I) reacts with CO2 but is thought to be less likely to react with Li inside the active material (I). Therefore, when a positive electrode plate is prepared using slurry (1) and this positive electrode plate is applied to a battery, gas generation from the battery can be suppressed.

[0015] The method for step (S1) is not particularly limited as long as it allows for the supply of CO2 into NMP. For example, NMP may be bubbled with CO2, or NMP may be stirred while CO2 is supplied. The amount of CO2 supplied to NMP in step (S1) may be determined depending on the type of active material (I), the type of modified PVDF, and their contents in the slurry (1). For example, CO2 may be supplied to NMP so that the concentration of CO2 contained in the NMP used for kneading in step (S2) falls within the range described below. The concentration of CO2 in NMP can be adjusted, for example, by the amount, rate, and time of CO2 supply.

[0016] The concentration of CO2 in the NMP used in step (S2) may be 70 ppm or more, preferably 100 ppm or more and 300 ppm or less, 120 ppm or more and 280 ppm or less, or 150 ppm or more and 250 ppm or less. When the concentration of CO2 in NMP is within the above range, gelation of the PVDF modified material is easily suppressed, and therefore, the slurry (1) is easily suppressed from becoming gelatinous.

[0017] The temperature condition for carrying out step (S2) is preferably 50° C. or lower, may be 45° C. or lower, or may be 40° C. or lower, and is usually 15° C. or higher. By carrying out step (S2) at a temperature condition within the above range, thermal degradation of the PVDF modified product can be suppressed.

[0018] The CO2 concentration in the atmosphere in which step (S2) is performed may be 10% by volume or more, 20% by volume or more and 100% by volume or less, or 30% by volume or more and 50% by volume or less. The atmosphere in which step (S2) is performed is, for example, the atmosphere in a kneading vessel in which kneading is performed. The CO2 concentration in the atmosphere in which step (S2) is performed can be adjusted, for example, by a method described in an embodiment below. The CO2 concentration can be measured using a carbon dioxide concentration meter.

[0019] Step (S2) may include a step of dry-mixing the active material (I) and the PVDF modified material to obtain a mixture (1), and a step of kneading the mixture (1) with NMP after step (S1) to obtain a kneaded product (1).

[0020] The method for producing the slurry (1) may further include a step of mixing NMP with a conductive material to obtain a conductive paste (1). In this case, the method for producing the slurry (1) may include a step of kneading the kneaded material (1) with the conductive paste (1). The NMP used to obtain the conductive paste (1) is preferably the NMP after the step (S1).

[0021] The active material (I) may be a compound represented by the above formula (I). In formula (I), x may be 0.01≦x≦0.09, 0.03≦x≦0.08, or 0.05≦x≦0.07. In formula (I), y may be 0.81≦y≦0.98, 0.82≦y≦0.95, or 0.83≦y≦0.90. In formula (I), Me is preferably two or more elements selected from the group consisting of Mn, Co, and Al, and more preferably contains three elements selected from Mn, Co, and Al, or two elements selected from Mn and Co. The composition of the active material (I) can be determined by inductively coupled plasma (ICP) emission spectroscopy.

[0022] The active material (I) is, for example, Li (1+0.07) Ni 0.81 Co 0.05 Mn 0.12 Al 0.02 O2 and Li (1+0.01) Ni 0.83 Co 0.12 Mn 0.05 O2.

[0023] The LiOH content in the active material (I) before the kneading in step (S2) may be, for example, 0.01% by mass to 0.4% by mass, 0.05% by mass to 0.25% by mass, or 0.1% by mass to 0.23% by mass. The LiOH content in the active material (I) can be determined by neutralization titration.

[0024] The slurry (1) may contain one or more active materials (I), and may also contain a positive electrode active material other than the active material (I). The content of the active material (I) in the slurry is preferably 95% by mass or more and 100% by mass or less, more preferably 95% by mass or more and 99% by mass or less, and may even be 96% by mass or more and 98% by mass or less, based on the total amount of the positive electrode active materials in the slurry. When two or more active materials (I) are contained, the content of the active materials (I) refers to the total amount thereof.

[0025] The content of the positive electrode active material (including the active material (I)) in the slurry (1) is, for example, 80% by mass or more and 99% by mass or less, or alternatively 85% by mass or more and 98% by mass or less, or 90% by mass or more and 97% by mass or less, relative to the solid content of the slurry (1). When two or more positive electrode active materials are contained, the content of the positive electrode active materials refers to the total amount thereof.

[0026] The weight-average molecular weight of the PVDF modified product may be 1,000,000 or more, 1,050,000 to 2,000,000 or less, or 1,100,000 to 1,500,000 or less. The weight-average molecular weight of the PVDF modified product can be measured by gel permeation chromatography (GPC). By using a PVDF modified product having a weight-average molecular weight within the above range, a small amount of the PVDF modified product can be imparted with thixotropy to the slurry (1), making it easy to disperse or dissolve the PVDF modified product in the slurry (1). Furthermore, the adhesion strength can be ensured when the slurry (1) is applied to the current collector.

[0027] Modified PVDF is obtained by defluorinating polyvinylidene fluoride (hereinafter also referred to as "PVDF") and introducing functional groups such as polar functional groups. PVDF may be a homopolymer or a copolymer of vinylidene fluoride with a fluorine compound other than vinylidene fluoride, such as tetrafluoroethylene, hexafluoropropylene, or chlorotrifluoroethylene. Examples of polar functional groups include one or more selected from the group consisting of a hydroxyl group (-OH), a carbonyl fluoride group (-COF), a carboxyl group (-COOH), an acid anhydride group, an acrylate group (-CH=CHCOOH), and a glycidyl methacrylate group (-CHC(CH)COOCH(CHCHO)).

[0028] The content of the PVDF modified material in the slurry (1) is, for example, 0.5% by mass or more and 10% by mass or less, or may be 1% by mass or more and 8% by mass or less, or may be 1% by mass or more and 5% by mass or less, based on the solid content of the slurry (1).

[0029] The content of NMP in the slurry (1) is, for example, 3% by mass or more and 40% by mass or less, or may be 5% by mass or more and 30% by mass or less, or may be 8% by mass or more and 20% by mass or less, relative to the total amount of the slurry (1).

[0030] The slurry (1) may contain, in addition to the positive electrode active material such as the active material (I), the above-mentioned conductive material, a binder other than the modified PVDF, and the like.

[0031] The conductive material may be, for example, a carbon material. The carbon material may be, for example, one or more selected from the group consisting of fibrous carbon, carbon black (acetylene black, ketjen black, etc.), coke, and activated carbon. The fibrous carbon may be, for example, carbon nanotubes (CNT). The CNT may be a single-walled carbon nanotube (SWCNT) or a multi-walled carbon nanotube such as a double-walled carbon tube (DWCNT). The conductive material may be, for example, 1% by mass or more and 5% by mass or less, or 1.5% by mass or more and 3% by mass or less, based on the solid content of the slurry (1).

[0032] The binder may be, for example, one or more selected from the group consisting of styrene butadiene rubber (SBR), unmodified PVDF, and polytetrafluoroethylene (PTFE).

[0033] [Embodiment 2] (Method for producing positive electrode mixture slurry (2)) 2 is a flowchart showing a method for producing a positive electrode mixture slurry according to another embodiment. The method for producing a positive electrode mixture slurry (hereinafter also referred to as "slurry (2)") according to this embodiment is a method for producing a positive electrode mixture slurry for a battery. The slurry (2) is used to form an active material layer of a positive electrode plate of a battery.

[0034] Slurry (2) contains active material (I) and a PVDF modified material having a weight-average molecular weight of 1,000,000 or more. Examples of active material (I) and PVDF modified material include those described above. The content of LiOH in active material (I) and the weight-average molecular weight of the PVDF modified material are, for example, within the ranges described above. The contents of active material (I), PVDF modified material, and NMP in slurry (2) can be within the ranges of those contents described for slurry (1).

[0035] The method for producing the slurry (2) includes a step (S3) of kneading NMP, the active material (I), and the modified PVDF in an atmosphere having a CO2 concentration of 10% by volume or more.

[0036] As described above, the use of the active material (I) tends to make the slurry alkaline, which can easily cause gelation of the PVDF modified material. In the method for producing the slurry (2), NMP, the active material (I), and the PVDF modified material are kneaded in an atmosphere with a CO concentration of 10% by volume or more, allowing LiOH generated in the slurry to react with CO. This consumes LiOH to produce LiCO and water, thereby preventing gelation of the PVDF modified material and preventing the slurry (1) from becoming gelatinous. Furthermore, gas generation can also be prevented in a battery cell having a positive electrode plate produced using the slurry (2).

[0037] The CO2 concentration in the atmosphere in which step (S3) is performed may be 10% by volume or more, 10% by volume to 100% by volume, 20% by volume to 90% by volume, 30% by volume to 50% by volume, or even 100% by volume. The atmosphere in which step (S3) is performed is, for example, the atmosphere in a kneading vessel in which kneading is performed. The CO2 concentration can be measured using a carbon dioxide concentration meter.

[0038] The atmosphere in which step (S3) is performed can be adjusted, for example, by reducing the pressure or creating a vacuum inside the kneading vessel and then supplying CO2 into the kneading vessel. The concentration of CO2 in the kneading vessel can be adjusted, for example, by adjusting the pressure inside the kneading vessel when the pressure inside the kneading vessel is reduced.

[0039] The temperature condition for carrying out step (S3) is preferably 50° C. or lower, may be 45° C. or lower, or may be 40° C. or lower, and is usually 15° C. or higher. By carrying out step (S3) at a temperature condition within the above range, thermal degradation of the PVDF modified product can be suppressed.

[0040] Step (S3) may include a step of dry-mixing the active material (I) and the PVDF modified material to obtain a mixture (2), and a step of kneading NMP with the mixture (2) to obtain a kneaded product (2). The NMP used in step (S3) may or may not be subjected to the treatment of supplying CO2 described in the above embodiment.

[0041] The method for producing the slurry (2) may further include a step of mixing NMP with a conductive material to obtain a conductive paste (2). In this case, the method for producing the slurry (2) may include a step of kneading the above-mentioned kneaded material (2) with the conductive paste (2). The NMP used to obtain the conductive paste (2) may or may not be subjected to the treatment of supplying CO2 described in the above embodiment.

[0042] The slurry (2) may contain, in addition to the positive electrode active material such as the active material (I), a conductive material, a binder other than the modified PVDF, etc. Examples of the conductive material and binder include those described above.

[0043] (Manufacturing method of positive electrode plate) The slurries (1) and (2) produced by the above-mentioned production method can be used to form an active material layer of a positive electrode plate. The method for producing a positive electrode plate is a method for producing a positive electrode plate having a current collector and an active material layer formed on the current collector, and includes, for example, A step of obtaining a slurry (1) or (2) produced by the above-mentioned method for producing a slurry; The method may include a step of applying the slurry (1) or (2) onto a current collector and drying the applied slurry.

[0044] The slurry (1) or (2) may be applied to one or both sides of the current collector. The slurry (1) or (2) applied to the current collector and dried may be compressed as necessary.

[0045] The slurries (1) and (2) are unlikely to gel and are unlikely to become highly viscous, and therefore are easy to handle when applied to a current collector. By using the positive electrode plate manufactured by the above manufacturing method, a battery cell with suppressed gas generation can be obtained.

[0046] (Nonaqueous electrolyte secondary battery) The battery may include a positive electrode plate formed using the slurry (1) or (2) produced by the above-described method for producing a slurry. The positive electrode plate may be produced by, for example, the above-described method.

[0047] A battery typically includes an electrode assembly and an electrolyte. The battery may further include a case that houses the electrode assembly and the electrolyte. A resin sheet serving as an electrode holder may be disposed between the electrode assembly and the case.

[0048] The electrode assembly may have the above-mentioned positive electrode plate, negative electrode plate, and a separator interposed between the positive electrode plate and the negative electrode plate. The electrode assembly may be of a wound type or a laminated type.

[0049] The negative electrode may have a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode current collector is, for example, a metal foil made of a copper material such as copper or a copper alloy. The negative electrode active material layer contains a negative electrode active material and may further contain one or both of a binder and a conductive additive.

[0050] Examples of negative electrode active materials include carbon-based active materials containing carbon (C) atoms, such as graphite; and metal-based active materials containing metal elements, such as simple metals or metal oxides containing elements selected from the group consisting of silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), and germanium (Ge). Examples of binders include cellulose-based binders such as carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), and the like. Examples of conductive additives include carbon materials such as fibrous carbon, carbon black (e.g., acetylene black, ketjen black), coke, and activated carbon. Examples of fibrous carbon include those described above.

[0051] The separator may be a porous sheet (film, nonwoven fabric, etc.) made of a resin such as polyethylene, polypropylene, polyester, cellulose, or polyamide. The porous sheet may have a single-layer structure or a multi-layer structure of two or more layers. The separator may have a functional layer on the surface of the porous sheet. The functional layer may be at least one of a heat-resistant layer and an adhesive layer for adhering to the positive electrode plate and the negative electrode plate.

[0052] The electrolyte may be a non-aqueous electrolyte, such as a non-aqueous solvent such as an organic solvent containing a supporting salt. Examples of non-aqueous solvents include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Examples of supporting salts include lithium perchlorate (LiClO4) and lithium hexafluorophosphate (LiPF6).

[0053] The case is a housing that houses the electrode assembly and is preferably made of metal, such as aluminum, an aluminum alloy, iron, or an iron alloy. [Example]

[0054] Hereinafter, the present disclosure will be described more specifically with reference to examples and comparative examples. Comparative Example 1 (Preparation of Slurry) As the active material (I), Li (1+0.07) Ni 0.81 Co 0.05 Mn 0.12 Al 0.02 O2 and Li (1+0.01) Ni 0.83 Co 0.12 Mn 0.05 O2 was prepared in a mass ratio of 7:3. This active material (I) and a modified PVDF (weight average molecular weight: 1.1 million) were dry-mixed in a sealed kneading vessel, and then NMP was added and kneaded at 25 to 50°C to achieve a solid content of 87 to 92%. Next, a conductive paste in which a conductive material was dispersed in NMP was added and kneaded to obtain a slurry.

[0055] (Preparation of positive electrode plate) The slurry prepared above was applied to both sides of a current collector, dried, and compressed to form an active material layer, thereby obtaining a positive electrode plate including the current collector and the active material layer.

[0056] (Preparation of negative electrode plate) A negative electrode active material layer containing natural graphite as a negative electrode active material was formed on a negative electrode current collector to obtain a negative electrode plate.

[0057] (Battery cell manufacturing) An electrode assembly was fabricated using the positive and negative electrode plates and separators prepared above. A battery cell with an energy density of 650 Wh / L was fabricated using the electrode assembly and electrolyte.

[0058] [Comparative Example 2 and Examples 1 to 3] NMP with the CO2 concentration shown in Table 1 was obtained by bubbling NMP with CO2. The CO2 concentration in NMP was adjusted by adjusting the bubbling time while supplying CO2 at 100 cc / min per 1 L of NMP. The bubbling time was 1 minute (Comparative Example 2), 2 minutes (Example 1), 5 minutes (Example 2), or 10 minutes (Example 3). A positive electrode mixture slurry was obtained, and a positive electrode plate, a negative electrode plate, and a battery were fabricated in the same manner as in Comparative Example 1, except that NMP adjusted to the CO2 concentration shown in Table 1 was used.

[0059] [Comparative Example 3 and Reference Example 1] The active material (I) was placed in a CO atmosphere to neutralize the LiOH on the surface of the active material (I), and the contents of LiOH and Li2CO3 were adjusted as shown in Table 1. A positive electrode mixture slurry was obtained in the same manner as in Comparative Example 1, except that the active material with the LiOH and Li2CO3 contents adjusted to those shown in Table 1 was used, and a positive electrode plate, a negative electrode plate, and a battery were fabricated.

[0060] [Comparative Example 4, Examples 4 and 5] After the pressure in the kneading vessel was reduced, CO was supplied to create a CO atmosphere inside the kneading vessel. The CO concentration in the kneading vessel was adjusted by adjusting the pressure inside the kneading vessel when the pressure in the kneading vessel was reduced to −1 kPa (Comparative Example 5), −10 kPa (Example 4, kneading vessel pressure: 90 kPa in absolute value), or −100 kPa (Example 5, kneading vessel pressure: 1 kPa in absolute value) relative to atmospheric pressure. A positive electrode mixture slurry was obtained, and a positive electrode plate, a negative electrode plate, and a battery were fabricated in the same manner as in Comparative Example 1, except that kneading was performed in a kneading vessel adjusted to the CO concentration shown in Table 1.

[0061] [Composition of active material] The composition of the active material (I) was determined by high-frequency inductively coupled plasma (ICP) emission spectrometry.

[0062] [LiOH and Li2CO3 content in active materials] The contents of LiOH and Li2CO3 were determined by neutralization titration for the active materials (active materials neutralized with CO2 for Comparative Example 3 and Reference Example 1) before being kneaded with the PVDF modified material and NMP. The results are shown in Table 1.

[0063] [Measurement of weight-average molecular weight] The weight average molecular weight of the modified PVDF was measured by GPC (gel permeation chromatography).

[0064] [CO2 concentration in NMP] The CO2 concentration in the NMP before kneading with the PVDF modified material and the active material (NMP after bubbling for Comparative Example 2 and Examples 1 to 3) was calculated using the following procedure. Hydrochloric acid was added to the NMP placed in a sealed container to desorb the CO2 dissolved in the NMP, and the gas in the space of the sealed container was analyzed using gas chromatography to calculate the CO2 concentration. At 25°C, the volume of the NMP in the sealed container was taken as 100, the volume of the remaining space in the sealed container (space not occupied by NMP) was taken as 500, and the volume of hydrochloric acid was taken as 1. The results are shown in Table 1.

[0065] [CO2 concentration in the kneading vessel] The CO2 concentration in the kneading vessel was measured using a carbon dioxide concentration meter. The results are shown in Table 1.

[0066] [Slurry evaluation] Using a spiral viscometer, the viscosity of the slurry immediately after preparation was measured at 25°C, and this was taken as the initial viscosity. Seven days after preparation, the viscosity of the slurry was measured in the same manner as above, and this was taken as the viscosity after storage. If the viscosity after storage was within ±20% of the initial viscosity, it was evaluated as "A," and if it exceeded ±20% of the initial viscosity, it was evaluated as "B." The results are shown in Table 1.

[0067] [Measurement of gas generation rate] The battery cells produced in the example and comparative examples were activated, and the internal pressure was measured while the battery cells were constrained to a fixed size. The internal pressure of the battery cells after a storage durability test in which the battery cells were stored for 60 days at a voltage of 4.2 V and a temperature of 60°C was calculated as a relative value, with the internal pressure of the battery cell after the storage durability test in Comparative Example 1 set as 100 (reference). The results are shown in Table 1.

[0068] [Table 1]

[0069] From Table 1, it is believed that the slurries obtained in Examples 1 to 3 were less likely to gel and thus suppressed the amount of gas generated from the battery because CO2 was supplied to NMP and then kneaded with the active material (I) and the PVDF modified product. In contrast, it is believed that gelation occurred in Comparative Example 1 because NMP to which CO2 had not been supplied was used, and in Comparative Example 2, CO2 was supplied but in a small amount. In Comparative Example 3 and Reference Example 1, a treatment was performed to neutralize LiOH in the active material (I) before kneading with CO2, which prevented the slurry from gelling in Reference Example 1, but gas was generated from the battery in both Comparative Example 3 and Reference Example 1.

[0070] From Table 1, it is considered that the slurries obtained in Examples 4 and 5 were less likely to gel and the amount of gas generated from the battery was reduced because they were kneaded in a kneading vessel with a CO2 concentration of 10% by volume or more. In contrast, it is considered that the slurry obtained in Comparative Example 4 was gelled because it was kneaded in a kneading vessel with a CO2 concentration of 1% by volume.

Claims

1. A method for producing a positive electrode mixture slurry, comprising: The slurry contains two or more active materials represented by the following formula (I) and a modified polyvinylidene fluoride having a weight average molecular weight of 1,000,000 or more: The two or more active materials include an active material in which Me in the following formula (I) is Mn, Co, and Al, and an active material in which Me in the following formula (I) is Mn and Co, The manufacturing method includes: A step (S1) of supplying carbon dioxide into N-methyl-2-pyrrolidone; and a step (S2) of kneading N-methyl-2-pyrrolidone, the two or more active materials, and the modified polyvinylidene fluoride after the step (S1), The method for producing a slurry, wherein the carbon dioxide concentration of the N-methyl-2-pyrrolidone used in the step (S2) is 70 ppm or more. Li （１＋ｘ） Ni ｙ Me （１－ｙ） O ２ (I) [In the formula, 0≦x≦0.1 and 0.8≦y<1, and Me includes two or more elements selected from the group consisting of Mn, Co, Al, and Ti.]

2. 2. The method for producing a slurry according to claim 1, wherein the carbon dioxide concentration of the N-methyl-2-pyrrolidone used in the step (S2) is 100 ppm or more and 300 ppm or less.

3. The method for producing a slurry according to claim 1 , wherein the step (S2) is carried out at a temperature of 50° C. or less.

4. The method for producing a slurry according to claim 1 , wherein the step (S2) is carried out in an atmosphere having a carbon dioxide concentration of 10% by volume or more.

5. 2. The method for producing a slurry according to claim 1, wherein the step (S1) is a step of bubbling N-methyl-2-pyrrolidone with carbon dioxide.

6. A method for producing a positive electrode mixture slurry, comprising: The slurry contains two or more active materials represented by the following formula (I) and a modified polyvinylidene fluoride having a weight average molecular weight of 1,000,000 or more: The two or more active materials include an active material in which Me in the following formula (I) is Mn, Co, and Al, and an active material in which Me in the following formula (I) is Mn and Co, The manufacturing method of the slurry includes a step (S3) of kneading N-methyl-2-pyrrolidone, the two or more active materials, and the modified polyvinylidene fluoride in a kneading vessel in an atmosphere having a carbon dioxide concentration of 10% by volume or more. Li （１＋ｘ） Ni ｙ Me （１－ｙ） O ２ (I) [In the formula, 0≦x≦0.1 and 0.8≦y<1, and Me includes two or more elements selected from the group consisting of Mn, Co, Al, and Ti.]

7. The method for producing a slurry according to claim 6, wherein the step (S3) is carried out at a temperature of 50°C or less.

8. A method for manufacturing a positive electrode plate having a current collector and an active material layer formed on the current collector, A step of producing a slurry by the method for producing a positive electrode mixture slurry according to any one of claims 1 to 7; and applying the slurry onto the current collector and drying the slurry.

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