Method for manufacturing positive electrodes for lithium-ion batteries

Abstract

To provide a manufacturing method of a positive electrode for a lithium ion battery with improved dispersibility of a conductive material and an active material by optimizing the pressure applied when kneading a first active material with a large particle size and a second active material with a small particle size while the active material granules are prevented from collapsing during kneading.SOLUTION: In a manufacturing method of a positive electrode for a lithium ion battery according to an embodiment, a positive electrode active material includes a first active material having a spherical shape obtained by granulating nanoscale primary particles, and a second active material having a particle diameter of 1 / 10 or less of the particle size distribution D50 of the first active material as a form at the time of manufacturing the positive electrode, and the method includes preparing a first slurry by kneading the first active material, a conductive material, a binder, and a solvent with a liquid kneading, preparing a second slurry by kneading the second active material, the conductive material, the binder and the solvent with a hard kneading, mixing the first slurry and the second slurry to prepare a mixed slurry in which the weight ratio of the first active material and the second active material is 95:5 to 70:30, and applying the mixed slurry to one or both sides of a current collector.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a positive electrode for a lithium ion battery. [Background technology]

[0002] In recent years, lithium-ion secondary batteries have become widely used due to their high energy density and other reasons, and are installed as power sources in small portable devices such as mobile phones, digital cameras, and laptop computers. Furthermore, in light of issues such as energy resource depletion and global warming, development of lithium-ion secondary batteries is being promoted for large-scale industrial applications, such as hybrid and electric vehicles, and for storing electricity generated by natural energy sources such as solar and wind power. Therefore, lithium-ion secondary batteries are required to have even higher density and longer life in order to expand the use of these power sources.

[0003] Such lithium-ion secondary batteries are charged and discharged by transferring lithium ions between the positive electrode and the negative electrode. The positive electrode comprises a positive electrode current collector and a positive electrode layer containing a positive electrode active material provided on one or both surfaces of the positive electrode current collector. Currently, lithium metal oxides, such as lithium cobalt oxide (LiCoO), lithium manganese oxide (LiMnO), lithium nickel oxide (LiNiO), and lithium iron phosphate (LiFePO), are used as positive electrode active materials, or metal phosphate oxides containing lithium are being developed for commercialization.

[0004] Of these, lithium iron phosphate is known as a highly safe material and is used in vehicles and stationary applications. The lithium iron phosphate typically produced by Japanese manufacturers has nanoparticles as primary particles and spherical granules as secondary particles. In addition, there is a tendency to make the particle size of the secondary particles relatively large to improve the handling of the powder itself.

[0005] Conventionally, in the manufacture of positive electrodes for lithium-ion secondary batteries, conductive materials with high conductivity, i.e., large specific surface area and low tap density, are generally used together with the positive electrode active material. Similar conductive materials are also used in the manufacture of positive electrodes using cobalt-based oxides and nickel-cobalt-manganese composite oxide-based positive electrode active materials.

[0006] On the other hand, when lithium iron phosphate is used as the positive electrode active material, the active material is a granular material that exists as secondary particles, so the particle size itself is similar to that of conventional active materials. However, because the primary particles are nanoscale, it exhibits a high specific surface area.

[0007] Therefore, to prepare a stable slurry and manufacture a positive electrode with guaranteed mechanical strength and battery characteristics, a large amount of binder and solvent is required even for the active material alone. Furthermore, since a material with a large specific surface area is used for the conductive material, even more binder and solvent are required.

[0008] When a positive electrode containing such a positive electrode active material is produced, even if the active material itself is closest packed in the vicinity of the current collector, large gaps will occur between adjacent active material particles and the current collector.

[0009] Furthermore, since a large amount of binder and solvent is used, the solid content of the slurry is low, and voids are likely to occur in the positive electrode layer when the positive electrode layer (positive electrode mixture layer) containing the active material on the current collector is dried.

[0010] If the slurry is applied and dried in the above state to prepare a positive electrode, the positive electrode layer may peel off from the current collector in the subsequent rolling step.

[0011] Patent Document 1 proposes a technique in which a first active material and a second active material made of lithium iron phosphate and having different particle sizes coexist. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] International Publication No. 2015 / 005228 Summary of the Invention [Problem to be solved by the invention]

[0013] However, in the method of Patent Document 1, the granulated active material collapses during kneading, resulting in a decrease in output characteristics and cycle characteristics, or the dispersion of the conductive material and the active material becomes uneven during kneading, resulting in a decrease in battery characteristics and a significant decrease in the strength of the positive electrode.

[0014] The present invention provides a method for producing a positive electrode for a lithium-ion battery, which prevents collapse of granules of the first active material during kneading and improves dispersibility of the conductive material and active material, by optimizing the load applied during kneading of a first active material having a large particle size and a second active material having a small particle size. [Means for solving the problem]

[0015] A method for manufacturing a positive electrode for a lithium-ion battery according to an embodiment includes a positive electrode active material comprising a first active material having a spherical shape formed by granulating nanoscale primary particles and a second active material having a particle size that is 1 / 10 or less of the particle size distribution D50 of the first active material as the form during production of the positive electrode, and includes the steps of: mixing the first active material, a conductive material, a binder, and a solvent by liquid kneading to prepare a first slurry; mixing the second active material, the conductive material, the binder, and the solvent by solid kneading to prepare a second slurry; mixing the first slurry and the second slurry to prepare a mixed slurry in which the weight ratio of the first active material to the second active material (weight ratio of first active material / second active material) is 95:5 to 70:30; and applying the mixed slurry to one or both surfaces of a current collector. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a method for producing a positive electrode for a lithium ion battery that is excellent in output characteristics and cycle characteristics and that can also ensure sufficient strength. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, the method for producing a positive electrode for a lithium ion battery according to the embodiment will be described in detail.

[0018] The method for producing a positive electrode for a lithium ion battery according to this embodiment includes: preparing a first slurry by liquid kneading a first active material having a spherical shape formed by granulating nanoscale primary particles, a conductive material, a binder, and a solvent; preparing a second slurry by solid kneading a second active material, a conductive material, a binder, and a solvent; mixing the first slurry and the second slurry to prepare a mixed slurry in which the weight ratio of the first active material to the second active material is 95:5 to 70:30; and applying the mixed slurry to one or both surfaces of a current collector.

[0019] (Cathode active material) The positive electrode active material is not particularly limited, and known or commercially available materials can be used, such as lithium-containing oxides such as lithium iron phosphate, lithium manganese oxide, lithium nickel oxide, lithium iron fluorophosphate, and lithium cobalt oxide.

[0020] The first active material has a spherical shape formed by granulating nanoscale primary particles, and preferably has a size of, for example, 8.0 μm to 12.0 μm.

[0021] The second active material has a particle size that is 1 / 10 or less of the particle size distribution D50 of the first active material in the form in which the positive electrode is produced, and is preferably, for example, 0.7 μm to 0.9 μm.

[0022] Here, "particle size distribution D50" refers to the following value: A sample in which positive electrode active material particles are dispersed in water is irradiated with laser light, and the light scattered by the positive electrode active material particles is captured by a light-scattering particle size distribution analyzer, which then processes the particle size distribution of the positive electrode active material particles in the sample to determine the particle size distribution of the positive electrode active material particles in the sample. The particle size distribution of the obtained positive electrode active material particles, for example, the particle size distribution of 200 positive electrode active material particles, is processed to sort the particle size values ​​of the positive electrode active material particles in ascending order, and the particle size of the 100th smallest positive electrode active material particle (the 50th smallest when 100 particles are used as the base) is defined as "D50."

[0023] (Conductive material) The conductive material is not particularly limited, and known or commercially available materials can be used, such as carbon black such as acetylene black or ketjen black, activated carbon, or graphite.

[0024] (binder) The binder is not particularly limited, and known or commercially available binders can be used, such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), or carboxymethyl cellulose (CMC).

[0025] (solvent) As the solvent for the organic slurry, N-methyl-2-pyrrolidone (NMP) or the like can be used, and as the solvent for the aqueous slurry, ion-exchanged water or the like can be used.

[0026] First slurry preparation process The first slurry can be prepared by kneading the first active material, the conductive material, the binder, and the solvent.

[0027] That is, a first slurry raw material containing a first active material, a conductive material, a binder, and a solvent is prepared. The first active material, conductive material, and binder are weighed so that the total solid content of the first active material, conductive material, and binder is a predetermined value relative to the solvent. After weighing, the binder is added to the solvent and stirred and mixed to prepare a binder solution. The conductive material is added to this binder solution and stirred and mixed to disperse the conductive material in the binder solution. After stopping the stirring and mixing, or during the stirring and mixing process, the first active material is added and stirred and mixed. It is preferable to keep this stirring force lower than when adding the conductive material to the binder solution and stirring it. A first slurry is prepared by this liquid kneading. The stirring and mixing can be performed using a stirring kneader such as a planetary disperser.

[0028] The total solid content relative to the solvent is preferably weighed to a predetermined value, for example, 40% by weight or more and less than 60% by weight.

[0029] The composition of the first slurry is preferably, for example, a mass composition of first active material:conductive material:binder=98-96:1-2:1-2.

[0030] Second slurry preparation process The second slurry can be prepared by kneading the second active material, the conductive material, the binder, and the solvent.

[0031] That is, a second slurry raw material containing a second active material, a conductive material, a binder, and a solvent is prepared. The second active material, conductive material, and binder are weighed out so that the total solid content of the second active material, conductive material, and binder is higher than the total solid content of the liquid kneading mixture. After weighing, the binder is added to the solvent and stirred and mixed to prepare a binder solution. The conductive material is added to this binder solution and stirred and mixed, and then the second active material is added and stirred and mixed. By this kneading process, a second slurry is prepared. The stirring and mixing can be performed using a stirring kneader such as a planetary dispenser.

[0032] The total solid content of the second active material, the conductive material, and the binder is preferably weighed out relative to the solvent so that the total solid content is higher than the total solid content of the kneaded mixture, for example, 60% by weight to 80% by weight when the total solid content of the kneaded mixture is 40% by weight to less than 60% by weight. The weighing of the total solid content relative to the solvent is more preferably 65% ​​by weight to 75% by weight.

[0033] The composition of the second slurry is preferably, for example, second active material:conductive material:binder=96-90:2-5:2-5 by mass.

[0034] The conductive material, binder, and solvent contained in the first and second slurries may be the same or different from each other.

[0035] Mixing the first slurry with the second slurry The obtained first and second slurries are mixed to prepare a mixed slurry. The mixing can be performed using a stirring kneader such as a planetary disperser. Here, the weight ratio of the first active material contained in the first slurry to the second active material contained in the second slurry is preferably 95:5 to 70:30, and more preferably 90:10 to 80:20. By specifying the mixing ratio of the first active material to the second active material in this way, it is possible to improve the peel strength of the electrode while maintaining excellent output characteristics and cycle characteristics.

[0036] The first active material and the second active material preferably account for 40 to 60% by weight of the solid content in the mixed slurry.

[0037] - Coating process of mixed slurry onto current collector The prepared mixed slurry is applied to at least one surface of a current collector.

[0038] (current collector) The current collector is, for example, a foil made of aluminum, nickel, stainless steel, titanium, or an alloy of any of these materials.The thickness of the current collector is preferably 10 μm to 30 μm.

[0039] The method for applying the mixed slurry to the current collector is not particularly limited, and can be any commonly used method. Examples of such methods that can be used include gravure coating, gravure reverse coating, roll coating, Mayer bar coating, blade coating, knife coating, air knife coating, comma coating, slot die coating, slide die coating, and dip coating. The coating may be performed on only one surface or both surfaces of the current collector.

[0040] Next, drying is carried out. The drying method is not particularly limited, and any commonly used method can be used. For example, oven drying can be used. For oven drying, it is preferable to dry the mixed slurry applied to the current collector under conditions of an oven temperature of 80 to 130°C for 2 to 10 minutes. By such a method, a positive electrode layer (positive electrode mixture layer) is formed on the current collector. Note that the drying temperature and time may be appropriately changed depending on the equipment used for the oven drying and the specifications of the slurry to be applied.

[0041] Pressing process The positive electrode layer of the current collector is rolled using a press.

[0042] The rolling may be performed using a common rolling technique for positive electrodes. For example, a press or the like can be used for such a method. The press preferably has a rolling width that is sufficiently larger than the width of the positive electrode layer produced in the coating process. The rolling width is, for example, 300 to 800 mm.

[0043] The positive electrode sheet obtained by the rolling process is cut to a desired size, and then an electrode terminal is welded to produce a positive electrode for a lithium ion battery. The positive electrode can be used as the positive electrode for, for example, a cylindrical, prismatic, or laminated battery.

[0044] According to the embodiment described above, it is possible to provide a method for producing a positive electrode for a lithium ion secondary battery that is excellent in output characteristics and cycle characteristics and has sufficient strength.

[0045] That is, the manufacturing method according to the embodiment is characterized in that when a first active material having a spherical shape formed by granulating nanoscale primary particles, a conductive material, a binder, and a solvent are kneaded to prepare a first slurry, the kneading is performed by "liquid kneading," and when a second active material having a particle size that is 1 / 10 or less of the particle size distribution D50 of the first active material as the form in which the positive electrode is produced, a conductive material, a binder, and a solvent are kneaded to prepare a second slurry, the kneading is performed by "solid kneading."

[0046] Because the first active material is a granule of nanoscale primary particles, there is a risk that the granules will collapse during kneading. For this reason, the first slurry is prepared by "liquid kneading." As described above, "liquid kneading" involves adding the conductive material and the first active material to the binder solution in this order and stirring, and the pressure after adding the first active material is lower than that during the addition of the conductive material. This "liquid kneading" method improves the mixing and dispersibility of the conductive material in the binder solution, while at the same time making it possible to prepare a first slurry in which the first active material is well mixed and dispersed in the binder solution and conductive material without causing the first active material to collapse.

[0047] The second active material, as long as it has a particle size of 1 / 10 or less of the particle size distribution D50 of the first active material during positive electrode preparation, does not need to maintain the granular structure of the first active material. Therefore, the second slurry is prepared by "stiff kneading." As described above, "stiff kneading" involves preparing a mixed powder of the second active material and conductive material, and a binder solution. To the mixed powder, a binder solution is added in an amount of, for example, 40 to 70 wt.% of the binder amount contained in the final slurry. The mixed powder is then stirred and mixed under a predetermined pressure in the presence of the binder solution, kneading the binder solution into the mixed powder. During this kneading process, the remaining binder solution is added once or in multiple batches, and the binder solution is again kneaded into the mixed powder to obtain a second slurry. This "stiff kneading" process allows the preparation of a second slurry with excellent mixing and dispersion between the second active material and conductive material, and between the second active material, conductive material, and binder solution.

[0048] Furthermore, when mixing the first slurry and the second slurry, by specifying the weight ratio of the first active material to the second active material to be (95:5 to 70:30), it is possible to prepare a mixed slurry in which the positive electrode active material consisting of the first and second active materials is well dispersed in the binder solution, the second active material is well dispersed between the first active materials without disintegration, and the conductive material is well dispersed between and in contact with the first and second active materials.

[0049] Such a mixed slurry is applied to at least one surface of a current collector, and the coating is then dried and rolled, thereby producing a positive electrode for a lithium ion secondary battery that has excellent output characteristics and cycle characteristics and also has sufficient strength.

[0050] The improved strength of the resulting positive electrode is due to the fact that the second active material, which is finer than the first active material, is well dispersed and in contact with the first active material (granules) whose disintegration is suppressed in the positive electrode layer (positive electrode mixture layer) formed on the current collector, and the first and second active materials are well mixed and dispersed in the binder.

[0051] Furthermore, the lithium-ion secondary battery equipped with the obtained positive electrode has excellent output characteristics and cycle characteristics because the first active material (granules) and the second active material, which are inhibited from collapsing, are well dispersed within the positive electrode layer, and at the same time, the conductive material is well dispersed and in contact between the first and second active materials, forming a conductive path and a conductive network to the current collector.

[0052] Examples of the present invention will be described in detail below.

[0053] (Examples 1 to 4, Comparative Examples 1 to 4) <Manufacturing positive electrodes for lithium-ion batteries> A positive electrode for a lithium ion battery was manufactured using the following materials. First active material: LCP420TU4 (Sumitomo Osaka Cement) as lithium iron phosphate Second active material: M121 (Aleees) as lithium iron phosphate Conductive material: Li400 (Denka) as acetylene black Binder: Polyvinylidene fluoride (PVDF) as KF Polymer #9100 (Kureha) Solvent: N-methyl-2-pyrrolidone

[0054] Preparation of the first slurry A first active material, lithium iron phosphate (manufactured by Sumitomo Osaka Cement Co., Ltd., trade name: LFP420-TU4) having a particle size distribution D50 of 8.7 μm, a conductive material, acetylene black (manufactured by Denka Co., Ltd., trade name: Li400), and a binder, polyvinylidene fluoride (manufactured by Kureha Corporation, trade name: KF Polymer #9100), were weighed out to achieve a mass composition of 96:2:2.

[0055] (Liquid kneading process) N-methyl-2-pyrrolidone (NMP) with a final solid content of 50% was weighed in advance into a mixer container, and the entire amount of binder (2 parts by weight) was added to this container and stirred to prepare a binder+NMP binder solution.

[0056] The binder solution was put into a planetary dispenser manufactured by Primix Corporation, and the entire amount (2 parts by weight) of the conductive material was added to the binder solution. The mixture was then kneaded by stirring with the planetary dispenser and dispersing with the dispenser to prepare a binder solution in which the conductive material was dispersed.

[0057] Next, the entire amount (96 parts by weight) of the first active material was added to the binder solution in which the conductive material was dispersed in the planetary dispenser, and the mixture was kneaded by stirring with the planetary dispenser and dispersing with the dispenser. After confirming that the mixture had become a paste, the liquid kneading process was completed and the first slurry was prepared. At this time, the pressure applied to the conductive material-dispersed binder solution and the first active material by the planetary dispenser was set to a pressure lower than the pressure applied to the binder solution and the conductive material.

[0058] (Check the liquid mixing condition) The first slurry prepared in the liquid-kneading process was applied to aluminum foil and dried to prepare a test electrode. The test electrode was observed using surface SEM and cross-sectional SEM to check for any deformation of the first active material. As a result, it was confirmed that the first active material had not collapsed. This is because the load applied to the first active material during the liquid-kneading process was kept as low as possible, thereby preventing the first active material, which is a granule, from collapsing.

[0059] Preparation of the second slurry A second active material, lithium iron phosphate (manufactured by Aleees, trade name: M121) with an average particle size of 4.0 μm, a conductive material, acetylene black (manufactured by Denka, trade name: Li400), and a binder, polyvinylidene fluoride (manufactured by Kureha, trade name: KF Polymer #9100), were weighed out to achieve a mass composition of 96:2:2.

[0060] (Kneading process) N-methyl-2-pyrrolidone (NMP) with a final solid content of 70% by weight was weighed out in advance into a mixer container, and the entire amount of binder (2 parts by weight) was added to this container, stirred, and mixed to prepare a binder solution of binder + NMP.

[0061] The binder solution was poured into a Primix Planetary Disperser, and the entire amount (2 parts by weight) of conductive material was added to the binder solution. The mixture was then kneaded by stirring with the planetary and dispersing with the Disperser, and the entire amount (96 parts by weight) of the second active material was then added, and the mixture was stirred, dispersed, and kneaded. At this time, the mixture became clay-like with high viscosity from the early stage of kneading after the addition of the second active material, making it possible to apply a strong shear to the second active material in the kneaded mixture.

[0062] After the kneading process was completed, NMP was added to the planetary dispenser so that the final solid content was 50% by weight, and further kneading was continued. Once it was confirmed that the mixture had become a paste, the kneading was stopped and a second slurry was prepared.

[0063] (Check the state of the mixture) The second slurry prepared in the kneading process was applied to aluminum foil and dried to prepare a test electrode. The state of the second active material was observed using surface and cross-sectional SEM. No aggregation of the second active material was observed, and it was confirmed that the conductive material was well dispersed around the second active material. The average particle size of the second active material observed in the positive electrode was 0.8 μm or less.

[0064] The obtained first and second slurries were mixed to prepare mixed slurries of Examples 1 to 4 and Comparative Examples 1 to 4, which differ in the mass ratio of the first active material to the second active material contained in the mixed slurry and in the kneading conditions, as shown in Table 1 below. Mixing was carried out using a planetary dispenser. In this mixing step, the first and second slurries were already in a highly fluid state, so no additional high-load kneading was carried out, and the mixture was stirred by the planetary and dispersed by the dispenser.

[0065] ·Cathode manufacturing Each of the four mixed slurries was applied to one side of an aluminum foil current collector using a comma coater to form a coating, which was then dried in a furnace at 120°C, with the coated current collector running through the furnace for 2 minutes.

[0066] The current collector with the coating was pressed in a rolling machine to produce a positive electrode having a positive electrode layer (positive electrode mixture layer) with a density of 2.0 g / cc on one side. The pressing machine used had an effective substrate width of 300 mm.

[0067] <Peel strength test> The obtained positive electrodes of Examples 1 to 4 and Comparative Examples 1 to 4 were tested by a 90° peel test method (based on JIS-C6481-1996) to measure the peel strength of the positive electrodes. The results are shown in Table 1 below.

[0068] <Unipolar evaluation> The positive electrodes of Examples 1 to 4 and Comparative Examples 1 to 4 were punched using a 13 mm diameter punching machine to prepare positive electrodes for single-electrode testing. A lithium metal foil processed to a diameter of 16 mm was used as the counter electrode. A separator consisting of a 25 μm thick, 20 mm diameter porous polypropylene film was interposed between the positive electrode and counter electrode. The positive electrode and counter electrode with the separator interposed were housed in a circular positive electrode can, and a circular counter electrode can was fitted to the positive electrode terminal. A gasket was placed between them, and the can was crimped using a crimping machine to assemble a 2032 coin cell. Prior to crimping, an electrolyte solution prepared by dissolving lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (weight ratio 3:7) to a concentration of 1.3 mol / L was poured into the positive electrode can. This assembly process was carried out in an environment with a dew point of −50°C or lower.

[0069] The assembled coin cell was placed in a dedicated charge-discharge test device and subjected to the specified initial charge at a current of 0.1 CA, after which it was stored for a specified period of time. It was then discharged at a current of 0.2 CA until the cell voltage reached 2.0 V, and finally, it was activated.

[0070] Cycle testing A total of 30 cycles were performed, each cycle consisting of CC-CV charging at a constant current of 0.5 CA to 4.2 V, then at a constant voltage of 4.2 V to 0.05 CA, and then discharging at 2.0 CA to 2.0 V. The capacity retention rate at the 30th cycle was calculated using the formula (discharge capacity at the 30th cycle / discharge capacity at the 1st cycle) × 100. The results are shown in Table 1 below.

[0071] [Table 1]

[0072] As can be seen from Table 1, in Comparative Example 1, both the slurry of the first active material and the slurry of the second active material were prepared by liquid kneading, and in Comparative Example 4, an electrode was prepared using only the slurry of the first active material (liquid kneaded), and as a result, for both levels, the charge-discharge retention rate showed a good result of 99%, but the peel strength was 0. Furthermore, in Comparative Example 2, both the slurry of the first active material and the slurry of the second active material were prepared by stiff kneading, and in Comparative Example 3, an electrode was prepared using a second active material slurry (stiff kneaded) of 40%, which is outside the range of the present invention, and as a result, the peel strength showed a good value of 30 mN / mm or more, but the charge-discharge retention rate showed a low value of 85% or less.

[0073] In Examples 1 to 4, electrodes were prepared with a ratio of the slurry (solid kneaded) of the second active material in the range of 5 to 30%. As a result, it was found that the peel strength was 0 mN / mm when prepared using only the slurry or liquid kneaded first active material, whereas the peel strength was 5 mN / mm or more in Examples 1 to 4. Furthermore, the positive electrodes of Examples 1 to 4 showed good charge-discharge retention rates of 95% or more.

[0074] <Electrode machining test> A positive electrode punching die for assembling laminated lithium-ion secondary batteries was used to punch out the positive electrodes of Examples 1 to 4 and Comparative Examples 1 to 4 to produce multiple processed positive electrodes. Ten processed positive electrodes were selected, and the processed cross sections of the punched positive electrodes were observed to visually check for chipping or peeling of the positive electrode layer. Those that showed neither chipping nor peeling of the positive electrode layer were judged to be "good," and those that showed either chipping or peeling of the positive electrode layer were judged to be "fail." The results are shown in Table 2 below.

[0075] [Table 2]

[0076] As shown in Table 2, no chipping or peeling of the positive electrode layer during electrode processing was observed for the positive electrodes of Examples 1 to 4. Similarly, no chipping or peeling during processing was observed for the positive electrode of Comparative Example 2, in which both the slurry of the first active material and the slurry of the second active material were prepared by stiff kneading, and the positive electrode of Comparative Example 3, in which the slurry of the second active material was prepared by stiff kneading.

[0077] On the other hand, chipping and peeling were observed during processing for the positive electrode of Comparative Example 1, which was prepared by mixing the slurry of the first active material and the slurry of the second active material at a ratio of 70:30 and then kneading both of them. Similarly, chipping and peeling were observed during processing for the positive electrode of Comparative Example 4, which was prepared by kneading only the slurry of the first active material. The inventions described in the original claims of this application are set forth below. [1] A method for producing a positive electrode for a lithium ion battery, comprising: the positive electrode active material comprises a first active material having a spherical shape obtained by granulating nanoscale primary particles, and a second active material having a particle size of 1 / 10 or less of the particle size distribution D50 of the first active material as a form during preparation of the positive electrode; preparing a first slurry by mixing the first active material, the conductive material, the binder, and the solvent by liquid kneading; preparing a second slurry by kneading the second active material, the conductive material, the binder, and the solvent with a high-pressure mixer; mixing the first slurry and the second slurry to prepare a mixed slurry in which the weight ratio of the first active material to the second active material is 95:5 to 70:30; and applying the mixed slurry to one or both surfaces of a current collector; A method for producing a positive electrode for a lithium ion battery, comprising: [2] The liquid kneading step includes preparing a first slurry raw material containing the first active material, the conductive material, the binder, and the solvent, weighing the first active material, the conductive material, and the binder so that their total solid contents are a predetermined value relative to the solvent, adding the binder to the solvent and stirring and mixing, adding the conductive material to the obtained binder solution and stirring and mixing, and further adding the first active material and stirring and mixing to prepare the first slurry; The method for producing a positive electrode for a lithium ion battery according to [1], characterized in that the kneading step includes preparing a second slurry raw material containing the second active material, the conductive material, the binder, and the solvent, weighing the second active material, the conductive material, and the binder so that the total solid content thereof is higher than the total solid content of the liquid kneading, adding the binder to the solvent and stirring and mixing, adding the conductive material to the obtained binder solution and stirring and mixing, and further adding the second active material and stirring and mixing, thereby preparing the second slurry. [3] The method for producing a positive electrode for a lithium ion battery according to [2], characterized in that the total solid content of the first active material, the conductive material, and the binder relative to the solvent is 40% by weight or more and less than 60% by weight. [4] The method for producing a positive electrode for a lithium ion battery according to any one of [1] to [3], wherein the first and second active materials are lithium iron phosphate. [5] the first and second active materials are lithium-containing oxides different from each other, The method for producing a positive electrode for a lithium ion battery according to any one of [1] to [3], wherein the first active material is lithium iron phosphate, and the second active material is a lithium-containing oxide different from lithium iron phosphate. [6] The method for producing a positive electrode for a lithium ion battery according to any one of [1] to [5], characterized in that after the step of applying the mixed slurry to one or both surfaces of the current collector, one or both surfaces of the current collector are further dried and pressed. [7] The method for manufacturing a positive electrode for a lithium ion battery according to any one of [1] to [6], wherein the first active material and the second active material in the mixed slurry account for 40 to 60% by weight of the total solid content in the mixed slurry.

Claims

1. A method for producing a positive electrode for a lithium ion battery including a positive electrode active material, comprising: the positive electrode active material comprises a first active material having a spherical shape obtained by granulating nanoscale primary particles, and a second active material having a particle size in a form at the time of fabricating the positive electrode that is 1 / 10 or less of the particle size distribution D50 of the first active material, preparing a first slurry by kneading the first active material, the conductive material, the binder, and the solvent by liquid kneading; preparing a second slurry by kneading the second active material, the conductive material, the binder, and the solvent with a high-pressure mixer; mixing the first slurry and the second slurry to prepare a mixed slurry in which the weight ratio of the first active material to the second active material is 95:5 to 70:30; and applying the mixed slurry to one or both surfaces of a current collector; A method for producing a positive electrode for a lithium ion battery, comprising:

2. The liquid kneading step includes preparing a first slurry raw material containing the first active material, the conductive material, the binder, and the solvent, weighing the first active material, the conductive material, and the binder so that their total solid contents are a predetermined value relative to the solvent, adding the binder to the solvent and stirring and mixing, adding the conductive material to the obtained binder solution and stirring and mixing, and further adding the first active material and stirring and mixing to prepare the first slurry; 2. The method for producing a positive electrode for a lithium ion battery according to claim 1, wherein the kneading step includes preparing a second slurry raw material containing the second active material, the conductive material, the binder, and the solvent, weighing the second active material, the conductive material, and the binder so that the total solid content thereof is higher than the total solid content of the liquid kneading mixture, adding the binder to the solvent and stirring and mixing, adding the conductive material to the obtained binder solution and stirring and mixing, and further adding the second active material and stirring and mixing to prepare the second slurry.

3. 3. The method for producing a positive electrode for a lithium ion battery according to claim 2, wherein a total solid content of the first active material, the conductive material, and the binder relative to the solvent is 40% by weight or more and less than 60% by weight.

4. 4. The method for producing a positive electrode for a lithium ion battery according to claim 1, wherein the first and second active materials are lithium iron phosphate.

5. The first and second active materials are lithium-containing oxides different from each other, 4. The method for producing a positive electrode for a lithium ion battery according to claim 1, wherein the first active material is lithium iron phosphate, and the second active material is a lithium-containing oxide different from lithium iron phosphate.

6. The method for producing a positive electrode for a lithium ion battery according to any one of claims 1 to 5, characterized in that after the step of applying the mixed slurry to one or both surfaces of the current collector, one or both surfaces of the current collector are further dried and pressed.

7. 7. The method for producing a positive electrode for a lithium ion battery according to claim 1, wherein the first active material and the second active material in the mixed slurry account for 40 to 60% by weight of a total solid content in the mixed slurry.

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