Secondary battery positive electrode, method for manufacturing same, and nonaqueous electrolyte secondary battery

Abstract

This secondary battery positive electrode comprises a positive electrode mixture containing a positive electrode active material, a binder, and a conductive assistant. The positive electrode active material contains a lithium-containing composite oxide. The binder contains a fluorine-containing polymer. The degree of polyene conversion in the fluorine-containing polymer is 0.2 or less. The conductive assistant contains carbon black.

Description

Positive electrode for secondary battery, method of manufacturing the same, and non-aqueous electrolyte secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-042681, filed on March 18, 2024 in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a positive electrode for a secondary battery, a method for producing the same, and a non-aqueous electrolyte secondary battery.

[0003] Patent Document 1 proposes "a sodium ion secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having sodium ion conductivity, the positive electrode comprising positive electrode active material particles, a conductive additive, and a binder, the positive electrode active material particles comprising oxide particles that occlude and release sodium ions and a coating layer that coats the oxide particles, the oxide particles comprising oxide A containing Ni and Mn, the coating layer comprising at least one material B selected from the group consisting of ceramics and carbonaceous materials, and the binder comprising a fluororesin."

[0004] Patent Document 2 proposes "a positive electrode for a sodium ion secondary battery, comprising a positive electrode active material that absorbs and releases sodium ions, a conductive additive, a binder, and a carboxylic acid, wherein the binder comprises a vinylidene fluoride polymer, and the carboxylic acid has at least one of a boiling point and a thermal decomposition point, the lower of which exceeds 150°C."

[0005] Patent Document 3 describes a layered lithium nickel composite oxide in which the nickel ratio in metals other than lithium is 80 mol % or more, LiOH, Li ２ CO ３ and a positive electrode having a positive electrode mixture layer containing a chlorine-containing polyvinylidene fluoride polymer, ２ CO ３ The contents of LiOH and Li in the positive electrode mixture layer are 0.1% by weight or more and 2.1% by weight or less,２ CO ３ The document proposes a lithium ion secondary battery in which the total content of chlorine in the positive electrode mixture layer is 0.2% by weight or more and 4.2% by weight or less, and the chlorine content in the positive electrode mixture layer is 30 μg / g or more and 120 μg / g or less.

[0006] JP 2017-107713 A International Publication No. 2016 / 021405 International Publication No. 2019 / 088171

[0007] A positive electrode for a secondary battery has a positive electrode composite layer formed by applying a slurry containing a positive electrode composite (positive electrode slurry) to the surface of a positive electrode current collector and drying the coating. When the positive electrode composite contains a lithium-containing composite oxide, alkaline components leach from the lithium-containing composite oxide into the liquid medium when the positive electrode composite is mixed with a liquid medium to prepare the positive electrode slurry. The alkaline components react with a fluorine-containing polymer contained as a binder in the positive electrode composite, converting the fluorine-containing polymer into a polyenic form. The polyenic fluorine-containing polymer forms coarse aggregates by entanglement with carbon black contained as a conductive additive in the positive electrode composite due to van der Waals forces. As a result, the positive electrode slurry thickens, the effect of the conductive additive is reduced, and the positive electrode resistance increases. The thickening of the positive electrode slurry can lead to reduced productivity due to poor liquid delivery, increased material loss, and variations in positive electrode quality.

[0008] It is possible to partially remove the alkaline component by washing the lithium-containing composite oxide with water. However, washing with water causes deterioration of the lithium-containing composite oxide. From the viewpoint of avoiding deterioration as much as possible, it is difficult to sufficiently remove the alkaline component by washing with water.

[0009] It is conceivable to form a "coating layer that coats the oxide particles" as in Patent Document 1, but there is room for improvement because the coating layer increases the resistance of the positive electrode.

[0010] One aspect of the present disclosure relates to a positive electrode for a secondary battery, including a positive electrode composite including a positive electrode active material, a binder, and a conductive additive, wherein the positive electrode active material includes a lithium-containing composite oxide, the binder includes a fluorine-containing polymer, the fluorine-containing polymer has a degree of polyenization of 0.2 or less, and the conductive additive includes carbon black.

[0011] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including the above-described positive electrode for a secondary battery, a negative electrode, a separator, and a non-aqueous electrolyte.

[0012] Yet another aspect of the present disclosure relates to a method for producing a positive electrode for a secondary battery, the method including: preparing a positive electrode slurry including a liquid medium and a positive electrode composite dispersed in the liquid medium; preparing a positive electrode current collector and forming a coating film of the positive electrode slurry on a surface of the positive electrode current collector; and heating the coating film to dry it at 200°C or less, wherein the positive electrode composite includes a positive electrode active material, a binder, a conductive additive, and an organic additive containing a heteroatom, the positive electrode active material includes a lithium-containing composite oxide, the binder includes a fluorine-containing polymer, and the conductive additive includes carbon black.

[0013] According to the present disclosure, it is possible to suppress thickening of the positive electrode slurry and reduce the positive electrode resistance. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0014] 1 is a partially cutaway schematic perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure.

[0015] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or greater and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not greater than the upper limit. In the following description, when examples of components or methods are listed, only one of the listed examples may be used, or multiple of the listed examples may be used in combination, unless otherwise specified.

[0016] The present disclosure encompasses any combination of two or more claims arbitrarily selected from the appended claims, i.e., any combination of two or more claims arbitrarily selected from the appended claims may be combined unless a technical contradiction arises.

[0017] Non-aqueous electrolyte secondary batteries include lithium ion secondary batteries that use a liquid non-aqueous electrolyte, solid state batteries that include a gel electrolyte, and all-solid state batteries that use a solid electrolyte.

[0018] A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and a separator is usually disposed between the positive electrode and the negative electrode.

[0019] (Positive Electrode) The positive electrode according to the present disclosure includes a positive electrode mixture containing a positive electrode active material, a binder, and a conductive additive. The positive electrode mixture is usually supported on a positive electrode current collector to form a positive electrode mixture layer. The positive electrode mixture layer is formed on the surface of the positive electrode current collector.

[0020] The positive electrode current collector is made of a sheet-like conductive material. A non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used as the positive electrode current collector. The positive electrode composite layer is supported in the form of a film on one or both surfaces of the positive electrode current collector. The material of the positive electrode current collector is not particularly limited, but examples thereof include stainless steel, aluminum, aluminum alloys, and titanium.

[0021] The positive electrode active material includes a lithium-containing composite oxide. From the viewpoint of increasing capacity and reducing cost, the lithium-containing composite oxide preferably includes at least Ni. In the lithium-containing composite oxide, the proportion of Ni to the metal elements other than Li may be 80 atomic % or more. Among these, the lithium-containing composite oxide is preferably a lithium nickel oxide (hereinafter also referred to as "composite oxide N") that contains Li and Ni, the proportion of Ni to the metal elements other than Li is 80 atomic % or more, and has a layered rock salt crystal structure. The proportion of Ni to the metal elements other than Li may be 88 atomic % or more, 90 atomic % or more, or 95 atomic % or more. In the composite oxide N, the proportion of Ni to the metal elements other than Li is less than 100 atomic %, may be 99 atomic % or less, or may be 98 atomic % or less.

[0022] The positive electrode active material may contain other materials, but the proportion of the composite oxide N in the positive electrode active material is, for example, 70% by mass or more, 90% by mass or more, 95% by mass or more, or even 100%.

[0023] Lithium-containing composite oxides contain alkaline components because they are synthesized using lithium compounds as raw materials. Composite oxides N with a high Ni content contain more alkaline components. Composite oxides N in which the proportion of Ni among metal elements other than Li is 90 atomic % or more are particularly alkaline. If the alkalinity of the synthesized lithium-containing composite oxide (before washing with water) is higher than the allowable range, some of the alkaline components may be removed by washing with water to an extent that allows for acceptable deterioration. However, lithium-containing composite oxides (particularly composite oxides N) may contain alkaline components regardless of whether they are washed with water or not. The alkaline components include, for example, LiOH, Li, ２ CO ３ However, it is mostly strong alkali (mainly LiOH) that causes thickening of the positive electrode slurry.

[0024] When the lithium-containing composite oxide is mixed with a liquid medium (dispersion medium), the alkaline component dissolves into the liquid medium. For example, when the lithium-containing composite oxide contains 0.2 mass % or more of the alkaline component, the viscosity of the positive electrode slurry increases significantly during preparation of the positive electrode slurry.

[0025] The thickening occurs when the fluorine-containing polymer contained as a binder in the positive electrode mixture reacts with the alkaline component and undergoes polyenization. Polyenization is a phenomenon in which the fluorine-containing polymer generates double bonds. The polyenized fluorine-containing polymer promotes gelation of the positive electrode slurry over time. Specifically, the polyenized fluorine-containing polymer aggregates and coarsens due to the action of van der Waals forces while entangled with the carbon black contained as a conductive additive. This aggregation phenomenon further increases the thickening, inhibits the formation of conductive paths by the conductive additive, and increases the positive electrode resistance.

[0026] It is necessary to avoid a significant increase in viscosity and an increase in positive electrode resistance due to gelation of the positive electrode slurry. For this reason, conventional lithium-containing composite oxides are subjected to treatment such as washing with water so that the content of alkaline components becomes less than 0.2 mass%.

[0027] In contrast, the lithium-containing composite oxide contained in the positive electrode mixture according to the present disclosure may contain 0.2 mass% or more of an alkaline component. The thickening of the positive electrode slurry is suppressed by an organic additive containing a heteroatom contained in the positive electrode mixture in the positive electrode slurry. The organic additive containing a heteroatom (hereinafter also referred to simply as "organic additive") adheres to the surface of the lithium-containing composite oxide via the heteroatom. As a result, contact between the surface of the lithium-containing composite oxide and the liquid medium is suppressed, elution of the alkaline component is suppressed, and reaction of the fluorine-containing polymer with the alkaline component is suppressed. Therefore, the polyenation degree of the fluorine-containing polymer contained in the positive electrode mixture layer (positive electrode mixture) peeled from the positive electrode current collector is small, i.e., 0.2 or less. The polyenation degree of the fluorine-containing polymer is preferably 0.17 or less, more preferably 0.15 or less.

[0028] The content of the alkaline component contained in the lithium-containing composite oxide may be 0.2% by mass or more, but is preferably 0.5% by mass or less, and may be, for example, 0.25% by mass or more and 0.4% by mass or less, or 0.25% by mass or more and 0.35% by mass or less. When the lithium-containing composite oxide contains more than 0.5% by mass of the alkaline component, it may be difficult to suppress the thickening of the positive electrode slurry even if an organic additive is used.

[0029] The degree of polyenation of the fluorine-containing polymer can be determined by the following method. A 20 g sample of the positive electrode composite layer (positive electrode composite) peeled from the positive electrode current collector is weighed and mixed with 10 mL of N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") to obtain a mixed solution. The mixed solution is centrifuged at 8,000 rpm for 0.5 hours to remove the positive electrode active material. Next, the conductive additive is removed by ultracentrifugation at 80,000 rpm for 11 hours and then at 80,000 rpm for 4 hours to obtain an NMP solution of the fluorine-containing polymer. A fluorine-containing polymer film is obtained from the NMP solution of the fluorine-containing polymer by a casting method. The film is dried under reduced pressure at 60°C for 2 hours. The obtained film is measured using a Raman spectrometer. After baseline correction, the peak intensity at 1,130 cm －１ The peak P1 due to the C═C bond observed near 2980 cm －１ CH observed nearby ２ The intensity ratio (P1 / P2) of the peak P1 derived from the polyenation product to the peak P2 derived from the polyenation product is calculated as the degree of polyenation.

[0030] The content of the alkaline component contained in the lithium-containing composite oxide can be determined by the following method. A sample equivalent to 1.0 g of the positive electrode composite layer (positive electrode composite) peeled from the positive electrode current collector is weighed, mixed with 30 mL of ion-exchanged water, and dissolved by shaking for 30 seconds to obtain a sample solution in which the alkaline component has been eluted. Thereafter, the sample solution is filtered and then subjected to a nitrogen atmosphere. ２ The sample is titrated with 1 mol / L hydrochloric acid under atmospheric pressure until the pH reaches 8.4. The mass of LiOH, which is an alkaline component, is calculated from the amount of hydrochloric acid required for the titration, and the ratio of the mass of LiOH to the total mass of the lithium-containing composite oxide and the alkaline component contained in the sample is calculated. Note that the pH range below 8.4 is determined by the ratio of the mass of LiOH to the total mass of the lithium-containing composite oxide and the alkaline component. ２ CO ３ This is the titration range for weak alkalis such as

[0031] Because the organic additive has a flexible organic chain, it is unlikely to inhibit the battery reaction even when attached to the surface of the lithium-containing composite oxide. Therefore, the organic additive has little effect on the resistance of the positive electrode. Furthermore, because the organic additive has low heat resistance, it can be removed from the positive electrode composite by thermal decomposition or the like during the manufacturing process of the positive electrode. The positive electrode composite layer (positive electrode composite) peeled off from the positive electrode current collector may or may not contain an organic additive containing a heteroatom. From the viewpoint of significantly suppressing an increase in the resistance of the positive electrode, the smaller the content of the organic additive contained in the positive electrode composite, the more preferable.

[0032] When the positive electrode composite layer (positive electrode composite) peeled off from the positive electrode current collector contains an organic additive, the mass ratio of the organic additive to the mass of the lithium-containing composite oxide is preferably 0.1 mass% or less, more preferably 0.07 mass% or less, still more preferably 0.05 mass% or less or 0.03 mass% or less, and may be 0.01 mass% or less. The mass ratio of the organic additive to the mass of the lithium-containing composite oxide may be, for example, 0.005 mass% or more.

[0033] The ratio of the mass of the organic additive to the mass of the lithium-containing composite oxide can be calculated from the measurement results of thermogravimetric analysis (TGA) of a sample of the positive electrode composite layer (positive electrode composite) peeled from the positive electrode current collector and the mass of the lithium-containing composite oxide contained in the sample. TGA may be performed in air at a rate of 5°C / min.

[0034] The decomposition temperature (thermal decomposition temperature) of the organic additive is preferably 200°C or lower, more preferably 180°C or lower. The decomposition temperature can be measured by TGA under atmospheric conditions at a rate of 5°C / min. Organic additives having a decomposition temperature of 200°C or lower are easily removed from the positive electrode mixture by thermal decomposition or the like during the manufacturing process of the positive electrode. This makes it easy to reduce the ratio of the mass of the organic additive to the mass of the lithium-containing composite oxide contained in the positive electrode mixture layer in the completed positive electrode.

[0035] The organic additive may be a Lewis base. Among Lewis bases, Lewis bases having nitrogen (N) as a heteroatom are particularly preferred. It is believed that the unshared electron pair of nitrogen adsorbs to the Lewis acid site on the surface of the lithium-containing composite oxide. Lewis bases having multiple nitrogen atoms with unshared electron pairs in the molecule are particularly preferred in terms of increasing the number of reaction sites. Among Lewis bases having nitrogen as a heteroatom, Lewis bases having a nitrogen-containing heterocycle are particularly preferred. The nitrogen-containing heterocycle is preferably, for example, a 5-membered ring, a 6-membered ring, or a 7-membered ring, and preferably has two or more nitrogen atoms. The heteroatom is not limited to N, but may be S, P, O, or the like. The organic additive may contain N and a heteroatom other than N as the heteroatom.

[0036] The nitrogen-containing heterocycle may be a saturated ring without a double bond, but preferably has aromaticity. The aromatic nitrogen-containing heterocycle may contain one or more nitrogen atoms in the ring, but may also contain two or more nitrogen atoms. Preferred examples of the nitrogen-containing heterocycle include a pyrazole ring, an imidazole ring, a diazine ring, a triazine ring, and a diazepine ring.

[0037] The organic additive preferably has a chain organic group to effectively prevent contact between the liquid medium and the surface of the lithium-containing composite oxide. For example, the organic additive may have a chain organic group bonded to a ring-constituting atom of the nitrogen-containing heterocycle. The number of carbon atoms in such an organic group may be, for example, 1 to 50, or 5 to 20.

[0038] The molecular weight of the organic additive may be, for example, from 50 to 1,000, from 100 to 800, or from 100 to 500. An organic additive having such a molecular weight has a low thermal decomposition temperature and a size sufficient to effectively suppress contact between the liquid medium and the surface of the lithium-containing composite oxide.

[0039] The lithium-containing composite oxide may contain Ni, Co, and Mn. Co and Mn contribute to stabilizing the crystal structure of the lithium-containing composite oxide. Since the lithium-containing composite oxide containing Co and Mn is highly alkaline and is significantly deteriorated by washing with water, the role of the organic additive is particularly important.

[0040] From the viewpoint of reducing costs and increasing capacity, the proportion of Co in the metal elements other than Li contained in the lithium-containing composite oxide is preferably 0 atomic % or more and 20 atomic % or less, and more preferably more than 0 atomic % and 5 atomic % or less.

[0041] From the viewpoint of cost reduction, the ratio of Mn to the metal elements other than Li contained in the lithium-containing composite oxide may be 1 atomic % or more and 10 atomic % or less, 2 atomic % or more and 5 atomic % or less, or 3 atomic % or more and 5 atomic % or less.

[0042] The lithium-containing composite oxide may further contain Al. Al contributes to stabilizing the crystal structure of the lithium-containing composite oxide. Since the lithium-containing composite oxide containing Co, Mn, and Al has a very high alkalinity, the role of the organic additive is particularly important.

[0043] The proportion of Al in the metal elements other than Li contained in the lithium-containing composite oxide may be 0.1 atomic % or more and 5 atomic % or less, 0.2 atomic % or more and 3 atomic % or less, or 0.5 atomic % or more and 1 atomic % or less.

[0044] The lithium-containing composite oxide is, for example, a compound represented by the formula: Li ｙ Ni ｘ M （１－ｘ） O ２－δ (0.8≦x≦1, 0<y≦1.2, and −0.05≦δ≦0.05). Here, the element M may include at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, W, and B. x, which indicates the atomic ratio of Ni, may be 0.98 or less, or 0.95 or less. x increases or decreases due to charging and discharging of the secondary battery.

[0045] The element M is represented by the formula: Co １－ｘ－ａ－ｂ Mn ａ Al ｂ(0<a<0.05 and 0<b<0.05).

[0046] The thickness of the positive electrode composite layer is not particularly limited, but may be, for example, 50 μm to 150 μm, or 75 μm to 125 μm. A single positive electrode active material layer may be formed by a plurality of layers having different morphologies. For example, two or more layers containing positive electrode active materials with different average particle sizes may be stacked, or two or more layers containing positive electrode active materials of different types or compositions may be stacked.

[0047] The average particle size (D50) of the lithium-containing composite oxide particles is, for example, 1 μm or more and 50 μm or less, and may be 5 μm or more and 25 μm or less. The average particle size (D50) refers to the median diameter at which the cumulative volume is 50% in a volume-based particle size distribution. The volume-based particle size distribution can be measured using a commercially available laser diffraction / scattering particle size distribution measuring device.

[0048] The average particle size of the lithium-containing composite oxide may be measured from a cross section of the positive electrode mixture layer by taking an SEM image of the cross section so that 10 or more particles of the lithium-containing composite oxide are observed, and the diameters of the circles having the same area as the cross sections of the 10 or more particles of the lithium-containing composite oxide are determined by image processing, and the average value thereof may be used as the average particle size.

[0049] Fluorine-containing polymers that function as binders include vinylidene fluoride polymers. Examples of vinylidene fluoride polymers include polymers of monomers containing vinylidene fluoride. The fluorine-containing polymer may be a combination of a vinylidene fluoride polymer and another fluorine-containing polymer. The vinylidene fluoride polymer may be a copolymer of vinylidene fluoride and another monomer. Examples of vinylidene fluoride polymers include polyvinylidene fluoride (PVDF) and vinylidene fluoride-hexafluoropropylene copolymer. The vinylidene fluoride polymer preferably contains 90 mol % or more of monomer units derived from vinylidene fluoride. The degree of polyenization of the fluorine-containing polymer before being mixed with the positive electrode slurry is usually 0.1 or less.

[0050] In the positive electrode composite, the amount of the fluorine-containing polymer relative to 100 parts by mass of the positive electrode active material may be 0.1 parts by mass or more, or 0.5 parts by mass or more, and may be 2.0 parts by mass or less, or 1.2 parts by mass or less (particularly 0.98 parts by mass or less).

[0051] The progress of the polyenation reaction of the fluorine-containing polymer and the degree of formation of coarse aggregates containing carbon black can be affected by the amount of fluorine-containing polymer present. From the viewpoint of reducing the frequency of contact between the fluorine-containing polymer and the alkaline component and effectively suppressing the polyenation reaction and the formation of aggregates, the amount of fluorine-containing polymer per 100 parts by mass of the positive electrode active material in the positive electrode mixture is preferably 0.5 parts by mass or more but less than 1 part by mass (e.g., 0.98 parts by mass or less), and more preferably 0.5 parts by mass or more but 0.8 parts by mass or less. In an environment where an organic additive suppresses the polyenation reaction of the fluorine-containing polymer, the amount of fluorine-containing polymer per 100 parts by mass of the positive electrode active material is sufficient, for example, even if it is 0.98 parts by mass or less. Furthermore, if the amount of fluorine-containing polymer per 100 parts by mass of the positive electrode active material is 0.5 parts by mass or more, it can fully function as a binder.

[0052] The weight-average molecular weight of the fluorine-containing polymer may be 800,000 or more, 1,000,000 or more, or 1,200,000 or more, or may be 2,000,000 or less, or 1,800,000 or less. By making the weight-average molecular weight 1,000,000 or more, a high effect as a binder can be obtained with a small amount.

[0053] The conductive additive contained in the positive electrode composite layer includes at least carbon black. The particle form of the carbon black is such that it is easily distributed near the surface of the lithium-containing composite oxide. The heteroatoms of the organic additive interact not only with the surface of the lithium-containing composite oxide but also with functional groups such as OH groups and COOH groups on the surface of the carbon black. Therefore, the organic additive promotes more uniform distribution of the carbon black on the surface of the lithium-containing composite oxide, thereby enhancing the effect of reducing the positive electrode resistance. In addition, the carbon black promotes adhesion of the organic additive to the surface of the lithium-containing composite oxide. By protecting the surface of the lithium-containing composite oxide with carbon black, Li elution due to a proton exchange reaction between the lithium-containing composite oxide and trace amounts of water contained in the positive electrode slurry is also suppressed, and viscosity increase due to Li elution is also suppressed.

[0054] The type of carbon black is not particularly limited, and known carbon blacks may be used. Examples of carbon black include acetylene black, ketjen black, thermal black, furnace black, and channel black. For example, it is preferable to use acetylene black.

[0055] The degree of formation of coarse aggregates containing carbon black can be affected by the amount of carbon black present. Meanwhile, carbon black plays an important role in reducing positive electrode resistance. From the viewpoint of effectively suppressing aggregate formation while reducing positive electrode resistance, the amount of carbon black per 100 parts by mass of positive electrode active material in the positive electrode composite is preferably, for example, 0.6 parts by mass or more but less than 1 part by mass (e.g., 0.95 parts by mass or less), and more preferably 0.6 parts by mass or more but 0.8 parts by mass or less. In an environment where an organic additive suppresses the polyenation reaction of the fluorine-containing polymer, carbon black more effectively contributes to the formation of conductive paths. Therefore, for example, an amount of carbon black per 100 parts by mass of positive electrode active material of 0.95 parts by mass or less is sufficient. Furthermore, if the amount of carbon black per 100 parts by mass of positive electrode active material is 0.6 parts by mass or more, it is possible to sufficiently reduce positive electrode resistance in an environment where an organic additive suppresses aggregate formation.

[0056] Carbon black may be used in combination with other conductive additives, such as carbon nanotubes (CNTs).

[0057] The ratio of the mass of CNTs contained in the positive electrode mixture layer to the mass of the positive electrode active material is, for example, 0.005% to 3%, or alternatively, 0.01% to 3%, or alternatively, 0.03% to 0.5%.

[0058] Carbon nanotubes (CNTs) are nanometer-sized carbon materials with a cylindrical structure in which a sheet of graphene (a six-membered ring network formed by carbon atoms) is rolled up, and they have excellent electrical conductivity. CNTs with a single graphene layer constituting the cylindrical structure are called single-walled carbon nanotubes (SWCNTs). CNTs with multiple graphene layers constituting the cylindrical structure are called multi-walled carbon nanotubes (MWCNTs).

[0059] The CNTs may be SWCNTs, MWCNTs, or a combination of both. Among them, SWCNTs are thin and flexible, and have a strong interaction with aromatic sulfonates (SAs) used as dispersants.

[0060] The average diameter of the CNTs is, for example, preferably 8 nm or less, more preferably 3 nm or less, more preferably 2 nm or less, and may be less than 1 nm. The average diameter of the CNTs is preferably 0.8 nm or more. The average length of the CNTs is preferably 1 μm or more and 100 μm or less, more preferably 5 μm or more and 20 μm or less.

[0061] The positive electrode slurry may contain other components as needed. For example, the positive electrode slurry may contain a binder other than the fluorine-containing polymer. Examples of such binders include hydrogenated nitrile butadiene rubber.

[0062] (Method for Manufacturing Positive Electrode) An example of a method for manufacturing a positive electrode for a secondary battery according to the present disclosure (hereinafter also referred to as "manufacturing method (M)") will be described. The manufacturing method (M) includes a first step of preparing a positive electrode slurry and a second step of forming a positive electrode mixture layer on a positive electrode current collector using the positive electrode slurry. The second step includes a step of preparing a positive electrode current collector, forming a coating film of the positive electrode slurry on the surface of the positive electrode current collector, and a step of heating and drying the coating film.

[0063] The positive electrode slurry prepared in the first step of manufacturing method (M) includes a liquid medium and a positive electrode composite dispersed in the liquid medium. The positive electrode composite is the same as the positive electrode composite described above. For example, the positive electrode slurry can be prepared by mixing a positive electrode active material, a fluorine-containing polymer, carbon black, an organic additive, and a liquid medium. The positive electrode active material includes a lithium-containing composite oxide (preferably composite oxide N). The positive electrode slurry can be prepared by mixing the positive electrode composite with the liquid medium. The organic additive may be dissolved in the liquid medium in advance as a solution and then mixed with other components. The mixing method is not particularly limited, and known mixing methods may be used.

[0064] The liquid medium may be an organic solvent or water. As the organic solvent, N-methyl-2-pyrrolidone (NMP) is preferred, but alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, ketones such as cyclohexanone, etc. may also be used.

[0065] The organic additive is used to suppress thickening of the positive electrode slurry. Therefore, it is sufficient that the organic additive is contained in the positive electrode slurry, and it is not necessary that the organic additive is contained in the positive electrode composite layer. In order to effectively suppress thickening of the positive electrode slurry, the mass ratio of the organic additive to the mass of the lithium-containing composite oxide in the positive electrode slurry is, for example, preferably 0.002 mass% or more and 0.3 mass% or less, more preferably 0.02 mass% or more and 0.3 mass% or less, and may be 0.1 mass% or more and 0.25 mass% or less.

[0066] In the second step, the step of forming a coating film of the positive electrode slurry on the surface of the positive electrode current collector can be performed using a coating device such as a bar coater, a gravure coater, a blade coater, a roll coater, a comma coater, a die coater, a lip coater, etc. Multiple positive electrode slurries with different compositions may be prepared, and two or more layers of these slurries may be coated one on top of the other.

[0067] In the second step, the coating film is preferably dried at 200° C. or lower, and may be dried at a temperature of, for example, 150° C. or higher, or even 180° C. or higher. The coating film drying step vaporizes the liquid medium to form an unrolled coating film. At least a portion of the organic additives can be decomposed or vaporized during the coating film drying step and removed from the unrolled coating film.

[0068] Typically, the unrolled coating film is then rolled to form a positive electrode composite layer. The conditions for the rolling are not particularly limited. The density of the positive electrode active material in the positive electrode composite layer is, for example, 3.3 g / cm ３ Above, 4.0g / cm ３ or less, and 3.5 g / cm ３ 4.0g / cm or more ３ The following is also acceptable.

[0069] The positive electrode mixture layer may be formed on only one surface of the positive electrode current collector, or on both surfaces of the positive electrode current collector, depending on the structure of the battery.

[0070] (Non-aqueous electrolyte secondary battery) A battery (hereinafter also referred to as "secondary battery (B)") that is an example of a non-aqueous electrolyte secondary battery according to the present disclosure will be described. The secondary battery (B) includes the above-described positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte.

[0071] The positive electrode mixture layer of the positive electrode of the secondary battery (B) may contain an organic additive attached to the surface of the lithium-containing composite oxide, but the amount is small. The ratio of the mass of the organic additive to the mass of the lithium-containing composite oxide is, for example, 0.1 mass% or less. Therefore, the organic additive remaining in the positive electrode mixture layer hardly increases the resistance of the positive electrode.

[0072] The components other than the positive electrode are not particularly limited, and components used in known non-aqueous electrolyte secondary batteries may be applied. Examples of the components of the secondary battery (B) are described below.

[0073] (Positive Electrode) The positive electrode described above is used as the positive electrode.

[0074] (Negative electrode) The negative electrode may have at least a negative electrode current collector. In the case of a lithium metal secondary battery, a negative electrode current collector on which lithium metal or a lithium alloy can be deposited is used as the negative electrode. In the case of a lithium ion secondary battery, the negative electrode typically includes a negative electrode composite containing a negative electrode active material. The negative electrode composite is supported on the negative electrode current collector to form a negative electrode composite layer. The negative electrode composite layer is formed on the surface of the negative electrode current collector.

[0075] The negative electrode mixture contains a negative electrode active material as an essential component. The negative electrode mixture may contain optional components such as a binder, a thickener, and a conductive additive. These optional components may include the components exemplified as the components of the positive electrode.

[0076] The negative electrode composite layer may be formed by applying a negative electrode slurry, in which the components of the negative electrode composite are dispersed in a liquid medium (dispersion medium), to the surface of the negative electrode current collector and drying the applied film. The dried coating may be rolled as necessary. The liquid medium may be any of the liquid media exemplified for the positive electrode slurry.

[0077] (Negative electrode active material) The negative electrode active material is selected depending on the type of secondary battery (B). An example of the negative electrode active material is a material capable of absorbing and releasing lithium ions. Examples of such materials include carbonaceous materials, Si-containing materials, and the like. The negative electrode active material may contain or be a Si-containing material. Metallic lithium, a lithium alloy, or the like may be used as the negative electrode active material. The negative electrode may contain one type of negative electrode active material or a combination of two or more types.

[0078] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination. Graphite is preferred because it has excellent charge / discharge stability and a small irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0079] Examples of Si-containing materials include simple Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed in a lithium ion conductive phase (matrix). Examples of silicon oxides include SiO ｘ The lithium ion conductive phase may include SiO particles. For example, x is 0.5≦x<2, and may be 0.8≦x≦1.6. ２ At least one selected from the group consisting of a silicate phase, a silicate phase, and a carbon phase may be used.

[0080] The negative electrode current collector may be a metal foil. The negative electrode current collector may be porous. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0081] (Non-aqueous electrolyte) The non-aqueous electrolyte includes a solvent (non-aqueous solvent) and a solute dissolved in the solvent. Examples of the solute include a lithium salt. Various additives may be added to the non-aqueous electrolyte.

[0082] Known materials can be used as the solvent. Examples of solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0083] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ etc.), lithium salts of fluorine-containing acids (LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ etc.), lithium salt of fluorine-containing acid imide (LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２and lithium halides (LiCl, LiBr, LiI, etc.). The lithium salts may be used alone or in combination of two or more.

[0084] The concentration of the lithium salt in the nonaqueous electrolyte may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By setting the lithium salt concentration within this range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained.

[0085] The non-aqueous electrolyte may contain known additives, examples of which include 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0086] (Separator) The separator is disposed between the positive electrode and the negative electrode. The separator preferably has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. Examples of separator materials include polyolefins (polypropylene, polyethylene, etc.) and other resins.

[0087] (Exterior Body) The exterior body (battery case) houses the electrode group and the non-aqueous electrolyte. The exterior body is not particularly limited, and a known exterior body may be used. The electrode group is composed of a positive electrode, a negative electrode, and a separator. The configuration of the electrode group is not particularly limited, and may be a wound type or a laminated type. A wound type electrode group is formed by winding a positive electrode and a negative electrode with a separator interposed therebetween. A laminated type electrode group is formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The shape of the non-aqueous electrolyte secondary battery is not particularly limited, and may be cylindrical, prismatic, coin-shaped, button-shaped, laminate-shaped, etc.

[0088] Fig. 1 is a schematic perspective view with a portion cut away of a secondary battery 10 according to an embodiment of the present disclosure. Fig. 1 shows a prismatic non-aqueous electrolyte battery as an example. The secondary battery 10 shown in Fig. 1 includes a battery case 4 in the shape of a rectangular cylinder with a bottom, and an electrode group 1 and a non-aqueous electrolyte (not shown) housed in the battery case 4.

[0089] The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator disposed therebetween. The positive electrode is the same as the positive electrode described above. The negative electrode current collector of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 is fitted into the open end of the battery case 4, and the fitting is laser welded. The sealing plate 5 has an injection hole for a non-aqueous electrolyte. The injection hole is closed with a sealing plug 8 after the non-aqueous electrolyte is injected.

[0090] (Additional Notes) The above description discloses the following technologies. (Technology 1) A positive electrode for a secondary battery, comprising a positive electrode composite including a positive electrode active material, a binder, and a conductive additive, wherein the positive electrode active material includes a lithium-containing composite oxide, the binder includes a fluorine-containing polymer, the fluorine-containing polymer has a degree of polyenization of 0.2 or less, and the conductive additive includes carbon black. (Technology 2) A positive electrode for a secondary battery according to Technology 1, wherein in the positive electrode composite, the amount of the carbon black per 100 parts by mass of the positive electrode active material is 0.6 parts by mass or more and 0.95 parts by mass or less. (Technology 3) A positive electrode for a secondary battery according to Technology 1 or 2, wherein in the positive electrode composite, the amount of the fluorine-containing polymer per 100 parts by mass of the positive electrode active material is 0.5 parts by mass or more and 0.98 parts by mass or less. (Technology 4) The lithium-containing composite oxide contains at least Ni, and in the lithium-containing composite oxide, a proportion of Ni in metal elements other than Li is 80 atomic % or more. (Technology 5) The lithium-containing composite oxide contains 0.2 mass % or more and 0.5 mass % or less of an alkaline component. (Technology 6) The lithium-containing composite oxide is a positive electrode for a secondary battery according to any one of Technologies 1 to 3, wherein the lithium-containing composite oxide has a formula: Li ｙ Ni ｘ M （１－ｘ） O ２－δ (0.8≦x≦1, 0<y≦1.2 and −0.05≦δ≦0.05), and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, W, and B. (Technology 7) A positive electrode for a secondary battery according to any one of Techniques 1 to 5, wherein M is represented by the formula: Co １－ｘ－ａ－ｂ Mn ａ Al ｂ(Technology 8) The positive electrode for a secondary battery according to any one of Techniques 1 to 7, wherein the positive electrode mixture further contains an organic additive containing a heteroatom, and the ratio of the mass of the organic additive to the mass of the lithium-containing composite oxide is 0.1 mass% or less. (Technology 9) The positive electrode for a secondary battery according to Technique 8, wherein the decomposition temperature of the organic additive is 200°C or less. (Technology 10) The positive electrode for a secondary battery according to Technique 8 or 9, wherein the organic additive is a Lewis base. (Technology 11) The positive electrode for a secondary battery according to any one of Techniques 8 to 10, wherein the molecular weight of the organic additive is 50 or more and 1000 or less. (Technology 12) A non-aqueous electrolyte secondary battery comprising the positive electrode for a secondary battery according to any one of Techniques 1 to 11, a negative electrode, a separator, and a non-aqueous electrolyte. (Technology 13) A method for manufacturing a positive electrode for a secondary battery, comprising the steps of: preparing a positive electrode slurry containing a liquid medium and a positive electrode composite dispersed in the liquid medium; preparing a positive electrode current collector and forming a coating film of the positive electrode slurry on the surface of the positive electrode current collector; and heating and drying the coating film at 200°C or less, wherein the positive electrode composite contains a positive electrode active material, a binder, a conductive additive, and an organic additive containing a heteroatom, wherein the positive electrode active material contains a lithium-containing composite oxide, the binder contains a fluorine-containing polymer, and the conductive additive contains carbon black. (Technology 14) The method for manufacturing a positive electrode for a secondary battery according to Technology 13, wherein the decomposition temperature of the organic additive is 200°C or less. (Technology 15) The method for manufacturing a positive electrode for a secondary battery according to Technology 13 or 14, wherein the organic additive is a Lewis base. (Technology 16) The method for producing a positive electrode for a secondary battery according to any one of Techniques 13 to 15, wherein the molecular weight of the organic additive is 50 or more and 1000 or less. (Technology 17) The method for producing a positive electrode for a secondary battery according to any one of Techniques 13 to 16, wherein the ratio of the mass of the organic additive to the mass of the lithium-containing composite oxide is 0.002 mass% or more and 0.3 mass% or less.

[0091] The present disclosure will be specifically described below based on examples, but the present disclosure is not limited to the following examples.

[0092] Example 1 (Preparation of Positive Electrode Slurry SA1) Positive electrode active material, polyvinylidene fluoride (PVDF, fluorine-containing polymer), pyrazole derivative having a chain organic group with 10 carbon atoms (PZ1, Lewis base), acetylene black (AB, conductive additive), and NMP (liquid medium) were mixed in a predetermined mass ratio to prepare positive electrode slurry SA1. The positive electrode active material, PVDF, PZ1, and AB are components of the positive electrode mixture. The positive electrode active material was a fluorine-containing polymer having the composition formula LiNi ０．９ Co ０．０４ Mn ０．０５ Al ０．０１ O ２ In the positive electrode composite, the positive electrode active material, PVDF, PZ1, and AB were mixed in a mass ratio of NCA:PVDF:PZ:AB=100:0.6:0.01:0.75.

[0093] The weight-average molecular weight of PVDF was 1.4 million. The molecular weight of the pyrazole derivative (PZ1) was about 200, and the thermal decomposition temperature of PZ1 was about 180°C.

[0094] [Evaluation 1] <Measurement of Viscosity Increase Rate> For the prepared positive electrode slurry SA1, the viscosity μ0 on the day of preparation and the viscosity μ1 7 days after preparation were measured. The viscosity increase rate was then calculated using the following formula. The viscosity increase rate of each positive electrode slurry is shown in Table 1. The lower the viscosity increase rate, the better the positive electrode slurry.

[0095] "Viscosity increase rate (%) = 100 × μ1 / μ0"

[0096] (1) Preparation of Positive Electrode The positive electrode slurry SA1 was applied to the surface of an aluminum foil (positive electrode current collector) to form a coating film, and a laminate of the aluminum foil and the coating film was obtained. Next, the coating film was dried at 180 ° C., and then the laminate was rolled. In this way, a positive electrode A1 including an aluminum foil and a positive electrode composite layer formed on both sides of the aluminum foil was prepared.

[0097] [Evaluation 2] <Measurement of Polyenation Degree> For the positive electrode A1, the positive electrode composite layer (positive electrode composite) was peeled off from the positive electrode current collector, and a 20 g sample of the positive electrode composite was weighed and mixed with 10 mL of NMP. From the mixed solution, an NMP solution of a fluorine-containing polymer (PVDF) was obtained by the method described above. A fluorine-containing polymer film was formed from the NMP solution of PVDF by the method described above, and a Raman spectrometer was used to measure the polyenation degree. －１ The peak P1 due to the C═C bond observed near 2980 cm －１ CH observed nearby ２ The intensity ratio (P1 / P2) of peak P1 derived from the positive electrode slurry SA1 to peak P2 derived from the positive electrode slurry SA2 was calculated as the polyenation degree. The polyenation degree of PVDF contained in positive electrode A1 was 0.13, as shown in Table 1. The polyenation degree of PVDF before being mixed with positive electrode slurry SA1 was less than 0.01.

[0098] <Measurement of Alkaline Component Content of Complex Oxide N> For the positive electrode A1, the positive electrode composite layer (positive electrode composite) was peeled from the positive electrode current collector, and a 1.0 g sample of the positive electrode composite was weighed and mixed with 30 mL of ion-exchanged water. The alkaline component was eluted as described above to prepare a sample solution, and the mass of the alkaline component was calculated by titration. The ratio of the mass of the alkaline component to the total mass of the complex oxide N and the alkaline component contained in the sample (the alkaline component content) was calculated to be 0.31 mass%.

[0099] (2) Preparation of Negative Electrode A negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), and water were mixed in a predetermined mass ratio to prepare a negative electrode slurry. Graphite was used as the negative electrode active material. Next, the negative electrode slurry was applied to the surface of copper foil (negative electrode current collector) to form a laminate including the copper foil and a coating film formed on the copper foil. Next, the coating film was dried, and the laminate was rolled. In this way, a negative electrode including copper foil and a negative electrode mixture layer formed on both sides of the copper foil was formed.

[0100] (3) Preparation of electrolyte (non-aqueous electrolyte) LiPF ６ The electrolyte solution was prepared by adding LiPF (lithium salt). ６The concentration of was 1.0 mol / L. As the non-aqueous solvent, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC=3:7 was used.

[0101] (4) Fabrication of Secondary Battery Leads were attached to the positive electrode A1 and the negative electrode. Next, the positive electrode, the negative electrode, and the separator were spirally wound together so that the separator was disposed between the positive electrode and the negative electrode, thereby fabricating an electrode assembly. Next, this electrode assembly and a non-aqueous electrolyte were housed in a cylindrical battery case. In this manner, a cylindrical battery A1 of the example equipped with the positive electrode A1 was fabricated.

[0102] Examples 2 to 5, Comparative Examples 1 to 7 (Preparation of Positive Electrode Slurries SA2, SA3, SC1) Positive electrode slurries SA2, SA3, and SC1 were prepared in the same manner and under the same conditions as for preparing positive electrode slurry SA1, except that the mass ratio of the organic additive to the composite oxide N was changed as shown in Table 1. Positive electrode slurry SC1 did not contain any organic additive.

[0103] (Preparation of Positive Electrode Slurries SC2 to SC5) Positive electrode slurries SC2 to SC5 were prepared in the same manner and under the same conditions as those for preparing the positive electrode slurry SA1, except that CNT was used instead of AB as the conductive additive and the mass ratio of the organic additive to the composite oxide N was changed as shown in Table 1.

[0104] The CNTs had an average diameter of 7 nm and an average length of 10 μm. The mass ratio of the positive electrode active material to the CNTs was positive electrode active material:CNT=100:0.4.

[0105] (Preparation of Positive Electrode Slurries SA4, SA5, SC6, and SC7) Positive electrode slurries SA4, SA5, SC6, and SC7 were prepared in the same manner and under the same conditions as those for preparing positive electrode slurry SA1, except that the mass ratios of the organic additive, AB, and PVDF to the composite oxide N were changed as shown in Table 1.

[0106] <Measurement of Viscosity Increase Rate> The viscosity increase rate of each positive electrode slurry was measured in the same manner as for slurry SA1. The relative values, with the viscosity increase rate of slurry SA1 set to 100, are shown in Table 1. The lower the relative value, the better the positive electrode slurry.

[0107] Using each positive electrode slurry, positive electrodes A2 to A5 of Examples 2 to 5 and positive electrodes C1 to C7 of Comparative Examples 1 to 7 were fabricated in the same manner as positive electrode A1.

[0108] Using each positive electrode, batteries A2 to A5 of Examples 2 to 5 and batteries C1 to C7 of Comparative Examples 1 to 7 were fabricated in the same manner as battery A1.

[0109] [Evaluation 3] The internal resistance of each battery was measured. Table 1 shows the relative values ​​of batteries A1 to A3 when the internal resistance of battery C1 was set to 100. Table 1 shows the relative values ​​of batteries C3 to C5 when the internal resistance of battery C2 was set to 100. Table 1 shows the relative value of battery A4 when the internal resistance of battery C6 was set to 100. Table 1 shows the relative value of battery A5 when the internal resistance of battery C7 was set to 100. The smaller the relative value of the internal resistance, the better.

[0110] <Measurement of internal resistance> In a temperature environment of 25°C, the battery was charged at a constant current of 0.3 It until the voltage reached 4.2 V, and then at a constant voltage of 4.2 V, the battery was charged at a constant voltage of 0.05 It. Using a frequency response analyzer, impedance measurements were performed on the battery at a state of charge (SOC) of 100% in a frequency range of 10 kHz to 0.02 Hz and an applied voltage of 10 mV. A Cole-Cole plot was created from the obtained results, and the Rs value (i.e., electronic resistance value) was obtained from the intersection of the plot with the real axis.

[0111]

[0112] As is clear from a comparison between Examples 1 to 3 (A1 to A3) and Comparative Example 1 (C1), the inclusion of an organic additive in the positive electrode slurry significantly reduced the viscosity increase rate and also reduced DCIR. Furthermore, although the viscosity increase rate decreases as the amount of organic additive increases, it can be understood that from the viewpoint of DCIR, it is desirable that the amount of organic additive is not too large.

[0113] From a comparison of Examples 1 to 3 (A1 to A3), Comparative Example 1 (C1), and Comparative Examples 2 to 5 (C2 to C5), it can be seen that the effect of the organic additive in reducing the viscosity increase rate of the positive electrode slurry and the effect in reducing DCIR are not significant when CNT is used as the conductive additive, but are particularly significant when carbon black is used as the conductive additive.

[0114] From a comparison between Example 4 (A4) and Comparative Example 6 (C6), it can be seen that the amount of each of the fluorine-containing polymer and the carbon black is preferably less than 1 part by mass, more preferably 0.8 parts by mass or less, relative to 100 parts by mass of the lithium-containing composite oxide.

[0115] In addition, a clear effect of adding the organic additive is also observed in Example 4 (A4) compared to Comparative Example 1 (C1). In Example 5 (A5), the amount of binder (fluorine-containing polymer) was large, which is thought to have increased the frequency of polyenation, resulting in an increase in viscosity.

[0116] The positive electrode for a secondary battery according to the present disclosure can be used in a high-performance non-aqueous electrolyte secondary battery.

[0117] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0118] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug, 10: secondary battery (nonaqueous electrolyte secondary battery)

Claims

1. A positive electrode for a secondary battery, comprising a positive electrode composite including a positive electrode active material, a binder, and a conductive additive, wherein the positive electrode active material includes a lithium-containing composite oxide, the binder includes a fluorine-containing polymer, the degree of polyenization of the fluorine-containing polymer being 0.2 or less, and the conductive additive includes carbon black.

2. The positive electrode for a secondary battery according to claim 1, wherein the amount of carbon black in the positive electrode mixture is 0.6 parts by mass or more and 0.95 parts by mass or less per 100 parts by mass of the positive electrode active material.

3. The positive electrode for a secondary battery according to claim 1, wherein the amount of the fluorine-containing polymer in the positive electrode mixture is 0.5 parts by mass or more and 0.98 parts by mass or less per 100 parts by mass of the positive electrode active material.

4. The positive electrode for a secondary battery according to claim 1, wherein the lithium-containing composite oxide contains at least Ni, and in the lithium-containing composite oxide, the proportion of Ni among metal elements other than Li is 80 atomic % or more.

5. The positive electrode for a secondary battery according to claim 1, wherein the lithium-containing composite oxide contains 0.2 mass % or more and 0.5 mass % or less of an alkaline component.

6. The lithium-containing composite oxide has the formula: Li ｙ Ni ｘ M （１－ｘ） O ２－δ (0.8≦x≦1, 0<y≦1.2 and −0.05≦δ≦0.05), wherein M includes at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Zr, W, and B. The positive electrode for secondary batteries according to claim 1, 7. M is a compound of the formula: Co １－ｘ－ａ－ｂ Mn ａ Al ｂ 7. The positive electrode for a secondary battery according to claim 6, wherein a is a number expressed by the formula (0<a<0.05 and 0<b<0.05).

8. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode mixture further contains an organic additive containing a heteroatom, and the ratio of the mass of the organic additive to the mass of the lithium-containing composite oxide is 0.1 mass % or less.

9. The positive electrode for a secondary battery according to claim 8, wherein the decomposition temperature of the organic additive is 200° C. or lower.

10. The positive electrode for a secondary battery according to claim 8, wherein the organic additive is a Lewis base.

11. The positive electrode for a secondary battery according to claim 8, wherein the molecular weight of the organic additive is 50 or more and 1,000 or less.

12. A non-aqueous electrolyte secondary battery comprising the positive electrode for a secondary battery according to claim 1, a negative electrode, a separator, and a non-aqueous electrolyte.

13. A method for manufacturing a positive electrode for a secondary battery, comprising: a step of preparing a positive electrode slurry containing a liquid medium and a positive electrode composite dispersed in the liquid medium; a step of preparing a positive electrode current collector and forming a coating film of the positive electrode slurry on the surface of the positive electrode current collector; and a step of heating the coating film at 200°C or less to dry it; wherein the positive electrode composite contains a positive electrode active material, a binder, a conductive additive, and an organic additive containing a heteroatom; the positive electrode active material contains a lithium-containing composite oxide; the binder contains a fluorine-containing polymer; and the conductive additive contains carbon black.

14. The method for producing a positive electrode for a secondary battery according to claim 13, wherein the decomposition temperature of the organic additive is 200°C or lower.

15. The method for producing a positive electrode for a secondary battery according to claim 13, wherein the organic additive is a Lewis base.

16. The method for producing a positive electrode for a secondary battery according to claim 13, wherein the molecular weight of the organic additive is 50 or more and 1,000 or less.

17. The method for producing a positive electrode for a secondary battery according to claim 13, wherein the ratio of the mass of the organic additive to the mass of the lithium-containing composite oxide is 0.002 mass % or more and 0.3 mass % or less.

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