Method for manufacturing positive electrode, and positive electrode

Abstract

To provide a positive electrode which can provide an output characteristic to a lithium ion secondary battery.SOLUTION: The method for manufacturing a positive electrode disclosed therein includes the steps of: applying a positive electrode slurry including a positive electrode active material, a carbon nanotube, and a dispersion medium to a positive electrode collector; drying the applied positive electrode slurry and forming a positive electrode active material layer; and pressing the positive electrode active material layer so that the void ratio of the positive electrode active material is in the range of 42.5% to 54.5%. The average length of the carbon nanotube is in the range of 0.3 μm to 0.7 μm. The velocity of the positive electrode slurry at the temperature of 25°C is in the range of 2.1 Pa s to 4.2 Pa s.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] In recent years, lithium ion secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, and the like, and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] As lithium-ion secondary batteries become more widespread, there is a demand for even higher output, particularly in applications as vehicle drive power sources. One known method for increasing the output of lithium-ion secondary batteries is to use carbon nanotubes as the conductive material in the positive electrode (see, for example, Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-37549 [Patent Document 2] Japanese Patent Publication No. 2022-55889 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-185229 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-195143 Summary of the Invention [Problem to be solved by the invention]

[0005] As a result of extensive research, the present inventors have found that there is still room for improvement in the positive electrodes of the prior art in terms of increasing the output of lithium ion secondary batteries.

[0006] Therefore, an object of the present invention is to provide a positive electrode that can impart excellent output characteristics to a lithium ion secondary battery. [Means for solving the problem]

[0007] The method for manufacturing a positive electrode disclosed herein includes the steps of: applying a positive electrode slurry containing a positive electrode active material, carbon nanotubes, and a dispersion medium to a positive electrode current collector; drying the applied positive electrode slurry to form a positive electrode active material layer; and pressing the positive electrode active material layer so that the porosity of the positive electrode active material layer is 42.5% to 54.5%. The average length of the carbon nanotubes is 0.3 μm to 0.7 μm. The viscosity of the positive electrode slurry at 25°C is 2.1 Pa·s to 4.2 Pa·s.

[0008] According to this configuration, it is possible to provide a positive electrode that can impart excellent output characteristics to a lithium ion secondary battery.

[0009] From another aspect, the present disclosure provides a positive electrode comprising a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material and carbon nanotubes. The positive electrode active material layer has a porosity of 42.5% to 54.5%. The carbon nanotubes have an average length of 0.3 μm to 0.7 μm. The positive electrode is formed using a positive electrode slurry having a viscosity at 25°C of 2.1 Pa·s to 4.2 Pa·s.

[0010] According to this configuration, it is possible to provide a positive electrode that can impart excellent output characteristics to a lithium ion secondary battery. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a flowchart showing the steps of a method for producing a positive electrode according to one embodiment of the present invention. [Figure 2] 1 is a schematic cross-sectional view of an example of a positive electrode obtained by a manufacturing method according to one embodiment of the present invention. [Figure 3] FIG. 3 is a cross-sectional view schematically showing the configuration of a lithium ion secondary battery constructed using the positive electrode of FIG. 2. [Figure 4] FIG. 4 is a schematic exploded view showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. [Figure 5] 1 is a graph plotting the value of initial output resistance for various slurry viscosities versus the porosity of the positive electrode active material layer when carbon nanotubes with an average length of 0.1 μm are used. [Figure 6] 1 is a graph plotting the value of initial output resistance for various slurry viscosities versus the porosity of the positive electrode active material layer when carbon nanotubes with an average length of 0.3 μm are used. [Figure 7] 1 is a graph plotting the value of initial output resistance for various slurry viscosities versus the porosity of the positive electrode active material layer when carbon nanotubes with an average length of 0.5 μm are used. [Figure 8] 1 is a graph plotting the value of initial output resistance for various slurry viscosities versus the porosity of the positive electrode active material layer when carbon nanotubes with an average length of 0.7 μm are used. [Figure 9] 1 is a graph plotting the value of initial output resistance for various slurry viscosities versus the porosity of the positive electrode active material layer when carbon nanotubes with an average length of 1.0 μm are used. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for implementing the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In this specification, a numerical range expressed as "A to B" includes A and B.

[0013] In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

[0014] In this specification, the term "slurry" refers to a mixture in which a part or all of the solid content is dispersed in a dispersion medium, and includes so-called "paste," "ink," and the like.

[0015] Hereinafter, as an example, an embodiment in which the positive electrode obtained by the manufacturing method disclosed herein is a positive electrode for a lithium ion secondary battery will be specifically described with reference to the drawings.

[0016] As shown in FIG. 1, the method for manufacturing a positive electrode according to this embodiment includes the steps of: applying a positive electrode slurry containing a positive electrode active material, carbon nanotubes (CNTs), and a dispersion medium to a positive electrode current collector (hereinafter also referred to as the "coating step") S101; drying the applied positive electrode slurry to form a positive electrode active material layer (hereinafter also referred to as the "drying step") S102; and pressing the positive electrode active material layer so that the porosity of the positive electrode active material layer is 42.5% to 54.5% (hereinafter also referred to as the "pressing step") S103. The average length of the carbon nanotubes is 0.3 μm to 0.7 μm. The viscosity of the positive electrode slurry at 25° C. is 2.1 Pa·s to 4.2 Pa·s.

[0017] Each step will be described below using as an example the case of producing the positive electrode shown in Fig. 2. Fig. 2 is a cross-sectional view of the positive electrode taken along the width and thickness directions.

[0018] 2 includes a positive electrode current collector 52 and a positive electrode active material layer 54 supported on the positive electrode current collector 52. In the illustrated example, the positive electrode active material layer 54 is provided on both sides of the positive electrode current collector 52, but it may be provided on one side. The positive electrode active material layer 54 is preferably provided on both sides of the positive electrode current collector 52.

[0019] As shown in the illustrated example, the positive electrode 50 may have a positive electrode active material layer-free portion 52a at at least one end, where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed. The positive electrode active material layer-free portion 52a functions as a current collector. However, the form of the current collector in the positive electrode 50 is not limited to this.

[0020] First, the coating step S101 will be described. In this step S101, a positive electrode slurry containing a positive electrode active material, carbon nanotubes (CNTs), and a dispersion medium is coated onto a positive electrode current collector. The average length of the carbon nanotubes is 0.3 μm to 0.7 μm. The viscosity of the positive electrode slurry at 25° C. is 2.1 Pa·s to 4.2 Pa·s.

[0021] Examples of the positive electrode current collector 52 used in the coating step S101 include a sheet or foil made of a metal such as aluminum, nickel, titanium, or stainless steel, and among these, aluminum foil is preferred. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0022] The positive electrode active material contained in the positive electrode slurry may be a known positive electrode active material used in lithium ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0023] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more. The positive electrode active material is preferably lithium nickel cobalt manganese composite oxide.

[0024] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0025] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0026] The above positive electrode active materials can be used alone or in combination of two or more.

[0027] The average particle diameter of the positive electrode active material is not particularly limited, but is, for example, 0.5 μm to 30 μm, preferably 1 μm to 20 μm, more preferably 1.5 μm to 10 μm, and even more preferably 2 μm to 5 μm. In this specification, the term "average particle diameter" refers to the median diameter (D50), and therefore refers to the particle diameter corresponding to a cumulative frequency of 50% by volume of particles with smaller particle diameters in a volume-based particle size distribution based on a laser diffraction / scattering method. Therefore, the average particle diameter (D50) can be determined using a known laser diffraction / scattering particle size distribution analyzer or the like.

[0028] The amount of the positive electrode active material used is not particularly limited, but is preferably 87 mass % or more, more preferably 90 mass % or more, and even more preferably 96 mass % or more of the total solid content of the positive electrode slurry (i.e., relative to the total mass of the solid content of the positive electrode slurry).

[0029] The type of CNT contained in the positive electrode slurry is not particularly limited, and examples thereof include single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT). These may be used alone or in combination of two or more. MWCNT is preferred as the CNT. The CNT may be produced by arc discharge, laser ablation, chemical vapor deposition, or the like.

[0030] The average length of the CNTs is 0.3 μm to 0.7 μm. When the average length of the CNTs is within this range, a good conductive path can be formed between the positive electrode active materials, which contributes to increasing the output of the lithium ion secondary battery. From the viewpoint of imparting a higher output to the lithium ion secondary battery, the average length of the CNTs is preferably 0.5 μm to 0.7 μm.

[0031] The average length of the CNTs can be determined as the average value of the equivalent spherical diameter (ESD) using a flow imaging microscope (for example, "FlowCam nano" manufactured by Yokogawa Electric Corporation).

[0032] The average diameter of the CNTs is not particularly limited and is, for example, 0.1 nm to 50 nm, preferably 1 nm to 40 nm, and more preferably 2 nm to 30 nm.

[0033] The average diameter of CNTs can be determined by taking an electron microscope photograph of the CNTs and averaging the diameters of 100 or more CNTs. Specifically, for example, a CNT dispersion is diluted and then dried to prepare a measurement sample. This sample is observed using a scanning electron microscope to determine the diameters of 100 or more CNTs, and the average is calculated. If the CNTs have re-aggregated, the diameter is determined for the bundle of aggregated CNTs.

[0034] The amount of CNT used is not particularly limited. If the amount of CNT used is too small, the effect as a conductive material may be reduced. On the other hand, if the amount of CNT used is too large, the viscosity of the positive electrode slurry may increase. Therefore, the amount of CNT used is preferably 0.1% by mass or more and 3.0% by mass or less, more preferably 0.3% by mass or more and 2.5% by mass or less, and even more preferably 0.5% by mass or more and 2.0% by mass or less, based on the total solid content of the positive electrode slurry (i.e., relative to the total mass of the solid content of the positive electrode slurry).

[0035] The dispersion medium contained in the positive electrode slurry is also referred to as a "solvent" in the art. The dispersion medium may be a dispersion medium (particularly, an organic solvent) used in known positive electrode slurries for lithium ion secondary batteries. A suitable example of the dispersion medium is N-methyl-2-pyrrolidone (NMP).

[0036] The amount of dispersion medium used is not particularly limited as long as it can disperse the positive electrode active material and CNTs. The viscosity of the positive electrode slurry can be easily controlled by adjusting the amount of dispersion medium used. Therefore, the dispersion medium is preferably used in an amount such that the viscosity of the positive electrode slurry at 25°C is 2.1 Pa·s to 4.2 Pa·s.

[0037] The positive electrode slurry may contain components (optional components) other than the positive electrode active material, CNT, and dispersion medium, such as a CNT dispersion medium and a binder.

[0038] A CNT dispersant is an additive that stably disperses or solubilizes CNTs in a dispersion medium, and is sometimes called a solubilizer. Examples of CNT dispersants that can be used include surfactant-type dispersants (also called low-molecular-weight dispersants), polymer-type dispersants, and inorganic-type dispersants. CNT dispersants may be anionic, cationic, amphoteric, or nonionic. Therefore, the CNT dispersant may have at least one functional group selected from the group consisting of anionic groups, cationic groups, and nonionic groups in its molecular structure. A surfactant is an amphiphilic substance that has a chemical structure in which hydrophilic and lipophilic moieties are covalently bonded within its molecular structure.

[0039] Specific examples of CNT dispersants include polycondensation-based aromatic surfactants such as sodium salt of naphthalenesulfonic acid-formaldehyde condensate, ammonium salt of naphthalenesulfonic acid-formaldehyde condensate, and sodium salt of methylnaphthalenesulfonic acid-formaldehyde condensate; polycarboxylic acids and their salts, such as polyacrylic acid and its salts, polymethacrylic acid and its salts; triazine derivative dispersants (preferably those containing a carbazolyl group or a benzimidazolyl group); polyvinylpyrrolidone (PVP); polymers having polynuclear aromatics in the side chain, such as pyrene and anthracene; polynuclear aromatic ammonium derivatives, such as pyrene ammonium derivatives (e.g., compounds in which an ammonium bromide group is introduced into pyrene) and anthracene ammonium derivatives; and the like. These CNT dispersants can be used alone or in combination. CNT dispersants containing polynuclear aromatics are preferred. Specifically, polymers having polynuclear aromatics in the side chain and polynuclear aromatic ammonium derivatives are preferred.

[0040] The ratio of CNT dispersant to CNT may be determined appropriately depending on the type of CNT and CNT dispersant. Here, if the ratio of CNT dispersant is too small, dispersibility may be insufficient. On the other hand, if the ratio of CNT dispersant is too large, excessive CNT dispersant may adhere to the CNT surface, causing an increase in resistance. When the CNTs are SWCNTs, the amount of CNT dispersant used is, for example, 1 to 400 parts by mass, preferably 20 to 200 parts by mass, per 100 parts by mass of CNTs. When the CNTs are MWNTs, the amount of CNT dispersant used is, for example, 1 to 100 parts by mass, preferably 4 to 40 parts by mass, per 100 parts by mass of CNTs.

[0041] As the binder, for example, polyvinylidene fluoride (PVdF) or the like can be used. The molecular weight of the binder may be the same as that of known binders used in the positive electrodes of secondary batteries. PVdF is typically a homopolymer, but may be copolymerized with other monomer units other than vinylidene fluoride within a range that does not impair the effects of the present invention (for example, 10 mol % or less of all monomer units).

[0042] The amount of binder used is not particularly limited, but is, for example, 0.1 mass % to 10 mass %, preferably 0.2 mass % to 5 mass %, and more preferably 0.3 mass % to 2 mass %, of the total solid content of the positive electrode slurry (i.e., relative to the total mass of the solid content of the positive electrode slurry).

[0043] The positive electrode slurry can be prepared by mixing the positive electrode active material, CNTs, a dispersion medium, and optional components according to a known method.

[0044] Here, the viscosity of the positive electrode slurry used in the coating step S101 at 25°C is 2.1 Pa·s to 4.2 Pa·s. When the viscosity of the positive electrode slurry is within this range, the CNTs can be uniformly dispersed in the positive electrode active material layer 54, and a good conductive path can be formed throughout the positive electrode active material layer 54. This also contributes to increasing the output of the lithium-ion secondary battery.

[0045] The viscosity of the positive electrode slurry can be controlled by adjusting the content of each component. In particular, adjusting the content of the dispersion medium in the positive electrode slurry is an easy way to control the viscosity. Alternatively, the viscosity can be adjusted by changing the molecular weight of the binder, the average length of the CNTs, or the content of the CNTs.

[0046] The viscosity of the positive electrode slurry was measured using an E-type viscometer at 25°C and a shear rate of 10 seconds. -1 It can be determined by measuring under the following conditions.

[0047] The order in which the components of the positive electrode slurry are mixed is not particularly limited. Because CNTs have low dispersibility, it is preferable to first disperse the CNTs in a dispersion medium from the viewpoint of preparing the positive electrode slurry in a short time. When dispersing, ultrasonic irradiation or the like is advantageous for dispersing the CNTs in a short time. Next, the positive electrode active material and optional components (e.g., binder, etc.) are added and mixed using a known stirring device (e.g., planetary mixer, etc.), whereby a slurry in which the components are uniformly dispersed can be easily prepared.

[0048] The positive electrode slurry can be applied by a known method. For example, the positive electrode slurry can be applied to the positive electrode current collector using a coating device such as a slit coater, die coater, comma coater, gravure coater, or dip coater. The amount of the positive electrode slurry to be applied may be determined appropriately taking into account the thickness, basis weight, etc. of the positive electrode active material layer 54.

[0049] When producing the positive electrode 50 shown in FIG. 2, the positive electrode slurry is applied to both sides of the positive electrode current collector 52. However, when the positive electrode active material layer 54 is formed on only one side of the positive electrode current collector 52 in the positive electrode 50, the positive electrode slurry is applied to one side of the positive electrode current collector. Furthermore, by applying the positive electrode slurry along one end in the width direction of the positive electrode current collector 52 while not applying the positive electrode slurry to the other end, a positive electrode active material layer-free portion 52a can be provided. The coating step S101 can be performed in this manner.

[0050] Next, the drying step S102 will be described. In the drying step S102, the coated positive electrode slurry is dried to form a positive electrode active material layer. The drying step S102 can be performed according to a known method.

[0051] Specifically, for example, the dispersion medium can be removed from the positive electrode slurry coated on the positive electrode current collector using a drying device such as a drying oven. The drying temperature and drying time are not particularly limited and may be appropriately determined depending on the type and amount of the dispersion medium contained in the positive electrode slurry. The drying temperature is, for example, 70°C to 200°C (particularly, 110°C to 180°C). The drying time is, for example, 20 seconds to 120 minutes.

[0052] Next, the pressing step S103 will be described. In the pressing step S103, the positive electrode active material layer 54 is pressed so that the porosity of the positive electrode active material layer 54 becomes 42.5% to 54.5%. This pressing can be performed according to a known method (for example, roll pressing).

[0053] Here, by adjusting the pressing conditions, the porosity of the positive electrode active material layer 54 can be controlled to 42.5% to 54.5%. For example, when the pressing process is performed by roll pressing, the porosity of the positive electrode active material layer 54 can be controlled by changing the linear pressure or the gap between the rolls.

[0054] Generally, from the viewpoint of a high energy density of the positive electrode, a low porosity of the positive electrode active material layer 54 is preferable (for example, in a high-energy density positive electrode, the porosity is usually set to 25% or less). However, in this embodiment, the porosity of the positive electrode active material layer 54 is intentionally set to be high. This allows the nonaqueous electrolyte to easily impregnate into the positive electrode active material layer 54, and allows Li ion flow paths to be suitably formed within the positive electrode active material during discharge of the lithium ion secondary battery. This also contributes to higher output. From the viewpoint of a higher output improvement effect, the porosity of the positive electrode active material layer 54 is preferably 45% to 54%, and more preferably 46% to 53%. The porosity of the positive electrode active material layer 54 can be determined by measurement using mercury intrusion porosimeter. This measurement can be performed using a commercially available mercury intrusion porosimeter according to a known method.

[0055] In this way, by using CNTs with an average length of 0.3 μm to 0.7 μm, using a positive electrode slurry with a viscosity at 25° C. of 2.1 Pa·s to 4.2 Pa·s, and adjusting the porosity of the positive electrode active material layer to 42.5% to 54.5% by pressing, it is possible to form a good conductive path throughout the positive electrode active material layer 54, while also forming a flow path for Li ions. As a result, it is possible to increase the output of a lithium-ion secondary battery using the positive electrode 50.

[0056] In the resulting positive electrode 50, the thickness of the positive electrode active material layer 54 per side is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0057] A protective layer may be further provided on the positive electrode active material layer non-forming portion 52a adjacent to the positive electrode active material layer 54. The protective layer is, for example, an insulating layer containing ceramic particles or the like. The protective layer can be formed according to a known method.

[0058] Furthermore, a step of cutting the portion 52a without the positive electrode active material layer into a predetermined shape using a laser or the like may be further performed. By cutting the portion 52a without the positive electrode active material layer into a predetermined shape, a current collecting tab or the like can be formed.

[0059] The positive electrode 50 obtained by the manufacturing method disclosed herein can be used as a positive electrode for a secondary battery (particularly a lithium ion secondary battery) according to known methods.

[0060] From another perspective, a positive electrode 50 disclosed herein includes a positive electrode current collector 52 and a positive electrode active material layer 54 supported by the positive electrode current collector 52. The positive electrode active material layer 54 contains a positive electrode active material and carbon nanotubes. The positive electrode active material layer 54 has a porosity of 42.5% to 54.5%. The carbon nanotubes have an average length of 0.3 μm to 0.7 μm. The positive electrode 50 is formed using a positive electrode slurry having a viscosity at 25°C of 2.1 Pa·s to 4.2 Pa·s.

[0061] As an example of a secondary battery using the positive electrode 50, a configuration example of a lithium ion secondary battery using the positive electrode 50 will be described below with reference to the drawings.

[0062] The lithium-ion secondary battery 100 shown in FIG. 3 is a sealed battery constructed by housing a flat wound electrode assembly 20 and a nonaqueous electrolyte 80 in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, as well as a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 also has an injection port (not shown) for injecting the nonaqueous electrolyte 80. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum. Note that FIG. 3 does not accurately represent the amount of nonaqueous electrolyte 80.

[0063] As shown in Figures 3 and 4, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62.

[0064] The positive electrode 50 has a positive electrode active material layer-free portion 52a where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed. The negative electrode 60 has a negative electrode active material layer-free portion 62a where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed. The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are formed so as to protrude outward from both ends of the wound electrode body 20 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

[0065] The positive electrode sheet 50 is a positive electrode obtained by the manufacturing method according to the embodiment described above.

[0066] The negative electrode current collector 62 constituting the negative electrode sheet 60 may be, for example, copper foil. The negative electrode active material layer 64 includes a negative electrode active material. Examples of the negative electrode active material that can be used include carbon materials such as graphite, hard carbon, and soft carbon. The negative electrode active material layer 64 may further include a binder, a thickener, and the like. Examples of the binder that can be used include styrene butadiene rubber (SBR). Examples of the thickener that can be used include carboxymethyl cellulose (CMC).

[0067] The separator 70 can be made of various microporous sheets similar to those conventionally used in lithium-ion secondary batteries, including microporous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such microporous resin sheets may have a single-layer structure or a multi-layer structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separator 70 may also have a heat-resistant layer (HRL).

[0068] The nonaqueous electrolyte 80 typically contains a nonaqueous solvent and an electrolyte salt (in other words, a supporting salt). As the nonaqueous solvent, various organic solvents used in electrolytes of general lithium ion secondary batteries, such as carbonates, ethers, esters, nitriles, sulfones, and lactones, can be used without any particular limitation. Of these, carbonates are preferred.

[0069] As the electrolyte salt, for example, lithium salts such as LiPF, LiBF, and lithium bis(fluorosulfonyl)imide (LiFSI) can be used, and among them, LiPF is preferred. The concentration of the electrolyte salt is not particularly limited, but is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0070] The nonaqueous electrolyte 80 may contain various additives other than the above-mentioned components, such as an oxalate complex, a film-forming agent such as vinylene carbonate (VC), a gas generating agent such as biphenyl (BP) or cyclohexylbenzene (CHB), a thickener, etc., as long as the effects of the present invention are not significantly impaired.

[0071] The lithium ion secondary battery 100 has excellent output characteristics. The lithium ion secondary battery 100 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices and the like. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of a plurality of batteries connected in series and / or parallel.

[0072] As an example, a rectangular lithium ion secondary battery 100 including a flat wound electrode assembly 20 has been described. However, the lithium ion secondary battery disclosed herein can also be configured as a lithium ion secondary battery including a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked). Furthermore, the nonaqueous electrolyte secondary battery disclosed herein can also be configured as a cylindrical lithium ion secondary battery, a laminated case lithium ion secondary battery, a coin-type lithium ion secondary battery, etc.

[0073] Also, a non-aqueous electrolyte secondary battery other than a lithium ion secondary battery can be constructed using the above-described positive electrode according to a known method. Furthermore, an all-solid-state secondary battery (particularly an all-solid-state lithium ion secondary battery) can be constructed using a solid electrolyte instead of the non-aqueous electrolyte 80 according to a known method.

[0074] Examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples.

[0075] [Preparation of Positive Electrode] LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3O2, CNT as a conductive material, and PVdF as a binder were prepared. MWCNTs with an average diameter of 13 nm were used as CNTs, and average lengths of 0.1 μm, 0.3 μm, 0.5 μm, 0.7 μm, and 1.0 μm were prepared. A copolymer with a molecular weight of approximately 1.4 million was used as PVdF. MWCNTs, N-methyl-2-pyrrolidone (NMP), and a pyrene ammonium derivative as a CNT dispersant were mixed by ultrasonic irradiation to prepare a CNT dispersion.

[0076] LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (LNCM), CNT dispersion, and PVdF were mixed in a mass ratio of LNCM:CNT:PVdF = 98.3:1.0:0.7. An appropriate amount of NMP was added to this mixture to obtain a positive electrode slurry. The viscosity of the positive electrode slurry was varied by changing the amount of NMP added. As shown in Table 1, three types of positive electrode slurries with different viscosities were prepared using CNTs with average lengths of 0.1 μm, 0.5 μm, 0.7 μm, and 1.0 μm. Four types of positive electrode slurries with different viscosities were prepared using CNTs with an average length of 0.3 μm.

[0077] [Table 1]

[0078] The positive electrode slurry prepared above was applied to both sides of a 12 μm thick aluminum foil serving as a positive electrode current collector. At this time, a positive electrode slurry-uncoated portion was provided on the aluminum foil as a lead connection portion. The amount of positive electrode slurry applied was adjusted so that the basis weight of the positive electrode active material layer formed on both sides was 11 mg / cm in total. 2 It was adjusted so that

[0079] The applied slurry was dried to form a positive electrode active material layer. The obtained sheet was pressed using a pressure roller. The rolling gap of the pressure roller was changed to adjust the porosity of the positive electrode active material layer. The obtained sheet was cut to a predetermined size to obtain a positive electrode in which a positive electrode active material layer was formed on both sides of the positive electrode current collector. In this way, several positive electrodes with different porosities of the positive electrode active material layer were produced.

[0080] [Measurement of CNT average length] The MWCNTs used were dispersed in NMP at a concentration of 0.035% by mass to prepare a measurement sample. Using this measurement sample and a flow imaging microscope "FlowCam nano" (manufactured by Yokogawa Electric Corporation), particle size distribution was measured, and the built-in software "VisualSpreadsheet" was used to determine the average equivalent spherical diameter (ESD) as the average length.

[0081] [Measurement of Viscosity of Positive Electrode Slurry] A cone plate was attached to a rheometer "HAAKE MARS" (manufactured by Thermo Scientific). Using this, the viscosity of the positive electrode slurry was measured at 25°C at a shear rate of 10 s -1 The measurement was carried out under the following conditions.

[0082] [Measurement of Porosity of Positive Electrode Active Material Layer] Test pieces of the positive electrodes produced in each example and comparative example were prepared. The porosity of the positive electrode active material layer was determined using a mercury porosimeter "Autopore V9260" (manufactured by Shimadzu Corporation).

[0083] [Preparation of Evaluation Battery] Graphite as a carbon-based negative electrode active material, carboxymethylcellulose sodium salt (CMC-Na), and a dispersion of styrene butadiene rubber (SBR) were mixed in a solids mass ratio of graphite:CMC-Na:CMC = 98:1:1. An appropriate amount of ion-exchanged water was added to prepare a negative electrode slurry. The negative electrode slurry was applied to both sides of an 8 μm-thick copper foil serving as a negative electrode current collector. An uncoated portion of the copper foil was left as a lead connection.

[0084] The applied paste was dried to form a negative electrode active material layer. The obtained sheet was pressed with a roller and then cut to a predetermined size to obtain a negative electrode in which a negative electrode active material layer was formed on both sides of the negative electrode current collector. The packing density of the negative electrode active material layer was 1.20 g / cm. 3 It was.

[0085] A lead was attached to each of the positive and negative electrodes prepared above. A single-layer polypropylene separator was prepared. The positive and negative electrodes were alternately stacked one by one with the separator interposed therebetween to prepare a laminated electrode assembly.

[0086] A mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 25:35:40 was prepared. Vinylene carbonate was dissolved in this mixed solvent at a concentration of 1% by mass, lithium bis(oxalato)borate was dissolved at a concentration of 0.8% by mass, and LiPF6 was dissolved as a supporting electrolyte at a concentration of 1.15 mol / L. This produced a nonaqueous electrolyte.

[0087] The laminated electrode body and the non-aqueous electrolyte solution were placed in a rectangular battery case and sealed to obtain a rectangular lithium ion secondary battery for evaluation. The amount of the non-aqueous electrolyte solution injected was 9.0 g / Ah.

[0088] <Output resistance measurement> Each evaluation lithium-ion secondary battery was adjusted to a SOC (State of charge) of 50% by constant current-constant voltage (CC-CV) charging and then placed in an environment at 25°C. Discharge was performed at a current value of 50 C for 10 seconds, and the voltage drop ΔV at this time was obtained. The initial output resistance value of each evaluation secondary battery was calculated using this voltage drop ΔV and the current value. The results are shown in Figures 5 to 9. Note that the smaller the initial output resistance value, the higher the output power of the secondary battery and therefore the better the output characteristics.

[0089] 5 to 9 are graphs plotting the initial output resistance (μΩ) versus the porosity (%) of the positive electrode active material layer when the average length of the CNTs is 0.1 μm, 0.3 μm, 0.5 μm, 0.7 μm, and 1.0 μm, respectively.

[0090] As shown in Figures 5 and 9, when the average CNT length was 0.1 μm and 1.0 μm, the initial output resistance tended to increase with increasing porosity, regardless of the viscosity of the positive electrode slurry. On the other hand, in Figure 6, where the average CNT length was 0.3 μm, the initial output resistance also tended to increase with increasing porosity when the viscosity of the positive electrode slurry was 0.7 Pa·s and 5.7 Pa·s. However, when the viscosity of the positive electrode slurry was 2.1 Pa·s and 4.2 Pa·s, a different trend was observed, with a decrease in initial output resistance observed in the porosity range of 42.5% to 54.5%, as shown in the elliptical dashed frame. Similarly to Figure 6, in Figure 7, where the average CNT length was 0.5 μm, when the viscosity of the positive electrode slurry was 3.2 Pa·s, a decrease in initial output resistance was observed in the porosity range of 42.5% to 54.5%, as shown in the elliptical dashed frame. In the graph of Figure 8, where the average length of the CNTs is 0.7 μm, similar to Figure 6, when the viscosity of the positive electrode slurry was 4.2 Pa·s, a decrease in the initial output resistance was observed in the porosity range of 42.5% to 54.5%, as shown in the elliptical dashed frame.

[0091] The above results show that the output resistance is specifically reduced when CNTs with an average length of 0.3 μm to 0.7 μm are used, a positive electrode slurry with a viscosity at 25°C of 2.1 Pa·s to 4.2 Pa·s is used, and the positive electrode active material layer is pressed so that the porosity is 42.5% to 54.5%.

[0092] Considering these results, the high output resistance when the CNT length was 0.1 μm is due to the CNTs being too short, resulting in insufficient formation of conductive paths between the positive electrode active material. The initial output resistance increases as the porosity increases, presumably because the positive electrode active material particles become sparser with increasing porosity, reducing the conductive paths.

[0093] The high output resistance when the CNT length is 1.0 μm is due to insufficient dispersion of the CNTs in the positive electrode active material layer. As the CNT length increases, dispersion decreases, and this is further reduced by re-aggregation. The initial output resistance increases with increasing porosity, likely because the positive electrode active material particles become sparser with increasing porosity, reducing the number of conductive paths.

[0094] When the CNT length is 0.3 μm to 0.7 μm and the viscosity is less than 2.1 Pa·s, the output resistance is high, presumably because CNT migration occurs during slurry application, resulting in non-uniform CNT dispersion.When the CNT length is 0.3 μm to 0.7 μm and the viscosity is greater than 4.2 Pa·s, the output resistance is high, presumably because the viscosity of the slurry is too high, resulting in non-uniform CNT dispersion in the positive electrode active material layer.

[0095] When the CNT length is 0.3 μm to 0.7 μm, the viscosity is 2.1 Pa·s to 4.2 Pa·s, and the porosity is 42.5% to 54.5%, the output resistance is low. This is thought to be because, even when the porosity is relatively high, the CNT length is appropriate, which allows for the formation of good conductive paths in the positive electrode active material layer, and the presence of a moderate amount of voids in the positive electrode active material layer facilitates the movement of Li ions in the electrolyte when the lithium-ion secondary battery is discharged.

[0096] From the above, it can be seen that the method for producing a positive electrode disclosed herein makes it possible to produce a positive electrode that can impart excellent output characteristics to a lithium ion secondary battery.

[0097] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0098] That is, the method for producing a positive electrode and the positive electrode disclosed herein are the following items [1] to [5]. [1] A step of applying a positive electrode slurry containing a positive electrode active material, carbon nanotubes, and a dispersion medium to a positive electrode current collector; drying the coated positive electrode slurry to form a positive electrode active material layer; and pressing the positive electrode active material layer so that the porosity of the positive electrode active material layer is 42.5% to 54.5%, The average length of the carbon nanotubes is 0.3 μm to 0.7 μm, The viscosity of the positive electrode slurry at 25°C is 2.1 Pa s to 4.2 Pa s. Manufacturing method. [2] The manufacturing method according to item [1], wherein the positive electrode active material layer is pressed so that the porosity of the positive electrode active material layer is 46% to 53%. [3] The method according to item [1] or [2], wherein the dispersion medium is N-methyl-2-pyrrolidone. [4] The method according to any one of items [1] to [3], wherein the positive electrode is a positive electrode of a lithium ion secondary battery. [5] A positive electrode comprising a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector, the positive electrode active material layer contains a positive electrode active material and carbon nanotubes, the porosity of the positive electrode active material layer is 42.5% to 54.5%, The average length of the carbon nanotubes is 0.3 μm to 0.7 μm, The positive electrode is formed using a positive electrode slurry having a viscosity at 25°C of 2.1 Pa·s to 4.2 Pa·s. [Explanation of symbols]

[0099] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator sheet (separator) 80 Non-aqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. a step of applying a positive electrode slurry containing a positive electrode active material, carbon nanotubes, and a dispersion medium to a positive electrode current collector; drying the coated positive electrode slurry to form a positive electrode active material layer; and pressing the positive electrode active material layer so that the porosity of the positive electrode active material layer is 42.5% to 54.5%, The average length of the carbon nanotubes is 0.3 μm to 0.7 μm, The viscosity of the positive electrode slurry at 25°C is 2.1 Pa s to 4.2 Pa s; Manufacturing method.

2. The method according to claim 1, wherein the positive electrode active material layer is pressed so that the porosity of the positive electrode active material layer is 46% to 53%.

3. The method according to claim 1, wherein the dispersion medium is N-methyl-2-pyrrolidone.

4. The method according to claim 1 , wherein the positive electrode is a positive electrode of a lithium ion secondary battery.

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