Positive electrode for nonaqueous electrolyte secondary battery, nonaqueous electrolyte secondary battery using same, and conductive material dispersion

Abstract

The positive electrode of the present disclosure is a positive electrode for a non-aqueous electrolyte secondary battery. The positive electrode includes a positive electrode mixture layer. The positive electrode mixture layer contains a positive electrode active material, a conductive material, and a binder. The conductive material includes both single-layer carbon nanotubes and multi-layer carbon nanotubes. The binder contains at least one selected from the group consisting of nitrile group-containing rubbers and cellulose derivatives.

Description

Technical Field

[0001] The present application relates to a positive electrode for a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte secondary battery using the positive electrode, and a conductive material dispersion. Background Art

[0002] Non-aqueous electrolyte secondary batteries have high power and high energy density, and are therefore used in a variety of fields, including consumer applications and automotive applications. In recent years, non-aqueous electrolyte secondary batteries have been sought to achieve higher performance, and various proposals have been made for non-aqueous electrolyte secondary batteries.

[0003] Patent document 1 (Japanese Patent Publication No. 2020-19705) discloses a dispersion used in the manufacture of an electrode: "A carbon nanotube dispersion, characterized in that it comprises bundle-type carbon nanotubes, a dispersion medium, and a partially hydrogenated nitrile rubber having a residual double bond (RDB) value of 0.5 to 40% by weight as calculated by the following mathematical formula 1, wherein the particle size distribution D of the dispersed particle size of the carbon nanotubes is 50 3 to 10 μm. [Mathematical formula 1] RDB (weight %) = BD weight / (BD weight + HBD weight) × 100. In the mathematical formula 1, BD means a structural unit derived from a conjugated diene, and HBD means a structural unit derived from a hydrogenated conjugated diene. ".

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2020-19705 Summary of the invention

[0007] Problem that the invention aims to solve

[0008] Carbon nanotubes are easily aggregated in the positive electrode slurry and the positive electrode mixture layer. When carbon nanotubes are aggregated in the positive electrode mixture layer, the internal resistance of the positive electrode becomes high and the battery characteristics are reduced. In such a case, one of the purposes of the present disclosure is to provide a positive electrode with low internal resistance.

[0009] Solutions for solving problems

[0010] One aspect of the present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery, the positive electrode comprising a positive electrode mixture layer, the positive electrode mixture layer comprising a positive electrode active material, a conductive material and a binder, the conductive material comprising both single-layer carbon nanotubes and multi-layer carbon nanotubes, and the binder comprising at least one selected from the group consisting of nitrile-containing rubber and cellulose derivatives.

[0011] Another aspect of the present disclosure relates to a nonaqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery includes the positive electrode of the present disclosure.

[0012] Another aspect of the present disclosure relates to a conductive material dispersion liquid comprising at least one selected from the group consisting of nitrile group-containing rubber and cellulose derivatives, a conductive material and a liquid medium, wherein the conductive material comprises both single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0013] Effects of the Invention

[0014] According to the present disclosure, a positive electrode with low internal resistance can be obtained. By using this positive electrode, a non-aqueous electrolyte secondary battery with low internal resistance can be obtained.

[0015] The novel features of the present invention are described in the appended claims, but the present invention can be better understood with reference to the following detailed description of the accompanying drawings in conjunction with other objects and features of the present invention, both in terms of structure and content. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic perspective view of a partially cutaway nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0017] Hereinafter, the embodiments of the present disclosure are described with examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials are sometimes exemplified, but other numerical values ​​and materials may also be applied as long as the effects of the present disclosure can be obtained. In this specification, a description such as "numerical value A to numerical value B" includes numerical value A and numerical value B, and may be renamed as "numerical value A or above and numerical value B or below". In the following description, when the lower limit and upper limit of numerical values ​​related to specific physical properties, conditions, etc. are exemplified, as long as the lower limit is not above the upper limit, any of the exemplified lower limits may be arbitrarily combined with any of the exemplified upper limits. In the following description, when examples of constituent elements and examples of methods are listed, as long as there is no special description, only one of the listed examples may be used, or a plurality of the listed examples may be used in combination.

[0018] (Positive electrode for non-aqueous electrolyte secondary battery)

[0019] The positive electrode of the present embodiment is a positive electrode for a non-aqueous electrolyte secondary battery. The positive electrode includes a positive electrode mixture layer. The positive electrode mixture layer includes a positive electrode active material, a conductive material, and a binder. The conductive material includes both single-layer carbon nanotubes and multi-layer carbon nanotubes. The binder includes at least one selected from the group consisting of nitrile-containing rubber and cellulose derivatives.

[0020] The binder may also contain both a nitrile-containing rubber and a cellulose derivative. Alternatively, the binder may contain a nitrile-containing rubber and not contain a cellulose derivative. Alternatively, the binder may contain a cellulose derivative (e.g., ethyl cellulose) and not contain a nitrile-containing rubber.

[0021] Carbon nanotubes have high conductivity and are easy to aggregate. Therefore, it is sometimes difficult to reduce the internal resistance of the positive electrode even with the use of carbon nanotubes. As a result of research, the inventors of the present application found that by combining two kinds of carbon nanotubes with nitrile-containing rubber and / or cellulose derivatives, the internal resistance of the positive electrode can be greatly reduced. The present invention is based on this new insight.

[0022] The reason why the internal resistance of the positive electrode is reduced by the above combination is still unclear. The physical properties of single-layer carbon nanotubes and multi-layer carbon nanotubes, such as the ease of aggregation, are different. Therefore, there is the possibility that the dispersibility of carbon nanotubes becomes higher due to the synergistic effect brought about by combining carbon nanotubes with different physical properties with rubber and / or cellulose derivatives containing nitrile groups.

[0023] (Positive electrode active material)

[0024] The positive electrode active material can use a substance that can absorb and release lithium ions. Examples of positive electrode active materials include composite oxides containing lithium and transition metals. The composite oxide can have a layered structure (e.g., a rock salt type crystal structure). As the positive electrode active material, a composite oxide represented by the above composition formula can be used.

[0025] In the positive electrode active material, the proportion of cobalt in the elements other than lithium and oxygen (hereinafter sometimes referred to as "cobalt ratio") can be 10 atomic % or less. Cobalt is expensive and has low supply stability, so it is preferred to reduce the cobalt ratio. However, when the cobalt ratio is reduced, the internal resistance of the positive electrode may increase. In the positive electrode of this embodiment, the internal resistance of the positive electrode can be reduced by adopting the above-mentioned structure. Therefore, the performance of the positive electrode can be maintained and the cobalt ratio can be reduced. The cobalt ratio can be less than 8 atomic % or less than 5 atomic %.

[0026] The positive electrode active material optionally comprises a composition formula of Li y Ni x M (1-x) O 2-δ (wherein, 0.8≤x≤1, 0<y≤1.2, 0≤δ≤0.05, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca and B.) A composite oxide represented by.

[0027] In the above composition formula, M can be at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, Si, Nb, Zr, Mo, Zn, W and B. Or M can also be at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca, and B. M preferably contains at least one element selected from the group consisting of Co, Mn, Al and Fe. It should be noted that in the above composition formula, the value y representing the composition ratio of lithium increases and decreases by charging and discharging. Specific examples of composite oxides include lithium-nickel-cobalt-aluminum composite oxides (LiNi 0.9 Co 0.05 Al 0.05 O2, etc.)

[0028] In the composition formula of the composite oxide, by setting x to 0.8 or more, the battery capacity can be increased. In the composition formula, 0.85≤x≤1 may also be satisfied. By setting x to 0.85 or more (for example, 0.9 or more), the battery capacity can be particularly increased.

[0029] The composite oxide is usually used in the form of particles. The average particle size of the composite oxide as a whole may be 1 μm or more, 5 μm or more, or 20 μm or less, 15 μm or less, 10 μm or less, 6 μm or less, or 5 μm or less.

[0030] In this specification, unless otherwise specified, the average particle size is the median particle size (D50) at which the cumulative volume becomes 50% in the volume-based particle size distribution. The median particle size can be obtained using a laser diffraction / scattering particle size distribution measuring device. In addition, by observing the cross section of the positive electrode mixture, the particle size of the particles contained in the positive electrode mixture can also be evaluated.

[0031] The positive electrode active material may also include particles of the above-mentioned composite oxide having an average particle size of 1 μm or more and 6 μm or less. When the particle size of the composite oxide is large, the particles are easily broken during charging. When the particles are broken, it is easy to cause the generation of gas from the grain boundary and the dissolution of the metal from the grain boundary, and the durability of the battery is reduced. Therefore, the positive electrode active material preferably includes particles with a small particle size.

[0032] The positive electrode active material may include: a first particle having an average particle size of 1 μm or more and 6 μm or less, and a second particle of the above-mentioned composite oxide having an average particle size of 8 μm or more and 20 μm or less. In this case, a peak in the range of about 1 to 6 μm (particle size) and a peak in the range of about 8 to 20 μm (particle size) may appear in the particle size distribution curve (volume basis) of the entire composite oxide particle. By using two kinds of active material particles with different average particle sizes, the capacity and durability of the battery can be improved.

[0033] When the positive electrode active material includes the first particles and the second particles, the ratio R1 of the mass M1 of the first particles in the above-mentioned composite oxide particles may be in the range of 10 to 40 mass % (e.g., in the range of 15 to 30 mass %). The ratio R2 of the mass M2 of the second particles in the above-mentioned composite oxide particles may also be in the range of 60 to 90 mass % (e.g., in the range of 70 to 85 mass %).

[0034] The content of elements constituting the composite oxide can be measured by an inductively coupled plasma atomic emission spectroscopy (ICP-AES), an electron probe microanalyzer (EPMA), or an energy dispersive X-ray spectroscopy (EDX).

[0035] (Carbon Nanotubes)

[0036] The positive electrode mixture layer includes both single-layer carbon nanotubes (SWCNT) and multi-layer carbon nanotubes (MWCNT). Multi-layer carbon nanotubes are carbon nanotubes with two or more layers. Examples of multi-layer carbon nanotubes include double-layer carbon nanotubes, triple-layer carbon nanotubes, and carbon nanotubes with four or more layers. The positive electrode mixture layer optionally includes multiple multi-layer carbon nanotubes with different numbers of layers.

[0037] In the positive electrode mixture layer, the amount of the carbon nanotubes may be 0.01 parts by mass or more and 1 part by mass or less, or 0.02 parts by mass or more and 0.5 parts by mass or less, relative to 100 parts by mass of the positive electrode active material.

[0038] The proportion of single-walled carbon nanotubes in the carbon nanotubes may be in the range of 0.1 to 20 mass % (e.g., 1 to 5 mass %). It is preferred that the proportion of single-walled carbon nanotubes in the carbon nanotubes is smaller than that of multi-walled carbon nanotubes from the viewpoint of forming conductive paths in the mixture layer.

[0039] The average length of the carbon nanotubes can be more than 1 μm. In this case, the aspect ratio of the carbon nanotubes (the ratio of the length of the fiber to the diameter) becomes extremely large. Carbon nanotubes with a large aspect ratio become easy to contact the positive electrode active material and the collector linearly. Furthermore, the conductivity of the carbon nanotubes is excellent. Therefore, by using carbon nanotubes, the direct current resistance (DCR) of the battery can be greatly reduced. For the carbon nanotubes present in the positive electrode, sometimes a plurality of carbon nanotubes exist in the positive electrode mixture layer in a bundle state. In this case, the length and diameter of the carbon nanotubes refer to the length and diameter of a single carbon nanotube present in the carbon nanotubes that form a bundle.

[0040] From the viewpoint of improving the conductivity of the composite layer, the average length of the carbon nanotubes is preferably 1 μm or more. On the other hand, there is no particular upper limit on the length of the carbon nanotubes, but the length of the carbon nanotubes is preferably not extremely large compared to the particle size of the positive electrode active material. The average length of the carbon nanotubes may be 1 μm or more or 5 μm or less, or 20 μm or less or 10 μm or less.

[0041] The average length of the carbon nanotubes can be obtained by image analysis using a scanning electron microscope (SEM). The average length of the carbon nanotubes can be obtained by randomly selecting 100 carbon nanotubes and measuring their lengths, and then calculating the arithmetic average of these. In addition, the length refers to the length of the carbon nanotubes when they are extended in a straight line.

[0042] The average diameter of the carbon nanotubes can be 20 nm or less, 15 nm or more, or 1 nm or more. By setting the average diameter to 20 nm or less, a high effect can be obtained with a small amount. The average diameter of the single-walled carbon nanotubes and the average diameter of the double-walled carbon nanotubes can be, for example, 5 nm or less, respectively.

[0043] The average diameter of carbon nanotubes is obtained by image analysis using a transmission electron microscope (TEM). The average diameter of carbon nanotubes can be measured by the following method. First, 100 carbon nanotubes are randomly selected and the diameter (outer diameter) of each of them is measured at any one point. Then, the measured diameters are arithmetic averaged to obtain the average diameter.

[0044] The positive electrode mixture layer may also include carbon materials (conductive materials) other than carbon nanotubes. Examples of such carbon materials include conductive carbon particles such as carbon black, and other conductive carbon materials. The proportion of carbon nanotubes in the total carbon material (conductive material) is, for example, 50% by mass or more, preferably in the range of 80 to 100% by mass (e.g., in the range of 90 to 100% by mass).

[0045] The BET specific surface area of ​​at least one carbon nanotube selected from the group consisting of single-walled carbon nanotubes and multi-walled carbon nanotubes may be 200 m 2 / g or more. With such a structure, it is easy to ensure the contact between the carbon nanotubes and the active material, and it is easy to achieve the effect of reducing resistance. The specific surface area of ​​the carbon nanotubes (single-layer carbon nanotubes and / or multi-layer carbon nanotubes) can be 100m 2 / g or more, 200m 2 / g or more, 210m 2 / g or above, or 400m 2 There is no particular upper limit on the BET specific surface area, and it may be 2000 m 2 / g or less. The specific surface area of ​​the entire carbon nanotube can be within the above range. The BET specific surface area of ​​carbon nanotubes can be measured by the BET method (nitrogen adsorption method) described in JIS (Japanese Industrial Standard) R1626. In addition, the BET specific surface area of ​​carbon nanotubes is usually correlated with the fiber diameter and fiber length. Specifically, if the fiber diameter is 10nm and the fiber length is 1μm, the BET specific surface area is 200m 2 / g and above and 250m 2 Therefore, even in the state of a battery, the BET specific surface area of ​​carbon nanotubes can be calculated with high accuracy.

[0046] (Cellulose derivatives)

[0047] In the positive electrode mixture layer, the cellulose derivative acts as a binder. In addition, the cellulose derivative also acts as a dispersant. Examples of cellulose derivatives include alkyl cellulose, hydroxyalkyl cellulose, carboxyalkyl cellulose and their salts (alkali metal salts, ammonium salts, etc.). Examples of alkyl cellulose include methyl cellulose, ethyl cellulose, ethyl methyl cellulose, etc. Examples of hydroxyalkyl cellulose include hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, etc. Examples of carboxyalkyl cellulose include carboxymethyl cellulose, carboxyethyl cellulose, etc. Examples of alkali metals that form alkali metal salts include potassium and sodium, etc. Among them, methyl cellulose, ethyl cellulose, and hydroxypropyl methyl cellulose are preferred. The cellulose derivative may be ethyl cellulose. The weight average molecular weight of the cellulose derivative may be in the range of 1000 to 1000000 (for example, in the range of 10000 to 1000000). Based on the viewpoint of the improved effect brought about by the configuration disclosed in the present invention, the weight average molecular weight of the cellulose derivative may be in the range of 10000 to 200000.

[0048] In the positive electrode mixture layer, the amount of the cellulose derivative per 100 parts by mass of the positive electrode active material may be 0.0001 parts by mass or more, or 0.001 parts by mass or more, or 2 parts by mass or less, or 1 part by mass or less.

[0049] In the positive electrode mixture layer, the ratio Wca / Wce of the mass Wca of the carbon nanotubes to the mass Wce of the cellulose derivative may be 1 or more, 2 or more, or 10 or less.

[0050] (Rubber containing nitrile group)

[0051] The positive electrode mixture layer may also contain a rubber containing a nitrile group. By using a rubber containing a nitrile group, the dispersibility of the carbon nanotubes can be significantly improved, and the internal resistance of the positive electrode can be reduced. In addition, the rubber containing a nitrile group acts as a binding material in the positive electrode mixture layer. The rubber containing a nitrile group contains a nitrile group. Examples of rubber containing a nitrile group include: copolymers containing monomers of acrylonitrile and a diene (such as butadiene). Specifically, examples of rubber containing a nitrile group include acrylonitrile rubber (NBR), hydrogenated nitrile rubber (H-NBR), and modified bodies thereof. The weight average molecular weight of the rubber containing a nitrile group may be in the range of 40,000 to 5,000,000.

[0052] In the positive electrode mixture layer, the amount of the nitrile group-containing rubber per 100 parts by mass of the positive electrode active material may be 0.01 parts by mass or more, or 0.05 parts by mass or more, or 1 part by mass or less, or 0.5 parts by mass or less.

[0053] The positive electrode mixture layer may contain components or compounds other than those described above. For example, the binder may contain a polyvinyl pyrrolidone-based polymer.

[0054] (Polyvinyl pyrrolidone polymer)

[0055] The polyvinyl pyrrolidone polymer is at least one selected from the group consisting of polyvinyl pyrrolidone and polyvinyl pyrrolidone derivatives. Examples of polyvinyl pyrrolidone derivatives include polymers obtained by replacing hydrogen atoms of polyvinyl pyrrolidone with other substituents, such as alkylated polyvinyl pyrrolidone. As the polyvinyl pyrrolidone polymer, only polyvinyl pyrrolidone can be used, or a copolymer of vinyl pyrrolidone and other monomolecules can be used. As other monomolecules, monomolecules of styrene and vinyl acetate can be listed.

[0056] The weight average molecular weight of the polyvinyl pyrrolidone polymer may be in the range of 1000 to 2000000. From the viewpoint of improving the effect of the configuration of the present disclosure, the weight average molecular weight of the polyvinyl pyrrolidone polymer may be in the range of 5000 to 1000000.

[0057] When the positive electrode mixture layer contains a cellulose derivative and a polyvinyl pyrrolidone polymer, the amount of the cellulose derivative relative to 100 parts by mass of the polyvinyl pyrrolidone polymer is preferably in the range of 30 to 400 parts by mass (e.g., 100 to 400 parts by mass, 300 to 400 parts by mass). By setting the amount in the range of 30 to 400 parts by mass, a particularly high effect can be obtained.

[0058] The proportion of the positive electrode active material in the positive electrode mixture layer is determined using a mixture sample. The mixture sample is obtained according to the following steps. First, the secondary battery in the discharged state is disassembled to take out the positive electrode. Next, the positive electrode is cleaned with an organic solvent and then vacuum dried. After that, only the positive electrode mixture layer is taken out, and the taken out positive electrode mixture layer is used as a mixture sample. By performing TG-DTA, NMR and other analyses on the mixture sample, the ratio of the binder and the conductive material other than the positive electrode active material can be calculated. In the case where the conductive material contains a plurality of carbon materials, the proportion of carbon nanotubes in the conductive material can be calculated by performing micro-Raman spectroscopy analysis on the cross section of the positive electrode mixture layer.

[0059] Positive electrode mixture layer (1 layer) per 1m 2 The mass of the positive electrode plate can be 200 g or more, preferably 250 g or more. By setting the mass to 250 g or more, a high capacity can be achieved as a lithium ion battery. As described above, by using the positive electrode plate disclosed in the present invention, the disadvantages caused by increasing the mass can be suppressed. The mass can be increased by thickening the positive electrode mixture layer or increasing the density of the positive electrode mixture layer.

[0060] The thickness of the positive electrode mixture layer is not particularly limited, and may be in the range of 50 μm to 250 μm. According to this embodiment, even if the positive electrode mixture layer is thickened, an increase in internal resistance can be suppressed.

[0061] (Positive electrode slurry and method for producing positive electrode)

[0062] The positive electrode slurry of this embodiment is a slurry for the positive electrode of a non-aqueous electrolyte secondary battery. This slurry is used to manufacture the above-mentioned positive electrode. Since the matters described for the positive electrode are applicable to the positive electrode slurry, repeated descriptions may be omitted.

[0063] The positive electrode slurry contains the components of the above-mentioned positive electrode mixture layer and a liquid medium (dispersion medium) in which they are dispersed. Specifically, the positive electrode slurry contains a positive electrode active material, a conductive material and a binder. The conductive material includes both single-layer carbon nanotubes and multi-layer carbon nanotubes. The binder includes at least one selected from the group consisting of nitrile-containing rubber and cellulose derivatives. For each component, as described above, repeated descriptions are omitted. The positive electrode slurry can further include any component as needed. For example, the positive electrode slurry optionally further includes a polyvinyl pyrrolidone-based polymer. The binder can include both nitrile-containing rubber and cellulose derivatives, or only any one of them.

[0064] There is no particular limitation on the liquid medium (dispersion medium), and water, an organic solvent, and a mixed solvent thereof may be used. Examples of organic solvents include alcohols (ethanol, etc.), ethers (tetrahydrofuran, etc.), amides (dimethylformamide, etc.), N-methyl-2-pyrrolidone (NMP), etc.

[0065] The ratio of the components contained in the positive electrode slurry is reflected in the ratio of the components in the positive electrode mixture layer in principle. Therefore, by changing the ratio of the components contained in the positive electrode slurry, the ratio of the components in the positive electrode mixture layer can be changed. The ratio of the components exemplified for the positive electrode mixture layer can also be applied to the ratio of the components in the positive electrode slurry.

[0066] There is no particular limitation on the method for manufacturing the positive electrode, and a known method can be used. For example, the positive electrode can be formed by the following method. First, prepare a positive electrode slurry obtained by dispersing the materials of the positive electrode mixture layer (positive electrode active material, conductive material, binder and other arbitrary components as needed) in a liquid medium. Then, after the positive electrode slurry is applied to the surface of the positive electrode collector to form a coating, the coating is dried to form a positive electrode mixture layer. The dried coating can also be rolled as needed. The positive electrode mixture layer can be formed on one surface of the positive electrode collector or on two surfaces.

[0067] (Conductive material dispersion)

[0068] The conductive material dispersion of this embodiment can be applied to the preparation of positive electrode slurry. The conductive material dispersion contains: at least one selected from the group consisting of nitrile-containing rubber and cellulose derivatives, a conductive material and a liquid medium (dispersion medium). The conductive material includes both single-layer carbon nanotubes and multi-layer carbon nanotubes. As described above, the conductive material, nitrile-containing rubber and cellulose derivatives are omitted. The conductive material dispersion may also contain any component contained in the positive electrode mixture layer (for example, any component as described above). For example, the conductive material dispersion may further contain a polyvinyl pyrrolidone-based polymer. The conductive material dispersion may contain both nitrile-containing rubber and cellulose derivatives, or may contain only any one of them.

[0069] The liquid medium of the conductive material dispersion may be the liquid medium described as the liquid medium of the positive electrode slurry, or a liquid medium different therefrom may be used. The conductive material dispersion does not substantially contain a positive electrode active material. By adding a positive electrode active material to the conductive material dispersion, a positive electrode slurry can be prepared. It is also possible to add any component of the positive electrode mixture layer and a liquid medium to the conductive material dispersion together with the positive electrode active material.

[0070] By changing the ratio of the components contained in the conductive material dispersion, the ratio of the components in the positive electrode mixture layer can be changed. The ratio of the components exemplified for the positive electrode mixture layer can be applied to the ratio of the components in the conductive material dispersion. Specifically, the ratio between the components can be obtained from the ratio of each component relative to 100 parts by mass of the positive electrode active material.

[0071] (Non-aqueous electrolyte secondary battery)

[0072] The non-aqueous electrolyte secondary battery of the present embodiment includes the positive electrode of the present embodiment. The secondary battery at least further includes a negative electrode and a non-aqueous electrolyte on the basis of the positive electrode. The secondary battery may also include a positive electrode, a negative electrode, a non-aqueous electrolyte, a separator and an outer shell. Examples of the secondary battery include lithium ion secondary batteries, lithium metal secondary batteries, etc. There are no particular limitations on the constituent elements other than the positive electrode mixture layer, and known constituent elements may also be used. Examples of the constituent elements of the secondary battery are described below.

[0073] (positive electrode)

[0074] As the positive electrode, the positive electrode of this embodiment can be used.

[0075] (Positive electrode collector)

[0076] The positive electrode may include a positive electrode collector. The positive electrode mixture layer may be disposed on the positive electrode collector. The shape and thickness of the positive electrode collector may be selected according to the application, and may be selected in a manner corresponding to the shape and thickness of the negative electrode collector. Examples of the material of the positive electrode collector include stainless steel, aluminum, aluminum alloy, titanium, etc. The positive electrode mixture layer may be formed only on one side of the positive electrode collector, or may be formed on both sides of the positive electrode collector.

[0077] (negative electrode)

[0078] The negative electrode typically includes a negative electrode mixture layer containing a negative electrode active material. The negative electrode may include a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. In the case of a lithium metal secondary battery, the negative electrode may use a negative electrode current collector on which lithium metal or a lithium alloy can be precipitated.

[0079] The negative electrode mixture layer contains a negative electrode active material as an essential component. The negative electrode mixture layer may also contain a binder, a thickener, a conductive material, etc. as an optional component. As these optional components, the components exemplified as positive electrode components may also be used.

[0080] The negative electrode mixture layer can be formed by applying the negative electrode slurry obtained by dispersing the constituent components of the negative electrode mixture layer in a liquid medium (dispersion medium) to the surface of the negative electrode collector and drying it. The dried coating film can also be rolled as needed. As the liquid medium, the liquid medium exemplified as the liquid medium of the positive electrode slurry can also be used. The negative electrode mixture layer can be formed only on one side of the negative electrode collector, or on both sides of the negative electrode collector.

[0081] (Negative electrode active material)

[0082] The negative electrode active material can be selected according to the type of secondary battery. An example of a negative electrode active material is a material that can adsorb and release lithium ions. Examples of such materials include carbon materials, Si-containing materials, etc. The negative electrode active material can include Si-containing materials, or it can be Si-containing materials. As the negative electrode active material, metallic lithium, lithium alloys, etc. can also be used. The negative electrode can include one negative electrode active material, or it can include a combination of two or more negative electrode active materials.

[0083] Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and difficultly graphitized carbon (hard carbon). One carbon material may be used alone or in combination of two or more. Graphite is preferred from the viewpoint of excellent charge and discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0084] Examples of Si-containing materials include Si monomers, 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 Particles. x is, for example, 0.5≤x<2, or 0.8≤x≤1.6. As the lithium ion conductive phase, at least one selected from the group consisting of SiO2 phase, silicate phase, and carbon phase can be used.

[0085] The negative electrode current collector may be made of metal foil. The negative electrode current collector may be porous. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0086] (Non-aqueous electrolyte)

[0087] The non-aqueous electrolyte (non-aqueous electrolytic solution) contains a solvent (non-aqueous solvent) and a solute dissolved in the solvent. Examples of the solute include lithium salts. Various additives may be added to the electrolytic solution.

[0088] As the solvent, known materials can be used. As the solvent, for example, cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, etc. can be used. As examples of cyclic carbonates, propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethylene carbonate (VC), etc. can be listed. As chain carbonates, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. are included. As examples of cyclic carboxylates, γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. are included. As examples of chain carboxylates, non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP), etc. are included. The non-aqueous solvent can be used alone or in combination of two or more.

[0089] Examples of lithium salts include lithium salts containing chloric acid (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing amides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salts may be used alone or in combination of two or more.

[0090] The concentration of the lithium salt in the electrolyte may be 1 mol / L to 2 mol / L, or 1 mol / L to 1.5 mol / L. By setting the concentration of the lithium salt within the above range, an electrolyte having excellent ion conductivity and moderate viscosity can be obtained.

[0091] The electrolyte solution may contain a known additive, and examples of the additive include 1,3-propane sultone, toluenesulfonic acid, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, and the like.

[0092] (Separator)

[0093] The separator is arranged between the positive electrode and the negative electrode. The separator preferably has high ion permeability and has appropriate mechanical strength and insulation. As the separator, a microporous film, a woven fabric, a non-woven fabric, etc. can be used. Examples of separator materials include polyolefins (polypropylene, polyethylene, etc.) and other resins.

[0094] (Outer shell)

[0095] The outer shell (battery shell) contains an electrode group and a non-aqueous electrolyte. There is no particular limitation on the outer shell, and a known outer shell can be used. The electrode group is composed of a positive electrode, a negative electrode and a separator. There is no particular limitation on the composition of the electrode group, and it can be a winding type or a stacked type. The wound electrode group is formed by winding the positive electrode and the negative electrode with a separator between them. The stacked electrode group is formed by stacking the positive electrode and the negative electrode with a separator between them. There is no particular limitation on the shape of the non-aqueous electrolyte secondary battery, and it can be cylindrical, square, coin-shaped, button-shaped, laminated, etc.

[0096] Figure 1 This is a schematic perspective view in which a portion of the secondary battery 10 according to one embodiment of the present disclosure is cut away. Figure 1 2 shows a square non-aqueous electrolyte battery as an example. Figure 1 The secondary battery 10 shown includes a battery case 4 in the shape of a square tube with a bottom, and an electrode group 1 and a non-aqueous electrolyte (not shown) housed in the battery case 4 .

[0097] The electrode group 1 includes: a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator arranged therebetween. The negative electrode collector of the negative electrode is electrically connected to the negative terminal 6 provided on the sealing plate 5 via the negative electrode lead 3. The resin gasket 7 insulates the negative terminal 6 from the sealing plate 5. The positive electrode collector of the positive electrode is electrically connected to the back of the sealing plate 5 via the positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4 which also serves as the positive terminal. The periphery of the sealing plate 5 is fitted with the open end of the battery case 4, and the fitting portion is laser welded. The sealing plate 5 has an injection hole for a non-aqueous electrolyte. The injection hole is plugged by a plug 8 after the non-aqueous electrolyte is injected.

[0098] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. As the positive electrode mixture layer, the above-mentioned positive electrode mixture layer is used.

[0099] (Note)

[0100] The above description discloses the following technology.

[0101] (Technology 1)

[0102] A positive electrode for a non-aqueous electrolyte secondary battery,

[0103] It includes a positive electrode mixture layer,

[0104] The positive electrode mixture layer comprises a positive electrode active material, a conductive material and a binder.

[0105] The conductive material includes both single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0106] The binder includes at least one selected from the group consisting of nitrile group-containing rubber and cellulose derivatives.

[0107] (Technique 2)

[0108] The positive electrode according to technology 1, wherein the cellulose derivative is ethyl cellulose.

[0109] (Technique 3)

[0110] The positive electrode according to technology 1 or 2, wherein the binder contains both the nitrile group-containing rubber and the cellulose derivative.

[0111] (Technique 4)

[0112] The positive electrode according to any one of Techniques 1 to 3, wherein the binder further includes a polyvinyl pyrrolidone-based polymer.

[0113] (Technique 5)

[0114] The positive electrode according to any one of techniques 1 to 4, wherein in the positive electrode active material, a ratio of cobalt to elements other than lithium and oxygen is 10 atomic % or less.

[0115] (Technique 6)

[0116] The positive electrode according to any one of techniques 1 to 5, wherein the positive electrode active material comprises a composition formula of Li y Ni x M (1-x) O 2-δ The composite oxide represented by the formula, wherein 0.8≤x≤1, 0<y≤1.2, 0≤δ≤0.05, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca and B.

[0117] (Technique 7)

[0118] The positive electrode according to any one of techniques 1 to 6, wherein the BET specific surface area of ​​at least one carbon nanotube selected from the group consisting of the single-walled carbon nanotube and the multi-walled carbon nanotube is 200 m 2 / g or above.

[0119] (Technology 8)

[0120] A non-aqueous electrolyte secondary battery comprising the positive electrode according to any one of techniques 1 to 7.

[0121] (Technique 9)

[0122] A conductive material dispersion comprising:

[0123] at least one selected from the group consisting of nitrile-containing rubber and cellulose derivatives, a conductive material, and a liquid medium,

[0124] The conductive material includes both single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0125] (Technology 10)

[0126] The conductive material dispersion according to technology 9, wherein the cellulose derivative is ethyl cellulose.

[0127] (Technology 11)

[0128] The conductive material dispersion according to technology 9 or 10, further comprising a polyvinyl pyrrolidone-based polymer.

[0129] Example

[0130] Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to the following examples. In this example, a plurality of nonaqueous electrolyte secondary batteries having different positive electrodes were prepared and evaluated.

[0131] (Production of Battery A1)

[0132] Battery A1 was produced in the following manner.

[0133] (1) Preparation of negative electrode

[0134] The silicon composite material and graphite are mixed in a mass ratio of silicon composite material: graphite = 5:95 and used as a negative electrode active material. The negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene butadiene rubber (SBR) and water are mixed in a specified mass ratio to prepare a negative electrode slurry. Next, the negative electrode slurry is applied to the surface of the copper foil (negative electrode collector) to form a laminate comprising copper foil and a coating formed on the copper foil. Then, after the coating is dried, the laminate is rolled. Thus, a negative electrode comprising a copper foil and a negative electrode mixture layer formed on both sides of the copper foil is formed.

[0135] (2) Preparation of positive electrode

[0136] First, the positive electrode active material, carbon nanotubes (conductive material), ethyl cellulose (cellulose derivative), hydrogenated nitrile rubber (rubber containing nitrile groups), polyvinyl pyrrolidone (PVP, and N-methyl-2-pyrrolidone (liquid medium) are mixed in a predetermined mass ratio to prepare a positive electrode slurry SA1. As the positive electrode active material, LiNi 0.90 Co 0.04 Mn 0.06As the conductive material, single-walled carbon nanotubes and multi-walled carbon nanotubes were mixed and used. The ratio of the single-walled carbon nanotubes and the specific surface area of ​​the carbon nanotubes are shown in Table 1.

[0137] Next, the positive electrode slurry is applied to the surface of the aluminum foil (positive electrode current collector) to form a coating film, thereby obtaining a laminate of the aluminum foil and the coating film. Next, after the coating film is dried, the laminate is rolled. In this way, a positive electrode PA1 including an aluminum foil and a positive electrode mixture layer formed on both sides of the aluminum foil is produced.

[0138] (3) Preparation of electrolyte (non-aqueous electrolyte)

[0139] The electrolyte solution was prepared by adding LiPF6 (lithium salt) to a non-aqueous solvent. The concentration of LiPF6 in the electrolyte solution was set to 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.

[0140] (4) Production of secondary batteries

[0141] Leads are installed on the positive electrode and negative electrode respectively. Next, the positive electrode, the negative electrode and the separator are wound into a spiral shape in a manner that a separator is arranged between the positive electrode and the negative electrode, thereby preparing an electrode group. Next, the electrode group is inserted into the outer casing. As the outer casing, an outer casing made of a laminated film containing an aluminum foil (barrier layer) is used. Next, after the outer casing with the electrode group inserted is vacuum dried at 105°C for 2 hours, a non-aqueous electrolyte is injected into the outer casing, and the opening of the outer casing is sealed. In this way, a secondary battery A1 is produced.

[0142] (Production of Batteries A2 to A7 and Batteries C1 to C6)

[0143] The positive electrode slurries SA2 to SA7 and positive electrode slurries SC1 to SC6 were prepared by the same method and conditions as those for preparing the positive electrode slurry SA1 of battery A1, except that the components contained in the positive electrode slurry were changed as shown in Table 1. As a composite oxide constituting the positive electrode active material, a composite oxide having a composition formula of Li y Ni x M (1-x) O2 represents a composite oxide in which the element M is Co and Mn. Using these positive electrode slurries, positive electrodes PA2 to PA7 and positive electrodes PC1 to PC6 were produced using the same method and conditions as the method and conditions for producing positive electrode PA1 of battery A1. Using these positive electrodes, batteries A2 to A7 and batteries C1 to C6 were produced using the same method and conditions as the method and conditions for producing battery A1.

[0144] (Battery Evaluation)

[0145] The internal resistance of the manufactured battery was measured.

[0146] Part of the manufacturing conditions and the evaluation results are shown in Table 1. It should be noted that the internal resistance of the battery is expressed as a relative value when the measured value of battery C1 is set to 100. In Table 1, "Active material Ni / Co / Mn (composition ratio)" represents the composition ratio of Ni / Co / Mn in the composite oxide. For example, the positive electrode active material of battery A6 has a composition formula of Li y Ni 0.80 Co 0.12 Mn 0.08 In this case, the composition formula Li y Ni x M (1-x) O 2-δ When expressed, x=0.80.

[0147] In Table 1, "the proportion of SWCNT (mass %)" indicates the proportion (mass %) of single-walled carbon nanotubes (SWCNT) in all carbon nanotubes. In Table 1, "specific surface area of ​​CNT" indicates the specific surface area of ​​the entire carbon nanotubes.

[0148] [Table 1]

[0149]

[0150] Batteries A1 to A7, and the positive electrode slurries and positive electrodes used in their production are the batteries, positive electrode slurries, and positive electrodes of the present embodiment. Batteries C1 to C6, and the positive electrode slurries and positive electrodes used in their production are comparative examples.

[0151] As shown in Table 1, the internal resistance of the battery using the positive electrode of this embodiment is low. This is because the internal resistance of the positive electrode of this embodiment is low.

[0152] Industrial Applicability

[0153] The present disclosure can be used for a positive electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery. The secondary battery of the present disclosure can be used for various applications, and is preferably used as a main power source for mobile communication equipment, portable electronic equipment, and the like.

[0154] The present invention has been described with respect to the currently preferred embodiments, but such disclosure is not interpreted as limiting. By reading the above disclosure, various modifications and changes should undoubtedly be obvious to those skilled in the art in the technical field of the present invention. Therefore, it should be interpreted that the attached claims include all modifications and changes without departing from the true spirit and scope of the present invention.

[0155] Description of Reference Numerals

[0156] 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 (non-aqueous electrolyte secondary battery)

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, It includes a positive electrode mixture layer, The positive electrode mixture layer comprises a positive electrode active material, a conductive material and a binder. The conductive material includes both single-walled carbon nanotubes and multi-walled carbon nanotubes. The binder includes at least one selected from the group consisting of nitrile group-containing rubber and cellulose derivatives.

2. The positive electrode according to claim 1, wherein The cellulose derivative is ethyl cellulose.

3. The positive electrode according to claim 1 or 2, wherein The binder contains both the nitrile group-containing rubber and the cellulose derivative.

4. The positive electrode according to claim 1 or 2, wherein The binder further includes a polyvinyl pyrrolidone-based polymer.

5. The positive electrode according to claim 1 or 2, wherein In the positive electrode active material, the ratio of cobalt to elements other than lithium and oxygen is 10 atomic % or less.

6. The positive electrode according to claim 1 or 2, wherein The positive electrode active material comprises a composition formula of Li y Ni x M (1-x) O 2-δ The composite oxide represented by the formula, wherein 0.8≤x≤1, 0<y≤1.2, 0≤δ≤0.05, and M contains at least one element selected from the group consisting of Co, Mn, Al, Fe, Ti, Sr, Ca and B.

7. The positive electrode according to claim 1 or 2, wherein The BET specific surface area of ​​at least one carbon nanotube selected from the group consisting of the single-walled carbon nanotube and the multi-walled carbon nanotube is 200 m 2 / g or above. 8 . A non-aqueous electrolyte secondary battery comprising the positive electrode according to claim 1 or 2 .

9. A conductive material dispersion comprising: at least one selected from the group consisting of nitrile-containing rubber and cellulose derivatives, a conductive material, and a liquid medium, The conductive material includes both single-walled carbon nanotubes and multi-walled carbon nanotubes.

10. The conductive material dispersion according to claim 9, wherein The cellulose derivative is ethyl cellulose. 11 . The conductive material dispersion according to claim 9 , further comprising a polyvinyl pyrrolidone-based polymer.

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