Positive electrode for nonaqueous electrolyte secondary battery, nonaqueous electrolyte secondary battery using same, and positive electrode slurry for positive electrode of nonaqueous electrolyte secondary battery

Abstract

The disclosed positive electrode is a positive electrode for a non-aqueous electrolyte secondary battery. The positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer contains active material particles having an average particle diameter of less than 5 [mu] m, a conductive material, a dispersant, and a binder. The active material particles include composite oxide particles and a surface modification layer formed on the surfaces of the composite oxide particles and containing a boron compound. The composite oxide particles are particles of a lithium transition metal composite oxide. The conductive material contains a carbon material. The dispersant contains nitrile group-containing rubber. The binder contains a fluorine-containing polymer.

Description

Technical Field

[0001] The present disclosure relates to a positive electrode for a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte secondary battery using the positive electrode, and a positive electrode slurry for a positive electrode of a nonaqueous electrolyte secondary battery. Background Art

[0002] Non-aqueous electrolyte secondary batteries have high output and high energy density, and therefore are widely used for consumer and vehicle applications. In recent years, non-aqueous electrolyte secondary batteries have been required to have higher performance. Various proposals have been made for non-aqueous electrolyte secondary batteries.

[0003] Patent Document 1 (Japanese Patent No. 7055476) states in Claim 1 that “a positive electrode comprises a current collector and a positive electrode active material layer disposed on the current collector, wherein the positive electrode active material layer comprises a positive electrode active material, carbon nanotubes, and a binder, wherein the binder comprises polyvinylidene fluoride having a weight average molecular weight of 720,000 to 980,000, and the carbon nanotubes have a BET specific surface area of ​​140 m 2 / g~195m 2 / g, the positive electrode satisfies the following formula 1:

[0004] [Formula 1] 1.3 ≤ B / A ≤ 3.4

[0005] In Formula 1, B is the content (weight %) of the polyvinylidene fluoride in the positive electrode active material layer, and A is the content (weight %) of the carbon nanotubes in the positive electrode active material layer.

[0006] Prior Art Literature

[0007] Patent Document 1: Japanese Patent No. 7055476 Summary of the invention

[0008] Problems to be solved by the invention

[0009] At present, it is required to improve the performance of non-aqueous electrolyte secondary batteries. However, if the capacity as a battery performance is to be improved, sometimes it will cause manufacturing to become difficult. One of the purposes of the present disclosure is to provide a positive electrode (positive electrode for non-aqueous electrolyte secondary batteries) with high capacity and easy manufacturing.

[0010] Means for solving problems

[0011] One aspect of the present disclosure relates to a positive electrode. The positive electrode is a positive electrode for a non-aqueous electrolyte secondary battery, comprising a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode mixture layer comprising active material particles having an average particle size of less than 5 μm, a conductive material, a dispersant, and a binder, the active material particles comprising composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles, the composite oxide particles are particles of a lithium transition metal composite oxide, the conductive material comprises a carbon material, the dispersant comprises a nitrile-containing rubber, and the binder comprises a fluorine-containing polymer.

[0012] 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.

[0013] Another aspect of the present disclosure relates to a positive electrode slurry for a positive electrode of a non-aqueous electrolyte secondary battery. The positive electrode slurry is a positive electrode slurry for a positive electrode of a non-aqueous electrolyte secondary battery, comprising active material particles with an average particle size of less than 5 μm, a conductive material, a dispersant, a binder, and a liquid medium, wherein the active material particles include composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles, the composite oxide particles are particles of lithium transition metal composite oxides, the conductive material contains a carbon material, the dispersant contains a nitrile-containing rubber, and the binder contains a fluorine-containing polymer.

[0014] Effects of the Invention

[0015] According to the present disclosure, a high-capacity and easily manufactured positive electrode (positive electrode for nonaqueous electrolyte secondary battery) can be obtained. In addition, according to the present disclosure, a nonaqueous electrolyte secondary battery using the positive electrode and a positive electrode slurry for manufacturing the positive electrode can be obtained.

[0016] The novel features of the present invention are set forth in the appended claims, both as to structure and content, together with other objects and features of the present invention, and will be better understood from the following detailed description with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic perspective view showing a portion of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure cut away. DETAILED DESCRIPTION

[0018] Hereinafter, the embodiments of the present disclosure will be described with examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​or materials are sometimes exemplified, but other numerical values ​​or materials may also be applied as long as the effects of the present disclosure can be obtained. In this specification, the description of "numerical value A to numerical value B" includes numerical value A and numerical value B, which may be replaced by "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 or conditions are exemplified, as long as the lower limit is not greater than 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 or 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.

[0019] (Positive electrode for non-aqueous electrolyte secondary battery)

[0020] The positive electrode involved in this embodiment is a positive electrode for a non-aqueous electrolyte secondary battery. The positive electrode includes a positive electrode collector and a positive electrode mixture layer arranged on the positive electrode collector. The positive electrode mixture layer contains active material particles (positive electrode active material particles) with an average particle size of less than 5μm, a conductive material, a dispersant and a binder. The active material particles include composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles. The composite oxide particles are particles of lithium transition metal composite oxides. The conductive material contains a carbon material. The dispersant contains a nitrile-containing rubber. The binder contains a fluorine-containing polymer.

[0021] At present, non-aqueous electrolyte secondary batteries are required to have high durability and high capacity. As a method for achieving this requirement, it is known to increase the charging voltage, but if the active material particles are continuously exposed to a high charging voltage, the particles will break, causing side reactions such as gas or metal dissolution from the particle boundary, resulting in deterioration of durability. Since the active material particles with a small particle size can suppress rupture, such degradation behavior can be suppressed, and the durability of the battery can be improved. In addition, if active material particles with a small particle size are used, the active material particles can be efficiently filled in a certain space, and the high capacity of the non-aqueous electrolyte secondary battery can be achieved.

[0022] On the other hand, if the particle size of the active material particles is reduced, the specific surface area of ​​the active material particles in the mixture increases, so there will be problems such as reduced adhesion with the collector, reduced conductivity of the positive electrode mixture, gas generation or capacity reduction caused by side reactions with the electrolyte, and deterioration of durability. If a binder or a conductive material is added to solve these problems, new problems such as reduced battery capacity, deteriorated durability, and difficulty in manufacturing the positive electrode will arise. For example, if a binder or a conductive material is added, the viscosity of the slurry used to manufacture the positive electrode will increase significantly.

[0023] As a result of the research, the inventors of the present application have newly discovered that the problems caused by using active material particles with a small particle size can be solved by combining positive electrode active material particles having a surface modification layer, a specific conductive material, a specific dispersant, and a specific binder. The present disclosure is based on this new finding.

[0024] The reason for the above effect is not yet clear. However, in the positive electrode involved in this embodiment, positive electrode active material particles with a surface modification layer are used. This can avoid the disadvantages caused by small particle size (such as side reactions caused by increased specific surface area).

[0025] In addition, in the positive electrode involved in the present embodiment, a further effect is shown by containing a nitrile-containing rubber. The side reaction that increases due to the increase in the specific surface area of ​​the active material occurs at the interface between the non-aqueous electrolyte and the active material, especially when the active material is in a charged state. In a secondary battery being charged, the potential of the positive electrode active material is not necessarily constant, and the side reaction is promoted around the active material that has a local high potential. In particular, positive electrode active material particles with a surface modification layer tend to have reduced electronic conductivity. Therefore, if a secondary battery is constructed using positive electrode active material particles with a surface modification layer, the potential during charging is prone to deviation. The nitrile-containing rubber has the effect of improving the dispersibility of the conductive material in the mixture and suppressing potential deviation. Therefore, it is possible to suppress side reactions in active materials with a large specific surface area, especially in positive electrode active material particles with a surface modification layer.

[0026] (Positive electrode active material)

[0027] The positive electrode mixture layer contains active material particles (positive electrode active material particles) having an average particle size of less than 5 μm. Hereinafter, the active material particles having an average particle size of less than 5 μm are sometimes referred to as "particles (P1)". Particles (P1) include composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles. The composite oxide particles are particles of lithium transition metal composite oxides. The lithium transition metal composite oxide may have a layered structure (e.g., a rock salt type crystal structure). Examples of lithium transition metal composite oxides will be described later.

[0028] In this specification, unless otherwise specified, the average particle size refers to the median particle size (D50) at which the cumulative volume becomes 50% in the volume-based particle size distribution. The median particle size is obtained using a laser diffraction / scattering particle size distribution measuring device. In addition, the particle size of the particles contained in the positive electrode mixture layer can also be evaluated by observing the cross section of the positive electrode mixture layer.

[0029] In the particle (P1), the molar amount of the boron compound in the surface modification layer may be in the range of 1.0 to 3.0% of the total molar amount of metal elements other than lithium in the lithium transition metal composite oxide. Within this range, side reactions such as gas generation can be suppressed and battery durability can be improved.

[0030] (Conductive materials)

[0031] The conductive material contains a carbon material. The carbon material preferably contains carbon nanotubes. By using carbon nanotubes as the conductive material, the deviation of the potential in the positive electrode can be particularly suppressed, and gas generation can be particularly suppressed. In addition, by using carbon nanotubes, the resistance of the positive electrode mixture layer can be reduced by adding a small amount. The positive electrode mixture layer may contain conductive materials other than carbon nanotubes. Examples of such conductive materials include conductive carbon materials such as graphene, carbon black (acetylene black, Ketjen black, furnace black, etc.), 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 (for example, in the range of 90 to 100% by mass).

[0032] In the positive electrode mixture layer, the amount of the carbon nanotubes may be 0.01 parts by mass or more or 0.04 parts by mass or more, and may be 1 part by mass or less or 0.5 parts by mass or less, relative to 100 parts by mass of the positive electrode active material.

[0033] The average length of carbon nanotubes can be more than 1 μm. In this case, the aspect ratio (ratio of fiber length to diameter) of carbon nanotubes becomes extremely large. Carbon nanotubes with large aspect ratios are easy to contact with positive electrode active materials and collectors in a linear form. In addition, carbon nanotubes have excellent electrical conductivity. Therefore, by using carbon nanotubes, the direct current resistance (DCR) of the battery can be greatly reduced.

[0034] From the viewpoint of improving the conductivity in the composite layer, the average length of the carbon nanotubes is preferably greater than 1 μm. On the other hand, there is no particular upper limit on the length of the carbon nanotubes, and the length of the carbon nanotubes is preferably not excessively greater than the particle size of the positive electrode active material. The average length of the carbon nanotubes may be greater than 1 μm or greater than 5 μm, and may be less than 20 μm or less than 10 μm. There are also cases where the carbon nanotubes present in the positive electrode exist in a bundle state. In the calculation of the above average length, the length of a single carbon nanotube present in the bundled carbon nanotubes is used.

[0035] The average length of the carbon nanotubes is obtained by image analysis using a scanning electron microscope (SEM). The average length of the carbon nanotubes is obtained by randomly selecting 100 carbon nanotubes, measuring their lengths, and averaging them. In addition, the length refers to the length of the carbon nanotubes when stretched into a straight line.

[0036] The average diameter of the carbon nanotubes may be 20 nm or less, or 15 nm or less, or may be 1 nm or more. When the average diameter is 20 nm or less, a high effect can be obtained with a small amount.

[0037] The average diameter of the carbon nanotubes is determined by image analysis using a transmission electron microscope (TEM). The average diameter of the carbon nanotubes can be measured by the following method. First, 100 carbon nanotubes are randomly selected and the diameter (outer diameter) of any one of them is measured. Then, the average diameter is obtained by taking the arithmetic average of the measured diameters.

[0038] The carbon nanotubes may be any of single-layer carbon nanotubes (SWCNT) and multi-layer carbon nanotubes (MWCNT). Examples of multi-layer carbon nanotubes include 2-layer carbon nanotubes, 3-layer carbon nanotubes, and 4 or more layers of carbon nanotubes. The positive electrode mixture layer preferably contains single-layer carbon nanotubes and / or multi-layer carbon nanotubes. The multi-layer carbon nanotubes contained in the positive electrode mixture layer may be one type of multi-layer carbon nanotubes or multiple types of multi-layer carbon nanotubes with different numbers of layers.

[0039] The BET specific surface area of ​​carbon nanotubes can be 200 m 2 / g or more, 250m 2 / g or above or 300m 2 There is no particular upper limit on the BET specific surface area, but it can be 1000 m 2 / g or less. The BET specific surface area is 200m 2 / g or more, even a small amount of addition can suppress the potential deviation of the active material in the mixture layer. The BET specific surface area of ​​carbon nanotubes can be measured by nitrogen adsorption. However, the BET specific surface area of ​​carbon nanotubes is usually related to 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 ​​the carbon nanotubes can be calculated with high accuracy.

[0040] (Dispersant)

[0041] The nitrile group-containing rubber contains a nitrile group. The nitrile group-containing rubber acts as a dispersant. Furthermore, in the positive electrode mixture layer, the nitrile group-containing rubber also acts as a binder. Examples of nitrile group-containing rubbers include copolymers containing monomers of acrylonitrile and diene (such as butadiene), or hydrogenated (hydrogenated) products thereof as raw materials. Specifically, examples of nitrile group-containing rubbers include acrylonitrile rubber (NBR), hydrogenated acrylonitrile rubber (H-NBR), and modified products thereof. The weight average molecular weight of the nitrile group-containing rubber may be in the range of 5,000 to 500,000.

[0042] In the positive electrode mixture layer, the amount of the nitrile group-containing rubber may be 0.01 parts by mass or more, or 0.05 parts by mass or more, or 2 parts by mass or less, or 1 part by mass or less, based on 100 parts by mass of the positive electrode active material.

[0043] (Binder)

[0044] Fluoropolymers function as binders. Fluoropolymers are polymers containing fluorine. Examples of fluorine-containing polymers include vinylidene fluoride polymers. Examples of vinylidene fluoride polymers include polymers of monomers containing vinylidene fluoride. Fluoropolymers can be a combination of vinylidene fluoride polymers and other fluorine-containing polymers. Vinylidene fluoride polymers can be copolymers of vinylidene fluoride and other monomers. Examples of vinylidene fluoride polymers include polyvinylidene fluoride (PVDF). In addition, polytetrafluoroethylene (PTFE) and the like are also present in fluorine-containing polymers, which can obtain the same effect.

[0045] In the positive electrode mixture layer, the amount of the fluorinated polymer may be 0.1 parts by mass or more or 0.5 parts by mass or more, and may be 2.0 parts by mass or less or 1.5 parts by mass or less, based on 100 parts by mass of the positive electrode active material.

[0046] The weight average molecular weight of the fluorine-containing polymer (e.g., vinylidene fluoride polymer) may be 1,000,000 or more, 1,100,000 or more, or 1,200,000 or less, or 2,000,000 or less, or 1,800,000 or less. By setting the weight average molecular weight to 1,000,000 or more, a high effect as a binder can be obtained with a small amount.

[0047] The positive electrode mixture layer may contain ingredients other than the above (e.g., thickeners) or compounds other than the above. For example, the positive electrode mixture layer may contain polyvinyl pyrrolidone, cellulose derivatives (e.g., alkyl cellulose, carboxyalkyl cellulose, and salts thereof), etc. Polymer materials such as polyvinyl pyrrolidone and cellulose derivatives may function as dispersants or binders.

[0048] The proportion of the positive electrode active material in the positive electrode mixture layer is obtained using a mixture sample. The mixture sample is obtained in the following order. First, the secondary battery in the discharged state is disassembled and the positive electrode is taken out. Next, the positive electrode is cleaned with an organic solvent and vacuum dried, and then 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, thermal decomposition GC-MS 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 analyzing the cross section of the positive electrode mixture layer using thermal analysis such as TG-DTA and micro-Raman spectroscopy.

[0049] Positive electrode mixture layer (1 layer) per 1m 2 The mass 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, according to the positive electrode plate involved in the present disclosure, the disadvantages caused by the increase in 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.

[0050] 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 thickness of the positive electrode mixture layer is increased, an increase in internal resistance can be suppressed.

[0051] (Details of positive electrode active material)

[0052] The composite oxide constituting the positive electrode active material may be a material capable of absorbing and releasing lithium ions. Specifically, the composite oxide constituting the positive electrode active material may be a lithium transition metal composite oxide containing lithium and a transition metal. The lithium transition metal composite oxide contained in the particle (P1) may be a material having a composition formula of Li y Ni x M (1-x) O 2-δ (wherein, 0.6≤x≤1, 0<y≤1.2, 0≤δ≤0.05, and M contains at least one element selected from Co, Mn, Al, Fe, Ti, Sr, Ca and B).

[0053] In the above composition formula, M may be at least one element selected from Co, Mn, Al, Fe, Ti, Sr, Ca, Si, Nb, Zr, Mo, Zn and B. Alternatively, M may be at least one element selected from Co, Mn, Al, Fe, Ti, Sr, Ca and B. M preferably contains at least one element selected from Co, Mn, Al and Fe. In addition, in the above composition formula, the value of y representing the composition ratio of lithium increases or decreases according to charge and discharge. Specific examples of composite oxides include lithium-nickel-cobalt-aluminum composite oxides (LiNi 0.9 Co 0.05 Al 0.05 O2, etc.)

[0054] As the positive electrode active material, the battery capacity can be increased by using a high Ni active material (a composite oxide with a high Ni content). For example, in the composition formula of the composite oxide, by setting x to 0.6 or more, the battery capacity can be increased. In this composition formula, 0.8≤x≤1 can be satisfied. By setting x to 0.8 or more, the battery capacity can be particularly increased.

[0055] By using a high Ni active material (a composite oxide with a high Ni content) as a positive electrode active material, the capacity of the secondary battery can be increased. However, the high Ni active material causes a large amount of lithium to be detached during charging, so the crystalline state may become unstable. In particular, it is possible to produce a potential deviation in the active material in the mixture layer. If it becomes a high potential locally, it becomes more unstable, and the side reaction with the non-aqueous electrolyte will also increase. On the other hand, if it is a positive electrode active material disclosed in the present invention having a surface modification layer, these effects can be alleviated.

[0056] In the particle size distribution curve of the entire positive electrode active material particles (e.g., volume basis), the peak from the particle (P1) may appear in the range of particle diameter less than 5 μm on the horizontal axis. Furthermore, the value of the average particle diameter will deviate from the position of the peak, so the peak from the particle (P1) may also appear at a position above 5 μm. For example, the peak from the particle (P1) may appear in the range below 7 μm. The average particle diameter of the particle (P1) may be above 1 μm. The positive electrode mixture layer may contain other positive electrode active material particles having an average particle diameter larger than that of the particle (P1). Hereinafter, the positive electrode active material particle is sometimes referred to as "particle (P2)". In this case, two or more peaks (e.g., two peaks) may appear in the particle size distribution curve of the entire positive electrode active material particles (e.g., volume basis). The average particle diameter of the particle (P2) may be above 8 μm and below 20 μm. For example, the positive electrode active material particles may include particles (P1) having an average particle size of 1 μm or more and less than 5 μm and particles (P2) having an average particle size of 8 μm or more and less than 20 μm. In this case, a peak in the range of about 1 to 5 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 positive electrode active material particles.

[0057] When the positive electrode active material particles are composed only of particles (P1), the average particle size of the positive electrode active material particles as a whole is less than 5 μm. When the positive electrode active material particles contain particles (P1) and particles (P2), the average particle size of the positive electrode active material particles as a whole can be greater than 5 μm. For example, the average particle size of the positive electrode active material particles as a whole can be in the range of 1 μm to 20 μm (for example, in the range of 4 μm to 17 μm).

[0058] The composite oxide particles may be composed of at least one of a single particle and a secondary particle formed by the aggregation of a plurality of single particles. A single particle is a primary particle in which no grain boundary exists inside the particle. A secondary particle is formed by the aggregation of 2 to 1000 single particles. Particle (P1) may be a single particle. Particle (P2) may be a secondary particle.

[0059] By using two kinds of active material particles having different average particle sizes, the filling property of the positive electrode mixture layer into the positive electrode current collector can be improved. When the filling property is improved, the amount of active material in the space increases, thereby improving the capacity of the secondary battery.

[0060] The particles (P2) may use the composite oxide particles described in the particles (P1). The composite oxide particles contained in the particles (P1) and the composite oxide particles contained in the particles (P2) are usually the same, but may be different. The particles (P2), like the particles (P1), may also include a surface modification layer containing a boron compound formed on the surface of the composite oxide particles. Alternatively, the particles (P2) may not contain the surface modification layer.

[0061] The positive electrode active material may be composed only of particles (P1). When the positive electrode active material contains particles (P1) and particles (P2), the ratio between the two is not particularly limited. The proportion of particles (P1) in the positive electrode active material may be in the range of 10 to 50% by mass (e.g., in the range of 15 to 30% by mass), and the proportion of particles (P2) may be in the range of 50 to 90% by mass (e.g., in the range of 70 to 85% by mass). The proportion of positive electrode active material particles having a particle size of less than 5 μm in the total positive electrode active material particles may be 10% by volume or more, 20% by volume or more, 30% by volume or more, 50% by volume or more, or 70% by volume or more, and may be 100% by volume or less, 80% by volume or less, 60% by volume or less, or 40% by volume or less. The proportion of positive electrode active material particles with a particle size less than 5 μm in the total positive electrode active material particles can be greater than 10 mass %, greater than 20 mass %, greater than 30 mass %, greater than 50 mass % or greater than 70 mass %, and can be less than 100 mass %, less than 80 mass %, 60 mass % or less than 40 mass %.

[0062] The content of the 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).

[0063] In the particle (P1), a surface modification layer containing a boron compound is formed on the surface of the composite oxide particle. The boron compound refers to a compound containing B (boron). By forming a surface modification layer containing a boron compound on the surface of the composite oxide particle, the charge and discharge efficiency is significantly improved. It is speculated that the boron compound inhibits the decomposition of the electrolyte and promotes the exchange of Li ions between the non-aqueous electrolyte and the positive electrode active material on the surface of the lithium transition metal composite oxide. When using active material particles with a small average particle size, the internal resistance of the battery tends to increase, so in the particle (P1), the presence of a surface modification layer becomes particularly important. Furthermore, in addition to the lithium transition metal composite oxide, the positive electrode active material may also contain LiF, Li2S, etc.

[0064] The thickness of the surface modification layer formed on the surface of the lithium transition metal composite oxide is preferably less than 100nm. By making the thickness of the surface modification layer less than 100nm, it is easy to achieve high battery capacity. The thickness of the surface modification layer is preferably less than 50nm, more preferably less than 30nm. The lower limit of the thickness of the surface modification layer is, for example, 1nm. The surface modification layer can exist in a point-like manner in the form of at least a portion of the surface of the coated lithium transition metal composite oxide, or it can exist in the form of the entire surface of the coated lithium transition metal composite oxide. At a depth of 30nm from the outermost surface of the surface modification layer, the ratio of the sum of the atomic concentration of the transition metal contained in the lithium transition metal composite oxide is preferably less than 0.6, more preferably less than 0.5. The atomic concentration of each element can be measured by X-ray photoelectron spectroscopy (XPS).

[0065] The boron compound in the surface modification layer is not particularly limited as long as it contains B, and is, for example, boron oxide, boron fluoride, boron chloride, and boron sulfide. The boron compound is preferably boron oxide. Boron oxides include, for example, boric acid (H3BO3), boron oxide (B2O3), and lithium borate (LiBO2, LiB3O5, Li2B4O7). The boron compound present on the surface of the lithium transition metal composite oxide can be confirmed by low-acceleration SEM or TEM-EDX.

[0066] The molar amount of the boron compound in the surface modification layer can be 0.1% or more and less than 7% by mole of the total molar amount of the metal elements other than lithium in the lithium transition metal composite oxide. By setting the molar amount to 0.1% or more, the effect brought by the boron compound becomes more significant. If the molar amount exceeds 7%, the resistance of the battery will increase. As described above, the molar amount can be 0.5% or more or 1.0%, and can be 5.0% or less or 3.0% or less. As described above, the molar amount is preferably in the range of 1.0 to 3.0%.

[0067] The positive electrode active material may contain at least one metal element selected from Ca, Sr, Fe, Cu, Zr, Mg, Si, Cr and Ti. These metal elements may be contained in the composite oxide particles, but are preferably present on the surface of the composite oxide particles. Thus, the side reaction between the composite oxide particles and the electrolyte can be suppressed, and the degradation of the battery can be suppressed. These metal elements may also be included in the surface modification layer together with B. In the positive electrode active material, these metal elements are preferably contained in a range of 0 mol% to 5 mol% (for example, a range of 0.01 mol% to 1 mol%) relative to the total amount of Ni, Mn and Fe. In the positive electrode active material, these metal elements are preferably contained in a range of 0.05 mol% to 0.2 mol% relative to the total amount of Ni, Mn and Fe.

[0068] Below, an example of a method for producing particles (P1) is described. When particles (P2) contain a surface modification layer, the same method as particles (P1) can be used for production. The method for producing particles (P1) includes, for example, a synthesis step, a cleaning step, a drying step, and an addition step. In addition, particles (P1) can also be produced by methods other than the following production methods.

[0069] In the synthesis step, a metal hydroxide containing a metal element constituting the composite oxide particles and a Li compound are mixed and calcined to obtain composite oxide particles (lithium transition metal composite oxide particles).

[0070] The metal hydroxide can be obtained, for example, by stirring a solution of a metal salt containing a metal element constituting the composite oxide particles while dropping an alkaline solution such as sodium hydroxide, adjusting the pH value to the alkaline side (e.g., 8.5 to 12.5), and allowing it to precipitate (coprecipitate). Furthermore, a metal oxide obtained by heat-treating a metal hydroxide can be used instead of the metal hydroxide. The smaller the particle size of the metal hydroxide, the easier it is for the primary particles to grow, so it is preferably 7 μm or less.

[0071] As Li compounds, for example, Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, LiF, etc. can be cited. Regarding the mixing ratio of metal hydroxide and Li compound, from the viewpoint of easily adjusting the above parameters to the above predetermined range, for example, the molar ratio of metal elements other than Li: Li is preferably in the range of 1:0.98 to 1:1.1. Furthermore, when mixing metal hydroxide and Li compound, Ca compound, Sr compound, Fe compound, Cu compound, Zr compound, Mg compound, Si compound, Cr compound, Ti compound, S compound, fluoride, etc. can be added. As Ca compound, for example, CaO, Ca(OH)2 and CaCO3 can be cited. As Sr compound, for example, SrO, Sr(OH)2 and SrCO3 can be cited. As for other elements, oxides, hydroxides, sulfides, and fluorides can also be used. In addition, alkaline earth metals and Zr have a high effect of reducing resistance and are useful.

[0072] A mixture of metal hydroxide and Li compound, etc. is fired, for example, in an oxygen atmosphere (gas with a flowing oxygen concentration of 80% or more). The firing conditions may be a temperature rise rate of 450°C or more and 680°C or less in a range of greater than 1.0°C / min and less than 5.5°C / min, and a maximum temperature reached in a range of 700°C or more and less than 850°C. The temperature rise rate from more than 680°C to the maximum temperature reached may be, for example, 0.1°C / min to 3.5°C / min. In addition, the holding time of the maximum temperature reached may be more than 1 hour and less than 30 hours. In addition, the firing process may be a multi-stage firing, and multiple temperature zones may be set as long as the first temperature rise rate and the second temperature rise rate are within the above-mentioned predetermined range. By adjusting the firing conditions, the particle size of the single particle can be adjusted. For example, by increasing the maximum temperature reached, the particle size of the single particle can be increased.

[0073] In the washing step, the composite oxide particles obtained in the synthesis step are washed with water and dehydrated to obtain a cake-like composition. The washing and dehydration can be carried out by known methods and conditions. These can be carried out as long as lithium is not dissolved from the composite oxide particles and the battery characteristics are not deteriorated. Furthermore, Ca compounds, Sr compounds, Fe compounds, Cu compounds, Zr compounds, Mg compounds, Si compounds, Cr compounds, Ti compounds, S compounds, fluorides, etc. can be added to the cake-like composition.

[0074] In the drying step, the cake-like composition obtained in the washing step is dried to obtain a powdery composition. The drying step can be performed in a vacuum atmosphere. The drying conditions are, for example, 150 to 400° C. and 0.5 to 15 hours.

[0075] In the adding step, a boron-containing compound such as boric acid (H3BO3) is added to the powdered composition obtained in the drying step, and the temperature is raised to 200°C to 400°C. Thus, a surface modification layer containing a boron compound can be formed on the surface of the lithium transition metal composite oxide. The amount of the boron-containing compound added is, for example, 0.1 mol% to 7 mol% relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide.

[0076] As described above, particles (P1) are obtained. The composite oxide particles may be produced by a method other than the above-mentioned synthesis step, and may be produced by a known method. The particles (P1) can be obtained by subjecting the produced composite oxide particles to the above-mentioned addition step.

[0077] (Positive electrode slurry and method for producing positive electrode)

[0078] 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. The matters described about the positive electrode can be applied to the positive electrode slurry, so repeated descriptions are sometimes omitted.

[0079] The positive electrode slurry contains the components of the above-mentioned positive electrode mixture layer and a liquid medium (dispersion medium) for dispersing them. Specifically, the positive electrode slurry contains active material particles (particles (P1)) having an average particle size of less than 5 μm, a conductive material, a dispersant, a binder and a liquid medium. As described above, the active material particles (particles (P1)) contain composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles. The composite oxide particles are particles of lithium transition metal composite oxides. The conductive material contains a carbon material. The dispersant contains a nitrile-containing rubber. The binder contains a fluorine-containing polymer. The above describes each component, so repeated descriptions are omitted. The positive electrode slurry may contain any of the above-mentioned components.

[0080] The liquid medium (dispersion medium) is not particularly limited, and water, an organic solvent, and a mixed solvent thereof can be used. Examples of organic solvents include alcohols (ethanol, etc.), ethers (tetrahydrofuran, etc.), amides (dimethylformamide, etc.), N-methyl-2-pyrrolidone (NMP), etc.

[0081] The ratio of the components contained in the positive electrode slurry is in principle reflected in the ratio of the components in the positive electrode mixture layer. 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 be applied to the ratio of the components in the positive electrode slurry.

[0082] The manufacturing method of the positive electrode is not particularly limited, and a known method can be applied. For example, the positive electrode can be formed by the following method. First, a positive electrode slurry is prepared by dispersing the materials of the positive electrode mixture layer (positive electrode active material, conductive material, dispersant, 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 be rolled as needed. The positive electrode mixture layer can be formed on the surface of one side of the positive electrode collector, or on the surfaces of both sides.

[0083] (Conductive material dispersion)

[0084] The present disclosure provides a conductive material dispersion. The conductive material dispersion can be used for the preparation of positive electrode slurry. The conductive material dispersion contains a carbon material (conductive material), a nitrile-containing rubber, a fluorine-containing polymer and a liquid medium. The carbon material, the nitrile-containing rubber and the fluorine-containing polymer are described above, so repeated descriptions are omitted. The conductive material dispersion may contain any component contained in the positive electrode mixture layer (for example, any of the above-mentioned components).

[0085] The liquid medium of the conductive material dispersion can use the liquid medium described for the positive electrode slurry. Alternatively, a liquid medium different from the positive electrode slurry can 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, the positive electrode slurry can be prepared. In the conductive material dispersion, any component of the positive electrode mixture layer and a liquid medium can be added together with the positive electrode active material.

[0086] 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 based on the ratio of each component exemplified in the description of the positive electrode mixture layer relative to 100 parts by mass of the positive electrode active material.

[0087] (Non-aqueous electrolyte secondary battery)

[0088] The non-aqueous electrolyte secondary battery involved in the present embodiment includes the positive electrode involved in the present embodiment. In addition to the positive electrode, the secondary battery also includes at least a negative electrode and a non-aqueous electrolyte. The secondary battery may include a positive electrode, a negative electrode, a non-aqueous electrolyte, a separator and an outer body. Examples of the secondary battery include lithium ion secondary batteries, lithium metal secondary batteries, etc. There is no particular limitation on the constituent elements other than the positive electrode mixture layer, and known constituent elements can be used. Examples of the constituent elements of the secondary battery are described below.

[0089] (positive electrode)

[0090] As the positive electrode, the positive electrode according to this embodiment is used.

[0091] (Positive electrode collector)

[0092] The shape and thickness of the positive electrode current collector can be selected according to the application, and can be selected corresponding to the shape and thickness of the negative electrode current collector. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, titanium, etc. can be exemplified. The positive electrode mixture layer can be formed only on one side of the positive electrode current collector, and can also be formed on both sides of the positive electrode current collector.

[0093] (negative electrode)

[0094] 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. However, in the case of a lithium metal secondary battery, a negative electrode current collector capable of precipitating lithium metal or lithium alloy is used as the negative electrode.

[0095] The negative electrode mixture layer contains a negative electrode active material as an essential component. The negative electrode mixture layer may contain a binder, a thickener, a conductive material, etc. as an optional component. Among these optional components, the components exemplified as components of the positive electrode can be used.

[0096] The negative electrode mixture layer can be formed by applying a negative electrode slurry formed by dispersing the constituent components of the negative electrode mixture layer in a liquid medium (dispersion medium) on the surface of the negative electrode collector and drying it. The dried coating can be rolled as needed. As the liquid medium, a liquid medium exemplified as a liquid medium of the positive electrode slurry can 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.

[0097] (Negative electrode active material)

[0098] The negative electrode active material is selected according to the type of secondary battery. An example of a negative electrode active material is a material that can absorb and release lithium ions. Examples of such materials include carbonaceous materials, Si-containing materials, etc. The negative electrode active material may contain Si-containing materials, or may be Si-containing materials. As the negative electrode active material, metallic lithium, lithium alloys, etc. may also be used. The negative electrode may contain one negative electrode active material, or may contain a combination of two or more.

[0099] Examples of carbonaceous materials include graphite, easily graphitized carbon (soft carbon), and difficultly graphitized carbon (hard carbon). One carbonaceous material may be used alone or in combination of two or more. Graphite is preferred in terms of excellent charge and discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0100] Examples of Si-containing materials include Si single substance, 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 a SiO2 phase, a silicate phase, and a carbon phase can be used.

[0101] 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.

[0102] (Non-aqueous electrolyte)

[0103] 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.

[0104] As a solvent, known materials can be utilized. As a solvent, for example, cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, etc. can be used. Examples of cyclic carbonates can include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), etc. As chain carbonates, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. are included. Examples of cyclic carboxylates include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. Examples of chain carboxylates include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP). Non-aqueous solvents can be used alone or in combination of two or more.

[0105] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorinated acids (LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorinated acid imides (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.

[0106] 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 to the above range, an electrolyte having excellent ion conductivity and appropriate viscosity can be obtained.

[0107] The electrolyte may contain a known additive. Examples of the additive include 1,3-propane sultone, toluene sulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, and the like.

[0108] (Diaphragm)

[0109] 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 the material of the separator include polyolefins (polypropylene, polyethylene, etc.) and other resins.

[0110] (outer body)

[0111] The electrode group and the non-aqueous electrolyte are housed in an outer casing (battery case). The outer casing is not particularly limited, and a known outer casing can be used. The electrode group is composed of a positive electrode, a negative electrode, and a separator. The structure of the electrode group is not particularly limited, and it can be a wound 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. The shape of the non-aqueous electrolyte secondary battery is not particularly limited, and it can be cylindrical, square, coin-shaped, button-shaped, laminated, etc.

[0112] Figure 1 This is a schematic perspective view in which a portion of a secondary battery 10 according to an embodiment of the present disclosure is cut away. Figure 1 2 shows a square nonaqueous electrolyte battery as an example. Figure 1 The secondary battery 10 shown includes a battery case 4 having a bottomed square tube shape, an electrode group 1 housed in the battery case 4 , and a non-aqueous electrolyte (not shown).

[0113] 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 negative terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. 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. The periphery of the sealing plate 5 is engaged with the open end of the battery case 4, and the engaged portion is laser welded. That is, the positive electrode is electrically connected to the battery case 4 which also serves as the positive terminal. There is an injection hole for a non-aqueous electrolyte on the sealing plate 5. The injection hole is sealed by a plug 8 after the non-aqueous electrolyte is injected.

[0114] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer uses the above-mentioned positive electrode mixture layer.

[0115] (Note)

[0116] Through the above records, the following technical solutions are disclosed.

[0117] (Technical Solution 1)

[0118] A positive electrode for a non-aqueous electrolyte secondary battery comprises a positive electrode collector and a positive electrode mixture layer arranged on the positive electrode collector, wherein the positive electrode mixture layer contains active material particles with an average particle size of less than 5 μm, a conductive material, a dispersant and a binder, the active material particles comprise composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles, the composite oxide particles are particles of a lithium transition metal composite oxide, the conductive material contains a carbon material, the dispersant contains a nitrile-containing rubber, and the binder contains a fluorine-containing polymer.

[0119] (Technical Solution 2)

[0120] According to the positive electrode described in Technical Solution 1, the carbon material is carbon nanotubes.

[0121] (Technical Solution 3)

[0122] According to the positive electrode described in technical solution 2, the BET specific surface area of ​​the carbon nanotubes is 200m 2 / g or above.

[0123] (Technical Solution 4)

[0124] In the positive electrode according to any one of claims 1 to 3, the weight average molecular weight of the fluorine-containing polymer is 1,000,000 or more.

[0125] (Technical Solution 5)

[0126] According to any one of technical solutions 1 to 4, in the active material particles, the molar amount of the boron compound in the surface modification layer is within the range of 1.0 to 3.0% of the total molar amount of metal elements other than lithium in the lithium transition metal composite oxide.

[0127] (Technical Solution 6)

[0128] A non-aqueous electrolyte secondary battery comprises the positive electrode according to any one of technical solutions 1 to 5.

[0129] (Technical Solution 7)

[0130] A positive electrode slurry for a positive electrode of a non-aqueous electrolyte secondary battery comprises active material particles having an average particle size of less than 5 μm, a conductive material, a dispersant, a binder and a liquid medium, wherein the active material particles comprise composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles, the composite oxide particles are particles of a lithium transition metal composite oxide, the conductive material comprises a carbon material, the dispersant comprises a nitrile-containing rubber, and the binder comprises a fluorine-containing polymer.

[0131] (Technical Solution 8)

[0132] According to the positive electrode slurry described in Technical Solution 7, the carbon material is carbon nanotubes.

[0133] Example

[0134] Hereinafter, the present disclosure will be specifically described based on examples, but the present disclosure is not limited to the following examples. In this example, a plurality of nonaqueous electrolyte secondary batteries having different positive electrodes were produced and evaluated.

[0135] (Production of Battery A1)

[0136] Battery A1 was produced by the following method.

[0137] (1) Preparation of negative electrode

[0138] 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 predetermined 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. In this way, a negative electrode comprising copper foil and a negative electrode mixture layer formed on both sides of the copper foil is formed.

[0139] (2) Preparation of positive electrode

[0140] (2-1) Preparation of positive electrode active material particles

[0141] Through the above synthesis process, LiNi 0.9 Co 0.05 Al 0.05O2 represents a composite oxide particle. Then, the remaining lithium of the obtained composite oxide particles is removed by washing with water and the particles are dried. Powdered boric acid (H3BO3) is added to the obtained particles (addition step). The molar amount of the added boric acid is 2% of the total molar amount of the metal elements other than Li in the composite oxide particles. By drying the composite oxide particles to which boric acid is added, a positive electrode active material (particles (P1)) is obtained. The average particle size (D50) of the particles (P1) is 4 μm.

[0142] (2-2) Preparation of positive electrode

[0143] First, the positive electrode active material, carbon nanotubes (conductive material), hydrogenated nitrile rubber (rubber containing nitrile groups), polyvinylidene fluoride (fluorinated polymer, PVDF) and N-methyl-2-pyrrolidone (liquid medium) are mixed at a predetermined mass ratio to prepare positive electrode slurry SA1. The average length and average diameter of the carbon nanotubes are 1 μm and 10 nm, respectively.

[0144] The amount of carbon nanotubes added was 0.5 parts by mass relative to 100 parts by mass of the positive electrode active material. The amount of hydrogenated nitrile rubber (H-NBR) added was 0.5 parts by mass relative to 100 parts by mass of the positive electrode active material. The amount of polyvinylidene fluoride (PVDF) added was 1 part by mass relative to 100 parts by mass of the positive electrode active material.

[0145] Next, a coating film is formed by applying the positive electrode slurry on the surface of the aluminum foil (positive electrode current collector) to obtain 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.

[0146] (3) Preparation of electrolyte (non-aqueous electrolyte)

[0147] An electrolyte was prepared by adding LiPF6 (lithium salt) to a non-aqueous solvent. The concentration of LiPF6 in the electrolyte was 1.0 mol / L. As a 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.

[0148] (4) Production of secondary batteries

[0149] Leads are installed on the positive electrode and the negative electrode, respectively. Next, the positive electrode, the negative electrode and the separator are spirally wound in a manner that the separator is arranged between the positive electrode and the negative electrode, thereby producing 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, 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.

[0150] (Production of Batteries A2 to A4 and Batteries C1 to C6)

[0151] Positive electrode slurries SA2 to SA4 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 and the average particle size of the positive electrode active material particles were changed as shown in Table 1. In addition, polyvinylidene fluoride (PVDF) with different weight average molecular weights was used as a binder. As a composite oxide constituting the positive electrode active material, the same composite oxide as that constituting the positive electrode active material used in the positive electrode slurry SA1 was used. In addition to using these positive electrode slurries, positive electrodes PA2 to PA4 and positive electrodes PC1 to PC6 were prepared by the same method and conditions as those for preparing the positive electrode PA1 used in battery A1. In addition to using these positive electrodes, batteries A2 to A4 and batteries C1 to C6 were prepared by the same method and conditions as those for preparing the battery A1. Particles with an average particle size of 4 μm and particles with an average particle size of 17 μm were mixed in the positive electrode active material of battery A3.

[0152] (Evaluation of Stability of Positive Electrode Slurry)

[0153] The stability of the positive electrode slurry was evaluated by the following method. The viscosity V0 on the day of slurry preparation and the viscosity V1 after the slurry was left to stand for 2 days were measured. The viscosity of the slurry was measured using a B-type viscometer. Then, the rate of change of viscosity was calculated by the following formula.

[0154] Viscosity change rate (%) = (V1 / V0) × 100

[0155] (Evaluation of Adhesion of Positive Electrode Mixture Layer)

[0156] Positive electrodes PA1-PA4 and PC1-PC5 were prepared by the above method, and the adhesion of the positive electrode mixture layer was evaluated. Regarding the adhesion, the bonding strength between the aluminum foil and the positive electrode mixture was evaluated by the method of JIS (Japanese Industrial Standard) 6854-1. In order to compare between samples, electrodes of all the same size were prepared for measurement.

[0157] (Evaluation of battery DC internal resistance and energy density)

[0158] The direct current internal resistance and energy density of the manufactured batteries were measured.

[0159] (Evaluation of battery capacity retention rate)

[0160] The capacity retention rate of the battery produced above was evaluated after 30 cycles of charge and discharge. Specifically, the discharge capacity E(0) before the charge and discharge cycle and the discharge capacity E(30) after 30 cycles were measured, and the capacity retention rate was calculated from the following formula.

[0161] Capacity retention rate (%) = 100 × E (30) / E (0)

[0162] Part of the components contained in the positive electrode mixture layer and the evaluation results are shown in Table 1. In Table 1, the adhesion of the positive electrode mixture layer, the DC internal resistance of the battery, and the battery energy density are relative values ​​when the measurement results of the positive electrode PC1 and the battery C1 are regarded as 100%.

[0163] [Table 1]

[0164]

[0165] Batteries A1 to A4, positive electrode slurries SA1 to SA4 and positive electrodes PA1 to PA4 used to produce them are batteries, positive electrode slurries and positive electrodes according to the present disclosure. Batteries C1 to C6, positive electrode slurries SC1 to SC6 and positive electrodes PC1 to PC6 used to produce them are comparative examples.

[0166] The higher the value of the viscosity change rate of the slurry shown in Table 1, the more the viscosity tends to increase. In addition, if the value is much lower than 100%, the positive electrode active material will precipitate and the production stability will decrease. The closer the change rate is to 100%, the better the dispersibility and dispersion stability. Regarding the adhesion of the positive electrode mixture layer, the higher the value, the higher the adhesion. When using conductive materials that are easy to agglomerate, such as carbon nanotubes, the dispersibility of the material in the slurry is likely to decrease. If the dispersibility is low, a uniform positive electrode mixture layer cannot be formed, and the battery characteristics are reduced. In addition, if the dispersibility is low, it is sometimes difficult to form a positive electrode mixture layer. In particular, if the value of the viscosity change rate is less than 70% or exceeds 120%, the productivity is significantly reduced.

[0167] As shown in Table 1, the positive electrode slurries SA1 to SA4 disclosed in the present invention have high dispersibility and are stable. Therefore, the positive electrode can be manufactured stably and easily. In addition, the batteries A1 to A4 disclosed in the present invention have low DC internal resistance and high capacity retention rate. It is believed that these are caused by the synergistic effect of the combination of the above-mentioned specific components. In addition, the positive electrode mixture layer of the positive electrodes PA1 and PA3 disclosed in the present invention has high adhesion.

[0168] Industrial Availability

[0169] 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 according to the present disclosure can be used for various applications, for example, preferably as a main power source for mobile communication devices, portable electronic devices, etc.

[0170] The present invention has been described in conjunction with the currently preferred embodiments, but this disclosure should not be interpreted as limiting. By reading the above disclosure, a person of ordinary skill in the art to which the present invention belongs can clearly understand various modifications and changes. Therefore, the appended claims should be interpreted as including all modifications and changes without departing from the true spirit and scope of the present invention.

[0171] Description of Reference Numerals

[0172] 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, comprising a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, The positive electrode mixture layer contains active material particles with an average particle size of less than 5 μm, a conductive material, a dispersant and a binder. The active material particles include composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles. The composite oxide particles are particles of lithium transition metal composite oxides, The conductive material contains a carbon material, The dispersant contains a nitrile-containing rubber, The binder contains a fluorine-containing polymer.

2. The positive electrode according to claim 1, The carbon material is carbon nanotubes.

3. The positive electrode according to claim 2, The BET specific surface area of ​​the carbon nanotubes is 200 m 2 / g or above.

4. The positive electrode according to claim 1 or 2, The weight average molecular weight of the fluorine-containing polymer is 1,000,000 or more.

5. The positive electrode according to claim 1 or 2, In the active material particle, the molar amount of the boron compound in the surface modification layer is within a range of 1.0 to 3.0% of the total molar amount of metal elements other than lithium in the lithium transition metal composite oxide. 6 . A non-aqueous electrolyte secondary battery comprising the positive electrode according to claim 1 or 2 .

7. A positive electrode slurry for a positive electrode of a non-aqueous electrolyte secondary battery, comprising active material particles having an average particle size of less than 5 μm, a conductive material, a dispersant, a binder and a liquid medium, The active material particles include composite oxide particles and a surface modification layer containing a boron compound formed on the surface of the composite oxide particles. The composite oxide particles are particles of lithium transition metal composite oxides, the conductive material contains a carbon material, The dispersant contains a nitrile-containing rubber, The binder contains a fluorine-containing polymer.

8. The positive electrode slurry according to claim 7, The carbon material is carbon nanotubes.

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