Non-aqueous electrolyte secondary-battery positive electrode, non-aqueous electrolyte secondary battery using the same, battery module, and battery system

Abstract

A non-aqueous electrolyte secondary-battery positive electrode (1) comprises a positive electrode current collector (11) and a positive electrode active material layer (12) existing on the positive electrode current collector (11). The positive electrode active material layer (12) contains positive electrode active material particles, with at least a part of the surface of the positive electrode active material particles being coated with a conductive material. The positive electrode current collector (11) comprises a positive electrode current collector body (14) and a collector coating layer (15) that covers a part of the surface on the positive electrode active material layer (12) side of the positive electrode current collector body (14). The positive electrode active material layer (12) includes particulate binder material.

Description

Positive electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery, battery module, and battery system using the same

[0001] The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery, a battery module, and a battery system using the same. This application claims priority to Japanese Patent Application No. 2022-140246, filed in Japan on September 2, 2022, the contents of which are incorporated herein by reference.

[0002] A non-aqueous electrolyte secondary battery is generally composed of a positive electrode, a non-aqueous electrolyte, a negative electrode, and a separator (hereinafter also referred to as "separator") disposed between the positive electrode and the negative electrode. Known positive electrodes for non-aqueous electrolyte secondary batteries are those in which a composition consisting of a positive electrode active material containing lithium ions, a conductive additive, and a binder is fixed to the surface of a metal foil current collector. Positive electrode active materials containing lithium ions include lithium transition metal composite oxides such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide, and lithium iron phosphate (LiFePO ). ４ Lithium phosphate compounds such as phosphate phosphate (Pt) and phosphate phosphate (Pt) have been put to practical use.

[0003] The positive electrode has a positive electrode current collector and a positive electrode active material layer present on the positive electrode current collector. To form the positive electrode active material layer on the positive electrode current collector, a positive electrode manufacturing composition containing positive electrode active material particles, a binder, and a solvent is applied onto the positive electrode current collector and dried to remove the solvent.

[0004] In order to improve battery characteristics, it is necessary to obtain a composition for manufacturing a positive electrode that has excellent dispersibility of positive electrode active material particles. Conventionally, a composition for manufacturing a positive electrode that has excellent dispersibility of positive electrode active material particles contains at least one of an electrode active material or a carbon material that is a conductive additive, an anionic dispersant, and water, in which the anionic dispersant has at least one of a carboxylic acid or a sulfonic acid as an anionic moiety, an acid value of 100 to 600 mgKOH / g, a hydroxyl value of 0 to 400 mgKOH / g, and a weight-average molecular weight of 5,000 or more (see, for example, Patent Document 1).

[0005] Patent No. 5252134

[0006] However, the positive electrode produced using the composition for producing a positive electrode of Patent Document 1 has high resistance, and non-aqueous electrolyte secondary batteries equipped with this positive electrode have problems such as low low-temperature output and low energy density after storage.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a positive electrode for a nonaqueous electrolyte secondary battery that realizes a nonaqueous electrolyte secondary battery with high low-temperature output after storage and high energy density, as well as a nonaqueous electrolyte secondary battery, a battery module, and a battery system that use the same.

[0008] The present invention has the following aspects. [1] A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode current collector; and a positive electrode active material layer present on the positive electrode current collector, wherein the positive electrode active material layer contains positive electrode active material particles, at least a portion of the surface of the positive electrode active material particles being coated with a conductive material, and the positive electrode current collector comprises a positive electrode current collector body and a current collector coating layer coating a portion of the surface of the positive electrode current collector body facing the positive electrode active material layer, and the positive electrode active material layer contains a particulate binder. [2] The positive electrode for a non-aqueous electrolyte secondary battery according to [1], wherein a content of the positive electrode active material particles is 93 mass% or more with respect to the total mass of the positive electrode active material layer. [3] The positive electrode for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein a content of the particulate binder is 4 mass% or less with respect to the total mass of the positive electrode active material layer. [3-1] The positive electrode for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the content of the particulate binder is less than 3.8% by mass relative to the total mass of the positive electrode active material layer. [3-2] The positive electrode for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the content of the particulate binder is 3% by mass or less relative to the total mass of the positive electrode active material layer. [3-3] The positive electrode for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the content of the particulate binder is 2% by mass or less or less than 2% by mass relative to the total mass of the positive electrode active material layer. [3-4] The positive electrode for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the content of the particulate binder is 1.5% by mass or less relative to the total mass of the positive electrode active material layer. [4] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [3-4], wherein the positive electrode active material layer contains a conductive additive, and the content of the conductive additive is 5% by mass or less relative to the total mass of the positive electrode active material layer. [4-1] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [3-4], wherein the positive electrode active material layer contains a conductive additive, and the content of the conductive additive is less than 1.8 mass% relative to the total mass of the positive electrode active material layer. [4-2] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [3-4], wherein the positive electrode active material layer contains a conductive additive, and the content of the conductive additive is 1.0 mass% or less relative to the total mass of the positive electrode active material layer.[4-3] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [3-4], wherein the positive electrode active material layer contains a conductive additive, and the content of the conductive additive is 0.5 mass % or less relative to the total mass of the positive electrode active material layer. [5] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [4-3], wherein the particulate binder has an average particle size of 50 nm or more. [5-1] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [5], wherein the particulate binder has an average particle size of less than 500 nm. [5-2] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [5], wherein the particulate binder has an average particle size of 400 nm or less. [5-3] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [5], wherein the particulate binder has an average particle size of 350 nm or less. [5-4] The positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [5], wherein the particulate binder has an average particle size of 300 nm or less. [6] A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery according to any one of [1] to [5-4], a negative electrode, and a non-aqueous electrolyte present between the positive electrode for a non-aqueous electrolyte secondary battery and the negative electrode. [7] The non-aqueous electrolyte secondary battery according to [6], wherein the non-aqueous electrolyte contains a lithium imide salt represented by the following formula (1): LiN(SO). ２ R) ２ (1) [wherein R is a fluorine atom or C ｘ F （２ｘ＋１） where x is an integer of 1 to 3.] [8] A battery module or a storage battery system including a plurality of the nonaqueous electrolyte secondary batteries according to [7].

[0009] According to the present invention, it is possible to provide a positive electrode for a nonaqueous electrolyte secondary battery that realizes a nonaqueous electrolyte secondary battery with high low-temperature output after storage and high energy density, as well as a nonaqueous electrolyte secondary battery, a battery module, and a battery system that use the same.

[0010] 1 is a cross-sectional view schematically showing an example of a positive electrode for a non-aqueous electrolyte secondary battery according to the present invention. 2 is a cross-sectional view schematically showing an example of a non-aqueous electrolyte secondary battery according to the present invention. 3 is a view for explaining a second measurement method for evaluating whether a binder is in a particulate state, and is an image of a scanning electron microscope image showing a cross section of a positive electrode active material constituting a positive electrode active material layer.

[0011] In this specification and claims, the symbol "to" indicating a range of numerical values ​​means that the numerical values ​​before and after it are included as the lower and upper limits. FIG. 1 is a schematic cross-sectional view showing one embodiment of a positive electrode for a nonaqueous electrolyte secondary battery of the present invention, and FIG. 2 is a schematic cross-sectional view showing one embodiment of a nonaqueous electrolyte secondary battery of the present invention. Note that FIGS. 1 and 2 are schematic views for easily explaining the configuration, and the dimensional ratios of each component may differ from the actual ones.

[0012] <Positive Electrode for Non-Aqueous Electrolyte Secondary Battery> The positive electrode for a non-aqueous electrolyte secondary battery (hereinafter also referred to as "positive electrode") 1 of this embodiment has a positive electrode current collector 11 and a positive electrode active material layer 12. The positive electrode active material layer 12 is present on at least one surface of the positive electrode current collector 11. The positive electrode active material layer 12 may be present on both surfaces of the positive electrode current collector 11. In the example of FIG. 1 , the positive electrode current collector 11 has a positive electrode current collector body 14 and a current collector coating layer 15 that coats a portion of the surface of the positive electrode current collector body 14 facing the positive electrode active material layer 12.

[0013] [Positive Electrode Active Material Layer] The positive electrode active material layer 12 contains positive electrode active material particles. The positive electrode active material layer 12 further contains a particulate binder. It can be assumed that in the positive electrode active material layer 12, the particulate binder is crushed and adhered to the surface of the positive electrode active material. The positive electrode active material layer 12 may further contain a conductive additive. The positive electrode active material particles contain a positive electrode active material. The positive electrode active material particles are so-called coated particles having a core of the positive electrode active material and a coating (active material coating) covering the core. A group of positive electrode active material particles contained in the positive electrode active material layer 12 are coated particles.

[0014] The positive electrode active material preferably contains a compound having an olivine crystal structure. The compound having an olivine crystal structure is represented by the general formula LiFe ｘ M （１－ｘ） P.O. ４(hereinafter also referred to as "general formula (1)") is preferred. In general formula (1), 0≦x≦1. M is Co, Ni, Mn, Al, Ti or Zr. A small amount of Fe and a part of M (Co, Ni, Mn, Al, Ti or Zr) can also be substituted with other elements to the extent that the physical properties are not changed. The compound represented by general formula (1) does not impair the effects of the present invention even if it contains a small amount of metal impurities. The compound represented by general formula (1) is LiFePO ４ Lithium iron phosphate represented by the formula (hereinafter, also referred to as "lithium iron phosphate") is preferred.

[0015] The positive electrode active material may contain other positive electrode active materials in addition to the compound having an olivine crystal structure. The other positive electrode active materials are preferably lithium transition metal composite oxides. For example, lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt oxide (LiNi ｘ Co ｙ Al ｚ O ２ , where x + y + z = 1), lithium nickel cobalt manganese oxide (LiNi ｘ Co ｙ Mn ｚ O ２ , where x+y+z=1), lithium manganate, lithium cobalt manganate, lithium manganese chromate, lithium vanadium nickel oxide, nickel-substituted lithium manganate (e.g., LiMn １．５ Ni ０．５ O ４ ), and lithium vanadium cobalt oxide (LiCoVO ４ ), and non-stoichiometric compounds in which a portion of these compounds is substituted with a metal element. The metal element may be one or more selected from the group consisting of Mn, Mg, Ni, Co, Cu, Zn, and Ge. The other positive electrode active material may be one type or two or more types. The other positive electrode active material may have the active material coating portion on at least a portion of its surface.

[0016] The content of the compound having an olivine crystal structure relative to the total mass of the positive electrode active material is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. The content of the compound having an olivine crystal structure relative to the total mass of the positive electrode active material particles may be 100% by mass. When coated lithium iron phosphate is used, the content of the coated lithium iron phosphate relative to the total mass of the positive electrode active material is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. It may even be 100% by mass.

[0017] The positive electrode active material particles of this embodiment are coated particles in which at least a portion of the surface of the positive electrode active material is coated with a conductive material. By using the coated particles as the positive electrode active material particles, the battery capacity and high-rate cycle characteristics can be further improved.

[0018] The conductive material of the active material coating portion preferably contains carbon. The conductive material may consist of only carbon, or may be a conductive organic compound containing carbon and other elements. Examples of other elements include nitrogen, hydrogen, and oxygen. In the conductive organic compound, the content of other elements is preferably 10 atomic % or less, and more preferably 5 atomic % or less.

[0019] The conductive material constituting the active material coating portion is preferably composed of carbon alone. The content of the conductive material is preferably 0.1 to 3.0 mass %, more preferably 0.5 to 1.5 mass %, and even more preferably 0.7 to 1.3 mass %, relative to the total mass of the positive electrode active material particles having the active material coating portion.

[0020] As the coated particles, coated particles having a core made of a compound having an olivine-type crystal structure are preferred, coated particles having a core made of a compound represented by general formula (1) are more preferred, and coated particles having a core made of lithium iron phosphate (hereinafter also referred to as "coated lithium iron phosphate") are even more preferred. These coated particles further improve battery capacity and cycle characteristics. In addition, it is particularly preferred that the entire surface of the core of the coated particles is coated with a conductive material.

[0021] Coated particles can be produced by known methods. The following describes a method for producing coated particles, using coated lithium iron phosphate as an example. Examples include heat treatment of iron phosphate particles at 600 to 1300°C using a graphitizable or non-graphitizable resin, naphthalene, coal tar, binder pitch, or the like as a precursor, or chemical vapor deposition (CVD) treatment of lithium iron phosphate particles in a fluidized state at 600 to 1300°C using hydrocarbon compounds such as methanol, ethanol, benzene, and toluene as the chemical vapor deposition carbon source, thereby forming a carbon coating on the surface. For example, the amount and resistivity of carbon coating the lithium iron phosphate powder can be adjusted by adjusting the heating time and temperature during the heat treatment process while supplying methanol vapor. Uncoated carbon particles are preferably removed by subsequent classification, washing, or other processes. Other positive electrode active materials may have the active material coating on at least a portion of their surfaces.

[0022] The thickness of the active material coating portion of the positive electrode active material is 1 to 100 nm, preferably 1 to 50 nm, and more preferably 1 to 10 nm. The thickness of the active material coating portion of the positive electrode active material can be measured by measuring the thickness of the active material coating portion in a transmission electron microscope (TEM) image of the positive electrode active material. The thickness of the active material coating portion present on the surface of the positive electrode active material does not have to be uniform. It is preferable that an active material coating portion with a thickness of 1 nm or more is present on at least a portion of the surface of the positive electrode active material, and that the maximum thickness of the active material coating portion is 100 nm or less.

[0023] For example, the active material coating portion is formed on the surface of the positive electrode active material particle in advance and is present on the surface of the positive electrode active material particle in the positive electrode active material layer. That is, the active material coating portion in this specification is not newly formed in a process subsequent to the preparation of the positive electrode manufacturing composition. In addition, the active material coating portion is not easily removed in a process subsequent to the preparation of the positive electrode manufacturing composition. For example, even when the coated particles are mixed with a solvent using a mixer or the like when preparing the positive electrode manufacturing composition, the active material coating portion still coats the surface of the core of the positive electrode active material particle. Furthermore, even if the positive electrode active material layer is peeled off from the positive electrode and placed in a solvent to dissolve the binder in the positive electrode active material layer, the active material coating portion still coats the surface of the positive electrode active material particle. Furthermore, even if an operation to loosen aggregated particles is performed when measuring the particle size distribution of the particles in the positive electrode active material layer by laser diffraction / scattering, the active material coating portion still coats the surface of the positive electrode active material particle. The active material coating portion is present in an amount of preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more of the total area of ​​the outer surface of the positive electrode active material particle. That is, the coated particle has a core portion which is a positive electrode active material and an active material coating portion which covers the surface of the core portion, and the area of ​​the active material coating portion relative to the surface area of ​​the core portion (coverage rate) is preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more.

[0024] The coverage can be measured by the following method. First, particles in the positive electrode active material layer are analyzed by energy dispersive X-ray spectroscopy (TEM-EDX) using a transmission electron microscope. Specifically, the outer periphery of the positive electrode active material particle in the TEM image is subjected to elemental analysis by EDX. The elemental analysis is performed on carbon to identify the carbon coating the positive electrode active material particle. The portion where the carbon coating is 1 nm or more thick is defined as the coated portion, and the proportion of the coated portion relative to the entire periphery of the observed positive electrode active material particle is determined, which can be used as the coverage. Measurements are performed on, for example, 10 positive electrode active material particles, and the average value of these can be used as the coverage.

[0025] The content of the positive electrode active material particles is preferably 93 mass % or more, more preferably 95 mass % or more, still more preferably more than 99 mass %, and particularly preferably 99.5 mass % or more, relative to the total mass of the positive electrode active material layer 12. When the content of the positive electrode active material particles is equal to or greater than the above lower limit, the battery capacity and cycle characteristics can be further improved.

[0026] The average particle diameter of the group of positive electrode active material particles (i.e., the powder of positive electrode active material particles) is, for example, preferably 0.1 to 20.0 μm, more preferably 0.2 to 10.0 μm. When two or more types of positive electrode active material particles are used, it is sufficient that the average particle diameter of each is within the above range. The average particle diameter is a volume-based median diameter measured using a particle size distribution measuring instrument based on a laser diffraction / scattering method.

[0027] The particulate binder contained in the positive electrode active material layer 12 is an organic substance, and examples thereof include an acrylic acid ester-acrylic acid copolymer, a polyvinyl alcohol-butylaldehyde copolymer, a styrene-butadiene copolymer, polyvinylidene fluoride, a vinylidene fluoride-hexafluoropropylene copolymer, etc. One type of particulate binder may be used alone, or two or more types may be used in combination.

[0028] The average particle diameter of the particulate binder is preferably 50 nm or more, more preferably 160 nm or more, and even more preferably 200 nm or more. It is believed that when the average particle diameter of the particulate binder is equal to or greater than the above-mentioned lower limit, the resistance is reduced and output performance is improved. The upper limit of the average particle diameter of the particulate binder may be 600 nm or less, preferably 500 nm or less, more preferably less than 500 nm, more preferably 400 nm or less, more preferably 350 nm or less, more preferably 300 nm or less, and even more preferably 200 nm or less. The particulate state of the binder can be evaluated, for example, by the following measurement method. FIG. 3 is a diagram illustrating the first measurement method, showing the cross section of the positive electrode active material constituting the positive electrode active material layer observed with a scanning electron microscope (SEM). In FIG. 3, reference numeral 100 denotes the positive electrode active material, and reference numeral 110 denotes the particulate binder. (First Measurement Method) As a pretreatment, the positive electrode is cut using an ion milling device to expose the cross section of the positive electrode. The positive electrode is cooled during cutting depending on the heat resistance of the positive electrode active material and binder. The cross section of the positive electrode is observed using a scanning electron microscope (SEM), and the carbon component attached to the surface of the positive electrode active material 100 is identified by elemental analysis using energy dispersive X-ray spectroscopy (EDS). When a particulate binder 110 is used, it is found that the surface of the positive electrode active material 100 is significantly covered with the carbon component compared to when a solution-type binder is used, and the particulate binder 110 is attached. The thickness of the active material coating portion is at most 100 nm or less from the surface of the positive electrode active material, preferably 10 nm or less. In the portion where the particulate binder is attached to the surface of the positive electrode active material, the thickness of the particulate binder is 1 μm or more from the surface of the positive electrode active material. Therefore, at the magnification required to check the thickness of the particulate binder, the thickness of the active material coating portion cannot be confirmed. As the microscope, for example, a scanning electron microscope (model name: S4800, manufactured by Hitachi High-Technologies Corporation) is used. (Second Measurement Method) As a pretreatment, the positive electrode is cut using a focused ion beam (FIB) device to expose a cross section of the positive electrode.The cross section of the positive electrode is observed using a transmission electron microscope (TEM), and the carbon component attached to the surface of the positive electrode active material 100 is identified by elemental analysis using energy dispersive X-ray spectroscopy (EDS). Similar to the first measurement method, it is determined that the binder is in a particulate state. For example, a focused ion beam device (model name: FB2200, manufactured by Hitachi High-Technologies Corporation) is used for pretreatment. For example, a transmission electron microscope (model name: HD2700, manufactured by Hitachi High-Technologies Corporation) is used as the microscope. (Third Measurement Method) As a pretreatment, the positive electrode is cut using an ultramicrotome device to expose the cross section of the positive electrode. Depending on the heat resistance of the positive electrode active material and the binder, the positive electrode is cooled during cutting. The cross section of the positive electrode is observed using an atomic force microscope (AFM), and it is determined that the binder is in a particulate state based on differences in physical properties such as elastic modulus and resistance value. Although most of the particulate binders present in the positive electrode active material layer 12 are deformed as shown in Fig. 3 due to adhesion to the surfaces of the positive electrode active material particles, the particle diameter of the particulate binder 110 in Fig. 3 can be determined by assuming a perfect circle having the same cross-sectional area as the particulate binder 110. Then, the particle diameters of 10 or more such particulate binders are determined, and the average particle diameter of the particulate binders can be determined.

[0029] The content of the particulate binder in the positive electrode active material layer 12 is preferably 4% by mass or less, more preferably less than 3.8% by mass, more preferably 3% by mass or less, more preferably 2% by mass or less, more preferably less than 2% by mass, more preferably 1.5% by mass or less, and even more preferably 1% by mass or less, relative to the total mass of the positive electrode active material layer 12. When the content of the binder is equal to or less than the above upper limit, the proportion of materials in the positive electrode active material layer 12 that do not contribute to lithium ion conduction is reduced, increasing the true density of the positive electrode active material layer 12. Furthermore, the proportion of binder covering the surface of the positive electrode 12 is reduced, further increasing lithium conductivity and further improving high-rate cycle characteristics. The lower limit of the content of the particulate binder in the positive electrode active material layer 12 is preferably 0.1% by mass or more relative to the total mass of the positive electrode active material layer 12.

[0030] Examples of the conductive additive contained in the positive electrode active material layer 12 include carbon materials such as graphite, graphene, hard carbon, ketjen black, acetylene black, and carbon nanotubes. One type of conductive additive may be used alone, or two or more types may be used in combination.

[0031] The content of the conductive additive in the positive electrode active material layer 12 is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, even more preferably less than 1.8% by mass, even more preferably 1.0% by mass or less, even more preferably 0.5% by mass or less, particularly preferably 0.2% by mass or less, and most preferably 0% by mass (i.e., no conductive additive) relative to the total mass of the positive electrode active material layer 12. If the content of the conductive additive is equal to or less than the above upper limit, the proportion of substances in the positive electrode active material layer 12 that do not contribute to lithium ion conduction is reduced, thereby increasing the true density of the positive electrode active material layer 12 and further improving the high-rate cycle characteristics. When a conductive additive is incorporated into the positive electrode active material layer 12, the lower limit of the conductive additive is determined appropriately depending on the type of conductive additive, and is, for example, greater than 0.1% by mass relative to the total mass of the positive electrode active material layer 12. Note that the positive electrode active material layer 12 "does not contain a conductive additive" means that the conductive additive is substantially not contained, and does not exclude conductive additives that are contained to an extent that does not affect the effects of the present invention. For example, if the content of the conductive additive is 0.1 mass % or less with respect to the total mass of the positive electrode active material layer 12, it can be determined that the conductive additive is substantially not contained.

[0032] Conductive additive particles that do not contribute to the conductive path can cause self-discharge in the battery or undesirable side reactions.

[0033] The positive electrode active material layer 12 may contain additives as needed. For example, the positive electrode active material layer 12 may contain carboxymethyl cellulose (CMC) or the like as a viscosity adjuster for the slurry.

[0034] [Positive Electrode Current Collector Body] Examples of materials constituting the positive electrode current collector body 14 include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. The thickness of the positive electrode current collector body 14 is, for example, preferably 8 μm to 40 μm, and more preferably 10 μm to 25 μm. The thickness of the positive electrode current collector body 14 and the thickness of the positive electrode current collector 11 can be measured using a micrometer. An example of a measuring device is a product named "MDH-25M" manufactured by Mitutoyo Corporation.

[0035] [Current Collector Coating Layer] The current collector coating layer 15 contains a conductive material. The conductive material in the current collector coating layer 15 preferably contains carbon, and more preferably a conductive material consisting solely of carbon. The current collector coating layer 15 is preferably a coating layer containing carbon particles such as carbon black and a binder. The binder of the current collector coating layer 15 is an organic substance, and examples thereof include polyacrylic acid, lithium polyacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, styrene butadiene rubber, polyvinyl alcohol, polyvinyl acetal, polyethylene oxide, polyethylene glycol, carboxymethyl cellulose, polyacrylonitrile, and polyimide. One type of binder may be used, or two or more types may be used in combination. The positive electrode current collector 11, in which the surface of the positive electrode current collector body 14 is coated with the current collector coating layer 15, can be produced, for example, by applying a current collector coating layer composition containing a conductive material, a binder, and a solvent to the surface of the positive electrode current collector body 14 using a known coating method, and then drying to remove the solvent. To achieve an area coverage of the current collector coating layer 15 on the surface of the positive electrode current collector body 14 facing the positive electrode active material layer 12 within the range described below, coating methods such as spraying and intermittent coating can be used. In the spraying method, the slurry is sprayed non-uniformly onto the surface of the positive electrode current collector body 14 facing the positive electrode active material layer 12 to achieve a predetermined area coverage. In the spraying method, the area coverage can be adjusted by adjusting the spray amount, droplet size, etc. In the intermittent coating method, for example, a mask with intermittent slits (portions through which the slurry passes) is placed on the surface of the positive electrode current collector body 14 facing the positive electrode active material layer 12, and the slurry is applied. In intermittent coating, the area coverage can be adjusted by adjusting the slit interval, etc.

[0036] The thickness of the current collector coating layer 15 is preferably 0.1 to 4.0 μm, more preferably 0.2 to 2.0 μm, and even more preferably 0.5 to 1.2 μm. A thickness equal to or greater than the lower limit of the above range provides an excellent impedance reduction effect, while a thickness equal to or less than the upper limit provides an excellent effect of improving peel strength. The thickness of the current collector coating layer can be measured by measuring the thickness of the coating layer in a transmission electron microscope (TEM) image or a scanning electron microscope (SEM) image of the cross section of the current collector coating layer. The thickness of the current collector coating layer does not have to be uniform. When current collector coating layers 15 are present on both sides of the positive electrode current collector body 14, the average thickness of both layers may be within the above range.

[0037] [Method for Manufacturing Positive Electrode] The positive electrode 1 of this embodiment can be manufactured by, for example, applying a positive electrode manufacturing composition containing positive electrode active material particles, a particulate binder, and a solvent onto a positive electrode current collector 11, drying the positive electrode manufacturing composition to remove the solvent, and forming a positive electrode active material layer 12 on the positive electrode current collector 11. In the active material layer forming step, a laminate in which the positive electrode active material layer 12 is formed on the positive electrode current collector 11 is pressed in the thickness direction against the surface of the positive electrode current collector 11 having the current collector coating layer 15 so that the ratio of the thickness of the current collector coating layer 15 to the thickness of the positive electrode active material layer 12 is greater than 0.000 and less than 0.020. The positive electrode manufacturing composition may contain a conductive additive. The thickness of the positive electrode active material layer 12 can be adjusted by sandwiching the laminate in which the positive electrode active material layer 12 is formed on the positive electrode current collector 11 between two flat-plate jigs and applying uniform pressure in the thickness direction. For example, a method of applying pressure using a roll press can be used.

[0038] The solvent for the positive electrode manufacturing composition is preferably an aqueous solvent. Examples of the aqueous solvent include water and alcohols such as methanol, ethanol, 1-propanol, and 2-propanol. One type of solvent may be used alone, or two or more types may be used in combination. In this embodiment, a particulate binder is used to prepare a positive electrode manufacturing composition using an aqueous solvent.

[0039] The particulate binder may be any of those described above. The binder content is preferably 3.2% by mass or less, more preferably 1.6% by mass or less, and even more preferably 0.8% by mass or less, relative to the total mass of the solids in the positive electrode manufacturing composition. When the binder content is equal to or less than the upper limit, the proportion of substances that do not contribute to lithium ion conduction in the resulting positive electrode active material layer 12 is reduced, increasing the true density of the positive electrode active material layer 12. Furthermore, the proportion of binder covering the surface of the positive electrode 1 is reduced, further increasing the lithium conductivity, thereby further improving the high-rate cycle characteristics.

[0040] When preparing a composition for manufacturing a positive electrode, the timing for adding a particulate binder to the solvent is preferably just before the preparation of the composition for manufacturing a positive electrode is completed. That is, in the preparation of the composition for manufacturing a positive electrode, the particulate binder is preferably added last. In general, in the preparation of a composition for manufacturing a positive electrode, a solvent is added to the composition for manufacturing a positive electrode from a state in which the solids concentration is high, and the concentration is reduced to a level that is easy to coat. However, if the binder is added when the solids concentration is high, the composition for manufacturing a positive electrode may aggregate and become impossible to coat.

[0041] The temperature during preparation of the positive electrode manufacturing composition is preferably 5 to 60°C, more preferably 10 to 50°C, and even more preferably 15 to 40°C. When the temperature is equal to or higher than the lower limit, the solvent freezes, preventing aggregation of the positive electrode manufacturing composition, promoting dispersion of the materials and resulting in a uniform coated surface. When the temperature is equal to or lower than the upper limit, heat generated by the positive electrode manufacturing composition or the apparatus during preparation causes the solvent to evaporate, increasing the solids concentration, and preventing aggregation of the positive electrode manufacturing composition.

[0042] When at least one of the conductive material and conductive additive coating the positive electrode active material contains carbon, the content of the conductive carbon is preferably 0.5 to 4.0 mass%, and more preferably 1.5 to 3.0 mass%, relative to the mass of the remainder after excluding the positive electrode current collector body 14 from the positive electrode 1. When the positive electrode 1 is composed of the positive electrode current collector body 14, the current collector coating layer 15, and the positive electrode active material layer 12, the mass of the remainder after excluding the positive electrode current collector body 14 from the positive electrode 1 is the total mass of the current collector coating layer 15 and the positive electrode active material layer 12. When the content of the conductive carbon is within the above range relative to the total mass of the positive electrode active material layer 12, the battery capacity is further improved, and a nonaqueous electrolyte secondary battery with better cycle characteristics can be realized.

[0043] The conductive carbon content relative to the mass of the remainder, excluding the positive electrode current collector body 14, from the positive electrode 1 can be measured by the <<Method for measuring conductive carbon content>> below, using a dried product (powder) obtained by peeling off all layers present on the positive electrode current collector body 14 and vacuum-drying it in a 120°C environment as the measurement target. The conductive carbon content measured by the <<Method for measuring conductive carbon content>> below includes carbon in the active material coating portion, carbon in the conductive additive, and carbon in the current collector coating layer 15. It does not include carbon in the binder.

[0044] The following method can be used, for example, to obtain the measurement object. First, the positive electrode 1 is punched out to a desired size, and the layer (powder) present on the positive electrode current collector body 14 is completely peeled off by immersing it in a solvent (for example, N-methyl-2-pyrrolidone (NMP)) and stirring it. Next, after confirming that no powder is adhering to the positive electrode current collector body 14, the positive electrode current collector body 14 is removed from the solvent, and a suspension containing the peeled powder and the solvent is obtained. The obtained suspension is dried at 120°C to completely volatilize the solvent, and the target measurement object (powder) is obtained.

[0045] <<Method for Measuring Conductive Carbon Content>> [Measurement Method A] The materials to be measured are uniformly mixed, and a sample (mass w1) is weighed out. Thermogravimetric differential heat (TG-DTA) measurement is performed in the following order of steps A1 and A2 to obtain a TG curve. From the obtained TG curve, the following first weight loss amount M1 (unit: mass%) and second weight loss amount M2 (unit: mass%) are determined. The conductive carbon content (unit: mass%) is obtained by subtracting M1 from M2. Step A1: In an argon flow of 300 mL / min, the temperature is increased from 30°C to 600°C at a heating rate of 10°C / min, and then the sample is held at 600°C for 10 minutes. From the mass w2, the first weight loss amount M1 is determined using the following formula (a1): M1 = (w1 - w2) / w1 x 100 ... (a1) Step A2: Immediately after step A1, the temperature is decreased from 600°C at a rate of 10°C / min, and the sample is held at 200°C for 10 minutes. Thereafter, the measurement gas is completely replaced from argon to oxygen, and the sample is heated from 200°C to 1000°C at a rate of 10°C / min in an oxygen stream of 100 mL / min, and held at 1000°C for 10 minutes. From the mass w3 obtained when the sample is held at 1000°C for 10 minutes, a second weight loss amount M2 (unit: mass%) is calculated using the following formula (a2): M2 = (w1 - w3) / w1 x 100 ... (a2)

[0046] [Measurement Method B] The measurement objects are mixed uniformly and 0.0001 mg of the sample is weighed out. The sample is combusted under the following combustion conditions, and the generated carbon dioxide is quantified using a CHN elemental analyzer to measure the total carbon amount M3 (unit: mass%) contained in the sample. The first weight loss amount M1 is also determined using the procedure of step A1 of Measurement Method A. The conductive carbon content (unit: mass%) is obtained by subtracting M1 from M3. [Combustion Conditions] Combustion furnace: 1150°C Reduction furnace: 850°C Helium flow rate: 200 mL / min Oxygen flow rate: 25-30 mL / min

[0047] [Measurement Method C] The total carbon amount M3 (unit: mass %) contained in the sample is measured in the same manner as in the above-mentioned Measurement Method B. In addition, the content M4 (unit: mass %) of carbon derived from the binder is determined by the following method. The content M4 (unit: mass %) of conductive carbon is obtained by subtracting M4 from M3. When the binder is polyvinylidene fluoride (PVDF:monomer (CH ２ CF ２) molecular weight 64), the fluoride ion (F － ) content (unit: mass%), the atomic weight of fluorine in the monomer constituting PVDF (19), and the atomic weight of carbon constituting PVDF (12) can be calculated using the following formula: PVDF content (unit: mass%) = fluoride ion content (unit: mass%) x 64 / 38 PVDF-derived carbon content M4 (unit: mass%) = fluoride ion content (unit: mass%) x 12 / 19 The fact that the binder is polyvinylidene fluoride can be confirmed by measuring the Fourier transform infrared spectrum of the sample or a liquid obtained by extracting the sample with N-N dimethylformamide solvent, and confirming the absorption derived from the C-F bond. Similarly, nuclear magnetic resonance spectroscopy of fluorine nuclei ( １９ This can also be confirmed by F-NMR measurement. If the binder is identified as something other than PVDF, the binder-derived carbon amount M4 can be calculated by determining the binder content (unit: mass%) and carbon content (unit: mass%) corresponding to its molecular weight. These methods are described in the following publicly known documents. Toray Research Center, The TRC News No. 117 (Sep. 2013), pp. 34-37, [searched February 10, 2021], Internet: <https: / / www.toray-research.co.jp / technical-info / trcnews / pdf / TRC117(34-37).pdf> Tosoh Analysis Center, Technical Report No. T1019 2017.09.20, [Retrieved February 10, 2021], Internet <http: / / www.tosoh-arc.co.jp / techrepo / files / tarc00522 / T1719N.pdf>

[0048] <<Conductive Carbon Analysis Method>> The conductive carbon constituting the active material coating portion of the positive electrode active material and the conductive carbon serving as a conductive additive can be distinguished by the following analytical method. For example, particles in a positive electrode active material layer are analyzed by transmission electron microscopy electron energy loss spectroscopy (TEM-EELS). Particles in which a carbon-derived peak near 290 eV is present only near the particle surface can be determined to be a positive electrode active material, while particles in which a carbon-derived peak is present even in the interior of the particle can be determined to be a conductive additive. Another method involves mapping analysis of particles in a positive electrode active material layer by Raman spectroscopy. Particles in which the carbon-derived G-band and D-band and peaks of oxide crystals derived from the positive electrode active material are simultaneously observed can be determined to be a positive electrode active material, while particles in which only the G-band and D-band are observed can be determined to be a conductive additive. Another method is to observe the cross section of the positive electrode active material layer using a scanning resistance microscope (SSRM). If there is a portion on the particle surface where the resistance is lower than inside the particle, the low resistance portion can be determined to be conductive carbon present in the active material coating portion. Portions that exist independently of such particles and have low resistance can be determined to be conductive additives. Note that trace amounts of carbon that are considered to be impurities or trace amounts of carbon that have unintentionally peeled off from the surface of the positive electrode active material during production are not determined to be conductive additives. Using these methods, it can be confirmed whether a conductive additive made of a carbon material is contained in the positive electrode active material layer.

[0049] The positive electrode 1 of this embodiment has a positive electrode current collector 11 and a positive electrode active material layer 12 present on the positive electrode current collector 11. The positive electrode active material layer 12 contains positive electrode active material particles, and at least a portion of the surface of the positive electrode active material particles is coated with a conductive material. The positive electrode current collector 11 has a positive electrode current collector body 14 and a current collector coating layer 15 that covers a portion of the surface of the positive electrode current collector body 14 facing the positive electrode active material layer 12. The positive electrode active material layer 12 contains a particulate binder, and therefore has a higher low-temperature output after storage and a higher energy density than conventional positive electrodes.

[0050] <Non-aqueous electrolyte secondary battery> The non-aqueous electrolyte secondary battery 10 of this embodiment shown in FIG. 2 includes the positive electrode 1 for the non-aqueous electrolyte secondary battery of this embodiment, a negative electrode 3, and a non-aqueous electrolyte. The non-aqueous electrolyte secondary battery 10 may further include a separator 2. Reference numeral 5 in the figure denotes an exterior body. In this embodiment, the positive electrode 1 includes a plate-shaped positive electrode current collector 11 and positive electrode active material layers 12 provided on both sides thereof. The positive electrode active material layers 12 are present on a portion of the surface of the positive electrode current collector 11. The edge of the surface of the positive electrode current collector 11 is a positive electrode current collector exposed portion 13 where the positive electrode active material layer 12 is not present. A terminal tab (not shown) is electrically connected to any location on the positive electrode current collector exposed portion 13. The negative electrode 3 includes a plate-shaped negative electrode current collector 31 and negative electrode active material layers 32 provided on both sides thereof. The negative electrode active material layers 32 are present on a portion of the surface of the negative electrode current collector 31. The edge of the surface of the negative electrode current collector 31 is a negative electrode current collector exposed portion 33 where the negative electrode active material layer 32 is not present. A terminal tab (not shown) is electrically connected to any location on the negative electrode current collector exposed portion 33. The shapes of the positive electrode 1, the negative electrode 3, and the separator 2 are not particularly limited. For example, they may be rectangular in plan view.

[0051] The nonaqueous electrolyte secondary battery 10 of this embodiment can be manufactured, for example, by preparing an electrode stack in which positive electrodes 1 and negative electrodes 3 are alternately stacked with separators 2 interposed therebetween, enclosing the electrode stack in an exterior (casing) 5 such as an aluminum laminate bag, and injecting and sealing a nonaqueous electrolyte (not shown). While FIG. 2 shows a typical structure in which negative electrode / separator / positive electrode / separator / negative electrode are stacked in this order, the number of electrodes can be changed as appropriate. One or more positive electrodes 1 are sufficient, and any number of positive electrodes 1 can be used depending on the desired battery capacity. The number of negative electrodes 3 and separators 2 is one more than the number of positive electrodes 1, and they are stacked so that the negative electrode 3 is the outermost layer.

[0052] [Negative Electrode] The negative electrode active material layer 32 contains a negative electrode active material. The negative electrode active material layer 32 may further contain a binder. The negative electrode active material layer 32 may further contain a conductive additive. The negative electrode active material is preferably in the form of particles. The negative electrode 3 can be manufactured, for example, by preparing a negative electrode manufacturing composition containing a negative electrode active material, a binder, and a solvent, applying the composition to the negative electrode current collector 31, and drying to remove the solvent to form the negative electrode active material layer 32. The negative electrode manufacturing composition may contain a conductive additive.

[0053] Examples of the negative electrode active material and the conductive additive include carbon materials such as graphite, graphene, hard carbon, ketjen black, acetylene black, carbon nanotubes (CNT), etc. The negative electrode active material and the conductive additive may each be used alone or in combination of two or more.

[0054] Examples of the material of the negative electrode current collector 31, and the binder and solvent in the composition for manufacturing a negative electrode, include the same materials as those of the material of the positive electrode current collector 11, and the binder and solvent in the composition for manufacturing a positive electrode. One type of binder and one type of solvent in the composition for manufacturing a negative electrode may be used alone, or two or more types may be used in combination.

[0055] The total content of the negative electrode active material and the conductive additive relative to the total mass of the negative electrode active material layer 32 is preferably 80.0 mass % to 99.9 mass %, more preferably 85.0 mass % to 98.0 mass %.

[0056] [Separator] The separator 2 is disposed between the negative electrode 3 and the positive electrode 1 to prevent short circuits and the like. The separator 2 may hold a non-aqueous electrolyte, which will be described later. The separator 2 is not particularly limited, and examples thereof include a porous polymer membrane, a nonwoven fabric, and glass fiber. An insulating layer may be provided on one or both surfaces of the separator 2. The insulating layer is preferably a layer having a porous structure in which insulating fine particles are bound by a binder for the insulating layer.

[0057] The separator 2 may contain various plasticizers, antioxidants, and flame retardants. Examples of antioxidants include phenolic antioxidants such as hindered phenolic antioxidants, monophenolic antioxidants, bisphenolic antioxidants, and polyphenolic antioxidants; hindered amine antioxidants; phosphorus-based antioxidants; sulfur-based antioxidants; benzotriazole-based antioxidants; benzophenone-based antioxidants; triazine-based antioxidants; and salicylic acid ester-based antioxidants. Phenol-based antioxidants and phosphorus-based antioxidants are preferred.

[0058] [Non-aqueous electrolyte] The non-aqueous electrolyte fills the space between the positive electrode 1 and the negative electrode 3. For example, a non-aqueous electrolyte known in lithium ion secondary batteries, electric double layer capacitors, etc. can be used. The non-aqueous electrolyte used in manufacturing the non-aqueous electrolyte secondary battery 10 contains an organic solvent, an electrolyte, and an additive. After manufacturing, particularly after initial charging, the non-aqueous electrolyte secondary battery 10 contains the organic solvent and the electrolyte, and may further contain residues or traces derived from the additive.

[0059] The organic solvent is preferably one that is resistant to high voltage, and examples thereof include polar solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, and methyl acetate, or mixtures of two or more of these polar solvents.

[0060] The electrolyte is not particularly limited, and examples thereof include lithium-containing salts such as lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium trifluoroacetate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide, or mixtures of two or more of these salts.

[0061] The non-aqueous electrolyte preferably contains a lithium imide salt represented by the following formula (1): LiN(SO ２ R) ２(1) [wherein R is a fluorine atom or C ｘ F （２ｘ＋１） where x is an integer of 1 to 3.

[0062] Examples of the lithium imide salt represented by the above formula (1) include lithium bis(fluorosulfonyl)imide (LIFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI).

[0063] The content of the lithium imide salt in the non-aqueous electrolyte is preferably 5% by mass or more and 90% by mass or less, more preferably 15% by mass or more and 80% by mass or less, and even more preferably 30% by mass or more and 75% by mass or less, based on the total mass of the non-aqueous electrolyte. When the content of the lithium imide salt is equal to or more than the lower limit, the output characteristics of the non-aqueous electrolyte secondary battery are improved. When the content of the lithium imide salt is equal to or less than the upper limit, the durability of the battery is improved.

[0064] The nonaqueous electrolyte secondary battery of this embodiment includes the positive electrode for the nonaqueous electrolyte secondary battery of the above embodiment, and therefore has excellent low-temperature output, low-temperature output after storage, and energy density. The nonaqueous electrolyte secondary battery of this embodiment can be used as a lithium ion secondary battery for various applications, such as industrial, consumer, automotive, and residential use. The use form of the nonaqueous electrolyte secondary battery of this embodiment is not particularly limited. For example, it can be used in a battery module configured by connecting multiple nonaqueous electrolyte secondary batteries in series or parallel, a battery pack including multiple electrically connected battery modules and a battery control system, or a battery system including multiple electrically connected battery modules and a battery control system.

[0065] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to these examples.

[0066] <Evaluation Method> [Method of Measuring Resistance] A cell was prepared so that the rated capacity was 1 Ah. The resulting cell was charged at a constant current of 0.2 C rate (i.e., 200 mA) at a cut-off voltage of 3.6 V in an environment of 25°C (room temperature). Then, the cell was charged at a constant voltage with a cut-off current (i.e., 20 mA) that was 1 / 10 of the charging current. The impedance of the cell was then measured at room temperature (25°C) and a frequency of 1 kHz, and the resistance value was recorded. The impedance was measured using a four-terminal method in which a current terminal and a voltage terminal were attached to each of the positive and negative electrode tabs. An impedance analyzer manufactured by BioLogic was used as an example to measure the impedance. The measurement results were evaluated based on the following criteria. The best battery characteristics were evaluated as "A," the next best battery characteristics after A as "B," the next best battery characteristics after B as "C," and the poorest battery characteristics as "D." The same evaluations were made in the following evaluations (measurements). A: 25mΩ or less B: 25mΩ or more but 30mΩ or less C: 30mΩ or more but 35mΩ or less D: Greater than 35mΩ

[0067] [Low-Temperature Output Evaluation] The obtained nonaqueous electrolyte secondary battery was fully charged (100% SOC) at 25°C, and then discharged at 1C from the fully charged state at -30°C. The potential was observed 30 seconds after the start of discharge (equivalent to 8 mAh). The potential of the nonaqueous electrolyte secondary battery was measured. The measurement results were evaluated based on the following criteria: A: 2.3 V or higher B: Less than 2.3 V and 1.9 V or higher C: Less than 1.9 V and 1.8 V or higher D: Less than 1.8 V

[0068] [Low-temperature output evaluation after storage] The low-temperature output evaluation after storage was performed according to the following procedure. The obtained nonaqueous electrolyte secondary battery was fully charged (100% SOC) at 25°C and then stored in a thermostatic chamber at 70°C. After 20 days, the battery was removed and low-temperature output evaluation was performed using the method described above. The degree of voltage maintenance relative to the value of the low-temperature output evaluation (initial low-temperature output evaluation) described above was calculated. The calculation results were evaluated based on the following criteria: A: 90% or more compared to the initial value B: Less than 90% and 80% or more compared to the initial value C: Less than 80% and 70% or more compared to the initial value D: Less than 70%

[0069] [Evaluation of Energy Density] The energy density was evaluated according to the following procedures (1) to (3). (1) A cell was prepared so that the rated capacity was 1 Ah, and the mass (unit: kg) of the cell was measured. (2) The obtained cell was charged at a constant current of 0.2 C rate (i.e., 200 mA) at a cut-off voltage of 3.6 V in a 25°C (room temperature) environment, and then charged at a constant voltage with a cut-off current (i.e., 20 mA) that was 1 / 10 of the charge current, followed by a 30-minute rest in an open-circuit state. (3) The cell was discharged at a constant current of 0.2 C rate to a cut-off voltage of 2.5 V. The total discharge power (unit: Wh) measured from the start to the end of discharge was divided by the mass (unit: kg) of the cell measured in (1) to calculate the gravimetric energy density (unit: Wh / kg). The calculation results were evaluated based on the following criteria. A: 175Wh / kg or more B: Less than 175Wh / kg and 165Wh / kg or more C: Less than 165Wh / kg and 150Wh / kg or more D: Less than 150Wh / kg

[0070] [Measurement of the size of coarse particles in the composition] The size of coarse particles in the composition during coating was measured. A grind gauge (measurement scale: 0 to 100 μm) manufactured by BYK was used as a device for measuring the size of coarse particles. The composition was cast into the groove of the measuring device, and the smallest scale number at which the composition was not cast due to agglomerates was recorded as the measured value.

[0071] [Electrode Evaluation] The coated electrodes were evaluated. The coated, dried, and pressed electrodes were cut into 150 mm squares. The electrode weight per area after cutting was measured as M1 (mg / cm ２) was then punched out from the center of the electrode to a predetermined size of 100 mm square. The electrode weight per area after punching was defined as M2. The peeling rate due to punching of the electrode layer was calculated using the following formula (2) and used as the electrode strength. (Peeling rate) = 100 - M2 / M1 x 100 A low peeling rate indicates high electrode strength. Factors that can cause a decrease in strength include insufficient dispersion of the active material, insufficient binder required for binding, or even if the binder is theoretically sufficient, aggregation may prevent the binder from performing its functions. Coating performance was evaluated according to the following criteria. A: Minimum scale number is 60 μm or less, and the peeling rate is 5% or less. B: Minimum scale number is 70 μm or less, or the peeling rate is 7% or less. C: Minimum scale number is 80 μm or less, or the peeling rate is 9% or less. D: Minimum scale number is 90 μm or more, or the peeling rate is 10% or more. (Note that the "minimum scale number" here is the value measured in the above item [Measurement of the size of coarse particles in the composition].)

[0072] <Production Example: Production of Negative Electrode> 100 parts by mass of artificial graphite as a negative electrode active material, 1.5 parts by mass of styrene-butadiene rubber as a binder, 1.5 parts by mass of carboxymethyl cellulose Na as a thickener, and water as a solvent were mixed to obtain a composition for producing a negative electrode with a solid content of 50% by mass. The obtained composition for producing a negative electrode was coated on both sides of copper foil (thickness: 8 μm), vacuum dried at 100° C., and then pressed under pressure to obtain a negative electrode sheet.

[0073] Example 1: Carbon-coated lithium iron phosphate (hereinafter also referred to as "carbon-coated active material"; 99 parts by mass, average particle diameter 1.5 μm, content 1.5% by mass) was used as the positive electrode active material. No conductive additive was added to the positive electrode manufacturing composition. First, a positive electrode current collector was fabricated by coating both the front and back surfaces of a positive electrode current collector body with current collector coating layers using the following method. Aluminum foil (thickness 15 μm) was used as the positive electrode current collector body. A slurry was obtained by mixing 100 parts by mass of carbon black, 40 parts by mass of polyvinylidene fluoride as a binder, and N-methyl-2-pyrrolidone (NMP) as a solvent. The obtained slurry was applied to both sides of the positive electrode current collector body by gravure coating to a coating thickness of 1 μm, and then dried to remove the solvent, yielding a positive electrode current collector having a current collector coating layer.

[0074] Next, a positive electrode active material layer was formed using the following method. 99 parts by mass of carbon-coated active material, 0.5 parts by mass of acrylic particles (acrylic ester-acrylic acid copolymer particles, average particle diameter: 300 nm) as a binder, 0.5 parts by mass of carboxymethyl cellulose, and water as a solvent were mixed in a mixer to obtain a positive electrode manufacturing composition. The amount of solvent used was the amount necessary for applying the positive electrode manufacturing composition. The positive electrode manufacturing composition was applied to both sides of a positive electrode current collector, pre-dried, and then vacuum-dried at 120°C to form a positive electrode active material layer with a thickness of 70 μm. The resulting laminate was pressed under a load of 10 kN to obtain a positive electrode sheet. SEM observation of the cross section of the obtained positive electrode sheet revealed an image in which the particulate binder appeared to be attached to the surface of the carbon-coated active material.

[0075] A non-aqueous electrolyte secondary battery having the configuration shown in Fig. 2 was manufactured by the following method: LiPF as an electrolyte was added to a solvent in which ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed in a volume ratio of EC:PC:DEC of 30:5:65. ６and LIFSI were dissolved in a mass ratio of 65:35 and 1 mol / L to prepare a nonaqueous electrolyte solution. The positive electrode obtained in this example and the negative electrode obtained in Production Example 1 were alternately stacked with a separator interposed therebetween to prepare an electrode laminate in which the outermost layer was a negative electrode. A polyolefin film (thickness: 15 μm) was used as the separator. In the process of preparing the electrode laminate, the separator and the positive electrode were first stacked, and then the negative electrode was stacked on the separator. Terminal tabs were electrically connected to the exposed positive electrode current collector and exposed negative electrode current collector portions of the electrode laminate, respectively. The electrode laminate was sandwiched between aluminum laminate films so that the terminal tabs protruded to the outside, and three sides were laminated and sealed. Next, a nonaqueous electrolyte solution was injected from the remaining unsealed side, and the laminate was vacuum-sealed to produce a nonaqueous electrolyte secondary battery (laminated cell).

[0076] Table 1 shows the evaluation of the resistance of the positive electrode, the evaluation of the low-temperature output of the nonaqueous electrolyte secondary battery, the evaluation of the low-temperature output of the nonaqueous electrolyte secondary battery after storage, the evaluation of the volumetric energy density of the nonaqueous electrolyte secondary battery, and the evaluation of the coatability of the composition for producing a positive electrode.

[0077] [Observation of Positive Electrode Active Material Layer] The positive electrode active material layer was observed using a scanning electron microscope (SEM). The results are shown in Fig. 3. Fig. 3 is an image of a scanning electron microscope image showing a cross section of the positive electrode active material constituting the positive electrode active material layer. As shown in Fig. 3, it can be assumed that the particulate binder 110 has been crushed and adhered to the surface of the positive electrode active material 100.

[0078] [Example 2] A positive electrode of Example 2 was produced in the same manner as in Example 1, except that the carbon-coated active material and acrylic particles were used in the positive electrode-producing composition in an amount of 98 parts by mass and 1.5 parts by mass, respectively. The above evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0079] [Example 3] A positive electrode of Example 3 was produced in the same manner as in Example 1, except that the carbon-coated active material in the composition for producing a positive electrode was 96.2 parts by mass and the acrylic particles serving as a binder were 3.3 parts by mass. The above evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0080] [Example 4] A positive electrode of Example 4 was prepared in the same manner as in Example 1, except that 0.4 parts by mass of carbon black as a conductive additive was added to the composition for preparing a positive electrode, relative to 100 parts by mass of the total mass of the carbon-coated active material, acrylic particles, and carboxymethyl cellulose. The above evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0081] [Example 5] A positive electrode of Example 5 was prepared in the same manner as in Example 1, except that 1.8 parts by mass of carbon black as a conductive additive was added to the composition for preparing a positive electrode, relative to 100 parts by mass of the total mass of the carbon-coated active material, acrylic particles, and carboxymethyl cellulose. The above evaluations were carried out in the same manner as in Example 1. The results are shown in Table 2.

[0082] [Example 6] A positive electrode of Example 6 was produced in the same manner as in Example 1, except that the average particle diameter of the acrylic particles was set to 150 nm. The above evaluations were carried out in the same manner as in Example 1. The results are shown in Table 2.

[0083] [Example 7] A positive electrode of Example 7 was produced in the same manner as in Example 1, except that the average particle diameter of the acrylic particles was set to 500 nm. The above evaluations were carried out in the same manner as in Example 1. The results are shown in Table 2.

[0084] [Comparative Example 1] A positive electrode of Comparative Example 1 was produced in the same manner as in Example 1, except that a composition for producing a positive electrode was prepared using 1 part by mass of carboxymethyl cellulose as a binder. When the cross section of the obtained positive electrode sheet was observed using an SEM, no image was obtained that indicated that the particulate binder had adhered to the surface of the active material. The above evaluation was carried out in the same manner as in Example 1. The results are shown in Table 3.

[0085] A positive electrode of Comparative Example 2 was produced in the same manner as in Example 1, except that a positive electrode current collector body consisting only of aluminum foil (thickness 15 μm) without a current collector coating layer was used as the positive electrode current collector. The above evaluations were carried out in the same manner as in Example 1. The results are shown in Table 3.

[0086]

[0087]

[0088]

[0089] The results shown in Tables 1 and 2 indicate that Examples 1 to 7 are excellent in the positive electrode resistance, low-temperature output of the nonaqueous electrolyte secondary battery, low-temperature output after storage of the nonaqueous electrolyte secondary battery, energy density of the nonaqueous electrolyte secondary battery, and applicability of the positive electrode manufacturing composition. The results shown in Table 3 indicate that Comparative Example 1 is inferior in the positive electrode resistance, low-temperature output of the nonaqueous electrolyte secondary battery, low-temperature output after storage of the nonaqueous electrolyte secondary battery, and applicability of the positive electrode manufacturing composition. Comparative Example 2 is inferior in the positive electrode resistance, low-temperature output of the nonaqueous electrolyte secondary battery, and low-temperature output after storage of the nonaqueous electrolyte secondary battery.

[0090] REFERENCE SIGNS LIST 1 Positive electrode (positive electrode for non-aqueous electrolyte secondary battery) 2 Separator 3 Negative electrode 5 Exterior body 10 Non-aqueous electrolyte secondary battery 11 Positive electrode current collector 12 Positive electrode active material layer 13 Positive electrode current collector exposed portion 14 Positive electrode current collector body 15 Current collector coating layer 31 Negative electrode current collector 32 Negative electrode active material layer 33 Negative electrode current collector exposed portion 100 Positive electrode active material 110 Particulate binder

Claims

1. It comprises a positive electrode current collector and a positive electrode active material layer present on the positive electrode current collector, The positive electrode active material layer includes positive electrode active material particles, and at least a portion of the surface of the positive electrode active material particles is coated with a conductive material. The positive electrode current collector comprises a positive electrode current collector body and a current collector covering layer that covers a portion of the surface of the positive electrode current collector body on the positive electrode active material layer side. The positive electrode active material layer contains particulate binder, The content of the positive electrode active material particles is 93% by mass or more relative to the total mass of the positive electrode active material layer. The positive electrode for a non-aqueous electrolyte secondary battery comprises a current collector coating layer containing a conductive material, and the conductive material containing carbon.

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the particulate binder is 4% by mass or less with respect to the total mass of the positive electrode active material layer.

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material layer contains a conductive additive, and the content of the conductive additive is 5% by mass or less relative to the total mass of the positive electrode active material layer.

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the average particle size of the particulate binder is 50 nm or more.

5. The positive electrode for a non-aqueous electrolyte secondary battery according to Claim 1, wherein the thickness of the current collector coating layer is 0.1 to 4.0 μm.

6. A non-aqueous electrolyte secondary battery comprising a positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, a negative electrode, and a non-aqueous electrolyte present between the positive electrode for a non-aqueous electrolyte secondary battery and the negative electrode.

7. A battery module or battery storage system comprising a plurality of non-aqueous electrolyte secondary batteries as described in claim 6.

8. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte present between the positive electrode and the negative electrode, The positive electrode comprises a positive electrode current collector and a positive electrode active material layer present on the positive electrode current collector. The positive electrode active material layer includes positive electrode active material particles, and at least a portion of the surface of the positive electrode active material particles is coated with a conductive material. The positive electrode current collector comprises a positive electrode current collector body and a current collector covering layer that covers a portion of the surface of the positive electrode current collector body on the positive electrode active material layer side. The positive electrode active material layer contains particulate binder, The non-aqueous electrolyte secondary battery comprises a lithium imide salt represented by the following formula (1). LiN(SO2R)2 (1) [However, R represents a fluorine atom or C x F (2x + 1), and x is an integer from 1 to 3.]

9. The non-aqueous electrolyte secondary battery according to claim 8, wherein the non-aqueous electrolyte contains a lithium imide salt, and the content of the lithium imide salt is 5% by mass or more and 90% by mass or less with respect to the total amount of the non-aqueous electrolyte.

10. A battery module or battery storage system comprising a plurality of non-aqueous electrolyte secondary batteries according to claim 8 or 9.