Positive electrode for lithium ion secondary battery and lithium ion secondary battery

Abstract

To provide a cathode for a lithium ion secondary battery, which is superior in lithium ion input and output characteristics, and a lithium ion secondary battery.SOLUTION: The cathode for a lithium ion secondary battery comprises: a collector; and a cathode active material layer on at least one face of the collector. The cathode active material layer has a plurality of cathode active materials, a plurality of coating layers which at least partially cover the plurality of cathode active materials respectively. The plurality of coating layers each contain a covering material having an isoelectric point of 7 or below. The cathode active material layer is divided into a first region, and a second region located farther away from the collector than the first region with respect to a center thereof in a thickness direction. The difference between an average weight concentration (wt.%) of the covering material in the first region, and an average weight concentration (wt.%) of the covering material in the second region is 1 wt.% or more.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery. [Background technology]

[0002] Lithium ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and notebook computers, as well as hybrid cars.

[0003] For example, Patent Document 1 describes a lithium ion secondary battery in which the positive electrode active material has a composite coating film of a specific oxide or fluoride and a phosphate compound. The composite coating film suppresses oxidative decomposition of the positive electrode active material, and the lithium ion secondary battery having the composite coating film has excellent thermal stability during overcharging. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2014 / 128903 Summary of the Invention [Problem to be solved by the invention]

[0005] The composite coating film described in Patent Document 1 is inactive in battery reactions. The composite coating film causes an increase in internal resistance and reduces the input / output characteristics of the lithium ion secondary battery.

[0006] The present disclosure has been made in consideration of the above problems, and has an object to provide a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery that are excellent in input / output characteristics. [Means for solving the problem]

[0007] The present inventors have found that by creating a concentration difference of the coating material in the thickness direction of the positive electrode active material layer, the impregnation of the positive electrode active material layer with an electrolyte solution can be improved, and the input / output characteristics of the lithium ion secondary battery can be improved.

[0008] (1) A positive electrode for a lithium ion secondary battery according to a first aspect includes a current collector and a positive electrode active material layer in contact with at least one surface of the current collector, the positive electrode active material layer having a plurality of positive electrode active materials and a plurality of coating layers respectively coating at least a portion of the plurality of positive electrode active materials, each of the plurality of coating layers including a coating material having an isoelectric point of 7 or less, the positive electrode active material layer being divided, based on the center in the thickness direction, into a first region and a second region located farther from the current collector than the first region, and a difference between an average weight percent concentration of the coating material in the first region and an average weight percent concentration of the coating material in the second region is 1 weight % or more.

[0009] (2) In the positive electrode for a lithium ion secondary battery according to the above aspect, the second region may have an average weight percent concentration of the coating material higher than that of the first region.

[0010] (3) In the positive electrode for a lithium ion secondary battery according to the above aspect, the coating material may be 0.1 parts by weight or more and 5.0 parts by weight or less per 100 parts by weight of the positive electrode active material.

[0011] (4) In the positive electrode for a lithium ion secondary battery according to the above aspect, the average thickness of the plurality of coating layers may be 25 μm or less.

[0012] (5) In the positive electrode for a lithium ion secondary battery according to the above aspect, the plurality of positive electrode active materials may include first particles and second particles having different average particle sizes.

[0013] (6) In the positive electrode for a lithium ion secondary battery according to the above aspect, an average particle size Da of the positive electrode active materials belonging to the first region among the plurality of positive electrode active materials, an average particle size Db of the positive electrode active material belonging to the second region among the plurality of positive electrode active materials, and a thickness h of the positive electrode active material layer may satisfy the relationship 2≦Db / Da≦2h.

[0014] (7) A lithium ion secondary battery according to a second aspect includes the positive electrode for lithium ion secondary batteries according to the above aspect. Effect of the Invention

[0015] The positive electrode for a lithium ion secondary battery and the lithium ion secondary battery according to the above aspects have excellent input / output characteristics of lithium ions. [Brief description of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a positive electrode according to the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and may be appropriately modified and implemented within the scope of the present invention.

[0018] "Lithium-ion secondary battery" Fig. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. The lithium ion secondary battery 100 shown in Fig. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of connected terminals 60, 62. The non-aqueous electrolyte is contained within the exterior body 50.

[0019] (Power generation element) The power generating element 40 includes a positive electrode 20, a negative electrode 30, and a separator 10.

[0020] <Positive electrode> The positive electrode 20 has, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22. Fig. 2 is an enlarged cross-sectional view of a characteristic portion of the positive electrode 20 according to this embodiment.

[0021] [Positive electrode current collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, etc. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0022] [Cathode active material layer] The positive electrode active material layer 24 includes, for example, a plurality of particles 70, 80, a conductive additive (not shown), and a binder (not shown). The conductive additive and the binder are present between the plurality of particles 70, 80.

[0023] Each of the particles 70 has a positive electrode active material 71 and a coating layer 72. The coating layer 72 coats at least a portion of the positive electrode active material 71. Each of the particles 80 has a positive electrode active material 81 and a coating layer 82. The coating layer 82 coats at least a portion of the positive electrode active material 81. The particles 70 and 80 have different average particle sizes, for example. The particle size is the sum of the long axis length and the short axis length of a particle divided by 2. The shapes of the particles 70 and 80 do not matter, and are, for example, irregular.

[0024] The positive electrode active materials 71 and 81 each contain an electrode active material capable of reversibly undergoing lithium ion occlusion and release, lithium ion desorption and intercalation, or doping and dedoping of lithium ions and counter anions. The positive electrode active material 71 and the positive electrode active material 81 are made of, for example, the same material.

[0025] The positive electrode active materials 71 and 81 are each, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMnO2), lithium manganese spinel (LiMn2O4), and the general formula: LiNi x Co y Mn z M a O2 compound (in the general formula, x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦ a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), lithium vanadium compound (LiV2O5), olivine type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). Each of the positive electrode active materials 71 and 81 may be an organic substance. For example, each of the positive electrode active materials 71 and 81 may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0026] The average particle diameter of the positive electrode active material 71 is different from the average particle diameter of the positive electrode active material 81. When measuring the particle size distribution of the positive electrode active material contained in the positive electrode active material layer 24, if two peaks can be confirmed, it can be regarded that the positive electrode active material has two types of positive electrode active materials 71 and 81 with different average particle diameters. The positive electrode active material 71 is an example of the first particle. The positive electrode active material 81 is an example of the second particle.

[0027] The positive electrode active material layer 24 can be divided into two regions in the thickness direction. Hereinafter, the region near the positive electrode current collector 22 with respect to the center in the thickness direction is referred to as a first region R1, and the region located farther from the positive electrode current collector 22 than the first region R1 is referred to as a second region R2.

[0028] Positive electrode active material 71 mainly belongs to first region R1. Positive electrode active material 81 mainly belongs to second region R2. The region to which positive electrode active materials 71, 81 belong is determined by the position of the center of gravity of positive electrode active materials 71, 81.

[0029] The average particle size Da of the positive electrode active material belonging to the first region R1, the average particle size Db of the positive electrode active material belonging to the second region R2, and the thickness h of the positive electrode active material layer 24 satisfy, for example, the relationship 2≦Db / Da≦2h. When these relationships are satisfied, a difference in concentration of the coating material constituting the coating layers 72, 82 is likely to occur between the first region R1 and the second region R2. The average particle size Da of the positive electrode active material belonging to the first region R1 approximately matches the average particle size of the positive electrode active material 71. The average particle size Db of the positive electrode active material belonging to the second region R2 approximately matches the average particle size of the positive electrode active material 81.

[0030] Each of the coating layers 72, 82 includes a coating material with an isoelectric point of 7 or less. The isoelectric point is the pH value at which the zeta potential is zero. The isoelectric point is a value measured at room temperature (25°C). The zeta potential of a substance varies depending on the system (e.g., pH value) in which the substance is present, and is zero in a particular system (particular pH). In a state in which the zeta potential is zero, the substance does not cause interfacial conductive phenomena such as electrophoresis.

[0031] Examples of materials with an isoelectric point of 7 or less include WO3, SiO2, V3O8, and TiO2. These materials can be used as coating materials.

[0032] The coating layers 72, 82 exhibit a positive zeta potential at a pH below the isoelectric point and a negative zeta potential at a pH above the isoelectric point. The coating layers 72, 82 exhibit a negative zeta potential in a non-aqueous electrolyte. This is because the pH of the non-aqueous electrolyte is around 7, which is a higher pH value than the pH at which the coating material has its isoelectric point. The coating layers 72, 82, which exhibit a negative zeta potential in a non-aqueous electrolyte, attract lithium ions in the non-aqueous electrolyte.

[0033] The average weight percent concentration of the coating material is different between the first region R1 and the second region R2. The absolute value of the difference in the average weight percent concentration of the coating material between the first region R1 and the second region R2 is 1 weight percent or more. For example, the average weight percent concentration of the coating material in the second region R2 is higher than that in the first region R1. The average weight percent concentration of the coating material is obtained by separating the first region R1 and the second region R2, measuring the weight of each region, and then separating the coating material from each of the first region R1 and the second region R2 and measuring the weight. The average weight percent concentration of the coating material may also be obtained by using energy dispersive X-ray analysis (EDS) using a scanning electron microscope (SEM). The atomic weight mapping of the EDS is used to calculate the atomic number concentration of the atoms constituting the coating materials 72 and 82 relative to the atomic number concentration of the atoms constituting the positive electrode active materials 71 and 81. For example, when the positive electrode active materials 71, 81 are LiCoO2 and the coating materials 72, 82 are WO3, the atomic concentration of Co atoms and W atoms is obtained.

[0034] The positive electrode active material layer 24 preferably contains 0.1 parts by weight or more and 5.0 parts by weight or less of the coating material with respect to 100 parts by weight of the positive electrode active material 71, 81. The weight ratio of the coating material to the positive electrode active material 71, 81 corresponds to the coating amount of the coating layer 72, 82 with respect to the positive electrode active material 71, 81. The coating layer 72, 82 does not absorb lithium ions and is therefore inactive in charge and discharge reactions. If the coating amount of the coating layer 72, 82 is too large, this causes an increase in the resistance of the lithium ion secondary battery 100. If the coating amount of the coating layer 72, 82 is too small, the positive electrode active material 71, 81 is easily oxidized and decomposed during overcharging.

[0035] The average thickness of each of the coating layers 72, 82 is, for example, 25 μm or less. The average thickness of each of the coating layers 72, 82 is, for example, 0.01 μm or more. The average thickness is measured using a scanning electron microscope (SEM). First, 10 particles 70, 80 are extracted from a cross-sectional image of the positive electrode active material layer 24 taken by a scanning electron microscope, and the thickness of each of the coating layers 72, 82 is measured. Then, the measured thicknesses of each of the coating layers 72, 82 are averaged to obtain the average thickness of each of the coating layers 72, 82. If the thickness of the coating layers 72, 82 is too thick, it causes an increase in the resistance of the lithium ion secondary battery 100. If the thickness of the coating layers 72, 82 is too thin, the positive electrode active materials 71, 81 are easily oxidized and decomposed during overcharging.

[0036] The conductive assistant enhances electronic conductivity between the particles 70 and 80. Examples of the conductive assistant include carbon powders such as carbon black and Ketjen black, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. The conductive assistant is preferably a carbon material such as carbon black or Ketjen black.

[0037] The binder binds the active materials together. Any known binder can be used. The binder is, for example, a fluororesin. Examples of the fluororesin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc.

[0038] In addition to the above, the binder may be, for example, vinylidene fluoride-based fluororubber such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinylether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), or vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber). The binder may be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, or the like.

[0039] <Negative electrode> The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[0040] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[0041] [Negative electrode active material layer] The negative electrode active material layer 34 contains a negative electrode active material and may further contain a conductive assistant, a binder, and a solid electrolyte, as necessary.

[0042] The negative electrode active material may be any compound capable of occluding and releasing ions, and known negative electrode active materials used in lithium ion secondary batteries can be used. Examples of the negative electrode active material include metallic lithium, lithium alloys, graphites (natural graphite, artificial graphite) capable of occluding and releasing ions, carbon nanotubes, graphitizable carbon, easily graphitizable carbon, carbon materials such as low-temperature calcined carbon, metals such as aluminum, silicon, tin, and germanium that can combine with metals such as lithium, SiO x (0 < x < 2), amorphous compounds mainly composed of oxides such as tin dioxide, particles containing lithium titanate (Li4Ti5O 12 ), etc.

[0043] The negative electrode active material layer 34 may contain silicon, tin, or germanium. Silicon, tin, or germanium may exist as a single element or as a compound. The compound is, for example, an alloy, an oxide, or the like. As an example, when the negative electrode active material is silicon, the negative electrode 30 may be called an Si negative electrode. The negative electrode active material may be, for example, a mixed system of a single substance or compound of silicon, tin, or germanium and a carbon material. The carbon material is, for example, natural graphite. Also, the negative electrode active material may be, for example, one in which the surface of a single substance or compound of silicon, tin, or germanium is coated with carbon. The carbon material and the coated carbon enhance the conductivity between the negative electrode active material and the conductive assistant. When the negative electrode active material layer contains silicon, tin, or germanium, the capacity of the lithium ion secondary battery 100 increases.

[0044] The negative electrode active material layer 34 may contain lithium, for example, as described above. The lithium may be metallic lithium or a lithium alloy. The negative electrode active material layer 34 may be metallic lithium or a lithium alloy. The lithium alloy is, for example, an alloy of one or more elements selected from the group consisting of Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, and Al, and lithium. As an example, when the negative electrode active material is metallic lithium, the negative electrode 30 may be called an Li negative electrode. The negative electrode active material layer 34 may be a sheet of lithium.

[0045] The negative electrode 30 may not have the negative electrode active material layer 34 during fabrication, and may comprise only the negative electrode current collector 32. When the lithium-ion secondary battery 100 is charged, metallic lithium is precipitated on the surface of the negative electrode current collector 32. Metallic lithium is elemental lithium from which lithium ions have been precipitated, and the metallic lithium functions as the negative electrode active material layer 34.

[0046] The conductive assistant and binder may be the same as those in the positive electrode 20. The binder in the negative electrode 30 may be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc., in addition to those listed in the positive electrode 20. The cellulose may be, for example, carboxymethyl cellulose (CMC).

[0047] <separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30 and prevents a short circuit between the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0048] The separator 10 has, for example, an electrically insulating porous structure. The separator 10 may be, for example, a monolayer or laminate of a film made of a polyolefin such as polyethylene or polypropylene, or a stretched film of a mixture of the above resins, or a fibrous nonwoven fabric made of at least one material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be, for example, a solid electrolyte. The solid electrolyte may be, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte.

[0049] (Terminal) The terminals 60 and 62 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 60 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 62 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60 and 62 are responsible for electrical connection to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. The terminals 60 and 62 are preferably protected with insulating tape to prevent short circuits.

[0050] (Exterior body) The power generating element 40 and the non-aqueous electrolyte are sealed inside the exterior body 50. The exterior body 50 prevents the non-aqueous electrolyte from leaking to the outside and prevents moisture and the like from entering the lithium-ion secondary battery 100 from the outside.

[0051] 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0052] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used as the resin layer 54. The materials constituting the resin layer 54 may be different between the inside and outside. For example, a polymer with a high melting point such as polyethylene terephthalate (PET) or polyamide (PA) can be used as the material for the outside, and polyethylene (PE), polypropylene (PP), etc. can be used as the material for the polymer film on the inside.

[0053] (Non-aqueous electrolyte) The non-aqueous electrolyte is sealed in the exterior body 50 and impregnates the power generating element 40. The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in the non-aqueous solvent.

[0054] The non-aqueous solvent contains, for example, a cyclic carbonate and a chain carbonate. The cyclic carbonate solvates the electrolyte. The cyclic carbonate is, for example, ethylene carbonate, propylene carbonate, and butylene carbonate. The cyclic carbonate preferably contains at least propylene carbonate. The chain carbonate reduces the viscosity of the cyclic carbonate. The chain carbonate is, for example, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent may also contain methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like.

[0055] The volume ratio of the cyclic carbonate to the chain carbonate in the non-aqueous solvent is preferably 1:9 to 1:1.

[0056] The electrolyte is, for example, a lithium salt. The electrolyte is, for example, LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, etc. The lithium salt may be used alone or in combination of two or more. From the viewpoint of the degree of ionization, it is preferable that the electrolyte contains LiPF6.

[0057] "Method of manufacturing lithium-ion secondary batteries" The positive electrode 20 is obtained by applying a paste-like positive electrode slurry (coating) to at least one surface of a positive electrode current collector 22 and drying it. As the positive electrode current collector 22, a commercially available product can be used.

[0058] When preparing the positive electrode slurry, first, the particles 70, 80, a conductive assistant, a binder, and a solvent are mixed. The positive electrode slurry contains a mixture of particles 70, 80 with different average particle sizes. The particles 70, 80 are obtained by coating the surfaces of positive electrode active materials 71, 81 with coating layers 72, 82. The particles 70, 80 may be commercially available products.

[0059] The method for applying the positive electrode slurry is not particularly limited. For example, a slit die coating method or a doctor blade method can be used as a method for applying the positive electrode slurry.

[0060] Next, the solvent is removed from the positive electrode slurry. For example, the positive electrode current collector 22 coated with the positive electrode slurry may be dried under an atmosphere of 80°C to 150°C. When the positive electrode slurry dries, the particles 70, 80 mixed in the positive electrode slurry undergo convection. Then, the particles 70 and the particles 80 are separated into layers, and the particles 70 with larger particle diameters are positioned closer to the positive electrode current collector 22 than the particles 80 with smaller particle diameters. As a result, the average weight percent concentration of the coating material in the second region R2 is higher than that in the first region R1. Through this procedure, the positive electrode 20 in which the positive electrode active material layer 24 is formed on the positive electrode current collector 22 is obtained.

[0061] The positive electrode on which the positive electrode active material layer 24 is formed may be pressed by a roll press device or the like as necessary. The linear pressure of the roll press varies depending on the material used, but is adjusted so that the density of the positive electrode active material layer 24 has a predetermined value. The relationship between the density of the positive electrode active material layer 24 and the linear pressure is determined in advance by considering the relationship with the ratio of materials constituting the positive electrode active material layer 24.

[0062] Next, the negative electrode 30 is prepared. The negative electrode 30 can be prepared in the same manner as the positive electrode 20. A paste-like negative electrode slurry is applied to at least one surface of the negative electrode current collector 32. The negative electrode slurry is prepared by mixing a negative electrode active material, a binder, a conductive assistant, and a solvent to form a paste. The negative electrode slurry is applied to the negative electrode current collector 32 and dried to obtain the negative electrode 30.

[0063] Next, the prepared positive electrode 20 and negative electrode 30 are laminated so that the separator 10 is located between them to prepare the power generating element 40. When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and one end side of the separator 10 are wound around the axis.

[0064] Finally, the power generating element 40 is enclosed in the exterior body 50. A non-aqueous electrolyte is injected into the exterior body 50. After the injection of the non-aqueous electrolyte, the pressure is reduced, heating, etc. is performed, so that the non-aqueous electrolyte is impregnated into the power generating element 40. The exterior body 50 is sealed by applying heat, etc., to obtain the lithium ion secondary battery 100.

[0065] In the lithium ion secondary battery 100 according to the first embodiment, there is a difference in the concentration of the coating material between the first region R1 and the second region R2. As a result, the charge in the positive electrode active material layer 24 becomes non-uniform, and a zeta potential gradient occurs in the positive electrode active material layer 24. The zeta potential gradient promotes the movement of the non-aqueous electrolyte toward the positive electrode collector 22. When lithium ions are efficiently supplied to the positive electrode active material 71 located on the positive electrode collector 22 side, an increase in concentration overvoltage is suppressed. Therefore, the lithium ion secondary battery 100 according to the first embodiment has excellent input / output characteristics.

[0066] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is merely an example, and addition, omission, substitution, and other modifications of the configuration are possible without departing from the spirit of the present invention.

[0067] For example, in the first embodiment, an example is shown in which the particles 70 having a larger particle size are on the positive electrode current collector 22 side, but the positional relationship between the particles 70 and the particles 80 may be reversed.

[0068] For example, in the first embodiment, the difference in concentration of the coating material is generated by utilizing the difference in particle size between the particles 70 and 80. The difference in concentration of the coating material may be generated, for example, by coating the first region R1 and the second region R2 with positive electrode slurries having different mixing ratios of the coating material in two stages. EXAMPLES

[0069] "Example 1" First, the positive electrode was prepared. First, a positive electrode slurry was applied to one side of an aluminum foil with a thickness of 15 μm. The positive electrode slurry was prepared by mixing two types of positive electrode active material with different particle sizes covered with a coating layer, a conductive additive, a binder, and a solvent.

[0070] LiCoO2 was used as the positive electrode active material. Two types of positive electrode active material were prepared, one with a median diameter (D50) of 9 μm and the other with a median diameter of 15 μm. The positive electrode active material was coated with tungsten oxide (WO3) to a thickness of 12.5 μm. The isoelectric point of tungsten oxide was 0.5. Carbon black (Super-P) was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. The mass ratio of the positive electrode active material, conductive additive, and binder was 96:2:2. The amount of positive electrode active material supported in the positive electrode active material layer after drying was 25 mg / cm. 2 It was decided.

[0071] Next, the positive electrode slurry was dried to prepare a positive electrode. Then, a weight analysis was performed on the coating material in a positive electrode prepared under the same conditions as the positive electrode. The weight analysis was performed by observing the cross section of the completed positive electrode by SEM-EDS analysis. The atomic concentration of W atoms relative to the atomic concentration of Co atoms was calculated by EDS atomic weight mapping, and the average weight percent concentration of the coating material WO3 relative to LiCoO2 in the first and second regions was obtained. As a result, the average weight percent concentration of the coating material in the first region R1 was 0.5 wt%, and the average weight percent concentration of the coating material in the second region R2 was 1.5 wt%. The difference in average weight percent concentration of the coating material between the first region R1 and the second region R2 was 1.0 wt%.

[0072] Next, a negative electrode was prepared. First, negative electrode slurry was applied to one side of a copper foil having a thickness of 10 μm. The negative electrode slurry was prepared by mixing a negative electrode active material, a conductive assistant, a binder, and a solvent. Silicon was used as the negative electrode active material. Carbon black (Super-P) was used as the conductive assistant. Polyvinylidene fluoride (PVDF) was used as the binder. The mass ratio of the negative electrode active material, conductive assistant, and binder was 89.5:2.4:8.1. The amount of negative electrode active material carried in the negative electrode active material layer after drying was 2.0 mg / cm. 2 It was decided.

[0073] (Preparation of evaluation lithium-ion secondary battery, full cell) The negative and positive electrodes thus prepared were alternately stacked with 10 μm thick polypropylene separators in between, and six negative electrodes and five positive electrodes were stacked to produce a laminate. Furthermore, in the negative electrode of the laminate, a nickel negative electrode lead was attached to the protruding end of the copper foil where the negative electrode active material layer was not provided. In the positive electrode of the laminate, an aluminum positive electrode lead was attached by an ultrasonic welding machine to the protruding end of the aluminum foil where the positive electrode active material layer was not provided.

[0074] This laminate was then inserted into an exterior body of a laminate film and heat-sealed except for one peripheral location to form a closed portion. A non-aqueous electrolyte was injected into the exterior body. The non-aqueous electrolyte was a solvent of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a volume ratio of 1:9, to which 1.0M (mol / L) LiPF6 was added as a lithium salt. The remaining location was then heat-sealed while reducing the pressure using a vacuum sealer to produce a lithium ion secondary battery (full cell).

[0075] The discharge capacity and rate characteristics of the lithium-ion secondary battery were obtained. These were measured using a secondary battery charge / discharge test device. The rate characteristics were evaluated in terms of the 2C discharge capacity retention rate (%), with the voltage range being 4.2V to 3.0V, and 1C=1000mAh per full cell design capacity. The 5C discharge capacity retention rate is the ratio of the discharge capacity when CCCV charging (constant current / constant voltage charging, end current value 0.05C) at a current value of 0.2C and discharging at a current value of 0.2C to the discharge capacity when CCCV charging (constant current / constant voltage charging, end current value 0.05C) at a current value of 0.2C and discharging at a current value of 0.2C, and is expressed by the following formula (1). (2C capacity retention rate (%))=(2C discharge capacity) / (0.2C discharge capacity)×100...(1)

[0076] The 0.2C discharge capacity of Example 1 was 161 mAh, the 2C discharge capacity was 125 mAh, and the 2C capacity retention rate was 78%.

[0077] "Example 2" Example 2 differs from Example 1 in that the relationship of the concentration of the coating material in the first region R1 and the second region R2 is reversed. In Example 2, the average weight percent concentration of the coating material in the first region R1 was 1.5 wt%, and the average weight percent concentration of the coating material in the second region R2 was 0.5 wt%. The average weight percent concentration difference of the coating material between the first region R1 and the second region R2 was 1.0 wt%. In Example 2, as the paint for the first region R1, a coating material was applied to the surface of LiCoO2 particles having a median diameter (D50) of 15 μm using a pot mill so that the coverage rate was 1.5 wt%, and as the paint for the second region R2, a coating material was applied to the surface of LiCoO2 particles having a median diameter (D50) of 9 μm using a pot mill so that the coverage rate was 0.5 wt%. By producing the first region R1 and the second region R2 in two stages using these paints, the concentration of the coating material in the first region R1 was made higher than the concentration of the coating material in the second region R2.

[0078] The 0.2C discharge capacity of Example 2 was 161 mAh, the 2C discharge capacity was 120 mAh, and the 2C capacity retention rate was 75%.

[0079] "Examples 3 and 4" The difference between Examples 3 and 4 is that the coating layer was changed from tungsten oxide to silicon oxide (SiO2), which had an isoelectric point of 2.0.

[0080] In Example 3, the average weight percent concentration of the coating material in the first region R1 was 0.5 wt%, and the average weight percent concentration of the coating material in the second region R2 was 1.5 wt%. In Example 4, the average weight percent concentration of the coating material in the first region R1 was 1.5 wt%, and the average weight percent concentration of the coating material in the second region R2 was 0.5 wt%. In both Example 3 and Example 4, the difference in average weight percent concentration of the coating material between the first region R1 and the second region R2 was 1.0 wt%.

[0081] The 0.2C discharge capacity of Example 3 was 161 mAh, the 2C discharge capacity was 115 mAh, and the 2C capacity retention rate was 71%. The 0.2C discharge capacity of Example 4 was 161 mAh, the 2C discharge capacity was 110 mAh, and the 2C capacity retention rate was 68%.

[0082] "Comparative Examples 1 and 2" Comparative Examples 1 and 2 differ in that the coating layer was changed from tungsten oxide to aluminum oxide (Al2O3). The isoelectric point of aluminum oxide was 9.0. In Comparative Example 1, the average weight percent concentration of the coating material in the first region R1 was 0.5 wt%, and the average weight percent concentration of the coating material in the second region R2 was 1.5 wt%. In Comparative Example 2, the average weight percent concentration of the coating material in the first region R1 was 1.5 wt%, and the average weight percent concentration of the coating material in the second region R2 was 0.5 wt%. In both Comparative Example 1 and Comparative Example 2, the difference in average weight percent concentration of the coating material between the first region R1 and the second region R2 was 1.0 wt%.

[0083] The 0.2C discharge capacity of Comparative Example 1 was 161 mAh, the 2C discharge capacity was 100 mAh, and the 2C capacity retention rate was 62%. The 0.2C discharge capacity of Comparative Example 2 was 161 mAh, the 2C discharge capacity was 96 mAh, and the 2C capacity retention rate was 60%.

[0084] "Comparative Examples 3 and 4" Comparative Examples 3 and 4 are different in that the coating layer was changed from tungsten oxide to boehmite (AlOOH). The isoelectric point of boehmite was 7.7. In Comparative Example 3, the average weight percent concentration of the coating material in the first region R1 was 0.5 wt%, and the average weight percent concentration of the coating material in the second region R2 was 1.5 wt%. In Comparative Example 4, the average weight percent concentration of the coating material in the first region R1 was 1.5 wt%, and the average weight percent concentration of the coating material in the second region R2 was 0.5 wt%. In both Comparative Example 3 and Comparative Example 4, the difference in average weight percent concentration of the coating material between the first region R1 and the second region R2 was 1.0 wt%.

[0085] The 0.2C discharge capacity of Comparative Example 3 was 161 mAh, the 2C discharge capacity was 94 mAh, and the 2C capacity retention rate was 58%. The 0.2C discharge capacity of Comparative Example 4 was 161 mAh, the 2C discharge capacity was 89 mAh, and the 2C capacity retention rate was 55%.

[0086] "Comparative Example 5" Comparative Example 5 differs from Example 1 in that no coating layer was formed on the surface of the positive electrode active material. The 0.2C discharge capacity of Comparative Example 5 was 161 mAh, and the 2C discharge capacity was 87 mAh. The 2C capacity retention rate was 54%.

[0087] "Comparative Example 6" Comparative Example 6 differs from Example 1 in that only LiCoO2 having a median diameter (D50) of 15 μm and coated with tungsten oxide is added to the positive electrode slurry.

[0088] In Comparative Example 6, the average weight percent concentration of the coating material in both the first region R1 and the second region R2 was 0.5 wt%. In Comparative Example 6, the difference in average weight percent concentration of the coating material between the first region R1 and the second region R2 was 0 wt%.

[0089] The 0.2C discharge capacity of Comparative Example 6 was 161 mAh, the 2C discharge capacity was 90 mAh, and the 2C capacity retention rate was 56%.

[0090] "Comparative Example 7" Comparative Example 6 differs from Example 1 in that only LiCoO2 having a median diameter (D50) of 15 μm and coated with aluminum oxide is added to the positive electrode slurry.

[0091] In Comparative Example 7, the average weight percent concentration of the coating material in both the first region R1 and the second region R2 was 9.0 wt%. In Comparative Example 7, the difference in average weight percent concentration of the coating material between the first region R1 and the second region R2 was 0 wt%.

[0092] The 0.2C discharge capacity of Comparative Example 7 was 161 mAh, the 2C discharge capacity was 84 mAh, and the 2C capacity retention rate was 52%.

[0093] The conditions and measurement results for Examples 1 to 4 and Comparative Examples 1 to 7 are summarized in Table 1.

[0094] [Table 1] [Explanation of symbols]

[0095] 10 Separator 20 positive electrode 22 Positive electrode current collector 24 Cathode active material layer 30 negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior body 52 Metal foil 54 Resin layer 60, 62 terminals 70, 80 particles 71, 81 Cathode active material 72, 82 Covering layer 100 Lithium-ion secondary battery R1 1st area R2 2nd area

Claims

1. A positive electrode active material layer is provided in contact with at least one surface of a current collector. the positive electrode active material layer has a plurality of positive electrode active materials and a plurality of coating layers that coat at least a portion of each of the plurality of positive electrode active materials, each of the plurality of coating layers includes a coating material having an isoelectric point of 7 or less; The positive electrode active material layer may be divided into a first region and a second region located farther from the current collector than the first region, based on a center in a thickness direction, the average particle size of the positive electrode active material belonging to the first region is larger than the average particle size of the positive electrode active material belonging to the second region, a difference between an average weight percent concentration of the coating material in the first region and an average weight percent concentration of the coating material in the second region is 1 weight percent or more; The second region has a higher average weight percent concentration of the coating material than the first region.

2. 2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the coating material is present in an amount of 0.1 parts by weight or more and 5.0 parts by weight or less with respect to 100 parts by weight of the positive electrode active material.

3. 3. The positive electrode for a lithium ion secondary battery according to claim 1, wherein an average thickness of the plurality of coating layers is 25 μm or less.

4. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the plurality of positive electrode active materials include first particles and second particles having different average particle sizes.

5. A lithium ion secondary battery comprising the positive electrode for lithium ion secondary batteries according to any one of claims 1 to 4.

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