Positive electrode for lithium-ion secondary batteries and lithium-ion secondary batteries

Abstract

This invention provides an electrode for a lithium-ion secondary battery with excellent input / output characteristics, using an olivine-type compound and a layered rock salt-type oxide as positive electrode active materials. [Solution] A positive electrode for a lithium-ion secondary battery is provided. This positive electrode comprises a positive electrode current collector layer and a positive electrode composite layer containing an olivine-type compound and a layered rock salt-type oxide as positive electrode active materials. The positive electrode composite layer has a first region containing the layered rock salt-type oxide at a high concentration on the side of the positive electrode composite layer that is close to the first surface, which is the surface opposite to the surface facing the positive electrode current collector layer.

Description

[Technical Field]

[0001] The technologies disclosed herein relate to cathodes for lithium-ion secondary batteries and lithium-ion secondary batteries. [Background technology]

[0002] The positive electrode of lithium-ion secondary batteries often uses a positive electrode active material that is a metal oxide containing lithium ions. For example, layered rock salt type oxides and olivine type compounds are used.

[0003] For example, layered rock salt oxides such as nickel-cobalt-manganese composite oxide (NCM) have the advantage of high energy density, enabling the construction of high-power and high-capacity secondary batteries. Olivine-type compounds such as manganese iron lithium phosphate (LMFP) also possess low cost and high energy density. The combination of these positive electrode active materials according to specific purposes is described (Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-138053 [Overview of the project] [Problems that the invention aims to solve]

[0005] Olivine-type compounds and layered rock salt-type oxides each possess excellent properties as positive electrode active materials. However, when the amount of olivine-type compounds used is increased, the input / output performance of lithium-ion secondary batteries can decrease significantly. This is thought to be due to the low electronic conductivity of olivine-type compounds, as well as the low diffusivity of lithium ions within LMFP particles.

[0006] This specification provides electrodes for lithium-ion secondary batteries with excellent input / output characteristics, using olivine-type compounds and layered rock salt-type oxides as positive electrode active materials. [Means for solving the problem]

[0007] This specification provides a positive electrode for a lithium-ion secondary battery. The positive electrode comprises a positive electrode current collector layer and a positive electrode composite layer containing an olivine-type compound and a layered rock salt-type oxide as positive electrode active material. The positive electrode composite layer has a first region containing the layered rock salt-type oxide at a high concentration on the side of the positive electrode composite layer that is close to the first surface, which is the surface opposite to the surface facing the positive electrode current collector layer.

[0008] This positive electrode has a first region containing a high concentration of layered rock salt oxide on the first surface, or in other words, on the side of the positive electrode closest to the separator. Layered rock salt oxide has superior electronic conductivity compared to olivine-type compounds. By containing a high concentration of layered rock salt oxide on the side closest to the first surface, the layered rock salt oxide can preferentially transfer electrons. As a result, the input / output characteristics of the positive electrode are improved.

[0009] Furthermore, this specification provides a lithium secondary battery comprising a positive electrode, a negative electrode, and a separator. This lithium-ion secondary battery provides a lithium-ion secondary battery with excellent input / output characteristics by including a positive electrode with excellent input / output characteristics. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows an example of a lithium-ion secondary battery. [Figure 2] This figure shows an example of a positive electrode composite layer in a lithium-ion secondary battery. [Figure 3] This figure shows a part of the structure of the positive electrode composite layer of the positive electrode fabricated in the example, along with its evaluation results. [Modes for carrying out the invention]

[0011] The positive electrode and the secondary battery of the lithium-ion secondary battery (hereinafter simply referred to as "secondary battery") disclosed herein will be described below with reference to the drawings as appropriate. Figure 1 shows an example of a cell of secondary battery 2.

[0012] In this specification, "secondary battery" refers to a battery that can be repeatedly charged and discharged by the movement of a charge carrier between a positive electrode and a negative electrode. Furthermore, in this specification, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and achieves charging and discharging through the movement of charge associated with lithium ions between the positive and negative electrodes.

[0013] Furthermore, the secondary battery is equipped with a positive electrode terminal and a negative electrode terminal on the electrode body and is housed in a battery case. The secondary battery is, for example, a lithium-ion secondary battery equipped with a non-aqueous electrolyte. The shape of the secondary battery is not particularly limited and may be cylindrical, laminated, or the like.

[0014] (Positive electrode for secondary batteries) As shown in Figure 1, the cell, as a unit structure of the secondary battery 2, comprises a positive electrode 4, a separator 6, and a negative electrode 8. The positive electrode 4 comprises a sheet-like positive electrode current collector 10 and a positive electrode composite layer 12. The positive electrode 4 has a configuration corresponding to the form of the secondary battery.

[0015] (Positive electrode current collector) The positive electrode current collector 10 is made of a conductive metal such as aluminum, aluminum alloy, nickel, or stainless steel, although this is not particularly limited. The positive electrode current collector 10 is generally in the form of a sheet with a thickness of about 100 μm or less. The positive electrode current collector 10 is typically a metal foil, more specifically an aluminum foil. The positive electrode current collector 10 is an example of a positive electrode current collector layer disclosed herein.

[0016] (Positive electrode composite layer) The positive electrode composite material layer 12 is provided by being fixed on at least one surface of the positive electrode current collector 10. The positive electrode composite material layer 12 may be provided on both surfaces of the positive electrode current collector 10. The positive electrode composite material layer 12 has a layered form along the surface of the positive electrode current collector 10.

[0017] The positive electrode composite material layer 12 is composed of a positive electrode composite material. The positive electrode composite material contains a positive electrode active material. As the positive electrode active material, an olivine-type compound and a layered rock salt-type oxide can be used. In the positive electrode composite material layer 12, the layered rock salt-type oxide is unevenly distributed on the side closer to a specific surface. Hereinafter, first, the layered rock salt-type oxide and the olivine-type compound will be described, and then the uneven distribution form of the layered rock salt-type oxide will be described.

[0018] (Layered rock salt-type oxide) As the layered rock salt-type oxide, for example, one or more selected from lithium-containing transition metal oxides represented by the following compositional formulas (1) and (2) can be used. This type of metal oxide adopts a layered rock salt structure. Also, this type of metal oxide has excellent electronic conductivity.

[0019] LiNi a Co b Mn c M 1 w O2 ··· Formula (1) (In formula (1), M 1 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, w are numbers satisfying 0.3 ≤ a < 1, 0 < b ≤ 0.7, 0 < c ≤ 0.7, 0 ≤ w ≤ 0.3, and 3a + 3b + 3c + (valence of M 1 ) × w = 3. )

[0020] LiNi d Co e Al f M 2 x O2 ··· Formula (2) (In formula (1), M 2represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. d, e, f, x are numbers satisfying 0.4 ≦ d < 1, 0 < e ≦ 0.5, 0 < f ≦ 0.3, 0 ≦ x ≦ 0.3, and 3a + 3b + 3c + (valence of M 2 ) × w = 3.

[0021] The layered rock salt type oxide represented by Formula (1) is a so-called Li-Ni-Co-Mn oxide (NCM type oxide), and the layered rock salt type oxide represented by Formula (2) is a so-called Li-Ni-Co-Al oxide (NCA type oxide).

[0022] In the NCM type oxide represented by Formula (1), Ni, Co, and Mn are known to have excellent electron conductivity and contribute to battery capacity and output characteristics. Also, from the viewpoint of cycle characteristics, it may be preferable that a part of such transition elements is substituted by another metal element M 1 .

[0023] The molar ratio of Ni in Formula (1), a, satisfies 0.3 ≦ a < 1. For example, it is 0.30 or more, 0.40 or more, 0.50 or more, 0.60 or more, 0.70 or more, and for example, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less. The range of a can be set by selecting these lower and upper limits. For example, it can be 0.40 or more and 0.90 or less, 0.50 or more and 0.80 or less, etc. Note that multiplying the molar ratio by 100 gives the mol% of atoms in the composition formula.

[0024] The molar ratio of Co in Formula (1), b, satisfies 0 < b ≦ 0.7. For example, it is 0.10 or more, 0.15 or more, 0.20 or more, and for example, 0.70 or less, 0.50 or less, 0.40 or less, 0.35 or less, 0.30 or less, 0.20 or less. The range of b can be set by selecting these lower and upper limits. For example, it can be 0.10 or more and 0.40 or less, 0.10 or more and 0.30 or less, etc.

[0025] In formula (1), c, which is the molar ratio of Mn, satisfies 0 < c ≤ 0.7. For example, c is 0.10 or more, 0.15 or more, and for example, 0.70 or less, 0.50 or less, 0.30 or less, 0.25 or less, 0.20 or less, 0.15 or less. The range of c can be set by selecting these lower and upper limits. For example, it can be 0.10 or more and 0.20 or less etc.

[0026] The NCM-based oxide represented by formula (1) is not particularly limited. For example, LiNi 0.33 Co 0.33 Mn 0.34 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2, LiNi 0.33 Co 0.31 Mn 0.33 Mg 0.03 O2, LiNi 0.33 Co 0.31 Mn 0.33 Zn 0.03 O2 can be mentioned. Among them, NCM-based acid compounds composed of compositions such as LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2 etc. may be preferable.

[0027] In the case of the NCA-based oxide represented by formula (2), similar to the NCM-based oxide, Ni and Co have excellent electron conductivity. From the viewpoint of cycle characteristics, it may be preferable that a part of such transition elements is substituted by another metal element M 2 The NCA-based oxide is excellent in battery capacity and output characteristics. In addition, due to the inclusion of Al, deterioration due to moisture in the atmosphere hardly occurs, and it is also excellent in safety.

[0028] d, which is the molar ratio of Ni in formula (2), satisfies 0.4 ≦ d < 1. For example, d can be 0.40 or more, 0.50 or more, 0.60 or more, 0.70 or more, and also, for example, less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less. It can be 0.50 or more and 0.90 or less, 0.60 or more and 0.80 or less, etc.

[0029] e, which is the molar ratio of Co in formula (2), satisfies 0 < e ≦ 0.6. For example, e can be 0.10 or more, 0.15 or more, 0.20 or more, and also, for example, 0.60 or less, 0.40 or less, 0.35 or less, 0.30 or less, 0.20 or less. The range of e can be set by selecting these lower and upper limits. For example, it can be 0.10 or more and 0.40 or less, 0.10 or more and 0.30 or less, 0.10 or more and 0.20 or less, etc.

[0030] f, which is the molar ratio of Al in formula (2), satisfies 0 < f ≦ 0.3. For example, f can be 0.01 or more, 0.02 or more, 0.05 or more, and also, for example, 0.30 or less, 0.20 or less, 0.15 or less, 0.10 or less. The range of c can be set by selecting these lower and upper limits. For example, it can be 0.01 or more and 0.10 or less, etc.

[0031] The NCA-based oxide represented by formula (2) is not particularly limited. For example, LiNi 0.33 Co 0.33 Al 0.34 O2, LiNi[[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

[0032] NCM and NCA compounds are generally spherical or irregularly shaped particles, depending on the manufacturing method, etc. The average particle size (D) of NCM and NCA compounds is... 50 The primary particle size (D) is not particularly limited. It is appropriately set within a range that allows for dispersion in the positive electrode composite layer 12 so that the intended properties can be obtained. For example, it may be 50 nm to 100 μm, 1 μm to 100 μm, 1 μm to 50 μm, or 1 μm to 20 μm. 50 In a volume-based particle size distribution (cumulative distribution) based on laser diffraction and scattering methods, the particle size can be measured as the particle size corresponding to a cumulative 50% of the total volume, starting from the smallest particle size (fine particle side).

[0033] As layered rock salt type oxides, NCM-type oxides and NCA-type oxides can be used, as well as other known lithium-containing transition metal oxides.

[0034] NCM-based oxides and NCA-based oxides can be used in combination as appropriate. They can be used individually, in mixtures of two or more, or in composite forms as needed.

[0035] (Olivine-type compounds) As the olivine-type compound, one or more compounds selected from the oxides represented by the following formula (3) can be used. This type of lithium manganese iron phosphate can contribute to safety because it adopts the olivine type and has excellent structural stability. It can also contribute to improving the energy density per unit area.

[0036] Li g Mn h Fe i M 3 y PO4···(3) (In formula (3), M 3represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. g, h, i, and y are 0≦g≦1.2, 0≦h≦1.2, 0≦i≦1.2, where h+i is not 0, and 0≦y≦0.3, g+(valence of Mn)×g+(valence of Fe)×h+(M 3 Show the number that satisfies (valence of x) × x = 3.

[0037] Examples of olivine-type compounds represented by formula (3) include MFP, LMP, and LFP. In formula (3), M 3 From the viewpoint of increasing the energy density per unit volume, Mg, Al, Ti, Zn, Nb, Co, Zr, or Gd are preferred.

[0038] Furthermore, the molar ratio of Li, g, in equation (3) is greater than 0 and less than or equal to 1.2, but for example, it is between 0.60 and 1.20, between 0.65 and 1.15, and between 0.70 and 1.10.

[0039] In equation (3), h, which is the molar ratio of Mn, is between 0 and 1.2. For LMFP, for example, it is between 0.20 and 0.70, or between 0.20 and 0.60, between 0.20 and 0.50, or between 0.20 and 0.40.

[0040] In equation (3), the molar ratio of Fe, i, is between 0 and 1.2. For LMFP, for example, it is between 0.40 and 0.90, or between 0.40 and 0.80, or between 0.60 and 0.80.

[0041] M in equation (3) 3 The molar ratio y is between 0 and 0.3, but for example, it can be between 0 and 0.20, between 0 and 0.15, or between 0 and 0.10.

[0042] Olivine-type compounds are not particularly limited, but examples include LiMnPO4, LiFePO4, LiMn 0.2 Fe 0.8 PO4, LiMn0.3 Fe 0.7 PO4, LiMn 0.4 Fe 0.6 PO4, LiMn 0.5 Fe 0.5 PO4, LiMn 0.6 Fe 0.4 PO4, Li 1.2 Mn 0.53 Fe 0.37 PO4 and the like can be mentioned.

[0043] Olivine-type compounds such as LMFP are generally spherical or irregularly shaped particles, although it depends on the manufacturing method and the like. The average particle diameter (primary particle diameter) and the particle diameter distribution range of olivine-type compounds such as LMFP are not particularly limited. In the positive electrode composite material layer 12, it is appropriately set within a range where dispersion is possible so as to obtain the intended characteristics. The average particle diameter of olivine-type compounds such as LMFP is, for example, 1 nm or more and 10 μm or less, 1 nm or more and 2 μm or less, 1 nm or more and 1 μm or less, or 1 nm or more and 0.5 μm or less. By having a small primary particle diameter, the lithium ion diffusivity within the particle can be improved. Also, olivine-type compounds such as LMFP may be granulated bodies in which primary particles are granulated. In this case, the granulated body may preferably be granulated from primary particles having a particle diameter of 100 nm or less. Regarding the average particle diameter of LMFP and the like, similar to the layered rock salt-type oxide, D in the volume-based particle size distribution (integrated distribution) based on the laser diffraction / scattering method 50 can be measured as.

[0044] In addition to the positive electrode active material, the positive electrode composite material layer 12 can appropriately contain a binder, a conductive assistant, and other additives. Examples of the binder include one or more of fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. Examples of the conductive assistant include one or more of carbon materials such as carbon black, acetylene black, ketjen black, graphite, and carbon nanotubes.

[0045] The positive electrode 4 and the positive electrode composite layer 12 can be manufactured by supplying a slurry prepared by mixing a positive electrode active material, a binder, a conductive additive, and the necessary solvent to the positive electrode current collector 10 in a known manner, and then drying, pressing, etc.

[0046] Next, with reference to Figure 2, the uneven distribution of layered rock salt oxides in the positive electrode composite layer 12 will be explained. In Figure 2, for the sake of explanation, the layered rock salt oxides are shown as particles 22. In the positive electrode composite layer 12, there is a region 20 containing a high concentration of layered rock salt oxide particles 22 on the side closer to surface 14, which is the surface opposite to surface 16 facing the positive electrode current collector 10. Surface 14 is an example of the first surface disclosed herein, and surface 16 is an example of the second surface disclosed herein. Region 20 is an example of the first region disclosed herein.

[0047] By unevenly distributing the layered rock salt oxide on surface 14, that is, on the separator 6 side of the secondary battery 2, the layered rock salt oxide preferentially receives and releases electrons in the electrolyte, allowing the positive electrode composite layer 12 to move rapidly. As a result, the low electronic conductivity of olivine-type compounds such as LMFP can be effectively compensated for.

[0048] Having such a region 20 means that, in the thickness direction of the positive electrode composite layer 12, the particles 22 are unevenly distributed, with a higher concentration on the side closer to the surface 14 than on the side closer to the surface 16. Therefore, in addition to cases where layered rock salt type oxides are contained only on the side closer to the surface 14, there are also cases where layered rock salt type oxides are contained at a first concentration on the side closer to the surface 14 and at a second concentration lower than the first concentration on the side closer to the surface 16. On the other hand, region 20 may also be a region in which the concentration of layered rock salt type oxides increases continuously or stepwise as the positive electrode composite layer 12 approaches the surface 14. Such region 20 may extend over the entire thickness of the positive electrode composite layer 12.

[0049] Furthermore, from the viewpoint of electronic conductivity, region 20 is preferably located in close proximity to surface 14, and preferably is provided in the positive electrode composite layer 12 including surface 14. In terms of the distribution of particles 22 in the thickness direction of the positive electrode composite layer 12, it is preferable that surface 14 or the area including surface 14 has the highest concentration.

[0050] Region 20 is a region that contains olivine-type compounds as well as layered rock salt-type oxides. Region 20 does not necessarily have to be layered in the positive electrode composite layer 12, but as will be described later, it is preferable that it exists in a layered manner over almost the entire surface 14 of the positive electrode composite layer 12. By doing so, the effect of unevenly distributed layered rock salt-type oxides can be obtained throughout the positive electrode 4.

[0051] The thickness of region 20 is not particularly limited, but can be 5% or more, 10% or more, 15% or more, 20% or more, 30% or more, or 40% or more of the thickness of the positive electrode composite layer 12, and can also be 60% or less, 50% or less, 40% or less, etc., and the thickness of region 20 can be 5% or more and 50% or less, 10% or more and 50% or less of the thickness of the positive electrode composite layer 12.

[0052] As shown in Figure 2, the positive electrode composite layer 12 may have a region 30 in addition to region 20, on the side closer to the surface 16, which does not contain or contains layered rock salt type oxides at the second concentration. Region 30 contains olivine-type compounds and does not contain or contains layered rock salt type oxides at a low concentration. Region 30 does not necessarily have to be layered in the positive electrode composite layer 12, but it is preferable that it exists in a layered manner over almost the entire surface 16. This makes it possible to obtain the effect of unevenly containing layered rock salt type oxides throughout the positive electrode 4. The thickness of region 30 is not particularly limited, but generally it is the remaining thickness of region 20 in the thickness direction of the positive electrode composite layer 12. Region 30 is an example of the second region disclosed herein.

[0053] In region 20, the mass ratio of layered rock salt type oxides to olivine type compounds is not particularly limited, and it is sufficient that the layered rock salt type oxides are concentrated on the side closer to surface 14. In region 20 where particles 22 are contained at a generally constant concentration, the content of layered rock salt type oxides relative to the total mass of layered rock salt type oxides and olivine type compounds is, for example, 5% to 40% by mass, 5% to 30% by mass, or 5% to 20% by mass.

[0054] Region 20 may consist of two or more regions with different layered rock salt type oxide concentrations in the thickness direction of the positive electrode composite layer 12.

[0055] Furthermore, if region 20 is a region in which the concentration of particles 22 changes continuously or stepwise along the thickness direction, the content of layered rock salt type oxide relative to the total mass is configured to increase in the thickness direction of the positive electrode composite layer 12 from surface 16 to surface 14 in the range of 0 to 40% by mass, 0 to 30% by mass, 0 to 20% by mass, etc.

[0056] On the other hand, in region 30, the concentration of layered rock salt type oxides should be lower than in region 20. For example, the content of layered rock salt type oxides relative to the total mass of layered rock salt type oxides and olivine type compounds may be, for example, 0% to 30% by mass, 0% to 20% by mass, 0% to 10% by mass, 0% to 5% by mass, or 0% by mass.

[0057] The amount of layered rock salt oxide relative to the total mass of layered rock salt oxide and olivine-type compound in the positive electrode composite layer 12 can be 2% to 15% by mass, or 2.5% to 10% by mass. By unevenly distributing the layered rock salt oxide towards the side closer to the surface 14, good power characteristics can be obtained even if the content of layered rock salt oxide in the positive electrode composite layer 12 is reduced. Since layered rock salt oxide contains cobalt and other elements, reducing its content is advantageous.

[0058] A positive electrode composite layer 12 in which layered rock salt oxide particles 22 are unevenly distributed on the surface 14 side can be manufactured by laminating slurries with different concentrations of layered rock salt oxide. For example, a composite slurry containing layered rock salt oxide as a positive electrode active material, either without or at a low concentration, and containing an olivine-type compound, is applied to the positive electrode current collector 10 to form a layer 34 corresponding to region 30. Then, a composite slurry containing layered rock salt oxide and an olivine-type compound at a higher concentration than the previously applied slurry is applied on top of layer 34 to form a layer 24 corresponding to region 20. Layers 34 and 24 may be dried and pressed together, or dried and pressed individually.

[0059] In this way, by unevenly distributing the layered rock salt oxide on the separator 6 side, i.e., the electrolyte side, in a secondary battery, the layered rock salt oxide, which has excellent electron conductivity, can preferentially accept electrons in reactions, etc., and electron conduction on the separator 6 side can be rapidly carried out. As a result, for example, a small amount of the layered rock salt oxide with excellent electron conductivity can compensate for the low electron conductivity of olivine-type compounds such as LMFP, thereby improving the output characteristics, especially the output characteristics in a short time.

[0060] (Secondary battery) The secondary battery 2 disclosed in this invention comprises a positive electrode 4, a negative electrode 8, and a separator 6. Because this secondary battery 2 includes a positive electrode 4 with excellent input / output characteristics, it is possible to provide a secondary battery with excellent input / output characteristics.

[0061] The secondary battery 2 consists of a positive electrode 4, a negative electrode 8, an electrolyte (liquid or solid), and a separator 6. The negative electrode 8, electrolyte, and separator 6 are not particularly limited, and known materials and configurations can be appropriately applied. For example, the negative electrode 8 can be made of lithium metal, graphite, silicon-based materials (Si, SiOx), lithium titanate, or amorphous carbon. The electrolyte solution is, for example, an organic solvent in which a supporting salt is dissolved. The organic solvent is not particularly limited as long as it is an organic solvent commonly used in the electrolyte of lithium-ion secondary batteries; for example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds, etc., can be used. The type of supporting salt is not particularly limited, but known organic salts such as LiPF6, LiBF4, or their derivatives can be used. As the separator 6, for example, a porous synthetic resin membrane, particularly a porous membrane of polyolefin polymers (polyethylene (PE), polypropylene (PP)), can be used. Furthermore, the solid electrolyte can be any material that electrically insulates the positive and negative electrodes and exhibits high lithium-ion conductivity; any known solid electrolyte can be used as appropriate.

[0062] The shape of the secondary battery 2 having the above configuration is not particularly limited and may be various shapes such as coin-type, cylindrical, or prismatic, or it may be an irregular shape enclosed in a laminated casing. [Examples]

[0063] The following describes examples that embody the disclosures of this specification, but the disclosures of this specification are not limited to the following examples.

[0064] (1) Preparation of the positive electrode A cathode composite slurry was prepared using LMFP (average particle size 0.5 μm), an olivine-type compound, as the first cathode active material, and NCM (average particle size 10 μm), a layered rock salt-type oxide, as the second cathode active material, and a cathode was fabricated.

[0065] As the LMFP, three types (LiMn 0.2 Fe 0.8 、LiMn 0.6 Fe 0.4 、LiMn 0.6 Fe 0.4 、with the ratios of Mn being 20 mol%, 40 mol% and 60 mol% respectively.) were used. As the NCM, two types (LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2, with the molar ratios of Ni, Co and Mn being 80 mol% / 10 mol% / 10 mol% and 50 mol% / 30 mol% / 20 mol% respectively.) were used.

[0066] Using three types of the first active material (LMFP) and three types of the second active material (NCM), carbon nanotubes (CNT) as the conductive assistant and polyvinylidene fluoride (PVDF) as the binder, a plurality of types of slurry I on the current collector side and slurry II on the separator side were prepared respectively. For each slurry, the contents of CNT and PVDF were fixed at 1.5% by mass and 4.0% by mass respectively, and the remainder was formulated to be the combination and mass ratio (%) of the first active material and the second active material shown in Table 1, and prepared as a slurry together with the solvent.

[0067]

Table 1

[0068] Using slurry I and slurry II in the combinations shown in Table 1, cathodes provided with cathode composite material layers of Examples 1 to 5 and Comparative Examples 1 to 2 were fabricated. First, a predetermined amount of slurry I was applied onto an AI foil with a thickness of 30 μm at a coating weight of 13 mg / cm 2 using a doctor blade, and then slurry II was applied onto slurry I in the same amount as above at a coating weight of 13 mg / cm 2The material was applied using a doctor blade to achieve the desired consistency. Then, it was dried at 100°C for 10 minutes, and each positive electrode was fabricated by pressing to a density of 2.3 g / cc.

[0069] (2) Fabrication of the negative electrode Artificial graphite (average particle size 22 μm) was used as the negative electrode active material, and styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC) were used as binders. The active material:SBR:CMC were mixed in a ratio of 96:3:1 (mass%) to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to a 15 μm Cu foil with a basis weight of 13 mg / cm² on one side. 2 The material was applied using a doctor blade so that the ratio of positive electrode capacity to negative electrode capacity was 1.1. It was then dried at 100°C for 10 minutes and pressed to a density of 1.25 g / cc.

[0070] (3) Manufacturing of secondary battery cells A PP / PE / PP three-layer laminate (16 μm) was used as the separator, and laminate cells were prepared by laminating the positive electrode, separator, and negative electrode from Examples 1-5 and Comparative Examples 1-2. A 1.1 M LiPF6 (ethylene carbonate (EC):dimethyl carbonate (DMC):ethyl methyl carbonate (EMC) (30:40:30 (vol%))) was used as the electrolyte, and the cell confinement pressure was set to 500 kPa.

[0071] (4) Activation and Characterization The initial charging was performed using a constant current-constant voltage method. Constant current charging was performed at a current of 0.1C until the voltage reached 4.30V. Constant voltage charging was then performed for 3 hours. Finally, the battery was discharged to 3.0V at a current of 0.1C using the constant current method to activate it. After the activation process, the battery was charged to 60% of its state of charge (SOC) at a current of 0.1C using the constant current-constant voltage method. The 10-second IV resistance was calculated from the cell voltage after discharging at 1.5C for 10 seconds. The results are shown in Table 1.

[0072] Figure 3 also illustrates the dispersion state of NCM, the second active material, in Examples 1-2 and Comparative Examples 1-2.

[0073] As shown in Table 1 and Figure 3, Examples 1 to 4 were configured such that the current collector foil side contained only LMFP, and the separator side contained 20% by mass of NCM (the entire positive electrode composite layer contained 10% by mass of NCM, and the entire amount was contained on the separator side). In all of these cells, the 10-second IV resistance value was 1.1 to 1.2 Ω.

[0074] In contrast, Comparative Example 1 had the same composition on both the current collector foil side and the separator side (the entire positive electrode composite layer contained 10% by mass of NCM, with the entire amount uniformly distributed on both the current collector foil side and the separator side), but the 10-second IV resistance was remarkably high at 2.6Ω. Similarly, Comparative Example 2 had a configuration where the current collector foil side contained 20% by mass of LCM and the separator side contained only LMFP (the entire positive electrode composite layer contained 10% by mass of NCM, with the entire amount distributed on the current collector side), but the 10-second IV resistance was remarkably high at 2.5Ω. From these results, it was found that by unevenly distributing NCM on the separator side, the reaction of NCM, which has excellent electronic conductivity, became dominant, and the 10-second IV resistance was reduced.

[0075] Furthermore, the results from Examples 1 and 2 showed that the molar ratios of Ni, Co, and Mn in LCM are sufficiently effective within the range of 80 mol% / 10 mol% / 10 mol% to 50 mol% / 30 mol% / 20 mol%, respectively.

[0076] Furthermore, Examples 3 and 4 differ from Examples 1 and 2 only in the Mn ratio of the LMFP used. Since the 10-second IV resistance values ​​are equivalent for Examples 1 and 2 and Examples 3 and 4, it was found that a Mn ratio of 20 mol% or higher for the LMFP, and 60 mol% or lower, is sufficiently effective in reducing the resistance value.

[0077] Example 5 has a configuration in which the current collector foil side contains only LMFP, and the separator side contains 5% by mass of NCM (the entire positive electrode composite layer contains 2.5% by mass of NCM, and the entire amount is contained in the separator side). In this cell as well, the 10-second IV resistance was 1.3Ω. From this, it was found that even 5% by mass of LCM contained in the separator side (2.5% by mass in the entire positive electrode composite layer) is sufficiently effective in reducing the 10-second IV resistance.

[0078] From the above, it was found that when using olivine-type compounds such as LMFP and layered rock salt-type oxides such as LCM as positive electrode active materials, the input / output characteristics can be effectively improved by unevenly distributing the layered rock salt oxides on the separator side of the positive electrode composite layer, thereby making the reaction by the layered rock salt-type oxides dominant on the separator side.

[0079] Furthermore, the disclosures in this specification include the following components. [1] A positive electrode for a lithium-ion secondary battery, Positive electrode current collector layer, The positive electrode active material is a positive electrode composite layer containing an olivine-type compound and a layered rock salt-type oxide, Equipped with, The positive electrode comprises a positive electrode having a first region containing the layered rock salt type oxide in high concentration on the side of the positive electrode composite layer that is closer to the first surface, which is the surface opposite to the surface facing the positive electrode current collector layer. [2] The positive electrode according to claim 1, wherein the positive electrode composite layer comprises the first region in a layered manner. [3] The positive electrode according to [1], wherein the positive electrode composite layer comprises a second region in the positive electrode composite layer that contains or does not contain the layered rock salt type oxide at a low concentration on the second side which is the surface facing the positive electrode current collector layer. [4] The positive electrode according to [2], wherein the positive electrode composite layer comprises a second region in the positive electrode composite layer that contains or does not contain the layered rock salt type oxide in a low concentration on the side of the positive electrode composite layer that is closer to the second surface facing the positive electrode current collector layer. [5] The positive electrode according to any one of claims [1] to [4], wherein the layered rock salt type oxide includes an oxide represented by the following formula (1) or formula (2). LiRing a Co b Mn c M 1 w O2...Equation (1) (In formula (1), M 1 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, w are 0.3 ≤ a < 1, 0 <b≦0.7、 0 <c≦0.7、0≦w≦0.3、かつ3a+3b+3c+(M 1 Show the number that satisfies (valence of) × w = 3. LiRing d Co e Al f M 2 x O2...Equation (2) (In formula (1), M 2 represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. d, e, f, x are 0.4 ≤ d < 1, 0 <e≦0.5、 0 <f≦0.3、0≦x≦0.3、かつ3a+3b+3c+(M 2 Show the number that satisfies (valence of) × w = 3. [6] The positive electrode according to [5], wherein the layered rock salt type oxide comprises an oxide represented by formula (1). [7] The oxide represented by formula (1) contains 50 mol% to 80 mol% of Ni, as described in [6]. [8] The olivine-type compound comprises a compound represented by the following formula (3) and is the positive electrode according to any one of [1] to [7]. Li g Mn h Fe i M 3 y PO4···(3) (In formula (3), M 3represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. g, h, i, and y are 0≦g≦1.2, 0≦h≦1.2, 0≦i≦1.2, where h+i is not 0, and 0≦y≦0.3, g+(valence of Mn)×g+(valence of Fe)×h+(M 3 Show the number that satisfies (valence of x) × x = 3. [9] The cathode according to [8], wherein the olivine-type compound contains 20 mol% or more of Mn. The positive electrode described in any of

[10] [1]~[9], The negative electrode and, Separator and, A lithium-ion secondary battery equipped with these features.

[0080] The specific examples of the technology disclosed in this specification have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples described above. The technical elements described in this specification or in the drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. The technology illustrated in this specification or in the drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives itself constitutes technical usefulness. [Explanation of symbols]

[0081] 2 lithium-ion secondary battery, 4 positive electrode, 6 separator, 8 negative electrode, 10 positive electrode current collector, 12 positive electrode composite layer, 14, 16 planes, 20, 30 regions, 24, 34 layers

Claims

1. A positive electrode for lithium-ion secondary batteries, Positive electrode current collector layer, The positive electrode active material is a positive electrode composite layer containing an olivine-type compound and a layered rock salt-type oxide, Equipped with, The positive electrode comprises a positive electrode having a first region containing the layered rock salt type oxide in high concentration on the side of the positive electrode composite layer that is closer to the first surface, which is the surface opposite to the surface facing the positive electrode current collector layer.

2. The positive electrode according to claim 1, wherein the positive electrode composite layer comprises the first region in a layered manner.

3. The positive electrode according to claim 1, wherein the positive electrode composite layer comprises a second region in the positive electrode composite layer that contains or does not contain the layered rock salt type oxide at a low concentration on the second side which is the surface facing the positive electrode current collector layer.

4. The positive electrode according to claim 2, wherein the positive electrode composite layer comprises a second region in which the layered rock salt type oxide is contained in a low concentration or not contained in the positive electrode composite layer, on the side of the positive electrode composite layer that is closer to the second surface facing the positive electrode current collector layer.

5. The positive electrode according to claim 1, wherein the layered rock salt type oxide includes an oxide represented by the following formula (1) or formula (2). LiNi ａ Co ｂ Mn ｃ M １ ｗ O ２ ・・・Formula (1) (In formula (1), M １ represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Al, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. a, b, c, and w are 0.3 ≤ a < 1, 0 < b ≤ 0.7, 0 < c ≤ 0.7, 0 ≤ w ≤ 0.3, and 3a + 3b + 3c + (M １ This shows the number that satisfies (valence of) × w = 3. LiNi ｄ Co ｅ Al ｆ M ２ ｘ O ２ ··· Formula (2) (In formula (1), M ２ represents one or more elements selected from Mg, Ti, Nb, Fe, Cr, Si, Ga, V, Zn, Cu, Sr, Mo, Zr, Sn, Ta, W, La, Ce, Pb, Bi, and Ge. d, e, f, and x satisfy 0.4 ≤ d < 1, 0 < e ≤ 0.5, 0 < f ≤ 0.3, 0 ≤ x ≤ 0.3, and 3a + 3b + 3c + (M ２ This shows the number that satisfies (valence of) × w = 3.

6. The positive electrode according to claim 5, wherein the layered rock salt type oxide includes an oxide represented by formula (1).

7. The positive electrode according to claim 6, wherein the oxide represented by formula (1) contains 50 mol% to 80 mol% of Ni.

8. The positive electrode according to claim 1, wherein the olivine-type compound comprises a compound represented by the following formula (3). Li ｇ Mn ｈ Fe ｉ M ３ ｙ PO ４ ・・・(3) (In formula (3), M ３ represents one or more elements selected from Mg, Al, Ti, Cu, Zn, Nb, Co, Ni, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, and Gd. g, h, i, and y are 0 ≤ g ≤ 1.2, 0 ≤ h ≤ 1.2, 0 ≤ i ≤ 1.2, where h + i is not 0, and 0 ≤ y ≤ 0.3, g + (valence of Mn) × g + (valence of Fe) × h + (M ３ Show the number that satisfies (valence of x) × x = 3.

9. The positive electrode according to claim 8, wherein the olivine-type compound contains 20 mol% or more of Mn.

10. A positive electrode according to any one of claims 1 to 9, The negative electrode and, Separator and, A lithium-ion secondary battery equipped with these features.

Images