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

Abstract

The present disclosure provides an electrode for a lithium ion secondary battery, which uses an olivine-type compound and a layered rock salt-type oxide as a positive electrode active material, and which has excellent input / output characteristics. Provided is a positive electrode for a lithium ion secondary battery. The positive electrode is provided with a positive electrode current collector layer and a positive electrode mixture layer containing an olivine-type compound and a layered rock salt-type oxide as positive electrode active materials. The positive electrode mixture layer is provided with a first region containing the layered rock salt-type oxide at a high concentration on the side close to a first surface, and the first surface is the surface of the positive electrode mixture layer on the opposite side from the surface facing the positive electrode current collector layer.

Description

Technical Field

[0001] The technology disclosed in this specification relates to positive electrodes for lithium-ion secondary batteries and lithium-ion secondary batteries. Background Technology

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

[0003] For example, layered rock-salt type oxides such as nickel-cobalt-manganese composite oxides (NCM) have the advantage of being able to form secondary batteries with high energy density, high output, and high capacity. Additionally, olivine-type compounds such as lithium manganese iron phosphate (LMFP) are low in cost and have high energy density. Patent Document 1 describes a composite formulation that combines these positive electrode active materials for a specific purpose.

[0004] Prior art literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2022-138053 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] Olivine-type compounds and layered rock salt oxides exhibit excellent properties as positive electrode active materials. However, increasing the amount of olivine-type compounds sometimes leads to a significant decrease in the input-output performance of lithium-ion secondary batteries. This is attributed to the low electronic conductivity of olivine-type compounds and the low diffusion of lithium ions within LMFP particles.

[0008] This specification provides an electrode for lithium-ion secondary batteries that uses olivine-type compounds and layered rock salt-type oxides as positive electrode active materials and exhibits excellent input-output characteristics.

[0009] Methods for solving problems

[0010] This specification provides a positive electrode for a lithium-ion secondary battery. The positive electrode has a positive current collector layer and a positive electrode additive layer containing an olivine-type compound and a layered rock salt-type oxide as positive electrode active materials. The positive electrode additive layer has a first region containing the layered rock salt-type oxide at a high concentration on a side near a first surface, the first surface being the side opposite to the surface of the positive electrode additive layer opposite to the positive current collector layer.

[0011] According to this positive electrode, a first region containing a high concentration of layered rock salt oxides is provided on the side near the first surface, or in other words, on the side near the positive electrode opposite to the separator. Layered rock salt oxides exhibit superior electronic conductivity compared to olivine-type compounds. By containing a high concentration of layered rock salt oxides on the side near the first surface, the layered rock salt oxides can preferentially perform electron donation and acceptance. As a result, the input-output characteristics of the positive electrode are improved.

[0012] Furthermore, this specification provides a lithium secondary battery comprising the positive electrode, negative electrode, and separator. According to this lithium-ion secondary battery, by having a positive electrode with excellent input-output characteristics, a lithium-ion secondary battery with excellent input-output characteristics can be provided. Attached Figure Description

[0013] Figure 1 This is a diagram illustrating an example of a lithium-ion secondary battery.

[0014] Figure 2 This is a diagram showing an example of the positive electrode binder layer in a lithium-ion secondary battery.

[0015] Figure 3 This is a diagram showing a portion of the structure of the positive electrode mixture layer of the positive electrode produced in the embodiment and its evaluation results.

[0016] Explanation of reference numerals in the attached figures

[0017] 2 Lithium-ion secondary battery, 4 Positive electrode, 6 Separator, 8 Negative electrode, 10 Positive current collector, 12 Positive flux layer, 14 and 16 surfaces, 20 and 30 regions, 24 and 34 layers. Detailed Implementation

[0018] The positive electrode and the secondary battery of the lithium-ion secondary battery (hereinafter referred to as the secondary battery) disclosed in this specification will be described below with appropriate reference to the accompanying drawings. Figure 1 This is an example of a cell representing a secondary battery 2.

[0019] In addition, in this specification, "secondary battery" refers to a battery that can be repeatedly charged and discharged by the movement of charge carriers between the positive and negative electrodes. Furthermore, in this specification, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging through the movement of lithium ions accompanied by charges between the positive and negative electrodes.

[0020] Furthermore, the secondary battery, relative to the electrode body, has a positive terminal and a negative terminal, which are housed within the battery casing. A secondary battery, for example, is a lithium-ion secondary battery that uses a non-aqueous electrolyte. Additionally, the shape of the secondary battery is not particularly limited; it can be cylindrical or laminated, etc.

[0021] (Positive electrode used in secondary batteries)

[0022] like Figure 1 As shown, the unit structure of the secondary battery 2 includes a positive electrode 4, a separator 6, and a negative electrode 8. The positive electrode 4 includes a sheet-like positive current collector 10 and a positive electrode flux layer 12. The positive electrode 4 has a shape corresponding to the shape of the secondary battery 2.

[0023] (Positive current collector)

[0024] There are no particular limitations on the positive current collector 10, which may be made of conductive metals such as aluminum, aluminum alloy, nickel, or stainless steel. The positive current collector 10 is typically a sheet with a thickness of about 100 μm or less. The positive current collector 10 is typically a metal foil, more specifically an aluminum foil. The positive current collector 10 is an example of a positive current collector layer disclosed in this specification.

[0025] (Positive electrode mixture layer)

[0026] The positive electrode flux layer 12 is provided by being fixed to at least one surface of the positive electrode current collector 10. The positive electrode flux layer 12 may also be provided on both surfaces of the positive electrode current collector 10. The positive electrode flux layer 12 has a layered morphology along the surface of the positive electrode current collector 10.

[0027] The positive electrode mixture layer 12 is composed of a positive electrode mixture. The positive electrode mixture contains a positive electrode active material. As the positive electrode active material, olivine-type compounds and layered rock salt-type oxides can be used. In the positive electrode mixture layer 12, the layered rock salt-type oxide is biased towards one side closer to a specific surface. Hereinafter, the layered rock salt-type oxide and the olivine-type compound will be explained first, and then the biased (non-uniform) morphology of the layered rock salt-type oxide will be explained.

[0028] (Layered rock salt type oxide)

[0029] As a layered rock salt type oxide, one or more lithium-containing transition metal oxides selected from the compositional formulas (1) and (2) below can be used. This metal oxide adopts a layered rock salt structure. In addition, this metal oxide has excellent electronic conductivity.

[0030] LiNi a Co b Mn c M 1 w O2 ・・・Form (1)

[0031] (In formula (1), M) 1This indicates that the element is selected from one or more of the following: 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 represent elements satisfying 0.3 ≤ a < 1, 0 < b ≤ 0.7, 0 < c ≤ 0.7, 0 ≤ w ≤ 0.3, and 3a + 3b + 3c + (M... 1 (The value of the number) × w = 3.

[0032] LiNi d Co e Al f M 2 x O2 ・・・Form (2)

[0033] (In equation (2), M) 2 This indicates that the element is selected from one or more of the following: 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 represent elements satisfying 0.4 ≤ d < 1, 0 < e ≤ 0.5, 0 < f ≤ 0.3, 0 ≤ x ≤ 0.3, and 3d + 3e + 3f + (M... 2 (The value of x) × x = 3.

[0034] The layered rock salt oxide represented by formula (1) is the so-called Li-Ni-Co-Mn oxide (NCM series oxide), and the layered rock salt oxide represented by formula (2) is the so-called Li-Ni-Co-Al oxide (NCA series oxide).

[0035] Among the NCM-based oxides represented by equation (1), Ni, Co, and Mn are known to have excellent electronic conductivity, which contributes to battery capacity and output characteristics. Furthermore, from a cycling performance perspective, it is sometimes preferable to have a portion of this transition element replaced by other metallic elements, such as Mn. 1 replace.

[0036] In formula (1), the molar ratio 'a' of Ni is 0.3 ≤ a < 1, for example, 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 choosing these lower and upper limits, for example, it can be set to 0.40 or more and 0.90 or less, 0.50 or more and 0.80 or less, etc. In addition, by multiplying the molar ratio by 100, the mol% of atoms in the composition formula is obtained.

[0037] In equation (1), the molar ratio b of Co is 0 < b ≤ 0.7, for example, above 0.10, above 0.15, above 0.20, or below 0.70, below 0.50, below 0.40, below 0.35, below 0.30, or below 0.20. The range of b can be set by these lower and upper limits, for example, it can be set to above 0.10 and below 0.40, above 0.10 and below 0.30, etc.

[0038] In equation (1), the molar ratio c of Mn is 0 < c ≤ 0.7, for example, above 0.10 or above 0.15, or below 0.70, below 0.50, below 0.30, below 0.25, below 0.20, or below 0.15. The range of c can be set by selecting these lower and upper limits, for example, it can be set to above 0.10 and below 0.20, etc.

[0039] There are no particular limitations on the NCM-based oxides represented by equation (1), 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. Sometimes, LiNi is preferred. 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 NCM-based oxides composed of O2 and other components.

[0040] In NCA-based oxides represented by equation (2), similar to NCM-based oxides, Ni and Co exhibit excellent electronic conductivity. From the perspective of cycling characteristics, it is sometimes preferable to have a portion of these transition elements replaced by other metallic elements M. 2It replaces [other materials]. NCA-based oxides offer excellent battery capacity and output characteristics. Furthermore, by containing Al, they are less prone to deterioration caused by humidity in the atmosphere, resulting in excellent safety.

[0041] The molar ratio d of Ni in formula (2) is 0.4≤d<1, for example, 0.40 or more, 0.50 or more, 0.60 or more, 0.70 or more, or less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less. It can be set to 0.50 or more and 0.90 or less, 0.60 or more and 0.80 or less, etc.

[0042] In equation (2), the molar ratio e of Co is 0 < e ≤ 0.6, for example, above 0.10, above 0.15, above 0.20, or below 0.60, below 0.40, below 0.35, below 0.30, or below 0.20. The range of e can be set by these lower and upper limits, for example, it can be set to above 0.10 and below 0.40, above 0.10 and below 0.30, above 0.10 and below 0.20, etc.

[0043] In equation (2), the molar ratio f of Al is 0 < f ≤ 0.3, for example, above 0.01, above 0.02, above 0.05, or below 0.30, below 0.20, below 0.15, or below 0.10. The range of f can be set by selecting these lower and upper limits, for example, it can be set to above 0.01 and below 0.10.

[0044] There are no particular limitations on the NCA-based oxides represented by equation (2), for example, LiNi 0.33 Co 0.33 Al 0.34 O2, LiNi 0.8 Co 0.1 Al 0.1 O2, LiNi 0.6 Co 0.2 Al 0.2 O2, LiNi 0.5 Co 0.3 Al 0.2 O2, LiNi 0.8 Co 0.15 Al 0.03 Mg 0.03 O2, LiNi 0.8 Co 0.15 Al 0.03 Zn 0.03 O2.

[0045] NCM and NCA compounds are typically 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 is not particularly limited. It is appropriately set within a range that allows for dispersion in the positive electrode mixture layer 12 to obtain the desired characteristics. For example, it can be 50 nm or more and 100 μm or less, 1 μm or more and 100 μm or less, 1 μm or more and 50 μm or less, or 1 μm or more and 20 μm or less. Average particle size D 50 It can be used as a volume-based particle size distribution (cumulative distribution) based on laser diffraction scattering method, equivalent to accumulating 50% of the particle size from the smaller particle side (microparticle side) for determination.

[0046] In addition to NCM-based and NCA-based oxides, other known lithium-containing transition metal oxides can also be used as layered rock salt oxides.

[0047] NCM-based oxides and NCA-based oxides can be used in appropriate combinations. They can be used individually, in combination of two or more, or in composite applications, depending on the need.

[0048] (Olivine-type compounds)

[0049] As an olivine-type compound, one or more compounds selected from those represented by oxides according to the compositional formula shown in formula (3) below can be used. This lithium manganese iron phosphate adopts the olivine type, which has excellent structural stability and thus can contribute to safety. In addition, it can sometimes contribute to the improvement of energy density per unit area.

[0050] Li g Mn h Fe i M 3 y PO4・・・(3)

[0051] (In equation (3), M) 3 This indicates that the element is selected from one or more of the following: 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 represent elements satisfying the following conditions: 0 ≤ g ≤ 1.2, 0 ≤ h ≤ 1.2, 0 ≤ i ≤ 1.2, h + i ≠ 0, 0 ≤ y ≤ 0.3, g + (valence of Mn) × h + (valence of Fe) × i + (Mn + γ) γ. 3 (The value of y) × y = 3.

[0052] Examples of olivine-type compounds represented by equation (3) include MFP, LMP, and LFP. In equation (3), from the viewpoint of increasing the energy density per unit volume, M... 3 The preferred materials are Mg, Al, Ti, Zn, Nb, Co, Zr, or Gd.

[0053] In addition, the molar ratio g of Li in formula (3) is 0 or more and 1.2 or less, for example, 0.60 or more and 1.20 or less, 0.65 or more and 1.15 or less, or 0.70 or more and 1.10 or less.

[0054] In equation (3), the molar ratio h of Mn is 0 or more and 1.2 or less. When it is LMFP, for example, it is 0.20 or more and 0.70 or less, and for other examples, it is 0.20 or more and 0.60 or less, 0.20 or more and 0.50 or less, or 0.20 or more and 0.40 or less.

[0055] The molar ratio i of Fe in equation (3) is 0 or more and 1.2 or less. When it is LMFP, for example, it is 0.40 or more and 0.90 or less, and for other examples, it is 0.40 or more and 0.80 or less, or 0.60 or more and 0.80 or less.

[0056] M in equation (3) 3 The molar ratio y is greater than or equal to 0 and less than 0.3, for example, greater than or equal to 0 and less than 0.20, greater than or equal to 0 and less than 0.15, or greater than or equal to 0 and less than 0.10.

[0057] There are no particular limitations on olivine-type compounds; examples include LiMnPO4, LiFePO4, and LiMn. 0.2 Fe 0.8 PO4, LiMn 0.3 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, etc.

[0058] olivine-type compounds such as LMFP are typically spherical or amorphous particles, depending on the manufacturing method, etc. There are no particular limitations on the average particle size (primary particle size) and particle size distribution range of olivine-type compounds such as LMFP. In the positive electrode compound layer 12, the particle size is appropriately set within a disperseable range to obtain the desired characteristics. For example, the average particle size of olivine-type compounds such as LMFP is 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. A small primary particle size improves the lithium-ion diffusion within the particle. Furthermore, olivine-type compounds such as LMFP can be granulated bodies formed by granulating primary particles. In this case, the granulated body is sometimes preferably formed from primary particles with a particle size of 100 nm or less. Additionally, regarding the average particle size of LMFP, similar to layered rock salt oxides, it can be used as D in the particle size distribution (cumulative distribution) based on the volume reference of laser diffraction scattering. 50 To measure.

[0059] In addition to containing the positive electrode active material, the positive electrode binder layer 12 may also contain appropriate binders, conductive additives, and other additives. Examples of binders include one or more of the following: fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefins. Examples of conductive additives include one or more of the following: carbon materials such as carbon black, acetylene black, Ketjen black, graphite, and carbon nanotubes.

[0060] The positive electrode 4 and the positive electrode mixture layer 12 can be manufactured by feeding a mixture slurry prepared by mixing positive electrode active material, binder and conductive additive and necessary solvent onto the positive electrode current collector 10 by a known method, and then drying, pressing or the like.

[0061] Next, refer to Figure 2 The bias (non-uniform distribution) of layered rock salt type oxides in the positive electrode mixture layer 12 is explained. Figure 2 For ease of explanation, layered rock salt oxides are referred to as particles 22. In the positive electrode mixture layer 12, on the side of surface 14, which is opposite to surface 16 opposite to the positive electrode current collector 10, there is a region 20 containing particles 22 of layered rock salt oxides at a high concentration. Surface 14 is an example of a first surface disclosed in this specification, and surface 16 is an example of a second surface disclosed in this specification. Region 20 is an example of a first region disclosed in this specification.

[0062] By biasing the layered rock salt oxide towards surface 14, i.e., the membrane 6 of the secondary battery 2, the layered rock salt oxide preferentially accepts or releases electrons from the electrolyte, enabling it to move rapidly within the positive electrode binder layer 12. As a result, for example, the low electronic conductivity of olivine-type compounds such as LMFP can be effectively compensated.

[0063] The presence of region 20 means that, in the thickness direction of the positive electrode mixture layer 12, particles 22 are distributed at a higher concentration on the side near surface 14 than on the side near surface 16. Therefore, besides the case where layered rock-type oxides are present only on the side near surface 14, it is also possible for layered rock-type oxides to be present at a first concentration on the side near surface 14 and at a second concentration lower than the first concentration on the side near surface 16. On the other hand, region 20 can also be a region where the concentration of layered rock-type oxides increases continuously or progressively with increasing proximity to surface 14 of the positive electrode mixture layer 12. This region 20 sometimes extends throughout the entire thickness of the positive electrode mixture layer 12.

[0064] Furthermore, from the viewpoint of electronic conductivity, region 20 is preferably disposed very close to surface 14, and preferably includes surface 14 disposed within the positive electrode mixture layer 12. As for the distribution of particles 22 in the thickness direction of the positive electrode mixture layer 12, surface 14 or the area including surface 14 preferably has the highest concentration.

[0065] Region 20 is a region containing olivine-type compounds and layered rock salt-type oxides. Region 20 is not necessarily layered in the cathode compound layer 12, but preferably, as described later, it exists in layers over approximately the entire surface 14 of the cathode compound layer 12. This allows for the non-uniform presence of layered rock salt-type oxides throughout the entire cathode 4.

[0066] There is no particular limitation on the thickness of region 20. It can be more than 5%, more than 10%, more than 15%, more than 20%, more than 30%, or more than 40% of the thickness of the positive electrode compound layer 12. Alternatively, it can be less than 60%, less than 50%, or less than 40%. The thickness of region 20 can be set to more than 5% and less than 50%, or more than 10% and less than 50% of the thickness of the positive electrode compound layer 12.

[0067] like Figure 2As shown, in addition to region 20, the positive electrode compound layer 12 sometimes has a region 30 on the side near surface 16 that does not contain layered rock salt oxides, or contains layered rock salt oxides at the second concentration. Region 30 is a region containing olivine-type compounds and does not contain or contains layered rock salt oxides at a low concentration. Region 30 is not necessarily layered in the positive electrode compound layer 12, but it is preferable to exist in layers over approximately the entire surface 16. Thus, the effect of unevenly containing layered rock salt oxides can be obtained throughout the positive electrode 4. Furthermore, the thickness of region 30 is not particularly limited, and is approximately the remaining thickness of region 20 in the thickness direction of the positive electrode compound layer 12. Region 30 is an example of the second region disclosed in this specification.

[0068] There is no particular limitation on the mass ratio of layered rock salt oxides to olivine compounds in region 20, as long as the concentration of layered rock salt oxides is high on the side closest to surface 14. In region 20, which contains particles 22 at a substantially constant concentration, the content of layered rock salt oxides relative to the total mass of layered rock salt oxides and olivine compounds is, for example, 5% by mass or more and 40% by mass or less, 5% by mass or more and 30% by mass or less, or 5% by mass or more and 20% by mass or less.

[0069] Region 20 can also be composed of two or more regions with different concentrations of layered rock salt oxides in the thickness direction of the positive electrode mixture layer 12.

[0070] Furthermore, in the case where region 20 is a region where the concentration of particle 22 changes continuously or in stages along the thickness direction, the content of layered rock salt type oxide increases relative to the total mass from surface 16 to surface 14 in the thickness direction of the positive electrode compound layer 12 within the range of 0% to 40% by mass, 0% to 30% by mass, 0 to 20% by mass, etc.

[0071] On the other hand, in region 30, the concentration of layered rock salt oxides can be lower than that in region 20. For example, relative to the total mass of layered rock salt oxides and olivine compounds, the content of layered rock salt oxides is, for example, 0% by mass or more and 30% by mass or less, 0% by mass or more and 20% by mass or less, 0% by mass or more and 10% by mass or less, 0% by mass or more and 5% by mass or less, or 0% by mass.

[0072] Relative to the total mass of layered rock salt oxides and olivine-type compounds in the positive electrode compound layer 12, the layered rock salt oxide content can be set to 2% to 15% by mass and 2.5% to 10% by mass. By distributing the layered rock salt oxides towards the side closest to surface 14, good output characteristics can be obtained even by reducing the content of layered rock salt oxides in the positive electrode compound layer 12. Since the layered rock salt oxides contain cobalt and the like, reducing their content is advantageous.

[0073] The cathode mixture layer 12, in which the particles 22 of layered rock salt oxide are biased towards the side of surface 14, can be manufactured by stacking slurries with different concentrations of layered rock salt oxide. For example, relative to the cathode current collector 10, a mixture slurry containing no or a low concentration of layered rock salt oxide as the cathode active material and containing olivine-type compounds is coated onto the cathode current collector 10 to form layer 34 corresponding to region 30. Then, a mixture slurry containing a higher concentration of layered rock salt oxide and olivine-type compounds than the previously coated mixture slurry is coated onto layer 34 to form layer 24 corresponding to region 20. Layers 34 and 24 can be dried and pressed together, or they can be dried and pressed separately.

[0074] In this way, by biasing the layered rock-salt oxide towards the 6th side of the membrane in the secondary battery, i.e., the electrolyte side, the layered rock-salt oxide with excellent electron conductivity can preferentially undergo electron-accepting reactions, thus enabling rapid electron conduction on the 6th side of the membrane. As a result, for example, a small amount of layered rock-salt oxide with excellent electron conductivity can compensate for the low electron conductivity of olivine-type compounds such as LMFP, thereby improving output characteristics, especially short-term output characteristics.

[0075] (Secondary battery)

[0076] The secondary battery 2 disclosed in this invention includes a positive electrode 4, a negative electrode 8, and a separator 6. According to this secondary battery 2, since it has a positive electrode 4 with excellent input-output characteristics, it is possible to provide a secondary battery with excellent input-output characteristics.

[0077] As a secondary battery 2, it consists of a positive electrode 4, a negative electrode 8, an electrolyte (liquid or solid), and a separator 6. There are no particular limitations on the negative electrode 8, the electrolyte, and the separator 6; known materials and structures can be appropriately used. For example, the negative electrode 8 can be made of lithium metal, graphite, or silicon-based materials (Si, SiO₂). xCarbon materials such as lithium titanate or amorphous carbon can be used. The electrolyte solution is, for example, an electrolyte solution in which the supporting salt is dissolved in an organic solvent. The organic solvent can be any organic solvent commonly used in electrolytes of lithium-ion secondary batteries, without particular limitation; for example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxacyclopentane compounds, etc., can be used. Furthermore, there is no particular limitation on the type of supporting salt; known organic salts such as LiPF6 and LiBF4 or their derivatives can be used. As the separator 6, for example, a porous synthetic resin membrane can be used, especially a porous membrane of polyolefin polymers (polyethylene (PE), polypropylene (PP)). In addition, any material that electrically insulates the positive and negative electrodes and exhibits high lithium-ion conductivity can be used; known solid electrolytes can be appropriately used.

[0078] The shape of the secondary battery 2 with the above structure is not particularly limited. It can be various shapes such as coin-shaped, cylindrical, or square, or it can be an indefinite shape sealed in a laminated outer casing.

[0079] Example

[0080] The following describes embodiments that embody the disclosure of this specification, but the disclosure of this specification is not limited to the following embodiments.

[0081] (1) Production of the positive electrode

[0082] The cathode was prepared by using LMFP (average particle size 0.5 μm), an olivine-type compound, as the first positive electrode active material, and NCM (average particle size 10 μm), a layered rock salt-type oxide, as the second positive electrode active material.

[0083] As LMFP, three different molar ratios (%) of Mn and Fe were used (LiMn) 0.2 Fe 0.8 LiMn 0.6 Fe 0.4 LiMn 0.6 Fe 0.4 The Mn ratios were 20 mol%, 40 mol%, and 60 mol%, respectively. Two NCMs with different Ni, Co, and Mn molar ratios (LiNi) were used. 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.3 Mn 0.2 The molar ratios of O2, Ni, Co, and Mn are 80 mol% / 10 mol% / 10 mol% and 50 mol% / 30 mol% / 20 mol%.

[0084] Using three primary active materials (LMFP) and three secondary active materials (NCM), and with carbon nanotubes (CNTs) as conductive additives and polyvinylidene fluoride (PVDF) as binders, various slurries were prepared for the current collector foil side (Paste I) and the separator side (Paste II). In each slurry, the contents of CNTs and PVDF were fixed at 1.5% by mass and 4.0% by mass, respectively, and the remainder was prepared according to the combination and mass ratio (%) of the primary and secondary active materials as shown in Table 1, and mixed with solvents to form the slurry.

[0085] Table 1

[0086]

[0087] Using slurry I and slurry II in the combinations shown in Table 1, positive electrodes having the positive electrode mixture layers of Examples 1-5 and Comparative Examples 1-2 were fabricated. First, a predetermined amount of slurry I was coated onto an AI foil with a thickness of 30 μm using a doctor blade, so that the weight per unit area was 13 mg / cm². 2 Then, using a scraper, apply the same amount of slurry II onto slurry I, so that the weight per unit area is 13 mg / cm². 2 Then, it was dried at 100°C for 10 minutes and pressed into a density of 2.3 g / cc to produce various positive electrodes.

[0088] (2) Fabrication of the negative electrode

[0089] Artificial graphite (average particle size 22 μm) was used as the negative electrode active material, and styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were used as binders. The active material:SBR:CMC was mixed at a ratio of 96:3:1 (mass%) to prepare the negative electrode slurry. This negative electrode slurry was then coated onto a 15 μm Cu foil using a doctor blade, achieving a single-sided unit area weight of 13 mg / cm². 2 (The ratio of positive electrode capacity to negative electrode capacity is 1.1). Then, it is dried at 100°C for 10 minutes and pressed into a product with a density of 1.25 g / cc.

[0090] (3) Fabrication of secondary battery units

[0091] As the separator, a laminated battery was fabricated by stacking the positive electrode, separator, and negative electrode of Examples 1-5 and Comparative Examples 1-2 using a PP / PE / PP3 laminate (16 μm). As the electrolyte, 1.1 M LiPF6 (ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) (30:40:30 (volume%)) was used to set the battery confinement pressure to 500 kPa.

[0092] (4) Activation and property evaluation

[0093] The initial charging was performed using a constant current-constant voltage method, charging to 4.30V at a constant current of 0.1C, followed by constant voltage charging for 3 hours. The battery was then discharged to 3.0V using a constant current of 0.1C for activation. After activation, the battery was charged to 60% SOC using a constant current-constant voltage method at 0.1C. The 10-second IV resistance was calculated based on the cell voltage after discharging at 1.5C for 10 seconds. The results are shown in Table 1.

[0094] in addition, Figure 3 The dispersion of NCM as the second active substance in Examples 1-2 and Comparative Examples 1-2 is illustrated.

[0095] As shown in Table 1 and Figure 3 As shown, in Examples 1-4, the current collector foil side is configured to contain only LMFP, and the separator side is configured to contain 20% by mass NCM (10% by mass NCM is contained in the entire positive electrode compound layer, the entire amount of which is included in the separator side). In these cells, the 10-second IV resistance value is 1.1~1.2Ω.

[0096] In contrast, in Comparative Example 1, the current collector foil side and the separator side were composed of the same material (containing 10% NCM by mass in the entire positive electrode flux layer, with its entire amount uniformly distributed on both the current collector foil side and the separator side), but the 10-second IV resistance was significantly higher than 2.6 Ω. Furthermore, in Comparative Example 2, the current collector foil side was configured to contain 20% LCM by mass, and the separator side was configured to contain only LMFP (containing 10% NCM by mass in the entire positive electrode flux layer, with its entire amount contained on the current collector side), but the 10-second IV resistance was significantly higher than 2.5 Ω. This demonstrates that by biasing NCM towards the separator side, the reaction of NCM with excellent electronic conductivity becomes dominant, resulting in a lower 10-second IV resistance.

[0097] Furthermore, the results of Examples 1 and 2 show that the mol ratios of Ni, Co, and Mn in LCM within the range of 80 mol% / 10 mol% / 10 mol% to 50 mol% / 30 mol% / 20 mol% are sufficiently effective.

[0098] Furthermore, in Examples 3-4, the only difference from Examples 1-2 was the Mn ratio of the LMFP used. Since the 10-second IV resistance values ​​of Examples 1-2 and Examples 3-4 were the same, it can be seen that if the Mn ratio of the LMFP is 20 mol% or more and 60 mol% or less, it is very effective in reducing the resistance value.

[0099] In Example 5, the current collector foil side is configured to contain only LMFP, and the separator side is configured to contain 5% by mass NCM (2.5% by mass NCM is contained in the entire positive electrode compound layer, and its entire amount is included in the separator side). In this unit, the 10-second IV resistance is also 1.3Ω. Therefore, it can be seen that even if the LCM contained in the separator side is 5% by mass (2.5% by mass in the entire positive electrode compound layer), the 10-second IV resistance can be reduced sufficiently and effectively.

[0100] As can be seen from the above, when using olivine-type compounds such as LMFP and layered rock salt-type oxides such as LCM as positive electrode active materials, by making the layered rock salt oxides biased on the membrane side of the positive electrode compound layer, the reaction caused by the layered rock salt-type oxides plays a dominant role on the membrane side, which can effectively improve the input and output characteristics.

[0101] In addition, the disclosure in this specification includes the following technical components.

[0102] [1] A positive electrode, which is used in lithium-ion secondary batteries, has a positive electrode current collector layer and a positive electrode flux layer.

[0103] The positive electrode mixture layer contains olivine-type compounds and layered rock salt-type oxides as positive electrode active materials.

[0104] The positive electrode mixture layer has a first region containing the layered rock salt type oxide at a high concentration on one side near the first surface, the first surface being the side opposite to the side of the positive electrode current collector layer in the positive electrode mixture layer.

[0105] [2] According to the positive electrode described in [1],

[0106] The positive electrode mixture layer has a layered first region.

[0107] [3] According to the positive electrode described in [1],

[0108] The positive electrode mixture layer has a second region containing the layered rock salt type oxide at a low concentration on one side near the second surface, the second surface being the surface of the positive electrode mixture layer opposite to the positive electrode current collector layer.

[0109] [4] According to the positive electrode described in [2],

[0110] The positive electrode mixture layer has a second region on the side near the second surface that contains or does not contain the layered rock salt type oxide at a low concentration, and the second surface is the surface of the positive electrode mixture layer opposite to the positive electrode current collector layer.

[0111] [5] The positive electrode according to any one of [1] to [4],

[0112] The layered rock salt type oxides comprise oxides represented by the following formula (1) or formula (2).

[0113] LiNi a Co b Mn c M 1 wO2 ・・・Form (1)

[0114] (In formula (1), M) 1 The expression 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 represent elements satisfying 0.3 ≤ a < 1, 0 < b ≤ 0.7, 0 < c ≤ 0.7, 0 ≤ w ≤ 0.3, and 3a + 3b + 3c + (M... 1 (The value of the number) × w = 3.

[0115] LiNi d Co e Al f M 2 x O2 ・・・Form (2)

[0116] (In equation (2), M) 2 The expression indicates that the element is selected from one or more elements chosen 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 represent elements satisfying 0.4 ≤ d < 1, 0 < e ≤ 0.5, 0 < f ≤ 0.3, 0 ≤ x ≤ 0.3, and 3d + 3e + 3f + (M... 2 (The value of x) × x = 3.

[0117] [6] According to the positive electrode described in [5],

[0118] The layered rock salt type oxide comprises oxides represented by the formula (1).

[0119] [7] According to the positive electrode described in [6],

[0120] The oxide represented by the formula (1) contains more than 50 mol% and less than 80 mol% Ni.

[0121] [8] The positive electrode according to any one of [1] to [7],

[0122] The olivine-type compounds comprise compounds represented by the following formula (3).

[0123] Li g Mn h Fei M 3 y PO4 ・・・(3)

[0124] (In equation (3), M) 3 This indicates that the element is selected from one or more elements among 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 represent elements satisfying 0 ≤ g ≤ 1.2, 0 ≤ h ≤ 1.2, 0 ≤ i ≤ 1.2, h + i ≠ 0, 0 ≤ y ≤ 0.3, and g + (valence of Mn) × h + (valence of Fe) × i + (Mn + (valence of Fe)). 3 (The value of y) × y = 3.

[0125] [9] According to the positive electrode described in [8],

[0126] The olivine-type compound contains more than 20 mol% Mn.

[0127]

[10] A lithium-ion secondary battery comprising a positive electrode, a negative electrode and a separator as described in any one of [1] to [9].

[0128] The above provides a detailed description of specific examples of the technology disclosed in this specification, but these are merely illustrative and do not limit the scope of the claims. The technology described within the scope of the claims includes technical solutions with various modifications and alterations to the specific examples described above. The technical elements illustrated in this specification or drawings achieve technical utility individually or in various combinations, and are not limited to the combinations described in the claims at the time of application. The technology illustrated in this specification or drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives is itself technically practical.

Claims

1. A positive electrode, used in lithium-ion secondary batteries, comprising a positive electrode current collector layer and a positive electrode flux layer. The positive electrode mixture layer contains olivine-type compounds and layered rock salt-type oxides as positive electrode active materials. The positive electrode mixture layer has a first region containing the layered rock salt type oxide at a high concentration on one side near the first surface, the first surface being the side opposite to the side of the positive electrode current collector layer in the positive electrode mixture layer.

2. The positive electrode according to claim 1, The positive electrode mixture layer has a layered first region.

3. The positive electrode according to claim 1, The positive electrode mixture layer has a second region containing the layered rock salt type oxide at a low concentration on one side near the second surface, the second surface being the surface of the positive electrode mixture layer opposite to the positive electrode current collector layer.

4. The positive electrode according to claim 2, The positive electrode mixture layer has a second region on the side near the second surface that contains or does not contain the layered rock salt type oxide at a low concentration, and the second surface is the surface of the positive electrode mixture layer opposite to the positive electrode current collector layer.

5. The positive electrode according to claim 1, The layered rock salt type oxides comprise oxides represented by the following formula (1) or formula (2), LiNi a Co b Mn c M 1 wO2 ・・・Form (1) In equation (1), M 1 The expression 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 represent elements satisfying 0.3 ≤ a < 1, 0 < b ≤ 0.7, 0 < c ≤ 0.7, 0 ≤ w ≤ 0.3, and 3a + 3b + 3c + (M... 1 The number whose valence (value) × w = 3, LiNi d Co e Al f M 2 x O2 ・・・Form (2) In equation (2), M 2 The expression indicates that the element is selected from one or more elements chosen 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 represent elements satisfying 0.4 ≤ d < 1, 0 < e ≤ 0.5, 0 < f ≤ 0.3, 0 ≤ x ≤ 0.3, and 3d + 3e + 3f + (M... 2 The number whose valence is x = 3.

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

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

8. The positive electrode according to claim 1, The olivine-type compounds comprise compounds represented by the following formula (3), Li g Mn h Fe i M 3 y PO4 ・・・(3) In equation (3), M 3 This indicates that the element is selected from one or more elements among 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 represent elements satisfying 0 ≤ g ≤ 1.2, 0 ≤ h ≤ 1.2, 0 ≤ i ≤ 1.2, h + i ≠ 0, 0 ≤ y ≤ 0.3, and g + (valence of Mn) × h + (valence of Fe) × i + (Mn + (valence of Fe)). 3 The number whose valence is 3 × y = 3.

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

10. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and a separator as described in any one of claims 1 to 9.

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