Positive electrode and secondary battery using the same

Abstract

To provide a positive electrode which can apply a capacity characteristic and an output characteristic to a secondary battery at a high level.SOLUTION: A positive electrode disclosed here, comprises: a positive electrode collector; and a positive electrode active material layer supported by the positive electrode collector. The positive electrode active material layer contains a first lithium composite oxide having a layer-shaped structure and a second lithium composite oxide having a layer-shaped structure. In the first lithium composite oxide, a mole ratio of Li to a metal other than Li is 0.90 to 0.97. In the second lithium composite oxide, a mole ratio of Li to the metal other than Li is 1.10 to 1.20. A crystallite diameter of a (104) surface of the first lithium composite oxide is 400Å to 600Å. The crystallite diameter of the (104) surface of the second lithium composite oxide is larger than the crystallite diameter of the (104) surface of the first lithium composite oxide.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode. The present invention also relates to a secondary battery using the positive electrode. [Background technology]

[0002] In recent years, secondary batteries such as lithium-ion secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] A positive electrode active material is generally used in the positive electrode of a secondary battery such as a lithium-ion secondary battery. A lithium composite oxide is often used as the positive electrode active material. Patent Document 1 discloses that by using a combination of a lithium composite oxide having a Li / Me ratio of 1 or less and a Li-excess lithium composite oxide (i.e., a lithium composite oxide having a Li / Me ratio exceeding 1) as the positive electrode active material, and by setting the crystallite diameter of the lithium composite oxide having a Li / Me ratio of 1 or less to 180 nm or more (1800 Å or more), the output characteristics, capacity characteristics, etc. of the secondary battery are improved. The Li / Me ratio of the lithium composite oxide is the molar ratio of Li to the metal other than Li (Me). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2021-518049 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as a result of intensive research by the present inventors, it has been newly discovered that the above-mentioned conventional technology has a problem in that it is not possible to achieve both high levels of capacitance characteristics and output characteristics.

[0006] Therefore, an object of the present invention is to provide a positive electrode that can impart high levels of both capacity characteristics and output characteristics to a secondary battery. [Means for solving the problem]

[0007] The positive electrode disclosed herein comprises a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector. The positive electrode active material layer contains a first lithium composite oxide having a layered structure and a second lithium composite oxide having a layered structure. In the first lithium composite oxide, the molar ratio of Li to the metal other than Li is 0.90 to 0.97. In the second lithium composite oxide, the molar ratio of Li to the metal other than Li is 1.10 to 1.20. The crystallite diameter of the (104) plane of the first lithium composite oxide is 400 Å to 600 Å. The crystallite diameter of the (104) plane of the second lithium composite oxide is larger than the crystallite diameter of the (104) plane of the first lithium composite oxide.

[0008] According to this configuration, it is possible to provide a positive electrode that can impart high levels of both capacity characteristics and output characteristics to a secondary battery.

[0009] From another aspect, a secondary battery disclosed herein includes the above-described positive electrode, a negative electrode, and an electrolyte. With this configuration, it is possible to provide a secondary battery having both high capacity characteristics and high output characteristics. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view schematically showing a positive electrode according to one embodiment of the present invention. [Figure 2] 1 is a cross-sectional view schematically showing the internal structure of a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 3] 1 is a schematic exploded view showing the configuration of a wound electrode body of a lithium ion secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for implementing the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In this specification, a numerical range expressed as "A to B" includes A and B.

[0012] In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

[0013] The present invention will be described in detail below using a positive electrode used in a lithium-ion secondary battery as an example, but it is not intended to limit the present invention to the embodiment described. Figure 1 is a schematic cross-sectional view of a positive electrode according to this embodiment, taken along the thickness and width directions.

[0014] As shown in the figure, the positive electrode 50 includes a positive electrode current collector 52 and a positive electrode active material layer 54 supported on the positive electrode current collector 52. In the illustrated example, the positive electrode active material layer 54 is provided on both sides of the positive electrode current collector 52. However, the positive electrode active material layer 54 may be provided on one side of the positive electrode current collector 52. Preferably, the positive electrode active material layer 54 is provided on both sides of the positive electrode current collector 52.

[0015] As shown in the illustrated example, a positive electrode active material layer non-forming portion 52a where no positive electrode active material layer 54 is provided may be provided at one end in the width direction of the positive electrode 50. In the positive electrode active material layer non-forming portion 52a, the positive electrode current collector 52 is exposed, and the positive electrode active material layer non-forming portion 52a can function as a current collecting portion. However, the configuration for collecting current from the positive electrode 50 is not limited to this.

[0016] A known positive electrode current collector used in lithium ion secondary batteries may be used as the positive electrode current collector 52, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0017] The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0018] The positive electrode active material layer 54 contains a positive electrode active material. In this embodiment, a first lithium composite oxide having a layered structure and a second lithium composite oxide having a layered structure are used in combination as the positive electrode active material. Therefore, the positive electrode active material layer contains at least the first lithium composite oxide and the second lithium composite oxide. In the following description, when the term "lithium composite oxide" is simply used, it basically refers to matters that apply to both the first lithium composite oxide and the second lithium composite oxide.

[0019] Examples of types of lithium composite oxides having a layered structure include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium iron nickel manganese composite oxides.

[0020] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide described above.

[0021] The first lithium composite oxide and the second lithium composite oxide may be the same type of lithium composite oxide, or may be different types of lithium composite oxides.

[0022] When different types of lithium composite oxides are used, it is easy to distinguish between the first lithium composite oxide and the second lithium composite oxide.

[0023] When the same type of lithium composite oxide is used, it is easy to adjust the characteristics of the lithium-ion secondary battery 100. When the same type of lithium composite oxide is used, it is preferable that both the first lithium composite oxide and the second lithium composite oxide are lithium nickel cobalt manganese-based composite oxides or lithium nickel cobalt aluminum-based composite oxides, and it is more preferable that they are lithium nickel cobalt manganese-based composite oxides. In this specification, "the same type of lithium composite oxide" refers to the lithium composite oxides containing the same constituent elements. Therefore, for example, if both the first lithium composite oxide and the second lithium composite oxide are lithium nickel cobalt manganese-based composite oxides, they are considered to be the same type of lithium composite oxide even if the elemental ratios of the constituent elements are different between the first lithium composite oxide and the second lithium composite oxide.

[0024] The positive electrode active material may be composed only of the first lithium composite oxide and the second lithium composite oxide. However, the positive electrode active material layer 54 may contain a positive electrode active material other than the first lithium composite oxide and the second lithium composite oxide within a range that does not significantly impair the effects of the present invention (for example, 10% by mass or less, 5% by mass or less, or 1% by mass or less, relative to the total mass of the positive electrode active material).

[0025] In the first lithium composite oxide, the molar ratio of Li to the metal other than Li (Me) (hereinafter also referred to as "Li / Me ratio") is 0.90 to 0.97, while in the second lithium composite oxide, the molar ratio of Li to the metal other than Li (Me) (Li / Me ratio) is 1.10 to 1.20.

[0026] Furthermore, the crystallite diameter of the (104) plane of the first lithium composite oxide is 400 Å to 600 Å. In addition, the crystallite diameter of the (104) plane of the second lithium composite oxide is larger than the crystallite diameter of the (104) plane of the first lithium composite oxide.

[0027] In this way, by setting the Li / Me ratio of the two types of lithium composite oxides within the above range and the crystallite size of the (104) plane within the above range, it is possible to improve the output characteristics and capacity characteristics of the lithium-ion secondary battery 100 using the positive electrode 50. The reason for this is believed to be as follows: By balancing the Li / Me ratio and limiting the crystallite size, when lithiation occurs in the lithium composite oxide (particularly the first lithium composite oxide), the formation of an excess resistive layer (Li2CO3 or LiOH) on the surface of the lithium composite oxide is suppressed, and rearrangement of the crystal structure occurs. These contribute to improving the output characteristics and capacity characteristics.

[0028] Specifically, if the Li / Me ratio in the first lithium composite oxide is outside the range of 0.90 to 0.97, the capacity characteristics will be insufficient. If the Li / Me ratio in the second lithium composite oxide is outside the range of 1.10 to 1.20, the capacity characteristics will also be insufficient. From the viewpoint of increasing the capacity of the lithium ion secondary battery 100, the Li / Me ratio in the second lithium composite oxide is preferably 1.10 to 1.15, and more preferably 1.10 to 1.12.

[0029] If the crystallite diameter of the (104) plane of the first lithium composite oxide is outside the range of 400 Å to 600 Å, the output characteristics will be insufficient. The crystallite diameter of the (104) plane of the first lithium composite oxide is preferably 500 Å to 600 Å, more preferably 550 Å to 600 Å, and even more preferably 580 Å to 600 Å.

[0030] If the crystallite diameter of the (104) plane of the second lithium composite oxide is equal to or smaller than the crystallite diameter of the (104) plane of the first lithium composite oxide, the output characteristics will be insufficient. The crystallite diameter of the (104) plane of the second lithium composite oxide is preferably at least 50 Å larger, more preferably at least 100 Å larger, even more preferably at least 150 Å larger, and particularly preferably at least 200 Å larger than the crystallite diameter of the (104) plane of the first lithium composite oxide. The crystallite diameter of the (104) plane of the second lithium composite oxide is preferably 500 Å to 900 Å, more preferably 600 Å to 800 Å.

[0031] The Li / Me ratio and crystallite size of the lithium composite oxide can be adjusted by known methods. For example, lithium composite oxides are generally produced by mixing a precursor containing a metal element other than Li (Me) (e.g., hydroxide containing Me, carbonate containing Me, etc.) with a Li source compound (e.g., lithium hydroxide, lithium carbonate, lithium chloride, etc.) and firing the mixture. Here, the Li / Me ratio can be controlled by adjusting the mixing ratio of the precursor and the Li source compound. Furthermore, the crystallite size can be controlled by adjusting the firing conditions. In particular, increasing the firing temperature tends to increase the crystallite size.

[0032] In addition, in used lithium-ion secondary batteries, the Li / Me ratio of the lithium composite oxide may decrease due to the desorption of Li. Furthermore, the expansion / contraction of the lithium composite oxide during charge / discharge cycles may destroy and reduce the crystallite size. Therefore, in used lithium-ion secondary batteries, a layered lithium composite oxide may be produced in which the Li / Me ratio is in the range of 0.90 to 0.97 and the crystallite diameter of the (104) plane is in the range of 400 Å to 600 Å.

[0033] Therefore, in this embodiment, a layered lithium composite oxide recovered from a used lithium ion secondary battery can be used as the first lithium composite oxide. This is advantageous from the viewpoint of reducing the environmental load. The lithium composite oxide can be recovered from the used lithium ion secondary battery by a known method. For example, the used lithium ion secondary battery is disassembled to remove the positive electrode, the positive electrode active material layer is peeled off from the positive electrode current collector, the binder is removed using a solvent, and the lithium composite oxide and the conductive material are separated by classification or the like, thereby obtaining the lithium composite oxide. On the other hand, the second lithium composite oxide may be a lithium composite oxide that has not been recycled.

[0034] The crystallite size of the (104) plane of the lithium composite oxide can be determined by subjecting the lithium composite oxide powder to X-ray diffraction (XRD) measurement. Specifically, the X-ray diffraction pattern is measured using a known X-ray diffraction (XRD) device, and the (104) crystallite size is determined by the following formula: 1 The crystallite size can be calculated using the half-width (FWHM) of the diffraction peak (2θ=44±1°) assigned to the 04 plane, the 2θ value, and the Scherrer equation. Note that if the positive electrode active material is already contained in the positive electrode, the positive electrode active material alone may be isolated according to a known method and used as the measurement sample.

[0035] When the first lithium composite oxide is a lithium nickel cobalt manganese composite oxide, the first lithium composite oxide has a composition represented by, for example, the following formula (I). Li x1 Ni y1 Co z1 Mn (1-y1-z1) M1 α1 O 2-β1 Q1 β1 (I)

[0036] In formula (I), x1, y1, z1, α1, and β1 satisfy 0.90 ≦ x1 ≦ 0.97, 0 < y1 ≦ 0.40, 0 < z1 ≦ 0.40, 0 ≦ α1 ≦ 0.10, and 0 ≦ β1 ≦ 0.5, respectively. M1 is at least one element selected from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Sn, B, and Al. Q1 is at least one element selected from the group consisting of F, Cl, and Br.

[0037] y1 preferably satisfies 0.05 ≦ y1 ≦ 0.40. z1 preferably satisfies 0.05 ≦ z1 ≦ 0.40. α1 preferably satisfies 0 ≦ α1 ≦ 0.05, more preferably 0 ≦ α1 ≦ 0.03. β1 preferably satisfies 0 ≦ β1 ≦ 0.1, more preferably 0.

[0038] When the second lithium composite oxide is a lithium nickel cobalt manganese composite oxide, the second lithium composite oxide has a composition represented by, for example, the following formula (II). Li x2 Ni y2 Co z2 Mn (1-y2-z2) M2 α2 O 2-β2 Q2 β2 (II)

[0039] In formula (II), x2, y2, z2, α2, and β2 respectively satisfy 1.10≦x≦1.20, 0<y2≦0.40, 0<z2≦0.40, 0≦α2≦0.10, and 0≦β2≦0.5. M2 is at least one element selected from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Sn, B, and Al. Q2 is at least one element selected from the group consisting of F, Cl, and Br.

[0040] x2 preferably satisfies 1.10≦x2≦1.15, and more preferably satisfies 1.10≦x2≦1.12. y2 preferably satisfies 0.05≦y2≦0.40. z2 preferably satisfies 0.05≦z2≦0.40. α2 preferably satisfies 0≦α2≦0.05, and more preferably α2 = 0≦α2≦0.03. β2 preferably satisfies 0≦β2≦0.1, and more preferably β2 = 0.

[0041] The compositions of the first lithium composite oxide and the second lithium composite oxide may be the same or different even if the compositions other than the Li / Me ratio are the same.

[0042] When the compositions of the first lithium composite oxide and the second lithium composite oxide are different, it is easy to distinguish between the first lithium composite oxide and the second lithium composite oxide.

[0043] When the compositions of the first lithium composite oxide and the second lithium composite oxide other than the Li / Me ratio are the same, it is easy to adjust the characteristics of the lithium ion secondary battery 100. In this case, for example, in the above formulas (I) and (II), x1 and x2 are different, but y1 = y2, z1 = z2, α1 = α2, β1 = β2, M1 = M2, and Q1 = Q2.

[0044] The average particle diameter (D50) of the first lithium composite oxide and the second lithium composite oxide is not particularly limited and is, for example, 0.05 μm to 25 μm. The average particle diameter (D50) of the first lithium composite oxide is preferably 1 μm to 15 μm, more preferably 2 μm to 12 μm, and even more preferably 3 μm to 8 μm. When the average particle diameter (D50) of the first lithium composite oxide is 3 μm to 8 μm, the output characteristics of the lithium ion secondary battery are particularly improved. The average particle diameter (D50) of the second lithium composite oxide is preferably 1 μm to 15 μm, more preferably 2 μm to 12 μm.

[0045] In this specification, the term "average particle diameter (D50)" refers to a particle diameter corresponding to a cumulative frequency of 50 volume % from the side of fine particles with small particle diameters in a volume-based particle size distribution based on a laser diffraction / scattering method. Therefore, the average particle diameter (D50) can be determined using a known laser diffraction / scattering particle size distribution measuring device or the like.

[0046] The mixing ratio of the first lithium composite oxide to the second lithium composite oxide is not particularly limited, and the mass ratio of the first lithium composite oxide to the second lithium composite oxide is, for example, 5:95 to 95:5, preferably 10:90 to 90:10, more preferably 30:70 to 70:30, and even more preferably 40:60 to 60:40.

[0047] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 87 mass% or more, more preferably 90 mass% or more, and even more preferably 96 mass% or more.

[0048] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB), carbon nanotubes (CNT), and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF). When CNT is used as the conductive material, the positive electrode active material layer 54 may contain a dispersant for the CNT.

[0049] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more and 99% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 15% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.4% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less.

[0050] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is usually 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is usually 300 μm or less, and preferably 200 μm or less.

[0051] In the positive electrode active material layer-free portion 52a of the positive electrode 50, an insulating protective layer (not shown) containing insulating particles (e.g., ceramic particles) may be provided at a position adjacent to the positive electrode active material layer 54. This protective layer can prevent a short circuit between the positive electrode active material layer-free portion 52a and the negative electrode.

[0052] The positive electrode 50 according to this embodiment can be produced by preparing a positive electrode slurry containing a positive electrode active material, optional components (e.g., a binder, a conductive material, etc.), and a solvent (dispersion medium), applying the slurry onto the positive electrode current collector 52, drying the slurry, and then pressing the slurry as necessary.

[0053] The positive electrode 50 according to this embodiment can provide a secondary battery with high levels of both capacity and output characteristics.

[0054] From another perspective, the secondary battery according to this embodiment includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode is the positive electrode according to this embodiment described above.

[0055] The secondary battery according to this embodiment will be described in detail below, taking as an example a flat prismatic lithium ion secondary battery having a flat wound electrode body and a flat battery case. However, the secondary battery according to this embodiment is not limited to the example described below.

[0056] The lithium-ion secondary battery 100 shown in FIG. 2 is a sealed battery constructed by housing a flat wound electrode assembly 20 and a nonaqueous electrolyte 80 in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, as well as a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 also has an injection port (not shown) for injecting the nonaqueous electrolyte 80. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum. Note that FIG. 2 does not accurately represent the amount of nonaqueous electrolyte 80.

[0057] As shown in Figures 2 and 3, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are stacked together with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction.

[0058] The positive electrode sheet 50 used is the positive electrode 50 according to the present embodiment described above. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62.

[0059] The positive electrode 50 has a positive electrode active material layer-free portion 52a where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed. The negative electrode 60 has a negative electrode active material layer-free portion 62a where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed. The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are formed so as to protrude outward from both ends of the wound electrode body 20 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

[0060] The negative electrode current collector 62 constituting the negative electrode sheet 60 may be a known negative electrode current collector used in lithium ion secondary batteries, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil is preferred as the negative electrode current collector 62.

[0061] The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0062] The negative electrode active material layer 64 contains a negative electrode active material. Examples of the negative electrode active material include carbon materials such as graphite, hard carbon, and soft carbon; Si; and composite materials of Si and carbon. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite, which is graphite coated with an amorphous carbon material.

[0063] The average particle diameter (D50) of the negative electrode active material is not particularly limited, but is, for example, 0.1 μm or more and 50 μm or less, preferably 1 μm or more and 25 μm or less, and more preferably 5 μm or more and 20 μm or less.

[0064] The negative electrode active material layer 64 may contain components other than the active material, such as a binder or a thickener. Examples of binders that may be used include styrene butadiene rubber (SBR) and polyvinylidene fluoride (PVDF). Examples of thickeners that may be used include carboxymethyl cellulose (CMC).

[0065] The content of the negative electrode active material in the negative electrode active material layer 64 is preferably 90% by mass or more, and more preferably 95% by mass or more and 99% by mass or less. The content of the binder in the negative electrode active material layer 64 is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer 64 is preferably 0.3% by mass or more and 3% by mass or less, and more preferably 0.5% by mass or more and 2% by mass or less.

[0066] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 300 μm or less, and preferably 20 μm or more and 200 μm or less.

[0067] Examples of separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such porous sheets may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of separator 70.

[0068] The nonaqueous electrolyte 80 typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). As the nonaqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones commonly used in electrolyte solutions for lithium-ion secondary batteries can be used without any particular limitation. Specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). One of these nonaqueous solvents can be used alone, or two or more can be used in appropriate combination.

[0069] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0070] The nonaqueous electrolyte 80 may contain various additives other than the above-mentioned components, such as film-forming agents such as vinylene carbonate (VC) and oxalate complexes; gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); and thickeners, as long as the effects of the present invention are not significantly impaired.

[0071] The lithium ion secondary battery 100 configured as described above is excellent in both capacity and output characteristics. That is, the lithium ion secondary battery 100 has a high capacity and a high output.

[0072] The lithium ion secondary battery 100 can be used for a variety of purposes. Specific applications include portable power sources for personal computers, portable electronic devices, portable terminals, etc.; power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); and storage batteries for small-sized power storage devices, with the battery being preferred as a power source for driving vehicles. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of a plurality of batteries connected in series and / or parallel.

[0073] As an example, a rectangular lithium ion secondary battery 100 including a flat wound electrode assembly 20 has been described. However, the lithium ion secondary battery disclosed herein can also be configured as a lithium ion secondary battery including a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked). Furthermore, the nonaqueous electrolyte secondary battery disclosed herein can also be configured as a cylindrical lithium ion secondary battery, a laminated case lithium ion secondary battery, a coin-type lithium ion secondary battery, etc.

[0074] In addition, secondary batteries other than lithium ion secondary batteries can be constructed using the above-described positive electrode according to known methods. Furthermore, all-solid-state secondary batteries (particularly all-solid-state lithium ion secondary batteries) can be constructed using a solid electrolyte instead of the nonaqueous electrolyte 80 according to known methods.

[0075] Examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples.

[0076] <Examples 1 to 14 and Comparative Examples 1 to 5> As a precursor of the positive electrode active material, Ni 1 / 3 Co 1 / 3 Mn 1 / 3 A nickel-cobalt-manganese composite hydroxide represented by (OH)2 was prepared.

[0077] The first lithium composite oxide was produced by the following procedure. First, the prepared composite hydroxide and a lithium source (lithium carbonate) were mixed to achieve the Li / Me ratio shown in Tables 1 and 2. This was fired for 12 hours at the temperature shown in Tables 1 and 2 to obtain the first lithium composite oxide (lithium nickel cobalt manganese composite oxide). In Examples 7, 13, and 14, a composite hydroxide having an average particle size (D50) of 7 μm was used; in Examples 9 and 10, a composite hydroxide having an average particle size (D50) of 3 μm was used; in Example 11, a composite hydroxide having an average particle size (D50) of 2 μm was used; in Example 12, a composite hydroxide having an average particle size (D50) of 11 μm was used; and in the other Examples and Comparative Examples, a composite hydroxide having an average particle size (D50) of 9 μm was used. Tables 1 and 2 show the average particle diameter (D50) of the first lithium composite oxide measured using a commercially available laser diffraction / scattering particle size distribution measuring device.

[0078] The second lithium composite oxide was prepared by the following procedure. First, the composite hydroxide and a lithium source (lithium carbonate) were mixed to obtain the Li / Me ratio shown in Tables 1 and 2. This mixture was calcined for 12 hours at the temperature shown in Tables 1 and 2 to obtain a second lithium composite oxide (lithium nickel cobalt manganese composite oxide). In Example 8, a composite hydroxide having an average particle size (D50) of 7 μm was used, in Example 13, a composite hydroxide having an average particle size (D50) of 3 μm was used, and in Example 14, a composite hydroxide having an average particle size (D50) of 11 μm was used. In the other Examples and Comparative Examples, a composite hydroxide having an average particle size (D50) of 9 μm was used. The average particle size (D50) values ​​of the second lithium composite oxides, measured using a commercially available laser diffraction / scattering particle size distribution analyzer, are shown in Tables 1 and 2.

[0079] The first lithium composite oxide and the second lithium composite oxide thus prepared were mixed in the mass ratios shown in Tables 1 and 2 to obtain positive electrode active materials.

[0080] The obtained positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in N-methylpyrrolidone (NMP) in a mass ratio of positive electrode active material:AB:PVDF = 85:10:5 to prepare a paste for forming a positive electrode active material layer. This paste was applied to a 15 μm thick aluminum foil and dried to produce a positive electrode sheet.

[0081] A paste for forming a negative electrode active material layer was prepared by mixing natural graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener in a mass ratio of C:SBR:CMC = 98:1:1 in ion-exchanged water. This paste was applied to a copper foil with a thickness of 10 μm and dried to prepare a negative electrode sheet.

[0082] In addition, a porous polyolefin sheet having a thickness of 20 μm and a three-layer structure of PP / PE / PP was prepared as a separator sheet.

[0083] The positive electrode sheet, negative electrode sheet, and separator sheet were stacked together, and electrode terminals were attached and housed in a laminate case. A nonaqueous electrolyte was then poured into the laminate case, which was then airtightly sealed. The nonaqueous electrolyte used was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, with LiPF6 dissolved as a supporting electrolyte at a concentration of 1.0 mol / L. In this manner, lithium ion secondary batteries for evaluation were obtained for each of the Examples and Comparative Examples.

[0084] <Crystallite size measurement> The first lithium composite oxide and the second lithium composite oxide prepared above were analyzed using an XRD device "smart Lab" (manufactured by Rigaku Co., Ltd.) and analysis software "PDXL2" (manufactured by Rigaku Co., Ltd.), respectively. 1The crystallite size was calculated using the half-width of the diffraction peak (2θ = 44 ± 1°) assigned to the 04 plane, the 2θ value, and the Scherrer equation. The results are shown in Tables 1 and 2.

[0085] Activation and initial capacity measurement Each of the lithium-ion secondary batteries for evaluation prepared above was placed in an environment of 25°C. Activation (initial charging) was performed using a constant current-constant voltage method, with each lithium-ion secondary battery for evaluation being charged at a constant current of 1 / 3C up to 4.2V, followed by constant voltage charging until the current reached 1 / 50C, and then fully charged. Each lithium-ion secondary battery for evaluation was then discharged at a constant current of 1 / 3C down to 3.0V. The discharge capacity at this time was measured to determine the initial capacity. The ratios of the initial capacities of the other comparative examples and examples, with the initial capacity of Comparative Example 1 taken as 1, were calculated. The results are shown in Tables 1 and 2.

[0086] <Output characteristic evaluation> After activation treatment, each evaluation lithium-ion secondary battery was adjusted to an SOC of 60% and placed in an environment at -10°C. Each evaluation lithium-ion secondary battery was discharged for 2 seconds at a current value of 15 C. The output (W) was calculated based on the voltage and current value at this time. When the output of the evaluation lithium-ion secondary battery using the positive electrode active material obtained in Comparative Example 1 was set to 1, the output ratios of the evaluation lithium-ion secondary batteries using the positive electrode active materials obtained in the other Comparative Examples and Examples were calculated. The results are shown in Tables 1 and 2.

[0087] [Table 1]

[0088] [Table 2]

[0089] The results in Tables 1 and 2 show that when the Li / Me ratio in the first lithium composite oxide is 0.90 to 0.97, the Li / Me ratio in the second lithium composite oxide is 1.10 to 1.20, the crystallite diameter of the (104) plane of the first lithium composite oxide is 400 Å to 600 Å, and the crystallite diameter of the (104) plane of the second lithium composite oxide is larger than the crystallite diameter of the (104) plane of the first lithium composite oxide, both the capacity and the output are increased.

[0090] Therefore, it is clear that the positive electrode disclosed herein can impart high levels of both capacity and output characteristics to a secondary battery.

[0091] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0092] That is, the positive electrode and secondary battery disclosed herein are the following items [1] to [6]. [1] A positive electrode comprising a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector, the positive electrode active material layer contains a first lithium composite oxide having a layered structure and a second lithium composite oxide having a layered structure, In the first lithium composite oxide, the molar ratio of Li to metals other than Li is 0.90 to 0.97; In the second lithium composite oxide, the molar ratio of Li to metals other than Li is 1.10 to 1.20; the crystallite diameter of the (104) plane of the first lithium composite oxide is 400 Å to 600 Å; A positive electrode in which the crystallite size of the (104) plane of the second lithium composite oxide is larger than the crystallite size of the (104) plane of the first lithium composite oxide. [2] The crystallite diameter of the (104) plane of the second lithium composite oxide is 100 Å or more larger than the crystallite diameter of the (104) plane of the first lithium composite oxide, and Item [1]: The positive electrode according to item [1], wherein the second lithium composite oxide has a crystallite diameter of the (104) plane of 600 Å to 800 Å. [3] The positive electrode according to item [1] or [2], wherein the first lithium composite oxide and the second lithium composite oxide are both lithium nickel cobalt manganese composite oxides. [4] The positive electrode according to any one of items [1] to [3], wherein the first lithium composite oxide is recovered from a used lithium ion secondary battery. [5] The positive electrode according to any one of items [1] to [4], wherein the first lithium composite oxide has an average particle size (D50) of 3 μm to 8 μm. [6] The positive electrode according to any one of items [1] to [5], a negative electrode; Electrolytes, A secondary battery comprising: [Explanation of symbols]

[0093] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator sheet (separator) 80 Non-aqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. A positive electrode comprising a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector, The positive electrode is used in a secondary battery, the positive electrode active material layer contains a first lithium composite oxide having a layered structure and a second lithium composite oxide having a layered structure, In the first lithium composite oxide, a molar ratio of Li to metals other than Li is 0.90 to 0.97; In the second lithium composite oxide, the molar ratio of Li to metals other than Li is 1.10 to 1.20; the crystallite diameter of the (104) plane of the first lithium composite oxide is 400 Å to 600 Å; A positive electrode in which the crystallite size of the (104) plane of the second lithium composite oxide is larger than the crystallite size of the (104) plane of the first lithium composite oxide.

2. the crystallite diameter of the (104) plane of the second lithium composite oxide is 100 Å or more larger than the crystallite diameter of the (104) plane of the first lithium composite oxide; and 2. The positive electrode according to claim 1, wherein the second lithium composite oxide has a crystallite diameter of the (104) plane of 600 Å to 800 Å.

3. 2. The positive electrode according to claim 1, wherein the first lithium composite oxide and the second lithium composite oxide are both lithium nickel cobalt manganese composite oxides.

4. 2. The positive electrode according to claim 1, wherein the first lithium composite oxide is recovered from a used lithium ion secondary battery.

5. 2. The positive electrode according to claim 1, wherein the first lithium composite oxide has an average particle diameter (D50) of 3 μm to 8 μm.

6. The positive electrode according to claim 1 ; a negative electrode; Electrolytes, A secondary battery comprising:

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