Lithium-ion secondary battery

Abstract

To provide a lithium ion secondary battery in which initial resistance and an increase in resistance during storage are suppressed while the cobalt content in a positive electrode active material is reduced.SOLUTION: In a lithium ion secondary battery 100 disclosed herein, a positive electrode active material 56 includes: a layer-structured lithium transition metal composite oxide 56a containing Li, Ni, and Mn; and a coating part 56c. The lithium transition metal composite oxide 56a has the content of Ni to the total of metal elements other than Li of 75 mol% or more, and is secondary particles which each are formed by aggregation of primary particles 56p, the average porosity of the secondary particles being 2% or more and 10% or less. The coating part 56c includes a boron compound. On the other hand, a negative electrode active material 66 contains a carbon material 66c and a Si-containing material 66s, and when the total amount of the negative electrode active material 66 is 100 mass%, the content ratio of a Si element in the negative electrode active material 66 is 5 mass% or more and 10 mass% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a lithium-ion secondary battery.

Background Art

[0002] Lithium-ion secondary batteries are suitably used as drive power sources mounted on vehicles such as hybrid vehicles (HEVs), plug-in hybrid vehicles (PHEVs), and battery electric vehicles (BEVs), and their demand is rapidly expanding. As the positive electrode active material used in lithium-ion secondary batteries, lithium transition metal composite oxides are used. Among them, lithium nickel cobalt manganese-based composite oxides containing Ni, Co, and Mn, as shown in Patent Document 1, can be mentioned.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] On the other hand, in recent years, with the expansion of the demand for lithium-ion secondary batteries, depletion of cobalt (Co) used in lithium-ion secondary batteries has been a concern. As a countermeasure, it is conceivable to reduce the Co content in the positive electrode active material. However, as a result of intensive studies by the present inventors, it has been found that when the Co content in the positive electrode active material is reduced, the initial resistance of the lithium-ion secondary battery and the increase in resistance during storage become remarkable.

[0005] The technology disclosed herein has been made in view of the above circumstances, and an object thereof is to provide a lithium-ion secondary battery that suppresses an increase in initial resistance and resistance during storage while reducing the cobalt content in the positive electrode active material.

Means for Solving the Problems

[0006] The technology disclosed herein relates to a lithium-ion secondary battery, which includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode includes a positive electrode active material layer containing a positive electrode active material. The positive electrode active material includes a layered lithium transition metal composite oxide containing Li, Ni, and Mn, and a coating portion disposed on at least a part of the surface of the lithium transition metal composite oxide. The lithium transition metal composite oxide is a secondary particle formed by aggregation of primary particles, and the average porosity of the secondary particle in the cross-sectional observation of the secondary particle by a scanning electron microscope is 2% or more and 10% or less. The coating portion contains a boron compound. The negative electrode includes a negative electrode active material layer containing a negative electrode active material. The negative electrode active material includes a carbon material and a Si-containing material. When the total amount of the negative electrode active material is 100% by mass, the content ratio of Si element in the negative electrode active material is 5% by mass or more and 10% by mass or less.

[0007] According to such a configuration, the composition in the positive electrode active material, specifically, the content ratio of Ni in the lithium transition metal composite oxide is within a predetermined range. The lithium transition metal composite oxide is a secondary particle having a porosity within a predetermined range, and a coating portion containing a boron compound is disposed on at least a part of the lithium transition metal composite oxide. Further, as the negative electrode active material, a predetermined amount of a Si-containing material is contained in addition to the carbon material. Thereby, it is possible to provide a lithium-ion secondary battery that suppresses an increase in initial resistance and resistance during storage while reducing the cobalt content in the positive electrode active material.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0009] Hereinafter, with reference to the drawings as appropriate, some preferred embodiments of the lithium-ion secondary battery disclosed herein will be described. Matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, the general configuration and manufacturing process of a lithium-ion secondary battery that do not characterize the present disclosure) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The lithium-ion secondary battery disclosed herein can be implemented based on the content disclosed herein and the common general knowledge in the relevant field.

[0010] In the following drawings, members and parts having the same function are denoted by the same reference numerals, and redundant descriptions may be omitted or simplified. Also, in this specification, the notation "A to B" indicating a range includes the meaning of "A or more and B or less" as well as the meaning of "preferably greater than A" and "preferably less than B". Further, in this specification, the "lithium-ion secondary battery" (hereinafter sometimes simply referred to as "battery") refers to all storage devices that can be repeatedly charged and discharged by the movement of charge associated with lithium ions between the positive and negative electrodes using lithium ions as charge carriers.

[0011] Hereinafter, the present disclosure will be described in detail by taking a flat rectangular lithium-ion secondary battery having a flat electrode body and a flat battery case as an example, but the present disclosure is not intended to be limited to those described in such embodiments.

[0012] FIG. 1 is a cross-sectional view schematically showing the internal structure of a lithium-ion secondary battery according to an embodiment. The lithium-ion secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat electrode body 20 and a non-aqueous 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, and a thin safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous 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. As the material of the battery case 30, for example, a lightweight and highly thermally conductive metal material such as aluminum is used.

[0013] FIG. 2 is an exploded schematic view showing the configuration of the electrode body of a lithium-ion secondary battery according to an embodiment. As shown in FIGS. 1 and 2, the electrode body 20 has a form in which a strip-shaped positive electrode sheet 50 and a strip-shaped negative electrode sheet 60 are overlapped via two strip-shaped separators 70 and wound in the longitudinal direction. 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 side or both sides (here, both sides) 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 side or both sides (here, both sides) of a long negative electrode current collector 62. The non-formed portion 52a of the positive electrode active material layer (i.e., the portion where the positive electrode current collector 52 is exposed without the formation of the positive electrode active material layer 54) and the non-formed portion 62a of the negative electrode active material layer (i.e., the portion where the negative electrode current collector 62 is exposed without the formation of the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction of the electrode body 20 (i.e., the sheet width direction orthogonal to the longitudinal direction). The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are joined to the non-formed portion 52a of the positive electrode active material layer and the non-formed portion 62a of the negative electrode active material layer, respectively. Note that the positive electrode sheet 50 is an example of the "positive electrode" disclosed herein, and the negative electrode sheet 60 is an example of the "negative electrode" disclosed herein.

[0014] <Positive electrode> The positive electrode (positive electrode sheet 50) disclosed herein includes a positive electrode active material layer 54. As shown in FIG. 2, the positive electrode (positive electrode sheet 50) includes, here, a positive electrode current collector 52 and a positive electrode active material layer 54 supported by the positive electrode current collector 52. In this embodiment, the positive electrode active material layer 54 is shown only on one surface of the positive electrode current collector 52, but the positive electrode active material layer 54 may be provided on both surfaces of the positive electrode current collector 52, respectively.

[0015] FIG. 3 is a schematic diagram showing the configuration of the positive electrode active material 56 according to one embodiment. The positive electrode active material layer 54 contains a positive electrode active material 56 capable of reversibly occluding and releasing charge carriers. The positive electrode active material 56 according to this embodiment includes a lithium transition metal composite oxide 56a serving as a base material and a coating portion 56c disposed on at least a part of the surface of the lithium transition metal composite oxide 56a. The coating portion 56c is typically disposed on the lithium transition metal composite oxide 56a by physical and / or chemical bonding.

[0016] The lithium transition metal composite oxide 56a according to this embodiment has a layered structure and contains lithium (Li), nickel (Ni), and manganese (Mn) as essential elements. The lithium transition metal composite oxide 56a according to this embodiment is a so-called high-Ni-containing lithium transition metal composite oxide in which the content of Ni with respect to metal elements other than Li is 75 mol% or more. The high-Ni-containing lithium transition metal composite oxide is an example of the "lithium transition metal composite oxide in which the content of Ni is 75 mol% or more with respect to the total of metal elements other than Li" disclosed herein.

[0017] The positive electrode active material may contain a positive electrode active material other than the lithium transition metal composite oxide 56a within a range that does not inhibit the effects of the present disclosure (for example, less than 10% by mass, preferably 5% by mass or less with respect to the total mass of the positive electrode active material). Also, the positive electrode active material may be composed only of the lithium transition metal composite oxide 56a.

[0018] From the perspective of the high volumetric energy density of a lithium-ion secondary battery, in the lithium transition metal composite oxide 56a, the content ratio of Ni to the total of metal elements other than Li is preferably 80 mol% or more, more preferably 90 mol% or more. On the other hand, from the perspective of high stability, the content ratio of Ni to the total of metal elements other than Li is preferably 95 mol% or less, more preferably 93 mol% or less.

[0019] Examples of the lithium transition metal composite oxide 56a having a layered structure include, for example, lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, lithium iron nickel manganese composite oxides, and the like. These may be used alone or in combination of two or more. Whether the particles of the high-Ni-containing lithium transition metal composite oxide have a layered structure (i.e., a layered crystal structure) can be confirmed by a known method (e.g., X-ray diffraction method).

[0020] In this specification, the term "lithium nickel cobalt manganese composite oxide" includes oxides containing one or two or more additional elements other than those in addition to the oxides composed of Li, Ni, Co, Mn, and O as constituent 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, Sn, etc. The additional elements may also be semi-metal elements such as B, C, Si, P, etc., or non-metal elements such as S, F, Cl, Br, I, etc. This also applies to the above-mentioned lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium nickel manganese composite oxides, lithium nickel cobalt aluminum composite oxides, lithium iron nickel manganese composite oxides, and the like.

[0021] In some preferred embodiments, the lithium transition metal composite oxide 56a preferably has a composition represented by the following general formula (i). Li α Ni xMn y M z O₂···Formula (i) In the above formula (i), x, y, z, and α satisfy 0.8 ≤ α ≤ 1.2, 0.75 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.25, 0 ≤ z ≤ 0.2, and x + y + z = 1, respectively. M in formula (i) is at least one element selected from the group consisting of Mg, Ca, Co, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.

[0022] α in the above formula (i) preferably satisfies 0.9 ≤ α ≤ 1.2, and more preferably satisfies 1.0 ≤ α ≤ 1.1. From the perspective of battery characteristics (such as energy density, cycle characteristics, and thermal stability), x preferably satisfies 0.8 ≤ x ≤ 0.95, and more preferably satisfies 0.8 ≤ x ≤ 0.9. From the perspectives of, for example, energy density, thermal stability, and cost, y preferably satisfies 0.1 ≤ y ≤ 0.25, and more preferably satisfies 0.17 ≤ y ≤ 0.25. z preferably satisfies 0 ≤ z ≤ 0.1, more preferably satisfies 0 ≤ z ≤ 0.03, and still more preferably is 0. In the above formula (i), "x + y + z = 1" includes not only the case where x + y + z is 1, but also the case where it is regarded as substantially 1 as long as the effects of the technology disclosed herein can be achieved. Typically, x + y + z can be 0.95 to 1.1, and preferably can be 0.99 to 1.05.

[0023] In some preferred embodiments, it is preferable that the lithium transition metal composite oxide 56a is doped with W as an additive element. Thereby, since the layered structure of the lithium transition metal composite oxide 56a is stabilized, not only the initial resistance is more preferably improved, but also the elution of Mn into the non-aqueous electrolyte 80 is suppressed, and a more preferable effect of suppressing the increase in resistance during storage can be obtained. When doping (solid-solution) W as an additive element into the lithium transition metal composite oxide 56a, it is preferably doped at 0.1 to 0.5 mol% with respect to the total of the transition metal elements of the lithium transition metal composite oxide 56a other than W, and more preferably doped at 0.1 to 0.3 mol%.

[0024] The lithium transition metal composite oxide 56a has the following relationship between the content (mole) of Co element and the content (mole) of Mn element: 0 ≦ Co / Mn ≦ 0.42 is preferred. Thereby, the content ratios of the Co element and the Mn element in the lithium transition metal composite oxide 56a are controlled. In the above relationship, the smaller the value of Co / Mn, the more preferably the cobalt content in the lithium transition metal composite oxide 56a can be reduced while suppressing the initial resistance and the increase in resistance during storage. Therefore, 0 ≦ Co / Mn ≦ 0.32 is preferred, 0 ≦ Co / Mn ≦ 0.25 is more preferred, and 0 ≦ Co / Mn ≦ 0.21 is even more preferred.

[0025] In the lithium transition metal composite oxide used in conventional lithium-ion batteries, the content ratio of the Co element to the metal elements other than Li is typically about 10 mol% to 40 mol%. However, since cobalt resources are limited, in this embodiment, it is preferable to reduce the Co content rate of the lithium transition metal composite oxide 56a compared with the conventional one. Therefore, in some preferred embodiments, the content ratio of Co to the total of the metal elements other than Li is preferably, for example, 5 mol% or less, more preferably 3 mol% or less, and even more preferably 0 mol% (that is, Co-free).

[0026] As shown in FIG. 3, the lithium transition metal composite oxide 56a is typically in the form of secondary particles in which a plurality of primary particles 56p are aggregated by physical or chemical binding forces. The number of primary particles 56p of the lithium transition metal composite oxide 56a that make up the secondary particles of the lithium transition metal composite oxide 56a is not particularly limited, but is, for example, approximately 10 or more, preferably approximately 30 or more, and more preferably approximately 50 or more. The number of primary particles 56p is not particularly limited, but can be, for example, approximately 120 or less. In the present specification, the “primary particle” refers to the smallest unit of the particles constituting the positive electrode active material, and specifically refers to the smallest unit determined from the external geometric form observed under electron microscope observation. Further, in the present specification, an aggregate formed by aggregation of 10 or more such primary particles 56p is referred to as a “secondary particle”.

[0027] The lithium transition metal composite oxide 56a has voids S inside the secondary particles, which are derived from the gaps between the aggregated primary particles 56p. Note that the voids S may or may not be open. When the voids S are open, one void S may have two or more openings. The voids S are located inside the virtual outer contour line OL of the secondary particles in a cross-sectional view, and are typically spaces surrounded by a plurality of primary particles 56p.

[0028] In this embodiment, the porosity of the lithium transition metal composite oxide 56a (secondary particles) is 2% or more and 10% or less. The porosity of the secondary particles may be, for example, 2% or more, 3% or more, or 5% or more. By setting the porosity of the secondary particles to be a predetermined value or more, the initial resistance of the lithium ion secondary battery 100 can be reduced. Also, an appropriate void S can be ensured inside the secondary particles of the lithium transition metal composite oxide 56a, and for example, the coating portion 56c described later can be evenly disposed up to the center portion of the secondary particles. On the other hand, when the porosity of the secondary particles is excessive (typically, when the porosity exceeds 10%), the increase in resistance during storage increases. Therefore, the porosity of the secondary particles may be, for example, 10% or less, 9% or less, or 8% or less. In this specification, the "average porosity of the lithium transition metal composite oxide" can be grasped from a cross-sectional electron microscope image of the lithium transition metal composite oxide, and the average value can be calculated by measuring the porosities of a plurality of arbitrarily selected secondary particles. The plurality of secondary particles can be, for example, 20 or more. First, a sample for cross-sectional observation of the lithium transition metal composite oxide 56a is prepared by cross-section polisher processing or the like. Next, an SEM image of the sample for cross-sectional observation is obtained using a scanning electron microscope (SEM: Scanning Electron Microscope). From the obtained SEM image, using image analysis software (for example, "ImageJ"), the area of the entire secondary particle and the total area of all voids inside the secondary particle are obtained respectively. Then, using the following formula (ii): Porosity (%) = (Total area of all voids / Area of the entire secondary particle) × 100... (ii) the porosity is obtained and the average value is calculated.

[0029] The porosity of the lithium transition metal composite oxide 56a can be adjusted, for example, by changing the synthesis conditions when synthesizing the hydroxide, which is a precursor of the lithium transition metal composite oxide 56a, by the crystallization method. Specifically, in the crystallization method, the hydroxide is synthesized by adding an aqueous raw material solution containing a metal element other than lithium and a pH adjusting solution to the reaction solution. By changing the pH value and the stirring speed of the reaction solution at this time, the porosity of the hydroxide can be adjusted. By mixing this hydroxide with a compound serving as a lithium source (for example, lithium hydroxide, etc.) and firing it, a lithium transition metal composite oxide 56a in the form of secondary particles with an adjusted porosity can be obtained.

[0030] A coating portion 56c is disposed on at least a part of the surface of the lithium transition metal composite oxide 56a. And in the present embodiment, the coating portion 56c contains at least a boron compound. By disposing a boron compound as the coating portion 56c on the surface of the lithium transition metal composite oxide 56a, the elution of Mn from the lithium transition metal composite oxide 56a into the non-aqueous electrolyte 80 can be suppressed, and the increase in resistance during storage can be suppressed. Further, in the positive electrode active material 56, by providing the coating portion 56c, the binding force between the primary particles 56p is increased. Thereby, the stress during expansion and contraction can be relaxed, and the particle cracking of the lithium transition metal composite oxide 56a can be suppressed. From this viewpoint as well, the increase in resistance during storage can be preferably suppressed. As the boron compound, for example, a boron-containing oxide or an oxide containing boron and lithium is preferable, and a lithium borate salt is particularly preferable. Specific examples of the boron compound may include LiBO2, LiB(OH)4, Li3BO3, B2O3, etc., and LiBO2 is particularly preferably used. The ratio of the boron compound as the coating portion 56c is, in terms of boron (B) conversion when the total amount of Ni and Mn in the positive electrode active material is 100 mol%, for example, 0.5 to 2.0 mol%, and preferably 0.7 to 1.5 mol%.

[0031] The coating portion 56c is preferably disposed at least on the surface of the secondary particles of the lithium transition metal composite oxide 56a. At this time, the coating rate of the positive electrode active material 56 by the coating portion 56c may be 60% or more. Note that the fact that the coating portion 56c is disposed on the surface of the secondary particles and the ratio (coating rate) of the portion covered by the coating portion 56c can be confirmed, for example, by XPS (X-ray Photoelectron Spectroscopy) analysis with respect to the positive electrode active material 56.

[0032] Preferably, in addition to or instead of on the surface of the secondary particles, the coating portion 56c is present inside the secondary particles. Specifically, it is preferable that the coating portion 56c is present on the surface of the primary particles 56p inside the secondary particles. The amount of the coating portion 56c present inside the secondary particles may be more or less than the amount of the coating portion 56c present on the surface of the secondary particles. Thereby, the effects of the technology disclosed herein can be exhibited at a higher level. Note that the fact that the coating portion 56c is present inside the secondary particles can be confirmed by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) analysis.

[0033] In some preferred embodiments, the coating portion 56c preferably further contains aluminum oxide (Al2O3) in addition to the boron compound. Specifically, aluminum oxide has a function of trapping hydrogen fluoride gas (hydrofluoric acid) generated by the decomposition of the non-aqueous electrolyte 80. Thereby, elution of Mn into the non-aqueous electrolyte 80 due to the reaction between the positive electrode active material 56 (lithium transition metal composite oxide 56a) and hydrogen fluoride gas can be suppressed. Therefore, by disposing aluminum oxide on the surface, an increase in resistance during storage can be more preferably suppressed. The ratio of aluminum oxide as the coating portion 56c is, in terms of aluminum (Al) when the total amount of Ni and Mn in the lithium transition metal composite oxide 56a is 100 mol%, for example, 0.1 to 0.5 mol%, and preferably 0.3 to 0.5 mol%.

[0034] The average particle diameter (D50) of the lithium transition metal composite oxide 56a (secondary particles) is not particularly limited. From the viewpoints of particularly high fillability of the positive electrode active material layer 54 and high volumetric energy density of the lithium ion secondary battery 100, the average particle diameter (D50) of the lithium transition metal composite oxide 56a (secondary particles) is preferably 5 μm to 30 μm, more preferably 10 μm to 20 μm.

[0035] In the present specification, the "average particle diameter (D50)" refers to the median diameter (D50), and in the volume-based particle size distribution based on the laser diffraction / scattering method, it means the particle diameter corresponding to a cumulative frequency of 50% by volume from the side of fine particles with a small particle diameter. Therefore, the average particle diameter (D50) of the secondary particles can be determined using a laser diffraction / scattering type particle size distribution measuring device or the like.

[0036] The average particle diameter of the primary particles 56p of the lithium transition metal composite oxide 56a is not particularly limited. For example, from the viewpoints of the energy density and output characteristics of the positive electrode active material 56, the average particle diameter of the primary particles 56p of the lithium transition metal composite oxide 56a is 0.05 μm to 2.5 μm, preferably 1.2 μm or more, more preferably 1.5 μm or more, and still more preferably 1.7 μm or more. On the other hand, from the viewpoint of higher cycle characteristics of the positive electrode active material 56, the average particle diameter of the primary particles 56p of the lithium transition metal composite oxide 56a is preferably 2.2 μm or less, more preferably 2.1 μm or less.

[0037] The "average particle diameter of the primary particles of the lithium transition metal composite oxide" refers to the average particle diameter of the major axis of the primary particles 56p of the lithium transition metal composite oxide 56a, which is grasped from the cross-sectional electron microscope image of the lithium transition metal composite oxide 56a and refers to the average value of the major axes of a plurality of arbitrarily selected primary particles 56p. The plurality of primary particles 56p can be, for example, 20 or more. Specifically, the average particle diameter of the primary particles 56p is obtained, for example, by preparing a cross-sectional observation sample of the lithium transition metal composite oxide 56a by cross-section polisher processing. Next, an SEM image of the cross-sectional observation sample of the lithium transition metal composite oxide 56a is acquired using a scanning electron microscope (SEM). Then, the major axes of a plurality of primary particles 56p arbitrarily selected from the SEM image are respectively determined using image analysis type particle size distribution measurement software (for example, "Mac-View"), and the average value thereof is calculated to obtain the average particle diameter.

[0038] The primary particles 56p of the lithium transition metal composite oxide 56a are typically substantially spherical. However, they may have an irregular shape or the like. In the present specification, "substantially spherical" means a form that can be generally regarded as a sphere as a whole, and the average aspect ratio (major axis / minor axis ratio) based on the cross-sectional observation image of the electron microscope is, for example, 1 to 1.5.

[0039] The average particle diameter (D50) of the lithium transition metal composite oxide 56a is not particularly limited. From the viewpoint of improving the energy density and output characteristics of the positive electrode active material layer, the average particle diameter (D50) of the lithium transition metal composite oxide 56a is preferably 5 μm to 30 μm, more preferably 10 μm to 20 μm. In the present specification, the "average particle diameter (D50)" refers to the median diameter (D50), and in the volume-based particle size distribution based on the laser diffraction / scattering method, it means the particle diameter corresponding to the cumulative frequency of 50 volume% from the side with a smaller particle diameter (fine particle side). Therefore, the average particle diameter (D50) can be obtained using a laser diffraction / scattering type particle size distribution measuring device or the like.

[0040] The BET specific surface area of the lithium transition metal composite oxide 56a is not particularly limited. Since it can impart excellent output characteristics to the lithium-ion secondary battery 100, the BET specific surface area of the lithium transition metal composite oxide 56a is preferably 0.50 m 2 / g to 0.85 m 2 / g, more preferably 0.55 m 2 / g to 0.80 m 2 / g. The BET specific surface area of the lithium transition metal composite oxide 56a can be measured by the nitrogen adsorption method using a commercially available specific surface area measuring device.

[0041] The content of the positive electrode active material 56 in the positive electrode active material layer 54 (that is, the content of the positive electrode active material 56 with respect to the total mass of the positive electrode active material layer 54) is not particularly limited, but is, for example, 80% by mass or more, preferably 87% by mass or more, more preferably 90% by mass or more, still more preferably 95% by mass or more, and most preferably 97% by mass or more.

[0042] As the positive electrode current collector 52, a known material used for the lithium-ion secondary battery 100 may be used. Examples thereof include a sheet or foil made of a metal having good conductivity (for example, aluminum, nickel, titanium, stainless steel, etc.). As the positive electrode current collector 52 according to the present embodiment, an aluminum foil is preferable.

[0043] The dimensions of the positive electrode current collector 52 are not particularly limited and can be appropriately determined according to the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness of the positive electrode current collector 52 is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0044] <Negative electrode> The negative electrode (negative electrode sheet 60) disclosed herein includes a negative electrode active material layer 64. As shown in FIG. 2, the negative electrode (negative electrode sheet 60) includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported on the negative electrode current collector 62. In this embodiment, the negative electrode active material layer 64 is shown only on one surface of the negative electrode current collector 62, but the negative electrode active material layer 64 may be provided on both surfaces of the negative electrode current collector 62, respectively.

[0045] The negative electrode active material layer 64 contains a negative electrode active material 66 that can reversibly occlude and release charge carriers. FIG. 4 is a schematic diagram showing the configuration of a negative electrode sheet 60 (negative electrode) according to an embodiment. The negative electrode active material 66 according to this embodiment essentially includes a carbon material 66c and a Si-containing material 66s. However, the negative electrode active material 66 may further contain other negative electrode active materials in addition to the carbon material 66c and the Si-containing material 66s. Also, for convenience of explanation, although not shown here, the negative electrode active material layer 64 may contain a conductive material, a binder, a thickener, etc. in addition to the negative electrode active material 66.

[0046] As the carbon material 66c, for example, graphite, hard carbon, soft carbon, etc. are used, and among them, graphite is preferably used. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in a form where the graphite is coated with an amorphous carbon material.

[0047] The average particle diameter of the carbon material 66c is not particularly limited, but for example, it is 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. The average particle diameter (D50) of the carbon material can be obtained, for example, by the laser diffraction scattering method.

[0048] The Si-containing material 66s is, for example, silicon (Si), SiO x (0.05 < x < 1.95) silicon oxide represented by, Si-C composite containing Si particles in carbon particles, lithium silicate (Li x Si y O z) and the like can be mentioned. As the Si-containing material 66s, silicon, silicon oxide, and Si-C composites can be preferably used. Further, as the Si-containing material 66s, an alloy composed of Si and an element other than Si can be used. Examples of the element other than Si include Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Sn, Ti, and the like. By including the Si-containing material 66s in the negative electrode active material 66, the formation of a coating layer starting from Mn eluted from the positive electrode active material on the surface of the negative electrode active material layer 64 can be suppressed. Therefore, an increase in resistance during storage can be preferably suppressed.

[0049] The average particle diameter of the Si-containing material 66s is not particularly limited, but for example, it is 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. The average particle diameter (D50) of the carbon material can be obtained, for example, by the laser diffraction scattering method.

[0050] In the present embodiment, when the total amount of the negative electrode active material 66 is 100% by mass, the content ratio of the Si element in the negative electrode active material 66 is 5% by mass or more and 10% by mass or less from the viewpoint of achieving both the effect of suppressing the increase in resistance during storage and the capacity retention rate. When the content ratio of the Si element is small (typically less than 5% by mass), the effect of suppressing the formation of the coating layer starting from Mn eluted from the positive electrode active material on the surface of the negative electrode active material layer 64 cannot be preferably obtained, and the effect of suppressing the storage resistance cannot be obtained well. On the other hand, when the content ratio of the Si element is too high (typically exceeding 10% by mass), the capacity retention rate during storage decreases.

[0051] Regarding the distribution of the Si-containing material 66s in the negative electrode active material layer 64, there is no particular limitation. The Si-containing material may be dispersed throughout the entire negative electrode active material layer 64, may be concentrated and dispersed in the upper layer of the negative electrode active material layer 64 (the surface layer side of the negative electrode active material layer 64), or may be concentrated and dispersed in the lower layer of the negative electrode active material layer 64 (the side of the negative electrode current collector 62). In some preferred embodiments, as shown in the second embodiment (FIG. 5) described later, the Si-containing material 66s is preferably concentrated and dispersed in the upper layer of the negative electrode active material layer 64 (corresponding to the negative electrode upper layer 164u in FIG. 5). As shown in FIG. 4, in this embodiment, the Si-containing material 66s is dispersed almost uniformly throughout the entire negative electrode active material layer 64. Note that "concentrated and dispersed in the upper layer of the negative electrode active material layer 64" is an example of "the content ratio of the Si element in the negative electrode upper layer is 90% by mass or more when the Si element of the entire negative electrode active material is 100% by mass" disclosed herein.

[0052] The negative electrode active material layer 64 may contain components other than the negative electrode active material 66, such as a conductive material, a binder, a thickener, etc. As the binder, for example, those conventionally used in lithium secondary batteries, such as styrene-butadiene rubber (SBR) and polyvinylidene fluoride (PVDF), can be used without particular limitation. As the thickener, for example, carboxymethyl cellulose (CMC) etc. can be used.

[0053] The negative electrode active material layer 64 preferably further contains a conductive material. As the conductive material, for example, carbon black (CB) such as acetylene black (AB) and ketjen black, carbon fiber such as carbon nanotube (CNT) and vapor grown carbon fiber (VGCF), activated carbon, carbon materials such as graphite can be preferably used, and among them, it is preferable to contain CNT. By containing CNT as the conductive material, the capacity retention rate can be preferably maintained. As the CNT, any of single-walled carbon nanotube (SWCNT), double-walled carbon nanotube (DWCNT), and multi-walled carbon nanotube (MWCNT) may be used, and these can be used alone or in combination of two or more. When using CNT as the conductive material in the negative electrode active material layer 64, it is preferable to use SWCNT. When using CNT as the conductive material in the negative electrode active material layer 64, it is preferable to contain 0.5% by mass or less of CNT with respect to the whole negative electrode active material layer.

[0054] The content of the negative electrode active material 66 in the negative electrode active material layer 64 is preferably 90% by mass or more, 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, 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, more preferably 0.5% by mass or more and 2% by mass or less.

[0055] The thickness of the negative electrode active material layer 64 is not particularly limited. However, when the negative electrode active material layer 64 is provided only on one surface of the negative electrode current collector 62, for example, it is 10 μm or more and 200 μm or less, preferably 20 μm or more and 100 μm or less. Further, when the negative electrode active material layer 64 is provided on both surfaces of the negative electrode current collector 62, for each side (the thickness of each of one surface and the other surface), for example, it is 10 μm or more and 200 μm or less, preferably 20 μm or more and 100 μm or less.

[0056] As the negative electrode current collector 62, a known negative electrode current collector used in a lithium-ion secondary battery may be used. Examples thereof include sheets or foils made of metals with good conductivity (for example, copper, nickel, titanium, stainless steel, etc.). As the negative electrode current collector 62, a copper foil is preferred.

[0057] The dimensions of the negative electrode current collector 62 are not particularly limited and can be appropriately determined according to the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness of the negative electrode current collector 62 is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0058] As the separator 70, various porous sheets similar to those conventionally used in lithium-ion secondary batteries can be used. Examples thereof include porous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such a porous resin sheet may have a single-layer structure or a multi-layer 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). The separator 70 may be provided with a heat-resistant layer (HRL).

[0059] As the non-aqueous electrolyte 80, the same as those used in conventional lithium-ion secondary batteries can be used, and typically, those containing a supporting salt in an organic solvent (non-aqueous solvent) can be used. As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, etc. can be used. Among them, carbonates are preferred because the effect of reducing the low-temperature resistance due to the positive electrode material is particularly high. Examples of carbonates include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), etc. Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, lithium bis(fluorosulfonyl)imide (LiFSI), etc. can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0060] In addition, as long as the effects of the present disclosure are not significantly impaired, the non-aqueous electrolyte 80 may contain components other than the above-mentioned non-aqueous solvent and supporting salt, for example, various additives such as a gas generator, a film-forming agent, a dispersant, a thickener, etc. Examples of the additives used in the non-aqueous electrolyte 80 include positive and negative electrode film-forming agents such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS); overcharge preventives such as biphenyl (BP), cyclohexylbenzene (CHB), t-butylbenzene, t-amylbenzene, etc. Among them, the non-aqueous electrolyte 80 preferably contains FEC. When the non-aqueous electrolyte 80 contains FEC as an additive, it is preferably contained in a proportion of 20 vol% or less with respect to the non-aqueous electrolyte 80.

[0061] The lithium-ion secondary battery 100 configured as described above includes a lithium transition metal composite oxide that is a high-Ni lithium transition metal composite oxide, and the lithium transition metal composite oxide is secondary particles formed by aggregation of primary particles, and a cathode active material having an average porosity of the secondary particles of 2% or more and 10% or less is used. Further, the cathode active material includes a boron compound on the surface of the lithium transition metal composite oxide and has a coating portion. In addition, the anode active material of the lithium-ion secondary battery 100 includes a carbon material and a Si-containing material, and when the total amount of the anode active material is 100% by mass, the content ratio of Si element in the anode active material is 5% by mass or more and 10% by mass or less. Thereby, while reducing the cobalt content in the cathode active material, an increase in the initial resistance can be suppressed, and an increase in the resistance during storage can be suppressed.

[0062] Specifically, when a layered lithium transition metal composite oxide containing Mn is used as the cathode active material, if the Co content in the lithium transition metal composite oxide is reduced, the stability of the lithium transition metal composite oxide decreases. Typically, since the interlayer of the lithium transition metal composite oxide becomes narrow, the conductivity of Li ions decreases, resulting in an increase in the initial resistance. In addition, due to the decrease in the stability of the lithium transition metal composite oxide, Mn elution from the cathode active material layer (specifically, the lithium transition metal composite oxide) becomes remarkable during storage. For example, Mn eluted from the cathode active material layer precipitates on the surface of the anode active material layer (typically the graphite surface), and further film growth starting from the precipitated Mn occurs. Such a film causes durability deterioration of the lithium-ion battery. Therefore, conventionally, in a lithium-ion secondary battery using, as the cathode active material, a layered lithium transition metal composite oxide containing Mn with a reduced Co content, there has been a problem that the deterioration of the initial resistance and the increase in the resistance during storage become large.

[0063] On the other hand, in the lithium-ion secondary battery 100 according to the present embodiment, in the lithium transition metal composite oxide 56a, since the content ratio of Ni to the total of metal elements other than Li is 75 mol% or more, the Mn ratio in the lithium transition metal composite oxide 56a can be suppressed. And by setting the porosity of the secondary particles of the lithium transition metal composite oxide 56a to 2 to 10%, the reaction area between the non-aqueous electrolyte 80 and the lithium transition metal composite oxide 56a increases. Thereby, the conductivity of Li ions is improved and the initial resistance is improved. By the way, by providing voids between the secondary particles of the lithium transition metal composite oxide 56a, while the initial resistance is improved, a contradiction occurs in that the elution of Mn from the lithium transition metal composite oxide 56a to the non-aqueous electrolyte 80 is promoted due to the increase in such a reaction area. Therefore, in the lithium-ion secondary battery 100 according to the present embodiment, a boron compound is disposed as a coating portion 56c on at least a part of the surface of the lithium transition metal composite oxide 56a. By being disposed on the surface of the lithium transition metal composite oxide 56a, the boron compound can, for example, suppress the reaction area between the non-aqueous electrolyte 80 and the lithium transition metal composite oxide 56a. Thereby, the elution of Mn from the lithium transition metal composite oxide 56a can be suppressed. Further, according to the Si-containing material 66s contained in the negative electrode active material 66, the precipitation of Mn on the surface of the negative electrode active material layer 64 is suppressed. Thereby, the formation of an Mn film on the surface of the negative electrode active material layer 64 can be suppressed. Therefore, even if the Co content in the lithium transition metal composite oxide is reduced, it is possible to provide a lithium-ion secondary battery 100 that suppresses an increase in the initial resistance and suppresses an increase in the resistance during storage.

[0064] The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power sources for driving mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Further, the lithium-ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium-ion secondary battery 100 can typically be used in the form of an assembled battery formed by connecting a plurality of them in series and / or in parallel.

[0065] Note that, as an example, a rectangular lithium-ion secondary battery 100 including an electrode body 20 having a flat wound structure has been described. However, the lithium-ion secondary battery disclosed herein can also be configured as a lithium-ion secondary battery including a laminated electrode body (i.e., an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). The laminated electrode body may include a plurality of separators such that one separator is interposed between each of the positive electrode and the negative electrode, and the positive electrode and the negative electrode may be alternately laminated while one separator is folded. Further, the lithium-ion secondary battery according to the present embodiment can also be constructed in forms such as a cylindrical lithium-ion secondary battery, a coin-type lithium-ion secondary battery, and a laminate-type lithium-ion secondary battery. Furthermore, an all-solid-state secondary battery using a solid electrolyte as the electrolyte can also be constructed.

[0066] <Second Embodiment> FIG. 5 is a corresponding view of FIG. 4 of the negative electrode sheet 160 (negative electrode) according to the second embodiment. In FIG. 5, it may be the same as the above-described lithium-ion secondary battery 100 except that a negative electrode active material layer 164 is provided instead of the negative electrode active material layer 64. As shown in FIG. 5, the negative electrode active material layer 164 has a so-called multi-layer structure including a negative electrode upper layer 164u and a negative electrode lower layer 164d.

[0067] In some preferred embodiments, the negative electrode active material layer 164 may include a lower negative electrode layer 164d in contact with the negative electrode current collector 62 and an upper negative electrode layer 164u farther from the negative electrode current collector 62 than the lower negative electrode layer 164d when viewed in the thickness direction. However, the negative electrode active material layer 164 may have a multi-layer structure of three or more layers. For example, a layer having a composition different from that of the upper negative electrode layer 164u and the lower negative electrode layer 164d may be formed between the upper negative electrode layer 164u and the lower negative electrode layer 164d.

[0068] The upper negative electrode layer 164u is a layer located on the surface side of the lower negative electrode layer 164d when viewed in the thickness direction of the negative electrode active material layer 164. The upper negative electrode layer 164u is located farther from the negative electrode current collector 62 than the lower negative electrode layer 164d. Here, the upper negative electrode layer 164u constitutes the outermost layer of the negative electrode active material layer 164. In other words, the upper negative electrode layer 164u is located closer to the positive electrode (positive electrode sheet 50) than the lower negative electrode layer 164d.

[0069] The negative electrode active material 66 related to the upper negative electrode layer 164u essentially includes an Si-containing material 66s. When the total amount of Si element in the negative electrode active material layer 164 is 100% by mass, the content ratio of Si element contained in the upper negative electrode layer 164u is preferably 90% by mass or more, more preferably 95% by mass or more, and still more preferably 98% by mass or more. Typically, the content of Si element contained in the upper negative electrode layer 164u is higher than the content of Si element contained in the lower negative electrode layer 164d. In other words, in the negative electrode active material layer 164, the Si element is concentrated and dispersed on the positive electrode (positive electrode sheet 50) side.

[0070] As described above, Mn eluted from the positive electrode active material layer 54 (typically, the lithium transition metal composite oxide 56a) precipitates from the positive electrode (positive electrode sheet 50) side of the negative electrode active material layer 64, in other words, from the surface layer of the negative electrode active material layer 164, to form a film. Here, by adopting a configuration in which the Si-containing material is concentrated and dispersed on the positive electrode (positive electrode sheet 50) side, that is, the upper negative electrode layer 164u side, of the negative electrode active material layer 164, the effect of suppressing the film formation on the surface of the negative electrode active material layer 164 by Mn can be more efficiently exhibited. Therefore, an increase in resistance during storage can be preferably suppressed.

[0071] In addition, the "content ratio (mass%) of Si element contained in the upper negative electrode layer when the amount of Si element in the entire negative electrode active material layer is 100%" in this specification can be calculated as follows. First, the thickness of the upper negative electrode layer is obtained by observing the cross-sectional SEM image of the negative electrode active material layer 164. Next, for a predetermined area of the negative electrode active material layer, a sample is collected by scraping off the negative electrode active material layer by the thickness of the obtained upper negative electrode layer, and the amount of Si element in the upper negative electrode layer is calculated by performing ICP (Inductively Coupled Plasma) analysis. Similarly, for the lower negative electrode layer, a sample is collected by scraping off a predetermined area of the negative electrode active material layer, and the amount of Si element in the lower negative electrode layer is calculated by ICP analysis. Then, from the values of the amount of Si element in the obtained upper negative electrode layer and lower negative electrode layer respectively, the content ratio of the Si element in the upper negative electrode layer to the amount of Si element in the entire negative electrode active material layer can be obtained.

[0072] The content of Si element contained in the upper negative electrode layer 164u is preferably 2% by mass or more, more preferably 5% by mass or more, when the total of the negative electrode active material 66 in the upper negative electrode layer 164u is 100% by mass.

[0073] The lower negative electrode layer 164d is a layer located closer to the negative electrode current collector 62 than the upper negative electrode layer 164u when viewed in the thickness direction of the negative electrode active material layer 64. Here, the lower negative electrode layer 164d is in contact with the negative electrode current collector 62.

[0074] The negative electrode active material 66 related to the lower negative electrode layer 164d essentially contains a carbon material 66c. The lower negative electrode layer 164d may or may not contain a Si-containing material 66s. The content of Si element contained in the lower negative electrode layer 164d is not particularly limited, but when the negative electrode active material of the lower negative electrode layer 164d is 100% by mass, for example, it can be 2% by mass or less, preferably 1% by mass or less. The content of C element contained in the lower negative electrode layer 164d is not particularly limited, but when the negative electrode active material of the lower negative electrode layer 164d is 100% by mass, it is preferably 98% by mass or more, more preferably 99% by mass or more.

[0075] The ratio (Tu / Ta) of the thickness Tu of the upper negative electrode layer 164u to the total thickness Ta of the negative electrode active material layer 164 is, for example, 0.8 or less, preferably 0.7 or less. Thereby, since the Si-containing material 66s (in other words, Si element) in the negative electrode active material layer 164 can be dispersed toward the positive electrode (positive electrode sheet 50) side, the effect of more efficiently suppressing the film formation by Mn is exhibited. Therefore, it is possible to suitably suppress the increase in resistance during storage. Further, from the viewpoint of fillability, the ratio of the thickness Tu of the upper negative electrode layer 164u to the total thickness Ta of the negative electrode active material layer 164 is, for example, 0.4 or more, preferably 0.5 or more. The "ratio of the thickness of the upper negative electrode layer to the total thickness of the negative electrode active material layer" in the specification is measured in the cross-sectional SEM image of the negative electrode active material layer 164.

[0076] Note that the upper negative electrode layer 164u and the lower negative electrode layer 164d as described above can be manufactured by a conventionally known method. Although not limited thereto, for example, a slurry for forming a negative electrode active material layer is prepared using two or more types of negative electrode active materials 66 having different compositions (typically, the content ratio of the Si-containing material), and the slurry for forming the negative electrode active material layer is applied to form the layers.

[0077] Hereinafter, examples related to the technology disclosed herein will be described, but it is not intended to limit the technology disclosed herein to those shown in such examples.

[0078] <Test Example 1: Examination of the Structure of the Positive Electrode Active Material and the Presence or Absence of Si in the Negative Electrode Active Material> (Example 1) [Fabrication of Positive Electrode Sheet] First, a positive electrode active material according to Example 1 was prepared. Specifically, when generating a hydroxide as a precursor by a crystallization method, a compound containing ammonium ions was used as an alkaline compound, the ammonium ion concentration in the reaction solution was 1 wt%, and when the hydroxide and a lithium source were mixed and fired, the firing temperature was 800 ° C and the firing time was 12 hours. Thereby, a lithium transition metal composite oxide (Li1Ni 0.83 Mn0.17 O2) was obtained. The porosity was measured by the method described above.

[0079] To the matrix of the obtained lithium transition metal composite oxide, H3BO4 was dry-mixed. At this time, the molar ratio of the total amount of Ni and Mn in the lithium transition metal composite oxide to boron was made 100:1.0. Then, after boron was complexed on the surface of the first lithium transition metal composite oxide, heat treatment was performed at 300 °C for 3 hours in an oxygen atmosphere. In this way, on the surface of the layered-structured lithium transition metal composite oxide with the composition (molar ratio) of Ni:Co:Mn = 83:0:17 (corresponding to 83 / 0 / 17 in the "Ni / Co / Mn" column of Table 1), as a coating part, 1 mol% of a boron compound (LiBO2) in terms of boron was arranged with respect to 100 mol% of the total amount of Ni and Mn in the lithium transition metal composite oxide, and the positive electrode active material according to Example 1 was obtained.

[0080] The positive electrode active material prepared above, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed at a solid content mass ratio of positive electrode active material:AB:PVDF = 100:1:1. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the obtained mixture and kneaded to prepare a slurry for forming a positive electrode active material layer. The obtained slurry for forming a positive electrode active material layer was applied to both sides of an aluminum foil positive electrode current collector with a thickness of 15 μm and dried to form a positive electrode active material layer. The obtained positive electrode active material layer was roll-pressed using a rolling roller and then cut into a predetermined size to produce a positive electrode sheet according to Example 1.

[0081] [Fabrication of negative electrode sheet] First, graphite as a carbon material and a Si-C composite as a Si-containing material were mixed so that when the total amount of the negative electrode active material was 100% by mass, the content ratio of Si element was 10% by mass and the content ratio of C element was 90% by mass, and the negative electrode active material according to Example 1 was produced.

[0082] The prepared negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and single-walled carbon nanotubes (SWCNT) as a conductive material were mixed in an ion-exchanged water at a solid content mass ratio of negative electrode active material:SBR:CMC:SWCNT = 100:1:1:0.5 to prepare a slurry for forming a negative electrode active material layer. The obtained slurry for forming a negative electrode active material layer was applied onto a negative electrode current collector made of a copper foil with a thickness of 10 μm and dried to form a negative electrode active material layer. The obtained negative electrode active material layer was roll-pressed using a rolling roller and then cut into a predetermined size. Thus, a negative electrode sheet according to Example 1 in which the Si-containing material was almost uniformly dispersed throughout the entire negative electrode active material was produced.

[0083] [Preparation of Separator] As the separator, a porous polyolefin sheet with a thickness of 20 μm having a three-layer structure of PP / PE / PE was prepared.

[0084] [Preparation of Non-aqueous Electrolyte] As the non-aqueous electrolyte, a mixture in which lithium hexafluorophosphate (LiPF6) as a supporting salt was dissolved at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FEC) at a volume ratio of EC:EMC:DMC:FEC = 25:40:30:5 was prepared.

[0085] [Fabrication of Lithium-ion Secondary Battery for Evaluation] The positive electrode sheet and the negative electrode sheet were overlapped with a separator interposed therebetween to obtain a laminate. Next, the laminate was wound to obtain a wound body, which was press-treated to have a flat shape to obtain a flat electrode body. Current collector terminals were attached to the electrode body, which was inserted into a battery case and welded, and then the non-aqueous electrolyte was injected. Thereafter, the battery case was sealed to fabricate a lithium-ion secondary battery for evaluation.

[0086] [Initial Charging Treatment] For the obtained lithium-ion secondary battery for evaluation according to Example 1, as an initial charging process under a temperature environment of 25°C, constant current charging was performed at a current value of 0.1C up to a voltage of 4.2V, and then constant voltage charging was performed until the current value reached 1 / 20C. Thereafter, constant current discharging was performed at a current value of 0.1C down to 3.0V.

[0087] (Example 2) Li1Ni 0.83 Mn 0.17 A lithium-ion secondary battery for evaluation according to Example 2 was fabricated and subjected to an initial charging process in the same manner as in Example 1, except that a lithium transition metal composite oxide doped with 0.3 mol% of W (tungsten) as an additive element in Li1NiMn 0.83 0.17 O2 was used.

[0088] (Example 3) A lithium-ion secondary battery for evaluation according to Example 3 was fabricated and subjected to an initial charging process in the same manner as in Example 1, except that the negative electrode active material was only graphite (i.e., not containing an Si-containing material).

[0089] (Example 4) A lithium-ion secondary battery for evaluation according to Example 4 was fabricated and subjected to an initial charging process in the same manner as in Example 1, except that the porosity of the lithium transition metal composite oxide (secondary particles) was 0% (i.e., a solid structure without voids). The porosity of the positive electrode active material was adjusted by changing the production conditions of the precursor hydroxide.

[0090] (Example 5) A lithium-ion secondary battery for evaluation according to Example 5 was fabricated and subjected to an initial charging process in the same manner as in Example 1, except that LiBO2 was not disposed on the surface of the lithium transition metal composite oxide (indicated by a dotted line in the "coated part" column of Table 1).

[0091] (Example 6) A lithium-ion secondary battery for evaluation according to Example 6 was fabricated and subjected to an initial charging process in the same manner as in Example 1, except that the porosity of the lithium transition metal composite oxide (secondary particles) was 0% (i.e., a solid structure without voids) and LiBO2 was not disposed on the surface of the lithium transition metal composite oxide.

[0092] (Example 7) A lithium transition metal composite oxide (secondary particle) has a porosity of 0% (i.e., a solid structure without voids being formed), and LiBO2 is not disposed on the surface of the lithium transition metal composite oxide, and an evaluation lithium ion secondary battery according to Example 7 was produced and an initial charging process was performed in the same manner as in Example 1 except that the negative electrode active material was only graphite (i.e., not containing an Si-containing material).

[0093] [Evaluation of initial resistance] For the evaluation lithium ion secondary batteries according to Examples 1 to 7, first, in a thermostatic bath at 25°C, constant current constant voltage (CCCV) charging was performed on each evaluation lithium ion secondary battery at a current value of 0.3C, and the state of charge (SOC) was adjusted to 50%. Then, a pulse discharge was performed at a current value of 5C for 10 seconds. The amount of voltage change (ΔV) during the pulse discharge was acquired, and the following formula (I): Resistance value = Amount of voltage change (ΔV) / Current value (5C) … (I) was used to calculate the resistance value before storage (initial resistance value). The results are shown in Table 1.

[0094] [Evaluation of storage resistance increase rate] For the evaluation lithium ion secondary batteries according to Examples 1 to 7 after measuring the initial resistance value, the SOC was adjusted to 100% under a temperature environment of 25°C. Each of these evaluation lithium ion secondary batteries was placed in a thermostatic layer at 60°C and stored for 14 days. Then, the resistance value after storage was calculated by the same method as the initial resistance. And the following formula (II): Resistance increase rate (%) = (Resistance value after storage / Resistance value before storage) × 100 … (II) was used to obtain the resistance increase rate (%). The results are shown in Table 1.

[0095] [Capacity retention rate] In parallel with the evaluation of the storage resistance increase rate described above, the capacity retention rate was evaluated. Specifically, for the evaluation lithium-ion secondary batteries according to Examples 1 to 7, the discharge capacity measured during the resistance increase rate evaluation was used as the initial capacity. Then, after storage was performed in the above-described storage resistance increase rate evaluation (i.e., after storage at 60 °C for 14 days), the discharge capacity of the evaluation lithium-ion secondary battery was determined in the same manner as the initial capacity. And, the following formula (III): Capacity retention rate (%) = Discharge capacity after 14-day storage / Initial capacity × 100…(III) was used to determine the capacity retention rate (%) for each example. The results are shown in Table 1.

[0096]

Table 1

[0097] From the results of Example 1 and Example 7, in Example 1 in which the secondary particles of the lithium transition metal composite oxide had 5% voids, a boron compound was disposed as a coating portion on the surface of the lithium transition metal composite oxide, and a negative electrode active material containing a carbon material and an Si-containing material was used, both reduction of the initial resistance and increase of the resistance during storage were favorably observed. On the other hand, in Examples 4, 6, and 7 using a positive electrode active material in which the porosity of the secondary particles of the lithium transition metal composite oxide was 0% (i.e., a solid structure), reduction of the initial resistance was not observed. From this, it is considered that by providing voids in the secondary particles of the lithium transition metal composite oxide, an effect of reducing the initial resistance can be obtained. Further, from the comparison of Examples 1, 3, and 5, it was found that by satisfying both the arrangement of a boron compound as a coating portion on the surface of the lithium transition metal composite oxide and the use of a negative electrode active material containing a carbon material and an Si-containing material, an effect of suppressing the increase of the resistance during storage can be favorably obtained.

[0098] In addition to the configuration of Example 1, in Example 2 where W was doped as an additive element in the lithium transition metal composite oxide, even better results were obtained for the initial resistance and the increase in resistance during storage. The reason for this is considered to be that the layered structure of the lithium transition metal composite oxide was stabilized by doping with W, resulting in improved Li-ion conductivity, and also because the elution of Mn into the non-aqueous electrolyte was suppressed.

[0099] <Test Example 2: Examination of Si Ratio> (Examples 8 to 10) An evaluation lithium ion secondary battery according to Examples 8 to 10 was fabricated and subjected to an initial charge treatment in the same manner as in Example 1, except that the content ratio of Si element was changed as shown in the "Si content ratio" column of Table 2 when the total amount of the negative electrode active material was 100% by mass, with graphite as the carbon material and the Si-C composite as the Si-containing material. The evaluation lithium ion secondary battery according to Examples 8 to 10 was evaluated in the same manner as in Test Example 1 by the method described above. The results are shown in Table 2.

[0100]

Table 2

[0101] As shown in the results of Table 2, for Example 9 and Example 1 where the content ratio of Si element in the negative electrode active material was 5% by mass or 10% by mass, the increase in resistance during storage was preferably suppressed in both cases, and no decrease in the capacity retention rate was observed. In Example 8 where the content ratio of Si element was 2% by mass, the effect of suppressing the increase in resistance during storage was not obtained sufficiently. The reason for this is considered to be that in Example 8, the Si-containing material that could sufficiently suppress the precipitation of Mn on the surface of the negative electrode active material was not contained in the negative electrode. Also, while the effect of suppressing the increase in battery resistance becomes better as the content ratio of Si element increases, in Example 10 where the content ratio of Si element was 13% by mass, a decrease in the capacity retention rate was observed compared to other examples. Therefore, from the viewpoint of achieving both the effect of suppressing the increase in resistance during storage and the capacity retention rate, the content ratio of Si element in the negative electrode active material is considered to be more preferably in the range of 5 to 10% by mass.

[0102] <Test Example 3: Examination of the Porosity of the Positive Electrode Active Material> (Examples 11 to 13) An evaluation lithium ion secondary battery according to Examples 11 to 13 was produced and subjected to an initial charging process in the same manner as in Example 1, except that the porosity of the lithium transition metal composite oxide (secondary particles) was set to the ratio shown in "Porosity (%)" in Table 3. The porosity of the positive electrode active material was adjusted by changing the production conditions of the hydroxide serving as the precursor. The evaluation lithium ion secondary batteries according to Examples 11 to 13 were evaluated in the same manner as in Test Example 1 by the method described above. The results are shown in Table 3.

[0103]

Table 3

[0104] In Example 11 where the porosity of the lithium transition metal composite oxide (secondary particles) was 2%, the initial resistance was preferably suppressed as compared with Example 4 where the porosity was 0%. When the porosity increases, good results are obtained for the improvement of the initial resistance. On the other hand, in Example 13 where the porosity was 13%, less suppression of the resistance increase during storage was observed as compared with Examples 1, 11 to 12. This is presumably because the influence of Mn elution due to the increased reaction area between the non-aqueous electrolyte and the positive electrode active material became larger due to the increased porosity.

[0105] <Test Example 4: Examination of the Coated Portion> (Example 14) An evaluation lithium ion secondary battery according to Example 14 was produced and subjected to an initial charging process in the same manner as in Example 1, except that a positive electrode active material in which Al2O3 was disposed in addition to LiBO2 as a coated portion was used on the surface of the lithium transition metal composite oxide. The ratios of LiBO2 and Al2O3 were set to a molar ratio of LiBO2:Al2O3 = 1:0.5 in terms of Li and Al, respectively, with respect to 100 mol% of the total amount of Ni and Mn of the lithium transition metal composite oxide.

[0106] (Example 15) An evaluation lithium ion secondary battery according to Example 15 was produced and subjected to an initial charging process in the same manner as in Example 1, except that a cathode active material in which 0.5 mol% of Al2O3 in terms of Al was arranged on the surface of the lithium transition metal composite oxide was used with respect to the total amount of Ni and Mn of 100 mol% of the lithium transition metal composite oxide.

[0107] For the evaluation lithium ion secondary batteries according to Examples 14 and 15, the same evaluation as in Test Example 1 was carried out by the method described above. The results are shown in Table 4.

[0108] [Table 4]

[0109] From the results of Example 1 and Example 14, in Example 14 in which Al2O3 was contained in addition to LiBO2 as the coating portion, better results were obtained for suppressing the increase in resistance during storage. On the other hand, for Example 15 in which only Al2O3 was arranged on the surface of the lithium transition metal composite oxide, the suppression of the increase in resistance during storage was not as good as in Example 1. The following reasons are considered for this. The boron compound has the function of suppressing the reaction area between the non-aqueous electrolyte and the lithium transition metal composite oxide and suppressing the elution of Mn by being arranged on the surface of the lithium transition metal composite oxide. Aluminum oxide has the function of trapping hydrogen fluoride gas generated by the decomposition of the non-aqueous electrolyte. Therefore, in Example 14, it is considered that the elution of Mn into the non-aqueous electrolyte was preferably suppressed because both the Mn elution suppressing effect of the boron compound and the hydrogen fluoride gas trapping effect of aluminum oxide were preferably exhibited. However, the elution of Mn from the lithium transition metal composite oxide is largely caused by the reaction between the non-aqueous electrolyte and the lithium transition metal composite oxide. Therefore, it is considered that simply arranging Al2O3 could not sufficiently suppress the elution of Mn from the lithium transition metal composite oxide.

[0110] <Test Example 5: Examination of Ni Mixing Ratio in Cathode Active Material> (Examples 18 to 24) An evaluation lithium-ion secondary battery according to Examples 18 to 24 was fabricated and subjected to an initial charging process in the same manner as in Example 1, except that the composition (molar ratio) of Ni, Co, and Mn in the lithium transition metal composite oxide was changed as shown in the "Ni / Co / Mn" column of Table 5. The same evaluation as in Test Example 1 was performed on the evaluation lithium-ion secondary batteries according to Examples 18 to 24 by the method described above. The results are shown in Table 5.

[0111]

Table 5

[0112] As shown in the results of Table 5, in Examples 1 and 18 to 20 where the Ni blending ratio of the lithium transition metal composite oxide is 83 mol%, even when the Co content of the lithium transition metal composite oxide is reduced, the reduction of the initial resistance, the resistance suppression effect during storage, and the capacity retention rate are stably compatible. Further, for Examples 21 to 22 where the Ni blending ratio of the lithium transition metal composite oxide is 95 mol% and Examples 23 to 24 where the Ni blending ratio of the lithium transition metal composite oxide is 75 mol%, similarly, the compatibility of the reduction of the initial resistance, the resistance suppression effect during storage, and the capacity retention rate was observed. Also, when viewed from the Co / Mn ratio of the lithium transition metal composite oxide, for Example 20 where the Co / Mn ratio is 0.42, the compatibility of the reduction of the initial resistance, the resistance suppression effect during storage, and the capacity retention rate was suitably obtained.

[0113] <Test Example 6: Examination of Si-containing material distribution in the negative electrode active material layer> (Examples 25 to 26) A first slurry for forming a negative electrode active material layer was prepared in the same manner as in Example 1, except that the negative electrode active material was only graphite (that is, when the total amount of the negative electrode active material was 100% by mass, the content ratio of Si element was 0% by mass and the content ratio of C element was 100% by mass). Similarly, a second slurry for forming a negative electrode active material layer was prepared in the same manner as in Example 1, except that graphite and an Si-C composite were used such that when the total amount of the negative electrode active material was 100% by mass, the content ratio of Si element was 20% by mass and the content ratio of C element was 80% by mass.

[0114] On the same copper foil negative electrode current collector as used in Example 1, a slurry for forming a first negative electrode active material layer was applied, dried, and roll-pressed using a rolling roller so as to have a predetermined density. Next, a slurry for forming a second negative electrode active material layer was applied onto the dried coating film of the slurry for forming the first negative electrode active material layer, dried, and roll-pressed using a rolling roller so as to have a predetermined density. At this time, the thickness of the slurry for forming the second negative electrode active material layer was set to 0.5 with respect to the thickness of the entire negative electrode active material layer. As a result, a negative electrode sheet of Example 25 in which a negative electrode lower layer formed by the slurry for forming the first negative electrode active material layer and a negative electrode upper layer formed by the slurry for forming the second negative electrode active material layer were supported on the negative electrode current collector was obtained. In the negative electrode sheet of Example 25, the content ratio of the Si element in the negative electrode upper layer was 99% by mass when the Si element of the entire negative electrode active material was 100% by mass. Similarly, on the same copper foil negative electrode current collector as used in Example 1, a slurry for forming a second negative electrode active material layer was applied, dried, and roll-pressed using a rolling roller so as to have a predetermined density. Next, a slurry for forming a first negative electrode active material layer was applied onto the dried coating film of the slurry for forming the second negative electrode active material layer, dried, and roll-pressed using a rolling roller so as to have a predetermined density. At this time, the thickness of the slurry for forming the first negative electrode active material layer was set to 0.5 with respect to the thickness of the entire negative electrode active material layer. As a result, a negative electrode sheet of Example 26 in which a negative electrode lower layer formed by the slurry for forming the second negative electrode active material layer and a negative electrode upper layer formed by the slurry for forming the first negative electrode active material layer were supported on the negative electrode current collector was obtained. In the negative electrode sheet of Example 26, the content ratio of the Si element in the negative electrode upper layer was 1% by mass when the Si element of the entire negative electrode active material was 100% by mass.

[0115] Evaluation batteries according to Example 25 and Example 26 were fabricated in the same manner as in Example 1 except that the above-described negative electrode sheets were used, and an initial charging treatment was performed. The same evaluations as in Test Example 1 were carried out on the evaluation lithium ion secondary batteries according to Example 25 and Example 26 by the method described above. The results are shown in Table 6.

[0116]

Table 6

[0117] As shown in the results of Table 6, for any of Example 1, Example 25, and Example 26, an effect of suppressing the increase in resistance during storage was observed as compared with Example 3 using only graphite as the negative electrode active material. Looking at Example 1, 25, and Example 26 where the Si blending ratio is 10% by mass in all cases, there was almost no difference in the capacity retention rate during storage due to the distribution of the Si-containing material. Further, when comparing Example 1, Example 25, and Example 26, in Example 1 where the Si-containing material (Si-C composite) was uniformly dispersed throughout the negative electrode active material layer, the effect of suppressing the rate of increase in resistance during storage was good. In Example 25 where the Si-containing material (Si-C composite) was concentrated and arranged in the upper layer, even better results were obtained for suppressing the rate of increase in resistance during storage. This is presumably because by concentrating and arranging the Si-containing material in the upper layer of the negative electrode active material, the effect of suppressing the precipitation of Mn by the Si-containing material was more efficiently exhibited on the surface of the negative electrode active material.

[0118] As described above, the preferred embodiments of the present disclosure have been explained, but the above embodiments are merely examples. The present disclosure can be implemented in various other forms. The present disclosure can be implemented based on the content disclosed herein and common general knowledge in the art. The technology described in the claims includes various modifications and changes of the above-exemplified embodiments. For example, it is also possible to replace a part of the above-described embodiments with other modified examples, and it is also possible to add other modified examples to the above-described embodiments. Further, if the technical features are not described as essential, they can be appropriately deleted.

[0119] As described above, specific aspects of the technology disclosed herein include those described in the following items. Item 1: A lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode active material layer containing a positive electrode active material, the positive electrode active material includes a layered lithium transition metal composite oxide containing Li, Ni, and Mn, and a coating portion disposed on at least a part of the surface of the lithium transition metal composite oxide, the lithium transition metal composite oxide has a Ni content of 75 mol% or more with respect to the total of metal elements other than Li, is secondary particles formed by aggregation of primary particles, and in cross-sectional observation of the secondary particles by a scanning electron microscope, the average porosity of the secondary particles is 2% or more and 10% or less, the coating portion contains a boron compound, the negative electrode includes a negative electrode active material layer containing a negative electrode active material, the negative electrode active material includes a carbon material and a Si-containing material, and when the total amount of the negative electrode active material is 100% by mass, the content ratio of Si element in the negative electrode active material is 5% by mass or more and 10% by mass or less. Item 2: The lithium-ion secondary battery according to Item 1, wherein the lithium transition metal composite oxide is represented by the following general formula: Li α Ni x Mn y M z O2 (where 0.8 ≤ α ≤ 1.2, 0.75 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.25, 0 ≤ z ≤ 0.2, x + y + z = 1, and M is one or more selected from Mg, Ca, Co, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W). Item 3: The lithium-ion secondary battery according to Item 1 or 2, wherein the lithium transition metal composite oxide contains W. Item 4: The lithium-ion secondary battery according to any one of Items 1 to 3, wherein in the lithium transition metal composite oxide, the content (mol) of Co element and the content (mol) of Mn element satisfy the following relationship: 0 ≤ Co / Mn ≤ 0.42. Item 5: The lithium-ion secondary battery according to any one of Items 1 to 4, wherein the boron compound includes a lithium borate salt. Item 6: The lithium-ion secondary battery according to any one of Items 1 to 5, wherein the coating portion further contains aluminum oxide. Item 7: The negative electrode further includes a negative electrode current collector, and the negative electrode active material layer includes, when viewed in the thickness direction, a negative electrode lower layer in contact with the negative electrode current collector and a negative electrode upper layer farther from the negative electrode current collector than the negative electrode lower layer. The lithium ion secondary battery according to any one of Items 1 to 6, wherein the content ratio of the Si element in the negative electrode upper layer is 90% by mass or more when the total Si element of the negative electrode active material is 100% by mass. Item 8: The lithium ion secondary battery according to Item 7, wherein the ratio of the thickness of the negative electrode upper layer to the total thickness of the negative electrode active material layer is 0.4 or more and 0.8 or less. Item 9: The lithium ion secondary battery according to any one of Items 1 to 8, wherein the Si-containing material includes one or more selected from Si, silicon oxide, and Si-C composites.

Explanation of Signs

[0120] 20 Electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Non-formation part of positive electrode active material layer 54 Positive electrode active material layer 56 Positive electrode active material 56a Lithium transition metal composite oxide 56c Coating part 56p Primary particle 60, 160 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Non-formation part of negative electrode active material layer 64, 164 Negative electrode active material layer 164u Negative electrode upper layer 164d Negative electrode lower layer 66 Negative electrode active material 66c Carbon material 66s Si-containing material 70 Separator 80 Non-aqueous electrolyte 100 Lithium-ion secondary battery

Claims

1. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode active material layer containing a positive electrode active material, the positive electrode active material includes a layered lithium transition metal composite oxide containing Li, Ni, and Mn, and a coating portion disposed on at least a part of the surface of the lithium transition metal composite oxide, the lithium transition metal composite oxide has a Ni content of 75 mol% or more with respect to the total of metal elements other than Li, in the lithium transition metal composite oxide, the content (mol) of Co element and the content (mol) of Mn element satisfy the following relationship: 0 ≤ Co / Mn ≤ 0.42 and is secondary particles formed by aggregation of primary particles, the average porosity of the secondary particles in cross-sectional observation of the secondary particles by a scanning electron microscope is 2% or more and 10% or less, the coating portion contains a boron compound, the negative electrode includes a negative electrode active material layer containing a negative electrode active material, the negative electrode active material includes a carbon material and a Si-containing material, when the total amount of the negative electrode active material is 100% by mass, the content ratio of Si element in the negative electrode active material is 5% by mass or more and 10% by mass or less, a lithium-ion secondary battery.

2. The lithium transition metal composite oxide has the following general formula: Li α Ni ｘ Mn ｙ M ｚ O ２ (where 0.8 ≤ α ≤ 1.2, 0.75 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.25, 0 ≤ z ≤ 0.2, x + y + z = 1, and M is one or more selected from Mg, Ca, Co, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W.) represented by The lithium-ion secondary battery according to Claim 1.

3. The lithium transition metal composite oxide contains W, The lithium-ion secondary battery according to Claim 1 or 2.

4. The boron compound includes a lithium borate salt, The lithium-ion secondary battery according to Claim 1 or 2.

5. The coating portion further includes aluminum oxide, The lithium-ion secondary battery according to Claim 1 or 2.

6. The negative electrode further includes a negative electrode current collector, the negative electrode active material layer includes a negative electrode lower layer in contact with the negative electrode current collector and a negative electrode upper layer farther from the negative electrode current collector than the negative electrode lower layer when viewed in the thickness direction, when the Si element of the entire negative electrode active material is 100% by mass, the content ratio of Si element in the negative electrode upper layer is 90% by mass or more, The lithium-ion secondary battery according to Claim 1 or 2.

7. The ratio of the thickness of the upper negative electrode layer to the total thickness of the negative electrode active material layer is 0.4 or more and 0.8 or less. The lithium ion secondary battery according to claim 6.

8. The Si-containing material contains one or more selected from Si, silicon oxide, and Si—C composites. The lithium ion secondary battery according to claim 1 or 2.

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