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.SOLUTION: The present disclosure relates to a lithium ion secondary battery 100 including a positive electrode 50, a negative electrode 60, a separator 70, and a nonaqueous electrolyte 80. The positive electrode 50 includes a positive electrode active material layer 54 containing a positive electrode active material 56. The positive electrode active material 56 includes a layer-structured lithium transition metal composite oxide 56a containing Li, Ni, and Mn, the content of Ni to metal elements other than Li being 75 mol% or more. The lithium transition metal composite oxide 56a is secondary particles which each are formed by aggregation of primary particles 56p with an aspect ratio of 1.9 or more. The negative electrode 60 includes a negative electrode active material layer 64 containing a negative electrode active material 66, and the negative electrode active material 66 contains a carbon material 66c and a Si-containing material 66s. When the total amount of the negative electrode active material 66 is 100 mass%, the content ratio of a Si element is 5 mass% or more and 10 mass% or less.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to lithium-ion secondary batteries. [Background technology]

[0002] Lithium-ion secondary batteries are suitable for use as driving power sources in vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (BEVs), and demand for them is rapidly expanding. Lithium-transition metal composite oxides are used as the positive electrode active material for lithium-ion secondary batteries, and among these, lithium-nickel-cobalt-manganese composite oxides containing Ni, Co, and Mn, as disclosed in Patent Document 1, are mentioned. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-191662 Summary of the Invention [Problem to be solved by the invention]

[0004] On the other hand, in recent years, with the expansion of demand for lithium-ion secondary batteries, there has been concern about the depletion of cobalt (Co) used in lithium-ion secondary batteries. One possible solution to this problem is to reduce the Co content in the positive electrode active material. However, as a result of extensive research, the present inventors have found a problem in that when the Co content in the positive electrode active material is reduced, the initial resistance and resistance during storage of the lithium-ion secondary battery increase significantly.

[0005] The technology disclosed herein has been made in consideration of the above circumstances, and aims to provide a lithium-ion secondary battery in which the cobalt content in the positive electrode active material is reduced while the initial resistance and the increase in resistance during storage are suppressed. [Means for solving the problem]

[0006] The technology disclosed herein relates to a lithium ion secondary battery, the battery comprising: a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte; the positive electrode comprises a positive electrode active material layer containing a positive electrode active material; the positive electrode active material comprises a lithium transition metal composite oxide having a layered structure containing Li, Ni, and Mn; the lithium transition metal composite oxide has a Ni content of 75 mol % or more relative to a total of metal elements other than Li, and is formed of secondary particles formed by aggregation of primary particles, the primary particles having an aspect ratio of 1.9 or more; and the negative electrode comprises a negative electrode active material layer containing a negative electrode active material; the negative electrode active material comprises a carbon material and a Si-containing material; and when the total amount of the negative electrode active material is taken as 100 mass %, a content ratio of Si element in the negative electrode active material is 5 mass % or more and 10 mass % or less.

[0007] According to this configuration, the composition of the positive electrode active material, specifically the Ni content in the lithium transition metal composite oxide, is set within a predetermined range. The lithium transition metal composite oxide is in the form of secondary particles, and the aspect ratio of the primary particles of the lithium transition metal composite oxide is 1.9 or greater. Furthermore, the negative electrode active material contains a predetermined amount of a Si-containing material in addition to a carbon material. This allows for a lithium-ion secondary battery to be provided that suppresses the initial resistance and the increase in resistance during storage while reducing the cobalt content in the positive electrode active material. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view that schematically shows the internal structure of a lithium-ion secondary battery according to one embodiment. [Figure 2] FIG. 2 is a schematic exploded view showing the configuration of an electrode body of a lithium ion secondary battery according to one embodiment. [Figure 3] FIG. 3 is a schematic diagram showing the configuration of a positive electrode active material according to one embodiment. [Figure 4] FIG. 4 is a schematic diagram showing the configuration of a negative electrode sheet (negative electrode) according to one embodiment. [Figure 5]FIG. 5 is a view corresponding to FIG. 4, showing the configuration of a negative electrode sheet (negative electrode) according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

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

[0010] In the following drawings, components and parts that perform the same function are denoted by the same reference numerals, and redundant explanations may be omitted or simplified. In this specification, the notation "A to B" indicating a range means not less than A and not more than B, and also encompasses the meanings of "preferably larger than A" and "preferably smaller than B." In this specification, a "lithium ion secondary battery" (hereinafter sometimes simply referred to as a "battery") refers to a general electricity storage device that uses lithium ions as a charge carrier and can be repeatedly charged and discharged by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

[0011] Hereinafter, the present disclosure will be described in detail using as an example a flat prismatic lithium ion secondary battery having a flat electrode body and a flat battery case, but it is not intended that the present disclosure be limited to the embodiments described therein.

[0012] FIG. 1 is a cross-sectional view schematically illustrating the internal structure of a lithium-ion secondary battery according to one embodiment. The lithium-ion secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat electrode assembly 20 and a nonaqueous electrolyte 80 in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, as well as a thin-walled safety valve 36 that is designed to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 also has an injection port (not shown) for injecting the nonaqueous electrolyte 80. The positive terminal 42 is electrically connected to a positive current collector plate 42a. The negative terminal 44 is electrically connected to a negative current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0013] Fig. 2 is a schematic exploded view showing the configuration of an electrode assembly of a lithium-ion secondary battery according to one embodiment. As shown in Figs. 1 and 2, the electrode assembly 20 has a configuration in which a strip-shaped positive electrode sheet 50 and a strip-shaped negative electrode sheet 60 are overlapped with two strip-shaped separators 70 interposed therebetween 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 or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62. The positive electrode active material layer-free portion 52a (i.e., a portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer-free portion 62a (i.e., a portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends of the electrode body 20 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction). A positive electrode current collector 42a and a negative electrode current collector 44a are joined to the positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a, respectively. The positive electrode sheet 50 is an example of a "positive electrode" disclosed herein, and the negative electrode sheet 60 is an example of a "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) here includes a positive electrode current collector 52 and a positive electrode active material layer 54 supported by the positive electrode current collector 52. Note that, although the positive electrode active material layer 54 is illustrated only on one surface of the positive electrode current collector 52 in this example, the positive electrode active material layer 54 may be provided on each of both surfaces of the positive electrode current collector 52.

[0015] 3 is a schematic diagram showing the configuration of a positive electrode active material 56 according to one embodiment. The positive electrode active material layer 54 includes a positive electrode active material 56 capable of reversibly absorbing 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 portion 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 lithium transition metal composite oxide, in which the Ni content relative to the total of metal elements other than Li is 75 mol % or more. The high-Ni lithium transition metal composite oxide is an example of the "lithium transition metal composite oxide in which the Ni content relative to the total of metal elements other than Li is 75 mol % or more" 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 impair the effects of the present disclosure (for example, less than 10 mass% of the total mass of the positive electrode active material, preferably 5 mass% or less). Alternatively, the positive electrode active material may be composed only of the lithium transition metal composite oxide 56a.

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

[0019] Examples of the lithium transition metal composite oxide 56a having a layered structure include lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These may be used alone or in combination of two or more. The fact that the particles of the high Ni content 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, etc.).

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

[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...Equation (i) In the above formula (i), x, y, z, and α respectively satisfy the following conditions: 0.8≦α≦1.2, 0.75≦x≦0.95, 0.05≦y≦0.25, 0≦z≦0.2, and x+y+z=1. 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] In the above formula (i), α preferably satisfies 0.9≦α≦1.2, and more preferably satisfies 1.0≦α≦1.1. From the viewpoint of battery characteristics (e.g., 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 viewpoint 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 even more preferably is 0. Note that 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 realized, and typically, x+y+z=0.95 to 1.1, and preferably x+y+z=0.99 to 1.05.

[0023] In some preferred embodiments, the lithium transition metal composite oxide 56a preferably contains W. In other words, the lithium transition metal composite oxide 56a is preferably doped with W as an additive element. This stabilizes the layered structure of the lithium transition metal composite oxide 56a, thereby more preferably improving the initial resistance and suppressing the elution of Mn into the non-aqueous electrolyte, thereby more preferably suppressing an increase in resistance during storage. When W is doped (solid-dissolved) as an additive element into the lithium transition metal composite oxide 56a, the doping amount is preferably 0.1 to 0.5 mol %, and more preferably 0.1 to 0.3 mol %, relative to the total transition metal elements of the lithium transition metal composite oxide 56a other than W.

[0024] The lithium transition metal composite oxide 56a has the following relationship between the content (mol) of Co element and the content (mol) of Mn element: 0≦Co / Mn≦0.42 It is preferable that the Co / Mn ratio satisfies the above-mentioned condition. This makes it possible to suitably control the content ratio of Co and Mn in the positive electrode active material. In the above-mentioned relationship, the smaller the Co / Mn value, the more preferably the cobalt content in the lithium transition metal composite oxide 56a can be reduced while suppressing an increase in resistance during storage. Therefore, it is more preferable that the Co / Mn ratio satisfies the condition 0≦Co / Mn≦0.25, and even more preferable that the Co / Mn ratio satisfies the condition 0≦Co / Mn≦0.21.

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

[0026] As shown in FIG. 3 , the lithium transition metal composite oxide 56a typically has a secondary particle form in which multiple primary particles 56p aggregate through physical or chemical bonding forces. The number of primary particles 56p of the lithium transition metal composite oxide 56a constituting the secondary particles of the lithium transition metal composite oxide 56a is not particularly limited, but may be, 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 may be, for example, approximately 120 or less. In this specification, the term "primary particle" refers to the smallest unit of particles constituting the positive electrode active material, specifically, the smallest unit determined from the geometric shape of the particle as observed under an electron microscope. In this specification, an aggregate formed by aggregation of 10 or more such primary particles 56p is referred to as a "secondary particle."

[0027] In this embodiment, the primary particles 56p have an aspect ratio (ratio of major axis to minor axis) of 1.9 or more, that is, a so-called elongated shape. This favorably improves the initial resistance. The primary particles 56p according to this embodiment typically have their major axes extending substantially parallel to the direction (a-axis) perpendicular to the stacking direction (c-axis) of the layered lithium transition metal composite oxide. The primary particles 56p according to this embodiment typically have their minor axes extending substantially parallel to the stacking direction (c-axis). There is no particular upper limit to the aspect ratio of the primary particles 56p, but for example, it is preferably 2.5 or less, more preferably 2.2 or less.

[0028] In this specification, the term "aspect ratio of primary particles" refers to the ratio of the major axis to the minor axis of primary particles 56p constituting the lithium transition metal composite oxide 56a. The aspect ratio of primary particles refers to the average value of the major axis to minor axis ratios of a plurality of arbitrarily selected primary particles 56p as determined from a cross-sectional electron microscope image of the lithium transition metal composite oxide 56a. The plurality of primary particles 56p may be, for example, 20 or more. Specifically, the average particle diameter of the primary particles 56p can be determined by, for example, preparing a cross-sectional observation sample of the lithium transition metal composite oxide 56a by cross-section polishing. Next, an SEM image of the cross-sectional observation sample of the lithium transition metal composite oxide 56a is obtained using a scanning electron microscope (SEM). Then, using image analysis particle size distribution measurement software (e.g., "Mac-View"), the aspect ratio (ratio of the major axis to the minor axis) is calculated based on the major axis and minor axis of a plurality of primary particles 56p arbitrarily selected from the SEM image, and the arithmetic average value of the calculated aspect ratios can be obtained.

[0029] The average particle size of the primary particles 56p of the lithium transition metal composite oxide 56a is not particularly limited. For example, from the viewpoint of high output characteristics and cycle characteristics of the positive electrode active material 56, the average particle size of the primary particles 56p of the lithium transition metal composite oxide 56a may be, for example, 0.1 μm to 2.5 μm, preferably 1.0 μm or more, more preferably 1.3 μm or more, and even more preferably 1.9 μm or more. On the other hand, from the viewpoint of high output characteristics and cycle characteristics of the positive electrode active material 56, the average particle size 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. Note that the "average particle size of the primary particles of the lithium transition metal composite oxide" refers to the average particle size of the major axes of the primary particles 56p of the lithium transition metal composite oxide 56a. The aspect ratio of the primary particles can be determined by determining the major axes of a plurality of primary particles 56p arbitrarily selected from an SEM image and calculating the average value using the same method as in the above-described method for determining the aspect ratio of the primary particles.

[0030] 3, the primary particles 56p according to this embodiment are typically arranged in the lithium transition metal composite oxide 56a (secondary particles) so as to extend from the center C toward the outer diameter OL (so as to be radially oriented). One end of the long side of the primary particle 56p is arranged on the outer diameter OL side, and the other end is arranged on the center C side. In other words, the primary particles 56p are arranged so that the a-axes extend from the center C toward the outer diameter OL.

[0031] The primary particles 56p and secondary particles according to this embodiment can be produced by a conventionally known method (e.g., a crystallization method, etc.). Although not particularly limited, the method for forming the primary particles 56p and secondary particles described above can be, for example, a conventionally known crystallization method. First, a hydroxide is prepared as a precursor to a lithium transition metal composite oxide in which primary particles are radially oriented. This hydroxide typically contains metal elements other than lithium among the metal elements contained in the lithium transition metal composite oxide. To produce this hydroxide, an alkaline compound containing ammonium ions (e.g., ammonia) is used. The aspect ratio (longer and shorter diameters of the primary particles 56p) and the arrangement of the primary particles 56p can be controlled by adjusting the pH, NH3 concentration, and reaction time. The NH3 concentration in the reaction solution is preferably 2 wt% or less, e.g., 1 to 2 wt%. The pH of the reaction solution is preferably approximately 10.5 to 12. The reaction time is preferably approximately 20 to 40 hours. The resulting hydroxide is then mixed with a lithium source compound (e.g., lithium hydroxide) and calcined. The calcination temperature and calcination time can be adjusted to suitably control the shape of the secondary particles of the lithium transition metal composite oxide 56a. The calcination temperature is not particularly limited, but may be, for example, 700°C to 800°C. The calcination time is also not particularly limited, but may be, for example, 10 hours to 20 hours. This allows for the production of secondary particles of the lithium transition metal composite oxide 56a according to this embodiment, in which elongated primary particles 56p are radially oriented, as shown in FIG. 3.

[0032] As shown in FIG. 3 , in some preferred embodiments, a coating portion 56c may be disposed on at least a portion of the surface of the lithium transition metal composite oxide 56a. However, the coating portion 56c is not essential and can be omitted in some embodiments. The coating portion 56c preferably contains a boron compound. By disposing the boron compound as the coating portion 56c on the surface of the lithium transition metal composite oxide 56a, Mn elution from the lithium transition metal composite oxide 56a into the nonaqueous electrolyte 80 can be suppressed, thereby suppressing an increase in resistance during storage. Furthermore, in the positive electrode active material 56, the coating portion 56c enhances the bonding strength between the primary particles 56p. This reduces stress during expansion and contraction and suppresses particle cracking of the lithium transition metal composite oxide 56a. From this perspective, an increase in resistance during storage can be suitably suppressed. Examples of the boron compound include boron-containing oxides and oxides containing boron and lithium, and lithium borate is particularly preferred. Specific examples of the boron compound include LiBO2, LiB(OH)4, Li3BO3, and B2O3, with LiBO2 being particularly preferred. The proportion of the boron compound in coating portion 56c, calculated as boron (B), may be, for example, 0.5 to 2.0 mol %, and preferably 0.7 to 1.5 mol %, when the total amount of metal elements other than Li in the positive electrode active material is taken as 100 mol %.

[0033] In some preferred embodiments, the coating portion 56c may further contain aluminum oxide (Al2O3) in addition to the boron compound. Specifically, aluminum oxide traps hydrogen fluoride gas (hydrofluoric acid) generated by decomposition of the non-aqueous electrolyte 80. This prevents Mn from leaching into the non-aqueous electrolyte 80 due to a reaction between the positive electrode active material 56 (lithium transition metal composite oxide 56a) and hydrogen fluoride gas. Therefore, by disposing the coating portion 56c on the surface, which further contains aluminum oxide in addition to the boron compound, an increase in resistance during storage can be more effectively prevented. The proportion of aluminum oxide in the coating portion 56c may be, for example, 0.1 to 0.5 mol %, preferably 0.3 to 0.5 mol %, calculated as aluminum (Al) when the total amount of metal elements other than Li in the lithium transition metal composite oxide 56a is taken as 100 mol %.

[0034] The coating portions 56c are preferably disposed at least on the surfaces of the secondary particles of the lithium transition metal composite oxide 56a. In this case, the coverage of the positive electrode active material 56 by the coating portions 56c may be 60% or more. The fact that the coating portions 56c are disposed on the surfaces of the secondary particles and the proportion of the portions covered by the coating portions 56c (coverage) can be confirmed by, for example, X-ray Photoelectron Spectroscopy (XPS) analysis of the positive electrode active material 56.

[0035] When the lithium transition metal composite oxide 56a has voids, the coating portions 56c are preferably present inside the secondary particles in addition to or instead of on the surfaces of the secondary particles. Specifically, the coating portions 56c are preferably present on the surfaces of the primary particles 56p inside the secondary particles. The amount of the coating portions 56c present inside the secondary particles may be greater or less than the amount of the coating portions 56c present on the surfaces of the secondary particles. This allows the effects of the technology disclosed herein to be exerted to a higher level. The presence of the coating portions 56c inside the secondary particles can be confirmed by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) analysis.

[0036] The average particle diameter (D50) of the lithium transition metal composite oxide 56a (secondary particles) is not particularly limited. From the viewpoint of particularly high packing efficiency 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, and more preferably 10 μm to 20 μm.

[0037] In this specification, the "average particle diameter (D50) of the lithium transition metal composite oxide (secondary particles)" refers to the median diameter (D50), and means the particle diameter corresponding to a cumulative frequency of 50 volume % from the smallest particle diameter side in a volume-based particle size distribution based on a laser diffraction / scattering method. Therefore, the average particle diameter (D50) of the lithium transition metal composite oxide (secondary particles) can be determined using a laser diffraction / scattering particle size distribution measuring device or the like.

[0038] In this embodiment, the lithium transition metal composite oxide 56a may have a so-called solid structure in which the primary particles 56p simply aggregate to form secondary particles. However, the present invention is not limited to this structure. The lithium transition metal composite oxide 56a may have voids resulting from gaps between the aggregated primary particles 56p inside the secondary particles. The voids may be open or closed. If the voids are open, each void may have two or more openings. In a cross-sectional view, the voids are located within a virtual outer diameter OL of the secondary particle and are typically spaces surrounded by multiple primary particles 56p.

[0039] When the lithium transition metal composite oxide 56a (secondary particles) has voids, the porosity is preferably 2% or more and 10% or less. The porosity of the secondary particles is, for example, 2% or more, and may be 3% or more, or 5% or more. By setting the porosity of the secondary particles at a predetermined level or more, the initial resistance of the lithium ion secondary battery 100 can be reduced. Furthermore, appropriate voids can be secured inside the secondary particles of the lithium transition metal composite oxide 56a. Furthermore, when the lithium transition metal composite oxide 56a has coating portions 56c, the coating portions 56c can be evenly disposed up to the center C of the secondary particles. On the other hand, if the porosity of the secondary particles is excessive (typically, if the porosity exceeds 10%), the increase in resistance during storage increases. Therefore, the porosity of the secondary particles is, for example, 10% or less, and may be 9% or less, or 8% or less. In this specification, the "average porosity of the lithium transition metal composite oxide" can be determined from a cross-sectional electron microscope image of the lithium transition metal composite oxide, and the average value can be calculated by measuring the porosity of multiple arbitrarily selected secondary particles. The multiple secondary particles can be, for example, 20 or more. First, a cross-sectional observation sample of the lithium transition metal composite oxide 56a is prepared by cross-section polishing or the like. Next, an SEM image of the cross-sectional observation sample is obtained using a scanning electron microscope (SEM). From the obtained SEM image, the area of ​​the entire secondary particles and the total area of ​​all voids inside the secondary particles are calculated using image analysis software (e.g., "ImageJ"). Then, using the following formula (ii): Porosity (%) = (total area of ​​all voids / total area of ​​secondary particles) × 100...(ii) The porosity is calculated using the formula and the average value is calculated.

[0040] The porosity of the lithium transition metal composite oxide 56a can be adjusted, for example, by changing the synthesis conditions when synthesizing a hydroxide, which is a precursor of the lithium transition metal composite oxide 56a, by a crystallization method. Specifically, in the crystallization method, a raw material aqueous solution containing metal elements other than lithium and a pH adjusting solution are added to a reaction solution to synthesize the hydroxide. The porosity of the hydroxide can be adjusted by changing the pH value of the reaction solution and the stirring speed. The hydroxide is mixed with a lithium source compound (e.g., lithium hydroxide) and calcined to obtain the lithium transition metal composite oxide 56a in the form of secondary particles with an adjusted porosity.

[0041] The BET specific surface area of ​​the lithium transition metal composite oxide 56a is not particularly limited. In order to provide the lithium ion secondary battery 100 with excellent output characteristics, the BET specific surface area of ​​the lithium transition metal composite oxide 56a is preferably 0.50 m 2 / g~0.85m 2 / g, more preferably 0.55m 2 / g~0.80m 2 The BET specific surface area of ​​the lithium transition metal composite oxide 56a can be measured by a nitrogen adsorption method using a commercially available specific surface area measuring device.

[0042] The content of the positive electrode active material 56 in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material 56 relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is, for example, 80 mass% or more, preferably 87 mass% or more, more preferably 90 mass% or more, even more preferably 95 mass% or more, and most preferably 97 mass% or more.

[0043] The positive electrode current collector 52 may be a known material used in the lithium ion secondary battery 100, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52 according to this embodiment.

[0044] The dimensions of the positive electrode current collector 52 are not particularly limited and can be determined appropriately depending on 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, and preferably 7 μm or more and 20 μm or less.

[0045] <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. Note that, in this example, the negative electrode active material layer 64 is illustrated only on one surface of the negative electrode current collector 62, but the negative electrode active material layer 64 may be provided on each of both surfaces of the negative electrode current collector 62.

[0046] The negative electrode active material layer 64 contains a negative electrode active material 66 that can reversibly absorb and release charge carriers. FIG. 4 is a schematic diagram showing the configuration of a negative electrode sheet 60 (negative electrode) according to one embodiment. The negative electrode active material 66 according to this embodiment essentially contains 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. Although not shown here for ease of explanation, the negative electrode active material layer 64 may also contain a conductive material, a binder, a thickener, and the like in addition to the negative electrode active material 66.

[0047] The carbon material 66c may be, for example, graphite, hard carbon, or soft carbon, with graphite being particularly preferred. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite, which is graphite coated with an amorphous carbon material.

[0048] 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. Note that the average particle diameter (D50) of the carbon material can be determined, for example, by the laser diffraction scattering method.

[0049] 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. The Si-containing material 66s can preferably use silicon, silicon oxide, and Si-C composite. Further, the Si-containing material 66s can use an alloy composed of Si and an element other than Si. 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.

[0050] 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. Note that the average particle diameter (D50) of the carbon material can be determined, for example, by the laser diffraction scattering method.

[0051] In this embodiment, the content of Si element in the negative electrode active material 66, when the total amount of the negative electrode active material 66 is taken as 100% by mass, is 5% by mass or more and 10% by mass or less, from the viewpoint of achieving both the effect of suppressing an increase in resistance during storage and the capacity retention rate. If the content of Si element is low (typically, less than 5% by mass), the effect of suppressing the formation of a coating layer originating from Mn eluted from the positive electrode active material on the surface of the negative electrode active material layer 64 cannot be suitably obtained, and the effect of suppressing storage resistance cannot be satisfactorily obtained. On the other hand, if the content of Si element is too high (typically, more than 10% by mass), the capacity retention rate during storage decreases, which is undesirable.

[0052] The distribution of the Si-containing material 66s in the anode active material layer 64 is not particularly limited. The Si-containing material may be dispersed throughout the entire anode active material layer 64, concentrated in an upper layer (the surface layer side of the anode active material layer 64) of the anode active material layer 64, or concentrated in a lower layer (the anode current collector 62 side) of the anode active material layer 64. In some preferred embodiments, the Si-containing material 66s is preferably concentrated in an upper layer (corresponding to the anode upper layer 164u in FIG. 5) of the anode active material layer 64, as shown in a second embodiment (FIG. 5) described later. As shown in FIG. 4, in this embodiment, the Si-containing material 66s is dispersed almost uniformly throughout the anode active material layer 64. Note that "concentrated and dispersed in the upper layer of the anode active material layer 64" is an example of "the content of the Si element in the anode upper layer is 90 mass% or more when the Si element content of the entire anode active material is 100 mass%" disclosed herein.

[0053] 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, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), or other materials conventionally used in lithium secondary batteries can be used without any particular limitation. As the thickener, for example, carboxymethyl cellulose (CMC) or the like can be used.

[0054] The negative electrode active material layer 64 preferably further contains a conductive material. Examples of suitable conductive materials include carbon black (CB) such as acetylene black (AB) or Ketjen black, carbon nanotubes (CNT), carbon fibers such as vapor-grown carbon fiber (VGCF), activated carbon, and carbon materials such as graphite. Among these, CNT is preferred. By including CNT as the conductive material, the capacity retention rate can be favorably maintained. The CNT may be any of single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT), and these may be used alone or in combination of two or more. When CNT is used as the conductive material in the negative electrode active material layer 64, SWCNT is preferred. When CNT is used as the conductive material in the negative electrode active material layer 64, the negative electrode active material layer preferably contains 0.5 mass% or less of CNT relative to the entire negative electrode active material layer.

[0055] The content of the negative electrode active material 66 in the negative electrode active material layer 64 is preferably 90% by mass or more, and more preferably 95% by mass or more and 99% by mass or less. The content of the binder in the negative electrode active material layer 64 is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer 64 is preferably 0.3% by mass or more and 3% by mass or less, and more preferably 0.5% by mass or more and 2% by mass or less.

[0056] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm to 200 μm, and preferably 20 μm to 100 μm, when the negative electrode active material layer 64 is provided on only one surface of the negative electrode current collector 62. When the negative electrode active material layer 64 is provided on both surfaces of the negative electrode current collector 62, the thickness per surface (each of the thicknesses of one surface and the other surface) is, for example, 10 μm to 200 μm, and preferably 20 μm to 100 μm.

[0057] A known negative electrode current collector used in lithium ion secondary batteries may be used as the negative electrode current collector 62, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.). Copper foil is preferred as the negative electrode current collector 62.

[0058] The dimensions of the negative electrode current collector 62 are not particularly limited and can be determined appropriately depending on 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, and preferably 7 μm or more and 20 μm or less.

[0059] The separator 70 can be made of various porous sheets similar to those conventionally used in lithium-ion secondary batteries, including porous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such porous resin sheets 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 also have a heat-resistant layer (HRL).

[0060] The nonaqueous electrolyte 80 can be the same as that used in conventional lithium-ion secondary batteries, and typically, a supporting salt can be used in an organic solvent (nonaqueous solvent). Examples of the nonaqueous solvent include aprotic solvents such as carbonates, esters, and ethers. Carbonates are particularly preferred because they are effective in reducing low-temperature resistance due to the positive electrode material. 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), and trifluorodimethyl carbonate (TFDMC). These nonaqueous solvents can be used alone or in appropriate combinations of two or more. Suitable supporting salts include lithium salts such as LiPF6, LiBF4, lithium bis(fluorosulfonyl)imide (LiFSI), etc. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0061] The nonaqueous electrolyte 80 may contain various additives other than the nonaqueous solvent and supporting salt, such as gas generating agents, film-forming agents, dispersants, and thickeners, as long as the additives do not significantly impair the effects of the present disclosure. Examples of additives used in the nonaqueous electrolyte 80 include positive and negative electrode film-forming agents such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), and 1,3-propane sultone (PS); and overcharge inhibitors such as biphenyl (BP), cyclohexylbenzene (CHB), t-butylbenzene, and t-amylbenzene. Among these, the nonaqueous electrolyte 80 preferably contains FEC. When the nonaqueous electrolyte 80 contains FEC as an additive, the FEC content is preferably 20 vol% or less relative to the nonaqueous electrolyte 80.

[0062] The positive electrode active material of 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 has a primary particle aspect ratio of 1.9 or more. The negative electrode active material of the lithium-ion secondary battery 100 includes a carbon material and a Si-containing material, and the negative electrode active material has a Si content of 5% to 10% by mass, where the total amount of the negative electrode active material is taken as 100% by mass. This reduces the cobalt content in the positive electrode active material while suppressing an increase in initial resistance and an increase in resistance during storage.

[0063] Specifically, lithium transition metal composite oxides with a layered structure typically have a layered structure in which lithium layers, oxygen layers, transition metal layers, and oxygen layers are repeatedly stacked. The lithium transition metal composite oxide absorbs and releases Li ions in the a-axis, which is perpendicular to the stacking direction (c-axis). When a lithium transition metal composite oxide with a layered structure containing Mn is used as a positive electrode active material, reducing the Co content in the lithium transition metal composite oxide reduces the stability of the lithium transition metal composite oxide. Typically, the interlayer spacing of the lithium transition metal composite oxide narrows, reducing Li ion conductivity and increasing initial resistance. Furthermore, the reduced stability of the lithium transition metal composite oxide leads to significant Mn elution from the positive electrode active material layer (specifically, the lithium transition metal composite oxide) during storage. For example, Mn eluted from the positive electrode active material layer precipitates on the surface of the negative electrode active material layer (typically the graphite surface), and a film grows from the precipitated Mn. This film reduces the durability of the lithium-ion secondary battery. Therefore, conventional lithium-ion secondary batteries that use a layered lithium transition metal composite oxide containing Mn with a reduced Co content as the positive electrode active material have had the problems of a deterioration in initial resistance and a large increase in resistance during storage.

[0064] In contrast, in the lithium-ion secondary battery 100 according to this embodiment, the Ni content relative to the total of the metal elements other than Li in the lithium transition metal composite oxide 56a is 75 mol % or more, thereby reducing the Mn ratio in the lithium transition metal composite oxide 56a. Furthermore, the primary particles 56p according to this embodiment have an elongated shape with an aspect ratio of 1.9 or more. With the primary particles 56p having such a configuration, the lithium transition metal composite oxide 56a includes multiple primary particles 56p arranged such that their major axes extend from the outer diameter OL toward the center C, thereby forming secondary particles. An efficient diffusion path for Li ions from the surface (the outer diameter OL side) of the lithium transition metal composite oxide 56a to the center C is formed, thereby reducing the length of the diffusion path. This improves the Li ion diffusibility of the lithium transition metal composite oxide 56a. Therefore, the initial resistance can be reduced. Furthermore, the Si-containing material 66s contained in the negative electrode active material 66 suppresses the deposition of Mn on the surface of the negative electrode active material layer 64. This suppresses the formation of a Mn coating on the surface of the negative electrode active material layer 64. Therefore, even when the Co content in the lithium transition metal composite oxide is reduced, it is possible to provide a lithium ion secondary battery 100 in which an increase in initial resistance is suppressed and an increase in resistance during storage is suppressed.

[0065] The lithium ion secondary battery 100 can be used for a variety of purposes. Suitable applications include a driving power source mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices and the like. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of a plurality of batteries connected in series and / or parallel.

[0066] As an example, a prismatic lithium ion secondary battery 100 including an electrode assembly 20 with 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 stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked). The stacked electrode assembly may include multiple separators, with one separator interposed between each of the positive and negative electrodes, or may be configured such that the positive and negative electrodes are alternately stacked with one separator folded back. The lithium ion secondary battery according to this embodiment can also be configured in the form of a cylindrical lithium ion secondary battery, a coin-type lithium ion secondary battery, a laminated lithium ion secondary battery, etc. Furthermore, an all-solid-state secondary battery using a solid electrolyte can also be constructed.

[0067] Second Embodiment Fig. 5 is a view of a negative electrode sheet 160 (negative electrode) according to the second embodiment, corresponding to Fig. 4. In Fig. 5, the negative electrode sheet 160 may be the same as the above-described lithium ion secondary battery 100, except that it has a negative electrode active material layer 164 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 that includes a negative electrode upper layer 164u and a negative electrode lower layer 164d.

[0068] In some preferred embodiments, the negative electrode active material layer 164 may include, in the thickness direction, a negative electrode lower layer 164d that is in contact with the negative electrode current collector 62, and a negative electrode upper layer 164u that is farther from the negative electrode current collector 62 than the negative electrode lower layer 164d. However, the negative electrode active material layer 164 may also have a multi-layer structure of three or more layers. For example, between the negative electrode upper layer 164u and the negative electrode lower layer 164d, an additional layer having a composition different from those of the negative electrode upper layer 164u and the negative electrode lower layer 164d may be formed.

[0069] The negative electrode upper layer 164u is a layer located closer to the surface than the negative electrode lower layer 164d when viewed in the thickness direction of the negative electrode active material layer 164. The negative electrode upper layer 164u is located farther from the negative electrode current collector 62 than the negative electrode lower layer 164d. Here, the negative electrode upper layer 164u constitutes the outermost layer of the negative electrode active material layer 164. In other words, the negative electrode upper layer 164u is located closer to the positive electrode (positive electrode sheet 50) than the negative electrode lower layer 164d.

[0070] The negative electrode active material 66 of the negative electrode upper layer 164u essentially contains a Si-containing material 66s. When the amount of Si element in the entire negative electrode active material layer 164 is taken as 100 mass%, the content ratio of Si element contained in the negative electrode upper layer 164u is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 98 mass% or more. Typically, the content of Si element contained in the negative electrode upper layer 164u is greater than the content of Si element contained in the negative electrode lower layer 164d. In other words, in the negative electrode active material layer 164, Si element is concentrated and dispersed on the positive electrode (positive electrode sheet 50) side.

[0071] As described above, Mn eluted from the positive electrode active material layer 54 (typically, the lithium transition metal composite oxide 56a) precipitates on the positive electrode (positive electrode sheet 50) side of the negative electrode active material layer 64, in other words, on the surface layer of the negative electrode active material layer 164, to form a coating. Here, by adopting a configuration in which the Si-containing material is concentrated and dispersed on the positive electrode (positive electrode sheet 50) side of the negative electrode active material layer 164, i.e., on the negative electrode upper layer 164u side, the effect of suppressing the formation of a coating by Mn on the surface of the negative electrode active material layer 164 is more efficiently exhibited. Therefore, an increase in resistance during storage can be suitably suppressed.

[0072] In this specification, the "content ratio (mass %) of the Si element contained in the negative electrode upper layer when the amount of Si element in the entire negative electrode active material layer is taken as 100%" can be calculated as follows. First, the thickness of the negative electrode upper layer is obtained by observing a cross-sectional SEM image of the negative electrode active material layer 164. Next, a sample is collected by scraping off a predetermined area of ​​the negative electrode active material layer by the obtained thickness of the negative electrode upper layer, and an ICP (Inductively Coupled Plasma) analysis is performed to calculate the amount of Si element in the negative electrode upper layer. Similarly, a sample is collected by scraping off a predetermined area of ​​the negative electrode lower layer by scraping off a predetermined area of ​​the negative electrode active material layer, and the amount of Si element in the negative electrode lower layer is calculated by ICP analysis. Then, the content ratio of the Si element in the negative electrode upper layer to the amount of Si element in the entire negative electrode active material layer can be calculated from the obtained values ​​of the Si element amounts in the negative electrode upper layer and negative electrode lower layer.

[0073] The content of Si element contained in the negative electrode upper layer 164u is preferably 2% by mass or more, and more preferably 5% by mass or more, when the total amount of the negative electrode active material 66 in the negative electrode upper layer 164u is taken as 100% by mass.

[0074] The negative electrode lower layer 164d is a layer located closer to the negative electrode current collector 62 than the negative electrode upper layer 164u when viewed in the thickness direction of the negative electrode active material layer 64. Here, the negative electrode lower layer 164d is in contact with the negative electrode current collector 62.

[0075] The negative electrode active material 66 of the negative electrode lower layer 164d essentially contains a carbon material 66c. The negative electrode lower layer 164d may or may not contain a Si-containing material 66s. The content of the Si element contained in the negative electrode lower layer 164d is not particularly limited, but may be, for example, 2% by mass or less, and preferably 1% by mass or less, when the negative electrode active material of the negative electrode lower layer 164d is taken as 100% by mass. The content of the C element contained in the negative electrode lower layer 164d is not particularly limited, but is preferably 98% by mass or more, and more preferably 99% by mass or more, when the negative electrode active material of the negative electrode lower layer 164d is taken as 100% by mass.

[0076] The ratio (Tu / Ta) of the thickness Tu of the negative electrode upper 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. This allows the Si-containing material 66s (in other words, Si element) in the negative electrode active material layer 164 to be dispersed toward the positive electrode (positive electrode sheet 50), thereby more efficiently suppressing the formation of a coating by Mn. This effectively suppresses an increase in resistance during storage. Furthermore, from the viewpoint of packing property, the ratio of the thickness Tu of the negative electrode upper 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. Note that the "ratio of the thickness of the negative electrode upper layer to the total thickness of the negative electrode active material layer" in the present specification is measured using a cross-sectional SEM image of the negative electrode active material layer 164.

[0077] The above-described negative electrode upper layer 164u and negative electrode lower layer 164d can be produced by a conventionally known method, but are not limited to this. For example, they can be formed by preparing a negative electrode active material layer-forming slurry using two or more types of negative electrode active materials 66 that differ in composition (typically, the content ratio of the Si-containing material), and applying the negative electrode active material layer-forming slurry.

[0078] Hereinafter, examples of the technology disclosed herein will be described, but it is not intended that the technology disclosed herein be limited to those shown in these examples.

[0079] <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) [Preparation of positive electrode sheet] First, a positive electrode active material according to Example 1 was prepared. Specifically, a hydroxide precursor was produced by a crystallization method. In this case, a compound containing ammonium ions was used as the alkaline compound, and the hydroxide was produced by adjusting the pH of the reaction solution to 11, the NH3 concentration to 1.0 to 1.5 wt%, and the reaction time to 40 hours. The obtained hydroxide was then mixed with a lithium source and fired at a firing temperature of 770°C for 15 hours. This produced a layered structure lithium transition metal composite oxide (LiNi) in the form of secondary particles, with the aspect ratio of the primary particles being 1.9. 0.83 Mn 0.17 O2) (corresponding to 83 / 0 / 17 in the "Ni / Co / Mn" column in Table 1). The aspect ratio of the primary particles was measured by the method described above.

[0080] The positive electrode active material prepared above, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a solid 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 resulting mixture, and the mixture was kneaded to prepare a slurry for forming a positive electrode active material layer. The resulting slurry for forming a positive electrode active material layer was applied to both sides of a 15 μm-thick aluminum foil positive electrode current collector and dried to form a positive electrode active material layer. The resulting positive electrode active material layer was roll-pressed using a rolling roller and then cut to a predetermined size to produce a positive electrode sheet according to Example 1.

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

[0082] The negative electrode active material prepared above, 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 ion-exchanged water at a solids mass ratio of negative electrode active material:SBR:CMC:SWCNT = 100:1:1:0.5 to prepare a negative electrode active material layer-forming slurry. The resulting negative electrode active material layer-forming slurry was applied to a 10 μm-thick copper foil negative electrode current collector and dried to form a negative electrode active material layer. The resulting negative electrode active material layer was roll-pressed using a rolling roller and then cut to a predetermined size. This produced a negative electrode sheet according to Example 1 in which the Si-containing material was dispersed almost uniformly throughout the negative electrode active material.

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

[0084] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by dissolving LiPF6 as a supporting electrolyte 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) in a volume ratio of EC:EMC:DMC:FEC = 25:40:30:5.

[0085] [Preparation of Lithium-ion Secondary Batteries for Evaluation] The positive electrode sheet and the negative electrode sheet were stacked with a separator interposed therebetween to obtain a laminate. The laminate was then wound to obtain a wound body, which was then pressed to obtain a flattened electrode body. A current collector terminal was attached to the electrode body, which was then inserted into a battery case and welded, after which a nonaqueous electrolyte was poured into the electrode body. The battery case was then sealed to produce a lithium-ion secondary battery for evaluation.

[0086] [Initial charging process] The obtained lithium ion secondary battery for evaluation according to Example 1 was subjected to a constant current charge at a current value of 0.1 C up to a voltage of 4.2 V as an initial charge treatment in a temperature environment of 25° C., and then to a constant voltage charge until the current value became 1 / 20 C. Thereafter, the battery was subjected to a constant current discharge at a current value of 0.1 C down to 3.0 V.

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

[0088] (Example 3) A lithium-ion secondary battery for evaluation according to Example 3 was produced and subjected to an initial charging process in the same manner as in Example 1, except that the aspect ratio of the primary particles of the lithium transition metal composite oxide was 1.5. The aspect ratio of the primary particles was adjusted by controlling the pH of the reaction solution, the NH3 concentration, and the reaction time during precursor production.

[0089] (Example 4) A lithium ion secondary battery for evaluation according to Example 4 was fabricated in the same manner as in Example 1, except that the negative electrode active material was graphite only (i.e., no Si-containing material was included), and an initial charging process was carried out.

[0090] (Example 5) A lithium-ion secondary battery for evaluation according to Example 5 was fabricated in the same manner as in Example 1, except that the aspect ratio of the primary particles of the lithium transition metal composite oxide was 1.5 and the negative electrode active material was graphite alone (i.e., no Si-containing material was included), and an initial charging process was performed.

[0091] [Evaluation of initial resistance] For the lithium ion secondary batteries for evaluation according to Examples 1 to 5, first, each lithium ion secondary battery for evaluation was charged at a constant current constant voltage (CCCV) at a current value of 0.3 C in a thermostatic chamber at 25°C, and adjusted to a state of 50% SOC (State of Charge). Then, pulse discharge was carried out at a current value of 5 C for 10 seconds. The voltage change (ΔV) during this pulse discharge was obtained and calculated using the following formula (I): Resistance value = voltage change (ΔV) / current value (5C)...(I) The resistance value before storage (initial resistance value) was calculated using the formula: The results are shown in Table 1.

[0092] [Evaluation of storage resistance increase rate] After measuring the initial resistance, the evaluation lithium ion secondary batteries according to Examples 1 to 5 were adjusted to SOC 100% in a temperature environment of 25°C. Each evaluation lithium ion secondary battery was placed in a thermostatic chamber at 60°C and stored for 14 days. Then, the resistance after storage was calculated using the same method as for the initial resistance. Then, the resistance was calculated using the following formula (II): Resistance increase rate (%) = (resistance value after storage / resistance value before storage) x 100...(II) The resistance increase rate (%) was calculated from the above data. The results are shown in Table 1.

[0093] [Capacity maintenance 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 5, the discharge capacity measured during the evaluation of the resistance increase rate was taken as the initial capacity. Thereafter, the discharge capacity of the evaluation lithium ion secondary batteries that had been stored in the above-described storage resistance increase rate evaluation (i.e., after storage at 60°C for 14 days) was determined in the same manner as the initial capacity. Then, the discharge capacity of the evaluation lithium ion secondary batteries was calculated using the following formula (III): Capacity retention rate (%) = discharge capacity after 14 days storage / initial capacity × 100 (III) The capacity retention rate (%) for each example was calculated using the following formula: The results are shown in Table 1.

[0094] [Table 1]

[0095] The results of Examples 1 and 5 show that Example 1, which used a negative electrode active material containing a lithium transition metal composite oxide with an aspect ratio of 1.9 for primary particles and a carbon material and a Si-containing material, exhibited both a favorable reduction in initial resistance and an increase in resistance during storage compared to Example 5. On the other hand, Example 3, which used a positive electrode active material containing a lithium transition metal composite oxide with an aspect ratio of 1.5 for primary particles, exhibited no reduction in initial resistance compared to Example 1. This suggests that the use of a lithium transition metal composite oxide with an aspect ratio of 1.9 or greater for primary particles is effective in reducing initial resistance. Furthermore, Example 4, which used only graphite (i.e., no Si-containing material) as the negative electrode active material, exhibited no reduction in the rate of resistance increase during storage compared to Example 1. It was found that the effect of suppressing the increase in resistance during storage can be favorably achieved by satisfying both the aspect ratio of the lithium transition metal composite oxide with a primary particle of 1.9 or greater and the use of a negative electrode active material containing a carbon material and a Si-containing material.

[0096] Furthermore, in Example 2, in which W was doped as an additive element into the lithium transition metal composite oxide in addition to the configuration of Example 1, even better results were obtained in terms of initial resistance and suppression of resistance increase during storage. This is thought to be because the layered structure of the lithium transition metal composite oxide was stabilized by doping with W, which further improved the conductivity of Li ions and also suppressed the elution of Mn into the non-aqueous electrolyte.

[0097] <Test Example 2: Examination of Si ratio> (Examples 6-8) Lithium ion secondary batteries for evaluation according to Examples 6 to 8 were fabricated and subjected to an initial charge treatment in the same manner as in Example 1, except that the content ratio of Si element when the total amount of graphite as the carbon material and the Si-C composite as the Si-containing material was taken as 100 mass% of the negative electrode active material was changed as shown in the "Si content ratio" column in Table 2. The lithium ion secondary batteries for evaluation according to Examples 6 to 8 were evaluated in the same manner as in Test Example 1 using the methods described above. The results are shown in Table 2.

[0098] [Table 2]

[0099] As shown in the results in Table 2, in Examples 7 and 1, in which the Si element content in the negative electrode active material was 5% by mass or 10% by mass, the resistance increase during storage was favorably suppressed and no decrease in capacity retention was observed. In Example 6, in which the Si element content was 2% by mass, the effect of suppressing the resistance increase during storage was not very strong. This is thought to be because in Example 6, the negative electrode did not contain enough Si-containing material to sufficiently suppress Mn precipitation on the surface of the negative electrode active material. Furthermore, while the effect of suppressing the increase in battery resistance was more favorable as the Si element content increased, Example 8, in which the Si element content was 13% by mass, exhibited a lower capacity retention rate than the other examples. Therefore, from the perspective of achieving both the effect of suppressing the resistance increase during storage and the capacity retention rate, a more favorable range of Si element content in the negative electrode active material is thought to be 5 to 10% by mass.

[0100] <Test Example 3: Study on the aspect ratio of primary particles> (Examples 9-10) Lithium ion secondary batteries for evaluation according to Examples 9 and 10 were fabricated in the same manner as in Example 1, except that the aspect ratio of the primary particles of the lithium transition metal composite oxide was set as shown in the "Primary particle aspect ratio" column in Table 3, and an initial charge treatment was performed. The aspect ratio of the primary particles was adjusted by controlling the pH of the reaction solution, the NH3 concentration, and the reaction time during precursor production. The aspect ratio of the primary particles was measured by the method described above. The lithium ion secondary batteries for evaluation according to Examples 9 and 10 were evaluated in the same manner as in Test Example 1 by the method described above. The results are shown in Table 3.

[0101] [Table 3]

[0102] The results in Table 3 show that compared to Example 3, in which the aspect ratio of the primary particles of the lithium transition metal composite oxide was 1.5, Examples 1, 9, and 10, in which the aspect ratio of the primary particles was 1.9 or greater, better results were obtained with respect to initial resistance. This is thought to be because, by setting the aspect ratio to 1.9 or greater, efficient diffusion paths for Li ions are formed in the secondary particle form, reducing the length of the diffusion path, thereby improving the Li ion diffusibility of lithium transition metal composite oxide 56a. Furthermore, the results of Examples 1, 9, and 10 show a tendency for better initial resistance to be obtained as the aspect ratio of the primary particles increases.

[0103] <Test Example 4: Examination of the Covered Part> (Example 11) The lithium-ion secondary battery for evaluation according to Example 11 was fabricated in the same manner as in Example 1, except that the lithium-transition metal composite oxide had the same configuration as in Example 1, and a positive electrode active material was used in which 1 mol % of LiBO2, calculated as boron, was disposed on the surface of the lithium-transition metal composite oxide, relative to 100 mol % of the total amount of metal elements other than Li in the lithium-transition metal composite oxide, and the initial charging treatment was carried out.

[0104] (Example 12) A lithium-ion secondary battery for evaluation according to Example 14 was fabricated 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 on the surface of a lithium transition metal composite oxide in addition to LiBO2 as a coating portion was used. The proportions of LiBO2 and Al2O3 were calculated as Li and Al, respectively, and the molar ratio of LiBO2:Al2O3 was 1:0.5 relative to 100 mol% of the total amount of metal elements other than Li in the lithium transition metal composite oxide.

[0105] The evaluation lithium ion secondary batteries according to Examples 11 and 12 were evaluated in the same manner as in Test Example 1 by the above-described method. The results are shown in Table 4.

[0106] [Table 4]

[0107] As shown in the results in Table 4, Example 11, in which LiBO2 was disposed on the surface of the lithium transition metal composite oxide as a coating, achieved better results in suppressing resistance increase during storage. Furthermore, Example 12, in which Al2O3 was contained in addition to LiBO2 as a coating, achieved even better results in suppressing resistance increase during storage. The reason for this is that the boron compound, when disposed on the surface of the lithium transition metal composite oxide, reduces the reaction area between the non-aqueous electrolyte and the lithium transition metal composite oxide, thereby suppressing Mn elution. Furthermore, aluminum oxide traps hydrogen fluoride gas generated by decomposition of the non-aqueous electrolyte. Therefore, it is believed that Examples 11 and 12 achieved better results in suppressing resistance increase during storage.

[0108] <Test Example 5: Investigation of Ni compounding ratio in positive electrode active material> (Examples 13-19) Lithium ion secondary batteries for evaluation according to Examples 13 to 19 were fabricated and subjected to an initial charge treatment 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 in Table 5. The molar ratio of Li to the total of the transition metal elements (Ni+Co+Mn) in the lithium transition metal composite oxides according to Examples 13 to 19 (Li / (Ni+Co+Mn)) was 1.0 mol % in all cases. The lithium ion secondary batteries for evaluation according to Examples 13 to 19 were evaluated in the same manner as in Test Example 1 using the method described above. The results are shown in Table 5.

[0109] [Table 5]

[0110] As shown in the results in Table 5, in Examples 1 and 13 to 15, in which the Ni content of the lithium transition metal composite oxide was 83 mol%, both the resistance suppression effect during storage and the capacity retention rate were stably achieved, even when the Co content of the lithium transition metal composite oxide was reduced. Similarly, in Examples 16 to 17, in which the Ni content of the lithium transition metal composite oxide was 95 mol%, and in Examples 18 to 19, in which the Ni content of the lithium transition metal composite oxide was 75 mol%, both the resistance suppression effect during storage and the capacity retention rate were also achieved. Furthermore, in terms of the Co / Mn ratio of the lithium transition metal composite oxide, Example 15, in which the Co / Mn ratio was 0.42, also achieved a favorable balance between the resistance suppression effect during storage and the capacity retention rate.

[0111] <Test Example 6: Examination of Si-containing material distribution in negative electrode active material layer> (Examples 20-21) A first negative electrode active material layer-forming slurry was prepared in the same manner as in Example 1, except that the negative electrode active material was graphite alone (i.e., 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 negative electrode active material layer-forming slurry was prepared in the same manner as in Example 1, except that the graphite and the Si-C composite were mixed so that the content ratio of Si element was 20 mass% and the content ratio of C element was 80 mass% when the total amount of the negative electrode active material was 100 mass%.

[0112] The first negative electrode active material layer-forming slurry was applied to the same copper foil negative electrode current collector as used in Example 1, dried, and roll-pressed using a rolling roller to achieve a predetermined density. Next, the second negative electrode active material layer-forming slurry was applied to the dried coating of the first negative electrode active material layer-forming slurry, dried, and roll-pressed using a rolling roller to achieve a predetermined density. The thickness of the second negative electrode active material layer-forming slurry was adjusted to 0.5 times the thickness of the entire negative electrode active material layer. This resulted in a negative electrode sheet of Example 20, in which a negative electrode lower layer formed from the first negative electrode active material layer-forming slurry and a negative electrode upper layer formed from the second negative electrode active material layer-forming slurry were supported on the negative electrode current collector. In the negative electrode sheet of Example 20, the Si element content of the negative electrode upper layer was 99% by mass, where the Si element content of the entire negative electrode active material was 100% by mass. Similarly, the second negative electrode active material layer-forming slurry was applied to the same copper foil negative electrode current collector as used in Example 1, dried, and roll-pressed using a rolling roller to achieve a predetermined density. Next, the first negative electrode active material layer-forming slurry was applied to the dried coating of the second negative electrode active material layer-forming slurry, dried, and roll-pressed using a rolling roller to achieve a predetermined density. The thickness of the first negative electrode active material layer-forming slurry was adjusted to 0.5 times the thickness of the entire negative electrode active material layer. This resulted in a negative electrode sheet of Example 21, in which a negative electrode lower layer formed from the second negative electrode active material layer-forming slurry and a negative electrode upper layer formed from the first negative electrode active material layer-forming slurry were supported on the negative electrode current collector. In the negative electrode sheet of Example 21, the Si element content of the negative electrode upper layer was 1 mass% when the Si element content of the entire negative electrode active material was taken as 100 mass%.

[0113] Except for using the above-mentioned negative electrode sheet, evaluation batteries according to Examples 20 and 21 were fabricated in the same manner as in Example 1, and an initial charging process was carried out. The evaluation lithium-ion secondary batteries according to Examples 20 and 21 were evaluated in the same manner as in Test Example 1 using the above-mentioned method. The results are shown in Table 6.

[0114] [Table 6]

[0115] As shown in Table 6, Examples 1, 20, and 21 all exhibited a greater effect in suppressing resistance increase during storage than Example 4, which used only graphite as the negative electrode active material. In Examples 1, 20, and 21, all of which had a Si content of 10% by mass, there was almost no difference in the capacity retention rate during storage due to the distribution of the Si-containing material. Furthermore, comparing Examples 1, 20, and 21, Example 1, in which the Si-C composite was uniformly dispersed throughout the negative electrode active material layer, exhibited a better effect in suppressing the rate of resistance increase during storage. Example 20, in which the Si-containing material was concentrated in the upper layer, exhibited even better results in suppressing the rate of resistance increase during storage. This is thought to be because the Si-containing material was concentrated in the upper layer of the negative electrode active material, which more efficiently suppressed the Mn precipitation on the surface of the negative electrode active material.

[0116] Although preferred embodiments of the present disclosure have been described above, 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 contents disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. For example, it is possible to replace part of the above-described embodiments with other modifications, and it is also possible to add other modifications to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it can be deleted as appropriate.

[0117] As described above, specific aspects of the technology disclosed herein include those described in the following sections. 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 comprises a positive electrode active material layer containing a positive electrode active material, the positive electrode active material comprising a lithium transition metal composite oxide having a layered structure containing Li, Ni, and Mn, the lithium transition metal composite oxide having a Ni content of 75 mol % or more relative to a total of metal elements other than Li, the lithium transition metal composite oxide being in the form of secondary particles formed by aggregation of primary particles, the primary particles having an aspect ratio of 1.9 or more, and the negative electrode comprises a negative electrode active material layer containing a negative electrode active material, the negative electrode active material comprising a carbon material and a Si-containing material, and a content ratio of Si element in the negative electrode active material being 5% by mass or more and 10% by mass or less when the total amount of the negative electrode active material is taken as 100% by mass. Item 2: The lithium transition metal composite oxide has the following general formula: Li α Ni x Mn y M z Item 1. The lithium ion secondary battery according to item 1, wherein α is 0.8≦α≦1.2, 0.75≦x≦0.95, 0.05≦y≦0.25, 0≦z≦0.2, and x+y+z=1, and M is one or more selected from Mg, Ca, Co, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W. Item 3: The lithium ion secondary battery according to Item 1 or 2, wherein the content (moles) of Co element and the content (moles) of Mn element in the lithium transition metal composite oxide satisfy the following relationship: 0≦Co / Mn≦0.42. Item 4: The lithium ion secondary battery according to any one of Items 1 to 3, wherein the lithium transition metal composite oxide contains W. Item 5: The lithium ion secondary battery according to any one of Items 1 to 4, wherein the primary particles have an aspect ratio of 2.5 or less. Item 6: The lithium ion secondary battery according to any one of Items 1 to 5, further comprising a coating portion disposed on at least a portion of the surface of the lithium transition metal composite oxide, the coating portion containing a boron compound. Item 7: The lithium ion secondary battery according to Item 6, wherein the coating further contains aluminum oxide. Item 8: The lithium ion secondary battery according to any one of Items 1 to 7, wherein the negative electrode further includes a negative electrode current collector, and 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 further from the negative electrode current collector than the negative electrode lower layer, as viewed in the thickness direction, and a content ratio of Si element in the negative electrode upper layer is 90 mass % or more when Si element in the entire negative electrode active material is 100 mass %. Item 9: The lithium ion secondary battery according to Item 8, 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 10: The lithium ion secondary battery according to any one of Items 1 to 9, wherein the Si-containing material includes at least one selected from the group consisting of Si, silicon oxide, and an Si—C composite. [Explanation of symbols]

[0118] 20 Electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 56 Cathode active material 56a Lithium transition metal composite oxide 56c Covering part 56p primary particle 60, 160 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64, 164 Negative electrode active material layer 164u negative electrode upper layer 164d Negative electrode lower layer 66 Negative electrode active material 66c Carbon Materials 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, the positive electrode includes a positive electrode active material layer containing a positive electrode active material, the positive electrode active material includes a lithium transition metal composite oxide having a layered structure containing Li, Ni, and Mn, The lithium transition metal composite oxide is The content of Ni relative to the total of metal elements other than Li is 75 mol% or more, The content of Co relative to the total of metal elements other than Li is 20 mol% or less, In the lithium transition metal composite oxide, the content (mol) of Co element and the content (mol) of Mn element have the following relationship: 0≦Co / Mn≦0.42 Fulfilling secondary particles formed by agglomeration of primary particles, The aspect ratio of the primary particles is 1.9 or more, the negative electrode comprises a negative electrode active material layer containing a negative electrode active material and a negative electrode current collector, the negative electrode active material includes a carbon material and a Si-containing material, the content of Si element in the negative electrode active material is 5% by mass or more and 10% by mass or less, with the total amount of the negative electrode active material being 100% by mass; the negative electrode active material layer includes, when viewed in a thickness direction, a negative electrode lower layer in contact with the negative electrode current collector and a negative electrode upper layer that is farther from the negative electrode current collector than the negative electrode lower layer, the content of Si element in the negative electrode upper layer is 90% by mass or more, based on 100% by mass of Si element in the entire negative electrode active material; 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 (wherein 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, and W.) 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 aspect ratio of the primary particles is 2.5 or less. The lithium ion secondary battery according to claim 1 or 2.

5. further comprising a coating portion disposed on at least a portion of the surface of the lithium transition metal composite oxide; The coating portion contains a boron compound. The lithium ion secondary battery according to claim 1 or 2.

6. The coating portion further contains aluminum oxide. The lithium ion secondary battery according to claim 5 .

7. a 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; The lithium ion secondary battery according to claim 1 or 2.

8. The Si-containing material includes one or more selected from the group consisting of Si, silicon oxide, and a Si-C composite. The lithium ion secondary battery according to claim 1 or 2.

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