Lithium-ion secondary battery

Abstract

To provide a lithium ion secondary battery in which an increase in resistance during storage is suppressed while the cobalt content in a positive electrode active material is reduced.SOLUTION: A technique disclosed herein 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. The positive electrode active material is a layer-structured lithium transition metal composite oxide containing Li, Ni, and Mn. In the lithium transition metal composite oxide, the content of Ni to metal elements other than Li is 75 mol% 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. The Co element content (mol) and the Mn element content (mol) in the positive electrode active material satisfy the following relationship: 0≤Co / Mn≤0.43.SELECTED DRAWING: Figure 1

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 reducing the Co content in the positive electrode active material significantly increases the resistance of lithium-ion secondary batteries during storage.

[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 suppressing an increase in resistance during storage. [Means for solving the problem]

[0006] The technology disclosed herein relates to 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 being a lithium transition metal composite oxide having a layered structure containing Li, Ni, and Mn, and wherein in the lithium transition metal composite oxide, a content of Ni relative to a total of metal elements other than Li is 75 mol % or more, the negative electrode comprises a negative electrode active material layer containing a negative electrode active material, the negative electrode active material including a carbon material and a Si-containing material, and wherein a content (mol) of Co element and a content (mol) of Mn element in the positive electrode active material satisfy the following relationship: 0≦Co / Mn≦0.43.

[0007] According to this configuration, the composition of the positive electrode active material, specifically the Ni content and the ratio of the Co content to the Mn content, are each within a predetermined range. Furthermore, the negative electrode active material contains a Si-containing material in addition to a carbon material. This allows for a lithium-ion secondary battery that suppresses resistance increases 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 negative electrode sheet (negative electrode) according to one embodiment. [Figure 4] FIG. 4 is a view corresponding to FIG. 3, 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] The positive electrode active material layer 54 contains a positive electrode active material capable of reversibly absorbing and releasing charge carriers. The positive electrode active material according to this embodiment essentially contains a high-Ni lithium transition metal composite oxide having a layered structure and containing lithium (Li), nickel (Ni), and manganese (Mn) as essential elements. The high-Ni lithium transition metal composite oxide is an example of the "lithium transition metal composite oxide in which the content of Ni relative to the total of metal elements other than Li is 75 mol % or more" disclosed herein.

[0016] The positive electrode active material may contain a positive electrode active material other than the high Ni lithium transition metal composite oxide within a range that does not impair the effects of the present disclosure (for example, less than 10 mass % relative to the total mass of the positive electrode active material, preferably 5 mass % or less). Alternatively, the positive electrode active material may be composed solely of the high Ni lithium transition metal composite oxide.

[0017] In the high Ni-containing lithium transition metal composite oxide according to this embodiment, the Ni content relative to the total metal elements other than Li is 75 mol% or more. From the viewpoint of high volumetric energy density of lithium ion secondary batteries, the Ni content relative to the total 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 Ni content relative to the total metal elements other than Li is preferably 95 mol% or less, more preferably 93 mol% or less.

[0018] Examples of high Ni-containing lithium transition metal composite oxides having a layered structure include lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium iron nickel manganese composite oxides. These may be used alone or in combination of two or more. The fact that particles of high Ni-containing lithium transition metal composite oxides have a layered structure (i.e., a layered crystal structure) can be confirmed by known methods (e.g., X-ray diffraction).

[0019] 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.

[0020] In some preferred embodiments, the high-Ni lithium transition metal composite oxide preferably has a composition represented by the following general formula (i): Li α Ni x Mn 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.

[0021] In the above formula (i), α preferably satisfies 0.9 ≤ α ≤ 1.2, more preferably satisfies 1.0 ≤ α ≤ 1.1. From the viewpoint of battery characteristics (such as energy density, cycle characteristics, thermal stability), x preferably satisfies 0.8 ≤ x ≤ 0.95, more preferably satisfies 0.​​​​​​​​​​​​​​​In the positive electrode active materials used in conventional lithium ion batteries, the content ratio of Co element to metal elements other than Li is typically about 10 mol % to 40 mol %. However, because Co resources are limited, it is preferable to reduce the Co content in the positive electrode active material compared to conventional materials. 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).

[0025] The average particle diameter (D50) of the lithium transition metal composite oxide is not particularly limited. From the viewpoint of improving the energy density and output characteristics of the positive electrode active material layer, the average particle diameter (D50) of the lithium transition metal composite oxide is preferably 5 μm to 30 μm, and more preferably 10 μm to 20 μm. In this specification, the term "average particle diameter (D50)" refers to the median diameter (D50) and also refers to the particle diameter corresponding to a cumulative frequency of 50 volume % from the smaller particle diameter side (fine particle side) in a volume-based particle size distribution based on a laser diffraction / scattering method. Therefore, the average particle diameter (D50) can be determined using a laser diffraction / scattering particle size distribution measuring device or the like.

[0026] Although not particularly limited, the lithium transition metal composite oxide is preferably substantially spherical. However, it may also have an irregular shape. In this specification, the term "substantially spherical" refers to a shape that can be generally regarded as a sphere overall, and an average aspect ratio (ratio of major axis to minor axis) based on a cross-sectional image observed with an electron microscope, for example, of 1 to 1.5. The lithium transition metal composite oxide is typically in the form of secondary particles formed by the aggregation of multiple primary particles due to physical or chemical bonding forces. In other words, the lithium transition metal composite oxide (i.e., secondary particles) is an aggregate of primary particles, in which a single particle is formed by the aggregation of many primary particles.

[0027] The BET specific surface area of ​​the lithium transition metal composite oxide is not particularly limited. The BET specific surface area of ​​the lithium transition metal composite oxide is preferably 0.50 m because it can provide excellent output characteristics to the lithium ion secondary battery. 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 can be measured by a nitrogen adsorption method using a commercially available specific surface area measuring device.

[0028] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is, 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.

[0029] A known positive electrode current collector used in lithium ion secondary batteries may be used as the positive electrode current collector 52, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0030] 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.

[0031] <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.

[0032] The negative electrode active material layer 64 contains a negative electrode active material 66 capable of reversibly occluding and releasing charge carriers. FIG. 3 is a schematic diagram showing the configuration of a negative electrode sheet 60 (negative electrode) according to an embodiment. The negative electrode active material 66 according to this embodiment essentially includes a carbon material 66c and a Si-containing material 66s. However, the negative electrode active material 66 may further contain other negative electrode active materials in addition to the carbon material 66c and the Si-containing material 66s. Also, for convenience of explanation, although not shown here, the negative electrode active material layer 64 may contain a conductive material, a binder, a thickener, etc. in addition to the negative electrode active material 66.

[0033] As the carbon material 66c, for example, graphite, hard carbon, soft carbon, etc. are used, and among them, graphite is preferably used. The graphite may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in a form where the graphite is coated with an amorphous carbon material.

[0034] The average particle diameter of the carbon material 66c is not particularly limited, but for example, it is 0.1 μm or more and 50 μm or less, preferably 1 μm or more and 25 μm or less, and more preferably 5 μm or more and 20 μm or less. The average particle diameter (D50) of the carbon material can be obtained, for example, by the laser diffraction scattering method.

[0035] The Si-containing material 66s is, for example, silicon (Si), SiO x (0.05 < x < 1.95), silicon oxide represented by, a Si-C composite containing Si particles in carbon particles, lithium silicate (Li x Si y O z) and the like. Silicon, silicon oxide, and Si-C composites can be suitably used as the Si-containing material 66s. Alternatively, an alloy composed of Si and an element other than Si can be used as the Si-containing material 66s. Examples of elements other than Si include Fe, Co, Sb, Bi, Pb, Ni, Cu, Zn, Ge, In, Sn, and Ti. By including the Si-containing material 66s in the negative electrode active material 66, it is possible to suppress 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. Therefore, it is possible to suitably suppress an increase in resistance during storage.

[0036] The average particle diameter of the Si-containing material 66s is not particularly limited, but is, for example, 0.1 μm to 50 μm, preferably 1 μm to 25 μm, and more preferably 5 μm to 20 μm. The average particle diameter (D50) of the carbon material can be determined, for example, by a laser diffraction scattering method.

[0037] Although not particularly limited, when the total amount of the negative electrode active material 66 is taken as 100% by mass, the content of Si element in the negative electrode active material 66 is 2% by mass or more, which can suppress 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, thereby achieving the effect of suppressing storage resistance. The higher the content of Si element, the better the effect of suppressing resistance increase during storage, but the lower the content of Si element, the better the capacity retention rate during storage. Therefore, from the viewpoint of achieving both the effect of suppressing resistance increase during storage and the capacity retention rate, the content of Si element in the negative electrode active material 66 is preferably 2% by mass or more and 13% by mass or less, and more preferably 5% by mass or more and 10% by mass or less.

[0038] 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. 4) of the anode active material layer 64, as shown in a second embodiment (FIG. 4) described later. As shown in FIG. 3, 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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).

[0046] 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.

[0047] 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, FEC is preferred. When FEC is included as an additive in the nonaqueous electrolyte 80, the amount of FEC is preferably 20 vol% or less relative to the nonaqueous electrolyte 80.

[0048] The lithium-ion secondary battery 100 configured as described above uses a high-Ni lithium transition metal composite oxide as the positive electrode active material, and the content (molar) of Co and Mn in the positive electrode active material satisfies the following relationship: 0≦Co / Mn≦0.43. The negative electrode active material contains a carbon material and a Si-containing material. This allows the cobalt content in the positive electrode active material to be reduced while suppressing an increase in resistance during storage.

[0049] Specifically, when a layered lithium transition metal composite oxide containing Mn is used as a positive electrode active material, reducing the Co content in the positive electrode active material reduces the stability of the positive electrode active material, resulting in significant Mn elution from the positive electrode active material layer during storage. The eluted Mn precipitates on the surface of the negative electrode active material layer (typically the graphite surface), and a film grows starting from the precipitated Mn. This film causes deterioration of the durability of the lithium ion battery. Therefore, conventional lithium ion secondary batteries using a layered lithium transition metal composite oxide containing Mn with a reduced Co content as a positive electrode active material have had the problem of a significant increase in resistance during storage.

[0050] In contrast, the lithium-ion secondary battery 100 according to this embodiment contains a high-Ni lithium transition metal composite oxide as the positive electrode active material. In other words, since the Ni content of the lithium transition metal composite oxide relative to the total of metal elements other than Li is 75 mol % or more, the Mn ratio in the lithium transition metal composite oxide can be reduced. Furthermore, the Co and Mn content ratios in the positive electrode active material are controlled by adjusting the Co / Mn content (mol) in the positive electrode active material to 0≦Co / Mn≦0.43. Furthermore, the Si-containing material contained in the negative electrode active material suppresses Mn deposition on the negative electrode surface. This suppresses the formation of a Mn coating on the negative electrode surface. Therefore, a lithium-ion secondary battery 100 can be provided that suppresses an increase in resistance during storage.

[0051] 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.

[0052] 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.

[0053] Second Embodiment Fig. 4 is a view of a negative electrode sheet 160 (negative electrode) according to the second embodiment, corresponding to Fig. 3. In Fig. 4, the negative electrode sheet 160 may be the same as the above-described lithium ion secondary battery 100, except that a negative electrode active material layer 164 is provided instead of the negative electrode active material layer 64. As shown in Fig. 4, the negative electrode active material layer 164 has a so-called multi-layer structure including a negative electrode upper layer 164u and a negative electrode lower layer 164d.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] As described above, Mn eluted from the positive electrode active material layer 54 deposits 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] <Test Example 1: Examination of the presence or absence of Si in the negative electrode active material> (Example 1) [Preparation of positive electrode sheet] A layered lithium transition metal composite oxide with a composition (molar ratio) of Ni, Co, and Mn of Ni:Co:Mn = 83:0:17 (corresponding to 83 / 0 / 17 in the "Ni / Co / Mn" column in Table 1) was prepared and used as the positive electrode active material of Example 1. The molar ratio of Li to the total transition metal elements (Ni, Co, and Mn) in the lithium transition metal composite oxide (Li / (Ni, Co, and Mn)) was 1.0 mol%. 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 prepare a positive electrode sheet.

[0066] [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 Si content was 10% by mass and the C content was 90% by mass, where the total amount of the negative electrode active material was 100% by mass. A negative electrode active material was prepared by mixing styrene butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and single-walled carbon nanotubes (SWCNT) as a conductive material in ion-exchanged water at a solids mass ratio of negative electrode active material:SBR:CMC:SWCNT = 100:1:1:0.5. A negative electrode active material layer-forming slurry was prepared by coating the resulting negative electrode active material layer-forming slurry on a 10 μm-thick copper foil negative electrode current collector and drying 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 the specified dimensions. In this way, a negative electrode sheet according to Example 1 was produced in which the Si-containing material was dispersed almost uniformly throughout the negative electrode active material.

[0067] [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.

[0068] [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.

[0069] [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.

[0070] [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.

[0071] (Examples 2-4) Lithium ion secondary batteries for evaluation according to Examples 2 to 4 were fabricated in the same manner as in Example 1, except that the composition (molar ratio) of Ni, Co, and Mn in the positive electrode active material was changed as shown in the "Ni / Co / Mn" column in Table 1, and an initial charging process was performed.

[0072] (Examples 5-8) Lithium ion secondary batteries for evaluation according to Examples 5 to 8 were fabricated in the same manner as in Example 1, except that the composition (molar ratio) of Ni, Co, and Mn in the positive electrode active material was changed as shown in the "Ni / Co / Mn" column in Table 1, and the negative electrode active material was graphite alone (i.e., no Si-containing material was included), and an initial charging process was performed.

[0073] (Reference example) Furthermore, as a reference example, a lithium ion secondary battery for evaluation was fabricated in the same manner as in Example 1, except that the composition (molar ratio) of Ni, Co, and Mn in the positive electrode active material was set to 83:5:12, and the negative electrode active material was graphite alone (i.e., no Si-containing material was included), and an initial charging process was performed.

[0074] [Evaluation of storage resistance increase rate] For the lithium ion secondary batteries for evaluation according to Examples 1 to 8 and Reference Example, 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 above formula. Thereafter, the SOC was adjusted to 100% in a temperature environment of 25°C. Each of the lithium ion secondary batteries for evaluation was placed in a thermostatic chamber at 60°C and stored for 14 days. The resistance value after storage was calculated using the same method as for the initial resistance. Then, the resistance value after storage 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.

[0075] [Table 1]

[0076] As shown in the results in Table 1, a comparison of the Reference Example and Examples 5 to 8 reveals that reducing the Co content in the positive electrode active material increases the resistance increase rate during storage, thereby deteriorating the resistance characteristics. However, in Examples 1 to 4, by including an Si-C composite in addition to graphite as the negative electrode active material, the resistance increase during storage was suppressed compared to Examples 5 to 8, and the resistance increase rate was close to that of the Reference Example.

[0077] <Test Example 2: Examination of Ni compounding ratio in positive electrode active material> (Examples 9-16) Lithium ion secondary batteries for evaluation according to Examples 9 to 16 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 positive electrode active material was changed as shown in the "Ni / Co / Mn" column in Table 2. The resistance increase rate during storage of the lithium ion secondary batteries for evaluation according to Examples 9 to 16 was evaluated in the same manner as described above. The results are shown in Table 2.

[0078] [Table 2]

[0079] As shown in the results in Table 2, a resistance suppression effect during storage was observed in Examples 9 to 12, in which the Ni compounding ratio in the positive electrode active material was 90 mol %, and in Examples 13 to 16, in which the Ni compounding ratio in the positive electrode active material was 75 mol %.

[0080] <Test Example 3: Examination of Si ratio> (Examples 17-19) Lithium-ion secondary batteries for evaluation according to Examples 17 to 19 were fabricated and subjected to initial charging 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 3. The lithium-ion secondary batteries for evaluation according to Examples 17 to 19 were evaluated for their resistance increase rate during storage in the same manner as described above. The results are shown in Table 3.

[0081] [Capacity maintenance rate] Furthermore, in this test example, evaluation of the capacity retention rate was carried out in parallel with evaluation of the resistance increase rate during storage. First, for the evaluation lithium ion secondary batteries of Examples 1, 5, and 17 to 19, the discharge capacity measured during the evaluation of the resistance increase rate during storage was taken as the initial capacity. Thereafter, the discharge capacity of the evaluation lithium ion secondary batteries stored in the above-mentioned evaluation of the resistance increase rate during storage (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) was used to determine the capacity retention rates (%) for Examples 1, 5, and 17 to 19. The results are shown in Table 3.

[0082] [Table 3]

[0083] As shown in the results in Table 3, even in Example 18, in which the Si content was 2% by mass, a resistance increase suppression effect during storage was observed compared to Example 5, in which only graphite was used as the negative electrode active material. Furthermore, the higher the Si content, the better the battery resistance increase suppression effect, while the lower the Si content, the better the capacity retention rate. Therefore, from the perspective of achieving both the resistance increase suppression effect during storage and the capacity retention rate, a more preferable range for the Si content in the negative electrode active material is thought to be 5 to 10% by mass.

[0084] <Test Example 4: 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., the content ratio of Si element was 0 mass% and the content ratio of C element was 100 mass% when the total amount of the negative electrode active material was 100 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%.

[0085] 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%.

[0086] 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 performed. The evaluation rate of storage resistance increase was evaluated for the evaluation lithium-ion secondary batteries according to Examples 20 and 21 in the same manner as described above. The results are shown in Table 4.

[0087] [Table 4]

[0088] As shown in the results in Table 4, Examples 1, 20, and 21 all exhibited a resistance increase suppression effect during storage compared to Example 5, which used only graphite as the negative electrode active material. Looking at Examples 1, 20, and 21, all of which had a Si compounding ratio of 10 mass%, Example 1, in which the Si-containing material (Si-C composite) was uniformly dispersed throughout the negative electrode active material layer, exhibited a good effect of suppressing the resistance increase rate during storage, while Example 20, in which the Si-containing material (Si-C composite) was concentrated in the upper layer, exhibited even better results in suppressing the resistance increase rate 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 by the Si-containing material on the surface of the negative electrode active material.

[0089] 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.

[0090] 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 being a lithium transition metal composite oxide having a layered structure containing Li, Ni, and Mn, and wherein in the lithium transition metal composite oxide, a content of Ni relative to a total of metal elements other than Li is 75 mol % or more; the negative electrode comprises a negative electrode active material layer containing a negative electrode active material, the negative electrode active material including a carbon material and a Si-containing material; and in the positive electrode active material, a content (mol) of Co element and a content (mol) of Mn element satisfy the following relationship: 0≦Co / Mn≦0.43. Item 2: The lithium ion secondary battery according to Item 1, wherein the content (moles) of Co element and the content (moles) of Mn element in the positive electrode active material satisfy the following relationship: 0≦Co / Mn≦0.32. Item 3: The positive electrode active material has the following general formula: Li α Ni x Mn y M z Item 3. The lithium ion secondary battery according to item 1 or 2, 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 4: The lithium ion secondary battery according to any one of Items 1 to 3, wherein the content of Si element in the negative electrode active material is 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 5: The lithium ion secondary battery according to any one of Items 1 to 4, 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 current collector and a negative electrode upper layer that is farther 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 6: The lithium ion secondary battery according to Item 5, 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 7: The lithium ion secondary battery according to any one of Items 1 to 6, 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]

[0091] 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 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 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 is a lithium transition metal composite oxide having a layered structure containing Li, Ni, and Mn, In the lithium transition metal composite oxide, the content of Ni relative to the total of metal elements other than Li is 83 mol% or more, the negative electrode includes a negative electrode active material layer containing a negative electrode active material, the negative electrode active material includes a carbon material and a Si-containing material, In the positive electrode active material, the content (mol) of Co element and the content (mol) of Mn element have the following relationship: 0≦Co / Mn≦0.25 Fulfilling The content of Mn relative to the total of metal elements other than Li is more than 5 mol%, The content of Co relative to the total of metal elements other than Li is 3 mol% or less; Lithium-ion secondary battery.

2. In the lithium transition metal composite oxide, the content of Ni relative to the total of metal elements other than Li is 90 mol% or more. The lithium ion secondary battery according to claim 1 .

3. In the positive electrode active material, the content (mol) of Co element and the content (mol) of Mn element satisfy the following relationship: 0≦Co / Mn≦0.21 fulfill, The lithium ion secondary battery according to claim 1 or 2.

4. The content of Mn relative to the total of metal elements other than Li is 10 mol% or more. The lithium ion secondary battery according to claim 1 or 2.

5. The positive electrode active material 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 or 2.

6. When the total amount of the negative electrode active material is taken as 100 mass%, the content ratio of Si element in the negative electrode active material is 2 mass% or more and 13 mass% or less. The lithium ion secondary battery according to claim 1 or 2.

7. When the total amount of the negative electrode active material is taken as 100 mass%, the content ratio of Si element in the negative electrode active material is 5 mass% or more and 10 mass% or less. The lithium ion secondary battery according to claim 1 or 2.

8. The negative electrode further includes a negative electrode current collector, the negative electrode active material layer includes, 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 when the content of Si element in the entire negative electrode active material is 100% by mass; The lithium ion secondary battery according to claim 1 or 2.

9. 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 8.

10. 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.

Images