Negative electrode for secondary battery and secondary battery using said negative electrode

Abstract

To provide a negative electrode using a doped Si-based negative electrode active material, the negative electrode being capable of suppressing self discharge of a secondary battery while reducing resistance of the secondary battery.SOLUTION: A negative electrode disclosed herein includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer includes a first layer located on a side of the negative electrode current collector, and a second layer located on a surface layer side. The first layer contains a first graphite particle and a first Si-C composite particle. The second layer contains a second graphite particle and a second Si-C composite particle. The total content of a group 15 element and a group 16 element in Si of the first Si-C composite particle is less than 0.1 atom%. The total content of the group 15 element and the group 16 element in Si of the second Si-C composite particle is 0.1-5 atom%. The thickness of the first layer is 60-90% of the thickness of the negative electrode active material layer, ant the thickness of the second layer is 10-40% of the thickness of the negative electrode active material layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

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

[0003] For applications as a power source for driving vehicles, particularly BEVs, secondary batteries are desired to have a higher capacity in order to extend the vehicle's driving range. One method for increasing the capacity of secondary batteries is to use a high-capacity active material in the electrodes, and a negative electrode active material containing Si (hereinafter also referred to as "Si-based negative electrode active material") is known as a high-capacity negative electrode active material (see, for example, Patent Documents 1 to 3). Patent Document 1 discloses a technique for using graphite particles in combination with a Si-based negative electrode active material. Patent Document 2 discloses that doping Si with 0.1 atomic % to 5 atomic % of sulfur can improve electronic conductivity and ionic conductivity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-38862 [Patent Document 2] Japanese Patent Application Publication No. 2022-507948 [Patent Document 3] Special Publication No. 2015-537347 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as a result of intensive research, the present inventors have newly discovered that when a doped Si-based negative electrode active material is used in a secondary battery, although the resistance of the secondary battery can be reduced, there is a problem in that the capacity decreases due to self-discharge.

[0006] In view of the above circumstances, an object of the present invention is to provide a negative electrode using a doped Si-based negative electrode active material, which can reduce the resistance of a secondary battery while suppressing self-discharge of the secondary battery. [Means for solving the problem]

[0007] The negative electrode disclosed herein comprises a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer includes a first layer located on the negative electrode current collector side and a second layer located on the surface layer side. The first layer contains first graphite particles and first Si-C composite particles. The second layer contains second graphite particles and second Si-C composite particles. The Si of the first Si-C composite particles may be doped with at least one element selected from the group consisting of Group 15 elements and Group 16 elements, and the Si of the second Si-C composite particles is doped with at least one element selected from the group consisting of Group 15 elements and Group 16 elements. The total content of the Group 15 element and the Group 16 element in the Si of the first Si-C composite particles is less than 0.1 atomic %. The total content of the Group 15 element and the Group 16 element in Si of the second Si-C composite particles is 0.1 atomic % to 5 atomic %. The thickness of the first layer is 60% to 90% of the thickness of the negative electrode active material layer, and the thickness of the second layer is 10% to 40% of the thickness of the negative electrode active material layer.

[0008] According to this configuration, it is possible to provide a negative electrode that uses a doped Si-based negative electrode active material and that can reduce the resistance of the secondary battery while suppressing self-discharge of the secondary battery.

[0009] From another aspect, the present disclosure provides a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode described above.

[0010] With this configuration, it is possible to provide a secondary battery in which the resistance is reduced and self-discharge is suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view schematically illustrating the configuration of a negative electrode of a secondary battery according to one embodiment of the present invention. [Figure 2] 1 is a cross-sectional view schematically showing the configuration of a lithium ion secondary battery constructed using a negative electrode of a secondary battery according to one embodiment of the present invention. [Figure 3] 3 is a schematic exploded view showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. 2. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for implementing the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In this specification, a numerical range expressed as "A to B" includes A and B.

[0013] In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

[0014] The negative electrode disclosed herein is used in a secondary battery, and is preferably used in a lithium-ion secondary battery. One embodiment of the negative electrode disclosed herein will be specifically described with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically illustrating an example of a negative electrode 60 according to this embodiment, taken along the thickness direction and the width direction. The negative electrode 60 according to this embodiment shown in FIG. 1 is a negative electrode for a lithium-ion secondary battery.

[0015] As shown in the figure, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes the negative electrode current collector 62 and the negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided on only one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. The negative electrode active material layer 64 is preferably provided on both sides of the negative electrode current collector 62.

[0016] As shown in the illustrated example, a negative electrode active material layer-free portion 62a where no negative electrode active material layer 64 is provided may be provided at one end in the width direction of the negative electrode 60. In the negative electrode active material layer-free portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer-free portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0017] In the illustrated example, the shape of the negative electrode current collector 62 is foil (or sheet), but is not limited thereto. The negative electrode current collector 62 may have various shapes such as a rod, a plate, or a mesh. As with conventional lithium-ion secondary batteries, the material of the negative electrode current collector 62 can be a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.), and copper is particularly preferred. Copper foil is particularly preferred as the negative electrode current collector 62.

[0018] The dimensions of the negative electrode current collector 62 are not particularly limited and may be determined appropriately depending on the battery design. When a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 6 μm to 20 μm.

[0019] As shown in FIG. 1, the negative electrode active material layer 64 has a multi-layer structure and includes a first layer 64a located on the negative electrode current collector 62 side and a second layer 64b located on the surface side of the negative electrode active material layer 64. In the example shown in FIG. 1, the first layer 64a forms the so-called lower layer of the negative electrode active material layer 64, and the second layer 64b forms the so-called upper layer of the negative electrode active material layer 64. Note that the negative electrode active material layer 64 may further include layers other than the first layer 64a and the second layer 64b, as long as the effects of the present invention are not significantly impaired. For example, the negative electrode active material layer 64 may include an intermediate layer between the first layer 64a and the second layer 64b, in which the components of these layers are mixed.

[0020] The negative electrode active material layer 64 contains a negative electrode active material. With respect to the negative electrode active material, the first layer 64a contains first graphite particles and first Si-C composite particles. The second layer 64b contains second graphite particles and second Si-C composite particles. Thus, the first layer 64a uses at least the first graphite particles and the first Si-C composite particles as the negative electrode active material, and the second layer 64b uses at least the second graphite particles and the second Si-C composite particles as the negative electrode active material. Si-C composite particles undergo large volume changes due to expansion / contraction during charge / discharge, but using them in combination with graphite particles can prevent disconnection of the conductive path caused by volume changes of the Si-C composite particles.

[0021] The graphite that constitutes the first graphite particles and the second graphite particles may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

[0022] The shape of the first graphite particles and the second graphite particles is not particularly limited, and may be flaky, spherical, or the like.

[0023] The average particle diameter (D50) of the first graphite particles and the second graphite particles is not particularly limited. The average particle diameter (D50) of the graphite particles 12 is, for example, 1 μm to 35 μm, preferably 5 μm to 30 μm, more preferably 10 μm to 25 μm, and even more preferably 12 μm to 23 μm.

[0024] In this specification, the term "average particle size (D50)" refers to the median size (D50), which is the particle size corresponding to a cumulative frequency of 50 volume percent from the smallest particle size side in a volume-based particle size distribution based on a laser diffraction / scattering method. The average particle size (D50) can be determined using a commercially available laser diffraction / scattering particle size distribution analyzer or the like.

[0025] The first graphite particles and the second graphite particles may be the same or different, but it is preferable to use the same graphite particles as the first graphite particles and the second graphite particles.

[0026] The first and second Si—C composite particles are composed of a Si—C composite material, which typically includes carbon domains and Si-containing domains.

[0027] The carbon domains are, for example, carbonized products of carbon precursors (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite, etc. The carbon domains preferably constitute a carbon matrix. Therefore, the Si-C composite material is preferably a material in which multiple Si-containing domains are dispersed in a carbon matrix. In this case, the carbon matrix is ​​advantageous because it can mitigate volume changes due to the expansion / contraction of the Si-containing domains.

[0028] The Si-containing domains contain Si. The Si-containing particles are, for example, Si nanoparticles. The average particle diameter of the Si-containing domains is, for example, 50 nm or less, and may be 5 nm to 50 nm. The "average particle diameter of the Si-containing domains" can be determined as follows. First, the negative electrode active material layer 64 is processed with a focused ion beam (FIB) to prepare a sample for observation with a scanning transmission electron microscope (STEM). Then, the sample is subjected to elemental analysis by EDX element mapping, and then a BF image (bright-field image) and a HAADF image (high-angle annular dark-field image) are obtained. The diameter of the Si-containing domains can be determined from the contrast and shape obtained from the BF image and the HAADF image. The diameters of 10 or more arbitrarily selected Si-containing domains are determined, and the average value thereof is defined as the "average particle diameter of the Si-containing domains" herein.

[0029] Examples of Si-C composite materials include those in which Si nanoparticles are dispersed inside a carbon material, and those in which Si nanoparticles are dispersed within the pores of a porous carbon material (e.g., granulated porous graphite). The Si-C composite material may also be one in which Si microparticles are attached to the surfaces of carbon particles, or one in which carbon microparticles are attached to the surfaces of Si particles. From the viewpoint of suppressing the volume change of Si, those in which Si nanoparticles are dispersed inside a carbon material and those in which Si nanoparticles are dispersed within the pores of a porous carbon material are preferred, and those in which Si nanoparticles are dispersed within the pores of a porous carbon material are more preferred.

[0030] The Si content in the Si-C composite material is not particularly limited. From the viewpoint of achieving a higher capacity of the secondary battery, the Si content in the Si-C composite material is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. From the viewpoint of suppressing excessive volume change of the Si-C composite material, the Si content in the Si-C composite material is preferably 60% by mass or less, more preferably 50% by mass or less.

[0031] In this embodiment, the Si of the second Si-C composite particles is doped with at least one element selected from the group consisting of Group 15 and Group 16 elements. On the other hand, the Si of the first Si-C composite particles may be doped with at least one element selected from the group consisting of Group 15 and Group 16 elements. The Si of the first Si-C composite particles is not doped with the above element, or if doped, only a small amount.

[0032] Therefore, the total content of Group 15 and Group 16 elements in the Si of the first Si-C composite particle is different from the total content of Group 15 and Group 16 elements in the Si of the second Si-C composite particle, with the former being lower.

[0033] Specifically, the total content of Group 15 and Group 16 elements in the Si of the first Si-C composite particle is less than 0.1 atomic % (including 0 atomic %). When this content is less than 0.1 atomic %, the first Si-C composite particle can exhibit a self-discharge suppression effect. The total content of Group 15 and Group 16 elements in the Si of the first Si-C composite particle is preferably 0 atomic % to 0.05 atomic %, more preferably 0 atomic % to 0.01 atomic %, and even more preferably 0 atomic % (i.e., the Si of the first Si-C composite particle is not doped with Group 15 and Group 16 elements).

[0034] On the other hand, the total content of Group 15 and Group 16 elements in the Si of the second Si-C composite particles is 0.1 atomic % to 5 atomic %. When this content is 0.1 atomic % or more, the effect of improving conductivity by doping is particularly high, thereby reducing the resistance of the secondary battery. On the other hand, when this content is less than 5 atomic %, compound formation between the doping element and Si can be suppressed, resulting in a particularly high effect of improving conductivity. The total content of Group 15 and Group 16 elements in the Si of the second Si-C composite particles is preferably 0.2 atomic % to 3 atomic %, more preferably 1 atomic % to 3 atomic %.

[0035] Examples of Group 15 elements include N, P, As, Sb, Bi, and Mc, of which N and P are preferred, and P is more preferred. Examples of Group 16 elements include O, S, Se, Te, Po, and Lv, of which S is preferred.

[0036] The type and amount of doping elements in the Si of the Si-C composite particles can be determined by a known method, specifically, by inductively coupled plasma optical emission spectroscopy (ICP-OEC).

[0037] The average particle diameter (D50) of the first Si-C composite particles and the second Si-C composite particles is not particularly limited, and may be, for example, 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 4 μm to 10 μm.

[0038] The first Si-C composite particles and the second Si-C composite particles can be produced according to known methods. For example, various methods for producing particles of Si-C composite materials are known (see, for example, JP 2015-38862 A, WO 2014 / 046144 A, and prior art documents cited in the WO 2014 / 046144 A, etc.). Therefore, the first Si-C composite particles can be produced according to these known methods. In addition, methods for doping Si with Group 15 and Group 16 elements are known, and the second Si-C composite particles can be produced by combining these known methods. Si doping may be performed before or after the production of the Si-C composite particles.

[0039] In this embodiment, the thickness of the first layer 64a is 60% to 90% of the thickness of the negative electrode active material layer 64, and the thickness of the second layer 64b is 10% to 40% of the thickness of the negative electrode active material layer 64.

[0040] Doping Si in Si-C composite particles can increase the conductivity, thereby reducing the resistance of secondary batteries. However, the inventors have found that when doped Si-C composite particles are used as the negative electrode active material of secondary batteries, the secondary batteries are prone to self-discharge. Furthermore, the inventors have found that the doped Si-C composite particles are more prone to self-discharge the closer they are to the negative electrode current collector.

[0041] Therefore, in this embodiment, doped second Si-C composite particles are arranged in a second layer 64b on the surface side of the negative electrode active material layer 64, and substantially undoped first Si-C composite particles are arranged in a first layer 64a located on the negative electrode current collector 62 side. The thickness of the first layer 64a is set to 60% or more of the thickness of the negative electrode active material layer 64 (hence, the thickness of the second layer 64b is 40% or less of the thickness of the negative electrode active material layer 64). With this configuration, the substantially undoped first Si-C composite particles of the first layer 64a are less likely to cause self-discharge, and therefore the first layer 64a functions as a barrier layer that suppresses self-discharge by the second Si-C composite particles of the second layer 64b.

[0042] On the other hand, in order to effectively obtain the battery resistance reducing effect of the doped second Si—C composite particles, the thickness of the second layer 64b is 10% or more of the thickness of the negative electrode active material layer 64. Therefore, the thickness of the first layer 64a is 90% or less of the thickness of the negative electrode active material layer 64.

[0043] Preferably, the thickness of the first layer 64a is 65% to 85% of the thickness of the negative electrode active material layer 64, and the thickness of the second layer 64b is 15% to 35% of the thickness of the negative electrode active material layer 64. More preferably, the thickness of the first layer 64a is 70% to 80% of the thickness of the negative electrode active material layer 64, and the thickness of the second layer 64b is 20% to 30% of the thickness of the negative electrode active material layer 64. From the viewpoint of a higher self-discharge suppression effect, it is even more preferable that the thickness of the first layer 64a is 75% to 80% of the thickness of the negative electrode active material layer 64, and the thickness of the second layer 64b is 20% to 25% of the thickness of the negative electrode active material layer 64. On the other hand, from the viewpoint of a higher battery resistance reduction effect, it is more preferable that the thickness of the first layer 64a is 70% to 75% of the thickness of the negative electrode active material layer 64, and the thickness of the second layer 64b is 25% to 30% of the thickness of the negative electrode active material layer 64.

[0044] In the first layer 64a, the mass ratio of the first Si-C composite particles to the total of the first graphite particles and the first Si-C composite particles is preferably 10 mass% to 60 mass%, more preferably 15 mass% to 50 mass%, and even more preferably 20 mass% to 40 mass%.

[0045] In the second layer 64b, the mass ratio of the second Si-C composite particles to the total of the second graphite particles and the second Si-C composite particles is preferably 10% to 60% by mass, more preferably 15% to 50% by mass, and even more preferably 20% to 40% by mass. The mass ratio of the first Si-C composite particles in the first layer 64a and the mass ratio of the second Si-C composite particles in the second layer 64b may be the same or different.

[0046] The first layer 64a may further contain a negative electrode active material other than the first graphite particles and the first Si-C composite particles, within a range that does not impair the effects of the present invention (for example, 10 mass% or less of the total amount of the negative electrode active material).

[0047] The second layer 64b may further contain a negative electrode active material other than the second graphite particles and the second Si-C composite particles, within a range that does not impair the effects of the present invention (for example, 10 mass % or less of the total amount of the negative electrode active material).

[0048] The first layer 64a and the second layer 64b may each contain components other than the negative electrode active material (hereinafter also referred to as "optional components"). Examples of optional components include a binder and a conductive material. Examples of binders that can be used include styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickener. Examples of conductive materials include carbon nanotubes (CNTs). When CNTs are used as the conductive material, the negative electrode active material layer 64 may contain a dispersant for the CNTs.

[0049] The content of the negative electrode active material in the first layer 64a (i.e., relative to the total mass of the first layer 64a) is preferably 90 mass% or more, and more preferably 95 mass% or more. The content of the binder in the first layer 64a is preferably 0.1 mass% to 8 mass% or less, and more preferably 0.5 mass% to 5 mass% or less. The content of the conductive material in the first layer 64a is preferably 0.01 mass% to 3 mass% or less, and more preferably 0.05 mass% to 1 mass% or less.

[0050] Similarly, the content of the negative electrode active material in the second layer 64b (i.e., relative to the total mass of the second layer 64b) is preferably 90 mass% or more, and more preferably 95 mass% or more. The content of the binder in the second layer 64b is preferably 0.1 mass% to 8 mass% or less, and more preferably 0.5 mass% to 5 mass% or less. The content of the conductive material in the second layer 64b is preferably 0.01 mass% to 3 mass% or less, and more preferably 0.05 mass% to 1 mass% or less.

[0051] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 400 μm or less, and preferably 20 μm or more and 300 μm or less.

[0052] The density of the negative electrode active material layer 64 is not particularly limited, but is, for example, 0.9 g / cm 3 or more, preferably 1.1 g / cm 3 More preferably, it is 1.2 g / cm or more. 3 On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm 3 is less than or equal to 2.0 g / cm 3 It may be the following:

[0053] The negative electrode 60 may include members other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) adjacent to the negative electrode active material layer 64 may be provided on the negative electrode active material layer non-forming portion 62a. The insulating layer contains, for example, an insulating inorganic filler.

[0054] The negative electrode 60 can be fabricated according to a known method. Specifically, for example, a slurry containing the components of the first layer 64a is prepared. This slurry is applied to the negative electrode current collector 62 and dried to form the first layer 64a on the negative electrode current collector 62. A slurry containing the components of the second layer 64b is prepared. This slurry is applied to the first layer 64a and dried to form the second layer 64b on the first layer 64a. In this manner, the negative electrode active material layer 64 can be formed on the negative electrode current collector 62. If necessary, the negative electrode active material layer 64 may be pressed.

[0055] The negative electrode 60 according to this embodiment uses a doped Si-based negative electrode active material, which can reduce the resistance of the secondary battery. Furthermore, despite the use of a doped Si-based negative electrode active material, the negative electrode 60 according to this embodiment can suppress self-discharge of the secondary battery. Furthermore, because the negative electrode 60 according to this embodiment uses a Si-based negative electrode active material, the capacity of the secondary battery can be increased.

[0056] Therefore, from another aspect, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. Hereinafter, one embodiment of the secondary battery disclosed herein will be described with reference to FIGS. 2 and 3, taking a lithium-ion secondary battery as an example. The following configuration example is a flat prismatic lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0057] The lithium-ion secondary battery 100 shown in FIG. 2 is a sealed lithium-ion secondary battery 100 constructed by housing a flat wound electrode assembly 20 and a nonaqueous electrolyte (not shown) in a flat, rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-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 inlet (not shown) for injecting the nonaqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0058] As shown in Figures 2 and 3, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped with two long separator sheets 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 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20. The positive electrode active material layer-free portion 52a and the negative electrode active material layer-free portion 62a are joined to the positive electrode current collector 42a and the negative electrode current collector 44a, respectively.

[0059] The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current collector used in lithium ion secondary batteries, and examples thereof include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0060] The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0061] The positive electrode active material layer 54 contains a positive electrode active material. The positive electrode active material may be a positive electrode active material of a known composition used in lithium-ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0062] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0063] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0064] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0065] These positive electrode active materials may be used alone or in combination of two or more. As the positive electrode active material, lithium nickel cobalt manganese composite oxide is particularly preferred because of its excellent properties such as initial resistance.

[0066] The average particle diameter (D50) of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0067] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB); carbon fibers such as vapor grown carbon fiber (VGCF) and carbon nanotubes (CNT); and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF).

[0068] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more and 99% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 15% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.4% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less.

[0069] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is usually 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, and preferably 300 μm or less.

[0070] As the negative electrode sheet 60, the above-mentioned negative electrode 60 is used.

[0071] The separator 70 may be a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0072] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability of the separator 70 measured by the Gurley test method is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0073] The nonaqueous electrolyte typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). As the nonaqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones commonly used in electrolytes for lithium-ion secondary batteries can be used without any particular limitation. Among these, carbonates are preferred, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). One of these nonaqueous solvents can be used alone, or two or more can be used in appropriate combination. For example, the nonaqueous solvent consists solely of carbonates. As another example, non-aqueous solvents include carbonates and esters such as methyl acetate.

[0074] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0075] The nonaqueous electrolyte may contain various additives other than the above-mentioned components, such as film-forming agents such as vinylene carbonate (VC) and oxalate complexes; gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB); and thickeners, as long as the effects of the present invention are not significantly impaired.

[0076] The lithium ion secondary battery 100 has low battery resistance and therefore excellent input / output characteristics. Furthermore, the lithium ion secondary battery 100 has suppressed self-discharge and therefore excellent storage characteristics. In addition, the lithium ion secondary battery 100 has a high capacity. The lithium ion secondary battery 100 can be used for a variety of applications. Suitable applications include a driving power source mounted on vehicles such as 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.

[0077] The above describes, as an example, a rectangular lithium ion secondary battery 100 equipped with a flat wound electrode assembly 20. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery equipped with a stacked electrode assembly (i.e., an electrode assembly in which multiple positive electrodes and multiple negative electrodes are stacked alternately). The lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery, a laminated case lithium ion secondary battery, etc.

[0078] Furthermore, the lithium ion secondary battery 100 can also be configured as an all-solid-state lithium ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte according to known methods.

[0079] Furthermore, although the negative electrode 60 according to this embodiment is suitable as a negative electrode for a lithium ion secondary battery, it can be constructed and used as a negative electrode for other secondary batteries, and the other secondary batteries can be constructed according to known methods.

[0080] Examples of the present invention will be described in detail below, but it is not intended that the present invention be limited to those shown in these examples.

[0081] <Preparation of negative electrode> Example 1 The following negative electrode active materials were prepared. The Si content of the first Si-C composite particles and the second Si-C composite particles, and the doping amount of the second Si-C composite particles relative to Si, were measured using a commercially available ICP-OEC device. The average particle diameter (D50) of the graphite particles was measured using a commercially available laser diffraction / scattering particle size distribution analyzer. First Si-C composite particles: Si-C composite material, Si content = 50 mass%, no element doping Second Si-C composite particles: Si-C composite material, Si content = 50 mass%, sulfur (S) 3 atomic % doped Graphite particles: average particle size (D50) = 15 μm

[0082] Single-walled carbon nanotubes (SWCNTs) were prepared as the conductive material. The SWCNTs were prepared in the form of an aqueous dispersion with a solid content of 2%. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as binders.

[0083] A paste for forming a first layer containing graphite particles, first Si—C composite particles, SWCNTs, CMC, PAA, and SBR in a mass ratio of 70:30:0.1:1:1:1.5 was prepared by the following procedure.

[0084] First, the first Si-C composite particles, SWCNT dispersion, and dispersion medium (water) were mixed using a disper at a rotation speed of 3000 rpm to prepare a pre-mixed paste. Meanwhile, graphite particles, CMC, and PAA were dry-blended using a planetary mixer. The pre-mixed paste and dispersion medium (water) were added to the obtained dry mixture and kneaded using the planetary mixer. The solid content at this time was set to 65% by mass. Furthermore, SBR and dispersion medium (water) were added to the planetary mixer and diluted and mixed to obtain a paste for forming the first layer.

[0085] The prepared first layer forming paste was applied to the surface of a copper foil with a thickness of 10 μm and dried to form the first layer, which is the lower layer portion of the negative electrode active material layer.

[0086] Next, a paste for forming a second layer containing graphite particles, second Si-C composite particles, SWCNTs, CMC, PAA, and SBR in a mass ratio of 70:30:0.1:1:1:1.5 was prepared using the same method as above, except that second Si-C composite particles were used instead of the first Si-C composite particles.

[0087] The prepared second layer-forming paste was applied to the surface of the formed first layer and dried to form the second layer, which is the upper layer of the negative electrode active material layer. The amount of the second layer-forming paste applied was adjusted so that the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer (the total thickness of the first and second layers) was 25%. The obtained sheet was roll-pressed and then processed to the specified dimensions to obtain a negative electrode sheet. It was confirmed that the above thickness ratio was maintained even after pressing.

[0088] Example 2 A negative electrode sheet of Example 2 was obtained in the same manner as in Example 1, except that the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer (total thickness of the first layer and second layer) was changed to 20%.

[0089] Example 3 A negative electrode sheet of Example 3 was obtained in the same manner as in Example 1, except that the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer (total thickness of the first layer and second layer) was changed to 30%.

[0090] Comparative Examples 1 to 5 A paste for forming a negative electrode active material layer containing graphite particles, Si-containing particles, SWCNTs, CMC, PAA, and SBR in a mass ratio of 70:30:0.1:1:1:1.5 was prepared using the same procedure as above. In Comparative Example 1, first Si-C composite particles were used as the Si-C composite particles. In Comparative Example 2, second Si-C composite particles were used as the Si-C composite particles. In Comparative Example 3, a mixture of first Si-C composite particles and second Si-C composite particles in a mass ratio of 30:70 was used as the Si-C composite particles. In Comparative Example 4, a mixture of first Si-C composite particles and second Si-C composite particles in a mass ratio of 50:50 was used as the Si-C composite particles. In Comparative Example 5, a mixture of first Si-C composite particles and second Si-C composite particles in a mass ratio of 70:30 was used as the Si-containing particles.

[0091] The paste for forming the negative electrode active material layer was applied to the surface of a copper foil having a thickness of 10 μm and dried to form a negative electrode active material layer. The thickness of the negative electrode active material layer at this time was set to be the same as the total thickness of the first and second layers in Example 1. The obtained sheet was roll-pressed and then processed to a predetermined size to obtain the negative electrode sheets of Comparative Examples 1 to 5.

[0092] Comparative Example 6 A negative electrode sheet of Comparative Example 6 was obtained in the same manner as in Example 1, except that the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer (total thickness of the first layer and second layer) was changed to 5%.

[0093] Comparative Example 7 A negative electrode sheet of Comparative Example 7 was obtained in the same manner as in Example 1, except that the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer (total thickness of the first layer and second layer) was changed to 45%.

[0094] Comparative Example 8 The negative electrode sheet of Comparative Example 8 was obtained in the same manner as in Comparative Example 6, except that the order of using the first layer-forming paste and the second layer-forming paste was changed so that the lower layer portion of the negative electrode active material layer was formed with the second layer-forming paste and the upper layer portion of the negative electrode active material layer was formed with the first layer-forming paste.

[0095] Comparative Example 9 The negative electrode sheet of Comparative Example 9 was obtained in the same manner as in Example 6, except that the order of using the first layer-forming paste and the second layer-forming paste was changed so that the lower layer portion of the negative electrode active material layer was formed with the second layer-forming paste and the upper layer portion of the negative electrode active material layer was formed with the first layer-forming paste.

[0096] Comparative Example 10 The negative electrode sheet of Comparative Example 10 was obtained in the same manner as in Comparative Example 7, except that the order of using the first layer-forming paste and the second layer-forming paste was changed so that the lower layer portion of the negative electrode active material layer was formed with the second layer-forming paste and the upper layer portion of the negative electrode active material layer was formed with the first layer-forming paste.

[0097] <Preparation of Lithium-ion Secondary Batteries for Evaluation> LiNi as a positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode paste was prepared by mixing O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF = 100:1:1. This paste was applied to the surface of a 15 μm thick aluminum foil and dried to form a positive electrode active material layer. After roll pressing the positive electrode active material layer, the resulting sheet was cut to the specified dimensions to obtain a positive electrode sheet.

[0098] A porous polyolefin separator was prepared. Leads were attached to the negative electrode sheet and positive electrode sheet prepared above, and they were stacked with a separator interposed between them to prepare an electrode assembly. This was housed in a case made of aluminum laminate film together with a non-aqueous electrolyte. The non-aqueous electrolyte used was a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 15:5:40:40, with LiPF6 dissolved as a supporting electrolyte at a concentration of 1.0 mol / L. The case was then sealed to obtain a lithium-ion secondary battery for evaluation.

[0099] <Self-discharge evaluation> Each of the prepared lithium-ion secondary batteries for evaluation was placed in an environment of 25°C. An activation process was performed on each of the lithium-ion secondary batteries for evaluation. Each activated lithium-ion secondary battery for evaluation was charged at a constant current of 0.4 C up to 4.2 V, and then charged at a constant voltage until the current reached 0.1 C. Next, each of the lithium-ion secondary batteries for evaluation was discharged at a constant current of 0.4 C down to 2.5 V. The discharge capacity at this time was measured to determine the initial capacity.

[0100] The initial capacity at this time was taken as SOC 100%, and each evaluation lithium-ion secondary battery was adjusted to SOC 95%. The charge capacity at this time was determined. Next, each evaluation lithium-ion secondary battery was stored in a 60°C environment for 4 weeks. Thereafter, each evaluation lithium-ion secondary battery was placed in a 25°C environment and discharged at a constant current of 0.4 C to 2.5 V, and the discharge capacity at this time was measured. The capacity retention rate was calculated by (discharge capacity after storage / charge capacity before storage) x 100. The results are shown in Tables 1 and 2. Note that a higher capacity retention rate indicates less self-discharge.

[0101] <Battery resistance evaluation> The initial capacity was considered to be 100% SOC, and each lithium-ion secondary battery for evaluation was adjusted to 50% SOC. Next, the batteries were discharged for 10 seconds at current values ​​of 0.1C, 0.2C, 0.5C, 1C, and 2C in a 25°C environment, and the battery voltage after discharge was measured. The IV characteristics during discharge were determined by plotting each current value against each battery voltage, and the IV resistance (Ω) during discharge was calculated as the battery resistance from the slope of the resulting line. The results are shown in Tables 1 and 2.

[0102] [Table 1]

[0103] [Table 2]

[0104] In Comparative Examples 1 to 5, the negative electrode active material layer has a single-layer structure. A comparison of Comparative Examples 1 and 2 reveals that, when S-doped Si-C composite particles are used, a resistance-reducing effect is obtained, but self-discharge increases. The results of Comparative Examples 3 to 5 reveal that simply using S-doped and undoped Si-C composite particles in combination does not achieve both a reduction in battery resistance and suppression of self-discharge.

[0105] In Examples 1 to 3 and Comparative Examples 6 to 10, the negative electrode active material layer had a two-layer structure, with one layer containing S-doped Si-C composite particles and the other layer containing undoped Si-C composite particles. In Comparative Examples 8 to 10, S-doped Si-C composite particles were placed in the first layer on the negative electrode current collector side. In these cases, self-discharge was large. This indicates that self-discharge is more likely to occur the closer the S-doped Si-C composite particles are to the negative electrode current collector.

[0106] On the other hand, in Examples 1 to 3 and Comparative Examples 6 and 7, S-doped Si-C composite particles were placed on the surface layer side. These comparisons reveal that if the thickness of the first layer is too large (if the thickness of the second layer is too small), the battery resistance increases, and if the thickness of the first layer is too small (if the thickness of the second layer is too large), self-discharge increases. Therefore, it can be seen that when the thicknesses of the first and second layers are within a predetermined range, it is possible to achieve both reduced battery resistance and suppressed self-discharge. From the above, it can be seen that the negative electrode disclosed herein can reduce the resistance of a secondary battery and suppress self-discharge of the secondary battery, even when using a doped Si-based negative electrode active material.

[0107] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above.

[0108] That is, the negative electrode of the secondary battery and the secondary battery disclosed herein are the following items [1] to [5]. [1] A negative electrode current collector; a negative electrode active material layer supported on the negative electrode current collector; A negative electrode comprising: the negative electrode active material layer includes a first layer located on the negative electrode current collector side and a second layer located on a surface layer side, the first layer contains first graphite particles and first Si—C composite particles; the second layer contains second graphite particles and second Si—C composite particles; The Si of the first Si-C composite particles may be doped with at least one element selected from the group consisting of Group 15 elements and Group 16 elements, the Si of the second Si-C composite particles is doped with at least one element selected from the group consisting of Group 15 elements and Group 16 elements; the total content of the Group 15 element and the Group 16 element in Si of the first Si-C composite particle is less than 0.1 atomic %; the total content of the Group 15 element and the Group 16 element in Si of the second Si-C composite particle is 0.1 atomic % to 5 atomic %; A negative electrode, wherein the thickness of the first layer is 60% to 90% of the thickness of the negative electrode active material layer, and the thickness of the second layer is 10% to 40% of the thickness of the negative electrode active material layer. [2] The negative electrode according to item [1], wherein in the first layer, a mass ratio of the first Si-C composite particles to the total of the first graphite particles and the first Si-C composite particles is 10 mass% to 60 mass%, and in the second layer, a mass ratio of the second Si-C composite particles to the total of the second graphite particles and the second Si-C composite particles is 10 mass% to 60 mass%. [3] The negative electrode according to item [1] or [2], wherein the thickness of the first layer is 65% to 85% of the thickness of the negative electrode active material layer, and the thickness of the second layer is 15% to 35% of the thickness of the negative electrode active material layer. [4] The negative electrode according to any one of items [1] to [3], wherein the Si of the second Si-C composite particles is doped with at least one element selected from the group consisting of N, P, and S. [5] A positive electrode, a negative electrode, an electrolyte, A secondary battery comprising: A secondary battery, wherein the negative electrode is the negative electrode according to any one of items [1] to [4]. [Explanation of symbols]

[0109] 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator sheet (separator) 100 Lithium-ion secondary battery

Claims

1. a negative electrode current collector; a negative electrode active material layer supported on the negative electrode current collector; A negative electrode comprising: the negative electrode active material layer includes a first layer located on the negative electrode current collector side and a second layer located on a surface layer side, the first layer contains first graphite particles and first Si—C composite particles; the second layer contains second graphite particles and second Si—C composite particles; the Si of the first Si—C composite particles is undoped; the Si of the second Si—C composite particles is doped with sulfur; the content of the sulfur element in Si of the second Si-C composite particles is 0.1 atomic % to 5 atomic %; a thickness of the first layer is 60% to 90% of the thickness of the negative electrode active material layer, and a thickness of the second layer is 10% to 40% of the thickness of the negative electrode active material layer.

2. 2. The negative electrode according to claim 1, wherein a mass ratio of the first Si—C composite particles to a total of the first graphite particles and the first Si—C composite particles in the first layer is 10 mass% to 60 mass%, and a mass ratio of the second Si—C composite particles to a total of the second graphite particles and the second Si—C composite particles in the second layer is 10 mass% to 60 mass%.

3. 2. The negative electrode according to claim 1, wherein a thickness of the first layer is 65% to 85% of a thickness of the negative electrode active material layer, and a thickness of the second layer is 15% to 35% of a thickness of the negative electrode active material layer.

4. a positive electrode, a negative electrode, an electrolyte, A secondary battery comprising: A secondary battery, wherein the negative electrode is the negative electrode according to claim 1.

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