Negative electrode for secondary battery, and secondary battery

Abstract

A negative electrode (12) is provided with a negative electrode current collector (40) and a negative electrode mixture layer (41) disposed on the negative electrode current collector (40). The negative electrode mixture layer (41) has, in the thickness direction, a first region (45) disposed on the negative electrode current collector (40) side and a second region (46) disposed on the opposite side from the negative electrode current collector (40) side. The second region (46) has a negative electrode active material containing graphite and a silicon-containing material. The ratio A of the mass of silicon element in the first region (45) to the mass of the first region (45) is 0-60 mass% inclusive, the ratio B of the mass of silicon element in the second region (46) to the mass of the second region (46) is 15 mass% or more but less than 100 mass%, and the ratio B is greater than the ratio A.

Description

Negative electrode for secondary battery, and secondary battery

[0001] This disclosure relates to a negative electrode for a secondary battery and a secondary battery.

[0002] In secondary batteries, silicon-containing materials are being considered as negative electrode active materials to increase battery capacity. However, silicon-containing materials undergo large volume changes (expansion and contraction) during charging and discharging. As a result of repeated charging and discharging, the large volume changes in the silicon-containing material can lead to problems such as uneven distribution of the electrolyte within the negative electrode mixture layer, which can easily degrade the charge-discharge cycle characteristics.

[0003] For example, Patent Document 1 proposes using a gradient electrode for a secondary battery, in which the ratio of silicon-containing material is increased by an appropriate amount from the current collector to the side opposite the positive electrode, in order to improve the charge-discharge cycle characteristics.

[0004] However, in negative electrodes using silicon-containing materials as the negative electrode active material, further improvements are still needed in terms of enhancing charge-discharge cycle characteristics.

[0005] Japanese Patent Publication No. 2018-063920

[0006] Therefore, the purpose of this disclosure is to provide a negative electrode for a secondary battery that can improve charge-discharge cycle characteristics even when a silicon-containing material is included as the negative electrode active material, and a secondary battery equipped with said negative electrode.

[0007] A negative electrode for a secondary battery according to one aspect of the present disclosure comprises a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer has, in the thickness direction, a first region disposed on the negative electrode current collector side and a second region disposed on the opposite side from the negative electrode current collector side, the second region having a negative electrode active material containing graphite and silicon-containing material, the ratio A of the mass of silicon elements in the first region to the mass of the first region being 0% by mass or more and 60% by mass or less, and the ratio B of the mass of silicon elements in the second region to the mass of the second region being 15% by mass or more and less than 100% by mass, wherein ratio B is greater than ratio A.

[0008] Furthermore, a negative electrode for a secondary battery according to one aspect of the present disclosure comprises a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer has, in the thickness direction, a first region disposed on the negative electrode current collector side and a second region disposed on the opposite side from the negative electrode current collector side, the second region having a negative electrode active material containing graphite and silicon-containing material, and when the ratio A of the mass of silicon elements in the first region to the mass of the first region is 0% by mass or more and less than 15% by mass, the ratio B of the mass of silicon elements in the second region to the mass of the second region is 15% by mass or more and less than 100% by mass, and when the ratio A is 15% by mass or more, the ratio of the ratio B to the ratio A (B / A) is 1.4 or more and the ratio B is less than 100% by mass.

[0009] Furthermore, a secondary battery according to one aspect of this disclosure includes a negative electrode for the secondary battery.

[0010] According to this disclosure, it is possible to provide a negative electrode for a secondary battery that can improve charge-discharge cycle characteristics even when a silicon-containing material is included as the negative electrode active material, and a secondary battery equipped with said negative electrode.

[0011] This is a cross-sectional view showing an example of a secondary battery according to the embodiment. This is a cross-sectional view showing an example of a negative electrode according to the embodiment.

[0012] The following describes an example of an embodiment of the negative electrode for a secondary battery and the secondary battery relating to this disclosure. This disclosure is not limited to the embodiments described below, and includes forms that are implemented without altering the gist of this disclosure.

[0013] (Secondary Battery) Figure 1 is a cross-sectional view showing an example of a secondary battery according to the embodiment. The secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, an electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, a battery case 15, a positive electrode lead 20, and a negative electrode lead 21. In addition to the wound electrode body 14, other forms of electrode bodies may be used, such as a laminated electrode body in which the positive electrode and negative electrode are alternately stacked with a separator. The battery case 15 is composed of an outer can 16 having an opening and housing the electrode body 14, etc., and a sealing body 17 that closes the opening of the outer can 16. Examples of the battery case 15 include metal cases such as cylindrical, square, coin-shaped, and button-shaped cases, and resin cases (so-called pouch type) formed by laminating resin sheets.

[0014] The electrolyte, for example, has lithium ion conductivity. The electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte.

[0015] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0016] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc. Although the electrolytes exemplified above are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may also be an aqueous electrolyte.

[0017] The outer casing 16 is, for example, a metal container in the shape of a bottomed cylinder. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to further ensure airtightness inside the battery. The outer casing 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17.

[0018] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, and the current path between the lower valve body 24 and the upper valve body 26 is interrupted. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0019] In the secondary battery 10 shown in Figure 1, one end of the positive electrode lead 20 is attached to the positive electrode 11. The positive electrode lead 20 attached to the positive electrode 11 extends through a through-hole in the insulating plate 18 towards the sealing body 17 and is connected by welding or other means to the lower surface of the internal terminal plate 23, which is the bottom plate of the sealing body 17. As a result, the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. In the secondary battery 10 shown in Figure 1, one end of the negative electrode lead 21 is attached to the negative electrode 12. The negative electrode lead 21 attached to the negative electrode 12 extends through the outside of the insulating plate 19 towards the bottom of the outer casing 16 and is connected by welding or other means to the inner surface of the bottom of the outer casing 16. As a result, the outer casing 16 becomes the negative electrode terminal.

[0020] The positive electrode 11, negative electrode 12, and separator 13 will be described below.

[0021] [Positive Electrode] The positive electrode 11 comprises, for example, a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer may be disposed on only one side of the positive electrode current collector or on both sides of the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable in the positive electrode potential range, such as aluminum or an aluminum alloy, or a film with the metal disposed on its surface. The positive electrode mixture layer includes, for example, a positive electrode active material, a binder, a conductive agent, etc. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture layer slurry containing a positive electrode active material, a binder, a conductive agent, etc., onto the positive electrode current collector, drying the coating, rolling it, and forming the positive electrode mixture layer on the positive electrode current collector.

[0022] The positive electrode active material contained in the positive electrode mixture layer can be exemplified by lithium transition metal composite oxides containing transition metal elements such as Co, Mn, and Ni. Examples of metal elements contained in lithium transition metal composite oxides include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Ca, Sb, Pb, Bi, and Ge. A suitable example of a lithium transition metal composite oxide is a composite oxide containing at least one of Ni, Co, Mn, and Al.

[0023] Examples of conductive agents included in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, Ketjen black, graphite, and carbon nanotubes. Examples of binders included in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA).

[0024] [Negative Electrode] Figure 2 is a cross-sectional view showing an example of a negative electrode according to the embodiment. The negative electrode 12 comprises a negative electrode current collector 40 and a negative electrode mixture layer 41 disposed on the negative electrode current collector 40. In the negative electrode 12 shown in Figure 2, the negative electrode mixture layer 41 is disposed on both sides of the negative electrode current collector 40, but is not limited to this, and may be disposed on only one side of the negative electrode current collector 40. The negative electrode current collector 40 can be made of a metal foil that is stable in the negative electrode potential range, such as copper or a copper alloy, or a film with the metal disposed on its surface. The negative electrode mixture layer 41 contains a negative electrode active material and may also contain a binder or the like.

[0025] The negative electrode active material contained in the negative electrode mixture layer 41 includes graphite and silicon-containing material. Graphite is a material that can electrochemically intercept and release lithium ions, and examples include natural graphite, artificial graphite, a mixture of natural and artificial graphite, and natural graphite coated with artificial graphite. These can be used individually or in combination of two or more. Silicon-containing material is a material that can electrochemically intercept and release lithium ions, and examples include Si, Si alloys, and Si compounds. Details of Si compounds will be described later.

[0026] The negative electrode mixture layer 41 has a two-layer structure consisting of a first region 45 located on the negative electrode current collector 40 side and a second region 46 located on the opposite side of the negative electrode current collector 40 in the thickness direction. The first region 45 is the lower layer, and the second region 46 is the upper layer. The lower layer of the first region 45 contains at least graphite, and may contain silicon-containing material, among graphite and silicon-containing material. The upper layer of the second region 46 contains graphite and silicon-containing material. The silicon element content in the first region 45 and the second region 46 is determined by the first or second embodiment described later. The silicon element content is derived from the silicon element constituting the silicon-containing material.

[0027] (First Embodiment) In the first embodiment of this embodiment, the ratio A of the mass of silicon elements in the first region 45 to the mass of the first region 45 is 0% by mass or more and 60% by mass or less, and the ratio B of the mass of silicon elements in the second region 46 to the mass of the second region 46 is 15% by mass or more and less than 100% by mass, characterized in that ratio B is greater than ratio A.

[0028] (Second Embodiment) In a second embodiment of this embodiment, when the ratio A of the mass of silicon elements in the first region 45 to the mass of the first region 45 is 0% by mass or more and less than 15% by mass, the ratio B of the mass of silicon elements in the second region 46 to the mass of the second region 46 is 15% by mass or more and less than 100% by mass, and when the ratio A is 15% by mass or more, the ratio of the ratio B to the ratio A (B / A) is 1.4 or more, and the ratio B is less than 100% by mass.

[0029] Ratios A and B are measured as follows: (1) The negative electrode is embedded in a thermosetting resin (epoxy resin), cured and molded, and then mechanically polished to expose the cross-section of the negative electrode mixture layer. (2) A scanning electron microscope (SEM) is used to observe the cross-section of the exposed negative electrode mixture layer and to obtain cross-sectional SEM images of the first region and the second region. (3) From the above cross-sectional SEM images, mapping analysis images of the first region and the second region are obtained by energy-dispersive X-ray (EDX) mapping analysis. An analyzer manufactured by JEOL Ltd. (JED-2300T) is used for the EDX analysis. (4) From the obtained mapping analysis images, the amount of Si elements (atomic %) in the first region and the amount of Si elements (atomic %) in the second region are obtained. (5) The amount of Si elements (atomic %) in the first region is converted to the amount of Si elements (mass %) in the first region and the ratio A is calculated. Similarly, the amount of Si elements (atomic %) in the second region is converted to the amount of Si elements (mass %) in the second region to calculate ratio B.

[0030] Furthermore, by satisfying the silicon element content specified in the first and second embodiments, the penetration of the liquid electrolyte into the negative electrode mixture layer 41 is promoted, and it is presumed that the fluidity of the liquid electrolyte within the negative electrode mixture layer 41 is improved. As a result, even with large volume changes in the silicon-containing material due to repeated charging and discharging, for example, the non-uniformity of the electrolyte distribution within the negative electrode mixture layer is suppressed, which is thought to lead to an improvement in charge-discharge cycle characteristics. In addition, even if the electrolyte is a solid electrolyte, by satisfying the silicon element content specified in the first and second embodiments, a large amount of silicon-containing material with high expansion and contraction is present on the negative electrode surface, which enables the reconstruction of the Li ion conduction path. Furthermore, since the graphite ratio is high at the interface between the negative electrode mixture layer 41 and the negative electrode current collector 40, the adhesion between the negative electrode mixture layer 41 and the negative electrode current collector 40 is improved, which is thought to lead to an improvement in charge-discharge cycle characteristics.

[0031] In the first aspect, the ratio A is preferably 0% by mass. That is, it is preferable that the first region 45 contains no silicon-containing material. This suppresses the expansion and contraction of the lower layer of the negative electrode mixture layer 41, improves the adhesion between the negative electrode mixture layer 41 and the negative electrode current collector 40, and can further improve the charge-discharge cycle characteristics.

[0032] Further, in the first aspect, when the ratio A is more than 0% by mass and 60% by mass or less, the ratio of the ratio B to the ratio A (B / A) is preferably 1.5 or more, and more preferably 1.5 or more and 5 or less. This suppresses the expansion and contraction of the lower layer of the negative electrode mixture layer 41, improves the adhesion between the negative electrode mixture layer 41 and the negative electrode current collector 40, and can further improve the charge-discharge cycle characteristics.

[0033] Further, in both the first aspect and the second aspect, the ratio B is preferably 20% by mass or more and less than 50% by mass, and more preferably 25% by mass or more and 40% by mass or less. This suppresses the expansion and contraction of the lower layer of the negative electrode mixture layer 41, improves the adhesion between the negative electrode mixture layer 41 and the negative electrode current collector 40, and can further improve the charge-discharge cycle characteristics.

[0034] Further, in both the first aspect and the second aspect, the ratio A is preferably 0% by mass or more and 10% by mass or less. This suppresses the expansion and contraction of the lower layer of the negative electrode mixture layer 41, improves the adhesion between the negative electrode mixture layer 41 and the negative electrode current collector 40, and can further improve the charge-discharge cycle characteristics.

[0035] Further, in both the first aspect and the second aspect, the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 is preferably 10% by mass or more, and more preferably 15% by mass or more. This enables a higher capacity of the secondary battery. The upper limit of the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 only needs to be less than 100% by mass, but for example, from the viewpoint of further improving charge-discharge cycle characteristics, it is preferably 25% by mass or less, and more preferably 20% by mass or less.

[0036] The mass of the first region 45 may be, for example, 50% by mass or more and less than 90% by mass, or 60% by mass or more and 80% by mass or less, relative to the mass of the negative electrode mixture layer 41. The mass of the second region 46 may be, for example, 10% by mass or more and 50% by mass or less, or 20% by mass or more and 40% by mass or less, relative to the mass of the negative electrode mixture layer 41. In other words, the mass ratio of the first region 45 to the second region 46 (second region 46 / first region 45) may be 0.1 or more and 1 or less, or 0.3 or more and 0.7 or less. This may lead to improved output characteristics and increased capacity of the secondary battery.

[0037] The packing density of the negative electrode mixture layer 41 is 1.65 g / cm³, for example, in terms of increasing the capacity of the secondary battery. ３ The above is sufficient, 1.65 g / cm³ ３ 1.75g / cm or more ３ The following is acceptable. The packing densities of the first region 45 and the second region 46 may be the same or different. The packing density of the second region 46 may be lower than, for example, the packing density of the first region 45. An example of the packing density of the second region 46 is 1.40 g / cm³. ３ 1.55g / cm or more ３ The following is an example of the packing density of the first region 45: 1.70 g / cm³ ３ 1.95g / cm or more ３ The following applies:

[0038] The thickness of the negative electrode mixture layer 41 is, for example, 30 μm to 100 μm on one side of the negative electrode current collector 40. The thicknesses of the first region 45 and the second region 46 may be the same or different. The thickness of the first region 45 may be greater than or less than the thickness of the second region 46. For example, the ratio of the thickness of the first region 45 to the thickness of the negative electrode mixture layer 41 may be 50% to 80%, and the ratio of the thickness of the second region 46 to the thickness of the negative electrode mixture layer 41 may be 20% to 50%. In addition, the negative electrode mixture layer 41 may include layers other than the first region 45 and the second region 46, as long as it does not impair the purpose of this disclosure.

[0039] The silicon-containing material preferably contains a Si compound, for example, from the viewpoint of further improving charge-discharge cycle characteristics. The Si compound is preferably composite particles including an ion conductive phase and a silicon phase dispersed in the ion conductive phase (silicon particles in one aspect), for example, from the viewpoint of further improving charge-discharge cycle characteristics.

[0040] The silicon phase includes at least one of elemental silicon and a silicon alloy, for example. The average particle diameter of the silicon phase is preferably 150 nm or less, and more preferably 50 nm or less, from the viewpoint of further improving charge-discharge cycle characteristics. Although the lower limit of the average particle diameter of the silicon phase is not particularly limited, it is preferably 0.1 nm or more, and more preferably 1 nm or more. The average particle diameter of the silicon phase can be measured by cross-sectional observation of particles of the silicon-containing material using SEM (scanning electron microscope) or TEM (transmission electron microscope). Specifically, it is obtained by averaging the maximum diameters of any 100 particulate silicon phases.

[0041] The ratio of the mass of the silicon phase to the mass of the silicon-containing material in the negative electrode mixture layer 41 is preferably, for example, 30% by mass or more and 60% by mass or less from the viewpoint of increasing the capacity of the secondary battery.

[0042] The ion conductive phase of the Si compound is a phase that conducts ions, and includes at least one of a silicate phase, a carbon phase, and a silicon oxide phase, for example.

[0043] The carbon phase may be composed of, for example, amorphous carbon. Examples of the amorphous carbon constituting the carbon phase include hard carbon, soft carbon, and other amorphous carbons. Amorphous carbon has an average interplanar spacing d of the (002) plane measured by X-ray diffraction ００２ that is a carbon material exceeding 0.34 nm.

[0044] The main component (for example, 95% by mass or more and 100% by mass or less) of the silicon oxide phase may be silicon dioxide. The composition of the composite particles including the silicon oxide phase and the silicon phase dispersed therein is generally SiO ｘ which can be represented by. SiO ｘThis is a silicon nanoparticle in amorphous SiO ２ It has a dispersed structure inside. The oxygen content ratio x to silicon is preferably, for example, 0.5 ≤ x < 2.0, and more preferably 0.8 ≤ x ≤ 1.5.

[0045] The silicate phase may satisfy the following conditions (1) and / or (2): (1) The silicate phase contains at least one element selected from the group consisting of alkali metal elements and Group 2 elements (Group 2 elements of the long-period periodic table). (2) The silicate phase contains element L. Element L is at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanides, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Lanthanides are a collective term for 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

[0046] Regarding the above condition (1), examples of alkali metal elements include lithium (Li), potassium (K), and sodium (Na). Examples of group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The inclusion of alkali metal elements and / or group 2 elements may reduce the irreversible capacity of the silicate phase. A lithium-containing silicate phase (hereinafter sometimes referred to as the "lithium silicate phase") is preferred, for example, for its low irreversible capacity and high initial charge-discharge efficiency.

[0047] The lithium silicate phase may be any oxide phase containing Li, Si, and O, and may also contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4.

[0048] The lithium silicate phase is given by formula: Li ２ｚ SiO （２＋ｚ）It may include a lithium silicate phase represented by (0 < z < 2), or may be composed of such lithium silicate phase. Preferably, z satisfies the relationship 0 < z < 1, and z = 1 / 2 (i.e., Li ２ Si ２ O ５ ) is preferable.

[0049] Furthermore, the Si compound may also include composite particles comprising an ionic conductive phase and a silicon phase dispersed within the ionic conductive phase, and a coating layer covering at least a portion of the surface of the composite particles.

[0050] The coating layer present on the surface of the composite particles includes, for example, a conductive layer. A conductive material containing carbon is preferred as the conductive material constituting the conductive layer. Examples of conductive materials containing carbon include conductive carbon materials. Examples of conductive carbon materials include carbon black, graphite, and amorphous carbon with low crystallinity. The thickness of the conductive layer may be, for example, in the range of 1 to 200 nm. The thickness of the conductive layer can be measured by cross-sectional observation of the Si-containing material using a SEM or TEM (transmission electron microscope).

[0051] The ratio of the mass of graphite to the mass of the first region 45 may be 90% by mass or more, for example, in that it can further improve the charge-discharge cycle characteristics. Also, the ratio of the mass of graphite to the mass of the second region 46 may be 50% by mass or more and 80% by mass or more, for example, in that it can further improve the charge-discharge cycle characteristics.

[0052] The negative electrode mixture layer 41 may contain, for example, graphite and silicon-containing materials, as well as other negative electrode active materials. Other negative electrode active materials may be substances that can electrochemically intercept and release lithium ions, such as amorphous carbon, Sn, Sn alloys, Sn compounds, lithium titanate, etc.

[0053] Examples of binders included in the negative electrode mixture layer 41 include those similar to those used in the positive electrode 11. The binder content in the negative electrode mixture layer 41 may be, for example, 0.5% by mass or more and 10% by mass or less relative to the mass of the negative electrode mixture layer 41. In addition, the negative electrode mixture layer 41 may also contain a conductive agent, similar to that used in the positive electrode 11.

[0054] The negative electrode 12 is manufactured, for example, by the following method: A first negative electrode mixture layer slurry for the first region 45 is prepared, containing graphite, a binder, etc. The first negative electrode mixture layer slurry may also contain a silicon-containing material. A second negative electrode mixture layer slurry for the second region 46 is prepared, containing graphite, a silicon-containing material, a binder, etc. The first negative electrode mixture layer slurry is then applied onto the negative electrode current collector 40, and the coating is dried to form the first region 45 on the negative electrode current collector 40. Next, the second negative electrode mixture layer slurry is applied onto the first region 45, and the coating is dried to form the second region 46 on the first region 45, after which the first region 45 and the second region 46 are compressed. In this way, a negative electrode 12 is obtained in which a negative electrode mixture layer 41 containing the first region 45 and the second region 46 is formed on the negative electrode current collector 40.

[0055] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include olefin resins such as copolymers containing polyethylene, polypropylene, ethylene, and propylene, and cellulose. The separator 13 may have a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 13.

[0056] The present disclosure will be further illustrated below with reference to examples, but the present disclosure is not limited to these examples.

[0057] <Example 1> [Positive electrode] LiNiCoAlO ２ A lithium transition metal composite oxide represented by was used. 95 parts by mass of the above positive electrode active material, 3 parts by mass of acetylene black (AB), and 2 parts by mass of polyvinylidene fluoride (PVDF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and after the coating film was dried, the coating film was rolled using a rolling roller and cut to a predetermined electrode size to produce a positive electrode.

[0058] [Negative electrode] In order to form the first region 45 of the negative electrode mixture layer 41, graphite, carboxymethylcellulose (hereinafter referred to as CMC), and a dispersion of styrene-butadiene copolymer (hereinafter referred to as SBR) were mixed in a solid content mass ratio of 100:1:5, and an appropriate amount of water was added to prepare the first negative electrode mixture layer slurry for the first region 45.

[0059] To form the second region 46 of the negative electrode mixture layer 41, graphite, a silicon-containing material in which silicon particles are dispersed in the carbon phase (hereinafter referred to as SiC), CMC, and an SBR dispersion were mixed in a solid content mass ratio of 40:60:1:5, and an appropriate amount of water was added to prepare a second negative electrode mixture layer slurry for the second region 46.

[0060] Next, the first negative electrode mixture layer slurry was applied to both sides of the negative electrode current collector 40, which is made of copper foil, and the coating was dried to form a first region 45 on both sides of the negative electrode current collector 40. Then, the second negative electrode mixture layer slurry was applied onto the first region 45 formed on both sides of the negative electrode current collector 40, and the coating was dried to form a second region 46. Finally, the coating was rolled using a rolling roller and cut to a predetermined electrode size to produce a negative electrode 12 in which a negative electrode mixture layer 41 including the first region 45 and the second region 46 was formed on both sides of the negative electrode current collector 40.

[0061] The silicon content was measured from the obtained negative electrode 12. The ratio A of the mass of silicon in the first region 45 to the mass of the first region 45 was 0 mass%, the ratio B of the mass of silicon in the second region 46 to the mass of the second region 46 was 30 mass%, and the ratio of the mass of silicon in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 (overall ratio) was 15 mass%. The measurement method was as described above.

[0062] [Non-aqueous electrolyte] A non-aqueous electrolyte was prepared by mixing ethylene carbonate (EC), fluorinated ethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 10:10:5:75, adding 4% by mass of vinylene carbonate (VC), and dissolving LiPF6 at a rate of 1.35 mol / L.

[0063] [Test Cell] A positive electrode lead was attached to the positive electrode 11 and a negative electrode lead to the negative electrode 12. An electrode body 14 was fabricated by spirally winding the positive electrode 11 and the negative electrode 12 with a separator 13 in between. A single-layer polypropylene separator was used for the separator 13. The fabricated electrode body 14 was inserted into an outer casing made of aluminum laminate sheet, and the opening of the outer casing was sealed to create a test cell.

[0064] <Example 2> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersion were mixed in a solid content mass ratio of 90:10:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersion were mixed in a solid content mass ratio of 50:50:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 5 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 25 mass%, and the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 (overall ratio) was 15 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0065] <Example 3> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 80:20:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 60:40:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 10 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 20 mass%, and the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 (overall ratio) was 15 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0066] <Example 4> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 76:24:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 64:36:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 12 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 18 mass%, and the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 (overall ratio) was 15 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0067] <Comparative Example 1> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersion were mixed in a solid content mass ratio of 70:30:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersion were mixed in a solid content mass ratio of 70:30:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 15 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 15 mass%, and the ratio (overall ratio) of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 was 15 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0068] [Evaluation of Charge / Discharge Cycle Characteristics] The test cells of Examples 1-4 and Comparative Example 1 were charged with a constant current of 0.5 It at a temperature of 25°C until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current value reached 0.02 It. After that, constant current discharge was performed with a constant current of 0.5 It until the voltage reached 2.5V. This charge / discharge cycle was considered one cycle, and 500 cycles were performed. The capacity retention rate in the charge / discharge cycle was then calculated using the following formula. The results are summarized in Table 1. In Table 1, the capacity retention rate of Comparative Example 1 is set as the baseline (100), and the capacity retention rates of Examples 1-4 are shown as relative values. Capacity Retention Rate = (Discharge Capacity at Cycle 500 / Discharge Capacity at Cycle 1) × 100

[0069]

[0070] As shown in Table 1, all of Examples 1 to 4 showed a higher capacity retention rate than Comparative Example 1. By using a negative electrode in which ratio A is 0% by mass or more and less than 15% by mass (60% by mass or less), and ratio B is 15% by mass or more, as in Examples 1 to 4, the charge-discharge cycle characteristics can be improved compared to the Comparative Example in which ratios A and B are the same.

[0071] <Example 5> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersion were mixed in a solid content mass ratio of 90:10:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersion were mixed in a solid content mass ratio of 10:90:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 5 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 45 mass%, and the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 (overall ratio) was 25 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0072] <Example 6> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 80:20:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 20:80:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 10 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 40 mass%, and the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 (overall ratio) was 25 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0073] <Comparative Example 2> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersion were mixed in a solid content mass ratio of 50:50:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersion were mixed in a solid content mass ratio of 50:50:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 25 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 25 mass%, and the ratio (overall ratio) of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 was 25 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0074] The test cells of Examples 5-6 and Comparative Example 2 were subjected to charge-discharge cycles under the same conditions as described above, and the capacity retention rate was determined. The results are summarized in Table 2. In Table 2, the capacity retention rate of Examples 5-6 is shown as a relative value, with the capacity retention rate of Comparative Example 2 set as the baseline (100).

[0075]

[0076] As shown in Table 2, Examples 5 and 6 all showed higher capacity retention rates than Comparative Example 2. By using a negative electrode in which ratio A is 0% by mass or more and less than 15% by mass (60% by mass or less), and ratio B is 15% by mass or more, as in Examples 5 and 6, the charge-discharge cycle characteristics can be improved compared to Comparative Example 2, in which ratios A and B are the same.

[0077] <Example 7> In preparing the second negative electrode mixture layer slurry for the second region 46, the negative electrode 12 was prepared under the same conditions as in Example 1, except that graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 60:40:1:5. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 0 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 20 mass%, and the ratio (overall ratio) of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 was 10 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0078] <Example 8> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 90:10:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 70:30:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 5 mass%, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 15 mass%, and the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 (overall ratio) was 10 mass%. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0079] <Comparative Example 3> In the preparation of the first negative electrode mixture layer slurry for the first region 45, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 80:20:1:5, and an appropriate amount of water was added to adjust the consistency. In the preparation of the second negative electrode mixture layer slurry for the second region 46, graphite, SiC, CMC, and SBR dispersions were mixed in a solid content mass ratio of 80:20:1:5. Otherwise, the negative electrode 12 was prepared under the same conditions as in Example 1. In the obtained negative electrode 12, the ratio A of the mass of silicon element in the first region 45 to the mass of the first region 45 was 10% by mass, the ratio B of the mass of silicon element in the second region 46 to the mass of the second region 46 was 10% by mass, and the ratio of the mass of silicon element in the negative electrode mixture layer 41 to the mass of the negative electrode mixture layer 41 (overall ratio) was 10% by mass. Using this negative electrode 12, a test cell was prepared in the same manner as in Example 1.

[0080] The test cells of Examples 7-8 and Comparative Example 3 were subjected to charge-discharge cycles under the same conditions as described above, and the capacity retention rate was determined. The results are summarized in Table 3. In Table 3, the capacity retention rate of Examples 7-8 is shown as a relative value, with the capacity retention rate of Comparative Example 3 set as the baseline (100).

[0081]

[0082] As shown in Table 3, Examples 7 and 8 all showed higher capacity retention rates than Comparative Example 3. By using a negative electrode in which ratio A is 0% by mass or more and less than 15% by mass (60% by mass or less), and ratio B is 15% by mass or more, as in Examples 7 and 8, the charge-discharge cycle characteristics can be improved compared to Comparative Example 3, in which ratios A and B are the same.

[0083] (Note) The present disclosure is further illustrated by the following embodiments. Configuration 1: A negative electrode for a secondary battery comprising a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer has, in the thickness direction, a first region disposed on the negative electrode current collector side and a second region disposed on the opposite side from the negative electrode current collector side, the second region having a negative electrode active material comprising graphite and silicon-containing material, the ratio A of the mass of silicon elements in the first region to the mass of the first region being 0% by mass or more and 60% by mass or less, and the ratio B of the mass of silicon elements in the second region to the mass of the second region being 15% by mass or more and less than 100% by mass, wherein ratio B is greater than ratio A. Configuration 2: The negative electrode for a secondary battery according to Configuration 1, wherein when ratio A is greater than 0% by mass and 60% by mass or less, the ratio of ratio B to ratio A (B / A) is 1.5 or more. Configuration 3: A negative electrode for a secondary battery comprising a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer has, in the thickness direction, a first region disposed on the negative electrode current collector side and a second region disposed on the opposite side from the negative electrode current collector side, the second region having a negative electrode active material containing graphite and silicon-containing material, where the ratio A of the mass of silicon elements in the first region to the mass of the first region is 0% by mass or more and less than 15% by mass, the ratio B of the mass of silicon elements in the second region to the mass of the second region is 15% by mass or more and less than 100% by mass, where the ratio A is 15% by mass or more, the ratio of ratio B to ratio A (B / A) is 1.4 or more, and the ratio B is less than 100% by mass. Configuration 4: A negative electrode for a secondary battery according to any one of Configurations 1 to 3, wherein the ratio B is 20% by mass or more and less than 50% by mass. Configuration 5: A negative electrode for a secondary battery according to any one of Configurations 1 to 4, wherein the ratio A is 0% by mass or more and 10% by mass or less. Configuration 6: A negative electrode for a secondary battery according to any one of Configurations 1 to 5, wherein the ratio of the mass of silicon element in the negative electrode mixture layer to the mass of the negative electrode mixture layer is 10% by mass or more. Configuration 7: A negative electrode for a secondary battery according to any one of Claims 1 to 6, wherein the ratio of the mass of silicon element in the negative electrode mixture layer to the mass of the negative electrode mixture layer is 25% by mass or less.Configuration 8: The negative electrode for a secondary battery according to any one of Configurations 1 to 7, wherein the silicon-containing material comprises an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. Configuration 9: The negative electrode for a secondary battery according to Configuration 8, wherein the ratio of the mass of the silicon phase to the mass of the silicon-containing material in the negative electrode mixture layer is 30% by mass or more and 60% by mass or less. Configuration 10: The negative electrode for a secondary battery according to Configuration 8 or 9, wherein the average particle size of the silicon phase is 150 nm or less. Configuration 11: A secondary battery comprising the negative electrode for a secondary battery according to any one of Configurations 1 to 10.

[0084] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 40 Negative electrode current collector, 41 Negative electrode mixture layer, 45 First region, 46 Second region.

Claims

1. A negative electrode for a secondary battery comprising a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer has, in the thickness direction, a first region disposed on the side of the negative electrode current collector and a second region disposed on the opposite side from the negative electrode current collector, the second region having a negative electrode active material containing graphite and silicon-containing material, the ratio A of the mass of silicon elements in the first region to the mass of the first region being 0% by mass or more and 60% by mass or less, and the ratio B of the mass of silicon elements in the second region to the mass of the second region being 15% by mass or more and less than 100% by mass, the ratio B being greater than the ratio A.

2. When the ratio A is greater than 0% by mass and 60% by mass or less, the ratio of the ratio B to the ratio A (B / A) is 1.5 or more, the negative electrode for a secondary battery according to claim 1.

3. A negative electrode for a secondary battery, comprising a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer has, in the thickness direction, a first region disposed on the negative electrode current collector side and a second region disposed on the opposite side from the negative electrode current collector side, the second region having a negative electrode active material containing graphite and silicon-containing material, where the ratio A of the mass of silicon elements in the first region to the mass of the first region is 0% by mass or more and less than 15% by mass, the ratio B of the mass of silicon elements in the second region to the mass of the second region is 15% by mass or more and less than 100% by mass, where the ratio A is 15% by mass or more, the ratio of ratio B to ratio A (B / A) is 1.4 or more, and the ratio B is less than 100% by mass.

4. The negative electrode for a secondary battery according to any one of claims 1 to 3, wherein the ratio B is 20% by mass or more and less than 50% by mass.

5. The negative electrode for a secondary battery according to any one of claims 1 to 3, wherein the ratio A is 0% by mass or more and 10% by mass or less.

6. The negative electrode for a secondary battery according to any one of claims 1 to 3, wherein the ratio of the mass of silicon element in the negative electrode mixture layer to the mass of the negative electrode mixture layer is 10% by mass or more.

7. The negative electrode for a secondary battery according to any one of claims 1 to 3, wherein the ratio of the mass of silicon element in the negative electrode mixture layer to the mass of the negative electrode mixture layer is 25% by mass or less.

8. The negative electrode for a secondary battery according to any one of claims 1 to 3, wherein the silicon-containing material comprises an ionic conductive phase and a silicon phase dispersed in the ionic conductive phase.

9. The negative electrode for a secondary battery according to claim 8, wherein the ratio of the mass of the silicon phase to the mass of the silicon-containing material in the negative electrode mixture layer is 30% by mass or more and 60% by mass or less.

10. The negative electrode for a secondary battery according to claim 8, wherein the average particle size of the silicon phase is 150 nm or less.

11. A secondary battery comprising a negative electrode for a secondary battery as described in any one of claims 1 to 3.

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