Negative electrode for secondary battery, secondary battery, and method for manufacturing negative electrode for secondary battery

Abstract

A secondary battery negative electrode according to one aspect of the present disclosure is characterized by comprising a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, and is characterized in that: the negative electrode mixture layer contains a negative electrode active material including a carbon material and a Si-based material; a pore diameter distribution of the negative electrode mixture layer as measured by a mercury press-in method has two peak values R1 and R2; the peak value R1 is in the range of 0.5-1.5 μm; the peak value R2 is in the range of 2-10 μm; an average particle diameter of the Si-based material is 4 μm or more; and the contained amount of the Si-based material with respect to the total amount of the negative electrode active material is 30 mass% or more.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode for a secondary battery, a secondary battery, and a method for manufacturing a negative electrode for a secondary battery.

Background Art

[0002] Currently, Si-based materials are attracting attention as materials that can achieve a higher capacity of batteries. Si-based materials are materials that can electrochemically occlude and release lithium ions, and can be charged and discharged with a much larger capacity compared to carbon materials such as graphite.

[0003] For example, Patent Document 1 discloses a negative electrode active material for a lithium ion secondary battery containing a Si-based material represented by SiOx (0 < x < 2) and a carbon material, wherein the negative electrode active material for a lithium ion secondary battery has voids inside.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, while Si-based materials can achieve a higher capacity of secondary batteries, problems such as the expansion of the negative electrode and further deterioration of the charge-discharge cycle characteristics occur. Even in the technology of Patent Document 1, although the deterioration of the charge-discharge cycle characteristics can be suppressed, further improvement is desired.

Means for Solving the Problems

[0007] A negative electrode for a secondary battery according to one aspect of the present disclosure includes a negative electrode current collector and a negative electrode composite layer disposed on the negative electrode current collector. The negative electrode composite layer has a negative electrode active material including a carbon material and a Si-based material. The pore size distribution of the negative electrode composite layer measured by mercury porosimetry has two peak values ​​R1 and R2, the peak value R1 being 0.5 μm or more and 1.5 μm or less, and the peak value R2 being 2 μm or more and 10 μm or less. The Si-based material has an average particle size of 4 μm or more, and a content of the Si-based material is 30 mass% or more relative to the total amount of the negative electrode active material.

[0008] A secondary battery according to one aspect of the present disclosure is characterized by including the negative electrode for secondary batteries described above.

[0009] A method for manufacturing a negative electrode for a secondary battery according to one aspect of the present disclosure includes a first step of applying a negative electrode paste containing a carbon material, a negative electrode active material containing a Si-based material, and a pore-forming material to a negative electrode current collector to form a coating film, and then rolling the coating film; and a second step of heat-treating the coating film after the first step to decompose and vaporize the pore-forming material, thereby forming a negative electrode composite layer, wherein the pore size distribution of the negative electrode composite layer measured by mercury porosimetry has two peak values ​​R1 and R2, wherein the peak value R1 is 0.5 μm or more and 1.5 μm or less, and the peak value R2 is 2 μm or more and 10 μm or less, the average particle size of the Si-based material is 4 μm or more, and the content of the Si-based material is 30 mass% or more with respect to the total amount of the negative electrode active material. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a negative electrode for a secondary battery, a secondary battery, and a method for manufacturing a negative electrode for a secondary battery, which are capable of increasing capacity, suppressing swelling of the negative electrode, and further suppressing deterioration of charge-discharge cycle characteristics. [Brief explanation of the drawings]

[0011] [Figure 1]1 is a cross-sectional view of a secondary battery according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0012] A negative electrode for a secondary battery according to one embodiment of the present disclosure includes a negative electrode current collector and a negative electrode composite layer disposed on the negative electrode current collector. The negative electrode composite layer contains a negative electrode active material including a carbon material and a Si-based material. The pore size distribution of the negative electrode composite layer measured by mercury porosimetry has two peak values ​​R1 and R2, where R1 is between 0.5 μm and 1.5 μm, and R2 is between 2 μm and 10 μm. The Si-based material has an average particle size of 4 μm or greater, and the content of the Si-based material is 30 mass% or greater with respect to the total amount of the negative electrode active material. A negative electrode composite layer having the two peak values ​​(R1, R2) in the pore size distribution that satisfy the above ranges is presumably capable of improving electrolyte permeability and absorbing the expansion and contraction of the Si-based material during charge and discharge, thereby enabling a high battery capacity and suppressing negative electrode swelling. However, even if R1 and R2 are set within appropriate ranges, if the average particle size of the Si-based material is too small, a side reaction between the Si-based material and the electrolyte may occur, preventing a sufficient reduction in the charge-discharge cycle performance. Furthermore, even if R1 and R2 are set within appropriate ranges, if the content of the Si-based material is too low, the battery capacity cannot be increased. Therefore, only by setting all of the parameters (i.e., the two peak values ​​R1 and R2 in the pore size distribution of the negative electrode composite layer measured by mercury porosimetry, the average particle size and content of the Si-based material) within the appropriate ranges can the capacity be increased, the swelling of the negative electrode be suppressed, and the charge-discharge cycle performance be reduced.

[0013] Hereinafter, embodiments of the negative electrode active material for a secondary battery and the secondary battery according to the present disclosure will be described in detail with reference to the drawings.

[0014] An example of a secondary battery according to one aspect of the present disclosure will be described below.

[0015] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment. The secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, an electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing member 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, an electrode assembly of another shape may be used, such as a laminated electrode assembly formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include a metal case having a cylindrical, prismatic, coin-like, or button-like shape, and a resin case formed by laminating a resin sheet (a so-called laminated type).

[0016] The electrolyte may be an aqueous electrolyte, but is preferably a nonaqueous electrolyte containing a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. Examples of the nonaqueous solvent include esters, ethers, nitriles, amides, and mixtures of two or more of these. The nonaqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are replaced with halogen atoms such as fluorine. The electrolyte salt may be a lithium salt such as LiPF6.

[0017] Case body 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between case body 16 and sealing body 17 to ensure airtightness inside the battery. Case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports sealing body 17. Protruding portion 22 is preferably formed in an annular shape along the circumferential direction of case body 16, and supports sealing body 17 on its upper surface.

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

[0019] In the secondary battery 10 shown in Fig. 1, a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in an insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside an insulating plate 19 and extends toward the bottom side of the case body 16. The positive electrode lead 20 is connected to the underside of a filter 23, which is the bottom plate of the sealing body 17, by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the case body 16 by welding or the like, and the case body 16 serves as the negative electrode terminal.

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

[0021] [Positive electrode] The positive electrode 11 has a positive electrode current collector and a positive electrode composite layer disposed on the positive electrode current collector. The positive electrode current collector can be a foil of a metal such as aluminum that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on its surface. The positive electrode composite layer is composed of, for example, a positive electrode active material, a binder, a conductive material, and the like. The positive electrode 11 can be produced, for example, by applying a positive electrode paste containing a positive electrode active material, a binder, a conductive material, and the like to the surface of the positive electrode current collector, drying the coating, and then rolling the coating to form a positive electrode composite layer on both sides of the positive electrode current collector.

[0022] Examples of conductive materials contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, graphite, carbon nanotubes, etc. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, styrene-butadiene rubber (SBR), cellulose derivatives such as carboxymethyl cellulose (CMC) or a salt thereof, and polyethylene oxide (PEO).

[0023] Examples of the positive electrode active material include lithium transition metal composite oxides. Metal elements contained in the 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, and W. Among these, it is preferable to contain at least one of Ni, Co, and Mn. An example of a suitable lithium transition metal composite oxide is a composite oxide represented by the general formula LiMO2 (wherein M is Ni and X, X is a metal element other than Ni, and the proportion of Ni is 50 mol% to 95 mol% based on the total number of moles of the metal elements excluding Li). Examples of X in the formula include Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W.

[0024] [Negative electrode] The negative electrode 12 has a negative electrode current collector and a negative electrode composite layer disposed on the negative electrode current collector. The negative electrode current collector can be a foil of a metal, such as copper, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on its surface. The negative electrode composite layer contains a negative electrode active material and may also contain a binder, a conductive material, etc. The negative electrode active material contains a carbon material and a Si-based material. In addition to the carbon material and the Si-based material, the negative electrode active material may also contain a substance that can reversibly absorb and release lithium ions. Examples of the binder and conductive material include those similar to those used for the positive electrode 11.

[0025] The pore size distribution of the negative electrode mixture layer measured by mercury porosimetry has two peak values ​​R1 and R2. Peak value R1 is 0.5 μm or more and 1.5 μm or less, and peak value R2 is 2 μm or more and 10 μm or less.

[0026] In mercury intrusion porosimetry, mercury is forced into the pores of a solid sample by pressurizing the mercury, and the diameter and volume of the pores are calculated from the pressure applied to the mercury and the amount of mercury forced into the pores. When mercury applied with pressure P can penetrate a pore of diameter D, the pore diameter D can be calculated from the pressure P, the contact angle θ of the mercury, and the surface tension σ of the mercury according to the following formula. The pore volume is also calculated from the amount of mercury forced into the pores. In this disclosure, the pore diameter refers to the diameter of the pore. -4σcosθ=PD

[0027] The pore size distribution measured by mercury intrusion porosimetry is expressed as the log differential pore volume (cm 3 / g) against the average pore diameter (μm) in each measurement section, with the horizontal axis representing the pore diameter (μm) and the vertical axis representing the Log differential pore volume (cm 3 / g). In the present disclosure, the peak value in the pore size distribution means the pore size at the apex of the peak in the pore size distribution.

[0028] The peak value R1 may be 0.5 μm or more and 1.5 μm or less, but preferably, for example, 0.8 μm or more and 1.2 μm or less. If the peak value R1 is less than 0.5 μm, many small voids exist between particles in the negative electrode composite layer, reducing the permeability of the electrolyte and preventing the secondary battery from achieving a high capacity. If the peak value R1 is more than 1.5 μm (less than 2 μm), the density of the negative electrode active material decreases, preventing the secondary battery from achieving a high capacity. If the peak value R2 is 2 μm or more and 10 μm or less, but preferably, for example, 4 μm or more and 8 μm or less. If the peak value R2 is less than 2 μm, there are few voids in the negative electrode composite layer that can absorb the expansion and contraction of the Si-based material during charge and discharge, preventing the expansion of the negative electrode. If the peak value R2 is 10 μm or more, many large voids exist in the negative electrode composite layer, reducing the density of the negative electrode active material and preventing the secondary battery from achieving a high capacity.

[0029] The pore size distribution measurement by mercury porosimetry is performed on the negative electrode composite layer before initial charging. For example, a measurement sample obtained by punching a secondary battery negative electrode into a predetermined shape before initial charging can be used to measure the pore size distribution of the negative electrode composite layer of the measurement sample by mercury porosimetry. Note that the measurement sample only needs to have at least the negative electrode composite layer on its surface, and may also have other components such as a negative electrode current collector.

[0030] Measurement of pore size distribution by mercury intrusion porosimetry can be performed using an apparatus such as the Autopore IV9500 series manufactured by Micromeltix. During measurement, a measurement sample is sealed in a sample container under an inert atmosphere, and mercury is injected into the sample container to apply pressure to the mercury. The pressure applied to the mercury is adjusted appropriately depending on the pore size that the measurement sample may have, and is not particularly limited. For example, it is preferable to perform measurement while varying the pressure from 0.5 psi (3.4 kPa) to 60,000 psi (413,400 kPa) in order to be able to measure pore sizes over a wide range.

[0031] As mentioned above, in order to increase the capacity of a secondary battery, suppress swelling of the negative electrode, and further suppress deterioration of charge-discharge cycle characteristics, it is necessary that the peak value R1 is 0.5 μm or more and 1.5 μm or less, and the peak value R2 is 2 μm or more and 10 μm or less, and that the content and average particle size of the Si-based material also fall within appropriate ranges. The Si-based material and carbon material contained in the negative electrode active material will be described below.

[0032] The Si-based material contained in the negative electrode active material is not particularly limited as long as it can reversibly absorb and release ions such as lithium ions, and examples thereof include Si particles, Si-containing alloy particles, Si compound particles, etc. These may be used alone or in combination of two or more.

[0033] Si particles can be obtained by a vapor phase method or by pulverizing silicon chips, but any method for producing them can be used. Examples of Si-containing alloy particles include alloys containing Si and a metal selected from alkali metals, alkaline earth metals, transition metals, rare earth metals, or combinations thereof. Examples of Si compound particles include Si compound particles having a silicate phase and Si particles dispersed therein, Si compound particles having a silicon oxide phase and Si particles dispersed therein, and Si compound particles having a carbon phase and Si particles dispersed therein. Among these, Si compound particles having a silicate phase and Si particles dispersed therein and Si compound particles having a carbon phase and Si particles dispersed therein are preferred in terms of increasing the capacity of secondary batteries and suppressing deterioration in charge-discharge cycle performance.

[0034] The silicate phase preferably contains at least one element selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium, for example, from the viewpoint of high lithium ion conductivity and the like. Among them, the silicate phase is preferably a silicate phase containing lithium (hereinafter sometimes referred to as a lithium silicate phase) from the viewpoint of high lithium ion conductivity and the like.

[0035] The lithium silicate phase is represented by, for example, the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., z preferably satisfies the relationship of 0 < z < 1, and z = 1 / 2 is more preferable.

[0036] Si compound particles in which Si particles are dispersed in the silicon oxide phase are represented by, for example, the general formula SiO x (a range of 0 < x < 2 is preferable, and a range of 0.5 ≦ x ≦ 1.6 is more preferable). Si compound particles in which Si particles are dispersed in the carbon phase are represented by, for example, the general formula SixC1y (a range of 0 < x ≦ 1 and 0 < y ≦ 1 is preferable, and a range of 0.3 ≦ x ≦ 0.45 and 0.7 ≦ y ≦ 0.55 is more preferable).

[0037] It is preferable that a conductive film made of a material with high conductivity is formed on the particle surface of the Si-based material. Examples of the conductive film include a carbon film, a metal film, and a metal compound film, etc. From the viewpoint of electrochemical stability and the like, a carbon film is preferable. The carbon film can be formed by, for example, a CVD method using acetylene, methane, etc., or a method of mixing coal pitch, petroleum pitch, phenol resin, etc. with a silicon-based active material and performing heat treatment. Also, a conductive film may be formed by fixing a conductive filler such as carbon black to the particle surface of the Si-based material using a binder.

[0038] The content of the Si-based material may be 30% by mass or more relative to the total amount of the negative electrode active material in order to increase the capacity of the secondary battery, etc. Furthermore, in order to increase the capacity of the secondary battery and further suppress swelling of the negative electrode, the content of the Si-based material is preferably 30% by mass or more and 60% by mass or less, and more preferably 35% by mass or more and 55% by mass or less, relative to the total amount of the negative electrode active material.

[0039] The average particle size of the Si-based material may be, for example, 4 μm or more in order to suppress deterioration of charge-discharge cycle characteristics due to side reactions with the electrolyte, etc. Furthermore, in order to suppress deterioration of charge-discharge cycle characteristics and further suppress swelling of the negative electrode, the average particle size of the Si-based material is, for example, preferably 4 μm or more and 12 μm or less, and more preferably 6 μm or more and 10 μm or less.

[0040] Examples of the carbon material contained in the negative electrode active material include conventionally known carbon materials used as negative electrode active materials for secondary batteries. However, in terms of further suppressing the deterioration of charge-discharge cycle characteristics, graphite such as natural graphite, such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite, such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), is preferred.

[0041] The average particle size of the carbon material is preferably 10 μm or more and 25 μm or less, and more preferably 12 μm or more and 20 μm or less, for example, in order to further suppress swelling of the negative electrode.

[0042] The average particle size of each material is the volume average particle size D50 at which the volume integrated value becomes 50% in the particle size distribution obtained by the laser diffraction scattering method.

[0043] The content of the carbon material is, for example, preferably 40% by mass or more and 70% by mass or less, and more preferably 45% by mass or more and 65% by mass or less, relative to the total amount of the negative electrode active material.

[0044] An example of a method for manufacturing the negative electrode 12 will now be described. The negative electrode 12 comprises a first step of applying a negative electrode paste containing a negative electrode active material including a carbon material and a Si-based material, a pore-forming material, and a binder, etc., to a negative electrode current collector to form a coating film, and then rolling the coating film; and a second step of, after the first step, heat-treating the coating film to decompose and vaporize the pore-forming material, thereby forming a negative electrode composite layer. The average particle size and content of the Si-based material and the carbon material are as described above, and therefore will not be described here.

[0045] Heating the coating decomposes and vaporizes (e.g., sublimates) the pore-forming material, removing it from the coating, forming not only small voids but also relatively large voids between particles in the negative electrode composite layer. Thus, by heat-treating the negative electrode paste containing the pore-forming material, the pore distribution of the negative electrode composite layer has a peak value R1 of 0.5 μm to 1.5 μm and a peak value R2 of 2 μm to 10 μm. On the other hand, when a negative electrode paste containing no pore-forming material is used, only small voids are formed between particles in the resulting negative electrode composite layer. Therefore, when a negative electrode paste containing no pore-forming material is used, the pore distribution of the negative electrode composite layer typically has only a peak value R1 of 0.5 μm to 1.5 μm.

[0046] The heat treatment temperature is not particularly limited as long as it is a temperature at which the pore-forming material is decomposed and vaporized. The heat treatment time may be, for example, 5 hours or more, as long as it is long enough to decompose and vaporize the pore-forming material in the coating film.

[0047] Known pore-forming materials can be used. Examples of pore-forming materials include metal oxalates, camphor, and naphthalene. Dicarboxylic acids such as fumaric acid, malonic acid, and malic acid may also be used. The average particle size of the pore-forming material is preferably 2 μm or more and 10 μm or less. By setting the average particle size of the pore-forming material within the above range, it becomes easy to control the peak value R2 in the pore size distribution of the negative electrode composite layer to a range of 2 μm or more and 10 μm or less. When using a pore-forming material with an average particle size outside the above range, the peak values ​​R2 and R1 in the pore size distribution of the negative electrode composite layer can be controlled by adjusting the average particle size of each material used in the negative electrode active material, adjusting the viscosity of the negative electrode paste by adding a solvent, adjusting the heating time and temperature, adjusting the linear pressure during coating film rolling, and the like.

[0048] When preparing a negative electrode paste, raw materials such as a negative electrode active material, a pore-forming material, and a binder can be mixed using, for example, a cutter mill, a pin mill, a bead mill, a microparticle compounding device (a device in which shear force is generated between a specially shaped rotor that rotates at high speed inside a tank and an impact plate), a granulator, or a kneading machine such as a twin-screw extrusion kneader or a planetary mixer.

[0049] The negative electrode paste can be applied using, for example, a slit die coater, a reverse roll coater, a lip coater, a blade coater, a knife coater, a gravure coater, or a dip coater.

[0050] When the negative electrode paste is applied to the negative electrode current collector to obtain a coating film, it is preferable to heat-dry the coating film. The temperature for heat-drying is preferably a temperature at which the pore-forming material does not decompose or vaporize, but heat-drying may cause a portion of the pore-forming material to decompose or vaporize.

[0051] The coating film may be rolled, for example, by a roll press machine at a predetermined line pressure several times until the coating film reaches a predetermined thickness.

[0052] [Separator] For example, a porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator. [Example]

[0053] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0054] Example 1 [Preparation of negative electrode] Graphite particles with an average particle size of 17 μm and a Si-based material with an average particle size of 8 μm, in which Si particles were dispersed within the carbon phase, were mixed in a mass ratio of 50:50. This mixture was used as the negative electrode active material. The mass ratio of the negative electrode active material: carboxymethyl cellulose (CMC): styrene-butadiene copolymer rubber (SBR): multi-walled carbon nanotubes: fumaric acid (average particle size 6 μm) was then mixed in a ratio of 100:1:1:1:12.5, and the mixture was mixed with water to prepare a negative electrode paste. This negative electrode paste was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried and then rolled using a rolling roller. The coating was then heat-treated at 200°C for 5 hours to produce a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode current collector.

[0055] The pore size distribution of the negative electrode mixture layer of the obtained negative electrode was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0056] [Preparation of positive electrode] LiCo 1 / 3 Mn 1 / 3 Ni 1 / 3The lithium transition metal composite oxide (O2), acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 98:1:1, and then N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode paste. This negative electrode paste was applied to both sides of an aluminum foil, the coating was dried, and then the coating was rolled with a rolling roller to produce a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode current collector.

[0057] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 1:3.

[0058] [Test cell construction] The positive electrode and the negative electrode were stacked facing each other with a polyolefin separator interposed therebetween and wound up to prepare an electrode assembly, which was then housed in a cylindrical battery case body with a bottom, and the nonaqueous electrolyte solution was poured into the battery case body. After that, the opening of the battery case body was sealed with a gasket and a sealing member to prepare a test cell.

[0059] <Example 2> A negative electrode was prepared in the same manner as in Example 1, except that fumaric acid having an average particle size of 2 μm was used, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 2, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, two peak values ​​R1 and R2 were observed, with the peak value R1 being 1 μm and the peak value R2 being 2 μm.

[0060] Example 3 A negative electrode was prepared in the same manner as in Example 1, except that fumaric acid having an average particle size of 10 μm was used, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 3, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, two peak values ​​R1 and R2 were observed, with the peak value R1 being 1 μm and the peak value R2 being 10 μm.

[0061] Example 4 A negative electrode was prepared in the same manner as in Example 1, except that graphite particles having an average particle size of 17 μm and a Si-based material having Si particles dispersed in a carbon phase and an average particle size of 8 μm were mixed in a mass ratio of 70:30, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 4, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0062] <Example 5> A negative electrode was prepared in the same manner as in Example 1, except that graphite particles having an average particle size of 17 μm and a Si-based material having Si particles dispersed in a carbon phase and an average particle size of 8 μm were mixed in a mass ratio of 40:60, and a test cell was prepared in the same manner as in Example 1, except that this negative electrode was used. In the negative electrode of Example 5, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0063] Example 6 A negative electrode was prepared in the same manner as in Example 1, except that a Si-based material having Si particles dispersed in a carbon phase and an average particle size of 4 μm was used, and a test cell was also prepared in the same manner as in Example 1. In the negative electrode of Example 6, the pore size distribution of the negative electrode composite layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0064] Example 7 A negative electrode was produced in the same manner as in Example 1, except that a Si-based material with Si particles dispersed in a carbon phase and an average particle size of 12 μm was used, and a test cell was produced in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 7, the pore size distribution of the negative electrode composite layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0065] Example 8 A negative electrode was prepared in the same manner as in Example 1, except that graphite particles having an average particle size of 10 μm were used, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 8, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, two peak values ​​R1 and R2 were observed, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0066] Example 9 A negative electrode was prepared in the same manner as in Example 1, except that graphite particles having an average particle size of 25 μm were used, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 8, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, two peak values ​​R1 and R2 were observed, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0067] Example 10 A negative electrode was prepared in the same manner as in Example 1, except that graphite particles having an average particle size of 17 μm and a Si-based material having Si particles dispersed in a carbon phase and an average particle size of 8 μm were mixed in a mass ratio of 30:70, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 10, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0068] Example 11 A negative electrode was prepared in the same manner as in Example 1, except that a Si-based material having Si particles dispersed in a carbon phase and an average particle size of 14 μm was used, and a test cell was also prepared in the same manner as in Example 1. In the negative electrode of Example 11, the pore size distribution of the negative electrode composite layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0069] Example 12 A negative electrode was prepared in the same manner as in Example 1, except that graphite particles having an average particle size of 8 μm were used, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 12, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, two peak values ​​R1 and R2 were observed, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0070] Example 13 A negative electrode was prepared in the same manner as in Example 1, except that graphite particles having an average particle size of 40 μm were used, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Example 12, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, two peak values ​​R1 and R2 were observed, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0071] <Comparative Example 1> A negative electrode was prepared in the same manner as in Example 1, except that fumaric acid with an average particle size of 12 μm was used, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Comparative Example 1, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, two peak values ​​R1 and R2 were observed, with the peak value R1 being 1 μm and the peak value R2 being 12 μm.

[0072] <Comparative Example 2> A negative electrode was prepared in the same manner as in Example 1, except that fumaric acid having an average particle size of 1 μm was used, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Comparative Example 2, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, one peak value R1 was found, and the peak value R1 was 1 μm.

[0073] <Comparative Example 3> A negative electrode was prepared in the same manner as in Example 1 except that fumaric acid was not used, and a test cell was prepared in the same manner as in Example 1 except that the negative electrode was used. In the negative electrode of Comparative Example 3, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, one peak value R1 was shown, and the peak value R1 was 1 μm.

[0074] <Comparative Example 4> A negative electrode was prepared in the same manner as in Example 1, except that graphite particles having an average particle size of 17 μm and a Si-based material having Si particles dispersed in a carbon phase and an average particle size of 8 μm were mixed in a mass ratio of 80:20, and a test cell was prepared in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Comparative Example 4, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0075] <Comparative Example 5> A negative electrode was produced in the same manner as in Example 1, except that a Si-based material having Si particles dispersed in a carbon phase and an average particle size of 3 μm was used, and a test cell was produced in the same manner as in Example 1, except that the negative electrode was used. In the negative electrode of Comparative Example 5, the pore size distribution of the negative electrode composite layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 1 μm and the peak value R2 being 6 μm.

[0076] <Comparative Example 6> Except for increasing the compressive force used in rolling the coating film, a negative electrode was produced in the same manner as in Example 1, and a test cell was produced in the same manner as in Example 1, except for using this negative electrode. In the negative electrode of Comparative Example 6, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and the result showed two peak values ​​R1 and R2, with the peak value R1 being 0.3 μm and the peak value R2 being 6 μm.

[0077] <Comparative Example 7> Except for the fact that the compressive force used in rolling the coating film was reduced, a negative electrode was produced in the same manner as in Example 1, and a test cell was produced in the same manner as in Example 1, except for using this negative electrode. In the negative electrode of Comparative Example 7, the pore size distribution of the negative electrode mixture layer was measured by mercury porosimetry, and as a result, two peak values ​​R1 and R2 were observed, with the peak value R1 being 1.7 μm and the peak value R2 being 6 μm.

[0078] [Battery capacity evaluation] The test cells of each example and comparative example were charged at a constant current of 0.5 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.05 C. They were then discharged at a constant current of 0.2 C until the battery voltage reached 2.5 V. The discharge capacity at this time was taken as the battery capacity, and the results are summarized in Table 1.

[0079] [Charge / discharge cycle test] The test cells of each example and comparative example were charged at a constant current of 0.5 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.05 C. Thereafter, the cells were discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 50 times, and the capacity retention rate during the charge / discharge cycle was calculated using the following formula. The results are summarized in Table 1. Capacity retention rate (%) = (discharge capacity at 50th cycle / discharge capacity at 1st cycle) x 100

[0080] [Evaluation of negative electrode swelling rate] The test cells of each example and comparative example were charged at a constant current of 1 / 3 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.05 C. After charging, the test cells were disassembled, the negative electrodes were removed, and the thickness of the negative electrodes after charging was measured with a microgauge. The negative electrode swelling rate was calculated using the following formula, and the results are summarized in Table 1. Negative electrode swelling rate (%) = (negative electrode thickness after charging / negative electrode thickness at the time of negative electrode preparation) × 100

[0081] [Table 1]

[0082] As shown in Table 1, all of Examples 1 to 13 exhibited a battery capacity of 500 mAh or more, a capacity retention rate after 50 cycles of 95% or more, and a negative electrode swelling rate of less than 140%. On the other hand, Comparative Examples 1 to 7 exhibited a battery capacity of less than 500 mAh, a capacity retention rate after 50 cycles of less than 90%, or a negative electrode swelling rate of 140% or more. That is, in a negative electrode composite layer having a negative electrode active material containing a carbon material and a Si-based material, the pore size distribution measured by mercury porosimetry has two peak values ​​R1 and R2, the peak value R1 being 0.5 μm or more and 1.5 μm or less, and the peak value R2 being 2 μm or more and 10 μm or less, the average particle size of the Si-based material being 4 μm or more, and the content of the Si-based material being 30 mass% or more with respect to the total amount of the negative electrode active material. By using a negative electrode, it is possible to increase the capacity of the battery, suppress swelling of the negative electrode, and suppress deterioration of the charge-discharge cycle characteristics. [Explanation of symbols]

[0083] 10 secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 battery case, 16 case body, 17 sealing body, 18 insulating plate, 18, 19 insulating plates, 20 positive electrode lead, 21 negative electrode lead, 22 protruding portion, 23 filter, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket.

Claims

1. a negative electrode current collector; and a negative electrode mixture layer disposed on the negative electrode current collector, the negative electrode mixture layer has a negative electrode active material including a carbon material and a Si-based material, the pore size distribution of the negative electrode mixture layer measured by mercury porosimetry has two peak values ​​R1 and R2; The peak value R1 is 0.5 μm or more and 1.5 μm or less, and the peak value R2 is 2 μm or more and 10 μm or less, The negative electrode for a secondary battery, wherein the average particle size of the Si-based material is 4 μm or more, and the content of the Si-based material is 30 mass % or more with respect to the total amount of the negative electrode active material.

2. 2. The negative electrode for a secondary battery according to claim 1, wherein the Si-based material includes a Si compound having a silicate phase and Si particles dispersed in the silicate phase.

3. 3. The negative electrode for a secondary battery according to claim 1, wherein the Si-based material comprises a Si compound having a carbon phase and Si particles dispersed in the carbon phase.

4. 3. The negative electrode for a secondary battery according to claim 1, wherein the content of the Si-based material is 30% by mass or more and 60% by mass or less with respect to the total amount of the negative electrode active material.

5. 3. The negative electrode for a secondary battery according to claim 1, wherein the average particle size of the Si-based material is 4 μm or more and 12 μm or less.

6. The negative electrode for a secondary battery according to claim 1 , wherein the carbon material includes graphite.

7. 3. The negative electrode for a secondary battery according to claim 1, wherein the carbon material has an average particle size of 10 μm or more and 25 μm or less.

8. A secondary battery comprising the negative electrode for secondary batteries according to claim 1 or 2.

9. a first step of applying a negative electrode paste containing a carbon material, a negative electrode active material containing a Si-based material, and a pore-forming material to a negative electrode current collector to form a coating film, and then rolling the coating film; a second step of, after the first step, heat-treating the coating film to decompose and vaporize the pore-forming material, thereby forming a negative electrode composite layer; the pore size distribution of the negative electrode mixture layer measured by mercury porosimetry has two peak values ​​R1 and R2; The peak value R1 is 0.5 μm or more and 1.5 μm or less, and the peak value R2 is 2 μm or more and 10 μm or less, the Si-based material has an average particle size of 4 μm or more, and the content of the Si-based material is 30 mass % or more relative to the total amount of the negative electrode active material.

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