Battery electrodes and batteries

Abstract

This electrode for batteries is provided with: a core material; and a mixture layer which contains an active material and a binder, while being arranged on the surface of the core material. If the mixture layer is trisected in the thickness direction and the divided sections are defined as the first region, the second region and the third region sequentially from the core material side, the void fraction of the second region is higher than the void fraction of the first region.

Description

[Technical Field]

[0001] The present disclosure relates to an electrode for a battery and a battery including the electrode. [Background technology]

[0002] Electrodes for lithium-ion batteries and the like are generally manufactured by a wet process in which an electrode composite slurry containing an active material, a binder, and the like is applied to the surface of a metal foil core material, and the resulting coating is then dried and compressed (see, for example, Patent Document 1). In this case, migration of the binder from the core material side to the surface occurs during drying of the coating, resulting in an increase in the amount of binder near the surface compared to near the core material, which is likely to result in uneven distribution of the binder in the thickness direction. Furthermore, voids exist in the coating, for example, between active material particles. These voids decrease as the active material particles move when the coating is compressed, but because the coating is fixed to the core material, the composite material is less likely to move closer to the core material, leaving more voids, which is likely to result in uneven distribution of voids in the thickness direction.

[0003] In recent years, methods have been proposed for producing electrodes by rolling an electrode mixture into a sheet and then bonding the sheet to a core material (see, for example, Patent Documents 2 and 3). Patent Document 2 discloses a method in which the pressure applied between a pair of rolls that pressurize a laminate of a compressed powder layer and a core material is greater than the pressure applied between a pair of rolls that pressurize the powder of the electrode mixture to form a compressed powder layer, and the compressed powder layer is then bonded to the core material and densified. Patent Document 3 also discloses a method in which a granulated product made from a mixture of an active material, a thickener, a solvent, and a binder is formed into a sheet and the sheet is then placed on a core material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-056743 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-77560 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-138658 Summary of the Invention

[0005] The methods disclosed in Patent Documents 2 and 3 can eliminate or simplify the drying process of the composite layer, which is expected to improve the problems associated with the wet process. However, as with the wet process, the methods of Patent Documents 2 and 3 are prone to uneven distribution of porosity in the thickness direction of the composite layer, leaving much room for improvement. If a porosity distribution is formed in which the porosity increases in the region closer to the core of the composite layer, in other words, the porosity decreases closer to the surface of the composite layer, the permeability of the electrolyte solution into the composite layer will be poor, leading to, for example, a decrease in high-rate characteristics. Furthermore, the specific process conditions of Patent Documents 2 and 3 make it difficult to densify the composite layer.

[0006] The battery electrode according to the present disclosure is a battery electrode comprising a core material and a composite layer containing an active material and a binder and provided on the surface of the core material, wherein when the composite layer is divided into three equal parts in the thickness direction and defined as a first region, a second region, and a third region in order from the core material side, the porosity of the second region is higher than the porosity of the first region.

[0007] A battery according to the present disclosure includes the battery electrode and an electrolyte solution.

[0008] According to one aspect of the present disclosure, a battery electrode including a composite layer with good electrolyte permeability can be provided. Furthermore, a battery including the electrode according to the present disclosure has, for example, excellent high-rate characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of a battery according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a positive electrode according to an embodiment of the present invention. [Figure 3] FIG. 3 is a diagram showing the relationship between the density of the positive electrode mixture layer and the permeability of the nonaqueous electrolyte. [Figure 4]FIG. 4 is a diagram showing a method for manufacturing a positive electrode according to the first embodiment. [Figure 5] FIG. 5 is a diagram showing a method for manufacturing a positive electrode according to the second embodiment. [Figure 6] FIG. 6 is a diagram showing a method for manufacturing a positive electrode according to the third embodiment. [Figure 7] FIG. 7 is a diagram showing a method for manufacturing a positive electrode according to the fourth embodiment. [Figure 8] FIG. 8 is a diagram showing a method for manufacturing a positive electrode according to the fifth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of a battery electrode according to the present disclosure will be described in detail. The embodiments described below are merely examples, and the present disclosure is not limited to the following embodiments. Furthermore, the drawings referred to in the description of the embodiments are schematic, and the dimensional ratios of the components depicted in the drawings should be determined in consideration of the following description.

[0011] The battery electrode according to the present disclosure is suitable for use as an electrode for a non-aqueous electrolyte secondary battery such as a lithium ion battery, but can also be applied to a battery containing an aqueous electrolyte solution. Furthermore, the application is not limited to secondary batteries, and can also be applied to primary batteries. The following description will be given taking a non-aqueous electrolyte secondary battery and an electrode (particularly a positive electrode) for a non-aqueous electrolyte secondary battery as examples.

[0012] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte solution, and an outer can 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0013] The battery exterior body is not limited to a cylindrical exterior can, and may be, for example, a rectangular exterior can or an exterior body made of a laminate sheet including a metal layer and a resin layer. Also, the electrode body may be a laminated electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0014] The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous 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.

[0015] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all strip-shaped, long bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the longitudinal direction and the width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0016] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 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 internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0017] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0018] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are layered. 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 at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, 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 breaks, and gas is discharged from the opening of the cap 27.

[0019] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, with the positive electrode 11 being particularly described in detail below.

[0020] [Positive electrode] FIG. 2 is a cross-sectional view of a positive electrode 11 according to an embodiment. As shown in FIG. 2, the positive electrode 11 includes a positive electrode core material 30 and a positive electrode composite layer 31 provided on the surface of the positive electrode core material 30. The positive electrode core material 30 may be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on the surface. The positive electrode composite layer 31 contains a positive electrode active material, a binder, and a conductive material, and is preferably provided on both sides of the positive electrode core material 30. Examples of the conductive material include carbon materials such as carbon black, acetylene black, ketjen black, graphite, and carbon nanotubes. The content of the conductive material is preferably 0.01 to 5 mass% relative to the mass of the positive electrode composite layer 31. The conductive material may be used alone or may be pre-combined with an active material or the like.

[0021] The positive electrode mixture layer 31 is formed by bonding positive electrode mixture sheets 43, 43x, 53, which are produced by a manufacturing method described later, to the surface of the positive electrode core material 30. When the positive electrode mixture layer 31 is made up of the positive electrode mixture sheets 43, 43x, the positive electrode mixture layer 31 contains, for example, a fibrous binder as the binder. By using the fibrous binder, the positive electrode mixture 40 (see the drawings described later) can be easily bonded to the positive electrode core material 30. 4 In this specification, the term "positive electrode composite layer 31" can be read as "positive electrode composite sheet 43, 43x, 53."

[0022] The positive electrode mixture layer 31 is composed mainly of a positive electrode active material (the component with the highest mass percentage). The content of the positive electrode active material is preferably 85 to 99 mass %, and more preferably 90 to 98 mass %, relative to the mass of the positive electrode mixture layer 31. The volume-based median diameter (D50) of the positive electrode active material is, for example, 1 to 30 μm, and preferably 2 to 15 μm. The thickness of the positive electrode mixture layer 31 is, for example, 30 to 300 μm, preferably 30 to 120 μm, and more preferably 50 to 100 μm.

[0023] A lithium transition metal composite oxide is used as the positive electrode active material. Metal elements contained in the lithium transition metal composite oxide 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. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al.

[0024] The fibrous binder is made of, for example, a resin whose main component is polytetrafluoroethylene (PTFE), and is obtained by fibrillating PTFE particles. The content of the fibrous binder is preferably 0.05 to 5 mass% with respect to the mass of positive electrode mixture layer 31. Positive electrode mixture layer 31 may also contain a non-fibrillated (non-fibrous) binder. An example of a non-fibrillated binder is polyvinylidene fluoride (PVdF).

[0025] Voids exist in the positive electrode mixture layer 31. The voids formed inside the positive electrode mixture layer 31 are, for example, connected to the surface of the positive electrode mixture layer 31. These voids serve as paths for the electrolyte, improving the permeability of the electrolyte into the positive electrode mixture layer 31. In the positive electrode 11, when the positive electrode mixture layer 31 is divided into three equal parts in the thickness direction and defined as a first region 31a, a second region 31b, and a third region 31c in that order from the positive electrode core material 30 side, the porosity (b) of the second region 31b is higher than the porosity (a) of the first region 31a. Here, the porosity refers to the proportion of voids in the positive electrode mixture layer 31.

[0026] That is, in the positive electrode mixture layer 31, there are more voids in the center of the layer in the thickness direction than in the vicinity of the positive electrode core material 30. When a positive electrode is manufactured using a conventional wet process, there are more voids in the positive electrode mixture layer near the positive electrode core material and fewer voids as the distance from the core material increases. However, according to the manufacturing method described below, more voids can be formed in the second region 31b than in the first region 31a. In this case, the permeability of the electrolyte into the positive electrode mixture layer 31 is improved, and the high-rate characteristics of the battery are improved.

[0027] The porosity of the positive electrode mixture layer is measured by the following method. (1) Using an ion milling device (for example, IM4000PLUS manufactured by Hitachi High-Technologies Corporation), the cross section of the positive electrode composite layer is exposed. (2) A backscattered electron image of the cross section of the exposed positive electrode composite layer is taken using a scanning electron microscope (SEM). The magnification for taking the backscattered electron image is 1000 to 5000 times. (3) The SEM image of the cross section of the positive electrode composite layer was imported into a computer and color-coded into three colors based on contrast using image analysis software (for example, ImageJ manufactured by the National Institutes of Health), with intermediate colors representing voids. (4) A measurement target area is selected from the processed image, the total area of ​​voids in that area is found, and the proportion of voids in the measurement target area (porosity) is calculated.

[0028] As described above, the porosity of positive electrode composite layer 31 is higher in second region 31b than in first region 31a, and for example, the difference (ba) between the porosity (b) of second region 31b and the porosity (a) of first region 31a is 0.5% or more. On the other hand, it is preferable that the porosity difference (ba) is not too large, preferably 10% or less, and more preferably 5% or less.

[0029] The porosity (b) of the second region 31b may be higher than the porosity (c) of the third region 31c. For example, the difference (bc) between the porosity (b) and the porosity (c) is 0.5% or more. The difference (bc) in porosity is preferably not too large, preferably 10% or less, and more preferably 5% or less.

[0030] Furthermore, the difference (ac) between the porosity (a) of the first region 31a and the porosity (c) of the third region 31c is, for example, within a range of ±1%. The porosity (a) and the porosity (c) may be substantially the same. In one example of a suitable positive electrode mixture layer 31, the porosity (a) is approximately equal to the porosity (c) and less than the porosity (b), and the voids are not concentrated in a portion of the positive electrode mixture layer 31 but are present evenly throughout the layer. In this case, the electrolyte solution can easily permeate the entire positive electrode mixture layer 31.

[0031] The porosity (c) of the third region 31c may be higher than the porosity (b) of the second region 31b, and the porosity of the positive electrode composite layer 31 may be such that porosity (a) < porosity (b) < porosity (c). In this case, good electrolyte permeability is also obtained. For example, the difference (cb) between the porosity (c) and the porosity (b) is 0.5% or more. The difference (cb) in porosity is preferably 10% or less, and more preferably 5% or less.

[0032] The overall porosity of positive electrode mixture layer 31 is preferably 40% or less, and more preferably 30% or less, from the viewpoint of increasing capacity, etc. On the other hand, the overall porosity of positive electrode mixture layer 31 is preferably 5% or more, and more preferably 10% or more, from the viewpoint of improving the permeability of the electrolyte solution, etc.

[0033] In the positive electrode mixture layer 31, it is preferable that the binder content be substantially uniform throughout the layer or be higher on the positive electrode core material 30 side. In this case, the bonding strength of the positive electrode mixture layer 31 to the positive electrode core material 30 is improved, and the permeability of the electrolyte is further improved. Note that when a positive electrode is manufactured using a conventional, general wet process, binder migration occurs during drying of the coating, and the amount of binder increases near the surface compared to near the core material.

[0034] In one example of a suitable positive electrode composite layer 31, the ratio ((ac) × 100 / (a+b+c)) of the difference between the binder content (a) in the first region 31a and the binder content (c) in the third region 31c to the content (a+b+c) is within a range of ±5%. The contents (a) and (c) may be substantially the same.

[0035] The ratio ((ab) × 100 / (a+b+c)) of the difference (ab) between the content (a) and the content (b) of the binder in the second region 31b to the content (a+b+c) is within the range of ±5%. ( a+b+c )The ratio ((bc) × 100 / (a+b+c)) to the total amount of the binder is within a range of ±5%. That is, in one example of a suitable positive electrode mixture layer 31, the content (a) is approximately equal to the content (b) and approximately equal to the content (c), and the binder is not unevenly distributed in a part of positive electrode mixture layer 31 but is evenly distributed throughout the entire layer.

[0036] In another example of a suitable positive electrode composite layer 31, the binder content increases in the order of third region 31c, second region 31b, and first region 31a (content (c)<content (b)<content (a)). However, it is preferable that the difference in content among the regions is not too large, and each of the above ratios ((ac)×100 / (a+b+c)) and ((bc)×100 / (a+b+c)) is preferably 20% or less, and more preferably 10% or less.

[0037] The density of the positive electrode mixture layer 31 is not particularly limited, but the effects of the present disclosure become more pronounced when the density is high. The density of the positive electrode mixture layer 31 is, for example, 3.5 g / cc or more, preferably 3.6 g / cc or more, and more preferably 3.8 g / cc or more. The upper limit of the density of the positive electrode mixture layer 31 is, for example, 4.3 g / cc.

[0038] FIG. 3 shows the relationship between the density of the positive electrode composite layer 31 (Example) and the permeability of the non-aqueous electrolyte. For comparison, the permeability of a positive electrode manufactured using a conventional wet process is shown. In this test, several samples with different composite layer densities were prepared, and propylene carbonate (PC) was used instead of the non-aqueous electrolyte. A predetermined amount of PC was dropped onto each composite layer, and the time until the PC permeated and disappeared was measured. The shorter this time, the better the permeability of the electrolyte. The porosity and binder content of the positive electrode composite layer in the Examples and Comparative Examples used in this test are as shown in Table 1 (the same positive electrode active material was used and the same amount was added).

[0039] [Table 1]

[0040] 3, it can be seen that the positive electrode mixture layer of the Example has a shorter liquid disappearance time and better electrolyte permeability than the positive electrode mixture layer of the Comparative Example. Furthermore, in the positive electrode mixture layer of the Comparative Example, the electrolyte permeability drops sharply as the density increases, but in the positive electrode mixture layer of the Example, the drop in permeability is suppressed and good permeability is maintained even at high density.

[0041] In the positive electrode 11, the positive electrode active material may be embedded in the positive electrode core material 30. The maximum penetration depth D of the positive electrode active material is, for example, 30% or more of the thickness of the positive electrode core material 30, specifically 6 μm or more. Here, the penetration depth D of the positive electrode active material refers to the length along the thickness direction of the positive electrode core material 30 from the surface of the positive electrode core material 30 to the part where the positive electrode active material is embedded the most. The penetration depth D can be measured by observing the cross section of the positive electrode 11 using an SEM. The maximum penetration depth D can be controlled, for example, by the softening temperature of the positive electrode core material 30, and the heating temperature and pressing pressure in the heat pressing step described below.

[0042] [Negative electrode] The negative electrode 12 comprises a negative electrode core material made of metal foil or the like, and a negative electrode composite layer provided on the surface of the negative electrode core material. Copper foil is generally used for the negative electrode core material. The negative electrode 12 may be a conventionally known electrode plate manufactured by a wet process, or may be an electrode plate provided with a negative electrode composite sheet manufactured by a method described later. The negative electrode 12 comprises a negative electrode composite layer in which the porosity of the second region is higher than the porosity of the first region, and 11 It may have a similar configuration.

[0043] The negative electrode active material may be a carbon-based active material, such as natural graphite (e.g., flake graphite, lump graphite, or amorphous graphite), or artificial graphite (e.g., massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB)). Alternatively, the negative electrode active material may be a Si-based active material that alloys with lithium. Since carbon-based active materials have higher electronic conductivity than positive electrode active materials, the negative electrode 12 may not contain a conductive material.

[0044] [Separator] 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 13.

[0045] [Cathode manufacturing method] The manufacturing method of the positive electrode 11 will be described in detail below. Although the manufacturing method of the positive electrode 11 including a conductive material will be exemplified below, the manufacturing method described below can also be applied to the manufacturing of a negative electrode. In the case of a negative electrode, a negative electrode active material is used instead of a positive electrode active material, and a conductive material does not need to be added to the composite sheet.

[0046] 4 is a diagram showing an example of a method for manufacturing positive electrode 11. As shown in FIG. 4, in the manufacturing process of positive electrode 11, powdered positive electrode mixture 40 is rolled and formed into a sheet to produce positive electrode mixture sheet 43, and positive electrode 11 is manufactured by bonding positive electrode mixture sheet 43 to positive electrode core material 30. In this process, before bonding positive electrode mixture sheet 43 to positive electrode core material 30, the sheet is strongly compressed to increase its density. This results in positive electrode mixture sheet 43 (positive electrode mixture layer 31) having the above-described void distribution.

[0047] The example shown in Fig. 4 is a dry process. The dry process is a process in which the active material and binder are mixed without using a solvent, and the active material and binder are mixed in a state where the solid content is substantially 100%. During mixing, a conductive material other than the active material and binder may be added, and even when a material other than the active material and binder is added, the solid content during mixing is substantially 100%.

[0048] The positive electrode mixture 40 can be obtained, for example, by putting a positive electrode active material, binder particles, and a conductive material into a mixer and mixing these materials while fibrillating the binder particles. The positive electrode mixture 40 contains a particulate active material and a fibrous binder, and the fibrous binder adheres to the particle surfaces of the active material and is entangled with the active material. In other words, the active material is held in place by the fibrous binder present in a mesh-like structure. This type of structure can also be seen in the positive electrode mixture sheet 43.

[0049] The binder particles are preferably particles whose main component is polytetrafluoroethylene (PTFE). PTFE is easily fibrillated and is suitable as a binder for the positive electrode mixture sheet 43. For example, PTFE particles with a volume-based median diameter (D50) of 5 to 100 μm are used and mixed with the active material and conductive material for a short period of time. In this case, particle cracking of the active material is suppressed, and a positive electrode mixture 40 can be produced in which the amount of conductive material incorporated into the fibrous binder is small. Examples of mixers that can be used include a cutter mill, a pin mill, a bead mill, a fine particle composite device, a granulator, and a kneader.

[0050] As shown in FIGS. 4(a) to 4(c), the positive electrode composite sheet 43 is produced through multiple rolling and compression steps. First, the positive electrode composite 40 is rolled by a pair of rolls 100 to form a sheet. The two rolls 100 are arranged with a predetermined gap (e.g., 1 to 3 mm) between them and rotate in the same direction (e.g., at a peripheral speed of 0.5 to 1.5 m / min). The positive electrode composite 40 is fed into the gap between the two rolls 100 and rolled to form the positive electrode composite sheet 41. Second, the positive electrode composite sheet 41 is further rolled to obtain a positive electrode composite sheet 42 that is thinner and more dense than the positive electrode composite sheet 41. Third, the positive electrode composite sheet 42 is further compressed to obtain a positive electrode composite sheet 43 that is thinner and more dense than the positive electrode composite sheet 42.

[0051] Positive electrode mixture sheet 41 is rolled using three rolls 101a, 101b, and 101c. In the example shown in FIG. 4, the three rolls are arranged side by side with a predetermined gap (for example, 50 to 200 μm) between them, with rolls 101a and 101b rotating in the same direction and roll 101c rotating in the opposite direction to the two rolls. Positive electrode mixture sheet 41 is rolled by passing between rolls 101a and 101b and between rolls 101b and 101c, and is formed into positive electrode mixture sheet 42. The peripheral speed ratios of the three rolls may be different; for example, the peripheral speed of roll 101b may be 1.5 to 3 times the peripheral speed of roll 101a, and the peripheral speed of roll 101c may be 1.2 to 2 times the peripheral speed of roll 101b.

[0052] The positive electrode mixture sheet 42 is compressed using a pair of rolls 102 (for example, with a gap set to 0 μm). The positive electrode mixture sheet 42 is compressed with a force greater than those in the first and second rolling steps, and is formed into a positive electrode mixture sheet 43 that constitutes the positive electrode mixture layer 31. The thickness, density, and void distribution of the positive electrode mixture sheet 43 are determined in this compression step and do not change substantially in the subsequent step of bonding to the positive electrode core material 30. The linear pressure applied by the rolls 102 is, for example, 10 times or more, preferably 15 to 25 times, the linear pressure applied in the first and second rolling steps, and specifically 1.0 to 3 t / cm. The positive electrode mixture sheet 42 may also be compressed while being heated at a temperature of 50 to 200°C.

[0053] As shown in FIG. 4(d), by bonding the positive electrode composite sheet 43 to the positive electrode core material 30, a positive electrode 11 is obtained in which a positive electrode composite layer 31 made of the positive electrode composite sheet 43 is provided on the surface of the positive electrode core material 30. In the example shown in FIG. 4, a pair of rolls 103 is used to hot-press the laminate of the positive electrode core material 30 and the positive electrode composite sheet 43, thereby bonding the positive electrode composite sheet 43 to the surface of the positive electrode core material 30. The pressure applied by the rolls 103 is preferably equal to or less than the pressure applied by the rolls 102. The positive electrode composite sheet 43 is preferably heated at a temperature equal to or less than the melting point of the binder, and may be heated at a temperature equal to or higher than the softening temperature of the positive electrode core material 30 and equal to or less than the melting point of the fibrous binder. The hot-press temperature is set to, for example, 150 to 250°C.

[0054] The positive electrode composite sheets 43 are bonded to both sides of the positive electrode core material 30. In the example shown in Fig. 4, one sheet is bonded to one side of the positive electrode core material 30, and then another sheet is bonded to the other side, but as shown in Fig. 5, two positive electrode composite sheets 43 may be bonded simultaneously to both sides of the positive electrode core material 30. In the example shown in Fig. 5, the positive electrode core material 30 and two positive electrode composite sheets 43 are supplied between a pair of rolls 103, and the two positive electrode composite sheets 43 are simultaneously heat-pressed.

[0055] The positive electrode mixture layer 31 of the positive electrode 11 manufactured through the above steps has a void distribution in which the porosity (a) of the first region 31a is approximately equal to the porosity (c) of the third region 31c < the porosity (b) of the second region 31b. The difference between the porosity (b) and the porosities (a, c) is not large, and voids are present throughout the positive electrode mixture layer 31, resulting in good electrolyte permeability. This void distribution is obtained by compressing the positive electrode mixture sheet 42 without being constrained by the positive electrode core material 30 or the like. Furthermore, because this step does not use a solvent to prepare the positive electrode mixture sheet 43, binder migration does not occur, and the binder is present substantially uniformly throughout the positive electrode mixture layer 31.

[0056] 6 and 7 are diagrams showing another example of a method for manufacturing positive electrode 11. As shown in Fig. 6 and 7, positive electrode 11 can be manufactured using positive electrode mixture sheet 53 produced by a wet process.

[0057] 6(a) to 6(c), positive electrode mixture sheet 53 is produced by applying positive electrode mixture slurry 50 containing a positive electrode active material, a binder, a conductive material, and a solvent onto release film 60 to form coating film 51, drying coating film 51 to form positive electrode mixture sheet 52, and then compressing this sheet. Then, as shown in FIG. 6(d), positive electrode mixture sheet 53 is bonded to positive electrode core material 30 to obtain positive electrode 11 in which positive electrode mixture layer 31 made of positive electrode mixture sheet 53 is provided on the surface of positive electrode core material 30. Compression of positive electrode mixture sheet 52 and heat pressing of the laminate of positive electrode core material 30 and positive electrode mixture sheet 53 can be performed under conditions similar to those of the manufacturing method shown in FIG.

[0058] In this process using the positive electrode mixture slurry 50, a drying process for volatilizing and removing the solvent is required, which causes migration of the binder. As a result, a binder distribution is formed in the thickness direction of the positive electrode mixture sheet 52, with the binder amount increasing the further away from the release film 60. In addition, in this process, as shown in FIG. 6(c), the positive electrode mixture sheet 52 is compressed in the state of a laminate arranged on the release film 60. In other words, because the positive electrode mixture sheet 52 is compressed while being restrained by the release film 60, a void distribution is formed in the positive electrode mixture sheet 52, with the voids increasing the closer to the release film 60.

[0059] 6, after the positive electrode composite sheet 53 is peeled from the release film 60, the positive electrode composite sheet 53 is placed on the positive electrode core material 30 with the surface of the positive electrode composite sheet 53 opposite to the release film 60 facing the positive electrode core material 30, and then hot pressing is performed. The positive electrode composite layer 31 of the positive electrode 11 manufactured through the process shown in FIG. 6 has a void distribution of porosity (a) of the first region 31a < porosity (b) of the second region 31b < porosity (c) of the third region 31c. Also, a binder distribution of binder content (a) in the first region 31a > binder content (b) in the second region 31b > binder content (c) in the third region 31c is formed.

[0060] 7(c), after the positive electrode mixture sheet 52 is peeled from the release film 60, the positive electrode mixture sheet 52 may be compressed to produce a positive electrode mixture sheet 53. In this case, the positive electrode mixture sheet 52 is compressed without being restrained by the release film 60, and therefore a void distribution is formed in the positive electrode mixture sheet 53 such that the porosity (a) of the first region 31a is approximately equal to the porosity (c) of the third region 31c < the porosity (b) of the second region 31b. Note that even in this case, migration of the binder occurs during the drying process.

[0061] As shown in FIG. 7(d), the positive electrode mixture sheet 53 may be placed on the positive electrode core material 30 with the surface of the positive electrode mixture sheet 53 facing the release film 60 toward the positive electrode core material 30, and then heat-pressed. Alternatively, the positive electrode mixture sheet 53 may be placed on the positive electrode core material 30 with the surface of the positive electrode mixture sheet 53 opposite the release film 60 facing the positive electrode core material 30, and then heat-pressed. In the former case, the positive electrode mixture layer 31 has a binder distribution in which the binder content (a) in the first region 31a < the binder content (b) in the second region 31b < the binder content (c) in the third region 31c. In the latter case, the binder distribution is content (a) > content (b) > content (c).

[0062] FIG. 8 is a diagram showing another example of a method for manufacturing positive electrode 11. The process shown in FIG. 8 differs from the process shown in FIG. 4 in that release film 60 is used in a dry process for producing a positive electrode composite sheet from powdered positive electrode composite 40. In the example shown in FIG. 8, release film 60 is supplied between rolls 101b and 101c and laminated with positive electrode composite sheet 42, and positive electrode composite sheet 42 is compressed by a pair of rolls 102 while placed on release film 60. In this case, positive electrode composite sheet 43x is produced having a void distribution in which the voids increase closer to release film 60.

[0063] 8, positive electrode mixture sheet 43x is placed on positive electrode core material 30 and hot pressed with the surface of positive electrode mixture sheet 43x opposite release film 60 facing the positive electrode core material 30. In this case, positive electrode mixture layer 31 has a void distribution where porosity (a) of first region 31a<porosity (b) of second region 31b<porosity (c) of third region 31c.

[0064] 8, release film 60 is peeled off after compressing positive electrode mixture sheet 42 and before heat-pressing positive electrode mixture sheet 43x, but release film 60 may be peeled off after heat-pressing. Also, in the process shown in FIG. 6, release film 60 may be peeled off after heat-pressing. [Explanation of symbols]

[0065] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 16 outer can 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Grooved part 23 Internal terminal board 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket 30 Positive electrode core material 31 Positive electrode mixture layer 31a 1st area 31b 2nd area 31c 3rd area 40 Positive electrode mixture 41, 42, 43, 43x Positive electrode composite sheet 50 Positive electrode mixture slurry 51 Paint film 52,53 Positive electrode composite sheet 60 Release film 100,101 a ,101b,101c, 102, 103 rolls

Claims

1. A core material and a composite layer including an active material and a binder and provided on a surface of the core material; A battery electrode comprising: When the composite layer is divided into three equal parts in the thickness direction and defined as a first region, a second region, and a third region in that order from the core material side, the porosity (b) of the second region is higher than the porosity (a) of the first region, The difference (b-a) between the porosity (b) of the second region and the porosity (a) of the first region is 0.5% or more and 10% or less, The difference (a-c) between the porosity (a) of the first region and the porosity (c) of the third region is within a range of ±1%. Electrodes for batteries.

2. A core material and a composite layer including an active material and a binder and provided on a surface of the core material; A battery electrode comprising: When the composite layer is divided into three equal parts in the thickness direction and defined as a first region, a second region, and a third region in that order from the core material side, the porosity (b) of the second region is higher than the porosity (a) of the first region, The porosity (b) of the second region is higher than the porosity (c) of the third region; A battery electrode, wherein the difference (b-c) between the porosity (b) of the second region and the porosity (c) of the third region is 0.5% or more and 10% or less.

3. The porosity (c) of the third region is higher than the porosity (b) of the second region; 2. The battery electrode according to claim 1, wherein the difference (c-b) between the porosity (c) of the third region and the porosity (b) of the second region is 0.5% or more and 10% or less.

4. 4. The battery electrode according to claim 1, wherein a ratio of a difference between a content (a) of the binder in the first region and a content (c) of the binder in the third region to a content (a+b+c) is within a range of ±5%.

5. 5. The battery electrode according to claim 1, wherein a ratio of a difference between a content (a) of the binder in the first region and a content (b) of the binder in the second region to a content (a+b+c) is within a range of ±5%.

6. 6. The battery electrode according to claim 1, wherein a ratio of a difference between a content (b) of the binder in the second region and a content (c) of the binder in the third region to a content (a+b+c) is within a range of ±5%.

7. The battery electrode according to any one of claims 1 to 3, wherein the content of the binder in the composite layer increases in the order of the third region, the second region, and the first region.

8. The battery electrode according to any one of claims 1 to 7, wherein the overall porosity of the composite layer is 40% or less.

9. A battery comprising the battery electrode according to any one of claims 1 to 8 and an electrolyte solution.

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