The present disclosure relates to a battery electrode and a battery that includes this electrode.
Electrodes for lithium ion batteries or other batteries are typically produced through a wet process which comprises applying electrode mixture slurry that contains, for example, an active material and a binder to the surface of a core which is metal foil, and drying and compressing the applied film (see, for example, Patent Literature 1). This causes migration in which the binder migrates from the core side toward the surface side during the drying of the applied film. As the amount of the binder in the vicinity of the surface is increased compared to that in the vicinity of the core, the binder tends to be unevenly distributed in the thickness direction. The applied film includes voids, for example, between particles of the active material. When the applied film is compressed, the voids decrease as the particles of the active material move. However, because the applied film is fixed to the core, the closer to the core, the more difficult for the mixture to move and the more voids remain; as such, the voids tend to be unevenly distributed in the thickness direction.
In recent years, there are also proposed methods of producing an electrode by rolling and molding an electrode mixture into a sheet and then laminating this sheet to a core (see, for example, Patent Literatures 2 and 3). Patent Literature 2 discloses a method of increasing the density after lamination of a compressed powder layer to a core, the method comprising applying greater pressure between a pair of rolls that press a laminate of the compressed powder layer and the core than pressure applied between a pair of rolls that press electrode mixture powder to prepare the compressed powder layer. Patent Literature 3 discloses a method comprising molding a granulated product composed of a mixture of an active material, a thickener, a solvent, and a binder into a sheet and disposing this sheet on a core.
The methods disclosed in Patent Literatures 2 and 3 can eliminate or simplify the step of drying the mixture layer, and are expected to address problems of the wet process described above. However, the methods disclosed in Patent Literatures 2 and 3 still have significant room for improvement, as they are similar to the wet process in that the porosity tends to be unevenly distributed in the thickness direction of a mixture layer. In response to the formation of a porosity distribution having more voids in regions of the mixture layer that are closer to the core or, in other words, less voids in regions of the mixture layer that are closer to the surface, the penetration of an electrolyte solution into the mixture layer is impaired, leading to, for example, a decrease in high rate characteristics. Under specific process conditions disclosed in Patent Literatures 2 and 3, it is difficult to increase the density of the mixture layer.
According to an aspect of the present disclosure, there is provided a battery electrode comprising a core; and a mixture layer containing an active material and a binder, the mixture layer being disposed on a surface of the core, wherein assuming that the mixture layer is divided into three equal parts in a thickness direction, the three equal parts being defined as a first region, a second region, and a third region in that order from a side on which the core is located, a porosity of the second region is higher than a porosity of the first region.
According to another aspect of the present disclosure, there is provided a battery comprising the above-described battery electrode and an electrolyte solution.
An embodiment of the present disclosure can provide a battery electrode that includes a mixture layer with good penetration of the electrolyte solution. A battery that includes an electrode according to the present disclosure has, for example, excellent high rate performance.
Embodiments of a battery electrode according to the present disclosure will now be described in detail. The embodiments described below are merely illustrative, and the present disclosure is not limited to the following embodiments. The drawings to which reference is made in the description of the embodiments are schematic, and the following description should be taken into consideration for determination of, for example, the size proportion of components depicted in the drawings.
A battery electrode according to the present disclosure is suitable as an electrode for a non-aqueous electrolyte secondary battery such as a lithium ion battery, but can also be used for a battery including an aqueous electrolyte solution. The electrode can be used not only for a secondary battery, but can also be used for a primary battery. In the following description, a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery electrode (especially, positive electrode) will be described by way of example.
The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound configuration in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 therebetween. The outer can 16 is a metal container having a cylindrical shape with a closed bottom, which is open on one side in an axial direction, the opening of the outer can 16 being closed by a sealing assembly 17. In the following description, for ease of description, the term “upper” refers to the side toward the sealing assembly 17 of the battery, and the term “lower” refers to the side toward the bottom of the outer can 16.
It should be noted that the outer housing of the battery is not limited to a cylindrical outer can. For example, the outer housing may be a rectangular outer can or may be composed of a laminate sheet including a metal layer and a resin layer. The electrode assembly may be a laminated electrode assembly that includes multiple positive electrodes and multiple negative electrodes alternately laminated with a separator therebetween.
The non-aqueous electrolyte solution includes 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 mixed solvents of two or more thereof. The non-aqueous solvent may contain a halogen-substituted product of these solvents in which hydrogens of the solvents are, at least in part, substituted with a halogen atom such as fluorine. Examples of the electrolyte salt include a lithium salt such as LiPF6.
The positive electrode 11, the negative electrode 12, and the separator 13 of the electrode assembly 14 are long strips of material that are laminated alternately in the radial direction of the electrode assembly 14 as they are spirally wound. To prevent precipitation of lithium, the negative electrode 12 has a size slightly larger than the positive electrode 11. More specifically, the negative electrode 12 has longer lengths than the positive electrode 11 both in the length direction and in the width direction (shorter length direction). Two separators 13, which have a size slightly larger than at least the positive electrode 11, are disposed so that, for example, the positive electrode 11 is interposed between the two separators 13. The electrode assembly 14 includes a positive electrode lead 20 that is connected to the positive electrode 11 by, for example, welding, and a negative electrode lead 21 that is connected to the negative electrode 12 by, for example, welding.
Insulating plates 18 and 19 are respectively disposed on upper and lower sides of the electrode assembly 14. In the example illustrated in
A gasket 28 is provided between the outer can 16 and the sealing assembly 17, thereby maintaining airtightness of the space inside the battery. The outer can 16 has a groove or inward projection 22, which is an inwardly protruding portion of the side surface of the outer can 16, and the groove or inward projection 22 supports the sealing assembly 17. The groove or inward projection 22 preferably has an annular shape extending along the circumference of the outer can 16, and supports the sealing assembly 17 on its upper surface. The sealing assembly 17 is fixed to an upper portion of the outer can 16 via the groove or inward projection 22 and an opening edge portion of the outer can 16 that is swaged to the sealing assembly 17.
The sealing assembly 17 has a configuration in which the internal terminal plate 23, a lower vent member 24, an insulating member 25, an upper vent member 26, and the cap 27 are stacked in that order from the side on which the electrode assembly 14 is located. The components of the sealing assembly 17 have, for example, either a disc shape or a ring shape and are, except for the insulating member 25, electrically connected to each other. The lower vent member 24 and the upper vent member 26 are connected to each other at their center portions, and the insulating member 25 is interposed between their peripheral portions. In response to an increase in internal pressure of the battery due to abnormal heat generation, the lower vent member 24 breaks as it is deformed so as to push the upper vent member 26 toward the cap 27, resulting in an interruption of the electric current path between the lower vent member 24 and the upper vent member 26. In response to a further increase in internal pressure, the upper vent member 26 breaks, letting gas escape through an opening of the cap 27.
The positive electrode 11, the negative electrode 12, and the separator 13 of the electrode assembly 14, especially the positive electrode 11, will now be described in detail.
The positive electrode mixture layer 31 is formed by bonding a positive electrode mixture sheet 43, 43x, or 53 prepared through a method of manufacture described later, to the surface of the positive electrode core 30. When the positive electrode mixture layer 31 is composed of a positive electrode mixture sheet 43 or 43x, the positive electrode mixture layer 31 contains, for example, a fibrous binder as the binder. The use of a fibrous binder makes it easy to roll and mold a positive electrode mixture 40 (see, for example,
The positive electrode mixture layer 31 contains a positive electrode active material as the main component (the component with the highest mass ratio). The content of the positive electrode active material is preferably 85% to 99% by mass and more preferably 90% to 98% by 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.
A lithium transition metal composite oxide is used as the positive electrode active material. Examples of 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. The lithium transition metal composite oxide preferably contains, among others, at least one of Ni, Co, and Mn. Suitable examples of the composite oxide include a lithium transition metal composite oxide that contains Ni, Co, and Mn and a lithium transition metal composite oxide that contains Ni, Co, and Al.
The fibrous binder is composed of a resin that contains, for example, polytetrafluoroethylene (PTFE) as the main component, and is obtained through fibrillation of PTFE particles. The content of the fibrous binder is preferably 0.05% to 5% by mass relative to the mass of the positive electrode mixture layer 31. The positive electrode mixture layer 31 may contain a non-fibrillated (non-fibrous) binder. Examples of the non-fibrillated binder include polyvinylidene fluoride (PVdF).
The positive electrode mixture layer 31 includes voids. Voids formed inside the positive electrode mixture layer 31 communicate to, for example, the surface of the positive electrode mixture layer 31. These voids serve as passages for the electrolyte solution and improve the penetration of the electrolyte solution into the positive electrode mixture layer 31. Assuming that the positive electrode mixture layer 31 of the positive electrode 11 is divided into three equal parts in the thickness direction, the three equal parts being defined as a first region 31a, a second region 31b, and a third region 31c in that order from the side on which the positive electrode core 30 is located, the second region 31b has a porosity (b) that is higher than a porosity (a) of the first region 31a. The term “porosity” as used herein represents the ratio of voids in the positive electrode mixture layer 31.
In other words, the positive electrode mixture layer 31 includes more voids in a center portion of the layer in the thickness direction than in the vicinity of the positive electrode core 30. Although a positive electrode mixture layer of a positive electrode produced through a conventional typical wet process includes more voids in the vicinity of a positive electrode core so that voids are fewer with distance from the core, if it is produced through a method of manufacture described later, more voids can be formed in the second region 31b than in the first region 31a. This increases the penetration of the electrolyte solution into the positive electrode mixture layer 31, thereby improving battery high rate performance.
The porosity of a positive electrode mixture layer is measured by the following method:
(1) A cross section of the positive electrode mixture layer is exposed using an ion milling system (for example, IM4000PLUS from Hitachi High-tech).
(2) A backscattered electron image of the exposed cross section of the positive electrode mixture layer is captured using a scanning electron microscope (SEM). The backscattered electron image is captured at 1,000 to 5,000-fold magnification.
(3) An SEM image of the cross section of the positive electrode mixture layer is imported into a computer and color-coded based on contrast into three colors, an intermediate color for voids, using image analysis software (for example, ImageJ from USA National Institutes of Health).
(4) A measurement target region is selected from the processed image, a total area of voids in this region is obtained, and the ratio of voids in the measurement target region (porosity) is calculated.
As described above, the positive electrode mixture layer 31 has a higher porosity in the second region 31b than in the first region 31a, and a difference (b−a) between the porosity (b) of the second region 31b and the porosity (a) of the first region 31a is, for example, 0.5% or greater. However, the difference (b−a) in porosity is preferably not too large: preferably 10% or less and more preferably 5% or less.
The porosity (b) of the second region 31b may be higher than a porosity (c) of the third region 31c. A difference (b−c) between the porosity (b) and the porosity (c) is, for example, 0.5% or greater. The difference (b−c) in porosity is preferably not too large: preferably 10% or less and more preferably 5% or less.
A difference (a−c) between the porosity (a) of the first region 31a and the porosity (c) of the third region 31c is, for example, within the range of ±1%. The porosity (a) and the porosity (c) may be substantially the same. In a suitable example, the positive electrode mixture layer 31 satisfies the relationship that the porosity (a) the porosity (c)<the porosity (b), and voids are present evenly throughout the positive electrode mixture layer 31 without too many somewhere in the layer or too few elsewhere. This facilitates penetration of the electrolyte solution throughout the positive electrode mixture layer 31.
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 mixture layer 31 may be such that the porosity (a)<the porosity (b)<the porosity (c). This also provides good penetration of the electrolyte solution. A difference (c−b) between the porosity (c) and the porosity (b) is, for example, 0.5% or greater. The difference (c−b) in porosity is preferably 10% or less and more preferably 5% or less.
In terms of, for example, increased capacity, the overall porosity of the positive electrode mixture layer 31 is preferably 40% or less and more preferably 30% or less. However, in terms of, for example, improved penetration of the electrolyte solution, the overall porosity of the positive electrode mixture layer 31 is preferably 5% or greater and more preferably 10% or greater.
The content of the binder in the positive electrode mixture layer 31 is preferably either substantially uniform throughout the layer or higher toward the side on which the positive electrode core 30 is located. This improves the bond strength of the positive electrode mixture layer 31 with the positive electrode core 30, and also further improves the penetration of the electrolyte solution. It should be noted that, when a positive electrode is produced through a conventional typical wet process, migration of the binder occurs during the drying of the applied film, resulting in an increase in the amount of the binder in the vicinity of the surface compared to that in the vicinity of the core.
In a suitable example of the positive electrode mixture layer 31, the ratio ((a−c)×100/(a+b+c)) of a difference between the content (a) of the binder in the first region 31a and the content (c) of the binder in the third region 31c to the total content (a+b+c) of the binder in each of the regions is within the range of ±5%. The content (a) and the content (c) may be substantially the same.
The ratio ((a−b)×100/(a+b+c)) of a difference (a−b) 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%. Similarly, the ratio ((b−c)×100/(a+b+c)) of a difference between the content (b) and the content (c) to the content ((a+b+c)) is within the range of ±5%. That is, in a suitable example, the positive electrode mixture layer 31 satisfies the relationship that the content (a) the content (b) the content (c), the binder is present evenly throughout the positive electrode mixture layer 31 without too much somewhere in the layer or too little elsewhere.
In another suitable example of the positive electrode mixture layer 31, the content of the binder increases in the order of the third region 31c, the second region 31b, and the first region 31a (the content (c)<the content (b)<the content (a)). However, the difference in content between the regions is preferably not too large: each of the above-described ratios ((a−c)×100/(a+b+c)) and ((b−c)×100/(a+b+c)) is preferably 20% or less and more preferably 10% or less.
The density of the positive electrode mixture layer 31 is not particularly limited but, when the density is higher, advantages obtainable by the present disclosure are more notable. The density of the positive electrode mixture layer 31 is, for example, 3.5 g/cc or greater, preferably 3.6 g/cc or greater, and more preferably 3.8 g/cc or greater. The maximum density of the positive electrode mixture layer 31 is, for example, 4.3 g/cc.
As a comparative, the penetration in a positive electrode produced through a conventional typical wet process is shown. In this test, multiple samples having different densities in the mixture layers were prepared, propylene carbonate (PC) was used in place of a non-aqueous electrolyte solution, and the time that elapsed after a predetermined amount of PC was dripped onto each of the mixture layers until PC had penetrated and disappeared was measured. The measured time indicates that the shorter it is, the better the penetration of the electrolyte solution. The porosity of the positive electrode mixture layer and the content of the binder in an example and a comparative example used in this test are shown in Table 1 (as the positive electrode active material, the same material was used, and the same amount of it was added).
33%
17%
33%
31%
34%
52%
It can be seen from
In the positive electrode 11, the positive electrode active material may bite into the positive electrode core 30. The maximum bite depth D of the positive electrode active material is, for example, 30% or greater of the thickness of the positive electrode core 30, and is 6 μm or greater in a specific example. The bite depth D of the positive electrode active material herein represents a length along the thickness direction of the positive electrode core 30 as measured from the surface of the positive electrode core 30 to the deepest point to which the positive electrode active material bites. The bite depth D can be measured by observing a cross section of the positive electrode 11 using an SEM. It should be noted that the maximum bite depth D can be controlled using, for example, the softening temperature of the positive electrode core 30 or the heating temperature and pressing pressure of a heat pressing process, which will be described later.
The negative electrode 12 includes a negative electrode core that is made of, for example, metal foil and a negative electrode mixture layer that is disposed on the surface of the negative electrode core. Typically, copper foil is used as the core of the negative electrode. The negative electrode 12 may be formed using a conventionally known electrode plate produced through a wet process, or may be formed using an electrode plate that includes a negative electrode mixture sheet produced through a method described later. The negative electrode 12 may have a structure that is similar to that of the above-described positive electrode 10 and includes a negative electrode mixture layer in which the porosity of the second region is higher than the porosity of the first region.
The negative electrode active material is, for example, a carbon-based active material including natural graphite such as flake graphite, massive graphite, and earthy graphite, and artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may be, for example, a Si-based active material that forms an alloy with lithium. It should be noted that carbon-based active materials have a higher electron conduction than positive electrode active materials; therefore, the negative electrode 12 may contain no conductive agent.
A porous sheet having ion permeability and insulating properties is used as the separator 13. Specific examples of the porous sheet include a microporous thin film, woven fabric, and non-woven fabric. Suitable examples of the material for the separator 13 include polyethylene, polypropylene, and other polyolefins and cellulose. The separator 13 may have either a single-layer structure or a multi-layer structure. The separator 13 may have, for example, a heat-resistant layer on its surface.
A method of manufacturing the positive electrode 11 will now be described in detail. In the following description, a method of manufacturing the positive electrode 11 that contains a conductive agent will be described by way of example, but the following method of manufacture may also be similarly used for the production of the negative electrode. For the negative electrode, the positive electrode active material is replaced with a negative electrode active material, and no conductive agent may be added to the mixture sheet.
The example illustrated in
The positive electrode mixture 40 is prepared by, for example, introducing a positive electrode active material, binder particles, and a conductive agent into a mixing machine, and mixing these materials together 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 surfaces of particles of the active material and is entangled with the active material. In other words, the active material is held by the fibrous binder that is present in the form of mesh. This configuration can also be observed in the positive electrode mixture sheet 43.
The binder particles are preferably particles that contain polytetrafluoroethylene (PTFE) as the main component. PTFE, which is easily fibrillated, is suitable as the binder for the positive electrode mixture sheet 43. The active material and the conductive agent are mixed together in a short time using PTFE particles having, for example, a volume-based median diameter (D50) of 5 to 100 μm. This reduces particle breakage of the active material, and the positive electrode mixture 40 with a small amount of the conductive agent incorporated in the fibrous binder can be prepared. For example, a cutter mill, a pin mill, a bead mill, a microparticle composite forming apparatus, a granulating machine, or a kneading machine can be used as the mixing machine.
As illustrated in
The positive electrode mixture sheet 41 is rolled using three rolls 101a, 101b, and 101c. In the example illustrated in
The positive electrode mixture sheet 42 is compressed using a pair of rolls 102 (for example, the gap is set to 0 μm). The positive electrode mixture sheet 42 is compressed by a force that is greater than those applied in the first and second rolling steps, and is molded into the positive electrode mixture sheet 43 that constitutes the positive electrode mixture layer 31. The thickness, density, and porosity distribution of the positive electrode mixture sheet 43 are determined through this compression process and remain substantially unchanged in the subsequent process in which it is bonded to the positive electrode core 30. The press linear pressure applied by the rolls 102 is, for example, 10 or more times, preferably 15 to 25 times, the press linear pressure applied in the first and second rolling steps, and is 1.0 to 3 t/cm in a specific example. The positive electrode mixture sheet 42 may be compressed while being heated at a temperature of 50 to 200° C.
As illustrated in
The positive electrode mixture sheet 43 is bonded to both sides of the positive electrode core 30. In the example illustrated in
The positive electrode mixture layer 31 of the positive electrode 11 produced through the above-described process has a porosity distribution in which the porosity (a) of the first region 31a 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, voids are present evenly throughout the positive electrode mixture layer 31, and the penetration of the electrolyte solution is good. Such a porosity distribution is obtained by compressing the positive electrode mixture sheet 42 while not being bound to, for example, the positive electrode core 30. In this process, as no solvent is used for preparation of the positive electrode mixture sheet 43, migration of the binder does not occur, and the binder is present substantially uniformly throughout the positive electrode mixture layer 31.
As illustrated in
In this process, in which the positive electrode mixture slurry 50 is used, as it is necessary to perform the drying step for volatilizing and removing the solvent, migration of the binder occurs. As a result, the positive electrode mixture sheet 52 has, in the thickness direction, a binder distribution in which the amount of the binder increases with distance from the releasing film 60. In this process, as illustrated in
In the example illustrated in
As illustrated in
As illustrated in
In the example illustrated in
It should be noted that, although, in the example illustrated in
Number | Date | Country | Kind |
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2020-049201 | Mar 2020 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2021/000959 | 1/14/2021 | WO |