Electrode body for secondary battery and secondary battery

Abstract

To provide an electrode body for a secondary battery and a secondary battery in which a variation in the amount of electrolyte inflow in the gravity direction is suppressed.SOLUTION: An electrode body for a secondary battery includes an electrode layer containing active material particles, and an orientation ratio (X / Y) calculated from an orientation degree X of the electrode layer located in a region below the center in the direction of gravity and an orientation degree Y of the electrode layer located in a region above the center in the direction of gravity is greater than 1.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] 2. Description of the Related Art Secondary batteries such as lithium ion secondary batteries are used in a wide range of fields, including electronic devices, electric vehicles, and power storage facilities. A typical secondary battery has a structure in which an electrolytic solution containing an electrolyte is impregnated into an electrode assembly including a positive electrode and a negative electrode. The negative electrode of the secondary battery contains graphite particles as an active material. Graphite particles contained as an active material in the negative electrode of a secondary battery expand due to the insertion of lithium ions during charging of the secondary battery, and contract due to the desorption of lithium ions during discharging. Therefore, when the secondary battery is charged, a part of the electrolyte flows out of the electrode body due to the expansion of the graphite particles contained in the negative electrode, and when the secondary battery is discharged, the electrolyte outside the electrode body flows into the electrode body due to the contraction of the graphite particles contained in the negative electrode. The amount of electrolyte that flows into the electrode assembly during discharge of the secondary battery is affected by gravity. Specifically, the amount of electrolyte that flows into the electrode assembly tends to be relatively small at the top in the direction of gravity and relatively large at the bottom. If the secondary battery is repeatedly charged and discharged in a state in which the amount of electrolyte flowing into the electrode assembly is not uniform, uneven electrolyte concentration will occur inside the electrode assembly, which may lead to a deterioration in battery characteristics.

[0003] As a method for controlling the outflow or inflow of the electrolyte from or into the electrode assembly, for example, Patent Document 1 proposes providing a liquid flow suppressing member on the end side of the wound electrode assembly. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2012 / 029144 Summary of the Invention [Problem to be solved by the invention]

[0005] The problem solved by the method described in Patent Document 1 is the suppression of fluctuations in the ion concentration of the electrolyte when repeated charging and discharging at a high current in a low-temperature environment, but it does not address the elimination of variations in the amount of electrolyte flowing into the electrode body in the direction of gravity. An object of the present disclosure is to provide an electrode body for a secondary battery and a secondary battery in which variation in the amount of electrolyte inflow in the gravity direction is suppressed. [Means for solving the problem]

[0006] Means for solving the above problems include the following means. <1> an electrode layer including active material particles; An electrode body for a secondary battery, in which an orientation ratio (X / Y) calculated from an orientation degree X of an electrode layer located in a region below the center in the direction of gravity and an orientation degree Y of an electrode layer located in a region above the center in the direction of gravity is greater than 1. <2> the aspect ratio of the active material particles is greater than 1; <1> The electrode body for a secondary battery according to claim 1. <3> The active material particles are graphite particles. <1> or <2> The electrode body for a secondary battery according to claim 1. <4> A wound body of an electrode layer or a laminate of an electrode layer, <1> ~ <3> 13. The electrode body for a secondary battery according to claim 12. <5> <1> ~ <4> A secondary battery comprising the electrode assembly according to any one of claims 1 to 5 and an electrolyte solution. Effect of the Invention

[0007] According to one embodiment of the present disclosure, there is provided an electrode body for a secondary battery and a secondary battery in which variation in the amount of electrolyte inflow in the gravity direction is suppressed. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is a diagram for explaining the concept of the degree of orientation of an electrode layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment that is an example of the present disclosure will be described. These descriptions and examples are merely illustrative of the embodiment, and are not intended to limit the scope of the invention. In the present specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the present specification, the upper or lower limit of the numerical range may be replaced with a value shown in the examples.

[0010] <Electrode body for secondary batteries> The electrode body for a secondary battery according to the present disclosure is an electrode layer including active material particles; The orientation ratio (X / Y) calculated from the orientation X of the electrode layer located in the region below the center in the gravity direction and the orientation Y of the electrode layer located in the region above the center in the gravity direction is greater than 1.

[0011] Hereinafter, the "secondary battery electrode body" may be referred to as the "electrode body", "below the center in the direction of gravity" may be referred to as the "below", and "above the center in the direction of gravity" may be referred to as the "above". In the present disclosure, the "direction of gravity" refers to the direction of gravity when a secondary battery including an electrode assembly is used in a specified manner.

[0012] In the present disclosure, the "degree of orientation of the electrode layer" is a concept indicating the degree to which the major axes of the active material particles contained in the electrode layer are oriented in the thickness direction of the electrode layer. The concept of the degree of orientation of the electrode layer is shown in Figure 1. As shown in Figure 1, when the long axis of the active material particle is completely oriented in the thickness direction of the electrode layer, the degree of orientation is 1, and when the long axis of the active material particle is completely oriented in the surface direction of the electrode layer, the degree of orientation is 0. When multiple active material particles are included within the electrode layer to be evaluated, the average value of the orientation degrees of each particle is calculated and used as the degree of orientation of the electrode layer. The closer the degree of orientation of the electrode layer is to 1, the greater the degree to which the long axis of the active material particle is oriented in the thickness direction of the electrode layer.

[0013] The electrolyte flows in from the end face of the electrode body (i.e., the cut surface of the electrode layer). The greater the degree of orientation of the electrode layer, the more difficult it is for the electrolyte to flow in from the end face of the electrode body. Since the electrolyte flows in along the surface direction of the electrode layer, when the electrode layer has a high degree of orientation, the long axis of the active material particles is perpendicular to the direction in which the electrolyte flows in, and the surface of the active material particle with the largest projected area faces the direction in which the electrolyte flows in. As a result, the flow path resistance in the electrode layer increases, making it difficult for the electrolyte to flow in.

[0014] In the electrode body of the present disclosure, the orientation degree X of the electrode layer located in the region below the center, where the electrolyte solution is likely to flow in due to the effect of gravity, is set to a value greater than the orientation degree Y of the electrode layer in the region above the center. This offsets the effect of gravity on the amount of electrolyte solution flowing in, and reduces the variation in the amount of electrolyte solution flowing in the direction of gravity of the electrode body.

[0015] The value of the degree of orientation X of the electrode layer located in the region below the center of the electrode body is not particularly limited, and can be selected depending on the shape, size, etc. of the electrode body. From the viewpoint of effectively suppressing the variation in the amount of electrolyte inflow in the gravity direction, the degree of orientation X of the electrode layer may be 0.4 or more, 0.5 or more, or 0.7 or more. From the viewpoint of ensuring the fluidity of the electrolyte, the degree of orientation X of the electrode layer may be 0.95 or less, 0.9 or less, or 0.8 or less.

[0016] The value of the degree of orientation Y of the electrode layer located in the region above the center of the electrode body is not particularly limited, and can be selected according to the structure of the secondary battery, etc. From the viewpoint of effectively suppressing the variation in the amount of electrolyte inflow in the gravity direction, the degree of orientation Y of the electrode layer may be 0.1 or more, 0.2 or more, or 0.3 or more. From the viewpoint of ensuring the fluidity of the electrolyte, the degree of orientation Y of the electrode layer may be 0.95 or less, 0.9 or less, or 0.8 or less.

[0017] In the electrode body of the present disclosure, all of the electrode layers located in the region below or above the center may satisfy the above-mentioned orientation condition, or only a portion of the electrode layers located in the region below or above the center may satisfy the above-mentioned orientation condition.

[0018] From the viewpoint of effectively suppressing the variation in the amount of electrolyte inflow in the direction of gravity, it is preferable that the orientation ratio (X / Y) calculated from the orientation degree X of the electrode layer located at a point corresponding to 1 / X from the bottom when the electrode body is equally divided into X regions along the direction of gravity and the orientation degree Y of the electrode layer located at a point corresponding to 1 / X from the top be greater than 1. X can be selected from 3 to 10, for example.

[0019] The value of the orientation ratio (X / Y) is not particularly limited as long as it is greater than 1, and can be selected depending on the shape, size, etc. of the electrode body. From the viewpoint of effectively suppressing the variation in the amount of electrolyte inflow in the gravity direction, the orientation ratio (X / Y) may be 1.1 or more, 1.2 or more, or 1.5 or more. From the viewpoint of ensuring the fluidity of the electrolyte, the orientation ratio (X / Y) may be 2.2 or less, 2.1 or less, or 2.0 or less.

[0020] In the present disclosure, the degree of orientation of the electrode assembly is measured as follows. FIB-SEM images of the cut surface of the electrode layer included in the electrode assembly are taken at a point located in a region below the center of the electrode assembly to be measured and at a point located in a region above the center. Next, a 3D model of the electrode layer is created from the obtained FIB-SEM image using 3D simulation software. Lines corresponding to the long axes of the active material particles are extracted from the created 3D model, and the degree of orientation is calculated from the ratio of the lengths of each component when the surface directions are the x-direction and y-direction, and the thickness is the z-direction. Orientation degree = length of the z component of the major axis of the active material particle / length of the major axis of the active material particle

[0021] In measuring the degree of orientation, the positions of the points located in the lower region and the upper region of the electrode body can be determined arbitrarily. For example, when the electrode body is equally divided into X regions along the direction of gravity, the points may be determined as 1 / X from the bottom and 1 / X from the top. X can be selected from the range of 3 to 10, for example.

[0022] The electrode layer used to measure the degree of orientation X and the electrode layer used to measure the degree of orientation Y may be the same or different.

[0023] The degree of orientation of the electrode layer can be adjusted by using particles having an aspect ratio of more than 1 as the active material particles contained in the electrode layer and by controlling the direction of the major axis of the particles in the electrode layer. In the present disclosure, the aspect ratio of a particle means the major axis dimension of the particle divided by the minor axis dimension. The aspect ratio of the active material particles contained in the electrode layer is not particularly limited as long as it is greater than 1, and can be selected depending on the shape, size, etc. of the electrode body. From the viewpoint of ease of adjusting the degree of orientation of the electrode layer, the aspect ratio of the active material particles may be 1.1 or more, 1.2 or more, or 1.5 or more. The upper limit of the aspect ratio of the active material particles is not particularly limited, and may be, for example, 2.0 or less.

[0024] Graphite particles are an example of the active material particles contained in the electrode layer and having an aspect ratio of greater than 1. The graphite particles are used as the active material of the negative electrode layer. The volume of graphite particles changes significantly due to the insertion or removal of lithium ions, so the variation in the amount of electrolyte inflow can be effectively suppressed by controlling the degree of orientation of the electrode layer containing the graphite particles.

[0025] Since the graphite particles are conductive, the direction of the long axis can be controlled by applying a magnetic field. A method of applying a magnetic field to the graphite particles contained in the electrode layer includes a method of placing the electrode layer between electromagnets arranged to sandwich the electrode layer and passing a current through the electromagnets. The magnetic field may be applied while the electrode layer is stationary or while the electrode layer is moving. The direction of the long axis of the active material contained in the electrode layer can be controlled by changing the strength of the magnetic field, the time of passage through the electrode layer, etc.

[0026] The electrode body may be a wound body of electrode layers or a laminate of electrode layers. A wound electrode layer body is an object obtained by winding one electrode layer, and a laminate of electrode layers is an object obtained by stacking a plurality of electrode layers of the same dimensions. When the electrode body is a wound body of electrode layers, the shape of the end face of the wound body is not particularly limited and may be circular, elliptical, rectangular, or the like. When the electrode body is a wound body of electrode layers, the axial direction of the wound body (direction perpendicular to the end face of the wound body) is not particularly limited, and may be the direction of gravity, the horizontal direction, or an oblique direction. When the electrode body is a laminate of electrode layers, the plane direction of the laminate (direction perpendicular to the thickness direction of the laminate) is not particularly limited, and may be the direction of gravity, a horizontal direction, or an oblique direction.

[0027] The electrode layer included in the electrode assembly may include a negative electrode layer, a separator, and a positive electrode layer. In this case, it is sufficient that either the negative electrode layer or the positive electrode layer satisfies the above-mentioned condition for the degree of orientation, and it is preferable that at least the negative electrode layer satisfies the above-mentioned condition for the degree of orientation.

[0028] The types of the negative electrode layer, separator, and positive electrode layer contained in the electrode layer are not particularly limited, and can be selected from negative electrode layers, separators, and positive electrode layers used in known secondary batteries. Examples of the negative electrode layer, the separator and the positive electrode layer are shown below.

[0029] The negative electrode layer includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. Specific examples of the negative electrode current collector include foils and meshes containing metals such as copper, copper alloys, nickel, titanium, and stainless steel. The negative electrode mixture layer may be a layer containing negative electrode active material particles, a binder, and a conductive assistant that is included as needed. Specific examples of the negative electrode active material particles include graphite particles such as natural graphite and artificial graphite, silicon particles, etc. The volume average particle size of the negative electrode active material particles is not particularly limited and can be selected from the range of 5 μm to 30 μm. Specific examples of binders include polyvinylidene fluoride, polyethylene, polypropylene, polyethylene terephthalate, cellulose, nitrocellulose, carboxymethyl cellulose, polyethylene oxide, polyepichlorohydrin, polyacrylonitrile, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), polyacrylate, and polymethacrylate. Specific examples of the conductive assistant include carbon black (acetylene black, thermal black, furnace black, etc.) and carbon materials such as carbon nanotubes.

[0030] The positive electrode layer includes a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. Specific examples of the positive electrode current collector include foils and meshes containing metals such as aluminum, aluminum alloys, nickel, titanium, and stainless steel. The positive electrode mixture layer may be a layer containing positive electrode active material particles, a binder, and an optional conductive assistant.

[0031] Specific examples of the positive electrode active material particles include particles of lithium transition metal composite oxides. Examples of the transition metal include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Specific examples of lithium transition metal composite oxides include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), and their composite oxides (LiCo x Ni y Mn z O2, x+y+z=1), and a double oxide containing an additive element M' (LiCo a Ni b Mn c M' d O2, a+b+c+d=1, M': Al, Mg, Ti, Zr or Ge), spinel-type lithium manganese oxide (LiMn2O4), olivine-type LiMPO4 (M: Co, Ni, Mn, Fe), etc. Among the above, lithium transition metal composite oxides containing at least one selected from Ni, Co, and Mn are preferred, and lithium transition metal composite oxides containing Ni, Co, and Mn (NCM, nickel cobalt manganese oxide) are more preferred. The volume average particle size of the positive electrode active material particles is not particularly limited and can be selected from the range of 5 μm to 30 μm. Specific examples of the binder and conductive assistant include the binder and conductive assistant contained in the negative electrode mixture layer described above.

[0032] Specific examples of the separator include nonwoven fabric, cloth, and microporous film that are mainly made of polyolefin such as polyethylene and polypropylene.

[0033] <Secondary battery> The secondary battery of the present disclosure includes the electrode assembly of the present disclosure described above and an electrolyte solution. As the electrolytic solution, any solution in which an electrolyte is dissolved in a non-aqueous solvent can be used without particular limitation. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, and cyclic sulfonates. The non-aqueous solvent may be a mixed solvent of two or more kinds of non-aqueous solvents, for example, a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0034] Examples of the electrolyte include inorganic lithium salts, fluorine-containing organic lithium salts, oxalatoborate salts, etc. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0035] Specific types of secondary batteries include lithium ion secondary batteries and sodium ion secondary batteries. The shape of the secondary battery is not particularly limited, and may be cylindrical, square, or the like. EXAMPLES

[0036] Examples will be described below, but the present invention is not limited to these examples in any way.

[0037] (1) Creating the electrode body Using 3D simulation software, a simulation model of an electrode body was created in which the degree of orientation increases from the top to the bottom of the electrode body (excluding Comparative Example 1), and the orientation degree X of the electrode layer in the region below the center of the electrode body and the orientation degree Y of the electrode layer in the region above the center have the values ​​shown in Table 1. The simulation model of the electrode body created was a wound body of electrode layers, with the end face shaped like a rectangle with a long side of 50 mm and a short side of 10 mm, and the axial dimension of 100 mm. In the simulation, the direction of the long side of the end face of the wound body corresponds to the direction of gravity.

[0038] (2) Analysis of flow velocity ratio Using a simulation model of the electrode body, the ratio (max / min) of the maximum flow velocity (max) to the minimum flow velocity (min) of the electrolyte in the electrode body during high-rate discharge (i.e., when the negative electrode active material particles contracted) was analyzed. The results are shown in Table 1.

[0039] [Table 1]

[0040] The degree of orientation X of the electrode layer shown in Table 1 is a value at a point corresponding to 1 / 10 from the bottom when the electrode body is equally divided into 10 regions along the direction of gravity. The degree of orientation Y of the electrode layer shown in Table 1 is a value at a point corresponding to 1 / 10 from the top when the electrode body is equally divided into 10 regions along the direction of gravity. The flow rate ratios shown in Table 1 are values ​​at the center of the electrode assembly.

[0041] As shown in Table 1, the electrode assemblies of Examples 1 to 3 in which the orientation ratio X / Y value is greater than 1 have a smaller flow rate ratio at the center of the electrode assembly than the electrode assembly of Comparative Example 1 in which the orientation ratio X / Y value is 1. The results shown in Table 1 indicate that by restricting the inflow of electrolyte in the region below the electrode assembly, it is possible to reduce the variation in the amount of electrolyte inflow in the direction of gravity of the electrode assembly.

Claims

1. an electrode layer including active material particles; An electrode body for a secondary battery, in which an orientation ratio (X / Y) calculated from an orientation degree X of an electrode layer located in a region below the center in the direction of gravity and an orientation degree Y of an electrode layer located in a region above the center in the direction of gravity is greater than 1.

2. The electrode body for a secondary battery according to claim 1 , wherein the active material particles have an aspect ratio of greater than 1.

3. The electrode body for a secondary battery according to claim 1 , wherein the active material particles are graphite particles.

4. 2. The electrode body for a secondary battery according to claim 1, which is a wound body of an electrode layer or a laminate of an electrode layer.

5. A secondary battery comprising the electrode assembly according to any one of claims 1 to 4 and an electrolyte solution.

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