Method for manufacturing a positive electrode for a lithium-ion secondary battery

Abstract

Method for manufacturing a positive electrode for a lithium-ion secondary battery, wherein the method is characterized in that it comprises: Manufacturing a current collector film with a first main surface and a second main surface, which are opposite sides of the current collector film; Obtaining a granulated body in which a solvent remains, by mixing a positive electrode active material, a conductive material, a binder and the solvent together to obtain a mixture, and granulating the mixture. wherein the solids content of the granulated body is 65 wt% to 80 wt%; Obtaining a first positive electrode mixture layer by pressing the granulated body into a flat shape; Arranging the first positive electrode mixture layer on the first main surface; and Heating the current collector film in a state where the first positive electrode mixture layer containing the granulated body is arranged on the first main surface, such that the temperature of the current collector film becomes a softening point of the current collector film or higher, and that the temperature of the first positive electrode mixture layer is lower than a melting point of the binder.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to a method for producing a positive electrode for a lithium ion secondary battery. 2. Description of the related art

[0002] Japanese Patent Application Publication No. 2009-064770 (JP 2009-064770 A) discloses a method for manufacturing a non-aqueous electrolyte secondary battery in which a heat treatment is performed on a positive electrode current collector foil and a positive electrode mixture after the positive electrode mixture is applied to the positive electrode current collector foil and dried.

[0003] In JP 2009-064770 A, the current collector foil is softened to improve safety in a compression test. Specifically, the heat treatment to soften the current collector foil is performed after the positive electrode mixture is applied to the current collector foil and dried to form a positive electrode mixture layer. However, according to this method, the positive electrode mixture layer and the current collector foil are subjected to heat treatment at the same temperature. As a result, a binder contained in the positive electrode mixture layer is melted, and the resistance increases.

[0004] The reason why the soft current collector foil is not used from the beginning is that it is difficult to accurately form the positive electrode mixture layer on the easily deformable soft current collector foil.

[0005] Furthermore, the published patent applications US 2011 / 0052954 A1, US 2011 / 0143199 A1, US 2014 / 0342225 A1 and JP 2014-017200 A disclose electrodes or batteries and their manufacturing methods from the prior art. SUMMARY OF THE INVENTION

[0006] The present invention provides a method for manufacturing a positive electrode for a lithium ion secondary battery capable of softening a current collector foil while suppressing an increase in resistance.

[0007] One aspect of the present invention is a method for manufacturing a positive electrode for a lithium-ion secondary battery. The method includes: preparing a current collector foil having a first main surface and a second main surface that are opposite sides of the current collector foil; obtaining a granulated body in which a solvent remains by mixing a positive electrode active material, a conductive material, a binder, and the solvent together to obtain a mixture, and granulating the mixture, wherein the solid content ratio of the granulated body is 65 mass % to 80 mass %; obtaining a first positive electrode mixture layer by pressing the granulated body into a sheet shape; disposing the first positive electrode mixture layer on the first main surface;and heating the current collector foil in a state where the first positive electrode mixture layer containing the granulated body is disposed on the first main surface so that a temperature of the current collector foil becomes a softening point of the current collector foil or higher, and that a temperature of the first positive electrode mixture layer is lower than a melting point of the binder;

[0008] In the manufacturing method described above, the current collector foil is heated to a softening point or above in a state where the first positive electrode mixture layer is disposed on the first main surface of the current collector foil. As a result, the current collector foil is softened. At this time, the current collector foil is supported by the first positive electrode mixture layer. Therefore, a decrease in productivity caused by the softening of the current collector foil is suppressed.

[0009] Furthermore, the current collector foil is heated such that the temperature of the first positive electrode mixture layer is lower than the melting point of the binder. As a result, melting of the binder is suppressed. For example, the current collector foil can be locally heated using heating means such as a heating roller or the induction heating described below, and a rise in the temperature of the first positive electrode mixture layer can be suppressed. According to the manufacturing method described above, during heating of the current collector foil, the first positive electrode mixture layer is not dried and is wet because it contains the granulated body in which the solvent remains.Therefore, heat applied to the current collector foil is consumed as the heat of vaporization of the solvent contained in the first positive electrode mixture layer, and an increase in the temperature of the first positive electrode mixture layer is suppressed. Accordingly, in the above-described manufacturing method, the temperature of the first positive electrode mixture layer can be controlled to be lower than the melting point of the binder.

[0010] In the above-described manufacturing method, a granulated body is used as the positive electrode mixture instead of a slurry. The granulated body is an aggregate of wet granulated particles. The amount of solvent in the granulated body is less than that in the slurry. By pressing the granulated body into a sheet shape, a separate positive electrode mixture layer is formed. This positive electrode mixture layer can support the current collector foil more strongly than a coating film formed from a slurry. When the current collector foil is heated in a state where the slurry is applied to the current collector foil, it is expected that marked binder migration occurs and the resistance increases. Accordingly, in the above-described manufacturing method, the mixture layer is formed from the granulated body.

[0011] A positive electrode for a lithium-ion secondary battery (hereinafter also referred to as "positive electrode") is manufactured, wherein the positive electrode includes: the current collector foil softened as described above; and the positive electrode mixture layer formed on the current collector foil, in which the melting of the binder is suppressed as described above. According to this positive electrode, the current collector foil can be softened and, at the same time, an increase in resistance can be suppressed.

[0012] In the aspect of the present invention, the current collector foil may be an aluminum alloy foil containing 1.2 mass% to 1.7 mass% of iron.

[0013] The aluminum (Al) alloy foil described above has a lower softening point than other Al alloy foils or pure Al foil. Accordingly, the amount of heat in the heating step can be reduced. As a result, the amount of heat applied to the binder can be reduced.

[0014] In the aspect of the present invention, a heated roller and the second main surface may be brought into contact with each other during heating of the current collector foil.

[0015] In the above-described aspect, the current collector foil can be heated directly. Furthermore, the first positive electrode mixture layer disposed on the first main surface is not heated directly. Accordingly, the amount of heat applied to the binder can be reduced. Hereinafter, the heated roller is also referred to as a "heating roller."

[0016] In the aspect of the present invention, a temperature of the roller during heating of the current collector foil may be 200 °C to 400 °C.

[0017] By adjusting the temperature of the roller to be within the temperature range described above, the current collector foil can be easily softened.

[0018] In the aspect of the present invention, a contact time between the roller and the second main surface during heating of the current collector foil may be 0.05 seconds to 2.0 seconds.

[0019] Because the heating roller has high heat transfer efficiency, the current collector foil can be softened within a short period of time. By setting a short contact time, melting of the binder can be prevented.

[0020] In the aspect of the present invention, a tapping density of the positive electrode active material may be 1.2 g / cm3 up to 1.9 g / cm 3 be.

[0021] If the tapped density is prevented from reaching 1.9 g / cm 3 or less, it is unlikely that the first positive electrode mixture layer will be dried during heating of the current collector foil. By setting the tapping density to 1.2 g / cm 3 or more, the density of the first positive electrode mixture layer can be increased.

[0022] The method according to the aspect of the present invention may further include disposing a second positive electrode mixture layer on the second main surface after heating the current collector foil.

[0023] According to the manufacturing method described above, the softened current collector foil is supported by the first positive electrode mixture layer. Accordingly, the second positive electrode mixture layer can be accurately formed and arranged on the second main surface. As a result, a positive electrode can be manufactured in which the positive electrode mixture layer is formed on both main surfaces of the current collector foil.

[0024] According to aspects of the present invention, it is possible to provide a method for manufacturing a positive electrode for a lithium ion secondary battery capable of softening a current collector foil while suppressing an increase in resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, in which like characters designate like elements. It shows: Fig. 1 is a flowchart showing the summary of a method for manufacturing a lithium ion secondary battery according to an embodiment of the present invention; Fig. 2 is a flowchart showing the summary of a method for manufacturing a positive electrode for a lithium ion secondary battery according to the embodiment of the present invention; Fig. 3 is a schematic diagram showing a configuration example of a positive electrode according to the embodiment of the present invention; Fig. 4 is a schematic diagram showing a pressing step, a first arranging step, and a film heating step according to the embodiment of the present invention; Fig. 5 is a schematic diagram showing a configuration example of a negative electrode according to the embodiment of the present invention; Fig. 6 is a schematic diagram showing a configuration example of an electrode body according to the embodiment of the present invention; Fig. 7 is a schematic diagram showing a configuration example of a lithium ion secondary battery according to the embodiment of the present invention; Fig. Figure 8 is a schematic cross-sectional view along the line VIII-VIII of the Fig. 7; and Fig. 9 is a table showing the manufacturing conditions of positive electrodes according to respective samples, experimental results of the positive electrodes according to respective samples, and the experimental results of batteries manufactured using the positive electrodes according to respective samples. DETAILED DESCRIPTION OF EMBODIMENTS

[0026] An embodiment of the present invention (hereinafter referred to as "the embodiment") will be described below. However, the embodiment is not limited to the following description.

[0027] Fig. 1 is a flowchart showing the summary of a method for manufacturing a lithium-ion secondary battery according to the embodiment. As shown in Fig. As shown in Figure 1, the manufacturing method includes a positive electrode manufacturing step S100, a negative electrode manufacturing step S200, an electrode body manufacturing step S300, a case insertion step S400, and a liquid injection step S500. Each step will be described below.

[0028] In the positive electrode manufacturing step S100, a Fig. 3 is manufactured. The positive electrode 10 is an elongated, ribbon-shaped sheet member. The positive electrode 10 includes: a current collector foil 11 having a first main surface 11a and a second main surface 11b, the first main surface 11a and the second main surface 11b being opposite sides of the current collector foil; a first positive electrode mixture layer 12a disposed on the first main surface 11a; and a second positive electrode mixture layer 12b disposed on the second main surface 11b. The positive electrode 10 includes an exposure portion Ep where the current collector foil 11 is exposed. The exposure portion Ep is provided for connection to an external terminal.

[0029] Fig. Figure 2 is a flow chart showing the summary of the positive electrode manufacturing step S100. As shown in Fig. As shown in Figure 2, the positive electrode manufacturing step S100, that is, the method for manufacturing a positive electrode for a lithium-ion secondary battery, includes a manufacturing step S101, a granulation step S102, a pressing step S103, a first arranging step S104, a film heating step S105, a first drying step S106, a second arranging step S107, and a second drying step S108. The respective steps are performed in this order. The first arranging step S104 and the film heating step S105 may be performed simultaneously. By performing them simultaneously, the steps can be simplified.

[0030] In the manufacturing step S101, the Fig. 3, the current collector foil 11 having the first main surface 11a and the second main surface 11b is manufactured, wherein the first main surface 11a and the second main surface 11b are opposite sides of the current collector foil 11. Examples of the current collector foil according to the embodiment include: a pure metal foil formed from a single metal element; and an alloy foil formed from multiple metal elements. The current collector foil may contain impurities other than the metal elements, which are inevitably incorporated during the manufacturing process. From the perspective of mechanical properties and corrosion resistance, it is preferable that the current collector foil be a pure Al foil or an Al alloy foil. When heated to a softening point or above, the current collector foil softens.The "softening point" described here refers to a temperature at which the elimination of point defects in crystals and / or the rearrangement of dislocations in a metal material begins. Even if the current collector foil is heated to a temperature lower than the softening point, the softening of the current collector foil is not significant.

[0031] The softening point of current collector foil varies depending on the type of metal element, processing history, and the like. The softening point of pure Al foil is approximately 200°C to 300°C. The softening point of Al alloy foil varies depending on the constituent element and its addition amount. Accordingly, by selecting an alloy composition with a low softening point, the amount of heat required for softening can be reduced. For example, an Al alloy foil containing 1.2 mass% to 1.7 mass% iron (Fe) and 97.9 mass% or more of Al is preferable. This Al alloy foil is called a low-temperature softening Al foil. The softening point of the low-temperature softening Al foil is lower than 200°C (for example, approximately 160°C).

[0032] The thickness of the current collector foil 11 is preferably 3 µm to 30 µm, more preferably 5 µm to 25 µm, and even more preferably 5 µm to 20 µm. It is preferable that the current collector foil 11 is not softened at this time. This is because productivity decreases when a soft current collector foil is used from the outset. From this point of view, the percentage elongation at break of the current collector foil 11 at this time is preferably 3.0% or less.

[0033] Here, "percent elongation at break" refers to a value measured and calculated according to "JIS Z 2241: 2011 Metal Material Tensile Testing Method." "JIS Z 2241" is defined by Japanese Industrial Standards. In "JIS Z 2241: 2011 Metal Material Tensile Testing Method," the percent elongation at break is defined as the ratio (in percent) of the percent residual elongation after break to the original gauge length L0. The gauge length refers to the length over which the elongation is measured in a parallel section of a test piece. The original gauge length L0 refers to the gauge length of the test piece measured at room temperature before the test and is indicated on the test piece. The percent residual elongation refers to the increase in the original gauge length L0 after the removal of a specified stress, expressed as a percentage of the original gauge length L0. The percent residual elongation at break is a value (L u-L0), which is calculated by subtracting the original measuring length L0 from a final measuring length after fraction L u The final gauge length after fracture is the gauge length of the test specimen measured at room temperature after fracture and is indicated on the test specimen. The final gauge length after fracture L u is measured after two broken test pieces have been joined together in such a way that their axes lie on a straight line.

[0034] In the granulation step S102, a granulated body is produced. The granulated body is an aggregate of granulated particles. The granulated body can be produced by mixing a positive electrode active material, a conductive material, a binder, and a solvent. A granulation method is not particularly limited. For example, the above materials can be mixed together in a predetermined solid content ratio using a general mixing device. For example, the mixing device can be a planetary mixer. The solid content ratio of the granulated body is 65 mass% to 80 mass%. A positive electrode mixture layer formed from the granulated body is homogeneous and can stably support the current collector foil.Furthermore, since the solvent remains in the granulated body, a rise in the temperature of the first positive electrode mixture layer 12a in the film heating step S105 can be suppressed. The lower limit of the solid content of the granulated body may be 70 mass %, and the upper limit may be 75 mass %. For example, N-methyl-2-pyrrolidone (NMP) can be used as the solvent.

[0035] The proportion of the positive electrode active material in the solid content of the granulated body is, for example, 85 mass% to 97 mass%, and preferably 87 mass% to 95 mass%, and more preferably 89 mass% to 93 mass%. Any material that can function as a positive electrode active material of a lithium-ion secondary battery can be used as the positive electrode active material. For example, powder of a Li-containing compound such as LiCoO2, LiNiO2, LiNi a Co bO2 (where a + b = 1, 0 < a < 1 and 0 < b < 1), LiMnO2, LiMn2O4, LiNi a Co b Mn cO2 (where a + b + c = 1, 0 < a < 1, 0 < b < 1, and 0 < c < 1) or LiFePO4 may be used. The positive electrode active material powder may contain hollow particles. The hollow particles are particles that have one or more voids. Because the positive electrode active material contains the hollow particles, the solvent is likely to remain in the voids. As a result, a rise in the temperature of the first positive electrode mixture layer 12a in the film heating step S105 can be suppressed. The positive electrode active material containing the hollow particles can be confirmed by observing a section of the first positive electrode mixture layer 12a using a scanning electron microscope (SEM). The proportion of the hollow particles in the positive electrode active material powder may be 5% or more, 50% or more, or 80% or more.

[0036] The tap density of the positive electrode active material powder can be 1.2 g / cm 3 up to 1.9 g / cm 3 According to the present inventors' study, as the tap density of the positive electrode active material increases, the positive electrode mixture layer is more easily dried in the film heating step S105. By preventing a tap density of 1.9 g / cm 3 or less, an increase in the temperature of the first positive electrode mixture layer 12a in the film heating step S105 can be prevented. On the other hand, by setting the tapping density to 1.2 g / cm 3 or more increase the mixture density of the positive electrode mixture layer. The lower limit of the tapped density of the positive electrode active material can be 1.3 g / cm 3 or 1.4 g / cm 3 The upper limit of the tapped or tamped density can be 1.8 g / cm 3 or 1.7 g / cm 3be.

[0037] For example, acetylene black (AB) or graphite can be used as the conductive material. The proportion of the conductive material in the solid content of the granulated body is, for example, 1 mass% to 10 mass%, and is preferably 3 mass% to 10 mass%, and more preferably 5 mass% to 10 mass%. For example, polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) can be used as the binder. The proportion of the binder in the solid content of the granulated body is, for example, 1 mass% to 10 mass%, and is preferably 1 mass% to 8 mass%, and more preferably 1 mass% to 5 mass%.

[0038] In the pressing step S103, the first positive electrode mixture layer 12a is produced by pressing the granulated body into a flat shape.

[0039] Fig. Fig. 4 is a schematic diagram showing the pressing step S103 and the first arranging step S104 and the film heating step S105 described below. The granulated body 4 obtained in the granulating step S102 is supplied to a feeder 95 of a Fig. 4 shown electrode manufacturing device 90. The granulated body 4 is guided by the feed device 95 onto an A-roller 91. In Fig. 4, arrows indicate the rotation directions of the respective roller elements. The granulated body 4 is transported along the A roller 91 in the direction of the arrow and reaches a gap between the A roller 91 and a B roller 92. In the gap, pressure is applied to the granulated body 4 by the A roller 91 and the B roller 92 such that the granulated body 4 is formed into a sheet-like first positive electrode mixture layer 12a. The width of the first positive electrode mixture layer 12a is adjusted by separating elements 94. The coating weight (mass per unit area) of the first positive electrode mixture layer 12a is adjusted by the gap between the A roller 91 and the B roller 92.

[0040] In the first arrangement step S104, the first positive electrode mixture layer 12a is arranged on the first main surface 11a of the current collector foil 11. As shown in Fig. 4, the first positive electrode mixture layer 12a formed in a sheet shape is transported along the B roller 92 in the arrow direction.

[0041] The current collector foil 11 is transported along a C-roller 93 in the direction of the arrow. In the gap between the B-roller 92 and the C-roller 93, pressure is applied by the B-roller 92 and the C-roller 93 to the first positive electrode mixture layer 12a and the current collector foil 11 such that the first positive electrode mixture layer 12a is transferred from the B-roller 92 to a predetermined position on the first main surface 11a. At the same time, the first positive electrode mixture layer 12a is pressure-bonded to the first main surface 11a.

[0042] In the Fig. In the electrode manufacturing apparatus 90 shown in Figure 4, the first disposing step S104 and the foil heating step S105 are performed simultaneously. That is, the C-roller 93 contacting the second main surface 11b of the current collector foil 11 is a heating roller heated to a predetermined temperature. As a result, the current collector foil 11 is heated by the second main surface 11b so that the current collector foil 11 is softened. At this time, the first positive electrode mixture layer 12a has already been disposed on at least a part of the first main surface 11a, and the current collector foil 11 is supported by the first positive electrode mixture layer 12a. As a result, the current collector foil 11 and the first positive electrode mixture layer 12a can be stably transported on the subsequent transport path even if the current collector foil 11 is softened.

[0043] According to this method, heat is not directly applied to the first positive electrode mixture layer 12a disposed on the first main surface 11a. Furthermore, the first positive electrode mixture layer 12a is wet. Therefore, heat transferred to the first positive electrode mixture layer 12a through the current collector foil 11 is consumed as the heat of vaporization of the solvent contained in the first positive electrode mixture layer 12a. A rise in the temperature of the first positive electrode mixture layer 12a is suppressed. As a result, the temperature of the first positive electrode mixture layer 12a can be controlled to be lower than the melting point of the binder.

[0044] Here, "binder melting point" refers to an upper peak temperature in differential scanning calorimetry (DSC) according to "JIS 7121:2012 Test Method for Transition Temperatures of Plastics." "JIS K 7121" is defined by Japanese Industrial Standards. Specifically, DSC includes power-compensated differential scanning calorimetry (power-compensated DSC) and heat-flux differential scanning calorimetry (heat-flux DSC). In power-compensated DSC, the difference between heat energy inputs applied to a test sample and a reference material per unit time such that the temperatures of the test sample and the reference material are equal is measured as a function of temperature while the temperatures of the test sample and the reference material are varied according to a set program.In heat flow DSC, a temperature difference between a test specimen and a reference material is measured as a function of temperature, while the temperatures of the test specimen and the reference material are varied according to a preset program. In heat flow DSC, a temperature difference between a test specimen and a reference material is proportional to the difference between the thermal energy input values ​​per unit time.

[0045] The temperature of the heating roller and the contact time between the heating roller and the current collector foil 11 are adjusted so that the current collector foil 11 is heated to a softening point or higher, and the temperature of the first positive electrode mixture layer 12a is lower than the melting point of the binder. Although the melting point of PVDF used as a binder of a battery depends on the molecular weight or the like, it is approximately 170°C. Therefore, in this case, the temperature of the first positive electrode mixture layer 12a is adjusted to be lower than 170°C.

[0046] When the current collector foil 11 is a pure Al foil or an Al alloy foil, the temperature of the heating roller is preferably 200°C to 400°C. Within the above-described temperature range, the current collector foil 11 can be heated to a softening point or above by contact within a short period of time. The lower limit of the temperature of the heating roller may be 250°C, and the upper limit thereof may be 350°C. The contact time can be adjusted by the rotation speed, diameter, and the like of the roller. The contact time is preferably 0.05 seconds to 2.0 seconds. As a result, the amount of heat applied to the first positive electrode mixture layer 12a can be reduced. The lower limit of the contact time may be 0.1 seconds or 0.2 seconds. The upper limit of the contact time may be 1.0 seconds or 0.4 seconds.

[0047] The percentage elongation at break of the current collector foil 11 that has undergone the foil heating step S105 is preferably 5.5% to 7.5%. The current collector foil having a percentage elongation at break of 5.5% or more can withstand the expansion and shrinkage of the positive electrode mixture layer in the battery. The current collector foil having a percentage elongation at break of 7.5% or less is easy to handle after softening. The lower limit of the percentage elongation at break of the current collector foil that has undergone the foil heating step S105 may be 5.9% or 6.5%. The upper limit of the percentage elongation at break of the current collector foil that has undergone the foil heating step S105 may be 7.4% or 7.3%.

[0048] The film heating step S105 and the first disposing step S104 do not need to be performed simultaneously, as long as the steps are performed in a state where the first positive electrode mixture layer 12a is disposed on the first main surface 11a. For example, the following configuration may be adopted in which, instead of using a heating roller as the C-roller 93, a heating roller is separately provided on a transport path after the C-roller 93, so that the second main surface 11b and the heating roller are brought into contact with each other.

[0049] In the foil heating step S105, the heating means is not limited to the heating roller. However, it is preferable to use a heating means that can directly apply heat to the current collector foil 11 without substantially applying heat to the first positive electrode mixture layer 12a. For example, the current collector foil can be heated by induction heating. The frequency of the induction heating can be appropriately adjusted according to the metal deformation of the current collector foil. According to the induction heating, the current collector foil itself can generate heat without contact. As a result, as in the case of the heating roller, a rise in the temperature of the first positive electrode mixture layer 12a can be suppressed.

[0050] After the film heating step S105, the first positive electrode mixture layer 12a is dried in a hot air drying oven (not shown) in the first drying step S106.

[0051] In the second disposition step S107, the second positive electrode mixture layer 12b is disposed on the second main surface 11b of the current collector foil 11. The second positive electrode mixture layer 12b is prepared using the same method as the first positive electrode mixture layer 12a and is disposed on the second main surface 11b using the same method as the first disposition step S104. At this time, the current collector foil 11 has already been softened, but is supported by the first positive electrode mixture layer 12a. Accordingly, the second positive electrode mixture layer 12b can be accurately disposed on the second main surface 11b. However, in this step, it is preferable that the Fig. 4 is not heated. The reason for this is as follows: In the second arrangement step S107, the first positive electrode mixture layer 12a has already been dried by being brought into contact with the C-roller 93, and it is not preferable that the first positive electrode mixture layer 12a be further heated.

[0052] After the second arranging step S107, the second positive electrode mixture layer 12b is dried in the second drying step S108 in a hot air drying oven as in the case of the first positive electrode mixture layer 12a.

[0053] Next, the first positive electrode mixture layer 12a and the second positive electrode mixture layer 12b are pressed to a predetermined thickness and cut to a predetermined size. In this way, the Fig. 3 shown positive electrode 10 is completed.

[0054] In the above-described embodiment, the positive electrode mixture layer is formed from the granulated body. As a result, binder migration is prevented, and the diffusion of lithium ions (Li + ) is promoted. The positive electrode active material containing the granulated particles can be detected by observing a surface or a section of the positive electrode mixture layer using an SEM. The distribution of the binder in the thickness direction of the positive electrode mixture layer can be evaluated based on, for example, a migration index (MI).

[0055] MI can be calculated by analyzing a cross-section of the positive electrode using scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX). The measurement procedure is as follows.

[0056] First, a sample is cut from the positive electrode for cross-sectional observation, and a section of the sample is cleaned using, for example, a cross-section polisher. At this time, the section is not necessarily perpendicular to the surface of the positive electrode mixture layer. For example, the section may be a surface inclined by about ±5° from a surface perpendicular to the surface of the positive electrode mixture layer. Next, the section is examined by SEM-EDX. For example, when the binder is a resin such as fluorine (F)-containing PVDF, F mapping can be performed. When the binder is not a fluororesin, an element is appropriately selected as an index.In surface analysis, the section is divided into two regions in the thickness direction, including: a first region located on the current collector foil side; and a second region located on the surface of the positive electrode mixture layer. MI can be calculated by dividing an integrated value of the detected intensity of F in the second region by an integrated value of the detected intensity of F in the first region. When the binder is uniformly distributed in the thickness direction of the positive electrode mixture layer, MI is close to 1.0.

[0057] Since the positive electrode mixture layer is formed from the granulated body in the embodiment, MI falls within a range of 0.7 to 1.3. In contrast, for a positive electrode mixture layer formed from a slurry containing a large amount of solvent, binder migration is significant, and MI exceeds 1.5.

[0058] In the negative electrode manufacturing step S200, a Fig. 5 is manufactured. The negative electrode 20 is an elongated, belt-shaped sheet member. The negative electrode 20 includes a negative electrode current collector foil 21; and a negative electrode mixture layer 22 disposed on both main surfaces of the negative electrode current collector foil 21. The negative electrode 20 includes an exposure portion Ep where the negative electrode current collector foil 21 is exposed. The exposure portion Ep is provided for connection to an external terminal. The negative electrode current collector foil 21 is, for example, copper (Cu) foil.

[0059] The negative electrode 20 can be manufactured, for example, as follows. A negative electrode active material, a thickener, and a binder are kneaded together in water to prepare a negative electrode mixture slurry. Using a die coater or the like, the negative electrode mixture slurry is applied to a predetermined position on the negative electrode current collector foil 21. After application, the negative electrode mixture slurry (coating film) is dried. In this way, the negative electrode mixture layer 22 can be formed. As in the case of the positive electrode mixture layer, the negative electrode mixture layer 22 can also be formed from a granulated body.

[0060] Any material that can function as a negative electrode active material of a lithium-ion secondary battery can be used as the negative electrode active material. For example, a negative electrode active material made of carbon, such as graphite or coke, or a negative electrode active material made of an alloy of silicon (Si), tin (Sn), or the like can be used. For example, carboxymethyl cellulose (CMC) can be used as the thickener. For example, styrene-butadiene rubber (SBR) can be used as the binder. The composition (mass ratio; negative electrode active material: thickener: binder) of the negative electrode mixture can be, for example, about 98:1:1.

[0061] In the electrode body manufacturing step S300, an electrode body 80 is manufactured. Fig. 6 is a schematic diagram showing a configuration example of the electrode body 80. As shown in Fig. As shown in Figure 6, the electrode body 80 is manufactured by arranging the positive electrode 10 and the negative electrode 20 so as to face each other with separators 40 interposed therebetween, and winding the components along a longitudinal direction thereof. At this time, the exposed portions Ep of the positive electrode 10 and the negative electrode 20 are arranged at opposite end portions on the winding axis AW. The electrode body 80 is wound into an elliptical shape and then pressed into a flat shape.

[0062] As the separator, a microporous membrane made of polyethylene (PE), polypropylene (PP), or the like is preferable. The thickness of the separator is, for example, 5 µm to 40 µm. The pore size and porosity of the separator can be appropriately adjusted to adjust the air permeability to a desired value. Multiple microporous membranes can be laminated or layered in the separator, or a heat-resistant layer containing an inorganic filler (e.g., alumina) can be formed on its surface.

[0063] In the housing insertion step S400, a Fig. 7 is manufactured. The battery case 50 includes a rectangular case body 52 with a bottom and a lid 54. The battery case 50 is formed, for example, from an aluminum alloy. A positive electrode terminal 70 and a negative electrode terminal 72 are provided on the lid 54.

[0064] Fig. Figure 8 is a schematic sectional view along the line VIII-VIII of Fig. 7. As in Fig. As shown in Figure 8, the exposure portion Ep (current collector foil 11) of the electrode body 80 on the positive electrode 10 side is electrically connected to the positive electrode terminal 70. Similarly, the exposure portion Ep (negative electrode current collector foil 21) on the negative electrode 20 side is electrically connected to the negative electrode terminal 72. Next, the electrode body 80 is inserted into the case body 52. ​​The case body 52 and the lid 54 are joined together by, for example, laser welding.

[0065] In the liquid injection step S500, an electrolyte solution is injected into the battery case 50. For example, the electrolyte solution is injected through a liquid injection hole (not shown) provided on the battery case 50. After the injection, the liquid injection hole is sealed by predetermined sealing means.

[0066] The electrolyte solution is prepared by dissolving a Li salt in an aprotic solvent. Examples of the usable solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and γ-butyrolactone (γBL); and chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Among these aprotic solvents, a combination of two or more types is preferable from the viewpoint of electrical conductivity and electrochemical stability. In particular, a mixture of a cyclic carbonate and a chain carbonate is preferably used. At this time, a volume ratio of the cyclic carbonate to the chain carbonate is preferably about 1:9 to 5:5.

[0067] Examples of lithium salts that can be used include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(trifluoromethanesulfonyl)imide (Li(CF3SO2)2N), or lithium trifluoromethanesulfonate (LiCF3SO3). Among these lithium salts, a combination of two or more types can be used. The concentration of the lithium salt in the electrolyte solution is preferably about 0.5 mol / L to 2.0 mol / L.

[0068] As described above, a Fig. 7 and Fig. 8 was manufactured. The embodiment was described using the rectangular battery as an example. However, the embodiment is not limited to the rectangular battery. The embodiment is applicable to, for example, a cylindrical battery or a laminate battery. The electrode body is not limited to a wound electrode body. The electrode body may be a laminated or layered (also referred to as "stacked") electrode group.

[0069] Below, the embodiment will be described in more detail using examples. However, the embodiment is not limited to the following examples.

[0070] Positive electrodes for a lithium-ion secondary battery according to Samples A1 to A10 and Samples B1 to B6 were prepared as follows. Samples A1 to A10 correspond to Examples, and Samples B1 to B6 correspond to Comparative Examples.

[0071] The following describes the manufacturing step S101 using Sample A1. In the manufacturing step S101, the following materials were prepared.

[0072] Current collector foil: A8021 foil with a thickness of 15 µm Positive electrode active material: LiNi powder 1 / 3 Co 1 / 3 Mn 1 / 3 O2 particles, the hollow particles with a tap density of 1.5 g / cm 3 Contains conductive material: acetylene black Binder: PVDF

[0073] "A8021" described here and "A1085" described below are foils with a chemical composition defined according to "JIS H 4160:2006 Aluminum and Aluminum Alloy Foils." "JIS H 4160" is defined by Japanese Industrial Standards. "A8021" is an aluminum alloy foil containing 1.2 to 1.7 wt% Fe. "A1085" is a so-called pure aluminum foil.

[0074] In the granulation step S102, NMP was converted to LiNi 1.3 Co 1 / 3 Mn1 / 3 O2 (100 parts by mass), acetylene black (8 parts by mass), and PVDF (2 parts by mass) were added. The components were mixed together using a planetary mixer to obtain a mixture, and the mixture was granulated. At this time, the solid content was 75% by mass. As a result, a granulated body was obtained.

[0075] Next, the pressing step S103, the first arranging step S104, the film heating step S105 and the first drying step S106 are described. Fig. The electrode manufacturing apparatus 90 shown in Figure 4 was manufactured. A temperature-adjustable heating roller was used as the C-roller 93. The temperature of the C-roller 93 was set to 180°C, and the rotation speed of the C-roller 93 was adjusted so that the contact time between the C-roller 93 and the current collector foil 11 (second main surface 11b) was 2 seconds.

[0076] As in Fig. As shown in Figure 4, the granulated body 4 was pressed in the gap between the A roller 91 and the B roller 92. As a result, the first positive electrode mixture layer 12a was obtained. Next, in the gap between the B roller 92 and the C roller 93, the first positive electrode mixture layer 12a was press-bonded to the first main surface 11a. During the press-bonding, the current collector foil 11 was heated by bringing the C roller 93 and the second main surface 11b into contact with each other. At this time, the maximum temperature of the first positive electrode mixture layer 12a was 160°C or less. The first positive electrode mixture layer 12a was dried in a hot-air drying oven in which the hot-air drying temperature was set to 150°C.

[0077] Next, the second assembly step S107 and the second drying step S108 are described. The second positive electrode mixture layer 12b was obtained using the same method as for the first positive electrode mixture layer 12a, except that the electrode manufacturing apparatus 90 including the unheated C-roller 93 was used. Next, the second positive electrode mixture layer 12b was pressure-bonded to the second main surface 11b. After drying the second positive electrode mixture layer 12b in a hot-air drying oven in which the hot-air drying temperature was set to 150°C, the first positive electrode mixture layer 12a and the second positive electrode mixture layer 12b were pressed to a predetermined thickness and then cut to a predetermined size. As a result, the positive electrode 10 according to Sample A1 was obtained. Manufacturing conditions of Sample A1 are shown in the table of Fig. 9 shown.

[0078] Samples A2 to A6 were obtained using the same procedure as for Sample A1, except that the temperature of the heating roller and the contact time were changed as in Fig. 9 shown.

[0079] Samples A7 to A9 were obtained using the same procedure as Sample A4, except that the tap density of the positive electrode active material changed as shown in Fig. 9 shown.

[0080] Sample A10 was obtained using the same procedure as Sample A1, except that: the A1085 foil with a thickness of 15 µm was used as the current collector foil as in Fig. 9; and the temperature of the heating roller and the contact time were changed as shown in Fig. 9 shown.

[0081] Sample B1 was obtained using the same procedure as Sample A4, except that: the A1085 foil with a thickness of 15 µm was used as the current collector foil as in Fig. 9 shown.

[0082] Sample B2 was obtained using the same procedure as Sample A1, except that the temperature of the heating roller and the contact time were changed as in Fig. 9 shown.

[0083] Sample B3 was obtained using the same procedure as Sample A1, except that the heating roller was not used, that is, the film heating step S105 was not performed.

[0084] Sample B4 was obtained using the same procedure as Sample A1, except that the temperature of the heating roller and the contact time were changed as in Fig. 9 shown.

[0085] Sample B5 was obtained using the same procedure as Sample B3, except that the positive electrode mixture layer was dried in a hot air drying oven in which the hot air drying temperature was set to 250 °C.

[0086] Sample B6 was prepared under the same conditions as those of Sample A4 except for the following conditions. In the granulation step S102, a positive electrode mixture slurry with a solid content of 52 mass% was prepared by increasing the amount of NMP. Using a die coater, the positive electrode mixture slurry was applied to the current collector foil (first main surface) to form a slurry coating film. In a state where this slurry coating film was formed on the first main surface, the heating roller and the second main surface were heated under Fig. 9. Next, the slurry coating film was dried in a hot air drying oven to form a first positive electrode mixture layer. Further, the positive electrode mixture slurry was applied to the second main surface to form a second positive electrode mixture layer.

[0087] Using each of the samples, a lithium-ion secondary battery with a nominal capacity of 4 Ah was fabricated for evaluation. The manufacturing process is as follows.

[0088] The negative electrode was prepared as in Fig. 5. Specifically, graphite (negative electrode active material), CMC (thickener), and SBR (binder) were kneaded together in water to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was coated on both main surfaces of a Cu foil (negative electrode current collector foil 21). The slurry coating film was dried to form the negative electrode mixture layer 22. The negative electrode mixture layer 22 was pressed to a predetermined thickness. The negative electrode mixture layer 22 and the negative electrode current collector foil 21 were machined to have a predetermined dimension. As a result, the negative electrode 20 was manufactured.

[0089] The assembly was carried out as in the Fig. 6 and Fig. 8. Specifically, as shown in Fig. 6, the separators 40, the positive electrode 10 and the negative electrode 20 were wound to produce the electrode body 80. As shown in Fig. As shown in Figure 8, the electrode body 80 was inserted into the battery case 50. An electrolyte solution having the following composition was injected into the battery case 50, and then the interior of the battery case 50 was sealed. LiPF6: 1.0 mol / l EC:DMC:EMC = 3:4:3 (volume ratio)

[0090] The battery fabricated using each of the samples was first charged and discharged as follows. First, the battery was charged at a current of 4 A until the voltage reached 4.1 V. Next, the battery was discharged at a current of 4 A until the voltage reached 3.0 V.

[0091] Each of the samples and each of the batteries was evaluated as follows.

[0092] The migration index (MI) was measured as follows. The battery with a voltage of 3.0 V was disassembled to remove the positive electrode. The positive electrode was washed with EMC. Using the method described above, a section of the sample was analyzed by SEM-EDX with F as an index to calculate MI. The results are shown in Fig. 9 shown.

[0093] The percentage elongation at break of the current collector foil was measured as follows. First, the positive electrode mixture layer was peeled off the washed positive electrode to obtain the current collector foil. The current collector foil was cut into a rectangular specimen with a length of 60 mm and a width of 15 mm. This specimen was placed in a holding fixture (chuck) of a tensile testing machine so that it would not be loosened or deformed. The distance between two measuring points was set to 20 mm, the clamping speed was set to 100 mm / min, and the specimen was stretched in a longitudinal direction. At this time, the percentage elongation at break was calculated from the following expression (i): (Percentage elongation at break)={(X1−Z0)÷X0}×100 where X0 represents the distance between two measuring points at the beginning of the application of a load to the specimen, and X1 represents the distance between two measuring points when the specimen has been broken in such a way that the applied load has been removed. The results are shown in Fig. 9 shown.

[0094] The battery resistance was measured as follows. First, the battery was charged to 3.7 V. At 0 °C, the battery was discharged for 10 seconds at a current value of 5 A. At this time, the resistance was obtained based on polarity. This resistance was taken as the low-temperature initial resistance. Specifically, the resistance R (Ω) was calculated from the following equation (ii) based on: a voltage drop amount ΔV (V) 10 seconds after the start of discharge; and a current value of 5 (A). The results are shown in Fig. 9 shown. R(Ω)=ΔV(V)÷5(A)

[0095] According to the study of the present inventors, an increase in resistance caused by repeated high-current charging and discharging is considered to proceed as follows.

[0096] During high-current charging and discharging, the expansion and contraction of the positive electrode active material and the positive electrode mixture layer containing the positive electrode active material are more pronounced than during normal-current charging and discharging. At this time, the current collector foil is hard and cannot withstand the expansion and contraction of the positive electrode mixture layer. Therefore, tensile stress or deformation occurs in the positive electrode mixture layer and the electrode body. As a result, the electrolyte solution initially held in the electrode body is forced to leak out of the electrode body.

[0097] Due to the deformation of the positive electrode mixture layer, a conductive network in the positive electrode mixture layer is disrupted, increasing the battery resistance. As the resistance increases, the amount of heat generated by Joule heating due to high-current charging and discharging increases. The temperature of the electrolyte solution rises due to Joule heating, and the electrolyte solution thermally expands. As a result, the electrolyte solution that cannot be contained in the electrode body leaks out of the electrode body.

[0098] As the amount of electrolyte solution held in the electrode body decreases, the battery resistance continues to increase. As a result, Joule heating becomes even more severe, promoting electrolyte leakage.

[0099] When a coating film formed from a slurry is dried, binder migration contributes to Joule heating. That is, when the coating film is dried, convection is generated in a solvent, and a binder migrates along with the solvent to the surface of the coating film. In a positive electrode mixture layer obtained as described above, the binder segregates onto its surface. As a result, the diffusion of Li is inhibited on the surface of the positive electrode mixture layer. + prevented, which promotes Joule heating.

[0100] In the positive electrode including the softened current collector foil according to the embodiment, the current collector foil can withstand the expansion and contraction of the positive electrode mixture layer. As a result, the expansion stress and deformation in the positive electrode mixture layer and the electrode body can be suppressed. Furthermore, the positive electrode mixture layer in the positive electrode is formed from the granulated body. As described above, the amount of solvent in the granulated body is small. Furthermore, the migration area of ​​the binder is limited to the interior of each of the granulated particles. Therefore, by using the granulated body, binder migration can be suppressed. Furthermore, in this positive electrode, melting of the binder can be suppressed.

[0101] It is assumed that the positive electrode according to the embodiment can prevent the electrolyte solution from leaking from the electrode body and the thermal expansion of the electrolyte solution caused by Joule heating. That is, it is assumed that an increase in resistance caused by repeated charging and discharging at high currents can be prevented. To verify these assumptions, the battery resistance was measured after high-current cycling.

[0102] At 25 °C, 1000 charge-discharge cycles were performed, in which consecutive charge → first stop → discharge → second stop was set as one cycle. Charging: 80 A x 30 seconds First stop: 30 seconds Discharging: 8 A x 300 seconds Second stop: 30 seconds

[0103] After 1000 cycles, the battery resistance was measured as described above and assumed to be a low-temperature resistance after high-current cycling. The results are shown in Fig. 9 shown.

[0104] The experimental results are described below and discussed with respect to each of the samples. Fig.9, for samples A1 to A10, the percentage elongation at break of the current collector foil was high, and the initial resistance and resistance after high-current cycling were low. The reason for this result is as follows: In these samples, the current collector foil was softened, and binder migration and binder melting could be prevented. That is, because binder migration and binder melting were prevented, the thermal expansion of the electrolyte solution was prevented. Furthermore, because the softened current collector foil was able to resist the expansion and shrinkage of the positive electrode mixture layer, the deformation of the positive electrode mixture layer and the electrode body was prevented. As a result, the leakage of the electrolyte solution from the electrode body was prevented.It is assumed that a combination of the above factors prevented an increase in resistance after high current cycles.

[0105] From samples A4 and A7 to A9, it was found that the effect of preventing an increase in resistance is high when the tapping density of the positive electrode active material is within a range of 1.2 g / cm 3 up to 1.9 g / cm 3 lies.

[0106] For Sample B1, the percent elongation at break was lower than that of Sample A10. Accordingly, it is assumed that the current collector foil in Sample B1 could not be heated to a softening point or above. As a result, the current collector foil in Sample B1 was unable to withstand the expansion and shrinkage of the positive electrode mixture layer, and the resistance after high-current cycling was high.

[0107] From the results of Sample B3, it was found that even the low-temperature softening Al foil (A8021) was not softened by hot air drying at 150 °C. For Sample B2, the foil heating step S105 was performed using a heating roller, but the percentage elongation at break was low. The reason for this result is as follows: Because the amount of heat in the foil heating step S105 was small, the current collector foil could not be heated to or above the softening point.

[0108] In samples B4 and B5, although the current collector foil could be softened, an increase in resistance could not be prevented. The reason for this result is the following: Because the amount of heat applied to the positive electrode mixture layer was large, the temperature of the positive electrode mixture layer was raised to or above the melting point of the binder.

[0109] In sample B6, which used the slurry, binder migration was severe and the MI value was high. As a result, an increase in resistance after high-current cycling was promoted.

[0110] The embodiments and examples of the present invention have been described above. However, the present invention is not limited to the embodiments and examples.

Claims

[1] A method for producing a positive electrode for a lithium ion secondary battery, the method comprising characterized by is that it includes: Producing a current collector foil having a first major surface and a second major surface that are opposite sides of the current collector foil; Obtaining a granulated body in which a solvent remains by mixing a positive electrode active material, a conductive material, a binder and the solvent together to obtain a mixture, and granulating the mixture, wherein the solid content of the granulated body is 65 mass% to 80 mass%; Obtaining a first positive electrode mixture layer by pressing the granulated body into a flat shape; Arranging the first positive electrode mixture layer on the first main surface; and Heating the current collector foil in a state where the first positive electrode mixture layer containing the granulated body is disposed on the first main surface so that a temperature of the current collector foil becomes a softening point of the current collector foil or higher, and that a temperature of the first positive electrode mixture layer is lower than a melting point of the binder. [2] A method for producing a positive electrode for a lithium ion secondary battery according to claim 1, wherein the current collector foil is an aluminum alloy foil containing 1.2 mass% to 1.7 mass% of iron. [3] A method for producing a positive electrode for a lithium ion secondary battery according to claim 1 or 2, wherein, during heating of the current collector foil, a heated roller and the second main surface are brought into contact with each other. [4] The method for manufacturing a positive electrode for a lithium ion secondary battery according to claim 3, wherein a temperature of the roller during heating of the current collector foil is 200°C to 400°C. [5] The method for manufacturing a positive electrode for a lithium ion secondary battery according to claim 3 or 4, wherein during heating of the current collector foil, a contact time between the roller and the second main surface is 0.05 seconds to 2.0 seconds. [6] A method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein a tap density of the positive electrode active material is 1.2 g / cm 3 up to 1.9 g / cm 3 amounts. [7] A method for manufacturing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 6, further comprising disposing a second positive electrode mixture layer on the second main surface after heating the current collector foil.

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