Method for manufacturing secondary battery

Abstract

To secure high binding force by suppressing excessive drying of an electrode mixture.SOLUTION: When manufacturing a secondary battery, an electrode sheet 35 is formed by applying an electrode mixture 37 to a base material 36 serving as a collector, and the electrode mixture 37 applied to the base material 36 is dried by arranging the electrode sheet 35 in a dry region α. Further, as the dry region α, a boundary coated part dry region α s for drying the boundary coated part 37s of the mixture 37 adjacent to the uncoated part 39 is set. Further, an inner coated part dry region α c for drying the inner coated part 37c of the mixture 37 is set at a position inside the boundary coated part dry region α s in the widthwise direction of the sheet 35. A dew point higher than that of the inner coated part dry region α c is set in the boundary coated part dry region α s.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a secondary battery. [Background technology]

[0002] When manufacturing a secondary battery such as a lithium-ion secondary battery, an electrode sheet is formed by applying an electrode composite to a substrate that serves as a current collector. The electrode sheet is then placed in a drying area, where the electrode composite applied to the substrate is dried. After this drying process, the electrode sheet is pressed and cut to form a secondary battery electrode plate.

[0003] Furthermore, during the drying process of the electrode sheet, when the solvent in the electrode mixture evaporates, the binder in the electrode mixture tends to migrate to the surface side of the electrode mixture along with the solvent. In other words, so-called migration occurs. Furthermore, this binder migration is promoted by the rapid drying of the electrode mixture. This reduces the binding strength of the electrode mixture, which leads to a problem of reduced peel strength of the electrode mixture layer formed on the substrate.

[0004] In light of this, for example, Patent Document 1 describes a configuration in which the humidity of the air introduced into a drying furnace that forms a drying region for an electrode sheet is adjusted to a constant level. Patent Document 2 also describes a configuration in which, in the drying process of an electrode sheet, humidified hot air is blown onto the electrode sheet, and then dry hot air is blown onto the electrode sheet. This configuration suppresses the migration of the binder during the drying process and ensures the binding strength of the electrode mixture coated on the substrate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-106175 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-319117 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in applications requiring high levels of battery performance, such as electric vehicles, further improvements in performance are being sought on a daily basis, and therefore further improvements are being sought in the above-mentioned conventional technologies to meet the evolving requirements. [Means for solving the problem]

[0007] Various aspects of the method for manufacturing a secondary battery that solves the above problems will be described. Aspect 1 is a method for manufacturing a secondary battery, comprising the steps of forming an electrode sheet by applying an electrode mixture to a substrate that serves as a current collector, and drying the electrode mixture applied to the substrate by placing the electrode sheet in a drying area, wherein the electrode mixture includes an electrode active material and a binder, and the electrode sheet has an uncoated area where the electrode mixture is not applied, and the drying area comprises a boundary coated area drying area that dries a boundary coated area of ​​the electrode mixture adjacent to the uncoated area, and an inner coated area drying area that dries an inner coated area of ​​the electrode mixture at a position more inward in the direction of extension of the electrode sheet than the boundary coated area drying area, and a higher dew point is set for the boundary coated area drying area than for the inner coated area drying area.

[0008] According to the above configuration, it is possible to effectively prevent excessive drying even at the boundary coating portion where the electrode mixture coated on the substrate is prone to drying, thereby suppressing migration of the binder contained in the electrode mixture and ensuring high binding strength.

[0009] Aspect 2 is the method for manufacturing a secondary battery according to Aspect 1, in which the electrode sheet passes through a plurality of drying ovens that form the drying region, and when the drying ovens are divided into an upstream side and a downstream side in the order in which the electrode sheet passes through, a lower dew point is set in the downstream drying oven than in the upstream drying oven.

[0010] That is, the electrode mixture coated on the substrate tends to dry more rapidly in the early stages of the drying process. Therefore, as described above, by setting the dew point of the drying region formed by the drying furnace on the early stage high, it is possible to effectively prevent the electrode mixture from drying excessively. Furthermore, by setting the dew point of the drying region formed by the drying furnace on the later stage low, it is possible to prevent undried portions from remaining. This makes it possible to more effectively suppress migration of the binder contained in the electrode mixture and ensure high binding strength.

[0011] A third aspect is the method for producing a secondary battery according to the first or second aspect, further comprising a blocking section that separates the boundary coated section dry region from the inner coated section dry region. According to the above configuration, the dew point set in the boundary coated section drying region can be maintained higher than the dew point in the inner coated section drying region.

[0012] Aspect 4 is a method for manufacturing a secondary battery according to any one of Aspects 1 to 3, in which the electrode sheet passes through a multi-stage drying oven that forms the drying region, and the boundary coated portion drying region and the inner coated portion drying region are formed at least in the first stage of the drying oven through which the electrode sheet first passes.

[0013] According to the above-mentioned configuration, excessive drying of the boundary coating portion can be effectively suppressed in the initial stage of the drying process where the electrode mixture is most likely to dry. Aspect 5 is a method for manufacturing a secondary battery according to aspect 4, in which a lower dew point is set in the drying furnace in which there is no distinction between the boundary coating drying area and the inner coating drying area than in the drying furnace in which the boundary coating drying area and the inner coating drying area are formed.

[0014] According to the above-described configuration, with a simple configuration, it is possible to effectively prevent the boundary coating portion from being over-dried while avoiding the remaining undried portion. [Effects of the Invention]

[0015] According to the present invention, excessive drying of the electrode mixture can be prevented, and a high binding strength can be ensured. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a perspective view of a secondary battery. [Figure 2] FIG. 2 is an exploded view of the electrode assembly. [Figure 3] FIG. 3 is a side view of the secondary battery. [Figure 4] FIG. 4 is a flowchart showing the procedure for manufacturing the electrode sheet. [Figure 5] FIG. 5 is a side view that schematically shows a drying furnace that forms a drying region and an electrode sheet passing through the drying furnace. [Figure 6] FIG. 6 is a plan view schematically showing a drying furnace that forms a drying region and an electrode sheet passing through the drying furnace. [Figure 7] FIG. 7 is an explanatory diagram of a drying furnace and the dew point set in the drying region thereof. [Figure 8] FIG. 8 is a graph comparing the likelihood of migration occurring when the electrode mixture is dried. [Figure 9] FIG. 9 is a graph comparing the peel strength after drying of the electrode mixture. [Figure 10] FIG. 10 is a table showing test results when the dew point inside the drying oven is changed between the front and rear sides. [Figure 11] FIG. 11 is a graph comparing the peel strength after drying of the electrode mixture. [Figure 12] FIG. 12 is a table showing the test results when the dew point was changed between the boundary coated drying area and the inner coated drying area. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of a method for manufacturing a secondary battery will be described with reference to the drawings. <Lithium-ion secondary battery> 1, the secondary battery 1 includes an electrode assembly 10 in which a positive electrode 3, a negative electrode 4, and a separator 5 are integrated, and a case 20 that houses the electrode assembly 10. The secondary battery 1 of this embodiment has a configuration as a lithium ion secondary battery in which the electrode assembly 10 inside the case 20 is impregnated with a non-aqueous electrolyte solution (not shown).

[0018] More specifically, in the secondary battery 1 of this embodiment, the positive electrode 3, the negative electrode 4, and the separator 5 are stacked in a sheet-like outer shape. Then, by winding up the stack of the positive electrode 3, the negative electrode 4, and the separator 5, an electrode assembly 10 is formed in which the positive and negative electrodes and the separators 5 are alternately arranged in the radial direction with the separator 5 sandwiched between the positive electrode 3 and the negative electrode 4.

[0019] The case 20 of this embodiment includes a case body 21 in the shape of a flat, generally rectangular box, and a lid member 22 that closes an open end 21x of the case body 21. The electrode body 10 of this embodiment has a flat outer shape that corresponds to the box shape of the case 20.

[0020] <Electrode sheet and electrode body> More specifically, as shown in FIG. 2, in the secondary battery 1 of this embodiment, the positive electrode 3 and the negative electrode 4 each have a configuration as an electrode sheet 35 including a current collector 31 having a sheet-like outer shape and an electrode active material layer 32 laminated on this current collector 31.

[0021] Specifically, for the electrode sheet 35P for the positive electrode 3, a paste-like positive electrode mixture 37P containing a lithium transition metal oxide as the positive electrode active material is applied to a substrate 36P made of aluminum or the like that constitutes the positive electrode current collector 31P. For the electrode sheet 35N for the negative electrode 4, a slurry-like negative electrode mixture 37N containing a carbon-based material as the negative electrode active material is applied to a substrate 36N made of copper or the like that constitutes the negative electrode current collector 31N. Furthermore, each of the electrode mixtures 37 for the positive electrode 3 and the negative electrode 4 contains a binder. In the secondary battery 1 of this embodiment, the electrode mixture 37 applied to the substrate 36 dries to form a corresponding positive electrode active material layer 32P and a corresponding negative electrode active material layer 32N on the positive and negative electrode sheets 35P and 35N, respectively.

[0022] Furthermore, in the secondary battery 1 of this embodiment, the positive and negative electrode sheets 35P, 35N are each shaped like a strip. The electrode assembly 10 of this embodiment has a configuration as a wound body in which the positive and negative electrode sheets 35P, 35N, stacked with the separator 5 sandwiched therebetween, are wound around a winding axis 10x extending in the width direction of the strip (the left-right direction in FIG. 2).

[0023] 2, the separator 5 and each electrode sheet 35 are wound in such a way that the electrode sheet 35P constituting the positive electrode 3 is wound on the inside. However, this figure is one example showing the structure of the electrode assembly 10, and the separator 5 and each electrode sheet 35 may also be wound in such a way that the electrode sheet 35N constituting the negative electrode 4 is wound on the inside. This determines whether the electrode sheet 35 arranged on the outermost shell of the electrode assembly 10 is the electrode sheet 35P constituting the positive electrode 3 or the electrode sheet 35N constituting the negative electrode 4.

[0024] 1 to 3, the lid member 22 of the case 20 is provided with a positive electrode terminal 38P and a negative electrode terminal 38N that protrude outside the case 20. Furthermore, each electrode sheet 35 has an uncoated portion 39 where the electrode active material layer 32 is not formed on the current collector 31. The secondary battery 1 of this embodiment is configured so that, utilizing these uncoated portions 39, the electrode sheet 35P constituting the positive electrode 3 and the positive electrode terminal 38P are electrically connected, and the electrode sheet 35N constituting the negative electrode 4 and the negative electrode terminal 38N are electrically connected.

[0025] Specifically, the electrode body 10 of this embodiment is housed in the case 20 with its winding axis 10x aligned along the longitudinal direction (left-right direction in FIG. 1 ) of the lid member 22, which is an elongated, generally rectangular plate. Furthermore, in this state, an uncoated portion 39P of an electrode sheet 35P constituting the positive electrode 3 is connected to a positive electrode terminal 38P via a connecting member 40P. Similarly, an uncoated portion 39N of an electrode sheet 35N constituting the negative electrode 4 is connected to a negative electrode terminal 38N via a connecting member 40N.

[0026] Furthermore, an electrolyte solution 45 is poured into the case 20. That is, the electrolyte solution 45 of the secondary battery 1 configured as a lithium ion secondary battery is one in which a lithium salt serving as a supporting salt is dissolved in an organic solvent. Thus, the secondary battery 1 of this embodiment is configured such that the electrode assembly 10 sealed in the case 20 is impregnated with the electrolyte solution 45.

[0027] <Electrode sheet manufacturing process> 4, in the manufacturing process of the electrode sheet 35 that forms the secondary battery 1 of this embodiment, first, the electrode active material, binder, thickener, etc. that are raw materials for the electrode mixture 37 are mixed (step 101). Then, the mixed electrode mixture 37 is kneaded (step 102).

[0028] Next, the electrode mixture 37 kneaded in step 102 is applied to the corresponding substrate 36 in the form of the mixture paste (step 103, see FIG. 2). The electrode mixture 37 applied in step 103 is then dried (step 104). In this embodiment, the application step in step 102 and the drying step in step 103 are performed consecutively. Furthermore, in a state where the electrode mixture 37 applied to the substrate 36 forms the corresponding positive and negative electrodes, i.e., electrode plates, these are pressed to increase the adhesive strength of the electrode mixture 37 to the substrate 36 (step 105). In the secondary battery 1 of this embodiment, a cutting step (step 106) is performed after the pressing step, and an electrode sheet 35 having an electrode active material layer 32 laminated on the substrate 36, which becomes the current collector 31, is manufactured (see FIG. 2).

[0029] <Electrode sheet drying process> As shown in Figures 5 and 6, in the secondary battery 1 of this embodiment, the drying process of the electrode sheet 35 (see Figure 4, step 104) is performed by placing the electrode sheet 35, having the electrode mixture 37 coated on its base material 36, in a drying area α.

[0030] More specifically, in the manufacturing process of the secondary battery 1, the electrode sheet 35 is wound around multiple rotating bodies (not shown) and transported along the longitudinal direction of its strip shape. In the example shown in FIGS. 5 and 6, the electrode sheet 35 is transported from left to right in each figure. Also, along this transport path, multiple drying ovens 50 are provided, each forming a drying region α therein. Specifically, two drying ovens 51, 52 are provided, one at a time and one at a time, aligned in the transport direction of the electrode sheet 35. The electrode sheet 35 passes through each of these drying ovens 51, 52 in sequence. The drying process of this embodiment is thus configured to dry the electrode mixture 37 applied to the substrate 36.

[0031] <Dew point setting> As shown in FIGS. 5 to 7 , in this embodiment, when comparing the two drying ovens 51 and 52, which are divided into an upstream side and a downstream side in the order in which the electrode sheet 35 passes through, the downstream drying oven 52 has a lower dew point T (T1 > T2) than the upstream drying oven 51. That is, the “dew point” is the temperature at which, when air is cooled, the water vapor contained in the air reaches a saturated water vapor content and becomes liquid, e.g., atomized. Therefore, under the same temperature conditions, “air with a high dew point” has a higher humidity, i.e., a higher moisture content, than “air with a low dew point.” This configuration allows the electrode composite 37 applied to the substrate 36 to dry more easily in the drying region α2 formed by the downstream drying oven 52 than in the drying region α1 formed by the upstream drying oven 51.

[0032] In the drying step of this embodiment, the electrode sheet 35 passes through each drying oven 50 with uncoated portions 39, 39 on both sides of the width direction of the strip shape (vertical direction in FIG. 6). In this embodiment, in the width direction of the electrode sheet 35, the drying region α1 formed in the drying oven 51 on the upstream side has multiple compartments.

[0033] Specifically, the drying region α1 includes a pair of boundary-coated drying regions αs, αs that dry boundary-coated regions 37s, 37s of the electrode composite 37 adjacent to uncoated regions 39, 39 provided on both sides of the width of the electrode sheet 35. The drying region α1 also includes an inner-coated drying region αc that dries an inner-coated region 37c of the electrode composite 37 at a position inward of each of the boundary-coated drying regions αs, αs in the width direction of the electrode sheet 35, i.e., at a position between the boundary-coated drying regions αs, αs. In this embodiment, the boundary-coated drying regions αs, αs have a higher dew point T than the inner-coated drying region αc (Ts1 > Tc1).

[0034] On the other hand, in the drying process of this embodiment, the downstream drying oven 52 does not distinguish between the boundary-coated-area drying zone αs and the inner-coated-area drying zone αc. Furthermore, the dew point T2 of the drying zone α2 formed by this downstream drying oven 52 is set to a value lower than the dew point Tc1 of the inner-coated-area drying zone αc formed in the upstream drying oven 51 (Tc1 > T2). As a result, the dew point T2 of this drying zone α2 is lower than the dew point T1 of the drying zone α1 formed by the upstream drying oven 51.

[0035] More specifically, as shown in FIG. 6 , each drying oven 50 of this embodiment has a plurality of air outlets 60 for blowing hot air into the drying oven 50. Specifically, the upstream drying oven 51 has a plurality of air outlets 60s aligned in the conveyance direction of the electrode sheet 35 at positions where both widthwise ends of the electrode sheet 35 are located, i.e., positions where the boundary-coated drying regions αs, αs are formed. Furthermore, the upstream drying oven 51 has a plurality of air outlets 60c aligned in the conveyance direction of the electrode sheet 35 at positions where the widthwise center of the electrode sheet 35 is located, i.e., positions where the inner-coated drying region αc is formed. The downstream drying oven 52 also has a plurality of air outlets 60 aligned in the widthwise and conveyance directions of the electrode sheet 35, similar to the air outlets 60s, 60c provided in the upstream drying oven 51.

[0036] Each drying oven 50 of this embodiment is configured so that hot air blown out from each of these air outlets 60 is blown onto the electrode sheet 35 passing through the drying oven 50, specifically onto the electrode composite 37 coated on the substrate 36. Furthermore, each drying oven 50 of this embodiment controls the dew point T of the hot air blown into the drying region α from each air outlet 60, i.e., the "blowing dew point." This controls the dew points T1 and T2 in the upstream and downstream drying ovens 51 and 52 in the drying process of this embodiment. Furthermore, the dew point T1 of the drying region α1 formed by the upstream drying oven 51 is set to the dew point Ts1 of the boundary-coated drying region αs and the dew point Tc1 of the inner-coated drying region αc.

[0037] Furthermore, in the drying process of this embodiment, blocking sections 70, 70 that separate the boundary-coated drying zones αs, αs from the inner-coated drying zone αc are provided in the upstream drying furnace 51. In the drying furnace 51 of this embodiment, these blocking sections 70, 70 each have a partition-like outer shape that extends along the conveyance direction of the electrode sheet 35. This allows the drying furnace 51 of this embodiment to maintain the dew point Ts1 set in each boundary-coated drying zone αs, αs at a value higher than the dew point Tc1 of the inner-coated drying zone αc.

[0038] <Example> In the secondary battery 1 of this embodiment, for example, in the drying process of the electrode sheet 35N constituting the negative electrode 4, the temperature of the air blown in from each outlet 60 of each drying oven 50 is controlled to, for example, about 170°C. Furthermore, in the upstream drying oven 51, the humidity of the air blown in from each outlet 60s provided at a position where the boundary coated portion drying region αs is formed is controlled to, for example, about 55% to 60%. Similarly, in the upstream drying oven 51, the humidity of the air blown in from each outlet 60c provided at a position where the inner coated portion drying region αc is formed is controlled to, for example, about 35% to 45%. Furthermore, the humidity of the air blown in from each outlet 60 provided in the downstream drying oven 52 is controlled to, for example, about 10% to 20%.

[0039] For the electrode sheet 35N for the negative electrode 4, for example, in the kneading step (see FIG. 4, step 102), an aqueous solvent is added to the negative electrode mixture 37N. The negative electrode mixture 37N contains, for example, about 1 wt % of styrene butadiene latex (SBR) as a binder.

[0040] Furthermore, in the drying process of this embodiment, the "blowing dew points" are controlled as described above for each of the upstream and downstream drying ovens 51, 52. As a result, when the dew point Ts1 in the boundary coated area drying region αs is set to "high" and the dew point T2 in the downstream drying oven 52 is set to "low," the dew point Tc1 in the inner coated area drying region αc is set to "medium."

[0041] <Loss of adhesive strength due to excessive drying of the coated boundary> That is, in the boundary-coated portion 37s adjacent to the uncoated portion 39, the electrode mixture 37 coated on the substrate 36 flows, and the thickness of the formed layer is likely to be thin. Furthermore, in such a thinned portion, the electrode mixture 37 coated on the substrate 36 is likely to dry out. In other words, the electrode mixture 37 is likely to become over-dried. This causes the binder contained in the electrode mixture 37 to move toward the surface, that is, promotes migration, which reduces the binding strength of the electrode mixture 37 coated on the substrate 36. As a result, the electrode mixture 37 after drying tends to have a reduced peel strength at the boundary-coated portion 37s.

[0042] In consideration of this, in the secondary battery 1 of this embodiment, the dew point T is set for each drying oven 50 used in the drying process of the electrode sheet 35 according to the order and position through which the electrode sheet 35 passes. This prevents the electrode mixture 37 from being over-dried and ensures a high binding strength.

[0043] <Verification 1: Uniformly low dew point> 8 and 9 are graphs comparing the amount of migration and the decrease in binding strength caused by drying of the electrode mixture 37 between the boundary-coated portion 37s and the inner-coated portion 37c when the dew point T in the drying furnace 50 is uniformly set to a low value during the drying process of the electrode sheet 35. In this test, the low "dew point T" in the drying furnace 50 is approximately equal to the dew point T2 in the subsequent drying furnace 52 in this embodiment (for example, air temperature of about 170°C and humidity of about 10% to 20%). Then, under these conditions, the drying process of the electrode sheet 35N for the negative electrode 4 was carried out.

[0044] In FIG. 8 , the “migration average” is a value representing the average degree of migration in the boundary-coated area 37s, with the amount of binder migration occurring in the electrode mixture 37 coated on the substrate 36 in the inner-coated area 37c being “100%.” That is, the “migration amount” of the binder occurring in the electrode mixture 37 can be determined by observing the cross-section of the electrode mixture 37 layered on the substrate 36 using a scanning electron microscope (SEM). Furthermore, when the cross-section of the electrode mixture 37 is divided into an upper layer region and a lower layer region in ascending order from the surface, the area occupied by the binder in each of these regions is measured in the analysis image. The “migration average” is a value based on the ratio of the area occupied by the binder in the upper layer region to the area occupied by the lower layer region.

[0045] In Fig. 9, "average peel strength" refers to the average force required to physically peel off the electrode active material layer 32 formed by the dried electrode mixture 37 from the electrode sheet 35 that forms the electrode plate. The unit of "peel strength" is, for example, "Newtons / millimeter squared." Fig. 9 is also a graph in which the value at the inner coated portion 37c is set to "100(%)."

[0046] 8 and 9, when the dew point T in the drying furnace 50 is uniformly set to a low value, it has been confirmed that the amount of binder migration caused by drying of the electrode mixture 37 tends to increase in the boundary-coated area 37s. This is thought to be because, as described above, the electrode mixture 37 reaches a so-called over-dried state in the boundary-coated area 37s. This has been confirmed to result in a decrease in the "average peel strength" when the electrode sheet 35 forms an electrode plate, that is, a decrease in the binding strength in the boundary-coated area 37s after drying.

[0047] <Verification 2: Changing the dew point in the front and rear stages> 10 is a table listing the test conditions and results of an evaluation test in which the dew points T1 and T2 of the drying regions α1 and α2 formed by the upstream and downstream drying furnaces 51 and 52 used in the drying process of the electrode sheet 35 were changed. In this verification test, the drying state and peel strength of the boundary coating portion 37s were evaluated after the electrode mixture 37 was dried. In this verification test, the level notations of the dew points T1 and T2 indicated as "high" and "low" in the table also indicate values ​​equivalent to the level notation of the dew point T in this embodiment as described above.

[0048] 10, "Pattern 1" in the table shows a case where the dew point T1 of the drying region α1 formed by the front-stage drying furnace 51 is set to "low" and the dew point T2 of the drying region α2 formed by the rear-stage drying furnace 52 is set to "low," i.e., the same conditions as in "Verification 1." In this case, as described above, the boundary coated portion 37s is in an over-dried state, resulting in a decrease in peel strength.

[0049] Furthermore, "Pattern 2" in the table corresponds to a case where the dew point T1 of the drying region α1 formed by the front-stage drying furnace 51 is set to "low" and the dew point T2 of the drying region α2 formed by the rear-stage drying furnace 52 is set to "high." In this case as well, the boundary coated portion 37s was found to be in an over-dried state, resulting in a decrease in peel strength. This is presumably because the electrode mixture 37 coated on the substrate 36 is easily dried in the front stage of the drying process.

[0050] Furthermore, "Pattern 3" in the table corresponds to a case where the dew point T1 of the drying region α1 formed by the drying furnace 51 on the front side is set to "high" and the dew point T2 of the drying region α2 formed by the drying furnace 52 on the rear side is set to "high." In this case, the inner coated portion 37c is in an undried state. As a result, in the pressing process after drying (see FIG. 4, step 105), it was confirmed that the electrode mixture 37 coated on the substrate 36 adheres to the press roll that crushes the electrode mixture 37.

[0051] In contrast, "Pattern 4" in the table shows a case where the dew point T1 of the drying region α1 formed by the front-stage drying furnace 51 is set to "high" and the dew point T2 of the drying region α2 formed by the rear-stage drying furnace 52 is set to "low." In other words, this is a combination pattern similar to the drying process of the present embodiment described above. In this case, it was confirmed that over-drying of the electrode mixture 37 was suppressed even in the boundary-coated portion 37s.

[0052] 11, a significant improvement in peel strength was confirmed in comparison with "Pattern 1" which is the same as "Verification 1" described above. In other words, it was confirmed that the high adhesive strength of the electrode mixture 37 after drying could also be ensured.

[0053] <Verification 3: Changing the dew point for each drying area in the boundary coating area and the inner coating area> 12 is a table listing the test conditions and results of evaluation tests in which the dew points Ts1 and Tc1 of the boundary coated area drying area αs and the inner coated area drying area αc of the drying area α1 formed by the upstream drying oven 51 were changed. The dew point T2 of the drying area α2 formed by the downstream drying oven 52 was set to "low" in all cases. In this verification test, the dew points Ts1 and Tc1 indicated as "high" and "medium" in the table each represent values ​​equivalent to the level notation for the dew point T in this embodiment, as described above.

[0054] 12, "Pattern 5" in the table corresponds to the case where the dew point Ts1 of the boundary coated drying region αs is set to "high" and the dew point Tc1 of each of the inner coated drying regions αc is set to "high." In this case, the inner coated region 37c of the electrode mixture 37 applied to the substrate 36 remains wet.

[0055] In contrast, "Pattern 6" in the table shows a case where the dew point Ts1 of the boundary-coated drying region αs is set to "high" and the dew points Tc1 of the inner-coated drying region αc are set to "medium." This is a combination pattern similar to the drying process of the present embodiment described above. In this case, no undried areas were observed in the inner-coated region 37c. Furthermore, in the boundary-coated region 37s, it was confirmed that the electrode mixture 37 was prevented from over-drying, thereby improving its peel strength.

[0056] <Operation of this embodiment> That is, by setting the dew point Ts1 of the boundary-coated drying region αs to a high value, over-drying of the electrode mixture 37 coated on the substrate 36 is suppressed even in the boundary-coated region 37s adjacent to the uncoated region 39 of the electrode sheet 35. Furthermore, by setting the dew point Tc1 of each of the inner-coated drying regions αc lower than the dew point Ts1 of the boundary-coated drying region αs (Ts1 > Tc1), remaining undried portions of the inner-coated region 37c are prevented. Furthermore, by setting the dew point T in the rear-stage drying furnace 52 lower than that in the front-stage drying furnace 51 (T1 > T2), it is possible to more effectively suppress over-drying of the electrode mixture 37 coated on the substrate 36 while avoiding remaining undried portions.

[0057] <Effects of this embodiment> Next, the effects of this embodiment will be described. (1) When manufacturing the secondary battery 1, an electrode sheet 35 is formed by coating a substrate 36, which will become the current collector 31, with an electrode mixture 37. The electrode sheet 35 is then placed in a drying region α, where the electrode mixture 37 coated on the substrate 36 is dried. In the drying process of the electrode sheet 35, a boundary-coated drying region αs is defined as the drying region α, which dries a boundary-coated region 37s of the electrode mixture 37 adjacent to an uncoated region 39. Furthermore, an inner-coated drying region αc is defined as the drying region α, which dries an inner-coated region 37c of the electrode mixture 37 at a position more inward in the width direction of the electrode sheet 35 than the boundary-coated drying region αs. A higher dew point T is set in the boundary-coated drying region αs than in the inner-coated drying region αc (Ts1 > Tc1).

[0058] According to the above configuration, it is possible to effectively prevent excessive drying even in the boundary-coated portion 37s where the electrode mixture 37 applied to the substrate 36 is likely to dry out. This makes it possible to suppress migration of the binder contained in the electrode mixture 37 and ensure high binding strength.

[0059] (2) In the drying process, the electrode sheet 35 passes through two drying ovens 51 and 52, which are divided into a front-stage side and a rear-stage side, in the order in which the electrode sheet 35 passes. A lower dew point T is set in the drying oven 52 on the rear stage than in the drying oven 51 on the front stage (T1>T2).

[0060] That is, the electrode mixture 37 applied to the substrate 36 is more likely to dry in the early stages of the drying process. Therefore, as described above, by setting the dew point T1 of the drying region α1 formed by the drying furnace 51 on the early stage high, it is possible to effectively prevent the electrode mixture 37 from being over-dried. Furthermore, by setting the dew point T2 of the drying region α2 formed by the drying furnace 52 on the later stage low, it is possible to prevent undried portions from remaining. This makes it possible to more effectively prevent the migration of the binder contained in the electrode mixture 37 and ensure high binding strength.

[0061] (3) In the drying step, a barrier 70 is provided to separate the boundary coated drying area αs from the inner coated drying area αc. According to the above configuration, the dew point Ts1 set in the boundary coated section drying region αs can be maintained preferably higher than the dew point Tc1 in the inner coated section drying region αc.

[0062] (4) In the drying furnace 51 on the upstream side, a boundary coated portion drying region αs and an inner coated portion drying region αc are formed. According to the above configuration, excessive drying of the boundary coated portion 37s can be effectively prevented in the early stage of the drying process where the electrode mixture 37 tends to dry out.

[0063] (5) The drying furnace 52 at the rear stage does not distinguish between the boundary coated portion drying region αs and the inner coated portion drying region αc. According to the above configuration, by combining it with the above configurations (2) and (4), it is possible to effectively prevent the boundary coated portion 37s from being over-dried while avoiding the remaining undried portion with a simple configuration.

[0064] <Another example> The above embodiment can be modified as follows: The above embodiment and the following modifications can be combined with each other within the scope of technical compatibility.

[0065] In the above embodiment, the temperature of the air blown into each drying oven 50 as hot air is controlled to, for example, about 170°C. Furthermore, the humidity in the boundary coated drying area αs, where the dew point T is indicated as "high," is controlled to, for example, about 55% to 60%, and the humidity in the boundary coated drying area αs, where the dew point T is indicated as "medium," is controlled to, for example, about 35% to 45%. The humidity in the downstream drying oven 52, where the dew point T is indicated as "low," is controlled to, for example, about 10% to 20%.

[0066] However, the temperature and humidity in each drying oven 50 may be changed as desired. In other words, the absolute value of the dew point T may be any value. It is sufficient that a higher dew point T is set for the boundary coated area drying region αs than for the inner coated area drying region αc (Ts1>Tc1). It is also preferable that a lower dew point T is set for the rear drying oven 52 than for the front drying oven 51 (T1>T2).

[0067] In the above embodiment, the downstream drying oven 52 does not have a distinction between the boundary coated area drying area αs and the inner coated area drying area αc. However, this is not limiting, and the downstream drying oven 52 may also have such a distinction between the boundary coated area drying area αs and the inner coated area drying area αc.

[0068] In the above embodiment, a barrier 70 is provided to separate the boundary coated drying region αs from the inner coated drying region αc, but such a barrier 70 is not necessarily provided. It is sufficient that the dew point Ts1 set for the boundary coated drying region αs can be maintained higher than the dew point Tc1 of the inner coated drying region αc. Specifically, it is sufficient that the dew point Ts1 of the boundary coated drying region αs, which is set to a higher value, can be maintained at the position where the boundary coated region 37s is located.

[0069] In the above embodiment, the drying process of the electrode sheet 35 is performed using two drying ovens 51, 52 arranged in the front and rear direction. However, this is not limited to this, and the drying process of the electrode sheet 35 may be performed using three or more drying ovens 50. In this case, it is preferable to divide each of these drying ovens 50 into a front-stage side and a rear-stage side by any number in the order in which the electrode sheet 35 passes, thereby setting the front-stage drying ovens 51 and the rear-stage drying ovens 52. It is preferable that the boundary-coated portion drying region αs and the inner-coated portion drying region αc are formed at least in the first-stage drying oven 50 through which the electrode sheet 35 passes first.

[0070] Furthermore, by dividing the interior of the drying oven 50 in the transport direction of the electrode sheet 35, it is possible to configure a single drying oven 50 so that, from the outside, a front-stage drying oven 51 and a rear-stage drying oven 52 are set up.

[0071] In the above embodiment, during the drying process, the electrode sheet 35 is transported along the longitudinal direction of its strip shape. The inner coated section drying region αc is set at a position more inward in the width direction of the electrode sheet 35 than the boundary coated section drying region αs. However, this is not limiting, and the invention may be applied to a configuration in which the electrode sheet 35 is not necessarily transported along the longitudinal direction of the strip shape. The inner coated section drying region αc may be set at a position more inward in the width direction of the electrode sheet 35 than the boundary coated section drying region αs so as to dry the inner coated section 37c of the electrode mixture 37.

[0072] In the above embodiment, the drying process of the electrode sheet 35N that constitutes the negative electrode 4 is exemplified, but it may also be applied to the drying process of the electrode sheet 35P that constitutes the positive electrode 3. In the above embodiment, the electrode assembly 10 of the secondary battery 1 has a configuration as a wound body, but the positive and negative electrode sheets 35P, 35N stacked with the separator 5 sandwiched therebetween do not necessarily have to be wound. Also, the secondary battery 1 does not necessarily have to be a lithium ion secondary battery, and may be applied to other non-aqueous electrolyte secondary batteries.

[0073] The terminal shapes of the positive electrode terminal 38P and the negative electrode terminal 38N are not limited to the shapes shown in FIG. 1 and may be changed arbitrarily. <Additional Notes> Next, the technical ideas that can be understood from the above-described embodiment and modified examples will be described.

[0074] (i) The electrode sheet is transported along the longitudinal direction of the electrode sheet, and the widthwise end portion of the electrode sheet is positioned relative to the boundary coating drying area, and the widthwise center portion of the electrode sheet is positioned relative to the inner coating drying area.

[0075] (b) The dew point is the blowing dew point of the hot air blown into the drying area. [Explanation of symbols]

[0076] 1…Secondary battery 31...Current collector 35...Electrode sheet 36...Base material 37…Electrode composite material 37s...Border coating section 37c...Inner coating section 39...Uncoated area α…Dry area αs: Drying area of ​​the boundary coating area αc: Inner coating drying area T…Dew point

Claims

1. A method for manufacturing a secondary battery, comprising: a step of forming an electrode sheet by applying an electrode mixture to a substrate that serves as a current collector; and a step of drying the electrode mixture applied to the substrate by placing the electrode sheet in a drying area, wherein the electrode mixture includes an electrode active material and a binder, and the electrode sheet has an uncoated portion that is not coated with the electrode mixture, The drying area includes: a boundary-coated-portion drying region for drying a boundary-coated portion of the electrode mixture adjacent to the uncoated portion; an inner coated portion drying region that dries an inner coated portion of the electrode mixture at a position more inward than the boundary coated portion drying region in the spreading direction of the electrode sheet; A higher dew point is set for the boundary coated section drying region than for the inner coated section drying region. A method for manufacturing a secondary battery.

2. The electrode sheet passes through a multi-stage drying oven that forms the drying region, and When each drying oven is divided into a front-stage side and a rear-stage side in the order in which the electrode sheets pass, a lower dew point is set in the rear-stage drying oven than in the front-stage drying oven. The method for manufacturing the secondary battery according to claim 1 .

3. A partition section is provided to separate the boundary coating drying area from the inner coating drying area. The method for manufacturing the secondary battery according to claim 1 or 2.

4. The electrode sheet passes through a multi-stage drying oven that forms the drying region, and The boundary coated portion drying region and the inner coated portion drying region are formed at least in the first stage of the drying furnace through which the electrode sheet first passes. The method for manufacturing the secondary battery according to claim 1 or 2.

5. 5. The method for manufacturing a secondary battery according to claim 4, wherein a lower dew point is set in the drying furnace in which there is no distinction between the boundary coating drying area and the inner coating drying area than in the drying furnace in which the boundary coating drying area and the inner coating drying area are formed.

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