JPH11185736A - Manufacture of sheet electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a distribution of uneven distortion, twisting or curving in a sheet surface by forming an electrode active material layer on both surfaces of metal foil current collector, pressure forming a sheet on which a collector active material is applied as coating. SOLUTION: Only a paste painted portion is pressed at the time of pressure forming while no edge side uncoated portion is pressed, thereby generating a distortion difference caused by elongation between both the portions, resulting in generation of bending in a sheet electrode. When this sheet electrode is taken up with a specified tension at the outside of a heating furnace while it is made to run in the furnace a tensile stress generates at first at the uncoated portion where there is no elongation at the time of pressure forming. Elongation generates and is liable to grow by the stress as the electrode is heated with the heating furnace. As a result, a same elongation distortion amount generates in the coated and uncoated regions, and accordingly the purpose is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a sheet electrode, and more particularly to a method for manufacturing a sheet electrode suitable for a positive electrode sheet for a lithium secondary battery.

[0002]

2. Description of the Related Art As shown schematically in FIG. 9, a lithium secondary battery generally has, for example, ethylene carbonate (E).
A positive electrode sheet 52 and a negative electrode sheet 54 are placed in a battery case 50 in which a mixed solvent of C) and diethylene carbonate (DEC) is filled with a non-aqueous electrolyte in which an electrolyte of lithium phosphofluoride (L _i PF ₆ ) is dissolved. A safety valve (explosion-proof valve) 58 which is spirally disposed with a separator 56 interposed therebetween and which is deformed when the internal pressure of the battery rises is provided at the opening on the upper surface of the battery case 50. A poly switch plate (PTC element) 60 for blocking the flow and a sealing cap (lid) 62 are mounted via an insulating gasket 64.

In such a configuration, a positive electrode sheet of a lithium secondary battery generally includes a positive electrode active material such as a lithium cobalt composite oxide or a lithium manganese composite oxide, an organic binder such as polyvinylidene fluoride, and acetylene black or graphite. And a mixture of such a conductive substance and a thin metal foil collector having a thickness of about 10 to 100 μm. Aluminum foil, stainless steel foil, nickel foil or the like is used for the current collector.

Conventionally, as shown in FIG. 10, the positive electrode sheet 30 of the lithium secondary battery has a positive electrode active material, an organic binder, and a conductive material on both surfaces of a metal foil current collector 32, as schematically shown in FIG. (Paste) 34 mixed with
After coating a and b, the coating is dried in a drying oven and then pressed by a roll press to form positive electrode active material layers a and b on both surfaces of the metal foil current collector 32. Then, the current collector active material coated sheet 36 on which the positive electrode active material layer is formed is cut into a predetermined length. At this time, the current collector active material coated sheet 3
In 6, when the active material uncoated area is cut off or a tab is attached to the edge side, the uncoated area on one edge side is cut off leaving a tab connection margin.

[0005] The sheet electrode 30 thus obtained is once wound up into a rolled positive electrode sheet. Next, the negative electrode sheet 38 supplied from the negative electrode sheet roll and the separator sheet 39 supplied from the separator roll are overlapped while supplying the positive electrode sheet 30 from the positive electrode sheet roll wound in a roll shape in this manner. The electrode body 41 is formed by winding the sheet together and connecting the tab 40 to the sheet electrode 30.

However, when such a conventionally known method is used, the elongation rate and the deformation amount with respect to the pressure by the roll press are different in each area such as a coated part and an uncoated part of the sheet electrode. There is a problem that a wrinkle or a wavy pattern occurs, or it is difficult to obtain a flat sheet electrode that is curved in the length direction and has no distortion after cutting.

[0007] The mechanism that causes the sheet surface to be curved or distorted is probably caused by the occurrence of strain distribution on the surface of the sheet electrode 30 after roll pressing as shown in FIG. . That is, the elongation of the positive electrode active material uncoated region (uncoated portion) 36a of the metal foil current collector 32 due to compression by the roll press is zero (0) because it is not compressed, whereas Work area (coating department) 3
In the case of 6b, since elongation and distortion occur, and a distribution of distortion occurs in the width direction due to the difference between them, it is considered that the surface may be curved at the time of winding after cutting.

In order to solve this problem, for example, Japanese Unexamined Patent Publication No. 7-192726 discloses a method in which a paste-like active material is first applied to only one side of a metal foil current collector, dried, and then uncoated. Provide multiple discontinuous linear cuts from
After that, the paste-like active material is applied again. There is disclosed a method of manufacturing a sheet electrode by drying and then press-molding with a roll press machine. According to this, there is a cut in the metal foil even if the density is increased,
It is said that flat sheet electrodes free from twisting and distortion can be manufactured by following the elongation of the active material on both surfaces.

In addition, as shown in FIG. 12, the uncoated portion 36a of the current collector active material coated sheet (sheet electrode) 30 to which the active material has not been applied after the pressure molding by the roll press.
There is also a method in which a slit is provided to reduce the curvature generated after cutting and then wound.

[0010]

However, these prior art countermeasures may damage the current collector because the metal foil current collector is provided with cuts or slits. For example, as described in Japanese Patent Application Laid-Open No. 7-192726, when a cut is made in the current collector, the cut is widened after pressure molding, stress concentration occurs at the cut end, cracks are generated, and durability as an electrode is increased. Affect. In addition, there is a problem that a system of a current collector and an active material or a system of only an active material exists in an electrode, and the electrode does not have a uniform density.

In the method disclosed in Japanese Unexamined Patent Publication No. 7-192726, since the active material coating step is performed twice on the surface of the metal foil current collector, the number of manufacturing steps is increased and productivity is deteriorated. There is a problem.

Further, as shown in FIG. 12, when a slit is provided in an uncoated portion of the sheet electrode, the curvature is reduced, but if the curvature is large, a crack is generated from the slit end as a starting point. Become. In addition, when connecting a tab to the uncoated portion of the active material to collect current, the tab cannot be connected to an arbitrary position because of the slit, and the degree of freedom in connecting the tab is reduced and the current collection efficiency is reduced. There was a problem of doing.

An object of the present invention is to provide a manufacturing method in which a sheet surface of a sheet electrode used in a lithium secondary battery or the like does not have a non-uniform strain distribution, twist, or curve. Thus, stabilization of quality such as charge / discharge cycle characteristics of the lithium secondary battery is achieved.

[0014]

In order to solve this problem, a method for manufacturing a sheet electrode according to the present invention comprises: an active material layer forming step of forming an electrode active material layer on both surfaces of a metal foil current collector; In the active material layer forming step, a pressure forming step of pressure forming a current collector active material coated sheet having an electrode active material layer formed on the surface of the metal foil current collector; A heating step of heating the formed current collector active material coated sheet while applying tension thereto is included in the gist.

According to the sheet electrode manufactured as described above, the strain distribution of the sheet electrode, which is mainly subjected to non-uniform strain in the metal foil current collector at the time of pressure molding, is made uniform by the heating step while applying tension. The defect of the sheet electrode caused by the non-uniform strain distribution (for example, a waving phenomenon on the sheet surface or a curving after cutting) is corrected. Therefore, a good sheet electrode that does not cause any trouble in winding in a later step or the like is manufactured.

In this case, as a material of the metal foil current collector, other materials such as aluminum (Al), stainless steel (SUS), nickel (Ni) foil and the like are applied. The sheet electrode is applied not only to the positive electrode sheet of a lithium secondary battery, but also to a negative electrode sheet. In the case of application to a positive electrode sheet, a positive electrode active material such as a lithium cobalt composite oxide or a lithium manganese composite oxide is mixed with an organic binder or graphite as an active material layer formed on the surface of the metal foil current collector. What is done is generally used. In the case of a negative electrode sheet, a coke-based or graphite-based material is used.

In the case of pressing by a roll press, the pressing force of the sheet coated with the current collector active material is such that the amount of curvature of the sheet surface increases with an increase in the linear pressure of the press. It is preferable that the temperature range from a temperature at which the strength of the current collector metal material does not decrease and an improvement in elongation is expected to a temperature at which the current collector metal material does not recrystallize or form a solution. In the case of an Al foil current collector, a temperature range of about 100 ° C. to 250 ° C. seems to be appropriate.

In the present invention, the pressure forming step of the current collector active material coated sheet and the heating step while applying tension may be performed individually, or both may be performed simultaneously, that is, at the time of pressure forming. Heating while applying tension may be used.

[0019]

Embodiments of the present invention will be described below in detail with reference to the drawings. First, FIG. 1 shows a manufacturing process of a sheet electrode (particularly, a positive electrode sheet for a lithium secondary battery) as an example of the present invention. FIG. 1 shows a manufacturing method of three embodiments. The first manufacturing method includes a paste manufacturing step 10, a coating step 12, a roll press forming step 1
4. Warm winding step 16, cutting and cutting step 18, winding step 2
Consists of zero.

The second manufacturing method is characterized in that a roll press forming step and a warm winding step 22 are performed simultaneously, and a roll press / warm winding step 22 is performed. Further, the third manufacturing method is characterized in that the warm winding step 16 and the cutting step 18 are reversed, and the cutting step 18 is performed first and the warm winding step 16 is performed later.

First, the first production method will be described. In the "paste production step", a positive electrode active material such as a lithium cobalt composite oxide or a lithium manganese composite oxide is mixed with an organic binder such as polyvinylidene fluoride and acetylene black. , Mix conductive materials such as graphite,
Mud is recommended (paste).

In the "coating process", the slurry of the positive electrode material obtained in the paste manufacturing process is applied to the surface (both surfaces) of a metal foil current collector such as an aluminum (Al) foil. In this embodiment, a positive electrode material having a thickness of 64 μm and a width of 160 mm per side is applied to an Al current collector (thickness: 15 μm, width: 200 mm). Accordingly, the uncoated portion has a width of 20 mm on each side of the current collector, and the entire thickness of the coated portion is 143 μm.

The schematic structure of the apparatus for the following "roll press forming step" and "warm winding step" will be described with reference to FIG. In the “roll press forming step”, the current collector active material coated sheet 36 coated with the above-described positive electrode material is passed between a pair of pressure rolls 24a and 24b in a pressurized state.
A positive electrode material slurry (paste) is pressed onto both surfaces of the metal foil current collector 32. In this example, the total load was 5 tonf, and the total thickness of the positive electrode material coated region on the current collector active material coated sheet 36 was 115 μm. At this time, only the positive electrode material application region is pressurized, and elongation strain occurs in the application region. At this time, since the uncoated region at the edge is not pressurized, the distortion amount is almost zero (0) in the uncoated region, so that the surface of the current collector active material coated sheet 36 has wrinkles. Rippling and the like will occur.

In the "warm-up winding step", the pressure-coated current collector active material coated sheet 36 is heated to a temperature of 100.degree.
While running through the heating furnace 26 at 250 ° C., the heating furnace 2
6. Winding is performed as needed with a predetermined tension. Although the traveling speed in the heating furnace and the winding tension outside the furnace depend on the material, in this embodiment, the traveling speed in the heating furnace is 0.85 m / min using a hot air fan having a temperature of 200 ° C. and an air flow of 2300 rpm. , And wound up as needed at a tension of 2.5 kgf outside the heating furnace.

Then, the current collector active material coated sheet 36 thus wound is cut to a predetermined length in the next cutting step 18. At this time, as described in the conventional process of FIG. 10, the active material uncoated area on one edge side of the current collector active material coated sheet 36 is cut off leaving a tab connection margin for tab connection. Then, the positive electrode sheet 40 thus obtained is wound into a roll in the next winding step. At this time, the positive electrode sheet 40 was not curved, and the electrode body could be manufactured without meandering even in the winding step. As a result, defects in the cutting and winding steps and the winding step are eliminated, and the yield is improved.

In order to form a current collecting structure on the entire end surface of the wound cylindrical electrode without using a tab, it is necessary that the electrode end surface of the battery maintain a flat surface as shown in FIG. It is. By adopting this manufacturing method, the sheet electrode can be wound without meandering, so that it is possible to provide a cylindrical electrode having a uniform end surface. Furthermore, when the curved sheet electrode is forcibly wound, there are places where each part is tightly tightened and places where it is not tight, and stress distribution occurs, but since the sheet electrode created by this method is uniform, Non-uniform distribution of stress does not occur, uniform charging and discharging can be performed, and an effect can be expected in durability (cycle characteristics).

FIG. 4 shows a mechanism for obtaining a sheet electrode free from such problems as bending and meandering. The paste-like positive electrode active material is applied to the surface (both surfaces) of the metal foil current collector. When the coated sheet electrode is passed in a pressurized state in the roll press molding process and the mud (paste) is pressed against both sides, only the coated portion of the mud is pressed, and the uncoated portion on the edge side is pressed. Since no pressure is applied in this case, distortion due to elongation occurs in the coated region, and no distortion occurs due to elongation in the uncoated region. This difference causes the sheet electrode to bend.

Then, while running the sheet electrode in which the deflection has occurred in the heating furnace and winding it with a predetermined tension outside the heating furnace as needed, the sheet electrode is pulled (a). First, a tensile stress is generated on the uncoated area side where no elongation has occurred during the forming step (b). Since the material is heated by the heating furnace, the uncoated region is elongated due to the generation of the stress, and the region is easily grown (c). As a result,
Finally, the same amount of elongation strain is generated in the coated region and the uncoated region (d). Therefore, it is possible to obtain a sheet electrode having a uniform strain distribution.

Next, a second manufacturing method will be described.
The second manufacturing method is a “roll press / warm winding step 22” in which the “roll press forming step” and the “warm winding step” described in the first manufacturing method are performed simultaneously. The details of the “paste manufacturing step” and the “coating step” in the second manufacturing method are the same as those of the first manufacturing method described above, and thus the description is omitted.

The above-mentioned “roll press / warm winding step 2”
The schematic configuration of the device 2 ”will be described with reference to FIG. In the “roll press / warm-up winding process”, since the heating furnace is incorporated in the roll press machine, the current collector active material coated sheet 36 is pressed between the pair of pressure rolls 24a and 24b. At 100 ° C to 250 ° C
In the heating furnace 26, and winds the sheet with a predetermined tension outside the heating furnace as needed. As a result, the action mechanism described with reference to FIG. 4 operates, and the same amount of elongation strain occurs in the width direction in the coating region and the uncoated region on the edge side, thereby obtaining a sheet electrode having a uniform strain distribution. .

Then, the thus wound sheet electrode 40 was cut into a predetermined length and width in the same manner as in the first embodiment. As a result, like the sheet electrode manufactured by the first manufacturing method, the cut sheet electrode 40 can perform an electrode forming operation without curving and meandering even in the winding step.

Next, a third manufacturing method will be described (not shown). In the third manufacturing method, as shown in the sheet electrode manufacturing process of FIG. 1, after passing through a “paste manufacturing process” and a “coating process”, a collector active material is applied by a “roll press forming process”. The positive electrode material is pressed on the sheet 36. In the first and second manufacturing methods, the sheet electrode 40 is cut and cut after the strain distribution of the sheet electrode 40 is made uniform in the “warm-up winding process”. However, in the third manufacturing method, the sheet electrode is bent. Cut as it is. A sheet electrode having a uniform strain distribution is obtained by performing a "warm-up winding process" for each of the cut sheet electrodes. The detailed description of each step in the third manufacturing method is omitted because it is described in the first manufacturing method.

In each of the above-described manufacturing methods, a sheet electrode having a uniform strain distribution is obtained by using any of the methods. Therefore, as shown in the first and second manufacturing methods, the “warm winding step” is performed. A method of performing a “cutting step” after passing through may be used, or a method of performing a “warm winding step” after passing through a “cutting step” as shown in the third manufacturing method may be used. It is also possible to select an arbitrary method according to the situation such as the manufacturing process and the product shape. Further, even when the sheet electrode having only the coated portion does not have the uncoated portion and has waving and warping, a flat sheet electrode having no waving and bending can be obtained by this manufacturing method.

Next, a so-called warm correction method in the "warm winding step" shown in the above embodiment has been studied and will be described. The following experiment was performed to determine the correction temperature for the warm correction method. The material used in the experiment was a positive electrode sheet composed of lithium manganate, carbon black, and polyvinylidene fluoride in a weight ratio of 90: 5: 5, and the current collector used was a hard Al foil (A1N30-H).
(15 μ × 200 mm) was used. Coating width is 160m
The thickness after both-side coating and drying is 179 μm including the current collector.
And Further, a negative electrode sheet (spherical graphite, polyvinylidene fluoride (weight ratio 90:10)) was used, and a Cu foil (H material) (10 μm × 200 mm) was used as the current collector. The coating width was 160 mm, and the thickness after both-side coating and drying was 162 μm including the current collector.

Then, first, a roll press is performed using the above-described material, and thereafter, the material is cut into a predetermined size, and changes in the amounts of curvature of the positive electrode sheet and the negative electrode sheet are measured. FIG.
2 shows the cutting dimensions of the sheet electrode when measuring the amount of bending, and the amount of bending required after cutting. First, after going through the "paste manufacturing process""coatingprocess",
The active material is pressed on the current collector by a “roll press forming step”. At this time, since the pressurized state is different between the coated portion and the uncoated portion, an elongation strain distribution occurs.

When the sheet electrode having the elongation strain distribution is cut into a predetermined size having an uncoated portion through a "cutting step", a curvature caused by the elongation strain distribution is generated in the sheet electrode. A height difference between an extension line through both end portions of the curved sheet and a point farthest on the sheet from the extension line is measured. This height difference is the amount of bending caused by the strain distribution generated by the “roll press step”.

Next, the change in the amount of bending with respect to the correction temperature was measured. In this measurement, the positive electrode sheet was 0.19 t.
onf / cm, and a negative electrode sheet formed by press linear pressure molding of 1.53 tonf / cm. Furthermore, at the time of winding, a tension of 2.5 kgf and a feed speed of 0.85 m / mi
n, the stress change when the tension transmission changes from only the uncoated portion of the active material to the entire sheet electrode at the beginning is 4.2 → 0.8 kg / mm ^{2 in} the positive electrode sheet, and 6.3 → 1.3 kg / mm ² .

FIG. 7 shows the measurement results of the change in the amount of bending of the positive electrode sheet with respect to the correction temperature. The horizontal axis indicates the correction temperature (° C.), and the vertical axis indicates the amount of bending (mm). First, the amount of bending shows a relatively large value (about 20 mm) in a temperature range where the correction temperature is lower than about 100 ° C., but it is understood that the correction amount tends to gradually decrease when the correction temperature exceeds 100 ° C. .

Since an aluminum material is used for the current collector metal foil of the positive electrode sheet, if the straightening temperature exceeds 250 ° C., recrystallization occurs, the softening of the work starts, and the elongation occurs rapidly. Therefore, the straightening temperature for obtaining a positive electrode sheet (curving amount of 10 mm or less) having no problem in the winding step is approximately 100 ° C. from which improvement in elongation can be expected without substantially decreasing strength, taking into account the change in tension. A temperature up to 250 ° C. below the recrystallization temperature of the material is considered appropriate.

FIG. 8 shows the measurement results of the change in the amount of bending with respect to the correction temperature of the negative electrode sheet. Again, the horizontal axis indicates the correction temperature (° C.), and the vertical axis indicates the amount of curvature (mm). In the case of the negative electrode sheet as well, the amount of bending tends to decrease as the correction temperature increases. However, the correction temperature is 100
When the temperature exceeds ℃, discoloration due to an oxide film is observed on the surface of the Cu foil. However, there was no problem in practical use, and it was confirmed that the effect of warming straightening was also obtained for the negative electrode sheet.

It should be noted that the present invention is not limited to the above-described embodiment at all, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, the positive electrode sheet or the negative electrode sheet for a lithium secondary battery has been described, but it goes without saying that the present invention can be applied to other sheet electrodes of a nickel-metal hydride storage battery and the like.

[0042]

According to the method for manufacturing a sheet electrode according to the present invention, a current collector active material coated sheet having an electrode active material layer formed on the surface of a metal foil current collector is pressed to form a metal current collector. After the electrode active material layer is pressed on the surface, the current collector active material coated sheet is heated while applying tension, thereby eliminating the uneven strain distribution generated on the sheet surface in the processing step, and the sheet surface deflection. Is also eliminated. Accordingly, a flat sheet electrode that does not meander in the subsequent “winding step” can be obtained, thereby providing a cylindrical electrode having a flat and flat electrode end surface. In addition, a secondary battery which can be uniformly charged and discharged and has excellent durability can be provided.

[Brief description of the drawings]

FIG. 1 is a view showing a manufacturing process of a sheet electrode of the lithium secondary battery shown in the present embodiment.

FIG. 2 is a view showing a schematic configuration of an apparatus in a “roll press forming step and a warm winding step” which is a first manufacturing method in FIG.

FIG. 3 is an external view of an electrode body having an end face current collecting structure.

FIG. 4 is a diagram illustrating an operation mechanism of a sheet electrode in which a problem such as bending or meandering does not occur.

FIG. 5 is a view showing a schematic configuration of an apparatus in a roll press / warm-up winding process which is a second manufacturing method in FIG.

FIG. 6 is a diagram illustrating a method for measuring a sheet electrode.

FIG. 7 is a diagram showing a measurement result of a change in a bending amount with respect to a correction temperature in a positive electrode sheet.

FIG. 8 is a diagram showing a measurement result of a bending amount with respect to a correction temperature in a negative electrode sheet.

FIG. 9 is a diagram showing a schematic configuration of a conventional lithium secondary battery.

FIG. 10 is a diagram schematically illustrating a process of manufacturing a sheet electrode which is generally performed in the related art.

FIG. 11 is a view showing a mechanism in which bending, bending, waving, and the like occur in a conventional sheet electrode.

FIG. 12 is a view showing a conventional mode in which a slit is formed in an uncoated portion of a sheet electrode to reduce a curve.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Paste manufacturing process 12 Coating process 14 Roll press molding process 16 Warm winding process 18 Cutting and cutting process 20 Winding process 22 Roll press / warm winding process 30 Sheet electrode (positive electrode sheet) 32 Metal foil current collector 34a , 34b paste 36 active material coated sheet 36a coating area (coating part) 36b uncoated area 38 negative electrode sheet

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01M 10/40 H01M 10/40 Z

Claims (1)

[Claims] 1. An active material layer forming step of forming electrode active material layers on both surfaces of a metal foil current collector, and the electrode active material layer is formed on the surface of the metal foil current collector in the active material layer forming step. The method includes a pressure forming step of pressure-forming the current collector active material coated sheet, and a heating step of heating the current collector active material coated sheet pressed in the pressure forming step while applying tension. A method for manufacturing a sheet electrode, comprising: