Secondary batteries

Abstract

This invention provides a technology to suppress the generation of temperature differences caused by temperature rise during the charging and discharging of secondary batteries. [Solution] A bipolar secondary battery comprising a plurality of electrode sheets and a plurality of separators, wherein the plurality of electrode sheets and the plurality of separators are stacked alternately along a first direction, and each electrode sheet comprises a current collector, a positive electrode active material layer provided on one side of the current collector, and a negative electrode active material layer provided on the other side of the current collector. In at least one electrode sheet, the ratio of the thickness of the current collector to the thickness of the electrode sheet in the first direction is greater at a first position than at a second position. In a second direction perpendicular to the first direction, the first position is closer to the center of the electrode sheet than to the edge of the electrode sheet, and in the second direction, the second position is closer to the edge of the electrode sheet than to the first position.

Description

Technical Field

[0001] The technology disclosed in this specification relates to a bipolar secondary battery.

Background Art

[0002] Patent Document 1 discloses a configuration of a laminated battery for suppressing a temperature rise inside the battery. According to the said Document 1, inside a cylindrical exterior body, a positive electrode and a negative electrode are laminated in the axial direction of the exterior body with a separator interposed therebetween. A conductive current collector penetrates the positive electrode, the negative electrode, and the separator in the axial direction, and either one of the positive electrode or the negative electrode is electrically connected to the exterior body, and the other electrode is electrically connected to the current collector.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The temperature of a secondary battery rises due to heat generation accompanying charge and discharge. In particular, in the central part of the battery, heat transfer to the outside is retarded, so heat tends to accumulate, the temperature rises, and a temperature difference occurs inside the battery. The occurrence of such a temperature difference can lead to deterioration and overcharging of the battery. Here, a bipolar secondary battery has a positive electrode current collector foil and a negative electrode current collector foil at both ends, and cells that are units constituting the battery are connected in series. Therefore, a configuration in which a current collector penetrating the center of the battery as in the said Document 1 cannot be adopted because there is a risk of generating a short-circuit path between the positive electrode and the negative electrode. In view of such a situation, a technology is provided for suppressing the occurrence of a temperature difference accompanying a temperature rise due to charge and discharge of a secondary battery without forming a short-circuit path.

Means for Solving the Problems

[0005] The bipolar secondary battery disclosed herein comprises a plurality of electrode sheets and a plurality of separators, wherein the plurality of electrode sheets and the plurality of separators are alternately stacked along a first direction. The electrode sheets comprise a current collector, a positive electrode active material layer provided on one side of the current collector, and a negative electrode active material layer provided on the other side of the current collector. In at least one of the electrode sheets, the ratio of the thickness of the current collector to the thickness of the electrode sheet in the first direction is greater at a first position than at a second position. In a second direction perpendicular to the first direction, the first position is closer to the center of the electrode sheet than to the edge of the electrode sheet, and the second position is closer to the edge of the electrode sheet than to the first position in the second direction.

[0006] According to the above configuration, the ratio of the thickness of the current collector to the thickness of the electrode sheet in the first direction is larger at the first position compared to the second position. As a result, the heat capacity is relatively increased at positions closer to the center of the electrode sheet in the second direction. Therefore, without forming a short circuit, the temperature rise due to charging and discharging of the secondary battery can be made uniform throughout the battery, and temperature differences within the battery can be suppressed. [Brief explanation of the drawing]

[0007] [Figure 1] Cross-sectional view of a secondary battery. [Figure 2] A cross-sectional view of a single cell according to the first embodiment. [Figure 3] A cross-sectional view of a single cell according to the second embodiment. [Figure 4] A cross-sectional view of a single cell according to the third embodiment. [Figure 5] A cross-sectional view of a single cell according to the fourth embodiment. [Figure 6] Cross-sectional view of multiple cells according to the fifth embodiment. [Figure 7] A diagram showing the effects of each embodiment in table format. [Figure 8] A cross-sectional view of one cell in the comparative example. [Modes for carrying out the invention]

[0008] The main features of the embodiments described below are listed. These features can be combined in any way.

[0009] According to the secondary battery disclosed herein, the ratio of the thickness of the current collector to at least one of the electrode sheets may be progressively larger in the second direction as one approaches the center of the electrode sheet from the edge of the electrode sheet. According to the above configuration, the ratio of the current collector's thickness can be increased in a simple manner as it approaches the center of the electrode sheet from the edge of the electrode sheet in the second direction.

[0010] According to the secondary battery disclosed herein, the ratio of the thickness of the current collector of at least one electrode sheet may be continuously increasing in the second direction as one approaches the center of the electrode sheet from the edge of the electrode sheet. According to the above configuration, the ratio of the thickness of the current collector can be smoothly increased in the second direction as it approaches the center of the electrode sheet from the edge of the electrode sheet.

[0011] According to the secondary battery disclosed herein, the ratio of the thickness of the current collector at the first position may be 20% or more greater than the ratio of the thickness of the current collector at the second position. According to the above configuration, the temperature difference within the battery can be suppressed more effectively.

[0012] According to the secondary battery disclosed herein, the current collector is formed by overlapping a positive electrode current collector foil and a negative electrode current collector foil. When the positive electrode current collector foil, the positive electrode active material layer, the separator, the negative electrode active material layer, and the negative electrode current collector foil, which are stacked sequentially along the first direction, are considered as one cell, the secondary battery is formed by connecting a plurality of such cells in series along the first direction. In each of the plurality of such cells, excluding a predetermined number of cells located at one end of the first direction and a predetermined number of cells located at the other end of the first direction and which are consecutive, the ratio of the thickness of the current collector at the first position may be greater than the ratio of the thickness of the current collector at the second position. According to the above configuration, in a configuration in which multiple cells are connected in series, the heat capacity of cells that tend to accumulate heat is increased at a position close to the center in the second direction, thereby suppressing temperature differences within the battery.

[0013] This embodiment will be described with reference to the drawings. Each figure is for illustrative purposes only, and this embodiment is not limited to what is shown. Also, since each figure is illustrative, some parts may be omitted.

[0014] Figure 1 shows a simplified cross-section of the main part of a bipolar secondary battery 10. The secondary battery 10 is installed in vehicles such as electric vehicles and hybrid vehicles. An XYZ coordinate system is defined in each figure. The secondary battery 10 is, for example, a rechargeable lithium-ion battery. The secondary battery 10 comprises a plurality of electrode sheets 20 and a plurality of separators 30. The plurality of electrode sheets 20 and the plurality of separators 30 are stacked alternately along the Z axis. The Z axis corresponds to the "first direction".

[0015] The electrode sheet 20 is a bipolar electrode sheet. The electrode sheet 20 comprises a current collector 21, a positive electrode active material layer 24 provided on one side of the current collector 21, and a negative electrode active material layer 25 provided on the other side of the current collector 21. In other words, the electrode sheet 20 is formed by laminating the positive electrode active material layer 24, the current collector 21, and the negative electrode active material layer 25.

[0016] The current collector 21 is formed by overlapping a positive electrode current collector foil 22 and a negative electrode current collector foil 23. The current collector 21 is formed by bonding two types of metal foils or by a film forming technique such as vapor deposition. The positive electrode current collector foil 22 is, for example, an aluminum foil. The negative electrode current collector foil 23 is, for example, a copper foil. Therefore, the positive electrode current collector foil 22 corresponds to one surface of the current collector 21 and is in contact with the positive electrode active material layer 24. The negative electrode current collector foil 23 corresponds to the other surface of the current collector 21 and is in contact with the negative electrode active material layer 25. The separator 30 is sandwiched between the positive electrode active material layer 24 and the negative electrode active material layer 25 in the Z-axis direction.

[0017] The positive electrode active material layer 24 contains a positive electrode active material. Examples of the positive electrode active material include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, etc. The negative electrode active material layer 25 contains a negative electrode active material. Examples of the negative electrode active material include carbon materials such as graphite (graphite), hard carbon, soft carbon, materials that form an alloy with lithium such as silicon (Si), and lithium alloys. Each active material may be composed of a single type of material or may be composed of a plurality of types of materials. Each of the active material layers 24 and 25 may further contain a conductive assistant, a binder, etc.

[0018] The separator 30 is formed of a material having both the permeability that enables the movement of charge carriers (here, lithium ions) accompanying charge and discharge, and the insulating property that prevents short circuit due to contact between the positive electrode and the negative electrode. Examples of the material of the separator 30 include polypropylene, polyethylene, etc. An electrolytic solution penetrates into the positive electrode active material layer 24, the negative electrode active material layer 25, and the separator 30.

[0019] The X-axis direction is orthogonal to the Z-axis direction. The Y-axis direction is orthogonal to both the X-axis direction and the Z-axis direction. The X-axis direction and / or the Y-axis direction correspond to the "second direction". Each of the electrode sheet 20 and the separator 30 extends along the X-axis direction and the Y-axis direction. The outer frame 50 is a resin member that surrounds the peripheries of the electrode sheet 20 and the separator 30 in the X-axis direction and the Y-axis direction. The current collector 21 of the electrode sheet 20 and the separator 30 are fixed to the outer frame 50 such that their respective edges are inserted into the outer frame 50.

[0020] A positive current collector foil 22, a positive electrode active material layer 24, a separator 30, a negative electrode active material layer 25, and a negative current collector foil 23, which are stacked in order along the + or - direction of the Z-axis direction, are regarded as one unit constituting the secondary battery 10, that is, one cell 40. The secondary battery 10 is formed by connecting a plurality of such cells 40 in series along the Z-axis direction. In the example of FIG. 1, only about three cells 40 are shown, but the secondary battery 10 is constituted by connecting, for example, several tens of cells 40 in series.

[0021] Next, the characteristics regarding the ratio of the thickness of the current collector 21 in the present embodiment will be described. Note that this characteristic is not shown in FIG. 1.

[0022] (First Embodiment) FIG. 2 shows a cross-section of one cell 40 according to the first embodiment. The cross-section shown in each figure is a cross-section parallel to the X-axis direction and perpendicular to the Y-axis direction. In the first embodiment, all or one or more of the plurality of cells 40 constituting the secondary battery 10 have a cross-section as shown in FIG. 2. FIG. 8 shows a cross-section of one conventional cell 4 according to the comparative example. The cell 4 is constituted by a positive current collector foil 2, a positive electrode active material layer 3, a separator 5, a negative electrode active material layer 6, and a negative current collector foil 7, which are stacked in order along the + or - direction of the Z-axis direction.

[0023] According to the first embodiment, in at least one electrode sheet 20, the ratio of the thickness of the current collector 21 to the thickness of the electrode sheet 20 in the Z-axis direction (hereinafter referred to as the current collector ratio) is greater at the first position P1 than at the second position P2. At the first position P1, in a second direction perpendicular to the Z-axis direction (the X-axis direction in Figure 2), the center C of the electrode sheet 20 is closer to the edge E of the electrode sheet 20 than at the edge E of the electrode sheet 20, and at the second position P2, in the second direction (the X-axis direction in Figure 2), the edge E of the electrode sheet 20 is closer to the first position P1 than at the first position P1. In other words, comparing the first position P1 and the second position P2, the first position P1 is closer to the center C in the second direction.

[0024] Hereinafter, the thickness in the Z-axis direction will simply be referred to as "thickness." The current collector ratio of the electrode sheet 20 can be understood as the ratio of the thickness of the current collector 21, which is the combined thickness of the positive electrode current collector 22 and the negative electrode current collector 23, to the combined thickness of the electrode sheet 20, which is the combined thickness of the positive electrode active material layer 24, the positive electrode current collector foil 22, the negative electrode current collector foil 23, and the negative electrode active material layer 25. Alternatively, the current collector ratio of the electrode sheet 20 can be understood as the ratio of the thickness of the positive electrode current collector foil 22 to the combined thickness of the positive electrode active material layer 24 and the positive electrode current collector foil 22, and / or the ratio of the thickness of the negative electrode current collector foil 23 to the combined thickness of the negative electrode active material layer 25 and the negative electrode current collector foil 23. According to Figure 2, the edge E of the electrode sheet 20 is the edge of the region where the current collector 21 (positive electrode current collector foil 22 and negative electrode current collector foil 23) and the active material layers 24 and 25 overlap.

[0025] In the first embodiment, the current collector ratio at the first position P1 > current collector ratio at the second position P2 holds true. On the other hand, in the comparative example, the thickness of the positive electrode current collector foil 2 and the negative electrode current collector foil 7 are constant at any position in the second direction perpendicular to the Z-axis direction (the X-axis direction in Figure 8), and the current collector ratio is constant. Furthermore, according to Figure 2, the current collector ratio increases in steps as you approach the center C from the end E in the second direction. According to Figure 2, the current collector ratio is largest in the central part including the center C.

[0026] When viewed from a viewpoint along the Z-axis, cell 40 is, for example, a square or a rectangle. The area of ​​cell 40 when viewed from a viewpoint along the Z-axis, that is, the area in the XY plane where the current collector 21 (positive electrode current collector foil 22 and negative electrode current collector foil 23) and the active material layers 24 and 25 overlap, is, for example, 0.2 square meters or more. Thus, the first embodiment assumes a relatively large cell 40.

[0027] When the current collector ratio at end E and second position P2 is taken as the baseline (100%), the current collector ratio at first position P1 is, for example, 20% or more greater than this baseline. Therefore, for example, if the current collector ratio at end E and second position P2 is 10%, then the current collector ratio at first position P1 is 12% or more. In Figure 2, for example, within a range of 30 cm from end E toward center C, the current collector ratio including second position P2 is approximately the same as that of end E, and in the range closer to center C than this 30 cm range, the current collector ratio is greater than that of end E.

[0028] A brief explanation of the manufacturing method for the secondary battery 10 is provided. First, a current collector 21 is prepared, which is formed by laminating a positive electrode current collector foil 22 and a negative electrode current collector foil 23, and is formed such that its thickness increases as it approaches the center C, for example, as shown in Figure 2. Next, a positive electrode active material layer 24 is formed on the positive electrode current collector foil 22 side of the current collector 21, and a negative electrode active material layer 25 is formed on the negative electrode current collector foil 23 side of the current collector 21. At this time, the active material layers 24 and 25 are formed so that the thickness of the electrode sheet 20, including the current collector 21 (positive electrode current collector foil 22 and negative electrode current collector foil 23) and the active material layers 24 and 25, is constant. Then, the secondary battery 10 is manufactured by alternately laminating the electrode sheet 20 thus produced with a separator 30.

[0029] Thus, in the first embodiment, the current collector 21 is molded such that, in principle, its thickness increases as it approaches the center C from the end E. However, the relationship between the magnitudes of the thicknesses may be reversed locally. For example, in at least one electrode sheet 20, the proportion of current collectors at a third position closer to the center C than at a first position P1 in the second direction may be smaller than the proportion of current collectors at the first position P1. Also, although Figure 2 illustrates a configuration in which the proportion of current collectors differs at different positions in the X-axis direction, in at least one electrode sheet 20, the cross-sectional shape shown in Figure 2 extends in the Y-axis direction. Alternatively, in at least one electrode sheet 20, the proportion of current collectors may differ at different positions in the Y-axis direction as well as in the X-axis direction.

[0030] (Second example) Figure 3 shows a cross-section of a single cell 40 according to the second embodiment. For the second to fourth embodiments described below, explanations common to the first embodiment will be omitted, and the differences from the first embodiment will be explained. As shown in Figure 3, in the second embodiment, the current collector ratio increases continuously as you approach the center C from the end E in the second direction (the X-axis direction in Figure 3). More specifically, using the current collector ratio at the end E as a reference, the current collector ratio increases continuously as you approach the center C from the end E, and the current collector ratio is largest at the center C. Therefore, in the second embodiment, the current collector ratio at the second position P2, which is far from the end E, is larger than the current collector ratio at the end E. Continuous can mean either linearly continuous or nonlinearly continuous.

[0031] (Third embodiment) Figure 4 shows a cross-section of a single cell 40 according to the third embodiment. In the third embodiment, similar to the first embodiment, the current collector ratio increases gradually from the end E towards the center C in the second direction (X-axis direction in Figure 4), but the degree of increase in the current collector ratio is suppressed overall compared to the first embodiment. According to Figure 4, when the current collector ratio at the end E and the second position P2 is taken as a baseline (100%), the current collector ratio at the first position P1 is one step higher than this baseline, and is, for example, 10% or more (but less than 20%) larger than this baseline. Also, according to Figure 4, the current collector ratio in the central part, including the center C, is smaller than in Figure 2.

[0032] (Fourth embodiment) Figure 5 shows a cross-section of a cell 40 according to the fourth embodiment. In the fourth embodiment, similar to the first and third embodiments, the current collector ratio increases gradually as you approach the center C from the end E in the second direction (the X-axis direction in Figure 5), but the range in which the current collector ratio is large is narrower than in the first and third embodiments. According to Figure 5, for example, in the range within 60 cm from the end E toward the center C, the current collector ratio is approximately the same as that of the end E, including the second position P2, and in the range closer to the center C than this 60 cm range, the current collector ratio is larger than that of the end E.

[0033] (Fifth example) Figure 6 shows cross-sections of multiple cells 40 according to the fifth embodiment. In the first to fourth embodiments, all cells 40 constituting the secondary battery 10 may have the characteristics shown in Figures 2 to 5. On the other hand, in the fifth embodiment, among the multiple cells 40 constituting the secondary battery 10, a predetermined number of cells 40 located at one end in the Z-axis direction (for example, the + side) and a predetermined number of cells 40 located at the other end in the Z-axis direction (the - side) are excluded from each of the cells 40, and the current collector ratio at the first position P1 is greater than the current collector ratio at the second position P2. Although omitted in Figure 6 as appropriate, in the fifth embodiment, the secondary battery 10 has 30 cells connected in series along the Z-axis direction. Of these 30 cells, the 5 cells located at one end in the Z-axis direction and the 5 cells located at the other end in the Z-axis direction are the cells 4 of the comparative example shown in Figure 8. The remaining 20 cells, excluding these 10 cells 4, are the cells 40 of the first embodiment shown in Figure 2. In other words, in the fifth embodiment, in a configuration in which multiple cells are connected in series in the Z-axis direction, the features of this embodiment regarding the current collector ratio were applied only to the cells in the intermediate part, which tend to accumulate heat, excluding a predetermined number of cells located at both ends in the Z-axis direction.

[0034] (effect) In this embodiment, the electrode sheet 20 has a larger current collector ratio at the first position P1, which is closer to the center C, compared to the second position P2. As a result, the heat capacity is relatively increased at the position of the electrode sheet 20 closer to the center C in the second direction, and the temperature rise is suppressed even in situations where heat tends to accumulate. Therefore, in a bipolar secondary battery 10, the temperature rise due to charging and discharging can be made uniform throughout the battery without forming a short circuit, and the temperature difference inside the battery can be suppressed.

[0035] Figure 7 shows the effects of each embodiment included in this embodiment in a table format. In Figure 7, the effects of the secondary battery 10 using cell 40 in the first embodiment (Figure 2), the effects of the secondary battery 10 using cell 40 in the third embodiment (Figure 4), the effects of the secondary battery 10 using cell 40 in the fourth embodiment (Figure 5), and the effects of the fifth embodiment (Figure 6) are listed side by side with the effects of the secondary battery using cell 4 in the comparative example (Figure 8). In Figure 7, the number of applicable cells is the number of cells to which this embodiment is applied out of the total number of cells connected in series to constitute the secondary battery (number of cells to which this embodiment is applied / total number of cells). The number of applicable cells is 30 / 30 for the first, third, and fourth embodiments, 20 / 30 for the fifth embodiment, and 0 / 30 for the comparative example.

[0036] In Figure 7, the current collector increase rate is the increase rate of the current collector ratio relative to the reference rate (100%), with the current collector ratio at end E and second position P2 being increased by one step relative to the reference rate. In the first embodiment, the current collector increase rate was 20% for all cells 40; in the third embodiment, it was 10% for all cells 40; in the fourth embodiment, it was 20% for all cells 40, the same as in the first embodiment; and in the fifth embodiment, it was 20% only for the 20 cells 40 in the middle section, the same as in the first embodiment.

[0037] In Figure 7, the current collector increase range indicates the range in which the current collector ratio is increased compared to the end E and the second position P2. In the first and third embodiments, the current collector increase range is the range excluding the area within 30 cm from the end E toward the center C in all cells 40, as described above. In the fourth embodiment, the current collector increase range is the range excluding the area within 60 cm from the end E toward the center C in all cells 40, as described above. In the fifth embodiment, the current collector increase range is limited to the 20 cells 40 in the middle section, excluding the area within 30 cm from the end E toward the center C.

[0038] In Figure 7, the temperature difference is the maximum temperature difference within the secondary battery obtained by measuring the temperature inside the secondary battery during repeated charging and discharging using a predetermined measuring means. According to Figure 7, the comparative example had the largest temperature difference, while the first, third, fourth, and fifth examples all had lower temperature differences than the comparative example. In particular, the first and fifth examples showed excellent results (○) with the temperature difference suppressed to 10 degrees or less, indicating that the setting of the current collector increase rate and current collector increase range was appropriate. The third and fourth examples showed a temperature difference suppression that was not as good as the first and fifth examples, but still better than the comparative example, indicating a good effect (△). It should be noted that the second example is also expected to have a temperature difference suppression effect within the battery that is at least as good as that of the third and fourth examples, or at least as good as that of the first and fifth examples.

[0039] The specific examples of the technologies disclosed herein have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples described above. Furthermore, the technical elements described herein or in the drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technologies illustrated herein or in the drawings achieve multiple objectives simultaneously, and achieving even one of these objectives constitutes technical usefulness in itself. [Explanation of Symbols]

[0040] 10: Secondary battery, 20: Electrode sheet, 21: Current collector, 22: Positive electrode current collector foil, 23: Negative electrode current collector foil, 24: Positive electrode active material layer, 25: Negative electrode active material layer, 30: Separator, 40: Cell, 50: Outer frame, C: Center, E: End, P1: First position, P2: Second position

Claims

1. A bipolar secondary battery, Multiple electrode sheets, Equipped with multiple separators, The plurality of electrode sheets and the plurality of separators are stacked alternately along the first direction. The electrode sheet comprises a current collector, a positive electrode active material layer provided on one side of the current collector, and a negative electrode active material layer provided on the other side of the current collector. In at least one of the electrode sheets, the ratio of the thickness of the current collector to the thickness of the electrode sheet in the first direction is greater at the first position than at the second position. The first position is closer to the center of the electrode sheet than to the edge of the electrode sheet in a second direction perpendicular to the first direction. The second position is a secondary battery in which, in the second direction, is closer to the edge of the electrode sheet than the first position.

2. The secondary battery according to claim 1, wherein the ratio of the thickness of the current collector of at least one electrode sheet increases in a stepwise manner in the second direction as it approaches the center of the electrode sheet from the edge of the electrode sheet.

3. The secondary battery according to claim 1, wherein the ratio of the thickness of the current collector of at least one electrode sheet increases continuously in the second direction as it approaches the center of the electrode sheet from the edge of the electrode sheet.

4. The secondary battery according to claim 1, wherein the ratio of the thickness of the current collector at the first position is 20% or more greater than the ratio of the thickness of the current collector at the second position, based on the ratio of the thickness of the current collector.

5. The current collector is formed by overlapping a positive electrode current collector foil and a negative electrode current collector foil. When the positive electrode current collector foil, the positive electrode active material layer, the separator, the negative electrode active material layer, and the negative electrode current collector foil are stacked in order along the first direction, the secondary battery is formed by connecting a plurality of these cells in series along the first direction. The secondary battery according to claim 1, wherein, among the plurality of cells, a predetermined number of cells located at one end of the first direction and a predetermined number of cells located at the other end of the first direction are excluded, and in each of the cells, the ratio of the thickness of the current collector at the first position is greater than the ratio of the thickness of the current collector at the second position.

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