Secondary battery

Abstract

The present application provides a secondary battery having a laminated structure in which a positive electrode active material layer coated on a positive electrode foil and a negative electrode active material layer coated on a negative electrode foil face each other with a separator interposed therebetween and an electrolyte is injected between the positive electrode foil and the negative electrode foil, and the periphery of the separator is continuously bonded to the edge of the positive electrode or the negative electrode over the entire circumference.

Description

Technical Field

[0001] This invention relates to a secondary battery, and more specifically, to the structure of a secondary battery. Background Technology

[0002] In short, secondary batteries such as lithium-ion batteries have a stacked structure in which a positive electrode active material layer and a negative electrode active material layer are placed opposite each other with an electrolyte injected between them, separated by a separator. The positive electrode active material layer is coated on a current collector (positive electrode foil), which may be a metal foil, and the negative electrode active material layer is coated on a current collector (negative electrode foil), which may also be a metal foil. Various structures have been proposed to address various problems that may arise in this type of secondary battery. For example, Patent Document 1 proposes a method as follows: a battery structure having an electrode stack and a primary sealing portion is fabricated; the interior of the battery structure is heated to reduce the pressure in the internal space of the battery structure; then, an electrolyte is injected into the internal space of the battery structure, thereby ensuring space for the electrolyte to enter in the separator; the electrode stack is formed by stacking bipolar electrodes having a positive electrode formed on the upper surface of a nickel foil and a negative electrode formed on the lower surface of a nickel foil, separated by a separator; and the primary sealing portion is arranged to surround the electrode stack and hold the nickel foil. Furthermore, in Patent Document 2, a secondary battery with good manufacturability is proposed with the following structure: having a power generation element and a sealing part, all of the sealing parts having a substantially identical shape, and each sealing part having a base that surrounds the outer periphery of the electrode and a cover that extends from the base and overlaps with the surface of the electrode, the power generation element being formed by stacking electrodes with a separator 45 in between, and the sealing part being used to seal at least a portion of the outer periphery of the power generation element.

[0003] Patent Document 1: Japanese Patent Application Publication No. 2019-087414

[0004] Patent Document 2: Japanese Patent Application Publication No. 2016-146269 Summary of the Invention

[0005] In a secondary battery as described above, a positive electrode active material layer coated on a positive electrode foil and a negative electrode active material layer coated on a negative electrode foil are positioned opposite each other across a separator, and an electrolyte is injected between the electrode foils (positive electrode foil and negative electrode foil). Figure 6 As schematically depicted in (A), the periphery 6a of the diaphragm 6 is fixed and held in place by either the edge of the positive electrode side or the edge of the negative electrode side (the edge 7a of the sealing portion or the edge of the electrode foils 2 and 4). In this case, generally, it is sufficient to maintain the stable position of the diaphragm 6, therefore... Figure 6 As described in (B), the edge of the diaphragm is bonded to the edge of one of the electrodes at multiple point regions 6c, and the electrolyte EL can flow easily between the bonding points.

[0006] However, with the injection of electrolyte, foreign matter can sometimes infiltrate between the electrode foils in the aforementioned layered structure. If this foreign matter enters the positive or negative active material layer, it can puncture the separator, causing a short circuit between the electrodes and potentially leading to battery malfunction. In structures where the periphery of the separator is intermittently fixed relative to the electrode edge, even on the side where the separator periphery is fixed, foreign matter can still infiltrate along with the electrolyte from the bonding points at the periphery of the separator. Therefore, short circuits caused by foreign matter can occur in the active materials of both electrodes. Thus, a structure that prevents foreign matter in the electrolyte from reaching either the positive or negative active material layer is advantageous.

[0007] In view of the above, the main objective of the present invention is to prevent foreign matter that can penetrate with the injection of electrolyte from reaching either the positive electrode active material layer or the negative electrode active material layer in a secondary battery having a stacked structure in which a positive electrode active material layer coated on a positive electrode foil and a negative electrode active material layer coated on a negative electrode foil are opposed to each other across a separator and an electrolyte is injected between the electrode foils.

[0008] Regarding the aforementioned issues, when the edge of the diaphragm is fixed to the edge of one of the electrodes (the sealing portion or the edge of the electrode foil), if the edge of the diaphragm is continuously bonded to the edge of the electrode in a linear fashion along its entire circumference, and the active material layer of one electrode is entirely covered by the diaphragm and the electrode foil, then even if foreign matter enters the space between the electrode foils during electrolyte injection, it can be prevented from reaching the active material layer covered by the diaphragm and the electrode foil. Furthermore, as described in the Embodiments section below, the electrolyte itself permeates the diaphragm; therefore, the space between the diaphragm and the electrode foil on the side where the edge of the diaphragm is bonded can also be filled with electrolyte. This insight is utilized in the present invention.

[0009] According to the present invention, the above-mentioned problem is achieved by a secondary battery having a stacked structure in which a positive electrode active material layer coated on a positive electrode foil and a negative electrode active material layer coated on a negative electrode foil are opposed to each other across a separator and an electrolyte is injected between the electrode foils, wherein the periphery of the separator is continuously bonded to the edge of the positive electrode or the negative electrode over its entire circumference.

[0010] In the above structure, the secondary battery can be a non-aqueous secondary battery, typically a lithium-ion secondary battery. The positive electrode foil (positive foil) and negative electrode foil (negative foil) can be current collectors made of conventionally formed metal foils, and the positive and negative active material layers can be conventionally coated onto the positive and negative electrode foils, respectively. The separator and electrolyte can also be conventionally formed separators and electrolytes. The edge of the positive or negative electrode can be the edge of the positive or negative electrode foil, or a sealing portion that holds the edge. The sealing portion holding the edge of the positive and negative electrode foil can be formed of a resin material commonly used in this field, such as polyethylene. In addition, in a secondary battery, a structure can be formed by stacking multiple layers of a positive electrode foil and a negative electrode foil as described above. The positive electrode foil and the negative electrode foil can be respectively attached to the negative electrode foil and the positive electrode foil of the adjacent stacked structure. Thus, except for the two ends of the battery, the separator is sandwiched between two bipolar electrode bodies (electrode bodies stacked in the order of negative electrode active material layer-negative electrode foil-positive electrode foil-positive electrode active material layer).

[0011] Furthermore, in the structure of the present invention described above, the periphery of the separator is continuously bonded to the edge of the positive or negative electrode along its entire circumference. The bonding of the periphery of the separator to the edge of the positive or negative electrode can be achieved by any method such as thermal welding or hot pressing. According to this structure, as described above, in the electrode on one side where the periphery of the separator is bonded to the edge, electrolyte flow from the periphery of the separator to the space between the electrode foil and the separator is prevented, thus preventing foreign matter from entering this space. This prevents foreign matter from reaching the active material layer and prevents short circuits caused by foreign matter. Additionally, as described above, the filling of the electrolyte between the electrode foil and the separator in the electrode on one side where the periphery of the separator is bonded to the edge is achieved by electrolyte permeation through the separator.

[0012] In the structure of the present invention described above, it is more preferable that the periphery of the diaphragm can be bonded to the electrode foil of the positive or negative electrode. In this structure, the electrode foil, which is a metal foil, has a small coefficient of thermal expansion, and therefore has an advantage in suppressing the formation of wrinkles or damage to the diaphragm caused by temperature changes.

[0013] In the structure described above, the active material layer covered by the diaphragm and electrode foil becomes either the positive or negative electrode. Regarding this, in the event of a foreign object reaching the active material layer, the side with fewer pores within the active material layer is more likely to have the foreign object puncture the diaphragm. The reason is simple: if a foreign object enters the active material layer, its volume is absorbed into the volume of the pores within the active material layer. However, when the pores are fewer, the volume of the foreign object cannot be completely absorbed by the pore volume, thus increasing the likelihood of pressure on the diaphragm. Therefore, it is preferable that the side with fewer pores in the active material layer of the two electrodes more reliably prevents the arrival of foreign objects. Thus, in the active material layer covered by the diaphragm and electrode foil, the side with fewer pores in the active material layer of the two electrodes can be selected. In this way, in the above structure, the periphery of the diaphragm can adhere to the edge of the electrode with lower porosity in the active material layer. Here, porosity is given by 1 - (electrode bulk density) / (true density of the active material layer). In addition, the porosity of the positive electrode is usually smaller than that of the negative electrode, so the periphery of the membrane can be bonded to the edge of the positive electrode.

[0014] Furthermore, the resistance of the positive electrode active material layer is typically greater than that of the negative electrode active material layer. Therefore, in the event of a short circuit caused by conductive foreign matter intruding into the positive electrode active material layer, the decrease in resistance between the electrodes is greater compared to the case of a short circuit caused by conductive foreign matter intruding into the negative electrode active material layer, potentially leading to a larger short-circuit current between the electrodes. In other words, the positive electrode active material layer should be more reliably protected from foreign matter intrusion than the negative electrode active material layer; therefore, the periphery of the separator can be adhered to the edge of the positive electrode.

[0015] Furthermore, in the structure of the present invention described above, the diaphragm, in addition to its periphery, can be further bonded to the surface of the electrode foil that is not coated with an active material layer. This allows the diaphragm to be held more stably.

[0016] Invention Effects

[0017] Thus, according to the structure of the present invention, in a secondary battery with a stacked structure in which a positive electrode active material layer coated on a positive electrode foil and a negative electrode active material layer coated on a negative electrode foil are opposed to each other across a separator and an electrolyte is injected between the electrode foils, either the positive electrode active material layer or the negative electrode active material layer is covered by the separator and the electrode foil. This prevents foreign matter accompanying the electrolyte from reaching the active material layer and minimizes short circuits caused by foreign matter. Furthermore, the structure of the present invention is substantially achieved by continuously bonding the periphery of the separator to the edge of one of the electrodes, which also has the advantage of not increasing the number of parts.

[0018] Other objects and advantages of the present invention will be further explained in the following description of preferred embodiments of the invention. Attached Figure Description

[0019] Figure 1 (A) is a schematic cross-sectional view near the periphery of an electrode in one embodiment of a stacked structure applicable to the secondary battery of this embodiment. Figure 1 (B) is a schematic top view of the separator and electrode active materials in the secondary battery applicable to this embodiment. Figure 1 (C) is a schematic cross-sectional view near the periphery of an electrode in another manner of the stacked structure applicable to the secondary battery of this embodiment.

[0020] Figure 2 (A) is a schematic cross-sectional view of the stacked structure of a secondary battery used in an experiment to confirm the permeability of the electrolyte to the diaphragm. Figure 2 (B) represents the measured values ​​of fluid resistance in each part of the stacked structure. The values ​​in the figure are the measured values ​​of fluid resistance.

[0021] Figure 3 (A) Figure 3 (B) is a schematic cross-sectional view of the stacked structure in the secondary battery applicable to this embodiment. Figure 3 (A) is the case where the diaphragm is fixed only at the periphery. Figure 3 (B) is the case where a diaphragm is fixed in the region inside the electrode foil where no active material layer is coated.

[0022] Figure 4 (A) is a schematic cross-sectional view showing a situation where foreign matter intrudes into the stacked positive electrode active material layer of a secondary battery, causing a short circuit between the electrodes. Figure 4 (B) is a schematic cross-sectional view showing a situation where foreign matter intrudes into the stacked negative electrode active material layer of a secondary battery, causing a short circuit between electrodes.

[0023] Figure 5 (A) ~ Figure 5 (D) is a schematic cross-sectional view of the stacked structure of the secondary battery during the process of stacking bipolar electrodes in the secondary battery applicable to this embodiment.

[0024] Figure 6 (A) is a schematic cross-sectional view of the periphery of the electrodes in a conventional secondary battery with a stacked structure. Figure 6 (B) is a schematic top view of the separator and electrode active materials in a conventional secondary battery.

[0025] Symbol Explanation

[0026] 1-Secondary battery, 2-Positive electrode foil, 3-Positive electrode active material, 4-Negative electrode foil, 5-Negative electrode active material layer, 6-Separator, 6a-Separator periphery, 6b, 6c-Adhesive part, 7-Sealing part, 7a-Sealing end, 10-Electrolyte injection port, v1, v2-Electrolyte filling space, EL-Electrolyte, X-Foreign object. Detailed Implementation

[0027] Hereinafter, several preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same symbols denote the same parts.

[0028] Stacked structure of secondary batteries

[0029] like Figure 1 (A) Figure 1 As schematically depicted in (C), in the secondary battery 1 applicable to this embodiment, a cell is formed by a stacked structure in which a positive electrode active material layer 3 coated on the surface of the positive electrode foil (positive electrode foil) 2 and a negative electrode active material layer 5 coated on the surface of the negative electrode foil (negative electrode foil) 4 are separated by a separator 6. This cell can be a structure formed by stacking multiple layers (see reference). Figure 5In this configuration, the positive electrode foil 2 of each cell is bonded to the negative electrode foil 4 of the cell adjacent to it on the upper side of the diagram, and the negative electrode foil 4 of each cell is bonded to the positive electrode foil 2 of the cell adjacent to it on the lower side of the diagram, thereby forming a series connection of multiple cells. Thus, the bonded positive electrode foil 2 and negative electrode foil 4 constitute a "bipolar electrode". The positive electrode foil 2 and negative electrode foil 4 can be commonly used metal foils with a thickness of tens of μm, such as aluminum foil or nickel foil. The positive electrode active material layer 3 can be a layer with a thickness of about 0.1 mm, formed by applying a mixture of conductive additives such as carbon black and binders such as PVdF (polyvinylidene fluoride) to commonly used positive electrode active materials such as NCM (nickel, cobalt, manganese oxide), LFP (lithium iron phosphate), and LMFP (lithium manganese iron phosphate). The negative electrode active material layer 5 can be a layer with a thickness of about 0.1 mm, formed by applying a mixture of binders such as SBR / CMC (styrene-butadiene rubber / carboxymethyl cellulose) to commonly used negative electrode active materials such as graphite (natural or artificial). The separator 6 can be formed from a three-layer lithium-ion permeable resin membrane of PP-PE-PP (polypropylene-polyethylene-polypropylene) with a thickness of about 20 μm. Then, the spaces v1 and v2 on both sides of the separator 6 between the positive electrode foil 2 and the negative electrode foil 4 are filled with electrolyte. The electrolyte is selected appropriately according to the type of battery. For example, in the case of a non-aqueous lithium-ion battery, an electrolyte solution containing LiPF6 (lithium hexafluorophosphate) dissolved at a concentration of 1M can be used, which is a non-aqueous solvent composed of EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DMC (dimethyl carbonate) in a 1:1:1 ratio. Furthermore, as shown in the figure, the active material layers 3 and 5 of the positive and negative electrodes are coated on the central regions of the corresponding electrode foils 2 and 4. The outer peripheries of the electrode foils 2 and 4 are fixed by being clamped by the end 7a of the sealing portion 7, thereby maintaining the spacing of the electrode foils 2 and 4 in the stacking direction. The sealing portion 7 can be formed of any resin with appropriate rigidity, such as polyethylene. Additionally, a portion of the sealing portion 7 located between the electrode foils 2 and 4 is opened to form an injection port 10 for injecting the electrolyte.

[0030] In the above structure, the periphery 6a of the diaphragm 6 can be as follows: Figure 1 (A) It can be fixed to the end 7a of the sealing part 7 as described in the example, or it can be fixed to the end 7a of the sealing part 7 as described in the example. Figure 1 (C) Fixed to either the positive electrode foil 2 or the negative electrode foil 4 as described in the example. At this time, if combined... Figure 6 (B) As previously described, conventionally, the periphery 6a of the diaphragm 6 is adhered to the edge of the electrode at multiple point regions 6c along the circumferential direction. However, in this case, foreign matter can enter between the point regions 6c along with the electrolyte and reach the active material. Then, if the foreign matter is conductive and penetrates the diaphragm, crossing the positive electrode active material 3 and the negative electrode active material 5, a short circuit occurs between the electrodes. Therefore, in this embodiment, in order to minimize the possibility of foreign matter reaching the active material, such as Figure 1As schematically depicted in (B), the periphery 6a of the diaphragm 6 is continuously bonded to the end 7a of the sealing portion 7 or either the positive electrode foil 2 or the negative electrode foil 4 along its entire circumference (symbol 6b represents the bonding area). Therefore, the electrolyte does not flow from the periphery 6a of the diaphragm 6 into the space v1 or v2 between the electrode foil 2 or 4 and the diaphragm 6 on the side where it is bonded to the edge, thus preventing foreign matter from entering. This protects the active material layer 3 or 5 of the electrode on the side where the periphery 6a of the diaphragm 6 is bonded to the edge from foreign matter. In this regard, the coefficient of thermal expansion of the electrode foils 2 and 4, which are metal foils, is smaller than that of the sealing portion 7, resulting in smaller dimensional changes due to temperature variations. Therefore, if... Figure 1 (C) By bonding the periphery 6a of the diaphragm 6 to the electrode foils 2 and 4 in this way, the formation of wrinkles or damage to the diaphragm 6 can be suppressed, which is an advantage.

[0031] In the above embodiment, if the periphery 6a of the diaphragm 6 is continuously bonded to the end 7a of the sealing portion 7 or the positive electrode foil 2 or the negative electrode foil 4 along the entire circumference, the electrolyte will not flow from the periphery 6a of the diaphragm 6 into the space v1 or v2 between the electrode foil and the diaphragm 6 on the side where it is bonded to the edge. However, the diaphragm 6 permeates the electrolyte, so the space v1 or v2 between the electrode foil and the diaphragm 6 on the side where the periphery 6a of the diaphragm 6 is bonded to the edge is filled with electrolyte. This was confirmed by the following experiment.

[0032] In the experiment, such as Figure 2 As shown in (A), the fluid resistance was measured in (a) a structure in which a membrane 6 is sandwiched between a conventional positive electrode active material 3 and a conventional negative electrode active material 5 (in which the former has greater fluid resistance) and (b) a structure in which a membrane 6 is sandwiched between a positive electrode active material 3 and a conventional negative electrode active material 5 with a thickness thinner than conventional but with increased fluid resistance. Figure 2 (B) represents the fluid resistance FR of the three layers (3) of structure (a) and structure (b) when the positive active material (+), negative active material (-), and electrolyte are injected from the edges of the positive and negative active materials. Figure 2 As shown in (B), the fluid resistance of the positive electrode active material is large in structure (b), but the fluid resistance of the three layers is lower in structure (b) than in structure (a). This indicates that in the case of three layers, the electrolyte first permeates into the negative electrode active material and then permeates from the negative electrode active material through the membrane to the positive electrode active material (if it does not pass through the membrane, in structure (b), the permeation of the electrolyte of the positive electrode active material with large fluid resistance becomes a rapid step, so the fluid resistance of the three layers (3) of structure (b) should be greater than that of structure (a). Thus, it is confirmed that even if the electrolyte does not flow directly from the end 6a of the membrane 6, the electrolyte can still permeate through the membrane 6 and fill the spaces v1 and v2 on both sides of the membrane 6 between the electrode foils.

[0033] In the above structure, such as Figure 3 As shown in (A), the diaphragm 6 can be fixed only to the periphery 6a, but as Figure 3 As shown in (B), when there are areas in the electrode foil 2 that are not coated with active material 3, except for the periphery, the diaphragm 6 (6d) can be bonded to these areas. This allows the diaphragm 6 to be held more stably.

[0034] In the structure of this embodiment described above, either the positive electrode active material layer 3 or the negative electrode active material layer 5 is used to prevent the entry of foreign matter by covering the membrane 6 and the electrode foil 2 or 4. Therefore, when foreign matter enters, it is preferable to prevent the entry of foreign matter by covering the one that is more affected.

[0035] Regarding this, firstly, the porosity ratios of the positive and negative electrodes in the active material layer are typically different. Secondly, the electrode with a smaller porosity and a smaller volume capable of accommodating foreign matter upon intrusion is more likely to compress the diaphragm. Therefore, it is preferable to more reliably prevent foreign matter intrusion in the active material layer with a smaller porosity. Thus, in this embodiment, the periphery of the diaphragm can be adhered to the edge of the electrode with lower porosity in the active material layer. Here, porosity is given by 1 - (electrode bulk density) / (true density of the active material layer). Typically, the porosity of the positive electrode active material layer is 30–42%, while that of the negative electrode active material layer is 45%, etc., with the positive electrode having less porosity than the negative electrode; therefore, the periphery of the diaphragm can be adhered to the edge of the positive electrode.

[0036] Furthermore, typically, the resistance (insulation) of the positive and negative electrodes differs within the active material layer. Then, as... Figure 4 As shown in (A), when a conductive foreign object enters the area with higher resistance, the area with higher resistance is replaced by the conductive foreign object, thus... Figure 4 Compared to when a conductive foreign object enters the side with lower resistance (as shown in (B)), the decrease in resistance is greater. Therefore, the short-circuit current Ia flowing when a conductive foreign object enters the side with higher resistance is greater than the short-circuit current Ib flowing when a conductive foreign object enters the side with lower resistance, and correspondingly, the impact is also greater. Thus, it is preferable to prevent the entry of foreign objects more reliably in the active material layer with higher resistance. In this embodiment, the periphery of the diaphragm can be adhered to the edge of the electrode with higher resistance in the active material layer. Typically, with respect to the resistance of the active material layer, the positive electrode is greater than the negative electrode, so the periphery of the diaphragm can be adhered to the edge of the positive electrode.

[0037] Formation process of stacked structures

[0038] The structure of this embodiment described above, except that the bonding area of ​​the periphery 6a of the diaphragm 6 extends continuously along the entire circumference, can be formed using conventional methods. Specifically, firstly, as... Figure 5As shown in (A), corresponding active material layers 3 and 5 are coated on both sides of the electrode foils where the positive electrode foil 2 and the negative electrode foil 4 are bonded, as follows: Figure 5 As shown in (B), a diaphragm 6 is laminated on one side, and its periphery 6a is continuously bonded to the end of the electrode (periphery of the electrode foil, etc.) by, for example, hot pressing p. Then, as... Figure 5 As shown in (C), with the periphery 6a of the diaphragm 6 and the electrode foils 2 and 4 clamped together, the sealing part 7 is heat-fused to it. This forms a bipolar electrode. Subsequently, as... Figure 5 As shown in (D), multiple bipolar electrodes are stacked and combined with the sealing part 7 to form a battery module in which multiple cells are connected in series.

[0039] Thus, in the structure of this embodiment described above, either the positive electrode active material layer or the negative electrode active material layer is covered by the diaphragm and the electrode foil, thereby preventing foreign matter that enters with the electrolyte from reaching the active material layer and suppressing short circuits caused by foreign matter as much as possible.

[0040] The above description is based on embodiments of the present invention. However, those skilled in the art can easily make various modifications and changes. The present invention is not limited to the embodiments illustrated above, and can obviously be applied to various devices without departing from the concept of the present invention.

Claims

1. A secondary battery comprising a laminated structure having a positive electrode active material layer coated on a positive electrode foil and a negative electrode active material layer coated on a negative electrode foil, the two layers facing each other across a separator, with an electrolyte injected between the electrode foils, characterized in that... The periphery of the diaphragm is continuously bonded to the edge of the positive or negative electrode along its entire circumference.

2. The secondary battery according to claim 1, characterized in that, The periphery of the diaphragm is bonded to the electrode foil of the positive or negative electrode.

3. The secondary battery according to claim 1, characterized in that, The periphery of the diaphragm is bonded to the edge of the electrode with lower porosity in the active material layer.

4. The secondary battery according to claim 1, characterized in that, The periphery of the diaphragm is bonded to the edge of the positive electrode.

5. The secondary battery according to claim 1, characterized in that, The diaphragm is also bonded to the surface of the electrode foil that is not coated with an active material layer.

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