Secondary battery

Abstract

This invention provides a structure in a secondary battery using a current collector consisting of a PPTC material layer sandwiched between a pair of metal foils, capable of suppressing current flowing between the two electrodes through the metal foils on the inner side of the current collector and the melted portion of the separator, even if the separator melts due to abnormal heating of the battery. A secondary battery is provided having an active material layer coated on the current collector of the positive electrode and an active material layer coated on the current collector of the negative electrode, facing each other across a separator, with an electrolyte impregnated between the electrodes. The current collector of at least one of the positive and negative electrodes is composed of a PPTC material layer sandwiched between a pair of electrode foils. A raised region is formed on the surface of the PPTC material layer on the active material layer side, and when the PPTC material layer expands, the raised region cuts off the electrode foil it abuts.

Description

Technical Field

[0001] The present invention relates to a secondary battery, and more specifically, to a secondary battery configured to suppress current flow through a current collector when abnormal heating occurs. Background Technology

[0002] In short, secondary batteries such as lithium-ion batteries, in the case of liquid batteries, have a stacked structure in which a positive electrode active material layer coated on the positive electrode current collector and a negative electrode active material layer coated on the negative electrode current collector are separated by a separator immersed in the electrolyte. Various structures have been proposed to address various problems that may occur in such secondary batteries. For example, Patent Document 1 proposes a structure where, in the event of an internal short circuit caused by a metallic foreign object, a short circuit occurs between the current collectors of the positive and negative electrodes. To restore the insulation between the current collectors of the positive and negative electrodes and eliminate the internal short circuit, a current collector is formed by a metal foil and a resin film bonded to one side thereof. When heat is generated in the battery due to the internal short circuit, the resin film shrinks, thereby separating the metal foil from the metallic foreign object and eliminating the short circuit. Patent document 2 proposes the following: To block the current flowing through the battery when it overheats abnormally, a structure is used as the current collector disposed on the electrodes, consisting of metal foils bonded to both sides of a polymer positive temperature coefficient (PPTC) material layer. In this case, under normal circumstances, the PPTC material layer is in a conductive state, and the metal foils on both sides are also conductive. When current flows through the external circuit connected between the battery electrodes, if the battery becomes hot, the PPTC material layer expands and becomes non-conductive, insulating the metal foils on both sides. This blocks the current flowing between the battery electrodes, thereby preventing the battery from overheating further.

[0003] Patent Document 1: Japanese Patent Application Publication No. 2017-16787

[0004] Patent Document 2: Chinese Patent Publication No. 106910897 Summary of the Invention

[0005] like Figure 4 (A) schematically depicts a secondary battery 1 in which a PPTC material layer 5+, 5- is sandwiched between a pair of electrode foils 6i, 6o, which can be metal foils, serving as current collectors 2+, 2-. As described in Patent Document 2, under normal conditions, the PPTC material layer 5+, 5- is in a conductive state, and current I flows through the current collectors 2+, 2-. Current flows between the electrodes through an external circuit 10 connecting the electrodes outside the battery. On the other hand, when the battery abnormally heats up, as... Figure 4As shown in (B), the PPTC material layers 5+ and 5- expand, becoming non-conductive. The conductive pathways within the PPTC material layers 5+ and 5- are blocked, as indicated by × in the figure. Therefore, the current between the electrodes of the external circuit 10 is blocked. However, if the battery becomes very hot due to abnormal heating, etc., as... Figure 4 As shown in (C), when the diaphragm 4 melts (4B), a surface short circuit occurs due to contact between the positive and negative active material layers 3+ and 3-. Even if the PPTC material layer becomes non-conductive, the electrode foil on the active material layer side of the PPTC material layer becomes conductive. Therefore, a current (overflow current) Ir may flow from the region of the positive active material layer 3+ that has never been in contact with the melted region 4B of the diaphragm 4 through the electrode foil 6i on the active material layer side of the PPTC material layer 5+ and through the melted region 4B of the diaphragm, and further through the electrode foil 6i on the active material layer side of the PPTC material layer 5- into the region of the negative active material layer 3- that has not been in contact with the melted region 4B of the diaphragm 4. The short circuit state continues, thus the heating state continues. Therefore, it is advantageous to have a structure that can also prevent the overflow current Ir when the diaphragm 4 melts, as described above.

[0006] In view of the above, the object of the present invention is to provide a structure that, in a secondary battery using a current collector consisting of a PPTC material layer sandwiched between a pair of metal foils, can suppress the circumferential current as described above, even if the separator melts due to abnormal heating of the battery.

[0007] According to the present invention, the above-mentioned problem is achieved by a secondary battery having an active material layer coated on a current collector of the positive electrode and an active material layer coated on a current collector of the negative electrode, which are placed opposite each other across a separator and an electrolyte is impregnated between the electrodes, wherein the current collector of at least one of the positive and negative electrodes is composed of a PPTC material layer sandwiched between a pair of electrode foils, wherein...

[0008] The surface of the PPTC material layer on the side of the active material layer is configured to form a raised region, and when the PPTC material layer expands, the raised region cuts off the electrode foil that it abuts.

[0009] In the above structure, the secondary battery is typically a liquid secondary battery such as a lithium-ion secondary battery. The active material layers of the positive and negative electrodes, the separator, and the electrolyte can each be formed from any material in a conventional manner. Furthermore, in the case of the present invention, at least one of the current collectors on which the active material layers of the positive and negative electrodes are respectively coated is configured such that a PPTC material layer is sandwiched between a pair of electrode foils, which can be metal foils. Typically, the PPTC material layer refers to a material in which conductive particles such as copper, aluminum, nickel, silver, carbon black, nanotubes, and nanofibers are dispersed in a resin matrix such as polyethylene, epoxy resin, polypropylene, polyamide, or polyvinylidene fluoride. As mentioned above, when it is a resin layer with dispersed conductive particles, it is in a conductive state because the conductive particles are connected to each other below a certain temperature (switching temperature). On the other hand, when it is above the switching temperature, it expands and the conductive particles separate from each other, changing to an insulating state. Therefore, if the battery temperature is below the switching temperature, the electrode foils on both sides of the current collector are conductive, and current flows through the battery. On the other hand, if the battery overheats abnormally and its temperature exceeds the switching temperature, the electrode foils on both sides of the current collector are insulated, and the current in the circuit between the external electrodes of the battery is blocked (see reference). Figure 4 (A) Figure 4 (B)).

[0010] However, as mentioned earlier, if the separator melts due to abnormal heating of the battery, the surfaces of the active material layers of the positive and negative electrodes come into contact. A significant "circumferential current" is then generated between the electrode foil on the active material layer side of the current collector and the melted portion of the separator, resulting in a short circuit and continuous heating (see reference). Figure 4 (C)).

[0011] Therefore, in this invention, as described above, a raised region is formed on the surface of the PPTC material layer on the active material layer side, and when the PPTC material layer expands, the raised region cuts the electrode foil on the active material layer side that it contacts. According to this structure, if the battery temperature exceeds the switching temperature of the PPTC material layer, it expands, and the raised region cuts the electrode foil on the active material layer side, blocking the conductive path within the electrode foil. In this way, if a portion of the separator melts and the surfaces of the active material layers of the positive and negative electrodes come into contact, the conductive path between the melted portion of the separator and the area on the other side of the cut portion of the electrode foil as viewed from that portion is blocked (while allowing some current flow between the melted portion of the separator and the cut portion of the electrode foil), thus significantly suppressing the current flow through the melted portion of the separator, thereby suppressing further battery heating. Furthermore, the electrode foil can be laminated onto the PPTC material layer using vapor deposition or sputtering, thereby reducing the stress required for cutting, which is advantageous from the viewpoint that cutting the electrode foil during the expansion of the PPTC material layer becomes easier and more reliable. The raised regions in the PPTC material layer can be formed only on the positive or negative electrode, or on both electrodes. The width of the raised regions is, for example, 1 mm or more, so that they can more reliably cut the electrode foil when the PPTC material layer expands. The thickness of the PPTC material layer can be 3–10 μm in the raised regions and 0.5–2 μm outside the raised regions. The switching temperature of the PPTC material layer can be set between 120 and 180 °C.

[0012] In the above structure, specifically, it is preferable to adjust the thickness of the raised region so that the expansion of the PPTC material layer during the expansion of the raised region is greater than the thickness of the electrode foil in order to reliably cut off the portion of the electrode foil that abuts the raised region when the temperature of the raised region rises. For example, the PPTC material layer may preferably be formed of a material whose expansion rate is more than 5 times that of the electrode foil.

[0013] Furthermore, in the above structure, the raised region of the PPTC material layer can be configured such that only a portion of the electrode foil that abuts the raised region separates from the surrounding electrode foil region when the temperature rises, thereby reliably separating the electrode foils on both sides (in the planar direction) of the raised region of the PPTC material layer when the temperature rises. That is, the portion of the electrode foil that abuts the raised region is displaced by the expansion of the raised region and separated from the surrounding electrode foil, thereby enabling the cutting of the electrode foil.

[0014] Furthermore, in the aforementioned PPTC material layer, raised regions can be formed at multiple locations. Therefore, when the battery overheats abnormally, regardless of where the separator melts, the distance between the melted portion and the cut portion of any electrode foil is minimized. Correspondingly, the capacity of the region between the melted portion and the nearest cut portion of the electrode foil can be reduced, and the magnitude of the current flowing through the melted portion of the separator can be suppressed to a minimum. For example, the positions of the raised regions, i.e., the intervals between the electrode foil portions that are cut when the PPTC material layer expands, can be adjusted so that the capacity of each region between the locations where raised regions are formed is less than 1 Ah. Additionally, in the locations where raised regions are formed by the PPTC material layer, when the thickness of the electrode foil decreases, the resistance of that portion of the electrode foil (when the PPTC material layer is not expanded) increases. Therefore, the number of raised region formation locations between each portion of the electrode foil and the connection terminal is preferably small. In this regard, when a connection terminal is provided at one corner of a rectangular current collector, by providing a raised area along the long side, the number of raised areas passing through the line connecting the corner of the connection terminal and its opposite corner can be further reduced.

[0015] Invention Effects

[0016] Thus, according to the structure of the present invention, the magnitude of the swirling current that may occur when the separator melts due to abnormal heating of the battery can be suppressed. Consequently, the heating caused by the internal short circuit of the battery due to the melting of the separator caused by abnormal heating of the battery can be suppressed.

[0017] Other objects and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention. Attached Figure Description

[0018] Figure 1 (A) Figure 1 (B) is an enlarged schematic partial cross-sectional view of one embodiment of the secondary battery to which this embodiment is applied. Figure 1 (A) is when the PPTC material layer is not expanded. Figure 1 (B) is the expansion of the PPTC material layer. Figure 1 (C) Figure 1 (D) is a schematic cross-sectional view of the secondary battery to which this embodiment is applicable. Figure 1 (C) represents the state when the PPTC material layer is not expanded. Figure 1 (D) is the expansion of the PPTC material layer.

[0019] Figure 2 (A) Figure 2 (B) is an enlarged schematic partial cross-sectional view of another embodiment of the secondary battery to which this embodiment is applied. Figure 2 (A) is when the PPTC material layer is not expanded. Figure 2(B) is the expansion of the PPTC material layer.

[0020] Figure 3 (A) ~ Figure 3 (C) is a schematic top view of the PPTC material layer of the secondary battery to which this embodiment is applied.

[0021] Figure 4 (A) ~ Figure 4 (C) is a schematic cross-sectional view of a conventional secondary battery. Figure 4 (A) is when the PPTC material layer is not expanded. Figure 4 (B) refers to the expansion of the PPTC material layer. Figure 4 (C) represents the state where the diaphragm further melts when the PPTC material layer expands.

[0022] Symbol Explanation

[0023] 1-Secondary battery, 2+-Positive current collector, 2--Negative current collector, 3+-Positive active material layer, 3-Negative active material layer, 4-Separator, 5+, -PPTC material layer, 6i-Electrode foil (inner side), 6o-Electrode foil (outer side), 6p-Electrode foil sheet, 7-Raised area, 10-External circuit, 11-Connecting terminal. Detailed Implementation

[0024] 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.

[0025] Structure of a secondary battery

[0026] like Figure 1 (A) Figure 1 As schematically depicted in (C), in the secondary battery 1 of the lithium-ion battery to which this embodiment is applicable, the positive electrode active material layer 3+ coated on the surface of the current collector 2+ of the positive electrode and the negative electrode active material layer 3- coated on the surface of the current collector 2- of the negative electrode form a stacked structure facing each other with a separator 4 in between. An electrolyte is filled between the current collectors 2+ and 2-. Although not shown, the periphery of each electrode can be surrounded by a sealing portion formed of any resin material or the like. The positive electrode active material layer 3+, the negative electrode active material layer 3-, the separator 4, and the electrolyte can be formed in a conventional manner. In short, the active material layers 3+ and 3- are typically formed by coating the current collectors 2+ and 2- with a slurry containing appropriately selected active materials, conductive additives, binders, etc., and then drying it to form layers approximately 0.1 mm thick. The separator 4 is formed of a resin film such as polypropylene that allows the passage of mobile substances such as lithium ions, and the electrolyte can be appropriately selected depending on the type of battery.

[0027] In this embodiment, the current collectors 2+ and 2- have electrode foils 6i and 6o, which can be metal foils, stacked on both sides of the PPTC material layers 5+ and 5-. The PPTC material layers 5+ and 5- are typically materials in which conductive particles such as copper, aluminum, nickel, silver, carbon black, nanotubes, and nanofibers are dispersed in a resin matrix such as polyethylene, epoxy resin, polypropylene, polyamide, or polyvinylidene fluoride. As described above, they are prepared such that below a suitable switching temperature, they are in a conductive state due to the interconnection of the conductive particles; on the other hand, above the switching temperature, they expand and the conductive particles separate, becoming in an insulating state. The expansion rate of the PPTC material layers 5+ and 5- can be 5 to 20 × 10⁻⁶. -5 / ℃, etc. Furthermore, the switching temperature of the PPTC material layer can be set between 120 and 180℃.

[0028] The electrode foils on both sides of the PPTC material layers 5+ and 5- can be aluminum foil, copper foil, etc., or they can be laminated on both sides of the PPTC material layers 5+ and 5- by vapor deposition or sputtering.

[0029] Furthermore, in this embodiment, as described in the "Summary of the Invention" section, raised regions 7 are formed on the surface of the PPTC material layers 5+ and 5- on the side of the active material layer, as illustrated. (As will be described later.) Figure 3 As shown, the raised region 7 can be integrally formed in a strip shape along the surfaces of the PPTC material layers 5+ and 5-. That is, the PPTC material layers 5+ and 5- are shaped to form the raised region 7. Furthermore, as shown, the raised region 7 can be formed at multiple locations on the PPTC material layers 5+ and 5-. The thickness of the PPTC material layer is, for example, 0.5 to 2 μm outside the raised region and 3 to 10 μm in the raised region, and the width of the raised region can be, for example, 1 mm or more. As explained later, the height of the raised region 7 is set as follows: when the PPTC material layers 5+ and 5- expand, the electrode foil 6i on the active material layer side is cut perpendicularly along the surface direction according to the difference in its expansion amount, thereby blocking the conduction in the surface direction of the electrode foil 6i.

[0030] Operation of the current collector when the battery heats up

[0031] exist Figure 1 (A) Figure 1 In the structure of battery 1 illustrated in (C), if abnormal heat generation occurs in battery 1 and the battery temperature exceeds the switching temperature of the PPTC material layers 5+ and 5-, then as follows: Figure 1 (B) Figure 1 As shown in (D), the PPTC material layers 5+ and 5- expand and become non-conductive, thereby the electrode foils 6i and 6o on both sides of the PPTC material layers 5+ and 5- become insulated from each other, and thus the current in the external circuit 10 connected to the outside of the electrode foil 6o is blocked.

[0032] Furthermore, in this embodiment, as described above, raised regions 7 are further formed on the PPTC material layers 5+ and 5-, such as... Figure 1 (B) Figure 1 As depicted in (D), the expansion of the raised regions 7 of the PPTC material layers 5+ and 5- exceeds the thickness of the electrode foil 6i. Consequently, the raised regions 7 cut the electrode foil 6i, causing it to be cut in a direction perpendicular to the surface direction. In this case, electrical conduction in the surface direction at the cut location is blocked. As in this case, Figure 1 As shown in (D), when a fusion point 4B is formed on the separator 4, and the positive electrode active material layer 3+ and the negative electrode active material layer 3- form a conductive path (electron flow path) through surface contact, a conductive path can be formed in the region of the fusion point 4B of the separator 4, up to the cut-off point of the nearest electrode foil 6i. Figure 4 (C) As described above, the "overflow current Ir" in connection with this is insulated from the portion of the electrode foil 6i in the region of the fusible part 4B, which is opposite to the cut portion of the nearest electrode foil 6i when viewed from the fusible part 4B. Therefore, no overflow current is generated in the region opposite to the cut portion of the nearest electrode foil 6i when viewed from the fusible part 4B. In this way, the overflow current flowing in the fusible part 4B can be suppressed, thereby suppressing further heating.

[0033] In the above structure, as described, it is preferable that raised regions 7 are formed at multiple locations in the PPTC material layers 5+ and 5-, so that the electrode foil 6i can be divided into multiple regions when the PPTC material layers 5+ and 5- expand. Therefore, even if a fusion break 4B is formed in any region of the separator 4, the distance between the fusion break 4B and the nearest cut portion of the electrode foil 6i becomes shorter, making the divided region of the electrode foil 6i, including the region of the fusion break 4B, smaller, reducing the capacity of that region, and thus reducing the magnitude of the current flowing through the fusion break 4B. In this regard, to appropriately reduce the current flowing through the separator 4 after fusion breakage, raised regions 7 can be formed in the PPTC material layers 5+ and 5- after the division based on the raised regions 7 of the electrode foil 6i, so that the capacity of the battery in each region divided by the cut portion of the electrode foil 6i is, for example, 1 Ah or less.

[0034] Another way of structuring a current collector

[0035] As described above, in cases where the battery reaches a high temperature due to abnormal heating, another method is to cut off the electrode foil 6i inside the current collectors 2+ and 2- by forming the raised regions 7 in the PPTC material layers 5+ and 5-. For example, Figure 2As shown in (A), an electrode foil 6p, which is part of the electrode foil 6i, can be abutted against the surface of the raised regions 7 of the PPTC material layers 5+ and 5-, and the height of the raised regions 7 can be adjusted such that the electrode foil 6p is in contact with the surrounding electrode foil 6i below the switching temperature of the PPTC material layers 5+ and 5-. Furthermore, the battery temperature T exceeds the switching temperature, as... Figure 2 As shown in (B), when the PPTC material layers 5+ and 5- expand, the electrode foil 6p is displaced by the expansion of the raised region 7 to separate it from the surrounding electrode foil 6i, thereby allowing the electrode foil 6i to be cut. Therefore, it is expected that the electrode foil 6i can be cut quickly.

[0036] Location of protrusions in PPTC material layer

[0037] As described above, in the PPTC material layers 5+ and 5-, the raised regions 7 can be formed in a strip shape at multiple locations. Regarding this, for example, as from... Figure 1 (A) Figure 1 (C) It is understood that in the region where the raised area 7 is formed, the electrode foil 6i is relatively thinner than its surroundings, and the resistance increases accordingly. It is preferable to suppress this increase in resistance as much as possible. Regarding this, for example, as... Figure 3 (A) ~ Figure 3 As schematically depicted in (C), in a structure where the battery is rectangular in planar shape and has an outwardly facing connection terminal 11 at one of its corners, and multiple raised areas 7 are formed in strips on the PPTC material layers 5+ and 5-, such as Figure 3 As shown in (A), if a raised region 7 is formed along the long side direction of PPTC material layers 5+ and 5-, then it will be similar to... Figure 3 (B) Figure 3 As shown in (C), compared to the case where the raised region 7 is formed along the short side direction of the PPTC material layers 5+ and 5-, the number of locations where the raised region 7 intersects with the line a connecting the connecting terminal 11 and the corner located opposite it (arrow in the figure) is reduced. Therefore, it is advantageous to further reduce the increase in resistance in the electrode foil 6i caused by the formation of the raised region 7.

[0038] Thus, according to this embodiment, if the battery temperature exceeds the switching temperature of the PPTC material layer, not only are the two sides of the current collector insulated, but the inner electrode foil is also cut off. This can suppress the magnitude of the swirling current that may be generated when the separator melts, and can also suppress the heat generated by the internal short circuit of the battery.

[0039] The above description relates to the embodiments of the present invention. However, it is easy for those skilled in the art to make numerous modifications and changes. The present invention is not limited to the embodiments illustrated above, and it is obviously applicable to various devices without departing from the concept of the present invention.

Claims

1. A secondary battery comprising an active material layer coated on a current collector of a positive electrode and an active material layer coated on a current collector of a negative electrode, the two layers being opposed to each other across a separator and with an electrolyte impregnated between the electrodes, wherein the current collector of at least one of the positive and negative electrodes is composed of a PPTC material layer sandwiched between a pair of electrode foils, characterized in that... The surface of the PPTC material layer on the side of the active material layer is configured to form a raised region, and when the PPTC material layer expands, the raised region cuts off the electrode foil that it abuts.

2. The secondary battery according to claim 1, characterized in that, The expansion of the raised area of ​​the PPTC material layer when the temperature rises is greater than the thickness of the electrode foil.

3. The secondary battery according to claim 1, characterized in that, The raised areas of the PPTC material layer expand when the temperature rises in such a way that only a portion of the electrode foil that abuts the raised areas separates from the area of ​​the electrode foil surrounding them.

4. The secondary battery according to claim 1, characterized in that, The raised areas are formed in multiple locations within the PPTC material layer.

5. The secondary battery according to claim 4, characterized in that, The capacity of each region between the locations forming the protruding region is less than 1 Ah.

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