Secondary batteries

Abstract

In a secondary battery 1 having a laminated structure in which a positive electrode active material layer 3 coated on a positive electrode foil 2 and a negative electrode active material layer 5 coated on a negative electrode foil 4 face each other with a separator 6 in between, and an electrolyte is injected between the electrode foils, the objective is to prevent foreign matter that may enter during the injection of the electrolyte from reaching either the positive electrode active material layer or the negative electrode active material layer. [Solution] The secondary battery 1 has a laminated structure in which a positive electrode active material layer 3 coated on the electrode foil 2 of the positive electrode and a negative electrode active material layer 5 coated on the electrode foil 4 of the negative electrode face each other with a separator 6 in between, and an electrolyte is injected between the positive electrode foil and the negative electrode foil, and the peripheral edge 6a of the separator is continuously bonded to the edge of the positive electrode or the negative electrode over its entire circumference.

Description

Technical Field

[0001] The present invention relates to a secondary battery, and more particularly to the structure of a secondary battery.

Background Art

[0002] A secondary battery such as a lithium-ion secondary battery, briefly stated, has a laminated structure in which a positive electrode active material layer coated on a current collector (positive electrode foil) that may be a metal foil and a negative electrode active material layer coated on a current collector (negative electrode foil) that may be a metal foil face each other with a separator interposed therebetween, and an electrolytic solution is injected therebetween. Various configurations have been proposed for various problems that can occur in such secondary batteries. For example, in Patent Document 1, an electrode laminate portion in which 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 the nickel foil are laminated via a separator, and a primary seal portion disposed so as to surround the electrode laminate portion and holding the nickel foil are provided, and the inside of the battery structure is heated, and after reducing the pressure in the internal space of the battery structure, an electrolytic solution is injected into the internal space of the battery structure, and a method of securing a space into which the electrolytic solution enters in the separator has been proposed. Further, in Patent Document 2, as a secondary battery having good productivity, it has a power generation element formed by laminating electrodes via a separator 45 and a seal portion for sealing at least a part of the outer peripheral portion of the power generation element, all of the seal portions have substantially the same outer shape, and each of the seal portions has a base portion surrounding the outer periphery of the electrode and a covering portion extending from the base portion and overlapping the surface of the electrode.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the laminated structure of the secondary battery described above, where a positive electrode active material layer coated on the positive electrode foil and a negative electrode active material layer coated on the negative electrode foil face each other with a separator in between, and an electrolyte is injected between the electrode foils (positive electrode foil and negative electrode foil), as schematically depicted in Figure 6(A), the separator 6 is held in place by fixing its peripheral edge 6a to either the positive electrode side or the negative electrode side edge (the edge 7a of the sealing portion or the edges of the electrode foils 2 and 4). In this case, since it is usually sufficient to maintain the stable position of the separator 6, the edge of the separator is bonded to the edge of one of the electrodes at several point regions 6c, as depicted in Figure 6(B), and the electrolyte EL can easily flow between the bonded points.

[0005] Incidentally, when electrolyte is injected between the electrode foils of the above-mentioned laminated structure, foreign matter may enter. If such foreign matter penetrates to the positive electrode active material layer or the negative electrode active material layer, it can penetrate the separator, short-circuit the electrodes, and cause a loss of battery function. In this regard, in the configuration in which the periphery of the separator is intermittently fixed to the edge of the electrode as described above, foreign matter can enter along with the electrolyte through the adhesive points of the separator's periphery on the electrode side to which the separator's periphery is fixed. As a result, short circuits due to foreign matter can occur in the active materials of both electrodes. Therefore, it is advantageous to have a configuration that prevents foreign matter in the electrolyte from reaching either the positive electrode active material layer or the negative electrode active material layer.

[0006] In view of the above circumstances, the main object of the present invention is to provide 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 in between, and an electrolyte is injected between the electrode foils, in such a case that foreign matter that may enter during the injection of the electrolyte cannot reach either the positive electrode active material layer or the negative electrode active material layer.

[0007] In addressing the above-mentioned problem, when fixing the edge of the separator to the edge of one electrode (seal portion or edge of electrode foil), if the edge of the separator is continuously and linearly adhered to the edge of the electrode along its entire circumference, and the entire active material layer of one electrode is wrapped by the separator and electrode foil, then even if foreign matter enters the space between the electrode foils when the electrolyte is injected, it can be prevented from reaching the active material layer wrapped by the separator and electrode foil. Furthermore, as described in the Embodiments section below, since the electrolyte itself permeates the separator, the space between the separator on the side where the separator edge is adhered and the electrode foil can also be filled with the electrolyte. This finding is utilized in the present invention. [Means for solving the problem]

[0008] According to the present invention, the above problems are solved by 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 in between, and an electrolyte is injected between the electrode foils, wherein the peripheral edge of the separator is continuously bonded to the edge of the positive electrode or the negative electrode over its entire circumference.

[0009] In the above configuration, the secondary battery may be a non-aqueous secondary battery, and is typically a lithium-ion secondary battery. The positive electrode foil (positive electrode foil) and the negative electrode foil (negative electrode foil) may be current collectors made of metal foil in a conventional manner, and the positive electrode active material layer and the negative electrode active material layer may be coated on the positive electrode foil and the negative electrode foil in a conventional manner, respectively. The separator and electrolyte may also be a separator and electrolyte in a conventional manner. The edge of the positive or negative electrode may be the edge of the positive electrode foil or the negative electrode foil or a sealing portion that holds it. The sealing portion that holds the edge of the positive electrode foil and the negative electrode foil may be formed of a resin material commonly used in this field, such as polyethylene. Furthermore, in a secondary battery, a structure may be formed in which multiple laminated structures, as described above, sandwiched between positive and negative electrode foils, are stacked on top of each other. The positive and negative electrode foils are bonded to the negative and positive electrode foils of adjacent laminated structures, and as a result, except for both 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).

[0010] Furthermore, in the above-described configuration of the present invention, the periphery of the separator is continuously bonded to the edge of the positive or negative electrode over its entire circumference. The bonding of the periphery of the separator to the edge of the positive or negative electrode may be achieved by any method such as heat welding or heat pressing. With this configuration, as already mentioned, in the electrode where the periphery of the separator is bonded to the edge, the flow of electrolyte from the periphery of the separator into the space between the electrode foil and the separator is prevented, and therefore, foreign matter cannot enter this space, thus preventing foreign matter from reaching the active material layer and preventing short circuits caused by foreign matter. As already stated, the filling of the space between the electrode foil and the separator in the electrode where the periphery of the separator is bonded to the edge is achieved by the electrolyte permeating through the separator.

[0011] In the above-described configuration of the present invention, more preferably, the periphery of the separator may be bonded to the electrode foil of the positive or negative electrode. In this configuration, the electrode foil, which is a metal foil, has a low coefficient of thermal expansion, which is advantageous in that it can suppress the occurrence of wrinkles and tears in the separator due to temperature changes.

[0012] In the configuration of the present invention described above, the active material layer enclosed by the separator and electrode foil is either the positive electrode or the negative electrode. In this regard, if foreign matter reaches the active material layer, the possibility of the foreign matter piercing the separator is higher when there are fewer voids within the active material layer. Simply put, if foreign matter enters the active material layer, its volume will be absorbed by the volume of the voids within the active material layer. When there are few voids, the volume of the foreign matter may not be fully absorbed by the voids, increasing the likelihood of it compressing the separator. Therefore, it is preferable to more reliably prevent foreign matter from reaching the active material layer of the two electrodes that has fewer voids. Thus, the active material layer of the two electrodes that has fewer voids may be selected to be enclosed by the separator and electrode foil. In this way, in the above configuration, the periphery of the separator may be bonded to the edge of the electrode with the smaller void ratio in the active material layer. Here, the porosity is given by 1 - (electrode density) / (true specific gravity of the active material layer). Generally, the porosity is smaller at the positive electrode than at the negative electrode, so the periphery of the separator may be bonded to the edge of the positive electrode.

[0013] Furthermore, since the positive electrode active material layer generally has a higher electrical resistance than the negative electrode active material layer, if a conductive foreign object enters the positive electrode active material layer and causes a short circuit, the reduction in resistance between the electrodes will be greater than if a conductive foreign object enters the negative electrode active material layer and causes a short circuit, potentially leading to a larger short-circuit current between the electrodes. In other words, the positive electrode active material layer should be protected from foreign object intrusion more reliably than the negative electrode active material layer, so the periphery of the separator may be bonded to the edge of the positive electrode.

[0014] Furthermore, in the above-described configuration of the present invention, the separator may be bonded not only to its periphery but also to the surface of the electrode foil that is not coated with an active material layer. This allows the separator to be held more stably. [Effects of the Invention]

[0015] Thus, according to the configuration of the present invention, in 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 in between, and an electrolyte is injected between the electrode foils, either the positive electrode active material layer or the negative electrode active material layer is placed in a state where it is wrapped by the separator and the electrode foil. This prevents foreign matter that may enter the active material layer along with the electrolyte from reaching it, thereby suppressing short circuits caused by foreign matter as much as possible. Furthermore, the configuration of the present invention is substantially achieved by continuously bonding the periphery of the separator to the edge of one of the electrodes, which is also advantageous in that it does not increase the number of parts.

[0016] Other objects and advantages of the present invention will become apparent from the following description of preferred embodiments of the present invention. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1(A) is a schematic cross-sectional view of the vicinity of the periphery of an electrode in one embodiment of the laminated structure in a secondary battery to which this embodiment is applied. Figure 1(B) is a schematic plan view of the separator and electrode active material in a secondary battery to which this embodiment is applied. Figure 1(C) is a schematic cross-sectional view of the vicinity of the periphery of an electrode in another embodiment of the laminated structure in a secondary battery to which this embodiment is applied. [Figure 2] Figure 2(A) is a schematic cross-sectional view of the stacked structure of a secondary battery used in an experiment to confirm the electrolyte permeability of the separator, and Figure 2(B) shows the measured fluid resistance values ​​of each part in this stacked structure. The numerical values ​​in the figures are the measured fluid resistance values. [Figure 3] Figures 3(A) and 3(B) are schematic cross-sectional views of the laminated structure in a secondary battery to which this embodiment is applied. (A) shows the case where the separator is fixed only at the periphery, and (B) shows the case where the separator is fixed even in the region inside the electrode foil where the active material layer is not coated. [Figure 4]FIG. 4(A) is a schematic cross-sectional view showing a state where a foreign object that has penetrated into the positive electrode active material layer of the laminated structure in the secondary battery has short-circuited between the electrodes, and FIG. 4(B) is a schematic cross-sectional view showing a state where a foreign object that has penetrated into the negative electrode active material layer of the laminated structure in the secondary battery has short-circuited between the electrodes. [Figure 5] FIGS. 5(A) to (D) are schematic cross-sectional views of the laminated structure of the secondary battery in the process of laminating the bipolar electrodes in the secondary battery to which the present embodiment is applied. [Figure 6] FIG. 6(A) is a schematic cross-sectional view near the peripheral edge of the electrode of the laminated structure in the conventional secondary battery. FIG. 6(B) is a schematic plan view of the separator and the electrode active material in the conventional secondary battery.

Explanation of Reference Numerals

[0018] 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... bonding part, 7... seal part, 7a... seal end part, 10... electrolyte injection port, v1, v2... electrolyte filling space, EL... electrolyte, X... foreign object

Best Mode for Carrying Out the Invention

[0019] The present invention will be described in detail below with reference to the accompanying drawings in several preferred embodiments. In the drawings, the same reference numerals indicate the same parts.

[0020] Stacked structure of secondary batteries As schematically depicted in Figures 1(A) and (C), in the secondary battery 1 to which this embodiment is applied, a single cell is formed by a laminated structure in which a positive electrode active material layer 3 coated on the surface of a positive electrode foil (positive electrode foil) 2 and a negative electrode active material layer 5 coated on the surface of a negative electrode foil (negative electrode foil) 4 face each other with a separator 6 in between. Multiple such cells may be stacked on top of each other (see Figure 5), in which case 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 in the figure, and the negative electrode side current collector 4 of each cell is bonded to the positive electrode side current collector 2 of the cell adjacent to it on the lower side in the figure, thereby forming a state in which multiple cells are connected in series. Thus, the positive electrode foil 2 and negative electrode foil 4 bonded to each other constitute a "bipolar electrode". The positive electrode foil 2 and negative electrode foil 4 may each be commonly used metal foils such as aluminum foil and nickel foil with a thickness of several tens of micrometers. The positive electrode active material layer 3 may be a mixture of commonly used positive electrode active materials such as NCM (nickel-cobalt-manganate lithium), LFP (lithium iron phosphate), and LMFP (lithium iron manganese phosphate) systems, along with a conductive additive such as carbon black and a binder such as PVdF (polyvinylidene fluoride), applied in a layer approximately 0.1 mm thick. The negative electrode active material layer 5 may be a mixture of commonly used negative electrode active materials such as graphite (natural or artificial) along with a binder such as SBR / CMC (styrene-butadiene rubber / carboxymethylcellulose), applied in a layer approximately 0.1 mm thick. The separator 6 may be formed from a three-layer lithium ion permeable resin film of PP-PE-PP (polypropylene-polyethylene-polypropylene) with a thickness of approximately 20 μm. 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 depending on the type of battery. For example, in the case of a non-aqueous lithium-ion battery, it may be a non-aqueous solvent in which LiPF6 (lithium hexafluoride phosphate) is dissolved at 1M in a non-aqueous solvent consisting of a 1:1:1 mixture of EC (ethyl carbonate), EMC (ethyl methyl carbonate), and DMC (dimethyl carbonate).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, and the outer edges of the electrode foils 2 and 4 are fixed in place by being sandwiched between the ends 7a of the seal portion 7, thereby maintaining the spacing between the electrode foils 2 and 4 in the stacking direction. The seal portion 7 may be made of any resin with appropriate rigidity, such as polyethylene. A part of the seal portion 7 between the electrode foils 2 and 4 is opened to form an injection port 10 for injecting the electrolyte.

[0021] In the above configuration, the peripheral edge 6a of the separator 6 may be fixed to the end 7a of the seal portion 7, as shown in Figure 1(A), or it may be fixed to either the positive electrode foil 2 or the negative electrode foil 4, as shown in Figure 1(C). Previously, as already mentioned in relation to Figure 6(B), the peripheral edge 6a of the separator 6 was bonded to the edge of the electrode at several point regions 6c along the circumferential direction. In that case, foreign matter could enter through the gaps between the point regions 6c along with the electrolyte and reach the active material. If the foreign matter is conductive and penetrates the separator and crosses over the positive electrode active material 3 and the negative electrode active material 5, a short circuit will occur between the electrodes. Therefore, in this embodiment, in order to avoid foreign matter from reaching the active material as much as possible, the peripheral edge 6a of the separator 6 is continuously bonded around its entire circumference to either the end 7a of the seal portion 7, or to either the positive electrode foil 2 or the negative electrode foil 4 (reference numeral 6b indicates the bonding area). As a result, the electrolyte does not flow from the peripheral edge 6a of the separator 6 to the space v1 or v2 between the electrode foil 2 or 4 on the side to which the separator 6 is bonded, so foreign matter cannot enter, and the active material layer 3 or 5 of the electrode on the side to which the peripheral edge 6a of the separator 6 is bonded is protected from foreign matter. In this regard, the electrode foils 2 and 4, which are metal foils, have a lower coefficient of thermal expansion than the sealing portion 7, and therefore experience less dimensional change due to temperature changes. As a result, bonding the peripheral edge 6a of the separator 6 to the electrode foils 2 and 4 as shown in Figure 1(C) is advantageous because it suppresses the occurrence of wrinkles and tears in the separator 6.

[0022] In the above embodiment, when the peripheral edge 6a of the separator 6 is continuously bonded along its entire circumference to the end 7a of the seal portion 7 or to the positive electrode foil 2 or the negative electrode foil 4, the electrolyte will not flow from the peripheral edge 6a of the separator 6 into the space v1 or v2 between the electrode foil on the side to which it is bonded and the separator 6. However, since the separator 6 is permeable to the electrolyte, the space v1 or v2 between the electrode foil on the side to which the peripheral edge 6a of the separator 6 is bonded and the separator 6 will be filled with the electrolyte. This was confirmed by the following experiment.

[0023] In the experiment, as shown in Figure 2(A), fluid resistance was measured in two configurations: (a) a configuration in which a separator 6 was sandwiched between a normal positive electrode active material 3 and a normal negative electrode active material 5 (the fluid resistance was greater in the former), and (b) a configuration in which a separator 6 was sandwiched between a positive electrode active material 3 that was thinner than normal but had increased fluid resistance, and a normal negative electrode active material 5. Figure 2(B) shows the fluid resistance FR of the three-layer (3) when electrolyte was injected from both the positive electrode active material (+), the negative electrode active material (-), and the edges between the positive and negative electrode active materials for each configuration (a) and configuration (b). As can be seen from this figure, the fluid resistance of the positive electrode active material was greater in configuration (b), but the fluid resistance of the three layers was reduced in configuration (b) compared to configuration (a). This indicates that, in the case of a three-layer structure, the electrolyte first permeates into the negative electrode active material, and then permeates from the negative electrode active material through the separator to the positive electrode active material (if it did not pass through the separator, the penetration of the electrolyte into the positive electrode active material, which has high fluid resistance, would be the rate-limiting factor in configuration (b), and the fluid resistance of the three layers (3) in configuration (b) would be greater than in configuration (a)). Thus, it is confirmed that even without direct inflow of electrolyte from the end 6a of the separator 6, the electrolyte can permeate the separator 6 and fill the spaces v1 and v2 on both sides of the separator 6 between the electrode foils.

[0024] In the above configuration, as shown in Figure 3(A), the separator 6 may be fixed only to the periphery 6a. However, as shown in Figure 3(B), if there are areas on the electrode foil 2 other than the periphery where the active material 3 is not coated, the separator 6 may be bonded to such areas (6d). This allows the separator 6 to be held more stably.

[0025] In the configuration of this embodiment described above, the positive electrode active material layer 3 and the negative electrode active material layer 5 are encased in the separator 6 and electrode foil 2 or 4, preventing the entry of foreign matter. Therefore, it is preferable that, if foreign matter does enter, the entry of foreign matter is prevented in the area where the impact is greater.

[0026] In this regard, firstly, generally, the proportion of voids in the active material layer differs between the positive electrode and the negative electrode. When the proportion of voids is small, and the volume that can accommodate foreign matter when it enters is small, the likelihood of the separator being compressed increases. Therefore, it is preferable to more reliably prevent the entry of foreign matter in the active material layer with the smaller proportion of voids. Thus, in this embodiment, the periphery of the separator may be bonded to the edge of the electrode with the smaller void ratio in the active material layer. Here, the void ratio is given by 1 - (electrode density) / (true specific gravity of the active material layer). Typically, the void ratio is 30-42% for the positive electrode active material layer and 45% for the negative electrode active material layer, and since the positive electrode has a smaller void ratio than the negative electrode, the periphery of the separator may be bonded to the edge of the positive electrode.

[0027] Furthermore, generally, the electrical resistance (insulation) of the active material layer differs between the positive and negative electrodes. As shown in Figure 4(A), when a conductive foreign substance enters the area with higher electrical resistance, the area that should have higher electrical resistance is replaced by the conductive foreign substance. As a result, as shown in Figure 4(B), the reduction in electrical resistance is greater compared to when the conductive foreign substance enters the area with lower electrical resistance. Therefore, the short-circuit current Ia that flows when the conductive foreign substance enters the area with higher electrical resistance is greater than the short-circuit current Ib that flows when the conductive foreign substance enters the area with lower electrical resistance, and thus the effect is greater. Thus, it is preferable to more reliably prevent the entry of foreign substances into the active material layer with higher electrical resistance. In this embodiment, the periphery of the separator may be bonded to the edge of the electrode with higher electrical resistance in the active material layer. Typically, the electrical resistance of the active material layer is greater at the positive electrode than at the negative electrode, so the periphery of the separator may be bonded to the edge of the positive electrode.

[0028] Layered structure formation process The configuration of this embodiment described above may be formed by a conventional method, except that the adhesive area of ​​the periphery 6a of the separator 6 extends continuously around its entire circumference. Specifically, first, as shown in Figure 5(A), corresponding active material layers 3 and 5 are coated on both sides of the electrode foil, which is formed by bonding a positive electrode foil 2 and a negative electrode foil 4 together. Then, as shown in Figure 5(B), the separator 6 is laminated on one side, and its periphery 6a is continuously bonded to the end of the electrode (the periphery of the electrode foil, etc.) by, for example, a heat press p. Then, as shown in Figure 5(C), the seal portion 7 is heat-welded to the periphery 6a of the separator 6 and the electrode foils 2 and 4, sandwiching them together. This forms a bipolar electrode. After that, as shown in Figure 5(D), multiple bipolar electrodes are stacked, the seal portions 7 are joined, and a battery module is formed in which multiple cells are connected in series.

[0029] Thus, in the configuration of this embodiment described above, either the positive electrode active material layer or the negative electrode active material layer is wrapped in a separator and electrode foil, thereby preventing foreign matter that may enter the active material layer along with the electrolyte from reaching it, and thus minimizing short circuits caused by foreign matter.

[0030] While the above description is made in relation to embodiments of the present invention, many modifications and changes are readily possible for those skilled in the art, and it will be clear that the present invention is not limited to the embodiments illustrated above, but can be applied to various devices without departing from the concept of the present invention.

Claims

1. 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 in between, 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.

2. A secondary battery according to claim 1, wherein the periphery of the separator is bonded to the electrode foil of the positive electrode or the negative electrode.

3. A secondary battery according to claim 1, wherein the periphery of the separator is bonded to the edge of the electrode with the smaller porosity of the active material layer.

4. A secondary battery according to claim 1, wherein the periphery of the separator is bonded to the edge of the positive electrode.

5. A secondary battery according to claim 1, wherein the separator is further bonded to the surface of an electrode foil that is not coated with an active material layer.

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