Secondary battery, separator for secondary battery, and method for producing separator for secondary battery

Abstract

This secondary battery is provided with a positive electrode layer having a positive electrode current collector and a positive electrode active material layer, a negative electrode layer having a negative electrode current collector and a negative electrode active material layer, and a separator disposed between the positive electrode layer and the negative electrode layer. The separator is provided with a first separator and a second separator laminated on one surface of the first separator, and the first separator is provided with: a first porous base material that is in contact with the second separator; and a conductive layer that covers the entire surface of the first porous substrate, said surface being on the opposite side from the second separator side. The second separator has a second porous base material in contact with the first separator, the second separator has a protruding portion protruding from an end portion of the first separator, and the separator is disposed such that the conductive layer of the first separator faces the negative electrode active material layer.

Description

Technical Field

[0001] The present invention relates to a secondary battery, a separator for a secondary battery, and a method for manufacturing the same. Background Art

[0002] In recent years, in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy, research and development related to secondary batteries that contribute to energy efficiency have been underway. For example, in a lithium-ion secondary battery, for the purpose of preventing metal precipitation on the negative electrode layer, a conductive porous body is interposed between the positive electrode layer and the negative electrode layer, and a separator is disposed between the positive electrode layer and the conductive porous body and between the negative electrode layer and the conductive porous body (Patent Document 1).

[0003] [Prior Art Documents]

[0004] (Patent Document)

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2015-141864 Summary of the Invention

[0006] [Problems to be Solved by the Invention]

[0007] In the technology related to secondary batteries, high capacity is an issue. For the high capacity of secondary batteries, it is desired to put into practical use a lithium metal secondary battery that uses lithium as a negative electrode active material. However, in a lithium metal secondary battery, due to repeated charge and discharge, a solid electrolyte interphase (SEI) layer accumulates at the interface between the current collector of the negative electrode and lithium, and lithium dendrites are likely to be generated during charging. If lithium dendrites are generated, there is a possibility that the lithium dendrites penetrate the separator and short-circuit the positive electrode layer and the negative electrode layer. In addition, if lithium dendrites are generated, there is a possibility that the density of the metallic lithium layer deposited on the negative electrode layer decreases, and the swelling of the lithium secondary battery during charging becomes excessive.

[0008] The present invention has been made in view of the above problems, and an object thereof is to provide a secondary battery, a separator useful for the secondary battery, and a method for manufacturing the same, in which the positive electrode layer and the negative electrode layer are not easily short-circuited even after repeated charge and discharge, and the density of the metallic lithium layer in the negative electrode layer in the charged state is not easily decreased.

[0009] [Technical Means for Solving the Problems]

[0010] The present inventors have found that, in order to solve the above problems, it is effective to provide a conductive layer on the surface of the separator facing the negative electrode active material layer. As a method for manufacturing a separator having a conductive layer, a method of strip-coating a coating liquid of a conductive material on the surface of a porous substrate, which is a substrate of the separator, can be considered. However, according to the research of the present inventors, it has been found that the method of coating the coating liquid of the conductive material has the following problems to be improved. First, there is a case where the dimensional accuracy of the obtained conductive layer decreases due to the seepage of the coating liquid of the conductive material on the surface of the porous substrate. Second, there is a case where the insulation property decreases when the coating liquid of the conductive material enters the pores of the porous substrate. In addition, there is the following case. When the pores of the porous substrate are made fine in order to prevent the coating liquid of the conductive material from entering the pores of the porous substrate, the electrolyte retention amount and the ionic conductivity of the separator decrease, which becomes a cause for increasing the internal resistance of the secondary battery. In addition, as a method for manufacturing a separator having a conductive layer, a sputtering method can be considered. However, when a method accompanied by heat such as the sputtering method is used to form the conductive layer, sometimes a part of the porous substrate is shut down, and sometimes the characteristics of the separator change due to the heat history during the formation of the conductive layer. Furthermore, regardless of the manufacturing method of the conductive layer, in the separator, it is desired to improve the effective shut-down function during abnormal heating of the secondary battery.

[0011] The present inventors have further studied and found that, in order to solve the above problems of the separator having a conductive layer, it is effective to use, as the separator, a separator having a first separator and a second separator laminated on one surface of the first separator. The first separator has: a first porous substrate in contact with the second separator; and a conductive layer covering the entire surface of the surface of the first porous substrate opposite to the second separator side. The second separator has a second porous substrate in contact with the first separator, and the second separator has a protruding portion protruding from an end portion of the first separator. Therefore, the present invention provides the following solutions.

[0012] (1) A secondary battery including a positive electrode layer, a negative electrode layer, and a separator disposed between the positive electrode layer and the negative electrode layer. The positive electrode layer has a positive electrode current collector and a positive electrode active material layer, the negative electrode layer has a negative electrode current collector, the separator has a first separator and a second separator laminated on one surface of the first separator. The first separator has: a first porous substrate in contact with the second separator; and a conductive layer covering the entire surface of the surface of the first porous substrate opposite to the second separator side. The second separator has a second porous substrate in contact with the first separator, and the second separator has a protruding portion protruding from an end portion of the first separator. The separator is disposed such that the conductive layer of the first separator faces the negative electrode layer.

[0013] The secondary battery according to (1), since the conductive layer of the first separator of the separator is arranged to face the negative electrode layer, during charging, lithium is likely to be evenly deposited on the negative electrode layer. In addition, since electrons are supplied to the conductive layer during charging, a large number of lithium deposition sites are also formed in the conductive layer, so the current density during charging can be reduced. Since the current density during charging decreases, the overvoltage is reduced, so the decomposition of the electrolyte is suppressed. In addition, since the second separator has a protrusion without a conductive layer, it is not easy for the positive electrode layer and the negative electrode layer to be short-circuited via the conductive layer. Furthermore, since the separator is divided into a first separator having a conductive layer and a second separator having a protrusion, it is easy to adjust the size of the conductive layer, and the dimensional accuracy of the conductive layer is improved. Therefore, it is easy to obtain the above effects brought by the conductive layer. Thus, even if the secondary battery according to (1) is repeatedly charged and discharged, the positive electrode layer and the negative electrode layer are not easily short-circuited, and the density of the metallic lithium layer of the negative electrode layer in the charged state is not easily decreased, thereby achieving a high capacity.

[0014] (2) The secondary battery according to (1), wherein the positive electrode current collector is connected to the positive electrode tab, and the positive electrode tab extends toward the protrusion side of the separator.

[0015] In the secondary battery according to (2), since the positive electrode tab contacts the protrusion of the separator, it is not easy for the positive electrode tab to contact the conductive layer. Therefore, it is not easy for the positive electrode layer and the negative electrode layer to be short-circuited.

[0016] (3) The secondary battery according to (1) or (2), wherein the average pore diameter of the first porous substrate is smaller than that of the second porous substrate.

[0017] In the secondary battery according to (3), the conductive material of the conductive layer is not easily introduced into the pores of the first porous substrate, and it is not easy for the insulation of the separator to decrease due to the introduction of the conductive material.

[0018] (4) The secondary battery according to (1) or (2), wherein the average pore diameter of the second porous substrate is larger than that of the first porous substrate.

[0019] In the secondary battery according to (4), since the electrolyte is easily permeated into the pores of the second porous substrate, the internal resistance caused by the decrease in the ionic conductivity inside the secondary battery becomes smaller.

[0020] (5) The secondary battery according to any one of (1) to (4), wherein the second porous substrate is made of polyethylene and has higher insulation than the first porous substrate.

[0021] The secondary battery according to (5), since the melting point of polyethylene is low, the closing of the pores of the second porous substrate is accelerated when the internal temperature of the secondary battery rises. In addition, since the second porous substrate has high insulation, it is more difficult for the positive electrode layer and the negative electrode layer to short-circuit.

[0022] (6) The secondary battery according to any one of (1) to (5), wherein the first porous substrate is made of a high heat-resistant resin having a higher heat resistance than the second porous substrate.

[0023] In the secondary battery according to (6), since the melting point of the first porous substrate is high, even when a method accompanied by heat such as sputtering is used to form the conductive layer, the first porous substrate is not easily deformed, and thus local pore closing is not likely to occur. Therefore, the range of selection of the manufacturing method of the conductive layer becomes wider.

[0024] (7) The secondary battery according to any one of (1) to (6), wherein the second separator has an inorganic insulating layer on the surface of the second porous substrate opposite to the first separator side.

[0025] In the secondary battery according to (7), since the insulation on the side in contact with the positive electrode active material layer is further improved, it is more difficult for the positive electrode layer and the negative electrode layer to short-circuit.

[0026] (8) A separator including a first separator and a second separator laminated on one surface of the first separator, the first separator having: a first porous substrate in contact with the second separator; and a conductive layer covering the entire surface of the surface of the first porous substrate opposite to the second separator side; the second separator having a second porous substrate in contact with the first separator, and the second separator having a protruding portion protruding from an end of the first separator.

[0027] With the separator according to (8), a secondary battery can be obtained in which the conductive layer of the first separator is arranged to face the negative electrode layer, so that lithium is easily deposited uniformly on the negative electrode layer during charging. In this secondary battery during charging, the overvoltage is reduced, so the decomposition of the electrolyte is suppressed. In addition, since the second separator has a protruding portion without a conductive layer, it is difficult for the positive electrode layer and the negative electrode layer to short-circuit via the conductive layer. Furthermore, since it is divided into a first separator having a conductive layer and a second separator having a protruding portion, it is easy to adjust the size of the conductive layer and the dimensional accuracy of the conductive layer is improved.

[0028] (9) A method for manufacturing a separator, comprising the following steps: a step of preparing a first laminate having a first porous sheet and a conductive layer covering one surface of the first porous sheet, and a second laminate having a second porous sheet; a step of laminating the first porous sheet of the first laminate and the second porous sheet of the second laminate such that at least one end of the second laminate protrudes from an end of the first laminate; and a step of joining the first laminate and the second laminate.

[0029] According to the method for manufacturing a separator of (9), the size of the conductive layer of the obtained separator can be adjusted by the size of the first laminate, so that a separator with high dimensional accuracy of the conductive layer can be advantageously manufactured industrially.

[0030] (10) According to the method for manufacturing a separator of (9), wherein the second laminate has an inorganic insulating layer on one surface of the second porous sheet.

[0031] According to the method for manufacturing a separator of (10), a separator with high insulation can be manufactured.

[0032] (Effects of the Invention)

[0033] According to the present invention, it is possible to provide a secondary battery, a separator useful for such a secondary battery, and a method for manufacturing the same, in which even when repeatedly charged and discharged, the positive electrode layer and the negative electrode layer are not easily short-circuited, and the density of the metallic lithium layer of the negative electrode layer in the charged state is not easily decreased. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a plan view of a separator according to an embodiment of the present invention.

[0035] Figure 2 is Figure 1 a sectional view taken along line II-II of

[0036] Figure 3 is a sectional view of a secondary battery according to an embodiment of the present invention.

[0037] Figure 4 is a plan view of a first laminate that can be used in the method for manufacturing a separator according to an embodiment of the present invention.

[0038] Figure 5 is a plan view of a second laminate that can be used in the method for manufacturing a separator according to an embodiment of the present invention.

[0039] Figure 6It is a plan view of a laminate of a first laminate sheet and a second laminate sheet obtained in a method for manufacturing a separator according to an embodiment of the present invention. Detailed Embodiment

[0040] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Among them, the embodiments shown below are illustrative of the present invention, and the present invention is not limited to the following.

[0041] Figure 1 It is a plan view of a separator according to an embodiment of the present invention. Figure 2 Is Figure 1 A sectional view taken along line II-II of

[0042] The separator 10 of the present embodiment includes a first separator 11 and a second separator 15 laminated on one surface of the first separator (the lower side in Figure 2 ).

[0043] The first separator 11 has: a first porous substrate 12 that contacts the second separator 15; and a conductive layer 13 that covers the entire surface of the surface of the first porous substrate 12 opposite to the second separator 15 side. The second separator 15 has: a second porous substrate 16 that contacts the first separator; and an inorganic insulating layer 17 disposed on the surface of the second porous substrate 16 opposite to the first separator 11 side. The inorganic insulating layer 17 may also cover the entire surface of the second porous substrate 16. The second separator 15 has a protruding portion 10a that protrudes from the end of the aforementioned first separator.

[0044] The first porous substrate 12 and the second porous substrate 16 may each be a porous resin sheet having a plurality of pores. The characteristics such as the average pore diameter, material, and thickness of the first porous substrate 12 and the second porous substrate 16 may be the same or different.

[0045] The average pore diameter of the first porous substrate 12 may, for example, also be in the range of 0.03 μm or more and 0.3 μm or less. The first porous substrate 12 may have a smaller average pore diameter than the second porous substrate 16. The ratio of the average pore diameter of the first porous substrate 12 to the average pore diameter of the second porous substrate 16 may, for example, also be 1.5 or more.

[0046] The first porous substrate 12 may also be made of a high heat-resistant resin having a higher heat resistance than the second porous substrate 16. For example, the melting point of the first porous substrate 12 may also be in the range of 150°C or higher and 250°C or lower. The melting point of the first porous substrate 12 may also be higher than the melting point of the second porous substrate 16 by, for example, 50°C or higher and 100°C or lower. As an example of the high heat-resistant resin, polypropylene, polyimide, etc. may be used. The thickness of the first porous substrate 12 may also be in the range of 5μm or more and 20μm or less, for example.

[0047] The average pore diameter of the second porous substrate 16 may also be in the range of 0.08μm or more and 0.5μm or less, for example. The second porous substrate 16 may have a larger average pore diameter than the first porous substrate 12. The average pore diameter of the second porous substrate 16 may also be, for example, 1.5 times or more that of the first porous substrate 12.

[0048] The second porous substrate 16 may be made of polyethylene. In addition, the second porous substrate 16 may have a higher insulation property than the first porous substrate 12. The withstand voltage (dielectric breakdown voltage) of the second porous substrate 16 may also be in the range of 500V or more, for example. The withstand voltage of the second porous substrate 16 may also be higher than that of the first porous substrate 12 by, for example, 300V or more. The thickness of the second porous substrate 16 may also be in the range of 5μm or more and 20μm or less, for example.

[0049] The withstand voltage (dielectric breakdown voltage) of the second porous substrate 16 may also be in the range of 500V or more, for example.

[0050] The conductivity of the conductive layer 13 is not particularly limited and may also be in the range of 1.0×10 1 ~1.0×10 5 S / cm. The surface resistivity may also be 200Ω / cm 2 or less, for example. As the material of the conductive layer 13, for example, conductive materials such as metals and carbon nanotubes (CNT) may be used. As an example of the metal, Cu, Zn, Ti, Sn may be cited. These conductive materials may be used alone or in combination of two or more. The thickness of the conductive layer 13 may also be in the range of 1nm or more and 5000nm (5μm) or less, for example.

[0051] The conductive layer 13 can be, for example, a layer of an aggregate of conductive material particles formed by coating a coating liquid of a conductive material, or a layer of a continuous film formed by a sputtering method or an evaporation method. In the case of an aggregate of conductive material particles, optionally, the average particle diameter of the conductive material particles is in the range of 5 nm or more and 100 nm or less, and the average size of the aggregate is 0.03 μm or more and 0.5 μm or less. The average pore diameter of the first porous substrate 12 can be the same as or smaller than the average size of the aggregate of conductive material particles.

[0052] The withstand voltage of the inorganic insulating layer 17 is not particularly limited. Optionally, the withstand voltage in combination with the porous substrate is 50 V. The inorganic insulating layer 17 can be, for example, a layer of an aggregate of inorganic particles formed by coating a coating liquid of inorganic particles, or a layer of a composite material in which inorganic particles are filled in a resin. The inorganic particles preferably have high heat resistance. As the inorganic particles, for example, alumina and boehmite can be used.

[0053] Next, a secondary battery using the separator 10 of the present embodiment will be described.

[0054] Figure 3 It is a cross-sectional view of a secondary battery according to an embodiment of the present invention.

[0055] The secondary battery 100 of the present embodiment includes: an electrode laminate 1 having a positive electrode layer 20, a negative electrode layer 30, and a separator 10 disposed between the positive electrode layer 20 and the negative electrode layer 30; an electrolytic solution (not shown); and an exterior body 50 that houses the electrode laminate 1 and the electrolytic solution. The exterior body 50 includes a positive electrode terminal 24 and a negative electrode terminal 34. The positive electrode layer 20 has a positive electrode current collector 21 and positive electrode active material layers 22 laminated on both surfaces of the positive electrode current collector 21. The positive electrode current collector 21 is connected to a positive electrode tab 23, and the positive electrode tab 23 is connected to the positive electrode terminal 24. The negative electrode layer 30 has a negative electrode current collector 31 and negative electrode active material layers 32 laminated on both surfaces of the negative electrode current collector 31. The negative electrode current collector 31 is connected to a negative electrode tab 33, and the negative electrode tab 33 is connected to the negative electrode terminal 34. The secondary battery 100 is configured as a lithium metal secondary battery. A lithium metal secondary battery is a secondary battery that uses metallic lithium as the negative electrode active material layer 32. During charging, lithium released from the positive electrode active material layer 22 is deposited on the surface of the negative electrode active material layer 32 to form a metallic lithium layer. Therefore, during charging, the thickness of the negative electrode layer 30 increases. On the other hand, during discharging, lithium is released from the metallic lithium layer and occluded by the positive electrode active material layer 22. Therefore, during discharging, the thickness of the negative electrode layer 30 decreases. Thus, the change in the thickness of the negative electrode layer 30 due to charge and discharge is large. Figure 3 The secondary battery 100 shown is in a discharged state.

[0056] The separator 10 is disposed such that the conductive layer 13 faces the negative electrode active material layer 32. The conductive layer 13 is in electrical contact with the negative electrode active material layer 32 in the discharged state and in electrical contact with metallic lithium deposited on the negative electrode active material layer 32 in the charged state. Thus, the potential of the conductive layer 13 is the same as that of the negative electrode active material layer 32, so that lithium can be deposited more uniformly between the conductive layer 13 and the negative electrode active material layer 32. Therefore, lithium dendrites are not easily formed, and the density of the deposited lithium layer is not easily decreased.

[0057] The conductivity of the conductive layer 13 may also be lower than the conductivity of the negative electrode current collector 31. Thereby, it is possible to prevent the concentrated deposition of lithium on the first porous base material 12 side of the conductive layer 13 during charging, and to suppress the shape change and breakage of the conductive layer 13 caused by the concentrated deposition of lithium. The conductivity of the conductive layer 13 may also be, for example, in the range of 1 / 10 to 1 / 100,000 of the conductivity of the negative electrode current collector 31.

[0058] The material of the positive electrode current collector 21 is not particularly limited, and for example, aluminum can be used. The material of the positive electrode tab 23 may be the same as or different from the material of the positive electrode current collector 21. The positive electrode tab 23 may also be integrally connected to the positive electrode current collector 21. In the present embodiment, the positive electrode tab 23 is formed by extending the positive electrode current collector 21 and is integrally connected to the positive electrode current collector 21. The material of the positive electrode terminal 24 may be the same as or different from the material of the positive electrode tab 23. The positive electrode terminal 24 may also be integrally connected to the positive electrode tab 23. In the present embodiment, the positive electrode terminal 24 and the positive electrode tab 23 are different components and are electrically connected.

[0059] The positive electrode active material layer 22 contains a positive electrode active material. Examples of the positive electrode active material include lithium cobaltate (LiCoO 2 ), lithium nickelate (LiNiO 2 ), LiNi p Mn q Co r O 2 (p + q + r = 1), LiNi p Al q Co r O 2 (p + q + r = 1), lithium manganate (LiMn 2 O 4 ), Li 1+x Mn 2－x－y M y O 4(x + y = 2, M is selected from at least one of Al, Mg, Co, Fe, Ni, and Zn) represents a heteroelement-substituted Li-Mn spinel, lithium titanate (including oxides of Li and Ti), lithium metal phosphate (LiMPO 4 , M is selected from at least one of Fe, Mn, Co, and Ni), etc. The positive electrode active material layer 22 may also contain various additives such as a binder and a conductive assistant, which are used as materials for the positive electrode active material layer.

[0060] The material of the negative electrode current collector 31 is not particularly limited, and for example, copper can be used. The material of the negative electrode tab 33 may be the same as or different from the material of the negative electrode current collector 31. The negative electrode tab 33 may also be integrally connected to the negative electrode current collector 31. In the present embodiment, the negative electrode tab 33 is formed by extending the negative electrode current collector 31 and is integrally connected to the negative electrode current collector 31. The material of the negative electrode terminal 34 may be the same as or different from the material of the negative electrode tab 33. The negative electrode terminal 34 may also be integrally connected to the negative electrode tab 33. In the present embodiment, the negative electrode terminal 34 and the negative electrode tab 33 are different components and are electrically connected.

[0061] As the negative electrode active material layer 32, lithium and metals that form an alloy with lithium can be used. Examples of metals that form an alloy with lithium include Mg, Si, Au, Ag, In, Ge, Sn, Pb, Al, and Zn. The thickness of the negative electrode active material layer 32 can also be, for example, 50 μm or less.

[0062] The electrolyte contains an organic solvent and an electrolyte. As the organic solvent, for example, cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, hydrofluoroethers, aromatic ethers, sulfones, cyclic esters, chain carboxylic acid esters, and nitriles can be used. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, etc. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, etc. Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, etc. Examples of chain ethers include 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, diethyl ether, etc. Examples of hydrofluoroethers include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, etc. Examples of aromatic ethers include anisole. Examples of sulfones include sulfolane, methyl sulfolane, etc. Examples of cyclic esters include γ-butyrolactone, etc. Examples of chain carboxylic acid esters include acetate, butyrate, propionate, etc. Examples of nitriles include acetonitrile, propionitrile, etc. The organic solvent can be used alone or in combination of two or more.

[0063] The electrolyte is a source of lithium ions as a charge transfer medium and contains a lithium salt. Examples of lithium salts include LiPF 6 , LiBF 4 , LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , LiC(CF 3 SO 2 ), 3 , LiN(CF 3 SO 2 ), 2 (LiTFSI), LiN(FSO 2 ), 2 (LiFSI), and LiBC 4 O 8 , etc. The lithium salt can be used alone or in combination of two or more. The concentration of the electrolyte is, for example, in the range of 1.5 to 4.0 mol / L.

[0064] The outer package 50 can expand and contract according to the change in the thickness of the electrode laminate 1 caused by charge and discharge (especially the change in the thickness of the negative electrode layer 30). As the material of the outer package 50, a laminated film can be used. As the laminated film, a laminated film having a three-layer structure in which an inner resin layer, a metal layer, and an outer resin layer are laminated in order from the inside can be used. As the material of the inner resin layer and the outer resin layer, for example, polyethylene terephthalate (PET), polyamide (nylon), polypropylene (PP) can be used. As the material of the metal layer, for example, aluminum can be used.

[0065] Next, a method for manufacturing the separator 10 of the present embodiment will be described.

[0066] The separator 10 can be manufactured, for example, by a method including a preparation step, a lamination step, and a bonding step.

[0067] The preparation step is a step of preparing a first laminated sheet and a second laminated sheet having a second porous substrate.

[0068] Figure 4 It is a plan view of the first laminated sheet that can be used in the method for manufacturing a separator according to an embodiment of the present invention. The first laminated sheet 111 has a first porous sheet 112 and a conductive layer 113 that covers one surface of the first porous sheet 112. As a method of covering the surface of the first porous sheet 112 with the conductive layer 113, there is no particular limitation. For example, a sputtering method and a coating method can be used. As the sputtering method, a direct current (DC) sputtering method or a radio frequency (RF) sputtering method can be used. The coating method is a method of coating a coating liquid of a conductive material on the surface of the first porous sheet 112 and drying it. As a method of coating the coating liquid, for example, a die coating method or an inkjet method can be used. The first laminated sheet 111 is cut into strips at an interval A. The interval A is twice the length of the first separator 11 in the direction in which the positive electrode tab 23 of the secondary battery 100 extends.

[0069] Figure 5This is a plan view of a second laminate sheet that can be used in a method for manufacturing a separator according to an embodiment of the present invention. The second laminate sheet 115 has a second porous sheet 116 and an inorganic insulating layer 117 that covers one surface of the second porous sheet 116. As a method for covering the surface of the second porous sheet 116 with the inorganic insulating layer 117, there is no particular limitation. For example, a method of coating a coating liquid of inorganic particles on the surface of a long strip-shaped insulating porous membrane 41 and drying it can be used. As a method for coating the coating liquid, for example, a die coating method or an inkjet method can be used. The second laminate sheet 115 is cut into strips at an interval B. The interval B is twice the length of the second separator 15 in the direction in which the positive electrode tab 23 of the secondary battery 100 extends.

[0070] The laminating step is a step of laminating the first porous sheet 112 of the strip-shaped first laminate sheet 111a and the second porous sheet 116 of the strip-shaped second laminate sheet 115a such that at least one end of the strip-shaped second laminate sheet 115a protrudes from the end of the strip-shaped first laminate sheet 111a.

[0071] Figure 6 This is a plan view of a laminate of the first laminate sheet and the second laminate sheet obtained in a method for manufacturing a separator according to an embodiment of the present invention. In the laminate 120, the strip-shaped first laminate sheet 111a and the strip-shaped second laminate sheet 115a are laminated on top of each other such that the center line 121 in the long side direction overlaps. As a result, both ends of the strip-shaped second laminate sheet 115a protrude equally from the ends of the strip-shaped first laminate sheet 111a.

[0072] The bonding step is to bond the first porous sheet 112 of the strip-shaped first laminate sheet 111a and the second porous sheet 116 of the two-layer laminate sheet. An adhesive can be used for bonding.

[0073] The bonded body thus obtained is cut along the center line 121 in the long side direction. Subsequently, cutting is performed in a direction orthogonal to the long side direction, whereby a separator 10 of a desired size can be manufactured.

[0074] According to the secondary battery 100 of the present embodiment having the above configuration, since the conductive layer 13 of the first separator 11 of the separator 10 is arranged to face the negative electrode active material layer 32, lithium is likely to be uniformly deposited on the negative electrode active material layer 32 during charging. In addition, since electrons are supplied to the conductive layer 13 during charging, a large number of lithium deposition sites are also formed in the conductive layer 13, so the current density during charging can be reduced. Since the current density during charging decreases, the overvoltage is reduced, so the decomposition of the electrolyte is suppressed. In addition, since the second separator 15 has the protrusion 10a without the conductive layer 13, it is not easy for the positive electrode layer 20 and the negative electrode layer 30 to be short-circuited via the conductive layer 13. Furthermore, since the separator 10 is divided into the first separator 11 having the conductive layer 13 and the second separator 15 having the protrusion 10a, it is easy to adjust the size of the conductive layer 13, and the dimensional accuracy of the conductive layer 13 is improved. Therefore, it is easy to obtain the above effects brought by the conductive layer 13. Thus, even if the secondary battery 100 is repeatedly charged and discharged, the positive electrode layer 20 and the negative electrode layer 30 are not easily short-circuited, and the density of the metallic lithium layer of the negative electrode layer 30 in the charged state is not easily decreased, resulting in a high capacity.

[0075] In the secondary battery 100 of the present embodiment, the positive electrode current collector 21 is connected to the positive electrode tab 23, and the positive electrode tab 23 extends toward the protrusion 10a side of the separator 10. As a result, the positive electrode tab 23 comes into contact with the protrusion 10a of the separator 10, so it is not easy for the positive electrode tab 23 to come into contact with the conductive layer 13. Therefore, it is even less likely for the positive electrode layer 20 and the negative electrode layer 30 to be short-circuited.

[0076] In the secondary battery 100 of the present embodiment, when the average pore diameter of the first porous substrate 12 is smaller than that of the second porous substrate 16, the conductive material of the conductive layer 13 is not easily introduced into the pores of the first porous substrate 12, and it is not easy for the insulation of the separator to decrease due to the introduction of the conductive material. In addition, when the average pore diameter of the second porous substrate 16 is larger than that of the first porous substrate 12, the electrolyte easily penetrates into the pores of the second porous substrate 16, so the internal resistance caused by the decrease in ion conductivity inside the secondary battery 100 becomes smaller.

[0077] In the secondary battery 100 of the present embodiment, when the second porous substrate 16 is made of polyethylene and has higher insulation than the first porous substrate 12, since the melting point of polyethylene is low, the closing of the pores of the second porous substrate 16 is accelerated when the internal temperature of the secondary battery 100 rises. In addition, since the second porous substrate 16 has high insulation, the positive electrode layer 20 and the negative electrode layer 30 are less likely to be short-circuited. Further, in the secondary battery 100 of the present embodiment, when the first porous substrate 12 is made of a high heat-resistant resin having higher heat resistance than the second porous substrate 16, even if a method accompanied by heat generation such as sputtering is used to form the conductive layer 13, the first porous substrate 12 is not easily deformed, and thus local pore closing is less likely to occur. Therefore, the range of selection of the manufacturing method of the conductive layer 13 is widened. Furthermore, in the secondary battery 100 of the present embodiment, when the second separator 15 has the inorganic insulating layer 17 on the surface opposite to the first separator 11 side of the second porous substrate 16, the insulation on the side in contact with the positive electrode active material layer 22 is further improved, so the positive electrode layer 20 and the negative electrode layer 30 are less likely to be short-circuited.

[0078] According to the separator 10 of the present embodiment, the conductive layer 13 of the first separator 11 is arranged so as to face the negative electrode active material layer 32, whereby a secondary battery 100 in which lithium is easily deposited uniformly on the negative electrode active material layer 32 during charging can be obtained. During charging of this secondary battery 100, the overvoltage is reduced, so the decomposition of the electrolyte is suppressed. In addition, since the second separator 15 has the protruding portion 10a without the conductive layer, the positive electrode layer 20 and the negative electrode layer 30 are less likely to be short-circuited via the conductive layer 13. Furthermore, since it is divided into the first separator 11 having the conductive layer 13 and the second separator having the protruding portion 10a, it is easy to adjust the size of the conductive layer 13, and the dimensional accuracy of the conductive layer is improved.

[0079] According to the manufacturing method of the separator 10 of the present embodiment, the size of the conductive layer 13 of the obtained separator 10 can be adjusted by the size of the strip-shaped first laminated sheet 111a, so a separator with high dimensional accuracy of the conductive layer 13 can be industrially advantageously manufactured. In the manufacturing method of the separator 10 of the present embodiment, since the strip-shaped second laminated sheet 115a has the inorganic insulating layer 117, a separator 10 with high insulation can be manufactured.

[0080] The embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments and can be appropriately changed.

[0081] For example, in the secondary battery 100 of the present embodiment, lithium and a metal that forms an alloy with lithium are used as the material of the negative electrode active material layer 32, but the material of the negative electrode active material layer 32 is not particularly limited. As the material of the negative electrode active material layer 32, a layer containing a negative electrode active material that occludes lithium during charging and releases lithium during discharging can also be used. As the negative electrode active material, for example, lithium transition metal oxides such as lithium titanate, TiO 2 、Nb 2 O 3 and WO 3 and other transition metal oxides, SiO, metal sulfides, metal nitrides, and carbon materials such as artificial graphite, natural graphite, graphite, soft carbon, and hard carbon.

[0082] In the secondary battery 100 of the present embodiment, the protruding portion 10a of the separator 10 is provided only at the end in the extending direction of the positive electrode tab 23, but the position of the protruding portion 10a is not limited thereto. The protruding portion 10a can also be provided at the end in the extending direction of the negative electrode tab 33. In addition, in the present embodiment, the negative electrode layer 30 has a negative electrode current collector 31 and negative electrode active material layers 32 laminated on both surfaces of the negative electrode current collector 31, but the structure of the negative electrode layer 30 is not limited thereto. For example, the negative electrode active material layer 32 can also be omitted. In this case, during charging, lithium is deposited on the surface of the negative electrode current collector 31 to form a lithium layer.

[0083] In the manufacturing method of the separator 10 of the present embodiment, the strip-shaped first laminated sheet 111a and the strip-shaped second laminated sheet 115a are joined, but the strip-shaped first laminated sheet 111a and the strip-shaped second laminated sheet 115a can also be joined after being cut into the size of the separator 10. In addition, the strip-shaped second laminated sheet 115a may not have the inorganic insulating layer 117.

[0084] [Experimental Example]

[0085] The effects brought about by providing the conductive layer 13 on the negative electrode layer 30 side of the separator 10 will be described by way of experimental examples.

[0086] [Experimental Example 1]

[0087] (Fabrication of Separator)

[0088] Prepare an insulating porous membrane (membrane thickness: 20 μm, porosity: 58%, air permeability: 92 sec / 100 mL). By RF sputtering method, a copper conductive layer with a thickness of 0.08 μm is formed on one surface of the insulating porous membrane. The insulating porous membrane with the copper conductive layer formed thereon is cut into a size of 40 mm × 50 mm to fabricate a separator.

[0089] (Fabrication of Positive Electrode Layer)

[0090] Pre-mix acetylene black (AB), which is an electron-conductive material, polyvinylidene fluoride (PVDF), which is a binder, and polyvinylpyrrolidone (PVP), which is a dispersant, in N-methyl-2-pyrrolidone (NMP), which is a dispersion solvent, and perform wet mixing using a planetary mixer to obtain a pre-mixed slurry. Subsequently, Li 1 Ni 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) and the pre-doped material are mixed with the obtained pre-mixed slurry, and dispersion treatment is performed using a planetary mixer to obtain a positive electrode active material paste. The median particle size of NCM811 is 12 μm. Next, the obtained positive electrode active material paste is coated on an aluminum positive electrode current collector without a primer layer and dried. After pressing using a roll press, drying is performed in a vacuum at 0019°C to form a positive electrode plate having a positive electrode active material layer. The obtained positive electrode plate is punched into a size of 30 mm × 40 mm to form a positive electrode layer.

[0091] (Fabrication of negative electrode layer)

[0092] Prepare a clad material obtained by bonding a copper foil with a thickness of 10 μm (negative electrode current collector, conductivity: 6.5×10 6 S / cm) and a lithium foil with a thickness of 20 μm (negative electrode active material layer). The clad material is punched into a size of 34 mm × 44 mm to form a negative electrode layer.

[0093] (Electrolyte)

[0094] Prepare an electrolyte obtained by dissolving LiFSI at a concentration of 4 mol / L in 1,2-dimethoxyethane (DME).

[0095] (Fabrication of lithium metal secondary battery)

[0096] On the surface of the lithium foil side of the negative current collector, the copper conductive layer of the separator was overlapped, and on the surface of the separator opposite to the copper conductive layer side, the positive electrode active material layer of the positive electrode layer was overlapped, and an electrode laminate in which the negative electrode layer, the separator, and the positive electrode layer were laminated in this order was produced. Subsequently, a positive electrode terminal was installed on the positive current collector of the obtained electrode laminate via a positive electrode tab, and a negative electrode terminal was installed on the copper foil of the negative current collector via a negative electrode tab. The laminate equipped with the positive electrode terminal and the negative electrode terminal was placed in a laminated film bag, and then an electrolytic solution was placed, and then the laminated film bag was sealed, thereby producing a lithium metal secondary battery.

[0097] [Experimental Example 2]

[0098] In the production of the separator, a zinc conductive layer with a thickness of 0.06 μm was formed by RF sputtering method to replace the copper conductive layer, and in other respects, a lithium metal secondary battery was produced in the same manner as in Experimental Example 1.

[0099] [Experimental Example 3]

[0100] In the production of the separator, a carbon nanotube (CNT) conductive layer with a thickness of 2.1 μm was formed by a coating method to replace the copper conductive layer, and in other respects, a lithium metal secondary battery was produced in the same manner as in Experimental Example 1. The formation of the carbon nanotube conductive layer was carried out as follows. First, N-methyl-N-pyrrolidinone (NMP) was used as a solvent, carbon nanotubes in an amount corresponding to a solid component concentration of 4% by mass were introduced, and PVDF (#9300, manufactured by Kureha Corporation) in an amount of 5 parts by mass relative to 95 parts by mass of the carbon nanotubes was introduced as a binder. Subsequently, using a rotation-revolution stirrer, dispersion treatment was carried out at 1000 rpm for 10 minutes to prepare a coating solution. The obtained coating solution was coated on the surface of the insulating porous membrane using a doctor blade and dried.

[0101] [Experimental Example 4]

[0102] In the production of the separator, a tin conductive layer with a thickness of 0.06 μm was formed by RF sputtering method to replace the copper conductive layer, and in other respects, a lithium metal secondary battery was produced in the same manner as in Experimental Example 1.

[0103] [Comparative Experimental Example 1]

[0104] In the production of the separator, the copper conductive layer was not formed, and in other respects, a lithium metal secondary battery was produced in the same manner as in Experimental Example 1.

[0105] [Evaluation]

[0106] The conductivity, surface resistivity, and peel strength of the conductive layer of the separator fabricated in each experimental example were measured by the following method. The results are shown in Table 1 together with the material, coating method, and thickness of the conductive layer of the separator.

[0107] In addition, the presence or absence of short circuit and the lithium thickness increase rate of the lithium metal secondary batteries fabricated in each experimental example and comparative experimental example were measured by the following method. The results are shown in Table 1.

[0108] (Conductivity and surface resistivity of the conductive layer)

[0109] The conductivity and surface resistivity were measured using a high-precision and high-performance resistivity meter (Analytech Co., Ltd., Loresta GP TCP-600).

[0110] (Peel strength)

[0111] A conductive layer with a length of 5.0 cm and a width of 2.5 cm was crimped onto an adhesive tape with a width of 2.5 cm that was crimped to a fixed plate. Subsequently, one end of the conductive layer was folded back 180 degrees, and using an electric measuring table (manufactured by IMADA Co., Ltd.), it was pulled up at a speed of 300 mm / min to peel the conductive layer from the insulating porous membrane. A digital force gauge (manufactured by IMADA Co., Ltd.) was used to measure the load required from the start to the end of peeling the conductive layer. The value obtained by dividing the average value of the obtained load by the width of the adhesive tape was used as the peel strength.

[0112] (Presence or absence of short circuit in the lithium metal secondary battery)

[0113] The newly fabricated lithium metal secondary battery was left standing at a measurement temperature of 25 °C for 24 hours.

[0114] For the lithium metal secondary battery after standing, the following first charge-discharge cycle was performed 3 times, and then the following second charge-discharge cycle was performed 50 times to confirm the presence or absence of short circuit in the lithium metal secondary battery. When the charging capacity of the lithium metal secondary battery was 105% or more relative to the discharge capacity before charging, it was determined that a short circuit had occurred and was determined to have a short circuit.

[0115] (First charge-discharge cycle)

[0116] Charging was carried out under the following conditions: constant current charging was performed at a current value of 2.2 mA until 4.300 V, and then constant voltage charging was continued at a voltage value of 4.300 V for 60 minutes. Discharging was carried out under the following conditions: constant current discharging was performed at a current value of 4 mA until 2.65 V. A 30-minute rest was taken between discharging and charging.

[0117] (Second charge-discharge cycle)

[0118] Charging is carried out under the following conditions: constant current charging is implemented at a current value of 74 mA until 3.823 V, at a current value of 52 mA until 4.051 V, at a current value of 46 mA until 4.173 V, at a current value of 22 mA until 4.300 V, and then constant voltage charging is implemented at a voltage value of 4.300 V for 90 minutes. Discharging is carried out under the following conditions: constant current discharging is implemented at a current value of 18 mA until 2.65 V. A 30-minute rest is provided between discharging and charging.

[0119] (Rate of increase in lithium thickness of lithium metal secondary battery)

[0120] Measure the thickness T1 (μm) of the negative electrode layer during the initial charging and the thickness (total thickness of the negative electrode current collector and the lithium metal layer) T2 (μm) of the negative electrode layer during charging after 50 cycles, and calculate the rate of increase in lithium thickness T (μm / cycle) using the following formula.

[0121] T (μm / cycle) = (T2 - T1) / 50

[0122] The thickness T1 (μm) of the negative electrode layer during the initial charging is measured as follows.

[0123] The lithium metal secondary battery just after fabrication is left to stand at a measurement temperature of 25 °C for 24 hours.

[0124] For the lithium metal secondary battery after standing, the above-mentioned first charge-discharge cycle is carried out 3 times. Then, constant current charging is implemented at a current value of 14.7 mA until 4.300 V, and constant voltage charging is continued at a voltage value of 4.300 V for 60 minutes to charge the lithium metal secondary battery. After the charged lithium metal secondary battery is left to stand for 30 minutes, it is disassembled, the negative electrode layer is taken out, and its thickness is measured as the thickness T1 of the negative electrode layer.

[0125] The thickness T2 of the negative electrode layer during charging after 50 cycles is measured as follows.

[0126] The lithium metal secondary battery just after fabrication is left to stand at a measurement temperature of 25 °C for 24 hours.

[0127] For the lithium metal secondary battery after standing, the above-mentioned first charge-discharge cycle is carried out 3 times, and then the above-mentioned second charge-discharge cycle is carried out 50 times. Then, constant current charging is implemented at a current value of 14.7 mA until 4.300 V, and constant voltage charging is continued at a voltage value of 4.300 V for 60 minutes to charge the lithium metal secondary battery. After the charged lithium metal secondary battery is left to stand for 30 minutes, it is disassembled, the negative electrode layer is taken out, and its thickness is measured as the thickness T2 of the negative electrode layer.

[0128] [Table 1]

[0129]

[0130] As shown in Table 1, it can be seen that for the lithium metal secondary batteries of Experimental Examples 1 to 4 having a separator with a conductive layer, even when repeatedly charged and discharged, short circuits are not likely to occur, the lithium thickness increase rate is low, and the metallic lithium layer formed by charging is dense and has a high density. In contrast, for the lithium metal secondary battery of Comparative Experimental Example 1 having a separator without a conductive layer, when repeatedly charged and discharged, short circuits are likely to occur, the lithium thickness increase rate is high, and thus a thick and low-density metallic lithium layer is formed by charging.

[0131] Reference Numerals

[0132] 1 Electrode laminate

[0133] 10 Separator

[0134] 10a Protrusion

[0135] 11 First separator

[0136] 12 First porous substrate

[0137] 13 Conductive layer

[0138] 15 Second separator

[0139] 16 Second porous substrate

[0140] 17 Inorganic insulating layer

[0141] 20 Positive electrode layer

[0142] 21 Positive electrode current collector

[0143] 22 Positive electrode active material layer

[0144] 23 Positive electrode tab

[0145] 24 Positive electrode terminal

[0146] 30 Negative electrode layer

[0147] 31 Negative electrode current collector

[0148] 32 Negative electrode active material layer

[0149] 33 Negative electrode tab

[0150] 34 Negative electrode terminal

[0151] 50 Outer package

[0152] 100 Secondary battery

[0153] 111 First laminated sheet

[0154] 111a Strip-shaped first laminated sheet

[0155] 112 First porous sheet

[0156] 113 Conductive layer

[0157] 115 Second laminated sheet

[0158] 115a Strip-shaped second laminated sheet

[0159] 116 Second porous sheet

[0160] 117 Inorganic insulating layer

[0161] 120 Laminate

[0162] 121 Center line.

Claims

1. A secondary battery includes a positive electrode layer, a negative electrode layer, and a separator disposed between the positive electrode layer and the negative electrode layer. The positive electrode layer has a positive electrode current collector and a positive electrode active material layer. The negative electrode layer has a negative electrode current collector. The separator includes a first separator and a second separator laminated on one surface of the first separator. The first separator has: a first porous substrate that contacts the second separator; and a conductive layer that covers the entire surface of the surface of the first porous substrate opposite to the second separator side. The second separator has a second porous substrate that contacts the first separator, and the second separator has a protruding portion that protrudes from an end of the first separator. The separator is disposed such that the conductive layer of the first separator faces the negative electrode layer.

2. The secondary battery according to claim 1, wherein, the positive electrode current collector is connected to a positive electrode tab, and the positive electrode tab extends toward the protruding portion side of the separator.

3. The secondary battery according to claim 1 or 2, wherein, the average pore diameter of the first porous substrate is smaller than that of the second porous substrate.

4. The secondary battery according to claim 1 or 2, wherein, the average pore diameter of the second porous substrate is larger than that of the first porous substrate.

5. The secondary battery according to claim 1 or 2, wherein, the second porous substrate is made of polyethylene and has higher insulation than the first porous substrate.

6. The secondary battery according to claim 1 or 2, wherein, the first porous substrate is made of a high heat-resistant resin with higher heat resistance than the second porous substrate.

7. The secondary battery according to claim 1 or 2, wherein, the second separator has an inorganic insulating layer on the surface of the second porous substrate opposite to the first separator side.

8. A separator includes a first separator and a second separator laminated on one surface of the first separator. The first separator has: a first porous substrate that contacts the second separator; and a conductive layer that covers the entire surface of the surface of the first porous substrate opposite to the second separator side. The second separator has a second porous substrate that contacts the first separator. The second separator has a protruding portion that protrudes from an end of the first separator.

9. A method for manufacturing a separator includes the following steps: preparing a first laminated sheet having a first porous sheet and a conductive layer covering one surface of the first porous sheet, and a second laminated sheet having a second porous sheet; laminating the first porous sheet of the first laminated sheet and the second porous sheet of the second laminated sheet such that at least one end of the second laminated sheet protrudes from an end of the first laminated sheet; and joining the first laminated sheet and the second laminated sheet.

10. The method for manufacturing a separator according to claim 9, wherein, the second laminated sheet has an inorganic insulating layer on one surface of the second porous sheet.

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