JP2011054492A - Collector for bipolar battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a collector for a bipolar secondary battery which follows the expansion/contraction of a negative electrode. <P>SOLUTION: The collector for a bipolar secondary battery contains an elastomer and polyolefin resin. A content of the elastomer at one surface side of the collector is greater than a content of the polyolefin resin at the other surface side of the collector. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a current collector for a bipolar secondary battery.

  In recent years, reduction of carbon dioxide emissions has been strongly desired for environmental protection. In the automobile industry, there is an expectation for reducing carbon dioxide emissions by introducing electric vehicles (EV) and hybrid electric vehicles (HEV), and the development of motor drive batteries that hold the key to their practical use has been carried out. ing. As batteries for automobiles, attention has been focused on lithium ion secondary batteries that can achieve high energy density and high output density. In particular, in the case of a bipolar battery, a current flows in the vertical direction (electrode stacking direction) through the current collector, so that the electron conduction path can be shortened and the output becomes high. Thereby, a battery with a high battery voltage can be constituted.

  The bipolar battery includes a current collector having a positive electrode active material layer and a negative electrode active material layer formed on each surface as a constituent member. A metal foil has been generally used as a current collector. However, for the purpose of reducing the weight of the current collector, Patent Document 1 proposes a current collector containing a conductive resin mixed with a carbon filler. Yes.

JP 2006-190649 A

  However, the current collector described in Patent Document 1 uses a resin having high crystallinity and high density. Therefore, the current collector cannot follow the expansion and contraction of the negative electrode accompanying charge / discharge, and the negative electrode may be peeled off. Accordingly, an object of the present invention is to provide a current collector that can sufficiently follow the expansion and contraction of the negative electrode.

  The inventors of the present invention tackled the above-mentioned problems and accumulated intensive studies. As a result, it has been found that the above problem can be solved by using a current collector having a larger amount of elastomer on one side of the current collector than the content of elastomer on the other side.

  The above problem can be improved simply by using a flexible resin (high elongation strain rate) corresponding to the expansion and contraction of the electrode, but such a resin has a structure having an unsaturated bond. Therefore, there is a problem that the resin decomposes at the positive electrode potential. This decomposition reaction occurs at the interface between the conductive carbon filler in the current collector foil and the resin at the positive electrode potential for accelerating the reaction, and reacts with an unsaturated bond portion having a low activation energy of the reaction. Therefore, such a problem can be solved by incorporating a polyolefin resin into the current collector.

  Since there is a large amount of elastomer on one side of the current collector for a bipolar secondary battery, if this surface is used on the negative electrode side, the elastomer can exist stably with respect to the potential, so the expansion and contraction of the negative electrode A current collector that can follow the above is provided.

1 is a schematic cross-sectional view showing the structure of a bipolar lithium ion secondary battery. It is the cross-sectional schematic of the bipolar electrode using the electrical power collector which is preferable embodiment. 1 is an external perspective view of a bipolar lithium ion secondary battery.

  First, a bipolar lithium ion secondary battery which is a preferred embodiment will be described. However, it is not limited only to the following embodiments. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

  FIG. 1 is a schematic cross-sectional view schematically showing the entire structure of a bipolar lithium ion secondary battery 10. The bipolar lithium ion secondary battery 10 shown in FIG. 1 has a structure in which a substantially rectangular power generation element 21 in which a charge / discharge reaction actually proceeds is sealed inside a battery exterior material 29.

  As shown in FIG. 1, the power generating element 21 of the bipolar lithium ion secondary battery 10 has a positive electrode active material layer 13 electrically coupled to one surface of the current collector 11, and is opposite to the current collector 11. It has a plurality of bipolar electrodes 23 formed with a negative electrode active material layer 15 electrically coupled to the side surface. Each bipolar electrode 23 is laminated via the electrolyte layer 17 to form the power generation element 21. The electrolyte layer 17 has a configuration in which an electrolyte is held at the center in the surface direction of a separator as a base material. At this time, the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the one bipolar electrode 23 face each other through the electrolyte layer 17. The bipolar electrodes 23 and the electrolyte layers 17 are alternately stacked. That is, the electrolyte layer 17 is interposed between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the one bipolar electrode 23. ing.

  The adjacent positive electrode active material layer 13, electrolyte layer 17, and negative electrode active material layer 15 constitute one unit cell layer 19. Therefore, it can be said that the bipolar lithium ion secondary battery 10 has a configuration in which the single battery layers 19 are stacked. In addition, for the purpose of preventing liquid junction due to leakage of the electrolytic solution from the electrolyte layer 17, an insulating portion 31 is disposed on the outer peripheral portion of the unit cell layer 19. A positive electrode active material layer 13 is formed only on one side of the positive electrode outermost layer current collector 11 a located in the outermost layer of the power generation element 21. The negative electrode active material layer 15 is formed only on one surface of the outermost current collector 11b on the negative electrode side located in the outermost layer of the power generation element 21. However, the positive electrode active material layer 13 may be formed on both surfaces of the outermost layer current collector 11a on the positive electrode side. Similarly, the negative electrode active material layer 15 may be formed on both surfaces of the outermost layer current collector 11b on the negative electrode side.

  Further, in the bipolar secondary battery 10 shown in FIG. 1, the positive electrode current collector plate 25 is disposed so as to be adjacent to the outermost layer current collector 11 a on the positive electrode side, and this is extended and led out from the battery exterior material 29. . On the other hand, the negative electrode current collector plate 27 is disposed so as to be adjacent to the outermost layer current collector 11 b on the negative electrode side, and is similarly extended and led out from the battery exterior material 29.

  In the bipolar lithium ion secondary battery 10 shown in FIG. 1, an insulating part 31 is usually provided around each single battery layer 19. The insulating part 31 prevents the adjacent current collectors 11 in the battery from coming into contact with each other and a short circuit caused by a slight irregularity at the end of the unit cell layer 19 in the power generation element 21. Is provided.

  Note that the number of stacks of the unit cell layers 19 is adjusted according to a desired voltage. In the bipolar lithium ion secondary battery 10, the number of stacks of the single battery layers 19 may be reduced if a sufficient output can be secured even if the thickness of the battery is reduced as much as possible. Even in the bipolar lithium ion secondary battery 10, in order to prevent external impact and environmental degradation during use, the power generation element 21 is sealed in the battery exterior material 29 under reduced pressure, and the positive current collector plate 25 and the negative current collector plate It is preferable to adopt a structure in which 27 is taken out of the battery exterior material 29.

  As mentioned above, although the bipolar lithium ion secondary battery was demonstrated as preferable embodiment, the battery which can apply the electrical power collector mentioned later is not restricted above. For example, when bipolar batteries are distinguished by structure and form, they are not particularly limited, such as stacked (flat) batteries and wound (cylindrical) batteries, and can be applied to any conventionally known structure.

  Similarly, there is no particular limitation even when distinguished by the form of electrolyte of the bipolar battery. For example, the present invention can be applied to any of a liquid electrolyte type battery in which a separator is impregnated with a nonaqueous electrolytic solution, a polymer gel electrolyte type battery also called a polymer battery, and a solid polymer electrolyte (all solid electrolyte) type battery. With respect to the polymer gel electrolyte and the solid polymer electrolyte, these can be used alone, or the polymer gel electrolyte or the solid polymer electrolyte can be used by impregnating the separator.

  Moreover, when it sees in the electrode material of a battery, or the metal ion which moves between electrodes, it does not restrict | limit in particular, It can apply to any well-known electrode material. For example, a lithium ion secondary battery, a sodium ion secondary battery, a potassium ion secondary battery, a nickel metal hydride secondary battery, a nickel cadmium secondary battery, a nickel metal hydride battery, and the like can be given. A lithium ion secondary battery is preferable. This is because in the lithium ion secondary battery, the voltage of the cell (single cell layer) is large, high energy density and high output density can be achieved, and it is excellent as a vehicle driving power source or an auxiliary power source.

(Current collector for bipolar secondary battery)
Hereinafter, the current collector for the bipolar secondary battery will be described in detail.

  FIG. 2 is a schematic sectional view of the bipolar electrode 23. In the current collector 11, the polyolefin resin layer 2 including a polyolefin resin and a conductive material is disposed on the side in contact with the positive electrode active material layer 13. An elastomer layer 3 containing an elastomer and a conductive material is disposed on the side in contact with the negative electrode active material layer 15, and the current collector 11 is composed of these two layers. The elastomer layer 3 is thicker than the polyolefin resin layer 2.

  In a conventional bipolar battery, particularly a lithium ion battery that is expected to be widely applied in the future, as a negative electrode active material constituting the negative electrode, a carbon / graphite negative electrode material, silicon (Si) or tin (which can be alloyed with lithium) An alloy negative electrode material such as Sn) is used. In particular, alloy-based negative electrode materials can achieve a higher energy density than carbon / graphite-based negative electrode materials, and are expected as candidates for vehicle batteries.

  The alloy-based negative electrode material has a large expansion and contraction associated with insertion and extraction of lithium ions. For example, volume expansion when lithium ions are occluded is about 1.2 times that of graphite, and about 4 times that of silicon-based materials. When the active material expands greatly as described above, the contact between the active materials or the adhesiveness between the active material layer and the current collector decreases as charge and discharge are repeated. As a result, it has been pointed out that the negative electrode active material is cracked, pulverized, or peeled off from the current collector, and desired cycle characteristics cannot be obtained.

  The current collector that contains the resin cannot follow the expansion and contraction of the negative electrode active material layer during charge and discharge because the conventional resin current collector uses a highly crystalline, high density resin with a high Young's modulus. is there. In order to suppress the peeling of the negative electrode active material layer, it can be simply improved by using a highly flexible resin corresponding to the expansion and contraction of the negative electrode active material layer. However, on the other hand, such a resin has a problem that the resin decomposes at the positive electrode potential. The present inventors have found that this decomposition reaction occurs at the interface between a filler such as conductive carbon in the current collector and the resin at the positive electrode potential that accelerates the reaction, and the activation energy of the reaction is low. It was found to react with the binding moiety. When such a decomposition reaction proceeds, the resin and the carbon filler fall off, and as a result, the battery performance decreases.

  The current collector for a bipolar secondary battery of the present embodiment includes an elastomer and a polyolefin resin, and the content of the elastomer on one surface side is larger than the content of the elastomer on the other surface side. Here, the one surface side or the other surface side refers to a region of a certain thickness in the current collector that is closer to one surface or the other surface of the current collector. When the current collector is applied to a bipolar electrode, the one surface is in contact with the negative electrode active material layer, and the other surface is in contact with the positive electrode active material layer. Preferably, the current collector has an elastomer layer including an elastomer and a conductive material on the surface on the one surface side. A polyolefin resin layer containing a polyolefin resin and a conductive material is present on the surface on the other side. More preferably, the current collector is composed of these two layers. In the following, preferred embodiments composed of two layers will be mainly described.

  By incorporating an elastomer on the negative electrode active material layer side of the current collector, the elastomer is flexible, and the elastomer can exist stably with respect to the potential. Can follow contraction. Therefore, when applied to a bipolar secondary battery, it is possible to prevent peeling of the negative electrode during charging / discharging due to the failure of the current collector to follow. Further, since the polyolefin resin is composed of a single bond, the decomposition reaction of the resin current collector at the positive electrode potential is suppressed by including the polyolefin resin on the positive electrode active material layer side of the current collector. Therefore, suppression of negative electrode peeling of the current collector and improvement in resistance at the positive electrode potential are realized. If such a current collector is used for a bipolar secondary battery, the cycle characteristics of the battery can be improved.

  The elastomer layer 3 preferably contains 50 to 99% by mass, more preferably 70 to 95% by mass of the elastomer. Examples of the elastomer contained in the negative electrode active material layer side of the current collector for a bipolar secondary battery include silicone rubber, silicone-modified heat-resistant rubber, natural rubber, chlorosulfonated polyethylene rubber, butyl rubber, butadiene rubber, and styrene-butadiene. Examples thereof include rubber, urethane rubber, polysulfide rubber, nitrile rubber, hydrogenated styrene-butadiene-styrene block copolymer (SEBS), ethylene propylene rubber, chloroprene rubber, acrylic rubber, and isoprene rubber. Of these, ethylene propylene rubber is preferable from the viewpoint of easy follow-up of expansion and contraction of the negative electrode active material layer. However, the present invention is not limited to these.

  The polyolefin resin layer 2 contains 50 to 99% by mass, more preferably 70 to 95% by mass of a polyolefin resin. The polyolefin resin means a homopolymer or copolymer of a linear or branched α-olefin or cyclic olefin, or a mixture thereof. For example, specific examples of linear or branched α-olefins include ethylene, propylene, 1-butene, pentene, methylbutene, hexene, methylpentene, methylpentene, ethylpentene, dimethylpentene, methylhexene, dimethylhexene. , Ethylhexene, ethylhexene, octene, decene, dodecene, tetradecene, hexadecene, octadecene, eicosene and the like, but are not limited thereto. Of these polymers and copolymers, polyethylene and polypropylene are more preferred, and polyethylene is particularly preferred from the viewpoint of preventing decomposition at the positive electrode potential. Similarly, a mixed resin obtained by mixing 1 to 40% by mass of polypropylene with polyethylene is also preferably used.

  The current collector for a bipolar secondary battery may contain a conductive material (conductive filler) together with the elastomer and the polyolefin resin. Examples of the conductive material include, but are not limited to, acetylene black, vulcan, black pearl, carbon nanofiber, ketjen black, carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. It is preferable to contain at least one kind. Of these, ketjen black is particularly preferable since it exhibits good conductive properties when added in a small amount. In addition to these carbon materials, aluminum particles, SUS particles, carbon particles, silver particles, gold particles, copper particles, titanium particles, and alloy particles thereof can also be used.

  The content of the conductive material is not particularly limited. In particular, when the resin includes a conductive polymer material and sufficient conductivity can be ensured, it is not always necessary to add a conductive material. However, when the resin is made of only a non-conductive polymer material, it is essential to add a conductive material in order to impart conductivity. The content of the conductive material at this time is preferably 5 to 35% by mass, more preferably 5 to 25% by mass, and further preferably 5 to 15% with respect to the total mass of the non-conductive polymer material. % By mass. By adding such an amount of conductive material to the resin, sufficient conductivity can be imparted to the non-conductive polymer material while suppressing an increase in the mass of the resin.

  The bipolar secondary battery current collector may be mixed with a non-conductive polymer material together with the elastomer or the polyolefin resin. Non-conductive polymer materials include polyethylene terephthalate (PET), polyether nitrile (PEN), polyimide (PI), polyamide imide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN). ), Polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), and polystyrene (PS). Such a non-conductive polymer material may have excellent potential resistance or solvent resistance.

  Moreover, you may use a conductive polymer material with the said elastomer and polyolefin resin. Examples of the conductive polymer material include polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylene vinylene, polyacrylonitrile, and polyoxadiazole. Since such a conductive polymer material has sufficient conductivity without adding a conductive filler, it is advantageous in terms of facilitating the manufacturing process or reducing the weight of the current collector.

  As described above, it is preferable that the current collector for a bipolar secondary battery is composed of two layers of an elastomer layer and a polyolefin resin layer because the production process becomes simple. When comprised by these two layers, it is preferable that the thickness of each layer is 1-200 micrometers, and it is more preferable that it is 10-100 micrometers. When the thickness of each layer is in the range of 1 to 200 μm, the resistance in the thickness direction can be suppressed sufficiently low.

  Regarding the relationship between the thicknesses of the layers, the thickness of the polyolefin resin layer is preferably equal to or less than the thickness of the elastomer layer. Specifically, the polyolefin resin layer is preferably 1/2 to 1/10 of the elastomer layer, and more preferably, the polyolefin resin layer is 1/5 to 1/10 of the elastomer layer. When the polyolefin resin layer has a thickness equal to or less than the elastomer layer, the polyolefin resin layer becomes thinner, which is preferable because the resistance of the current collector is reduced. Furthermore, since the elastomer layer becomes thicker, it is possible to improve the elongation strain of the current collector due to the expansion and contraction of the negative electrode active material layer, and the current collector follows the expansion and contraction of the negative electrode active material layer more sensitively. it can.

  Regarding the resistance in the direction perpendicular to the surface of each layer, the resistance of the polyolefin resin layer is preferably larger than the resistance of the elastomer layer. By making the resistance in the perpendicular direction of the polyolefin resin layer greater than that of the elastomer layer, reaction at the interface where the decomposition reaction occurs on the positive electrode active material layer side of the current collector is less likely to occur, and the current to the positive electrode potential of the current collector is reduced. Resistance can be improved. Specifically, the resistance of the polyolefin resin layer is preferably 2 to 10 times that of the elastomer layer, and more preferably, the resistance of the polyolefin resin layer is 5 to 10 times that of the elastomer layer.

Further, as a whole collector, the resistance of the orthogonal directions, preferably at 1000M · cm ² or less, more preferably 100~300mΩ · cm ^2. Further, the planar direction of the resistance of the current collector is preferably 10 [Omega · cm ² or more, more preferably 100~1000Ω · cm ^2.

  The single-layer bipolar secondary battery current collector or the polyolefin resin layer and the elastomer layer can each be produced by a conventionally known method. For example, it can be manufactured by using a spray method or a coating method. Specifically, a method of preparing a slurry containing materials such as a resin, a conductive material, and an additive, and applying and curing the slurry is exemplified. Alternatively, it can be obtained by mixing the materials by a conventionally known mixing method and molding the obtained mixture into a film. Further, for example, as in the method described in JP-A-2006-190649, the current collector or each layer constituting the current collector may be produced by an inkjet method.

As a method for laminating the polyolefin resin layer and the elastomer layer, it is possible to use an appropriate combination of existing resin thin film deposition techniques, lamination techniques, and lamination techniques. That is, a method including laminating a polyolefin resin layer and an elastomer layer by thermocompression bonding, or applying a slurry containing a polymer material or a conductive polymer material in which a conductive material is added to the surface of the ion blocking layer, and drying. Alternatively, a method of forming a resin layer may be used. Preferably, heat fusion is performed at 120 to 160 ° C. A current collector having a laminated structure of three or more layers can be produced by repeating these methods. However, when an adhesive is used, the resistance in the direction perpendicular to the surface of the current collector prepared by bonding is preferably 1 Ω · cm ² or less.

  As described above, the bipolar secondary battery current collector composed of two layers has been described, and if the physical or chemical function as described above can be achieved on the negative electrode active material side and the positive electrode active material layer side, The current collector may be composed of a single layer. As long as the side of the current collector that is in contact with the negative electrode active material layer contains an elastomer to the extent that it follows the expansion and contraction of the negative electrode active material layer, it may contain other resins. Similarly, the positive electrode active material layer side may also contain other resins as long as the polyolefin resin is contained to such an extent that the decomposition reaction can be suppressed. The elastomer and the polyolefin resin may have opposite concentration gradients in the thickness direction of the current collector. Further, when the current collector includes the elastomer layer and the polyolefin resin layer as described above, one or more intermediate layers may be included therebetween.

The effects brought about by the bipolar secondary battery current collector of the present embodiment will be described.
(A) By disposing the elastomer layer on the negative electrode active material layer side of the current collector, the current collector can follow the expansion and contraction of the negative electrode during charge and discharge, and the peeling of the negative electrode active material layer can be suppressed.
(B) By disposing the polyolefin resin layer on the side of the current collector that is in contact with the positive electrode active material, the resin current collector does not decompose even at the positive electrode potential and exists stably.
(C) By making the resistance in the perpendicular direction of the polyolefin resin layer greater than that of the elastomer layer, reaction at the interface where the decomposition reaction occurs is less likely to occur, and the resistance of the positive electrode potential can be improved.
(D) When the thickness of the polyolefin resin layer is thinner than the thickness of the elastomer layer, the resistance is lowered and the battery characteristics can be improved. Further, by making the elastomer layer thicker, the resin current collector becomes easier to follow due to the expansion and contraction of the negative electrode active material layer.

  The bipolar lithium ion secondary battery described above is characterized by the structure of the current collector. Hereinafter, other main components will be described.

(Active material layer)
[Positive electrode active material layer and negative electrode active material layer]
The active material layer contains an active material, and further contains other additives as necessary.

The positive electrode active material layer includes a positive electrode active material. As the positive electrode active material, for example, LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , Li (Ni—Co—Mn) O _2, and lithium-such as those in which a part of these transition metals are substituted with other elements Examples include transition metal composite oxides, lithium-transition metal phosphate compounds, and lithium-transition metal sulfate compounds. In some cases, two or more positive electrode active materials may be used in combination. Preferably, a lithium-transition metal composite oxide is used as the positive electrode active material from the viewpoint of capacity and output characteristics. Of course, positive electrode active materials other than those described above may be used.

The negative electrode active material layer includes a negative electrode active material. Examples of the negative electrode active material include carbon materials such as graphite, soft carbon, and hard carbon, lithium-transition metal composite oxides (for example, Li ₄ Ti ₅ O ₁₂ ), metal materials, lithium alloy negative electrode materials, and the like. . In some cases, two or more negative electrode active materials may be used in combination. Preferably, from the viewpoint of capacity and output characteristics, a carbon material or a lithium-transition metal composite oxide is used as the negative electrode active material. Of course, negative electrode active materials other than those described above may be used.

  The average particle diameter of each active material contained in each active material layer is not particularly limited, but is preferably 1 to 20 μm from the viewpoint of increasing the output.

  The positive electrode active material layer and the negative electrode active material layer may include a binder. Although it does not specifically limit as a binder used for an active material layer, For example, the following materials are mentioned. Polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile (PEN), polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethyl cellulose (CMC), ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR) ), Isoprene rubber, butadiene rubber, ethylene / propylene rubber, ethylene / propylene / diene copolymer, styrene / butadiene / styrene block copolymer and hydrogenated product thereof, styrene / isoprene / styrene block copolymer and hydrogenated product thereof Thermoplastic polymers such as products, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene / hexafluoropropylene copolymer (FE) ), Tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / chlorotrifluoroethylene copolymer (ECTFE) , Fluororesin such as polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-) TFE fluorine rubber), vinylidene fluoride-pentafluoropropylene fluorine rubber (VDF-PFP fluorine rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene Fluoro rubber (VDF-PFP-TFE fluoro rubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluoro rubber (VDF-PFMVE-TFE fluoro rubber), vinylidene fluoride-chlorotrifluoroethylene fluoro rubber Examples thereof include vinylidene fluoride-based fluororubber such as (VDF-CTFE-based fluororubber), an epoxy resin, and the like. Among these, polyvinylidene fluoride, polyimide, styrene / butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable. These suitable binders are excellent in heat resistance, have a very wide potential window, are stable at both the positive electrode potential and the negative electrode potential, and can be used for the active material layer. These binders may be used alone or in combination of two or more.

  The amount of the binder contained in the active material layer is not particularly limited as long as it is an amount capable of binding the active material, but is preferably 0.5 to 15% by mass with respect to the active material layer. Yes, more preferably 1 to 10% by mass.

  Examples of other additives that can be included in the active material layer include a conductive additive, an electrolyte salt (lithium salt), and an ion conductive polymer.

  The conductive assistant refers to an additive that is blended in order to improve the conductivity of the positive electrode active material layer or the negative electrode active material layer. Examples of the conductive assistant include carbon materials such as carbon black such as acetylene black, graphite, and vapor grown carbon fiber. When the active material layer contains a conductive additive, an electronic network inside the active material layer is effectively formed, which can contribute to improvement of the output characteristics of the battery.

Examples of the electrolyte salt (lithium salt) include Li (C ₂ F ₅ SO ₂ ) ₂ N, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO _{3 and the} like.

  Examples of the ion conductive polymer include polyethylene oxide (PEO) -based and polypropylene oxide (PPO) -based polymers.

  The compounding ratio of the components contained in the positive electrode active material layer and the negative electrode active material layer is not particularly limited. The mixing ratio can be adjusted by appropriately referring to known knowledge about the non-aqueous solvent secondary battery. The thickness of each active material layer is not particularly limited, and conventionally known knowledge about the battery can be appropriately referred to. For example, the thickness of each active material layer is about 2 to 100 μm.

(Electrolyte layer)
As the electrolyte constituting the electrolyte layer, a liquid electrolyte or a polymer electrolyte can be used. The liquid electrolyte has a form in which a lithium salt as a supporting salt is dissolved in an organic solvent as a plasticizer. Examples of the organic solvent that can be used as the plasticizer include carbonates such as ethylene carbonate (EC) and propylene carbonate (PC). Further, as the supporting salt (lithium salt), a compound that can be added to the active material layer of the electrode, such as LiBETI, can be similarly employed.

  On the other hand, the polymer electrolyte is classified into a gel electrolyte containing an electrolytic solution and an intrinsic polymer electrolyte containing no electrolytic solution.

  The gel electrolyte has a configuration in which the above liquid electrolyte is injected into a matrix polymer made of an ion conductive polymer. Examples of the ion conductive polymer used as the matrix polymer include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof. In such polyalkylene oxide polymers, electrolyte salts such as lithium salts can be well dissolved.

  In addition, when an electrolyte layer is comprised from a liquid electrolyte or a gel electrolyte, you may use a separator for an electrolyte layer. Specific examples of the separator include a microporous film made of polyolefin such as polyethylene or polypropylene.

  The intrinsic polymer electrolyte has a structure in which a supporting salt (lithium salt) is dissolved in the matrix polymer, and does not include an organic solvent that is a plasticizer. Therefore, when the electrolyte layer is composed of an intrinsic polymer electrolyte, there is no fear of liquid leakage from the battery, and the reliability of the battery can be improved.

  The matrix polymer of the gel electrolyte or the intrinsic polymer electrolyte can exhibit excellent mechanical strength by forming a crosslinked structure. In order to form a crosslinked structure, thermal polymerization, ultraviolet polymerization, radiation polymerization, electron beam polymerization, etc. are performed on a polymerizable polymer (for example, PEO or PPO) for forming a polymer electrolyte using an appropriate polymerization initiator. A polymerization treatment may be performed.

(Outermost layer current collector)
As the material of the outermost layer current collector, for example, a metal or a conductive polymer can be adopted. From the viewpoint of ease of taking out electricity, a metal material is preferably used. Specifically, metal materials, such as aluminum, nickel, iron, stainless steel, titanium, copper, are mentioned, for example. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or a plating material of a combination of these metals can be preferably used. Moreover, the foil by which aluminum is coat | covered on the metal surface may be sufficient. Of these, aluminum and copper are preferable from the viewpoints of electron conductivity and battery operating potential.

(Tabs and leads)
A tab may be used for the purpose of taking out the current outside the battery. The tab is electrically connected to the outermost layer current collector or current collector plate, and is taken out of the laminate sheet which is a battery exterior material.

  The material which comprises a tab in particular is not restrict | limited, The well-known highly electroconductive material conventionally used as a tab for lithium ion secondary batteries can be used. As a constituent material of a tab, metal materials, such as aluminum, copper, titanium, nickel, stainless steel (SUS), these alloys, are preferable, for example. More preferred are aluminum, copper and the like from the viewpoints of light weight, corrosion resistance, and high conductivity. Note that the same material may be used for the positive electrode tab and the negative electrode tab, or different materials may be used.

  The positive terminal lead and the negative terminal lead are also used as necessary. As the material of the positive terminal lead and the negative terminal lead, a terminal lead used in a known lithium ion secondary battery can be used. In addition, the parts removed from the battery exterior material should be heat-insulating so that they do not affect products (for example, automobile parts, especially electronic devices) by touching peripheral devices or wiring and causing leakage. It is preferable to coat with a heat shrinkable tube or the like.

(Battery exterior material)
As the battery exterior material, a known metal can case can be used, and a bag-like case using a laminate film containing aluminum that can cover the power generation element can be used. As the laminate film, for example, a laminate film having a three-layer structure in which PP, aluminum, and nylon (registered trademark) are laminated in this order can be used, but the laminate film is not limited thereto. A laminate film is desirable from the viewpoint that it is excellent in high output and cooling performance, and can be suitably used for a battery for large equipment for EV and HEV.

(Insulation part)
The insulating part prevents liquid junction due to leakage of the electrolyte from the electrolyte layer. In addition, the insulating portion is provided for the purpose of preventing the adjacent current collectors in the battery from contacting each other and the occurrence of a short circuit due to a slight irregularity of the end of the cell layer in the power generation element. .

  As the material constituting the insulating portion, as long as it has insulating properties, sealing performance against falling off of solid electrolyte, sealing performance against moisture permeation from the outside (sealing performance), heat resistance under battery operating temperature, etc. Good. For example, urethane resin, epoxy resin, polyethylene resin, polypropylene resin, polyimide resin, rubber and the like can be used. Among these, polyethylene resin and polypropylene resin are preferably used as the constituent material of the insulating portion from the viewpoints of corrosion resistance, chemical resistance, ease of production (film forming property), economy, and the like.

  In addition, said bipolar lithium ion secondary battery can be manufactured by a conventionally well-known manufacturing method.

<Bipolar electrode>
As described above, the bipolar electrode has the structure shown in FIG. That is, the elastomer layer is disposed on the negative electrode active material layer side of the current collector, and the polyolefin resin layer is disposed on the positive electrode active material layer side. Thereby, when used in a bipolar secondary battery, peeling of the negative electrode during charge / discharge is prevented, and decomposition of the resin current collector at the positive electrode potential is also suppressed. Therefore, a battery with improved cycle characteristics can be provided.
(E) Since the bipolar electrode of this embodiment suppresses peeling of the negative electrode active material during charging and discharging and prevents the current collector from being decomposed at the positive electrode potential, it can constitute a bipolar battery having excellent cycle characteristics. .

<Appearance structure of bipolar lithium ion secondary battery>
FIG. 3 is a perspective view showing an appearance of a stacked flat bipolar lithium ion secondary battery, which is a typical form of the bipolar battery. As shown in FIG. 3, the stacked flat lithium ion secondary battery 50 has a rectangular flat shape, and a positive electrode tab 58 and a negative electrode tab 59 for taking out electric power from both sides thereof. Has been pulled out. The power generation element 57 is encased by the battery outer packaging material 52 of the lithium ion secondary battery 50, and the periphery thereof is heat-sealed. The power generation element 57 is sealed with the positive electrode tab 58 and the negative electrode tab 59 pulled out to the outside. Has been. Here, the power generation element 57 corresponds to the power generation element 21 of the bipolar lithium ion secondary battery 10 shown in FIG. 1 described above, and includes the positive electrode active material layer 13, the electrolyte layer 17, and the negative electrode active material layer. A plurality of single battery layers (single cells) 19 composed of 15 are stacked.

  The lithium ion battery is not limited to a stacked flat shape, and a wound lithium ion battery may have a cylindrical shape. Further, such a cylindrical shape may be deformed to form a rectangular flat shape. In the said cylindrical shape thing, a laminate film may be used for the exterior material, and the conventional cylindrical can (metal can) may be used, for example, It does not restrict | limit. Preferably, the power generation element is covered with an aluminum laminate film. With this configuration, weight reduction can be achieved.

  3 is not particularly limited, and the positive electrode tab 58 and the negative electrode tab 59 may be pulled out from the same side, or the positive electrode tab 58 and the negative electrode tab 59 may be pulled out. May be divided into a plurality of parts and taken out from each side. Further, in a wound type lithium ion battery, instead of a tab, for example, a terminal may be formed using a cylindrical can (metal can).

The lithium ion battery is suitable as a power source for driving a vehicle or an auxiliary power source that requires a high volume energy density and a high volume output density as a large-capacity power source for electric vehicles, hybrid electric vehicles, fuel cell vehicles, hybrid fuel cell vehicles, Can be used.
(F) The bipolar lithium ion secondary battery of the present embodiment is a bipolar lithium ion excellent in cycle characteristics because peeling of the negative electrode active material is suppressed during charging and discharging and decomposition of the current collector at the positive electrode potential is prevented. It becomes an ion secondary battery.

  Hereinafter, a current collector for a bipolar secondary battery will be described through examples.

Example 1
In Example 1, a current collector for a bipolar secondary battery was produced as a two-layer structure of a polyolefin resin layer and an elastomer layer.

  Polyethylene as the polyolefin resin and ketjen black as the conductive material are kneaded at 5% by mass with respect to the total amount of polyethylene and ketjen black and extruded to a polyolefin resin layer having a thickness of 20 μm (volume resistivity 10 mΩ · m, in the direction perpendicular to the surface) Resistance 0.02Ω). Ethylene propylene rubber as the elastomer, Ketjen black as the conductive material, 7% by mass with respect to the total amount of ethylene propylene rubber and Ketjen black, and an elastomer layer with a thickness of 80 μm (volume resistivity 1 mΩ · m , Resistance in the direction perpendicular to the surface is 0.008Ω). The polypropylene layer and the ethylene propylene rubber layer were heated to 120 to 160 ° C. and bonded together by heat fusion to complete the current collector. The resistance of the current collector was 0.028Ω (excluding the contact resistance with the electrode).

(Comparative Example 1)
In Comparative Example 1, a single-layer current collector was produced as follows. Ethylene propylene rubber and ketjen black as a conductive material are mixed in an amount of 7% by mass with respect to the total amount of ethylene propylene rubber and ketjen black, and a 100 μm thick current collector (volume resistivity 1 mΩ · m , Resistance in the direction perpendicular to the surface is 0.01Ω).

(Potential test)
With respect to the current collectors of Example 1 and Comparative Example 1 manufactured as described above, a potential resistance test was performed as follows. A triode type glass cell was used, lithium was used as a counter electrode, lithium was used as a reference electrode, current collectors of Example 1 and Comparative Example 1 were used as working electrodes, lead wires were attached and immersed in an electrolyte solution. A constant voltage of 4.2 V was applied to this system, and the current value flowing at that time was measured. The measurement results are shown in Table 1 below. A 1M LiPF ₆ solution was used as the electrolyte. The working electrode had a geometric surface area of 1.0 cm ² .

(Comparison)
From the results shown in Table 1, it was found that the capacity (current amount mAh) was reduced to about 1/5 by providing the polyolefin resin layer on the positive electrode side. This indicates that the resin decomposition due to charging / discharging of the current collector of Example 1 was suppressed as compared with the current collector of Comparative Example 1. This is because electrons move along with the decomposition reaction of the resin, so that when the decomposition occurs, the current value and the amount of current increase. The polyolefin resin has a higher reaction activation energy than the elastomer, and it can be seen from this result that the resin itself is not easily decomposed.

(Example 2)
In Example 2, a bipolar secondary battery was manufactured using a current collector produced in the same manner as in Example 1, and a cycle test described later was performed.

[electrode]
Using acetylene black (AB) (conducting aid), polyvinylidene fluoride (PVDF) (binder), lithium nickel oxide-based active material (LiNiO ₂ ) having an average particle size of 5 μm and N-methyl-2-pyrrolidone (NMP) Thus, a slurry for the positive electrode active material layer was produced. The compounding ratio of each component was lithium nickel oxide based active material: AB: PVDF = 86: 6: 8 (mass ratio). Moreover, the slurry for negative electrode active material layers was produced using acetylene black (AB), polyvinylidene fluoride (PVdF), hard carbon, and NMP. The compounding ratio of each component was set to hard carbon: AB: PVDF = 85: 5: 10 (mass ratio). The prepared slurry for positive electrode active material layer was applied on the polyolefin resin layer of the current collector for a bipolar secondary battery and dried to form a positive electrode active material layer having a thickness of 30 μm. Also, a negative electrode active material layer slurry was applied on the elastomer layer of the current collector and dried to form a negative electrode active material layer having a thickness of 20 μm. In this way, a laminate in which the positive electrode active material layer was formed on the polyolefin resin layer of the current collector and the negative electrode active material layer was formed on the elastomer layer was obtained.

[Production of battery]
The obtained positive electrode active material layer / current collector / negative electrode active material layer laminate was cut into a size of 160 × 130 mm. Next, the positive electrode active material layer and the negative electrode active material layer formed on the outer peripheral portion of 10 mm from the outer edge were peeled off to expose the surfaces of the polyolefin resin layer and the elastomer layer as current collectors. As a result, a bipolar electrode in which each active material layer surface was 140 × 110 mm and the current collector on the outer peripheral portion 10 mm from the outer edge was exposed was obtained.

  A sheet for sealing (a three-layer structure in which PP / PEN / PP is laminated) is applied to the exposed portion (active material uncoated portion) of the polyolefin resin layer on the surface where the positive electrode active material layer is formed of the obtained bipolar electrode. Arranged. Next, a 170 × 140 mm resin separator (a microporous membrane manufactured by Ube Industries) was disposed so as to cover all the bipolar electrodes on the side where the positive electrode active material layer was formed. Then, the sheet | seat for sealing was arrange | positioned on the separator corresponding to an active material non-application part, and the negative electrode active material layer surface of the bipolar electrode was piled up from it. Thus, a bipolar electrode (negative electrode active material layer / current collector / positive electrode active material layer) / separator / bipolar electrode (negative electrode active material layer / current collector / positive electrode active material layer)... Separator / bipolar The stacking was performed in the order of the type electrode (negative electrode active material layer / current collector / positive electrode active material layer). Finally, 13 bipolar electrodes were laminated through a separator and sealed to produce a bipolar secondary battery structure in which 12 cell layers were laminated.

As an electrolytic solution, a solution in which LiPF _{6 as a} lithium salt was dissolved at a concentration of 1.0 M in an equal volume mixed solution of ethylene carbonate (EC) and propylene carbonate (PC) was prepared. And the electrolyte layer was formed by impregnating said bipolar secondary battery structure in electrolyte solution, and making it hold | maintain on the separator distribute | arranged between bipolar electrodes.

  After forming the electrolyte layer in this manner, four bipolar secondary battery structures are stacked in series, sandwiched between aluminum tabs for current extraction, and vacuum sealed using an aluminum laminate film as an exterior material, 48 series bipolar lithium ion secondary batteries were produced.

(Comparative Example 2)
A bipolar lithium ion secondary battery was produced in the same manner as in Example 2 except that a current collector produced by the same method as in Comparative Example 1 was used as the current collector.

(Cycle test)
The bipolar lithium ion secondary batteries of Example 2 and Comparative Example 2 manufactured by the above method were charged to 200 V by a constant current method (CC, current: 5C) in an atmosphere at 25 ° C. Then, after resting for 10 minutes, it was discharged to 120 V with a constant current (CC, current: 5C), and rested for 10 minutes after the discharge. This charge / discharge process was defined as one cycle, a 20-cycle charge / discharge test was performed, and the discharge capacity retention rate was examined. The results are shown in Table 2 below. In Table 2, “discharge capacity maintenance ratio” represents the ratio of the discharge capacity at the first, fifth, tenth, and 20th cycles to the discharge capacity at the first cycle (percentage display).

  As can be seen from the results in Table 2, in Comparative Example 2, the capacity retention rate decreased to nearly ½ after 20 cycles, but in Example 2, it was maintained at a high rate of 81.4%. This indicates that the use of the current collector of the present invention suppressed the peeling of the negative electrode active material layer and the decomposition of the resin on the positive electrode active material layer side. In Comparative Example 2, the resin on the positive electrode active material layer side is decomposed, and the decomposition current flows, so that the filler in the resin is desorbed, and accordingly, the electrode on the positive electrode side is also peeled off. As a result, a decrease in battery performance was observed.

2 Polyolefin resin layer,
3 Elastomer layer,
10, 50 Bipolar lithium ion secondary battery,
11 Current collector,
11a The outermost layer current collector on the positive electrode side,
11b The outermost layer current collector on the negative electrode side,
13 positive electrode active material layer,
15 negative electrode active material layer,
17 electrolyte layer,
19 cell layer,
21, 57 power generation element,
23 Bipolar electrode,
25 positive current collector,
27 negative current collector,
29, 52 Battery exterior material,
31 insulation,
58 positive electrode tab,
59 Negative electrode tab.

Claims (7)

A current collector for a bipolar secondary battery comprising an elastomer and a polyolefin resin,
A current collector for a bipolar secondary battery in which the content of the elastomer on one side is larger than the content of the elastomer on the other side.   The current collector for a bipolar secondary battery according to claim 1, wherein an elastomer layer containing an elastomer and a conductive material is present on the surface on the one surface side.   The current collector for a bipolar secondary battery according to claim 1, wherein a polyolefin resin layer containing a polyolefin resin and a conductive material is present on the surface on the other surface side.   The current collector for a bipolar secondary battery according to claim 3, wherein a resistance in a direction perpendicular to the plane of the polyolefin resin layer is larger than that of the elastomer layer.   The current collector for a bipolar secondary battery according to claim 3 or 4, wherein a thickness of the polyolefin resin layer is equal to or less than a thickness of the elastomer layer.   A current collector for a bipolar secondary battery according to any one of claims 1 to 5, wherein a negative electrode active material layer is formed on the one surface, and a positive electrode active material layer is formed on the other surface. The formed bipolar electrode.   A bipolar battery using the bipolar electrode according to claim 6.

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