JP2014086179A - Positive electrode for lithium ion secondary battery - Google Patents

Abstract

A means for improving the life of a lithium ion secondary battery is provided.
A positive electrode for a lithium ion secondary battery having a current collector and a positive electrode active material layer formed on the current collector, wherein the positive electrode active material layer is a tap as a positive electrode active material. The ratio of the density to the average particle diameter of the secondary particles is tap density (g / ml) / average particle diameter of the secondary particles (μm) = 0.07 to 0.45, and the true specific gravity is 4.19. The positive electrode for lithium ion secondary batteries containing the lithium manganate which is -4.25.
[Selection figure] None

Description

  The present invention relates to a positive electrode for a lithium ion secondary battery.

  In recent years, in order to cope with global warming, reduction of the amount of carbon dioxide is eagerly desired. In the automobile industry, there is a great expectation for reducing carbon dioxide emissions by introducing electric vehicles (EV) and hybrid electric vehicles (HEV). Electric devices such as secondary batteries for motor drive that hold the key to their practical application. Is being actively developed.

  As a secondary battery for driving a motor, a lithium ion secondary battery having the highest theoretical energy among all the batteries is attracting attention, and is currently being developed rapidly. Generally, a lithium ion secondary battery has a configuration in which a positive electrode and a negative electrode having an active material layer in which an active material or the like is applied to a current collector together with a binder are connected via an electrolyte layer and stored in a battery case. doing.

  The non-aqueous electrolyte secondary battery used as a motor drive power source for automobiles and the like as described above has extremely high output characteristics compared to consumer non-aqueous electrolyte secondary batteries used in mobile phones, notebook computers, etc. It is required to have. At present, research and development is underway to meet such demands.

Patent document 1 is disclosing the positive electrode active material for nonaqueous electrolyte secondary batteries which consists of lithium manganate particle | grains. The average primary particle size of the lithium manganate particles is 3.0 to 20.0 μm, the average secondary particle size D ₅₀ is 2.5 to 40.0 μm, and the average primary particle size and the average secondary particle The ratio with the diameter is 0.5 to 1.2. By using lithium manganate particles having such properties, a nonaqueous electrolyte secondary battery having excellent cycle characteristics can be obtained.

JP 2003-272629 A

  However, the positive electrode active material described in Patent Document 1 has a problem in that secondary particles collapse due to elution of manganese from the active material, and the life of the battery is deteriorated.

  Then, an object of this invention is to provide the means to improve the lifetime of a lithium ion secondary battery.

  The inventors of the present invention have accumulated extensive research. As a result, it has been found that the above problem can be solved by setting the ratio of the tap density of lithium manganate contained in the positive electrode active material to the average particle diameter of the secondary particles and the true density within a predetermined range.

  Since the bond between primary particles of lithium manganate is strong, the secondary particles can be prevented from collapsing due to elution of manganese, and the life of the lithium ion secondary battery is improved.

1 is a schematic cross-sectional view showing a basic configuration of a non-aqueous electrolyte lithium ion secondary battery that is not a flat type (stacked type) bipolar type, which is an embodiment of a lithium ion secondary battery. It is the cross-sectional schematic which shows the basic composition of the bipolar lithium ion secondary battery which is other embodiment of a lithium ion secondary battery. It is the perspective view showing the external appearance of the flat lithium ion secondary battery which is another one Embodiment of a lithium ion secondary battery.

  First, although a nonaqueous electrolyte lithium ion secondary battery will be described as a preferred embodiment of a lithium ion secondary battery, it is not limited to only 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.

  There is no particular limitation when distinguished by the electrolyte form of the lithium ion secondary 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.

  FIG. 1 is a schematic cross-sectional view schematically showing a basic configuration of a non-aqueous electrolyte lithium ion secondary battery (hereinafter also simply referred to as “stacked battery”) that is not a flat (stacked) bipolar type. As shown in FIG. 1, the stacked battery 10 a of this embodiment 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 that is an exterior body. Have. Here, the power generation element 21 has a configuration in which a positive electrode, an electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which the positive electrode active material layer 13 is disposed on both surfaces of the positive electrode current collector 11. The negative electrode has a structure in which the negative electrode active material layers 15 are disposed on both surfaces of the negative electrode current collector 12. Specifically, the negative electrode, the electrolyte layer, and the positive electrode are laminated in this order so that one positive electrode active material layer 13 and the negative electrode active material layer 15 adjacent thereto face each other with the electrolyte layer 17 therebetween. . Thereby, the adjacent positive electrode, electrolyte layer, and negative electrode constitute one unit cell layer 19. Therefore, it can be said that the stacked battery 10a shown in FIG. 1 has a configuration in which a plurality of single battery layers 19 are stacked and electrically connected in parallel.

  In addition, although the positive electrode active material layer 13 is arrange | positioned only at one side in the outermost layer positive electrode collector located in both outermost layers of the electric power generation element 21, an active material layer may be provided in both surfaces. That is, instead of using a current collector dedicated to the outermost layer provided with an active material layer only on one side, a current collector having an active material layer on both sides may be used as it is as an outermost current collector. In addition, the arrangement of the positive electrode and the negative electrode is reversed from that in FIG. 1, so that the outermost layer negative electrode current collector is positioned on both outermost layers of the power generation element 21, and the outermost layer negative electrode current collector is disposed on one or both surfaces. A negative electrode active material layer may be disposed.

  The positive electrode current collector 11 and the negative electrode current collector 12 are respectively attached with a positive electrode current collector plate 25 and a negative electrode current collector plate 27 that are electrically connected to the respective electrodes (positive electrode and negative electrode), and are sandwiched between end portions of the battery exterior material 29. Thus, it has a structure led out of the battery exterior material 29. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 are ultrasonically welded to the positive electrode current collector 11 and the negative electrode current collector 12 of each electrode, respectively, via a positive electrode lead and a negative electrode lead (not shown) as necessary. Or resistance welding or the like.

  FIG. 2 is a schematic cross-sectional view schematically showing the basic configuration of a bipolar nonaqueous electrolyte lithium ion secondary battery (hereinafter also simply referred to as “bipolar battery”) 10b. The bipolar battery 10b shown in FIG. 2 has a structure in which a substantially rectangular power generation element 21 in which a charge / discharge reaction actually proceeds is sealed inside a laminate film 29 that is a battery exterior material.

  As shown in FIG. 2, the power generation element 21 of the bipolar battery 10 b has a positive electrode active material layer 13 that is electrically coupled to one surface of the current collector 11, and is formed on the opposite surface of the current collector 11. It has a plurality of bipolar electrodes 23 in which a negative electrode active material layer 15 that is electrically coupled is formed. 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 battery 10b has a configuration in which the single battery layers 19 are stacked. Further, for the purpose of preventing liquid junction due to leakage of the electrolytic solution from the electrolyte layer 17, a seal portion (insulating layer) 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.

  Furthermore, in the bipolar battery 10b shown in FIG. 2, the positive electrode current collector plate 25 is disposed so as to be adjacent to the outermost layer current collector 11a on the positive electrode side, and this is extended and led out from the laminate film 29 which is a battery exterior material. ing. On the other hand, the negative electrode current collector plate 27 is disposed so as to be adjacent to the outermost layer current collector 11b on the negative electrode side, and similarly, this is extended and led out from the laminate film 29 which is an exterior of the battery.

  In the bipolar battery 10 b shown in FIG. 2, a seal portion 31 is usually provided around each single cell layer 19. The purpose of the seal portion 31 is to prevent 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. By installing such a seal portion 31, long-term reliability and safety can be ensured, and a high-quality bipolar battery 10b can be provided.

  Note that the number of stacks of the unit cell layers 19 is adjusted according to a desired voltage. Further, in the bipolar battery 10b, the number of stacks of the single battery layers 19 may be reduced if a sufficient output can be ensured even if the battery is made as thin as possible. Even in the bipolar battery 10b, it is necessary to prevent external impact and environmental degradation during use. Therefore, the power generation element 21 is preferably sealed in a laminate film 29 that is a battery exterior material, and the positive electrode current collector plate 25 and the negative electrode current collector plate 27 are taken out of the laminate film 29.

  In addition, in this specification, when describing as “current collector”, it may indicate all of the positive electrode current collector, the negative electrode current collector, and the bipolar battery current collector, or may refer to only one. is there. Similarly, when describing as "active material layer", both a positive electrode active material layer and a negative electrode active material layer may be pointed out, and only one side may be pointed out. Similarly, when describing as "active material", both a positive electrode active material and a negative electrode active material may be pointed out, and only one side may be pointed out.

  In Patent Document 1, a non-aqueous electrolyte secondary battery comprising lithium manganate particles having an average primary particle diameter, an average secondary particle diameter, and a ratio of the average primary particle diameter to the average secondary particle diameter in a specific range. A positive electrode active material is disclosed. However, the lithium manganate particles having such properties have weak bonds between primary particles and gaps, and manganese is eluted from the gaps when charging and discharging are repeated. Therefore, there is a problem that the primary particles of lithium manganate slip off, the secondary particles collapse, and the life of the secondary battery deteriorates.

  On the other hand, in the positive electrode for a lithium ion secondary battery of this embodiment, the ratio between the tap density (g / ml) of lithium manganate contained in the positive electrode active material and the average particle diameter (μm) of secondary particles is 0. The range is from .07 to 0.45. The true specific gravity of the lithium manganate is in the range of 4.19 to 4.25. Lithium manganate having such properties has very strong bonds between primary particles and almost no gaps, and manganese elution hardly occurs or does not occur at all. Therefore, the primary particles of lithium manganate are prevented from slipping and the secondary particles of lithium manganate can be prevented from collapsing. And the life of a lithium ion secondary battery having such a positive electrode is improved.

  Hereinafter, the active material layer used for the lithium ion secondary battery will be described in more detail.

[Active material layer]
The positive electrode active material layer or the negative electrode active material layer contains an active material, and if necessary, a conductive aid, a binder, an electrolyte (polymer matrix, ion conductive polymer, electrolyte, etc.), and a lithium salt for enhancing ion conductivity. And other additives.

(Positive electrode active material)
The positive electrode active material layer includes lithium manganate as the positive electrode active material. Lithium manganate is a composite oxide composed of lithium, manganese, and oxygen. Specifically, for example, lithium manganate having a spinel structure such as LiMn ₂ O ₄ , LiMnO _2, and Li ₂ MnO ₃ can be mentioned. it can. Further, as the positive electrode active material, not only the above lithium manganate, but also a material in which some of the manganese atoms in these materials are substituted with other metal atoms may be included. At this time, the atom replacing the manganese atom is an atom of an element selected from lithium, aluminum, magnesium, cobalt, chromium and the like. Among the lithium manganates, those having a spinel structure such as LiMn ₂ O ₄ are preferable. Further, for example, lithium cobalt oxide such as LiCoO ₂ , lithium nickel oxide such as LiNiO ₂ , lithium iron oxide such as LiFeO ₂ , lithium iron phosphate such as LiFePO _4, and the like may be used in combination with lithium manganate. .

  Among these, Preferably, it is at least 1 sort (s) selected from the group which consists of lithium cobaltate, lithium nickelate, and lithium iron phosphate. By using these active materials, the life and safety of the battery are further improved.

When the positive electrode active material other than lithium manganate is used, the total amount of the positive electrode active material is 100% by weight, preferably 10 to 70% by weight, more preferably 15 to 50% by weight. The ratio of the tap density of lithium acid to the average particle diameter of secondary particles is tap density (g / ml) / average particle diameter of secondary particles (μm) = 0.07 to 0.45. The ratio between the tap density and the average particle diameter of the secondary particles is preferably 0.08 to 0.35.

  The tap density of the lithium manganate is preferably 1.6 to 2.7 g / ml, and more preferably 1.7 to 2.6 g / ml. If it is this range, the decay | disintegration of the lithium manganate secondary particle accompanying manganese elution can be prevented further. The tap density can be measured by the method described in the examples.

  The average particle diameter of the secondary particles of lithium manganate is preferably 6 to 24 μm, and more preferably 7 to 20 μm. If it is this range, the decay | disintegration of the lithium manganate secondary particle accompanying manganese elution can be prevented further. The average particle diameter of the secondary particles of lithium manganate can be measured by the method described in the examples.

  The true density of the lithium manganate is 4.19 to 4.25. This range of true density means that impurities such as metal manganese are hardly contained or not contained at all. When the true density is within this range, the above effect can be obtained efficiently. The true density is preferably 4.20 to 4.24. The true density can be measured by the method described in the examples.

  Moreover, it is preferable that the average particle diameter of the primary particle of this lithium manganate is 1-5 micrometers, and it is more preferable that it is 1.5-4.5 micrometers. When the average particle diameter of the primary particles is 1 μm or more, side reactions of charge / discharge reactions accompanying an increase in the specific surface area of the active material can be suppressed. If it is 5 μm or less, there is little or no extreme delay in lithium diffusion in the active material particles, and the life of the lithium ion secondary battery is improved. In addition, the average particle diameter of the primary particles of the lithium manganate can be measured by the method described in Examples.

  As a method of producing lithium manganate having the above properties, a method of firing lithium manganate using a firing furnace or the like is preferable. This is because it is possible to efficiently obtain lithium manganate having the above properties in which the bonds between the primary particles are enhanced. By controlling the conditions of this firing treatment, the average particle diameter of primary particles of lithium manganate, the ratio of the tap density to the average particle diameter of secondary particles, and the true density can be set to desired values.

  The firing treatment of the lithium manganate is preferably performed in two stages. This is because lithium manganate having the above properties can be obtained more efficiently by the two-stage baking treatment. In the two-stage baking process, it is preferable that the baking temperature of the first-stage baking process is 700 to 900 ° C., and the baking temperature of the second-stage baking process is more than 1100 ° C. and less than 1300 ° C. If it is such a calcination temperature, the lithium manganate which has the above desired properties will be obtained efficiently.

  The firing time is not particularly limited, but the first stage firing time is preferably 4 to 10 hours, and the second stage firing time is preferably 4 to 8 hours.

(Negative electrode active material)
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 (graphite), soft carbon, and hard carbon, lithium transition metal composite oxides such as lithium titanium composite oxide, metal materials, and lithium alloy negative electrode materials.

  The average particle diameter of the negative electrode active material contained in the negative electrode active material layer is not particularly limited, but is preferably 1 to 100 μm, more preferably 1 to 20 μm from the viewpoint of increasing the output.

  The “particle diameter” of the negative electrode active material means the maximum distance L among the distances between any two points on the particle outline. As the value of the “average particle size” of the negative electrode active material, the particle size of particles observed in several to several tens of fields using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The value calculated as the average value of is assumed to be adopted.

  The positive electrode active material layer and the negative electrode active material layer can 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, 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 its hydrogenated product, styrene / isoprene / styrene block copolymer and its hydrogenated product, etc. Thermoplastic polymer, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), Tet Fluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / chlorotrifluoroethylene copolymer (ECTFE), polyfluorinated Fluorine resin such as vinyl (PVF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluorine) Rubber), vinylidene fluoride-pentafluoropropylene-based fluoro rubber (VDF-PFP-based fluoro rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluoro rubber (VDF-PFP-TFE fluorine rubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluorine rubber (VDF-PFMVE-TFE fluorine rubber), vinylidene fluoride-chlorotrifluoroethylene fluorine rubber (VDF) Vinylidene fluoride fluororubbers such as (-CTFE fluororubber) and epoxy resins. 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 independently and may use 2 or more types together.

  Examples of other additives that can be included in the active material layer include a conductive additive, an electrolyte, 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 auxiliary agent include carbon materials such as carbon black such as acetylene black, graphite, and 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.

  The lithium ion secondary battery described above has the following effects.

  The lithium ion secondary battery includes lithium manganate having a ratio of a tap density to an average particle diameter of secondary particles and a true density in a specific range as a positive electrode active material. Lithium manganate having such properties has very strong bonds between primary particles and almost no gaps, and manganese elution hardly occurs or does not occur at all. Thus, the primary particles of lithium manganate are prevented from slipping, the secondary particles of lithium manganate can be prevented from collapsing, and the life of the lithium ion secondary battery is improved.

  The lithium ion secondary battery described above is characterized in that the ratio between the tap density of the lithium manganate used as the positive electrode active material and the average particle diameter of the secondary particles, and the true density are in a specific range. Hereinafter, other main components will be described.

[Current collector]
There is no particular limitation on the material constituting the current collector, but a metal is preferably used.

  Specifically, examples of the metal include aluminum, nickel, iron, stainless steel, titanium, copper, and other alloys. 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, stainless steel, and copper are preferable, and aluminum is more preferable from the viewpoint of electron conductivity and battery operating potential.

  The size of the current collector is determined according to the intended use of the battery. For example, if it is used for a large battery that requires a high energy density, a current collector having a large area is used. There is no particular limitation on the thickness of the current collector. The thickness of the current collector is usually about 1 to 100 μm.

[Electrolyte layer]
The electrolyte constituting the electrolyte layer has a function as a lithium ion carrier. The electrolyte is not particularly limited as long as it can exhibit such a function, but a liquid electrolyte or a polymer electrolyte is 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 used include carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). As the lithium _{salt, Li (CF 3 SO 2)} 2 N, Li (C 2 F 5 SO 2) 2 N, LiPF 6, LiBF 4, LiClO 4, LiAsF 6, LiTaF such 6, LiCF ₃ SO ₃ A compound that can be added to the active material layer of the electrode can be similarly employed.

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

  The gel polymer electrolyte has a configuration in which the liquid electrolyte is injected into a matrix polymer (host polymer) made of an ion conductive polymer. The use of a gel polymer electrolyte as the electrolyte is superior in that the fluidity of the electrolyte is lost and the ion conductivity between the layers is easily cut off. Examples of the ion conductive polymer used as the matrix polymer (host 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.

  The intrinsic polymer electrolyte has a structure in which a lithium salt is dissolved in the above matrix polymer and does not contain an organic solvent. Therefore, by using an intrinsic polymer electrolyte as the electrolyte, there is no fear of liquid leakage from the battery, and the battery reliability 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.

  These electrolytes may be used alone or in combination of two or more.

  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 a polyolefin such as polyethylene or polypropylene, a hydrocarbon such as polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), glass fiber, or the like.

[Seal part]
The seal part 31 is a member peculiar to the bipolar battery 10 b shown in FIG. 2, and is disposed on the outer peripheral part of the single battery layer 19 for the purpose of preventing leakage of the electrolyte layer 17. In addition to this, it is possible to prevent current collectors adjacent in the battery from coming into contact with each other and a short circuit due to a slight unevenness at the end of the laminated electrode. In the form shown in FIG. 2, the seal portion 31 is sandwiched between the respective current collectors 11 constituting the two adjacent single battery layers 19 so as to penetrate the outer peripheral edge portion of the separator that is the base material of the electrolyte layer 17. Further, it is arranged on the outer peripheral portion of the unit cell layer 19. Examples of the constituent material of the seal portion 31 include polyolefin resins such as polyethylene and polypropylene, epoxy resins, rubber, and polyimide. Of these, polyolefin resins are preferred from the viewpoints of corrosion resistance, chemical resistance, film-forming properties, economy, and the like.

[Positive electrode current collector and negative electrode current collector]
The material which comprises a current collector plate (25, 27) is not restrict | limited in particular, The well-known highly electroconductive material conventionally used as a current collector plate for lithium ion secondary batteries can be used. As a constituent material of the current collector plate, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. Note that the positive electrode current collector plate 25 and the negative electrode current collector plate 27 may be made of the same material or different materials. Further, as shown in FIG. 2, the outermost layer current collector (11a, 11b) may be extended to form a current collector plate, or a separately prepared tab may be connected to the outermost layer current collector.

[Positive lead and negative lead]
Moreover, although illustration is abbreviate | omitted, you may electrically connect between the collector 11 and the current collector plates (25, 27) via a positive electrode lead or a negative electrode lead. As a constituent material of the positive electrode and the negative electrode lead, materials used in known lithium ion secondary batteries can be similarly employed. In addition, heat-shrinkable heat-shrinkable parts are removed from the exterior 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 tube or the like.

[Battery exterior materials]
As the battery exterior material 29, 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. For example, a laminate film having a three-layer structure in which PP, aluminum, and nylon are laminated in this order can be used as the laminate film, 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.

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

[Appearance structure of lithium ion secondary battery]
FIG. 3 is a perspective view showing the appearance of a flat lithium ion secondary battery which is a typical embodiment of the secondary battery.

  As shown in FIG. 3, the 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 are drawn out from both sides thereof. Yes. 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 lithium ion secondary battery 10 shown in FIGS. 1 and 2 described above. The power generation element 57 is formed by laminating a plurality of single battery layers (single cells) 19 including a positive electrode (positive electrode active material layer) 13, an electrolyte layer 17, and a negative electrode (negative electrode active material layer) 15.

  The lithium ion secondary battery is not limited to a stacked flat shape. The wound lithium ion secondary battery may have a cylindrical shape, or may have a shape that is a flattened rectangular shape by deforming such a cylindrical shape. There is no particular limitation. 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.

  Also, the removal of the tabs 58 and 59 shown in FIG. 3 is not particularly limited. The positive electrode tab 58 and the negative electrode tab 59 may be drawn from the same side, or the positive electrode tab 58 and the negative electrode tab 59 may be divided into a plurality of parts and taken out from each side, as shown in FIG. It is not limited to things. 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 secondary battery is used 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 an electric vehicle, a hybrid electric vehicle, a fuel cell vehicle, and a hybrid fuel cell vehicle It can be suitably used.

  Although the said positive electrode for lithium ion secondary batteries is demonstrated still in detail using a following example and a comparative example, it is not necessarily limited only to a following example. The average particle diameter of primary particles of the positive electrode active material, the average particle diameter of secondary particles, the true density, and the tap density were measured by the following methods.

Average particle diameter of primary particles: Measured average particle diameter of 400 primary particles of a cross section of particles using a high performance focused ion beam apparatus (model number: SMI3050R) manufactured by Seiko Instruments Inc. Average particle diameter of secondary particles: Made by Shimadzu Corporation, measured by a laser diffraction particle size distribution analyzer (model number: SALD-3100) True density: Made by Shimadzu Corporation, dry automatic densitometer Measured by Accupic II 1340 Tap density: After 500 taps Measure bulk density.

Example 1
-Production of positive electrode Using a firing furnace, lithium manganate which was fired at 800 ° C for 5 hours in the first stage and then fired at 1200 ° C for 6 hours in the second stage was used as the positive electrode active material.

  A positive electrode active material, PVdF (polyvinylidene fluoride) as a binder, and carbon powder as a conductive auxiliary agent are each dispersed in NMP (N-methyl-2-pyrrolidone) at 90: 5: 5 (weight ratio) to prepare a positive electrode slurry. did. The positive electrode slurry was applied to an aluminum foil using a die coater and dried to obtain a positive electrode. At that time, the amount of the positive electrode slurry applied was such that the positive electrode capacity was 90% of the negative electrode capacity.

-Preparation of negative electrode A negative electrode slurry was prepared by dispersing graphite powder as a negative electrode active material and PVdF (polyvinylidene fluoride) as a binder in NMP at 95: 5 (weight ratio). The negative electrode slurry was applied to a copper foil using a die coater and dried to obtain a negative electrode.

-Battery preparation As a separator, a polyethylene microporous membrane (thickness 25 µm) was prepared. Further, as the _{electrolyte, 1M LiPF 6 / (EC:} DEC) (EC: DEC = 1: 1 volume ratio) was prepared.

  Each of the positive electrode, negative electrode, and separator prepared above was prepared and laminated in the order of negative electrode / separator / positive electrode to prepare a power generation element.

  The obtained power generation element was placed in a bag made of an aluminum laminate sheet as an exterior, and the electrolytic solution prepared above was injected. Under vacuum conditions, the opening of the aluminum laminate sheet bag was sealed so that the current extraction tabs connected to both electrodes were led out to complete the test cell.

(Example 2)
A test cell was produced in the same manner as in Example 1 except that the firing temperature of the second stage of lithium manganate was 1250 ° C.

(Example 3)
A test cell was produced in the same manner as in Example 1 except that the firing temperature of the second stage of lithium manganate was 1150 ° C.

(Example 4)
A test cell was produced in the same manner as in Example 1 except that the first stage calcination temperature of lithium manganate was 750 ° C.

(Example 5)
A test cell was produced in the same manner as in Example 1 except that the first stage calcination temperature of lithium manganate was 850 ° C.

(Example 6)
For the test in the same manner as in Example 1, except that a mixture of lithium manganate and lithium cobaltate prepared in the same manner as in Example 1 at 50:50 (weight ratio) was used as the positive electrode active material. A cell was produced.

(Example 7)
For the test in the same manner as in Example 1, except that a mixture of lithium manganate and lithium nickelate prepared in the same manner as in Example 1 at 50:50 (weight ratio) was used as the positive electrode active material. A cell was produced.

(Example 8)
Tested in the same manner as in Example 1 except that a mixture of lithium manganate and lithium iron phosphate prepared in the same manner as in Example 1 at 50:50 (weight ratio) was used as the positive electrode active material. A cell was prepared.

Example 9
A test cell was produced in the same manner as in Example 1 except that the first stage baking temperature of lithium manganate was set to 700 ° C.

(Example 10)
A test cell was produced in the same manner as in Example 1 except that the first stage baking temperature of lithium manganate was set to 900 ° C.

(Comparative Example 1)
A test cell was produced in the same manner as in Example 1 except that the first stage baking temperature of lithium manganate was 900 ° C. and the second stage baking temperature was 1300 ° C.

(Comparative Example 2)
A test cell was prepared in the same manner as in Example 1 except that the first stage baking temperature of lithium manganate was 700 ° C. and the second stage baking temperature was 1100 ° C.

(Comparative Example 3)
A test cell was produced in the same manner as in Example 1 except that the firing temperature of the first stage of lithium manganate was 950 ° C.

(Comparative Example 4)
A test cell was produced in the same manner as in Example 1 except that the first stage calcination temperature of lithium manganate was 650 ° C.

[Evaluation]
About the test cell obtained by each Example and each comparative example, 0.2C / 4.2V and CC / CV charge were performed for 7 hours at 25 degreeC. Next, after a 10-minute rest, the battery was discharged to 2.5 V with 0.2 C CC discharge. After that, the cycle of 1C / 4.2V CC / CV charge (0.015C cut) and 1C CC discharge (2.5V voltage cut) was repeated at 55 ° C. The value was calculated as a capacity retention rate. The results are shown in Table 1 below.

  As apparent from Table 1 above, as the positive electrode active material, the ratio of the tap density to the average particle diameter of the secondary particles, and the test cell of the example including lithium manganate having a true specific gravity within a predetermined range, It can be seen that the capacity retention rate is excellent.

10a, 10b, 50 lithium ion secondary battery,
11 positive electrode current collector,
12 negative electrode current collector,
13 positive electrode active material layer,
15 negative electrode active material layer,
17 electrolyte layer,
19 cell layer,
21, 57 power generation element,
25 positive current collector,
27 negative current collector,
29, 52 Battery exterior material,
31 seal part,
58 positive electrode tab,
59 Negative electrode tab.

Claims (5)

A positive electrode for a lithium ion secondary battery having a current collector and a positive electrode active material layer formed on the current collector,
As the positive electrode active material, the positive electrode active material layer has a ratio between the tap density and the average particle diameter of the secondary particles as follows: tap density (g / ml) / secondary particle average particle diameter (μm) = 0.07 A positive electrode for a lithium ion secondary battery, comprising lithium manganate having a true specific gravity of 0.45 and 4.19 to 4.25.   2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the lithium manganate secondary particles have an average particle diameter of 6 to 24 μm.   The positive electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the tap density of the lithium manganate is 1.6 to 2.7 g / ml.   The lithium ion secondary according to any one of claims 1 to 3, wherein the positive electrode active material further includes at least one selected from the group consisting of lithium cobaltate, lithium nickelate, and lithium iron phosphate. Battery positive electrode.   The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein an average particle diameter of primary particles of the lithium manganate is 1 to 5 µm.

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