JP2007157502A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for improving an output property of a lithium secondary battery while restraining the lowering of an output per cubic volume accompanied by the increased amount of a conductive assistant to the minimum. <P>SOLUTION: A cathode activator layer of the lithium ion secondary battery contains a cathode activator and a fibrous conductive assistant having branched structure. The ratio of the total surface area a1 of the conductive assistant with respect to the total surface area a2 of the cathode activator is 1.7≤a1/a2≤5.4. The lithium ion secondary battery or an anode activator of the lithium ion secondary battery contains an anode activator and a fibrous conductive assistant having branched structure. The ratio of the total surface area b1 of the conductive assistant with respect to the total surface area b2 of the cathode activator is 0.5≤b1/b2≤2.7. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery with improved output characteristics.

  In recent years, in order to cope with air pollution and global warming, reduction of the amount of carbon dioxide has been strongly 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), and the development of secondary batteries for motor drive that holds the key to commercialization of these is thriving. Has been done.

  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 includes a positive electrode in which a positive electrode active material or the like is applied to both surfaces of a positive electrode current collector using a binder, and a negative electrode in which a negative electrode active material or the like is applied to both surfaces of a negative electrode current collector using a binder. However, it has the structure connected through an electrolyte layer and accommodated in a battery case.

  The lithium ion secondary battery used as a motor drive power source for automobiles as described above has extremely high output characteristics as compared with consumer lithium ion secondary batteries used in mobile phones, laptop computers, etc. The current situation is that research and development is underway to meet such demands.

Here, as a technique for improving the output characteristics of a lithium ion secondary battery in consideration of mounting in an automobile, for example, a positive electrode active material for a lithium secondary battery containing a Li—Mn—Ni composite oxide, A positive electrode for a lithium secondary battery, wherein an average diameter of primary particles of Li—Mn—Ni composite oxide is 2.0 μm or less and a BET specific surface area is 0.4 m ² / g or more. An active material is disclosed (see Patent Document 1).
JP 2003-68299 A

  According to the technique described in Patent Document 1, a lithium ion secondary battery having excellent discharge capacity characteristics and cycle durability can be provided. This is because, when the average diameter and specific surface area of the Li—Mn—Ni composite oxide, which is the positive electrode active material, are controlled to the above values, the surface area of the positive electrode active material that can come into contact with the electrolytic solution increases. This is probably because the reaction can proceed sufficiently.

  Thus, it is preferable to reduce the particle size of the active material contained in the active material layer of the electrode from the viewpoint of increasing the output of the battery. However, in order to efficiently build a conductive network that contributes to higher output, an excessive amount of conductive aid is required, which increases the volume of the electrode and lowers the output per volume. It was.

  Accordingly, an object of the present invention is to provide a means capable of improving output characteristics while minimizing a decrease in output per volume due to an increase in the amount of conductive aid used in a lithium ion secondary battery. To do.

  In view of the above problems, the present inventors have conducted intensive research. As a result, when the ratio of the total surface area of the fibrous conductive additive having a branched structure to the total surface area of the active material is within a specific range, the electrode It was found that the volume of the electrode can be reduced and the output per electrode volume can be increased.

  That is, according to the present invention, the positive electrode active material layer of the lithium ion secondary battery includes a positive electrode active material and a fibrous conductive additive having a branched structure, and the total surface area a1 of the conductive auxiliary agent is the positive electrode active material. The lithium ion secondary battery is characterized in that the ratio is 1.7 ≦ a1 / a2 ≦ 5.4 with respect to the total surface area a2.

  Further, in the present invention, the negative electrode active material layer of the lithium ion secondary battery includes a negative electrode active material and a fibrous conductive additive having a branched structure, and the total surface area b1 of the conductive auxiliary agent is the negative electrode active material. The lithium ion secondary battery is characterized in that the ratio is 0.5 ≦ b1 / b2 ≦ 2.7 with respect to the total surface area b2.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery which improved the output characteristic can be provided, suppressing the fall of the output per volume accompanying the increase in the usage-amount of a conductive support agent to the minimum.

  Embodiments of the present invention will be described below.

(First embodiment)
(Constitution)
In the first aspect of the present invention, the positive electrode active material layer of the lithium ion secondary battery includes a positive electrode active material and a branched fibrous conductive auxiliary agent, the surface area a1 of the conductive auxiliary agent, and the surface area of the positive electrode active material. The lithium ion secondary battery is characterized in that the ratio of a2 is a ratio of 1.7 ≦ a1 / a2 ≦ 5.4.

  In the second aspect of the present invention, the negative electrode active material layer of the lithium ion secondary battery includes a negative electrode active material and a branched fibrous conductive assistant, and the surface area b1 of the conductive assistant and the surface area of the negative active material. The lithium ion secondary battery is characterized in that b2 is a ratio of 0.5 ≦ b1 / b2 ≦ 2.7.

  First, the structure of the unit cell used for the lithium ion secondary battery of this invention is demonstrated with reference to drawings. In the present invention, the drawings are exaggerated for convenience of explanation, and the technical scope of the present invention is not limited to the forms shown in the drawings. Also, embodiments other than the drawings may be employed.

  FIG. 1 is a cross-sectional view showing one embodiment of a unit cell used in the lithium ion secondary battery of the present invention. As shown in FIG. 1, the cell 1 includes a positive electrode in which a positive electrode active material layer 13 is formed on a current collector 11, and a negative electrode in which a negative electrode active material layer 15 is formed on another current collector 11. Are stacked with the electrolyte layer 17 interposed therebetween.

  Hereinafter, an embodiment of the configuration of the lithium ion secondary battery of the present invention will be described with reference to FIG. The electrode of this embodiment is characterized in that, for each of the positive electrode and the negative electrode, the ratio of the total surface area of the conductive additive described later and the total surface area of the active material described below is the predetermined value described above. There is no particular limitation on the selection of the current collector, the type of active material, the binder, the supporting salt (lithium salt), the electrolyte, and other compounds added as necessary. Depending on the intended use, it may be selected by appropriately referring to conventionally known knowledge. Hereafter, the member which comprises the electrode used for this invention is demonstrated in detail.

[Current collector]
The current collector 11 is made of a conductive material such as an aluminum foil, a nickel foil, a copper foil, or a stainless steel (SUS) foil. The general thickness of the current collector is 1 to 30 μm. However, a current collector having a thickness outside this range may be used.

  The size of the current collector is determined according to the intended use of the battery. If a large electrode used for a large battery is manufactured, a current collector having a large area is used. If a small electrode is produced, a current collector with a small area is used.

[Active material layer]
An active material layer (13, 15) is formed on the current collector 11. The active material layers (13, 15) are layers containing an active material that plays a central role in the charge / discharge reaction. In the present invention, in either one or both of the positive electrode active material layer 13 and the negative electrode active material layer 15, the ratio of the total surface area of the conductive additive to the total surface area of the active material satisfies the predetermined value described above.

  The active material layers (13, 15) include an active material. When the electrode of the present invention is used as a positive electrode, the active material layer contains a positive electrode active material. On the other hand, when the electrode of the present invention is used as a negative electrode, the active material layer contains a negative electrode active material.

  Examples of the positive electrode active material include lithium-transition metal 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.

  Examples of the negative electrode active material include carbon materials, lithium-transition metal compounds, metal materials, and lithium-metal alloy materials. In some cases, two or more negative electrode active materials may be used in combination.

  In the unit cell of the lithium ion secondary battery of this embodiment, the average particle diameter of the positive electrode active material contained in the positive electrode active material layer 13 is 5 μm or less. The average particle size is preferably 3 μm or less, more preferably 1 μm or less. From the viewpoint of obtaining the effects of the present invention, the lower limit of the average particle diameter of the positive electrode active material is not particularly limited, but from the viewpoint of sufficiently forming a conductive network in the electrode, the average of the positive electrode active material The particle size is preferably 0.01 μm or more, more preferably 0.1 μm or more.

  On the other hand, in the unit cell of the lithium ion secondary battery of this embodiment, the average particle diameter of the negative electrode active material contained in the negative electrode active material layer 15 is 10 μm or less. The average particle size is preferably 5 μm or less, and more preferably 1 μm or less. From the viewpoint of obtaining the effects of the present invention, the lower limit of the average particle diameter of the negative electrode active material is not particularly limited, but from the viewpoint of sufficiently forming a conductive network in the electrode, the average of the negative electrode active material The particle size is preferably 0.01 μm or more, more preferably 0.1 μm or more.

  The active material layer also includes a fibrous conductive additive having a branched structure.

  A conductive support agent means the additive mix | blended in order to improve the electroconductivity of an active material layer. In the present invention, a fibrous conductive substance having a branched structure is used as the conductive assistant. Although there is no restriction | limiting in particular about the specific form, Carbon fiber is preferable from viewpoints, such as electroconductivity and intensity | strength.

  The method for producing the carbon fiber is not particularly limited, and examples thereof include a spinning method and a vapor phase growth method. The obtained carbon fiber has high crystallinity, high conductivity, high strength, and good chemical stability. Vapor phase epitaxy that can have

  The present invention is characterized in that the total surface area of the conductive additive is a specific ratio with respect to the total surface area of the active material.

  In the positive electrode, the total surface area a1 of the conductive additive is a ratio of 1.7 ≦ a1 / a2 ≦ 5.4 with respect to the total surface area a2 of the active material. From the viewpoint of forming a conductive network in the electrode, the total surface area ratio is more preferably 1.8 ≦ a1 / a2 ≦ 3.0. When the total surface area ratio is less than 1.7, the conductive network is not sufficiently formed. When the total surface area ratio exceeds 5.4, the conductive auxiliary agent becomes excessive, which is not preferable.

  In the negative electrode, the total surface area b1 of the conductive auxiliary agent is a ratio of 0.5 ≦ b1 / b2 ≦ 2.7 with respect to the total surface area b2 of the active material. When the total surface area ratio is less than 0.5, the conductive network is not sufficiently formed, and when it exceeds 2.7, the conductive aid becomes excessive, which is not preferable.

  The calculation method of the total surface area of the conductive assistant follows Formula 1 below.

  Here, the main chain and side chain of the conductive auxiliary agent have a cylindrical structure, and the fiber diameter and fiber length of the main chain and side chain necessary for calculating the volume and surface area are measured by a scanning electron microscope (SEM).

  The total surface area of the active material is calculated according to a method for calculating the total surface area of the main chain of the conductive additive.

  Furthermore, as the shape of the conductive auxiliary agent, it is preferable that at least one of the main chain and the side chain has a length equal to or larger than the particle diameter of the active material. By having this structure, it is possible to contact two or more active materials with respect to one conductive auxiliary agent, and a more efficient conductive network can be formed in the electrode. In addition, the smaller the fiber diameter of the conductive auxiliary agent, the more the contact characteristics between the active material per volume and the conductive auxiliary agent, so that the output characteristics are improved, which is more preferable.

  In the embodiment shown in FIG. 1, the ratio of the total surface area of the conductive additive and the total surface area of the active material is the above-mentioned predetermined value in both the positive electrode and the negative electrode, but the technical scope of the present invention is such a form. However, the present invention is not limited thereto, and a mode in which only one of the positive electrode and the negative electrode satisfies the above-described predetermined value can also be included.

  In addition, although the electrode material used for the battery of this invention contains the fibrous conductive support agent which has a branched structure as an essential component, you may also contain other conventionally well-known conductive support agents. For example, carbon black such as acetylene black, carbon powder such as graphite, and the like can be included.

  The active material layers (13, 15) may contain other materials if necessary. For example, a binder, a supporting salt (lithium salt), an ion conductive polymer, and the like can be included. When an ion conductive polymer is included, a polymerization initiator for polymerizing the polymer may be included.

  Examples of the binder include polyvinylidene fluoride (PVdF) and a synthetic rubber binder.

Examples of the supporting 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. Here, the polymer may be the same as or different from the ion conductive polymer used in the electrolyte layer of the battery in which the electrode of the present invention is employed, but is preferably the same.

  The polymerization initiator is added to act on the crosslinkable group of the ion conductive polymer to advance the crosslinking reaction. Depending on the external factor for acting as an initiator, it is classified into a photopolymerization initiator, a thermal polymerization initiator and the like. Examples of the polymerization initiator include azobisisobutyronitrile (AIBN), which is a thermal polymerization initiator, and benzyl dimethyl ketal (BDK), which is a photopolymerization initiator.

  The compounding ratio of the components contained in the active material layers (13, 15) is not particularly limited. The blending ratio can be adjusted by appropriately referring to known knowledge about lithium ion secondary batteries.

  The thickness of the active material layers (13, 15) is not particularly limited, and conventionally known knowledge about the lithium ion secondary battery can be appropriately referred to. As an example, the thickness of the active material layers (13, 15) is preferably about 10 to 100 μm, and more preferably 20 to 50 μm. When the active material layers (13, 15) are about 10 μm or more, the battery capacity can be sufficiently secured. On the other hand, when the active material layers (13, 15) are about 100 μm or less, it is possible to suppress the occurrence of the problem of an increase in internal resistance due to the difficulty in diffusing lithium ions in the electrode deep part (current collector side).

(Production method)
Then, the manufacturing method of the lithium ion secondary battery of this invention is demonstrated.

  First, the manufacturing method of the cell 1 of the form shown in FIG. 1 is demonstrated.

  The electrode used in the present invention is prepared by, for example, adding an active material to a solvent to prepare an active material slurry (active material slurry preparation step), applying the active material slurry to the surface of the current collector, and drying. It can be manufactured by forming a coating film by applying (coating film forming process) and pressing the laminate produced through the coating film forming process in the laminating direction (pressing process). When an ion conductive polymer is added to the active material slurry and a polymerization initiator is further added for the purpose of crosslinking the ion conductive polymer, it is simultaneously with drying in the coating film forming process, or before or after the drying. A polymerization treatment may be performed (polymerization step).

  Hereinafter, although such a manufacturing method is demonstrated in detail in order of a process, it is not restrict | limited only to the following form.

[Active material slurry preparation process]
In this step, a desired active material, a fibrous conductive assistant having a branched structure, and other components as necessary (for example, other conductive assistants, binders, ion conductive polymers, supporting salts (lithium salts) ), A polymerization initiator, etc.) are mixed in a solvent to prepare an active material slurry. Since the specific form of each component blended in the active material slurry is as described in the column of the configuration of the electrode of the present invention, detailed description is omitted here.

  The kind of solvent and the mixing means are not particularly limited, and conventionally known knowledge about electrode production can be appropriately referred to. As an example of the solvent, N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methylformamide and the like can be used. When adopting polyvinylidene fluoride (PVdF) as a binder, NMP is preferably used as a solvent.

[Coating film forming process]
Subsequently, a current collector is prepared, and the active material slurry prepared above is applied to the surface of the current collector and dried. Thereby, the coating film which consists of an active material slurry is formed on the surface of an electrical power collector. This coating film becomes an active material layer through a pressing step described later.

  The specific form of the current collector to be prepared is as described in the column of the configuration of the electrode of the present invention, and thus detailed description thereof is omitted here.

  The application means for applying the active material slurry is not particularly limited. For example, a commonly used means such as a self-propelled coater can be adopted.

  A coating film is formed according to the desired arrangement | positioning form of the electrical power collector and active material layer in the electrode manufactured. For example, when the manufactured electrode is a bipolar electrode, a coating film containing a positive electrode active material is formed on one surface of the current collector, and a coating film containing a negative electrode active material is formed on the other surface. On the other hand, when manufacturing a non-bipolar electrode, a coating film containing either the positive electrode active material or the negative electrode active material is formed on both surfaces of one current collector.

  Thereafter, the coating film formed on the surface of the current collector is dried. Thereby, the solvent in a coating film is removed.

  The drying means for drying the coating film is not particularly limited, and conventionally known knowledge about electrode production can be appropriately referred to. For example, heat treatment is exemplified. Drying conditions (drying time, drying temperature, etc.) can be appropriately set according to the application amount of the active material slurry and the volatilization rate of the solvent of the slurry.

  When a coating film contains a polymerization initiator, the ion conductive polymer in a coating film is bridge | crosslinked by a crosslinkable group by performing a superposition | polymerization process further.

  The polymerization treatment in the polymerization step is not particularly limited, and conventionally known knowledge may be referred to as appropriate. For example, when the coating film contains a thermal polymerization initiator (AIBN or the like), the coating film is subjected to heat treatment. Moreover, when a coating film contains a photoinitiator (BDK etc.), light, such as an ultraviolet light, is irradiated. In addition, the heat processing for thermal polymerization may be performed simultaneously with said drying process, and may be performed before or after the said drying process.

[Pressing process]
Then, the laminated body produced through the said coating-film formation process is pressed to the lamination direction. Thereby, the battery electrode of the present invention is completed. Specific examples of the press process include a hot press machine and a calendar roll press machine. Also, the press conditions (temperature, pressure, etc.) are not particularly limited, and conventionally known knowledge can be referred to as appropriate.

  A lithium ion secondary battery is configured using the electrode of the first embodiment. The electrode can be applied to any of a positive electrode, a negative electrode, and a bipolar electrode. A lithium ion secondary battery including the electrode as at least one electrode belongs to the technical scope of the present invention. However, preferably, all of the electrodes constituting the lithium ion secondary battery are the electrodes of the present invention. By adopting such a configuration, the output characteristics of the lithium ion secondary battery can be effectively improved.

  The battery of the present invention may be a bipolar lithium ion secondary battery (hereinafter also referred to as “bipolar battery”). FIG. 2 is a cross-sectional view showing a lithium ion secondary battery according to an embodiment of the present invention, which is a bipolar battery. Hereinafter, the bipolar battery shown in FIG. 2 will be described in detail as an example, but the technical scope of the present invention is not limited to such a form.

  The bipolar battery 10 of this embodiment shown in FIG. 2 has a structure in which a substantially rectangular battery element 21 in which a charge / discharge reaction actually proceeds is sealed inside a laminate sheet 29 that is an exterior.

  As shown in FIG. 2, the battery element 21 of the bipolar battery 10 of this embodiment includes a plurality of bipolar electrodes in which a positive electrode active material layer 13 and a negative electrode active material layer 15 are formed on each surface of a current collector 11. Have one. Each bipolar electrode is laminated via an electrolyte layer 17 to form a battery element 21. At this time, each bipolar electrode and electrolyte layer are arranged such that the positive electrode active material layer 13 of one bipolar electrode and the negative electrode active material layer 15 of another bipolar electrode adjacent to the one bipolar electrode face each other through the electrolyte layer 17. 17 are stacked.

  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 10 has a configuration in which the single battery layers 19 are stacked. In addition, an insulating layer 31 for insulating adjacent current collectors 11 is provided on the outer periphery of the unit cell layer 19. The current collector (outermost layer current collector) (11a, 11b) located in the outermost layer of the battery element 21 has a positive electrode active material layer 13 (positive electrode side outermost layer current collector 11a) or a negative electrode only on one side. One of the active material layers 15 (negative electrode side outermost layer current collector 11b) is formed.

  Furthermore, in the bipolar battery 10 shown in FIG. 2, the positive electrode side outermost layer current collector 11a is extended to be a positive electrode output terminal 25, which is led out from a laminate sheet 29 which is an exterior. On the other hand, the negative electrode side outermost layer current collector 11 b is extended to form a negative electrode output terminal 27, which is similarly derived from the laminate sheet 29.

  Hereinafter, members constituting the bipolar battery 10 of the present embodiment will be briefly described. However, since the components constituting the electrode are as described above, the description thereof is omitted here. Further, the technical scope of the present invention is not limited to the following forms, and conventionally known forms can be similarly adopted.

[Electrolyte layer]
A liquid electrolyte or a polymer electrolyte can be used as the electrolyte constituting the electrolyte layer 17.

  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 the case where the electrolyte layer 17 is composed of a liquid electrolyte or a gel electrolyte, a separator may be used for the electrolyte layer 17. 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 17 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.

[Insulation layer]
In the bipolar battery 10, an insulating layer 31 is usually provided around each unit cell layer 19. The insulating layer 31 is provided for the purpose of preventing the adjacent current collectors 11 in the battery from contacting each other and short-circuiting due to a slight unevenness at the end of the battery cell layer 19 in the battery element 21. It is done. The installation of such an insulating layer 31 ensures long-term reliability and safety, and can provide a high-quality bipolar battery 10.

  The insulating layer 31 only needs to have insulating properties, sealing properties against falling off of the solid electrolyte, sealing properties against moisture permeation from the outside (sealing properties), heat resistance under battery operating temperature, etc. Urethane resin, epoxy resin, polyethylene resin, polypropylene resin, polyimide resin, rubber and the like can be used. Of these, urethane resins and epoxy resins are preferred from the viewpoints of corrosion resistance, chemical resistance, ease of production (film forming properties), economy, and the like.

[Terminal]
In the bipolar battery 10, terminals (positive terminal 25 and negative terminal 27) electrically connected to the outermost current collector (11 a, 11 b) are taken out of the exterior for the purpose of taking out current outside the battery. Specifically, a positive electrode terminal 25 electrically connected to the positive electrode outermost layer current collector 11a and a negative electrode terminal 27 electrically connected to the negative electrode outermost layer current collector 11b are provided outside the exterior. It is taken out.

  The material of the terminals (the positive electrode terminal 25 and the negative electrode terminal 27) is not particularly limited, and a known material conventionally used as a terminal for a bipolar battery can be used. Examples thereof include aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof. The positive electrode terminal 25 and the negative electrode terminal 27 may be made of the same material or different materials. As in the present embodiment, the outermost layer current collectors (11a, 11b) may be extended to form terminals (25, 27), or a separately prepared terminal may be connected to the outermost layer current collector. Good.

[Exterior]
In the bipolar battery 10, the battery element 21 is preferably housed in an exterior such as a laminate sheet 29 in order to prevent external impact and environmental degradation during use. The exterior is not particularly limited, and a conventionally known exterior can be used. A polymer-metal composite laminate sheet or the like excellent in thermal conductivity can be preferably used in that heat can be efficiently transferred from a heat source of an automobile and the inside of the battery can be rapidly heated to the battery operating temperature.

(Second Embodiment)
In the second embodiment, an assembled battery is configured by connecting a plurality of the bipolar batteries of the first embodiment in parallel and / or in series.

  FIG. 3 is a perspective view showing the assembled battery of the present embodiment.

  As shown in FIG. 3, the assembled battery 40 is configured by connecting a plurality of bipolar batteries described in the second embodiment. Each bipolar battery 10 is connected by connecting the positive electrode terminal 25 and the negative electrode terminal 27 of each bipolar battery 10 using a bus bar. On one side surface of the assembled battery 40, electrode terminals (42, 43) are provided as electrodes of the entire assembled battery 40.

  The connection method when connecting the plurality of bipolar batteries 10 constituting the assembled battery 40 is not particularly limited, and a conventionally known method can be appropriately employed. For example, a technique using welding such as ultrasonic welding or spot welding, or a technique of fixing using rivets, caulking, or the like can be employed. According to such a connection method, the long-term reliability of the assembled battery 40 can be improved.

  According to the assembled battery 40 of the present embodiment, each bipolar battery 10 constituting the assembled battery 40 is excellent in output characteristics, and therefore, an assembled battery excellent in output characteristics can be provided.

  The connection of the bipolar batteries 10 constituting the assembled battery 40 may be all connected in parallel, all may be connected in series, or a combination of series connection and parallel connection may be used. Also good.

(Third embodiment)
In the third embodiment, the bipolar battery 10 of the first embodiment or the assembled battery 40 of the second embodiment is mounted as a motor driving power source to constitute a vehicle. As a vehicle using the bipolar battery 10 or the assembled battery 40 as a motor power source, for example, a complete electric vehicle that does not use gasoline, a hybrid vehicle such as a series hybrid vehicle or a parallel hybrid vehicle, and a fuel cell vehicle, etc., wheels are driven by a motor. A driving car can be mentioned.

  For reference, FIG. 4 shows a schematic diagram of an automobile 50 on which the assembled battery 40 is mounted. The assembled battery 40 mounted on the automobile 50 has the characteristics as described above. For this reason, the automobile 50 equipped with the assembled battery 40 has excellent output characteristics, and can provide sufficient output even under high output conditions.

  As described above, some preferred embodiments of the present invention have been described. However, the present invention is not limited to the above embodiments, and various modifications, omissions, and additions can be made by those skilled in the art. . For example, in the first embodiment, the bipolar lithium ion secondary battery (bipolar battery) has been described as an example, but the technical scope of the battery of the present invention is not limited to the bipolar battery, For example, a non-bipolar lithium ion secondary battery may be used. For reference, FIG. 5 is a cross-sectional view showing an outline of a lithium ion secondary battery 60 that is not bipolar.

  The effects of the present invention will be described using the following examples and comparative examples. However, the technical scope of the present invention is not limited only to the following examples.

  The carbon fibers having a branched structure used in the examples are as shown in Table 1. As shown in FIG. 6, R and L in Table 1 represent the fiber diameter and fiber length of the carbon fiber, respectively, and were measured using a scanning electron microscope (SEM).

<Example 1>
<Preparation of positive electrode>
Spinel-type lithium manganate (average particle size: 1.0 μm) (80% by mass) as a positive electrode active material, carbon fiber having a branched structure as a conductive assistant (Type 1 shown in Table 1) (10% by mass), and An appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjusting solvent was added to a solid content of polyvinylidene fluoride (PVdF) (10% by mass) as a binder to prepare a positive electrode active material slurry. .

The positive electrode active material slurry prepared above was applied on an aluminum foil (thickness: 20 μm) as a positive electrode current collector at a weight per unit area of 6.0 mg / cm ² using a self-propelled die coater, dried, and laminated. Got the body. Next, the obtained laminate was pressed using a press machine, and an output terminal was connected to the current collector to produce a test positive electrode.

<Production of negative electrode>
Hard carbon (average particle diameter: 9 μm) (85% by mass) as a negative electrode active material, vapor grown carbon fiber (VGCF; registered trademark) (5% by mass) as a conductive additive, and polyvinylidene fluoride as a binder ( An appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, was added to the solid content of PVdF) (10% by mass) to prepare a negative electrode active material slurry.

The negative electrode active material slurry prepared above was applied onto a copper foil (thickness: 10 μm) as a negative electrode current collector at a basis weight of 1.5 mg / cm ² using a self-propelled die coater, dried, and laminated. Got the body. Next, the obtained laminate was pressed using a press so that the porosity of the negative electrode active material layer was 55%, and an output terminal was connected to the current collector to produce a test negative electrode.

<Preparation of electrolyte>
Ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed at a volume ratio of 2: 2: 6 to obtain a plasticizer (organic solvent) of the electrolytic solution. Then, the plasticizer, the addition of LiPF ₆ as lithium salt to a concentration of 1M, to prepare an electrolytic solution.

<Production of laminated battery>
A polyethylene microporous membrane (thickness: 25 μm) as a lithium ion battery separator was sandwiched between the test positive electrode and the test negative electrode prepared above. Next, the obtained sandwiched body was inserted into an aluminum laminate pack which is a three-side sealed exterior material. Thereafter, the electrolytic solution prepared above was injected into the aluminum laminate pack, and the pack was vacuum-sealed so that the output terminals were exposed from the pack, thereby completing a laminate battery.

<Example 2>
The same as Example 1 except that the mass ratio of the spinel type lithium manganate, which is a solid content in the positive electrode active material slurry, the carbon fiber having a branched structure, and the polyvinylidene fluoride was 83: 7: 10. A laminated battery was prepared by the method described above.

<Example 3>
Mass ratio of spinel type lithium manganate which is solid content in the positive electrode active material slurry, carbon fiber having a branched structure, and polyvinylidene fluoride using a carbon fiber having a branched structure shown as Type 2 in Table 1 as a conductive additive A laminated battery was produced in the same manner as in Example 1 except that was set to 87: 3: 10.

<Example 4>
Mass ratio of spinel type lithium manganate which is solid content in the positive electrode active material slurry, carbon fiber having a branched structure, and polyvinylidene fluoride using a carbon fiber having a branched structure shown as Type 3 in Table 1 as a conductive additive. A laminated battery was manufactured in the same manner as in Example 1 except that the ratio was 89: 1: 10.

<Comparative example>
Instead of the carbon fiber, spherical acetylene black having a particle size of 0.5 μm is used, and the mass ratio of spinel type lithium manganate, acetylene black, and polyvinylidene fluoride, which are solid components in the positive electrode active material slurry, is determined. A laminated battery was produced in the same manner as in Example 1 except that the ratio was 89: 1: 10.

<Evaluation of battery characteristics of laminated battery>
Using the laminate batteries prepared in each of the above examples and comparative examples, a charge / discharge test was performed by the following method.

  Specifically, each laminate battery was initially charged at a constant current of 0.2 C, discharged at a constant current of 0.5 C, and then subjected to a 10-cycle charge / discharge test at a constant current of 1 C.

Thereafter, the DC resistance of a 5-second value at 100 C (20 mA / cm ² or more) in a fully charged state (4.2 V) was evaluated. The results are shown in Table 2 below.

  As shown in Table 2 above, it was confirmed that when a fibrous conductive additive having a branched structure as an example was used, compared with a spherical conductive additive as a comparative example, it had excellent output characteristics. It was done. Further, as shown in the examples, it can be seen that when the surface area ratio is the same, the smaller the fiber diameter, the better the output characteristics.

  From the above, the lithium ion secondary battery of the present invention has high output characteristics.

It is sectional drawing which shows the cell by one Embodiment of this invention. It is sectional drawing which shows one preferable embodiment of the bipolar battery of 1st Embodiment. It is a perspective view which shows the assembled battery of 2nd Embodiment. It is the schematic of the motor vehicle of 3rd Embodiment carrying the assembled battery of 2nd Embodiment. It is sectional drawing which shows the outline | summary of the lithium ion secondary battery which is not a bipolar type. It is a schematic diagram of the carbon fiber used for this invention.

Explanation of symbols

1 cell,
10 Bipolar battery,
11 Current collector,
13 positive electrode active material layer,
15 negative electrode active material layer,
17 electrolyte layer,
19 cell layer,
21 battery elements,
25 positive terminal,
27 negative terminal,
29 Laminate sheet,
31 insulating layer,
33 positive electrode current collector,
35 negative electrode current collector,
40 battery packs,
42, 43 electrode terminals,
50 cars,
60 Lithium ion secondary battery that is not bipolar.

Claims (17)

  The positive electrode active material layer of the lithium ion secondary battery includes a positive electrode active material and a fibrous conductive additive having a branched structure, and the total surface area a1 of the conductive auxiliary agent is larger than the total surface area a2 of the positive electrode active material. The lithium ion secondary battery is characterized by having a ratio of 1.7 ≦ a1 / a2 ≦ 5.4.   The negative electrode active material layer of the lithium ion secondary battery includes a negative electrode active material and a fibrous conductive additive having a branched structure, and the total surface area b1 of the conductive auxiliary agent is smaller than the total surface area b2 of the negative electrode active material. The lithium ion secondary battery is characterized in that the ratio is 0.5 ≦ b1 / b2 ≦ 2.7.   The ratio of the total surface area a1 of the conductive additive and the total surface area a2 of the positive electrode active material is 1.8 ≦ a1 / a2 ≦ 3.0. Next battery.   4. The lithium ion secondary battery according to claim 1, wherein a length of a main chain of the conductive additive is equal to or greater than a particle size of the active material. 5.   5. The lithium ion secondary battery according to claim 1, wherein a length of a side chain of the conductive auxiliary agent is not less than a particle size of the active material.   The lithium ion secondary battery according to claim 1, wherein the conductive auxiliary agent is a carbon-based material.   The lithium ion secondary battery according to claim 6, wherein the carbon-based material is a material manufactured by a vapor deposition method.   8. The lithium ion secondary battery according to claim 1, wherein an average particle diameter of the positive electrode active material is 5 μm or less. 9.   The lithium ion secondary battery according to claim 8, wherein an average particle diameter of the positive electrode active material is 1 μm or less.   The lithium ion secondary battery according to any one of claims 2 to 7, wherein an average particle size of the negative electrode active material is 10 µm or less.   The lithium ion secondary battery according to claim 10, wherein an average particle diameter of the negative electrode active material is 5 μm or less.   The said positive electrode active material contains at least one selected from the group which consists of a lithium transition metal oxide, a lithium transition metal phosphate compound, and a lithium transition metal sulfate compound, The any one of Claims 1-11 characterized by the above-mentioned. 2. The lithium ion secondary battery according to item 1.   The negative electrode active material includes at least one selected from the group consisting of a carbon material, a lithium transition metal compound, a metal material, and a lithium-metal alloy material. The lithium ion secondary battery according to item.   The lithium ion secondary battery according to any one of claims 1 to 13, wherein the electrolyte of the lithium ion secondary battery is selected from the group consisting of a liquid electrolyte, a polymer electrolyte, and a gel electrolyte. .   The lithium ion secondary battery according to claim 14, wherein the electrolyte is a polymer electrolyte or a gel electrolyte.   The assembled battery using the lithium ion secondary battery of any one of Claims 1-15.   A vehicle equipped with the lithium ion secondary battery according to any one of claims 1 to 15 or the assembled battery according to claim 16 as a motor driving power source.

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