JP2005235695A - Lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To shorten impregnation time of an electrolyte into a group of electrodes than that of a conventional one when current collection parts are formed in which electrode core exposed parts protruding from one and the other end surface of a column-shape electrode plate group are bent toward the center axis direction of the electrode plate group. <P>SOLUTION: The lithium-ion secondary battery is equipped with: a positive electrode 11 and a negative electrode 12 made of a complex lithium oxide; a separator 13, a non-aqueous electrolyte; a porous membrane having electron insulation nature adhered on at least one surface of the positive electrode and the negative electrode; a positive electrode current collector 18; and a negative electrode current collector 19. The negative electrode and the positive electrode constitute a column-shaped electrode group in a wound state through the separator. The positive electrode and the negative electrode have a core exposed parts at their one end parts along the longitudinal direction respectively. The core exposed parts protrude further from the end part of the other electrode at the one and the other end surfaces of the column-shape electrode group respectively, and form current collection parts 21, 22 by bending toward the center axis direction of the electrode group, and the current collectors 18, 19 are jointed to the parts. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a lithium ion secondary battery excellent in safety and productivity.

  A lithium ion secondary battery generally includes a positive electrode core material, a positive electrode made of a composite lithium oxide supported on the positive electrode core material, a negative electrode core material, and a negative electrode made of a material capable of entering and exiting lithium ions supported on the negative electrode core material. And a separator and a non-aqueous electrolyte. The positive electrode and the negative electrode are wound through a separator, and constitute a columnar electrode plate group. A core material exposed portion is formed at one end portion along the longitudinal direction of the positive electrode and the negative electrode, respectively, and these exposed portions are located at one end and the other end face of the columnar electrode plate group from the end of the other electrode. Is also protruding.

  In recent years, from the viewpoint of increasing the current collection efficiency of a lithium ion secondary battery and reducing the resistance at the time of charge and discharge, the core material exposed portion protruding from one and the other end surface of the columnar electrode plate group It has been proposed to bend in the direction of the central axis to form flat positive and negative current collectors (Patent Document 1). This is because by joining the flat current collecting part to the current collecting plate, the structure is stable against vibration and the like, the current collecting efficiency can be increased, and the resistance during charging and discharging is reduced.

  The separator interposed between the positive electrode and the negative electrode serves to electronically insulate between the electrode plates and to hold the electrolytic solution. For the separator, a microporous sheet mainly made of polyethylene resin is used. However, sheet-like separators such as microporous sheets generally tend to shrink even at temperatures of 150 ° C. or less, and easily cause short circuits. Further, when a sharply shaped protrusion such as a nail penetrates the battery (for example, during a nail penetration test), the short-circuit portion is expanded by a short-circuit reaction heat that is instantaneously generated. Then, further reaction heat is generated and the battery may be abnormally overheated.

  On the other hand, in order to suppress the dendrite generation of a lithium ion secondary battery, a technique for forming a porous film containing an inorganic filler on the surface of a sheet-like separator has been proposed (see Patent Document 2). Furthermore, in order to improve the electrolyte solution holding | maintenance capability between the electrode plates of a lithium ion secondary battery, the technique which forms the porous film containing an inorganic filler on the surface of a sheet-like separator is proposed (refer patent document 3). .

  In addition, from the viewpoint of preventing internal short circuit without reducing the capacity during high rate discharge, a technique for forming a protective layer made of an inorganic filler such as alumina and a water-soluble polymer on an electrode has been proposed (Patent Document 4). reference).

  In addition, a technology has been proposed in which a porous film made of a resin having a low glass transition point is formed on an electrode in order to give a lithium ion secondary battery a shutdown function that suppresses ion migration between electrode plates when an internal short circuit occurs. (See Patent Document 5).

  In recent years, a rubbery polymer made of a styrene-butadiene copolymer (SBR) has been used as a negative electrode binder for a negative electrode of a general lithium ion secondary battery. This is because the rubber-like polymer can be used in a smaller amount than polyvinylidene fluoride (PVDF) that has been conventionally used as a negative electrode binder, so that the lithium ion acceptability of the negative electrode is improved. When a rubber-like polymer is used as a negative electrode binder, a thickener composed of a water-soluble polymer is used in combination with the rubber-like polymer in order to support the negative electrode mixture layer on an electrode core material (such as copper foil). There is a need. As the water-soluble polymer, a cellulose-based resin is mainly used.

JP 2000-294222 A JP 2001-319634 A JP 2002-8730 A JP-A-9-147916 JP-A-11-144706

  When bending the core material exposed portion projecting from one and the other end surfaces of the columnar electrode plate group in the direction of the central axis of the electrode plate group, the core material exposed portions overlap to reduce the gap between the end surfaces of the electrode plate group. As a result, when the electrode plate group is impregnated with the electrolytic solution, the current collector becomes an obstacle, so that the impregnation takes time and the productivity is greatly reduced.

  In the technique of forming a porous film containing an inorganic filler on the surface of a sheet-like separator, there is a drawback that when the separator contracts, the porous film also contracts. Therefore, such a technique cannot guarantee the safety at the time of internal short circuit or nail penetration.

  In the technique of forming a protective layer composed of an inorganic filler and a water-soluble polymer on an electrode, when the negative electrode includes a water-soluble polymer, the thickener in the negative electrode is included in the protective layer before drying. Swells with water. Therefore, the malfunction that a negative electrode deform | transforms arises. Although the negative electrode that is free from deformation can be put to practical use, the production yield is greatly reduced.

  A technique for producing a shutdown effect by softening a resin having a low glass transition point cannot be an absolute safety mechanism against an internal short circuit. This is because in the nail penetration test, the heat generation temperature at the time of an internal short circuit may locally exceed several hundred degrees Celsius. In that case, the resin having a low glass transition point is excessively softened or burned out. When the nail penetrates the positive electrode and the negative electrode and at the same time the porous film is damaged, there is a high possibility that abnormal overheating will be caused.

  In view of the above, the present invention impregnates the electrode plate group with an electrolytic solution when bending the core exposed portion protruding from one and the other end faces of the columnar electrode plate group in the central axis direction of the electrode plate group. The purpose is to shorten the time and suppress the decrease in productivity. Another object of the present invention is to improve the safety of a lithium ion secondary battery by simultaneously using the porous film having excellent heat resistance and a sheet-like separator in a preferred mode simultaneously with the achievement of the object.

The present invention includes: (a) a positive electrode core material and a positive electrode comprising a composite lithium oxide supported on the positive electrode core material; (b) a negative electrode core material and a material capable of entering and exiting lithium ions supported on the negative electrode core material. A negative electrode, (c) a separator, (d) a nonaqueous electrolyte, (e) a porous film having an electronic insulating property adhered to at least one surface of the positive electrode and the negative electrode, (f) a positive electrode current collector plate, and (g ) A lithium ion secondary battery comprising a negative electrode current collector plate, wherein the positive electrode and the negative electrode are wound through a separator and constitute a columnar electrode plate group. Each of which has a core exposed portion at one end along the longitudinal direction thereof, and the core exposed portions of the positive electrode and the negative electrode are the end portions of the other electrode on one and the other end surfaces of the columnar electrode plate group, respectively. Projecting more than the electrode group A positive electrode current collector and a negative electrode current collector are bent in the axial direction, and the positive electrode current collector and the negative electrode current collector are lithium ions joined to the positive electrode current collector and the negative electrode current collector, respectively. The present invention relates to a secondary battery.

  The negative electrode contains a negative electrode binder and a thickener, the negative electrode binder is made of a rubber-like polymer, the thickener is made of a water-soluble polymer, and a porous film having an electronic insulating property is an inorganic filler and a film binder. When it is made of an agent, the membrane binder is preferably made of a resin material that is insoluble in water, does not have a crystal melting point of less than 200 ° C., and has a thermal deformation start temperature and a decomposition start temperature of 200 ° C. or higher. It is.

  In the case where the core material exposed part protruding from one and the other end face of the columnar electrode plate group is bent in the central axis direction of the electrode plate group by bonding the porous film to at least one surface of the positive electrode and the negative electrode However, the time for impregnating the electrode group with the electrolyte is shorter than before, and the productivity of the lithium ion secondary battery is greatly improved. Such an effect is considered to be based on the capillary action of the porous membrane.

  Further, the porous film is adhered to at least one surface of the positive electrode and the negative electrode, so that the porous film can be prevented from shrinking together with the sheet-like separator even when heat is generated. Therefore, according to the present invention, a highly safe lithium ion secondary battery can be efficiently produced.

  When the porous film is made of an inorganic filler and a film binder, and the film binder is made of a water-insoluble resin material, the negative electrode containing the water-soluble polymer is not deformed and the yield is not lowered.

  Further, when the membrane binder does not have a crystalline melting point of less than 200 ° C. and is made of a resin material having a thermal deformation start temperature and a decomposition start temperature of 200 ° C. or higher, the porous membrane has resistance to thermal decomposition and thermal deformation. improves. Therefore, deformation of the porous film and burnout of the porous film due to softening of the film binder are unlikely to occur.

  The lithium ion secondary battery of the present invention comprises a positive electrode comprising a positive electrode core material and a composite lithium oxide supported on the positive electrode core material, and a negative electrode core material and a material capable of entering and exiting lithium ions supported on the negative electrode core material. A negative electrode is provided. Each of the positive electrode and the negative electrode has a strip shape, and has a core material exposed portion at one end portion along the longitudinal direction thereof. An aluminum foil or the like is preferably used for the positive electrode core material, but is not limited thereto. Moreover, although copper foil etc. are used preferably for a negative electrode core material, it is not limited to this.

  In order to obtain such an electrode plate, first, an electrode mixture paste is prepared by mixing a composite lithium oxide or a material capable of entering and exiting lithium ions with a dispersion medium, and the electrode mixture paste is applied to a predetermined end. It can be obtained by applying to both surfaces of the strip-shaped core material except for drying. The end portion of the core material to which the electrode mixture paste is not applied may be only one end portion along the longitudinal direction or both end portions along the longitudinal direction. The width of the exposed core material is preferably 2 to 7 mm for the positive electrode and 1 to 4 mm for the negative electrode.

  The columnar electrode plate group is configured by winding a positive electrode and a negative electrode through a separator. In the winding step, the core material exposed portions of the positive electrode and the negative electrode are arranged so as to protrude from the end portions of the other electrode on one and the other end surfaces of the columnar electrode plate group, respectively. The length of the exposed core material protruding from the end of the other electrode is preferably about 0.5 to 2 mm, for example.

  On one and the other end faces of the columnar electrode plate group, the core material exposed portion protruding from the end portion of the other electrode is bent in the central axis direction of the electrode plate group. In this way, a positive electrode current collector and a negative electrode current collector formed of the bent core material exposed part are formed. A flat positive current collector plate and a negative current collector plate are joined to the positive current collector and the negative current collector, respectively, by a method such as welding.

  A porous film having electronic insulating properties is bonded to at least one surface of the positive electrode and the negative electrode. Since the porous membrane has an action of transferring the electrolytic solution by capillary action, the electrode plate whose surface is covered with the porous membrane is quickly impregnated with the electrolytic solution.

  In addition, since the porous film adheres to at least one surface of the positive electrode and the negative electrode, even if the separator contracts due to a large amount of heat generated due to the internal short circuit portion, it has electronic insulation properties together with the separator. The porous membrane is not deformed. Therefore, improvement in safety can be expected.

  As the porous film, one made of an inorganic filler and a film binder can be used. However, in the case of using a high-performance negative electrode excellent in lithium acceptability, the membrane binder needs to be made of a water-insoluble resin material. The high performance negative electrode includes a negative electrode binder and a thickener, the negative electrode binder is made of a rubbery polymer, and the thickener is made of a water-soluble polymer such as a cellulose resin. If a water-soluble resin material is used for the porous membrane, it is necessary to dissolve or disperse the raw material for the porous membrane in water. Therefore, in the production process, the water-soluble polymer in the negative electrode is swollen by the water contained in the porous film before drying, the negative electrode is deformed, and the yield is greatly reduced.

  Here, “the resin material is water-insoluble” means that a substantially uniform solution cannot be obtained even when the resin material is mixed with water. On the contrary, it is desirable that the resin material is one that can be uniformly dissolved in an organic solvent.

  Further, the film binder needs to be made of a resin material that does not have a crystal melting point of less than 200 ° C. and has a heat deformation start temperature and a decomposition start temperature of 200 ° C. or more. The reason is that, in the nail penetration test for evaluating the battery behavior at the time of internal short circuit, the heat generation temperature at the time of internal short circuit locally exceeds several hundred degrees Celsius depending on the conditions. At such a high temperature, a resin material having a crystal melting point of less than 200 ° C. or a resin material having a thermal deformation start temperature and a decomposition start temperature of less than 200 ° C. causes excessive softening and burning, and deforms the porous film. It will be.

  The membrane binder may be a combination of a plurality of types of resin materials. However, at least one kind of resin material does not have a crystal melting point of less than 200 ° C., and needs to have a thermal deformation start temperature and a decomposition start temperature of 200 ° C. or more. The resin material constituting the membrane binder is particularly preferably an amorphous material having no crystalline melting point.

  In addition, it is not realistic from the viewpoint of battery characteristics and design capacity to form the porous film alone without adhering the porous film to the electrode surface. This is because if the porous membrane alone is to be formed into a sheet shape, the thickness of the porous membrane needs to be considerably increased from the viewpoint of maintaining the sheet shape, and a large amount of membrane binder is required.

  The resin material constituting the membrane binder preferably contains at least a rubbery polymer having rubber elasticity. This is because a porous film containing such a film binder is excellent in impact resistance. Therefore, when the positive electrode and the negative electrode are wound through the separator, cracks and the like are hardly generated, and the production yield of the battery can be maintained higher.

  As the rubbery polymer constituting the membrane binder, a rubbery polymer containing an acrylonitrile unit is particularly excellent in that it is amorphous and has high heat resistance. A preferred example of the rubbery polymer containing acrylonitrile units is acrylonitrile rubber or a modified product thereof. Examples of commercially available rubbery polymers containing acrylonitrile units include BM-720H manufactured by Nippon Zeon Co., Ltd.

  The inorganic filler used for the porous membrane preferably has the same degree of heat resistance as the membrane binder, and is desirably thermally stable even at 200 ° C. or higher. In addition, the inorganic filler is desired to be electrochemically stable under the usage environment of the lithium ion secondary battery. The inorganic filler is preferably a material suitable for pasting (painting) the raw material of the porous film.

The BET specific surface area of the inorganic filler is 0.9 m ² / g or more, more preferably 1.5 m ² / g or more from the viewpoint of facilitating the impregnation of the electrolyte solution into the electrode plate group and improving battery performance and life. It is preferable that When the BET specific surface area is less than 0.9 m ² / g, the effect of increasing the bonding force between the inorganic fillers by the film binder is reduced. Further, from the viewpoint of suppressing the aggregation of the inorganic filler and optimizing the fluidity of the raw material paste (paint) of the porous membrane, the BET specific surface area is not too large, and is preferably 150 m ² / g or less, for example.

  The average particle diameter (number-based median diameter) of the inorganic filler is preferably 0.1 to 5 μm. A plurality of kinds of inorganic fillers may be mixed and used. In that case, it is also possible to obtain a dense porous film by mixing and using inorganic fillers having different average particle diameters.

  From the above viewpoint, the inorganic filler is preferably an inorganic oxide, and for example, alumina, titanium oxide, or the like can be preferably used. In particular, α-alumina is preferably used. Resin fine particles are also generally used as fillers. However, since the fillers used in the present invention require heat resistance and high electrochemical stability, resin fine particles are unsuitable.

  The content of the inorganic filler in the total of the inorganic filler and the membrane binder is preferably 50% by weight or more and 99% by weight or less. When the content of the inorganic filler is less than 50% by weight, the amount of the membrane binder becomes excessive, and it becomes difficult to control the pore structure constituted by the gaps between the inorganic filler particles. Moreover, when the content rate of an inorganic filler exceeds 99 weight%, the quantity of a film | membrane binder will become small and the adhesiveness with respect to the electrode-plate surface of a porous film will fall.

  The thickness of the porous membrane is not particularly limited, but from the viewpoint of sufficiently promoting the transfer of the electrolyte solution by the porous membrane and the effect of improving the safety, and maintaining the design capacity of the battery, from 0.5 to It is preferably 20 μm. In this case, it is possible to control the sum of the thickness of the separator and the thickness of the porous film that are currently used generally to 15 to 30 μm.

The negative electrode includes at least a negative electrode active material made of a material that allows lithium ions to enter and exit, a negative electrode binder, and a thickener.
As the negative electrode active material, it is possible to use various natural graphites, various artificial graphites, petroleum coke, carbon fibers, organic polymer fired products, silicon-containing composite materials such as oxides and silicides, various metals or alloy materials. it can.

  As the negative electrode binder, a rubbery polymer is used from the viewpoint of improving the lithium ion acceptability as described above. As such a rubbery polymer, those containing styrene units and butadiene units are preferably used. For example, a styrene-butadiene copolymer (SBR), a modified SBR, or the like can be used, but it is not limited thereto. However, the negative electrode binder needs to be able to be dissolved or dispersed in water. Even if it is the same as the water-insoluble rubbery polymer used as the membrane binder of the porous membrane, it can be used as the negative electrode binder as long as it can be dispersed in water.

  The rubbery polymer constituting the negative electrode binder needs to be used in combination with a thickener made of a water-soluble polymer. Here, as the water-soluble polymer, a cellulose-based resin is preferable, and carboxymethyl cellulose (CMC) is particularly preferable.

  The amount of the negative electrode binder composed of a rubber-like polymer and the thickener composed of a water-soluble polymer contained in the negative electrode is 0.1 to 5 parts by weight and 0.1 to 5 parts per 100 parts by weight of the negative electrode active material, respectively. It is preferable that it is a weight part.

The positive electrode includes at least a positive electrode active material made of a composite lithium oxide, a positive electrode binder, and a conductive agent.
Examples of the composite lithium oxide include lithium cobaltate (LiCoO ₂ ), lithium cobaltate modified, lithium nickelate (LiNiO ₂ ), lithium nickelate modified, lithium manganate (LiMn ₂ O ₄ ), lithium manganate Preferred are those obtained by substituting a part of Co, Mn or Ni of these oxides with other transition metal elements. Some modified bodies contain elements such as aluminum and magnesium. There are also those containing at least two of cobalt, nickel and manganese. Mn-based lithium-containing transition metal oxides such as LiMn ₂ O ₄ are particularly promising because they exist abundantly on the earth and are inexpensive.

  The positive electrode binder is not particularly limited, and polytetrafluoroethylene (PTFE), modified acrylonitrile rubber particles (such as BM-500B manufactured by Nippon Zeon Co., Ltd.), polyvinylidene fluoride (PVDF), and the like can be used. PTFE and BM-500B may be used in combination with CMC, polyethylene oxide (PEO), modified acrylonitrile rubber (such as BM-720H manufactured by Nippon Zeon Co., Ltd.), which is a thickener for the raw material paste of the positive electrode mixture layer. preferable. PVDF is single and has a function as a positive electrode binder and a function as a thickener.

  As the conductive agent, acetylene black, ketjen black, various graphites and the like can be used. These may be used alone or in combination of two or more.

As the non-aqueous electrolyte, it is preferable to use a non-aqueous solvent that dissolves a lithium salt as a solute. As the lithium salt, it is preferable to use lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium borofluoride (LiBF ₄ ), etc., and the nonaqueous solvent is ethylene carbonate (EC). , Propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC) and the like are preferably used. Although a nonaqueous solvent can also be used individually by 1 type, it is preferable to use 2 or more types in combination. The solute concentration dissolved in the non-aqueous solvent is generally 0.5 to 2 mol / L.

  Use vinylene carbonate (VC), cyclohexylbenzene (CHB), modified products of VC and CHB, etc. to form a good film on the positive electrode and / or negative electrode and to ensure stability during overcharge. You can also.

  The separator is not particularly limited as long as it is made of a material that can withstand the usage environment of the lithium ion battery, but a microporous sheet made of a polyolefin resin is generally used. In addition, as the polyolefin resin, polyethylene, polypropylene, or the like is used. The microporous sheet may be a single layer film made of one kind of polyolefin resin or a multilayer film made of two or more kinds of polyolefin resins. Although the thickness of a separator is not specifically limited, From a viewpoint of maintaining the design capacity of a battery, it is preferable that it is 8-30 micrometers.

  FIG. 1 is a schematic vertical cross-sectional view of a main part of an example of an electrode plate group before forming a current collector. The positive electrode includes a positive electrode core material 31 and a positive electrode mixture layer 32 supported on both surfaces thereof, and the negative electrode includes a negative electrode core material 33 and a negative electrode mixture layer 34 supported on both surfaces thereof. A porous film 35 is bonded to the surface of the negative electrode. A separator 36 is interposed between the porous film 35 on the negative electrode surface and the positive electrode mixture layer 32. Each of the positive electrode and the negative electrode has a core material exposed portion at one end, and these are disposed in opposite directions.

  FIG. 2 is a schematic vertical cross-sectional view of the main part of an example of the electrode plate group after the current collector is formed. The exposed portions of the positive electrode core material 31 and the negative electrode core material 33 are bent in the direction of the central axis of the electrode plate group by plastic deformation, respectively, to form a substantially flat positive electrode current collector 31a and negative electrode current collector 33a. The core exposed portion bent in the central axis direction forms a flat current collecting portion as a whole while producing some wrinkles. Since the positive electrode and the negative electrode are wound in a spiral shape, it is difficult to bend in the direction opposite to the central axis direction.

Next, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to these.
<< Comparative Example 1 >>
This will be described with reference to FIG.

(A) Production of positive electrode 3 kg of lithium cobaltate, 1 kg of PVDF # 1320 (N-methyl-2-pyrrolidone (NMP) solution containing 12% by weight of PVDF) manufactured by Kureha Chemical Co., Ltd. as a positive electrode binder, 90 g of acetylene black and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a positive electrode mixture paste. This paste was applied to both surfaces of a 15 μm-thick aluminum foil 11b as a positive electrode core material, dried and then rolled to form a positive electrode mixture layer 11a. At this time, a core material exposed portion having a width of 5 mm was left at one end portion along the longitudinal direction of the aluminum foil 11b. Further, the thickness of the electrode plate made of the aluminum foil 11b and the positive electrode mixture layer 11a was controlled to 160 μm. Thereafter, the electrode plate was slit to a width that can be inserted into a can-shaped battery case of a cylindrical battery (Part No. 18650) to obtain a hoop of the positive electrode 11.

(B) Production of negative electrode 3 kg of artificial graphite, 75 g of BM-400B (an aqueous dispersion containing 40% by weight of a styrene-butadiene copolymer (rubber particles)) manufactured by Nippon Zeon Co., Ltd. as a negative electrode binder, 30 g of CMC as a thickener and an appropriate amount of water were stirred with a double-arm kneader to prepare a negative electrode mixture paste. This paste was applied to both sides of a 10 μm-thick copper foil 12b as a negative electrode core material, dried and rolled to form a negative electrode mixture layer 12a. At this time, a core material exposed portion having a width of 5 mm was left at one end portion along the longitudinal direction of the copper foil 12b. Further, the thickness of the electrode plate made of the copper foil 12b and the negative electrode mixture layer 12a was controlled to 180 μm. Thereafter, the electrode plate was slit to a width that can be inserted into a can-shaped battery case of a cylindrical battery (product number 18650) to obtain a hoop of the negative electrode 12.

(C) Preparation of non-aqueous electrolyte solution A mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (MEC) at a volume ratio of 2: 3: 3, and a concentration of 1 mol / L of LiPF ₆ Then, 3% by weight of vinylene carbonate (VC) was added to prepare an electrolytic solution.

(D) Configuration of electrode plate group Each of the positive electrode 11 and the negative electrode 12 is cut to a predetermined length, and a microporous sheet 13 made of polyethylene resin and having a thickness of 20 μm is wound as a separator to form a columnar electrode plate. Groups were made up. However, in the winding process, the core material exposed portions of the positive electrode 11 and the negative electrode 12 were protruded by about 2 mm from the end portions of the other electrode on one and the other end surfaces of the columnar electrode plate group, respectively.

(E) Production of current collector part While rotating the electrode plate group around the winding axis, while pressing a predetermined jig against the exposed portion of the positive electrode core member on one end face of the electrode plate group, The jig was moved from the side toward the center. As a result, the core material exposed portion was sequentially bent from the outer peripheral side toward the inner peripheral side, and a flat positive electrode current collector 21 was formed. Similarly, while pressing the jig against the exposed portion of the negative electrode core material on the other end face of the electrode plate group, the jig is moved from the outer peripheral side of the end face toward the center to form a flat negative electrode current collector 22. did.

(F) Attaching the current collector plate The flat positive electrode current collector 21 and negative electrode current collector 22 were pressed against the flat positive electrode current collector 18 and negative electrode current collector 19, respectively. In this state, the positive electrode current collector 18 and the negative electrode current collector 19 were welded to the positive electrode current collector 21 and the negative electrode current collector 22 by irradiating a laser beam radially from the outside of each current collector.

(G) Battery Assembly One end of the positive electrode lead 18 a was welded to the positive electrode current collector plate 18, and one end of the negative electrode lead 19 a was welded to the negative electrode current collector plate 19. Next, the electrode plate group was accommodated in the inner space of the battery can 15. A separator was interposed between the electrode plate group and the inner surface of the battery can 15. The other end of the positive electrode lead 18 a was welded to the back surface of the battery lid 6. The other end of the negative electrode lead 19 a was welded to the inner bottom surface of the battery can 15. Thereafter, 5.5 g of a nonaqueous electrolytic solution was injected into the inner space of the battery can 15. Then, the electrode group was vacuum impregnated with the electrolytic solution. Then, the opening of the battery can 15 was closed with the battery lid 6, and the opening end of the battery can 15 was caulked and sealed with an insulating packing 17 disposed on the periphery of the battery lid 6. Thus, a cylindrical lithium ion secondary battery (Part No. 18650) was completed. It was confirmed that the obtained lithium ion secondary battery had a high current collection efficiency and a small temperature rise during charging and discharging.

<< Comparative Example 2 >>
(A) Preparation of porous membrane raw material paste 970 g of AKP50 (α-alumina with a median diameter of 0.3 μm) manufactured by Sumitomo Chemical Co., Ltd. as an inorganic filler, and manufactured by Nippon Zeon Co., Ltd. as a membrane binder BM-720H (NMP solution containing 8% by weight of a rubbery polymer containing an acrylonitrile unit) and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a porous film raw material paste.

(B) Battery assembly The raw material paste for the porous film was applied to both sides of the separator made of a microporous sheet and dried to form a porous film having a thickness of 5 μm per side adhered to the surface of the separator. A cylindrical lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the separator having the porous film thus obtained was used.

The physical properties of AKP50 (α-alumina) are shown below.
<1> BET specific surface area: about 10 m ² / g
<2> Heat resistance: 250 ° C. or higher It is known that the heat resistance of α-alumina is 250 ° C. or higher.

The physical properties of BM-720H (rubbery polymer containing acrylonitrile units) are shown below.
<1> Crystal melting point: None (non-crystalline)
<2> Decomposition start temperature: 320 ° C
<3> Thermal deformation start temperature: 320 ° C
<4> Affinity with water: water-insoluble

Example 1
The same raw material paste having the same porous film as that prepared in Comparative Example 2 was applied to both sides of the positive electrode hoop and dried to form a porous film having a thickness of 5 μm per side adhered to the surface of the positive electrode. A cylindrical lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the positive electrode having the porous film thus obtained was used.

<< Examples 2 to 8 >>
The same porous membrane raw material paste as prepared in Comparative Example 2 was applied to both sides of the negative electrode hoop, dried, and the thickness per side adhered to the negative electrode surface was 0.5 μm (Example 2). A porous film of 1 μm (Example 3), 5 μm (Example 4), 10 μm (Example 5), 15 μm (Example 6), 20 μm (Example 7) or 30 μm (Example 8) was formed. A cylindrical lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the negative electrode having the porous film thus obtained was used.

<< Examples 9 to 14 >>
The thickness of the separator made of the microporous sheet is 8 μm (Example 9), 10 μm (Example 10), 15 μm (Example 11), 25 μm (Example 12), 30 μm (Example 13) or 40 μm (implemented). A cylindrical lithium ion secondary battery in which the thickness of the porous film adhered to the negative electrode surface was 5 μm per side was produced in the same manner as in Example 4 except that Example 14) was adopted.

<< Examples 15 to 20 >>
(A) Preparation of raw material paste for porous film The content of the inorganic filler in the total of the inorganic filler and the film binder was 30% by weight (Example 15), 50% by weight (Example 16), 70% by weight. (Example 17), 90% by weight (Example 18), 95% by weight (Example 19) or 99% by weight (Example 20) Was prepared.

(B) Battery assembly The raw material paste for the porous film was applied to both sides of the negative electrode hoop and dried to form a porous film having a thickness of 5 μm per side adhered to the surface of the negative electrode. A cylindrical lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the negative electrode having the porous film thus obtained was used.

<< Example 21 >>
(A) Preparation of raw material paste for porous film 970 g of AKP50 (α-alumina having a median diameter of 0.3 μm) manufactured by Sumitomo Chemical Co., Ltd. as an inorganic filler, 30 g of water-soluble CMC, and an appropriate amount of water Was stirred with a double-arm kneader to prepare a raw material paste for a porous film.
CMC had no crystalline melting point, was amorphous, had a decomposition start temperature of 245 ° C., and a deformation start temperature of 198 ° C.

(B) Battery assembly The raw material paste for the porous film was applied to both sides of the negative electrode hoop and dried to form a porous film having a thickness of 5 μm per side adhered to the surface of the negative electrode. A cylindrical lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the negative electrode having the porous film thus obtained was used.

Example 22
(A) Preparation of raw material paste of porous film 970 g of AKP50 (α-alumina having a median diameter of 0.3 μm) manufactured by Sumitomo Chemical Co., Ltd. as an inorganic filler, 30 g of water-insoluble PVDF, and an appropriate amount of NMP And a two-arm kneader to prepare a raw material paste for a porous film.
The crystal melting point of PVDF was 174 ° C., the decomposition start temperature was 360 ° C., and the thermal deformation start temperature was 153 ° C.

(B) Battery assembly The raw material paste for the porous film was applied to both sides of the negative electrode hoop and dried to form a porous film having a thickness of 5 μm per side adhered to the surface of the negative electrode. A cylindrical lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the negative electrode having the porous film thus obtained was used.

<< Example 23 >>
(A) Preparation of Porous Film Raw Material Paste Porous film raw material paste in the same manner as Comparative Example 2 except that titania (titanium oxide) having the same median diameter was used instead of AKP50 (α-alumina). Was prepared. That is, 970 g of titania as an inorganic filler, 375 g of BM-720H (NMP solution containing 8% by weight of a rubbery polymer containing an acrylonitrile unit) as a membrane binder, and an appropriate amount of NMP And a two-arm kneader to prepare a raw material paste for a porous film.
As titania, TA300 (anatase type) manufactured by Fuji Titanium Industry Co., Ltd. was used. The BET specific surface area of titania was 8 m ² / g.

(B) Battery assembly The raw material paste for the porous film was applied to both sides of the negative electrode hoop and dried to form a porous film having a thickness of 5 μm per side adhered to the surface of the negative electrode. A cylindrical lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the negative electrode having the porous film thus obtained was used.

Example 24
A negative electrode was produced in the same manner as in Comparative Example 1 except that PVDF was used instead of BM400B as the negative electrode binder and CMC as the thickener. That is, 3 kg of artificial graphite, 240 g of PVDF as a negative electrode binder (8% by weight with respect to artificial graphite), and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a negative electrode mixture paste. Prepared. A negative electrode was produced in the same manner as in Comparative Example 1 except that this paste was used. Here, PVDF # 1320 used as a positive electrode binder was used for PVDF. Other than the above, in the same manner as in Example 4, a cylindrical lithium ion secondary battery in which the thickness of the porous film adhered to the surface of the negative electrode was 5 μm per side was produced.

Here, the physical property evaluation method of an inorganic filler is demonstrated.
[1] BET specific surface area of inorganic filler The BET specific surface area was measured based on the BET single point method using a direct reading specific surface area measuring device. First, a sample of 0.5 to 1 g of an inorganic filler is placed in a glass cell, and cleaning is performed at 250 ° C. for 20 to 30 minutes under the flow of a mixed carrier gas of nitrogen and helium (volume ratio N ₂ : He = 30: 70). Carried out. Next, N ₂ in the carrier gas was adsorbed while cooling the inorganic filler sample with liquid nitrogen. Thereafter, the temperature of the inorganic filler sample was raised to room temperature, the desorption amount of N ₂ was detected with a heat conduction detector, and the specific surface area was calculated from the surface area corresponding to the desorption amount and the measured sample mass. For the calculation, NOVA2000 manufactured by Yuasa Ionics Co., Ltd. was used.

[2] Heat resistance of inorganic fillers Differential scanning calorimetry (DSC) and thermogravimetry-differential thermal analysis (TG-DTA) of samples of inorganic fillers are performed, and changes in DSC measurement are performed. The heat resistance was evaluated by the temperature of the bending point or the temperature of the starting point of the weight change in the TG-DTA measurement.

Here, a method for evaluating physical properties of the membrane binder will be described.
[3] Crystal melting point or decomposition start temperature of membrane binder Perform DSC measurement and TG-DTA measurement of a sample of resin material, and determine the temperature at the inflection point in DSC measurement or the temperature at the start of weight change in TG-DTA measurement. Crystal melting point or decomposition start temperature.

[4] Thermal deformation start temperature of the membrane binder The thermal expansion of the membrane binder sample was measured with a thermal expansion measuring device (8141H manufactured by Rigaku Corporation), and the dimensional change became 10% of the initial value. This temperature was taken as the thermal deformation start temperature.

[5] Affinity between membrane binder and water The solubility of the membrane binder in water was measured at room temperature and normal pressure, and when the solubility was 1% by weight or less, it was judged to be “water-insoluble”. .

  The structure of the porous membrane is summarized in Table 1. Table 2 summarizes the thickness of the separator and the type of the negative electrode binder.

Next, the produced battery was evaluated by the following method.
[Electrolytic solution impregnation by electrode plate group]
Five battery cans each having each electrode plate group inserted therein were prepared. Each of the five battery cans was injected with 5.5 g of an electrolytic solution, and the electrode plate group was taken out from one battery can at regular intervals, and the wetness of the electrodes was visually observed. Table 2 shows the time required from the time of injecting the electrolyte to the time when it is determined that the entire electrode plate group is sufficiently wetted with the electrolyte.

[Porosity adhesion]
After coating on the positive electrode, negative electrode or separator, it was dried, and the state of the porous film immediately after obtained was visually observed. Table 2 shows that “OK” indicates that there is no trace of chipping, cracking, or dropout, and the state is good.

[Negative electrode appearance]
After applying the raw material paste for the porous film on the negative electrode, it was dried and the state of the negative electrode immediately after the porous film was formed was visually observed. Table 2 shows a case where a defect such as a dimensional change is observed as “changed” and the other as “no change”.

[Porosity of porous membrane]
When the positive electrode and the negative electrode were wound through a separator made of a microporous sheet, the state of the porous film formed on any one of the positive electrode, the negative electrode, and the separator was mainly visually observed. Table 2 shows the number of wound electrode plates each having 10 pieces attached to each battery, and the number of electrode plates that were chipped, cracked or dropped out by winding.

[Battery design capacity]
The diameter of the wound electrode plate group was set to 16.5 mm with emphasis on the insertability with respect to the battery case diameter of 18 mm. In this case, assuming that the capacity per 1 g of the positive electrode active material is 142 mAh, the battery design capacity is obtained from the weight of the positive electrode and is shown in Table 3.

[Battery charge / discharge characteristics]
The completed battery having the electrode plate group free from chipping, cracking, or falling off by winding was subjected to pre-charging / discharging twice and stored at 45 ° C. for 7 days. Thereafter, the following two patterns of charging and discharging were performed in a 20 ° C. environment.

(1) First pattern constant current charge: 1400 mA (end voltage 4.2 V)
Constant voltage charge: 4.2V (end current 100mA)
Constant current discharge: 400mA (end voltage 3V)
(2) Second pattern constant current charge: 1400 mA (end voltage 4.2 V)
Constant voltage charge: 4.2V (end current 100mA)
Constant current discharge: 4000 mA (final voltage 3 V)
The charge / discharge capacity at this time is shown in Table 3.

[Nail penetration safety]
The battery after evaluation of charge / discharge characteristics was charged as follows.
Constant current charging: 1400mA (end voltage 4.25V)
Constant voltage charge: 4.25V (end current 100mA)
From the side of the battery after charging, an iron round nail having a diameter of 2.7 mm was penetrated at a speed of 5 mm / second or 180 mm / second in a 20 ° C. environment, and the heat generation state at that time was observed. Table 3 shows the temperatures reached after 1 second and 90 seconds after the battery penetration.

Hereinafter, the evaluation results will be described.
(1) About the impregnation property of the electrolyte solution by the electrode plate group In the case of Comparative Example 1, the time required to impregnate the electrode plate group with the electrolyte solution was about 40 minutes. The reason why such a long time is required is that a flat current collecting portion formed by the exposed portion of the electrode core material formed on the end face of the electrode plate group becomes an obstacle. When a flat current collector is formed, it takes about twice as long as when a flat current collector is not formed. On the other hand, when the porous film was formed on the electrode surface as in each of the examples, the time for impregnating the electrode group with the electrolyte solution could be greatly shortened. For example, in Example 1, the impregnation was completed in about 5 minutes. In Example 4, the impregnation was completed in about 3 minutes.

(2) About the adhesion | attachment location of a porous film In the comparative example 1 which does not exist a porous film, the heat_generation | fever after 1 second is remarkable regardless of the nail penetration speed. On the other hand, in each Example in which the porous film was formed on the positive electrode or the negative electrode, heat generation after nail penetration was greatly suppressed.

  When all the batteries after the nail penetration test were disassembled and examined, the separators were melted over a wide range in all the batteries. However, for each example, the porous membrane retained its original shape. From this, it is considered that when the heat resistance of the porous film is sufficient, the film structure is not destroyed with respect to the heat generated by the short circuit that occurs after nail penetration, the expansion of the short-circuited portion can be suppressed, and the excessive heat generation can be prevented.

  On the other hand, in Comparative Example 2 in which the porous film is formed on the separator, it can be seen that heat generation is promoted when the nail penetration speed is low. When the battery of Comparative Example 2 was disassembled and examined, it was confirmed that the porous film was also deformed as the separator was melted. Regardless of the heat resistance of the porous film itself, it is considered that when the separator adhered to the porous film contracts or melts, the porous film follows the change in shape of the separator, and the porous film is damaged.

(3) About the nail penetration test The cause of the heat generated by the nail penetration can be explained as follows from the past experimental results.
When the positive electrode and the negative electrode come into contact (short circuit) by nail penetration, Joule heat is generated. Then, the low heat resistance material (separator) is melted by Joule heat to form a strong short-circuit portion. As a result, generation of Joule heat continues and the temperature is raised to a temperature range (160 ° C. or higher) where the positive electrode becomes thermally unstable. This causes a thermal runaway.

When the nail penetration speed was reduced, local enhancement of fever could be observed. When the nail penetration speed is reduced and the short-circuit area per unit time is limited, a considerable amount of heat is concentrated in the limited part. For this reason, it is considered that the positive electrode reaches a temperature range where it becomes thermally unstable.
On the other hand, when the nail penetration speed is increased and the short-circuit area per unit time is expanded, heat is dispersed over a large area. Therefore, it is thought that it becomes difficult to reach the temperature region where the positive electrode becomes thermally unstable.

(4) Thickness of porous film In Example 8 where the thickness of the porous film layer is too large, the design capacity is reduced because the length of the electrode plate constituting the electrode plate group is shortened, and high rate discharge is performed. The capacity has also declined. Therefore, in order to sufficiently obtain the effects of the present invention, it is desirable that the thickness of the porous film is 0.5 to 20 μm.

(5) About the thickness of the separator In Example 14 where the thickness of the separator is too large, since the length of the electrode plate constituting the electrode plate group is shortened, the design capacity is greatly reduced, and high-rate discharge is achieved. The capacity is also decreasing. Therefore, in order to sufficiently obtain the effects of the present invention, it is desirable that the thickness of the separator is 8 to 30 μm.

(6) Content of inorganic filler in porous membrane In Example 15, the content of the inorganic filler in the total of the inorganic filler and the membrane binder is small (the membrane binder is large). Decrease is observed. This is presumably because the gap between filler particles cannot be secured sufficiently because the membrane binder is excessive, and the ionic conductivity of the porous membrane is reduced. Therefore, in order to sufficiently obtain the effects of the present invention, the content of the inorganic filler is desirably 50 to 99% by weight.

(7) Types of membrane binder in the porous membrane In Example 21 using CMC and Example 22 using PVDF as the membrane binder, sufficient heat generation was achieved when the nail penetration speed was reduced. It cannot be suppressed. When these batteries were disassembled and examined, it was confirmed that not only the separator but also the porous film was deformed.

  In Example 21, it is considered that CMC (deformation start temperature 198 ° C.) was deformed due to Joule heat due to short circuit, and deformation of the porous film occurred. In Example 22, it is considered that deformation of the porous film occurred due to melting of PVDF (crystal melting point 174 ° C.). Furthermore, in Examples 21 and 22, it is considered that a strong short-circuit portion was formed by the penetration of the nail, and heat generation could not be suppressed.

  Therefore, the porous membrane does not have its own film binder that hardly burns down or melts or deforms. Specifically, it does not have a crystal melting point of less than 200 ° C., and has a thermal deformation start temperature and a decomposition start temperature of 200 ° C. or higher. It is essential to use at least one membrane binder having From the results of Table 1, it can be understood that a rubbery polymer (thermal decomposition start temperature 320 to 340 ° C., heat deformation start temperature 320 to 334 ° C.) containing an acrylonitrile unit that is amorphous and has high heat resistance can be preferably used. .

  The rubbery polymer containing acrylonitrile units has rubber elasticity. This property is very advantageous in the configuration of the wound electrode group. For example, in an embodiment in which the membrane binder has rubber elasticity, the wound porous membrane has a sufficient shape and has no defects. On the other hand, in Examples 21 and 22, many defects occurred in the evaluation of the flexibility of the porous film.

(8) Negative electrode appearance In Example 21, after the formation of the porous film, poor appearance due to deformation of the negative electrode was observed. As described above, this is considered to be a result of the thickener in the negative electrode being swollen by water contained in the porous film before drying. In order to avoid such a low yield production, it is essential to use a water-insoluble film binder for the porous film and not use water as a dispersion medium for the raw material paste of the porous film. More generally, it can be said that it is essential to form a porous film using a dispersion medium different from the dispersion medium used in the raw material paste (negative electrode paste) of the negative electrode active material layer.

(9) Types of inorganic filler In Example 23 using titania instead of α-alumina as the inorganic filler, it was confirmed that titania performed almost the same functions as α-alumina.

(10) About the configuration of the negative electrode As shown in Example 24, when PVDF is used as the negative electrode binder, the content of the negative electrode binder must be increased, and the lithium ion acceptability of the negative electrode is reduced. Charge capacity gradually decreases. In addition, the negative electrode plate becomes hard due to the properties of PVDF, and the flexibility of the porous film cannot be utilized. Therefore, it is desirable to use in combination with a water-soluble thickener (such as CMC) a negative electrode binder that has rubber elasticity, such as SBR, and can provide sufficient adhesion to the negative electrode active material layer even in a small amount.

  As described above, according to the present invention, in order to increase the current collection efficiency and suppress the heat generation during charging and discharging, the core material exposed portions protruding from one and the other end surfaces of the columnar electrode plate group are directed toward the end surfaces. In the case of bending the electrode plate, the time for impregnation of the electrolyte into the electrode plate group can be shortened compared to the conventional case, and the decrease in productivity can be suppressed.

  Moreover, this invention can improve the safety | security of a lithium ion secondary battery by using together the porous film excellent in heat resistance, and a sheet-like separator in a preferable aspect simultaneously with achievement of the said objective. Currently, in various applications, safety standards for lithium ion secondary batteries are becoming stricter. Under such circumstances, it can be said that the present invention capable of suppressing thermal runaway regardless of the nail penetration speed (short circuit state) is extremely practical.

It is a longitudinal cross-sectional schematic diagram of the principal part of an example of the electrode group before forming a current collection part. It is the longitudinal cross-sectional schematic of the principal part of an example of the electrode group after forming a current collection part. 4 is a longitudinal sectional view of a lithium ion secondary battery of Comparative Example 1. FIG.

Explanation of symbols

31 Positive electrode core material 31a Positive electrode current collector 33a Negative electrode current collector 32 Positive electrode mixture layer 33 Negative electrode core material 34 Negative electrode mixture layer 35 Porous film 36 Separator

6 Battery cover 11 Positive electrode 11a Positive electrode mixture layer 11b Aluminum foil 12 Negative electrode 12a Negative electrode mixture layer 12b Copper foil 13 Microporous sheet 15 Battery can 17 Insulation packing 18 Positive electrode current collector plate 18a Positive electrode lead 19 Negative electrode current collector plate 19a Negative electrode lead 21 Positive electrode current collector 22 Negative electrode current collector

Claims (7)

(A) a positive electrode comprising a positive electrode core material and a composite lithium oxide supported on the positive electrode core material;
(B) a negative electrode comprising a negative electrode core material and a material capable of entering and exiting lithium ions carried on the negative electrode core material;
(C) separator,
(D) a non-aqueous electrolyte,
(E) an electronic insulating porous film adhered to at least one surface of the positive electrode and the negative electrode;
(F) a positive electrode current collector plate, and (g) a lithium ion secondary battery comprising a negative electrode current collector plate,
The positive electrode and the negative electrode are wound through the separator and constitute a columnar electrode plate group,
The positive electrode and the negative electrode each have a core exposed portion at one end along the longitudinal direction thereof,
The core material exposed portions of the positive electrode and the negative electrode protrude from the end portions of the other electrode at one and the other end surfaces of the columnar electrode plate group, respectively, and bend in the central axis direction of the electrode plate group. Forming a positive current collector and a negative current collector,
The positive electrode current collector and the negative electrode current collector are lithium ion secondary batteries joined to the positive electrode current collector and the negative electrode current collector, respectively. The negative electrode includes a negative electrode binder and a thickener, the negative electrode binder is made of a first rubbery polymer, the thickener is made of a water-soluble polymer,
The electronic insulating porous membrane is composed of an inorganic filler and a membrane binder, and the membrane binder is water-insoluble and does not have a crystalline melting point of less than 200 ° C. and has a heat of 200 ° C. or more. The lithium ion secondary battery according to claim 1, comprising a resin material having a deformation start temperature and a decomposition start temperature.   The lithium ion secondary battery according to claim 2, wherein the resin material is made of a second rubber-like polymer.   The lithium ion secondary battery according to claim 3, wherein the second rubbery polymer contains an acrylonitrile unit.   The inorganic filler is made of alumina, and the content of the inorganic filler in the total of the inorganic filler and the membrane binder is 50 wt% or more and 99 wt% or less. The lithium ion secondary battery as described.   The lithium ion secondary battery according to claim 1, wherein the porous film has a thickness of 0.5 μm or more and 20 μm or less.   The lithium ion secondary battery according to claim 1, wherein the separator has a thickness of 8 μm or more and 30 μm or less.

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