JP2005222780A - Lithium ion secondary battery - Google Patents

Abstract

[PROBLEMS] To secure sufficient safety of a lithium ion secondary battery even when a negative electrode having high lithium ion acceptability is used.
A positive electrode made of a composite lithium oxide, a negative electrode through which lithium ions can enter and exit, a separator, an electrolyte solution made of a nonaqueous solvent, and a porous film bonded to at least one surface of the positive electrode and the negative electrode. The membrane is composed of an inorganic filler and a membrane binder. The membrane binder is water-insoluble and 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 higher. The resin material has an acidic group, the negative electrode includes a negative electrode binder and a thickener, the negative electrode binder is formed of the first rubbery polymer, and the thickener is Lithium ion secondary battery made of water-soluble polymer.
[Selection figure] None

Description

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

  Between the positive electrode and the negative electrode of the lithium ion secondary battery, there is a separator that electronically insulates the electrode plates and further holds the electrolyte. Currently, microporous sheets mainly made of polyethylene resin are used as separators for lithium ion secondary batteries.

  However, sheet-like separators such as microporous sheets generally tend to shrink even at temperatures of 150 ° C. or lower, 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.

Therefore, in order to improve the safety of the lithium ion secondary battery, techniques for forming a porous film containing an inorganic filler on the surface of a sheet-like separator have been proposed (see Patent Documents 1 and 2). Moreover, the technique which forms the porous film which consists of resin with a low glass transition point on an electrode is proposed (refer patent document 3). Furthermore, although the purpose is different, a technique has been proposed in which an inorganic filler such as alumina and a protective layer made of a water-soluble polymer are formed on an electrode (see Patent Document 4).
JP 2001-319634 A JP 2002-8730 A JP-A-11-144706 JP-A-9-147916

  However, in the proposals of Patent Documents 1 and 2, a porous film is formed on the surface of the sheet-like separator. Therefore, when the separator contracts, the porous film also contracts accordingly. These technologies are originally aimed at suppressing the growth of lithium dendrite and improving the high rate discharge characteristics. Therefore, the safety at the time of internal short circuit or nail penetration cannot be guaranteed.

  The proposal of Patent Document 3 intends to develop a shutdown effect that blocks the movement of ions between electrode plates by softening a resin having a low glass transition point during short-circuit heat generation. In the nail penetration test, the heat generation temperature during 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. Therefore, the resin shutdown function cannot be an absolute safety mechanism against an internal short circuit.

  In Patent Document 4, since the protective layer contains an inorganic filler having excellent heat resistance and a water-soluble polymer (polyacrylic acid derivative, cellulose derivative, etc.), it can be expected to suppress deformation of the protective layer itself during short-circuit heat generation. However, 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. The rubbery polymer can be used in a smaller amount than polyvinylidene fluoride (PVDF) that has been conventionally used as a negative electrode binder. Therefore, 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. When the protective layer containing the water-soluble polymer of Patent Document 4 is applied to such a negative electrode, the thickener in the negative electrode swells with water contained in the protective layer before drying. 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.

  Moreover, in the case of the porous film containing the above-mentioned inorganic filler, there is also a problem that the binding force of the resin material contained in the porous film is poor. The amount of resin material with poor binding power is inevitably increased. This is because if the amount of the resin material used is reduced, sufficient strength of the porous film cannot be secured. However, if the amount of the resin material used is large, the internal resistance due to the porous film increases, leading to a decrease in battery characteristics.

  Even when a negative electrode having high lithium ion acceptability is used, the present invention prevents the deformation of the negative electrode and uses a porous film excellent in heat resistance and a sheet-like separator in combination in a preferred embodiment, thereby providing lithium ion secondary. The purpose is to increase the safety of the secondary battery. Another object of the present invention is to suppress an increase in internal resistance and a decrease in production yield due to the porous film as much as possible.

That is, the present invention
(A) a positive electrode comprising a composite lithium oxide;
(B) a negative electrode through which lithium ions can enter and exit;
(C) separator,
(D) a lithium ion secondary battery comprising an electrolyte solution made of a non-aqueous solvent, and (e) a porous film adhered to at least one surface of the positive electrode and the negative electrode,
The porous membrane is composed of an inorganic filler and a membrane binder, and the membrane binder is water-insoluble and does not have a crystal melting point of less than 200 ° C., and has a thermal deformation initiation temperature and decomposition of 200 ° C. or more. Consisting of a resin material having an onset temperature, the membrane binder has an acidic group,
The negative electrode includes a negative electrode binder and a thickener, the negative electrode binder is made of a first rubber-like polymer, and the thickener is a lithium ion secondary battery made of a water-soluble polymer. .

  By adhering the porous film to at least one surface of the positive electrode and the negative electrode, it is possible to prevent the porous film from shrinking together with the sheet-like separator even when heat is generated. Moreover, since the membrane 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, since the membrane binder is made of a resin material that does not have a crystalline melting point of less than 200 ° C. and has a thermal deformation start temperature and a decomposition start temperature of 200 ° C. or higher, resistance to thermal decomposition and thermal deformation of the porous membrane Will improve. Therefore, deformation of the porous film and burnout of the porous film due to softening of the film binder are unlikely to occur. Furthermore, since the membrane binder has an acidic group, the adhesive force between the porous membrane and the electrode surface and the adhesive force between the inorganic fillers can be sufficiently ensured even if the amount of the membrane binder used is small. Therefore, the usage-amount of a film | membrane binder can be reduced and the raise of the internal resistance by a porous film can be suppressed.

  The porous film according to the present invention must be formed on the surface of at least one of the positive electrode and the negative electrode described in detail below and adhered to the surface. This is because when the porous film is formed on the separator, as described above, the porous film shrinks together with the separator due to a large amount of heat generated due to the internal short-circuit portion regardless of the heat resistance of the porous film itself.

  In addition, if the porous film alone is formed into a sheet shape without adhering the porous film to the electrode surface, the thickness of the porous film needs to be considerably increased from the viewpoint of maintaining the sheet shape, and a large amount of bonding is required. Need a dressing. Therefore, from the viewpoint of battery characteristics and design capacity, a sheet made of a porous film alone is not practical.

  The porous membrane according to the present invention comprises an inorganic filler and a membrane binder, and the membrane binder is water-insoluble and does not have a crystal melting point of less than 200 ° C., and a thermal deformation start temperature of 200 ° C. or more. And a resin material having a decomposition start temperature.

  The reason for using the water-insoluble resin material is that, as described above, most of the high-performance negative electrodes excellent in lithium acceptability contain a water-soluble polymer such as a cellulose resin as a thickener. 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 thickener in the negative electrode is swollen by water contained in the porous film before drying, the negative electrode is deformed, and the yield is significantly 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.

  The reason for using a resin material that does not have a crystalline melting point of less than 200 ° C. and that has a thermal deformation start temperature and a decomposition start temperature of 200 ° C. or higher is, as described above, in the nail penetration test, which is a substitute evaluation of internal short circuit, Depending on the conditions, the heat generation temperature at the time of internal short circuit is locally over several hundred degrees Celsius. 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, it is essential that the at least one resin material 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 resin material constituting the membrane binder is particularly preferably an amorphous material having no crystalline melting point.

  The reason for using the membrane binder having an acidic group is to sufficiently secure the adhesive strength between the porous membrane and the electrode surface and the adhesive strength between the inorganic fillers. Here, the acidic group is preferably composed of a hydroxyl group. This is because the hydroxyl group is easy to maintain the polar balance in the molecule and can be easily introduced into the resin material. An acidic group larger than the hydroxyl group may be introduced into the resin material, but the steric hindrance increases. Therefore, the number of large acidic groups introduced is generally reduced as compared to the case of introducing hydroxyl groups.

  The membrane binder preferably exhibits an acidity of pH 3 or more and less than 7, more preferably an acidity of pH 3 or more and less than 6. If the pH is too close to neutrality, it will be difficult to ensure sufficient adhesion between the porous membrane and the electrode surface and between the inorganic fillers when the amount of the membrane binder used is small. . It is not necessary to make the pH of the membrane binder too small. For example, a pH of 3 or higher is sufficient to ensure adhesive strength. The pH of the membrane binder is preferably measured in a state where it is dissolved or dispersed in a solvent (dispersion medium) used when preparing the raw material paste for the porous membrane.

  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. Production of batteries equipped with a wound-type electrode plate group, especially when a membrane binder containing a rubbery polymer is used, so that cracking and the like are unlikely to occur when the positive electrode and the negative electrode are wound through a separator. Yield can be kept high.

  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.

Examples of the method for introducing an acidic group into the membrane binder include the following.
First, there is a method of mixing a rubbery polymer containing an acrylonitrile unit and a resin material containing a hydroxyl group. In addition, a method in which a rubbery polymer and an organic acid are mixed is also exemplified. Examples of organic acids that can be used include benzoic acid and formic acid.

  From the viewpoint of increasing the potential resistance and the electrolyte solution resistance of the rubber-like polymer, it is preferable that the rubber-like polymer has an association point. Since a polymer having an association point can form a block state in which molecular chains are folded, the molecular structure is not easily destroyed by voltage application or an electrolytic solution. Therefore, rubber elasticity is easily maintained. Rubbery polymers are generally electrochemically unstable. However, a rubber-like polymer that forms a blocked state by intramolecular association can prevent the molecular chain from being damaged because the molecular chain is less likely to be attacked by electric charges. Further, such a rubbery polymer is unlikely to swell due to the electrolyte.

  The association point of the rubbery polymer often consists of a functional group capable of hydrogen bonding. The functional group capable of hydrogen bonding is most commonly a hydroxyl group, but is not limited thereto. When the rubbery polymer used as the membrane binder has a hydroxyl group, it is preferable to control the length of the molecular chain of the rubbery polymer so that the hydroxyl group can function effectively as an association point. The length of the molecular chain can be controlled by the synthesis conditions of the rubbery polymer. For example, the length of the molecular chain can be controlled by changing the molecular weight of the monomer or changing the monomer concentration of the monomer solution used during polymerization. When graft polymerization is performed, the number of association points can be increased or decreased by changing the size of the side chain.

  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, from the viewpoint of facilitating the injection of the electrolyte into the electrode plate group and improving the battery performance and life. The above is preferable. 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.

  Some inorganic fillers have basic sites on the surface. When the inorganic filler has a site showing basicity on the surface, the binding force between the membrane binder having an acidic group and the surface of the inorganic filler is enhanced. Therefore, the pH of the surface of the inorganic filler is preferably greater than 7 and 12 or less.

  The basic site of the inorganic filler is generally derived from a cation. For example, when the inorganic filler contains aluminum, there is a high possibility that basic sites derived from aluminum ions are formed. From the above viewpoint, alumina, titanium oxide, or the like can be preferably used as the inorganic filler. In particular, α-alumina is preferably used. Basic sites exist on the surface of α-alumina. When α-alumina is used as the inorganic filler, a bonding force based on an acid-base reaction is generated between the acidic group of the membrane binder and the basic site of the inorganic filler. Therefore, adhesion between inorganic fillers is easily maintained.

  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.

  In the present invention, since the membrane binder having an acidic group is used, the bonding strength between the inorganic filler and the membrane binder is high. Therefore, even when the content of the inorganic filler in the total of the inorganic filler and the membrane binder is as high as 97% by weight or more, for example, a porous film having sufficient strength can be obtained. That is, the present invention is particularly suitable when it is strongly desired to reduce the content of the membrane binder in the porous membrane and reduce the battery internal resistance due to the porous membrane.

  The thickness of the porous film is not particularly limited, but is preferably 0.5 to 20 μm from the viewpoint of sufficiently exerting the function of improving the safety by the porous film and maintaining the design capacity of the battery. 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, a negative electrode binder, and a thickener.
As the negative electrode active material, various natural graphites, various artificial graphites, silicon-containing composite materials such as silicide, and various alloy materials can be used.

  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, a positive electrode binder, and a conductive agent.
An example of the positive electrode active material is a composite oxide. As the composite oxide, lithium cobaltate, modified lithium cobaltate, lithium nickelate, modified lithium nickelate, lithium manganate, modified lithium manganate, and the like are preferable. Some modified bodies contain elements such as aluminum and magnesium. There are also those containing at least two of cobalt, nickel and manganese.

  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.

Various lithium salts such as LiPF ₆ and LiBF ₄ can be used as solutes in the electrolyte solution composed of a non-aqueous solvent. As the non-aqueous solvent, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), and the like are preferably used, but are not limited thereto. . Although a nonaqueous solvent can also be used individually by 1 type, it is preferable to use 2 or more types in combination.

  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.
Next, the present invention will be described more specifically based on examples, but the present invention is not limited to the following examples.

<< Comparative Example 1 >>
(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 a 15 μm thick aluminum foil, dried and then rolled to form a positive electrode mixture layer. Under the present circumstances, the thickness of the electrode plate which consists of aluminum foil and a mixture layer was controlled to 160 micrometers. Thereafter, the electrode plate was slit to a width that could be inserted into a can battery case of a cylindrical battery (Part No. 18650) to obtain a positive electrode hoop.

(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. The paste was applied to a 10 μm thick copper foil, dried and rolled to form a negative electrode mixture layer. Under the present circumstances, the thickness of the electrode plate which consists of copper foil and a mixture layer was controlled to 180 micrometers. Thereafter, the electrode plate was slit to a width that could be inserted into a can battery case of a cylindrical battery (Part No. 18650) to obtain a negative electrode hoop.

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

(D) Battery assembly The above-described positive electrode and negative electrode were each cut to a predetermined length, wound with a microporous sheet made of polyethylene resin having a thickness of 20 μm as a separator, and inserted into the battery case. Next, 5.5 g of the above electrolyte was weighed and poured into the battery case, and the opening of the case was sealed. Thus, a cylindrical lithium ion secondary battery was produced.

<< 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.

  When α-alumina contained in the raw material paste for the porous film was 890 g and BM-720H was 110 g, a part of the porous film might peel off from the separator. From this, it was found that when the content of the membrane binder in the porous membrane is less than 1% by weight, the strength of the porous membrane is lowered.

(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> Surface basicity: pH = 9
<2> BET specific surface area: about 10 m ² / g
<3> Heat resistance: 250 ° C. or higher As described above, since AKP50 had pH = 9, it was confirmed that AKP50 has a basic site on the surface. Moreover, 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 <5> pH: 6

<< Comparative Example 3 >>
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.

Example 1
(A) Preparation of a membrane binder comprising an acrylonitrile unit and containing an acidic group Polyethersulfone (PES) rich in hydroxyl groups is added to BM-720H manufactured by Nippon Zeon Co., Ltd. , Well mixed. PES is a linear polymer having heat resistance, a crystal melting point is 225 ° C., a thermal decomposition start temperature is 270 ° C., and a thermal deformation start temperature is 265 ° C. Here, Sumika Excel 5003P manufactured by Sumitomo Chemical Co., Ltd. was used as PES. The weight ratio of the rubbery polymer containing acrylonitrile units in BM-720H and PES was 15: 5. The pH of the membrane binder consisting of the obtained composite (PES / BM-720H) was 5.7.

(B) Preparation of raw material paste for porous membrane 980 g of AKP50 (α-alumina having a median diameter of 0.3 μm) manufactured by Sumitomo Chemical Co., Ltd. as an inorganic filler and the above-mentioned membrane binder (PES) having an acidic group / BM-720H) with a resin weight of 20 g and an appropriate amount of NMP were stirred in a double-arm kneader to prepare a raw material paste for a porous film.

(C) Battery assembly The raw material paste for the porous film 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.

  In this example, 980 g of α-alumina is mixed with 20 g of the membrane binder, and the ratio of the membrane binder to α-alumina is considerably small. However, the strength of the porous membrane did not decrease, and it had sufficient strength. Therefore, no peeling occurred in a part of the porous film in the winding step of the electrode plate. Also from this, it can be understood that an extremely high performance porous membrane can be obtained by using a membrane binder having an acidic group.

Example 2
(A) Preparation of a film binder having an acidic group and comprising an acrylonitrile unit having an association point and having an acidic group In the binder in the porous film paste, the molecular length of the monomer part of the polyacrylonitrile-modified rubber A paste was prepared as a rubber material having an association point as a material adjusted so as to be about 15% per unit length, and further provided with a hydroxyl group.

Here, polyethersulfone (PES) containing a large amount of hydroxyl groups was added to the rubber-like polymer having an association point and sufficiently mixed. The PES used is a linear polymer having heat resistance, the crystal melting point is 225 ° C., the thermal decomposition start temperature is 270 ° C., and the thermal deformation start temperature is 265 ° C. Here, Sumika Excel 5003P manufactured by Sumitomo Chemical Co., Ltd. was used as PES. The weight ratio of the rubbery polymer having an association point to PES was 15: 5.
The obtained membrane binder made of the composite (PES / AN *) had a pH of 5.8, no crystal melting point, a decomposition start temperature of 340 ° C., and a thermal deformation start temperature of 334 ° C. .

(B) Preparation of porous membrane raw material paste 970 g of AKP50 (α-alumina having a median diameter of 0.3 μm) manufactured by Sumitomo Chemical Co., Ltd. as an inorganic filler, and the above-described membrane binding having an acidic group and an association point An agent (PES / AN *) 30 g by weight of resin and an appropriate amount of NMP were stirred with a double-arm kneader to prepare a raw material paste for a porous film.

(C) Battery assembly The raw material paste for the porous film 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 3 to 9 >>
The raw material paste of the same porous film as prepared in Example 2 was applied on the negative electrode hoop and dried, and 0.5 μm (Example 3), 1 μm (Example 4), and 5 μm (Example 5) per side. ) A porous film having a thickness of 10 μm (Example 6), 15 μm (Example 7), 20 μm (Example 8) or 30 μm (Example 9) was formed in a state of being 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.

<< Examples 10 to 15 >>
The thickness of the separator made of the microporous sheet is 8 μm (Example 10), 10 μm (Example 11), 15 μm (Example 12), 25 μm (Example 13), 30 μm (Example 14) 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 5 except that Example 15) was used.

<< Examples 16 to 21 >>
(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 16), 50% by weight (Example 17), 70% by weight. (Example 18), 90 wt% (Example 19), 95 wt% (Example 20) or 99 wt% (Example 21) 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.

<< Comparative Example 4 >>
(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.

<< Comparative Example 5 >>
(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 22
(A) Preparation of Porous Film Raw Material Paste Porous film raw material paste in the same manner as in 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, 30 g of the same membrane binder prepared in Example 2 as a resin weight, and an appropriate amount of NMP were stirred with a double-arm kneader to obtain a porous membrane. A raw material paste was prepared.
As titania, TA300 (anatase type) manufactured by Fuji Titanium Industry Co., Ltd. was used. The titania BET specific surface area was 8 m ² / g, and the titania surface had pH = 8.

(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.

<< Comparative Example 6 >>
(A) Production of negative electrode A negative electrode was produced in the same manner as in Comparative Example 1 except that PVDF was used instead of BM400B as a negative electrode binder and CMC as a 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, 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 5.

Here, the physical property evaluation method of an inorganic filler is demonstrated.
[1] pH of inorganic filler
For the pH measurement, a boiling extraction glass electrode measurement method was used, and the measurement was performed based on JIS R6129-1976 and JIS28802-1984. Moreover, pH measurement by ELS-8000 (pH titration apparatus) of Otsuka Electronics Co., Ltd. was also implemented.

[2] BET specific surface area of inorganic filler The BET specific surface area was measured based on the BET one-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.

[3] Heat resistance of inorganic filler Perform differential scanning calorimetry (DSC) and thermogravimetry-differential thermal analysis (TG-DTA) of a sample of inorganic filler, and change in DSC measurement. 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.
[4] 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.

[5] Thermal deformation start temperature of the membrane binder The thermal expansion of the membrane binder sample was measured by 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.

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

[6] pH of membrane binder
A sample of the membrane binder was dissolved or dispersed in NMP, and the pH of the obtained sample solution was measured with a pH measuring device.

  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.
[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.

[Storage characteristics]
The storage characteristics of the batteries of Comparative Example 3, Example 1, and Example 2 were evaluated. That is, after performing preliminary charging / discharging twice for each battery, the battery was stored in a 60 ° C. environment for 20 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 4.

Hereinafter, the evaluation results will be described.
(1) About the adhesion | attachment location of a porous film In the comparative example 1 without a porous film, the heat_generation | fever after 1 second is remarkable regardless of the nail penetration speed. On the other hand, in Comparative Example 3 and each example in which the porous film was formed on the positive electrode or the negative electrode, heat generation after nail penetration was significantly 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.

(2) 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.

(3) Thickness of porous membrane In Example 9 where the thickness of the porous membrane 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.

(4) About the thickness of the separator In Example 15 where the thickness of the separator is too large, the design capacity is greatly reduced because the length of the electrode plate constituting the electrode plate group is shortened. 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.

(5) About the content of the inorganic filler in the porous film In Example 16, the content of the inorganic filler in the total of the inorganic filler and the film binder is small (the film 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.

(6) Types of membrane binder in the porous membrane In Comparative Example 4 using CMC and Comparative Example 5 using PVDF as the membrane binder, heat generation is suppressed when the nail penetration speed is reduced. I can't. When these batteries were disassembled and examined, it was confirmed that not only the separator but also the porous film was deformed.

  In Comparative Example 4, it is considered that deformation of the porous film occurred due to deformation of CMC (deformation start temperature 198 ° C.) due to Joule heat due to short circuit. In Comparative Example 5, it is considered that the porous film was deformed by melting PVDF (crystal melting point 174 ° C.). Further, in Comparative Examples 4 and 5, 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 Comparative Examples 4 and 5, many defects occurred in the evaluation of the flexibility of the porous film.

(7) Presence or absence of acidic groups in membrane binder As described in Comparative Examples 2-3, when the membrane binder has no acidic groups, the content of the inorganic filler in the porous membrane is 97% by weight. Above this, the production yield of the porous membrane decreased. On the other hand, according to each example using the membrane binder having an acidic group, as described in Example 1, even when the content of the inorganic filler in the porous membrane exceeds 97% by weight, the production yield is increased. Good and battery characteristics are also good. Furthermore, in Example 2 where the membrane binder is made of a resin material having an association point and has an acidic group, the storage characteristics are improved (see Table 4). Therefore, it is considered that the life of the battery can be extended.

  Moreover, in Example 1, since the content rate of the film | membrane binder in a porous membrane is as small as 2 weight%, it is thought that the resistance with respect to a movement of lithium ion is small. Therefore, the discharge characteristic at a high rate (high rate) shows a good value. Furthermore, in Example 1, it is thought that many voids of the porous film are secured, and it was revealed that the permeability of the electrolytic solution is good. For example, when the injection time of the electrolytic solution was compared between Example 1 and Comparative Example 1, the time was about 1/5 in Example 1 compared to Comparative Example 1.

(8) Negative electrode appearance In Comparative Example 4, a poor appearance due to deformation of the negative electrode was observed after the formation of the porous film. 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 22 in which titania was used 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 Comparative Example 6, 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, it is possible to prevent deformation of the negative electrode even when a negative electrode having high lithium ion acceptability is used. Moreover, since this invention uses together the porous film excellent in heat resistance, and a sheet-like separator in a preferable aspect, it becomes possible to improve the safety | security of a lithium ion secondary battery. In addition, the present invention is effective as an improvement measure against an increase in internal resistance of a battery and a decrease in production yield. Furthermore, according to the present invention, it is possible to improve the storage characteristics of the lithium ion secondary battery.

  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.

Claims (10)

(A) a positive electrode comprising a composite lithium oxide;
(B) a negative electrode through which lithium ions can enter and exit;
(C) separator,
(D) a non-aqueous electrolyte, and (e) a lithium ion secondary battery comprising a porous film adhered to at least one surface of the positive electrode and the negative electrode,
The porous membrane is composed of an inorganic filler and a membrane binder, and the membrane binder is water-insoluble and does not have a crystal melting point of less than 200 ° C., and has a thermal deformation start temperature and decomposition of 200 ° C. or more. Consisting of a resin material having an onset temperature, the membrane binder has an acidic group,
The lithium ion secondary battery, wherein the negative electrode includes a negative electrode binder and a thickener, the negative electrode binder is made of a first rubbery polymer, and the thickener is a water-soluble polymer.   The lithium ion secondary battery according to claim 1, wherein the acidic group is a hydroxyl group.   The lithium ion secondary battery according to claim 1, wherein the resin material comprises a second rubber-like polymer.   The lithium ion secondary battery according to claim 3, wherein the second rubbery polymer contains an acrylonitrile unit.   The lithium ion secondary battery according to claim 3, wherein the second rubbery polymer has an association point.   The lithium ion secondary battery according to claim 1, wherein the positive electrode and the negative electrode are wound through the separator.   The lithium ion according to claim 1, wherein 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. Secondary battery.   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.   The lithium ion secondary battery according to any one of claims 1 to 9, wherein the membrane binder exhibits an acidity of pH 3 or more and less than 6.