JP5214364B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery having a laminate film as an outer case.

Due to rapid progress in the electronics field in recent years, electronic devices have become more sophisticated, smaller and more portable, and the demand for rechargeable high energy density secondary batteries used in these electronic devices has increased. Conventionally, secondary batteries mounted on these electronic devices include lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, etc. Recently, carbon materials that can occlude / release lithium ions, lithium alloys, etc. are used as active materials. A lithium ion secondary battery in which the negative electrode used and a positive electrode using a lithium-containing composite oxide or the like as an active material is researched, developed, and put into practical use. This type of battery has a high battery voltage and a large energy density per unit weight and volume as compared with the conventional battery, and is the most expected secondary battery in the future.

Recently, it has been found that lithium ion secondary batteries are excellent not only in energy density but also in output characteristics due to their device structure, and are beginning to be put into practical use as batteries suitable for power tools such as electric tools. On the other hand, as a technique for improving the energy density of this type of battery, a sheet-like battery using a lightweight and thin laminated film for an outer case, which is formed by laminating and integrating a metal foil such as aluminum on a pair of insulating resin films. Is being considered. This battery has been studied and partially put into practical use as a battery for portable devices, a battery for hybrid automobiles, and a battery for electric assist bicycles.

As a laminate film used for the exterior case of this sheet-like battery, a configuration is generally employed in which a protective layer, a metal foil layer, an adhesive layer, and a heat-fusible resin layer are sequentially laminated from the outside. For the protective layer, a stretched polyester resin or nylon resin having insulating properties and excellent mechanical strength is used. The metal foil layer is a layer for preventing water vapor from entering from the outside, and a soft aluminum foil or the like is used. The adhesive layer is used to bond the metal foil layer and the heat-fusible resin layer. The heat-sealing resin layer is for housing the power generation element and sealing the outer case to form a package, and generally a polyolefin resin is used.

As this laminate film, a laminated film in which a base material layer, an adhesive layer, an aluminum foil layer, and a thermoadhesive resin layer are laminated in order from at least the outside are laminated so that the thermoadhesive resin layers face each other. A device (Patent Document 1) is proposed that is formed by heat-sealing the edge portion in a bag shape and used as a battery container such as a lithium polymer battery.

In addition, in a laminate in which an aluminum foil, an adhesive layer, and a sealant layer are sequentially laminated on a single layer or multilayer base film, at least the surface of the aluminum foil on the adhesive layer side is subjected to a boehmite treatment. The thing (patent document 2) used as a material is proposed.

JP 2001-176466 A JP 2002-110111 A

This type of sheet battery is desired to improve not only energy density but also life, and lithium hexafluorophosphate (LiPF ₆ ) is often used as an electrolyte salt in terms of charge / discharge characteristics. However, those using lithium hexafluorophosphate (LiPF ₆ ) as the electrolyte salt of the electrolytic solution have a problem in that lithium fluoride (LiF) is deposited on the negative electrode to deactivate the negative electrode with use. That is, this battery has low moisture stability and thermal stability. For example, water mixed in a small amount in the battery, electrolyte anion decomposes even at a weak high temperature of about 60 ° C., and hydrogen fluoride (HF) or similar in the battery. The acid is purified, and it combines with Li ions present in the battery to produce lithium fluoride (LiF), and a passive film that inhibits the charge / discharge reaction is deposited on the negative electrode surface. This phenomenon is a major cause of performance deterioration of this type of battery.

Immediate fixing after producing purified hydrogen fluoride (HF) or the like in the early stage of battery operation is important for extending the life of this type of battery. However, the exterior cases of the inventions described in the above-mentioned literature 1 and the above-mentioned literature 2 cannot cope with this problem.

The present invention has been made in view of the above-described problems of the prior art, and an electrode group formed by laminating a positive electrode and a negative electrode through a separator includes at least lithium hexafluorophosphate (LiPF ₆ ) in an electrolyte salt. In a lithium ion secondary battery housed in an outer case made of a laminate film together with a non-aqueous electrolyte, a laminate film capable of quickly fixing hydrogen fluoride (HF) generated with use is provided. An object of the present invention is to provide a lithium ion secondary battery having a lifetime.

In order to solve the above-mentioned problems, the present inventors have studied in detail the configuration of a laminate film used as an outer case of a sheet-like battery. As a result, the innermost layer in contact with the power generation element in the outer case is dispersed with polyamide (PA). It was found that the life of the battery can be greatly extended by using the polyolefin resin layer. That is, the lithium secondary battery according to the present invention includes, as described in claim 1, an electrode group formed by laminating a positive electrode and a negative electrode with a separator interposed between an electrolyte salt and at least lithium hexafluorophosphate (LiPF _6). In a lithium ion secondary battery housed in an outer case made of a laminate film together with a non-aqueous electrolyte solution containing a non-aqueous electrolyte, wherein the innermost layer of the laminate film is a polyolefin resin layer in which polyamide (PA) is dispersed Is.

Further, as described in claim 2, the laminate film is a laminate film in which a protective layer, a metal foil layer, an adhesive layer, and a heat-fusible resin layer are laminated in order from at least the outer side, The polyolefin resin layer in which polyamide (PA) is dispersed is formed in the inside of the conductive resin layer.

Further, as described in claim 3, the laminate film is a laminate film in which a protective layer, a metal foil layer, an adhesive layer, and a heat-fusible resin layer are laminated in order from at least the outer side, The conductive resin layer is a polyolefin resin layer in which polyamide is dispersed.

In the present invention, as described above, the outermost layer of a laminate film used as an outer case has a polyolefin resin layer in which polyamide (PA) is dispersed, so that hydrogen fluoride (HF) generated in the battery is removed from the outer case. In the vicinity of the film, lithium fluoride (LiF) can be prevented from being deposited on the negative electrode. This is because the polyamide dispersed in the innermost layer of the film has strong interaction with hydrogen fluoride (HF). Here, F of hydrogen fluoride (HF) only if scavenger were charged hydrogen fluoride catch (HF) ^- there is a method to catch, this time, reduced battery safety hydrogen gas is generated in the battery To do. What is important here is not to react hydrogen fluoride (HF) but to collect hydrogen fluoride (HF) at a site having little influence on battery performance by using interaction.

Therefore, the vicinity of the film is suitable as the site having little influence on the battery performance. Since deposition of lithium fluoride (LiF) occurs on the negative electrode, it is not preferable to collect hydrogen fluoride (HF) in the vicinity of the negative electrode, and the same applies to the separator. Further, the positive electrode active material used in the lithium ion secondary battery is not preferable because it is stable in an alkaline environment, and in the acidic environment, the positive electrode active material is decomposed and another deterioration mode starts. Therefore, the vicinity of the film is the only preferable place.

It should be noted that it is not preferable to use polyamide (PA) for all materials of the polyolefin resin layer in which polyamide (PA), which is the innermost layer of the film, is dispersed. This is because polyamide (PA) swells with this type of electrolytic solution, and therefore, when a film having only the innermost layer of polyamide (PA) is heat-sealed, a problem arises in the fusion strength. Therefore, it is important that the innermost layer of the film is a polyolefin resin layer in which polyamide (PA) is dispersed. Therefore, the amount of polyamide (PA) dispersed in the polyolefin resin as the main material is preferably about 0.1 to 10% by volume with respect to the amount of the main material. If the amount is less than 0.1% by volume, the addition effect is not obtained. If the amount is more than 10% by volume, the heat-sealing strength when sealing the films as described above is affected.

In the present invention, an electrode group formed by laminating a positive electrode and a negative electrode through a separator is housed in an outer case made of a laminate film together with a nonaqueous electrolytic solution containing at least lithium hexafluorophosphate (LiPF ₆ ) in an electrolyte salt. In the lithium ion secondary battery, the innermost layer of the laminate film is a polyolefin resin layer in which polyamide is dispersed, so that it is suitable for continuous large discharge without impairing its high energy density, A safe and long-life lithium secondary battery can be provided.

Below, the lithium ion secondary battery of this invention is demonstrated.

First, the laminate film constituting the battery outer case will be described in detail. In the laminate film according to the present invention, the outermost layer is formed of a protective layer. The protective layer is used for improving the mechanical strength and improving the anti-pinhole characteristics and the insulation as the battery outer case. As a configuration, even one layer may be used.In particular, in order to improve the anti-pinhole characteristics, even if a pinhole is formed in the protective layer, so that the pinhole does not penetrate across the front and back of the protective layer, Two or more layers may be laminated. For example, when two layers are used, the same type and the same material may be laminated such as stretched polyester and stretched polyester, or different and different materials such as stretched polyester and nylon film may be laminated. Moreover, as preferable thickness, it is 5-100 micrometers.

However, if the protective layer is excessively multi-layered, the mechanical strength is improved, but the heat dissipation characteristics are lowered, and accordingly, the battery life may be shortened. In addition, as a material of a protective layer, a stretched polyester resin or a nylon film is mentioned. Here, examples of the stretched polyester resin include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), copolymerized polyester, and polycarbonate (PC). . Examples of the nylon film include nylon 6, nylon 6,6, a copolymer of nylon 6 and nylon 6,6, and nylon 6,10.

Next, a metal foil layer is formed inside the protective layer. This metal foil layer is a layer for preventing water vapor from entering the inside of a sheet-like battery, that is, a lithium ion secondary battery. As a material, a metal such as aluminum or nickel is used in order to provide workability and pinhole resistance with a single metal foil layer. Moreover, as preferable thickness, it is 5-50 micrometers.

Next, an adhesive layer is formed inside the metal foil layer. This adhesive layer is provided to adhere the metal foil layer and a heat-fusible resin layer described later. As a material, it selects from the material used for the heat-fusible resin layer mentioned later. Specifically, for example, acid-modified polyolefin resin, polyolefin resin graft-modified with unsaturated carboxylic acid, copolymer of ethylene or propylene and acrylic acid or methacrylic acid (EAA, EMAA, PAA, PMAA), or metal Cross-linked polyolefin resins can be used. Moreover, as preferable thickness, it is 5-50 micrometers.

Next, a heat-fusible resin layer is formed inside the adhesive layer. This heat-fusible resin layer is provided for heat-sealing and sealing the end portion when a power generation element and an electrolytic solution are housed in a sheet-like battery. Moreover, since the electrolyte solution is accommodated in addition to the power generation element, it is necessary to have the electrolyte solution resistance in addition to the moisture resistance. Therefore, as a material, polyethylene (PE), ethylene-α-olefin copolymer, polypropylene (PP), polyolefin resin such as propylene-α-olefin copolymer, ethylene-vinyl acetate copolymer (EVA), Acid-modified products and silane-modified products of ethylene copolymers such as ethylene-α, β unsaturated carboxylic acid copolymers are used. Moreover, as preferable thickness, it is 5-50 micrometers.

Next, a polyolefin resin layer in which polyamide (PA) is dispersed is formed inside the heat-fusible resin layer. As described above, this layer is provided to fix hydrogen fluoride (HF) generated with use in a lithium secondary battery using lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte salt. It is. Examples of the polyamide (PA) material include, but are not limited to, nylon 6, nylon 6,6, a copolymer of nylon 6 and nylon 6,6, and nylon 6,10. The polyolefin material is the same as the polyolefin used for the heat-fusible resin layer, such as polyethylene (PE), ethylene-α-olefin copolymer, polypropylene (PP), propylene-α-olefin copolymer, etc. Is mentioned. A preferred thickness is 5 to 50 μm.

In addition, when the polyolefin resin layer in which polyamide (PA) is dispersed is formed inside the heat-fusible resin layer, since both have the heat-fusibility, the thickness of the heat-fusible resin layer is conventionally It is preferable to make it thinner than the case of the heat fusible resin layer alone.

In addition, the polyolefin resin layer in which polyamide (PA) is dispersed is formed to the same size as the laminate film, but it is sufficient that it is formed to cover at least the negative electrode. Specifically, the heat seal of the laminate film There is no problem if it is formed excluding the peripheral portion.

Further, when the heat-fusible resin layer itself is a polyolefin resin layer in which polyamide (PA) is dispersed, the polyamide (PA) dispersed to have the same thickness as that of a conventional heat-fusible resin alone Since the amount of the heat-fusible resin is reduced by that amount and the fusing strength is affected, it is preferable to form a thicker film than the conventional heat-fusible resin layer alone.

The positive electrode active material of a lithium ion secondary battery using the laminated film, the lithium-containing transition metal composite oxide having or spinel structure compound such as LiMn ₂ O _4, generally alpha-NaFeO ₂ structure which is expressed by LiMO ₂ Materials (where M is a single or two or more metal elements selected from Co, Ni, Al, Mn, Ti, Fe, etc.), lithium-containing phosphate compounds, and the like can be used. Furthermore, if the battery manufacturing method is devised, metal oxides such as MnO ₂ and V ₂ O ₅ into which lithium can be inserted, metal sulfides such as TiS ₂ and ZnS _2, and polyaniline having electrochemical redox activity It is also possible to use a π-conjugated polymer such as polypyrrole or a disulfide compound utilizing the formation-cleavage of a sulfur-sulfur bond in the molecule.

On the other hand, as the negative electrode, metallic lithium or various lithium alloys, various metal oxides such as SnO ₂ , or a carbon material capable of occluding and releasing lithium can be used. As the carbon material, naturally produced graphite or baked raw material is fired at a high temperature of 2000 ° C. or higher, and a graphite-based carbon material having a flat potential characteristic with a developed graphite structure, or an organic raw material is relatively heated to 1000 ° C. or lower. A coke-based carbon material that is fired at a low temperature and can be expected to have a larger charge / discharge capacity than a graphite-based material is used.

The positive electrode current collector is preferably an aluminum foil having a thickness of 5 to 60 μm. The positive electrode active material, a conductive aid such as scaly graphite or carbon black, and a binder such as polyvinylidene fluoride are provided on at least one surface of the current collector. A paste formed with a solvent can be applied and dried to form a positive electrode active material-containing coating film having a thickness of 30 to 300 μm.

As the negative electrode current collector, a copper foil having a thickness of 5 to 60 μm is preferable, and the negative electrode active material and a binder such as polyvinylidene fluoride are pasted into a paste with a solvent on at least one surface of the current collector and dried. And what formed the 30-300 micrometer-thick negative electrode active material containing coating film can be used.

As a solvent for the electrolytic solution, a solvent usually used in an electrolytic lithium ion secondary battery, for example, ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), sulfolane (SL), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), dimethoxyethane (DME), diethoxyethane (DEE), 2-methyl-tetrahydrofuran (2MeTHF), various glymes, etc. alone or in a mixed system Although it can be used, it is necessary to have a solvent structure capable of dissolving LiBETI up to 2.0. Also, it is desirable to avoid DMC, DME, DEE, etc. because the flash point is below room temperature.

The lithium salt used as the solute of the electrolytic solution is usually a lithium salt used in an electrolytic lithium ion secondary battery, such as lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO _4). ), Inorganic lithium salts such as lithium tetrafluoroborate (LiBF ₄ ), lithium trifluoromethylsulfonate (LiOSO ₂ CF ₃ ), lithium bis (trifluoromethylsulfonyl) imide (LiN (CF ₃ SO ₂ ) ₂ ) An organic lithium salt such as bis (perfluoroethylsulfonyl) imidolithium (LiN (CF ₅ SO ₂ ) ₂ ) can be appropriately selected and used in accordance with the synthesis method of the electrolyte gel.

As the separator, a nonwoven fabric or a porous sheet made of a polyolefin-based synthetic resin is suitable.

Next, the effects of the present invention will be described in detail by way of examples.

[Production of positive electrode]
In the dispersing machine, 91 parts by weight of LiCoO ₂ powder as the positive electrode active material, 4.0 parts by weight of polyvinylidene fluoride resin as the binder, 5.0 parts by weight of graphite powder as the conductive agent, and N-methylpyrrolidone as the dispersing agent were used. Then, a slurry for coating the positive electrode active material mixture was prepared by stirring and mixing.

Next, the positive electrode active material mixture coating slurry is applied simultaneously on both sides to a current collector made of aluminum foil using a die coater, and dried in an oven to remove the dispersant, thereby removing the positive electrode active material mixture. An agent coating was formed. This was pressed to a predetermined density, cut to a predetermined size, and a positive electrode plate having a plain part with a predetermined width was obtained.

[Production of negative electrode]
Application of negative electrode active material mixture by mixing 90 parts by weight of artificial graphite powder, 10 parts by weight of polyvinylidene fluoride resin as a binder, and N-methylpyrrolidone as a dispersant by stirring and mixing with a disperser A slurry was prepared.

Next, the negative electrode active material mixture coating slurry is simultaneously coated on both sides of a copper foil current collector using a die coater, and dried in an oven to remove the dispersant, thereby removing the negative electrode active material mixture. An agent coating was formed. This was pressed to a predetermined density and cut to a predetermined size to obtain a negative electrode plate having a plain part with a predetermined width.

[Battery assembly]
A separator made of two polyethylene porous sheets is partially heat-sealed to produce an envelope-like separator bag, and the produced positive electrode plate is inserted into the separator, leaving the plain portion, A positive electrode plate was obtained.

Next, the negative electrode plate and the separator-covered positive electrode plate are alternately stacked so that the plain portions of the respective electrode plates are in opposite directions to produce an electrode group of 10 positive electrode plates and 11 negative electrode plates. did.

Next, a positive electrode terminal made of an aluminum plate having a thickness of 0.2 mm and a width of 40 mm is formed on the uncoated positive electrode plate of the electrode group thus produced, and a nickel plate having a thickness of 0.2 mm and a width of 40 mm is formed on the uncoated negative electrode plate. A negative electrode terminal to be prepared was attached by an ultrasonic welding method under respective predetermined conditions to produce a battery element.

Example 1
Next, the battery element produced in this way was stored in an outer case made of a laminate film having the configuration shown in FIG. In FIG. 1, 1 is a protective layer A made of polyethylene terephthalate (PET) which is the outermost layer, and 2 is a protective layer B made of nylon. A metal foil layer 3 made of aluminum foil is formed inside these protective layers, and a double-sided adhesive polypropylene (PP) adhesive layer 4 is sandwiched inside the protective layer, and a heat-fusible resin layer made of polypropylene (PP). 5 is formed. The innermost layer is formed with a polyolefin resin layer 6 in which 5% by volume of nylon 6 is dispersed in polypropylene (PP). Specifically, the two laminated films are sandwiched so that the polyolefin resin layer 6 comes to the battery element side, and two laminated films are overlapped at the edge, and the positive electrode terminal and the negative electrode terminal are laminated films. Then, the exposed side and the other side were heat-sealed to form a heat-welded region, and a dry cell was produced using the remaining side as an opening.

Next, this dry cell is vacuum-dried under predetermined conditions, and then lithium hexafluorophosphate is dissolved in ethylene carbonate and diethyl carbonate in a weight mixing ratio of 3: 7 from the opening to 1.3 mol / l. The organic electrolyte solution was injected, the inside of the cell was decompressed, and the opening was heat sealed and sealed, followed by a predetermined initial charge at a current of 0.1 CA, storage for a predetermined time, and then 0.2 CA The cell was discharged with current until the cell voltage reached 2.75 V, and finally the activation process was performed to produce five sheet-like lithium ion secondary batteries with a rated capacity of 15 Ah. This battery is referred to as Example 1.

(Example 2)
The battery element was housed in an outer case made of a laminate film having the configuration shown in FIG. In FIG. 2, the configuration from the outside to the protective layers 1 and 2, the metal foil layer 3, and the adhesive layer 4 is the same as that of the first embodiment. The next heat-fusible resin layer is the innermost layer, and differs from the case of Example 1 in that it is a polyolefin resin layer 6 in which 5% by volume of nylon 6 is dispersed in polypropylene (PP). Except this, it carried out similarly to Example 1, and produced five sheet-like lithium ion secondary batteries with a rated capacity of 15 Ah. This battery is referred to as Example 2.

(Comparative Example 1)
The battery element was housed in an outer case made of a laminate film having the configuration shown in FIG. In FIG. 3, the protective layers 1 and 2, the metal foil layer 3, the adhesive layer 4, and the heat-fusible resin layer 5 from the outside are the same as the configuration of Example 1, and nylon 6 is applied to polypropylene (PP). The difference from Example 1 is that there is no polyolefin resin layer 6 dispersed by 5% by volume. Except this, it carried out similarly to Example 1, and produced five sheet-like lithium ion secondary batteries with a rated capacity of 15 Ah. This battery is referred to as Comparative Example 1.

[Confirmation test]
In order to compare the life characteristics of the sheet-like lithium ion secondary battery using the outer case made of the laminate film according to Examples 1 and 2 and Comparative Example 1, a cycle test was performed under the following conditions. That is, a cycle in which each battery was discharged for 2 hours at a current of 0.5 CA with a 4.2 V voltage regulation and discharged until the cell voltage reached 2.5 V at a current of 2 CA was repeated 1000 cycles. The ratio of the 2CA capacity at the 1000th cycle to the 2CA discharge capacity at the 10th cycle was compared as the discharge capacity retention rate (%). The results (average value of 5) are shown in Table 1.

As a result of the evaluation, the batteries of Examples 1 and 2 have a polyolefin resin layer in which polyamide (PA) is dispersed in the innermost layer of the laminate film, and the batteries of Examples 1 and 2 according to the present invention do not have the above resin layer. Compared to the battery of Example 1, it was found that the capacity retention rate was excellent by about 30 to 40%. This fixes hydrogen fluoride (HF), which causes generation of lithium fluoride (LiF), which becomes a passive film that inhibits charge / discharge reaction, on the negative electrode surface of the lithium ion secondary battery in the vicinity of the laminate film. It is guessed that it was possible.

The sheet-like lithium ion secondary battery using the laminate film having the structure according to the present invention for the outer case can perform continuous large discharge without impairing the high energy density, and can be used for motorcycles, electric vehicles, and hybrid electric vehicles. Although it can be used for a power source of an automobile or the like, the application is not particularly limited.

It is sectional drawing of the laminate film which comprises the battery outer case which concerns on Example 1 of this invention. It is sectional drawing of the laminate film which comprises the battery outer case which concerns on Example 2 of this invention. It is sectional drawing of the laminate film which comprises the battery exterior case which concerns on the conventional comparative example 1. FIG.

Explanation of symbols

1 Protective layer A
2 Protective layer B
3 Metal foil layer 4 Adhesive layer 5 Heat-sealable resin layer 6 Polyolefin resin layer in which polyamide is dispersed

Claims (3)

Lithium in which an electrode group formed by laminating a positive electrode and a negative electrode through a separator is housed in an outer case made of a laminate film together with a non-aqueous electrolyte containing at least lithium hexafluorophosphate (LiPF ₆ ) in an electrolyte salt The lithium ion secondary battery according to the ion secondary battery, wherein the innermost layer of the laminate film is a polyolefin resin layer in which polyamide is dispersed. The laminate film is a laminate film in which a protective layer, a metal foil layer, an adhesive layer, and a heat-fusible resin layer are laminated in order from at least the outside, and polyamide is dispersed inside the heat-fusible resin layer. The lithium ion secondary battery according to claim 1, wherein a polyolefin resin layer is formed. The laminate film is a laminate film in which a protective layer, a metal foil layer, an adhesive layer, and a heat-sealable resin layer are laminated in order from at least the outside, and the polyolefin resin in which the heat-sealable resin layer disperses polyamide. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is a layer.