JP3752913B2 - Secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a secondary battery used in a portable electronic device or the like. In particular, the present invention relates to a lithium ion secondary battery that can have a thin shape.
[0002]
[Prior art]
There is a great demand for miniaturization and weight reduction of portable electronic devices, but in order to realize this, it is indispensable to improve the performance and miniaturization of the battery. To that end, various battery developments and improvements are currently underway. Yes. The characteristics required for the battery include high voltage, high energy density, reliability, and arbitrary shape. The lithium ion secondary battery is a secondary battery that is expected to achieve the highest voltage and high energy density among the conventional batteries, and its improvement is actively promoted even now.
[0003]
A lithium ion secondary battery has, as main components, a positive electrode and a negative electrode, and an ion conductive layer sandwiched between the positive electrode and the negative electrode. In the lithium ion secondary battery currently in practical use, the positive electrode is made by applying a powder of lithium cobalt oxide or the like as an active material to a current collector, and the negative electrode is made of a carbon-based material as an active material. A powder is applied to a current collector to form a plate. As the ion conductive layer, a separator, which is a porous film made of polyethylene or polypropylene, filled with a non-aqueous electrolyte is used.
[0004]
In a conventional lithium ion secondary battery, for example, as shown in Japanese Patent Laid-Open No. 8-83608, in order to maintain the electrical contact of the positive electrode, the separator, and the negative electrode, which are constituent elements, By applying pressure from the outside with a strong outer can such as metal, contact between the surfaces of all the components is maintained.
[0005]
Further, in the example of the solid secondary battery described in, for example, JP-A-5-159802, the ion conductive solid electrolyte layer and the electrode layer are heated and bonded with a thermoplastic resin binder, whereby the electrode layer and A manufacturing method for integrating the electrolyte layer is shown. In this case, since the electrical contact is maintained by integrating the electrode layer and the electrolyte layer, it functions as a battery without applying pressure from the outside.
[0006]
In addition, a thin battery using a polymer gel as an ion conductor is known. In this thin battery, the positive electrode, the separator, and the negative electrode are integrated by using polyvinylidene fluoride as the polymer gel. ing.
[0007]
In order to solve the above-described problems, there is a method in which an electrode active material layer and a separator layer are bonded and integrated by a porous adhesive layer as disclosed in, for example, JP-A-10-172537. This publication is characterized in that after the adhesive resin solution is applied to the separator, the electrode active material layer is bonded, and the solvent is dried to make the adhesive layer porous.
[0008]
[Problems to be solved by the invention]
Since the conventional battery is configured as described above, in order to make the positive electrode and the separator, and the negative electrode and the separator sufficiently in electrical contact, a strong outer can made of a metal that can be pressurized from the outside is used. As a result, the ratio of the outer can other than the power generation unit to the volume and weight of the battery increases, which is disadvantageous for forming a battery having a high energy density with respect to the volume and weight. was there.
[0009]
In addition, in a battery in which the electrode layer and the electrolyte layer are joined with a binder or a polymer gel, the interface between the electrode layer and the electrolyte is covered with a solid binder, so that the interface between the electrode layer and the electrolyte is From the viewpoint of ion conductivity in the case, for example, it is disadvantageous compared to a battery of the type in which pressure is applied from the outside with an outer can using a liquid electrolyte as described above. Even when a binder or polymer gel is used, a binder or polymer gel having an ionic conductivity equal to or higher than that of a liquid electrolyte is not generally found, and is equivalent to a battery using a liquid electrolyte. Ion conductivity cannot be obtained.
[0010]
In the method of bonding and integrating the electrode active material layer and the separator layer with a porous adhesive layer as disclosed in JP-A-10-172537, the electrode active material layer integrated with an adhesive Since both of the separator and the separator have a porous structure, the adhesive solution layer that exists at the interface between the electrode active material layer and the separator and should contribute to the expression of the adhesive force has a porous structure when the solvent is dried. Many of the applied adhesives cannot be contributed to the expression of the adhesive force because they are sucked by the layer or the separator. On the other hand, it is desirable to reduce the coating amount of adhesive resin with poor ion conductivity from the viewpoint of improving battery characteristics, but for the above reasons, it is difficult to reduce the coating amount of adhesive to have a certain level of adhesive strength. There was a problem.
[0011]
The present invention has been made in order to solve the above-described problems, and by bonding an electrode active material layer and a separator with a smaller amount of adhesive, a strong outer can for applying pressure from the outside is provided. An object of the present invention is to form a battery body having good electrical contact between an electrode layer and an electrolyte layer without using it, and to obtain a battery having good charge / discharge characteristics and high safety.
[0012]
[Means for Solving the Problems]
A first secondary battery according to the present invention is a secondary battery in which a separator is bonded and integrated by an adhesive layer between the active material layers of a positive electrode and a negative electrode in which an active material layer is bonded to a current collector. , what polyolefin porous membrane der of porosity 45% to 55%, and those maximum pore diameter of the porous membrane is 100 to 300 nm.
[0014]
Second secondary battery according to the present invention is a secondary battery separator in the active material layers of adhered positive electrode and a negative electrode active material layer on the current collector is integrated is bonded by an adhesive layer, said separator A porous membrane made of polyolefin having a porosity of 45 to 55%, and the thermal shrinkage rate at 100 ° C. of the porous membrane is 10% or less.
[0015]
A third secondary battery according to the present invention is a secondary battery in which a separator is bonded and integrated by an adhesive layer between the active material layers of the positive electrode and the negative electrode in which an active material layer is bonded to a current collector. A porous film made of polyolefin having a porosity of 45 to 55%, and the porous film has a multilayer film structure in which two or more kinds of polyolefin resins are laminated.
[0016]
A fourth secondary battery according to the present invention has a plurality of units, wherein one of the first to third secondary batteries is a unit in which a separator is disposed between a positive electrode and a negative electrode. Is.
[0017]
A fifth secondary battery according to the present invention is the fourth secondary battery in which a plurality of units are formed by stacking the separated positive electrode, separator, and negative electrode.
[0018]
A sixth secondary battery according to the present invention is the fourth secondary battery in which a plurality of units are formed by winding up a continuous positive electrode, separator, and negative electrode.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
A feature of the present invention resides in the physical properties of a porous film used for a separator in a battery in which an electrode active material layer and a separator are bonded and integrated by a porous adhesive layer, and has a specific porosity, pore diameter, and heat. A porous film having a shrinkage rate is used as a separator.
[0020]
In the secondary battery in which the electrode active material layer and the separator are bonded and integrated by the porous adhesive layer, the present inventors have various ways to improve charge / discharge efficiency and improve safety. As a result of repeated research, the electrode active material layer and the separator are firmly bonded with a smaller amount of adhesive by using a porous film having a specific porosity, hole diameter, and heat shrinkage rate as described above. It was found that a secondary battery having good charge / discharge characteristics and high safety can be obtained.
[0021]
That is, according to the study by the present inventors, by using a polyolefin porous film having a porosity of 45 to 55% as a separator, the electrode active material layer and the separator can be firmly bonded with a smaller amount of adhesive, which is good. It was found that a secondary battery having excellent charge / discharge characteristics and high safety can be obtained. Thus, the reason why the electrode active material layer and the separator can be firmly bonded with a smaller amount of the adhesive and the secondary battery having good charge / discharge characteristics and high safety was obtained is that the electrode active material layer and the separator layer The most influential part of the adhesive is the interface between the adhesive layer and the separator, and the adhesive force at the interface between the adhesive layer and the separator is due to the anchoring effect that part of the adhesive enters the separator hole. It is considered that the anchoring effect was particularly effective when the porosity was 45 to 55%.
[0022]
Further, according to the research of the present inventors, when the pore diameter of the porous membrane used in the separator is in the range of 1000 to 300 nm, the adhesive force between the electrode active material layer and the separator is particularly strong, and even a smaller amount It has been found that an electrode active material layer and a separator layer can be firmly bonded with an adhesive, and a secondary battery having good charge / discharge characteristics and high safety can be obtained. The details of the effect of the pore size of this porous membrane are unknown, but after applying the adhesive solution, the electrode active material layer and the separator are bonded together, and when the solvent is dried, the adhesive solution layer has a porous structure. When the pore diameter of the porous membrane is in the range of 100 to 300 nm for the problem of being sucked by a certain electrode active material layer or separator, the suction force of the electrode active material layer based on the capillary force is balanced with the suction force of the separator, It is considered that as a result of more adhesive solution remaining at the interface between the electrode active material layer and the separator, the electrode active material layer and the separator were firmly bonded.
[0023]
The porous film used for the separator preferably has a heat shrinkage rate at 100 ° C. of 10% or less. In general, a porous film used for a separator is often stretched during production, and the stretched film shrinks by heating and attempts to restore its original shape. When the battery generates heat due to some cause such as a short circuit, in the case of a separator having a large heat shrinkage rate, there is a risk that the positive electrode and the negative electrode are short-circuited at the end face portion of the battery body due to contraction and further the heat generation of the battery is promoted. In order to avoid such a risk, it is desired that the separator is firmly bonded to the electrode and a porous film having a small heat shrinkage rate is used for the separator.
[0024]
The polyolefin porous film used for the separator is a single-layer porous film made of a resin such as polyethylene, polypropylene, polybutene, and a copolymer containing at least one of them, or a laminated porous film made of two or more kinds of resins. A membrane can be used. A shutdown characteristic in which the separator closes when the battery temperature rises to block the flow of ions is expected, but the temperature at which the shutdown function appears depends on the melting point of the porous membrane material used for the separator. From the viewpoint of the shutdown function, it is particularly desirable to use a porous film including at least one polyethylene or polypropylene layer as the separator.
[0025]
In manufacturing the battery, a non-aqueous solvent solution of lithium salt is used as the electrolytic solution.
[0026]
Lithium salts can include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiPF ₆ , LiI, LiBr, LiSCN, Li ₂ B ₁₀ Cl ₁₀ , LiCF ₃ CO ₂ and mixtures thereof.
[0027]
Further, as a non-aqueous solvent, propylene carbonate, γ-butyl lactone, ethylene carbonate, tetrahydrofuran, 2-tetrahydrofuran, 1,3-dioxolane, 4,4-dimethyl-1,3-dioxolane, diethyl carbonate, dimethyl carbonate, sulfolane, 3-Methylsulfolane, tert-butyl ether, iso-butyl ether, 1,2-dimethoxyethane, 1,2-ethoxymethoxyethane and mixtures thereof can be used.
[0028]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. In addition, the battery charging / discharging characteristic and overcharge characteristic of the battery obtained by this invention were performed in accordance with the following method.
[0029]
<Battery charge / discharge characteristics>
A battery was manufactured by the method described in the following examples, and the charge / discharge characteristics at a current value of 2C were described in the battery manual (Battery Manual Editorial Board edited by Maruzen, published in 1990). According to the method, it measured on condition of the following.
Charging: Constant current + constant voltage method, upper limit voltage 4.2V
Discharge: Constant current Lower limit voltage 2.5V
Charge / discharge efficiency (%) = discharged electric capacity / charged electric capacity × 100
[0030]
<Overcharge characteristics>
A battery was prepared by the method described in the following examples, and the battery thus manufactured was charged up to 10 V at a current value of 2 C. After that, the battery appearance change, the maximum temperature reached on the surface of the battery container, and the separator shut down The flowing current value was measured.
[0031]
<Physical properties of porous membrane>
The porosity, pore diameter, and heat shrinkage at 100 ° C. of the porous film used for the separator were determined as follows.
(1) From the true specific gravity D of the porous membrane determined by the porosity liquid replacement method and the bulk density d determined by measuring the weight and dimensions of the porous membrane, the following formula was used.
Porosity (%) = (1−d / D) × 100
(2) Pore diameter The maximum pore diameter of the porous membrane determined by the mercury intrusion method was adopted.
(3) After exposing a porous film cut out to a 10 cm square heat shrinkage (area 100 cm ² ) in a 100 ° C. atmosphere for 3 hours, the area S was measured and calculated using the following formula.
Thermal shrinkage (%) = (100−S / 100) × 100
[0032]
<Preparation of positive electrode>
An active material thin film was formed by applying a positive electrode active material paste adjusted to 87% by weight of LiCoO ₂ , 8% by weight of graphite powder and 5% by weight of polyvinylidene fluoride while adjusting the thickness to 300 μm by the doctor blade method. . An aluminum net with a thickness of 30 μm serving as a positive electrode current collector was placed on the active material thin film, and a positive electrode active material paste was applied onto the aluminum net with a doctor blade method adjusted to a thickness of 300 μm. This was left in a drier at 60 ° C. for 60 minutes to be in a semi-dry state to obtain a laminate electrode. The produced laminate electrode was rolled to 400 μm to produce a positive electrode.
[0033]
<Preparation of negative electrode>
A negative electrode active material paste in which mesophase microbead carbon was adjusted to 95% by weight and polyvinylidene fluoride was adjusted to 5% by weight was applied to a thickness of 300 μm by a doctor blade method to form an active material thin film. A 20 μm-thick belt-shaped copper net serving as a negative electrode current collector was placed on the active material thin film, and a negative electrode active material paste was applied to the upper part of the copper net by adjusting the thickness to 300 μm by a doctor blade method. This was left in a drier at 60 ° C. for 60 minutes to be in a semi-dry state to obtain a laminate electrode. The produced laminate electrode was rolled to 400 μm to produce a negative electrode.
[0034]
<Preparation of adhesive>
An adhesive was obtained by uniformly dispersing 20 parts by weight of alumina particles in a uniform solution comprising 10 parts by weight of polyvinylidene fluoride and 70 parts by weight of N-methylpyrrolidone (hereinafter abbreviated as NMP).
[0035]
<Preparation of laminated battery assembly>
FIG. 1 is a cross-sectional view schematically showing one embodiment of a laminated structure battery body of the present invention. In the figure, 1 is a positive electrode, 2 is a negative electrode, 3 is a separator, 4 is an adhesive layer, and 5 is a unitary laminate in which the positive electrode 1, the separator 3 and the negative electrode are joined by an adhesive layer 4.
[0036]
A method for producing the laminated structure battery body shown in FIG. 1 will be described below.
First, an adhesive is applied to one side of each of the two separators, and the negative electrode is sandwiched between the two separators coated with the adhesive, adhered and bonded, and then heated and dried to bond the separator and the negative electrode. It was.
[0037]
Next, the negative electrode with a separator was set to a predetermined size, an adhesive was applied to one surface of the separator, and a positive electrode having a predetermined size was adhered to and bonded to the adhesive-coated surface. Next, a new negative electrode with a separator was prepared, an adhesive was applied to one surface of the separator, and the negative electrode was adhered to the other surface of the positive electrode that had been adhered to the first surface. By repeating this step a predetermined number of times, a battery body in which a plurality of units are stacked is constructed, and then the battery body is dried while being heated, and the positive electrode and the negative electrode with a separator are bonded to each other to form a flat plate shape as shown in FIG. The laminated structure battery body was obtained.
[0038]
The obtained battery body was impregnated with an electrolytic solution prepared using LiPF ₆ as a lithium salt and a mixed solvent of ethylene carbonate and diethyl carbonate as a non-aqueous solvent so that the lithium salt concentration was adjusted to 1.2 mol / l. Thereafter, the battery body was inserted into a container made of an aluminum laminate film, and sealed by heat sealing to obtain a lithium ion secondary battery having a laminated structure.
[0039]
<Creation of a wound structure battery body>
FIG. 2 is a cross-sectional view schematically showing one embodiment of the wound structure battery body of the present invention. In the figure, 1 is a positive electrode, 2 is a negative electrode, 3 is a separator, 4 is an adhesive layer, and 5 is a unitary laminate in which the positive electrode 1, the separator 3 and the negative electrode are joined by an adhesive layer 4.
[0040]
A method for manufacturing the wound structure battery body shown in FIG. 2 will be described below.
Prepare two strip-shaped separators, apply an adhesive to each side of the separator, sandwich the strip-shaped negative electrode between the two separators coated with the adhesive, adhere them together, and then dry by heating. Thus, the separator and the negative electrode were bonded. Next, an adhesive was applied to one of the strip-shaped separator surfaces of the separator-attached negative electrode, and the strip-shaped positive electrode was brought into close contact with and bonded to the adhesive-coated surface. Next, an adhesive was applied to the surface of the negative electrode with a separator opposite to the surface on which the adhesive was previously applied, and was wound into an oval shape. The rolled oval battery body was dried while being pressurized, and the positive electrode and the negative electrode with a separator were bonded together to obtain a flat wound structure battery body as shown in FIG. The obtained battery body was impregnated with an electrolytic solution in the same manner as in the laminated battery, inserted into a container, and sealed to obtain a lithium ion secondary battery having a wound structure.
[0041]
Examples 1-42. And Comparative Examples 1-17.
In the battery produced as described above, using the porous film shown in FIGS. 3 to 7 as a separator, the batteries shown in Examples 1-42 and Comparative Examples 1-17 were produced, and the battery characteristics were evaluated. . The evaluation results are also shown in FIGS.
[0042]
The battery characteristics were evaluated by measuring the charge / discharge characteristics at room temperature and judging the following criteria. As a result, the batteries of Examples 1 to 42 showed good charge / discharge characteristics as shown in FIGS. It was.
◯: Charging / discharging efficiency is 90% or more ◯: Charging / discharging efficiency is 80% or more and less than 90% Δ: Charging / discharging efficiency is 70% or more and less than 80% ×: Charging / discharging efficiency is 70% or less
Moreover, as a result of examining the overcharge characteristic at room temperature using the battery obtained by the comparative example and determining the following items (1) to (3) according to the following criteria, the results are shown in FIGS. As described above, the batteries of Examples 1 to 42 all showed good overcharge characteristics with no change in battery appearance due to overcharge, no heat generation at high temperatures, and almost no current flow after shutdown.
(1) Battery appearance After overcharging, the battery appearance was visually observed and judged according to the following criteria.
○: No change due to overcharge ×: Some change in battery appearance due to overcharge (2) Maximum battery temperature ○: Maximum temperature reached below 120 ° C Δ: Maximum temperature reached from 120 ° C to below 140 ° C ×: Maximum reached Temperature is 140 ° C or higher (3) Current value after shutdown ○: Current value 0
Δ: Current value is less than 50 mA ×: Current value is 50 mA or more
In a secondary battery in which an electrode active material layer and a separator are bonded and integrated with a porous adhesive layer, and a porous film having a specific porosity, pore diameter, and heat shrinkage rate is used for the separator, The electrode active material layer and the separator layer could be firmly bonded with a small amount of adhesive, and a secondary battery having good charge / discharge characteristics and high safety was obtained.
[0045]
That is, it was found that a secondary battery having good charge / discharge characteristics and high safety can be obtained by using a polyolefin porous film having a porosity of 45 to 55% as a separator. As shown in Comparative Examples 3, 6 and 9, when the porosity of the porous film was larger than 55%, the charge / discharge characteristics were improved, but the overcharge characteristics were lowered and the safety of the battery was lowered. Further, according to Comparative Examples 1, 2, 4, 5, 7, and 8, when the porosity was lower than 45%, the charge / discharge characteristics were lowered.
[0046]
Moreover, when the pore diameter of the porous film used for a separator exists in the range of 100-300 nm, it discovered that the secondary battery which has especially favorable charging / discharging characteristics and high safety | security is obtained. When the pore diameter of the porous film was less than 100 nm as in Comparative Examples 10, 12, and 14, the charge / discharge characteristics were lowered. In addition, as shown in Comparative Examples 11, 13, and 15, when the pore diameter of the porous film was larger than 300 nm, the charge / discharge characteristics were improved, but the overcharge characteristics were lowered and the safety of the battery was lowered.
[0047]
Further, from the results of Comparative Examples 16 and 17, it was found that the separator having a thermal shrinkage rate of 10% or less is more desirable from the viewpoint of overcharge characteristics.
[0048]
From the comparison of the results of Examples 3, 12 and 21 and Examples 6, 15 and 24, even though the physical properties of the porous film were the same, the film material was a polyethylene and polypropylene multilayer film rather than a polyethylene single layer film. It was found that the higher overcharge characteristics were exhibited.
[0049]
【The invention's effect】
According to the first secondary battery of the present invention, in the secondary battery in which the separator is adhered and integrated with the adhesive layer between the active material layers of the positive electrode and the negative electrode in which the active material layer is adhered to the current collector, separator, porosity I 45% to 55% of the polyolefin porous membrane der, and the porous film of the maximum pore diameter is 100~300nm der Runode, firmly the electrode active material layer and the separator by a small amount of adhesive A secondary battery having good charge / discharge characteristics and high safety can be obtained.
[0051]
According to the second or third secondary battery of the present invention, it is possible to prevent the positive electrode and the negative electrode from being short-circuited at the battery body end face portion, and further to avoid the risk of promoting the heat generation of the battery.
[0052]
According to any one of the fourth to sixth secondary batteries according to the present invention, the unit in which the separator is disposed between the positive electrode and the negative electrode is defined as one unit, and has a plurality of units, so that the battery capacity can be increased. .
[Brief description of the drawings]
FIG. 1 is a cross-sectional view illustrating a configuration of a main part of a battery according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view showing a configuration of a main part of a battery according to another embodiment of the present invention.
FIG. 3 shows the battery characteristic evaluation results of Examples 1 to 15 of the present invention.
FIG. 4 shows the battery characteristic evaluation results of Examples 16 to 28 of the present invention.
FIG. 5 shows the battery characteristic evaluation results of Examples 29 to 40 of the present invention.
6 shows battery characteristic evaluation results of Examples 41 and 42 of the present invention and Comparative Examples 1 to 11. FIG.
7 shows battery characteristic evaluation results of Comparative Examples 12 to 17. FIG.
[Explanation of symbols]
1 positive electrode, 2 negative electrode, 3 separator, 4 adhesive layer, 5 laminate

Claims (6)

In a secondary battery in which a separator is adhered and integrated with an adhesive layer between the active material layers of a positive electrode and a negative electrode in which an active material layer is bonded to a current collector, the separator is made of a polyolefin having a porosity of 45 to 55%. secondary batteries I membrane der, and maximum pore size of the porous membrane is characterized in that it is a 100 to 300 nm. In a secondary battery in which a separator is adhered and integrated with an adhesive layer between the active material layers of a positive electrode and a negative electrode in which an active material layer is bonded to a current collector, the separator is made of a polyolefin having a porosity of 45 to 55%. A secondary battery comprising a porous film and having a heat shrinkage rate of 10% or less at 100 ° C. of the porous film. In a secondary battery in which a separator is adhered and integrated with an adhesive layer between the active material layers of a positive electrode and a negative electrode in which an active material layer is bonded to a current collector, the separator is made of a polyolefin having a porosity of 45 to 55%. secondary battery, characterized by a porous membrane, and a multilayer film structure in which the porous film is formed by laminating two or more kinds of polyolefin resins. The secondary battery according to any one of claims 1 to 3 , wherein a unit in which a separator is disposed between a positive electrode and a negative electrode is defined as one unit, and the unit includes a plurality of units. The secondary battery according to claim 4, wherein the plurality of units are formed by stacking the separated positive electrode, separator, and negative electrode. The secondary battery according to claim 4, wherein a plurality of units are formed by rolling up a continuous positive electrode, separator, and negative electrode.

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