JP2004111157A - Secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a secondary battery of excellent safety and reliability capable of more surely inhibiting battery temperature from arising even if the battery temperature abruptly rises due to short-circuiting, overcharge, or the like. <P>SOLUTION: This secondary battery is equipped with a positive electrode made by disposing a positive electrode active material layer on a first current collector, a negative electrode made by disposing a negative electrode active material layer on a second current collector, and a first layer including particles. The first layer is disposed between the positive electrode active material layer and the negative electrode active material layer. The particles expand at a prescribed temperature or above, and the prescribed temperature is 80°C or above. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a secondary battery and a method for manufacturing the same.
[0002]
[Prior art]
2. Description of the Related Art In recent years, there has been a great demand for reduction in size and weight of portable electronic devices. In order to realize this, further improvement in performance and life of the secondary battery is required, and various developments and improvements are being promoted in response to this. Typical characteristics required for a secondary battery include high voltage, high energy density, safety, longevity reliability, low cost, and the like. Among secondary batteries, a lithium ion secondary battery is a secondary battery that has high weight energy density and volume energy density and is highly expected, and its improvement is being actively promoted at present.
[0003]
A lithium ion secondary battery generally includes a positive electrode and a negative electrode, and a separator sandwiched between the positive electrode and the negative electrode. Although the separator is electrically insulating, it is necessary that the separator has lithium ion conductivity. (It is not necessary that the separator alone have lithium ion conductivity. If the separator is impregnated with an electrolyte, lithium ion conductivity can be obtained. Just fine). As the separator, a film sheet or the like obtained by making a polymer such as polypropylene or polyethylene porous by a biaxial stretching method is mainly used.
[0004]
The separator serving as an ion conductive layer is required to have as low an ion conduction resistance as possible in terms of battery performance. Therefore, it is required to make the thickness of the separator as thin as possible. However, when the thickness of the separator is reduced, the possibility that a short circuit occurs between the positive electrode and the negative electrode during repeated charge / discharge cycles increases. In addition, even when a sharp foreign object is stabbed, the separator may be damaged and a short circuit may occur. In particular, a lithium ion secondary battery has a high energy density, and if a short circuit occurs, a large current may concentrate, and the battery temperature may rise rapidly. In addition, overcharging or overdischarging may cause a rapid rise in battery temperature.
[0005]
Until now, when the temperature of an electrode plate has risen due to a short circuit, overcharge, or the like, a separator has been developed that melts by heat to insulate the positive electrode and the negative electrode and prevent further temperature rise (for example, And Patent Document 1). The separator is made of a porous film of polyolefin, and has ion conductivity in a normal use state by containing an electrolyte. When the battery temperature rises sharply due to a short circuit or the like, the generated heat melts and closes the pores of the separator itself, losing ionic conductivity and stopping the electrochemical reaction.
[0006]
[Patent Document 1]
JP-A-5-62662
[0007]
[Problems to be solved by the invention]
According to the present invention, a method different from the above-described conventional technique can reliably suppress the temperature rise of the battery even if the temperature of the battery suddenly rises due to a short circuit or the like. An object is to provide an excellent secondary battery.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the secondary battery of the present invention is
A positive electrode having a positive electrode active material layer disposed on a first current collector, a negative electrode having a negative electrode active material layer disposed on a second current collector, and a first layer containing particles;
The first layer is disposed between the positive electrode active material layer and the negative electrode active material layer,
The particles expand above a predetermined temperature,
The predetermined temperature is 80 ° C. or higher.
[0009]
With the secondary battery as described above, even if the temperature of the battery suddenly rises due to a short circuit or the like, the particles expand and maintain or increase the distance between the positive electrode active material layer and the negative electrode active material layer. It is possible to suppress a short circuit between the electrode plates and / or an electrochemical reaction. Therefore, a further increase in battery temperature can be prevented, and a secondary battery with higher safety and reliability can be obtained. Note that 80 ° C. is a temperature that reflects the lower limit temperature range in which the battery generally starts generating heat due to an internal short circuit or overcharge due to an abnormality.
[0010]
Further, the method for manufacturing a secondary battery of the present invention includes:
A first step of preparing a solution containing a polymer, a good solvent for the polymer, a poor solvent for the polymer, and particles that expand at a predetermined temperature or higher;
A second step of applying the solution on at least one electrode plate selected from a positive electrode plate and a negative electrode plate to form a coating film;
A third step of forming a porous film containing the particles on the at least one electrode plate by removing the poor solvent after the removal of the good solvent in the coating film;
A fourth step of forming an electrode plate group so that the porous film is placed between the positive electrode plate and the negative electrode plate,
The predetermined temperature is 80 ° C. or higher.
[0011]
By employing the method for manufacturing a secondary battery as described above, a porous film made of a polymer containing particles that expand at a predetermined temperature or higher can be arranged between the positive electrode plate and the negative electrode plate. Therefore, even if the temperature of the battery rises sharply due to a short circuit or the like, the particles expand and the distance between the positive electrode active material layer and the negative electrode active material layer can be maintained or increased. And / or an electrochemical reaction can be suppressed. Therefore, a further increase in battery temperature can be prevented, and a secondary battery with higher safety and reliability can be obtained. In addition, since the porous film is obtained by directly applying and drying the electrode on the electrode plate, the occurrence of wrinkles and the like on the porous film is suppressed. Can be kept constant. Further, since the contraction of the porous film when the battery temperature rises can be suppressed, a battery with higher reliability can be obtained.
[0012]
In this specification, a good solvent is a solvent that dissolves the polymer more than the θ solvent of the polymer, and a poor solvent is a solvent that does not dissolve the polymer more than the θ solvent of the polymer. ing.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
(Embodiment 1)
FIG. 1 is a schematic cross-sectional view showing a structural example of a part of a secondary battery according to the present invention.
[0014]
In the example shown in FIG. 1, a positive electrode 1 in which a positive electrode active material layer 1a is laminated on a first current collector (positive electrode current collector) 1b and a second current collector (negative electrode current collector) 2b And a negative electrode 2 on which a negative electrode active material layer 2a is laminated. Further, a first layer 3 containing particles 4 is sandwiched between the positive electrode active material layer 1a and the negative electrode active material layer 2a. In the example shown in FIG. 1, the first layer 3 also serves as a separator.
[0015]
Here, the particles may be particles that expand at a predetermined temperature or higher. However, the predetermined temperature is any temperature of 80 ° C. or higher. Since the first layer disposed between the positive electrode active material layer and the negative electrode active material layer contains the particles, the particles expand even if the temperature of the battery suddenly rises due to a short circuit or the like. Thus, the distance between the positive electrode active material layer and the negative electrode active material layer can be maintained or increased, and a short circuit between the electrode plates and / or an electrochemical reaction can be suppressed. Further, even when the separator in the battery shrinks due to a rapid rise in battery temperature, the distance between the positive electrode active material layer and the negative electrode active material layer can be maintained or increased by expanding the above particles, Short circuits between the plates and / or electrochemical reactions can be suppressed. Therefore, a further increase in battery temperature can be prevented, and a secondary battery with higher safety and reliability can be obtained.
[0016]
Volume V of the particles before expansion ₁ And the volume V of the particles after expansion ₂ And V ₂ ≧ 3 × V ₁ May be satisfied. When the particles satisfy the above relationship, the distance between the positive electrode active material layer and the negative electrode active material layer can be more reliably maintained or increased by the expansion of the particles. Above all, V ₂ ≧ 5 × V ₁ It is preferable to satisfy the following relationship.
[0017]
Further, the volume V of the particles in the range of 15 ° C. to 35 ° C. ₃ And the volume V in the range of 110 ° C. to 130 ° C. ₄ And V ₄ ≧ 3 × V ₃ May be satisfied. When the particles satisfy the above relationship, the above-described effects can be obtained more reliably. Above all, V ₄ ≧ 5 × V ₃ It is preferable to satisfy the following relationship. The temperature range of 15 ° C. to 35 ° C. is a temperature range included in the normal operating temperature range of the battery, and the temperature range of 110 ° C. to 130 ° C. is a temperature range included in a state where the inside of the battery is abnormally heated.
[0018]
The average particle size of the particles before expansion may be in the range of 3 μm to 20 μm. When the particles are within the above range, the distance between the positive electrode active material layer and the negative electrode active material layer during normal use of the battery can be further optimized, and a secondary battery having more excellent battery characteristics can be obtained. Further, the particles may be contained in the first layer in a range of 5% by volume to 35% by volume.
[0019]
The particles may be hollow. In this case, if necessary, various materials (for example, gas, liquid, solid, gel, etc.) can be included therein, and the expansion characteristics of the particles can be arbitrarily changed.
[0020]
For example, the hollow particles may include at least one material selected from a solid material and a liquid material that evaporates at the predetermined temperature or higher. In this case, when the particles are exposed to a temperature equal to or higher than the predetermined temperature, the encapsulated at least one material is vaporized, so that the particles can expand quickly. Therefore, even when the battery temperature rises sharply, further increase in the battery temperature can be suppressed more reliably.
[0021]
Further, by changing the at least one kind of material to be included, the expansion rate of the particles can be controlled. If necessary, a plurality of materials having different vaporizing temperatures can be included. Further, in addition to the at least one material, any material may be included as necessary. For example, if a flame retardant such as ammonium phosphate or ammonium hydroxide is included inside the particles, it is possible to extinguish the flame even if the battery is ignited and the particles are damaged.
[0022]
When the particles 4 are hollow, the material is not particularly limited as long as the material does not rupture or damage when expanded and can withstand the environment in the battery. For example, melamine, gelatin, a conjugate of melamine and urea, a conjugate of gelatin and urea, a copolymer such as urethane and acrylonitrile, and silica can be used.
[0023]
When the particle 4 is a hollow body, the material contained therein is not particularly limited as long as it does not damage the material constituting the particle 4. The material can be arbitrarily selected according to the properties required for the particles 4. For example, if hydrocarbon-based materials such as benzene and toluene are used, the particles 4 can be expanded at 80 ° C. to 110 ° C. Further, the ratio of the hollow portion in the particle 4 and the ratio of the volume of the hollow portion in the particle 4 containing the above material are not particularly limited. What is necessary is just to set arbitrarily according to the characteristic required as the particle 4.
[0024]
FIG. 2 shows an example of the expanded particles. The example shown in FIG. 2 is a photograph of an electron microscope (SEM) capturing a state in which the particles are expanded under an atmosphere of about 100 ° C. At this time, the particles are hollow bodies made of melamine containing toluene, and are embedded in a porous material made of polyvinylidene fluoride which is assumed to be a separator. The volume of the particles before expansion in the range of 15 ° C. to 35 ° C. is about 523.3 μm ³ (Particle size: about 10 μm), but at this time, it can be seen that it has expanded to about four times (particle size: 45 μm). No breakage or rupture was observed during the expansion of the particles.
[0025]
The material used for the positive electrode current collector 1b and the negative electrode current collector 2b is not particularly limited as long as it is a conductor that is stable in the battery. For example, aluminum can be used as the positive electrode current collector 1b and copper can be used as the negative electrode current collector 2b.
[0026]
In addition, as the shape of the positive electrode current collector 1b and the negative electrode current collector 2b, a current collector having an arbitrary shape such as a foil shape, a net shape, and an expanded metal can be used. When a current collector having a large surface area, such as a net or an expanded metal, is used, the adhesive strength with an active material disposed on the current collector can be further improved.
[0027]
The material of the positive electrode active material layer 1a is not particularly limited as long as it can reversibly occlude and release electrolyte ions (lithium ions in the case of a lithium ion secondary battery). In the case of a lithium ion secondary battery, for example, a composite oxide or a chalcogen compound of a transition metal such as cobalt, manganese, or nickel can be used. Alternatively, a composite compound of the above-described compounds or a compound to which various elements are added as necessary can be used without particular limitation.
[0028]
The material of the negative electrode active material layer 2a is not particularly limited as long as it has a lower potential than the positive electrode active material and can reversibly occlude and release electrolyte ions. In the case of a lithium ion secondary battery, for example, a carbonaceous material represented by graphite or the like can be used.
[0029]
As the shape of the above both active materials, for example, a granular active material may be used. In this case, the average particle size of the active material is in a range of, for example, 0.1 μm to 50 μm. In the above range, the active material layer can be easily formed into a thin film, and the packing density of the two active materials can be controlled to a more appropriate value. Above all, the average particle size of the active material is preferably in the range of 3 μm to 8 μm from the viewpoint of ion doping and deion doping efficiency during charge and discharge.
[0030]
The first layer 3 is not particularly limited as long as it is an electrically insulating thin film having a high electrolyte ion permeability, not subject to corrosion or the like inside the battery, and having a certain mechanical strength. For example, a porous film having the above characteristics, which is generally used for a secondary battery, may be used. Among the above porous films, a porous film made of a polymer is preferable. Further, it is preferable to have flexibility enough to follow volume expansion and contraction of the active material due to charge and discharge.
[0031]
As a material used for the first layer 3, for example, an olefin polymer containing at least one polymer selected from polypropylene and polyethylene, a sheet made of glass fiber, a nonwoven fabric, a woven fabric, or the like can be used. In addition, polyethers such as polyethylene oxide and polypropylene oxide, polyolefins such as polyethylene and polypropylene, and others, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyacrylonitrile, polyvinyl chloride, polyvinylidene chloride, poly Methyl methacrylate, polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinyl pyrrolidone, polycarbonate, polyethylene terephthalate, polyhexamethylene adipamide, polycaprolactam, polyurethane, polyethylene imine, polybutadiene, polystyrene, polyisoprene, and Derivatives of the above polymers can be used alone or in combination. That. Moreover, you may use the copolymerized polymer of various monomers which comprise the said polymer.
[0032]
The thickness of the first layer 3 is, for example, in the range of 10 μm to 25 μm when also serving as a separator as shown in FIG. The average pore size is preferably in a range where the active material detached from the electrode does not pass through, for example, in a range of 0.1 μm to 1.0 μm. Further, the porosity of the separator is determined by the electrical insulation properties and electrolyte ion permeability of the material constituting the separator, the thickness of the separator, and the like, and is, for example, in the range of 50 vol% to 75 vol%.
[0033]
The particles 4 can also serve as fillers for keeping the distance between the positive electrode 1 and the negative electrode 2 constant. For example, as shown in FIG. 3, particles 4 having an average particle diameter substantially equal to the thickness of the first layer 3 may be arranged in the first layer 3. That is, in a normal use temperature range, the particles 4 can serve as a filler that keeps the distance between the positive electrode 1 and the negative electrode 2 (the distance between the positive electrode active material layer 1a and the negative electrode active material layer 2a) constant.
[0034]
In the examples shown in FIGS. 1 and 3, the first layer 3 also served as a separator. However, separately from the separator, the first layer containing the above particles was used as the positive electrode active material layer 1a and the negative electrode active material. You may arrange | position between the layers 2a. At this time, the separator may or may not contain the particles. In this case, the material used for the first layer may be the same as that shown in FIGS. 1 and 3 when the first layer also serves as a separator.
[0035]
(Embodiment 2)
Embodiment 2 describes an example of a method for manufacturing a secondary battery according to the present invention.
[0036]
The method of manufacturing a secondary battery according to the present invention includes a first step of preparing a solution containing a polymer, a good solvent for the polymer, a poor solvent for the polymer, and particles that expand at a predetermined temperature or higher; A second step of applying a solution to at least one electrode plate selected from a positive electrode plate and a negative electrode plate to form a coating film, and delaying the removal of the good solvent in the coating film to form the poor solvent. By removing, a third step of forming a porous film containing the particles on the at least one electrode plate, so that the porous film is placed between the positive electrode plate and the negative electrode plate A fourth step of forming an electrode plate group, wherein the predetermined temperature is 80 ° C. or higher.
[0037]
By adopting such a manufacturing method, a porous film made of a polymer containing particles that expand at a predetermined temperature can be disposed between the positive electrode plate and the negative electrode plate. However, the predetermined temperature is any temperature of 80 ° C. or higher. Therefore, even if the temperature of the battery rises sharply due to a short circuit or the like, the particles expand and the distance between the positive electrode active material layer and the negative electrode active material layer can be maintained or increased. And / or an electrochemical reaction can be suppressed. Therefore, a further increase in battery temperature can be prevented, and a secondary battery with higher safety and reliability can be obtained. In addition, since the porous film is formed by directly applying and drying the electrode plate, the occurrence of wrinkles and the like on the porous film is suppressed. Can be kept constant, and a battery with higher reliability can be obtained. Note that the porous membrane is electrically insulating and has electrolyte ion conductivity by containing an electrolytic solution, and thus can serve as a separator. In this case, it is possible to suppress the contraction of the separator in the battery due to the rapid rise in the battery temperature, and it is possible to obtain a secondary battery having more safety and reliability.
[0038]
Examples of the polymer include polyethers such as polyethylene oxide and polypropylene oxide, polyolefins such as polyethylene and polypropylene, and others, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyacrylonitrile, polyvinyl chloride, and polyvinyl chloride. Vinylidene chloride, polymethyl methacrylate, polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinyl pyrrolidone, polycarbonate, polyethylene terephthalate, polyhexamethylene adipamide, polycaprolactam, polyurethane, polyethylene imine, polybutadiene, polystyrene, poly Isoprene and derivatives of the above polymers, alone or mixed It can be used Te. Further, a copolymer of various monomers constituting the above polymer can also be used.
[0039]
Among them, polyethers such as polyethylene oxide and polypropylene oxide, polyvinylidene fluoride (PVdF), polyvinyl chloride, polyacrylonitrile, polyacrylonitrile, polyvinylidene fluoride, polyvinylidene chloride, polymethyl methacrylate, polymethyl acrylate, polyvinyl Alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone, polyethyleneimine, polybutadiene, polystyrene, polyisoprene, and derivatives of the above polymers are preferred. Mixtures of multiple polymers can also be used. Further, a copolymer of various monomers constituting the above polymer, for example, vinylidene fluoride / hexafluoropropylene copolymer (P (VdF / HFP)) can also be used. These polymers are more flexible in a normal operating temperature range of the battery, and can more easily follow a change in volume of the active material due to charge and discharge.
[0040]
Any good solvent may be used as long as it dissolves the polymer to be used. Depending on the polymer used, for example, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, dimethyl ether, diethyl ether, ethyl Ethers such as methyl ether and tetrahydrofuran, dimethylacetamide, 1-methyl-pyrrolidinone, n-methyl-2-pyrrolidone (NMP) and the like can be used.
[0041]
Any poor solvent may be used as long as it is compatible with the good solvent used. Depending on the polymer used, for example, acetone, methyl ethyl ketone, alcohol, ethanol, cyclohexanol, n-butyl acetate, octanol, n-decanol, undecanol, n-dodecanol and the like can be used. The magnitude relationship between the boiling point of the good solvent and the boiling point of the poor solvent is not particularly limited.
[0042]
As the particles, the particles described in Embodiment 1 can be used as long as they do not maintain the shape as particles due to dissolution in the good solvent and the poor solvent.
[0043]
Further, the average particle diameter of the particles before expansion may be in a range of 3 μm to 20 μm. When the particles are within the above range, the distance between the positive electrode active material layer and the negative electrode active material layer during normal use of the battery can be further optimized, and a secondary battery having more excellent battery characteristics can be obtained.
[0044]
The average particle size of the particles may be arbitrarily adjusted according to the distance between the electrode plates of the positive electrode and the negative electrode. In this case, the particles can also serve as fillers for keeping the distance between the electrode plates constant.
[0045]
A solution containing a polymer, a good solvent, a poor solvent, and the above particles may be prepared by using a generally known method, for example, a disper or a ball mill. In preparing the solution, the order of mixing is not particularly limited. The temperature during preparation is preferably 50 ° C. or lower.
[0046]
The proportion of the polymer in the solution is, for example, in the range of 5% by mass to 40% by mass, and preferably in the range of 10% by mass to 40% by mass, based on the solution. The proportion of the good solvent in the solution is, for example, in the range of 50% by mass to 90% by mass, and preferably in the range of 60% by mass to 90% by mass, based on the solution. The proportion of the poor solvent in the solution is, for example, in the range of 1% by mass to 20% by mass, and preferably in the range of 5% by mass to 18% by mass, based on the solution. The proportion of the particles added is, for example, in the range of 10% by mass to 60% by mass, and preferably in the range of 10% by mass to 25% by mass, based on the mass of the polymer.
[0047]
The preparation of the solution may be performed under a low humidity atmosphere. By preparing the solution in a low humidity atmosphere, the strength of the porous film after coating and drying can be further improved. Here, the low humidity means a range of about 10% or less in relative humidity. When controlling based on the dew point temperature, for example, the dew point temperature can be controlled in the range of 0 ° C. to −80 ° C.
[0048]
In order to realize such a low-humidity atmosphere, for example, moisture adsorption using silica may be performed. Further, in a low humidity atmosphere, static electricity is easily generated, but in an inert gas atmosphere, the safety of each process can be further enhanced. As the inert gas, for example, nitrogen, argon, or the like can be used.
[0049]
In preparing the solution, a dehydrated polymer may be used as the polymer. As in the case where the solution is prepared in a low humidity atmosphere, the strength of the porous film after coating and drying can be further improved. In this case, it is preferable that the solution is prepared under a low humidity atmosphere.
[0050]
The method for applying the solution prepared as described above to an electrode plate to form a coating film is not particularly limited. For example, when using a coater, a generally used method such as a die nozzle method, a gravure method, a comma method, a spray method, a doctor blade method, etc. may be used. If necessary, a plurality of coating methods may be used.
[0051]
As the positive electrode plate and the negative electrode plate on which the coating film is formed, the positive electrode and the negative electrode described in Embodiment 1 may be used. The coating thickness of the solution on the electrode plate may be arbitrarily adjusted according to the thickness of the porous film required at the time of drying.
[0052]
When drying the coating film, the poor solvent may be removed after the removal of the good solvent in the coating film. A porous film containing the particles can be formed on the at least one electrode plate.
[0053]
As a method for removing each solvent from the coating film, a method of sequentially removing each solvent by a solvent replacement extraction method, and then evaporating the solvent used for replacement and drying may be used. A method of drying without solvent replacement can be used. From the viewpoint of productivity, a method of drying the coating film without replacing the solvent is preferable. As a method of drying without solvent replacement, a generally used method such as a hot air drying method or an IR (infrared) drying method may be used. The drying temperature and drying time at that time may be adjusted according to the thickness of the coating film and the type of each solvent.
[0054]
In the coating film applied on the electrode plate, the polymer is in a state of being dissolved in a good solvent, and the poor solvent is considered to be uniformly present in the coating film because it is compatible with the good solvent. . However, the polymer was not dissolved in the poor solvent. When such a coating film is dried, first, it is considered that a good solvent for dissolving the polymer evaporates and evaporates. Since the good solvent dissolves the polymer, it can move to the surface of the coating film one after another, and the evaporation from the surface of the coating film can be continued. On the other hand, since the poor solvent does not dissolve the polymer, the poor solvent is trapped in the polymer, and cannot be easily moved to the surface of the coating film and cannot be evaporated. However, if the drying is further continued, the poor solvent evaporates in the polymer, and thus it is considered that a porous layer made of the polymer is obtained using the portion where the poor solvent is confined as pores. The particles are considered to be in a state of being uniformly dispersed in the porous layer because they do not interact with the good solvent and the poor solvent.
[0055]
If the porous film obtained as described above is arranged between the positive electrode plate and the negative electrode plate (between the positive electrode active material layer and the negative electrode active material layer) and the porous film is filled with an electrolyte, A secondary battery with excellent safety and reliability can be obtained.
[0056]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples. Note that the present invention is not limited to the following embodiments.
[0057]
(Example 1)
Using polyvinylidene fluoride (PVdF) as a polymer and n-methyl-2-pyrrolidone (NMP) as a good solvent, 100 g of PVdF was dissolved in 900 g of NMP. After the PVdF was completely dissolved, octanol as a poor solvent was added in an amount of 100 g equivalent to PVdF, and the mixture was stirred for 120 minutes at a rotation speed of 1500 rpm using a disper to obtain a homogeneous state. Thereafter, 100 g of melamine particles (manufactured by Matsumoto Yushi Kogyo Co., Ltd.) were added, and the mixture was stirred with a disper at a rotation speed of 1500 rpm for 180 minutes to prepare a solution in which the above particles were uniformly dispersed.
[0058]
The particles are hollow bodies made of melamine, contain toluene (boiling point 110 ° C.), and have the property of expanding at 100 ° C. or higher. The average particle size before expansion (15 ° C. to 35 ° C.) is about 10 μm. Melamine is a material that is stable in NMP and octanol, and is also stable to encapsulated toluene.
[0059]
The preparation of the above solution was performed in an atmosphere having a relative humidity of 10% to 7%. Further, the atmosphere was an inert gas (argon gas) atmosphere, and the ambient temperature was 23 ° C.
[0060]
The PVdF used had been subjected to a dehydration treatment by previously performing vacuum drying (pressure: 5 kPa (40 mmHg), 60 ° C., 20 hours). As shown in FIG. 4, while the water content of PVdF in a general environment at room temperature of 25 ° C. to 30 ° C. and relative humidity of 50% to 70% is about 125 ppm, It can be seen that the water content of PVdF after vacuum drying has dropped to about 30 ppm. NMP and octanol were also used, which could not be dried in vacuum, but were dehydrated as much as possible by being left for 15 hours in an inert gas (argon gas, relative humidity 10%) atmosphere.
[0061]
The solution prepared as described above was applied to a polyethylene terephthalate (PET) film using a comma coater to form a coating film. Thereafter, the formed coating film was dried at 60 ° C. for 30 minutes to obtain a 20 μm-thick porous film made of PVdF containing the particles (sample 1a).
[0062]
Next, the dehydration treatment of PVdF, NMP and octanol is not performed in advance, and the preparation of the solution is performed in a general environment, that is, an atmosphere of a temperature of 25 ° C. to 30 ° C. and a relative humidity of 50% to 70%. A membrane was prepared (Sample 1b). Except for the atmosphere for dehydrating PVdF and the like and for preparing the solution, the materials and methods shown in the preparation of the sample 1a are completely the same.
[0063]
For each of the porous films of the samples 1a and 1b prepared as described above, a tensile strength measurement and a surface observation by an electron microscope (SEM) were performed.
[0064]
The tensile strength was measured as follows. Each of the porous films of Sample 1a and Sample 1b was shaped into a strip having a length of 30 mm and a width of 10 mm, and was set in a tensile tester to measure the maximum strength before breaking. The number of samples was set to 10, and the average value was determined. FIG. 5 shows the results. 6 and 7 show the results of surface observation by SEM.
[0065]
As a result, the tensile strength of the sample 1b that was not subjected to the dehydration treatment was 1.96 N / mm. ² (0.2kgf / mm ² On the other hand, in the sample 1a in which the dehydration treatment such as PVdF and the preparation of the solution under the low humidity atmosphere were performed, the tensile strength was 24.5 N / mm. ² (2.5kgf / mm ² ) And improved by 10 times or more. According to the result of the surface observation by SEM, in the sample 1b shown in FIG. 6, the PVdF is aggregated, and the aggregated PVdF has a shape in which the bundles are connected to each other by thin bundles. On the other hand, the sample 1a shown in FIG. 7 did not show aggregation of PVdF, and had a structure in which innumerable pores due to evaporation of the poor solvent were present on the surface of the uniformly formed film. Both sample 1a and sample 1b can be used for a secondary battery without any problem. However, by performing dehydration of a polymer and preparation of a solution under a low humidity atmosphere, a porous membrane having higher strength can be obtained. It is understood that it can be done.
[0066]
In both Sample 1a and Sample 1b, the particles were evenly dispersed in the porous film, and it was confirmed that the particles expanded about three times in a temperature range of 100 ° C. or more.
[0067]
Similar results were obtained when vinylidene fluoride / hexafluoropropylene copolymer (P (VdF / HFP)) or the like was used as the polymer.
[0068]
(Example 2)
In this example, a lithium ion secondary battery was manufactured using the sample used in Example 1 in which the solution was prepared in a low humidity atmosphere. However, the application of the solution was performed on the positive electrode active material layer.
[0069]
First, as in Example 1, a solution containing particles composed of dehydrated PVdF, NMP, octanol, and melamine was prepared. The particles are the same as those used in Example 1. .
[0070]
The positive electrode and the negative electrode were prepared as follows. Aluminum foil (thickness 15 μm) as a positive electrode current collector, and a composite oxide of cobalt transition metal (LiCoO ₂ ), The positive electrode active material was applied to both surfaces of the positive electrode current collector to form a positive electrode active material layer (one-sided thickness: 70 μm), thereby producing a positive electrode. Further, a copper foil (thickness: 15 μm) was used as a negative electrode current collector, and artificial graphite was used as a negative electrode active material. The negative electrode active material was applied to both surfaces of the negative electrode current collector, and a negative electrode active material layer (one-side thickness) 80 μm) to form a negative electrode. The average particle size of the positive electrode active material was 3 μm, and the average particle size of the negative electrode active material was 30 μm.
[0071]
On one surface of the positive electrode prepared as described above, the above solution was applied using a comma coater to form a coating film. Thereafter, the formed coating film was dried at 60 ° C. for 30 minutes to obtain a 20 μm thick porous film made of PVdF containing the particles on the positive electrode. The average pore diameter of the porous membrane was 1 μm, and it was confirmed that the particles were uniformly dispersed in the porous membrane.
[0072]
Next, the negative electrode was stacked on the porous film formed on the positive electrode to form a laminate including the positive electrode, the porous film, and the negative electrode, and the laminate was wound and housed in a case. Subsequently, a cylindrical lithium secondary battery having a diameter of 1.8 cm and a height of 6.5 cm was prepared by filling the case with a nonaqueous electrolyte (sample 2a).
[0073]
Separately, the following lithium secondary batteries were prepared.
[0074]
For the positive electrode and the negative electrode, exactly the same thing as the above-mentioned example sample was prepared. As a separator, a polyethylene porous membrane (average pore diameter: 0.1 μm) having a thickness of 20 μm was prepared. The separator is sandwiched between the positive electrode and the negative electrode to form a laminate, and the laminate is wound and stored in a case. The case is filled with a nonaqueous electrolyte in the same manner as in Sample 2a, and has a diameter of 1.8 cm. Then, a cylindrical lithium secondary battery having a height of 6.5 cm was produced (Sample 2b).
[0075]
For each of the samples 2a and 2b prepared as described above, the change in battery characteristics when the battery temperature was increased was measured.
[0076]
First, each of the samples 2a and 2b was charged until the terminal voltage of each sample became 4.2V. After the above charging, each sample was placed in a thermostat, and the temperature was raised from 0 ° C. to 150 ° C. at 5 ° C./min. After the start of the temperature rise, the change in the terminal voltage of each sample was measured. FIG. 8 shows the result. The horizontal axis of the graph shown in FIG. 8 is the time after the start of the temperature rise, and the temperature reached 150 ° C. at about 30 minutes.
[0077]
As shown in FIG. 8, it can be seen that in Sample 2b, the terminal voltage sharply decreases as the temperature increases, and after reaching 150 ° C., the terminal voltage further sharply decreases, losing the function as a battery. . On the other hand, although the terminal voltage tends to decrease in sample 2a, the decrease tendency is much smaller than in sample 2b, and it can be seen that the terminal voltage of about 3 V is maintained even 125 minutes after the start of the temperature rise. When the temperature of the case surface was measured at the same time, further heat generation was hardly observed. This is because the sample 2a contains the above-mentioned particles in the battery, and the particles expand when exposed to a high temperature, so that the positive electrode and the negative electrode (the positive electrode active material layer and the negative electrode active material layer) This is considered to be because the distance is maintained or increased and the function as a battery is maintained. On the other hand, in sample 2b, it is considered that the function as a battery has been lost due to exposure to a high temperature.
[0078]
In addition, as described above, not only when the porous film is formed on the positive electrode, but also when the porous film is formed on the negative electrode, and when the porous film is formed on both the positive electrode and the negative electrode. Was obtained.
[0079]
【The invention's effect】
As described above, according to the present invention, even when the battery temperature rises rapidly due to a short circuit, overcharging, or the like, the battery temperature rise can be more reliably suppressed, and more safety and reliability can be achieved. And an excellent secondary battery can be provided. Further, a method for manufacturing the secondary battery can be provided.
[0080]
In this specification, a lithium ion secondary battery has been described as an example, but similar effects can be obtained with other secondary batteries, for example, a nickel-metal hydride battery.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view illustrating a structural example of a secondary battery according to the present invention.
FIG. 2 is a diagram showing an example of particles contained in a secondary battery according to the present invention.
FIG. 3 is a cross-sectional view illustrating a structural example of a secondary battery according to the present invention.
FIG. 4 is a graph showing the water content of PVdF measured before and after dehydration treatment, measured in the examples.
FIG. 5 is a diagram showing the results of tensile strength tests of Examples and Comparative Examples measured in Examples.
FIG. 6 is a diagram showing a structure of a porous film obtained in a comparative example.
FIG. 7 is a diagram showing a structure of a porous film obtained in an example.
FIG. 8 is a graph showing the results of battery characteristics of an example and a conventional example measured in the example.
[Explanation of symbols]
1 positive electrode
1a Positive electrode active material layer
1b Positive electrode current collector
2 Negative electrode
2a Negative electrode active material layer
2b Negative electrode current collector
3 First layer
4 particles

Claims (11)

A positive electrode having a positive electrode active material layer disposed on a first current collector, a negative electrode having a negative electrode active material layer disposed on a second current collector, and a first layer containing particles;
The first layer is disposed between the positive electrode active material layer and the negative electrode active material layer,
The particles expand above a predetermined temperature,
The secondary battery, wherein the predetermined temperature is 80 ° C. or higher. _{2. The} secondary battery according to claim 1, wherein a volume V _{1 of the} particles before expansion and a volume V _{2 of the} particles after expansion satisfy a relationship of V ₂ ≧ 3 × V _1. 3. The secondary battery according to claim 1, wherein an average particle size of the particles before expansion is in a range of 3 μm or more and 20 μm or less. The secondary battery according to claim 1, wherein the particles are contained in the first layer in a range of 5% by volume to 35% by volume. The secondary battery according to claim 1, wherein the particles are hollow bodies. The secondary battery according to claim 5, wherein the hollow body contains at least one material selected from a solid material and a liquid material that evaporates at the predetermined temperature or higher. The secondary battery according to claim 1, wherein the first layer is a porous film made of a polymer. A first step of preparing a solution containing a polymer, a good solvent for the polymer, a poor solvent for the polymer, and particles that expand at a predetermined temperature or higher;
A second step of applying the solution on at least one electrode plate selected from a positive electrode plate and a negative electrode plate to form a coating film;
A third step of forming a porous film containing the particles on the at least one electrode plate by removing the poor solvent after the removal of the good solvent in the coating film;
A fourth step of forming an electrode plate group so that the porous film is placed between the positive electrode plate and the negative electrode plate,
A method for manufacturing a secondary battery, wherein the predetermined temperature is 80 ° C. or higher. 9. The method according to claim 8, wherein the first step is performed in an atmosphere having a relative humidity of 10% or less. 10. The method according to claim 8, wherein the polymer is a dehydrated polymer. The method for manufacturing a secondary battery according to claim 8, wherein a ratio of the polymer in the solution is in a range of 10% by mass to 40% by mass.

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