JP5311317B2 - Lithium ion secondary battery separator and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a separator for a lithium ion secondary battery and a lithium ion secondary battery. In particular, the present invention relates to a separator capable of producing a lithium ion secondary battery excellent in short circuit prevention and high rate discharge characteristics due to lithium dendrite, and a lithium ion secondary battery using this separator.

  With recent advances in electronic technology, small portable electronic devices such as camera-integrated VTRs, mobile phones, laptop computers, etc. have been developed. There is a strong demand for the development of secondary batteries.

  As a secondary battery that meets such a demand, a non-aqueous electrolyte secondary battery that uses a light metal such as lithium, sodium, and aluminum as a negative electrode active material that can theoretically generate a high voltage and has a high energy density is expected. ing. In particular, lithium ion secondary batteries that charge and discharge lithium ions via non-aqueous electrolytes have higher output and higher energy compared to nickel-cadmium batteries and lead-acid batteries that are aqueous electrolyte secondary batteries. Research and development is actively underway to achieve density.

In this lithium ion secondary battery, since the inherent energy is large, high safety is required in the event of an abnormality such as an internal short circuit or an external short circuit. For this safety measure, polyolefin microporous membranes are used. Yes. This is because it is considered to have a function (shutdown function) that does not pass electricity when abnormal heat is generated and does not flow electricity. Even if such safety measures were taken, abnormal heat generation did not stop, and the polyolefin microporous membrane contracted or melted, and the electrodes contacted each other to cause a short circuit, resulting in ignition.
As a separator that can be expected to prevent short circuit due to shrinkage or melting, such as a polyolefin-based microporous film, a separator provided with a ceramic film or the like has been proposed (Patent Documents 1 to 11).

JP 2005-502177 A JP 2005-518272 A JP 2005-525224 A JP 2005-536658 Gazette JP 2005-536857 A JP 2005-536858 A JP 2005-536860 Publication JP-T-2006-504228 JP 2006-505100 Gazette JP 2006-507635 A JP-T-2006-507636

  The methods disclosed in these patent documents basically apply a mixed solution obtained by mixing ceramic powder or the like with a sol solution to a support to form a ceramic film. However, as a result of the study by the present inventors, the separator obtained by such a method has a high porosity and a low air permeability resistance (Gurley) value (easy to be ventilated). When used as a battery separator, short-circuiting due to lithium dendrite was likely to occur. Therefore, when the amount of the mixed solution applied to the support was increased and the air resistance (Gurley) was increased (difficult to ventilate) to form a dense structure, the high rate discharge characteristics were poor. .

  The present invention has been made under such circumstances, and it is possible to prevent short-circuiting due to shrinkage or melting, as well as lithium-ion secondary batteries that are less prone to short-circuiting due to lithium dendrites and that have excellent high-rate discharge characteristics. It is an object of the present invention to provide a separator capable of producing a lithium ion secondary battery using the separator.

The invention according to claim 1 of the present invention is “a separator for a lithium ion secondary battery having a structure in which inorganic particles are filled in voids formed by bonding of fibers, and has a porosity of 40% or more and a transparent air resistance of (Gurley) of Ri der least 100 seconds, as a fiber, a lithium ion core component comprises polypropylene, fiber diameter sheath component consists of polyethylene is characterized Rukoto contains the following ultrafine fibers 5μm Secondary battery separator. "
The invention according to claim 2 of the present invention is “a lithium ion secondary battery using the separator for lithium ion secondary battery according to claim 1 ”.
The separator for a lithium ion secondary battery of the present invention preferably contains inorganic particles having an average primary particle size of 10 nm or less.
Moreover, it is preferable that the separator for lithium ion secondary batteries of this invention contains polyvinyl alcohol.

The invention according to claim 1 of the present invention is a lithium ion secondary battery separator having a structure in which voids formed by bonding of fibers are filled with inorganic particles, the porosity is 40% or more, and It has been found that when the air permeability resistance (Gurley) is 100 seconds or more, it does not shrink or melt even in a high temperature state, does not cause a short circuit due to lithium dendrite, and is excellent in high-rate discharge characteristics. is there. In addition, since it contains ultrafine fibers having a fiber diameter of 5 μm or less, the porosity is 40% or more, the air permeability resistance (Gurley) is easily set to 100 seconds or more, and the average primary particle diameter is 10 nm or less. When it contains, it is easy to hold | maintain this inorganic particle, and omission of an inorganic particle does not arise easily.
Since the invention according to claim 2 of the present invention uses the lithium ion secondary battery separator, the separator does not shrink or melt and / or short-circuit due to lithium dendrite does not occur, and is excellent in high-rate discharge characteristics. It is a lithium ion secondary battery.
In the preferred embodiment of the present invention, which includes “inorganic particles having an average primary particle diameter of 10 nm or less”, since the inorganic particles having an average primary particle diameter of 10 nm or less are included, the porosity is 40% or more, and the air permeability resistance ( Gurley) is easily set to 100 seconds or more. Further, the affinity with the electrolytic solution is increased, the internal resistance can be lowered, and the battery can be efficiently manufactured.
In a preferred embodiment of the present invention, “containing polyvinyl alcohol”, when polyvinyl alcohol is contained, the battery voltage at a high temperature state can be lowered, and after being exposed to a high temperature state, it cannot be recharged. can do.

  The separator for lithium ion secondary batteries of the present invention (hereinafter simply referred to as “separator”) has a structure in which voids formed by bonding of fibers are filled with inorganic particles, so that the fibers have no heat resistance. However, due to the heat resistance of the inorganic particles, a separator that does not shrink or melt even at high temperatures can be obtained.

  Although the fiber which comprises this separator is not specifically limited, It is preferable that the fiber diameter contains the ultrafine fiber of 5 micrometers or less. By including such ultrafine fibers, the individual voids formed by the bonding of the fibers are reduced, and as a result, the porosity is 40% or more and the air resistance (Gurley) is 100 seconds or more. This is because it is easy. In addition, the inclusion of ultrafine fibers increases the specific surface area of the separator and improves the retention of inorganic particles and the retention of electrolyte. Since the smaller the fiber diameter, the smaller the individual voids, the fiber diameter is preferably 3 μm or less, and more preferably 2 μm or less. The lower limit of the fiber diameter is not particularly limited, but it is appropriate that it is 0.01 μm or more from the viewpoint of the dispersibility of the ultrafine fibers.

  In addition, as long as the fiber diameter is 5 μm or less, such an ultrafine fiber may be produced in any way, and is not particularly limited. For example, sea components are removed from the sea-island type composite fiber. By applying external force to the composite fiber composed of two or more kinds of resins with poor compatibility, manufactured by peeling, fibers manufactured by the melt blow method, external force from a beating machine, etc. Examples thereof include a fiber having a fibril of 5 μm or less and a fiber manufactured by a flash spinning method. Among these, the island component fiber produced by removing the sea component from the sea-island type composite fiber has a relatively high fiber strength and can be suitably used because it has a uniform fiber diameter.

  In addition, the resin component constituting the ultrafine fiber is not particularly limited, but is an electrolyte solution-resistant and oxidation-resistant polyolefin resin, polyester resin, nylon resin (particularly wholly aromatic polyamide resin). It can be comprised from 1 or more types of resin chosen from the inside. Among these, polyolefin resins are particularly suitable because they are particularly excellent in electrolytic solution resistance and oxidation resistance and do not absorb moisture.

  In addition, although the ultrafine fiber is composed of one or more kinds of resins, it is composed of two or more kinds of resins having different melting points, and when the resin having a low melting point constitutes at least one part of the fiber surface, It is suitable because it can be fused with a resin and has excellent separator shape maintainability. In particular, a core-sheath type in which a resin having a low melting point occupies the entire fiber surface excluding both ends is particularly preferable because of excellent fusion power. As described above, since the ultrafine fibers are preferably made of a polyolefin resin, it is particularly preferable that the core component is made of polypropylene and the sheath component is made of polyethylene. In addition, it is possible to perform an affinity imparting treatment such as a plasma treatment on the ultrafine fibers so that the affinity of the ultrafine fibers with the electrolyte is increased, the internal resistance is lowered, and the battery can be efficiently manufactured. .

  Further, the fiber length of the ultrafine fiber is not particularly limited, but is preferably 10 mm or less, more preferably 5 mm or less, and even more preferably 3 mm or less so that the dispersibility is excellent. . Although the minimum of fiber length is not specifically limited, It is preferable that it is 0.1 mm or more.

  By including such ultrafine fibers, it is possible to reduce individual voids formed by bonding of the fibers, and as a result, the porosity is 40% or more and the air resistance (Gurley) is 100 seconds or more. It is preferable that it accounts for 5% or more of the total mass of the fiber, more preferably 10% or more, and even more preferably 15% or more.

  Although the fiber which comprises the separator of this invention can be comprised from the above-mentioned ultrafine fiber 100%, it can also contain the regular fiber in which a fiber diameter exceeds 5 micrometers. This regular fiber can be the same as the ultrafine fiber except that the fiber diameter exceeds 5 μm, that is, the resin component, the number of resin components, the arrangement state of the resin component, the fiber length, and the like. In addition, since it is preferable to contain ultrafine fibers, the regular fibers preferably occupy 95% or less of the total mass of the fibers, more preferably 90% or less, and 85% or less. More preferably. Moreover, it is preferable that the upper limit of the fiber diameter of a regular fiber is 20 micrometers or less. Furthermore, in order to increase the affinity with the electrolytic solution, an affinity imparting treatment such as plasma treatment can be performed on the regular fiber.

  A void is formed by bonding such fibers. The bonding between the fibers is not particularly limited, and examples thereof include bonding by fiber fusion, bonding by a binder, and bonding by fiber entanglement. These bonds can be used in combination.

  In addition, when couple | bonding with a binder, it is preferable that polyvinyl alcohol is included. This is because when polyvinyl alcohol is contained as a binder, the battery voltage at a high temperature can be lowered, and the battery can be prevented from being charged again after being exposed to a high temperature. As this polyvinyl alcohol, for example, those having a polymerization degree of 100 to 10000, preferably 200 to 5000, more preferably 300 to 3000 can be used. As for the degree of saponification, for example, 60 to 98%, preferably 70 to 98%, more preferably 80 to 98% polyvinyl alcohol can be used. In addition, it is preferable that content of polyvinyl alcohol is 0.1 mg or more per 1 mAh of battery capacity, More preferably, it is 0.5 mg or more, More preferably, it is 1 mg or more. The upper limit is not particularly limited, but it is reasonable that the upper limit is 100 mg or less per 1 mAh of battery capacity. Polyvinyl alcohol does not need to be contained as a binder, and even if it is contained as one kind of powder, the same effect is obtained.

  In the separator of the present invention, the voids thus formed are filled with inorganic particles, the porosity is 40% or more, and the air permeability resistance (Gurley) is 100 seconds or more. In addition to preventing short circuit due to lithium, it prevents short circuit due to lithium dendrite and has excellent high-rate discharge characteristics.

  The inorganic particles are not particularly limited as long as the porosity and air permeation resistance can be obtained, and include inorganic particles having an average primary particle diameter of 10 nm or less (preferably 8 nm or less). It is preferable. This is because when such inorganic particles are included, the porosity and the ventilation resistance can be easily obtained. In addition, the wettability of the electrolytic solution can be improved. In particular, when the ultrafine fibers as described above are included, the inorganic particles having an average primary particle diameter of 10 nm or less are difficult to drop off because they are supported by the ultrafine fibers. In addition, the lower limit of the average primary particle diameter of the inorganic particles is not particularly limited, but it is appropriate that it is 0.1 nm or more. The content of the inorganic particles having an average primary particle diameter of 10 nm or less is an amount that can be set as the porosity and the air resistance, and varies depending on the fiber. Find the amount that can be resistance. The “average primary particle diameter” refers to a value obtained by measurement by a laser diffraction / scattering method.

In this invention, as long as it can be set as the said porosity and air permeability resistance, the average primary particle diameter can contain the inorganic particle exceeding 10 nm.
Such a material constituting the inorganic particles is not particularly limited as long as it is excellent in electrolytic solution resistance and oxidation resistance regardless of the particle diameter. For example, silicon dioxide, titanium oxide, Examples thereof include metal oxides such as zinc oxide, yttrium oxide, ytterbium oxide, aluminum oxide, magnesium oxide, cerium oxide, and zirconium oxide. These inorganic particles can be used alone or in combination of two or more. Note that when the inorganic particles are subjected to a treatment that imparts or improves the affinity with the electrolytic solution, the internal resistance is reduced, and the battery can be efficiently manufactured.

  The separator of the present invention has a structure in which inorganic particles as described above are filled in voids formed by bonding of fibers, but the filling state is not limited as long as the porosity and the air resistance are the above. For example, the inorganic particles may be unevenly distributed and filled in the thickness direction of the separator, or may be filled uniformly throughout the thickness direction of the separator. Further, the inorganic particles may be fixed by fiber fusion, may be fixed to the fiber by a binder, or may be fixed to the fiber by electrostatic force or van der Waals force. The state is not particularly limited.

  As described above, the separator of the present invention has a porosity of 40% or more and an air resistance (Gurley) of 100 seconds or more by filling the voids formed by bonding of fibers with inorganic particles. is there. It was found that by satisfying such a porosity and air permeability resistance at the same time, a short circuit due to lithium dendrite does not occur and the high-rate discharge characteristics are excellent.

When the porosity of the separator is less than 40%, the high-rate discharge characteristics tend to deteriorate, so that it is 40% or more, preferably 42% or more, more preferably 44% or more, and further preferably 45. % Or more. On the other hand, the upper limit of the porosity is not particularly limited, but it is preferably 90% or less so that the air permeability resistance is easily set to 100 seconds or more and the shape maintaining property as a separator is excellent. % Or less is more preferable, 70% or less is further preferable, and 60% or less is still more preferable. The porosity P (unit:%) is a value obtained from the following formula.
P = 100− (Fr ₁ + Fr ₂ +. + Fr _n )
Here, Fr _n indicates the filling rate (unit:%) of the n component constituting the separator, and is a value obtained from the following equation.
Fr _n = (M × Pr _n / T × SG _n ) × 100
Here, M is the basis weight of the separator (unit: g / cm ² ), T is the thickness of the separator (cm), Pr _n is the mass ratio of the n component in the separator, and SG _n is the specific gravity of the component n (unit: g / Cm ³ ) respectively.

  On the other hand, if the air resistance is less than 100 seconds, a short circuit due to lithium dendrite tends to occur, and therefore it is 100 seconds or more, preferably 110 seconds or more, more preferably 120 seconds or more. More preferably, it is 128 seconds or more. On the other hand, the upper limit of the air resistance is not particularly limited, but is preferably 1000 seconds or less, more preferably 600 seconds or less, so that the porosity can be 40% or more. More preferably, it is 500 seconds or less. The air resistance refers to the air resistance measured in accordance with the method defined in JIS P 8117: 1998 (paper and paperboard-air permeability test method-Gurley test machine method).

Although the thickness of the separator of this invention is not specifically limited, It is preferable that it is 10-80 micrometers. When the thickness is less than 10 μm, the mechanical strength is weak and the handleability is poor, so that the thickness is preferably 15 μm or more. On the other hand, if the thickness exceeds 80 μm, it tends to be too thick to prevent high-rate discharge. Therefore, the thickness is preferably 60 μm or less, and more preferably 50 μm or less. The thickness of the separator micrometers (diameter: 6 mm) using, refers to a value at 13.7 N / cm ² load.

Such a separator of the present invention can be manufactured, for example, as follows.
First, fibers and inorganic particles constituting the separator are prepared. As the fibers, it is preferable to prepare ultrafine fibers so that the inorganic particles can be supported. Moreover, it is preferable to prepare a fusible composite fiber (whether or not it is an ultrafine fiber) so that the fibers can be bonded to each other. In addition, it is preferable to prepare inorganic particles having an average primary particle diameter of 10 nm or less as inorganic particles.
Subsequently, these fibers and inorganic particles are mixed to form a particle-containing web. The particle-containing web can be formed by a dry method such as an air array method or a wet method.

  And the separator of this invention can be manufactured by couple | bonding fibers. In addition, as a bonding method of fibers, for example, a method of fusing using the fusing property of fibers and a method of bonding using a binder can be exemplified. In addition, since the inorganic particles can be fixed to the fibers when the fibers are bonded to each other, a separator that hardly causes the inorganic particles to fall off can be manufactured. According to this manufacturing method, it is possible to manufacture a separator in which inorganic particles are uniformly distributed throughout the thickness direction of the separator. Furthermore, when polyvinyl alcohol is mixed as a binder or as a powder, the battery voltage at a high temperature can be lowered, and after being exposed to a high temperature, it cannot be recharged. Are particularly preferred. In addition, after bonding fibers and fibers and inorganic particles by utilizing the fusibility of the fibers, these bonds may be further strengthened by a binder.

The separator of the present invention can also be produced by another method as follows.
First, the fiber which comprises a separator is prepared. As the fibers, it is preferable to prepare ultrafine fibers so that the inorganic particles can be supported. Moreover, it is preferable to prepare a fusible composite fiber (whether or not it is an ultrafine fiber) so that the fibers can be bonded to each other.
Next, the fiber is opened to form a fiber web. The fiber web can be formed by a dry method such as an air array method or a wet method. In some cases, the fiber web can be directly formed by a melt blow method.

Subsequently, a nonwoven fabric is formed by bonding the fibers together. Examples of a method for bonding fibers include a method of fusing using the fusing property of fibers, a method of bonding using a binder, a method of entanglement with a fluid flow, and the like.
On the other hand, inorganic particles are prepared. As the inorganic particles, it is preferable to prepare inorganic particles having an average primary particle diameter of 10 nm or less.
And this inorganic particle is mixed with a binder, it apply | coated to the above-mentioned nonwoven fabric formed, and it can dry and can manufacture the separator of this invention. The binder is not particularly limited, but when polyvinyl alcohol is used, the battery voltage at a high temperature can be lowered, and the battery cannot be recharged after being exposed to a high temperature. Is particularly preferable. According to this manufacturing method, a separator in which inorganic particles are relatively unevenly distributed near the surface of the separator can be manufactured. In addition, without using a binder, inorganic particles can be dispersed on a non-woven fabric, and a separator can be produced using the fusing property of non-woven fabric constituent fibers.

  Regardless of the manufacturing method, the separator having a porosity of 40% or more and an air permeability resistance of 100 seconds or more has a thickness, a fiber amount, an inorganic particle amount, an ultrafine fiber amount, an average primary particle size of inorganic particles, and the like. It can be manufactured by adjusting appropriately, and in particular, it is easy to manufacture by using ultrafine fibers and including inorganic particles having an average primary particle diameter of 10 nm or less.

  Since the lithium ion secondary battery of the present invention uses the separator of the present invention as described above, lithium ion that does not cause shrinkage or melting of the separator and / or short-circuit due to lithium dendrite and has excellent high-rate discharge characteristics. It is a secondary battery.

The lithium ion secondary battery of the present invention can have the same configuration as that of a conventional lithium ion secondary battery except that the above separator is used. For example, a positive electrode in which a paste of a lithium-containing metal compound is carried on a current collector can be used as a positive electrode, and a lithium metal or a lithium alloy, and a carbon material containing carbon or graphite capable of occluding and releasing lithium (for example, coke) as a negative electrode. Non-aqueous electrolysis in which LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate as an electrolytic solution. Liquid can be used. Further, the cell structure of the lithium ion secondary battery is not particularly limited, and may be, for example, a stacked type, a cylindrical type, a square type, a coin type, or the like.

  Examples of the present invention will be described below, but the present invention is not limited to the following examples.

<< Examples 1-3 >>
A core-sheath type composite ultrafine fiber manufactured by removing the sea component of the sea-island type composite fiber as the fiber, wherein the core component is made of polypropylene (melting point: 160 ° C.) and the sheath component is made of high-density polyethylene (melting point: 135 ° C.) (Fiber diameter = 2 μm, fiber length = 1 mm) and hydrophilic silica particles (Aerosil (registered trademark), manufactured by Nippon Aerosil Co., Ltd.) having an average primary particle diameter of 7 nm were prepared.
Subsequently, the core-sheath type composite ultrafine fiber and the hydrophilic silica particles were mixed at a mass ratio shown in Table 1, and an inorganic particle-containing web was formed on the conveyor net by the air lay method.
Next, the inorganic particle-containing web was supplied to a compliant press machine (set temperature = 135 ° C.), heated and pressurized for 20 seconds, allowed to cool, and the sheath component of the core-sheath composite ultrafine fiber was fused. An inorganic particle-containing fusion web was prepared. Then, thickness adjustment was performed with the calendar roll press machine, and the separator sheet of this invention was manufactured. This separator sheet was in a state where inorganic particles were uniformly filled in the entire thickness direction. The physical properties of this separator sheet were as shown in Table 1.

Example 4
The separator sheet of Example 3 was immersed in an aqueous solution of 10% polyvinyl alcohol (degree of saponification: 98%, degree of polymerization: 500), squeezed with a mangle, dried, and then the separator sheet of the present invention (containing polyvinyl alcohol) the amount: 1 g / ^{m 2,} a basis weight: 19.1 g / ^{m 2,} thickness: 28 .mu.m, air resistance: 493 seconds, porosity: 46%) was prepared.

<< Comparative Examples 1-3 >>
Except that the core-sheath type composite ultrafine fiber and the hydrophilic silica particles were mixed at a mass ratio shown in Table 2 and the inorganic particle-containing web was formed on the conveyor net by the air lay method, exactly as in Examples 1 to 3. Thus, a separator sheet was manufactured. Table 2 shows the physical properties of this separator sheet.

<< Comparative Example 4 >>
A nonwoven fabric made of polyester fibers (weight per unit area: 12 g / m ² , thickness: 15 μm, average fiber diameter: 6 μm) was prepared.
A sol solution having the following composition was prepared.
Ethanol 85%
5wt% hydrochloric acid 8%
Tetraethoxysilane 5%
Methyltriethoxysilane 0.2%
Next, alumina particles were mixed with the sol solution so that the solid content mass ratio between the sol solution and α-alumina particles (average primary particle size = 1 μm) was 60:40 to prepare a binding paste.
Subsequently, after impregnating the bonded paste into the nonwoven fabric and drying with a dryer set at a temperature of 200 ° C., the thickness was adjusted with a calender roll press, and a separator sheet (inorganic component amount: 22.8 g / m ^2). , Basis weight: 34.8 g / m ² , thickness: 34 μm, air resistance: 101 seconds, porosity: 35.2%).

<< Comparative Example 5 >>
A polypropylene microporous membrane (celgard 2400; manufactured by Celgard, basis weight: 14.3 g / m ² , thickness: 26 μm, air resistance: 730 seconds, porosity: 37%) was prepared as a separator sheet.

(Production of lithium ion secondary battery)
(1) Preparation of positive electrode 87 parts by mass of lithium cobaltate (LiCoO ₂ ) powder, 6 parts by mass of acetylene black, and 7 parts by mass of polyvinylidene fluoride (PVdF) were dissolved in N-methyl-pyrrolidone (NMP), and polyfluorinated. A positive electrode paste having a vinylidene concentration of 13% was prepared. Next, this paste was applied to an aluminum foil having a thickness of 20 μm, dried, and pressed to prepare a positive electrode having a thickness of 90 μm.
(2) Preparation of negative electrode 90 parts by weight of natural graphite powder and 10 parts by weight of polyvinylidene fluoride (PVdF) are dissolved in N-methyl-pyrrolidone (NMP) as a negative electrode active material, and the negative electrode agent has a polyvinylidene fluoride concentration of 13%. A paste was prepared. This paste was applied onto a copper foil having a thickness of 15 μm, dried and then pressed to prepare a negative electrode having a thickness of 70 μm.
(3) Preparation of non-aqueous electrolyte As a non-aqueous electrolyte, a 1M solution (manufactured by Kishida Chemical Co., Ltd.) prepared by dissolving LiPF ₆ in a mixed solvent of ethylene carbonate and diethyl carbonate (ratio = 50: 50) is prepared. did.
(4) Production of Lithium Ion Secondary Battery A negative electrode (diameter: 12 mm), each separator sheet (diameter: 16 mm), and positive electrode (diameter: 12 mm) were laminated in this order on a CR-2032 type coin cell, and then a non-aqueous electrolyte was injected. After lysing and capping with a spacer, packing was performed using a coin cell caulking machine to prepare lithium ion secondary batteries. The mass ratio of the positive electrode to the negative electrode was 1: 1.1.

(Battery performance test)
After making a lithium ion secondary battery, let it stand at room temperature for a day, then charge at 0.2C, 4.2V constant current / constant voltage (6 hours), then discharge at 0.2C constant current. 5 cycles of charging and discharging were performed. When the presence or absence of an internal short circuit after one cycle in this charge / discharge test and the presence or absence of an internal short circuit after five cycles were confirmed, the results shown in Table 3 were obtained.
A high-rate discharge test was performed on the batteries using the separator sheets of Examples 1 to 4 and Comparative Examples 3 to 5 that were regarded as having no internal short circuit after 5 cycles. The high-rate discharge test consists of 5 cycles of charge / discharge, with a constant current / constant voltage charge (6 hours) of 0.2C and 4.2V (6 hours) followed by a constant current discharge at 8C. The average value of the capacity retention ratios after the measurement was taken as the capacity retention ratio of the lithium ion secondary battery. These results are shown in Table 3.

(Recharge test)
A theoretical capacity of a CR-2032 type coin cell (lithium ion secondary battery) manufactured in the same manner as described above is 1.6 mAh. The batteries using the separator sheets of Examples 1 to 4 and Comparative Examples 3 to 5 were 0.2 C, 4.2 V constant current / constant voltage charging (6 hours), and then 0.2 C, 3 V cut-off constant current. It was confirmed that a predetermined battery capacity was obtained after 5 cycles of discharge.
Next, the battery was fully charged with constant current / constant voltage charging (6 hours) at 0.2 C and 4.2 V, and then stored in a 120 ° C. hot air oven.
Then, after discharging the battery at 0.2 C, constant current charging at 0.2 C was performed for 6 hours to confirm whether the battery was operable. And the capacity | capacitance (%) with respect to the initial capacity after recharging was computed, and it was set as the recharge rate. A battery that has been exposed to a high temperature is likely to have deteriorated or decomposed electrolyte or active material, and it is desirable that the battery does not operate even if it is charged again from the viewpoint of safety. The case of less than% was judged as ◯, and the case of exceeding 1% was judged as ×. The results are shown in Table 3.

(Measurement of shrinkage rate)
The circular test pieces (diameter: 16 mm) of the separators of Example 2 and Comparative Example 5 were left in an oven set at a temperature of 160 ° C. for 20 minutes, and the contraction rate in the longitudinal direction (flow direction during production) before and after being left. Was calculated respectively. As a result, the shrinkage percentage of the separator of Comparative Example 5 was 37.5%, whereas that of Example 2 was 3%.

  As described above, from comparison between Examples 1 to 3 and Comparative Examples 1 to 3, when the air permeability resistance is 100 seconds or more, not only the initial short circuit due to lithium dendrite does not occur, but also high rate discharge. In some cases, short-circuit due to lithium dendrite does not occur, and comparison between Examples 1 to 3 and Comparative Examples 4 to 5 shows that the porosity is 40% or more, which means excellent high-rate discharge characteristics. From the comparison between No. 3 and Example 4, it can be recharged after being exposed to a high temperature state by containing polyvinyl alcohol, and from the comparison between Example 2 and Comparative Example 5, inorganic particles It has been found that short-circuiting can be prevented without shrinkage by containing selenium.

Claims (2)

A separator for a lithium ion secondary battery having a structure in which voids formed by bonding between fibers are filled with inorganic particles, the porosity is 40% or more, and the air resistance (Gurley) is 100 seconds or more. Ri, fibers as the core component comprises polypropylene, a lithium ion secondary battery separator fiber diameter sheath component consists of polyethylene is characterized Rukoto contains the following ultrafine fibers 5 [mu] m. The lithium ion secondary battery using the separator for lithium ion secondary batteries of Claim 1 .