JP2006019191A - Separator for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a lithium ion secondary battery having uniform hole diameters and excellent ion permeability, capable of preventing short-circuiting even when temperature rises to such high temperatures exceeding 180°C. <P>SOLUTION: This separator for the lithium-ion secondary battery comprises a nonwoven fabric containing heat resistant pulp fiber having a melting point or a carbonization temperature of 300°C or higher, and thermoplastic fiber having a melting point of 200°C or higher, and the percentage of void of the nonwoven fabric is not more than 60%, and none of the nonwoven fabric composing material is formed into a film. The lithium-ion secondary battery is provided with this separator for the lithium-ion secondary battery. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a separator for a lithium ion secondary battery and a lithium ion secondary battery.

  In recent years, along with the reduction in size and weight of electric devices, there is a strong demand for a battery that is a power source for reduction in size, weight, and high energy density. Since lithium ion secondary batteries have high energy density, they are expected to satisfy such demands.

  As a separator of such a lithium ion secondary battery, a polyolefin microporous membrane has been generally used. This is because the battery temperature rises markedly when an abnormal large current flows due to an external short circuit of the battery, etc., and the polyolefin microporous membrane contracts due to the heat to prevent the generation of flammable gas, battery rupture or ignition. Or it is because it is thought that it has the function (shutdown function) which melt | dissolves and obstruct | occludes a micropore and interrupts | blocks ion permeability. However, when the temperature rises to a high temperature exceeding 180 ° C., the polyolefin microporous film is melted and a sufficient short-circuit prevention function cannot be exhibited. Also, as the temperature rises, the separator heat shrinks in the width direction, the width dimension decreases, and the electrodes that are in contact with the width direction end of the separator are exposed, which may cause a short circuit. .

  As a separator that may be able to prevent a short circuit in such a lithium ion secondary battery, “the melting point or the thermal decomposition temperature is 250 ° C. or higher, the average fiber length is 0.3 mm to 2 mm, and at least part of the fiber diameter is 1 μm or less. There has been proposed "a separator for electrochemical elements" comprising a wet nonwoven fabric having a porosity of 68% to 85%, which contains fibrillated liquid crystalline polymer fibers (Patent Document 1).

JP 2002-266281 A (Claim 1, Claim 11, etc.)

  Since the separator for an electrochemical element as disclosed in Patent Document 1 is made of a wet nonwoven fabric, the pore diameter is uniform to some extent. However, it is desired that the pore diameter is further uniform and the ion permeation uniformity is further improved. It was.

  The present invention is a lithium ion secondary battery separator that can prevent a short circuit even when the temperature rises to a high temperature exceeding 180 ° C., has a uniform pore diameter, and is excellent in ion permeability, And it aims at providing a lithium ion secondary battery.

  The invention according to claim 1 of the present invention is “consisting of a nonwoven fabric containing a heat-resistant pulp fiber having a melting point or a carbonization temperature of 300 ° C. or higher and a thermoplastic fiber having a melting point of 200 ° C. or higher. The separator for a lithium ion secondary battery is characterized in that it is 60% or less, and none of the non-woven fabric constituting material is formed into a film. It has been found that when the non-woven fabric has a porosity of 60% or less, the proportion of fibers in a certain volume is high, and the pore diameter of the non-woven fabric (lithium ion secondary battery separator) can be made uniform. Further, since none of the nonwoven fabric constituting materials is made into a film, the ion permeability is not hindered and the ion permeability is excellent. Moreover, since the heat resistant pulp fiber in the form of pulp is contained, the pore diameter can be made smaller and larger (that is, the pore diameter can be made uniform), so that the ion permeability is excellent in uniformity. Furthermore, by containing a heat-resistant pulp fiber and a thermoplastic fiber having a melting point of 200 ° C. or higher, even if the temperature rises to a high temperature exceeding 180 ° C., it does not shrink, and a short circuit is caused. It can be prevented.

  The invention according to claim 2 of the present invention is “a separator for a lithium ion secondary battery according to claim 1, wherein the nonwoven fabric has a thickness of 10 to 30 μm”. Thus, even if the absolute thickness of the nonwoven fabric which is a separator for a lithium ion secondary battery is thin, the ion permeability is excellent.

  The invention according to claim 3 of the present invention is “a separator for a lithium ion secondary battery according to claim 1 or 2, wherein the heat-resistant pulp fiber is an aramid pulp fiber”. Since aramid pulp is excellent in heat resistance, it is excellent in short circuit prevention.

  The invention according to claim 4 of the present invention is “a separator for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the thermoplastic fiber is a polyester fiber”. Polyester fibers are excellent in thermal stability because they are excellent in short circuit prevention, and have a tendency to swell slightly due to the electrolytic solution, so that they are excellent in ion permeability.

  The invention according to claim 5 of the present invention is “a separator for a lithium ion secondary battery according to any one of claims 1 to 4, wherein an average fiber diameter of the thermoplastic fibers is 4 μm or less. Is. Since the thermoplastic fibers can be uniformly dispersed, the short circuit prevention is excellent, and the pore diameter can be made smaller and larger, so that the ion permeability is excellent in uniformity.

  The invention according to claim 6 of the present invention is as described in any one of claims 1 to 5, wherein the mass ratio of heat-resistant pulp fiber to thermoplastic fiber is 30:70 to 80:20. The separator for a lithium ion secondary battery described in the above. When the mass ratio of the heat-resistant pulp fiber and the thermoplastic fiber is within this range, even if the temperature rises to a high temperature, it does not shrink, and the short-circuit prevention reliability is high.

  The invention according to claim 7 of the present invention is the lithium ion according to any one of claims 1 to 6, wherein the lithium ion is made of a wet laminated nonwoven fabric produced by laminating two or more wet fiber webs. Secondary battery separator ". In the case of a wet laminated nonwoven fabric, the nonuniformity of the pore diameter can be further improved and the pore diameter can be further reduced, and thus the ion permeability is excellent in uniformity.

  The invention according to claim 8 of the present invention is “a lithium ion secondary battery including the lithium ion secondary battery separator according to any one of claims 1 to 7”. Therefore, even when the temperature rises to a high temperature exceeding 180 ° C., the short circuit can be prevented and the reliability is high and the internal resistance is low.

  The separator for a lithium ion secondary battery of the present invention can prevent a short circuit even when the temperature rises to a high temperature exceeding 180 ° C., has a uniform hole diameter, and is ion permeable. Are better.

  The lithium ion secondary battery of the present invention has high reliability and low internal resistance that can prevent a short circuit even when the temperature rises to a high temperature exceeding 180 ° C.

  The nonwoven fabric which is a separator for lithium ion secondary batteries of the present invention (hereinafter simply referred to as “separator”) does not melt or shrink even when exposed to a high temperature exceeding 180 ° C., and has excellent dimensional stability. In addition, heat-resistant pulp fibers having a melting point or a carbonization temperature of 300 ° C. or higher are included so that the heat resistance and the uniformity of ion permeability are excellent. In the present invention, “melting point” refers to a temperature obtained from a differential thermal analysis curve (DTA curve) obtained by differential thermal analysis defined in JIS K 7121, and “carbonization temperature” refers to JIS K 7120. This is the temperature obtained by thermogravimetry.

  Specifically, examples of the heat-resistant pulp fiber having a melting point of 300 ° C. or higher include polytetrafluoroethylene pulp fiber, polyphenylene sulfide pulp fiber, and the like. Aromatic polyamide pulp fibers, para-aromatic polyamide pulp fibers, polyamideimide pulp fibers, aromatic polyetheramide pulp fibers, polybenzimidazole pulp fibers, wholly aromatic polyester pulp fibers, and the like can be mentioned. Among these, meta-aromatic polyamide pulp fibers or para-aromatic polyamide pulp fibers (aramid pulp fibers) are excellent in heat resistance, and therefore have excellent short circuit prevention properties and excellent compatibility with electrolytes. Therefore, a para-aromatic polyamide pulp fiber having a higher carbonization temperature is more preferable. The “heat-resistant pulp fiber” generated fine fibers (fibrils) in the fiber length direction by applying mechanical shearing force or the like to the heat-resistant fiber made of the same resin as the heat-resistant pulp fiber. Is.

  The freeness of the heat resistant pulp fiber is preferably 200 ml CSF or less, more preferably 150 ml CSF or less, and more preferably 120 ml CSF or less so as to have a dense structure and excellent separation between electrodes. Is more preferable. In addition, it is preferable that the freeness of heat-resistant pulp fiber is 50 mlCSF or more. This “freeness” means a value measured by a JIS P8121 Canadian standard freeness tester. Further, the average fiber length of the heat-resistant pulp fiber is preferably 1 mm or more so as to be excellent in strength, and is preferably 2 mm or less so that the deterioration of the texture due to the entanglement between the fibers hardly occurs. In addition, the average fiber length of heat-resistant pulp fiber means the value measured using Kajaani Fiber Lab about ten heat-resistant pulp fibers.

  The nonwoven fabric which is a separator of the present invention can contain two or more types of heat-resistant pulp fibers that differ in terms of resin composition, freeness, and / or average fiber length.

  The nonwoven fabric which is the separator of the present invention contains thermoplastic fibers having a melting point of 200 ° C. or higher in addition to the heat-resistant pulp fibers as described above. By including such a thermoplastic fiber, even if the temperature rises to a high temperature exceeding 180 ° C., it does not shrink and can prevent a short circuit, and also forms a nonwoven fabric skeleton. In addition, strength can be imparted. Moreover, when a nonwoven fabric consists of a wet nonwoven fabric, the intensity | strength at the time of wet nonwoven fabric manufacture is provided, and the continuous production of a wet nonwoven fabric is enabled.

  The melting point of the thermoplastic fiber is 200 ° C. or higher so as not to be short-circuited even when the temperature becomes high. However, the higher the melting point, the better the effect is, and therefore it is preferably 220 ° C. or higher. More preferably, the temperature is 240 ° C. or higher. On the other hand, the upper limit of the melting point of the thermoplastic fiber is not particularly limited, but it is preferable to fix the heat-resistant pulp fiber by deformation of the thermoplastic fiber and impart the shape stability of the nonwoven fabric (separator). It is preferably not higher than ° C., more preferably not higher than 280 ° C.

  Specifically, examples of the thermoplastic fiber include polyester fibers such as polyethylene terephthalate fibers and polybutylene terephthalate fibers, polyamide fibers such as nylon 6 fibers and nylon 66 fibers, polyvinyl chloride fibers, and polyurethane fibers. Among these, polyester fibers are suitable because they are excellent in thermal stability and excellent in short circuit prevention properties, tend to swell slightly with the electrolyte, and are excellent in ion permeability.

  The average fiber diameter of the thermoplastic fiber is preferably 4 μm or less. This is because the thermoplastic fibers can be uniformly dispersed, so that the short circuit prevention property is excellent, and the pore diameter can be made smaller and larger, so that the ion permeability uniformity is excellent. A more preferable average fiber diameter is 3.6 μm or less, and a still more preferable average fiber diameter is 3.2 μm or less. The lower limit of the average fiber diameter of the thermoplastic fiber is not particularly limited, but about 10 nm is appropriate. In addition, the average fiber diameter of a thermoplastic fiber takes an enlarged photograph 500 times with a microscope, and says the arithmetic mean value of the fiber diameter of ten thermoplastic fibers.

  The average fiber length of the thermoplastic fiber is not particularly limited, but is preferably about 1 to 25 mm so that it can be uniformly dispersed and has excellent separation between electrodes. More preferably, it is about ˜20 mm. Further, the cross-sectional shape of the thermoplastic fiber is not necessarily circular, and may be non-circular (for example, an ellipse, an ellipse, a star, an alphabet such as Y or X, or a plus). Further, the thermoplastic fiber may or may not be fibrillated, but preferably contains a non-fibrillated thermoplastic fiber so as to be excellent in strength. In addition, two or more types of thermoplastic fibers that differ in at least one of the melting point, the average fiber diameter, the fiber length, the cross-sectional shape, and / or the presence or absence of fibrils may be included. In addition, the average fiber length of a thermoplastic fiber means the arithmetic average value of the fiber length of ten thermoplastic fibers, taking a 10 times magnified photograph with a microscope.

  The nonwoven fabric (separator) of the present invention contains the heat-resistant pulp fiber and the thermoplastic fiber as described above, and the mass ratio thereof is (heat-resistant pulp fiber) :( thermoplastic fiber) = 30: 70-80: 20 is preferred. If the amount of heat-resistant pulp fiber is less than 30 mass% of the whole nonwoven fabric, it tends to melt at a high temperature or shrink and easily short-circuit, and if it exceeds 80 mass%, the amount of thermoplastic fiber tends to decrease and the nonwoven fabric tends to be weak. For this reason, (heat resistant pulp fiber) :( thermoplastic fiber) = 50: 50 to 70:30 is more preferable.

  The nonwoven fabric (separator) of the present invention is preferably composed of the heat-resistant pulp fiber and the thermoplastic fiber as described above, but can also contain fibers other than these fibers. However, the fibers contained in addition to these fibers are preferably organic fibers, not cellulose fibers or glass fibers.

  Since the nonwoven fabric which is the separator of the present invention is excellent in the retention of the electrolytic solution, it can produce a lithium ion secondary battery that can stably exhibit performance, but the porosity is 60% or less. It has been found that when the nonwoven fabric has a porosity of 60% or less, the proportion of fibers in a certain volume is high, and the pore diameter of the nonwoven fabric (lithium ion secondary battery separator) can be made uniform. The lower the porosity of the nonwoven fabric (that is, the higher the proportion of fibers in a certain volume), the more uniform the pore diameter of the nonwoven fabric (separator), so the porosity is preferably 58% or less, 55% More preferably, it is more preferably 50% or less. On the other hand, if the porosity of the nonwoven fabric is too low (that is, the proportion of fibers in a certain volume becomes too high), the ion permeability tends to deteriorate, so the porosity is preferably 30% or more. 35% or more is more preferable, and 40% or more is still more preferable.

  In this way, when the non-woven fabric has a porosity of 60% or less and the proportion of fibers in a certain volume increases, the ion permeability tends to deteriorate, so the non-woven fabric constituent material (for example, heat-resistant pulp fiber, thermoplastic fiber, etc. ) Is not made into a film, so that ion permeability is not impaired. “Film” refers to a state in which a film is stretched around the intersection of fibers, for example, when thermoplastic fibers are fused or fibers are bonded with a binder such as an emulsion. Is done. Therefore, the nonwoven fabric (separator) of the present invention is neither in a state where thermoplastic fibers are fused nor in a state where the fibers are bonded by a binder.

In the present invention, “porosity (P)” refers to a value obtained by the following equation.
Porosity (P) = {1−W / (T × d)} × 100
Here, W means the basis weight (g / m ² ) of the nonwoven fabric (separator), T means the thickness (μm) of the nonwoven fabric (separator), and d means the density (g / m) of the nonwoven fabric (separator) constituent material. cm ³ ). In addition, since at least heat-resistant pulp fiber and thermoplastic fiber exist as a nonwoven fabric (separator) constituent material, the density of the constituent material means the mass average of each constituent material. For example, when the heat-resistant pulp fiber having the density d ₁ is a (mass%) and the thermoplastic fiber having the density d ₂ is b (mass%), the density (d) of the constituent material is obtained by the following equation. Value.
Density (d) = d ₁ × a / 100 + d ₂ × b / 100

  As can be seen from the formula for calculating the porosity, as a method of reducing the porosity of the nonwoven fabric (separator) of the present invention, a method of reducing the thickness of the nonwoven fabric, a method of using a constituent material having a low density, or a nonwoven fabric There is a way to increase the basis weight. The thinner the separator, the higher the capacity because the amount of electrode material can be increased, and the lithium ion secondary battery can be made compact. 60% or less is preferable. More specifically, the thickness of the nonwoven fabric (separator) is preferably 10 to 30 μm. When the thickness is 30 μm or less, the ion permeability is excellent, and when the thickness is 10 μm or more, the mechanical strength is excellent and the handling property is excellent. A more preferred thickness is 13 to 28 μm. The “thickness” means a value measured by a method defined in JIS B 7502, that is, a value measured by an outer micrometer at a load of 5N.

It is preferable that the fabric weight of the nonwoven fabric (separator) of this invention is 10-30 g / m < ² >. If it exceeds 30 g / m ² , the ion permeability is deteriorated, and if it is less than 10 g / m ² , it is difficult to obtain uniformity, and pinholes tend to be generated and a short circuit tends to occur. Is 14 to 28 g / m ² . “Weight weight” means the basis weight based on the method specified in JIS P 8124 (paper and paperboard—basis weight measurement method).

  The nonwoven fabric which is the separator of the present invention is excellent in the uniform dispersibility of the fiber, and is preferably made of a wet nonwoven fabric which is less likely to cause a short circuit and has high reliability. In particular, it is preferably a wet laminated nonwoven fabric produced by laminating two or more wet fiber webs, which are excellent in denseness, short circuit prevention, ion permeability and strength. Furthermore, a wet laminated nonwoven fabric produced by laminating two or more wet fiber webs having different fiber orientations is more preferable because it is excellent in short circuit prevention.

  Such a nonwoven fabric as the separator of the present invention can be produced by a conventional method. For example, a separator made of a suitable wet nonwoven fabric can be manufactured as follows.

  First, at least heat-resistant pulp fibers (preferably aramid pulp fibers) and thermoplastic fibers (preferably polyester fibers) are prepared. Then, using these fibers, a conventional wet method (for example, a horizontal long net method, an inclined wire type short net method, a circular net method, a forward flow net / reverse flow net combination method, a forward flow net / circular net) A wet fiber web is formed by a former combination method, a reverse flow circular net / circular net former combination method, a short net / circular net combination method, or a long net / circular net combination method. In order to laminate two or more wet fiber webs suitable in the present invention, for example, wet fiber webs made by the same type of net are laminated, or different types of nets (for example, short net and circular net, long net) And a wet fiber web made by using a circular mesh). According to the former method, a wet fiber web having two or more layers having the same fiber orientation can be formed, and according to the latter method, a wet fiber web having two or more layers having different fiber orientations can be formed. It may be laminated after the wet fiber web is dried, but if the wet fiber web in the wet state is laminated with the same basis weight, the same thickness, and the same fiber orientation, the wet nonwoven fabric is superior in ion permeability ( Separator) can be manufactured. In addition, the wet fiber web having three or more layers having different fiber orientations can be formed by appropriately combining a forward flow net, a reverse flow net, a circular net former, a long net, and a short net. For example, a wet fiber having a three-layer structure The web can be formed by combining a circular net, a circular net former and a circular net.

  The wet fiber web can then be dried to produce a wet nonwoven fabric (separator). The drying temperature is preferably a temperature at which the thermoplastic fiber constituting the wet fiber web does not melt. It is preferable to apply pressure to the wet nonwoven fabric produced in this way with a calender or the like to adjust the thickness, reduce the thickness, or make the thickness uniform. However, it is preferable to pressurize at a temperature at which the thermoplastic fiber does not form a film (temperature lower by 20 ° C. or more than the melting point of the thermoplastic fiber).

Since the lithium ion secondary battery of the present invention includes the separator as described above, even when the temperature rises to a high temperature exceeding 180 ° C., the short circuit can be prevented and the reliability is high. The internal resistance is low. The lithium ion secondary battery of the present invention is exactly the same as the conventional lithium ion secondary battery except that it includes the separator as described above. For example, a positive electrode having a lithium-containing metal compound paste supported on a current collector is used, and a negative electrode is a lithium metal or lithium alloy, and a carbon material containing carbon or graphite capable of inserting and extracting lithium (for example, Coke, carbon materials such as natural graphite and artificial graphite), those in which composite tin oxide is supported on a current collector, etc. are used, and LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate as an electrolyte. A water electrolyte or the like can be used.

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

Example 1
Para-aramid pulp fiber (registered trademark: Twaron, manufactured by Teijin, carbonization temperature: 500 ° C. or higher, freeness: 120 ml CSF, average fiber length: 1.4 mm) and polyethylene terephthalate fiber (registered trademark: Tepyrus, manufactured by Teijin, average) Fiber diameter: 3.2 μm, fiber length: 3 mm, melting point: 260 ° C., cross-sectional shape: circular, not fibrillated) were prepared.

  Next, after forming a slurry containing a para-aramid pulp fiber and a polyethylene terephthalate fiber in a mass ratio of 70:30, two wet fiber webs are formed by a circular net method, and the two wet fiber webs are laminated. Then, a two-layer laminated wet fiber web (same fiber orientation) was formed.

Next, this two-layer laminated wet fiber web was dried by a hot air circulation dryer set at a temperature of 120 ° C. and then pressed (linear pressure: 4.7 kN / cm) by a pair of thermal calenders set at a temperature of 170 ° C. A wet nonwoven fabric having a basis weight of 20 g / m ² and a thickness of 26 μm, that is, a separator was produced. The porosity of this wet nonwoven fabric was 45%, and the polyethylene terephthalate fiber was somewhat pressure-bonded but was not formed into a film.

(Comparative Example 1)
Para-aramid pulp fiber (registered trademark: Twaron, manufactured by Teijin, carbonization temperature: 500 ° C. or higher, freeness: 120 ml CSF, average fiber length: 1.4 mm) and polypropylene fiber (average fiber diameter: 2.0 μm, fiber length) : 2 mm, melting point: 170 ° C., cross-sectional shape: circular, not fibrillated).

  Next, after forming a slurry containing a para-aramid pulp fiber and a polypropylene fiber in a mass ratio of 70:30, two wet fiber webs are formed by a circular mesh method, and the two wet fiber webs are laminated. A two-layer laminated wet fiber web (same fiber orientation) was formed.

Next, this two-layer laminated wet fiber web was dried with a hot air circulation dryer set at a temperature of 120 ° C., and then pressed (linear pressure: 4.7 kN / cm) with a pair of thermal calenders set at a temperature of 120 ° C. A wet nonwoven fabric having a basis weight of 20 g / m ² and a thickness of 26 μm, that is, a separator was produced. The porosity of this wet nonwoven fabric was 45%, and the polypropylene fibers were not crimped although they were somewhat pressure-bonded.

(Comparative Example 2)
A two-layer laminated wet fiber web formed in exactly the same manner as in Example 1 except that the basis weight is 12 g / m ² is dried by a hot air circulation dryer set at a temperature of 120 ° C., and then set at a temperature of 60 ° C. A wet nonwoven fabric having a basis weight of 12 g / m ² and a thickness of 27 μm, that is, a separator, was produced by pressing with a pair of heat calenders (linear pressure: 0.8 kN / cm). The wet nonwoven fabric had a porosity of 69%, and the polyethylene terephthalate fiber was somewhat pressure-bonded but was not formed into a film.

(Comparative Example 3)
A two-layer laminated wet fiber web formed in exactly the same manner as in Example 1 except that the basis weight is 16 g / m ² is dried by a hot-air circulating dryer set at a temperature of 120 ° C., and then set at a temperature of 280 ° C. A wet nonwoven fabric having a basis weight of 16 g / m ² and a thickness of 18 μm, that is, a separator was manufactured by pressing with a pair of heat calenders (linear pressure: 4.7 kN / cm). The wet nonwoven fabric had a porosity of 36%, and the polyethylene terephthalate fiber was melted to form a film.

(Production of lithium ion secondary battery)
100 parts by weight of LiCo ₂ O ₄ , 8 parts by weight of acetylene black and 3 parts by weight of polyvinylidene fluoride (PVDF) were mixed with 100 parts by weight of N-methylpyrrolidone (NMP) to obtain a positive electrode mixture slurry. The slurry was applied to an aluminum foil having a thickness of 20 μm serving as a current collector, dried, and then pressed to obtain a positive electrode (30 mm square). Thereafter, a hole was made in the ear portion where the positive electrode mixture was not applied, and a terminal was attached.

  On the other hand, 100 parts by weight of graphitized mesocarbon microbeads, 10 parts by weight of PVDF, and 90 parts by weight of NMP were mixed to obtain a negative electrode mixture slurry. The slurry was applied to a 14 μm-thick copper foil serving as a current collector, dried, and then pressed to obtain a negative electrode (30 mm square). Thereafter, a hole was made in the ear portion where the negative electrode mixture was not applied, and a terminal was attached.

  Moreover, the separator of Example 1 and Comparative Examples 1-3 was cut | judged to 40 square mm, and prepared 3 sheets each, respectively.

  Subsequently, it laminated | stacked in order of the positive electrode, the separator, the negative electrode, and the separator in the center (the distance from each side is all 5 mm) of the separator of Example 1 and Comparative Examples 1-3.

And, while inserting the electrode terminal into the aluminum foil-modified polypropylene laminate film pack bag having a thickness of 0.08 mm so that the terminal of the electrode protrudes from the film pack bag, the electrolyte (ethylene carbonate and diethyl carbonate was 1: 1). Injecting LiPF ₆ in a mixed solution mixed at a weight ratio so as to have a concentration of 1 mol), the protruding edge of the terminal in the film pack bag was heat sealed, and the lithium ion secondary Each battery was manufactured.

(High temperature short circuit prevention)
These lithium ion secondary batteries were charged to 4.2 V with a current of 5 A, and then a heating test up to 180 ° C. was performed according to UL1642. Measure the voltage drop and battery surface temperature when the battery surface temperature reaches 180 ° C. If there is no voltage drop and the battery surface temperature does not rise, a short circuit has not occurred and the result is “pass”. In this case, it was judged as “failed” because a short circuit occurred.

  As a result, since the lithium ion secondary battery using the separator of Comparative Example 1 was short-circuited, the voltage dropped, the battery surface temperature rose, smoked, and was rejected. Lithium ion secondary batteries using other separators were acceptable as neither voltage drop nor battery surface temperature was observed.

(Charge / discharge characteristics)
These lithium ion secondary batteries were charged to 4.2 V with a current of 5 A, and then discharged to 3 V. If the hole diameter is uniform, charging / discharging can be performed without any problem, and if the hole diameter is not uniform, a short circuit occurs and charging / discharging cannot be performed. .

  As a result, the lithium ion secondary battery using the separator of Comparative Example 2 did not reach the specified voltage by charging and was rejected because a micro short circuit occurred. The lithium ion secondary battery using other separators was able to be charged / discharged without any problem, and passed.

(Ion permeability)
The discharge capacity at a discharge rate 0.2C current value of each of the lithium ion secondary battery _{(Cp 0.2),} the discharge capacity (Cp ₃₎ of 3C current value and the measured discharge capacity of 3C current value (Cp ₃ ) To the discharge capacity (Cp _0.2 ) at a current value of 0.2 C (= (Cp ₃ / Cp _0.2 ) × 100). The closer this percentage is to 100%, the lower the resistance and the better the ion permeability.

  As a result, the lithium ion secondary battery using the separator of Example 1 was 88%, the lithium ion secondary battery using the separator of Comparative Example 1 was 80%, and the lithium ion secondary battery using the separator of Comparative Example 2 The lithium ion secondary battery using the separator of Comparative Example 3 was 64%. Therefore, it was found that the separator of the present invention is excellent in ion permeability.

From the above, it was found that the separator of the present invention can prevent a short circuit even when the temperature rises to a high temperature, has a uniform pore diameter, and is excellent in ion permeability.

Claims (8)

It consists of a non-woven fabric containing a heat-resistant pulp fiber having a melting point or carbonization temperature of 300 ° C. or higher and a thermoplastic fiber having a melting point of 200 ° C. or higher. A separator for a lithium ion secondary battery, characterized by not being formed. The separator for a lithium ion secondary battery according to claim 1, wherein the nonwoven fabric has a thickness of 10 to 30 µm. The separator for a lithium ion secondary battery according to claim 1 or 2, wherein the heat-resistant pulp fiber is an aramid pulp fiber. The separator for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the thermoplastic fiber is a polyester fiber. The separator for a lithium ion secondary battery according to any one of claims 1 to 4, wherein an average fiber diameter of the thermoplastic fibers is 4 µm or less. The mass ratio of heat-resistant pulp fiber and thermoplastic fiber is 30: 70-80: 20, The separator for lithium ion secondary batteries in any one of Claims 1-5 characterized by the above-mentioned. The separator for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the separator is a wet laminated nonwoven fabric produced by laminating two or more wet fiber webs. The lithium ion secondary battery provided with the separator for lithium ion secondary batteries in any one of Claims 1-7.