JPH1121371A - Porous film and lithium ion secondary battery using the same - Google Patents

Abstract

(57) [Problem] A porous film made of polypropylene and polyethylene that can be used as a battery separator,
Provided is a highly safe porous film in which the deformation of the film surface pore structure due to an external force is small, thereby suppressing the amount of metallic lithium generated during overcharging, thereby reducing the frequency of thermal runaway of the battery. SOLUTION: When a polyolefin resin is contained as an essential component and a compressive load is applied to at least one surface of the porous film at a surface pressure of 80 kg / cm ² , the closed area of the opening due to film surface deformation is 30%. % Or less, and a lithium ion secondary battery using the same.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a polyolefin porous film useful as a separator for a lithium ion secondary battery, and a lithium ion secondary battery using the same.

[0002]

2. Description of the Related Art In recent years, cordless use has been greatly expanded with the miniaturization and high performance of electronic devices. In order to cope with this, lithium-ion secondary batteries having a high open-circuit voltage and a high energy density are widely used. Advantages of this lithium ion secondary battery include high safety because metal Li is not used, and good cycle characteristics due to a small change in electrode shape during the charging / discharging process.

This lithium ion secondary battery has a negative electrode made of carbon and graphite capable of inserting and extracting lithium, a positive electrode made of an oxide such as cobalt, nickel, manganese, and vanadium, and an electrical insulation between the positive and negative electrodes. A separator that secures a family name and has ion permeability, and LiPF ₆ , LiCF ₃ SO ₃ , LiC in a non-aqueous solvent
It is composed of an electrolytic solution in which 10 ₄ , LiBF _{4 and the} like are dissolved.

A lithium ion secondary battery made of such a material is short-circuited by an external short circuit or a dendritic lithium metal formed in an overcharged state due to a failure of an external charging device, or a Joule heat caused by an increase in internal resistance. The temperature may rise significantly and the equipment containing the battery may be damaged by blowing out and heat of the battery contents.

[0005]

SUMMARY OF THE INVENTION A lithium ion secondary battery is manufactured by laminating a strip-shaped positive electrode, a porous separator and a strip-shaped negative electrode, winding this into a roll, and enclosing it in a metal cylindrical body. Can be. However, the present inventors have found that the pore structure on the surface of the porous separator is deformed due to friction generated when the film comes into contact with the support during winding, and that the positive electrode ion discharge and negative electrode ion occlusion during charging occur. It has been confirmed that as a result of the electrodes swelling and metallic lithium being deposited, the occupied capacity increases, and as a result, the pore structure on the surface of the porous separator is deformed due to an increase in the contact pressure caused by an increase in the battery internal pressure. .

When the pore structure of these surfaces changes, that is, when the apparent aperture ratio is reduced and partial blockage occurs, current concentration in the aperture and an increase in internal resistance during overcharge are caused. This increases the amount of deposited lithium. Here, in the lithium ion secondary battery, the temperature inside the battery rises during overcharge and rapid charge, and the heat causes a chemical reaction between the deposited lithium and the electrolyte solvent. Therefore, when the amount of deposited lithium is large, the calorific value increases, thereby increasing the risk of abnormal temperature rise. Therefore, it is preferable to suppress the amount of precipitated lithium as much as possible.

[0007] It is an object of the present invention to provide a porous film in which the amount of deposited lithium generated during overcharging is reduced because the pore structure hardly changes during battery assembly or overcharging.

[0008]

In order to achieve the above object, a porous film of the present invention contains a polyolefin resin as an essential component, and has a surface pressure of at least 80 kg on at least one surface of the porous film. / Cm ² , when the compressive load is applied, 3
0% or less.

Further, in the porous film of the present invention, the polyolefin-based resin is a low-crystalline polyolefin-based resin.
It is preferably contained in an amount of from 01 to 40% by weight.

The porous film of the present invention is a composition containing polyethylene and polypropylene as essential components, has a layer structure of three or more layers, has a surface layer of polypropylene, and has at least one of an intermediate layer. The layer is preferably a layer having an alloy structure of polyethylene and polypropylene.

Further, a lithium ion secondary battery of the present invention is characterized in that the above-mentioned porous film is used as a separator.

Further, the lithium ion secondary battery of the present invention has a current value capable of charging / discharging the battery capacity in 30 minutes. Integrated value QL of heat generation in the range of -180 ° C and integrated value Q of heat generation in the range of 200-280 ° C
The heat generation ratio QL / QH of H is 0.4 or less.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The porous film of the present invention has a surface pressure of 80 kg against at least one surface of the porous film.
When a compressive load is applied at a pressure of / cm ² , the area closed by the opening due to film surface deformation is 30% or less. Opening closure area is 3
If it exceeds 0%, the probability of the above-mentioned heat generation ratio QL / QH becoming 0.4 or more increases due to an increase in the amount of precipitated lithium, thereby increasing the frequency of abnormal temperature rise in the overcharged state.

The porous film of the present invention contains a polyolefin resin as an essential component, and is particularly preferably a polyolefin resin containing 0.01 to 40% by weight of a low crystalline polyolefin resin. When the compounding ratio of the low-crystalline polyolefin-based resin is within this range, it is easy to obtain the above porous film without deteriorating the physical properties of the film. If less than this content range, it is difficult to obtain the above porous film,
Conversely, if the content is higher than this, the elastic modulus and the breaking strength of the porous film decrease, and it becomes difficult to use the porous film as a separator.

In the present invention, the low-crystalline polyolefin resin refers to a resin having a lower degree of crystallinity than polyethylene and polypropylene, which are the main components. Here, the crystallinity means the degree of crystallinity measured by differential scanning calorimetry. Preferred low-crystalline polyolefin resins include, for example, atactic polypropylene (polypropylene having a structure in which the stereoregularity of a methyl group, which is a side chain of polypropylene, is disordered), polypropylene-
Polyethylene block copolymer (copolymer consisting of molecules formed by bonding polypropylene and polyethylene at the ends), polypropylene-polyethylene random copolymer (polypropylene-polyethylene unit of the monomer constituting the copolymer) Are randomly arranged), ethylene-propylene rubber (a rubber-like elastic body polymerized as a main component of ethylene and propylene), and the like. Particularly preferred is atactic polypropylene.

The high-crystalline polyolefin resin, which is another component of the polyolefin resin in the present invention, refers to a resin having a higher degree of crystallinity than the low-crystalline polyolefin resin.

Further, the porous film of the present invention includes:
A composition containing polypropylene and polyethylene as essential components, having a layer structure of three or more layers, a surface layer having polypropylene, and at least one intermediate layer having a polypropylene and polyethylene alloy structure. Is preferred. The ratio of the polypropylene in the intermediate layer is 10 to 90.
% By weight and the proportion of polyethylene is preferably 90 to 10% by weight. In the case of three or more layers,
The effect is exhibited if the low-crystalline polyolefin resin is contained at least in the surface layer. Therefore,
In this case, 0.01 to 4
It is preferable to contain 0% by weight of a low-crystalline polyolefin resin.

Further, when a lithium ion secondary battery was manufactured using the porous film of the present invention as a constituent material, the battery was charged to 200% of the battery capacity at a current value capable of charging and discharging the battery in 30 minutes. Integrated value QL of heat generation in the range of 100 to 180 ° C and integrated value QH of heat generation in the range of 200 to 280 ° C for the negative electrode including the precipitate on the negative electrode of the battery and the separator.
Has a heat generation ratio QL / QH of 0.4 or less.

The porous film of the present invention can be formed by adding a low-crystalline polyolefin resin to polypropylene as a main component to form a portion having a non-uniform crystal grain size of polypropylene at the time of forming a crystal nucleus, and to form an amorphous chain. Structural changes in site increments occur. As a result, when the film is stretched, the cleavage of the holes is selectively generated from such an uneven portion, so that the formed hole structure has an increased thickness of the resin portion in the stretching direction,
In addition, the diameter of the formed hole increases. As a result, when an external pressure is applied due to an increase in surface pressure due to an increase in the internal pressure of the battery due to friction during film formation or an expansion of the electrode during charging, and an increase in surface pressure due to volume expansion due to deposition of metallic lithium during overcharge,
Since the resin portion is thick, it is difficult to buckle and the hole diameter is large, so that even if the resin portion is deformed, the hole is hardly blocked. As described above, since the change in the surface structure is small, current concentration in the opening and increase in internal resistance are suppressed during overcharge, so that the amount of deposited lithium can be reduced.

In a cell incorporating the porous film of the present invention, a current value at which the capacity of the battery can be charged and discharged in 30 minutes,
The amount of metallic lithium deposited on the negative electrode including the precipitate on the negative electrode and the separator of the battery charged to 200% of the battery capacity can be determined by differential scanning calorimetry. Specifically, an arbitrary part of the negative electrode including the above-described precipitate on the negative electrode and the separator is collected, and an electrolytic solution is added thereto to perform differential scanning calorimetry.

When the negative electrode and the separator on which metallic lithium is deposited by rapid charging or overcharging are subjected to differential scanning calorimetry analysis together with an electrolytic solution, the negative electrode usually has a temperature in the range of 100 to 180 ° C. and 200 to 2 ° C.
An exothermic peak is observed in the range of 80 ° C. The former is due to the reaction between the deposited metallic lithium and the electrolyte,
The latter is due to the reaction between the lithium ions occluded in the negative electrode and the electrolyte. Therefore, the negative electrode and the separator after the test are subjected to differential scanning calorimetry together with the electrolytic solution, and by measuring the calorific value, the amount of deposited metallic lithium and the amount of lithium ions occluded in the negative electrode can be measured. .

The reaction rate between the deposited metallic lithium and the lithium ions occluded in the negative electrode is different from that of the electrolytic solution as described above, and the deposited metallic lithium is much less safe than the occluded lithium ions. Therefore, in the differential scanning calorimetry of the negative electrode and the separator after the test, the integrated value of the calorific value on the low temperature side (QL) and the integrated value of the calorific value on the high temperature side (QH)
It is safer that the ratio QL / QH of the calorific values of is as small as possible.

If the heat generation ratio is 0.4 or less, it is not more than the amount of heat that causes a transition to a higher temperature exothermic reaction.
Further thermal runaway can be controlled. If the amount of deformation with respect to the above-mentioned pressure is small, the amount of deposited metallic lithium decreases, and if the amount of deformation is 30% or less, the heat generation ratio is 0.4 or less, which is safe.

In the method for producing a porous film according to the present invention, a thermoplastic resin as a constituent member can be formed into a film by melt extrusion and then stretched uniaxially to make it porous.

By subjecting the extruded film to a heat treatment, the orientational crystallization of the amorphous chains and the rearrangement of the crystals at the other position occur, and the orientation of the film rises. It is known to be promoted. As the heat treatment method, a method in which a film-like material is brought into contact with a heating roll or a metal plate, a tenter method, or a method in which a roll-like material is heated in a gaseous atmosphere can be used. Usually, the temperature is set to 120-160 ° C., and the processing time is 1 second to 50 hours.

The stretching method for making the film-like material porous is not particularly limited. However, in order to obtain a porous film having good ion permeability by increasing air permeability and porosity, after stretching at a low temperature, the film is further stretched. It is preferable to perform a multi-stage stretching method in which stretching is performed at a high temperature. This stretching is usually performed at a low temperature of 0 to 60 ° C. and the stretching ratio is 20 to 200% of the initial size. The stretching at a high temperature is performed at a processing temperature of 90 to 130 ° C, and the stretching ratio is 10 to 500% of the initial size.

It has been confirmed that the porous film obtained by such a stretching treatment has a residual stress and is deformed by shrinkage in the stretching method. Therefore, it is effective to suppress shrinkage deformation by performing heat shrinkage at a temperature equal to or higher than the stretching temperature to improve dimensional stability due to shrinkage. Dimensional change due to heat shrinkage is achieved in a range where the film length after stretching is reduced by 10 to 30%, whereby good dimensional stability is exhibited.

The same effect as the above heat shrinkage treatment can be expected by heat setting in which the dimensions of the stretched porous film in the stretching direction are fixed and this is heated at a higher temperature. A similar effect can be obtained by using the heat shrinking treatment and the heat setting treatment together.

The porous film thus obtained has a network structure in which the lamellar laminate is connected by fibrous bodies called microfibrils, and the lamellar-fibrils form micropores. The feature is that there is.

Further, the porous film of the present invention is subjected to a hydrophilic treatment such as a corona treatment, a surfactant impregnation, a drying treatment, and a graft polymerization treatment of a hydrophilic monomer in order to improve the wettability with respect to an electrolytic solution. It may be incorporated in

[0031]

The present invention will be described below with reference to examples.
The present invention is not limited to this embodiment.

Example 1 A melt index (hereinafter referred to as MI) 0.5 containing 3% by weight of a low-crystalline polyolefin resin (atactic polypropylene, density 0.82 g / cc), a density 0.90 g / cc, Isotactic polypropylene with 70% crystallinity, isotactic polypropylene 1
0 parts by weight and a mixture of 90 parts by weight of a high-density polyethylene having an MI of 0.3, a density of 0.964 g / cc, and a crystallinity of 72% were melted at a resin melting temperature of 250 ° C., respectively.
By using a T-die extruder and extruding three layers simultaneously,
12 μm thick polypropylene layer on the outer layer, 8 mm thick on the inner layer
A laminated film composed of a polypropylene / polyethylene mixed layer having a thickness of μm is extruded and cured and oriented by a cooling roll controlled at 80 ° C. to obtain a film-shaped molded product having a total thickness of 32 μm.

This film is heated in a rolled state at a temperature of 130 ° C. for 48 hours, then cooled to 125 ° C. in 10 minutes, and then heated at 125 ° C. for 12 hours. This is subjected to low-temperature stretching at a temperature of 25 ° C. in the machine direction at a stretching ratio of 40%, and then to high-temperature stretching at a temperature of 125 ° C. at a stretching ratio of 110% in the same direction. Then, it is heated at 110 ° C. for 1 minute to shrink the size in the stretching direction by 20%. The film thickness after stretching was 27 μm, and the porosity was 41%.

The surface roughness and heat generation ratio of this porous film were tested by the following measuring methods, and the results obtained are shown in Table 1.

(A) Surface rubbing property A stainless steel needle having a diameter of 1 mm and a hemispherical tip is contact-pressed from the film surface at a surface pressure of 80 kg / cm ² ,
After fixing for 2 seconds, the stylus was removed, and the morphological change of the film surface generated at the contact site was observed by FE-SEM. At the time of observation, the object is dyed with ruthenic acid for 2 hours in order to reduce the thermal damage caused by the electron beam and to improve the resolution by efficiently emitting secondary electrons from the object. Was. The image thus obtained was subjected to image processing such as shadowing, sharpening, binarization, and the like, and the area of the hole portion was calculated to quantify the closed area of the hole opening.

(B) Amount of deposited lithium The amount of deposited lithium was measured by a differential scanning calorimeter (DSC). Using lithium cobaltate for the positive electrode and graphite for the negative electrode, using the obtained porous film as a separator, and further adding lithium hexafluorophosphate to a solvent obtained by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1. Was dissolved so as to have a concentration of 1 mol / liter to prepare a rocking chair type lithium ion secondary battery using as an electrolytic solution. This battery was charged at a current density of 5.2 mA / cm ² for 60 minutes, and
After the battery was charged to a state of 00%, the negative electrode including the precipitate on the negative electrode and the separator were sampled at a size of 3.5 mmφ, and 20 μl of the same solution as the above-mentioned electrolyticity was added and sealed in a SUS pressure-resistant container. When the temperature is increased at a rate of 5 ° C./min by DSC7 manufactured by Seiko Electronic Industry Co., Ltd., the integrated amount of the exothermic peak appearing in the range of 100 to 180 ° C. is defined as QL. The integrated amount of heat generation was QH, and QL / QH was the heat generation ratio.

(C) Evaluation of actual battery The electrodes used were the same active materials as those used in the evaluation of (b). Using lithium cobalt oxide as a positive electrode active material, both sides of this were coated on an aluminum current collector, dried and compression molded, and coated on both sides of a copper current collector using graphite as a negative electrode,
After drying, compression molded was used. A negative electrode, a positive electrode strip and a porous film are sequentially laminated to form a wound body,
A spiral electrode body was manufactured. And this electrode body is SUS
The battery was housed in a 18650 battery can, and insulators were inserted on both upper and lower surfaces. Leads for current collection were led out from various electrodes and welded to the battery can. An electrolytic solution was poured into a battery obtained by dissolving lithium hexafluorophosphate at a concentration of 1 mol / liter in a solvent obtained by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 in the above battery can. did. Then, the battery was caulked to the battery can via a gasket, and the battery lid was fixed to produce a battery. After charging the battery thus manufactured at a charging current of 1.2 A and a voltage of 4.1 V throughout, overcharging was further performed at a charging voltage of 2.4 A to 120% of the battery capacity, and the outer wall temperature at that time was measured. did.

Comparative Example 1 Using isotactic polypropylene having an MI of 2, density of 0.92 g / cc and high density polyethylene of 1.2, MI having a density of 0.96 g / cc, 60 parts by weight of polypropylene and 40 parts by weight of polyethylene The mixture having the compounding ratio of the parts was extruded simultaneously with a single layer using a T-die to obtain a film having a thickness of 32 μm. This film was subjected to the same heat treatment and stretching as in Example 1 to obtain a porous film having a thickness of 27 μm and a porosity of 45%. This porous film was evaluated in the same manner as in Example 1, and as a result, various characteristics were as shown in Table 1.

Comparative Example 2 An isotactic polypropylene having an MI of 0.4 and a density of 0.9 was used without using a low-crystalline polyolefin resin.
1 g / cc of isotactic polypropylene was used, MI was 1.2 as high-density polyethylene, and density was 0.1.
Except that 96 g / cc of high-density polyethylene was used, three layers were extruded at the same compounding ratio as in Example 1 to a thickness of 32 μm.
Was obtained. This film was processed in the same manner as in Example 1 to obtain a porous film having a thickness of 27 μm and a porosity of 47%. This porous film was evaluated in the same manner as in Example 1, and as a result, various characteristics were as shown in Table 1.

[0040]

[Table 1]

As shown in Table 1, the porous film of the present invention had a lower fogging property and a lower heat generation ratio than the comparative porous film. Comparative Examples 1 and 2
The actual battery manufactured in Example 1 caused abnormal heat generation in the overcharge test, but the actual battery manufactured in Example 1 did not. Here, the abnormal heat generation refers to (c) a case where the outer wall temperature rises to 140 ° C. or more in the actual battery evaluation. It is considered that generation of precipitated lithium was suppressed due to good anti-fog resistance.

[0042]

As described above, since the porous film of the present invention has a small deformation of the surface pore structure, when used as a battery separator, the internal resistance increases due to the deformation and the extreme current concentration due to the deformation. , The amount of precipitated lithium generated by the heat generation can be suppressed, and thereby the occurrence of abnormal heat generation can be suppressed.

──────────────────────────────────────────────────続 き Continued on front page (51) Int.Cl. ⁶ Identification symbol FI H01M 10/40 H01M 10/40 Z (72) Inventor Souji Nishiyama 1-2-1, Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation In-company (72) Inventor Kiichiro Matsushita 1-2-1, Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation

Claims (5)

[Claims] Claims: 1. A polyolefin resin is contained as an essential component, and when a compressive load is applied to at least one surface of a porous film at a surface pressure of 80 kg / cm ² , the closed area of an opening due to film surface deformation is reduced. A porous film having a content of 30% or less. 2. The porous film according to claim 1, wherein the polyolefin-based resin contains 0.01 to 40% by weight of the low-crystalline polyolefin-based resin. 3. A composition comprising polyethylene and polypropylene as essential components, having a layer structure of three or more layers,
And the surface layer is polypropylene and at least one of the intermediate layers
3. The porous film according to claim 1, wherein the layer is a layer having an alloy structure of polyethylene and polypropylene. 4. A lithium ion secondary battery using the porous film according to claim 1 as a separator. 5. An integrated value of heat generation in a range of 100 to 180 ° C. of a negative electrode including a deposit on a negative electrode and a separator of a battery charged to 200% of the battery capacity at a current value capable of charging and discharging the battery capacity in 30 minutes. The lithium ion secondary battery according to claim 4, wherein a heat generation ratio QL / QH of QL and an integrated value QH of heat generation in the range of 200 to 280 ° C is 0.4 or less.