JPH04126352A - Separator for battery, manufacture thereof and battery - Google Patents

Abstract

PURPOSE:To obtain excellent resistance against mechanical stress while circuit breaking ability at a low temperature is maintained by using a micro-porous film comprising mixture for which PP is also added to two kinds PEs of specific molecular weight. CONSTITUTION:A micro porous film comprising polyethylene of viscosity average molecular weight of no more than 300, 000,. polyethylene of viscosity average molecular weight of no less than 1,000,000, and mixture including polypropylene, is used. Regarding the mixture ratio of low molecular weight polyethylene PE, ultra-high molecular weight polyethylene UHPE, and of polypropylene PP, the ratio of PE included in the mixture, i.e., in the total amount of three species is 5-50% weight, or preferably 10-40% weight, while the ratio of UHPE is 10-90% weight or preferably 35-80% weight, and the ratio of PP is 5-40 or preferably 10-25% weight. Circuit breaking ability is seen at a relatively low temperature, and excellent resistance against mechanical stress can thus be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a battery separator, a method for manufacturing the separator, and a battery incorporating the separator.

(Prior art) As a battery separator, it has the original function of separating positive and negative electrodes, has sufficient resistance to electrolyte, and has good ionic conductivity that does not increase the internal resistance of the battery. It is also necessary to prepare.

Although there are many types of batteries, non-aqueous electrolyte batteries using lithium or the like as a negative electrode have high energy density and little self-discharge, and have recently attracted attention as batteries for large currents.

This nonaqueous electrolyte battery uses dimethylformamide, propylene carbonate, acetonitrile, butyrolactone,
Li P F @, LiC1a, L in an aprotic solvent such as dimethylformamide and a liquid with a high dielectric constant.
An electrolytic solution containing tCloa, LiBFs, etc. dissolved therein is used as an electrolytic solution.

This nonaqueous electrolyte has low electrical conductivity, and therefore, in order to extract a large current, it is necessary to make the areas of the positive and negative electrodes as large as possible.

Therefore, a non-aqueous electrolyte battery has a structure in which the materials constituting the positive and negative electrodes are overlapped with each other via a separator and wound into a spiral shape, and the spiral body is housed in a metal case.

By the way, in large current batteries, an abnormal current flows due to an external short circuit caused by misuse such as forced discharge, resulting in a significant rise in internal temperature, which can eventually lead to a serious accident such as fire or explosion. There is sex. In order to avoid this, various measures have been taken, such as making the battery structure itself explosion-proof, but these have not provided any essential solutions from the perspective of preventing abnormal current and the heat generated by it.

On the other hand, as a separator for batteries,
It is known to use a microporous film made of polypropylene (hereinafter referred to as PP) as described in Japanese Patent No. 9.

This microporous PP film exhibits good electrical conductivity near room temperature, but has the property of increasing electrical resistance at high temperatures. Therefore, if an abnormal current flows in a battery using this as a separator, the internal electrical resistance of the battery will increase as the temperature rises due to the heat generated by the current, and as a result, a current cutoff function will be activated and danger can be avoided. I can also hope for that. However, it has been found that the electrical resistance of the PP microporous film does not increase sufficiently to interrupt abnormal currents unless the temperature is considerably high, for example, 180° C. or higher.

The present inventor conducted various studies to ensure safety by interrupting the current at a lower temperature when abnormal current flows inside the battery, and found that polyethylene (hereinafter referred to as PE) with a viscosity average molecular weight of 300,000 or less , PE with a viscosity average molecular weight of 1 million or more
We previously proposed the use of a microporous film consisting of a mixture of the
Publication No.). This battery separator has a lower temperature (for example, about 130°C) than conventional products made of microporous PP film.
), it exhibits a current cutoff function, greatly improving safety.

(Problem to be Solved by the Invention) By the way, when manufacturing a type of battery, such as a non-aqueous electrolyte battery, in which both electrodes are spirally wound through a separator and then housed in a case made of metal, etc., it is necessary to From the point of view of production efficiency (yield), it is desirable that the material be resistant to the mechanical stress exerted during winding work and storage work in a case and not be damaged.

However, the above-mentioned separator made of a mixture of PEs with different molecular weights may not have sufficient resistance to mechanical stress. Sometimes it gets torn or torn when stored.

Therefore, an object of the present invention is to provide a separator that has excellent resistance to mechanical stress while maintaining its current interrupting function at low temperatures.

(Means for Solving the Problems) In order to solve the above-mentioned problems of the prior art, the present inventor has made a microporous film made of a mixture of two types of PE with specific molecular weights and PP. has a current blocking function at low temperatures and exhibits excellent resistance to mechanical stress; this microporous film can be easily produced by a specific method using good and poor solvents for PE and PP; It was discovered that batteries using the microporous film as a separator have excellent safety, and the present invention was completed.

That is, the first aspect of the present invention is PE having a viscosity average molecular weight of 300,000 or less.
This invention relates to a battery separator made of a microporous film made of a mixture of PE having a viscosity average molecular weight of 1 million or more and PP as essential components.

The second aspect of the present invention is the following four steps: (a) a step of dissolving PE with a viscosity average molecular weight of 300,000 or less, PE with a viscosity average molecular weight of 1,000,000 or more, and PP in a good solvent; (b) the above step; (c) immersing the film-like material obtained in the above step in a poor solvent; (d) stretching the film-like material after the immersion treatment. This relates to a method of manufacturing a separator for use in a commercial vehicle.

Furthermore, the third aspect of the present invention provides a positive electrode, a negative electrode, and a separator interposed between these two electrodes, and the separator is made of PE having a viscosity average molecular weight of 300,000 or less, and a PE having a viscosity average molecular weight of 100,000 or less.
It is characterized by being made of a microporous film made of a mixture containing PE with a strength of 0,000 or more and PP as essential components.

One of the PEs used in the present invention has a viscosity average molecular weight (molecular weight measured by a viscosity method) of 300,000 or less, preferably about 1,000 to 250,000. This low molecular weight PE contributes to the current interrupting function when the temperature rises. In addition, the density of this PE is 0.90 to 0.9
7. Melt index is 0.1-100°g/
It is preferable to use min.

In addition, the other PE has a viscosity average molecular weight of 1 million or more, preferably about 1.2 million to 5 million, and is generally called ultra-high molecular weight polyethylene (hereinafter, PE with a viscosity average molecular weight of 1 million or more is referred to as (referred to as UHPE). Commercial products of this UHPE include Hyzex Million, manufactured by Mitsui Petrochemical Industries, Ltd., and Hostalen GUR, manufactured by Hoechst. If this UHPE is not used, it will be difficult to obtain a strong microporous ilm that has the function of isolating positive and negative electrodes, has resistance to electrolyte, and exhibits good ionic conductivity.

In the present invention, PE and UHP having different molecular weights are used.
In addition to E, PP is also used. By using PP, it is possible to obtain a separator that is not easily damaged during the operation of placing the separator between the positive and negative poles and winding it in a spiral shape, or during the operation of storing the spiral body in a case. As this PP, one having a viscosity average molecular weight of 1 million or less, preferably 200,000 to 600,000 is used.

In the present invention, the mixing ratio of PE, UHPE, P, and P is such that PE accounts for 5 to 50% by weight, preferably 10 to 40% by weight,
The proportion of PE is 10 to 90% by weight, preferably 35 to 80% by weight, and the proportion of PP is 5 to 40% by weight, preferably 10 to 2.
It is preferable to adjust the amount to 5% by weight. If the proportion of PE is too small, the current interrupting function is likely to become uncertain when the temperature rises due to abnormal current, and conversely, if the proportion of PE is too small, the tensile strength of the separator will decrease.In addition, if the proportion of PP is too small, A separator with excellent resistance to mechanical stress cannot be obtained, and on the other hand, if the amount is too large, the tensile strength of the separator will decrease.

A battery separator made of a microporous film made of PE, UHPE, and PP can be manufactured by various methods known as microporous film manufacturing methods, but the present invention including the four steps described above can be used. By adopting this method, it can be obtained easily and reliably.

The good solvent used in step (a) of the method according to the present invention is one that can dissolve or swell PE, UHPE, and PP (hereinafter, in the present invention, "dissolution" is used as a general term for "dissolution or swelling"). For example, one or more of xylene, decalin, 0-dichlorobenzene, trichlorobenzene, etc. may be used.

A heating dissolution method can be adopted as a method for dissolving the three components in a good solvent. The final temperature at this time is usually about 10
0 to 180°C, and the total time required for dissolution is approximately 10
minutes to 10 hours. When melting by heating, it is preferable to first dissolve PE and PP with a viscosity average molecular weight of 300,000 or less, and then dissolve UHPE, since this reduces the working time and makes the mixed state of the dissolved components more uniform. In addition, in this step, if the total concentration of PE, UHPE and PP contained in the solution obtained is 1 to 50% by weight, a separator with particularly excellent resistance to mechanical stress can be obtained. It is clear that

The solution obtained by such step (a) is then subjected to (b)
) process. The step (b) is a film forming step, and various film forming techniques such as the T-die method and the inflation method can be arbitrarily employed.

The film-like material obtained in the above step (b) contains a good solvent. Therefore, in the present invention, in the step (c), the film-like material is immersed in a poor solvent to remove the good solvent. The immersion time of the film-like material in the poor solvent is set depending on various conditions, and is usually 5 seconds to 30 minutes. The poor solvents used in this step are PE and UHP.
Any solvent may be used as long as it does not dissolve E and PP and is compatible with the poor solvent, such as water or various organic solvents, but alcohols, particularly methanol, are preferred.

When the good solvent is removed by immersing the film-like material in a poor solvent, the areas where the good solvent exists in the film-like material become fine voids. Then, these voids are made to grow into micropores by stretching in step (d), which is carried out later. Therefore, step (c) can be said to be a step for forming the basic structure of a microporous film.

Note that when the film-like material containing a good solvent obtained in the step (b) is subjected to the step (c), if the film-like material is at a high temperature, it can be cooled before being subjected to the step (c). The cooling may be carried out naturally or by immersion in a poor solvent such as that used in step (c). When cooling is performed by the latter method, cooling and removal of the good solvent can be performed as a continuous process.

In the step (d), a film-like material having countless fine voids is stretched (stretching may be either uniaxial stretching or multiaxial stretching). As a result of this stretching, the fine voids grow and expand to become fine pores. Of course, new micropores may be formed. The stretching temperature is below the melting point of the film-like material, usually about 10-130°C. Further, the stretching ratio is usually 1.
2 times or more, preferably 1. It is 3 to 6 times.

The battery separator thus obtained is a microporous film, and although its physical properties may vary depending on the manufacturing conditions, it usually has a thickness of about 5 to 300 μm and a micropore diameter of about 0.5 μm.
1-20 μm 1 porosity is about 20-90%.

In addition, in this battery separator, at least one of PE, UHPE, or PP may be crosslinked.

Furthermore, in order to improve fishing characteristics, the composition may contain arbitrary additives such as antioxidants, flame retardants, and fillers.

Furthermore, this battery separator is made of PE, UHPE and P
It may be a laminate of two or more microporous films containing P as an essential component. The laminated type separator is
For example, by superimposing two or more film-like materials that have been subjected to the poor solvent immersion treatment in step (c),
θ ~ 40℃ lower temperature, linear pressure approximately o, oi ~ 10kg/an
It can be obtained by a method of making the film-like material microporous and integrating the film-like products by carrying out pressure stretching under the following conditions. However, the laminated strength of the laminated type separator produced by this method is not so great, and care should be taken in handling after manufacture to prevent peeling.

The separator of the present invention can be interposed between a positive electrode and a negative electrode to assemble a battery in the same manner as conventional separators. At this time, the materials of the positive electrode, negative electrode, battery case, electrolyte, etc., and the arrangement structure of these components are not required to be anything special and may be the same as conventional ones.

(Effects of the Invention) The present invention is constructed as described above, and is made of a microporous film made of PE, UHPE, and PP, so it exhibits a current interrupting function at a relatively low temperature and has resistance to mechanical stress. It has excellent durability and is difficult to cause damage such as tearing or cracking due to the stress that is applied when it is placed between the positive and negative poles and wound into a spiral shape or when the spiral body is stored in a case, increasing the production efficiency of the battery. can be improved. Furthermore, according to the method of the present invention, battery separators can be produced easily and reliably. Furthermore, a battery incorporating this separator has excellent safety.

(Example) Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 1000 parts by weight of xylene and 100 parts by weight of decalin in a glass container
0 parts by weight, 20 parts by weight of PE powder with a viscosity average molecular weight of 200,000, density 0.95, and melt index of 20 g/min and 50 parts by weight of PP powder with a viscosity average molecular weight of 300,000, and stirred with a stirrer. Increase the liquid temperature from 25°C to 1
The temperature is raised to 40°C and maintained at the same temperature for 1 hour to dissolve PE and PP.

Next, 150 parts by weight of UHPE powder having a viscosity average molecular weight of 3 million is added to this solution (liquid temperature maintained at 140° C.) and stirring is continued for 1 hour to dissolve the UHPE.

This solution is extruded into a film having a thickness of 75 μm using a T-die extruder, immersed in methanol for 1 minute, cooled, and wound onto a roll-shaped core. Note that the extrusion temperature was 150°C, and the discharge rate was 100 g/min.

Next, this film-like material is immersed in methanol for 10 minutes to extract and remove xylene and decalin contained in the film-like material, pulled up, and air-dried.

Thereafter, by uniaxially stretching at a temperature of 120°C and a speed of 0.8 m/min at a stretching ratio of 2 times, a thickness of 25 μm was obtained.
A microporous battery separator (sample 1) with a porosity of 64% was obtained.

In addition, the porosity is the size of the film-like material, both vertically and horizontally 35m.
m, and its thickness (D) and weight (E
), and calculate the apparent density (G) by formula (I),
Furthermore, it was calculated by formula (I[) using the true specific gravity (F) of UHPE.

Porosity (%J=XIUU"・(U)
The units of (D) and (E) in formula (1) are "μm
” and “g”.

Example 2 PE, UHPE and PP were prepared in the same manner as in Example I except that the extrusion thickness by the T-die extruder was 50 μm.
Dissolve in a solvent, form a film, extract and remove the solvent, and air dry.

Next, two film-like materials were superimposed at a temperature of 120°C, a speed of 0.8 m/min, and a linear pressure of 1. By uniaxially stretching at a stretching ratio of 2 times under the conditions of 3 kg/an, a thickness of 25 μm was obtained.
A microporous and laminated type battery separator (sample 2) with a porosity of 60% was obtained.

Example 3 The blending ratio (parts by weight) of PE, UHPE and PP was
Three types of microporous battery separators (Samples 3 to 5) were obtained in the same manner as in Example 1 except as shown in the table. Note that the thickness of these separators is 25 μm.
The porosity was 62% for sample 3, 66% for sample 4, and 65% for sample 5.

Comparative Example I in Table 1 A microporous battery separator (sample 6) having a thickness of 25 μm and a porosity of 60% was obtained in the same manner as in Example 1 except that PP was not used.

Comparative Example 2 A microporous PP film with a thickness of 25 μm and a porosity of 49% was used as a battery separator (sample 7).

In order to understand the performance of the separators obtained in these Examples and Comparative Examples, we used lithium as the negative electrode, an active material mainly composed of manganese dioxide as the positive electrode, and an electrolyte containing a mixture of equal weights of propylene carbonate and dimethoxyethane (this I CI Oa as an electrolyte at a concentration of 1 mol/
A non-aqueous electrolyte battery was prepared by using a spiral electrode and a spiral electrode, and was tested in the following manner. The results obtained are shown in Table 2.

[Can wall temperature]

A thermocouple was attached to the wall of the battery can to record the maximum temperature.

[Battery capacity]

The battery capacity was determined by discharging at a temperature of 25° C. and a resistance of 25Ω. In this case, the end point was when the voltage reached 2°5V.

[Involved defect rate]

A voltage of 250V is applied between both electrodes of the battery, and a resistance of 10V is applied between both electrodes of the battery.
Those below MΩ were considered defective.

Table 2

Claims (3)

[Claims] (1) A microporous battery separator made of a mixture containing polyethylene with a viscosity average molecular weight of 300,000 or less, polyethylene with a viscosity average molecular weight of 1 million or more, and polypropylene. (2) The following four steps: (a) Dissolving polyethylene with a viscosity average molecular weight of 300,000 or less, polyethylene with a viscosity average molecular weight of 1 million or more, and polypropylene in a good solvent; (b) Dissolving the solution obtained in the above step. (c) immersing the film-like material obtained in the above step in a poor solvent; (d) stretching the film-like material after the above-mentioned immersion treatment. manufacturing method. (3) Micropores having a positive electrode, a negative electrode, and a separator interposed between these two electrodes, the separator being made of a mixture containing polyethylene with a viscosity average molecular weight of 300,000 or less, polyethylene with a viscosity average molecular weight of 1 million or more, and polypropylene. 1. A battery characterized by being a transparent film.