JPH04206257A - Separator for battery, its manufacture and battery - Google Patents

Abstract

PURPOSE:To satisfy practicability both in SD start temperature and resisting temperature by using in a battery separator a porous film made of polymer alloy wherein polyethylene (PE) and polypropylene (PP) are mixed at a specific ratio. CONSTITUTION:A separator is made of a porous film being mixture of PE and PP, which is formed of 10 to 90wt.% of PE and 90 to 10wt.% of PP. Thus if a temperature rises to a melting point of PE, only PE constituting the porous film is melted to have pores closed, and as a result an increase in a resistance value shuts current and prevents excessive rise in the temperature. Until a melting point of PP is reached, a film shape is maintained by PP being one of separator constituents and sufficient heat resisting temperature is exhibited. Thus shutdown (SD) start temperature is approximately 115 to 130 deg.C and the heat resisting temperature is approximately 150 deg.C or higher, so that practicability can be satisfied.

Description

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

[Conventional technology]

Various types of batteries are in practical use, and in these batteries, a separator is interposed between the positive and negative electrodes to prevent short circuits between the two electrodes.

BACKGROUND ART Recently, lithium batteries have been attracting attention as batteries for use in cordless electronic devices due to their high energy density, high electromotive force, and low self-discharge.

For lithium batteries, for example, metal lithium is used as the negative electrode,
Examples include alloys of lithium and metals such as aluminum, organic materials such as carbon and graphite that have the ability to adsorb lithium ions or the ability to occlude them with intercalated silane, and materials formed from conductive polymers doped with lithium ions. It is being

In this lithium battery, lithium as an electrode constituent material has strong reactivity, and LiPFi, LiPFi, LiCF35O
Since the electrolyte is a solution containing 1, Licl4, LiBFn, etc. as an electrolyte, if an abnormal current flows due to an external short circuit or incorrect connection, the internal temperature will rise significantly and eventually cause a fire or explosion. There is a risk of causing a serious accident.

To avoid such dangers, polyethylene (hereinafter referred to as
porous film (referred to as PE) or polypropylene (referred to as
It has been proposed to use a porous film (hereinafter referred to as PP) as a separator (Japanese Patent Laid-Open No. 60-23954
(Japanese Patent Application Laid-open No. 2-75151, etc.).

The purpose of using this porous separator is to prevent a short circuit between the positive and negative electrodes by being located between the positive and negative electrodes during normal energization, and also to reduce the electrical resistance between the two electrodes due to its porous structure, contributing to the energization efficiency. On the other hand, if the internal temperature of the battery rises due to abnormal current, it melts at a predetermined temperature and transforms from a porous structure to a non-porous structure, increasing its electrical resistance and interrupting the current. The goal is to prevent temperature rises and ensure safety.

When there is an abnormal rise in temperature in this way, the iiE current is cut off due to an increase in electrical resistance, and the function that ensures the safety of the battery by avoiding fire and explosion is generally activated by the shut-down characteristic (hereinafter referred to as "shut-down"). This is called the SD characteristic) and is an essential characteristic for lithium battery separators and the like.

It is important for a separator for lithium batteries to have this SD characteristic, but it is also important that the increase in electrical resistance starts at an appropriate temperature (hereinafter, the temperature at which the increase in electrical resistance starts is referred to as the SD start temperature) and It is also required that the increased electrical resistance be maintained up to a suitable temperature.

If the SD starting temperature is too low, an increase in electrical resistance will start with a slight temperature rise, which is impractical; if it is too high, safety will not be ensured sufficiently. at present,
It has been recognized that this SD initiation temperature is preferably about 115-130''C.

Additionally, it is recognized that fires and explosions can be almost prevented if the increased electrical resistance can be maintained up to around 140"C (of course, it would be better if it could be maintained at a higher temperature).The upper limit at which the increased electrical resistance can be maintained. In the present invention, the temperature will hereinafter be referred to as "heat-resistant temperature". The heat resistance temperature can be seen as the function of maintaining the film shape of the separator; if the separator melts due to an excessive rise in temperature and cannot maintain its film shape, the electrical resistance decreases and SD
Characteristics are lost. And when the SD characteristic is lost,
In a lithium battery, the positive and negative electrodes come into contact and the temperature rises rapidly, leading to ignition.

[Problem to be solved by the invention]

By the way, according to the inventor's experiments, PE porous film has a low heat resistance temperature of about 140 to 155°C.
It was found that the SD characteristics were lost relatively early, while the SD onset temperature of the porous PP film was as high as approximately 155°C (both of which still need improvement in terms of ensuring safety).

Therefore, an object of the present invention is to provide a novel separator which is rich in practicality and has an SD starting temperature of about 115 to 130°C and a heat resistance temperature of about 150°C or higher.

[Means to solve the problem]

As a result of various studies to solve the above-mentioned problems of the prior art, the present inventor has found that a porous film made of a polymer alloy made by mixing PE and PP in a specific ratio satisfies practicality in both the SD start temperature and the heat resistance temperature. We have discovered that this is the case, and have completed the present invention.

That is, the battery separator according to the present invention has a PEl of 0 to 90.
It is a mixture of PP9O-10% by weight and is characterized by having a porous film shape.

The separator of the present invention is a porous film made of a mixture of PE and PP, with 0 to 90% by weight of PE1 and 90 to 90% by weight of PP.
It consists of 10% by weight.

Although the separator of the present invention has a porous structure in common with the above-mentioned conventional PE porous film or PP porous film, it differs from these in the mechanism by which it exhibits the SD characteristic.

The above conventional PE porous film or PP polyTL'!
When a separator made of a film reaches the melting point of PE or PP, the separator melts and changes into a non-porous structure, causing an increase in resistance and cutting off the current. Therefore, its SD characteristics depend on the melting point of PE or PP constituting the separator. Therefore, a PE porous film separator has a low heat resistance temperature and is not practical, while a PPP porous film separator has an SD starting temperature that is too high and there are concerns about ensuring safety.

However, in the porous film made of a mixture of PE and PP of the present invention, when the temperature rises and reaches the melting point of PE, only the PE constituting the porous film melts and closes the pores, resulting in a decrease in resistance. The increase in value blocks the current,
Excessive temperature rise is prevented. The film shape is maintained by PP, which is one of the constituent materials of the separator, until the melting point of PP is reached, and the film exhibits sufficient heat resistance. That is, in the separator of the present invention, the PE constituting it contributes to an increase in the SD start temperature and resistance value,
Since PP contributes to the heat resistance temperature, a separator with excellent practicality can be provided. in this way,
The present invention is the first to develop a separator that exhibits excellent practicality by having two essential constituent materials share different functions.

In the present invention, PE is 9% in the total weight of both PE and PP.
The content of PP is 0 to 10% by weight, and PP is blended in a proportion of 10 to 90% by weight. If the amount of PE blended is too small, the pores will not be sufficiently closed even when the temperature rises due to abnormal current, and the degree of increase in resistance will be small, resulting in poor SD characteristics. If the amount of PE blended is too large, the SD starting temperature and heat resistance temperature will decrease Both are unfavorable because they result in a low value. In addition, if desired, a filler,
Appropriate amounts of additives such as colorants, anti-aging agents, and flame retardants can also be provided.

The separator of the present invention has a porous structure as described above, and its pore diameter and porosity are set depending on the battery in which it is incorporated, but usually the average pore diameter is 0.
01 to 5 μm, and the porosity is 20 to 80%. and,
Separators with pore diameter and porosity set in this way usually have a pore size of 20Ω・ctA/piece or less (8000Ω・cm or less)
shows the resistance value (value including solution resistance).

Note that the resistance value R, (Ω) of the separator is the volume specific resistance value ρ (Ω・cm), and the separator thickness jl! (cm) and separator area S (d) using the following formula % formula % The resistance value R0 is proportional to the thickness of the separator and inversely proportional to the area. Then, the resistance value R1 (Ω・47 pieces) per sheet without considering the thickness of the separator is expressed by the following formula (II), and the volume specific resistance value R1 considering the thickness of the separator
(Ω·C11l) is expressed by the following formula (% formula % ([[) The above resistance value R0 is the sum of the resistance value Re of the electrolytic solution and the resistance value Rs of the separator.

Here, the safety of the separator of the present invention will be explained.

Organic electrolyte (conductivity approximately 101 m S/c) near room temperature
The electrical resistance of a lithium battery separator measured in m) is generally about 20 Ω·d or less at a thickness of 25 μm.

In order to suppress excessive temperature rise due to abnormal current, it is recognized that the electrical resistance value needs to increase by at least two to three orders of magnitude compared to the value near room temperature in order to interrupt the current.

When the present inventor tested the separator according to the present invention, the electrical resistance of the separator at around room temperature was 2
It is approximately 20 Ω・d or less at 5 μm, but it is approximately 10′ to 10 hΩ・d at the SD characteristic development temperature (SD starting temperature to heat resistant temperature).
d, and it was found that the resistance increased by 4 to 5 orders of magnitude (
(See Examples below), this large increase in resistance indicates that the separator of the present invention has an excellent current interrupting function, that is, an SD characteristic, and that a battery incorporating this separator has excellent safety. be.

Such a 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. In this case, the materials of the positive electrode, negative electrode, battery case, electrolyte, etc., and the arrangement structure of these components do not need to be special at all, and may be the same as those of conventional batteries.

The present invention also provides a novel method for manufacturing separators.

That is, the method for manufacturing a battery separator according to the present invention is based on PE (
This melting point is defined as TlIb'C) 10 to 90% by weight and P
A film-like material consisting of 90 to 10% by weight of P is heated to -20°C.
(Tmb, b-30) uniaxially stretched in the low temperature range of “C”,
Next, the stretched film is (Tmb-30)℃~(T,,-
2) It is characterized in that it is made porous by stretching in the same direction as the low temperature stretching in a high temperature range of °C.

The film-like material used in the method of the present invention contains PE and PP as essential components, and the proportion of PE in the total weight of both is 10
It consists of a mixture in which the proportion of PP is 90 to 10% by weight. The PE forming this film-like material is not particularly limited, and low-density, medium-density, or high-density PE, linear low-density PE, etc. can be used. Although PP is not limited, isotactic PP is preferable in order to obtain a separator with high porosity, and isotactic PP is expressed as the weight fraction of the part that is not extracted with boiling n-hebutane. Isotactic PP having an index of 90% or more, preferably 95% or more is suitable.

Such a film-like material can be obtained by a known method such as a T-die extrusion method or an inflation method.

The die temperature when using the T-die extrusion method is the melting point of PP (
Hereinafter, this temperature is preferably set to 10°C higher than T (referred to as T, +150)C.

In the method of the present invention, the film is stretched at a low temperature by JjyJ, but prior to this stretching, the film is (Tmb
-30) "C to CT, b-2)" Annealing can be performed by heating in the temperature range of "C" for a predetermined period of time (usually several seconds to several hours). This annealing can increase the porosity of the porous film obtained by the two subsequent stretching steps.

The film-like material or the annealed film-like material is first heated at -20°C to (Tmb, b-30)°C.
L-axis stretching is carried out in a low temperature region by a method such as roll stretching or tenter stretching. If the temperature is too low, stretching becomes difficult, and if the temperature is too high, the desired porous film cannot be obtained even if high-temperature stretching is performed in the next step, so neither is preferable.

The stretching rate during this low-temperature stretching is usually about 25 to 200%, preferably about 10 to 100%, and the stretching speed is usually about 50 to 5000%/min. The stretching ratio is a value calculated by the following formula (IV) using the length Lo of the film-like material before stretching and the length Ll after stretching.

In the method of the present invention, after this low temperature stretching (T.

-30) In a high temperature region of ``C~('r,b-2)℃, uniaxial stretching is carried out in the same direction as the stretching direction during the low temperature stretching. The extremely fine pores formed in the film are enlarged and the film becomes porous.If the temperature during high-temperature stretching is outside the above range, the degree of porosity will be insufficient, which is not preferable.

This high-temperature stretching can also be performed by roll stretching, tenter stretching, etc., like the low-temperature stretching. In addition, the stretching ratio during high-temperature stretching is usually about 10 to 400%, preferably about 50 to 2
00%, and the stretching speed is usually about 50-5000%/m
It is in. Note that this stretching rate is the length Lo before low-temperature stretching.
This is a value calculated using the following (V) 2 using the length Ll of the low-temperature stretched film material (that is, the length before high-temperature stretching) and the length L2 after high-temperature stretching.

The porous separator that is used may have built-in stretching strain, and in order to remove this strain, the separator is kept in a tensioned or relaxed state after high-temperature stretching and heated to a predetermined temperature, usually approximately the same temperature as during the high-temperature stretching. Heating can be referred to as heat setting).

The heating time for removing distortion is set depending on the temperature, the amount of distortion remaining in the separator, etc., but is usually about 5 seconds to
It is 2 minutes.

[Effects of the Invention] The present invention is configured as described above and is a porous separator made of a mixture of PE and PP, so when the temperature rises abnormally, the PE melts and closes the pores, increasing the resistance. On the other hand, PP functions to maintain the film shape of the separator, so it has the advantage of being practical and highly safe.

Moreover, according to the method of the present invention, low temperature stretching and.

A separator with excellent properties can be obtained by high-temperature stretching, and since no organic solvent is used in the process, there is no adverse effect on the manufacturing site or the global environment.

[Example] Hereinafter, the present invention will be explained in more detail with reference to Examples. In the Examples and Comparative Examples, all "parts" indicating the number of parts of the materials used are 11 parts.

Example 1 50 parts of PE with melt index (hereinafter referred to as Ml) 0.7, melting point 134°C and MT 2.5, melting point 158
Melt and knead 50 parts of PP at a die temperature of 24 °C.
It is extruded from a T-die extruder at 0°C to obtain a long film with a thickness of 27 μm.

This film-like material was annealed by heating at a temperature of 120°C for 60 minutes, then uniaxially stretched at a temperature of 25°C to a stretching ratio of 35% in the longitudinal direction, and then at a temperature of 120°C.
The separator (sample 1) was obtained by uniaxially stretching the film in the same direction as the above at a temperature of 65% to make it porous, and then heating it for 1 minute at 120°C for heat setting. Note that during heat setting, the length in the stretching direction was kept unchanged.

The thickness of this separator is 25 μm, and the average pore size is 0.
．． 14μm, porosity 40%, resistance 15Ω, -c11
It was 1/piece.

Further, its SD characteristic was as shown as curve A in FIG. As can be seen from curve A, this separator has a resistance value of 4 to 5 at around 130°C (SD starting temperature).
The resistance value rapidly increases by an order of magnitude, this large resistance value is maintained, and then the resistance value decreases from around 165°C (allowable temperature).

The dramatic increase in resistance around 130°C is a phenomenon that occurs because PE, which is one of the materials forming the separator, melts and closes the pores. Since the temperature is maintained at around 165°C,
It can be seen that this separator has excellent SD characteristics. Further, the reason why the resistance value decreases from around 165° C. is a phenomenon that occurs because PP, which is another forming material of the separator, also melts and no longer maintains its film shape.

The Ml, melting point, and separator properties of the resin used were measured in the following manner.

(MT) Measured by the method specified in JIS K 7210.

(Melting point) PE or PP is heated at a temperature of 230°C for 5 minutes to melt it, and then cooled to 25°C at a rate of 5°C/min. Next, the temperature is raised at a rate of 5°C/min, and the temperature at the endothermic peak is defined as the melting point.

In addition, for measurement of the endothermic peak, D
SC200 was used.

(Average pore diameter) Measured using a mercury intrusion porosimeter model 2000 manufactured by Carlo Erba.

(Porosity) The density (ρ.) of the unstretched film-like material is determined from the thickness, area, and weight of the unstretched film-like material.

Next, the apparent density (ρ
, ).

Then, the porosity is calculated using the following formula (Vl).

(SD characteristics) As shown in Fig. 4, platinum electrodes 1.1 with a diameter of 20 mm are placed facing each other, and a separator 2 is placed between them, silicone rubber 3.3 is used as a pad, and a polytetrafluoroethylene plate 4 Tighten the whole thing from both sides with .4.

As an electrolytic solution, propylene carbonate and dimexymethane are mixed in equal weights, and LiBF4 is added in an amount of 1 mol/ml.
Use a solution that has been dissolved to a concentration of
It was impregnated into the PP nonwoven fabric 5 filled in between. Although not shown, a resistance meter and a thermocouple were connected to the platinum plate electrode.

Set the measuring cell with such a structure on a dry cloth and dry it for 5 to 7
The temperature is raised at a rate of °C/min, and the electrical resistance value (Ω·d) of one separator at each temperature is measured.

The electrical resistance was measured using a resistance meter, LCR meter KC-532 model manufactured by Kokusan Denki Kogyo Co., Ltd., as an AC resistance at IKHz, and was converted using the above formula (II).

Example 2 The work was carried out in the same manner as in Example 1, except that the conditions for low-temperature stretching and high-temperature stretching were set as shown in Table 1, and the heat set temperature was the same as the temperature during high-temperature stretching. Five types of porous separators (Samples 2 to 6) were obtained. Note that the units of temperature and stretching ratio in Table 1 are "°C and 1%".

The properties of these separators were as shown in Table 6. Note that "1 resistance" in Table 6 is the resistance value of the separator at a temperature of 25°C.

(Hereinafter, blank spaces) Table 1 Example 3 A film-like product was obtained in the same manner as in Example 1 except that a mixture of 30 parts by weight of PE and 70 parts by weight of PP was used, and this was heated at a temperature of 120°C for 30 minutes. annealing.

Using this annealed film material, low-temperature stretching and high-temperature stretching were performed sequentially under the conditions shown in Table 2, and then heat-setting was performed by heating for 1 minute at the same temperature as during high-temperature stretching, resulting in four types of porous Separators (samples 7 to 10) were obtained. These properties are also listed in Table 6.

(Hereinafter, blank space) Table 2 Example 4 (Production of film-like product) PE used in Example 1 (hereinafter referred to as B2 in this example) and PP (hereinafter referred to as B2 in this example) , A2), PE and PP having physical property values shown in Table 3 below are prepared. P shown in Table 3
P is all isotactic polypropylene, PE
Of these, B+ and B3 are high density polyethylene, and B4 is linear low density polyethylene.

(Hereinafter, blank spaces) Table 3 Using a mixture of PE and PP as shown in Table 4, seven types of film-like products (symbol C-1) were molded using a T-die extruder. Note that the thickness of each of these film-like materials was 25 to 30 μm.

(Hereinafter, blank space) Table 4 (Manufacture of porous separator) Using film material C-1, low temperature stretching and high temperature stretching were performed sequentially under the conditions shown in Table 5 below, and further, the temperature was the same as that during high temperature stretching. Eight types of porous separators (samples 11 to 18) were obtained by heat setting by heating at a high temperature for 1 minute.

The properties of these separators were as shown in Table 6.

(Hereinafter, blank space) Table 5 In addition, in the case of samples 11 and 15, the temperature was 115°C.
0 minutes, temperature 12 for sample 12.13.14.16
Annealing was performed by heating at 0°C for 30 minutes, and in the case of sample 17, heating at 105°C for 60 minutes, followed by stretching. Moreover, in the case of sample 18, it was stretched without annealing.

Table 6 shows the properties of the separators obtained in Examples 1 to 4 above. In addition, the units of porosity, pore diameter (average pore diameter), and resistance in this table are "%", "μm", and "Ω・cwt".
/ sheet", and the units of both the SD start temperature and the heat-resistant temperature are "°C".

(Hereinafter, blank space) Table 6 From Table 6, the porous separators according to the present invention all begin to exhibit SD characteristics at a temperature of approximately 115 to 130°C (that is, resistance begins to increase at this temperature). ), and it can be seen that the SD characteristics persist up to temperatures of about 140°C or higher.

Note that the curves showing the temperature dependence of the resistance value in each of the separators of Samples 2 to 18 are not shown;
It has been confirmed that, as in the case of , a rapid increase in resistance value of 4 to 5 orders of magnitude occurs from the SD start temperature, and that this increased resistance value is maintained up to the heat-resistant temperature.

Therefore, the separator according to the present invention has practical SD characteristics and is excellent in safety.

Comparative Example I A porous separator was obtained by working in the same manner as in Example 1 except for using a mixture of 8 parts PE and 92 parts PP
Sample 19).

Further, a porous separator was obtained (Sample 20) in the same manner as in Example 1 except for using a mixture of 91 parts of PE and 9 parts of PP.

The SD characteristics of these separators are shown in FIG. 2 as curves B (sample 19) and C (sample 20), and other characteristics are shown in Table 8.

Comparative Example 2 Three types of separators (Samples 21 to 23) were produced in the same manner as in Example 1, except that low-temperature stretching and high-temperature stretching were carried out sequentially under the conditions shown in Table 7. In sample 21, in which the low-temperature stretching temperature was low, the film broke during the low-temperature stretching.

The properties of these separators are also listed in Table 8.

Table 7 Comparative Example 3 A commercially available PE porous film (Sample 24) and a PP porous film (Sample 25) are used as a comparative example.

These SD characteristics are shown in FIG. 3 as curves D (sample 24) and E (sample 25). Other characteristics are also listed in Table 8.

Table 8 From Table 8, it can be seen that the separators of the comparative examples have drawbacks such as the SD start temperature being too high and the heat resistance temperature being too low, making them inferior in safety.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the SD characteristics of the separator according to the present invention, Figs. 2 and 3 are graphs showing the SD characteristics of separators of comparative examples, and Fig. 4 is a graph showing the SD characteristics of the separator. FIG. 2 is a schematic cross-sectional view. 1...Platinum plate electrode 2...Separator 3...
·silicone rubber

Claims (3)

[Claims] (1) A battery separator characterized by being a mixture of 10 to 90% by weight of polyethylene and 90 to 10% by weight of polypropylene and having a porous film shape. (2) Polyethylene (this melting point is T_m_b℃)
A film-like material consisting of 10 to 90% by weight and 90 to 10% by weight of polypropylene is heated to -20°C (T_m_b-30)
Uniaxially stretched in a low temperature range of ℃, then the stretched film (
A method for producing a battery separator, characterized in that it is made porous by stretching in the same direction as the low temperature stretching in a high temperature range of T_m_b-30)°C to (T_m_b-2)°C. (3) It has a positive electrode, a negative electrode, and a separator interposed between these two electrodes, and this separator is made of polyethylene
A battery characterized in that it is a porous film made of a mixture of 0 to 90% by weight of polypropylene and 90 to 10% by weight of polypropylene.