JPH1064501A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

(57) [Summary] [PROBLEMS] When a non-aqueous electrolyte battery causes an internal short circuit,
It is an object of the present invention to suppress a rapid temperature rise of a battery by the effect of a separator and to ensure safety. A strip-shaped polymer microporous membrane is used as a separator, the tensile elongation in the width direction is 1000 to 3000%, and the value of air permeability at room temperature is 700 to 1200 sec / 100.
cc, and the value of the air permeability when held at 85 ° C for 3 minutes in the atmosphere is 700-3000sec / 100cc, and also 95 ° C in the atmosphere.
When the air permeability is held at 3 min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of safety of a non-aqueous electrolyte secondary battery, and more particularly to a battery separator.

[0002]

2. Description of the Related Art In a non-aqueous electrolyte battery, particularly a secondary battery, in order to discharge a large current, it is generally necessary to spirally wind a thin sheet-like electrode to increase the facing area between the positive and negative electrodes. It is effective. Accordingly, the separator used is preferably a thin film.

However, in such a battery structure, while a large current discharge is possible, an excessive current flows when the battery is charged and an external short circuit occurs, and the battery generates heat due to the Joule heat. The temperature may become abnormally high.

[0004] In order to ensure the safety in the event of an external short circuit, separators have been improved so far, and it has been proposed to use a microporous membrane provided with a myriad of fine holes instead of a conventional nonwoven fabric or the like ( For example, JP-A-60-2395
No. 4) has been implemented.

When such a microporous membrane separator is used, when the battery temperature rises due to Joule heat caused by a short-circuit current at the time of an external short-circuit, the micropores of the separator are melted and closed, so that the movement of ions can be prevented. Thereby, the current in the battery is cut off, and it is possible to suppress a rapid rise in the battery temperature.

Further, from the viewpoints of ensuring safety in the event of an external short circuit in any environment, and further improving battery performance, productivity and reliability, the physical properties of the microporous membrane and the manufacturing method have been studied in detail. I have.

Japanese Patent Application Laid-Open No. 63-308866 proposes the use of a microporous film formed of a plurality of types of materials having different melting points, and Japanese Patent Application Laid-Open No. 1-267951 discloses a method of using a microporous film. Regulations on temper and film thickness are made. In addition,
Japanese Patent Application Laid-Open No. 3-219552 proposes to use a microporous membrane provided with micropores by biaxial stretching.

Furthermore, Japanese Patent Application Laid-Open No. 4-212265 discloses that a separator having a finely regulated average molecular weight, average pore size, maximum pore size, porosity and the like of a microporous membrane is used.

[0009]

As described above, the safety of a non-aqueous electrolyte battery against an external short circuit has been improved by the separator using a microporous membrane proposed so far. When considering safety, there is a possibility that an internal short circuit may occur in addition to an external short circuit. As an evaluation method assuming an actual internal short circuit, a nail penetration test in which a nail penetrates a battery is generally performed.

In addition, a large force is externally applied to the battery,
There is a crush test in which a battery is crushed and an internal short circuit occurs. Unlike the case of the external short-circuit, these internal short-circuits cause a short-circuit first in a minute part in the electrode, and the short-circuit current concentrates in that part. For this reason, abnormal heat generation of the battery is more likely to occur than in the case of an external short circuit, and it is difficult to suppress a rapid rise in temperature of the battery even when the separator made of the microporous film is used.

The present invention has been made in consideration of the above problems, and suppresses a rapid rise in temperature of a battery by the effect of a separator even when an internal short circuit occurs in the battery.
It is an object to provide a non-aqueous electrolyte secondary battery excellent in safety.

[0012]

In order to solve the above-mentioned problems, the present invention provides a positive electrode comprising a lithium-containing composite oxide, a carbon material capable of occluding and releasing lithium, a metal oxide, or a lithium metal or lithium metal. A negative electrode selected from an alloy and a non-aqueous electrolyte are used, and a strip-shaped polymer microporous film having a thickness of 15 to 40 μm is used as a separator, and its tensile elongation in the width direction is 1000 to 3000%, and Air permeability at room temperature is 700-1200sec / 100cc
Air permeability of 10,000sec / 10 when held at ~ 95 ° C for 1min. Or more
This is a non-aqueous electrolyte secondary battery of 0 cc or more.

According to the present invention, even when an external short circuit or an internal short circuit such as a nail penetration test or a crush test occurs, the battery is prevented from abrupt temperature rise, and the nonaqueous electrolyte secondary battery excellent in safety is provided. A battery is obtained.

The inventors of the present invention have analyzed the cause of abnormal heat generation during an internal short circuit of a charged battery, and have found the following. If an internal short circuit occurs at a part between the electrodes,
The short-circuit current concentrates on the short-circuited portion, and the positive electrode and the negative electrode cause an exothermic reaction by Joule heat.

When a lithium-containing oxide such as LiNiO2 or LiCoO2 is used as the positive electrode active material, the active material thermally decomposes and releases oxygen gas. The released oxygen gas permeates through the separator, reacts with the heated negative electrode, and causes a further rapid temperature rise. Therefore, in order to suppress such abnormal heat generation, it has been found effective to instantaneously block the permeation of the oxygen gas released by the thermal decomposition of the positive electrode active material and inhibit the reaction with the negative electrode. It is a thing.

The separator having air permeability according to the present invention causes a sharp rise in air permeability at a relatively low temperature, so that the oxygen gas is effectively shut off and abnormal heat generation of the battery can be prevented. In the case of an internal short-circuit due to crushing, a force is applied from the outside in the width direction of the separator, and the separator is short-circuited in a stretched state.

Since the separator of the present invention has a relatively high elongation, it does not easily break even during crushing, has a large effect of blocking oxygen, and can suppress abnormal heat generation of the battery. .

Here, the method of calculating the tensile elongation in the width direction of the separator was measured and calculated according to the following method. First, place one separator 45mm in the width direction and 3
It was cut out into a 0 mm sample piece. And 10mm each at both ends in the width direction
After a tape was applied to the part to reinforce it, it was set in a universal tensile tester. The separator was pulled in the width direction at room temperature at a tensile speed of 10 mm / min., And the elongation until the sample piece was broken was measured.

Since the original length of the separator was 25 mm, the tensile elongation was calculated by calculation. The same measurement was repeated three times and the average value was taken as the tensile elongation of the separator.

Next, a method for measuring the air permeability of the separator is as follows. The measurement is performed by Oken type air permeability smoothness tester EG1 manufactured by Asahi Seiko Co., Ltd.
Using S, the evaluation was performed as the time (sec) required for 100 cc of air to pass through the sample separator at room temperature.

The air permeability of the separator after the heat treatment was measured as follows. First, the sample separator was cut into a size of 45 mm in the width direction and 100 mm in the length direction. Then, the separator was placed in a constant temperature bath maintained at a constant temperature of 90 to 95 ° C. so as not to be in direct contact with others, and held for 3 minutes. Thereafter, the sample was returned to room temperature, and the air permeability was measured by the method described above. Each of the three separators was measured, and the average value was defined as the air permeability of the separator.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The invention described in claim 1 of the present invention relates to a positive electrode comprising a lithium-containing composite oxide and a carbon material, a metal oxide, or a lithium metal or lithium alloy capable of occluding and releasing lithium. Equipped with a selected negative electrode and a non-aqueous electrolyte, a belt-like film thickness of 15 ~ 40μ as a separator
m, a tensile elongation in the width direction of 1
000-3000% and air permeability at room temperature is 700-
This is a non-aqueous electrolyte secondary battery that has 1200 sec / 100 cc and air permeability of 10,000 sec / 100 cc or more when held at 90 to 95 ° C. for 1 min.

The value of the air permeability of the separator used in the nonaqueous electrolyte secondary battery of the present invention is important.
It is required to be 0 to 1200 sec / 100cc, and preferably 900 to 1100sec / 100cc. If it is less than 700, it is difficult to ensure sufficient safety, and if it exceeds 1200, the high-rate charge / discharge characteristics tend to decrease, which is not preferable.

The rate of increase in air permeability when the separator is kept at a high temperature is important, and the air permeability when kept at 90 to 95 ° C. for 1 min. Or more in the atmosphere is 10,000 sec / 100 cc or more. Is required. If the temperature of 10,000sec / 100cc or more exceeds 100 ° C, it is difficult to completely suppress abnormal heat generation due to internal short circuit.
In the case of / 100cc or more, although the safety can be ensured, the performance of the battery deteriorates greatly at the time of high temperature storage, and it is not enough for practical use.

In consideration of the internal short circuit due to the crushing of the battery, the value of the tensile elongation in the width direction of the separator is important, and the elongation of 1000 to 3000% is required.
~ 2200%. If the elongation is less than 1000%, the separator is easily broken at the time of crushing, and it is difficult to obtain an effect of blocking oxygen gas at the time of an internal short circuit.

On the other hand, a separator exceeding 3000% is not preferable because it is difficult to obtain sufficient strength, and the productivity of the battery is reduced. The thickness of the separator is preferably from 15 to 40 μm, more preferably from 20 to 30 μm. If it exceeds 40 μm, the capacity of the battery will be reduced. Conversely, if it is less than 15 μm, it will be difficult to secure the strength of the separator, and the productivity of the battery will be reduced.

According to a second aspect of the present invention, the material of the separator is polyethylene, which has a low melting point in the microporous polymer membrane, and has an air permeability when the separator is kept at a high temperature. The rate of increase of the degree is the largest, and the effect of the present invention is easily obtained.

According to a third aspect of the present invention, the membrane is provided with a large number of micropores by uniaxial stretching. This is because, in the manufacturing process of the separator, if the uniaxial stretching is performed when the micropores are provided, the directionality of the separator is reduced. It is possible to obtain a large anisotropy and control the tensile elongation in the width direction. On the other hand, in the case of biaxial stretching, it is possible to secure the strength in the width direction, but it is difficult to obtain a separator having a large elongation. Therefore, in order to secure the elongation of the present invention, it is desirable to manufacture the separator by uniaxial stretching.

[0029]

【Example】

(Example 1) Hereinafter, the present invention will be described in detail with reference to examples. FIG. 1 shows a vertical cross-sectional view of the cylindrical battery used in this example. In FIG. 1, reference numeral 1 denotes a battery case formed by processing a stainless steel plate having resistance to organic electrolyte, reference numeral 2 denotes a sealing plate provided with a safety valve, and reference numeral 3 denotes an insulating packing. Reference numeral 4 denotes an electrode group, in which the positive electrode and the negative electrode are spirally wound a plurality of times via a separator and housed in the case 1. A positive electrode lead 5 is drawn out from the positive electrode and connected to the sealing plate 2, and a negative electrode lead 6 is drawn out from the negative electrode and connected to the bottom of the case 1. Reference numeral 7 denotes an insulating ring provided at the upper and lower portions of the electrode plate group 4, respectively. Hereinafter, a method for manufacturing the positive and negative electrode plates will be described in detail.

LiNiO ₂ was used as the positive electrode active material. First, nickel hydroxide and lithium hydroxide were weighed so that the atomic ratio of Ni: Li was 1: 1 and sufficiently mixed by a ball mill. And put this mixture in an alumina crucible,
Heat treatment was performed at 750 ° C. for 10 hours in dry air.

After natural cooling, pulverization and classification were performed to obtain a positive electrode active material powder having an average particle size of 10 μm. 6 parts by weight of artificial graphite powder was added to 100 parts by weight of this active material, and N-
A solution in which polyvinylidene fluoride (PVDF) as a binder was dissolved in a solvent of methylpyrrolidone (NMP) was kneaded to form a paste. The amount of the added PVDF was adjusted to 4 parts by weight with respect to 100 parts by weight of the active material.

Next, this paste is applied to both sides of an aluminum foil, dried and rolled to a thickness of 0.14 mm and a width of 37 m.
m and a length of 380 mm. In addition, in producing the positive electrode plate, a series of steps after kneading were performed in dry air.

As the negative electrode, a mesophase small sphere having an average particle size of 6.0 μm which was heat-treated at 2800 ° C. and graphitized was used. 100 parts by weight of the graphitized mesophase was mixed with 3 parts by weight of styrene / butadiene rubber as a binder, and an aqueous carboxymethyl cellulose solution was added and kneaded to form a paste. The paste was applied to both surfaces of a copper foil, dried, and rolled to obtain a negative electrode plate having a thickness of 0.20 mm, a width of 39 mm, and a length of 420 mm.

Then, the leads were attached to the positive and negative electrode plates, and spirally wound through a separator having a thickness of 0.025 mm, a width of 45 mm, and a length of 1000 mm and having an air permeability shown in Table 1 and having a diameter of
It was stored in a battery case 1 having a height of 17.0 mm and a height of 50 mm. (Each corresponds to battery A to battery J.) 1 mol / l LiPF ₆ is dissolved as an electrolyte in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 20:80 as an electrolyte. The solution was injected. Then, the battery was sealed to obtain a completed battery.
Each of these batteries was produced in 15 cells and charged and discharged. Charging was performed at a constant current and constant voltage, and the battery was charged up to 4.2 V with a constant current of 630 mA. After the voltage reached 4.2 V, the charge was converted to a constant voltage and the charging was completed in a total of 2 hours. Discharge was performed at a constant current of 900 mA, and the discharge end voltage was set to 2.5 V. Such charge / discharge was performed for 10 cycles in a 20 ° C. environment, and the capacity at the 9th cycle was defined as the initial capacity. Then, in the charged state, each 5 cells is replaced by 8
It was stored for 3 days in an environment of 0 ° C. Thereafter, the temperature was returned to 20 ° C., and the cycle was repeated 5 times. The capacity of the fourth cycle was determined as the recovery capacity after storage, and the capacity recovery rate was determined. Figure 2 for the remaining 10 cells
Attach a thermocouple to the battery surface as shown in
A nail piercing test was performed in which a short was made to penetrate the nail. If the battery shows smoke or other abnormalities or the battery surface temperature
The ratio of 150 ° C. or higher was determined. Table 1 shows the battery capacity and the results of the nail penetration test.

[0035]

[Table 1]

According to Table 1, in order to ensure sufficient safety in the nail penetration test, the value of the air permeability of the separator used, especially the value of the air permeability when heat-treated at 95 ° C. is approximately 10%.
It turns out that it is necessary to be 000sec / 100cc or more.
However, in the case of the battery A, although it was 12000 sec / 100 cc, some batteries reached 150 ° C. or higher. This is because the value of the air permeability at room temperature of the used separator is 560 sec /
It is considered that it is difficult to completely suppress the temperature rise due to the small size of 100cc sheets.

On the other hand, the value of the air permeability at room temperature is 1200 sec.
It can be seen that the initial capacity of battery I and battery J exceeding / 100 cc tend to decrease. Although the battery F can secure safety, the recovery rate is extremely low. This is because the used separator is already 10,000sec / 100cc at 85 ℃
It is considered that the storage characteristics at high temperature cannot be satisfied because the number of sheets has changed to more than one.

From the above, in order to satisfy the initial capacity of the battery and the capacity recovery rate during high-temperature storage, and to ensure sufficient safety in a nail penetration test, the separator to be used has an air permeability of 700 at room temperature. ~ 1200 sec / 100cc sheet, no sharp increase in air permeability at 85 ° C, air permeability at 95 ° C is 1000
It is clear that a sheet having 0 sec / 100cc or more is desirable.

(Example 2) Air permeability at room temperature is 800 to 1
For a 100 sec / 100 cc polyethylene separator, the battery K whose tensile elongation value in the width direction is as shown in Table 2
~ Battery R was produced. The constituent materials other than the separator, such as the positive electrode, the negative electrode, and the electrolytic solution, and the manufacturing methods were all the same as in (Example 1).

These batteries were charged and discharged in the same manner as in Example 1 in an environment of 20 cells at 10 cells each. Then, a thermocouple was attached to the surface of the battery in the state of charge at the 10th cycle, and a crush test was performed in which the center of the battery was crushed with a metal round bar having a diameter of 15 mm as shown in FIG. Then, as in Example 1, the battery smokes or the battery surface temperature becomes 1
The ratio at which the temperature became 50 ° C. or higher was determined and shown in (Table 2).

[0041]

[Table 2]

From the results shown in Table 2, it is difficult to secure sufficient safety in the crush test for the batteries K and L having a small value of the tensile elongation in the width direction. Therefore, it is considered that the effect of blocking oxygen is hardly obtained. Tensile elongation value of 10
Although the battery M to the battery R, which is not less than 00%, can almost guarantee the safety, the battery P having a small increase rate of the air permeability at 95 ° C.
Then, it is difficult to suppress a rapid temperature rise.

From the above, in order to ensure sufficient safety during the crush test, the rate of increase in the air permeability of the separator at a high temperature is sufficient, and the value of the tensile elongation in the width direction is 1000.
%.

Some of the batteries R having a tensile elongation of 3150% have a temperature exceeding 150 ° C., which is caused by a decrease in the strength of the separator. It is considered easy to do.

Therefore, the value of the tensile elongation is 1000%,
It turns out that it is necessary to be preferably in the range of 1300% to 3000%.

Although LiNiO ₂ was used as the positive electrode active material in Examples 1 and 2, LiNi _{1 -x} M _x O ₂ (M is, for example, Co, Mn,
Similar effects were obtained when Mg, Sn), LiCoO ₂ , or LiMn ₂ O ₄ was used.

Although the negative electrode is made of graphitized mesophase spheres, other carbon materials, such as artificial graphite, cokes, carbon fibers, etc., and lithium such as metal oxides can be inserted and released. When these were used, almost the same effect was obtained.

[0048]

As described above, according to the present invention, the battery characteristics can be satisfied, and sufficient safety can be ensured even when an internal short circuit occurs in a battery such as a nail penetration test or a crush test. An advantageous effect is obtained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a cylindrical battery used in this example.

FIG. 2 is a conceptual diagram showing a nail penetration test of a battery.

FIG. 3 is a conceptual diagram showing a battery crush test.

[Explanation of symbols]

 1. Battery case 2. Sealing plate 3. Insulation packing 4. Electrode group 5. Positive electrode lead 6. Negative electrode lead 7. Insulation ring 8. Battery 9. Nail 10. Battery 11. Metal rod

Claims (3)

[Claims] A positive electrode comprising a lithium-containing composite oxide,
A carbon microporous film capable of occluding and releasing lithium, a metal oxide, or a negative electrode selected from lithium metal and lithium alloy, and a non-aqueous electrolyte, and a strip-shaped polymer microporous membrane having a thickness of 15 to 40 μm as a separator The tensile elongation in the width direction is 1000 to 3000%, the air permeability at room temperature is 700 to 1200 sec / 100 cc, and 90 to 95 in the atmosphere.
Air permeability of 10000sec / 100cc when kept at more than 1min.
Non-aqueous electrolyte secondary batteries. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the material of the separator is polyethylene. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the separator has a number of micropores formed in the membrane by uniaxial stretching.