JP3307231B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art In recent years, portable and cordless electronic devices such as AV devices and personal computers have been rapidly advanced, and there is a demand for a small, lightweight, high energy density secondary battery as a power supply for driving these devices. high. In this respect, there is great expectation for non-aqueous secondary batteries, especially non-aqueous electrolyte lithium secondary batteries having high voltage and high energy density.

[0003] High energy density is expected for lithium secondary batteries.
The reason for this is that a suitable positive electrode,
By selecting a positive electrode with high voltage, high voltage and high energy
The point is that you can obtain a high density. This request
LiCoO as filling _TwoAnd LiMn_TwoO_FourOf the system
A material exhibiting a high voltage of about 4 V may be used. Meanwhile, the negative electrode
As metal lithium, lithium alloys and lithium
Carbon materials that can store and release ions are being studied
However, metallic lithium is called a dendrite due to charging and discharging.
There is a problem of short circuit due to resinous products,
Carbon materials that do not cause these problems are lithium secondary batteries.
It is promising.

In a battery using a non-aqueous electrolyte such as a lithium secondary battery, the conductivity of the non-aqueous electrolyte is only about 1/10 of the conductivity of the aqueous electrolyte, so that it is generally difficult to extract a current. Further, the electrode plate of the non-aqueous electrolyte secondary battery is made thin and long so as to have current characteristics equivalent to those of the aqueous secondary battery, and the electrode area is increased. If you do,
Causes safety issues. Since an organic solvent is often used as a solvent for the non-aqueous electrolyte, when the battery is short-circuited for some reason, the inside of the battery is heated by Joule heat due to a large short-circuit current, and various chemical reactions and heat caused by the heat are generated. It can cause a runaway and result in a fire or even an explosion.

In particular, LiCoO ₂ or LiM
When an n ₂ O ₄ -based material showing a high voltage of about 4 V is used, the potential difference is large, so that the short-circuit current is further increased and safety is reduced.

In order to solve these problems, 110-110
Polyethylene composite with a so-called shutdown porous function, which has a so-called shutdown function that closes the micropores and loses porosity by thermal melting at a temperature near 130 ° C, and a layer structure like polyethylene and polypropylene Porous membranes are often used as separators.

[0007]

However, a porous membrane of polyethylene alone or a microporous polyolefin composite membrane containing polyethylene is accompanied by a remarkable thermal shrinkage when the temperature becomes close to a shutdown temperature, which is a heat closing temperature of pores. If the contraction is remarkable, even if the separator is shut down at the time of short circuit, the electrode plate group is deformed due to the contraction, and the positive electrode plate and the negative electrode plate come into contact with each other, resulting in a short circuit state again and a decrease in safety.

An object of the present invention is to provide a non-aqueous electrolyte secondary battery which maintains the current characteristics of the related art and is excellent in safety during a short circuit.

[0009]

In order to achieve the above object, a non-aqueous electrolyte secondary battery according to the present invention comprises a single-piece of polyethylene between a positive electrode made of a lithium composite oxide and a negative electrode made of a carbon material. An electrode body formed from a microporous film of polyolefin or a microporous polyolefin composite film containing polyethylene and spirally wound with a separator impregnated and held with a non-aqueous electrolyte. Heat treatment is performed at a temperature lower than the heat blocking temperature of the porous membrane, and the air permeability after this heat treatment increases compared to before the heat treatment.
And the value is 400 sec / 100 cc or more,
One having a range of 1100 sec / 100 cc or less and a shrinkage ratio at the heat closing temperature of 50% or less is used.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a microporous membrane made of polyethylene alone or polyethylene as described in claim 1.
Polyolefin composite microporous membranes containing
After heat treatment at a temperature lower than the
The separator has an air permeability of 400 sec / 100 cc or more and 1100 sec / 10
0 cc or less , and the shrinkage at the heat closing temperature is 50% or less . Here, the separator
The heat blocking temperature of the resulting microporous membrane is described in [0006].
At the temperature at which the pores are closed and the porosity disappears as described
And 110 to 13 for a polyolefin-based microporous membrane.
The temperature will be close to 0 ° C. At this temperature the air permeability of the separator
The degree is about 10,000 sec / 100 cc. above
By performing heat treatment, the charge / discharge cycle characteristics, etc., during normal times
And the shutdown function operates.
When made, the contraction rate of the separator is small,
Since a short circuit due to contact is prevented , a non-aqueous electrolyte secondary battery with excellent safety can be provided.

[0011]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. In this example, a cylindrical battery was constructed and evaluated.

FIG. 1 is a longitudinal sectional view of the cylindrical battery.
In the figure, reference numeral 1 denotes a positive electrode, LiCoO ₂ , which is an example of a lithium composite oxide, as an active material, and an aqueous dispersion of polytetrafluoroethylene in a weight ratio of 100: 4 using carbon black as a binder as a conductive material. : A mixture mixed at a ratio of 7 was applied to both sides of an aluminum foil, dried, rolled, and then cut into a predetermined size. And
A positive electrode lead plate 2 made of titanium is spot-welded to the positive electrode 1. The mixing ratio of the aqueous dispersion of polytetrafluoroethylene as the binder is calculated based on the solid content. Reference numeral 3 denotes a negative electrode, which is a mixture of a carbonaceous material and a fluorine-based binder at a weight ratio of 100: 5, applied to both sides of a copper foil, dried, rolled, and cut into a predetermined size. It was done. The negative electrode 3 is also spot-welded with a negative electrode lead plate 4 made of copper. Reference numeral 5 denotes a separator, which is interposed between the positive electrode 1 and the negative electrode 3, and is entirely spirally wound to form an electrode plate group. An upper insulating plate 6 and a lower insulating plate 7 made of polypropylene are arranged on the upper and lower sides of the electrode plate group, respectively, inserted into a case 8 in which nickel is plated on iron, and the positive electrode lead 2 is inserted into a sealing plate 9 made of titanium. Negative lead 4 in case 8
After spot welding to the bottom of each, an electrolytic solution in which 1 mol of LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate at a volume ratio of 1: 1 was injected, and the battery was sealed via a pocket 10. To complete batteries. The dimensions of this battery are 17mm in diameter and 50mm in height.
It is. Reference numeral 11 denotes a positive electrode terminal of the battery, and the battery case 8 also serves as a negative electrode terminal.

As shown in Table 1, the separator 5
Of polyethylene alone with different stretch ratio of resin
A porous membrane was used. In this embodiment, 100 sec / 100
Separators A to G were obtained by using polyethylene microporous membranes having air permeability of cc to 1700 sec / 100 cc and cutting them into predetermined dimensions . Furthermore, these microporous membranes are kept below the heat blocking temperature (110-130 ° C.).
100 ℃ is the temperature, was subjected to a heat treatment of 24 hours, before
Separators A ′ to G ′ cut to the same dimensions as described above were obtained. So
These separators A to G and separators A ′ to
G ′ was used for producing the above cylindrical battery.

[0014]

[Table 1]

[0015]

[Table 2]

A comparison between Tables 1 and 2 shows that each separator is 1
The heat treatment at 00 ° C. for 24 hours was accompanied by a slight increase in air permeability, but a significant decrease in the shrinkage at the closing temperature was observed.

The air permeability here is ASTM D72.
6, based on method A, membrane area 6.4 cm ² , pressure 124 m
It is measured by the time (sec) required for 100 cc of air to pass through in mH ₂ O. The shrinkage was measured by subjecting the separator alone to a heat treatment at a closing temperature for 10 minutes and calculating the change in area.

The produced cylindrical battery is placed in an environment of 20 ° C.
The charge / discharge current was 750 mA, the charge end voltage was 4.2 V, and the constant current charge / discharge at the discharge end voltage 3.0 V was repeated 10 cycles, stopped in the charged state, and a short circuit test was performed at room temperature. Tables 3 and 4 show the discharge capacity at the 10th cycle and the results of the short-circuit test.
It was shown to.

[0019]

[Table 3]

[0020]

[Table 4]

As can be seen from Table 3, the separators A,
In the batteries using B, C, D, and E, the battery surface temperature became 135 ° C. or higher in the short-circuit test, and gas was ejected from the sealing portion. After the short-circuit test, the electrode group was observed by X-ray CT scan. However, in the batteries using separators A, B, C, D, and E, the electrode group was deformed, and the positive and negative electrode plates were in contact. It was confirmed that a short circuit occurred again. Separator F, G
In the battery using, no deformation of the electrode plate group was observed, and no contact portion between the positive and negative electrode plates could be confirmed.

Since the separators A, B, C, D, and E have a higher shrinkage at the closing temperature than the separators F and G, the shrinkage causes deformation of the electrode plate group, and the positive and negative electrode plates come into contact and re-short-circuit. It is considered that this occurred. It can be seen that it is desirable to use a separator having a shrinkage ratio of 50% or less at the closing temperature in order to prevent the electrode group from being deformed during the heat generation during a short circuit and to prevent the re-short circuit due to the contact of the positive and negative electrodes with each other. .

Next, as can be seen from Table 4, the air permeability is 20
The battery using the 0 sec / 100 cc separator A 'had a battery surface temperature of 131 ° C. in the short-circuit test. After the short-circuit test, the electrode group was observed by an X-ray CT scan, but no deformation of the electrode group was observed, and no contact point between the positive and negative electrode plates was confirmed.

FIG. 2 shows the short-circuit behavior of a battery using separators B ′ and F having the same 50% shrinkage at the closing temperature as that of the separator A ′.

As can be seen from FIG. 2, it is considered that the separator A ′ has a low air permeability and therefore a large short-circuit current, which also increases the Joule heat, causing rapid heat generation inside the battery, leading to thermal runaway.

As described above, the air permeability is 400 sec / 100
Above 1 cc, air permeability is 1 with a polyethylene microporous membrane
It can be seen that by using a separator having a shrinkage ratio of 50% or less at a closing temperature of 00000 sec / 100 cc, safety in a short circuit is excellent.

Next, Table 3, Table 4 shows, when comparing the discharge capacity of the 10th cycle, the heat treatment is not subjected separator
Data F, separator G, and heat-treated
Regardless, the separator F ′ whose air permeability is out of the range of the present invention,
Discharge capacity of separator G 'is significantly reduced
Was confirmed.

From these results, the heat treatment was applied to the separator.
Air permeability after 400sec / 100cc or more , 1100
sec / 100cc or less , and thermal plugging temperature
It can be seen that it is desirable to use one having a shrinkage ratio of 50% or less.

In the present embodiment, a microporous membrane of polyethylene alone was used as the separator, but similar results were obtained with a polyolefin composite porous membrane containing polyethylene.

[0030]

As is apparent from the above description, the present invention can provide a non-aqueous electrolyte secondary battery excellent in safety at the time of short circuit.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a cylindrical battery according to an embodiment of the present invention.

FIG. 2 is a diagram showing a behavior during a short-circuit test in an example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Positive electrode lead plate 3 Negative electrode 4 Negative electrode lead plate 5 Separator 6 Upper insulating plate 7 Lower insulating plate 8 Case 9 Sealing plate 10 Gasket 11 Positive terminal

────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masaharu Hamada 1006 Kazuma Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-7-29563 (JP, A) JP-A-5-95 310989 (JP, A) JP-A-8-171934 (JP, A) JP-A-4-332479 (JP, A) JP-A-4-237971 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 2/16 H01M 10/40

Claims (1)

(57) [Claims] A non-aqueous electrolyte formed between a positive electrode made of a lithium composite oxide and a negative electrode made of a carbon material, formed of a microporous film of polyethylene alone or a polyolefin composite microporous film containing polyethylene. the a nonaqueous electrolyte secondary battery including the interposed a separator impregnated held spirally wound electrode body, wherein the separators, at a temperature lower than the thermal closure temperature of the microporous membrane The heat treatment is performed , and the air permeability is increased as compared with before the heat treatment, and the air permeability of the separator after the heat treatment is 400 sec / 100.
a non-aqueous electrolyte secondary battery, wherein the shrinkage at the heat closing temperature is 50% or less.