GB2160705A

GB2160705A - Nonaqueous electrolyte cell

Info

Publication number: GB2160705A
Application number: GB08515550A
Authority: GB
Inventors: Shinji Soh; Satoshi Narukawa; Tooru Amazutsumi
Original assignee: Sanyo Electric Co Ltd
Current assignee: Sanyo Electric Co Ltd
Priority date: 1984-06-22
Filing date: 1985-06-19
Publication date: 1985-12-24
Also published as: FR2566587A1; CA1248173A; JPH0560233B2; DE3522261A1; GB2160705B; DE3522261C2; FR2566587B1; JPS618852A; GB8515550D0

Abstract

A nonaqueous electrolyte cell comprising a negative electrode which contains as active material a light metal such as lithium or sodium or an alloy of these metals. The cell comprises a positive electrode of a material with the apparent volume thereof increased with a discharge reaction e.g. carbon fluorides, silver chromate, MnO2, metal sulphides, a separator of a microporous resin film disposed on the surface of either electrode facing the other electrode, and an electrolyte layer formed between the above-described other electrode and the separator.

Description

SPECIFICATION Nonaqueous electrolyte cell BACKGROUND OF THE INVENTION The present invention relates to a nonaqueous electrolyte cell having a positive electrode of a material with the apparent volume thereof increased with the discharge reaction.

There are materials with the apparent volume thereof increased with the discharge reaction, e.g., carbon fluorides, silver chromate and manganese dioxide as disclosed in Japanese Patent Publication 54-35,653 published Nov. 5, 1979 and metal sulphides (such as FeS, FeS2, CuS, Cu2S) or copper oxide and bismuth oxide, as disclosed in Japanese Laid Open Patent Publication No. 57-174871, published oct. 27, 1982. Where such materials are used as positive electrode active material for a nonaqueous electrolyte cell, the following problem arises.

As the separator for this type of cell, unwoven fabric of polypropylene is generally used as disclosed in the literatures described above. However, as the volume of the positive electrode is increased with the discharge reaction, the separator of polypropylene unwoven fabric disposed between the positive and negative electrodes is compressed to squeeze out the retained electrolyte from the separator.

Consequently, a local portion of polypropylene unwoven fabric where there is substantially no electrolyte contained therein is present between the positive and negative electrodes, so that the internal resistance is sharply increased, resulting in deterioration of cell characteristics.

SUMMARY OF THE INVENTION An object of the present invention is to provide an improved nonaqueous electrolyte cell which has desirous discharge voltage characteristics, without a sharp increase of the internal resistance.

Another object of the present invention is to provide an improved nonaqueous electrolyte cell which can suppress a sharp increase of the internal resistance thereof, the cell having a positive electrode with the apparent volume thereof increased with the discharge reaction in the process of discharge.

A further object of the present invention is to afford a new nonaqueous electrolyte cell which provides improved cell characteristics with a stable internal resistance and a discharge voltage.

According to the present invention, there is provided a nonaqueous electrolyte cell, which comprises a negative electrode containing as active material a light metal such as lithium or sodium or an alloy of these light metals, and a positive electrode consisting substantially of a material with the apparent volume thereof increasing by a discharge reaction. The cell has a separator formed substantially of a microporous resin film on the surface of either electrode facing the other electrode, and an electrolyte layer formed between the abovedescribed other electrode and the separator.

In the nonaqueous electrolyte cell according to the present invention the apparent volume increase of the positive electrode caused with the discharge reaction is substantially absorbed by the electrolyte layer. In addition, since the separator provided between the positive and negative electrodes is formed of a very thin synthetic resin film or films, even if the volume of the positive electrode is increased until the positive and negative electrodes are in contact with the separator where there is locally substantially no electrolyte present, the distance between the positive and negative electrodes is very small, and the internal resistance will never be sharply increased.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view of a nonaqueous electrolyte cell according to the present invention; Figure 2 is a sectional view of a comparative cell which is known in the art; Figure 3 is a graph showing cell voltage characteristics and internal resistance characteristics plotted relative to a discharge time, with respect to the inventive cell of Fig. 1 and the comparative cell of Fig. 2; and Figure 4 is a sectional view of a cell according to another embodiment of the invention.

PREFERRED EMBODIMENT OF THE INVEN TION Referring to Fig. 1 of the drawing, a cell has a positive electrode 1, a negative electrode 2, a separator 3 and an electrolyte layer 4. The positive electrode 1 is obtained by adding 10 wt% of graphite as electrical conductor and 5 wt% of fluorine resin powder as binder to 85wt% of iron disulfide (FeS2) as active material, press molding the mixture with a pressure of 2 tons/cm2 to produce pellets with a diameter of approximately 11.0 mm and a thickness of approximately 1.8 mm, and sintering the pellets at a temperature of 200 to 300 C. The negative electrode 2 is a stamped lithium sheet with a diameter of approximately 7.5 mm and a thickness of approximately 2.2 mm. The separator 3 is a stamped microporous polypropylene film with a diameter of approximately 11.0 mm and a thickness of approximately 0.025 mm.The electrolyte layer 4 is formed by filling the space between the negative electrode 2 and the separator 3 with liquid electrolyte. The microporous resin film may be a polyethylene film instead of the polypropylene film as in the above embodiment.

An assembly operation of the nonaqueous electrolyte cell illustrated in Fig. 1 will be explained. First, the lithium negative electrode 2 is press bonded to a negative electrode collector 7, which is secured to the inner surface of a seal cap 6 also serving as a negative electrode terminal with an annular insulating gasket 5 provided along the edge by insert molding, so that the lithium negative electrode 2 will not fall when the cell is inverted. At this time, a space is defined by the lithium negative electrode 2 and the annular insulating gasket 5.

Meanwhile, the positive electrode 1 is held in forced contact with a positive electrode collector 9 secured to the inner surface of a container 8 also serving as a positive electrode terminal. Then, the separator 3 is placed in position on the positive electrode 1. In this state, the seal cap 6 is fitted in the open top of the container 8.

The battery thus assembled is then put in a sealed vessel (not shown and the sealed vessel is vacuumized. Then it is immersed in an electrolyte which is obtained by dissolving 1 mole/l of lithium tetrafluoroborate in a mixed solution of propylene carbonate and 1,2-dimethoxyethane to fill the space described above with the electrolyte, thus forming the electrolyte layer 4. Thereafter, the open edge of the container 8 is sealed on the insulating gasket 5 to complete the cell.

Fig. 2 is a sectional view showing a comparative cell which has a generally known structure. The cell illustrated in Fig. 2 has no electrolyte layer and the separator 13 thereof is different from the separator 3 of the cell of the invention shown in Fig. 1. The separator 13 in the comparative cell consists of an unwoven fabric of polypropylene with a thickness of approximately 0.5 mm.

Fig. 3 shows the voltage and internal resistance of the cell of Fig. 1, indicated at "A" and the comparative cell of Fig. 2, indicated at "B" plotted relative to the time of discharge under a constant load of 5.6 K52 at a temperature of 20 C.

As is apparent from Fig. 3, the cell "A" according to the present invention provided a constant internal resistance and no sharp increase in the internal resistance was examined. Thus, desirable discharge voltage characteristics were obtained. By contrast, the comparative "B" showed that the internal resistance was abruptly and sharply increased in a certain stage of the discharge and, therefore, the cell "B" has a "two-stage" discharge voltage characteristic as shown.

The "two-stage" characteristic of the comparative cell "B" is considered to stem from the following ground. With the process of the discharge, the electrolyte that has been held by the polypropylene unwoven fabric as separator is squeezed out and, consequently, the separator has a portion or portions which include substantially no electrolyte between the positive and negative electrodes. This portion of the polypropylene unwoven fabric, which has a considerable thickness, will function as a sort of insulator to sharply increase the internal resistance. As the discharge proceeds further, however, the thickness of the polypropylene unwoven fabric is reduced, so that the internal resistance increase curve becomes much gentler and the discharge proceeds with a low cell voltage.

With the cell "A" according to the invention, which uses the thin microporous film as separator, even when the volume of the positive electrode is increased, the distance between the positive and negative electrodes is small, so that the internal resistance is not increased rapidly. Further, although the microporous polypropylene film has less liquid holding capacity compared to the polypropylene unwoven fabric, this gives rise to no problem with the cell "A" since the electrolyte layer consisting of the electrolyte only is formed between the separator and the negative electrode.

Fig. 4 shows a modified structure of the nonaqueous electrolyte cell according to the present invention. In the embodiment of Fig.

4, the separator 3 has three separator elements and the positive electrode 1 has a conductive ring 10 therearound. A first or lower separator element 3A, which is placed on the positive electrode 1 with the ring 10 is made of a microporous polyethylene film having a thickness of 0.05 mm. A desired microporous polyethylene film for the separator element 3A is "HIPORE 3050" produced by Asahi Kasei Co., Ltd., Tokyo, Japan, which has desired characteristics for holding the electrolyte, that is, 500% and more, and provides excellent extensibility toward the negative electrode 2 when the volume of the positive electrode 1 is increased. Although the first separator element 3A has two "HIPORE 3050" films simply superposed in the illustrated embodiment, these films can be lami nated together.

Above the first separator element 3A is disposed a second or upper separator element 3B contacted with a lower surface of the electrolyte layer 4 with a space therebetween.

The second separator element 3B is a micro porous polypropylene film having a thickness of 0.03 mm, and for this purpose "DURA GARD 2400" produced by Asahi Kasei Co., Ltd. is found to be desirable. The second separator element 3B of "DURAGARD 2400", which provides lower property for holding the electrolyte but has smaller pore opening than the first separator element 3A of "HIPORE 3050", can prevent the positive electrode powder from adhering to the lithium surface of the negative electrode 2, and consequently cell characteristics are improved.

A third or middle separator element 3C is disposed in the space confined between the first or lower separator element 3A and the second or upper separator element 3B in a closely contacted relation to form a threelayered separator 3. The third separator element 3C is a microporous polyethylene film having a thickness of 0.1 mm, and for this purpose "HIPORE 2100" produced by Asahi Kasei Co., Ltd. is desirable. The third separator element 3C of "HIPORE 2100" has an excellent property of holding the electrolyte, but may be omitted if necessary.

Other elements and structure may be considered to be substantially similar to those of the embodiment of Fig. 1, and a detailed description is omitted.

The cell according to the present invention has a separator of a microporous resin film or film and an electrolyte layer consisting of an electrolyte only between the separator and either electrode, while the positive electrode consists of a material with the apparent volume thereof increased by the discharge reaction. Thus, it is possible to suppress a sharp increase of the internal resistance with the progress of the discharge and provide a flat or constant discharge voltage characteristic, which is remarkably beneficial in industries.

The aforementioned Japanese Laid-Open Patent Publication No. 57-174871 discloses formation of an electrolyte layer between a separator and a negative electrode. In this case, however, the separator consists of an unwoven fabric of polypropylene. Therefore, the internal resistance will increase sharply like the comparative cell which has been described above with reference to Fig. 2.

Although the present invention has been described with reference to the preferred embodiments thereof, many modifications and alterations can be made within the spirit of the invention.

Claims

1. A nonaqueous electrolyte cell comprising a negative electrode containing an active material of at least a single light metal or its alloy, a positive electrode composed of a material with an apparent volume thereof increased with a discharge reaction, a separator having a microporous resin film disposed on a surface of one of the positive and negative electrodes, and an electrolyte layer formed between said separator and an other one of the positive and negative electrodes.

2. A nonaqueous electrolyte cell according to claim 1, wherein said separator is disposed on the surface of said positive electrode and said electrolyte layer is formed between said separator and said negative electrode.

3. A nonaqueous electrolyte cell according to claim 1, wherein said microporous resin film is a polypropylene film.

4. A nonaqueous electrolyte cell according to claim 1, wherein said separator comprises a first separator element of a polyethylene film and a second separator element of a polypropylene film, and wherein said first separator element is closely contacted with said positive electrode and said second separator is closely contacted with said electrolyte layer.

5. A nonaqueous electrolyte cell according to claim 4, further comprising a third separator element of a polyethylene film between said first separator element and said second separator element in a closely contacted threelayer structure.

6. A nonaqueous electrolyte cell according to claim 4, wherein said first separator element consists of two polyethylene films closely contacted together.