JPH03152881A - Rectangular type lithium secondary battery - Google Patents

Abstract

PURPOSE:To realize an excellent charge and discharge cycle property by making a strip type cathode area larger than a strip type anode area, and placing the cathode peripheral part outer than the anode peripheral part on the opposite side. CONSTITUTION:Strip type positive plates 1 and negative plates 3 are placed face to face through separaters 2 to form a rectangular type of battery. The negative plate 3 area is made larger than the positive plate 1 area while the negative plate 3 peripheral part is placed outer than the positive plate 1 peripheral part on the opposite side. Further preferably it is desirable that the outmost parts have cathodes, which necessitates more cathode sheets by one in number than anode sheets. This prevents dendrite from being produced to improve a charge and discharge property. The anode active materials are expected to be manganese dioxide, etc., chemically stable and reversibly excellent.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to the structure of an organic electrolyte lithium secondary battery, particularly a prismatic lithium secondary battery, as a power source for driving portable electronic equipment.

Conventional technology Lithium secondary batteries with features such as high energy density, excellent storage stability, self-discharge characteristics, and leakage resistance are already available in the form of graphite fluoride/lithium batteries, manganese dioxide/lithium batteries, thionyl chloride/lithium batteries, etc. Batteries are being put into practical use.

On the other hand, with the recent miniaturization and portability of electronic devices,
Batteries used as a power source are required to be smaller and lighter, but on the other hand, conventional secondary batteries do not have sufficient electrical capacity. Expectations are growing for lithium secondary batteries, which can be used over and over again as long as they are used properly.

As for lithium secondary batteries, batteries that use molybdenum disulfide as the positive electrode active material have already been put into practical use, and other batteries that use manganese dioxide, niobium selenide, etc. are also being researched for practical use. There is.

The biggest problem for the practical application of lithium secondary batteries is the formation of resinous lithium (dendrites) on the negative electrode during charging, which may lead to inactivation of the negative electrode, or may penetrate the separator and come into contact with the positive electrode, causing a short circuit. This has a negative effect on the charging and discharging of the battery, and its cycle life is not extended.

The causes of dendrite formation are: 1. 2. The degree of generation varies depending on the type of organic electrolyte. Generated when the charging current density exceeds a certain value; 3. 4. Easily generated when free electrolytes are present. For example, this is likely to occur if the current distribution to the negative electrode lithium differs during charging.

As for the organic electrolyte, a somewhat satisfactory result has been obtained using the combination of solvent and solute that is considered to be optimal. Regarding the charging current density, although the energy density decreases, it can be solved by making the electrode area narrower. Free electrolytes can also be dealt with by increasing the accuracy of battery construction, regulating the amount of liquid, etc.

The problem is how to keep the current distribution constant during charging of the negative electrode lithium.

Current distribution in the strict sense depends on whether the surface condition of the negative electrode is constant, whether it faces the positive electrode properly, and whether it is in tight contact with the separator. In this sense, cylindrical batteries allow thin, large-area positive and negative electrode plates to be tightly stacked and wound with increased tension, so all lithium secondary batteries that have been put into practical use or are close to being put into practical use have a cylindrical structure. We are hiring.

Problems to be Solved by the Invention On the other hand, as a request from manufacturers of devices that use batteries, there is a desire for a battery shape that matches the shape of the device. That is, as devices become thinner and smaller, batteries are also required to be thinner and smaller. Devices are generally rectangular in shape, and in order to effectively utilize the space in the device, it is desirable that the battery also have a rectangular shape.

When using a square-shaped battery case, that is, a rectangular parallelepiped battery case, what kind of structure can be considered for the electrode plate structure to ensure that the positive and negative electrodes face each other properly?1. Wrap the positive electrode plate and negative electrode plate through the separator, and insert them into the battery case while pressing the sides.
2. 3. Fold the group of electrode plates stacked one on top of the other with a separator in between into a folding screen shape and insert into the battery case; 3. One possible method is to cut out one or both of the positive and negative electrodes into strips, wrap them in a separator, stack them alternately, and insert them into the battery case. However, when the wound electrode group 1 is inserted into the case, the degree of compression of the electrodes differs in the vertical and horizontal directions of the case, and the electrolyte absorption state of the electrode plates differs. Dendrites are likely to occur due to liquid accumulation, and good charge/discharge cycle characteristics cannot be expected. Also, when inserting the folding screen-shaped electrode plate in step 2 into the battery case, the bent part of the electrode will not be even.
Similarly good charge/discharge cycle characteristics cannot be expected because liquid accumulates or dendrites are likely to occur because the current density is different from other parts.

Therefore, it can be said that the electrode structure of a prismatic lithium secondary battery has no choice but to have a structure in which strip-shaped electrodes are stacked one on top of the other, increasing the tension of the electrode group. Although a state of high stress or a state of proper electrode configuration is technically possible, there is still one problem. That is, as described above, if the current distribution differs on the negative electrode lithium during charging, dendrites are likely to be generated, but it can be considered that the current distribution is constant in the part where the positive electrode and negative electrode are properly opposed. Therefore, it can be said that the current distribution tends to vary at the ends of the electrodes, that is, at the periphery. In a cylindrical battery, the electrode is long and the peripheral edge is relatively constant, and its area is limited, but when a rectangular battery uses strip-shaped electrodes, the electrodes themselves are small and there are many, so the peripheral area is large. . As a result, dendrites are more likely to occur and the charge/discharge cycle characteristics of the battery may deteriorate.

Means for Solving the Problems The present invention solves the problems described above, and consists of a plurality of facing positive electrodes, a lithium negative electrode, and an organic electrolyte, each of which is inserted alternately into a rectangular battery case. The area of the negative electrode is more than the area of the opposing positive electrode,
The present invention provides a prismatic lithium secondary battery characterized in that the peripheral edge of the negative electrode is always located outside the peripheral edge of the opposing positive electrode.

In order to obtain good charge-discharge cycle characteristics in a functional lithium secondary battery, it is important to prevent dendrites from being generated as much as possible during battery charging. However, when a strip-shaped electrode is used as described above, dendrites are likely to be formed on the periphery of the lithium negative electrode. That is, when strip-shaped positive and negative electrodes are opposed to each other via a separator, the distance from the opposing positive electrode is constant no matter which part of the central part of the negative electrode is picked up, and therefore the current density during charging is also constant. - On the other hand, at the end of the negative electrode, several small strip-shaped positive and negative electrodes of the same size are stacked on top of each other, so there is a misalignment, and the end of the positive electrode protrudes outside the end of the negative electrode. There is also a possibility that there will be dogs. At this time, the end of the negative electrode not only reacts with the opposing positive electrode part, but also preferentially reacts with the positive electrode part protruding outside because it is close in distance, resulting in a high current density and a high possibility of dendrite formation. This results in a shortened charge/discharge cycle life.

In the present invention, in consideration of this, the area of the negative electrode is made in advance to be smaller than the opposing positive electrode, so that there will be no misalignment in the overlapping of the positive and negative electrodes, and even if misalignment occurs, the positive electrode will never overlap the negative electrode. By preventing dendrites from coming out, the purpose is to suppress the formation of dendrites and provide a prismatic lithium secondary battery with good charge-discharge cycle characteristics. This is based on the fact that the reaction of the negative electrode preferentially reacts with the part of the positive electrode that is closest to it, and from this point of view, if multiple positive and negative electrodes are stacked and the positive electrode is placed on the outermost side, the positive electrode The amount of reaction increases in proportion to the increase in thickness up to a certain thickness, so in order to not increase the current density of the reaction on the opposing surface of the inner negative electrode, the thickness of the outermost positive electrode must be increased. 1/ of the thickness of the positive electrode in
However, in the case of lithium negative electrodes, the current density decrease is not a problem, so When using stacked rectangular electrodes in a prismatic lithium secondary battery, it is desirable that the negative electrode be placed on the outermost side. The details will be explained below in Examples.

Example FIG. 1 is a structural diagram of a battery in an embodiment of the present invention.

In Fig. 1, 1 is a positive electrode plate, which includes manganese dioxide as a positive electrode active material, carbon powder as a conductive material, and poly 4 as a binder.
Aqueous dispersion of fluorinated ethylene at a weight ratio of 10
The mixture was kneaded into a paste at a ratio of 0 to 0.0 nia, applied to both sides of an aluminum foil with a thickness of 30 μm, dried, rolled, and cut into a predetermined size. Among these materials, poly 4
The weight percentage of 7-isoethylene is calculated as the solid content in the dispersion. Also, the size of the electrode is 12.5X
It has a thickness of 4811 ff and a thickness of 0.3 mm. The theoretical amount of electricity charged in one positive electrode is 120 mAh assuming that manganese dioxide performs a monovalent reaction. 2 uses a porous polypropylene film as a separator. 3 is a lithium negative electrode, which has a thickness of 14×501 RM and a thickness of 0.18 MM. Theoretically, the amount of electricity charged in one negative electrode is 26 ohm. The positive and negative electrode configurations are 6 positive electrodes and 6 negative electrodes, so the overall battery capacity is 600 mAh for the positive electrode and 1500 mAh for the negative electrode, but assuming that the outermost lithium reacts only on one side, it is 12e.
It becomes iOmAh.

Further, the positive electrode is actually charged and discharged at a valence of about 0.4. After inserting these electrode groups into an iron-nickel plated battery case 8 with a polypropylene insulating plate 10 laid on the bottom, the titanium glue 4 taken out from each positive electrode and bundled is placed on a stainless steel sealing plate 5. glass seal 6
Spot welding is performed to the embedded hermetic terminal 7 via the . In addition, nickel negative 11J- was taken out from each negative electrode.
The cables are bundled and spot welded to the case 8. After these operations, an electrolyte containing lithium hexafluorophosphate (LiPF6) dissolved in propylene carbonate at a ratio of 1 mol/l is injected, the sealing plate 6 is fitted into the case 8, and the surrounding area is laser welded to form a completed battery. do. The finished dimensions of this battery are 4x16x60ff excluding the positive terminal. This battery will be referred to as battery 11.

The material, battery configuration, and manufacturing method shown in Figure 2 are exactly the same as the battery manufacturer, but only the size of the positive electrode plate is 14 x 5.
It has the same size as 0ffl11 and the negative electrode. The thickness is 3mttt, same as the battery man. Therefore, the theoretical amount of electricity charged in the positive electrode is much higher than that of the battery, and as a monovalent reaction, the total amount of electricity in the battery is 670 mAh. This battery is battery B
shall be.

What is shown in Figure 3 is that the size of the positive and negative electrodes is exactly the same as that of battery B, but the configuration is different from that of battery B.
This is the reverse. That is, as seen in FIG. 3, the positive electrode is located on the outside of the electrode plate group, and as a result, there are six positive electrodes and six negative electrodes. The theoretical charging amount of electricity is 800 mAh for the positive electrode and 1250 mAh for the negative electrode. However, assuming that the two outermost positive electrodes work only on one side, the amount of electricity charged in the positive electrodes will be 670 mAh. This battery will be referred to as battery C. These batteries were charged at 20°C with a charging current of 20mA.
．． 2. up to 8V with a discharge current of eomA. Charge and discharge were repeated in the voltage range up to OV. FIG. 4 shows the relationship between the discharge capacity and the number of cycles at that time.

As is clear from FIG. 4, the battery according to the present invention exhibits excellent charge-discharge characteristics. On the other hand, batteries B and C exhibit a larger discharge capacity than the battery, but variations in discharge capacity are observed in battery B at around 8o psi and in battery C at around 50 cycles, after which charging and discharging becomes impossible. When these batteries were disassembled and observed, it was found that in both batteries B and O, dendrites were generated in some places around the periphery of the negative electrode lithium, which caused a short circuit in the batteries.

Moreover, in Battery C, this phenomenon was particularly remarkable in the outermost negative electrode lithium. In contrast, almost no dendrites were observed even when the battery was disassembled in the battery man. Therefore, the reason why the capacity of battery A decreased after 200 cycles was not due to a short circuit in the battery, but because the charging and discharging efficiency of the negative electrode lithium was
This is thought to be because it is about 98%.

Effects of the Invention As is clear from the above, in the prismatic lithium secondary battery according to the present invention, a rectangular electrode is used, and the peripheral edge of the negative FMIJ lithium is always located outside of the opposing positive electrode. By configuring the positive and negative electrodes as shown, when charging the battery, the charging current on the negative electrode lithium facing the positive electrode can be kept constant, and the current on the lithium outside the positive electrode can be kept below that level. This has the effect of suppressing the generation of dendrites in the negative lithium electrode and producing a prismatic lithium secondary battery with good charge-discharge cycle characteristics. Note that this good property is due to the composition of the positive electrode and negative electrode, and is not restricted by the positive electrode combined with negative electrode lithium, but it goes without saying that as a positive electrode active material, it is chemically stable and reversible. A material with excellent properties is desirable, and in this sense, manganese dioxide, vanadium oxide, titanium disulfide, molybdenum sulfide, LiCoO2, LiMn2O4, etc. are suitable.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the structure of a battery according to an embodiment of the present invention, FIGS. 2 and 3 are cross-sectional views of comparative batteries, and FIG.
The figure is a diagram showing the relationship between discharge capacity and cycle number. 1...Positive electrode plate, 2...Separator, 3
...Negative electrode plate, 4...Positive 1i 1J-de, 5... Sealing plate, 6... Glass seal 7... Positive electrode terminal , 8...Battery case,
9...Negative electrode lead, 1o...Insulating plate.

Claims (3)

[Claims] (1) In a battery consisting of a plurality of opposing positive electrodes, a lithium negative electrode, and an organic electrolyte that are inserted alternately into a rectangular battery case, the area of each negative electrode is larger than the area of the opposing positive electrode, and the negative electrode A prismatic lithium secondary battery characterized in that the periphery of the electrode is located outside the periphery of the opposing positive electrode. (2) The prismatic lithium secondary battery according to claim 1, wherein the number of negative electrodes is one more than the number of positive electrodes. (3) The prismatic lithium according to claim 1 or 2, wherein the active material of the positive electrode is one selected from the group of manganese dioxide, vanadium oxide, titanium disulfide, molybdenum sulfide, LiCoO_2, and LiMn_2O_4. Secondary battery.