JPH02174078A - Organic electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To prevent the deterioration in battery characteristic due to a long- term overdischarge at a high temperature by using stainless steel containing specific quantities of molybdenum and chromium respectively for the sealing plate of a battery. CONSTITUTION:A case 1 concurrently serving as a positive electrode terminal is made of stainless steel with excellent corrosion resistance, and a sealing plate 2 concurrently serving as a negative electrode terminal is made of stainless steel containing molybdenum 1-3wt.% and chromium 15-18wt.% as the material. A positive electrode active material made of a metal oxide forming an inter- layer compound with lithium ions, a negative electrode active material, and an organic electrolyte are provided. The deterioration of a battery due to the corrosion of the sealing plate 2 can be suppressed ever in long-term overdischarge.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an organic electrolyte secondary battery used as a mobile DC power source, a backup power source, or the like.

Conventional technology In general, organic electrolyte batteries have high energy density, can be made smaller and lighter, and are less prone to leakage, so they are used as a power source for precision equipment such as small calculators and electronic watches in place of other batteries. Widely used.

These organic electrolyte batteries include primary batteries and rechargeable secondary batteries. As a rechargeable secondary battery, the negative electrode is a metal oxide that has formed an intercalation compound with lithium, such as niobium pentoxide, and the positive electrode is a metal oxide that can form an intercalation compound with lithium at a higher potential than the negative electrode, such as vanadium pentoxide. It is also known to use an organic electrolyte solution in which a lithium salt is dissolved in an organic solvent. The reason why metal oxides capable of forming intercalation compounds with lithium are used for both the positive and negative electrodes is as follows.

That is, in general, when only metallic lithium is used for the negative electrode, lithium ions in the electrolyte are deposited on the lithium surface in the form of branches during charging, which penetrate the separator and come into contact with the positive electrode, causing an internal short circuit. As a result, the battery deteriorates and its cycle life is significantly reduced. On the other hand, if a metal oxide that has formed an intercalation compound with lithium, such as niobium pentoxide, is used for the negative electrode, even after repeated discharging and charging, the negative electrode will undergo a repeated reaction of releasing lithium ions and forming an intercalation compound. By simply performing this, the above-mentioned precipitation of lithium on the negative electrode surface does not occur. Further, this reaction is generally called a topochemical reaction, and the crystal structure and volume expansion of niobium pentoxide hardly change due to the addition and removal of lithium. Furthermore, if a metal oxide with similar properties to the negative electrode, such as vanadium pentoxide, is used for the positive electrode, there will be little deterioration of the positive and negative electrodes even after repeated charging and discharging, and lithium consumption will be minimal. Who? As a result, the lithium secondary battery has an extremely excellent cycle life.

Problems to be Solved by the Invention In this type of battery, it is difficult to extract high voltage. This is due to the use of metal oxide for the negative electrode. In other words, metal oxides are generally used as positive electrodes in primary batteries, and their potential is a liability. Batteries with higher energy density are better and have greater industrial value. Since energy density is proportional to battery voltage, higher voltage is advantageous. Therefore,
In order to obtain a high battery voltage in this type of battery, a material with a higher potential than the negative electrode is used for the positive electrode.

As a result, similar to organic electrolyte secondary batteries with high voltage, the case that is in electrical contact with the positive electrode either directly or through a lead is made of a material with excellent corrosion resistance that has a higher potential than the positive electrode active material. is used.

This is to prevent the case from corroding due to high potential while the battery is being stored, resulting in deterioration of battery performance. The specific case material used is stainless steel, which contains a lot of chromium, which is effective in corrosion resistance, and contains almost no nickel. On the other hand, the sealing plate, which is in electrical contact with the negative electrode either directly or through a lead, has a much lower potential than the positive electrode, so corrosion resistance does not need to be given as much consideration as the positive electrode case; Stainless steel, which is often used in general organic electrolyte secondary batteries, was used, for example, JIS standard (7) SUS304, which contains about 18% Cr by weight and 8% N1 by weight.

As a result of various characteristic evaluations of this conventional organic electrolyte secondary battery, we found that the battery was kept in a discharged state (the battery voltage was It has been found that when the battery is maintained at approximately ○V (hereinafter referred to as long-term overdischarge), the recovery performance upon charging deteriorates and the battery performance significantly deteriorates. As a result of disassembling and analyzing this deteriorated battery, it was found that the surface of the sealing plate in contact with the negative electrode active material corroded, and dissolved metal precipitated on the surface of each component material due to the difference in ionization tendency. As a result, it was found that the reaction of each active material was significantly impaired and the battery deteriorated. This corrosion is due to the following reasons.

That is, when the battery is discharged, the unipolar potentials of the positive and negative electrodes are as shown in FIG. As the discharge progresses, the potential of the positive electrode hardly changes, but the potential of the negative electrode gradually rises, and at the end of the discharge, it reaches a very high potential state like the positive electrode. In some cases, the polarity may be reversed, and the negative electrode may have a higher potential than the positive electrode. Therefore, if such a state continues due to long-term overdischarge, it is considered that the sealing plate made of 5US304, which has low corrosion resistance, will corrode. It was also found that this phenomenon is more pronounced at higher temperatures.

Generally, in organic electrolyte secondary batteries exhibiting high voltage, the electrolyte is consumed and depleted at the end of discharge, so corrosion of the sealing plate does not occur even if the potential of the negative electrode increases. Therefore, the above-mentioned problem is a problem specific to secondary batteries that use an active material having a very negative potential in the positive electrode.

An object of the present invention is to improve the deterioration of a battery due to long-term overdischarge described above.

Means for Solving the Problem In order to solve this problem, it is necessary to use a material with excellent corrosion resistance as well as the case for the constituent members of the sealing plate that electrically contact the negative electrode active material.

Specifically, the sealing plate needs to be made of chromium steel, which is a component of the case described above and contains almost no nickel. However, steel materials containing large amounts of chromium have the disadvantage of being hard and having poor workability. In particular, sealing plates are unsuitable because the structure around them is complicated. Regarding corrosion resistance, the addition of a small amount of molybdenum along with chromium has a large effect, so as long as the molybdenum is contained in a numerical coefficient and the chromium content is about 15% or more, there is almost no problem in corrosion resistance even if some nickel is present. There was found. Furthermore, since the content of chromium can be lowered, the hardness is also lowered and the workability is also good.

In light of the above, the present invention is characterized by using stainless steel containing 1 to 3% by weight of molybdenum and 15 to 18% by weight of chromium as the sealing plate material.

Effect: According to this configuration, in an organic electrolyte secondary battery using a material having a very negative potential as a positive electrode active material, deterioration of the battery due to corrosion of the sealing plate can be suppressed even after long-term overdischarge. Can be done.

Example The present invention will be explained below with reference to figures and tables.

FIG. 2 is a sectional view of the organic electrolyte secondary battery of the present invention.

Reference numeral 1 in the figure denotes a case which also serves as a positive terminal, and is made of the aforementioned stainless steel with excellent corrosion resistance.

Reference numeral 2 denotes a sealing plate which also serves as a negative electrode terminal and is made of the composition according to the present invention described above. An example of its composition in weight percentage is as shown in Table 1, which corresponds to 5US444 of stainless steel in the JIS standard. 3 is a polypropylene gasket that insulates the case and the sealing plate, 4 is a positive electrode, which is made by mixing vanadium pentoxide, acetylene black, which is a conductive material, and 7-based resin powder, and press-molding it into a pellet. Dry under vacuum at 0C for 12 hours.

5 is a negative electrode, in which niobium pentoxide, acetylene plaque as a conductive material, and fluororesin powder were mixed, formed into a pellet shape in the same way as the positive electrode, and dried under vacuum at 200° C. for 12 hours. 6 is a separator made of polypropylene nonwoven fabric, and 7 is metal lithium, which is located between the negative electrode 5 and the separator 6. After forming the battery, this lithium comes into contact with the negative electrode (
(short circuit), an intercalation compound is formed with niobium pentoxide, which is the negative electrode active material. Reference numerals 8 and 9 represent current collectors for the positive and negative electrodes, respectively, which are conductive carbon films. Also electrolytic ild
, 1 mol/l of lithium perchlorate was added to an equal volume mixed solvent of propylene carbonate and 12-dimethoxyethane.
A solution dissolved at the following ratio was used.

This battery was made into a person. For comparison, the JIS standard S
US 304 stainless steel (composition shown in table middle)
A battery with a conventional configuration using a sealing plate made of B was designated as B. Further, a battery having a configuration using a sealing plate made of stainless steel having the composition shown in C in the table was designated as C.

Each battery has a diameter of 20 mm, a thickness of 2.5 mm, and a capacity of approximately 20 mAh in the range of 2 V to 1 V.

(Left below) Characteristics of these batteries were measured at room temperature immediately after assembly and after two months of constant resistance discharge of 10Ω in an atmosphere at 60° C., and are shown in Table 2. In addition, the discharge capacity of the battery is 2V at a voltage of 2V.
After performing constant voltage charging for 4 hours, constant resistance discharge of 10 Ω was performed, and the capacitance was measured with a 1 V cut.

(Hereafter, extra white) Moreover, each value is an average value of 20 batteries.

As is clear from the table, the battery of the present invention using a sealing plate made of stainless steel containing chromium and molybdenum has no abnormal increase in internal resistance compared to conventional product B even after long-term overdischarge at high temperatures. Moreover, by charging, it is possible to obtain a discharge capacity equivalent to that at the initial stage. Further, after the above tests, batteries B and Q were disassembled and investigated, and the inner surfaces of the sealing plates of batteries B and C were found to be corroded. However, no corrosion of the sealing plate was observed for the battery man. Furthermore, from the comparison between Battery Man and C, it can be seen that the addition of molybdenum has a large effect on corrosion resistance.

Next, Figure 3 shows the degree of corrosion when steels with different contents of molybdenum and chromium are stored in the above-mentioned electrolyte in an atmosphere of 7oC for one month with a voltage of 2.5V applied to lithium. show.

In FIG. 3, the amount of molybdenum added is shown as 0 to 1.6% by weight, but if it exceeds 3% by weight, the steel becomes extremely brittle and processing such as rolling of the steel material becomes extremely difficult. As a result, the preferred amount of molybdenum added was 1 to 3% by weight.

The steel used in this test was commercially available stainless steel or prototype steel, and the amount of impurities other than chromium and molybdenum, such as manganese, carbon, nickel, and licon, was small. not the same,
They are slightly different.

Effects of the Invention As is clear from the above explanation, the battery sealing plate contains a highly corrosion-resistant material similar to the case constituent material, particularly molybdenum in an amount of 1 to 3% by weight, and chromium in an amount of 15 to 18% by weight.
By using stainless steel containing stainless steel, it is possible to prevent deterioration of battery characteristics due to long-term overdischarge, especially long-term overdischarge at high temperatures.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the potential change of each single pole of the battery during constant resistance discharge, Fig. 2 is a vertical cross-sectional view of the battery in the embodiment of the present invention, and Fig. 3 is the test result of the battery in the embodiment of the present invention. FIG. 1...Case, 2...Sealing plate, 3...River/Gasket, 4...Positive electrode, 6...Negative electrode, 6...Mark/Severe... -ta, 7...Lithium, 8...Positive electrode current collector, 9...Negative electrode current collector. Name of agent Patent attorney Shigetaka Awano 1 hand drawing 1-Case 2--3-J'ske on one stroke, Y 6'--Biha'L-Y7- Lithium

Claims (2)

[Claims] (1) Molybdenum is used as a sealing plate component that has a positive electrode active material and a negative electrode active material made of a metal oxide capable of forming an interlayer compound with lithium ions, and an organic electrolyte, and is in direct or indirect contact with the negative electrode active material. An organic electrolyte secondary battery using steel containing 1 to 3% by weight of chromium and 15 to 18% by weight of chromium. (2) The organic electrolyte secondary battery according to claim 1, wherein the positive electrode active material and the negative electrode active material are vanadium pentoxide and niobium pentoxide, respectively.