JPH01157068A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To make it resistible to over charge as well as to make improvements in cycle life by using niobium pentoxide mainly for a positive electrode and a lithium occlusion alloy for a negative electrode respectively, and specifying a filling quantity of lithium and a quantity of the niobium pentoxide. CONSTITUTION:In a charging state, a filling quantity of lithium in a lithium occlusion alloy of a negative electrode 6 is set to less than 8% of weight of niobium pentoxide for a positive electrode 4, and a lithium quantity is set to more than 1wt.% to the lithium occlusion alloy. With this constitution, when the niobium pentoxide is discharged, battery voltage is zeroed by a potential rise in the negative electrode before it goes down to 1V with pure lithium. Consequently, even if over charge takes place, electric potential of the niobium pentoxide will not come to less than 1V to the pure lithium, and such a fear that an electrolyte is decomposed and the niobium pentoxide is broken is brought to nothing. Accordingly, it is resistible to the over charge and, what is more, cycle life is thus improved.

Description

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

BACKGROUND OF THE INVENTION Secondary batteries using lithium as a negative electrode are expected to have high energy density and high reliability, and have been developed by many research institutes in recent years. For example, many materials are being considered as positive electrodes, including vanadium oxide, chromium oxide, manganese oxide, chalcogen compounds such as titanium disulfide and molybdenum disulfide, and conductive polymers such as polyacetylene and polyaniline. .

In addition, if lithium alone is used as the negative electrode, the charge/discharge cycle life will be significantly degraded due to dendrites generated during charging/discharging, so aluminum alloys that occlude lithium, lead, or bismuth are recommended.

Fusible metals such as cadmium, indium, and tin have been tried.

Problems to be Solved by the Invention When discharging using niobium pentoxide as the positive electrode, pure lithium as the counter electrode, and a non-aqueous solvent in which lithium perchlorate is dissolved as the electrolyte, the discharge starts at around 2V, and the voltage is 1. The discharge mode is such that the discharge occurs linearly and gently until it reaches around 0V, and then after a while it descends in a sharp curve to around oV. Then, at around 1.5 cm, if the non-aqueous solvent is, for example, glopylene carbonate, it decomposes and generates gas. Furthermore, near oV, the crystals of niobium pentoxide are destroyed and become fine powder, and even if the battery is charged, it will not return to its original state. In addition to these, when niobium pentoxide is used for the positive electrode, it is susceptible to overdischarge of 1 V or less, and drastic measures against overdischarge are required.

The present invention aims to solve the above problems in systems using niobium pentoxide (Nb2o6) as a positive electrode and a lithium storage alloy as a negative electrode, and to provide a practical lithium secondary battery.

Means for Solving the Problems In the present invention, in order to solve the above problems and improve the charge/discharge cycle life, various studies were conducted, and it was found that a lithium storage alloy was used for the negative electrode, and the amount of lithium filled was increased by increasing the amount of lithium pentoxide. It has been found that the amount of lithium should be 8% or less of the weight of niobium, and the amount of lithium should be 1 weight or more with respect to the lithium storage alloy.

Function The idea of this invention is to regulate the amount of lithium on the negative electrode side,
When niobium oxide is discharged, the battery voltage becomes Ov due to an increase in the potential of the negative electrode before reaching 1V relative to pure lithium. In this way, no matter how much it is over-discharged, the potential of niobium pentoxide will be 1v compared to pure lithium.
The electrolyte will not decompose, and the electrolyte will not decompose.
Moreover, niobium pentoxide is not destroyed. The reaction of niobium hexoxide is generally considered to be a two-electron reaction as shown below.

265.8y per mole of Nb206, and according to the above formula, the electric capacity per 91 mole is 26.8 X 2 = 53.6
Ah, and the theoretical capacitance per unit weight of Nb2O5 is approximately 201°6 mAh/y. On the other hand, the theoretical electric capacity per unit weight of lithium is approximately 386
1.7mAh/y, and the equivalent electric capacity weight ratio is as follows.

Nb2O6: Li = 100: 5.2 From this, theoretically, it is sufficient that the lithium amount is 6.2 inches or less. However, in reality, lithium is consumed by trace amounts of water contained in the electrolyte, water in the positive electrode mixture, and conductive agents, and a considerable amount of lithium also penetrates deeply into the negative electrode alloy and does not participate in the reaction. . Regarding this, experimentally, the appropriate amount of lithium is 8% by weight or less of niobium pentoxide.

In addition, the negative electrode alloy is an aluminum alloy.

There are fusible alloys such as lead, cadmium, and indium alloys, but in each case, even if lithium is occluded and discharged, some lithium remains in the alloy and does not come out. This amount varies considerably depending on the conditions, but in the case of long-term overdischarge, it is generally within about 1 inch, and is simply a battery voltage of 1 V.
If discharged to a maximum of 1,000,000 yen, there may be more than a few.

Furthermore, if the amount of lithium in the alloy is less than 1% by weight, over-discharge over a long period of time may cause the electrolyte to decompose and generate gas. This is thought to be due to the small amount of lithium in the alloy itself dissolving a small amount and causing side reactions.

From these facts, a lithium secondary battery that is resistant to overdischarge can be produced by setting the amount of lithium filled to 8% by weight or less of niobium hexoxide and 1% by weight or more of the lithium storage alloy.

The example diagram is a longitudinal sectional view of a lithium secondary battery of the present invention using niobium pentoxide for the positive electrode and a lithium storage alloy for the negative electrode. In the figure, 1 is a case that also serves as a positive electrode terminal, 2 is a sealing plate that is punched out of the same material as the case and also serves as a negative electrode terminal, and 3 is a polypropylene gasket that insulates the case and the sealing plate. 4 is a positive electrode, which is made by kneading 86 parts by weight of niobium pentoxide, 6 parts by weight of carbon black as a conductive agent, and 10 parts by weight of a solid content of fluororesin dispersion, drying and pulverizing it to make niobium pentoxide to 260 tnLj. It was weighed and molded into pellets with a diameter of 14.0M. 6 is a positive electrode current collector made of a carbon coating layer. 6 is a lithium storage alloy, which has a diameter of 14.0mm and has a diameter of 260mm including lithium.

7 is a negative electrode current collector, which is a 60-mesh stainless steel net with a wire diameter of 0.1 m+11. 8 is a separator made of a microporous membrane made of polypropylene.

The electrolytic solution used was one in which 1 mol/l of lithium perchlorate was dissolved in a mixture of propylene carbonate and 1,2-dimethoxyethane at a volume ratio of 1:1.

Now, the negative electrode alloy composition was adjusted as shown in Table 1.

In addition, lithium was occluded into the alloy by pressing a desired amount of lithium foil onto the alloy and charging it in an electrolyte.

Table 1 Using these negative electrodes, a battery with a diameter of 20 m and a thickness of 2.0 m was fabricated. battery! is in Table 1! corresponds to

These batteries were continuously overdischarged for 10 days at 60''C under a constant resistance load of 3Ω, and the internal resistance of the batteries at 1 KHz and the change in the height of the batteries were measured.

After that, charge it at 1 mA until it reaches 2V, and then discharge it for 3 hours at 1 mA current (but cut off 1V).
Charging and discharging at the same current for 3 hours was repeated 300 times, and discharged again from 2V to 1V at 3Ω to examine the rate of deterioration from the initial stage. These results are shown in Table 2.

As is clear from Table 2, when the lithium concentration exceeds 10 g (jσ6.12), the battery swells abnormally, the internal resistance increases significantly, and charging and discharging becomes impossible. This is considered to be because the amount of lithium was large, so that niobium hexoxide was discharged to 1 V or less with respect to pure lithium, and the electrolytic solution was decomposed. On the other hand, when the amount of lithium is less than 1% as in 1,7, the change in height becomes a little large, but this is because a small amount of metal ions other than lithium are dissolved, and the electrolyte decomposes due to some side reaction. It is thought that this was done. For these reasons, a lithium concentration of 1 to 8 parts by weight is appropriate.

On the other hand, Pb-Cd-In alloy and Al-Mn-I
When comparing n alloys, the height change of the battery is Al-M
The use of the n-I n alloy is smaller, and the internal resistance tends to be the same. In the case of Pb-Cd-In alloy, this is
It is thought that this is because only a very small amount of PB and Cd ions were dissolved after over-discharge at e o 'c. For this reason, aluminum alloys tend to be more resistant to overdischarge. However, under the same overdischarge conditions at room temperature, there was almost no change in height even in the Pb-Cd-In alloy at 1 to 8% by weight of lithium.

In addition, a. Al-Mn-I as the aluminum alloy
Although an n-alloy was used, alloys in which Mq, Aq, etc. were added to Al had exactly the same effect.

Further, in the examples, the niobium hexaoxide and the lithium storage alloy negative electrode were made to have the same weight, but this is not necessarily necessary, and a desired ratio may be used depending on the purpose.

Effects of the Invention As described above, the present invention provides a lithium secondary battery with excellent overdischarge characteristics.

[Brief explanation of the drawing]

The figure is a longitudinal cross-sectional view of a lithium secondary battery in an example of the present invention. 4...Positive electrode, 6...Negative electrode, 8...Separator. Name of agent: Patent attorney Toshio Nakao and 1 other person Key:

Claims (3)

[Claims] (1) It is composed of a positive electrode mainly made of niobium pentoxide, a negative electrode made of a lithium storage alloy, and an electrolyte made of a non-aqueous solvent in which lithium salt is dissolved, and in a charged state, the amount of lithium in the lithium storage alloy is is 8% or less of the weight of the niobium pentoxide, and 1% of the weight of the niobium pentoxide is
% or more by weight. (2) The lithium secondary battery according to claim 1, wherein the alloy that occludes lithium is composed of lead, cadmium, and indium. (3) The lithium secondary battery according to claim 1, wherein the alloy that occludes lithium is an aluminum alloy.