JPS6161376A - Rechargeable electrochemical device - Google Patents

Abstract

PURPOSE:To obtain an alloy negative electrode having good cycle life in overdischarge by using an alloy selected from a group comprising a specified wt% of bismuth, cadmium, tin, and lead as a negative electrode. CONSTITUTION:In a rechargeable electrochemical device having a negative electrode comprising an alloy which reversibly absorbs and desorbs alkali metal ion, a negative electrode alloy is selected from a group comprising 20-75wt% bismuth, 15-80wt% cadmium, an the balance tin and lead. The alloy comprising a specified composition of tin, lead, bismuth, and cadmium is melted, and a nickel expanded metal current collector is immersed in the melted alloy, then taken out, and dried. The current collector is rolled to the thickness of 0.2mm, and a part of expanded metal is exposed and a lead 6 comprising nickel ribbon is welded thereon. Thereby, an alloy negative electrode having good cycle life in overdischarge is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to rechargeable electrochemical devices such as secondary batteries using non-aqueous electrolytes, and in particular to improvements in their negative electrodes.

Conventional configurations and their problems Various secondary batteries that use alkali metals such as lithium as negative electrodes have been researched in the past, but there was a problem with dendrites that occur when charging the negative electrode. In order to solve the problem, the present inventors developed bismuth, tin, lead,
They proposed using alloys such as cadmium for the negative electrode.

By using these alloys for the negative electrode, during charging,
Lithium ions in the electrolyte can be occluded without generating dendrites. This is due to the formation of intermetallic compounds between tin, lead 2 bismuth and lithium in the alloy. Furthermore, when this negative electrode is discharged, tin, lead,
Lithium is released into the electrolyte as lithium ions from the intermetallic compound of bihimastrilithium. Charging and discharging can be performed by the above mechanism.

Cadmium in the alloy acts as a binder to prevent the negative electrode from becoming pulverized when the alloy absorbs lithium. In other words, single metals and alloys such as tin, lead, and bismuth become pulverized when they absorb lithium and create intermetallic compounds, making it impossible to maintain the shape of the negative electrode, but by using an alloy containing cadmium, , the shape of the negative electrode can be maintained.

Wear.

However, tin containing cadmium as mentioned above.

Even when an alloy of lead and bismuth is used, it is possible to prevent the negative electrode from becoming pulverized during charging, but there is a problem in that the cycle characteristics of the negative electrode deteriorate when overdischarged. In particular, when constructing a secondary battery, when the capacitance of the negative electrode is made smaller than the capacitance of the positive electrode, the negative electrode is always in an overdischarge state. The reason for reducing the capacitance of the negative electrode is as follows:
Since the carbon material used as the conductive agent in the positive electrode has a catalytic effect on the decomposition of the solvent used in the electrolyte in a base potential range, it is necessary to prevent the potential of the positive electrode from becoming too base. This is because there is.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a superior alloy negative electrode that exhibits good cycle life against over-discharge as a negative electrode for rechargeable electrochemical devices.

Structure of the Invention The present invention provides a rechargeable electrochemical device including a negative electrode made of an alloy that reversibly occludes and releases alkali metal ions upon charging and discharging, wherein the alloy is more than 20% by weight and 75% by weight. % undropped bismuth, 15 to 80% by weight of cadmium, and the balance is at least one member selected from the group consisting of tin and lead.

Explanation of Examples First, to explain the method for manufacturing the negative electrode, an alloy consisting of tin, lead, bismuth, and cadmium is melted to have a predetermined composition, and a nickel extracted band metal as a current collector is immersed in the melt. , pulled up, cooled, and then rolled to a thickness of 0.2 fl. A part of the nickel expanded band metal was exposed, and a nickel ribbon as a lead was spot-welded thereto, and then cut into a size of 1cIItXIGI, which was used as a negative test electrode.

The positive electrode active material may be any material as long as it has reversibility with respect to charging and discharging. In this example, MoO.

was used. A mixture consisting of 100 parts by weight of MOo, 20 parts by weight of acetylene black as a conductive agent, and 15 parts by weight of tetra7ethylene resin as a binder was prepared in a size 10×1.
α, press molded to a thickness of 34311 mm. Note that titanium expanded metal was used as the current collector, and the mixture was molded so as to embed the current collector. A part of the titanium extracted band metal was exposed and a titanium ribbon as a lead was spot welded. Thereafter, vacuum drying was performed, and a 30 mAh current was applied in the cathode direction in propylene carbonate in which 1 mol/β of LiCl0a had been dissolved and used as a positive electrode.

Construct a cell as shown in Figure 1 using the above electrodes,
The characteristics of the negative electrode were investigated. In the figure, 1 is the test negative electrode, 2
is a positive electrode, 3 is a lithium reference electrode for measuring the potential of the test electrode, 4 is a nickel lead, 6 is a titanium lead, 6 is a lead made of nickel ribbon, 7 is 1 mol/4 Li
A nonaqueous electrolyte made of propylene carbonate in which CIO4 is dissolved, and 8 is a liquid junction bridge.

The cycle characteristics of the test electrode were measured as follows.

First, the test electrode 1 made of various alloys was charged with a constant current of 1 m in the direction of the cand. Under these conditions, lithium does not precipitate on the test electrode, but is occluded in the alloy. Thereafter, the battery was discharged toward the anode until the voltage reached 2V with respect to the reference electrode 3, and then charging and discharging were repeated. .

Example 1 The cycle characteristics of alloys such as bismuth (Bi), lead (pb), and cadmium (Cd) as negative electrodes were investigated.

FIG. 2 shows changes in the amount of discharged electricity during charge and discharge cycles when using the various alloys shown in the following table.

From Figure 2, it can be seen that Alloy Man is superior.

People and B have a relationship in which the ratio of B1 and Pb is approximately equal and the Cd content is different, but sowing B with a low Cd content has poor cycle characteristics. This is because the charging ζ electrode is likely to become pulverized.

In addition, when comparing C with people with the same Cd content, ゛B
People with higher i content have better cycle characteristics. This becomes even clearer when compared with C. However, as is clear from the comparison between humans and D, it is clear that simply having more Bt does not result in better cycle characteristics.

Therefore, in order to evaluate the cycle characteristics of each alloy, the discharge capacity at the third cycle was used as a reference, and the number of cycles until the discharge capacity reached half of that was considered as the cycle characteristics. FIG. 3 shows the cycle characteristics when the amount of Cd is constant and the amount of Bi is plotted on the horizontal axis. Figure 3, Ca
is 15% by weight or more, Bi is more than 20% by weight and 70% by weight
It can be seen that good cycle characteristics can be obtained when the composition is less than 10%. Also, since the curve in the figure is convex near the center, the cycle characteristics are affected by Bi and P in the alloy.
It seems that there is some kind of interaction between b.

Regarding the amount of CaO, as mentioned above, if it is less than 15% by weight, part of the negative electrode starts to be pulverized during charging, so at least 15% by weight is required. However, if the amount of CaO is increased, a practical amount of discharge electricity cannot be obtained.From this point of view, the appropriate upper limit for Cd is 80% by weight, and if it exceeds this, the discharge capacity will decrease. The amounts of Pb and Bi involved are reduced.

Next, regarding the amount of Pb, the cycle characteristics can be improved by adding about 1% by weight. That is, in Fig. 3, the right end of each curve shows the characteristics of the Cd-B1 alloy υ, and adding PB to this means tracing each curve in the direction of Bi amount reduction, and a small amount of PB. It can be seen that the addition improves cycle characteristics.

In conclusion, the Pb-B1-Cd alloy has good cycle characteristics when Cd is 15% by weight or more and Bi is more than 20% by weight and less than 75% by weight.

Example 2 An alloy using tin (Sn) instead of pb was studied in the same manner as in Example 1. FIG. 4, like FIG. 3, shows the cycle characteristics when the amount of B1 in the alloy was varied while keeping the amount of CaO constant.

In the case of this alloy as well, s
Due to the interaction between n and Bi, Cd is 15% by weight or more, B
When 1 is more than 20% by weight and less than 75% by weight, the cycle characteristics are good. Sn.

Regarding the amount, the cycle characteristics can be improved by adding about 1% by weight, as in the case of pb. In order to obtain a practical amount of discharged electricity, the amount of Qd needs to be within 80% by weight.

Example 3 The same test as in Example 1 was conducted on a four-thick alloy of Sn-Pb-Bi-Cd. In this case, the amount of electricity for charging and discharging each cycle was larger than in Examples 1 and 2. As in Examples 1 and 2, good results were obtained when Qd was 15 double trays or more and bismuth Bi was more than 20% by weight and less than 75% by weight.

The above results are for the case where M2O3 is used as the positive electrode,
In addition to Mob, other reversible positive electrodes such as MnO□, TiS2, or carbon materials used in capacitors can be used. Also as an electrolyte,
In addition to those shown in the examples, LiBF4. LiAs7
5 and other alkali metal salts, γ-butyrolactone as a solvent,
Electrolytes using aprotic solvents such as dimethoxyethane and tetrahydro7ran are also effective.

Effects of the Invention As described above, according to the present invention, it is possible to obtain an electrochemical device with stable cycle characteristics even against overdischarge.

[Brief explanation of drawings]

Figure 1 is a schematic vertical cross-sectional view showing the configuration of the test cell used in the examples, Figure 2 is a diagram plotting the amount of discharged electricity against the number of cycles when various alloy negative electrodes are over-discharged, Figure 3 and Figure 4 shows Bi-Fb-Cd alloy and B knee S, respectively.
FIG. 3 is a diagram showing cycle characteristics when the BiO content is changed while keeping the Cd content constant in an n-Cd alloy. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 3: Eclipse-f Bi Kaoru (Senior! 2)

Claims (1)

[Claims] A non-aqueous electrolyte containing alkali metal ions, a reversible positive electrode, and a negative electrode made of an alloy that reversibly occludes and releases alkali metal ions upon charging and discharging, wherein the alloy contains more than 20% by weight and less than 75% by weight. Bismuth, 15-80% by weight
cadmium, the balance being tin and lead.