JPH0782838B2 - Rechargeable negative electrode for electrochemical device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a negative electrode of a rechargeable electrochemical device such as a secondary battery or a capacitor used as a portable power supply or a memory backup power supply.

Configuration of Conventional Example and Problems Thereof A non-aqueous electrolyte secondary battery using an alkali metal such as lithium as a negative electrode is under development. In a non-aqueous electrolyte secondary battery using metal lithium as a negative electrode, dendrite is generated on the lithium of the negative electrode during charging, short-circuiting with the positive electrode,
Alternatively, lithium is deposited in the form of small spheres and desorbs from the negative electrode, which causes a decrease in charge / discharge efficiency and a decrease in cycle characteristics.

In order to overcome this problem, it has been proposed to use tin or an alloy of lead and cadmium for the negative electrode. For example, when an alloy of lead and cadmium is used for the negative electrode, during charging, lithium ions in the non-aqueous electrolyte are occluded by the negative electrode lead-cadmium alloy to form an alloy with lithium, and during discharge,
The stored lithium is released as lithium ions into the non-aqueous electrolyte. As described above, the dendrite of lithium is not generated, and the lithium does not become a small spherical shape and is desorbed from the negative electrode, and the negative electrode has high charge / discharge efficiency and excellent cycle characteristics. In addition, since cadmium is included in the alloy of the negative electrode, it becomes an excellent negative electrode in which powdering of the negative electrode does not occur even when occluding lithium.

Here, based on the cycle characteristics and the charge / discharge amount at low rate charge / discharge, that is, the result of the examination under the condition that a larger charge / discharge amount per unit volume or weight is obtained, the content of tin or lead is 45% or less. It has been said that alloys with a weight percentage or more are good. this is,
This is because tin and lead in the alloy are involved in the storage and release of lithium during charge and discharge, and cadmium plays the role of a binder that prevents pulverization of the negative electrode.

However, even when this alloy is used, there is a problem that gas is generated during charging and discharging. Due to this generated gas, in a sealed battery, the battery case may swell, or it may accumulate between the negative electrode and the positive electrode, leading to deterioration of charge / discharge characteristics. When the generated gas is analyzed, most of it is air components such as oxygen and nitrogen, but only a small amount,
There is propylene which is presumed to be a decomposition product of propylene carbonate used as a solvent for the non-aqueous electrolyte.

To solve this gas generation problem, the alloy used for the negative electrode should be vacuumed in the molten state to remove oxygen and nitrogen from the alloy, or lithium should be stored in the alloy by charging and then degassed to vacuum. There is a way to get rid of it. By this method, most of the gas generated from the negative electrode can be prevented from being generated, such as oxygen and nitrogen, and the cycle characteristics are improved even in the sealed battery. However, there still remains a problem regarding the gas generated by decomposing the solvent of the non-aqueous electrolyte.

In particular, a rechargeable electrochemical device that uses activated carbon for the positive electrode and the above alloy for the negative electrode has a small charge / discharge capacity because the activated carbon has a small capacity, but the charge / discharge cycle characteristics are several thousand cycles. The above is required. In order to satisfy such cycle characteristics, it is necessary to prevent gas generation due to decomposition of the solvent of the non-aqueous electrolyte in the negative electrode.

OBJECT OF THE INVENTION It is an object of the present invention to provide an alloy which can reduce gas generation due to decomposition of a non-aqueous electrolyte used as a negative electrode of a rechargeable electrochemical device even when charging and discharging are repeated. Therefore, it becomes possible to obtain an electrochemical device having excellent cycle characteristics.

The negative electrode of the present invention constitutes a rechargeable electrochemical device in combination with a positive electrode having charge-discharge reversibility and a non-aqueous electrolyte containing an alkali metal ion. It consists of an alloy that occludes metal ions to form an alloy with an alkali metal and discharges it into the non-aqueous electrolyte as an alkali metal ion upon discharge. This alloy is at least 10% by weight selected from the group consisting of tin and lead. % Or more 45
Consists of less than wt% and cadmium. Furthermore, this alloy
It may contain at least one selected from the group consisting of bismuth, indium, calcium, antimony, mercury and silver in a range of 20% by weight or less. By using these alloys, it is possible to reduce gas generation due to decomposition of the electrolyte even when charging and discharging are repeated.

Alloys of the invention, eg 30% lead, 70% cadmium
The charge-discharge reaction when the alloy (hereinafter referred to as [Pb (30) -Cd (70)]) is used as a negative electrode in a non-aqueous electrolyte containing lithium ions can be expressed as in equation (1). .

In the formula, [Pb (30) -Cd (70)] Li _x represents a lithium alloy produced by occluding lithium in the alloy. In addition, as the range that can be charged and discharged, as shown in equation (1),
It is clear that it is not necessary to discharge all the lithium occluded in the alloy to discharge it, and it is clear that the amount of lithium in the alloy can be changed and the charge and discharge can be performed as shown in the formula (2). in this case,
The alloy of the negative electrode contains lithium even in the discharged state.

Description of Examples The alloys used in the following examples were all vacuumed during melting to remove oxygen, nitrogen and the like dissolved in the alloys. This is because the amount of gas due to the decomposition of the electrolyte is clearly known, excluding the influence of the dissolved gas in the alloy generated when charging and discharging the alloy.

Example 1 The following experiment was conducted for lead-cadmium alloys with different compositions.

FIG. 1 shows the configuration of a cell used for measuring the gas generated by decomposition of the electrolyte. 1 is an electrode made of various alloys for evaluating the characteristics as a negative electrode, and has a size of 1 × 1c
It is made by crimping expanded metal of nickel on both sides of an alloy of m and 0.2 mm in thickness. Reference numeral 2 denotes a lead of the electrode 1, which is made of nickel. Reference numeral 3 is a counter electrode, which is formed by compression-molding a mixture of MoO _{3 as} an active material, acetylene black as a conductive material, and polytetrafluoroethylene as a binder. This electrode 3
The current collector was titanium, and the lead used the titanium ribbon 4. A lithium plate was used for the reference electrode 5, and a nickel ribbon 6 was used for the lead. As the electrolyte 7, propylene carbonate in which 1 mol / liter of lithium perchlorate was dissolved was used. The alloy electrode 1 as the test electrode and the reference electrode 5 were covered with a conical glass container 9 having a hole 8 on the side surface so that the ions of the electrolyte could be transferred between the counter electrode and the counter electrode through the hole. . 10 is a glass cell containing these electrodes, and 11 is a silicone rubber stopper used for sealing. At the beginning of the experiment, there was no gas at the top of the conical glass container 9 for collecting gas.

After the measurement cell was set as described above, it was charged with 2 mA until the potential of the test electrode reached 0.05 V with respect to the reference electrode lithium and discharged with 2 mA until it reached 0.8 V after completion of charging. Further, this charging / discharging was repeated. At this time, the amount of gas accumulated at the apex of the glass container 9 for collecting gas was measured.

In the above charge / discharge cycle, the charge / discharge amount changes depending on the composition of the alloy. Generally, the smaller the amount of cadmium in the alloy, the larger the charge / discharge amount.

Therefore, in order to compare the gas generation amount from the alloys, the generated cumulative gas amount was compared with the cumulative discharge amount which is the sum of the discharge amounts in each cycle. As a result, the same cumulative discharge amount, that is, the amount of gas generated by the same discharge time by the constant current is compared.

FIG. 2 compares the amount of cadmium contained in the lead-cadmium alloy used with the cumulative amount of gas generated when the cumulative amount of discharge was 3 Ah. From this, it can be seen that when the lead content in the alloy is 50% by weight or less, particularly less than 45% by weight, gas generation is reduced.

Example 2 A tin-cadmium alloy was used as the alloy, and the same examination as in Example 1 was conducted. The electrolyte used was γ-butyrolactone in which 1 mol / liter of lithium borofluoride (LiBF ₄ ) was dissolved. Since the generated gas has carbon and hydrogen in the molecule, it is presumed that the solvent γ-butyrolactone is decomposed.

FIG. 3 is a plot showing the cumulative gas generation amount and the cadmium content in the alloy when the cumulative discharge amount is 3 Ah. It can be seen that even with this alloy, gas generation is reduced when the tin content is less than 45% by weight.

Example 3 A tin-lead-cadmium ternary alloy was used as the alloy. The amounts of tin and lead in the alloy were the same, and the amount of cadmium was changed and examined. Propylene carbonate and 1,2-
A solution obtained by dissolving 1 mol / liter of lithium perchlorate in a mixed solvent having a volume ratio of dimethoxyethane of 1: 1 was used. The generated gas is presumed to be a solvent decomposed because it has carbon and hydrogen in the molecule.

Fig. 4 shows the cadmium content in the alloy and the cumulative discharge capacity.
The cumulative gas generation amount at 3 Ah is plotted. It can be seen that even with this alloy, the amount of gas generated is reduced when the content of lead and tin is less than 45% by weight.

From the above, it is understood that tin and lead are mainly involved in the decomposition of the solvent of the non-aqueous electrolyte, and the gas generation amount is reduced by making the amount of tin or lead less than 45%. Further, in FIGS. 2 to 4, the gas generation is remarkably reduced when the amount of cadmium is around 55% by weight. From this, it is estimated that cadmium has some inhibitory effect on gas generation.

Since cadmium is a pollutant, it is better to use as little as possible. The results of investigating the effect on gas generation by replacing a part of cadmium with another metal will be described below.

Example 4 At least one selected from the group consisting of tin and lead, and cadmium are essential elements, and mercury, silver, indium, and
An alloy containing at least one selected from the group consisting of calcium, antimony and bismuth was studied. As an alloy shown as an example, tin or lead was 40% by weight, an amount of one selected from the group of bismuth, mercury, silver, indium, calcium and antimony was 20% by weight, and the balance was cadmium.

The test was conducted under the same conditions as in Example 1. The following table shows the cumulative gas generation rate when the cumulative discharge rate reached 3 Ah. From this, gas generation is reduced by adding bismuth, mercury, indium, and silver. It is also found that the addition of calcium and antimony reduces gas generation, albeit slightly.

Addition of mercury, silver, antimony, calcium, and indium to the alloy increases the charge / discharge amount in each cycle when charge / discharge is repeated. Therefore, it was found that when cadmium in the alloy was replaced with indium, bismuth, mercury, silver, antimony, and calcium, the charge / discharge amount increased and the gas generation amount also decreased. This can be presumed to be because these added metals have a suppressing effect on gas generation. However, when the amount of lead and tin is large, the effect of suppressing gas generation by adding indium, bismuth, mercury, silver, antimony, and calcium was small. The effect of adding these metals appears when the amount of tin and lead is less than 45% by weight.

In the above examples, MoO ₃ was used as the counter electrode, but it is also effective when activated carbon is used, and a positive electrode having reversibility of charge / discharge can be used. As described above, by reducing the amount of tin and lead in the alloy, it is possible to suppress gas generation, but as mentioned earlier, when the amount of tin and lead, which are directly involved in charging and discharging, decreases. The amount of discharge decreases.

The charge / discharge amount of the rechargeable charge / discharge device is determined by the charge / discharge amount of the positive electrode and the negative electrode. In particular, in the case of the positive electrode made of MoO ₃ described in the examples, the charge / discharge amount of the positive electrode is as large as 100 mAh / cc or more. On the other hand, when activated carbon is used for the positive electrode, the charge / discharge amount is as small as 2 mAh / cc or less. Especially when activated carbon is used for the positive electrode, the charge and discharge amount of the positive electrode is small, so if the charge and discharge amount of the negative electrode is 250 mAh / cc or more even at high rate discharge, the charge and discharge amount of the electrochemical device is: Almost determined by the charge and discharge capacity of the positive electrode. In other words, a value close to the charge / discharge amount when the inside of the electrochemical device is filled with activated carbon can be obtained.

As a result, the alloy composition of the negative electrode can be used even if the amount of lead or tin is 10% by weight.

EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to obtain a negative electrode with less gas generation due to the decomposition of the solvent of the non-aqueous electrolyte. Therefore, it is particularly useful for a closed electrochemical device having a long charge / discharge life.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a cell using an embodiment of the present invention, and FIG.
FIGS. 3, 3 and 4 are diagrams showing the relationship between the Cd content of various alloys and the cumulative gas generation amount.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takashi Iijima 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP 55-43737 (JP, A) Nikkei Electronics, No. 339 (Showa 59-3-26) P. 93-94

Claims (1)

[Claims] 1. A rechargeable negative electrode for an electrochemical device comprising an alloy which absorbs an alkali metal ion in a non-aqueous electrolyte to form an alloy with an alkali metal by charging and which is released into the electrolyte as an alkali metal ion by discharging. The alloy of the negative electrode is at least 10% by weight or more and less than 45% by weight of at least one selected from the group consisting of tin and lead, and indium,
A rechargeable negative electrode for an electrochemical device, which is an alloy composed of 20% by weight or less (not including 0) of at least one selected from the group consisting of calcium, antimony, mercury, and silver, and the balance of cadmium.