JPH01241756A - Nonaqueous secondary cell - Google Patents

Abstract

PURPOSE:To reduce the disintegration speed of a electrode when the charge and the discharge are repeated, and to extend the service life, by making the negative electrode of a secondary cell with an alloy of lithium, lead, and an alkaline earth metal, and specifying the atomic ratio of the alloy components. CONSTITUTION:A secondary cell is composed of a positive electrode 3, a negative electrode 1 which consists of a negative electrode substance to absorb and release lithium inversely, and a nonaqueous electrolyte including a lithium salt, and the negative electrode 1 of the secondary cell is composed of an alloy of lithium, lead, and an alkaline earth metal, making the atomic ratio of the alloy 1-8:0.01-0.05. That is, by adding an alkaline earth metal to a Li-Pb alloy, the strength of the alloy, under the condition to remove the lithium from the alloy by the discharge, is improved, and the disintegration of the electrode is suppressed accordingly. Consequently, a secondary cell with little deterioration of activeness owing to the disintegration of the electrode, and with a long service life, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a secondary battery using a nonaqueous electrolyte, and particularly to a high energy density secondary battery using a lithium alloy as a negative electrode active material.

[Conventional technology]

BACKGROUND ART Conventionally, high energy density secondary batteries using Li as a negative electrode active material and an organic electrolyte have been developed. In general, such batteries use metal Li as the negative electrode active material, various intercalation compounds or conductive polymers as the positive electrode active material, and a solution of Li salt dissolved in a stable organic solvent as the electrolyte. There is.

However, when Li is used as a single metal as a negative electrode, the current efficiency of Li precipitation and dissolution reaction during charging and discharging is poor, and when charging and discharging are repeated, Li in the electrode grows into a dendrite shape, which can cause short circuits in the battery. It is known to be easily triggered.

Therefore, various methods are being considered to avoid this.
As one of the methods, it is known that instead of using Li alone, it is effective to use Li as an alloy with other metals such as AQ (Japanese Patent Publication No. 12044/1982).

In this alloy electrode, when Li is deposited during charging,
It does not precipitate as metal Li, but directly as an alloy with the base metal. Therefore, it is electrochemically more stable than when Li is deposited as a single substance, and the current efficiency is improved. At the same time, dendrite-like precipitation is also suppressed.

As an alloying element that can be used as an electrode in this way, L
i and Mg, AQ, Si, Ga, Ge.

Metals such as In, Ag, Sn, Sb, Pb, and Bi are known.

However, in these alloy electrodes, during charging, alloying of the metal with Li occurs, and during discharging, a dealloying reaction occurs, which causes a significant change in the volume of the electrode. Therefore, when charging and discharging are repeated, the electrode expands.
This is a major problem when using a Li alloy as an electrode, as it tends to collapse and reduce the capacity of the electrode, that is, cause the electrode to deteriorate.

Deterioration of electrodes due to such a mechanism can occur in any alloy system, but among them, Li-Pb alloys are known to have a relatively long charge-discharge cycle life (Japanese Patent Laid-Open No. 57-141869). (No. Publication), and in addition, Mg
, Ca, Ga, In, Si.

By adding 5 atomic % or more of metals such as Ge and Sn to pb, the diffusion of Li in the electrode becomes easier,
It is known that the charging and discharging characteristics of the electrode are further improved (Japanese Patent Application Laid-Open No. 61-66370).

However, even in these alloy systems, it was difficult to say that the lifespan was sufficient compared to the lifespan required for negative electrodes of secondary batteries.

[Problem to be solved by the invention]

The electrodes of secondary batteries are required to operate stably over hundreds of charging and discharging cycles. The collapse of the electrode as described above is considered to be inevitable to some degree since the volume change of the electrode occurs. Therefore, in order to extend the life of the electrode, it is necessary to suppress the rate of disintegration, or even if disintegration occurs, maintain electrical contact with the current collector in that part so that it can be used as an electrode. It seems necessary.

An object of the present invention is to provide a long-life secondary battery with little reduction in activity due to electrode collapse.

[Means to solve the problem]

The above problem is solved by a secondary battery having a positive electrode, a negative electrode made of a negative electrode active material that reversibly intercalates and desorbs lithium, and a non-aqueous electrolyte containing a lithium salt. It consists of an alloy with similar metals, and the atomic ratio of the alloy components is 1 to 8: 1: 0.01 to 0.0.
This problem is solved by a non-aqueous secondary battery.

[Effect]

The negative electrode of a secondary battery includes a positive electrode, a negative electrode made of a negative electrode material that reversibly intercalates and desorbs lithium, and a non-aqueous electrolyte containing a lithium salt. The atomic ratio of the alloy components is 1 to 8.
:1:0.01~O,OS to slow down the rate of electrode disintegration.

〔Example〕

Examples according to the present invention will be described below.

First, a summary of the contents of the examples and the results obtained will be described.

First, we investigated the causes of a decrease in activity due to repeated charging and discharging of Li alloy electrodes, as well as the types of alloys that are less likely to cause a decrease in activity. The electrodes were studied in detail. As a result, for electrodes using any alloy, the chemical change in the electrode itself after a decrease in activity is small. It was found that the problem was caused by swelling and high resistance, resulting in loss of electrical contact with the current collector. It has also been found that such electrode collapse largely depends on properties such as the mechanical strength of the electrode and the adhesion between alloy particles.

Therefore, from the perspective of improving the strength of the electrode, we decided to use L, which has a relatively long cycle life among these alloy electrodes.
We investigated more effective additive components for i-Pb alloys.9 As a result, we found that long-life electrodes can be obtained by adding alkaline earth metals to Li and Pb alloys. Ta.

In this case, Mg, Ca, etc. are particularly effective as additives, and the optimum composition at that time is that the atomic ratio of Li to Pb is in the range of 1 to 8, more preferably 2. The content of alkaline earth metal is in the range of 0.01 to 0.01 to O,OS in atomic ratio to pb.
It was within the range of

When the ratio of Li to pb is smaller than 1, Li in the alloy cannot be used effectively as an electrode active material,
When it is larger than 8, stabilization of Li by alloying is insufficient, and redentite is precipitated by charging and discharging.

In addition, the ratio of alkaline earth metal additive to pb is 0.
．． If it was smaller than 01, the effect of the addition was not sufficiently exhibited, and no difference was observed from that without the addition. 0.0
When it was larger than 5, the moldability of the electrode deteriorated, and the cycle life of the electrode decreased on the contrary.

The electrode obtained in this way uses a layered compound such as titanium disulfide or a conductive polymer such as polyaniline as the positive electrode.
It can be applied to batteries using a propylene carbonate solution of lithium borofluoride as an electrolyte, and even in such batteries, a good lifespan could be obtained.

The reason why the cycle life of the electrode has improved in this way is that by adding alkaline earth metals to the Li-Pb alloy, the strength of the alloy is improved when lithium is removed from these alloys during discharge. This is thought to be because the collapse of the electrode was suppressed. In order to improve this strength, there is no effect unless the alkaline earth metal is added in an atomic ratio of 0.01 or more to pb, and if the addition is made in an atomic ratio of 0.05 or more, Li-Pb-alkaline earth metal The material itself became brittle and its moldability was extremely poor.

Therefore, in order to maintain moldability and improve the strength during discharge, the addition ratio of alkaline earth metal must be between 0.01 and 0.05 in terms of atomic ratio to PB.

Next, the performance will be explained in detail in Examples according to the present invention and Comparative Examples.

Example I Li, Pb, and Mg were weighed out in an atomic ratio of 4:1:0.03 so that the total amount was 20 g, placed in an iron crucible, and heated and melted at 800°C in an Ar gas atmosphere. After cooling, the 0 alloy prepared by adjusting the Li-Pb-Mg alloy was crushed into 100 meshes or less in a mortar, and a pressure of 2 tons/d was applied using a press to form a powder with a diameter of 15 cm and a thickness of 0.4 mm. ■ It was molded into a disk shape.

A coin-shaped battery as shown in FIG. 1 was assembled using the above disk-shaped alloy as a negative electrode and titanium disulfide as a positive electrode. This battery has a negative electrode 1. made of the above disc-shaped alloy. Separator made of polypropylene nonwoven fabric2. A positive electrode 3 formed into a disk shape by adding graphite and polytetrafluoroethylene to titanium disulfide powder and a separator 2 were impregnated with the powder.

L i B F+ propylene-carbonate and 1
゜2-dimethoxyethane solution, which is used in case 4 on the negative electrode side. It is sealed by a gasket 5 and a positive electrode side case 6.

Charge the above battery to i, s v with a current of 1 mA,
When the same operation of discharging to 1V at a current of 1mA was repeated, the discharge capacity of the battery changed as shown in Figure 2A,
500 cycles of charging and discharging were possible before the capacity decreased to 3 mAh or less.

Example 2 A battery having the same configuration as Example 1 was assembled except that the positive electrode 3 was made by mixing polyaniline, graphite, and polytetrafluoroethylene and molded into a diameter of 1511 W+ and a thickness of 0.4 mm. Ta. When this battery was repeatedly charged to 4 V at a current of 1 mA and discharged to 1.5 V at the same <1 mA, 600 charging/discharging cycles were possible before the capacity decreased to 3 mAh or less.

Example 3 Li:Pb:Ca= by almost the same method as Example 1
Li-Pb-Ca with a composition of 4:1:0.03
An alloy was prepared, and a coin-type battery having the same structure as in Example 1 was fabricated using this alloy as a negative electrode.

When this battery was repeatedly charged to 3.5 V at a current of 1 mA and discharged to 1 V at a current of 1 mA, 450 cycles of charging and discharging were possible before the capacity decreased to 3 mAh or less.

Comparative Example In almost the same manner as in Example 1, Li:Pb==4:
Li-Pb alloy having a composition of 1 (Comparative Example 1), Mg added Li:Pb:Mg=4:1:0,
05 (Comparative Example 2) and Li:Pb:Mg=4:
Li-Pb-M with a composition of 1:0.2
g alloy (Comparative Example 3) was prepared, and a coin-shaped battery having the same structure as Example 1 using these alloys as a negative electrode was also fabricated. The moldability of the electrode of Comparative Example 2 was extremely poor.

When these batteries were repeatedly charged to 3.5V at a current of 1mA and discharged to 1V at a current of 1mA, the discharge capacity of the batteries changed as shown in Figure 2 B, C, and D, and the capacity increased. It took only 300 cycles for Comparative Example 1, 320 cycles for Comparative Example 2, and 250 cycles for Comparative Example 3 to reduce the Mg content to 3 mAh or less, which clearly indicates that there is an effective range for the effect of Mg addition. It became.

In FIG. 2, the curve A is for Example 1, the curve B is for Comparative Example 1, and the curve C is for Comparative Example 2. Curve D shows the change in discharge capacity of each battery in Comparative Example 3.

〔Effect of the invention〕

According to the present invention, the negative electrode of the secondary battery is made of an alloy of lithium, lead, and an alkaline earth metal, and the atomic ratio of the alloy components is set to 1.
By setting the ratio to 8:1:0.01 to 0.05, the disintegration rate of the electrode during repeated charging and discharging can be slowed down, resulting in an excellent effect that a long-life secondary battery can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of the secondary battery prepared in the example of the present invention, and Fig. 2 shows the discharge capacity of the secondary battery prepared in Example 1 and Comparative Examples 1 to 3 of the present invention as a function of the number of charge/discharge cycles. This is a graph showing changes. 1... Negative electrode, 2... Separator, 3... Positive electrode, 4
... Negative electrode case, 5... Gasket, 6... Positive electrode case.・Miy-

Claims (1)

[Claims] 1. A positive electrode, a negative electrode comprising a negative electrode active material that reversibly intercalates and desorbs lithium, and a non-aqueous electrolyte containing a lithium salt;
In the secondary battery, the negative electrode is made of lithium, lead,
A non-aqueous secondary battery comprising an alloy with an alkaline earth metal and having an atomic ratio of alloy components of 1 to 8:1:0.01 to 0.05. 2. The non-aqueous secondary battery according to claim 1, wherein the alkaline earth metal is either magnesium or calcium.