JPH04179068A - Secondary battery with non-aqueous electrolytic solution - Google Patents

Abstract

PURPOSE:To provide a secondary battery having excellent safety and charging/ discharging cyclic characteristic by mixing carbon with powder of a neg. electrode material which can occlude and release Li electrochemically and on whose surface an oxide film is formed. CONSTITUTION:Through a heat treatment in a gas atmosphere including oxygen, an oxide film is formed on the surface of a neg. electrode material such as a fusible alloy which can occlude and release Li electrochemically and Al, and this is mixed with carbon. Even through this fusible alloy, Al, makes repetition of expansion and contraction associated with charging/discharging to cause change in the shape, slipping-off from the core 3 is prevented and electricity collection held practicable by pulverizing them, forming such an oxide film, and mixing with carbon. This prevents drop of the charging/discharging capacity with cycles and also generation of internal shortcircuiting and firing.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to improving the safety and charge/discharge cycle characteristics of lithium secondary batteries.

BACKGROUND OF THE INVENTION In recent years, as electronic devices such as AV devices have become portable and cordless, there are great expectations for small, lightweight, and high energy density lithium secondary batteries as power sources for driving them.

However, there are still a number of issues remaining before it can be put into practical use. One example of this is the improvement of the charge/discharge reversibility of the negative electrode.

For example, when metallic lithium is used as the negative electrode material, lithium is dissolved as ions from the negative electrode into the electrolytic solution by discharging, and deposited on the negative electrode again by charging. The shape of the deposit varies depending on the composition of the electrolyte and charging conditions, but it is mainly needle-shaped, and it detaches from the negative electrode or penetrates the separator and comes into contact with the positive electrode, causing an internal short circuit and ignition. .

Therefore, a method has been proposed in which a metal or alloy that has the function of absorbing lithium ions in the electrolytic solution during charging to form an alloy with lithium and releasing lithium as ions into the electrolytic solution during discharging is used as the negative electrode material. . That is,
In this case, lithium ions quickly absorb and absorb during charging.
Since an alloy is formed through diffusion, acicular lithium does not precipitate on the negative electrode, improving safety.

Problems to be Solved by the Invention Typical metals or alloys that can electrochemically absorb and release lithium include (1) lead as an essential component, either alone or with tin, cadmium, zinc, indium, and bismuth; A small amount selected from the following group (alloy with one, (2)) tin as a component, either alone or with lead, cadmium, and zinc.

An alloy (3) aluminum with at least one selected from the group consisting of indium and bismuth has been proposed. (1) and (2) are called fusible alloys because of their low melting point properties. However, both expand and contract as they absorb and release lithium. In the case of a fusible alloy, it was observed that when charging and discharging were repeated, cracks appeared on the surface from the beginning of the cycle, and as the cycle progressed, it became pulverized and collapsed along the cracks, falling off from the core material.

In addition, in the case of aluminum, it was observed that the pulverization became extremely muddy and could not be held in the core material. The performance of a fusible metal as a negative electrode material (the ability to absorb and release lithium, the ability to suppress shape changes due to cycling) depends on the components forming the fusible metal and their composition ratios. In the type of fusible alloy, gadmium has a poor ability to absorb and release lithium, and does not expand or contract much, so if it is contained in an amount of 20% by weight or more, changes in shape due to cycling can be suppressed to some extent. However, although there was no collapse from the beginning of the cycle, it was observed that cracks eventually appeared on the surface, and as the cycle progressed, it became pulverized, collapsed along the cracks, and fell off from the core material. This not only reduces the charge/discharge capacity due to the inability to collect current, but also has the risk of penetrating the separator and coming into contact with the positive electrode, causing an internal short circuit and ignition.

The present invention solves these problems and aims to provide a Chilam secondary battery with excellent safety and charge/discharge cycle characteristics. Fusion money.

After forming an oxide film on the surface of a negative electrode material such as aluminum by, for example, heat treatment in a gas atmosphere containing oxygen,
This is a mixture of this and carbon.

Function: According to the present invention, fusible alloys and aluminum repeatedly expand and contract as they are charged and discharged, resulting in shape changes (in the case of fusible alloys, cracks occur and become pulverized and disintegrate along the cracks; in the case of aluminum, it becomes muddy). Even if this occurs, the powder is made into powder, heat treated in an oxygen-containing gas atmosphere to form an oxide film on the surface, and then mixed with carbon to prevent it from falling off from the core material. At the same time, it is possible to collect current, thereby solving the problems of a decrease in charge/discharge capacity due to cycling, internal short circuits, and the occurrence of fire.

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Table 1 summarizes the compositions of fusible metals (1) to (8) used in the present invention.

For fusible metals (1) to (8), each component was mixed at a predetermined weight ratio, heated and melted in a stainless steel container, cooled to form an ingot, and this was rolled to form a fusible metal sheet. Further, after melting, a fusible metal powder was obtained by a so-called atomization method in which the powder was atomized in a high-speed inert gas stream. As aluminum (9), an aluminum alloy sheet classified into JIS standard H4160-IN30 was used. It was also ground into aluminum alloy powder. Hereinafter, these will be simply referred to as alloy sheets and alloy powders.

The above alloy sheet can be used to store and discharge lithium by charging and discharging.
A test electrode for emission was fabricated.

FIG. 1(A) is a cross-sectional view of the structure of the test electrode.

The above alloy sheet 1 has a diameter of 20 cm, a length of 25-2, a thickness of 0.2
-, a nickel expanded band was press-fitted as the core material 3, and a nickel lead plate 4 was spot welded to the core material 3 to prepare a test electrode. Note that one side of the alloy sheet 1 was coated with polyethylene 5 so as not to participate in the charge/discharge reaction.

A test electrode for intercalating and deintercalating lithium through charging and discharging was prepared using the above alloy powder. FIG. 1(B) is a cross-sectional view of the structure of the test electrode.

3% by weight of acetylene black as carbon is mixed with the above alloy powder 2, 1% by weight of carboxymethyl cellulose aqueous solution is added as a thickener, and 5% by weight of aqueous dispersion of polytetrafluoroethylene resin is further kneaded as a binder. Make it into a paste. Nickel foil was used as the core material 3, and it was coated on one side, dried, and rolled. Thereafter, test electrodes were produced in the same manner.

Using this test electrode, we constructed a device for intercalating and deintercalating lithium through charging and discharging. FIG. 2 is a sectional view of the structure of the charge/discharge test apparatus.

The test electrode 6, together with a counter electrode 7 and a reference electrode 8, is configured in an H-type cell 10 partitioned by a separator 9, and a nonaqueous electrolyte is injected into the cell 10 to charge and discharge at a constant current of 5 mA.

The reaction in which the test electrode absorbs lithium is called a charge reaction, and the reaction in which it releases lithium is called a discharge reaction.
0 mV, and the discharge end voltage was 800 mV. In addition, metallic lithium was used for the counter electrode and the reference electrode, and a nickel expanded band was press-fitted as a core material in each electrode. The counter electrode metal lithium was consumed during cycles due to its low charge/discharge reversibility, so it was filled in excess compared to the expected charge/discharge capacity. As the non-aqueous electrolyte, a mixture of propylene carbonate and ethylene carbonate at a volume ratio of 1:1 and IM/e dissolution of lithium hexafluorophosphate was used.

Test electrodes (1) using the above alloy sheets (1) to (9)
(9) was charged and discharged. Figure 3 shows the change in charge/discharge capacity with cycles.

As a result of observing the shape change of the test electrode due to the cycle, it was found that the test electrode (9) had already become muddy and was flowing out from the core material at the time of the 5th cycle. Test electrodes (1) and (5) already had cracks on their surfaces, and after 10 cycles,
It disintegrated along the cracks and became pulverized, and some of it fell off from the core material, and when a light impact was applied at this point, more flaking occurred.

Test electrodes (2) and (6) are at 25 cycles, test electrode (3) is
), (7) are at 35 cycles, test electrodes (4), (8)
After 40 cycles, some parts of the core material fell off. This corresponds to the results shown in FIG. 3, where changes in shape due to cycling, especially falling off from the core material, were observed, and at the same time, the charge/discharge capacity significantly decreased.

In fact, if a battery using such alloy sheets (1) to (9) as the negative electrode material is subjected to an impact such as being dropped, the fragments generated by such falling will penetrate the separator and come into contact with the positive electrode, causing internal damage. It is presumed to cause short circuit and ignition.

Example 1 Test electrodes (1') to (9') of the present invention (1) in which carbon was mixed with the alloy powders (1) to (9) described above were charged and discharged.

Figure 4 shows the change in charge/discharge capacity accompanying the cycle.

As a result of observing the shape change of the test electrode due to the cycle, 10
Even at 0 cycle, the test poles (1') to (9') are misaligned.
No falling off from the core material was observed. This corresponds well with the results shown in FIG. 4, and since there is no change in shape due to cycling, especially no falling off from the core material, the charge/discharge capacity does not decrease much.

Example 2 The above alloy powder (9) was heat-treated in an oxygen-containing gas atmosphere to form an oxide film on the surface, and then the test electrode (9'') of the present invention (2) mixed with carbon was charged and discharged. Invention (1)
) test electrode (9'). Heat treatment in air 300℃
℃ for 1 hour. FIG. 5 shows the change in charge/discharge capacity accompanying the cycle.

As a result of observing the shape change of the test electrode due to the cycle, 10
Even during the 0 cycle, neither of the test electrodes (9') and (9') were observed to fall off from the core material.

This corresponds well to the results shown in FIG. 5, and since there is no change in shape due to cycling, especially no falling off from the core material, the charge/discharge capacity hardly decreases.

When carbon is mixed into the alloy powder, the alloy powder may aggregate, and although the charge/discharge capacity does not decrease much in the test electrode (9'), the degree varies. but,
When alloy powder is heat-treated in an oxygen-containing gas atmosphere to form an oxide film on the surface and then mixed with carbon, the alloy powder does not aggregate and can be mixed uniformly with carbon, resulting in a test electrode (9″). In this case, the charge/discharge capacity hardly decreases, and there is no variation in the degree of decrease.

Regarding aluminum powder, JIS standard H41
No difference was observed even when aluminum alloys with other compositions classified as 60 were used.

Further, regarding the fusible metal powder, no difference was observed even when an alloy having a composition in which 5% by weight of lead or tin was replaced with indium was used.

Further, no difference was observed when an alloy having a composition in which 20% by weight of lead or tin was replaced with bismuth was used.

In addition to nickel, the core material is stainless steel,
No difference was observed when any copper material was used.

Effects of the Invention As described above, according to the present invention, in a non-aqueous electrolyte secondary battery using a fusible alloy, aluminum, etc. that can electrochemically intercalate and desorb lithium as a negative electrode material, it is possible to use these materials in powder form on the surface. After forming an oxide film by heat treatment in a gas atmosphere containing oxygen, for example, by mixing carbon, the fusible metal, aluminum, expands and expands as it charges and discharges.
Even if repeated shrinkage causes a change in shape (in the case of a fusible metal, cracks occur, it becomes pulverized, and collapses along the crack; in the case of aluminum, it becomes mud-like), it will not fall off from the core material. In addition to preventing this, it is possible to collect current, and the decrease in charge/discharge capacity due to cycling is small (and the effect is that internal short circuits and ignition do not occur).

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the configuration of the test electrode, Figure 2 is a cross-sectional view of the charge/discharge test device, Figure 3 is a diagram showing changes in charge/discharge capacity with cycles of a conventional test electrode, Figures 4 and 2 are FIG. 5 is a diagram showing the change in charge/discharge capacity of the test electrode of the present invention as it cycles. 1... Alloy sheet, 2... Alloy powder,
3...core material, 4...lead plate, 5...
...Polyethylene coat, 6...Test electrode,
7... Counter electrode, 8... Reference electrode, 9...
Figure 3: A (v cycle 1 missing (area)

Claims (4)

[Claims] (1) A non-aqueous electrolyte secondary battery characterized by mixing carbon with negative electrode material powder that can electrochemically absorb and release lithium and has an oxide film formed on its surface. (2) Claims characterized in that the negative electrode material powder contains lead as an essential component, and is either alone or in an alloy with at least one selected from the group consisting of tin, cadmium, zinc, indium, and bismuth. The non-aqueous electrolyte secondary battery according to item 1. (3) Claims characterized in that the negative electrode material powder contains tin as an essential component, and is either alone or in an alloy with at least one selected from the group consisting of lead, cadmium, zinc, indium, and bismuth. The non-aqueous electrolyte secondary battery according to item 1. (4) The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode material powder is aluminum.