JPH0521091A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To achieve long life by forming a negative electrode into a porous electrode. CONSTITUTION:A porous electrode formed out of a metal that reacts with lithium and a metal that does not react with lithium, is used for a negative electrode. In this structure, change in the apparent thickness of the negative electrode is reduced thanks to the porous structure, while the performance and life of a battery are improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary battery, especially a lithium secondary battery.

[0002]

2. Description of the Related Art Secondary batteries using lithium as a negative electrode active material, so-called lithium secondary batteries, have been attracting attention because of their high operating voltage and high theoretical energy density. In a conventionally known lithium secondary battery, lithium metal is used as a negative electrode active material in a flat plate shape. Further, in Japanese Patent Application Laid-Open No. 1-97373, an alloy of lithium and aluminum is stuck as a flat plate to form a negative electrode active material.

[0003]

When a secondary battery using lithium metal as a negative electrode is discharged, the negative electrode reaction causes lithium ions to dissolve into the electrolytic solution from the negative electrode. In a flat plate-shaped electrode, lithium ions are dissolved from the surface. On the other hand, during charging, contrary to discharging, lithium ions are deposited on the surface from the electrolytic solution as metallic lithium. At this time, the deposited lithium does not necessarily have a flat plate shape, but most of the deposited lithium has a granular shape or a string shape. Therefore, when charge and discharge are repeated, the surface of the negative electrode gradually becomes rough and becomes brittle. Thus, in the conventional flat plate-shaped negative electrode, roughening of the electrode surface due to charge and discharge is unavoidable, and therefore, the apparent thickness of the negative electrode is increased, and the electrode performance is fragile and easily collapsed. The phenomenon of deterioration of mechanical strength characteristics that limits the life has emerged.

When a lithium-aluminum alloy is used as the negative electrode, it is considered that lithium ions are dissolved during discharge, and during charging, precipitated metal lithium is re-alloyed with aluminum remaining in the negative electrode. Since it is still granular and string-like, the surface of the negative electrode becomes fragile even if the surface of the negative electrode is gradually roughened after repeated charging and discharging. As described above, when the lithium aluminum alloy is used as the negative electrode, it has a characteristic that the charge and discharge efficiency is higher than that of the pure lithium electrode, but its mechanical strength is weak, so it is difficult to make it thin,
It was difficult to form a thin flat plate electrode for a cylindrical battery, which is a representative of large-capacity batteries.

The present invention has been made to solve the above problems, and an object thereof is to obtain a lithium secondary battery having a long life.

Another object of the present invention is to obtain a lithium secondary battery which can be used over the entire area and has further enhanced mechanical strength characteristics.

[0007]

In the lithium secondary battery of the present invention, the negative electrode is a porous electrode.

The lithium secondary battery of another invention of the present invention is
The negative electrode is a porous electrode formed of a metal having reactivity with lithium and a metal having no reactivity with lithium.

[0009]

In the present invention, the negative electrode of the lithium secondary battery using the porous lithium or lithium alloy as the negative electrode active material acts as follows. That is, in the discharge reaction, an amount of lithium corresponding to the magnitude of the electric current taken out becomes cations and is dissolved in the electrolytic solution, and the electrons left in the negative electrode flow through the external circuit toward the positive electrode. This reaction is
Although it occurs at the interface between the electrolytic solution and the negative electrode, in the porous negative electrode, the electrolytic solution permeates into the negative electrode, so that the reaction proceeds in the entire negative electrode. Therefore, the decrease in the apparent thickness of the negative electrode due to the dissolution of lithium is smaller than that of the flat plate electrode.

In the charging reaction, an amount of lithium corresponding to the charging current value is deposited on the interface between the electrolytic solution and the negative electrode, but the area of the interface between the electrolytic solution and the negative electrode is larger than that in the case of a flat negative electrode. Since the negative electrode is larger, the current value per unit area, so-called current density, becomes smaller and the amount of charge per unit area becomes smaller at the same charging current. If the charge amount is small, the size of the deposited lithium will be small, so
The increase in the apparent thickness of the negative electrode due to precipitation is reduced.

As described above, since the change in the apparent thickness of the negative electrode due to charging and discharging can be reduced by the present invention, the deterioration of the mechanical strength characteristics of the negative electrode that limits the battery performance such as the electrode being brittle and easy to collapse and the service life. The phenomenon could be reduced and eliminated, and the performance life of the battery could be improved.

[0012]

【Example】

Example 1. Aluminum foil with a thickness of 20 microns is 100 in width
A shredded product having a micron size was entangled into a cotton shape, which was pressed to form a 100-micron-thick aluminum porous body. Using this porous body as a cathode and lithium as an anode, 1
Using a 1: 1 mixed solvent of propylene carbonate (hereinafter abbreviated as PC) containing 1 mol of lithium perchlorate per liter and dimethoxyethane (hereinafter abbreviated as DME) as an electrolytic solution, the amount of lithium per apparent unit area is converted into the amount of electricity. A negative electrode was obtained by electrodeposition of lithium so as to obtain 20 mAh. A positive electrode using lithiated manganese dioxide as an active material was opposed to the negative electrode via a polypropylene porous membrane separator, and this was incorporated into a 2025 size coin battery. FIG. 1 is a characteristic diagram showing a change in discharge capacity (ratio where the initial discharge amount is 1) according to the number of charge / discharge cycles comparing the present invention with a conventional example, in which 1 is a battery of one embodiment of the present invention. At a current density of 3 mA, a discharge lower limit voltage of 2 V, and a charge upper limit voltage of 3.8 V,
The change of discharge capacity for every cycle is shown.

Example 2. Lithium-aluminum alloy powder in which the atomic ratio of aluminum and lithium is 1: 1 and polyethylene oxide (hereinafter abbreviated as PEO) are mixed at a weight ratio of 9: 1, and then hot pressed to a thickness of 100 μm. A negative electrode was prepared by impregnating a 1: 1 mixed solvent of PC and DME containing 1 mol of lithium perchlorate per liter. In the same manner as in Example 1, a positive electrode using lithiated manganese dioxide as an active material was made to face the negative electrode through a polypropylene porous membrane separator, which was rolled up to form a 2/3 A size cylindrical type. Built into the battery. In FIG. 1, 2 indicates the change in the discharge capacity for each cycle when the battery of the other example of the present invention was charged and discharged at a current density of 400 mA, a discharge lower limit voltage of 2 V, and a charge upper limit voltage of 3.8 V. .

Example 3. A 20-micron-thick aluminum foil cut into 100-micron-width pieces, and a 20-micron-thick titanium foil cut into 100-micron-width pieces were mixed in a weight ratio of 3: 1 and entangled in cotton. Then, this was pressed to form an aluminum-titanium porous body having a thickness of 100 μm. This porous body is used as a cathode,
Propylene carbonate containing 1 mol of lithium perchlorate per liter (hereinafter PC)
Abbreviated) and dimethoxyethane (hereinafter abbreviated as DME) as a 1: 1 mixed solvent as an electrolytic solution, and lithium is electrodeposited so that the amount of lithium per apparent unit area is 20 mAh in terms of electricity. And A positive electrode using lithiated manganese dioxide as an active material was opposed to the negative electrode via a polypropylene porous membrane separator, and this was incorporated into a 2025 size coin battery. In FIG. 1, 3 is a battery of another embodiment of the present invention, which has a current density of 3 mA, a discharge lower limit voltage of 2 V,
The change in discharge capacity for each cycle when charging / discharging at a charge upper limit voltage of 3.8 V is shown.

Comparative Example 1. A 100-micron-thick lithium plate was used as a negative electrode, and a positive electrode using lithiated manganese dioxide as an active material was made to face the negative electrode through a polypropylene porous membrane separator, and this was incorporated into a 2025 size coin battery. It is. In FIG. 1, A indicates the change in discharge capacity for each cycle when this battery was charged and discharged at a current density of 3 mA, a discharge lower limit voltage of 2 V, and a charge upper limit voltage of 3.8 V.

Comparative Example 2. Using a 100-micron-thick lithium plate as a negative electrode, a positive electrode using lithiated manganese dioxide as an active material is opposed to the negative electrode via a polypropylene porous membrane separator, and rolled up to form a 2/3 A size sheet. It was incorporated into a cylindrical battery. In FIG. 1, B
This battery, current density 400mA, discharge lower limit voltage 2
V shows the change in discharge capacity for each cycle when charged and discharged at a charging upper limit voltage of 3.8 V.

As shown in FIG. 1, the battery using the negative electrode of the embodiment of the present invention is of coin type or cylindrical type.
Compared to the conventional example, the capacity was reduced less every cycle and the life was long.

In this example, lithiated manganese dioxide was used for the positive electrode, but it goes without saying that the use of other reversible positive electrode active material as a secondary battery is not prohibited. The electrolytic solution used in the same manner is not limited to the electrolytic solution used in the examples, and may be any electrolytic solution having lithium ion conductivity.

Even when a solid electrolyte having a lithium ion conductivity is used, the same effect can be obtained if the solid electrolyte is present in the pores of the porous negative electrode according to the present invention. Alternatively, the same effect as that of the embodiment can be obtained by putting the liquid electrolyte as used in the embodiment in the pores of the porous negative electrode according to the present invention and bringing it into contact with the lithium ion conductive solid electrolyte.

The porous electrode according to the present invention is a porous body formed by intertwining fibrous metal or powdery metal, binding with a binder, pressure bonding, welding, or sintering. Can be obtained in the same manner as in the above embodiment. Further, as the above-mentioned metal, for example, aluminum, aluminum alloys, and metals having no reactivity with lithium can be used together.

[0021]

As described above, according to the present invention, a lithium secondary battery having a long life can be obtained by using a negative electrode having a porous electrode, and another invention of the present invention is: By using a negative electrode that is a porous electrode formed of a metal that has reactivity with lithium and a metal that does not have reactivity with lithium, it is possible to use the whole area in addition to the above effects, and to improve mechanical properties. A lithium secondary battery with enhanced strength characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a change in discharge capacity according to the number of charge / discharge cycles comparing the present invention with a conventional example.

[Explanation of symbols]

1 Characteristics of Example 1 2 Characteristics of Example 2 3 Characteristics of Example 3

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[Procedure amendment]

[Submission date] September 27, 1991

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

[0012]

EXAMPLES Example 1. Aluminum foil with a thickness of 20 microns has a width of 100
A shredded product having a micron size was entangled into a cotton shape, which was pressed to form a 100-micron-thick aluminum porous body. Using this porous body as a cathode and lithium as an anode, 1
Using a 1: 1 mixed solvent of propylene carbonate (hereinafter abbreviated as PC) containing 1 mol of lithium perchlorate per liter and dimethoxyethane (hereinafter abbreviated as DME) as an electrolytic solution, the amount of lithium per apparent unit area is converted into the amount of electricity. A negative electrode was obtained by electrodeposition of lithium so as to obtain 20 mAh. A positive electrode using lithiated manganese dioxide as an active material was opposed to the negative electrode via a polypropylene porous membrane separator, and this was incorporated into a 2025 size coin battery. FIG. 1 is a characteristic diagram showing a change in discharge capacity (ratio of initial discharge amount to 1) according to the number of charge / discharge cycles comparing the present invention with a conventional example. Current 3mA, discharge lower limit voltage 2
V shows the change in discharge capacity for each cycle when charged and discharged at a charging upper limit voltage of 3.8 V.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

Example 2. Lithium-aluminum alloy powder in which the atomic ratio of aluminum and lithium is 1: 1 and polyethylene oxide (hereinafter abbreviated as PEO) are mixed at a weight ratio of 9: 1, and then hot pressed to a thickness of 100 μm. A negative electrode was prepared by impregnating a 1: 1 mixed solvent of PC and DME containing 1 mol of lithium perchlorate per liter. In the same manner as in Example 1, a positive electrode using lithiated manganese dioxide as an active material was made to face the negative electrode through a polypropylene porous membrane separator, which was rolled up to form a 2/3 A size cylindrical type. Built into the battery. In Figure 1, 2 a battery of another embodiment of the present invention, electrostatic
Current 400 mA, discharge lower limit voltage 2 V, charge upper limit voltage 3.8
The change in discharge capacity for each cycle when charged and discharged with V is shown.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

Example 3. A 20-micron-thick aluminum foil cut into 100-micron-width pieces, and a 20-micron-thick titanium foil cut into 100-micron-width pieces were mixed in a weight ratio of 3: 1 and entangled in cotton. Then, this was pressed to form an aluminum-titanium porous body having a thickness of 100 μm. This porous body is used as a cathode,
Propylene carbonate containing 1 mol of lithium perchlorate per liter (hereinafter PC)
Abbreviated) and dimethoxyethane (hereinafter abbreviated as DME) as a 1: 1 mixed solvent as an electrolytic solution, and lithium is electrodeposited so that the amount of lithium per apparent unit area is 20 mAh in terms of electricity. And A positive electrode using lithiated manganese dioxide as an active material was opposed to the negative electrode via a polypropylene porous membrane separator, and this was incorporated into a 2025 size coin battery. In FIG. 1, 3 is a battery according to another embodiment of the present invention, which is discharged at each cycle when the battery is charged and discharged at a current of 3 mA, a discharge lower limit voltage of 2 V, and a charge upper limit voltage of 3.8 V. The change in capacity is shown.

[Procedure amendment 4]

[Document name to be amended] Statement

[Name of item to be corrected] 0015

[Correction method] Change

[Correction content]

Comparative Example 1. A 100-micron-thick lithium plate was used as a negative electrode, and a positive electrode using lithiated manganese dioxide as an active material was made to face the negative electrode through a polypropylene porous membrane separator, and this was incorporated into a 2025 size coin battery. It is. In FIG. 1, A indicates that this battery has a current of 3 mA, a discharge lower limit voltage of 2 V, a charge upper limit voltage of 3.
The change in discharge capacity for each cycle when charged and discharged at 8 V is shown.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

Comparative Example 2. Using a 100-micron-thick lithium plate as a negative electrode, a positive electrode using lithiated manganese dioxide as an active material is opposed to the negative electrode via a polypropylene porous membrane separator, and rolled up to form a 2/3 A size sheet. It was incorporated into a cylindrical battery. In FIG. 1, B
Shows the change in discharge capacity for each cycle when this battery was charged and discharged at a current of 400 mA, a discharge lower limit voltage of 2 V, and a charge upper limit voltage of 3.8 V.

Claims (2)

[Claims] 1. A lithium secondary battery in which the negative electrode is a porous electrode. 2. A lithium secondary battery in which the negative electrode is a porous electrode formed of a metal having reactivity with lithium and a metal having no reactivity with lithium.