JPH09134720A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery which is capable of taking-out of a large current and excellent in the charge/discharge cyclic characteristics. SOLUTION: A lithium secondary battery concerned is composed of a negative electrode 2 made of lithium metal, a positive electrode 3, separator 5 installed between the negative and positive electrodes, and an electrolyte 6. The surface of the negative electrode 2 is covered with a porous substance 1 having communication holes of mean diameter 0.3nm to 1μm. It is preferable that the porous substance 1 is installed on the surface of negative electrode 2 on the separator 5 side. The specific surface area of this porous substance 1 is 100-3000m<2> /g. Examples of porous substance 1 are activated coal, method porous crystal, zeolite, zeolite-series compound, etc.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a lithium secondary battery, and more particularly, to a negative electrode for a lithium secondary battery which is capable of extracting a large current and has excellent charge / discharge cycle characteristics.

[0002]

2. Description of the Related Art Lithium metal is used as a negative electrode material of a lithium battery because it is electronegative and highly active. A lithium battery using such lithium metal has a high energy density (large capacity), can be made compact and lightweight, and is widely put into practical use as a primary battery. Also, the use of lithium batteries as secondary batteries is being put to practical use.

[0003]

However, when lithium metal is used as a negative electrode for a secondary battery, lithium dendrite (dendritic crystal) is deposited by charging and discharging.
This dendrite has a width of several μm [36th Symposium on Battery Symposium, p. 151 (1995) Keiichi Saito, Yasue Nemoto, Shinichi Tobishima, Junichi Yamamoto], causing a short circuit. In addition, there is a problem such as falling off from the negative electrode.

In order to cope with lithium dendrites, a battery using a lithium-aluminum alloy for the negative electrode has been proposed (Japanese Patent Laid-Open No. 52-5423). Here, since the lithium-aluminum alloy has a high energy density in the electrochemical cell, the overvoltage at the time of charging / discharging becomes low, so that the growth of dendrites can be suppressed. However, this alloy is hard and difficult to work, and there is a problem that sufficient charge / discharge cycle life cannot be obtained due to deterioration due to repeated charge / discharge.

Further, as a negative electrode for a lithium secondary battery,
Using porous carbon with a pore size of 10 μm to 1 mm as a base material,
It is known that lithium is occluded on its surface or inside (JP-A-3-216960). However, even when this negative electrode is used, since the pore size of the base material is large, problems such as lithium dendrite growing and short-circuiting due to repeated charging and discharging and the base material falling off from the negative electrode may occur. It was

In view of the above conventional problems, the present invention is to provide a lithium secondary battery capable of extracting a large current and excellent in charge / discharge cycle characteristics.

[0007]

According to a first aspect of the present invention, in a lithium secondary battery including a negative electrode made of lithium metal, a positive electrode, a separator provided between the negative electrode and the positive electrode, and an electrolyte, the surface of the negative electrode. Has an average pore diameter of 0.3 nm to 1 μm
The lithium secondary battery is characterized in that it is covered with a porous body having a communication hole.

The average pore diameter of the communicating pores of the porous body is 0.3 n.
If less than m, lithium ion (size 0.1n
Approximately m) may be blocked. On the other hand, 1μ
If it exceeds m, lithium dendrites are likely to grow.

The lithium secondary battery of the present invention comprises a negative electrode made of lithium metal and a porous body coated on the surface of the negative electrode. Therefore, the lithium ions generated in the negative electrode pass through the communication holes of the porous body and reach the electrolyte. When passing through the porous body, lithium ions pass through the communication holes (pores) of the porous body, while lithium dendrites cannot pass through. Therefore, the growth of lithium dendrite can be suppressed, and excellent charge / discharge cycle characteristics can be exhibited. In addition, since lithium metal is used for the negative electrode and the growth of lithium dendrite is suppressed, it is possible to stably extract a large current.

Further, it is preferable that the porous body is provided on the surface of the negative electrode on the side of the separator. The reason is that since lithium ions are deposited as metal on the surface of the negative electrode on the separator side during charging, the lithium dendrite generated from the negative electrode tends to grow in the direction of the separator. Therefore, the formation of dendrites can be prevented by providing the porous body on the separator side of the negative electrode.

As described in claim 3, the porous body preferably has a specific surface area of 100 to 3000 m ² / g. If it is less than 100 m ² / g, the proportion of the electrolytic solution in the porous body may decrease, which may hinder the movement of lithium ions. On the other hand, when it exceeds 3000 m ² / g, the strength of the porous body is lowered and lithium dendrite may grow while destroying the porous body.

As the porous body having the above-mentioned average pore size and specific surface area, for example, as described in claim 4,
One or more selected from the group of activated carbon, mesoporous crystal, zeolite, and zeolite-related compounds can be used. The mesoporous crystal is a crystalline silica having pores of meso-order (1 to 10 nm) in pore size and forms a three-dimensional structure.

The thickness of the porous body is 0.001 mm to 1
It is preferably 0 mm. If the thickness is less than 0.001 mm, lithium dendrite may grow, short circuit may occur during charging / discharging, or the porous body may drop off from the negative electrode surface. On the other hand, when it exceeds 10 mm, the migration of lithium ions generated in the negative electrode to the electrolyte is hindered, and a large amount of current may not be able to be taken out.

As a method for coating the surface of the lithium metal with the porous body, for example, a method in which a small amount of a binder is added to the porous body and mixed and press-molded is stacked on the lithium metal, or the binder is dissolved in a solvent. There is a method in which the solvent is evaporated after being mixed with the porous body and coated on the lithium metal. As the binder, for example, polyethylene, polytetrafluoroethylene, polypropylene, or polyvinylidene fluoride is used.

Examples of the positive electrode include LiMO
₂ (M represents at least one of Mn, Co, and Ni), or a composite metal oxide composed of LiMn ₂ O ₄ is used. In addition, it is preferable to add a conductive material such as acetylene black or ketjen black to the positive electrode in order to improve conductivity. Also, a binder such as polytetrafluoroethylene or polyethylene can be added. The positive electrode can be molded by a method such as cast molding, compression molding, roll molding and the like.

As the electrolyte, for example, an electrolyte solution in which salts soluble in an organic solvent are dissolved, or a solid electrolyte can be used. The salt is dissolved in an organic solvent and adjusted to a concentration of 0.1 to 3 mol / liter before use.
Examples of the salts include, for example, LiClO ₄ , LiB
F ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ ,
At least one selected from the group consisting of LiAlCl ₄ and Li (CF ₃ SO ₂ ) ₂ N is used.

Examples of the organic solvent include ethylene carbonate, propylene carbonate, dimethyl sulfoxide, sulfolane, γ-butyrolactone, 1,2-
One or more selected from the group of dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1,3-dioxolane, 2-methyltetrahydrofuran, diethyl ether and mixtures thereof are used.

When the electrolyte is a solid electrolyte, the above salts are added to polyethylene oxide, polypropylene oxide,
It can be used by adding it to an organic compound such as polyphosphazene, polyaziridine, or polyethylene sulfide, or a derivative thereof, or an ion conductive polymer such as a mixture thereof.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

First Exemplary Embodiment A lithium secondary battery according to an exemplary embodiment of the present invention will be described with reference to FIGS. 1 and 2. As shown in FIG. 1, the lithium secondary battery 7 of this example includes a negative electrode 2 made of lithium metal, a positive electrode 3, a separator 5 provided between the negative electrode 2 and the positive electrode 3, and an electrolytic solution 6. The surface of the negative electrode 2 on the separator 5 side is covered with the porous body 1 having an average pore diameter of 0.3 nm to 1 μm. Porous body 1 is activated carbon,
Its specific surface area is 100 to 3000 m ² / g and its thickness is 0.8 mm.

An electrolytic solution 6 is filled between the negative electrode 2 and the positive electrode 3. A negative electrode current collector 21 and a positive electrode current collector 31 are provided on the surfaces of the negative electrode 2 and the positive electrode 3. Both the negative electrode current collector 21 and the positive electrode current collector 31 are made of stainless steel.

Next, a method of manufacturing the above lithium secondary battery will be described. First, activated carbon as a porous material (having continuous pores with an average pore diameter of 2 nm, specific surface area of 3000 m ²
/ G, manufactured by Kansai Thermochemical Co., Ltd.) was mixed with polytetrafluoroethylene (manufactured by Daikin Industries, Ltd.) as a binder. The mixing ratio of the binder is 4% by weight based on the total weight of the porous body and the binder. Next, add these to 0.3G
A porous body 1 was obtained by molding with a pressure of Pa into a disk shape having a diameter of 15 mm. Next, on the surface of the porous body 1, a lithium metal as the negative electrode 2 and a stainless steel mesh (SUS304) as the negative electrode current collector 21 were sequentially laminated, and these were pressure-bonded.

Also, as a positive electrode, LiMn ₂ O ₄ 50m
g, acetylene black 3.3 mg as a conductive material, and polytetrafluoroethylene 2.3 as a binder
mg was mixed well. Then, the above mixture was molded at a pressure of 0.27 GPa on a stainless steel mesh (SUS304) serving as the positive electrode current collector 31, and the disk-shaped positive electrode 3 was formed.
I got

Further, 1 mol / liter of LiPF ₆ was added to a mixture of propylene carbonate (manufactured by Mitsubishi Chemical) and 1,2-dimethoxyethane (manufactured by Mitsubishi Chemical) at a volume ratio of 1: 1.
Was dissolved to obtain an electrolytic solution 6. Further, polypropylene (Celgard # 2400 manufactured by Hoechst Japan) was punched into a diameter of 15 mm to obtain a separator 5.

Next, the positive electrode 2 and the positive electrode current collector 21 were arranged on one side of the separator 5, and the negative electrode 3 and the negative electrode current collector 31 were arranged on the opposite side. An electrolyte solution was injected into these. As a result, a lithium secondary battery 7 was obtained.

Embodiment 2 In the lithium secondary battery of this embodiment, a mesoporous crystal body (hereinafter referred to as FSM) as a porous body is prepared. This FSM
Have communication holes with an average pore diameter of 3 nm. The specific surface area of FSM is 1000 m ² / g. And this FSM50
Polytetrafluoroethylene (manufactured by Daikin Industries, Ltd.) as a binder was mixed with mg at a ratio of 4% by weight to obtain 0.0
It was molded into a disk shape having a diameter of 15 mm and a thickness of 0.4 mm under a pressure of 5 GPa to obtain a porous body. Lithium metal as a negative electrode was pressure-bonded to this porous body. The lithium secondary battery of this example is the same as that of the first embodiment except that the above porous body is used.

Comparative Example 1 The lithium secondary battery of this example is
This is different from Embodiment 1 in that the surface of the negative electrode is not covered with the porous body. Others are the same as those in the first embodiment.

Comparative Example 2 The lithium secondary battery of this example is
As the porous body, porous carbon having an average pore diameter of the communication holes of 300 μm is used. A lithium metal layer, which is a negative electrode material, is formed in the pores of this porous body to the extent that the pores are not blocked. Others are the same as those in the first embodiment.

(Experimental Example) In this example, the charge / discharge cycle characteristics of the lithium secondary batteries of Embodiments 1 and 2 and Comparative Examples 1 and 2 were measured. When measuring
Each of the above lithium secondary batteries was assembled in a battery case for testing.

As shown in FIG. 2, the battery case for the test includes a T-shaped positive electrode support 731 and a cylindrical negative electrode support 7 for supporting the lithium secondary battery 7 from the positive electrode side and the negative electrode side.
21 and the lithium secondary battery 7 through the positive electrode support 731.
And a negative electrode terminal 724 for extracting the electricity of the lithium secondary battery 7 through the negative electrode support 721 and the container 723.

The negative electrode support 721 and the negative electrode terminal 724 are
It is fixed to the lower insulator 763. Positive electrode support 731
The positive electrode terminal 732 is fixed to the upper insulator 764. The center of the T-shaped positive electrode support 731 is inserted into a tubular intermediate insulator 762 provided between the lower insulator 763 and the upper insulator 764.

The intermediate insulator 762 and the lower insulator 763 are integrally arranged in the container 723.
The upper insulator 764 is screwed and fixed to the container 723 by the cap 75. The reference numeral 765 in FIG. 2 is a positioning pin.

The upper insulator 764, the intermediate insulator 762, the lower insulator 763 and the positioning pin 765 are made of polypropylene. Negative electrode support 721, positive electrode support 731,
The cap 75, the container 723, the positive electrode terminal 732, and the negative electrode terminal 724 are made of stainless steel (SUS316).

A lithium secondary battery was assembled in the above battery case, the current density was 2.0 mA / cm ² , and the charging voltage was 4.
Charging and discharging were repeated by setting the discharge voltage to 1V and the discharge voltage to 2V. The results are shown in Fig. 3.

As can be seen from the figure, the discharge capacity after 10 cycles is 149 mAh / g when activated carbon is used as the porous body (Embodiment 1), which is higher than that at the time of 1 cycle. It wasn't too expensive. In addition, when FSM is used as the porous body (Embodiment 2),
It did not change much with mAh / g.

On the other hand, in the case where the porous body was not provided (Comparative Example 1), it was greatly reduced to 73 mAh / g. Further, when a porous body having an average pore diameter of the communicating pores outside the present invention of 300 μm was used (Comparative Example 2), it was significantly reduced to 82 mAh / g. Further, as is known from the figure, in the first and second embodiments of the present invention, the discharge capacity hardly changed after the third or fifth time even if the number of cycles increased.
On the other hand, in Comparative Examples 1 and 2, the discharge capacity greatly decreased as the number of cycles increased. As described above, it is known that the lithium secondary battery of the present invention has a large discharge capacity and excellent charge / discharge cycle characteristics.

[0036]

According to the present invention, it is possible to provide a lithium secondary battery capable of extracting a large current and having excellent charge / discharge cycle characteristics.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a main part of a lithium secondary battery according to a first embodiment.

FIG. 2 is an explanatory diagram showing the overall structure of a test battery case in an experimental example.

FIG. 3 is a graph showing the relationship between the discharge capacity and the number of cycles of a lithium secondary battery in an experimental example.

[Explanation of symbols]

 1. . . Porous body, 2. . . Negative electrode, 3. . . Positive electrode, 5. . . Separator, 6. . . Electrolyte, 7. . . Lithium secondary battery,

Claims (4)

[Claims] 1. A negative electrode made of lithium metal, and a positive electrode,
In a lithium secondary battery including a separator provided between the negative electrode and the positive electrode and an electrolyte, the surface of the negative electrode is covered with a porous body having communication holes with an average pore diameter of 0.3 nm to 1 μm. Characteristic lithium secondary battery. 2. The lithium secondary battery according to claim 1, wherein the porous body is provided on the surface of the negative electrode on the separator side. 3. The lithium secondary battery according to claim 1, wherein the porous body has a specific surface area of 100 to 3000 m ² / g. 4. The method according to claim 1, wherein
The porous material is activated carbon, mesoporous crystal, zeolite,
And one or more selected from the group of zeolite-related compounds, a lithium secondary battery.