JP3152307B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a negative electrode plate of a lithium secondary battery as a power supply for driving electronic equipment or a power supply for holding a memory.

[0002]

2. Description of the Related Art As electronic devices have rapidly become smaller and lighter,
There is an increasing demand for the development of a secondary battery that is small, lightweight, has a high energy density, and that can be repeatedly charged and discharged with respect to the battery as the power source. As a secondary battery satisfying these requirements, a lithium secondary battery is most promising. This is because a lithium secondary battery can provide a high energy density secondary battery because the voltage of the battery is high and the volume and weight energy density of lithium are high because the potential of the negative electrode lithium is extremely low. This is because it has advantages.

Various materials have been proposed for the positive electrode active material of a lithium secondary battery based on the results of research. As a result, for example, an inorganic compound-based active material such as TiS ₂ , MnO ₂ , V ₂ O ₅ , and LiCoO ₂ or an organic compound-based active material such as polyaniline and polypyrrole is almost completed as having excellent charge / discharge reversibility. is there. Nevertheless, one reason that lithium secondary batteries are not put into practical use at present is that lithium, which is a negative electrode active material, has low charge / discharge reversibility.

The cause of the low charge / discharge efficiency of the lithium electrode is a so-called dissolution / deposition type charge / discharge reaction in which lithium dissolves as ions during discharging and lithium ions deposit as lithium on the negative electrode plate during charging. This is due to the formation of lithium dendrites (dendrites). The reason why dendrites are generated is that, during charging (lithium electrodeposition), lithium is unevenly distributed on the surface of lithium and electrodeposited, and lithium is preferentially deposited one after another on convexities which are more likely to be precipitated. Is thought to result from abnormal growth. The grown dendrites are easily peeled off from the negative electrode plate, and the peeled lithium reacts with the electrolytic solution to form an electrically insulating film on the surface, so that the lithium becomes electrically inactive and loses the negative electrode capacity.

[0005] When the charge / discharge cycle is repeated, the amount of inactive lithium increases, the chargeable / dischargeable lithium is consumed, and the life of the battery is exhausted. In addition, even if lithium that can be charged and discharged remains, the dendrite becomes spongy and absorbs the electrolyte, so the electrolyte between the electrodes is exhausted and the battery life is exhausted, or the dendrite penetrates through the separator and shorts out The battery life may be exhausted.

[0006]

As described above, it can be seen that the life of a lithium secondary battery is greatly affected by the generation of lithium dendrites. In order to put a lithium secondary battery into practical use, prevention of dendritic crystals generated during lithium electrodeposition is the most important issue.

An object of the present invention is to solve this problem and to provide a lithium secondary battery having a high energy density and a long charge / discharge cycle life.

[0008]

According to the present invention, in a lithium secondary battery comprising a positive electrode and a lithium or lithium alloy negative electrode, the material of the negative electrode current collector is brass, phosphor bronze, aluminum bronze, and lithium or lithium bronze is used. An object of the present invention is to provide a lithium secondary battery in which a lithium alloy is directly press-bonded to the negative electrode current collector to solve the above-mentioned problem.

[0009]

A battery was prepared using various metal materials for the negative electrode current collector, and a charge / discharge cycle test was performed. As a result, the mechanism of dendrite formation was speculated as follows.

First, it has been found that when stainless steel, nickel, or the like is used for the negative electrode current collector, dendrite is easily generated and the life of the battery is shortened. This is thought to be due to the nature of the oxide film present on the metal surface.Especially in the case of stainless steel and nickel, which have a large electric resistance, when lithium is electrodeposited, the film is relatively thin and concentrates on places with low electric resistance. It is considered that a projection was formed on the electrode plate for electrodeposition, and this was selectively grown to become a dendrite.

[0011] Therefore, when a negative electrode current collector of Cu having a relatively small electric resistance of the oxide film is used as the negative electrode current collector, dendrite is relatively hardly generated and the life of the battery is prolonged as compared with the case of using stainless steel or nickel.

As described above, it is considered that the properties of the oxide film on the metal surface are correlated with the formation of lithium dendrite. To prevent dendrite, the oxide film is removed or the oxide film has good electric conductivity. It has been found that the substrate having the current collector may be used.

[0013] However, even if a current collector having the above conditions is used, dendrite may be generated when the charging current is high, and it is not yet sufficient. Therefore, various metal materials were added to Cu for alloying, and an experiment was performed to check the correlation between the alloy composition of the negative electrode current collector and the battery life.

As a result, brass obtained by adding Zn, Sn, Al or the like which is a metal which easily alloys with lithium to Cu,
The use of phosphor bronze or aluminum bronze for the current collector greatly improved the battery life. In particular, remarkable effects were observed with brass and phosphor bronze.

The reason for this is not clear, but by adding a metal that easily alloys with lithium, the binding force between the dendrite and the current collector is improved as compared with the case where no metal is added, and the dendrite from the current collector is removed. This is probably due to the fact that the falling off of the steel became difficult. The remarkable effect was observed with brass and phosphor bronze because the oxides of Zn and Sn in the alloy have good electron conductivity, so that the surface film of the substrate was modified and synergistic with the above effects. It is considered that the effect appeared.

[0016]

The present invention will be described below with reference to preferred embodiments. Example 1 As a positive electrode active material, manganese dioxide prepared by subjecting to a vacuum drying treatment at 375 ° C. for 5 hours under a reduced pressure of 50 mmHg or less, followed by additional heat treatment at 375 ° C. for 20 hours in air in an electric furnace was used.

To 100 parts by weight of this manganese dioxide, 5 parts by weight of acetylene black (conductive agent) and 2 parts by weight of polytetrafluoroethylene (binder) were added and kneaded well. The mixture was dried with hot air for 4 hours to prepare a positive electrode mixture. Then, weigh 108mg of the positive electrode mixture
Wrapped in 325 mesh SUS316 wire mesh, 2 tons / cm
Press molding was performed in ² to obtain a positive electrode. The dimensions of the positive electrode are 15.0 in diameter
The thickness is about 0.6 mm. Before incorporating this positive electrode into the battery, hot air drying treatment was performed again at 120 ° C. for 3 hours.

For the negative electrode current collector, brass (JIS H 3100 C260
0) using a disk with a diameter of 16 mm and a thickness of 0.1 mm
Spot spot welding was performed.

The components of the brass are about 70% of Cu, about 30% of Zn, and 0.05% of Pb and Fe as impurities.

Pure lithium was used for the negative electrode. The size of the negative electrode is about 16 mm in diameter and about 0.1 mm in thickness, and the theoretical capacity is about 45 mAh. This lithium was directly pressed on the current collector at a pressure of about 100 kg / cm ² to obtain a negative electrode plate.

As a separator, a polypropylene microporous separator (trade name: Celgard K274) and a polypropylene non-woven fabric are used in layers.
A 2.0 mm high battery was created.

As the electrolyte, ethylene carbonate (E
At a volume ratio of C) and 4-methyl dioxolane (4-MeDOL) and 1,3-dioxolane 5: 3: 2.7 1mol lithium perchlorate (LiClO ₄₎ in a mixed solvent of
/ L dissolved.

FIG. 1 is a longitudinal sectional view of the battery. In the figure, 1 is a case also serving as a positive electrode terminal punched out of a stainless steel sheet having resistance to organic electrolyte by pressing, and 2 is a sealing plate also serving as a negative electrode terminal punched out of the same kind of material. The current collector 3 'is spot-welded, and the active material 3 is directly pressed on the current collector. Reference numeral 5 denotes a separator made of polypropylene impregnated with an organic electrolyte, 6 denotes a positive electrode mixture, and the inside of the sealing plate 2 which also serves as a negative electrode terminal is caulked by inwardly caulking an opening end of the case 1 which also serves as a positive electrode terminal. It is hermetically sealed by tightening the circumference.

The battery of the present invention using the negative electrode current collector made of brass is referred to as battery (A).

Example 2 A battery having the same battery configuration as that of Example 1 was prepared except that phosphor bronze (JIS H 3270 C5191) was used as the material of the negative electrode current collector of the battery. This battery of the present invention is referred to as battery (B).

The above phosphor bronze contains about 93% of Cu, Sn
About 6%, P about 0.03 to 0.35%.

Example 3 A battery having the same battery configuration as that of Example 1 was prepared except that aluminum bronze (JIS H 3100 C6280) was used as the material of the negative electrode current collector of the battery. This battery of the present invention is referred to as battery (C).

The component of the aluminum bronze is about 8 Cu.
0%, Al about 10%, Ni about 5%, Fe about 2.5%, M
n is about 1%.

Comparative Example 1 A battery having the same battery configuration as that of Example 1 was prepared except that stainless steel (SUS304) was used as the material of the negative electrode current collector of the battery. This comparative battery is referred to as battery (A).

Comparative Example 2 A battery having the same battery configuration as that of Example 1 was prepared except that nickel was used as the material of the negative electrode current collector of the battery. This comparative battery is referred to as battery (a).

COMPARATIVE EXAMPLE 3 The battery configuration was the same as that of Example 1 except that copper was used as the material of the negative electrode current collector of the battery.
A battery similar to the above was prepared. This comparative battery is referred to as a battery (C).

The above various batteries were placed in a thermostat at 25 ° C.
Charge and discharge were repeated at a constant current of 1.8 mA between 3.4 V and 2.0 V, and the discharge capacity in each cycle was measured.

FIG. 2 shows the type of battery and the change in discharge capacity as the charge / discharge cycle progresses.

The batteries (A), (B) and (C) of the present invention
Has a longer cycle life than the comparative batteries (A), (A) and (C). In particular, a remarkable effect was observed in (A) and (B).

When the batteries after the test were disassembled, it was found that all the batteries had the negative electrode powdered. When the batteries were assembled again using the positive electrodes of these batteries as they were and a charge / discharge test was performed, the discharge capacity was restored to the initial capacity. From these facts, it was found that the lifetimes of these batteries were all limited by the negative electrode. Although it is not clear why the cycle life of the negative electrode of the present invention has been extended, at least Zn, Cu,
This is presumably because the use of an alloy to which Sn, Al, or the like was added effectively prevented the generation of lithium dendrite and prevented the dendrite from falling off the current collector.

In the above-described embodiment, the case where Zn, Sn, and Al are added to Cu as the material of the negative electrode current collector has been described.
It is considered that a similar effect can be obtained when b, Bi, Cd, or the like is added to Cu, or when one or two or more metals selected from these added metals are combined.

In the above embodiment, the thickness of the current collector is 0.1
mm disk was used, but in view of the strength as a current collector, 5 mm
A similar effect can be obtained with a thickness of at least μm, and a thickness of 0.1 mm or less is desirable from the viewpoint of energy density, but a thickness greater than that does not affect the battery life.

As for the shape of the current collector, a higher effect can be obtained by using a porous body such as a net, a foam, an expanded net, a nonwoven fabric or an etched plate in addition to the above-mentioned disc. When taking, it is better that the mesh is fine,
Practically, a mesh of about 150 to 300 mesh seems to be optimal.

The positive electrode active material used in the lithium secondary battery according to the present invention is not particularly limited, and the positive electrode active material used in the conventional lithium secondary battery, ie, lithium ion or anion, and A substance that causes a reversible reaction can be used. For example, M
nO ₂ , LiMn ₂ O ₄ , LiCo _X Mn _2-X O ₂ , LiCo _X Ni _1-X O
₂ , CoO ₂ , TiS ₂ , V ₂ O ₅ and polyaniline.

The negative electrode active material is not basically limited either, and in addition to pure lithium, lithium and A
Alloys with one or more of l, Pb, Sn, Bi, Cd, P and the like can be used.

The organic solvent and the solid ionic conductor which are lithium ion conductive materials are not particularly limited, and those used in conventional lithium secondary batteries can be used. For example, examples of the organic solvent include cyclic esters such as ethylene carbonate, which are aprotic solvents, and ethers such as tetrahydrofuran and dioxolan, and a single or a mixture of two or more thereof can be used. As the solid ionic conductor, any one having lithium ion conductivity can be used. A typical example is polyethylene oxide.

The supporting electrolyte dissolved in such an organic solvent or solid ionic conductor is not basically limited. For example, LiAsF ₆ , LiClO ₄ , Li
One or more of BF ₄ , LiPF ₆ , LiCF ₃ SO _{3 and the} like can be used.

Although the batteries according to the above embodiments are button batteries, the same effects can be obtained by applying the present invention to cylindrical, square or paper batteries.

[0044]

According to the present invention, it is possible to provide a high energy density lithium secondary battery having excellent cycle life performance as compared with conventional batteries.

[Brief description of the drawings]

FIG. 1 is a diagram showing an internal structure of a button battery as an example of a lithium secondary battery.

FIG. 2 is a diagram showing an effect of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Lithium 3 'Negative electrode collector 4 Gasket 5 Separator 6 Positive electrode mixture

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 4/02 H01M 4/38 H01M 4/66 H01M 10/40

Claims (1)

(57) [Claims] 1. A lithium secondary battery comprising a positive electrode and lithium or lithium alloy negative electrode, a material of the negative electrode current collector, brass, phosphor bronze, an aluminum bronze, Li
Titanium or lithium alloy is pressed directly to the negative electrode current collector
Lithium secondary battery, characterized in that it is.