JPH0750162A - Negative electrode for lithium secondary battery - Google Patents

Abstract

(57) [Abstract] [Purpose] By suppressing the formation of dendritic lithium (dendrites) on the negative electrode during charge and discharge, the generation of lithium that is not used in the charge and discharge reaction is prevented and the charge and discharge cycle life is improved. The purpose is to prevent the safety of the lithium secondary battery from being lowered due to repeated charging and discharging. The negative electrode has a multi-layered structure in which two or more layers of lithium metal and layers of a substance other than lithium metal are alternately laminated. [Effect] By adopting a layered structure in which lithium metal and another substance are alternately present in the negative electrode, a lithium secondary battery having a long charge / discharge life and high safety can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode for a lithium secondary battery, more specifically, a lithium secondary battery using a non-aqueous electrolyte as a negative electrode active material and a positive electrode into which lithium ions can be inserted and released. Of the negative electrode.

[0002]

2. Description of the Related Art As electronic devices are becoming smaller and lighter and more portable, development of high energy density batteries is required as a power source for the electronic devices. As a battery that meets such demands, it is expected to develop a high performance rechargeable battery that uses lithium as an active material for the negative electrode and can be charged and discharged. Examples of the negative electrode using lithium as an active material include, for example, lithium metal, lithium metal alloys, or chemical substances capable of inserting and releasing lithium ions (for example, various carbon materials, Nb ₂ O ₅ , WO ₃
However, the negative electrode that enables the highest energy density in principle is a battery using lithium metal as the negative electrode. In the present specification, hereinafter, a lithium metal is used for a negative electrode, a positive electrode capable of inserting and releasing lithium ions and an electrolytic solution in which an ion dissociable lithium salt is dissolved in a non-aqueous solvent, and a chargeable / dischargeable battery is a lithium battery. It is called a secondary battery.

Lithium secondary batteries are basically various types of commercially available secondary batteries such as nickel-cadmium batteries,
It has higher performance than lead-acid batteries, but it has been confirmed that the discharge characteristics deteriorate and the safety deteriorates as the number of charge and discharge increases. It is considered that this is because in a lithium secondary battery, when charge and discharge are repeated, dendritic lithium (dendrites) grows on the negative electrode, and the dendritic lithium peels off and is not used for charge and discharge. In addition, the growth of the dendritic lithium increases the risk of breaking through the separator that electrically insulates the positive electrode and the negative electrode and causing an internal short circuit, which is also a safety problem. The reason why dendritic lithium (dendrites) is generated is that during battery charging, deposition occurs preferentially on the convex portions of the lithium surface, and this phenomenon is repeated and promoted by repeated charging and discharging. It is believed that

As a measure for preventing the generation of the dendritic lithium (dendrites), Japanese Patent Laid-Open No. 132567/1984 has been adopted.
JP-A-61-245475, JP-A-62
As described in US Pat. No. 1,403,558, attempts have been made to alloy lithium metal or compound conductive polymers, but they are still insufficient. Moreover, in the case of a lithium aluminum alloy, there is a problem that the negative electrode itself is destroyed by expansion and contraction of the alloy by repeating charging and discharging, and furthermore, the diffusion speed of lithium in the alloy during charging and discharging is slow, so the current obtained by the battery There was a problem that was low. Further, when the conductive polymer is compounded, there is a problem that the volumetric efficiency of the negative electrode is deteriorated.

As a measure for preventing the deterioration of the negative electrode, an additive such as Journal of Power Sources, 20 (1987) p253-
By mixing hexadecane, dicyclohexylethane, etc. in an electrolytic solution as described in p258 and the like, and forming a film composed of a reaction product of lithium and the above substance, or an adsorption film of the above substance on the lithium metal surface, Methods have been taken to suppress the formation of dendritic lithium (dendrites). However, if charging and discharging are repeated for a long time,
For example, the reaction between the additive and the electrolytic solution, the reaction between the additive and lithium, the reaction between the electrolytic solution and the compound with lithium, or the electrolysis of the additive itself occurs, and the effect as the additive is connected until the end of charge and discharge. There was a problem that was not done.

The present invention has been made in view of the above circumstances, and an object thereof is to suppress the formation of dendritic lithium (dendrites) on the negative electrode during charge and discharge,
The purpose of the present invention is to prevent the generation of lithium that is not used in the charge / discharge reaction, improve the charge / discharge cycle life, and prevent the safety of the lithium secondary battery from decreasing due to repeated charge / discharge.

[0007]

In order to solve the above problems, the negative electrode for a lithium secondary battery according to the present invention is a negative electrode for a lithium secondary battery in which the negative electrode active material is lithium. It is characterized in that it has a multi-layer structure in which two or more layers of metal layers and layers of materials other than lithium metal are alternately laminated.

The present invention will be described in more detail.

The lithium secondary battery according to the present invention can realize a safe lithium secondary battery having a long charge / discharge life by adopting the structure described above for the negative electrode. The negative electrode of the lithium secondary battery of the present invention is specifically an inorganic and organic solvent for the negative electrode, and a solution prepared by dissolving a metal salt in these is applied to the lithium metal surface, or a gas is flowed to the lithium surface to form lithium. Of the reaction substance film of, or a solution and gas in which an inorganic or organic solvent adsorbed to lithium and a metal salt is dissolved, or a layer in which the reaction substance or the adsorbed substance is mixed is formed. By forming a layered structure in which is alternately present, connection of the additive effect is expected even when charging and discharging are repeated, and the improvement of charge and discharge cycle life can be realized.

Examples of the substance other than lithium metal to be reacted with lithium of the present invention include, but are not limited to, benzene, toluene, decalin, naphthalene, xylene, benzylnaphthalene, thiourea, diethylthiourea and rhodamine. Acid B, thiazine paint, triphenyl paint, safranine paint, tetraglyme, diethyl ether, diethylene glycol, acetylcholine,
Triphenylmethane, potassium carbonate, nickel sulfate, nickel chloride, dibasic ammonium phosphate, dibasic sodium phosphate, trithiocarboxylic acid ester, polyglycol ester, dithiocarbamate, sulfonate, sulfone, carboxylic acid, alkylenecarboxyester , Alkylene aldehyde, allyl aldehyde, azine, thiazine, quinidine, pyrimidine, imidazole,
Pyridinium, potassium pyrophosphate, mercaptobenzidiimidazole, carbon monooxide 2-methylthiophene, thiophene, 2-methylfuran, pyrrole, 4
-Methylthiazole, diethyl ether, crown ether, hexadecane, dicyclohexylethane and the like can be used.

Particularly, a solvent having a large polarization can be preferably used. When it is present as a solid at room temperature, it is dissolved in the electrolyte solution used when used as a battery, and then the whole solution is applied onto the lithium metal negative electrode. Particularly preferred are, for example, benzene, toluene, and xylene which have an adsorbing property to lithium metal. As a material for the negative electrode, the above substance is applied onto lithium metal and rolled with a folding roll or a press, and this (roll, rolling) operation is repeated a plurality of times, or the above substance is added to molten lithium metal. After that, it is preferable to perform extrusion molding, fold and roll again to form the negative electrode.

As the positive electrode of the lithium secondary battery of the present invention, any positive electrode can be used, and preferred materials are, for example, Li _x CoO ₂ (0 ≦ x ≦ 1) and Li _x Ni.
O ₂ (0 ≦ x ≦ 1), Li _x Mn ₂ O ₄ (0 ≦ x ≦ 1), crystalline or amorphous V ₂ O ₅ , Li _x V ₃ O ₈ (0 <x ≦
1), TiS ₂ , NbSe ₃ or the like can be used. The lithium salt used in the electrolytic solution is not particularly limited, but examples thereof include LiAsF ₆ , LiPF ₆ , and LiSbF.
₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC
(CF ₃ SO ₂ ) ₃ , LiClO ₄ , LiBF ₄ , LiAl
Cl ₄ or the like can be used. The non-aqueous solvent used for the electrolytic solution is not particularly limited, but propylene carbonate, ethylene carbonate, cyclic ester such as Γ-butyl lactone, non-cyclic ester such as dimethyl carbonate and diethyl carbonate, tetrahydrofuran, 2-
Methyl tetrahydrofuran, 1,3-dioxolane, 4
A cyclic ether such as -methyl-1,3-dioxolane, an acyclic ether such as dialkoxyethane or glymes, or a sulfur compound such as sulfolane can be used alone or in combination of two or more kinds.

[0013]

Comparative Example 1 A negative electrode was prepared by dissolving a lithium metal thin film having a thickness of 150 μm and an electrolyte solution of 1 mol / l LiAsF ₆ in a mixed solvent of ethylene carbonate and propylene carbonate (volume mixing ratio: 1: 1). A coin battery (diameter 23 mm, thickness 2 mm) was produced. A lithium metal thin film was used as the positive electrode. The structure of the coin battery is shown in FIG. In the figure, 1 is a negative electrode case, 2 is a negative electrode, 3
Is an electrolytic solution, 4 is a separator, 5 is a positive electrode case, 6 is a positive electrode, and 7 is a gasket.

This battery is set to 0.4 mA (0.176 mA).
/ MA ² ) discharge current for 18 hours, 0.8mA
1 charging operation at (0.352mA / cm ² ) for 9 hours
As a cycle, after repeating the charge / discharge cycle 10 times, −2.0 V at 6 mA (2.65 mA / cm ² ).
Discharged up to. The charging / discharging efficiency in that case is shown in Table 1.

[0015]

Example 1 Similar to Comparative Example 1 except that a negative electrode (thickness 150 μm) prepared by applying benzene to lithium metal, folding the coated surface inside, and rolling the lithium metal 10 times was used. A battery was prepared and the charging / discharging efficiency was determined by the same method. The negative electrode was a metal thin film having a layer structure of benzene and a reaction product of benzene and lithium metal, and lithium metal. Table 1 shows the average charging / discharging efficiency for each charging / discharging cycle obtained at that time. It is apparent that the charging / discharging efficiency of Example 1 is dramatically improved as compared with Comparative Example 1.

[0016]

Example 2 A battery was produced in the same manner as in Comparative Example 1 by the same method as in Example 1 except that lithium metal was coated with toluene, and the charge / discharge efficiency was determined by the same method. The negative electrode was a metal thin film having a layer structure of toluene and a reaction product of toluene and lithium metal, and lithium metal. The charging / discharging efficiency in that case is shown in Table 1. Example 2 compared to Comparative Example 1
It is clear that the charge and discharge efficiency is dramatically improved.

[0017]

Example 3 A negative electrode (thickness 150 μm) prepared by repeatedly folding lithium metal in a carbon dioxide atmosphere 10 times.
A battery was prepared in the same manner as in Comparative Example 1 except that m) was used, and the charge / discharge efficiency was determined by the same method. The charging / discharging efficiency in that case is shown in Table 1. It is apparent that the charging / discharging efficiency of Example 3 is dramatically improved as compared with Comparative Example 1.

[0018]

EXAMPLE 4 1 mol / l of LiAsF ₆ and 0.1 mol / l of thiourea dissolved in a mixed solvent of ethylene carbonate and propylene carbonate (volume mixing ratio: 1: 1) were applied to lithium metal. A battery was prepared in the same manner as in Comparative Example 1 except that the negative electrode (thickness 150 μm) prepared by repeating folding 10 times was used, and the charge / discharge efficiency was determined by the same method. The charging / discharging efficiency in that case is shown in Table 1. It is clear that the charging / discharging efficiency of Example 4 is improved as compared with Comparative Example 1.

[0019]

Example 5 1 mol / l of LiAsF ₆ and 0.01 mol / l of nickel chloride dissolved in a mixed solvent of ethylene carbonate and propylene carbonate (volume mixing ratio: 1: 1) were applied to lithium metal. , Folding 1
A battery was prepared in the same manner as in Comparative Example 1 except that the negative electrode (thickness 150 μm) repeatedly prepared 0 times was used, and the charge / discharge efficiency was determined by the same method. The charging / discharging efficiency in that case is shown in Table 1. It is apparent that the charging / discharging efficiency is improved in Example 5 as compared with Comparative Example 1.

[0020]

[0021]

As is apparent from the above description, according to the present invention, since the negative electrode has a layered structure in which lithium metal and another substance are alternately present, the charge / discharge life is long and the safety is high. A secondary battery can be realized.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the structure of a coin battery.

[Explanation of symbols]

 1 Negative electrode case 2 Negative electrode 3 Electrolyte 4 Separator 5 Battery case 6 Positive electrode 7 Gasket

Front page continuation (72) Inventor Junichi Yamaki 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. A negative electrode for a lithium secondary battery, wherein the negative electrode active material is lithium. In the negative electrode, layers of lithium metal and layers of a substance other than lithium metal are alternately formed.
A negative electrode for a lithium secondary battery, which has a multilayer structure in which at least one layer is laminated. 2. The substance other than lithium metal is selected from lithium metal, a gas, an organic solvent, an inorganic solvent in which a metal salt is dissolved, an organic solvent alone in which a metal salt is dissolved, or a mixture with an inorganic solvent. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode is a reaction product substance with at least one of the above, or a mixture of the reaction product substances.