JPH097592A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To provide a lithium secondary battery which obtains improving a charge/discharge life, by using a lithium metal containing powder of a boron system compound in a negative electrode, so as to prevent decreasing safety generated by repeating a charge/discharge. CONSTITUTION: By including a boron system compound in a negative electrode, depositing lithium is generated preferentially from a contact surface between powder of the boron system compound and a lithium metal, to grow a crystal with powder of the boron system compound serving as a nuleus even when deposited. From this fact, a uniform deposition form can be realized as a negative electrode main unit, to prevent easily generating branch-shaped lithium even when repeated a long period charge/discharge. As the negative electrode, foil of 150μm thickness, containing uniformly 30vol.% TiB2 powder of 20μm grain size in a lithium metal, is used. Even in the case of separating by forming branch-shaped lithium, by making contact with powder of the boron system compound, since reusing in a charge/discharge can be made, improving a charge/ discharge life 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 lithium secondary battery which uses lithium as a negative electrode active material, a positive electrode into which lithium ions can be inserted and desorbed, and which uses a non-aqueous electrolyte.

[0002]

2. Description of the Related Art As electronic devices are becoming smaller and lighter and portable,
Development of a high energy density battery is required as the power source. As a battery that meets such demands, it is expected to develop a high-performance secondary battery that uses lithium as an active material for the negative electrode and can be charged and discharged. As the negative electrode using lithium as an active material, for example, lithium metal, a lithium metal alloy, or a chemical substance capable of inserting and releasing lithium ions (for example, various carbon materials, Nb ₂ O ₅ , WO ₃ etc.) is used. 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 are used, and a chargeable / dischargeable battery is a lithium battery. It is called a secondary battery.
Lithium secondary batteries are basically higher in performance than various commercially available secondary batteries, such as nickel-cadmium batteries and lead-acid batteries, but when the number of charging and discharging increases, the discharge characteristics deteriorate and Degradation has been confirmed. It is believed that this is because in a lithium secondary battery, when charging and discharging are repeated, lithium that is separated from the negative electrode and is not used for charging and discharging is deposited on the negative electrode. This peeling becomes remarkable especially when rapid charging is performed. As a measure for preventing the deterioration of the negative electrode, Japanese Patent Laid-Open Nos. 59-132567, 61-61, and 61-
As described in JP-A-245475 and JP-A-62-1403558, attempts have been made to alloy a lithium metal or compound a conductive polymer, but this is still insufficient. Moreover, in the case of a lithium aluminum alloy, there is a problem that the electrode is destroyed by expansion and contraction of the alloy by repeating charging and discharging, and furthermore, the diffusion current of lithium in the alloy during charging and discharging is slow, so the current obtained by the battery is low. There is a problem. Further, in the case of compounding a conductive polymer, there is a problem that the volumetric efficiency of the negative electrode deteriorates. Further, as described in Japanese Patent Application No. 6-103239, when a negative electrode is made of lithium metal containing a conductive powder capable of inserting and releasing lithium ions,
There is a problem that very fine lithium is deposited on the powder and the safety of the battery cannot be maintained. Further, as disclosed in JP-A-6-84512, a metal element,
Alternatively, when using a lithium negative electrode having a structure in which those clusters are present at least on the substrate surface part,
Weight is increased due to mixing of metal elements, which is an advantage of lithium batteries, lightness is impaired, it is difficult to form a thin film, and the sealed battery is exposed to the outside air by crushing, nailing, etc. In this case, there is a problem in that the metal itself generates heat by oxidation and the thermal safety of the battery is unavoidably deteriorated.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and an object thereof is to prevent the generation of lithium which is separated from the negative electrode during charging / discharging and is not used for charging / discharging. It is an object of the present invention to provide a lithium secondary battery capable of achieving improvement in discharge life and prevention of reduction in safety caused by repeated charging / discharging.

[0004]

Means for Solving the Problems The present invention will be described in brief. The present invention relates to a negative electrode using lithium as an active material, a positive electrode capable of inserting and releasing lithium ions, and a lithium salt ionically dissociable in a non-aqueous solvent. In a lithium secondary battery having an electrolytic solution in which is dissolved, the negative electrode is a lithium metal containing a powder of a boron-based compound.

The present invention will be described in more detail below. The lithium secondary battery according to the present invention is characterized in that the negative electrode contains a powder of a boron-based compound in the negative electrode, and by adopting this structure, a lithium secondary battery having a long charge / discharge life and safety can be realized. . That is, in the lithium secondary battery of the present invention, by containing the powder of the boron-based compound in the lithium metal, precipitation of lithium preferentially occurs from the contact surface between the powder of the boron-based compound and the lithium metal, and even during the deposition. The boron-based compound powder serves as nuclei for crystal growth.
From this, a uniform deposition morphology can be realized as the whole negative electrode, and branched lithium is unlikely to be generated even after repeated charge / discharge over a long period of time, and even when the branched lithium is formed and peeled off, it contacts the powder of the boron-based compound. By doing so, it can be used again for charging and discharging. From these effects, it is possible to improve the charge / discharge life.

As the powder of the boron compound, Gd
B ₂ , YB ₂ , TbB ₂ , DyB ₂ , HoB ₂ , ErB
₂ , LuB ₂ , ScB ₂ , PuB ₂ , ZrB ₂ , Mn
B, MnB ₂ , HfB ₂ , UB ₂ , TaB ₂ , NbB,
_{NbB 2, TiB 2, MoB 2} , Mo 3 B, Mo
₃ B ₅ , WB, WB ₂ , W ₃ B ₅ , AlB ₂ , VB ₂ ,
CrB ₂ , SiB ₂ , SiB ₄ , SiB ₆ , Ni ₂ B,
Ni ₃ B, NiB, FeB, Fe ₂ B, etc. can be used. Preferably, the particle size is 1 to 100 μm. When the particle size exceeds 100 μm, the area is large as a crystal growth nucleus during lithium deposition, and when the particle size is less than 1 μm, the effect as a crystal growth nucleus is expected,
The contact area for electrically contacting the separated branched lithium becomes small, and the contact effect with the separated lithium becomes poor. For the negative electrode, the above boron compound powder is sandwiched between lithium metals and rolled by a roll or a pressing machine.
It is preferable that the boron-based compound powder is added to molten lithium metal which is repeated 0 times or more, and then extrusion molding is performed to form a thin film, and the boron-based compound powder is dispersed and present in the negative electrode.

FIG. 5 shows the effect of the amount of the boron compound powder added according to the present invention on the charge / discharge efficiency. That is, FIG. 5 shows that the particle diameter of the boron-based compound powder is 20 mm.
3 is a graph showing the relationship between the powder mixture amount (volume%, horizontal axis) and the charge / discharge efficiency (%, vertical axis) when the TiB ₂ powder of No. ₂ was used. As is clear from FIG. 5, the optimum range of the powder mixing amount is 10 to 45% by volume.

Examples of the positive electrode of the lithium secondary battery of the present invention include 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. Further, as the lithium salt used in the electrolytic solution,
For example, LiAsF ₆ , LiPF ₆ , LiSbF ₆ , Li
_{CF 3 SO 3, LiN (CF} 3 SO 2) 2, LiC (C
F ₃ SO ₂ ) ₃ , LiClO ₄ , LiBF ₄ , LiAl
Cl ₄ or the like can be used. Examples of the non-aqueous solvent used for the electrolytic solution include cyclic esters such as propylene carbonate, ethylene carbonate and γ-butyl lactone, non-cyclic esters such as dimethyl carbonate and diethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, and 4 Cyclic ethers such as -methyl-1,3-dioxolane, dialkoxyethanes, acyclic ethers such as glymes, and sulfur compounds such as sulfolane can be used alone or in admixture of two or more.

[0009]

EXAMPLES The present invention will be described in more detail below with reference to examples and reference examples, but the present invention is not limited to these examples.

Reference Example 1 A negative electrode was prepared by dissolving a lithium metal thin film having a thickness of 150 μm and 1 mol / liter of LiAsF ₆ as an electrolytic solution 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 using the coin battery. The structure of the coin battery is shown in FIG. Battery discharge capacity at that time (mA
h, vertical axis) and the number of cycles (horizontal axis) are shown in FIGS.
As shown in FIG. That is, FIG. 1 is a diagram showing the structure of the coin battery. In FIG. 1, reference numeral 1 is a negative electrode case, 2 is a negative electrode, 3 is an electrolytic solution, 4 is a separator, 5 is a battery case,
6 is a positive electrode and 7 is a gasket. The negative electrode 2 is different in each example, but the positive electrode 6 is a lithium metal thin film.

Example 1 As a negative electrode, TiB ₂ having a particle size of 20 μm in lithium metal was used.
A battery was prepared in the same manner as in Reference Example 1 except that a foil having a thickness of 150 μm and containing 30% by volume of the powder uniformly was used. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. Compared to Reference Example 1, Example 1
It is clear that the cycle life of the battery has been dramatically improved.

Example 2 As a negative electrode, MnB ₂ having a particle size of 20 μm was added to lithium metal.
A battery was prepared in the same manner as in Reference Example 1 except that a foil having a thickness of 150 μm and containing 30% by volume of the powder uniformly was used. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. Compared to Reference Example 1, Example 2
It is clear that the cycle life of the battery has been dramatically improved.

Example 3 As a negative electrode, TiB ₂ having a particle size of 20 μm in lithium metal was used.
A battery was produced in the same manner as in Reference Example 1 except that a foil having a thickness of 150 μm and containing 10% by volume of the powder uniformly was used. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. Compared with Reference Example 1, Example 3 has
It is clear that the cycle life of the battery has been dramatically improved.

Example 4 As a negative electrode, MnB ₂ having a grain size of 20 μm in lithium metal was used.
A battery was produced in the same manner as in Reference Example 1 except that a foil having a thickness of 150 μm and containing 10% by volume of the powder uniformly was used. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. In comparison with Reference Example 1, Example 4 has
It is clear that the cycle life of the battery has been dramatically improved.

Example 5 A battery was prepared in the same manner as in Reference Example 1 except that a 150 μm-thick foil containing 30% by volume of FeB powder having a particle size of 20 μm uniformly contained in lithium metal was used as the negative electrode. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. It is apparent that in Example 5, the cycle life of the battery is dramatically improved as compared with Reference Example 1.

Example 6 A battery was prepared in the same manner as in Reference Example 1 except that a 150 μm-thick foil in which 30% by volume of FeB powder having a particle size of 5 μm was uniformly contained in lithium metal was used as the negative electrode. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. It is apparent that in Example 6, the cycle life of the battery is dramatically improved as compared with Reference Example 1.

Example 7 As a negative electrode, Ni ₃ B having a particle size of 20 μm was added to lithium metal.
A battery was produced in the same manner as in Reference Example 1 except that a foil having a thickness of 150 μm and containing 10% by volume of the powder uniformly was used. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. Compared to Reference Example 1, Example 7
It is clear that the cycle life of the battery has been dramatically improved.

Example 8 A battery was prepared in the same manner as in Reference Example 1 except that a 150 μm-thick foil in which 10% by volume of Ni ₃ B powder having a particle size of 5 μm was uniformly contained in lithium metal was used as the negative electrode. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. It is clear that in Example 8, the cycle life of the battery is dramatically improved as compared with Reference Example 1.

Example 9 A battery was prepared in the same manner as in Reference Example 1 except that a foil having a thickness of 150 μm containing 10% by volume of WB powder having a particle size of 20 μm uniformly contained in lithium metal was used as the negative electrode. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. It is apparent that in Example 9, the cycle life of the battery is dramatically improved as compared with Reference Example 1.

Example 10 As a negative electrode, SiB ₂ having a particle diameter of 10 μm was added to lithium metal.
A battery was produced in the same manner as in Reference Example 1 except that a foil having a thickness of 150 μm and containing 10% by volume of the powder uniformly was used. The structure of the coin battery is shown in FIG. The relationship between the discharge capacity and the number of cycles in that case is shown in FIG. Example 10 compared to Reference Example 1
It is clear that the cycle life of the battery is improved.

[0021]

As is apparent from the above description, according to the present invention, by using lithium metal containing a powder material of a boron-based compound for the negative electrode, the charge and discharge life is long,
A lithium secondary battery with high safety can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of a coin battery.

FIG. 2 is a diagram showing the relationship between the discharge capacity of a lithium secondary battery during a cycle and the number of cycles.

FIG. 3 is a diagram showing the relationship between the discharge capacity of a lithium secondary battery during a cycle and the number of cycles.

FIG. 4 is a diagram showing the relationship between the discharge capacity and the number of cycles during a cycle of a lithium secondary battery.

FIG. 5 is a diagram showing the relationship between the amount of boron compound powder mixed and the charge / discharge efficiency according to the present invention.

[Explanation of symbols]

1: negative electrode case, 2: negative electrode, 3: electrolytic solution, 4: separator, 5: battery case, 6: positive electrode, 7: gasket

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

Claims (1)

[Claims] 1. A lithium secondary battery comprising a negative electrode using lithium as an active material, a positive electrode into which lithium ions can be inserted and desorbed, and an electrolytic solution in which an ion dissociable lithium salt is dissolved in a non-aqueous solvent. Is a lithium metal containing a powder of a boron compound, and a lithium secondary battery.