JPH07296795A - Negative electrode for lithium secondary battery, and lithium secondary battery using it - Google Patents

Abstract

PURPOSE:To provide a negative electrode for lithium secondary battery minimized in the deterioration by repeat of charge and discharge and having a high charge and discharge capacity and a high energy density, and a lithium secondary battery using it. CONSTITUTION:A negative electrode for lithium secondary battery has a negative electrode material containing a hollow carbon fiber, preferably, a hollow carbon fiber having a cylindrical structure 1-500nm in diameter and 0.05-500mum in length consisting of a carbon atomic membrane in which carbon atoms are arranged in honeycomb form. A lithium secondary battery has this negative electrode, a positive electrode, and an electrolyte.

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 having excellent cycle life and high electromotive force and high energy density, and a lithium secondary battery using the same.

[0002]

2. Description of the Related Art Generally, secondary batteries are required to have high energy density, high output density, and low self-discharge rate.
It is inexpensive, has high energy efficiency, and has a long cycle life. As a secondary battery having such performance, a non-aqueous electrolyte battery utilizing electric energy due to movement of lithium ions, a so-called lithium secondary battery is known to have a high electromotive voltage and a high energy density.

In this lithium secondary battery, when pure lithium is used for the negative electrode, a negative electrode having a high energy density can be obtained, but on the other hand, there is a drawback that lithium tends to take a deposition form called dendrite during charging. This dendrite is a dendritic crystal, and once it is formed, it grows rapidly. Therefore, the dendrite penetrates through the separator and short-circuits with the positive electrode, and the cycle life of the battery is significantly shortened. Further, ignition may occur due to a short circuit between the dendrite and the positive electrode. Further, there is a problem that the capacity of the negative electrode decreases when the dendrite is cut off on the way and loses its activity.

Therefore, it is known to use a carbon material instead of pure lithium as the negative electrode material.
This carbon-based material has a graphite structure, that is, a crystal structure in which layers having a hexagonal network structure formed by carbon atoms are stacked. When the negative electrode is composed of this carbon material, lithium is inserted into these layers through these layers during charging, so that dendrite is rarely generated on the electrode surface.

However, in the crystal structure of the carbon material, the interlayer distance is about 3.54 to 3.9 Å, so the amount of lithium that can enter and leave is small. Therefore,
The negative electrode using this carbon material has a problem that charge / discharge capacity and energy density are lowered.

An object of the present invention is to solve the above problems and to provide a negative electrode for a lithium secondary battery which has a high charge / discharge capacity and a high energy density and is less likely to deteriorate due to repeated charge / discharge. . Another object of the present invention is to provide a lithium secondary battery having excellent cycle life, high electromotive force, high charge / discharge capacity, and high energy density.

[0007]

The present inventors have found that the above object can be achieved by using a specific carbon-based material, and have completed the present invention. That is, the negative electrode for a lithium secondary battery of the present invention has a diameter of 1 to 500 nm and a length of 0.05 to 500 formed of a hollow carbon fiber, preferably a carbon atom film in which carbon atoms are arranged in a honeycomb shape.
It has a negative electrode material containing hollow carbon fibers having a cylindrical structure of μm. The lithium secondary battery of the present invention has the above-mentioned negative electrode, positive electrode, and electrolyte.

[0008]

In the present invention, a hollow carbon fiber is used as the negative electrode material. Among the hollow carbon fibers, a particularly preferable one is that the carbon atoms are arranged in a honeycomb shape as shown in FIG. It has a cylindrical crystal structure consisting of a carbon atom film, and has a diameter of about 1 to 500 nm and a length of about 0.05 to
It has a thickness of 500 μm and is generally called a carbon nanotube or a bucky tube (referred to as a carbon nanotube in the present specification).

When a negative electrode is made of the above-mentioned hollow carbon fiber-containing negative electrode material, for example, as shown in the enlarged cross-sectional view of FIG. 3, even if dendrites grow during charging, they are hindered by the wall surface of this hollow carbon fiber. Therefore, it does not grow significantly.

Further, in the negative electrode material containing the hollow carbon fiber, lithium ions can enter and leave the hollow portion of the hollow carbon fiber. Since the diameter of this hollow carbon fiber is, for example, about 1 to 500 nm as described above, a large amount of lithium ions can enter and leave. Therefore, when the negative electrode is made of the negative electrode material containing this hollow carbon fiber, the decrease in charge / discharge capacity and energy density is suppressed.

The hollow carbon fiber used in the present invention may have a structure in which tubular bodies are arranged in a nested manner as shown in FIG. 2, but a large amount of lithium ions can come and go. It is desirable that the weight is 1 to 20 layers, preferably 1 to 5 layers so as to be possible.

Further, for example, in the case of carbon nanotubes, there are generally those with closed tips and those with open tips.
In the present invention, in order to allow lithium ions to move in and out as described above, it is preferable to use one having an open tip as shown in FIG.

The diameter of the hollow carbon fibers used in the present invention is usually 1 to 500 nm in the case of carbon nanotubes, but in order to allow a large amount of lithium ions to enter and exit, The thickness is preferably 500 nm, more preferably 20 to 100 nm.
If the diameter of the hollow carbon fiber is 500 nm or less,
As described above, the hollow carbon fiber can sufficiently suppress the growth of dendrite. The length of the hollow carbon fiber is usually 0.05 in the case of carbon nanotube.
˜500 μm, but in order to achieve the effect of suppressing the growth of the dendrite and the ability to be incorporated into the negative electrode material sufficiently, the thickness is 0.1 to 100 μm, and more preferably 0.5 to 50 μm. Is desirable.

The carbon nanotube has, for example, 1
By causing a direct current arc discharge between the carbon electrodes to grow a carbon deposit on the negative electrode in a container filled with an inert gas (He, Ar, etc.) of about 00 to 500 Torr, and collecting from the carbon deposit. Is what you get.

The hollow carbon fiber is contained in an amount of 1 to 10 parts by weight, preferably 2 to 5 parts by weight, per 100 parts by weight of the negative electrode material. When the hollow carbon fiber is contained in the negative electrode material in an amount of 1 part by weight or more, the negative electrode material has a porous structure due to a large number of hollow carbon fibers, and dendrite generation is more efficiently prevented, and Since a larger amount of lithium ions can enter and leave the hollow carbon fiber, the capacity of the negative electrode increases,
When the hollow carbon fibers are fixed with a binder, the binding property is sufficient. On the other hand, when the amount is 10 parts by weight or less, the degree of capacity decrease of the negative electrode due to the inclusion of the hollow carbon fiber which is an inert material is small.

In the negative electrode material, in addition to the hollow carbon fibers, 0.01 to 30 carbon particles, fullerenes (hollow carbon molecules) and the like are added to 100 parts by weight of the negative electrode material.
About 0 parts by weight may be contained.

The negative electrode for a lithium secondary battery of the present invention comprises, for example, the above hollow carbon fibers made of Cu tape, Al tape, Ni.
It is obtained by adhering it onto a current collector such as tape or Ag tape. More specifically, for example, a method in which hollow carbon fibers are pressure-bonded onto the current collector, or hollow carbon fibers are pasted It is obtained by a method such as coating on a current collector and hardening with a binder such as PVDF (polyvinylidene fluoride), polytetrafluoroethylene, or polyethylene.

The size and shape of the negative electrode are arbitrarily determined according to the size and shape of the battery (cylindrical type, square type, laminated type, coin type, button type, etc.).

FIG. 4 is a schematic view showing an embodiment of the lithium secondary battery of the present invention. In the figure, D is a lithium secondary battery, in which a separator 3 is interposed between a negative electrode 1 and a positive electrode 2, and a negative electrode can 5 that is in pressure contact with the outer surface of the negative electrode 1 and a positive electrode that is in pressure contact with the outer surface of the positive electrode 2. The cap 6 and the cap 6 are sealed with an insulator 7. In addition, 4 is a collector.

In the present invention, the negative electrode for the lithium secondary battery is used as the negative electrode 1.

On the other hand, the positive electrode of the lithium secondary battery D of the present invention
The positive electrode material that constitutes 2 is usually a lithium secondary battery.
The positive electrode material used for the positive electrode can be used, for example, V₂O_Five,
MnO₂, LiMn₂O_Four, LiCoO₂, LiNi
_0.5Co_0.5O₂, LiNiO ₂, Li-Co-P system compound
Compound oxide (LiCo_0.5P_0.5O₂, LiCo_0.4P_0.
6O₂, LiCo_0.6P_0.4O₂, LiCo_0.3Ni
_0.3P_0.4O₂, LiCo _0.2Ni_0.2P_0.6O₂etc)
A positive electrode material with an active material such as
Can increase the electromotive force and charge / discharge voltage of the battery
Li-Co-P type composite oxide can be used conveniently.

In the present invention, since hollow carbon fiber is basically used as the negative electrode material as described above, one of the above positive electrode materials that does not contain Li (V ₂ O ₅ , MnO _2).
2 ), it is necessary that Li is contained in the negative electrode material. On the other hand, when a positive electrode material containing Li (LiCoO ₂ , LiNiO ₂ , Li-Co-P-based composite oxide, etc.) is used, it is not necessary to contain Li in the negative electrode material, but a small amount (negative electrode material When 0.1 to 10 parts of Li is contained in 100 parts), even if a part of Li reacts with the electrolyte or the like and becomes inactive, it is contained in the negative electrode material. It is preferable because it can be supplemented with Li. As described above, in order to contain Li in the negative electrode material, for example, the heated and melted lithium is applied onto the current collector to which the hollow carbon fibers are pressure-bonded, and the inside of the hollow carbon fibers and the gaps between the hollow carbon fibers are filled. Li may be impregnated or the negative electrode material may be electrochemically doped with Li in an electrolytic solution.

The positive electrode material is mixed with a conductive material such as acetylene black, Ketjen black and graphite, and a binder such as polytetrafluoroethylene, polyvinylidene fluoride and polyethylene.

The positive electrode mixture obtained by blending a conductive material and a binder with the above positive electrode material is molded into an appropriate shape and size by any method such as casting, compression molding, roll molding, and the like. Positive electrode 2 of the lithium secondary battery D of the invention
Used as.

As the electrolyte used in the lithium secondary battery D of the present invention, an electrolytic solution in which salts are dissolved in an organic solvent or a solid electrolyte can be used. When the electrolyte is an electrolytic solution, the salts include LiClO ₄ , LiBF ₄ , LiPF ₆ ,
LiAsF ₆ , LiAlCl ₄ , Li (CF ₃ SO ₂ )
₂ N, etc. can be used, ethylene carbonate, propylene carbonate, dimethyl sulfoxide, sulfolane, γ
-Butyrolactone, 1,2-dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran, 1,3-dioxolane, 2-methyltetrahydrofuran, diethyl ether,
Dissolving it in an organic solvent such as dimethyl carbonate, diethyl carbonate and a mixture thereof to give a concentration of 0.1 to 3
It is used by adjusting to mol / l. This electrolyte is
Usually, it is used by impregnating or filling a separator such as a porous polymer or a glass filter.

When the electrolyte is a solid electrolyte, the above-mentioned salts are used by being mixed with polyethylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol and the like, their derivatives, mixtures and complexes. This solid electrolyte also serves as a separator for the negative electrode 1 and the positive electrode 2.

By forming the negative electrode 1, the separator (or solid electrolyte) 3, the positive electrode 2 and the like into a sheet and winding them into a spiral structure, a lithium secondary battery having a higher electric capacity can be obtained. .

[0028]

EXAMPLES Examples of the present invention will now be described in more detail. It should be noted that the present invention is by no means limited to these examples.

Example 1 (Preparation of Carbon Nanotubes) Two carbon rod electrodes having a diameter of 1 cm were placed in a glass reaction vessel, the vessel was evacuated, and then 500 Torr of He gas was introduced into the vessel. . Then, a DC voltage of 25 V was applied to the electrode (current at this time was about 200 A) to cause arc discharge, and this discharge was continued for 1 hour, and then carbon nanotubes were removed from the carbon deposit grown on the negative electrode. I collected it.

(Preparation of Negative Electrode) 100 mg of the above powdery aggregate of carbon nanotubes was pressure-bonded to a nickel expanded metal having a diameter of 18 mm, and a diameter of 18 mm and a thickness of 1
After a lithium sheet having a thickness of 00 μm was lightly pressure-bonded, it was heated to 200 ° C. under vacuum to melt lithium, and Li was impregnated inside the carbon nanotubes and in the gaps between the carbon nanotubes to obtain a negative electrode body.

(Production of Positive Electrode) Commercially available crystalline vanadium pentoxide (purity: 99.9% or more) was crushed by a ball mill and adjusted to a particle size of 20 μm or less by a sieve to obtain a positive electrode active material. Next, 80 mg of this positive electrode active material, 10 mg of acetylene black and 10 mg of polytetrafluoroethylene.
After thoroughly mixing mg, the mixture was molded into a disk shape having a diameter of 15 mm and a thickness of 0.2 mm to obtain a positive electrode body.

(Preparation of Electrolyte Solution and Separator) Volume ratio of propylene carbonate and diethyl carbonate 5
An electrolyte was prepared by dissolving 1 mol / liter of lithium perchlorate in a solution of 0:50. A 25 μm-thick porous polypropylene film was punched to a diameter of 19.0 mm to prepare a separator.

(Production of Lithium Secondary Battery) The above negative electrode body,
Using the positive electrode body and the separator, the separator 3 is provided between the negative electrode body 1 and the positive electrode body 2 so as to have the same configuration as that in FIG.
The anode case 5 that is pressed against the outer surface of the negative electrode body 1 and the stainless cathode cap 6 that is pressed against the outer surface of the positive electrode body 2 are sealed with a gasket 7 after injecting an electrolytic solution into the inside. A coin-type test lithium secondary battery D was produced.

(Charge / Discharge Test) Since the test lithium secondary battery is in a charged state, discharge is first performed at a constant current value (1 mA) up to 2.8 V, and after resting for 1 hour, a constant current value (1 mA) is set. ) And charged to 3.8 V, and then rested for 1 hour. This discharge and charge are regarded as one cycle, and this is 10
Repeated until 0 cycle.

Examples 2 to 3 The same procedure as in Example 1 was repeated except that the current value during charging / discharging was 5 mA (Example 2) and 10 mA (Example 3). A discharge test was conducted.

Comparative Example 1 In the above Example 1, a negative electrode body was prepared by pressing a lithium sheet having a diameter of 18 mm and a thickness of 100 μm onto an expanded metal made of nickel having a diameter of 18 mm without using the powdery aggregate of carbon nanotubes. A test lithium secondary battery was prepared and a charge / discharge test was conducted in the same manner except that this was used.

Comparative Examples 2 to 3 The same procedure as in Comparative Example 1 was repeated except that the current value during charging / discharging was 5 mA (Comparative Example 2) and 10 mA (Comparative Example 3). A discharge test was conducted.

Example 4 (Preparation of Negative Electrode) 50 parts by weight of the carbon nanotube powder aggregate obtained in Example 1 above, PVDF (binder)
A paste was prepared by mixing 2 parts by weight and 48 parts by weight of N-methyl-2-pyrrolidone, and
0 mg was applied on a Ni plate having a diameter of 15 mm and a thickness of 0.05 mm. Then, this was naturally dried and then vacuum dried at 120 ° C. to obtain a negative electrode body.

(Production of Positive Electrode) Lithium carbonate and basic cobalt carbonate were mixed so that the atomic ratio of Li and Co was 1: 1 and the mixture was calcined in air at 900 ° C. for 24 hours. This is further crushed in a mortar and sieved to a particle size of 20
The positive electrode active material was prepared by adjusting the thickness to less than μm. When this positive electrode active material was analyzed by X-ray diffraction, it was confirmed to be LiCoO ₂ . Then, 400 mg of this positive electrode active material,
50 mg of acetylene black and 50 mg of polytetrafluoroethylene powder were sufficiently mixed, and this mixture was molded into a disk shape having a diameter of 15 mm by a pressing machine to obtain a positive electrode body.

(Preparation of Lithium Secondary Battery and Charge / Discharge Test) A lithium secondary battery for test was prepared in the same manner as in Example 1 except that the negative electrode body and the positive electrode body were changed to those described above. Since this test lithium secondary battery is in a discharged state, first charge it at a constant current value (1 mA).
The voltage was maintained up to 2 V, and after resting for 1 hour, the battery was discharged to 2.8 V at a constant current value (1 mA) and rested for 1 hour. This charging and discharging was defined as one cycle, and this was repeated up to 100 cycles.

Comparative Example 4 A test lithium secondary battery was prepared in the same manner as in Example 4 except that natural graphite powder was used instead of the powdery aggregate of carbon nanotubes, and its charge / discharge test was conducted. It was

Evaluation of Lithium Secondary Battery In each of the above Examples 1 to 3 and Comparative Examples 1 to 3, the change in discharge capacity of the test lithium secondary battery with repeated charging and discharging is shown in FIG. And about Comparative Example 4, it was as shown in FIG. The discharge capacity is shown per 1 g of the positive electrode active material. Furthermore, when each test lithium secondary battery was disassembled after completion of charge / discharge and the inside was observed, no abnormality was observed in any of the batteries of Examples 1 to 4, whereas the batteries of Comparative Examples 1 to 4 were found. In each case, it was confirmed that dendrite grew on the negative electrode surface and penetrated the separator, and the positive electrode and the negative electrode were short-circuited.

As is clear from FIG.
In No. 3, the discharge capacity decreases as the current value increases, but the decrease in discharge capacity due to repeated charging and discharging is small. On the other hand, in Comparative Examples 1 to 3, the initial discharge capacity was almost the same as that of the Example, but the discharge capacity was remarkably decreased with the repetition of charging and discharging, and the charging and discharging did not occur at the 20th to 60th cycles. It has become possible. It is considered that this is due to a short circuit between the positive electrode and the negative electrode due to the growth of the dendrite. Further, in Comparative Example 4, although the upper limit of the number of cycles that can be charged and discharged is larger than that in Comparative Example 1,
Only about 80 cycles, which is not satisfactory.

[0044]

As described in detail above, since the negative electrode for a lithium secondary battery of the present invention is composed of the negative electrode material containing hollow carbon fibers, even if dendrite grows during charging, the hollow Since it is hindered by the wall surface of the filamentous carbon fiber, it does not grow greatly. Therefore, the negative electrode is less likely to deteriorate due to repeated charging and discharging.

In the negative electrode material containing the hollow carbon fiber, lithium ions can enter and leave the hollow portion of the hollow carbon fiber, and the diameter of the hollow carbon fiber is, for example, about 1 to 500 nm. Therefore, a large amount of lithium ions can enter and leave this hollow portion. As a result, the negative electrode has a high charge / discharge capacity and a high energy density.

Therefore, the lithium secondary battery of the present invention using the above negative electrode has excellent cycle life, high electromotive force, high charge / discharge capacity and high energy density.

[Brief description of drawings]

FIG. 1 is a schematic view showing a structure of a carbon nanotube.

FIG. 2 is a schematic view of a carbon nanotube having a nested structure.

FIG. 3 is a schematic partial enlarged cross-sectional view showing a state of a surface of a negative electrode for a lithium secondary battery of the present invention which faces a positive electrode during charging.

FIG. 4 is a schematic cross-sectional view of a lithium secondary battery showing an example of the present invention.

FIG. 5 is a graph showing changes in discharge capacity with respect to the number of cycles in charging and discharging of the test lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 3.

FIG. 6 is a graph showing changes in discharge capacity with respect to the number of cycles during charge / discharge of the test lithium secondary batteries of Example 4 and Comparative Example 4.

[Explanation of symbols]

 1 Negative electrode 2 Positive electrode 3 Separator D Lithium secondary battery

Claims (4)

[Claims] 1. A negative electrode for a lithium secondary battery, which has a negative electrode material containing hollow carbon fibers. 2. The hollow carbon fiber comprises a carbon atom film in which carbon atoms are arranged in a honeycomb shape and has a diameter of 1 to 500.
The negative electrode for a lithium secondary battery according to claim 1, which has a cylindrical structure having a length of nm and a length of 0.05 to 500 μm. 3. The negative electrode for a lithium secondary battery according to claim 1, wherein 1 to 10 parts by weight of the hollow carbon fiber is contained in 100 parts by weight of the negative electrode material. 4. A lithium secondary battery comprising the negative electrode for a lithium secondary battery according to claim 1, 2 or 3, a positive electrode and an electrolyte.