JP2000058066A - Secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the conductivity of a positive electrode. SOLUTION: This secondary battery has a positive electrode, containing carbon black, nonfibrous graphite particle and fibrous carbon, is equipped with a negative electrode composed mainly of a carbon material capable of storing/ emitting, for example, lithium, the positive electrode composed mainly of a transition metal multiple oxide, and an electrolytic solution made by dissolving an electrolyte of a lithium salt in a nonaqueous solvent. The positive electrode contains acetylene black, flake graphite, vapor phase epitaxy fibrous graphite, and a binder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary battery such as a lithium secondary battery provided with a positive electrode characterized by a conductive agent contained therein.

[0002]

2. Description of the Related Art Generally, in order to make a battery having good characteristics,
It is necessary to increase the conductivity of the electrodes constituting the battery and reduce the internal resistance. In particular, in the case of a positive electrode, an oxide having poor conductivity is often used as a positive electrode active material, and thus, a positive electrode is often produced using a positive electrode mixture in which a positive electrode active material and a conductive agent are mixed.

For example, since transition metal composite oxides such as LiCoO ₂ used as a positive electrode active material of a lithium secondary battery have low conductivity, in a cathode using these, acetylene black or the like is used as a conductive agent in the production thereof. Are mixed and used (for example, JP-A-6-333558 and JP-A-7-19271).
8 etc.). In this case, two types of carbon-based conductive agents are used as a mixture because carbon black alone does not provide sufficient conductivity, and graphite powder alone improves conductivity but adds a new porosity. The problem is that it is not possible to obtain sufficient ion conductivity due to small size, and furthermore, it is difficult to obtain sufficient cycle characteristics by itself, and so on. By mixing with graphite powder, improvement in conductivity of the positive electrode has been achieved while maintaining cycle characteristics, ion conductivity, and the like.

[0004]

However, the current batteries are required to further improve their performance, and the positive electrodes are also required to further improve the conductivity.

The present invention has been made in view of the above,
An object of the present invention is to provide a secondary battery including a positive electrode with improved conductivity.

[0006]

The present invention provides a conventional positive electrode to which carbon black and non-fibrous graphite particles are added,
It was made by finding that the conductivity can be further increased by adding fibrous carbon.

That is, the present invention relates to a battery comprising a positive electrode containing carbon black, non-fibrous graphite particles and fibrous carbon. In particular, acetylene black is used as carbon black. It is preferable that the fibrous graphite particles contain flaky graphite, and the fibrous carbon contains fibrous graphite, especially fibrous graphite obtained by vapor phase growth.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be specifically described by describing an embodiment applied to a nonaqueous electrolyte lithium secondary battery which is a preferred embodiment of the present invention.

In a non-aqueous electrolyte lithium secondary battery according to the present invention, for example, as shown in a schematic perspective view for explaining the structure of the battery in FIG. The non-aqueous electrolytic solution and the power generating element 1 whose cross section is formed into an elliptical shape and the nonaqueous electrolytic solution are housed in a rectangular battery container 2. In the battery shown in the figure, 3 is a safety valve, 4 is a positive electrode terminal, and the negative electrode is also used by the battery container 2.

The positive electrode used for the non-aqueous electrolyte lithium secondary battery includes, for example, a positive electrode active material, carbon black and graphite particles on a metal current collector such as an Al sheet or a stainless sheet.
It can be prepared by applying and forming a mixture of fibrous carbon and a binder, but the only difference is that fibrous carbon is added. It can be produced by a method.

As the positive electrode active material, for example, Li _X MO ₂
(M is a transition metal) (at least one of M a transition metal, N is the at least one non-transition metal) complex oxide and Li _x M _y N _z O ₂ with lithium and a transition metal represented by the represented by Composite oxides of lithium, a transition metal, and a non-transition metal, metal sulfides such as titanium disulfide, and metal oxides such as vanadium pentoxide and molybdenum trioxide can be used. In particular, lithium cobalt composite oxides ( LiCoO ₂ ), lithium-containing nickel-cobalt composite oxide (LiNi _X Co _{1 -X} O ₂ ), and spinel type lithium manganese oxide (LiMn ₂ O ₄ ) are preferable.

As the binder, for example, polytetrafluoroethylene, a rubber-based polymer, a mixture of these with a cellulose-based polymer, a copolymer mainly containing polyvinylidene fluoride, and the like can be used.

As carbon black, any of furnace black, channel black, thermal black and lamp black can be used. Among them, thermal black such as Ketjen black and acetylene black is preferred, and acetylene black is particularly preferred. It is preferably used.

[0014] Carbon black generally has a small particle size, and when added to a positive electrode, can be uniformly dispersed even in a fine portion inside. For this reason, in the present invention, carbon black mainly plays a role of forming a conductive path to details in the electrode, and plays a role of improving discharge characteristics under a large current. Also, by adding this, the addition amount of non-fibrous graphite particles and fibrous carbon can be reduced, and the total amount of carbon black, non-fibrous graphite particles, and fibrous carbon that do not work as an active material can be reduced. It becomes possible and plays a role in increasing the capacity density of the positive electrode. Therefore, the size thereof is preferably smaller than that of non-fibrous graphite particles or fibrous carbon.

As the non-fibrous graphite particles, scaly, massive, hammer-shaped particles and the like can be used, and among them, scaly particles are preferably used. Non-fibrous graphite particles have good crystallinity and high conductivity as compared with carbon black, and therefore play a role of mainly increasing the conductivity of the positive electrode when added to the positive electrode. Therefore, the higher the crystallinity, the better, but it does not need to be perfect graphite crystals, and may be natural graphite or artificial graphite.

The fibrous carbon is, like the non-fibrous graphite particles,
It has good crystallinity and high conductivity compared to carbon black, and plays a role of mainly increasing the conductivity of the positive electrode by being added to the positive electrode. However, in addition to the above non-fibrous graphite particles, it is necessary to further add this. This serves to further increase the conductivity of the positive electrode. In addition, fibrous graphite has a large fiber length of, for example, 300 μm, and one fiber can carry electronic conductivity over a long distance, and a conductive path between particles is formed by movement of an active material due to charge and discharge. It is less affected by being cut off, and acts to always maintain a constant conductivity regardless of repetition of charging and discharging. Further, the fibrous carbon also plays a role of suppressing the swelling of the positive electrode due to repeated charging and discharging of the secondary battery, and contributes to the improvement of the repetitive use characteristics of the secondary battery.
As for the crystallinity, as in the case of the non-fibrous graphite particles, high crystallinity is preferable, and fibrous graphite is preferable. Again, the fibrous graphite need not be perfect graphite crystals.
In addition, since fibrous graphite is not naturally available, it is preferable to use, for example, fibrous graphite heat-treated at 2400 ° C. or higher, and more preferably fibrous graphite obtained by vapor-phase growth. .

The negative electrode, the separator, the electrolyte and the like are not particularly different from those conventionally used, and those usually used can be used.

For example, as the negative electrode, lithium metal,
A lithium alloy such as a Li-Al alloy, a carbon material capable of inserting and extracting lithium, and a metal oxide can be used.

As the electrolytic solution, a solution obtained by dissolving an electrolyte which is a lithium salt in a non-aqueous solvent can be used. For example, a solvent having a high dielectric constant such as propylene carbonate, ethylene carbonate, γ-butyrolactone, or sulfolane can be used. , 2-dimethoxyethane, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, methyl propionate,
Use a mixture of low-viscosity solvents such as ethyl propionate, and use LiClO ₄ , LiBF ₄ , LiP
F ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN
(CnF _{2n + 1} SO ₂ ) ₂ (where n is 1, 2, 3, or 4 independently) can be used.

Incidentally, a solid polymer electrolyte is used in place of the electrolyte,
Alternatively, a gel electrolyte can be used.For example, a polymer solid electrolyte obtained by adding the above-mentioned mixed solvent or solution to a fluororubber-based polymer material, or a gel obtained by adding an acrylic monomer to the above-mentioned solvent or solution and polymerizing the same Electrolyte and the like can be used. In this case, the separator can be omitted.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a negative electrode mainly composed of a carbon material capable of inserting and extracting lithium, which is a more preferred embodiment of the present invention,
A positive electrode mainly composed of a transition metal composite oxide, and an electrolytic solution in which an electrolyte that is a lithium salt is dissolved in a non-aqueous solvent; and the positive electrode includes acetylene black, flaky graphite, and vapor-grown fibrous graphite. The present invention will be described in more detail by showing examples of a secondary battery having a configuration containing a binder and a binder.

Using LiCoO ₂ as a positive electrode active material,
89% by weight of iCoO _{2 were} mixed with ₂ % by weight of carbon black, 2% by weight of flaky graphite, 1% by weight of vapor-grown fibrous graphite, and 6% by weight of polyvinylidene fluoride (PVdF). Methyl-2-pyrrolidone (NMP) was appropriately added to obtain a slurry (paste). Next, this positive electrode mixture slurry was uniformly applied to a 20-μm-thick strip-shaped aluminum foil, dried, and roll-pressed to prepare a strip-shaped positive electrode. This positive electrode is used as a positive electrode of the example.

For comparison, a positive electrode was prepared in which the vapor-grown fibrous graphite in the mixture composition of the positive electrode of Example 1 was 0 and the flaky graphite was 3% by weight.

As a result of measuring the surface resistance of these positive electrodes, it was 35 Ωcm for the positive electrode of the example and 75 ohm cm for the positive electrode of the comparative example, indicating that the surface resistance was reduced by the addition of the vapor-grown fibrous graphite. confirmed. In addition, if the scale-like graphite particles are replaced with massive graphite particles, since the surface resistance becomes larger than in the case of the scale-like graphite particles,
As the non-fibrous graphite, flaky graphite particles are preferable.

Next, a monopolar test was performed on these positive electrodes. The test conditions are as follows.

Charge / discharge current density: 0.6 mA / cm ² , charge potential: 3.0 V-4.3 V, counter electrode: Li metal, electrolyte:
1M LiClO ₄ , EC: DEC = 1: 1, temperature: 25
° C As a result, the capacity retention at the 53rd cycle was
The comparative example positive electrode was the same, and there was no difference in the capacity retention between the example positive electrode and the comparative example positive electrode. The capacity retention was the same even with acetylene black alone, but the capacity retention was low at 50% with flake graphite only.

Also, the expansion coefficient in the thickness direction of the positive electrode after the cycle test was measured. As a result, the positive electrode of the example was 1.5%, whereas the positive electrode of the comparative example was 5.5%, which was large. It was found that the addition of the phase-grown fibrous graphite reduced the expansion of the positive electrode due to repeated use.

Further, a battery having the same structure as that shown in FIG. 1 was manufactured using the above positive electrode. As the negative electrode,
As a negative electrode active material, a mixture of mesocarbon microbeads made of mesophase microspheres generated in the carbonization process of pitch and flaky artificial graphite at a weight ratio of 80:20 is used.
A paste obtained by appropriately adding N-methylpyrrolidone using styrene-butadiene rubber as a binder, and forming a paste
Using a copper foil substrate having a thickness of m and dried and dried, a thickness of 25 μm, a porosity of 40%, an average through-hole diameter of 0.01 μm,
A polyethylene microporous membrane having a 10 mm width and a breaking strength of 0.7 kg was used as a separator. As the electrolyte,
A mixture of 1 mol / liter of LiPF ₆ dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 was used.

The battery was subjected to a charge / discharge cycle test at 45 ° C., and the capacity retention and internal resistance of the battery after 300 cycles were measured. As a result, there was no difference in the capacity retention between the positive electrode of the example and the positive electrode using the comparative example, but there was a difference in the internal resistance, and 70 mΩ was obtained using the positive electrode of the example and using the positive electrode of the comparative example. 110mΩ
Met.

In the above embodiment, the total amount of carbon black, flaky graphite and vapor-grown fibrous graphite in the positive electrode was 5%.
However, the total amount thereof is preferably further reduced for the purpose of increasing the capacity density of the positive electrode, and the total amount is preferably 3.5% by weight or less, 2% by weight or more, and more preferably about 3.5% by weight or more. The content is preferably 3% by weight. As described above, when the total amount of the carbon materials is reduced, the effect of adding the three types of carbon materials is remarkably exhibited by the resistance, the expansion coefficient of the positive electrode, and the high-rate characteristics of the battery.

[0031]

According to the present invention, a secondary battery having a positive electrode with improved conductivity can be manufactured, and a secondary battery excellent in high-rate characteristics can be manufactured.
Furthermore, it becomes possible to manufacture a secondary battery in which the positive electrode expands little with repeated use, and it is possible to manufacture a secondary battery with little decrease in battery performance and safety due to repeated use.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view for explaining the structure of a battery.

[Explanation of symbols]

 1: Power generation element 2: Battery container

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuhiro Nakamitsu 5 Kichijoin Nitta Ichidantancho, Minami-ku, Kyoto City Inside S-Melcotec Co., Ltd. (72) Inventor Minoru Mizutani Minami-ku, Kyoto No. 1 Ino Babacho, No. 1 F-term in Nippon Battery Co., Ltd. (reference) DJ08 DJ15 EJ04 EJ12

Claims (2)

[Claims] 1. A secondary battery comprising a positive electrode containing carbon black, non-fibrous graphite particles, and fibrous carbon. 2. A negative electrode mainly composed of a carbon material capable of occluding and releasing lithium, a positive electrode mainly composed of a transition metal composite oxide, and an electrolytic solution in which a lithium salt electrolyte is dissolved in a non-aqueous solvent. A secondary battery, wherein the positive electrode contains acetylene black, flaky graphite, vapor-grown fibrous graphite, and a binder.