JP2000188134A - Lithium ion secondary battery and method for producing negative electrode active material or positive electrode conductive material used therein - Google Patents

Abstract

(57) [Abstract] [Problem] To improve the conductivity of a positive electrode and a negative electrode,
Provided are a lithium ion secondary battery capable of improving energy density and a method for producing a negative electrode active material or a positive electrode conductive material used for the same. SOLUTION: A non-graphite carbon material such as amorphous carbon is mixed with TiO ₂ or the like, and this is mixed in a nitrogen atmosphere for 2 hours.
It is baked at 800 ° C. to chemically bond a titanium atom to a surface functional group of the carbon material to perform TiC coating and graphitize the carbon material. Thereby, a carbon material used for an electrode for a lithium ion secondary battery, which has high conductivity and does not increase much in weight, can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery, and more particularly, to an improvement in a negative electrode active material and a positive electrode conductive material used in a lithium ion secondary battery, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, in a lithium ion secondary battery, a carbon material has been used as a material constituting an electrode in order to reduce internal resistance and increase output. For example, graphite, carbon black, or the like is used as the positive electrode conductive material, and graphite or amorphous carbon that can be charged to near the lithium metal potential while improving conductivity is used as the negative electrode active material. For example,
Japanese Patent Application Laid-Open No. 9-27316 also discloses a lithium ion secondary battery having graphite-based carbon and amorphous carbon as a negative electrode active material.

[0003]

However, the electrode materials using the conventional carbon materials described above are not yet sufficiently conductive, and it is necessary to further improve the conductivity in order to further increase the output. is there.

In order to improve the conductivity, a method of mixing a metal material having higher conductivity than a carbon material is conceivable. However, since metal has a large specific gravity, there is a problem that the energy density is reduced. For this reason, a method of coating the surface of the carbon material with a metal is also conceivable. However, if the entire surface of the carbon material is covered, there is a problem that the lithium ion conductivity is deteriorated and the capacity is rather reduced. In this case, when charging and discharging are repeated, there is a problem that the metal coating on the surface of the carbon material is peeled off due to expansion and contraction of the carbon material due to intercalation of lithium.

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a lithium-ion secondary battery capable of improving the conductivity of a positive electrode and a negative electrode, and improving an output density and an energy density, and a use thereof. To provide a method for producing a negative electrode active material or a positive electrode conductive material.

[0006]

In order to achieve the above object, the present invention provides a lithium ion secondary battery characterized in that the surface of a carbon material constituting a negative electrode active material is coated with TiC. I do.

A lithium ion secondary battery,
The carbon material constituting the positive electrode conductive material is coated with TiC.

In the above-mentioned lithium ion secondary battery, ZrC is used instead of TiC.

A method for producing a negative electrode active material or a positive electrode conductive material, comprising the steps of: mixing a non-graphitic carbon material with a powder of Ti or Ti oxide;
The non-graphitic carbon material is graphitized by heating at a temperature of 00 ° C. or more, and TiC is formed on the surface thereof.

In the above-mentioned method for producing a negative electrode active material or a positive electrode conductive material, Zr or Zr oxide is used instead of Ti or Ti oxide.

[0011]

Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

In order to improve the conductivity of the electrode of the lithium ion secondary battery, as described above, the surface of the carbon material constituting the electrode is coated with a metal material firmly so as not to cover the entire surface. Is valid.

On the other hand, as shown in FIG. 1A, surface functional groups such as -COOH and -OH exist on the surface of the carbon material constituting the negative electrode active material or the positive electrode conductive material. For this reason, if a predetermined metal atom is chemically bonded to the surface functional group, the surface of the carbon material can be coated with the metal material firmly so as not to cover the entire surface.

Therefore, the inventors of the present invention have proposed a carbon material and TiO.
₂ and sintering the mixture to obtain titanium (T
i) A method of reacting atoms with surface functional groups to coat the surface of a carbon material in the form of TiC has been completed. In FIG. 1B,
According to the present invention, a carbon material whose surface is coated with TiC is shown. In this case, a non-graphitic carbon material such as amorphous carbon was used as the carbon material, and a method was employed in which the carbon material was calcined to be graphitized. In addition, FIG.
In the method shown in (b), the firing temperature is 2800 ° C. However, since the non-graphitic carbon material is graphitized at a temperature of 1500 ° C. or higher, the firing temperature may be 1500 ° C. or higher. In addition, if oxygen is present during firing, the carbon material reacts with oxygen. Therefore, the firing is performed in an inert atmosphere such as nitrogen.

By forming TiC on the surface of the graphitized carbon material as described above, the conductivity of the carbon material can be improved. Thereby, when used as an electrode of a lithium ion secondary battery, high output can be obtained. Further, since titanium is bonded only to the surface of the carbon material, the weight of the carbon material is not affected so much, and even when an electrode of a lithium ion secondary battery is formed, an increase in the weight can be suppressed. Therefore, both the output density and the energy density can be improved. Furthermore, according to the above method, the surface functional group present on the surface of the carbon material reacts with the titanium atom, so that the capacity reduction and the deterioration of the cycle characteristics due to the attachment of the lithium compound to the surface functional group can be suppressed.

Hereinafter, a specific example of the manufacturing method of FIG. 1 will be described as a first embodiment.

[0017]

Example 1 As a non-graphite carbon material, amorphous carbon obtained by firing mesocarbon microbeads (MCMB) manufactured by Osaka Gas Co., Ltd. at 1000 ° C. was used. 80% of this amorphous carbon and 20% of TiO ₂ powder were mixed and fired in a nitrogen atmosphere at a temperature of 2800 ° C. for 10 hours. The graphite thus produced has an X-ray diffraction spectrum as shown in FIG. In FIG. 2, the peak of graphite (C) and the peak of TiC are seen. As a result, it is considered that titanium was bonded to carbon in the surface functional group portion of the carbon material, and that graphite with strong TiC coating was obtained.

[0018]

Example 2 The graphite coated with TiC manufactured as in Example 1 was used as a negative electrode active material, and 10% P
VdF was mixed as a binder, formed into a paste, applied to a copper foil, and dried to form a negative electrode. E using lithium foil for the counter electrode and containing 1 mol / l LiPF ₆ as an electrolyte
C: Using a solution of DEC = 1: 1, the Ti on the graphite surface
The specific capacity when the C coating amount was changed was measured. The result is shown in FIG. This specific capacity is
1C = 1C when 1C = 300 mAh / g, 1C,
The discharge capacity was determined by constant current charging and discharging in three stages of 3C.

As can be seen from FIG. 3, the amount of TiC coating is preferably 20% or less. If the content is 20% or more, the amount of TiC that does not contribute to the capacity increases, and therefore the capacity decreases. At 20% or less, the capacity increase due to the decrease in IR drop due to the improvement in conductivity contributes more than the capacity decrease due to TiC, and the capacity at the time of high current density charge / discharge is particularly large. However, in order to maintain the effect of the TiC coating, a coating amount of at least 0.01% is necessary.

FIG. 4 shows the cycle characteristics when the above-described graphite coated with TiC is used as a negative electrode active material, LiMn ₂ O ₄ is used as a positive electrode active material, and the coin-type battery is operated. The cycle characteristics were evaluated by performing a 1 C constant current charge / discharge between 3 V and 4.2 V at 60 ° C., and as a capacity decrease per cycle from the difference between the initial capacity and the capacity after 20 times of charge / discharge. . As shown in FIG. 4, when the cycle deterioration rate of the sample without the TiC coating was 1.00, the cycle deterioration rate of the coating with 8% was 0.78, and the cycle deterioration of the coating with 20% was not. It was 0.85. Therefore, in any case, it is understood that the cycle deterioration rate is smaller when coating with TiC.
This is due to the effect of reducing IR drop due to the improvement in negative electrode conductivity due to TiC coating and the suppression of film growth of Li ₂ CO ₃ etc. due to the inactivation of the functional groups on the negative electrode carbon material surface by TiC coating. Probably because of the effect.

[0021]

Embodiment 3 FIG. 5 shows the above-mentioned T as the positive electrode conductive material.
The results of evaluation of the output density and energy density of a lithium ion secondary battery when using iC-coated graphite are shown. In this case, TiC is 1
A mixture of 10% of graphitized carbon black coated with 0% as a positive electrode conductive material, 90% of LiMn ₂ O ₄ as a positive electrode active material, and 7% of PVdF as a binder is formed into a paste, applied on an aluminum foil, and dried. Thus, a positive electrode was formed. The basis weight in this case was 30 mg / cm ² . Further, as a negative electrode material, 90% of natural graphite was used as an active material, and 10% of PVdF was mixed as a binder, made into a paste, applied on a copper foil, and dried to form a negative electrode. The basis weight in this case was 13 mg / cm ² . These positive and negative electrodes are P
Oppositely disposed with an E separator in between, 10 positive electrodes, 1 negative electrode
One was laminated. In this case, the area of one electrode is about 20c.
m ² . The electrolyte was 1 mol / l LiPF ₆
An EC: DEC = 1: 1 solution containing was used. As a comparative example, carbon black and TiC as shown in Table 1 below were also evaluated as conductive materials.

[0022]

[Table 1] The output was determined from a current value capable of discharging to 3 V in 3 seconds at 50% SOC (50% charged state) in 10 seconds. The energy density was determined from the average voltage and the capacity during charge and discharge between 3 V and 4.2 V. In this case, the output density and the energy density are values per weight of the entire contents excluding the container.

As shown in FIG. 5, the output density and the energy density have the highest values when 10% of graphite coated with 10% TiC is added to the positive electrode material.
This is because, by coating with TiC, the conductivity is improved as compared with ordinary graphitized carbon black, and
This is probably because the electrode is lighter than iC alone, so that higher conductivity can be obtained with a lighter electrode.

In each of the above embodiments, an example in which titanium is coated on the surface of a carbon material has been described. However, it is also preferable to use zircon (Zr) instead of titanium. In this case, what is coated on the surface of the carbon material is not TiC but ZrC. Such Zr
C can also improve conductivity.

Although improvement in conductivity cannot be expected, it is desired to react with a surface functional group and stabilize it to prevent the capacity from being reduced by coating the carbon material surface with a lithium compound. It is considered that coating with SiC using silicon (Si) is also effective.

[0026]

As described above, according to the present invention,
By coating the surface of the carbon material with TiC, the conductivity of the carbon material can be improved.
Both the output density and the energy density of the secondary battery can be improved.

Further, since the coating of TiC is performed by chemically bonding a titanium atom to a functional group on the surface of the carbon material, a strong coating can be formed.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a method of coating a surface of a carbon material constituting a negative electrode active material and a positive electrode conductive material used in a lithium ion secondary battery according to the present invention with TiC coating.

FIG. 2 is an X-ray diffraction spectrum of graphite coated with TiC by the method shown in FIG.

FIG. 3 is a diagram showing a relationship between a TiC coating amount and a specific capacity when the graphite coated with TiC shown in FIG. 1 is used as a negative electrode active material.

FIG. 4 is a diagram showing a cycle deterioration rate when the graphite coated with TiC shown in FIG. 1 is used as a negative electrode active material.

FIG. 5 is a diagram showing an output density and an energy density when the graphite coated with TiC shown in FIG. 1 is used as a positive electrode conductive material.

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Claims (5)

[Claims] 1. A carbon material constituting a negative electrode active material has T
A lithium ion secondary battery coated with iC. 2. A lithium ion secondary battery in which TiC is coated on a surface of a carbon material constituting a positive electrode conductive material. 3. The lithium ion secondary battery according to claim 1, wherein ZrC is used instead of TiC.
A lithium ion secondary battery characterized by using: 4. A step of mixing the non-graphitic carbon material and a powder of Ti or Ti oxide, and heating the non-graphite carbon material to a temperature of 1500 ° C. or more under an inert atmosphere to graphitize the non-graphite carbon material. ,
A method for producing a negative electrode active material or a positive electrode conductive material, comprising forming TiC on the surface. 5. The method for producing a negative electrode active material or a positive electrode conductive material according to claim 4, wherein Zr or Zr oxide is used instead of said Ti or Ti oxide. Material manufacturing method.