JP2001006683A - Active material for lithium battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an active material extending the lifetime of a lithium battery by consisting of a carbon material storing/releasing a lithium and a coating film of a silane coupling agent containing a fluorine atom in a molecule for covering the surface of the carbon material. SOLUTION: A carbon material in an active material for a lithium secondary battery is a material capable of storing/releasing a lithium and to be a base of an electromotive reaction. Generally, as a powdered element is used, the powdered element is also used for the active material. When using the powdered element, a coating film of a silane coupling agent covers each of powdered particles. A natural graphite, artificial graphite in a sphere or fiber shape, hardly graphitized carbon and easily graphitized carbon such as a coke are used for the carbon material. The silane coupling agent forms the coating film on at least a part of the surface of the carbon material and easily reacts with a reactive functional group such as a hydroxyl group, vinyl group, amino group, epoxy group, alkoxy group and isocyanate group to form a stable protecting film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for storing and storing lithium.
The present invention relates to an active material that can be used for a lithium battery utilizing a release phenomenon.

[0002]

2. Description of the Related Art A non-aqueous electrolyte battery having a non-aqueous electrolyte in which lithium or a lithium compound is used as an active material and a lithium salt serving as a supporting electrolyte is dissolved in an organic solvent, that is, a so-called lithium battery, has a high energy density. ,computer,
With the miniaturization of mobile phones and the like, they have already been put to practical use in the fields of information-related equipment and communication equipment, and have come into widespread use.
On the other hand, interest in electric vehicles has increased due to resource problems and environmental problems, and the use of lithium batteries as power sources for electric vehicles has been studied.

[0003] Among various lithium batteries, a rocking chair type lithium secondary battery using a lithium-containing metal composite oxide as a positive electrode active material and a carbon material as a negative electrode active material can obtain a high voltage of 4 V. This is the most promising lithium battery because it has little problem of dendrite deposition on the negative electrode surface.

However, in a lithium battery using a carbon material as the negative electrode active material, the lithium occluded in the negative electrode during the initial charge is trapped in the negative electrode, and not all of the lithium returns to the positive electrode by the subsequent discharge. Irreversible capacity (retention) was present, leaving a problem in terms of lithium utilization. The cause of this irreversible capacity is that at the interface between the carbon material and the electrolyte, lithium and the electrolyte,
Water reacts with carbon surface adsorbed water, carbon surface functional groups, etc.,
It is considered that this is because it does not contribute to the subsequent charge / discharge reaction. In the case of a secondary battery, the irreversible capacity is also accumulated during a subsequent charge / discharge cycle of the battery, so that the capacity of the battery is deteriorated and the battery life is shortened.

In order to solve the problem of the irreversible capacity caused by the carbon material negative electrode, conventionally, as shown in JP-A-8-111243, a film is formed on the surface of the carbon material by a surface treatment using a silane coupling agent. By forming
Techniques for protecting the surface of a carbon material from an electrolytic solution, moisture, and the like, suppressing substances deposited on the surface during charging and discharging of the battery, and reducing the capacity deterioration of the battery have been studied.

[0006]

However, Japanese Patent Application Laid-open No.
When the present inventors performed additional tests on the technique disclosed in JP-A-1111143, no sufficient irreversible capacity reduction effect was obtained, and it could not be an effective problem solving means. The inventor has conducted further experiments and found that an active material having a small irreversible capacity can be obtained by using an appropriate silane coupling agent as a coating agent.

[0007] The present invention is based on this finding,
By suppressing the irreversible capacity generated in the carbon material without impairing lithium ion conductivity, capacity deterioration is reduced,
It is an object to provide an active material that can form a lithium battery having a long charge and discharge life.

[0008]

The active material for a lithium battery according to the present invention comprises a carbon material capable of occluding and releasing lithium and a fluorine atom covering at least a part of the surface of the carbon material in a molecule. And a film made of a silane coupling agent. In other words, the carbon material is coated with a silane coupling agent containing a fluorine atom in its molecule, instead of a mere silane coupling agent.

The film formed by the silane coupling agent exhibits strong hydrophobicity, and the hydrophilic component such as the electrolyte and water does not directly touch the surface of the carbon material, so that the irreversible capacity is greatly reduced. Since the film formed by the silane coupling agent has lithium ion conductivity, the battery performance is not impaired. Further, in the active material of the present invention, the fluorine atom contained in the silane coupling agent has a function such that the atom itself exhibits strong hydrophobicity and excludes a hydrophilic component. Therefore, the film formed by the silane coupling agent has an advantage that due to its action, it exhibits a very strong hydrophobic action as compared with the film formed by the conventional silane coupling agent.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of an active material for a lithium battery according to the present invention will be described in terms of a carbon material, a silane coupling agent, a coating method, and a lithium ion battery using the same. This will be described separately in detail.

<Carbon Material> The carbon material in the active material for a lithium secondary battery of the present invention is a substance capable of occluding and releasing lithium (lithium ions), and is a source of an electromotive reaction (charge / discharge reaction). Substance. Generally, a powder is used, so that the active material of the present invention may be a powder. In the case of using a powdery material, the coating with the silane coupling agent described below covers each of the powder particles.

Carbon materials that can be used include natural graphite, spherical or fibrous artificial graphite, non-graphitizable carbon,
Further, fired organic compounds such as a phenol resin, and easily graphitizable carbon such as coke can be used. These carbon materials have their respective advantages, and may be selected according to the characteristics of the lithium secondary battery to be manufactured. In addition, these carbon materials can be used individually by 1 type, and can also be used in mixture of 2 or more types.

Of these, natural and artificial graphites have the advantage of being capable of forming a lithium secondary battery having a large capacity (high energy density) and good power characteristics because of its high true density and excellent conductivity. There is. When a lithium secondary battery utilizing this advantage is manufactured, it is desirable that the graphite used has high crystallinity, the (002) plane spacing d ₀₀₂ is 3.4 ° or less, and the crystallite thickness in the c-axis direction. It is preferable to use one having Lc of 1000 ° or more. In addition, artificial graphite is, for example, 2800 ° C.
It can be manufactured by heat treatment at the above high temperature. In this case, the easily graphitizable carbon as a raw material includes coke and optically anisotropic small spheres (mesocarbon microbeads: MCMB) obtained in the process of heating pitches at about 400 ° C. .

[0014] Graphitizable carbon is generally obtained from tar pitch obtained from petroleum or coal, and includes coke,
MCMB, mesophase pitch-based carbon fiber, pyrolytic vapor growth carbon fiber, and the like. An organic compound fired body such as a phenol resin can also be used. Since graphitizable carbon is an inexpensive carbon material, it can be an active material that can form a lithium secondary battery that is excellent in cost. Among them, coke has the advantages of low cost, relatively large capacity, and good cycle characteristics of the secondary battery that constitutes it. Considering this point, it is desirable to use coke. When using coke,
The (002) plane spacing d ₀₀₂ is 3.4 ° or more; c
It is preferable to use one having an axial crystallite thickness Lc of 30 ° or less.

[0015] The non-graphitizable carbon is a so-called hard carbon, and is a carbon material having a structure close to amorphous such as glassy carbon. Generally, it is a material obtained by carbonizing a thermosetting resin, and does not develop a graphite structure even when the heat treatment temperature is increased. Non-graphitizable carbon has the advantages of high safety, relatively low cost, and good cycle characteristics of the secondary batteries that make it up. It is desirable to do. Specifically, for example, a phenol resin fired body, polyacrylonitrile-based carbon fiber, pseudo-isotropic carbon,
Furfuryl alcohol resin fired bodies and the like can be used. More preferably, the (002) plane spacing d ₀₀₂ is 3.6 ° or more, and the crystallite thickness Lc in the c-axis direction is 10 °.
It is preferable to use the one having 0 ° or less.

When a powdery carbon material is used, the particle size of the carbon material powder affects battery characteristics such as output density and cycle characteristics. If the particle size is too large, there is a problem that coatability of the paste is remarkably deteriorated. If the particle size is too small, there is a problem that the silane coupling treatment efficiency is significantly reduced because the specific surface area is too large. Therefore, the average particle diameter of the carbon material powder used as the negative electrode active material is desirably 1 to 100 μm, and more desirably 5 to 60 μm.

<Silane Coupling Agent> In the active material for a lithium battery of the present invention, the silane coupling agent forms a film on at least a part of the surface of the carbon material. The silane coupling agent is a reactive functional group on the solid surface, such as a hydroxyl group, a vinyl group, an amino group, an epoxy group,
It reacts easily with an alkoxy group, an isocyanate group, etc. to form a stable protective film, and is a general term for a crosslinkable compound represented by the following formula.

R _n SiX _4-n (n = 1 to 2) In the formula, X represents a hydrolyzable group such as a lower alkoxy group. In the silane coupling agent used in the active material for a lithium battery of the present invention, R Represents a fluorine atom or an alkyl group having 1 to 16 carbon atoms containing a fluorine atom. Examples of the silane coupling agent containing a fluorine atom that can be used include fluorotriethoxysilane, fluorotrimethoxysilane, 3,3,3-trifluoropropyltrimethoxysilane, and the like.

<Coating method> As a method of forming a coating, that is, a method of treating the surface of the carbon material with a silane coupling agent containing a fluorine atom, for example, a silane coupling agent containing a fluorine atom is treated with water. Alternatively, the carbon material is immersed in a solution dissolved in an organic solvent or suspended in the case of a powdery material, and then the solid content is taken out.
The crosslinking reaction may be performed by drying at 0 to 200 ° C. After an electrode is formed using a carbon material by a method described in detail below, the electrode may be immersed in the above solution and then dried to treat the surface of the carbon material.

The amount of the protective film formed on the surface of the carbon material varies depending on the concentration of the solution to be treated. The concentration of the silane coupling agent in the solution to be treated is 0.5
It is preferable to set it to 10 wt%. If the amount is less than 0.5 wt%, the formation of the protective film may be insufficient, and if the amount exceeds 10 wt%, the conductivity of lithium ions may be reduced.

Incidentally, the coating may cover the entire carbon material, for example, the entire particle if it is in the form of particles. When a film is formed after an electrode is formed using a carbon material, the film is not necessarily coated on the entire carbon material, but may be formed on a part of the surface of the carbon material. If the portion of the active material that comes into contact with the electrolyte is efficiently covered, the irreversible capacity generated in the carbon material is reduced.

<Lithium rechargeable battery> A lithium battery using the active material for a lithium battery of the present invention uses the present active material as an active material of one of a positive electrode and a negative electrode.
The other electrode is formed using lithium or a lithium compound as an active material. For example, a lithium battery in which the active material of the present invention is used as the positive electrode active material and metallic lithium is used as the negative electrode active material can be obtained. A lithium secondary battery in which a lithium-containing metal composite oxide is used as a substance can be obtained. Further, the active material of the present invention,
It is also possible to configure a lithium battery in which a mixture of another active material, for example, a carbon material having no coating formed thereon is used as an active material. Further, the lithium battery is not limited to a rechargeable secondary battery, and may be used as a primary battery.

A lithium battery using the active material for a lithium battery of the present invention, in which various aspects can be considered, is used as a negative electrode active material, and a lithium-containing metal composite oxide such as LiCoO ₂ is used as a positive electrode active material. The used lithium secondary battery is a secondary battery having a high energy density of 4V class. Hereinafter, a secondary battery of this aspect will be described as one embodiment of a lithium battery using the active material for a lithium battery of the present invention. However, the invention is not limited to this embodiment.

The lithium secondary battery shown as an example has a negative electrode using the above active material as a negative electrode active material, a positive electrode using a lithium-containing metal composite oxide as a positive electrode active material, and a lithium salt dissolved in an organic solvent. A non-aqueous electrolyte is constituted as a main component.

The negative electrode is prepared by mixing the above-mentioned powdery active material material serving as the negative electrode active material with a binder, adding an appropriate solvent, and forming a paste into a negative electrode material. Can be formed by coating and drying on the surface of the substrate and compressing it to increase the electrode density. As described above, after a negative electrode is formed using a carbon material that is not coated with a silane coupling agent, the negative electrode is immersed in a solution in which the silane coupling agent is dissolved, so that the carbon material is surface-treated. May be applied.

The binder plays a role of binding the active material particles, and may be a fluorinated resin such as polytetrafluoroethylene, polyvinylidene fluoride or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. . As a solvent for dispersing the active material and the binder, an organic solvent such as N-methyl-2-pyrrolidone can be used.

The positive electrode is obtained by mixing a conductive material and a binder with a powder of a lithium-containing metal composite oxide serving as a positive electrode active material,
A paste-like positive electrode mixture obtained by adding an appropriate solvent can be applied to the surface of a current collector made of aluminum foil or the like, dried, and then compressed to increase the electrode density.

As the lithium transition metal composite oxide serving as the positive electrode active material, Lithium transition metal
CoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O _{4 and the} like can be used. Among them, Li with layered rock salt structure
Although CoO ₂ has a high raw material cost, it is easy to synthesize and handle, and is a positive electrode active material that can constitute a battery having good cycle characteristics and the like. On the other hand, LiMnO ₂ having a layered rock salt structure and LiMn ₂ O ₄ having a spinel structure have low raw material costs, and are used in applications where a large amount of active materials are used, such as electric vehicles and secondary batteries used in power storage systems. , Which is advantageous.

The conductive material is for ensuring the electrical conductivity of the positive electrode, and may be a mixture of one or more powdered carbon materials such as carbon black, acetylene black and graphite. it can. As in the case of the negative electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as the binder, and N-methyl- as a solvent for dispersing the active material, the conductive material, and the binder is used. An organic solvent such as 2-pyrrolidone can be used.

When a lithium secondary battery is formed, a separator is interposed between the positive electrode and the negative electrode for the purpose of separating the positive electrode and the negative electrode and holding an electrolyte. As this separator, a thin microporous film such as polyethylene or polypropylene can be used.

The non-aqueous electrolyte is obtained by dissolving a lithium salt as an electrolyte in an organic solvent. The lithium salt is dissociated by dissolving in an organic solvent and forms lithium ions in the electrolyte. Examples of usable lithium salts include LiBF ₄ , LiPF ₆ , LiClO ₄ , and LiCF ₃
SO ₃ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiN
(C ₂ F ₅ SO ₂ ) _{2 and the} like. Each of these lithium salts may be used alone, or two or more of these lithium salts may be used in combination.

An aprotic organic solvent is used as the organic solvent for dissolving the lithium salt. For example, a mixed solvent of one or more of cyclic carbonate, chain carbonate, cyclic ester, cyclic ether or chain ether can be used. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate.Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.Examples of the cyclic ester include gamma butyl lactone and gamma. Examples of barrel lactone include cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, and examples of chain ether include dimethoxyethane and ethylene glycol dimethyl ether.

The lithium secondary battery as one embodiment constituted by the above-mentioned components has a coin shape,
Various types such as a cylindrical type and a laminated type can be used. Regardless of the shape used, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and a current collecting lead extends from the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal that lead to the outside. The electrode body is impregnated with a non-aqueous electrolyte and sealed in a battery case to complete a lithium secondary battery.

[0034]

[Example] Artificial graphite was used as a carbon material,
Active material materials of the present invention each coated with a silane coupling agent containing a kind of fluorine atom were actually produced, and a secondary battery using these active material materials as a negative electrode active material was produced.
In addition, an active material coated with a silane coupling agent containing no fluorine atom was also manufactured, and a secondary battery using this as a negative electrode active material was also manufactured. A charge / discharge cycle test was performed on these secondary batteries, and the active material of the present invention was evaluated by comparing the battery capacity retention rates. The following is an example.

Example 1 First, 100 g of graphitized mesophase microspheres (MCMB25-28: manufactured by Osaka Gas Chemical) as artificial graphite were dissolved in a solution of 10 ml of fluorotriethoxysilane (manufactured by Chisso) in 500 ml of distilled water. Was suspended for 15 minutes. Remove the solids by filtration
By drying at 50 ° C. for 12 hours, an active material having a film formed thereon was produced.

To 100 parts by weight of this active material, 10 parts by weight of polyvinylidene fluoride powder (manufactured by Kureha Chemical Co., Ltd.) was added to N-methyl-2-pyrrolidone (manufactured by Wako Pure Chemical Industries)
By sufficiently mixing 100 parts by weight of the solution dissolved in parts by weight, a paste-like negative electrode mixture was obtained. This negative electrode mixture was applied to the surface of a 10 μm-thick copper foil negative electrode current collector using a coating machine, dried and pressed to form a 60 μm-thick negative electrode mixture layer having a negative electrode mixture layer formed on one surface. A negative electrode sheet was produced.

Next, LiMn ₂ O ₄ (manufactured by Honjo Chemical)
18.5 parts by weight, acetylene black (Tokai Carbon) 1.5 parts by weight, polyvinylidene fluoride powder (Kureha Chemical) 8 parts by weight, and N-methyl-2-pyrrolidone (Wako Pure Chemical Industries) 72 The mixture was sufficiently mixed with the parts by weight to obtain a paste-like positive electrode mixture. This positive electrode mixture is
Using a coating machine, it was applied to the surface of a 20-μm-thick aluminum foil positive electrode current collector, dried, and pressed to produce a 90-μm-thick positive electrode sheet having a positive electrode mixture layer formed on one surface.

The positive electrode sheet was punched into a disk having a diameter of 15 mmφ to form a positive electrode.
The negative electrode punched into 7 mmφ was used as the negative electrode, and the positive electrode and the negative electrode were opposed to each other with a 19 mmφ polyethylene separator (manufactured by Tonen Chemical) between them. Then, the battery was assembled in a coin-type battery case, and impregnated with an electrolyte (manufactured by Mitsubishi Chemical Corporation) in which LiPF ₆ was dissolved at a concentration of 1 M in an organic solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1. The next battery was completed. The manufactured coin-type secondary battery was used as the secondary battery of Example 1.

For comparison with the secondary battery of Example 1, an active material whose surface was treated with 3-aminopropyltriethoxysilane (manufactured by Wako Pure Chemical Industries) in place of the above-mentioned fluorotriethoxysilane was used. The secondary battery was manufactured, and a coin-type secondary battery was manufactured using the manufactured secondary battery. Except for the silane coupling agent, a method for producing an active material,
The configuration of the coin-type secondary battery was the same as that of the secondary battery of Example 1.

The secondary batteries of Example 1 and Comparative Example were subjected to a charge / discharge cycle test. The charge / discharge cycle test was conducted at a constant current density of 1 mA / cm ² and a battery voltage of 4.2 V.
And then charging at a constant voltage until the total charging time reaches 6 hours, and then discharging at a constant current density of 0.5 mA / cm ^{2 to} a battery voltage of 3.0 V. It was to be repeated. FIG. 1 shows the battery capacity retention ratio (discharge capacity in each cycle / discharge capacity in first cycle × 100%) in each cycle of the secondary batteries of Example 1 and Comparative Example as a result of the charge / discharge cycle test.

As is apparent from FIG. 1, the secondary battery of Example 1 using an active material composed of a carbon material whose surface is coated with fluorotriethoxysilane has its surface coated with 3-aminopropyltriethoxysilane. It can be seen that this is a secondary battery in which capacity deterioration is suppressed as compared with a secondary battery of a comparative example using an active material made of a carbon material.

<Example 2> An active material having a surface treated with 3,3,3-trifluoropropyltrimethoxysilane (manufactured by Chisso) instead of fluorotriethoxysilane in the case of Example 1 was produced. Using this, a coin-type secondary battery was manufactured, and a secondary battery of Example 2 was obtained. Except for the silane coupling agent, the manufacturing method of the active material and the configuration of the coin-type secondary battery were the same as those of the secondary battery of Example 1.

The secondary battery of Example 2 was also subjected to a charge / discharge cycle test under the same conditions as in Example 1. FIG. 2 shows the battery capacity retention ratio of each cycle of the secondary battery of Example 2 as a result of the cycle charge / discharge test. For comparison, FIG. 2 also shows the capacity retention ratio of the secondary battery of the comparative example.

As is apparent from FIG. 2, the secondary battery of Example 1 using an active material composed of a carbon material whose surface is coated with 3,3,3-trifluoropropyltrimethoxysilane has a 3-amino It can be seen that the secondary battery has a suppressed capacity deterioration as compared with the secondary battery of the comparative example using an active material composed of a carbon material whose surface is coated with propyltriethoxysilane.

Judging from the test results of Examples 1 and 2 above, it was found that the secondary material using the active material of the present invention consisting of a carbon material coated with a silane coupling agent containing a fluorine atom was used. The battery is a secondary battery with small capacity degradation, that is, good cycle characteristics, as compared with a secondary battery using an active material made of a carbon material coated with a silane coupling agent that does not contain a fluorine atom. Can be confirmed.

[0046]

The active material of the present invention is a carbon material coated with a silane coupling agent containing a fluorine atom in the molecule. A lithium battery using an active material having such a configuration as an active material is a good battery in which irreversible capacity due to a carbon material is significantly reduced and capacity deterioration is suppressed.

[Brief description of the drawings]

FIG. 1 shows a secondary battery of Example 1 using an active material composed of a carbon material whose surface is coated with fluorotriethoxysilane, and an active material composed of a carbon material whose surface is coated with 3-aminopropyltriethoxysilane. 4 shows a comparison of a capacity retention ratio with a secondary battery of a comparative example using a material.

FIG. 2 shows a secondary battery of Example 2 using an active material made of a carbon material whose surface is coated with 3,3,3-trifluoropropyltrimethoxysilane, and a surface of which is coated with 3-aminopropyltriethoxysilane. 4 shows a comparison of a capacity retention ratio with a secondary battery of a comparative example using an active material made of a coated carbon material.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H003 AA02 AA04 BB04 BC01 BC02 BC05 5H024 AA01 AA02 DD14 DD17 EE09 5H029 AJ03 AJ05 AK03 AL07 AL08 AM03 AM04 AM05 AM07 BJ13 CJ22 DJ08 DJ15 DJ16 EJ11

Claims (1)

[Claims] An active material for a lithium battery, comprising: a carbon material capable of occluding and releasing lithium; and a film of a silane coupling agent containing a fluorine atom in a molecule covering at least a part of the surface of the carbon material. .