JP3367060B2 - Negative electrode for lithium secondary battery - Google Patents

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 initial charge / discharge characteristics.

[0002]

2. Description of the Related Art In a lithium secondary battery using a carbon material as a negative electrode, the lithium in the positive electrode is occluded in the carbon material through the electrolytic solution during charging, and the lithium in the carbon material is released during discharging to remove the electrolytic solution. It utilizes an electrochemical reversible reaction of being occluded in the positive electrode via The characteristics required of the carbon material as the negative electrode material are that it has a large capacity for absorbing and desorbing lithium into the carbon material, that the voltage at the time of discharging lithium is low, and that there is little capacity deterioration during storage and desorption cycles. Is. From this point of view, various carbon materials have been proposed in the past, and specifically, a graphite negative electrode in which chargeable / dischargeable lithium is mixed in the crystal (Japanese Patent Laid-Open No. 57-208079) and graphitizable. Negative electrode made of a graphite material composed of spherical particles described in JP-A-4-115.
No. 457), a negative electrode obtained by doping lithium into graphitized carbon of a mesophase material generated by carbonization of pitch (Japanese Patent Laid-Open No. Hei.
115458), a carbon negative electrode having a pseudo-graphite structure in which an organic polymer compound or the like is carbonized (JP-A-62-1220).
66), a carbon negative electrode having a specific structure (Japanese Patent Laid-Open No. 62-90).
No. 863), a carbon negative electrode having a turbostratic structure (Japanese Patent Application Laid-Open No. HEI 2)
No. 66856), a wide range from graphite materials to turbostratic carbon materials.

[0003]

According to the experiments conducted by the present inventors using a wide range of carbon materials, lithium storage capacity and release capacity are large, and the voltage at the time of lithium release is low. Among them, it was found that a graphite material is suitable as a negative electrode material for lithium secondary batteries. However, it has also been found that a simple graphite material has insufficient initial charge / discharge characteristics and a high capacity battery cannot be obtained.

That is, in a battery structure in which an electrode composed of an active material containing chargeable / dischargeable lithium and an electrode composed of a graphite material are combined, 100% of lithium occluded in the graphite material is not released at an initial stage. However, the utilization efficiency of chargeable / dischargeable lithium is poor and a high capacity battery cannot be obtained. In other words, the graphite material has many initial irreversible capacity components (irreversible capacity component = lithium charge capacity to graphite material electrode−lithium discharge capacity from graphite material electrode).

Specifically, the graphite material has a large capacity of occluding and releasing lithium per unit weight of 200 mAH / g or more, and the specific graphite material reaches 300 mAH / g or more. However, the irreversible capacity during initial charge / discharge is also 150
Since it is as large as ˜580 mAH / g, the use efficiency of the chargeable / dischargeable lithium in the positive electrode is low in the battery configuration in which the positive electrode made of the active material containing chargeable / dischargeable lithium and the negative electrode made of the graphite material are combined. Therefore, a problem was found that a high capacity battery could not be obtained.

An object of the present invention is to reduce the irreversible capacity of a graphite negative electrode during initial charge / discharge, improve the utilization efficiency of chargeable / dischargeable lithium in the positive electrode, and improve the initial charge / discharge characteristics of a lithium secondary battery. It is to provide a negative electrode for use.

[0007]

The present inventors have conducted various experiments and studies in order to solve these problems, and as a result,
The inventors have found that the use of the negative electrode material containing the graphite material and the pseudographitic carbon black can significantly reduce the irreversible capacity as compared with the case where each of the individual materials is used as the negative electrode material, and completed the present invention.

That is, the present invention comprises the following inventions. (1) containing a graphite material and擬黒lead carbon black, pseudo
The lattice plane spacing of the graphite carbon black is 3.38 to 3.
The lithium secondary battery is 46Å, the true specific gravity is 1.9 to 2.1, and the ratio of the two is 70 to 99% by weight of the graphite material and 30 to 1% by weight of the pseudographitic carbon black. Negative electrode. (2) The negative electrode for a lithium secondary battery according to (1), wherein the graphite material is scale-like natural graphite or scale-like artificial graphite. (3) The volatile component of pseudo-graphitic carbon black is 0.5
The amount average primary particle size is 10 to 100 nm.
And has a specific surface area of 10 to 300 m ² by the nitrogen adsorption method.
/ G , The negative electrode for a lithium secondary battery according to (1).

The present invention will be described in detail below. The negative electrode for a lithium secondary battery in the present invention contains a graphite material and pseudographitic carbon black as active materials. The graphite material used in the present invention may be any one capable of inserting and extracting lithium by charge and discharge, and may be any of scaly graphite, fibrous graphite, spherical graphite, etc., but scaly graphite is particularly preferable, and specifically, Examples include scaly natural graphite and artificial graphite. Furthermore, it is also possible to mix and use fibrous graphite or spherical graphite with scaly graphite. The graphite material used in the present invention is preferably a graphite material having a lattice spacing d ₀₀₂ in X-ray diffraction of 3.37 Å or less and a true specific gravity of 2.23 or more, and more preferably a lattice spacing d ₀₀₂ in X-ray diffraction. It is a graphite material with a specific gravity of 3.36Å or less and a true specific gravity of 2.24 or more. Here, the lattice spacing d ₀₀₂ is an X-ray diffraction method using CuKα rays as X-rays and using high-purity silicon as a standard substance [Sugiro Otani, carbon fiber, P733-742].
(1986) Modern Editing Company]. The ash content of the graphite material used in the present invention is preferably 0.5% by weight or less, more preferably 0.1% by weight.
It is the following. In the case of natural graphite, although it varies depending on the place of origin, since the ash content contained is as large as several percent or more, it is preferably treated at a high temperature of 2500 ° C or higher, more preferably 2800 ° C or higher, and the ash content is preferably 0.5% by weight. %Less than,
More preferably, it is 0.1 wt% or less.
Here, the ash content means a value according to JISM8812.
The particle size of the graphite material used in the present invention is not particularly limited, but it is generally preferable that the number average particle size is about 1 to 50 μm.

The pseudographitic carbon black used in the present invention has a lattice spacing d ₀₀₂ of 3.3 in X-ray diffraction.
It is preferably 8 to 3.46Å and has a true specific gravity of 1.9 to 2.1. The pseudo-graphitic carbon black preferably has a volatile content of 0.5% by weight or less. Here, the true specific gravity is the value according to JIS R7222, and the volatile content is JIS M
8812 means a value. The number average primary particle size of the pseudographitic carbon black is preferably about 10 to 100 nm, and the specific surface area by the nitrogen adsorption method is usually 10 to 10.
It is preferably about 300 m ² / g. The pseudographitic carbon black is obtained by subjecting carbon black to graphitization treatment. Specifically, the pseudo-graphitic carbon black is obtained by heat treating carbon black at a temperature of about 1500 to 3000 ° C. Especially about 2500-3
What was heat-treated at 000 ° C. is preferable. Even if such carbon black is graphitized, the lattice spacing d ₀₀₂ in X-ray diffraction is 3.37Å or less and the true specific gravity is 2.
It does not become a graphite material of 23 or more. As the pseudo-graphitic carbon black, for example, carbon black such as furnace black made from creosote oil, ethylene bottom oil, natural gas or the like and acetylene black made from acetylene are heat-treated at a high temperature of about 2500 to 2800 ° C. The ones that have been done are listed. With or without graphitization, the lattice spacing d ₀₀₂ in X-ray diffraction is less than 3.38Å, the true specific gravity is less than 1.9, and the volatile content is 0.5% by weight. When a carbon black exceeding the above range is used, the effect of decreasing the irreversible capacity is not or small, and the potential at the time of releasing lithium is higher than the lithium potential, which is not preferable.

Next, the ratio of the graphite material and the pseudo-graphitic carbon black will be described. The blending amount of the graphite material and the pseudographitic carbon black is 30 to 1% by weight of the pseudographitic carbon black with respect to 70 to 99% by weight of the graphite material. The graphite material is preferably 80 to 97% by weight, and the pseudographitic carbon black is 20 to 3% by weight, more preferably the graphite material 9
The content of the pseudographite-treated carbon black is 10 to 4% by weight with respect to 0 to 96% by weight. If the blending amount of the pseudo-graphitic carbon black is too large, the discharge capacity of the negative electrode active material will decrease, whereas if the blending amount is too small, the irreversible capacity of the negative electrode active material will increase, which is not preferable.

Next, a method for manufacturing the negative electrode for a lithium secondary battery of the present invention will be described. The graphite material powder as the active material and the pseudo-graphitic carbon black powder, and the binder for binding the powders are uniformly mixed and then pressure-molded, or made into a paste using an organic solvent or the like to form a current collector. It can be manufactured by a known method, such as applying and drying to and pressing. Here, the graphite material powder and the pseudo-graphitic carbon black powder as the active material are preferably treated at a temperature of about 1000 ° C. or lower before being mixed in order to remove adhering water and the like. The binder for binding the graphite material powder and the pseudo-graphitic carbon black powder may be any binder as long as it has a binding effect and has resistance to the non-aqueous electrolyte used, and examples thereof include fluororesin powder and polyethylene powder. To be The amount of the binder is preferably about 1 to 20 parts by weight based on 100 parts by weight of the total amount of the graphite material powder and the pseudographitic carbon black powder.

The negative electrode for a lithium secondary battery of the present invention constitutes a lithium secondary battery in combination with a positive electrode composed of an active material containing chargeable / dischargeable lithium. Examples of the positive electrode active material used here include a composite oxide of lithium and a transition metal. Here, the transition metal can be selected from, for example, Co, Ni, Mn, Fe and the like. In addition, MnO ₂ , MoO as an active material is further added to the positive electrode.
Inorganic compounds such as ₃ , V ₂ O ₅ , TiO ₂ , TiS ₂ , FeS and activated carbon, and polymer compounds such as polyaniline can also be selected. In this case, a predetermined amount of lithium may be stored in the negative electrode in advance, or a predetermined amount of lithium may be pressure-bonded to the negative electrode before use.

The non-aqueous electrolyte solution used in the lithium secondary battery using the negative electrode of the present invention is preferably a solution prepared by dissolving a lithium salt in an organic solvent having a high dielectric constant. There is no particular limitation on the type of lithium salt, and for example, LiCl
O ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ or the like can be used. The concentration of the lithium salt is usually selected to be about 0.5 mol / l to 1.5 mol / l. Further, the organic solvent may be any one as long as it dissolves a lithium salt to give electrical conductivity and is electrochemically stable with respect to the constituent negative electrode / positive electrode material. Examples thereof include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, acetonitrile, sulfolane, and γ-butyrolactone. Normally,
Two or more kinds are mixed and used as a mixed solvent.

In the lithium secondary battery using the negative electrode of the present invention, in addition to the positive electrode, the negative electrode and the electrolyte solution, generally, there is a function of preventing contact between both electrodes, holding the electrolyte solution and allowing passage of lithium ions. It is preferable to use a separator and a current collector having a function of holding an electrode material and collecting current in combination. Examples of the separator include porous films such as polyethylene, polypropylene or polytetrafluoroethylene, non-woven fabrics and woven fabrics. The thickness of the separator is preferably about 20 to 200 μm.

As the current collector, a conductor which is electrochemically stable with respect to the positive electrode / negative electrode active material and the electrolytic solution can be used. For example, nickel, titanium, stainless steel, copper, aluminum and the like can be mentioned. Further, the lithium secondary battery using the negative electrode of the present invention, a cylindrical type, box type, coin type, button type, paper type, card type, etc.
It can have various shapes.

[0017]

EXAMPLES Hereinafter, the effects of the present invention will be specifically described by showing Examples and Comparative Examples, but the present invention is not limited to the following Examples. Generally, a negative electrode for a lithium secondary battery (referred to as an electrode here) is an electrochemical reaction of a charge reaction in which lithium is occluded in the electrode through an electrolyte solution and a discharge reaction in which lithium in the electrode is released into the electrolyte solution. The reversible reaction is utilized, and the electrode characteristics can be evaluated by constructing a secondary battery for evaluation using lithium metal as the counter electrode and carrying out this charge / discharge reaction. The evaluation method is generally performed (Examples 1 to 4 and Comparative Examples 1 to 3). In addition, Examples 5 and Comparative Example 4 show examples in which an electrode containing lithium nickel oxide as an active material containing chargeable / dischargeable lithium was used as a counter electrode.

Example 1 Heat treated at 3000 ° C., the number average particle size was 10 μm, the true specific gravity was 2.26, and the lattice spacing d ₀₀₂ in X-ray diffraction was 3.
36Å, 75 parts by weight of natural graphite (made in Madagascar) with an ash content of 0.05% by weight, a true specific gravity of 2.04 graphitized at 2800 ° C, a volatile content of 0.1% by weight, and a number average primary particle size of 66 nm, specific surface area by nitrogen adsorption method of 30 m ² /
g pseudo-graphitic carbon black [Tokai Carbon Co., Ltd.
Manufactured by the trade name TB3800] 25 parts by weight, and further 5.4 parts by weight of polyvinylidene fluoride using N-methylpyrrolidone as a binder as a solvent and sufficiently kneaded in an agate mortar, and then a part of stainless steel (hereinafter, SUS It was applied to a mesh made of carbon and then dried under vacuum overnight to obtain an electrode containing the active material of 48.7 mg of the mixed carbon material.

In order to evaluate the lithium charge / discharge characteristics of the active material obtained here, a lithium foil was used as the counter electrode, and lithium perchlorate was used as a non-aqueous electrolyte solution with ethylene carbonate (EC) and 1, Solution dissolved in an equal volume mixture with 2-dimethoxyethane (DME) (concentration 1 mol / l)
Was stored in a polypropylene separator having a thickness of 175 μm to prepare a secondary battery for evaluation and left overnight. The open circuit voltage of this evaluation battery before the test was 3.00V. Then, lithium was absorbed into the electrode (hereinafter referred to as charging) at a constant current of 0.5 mA until the voltage reached 0.00 V, and then the electrode was charged at a constant current of 0.5 mA until the voltage reached 0.6 V. Lithium was discharged from the battery (hereinafter referred to as discharge), and the initial charge / discharge characteristics of the electrode containing the mixed carbon material were evaluated. The results are shown in Table 1.

Example 2 Example 2 was repeated except that 90 parts by weight of natural graphite, 10 parts by weight of pseudographitic carbon black and 4.1 parts by weight of polyvinylidene fluoride were used. After preparing an electrode containing 51.1 mg of the mixed carbon material as an active material, a lithium counter electrode evaluation battery was prepared and the initial charge / discharge characteristics of the electrode containing the mixed carbon material were evaluated. The results are shown in Table 1.

Example 3 The same as Example 1 except that 95 parts by weight of natural graphite, 5 parts by weight of pseudo-graphitic carbon black and 3.7 parts by weight of polyvinylidene fluoride were used. After preparing an electrode containing 49.9 mg of the mixed carbon material as an active material, a battery for evaluation of a lithium counter electrode was prepared and the initial charge / discharge characteristics of the electrode containing the mixed carbon material were evaluated. The results are shown in Table 1.

Example 4 In Example 1, 1000 ° C. in an argon atmosphere before kneading the natural graphite and the pseudographitic carbon black.
The initial charge and discharge characteristics of the electrode containing the mixed carbon material were evaluated in the same manner as in Example 1 except that the heat treatment was performed.
The results are shown in Table 1.

Example 5 In the same manner as in Example 1, an electrode (1) containing 25.8 mg of a mixed carbon material of natural graphite and pseudographitic carbon black as an active material was prepared. On the other hand, lithium nickel oxide powder, acetylene black as a conductive material, and N as a binder.
After mixing with polyvinylidene fluoride in a solvent of methylpyrrolidone in a weight ratio of 91: 6: 3,
An electrode (2) containing an active material containing 47.9 mg of lithium nickel oxide was prepared by applying the composition on an S mesh, press-bonding it, and drying it in a vacuum overnight. Next, using the electrode (1) and the electrode (2) obtained here, lithium perchlorate was used as an electrolytic solution in an equal volume of ethylene carbonate (EC) and 1,2-dimethoxyethane (DME). Solution dissolved in mixture (concentration 1m
ol / l) was retained in a polypropylene separator having a thickness of 175 μm to prepare a battery and left overnight. The open circuit voltage of this battery before the test was 0.12V. Then, using the electrode (2) as a positive electrode and the electrode (1) as a negative electrode, charging was performed at a constant current of 0.5 mA until the voltage reached 4.15 V (lithium occlusion in the electrode (1)), and then a constant current was applied. Discharge (lithium release from the electrode (1)) was performed at 0.5 mA until the voltage reached 2.5 V, and the initial charge / discharge characteristic of the electrode (1) containing the mixed carbon material was evaluated. The results are shown in Table 1.

Comparative Example 1 In the same manner as in Example 1 except that 100 parts by weight of natural graphite and 3.1 parts by weight of polyvinylidene fluoride were used without using the pseudographitic carbon black. After preparing an electrode containing 49.0 mg of the carbon material, a lithium counter electrode evaluation battery was prepared and the initial charge / discharge characteristics of the electrode containing the carbon material were evaluated. The results are shown in Table 1.

Comparative Example 2 The same as Example 1 except that 100 parts by weight of pseudo-graphitic carbon black and 11.1 parts by weight of polyvinylidene fluoride were used without using natural graphite. After preparing an electrode containing 31.0 mg of the carbon material as an active material, a battery for evaluation of a lithium counter electrode was prepared and an evaluation test was conducted to evaluate the initial charge / discharge characteristics of the electrode containing the carbon material. The results are shown in Table 1.

Comparative Example 3 In Example 1, natural graphite was 75 parts by weight, polyvinylidene fluoride was 6.4 parts by weight, and true specific gravity was 1.82.
Carbon black having a lattice spacing d ₀₀₂ of 3.63Å in X-ray diffraction, a volatile content of 0.6% by weight, a number average primary particle diameter of 40 nm, and a specific surface area of 60 m ² / g measured by a nitrogen adsorption method (Tokai Carbon Co., Ltd. ), Trade name TB4500] 25
After preparing an electrode containing the active material of 45.3 mg of mixed carbon material in the same manner as in Example 1 except that the parts by weight were used as they were without heat treatment, a battery for evaluation of a lithium counter electrode was prepared and an evaluation test was conducted. Then, the initial charge / discharge characteristics of the electrode containing the mixed carbon material were evaluated. The results are shown in Table 1.

Comparative Example 4 An electrode (3) containing 26.5 mg of active material of natural graphite was prepared in the same manner as in Comparative Example 1. On the other hand, an electrode (4) containing an active material of lithium nickel oxide 48.7 mg was prepared in the same manner as in Example 5, and a battery having the electrode (4) as a positive electrode and the electrode (3) as a negative electrode was prepared in the same manner as in Example 5. Then, the initial charge and discharge characteristics of the electrode containing the carbon material were evaluated. The results are shown in Table 1.
Shown in.

[0028]

[Table 1]

[0029]

EFFECTS OF THE INVENTION According to the present invention, the irreversible capacity of the negative electrode for a lithium secondary battery at the time of initial charge / discharge is reduced to improve the utilization efficiency of chargeable / dischargeable lithium in the counter electrode or the positive electrode, thereby improving the initial charge / discharge. A negative electrode for a lithium secondary battery having excellent characteristics can be provided.

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Claims (3)

(57) [Claims] 1. A graphite material and pseudographitic carbon black are contained, and the lattice spacing of the pseudographitic carbon black is 3.38.
~ 3.46Å, the true specific gravity is 1.9 ~ 2.1,
A negative electrode for a lithium secondary battery, characterized in that the proportion of both is 70 to 99% by weight of graphite material and 30 to 1% by weight of pseudographitic carbon black. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the graphite material is scaly natural graphite or scaly artificial graphite. 3. The volatile component of pseudo-graphitic carbon black is
0.5% by weight or less and a number average primary particle size of 10 to 1
00 nm, and the specific surface area by the nitrogen adsorption method is 10 to 3
The negative electrode for a lithium secondary battery according to claim 1, wherein the negative electrode is 00 m ² / g .