JPH10116605A - Lithium secondary battery negative electrode material and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery negative electrode material and its manufacturing method in which charge and discharge capacity is large, and battery performance having excellent cycle characteristics is equipped. SOLUTION: Lithium secondary battery negative electrode material is made of carbon material in which its carbon fiber surface is coated by the carbide of thermosetting resin, a lattice spacing (d002 ) value by X-ray diffraction is d002 <0.343nm at a carbon fiber portion, d002 <0.340nm at a resin carbonized portion, and weight ratios of carbon fiber and resin carbide are 1:0.5 to 2. Solution prepared by dissolving the thermosetting resin in organic solvent is impregnated into a carbon fiber sheet, is cured so as to coat the carbon fiber surface by the thermosetting resin, and the same is carbonized in temperatures from 2,000 to 3,000 deg.C in a non-oxidizing atmosphere so as to manufacture the negative electrode material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium battery having a large charge / discharge capacity and excellent cycle characteristics using a carbon material having a composite composition structure in which a carbon fiber surface is coated with a carbide of a thermosetting resin as a lithium carrier. The present invention relates to a negative electrode material for a secondary battery and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries having a high energy density have attracted attention and are being developed as batteries for power supply or power storage of small electronic devices. Lithium metal was initially used for the negative electrode of a lithium secondary battery, but dendritic active lithium called dendrite was deposited on the negative electrode during repeated charging and discharging, and the positive electrode was formed by peeling and growing of the deposited dendrite. Short circuit occurs, and the cycle characteristics deteriorate.

There is a method of using a lithium alloy as a negative electrode material in order to prevent the precipitation of dendrite, but there is a disadvantage that the battery performance is not sufficient, and it is clear that the use of a carbon material prevents the formation of dendrite. A great deal of research has been conducted to make lithium, which is a negative electrode active material, supported on a certain type of carbon material to produce a negative electrode material. Many proposals have also been made regarding the properties of the carbon material to be supported (for example, Japanese Patent Application Laid-Open No. Nos. 62-193463, 63-236259, 64-64.
No. 2258, JP-A-1-274360, JP-A-2-44644, JP-A-2-66856, JP-A-2-230660, and 3
No.-93162).

[0004] Highly crystalline graphite represented by natural graphite as a carbon material has a battery capacity of 300 mAh / g or more theoretically and a flatness of potential, since a graphite layer having a hexagonal mesh structure is developed. Is also excellent. Further, when carbon fiber is used as the negative electrode, since the carbon fiber is fibrous, the contact between the fibers becomes good and the contact resistance is reduced, so that it is attracting attention as a negative electrode material having a relatively large battery capacity.

For example, as a negative electrode material for a lithium secondary battery using carbon fibers, a negative electrode for a lithium secondary battery having an electrode surface formed by a cross section perpendicular to the fiber axis of a carbon fiber assembly whose fiber axes are aligned in parallel. (JP-A-4-359862) or a sheet-like carbon fiber-containing carbon material in which carbon fibers are oriented parallel to the surface of the sheet (JP-A-7-22022). And so on.

When the degree of graphitization of carbon fiber is increased, 300 m
While a high battery capacity of about Ah / g can be obtained, the degree of exposure of the graphite structure edge surface to the electrode surface becomes large, and the initial charge / discharge efficiency decreases. There are difficulties that cannot be met.

Therefore, a carbon material having a multiphase structure using carbon fibers or the like as a negative electrode material has been developed. For example, a lithium secondary material in which the surface of a carbon material used as a component of a negative electrode is coated with a thin film of amorphous carbon is used. A negative electrode for a battery (Japanese Patent Application Laid-Open No. 5-275076), a multiphase structure comprising at least two phases of a carbonaceous material forming a nucleus and a surface carbonaceous material formed on the surface of the nucleus; ) The plane spacing d ₀₀₂ is 3.35 Å or more and 3.4.
A peak less than 3 angstroms and a _d002 of 3.43
Carbonaceous material having at least two peaks or more peaks Å (JP-A-5-290889), also d ₀₀₂ is less 3.37 Angstrom particles of carbonaceous material d ₀₀₂ is in carbonaceous material above 3.38 angstroms An electrode material having a coated multiphase structure (JP-A-6-267531) has been proposed.

[0008]

All of these carbon materials having a multi-phase structure have carbon having a high degree of graphitization as an inner core and carbon having a low degree of graphitization is formed on the surface layer. JP-A-5-290889 discloses a model of a multiphase structure in which a plurality of fibrous carbons having a high degree of graphitization are used as an inner core and carbons having a lower degree of graphitization are included.

Such a carbon material having a high degree of graphitization is used as an inclusion, and a carbon material having a two-phase structure coated with an amorphous carbon material having a lower degree of graphitization is used as a negative electrode material for a lithium secondary battery. And cycle characteristics can be improved. However, the adsorption and desorption of lithium ions during charging and discharging, especially the doping of lithium ions during charging, expands the graphite layers inside the high degree of graphitization, and conversely contracts during discharging, causing repeated charging and discharging. As a result, damage or destruction occurs, and there is a limit to improving the battery capacity.

The present inventor has conducted extensive studies on a negative electrode material for a lithium secondary battery having a large charge / discharge capacity and excellent cycle characteristics while maintaining excellent moldability of carbon fibers. It has been found that when coated with a curable thermosetting resin and fired, the carbonized portion of the resin becomes a carbon layer having a relatively high degree of graphitization, and the charge / discharge capacity and cycle characteristics are improved.

The present invention has been developed based on the above findings, and an object thereof is to provide a negative electrode material for a lithium secondary battery having a large charge / discharge capacity and excellent cycle characteristics, and a method for producing the same. It is in.

[0012]

According to the present invention, there is provided a negative electrode material for a lithium secondary battery according to the present invention, in which a carbon fiber surface is coated with a carbide of a thermosetting resin, and a lattice spacing is determined by X-ray diffraction. The value of (d ₀₀₂ ) is d ₀₀₂ <0.
343 nm, d ₀₀₂ <0.340 nm in the resin carbonized part,
In addition, it is characterized in that it is made of a carbon material having a weight ratio of carbon fiber to resin carbide of 1: 0.5 to 2.

[0013] Further, the production method is such that a solution obtained by dissolving a thermosetting resin in an organic solvent is impregnated into a carbon fiber sheet, cured, and the thermosetting resin is applied to the carbon fiber surface. By carbonizing at a temperature of 2000 to 3000 ° C., the carbon fiber surface is covered with a carbide of a thermosetting resin, and the value of lattice spacing (d ₀₀₂ ) by X-ray diffraction is d ₀₀₂ <0.343 nm in the carbon fiber portion. , D ₀₀₂ <
A structural feature is to obtain a carbon material having 0.340 nm and a weight ratio of carbon fiber to resin carbide of 1: 0.5 to 2.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The negative electrode material for a lithium secondary battery according to the present invention has a two-phase structure in which the surface of a carbon fiber is coated with a carbide of a thermosetting resin, and has a value of lattice spacing (d ₀₀₂ ). Is d ₀₀₂ <0.343 nm in the carbon fiber portion and d in the resin carbonized portion.
It is characterized by having graphite crystal properties of ₀₀₂ <0.340 nm. That is, it is characterized by the fact that the degree of graphitization of the carbide of the thermosetting resin coating the surface thereof is relatively higher than the degree of graphitization of the carbon fiber portion, and the interaction between the two causes a new occlusion site for lithium ions. Is generated, thereby improving the charge / discharge capacity, and further improving the initial current efficiency and improving the flatness of the potential of the charge / discharge curve. Graphitization degree (d
_When the value of ( ₀₀₂ ) is 0.343 nm or more in the carbon fiber portion and 0.340 nm or more in the resin carbonized portion, the effect of such interaction is small and the battery capacity cannot be increased. The lattice spacing (d ₀₀₂ ) representing the degree of graphitization is X
It is determined by separating peaks of 002 diffraction lines corresponding to carbon layers by line diffraction.

The weight ratio between the carbon fiber and the resin carbide must be in the range of 1: 0.5 to 2. When the weight ratio of the resin carbide is less than 0.5, the effect of the interaction is small because the coating of the resin carbide is not sufficient. On the other hand, when the weight ratio exceeds 2, the ratio of the carbon fiber portion is reduced and the charge / discharge capacity is reduced. The current efficiency also decreases.

In the negative electrode material for a lithium secondary battery according to the present invention, a carbon fiber sheet is impregnated with a solution obtained by dissolving a thermosetting resin in an organic solvent, and then heat-cured to apply the thermosetting resin to the carbon fiber surface. Then, in a non-oxidizing atmosphere, 2000 to 3000
It is produced by carbonizing and graphitizing by heat treatment at a temperature of ° C. If the heat treatment temperature is lower than 2000 ° C., the degree of graphitization becomes insufficient, and even if the heat treatment is performed at a temperature exceeding 3000 ° C., the graphitization degree is hardly improved.

The carbon fiber used in the present invention is not particularly limited, and any of polyacrylonitrile-based carbon fiber, rayon-based carbon fiber, and pitch-based carbon fiber can be used. Is preferably a radial (radial) structure because the diffusion rate of lithium ions increases.

As the thermosetting resin, a thermosetting resin such as a non-graphitizable phenol resin, a furan resin, a polyimide resin and a polycarbodiimide resin having a residual carbon ratio of 50% or more is used. Furan resin is used. If the residual carbon ratio is less than 50%, the carbon fiber surface cannot be sufficiently covered with the resin carbide. These thermosetting resins are methanol, ethanol,
A solution dissolved in an organic solvent such as benzene, acetone, or toluene at an appropriate concentration is applied to the carbon fiber sheet, or impregnated by a method such as immersing the carbon fiber sheet in the solution, heated, cured, and dried. A thermosetting resin is applied to the carbon fiber surface. The amount of impregnation is set so that the weight of the resin carbide after carbonization and graphitization is in the range of 0.5 to 2 with respect to carbon fiber 1 in weight ratio.

Then, the surface of the carbon fiber is heated by heating to a temperature of 2000 to 3000 ° C. in a non-oxidizing atmosphere such as argon or nitrogen gas, and carbonizing and graphitizing the coated thermosetting resin. Coated with carbide of thermosetting resin. Thermosetting resin undergoes volume shrinkage during carbonization and graphitization, but the volume shrinkage of carbon fiber is smaller than that of thermosetting resin. Without delamination, the degree of graphitization of the carbon fiber portion and the resin carbide was measured by the graphite crystalline property of the present invention, that is, the value of the lattice spacing (d ₀₀₂ ) was d ₀₀₂ <0.3 in the carbon fiber portion.
The graphite crystallinity can be made to be 43 nm and d ₀₀₂ <0.340 nm in the resin carbonized portion. The heat treatment temperature is 2000
If the temperature is lower than ℃, carbonization and graphitization are insufficient.
This is because heat treatment at a temperature exceeding ℃ is unnecessary. The carbon material obtained by coating the surface of the carbon fiber thus obtained with the resin carbide is pulverized to an appropriate size to produce the negative electrode material for a lithium secondary battery of the present invention.

In the negative electrode material for a lithium secondary battery of the present invention, since the resin carbide coated on the carbon fiber surface has an appropriate degree of graphitization, the edge surface on the surface is less exposed, so that the reaction with the electrolytic solution is suppressed. This prevents a decrease in current efficiency in the initial stage. In addition, the interaction between the carbon fiber and the resin carbide coated on the surface increases the number of lithium ion occlusion sites, thereby increasing the battery capacity during charge and discharge, improving the flatness of the potential during charge and discharge, and improving the charge and discharge. The difference increases, and the irreversible capacity can be suppressed. Further, the carbon fibers are prevented from being broken by expansion and contraction due to adsorption and desorption of lithium ions during charge and discharge, and cycle characteristics can be improved.

As described above, the negative electrode material for a lithium secondary battery of the present invention comprises a carbon material having a carbon fiber surface coated with a carbide of a thermosetting resin and a carbon material having a composite composition structure having a specified degree of graphitization. Therefore, the charge / discharge capacity can be increased, the cycle characteristics can be improved, and the contact resistance can be maintained at a low level because the fiber form is maintained.

[0022]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples.

Examples 1 to 9 and Comparative Examples 1 to 3 Pitch-based carbon fibers chopped to a length of 3 mm (XN-40, manufactured by Nippon Graphite Fiber Co., Ltd.) as carbon fibers,
And polyacrylonitrile-based carbon fiber [Toray Industries, Inc.
, T300], these chopped carbon fibers were dispersed in ethanol, filtered and dried to prepare a carbon fiber sheet. A solution (resin concentration of 5 to 40% by weight) of a phenol resin having a residual carbon ratio of 50% (PR940, manufactured by Sumitomo Durez Co., Ltd.) dissolved in acetone was added to the carbon fiber sheet. It was impregnated so that the ratio was 0.6 to 2.5, and was heated and cured and dried. The carbon fiber chop to which the phenolic resin was applied in this manner was heat-treated in a nitrogen atmosphere at a temperature in the range of 1800 to 3000 ° C., and carbonized and graphitized. With respect to the carbon fiber chop coated with the carbide of the phenolic resin produced in this way, the lattice spacing (d
₀₀₂ ) was measured by X-ray diffraction, and the graphitization characteristics of the obtained carbon material are shown in Table 1 in comparison with the production conditions.

[0024]

[Table 1] Table Note) * 1 Ph-1; phenol resin with residual carbon ratio of 50% * 2 Weight ratio of carbonized resin to carbon fiber

The carbon fiber chop coated with the carbide of the phenolic resin is pulverized so that the average fiber length becomes 100 μm, and these carbon materials are used as a lithium carrier, and 88 parts by weight of the carbon material is mixed with polyvinylidene fluoride powder (condensed matter). Adhesive)
12 parts by weight, and N-methylpyrrolidone (organic solvent)
100 parts by weight were added and kneaded to prepare a homogeneous mixed paste. This paste was uniformly applied to a copper foil having a thickness of 50 μm, heated to a temperature of 150 ° C. to remove the organic solvent and dried, and then pressed to produce a negative electrode material having a thickness of 50 μm.

This negative electrode material was prepared by using a 1: 1 mixed solution of ethylene carbonate and diethyl carbonate as an electrolyte.
A three-electrode cell was prepared using vanadium oxide as a counter electrode and metallic lithium as a reference electrode, and the performance of the negative electrode material was evaluated by a charge / discharge test. The charge / discharge test is performed at a constant current density of 50 mA / g.
Charge stop potential and discharge stop potential are 0 V and 1 V, respectively.
The discharge capacity at the 1st cycle and the 100th cycle was measured to evaluate the performance as a negative electrode material.
It was shown to.

[0027]

[Table 2]

Comparative Examples 4 to 6 Carbon fibers areotropic carbon fibers [Kureha Chemical Industry Co., Ltd.
C-199T], the production of a carbon material, the production of a negative electrode material, and the evaluation of battery performance were performed in the same manner as in Examples 1 to 9, and the results are shown in Tables 3 and 4. .

Comparative Examples 7 and 8 Example 1 was repeated except that the residual carbon ratio of the phenol resin was changed to 40%.
Production of a carbon material, production of a negative electrode material, and evaluation of battery performance were performed in the same manner as in Examples 9 to 9, and the results are shown in Tables 3 and 4.

Comparative Examples 9 to 10 Except for using petroleum pitch in place of the phenolic resin, the same method as in Examples 1 to 9 was used to produce a carbon material, produce a negative electrode material, and evaluate battery performance. Are shown in Tables 3 and 4.

Comparative Examples 11 to 12 Carbon materials were produced using only pitch-based carbon fibers (Nippon Graphite Fiber Co., Ltd., XN-40) without applying a thermosetting resin. Production of a negative electrode material and evaluation of battery performance were performed in the same manner as described above. Tables 3 and 4 show the obtained results.

Comparative Examples 13 and 14 A phenol resin having a residual carbon ratio of 50% [Sumitomo Durez Co., Ltd.
And PR940] were used to manufacture the negative electrode material and the battery performance was evaluated in the same manner as in Examples 1 to 9, and the obtained results are shown in Tables 3 and 4. Was.

[0033]

[Table 3] Table Note) * 1 Ph-1: Phenolic resin with residual carbon ratio of 50% Ph-2; Phenolic resin with residual carbon ratio of 40% Pitch; Petroleum pitch resin * 2 Weight ratio of carbonized resin to carbon fiber

[0034]

[Table 4]

From the results of Tables 1 to 4, all of the negative electrode materials of Examples satisfying the requirements of the present invention have a discharge capacity of 300 mAh /
g, the initial discharge capacity is also large. In addition, it is recognized that the discharge capacity is less reduced even after repeated charge and discharge, and that the battery performance is excellent in battery capacity and cycle characteristics. On the other hand, it can be seen that the negative electrode material of the comparative example which does not meet the requirements of the present invention has a small discharge capacity and a low initial discharge capacity.

[0036]

As described above, according to the negative electrode material of the present invention, the surface of the carbon fiber is covered with the carbide of the thermosetting resin.
Is composed of a phase structure, d value of the lattice spacing (d _{00 2)} is a carbon fiber unit ₀₀₂ <0.343 nm, a resin carbonized part d ₀₀₂ <
By providing graphite crystal properties of 0.340 nm, and by setting the coating amount of carbide to the weight ratio with carbon fiber in the range of 1: 0.5 to 2, the discharge capacity is high, and even if charge / discharge is repeated. It is possible to provide a negative electrode material for a lithium secondary battery having excellent battery performance and cycle characteristics with little decrease in discharge capacity. Further, according to the production method of the present invention, it is possible to efficiently produce a negative electrode material having the above-mentioned performance, and it is extremely useful as an industrial production method of a negative electrode material for a lithium secondary battery.

Claims (3)

[Claims] 1. A carbon fiber surface is coated with a carbide of a thermosetting resin, and the value of lattice spacing (d ₀₀₂ ) by X-ray diffraction is d ₀₀₂ <0.343 nm in the carbon fiber portion and d in the resin carbonized portion.
₀₀₂ <0.340 nm, and a negative electrode material for a lithium secondary battery, comprising a carbon material having a carbon fiber to resin carbide weight ratio of 1: 0.5 to 2. 2. A carbon fiber sheet is impregnated with a solution obtained by dissolving a thermosetting resin in an organic solvent and cured to apply the thermosetting resin to the carbon fiber surface.
By carbonizing at a temperature of ３3000 ° C., the carbon fiber surface is coated with a carbide of a thermosetting resin, and the value of lattice spacing (d ₀₀₂ ) by X-ray diffraction is d ₀₀₂ <0.3 in the carbon fiber portion.
A method for producing a negative electrode material for a lithium secondary battery, comprising: obtaining a carbon material having a diameter of 43 nm, a d ₀₀₂ <0.340 nm in a resin carbonized portion, and a weight ratio of carbon fiber to resin carbide of 1: 0.5 to 2. . 3. The method for producing a negative electrode material for a lithium secondary battery according to claim 2, wherein the thermosetting resin is a phenol resin or a furan resin having a residual carbon ratio of 50% or more.