JP2006228640A - Silicon-added graphite cathode material for lithium-ion secondary battery, and manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery negative electrode material which is superior in cycle characteristics and battery efficiency with high capacity exceeding a graphite material. <P>SOLUTION: This is a manufacturing method of the lithium secondary battery negative electrode material by calcining a mixture produced after graphite powders, carbon precursors, silicon fine powders, and a void forming agent are heated and blended. Furthermore, in the manufacturing method like this, this is the manufacturing method for coating the void forming agent on the silicon fine powders in advance. This is the negative electrode material for the secondary battery which is formed by embedding the silicon fine powders into the inner part of the graphite base material obtained by the above manufacturing method. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a negative electrode material for a lithium secondary battery, and relates to a negative electrode material excellent in cycle characteristics and a manufacturing method obtained by adding silicon to a graphite substrate.

Lithium secondary batteries are often used in portable devices such as mobile phones, personal computers and PDAs as high-power, high-capacity secondary batteries, and it is expected that the demand will continue to increase in the future.
In response to the trend toward smaller and lighter portable devices, there is an increasing demand for smaller and lighter lithium secondary batteries.
In order to meet this demand, high performance of materials used for lithium secondary batteries has been actively attempted, and the development of negative electrode materials has been increasing in importance as it affects battery performance.

Currently, carbon (graphite) is mainly used as the negative electrode material, and about 350 to 360 mAh / g, which is about the same as the theoretical capacity of graphite, 372 mAh / g, has been developed and is in practical use.
In recent years, in order to further increase the discharge capacity, a negative electrode material in which silicon and graphite powder are mixed, or a pitch obtained by mixing silicon powder on the surface of carbon powder or graphite powder has been proposed.
For example, Japanese Patent No. 3268770 proposes a carbon material and silicon powder mixed and heat-treated, but only 10 cycles are evaluated, which is insufficient for practical use. (Patent Document 1)

  Japanese Patent No. 3285246 proposes a silicon intermetallic compound in place of the silicon powder, but only the cycle characteristics are examined and the discharge capacity and the battery efficiency are not considered. (Patent Document 2)

  Alternatively, electrode materials in which silicon or silicon-cobalt composite plating is directly applied onto a copper foil as a current collector have been actively studied, but they absorb volume changes associated with lithium doping and doping. However, there is a big problem in cycle characteristics.

Furthermore, in the invention described in JP-A No. 2002-270170, reference is made to a negative electrode material containing silicon, other metals, or an alloy thereof, but it is an evaluation in 20 cycles and it cannot be said that the initial charge / discharge is sufficient. Efficiency is also 80% or less. (Patent Document 3)

Japanese Patent No. 3268770 Japanese Patent No. 3268770 JP 2002-270170 A

In this way, many developments of high-capacity negative electrode materials that exceed the current mainstream graphite have been studied, but it is difficult to develop a negative-electrode material that has high capacity and excellent cycle characteristics and battery efficiency and that can be put to practical use. is there.

In view of the above situation, the present inventor provides a lithium ion secondary battery negative electrode material having a high capacity exceeding that of a graphite material and excellent in cycle characteristics and battery efficiency.
The present inventor conducted intensive research on the improvement of cycle characteristics, especially for high capacity negative electrode materials in which silicon fine powder was added to graphite material. As the cycle progressed, the silicon surface was activated and reacted with the electrolyte. I noticed that the discharge capacity was reduced by.
Further, the volume change of the silicon fine powder mixed with graphite occurs with the lithium doping and doping, but it is necessary to absorb this volume expansion in order to improve the battery characteristics.
The present invention is intended to prevent activation of the silicon surface and to suppress volume expansion.

As a result, it is possible to reduce the reaction with the electrolytic solution by making the silicon powder into an optimum particle size and making it into a negative electrode material embedded in a graphite base material, and to form a void forming agent such as a chain polymer. It is effective in absorbing the volume change to cover the silicon fine powder and form voids around the silicon fine powder by completely eliminating the void forming agent by firing or leaving some residue to disappear. The present invention was completed with the knowledge that there was.
That is, the present invention is a method for producing a negative electrode material for a lithium ion secondary battery obtained by heating and mixing a graphite powder, a carbon precursor, a silicon fine powder, and a void forming agent. Moreover, it is a lithium ion secondary battery negative electrode material by which silicon fine powder is embed | buried under the graphite base material obtained by this manufacturing method.
The lithium ion secondary battery negative electrode material and the production method of the present invention will be described in detail below.

  The negative electrode material of the present invention uses graphite powder as a base material, a carbon precursor as a binder, and silicon fine powder as an additive for increasing the capacity. In addition to these, a void forming agent is used. is there.

First, graphite powder as a base material can be used as a graphitized product of coke or raw coke, a graphitized product of mesophase pitch powder, natural graphite, and a granulated product of natural graphite. Any mixture of these may be used.
The average particle diameter of the graphite powder is not a problem as long as it is about the same as that of a commercially available graphite negative electrode material, but about 1 to 50 μm is appropriate.
When the particle size is 50 μm or more, the particle diameter obtained after granulation of the particles is unfavorable because the particle size distribution includes many particles of 80 μm or more exceeding the thickness of the negative electrode sheet.
The degree of graphitization is preferably such that the distance d (002) between the carbon crystal faces is 0.337 nm or less in order to obtain a negative electrode material excellent in discharge capacity, battery efficiency, and the like.
Moreover, in order to make it easy to embed silicon fine powder, it is more preferable that the graphite powder has irregularities on the edge portion of the surface.

The carbon precursor uses the following pitch or resin.
As the pitch, any of petroleum-based, coal-based amorphous (isophase pitch) and crystalline (mesophase pitch) can be used.
The melting point of the pitch is preferably 360 ° C. or lower, and if it is higher than this, inconvenience is likely to occur during mixing and coating.
As the resin, phenol resin, furan resin or the like is used. These resins preferably have an oxygen content of 20% or less. If excessive oxygen is contained after the baking heat treatment, the discharge capacity and battery efficiency of the resulting negative electrode material are lowered, which is not preferable.
The amount of these carbon precursors used varies slightly depending on the specific surface area and oil absorption of the graphite powder as the base material, but is generally about 5 to 30 parts by weight with respect to 100 parts by weight of graphite powder. It is necessary to adjust according to the characteristics.
If it is less than 5 parts by weight, the effect is not obtained in a small amount, and if it exceeds 30 parts by weight, the efficiency is decreased, which is not preferable.

The silicon fine powder, which is an additive for increasing the capacity, is required to be fine particles to be embedded in the graphite powder of the base material, and preferably has an average particle diameter of 0.1 to 0.5 μm. Basically, it is desirable to be fine particles, but it is inconvenient in handling because it aggregates below 0.1 μm.
It is necessary to pay attention not only to the average particle size but also the maximum particle size, and the maximum particle size is preferably 1 μm or less. If one having a thickness of 1 μm or more is present, the cycle characteristics are liable to be adversely affected.
The amount of silicon fine powder added is preferably 1 to 20 parts by weight per 100 parts by weight of graphite powder. If it is 1 part by weight or less, the effect of increasing the discharge capacity is poor, and if it exceeds 20 parts by weight, the cycle characteristics are deteriorated.

In the present invention, a chain polymer material is further added as a void forming agent. This chain polymer material is a material that does not remain as residual charcoal after firing, and almost disappears upon firing.
As a result, voids are formed around or near the silicon fine powder. The void absorbs the volume expansion of the silicon fine powder, thereby preventing the electrode from being broken and exhibiting an excellent effect in improving the cycle characteristics.
Specific examples of the void forming agent include polyvinyl alcohol, polyethylene glycol, methyl cellulose, carboxymethyl cellulose, polycarbosilane and the like.

The production method of the present invention is as follows.
First, graphite powder, carbon precursor, silicon fine powder, and void forming agent are heated and mixed at 100 to 200 ° C. The apparatus used for mixing is generally suitable for a heating kneader, but is not limited thereto.
The mixture after the heating and mixing treatment is fired at 800 to 1300 ° C. in a non-oxidizing atmosphere or reducing atmosphere such as nitrogen to obtain the negative electrode material of the present invention.
Moreover, you may perform an infusibilization process after heat mixing. This is effective in preventing fusion of particles during firing.
Further, it is desirable that the silicon fine powder is previously coated with a void forming agent and then heated and mixed with the graphite powder and the carbon precursor.

The negative electrode material of the present invention obtained by the production method as described above has the following characteristics.
The silicon fine powder is contained in about 1 to 20% in one particle, and these silicon fine powders are embedded in the graphite base material.
In addition, there are voids formed by the void forming agent disappearing by baking heat treatment around or in the vicinity of the silicon fine powder.
The presence of this void is an effective means for absorbing the expansion and contraction of silicon accompanying charge / discharge.

According to the negative electrode material of the lithium secondary battery of the present invention, the degradation of cycle characteristics due to the reaction between silicon and the electrolyte is suppressed by adopting a structure in which finely divided silicon powder is embedded in a graphite base material. can do.
The voids formed around the silicon powder absorb the volume expansion of silicon accompanying the lithium ion doping and doping, and exhibit an excellent effect in preventing the destruction of the electrode.
These functions and effects can provide a negative electrode material that exhibits a high capacity exceeding that of a conventional graphite negative electrode material and is excellent in cycle characteristics and battery efficiency.

  Next, embodiments of the present invention will be described with reference to the following examples.

A mesophase pitch having an average particle size of 15.1 μm with a softening point of 340 ° C. and a mesophase amount of 95% was heat-treated in air at 360 ° C. to make it infusible. After crushing, it was heat-treated and fired at 1000 ° C. in an inert atmosphere, further transferred to a graphitization furnace, and heat-treated at 3000 ° C. in an argon atmosphere for graphitization. This was granulated, the average layer spacing d ₀₀₂ in average particle diameter 14.1μm to obtain a graphite powder 0.3357Nm.
Further, silicon fine powder having an average particle diameter of 0.2 μm and a maximum particle diameter of 1 μm or less was prepared by dipping in an alcohol solution in which 5% polyvinyl alcohol was previously dissolved as a void forming agent.
100 parts by weight of the above graphite powder and 15 parts by weight of silicon fine powder were mixed with a kneader at room temperature for 5 hours.
To this powder, 10 parts by weight of coal tar pitch having a softening point of 120 ° C. was added, and mixed and heat-treated at 150 ° C. for 1 hour using a kneader. The obtained powder was further heat-treated at 1200 ° C. in a nitrogen atmosphere, and then pulverized and sized to obtain a negative electrode material for a lithium ion secondary battery having an average particle size of 21.8 μm.

  Lithium ions having an average particle size of 19.1 μm were the same as in Example 1, except that the particle size of the graphite powder was 10 μm, the silicon fine powder was 5 parts by weight, and the coal pitch was 15 parts by weight. A secondary battery negative electrode material was obtained.

Granule-molding was performed on 100 parts by weight of scaly natural graphite having an average particle size of 14 μm and d ₍₀₀₂₎ of 0.336 nm using an impregnated pitch as a binder, and this was heat-treated at 3000 ° C. in an argon atmosphere to obtain an average particle size of 16 μm, A substantially spherical formed body having a tap density of 0.92 g / cm ³ (the ratio of the major axis to the minor axis is distributed in the range of about 1.1 to 2.0).
In a kneader heated at 200 ° C. in advance by adding 8 parts by weight of silicon fine powder having an average particle diameter of 0.25 μm and a maximum particle diameter of 1 μm or less and 5 parts by weight of polyvinyl alcohol to 100 parts by weight of the graphite compact. Mixed heat treatment was performed for 1 hour. The kneader temperature was kept at 150 ° C., and 10 parts by weight of the tar tar pitch used in Example 1 was added and mixed and heat-treated for 2 hours.
A fired product obtained by heat-treating the obtained powder at 1200 ° C. in a nitrogen atmosphere was pulverized with a quick mill manufactured by Seishin Enterprise Co., Ltd. and sized to obtain a negative electrode material for a lithium ion secondary battery having an average particle size of 22 μm. .

20 parts by weight of coal tar pitch with a softening point of 80 ° C. dissolved in excess of xylene in advance, 10 parts by weight of polyethylene glycol, 5 parts by weight of silicon fine powder having an average particle diameter of 0.3 μm and a maximum particle diameter of 1 μm or less To obtain a dispersion.
After adding 100 parts by weight of the natural graphite molded body used in Example 3 to the kneader and mixing, the above dispersion was applied to the surface by spraying at room temperature, and then the kneader treatment temperature was set to 150 ° C. and dispersed. After volatilizing the agent, it was crushed lightly, and the dried product was heat-treated at 1200 ° C. for 6 hours in a nitrogen atmosphere. This heat-treated product was crushed by the quick mill of Example 3 to obtain a negative electrode material for a lithium ion secondary battery having an average particle size of 20.6 μm.

  With respect to 100 parts by weight of graphite powder used in Example 1, 5 parts by weight of silicon fine powder having an average particle diameter of 0.25 μm and a maximum particle diameter of 1 μm and 5 parts by weight of polyethylene glycol were agitated at room temperature for 1 hour. After premixing, the mixture was heat-treated at 200 ° C. for 1 hour. Furthermore, 18 parts by weight of an amorphous pitch having a softening point of 110 ° C. was put into a kneader and mixed and heat-treated for 1 hour, and then the fired product obtained by heat treatment at 1000 ° C. in a nitrogen atmosphere was crushed, sized and averaged A negative electrode material for a lithium ion secondary battery having a particle size of 19.1 μm was obtained.

Silicon fine powder having an average particle diameter of 0.25 μm and a maximum particle diameter of 1 μm was dispersed in ethanol with respect to 100 parts by weight of the graphite powder used in Example 1 to prepare a dispersion having a silicon concentration of 40%. . After 250 parts by weight of this dispersion and 10 parts by weight of cellulose were mixed and stirred at room temperature with a kneader, 15 parts by weight of coal tar pitch dissolved in an excess amount of xylene was added and mixed for 1 hour. . After mixing, the temperature was raised to 150 ° C. as it was, xylene and ethanol were completely removed, and the obtained powder was heat-treated at 1200 ° C. under a nitrogen atmosphere. A negative electrode material for a lithium ion secondary battery having a particle size of 19.2 μm was obtained.
(Comparative Example 1)

For 100 parts by weight of natural graphite powder having an average particle diameter of 19.4 μm, 8 parts by weight of silicon fine powder having an average particle diameter of 0.3 μm and a maximum particle diameter of 3 μm and 20 parts by weight of the cold pitch used in Example 1 were added. -The mixture was put into a drier and stirred and mixed for 3 hours or more.
The obtained mixed powder was heat-treated at 1000 ° C. for 6 hours under a nitrogen atmosphere and baked, and then crushed and sized by a quick mill to obtain a negative electrode material for a lithium ion secondary battery having an average particle size of 20.3 μm. .
(Comparative Example 2)

To 100 parts by weight of the natural graphite powder used in Comparative Example 1, 8 parts by weight of silicon fine powder having an average particle size of 0.3 μm and a maximum particle size of 3 μm is put into a kneader, and stirred and mixed at room temperature for 3 hours or more. A negative electrode material for a lithium ion secondary battery in which graphite powder and silicon powder were mixed was obtained.
(Comparative Example 3)

The weight part of the coal pitch used as the binder in Example 1 was changed to 40 parts by weight, and 35 parts by weight of silicon fine powder having an average particle diameter of 0.3 μm and coal tar pitch were charged into the kneader. The mixture was mixed at room temperature for 1 hour. This powder was heat-treated and fired at 1000 ° C. in a nitrogen atmosphere to obtain a lithium ion secondary battery negative electrode material having an average particle size of 23.6 μm.
(Comparative Example 4)

  Using the same graphite powder as in Comparative Example 1, with respect to 100 parts by weight of the graphite powder, 10 parts by weight of the polyethylene glycol used in Example 5 and 15 parts by weight of a mesophase pitch having a softening point of 150 ° C. Then, the mixture was heat-treated at 180 ° C. for 1 hour and then heat-treated and fired at 1000 ° C. in a nitrogen atmosphere to obtain a negative electrode material having an average particle size of 18.9 μm for lithium ion secondary batteries.

After mixing N-methyl-2-pyrrolidone with 100 parts by weight of each of the negative electrode materials obtained in Examples 1 to 6 and Comparative Examples 1 to 4 and a conductive auxiliary agent and 8 parts by weight of polyvinylidene fluoride. After pasting and applying to a copper foil using a doctor blade, this was dried at 150 ° C. for 1 hour and then pressed at ² t / cm ² to obtain an electrode sheet.
Each coin cell was prepared using Li metal for the counter electrode and the reference electrode, and 1M LiPF ₆ -EC / MEC (volume ratio 1: 1) as the electrolyte, and the charge / discharge test was performed under the conditions described below.
The charging conditions were charging at a current density of 0.5 mA / cm ² until 10 mV, switching to constant voltage charging when the voltage reached 10 mV, and charging until the current value reached 0.001 mA. The discharge conditions were a discharge to 1.5 V at a current density of 0.5 mA / cm ² . The measurement environment temperature is 30 ° C., and the measurement range is 0.01 to 1.5V.
Table 1 shows the measurement results.

In the examples of the present invention, in any evaluation, the discharge capacity was 430 mAh / g or higher, the charge / discharge efficiency was as high as 88%, and the discharge capacity retention after 50 cycles was 84% or more. A lithium battery having good cycle characteristics was obtained.

According to the present invention, silicon fine powder is embedded in a graphite base material, and a low carbon residue polymer or chain polymer that is a void forming agent is used to form silicon fine powder and a graphite base material. By creating a gap between the electrodes, the expansion and contraction of silicon during charging and discharging can be absorbed, and the electrode can be prevented from being broken. Further, it is possible to improve efficiency by covering the silicon fine powder by coating with a pitch or the like and heat-treating it.
According to the present invention, a negative electrode material having high capacity, high efficiency and excellent cycle characteristics can be obtained, and high performance of a lithium ion secondary battery can be expected.

Claims (8)

A method for producing a negative electrode material for a lithium secondary battery, comprising heating and mixing a graphite powder, a carbon precursor, silicon fine powder, and a void forming agent, followed by firing. 2. The method for producing a negative electrode material for a lithium secondary battery according to claim 1, wherein the silicon fine powder is previously coated with a void forming agent. 3. The method for producing a negative electrode material for a lithium secondary battery, wherein the mixture is heated and mixed in claim 1 and then infusible. 4. The method for producing a negative electrode material for a lithium secondary battery according to claim 1, wherein the carbon precursor is a coal-based or petroleum-based amorphous pitch and / or a crystalline pitch. 5. The method for producing a negative electrode material for a lithium secondary battery according to claim 1, wherein the silicon fine powder has an average particle size of 0.1 to 0.5 [mu] m and a maximum particle size of 1 [mu] m or less. 6. The negative electrode for a lithium secondary battery according to claim 1, wherein the void forming agent is selected from polyvinyl alcohol, polyethylene glycol, polycarbosilane, and cellulose, and the firing rate is 20% or less. Method of manufacturing the material. A negative electrode material for a lithium secondary battery, wherein silicon fine powder is embedded in the negative electrode material obtained by the production method according to any one of claims 1 to 6. 8. The negative electrode material for a lithium secondary battery according to claim 7, wherein the silicon fine powder has an average particle size of 0.1 to 0.5 μm and a maximum particle size of 1 μm or less.