JP2012206923A - Method for producing silicon fine powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method for producing efficiently clean and fine silicon to be used mainly as a negative electrode of a lithium ion battery.SOLUTION: In this method for producing silicon fine powder, crystalline silicon pulverized by cutting and grinding is cracked and dispersed in a dispersion liquid to heighten the degree of dispersion, and then fine silicon whose apparent particle size is 0.1-20 μm is collected as silicon dispersion liquid from the dispersion liquid by a gravity classification method, and at least a part of a coarse part is returned into the dispersion liquid, and further cracked and dispersed.

Description

The present invention mainly relates to a method for producing clean and fine silicon for use in a negative electrode of a lithium ion secondary battery.

In the last 20 years, lithium-ion secondary batteries have been industrialized for the first time, and have rapidly developed along with the development of mobile phones, etc. Now, the largest amount and amount of money among all batteries including primary batteries and secondary batteries It was developed to become. In recent years, it has been attracting attention as a power source for electric vehicles and hybrid cars, which originated from environmental problems.
Furthermore, there are great expectations for taking advantage of its relatively small size and large capacity for backup of solar cells and wind power generation. In order to deal with these problems, extensive studies and improvements have been made on all lithium ion secondary battery technologies starting from the raw materials themselves. In particular, research on the positive electrode and research on electrolytes have been carried out remarkably, but there has been little attention regarding the negative electrode, and carbon-based materials have been widely used.

However, recently, a new negative electrode as a material capable of rapid charging / discharging and capable of withstanding a large capacity has come to have great expectations, and a silicon capable of large capacity has attracted a great deal of attention. It was. However, even with silicon that can be expected to be several times larger in capacity than carbon, it has problems such as volume expansion when holding lithium, due to its crystal structure. How to obtain fine silicon was a big problem.
For example, Japanese Patent Application Laid-Open No. 2004-349097 discloses an electrode structure of a lithium ion secondary battery using silicon fine powder.

In Japanese Patent Application Laid-Open No. 2004-525841, SiF4 or SiH4 is decomposed in plasma in an argon atmosphere and trapped on the surface of fine silicon powder to obtain active silicon particles for the purpose. Japanese Patent Application Laid-Open No. 2011-18575 proposes a negative electrode material in which the surface of silicon fine powder is covered with C.
Japanese Patent Application Laid-Open No. 2007-308774 discloses a method of forming a thin film containing Si carbon from a silane compound and carbon by a CVD method. Although there is a feature that fine silicon can be obtained, the manufacturing conditions are difficult, the raw materials are expensive, and the manufacturing equipment becomes expensive, which is an economic problem.

As shown in these figures, it is premised that all the technologies have silicon microcrystals or fine particles. Otherwise, it is difficult to handle SiF4 or FiH4 to form a thin film or a compound with others. The use of various silane compounds has been performed.

On the other hand, ordinary high-purity silicon, which is easy to obtain, is contrary to this, as represented by an electronic device and as represented by a solar cell. Efforts have been made to increase the diameter to reduce grain boundaries and to obtain larger crystals. For this purpose, therefore, it has been rarely possible to obtain fine particles of silicon that are suitable for the purpose, and generally, these large crystals have been mainly pulverized. However, the conditions for obtaining it were not used so much and it was not known to be a special miniaturization method.

On the other hand, when silicon is used for electronic devices and solar cells, some silicon is crushed. Some are cut with or without abrasives. However, the silicon used for cutting in this way is kept relatively clean although it is contaminated by the lubricant and cutting means used at the same time. However, since the silicon grains used for these differ greatly depending on the cutting method and site, they are not sufficient for use as, for example, a negative electrode of a lithium ion battery that requires fine and uniform grains.
In addition, the required silicon particles are 10 μm or less, usually 5 μm or less, which is insufficient for use, and that the conventional sieving as in the conventional method makes the yield very low. Since it is small, the time required for sieving becomes very long.

JP 2004-349097 A JP 2004-525841 A JP 2011-18575 A JP 2007-308774 A

An object of the present invention is to provide a production method for efficiently producing clean and fine silicon mainly for use as a negative electrode of a lithium ion battery.

In the present invention, the crystalline silicon finely divided by cutting or grinding is pulverized and dispersed in a dispersion to further increase the degree of dispersion. This is a method for producing fine silicon, in which fine silicon of 20 μm is collected as a silicon dispersion, and at least part of the coarse portion is returned to the dispersion and further pulverized and dispersed. In addition, silicon particles crushed as primary particles can be obtained with high efficiency.
This will be described in detail below.

Silicon is a single crystal silicon ingot or polycrystalline silicon, and is usually a very large mass. This is finely pulverized, but when it is usually crushed or refined by hammering, it inevitably scatters, or extra force is applied and it may jump out in other directions. Tend to pick up. Or even in the case where this is not the case, there is a problem that the yield becomes very bad this time if the scattered and contaminated silicon is removed. On the other hand, here, in order to obtain a powder, the silicon is powdered while being cut in the same manner as a cutting operation using a saw, a wire saw or a diamond saw. Alternatively, surface grinding is performed using a grinding blade to make fine powder. With this method, it is possible to concentrate at a certain place and to proceed with cutting and grinding at a considerably high speed, and it is possible to collect silicon in a fine powder form at a predetermined place in a state like a so-called chip.

Although this fine silicon certainly has primary particles, it exists as secondary particles to some extent through such grinding and cutting. In some cases, it may become a certain amount of agglomeration together with the abrasive used at the same time. These are mainly stirred in pure water as a dispersion medium, and are kept dispersed in the dispersion while being crushed. If necessary, so-called hydrogen water or reduced water can be used as a dispersion to keep the ORP of pure water low and improve the degree of dispersion. In this case, the dispersed particles as a whole are in the same direction. As a result of being charged, the agglomeration does not occur and sufficient dispersion can be maintained. It is also possible to stir the dispersion for this dispersion, but on the other hand, it is also recommended to use a wet jet mill to more strongly and actively disperse. This allows for a tighter dispersion. It is also possible to perform ball mill pulverization / disintegration in a wet manner using balls or silicon to prevent contamination.

The silicon that has been crushed and dispersed in this manner is extracted by separating a portion having an apparent particle size of 0.1 to 20 μm as a target particle by the gravity classification method. Here, sedimentation separation is typically used as the gravity classification method, but in addition to this, a separation method using a wet cyclone method and a separation method using centrifugal force instead of gravity are also included. In other words, in the case of relatively small particles such as silicon, for example, when the particle size is 50 μm or more, sedimentation / separation can be easily performed by gravity, but the smaller sedimentation rate tends to be significantly slowed down. In the case of 20 μm or less, particularly 10 μm or less, which is mainly used here, it takes time for gravity separation, so it is desirable to use acceleration by centrifugal force, that is, to use a centrifugal separation method or to perform cyclone separation.

The apparent particle size corresponds to 10 to 40 μm on both sides excluding the thickness of the base copper foil with respect to the thickness of the negative electrode of a normal lithium ion secondary battery, 20 to 50 μm, and the particle size is 0 It is necessary that the thickness is 5 to 20 μm, and if it is more than that, it becomes difficult to maintain the thickness of the negative electrode. In addition, fine particles having a large surface area and active are more effective for silicon particles. However, when the apparent particle size is 0.5 microns or less, it is difficult to agglomerate, and the bulk specific gravity is reduced. There is a problem that a sufficient amount of silicon does not enter the volume designated as a volume battery, and the battery capacity becomes rather small. From these, the desirable particle size range is the above-mentioned numerical value, and it is collected to match it. In addition, it is better if the particle size has a normal distribution, which makes it more compact and an increase in capacity can be expected.

In this way, silicon is collected, and the larger particles separated thereby are returned to the dispersed portion and further dispersed, or refined to perform gravity classification together with other new silicon. This improves the yield for obtaining fine particles. Also, the larger particles removed thereby can be separated and used for other purposes, or further refinement means, that is, as shown here, dispersion means can be further utilized. .

Here, the collection by classification is not necessarily performed in one stage, and can be performed in two stages or more. This can be obtained from the viewpoint of improvement in yield and efficiency. The above-mentioned means used for wet classification, that is, jet mills and ball mills, have a slight decrease in efficiency due to the wet process, but in addition to crushing / dispersion in a clean state, the pulverization effect is sufficient. Yes, it becomes possible to obtain fine particles for the purpose more efficiently.

In order to make the particle diameters uniform, it is possible to collect fine silicon particles more reliably and more efficiently by repeating a plurality of operations as described above.
In other words, silicon particles in cutting or grinding as shown here can usually be considerably miniaturized in combination with their brittleness.

Here, silicon that has been pulverized by cutting or grinding may be agglomerated to form secondary particles, and in order to disperse the silicon, it is crushed and dispersed when using a jet mill or ball mill. Since it has an effect of pulverization, the desired particles can be obtained more efficiently. The jet mill lining used here does not need to create very strong crushing conditions in order to crush silicon, but the material is moderately hard and sticky in order to prevent impurities from entering. It is desirable to use certain cocoons and similar materials. The same is true for ball milling, and the container itself is preferably a resin lining. By using a basket as the ball, the ball will not be worn away and will not enter as an impurity. High-purity silicon can also be used for the balls. In other words, even if high-purity silicon itself is partially crushed in such a case, it may be used as it is as a silicon raw material.

If other commonly used materials, such as super hard materials such as α-alumina and tungsten carbide, are used, the grinding effect is very high.

In this way, while the silicon powder is crushed in a liquid such as pure water, the secondary particles are physically separated and brought closer to the primary particles, but the conditions are not particularly specified.
If the conditions as described above are maintained, mixing of impurities can be prevented, so that there are few problems. The particles dispersed in the pure water or in a suitable medium in this way are separated by sedimentation, or a fine particle is separated together with a part of the pure water by a cyclone or centrifugal separation, and the liquid is removed and separated. The large particles thus separated are returned to the crushing and grinding mechanism for further fine grinding. In gravity classification, large particles that are considered to be sufficiently fine particles even if pulverized again are taken out separately and separated for use in other applications. Further, while the silicon corresponding to this amount is newly replenished, the amount of silicon held in the crushing / pulverizing mechanism is adjusted to be substantially constant.

The fine particles here may be used as they are, but in order to improve efficiency and improve the particle size and particle size distribution accuracy, this fine portion is further crushed and dispersed. In addition to miniaturization, it is possible to take out only a more appropriate portion by the second-stage classification mechanism. Naturally, in the case of crushing / dispersion / classification of two or more stages from the beginning, it is natural to change the classification standard for each stage to improve the utilization rate of silicon more efficiently. I can do it.

In this way, the silicon is cut and ground to obtain fine particles, and while being crushed and further pulverized in a wet clean, classification is performed, and only a desired portion is taken out. A portion that is not sufficiently finely powdered is repeatedly crushed and classified, and a desired silicon can be obtained in a high yield by repeating this step as necessary. Further, in the present technology, as long as impurities are prevented from being mixed, it is possible to use a chip cut from a wafer or the like.

The present invention will be described below with reference to the flowchart of FIG. In other words, this figure shows a case of a system in which classification is performed in two stages using a wet jet mill as a crushing / dispersing / crushing mechanism. In other words, the silicon powder refined by the cutting / grinding method is washed with a dilute hydrofluoric acid solution of about 2% as necessary, and then sent to the first stage crushing / pulverizing mechanism together with pure water. (1) Medium gas or liquid , (50), after sufficiently stirring and pulverizing with the first stage jet mill (100), the dispersion liquid (2) containing fine silicon particles is separated from the outlet through the first stage gravitational grain separation. Send to device (200). In a gravity separation device, for example, a wet cyclone, a portion having a large particle size falls downward or moves to a wall surface and falls downward along the wall surface. On the other hand, the target fine particles are taken out from above without dropping (6). Here, the part (5) falling downward is returned to the crushing / pulverizing part.

The fine particle part is further separated by (6) through the second-stage crushing / dispersing mechanism (110) and through the gravity separation device (210), and as fine particles, together with the aqueous medium (10). Take out. Here, a drain (3) is provided in the first crushing / dispersing part, and the non-fine part is continuously taken out, and a new amount of silicon fine powder corresponding thereto is added. The liquid in which the silicon taken out from the drain of the second stage crusher / disperser is returned to the first stage (8). As a matter of course, as shown in this flow, a large portion of the particles dropped by the second stage separation is returned to the entrance of the crushing / pulverizing machine (210) by (9).

Further, the portion that could not be sufficiently crushed and dispersed by the first crushing / dispersing mechanism portion (100) comes out from the bottom of the separation device (200) as a drain (5) and is returned to the original silicon raw material supply unit 1 return. The part dropped by the first stage crushing and crushing is taken out as a drain (3), and a part thereof is returned to the processing entrance part of the crushing and dispersing mechanism and reprocessed. A medium using a jet mill for crushing / dispersing is introduced from the inlet (50). Usually, a gas such as air or argon is sent, but a liquid such as pure water may be used. The conditions in this case are not particularly specified, and normal conditions may be used.

In this figure, as an example, pulverization is performed by a jet mill and classification is performed by a cyclone. However, as described above, the pulverization may be performed by a ball mill method, and the classification part may be a simple sedimentation method. But it ’s okay.
Further, although a two-stage system is shown here, it may be a single stage, or it may be three or more stages in order to collect even finer parts efficiently.
Examples will be described below, but it goes without saying that the present invention is not limited thereto.

Fine silicon particles were produced using silicon chips produced by cutting to obtain a silicon wafer from a silicon ingot by a multi-wire saw carrying diamond as a silicon raw material. The wire diameter of the multi-wire saw is 0.1 mm, and pure water is used as the lubricating cooling medium, so the silicon particles obtained do not contain impure parts from other parts, and the maximum particle size is the wire thickness. It was found that the particles consisted of particles having a maximum of about 100 μm and a minimum of about 1 μm. This material is dispersed in pure water and put together with clean air into a jet mill (Sugino Machine's jet mill device, starburst system) for crushing. The crushed silicon is classified with a cyclone classifier, and the particle size is classified up to an upper limit of 5 μm. The large silicon portion of the particles falling in the cyclone is returned to the jet mill, and the suspended portion that did not fall is collected. And collected by suction filtration on a filter having an opening of 1 μm. With 10 hours of continuous operation, 28% of the input silicon was collected on the filter. The particle size distribution of the silicon collected on the 1 μm mesh filter obtained as a result of the measurement showed an almost normal distribution centered on a particle size of 3 microns, but the tail was slightly smaller. I found that I was pulling.

Using a disk-shaped diamond saw with a thickness of 0.2 mm, the powder was cut out from the silicon ingot while being cooled with pure water. As a result, sludge-like silicon containing a large amount of moisture containing particles having an apparent maximum particle size of about 300 μm was obtained. This sludge-like silicon is placed in a stainless steel ball mill container with a PTFE liner together with pure water and smoked balls, crushed for 30 minutes, and then classified in a wet cyclone. The first stage limits the particle size to 10 μm. As for more than that, it is returned to the ball mill device, and new sludge-like silicon is added and crushed and pulverized. Parts with a particle size of 10 microns or more are crushed for another 30 minutes with a new second-stage ball mill. Grinding was performed. The suspension containing fine silicon thus obtained is separated by gravity sedimentation at a particle size of 5 μm, the portion having a particle size of 5 μm or more is returned to the second stage ball mill, and the portion having a particle size of 5 μm or less is In the same manner as in Example 1, it was collected with a filter having an opening of 1 μm. The recovered silicon was 70% of the silicon transferred to the second stage, and 32% of the total from the first stage.

With the rapid expansion of EV battery demand as a negative electrode for lithium ion secondary batteries that require large size and large capacity, the demand is rapidly expanding. Silicon there is a size that is the opposite of conventional size increase, especially 20 μm or less, preferably 3 μm or less, and the corresponding miniaturization technology that maintains high purity is rapidly emerging as a new market. I think it will expand.

It is the flow of the process concerning the manufacturing method of this invention.

DESCRIPTION OF SYMBOLS 1 Silicon powder supply 2 Silicon grain after the 1st stage crushing 3 Silicon coarse-grain part drain after 1st stage crushing 4 1st stage coarse-grain silicon return 5 1st stage grain size separation coarse grain 6 Silicon fine particles after second stage cracking 8 Silicon coarse grains after second stage cracking 9 Second stage particle size separation coarse grain 10 Product silicon fine particle extraction 50 Jet mill supply air 100 First stage jet mill 110 Second stage jet mill 200 First Cyclone 210 for stage particle size classification Cyclone for second stage particle size classification

Claims (11)

After the crystalline silicon finely divided by cutting or grinding is pulverized and dispersed in a dispersion, the degree of dispersion is further increased, and then the dispersion is finely divided into particles having an apparent particle size of 0.1 to 20 μm by a gravity classification method. A method for producing fine silicon, in which silicon is collected as a silicon dispersion, and at least a part of a coarse portion is returned to the dispersion to be further pulverized and dispersed. 2. The method for producing fine silicon according to claim 1, wherein the gravity classification is repeated a plurality of times to collect fine silicon having a particle size of 0.1 to 20 [mu] m as a silicon dispersion. 3. The method for producing fine silicon according to claim 1, wherein the dispersion medium is pure water / deionized water. The method for producing fine silicon according to any one of claims 1 to 3, wherein the crushing is performed by a wet jet mill. 4. The method for producing fine silicon according to claim 1, wherein the crushing is performed by a wet ball mill. 6. The method for producing fine silicon according to claim 5, wherein, when the crushing is performed by a wet ball mill, the ball material is a soot. 6. The method for producing fine silicon according to claim 5, wherein the ball material is high-purity silicon when the crushing is performed by a wet ball mill. 3. The method for producing fine silicon according to claim 1, wherein the gravity classification method is a centrifugal separation method. 3. The method for producing fine silicon according to claim 1, wherein the gravity classification method is a wet cyclone method. 3. The method for producing fine silicon according to claim 1, wherein the gravity classification method is a sedimentation separation method. 3. The method for producing fine silicon according to claim 1, wherein the finely divided crystalline silicon is a cutting waste for producing a silicon wafer.

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