A method of fabricating structured particles composed of silicon or a silicon-based material

The present invention relates to a method for producing particles containing pillars etched on the surface, a method for producing silicon fibers by peeling the pillars from such particles, an electrode containing such particles or fibers as an active material, an electrochemical battery, and a lithium regenerator. Regarding the anode of rechargeable batteries.

Recently, graphite-based anodes have been used for lithium-ion rechargeable battery cells. Silicon is known to be able to be used as an active anode material instead of graphite (eg, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, JO Besenhard, ME Spahr, and P. Novak in Adv. Mater. 1998. , 10, No. 10), it has been proposed that the silicon anode material can be in the form of silicon pillars (or fibers).

Methods for manufacturing such silicon pillars include Peng KQ, Yan, YJ, Gao SP, and Zhu J., Adv. Materials, 14 (2002), 1164-1167, Adv. Functional Materials, (2003), 13, No 2 February. , 127-132 and Adv. Materials, 16 (2004), 73-76. Peng et al. Showed a method for producing nanopillars from silicon by a chemical method. According to this method, a silicon wafer (exposing an n or p-type {1,1,1} surface to a solution) is placed at 50 ° C to the next solution: 5 M HF and 20 mM AgNO.₃Etch using. The mechanism assumed in this paper is that in the first stage (nucleation), silver-separated nanoclusters are non-electrically deposited on the surface. In the next step (etching), the silver nanoclusters and the silicon region surrounding them act as local electrodes, causing electrical oxidation of the silicon in the region surrounding the silver nanoclusters, SiF.₆It forms ions, diffuses away from the etching site, and leaves the silicon underneath the silver nanoclusters in a pillar shape.

K. Peng et al., Angew. Chem. Int. Ed., 44 (2005), 2737-2742; and K. Peng et al., Adv. Funct. Mater., 16 (2006), 387-394, Peng et al. It relates to a silicon wafer etching method similar to that described in previous papers, but the nucleation / silver nanoparticle deposition and etching steps are performed in different solutions. In the first step (nuclearization), the silicon chip is 4.6M HF and 0.01M AgNO.₃Place in the solution of 1 minute. The next step (etching) is a different solution: 4.6M HF and 0.135M Fe (NO)₃)₃Performed in solution for 30 to 50 minutes. This paper proposes a different mechanism for the etching step from previous papers. That is, the silicon beneath the silver (Ag) nanoparticles is moved, and the nanoparticles gradually sink into the bulk silicon, leaving columns of silicon in that region (not directly under the silver particles).

In order to increase the uniformity and density and growth rate of pillars on silicon wafers, WO2007 / 083152 treats in the presence of alcohol.

WO2009 / 010758 discloses the etching of silicon powder instead of wafers to make the silicon material used in lithium won batteries. The resulting pillar particles, for example shown in FIG. 2, contain pillars on the surface and the entire resulting particles can be used as the anode material of the battery. Alternatively, the pillars can be stripped from the particles to form silicon fibers. And only silicone fibers are used to make the anode. The etching method used is the same as that disclosed in WO2007 / 083152.

<p> The present invention relates to a method for producing particles containing pillars whose surface has been etched, a method for producing silicon fibers by peeling the pillars from such particles, an electrode containing such particles or fibers as an active material, an electrochemical battery, and a lithium regenerator. An object of the present invention is to provide an anode of a rechargeable battery.</p>

<p> A first aspect of the present invention is to provide a silicon etching process for pillar formation, which process involves silver deposition as part of the etching procedure. Silver is still present after the silicon has been etched and can be removed by nitric acid treatment. The process of the present invention allows the recycling of this removed silver and thus reduces the cost of the entire process.</p><p> Applicants have found that silver from silicon is dissolved in nitric acid and recycled, and the recovered silver solution can be reused directly for further etching of silicon, and such a recycling procedure can be repeated.</p><p> The process of the present invention is the next step: For example, granular or bulk material silicon, HF, Ag⁺It was treated with an etching solution containing ions and nitrate ions, thereby etching the silicon into silicon containing pillars etched on the surface, said silicon containing silver surface deposits. The etched silicon is separated from the used etching solution and separated. The etched silicon is dissolved in silver from the etched silicon using nitric acid to obtain Ag.⁺Form a solution containing ions and nitrate ions, Ag⁺The solution containing ions and nitrate ions is further mixed with HF to form an etching solution. The additional etching solution is used to further process the silicon.</p><p> The used solution can also be recycled. That is, the solution used is mixed with HF, and then Ag⁺It is mixed with a solution containing ions and nitrate ions and regenerated to make a further etching solution.</p>

<figref num="1">FIG. 1 is an electron micrograph of pillar particles.</figref>

In the following specification, the present invention will be described with reference to etching particulate silicon to form etched silicon particles. However, the same idea applies equally to silicon in the form of bulk material, such as silicon wafers.

This process takes place in two stages, nucleation and etching. In nuclearization, silver islands are non-electrically deposited in the form of silicon particles by the following reaction. 4Ag⁺ + 4e^― -&gt; 4Ag (metal) Nuclearization usually occurs by about 1 minute.

Etching mainly occurs along a specific crystal plane, and silicon etching is etched in columns. Silicon is etched according to the following equation. Si + 6F^- -&gt; SiF₆^2- + 4e^― Half-reaction (1) The electrons generated in the half-reaction (1) are transferred to the silver deposited through silicon, and the opposite reaction occurs in which the silver ions in the solution are reduced to elemental silver. 4Ag⁺ 4e-&gt; 4Ag (metal) half-reaction (2) The deposited elemental silver extends from the first deposited silver island to form dendrites. The dendrites are fixed to each other with dendrites of the same particle and other particles to form a mat. Each other's immobilization of dendrites increases the speed of the electrical process. This is because the reduction half-reaction (2) can occur, creating more sites and delocalizing the charge. Gas may be generated in this process, which causes the mat to float in solution.

The process may be agitated, but it is not necessary and is rather disadvantageous if agitation breaks the mat.

Particley silicon starting materials include undoped silicon, or doped p or n-type or a mixture thereof, such as silicon-aluminum-doped silicon. Preferably silicon is doped, thus improving conductivity during the etching process. We are p-doped silicone 10¹⁹From 10²⁰It was found that the carrier / cc one works well. Such a substance can be obtained, for example, by polishing doped silicon from silicon of the IC industry and sieving the obtained particles into particles of a desired size.

Alternatively, the particles may be commercially available, relatively low-purity metallurgical grade silicon. Metallurgical grade silicon is particularly preferred. This is because the defects are relatively denser (compared to the silicon wafers used in the semiconductor industry), which results in lower resistance and higher conductivity, allowing the silicon pillar particles or fibers to be rechargeable. This is because it is advantageous when it is used as an anode substance of a simple battery. Such silicon may be ground and graded. An example of metallurgical grade silicon is the Norwegian Elkem "Silgrain", which has an average particle size in the range of 5 to 500 μm, for example 15 to 500 μm, preferably 15 for pillar particle production. From 50 to 40 μm, and to make fibers, in the range of 50 to 500 μm. It can be ground (if necessary) and sieved.

The particles may have a normal or special cross section. When making silicon fibers, the particles remaining after the fibers have been removed can be recycled for further etching.

The particles have a silicon purity of 90.00% by mass or higher, preferably 99.0% to 99.99%. Silicon may be doped with any substance such as germanium, phosphorus, aluminum, boron and / or zinc.

The particles used for etching may be crystalline, such as single crystal or polycrystalline, and the crystallite size may be as high as or greater than the desired pillar height. Polycrystalline particles may contain any number, eg, two or more crystals.

The process can be carried out at temperatures from 0 ° C to 70 ° C. However, it is easiest to do it at room temperature. This is because only containers that are very expensive at temperatures near the upper limit of such a temperature range can withstand highly corrosive HF. For this reason, the temperature usually does not exceed 40 ° C. If necessary, the reaction mixture may need to be cooled during the process. This is because this reaction is an exothermic reaction.

The material of the reaction vessel is preferably polypropylene, but other HF resistant materials can be used instead.

The etching procedure is stopped when sufficient silicon has been etched to give a well-defined pillar with a height in the range of 1 to 100 μm, eg 3 to 100 μm, more preferably 5 to 40 μm. The pillar height of the pillar particles is usually 5 to 15 μm and is larger when making fibers, for example in the range of 10 to 50 μm. The optimum time for the process depends on the concentration of the substance in the solution, the conductivity of the silicon, the temperature and the amount of the etching solution relative to the amount of silicon.

Pillars usually taper away from their base (ie, the pillars are attached to the underlying silicon). Pillar diameters on the base are generally on the order of 0.08 to 0.70 μm, eg 0.1 to 0.5 μm, eg 0.2 to 0.4 μm, eg 0.3 μm or larger. Pillars therefore generally have an aspect ratio greater than 10: 1. Pillars can, but are not limited to, a substantially circular cross section.

Pillar surface density can be used to define the density of pillars on the surface of the particles. Here, it is defined as F = P / [R + P], where F represents the surface density of the pillars, P is the total surface area of the particles occupied by the pillars, and R is the total surface area of the particles not occupied by the pillars.

The larger the pillar surface density, the larger the lithium permissible amount per unit area of the silicon particle electrode, and a larger amount of pillars for making fibers can be obtained. For example, using the silicon powder obtained from Elken, Norway, with an average pre-etched particle diameter of 400 μm, the entire surface has pillars with a height of about 10 to 30 μm and a diameter of about 0.2 to 0.5 μm, and the pillar surface density. F can produce pillars that are 10 to 50%, more usually 30%.

In another example, when particles with a pre-etch particle diameter of about 63 to 80 μm were used, the entire surface had pillars with a height of about 10 to 15 μm and a diameter of about 0.2 to 0.5 μm, and the pillar surface density. A pillar with F of 30% is obtained.

The presence of silver in solution is required for the nuclearization stage and the growth of dendrites, but when these are completed, etching requires only the presence of ions that can be reduced in solution. This may be silver (half-reaction (2)), but it is not always necessary and it is preferable to use certain other reactions other than silver. In WO2007 / 083152, Applicants suggested adding ferrous nitrate, which can be reduced to ferrous iron in the counter solution, to ferrous nitrate. We also suggested the reduction of hydrogen ions as a possible alternative counter reaction.

We have found that the optimized counter reaction is the reduction of nitrate ions in solution, which is exactly suitable for the recycling of silver nitrate to form additional etching solutions. WO2007 / 083152 suggests that nitrate ions are added during the etching process to form silver nitrate or ferric nitrate. These are expensive, especially the former. In addition to the nitrate in the recycled silver solution, it is also possible to add more nitrate to the etching solution at the beginning of or during the etching. It is preferable to add additional nitrate moieties during the etching process, and we add nitrates to the etching solution as alkali metal nitrates, in particular sodium nitrate or aluminum nitrate. This is because these substances are highly soluble, cheaper than ferric nitrate, and are also inert cations (Na).⁺And NH4⁺) Is not harmful even if it is deposited in a solution that has been recycled several times.

According to one embodiment, the etching solution is substantially free of iron ions (ferric, ferric). "Substantially absent" means a concentration that is not sufficient to be effective in this process and is generally less than 0.05% by weight, less than 5 mM, for example less than 2.5 mM. ..

It is a constitution of WO2007 / 083152 that alcohol should be present in nucleation in an amount of 1-40%. Although the process of WO2007 / 083152 is carried out on a chip or wafer, we do not need the presence of alcohol in the content of the process of the invention carried out in the form of silicon particles, but rather its presence complicates the process. I found. This is because other components should be considered when controlling the concentration in solution. Therefore, the solution used in the present invention is substantially free of alcohol according to one embodiment of the present invention. This means that the amount of any alcohol is less than the concentration that produces the meaningful effect of the invention and can be less than 0.5% by volume.

The solution used at the beginning of the etching process of the present invention comprises HF at a concentration of 5 to 10 M, for example 7 M to 9 M and usually 6 to 8 M such as about 7 or 7.5 M. No additional HF needs to be added during the process, but may be added if the solution is etched with a large amount of material relative to volume.

Ag to deposit silver islands and silver dendrites⁺Concentrations range from 0.01M to 0.1M, such as 0.02 to 0.06M, and typically about 0.03M. Ag⁺The amount of ions is preferably insufficient to be involved in the etching of all silicon in the process, but should rather be limited to an amount that causes the formation of islands and dendrites. The half-reaction that opposes the etching half-reaction is therefore triggered by the reduction of nitrate ions. Silver is preferably not added to the solution after the etching reaction has begun.

As pointed out, NO₃^-The reduction provides a counter reaction to the etching of silicon (half-reaction (1)) and can be present in a concentration range of 0.02M to 0.2M, such as 0.04M to 0.08M, for example about 0.06M. Silver is generally added in the form of its nitrate. This is because other salts are usually poorly soluble. This may require specific nitrate ions, but the balance is achieved by the addition of alkali metal nitrates such as sodium nitrate or aluminum nitrate. Nitrate ions may be added during the etching process to provide additional nitrate ions for etching.

SiF₆^2-Is present in the etching solution as a result of silicon etching. The etching solution also contains nitric acid from a recycled silver nitrate solution.

Prior to etching, the components of the etching solution are bases, preferably NaOH or NH.₄It may be adjusted by adding OH. This is because they are inexpensive and their cations are very soluble. Nitric acid may be used to acidify the solution. According to one embodiment of the present invention, apart from water, other components may not be contained. The solution at the beginning of such a process is: 5 to 10M (eg 6 to 8M) HF, 01-0.1M Ag⁺ion, 02 to 0.2M NO₃^-ion, Water, hydrogen and hydroxyl ions, and Contains components and impurities added by chance.

After the etching is complete, the etched particles are separated from the etching solution used. Silver remains deposited during the etching process. This silver is dissolved from the etched particles with nitric acid and Ag⁺Form a solution containing ions and nitrate ions. This solution can be further mixed with HF and recycled directly to prepare additional etching solutions. Further, silver nitrate may be added.

In addition to silver recycling, the etching solution used is an additional HF also Ag⁺A recycled solution containing ions and nitrate ions is added and regenerated to prepare additional etching solutions.

SiF if the used etching solution is recycled₆^2-Ions can accumulate in the etching solution, but this is not always the case. This is because some of the etching solution is carried with the etched silicon product and is usually discarded after cleaning the silicon product.

The process described herein discloses the addition of sodium salts, such as hydroxides and nitrates, to the etching solution. These sodium salts are advantageous as they are interchangeable with the corresponding ammonium salts. Because (NH₄)₂SiF₆Is Na₂SiF₆Much more soluble than, therefore SiF₆^2-This is because if there is an accumulation of ions, a significant amount will remain in solution without precipitation.

The etched particles separated from the etched solution used are washed and completely dehydrated before dissolving the silver from the etched particles.

The amount of etching solution used relative to the amount of silicon particles should be sufficient to etch the required pillars. We found that for 20 g of silicon particles, 3 liters of etching solution gave good results. However, such relative ratios may need to be adjusted if scaled up or down.

Hereinafter, the present invention will be described according to one or more of the following non-limiting examples.

<p> Manufacture of pillar particles The reaction was carried out in a polyethylene container having a volume of 8 liters. A lid with holes for adding components and stirring was placed. The following reaction materials were used.</p><p><tables num="1"><img file="JP5000787B2_D0001.tif" /></tables> The reaction was carried out at room temperature (10-25 ° C). (1st cycle) 2.56M and 3.65M AgNO, respectively₃/ HNO₃AgNO including₃/ HNO₃35 ml of solution was placed in a reaction vessel with 3 liters of 7 M HF solution and mixed, then 5.1 g NaOH (or 4.5 g NH) dissolved in 30 ml water.₄OH) was added. The reaction mixture is 0.0299 M AgNO₃including. AgNO₃/ HNO₃The solution may be recycled from a previous etching process (see below), or in the absence of it, it may be prepared from commercially available silver nitrate and nitric acid.</p><p> 20 g of sieved and washed Si powder (40 μm) was added using a funnel through the hole in the lid of the reaction vessel, after which the mixture was manually stirred through the hole in the lid slowly for 1 minute using a rod. The reaction mixture was left for 40 minutes. A silicon plus silver "mat" was formed on the surface of the etching solution in the first 1-2 minutes.</p><p> After 40 minutes, 15g of NaNO₃(Or 13g NH₄NO₃) Is added. NaNO₃(Or 13g NH₄NO₃) Was dissolved in 50 ml of water and added through a funnel. NaNO₃(Or NH₄NO₃) After the addition was completed, the solution was stirred for about 1 minute. The mixture was left for an additional 50 minutes. Then, 90 minutes after the start of the process, the etching was almost complete and the etching solution used was transferred to the storage chamber for about 4,5 minutes, for a total time of about 95 minutes.</p><p> The mat was washed 3 times with 3-4 liters of water. The first two times were contacted with water for 5 minutes and the third time was contacted for 1 minute for washing.</p><p> Wet mats containing etched particles and silver were dehydrated as quickly as possible and then transferred to a 300 ml glass beaker. After adding 20 ml of water and 20 ml of concentrated nitric acid (68%, GPR RECTAPUR, VWR) and leaving for several hours, the solution split into two layers. The upper layer is a transparent solution (AgNO)₃Diluted HNO₃Below "AgNO₃/ HNO₃The solution) and the bottom layer were etched Si particles. Upper AgNO₃/ HNO₃The solution was carefully decanted and stored for further use. The total solution was about 120 ml. (2nd cycle) 35 ml of AgNO, the etching solution used from the first cycle₃/ HNO₃5.1 g NaOH (or 4.5 g NH) added with the solution and 150 ml 40% HF and dissolved in 30 ml water.₄By adding OH), the activity was completely restored and reused.</p><p> Then 20 g of silicon particles were added to the reactivated etching solution as in the first cycle. After 40 minutes, 15g of NaNO₃(Or 13g NH₄NO₃) Was added. The mixture was left for another 50 minutes. Etching was almost completed 90 minutes after the addition of silicon. The solution used was pumped into the storage chamber for 4-5 minutes and the total etching time was about 95 minutes.</p><p> The mat was washed 3 times with 3-4 liters of water. The first two times were contacted with water for 5 minutes and the third time was contacted for 1 minute for washing.</p><p> Wet mats containing etched silicon particles and silver were quickly treated with nitric acid to remove silver, then washed and stored wet. The nitric acid treatment was similar to the first cycle, but with 20 ml of HNO to remove silver from the etched particles.₃+ 20ml H₂Instead of adding O, 20 ml of HNO from the first batch₃+ 20ml AgNO₃/ HNO₃The solution was reused AgNO₃/ HNO₃Used to reduce the amount of water in the solution (ie AgNO in the reused solution)₃And HNO₃To increase the concentration of).</p><p> The silver nitrate / nitric acid solution is reused in the next cycle. (Third and higher cycles) Same as the second cycle. (Modified) AgNO after each cycle₃/ HNO₃Instead of recovering the solution and using it for the next cycle, the HNO3 treated product (etched silicon and AgNO)₃/ HNO₃Batches of solution) can be grouped together and used together for further etching of silicon particles.</p><p> HNO₃Several batches of treatment product (eg 10 batches, volume of about 1000 ml) were collected and placed in a centrifuge vessel. Centrifuge the batch mixture for 15 minutes and AgNO as the top layer₃/ HNO₃Collect the solution (# 1). Then 200 ml of deionized water was placed in the bottom etched silicon layer in a centrifuge and the mixture was left for 30 minutes. Then, it was centrifuged for 15 minutes. The upper clear solution (# 2) was recovered and mixed with the previous solution (# 1) for recycling.</p><p> Two additional washes and centrifugation may be used to remove the solution residue from the etched silicon product. The solutions from the further treatment are referred to as # 3 and # 4, respectively. Usually 95% or a little more silver is present in # 1 and # 2. # 3 and # 4 are stored together and treated with NaCl for recovery to precipitate silver as AgCl (not reused). (Control) Silver and HNO in a mixture of # 1 and # 2₃Concentrations of may be analyzed respectively. The silver ion concentration is Fe (NO)₃)₃NH as an indicator₄Obtained by titration with SCN. HNO₃The concentration is determined by titrating with NaOH using methyl red as an indicator. AgNO to be recycled last₃/ HNO₃The components of the solution can be adjusted as described above. AgNO recycled if necessary₃/ HNO₃AgNO in solution₃And HNO₃Fixed AgNO to keep the concentration of₃And / or concentrated nitric acid can be adjusted by adding each time.</p>

<p> Fiber production The reaction vessel and reaction reagent are the same as in Example 1. The reaction was carried out at room temperature. 40ml AgNO₃/ HNO₃The solution was mixed with 3 liters of 7M HF solution in a reaction vessel. Then 5.9 g NaOH (or 5.2 g MH) dissolved in 30 ml water₄OH) was added. The final solution is 0.0033M AgNO₃including.</p><p> 20 g of Si powder (J272.1) was added from above the container using a funnel, and the whole was slowly stirred with a stick manually through the hole in the lid. The reaction mixture was left for 40 minutes. A "mat" of silicon plus silver was formed on the surface of the solution in the first 1-2 minutes.</p><p> After 40 minutes, 14g of NaNO₃(Or 12g NH₄NO₃) Is added. NaNO₃(Or NH₄NO₃) Was dissolved in 50 ml of water and added through a funnel. NaNO₃(Or NH₄NO₃) After the addition was completed, the solution was stirred for about 1 minute. The mixture was left for an additional 50 minutes. Then, 90 minutes after the start of the process, the etching was almost complete and the etching solution used was transferred to the storage chamber for about 4-5 minutes, for a total time of about 95 minutes.</p><p> The mat was washed 3 times with 3-4 liters of water. The first two times were contacted with water for 5 minutes and the third time was contacted for 1 minute for washing. Wet mats containing etched silicon particles and silver were rapidly treated with nitric acid to remove silver, as described in Example 1. The particles were then separated from the silver / nitric acid solution (again according to the procedure of Example 1) and the etched silicon was further washed and stored wet. The silver nitrate / nitric acid solution is recyclable. (2nd cycle) 35 ml of AgNO, the etching solution used from the first cycle₃/ HNO₃5.1 g NaOH (or 4.5 g NH) added with the solution and 150 ml 40% HF and dissolved in 30 ml water.₄By adding OH), the activity was completely restored and reused.</p><p> Then 20 g of silicon particles were added to the reactivated etching solution as in the first cycle. After 40 minutes, 14g of NaNO₃(Or 12g NH₄NO₃) Was added. The mixture was left for another 50 minutes. Etching was almost completed 90 minutes after the addition of silicon. The solution used was pumped into the storage chamber for 4-5 minutes and the total etching time was about 95 minutes.</p><p> The mat was washed 3 times with 3-4 liters of water. The first two times were contacted with water for 5 minutes and the third time was contacted for 1 minute for washing. As described in conjunction with the first cycle, wet mats containing etched silicon particles and silver were quickly treated with nitric acid to remove silver, then washed and stored wet. (Third and higher cycles) Same as the second cycle.</p><p> The fibers are obtained from the obtained particles with pillars, which are placed in a beaker or a suitable container, covered with an inert liquid such as ethanol or water, and exposed to ultrasonic agitation by ultrasonic vibration. Can be done. The liquid was found to become cloudy in a few minutes. You can also observe with an electron microscope that the pillars have been removed from the particles at this stage.</p><p> Pillars can be removed from the particles in two steps. In the first stage, the particles are washed with water several times and, if necessary, dried in a low vacuum system to remove the water. In the next step, the particles are agitated in the ultrasonic vessel and the pillars are desorbed. These are dispersed in water and separated using centrifugation.</p>