JPWO2020129499A1 - Silicon fine particles and their manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide silicon fine particles which are less likely to be broken or deformed due to expansion and contraction when alloying with lithium and have excellent safety (oxidation resistance). SOLUTION: Polycrystalline primary particles having an average crystallite diameter in the range of 20 to 40 nm and an average diameter of 80 to 900 nm are composed of partially fused amorphous agglomerated particles, and the chlorine concentration is 0 with respect to the particle weight. . Silicon fine particles characterized by 1 to 1.0% by mass.

Description

The present invention relates to novel silicon fine particles and a method for producing the same. More specifically, the present invention provides silicon fine particles suitable as a negative electrode material for a lithium ion secondary battery, and a method for producing the fine particles.

Currently, silicon is used for various purposes including an electrode material (negative electrode material) of a lithium ion secondary battery, or its use is proposed.
Conventionally, carbon-based materials such as graphite and graphite are generally used as the negative electrode material of lithium ion secondary batteries, but the theoretical capacity is _{as low as 372 mAh / g (when lithium is converted to LiC 6} ), which is higher. A capacity negative electrode material is desired. Compared to carbon materials, silicon has a large occlusion of lithium per unit mass _{and has a very high theoretical capacity of 3,579 mAh / g (when lithium is converted to Li 15} Si ₄ ), and is a next-generation negative electrode. It is being considered as a material.

When silicon is used as a negative electrode material for a lithium ion secondary battery, the problem is that the volume expansion is large when silicon and lithium form an alloy to occlude lithium, and strain energy is generated by repeated expansion and contraction due to charging and discharging. It is possible that the charge capacity of the negative electrode is lowered by accumulating inside and breaking the silicon into pieces to generate voids and losing electrical conductivity and ionic conductivity.

Therefore, the present inventors have focused on using silicon fine particles that are polycrystalline with a small crystallite diameter and have voids that alleviate volume expansion.
As silicon fine particles, Japanese Patent No. 5618113 (Patent Document 1) states that it is a silicon powder having an average primary particle size of 20 nm to 200 nm, and the powder has a SiOx surface layer of 0 <x <2. A silicon powder is disclosed in which the surface layer has an average thickness of 0.5 nm to 10 nm, and the powder has a total oxygen content of 3% by mass or less at room temperature.

Further, in Japanese Patent No. 5338676 (Patent Document 2), the diffraction that the particle size of the crystallite of polycrystalline silicon is attributed to Si (111) in the vicinity of 2θ = 28.4 ° in the analysis of the X-ray diffraction pattern. Silicon particles having a crystallite size of 20 nm or more and 100 nm or less and a true specific gravity of 2.300 to 2.320, which are obtained by the Scherrer method from the half-value full width of the line, are disclosed. Patent Document 2 discloses that the polycrystalline silicon particles are produced by thermal decomposition at 1,000 ° C. or lower using chlorosilane gas as a raw material.

Furthermore, Japanese Patent No. 4607122 and Japanese Patent Application Laid-Open No. 2007-511460 (Patent Documents 3 and 4) have a BET surface area of 20 to ^{150 m 2 / g in the aggregated crystalline silicon powder, and phosphorus and arsenic.} , (Omitted), agglomerated crystalline silicon powder characterized by being doped with copper, silver, gold or zinc has been disclosed, and as a method for producing the aggregated crystalline silicon powder, steam or gaseous silane (chlorosilane is also included in the silane). Included) and the vapor or gaseous dope material, the inert gas, and hydrogen are heated and reacted in a hotwall reactor.

However, in these patent documents, a method for effectively controlling the crystallite diameter of silicon fine particles, particularly the significance and method of adjusting the chlorine content when chlorosilane is used as a raw material, and a void for relaxing the expansion of silicon are retained. There is no suggestion on how to do this.

On the other hand, as the porous silicon particles having voids, in Japanese Patent No. 5598861 (Patent Document 5), the porous silicon particles are formed by bonding a plurality of silicon fine particles, and the average particle size or the average strut of the silicon fine particles. The diameter is 10 nm to 500 nm, the average particle size of the porous silicon particles is 0.1 μm to 1000 μm, the porous silicon particles have a three-dimensional network structure with continuous voids, and the average of the porous silicon particles. Those having a void ratio of 15 to 93% are disclosed. For the porous silicon particles having a three-dimensional network structure, an alloy of silicon and other intermediate alloy elements is prepared, the silicon fine particles and the second phase are separated, and then the second phase is removed. , It is disclosed that it can be manufactured.

Further, in Japanese Patent No. 3827642 (Patent Document 6), a large number of voids composed of an aggregate of porous particles composed only of Si and having an average pore diameter in the range of 10 nm or more and 10 μm or less are contained inside the porous particles. The average particle size of the aggregate is in the range of 1 μm or more and 100 μm or less, a part of the structure of the porous particles is an amorphous phase of Si, and the rest is a crystalline phase of Si. It is disclosed. Patent Document 6 discloses that porous particles are produced by eluting and removing other elements from an alloy containing silicon with an acid or an alkali.

However, the method disclosed in these patent documents, that is, a method for producing porous particles in which a silicon alloy is phase-separated and the separated second phase is removed, is industrially and inexpensively voided because the production method is complicated. It is difficult to mass-produce polycrystalline silicon fine particles having the above.

As described above, an effective method for industrially producing silicon fine particles which are polycrystalline type having a small crystallite diameter and have voids for relaxing volume expansion has not been found so far.

Japanese Patent No. 5618113 Japanese Patent No. 5338676 Japanese Patent No. 4607122 Japanese Patent Application Laid-Open No. 2007-511460 Japanese Patent No. 5598861 Japanese Patent No. 3827642

Therefore, an object of the present invention is silicon fine particles which are less likely to be broken or deformed due to expansion and contraction when alloying with lithium, have voids to relax expansion and contraction, and are excellent in safety (oxidation resistance), and a method for producing the same. Is to provide.

In order to solve the above problems, the present inventors have stated that silicon fine particles having a chlorine content within a predetermined range together with the average crystallite diameter and the average diameter of the polycrystalline primary particles can solve all of the above problems. We have found and completed the present invention.

The silicon fine particles according to the present invention are composed of amorphous agglomerated particles in which polycrystalline primary particles having an average crystallite diameter in the range of 20 to 40 nm and an average diameter of 80 to 900 nm are partially fused, and have a chlorine concentration of particles. It is characterized in that it is 0.1 to 1.0% by mass with respect to the weight.

The average diameter of the polycrystalline primary particles is preferably 130 to 850 nm. It is preferable that the silicon fine particles have a bulk density of 1.3 g / cm ³ or less ^{when a load of 10 kN / cm 2 is applied.} Further, it is preferable that the bulk density when a load of 50 kN / cm ^{2 is applied is 1.8 g / cm 3 or less.}

Further, it is a preferable embodiment that the ratio Co / S of the concentration Co [mass%] of oxygen contained as an impurity in the particles and the specific surface area S [m ^{2 / g] is less than 0.05.}
The silicon fine particles having the above-mentioned characteristics are silicon fine particle precursors obtained by thermally decomposing silicon chloride gas at a temperature of 600 to 900 ° C. at 900 ° C. under the flow of a gas containing no oxygen atom in the composition or under reduced pressure. It can be produced by heating above 1200 ° C. or lower.

The silicon fine particles of the present invention are composed of amorphous agglomerated particles in which polycrystalline primary particles having an average crystallite diameter in a predetermined range are partially fused and are charged and discharged. At that time, the volume change due to alloying with lithium is small, and the influence of surface oxide is also small.
Further, the one in which the ratio of the amount of oxygen to the specific surface area is adjusted to a specific range is preferable because the influence of oxygen can be extremely reduced and the decrease in charge capacity can be prevented.

In the production method of the present invention, silicon fine particles having the above characteristics are obtained by adopting a two-step heating step consisting of a thermal decomposition step of trichlorosilane and a dechlorination step of the obtained silicon fine particle precursor at a predetermined temperature. Can be produced efficiently.

Such silicon fine particles of the present invention have a small bulk density when a load is applied and can retain the voids even when press-molded. Therefore, the voids absorb the volume expansion of the electrode during charging to change the volume. Can be suppressed.

The silicon fine particles of the present invention are in the form of a composite mixed with a carbon material such as graphite or graphite or a known negative electrode material such as silicon oxide, tin, antimony, magnesium or bismuth, or silicon alone in the form of an all-solid-state battery or gel. It can be used as an active material for the negative electrode of a lithium ion secondary battery including a battery using a state-like electrolyte.

The scanning electron micrograph of the silicon fine particle of this invention is shown. The manufacturing process of the silicon fine particles of the present invention is schematically shown. The change in bulk density with respect to the load evaluated in Examples, Comparative Examples and Reference Examples is shown.

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these descriptions.
<Silicon fine particles>
The silicon fine particles according to the present invention are composed of amorphous agglutinated particles in which polycrystalline primary particles are partially fused.

When silicon fine particles having a large crystallite diameter and close to a single crystal are used as a negative electrode material for a lithium ion battery, the expansion rate of the negative electrode becomes large because the phase change when reacting with lithium to form an alloy is large. It is known. The silicon fine particles of the present invention are amorphous or polycrystalline or a mixture thereof, preferably polycrystalline, and the average crystallite diameter of the polycrystalline primary particles is in the range of 20-40 nm, preferably 25-35 nm. .. The average crystallite diameter can be analyzed from the X-ray diffraction pattern by a method such as the Scherrer method, the Willamson-Hall method, or the Hander-Wagner method.

It is difficult to control the crystallinity of silicon fine particles by itself, and the presence of a second component is important. In the silicon fine particles of the present invention, a chlorine atom plays this role. In general, chlorine groups bonded to silicon atoms have high reaction activity and rapidly react with moisture in the air to release hydrogen chloride gas and oxidize, so it is difficult to safely handle silicon fine particles with residual chlorine. It is believed that. However, according to the research by the present inventors, only the chlorine groups exposed near the particle surface are highly active in the chlorine remaining in the silicon fine particles, and the chlorine existing inside the particles has almost no reactivity. Further, when a predetermined amount of chlorine is contained in the silicon fine particles, crystallization of silicon can be inhibited and the crystallite diameter can be kept small. The chlorine concentration in the silicon fine particles of the present invention is 0.1 to 1.0% by mass, preferably 0.3 to 0.8% by mass, based on the particle weight. When chlorine is contained in a predetermined range, it has the effect of suppressing the increase in crystallite diameter by hindering the crystallization of silicon and suppressing the expansion when lithium and silicon are alloyed. If the amount of chlorine is too small, the effect of containing chlorine becomes weak and the effect of suppressing expansion cannot be obtained. If the amount of chlorine is too large, the highly reactive chlorine groups remaining on the particle surface react with the moisture in the air to generate oxygen impurities. It is not preferable because it may increase or react with other battery materials to produce chloride.

Further, when the silicon fine particles are used as the negative electrode, the diameter of the primary particles is also important. It is known that when the diameter of the primary particles is too large, the silicon fine particles break due to expansion and contraction accompanying the reaction with lithium, resulting in loss of conductivity and deterioration of cycle characteristics in battery performance. On the contrary, when the diameter of the primary particles is too small, the amount of decomposition products (SEI) of the electrolytic solution generated on the surface of the silicon fine particles increases due to the increase in the specific surface area, and the initial charge / discharge efficiency in the battery performance decreases. It is known to do. The average diameter of the primary particles in the silicon fine particles of the present invention is in the range of 80 to 900 nm, preferably 130 to 850 nm, and more preferably 150 to 300 nm. The average diameter of the primary particles in the silicon fine particles is obtained from the specific surface area.

Those having such an average diameter of the primary particles do not break due to the expansion and contraction of the particles due to charge and discharge, and the formation of the SEI layer on the particle surface does not occur excessively.

The secondary particles are composed of aggregated particles in which polycrystalline primary particles are partially fused. In partial fusion, the primary particles are connected in a beaded or mesh-like shape, and the connecting portion constitutes a narrowed neck portion. Since the secondary particles themselves, which are aggregated particles, are irregular in shape, the particle size and shape cannot be specified, but when using them as the negative electrode of a battery, it is important that the size of the secondary particles does not exceed the thickness of the negative electrode formed. It is desirable that the average diameter (the longest diameter in the case of amorphous particles) is 20 μm or less, preferably 10 μm or less, and more preferably 5 μm or less.

The silicon fine particles of the present invention have voids in the secondary particles and also have voids between the secondary particles and the other particles so that other particles cannot enter. Such voids are maintained during pressure molding during electrode production. Normally, when used as a negative electrode of a lithium ion battery, the volume expands greatly when silicon is alloyed by the reaction of silicon and lithium, but in the present invention, since aggregated particles containing voids are used, the volume expansion of silicon is performed. By relaxing the voids of the agglomerated particles, the volume expansion of the negative electrode can be effectively suppressed.

The silicon fine particles of the present invention have a small bulk density when a particularly high load is applied, and have a property of retaining voids. When manufacturing a battery electrode, it is generally molded by applying a strong load to improve the adhesion between the electrode and the current collector and the electrical conductivity of the electrode. It has been increasing in recent years. The silicon fine particles obtained by the conventional manufacturing method have a large bulk density because the voids are clogged and the fine particles are filled when a stronger load is applied, but the silicon fine particles of the present invention retain the voids. , The bulk density is kept small. The silicon fine particles of the present invention preferably have a bulk density of 1.3 g / cm ³ or less, preferably 1.1 g / cm ³ or less ^{when a load of 10 kN / cm 2 is applied.} Further, it is preferable that the bulk density when a load of 50 kN / cm ^{2 is applied is 1.8 g / cm 3} or less, preferably 1.5 g / cm ^{3 or less.} Incidentally, the bulk density in the present invention, a load of 10 kN / cm ^2, although the bulk density of the load of 50 kN / cm ² is good if the range, if the same sample, when applying a load of 50 kN / cm ² The bulk density will not be lower than the bulk density when a load of ^{10 kN / cm 2 is applied.}

Also, the bulk density (BD ₅₀₎ when applying a load of 50 kN / cm ^2, between the bulk density (BD ₁₀₎ when applying a load of _{^{10kN / cm 2, (BD 50}} -BD 10) It is preferable that there is a relationship of / (50-10) <0.0130.

The ratio ( _Co / S) of the concentration of oxygen contained as impurities in the silicon fine particles ( _Co : mass%) to the specific surface area (S: m ² / g) is less than 0.05, preferably 0.03. Is less than. Since the oxygen concentration of silicon is mainly caused by the surface oxide layer on the surface of the silicon fine particles, the smaller the particle size of the silicon fine particles, the larger the specific surface area and the higher the oxygen concentration. Therefore, it is difficult to define the specific surface area and the oxygen concentration, respectively. Therefore, in the present invention, the range in which the influence of oxygen impurities in silicon is small is defined by the _{ratio (Co / S).} Oxygen impurities in silicon cause a decrease in charge capacity in battery performance by irreversibly binding with lithium, but _{by adjusting the ratio (Co} / S) to the predetermined ratio of the present invention, oxygen The influence of is extremely small and can be suppressed.

A scanning electron micrograph of the silicon fine particles of the present invention is shown in FIG.
In FIG. 1, a plurality of spherical fine particles having a size of about 50 to 300 nm form agglomerated particles in which a plurality of fine particles are connected in a beaded shape, but the shape of the secondary particles is not particularly limited.

<Manufacturing method of silicon fine particles>
The present invention also provides a suitable production method for producing the silicon fine particles.
That is, according to the production method according to the present invention.
In the reactor, it includes a thermal decomposition step of thermally decomposing trichlorosilane to produce a silicon fine particle precursor and a dechlorination step of heating the collected silicon fine particle precursor to dechlorinate.

-Pyrolysis step In the present invention, silicon chloride gas is used as the Si source, and trichlorosilane is used as the main component. Further, as a Si source other than trichlorosilane, dichlorosilane, silicon tetrachloride, etc. may be contained, and when these are contained, it is desirable to use an amount of 30 mol% or less in all the moles in the Si source. ..

A gas that is essentially inert to the reaction of the production method of the present invention, such as nitrogen, argon, and helium, can be mixed with the Si source in the reaction vessel as an accompanying gas. Trichlorosilane, which is the main source of Si, has a high boiling point of about 32 ° C. and is easily liquefied. However, by mixing the accompanying gas, the gas state can be maintained and a fixed amount can be easily supplied. The amount of the accompanying gas is not particularly limited, and it is desirable to use it in the range of 5 to 80% by volume with respect to trichlorosilane in order to stabilize the vaporization of trichlorosilane. In addition, by appropriately implementing the vaporization conditions and heating of the gas pipe, it is not necessary to use the accompanying gas industrially.

In the thermal decomposition step of the present invention, trichlorosilane is thermally decomposed as described below to produce SiCl _x (x is generally 0.1 to 0.3) as an intermediate product as a silicon fine particle precursor. do. A typical reaction in this thermal decomposition step is represented by the following formula (2).

SiHCl ₃ → (1-n) SiCl ₄ + nSiCl _x + (1/2) H ₂ (2)
In addition to silicon tetrachloride, by-products also include dichlorosilane and polymer-like silane.

In the pyrolysis step, the Si source is heated to a temperature of 600 to 900 ° C., preferably 650 to 900 ° C., more preferably 700 to 850 ° C. The heating temperature is important for producing the silicon fine particle precursor, and when the heating temperature is higher than the predetermined temperature, the reactant is fused to the inner wall of the reactor to block the reactor, and the temperature is higher than the predetermined temperature. If it is low, the desired silicon fine particle precursor cannot be obtained. As the reaction vessel, a tubular reaction vessel whose inner wall is made of a material such as carbon is usually used, and a heating device capable of heating the inner wall to a predetermined temperature is provided.

By going through a reaction for producing a silicon fine particle precursor represented by the formula (2), the crystallite diameter, which was not conventionally found in the past, was small, and the amount of oxygen and chlorine was adjusted to a predetermined range. Silicon fine particles can be obtained.

The temperature inside the reaction vessel is not particularly limited as long as it can be heated within the above range, and the temperature may be changed stepwise. The gas flow velocity and residence time of the Si source are also appropriately selected according to the size of the reaction vessel and the heat transfer area (efficiency).

The Si source is preferably preheated to a temperature of 40 ° C. or higher and lower than 600 ° C. in advance before being introduced into the reaction vessel, and then it is desirable to raise the temperature to the above temperature. When a low-temperature Si source is to be rapidly heated to the above temperature in the reaction vessel, the temperature of the inner wall (heater) of the reaction vessel becomes higher than the above temperature, and the Si source is locally generated near the inner wall of the reaction vessel. If the temperature exceeds the above temperature range, it is likely to cause fusion of the reactants to occur on the wall surface of the reaction vessel. By preheating, heating to the above temperature can be performed slowly, and the temperature of the inner wall of the reaction vessel can be appropriately maintained within the range of the thermal decomposition temperature of the Si source. In addition, the region required for preheating the Si source in the reaction vessel can be reduced. Furthermore, since it is easy to keep the temperature of the Si source uniform, it is possible to suppress variations in the particle size of the silicon fine particles obtained.

The silicon fine particle precursor of the reaction product is collected and separated from hydrogen, silicon tetrachloride, nitrogen, unreacted trichlorosilane, by-product dichlorosilane, polymer-like silane, and the like. The collection method is not particularly limited, and for example, a known method such as a cyclone type collection means, a bag filter, or an electrostatic precipitator can be used.

Unreacted trichlorosilane, silicon tetrachloride, and nitrogen gas are recovered from the reaction exhaust gas from which the silicon fine particle precursor is separated, and silicon tetrachloride is reacted with metallic silicon and hydrogen to convert it to trichlorosilane, and the reaction raw material is again. It can also be used as.

Separation of silanes and other gas components from the reaction exhaust gas can be performed by deep cooling or the like. Deep cooling is performed by cooling under pressure, generally under a pressure of about 500 to 800 kPaG, to about −30 to −50 ° C. by a heat exchanger or the like. By such deep cooling, trichlorosilane and silicon tetrachloride are condensed and separated from gas components such as nitrogen gas, hydrogen gas and hydrogen chloride gas. On the other hand, as for the gas component, hydrogen chloride gas may be removed by an adsorption tower filled with an adsorbent such as activated carbon, and then the separated and recovered hydrogen-containing nitrogen gas may be reused as an accompanying gas.

Trichlorosilane can be recovered from the condensate by distillation or the like and reused in the above reaction.
It is preferable that the separated silicon tetrachloride is reacted with hydrogen and metallic silicon (formula (3)), converted to trichlorosilane, and reused as a Si source.

Si + 2H ₂ + 3SiCl ₄ → 4SiHCl ₃ (3)
The obtained reaction product containing trichlorosilane is distilled to recover trichlorosilane and reuse it as a reaction raw material. Trichlorosilane may be recovered by mixing the condensate recovered by the deep cooling with the reaction product of the above reaction. In the reaction formula of formula (3), unreacted silicon tetrachloride can be converted to trichlorosilane by the above reaction again, and this loop is repeated to suppress the emission loss of by-products and make the raw material effective. It can be utilized. In addition, trichlorosilane may be further purified by making the distillation step multi-stage.

On the other hand, the collected silicon fine particle precursor is sent to the dechlorination step. The means of transport is not particularly limited as long as it does not come into contact with oxygen and moisture and there is no contamination from the container.
For example, the silicon fine particle precursor may be filled in a container made of carbon, aluminum, nickel-coated SUS or the like after nitrogen substitution, and transferred to a dechlorination step. A carbon container is preferable because it is less affected by metallic contamination, which is a problem especially when used as a battery material, and is not denatured even when filled with high-temperature particles. Alternatively, the silicon fine particle precursor collected by the known collecting means can be accumulated in a hopper or the like, and this can be accompanied by a gas containing no oxygen such as nitrogen and water and sent by air through a pipe.

-Dechlorination step Next, the collected silicon fine particle precursor is charged into a dechlorination reaction vessel, and at a temperature exceeding 900 ° C., a temperature up to 1200 ° C., preferably a temperature exceeding 1050 ° C. to 1180 ° C. Heat to dechlorinate. The dechlorination treatment is carried out in a dechlorination reaction vessel under the flow of a gas containing no oxygen atom or under reduced pressure. The gas supplied to the dechlorination reaction vessel and containing no oxygen atom is not particularly limited as long as it does not react with the silicon fine particle precursor. The gas is not limited as long as it does not contain an oxygen atom, and a gas such as nitrogen, argon or helium is preferably used. The gas is preferably a gas in which the water content is reduced as much as possible, and a gas having a dew point of −50 ° C. or lower is particularly preferable. Further, when the pressure is reduced, it is preferable to exhaust the gas from the dechlorination reaction vessel so that the pressure is 1 kPa or less.

By the dechlorination treatment, the reaction of the formula (4) proceeds, and silicon fine particles are obtained.
SiCl _x → (1-x / 4) Si + (x / 4) SiCl ₄ (4)
In the heating of the dechlorination treatment, in order to uniformly heat the silicon fine particle precursor, it is preferable to heat the precursor while stirring. The stirring may be performed by any known method such as one in which the reactor rolls, one in which the reactor is provided with a stirring blade, or one in which the stirring is performed by an air flow, and further, a baffle plate is provided to improve the stirring efficiency. You may let me.

The time (retention time) for heating the silicon fine particle precursor at the predetermined heating temperature is not particularly limited as long as it reaches the target chlorine concentration, but is generally about 5 to 60 minutes.

The heating temperature is important for the dechlorination treatment, and by heating in the temperature range, highly reactive chlorine near the surface of the fine particles is removed, and the polycrystalline primary particles have a predetermined chlorine content. Is partially fused to produce silicon fine particles composed of amorphous agglomerated particles having a predetermined specific surface area.

When the heating temperature is high, the chlorine concentration tends to be low, the crystallite diameter is large, and the specific surface area tends to be small.
Since the silicon fine particles of the present invention are adjusted to a predetermined amount of oxygen and chlorine, have a small crystallite diameter, and have a secondary aggregation structure that retains voids, the volume change during occlusion of lithium ions changes. It is possible to construct a negative electrode that can sustain a high charge / discharge capacity for a long period of time without breaking the particles due to a small amount and volume change. In addition, the silicon fine particles of the present invention can be handled extremely safely because rapid oxidation by highly active chlorine groups is effectively suppressed.

The present invention will be described with reference to the following examples and comparative examples. -Evaluation method of physical properties (1) Reaction rate of trichlorosilane The reaction rate of trichlorosilane is calculated from the ratio of trichlorosilane, silicon tetrachloride, and dichlorosilane detected by analyzing the composition of the exhaust gas after the reaction with a gas chromatograph. bottom.

(2) Chlorine concentration in the silicon fine particle precursor and the silicon fine particles The sample was measured and obtained by fluorescent X-ray analysis.
(3) Ratio of oxygen concentration in silicon fine particle precursor and silicon fine particle to specific surface area ( _CO / S)
^{The specific surface area (S [m 2} / g]) is determined by measuring the sample with a specific surface area measuring device using the nitrogen gas BET adsorption method, and the sample is measured with an oxygen nitrogen concentration analyzer (TC-600 manufactured by LECO). The oxygen concentration ( _CO [mass%]) was determined. The ratio of oxygen concentration to specific surface area ( _Co / S) was calculated by dividing the oxygen concentration by the specific surface area.

(4) Average diameter of silicon fine particles The specific surface area was obtained by measuring the sample with a specific surface area measuring device using the nitrogen gas BET adsorption method.
d = 6 / ρ · S
The average diameter was calculated by. In addition, d represents the average diameter, ρ represents the density of silicon, and S represents the specific surface area.

(5) Average crystallite diameter of silicon fine particles The diffraction profile obtained by X-ray diffraction of the sample was determined by analysis by the Hander-Wagner method.

(6) Bulk Density of Silicon Fine Particles A sample of a specified weight is filled in a powder press molding die made of super steel, and this is compressed using a precision universal testing machine (Autograph manufactured by Shimadzu Corporation), and the compression load and compression head The correlation of displacement of was measured. The volume of the green compact was calculated from the inner diameter of the die and the displacement of the head, and the bulk density was calculated from the sample weight and the volume of the green compact.

Example 1
-Synthesis of silicon fine particle precursor A graphite reaction cylinder with an inner diameter of 80 mm and a length of 2500 mm is heated to 750 ° C., and trichlorosilane is supplied to 900 g / min and accompanying nitrogen is supplied at a rate of 37 NL (L is liter) / min. The silicon fine particle precursor was synthesized, and separated and collected from the unreacted gas with a bag filter. The reaction rate of trichlorosilane was about 40%, and about 70% of the produced silicon fine particle precursor was collected by a bag filter. The collected silicon particulate precursor accumulated the atmosphere in a nitrogen-substituted storage vessel.

When a part of the collected silicon fine particle precursor was opened to the atmosphere, it reacted with the moisture in the air to generate white smoke composed of hydrogen chloride and oxidized. Analysis of the silicon particles precursor after air release, the ratio of the oxygen concentration and the specific surface area (C _o / S) became 0.192. The average crystallite diameter was 3 nm.

-Dechlorination of silicon fine particle precursors The silicon fine particle precursors (those that are not open to the atmosphere) accumulated in the storage container were supplied to a nitrogen-substituted graphite heating crucible while being careful not to touch the atmosphere. .. While stirring the silicon fine particle precursor in the crucible with a stirring blade made of graphite, an appropriate amount of nitrogen was supplied to the inside of the heating crucible and heated to 1150 ° C. while being circulated. Immediately after reaching 1150 ° C., heating was stopped and natural cooling was performed.

After cooling, the heating crucible was opened to the atmosphere, and silicon fine particles were taken out from the inside. Even when the obtained silicon fine particles were exposed to the atmosphere, no odor due to hydrogen chloride gas was felt. Chlorine concentration 0.7% by weight of the obtained silicon particles, the ratio of the oxygen concentration and the specific surface area (C _o / S) became 0.043. The average diameter of the primary particles was 159 nm, and the average crystallite diameter was 30 nm.

Example 2
In Example 1, trichlorosilane was supplied at a rate of 540 g / min and accompanying nitrogen was supplied at a rate of 22 NL (L is liter) / min to synthesize a silicon fine particle precursor, and dechlorination was performed to obtain silicon fine particles. Even when the obtained silicon fine particles were exposed to the atmosphere, no odor due to hydrogen chloride gas was felt. Chlorine concentration 0.8% by weight of the obtained silicon particles, the ratio of the oxygen concentration and the specific surface area (C _o / S) became 0.038. The average diameter of the primary particles was 486 nm, and the average crystallite diameter was 36 nm.

Example 3
In Example 1, trichlorosilane was supplied at a rate of 540 g / min and accompanying nitrogen was supplied at a rate of 83 NL (L is liter) / min to synthesize a silicon fine particle precursor, and dechlorination was performed to obtain silicon fine particles. Even when the obtained silicon fine particles were exposed to the atmosphere, no odor due to hydrogen chloride gas was felt. Chlorine concentration 0.4% by weight of the obtained silicon particles, the ratio of the oxygen concentration and the specific surface area (C _o / S) became 0.049. The primary particles were polycrystals having an average diameter of 84 nm and an average crystallite diameter of 30 nm.

Example 4
In Example 1, the dechlorination of the silicon fine particle precursor was heated to 1050 ° C. Immediately after reaching 1050 ° C., heating was stopped and natural cooling was performed.

After cooling, the heating crucible was opened to the atmosphere, and silicon fine particles were taken out from the inside. Even when the obtained silicon fine particles were exposed to the atmosphere, no odor due to hydrogen chloride gas was felt. Chlorine concentration 1.0% by weight of the obtained silicon particles, the ratio of the oxygen concentration and the specific surface area (C _o / S) became 0.038. The average diameter of the primary particles was 141 nm, and the average crystallite diameter was 20 nm.

Reference example 1
In Example 1, the dechlorination of the silicon fine particle precursor was heated to 800 ° C. Immediately after reaching 800 ° C., heating was stopped and natural cooling was performed.

After cooling, the heating crucible was opened to the atmosphere, and silicon fine particles were taken out from the inside. Even when the obtained silicon fine particles were exposed to the atmosphere, no odor due to hydrogen chloride gas was felt. Chlorine concentration 4.8% by weight of the obtained silicon particles, the ratio of the oxygen concentration and the specific surface area (C _o / S) became 0.029. The primary particles were polycrystalline with an average diameter of 123 nm and an average crystallite diameter of 7 nm.

Comparative Example 1
A powder obtained by crushing polycrystalline silicon (reagent manufactured by High Purity Chemical Laboratory, average diameter 5 μm) was used.

Comparative Example 2
The silicon powder of Comparative Example 1 was pulverized into a plate having an average diameter of about 200 nm using a bead mill.

Comparative Example 3
Silicon nanoparticles (reagent manufactured by Aldrich) having an average diameter of 100 nm synthesized by the plasma method were used.

Above embodiments, the silicon fine particles prepared in Reference Examples and Comparative Examples, the bulk density in a change in the bulk density in Fig. 3, in which a load of 10 kN / cm ² and 50 kN / cm ² when a load is applied (BD ₁₀ , BD ₅₀ ) are shown in Table 1.

Experimental Example 1
Using the silicon fine particles obtained in Example 1 as an active material, the active material, the conductive auxiliary agent (acetylene black) and the binder (polyimide) were kneaded so as to have a weight ratio of 7: 1: 2, and an NMP solvent was added. In addition, a paste having a viscosity of 0.8 to 1.5 Pa · S was obtained. This paste was applied onto a current collector (copper foil), dried and pressed, and then heated at a temperature of 350 ° C. for 0.5 hours under nitrogen flow to obtain a negative electrode sheet.

A half cell was prepared using an electrolytic solution containing 10 vol% each of vinylene carbonate and fluoroethylene carbonate with this negative electrode sheet as the negative electrode and the lithium foil as the counter electrode, and a cycle test was carried out at a charge / discharge rate of 0.05 C.
As a result, it showed a high charge capacity of 2,100 mAh / g even after 50 cycles.

Experimental Example 2
Using the silicon powder of Comparative Example 1 as an active material, a half cell was prepared in the same manner as in Experimental Example 1 and a cycle test was carried out. As a result, the charging capacity after 50 cycles was 230 mAh / g, and the charging capacity was greatly reduced.

Experimental Example 3
Using the silicon powder crushed by the bead mill of Comparative Example 2 as an active material, a half cell was prepared by the same method as in Experimental Example 1 and a cycle test was carried out. As a result, the charging capacity after 50 cycles was about 1,560 mAh / g, which was inferior to that of Experimental Example 1.

Claims (6)

Polycrystalline primary particles having an average crystallite diameter of 20 to 40 nm and an average diameter of 80 to 900 nm are composed of partially fused amorphous agglomerated particles, and the chlorine concentration is 0.1 to 1 with respect to the particle weight. Silicon fine particles, characterized by being 0.0% by mass. The silicon fine particles according to claim 1, wherein the polycrystalline primary particles have an average diameter of 130 to 850 nm. The silicon fine particles according to claim 1 or 2, wherein the bulk density when a load of 10 kN / cm ^{2 is applied is 1.3 g / cm 3 or less.} The silicon fine particles according to claim 1 or 2, wherein the bulk density when a load of 50 kN / cm ^{2 is applied is 1.8 g / cm 3 or less.} The invention according to any one of claims 1 to 4, wherein the ratio Co / S of the concentration Co [mass%] of oxygen contained as an impurity in the particles and the specific surface area S [m ^{2 / g] is less than 0.05.} Silicon fine particles. A silicon fine particle precursor obtained by thermally decomposing silicon chloride gas at a temperature of 600 to 900 ° C. is heated to more than 900 ° C. and 1200 ° C. or less under the flow of a gas containing no oxygen atom in its composition or under reduced pressure. Polycrystalline primary particles having a crystallite diameter in the range of 20 to 40 nm and a diameter of 80 to 900 nm are composed of atypical aggregated particles partially fused, and the chlorine concentration is relative to the particle weight. A method for producing silicon fine particles of 0.1 to 1.0% by mass.

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