Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/05—Preparation or purification of carbon not covered by groups C01B32/15, C01B32/20, C01B32/25, C01B32/30
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/02—Silicon
  • C01B33/021—Preparation
  • C01B33/027—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material
  • C01B33/029—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material by decomposition of monosilane
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• the present invention relates to a method for producing a Si—C composite (silicon-carbon composite).
• Silicon is an attractive negative electrode material for lithium-ion secondary batteries due to its high capacity, but its volume change is large during charging and discharging, and its cycle characteristics are poor.
• a technique for effectively using silicon as a negative electrode material for lithium ion secondary batteries by depositing elemental silicon in the pores of a porous carbon material, as disclosed in Patent Document 1 (U.S. Pat. No. 1,045,4103), One method is to use it as a silicon-carbon (Si--C) composite.
• silicon-carbon composite By making the silicon-carbon composite have a structure as disclosed in Patent Document 2 (Japanese translation of PCT publication No. 2018-534720), silicon can be fixed at a single position. Porous carbon materials are also believed to play an important role as a framework for expansion/contraction as well as contributing to the overall electronic and ionic conduction capabilities of the composite particles. For this reason, such a composite is considered to have much better cycle characteristics than when silicon is used alone as an active material.
• a silicon-containing gas such as silane or disilane is applied to porous carbon particles with pores in a furnace maintained at a high temperature. Then, the silicon-containing gas is thermally decomposed on the surface of the porous carbon particles and in the pores, and silicon is produced there. At this time, since the porous carbon contains pores such as micropores, mesopores, and macropores, it is considered that silicon having the size of these pores or smaller than them is deposited.
• CVD chemical vapor deposition
• CVI chemical vapor infiltration
• silicon may not precipitate uniformly over the entire porous carbon particles. Moreover, in the method of Patent Document 1, it may precipitate not only in the pores of the porous carbon, but also on the surface of the porous carbon particles and everywhere in the furnace.
• the main object of the present invention is to provide a method for producing a Si--C composite that deposits silicon uniformly over the entire porous carbon particles.
• the present inventors By forming a temperature gradient from upstream to downstream of the silane gas, the present inventors succeeded in eliminating variations in the silicon content in the Si—C composite and surface deposition Si depending on the location in the furnace. By performing chemical vapor infiltration (Si-CVI) of silicon at a lower temperature than before, the inventors succeeded in selectively depositing silicon in the pores of porous carbon particles, and completed the present invention.
• Si-CVI chemical vapor infiltration
• the configuration of the present invention is as follows. [1] Si—C, in which a silane-containing gas is passed through a reactor charged with porous carbon particles to thermally decompose silane in the silane-containing gas to deposit silicon in the pores of the porous carbon particles.
• a method for manufacturing a composite The reactor includes a silane-containing gas flow path from a silane-containing gas inlet to an exhaust gas outlet, and the porous carbon particles are loaded in the silane-containing gas flow path,
• the reaction temperature in the silane-containing gas flow path is 330° C. or higher and 450° C.
• Tup is the reaction temperature at the silane-containing gas inlet side end of the porous carbon particles in the silane-containing gas flow passage, and the middle point between the silane-containing gas inlet side end and the exhaust gas outlet side end
• Tdown 0° C. ⁇ Tmid ⁇ Tup ⁇ 70° C. and 0° C. ⁇ Tdown ⁇ Tmid ⁇ 50° C.
• the exhaust gas discharged from the exhaust gas outlet is sampled to monitor the composition of the exhaust gas, and from the mass flow rate of the silane gas at the reactor inlet and the concentration and reaction time of the silane gas in the exhaust gas at the reactor outlet, The method for producing a Si—C composite according to any one of [1] to [3], wherein the reaction amount is estimated.
• step (A) The step of obtaining a Si-C composite by the method for producing a Si-C composite according to any one of [1] to [5] (step (A)) is followed by forming an unsaturated bond.
• a method for producing a Si—C composite comprising the step of contacting the Si—C composite with a gas containing hydrocarbons at 400° C. or lower (step (B)).
• the expression "the porous carbon particles contained in the porous carbon particles have similar silicon deposition patterns” does not mean that there is little variation in the silicon deposition patterns among the porous carbon particles. This means that there is little variation in the way silicon precipitates from the porous carbon particles among the products obtained, regardless of the sampling location of the product (Si—C composite).
• the production method of the present invention since it has the above configuration, regardless of the location of the raw material in the reactor, It is considered that the method of silicon deposition is similar in the contained porous carbon particles (hereinafter sometimes referred to as "silicon is uniformly deposited over the entire porous carbon particles").
• the production method of the present invention can easily obtain a Si—C composite with little silicon deposition on the surface of the porous carbon particles, it is possible to obtain a negative electrode material excellent in cycle characteristics and initial coulomb efficiency.
• the method for producing the Si—C composite (that is, the silicon-carbon composite) of the present invention will be described in detail.
• a silane-containing gas is passed through a reactor charged with porous carbon particles, silane in the silane-containing gas is thermally decomposed, and silicon is deposited in the pores of the porous carbon particles.
• Reactor The reactor is provided with a silane-containing gas flow path extending from the silane-containing gas inlet to the exhaust gas outlet.
• the porous carbon particles used in the production method of the present invention are loaded in the silane-containing gas flow passage.
• the silane-containing gas is introduced into the silane-containing gas flow path from the silane-containing gas inlet of the reactor, and the silane-containing gas flow path is heated to thermally decompose silane in the silane-containing gas.
• silicon is deposited in the pores of the porous carbon particles loaded in the silane-containing gas flow passages to form a Si—C composite.
• the exhaust gas generated from the silane-containing gas by the reaction and the unreacted silane-containing gas are discharged from the exhaust gas outlet.
• the reactor is preferably equipped with a heater capable of controlling a predetermined temperature at a predetermined point in the silane-containing gas flow passage.
• a heater capable of controlling a predetermined temperature at a predetermined point in the silane-containing gas flow passage.
• the reactor may have three heaters, or four or more heaters. The greater the number of heaters, the easier it is to control each location in the silane-containing gas flow path at a predetermined temperature.
• the reactor is not limited to a stationary furnace, and may be a rotary furnace such as a rotary kiln.
• the shape of the reactor is not particularly limited as long as the above functions can be realized, and various shapes such as a straight tubular shape, a disk shape, and a spiral circular tubular shape can be appropriately selected.
• the shape of the silane-containing gas flow path and the like is appropriately determined according to the shape of the reactor. For example, when the reactor has a straight tube shape, the inner space of the straight tube serves as the silane-containing gas flow passage.
• the size of the reactor is not particularly limited as long as the functions can be realized, and can be appropriately determined according to production scale and the like.
• Porous carbon particles used in the production method of the present invention preferably have a pore volume of 0.2 cm 3 /g or more. Since the porous carbon particles have a pore volume of 0.2 cm 3 /g or more, they can be impregnated with a sufficient amount of silicon, so that a sufficiently large specific capacity can be obtained as a negative electrode material for batteries. From this point of view, the pore volume of the porous carbon particles is more preferably 0.4 cm 3 /g or more, more preferably 0.6 cm 3 /g or more.
• the porous carbon particles used in the production method of the present invention preferably have a pore volume of, for example, 2.2 cm 3 /g or less.
• the pore volume of the porous carbon particles is 2.2 cm 3 /g or less, the volume change due to expansion/contraction during charge/discharge can be absorbed by the porous carbon particles as a carrier.
• the pore volume of the porous carbon particles is more preferably 2.0 cm 3 /g or less, more preferably 1.8 cm 3 /g or less.
• the pore volume of porous carbon particles can be measured by a nitrogen adsorption test.
• the porous carbon particles used in the production method of the present invention preferably have a BET specific surface area of 800 m 2 /g or more.
• the BET specific surface area of the porous carbon particles is 800 m 2 /g or more, a large amount of silicon can be deposited mainly on the inner surfaces of the porous carbon particles, so that a sufficiently high specific capacity can be obtained as a negative electrode material. Obtainable.
• the BET specific surface area of the porous carbon particles is more preferably 900 m 2 /g or more, more preferably 1000 m 2 /g or more.
• the porous carbon particles used in the production method of the present invention preferably have a 50% particle diameter (hereinafter also referred to as “D V50 ”) in volume-based cumulative particle size distribution of 2.0 ⁇ m or more. If the D V50 of the porous carbon particles is 2.0 ⁇ m or more, the resulting Si—C composite has excellent handleability, can be easily prepared into a slurry having a viscosity and density suitable for coating, and can be used when forming an electrode. The density of the porous carbon particles is likely to increase. From this point of view, D V50 is more preferably 3.0 ⁇ m or more, further preferably 4.0 ⁇ m or more.
• D V50 of the porous carbon particles is preferably 30.0 ⁇ m or less. If the D V50 of the porous carbon particles is 30.0 ⁇ m or less, the diffusion length of lithium in each particle is short, so the rate characteristics of the lithium ion battery are excellent, and when the slurry is applied to the current collector, In addition, it is possible to suppress the occurrence of streaks due to the porous carbon particles and the occurrence of abnormal irregularities derived from the porous carbon particles. From this point of view, D V50 is more preferably 25.0 ⁇ m or less, more preferably 20.0 ⁇ m or less.
• D V50 can be measured by laser diffraction.
• D V10 and D V90 refer to the 10% particle size and 90% particle size in the volume-based cumulative particle size distribution, respectively.
• D V10 , D V90 can also be measured by laser diffraction.
• the porous carbon particles having the pore volume as described above, or additionally the BET specific surface area and particle size, may be purchased and used, or a known technique may be used to synthesize a resin, which is then carbonized and activated. It may be obtained by adjusting the particle size.
• a well-known technique can be used for adjusting the particle size.
• a method for adjusting the particle size for example, a pulverizer such as a jet mill, a pin mill, or a vibration mill, or an air classifier can be used. Even from the state of the Si—C composite, for example, by eluting silicon using an aqueous potassium hydroxide solution, the pore volume and BET specific surface area of the porous carbon particles used as the raw material, and the volume-based cumulative particle size distribution of 50 Physical properties such as % particle size and true density can be measured.
• Porous carbon particles are loaded into the silane-containing gas flow path of the reactor as described above.
• the method of filling the porous carbon is not particularly limited, and the sagger may be filled with the porous carbon particles, or the porous carbon particles may be placed on a structure such as a shelf.
• the porous carbon particles are preferably loaded into the silane-containing gas flow path with a uniform thickness from the silane-containing gas inlet toward the exhaust gas outlet.
• the amount of porous carbon particles to be charged is not particularly limited, and can be appropriately determined according to the size of the reactor, production scale, and the like.
• the silane-containing gas is introduced into the silane-containing gas flow path of the reactor, and undergoes a reaction to deposit silicon in the pores of the porous carbon particles (hereinafter also referred to as “Si-CVI reaction”). ).
• the silane-containing gas is preferably silane gas or a mixed gas of silane gas and inert gas.
• the molar fraction of silane gas in the mixed gas is preferably 0.001-1.
• the inert gas contained in the mixed gas is not particularly limited, but is preferably nitrogen or argon.
• the silane gas is represented by the general formula Si n H 2n+2 and specifically includes monosilane (SiH 4 ), disilane, trisilane, tetrasilane, etc. Monosilane is preferred.
• the silane-containing gas flow path before carrying out the Si-CVI reaction, it is preferable to flow an inert gas through the silane-containing gas flow path and raise the temperature to a predetermined reaction temperature, and then appropriately at the set temperature. You can keep warm.
• the rate at which the temperature is raised is not particularly limited, and can be freely selected in consideration of productivity, damage to the furnace, and the like.
• the gas may be switched from an inert gas to a reaction gas (silane-containing gas) when starting the Si-CVI reaction.
• the gauge pressure in the reactor during the Si-CVI reaction is preferably ⁇ 10 kPa or more and +10 kPa or less.
• the gas flow rate of silane-containing gas is not particularly limited. It can be determined as appropriate according to the scale and the like.
• the Si-CVI reaction takes place in the silane-containing gas flow path.
• the reaction temperature in the silane-containing gas flow path is 330° C. or higher and 450° C. or lower. That is, in the silane-containing gas flow path, the Si-CVI reaction with respect to the porous carbon particles takes place in the range of 330°C or higher and 450°C or lower.
• the reaction temperature is 330°C or higher, the silane gas is adsorbed in the pores of the porous carbon particles, and decomposition can occur only at the temperature reached by the temperature rise due to the heat generated at that time. Therefore, silicon can be selectively deposited in the pores.
• the reaction temperature is preferably 340°C or higher, more preferably 350°C or higher.
• reaction temperature is 450°C or lower, most of the silane gas will not decompose by itself, so it will hardly decompose outside the pores of the porous carbon particles. From this point of view, the reaction temperature is preferably 445°C or lower, more preferably 440°C or lower.
• the reaction temperature in the silane-containing gas flow path has a temperature distribution that rises from the silane-containing gas inlet toward the exhaust gas outlet. That is, the reaction temperature in the silane-containing gas flow path has a temperature gradient, and the reaction temperature must rise within the range of 330° C. or higher and 450° C. or lower from the silane-containing gas inlet toward the exhaust gas outlet. Alternatively, there may be a portion at a constant temperature from the silane-containing gas inlet to the exhaust gas outlet.
• the reaction temperature at the silane-containing gas inlet side end of the porous carbon particles in the silane-containing gas flow passage (hereinafter also referred to as "upstream end") is Tup, and the silane-containing gas inlet side end is Tup.
• Tmid is the reaction temperature at the midpoint between the part and the exhaust gas outlet side end (hereinafter also referred to as the “intermediate portion”), and the reaction temperature at the exhaust gas outlet side end (hereinafter also referred to as the “downstream end”). is Tdown, the temperature is controlled so that the following equation holds. 0°C ⁇ Tmid ⁇ Tup ⁇ 70°C 0°C ⁇ Tdown-Tmid ⁇ 50°C
• reaction temperature Tup at the upstream end is lower than the reaction temperature Tmid at the intermediate portion, the reaction activity on the upstream side and the midstream portion can be matched, and the physical properties of the product will be uniform.
• Tup is lower than Tmid by more than 70° C., the thermal decomposition reaction of silane is not promoted and Si deposition is greatly suppressed.
• the temperature difference between Tmid and Tup is preferably greater than 5°C, more preferably greater than 15°C.
• the temperature difference between Tmid and Tup (Tmid-Tup) is preferably 60°C or less, more preferably 50°C or less.
• reaction temperature Tdown at the downstream end equal to or higher than the reaction temperature Tmid at the intermediate portion, it is possible to compensate for the disadvantage of the downstream side that the concentration of silane gas is lower than that at the midstream portion.
• Tdown exceeds Tmid by more than 50° C., reactions in the downstream portion proceed locally, resulting in non-uniform loading.
• the temperature difference between Tdown and Tmid is preferably 0 to 45°C, more preferably 0 to 40°C.
• the reaction temperature Tup at the upstream end is preferably less than 400°C.
• Tmid may be less than 400° C. or 400° C. or more as long as 0° C. ⁇ Tmid ⁇ Tup ⁇ 70° C., and Tdown as long as 0° C. ⁇ Tdown ⁇ Tmid ⁇ 50° C. It may be below 400°C or above 400°C.
• the temperature distribution as described above can be achieved, for example, by a heater installed on the silane-containing gas inlet side along the silane-containing gas flow path of the reactor (hereinafter also referred to as an "upstream heater”), and a heater installed on the exhaust gas outlet side. (hereinafter also referred to as “downstream heater”), and a heater installed between the upstream heater and the downstream heater (hereinafter also referred to as “midstream heater”). It can be obtained by setting the set temperature of the heater to be lower than the set temperatures of the midstream heater and the downstream heater, and setting the set temperature of the downstream heater to be the same as the set temperature of the midstream heater or higher than the set temperature of the midstream heater. can.
• the time for the Si-CVI reaction is not particularly limited, and may be, for example, 1 second to 30 hours.
• the silicon content can be freely changed by adjusting the Si-CVI reaction time.
• the reaction SiH 4 ⁇ Si+2H 2 occurs.
• the gas composition at the outlet can be monitored by sampling the exhaust gas discharged from the exhaust gas outlet and measuring the concentration of silane gas or hydrogen gas. If the mass flow rate of silane gas at the reactor inlet and the concentration of silane gas in the exhaust gas at the reactor outlet are known, the amount of silane gas used for deposition on the porous carbon particles per unit time can be determined. Based on this, the amount of silicon deposited on the porous carbon particles per unit time can also be calculated.
• the reaction amount By accumulating the obtained values over the reaction time, the total amount of silicon deposited on the porous carbon particles during the reaction, that is, the reaction amount can be estimated. Also, if the flow rate of the silane gas introduced into the reactor is adjusted by the mass flow rate, the mass flow rate at the inlet of the reactor can be known. A method for measuring this may be gas chromatography or a gas detector, but a real-time composite gas mass spectrometer is preferable because it allows monitoring during the reaction.
• silicon can be selectively deposited in the pores of the porous carbon particles. Also, using the density of silicon, the total amount of silicon deposited on the porous carbon particles can be converted from a unit of mass to a unit of volume. From the pore volume of the porous carbon particles and the volume of the total amount of silicon deposited in the pores of the porous carbon particles by the above reaction, the filling rate of silicon in the pores of the porous carbon particles is obtained by the following equation. can be done. Although it is difficult to strictly distinguish between the silicon inside the pores of the porous carbon particles and the silicon outside the pores, it is difficult to measure and calculate the filling ratio obtained by the following equation. is the filling rate of silicon into the pores of
• (Filling rate ⁇ ) (%) 100 x w Si / (V p x w C x ⁇ Si )
• w Si is the total amount of silicon deposited on the porous carbon particles by the Si-CVI reaction [g]
• ⁇ Si is the density of silicon [g/cm 3 ]
• w C is the mass of the porous carbon particles used [ g]
• V p is the pore volume [cm 3 /g] of the porous carbon particles.
• the unit of the filling rate ⁇ is %.
• the filling rate (%) of silicon can be obtained with sufficient accuracy. can be estimated. By sampling, estimating the amount of deposited silicon, and calculating the fill factor while the reaction is running, the reaction can be stopped when the desired fill factor is obtained. In this way, the filling rate of silicon into the pores of the porous carbon particles can be adjusted from the reaction amount and the pore volume.
• Step (B) In a method for producing a Si—C composite according to another embodiment of the present invention, the step of obtaining a Si—C composite by the method for producing Si—C described above (step (A)) is followed by The step (step (B)) of contacting the Si—C composite at 400° C. or lower with a gas containing a hydrocarbon having In the present invention, not only the Si—C composite obtained in step (A), but also the Si—C composite obtained by performing steps (A) and (B) are produced according to the present invention. contained in the Si—C composite obtained by the method.
• the Si—H groups on the surface of the Si—C composite react with hydrocarbons having unsaturated bonds to form a layer containing hydrocarbons on the surface of the Si—C composite. Therefore, after obtaining the Si—C composite obtained in the step (A), it is preferable to perform the step (B) while suppressing oxidation of the surface of the Si—C composite. Therefore, as long as the oxidation of the surface of the Si—C composite is suppressed, the embodiment of the present invention also includes another step or operation between step (A) and step (B).
• an inert gas such as helium, argon or nitrogen, or a reducing gas such as hydrogen is used to remove the unreacted silane-containing gas in step (A).
• a method of removing gas is mentioned.
• the layer containing hydrocarbons may contain a substance obtained by reacting hydrocarbons with each other.
• hydrocarbon gas having an unsaturated bond a hydrocarbon gas having a double bond or a triple bond can be used.
• Hydrocarbons that have a low vapor pressure and hardly vaporize under normal pressure (101,325 pa) can be used by lowering the pressure below normal pressure.
• Hydrocarbons preferably used include acetylene, ethylene, propylene and 1,3-butadiene, which are gases at normal pressure, with acetylene and ethylene being more preferred.
• step (B) multiple types of hydrocarbons may be used.
• an inert gas such as helium or argon, or a reducing gas such as hydrogen may be mixed with the hydrocarbon.
• Step (B) is a treatment of contacting the Si—C composite at 400° C. or less from the viewpoint of reacting the Si—H groups with the unsaturated bonds in the hydrocarbon. If the temperature at which the Si—C composite is contacted (hereinafter also referred to as “contact temperature”) is 400° C. or less, the decomposition of the Si—H group is likely to be suppressed, and the Si— on the surface of the Si—C composite Reaction of H groups with unsaturated bonds in hydrocarbons is more likely.
• the contact temperature in the step (B) is not limited as long as it is a temperature at which the hydrocarbon having an unsaturated bond reacts on the surface of the Si—C composite. 100° C. or higher is preferable, and 150° C. or higher is more preferable.
• the thickness of the hydrocarbon-containing layer formed on the surface of the Si—C composite may be as thick as a monomolecular layer of hydrocarbon, or as thick as several molecular layers. Some of the hydrocarbons may be decomposed. Even if some of the hydrocarbons are decomposed into carbon, the layer containing the hydrocarbons is preferably a thin film because it is a material with high resistance unlike ordinary coated carbon. Therefore, it is preferable that the change in mass of the Si—C composite be small before and after the step (B).
• the mass of the Si—C composite before performing the step (B) is W B, before, and the mass of the Si—C composite having a layer containing a hydrocarbon after performing the step (B) is W B, after Then, the rate of increase in the mass of the Si—C composite due to step (B), ⁇ W B , is given by the following equation.
• ⁇ W B (%) 100 ⁇ (W B, rear - W B, front) / W B, front ⁇ W B is more preferably 1.0% or less, more preferably 0.5% or less. more preferred.
• the effects of the step (B) hydrocarbon treatment include, for example, suppression of oxidation of silicon inside the Si—C composite over time, suppression of hydrogen generation during preparation of slurry for electrode coating, increase in initial coulomb efficiency in the battery, cycle improvement of characteristics.
• the presence of the protective layer on the surface of the Si--C composite can prevent oxygen and moisture from entering the interior of the Si--C composite.
• the Si—C composite is brought into contact with an inert gas atmosphere containing oxygen,
• the silicon surface may be oxidized. Pure silicon, in particular, has high activity, so rapid deterioration can be suppressed by oxidizing the surface.
• the surface of the Si--C composite may be coated separately.
• a specific example is a carbon coating.
• Methods of carbon coating include chemical vapor deposition (CVD (Chemical Vapor Deposition), etc.) and the like. Carbon coating by CVD is referred to herein as "C-CVD.”
• the effects of the coating include, for example, suppressing the oxidation of silicon inside the Si—C composite over time, increasing the initial coulombic efficiency, and improving cycle characteristics.
• the presence of the coat layer on the surface of the Si—C composite can suppress the intrusion of oxygen and moisture into the interior of the Si—C composite.
• a Si—C composite in which silicon is uniformly deposited over the entire porous carbon particles is obtained.
• the obtained powder (Si—C composite) is divided into three equal parts along the length in the gas introduction direction, and the part on the side of the silane-containing gas inlet (upstream part) and the part on the side of the silane-containing gas outlet.
• the product is sampled from each portion (downstream portion) and the portion (midstream portion) sandwiched between the upstream portion and the downstream portion, and the products are uniformly mixed and then analyzed or evaluated for the battery. , and compare whether the products of each part have similar physical property values or battery characteristics.
• the physical properties of the Si—C composite and the method for measuring them will be described below. Further, hereinafter, variation indicates the difference between the highest value and the lowest value among the analysis values for each sampling position. For example, among the values obtained by analyzing the upstream portion, midstream portion, and downstream portion of the Si—C composite obtained as described above, the difference between the highest value and the lowest value show.
• R value of Raman spectrum In general, the R value of Raman spectrum reflects the amount of defects contained in the crystal structure of carbon. Therefore, the R value of the Raman spectrum of the Si--C composite obtained by the production method of the present invention is information on the amount of defects contained in the crystal structure of the porous carbon particles in the Si--C composite.
• the R value is the ratio (I D /I G ) of the peak intensity I D near 1360 cm ⁇ 1 and the peak intensity I G near 1580 cm ⁇ 1 in the Raman spectrum.
• the peak intensity is the height from the baseline to the peak apex after correcting the baseline.
• the Si—C composite obtained by the production method of the present invention preferably has a Raman spectrum R value of 0.50 or more.
• the R value is 0.50 or more, the reaction resistance is sufficiently low, leading to an improvement in the rate characteristics of the battery. From this point of view, the R value is more preferably 0.70 or more, more preferably 0.80 or more.
• the R value of the Raman spectrum of the Si—C composite obtained by the production method of the present invention is preferably less than 1.50.
• the R value is more preferably 1.40 or less, even more preferably 1.35 or less, and particularly preferably 1.30 or less.
• the Si—C composite obtained by the production method of the present invention has a silicon-derived peak in the Raman spectrum, and the peak exists at 450 to 495 cm ⁇ 1 . It is generally known that silicon having a peak in this wavenumber range is amorphous. In contrast, crystalline silicon is known to have a peak near 520 cm ⁇ 1 in the Raman spectrum. It is believed that amorphous silicon is formed in the Si—C composite obtained by the manufacturing method of the present invention. In the present invention, the peak intensity of amorphous silicon is denoted as I Si .
• I Si /I G in Raman spectrum IG is the intensity of the aforementioned peak near 1580 cm -1 , and this peak originates from porous carbon particles in the Si—C composite.
• the variation in the value of I Si /I G of the Raman spectrum of the Si—C composite due to the sampling position in the reactor is preferably 0.25 or less, more preferably 0.20 or less, and 0.10 or less. More preferred. As will be described later, variations in the value of I Si /I G are thought to be due to variations in the amount of Si deposited on the surface, which is thought to lead to variations in the cycle characteristics of the battery.
• the I Si /I G of the Raman spectrum of the Si—C composite obtained by the production method of the present invention is preferably 0.80 or less, more preferably 0.70 or less, and less than 0.30. is more preferred.
• the appearance of a silicon peak in the Raman spectrum indicates that silicon is deposited near the surface of the Si—C composite, and I Si /I G is 0.80 or less (more preferably 0 .70 or less, more preferably less than 0.30), it can be estimated that silicon is mainly deposited inside the pores of the porous carbon particles and is hardly precipitated on the surfaces of the porous carbon particles. can.
• I Si /I G is more preferably 0.25 or less, and even more preferably 0.20 or less.
• the lower limit of I Si /I G is not particularly limited, I Si /I G is preferably 0.03 or more, more preferably 0.06 or more. When I Si /I G is 0.03 or more, it is presumed that a sufficient amount of silicon is deposited, and a sufficiently large specific capacitance can be obtained.
• Si content rate is defined by the following formula.
• the unit is % by mass.
• Si content 100 ⁇ (mass of silicon contained in Si—C composite)/(mass of Si—C composite)
• the variation in the Si content of the Si—C composite due to the sampling position in the reactor is preferably 9% by mass or less, more preferably 6% by mass or less, and 3% by mass. preferable. This is because variations in the Si content lead to variations in the capacity of the battery.
• the Si content of the Si--C composite obtained by the production method of the present invention is preferably 15% by mass or more. From this point of view, the Si content is more preferably 20% by mass or more, and even more preferably 25% by mass.
• the Si content of the Si—C composite obtained by the production method of the present invention is preferably 85% by mass or less. From this point of view, the Si content is more preferably 75% by mass or less, and even more preferably 65% by mass or less. At this time, ideally, silicon should be present only in the pores of the porous carbon particles, but in reality, silicon may also be deposited on the outermost surfaces of the porous carbon particles. In addition, silicon may be deposited on the outermost surface of the porous carbon particles without completely filling the pore volume of the porous carbon particles with silicon.
• the Si content can be obtained by subjecting the Si—C composite to fluorescent X-ray analysis using a fundamental parameter (FP) method.
• FP fundamental parameter
• the Si—C composite obtained by the production method of the present invention is preferably one in which silicon is selectively deposited in pores of porous carbon particles. Since the pores of the raw material porous carbon particles are filled with silicon or the entrances of the pores are blocked with silicon, the BET specific surface area of the Si—C composite is the BET specific surface area of the porous carbon particles. less than As a result, it becomes a value suitable for use as a negative electrode material for a lithium ion secondary battery.
• Variation in the BET specific surface area of the Si—C composite due to sampling positions in the reactor is preferably 30 m 2 /g or less, more preferably 20 m 2 /g or less, and 15 m 2 /g or less. is more preferred.
• the variation in the BET specific surface area is 30 m 2 /g or less, the extent of the side reactions becomes equal, so the variation in the initial coulombic efficiency is reduced.
• the BET specific surface area of the Si—C composite obtained by the production method of the present invention is preferably 40 m 2 /g or less. If it is 40 m 2 /g or less, the decomposition reaction of the electrolytic solution, which is a side reaction, does not become remarkable, so the initial coulombic efficiency can be increased. From this point of view, the BET specific surface area is more preferably 20 m 2 /g or less, more preferably 15 m 2 /g or less.
• Porous carbon particles 1 Porous carbon particles having a pore volume of 0.85 cm 3 /g, D V10 of 6.9 ⁇ m, D V50 of 14.4 ⁇ m, D V90 of 26.2 ⁇ m, and a BET specific surface area of 1770 m 2 /g
• Porous carbon particles 2 Porous carbon particles having a pore volume of 0.60 cm 3 /g, D V10 of 5.3 ⁇ m, D V50 of 10.7 ⁇ m, D V90 of 19.6 ⁇ m, and a BET specific surface area of 1550 m 2 /g
• Porous carbon particles 3 Pores Porous carbon particles having a volume of 1.1 cm 3 /g, D V10 of 3.2 ⁇ m, D V50 of 4.4 ⁇ m, D V90 of 6.1 ⁇ m, and a BET specific surface area of 2100 m 2 /g. went like
• the BET specific surface area of the porous carbon material was calculated by the BET multipoint method from the adsorption isotherm data at a relative pressure of about 0.005 to less than 0.08.
• the BET specific surface area of the composite particles was calculated by the BET multipoint method from the adsorption isotherm data at three points near relative pressures of 0.1, 0.2 and 0.3.
• the pore volume was obtained by calculating the adsorption amount at a relative pressure of 0.99 by linear approximation from the adsorption isotherm data at two points around the relative pressure of 0.99. At this time, calculations were made assuming that the liquid density of nitrogen is 0.808 g/cc, the volume of 1 mol of nitrogen in the standard state is 22.4133 L, and the atomic weight of nitrogen is 14.0067.
• the set value of the relative pressure at the time of actual measurement is as follows. 0.005, 0.007, 0.010, 0.020, 0.050, 0.075, 0.100, 0.200, 0.300, 0.800, 0.900, 0.925, 0.950, 0.975, 0.990, 0.995
• ⁇ Microscopic Raman spectrometer manufactured by Horiba, Ltd., product name: LabRAM HR Evolution ⁇ Excitation wavelength: 532 nm ⁇ Exposure time: 10 seconds ⁇ Number of integration times: 2 ⁇ Diffraction grating: 300 lines/mm (600 nm) ⁇ Measurement range: length 60 ⁇ m x width 60 ⁇ m Number of points: Measurement was performed at 30 points with a longitudinal feed of 12 ⁇ m and a lateral feed of 15 ⁇ m, and the spectrum obtained by averaging them was obtained and the following analysis was performed.
• the intensity was defined as the height of the peak top from the baseline. From the measured spectrum, the ratio (ID/IG) of the peak intensity ID (derived from the amorphous component) near 1350 cm -1 to the peak intensity IG (derived from the graphite component) near 1600 cm -1 was calculated. This ratio (ID/IG) was measured at two points, and the average value was taken as the R value (ID/IG).
• the ratio (I Si /I G ) between the intensity ISi of the peak derived from amorphous silicon appearing at 450 to 495 cm ⁇ 1 and the IG was calculated. This ratio (I Si /I G ) was measured at two points, and the average value was taken as I Si /I G .
• Si-CVI reaction was carried out using a reactor (manufactured by Koyo Thermo Systems Co., Ltd.) equipped with a horizontal tubular reaction tube.
• a heater having a length of 70 cm was installed in the tubular reaction tube, and the temperature was controlled for each of three zones obtained by dividing the heater into three equal parts.
• a graphite plate having a length of 30 cm was placed in the tubular reaction tube so that the center of the heater in the extending direction of the reaction tube coincided with the center of the graphite plate in the extending direction of the reaction tube.
• the reaction temperature at the silane-containing gas inlet side end (upstream end) of the porous carbon particles 1, the midpoint (middle part) between the silane-containing gas inlet side end and the exhaust gas outlet side end and the reaction temperature at the exhaust gas outlet side end (downstream end) were adjusted to 375° C., 400° C., and 410° C., respectively.
• Argon gas was passed through the silane-containing gas flow path from the silane-containing gas inlet, and the temperature was raised and maintained in the argon gas atmosphere as described above. After that, the argon gas was switched to 100% monosilane gas at 130 sccm, and the monosilane gas was circulated from the silane-containing gas inlet to the silane-containing gas flow passage. The pressure in the reactor at this time was +1.6 kPa in gauge pressure. A Si-CVI reaction was carried out for 250 minutes, after which the monosilane gas was switched to argon gas, heating of the reactor was stopped, and the reactor was cooled to room temperature.
• the powder (Si—C composite) obtained by this Si—CVI reaction was divided into three equal parts in the direction of elongation of the reaction tube.
• the portion (downstream portion) and the portion (midstream portion) sandwiched between the upstream portion and the downstream portion are each sampled and uniformly mixed, and then each portion is subjected to Raman spectroscopic analysis, fluorescent X-ray analysis, and nitrogen Adsorption measurements were performed to determine the R value, Si peak position, I Si /I G , Si content, and BET specific surface area, and the numerical values and variations thereof were compared. Table 1 shows the results.
• Si-CVI was performed in the same manner as in Example 1, except that the reaction temperature and silane treatment time at the upstream end, intermediate portion, and downstream end of the silane-containing gas flow passage were adjusted as shown in Table 1, respectively. -C conjugate was obtained. Physical property values were measured in the same manner as in Examples. Table 1 shows the results.
• Example 4 Si--CVI reaction was carried out in the same manner as in Example 1 except that porous carbon particles 2 were used instead of porous carbon particles 1 and the silane treatment time was changed to obtain a Si--C composite. Physical property values were measured in the same manner as in Examples. Table 1 shows the results.
• Example 5 Si--CVI reaction was carried out in the same manner as in Example 1 except that porous carbon particles 3 were used instead of porous carbon particles 1 and the silane treatment time was changed to obtain a Si--C composite. Physical property values were measured in the same manner as in Examples. Table 1 shows the results.
• Example 6 Si—CVI reaction was carried out in the same manner as in Example 1 except that the pressure in the reactor was set to 200 kPa in gauge pressure and the silane treatment time was changed to obtain a Si—C composite. Physical property values were measured in the same manner as in Examples. Table 1 shows the results.
• the pore volume of the porous carbon particles used was rather small, and the obtained Si content was somewhat small. It was found that the physical property values of the obtained Si--C composites all showed little variation, and silicon was uniformly deposited over the entire porous carbon particles of the obtained Si--C composites.
• the porous carbon particles used had a rather large pore volume, and the resulting Si content was also large. It was found that the physical property values of the obtained Si--C composites all showed little variation, and silicon was uniformly deposited over the entire porous carbon particles of the obtained Si--C composites.
• the value of the gauge pressure during the reaction was as high as +200 kPa, but the variations in the physical properties of the obtained Si—C composites were all small, and the obtained Si—C composites were homogeneously siliconized throughout the porous carbon particles. was found to precipitate.
• the Si—C composite produced in the Si—CVI reaction exhibits uniform physical properties regardless of the sampling position, and porous carbon particles It can be seen that silicon is deposited uniformly over the entire surface.

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