Porous silicon and method for producing porous silicon

Abstract

To provide porous silicon having a high pore volume even under pressure. The crystallite diameter is 20 nm or more and 45 nm or less, and the pore volume is 0.60 cm in the pore diameter range of 5 nm or more and less than 300 nm in the pore distribution measured by mercury intrusion porosimetry. 3 / g or more 1.60cm 3 / g or less porous silicon.

Description

[Technical Field]

[0001] The present invention relates to porous silicon and a method for producing porous silicon. [Background technology]

[0002] Anodes made of silicon have a high theoretical energy capacity density and are therefore expected to be anode materials for next-generation batteries.

[0003] On the other hand, there are concerns that the anode made of silicon may suffer from deterioration of cycle characteristics due to significant volume changes during charging and discharging. One solution to this problem is to make the silicon porous.

[0004] Patent Document 1 describes a particle material consisting of a plurality of porous particles containing an electroactive material selected from silicon, germanium, or a mixture thereof, wherein the D of the porous particles 50 A particle material is disclosed having a particle diameter in the range of 0.5 μm to 7 μm, an intra-particle porosity in the range of 50% to 90%, and a pore size distribution measured by mercury porosimetry that has at least one peak in the range of 30 nm to less than 400 nm (see claim 1).

[0005] In Patent Document 2, the Raman spectrum before lithium absorption shows a 480 cm -1 More than 550cm -1 The present invention discloses a porous silicon material in which the half width of the peak in the Raman spectrum after lithium absorption is smaller than the half width of the peak appearing in the range (see claim 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2017-533533 [Patent Document 2] Japanese Patent Publication No. 2022-126068 Summary of the Invention [Problem to be solved by the invention]

[0007] In Patent Document 1, porous silicon is obtained by rapidly cooling a molten Al-Si alloy using an atomization method to precipitate silicon, and then dissolving the aluminum in the alloy with hydrochloric acid. Although the porous silicon has a high pore volume, due to the manufacturing process, a large amount of aluminum remains, and the silicon crystallite diameter is large. Aluminum has a different expansion rate from silicon during charge and discharge, and silicon with large crystallites produces a non-uniform Li-Si alloy during charging, so the material is thought to be insufficiently resistant to volume changes.

[0008] Patent Document 2 shows the pore distribution, which suggests that the material is porous to some extent. For example, from the pore distribution measured by mercury intrusion in Experimental Example 4 shown in Figure 6(b), the maximum Log differential pore volume for pores with diameters of 5 nm or more and less than 300 nm is estimated to be 0.5 cm 3 / g, which is not considered to have a sufficiently high pore volume.

[0009] On the other hand, all-solid-state batteries, which are attracting attention as next-generation batteries, are operated under a certain pressure in order to obtain a good solid-solid interface. As a result of intensive research, the present inventors have found that even porous silicon with a high pore volume can lose its porosity due to the pores being crushed by pressure.

[0010] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide porous silicon that has a high pore volume even under pressure, and to provide a method for producing such porous silicon. [Means for solving the problem]

[0011] The present invention provides the following: [1] The crystallite size is 20 nm or more and 45 nm or less, In the pore size distribution obtained by mercury intrusion porosimetry, the pore volume is 0.60 cm in the pore diameter range of 5 nm or more and less than 300 nm. 3 / g or more 1.60cm 3 / g or less.

[0012] According to the above-mentioned configuration, since the crystallite diameter is 20 nm or more and 45 nm or less, the pores are unlikely to collapse even when pressure is applied. The inventors speculate that the reason for this is that when the crystallite diameter is 20 nm or more and 45 nm or less, pressure is applied uniformly throughout the porous silicon when pressure is applied, making the pores unlikely to collapse even when pressure is applied.

[0013] In addition, pores with a pore diameter in the range of 5 nm or more and less than 300 nm are pores that are difficult to collapse when pressurized. According to the above configuration, in the pore diameter range of 5 nm or more and less than 300 nm in the pore distribution obtained by measurement using mercury intrusion porosimetry, the pore volume is 0.60 cm 3 / g or more, and the pore volume of pores having a pore diameter in the range of 5 nm or more and less than 300 nm, which are difficult to collapse when pressurized, is large.

[0014] Thus, according to the above-mentioned configuration, the crystallite diameter is 20 nm or more and 45 nm or less, and the pore volume in the range of the pore diameter is 5 nm or more and less than 300 nm is 0.60 cm 3 / g or more, a high pore volume can be maintained even under pressure.

[0015] Furthermore, the present invention provides the following: [2] The pore size distribution measured by mercury intrusion porosimetry shows that the total pore volume is 2.0 cm 3 / g or more 4.5cm 3 / g or less.

[0016] The total pore volume after pressurization will not be higher than the total pore volume before pressurization. Therefore, the total pore volume before pressurization is preferably high. 3 When the pore size is 1 / g or more, the total pore volume after pressure can be made higher.

[0017] Furthermore, the present invention provides the following: [3] Particle diameter D 50 The porous silicon according to [1] or [2], wherein the average particle size is 0.3 μm or more and 5.0 μm or less.

[0018] Particle diameter D 50 When the thickness is 0.3 μm or more and 5.0 μm or less, an electrode (particularly, a negative electrode) can be suitably produced. Specifically, particle diameter D 50 When the particle size is 0.3 μm or more, the aggregation of the Si particles is not too strong and is moderate, making it easy to produce a uniform electrode. In addition, the particle diameter D 50 When the particle size is 5.0 μm or less, a good Li conductive path is likely to be obtained when used as the negative electrode of a lithium battery. In addition, the particle diameter D 50 If the thickness is 5.0 μm or less, the structure of Si is less likely to be destroyed when a volume change occurs. This point will be explained below. The amount of volume change often differs between the inside and outside of a particle. If the amount of volume change differs between the inside and outside of a particle, stress will be generated due to the difference in the amount of volume change. Particle diameter D 50 When the particle diameter D 50 When the thickness is 5.0 μm or less, the stress due to the difference in the amount of volume change can be kept small, and the destruction of the Si structure can be made less likely to occur. In addition, the particle diameter D 50 When the thickness is 5.0 μm or less, the resistance can be reduced when an electrode is fabricated.

[0019] Furthermore, the present invention provides the following: [4] The porous silicon according to any one of [1] to [3], wherein the silicon content is 70% by mass or more and 95% by mass or less, when the entire porous silicon is taken as 100% by mass.

[0020] If the silicon content is 70 mass % or more, a higher capacity can be achieved when used as an electrode.

[0021] Furthermore, the present invention provides the following: [5] A step A includes oxidizing magnesium silicide in the atmosphere by raising the temperature from room temperature to 700°C or higher to obtain a porous silicon precursor, which is a composite particle of magnesium oxide and silicon; The method for producing porous silicon according to any one of [1] to [4], wherein the temperature increase in step A is at a rate of 5°C / hour or more and 100°C / hour or less in the temperature range of 450°C or more and 700°C or less.

[0022] As a result of intensive research, the present inventors have found that in the baking process of magnesium silicide, the slower the heating rate from 450°C to 700°C, the higher the pore volume of the resulting porous silicon with a pore diameter of 5 nm to less than 300 nm and the smaller the crystallite diameter of the resulting porous silicon tends to be.Then, if the heating rate in the temperature range of 450°C to 700°C is set to 5°C / hour to 100°C / hour, the porous silicon of [1] above, i.e., the porous silicon with a crystallite diameter of 20 nm to 45 nm and a pore volume of 0.60 cm3 in the pore diameter range of 5 nm to less than 300 nm, can be obtained. 3 / g or more 1.60cm 3 It has been found that porous silicon having a SiO 2 content of 1 / g or less can be obtained. According to the configuration [5], in the temperature increase in the step A, the temperature increase rate is 5°C / hour or more and 100°C / hour or less in the temperature range of 450°C or more and 700°C or less, so that the crystallite diameter is 20 nm or more and 45 nm or less, and the pore volume in the pore diameter range of 5 nm or more and less than 300 nm is 0.60 cm 3 / g or more 1.60cm 3 Porous silicon having a density of 1 / g or less can be obtained.

[0023] Furthermore, the present invention provides the following: [6] The method for producing porous silicon according to [5], wherein the molar ratio of magnesium to silicon (Mg / Si) in the magnesium silicide is 2.0 or more and 3.0 or less.

[0024] As a result of extensive research, the inventors have found that the larger the molar ratio (Mg / Si), the lower the distribution density of Si atoms in the magnesium silicide particles, and therefore the more improved the porosity of the resulting porous silicon. 50 However, it was found to be smaller. Therefore, when the molar ratio (Mg / Si) is 2.0 or more and 3.0 or less, the crystallite diameter is 20 nm or more and 45 nm or less, and the pore volume in the pore diameter range of 5 nm or more and less than 300 nm is 0.60 cm 3 / g or more 1.60cm 3 / g or less can be more suitably obtained.

[0025] Furthermore, the present invention provides the following: [7] The method for producing porous silicon according to [5] or [6], further comprising a step B of adding an acid to the porous silicon precursor obtained in the step A and removing magnesium oxide.

[0026] When an acid is added to the porous silicon precursor obtained in the step A to remove the magnesium oxide, porous silicon from which the magnesium oxide has been removed (porous silicon) can be obtained. [Effects of the Invention]

[0027] According to the present invention, it is possible to provide porous silicon that has a large pore volume even under pressure, and also to provide a method for producing such porous silicon. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is an SEM image of the powder of Example 1. [Figure 2] 1 is an SEM image of the powder of Example 4. [Figure 3] 1 shows the pore size distribution of the powder of Example 1. [Figure 4] 1 shows the pore size distribution of the powder of Example 5. [Figure 5]1 shows the pore size distribution of the powder of Example 10. [Figure 6] 1 shows the pore distribution of the powder of Comparative Example 1. [Figure 7] 1 shows the pore distribution of the powder of Example 1 after pressing. [Figure 8] 1 shows the pore distribution of the powder of Example 5 after pressing. [Figure 9] 1 shows the pore size distribution of the powder of Example 10 after pressing. [Figure 10] 1 shows the pore distribution of the powder of Comparative Example 1 after pressing. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to these embodiments. In this specification, the expressions "contain" and "comprise" include the concepts of "contain," "comprise," "substantially consist," and "consist only of."

[0030] [Porous silicon] An example of the porous silicon according to this embodiment will be described below, but the porous silicon of the present invention is not limited to the following example.

[0031] The porous silicon according to this embodiment is The crystallite diameter is 20 nm or more and 45 nm or less, In the pore size distribution obtained by mercury intrusion porosimetry, the pore volume is 0.60 cm in the pore diameter range of 5 nm or more and less than 300 nm. 3 / g or more 1.60cm 3 / g or less.

[0032] The porous silicon has a crystallite diameter of 20 nm or more and 45 nm or less, and therefore the pores are unlikely to collapse even when pressure is applied.

[0033] The crystallite size is preferably 21 nm or more, and more preferably 22 nm or more. The crystallite size is preferably 40 nm or less, more preferably 38 nm or less. The crystallite size is preferably 21 nm or more and 40 nm or less, and more preferably 22 nm or more and 38 nm or less.

[0034] Pores with a diameter in the range of 5 nm or more and less than 300 nm are pores with a size that are difficult to collapse when pressure is applied. The porous silicon has a pore volume of 0.60 cm within a pore diameter range of 5 nm or more and less than 300 nm in a pore distribution determined by measurement using mercury intrusion porosimetry. 3 / g or more, and the pore volume of pores having a pore diameter in the range of 5 nm or more and less than 300 nm, which are difficult to collapse when pressurized, is large.

[0035] The pore volume in the pore diameter range of 5 nm or more and less than 300 nm is preferably 0.70 cm 3 / g or more, more preferably 0.80 cm 3 / g or more. The larger the pore volume in the pore diameter range of 5 nm or more and less than 300 nm, the more preferable. 3 / g or less, 1.60cm 3 / g or less, 1.50cm 3 / g or less. The pore volume in the pore diameter range of 5 nm or more and less than 300 nm is preferably 0.70 cm 3 / g or more 1.70cm 3 / g or less, more preferably 0.80 cm 3 / g or more 1.60cm 3 / g or less, more preferably 0.80 cm 3 / g or more 1.50cm 3 / g or less.

[0036] The porous silicon has a total pore volume of 0.50 cm after being pressed at 2.5 kN. 3 / g or more 0.80cm 3 / g or less. After compaction at 2.5 kN, the total pore volume is preferably 0.50 cm 3 / g or more, it can be said that the pore volume is high even under pressure. The total pore volume after the above-mentioned pressure molding at 2.5 kN is the total pore volume in the pore distribution determined by measurement using mercury intrusion porosimetry. In addition, in this specification, the total pore volume refers to the volume of pores in the pore diameter range of 5 nm to 94,890 nm. The specific method for producing a molded body for measuring the total pore volume after molding under pressure at 2.5 kN is the method described in the Examples.

[0037] The total pore volume after pressing at 2.5 kN is preferably 0.55 cm 3 / g or more, and more preferably 0.60 cm 3 / g or more. The larger the total pore volume after compaction at 2.5 kN, the more preferable. For example, 3 / g or less, 0.75cm 3 / g or less. The total pore volume after pressing at 2.5 kN is preferably 0.55 cm 3 / g or more 0.80cm 3 / g or less, and more preferably 0.60 cm 3 / g or more 0.75cm 3 / g or less.

[0038] The porous silicon has a total pore volume of 2.0 cm in a pore distribution determined by mercury intrusion porosimetry. 3 / g or more 4.5cm 3 / g or less is preferable. The total pore volume after pressurization will not be higher than the total pore volume before pressurization. Therefore, the total pore volume before pressurization is preferably high. 3 When the pore size is 1 / g or more, the total pore volume after pressure can be made higher.

[0039] The total pore volume before pressing is preferably 2.2 cm 3 / g or more, more preferably 2.5cm 3 / g or more. The larger the total pore volume before pressure is, the more preferable. For example,3 / g or less, 3.5cm 3 / g or less. The total pore volume before pressing is preferably 2.2 cm 3 / g or more 4.1cm 3 / g or less, more preferably 2.5 cm 3 / g or more 3.5cm 3 / g or less.

[0040] The porous silicon has a pore volume of 1.30 cm within a pore diameter range of 5 nm or more and less than 2000 nm in a pore distribution determined by measurement using mercury intrusion porosimetry. 3 / g or more 2.60cm 3 / g or less is preferable. As described above, the total pore volume after pressurization will not be higher than the total pore volume before pressurization. Therefore, it is preferable that the pore volume of pores having a diameter of 5 nm or more and less than 2000 nm before pressurization is also high. When the pore volume of pores having a diameter of 5 nm or more and less than 2000 nm before pressurization is 1.30 cm 3 When the pore size is 1 / g or more, the total pore volume after pressure can be made higher.

[0041] The pore volume of the pores having a diameter of 5 nm or more and less than 2000 nm before pressure is preferably 1.50 cm 3 / g or more, more preferably 1.70 cm 3 / g or more. The pore volume of the pores in the range of pore diameters of 5 nm or more and less than 2000 nm before pressure is preferably larger. For example, 3 / g or less, 2.20cm 3 / g or less. The pore volume of the pores having a diameter of 5 nm or more and less than 2000 nm before pressure is preferably 1.50 cm 3 / g or more 2.60cm 3 / g or less, more preferably 1.70 cm 3 / g or more 2.20cm 3 / g or less.

[0042] The porous silicon has a particle diameter D 50It is preferable that the particle diameter D is 0.3 μm or more and 5.0 μm or less. 50 When the thickness is 0.3 μm or more and 5.0 μm or less, an electrode (particularly, a negative electrode) can be suitably produced. Specifically, particle diameter D 50 When the particle size is 0.3 μm or more, the aggregation of the Si particles is not too strong and is moderate, making it easy to produce a uniform electrode. In addition, the particle diameter D 50 When the particle size is 5.0 μm or less, a good Li conductive path is likely to be obtained when used as the negative electrode of a lithium battery. In addition, the particle diameter D 50 If the thickness is 5.0 μm or less, the structure of Si is less likely to be destroyed when a volume change occurs. This point will be explained below. The amount of volume change often differs between the inside and outside of a particle. If the amount of volume change differs between the inside and outside of a particle, stress will be generated due to the difference in the amount of volume change. Particle diameter D 50 When the particle diameter D 50 When the thickness is 5.0 μm or less, the stress due to the difference in the amount of volume change can be kept small, and the destruction of the Si structure can be made less likely to occur. In addition, the particle diameter D 50 When the thickness is 5.0 μm or less, the resistance can be reduced when an electrode is fabricated.

[0043] The particle diameter D 50 is more preferably 0.35 μm or more, and even more preferably 0.40 μm or more. The particle diameter D 50 is more preferably 4.50 μm or less, and even more preferably 3.50 μm or less. The particle diameter D 50 is more preferably 0.35 μm or more and 4.50 μm or less, and further preferably 0.40 μm or more and 3.50 μm or less.

[0044] The porous silicon has a BET specific surface area of ​​20 m 2 / g or more 100m 2 / g or less. 2 / g or more, the pore volume in the pore diameter range of 5 nm or more and less than 300 nm and the total pore volume can be made higher. 2 / g or more is preferable because when the porous silicon is used to prepare a negative electrode for a lithium ion battery, lithium insertion and desorption occurs more easily and uniformly.

[0045] The BET specific surface area is more preferably 40 m 2 / g or more, more preferably 50m 2 / g or more. The BET specific surface area is more preferably 98 m 2 / g or less, more preferably 95m 2 / g or less. The BET specific surface area is more preferably 40 m 2 / g or more 98m 2 / g or less, more preferably 50m 2 / g or more 95m 2 / g or less.

[0046] The porous silicon preferably has a silicon content of 70% by mass or more and 95% by mass or less, when the total mass of the porous silicon is taken as 100% by mass. A silicon content of 70% by mass or more can achieve a higher capacity when used as an electrode.

[0047] The silicon content in the porous silicon is more preferably 75% by mass or more, and even more preferably 80% by mass or more. The higher the silicon content in the porous silicon, the better, and it is, for example, 92 mass % or less, 90 mass % or less, etc. The silicon content in the porous silicon is more preferably 75% by mass or more and 92% by mass or less, and even more preferably 80% by mass or more and 90% by mass or less.

[0048] The components of the porous silicon other than silicon include oxygen that forms an oxide film, and water adsorbed on the particle surface.

[0049] The porous silicon according to this embodiment can be obtained, for example, by the following method for producing porous silicon. However, the method for producing porous silicon according to the present invention is not limited to the following method.

[0050] [Method of manufacturing porous silicon] An example of a method for producing porous silicon will now be described.

[0051] The method for producing porous silicon according to this embodiment includes the steps of: The method includes a step A of oxidizing magnesium silicide in the atmosphere by raising the temperature from room temperature to 700°C or higher to obtain a porous silicon precursor, which is a composite particle of magnesium oxide and silicon; The temperature increase in the step A is at a rate of 5°C / hour to 100°C / hour in the temperature range of 450°C to 700°C.

[0052] In the method for producing porous silicon according to this embodiment, magnesium silicide (Mg2Si) is first prepared. The magnesium silicide can be obtained by mixing magnesium powder and silicon powder and firing the mixture.

[0053] The magnesium powder is not particularly limited, and may have a particle diameter of, for example, 50 The thickness of the particle may be 1 μm or more and 500 μm or less. The purity of the magnesium powder is preferably high, for example, 99% by mass or higher.

[0054] The silicon powder is not particularly limited, and may have a particle diameter of, for example, 50 The thickness of the particle may be 1 μm or more and 10 μm or less. The purity of the silicon powder is preferably high, for example, 99% by mass or higher.

[0055] When a mixed powder of magnesium powder and silicon powder is fired (for example, at 900°C or higher and 1000°C or higher), these raw materials melt, so the particle size of the starting raw materials (magnesium powder, silicon powder) does not significantly affect the properties of the magnesium silicide or the porous silicon.

[0056] When the magnesium powder and the silicon powder are mixed, the ratio of the magnesium powder to the silicon powder is preferably such that the molar ratio of Mg to Si (Mg / Si) is in the range of 2.0 to 3.0, i.e., the molar ratio of magnesium to silicon (Mg / Si) in the magnesium silicide is preferably 2.0 to 3.0. As a result of extensive research, the inventors have found that the larger the molar ratio (Mg / Si), the lower the distribution density of Si atoms in the magnesium silicide particles, and therefore the more improved the porosity of the resulting porous silicon. 50 However, it was found to be smaller. Therefore, when the molar ratio (Mg / Si) is 2.0 or more and 3.0 or less, the crystallite diameter is 20 nm or more and 45 nm or less, and the pore volume in the pore diameter range of 5 nm or more and less than 300 nm is 0.60 cm 3 / g or more 1.60cm 3 / g or less can be more suitably obtained.

[0057] The molar ratio (Mg / Si) is more preferably 2.05 or more, and even more preferably 2.10 or more. The molar ratio (Mg / Si) is more preferably 2.90 or less, and even more preferably 2.80 or less. The molar ratio (Mg / Si) is more preferably 2.05 or more and 2.90 or less, and even more preferably 2.10 or more and 2.80 or less.

[0058] The magnesium powder and silicon powder may be mixed for a certain period of time so long as they are mixed uniformly to a certain extent, for example, by mixing them in a mortar for about 5 to 10 minutes.

[0059] The firing conditions for firing the mixed powder of magnesium powder and silicon powder are not particularly limited, but are preferably, for example, about 900°C to 1000°C or more and about 1 hour to 3 hours. The pressure during firing is preferably 1013.25 hPa to 10130.25 hPa. The atmosphere during firing is preferably an inert gas atmosphere such as a rare gas (e.g., argon).

[0060] The magnesium silicide obtained by firing may be in the form of a mass, in which case it may be crushed as necessary. Magnesium silicide particle diameter D 50 is not particularly limited, but is preferably 5 μm or more and 30 μm or less, and more preferably 10 μm or more and 20 μm or less. The particle diameter D 50 The grinding method for obtaining magnesium silicide having the formula (I) is not particularly limited, and may be, for example, grinding using a mortar for about 10 to 20 minutes.

[0061] <Process A> In step A, the magnesium silicide is oxidized in the atmosphere by raising the temperature from room temperature (25° C.) to 700° C. or higher to obtain a porous silicon precursor, which is composite particles of magnesium oxide and silicon. In this specification, "in the atmosphere" refers to an air atmosphere and atmospheric pressure (1013.25 hPa).

[0062] The temperature is increased at a rate of 5°C / hour to 100°C / hour in the temperature range of 450°C to 700°C. As a result of intensive research, the present inventors have found that in the baking process of magnesium silicide, the slower the heating rate from 450°C to 700°C, the higher the pore volume of the resulting porous silicon with a pore diameter of 5 nm to less than 300 nm and the smaller the crystallite diameter of the resulting porous silicon tends to be.Then, when the heating rate in the temperature range of 450°C to 700°C is set to 5°C / hour to 100°C / hour, the porous silicon obtained, i.e., the porous silicon with a crystallite diameter of 20 nm to 45 nm and a pore volume in the pore diameter range of 5 nm to less than 300 nm, tends to be 0.60 cm. 3 / g or more 1.60cm 3 It has been found that porous silicon having a SiO 2 content of 1 / g or less can be obtained. In the temperature increase in the step A, the temperature increase rate is 5°C / hour or more and 100°C / hour or less in the temperature range of 450°C or more and 700°C or less, so that the crystallite diameter is 20 nm or more and 45 nm or less, and the pore volume in the pore diameter range of 5 nm or more and less than 300 nm is 0.60 cm 3 / g or more 1.60cm 3 Porous silicon having a density of 1 / g or less can be obtained.

[0063] The temperature rise rate in the temperature range of 450° C. or higher and 700° C. or lower is preferably 7° C. / hour or higher, and more preferably 10° C. / hour or higher. The temperature rise rate in the temperature range of 450° C. or higher and 700° C. or lower is preferably 80° C. / hour or lower, more preferably 50° C. / hour or lower. The temperature rise rate in the temperature range of 450° C. to 700° C. is preferably 7° C. / hour to 80° C. / hour, more preferably 10° C. / hour to 50° C. / hour.

[0064] The temperature rise rate in the temperature range of 450° C. to 700° C. may be constant during the temperature rise period, or may vary within the numerical range of the temperature rise rate.

[0065] The temperature increase does not affect the crystallite size or pore distribution in the temperature range from room temperature to 450°C, but from the viewpoint of production efficiency, the temperature increase rate is preferably 50°C / hour or more and 200°C / hour or less.

[0066] The temperature rise rate in the temperature range from room temperature to 450°C is more preferably 80°C / hour or more, and even more preferably 100°C / hour or more. The temperature rise rate in the temperature range from room temperature to 450°C is more preferably 180°C / hour or less, and even more preferably 150°C / hour or less. The temperature rise rate in the temperature range from room temperature to 450°C is more preferably 80°C / hour to 180°C / hour, and even more preferably 100°C / hour to 150°C / hour.

[0067] The temperature rise rate in the temperature range from room temperature to 450° C. may be constant during the temperature rise period, or may vary within the numerical range of the temperature rise rate.

[0068] The temperature may be increased by maintaining a constant temperature for 1 hour to 10 hours in the temperature range of 450° C. to 550° C. If the temperature is maintained for 1 hour to 10 hours in the temperature range of 450° C. to 550° C., the crystallite size becomes smaller compared to when the temperature is not maintained.

[0069] The time for which the constant temperature is maintained is more preferably 2 hours or more, and even more preferably 3 hours or more. The time for which the constant temperature is maintained is more preferably 8 hours or less, and even more preferably 5 hours or less. The time for maintaining the constant temperature is more preferably 2 hours or more and 8 hours or less, and even more preferably 3 hours or more and 5 hours or less.

[0070] The temperature (temperature range) at which the constant temperature is maintained is more preferably 480°C or higher, and even more preferably 500°C or higher. The temperature at which the constant temperature is maintained is more preferably 540°C or lower, and even more preferably 520°C or lower. The temperature at which the constant temperature is maintained is more preferably 480°C or higher and 540°C or lower, and even more preferably 500°C or higher and 520°C or lower.

[0071] The temperature reached in the final stage of the temperature increase is not particularly limited as long as it is equal to or higher than 700° C. Since the final temperature reached in the temperature increase is equal to or higher than 700° C., the magnesium silicide can be suitably oxidized, and a porous silicon precursor, which is composite particles of magnesium oxide and silicon, can be suitably obtained. The final temperature reached in the temperature increase may be 700°C or higher, but is preferably 800°C or lower because impurities such as silicates are generated at temperatures around 800°C.

[0072] The final temperature reached in the temperature increase is more preferably 710°C or higher, and even more preferably 720°C or higher. The final temperature reached in the temperature increase is more preferably 750°C or lower, and even more preferably 740°C or lower. The final temperature reached in the temperature increase is more preferably 710°C or higher and 750°C or lower, and even more preferably 720°C or higher and 740°C or lower.

[0073] The calcination time after the final temperature is reached is not particularly limited, but is preferably 1 hour or longer. When the calcination time after the final temperature is reached is 1 hour or longer, more sufficient calcination is achieved, and a porous silicon precursor can be suitably obtained.

[0074] The firing time after the final temperature is reached is more preferably 2 hours or more, and even more preferably 3 hours or more. The firing time after the final temperature is reached is more preferably 10 hours or less, and even more preferably 5 hours or less. The firing time after the final temperature is reached is more preferably 2 hours or more and 10 hours or less, and even more preferably 3 hours or more and 5 hours or less.

[0075] After firing, the product is cooled to room temperature. The method for cooling to room temperature is not particularly limited, and may be, for example, natural cooling (natural cooling to room temperature without removing from the furnace).

[0076] <Process B> In step B, an acid is added to the porous silicon precursor obtained in step A to remove magnesium oxide.

[0077] The acid is not particularly limited as long as it can dissolve and remove magnesium oxide, and examples thereof include hydrochloric acid, nitric acid, etc. When hydrochloric acid is used as the acid, the concentration is not particularly limited as long as it is within a range that can remove magnesium oxide, and may be, for example, within a range of 0.1 mol / L to 7.7 mol / L.

[0078] The acid is added to the porous silicon precursor, and the magnesium oxide is removed, whereby the magnesium oxide dissolves in the acid to obtain a porous silicon dispersion. Thereafter, porous silicon (powder) can be obtained by a conventional method (e.g., filtration) as needed.

[0079] The method for producing porous silicon according to this embodiment has been described above. [Example]

[0080] The present invention will be described in detail below using examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention.

[0081] The maximum and minimum contents of each component shown in the following examples should be considered as the preferred minimum and maximum contents of the present invention, regardless of the contents of other components. Furthermore, the maximum and minimum values ​​of the measured values ​​shown in the following examples should be considered to be the preferred minimum and maximum values ​​of the present invention, regardless of the content (composition) of each component.

[0082] [Fabrication of porous silicon] Example 1 6.3 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 3.7 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=2.0). The resulting mixed powder was fired at 1000°C in an argon atmosphere to obtain magnesium silicide in the form of a lump. The obtained magnesium silicide was cooled naturally to room temperature and then crushed in a mortar to obtain magnesium silicide powder.

[0083] 10 g of the obtained magnesium silicide powder was placed in a magnesia sheath, and the temperature was raised in the air according to the following procedure to obtain a porous silicon precursor. <Heating procedure> 1: Heat from room temperature to 450°C at a rate of 100°C / hour 2: Heat from 450℃ to 500℃ at a rate of 20℃ / hour 3. Baked at 500°C for 2 hours 4: Heat from 500℃ to 700℃ at a rate of 20℃ / hour 5: Firing at 700℃ for 3 hours 6: After firing in step 5, let the temperature drop naturally to room temperature without removing it from the furnace.

[0084] The obtained porous silicon precursor was added to 600 mL of 1 mol / L hydrochloric acid to dissolve the magnesium oxide, thereby obtaining a porous silicon dispersion liquid. The dispersion liquid was filtered to obtain the porous silicon (powder) according to Example 1.

[0085] Example 2 Porous silicon according to Example 2 was obtained in the same manner as in Example 1, except that the temperature-raising procedure was changed as follows. <Heating procedure> 1: Heat from room temperature to 450°C at a rate of 100°C / hour 2: Heat from 450℃ to 700℃ at a rate of 10℃ / hour 3: Firing at 700℃ for 3 hours 4: After firing in step 3, let the temperature drop naturally to room temperature without removing it from the furnace.

[0086] Example 3 Porous silicon according to Example 3 was obtained in the same manner as in Example 2, except that step 2 of the temperature increase procedure in Example 2, "heat increase from 450°C to 700°C at a rate of 10°C / hour," was changed to "heat increase from 450°C to 700°C at a rate of 20°C / hour."

[0087] Example 4 Porous silicon according to Example 4 was obtained in the same manner as in Example 2, except that step 2 of the temperature increase procedure in Example 2, "heat increase from 450°C to 700°C at a rate of 10°C / hour," was changed to "heat increase from 450°C to 700°C at a rate of 50°C / hour."

[0088] Example 5 Porous silicon according to Example 5 was obtained in the same manner as in Example 2, except that step 2 of the temperature increase procedure in Example 2, "heat increase from 450°C to 700°C at a rate of 10°C / hour," was changed to "heat increase from 450°C to 700°C at a rate of 80°C / hour."

[0089] Example 6 Porous silicon according to Example 6 was obtained in the same manner as in Example 2, except that 6.5 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 3.5 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=2.1).

[0090] Example 7 Porous silicon according to Example 7 was obtained in the same manner as in Example 2, except that 6.7 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 3.3 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=2.3).

[0091] Example 8 Porous silicon according to Example 8 was obtained in the same manner as in Example 2, except that 6.8 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 3.2 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=2.5).

[0092] Example 9 Porous silicon according to Example 9 was obtained in the same manner as in Example 2, except that 6.9 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 3.1 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=2.6).

[0093] Example 10 Porous silicon according to Example 10 was obtained in the same manner as in Example 2, except that 7.0 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 3.0 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=2.7).

[0094] Example 11 Porous silicon according to Example 11 was obtained in the same manner as in Example 2, except that 7.1 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 2.9 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=2.8).

[0095] Example 12 Porous silicon according to Example 12 was obtained in the same manner as in Example 2, except that 7.2 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 2.8 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=3.0).

[0096] Example 13 Porous silicon of Example 13 was obtained in the same manner as in Example 10, except that step 2 of the temperature increase procedure in Example 10, "heat increase from 450°C to 700°C at a rate of 10°C / hour," was changed to "heat increase from 450°C to 700°C at a rate of 80°C / hour."

[0097] (Comparative Example 1) Silicon according to Comparative Example 1 was obtained in the same manner as in Example 1, except that the temperature-raising procedure was changed as follows. <Heating procedure> 1: Heat from room temperature to 700°C at a rate of 120°C / hour 2. Baked at 700℃ for 3 hours

[0098] (Comparative Example 2) Silicon according to Comparative Example 2 was obtained in the same manner as in Example 2, except that 6.0 g of commercially available metallic magnesium powder (Mitsuwa Chemical, purity 99%) and 4.0 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes (molar ratio Mg / Si=1.7).

[0099] (Comparative Example 3) 8.8 g of commercially available aluminum powder (Wako Pure Chemical Industries, purity 99.5%) and 1.2 g of commercially available silicon powder (Mitsuwa Chemical, purity 99.9%) were mixed in a mortar for 5 minutes. The obtained mixed powder was placed in a crucible and fired at 1200°C for 3 hours in an air atmosphere (heating rate up to 1200°C: 100°C / hour), and then rapidly cooled at room temperature (the sample was removed from the furnace and cooled at room temperature) to obtain a massive Al-Si alloy. The resulting Al-Si alloy was pulverized in a mortar and added to 1000 mL of 1 mol / L hydrochloric acid to dissolve the aluminum, yielding a porous silicon dispersion. The dispersion was filtered, and the resulting powder was added again to 1000 mL of 1 mol / L hydrochloric acid to completely dissolve the aluminum, yielding a porous silicon dispersion. The final dispersion was filtered to obtain porous silicon according to Comparative Example 3.

[0100] Comparative Example 4 A commercially available metal silicon powder (Mitsuwa Chemical, purity: 99.9%) was prepared as silicon according to Comparative Example 4.

[0101] [SEM observation] The surfaces of the powders of Examples 1 and 4 were observed using an SEM (scanning electron microscope, manufactured by JEOL Ltd., product name: JSM-6700F). Figure 1 shows an SEM image of the powder (porous silicon) of Example 1. Figure 2 shows an SEM image of the powder (porous silicon) of Example 4.

[0102] [Measurement of pore volume before pressure application] For the powders of the examples and comparative examples, the pore size distribution was measured by mercury intrusion porosimetry using a pore size distribution measuring device ("Autopore IV9500" manufactured by Micromeritics). The measurement conditions were as follows: <Measurement conditions> Measurement equipment: Pore size distribution measurement equipment (Micromeritics Autopore IV9500) Measurement range: 5nm to 94890nm Number of measurement points: 120 points Mercury contact angle: 140degrees Mercury surface tension: 480dyne / cm

[0103] Using the obtained pore distribution, the total pore volume, the pore volume of pores with a pore diameter of 5 nm or more and less than 300 nm, and the pore volume of pores with a pore diameter of 5 nm or more and less than 2000 nm were calculated. The results are shown in Table 1. 3 shows the pore distribution of the powder of Example 1, FIG. 4 shows the pore distribution of the powder of Example 5, FIG. 5 shows the pore distribution of the powder of Example 10, and FIG. 6 shows the pore distribution of the powder of Comparative Example 1.

[0104] The results of Examples 2 to 5 show that the faster the heating rate from 450°C to 700°C, the smaller the pore volume of pores with a diameter of 5 nm or more and less than 300 nm tends to be. In Example 2 and Example 3, the pore volume of pores with a diameter of 5 nm or more and less than 300 nm in Example 3 is slightly larger than the pore volume of pores with a diameter of 5 nm or more and less than 300 nm in Example 2. This is thought to be within the margin of error due to the similar heating rates in Example 2 and Example 3. Comparing Examples 2 and 3 with Examples 4 and 5, it can be seen that the faster the heating rate from 450°C to 700°C, the smaller the pore volume of pores with diameters of 5 nm or more and less than 300 nm tends to be.

[0105] Furthermore, the results of Examples 6 to 12 show that the larger the molar ratio (Mg / Si), the larger the volume of pores with a pore diameter of 5 nm or more and less than 300 nm tends to be. Although Examples 11 and 12 have a larger molar ratio (Mg / Si) than Example 10, the pore volume of pores with a pore diameter of 5 nm or more and less than 300 nm is smaller than that of Example 10. This is because when the molar ratio (Mg / Si) is excessively large, for example, when Mg / Si exceeds 2.7, the oxidation reaction is locally promoted by the oxidation heat of excess Mg during the magnesium silicide firing process, making it difficult to uniformly oxidize the magnesium silicide by controlling the temperature rise rate.

[0106] [Measurement of pore volume after pressure] The powders of the examples and comparative examples were pressed into pellets at 2.5 kN (125 MPa). The pore size distribution of the resulting pellets was measured by mercury intrusion porosimetry using a pore size distribution analyzer (Autopore IV9500, manufactured by Micromeritics). The measurement conditions were the same as those for "Measurement of pore volume before pressing."

[0107] The total pore volume after pressing was calculated using the obtained pore distribution. The results are shown in Table 1. Fig. 7 shows the pore distribution of the powder of Example 1 after pressing, Fig. 8 shows the pore distribution of the powder of Example 5 after pressing, Fig. 9 shows the pore distribution of the powder of Example 10 after pressing, and Fig. 10 shows the pore distribution of the powder of Comparative Example 1 after pressing.

[0108] [Crystallite size measurement] Powder X-ray diffraction measurements were performed using a Rigaku Ultima IV X-ray diffractometer using Cu-Kα, and the diffraction peak at half-width at 2θ = 28.4°, which is the diffraction peak assigned to the Si(111) crystal plane, was calculated using the Scherrer equation. The results are shown in Table 1.

[0109] The results of Examples 2 to 5 show that the slower the rate of temperature increase from 450° C. to 700° C., the smaller the crystallite diameter of the resulting porous silicon.

[0110] Furthermore, from the results of Examples 6 to 12, it can be seen that the larger the molar ratio (Mg / Si), the smaller the crystallite size tends to be. In addition, the crystallite diameter of Example 8 is slightly larger than that of Example 7. This is considered to be within the error range due to the fact that the molar ratios (Mg / Si) of Examples 7 and 8 are similar. Furthermore, the crystallite diameters are close to each other in Examples 10 to 12. This is thought to be within the error range due to the fact that the molar ratios (Mg / Si) in Examples 10 to 12 are close to each other. Comparing Example 6, Example 7, Example 8, Example 9, and Examples 10 to 12, it can be seen that the larger the molar ratio (Mg / Si), the smaller the crystallite size tends to be.

[0111] [Particle diameter D 50 Measurement of 0.10 g of the powder of each of the Examples and Comparative Examples and 40 ml of a 0.2% aqueous solution of sodium hexametaphosphate were placed in a 50 ml beaker and dispersed for 3 minutes using an ultrasonic homogenizer "Advanced Digital Sonifier" (Emerson Japan Co., Ltd.), and then the mixture was placed in a laser diffraction particle size distribution analyzer ("LA-950" manufactured by Horiba, Ltd.) and measured. The results are shown in Table 1.

[0112] From the results of Examples 6 to 12, it was found that the larger the molar ratio (Mg / Si), the smaller the particle diameter D 50 It can be seen that there is a tendency for it to become smaller. In Examples 9 to 12, the particle diameter D 50 This is thought to be within the error range due to the fact that the molar ratios (Mg / Si) in Examples 9 to 12 are close to each other. Comparison of Example 6, Example 7, Example 8, and Example 9 to Example 12 shows that the particle diameter D 50 It can be seen that there is a tendency for it to become smaller.

[0113] [Measurement of specific surface area] The specific surface areas of the powders of the examples and comparative examples were measured by the BET method using a specific surface area meter ("Macsorb" manufactured by Mountec). The results are shown in Table 1.

[0114] [Silicon content measurement] The powders of the examples and comparative examples were dissolved in a sodium hydroxide solution, and then the compositions were analyzed using an ICP emission spectrometer (Hitachi High-Technologies PS-3520). The Si content (mass%) is shown in Table 1.

[0115] [Measurement of confining pressure increase (rate of electrode thickness increase)] 1. Creating a solid electrolyte layer 0.4 g of sulfide solid electrolyte (Li2S-P2S5-based glass ceramic), 0.05 g of a heptane solution containing 5 wt% butylene rubber binder (butadiene rubber), and heptane were added to a polypropylene container and stirred for 30 seconds using an ultrasonic disperser (UH-50, manufactured by SMT). The container was then shaken for 30 minutes using a shaker (TTM-1, manufactured by Shibata Scientific Co., Ltd.). The mixture was applied to a release sheet (Al foil) using an applicator by the blade method and dried on a hot plate at 100°C for 30 minutes. This resulted in a transfer member having a release sheet and a solid electrolyte layer.

[0116] 2. Preparation of the Positive Electrode The positive electrode active material (LiNi 1 / 3 Co 1 / 3 Al 1 / 3 2 g of O2), 0.3 g of a sulfide solid electrolyte (Li2S-P2S5-based glass ceramic), 0.03 g of a conductive material (VGCF (registered trademark) (manufactured by Resonac)), 0.3 g of a butyl butyrate solution containing 5 wt% PVDF (Polyvinylidene Fluoride), and butyl butyrate were added and stirred for 30 seconds using an ultrasonic disperser (UH-50, manufactured by SMT). Next, the container was shaken for 3 minutes using a shaker (TTM-1, manufactured by Shibata Scientific Co., Ltd.), further stirred for 30 seconds using the ultrasonic disperser, and then shaken for 3 minutes using the shaker. Using an applicator, the mixture was applied to a positive electrode current collector (Al foil, manufactured by Showa Denko) using the blade method and dried for 30 minutes on a hot plate at 100 °C. This resulted in a positive electrode comprising a positive electrode current collector and a positive electrode layer. The area of ​​the positive electrode was smaller than that of the negative electrode.

[0117] 3. Preparation of the Negative Electrode 0.8 g of the obtained electrode active material (porous silicon obtained in the Examples and Comparative Examples), 0.6 g of a sulfide solid electrolyte (Li2S-P2S5-based glass ceramic), 0.03 g of a conductive material (VGCF), 0.3 g of a butyl butyrate solution containing 5 wt% PVDF, and butyl butyrate were added to a polypropylene container and stirred for 30 seconds using an ultrasonic disperser (UH-50, manufactured by SMT). Next, the container was shaken for 30 minutes using a shaker (TTM-1, manufactured by Shibata Scientific Co., Ltd.). Using an applicator, the mixture was applied to a negative electrode current collector (Cu foil, manufactured by UACJ) by the blade method and dried on a hot plate at 100 °C for 30 minutes. This resulted in a negative electrode having a negative electrode current collector and a negative electrode layer.

[0118] 4. Battery Fabrication A transfer member was placed on the positive electrode layer of the positive electrode, and the transfer member was set in a roll press and pressed at 100 kN / cm and 165° C. This produced a first laminate having a configuration of “positive electrode / solid electrolyte layer / release sheet.”

[0119] Next, the negative electrode was set in a roll press and pressed at 60 kN / cm and 25°C. This resulted in a pressed negative electrode. After that, a solid electrolyte layer for bonding and a transfer member were arranged in this order from the negative electrode layer side. At this time, the solid electrolyte layer for bonding and the solid electrolyte layer on the transfer member were arranged so as to face each other. The obtained laminate was set in a planar uniaxial press and temporarily pressed at 100 MPa and 25°C for 10 seconds. Then, the release sheet was peeled off from the solid electrolyte layer. This resulted in a second laminate having a configuration of "negative electrode / solid electrolyte layer / release sheet."

[0120] Next, the solid electrolyte layer for bonding in the first laminate and the solid electrolyte layer in the second laminate were arranged to face each other, and then set in a flat uniaxial press and pressed at 200 MPa and 120°C for 1 minute. This resulted in an all-solid-state battery.

[0121] (measurement) The evaluation batteries of Examples 1 to 3, 5, and 10 and Comparative Examples 1 and 4 were CC / CV charged (constant current, constant voltage charged) at 0.245 mA to 4.55 V, and then CC / CV discharged (constant current, constant voltage discharged) at 0.245 mA to 3.0 V. During the initial charge, the confining pressure of the battery was monitored, and the confining pressure at 4.55 V was measured. The increase in confining pressure in the evaluation battery of Comparative Example 4 was set as 100%, and the results of a relative evaluation of the evaluation batteries of Examples 1 to 3, 5, 10, and 12 and Comparative Example 1 are shown in Table 1. In the examples, it can be seen that the increase in confining pressure (increase in electrode thickness) is less than 75%, and the volume change due to charge and discharge (expansion due to charging) is kept below a certain level.

[0122] [Table 1]

Claims

1. The crystallite diameter is 20 nm or more and 45 nm or less, In the pore size distribution determined by mercury intrusion porosimetry, the pore volume is 0.60 cm in the pore size range of 5 nm or more and less than 300 nm. ３ / g or more 1.60cm ３ / g or less.

2. In the pore distribution determined by mercury intrusion porosimetry, the total pore volume is 2.0 cm ３ / g or more 4.5cm ３ 2. The porous silicon according to claim 1, wherein the SiO 2 content is 0.15 / g or less.

3. Particle diameter D ５０ 3. The porous silicon according to claim 1, wherein the average particle size is 0.3 μm or more and 5.0 μm or less.

4. 3. The porous silicon according to claim 1, wherein the silicon content is 70% by mass or more and 95% by mass or less when the entire porous silicon is taken as 100% by mass.

5. The method includes a step A of oxidizing magnesium silicide by raising the temperature from room temperature to 700°C or higher in the atmosphere to obtain a porous silicon precursor, which is a composite particle of magnesium oxide and silicon; 2. The method for producing porous silicon according to claim 1, wherein the temperature increase in step A is at a rate of 5°C / hour to 100°C / hour in the temperature range of 450°C to 700°C.

6. 6. The method for producing porous silicon according to claim 5, wherein the molar ratio of magnesium to silicon (Mg / Si) in the magnesium silicide is 2.0 or more and 3.0 or less.

7. 7. The method for producing porous silicon according to claim 5, further comprising a step B of adding an acid to the porous silicon precursor obtained in the step A to remove magnesium oxide.

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