WO2016201611A1

WO2016201611A1 - Porous silicon particles and a method for producing silicon particles

Info

Publication number: WO2016201611A1
Application number: PCT/CN2015/081495
Authority: WO
Inventors: Jun Yang; Rongrong MIAO; Xiaolin Liu; Rongrong JIANG; Jingjun Zhang; Yuqian DOU
Original assignee: Robert Bosch Gmbh
Priority date: 2015-06-15
Filing date: 2015-06-15
Publication date: 2016-12-22
Also published as: CN107710464A; CN107710464B

Abstract

Porous silicon particles, a method for producing silicon particles, a silicon-carbon composite, an electrode material and a battery comprising said composite, and the use of the silicon-carbon composite as an electrode active material.

Description

POROUS SILICON PARTICLES AND A METHOD FOR PRODUCING SILICON PARTICLES

Technical Field

The present invention relates to porous silicon particles, a method for producing silicon particles, a silicon-carbon composite, an electrode material and a battery comprising said composite, and the use of the silicon-carbon composite as an electrode active material.

Background Art

As the exigent demand for high energy lithium ion batteries (LIB) used in portable devices, (hybrid) electric vehicles (HEV) and grid-scale stationary energy storage, silicon (Si) has attracted tremendous attention due to its ten times higher theoretical capacity than traditionally used graphite anodes. However, the main impediment of silicon as anode material is huge volumetric change during repeated lithiation/delithiation processes. Repeated huge volumetric change leads to Si pulverization, cracks of electrode and continuous solid electrolyte interface (SEI) growth, which lead to loss of electronic and ionic conductivity. To address such a problem, extensive research has been devoted. For instance, silicon with nano-scaled size could buffer the large (de) lithiation strains without fracture to some extent； other well-defined Si nanostructures including nanowires, nanotubes, porous structures and their composites with carbon materials have been proposed to alleviate volume expansion as well. However, except for pursuing long cycling life and excellent specific capacity of silicon, the cost for Si production is also a critical parameter to consider for its widespread application as anode material. As we all know, with the increase of hybrid electrical vehicles (HEVs) and electrical vehicles (EV) markets, the price appears to be another great challenge for LIB production. Therefore, in order to develop large-scale silicon as anode material, choosing cheap raw materials and scalable manufacturing processes is becoming a major focus of recent battery research.

Among the diversity of silicon manufacturing processes, magnesiothermic reduction method is of great potential for large-scalable production based on its cheap raw material Mg powder and simple devices. Diverse silicon with porous structure has been synthesized by magnesiothermic reduction method and exhibits good electrochemical performance. Nevertheless, owing to the exothermic nature of magnesiothermic reduction reaction, massive heat will be released during the reactive process and results in a much higher reaction temperature than the set one. In this case, once the reaction is large-scaled, the architectures of the silica precursor will be easily collapsed and agglomerated products will be simultaneously formed under a too high temperature. Meanwhile, some side reaction products Mg₂Si and Mg₂SiO₄ will significantly influence the electrochemical performance of the synthesized silicon. Therefore, it is highly critical to control the temperature in the magnesiothermic reduction reaction with an efficient but cost-effective method when large scale production is developed. Based on the problem mentioned above, many research groups focus on inducing salts as heat absorber to produce silicon via the magnesiothermic reduction method. Following are some prior art results:

Xianbo Jin, et al., “Electrochemical preparation of silicon and its alloys from solid oxides in molten calcium chloride” , Angewandte Chemie, 2004. 116 (6) : p. 751-754 firstly reported the electrochemical preparation of silicon from solid oxides in molten calcium chloride. In this report, the molten CaCl₂ serves as electrolyte at 850℃and the silica could be directly electroreduced to silicon by fabricating SiO₂ powder into a porous electrode.

Liu, X., et al., “A molten-salt route for synthesis of Si and Ge nanoparticles: chemical reduction of oxides by electrons solvated in salt melt” , Journal of Materials Chemistry, 2012. 22(12) : p. 5454-5459 reported a molten-salt route for synthesis of Si nanoparticles, in which the LiCl/KCl and NaCl/MgCl₂ eutectic molten salts act as a reaction “solvent” and provided a salt-melt liquid environment in the magnesiothermic reduction of silica. The growth of Si nanocrystals could be controlled by adjusting the temperature and the type of salt.

Luo, W., et al., “Efficient Fabrication of Nanoporous Si and Si/Ge Enabled by a Heat Scavenger in Magnesiothermic Reactions” , Scientific Reports, 2013. 3 reported an efficient method to fabricate nanoporous Si by employing NaCl as heat scavenger in magnesiothermic reactions with a weight ratio of silica to NaCl of 1: 10. The collapse of original porous structure of silica precursor will be minimized by the fusion of NaCl, which consumes a large amount of heat released by the magnesiothermic reaction.

Summary of Invention

Inspired by the above researches, the inventors of the present invention successfully develop a large-scale silicon production process to achieve a high reversible capacity by using an appropriate salt or composite salt as a heat absorbent and using the extremely price advantageous silica precursor in a thermal reduction process.

The present invention provides a method for producing silicon particles, said method including the following steps:

1) preparing a mixture of a silica source material, magnesium and/or aluminium powder as a reducing agent, and a salt or composite salt as a heat absorbent；

2) heating the mixture obtained from step 1) at a heating temperature of from the melting point of said reducing agent to lower than 800℃under a protective atmosphere；

3) removing said heat absorbent and the oxidation products of said reducing agent；

wherein the melting temperature of said salt or the liquidus temperature of said composite salt ranges from a temperature higher than the heating temperature of step 2) to 800℃.

The present invention, according to another aspect, relates to porous silicon particles, which have a bimodal pore size distribution of < 2 nm and 10–30 nm.

The present invention, according to another aspect, relates to a silicon-carbon composite, which comprises a carbon coating layer as well as the silicon particles according to the present invention.

The present invention, according to another aspect, relates to an electrode material, which comprises the silicon-carbon composite according to the present invention.

The present invention, according to another aspect, relates to a battery, which comprises the electrode material according to the present invention.

The present invention, according to another aspect, relates to the use of the silicon-carbon composite according to the present invention as an electrode active material.

Brief Description of Drawings

Each aspect of the present invention will be illustrated in more detail in conjunction with the accompanying drawings, wherein:

Figure 1 shows the XRD pattern of the silicon particles of Example 1；

Figure 2 shows the cycling performances of the silicon particles of Example 1 (E1) and Comparison Example 1 (CE1) ；

Figure 3 shows the cycling performances of the silicon particles of Examples 1 ～ 7 (E1 ～ E7) and Comparison Examples 1 ～ 3 (CE1 ～ CE3) ；

Figure 4 shows the XRD patterns of the silicon particles of Example 3 (E3) , Example 6 (E6) , Example 5 (E5) , Example 2 (E2) , and Comparison Example 2 (CE2) ；

Figure 5 shows the SEM images of the silicon particles of Comparison Example 2 (CE2) , Example 2 (E2) , Example 5 (E5) , Example 6 (E6) , and Example 3 (E3) ；

Figure 6 shows the XRD patterns of the silicon particles of Example 7 (E7) and Comparison Example 3 (E3) ；

Figure 7 shows (a) the SEM and (b) the TEM images of the silicon particles of Example 8；

Figure 8 shows the N₂-sorption isotherms of (a) the raw material porous SiO₂ and (b) the silicon particles of Example 8；

Figure 9 shows the pore size distribution of (a) the raw material porous SiO₂ and (b) the silicon particles of Example 8；

Figure 10 shows the cycling performances of the silicon particles of Examples 8 (E8) and the silicon-carbon composite of Example 9 (E9) ；

Figure 11 shows the rate capability of the silicon-carbon composite of Example 9.

Detailed Description of Preferred Embodiments

All publications, patent applications, patents and other references mentioned herein, if not otherwise indicated, are explicitly incorporated by reference herein in their entirety for all purposes as if fully set forth.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, will control.

When an amount, concentration, or other value or parameter is given as either a range, preferred range or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

The present invention relates to a method for producing silicon particles, said method including the following steps:

1) Preparing a mixture of a silica source material, a reducing agent, and a heat absorbent The way of preparing a mixture of a silica source material, a reducing agent, and a heat absorbent is not particularly limited. For example, the silica source material can be dispersed in an aqueous solution of the heat absorbent under stirring at room temperature. The mixture can be heated to 80℃under vigorous stirring, dried under vacuum at 90℃to remove water, and then homogenized by hand-milling in an agate mortar. Then the mixture and magnesium and/or aluminium powder can be ground together in an agate mortar.

In accordance with an embodiment of the method according to the present invention, the melting temperature of said salt or the liquidus temperature of said composite salt can be 660–800℃, preferably 665–790℃, more preferably 670–780℃, for example 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, or 770℃. In addition to the melting temperature or liquidus temperature, the type of said salt or composite salt is not particularly limited. For example, said salt or composite salt shall not be decomposed under the heating temperature of step 2) , and preferably can be inorganic salts or composite salts, more preferably inorganic halides. Said salt or composite salt preferably does not contain water of crystallization, or is not susceptible to be hydrated.

In accordance with another embodiment of the method according to the present invention, said heat absorbent can be one or more selected from the group consisting of KCl；

KCl/LiCl with a LiCl content of ≤ 25 mol. ％, preferably ≤ 20 mol. ％, more preferably ≤ 10 mol. ％, particular preferably ≤ 5 mol. ％； and

KCl/NaCl with a NaCl content of ≤ 30 mol. ％or 66–98 mol. ％, preferably ≤ 10 mol. ％or 85–95 mol. ％.

In accordance with another embodiment of the method according to the present invention, the weight ratio of said silica source material to said heat absorbent can be 3 : 7–7 : 3, preferably 2 : 3–3 : 2, more preferably 4 : 5–1 : 1, calculated on the basis of SiO₂ in said silica source material.

In accordance with another embodiment of the method according to the present invention, said silica source material can be one or more selected from the group consisting of zeolite, diatom, SiO₂ nanopowder, and porous SiO₂, preferably porous SiO₂, such as

350, available from EVONIK.

In accordance with another embodiment of the method according to the present invention, the amount of said reducing agent used can be 1–1.5 times, preferably 1–1 : 1.3 times, more preferably 1–1.1 times the stoichiometry according to the reaction between SiO₂ and said reducing agent.

2) Heating the mixture

In accordance with another embodiment of the method according to the present invention, in step 2) the mixture obtained from step 1) can be heated at a heating temperature of at least 2℃, preferably 5℃, more preferably 10℃higher than the melting point of said reducing agent, for 1–6 hours, preferably 2–3 hours, for example at a heating rate of 2℃/min, 5℃/min, or 10℃/min, under a protective atmosphere, for example Ar/H₂ (5 vol. ％) . The heating rate and the heating duration are not particularly limited. The furnace used here is not particularly limited. For examle an ordinary tube furnace can be used for heating the mixture obtained from step 1) . In case of large-scale production of silicon particles, it would be preferred to use a tube furnace rotatable along its longitudinal axis, namely a rotation furnace.

3) Removing said heat absorbent and the oxidation products of said reducing agent

Firstly the heat absorbent can be removed by immersing the product of step 2) into water and filtering, and can also be recycled by drying the filtrate. And then the oxidation products of said reducing agent can be removed by immersing the filter residue into 2 M HCl solution and stirred for 12 hours.

In accordance with another embodiment of the method according to the present invention, after step 3) HF can be used to rinse the product obtained from step 3) , so as to remove unreacted SiO₂ and/or SiO₂ newly grown on the surface of the silicon particles during step 3) . Especially in case that the secondary particle size of the silicon particles obtained from step 2) is relatively small, for example less than 0.5 μm, so that the surface of the silicon particles might be oxidized again during step 3) , it would be preferred to use HF to rinse the product obtained from step 3) . For example, the product obtained from step 3) can be immersed into 1 wt. ％HF/EtOH (10 vol. ％) solution and stirred for 15 min. Finally, the product can be washed with distilled water and ethanol until pH ＝ 7 and then dried under vacuum at 65℃for 10 hours. Otherwise, it would be not very necessary to use HF to rinse the product obtained from step 3) in case that the secondary particle size of the silicon particles obtained from step 2) is relatively large, for example greater than or equal to 0.5 μm.

The present invention further relates to porous silicon particles, which have a bimodal pore size distribution of < 2 nm and 10–30 nm.

In accordance with an embodiment of the porous silicon particles according to the present invention, said porous silicon particles have a BET specific surface area of greater than 300 m²/g, preferably greater than 400 m²/g, more preferably greater than 500 m²/g.

The micropores of < 2 nm in the porous silicon particles are supposed to be derived from porous SiO₂ as the silica source material. In addition, the BET specific surface area of the porous silicon particles is one order of magnitude greater than that of porous SiO₂ as the silica source material. The inventors of the present invention believe that porous SiO₂ as the silica source material may contain quite a number of closed micropores, and these closed micropores may be opened during the vigorous reduction reaction and may contribute to the specific surface area.

In accordance with another embodiment of the porous silicon particles according to the present invention, the primary particle size of said porous silicon particles can be 30–100 nm, preferably 35–80 nm； and the secondary particle size (agglomerate particle size) of said porous silicon particles can be 1–10 μm, prefaerably 3–6 μm.

In accordance with another embodiment of the porous silicon particles according to the present invention, the pore volume of said porous silicon particles can be 0.1–1.5 cm³/g.

In accordance with another embodiment of the porous silicon particles according to the present invention, said porous silicon particles can be prepared by the method according to the present invention, in case that porous SiO₂ is used as the silica source material.

In accordance with an embodiment of the silicon-carbon composite according to the present invention, the thickness of the carbon coating layer can be 1–10 nm.

Example 1 (E1) :

KCl (melting temperature: 771℃) was used as the heat absorbent. In particular, 0.5 g of nano-SiO₂ power (Aladdin Chemical, 15 nm) was firstly dispersed to an aqueous KCl solution (0.1 g/mL) under stirring at room temperature. The ratio of silica to KCl in weight was 30: 70. The mixture was heated to 80℃under vigorous stirring followed by drying under vacuum at 90℃to remove water. Dried nano-SiO₂/KCl powder was then homogenized by hand-milling in an agate mortar.

A mixture of above nano-SiO₂/KCl powder and magnesium powder (Sinopharm Chemical Reagent Co. Ltd, 100 ～ 200 mesh) was ground together in the agate mortar at a molar ratio of Mg/SiO₂ ＝ 2.0. Next, the obtained mixture was loaded in an alundum boat and placed in the constant temperature zone of a tube furnace. And then the furnace was heated from room temperature to 650℃at a rate of 2℃/min and kept at 650℃for 4 hours under Ar (95 vol. ％) /H₂ (5 vol. ％) mixed atmosphere. Finally, after cooling to room temperature, a uniform powder in yellow color was obtained.

The obtained product after magnesiothermic reduction was firstly immersed in water and filtered, where KCl can be recycled by drying the filtrate. And then the residue was immersed into 2 M HCl solution and stirred for 12 hours to remove MgO. To further remove small amount of unreacted and surface-grown SiO₂, 1 wt. ％HF/EtOH (10 vol. ％) solution was used and stirred for 15 min. Finally, silicon products were washed with distilled water and ethanol until pH ＝ 7 and then vacuum dried at 65℃for 10 hours.

Structural evaluation:

X-Ray Diffraction (XRD) was used to analyse the composition, the crystallinity and the crystal size of the products. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were employed to characterize the size and structure of the products. N₂-sorption isotherms were used to analyse the pore size distribution of the products.

Figure 1 shows the XRD pattern of the silicon particles of Example 1. It can be seen that there was no impurity in the silicon particles of Example 1.

Cells assembling and electrochemical evaluation:

The electrochemical performances of the as-prepared composites were tested using two-electrode coin-type cells. The working electrodes were prepared by pasting a mixture of active material, Super P conductive carbon black (40 nm, Timical) and styrene butadiene rubber/sodium carboxymethyl cellulose (SBR/SCMC, 3: 5 by weight) as binder at a weight ratio of 60: 20: 20. After coating the mixture onto pure Cu foil, the electrodes were dried, cut to Φ12 mm sheets, and then further dried at 60℃in vacuum for 4 hours. The CR2016 coin cells were assembled in an argon-filled glove box (MB-10 compact, MBraun) using 1 M LiPF₆/EC+DMC (1: 1 by volume, ethylene carbonate (EC) , dimethyl carbonate (DMC)) as electrolyte, including 10％Fluoroethylene carbonate (FEC) , ENTEK ET20-26 as separator, and pure lithium foil as counter electrode. The cycling performances were evaluated on a LAND battery test system (Wuhan Kingnuo Electronics Co., Ltd., China) at 25℃, wherein in the cycling performance test the coin cells were discharged at 100 mA g^-1 for the initial two cycles and at 300 mA g^-1 (Examples 1 ～ 7 and Comparison Examples 1 ～ 3) or 1000 mA g^-1 (Examples 8 ～ 9) for the following cycles. The cut-off voltage was 0.01 V versus Li/Li⁺ for discharge (Li insertion) and 1.2 V versus Li/Li⁺ for charge (Li extraction) .

Figures 2 and 3 show the cycling performance of the silicon particles of Example 1.

Comparative Example 1 (CE1) :

Comparative Example 1 was carried out similar to Example 1, except that no KCl was used as the heat absorbent.

Figures 2 and 3 show the cycling performance of the silicon particles of Comparison Example 1. It can be seen that the electrochemical performance of the silicon particles can be greatly enhanced by using a salt or composite salt as the heat absorbent.

Example 2 (E2) :

Example 2 was carried out similar to Example 1, except that 0.54 g of nano-SiO₂ power was used, and the weight ratio of silica to the heat absorbent was 45: 55.

Figure 3 shows the cycling performance of the silicon particles of Example 2. Figure 4 shows the XRD patterns of the silicon particles of Example 2. Figure 5 shows the SEM images of the silicon particles of Example 2.

Comparative Example 2 (CE2) :

Comparative Example 2 was carried out similar to Example 2, except that NaCl (melting temperature: 801℃) was used as the heat absorbent.

Figure 3 shows the cycling performance of the silicon particles of Comparative Example 2.

Figure 4 shows the XRD patterns of the silicon particles of Comparison Example 2. Figure 5 shows the SEM images of the silicon particles of Comparison Example 2.

It can be seen that the particle size of the silicon particles of Comparison Example 2 was too small, and the crystallinity and the crystal size of the silicon particles of Comparison Example 2 were relatively low.

The inventors of the present invention believed that the surface of the silicon particles of Comparison Example 2 was too active and might be oxidized before being used as the electrode material, even though HF was used to rinse the product.

Example 3 (E3) :

Example 3 was carried out similar to Example 2, except that KCl/NaCl with a NaCl content of 10 mol. ％was used as the heat absorbent, the liquidus temperature of which can be determined from the binary diagram of KCl/NaCl as about 720℃.

Figure 3 shows the cycling performance of the silicon particles of Example 3. Figure 4 shows the XRD patterns of the silicon particles of Example 3. Figure 5 shows the SEM images of the silicon particles of Example 3.

Example 4 (E4) :

Example 4 was carried out similar to Example 2, except that KCl/NaCl with a NaCl content of 72 mol. ％was used as the heat absorbent, the liquidus temperature of which can be determined from the binary diagram of KCl/NaCl as about 670℃.

Figure 3 shows the cycling performance of the silicon particles of Example 4.

Example 5 (E5) :

Example 5 was carried out similar to Example 2, except that KCl/NaCl with a NaCl content of 90 mol. ％was used as the heat absorbent, the liquidus temperature of which can be determined from the binary diagram of KCl/NaCl as about 715℃.

Figure 3 shows the cycling performance of the silicon particles of Example 5. Figure 4 shows the XRD patterns of the silicon particles of Example 5. Figure 5 shows the SEM images of the silicon particles of Example 5.

Example 6 (E6) :

Example 6 was carried out similar to Example 2, except that KCl/LiCl with a LiCl content of 5 mol. ％was used as the heat absorbent, the liquidus temperature of which can be determined from the binary diagram of KCl/LiCl as about 750℃.

Figure 3 shows the cycling performance of the silicon particles of Example 6. Figure 4 shows the XRD patterns of the silicon particles of Example 6.

The crystal size of the silicon particles can be calculated based on the XRD patterns according to Scherrer formula. The sequence of the crystal size of the silicon particles were: CE2 < E2≈E5 < E6 < E3.

Figure 5 shows the SEM images of the silicon particles of Example 6. The particle size can be measured based on the SEM images of the silicon particles.

Table 1

	E2	E3	E5	E6	CE2
	E2	E3	E5	E6	CE2	Reversible Capacity (mAh/g)	2101	2951	2819	2716	1809
1^st Coulombic Efficiency	67.5％	86.2％	71.0％	79.0％	56.0％	Reversible Capacity (mAh/g)	2101	2951	2819	2716	1809
1^st Coulombic Efficiency	67.5％	86.2％	71.0％	79.0％	56.0％	Particle Size (nm)	30	100	35	70	10

Example 7 (E7) :

Example 7 was carried out similar to Example 2, except that 3.83 g of nano-SiO₂ power was used and a rotation furnace was used instead of the tube furnace.

Figure 3 shows the cycling performance of the silicon particles of Example 7. Figure 6 shows the XRD patterns of the silicon particles of Example 7.

Comparative Example 3 (CE3) :

Comparative Example 3 was carried out similar to Example 1, except that 2 g of nano-SiO₂ power was used, NaCl (melting temperature: 801℃) was used as the heat absorbent and the weight ratio of silica to the heat absorbent was 1: 10. Such a weight ratio was used according to Luo, W. ’s synthesis method. The high content of NaCl resulted in a low capacity.

Figure 3 shows the cycling performance of the silicon particles of Comparative Example 3.

Figure 6 shows the XRD patterns of the silicon particles of Comparison Example 3. It can be seen that the product of Comparative Example 3 contained SiO₂ impurity. The peaks at 69° and 76° were too weak, which demonstrated that the crystallinity of the product of Comparative Example 3 was relatively low. In addition, the FWHM of the product of Comparative Example 3 was broader than that of Example 7, which demonstrated that the crystal size of the product of Comparative Example 3 was smaller than that of Example 7.

The inventors of the present invention believed that the surface of the silicon particles of Comparison Example 3 was too active and might be oxidized before being used as the electrode material, even though HF was used to rinse the product.

Example 8 (E8) :

Example 8 was carried out similar to Example 7, except that porous SiO₂ (

350, available from EVONIK) was used as the silica source material to obtain porous silicon particles as the product and that HF was not used to rinse the product.

350 is a macroporous silica with low surface area and an average pore size in the 150 nm range. Its specific surface area (N₂, multipoint, following ISO 9277) is 55 m²/g. Its particle size (d50, laser diffraction, following ISO 13320-1) is 4.5 μm.

Figure 7 shows (a) the SEM and (b) the TEM images of the silicon particles of Example 8.

Figure 8 shows the N₂-sorption isotherms of (a) the raw material porous SiO₂ and (b) the silicon particles of Example 8. Figure 9 shows the pore size distribution of (a) the raw material porous SiO₂ and (b) the silicon particles of Example 8. It can be seen that the porous silicon particles of Example 8 have a bimodal pore size distribution of < 2 nm and 10–30 nm. Figure 10 shows the cycling performances of the silicon particles of Examples 8.

Example 9 (E9) :

A carbon coating was applied on the porous silicon particles obtained from Example 8 by CVD, and the carbon content was 26 wt. ％and the carbon layer thickness is ca. 6nm.

Figure 10 shows the cycling performances of the silicon-carbon composite of Example 9.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. The attached claims and their equivalents are intended to cover all the modifications, substitutions and changes as would fall within the scope and spirit of the invention.

Claims

A method for producing silicon particles, said method including the following steps:

1) preparing a mixture of a silica source material, magnesium and/or aluminium powder as a reducing agent, and a salt or composite salt as a heat absorbent；

2) heating the mixture obtained from step 1) at a heating temperature of from the melting point of said reducing agent to lower than 800℃ under a protective atmosphere；

3) removing said heat absorbent and the oxidation products of said reducing agent； characterized in that the melting temperature of said salt or the liquidus temperature of said composite salt ranges from a temperature higher than the heating temperature of step 2) to 800℃.
The method of claim 1, characterized in that the melting temperature of said salt or the liquidus temperature of said composite salt is 660–800℃, preferably 665–790℃, more preferably 670–780℃.
The method of claim 1 or 2, characterized in that the weight ratio of said silica source material to said heat absorbent is 3: 7–7: 3, preferably 2: 3–3: 2, more preferably 4: 5–1: 1, calculated on the basis of SiO₂ in said silica source material.
The method of any one of claims 1 to 3, characterized in that said silica source material is one or more selected from the group consisting of zeolite, diatom, SiO₂ nanopowder, and porous SiO₂.
The method of any one of claims 1 to 4, characterized in that said heat absorbent is one or more selected from the group consisting of KCl； KCl/LiCl with a LiCl content of ≤ 25 mol. ％, preferably ≤ 20 mol. ％, more preferably ≤ 10 mol. ％； and KCl/NaCl with a NaCl content of ≤ 30 mol. ％or 66–98 mol. ％, preferably ≤ 10 mol. ％or 85–95 mol. ％.
The method of any one of claims 1 to 5, characterized in that the amount of said reducing agent used is 1–1.5 times the stoichiometry according to the reaction between SiO₂ and said reducing agent, preferably 1–1: 1.3 times, more preferably 1–1.1 times.
The method of any one of claims 1 to 6, characterized in that in step 2) the mixture obtained from step 1) is heated at a heating temperature of at least 2℃, preferably 5℃, more preferably 10℃ higher than the melting point of said reducing agent, for 1–6 hours, preferably 2–3 hours.
The method of any one of claims 1 to 7, characterized in that after step 3) HF is used to rinse the product obtained from step 3) .
Porous silicon particles, characterized in that said porous silicon particles have a bimodal pore size distribution of < 2 nm and 10–30 nm.
Porous silicon particles of claim 9, characterized in that said porous silicon particles have a BET specific surface area of greater than 300 m²/g, preferably greater than 400 m²/g, more preferably greater than 500 m²/g.
Porous silicon particles of claim 9 or 10, characterized in that the primary particle size of said porous silicon particles is 30–100 nm, preferably 35–80 nm； and the secondary particle size of said porous silicon particles is 1–10 μm, prefaerably 3–6 μm.
Porous silicon particles of any one of claims 9 to 11, characterized in that the pore volume of said porous silicon particles is 0.1–1.5 cm³/g.
Porous silicon particles of any one of claims 9 to 12, characterized in that said porous silicon particles are prepared by the method of any one of claims 1 to 8, and the silica source material is porous SiO₂.
A silicon-carbon composite, characterized in that said silicon-carbon composite comprises a carbon coating layer as well as the porous silicon particles of any one of claims 9 to 13 or the silicon particles prepared by the method of any one of claims 1 to 8.
The silicon-carbon composite of claim 14, characterized in that the thickness of the carbon coating layer is 1–10 nm.
An electrode material, characterized in that said electrode material comprises the silicon-carbon composite of claim 14 or 15.
A battery, characterized in that said battery comprises the electrode material of claim 16.
The use of the silicon-carbon composite of claim 14 or 15 as an electrode active material.