Molten salt electrolysis in-situ synthesized silicon nanowire / graphite composite material as well as preparation method and application thereof

Abstract

The invention belongs to the field of new energy materials, and relates to a silicon nanowire / graphite composite material in-situ synthesized by a fused salt electrolysis method and a preparation method and application thereof, in particular to a preparation method of a silicon nanowire / graphite composite material in-situ synthesized by adopting a diaphragm electrolytic cell system, taking metal molybdenum as an anode, taking porous graphite as a cathode, taking high-alumina ceramic as a diaphragm, and taking a fused salt electrolyte system comprising metal chloride and a silicon source. A silicon source is selected from at least one of silicon dioxide and silicate, constant-voltage electrolysis is performed under the protection atmosphere of inert gas until 80% of theoretical electric quantity of the silicon source, silicate ions are incompletely reduced through electrochemistry, and the silicon nanowire / graphite composite material is synthesized in a cathode graphite pore channel. The preparation process is simple, the structure and morphology of the silicon nanowire / graphite composite material are easy to control, the specific capacity of the silicon nanowire / graphite composite material serving as a lithium ion battery negative electrode material is as high as 872 mAh / g, the cycle performance is excellent, and the silicon nanowire / graphite composite material has good industrial development and application prospects.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of new energy material preparation and electrochemical synthesis, and particularly relates to a method for preparing silicon nanowire / graphite composite material by molten salt electrolysis, which is particularly suitable for the controllable synthesis of high-capacity negative electrode material of lithium ion battery. BACKGROUND

[0002] Silicon material has a theoretical specific capacity of up to 4200 mAh / g, which is a potential negative electrode material of lithium ion battery, but its volume expansion of about 300% during charging and discharging is easy to cause electrode structure collapse, leading to active material shedding and repeated rupture and regeneration of solid electrolyte interface (SEI) film; although graphite material has excellent electrical conductivity and cycle stability, its theoretical capacity is only 372 mAh / g, which is difficult to meet the demand of high energy density. The silicon nanowire / graphite composite material combines the advantages of both: the one-dimensional structure of silicon nanowire can relieve the volume expansion along the axial direction, and the three-dimensional porous structure of graphite matrix provides efficient electron / ion transmission channels, and the strong interface coupling formed by Si-C covalent bond can synergistically improve the specific capacity and cycle stability of the material.

[0003] The existing synthesis method of silicon nanowire / graphite composite material has significant bottlenecks: (1) mechanical ball mixing method lacks chemical bonding, resulting in high silicon / graphite interface contact resistance (>200 Ω·cm²), and silicon nanowires are easy to aggregate due to mechanical stress; (2) chemical vapor deposition (CVD) method requires high temperature (>900 o C) vacuum environment, and the diffusion of silicon precursor (such as SiH4) in graphite pores is limited, resulting in uneven growth density of nanowires; (3) liquid phase chemical method (such as hydrothermal reduction) introduces impurity phase (Na2SiO3) due to the use of strong reducing agent (NaBH4, etc.), and the reaction kinetics is slow. In contrast, the molten salt electrolysis method has the advantages of green (no organic solvent) and scalability (controllable current density), but the traditional single-tank molten salt electrolysis faces problems such as anode chlorine (Cl - →Cl2↑) corrosion of equipment, cathode hydrogen (H + →H2↑) reduction of current efficiency, etc.

[0004] Therefore, the present application proposes a method for in-situ synthesis of silicon nanowire / graphite composite material by molten salt electrolysis, which uses a porous ceramic diaphragm to block >99% of anode by-products (Cl2 / O2), ensuring that the purity of silicon at the cathode is >99.5 wt%; under a voltage of 1.8-3.1 V, the cathode interface undergoes incomplete reduction of silicate, generating 1-3 nm SiO X layer wrapped around the silicon crystal nucleus, and through radial growth, silicon atoms form nanowires with a diameter of 20-200 nm and an aspect ratio of >100; during growth, silicon atoms and graphite sp 2Carbon forms low-resistance Si-C bonds through solid-phase diffusion, and cooperatively realizes high capacity and long cycle stability. The silicon nanowire / graphite composite material can be used as a negative electrode material of a lithium ion battery, and can significantly improve the specific capacity and cycle performance of the negative electrode, improve the conductivity of the silicon negative electrode, and effectively alleviate the volume expansion effect of the silicon-based negative electrode of the lithium ion battery. SUMMARY

[0005] The present application aims at the industrialization bottleneck caused by poor conductivity of the silicon negative electrode, uncontrollable volume expansion, weak interface combination, and high energy consumption process, and innovatively uses a porous graphite cathode etched by KOH and sintered to provide a SiOx / Si nanowire / graphite composite material based on SiOx / Si nanowire in-situ synthesis. X The diaphragm method molten salt electrolysis system of the self-constrained electrochemical directional growth system. The system uses a high-alumina ceramic material resistant to chloride molten salt as a diaphragm of a molten salt electrolytic cell, avoiding the influence of anode oxygen and chlorine on the growth of silicon nanofibers on the cathode. The pores of the porous graphite electrode provide a directional growth space for the silicon nanowires, and through precise voltage control (1.8-3.1 V), an SiOx constraint layer is formed in-situ on the graphite substrate, realizing the controllable growth of the diameter (20-200 nm) and aspect ratio (>100) of the silicon nanowires. Therefore, the in-situ synthesis of the silicon nanowire / graphite composite material by the molten salt method has the characteristics of in-situ generation, controllable growth of silicon nanowires, high purity of the product, simple process, high conversion efficiency, low energy consumption and cost, and easy industrialization, and is a silicon nanowire / graphite composite material preparation technology with development and application prospect.

[0006] To achieve the above application purposes, the present application provides the following technical solutions: A preparation method for in-situ synthesis of silicon nanowire / graphite composite material by molten salt electrolysis, which adopts a diaphragm electrolytic cell system, uses molybdenum as an anode, porous graphite as a cathode, high-alumina ceramic as a diaphragm, and a molten salt electrolyte system as an electrolyte, including at least one of silicon dioxide and silicate as a silicon source, and ends the constant voltage electrolysis under an inert gas protective atmosphere to 80-95% of the theoretical electric quantity of the silicon source, and synthesizes the silicon nanowire / graphite composite material in the cathode graphite pore by electrochemical incomplete reduction of silicate ions.

[0007] Preferably, the molybdenum anode is at least one of molybdenum wire, molybdenum plate and molybdenum rod, and more preferably the molybdenum wire is folded into a spiral shape to increase the electrode area.

[0008] Preferably, the porous graphite cathode is a three-dimensional porous structure graphite with a pore size of 50-500 nm and a porosity of 30-70%; And / or, the porous graphite is a three-dimensional porous structure graphite cathode material obtained by etching graphite with potassium hydroxide solution and high-temperature sintering treatment in an inert atmosphere; preferably, the graphite is at least one of graphite rod and graphite plate; And / or, the porous graphite is prepared by immersing a graphite rod in a potassium hydroxide solution with a molar concentration of 7-18 mol / L, etching the graphite rod under the condition of 60-120 o C for 0.5-6 h, and then sintering the etched graphite rod in an inert gas at a temperature rising rate of 1-10 ℃ / min to 500-900 o C for 1-3 h, to obtain a three-dimensional porous structure with a pore size of 50-500 nm and a porosity of 30-70%. By selecting the three-dimensional porous structure of the porous graphite with a network of interconnected channels formed in a three-dimensional space, abundant growth space can be provided for the silicon nanowires.

[0009] Preferably, the electrode spacing of the cathode and the anode is 1-5 cm. By using the electrode spacing in the range, the risk of electric field distortion and short circuit can be further prevented, and the energy consumption can be reduced.

[0010] Preferably, the diaphragm is a tubular porous high-alumina ceramic diaphragm with a chemical composition of a Al2O3·b CaO·cSiO2·d MgO, a=0.92-0.96, b=0.01-0.06, c=0.01-0.06, d=0.01-0.06, b+c+d=0.04-0.08, a+b+c+d=1.0, a porosity of 35-40%, and a pore size of 0.2-2.0 μm. The diaphragm has a good barrier effect for the oxygen evolution product of the anode, can obtain ultra-long silicon nanofibers, and can improve the purity of the silicon nanofibers.

[0011] Preferably, the metal chloride of the molten salt electrolyte is MClx, wherein M is at least one metal element selected from Li, Na, K, Mg, and Ca, and x is the valence of the corresponding metal; more preferably, the metal chloride is at least one selected from LiCl, NaCl, KCl, MgCl2, and CaCl2. And / or, the metal chloride of the molten salt electrolyte includes, in terms of mass percentage, LiCl 20-50%, NaCl 10-40%, KCl 10-30%, MgCl2 0-20%, and CaCl2 0-15%. And / or, the silicon source is SiO2 or CaSiO3, and the content of the silicon source is 0.5-5.0 wt% of the total mass of the molten salt electrolyte, in terms of the mass of silicon element.

[0012] Preferably, the temperature of the molten salt electrolyte is controlled to be 700-1000 o C during constant-voltage electrolysis. And / or, the regulated voltage is 1.8-3.1 V, and more preferably, the electrolysis is ended when the theoretical electric quantity of the silicon source is 80-92%, and more preferably, the electrolysis time is 1-24 h.

[0013] Preferably, the electrolysis product of the electrolysis is soaked in a dilute hydrochloric acid solution, washed with deionized water, and vacuum dried to obtain a silicon nanowire / graphite composite product; preferably the product is soaked and washed in a dilute hydrochloric acid solution with a molar concentration of 1 mol / L, the soaking time is 5-30 minutes, then washed with deionized water, and vacuum dried at 80°C.

[0014] Preferably, the silicon nanowire / graphite composite product has a silicon nanowire diameter of 20-200 nm, an aspect ratio >100, a silicon purity >75 %, more preferably >90 %, and a silicon nanowire mass ratio of 15-20 %, and can be used as a high-performance negative electrode material for lithium ion batteries. By using the technical solution, the silicon nanowire with a diameter of 20-200 nm has excellent mechanical stability, and a diameter that is too small will cause agglomeration, and a diameter that is too large will cause fracture. At the same time, an aspect ratio greater than 100 can further ensure the continuity of long-range electron conduction. In addition, a silicon mass ratio in the range of 15-20 % can provide higher capacity, and graphite can also provide better expansion constraint.

[0015] Preferably, the inert gas in the present application is a gas that does not react with reactants, products, and electrolytes, and is preferably any one or more of helium, nitrogen, and argon. More preferably, it is high-purity argon.

[0016] The present application also provides a silicon nanowire / graphite composite material synthesized in situ by a molten salt electrolysis method prepared by any one of the above preparation methods.

[0017] The present application also provides a silicon nanowire / graphite composite material synthesized in situ by a molten salt electrolysis method prepared by any one of the above preparation methods for use in the field of lithium ion batteries.

[0018] Compared with the prior art, the present application has the following beneficial effects: in view of the complex process, low conversion efficiency, high energy consumption and cost of lithium ion battery silicon-carbon negative electrode materials, a method and system for synthesizing silicon nanowire / graphite composite materials in situ by a diaphragm method of electrochemical molten salt electrolysis based on a porous graphite cathode are innovatively proposed. The three-dimensional porous graphite etched by KOH replaces the traditional dense graphite cathode, the through pores thereof provide a directional growth space for silicon nanowires, and the silicon atoms and graphite sp 2Carbon forms low-resistance Si-C bonds through solid-phase diffusion, and cooperatively realizes high capacity and cycle stability. The porous ceramic diaphragm is introduced to block the anode by-product oxygen, reduce the formation of silicon oxide film on the surface of silicon nanowires, and make the purity of silicon nanowires in the cathode region >99.5 wt%. The method integrates silicon nanowire synthesis, graphite matrix composite and interface optimization into a one-step electrolysis synthesis process, and outputs silicon nanowire / graphite composite materials that can be directly used as negative electrodes of lithium batteries. The method has the advantages of simple process and high purity of products, and has significant industrial cost advantages. At the same time, the structure and morphology of the composite material are easy to control, and the composite material has excellent electrochemical performance, and has good industrial development and application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic diagram of an electrolysis reaction system for in-situ electrochemical generation of silicon nanowire / graphite composite materials in an inert gas protection atmosphere according to the present application; Figure 1 In the figure, 1 is a quartz glass reactor, 2 is a molten salt electrolyte, 3 is an electrolytic cell (alumina), 4 is an air inlet, 5 is an air outlet, 6 is a spiral metal molybdenum anode, 7 is a porous graphite cathode, and 8 is a high-alumina ceramic diaphragm. Figure 2 is an XRD characterization diagram of the product of Example 1; Figure 3 is a SEM characterization diagram of the product of Example 1 at 200,000 times; Figure 4 is a SEM characterization diagram of the product of Example 2 at 50,000 times; Figure 5 is a cycle performance diagram of the silicon nanowire / graphite composite negative electrode material of Example 1; Figure 6 is a rate performance diagram of the silicon nanowire / graphite composite negative electrode material of Example 1. DETAILED DESCRIPTION

[0020] In order to better clarify and understand the purpose, process scheme and advantages of the present application, the technical scheme and implementation manner of the present application are further clearly, completely and specifically described below in combination with the drawings through specific examples. It should be known that the described examples of the present application are implemented on the premise of the technical scheme of the present application, and give detailed implementation manners and specific operation processes, but only part of the examples of the present application, not all the examples. The described specific implementation manner is limited to the description and explanation of the present application, and does not limit the present application. Based on the examples in the present application, all other implementation manners obtained by those skilled in the art without creative labor belong to the protection scope of the present application.

[0021] Unless otherwise specified, the experimental methods and conditions used in the embodiments of this invention are conventional methods and conditions. The materials, reagents, instruments, and equipment used in the embodiments, unless otherwise specified, are all conventional substances or equipment known to those skilled in the art and can be obtained commercially or prepared by conventional methods. The reaction conditions described in the invention's content can all achieve the stated reactions and obtain the desired products. Due to space limitations, some embodiments are listed below to further illustrate the advantages of the technical solution of this invention.

[0022] Example 1

[0023] Reference Figure 1 ,like Figure 1 This is a system for in-situ molten salt electrolysis synthesis of silicon nanowire / graphite composite materials using a diaphragm method. The anode is a spiral molybdenum anode 6, and the cathode is a porous graphite cathode 7. The porous graphite electrode is formed by impregnating a graphite rod in a 16 mol / L potassium hydroxide solution at 110 °C. o Etching was performed at C conditions for 6 h, and the etched graphite rod was then heated to 900 °C at a rate of 5 °C / min in an argon atmosphere. o C. After holding at this temperature for 3 hours, the porous graphite electrode was obtained by cooling it to room temperature in the furnace. The high-alumina ceramic membrane 8 has the following composition: a Al2O3·b CaO·c SiO2·d MgO, where a = 0.95, b = 0.02, c = 0.02, d = 0.01, porosity is 38%, and pore size is 0.2-2.0 μm. This high-alumina ceramic membrane was prepared by Yixing Xingguang Ceramics Research Institute. The molten salt electrolyte consisted of CaCl2 and the precursor silicon source CaSiO3, with the silicon source accounting for 5 wt% of the molten salt electrolyte mass. The electrolysis temperature was 880 °C. o C, electrode spacing 1 cm. The electrolyte CaCl2 and precursor CaSiO3 were heated to 900℃ at 5℃ / min under a protective atmosphere of flowing argon, held at this temperature for 1 h to ensure complete dissolution of CaCl2, and then cooled to 880℃ at 5℃ / min. o C. Electrolysis is performed between the cathode and anode using a constant voltage of U = 2.2V for t = 3 hours. The charge at this point is 80% of the theoretical charge required for the complete reduction of the silicon source to silicon (theoretical charge refers to the theoretical charge required to convert the silicon dioxide or calcium silicate used as the silicon source to silicon according to Faraday's law). After electrolysis, the porous graphite cathode and the resulting product are washed with 1 mol / L dilute hydrochloric acid and deionized water. o Vacuum dried at C. XRD characterization results are as follows: Figure 2 As shown, the main products are silicon nanowires and graphite. SEM characterization results are as follows: Figure 3 The results indicate that the obtained silicon nanofibers have a diameter of approximately 88 nm, an aspect ratio of approximately 131, a silicon purity of 97%, and a mass ratio of approximately 17% for silicon nanowires.

[0024] The calculation method of the silicon nanowire mass ratio is as follows: The obtained silicon nanowire / graphite composite material is weighed to obtain a total mass M1, and then is heated under air by TGA (thermal gravimetric analysis) at 600 o The graphite reacts with oxygen to become CO2 under C, and the remaining substance is SiO2. The mass of the remaining SiO2 is weighed as M2. .

[0025] Calculation of the silicon ratio: (coefficient 0.467 = Si / SiO2).

[0026] Example 2-20

[0027] According to the method of Example 1, the molten salt electrolyte CaCl2 in Example 1 is adjusted in composition, and is replaced by NaCl+CaCl2 (molar ratio 1:2) to reduce the melting point of the molten salt, and the electrolysis temperature is reduced to 750 o C, and the remaining parameters remain the same as in Example 1. After the electrolysis is completed, the obtained product is washed and dried. The electrolysis product obtained at the cathode is characterized by SEM as shown in Figure 4 The obtained product is silicon nanowires with a diameter of about 55 nm and an aspect ratio of about 125, and the silicon purity is 92%, and the silicon nanowire ratio is 5%.

[0028] After adjusting the electrolyte of Example 1, the electrolysis temperature is changed. Other process parameters are the same as in Example 1, and the results are shown in Table 1.

[0029]

[0030] It can be seen that adjusting the composition of the molten salt electrolyte can effectively reduce the electrolysis temperature. Because the solubility of different silicon sources in different molten salt compositions is different, and the diffusion coefficient of silicon ions in different molten salts is also different, different silicon and graphite ratios in the obtained silicon nanowire / graphite composite material will be obtained. Because the behavior of anode by-products and impurity ions is different under different molten salt compositions, the purity of the obtained silicon nanowires is also different.

[0031] Example 21-26

[0032] According to the method of Example 1, the conditions for preparing the porous graphite cathode in Example 1 are changed, and after the electrolysis is completed, the obtained product is washed and dried. The morphology of the product is characterized by SEM, and the silicon nanowire ratio is calculated. The results are shown in Table 2. Among them, the sintering time has little effect on the porous graphite rod, so it is omitted.

[0033]

[0034] The results in Table 2 show that the increase of the concentration of KOH solution does not affect the purity of the silicon nanowires, but the proportion of the silicon nanowires increases. This phenomenon is due to the concentration-dependent etching kinetics: high concentration of KOH (7-18 mol / L) generates a higher density of nanoscale etch pits (50-500 nm) on the surface of the graphite rod, and the edges of the etch pits expose abundant sp2carbon defect sites, providing more nucleation active centers for the electrochemical reduction of the silicon precursor. This process follows a positive correlation mechanism of etching intensity-active sites-silicon nanowire growth, thereby optimizing the silicon loading and electrochemical performance of the composite material. The sintering temperature (800-900°C) of the etched graphite rod has no significant effect on the loading of the silicon nanowires. This is because the sp2carbon defect structure formed by KOH etching has thermal stability at ≥300°C, and the surface active site density and chemical reduction remain essentially unchanged during the sintering process.

[0035] Examples 27-32

[0036] According to the method of Example 1, the electrolysis time t and voltage U in Example 1 were changed, and after the electrolysis was completed, the obtained product was washed and dried, and the morphology of the product was characterized by SEM, the current efficiency and the proportion of silicon nanofibers were calculated, and the results are shown in Table 3.

[0037]

[0038] Table 3 shows that the longer the electrolysis time, the larger the size of the silicon nanowires, and the purity and proportion of the silicon nanowires decrease, and the current efficiency decreases; below the minimum decomposition voltage of the molten salt component, the larger the electrolysis voltage, the larger the size of the obtained silicon nanowires, and the electrolysis time and electrolysis voltage are the main factors for controlling the size and length of the silicon nanofibers; below and above 80-95% of the theoretical electric quantity, the purity of the obtained silicon nanowires decreases; which mainly affects the nucleation rate and density distribution of the silicon nanofiber growth catalyst, and is also a key factor affecting the current efficiency.

[0039] Examples 33-38

[0040] According to the method of Example 1, the composition of the separator material a Al2O3·b CaO·c SiO2·d MgO in Example 1 was adjusted, the porosity and pore size remained basically unchanged, and after the electrolysis was completed according to the method of Example 1, the obtained product was washed and dried, the SEM characterization results were analyzed, and the service life of the separator was investigated, and the results are shown in Table 4.

[0041]

[0042] The results show that silicon nanowires can also be grown without the separator, but the diameter of the silicon nanowires becomes thicker, the length of the silicon nanowires becomes shorter, the purity decreases, and the proportion of the silicon nanowire / graphite composite is lower. The composition of the separator does not affect the growth of the silicon nanowires, but only affects the service life of the separator, and has no effect on the final product, the silicon nanowire / graphite composite.

[0043] Example 39

[0044] The product obtained in Example 1 was assembled into a CR2032 button cell to test the electrochemical performance of the silicon nanowire / graphite composite.

[0045] First, the electrode sheet was prepared. Polyacrylic acid (PAA) and deionized water were mixed in a mass ratio of 1:1 and dissolved at 60 o C to obtain a binder. Then the silicon nanowire / graphite composite, conductive agent acetylene black (Super-P), and polyacrylic acid (PAA) binder were mixed in a mass ratio of 6:2:2, and fully stirred and mixed in a homogenizer. The above viscous slurry was uniformly coated on a copper sheet (diameter 12 mm, thickness 0.1 mm), and the slurry on each electrode sheet was about 1 mg. The slurry was dried in a vacuum oven at 80 o C for 12 h, and then pressed at a pressure of 18 MPa, and then placed back in the vacuum oven at 80 o C for 8 h of continuous drying.

[0046] The battery assembly process was completed in an argon-filled glove box (O2<0.1 ppm, H2O<0.1 ppm). The electrode sheet was placed in the middle of the positive electrode shell, a Cellgard 2500 separator with a diameter of 20 mm was placed on the surface of the electrode sheet, and electrolyte (1 M lithium hexafluorophosphate (LiPF6) dissolved in equal volumes of ethylene carbonate (EC), dimethyl carbonate (DEC), and ethylene carbonate (EMC)) was added. Then a fresh piece of metallic lithium was placed on the separator, and after adding an appropriate amount of electrolyte, the negative electrode shell was sealed with a sealing machine and placed in a 30 o C environment for 24 h. The half-cell after standing was tested in a 30 o C environment using a CT-3008W Neware battery test system from Shenzhen Neware Electronics Co., Ltd. The long cycle performance test was carried out at a current density of 0.2 A g -1 , 0.5 A g -1 , 1 A g -1 , 2 A g -1 , 5 A g -1 , and 5 A g -1 The rate performance of the battery was tested. The voltage range for testing was 0.01-1.5 V. The electrochemical performance was as follows: Figure 5and Figure 6 As shown, after 100 cycles, the discharge specific capacity stabilized at 872 mAh / g, at 0.5 A g. -1 1 A g -1 2 A g -1 5 A g -1 At current densities, it has 770 mAh / g, 696 mAh / g, 638 mAh / g, and 506 mAh / g.

[0047] Following the method of Example 39, the product of Example 1 used in Example 39 was replaced with products from other examples and assembled into CR2032 button cells. The cells were then tested using a CT-3008W Neware battery testing system from Shenzhen Neware Electronics Co., Ltd. at 30°C. o Tested in a C environment, at 0.2A g -1 Electrochemical performance was tested at current densities ranging from 0.01 to 1.5 V; at 0.2 A g... -1 0.5 A g -1 1 A g -1 2 A g -1 5 A g -1 The rate performance of the battery was tested at different current densities. The electrochemical performance is shown in Table 5.

[0048]

[0049] The results show that the product obtained under the conditions used in Example 1 has the best cycle performance and rate performance of the battery prepared therefrom; the batteries prepared from the silicon nanowire / graphite composite products obtained in Comparative Examples 2-17 have poorer electrochemical performance than the batteries prepared from the silicon nanowire / graphite composite products obtained from the molten salt components in Example 1, because the silicon purity of the silicon nanowire / graphite composite products obtained from the molten salt components in Comparative Examples 2-17 is lower; the batteries prepared from the silicon nanowire / graphite composite products obtained in Comparative Examples 18-20 have poorer electrochemical performance, because the proportion of silicon nanowires in the products obtained at too low a concentration of KOH solution is too low, and the battery capacity is also reduced, and at too high a concentration of KOH solution, the proportion of silicon nanowires in the products obtained is too high, the volume expansion of silicon is out of control, leading to material pulverization and unstable structure, and the electrochemical performance is poor. The batteries prepared from the silicon nanowire / graphite composite products obtained in Comparative Examples 21 and 22 have poorer electrochemical performance, because too high or too low electrolysis time and electrolysis voltage will lead to a decrease in the purity and aspect ratio of the silicon nanowires, and thus the electrochemical performance is poor. Compared with the product prepared in Comparative Example 23 without a separator, the silicon nanowires obtained after using a separator are finer and longer, because the separator can hinder the by-products of the anode and improve the purity of silicon. Comparative Examples 24 and 25 only change the content of the chemical components of the separator, and the samples obtained are similar to the sample in Example 39, so under the condition that other conditions remain unchanged, the electrochemical performance of the battery is also similar to that of Example 39.

[0050] The above-described examples are only the preferred schemes of the present application, and do not limit the present application in any form. Other variants and modifications can be made without departing from the technical solutions recited in the claims.

Claims

1. A method for in-situ synthesis of silicon nanowire / graphite composite materials via molten salt electrolysis, characterized in that, The method employs a diaphragm electrolytic cell system, with molybdenum metal as the anode, porous graphite as the cathode, high-alumina ceramic as the diaphragm, and a molten salt electrolyte system including a metal chloride and a silicon source. The silicon source is selected from at least one of silicon dioxide and silicates. Electrolysis is performed under constant voltage in an inert gas protective atmosphere until 80%-95% of the theoretical charge of the silicon source is reached, thereby obtaining a silicon nanowire / graphite composite material.

2. The method for preparing silicon nanowire / graphite composite material by in-situ synthesis via molten salt electrolysis according to claim 1, characterized in that, The molybdenum anode is at least one of molybdenum wire, molybdenum plate, and molybdenum rod.

3. The method for preparing silicon nanowire / graphite composite materials by in-situ synthesis via molten salt electrolysis according to claim 1, characterized in that, The porous graphite cathode is a three-dimensional porous graphite with a pore size of 50-500 nm and a porosity of 30-70%. And / or, the porous graphite is obtained by etching graphite with potassium hydroxide solution and sintering it at high temperature in an inert atmosphere to obtain graphite with a three-dimensional porous structure. And / or, the porous graphite is obtained by immersing graphite rods in a potassium hydroxide solution with a molar concentration of 7-18 mol / L at a temperature of 60-120 °C. o Etching was performed at C conditions for 0.5-6 h. After etching, the graphite rod was heated to 500-900 °C in an inert gas atmosphere at a heating rate of 1-10 °C / min. o Sintering at C for 1-3 h yields a three-dimensional porous graphite structure with a substrate pore size of 50-500 nm and a porosity of 30-70%.

4. The method for preparing silicon nanowire / graphite composite materials by in-situ synthesis via molten salt electrolysis according to claim 1, characterized in that, The distance between the cathode and anode electrodes is 1-5 cm.

5. The method for preparing silicon nanowire / graphite composite materials by in-situ synthesis via molten salt electrolysis according to claim 1, characterized in that, The diaphragm is a tubular porous high-alumina ceramic diaphragm with the following chemical composition: aAl2O3·bCaO·cSiO2·dMgO, where a = 0.92-0.96, b = 0.01-0.06, c = 0.01-0.06, d = 0.01-0.06, b+c+d = 0.04-0.08, a+b+c+d = 1.0, porosity of 35-40%, and pore size of 0.2-2.0 μm.

6. The method for preparing silicon nanowire / graphite composite materials by in-situ synthesis via molten salt electrolysis according to claim 1, characterized in that, The metal chloride of the molten salt electrolyte is MClx, where M is selected from at least one metal element selected from Li, Na, K, Mg, and Ca, and x is the valence of the corresponding metal. And / or, the metal chlorides of the molten salt electrolyte are: by molar percentage, including: LiCl 20-50%, NaCl 10-40%, KCl 10-30%, MgCl2 0-20%, and CaCl2 0-15%; And / or, the silicon source is SiO2 or CaSiO3, and the content of the silicon source, based on the mass of silicon element, is 0.5-5.0 wt% of the total mass of the molten salt electrolyte.

7. The method for preparing silicon nanowire / graphite composite materials by in-situ synthesis via molten salt electrolysis according to claim 1, characterized in that, During constant voltage electrolysis, the temperature of the molten salt electrolyte is controlled at 700-1000°C. o C; And / or, adjust the voltage to 1.8-3.1V, And / or, the electrolysis time is 1-24 h.

8. The method for preparing silicon nanowire / graphite composite material by in-situ synthesis via molten salt electrolysis according to claim 1, characterized in that, The obtained silicon nanowire / graphite composite material has silicon nanowires with diameters of 20-200 nm, aspect ratios >100, silicon purity >75%, and a mass percentage of 15-20%.

9. A silicon nanowire / graphite composite material prepared by the preparation method according to any one of claims 1-8 via molten salt electrolysis in situ synthesis.

10. The application of the in-situ synthesis of silicon nanowire / graphite composite material by molten salt electrolysis as described in claim 9 in the field of lithium-ion batteries.

Images