CN121035166A

CN121035166A - A method for in-situ synthesis of silicon nanowire/graphite composite materials by molten salt electrolysis, its preparation method, and its applications.

Info

Publication number: CN121035166A
Application number: CN202511089475.7A
Authority: CN
Inventors: 甘永平; 刘董; 计鑫; 陈举; 黄辉; 张俊; 夏阳; 贺馨平; 夏新辉; 张文魁; 方如意; 范敏
Original assignee: Zhejiang University of Technology ZJUT
Current assignee: Zhejiang University of Technology ZJUT
Priority date: 2025-08-05
Filing date: 2025-08-05
Publication date: 2025-11-28

Abstract

This invention belongs to the field of new energy materials, and relates to an in-situ synthesis of silicon nanowire/graphite composite materials by molten salt electrolysis, its preparation method, and its application. Specifically, it employs a membrane electrolytic cell system with molybdenum as the anode, porous graphite as the cathode, and high-alumina ceramic as the separator. The molten salt electrolyte system includes a metal chloride and a silicon source, with the silicon source selected from at least one of silicon dioxide and silicates. Electrolysis is performed under an inert gas atmosphere at a constant voltage until 80% of the theoretical charge of the silicon source is reached. The silicon nanowire/graphite composite material is synthesized through incomplete electrochemical reduction of silicate ions within the pores of the cathode graphite. This invention features a simple preparation process, easily controllable morphology of the silicon nanowire/graphite composite material, and a high specific capacity (872 mAh/g) as a lithium-ion battery anode material, exhibiting excellent cycle performance and promising prospects for industrial development and application.

Description

Fused salt electrolysis in-situ synthesis silicon nanowire/graphite composite material and preparation method and application thereof

Technical Field

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

Background

The silicon material has theoretical specific capacity up to 4200 mAh/g, is a lithium ion battery cathode material with great potential, but the volume expansion of about 300% in the charge and discharge process is easy to cause collapse of an electrode structure, so that an active substance is dropped off and a Solid Electrolyte Interface (SEI) film is repeatedly broken and regenerated, and the graphite material has excellent conductivity and cycle stability, but the theoretical capacity is only 372 mAh/g, and is difficult to meet the requirement of high energy density. The silicon nanowire/graphite composite material combines the advantages that the one-dimensional structure of the silicon nanowire can axially relieve volume expansion, the three-dimensional porous structure of the graphite matrix provides a high-efficiency electron/ion transmission channel, and the two can cooperatively improve the specific capacity and the cycling stability of the material through strong interface coupling formed by Si-C covalent bonds.

The existing synthesis method of the silicon nanowire/graphite composite material has obvious bottlenecks that (1) a mechanical ball mixing method is lack of chemical bonding to cause high contact resistance of a silicon/graphite interface (> 200Ω cm < 2 >), the silicon nanowire is easy to agglomerate due to mechanical stress, (2) a Chemical Vapor Deposition (CVD) method needs a high-temperature (> 900 ^o C) vacuum environment, silicon precursors (such as SiH ₄) are limited in diffusion in graphite pores to cause uneven growth density of the nanowire, and (3) a liquid phase chemical method (such as hydrothermal reduction) introduces an impurity phase (Na ₂SiO₃) due to the use of a strong reducing agent (NaBH ₄ and the like), and reaction kinetics is slow. In contrast, the fused salt electrolysis method has the advantages of being green (no organic solvent is needed) and being capable of being scaled (current density is controllable), but the traditional single-tank fused salt electrolysis method has the problems that anode chlorine evolution (Cl ^-→Cl₂ +%) corrosion equipment and cathode hydrogen evolution (H ⁺→H₂ +%) reduce current efficiency and the like.

Based on the method, the invention provides a method for in-situ synthesis of a silicon nanowire/graphite composite material by a fused salt electrolysis method, wherein a porous ceramic diaphragm is used for blocking up anode byproducts (Cl ₂/O₂) with the purity of 99% to ensure that cathode silicon is 99.5 to wt%, under the voltage of 1.8 to 3.1 to V, the cathode interface is subjected to incomplete reduction of silicate radicals to generate 1 to 3 nm SiO _X layers of wrapped silicon crystal nuclei, the silicon atoms are induced to form nanowires with the diameter of 20 to 200 nm and the length-diameter ratio of >100 through radial growth, and the growing silicon atoms and graphite sp ² carbon form low-resistance Si-C bonds through solid phase diffusion to cooperatively realize high capacity and long-cycle stability. The silicon nanowire/graphite composite material is used as the negative electrode material of the lithium ion battery, so that the specific capacity and the cycle performance of the negative electrode can be obviously improved, the conductivity of the silicon negative electrode is improved, and the volume expansion effect of the silicon-based negative electrode of the lithium ion battery is effectively relieved.

Disclosure of Invention

Aiming at the industrialization bottleneck caused by poor conductivity, uncontrollable volume expansion, weak interface combination and high process energy consumption of the silicon cathode, the invention innovatively adopts a KOH etched and sintered porous graphite cathode, and provides a diaphragm method molten salt electrolysis system based on a SiO _X self-constrained electrochemical directional growth system. The system adopts a high-alumina ceramic material resistant to chloride molten salt as a diaphragm of the molten salt electrolysis cell, so that the influence of anode oxygen and chlorine on the growth of silicon nanofibers at the cathode is avoided. The pores of the porous graphite electrode provide directional growth space for the silicon nanowire, and an SiOx constraint layer is formed in situ on a graphite substrate through accurate voltage control (1.8-3.1V), so that the controllable growth of the diameter (20-200 nm) and the length-diameter ratio (100) of the silicon nanowire is realized. 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 the silicon nanowire, high product purity and the like, and simultaneously has the characteristics of simple process, high conversion efficiency, low energy consumption and cost, easiness in industrialization and the like, and is a preparation technology of the silicon nanowire graphite composite material with development and application prospects.

In order to achieve the above object, the present invention provides the following technical solutions:

The method adopts a diaphragm electrolytic tank system, takes metal molybdenum as an anode, porous graphite as a cathode, high-alumina ceramic as a diaphragm, and electrolyte as a molten salt electrolyte system, and comprises metal chloride and a silicon source, wherein the silicon source is selected from at least one of silicon dioxide and silicate, and is electrolyzed under the protection of inert gas under constant voltage until 80-95% of theoretical electric quantity of the silicon source is ended, silicate ions are incompletely reduced through electrochemistry, and the silicon nanowire/graphite composite is synthesized in a cathode graphite pore channel.

Preferably, the metal molybdenum anode is at least one of molybdenum wires, molybdenum plates and molybdenum rods, and the molybdenum wires are more preferably folded into a spiral shape, so that the electrode area is increased.

Preferably, the porous graphite cathode is graphite with a three-dimensional porous structure, wherein the pore diameter of the graphite is 50-500 nm, and the porosity of the graphite is 30-70%;

And/or the porous graphite is a graphite cathode material with a three-dimensional porous structure obtained by etching graphite by potassium hydroxide solution and sintering at high temperature in inert atmosphere, wherein the preferred graphite is at least one selected from a graphite rod and a graphite plate;

And/or immersing the graphite rod in potassium hydroxide solution with the molar concentration of 7-18 mol/L, etching at the temperature of 60-120- ^o ℃ to treat 0.5-6 h, heating the etched graphite rod to 500-900- ^o ℃ in inert gas at the heating rate of 1-10 ℃ per min, and sintering to 1-3 h to obtain the three-dimensional porous structure with the substrate pore diameter of 50-500 nm and the porosity of 30-70%. By selecting porous graphite with a three-dimensional porous structure forming a network of mutually communicated channels in a three-dimensional space, a rich growth space can be provided for the silicon nanowires.

Preferably, the electrode spacing between the cathode and anode is 1-5cm. By adopting the electrode spacing in the above range, the risk of distortion and short circuit of the electric field can be further prevented and the power consumption can be reduced.

Preferably, the diaphragm is a tubular porous high-alumina ceramic diaphragm, the chemical composition of the diaphragm is a Al₂O₃·b CaO·c SiO₂·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,, the porosity is 35-40%, and the pore diameter is 0.2-2.0 μm. The membrane has better function of blocking the anode oxygen evolution product, can obtain the ultra-long silicon nanofiber and has the function of improving the purity of the silicon nanofiber.

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, ca, and x is the valence of the corresponding metal;

And/or the metal chloride of the molten salt electrolyte comprises, by mass, 20-50% of LiCl, 10-40% of NaCl, 10-30% of KCl, 0-20% of MgCl ₂ and 0-15% of CaCl ₂;

and/or the silicon source is SiO ₂ or CaSiO ₃, and the content of the silicon source is 0.5-5.0wt% of the total mass of the molten salt electrolyte based on the mass of the silicon element.

Preferably, the temperature of the molten salt electrolyte is controlled to be 700-1000 ^o ℃ during constant voltage electrolysis;

And/or the regulating voltage is 1.8-3.1V, more preferably the electrolysis is ended to 80-92% of the theoretical electric quantity of the silicon source, and more preferably the electrolysis time is 1-24 h.

Preferably, the electrolytic product is soaked in dilute hydrochloric acid solution, washed with deionized water and vacuum dried to obtain the silicon nanowire/graphite composite product, and the preferable product is soaked in dilute hydrochloric acid solution with the molar concentration of 1 mol/L for 5-30 min, washed with deionized water and vacuum dried at 80 ℃.

Preferably, the silicon nanowire/graphite composite product has the diameter of 20-200nm, the length-diameter ratio of >100, the silicon purity of >75 percent, more preferably >90 percent, and the mass ratio of the silicon nanowire is 15-20 percent, and can be used as a high-performance anode material of a lithium ion battery. By adopting the technical scheme, the silicon nanowire with the diameter in the range of 20-200nm has excellent mechanical stability, and the diameter is too small to be agglomerated and too large to be broken. Meanwhile, the length-diameter ratio is more than 100, so that the continuity of long-range electron conduction can be further ensured. In addition, silicon can provide higher capacity in the 15-20% mass ratio range, and graphite can also provide better expansion constraints.

Preferably, the inert gas in the present invention is a gas that does not react with the reactants and products and the electrolyte, and is preferably selected from any one or more of helium, nitrogen and argon. More preferably high purity argon.

The invention also provides a fused salt electrolysis method in-situ synthesized silicon nanowire/graphite composite material prepared by any one of the preparation methods.

The invention also provides an application of the molten salt electrolysis in-situ synthesis silicon nanowire/graphite composite material prepared by any one of the preparation methods in the field of lithium ion batteries.

Compared with the prior art, the method and the system have the beneficial effects that a method and a system for synthesizing the silicon nanowire/graphite composite material by in-situ electrochemical molten salt electrolysis based on a diaphragm method of a porous graphite cathode are innovatively provided aiming at the problems of complex process, low conversion efficiency, high energy consumption and high cost of the silicon-carbon negative electrode material of the lithium ion battery. The KOH etched three-dimensional porous graphite is adopted to replace the traditional compact graphite cathode, the through holes of the KOH etched three-dimensional porous graphite provide directional growth space for the silicon nanowires, and silicon atoms and graphite sp ² carbon form low-resistance Si-C bonds through solid phase diffusion in growth, so that high capacity and cycle stability are cooperatively realized. And introducing a porous ceramic diaphragm to block oxygen as an anode byproduct, so that a silicon oxide film is formed on the surface of the silicon nanowire, and the purity of the silicon nanowire in the cathode region is higher than 99.5 wt%. The method integrates the synthesis of the silicon nanowire, the combination of the graphite matrix and the interface optimization into one-step electrolytic synthesis process, and the silicon nanowire/graphite composite material which can be directly used for the negative electrode of the lithium battery is produced, so that the method has the advantages of simple process, high product purity and obvious industrialized cost. Meanwhile, the structure and the shape of the composite material are easy to control, and the composite material has excellent electrochemical performance and good industrial development and application prospects.

Drawings

FIG. 1 is a schematic illustration of an electrolytic reaction system for electrochemically generating silicon nanowire/graphite composite material in situ in an inert gas atmosphere according to the present invention;

In FIG. 1, a 1-quartz glass reactor, 2-molten salt electrolyte, 3-electrolytic cell (alumina), 4-gas inlet, 5-gas outlet, 6-spiral metallic molybdenum anode, 7-porous graphite cathode, 8-high alumina ceramic membrane;

FIG. 2 is an XRD characterization of the product of example 1;

FIG. 3 is a SEM characterization of the product of example 1 at 200000 x;

FIG. 4 is a 50000 SEM characterization of the product of example 2;

FIG. 5 is a graph of the cycling performance of the silicon nanowire/graphite composite anode material of example 1;

fig. 6 is a graph of the rate performance of the silicon nanowire/graphite composite anode material of example 1.

Detailed Description

For a better understanding of the objects, process schemes and advantages of the present invention, the technical scheme and embodiments of the present invention will be further clarified, fully described and explained by the following specific examples, with reference to the accompanying drawings, it being understood that the described examples of the present invention are implemented on the premise of the technical scheme of the present invention, and detailed embodiments and specific operation procedures are given, but only some, but not all, examples of the present invention, and the described embodiments are limited to the description and illustration of the present invention and are not limited to the present invention. All other embodiments, which can be made by those skilled in the art without making any inventive effort, are intended to be within the scope of the present invention, based on the examples herein.

The experimental methods and conditions used in the examples of the present invention are conventional methods and conditions unless otherwise indicated, and the materials, reagents or instrumentation used in the examples, etc., are conventional materials or equipment known to those skilled in the art and are commercially available or prepared by conventional methods. The reaction conditions embodied in the inventive content of the present invention are all such that the reaction is achieved and the desired effect of the product is obtained. Some examples are listed below for the sake of space limitation to further illustrate the advantages of the solution according to the invention.

Example 1

Referring to FIG. 1, FIG. 1 shows a system for synthesizing a silicon nanowire/graphite composite material by in-situ molten salt electrolysis through a diaphragm method, wherein an anode is a spiral metal molybdenum anode 6, a cathode is a porous graphite cathode 7, the porous graphite electrode is prepared by immersing a graphite rod in a potassium hydroxide solution with the molar concentration of 16 mol/L, etching the porous graphite electrode under the condition of 110 ^o C to 6 h, heating the etched graphite rod to 900 ^o ℃ at 5 ℃ per minute in an argon atmosphere, preserving heat for 3 h, and then cooling the etched graphite rod to room temperature along with a furnace to obtain the porous graphite electrode, and a high-aluminum ceramic diaphragm 8 is prepared from a ceramic diaphragm with the components of aAl ₂O₃·b CaO·c SiO₂.dMgO, a=0.95, b=0.02, c=0.02, d=0.01, the porosity is 38%, and the pore diameter is 0.2-2.0 mu m. The molten salt electrolyte is CaCl ₂ and a precursor silicon source CaSiO ₃, the silicon source accounts for 5wt% of the mass of the molten salt electrolyte, the electrolysis temperature is 880 ^o C, and the electrode spacing is 1 cm. Electrolyte CaCl ₂ and precursor CaSiO ₃ are heated to 900 ℃ at 5 ℃ per min under the protection atmosphere of flowing argon, and after heat preservation for 1h, caCl ₂ is ensured to be completely melted, and then cooled to 880 ^o ℃ at 5 ℃ per min. and (3) applying constant voltage of U=2.2V between the cathode and the anode for electrolysis for time t=3h, wherein the electric quantity is 80% of the theoretical electric quantity of the silicon source completely reduced to silicon (the theoretical electric quantity refers to the theoretical electric quantity required for completing conversion of silicon dioxide or calcium silicate serving as the silicon source into silicon according to Faraday law). And washing the cathode porous graphite and the obtained product by 1mol/L dilute hydrochloric acid and deionized water after the electrolysis is finished, and vacuum drying at 80 ^o ℃. The XRD characterization result is shown in figure 2, the main products are silicon nanowires and graphite, and the SEM characterization result is shown in figure 3, and the obtained silicon nanofiber has the diameter of about 88nm, the length-diameter ratio of about 131, the silicon purity of 97% and the mass ratio of the silicon nanowires of about 17%.

The method for calculating the mass ratio of the silicon nanowire comprises the following steps:

The resulting silicon nanowire/graphite composite material was weighed to total mass M ₁, heated with TGA (thermogravimetric analysis) under air, graphite reacted with oxygen at 600 ^o C to CO ₂, the remainder being SiO ₂. Weighing the residual SiO ₂ as the mass 。

Silicon ratio calculation: (coefficient 0.467=si/SiO ₂).

Examples 2 to 20

The molten salt electrolyte CaCl ₂ of example 1 was modified to nacl+cacl ₂ (molar ratio 1:2) in the same manner as in example 1, the melting point of the molten salt was lowered, the electrolysis temperature was lowered to 750 ^o C, and the remaining parameters were kept consistent with example 1. And washing and drying the obtained product after the electrolysis is finished. The electrolytic product obtained at the cathode was found to be a silicon nanowire having a diameter of about 55nm and an aspect ratio of about 125, a silicon purity of 92% and a silicon nanowire ratio of 5% by SEM characterization as shown in fig. 4.

After adjusting the molten salt composition of the electrolyte in example 1, the electrolysis temperature was changed. Other process parameters were the same as in example 1 and the results are shown in Table 1.

It can be seen that the components of the molten salt electrolyte are adjusted to effectively reduce the electrolysis temperature, and as the solubilities of different silicon sources in different molten salt components are different and the diffusion coefficients of silicon ions in different molten salts are also different, the silicon to graphite ratios in different silicon nanowire/graphite composite materials can be obtained. Because the anode byproducts and impurity ions have different behaviors under different molten salt components, the purity of the obtained silicon nanowires is different.

Examples 21 to 26

The conditions for preparing the porous graphite cathode in example 1 were changed according to the method of example 1, the obtained product was washed and dried after the electrolysis was completed, the morphology of the product was characterized by SEM, and the silicon nanofiber ratio was calculated, and the results are shown in table 2. Among them, the sintering time has little influence on the porous graphite rod, and thus the embodiment is omitted.

The results in table 2 show that the increase in KOH solution concentration does not affect the purity of the silicon nanowires, but the silicon nanowire duty cycle increases. The phenomenon is derived from the concentration-dependent etching kinetics that high concentration KOH (7-18 mol/L) generates higher density nano-scale pits (50-500 nm) on the surface of the graphite rod, and the edges of the pits expose rich sp2 carbon defect sites to provide more polymorphonuclear active centers for electrochemical reduction of the silicon precursor. This procedure follows a positive correlation mechanism of etch strength-active site-silicon nanowire growth, optimizing the silicon loading and electrochemical performance of the composite. The sintering temperature (800-900 ℃) of the etched graphite rod has no obvious influence on the loading capacity of the silicon nanowire. This is because the sp2 carbon defect structure formed by KOH etching has thermal stability at a temperature of 300 ℃ or more, and the density of the surface active sites and the chemical reducibility thereof remain substantially unchanged during sintering.

Examples 27 to 32

The electrolysis time t and the voltage U in example 1 were changed according to the method of example 1, the obtained product was washed and dried after the electrolysis was completed, the morphology of the product was characterized by SEM, the current efficiency and the silicon nanofiber ratio were calculated, and the results are shown in table 3.

Table 3 shows that the longer the electrolysis time, the coarser the size of the silicon nanowires, the lower the purity and the duty ratio of the silicon nanowires, and the current efficiency, the coarser the electrolysis voltage, the larger the obtained silicon nanowires, the main factors for regulating the size and the length of the silicon nanofibers, the lower the theoretical electric quantity, the lower the purity of the obtained silicon nanowires, and the key factors for mainly affecting the nucleation rate and the density distribution of the silicon nanofiber growth catalyst, and the current efficiency.

Examples 33 to 38

The composition of the separator material aAl ₂O₃·b CaO·c SiO₂.dMgO of example 1 was adjusted as in example 1, the porosity and pore size were kept substantially unchanged, the resultant product after completion of electrolysis as in example 1 was washed and dried, SEM characterization results were analyzed, and the separator service life was examined, and the results are shown in Table 4.

The results show that the silicon nanowires can be grown without the diaphragm, but the diameter of the silicon nanowires becomes thicker, the length of the silicon nanowires becomes shorter, the purity is reduced, and the ratio of the silicon nanowires to the graphite composite material is lower. The composition of the diaphragm does not affect the growth of the silicon nanowire, only the service life of the diaphragm, and the final product silicon nanowire/graphite composite material.

Example 39

The electrochemical performance of the silicon nanowire/graphite composite was tested using the product obtained in example 1 assembled into a CR2032 button cell.

Firstly, preparing an electrode slice, mixing polyethylene acid (PAA) and deionized water according to the mass ratio of 1:1, and dissolving at 60 ^o ℃ until no particles and no bubbles exist, thus obtaining the adhesive. And mixing the silicon nanowire/graphite composite material, the conductive agent acetylene black (Super-P) and the polyethylene acid (PAA) binder in a mass ratio of 6:2:2, and fully stirring and uniformly mixing in a refiner. The adhesive paste was uniformly applied to copper sheets (diameter 12 mm, thickness 0.1 mm), the paste on each sheet was about 1 mg, dried 12h in a vacuum oven at 80 ^o C, compacted with 18 MPa pressure, and returned to the vacuum oven at 80 ^o C for further drying 8 h.

The battery assembly process was completed in an argon filled glove box (O ₂<0.1 ppm,H₂ O <0.1 ppm), the pole piece was placed in the middle of the positive electrode case, the Cellgard separator with a diameter of 20 mm was placed on the surface of the electrode piece, the electrolyte (1M lithium hexafluorophosphate (LiPF ₆) was dissolved in equal volumes of Ethylene Carbonate (EC), dimethyl carbonate (DEC) and ethylene carbonate (EMC)) was added dropwise, then fresh pieces of metallic lithium were placed on the separator, after adding a suitable amount of electrolyte, the negative electrode case was capped and sealed with a sealer, and 24h was left at 30 ^o C. The half-cell after being placed is tested by using a Shenzhen Neware electronic Co Ltd CT-3008W Neware battery test system under the environment of 30 ^o C, the long-cycle performance test is carried out under the current density of 0.2A g ^-1, and the multiplying power performance of the battery is tested under the current density of 0.2A g ^-1、0.5 A g^-1、1 A g^-1、2 A g^-1、5 A g^-1. The voltage interval tested was 0.01-1.5V. The electrochemical performance is shown in fig. 5 and 6, and after 100 circles of circulation, the specific discharge capacity is stable at 872, 872 mAh/g, and at the current density of 0.5, 0.5A g ^-1、1 A g^-1、2 A g^-1、5 A g^-1, 770 mAh/g,696, 696 mAh/g,638 mAh/g and 506, 506 mAh/g are possessed.

The product of example 1 used in example 39 was changed to other example products and assembled into a CR2032 button cell according to the method of example 39, and the electrochemical performance was tested under the environment of 30 ^o C using a Shenzhen Neware electronic Co., ltd. CT-3008W Neware cell test system, with a voltage interval of 0.01-1.5V at a current density of 0.2A g ^-1, and the rate performance of the cell was tested under a current density of 0.2A g ^-1、0.5 A g^-1、1 A g^-1、2 A g^-1、5 A g^-1. The electrochemical properties are shown in Table 5.

The results show that in the system for synthesizing the silicon nanowire/graphite composite material by the diaphragm method through in-situ molten salt electrolysis, the cycle performance and the multiplying power performance of a battery prepared by a product obtained under the condition of using the diaphragm method are best, the electrochemical performance of a battery prepared by the silicon nanowire/graphite composite product obtained by using different molten salt components in comparative examples 2-17 is lower than that of a battery prepared by using the silicon nanowire/graphite composite product obtained by using the molten salt components in example 1, the electrochemical performance of the prepared battery is poor, the electrochemical performance of the battery prepared by synthesizing the silicon nanowire/graphite composite material in comparative examples 18-20 is lower than that of the battery prepared by synthesizing the silicon nanowire/graphite composite material under different KOH solution concentrations, the proportion of the silicon nanowire of the product obtained under the condition of excessively low KOH solution concentration is lower, the battery capacity is correspondingly reduced, the proportion of the silicon nanowire of the product obtained under the condition of excessively high KOH solution concentration is higher, the silicon volume expansion is out of control, the material pulverization structure is unstable, and the electrochemical performance is poor. Comparative examples 21 and 22 are electrochemical properties of batteries prepared by synthesizing silicon nanowire/graphite composite materials under different electrolysis process conditions, and too high and too low electrolysis voltages at electrolysis time lead to degradation of parameters such as purity and aspect ratio of the silicon nanowire, thus resulting in deterioration of electrochemical properties. The silicon nanowires obtained after using the separator were thinner and longer because the separator can hinder the byproducts of the anode, improve the purity of silicon, and the sample obtained was comparable to that of example 39, except that the other conditions were unchanged, because the contents of the chemical components of the separator were changed only in comparative examples 24 and 25, and the electrochemical performance of the battery was comparable to that of example 39, as compared with the product prepared under the condition of comparative example 23 without the separator.

The above-described embodiments are only preferred embodiments of the present invention, and are not intended to limit the invention in any way, but other variations and modifications are possible without exceeding the technical solutions described in the claims.

Claims

1. A preparation method of a silicon nanowire/graphite composite material synthesized in situ by a fused salt electrolysis method is characterized in that a diaphragm electrolysis tank system is adopted, metal molybdenum is used as an anode, porous graphite is used as a cathode, high-alumina ceramic is used as a diaphragm, electrolyte is a fused salt electrolyte system, the fused salt electrolysis method comprises metal chloride and a silicon source, the silicon source is selected from at least one of silicon dioxide and silicate, and constant voltage electrolysis is performed under the protection of inert gas until 80% -95% of theoretical electric quantity of the silicon source is finished, so that the silicon nanowire/graphite composite material is obtained.

2. The method for preparing the silicon nanowire/graphite composite material by in-situ synthesis through a molten salt electrolysis method according to claim 1, wherein the metal molybdenum anode is at least one of molybdenum wires, molybdenum plates and molybdenum rods.

3. The method for preparing the silicon nanowire/graphite composite material synthesized in situ by a molten salt electrolysis method according to claim 1, wherein the porous graphite cathode is graphite with a three-dimensional porous structure, the pore diameter of which is 50-500 nm and the porosity of which is 30-70%;

And/or the porous graphite is graphite with a three-dimensional porous structure obtained by etching graphite with potassium hydroxide solution and sintering the graphite at high temperature in inert atmosphere;

And/or immersing the graphite rod in potassium hydroxide solution with the molar concentration of 7-18 mol/L, etching at the temperature of 60-120- ^o ℃ to treat 0.5-6 h, heating the etched graphite rod to 500-900- ^o ℃ in inert gas at the heating rate of 1-10 ℃ per min, and sintering to 1-3 h to obtain the graphite with the three-dimensional porous structure, wherein the pore diameter of the substrate is 50-500 nm, and the porosity is 30-70%.

4. The method for preparing the silicon nanowire/graphite composite material by in-situ synthesis by a molten salt electrolysis method according to claim 1, wherein the electrode spacing between the cathode and the anode is 1-5cm.

5. The method for preparing the silicon nanowire/graphite composite material synthesized in situ by a fused salt electrolysis method according to claim 1, wherein the diaphragm is a tubular porous high-alumina ceramic diaphragm, the chemical composition of the diaphragm is a Al₂O₃·b CaO·c SiO₂·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,, the porosity is 35-40%, and the pore diameter is 0.2-2.0 μm.

6. The method for preparing the silicon nanowire/graphite composite material synthesized in situ by a molten salt electrolysis method according to claim 1, wherein the metal chloride of the molten salt electrolyte is MClx, wherein M is at least one metal element selected from Li, na, K, mg, ca, and x is the valence of corresponding metal;

7. The method for preparing the silicon nanowire/graphite composite material by in-situ synthesis by a fused salt electrolysis method, which is characterized in that the temperature of a fused salt electrolyte is controlled to be 700-1000 ^o ℃ during constant-voltage electrolysis;

and/or the regulated voltage is 1.8-3.1V,

And/or the electrolysis time is 1-24 h.

8. The method for preparing the silicon nanowire/graphite composite material synthesized in situ by the molten salt electrolysis method according to claim 1, wherein the diameter of the obtained silicon nanowire/graphite composite material is 20-200 nm, the length-diameter ratio of the silicon nanowire is >100, the silicon purity is > 75%, and the mass ratio of the silicon nanowire is 15-20%.

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

10. Use of the molten salt electrolysis method of claim 9 for in-situ synthesis of silicon nanowire/graphite composite material in the field of lithium ion batteries.