A composite material for secondary lithium batteries and a preparation method and application thereof

Abstract

The present application relates to a kind of composite material for secondary lithium battery and preparation method and application;The composite material includes: composite material includes: porous carbon matrix, and SiO x Particle;0<x<1.5;The SiO x Particle is uniformly dispersed in the porous structure of porous carbon matrix, SiO x Particle size size is 2-80nm, is deposited on the porous carbon matrix by siloxane reduction preparation by electrolytic method;The pore size range in porous carbon matrix is 2-100nm;The SiO x The size of the void structure of the outer layer of particle is adjusted by controlling the electrolytic process to the siloxane, the width of the void is 1-100nm, the void structure is used to relieve the SiO x Particle in the volume effect in the secondary lithium battery charge-discharge.

Description

Technical Field

[0001] This invention relates to the field of materials technology, and in particular to a composite material for secondary lithium batteries, its preparation method, and its application. Background Technology

[0002] Today, with continuous economic and social development, humanity has entered an era of urgent energy demand. However, fossil fuels not only cause severe pollution and the greenhouse effect, but also have limited reserves, so we cannot rely solely on fossil fuels for energy. To reduce dependence on fossil fuels, significant efforts have been made in vehicle electrification, and corresponding energy storage devices are still under exploration. Among these newly developed energy storage devices, lithium-ion batteries, with their high energy density, have become the most widely used. Currently, the theoretical capacity of graphite anode materials for lithium-ion batteries is approximately 372 mAh g⁻¹. -1 The development of lithium-ion batteries has been hampered by limitations in electrode capacity. As the energy demands of lithium-ion batteries continue to increase, the limited capacity of commercially available electrodes has become a bottleneck for researchers.

[0003] Silicon-based materials are among the most promising anode materials for lithium-ion batteries. Firstly, silicon possesses an extremely high theoretical capacity of 4200 mAh g / g. -1 The value is far higher than that of graphite. Secondly, silicon-based materials have a lower operating potential (relative to Li / Li). + (≈0.37V) This helps to broaden the operating voltage range of lithium-ion batteries. Furthermore, silicon resources are abundant in mineral form, reducing raw material costs. However, the formation of silicon-lithium alloys during discharge can cause volume expansion of up to 300%. Moreover, the repeated contraction and expansion of the silicon anode material can lead to crack formation and even material pulverization, thereby disrupting the electrical contact between the active material and the current collector, resulting in rapid capacity decay.

[0004] Encapsulating small-sized silicon within carbon materials is one of the most effective methods, as it not only reduces surface cracking but also combines the advantages of silicon (high capacity) and carbon (good conductivity). However, due to their large specific surface energy, uniformly dispersing the nano-silicon and carbon materials presents a challenge. Patent CN 104269521B proposes extruding silicon powder into the pores on the surface of bulk graphite particles, followed by composite formation using pitch as a coating agent, ultimately yielding a carbon / silicon / bulk graphite anode material. This material alleviates, to some extent, the poor cycle stability caused by volume changes during lithium insertion / extraction in silicon-based anodes. However, this method still fails to solve the problem of uniform dispersion of silicon and carbon materials, resulting in limited performance improvement. Summary of the Invention

[0005] This invention provides a composite material for secondary lithium batteries, its preparation method, and its application. This material fully utilizes the advantages of silicon suboxide, solving the problem of easy agglomeration and difficulty in dispersion. Simultaneously, during electrolysis, the degree of reduction and the size of the voids formed around the silicon-oxygen material can be controlled, providing a suitable environment for SiO₂. x The volume expansion allows for space, effectively mitigating the damage to the carbon shell caused by volume effects, thereby maintaining the integrity of the overall structure. The composite material of this invention, when used as the negative electrode of a lithium-ion battery, can effectively improve the battery's rate performance and cycle performance.

[0006] In a first aspect, embodiments of the present invention provide a composite material for secondary lithium batteries, the composite material comprising: a porous carbon matrix and SiO2. x Particles; 0 <x<1.5;

[0007] The SiO x The particles are uniformly dispersed in the porous structure of the porous carbon matrix, SiO x The particle size is 2-80 nm, and it is prepared by depositing siloxane on the porous carbon matrix and then reducing it by electrolysis; the pore size in the porous carbon matrix is ​​2-100 nm.

[0008] The SiO x The size of the void structure on the outer layer of the particles is adjusted by controlling the electrolysis process of the siloxane. The width of the voids is 1-100 nm, and the void structure is used to alleviate the SiO2 content. x The volume effect of particles during the charging and discharging of the secondary lithium battery.

[0009] Preferably, the composite material further has a carbon coating layer on the outside; the mass of the carbon coating layer accounts for 0-20% of the mass of the composite material.

[0010] In a second aspect, embodiments of the present invention provide a method for preparing the composite material for secondary lithium batteries as described in the first aspect, the method comprising:

[0011] Liquid siloxane is vapor-deposited onto porous carbon by bubbling, so that the siloxane is deposited in the porous structure of the porous carbon matrix to obtain matrix A; the gas flow rate is 0.1-1 L / min, the deposition temperature is 500-1000℃, and the deposition time is 1-10 hours.

[0012] The matrix A was subjected to molten salt electrolysis to obtain an electrolyzed sample.

[0013] The electrolyzed sample is washed with water and then dried in a forced-air drying oven to obtain the composite material for secondary lithium batteries.

[0014] Preferably, the liquid siloxane is one or more of trimethoxysilane, tetramethoxysilane, triethoxysilane, and tetraethoxysilane.

[0015] Preferably, the method of molten salt electrolysis of the substrate A specifically includes:

[0016] The dried brine is placed in a crucible, and argon gas is introduced at a flow rate of 0.5-2 L / min. The temperature is raised to 800-900℃ at a heating rate of 2-10℃ / min. After holding at this temperature for half an hour, two graphite sheets, which are connected to the positive and negative electrodes respectively and suspended above the crucible, are placed in the crucible. The conductive wire is one of molybdenum wire, copper wire, or iron wire, and has a diameter of 0.2 mm-1 mm.

[0017] The positive and negative electrode plates formed by two graphite sheets are spaced 2-6 cm apart, and a constant voltage of 2.5-3.0V is applied for pre-electrolysis for 1-2 hours.

[0018] After the pre-electrolysis is completed, continue to heat to 900-1000℃, remove the graphite sheet connected to the negative electrode, connect the conductive wire connected to the negative electrode to the substrate A, with a distance of 2-6cm between the two electrodes, apply a constant voltage of 2.2-3.0V between the two electrodes, and start electrolysis for 5-20 hours.

[0019] Preferably, the water washing specifically includes: ultrasonic washing with ultrapure water 3-8 times to remove molten salt adhering to the sample surface, and removing the detached powder by filtration or centrifugation after each washing;

[0020] The process of drying the sample in a forced-air drying oven specifically includes: placing the sample in a forced-air drying oven and drying it at 50-80℃ for 8-20 hours.

[0021] Preferably, after drying the sample in a forced-air drying oven, the preparation method further includes: applying carbon coating to the surface of the dried material; the carbon coating specifically includes at least one of gas phase coating, liquid phase coating, and solid phase coating.

[0022] Preferably, the conductive wire is one of molybdenum wire, copper wire or iron wire, with a diameter of 0.2-1 mm.

[0023] Thirdly, embodiments of the present invention provide a negative electrode sheet comprising the composite material for secondary lithium batteries described in the first aspect above.

[0024] Fourthly, embodiments of the present invention provide a lithium battery comprising the composite material for secondary lithium batteries described in the first aspect above.

[0025] This invention provides a composite material for secondary lithium batteries. It involves vapor deposition of silicon oxide materials uniformly into porous carbon via bubbling of liquid siloxane, followed by molten salt electrolytic reduction to obtain the composite material. This material fully utilizes the advantages of silicon suboxide, solving the problem of easy agglomeration and difficult dispersion. Simultaneously, the degree of reduction and the size of the voids formed around the silicon oxide materials can be controlled during electrolysis, providing a suitable environment for SiO₂. x The volume expansion allows for sufficient space, effectively mitigating the damage to the carbon shell caused by volume effects, thereby maintaining the integrity of the overall structure. The composite material of this invention, when used as the negative electrode of a lithium-ion battery, can effectively improve the battery's rate performance and cycle performance. Attached Figure Description

[0026] The technical solutions of the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples.

[0027] Figure 1 This is a schematic diagram of the structure of the composite material used in a secondary lithium battery according to an embodiment of the present invention;

[0028] Figure 2 This is a flowchart of a method for preparing a composite material for a secondary lithium battery according to an embodiment of the present invention. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, it should be understood that these embodiments are only for more detailed description and should not be construed as limiting the present invention in any way, that is, not intended to limit the scope of protection of the present invention.

[0030] This invention proposes a composite material for secondary lithium batteries, comprising a porous carbon matrix and SiO₂. x Particles, 0 <x<1.5。 Figure 1 This is a schematic diagram of the composite material used in a secondary lithium battery according to an embodiment of the present invention. Figure 1 As shown, SiO x Particles 2 are uniformly dispersed in the porous structure of the porous carbon matrix 1. The pore size in the porous carbon matrix ranges from 2 to 100 nm. The porous structure is shown as number 5 in the figure. x The particles, ranging in size from 2 to 80 nm, were prepared by depositing siloxanes onto a porous carbon matrix followed by electrolytic reduction; SiO x The size of the void structure 3 on the outer layer of particle 2 is adjusted by controlling the electrolysis process of siloxane. The width of the void is preferably 1-100 nm. The void structure can be used to alleviate the SiO2 degradation. x The volume effect of particle 2 during the charging and discharging of secondary lithium batteries. The porosity structure 3 in... Figure 1 This is for illustrative purposes only; its actual structure can be based on SiO2.x Particle 2 has uniformly or unevenly distributed voids around its outer layer.

[0031] Furthermore, the exterior of the composite material may also have a carbon coating layer 4; the mass of the carbon coating layer accounts for 0-20% of the mass of the composite material.

[0032] This invention provides a composite material for secondary lithium batteries. This material fully utilizes the advantages of silicon suboxide, solving the problem of easy agglomeration and difficult dispersion. Simultaneously, during electrolysis, the degree of reduction and the size of the voids formed around the silicon oxide material can be controlled, providing a suitable environment for SiO₂. x The volume expansion allows for space, effectively mitigating the damage to the carbon shell caused by volume effects, thereby maintaining the integrity of the overall structure. The composite material of this invention, when used as the negative electrode of a lithium-ion battery, can effectively improve the battery's rate performance and cycle performance.

[0033] The composite material for secondary lithium batteries of this invention can be obtained by the following preparation method, the main process steps of which are as follows: Figure 2 As shown, it includes:

[0034] Step 110: Liquid siloxane is vapor-deposited onto porous carbon by bubbling, so that the siloxane is deposited in the porous structure of the porous carbon matrix to obtain matrix A;

[0035] The carrier gas can be an inert gas such as nitrogen or argon, the gas flow rate is 0.1-1 L / min, the deposition temperature is 500-1000℃, and the deposition time is 1-10 hours.

[0036] Step 120: The matrix A is subjected to molten salt electrolysis to obtain the electrolyzed sample;

[0037] Specifically, dried brine salts, such as one or more of calcium chloride, magnesium chloride, sodium chloride, and potassium chloride, can be placed in a crucible. Argon gas with a flow rate of 0.5-2 L / min is introduced, and the temperature is raised to 800-900℃ at a heating rate of 2-10℃ / min to form molten salt. After holding at this temperature for half an hour, two graphite sheets are placed above the crucible and suspended by conductive wires connected to the positive and negative electrodes, respectively. The conductive wires are one of molybdenum wire, copper wire, or iron wire, with a diameter of 0.2 mm-1 mm.

[0038] The positive and negative electrode plates formed by two graphite sheets are spaced 2-6 cm apart, and a constant voltage of 2.5-3.0V is applied for pre-electrolysis for 1-2 hours; the pre-electrolysis removes impurities and moisture from the molten salt.

[0039] After pre-electrolysis, the temperature is raised to 900-1000℃. The graphite sheet connected to the negative electrode is removed, and the conductive wire connected to the negative electrode is connected to the substrate A. The distance between the two electrodes is 2-6cm. A constant voltage of 2.2-3.0V is applied to start electrolysis. The electrolysis time is 5-20 hours to obtain the electrolyzed sample.

[0040] During the electrolysis process described above, the siloxane in matrix A undergoes controlled electrolytic reduction, resulting in the removal of some O ions that react with the graphite sheet connected to the positive electrode to form CO2, which is then released. Simultaneously, this process increases the porosity around the silicon-oxygen material. This porosity helps to mitigate the formation of SiO2 from the electrolytic reduction of siloxane. x The volume effect of particles during the charging and discharging of secondary lithium batteries.

[0041] Step 130: Wash the electrolyzed sample with water, and then dry the sample in a forced-air drying oven to obtain the composite material for secondary lithium batteries.

[0042] The water washing process specifically includes: ultrasonic washing with ultrapure water 3-8 times to remove molten salts adhering to the sample surface, and removing the detached powder by filtration or centrifugation after each wash;

[0043] The specific steps for drying the sample in a forced-air drying oven include: placing the sample in a forced-air drying oven and drying it at 50-80℃ for 8-20 hours.

[0044] Furthermore, after drying the sample in a forced-air drying oven, the dried material can be coated with carbon on its surface; carbon coating can specifically include at least one of gas phase coating, liquid phase coating, and solid phase coating.

[0045] This invention utilizes the above-described method to uniformly deposit silicon-oxygen materials into porous carbon through vapor deposition using liquid siloxane bubbling. Then, it achieves controllable reduction (i.e., SiO₂) via molten salt electrolytic reduction with controlled reduction degree. x The composite material (with controllable x) simultaneously forms voids, resulting in a porous structure.

[0046] The composite material for secondary lithium batteries proposed in this invention can be used to prepare negative electrode sheets for use in secondary lithium batteries such as lithium-ion batteries and solid-state lithium batteries. Using the composite material of this invention as the negative electrode of a lithium-ion battery can effectively improve the rate performance and cycle performance of the battery.

[0047] To better understand the technical solution provided by the present invention, the following uses several specific examples to illustrate the specific process of preparing composite materials for secondary lithium batteries using the methods provided in the above embodiments of the present invention, as well as the method and battery characteristics of applying them to secondary batteries.

[0048] To better understand the technical solution provided by the present invention, the following uses several specific examples to illustrate the specific process of preparing composite materials using the method provided in the above embodiments of the present invention.

[0049] Example 1

[0050] This embodiment provides a method for preparing a composite material and the material itself. The preparation method specifically includes the following steps:

[0051] (1) Liquid siloxane trimethoxysilane was vapor-deposited on porous carbon by N2 bubbling. The gas flow rate was 1L / min, the deposition temperature was 1000℃, and the deposition time was 10 hours to obtain matrix A.

[0052] (2) Molten Salt Electrolysis: Dry anhydrous calcium chloride was placed in a crucible. Two graphite sheets were suspended above the crucible by 0.2 mm molybdenum wires connected to the positive and negative electrodes, respectively. Argon gas was introduced at a flow rate of 0.5 L / min, and the temperature was raised to 800 °C at a rate of 2 °C / min. After holding at this temperature for half an hour, the two graphite sheets were removed. A constant voltage of 2.5 V was applied between the two graphite sheets for pre-electrolysis, which lasted for 1 hour. After pre-electrolysis, the temperature was further increased to 900 °C, and the pre-electrolyzed negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by 0.2 mm molybdenum wires, was then placed into the molten salt. The distance between the two electrodes was 2 cm. A constant voltage of 2.2 V was applied between the electrodes to begin electrolysis, which lasted for 20 hours.

[0053] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed three times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. Then it was dried at 50°C for 8 hours and then coated with carbon by vapor phase coating to obtain silicon-carbon composite material.

[0054] Example 2

[0055] This embodiment provides a method for preparing a composite material and the material itself. The preparation method specifically includes the following steps:

[0056] (1) Liquid siloxane tetramethoxysilane was vapor-deposited on porous carbon by N2 bubbling. The gas flow rate was 0.9 L / min, the deposition temperature was 950 °C, and the deposition time was 9 hours to obtain matrix A.

[0057] (2) Molten Salt Electrolysis: Dry anhydrous sodium chloride was placed in a crucible. Two graphite sheets, each connected to the positive and negative electrodes by 0.3 mm copper wires, were suspended above the crucible. Argon gas was introduced at a flow rate of 1 L / min, and the temperature was raised to 850°C at a rate of 3°C / min. After holding at this temperature for half an hour, the two graphite sheets were removed. A constant voltage of 2.6 V was applied between the two graphite sheets for pre-electrolysis for 2 hours. After pre-electrolysis, the temperature was raised to 950°C, and the pre-electrolyzed negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by 0.3 mm copper wires, was then placed into the molten salt. The distance between the two electrodes was 3 cm. A constant voltage of 2.3 V was applied between the electrodes, and electrolysis began for 18 hours.

[0058] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed four times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 55°C for 9 hours and then coated with carbon through solid-phase coating to obtain a silicon-carbon composite material.

[0059] Example 3

[0060] This embodiment provides a method for preparing a composite material and the material itself. The preparation method specifically includes the following steps:

[0061] (1) Liquid siloxane triethoxysilane was vapor-deposited on porous carbon by bubbling. The gas flow rate was 0.8 L / min, the deposition temperature was 900 °C, and the deposition time was 8 hours to obtain matrix A.

[0062] (2) Molten Salt Electrolysis: Dry anhydrous magnesium chloride was placed in a crucible. Two graphite sheets were each suspended above the crucible by 0.4 mm iron wires connected to the positive and negative electrodes, respectively. Argon gas was introduced at a flow rate of 2 L / min, and the temperature was raised to 900°C at a rate of 4°C / min. After holding at this temperature for half an hour, the two graphite sheets were removed. A constant voltage of 2.7 V was applied between the two graphite sheets for pre-electrolysis for 2 hours. After pre-electrolysis, the temperature was raised to 1000°C, and the pre-electrolyzed negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by a 0.4 mm copper wire, was then placed into the molten salt. The distance between the two electrodes was 4 cm. A constant voltage of 2.4 V was applied between the two electrodes to begin electrolysis, which lasted for 18 hours.

[0063] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed 5 times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 60°C for 10 hours and then coated with carbon by liquid phase coating to obtain silicon-carbon composite material.

[0064] Example 4

[0065] This embodiment provides a method for preparing a composite material and the material itself. The preparation method specifically includes the following steps:

[0066] (1) Liquid siloxane tetraethoxysilane was vapor-deposited on porous carbon by bubbling. The gas flow rate was 0.7 L / min, the deposition temperature was 850 °C, and the deposition time was 7 hours to obtain matrix A.

[0067] (2) Molten Salt Electrolysis: Dry anhydrous potassium chloride was placed in a crucible. Two graphite sheets, each connected to the positive and negative electrodes by 0.5 mm molybdenum wires, were suspended above the crucible. Argon gas was introduced at a flow rate of 2 L / min, and the temperature was raised to 850°C at a rate of 5°C / min. After holding at this temperature for half an hour, the two graphite sheets were removed. A constant voltage of 2.8 V was applied between the two graphite sheets for pre-electrolysis for 2 hours. After pre-electrolysis, the temperature was further increased to 1000°C, and the pre-electrolyzed negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by 0.5 mm molybdenum wires, was then placed into the molten salt. The distance between the two electrodes was 5 cm. A constant voltage of 2.5 V was applied between the electrodes to begin electrolysis, which lasted for 20 hours.

[0068] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed 6 times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 65°C for 12 hours and then coated with carbon through solid-phase coating to obtain a silicon-carbon composite material.

[0069] Example 5

[0070] This embodiment provides a method for preparing a composite material and the material itself. The preparation method specifically includes the following steps:

[0071] (1) Liquid siloxanes trimethoxysilane and tetramethoxysilane were vapor-deposited on porous carbon by bubbling. The gas flow rate was 0.6 L / min, the deposition temperature was 800 °C, and the deposition time was 6 hours to obtain matrix A.

[0072] (2) Molten Salt Electrolysis: Dry anhydrous calcium chloride and sodium chloride were placed in a crucible. Two graphite sheets, each connected to the positive and negative electrodes by 0.6 mm molybdenum wires, were suspended above the crucible. Argon gas was introduced at a flow rate of 1 L / min, and the temperature was raised to 800°C at a rate of 6°C / min. After holding at this temperature for half an hour, the two graphite sheets were lowered. A constant voltage of 2.9 V was applied between the two graphite sheets for pre-electrolysis for 1 hour. After pre-electrolysis, the temperature was raised to 900°C, and the pre-electrolyzed negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by 0.6 mm molybdenum wires, was then placed into the molten salt. The distance between the two electrodes was 6 cm. A constant voltage of 2.6 V was applied between the electrodes to begin electrolysis, which lasted for 15 hours.

[0073] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed 7 times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 70°C for 12 hours and then coated with carbon by vapor phase coating to obtain a silicon-carbon composite material.

[0074] Example 6

[0075] This embodiment provides a method for preparing a composite material, including the following steps:

[0076] (1) Liquid siloxane trimethoxysilane was vapor-deposited on porous carbon by bubbling. The gas flow rate was 0.5 L / min, the deposition temperature was 750 °C, and the deposition time was 5 hours to obtain matrix A.

[0077] (2) Molten Salt Electrolysis: Dry anhydrous calcium chloride and potassium chloride were placed in a crucible. Two graphite sheets, each connected to the positive and negative electrodes by 0.7 mm copper wires, were suspended above the crucible. Argon gas was introduced at a flow rate of 1 L / min, and the temperature was raised to 900°C at a rate of 6°C / min. After holding at this temperature for half an hour, the two graphite sheets were lowered. A constant voltage of 2.9 V was applied between the two graphite sheets for pre-electrolysis for 1 hour. After pre-electrolysis, the temperature was raised to 1000°C. The negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by 0.7 mm copper wires, was then placed into the molten salt. The distance between the two electrodes was 2 cm. A constant voltage of 2.7 V was applied between the electrodes to begin electrolysis, which lasted for 10 hours.

[0078] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed 8 times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 50°C for 16 hours and then coated with carbon by liquid phase coating to obtain silicon-carbon composite material.

[0079] Example 7

[0080] This embodiment provides a method for preparing a composite material and the material itself. The preparation method specifically includes the following steps:

[0081] (1) Liquid siloxane trimethoxysilane was vapor-deposited on porous carbon by bubbling. The gas flow rate was 0.4 L / min, the deposition temperature was 700 °C, and the deposition time was 4 hours to obtain matrix A.

[0082] (2) Molten Salt Electrolysis: Dry anhydrous calcium chloride and magnesium chloride were placed in a crucible. Two graphite sheets were each suspended above the crucible by 0.8 mm iron wires connected to the positive and negative electrodes, respectively. Argon gas was introduced at a flow rate of 1 L / min, and the temperature was raised to 800°C at a rate of 8°C / min. After holding at this temperature for half an hour, the two graphite sheets were removed. A constant voltage of 2.5 V was applied between the two graphite sheets for pre-electrolysis for 1 hour. After pre-electrolysis, the temperature was further increased to 900°C, and the pre-electrolyzed negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by 0.8 mm iron wires, was then placed into the molten salt. The distance between the two electrodes was 4 cm. A constant voltage of 2.8 V was applied between the two electrodes to begin electrolysis, which lasted for 8 hours.

[0083] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed 5 times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 60°C for 14 hours and then coated with carbon through solid-phase coating to obtain a silicon-carbon composite material.

[0084] Example 8

[0085] This embodiment provides a method for preparing a composite material and the material itself. The preparation method specifically includes the following steps:

[0086] (1) Liquid siloxane trimethoxysilane was vapor-deposited on porous carbon by bubbling. The gas flow rate was 0.3 L / min, the deposition temperature was 650 °C, and the deposition time was 3 hours to obtain matrix A.

[0087] (2) Molten Salt Electrolysis: Dry anhydrous sodium chloride and magnesium chloride were placed in a crucible. Two graphite sheets, each connected to the positive and negative electrodes by 1mm molybdenum wires, were suspended above the crucible. Argon gas was introduced at a flow rate of 2L / min, and the temperature was raised to 900℃ at a rate of 5℃ / min. After holding at this temperature for half an hour, the two graphite sheets were removed. A constant voltage of 2.5V was applied between the two graphite sheets for pre-electrolysis for 1.5 hours. After pre-electrolysis, the temperature was further increased to 1000℃, and the pre-electrolyzed negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by 1mm molybdenum wires, was then placed into the molten salt. The distance between the two electrodes was 5cm. A constant voltage of 2.5V was applied between the two electrodes to begin electrolysis, which lasted for 7 hours.

[0088] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed 5 times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 60°C for 18 hours and then coated with carbon by vapor phase coating to obtain a silicon-carbon composite material.

[0089] Example 9

[0090] This embodiment provides a method for preparing a composite material and the material itself.

[0091] The preparation method specifically includes the following steps:

[0092] (1) Liquid siloxane trimethoxysilane was vapor-deposited on porous carbon by bubbling. The gas flow rate was 0.2 L / min, the deposition temperature was 600 °C, and the deposition time was 2 hours to obtain matrix A.

[0093] (2) Molten Salt Electrolysis: Dry anhydrous sodium chloride and potassium chloride were placed in a crucible. Two graphite sheets, each connected to the positive and negative electrodes by 0.9 mm molybdenum wires, were suspended above the crucible. Argon gas was introduced at a flow rate of 2 L / min, and the temperature was raised to 800°C at a rate of 5°C / min. After holding at this temperature for half an hour, the two graphite sheets were lowered. A constant voltage of 2.8 V was applied between the two graphite sheets for pre-electrolysis for 1 hour. After pre-electrolysis, the temperature was raised to 900°C, and the pre-electrolyzed negative electrode graphite sheet was lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by 0.9 mm molybdenum wires, was then placed into the molten salt. The distance between the two electrodes was 6 cm. A constant voltage of 2.8 V was applied between the electrodes to begin electrolysis for 6 hours.

[0094] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed 6 times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 75°C for 18 hours and then coated with carbon by liquid phase coating to obtain silicon-carbon composite material.

[0095] Example 10

[0096] This embodiment provides a method for preparing a small-sized, highly dispersed silicon-carbon composite material with a porous structure, including the following steps:

[0097] (1) Liquid siloxane trimethoxysilane, tetramethoxysilane, triethoxysilane and a mixture of tetraethoxysilane were vapor-deposited on porous carbon by bubbling. The gas flow rate was 0.1 L / min, the deposition temperature was 500 °C and the deposition time was 1 hour to obtain matrix A.

[0098] (2) Molten Salt Electrolysis: Dry anhydrous calcium chloride, sodium chloride, and potassium chloride are placed in a crucible. Two graphite sheets are each wound with 1mm molybdenum wires connected to the positive and negative electrodes, respectively, and suspended above the crucible. Argon gas is introduced at a flow rate of 2L / min, and the temperature is raised to 900℃ at a rate of 10℃ / min. After holding at this temperature for half an hour, the two graphite sheets are lowered. A constant voltage of 3.0V is applied between the two graphite sheets for pre-electrolysis, which lasts for 2 hours. After pre-electrolysis, the temperature is raised to 1000℃, and the pre-electrolyzed negative electrode graphite sheet is lifted from the molten salt and suspended in the air. The substrate A, connected to the negative electrode by a 1mm molybdenum wire, is then placed into the molten salt. The distance between the two electrodes is 6cm. A constant voltage of 3.0V is applied between the two electrodes, and electrolysis begins for 5 hours.

[0099] (3) After electrolysis, the electrolyzed sample was taken out and ultrasonically washed 8 times with ultrapure water to remove the molten salt adhering to the sample surface. The powder that fell off after each washing was collected. The sample was dried at 80°C for 20 hours to obtain a silicon-carbon composite material, without carbon coating.

[0100] To better demonstrate the characteristics of the composite material prepared by the present invention, a comparison is made using Comparative Example 1 as follows.

[0101] Comparative Example 1

[0102] This comparative example provides a method for preparing silicon-carbon composite materials under existing technology, including the following steps:

[0103] (1) In an ethanol system, silicon particles, carbon source precursor polyvinylpyrrolidone, graphite and antioxidant citric acid are added and milled in a mass ratio of 1:1:1:0.1 to obtain a dispersion.

[0104] (2) The dispersion was spray-dried to obtain silicon carbide powder.

[0105] (3) The powder is then vapor-coated to obtain a silicon-carbon composite material.

[0106] The silicon-carbon composite materials obtained in the above embodiments and comparative examples were compounded with commercial graphite in a certain proportion to form a composite material with a capacity of 450 mAh / g. This composite material was then assembled with lithium cobalt oxide to form a coin cell, and its cycle performance was evaluated by cycling at 1C. The data are recorded in Table 1.

[0107]

[0108] Table 1

[0109] As shown in Table 1, the present invention can further improve the first-efficiency and cycle performance of the material by adjusting the deposition time and the molten salt electrolysis time, especially the molten salt electrolysis time. When the molten salt electrolysis time is short, the silicon oxide content is high, resulting in a lower specific charge capacity and coulombic efficiency. With the extension of the electrolysis time, the specific charge capacity and coulombic efficiency continuously increase. When the electrolysis time exceeds a certain period, the specific charge capacity continues to rise; however, due to the higher silicon content, its volume expansion becomes more pronounced, thus its coulombic efficiency begins to decrease. Furthermore, by adjusting the carbon coating layer, the composite material with carbon coating exhibits higher first-efficiency and cycle performance compared to that without carbon coating. Moreover, in the comparative examples, the silicon-carbon composite material prepared by conventional mechanical mixing has a higher first-efficiency but poorer cycle performance; the composite materials obtained in the embodiments of the present invention have better cycle performance.

[0110] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite material for secondary lithium batteries, characterized in that, The composite material comprises: a porous carbon matrix and SiO2. x Particles; 0 <x<1.5; The Si O x The particles are uniformly dispersed in the porous structure of the porous carbon matrix, SiO x The particle size is 2-80 nm, and it is prepared by depositing siloxane on the porous carbon matrix and then reducing it by electrolysis; the pore size in the porous carbon matrix is ​​2-100 nm. The Si O x The size of the void structure on the outer layer of the particles is adjusted by controlling the electrolysis process of the siloxane. The width of the voids is 1-100 nm. The void structure is used to alleviate the SiO2 content. x The volume effect of particles during the charging and discharging of the secondary lithium battery; The siloxane deposition on the porous carbon matrix specifically involves vapor deposition of liquid siloxane via bubbling, thereby depositing the siloxane into the porous structure of the porous carbon matrix. The liquid siloxane specifically includes one or more of trimethoxysilane, tetramethoxysilane, triethoxysilane, and tetraethoxysilane.

2. The composite material for secondary lithium batteries according to claim 1, characterized in that, The composite material also has a carbon coating layer on its exterior; the mass of the carbon coating layer accounts for 0-20% of the mass of the composite material.

3. A method for preparing the composite material for secondary lithium batteries according to claim 1, characterized in that, The preparation method includes: Liquid siloxane is vapor-deposited onto porous carbon by bubbling, so that the siloxane is deposited in the porous structure of the porous carbon matrix to obtain matrix A; the gas flow rate is 0.1-1 L / min, the deposition temperature is 500-1000℃, and the deposition time is 1-10 hours. The matrix A was subjected to molten salt electrolysis to obtain an electrolyzed sample. The electrolyzed sample is washed with water and then dried in a forced-air drying oven to obtain the composite material for secondary lithium batteries.

4. The preparation method according to claim 3, characterized in that, The liquid siloxane specifically includes one or more of trimethoxysilane, tetramethoxysilane, triethoxysilane, and tetraethoxysilane.

5. The preparation method according to claim 3, characterized in that, The method of molten salt electrolysis of the substrate A specifically includes: The dried brine is placed in a crucible, and argon gas is introduced at a flow rate of 0.5-2 L / min. The temperature is raised to 800-900℃ at a heating rate of 2-10℃ / min. After holding at this temperature for half an hour, two graphite sheets, which are connected to the positive and negative electrodes respectively and suspended above the crucible, are placed in the crucible. The conductive wire is one of molybdenum wire, copper wire, or iron wire, and has a diameter of 0.2 mm-1 mm. The positive and negative electrode plates formed by two graphite sheets are spaced 2-6 cm apart, and a constant voltage of 2.5-3.0V is applied for pre-electrolysis for 1-2 hours. After the pre-electrolysis is completed, continue to heat to 900-1000℃, remove the graphite sheet connected to the negative electrode, connect the conductive wire connected to the negative electrode to the substrate A, with a distance of 2-6cm between the two electrodes, apply a constant voltage of 2.2-3.0V between the two electrodes, and start electrolysis for 5-20 hours.

6. The preparation method according to claim 3, characterized in that, The water washing specifically includes: ultrasonic washing with ultrapure water 3-8 times to remove molten salts adhering to the sample surface, and removing the detached powder by filtration or centrifugation after each washing. The process of drying the sample in a forced-air drying oven specifically includes: placing the sample in a forced-air drying oven and drying it at 50-80℃ for 8-20 hours.

7. The preparation method according to claim 3, characterized in that, After drying the sample in a forced-air drying oven, the preparation method further includes: applying carbon coating to the surface of the dried material; the carbon coating specifically includes at least one of gas phase coating, liquid phase coating, and solid phase coating.

8. The preparation method according to claim 5, characterized in that, The conductive wire is one of molybdenum wire, copper wire or iron wire, with a diameter of 0.2-1mm.

9. A negative electrode comprising the composite material for a secondary lithium battery as described in claim 1.

10. A lithium battery comprising the composite material for a secondary lithium battery as described in claim 1.

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