Silicon-carbon composite material as well as preparation method and application thereof

Abstract

The invention provides a silicon-carbon composite material and a preparation method and application thereof.The silicon-carbon composite material comprises a carbon matrix and nanometer silicon particles, the nanometer silicon particles are located in pores of the carbon matrix, the Raman spectrum of the silicon-carbon composite material meets the condition that IA is larger than or equal to 1.2 IB, IA is the peak intensity of the Raman spectrum of the silicon-carbon composite material at 520 cm <-1 > + / -5 cm <-1 >, and IB is the peak intensity of the Raman spectrum of the silicon-carbon composite material at 520 cm <-1 > + / -5 cm <-1 >. And IB is the peak intensity of the Raman spectrum of the silicon-carbon composite material at 480cm <-1 > + / -5cm <-1 >. According to the silicon-carbon composite material provided by the invention, the gas production rate in the anode slurry preparation process can be reduced, the specific capacity of the silicon-carbon composite material is improved, and the high-temperature cycle performance of a secondary battery is improved.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, and in particular to a silicon-carbon composite material, a preparation method of the silicon-carbon composite material, a secondary battery using the silicon-carbon composite material, and an electronic device using the secondary battery. Background Art

[0002] Lithium-ion batteries have been widely used in the fields of electric vehicles, consumer electronics, and large-scale energy storage due to their ultra-high energy density, long cycle life, and low self-discharge characteristics. In recent years, silicon-based materials have gradually entered the industrialization track of lithium-ion battery anodes with their high specific capacity (Li 15 Si4, 3579 mAh g -1 ), rich resource reserves, and reasonable lithiation platform (<0.4V vs. Li / Li + ), and are considered to be the most promising anode materials for the next generation.

[0003] Although silicon-based materials have the above advantages, in the anode pulping process, the active materials formed by silicon are prone to react with water and generate gas, making it easy for bubbles to appear on the surface of the prepared negative electrode sheet, and also reducing the specific capacity of the active materials. Summary of the Invention

[0004] This application provides a silicon-carbon composite material, a preparation method thereof, and an application.

[0005] In the first aspect of this application, a silicon-carbon composite material is provided, which includes a carbon matrix and nano-silicon particles. The nano-silicon particles are located in the pores of the carbon matrix, and the Raman spectrum of the silicon-carbon composite material satisfies: I A ≥1.2I B , where I A is the peak intensity of the Raman spectrum of the silicon-carbon composite material at 520 cm -1 ±5 cm -1 , and I B is the peak intensity of the Raman spectrum of the silicon-carbon composite material at 480 cm -1 ±5 cm -1 .

[0006] In the silicon-carbon composite material provided in this application, the A peak represents the photon acoustic vibration peak of crystalline silicon, and the higher its I A peak intensity, the higher the proportion of crystalline silicon content. The B peak represents the Si-Si stretching vibration peak of amorphous silicon, and the higher its I B peak intensity, the higher the proportion of amorphous silicon content. The inventor found that within a specific range of I A / I BThe ratio can reflect the crystallinity of the silicon domain (the crystallinity is the ratio of the Raman peak intensity of the crystalline region to the Raman peak intensity of the amorphous silicon region in the silicon-carbon composite material), where the silicon domain is composed of crystalline silicon and amorphous silicon in the silicon-carbon composite material. I A and I B Satisfy the above relationship so that the proportions of crystalline silicon and amorphous silicon in the silicon-carbon composite material are both maintained within a suitable range, which is beneficial to reducing the content of amorphous silicon in the silicon-carbon composite material. The main component of amorphous silicon is silicon hydride, and due to its lack of an ordered lattice, silicon hydride is more likely to react with water. Therefore, by controlling the I A / I B ratio can reduce the reaction of silicon hydride with water, thereby reducing the gas generation during the preparation of the anode slurry, reducing the consumption of silicon during the preparation of the anode slurry, and thus facilitating the improvement of the silicon capacity contribution in the silicon-carbon composite material and the specific capacity of the silicon-carbon composite material.

[0007] Based on the first aspect, in some possible implementation manners, 5.3I B ≥I A ≥2.4I B . Compared with crystalline silicon, amorphous silicon has more defect pores inside, which is beneficial to improving the expansion of the silicon-carbon composite material and the cycling performance of the secondary battery. Making 5.3I B ≥I A ≥2.4I B is beneficial to further increasing the proportion of crystalline silicon in the silicon domain of the silicon-carbon composite material. Its ordered lattice will reduce the reaction ability with water, thereby reducing the gas generation during the anode slurry process, and at the same time being beneficial to improving the expansion of the silicon-carbon composite material and the cycling performance of the secondary battery.

[0008] Based on the first aspect, in some possible implementation manners, the silicon-carbon composite material and an aqueous potassium hydroxide solution are mixed to obtain a mixture, and the mixture is kept at T1 for 6 hours and then subjected to an alkali solubility test. Among them, 53°C ≥ T1 ≥ 47°C, the measured alkali solubility of the silicon-carbon composite material is α, α ≤ 20%, the concentration of the aqueous potassium hydroxide solution is 1.78 mol / L, and the mass ratio of potassium hydroxide to the silicon-carbon composite material in the mixture is 4:1. The magnitude of the alkali solubility can reflect the decomposition reaction energy barrier of silicon in the silicon-carbon composite material. In this application, controlling the alkali solubility of the silicon-carbon composite material within a suitable range is beneficial to keeping the silicon-carbon composite material with a certain inertness, so that the silicon in the silicon-carbon composite material has a higher decomposition reaction energy barrier when reacting with water, thereby further reducing the gas generation during the preparation of the anode slurry of the silicon-carbon composite material and increasing the specific capacity of the silicon-carbon composite material under the condition that the I A / I B ratio of the silicon-carbon composite material satisfies a certain relationship.

[0009] Based on the first aspect, in some possible embodiments, 0.1% < α < 15%. This is beneficial for further reducing the gas generation amount during the preparation of the silicon-carbon composite anode slurry and improving the specific capacity of the silicon-carbon composite.

[0010] Based on the first aspect, in some possible embodiments, at T2, the silicon-carbon composite and water are mixed in a mass ratio of 1:8 to obtain a dispersion system, where 27°C ≥ T2 ≥ 23°C. After the dispersion system is sealed and left standing for 48 h, the gas generation of the dispersion system is tested. Based on the total volume of the generated gas, the volume fraction of H2 is less than 3%. When the volume fraction of H2 in the dispersion system is within the above range, silicon in the silicon-carbon composite has greater inertness, which is beneficial for reducing the reaction degree between the silicon-carbon composite and water and for reducing the gas generation amount during the preparation of the anode slurry.

[0011] Based on the first aspect, in some possible embodiments, based on the total volume of the generated gas, the volume fraction of H2 is greater than 0.1% and less than 3%. Within this range, silicon in the silicon-carbon composite has greater inertness, which is beneficial for further reducing the reaction degree between the silicon-carbon composite and water, reducing the gas generation amount during the preparation of the anode slurry, and improving the specific capacity of the silicon-carbon composite.

[0012] Based on the first aspect, in some possible embodiments, the nano-silicon particles include silicon grains, and the average diameter of the silicon grains is 3 nm to 8 nm. When the average grain size of silicon is within the above range, it indicates that the content of crystalline silicon in the silicon domain is high, forming a silicon domain dominated by crystalline silicon, which is beneficial for increasing the decomposition reaction energy barrier of the reaction between silicon and water in the silicon-carbon composite, and thus is beneficial for further reducing the gas generation amount during the preparation of the anode slurry and improving the specific capacity of the silicon-carbon composite.

[0013] Based on the first aspect, in some possible embodiments, the average diameter of the silicon grains is 3 nm to 6 nm. This is beneficial for further increasing the content of crystalline silicon in the silicon domain, further reducing the gas generation amount during the preparation of the anode slurry, and improving the specific capacity of the silicon-carbon composite.

[0014] Based on the first aspect, in some possible embodiments, the silicon-carbon composite contains silicon element, and based on the mass of the silicon-carbon composite, the mass fraction of the silicon element is 42% to 50%. This is beneficial for the silicon-carbon composite to have good specific capacity and energy density while also being beneficial for minimizing the content of the silicon element as much as possible, thereby reducing the contact between the silicon-carbon composite and water and reducing the gas generation amount during the preparation of the anode slurry.

[0015] The second aspect of the present application provides a method for preparing a silicon-carbon composite material, comprising: introducing a silane mixed gas into a carbon matrix, maintaining the temperature at 400°C to 480°C for 2h to 30h to obtain a first intermediate, the silane mixed gas comprising silane gas and an inert gas; placing the first intermediate in an inert gas atmosphere and heat-treating it at 650°C to 750°C for 1h to 6h to obtain a second intermediate; and introducing oxygen into the second intermediate at a temperature of 25°C to 200°C to obtain the silicon-carbon composite material.

[0016] In the preparation method provided by the present application, by heat-treating the first intermediate, amorphous silicon in the first intermediate dehydrogenates to form crystalline silicon, increasing the content of crystalline silicon in the silicon domain, which is beneficial to reducing the content of amorphous silicon in the silicon-carbon composite material, reducing the reaction of the silicon-carbon composite material with water, reducing the gas generation amount during the preparation of the anode slurry, and reducing the consumption of silicon during the preparation of the anode slurry, thereby facilitating the improvement of the capacity contribution of silicon in the silicon-carbon composite material and the specific capacity of the silicon-carbon composite material.

[0017] Based on the second aspect, in some possible embodiments, the carbon matrix includes mesopores, where mesopores refer to pores with a pore diameter of 2nm to 10nm. Based on the sum of the pore volumes of pores with a pore diameter of 0.5nm to 10nm, the pore volume ratio of the mesopores is 40% to 70%. The pore volume ratio of the mesopores within the above specific range is beneficial to improving the high-temperature calcination tolerance of the carbon matrix, and within the specific pore volume and pore diameter of the mesopores in the specific range, it is not only beneficial to the formation of large-sized crystalline silicon by silicon, thus facilitating the formation of a silicon-carbon composite material dominated by crystalline silicon, but also beneficial to reducing the formation of silicon carbide during the high-temperature heating process, and beneficial to improving the specific capacity and first Coulomb efficiency of the silicon-carbon composite material.

[0018] The third aspect of the present application provides a secondary battery, comprising a positive electrode plate, a separator, and a negative electrode plate. The negative electrode plate includes a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material, the negative electrode active material includes graphite, and the negative electrode active material also includes a silicon-carbon composite material. Based on the mass of the negative electrode active material, the mass ratio of the silicon-carbon composite material is 5% to 30%. By adjusting the mass ratio of the silicon-carbon composite material within the above range, the high surface stability of graphite is utilized to protect the interface of the silicon-carbon composite material to a certain extent, thereby reducing the generation of an excessive SEI film, and further improving the specific capacity and cycling performance of the secondary battery.

[0019] Based on the third aspect, in some possible embodiments, the electrolyte includes fluoroethylene carbonate, and based on the mass of the electrolyte, the mass percentage content of fluoroethylene carbonate is 11.5% to 19.5%. By making the secondary battery include an electrolyte satisfying the above settings, the solid electrolyte interface film on the surface of the silicon-carbon composite material can be further stabilized, which is beneficial to improving the high-temperature cycling performance of the secondary battery.

[0020] A third aspect of the present application provides an electronic device, including a secondary battery that powers the electronic device. When the secondary battery has excellent specific capacity and cycling performance, it is beneficial to improve the cycling performance and service life of the electronic device. Detailed implementation manners

[0021] The technical solutions in the embodiments of the present application will be clearly and detailedly described below. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs. The terms used in the specification of this application are only for the purpose of describing specific embodiments, and are not intended to limit this application.

[0022] In the related art, the severe volume change of silicon-based materials during lithiation / delithiation leads to poor cycling performance, and then the electrode pulverizes and fails, which hinders the full play of the electrochemical performance of silicon-based materials. To address this problem, researchers have found that silicon-based materials can be nano-sized, form composite materials with other substances, and change the battery matching system to relieve structural strain, shorten the lithium-ion diffusion distance, and improve the effective electrochemical contact of active substances.

[0023] Carbon materials are considered to be one of the best composite materials for compounding with silicon due to their excellent conductivity and tough strength. Silicon-carbon composite materials have been widely studied and developed, but there are still certain problems at present. For example, in the anode pulping process, silicon-carbon materials are prone to react with water in the high-speed stirring process to generate hydrogen, resulting in gas generation in the slurry, easy appearance of bubbles in the electrode sheet, and loss of specific capacity of active substances.

[0024] Si + 2H2O = SiO2 + 2H2↑

[0025] Therefore, how to enhance the water reaction stability of silicon-carbon materials to ensure the normal operation of the electrode manufacturing process still needs to be further solved.

[0026] In view of the above problems, a silicon-carbon composite material, a preparation method thereof, a secondary battery, and an electronic device are provided in the present application.

[0027] An embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located inside the housing.

[0028] The housing can be a packaging bag obtained by encapsulating with a packaging film (such as an aluminum-plastic film). For example, the secondary battery is a soft-pack battery. In some other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0029] The electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator. The separator is disposed between the positive electrode plate and the negative electrode plate. The electrode assembly may be a stacked structure, which is formed by alternately stacking the positive electrode plate, the separator, and the negative electrode plate. In some other embodiments, the electrode assembly may also be a wound structure, which is formed by winding the stacked positive electrode plate, separator, and negative electrode plate.

[0030] Negative electrode plate

[0031] The negative electrode plate includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector may use at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector, etc., and may also be a composite current collector disclosed in any prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and a polymer substrate. The negative electrode material layer contains a negative electrode active material, and the negative electrode active material includes a silicon-carbon composite material.

[0032] The silicon-carbon composite material includes a carbon matrix and nano-silicon particles. The nano-silicon particles are located in the pores of the carbon matrix. The Raman spectrum of the silicon-carbon composite material satisfies: I A ≥1.2I B where I A is the peak intensity of the Raman spectrum of the silicon-carbon composite material at 520 cm -1 ±5 cm -1 and I B is the peak intensity of the Raman spectrum of the silicon-carbon composite material at 480 cm -1 ±5 cm -1 .

[0033] In this application, the A peak is the peak of the Raman spectrum of the silicon-carbon composite material at 520 cm -1 ±5 cm -1 , and the B peak is the peak of the Raman spectrum of the silicon-carbon composite material at 480 cm -1 ±5 cm -1 . The A peak represents the photon acoustic vibration peak of crystalline silicon. The higher the I A peak intensity, the higher the proportion of crystalline silicon content. The B peak represents the Si-Si stretching vibration peak of amorphous silicon. The higher the I B peak intensity, the higher the proportion of amorphous silicon content. The inventors found that within a specific range of the ratio of I A / I B can reflect the crystallinity of the silicon domain (the crystallinity is the ratio of the Raman peak intensity of the crystalline region to the Raman peak intensity of the amorphous silicon region in the silicon-carbon composite material), where the silicon domain is composed of crystalline silicon and amorphous silicon in the silicon-carbon composite material. I A and I BSatisfy the above relational expression so that the proportions of crystalline silicon and amorphous silicon in the silicon-carbon composite material are both maintained within a suitable range, which is conducive to reducing the content of amorphous silicon in the silicon-carbon composite material. The main component of amorphous silicon is silicon hydride. Due to its lack of an ordered lattice, silicon hydride is more likely to react with water. By controlling the ratio of I A / I B to reduce the reaction of silicon hydride with water, thereby reducing the gas generation amount during the preparation of the anode slurry, reducing the consumption of nano-silicon particles during the preparation of the anode slurry, and thus facilitating the improvement of the capacity contribution of nano-silicon particles in the silicon-carbon composite material and increasing the specific capacity of the silicon-carbon composite material.

[0034] If I A <1.2I B , the proportion of amorphous silicon in the silicon-carbon composite material is relatively large, and there will be pores in the amorphous silicon region (such as pores of ~0.5 nm). The silicon hydride in amorphous silicon has a high reactivity with water and is more likely to react with water, increasing the gas generation amount during the anode slurry process and reducing the specific capacity of the silicon-carbon composite material. In some embodiments, the ratio of I A / I B can be 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 2, 2.1, 2.2, 2.4, 2.5, 2.8, 3, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5, 5.2, 5.3 or any value within the range formed by any two of the above values.

[0035] In some embodiments, 5.3I B ≥I A ≥2.4I B . Compared with crystalline silicon, amorphous silicon has more defect pores inside, which is conducive to improving the expansion of the silicon-carbon composite material and enhancing the cycling performance of the secondary battery. When the relationship between I A and I B is within this range, it is beneficial to further increase the proportion of crystalline silicon in the silicon domain. Its ordered lattice will reduce the reaction ability with water, thereby reducing the gas generation amount during the anode slurry process, further increasing the specific capacity of the silicon-carbon composite material, and at the same time being conducive to improving the expansion of the silicon-carbon composite material and enhancing the cycling performance of the secondary battery.

[0036] In some embodiments, the silicon-carbon composite material and an aqueous potassium hydroxide solution are mixed to obtain a mixture, and the mixture is kept at T1 for 6 h and then subjected to an alkali solubility test. Among them, 53°C ≥ T1 ≥ 47°C, the measured alkali solubility of the silicon-carbon composite material is α, α ≤ 20%, the concentration of the aqueous potassium hydroxide solution is 1.78 mol / L, and the mass ratio of potassium hydroxide to the silicon-carbon composite material in the mixture is 4:1.

[0037] The magnitude of the alkali solubility can reflect the decomposition reaction energy barrier of the nanosilicon particles in the silicon-carbon composite material. As shown in the following chemical reaction:

[0038] Si + H2O + 2KOH = K2SiO3 + 2H2↑

[0039] In the above chemical reaction formula, if the alkali solubility of the silicon-carbon composite material is too high, it indicates that the energy barrier for the reaction of the silicon-carbon composite material with water is low; if the alkali solubility of the silicon-carbon composite material is too low, it indicates that the energy barrier for the reaction of the silicon-carbon composite material with water is high; the magnitude of the alkali solubility can reflect the inertness degree of the silicon-carbon composite material in reacting with water. A lower alkali solubility makes the silicon-carbon composite material have inertness in reacting with water, which is beneficial to reducing the gas generation amount during the preparation of the anode slurry and increasing the specific capacity of the silicon-carbon composite material. In this application, the alkali solubility of the silicon-carbon composite material is set within an appropriate range, which is beneficial for the silicon-carbon composite material to maintain a certain degree of inertness, so that the silicon in the silicon-carbon composite material has a higher decomposition reaction energy barrier in reacting with water, thereby under the condition that the ratio of I A / I B satisfies a certain relational expression, further reducing the gas generation amount during the preparation of the anode slurry and increasing the specific capacity of the silicon-carbon composite material.

[0040] In some embodiments, the alkali solubility of the silicon-carbon composite material can be 0.05%, 0.1%, 0.3%, 0.5%, 1%, 2%, 3%, 4%, 5%, 7%, 9%, 11%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% or any value within the range formed by any two of the above values. In some embodiments, T1 can be 47°C, 48°C, 49°C, 50°C, 51°C, 52°C, 53°C or any value within the range formed by any two of the above values.

[0041] In some embodiments, 0.1% ≤ α ≤ 15%. When the alkali solubility of the silicon-carbon composite material is within this range, the nanosilicon particles have a relatively high reaction inertness in reacting with water, which is beneficial to further reducing the gas generation amount during the preparation of the anode slurry and increasing the specific capacity of the silicon-carbon composite material.

[0042] In some embodiments, at T2 °C, a silicon-carbon composite material and water are mixed in a mass ratio of 1:8 to obtain a dispersion system, where 27 °C ≥ T2 ≥ 23 °C. After the dispersion system is sealed and placed for 48 h, the gas production of the dispersion system is tested. Based on the total volume of the gas production, the volume fraction of H2 is less than 3%. In this application, the volume fraction of H2 in the gas production test of the dispersion system can be controlled to further reflect the reaction energy barrier between the nano-silicon particles and water. At the above mass ratio, when the volume fraction of H2 in the dispersion system formed by the combination of the silicon-carbon composite material and water is within the above range, silicon in the silicon-carbon composite material has greater inertness, which is beneficial to reducing the reaction degree between the silicon-carbon composite material and water and reducing the gas production during the preparation of the anode slurry. When the volume fraction of H2 is relatively large, the reaction degree between the silicon-carbon composite material and water is relatively large, which is not conducive to reducing the gas production during the preparation of the anode slurry and not conducive to improving the specific capacity of the silicon-carbon composite material. In some embodiments, the volume fraction of H2 can be 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 1%, 1.5%, 2%, 2.2%, 2.5%, 2.9% or any value within the range composed of any two of the above values. In some embodiments, T2 can be 23 °C, 24 °C, 25 °C, 26 °C, 27 °C or any value within the range composed of any two of the above values. In some embodiments, when testing the gas production of the dispersion system, based on the total volume of the gas production, the volume fraction of H2 is greater than 0.1% and less than 3%. Within the above range, silicon in the silicon-carbon composite material has greater inertness, which is beneficial to further reducing the reaction degree between the silicon-carbon composite material and water, reducing the gas production during the preparation of the anode slurry, and improving the specific capacity of the silicon-carbon composite material.

[0043] In some embodiments, the nano-silicon particles include silicon crystallites, and the average diameter of the silicon crystallites is 3 nm to 8 nm. The average diameter of the nano-silicon particles can be measured by X-ray diffraction and obtained by calculation. The specifically measured average diameter of the silicon crystallites is the average grain size of crystalline silicon. When the average diameter of the silicon crystallites is within the above specific range, it can reflect the distribution of crystalline silicon in the silicon domain. When the average diameter of the silicon crystallites is within the above range, it indicates that the content of crystalline silicon in the silicon domain is high, forming a silicon domain dominated by crystalline silicon, which is beneficial to increasing the decomposition reaction energy barrier of the reaction between the silicon-carbon composite material and water, thereby further reducing the gas production during the preparation of the anode slurry and increasing the specific capacity of the silicon-carbon composite material. If the average diameter of the silicon crystallites is small, such as less than 3 nm, it indicates that the proportion of amorphous silicon in the silicon domain is relatively large, and amorphous silicon is easy to react with water, which is not conducive to reducing the gas production during the preparation of the anode slurry and not conducive to increasing the specific capacity of the silicon-carbon composite material. If the average grain diameter of the nano-silicon particles is large, such as greater than 8 nm, it will cause the silicon-carbon composite material to generate crystalline Li at the end stage of lithiation. 15Si4 affects the cycle stability of secondary batteries. In some embodiments, the average grain diameter size of the nano-silicon particles in the silicon-carbon composite material can be 3nm, 3.5nm, 4nm, 4.5nm, 5nm, 5.5nm, 6nm, 6.5nm, 7nm, 7.5nm, 8nm, or any value within the range formed by any two of the above values.

[0044] In some embodiments, the average diameter of the silicon grains is 3nm to 6nm. When the average diameter of the silicon grains is within the above range, it is beneficial to further make the silicon domain contain more crystalline silicon, further reduce the gas generation amount during the preparation of the anode slurry, and improve the specific capacity of the silicon-carbon composite material.

[0045] In some embodiments, the silicon-carbon composite material contains silicon element. Based on the mass of the silicon-carbon composite material, the mass ratio of the silicon element is 42% to 50%. When the mass ratio of the silicon element is within the above range, it is beneficial for the silicon-carbon composite material to have good specific capacity and energy density, and at the same time, it is beneficial to minimize the content of the silicon element as much as possible, thereby reducing the contact between the silicon-carbon composite material and water and reducing the gas generation amount during the preparation of the anode slurry. In some embodiments, based on the mass of the silicon-carbon composite material, the mass ratio of the silicon element can be 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or any value within the range formed by any two of the above values.

[0046] In some embodiments, the silicon-carbon composite material further contains carbon element and oxygen element. Based on the mass of the silicon-carbon composite material, the mass ratio of the carbon element is 45% to 57%, and the mass ratio of the oxygen element is 1% to 5%. In the silicon-carbon composite material, the mass ratio of the carbon element within the above specific range is beneficial to reducing the contact between silicon and water, beneficial to reducing the gas generation amount during the preparation of the anode slurry, and improving the specific capacity of the silicon-carbon composite material.

[0047] The mass ratio of the oxygen element within the above specific range is beneficial to improving the stability of the silicon-carbon composite material and improving the cycle stability. In some embodiments, based on the mass of the silicon-carbon composite material, the mass ratio of the carbon element can be 45%, 48%, 50%, 52%, 54%, 56%, 57%, or any value within the range formed by any two of the above values. The mass ratio of the oxygen element can be 1%, 2%, 3%, 4%, 5%, or any value within the range formed by any two of the above values.

[0048] This application also provides a preparation method of a silicon-carbon composite material, including:

[0049] Step S1. Pass a silane mixed gas into the carbon matrix to deposit nano-silicon particles in the carbon matrix to obtain a first intermediate. The silane mixed gas contains silane gas and inert gas.

[0050] By chemical vapor deposition, nanosilicon particles are deposited in the pores of the carbon matrix. Specifically, it includes: in an inert gas atmosphere, the carbon matrix is heated from room temperature to 400 °C to 480 °C at a rate of 5 °C / min to 20 °C / min and then held at this temperature. At this temperature, a silane gas mixture is introduced, the introduction time is 2 h to 30 h, and the flow rate is 100 sccm to 500 sccm. The silane gas mixture deposits silicon in the pores and on the surface of the carbon matrix to obtain a first intermediate.

[0051] In some embodiments, the temperature for holding and heating the carbon matrix can be 400 °C, 420 °C, 440 °C, 460 °C, 480 °C or any value within the range composed of any two of the above values. The heating rate can be 5 °C / min, 8 °C / min, 10 °C / min, 15 °C / min, 20 °C / min or any value within the range composed of any two of the above values. The introduction time of the silicon-carbon gas mixture can be 2 h, 4 h, 8 h, 10 h, 15 h, 20 h, 25 h, 30 h or any value within the range composed of any two of the above values. The flow rate of the silicon-carbon gas mixture can be 100 sccm, 200 sccm, 300 sccm, 400 sccm, 500 sccm or any value within the range composed of any two of the above values.

[0052] The silane gas mixture contains silane gas and inert gas. Based on the mass of the silane gas mixture, the mass ratio of the silane gas is 10% to 50%, and the mass ratio of the inert gas is 50% to 90%. Within the above range, introducing a specific amount of inert gas into the silane gas is beneficial to improving the uniformity of silicon deposition, contributing fully to the silicon capacity in the silicon-carbon composite material, and increasing the specific capacity of the silicon-carbon composite material. In some embodiments, based on the mass of the silane gas mixture, the mass ratio of the silane gas can be 10%, 20%, 30%, 40%, 50% or any value within the range composed of any two of the above values.

[0053] In some embodiments, the silane gas can include but is not limited to at least one of silane, disilane, trisilane, tetrasilane, chlorosilane, dichlorosilane, trichlorosilane or tetrachlorosilane.

[0054] The inert gas includes at least one of nitrogen and argon.

[0055] In some embodiments, the carbon matrix includes mesopores, where mesopores refer to pores with a pore diameter of 2 nm to 10 nm. Based on the sum of the pore volumes of pores with a pore diameter of 0.5 nm to 10 nm, the pore volume ratio of the mesopores is 40% to 70%. The pore volume of the mesopores of the carbon matrix within the above specific range is beneficial to improving the high-temperature calcination tolerance of the carbon matrix. Among the pore volume of the mesopores and the pore diameter of the mesopores within the specific range, it is not only beneficial for the nano-silicon particles to form crystalline silicon with a large particle size, thus facilitating the formation of a silicon-carbon composite material dominated by crystalline silicon, but also beneficial for reducing the silicon carbide generated during the high-temperature heating process, and beneficial for improving the specific capacity and first Coulomb efficiency of the silicon-carbon composite material. If the pore volume ratio of the mesopores is relatively low and the micropore ratio is relatively high, more silicon carbide may be generated in the silicon-carbon composite material during the high-temperature process, reducing the specific capacity and first Coulomb efficiency of the silicon-carbon composite material. If the pore diameter of the mesopores is small, the silicon particles in the micropores may be too small and prone to react with water, and the effect on reducing the gas generation amount during the preparation of the anode slurry is not obvious; if the pore diameter of the mesopores is large, the deposited silicon particle size is large, and the nano-silicon particles are prone to generate Li 15 Si4, reducing the cycle stability of the silicon-carbon composite material. In some embodiments, the pore volume ratio of the mesopores of the carbon matrix can be 40%, 45%, 50%, 55%, 60%, 65%, 70% or any value within the range composed of any two of the above values.

[0056] Step S2. In an inert gas atmosphere, heat-treat the first intermediate at 650 °C to 750 °C for 1 h to 6 h to obtain a second intermediate.

[0057] The inert gas includes at least one of nitrogen and argon.

[0058] Subjecting the first intermediate to secondary heating crystallization at 650 °C to 750 °C is beneficial for the dehydrogenation of amorphous silicon in the first intermediate to form crystalline silicon and increasing the content of crystalline silicon in the silicon domain.

[0059] The crystallization heat-treatment time of the first intermediate is 1 h to 6 h, which is beneficial for obtaining silicon grains with an average diameter within a specific range to adjust the average diameter of the silicon grains in the nano-silicon particles of the first intermediate. In some embodiments, the heat-treatment temperature of the first intermediate can be 650 °C, 670 °C, 690 °C, 710 °C, 720 °C, 740 °C, 750 °C or any value within the range composed of any two of the above values. The heat-treatment time of the first intermediate can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or any value within the range composed of any two of the above values.

[0060] Step S3. Reduce the heat-treatment temperature in Step S2 to 25 °C to 200 °C, and introduce oxygen into the second intermediate to obtain a silicon-carbon composite material.

[0061] In some embodiments, an inert gas and oxygen are mixed and introduced into the second intermediate, and oxygen reacts with silicon on the surface of the second intermediate to form silicon oxide, so as to passivate the surface of the second intermediate. In the oxygen mixture formed by the inert gas and oxygen, based on the mass of the oxygen mixture, the mass fraction of oxygen is 2% to 20%, and the mass fraction of the inert gas (such as nitrogen) is 80% to 98%. When the contents of oxygen and the inert gas are within the above ranges, it is beneficial for the oxygen mixture to passivate the surface of the second intermediate uniformly, so as to stabilize the surface of the second intermediate. In some embodiments, the mass fraction of oxygen can be 2%, 5%, 8%, 10%, 13%, 15%, 20% or any value within the range formed by any two of the above values.

[0062] In some embodiments, the reaction temperature of step S3 can be 25°C, 50°C, 75°C, 100°C, 125°C, 150°C, 175°C, 200°C or any value within the range formed by any two of the above values.

[0063] In some embodiments, the reaction time for oxygen to passivate the second intermediate in step S3 is 1 h to 6 h, and the flow rate of the introduced oxygen mixture is 100 sccm to 500 sccm, so that the oxygen mixture reacts fully with the surface of the second intermediate. The reaction time can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or any value within the range formed by any two of the above values. The flow rate of the introduced oxygen mixture can be 100 sccm, 200 sccm, 300 sccm, 400 sccm, 500 sccm or any value within the range formed by any two of the above values.

[0064] In the method for preparing the silicon-carbon composite material provided by this application, by heating the first intermediate, dehydrogenation of amorphous silicon in the first intermediate forms crystalline silicon, increasing the content of crystalline silicon in the silicon domain, which is beneficial to reducing the content of amorphous silicon in the silicon-carbon composite material, reducing the reaction of the silicon-carbon composite material with water, reducing the gas generation amount during the preparation of the anode paste, and reducing the consumption of silicon during the preparation of the anode paste, thereby facilitating the improvement of the capacity contribution of silicon in the silicon-carbon composite material and the specific capacity of the silicon-carbon composite material.

[0065] In this application, by adjusting the mesopore volume fraction, the pore diameter of the mesopores, the temperature and time for pyrolyzing the first intermediate of the carbon matrix, so that in the Raman spectrum of the silicon-carbon composite material, I A / I BMeet specific relationships, the silicon-carbon composite material maintains an appropriate alkali solubility, the volume fraction of H2 in the gas generated by the dispersion system is within a specific range, and the average diameter of silicon grains in the nano-silicon particles is within a specific range. Based on the appropriate ranges provided in this application, when the pore volume of the mesopores in the carbon matrix is less than 40% and / or the pore diameter of the mesopores is less than 2 nm, it may affect the size of silicon grains in the nano-silicon particles deposited in the carbon matrix, reduce the content of crystalline silicon in the silicon domain, making I A / I B The ratio becomes smaller, the alkali solubility increases, the volume fraction of H2 in the gas generated by the dispersion system increases, and the average diameter of the nano-silicon particles becomes smaller, thereby increasing the gas generation amount during the anode slurry process and reducing the specific capacity of the silicon-carbon composite material. When the pore volume of the mesopores in the carbon matrix is greater than 70% and / or the pore diameter of the mesopores is greater than 10 nm, I A / I B The ratio becomes smaller, the alkali solubility decreases, the volume fraction of H2 in the gas generated by the dispersion system decreases, and the average diameter of the silicon grains becomes larger, but the swelling performance of the secondary battery during the cycling process will increase, reducing the cycling performance of the secondary battery.

[0066] Based on the appropriate ranges provided in this application, when the temperature for pyrolyzing the first intermediate increases, I A / I B The ratio increases, the alkali solubility decreases, the volume fraction of H2 in the gas generated by the dispersion system decreases, and the average diameter of the silicon grains in the nano-silicon particles becomes larger. However, at higher pyrolysis temperatures, more silicon carbide will be generated in the silicon-carbon composite material, reducing the specific capacity of the silicon-carbon composite material. When the temperature for pyrolyzing the first intermediate decreases, I A / I B The ratio decreases, the alkali solubility increases, the volume fraction of H2 in the gas generated by the dispersion system increases, and the average diameter of the silicon grains in the nano-silicon particles becomes smaller, thereby increasing the gas generation amount during the anode slurry process and reducing the specific capacity of the silicon-carbon composite material.

[0067] Based on the appropriate ranges provided in this application, when the time for pyrolyzing the first intermediate increases, I A / I B The ratio increases, the alkali solubility decreases, the volume fraction of H2 in the gas generated by the dispersion system decreases, and the average diameter of the silicon grains in the nano-silicon particles becomes larger. The effect on reducing the gas generation amount during the anode slurry process is not obvious, which is not conducive to improving the specific capacity of the silicon-carbon composite material. When the time for pyrolyzing the first intermediate decreases, it may cause incomplete reaction of the first intermediate, reducing the content of crystalline silicon in the silicon domain, making I A / I BThe ratio decreases, the alkali solubility increases, the volume fraction of H2 in the gas generated by the dispersion system increases, and the average diameter of the silicon crystallites in the nano-silicon particles becomes smaller, which will increase the gas generation amount during the anode slurry process and reduce the specific capacity of the silicon-carbon composite material.

[0068] In some embodiments, the negative electrode active material further includes graphite, and the graphite includes at least one of artificial graphite or natural graphite. Based on the mass of the negative electrode active material, the mass fraction of the silicon-carbon composite material is 5% to 30%, and the mass fraction of the graphite is 70% to 95%. By adjusting the mass fractions of the silicon-carbon composite material and the graphite within the above ranges, and utilizing the high surface stability of the graphite, to protect the interface of the silicon-carbon composite material to a certain extent, thereby reducing the formation of an excessive SEI film, and further improving the specific capacity and cycle performance of the secondary battery. At the same time, using both the silicon-carbon composite material and the graphite as the negative electrode active material can also make full use of the advantages of both the silicon-carbon composite material and the graphite to achieve better electrochemical performance. In some embodiments, the mass fraction of the silicon-carbon composite material can be 5%, 10%, 15%, 20%, 25%, 30% or any value within the range composed of any two of the above values. The mass fraction of the graphite can be 70%, 75%, 80%, 85%, 90%, 95% or any value within the range composed of any two of the above values.

[0069] The negative electrode material layer further contains a binder for binding the negative electrode active material particles to facilitate the formation of a film layer, and at the same time can also improve the bonding force between the negative electrode material layer and the negative electrode current collector. In some embodiments, the binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0070] The negative electrode material layer may further include a conductive material, and the conductive material includes but is not limited to carbon-based materials, metal-based materials, conductive polymers or any combination thereof. In some embodiments, the carbon-based materials may include but are not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, the metal-based materials may include but are not limited to metal powders or metal fibers, such as copper, nickel, aluminum or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0071] Separator

[0072] There are no particular limitations on the material and shape of the separator used in the secondary battery of the present application, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or an inorganic substance formed of a material stable to the electrolyte of the present application.

[0073] For example, the separator may include a base material layer and a surface treatment layer. The base material layer is a non-woven fabric, a film or a composite film having a porous structure, and the material of the base material layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film can be selected.

[0074] A surface treatment layer is provided on at least one surface of the base material layer. The surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from at least one of alumina, silica, magnesia, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinyl pyrrolidone, polyethylene alkoxide, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinyl pyrrolidone, polyethylene alkoxide, polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene).

[0075] Electrolyte

[0076] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and an optional additive. The organic solvent in the electrolyte of the present application can be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. There is no limitation on the electrolyte used in the electrolyte according to the present application, and it can be any electrolyte known in the prior art. The additive of the electrolyte according to the present application can be any additive known in the prior art that can be used as an electrolyte additive. In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene sulfite, or ethyl propionate. In some embodiments, the organic solvent includes an ether solvent, for example, includes at least one of 1,3-dioxolane (DOL) and dimethoxyethane (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalato)borate LiB(C2O4)2 (LiBOB), or lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB).

[0077] In some embodiments, the electrolyte includes fluoroethylene carbonate, and based on the mass of the electrolyte, the mass percentage content of fluoroethylene carbonate is 11.5% to 19.5%. By making the secondary battery include an electrolyte that meets the above settings, the solid electrolyte interface film on the surface of the silicon-carbon composite material can be further stabilized, which is beneficial to improving the high-temperature cycle performance of the secondary battery.

[0078] Positive electrode sheet

[0079] The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on the positive electrode current collector. The positive electrode current collector can be made of aluminum foil, nickel foil, etc., or can be a composite current collector disclosed in any prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foil and polymer substrate. The positive electrode active layer contains a positive electrode active material, and the positive electrode active material includes a compound that can reversibly intercalate and deintercalate lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material can include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material can include but not limited to at least one of lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganate, spinel-type lithium nickel manganate, and lithium titanate.

[0080] The positive electrode active layer also contains a binder, which is used to bond the positive electrode active material particles to facilitate the formation of a film layer and at the same time can also improve the bonding force between the positive electrode active layer and the positive electrode current collector. In some embodiments, the binder can include but not limited to at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0081] The positive electrode active layer may also contain a conductive material, and the conductive material includes but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials can include but not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials can include but not limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0082] The above secondary battery is applied to an electronic device to supply power to a load in the electronic device. Moreover, the proportions of crystalline silicon and amorphous silicon in the silicon-carbon composite material in the above secondary battery are both maintained within a suitable range, which is beneficial to improving the specific capacity of the secondary battery, and thus conducive to extending the service life of the electronic device. Among them, the electronic device may include but is not limited to laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, portable cleaners, portable CD players, mini discs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, motorized bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household storage batteries, and lithium-ion capacitors, etc.

[0083] The present application is described below through specific examples and comparative examples. Those skilled in the art should understand that the preparation methods described in the present application are only examples, and any other suitable preparation methods are within the scope of the present application.

[0084] Example 1-1

[0085] Preparation of silicon-carbon composite material:

[0086] 100 g of a porous carbon skeleton (i.e., a porous carbon matrix, with the pore volume ratio of mesopores being 40%) is heated to 450 °C at a rate of 10 °C / min in a pure argon atmosphere.

[0087] Then, the pure argon atmosphere is switched to a silane mixture gas (by volume percentage, 20% silane and 80% argon), the flow rate of the silane mixture gas is 200 sccm, and the porous carbon skeleton is deposited at 450 °C for 10 h to obtain a first intermediate.

[0088] Pure argon is introduced into the first intermediate to evacuate the silane mixture gas, and then in a pure argon atmosphere, it is heated from 450 °C to 700 °C at a rate of 5 °C / min and reheated at 700 °C for 6 h to obtain a second intermediate.

[0089] Then, after the second intermediate naturally cools to 100 °C in a pure argon atmosphere, an oxygen mixture gas (by volume percentage, 10% oxygen and 90% nitrogen) is introduced, the flow rate of the oxygen mixture gas introduced is 200 sccm, and the second intermediate is passivated in the oxygen mixture gas atmosphere for 2 h to obtain the silicon-carbon composite material.

[0090] Preparation of lithium-ion coin half-cell:

[0091] A mixture of graphite and the silicon-carbon composite material in a weight ratio of 90:10 was used as the negative electrode active material, and the negative electrode active material, polymethyl acrylate and sodium carboxymethyl cellulose were fully stirred and mixed in deionized water in a weight ratio of 97:2:1 to form a uniform negative electrode slurry, wherein the solid content of the negative electrode slurry was 40wt%. The slurry was coated on the negative electrode current collector copper foil, dried at 85°C, and then cold pressed, cut and slit, and dried under vacuum conditions at 120°C for 12 hours to obtain a negative electrode sheet.

[0092] The electrolyte was prepared by mixing LiPF6 ethylene carbonate and diethyl carbonate solution with fluoroethylene carbonate (FEC). Based on the mass of the electrolyte, the mass of LiPF6 accounted for 12.5%, the mass of FEC accounted for 12%, and the remainder was ethylene carbonate and diethyl carbonate in a mass ratio of 1:1. A 7μm polyethylene isolation membrane was used, and lithium sheets were used as counter electrodes to assemble button-type half-cells in a glove box.

[0093] Preparation of lithium-ion full battery:

[0094] Preparation of negative electrode sheet: negative electrode active material (graphite and the above silicon-carbon composite material are mixed in a weight ratio of 80:20), polymethyl acrylate and sodium carboxymethyl cellulose in a weight ratio of 97:2:1, deionized water is added as a solvent, and a negative electrode slurry with a solid content of 40wt% is prepared. The negative electrode slurry is uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 12μm, and dried at 85°C to obtain a negative electrode sheet with a negative electrode material layer coated on one side. The coating weight of the negative electrode material layer during coating is 10mg / cm 2 Then repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet with a negative electrode material layer coated on both sides. Then, after cold pressing, cutting, slitting, and drying at 120°C under vacuum conditions for 12 hours, a negative electrode sheet with a specification of 78mm×875mm is obtained, which is welded to the pole ear for standby use. Among them, the compaction density of the negative electrode material layer after cold pressing is 1.7g / cm 3 ; The negative electrode active material includes artificial graphite and the silicon-carbon composite material prepared above, and the mass ratio of artificial graphite to silicon-carbon composite particles is 80:20.

[0095] Preparation of the positive electrode sheet: The positive active material lithium cobaltate, conductive carbon black Super P, and positive electrode binder polyvinylidene fluoride (PVDF) are mixed according to a mass ratio of 97:1.4:1.6, and N-methylpyrrolidone (NMP) is added as a solvent to prepare a positive electrode slurry with a solid content of 72 wt%. After vacuum stirring evenly, the positive electrode slurry is obtained. The positive electrode slurry is uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 12 μm and dried at 85°C to obtain a positive electrode sheet with a single-sided coated positive electrode material layer. When coating, the coating weight of the positive electrode material layer is 19 mg / cm 2 . Then repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. Then, through cold pressing, slicing, and slitting, it is dried under vacuum conditions at 85°C for 4 h to obtain a positive electrode sheet with a specification of 74 mm × 867 mm. After welding the electrode tab, it is ready for use. Among them, the compaction density of the positive electrode material layer after cold pressing is 4.2 g / cm 3 .

[0096] Preparation of the electrolyte: In a glove box with an argon atmosphere where the water content is less than 10 ppm, fluoroethylene carbonate (FEC), ethylene carbonate (EC), and diethyl carbonate (DEC) are mixed evenly to obtain a basic solvent. Then, lithium salt lithium hexafluorophosphate (LiPF6) is added to the above basic solvent and mixed evenly to obtain the electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of LiPF6 is 12.5%, the mass percentage content of FEC is 15%, the balance is EC and DEC, and the ratio of the mass percentage content of EC and DEC is 1:1.

[0097] Preparation of the separator: A porous polyethylene film with a thickness of 7 μm is used as the separator.

[0098] Preparation of the lithium-ion battery: The positive electrode sheet, separator, negative electrode sheet, and separator are stacked in sequence, and the separator is placed in the middle of the positive electrode sheet and the negative electrode sheet to play a role in isolation, and then wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried at 80°C, and then injected with the electrolyte. After vacuum packaging, standing, formation, shaping, and capacity testing processes, a soft-pack lithium-ion battery is obtained.

[0099] Examples 1-2 to 1-14

[0100] The differences between Examples 1-2 to 1-14 and Example 1-1 are that the temperature and / or time of pyrolyzing the first intermediate are adjusted. Except for adjusting the relevant preparation parameters according to Table 1, the remaining steps are the same as those in Example 1-1.

[0101] Examples 1-15 to 1-17

[0102] Examples 1-15 to 1-17 are different from Example 1-1 in that the pyrolysis time of the first intermediate is adjusted, and the pore volume ratio and average pore diameter of the mesopores of the carbon matrix are changed. Except for adjusting the relevant preparation parameters according to Table 2, the remaining steps are the same as those in Example 1-1.

[0103] Examples 1-18 to 1-20

[0104] Examples 1-18 to 1-20 are different from Example 1-1 in that the deposition time of silicon after introducing the silane mixture gas and the pyrolysis time of the first intermediate are adjusted. Except for adjusting the relevant preparation parameters according to Table 3, the remaining steps are the same as those in Example 1-1.

[0105] Examples 2-1 to 2-4

[0106] Examples 2-1 to 2-4 are different from Example 1-1 in that the content of the electrolyte components is adjusted so that the content of FEC is as shown in Table 4. The sum of the mass percentages of EC and DEC is adjusted correspondingly with the content of FEC, and the ratio of the mass percentages of EC and DEC remains unchanged.

[0107] Comparative Examples 1 to 3

[0108] Comparative Examples 1 to 3 are different from Example 1-1 in that the temperature of pyrolyzing the first intermediate is adjusted. Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as those in Example 1-1.

[0109] Performance test method for silicon-carbon composite materials

[0110] (1) Test and calculation of alkali solubility

[0111] Using a 500 ml volumetric flask, 10 g of potassium hydroxide and an appropriate amount of deionized water are used to prepare a potassium hydroxide aqueous solution with a concentration of 1.78 mol / L, and stirred evenly to obtain a potassium hydroxide solution. 0.5 g of silicon-carbon composite material powder is added to an aluminum-plastic bag, and 20 ml of the pre-prepared potassium hydroxide solution is added. Finally, the aluminum-plastic bag is sealed to ensure that the aluminum-plastic bag is completely sealed and there is no air leakage. The rectangular aluminum-plastic bag is placed in a constant temperature water bath at 50 °C for heat preservation for 6 h. During the reaction process, the volume change is recorded by the drainage method, and the alkali solubility is calculated.

[0112] Alkali solubility calculation formula:

[0113] Among them, the total volume of the aluminum-plastic bag and water at the beginning of immersion in the water bath is V1, and the total volume of the aluminum-plastic bag and water after 6 hours in the water bath is V2. ΔV is the change in the system volume, and ΔV = V2 - V1. P is the ambient pressure (101.325 kPa). R is the molar gas constant, which is 8.314 J / (mol·K). -1 . T is the water bath temperature, which is 323 K. m is the mass of the silicon-carbon composite material; f Si is the mass fraction of Si in the silicon-carbon composite material; Msi is the molar mass of Si.

[0114] (2) Powder gas production test

[0115] At 25 °C, 0.5 g of the silicon-carbon composite material powder and 4 g of deionized water were mixed and placed in a 10 ml sealed bottle. After standing at room temperature (25 °C) for 48 h, chemical gas chromatography analysis (Aglient 7890A, GC) was carried out. The gas production components and the volume ratio of H2 can be obtained.

[0116] (3) Raman test and calculation

[0117] The silicon-carbon composite material was loaded on a flat glass slide for Raman test, and the test range was from 50 cm -1 to 700 cm -1 . After the test was completed, the A peak (520 cm -1 to 700 cm -1 ) of crystalline silicon and the B peak (480 cm -1 ±5 cm -1 ) of amorphous silicon between were observed. The Raman spectrum was background-subtracted by Origin, and then the ratio of the intensity I -1 of the A peak and the intensity I -1 of the B peak was calculated. A and the intensity I B of the B peak.

[0118] (4) Test of the average grain diameter size of silicon in the silicon-carbon composite material

[0119] Using a D 8 Advance device, with a Cu target as the target material, the test was carried out in the angular range from 2θ = 10° to 80° at a voltage of 60 kV. After obtaining the XRD pattern of the silicon-carbon composite material by using Highscore for diffraction pattern analysis, the Debye-Scherrer formula: D = Kλ / βcosθ was used at 2θ = 28.4° to calculate the average diameter size of the silicon grains in the silicon-carbon composite material. Among them, K is the Scherrer constant, D is the silicon grain diameter size, B is the full width at half maximum of the diffraction peak of the measured sample, θ is the Bragg diffraction angle, and γ is the X-ray wavelength.

[0120] (5) Element mass fraction test

[0121] The mass percentage content of each element in the silicon-carbon composite material was measured by means of alkali etching, high-temperature oxidation and thermogravimetric analysis. Specifically, it includes: adding 5 g of the silicon-carbon composite material and 30 g of sodium hydroxide into a crucible, heating to 400 °C and holding for 1 h, then cooling to below 100 °C, putting the reactants into deionized water at 80 °C, washing multiple times with deionized water, filtering and drying, and weighing the mass of the product as m1, which is the mass of carbon. Collect the filtrate and then perform ICP element testing to obtain the mass fraction of Si.

[0122] The element proportion of oxygen was obtained by detecting with an oxygen, nitrogen and hydrogen content analyzer.

[0123] (6) Pore volume / pore size test

[0124] The silicon in the silicon-carbon composite material was removed by the alkali dissolution method. Add 5 g of the silicon-carbon composite material and 30 g of sodium hydroxide into a crucible, mix evenly, heat the mixture to 400 °C and then cool to below 100 °C, then put the reactants into 500 mL of deionized water at 80 °C, wash and filter to obtain a solid product. Then add the solid product into a container containing 50 mL of hydrochloric acid and 450 mL of deionized water and ultrasonically treat for 10 minutes, and then wash and filter the solid with deionized water until neutral, and dry at 80 °C to obtain the carbon skeleton material (i.e., the carbon matrix). The pore volume of the carbon skeleton material of the silicon-carbon negative electrode material was measured by the argon gas adsorption method. After obtaining the adsorption / desorption data, the pore structure was fitted with the NRDFT model to obtain the pore volume data from 0.5 nm to 10 nm and the pore volume data of mesopores respectively.

[0125] (7) Gas production test during the pulping process

[0126] Put 10 g of the negative electrode slurry used to prepare the lithium-ion full battery into a 20 ml glass bottle, seal the glass bottle with a rubber ring and a metal seal, shake the above glass bottle well and ultrasonically for 5 minutes, then let it stand at room temperature (25 °C) for 48 h, and then perform chemical gas chromatography analysis (Aglient 7890A, GC). The gas production components and ratios can be obtained, and thus the volume percentage of hydrogen can be obtained.

[0127] Performance test of lithium-ion coin half-cell / lithium-ion full battery

[0128] (1) Specific capacity

[0129] The coin-type half-cell was first discharged at a small current of 0.05C / 50μA / 10μA in three stages to 0.005V, and then the first discharge capacity of the coin-type half-cell was recorded; then it was charged at a constant current of 0.1C to 0.8V, and the first charge capacity of the coin-type half-cell was recorded. The first efficiency = first charge capacity / first discharge capacity × 100%; the first reversible specific capacity of the negative electrode active material from 0.005V to 0.8V = first charge capacity of the coin-type half-cell / mass of the negative electrode active material.

[0130] (2) High-temperature cycle test

[0131] The lithium-ion battery was placed in an incubator at 45°C, charged at a constant current of 1.5C to 4.4V, charged at a constant voltage of 4.4V to 0.05C, and then discharged at a constant current of 1.0C to 3.0V. This was the first charge-discharge cycle. The charge-discharge cycle test was carried out 300 times in the above manner, and the capacity retention rate was monitored. The capacity retention rate = remaining discharge capacity / initial discharge capacity * 100%.

[0132] The preparation conditions of each example and comparative example are recorded in Tables 1 to 4. And the secondary batteries assembled with the silicon-carbon composite materials prepared in Examples 1-1 to 1-20, Examples 2-1 to 2-4, and Comparative Examples 1 to 3 were tested.

[0133] Table 1

[0134]

[0135]

[0136] In Table 1 above, compared with Comparative Examples 1 to 3, in Examples 1-1 to 1-8, when the Raman spectrum of the silicon-carbon composite material satisfies a specific range, the gas generation amount during the preparation of the negative electrode slurry can be reduced, and the specific capacity of the silicon-carbon composite material can also be improved, thus being beneficial to the service life of the secondary battery.

[0137] Combined with Examples 1-9 to 1-14, when the alkali solubility of the silicon-carbon composite material satisfies an appropriate range, it is beneficial to further reduce the gas generation amount during the preparation of the negative electrode slurry and improve the specific capacity of the silicon-carbon composite material.

[0138] Table 2

[0139]

[0140]

[0141] In Table 2 above, in Examples 1-15 to 1-17, when the average diameter of the silicon crystallites in the nano-silicon particles is within a specific range, it is beneficial for the silicon-carbon composite material to reduce the gas generation amount during the preparation of the negative electrode slurry and is beneficial to improve the specific capacity of the silicon-carbon composite material.

[0142] Table 3

[0143]

[0144] In Table 3 above, in Examples 1-18 to 1-20, when the mass percentage of silicon element in the silicon-carbon composite material is within a specific range, it is beneficial for the silicon-carbon composite material to reduce the gas generation amount during the preparation of the negative electrode slurry and is beneficial to improve the specific capacity of the silicon-carbon composite material.

[0145] Table 4

[0146]

[0147]

[0148] In Table 4, on the basis of Example 1-1, in Examples 2-1 to 2-4, the mass percentage content of vinylene carbonate fluoride in the electrolyte is adjusted. When the mass percentage content of vinylene carbonate fluoride in the electrolyte satisfies the range of 11.5% to 19.5%, it further improves the high-temperature cycle capacity retention rate of the secondary battery.

[0149] The above-disclosed is only the preferred embodiment of the present application. Of course, the present application cannot be limited by this. Therefore, the equivalent changes made according to the present application still fall within the scope covered by the present application.

Claims

1. A silicon-carbon composite material, characterized in that: The invention comprises a carbon matrix and nano-silicon particles, wherein the nano-silicon particles are located in the pores of the carbon matrix, and the Raman spectrum of the silicon-carbon composite material satisfies: I A ≥ 1.2I B , I A The Raman spectrum of the silicon-carbon composite material is at 520 cm -1 ±5cm -1 The peak intensity at I B The Raman spectrum of the silicon-carbon composite material is at 480 cm -1 ±5cm -1 The peak intensity at .

2. The silicon-carbon composite material according to claim 1, characterized in that: 5.3I B ≥I A ≥2.4I B 。 3. The silicon-carbon composite material according to claim 1, characterized in that: The silicon-carbon composite material and a potassium hydroxide aqueous solution are mixed to obtain a mixture, and the mixture is kept warm at T1 for 6 hours and then an alkali solubility test is performed, wherein 53°C ≥ T 1 ≥ 47°C, the alkali solubility of the silicon-carbon composite material is measured to be α, α ≤ 20%, the concentration of the potassium hydroxide aqueous solution is 1.78 mol / L, and the mass ratio of potassium hydroxide to the silicon-carbon composite material in the mixture is 4:

1.

4. The silicon-carbon composite material according to claim 3, characterized in that: 0.1％≤α≤15％。 5. The silicon-carbon composite material according to any one of claims 1 to 4, characterized in that: At T2, the silicon-carbon composite material and water are mixed in a mass ratio of 1:8 to obtain a dispersion system, wherein 27°C ≥ T2 ≥ 23°C, and the dispersion system is sealed and allowed to stand for 48 hours. The gas production of the dispersion system is tested, and based on the total volume of the gas produced, the volume proportion of H2 is less than 3%.

6. The silicon-carbon composite material according to claim 5, characterized in that: Based on the total volume of the produced gas, the volume proportion of H2 is greater than 0.1% and less than 3%.

7. The silicon-carbon composite material according to any one of claims 1 to 4, characterized in that: The nano silicon particles include silicon grains, and the average diameter of the silicon grains is 3nm to 8nm.

8. The silicon-carbon composite material according to claim 7, characterized in that: The average diameter of the silicon grains is 3 nm to 6 nm.

9. The silicon-carbon composite material according to any one of claims 1 to 4, characterized in that: The silicon-carbon composite material contains silicon element, and based on the mass of the silicon-carbon composite material, the mass proportion of the silicon element is 42% to 50%.

10. A method for preparing a silicon-carbon composite material according to any one of claims 1 to 9, characterized in that: include: A silane mixed gas is introduced into the carbon matrix, and the mixture is kept at 400° C. to 480° C. for 2 to 30 hours to obtain a first intermediate, wherein the silane mixed gas comprises silane gas and an inert gas; In an inert gas atmosphere, heat-treating the first intermediate at 650° C. to 750° C. for 1 to 6 hours to obtain a second intermediate; as well as At a temperature of 25° C. to 200° C., oxygen is introduced into the second intermediate to obtain the silicon-carbon composite material.

11. The preparation method according to claim 10, characterized in that: The carbon matrix includes mesopores, wherein the mesopores refer to pores with a pore diameter of 2 nm to 10 nm. Based on the sum of the volumes of pores with a pore diameter of 0.5 nm to 10 nm, the pore volume of the mesopores accounts for 40% to 70%.

12. A secondary battery, comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises a negative electrode active material, wherein the negative electrode active material comprises graphite, characterized in that: The negative electrode active material further comprises the silicon-carbon composite material according to any one of claims 1 to 9 or the silicon-carbon composite material obtained by the preparation method according to claim 10, and the mass proportion of the silicon-carbon composite material is 5% to 30% based on the mass of the negative electrode active material.

13. The secondary battery according to claim 12, characterized in that: The electrolyte includes fluoroethylene carbonate, and based on the mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is 11.5% to 19.5%.

14. An electronic device, characterized in that: A secondary battery comprising the secondary battery according to claim 12 or 13.

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