Silicon-carbon composite material, and preparation method therefor and use thereof

Abstract

A silicon-carbon composite material, and a preparation method therefor and the use thereof. The silicon-carbon composite material comprises a porous silicon core and a coating carbon layer that coats the porous silicon core, wherein each of the porous silicon core and the coating carbon layer comprises a first doping element and a second doping element; the first doping element comprises at least one of silver and copper; the second doping element comprises at least one of boron, aluminum and gallium; the first doping element accounts for 0.2-5% by mass of the silicon-carbon composite material; and the second doping element accounts for 0.5-4% by mass of the silicon-carbon composite material.

Description

Silicon-carbon composite material, preparation method and application thereof

[0001] The present application claims priority to the Chinese patent application No. 2024110987545 filed on August 9, 2024 with the China Patent Office, the whole content of the above application being incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium batteries, in particular to a silicon-carbon composite material, a preparation method and application thereof. BACKGROUND

[0003] In the development process of new energy vehicles, in order to alleviate the problem of long charging time, developing fast charging technology has become one of the main directions of the industry. Under this background, using high power to charge the vehicle power battery has become a trend in market application, and since the high-rate charging method can further reduce the required charging time, the research of high-rate and high-performance power batteries has become a key element to promote the development of the industry.

[0004] Compared with many industries, it is found that the charging rate of small power batteries in the consumer field can often reach more than 10C, while in the automotive field, the charging rate of power batteries is mainly 1C. In addition, the current commercial fast charging is mainly concentrated in the graphite system. TECHNICAL PROBLEM

[0005] When the graphite negative electrode is directly applied to the fast-charging lithium ion battery, due to the lamellar structure of graphite, the diffusion distance of lithium ions in the graphite layer is long, which to some extent affects the rate performance of the lithium battery; in addition, under the condition of fast charging, the lithium intercalation potential in the graphite layer is close to the potential of lithium deposition, which greatly reduces the fast charging cycle performance of the lithium battery. TECHNICAL SOLUTION

[0006] In a first aspect, the present application provides a silicon-carbon composite material, which adopts the following technical solution:

[0007] A silicon-carbon composite material, comprising a porous silicon core and a carbon coating layer coated outside the porous silicon core; the porous silicon core and the carbon coating layer both comprise a first doping element and a second doping element; wherein the first doping element comprises at least one of silver and copper; the second doping element comprises at least one of boron, aluminum and gallium;

[0008] The mass fraction of the first doping element in the porous silicon core is 0.1%-4%, and the mass fraction of the second doping element in the porous silicon core is 0.4%-3%;

[0009] The mass percentage of the first doping element in the carbon coating layer is 2-9%, and the mass percentage of the second doping element in the carbon coating layer is 4-10%.

[0010] In a second aspect, the application provides a preparation method of a silicon-carbon composite material, which adopts the following technical scheme:

[0011] A preparation method of a silicon-carbon composite material, comprising the following steps:

[0012] S1, providing a silicon material, and etching the silicon material to obtain porous silicon powder;

[0013] S2, mixing the porous silicon powder, a first doping element source and a second doping element source, and performing ball milling to obtain the precursor;

[0014] S3, placing the precursor in an inert atmosphere, heating and introducing a carbon source gas, and then cooling to obtain the silicon-carbon composite material.

[0015] In a third aspect, the application provides a negative electrode sheet, which adopts the following technical scheme:

[0016] A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer comprises the silicon-carbon composite material as described above.

[0017] In a fourth aspect, the application provides a lithium ion battery, which adopts the following technical scheme:

[0018] A lithium ion battery, comprising a positive electrode, a separator, an electrolyte and the negative electrode sheet as described above. Advantages

[0019] The application adopts a structure of a carbon coating layer coating a porous silicon core as a negative electrode active material, and controls the mass percentages of the first doping element and the second doping element in the porous silicon core and the carbon coating layer within a suitable range.

[0020] Firstly, since the electronic conductivity of the silicon negative electrode material is low, the introduction of the first doping element and the second doping element can improve the electrical conductivity and thermal conductivity of the silicon negative electrode material, thereby obtaining a negative electrode active material with excellent rate performance and low heat generation temperature rise, which can greatly alleviate the heat generation during fast charging and discharging; in addition, since the second doping element lacks one electron when forming a covalent bond with the four silicon atoms around it, a vacancy will occur at this position, and the corresponding energy state of the vacancy belongs to an impurity energy level, which is usually located below the valence band, thereby greatly reducing the resistivity of the negative electrode active material, and thus helping to improve the rate performance of the negative electrode active material.

[0021] Secondly, the coating of the carbon layer on the porous silicon core can isolate the electrolyte from contacting the porous silicon core, reduce the generation of unstable SEI film, and thus improve the structural stability and electrochemical stability of the negative electrode active material; on the one hand, the first doping element and the second doping element cooperate to not only produce a "buffer skeleton" with a buffer expansion in the porous silicon core, improve the volume stability of the negative electrode active material, and improve the cycle stability of the negative electrode active material, but also reduce the internal resistance of the porous silicon core, which is helpful to improve the fast charging performance of the negative electrode active material; on the other hand, the cooperation of the first doping element and the second doping element can not only improve the expansion buffer effect of the carbon layer on the porous silicon core and improve the cycle stability of the negative electrode active material, but also enable the carbon layer to have better conductivity than ordinary carbon coating layers, which is helpful to improve the conductivity and rate performance of the negative electrode active material. Embodiments of the present application

[0022] In some embodiments, the mass of the first doping element in the porous silicon core is denoted as a, the mass of the first doping element in the carbon coating layer is denoted as b, and a / b satisfies 2-9:1.

[0023] In some embodiments, the mass of the second doping element in the porous silicon core is denoted as c, the mass of the second doping element in the carbon coating layer is denoted as d, and c / d satisfies 3-8:1.

[0024] By controlling the values of a / b and c / d, the distribution of the first doping element and the second doping element in the porous silicon core and the carbon coating layer can be adjusted, which is helpful to obtain a porous silicon core with good rate performance and small volume expansion effect in the fast charging cycle, and a carbon coating layer with better conductivity, which not only improves the conductivity of the negative electrode active material, but also improves the thermal conductivity of the negative electrode active material, and thus improves the fast charging cycle stability and safety of the lithium battery.

[0025] In some embodiments, the particle size of the porous silicon core is 6-12 μm, and / or the thickness of the carbon coating layer is 2-30 nm.

[0026] By controlling the particle size of the porous silicon core and the thickness of the carbon coating layer, both of which cooperate to ensure high lithium ion transmission characteristics while also taking into account the buffer characteristics of the carbon coating layer on the volume expansion of the porous silicon core, the structural stability and electrochemical performance of the negative electrode active material can be improved, and thus the fast charging performance and fast charging cycle stability of the lithium battery can be improved.

[0027] In some embodiments, during the ball milling process in S2, the ball milling speed is 200-400 r / min, and the ball milling time is 2-10 h; in S3, the temperature is raised to 550-1200℃, and the holding time is 0.3-12 h.

[0028] By controlling the ball milling process and the control in the heating process, the diffusion distribution of the first doping element and the second doping element in the porous silicon core and the carbon coating layer is promoted, and a silicon-carbon composite material with excellent fast charging performance is formed.

[0029] In some embodiments, the first doping element source includes at least one of silver element and copper element; and the second doping element source includes at least one of boron element, aluminum element and gallium element.

[0030] In some embodiments, the inert atmosphere includes at least one of nitrogen, helium and argon; and the carbon source gas includes at least one of methane, acetylene, ethylene and ethane.

[0031] In some embodiments, the preparation process of the porous silicon powder includes the following steps:

[0032] A platinum sheet is used as a cathode, a silicon sheet is used as an anode, and electrochemical etching is performed in an etching solution, and after cleaning, a porous silicon powder is obtained;

[0033] The etching solution includes a hydrofluoric acid ethanol solution, and the volume ratio of hydrofluoric acid to ethanol in the hydrofluoric acid ethanol solution is 1:1-8.

[0034] In some embodiments, the electrochemical etching process includes the following steps: etching at a first current density of 550-800 mA·cm -2 for 2-5 min; and continuing to etch at a second current density of 300-500 mA·cm -2 for 1-8 min.

[0035] The porous silicon powder obtained by the etching method has a uniform and dense tubular porous structure, so that the volume expansion generated in the charging and discharging process mainly occurs in the radial direction of the tubular structure, thereby being able to reduce the structural damage of the porous silicon powder caused by the volume expansion in the fast charging process to the greatest extent.

[0036] Example 1

[0037] 1. Preparation of silicon-carbon composite material

[0038] The specific steps for preparing the porous silicon powder in S1 are as follows:

[0039] A platinum sheet is used as a cathode, a silicon sheet is used as an anode, a hydrofluoric acid ethanol solution (the volume ratio of hydrofluoric acid to ethanol is 1:4) is used as an etching solution, and electrochemical etching is performed at a current density of 700 mA·cm -2 for 4 min, and then the electrochemical etching is continued at a current density of 400 mA·cm -2 for 5 min, and after ultrasonic peeling and water washing, a porous silicon powder is prepared.

[0040] S2, mixing the above porous silicon powder, silver element, boron element, ball milling at a speed of 300 r / min for 6h to obtain a porous silicon core;

[0041] S3, placing the above porous silicon core in nitrogen, heating to 800℃ and passing in methane, holding for 6h, then cooling to 25±2℃ to obtain a silicon-carbon composite material.

[0042] The silicon-carbon composite material comprises a 9μm porous silicon core and a carbon coating layer with a thickness of 16nm;

[0043] The mass percentage of silver in the porous silicon core is 2%, and the mass percentage of boron in the porous silicon core is 1.5%;

[0044] The mass percentage of silver in the carbon coating layer is 5%, and the mass percentage of boron in the carbon coating layer is 6%.

[0045] 2. Preparation of positive electrode sheet

[0046] Mixing and stirring the ternary material positive electrode active material (NCM811), polyvinylidene fluoride (PVDF) and conductive carbon black (SP) in a mass ratio of 94:3:3 to obtain a positive electrode slurry, then coating the positive electrode slurry on an aluminum foil through a coating process, and drying and cold pressing to obtain a positive electrode sheet.

[0047] 3. Preparation of negative electrode sheet

[0048] Dissolving the above silicon-carbon composite material, conductive agent SP, conductive agent SWCNT and binder PAA in a solvent in a mass percentage of 80:9:1:10, controlling the solid content to be 30%, coating on a copper foil current collector, vacuum drying to obtain a negative electrode sheet.

[0049] 4. Preparation of lithium battery

[0050] Using the above negative electrode sheet, the above positive electrode sheet, 1mol / L LiPF6 / EC+DMC+EMC (volume ratio of EC:DMC:EMC=1:1:1) electrolyte, PE+aluminum oxide separator, and using a conventional production process to assemble a soft package battery.

[0051] Example 2

[0052] 1. Preparation of silicon-carbon composite material

[0053] The specific steps for preparing the porous silicon powder in S1 are as follows:

[0054] Using a platinum sheet as the cathode, a silicon sheet as the anode, and a hydrofluoric acid ethanol solution (volume ratio of hydrofluoric acid to ethanol is 1:1) as the electrolyte, electrochemical etching was carried out at a current density of 550mA.cm -2 for 5min, and then at a second current density of 450mA.cm -2After etching for 8 min, the porous silicon powder is prepared by ultrasonic stripping and water washing;

[0055] S2, mixing the porous silicon powder, copper and aluminum, and ball milling at a speed of 200 r / min for 10 h to obtain a porous silicon core;

[0056] S3, placing the porous silicon core in helium, heating to 550℃ and introducing acetylene, and then cooling to 25±2℃ after holding for 12 h to obtain a silicon-carbon composite material.

[0057] The silicon-carbon composite material comprises a porous silicon core of 11 μm and a carbon coating layer of 30 nm in thickness;

[0058] The mass percentage of copper in the porous silicon core is 0.5%, and the mass percentage of aluminum in the porous silicon core is 0.8%;

[0059] The mass percentage of copper in the carbon coating layer is 2.5%, and the mass percentage of aluminum in the carbon coating layer is 4.8%.

[0060] 2. Preparation of positive electrode sheet

[0061] The ternary material positive electrode active material (NCM811), polyvinylidene fluoride (PVDF) and conductive carbon black (SP) are mixed and stirred uniformly at a mass ratio of 94:3:3 to obtain a positive electrode slurry, and then the positive electrode slurry is coated on an aluminum foil through a coating process, and then dried and cold-pressed to obtain a positive electrode sheet.

[0062] 3. Preparation of negative electrode sheet

[0063] The silicon-carbon composite material, conductive agent SP, conductive agent SWCNT and binder PAA are dissolved in a solvent at a mass percentage of 80:9:1:10, the solid content is controlled at 30%, and then coated on a copper foil current collector, vacuum dried to obtain a negative electrode sheet.

[0064] 4. Preparation of lithium battery

[0065] The above negative electrode sheet, the above positive electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (volume ratio of EC:DMC:EMC=1:1:1) electrolyte, PE+aluminum oxide separator, and shell are assembled into a soft package battery by using a conventional production process.

[0066] Example 3

[0067] 1. Preparation of silicon-carbon composite material

[0068] The specific steps for preparing the porous silicon powder in S1 are as follows:

[0069] The porous silicon powder is prepared by electrochemical etching with a platinum sheet as a cathode, a silicon sheet as an anode, and a hydrofluoric acid ethanol solution (volume ratio of hydrofluoric acid to ethanol is 1:8) as an electrolyte, under a current density of 800 mA·cm -2 for 2 min, and then under a second current density of 400 mA·cm -2 for 2 min, ultrasonic stripping, and water washing.

[0070] S2, mixing the porous silicon powder, silver, and gallium to obtain a porous silicon core by ball milling at a speed of 400 r / min for 2 h.

[0071] S3, placing the porous silicon core in argon, heating to 1200℃, and introducing ethylene and ethane, and then cooling to 25±2℃ after 1 h of heat preservation, to obtain a silicon-carbon composite material.

[0072] The silicon-carbon composite material includes a porous silicon core with a particle size of 7 μm and a carbon coating layer with a thickness of 2 nm.

[0073] The mass percentage of silver in the porous silicon core is 4%, and the mass percentage of gallium in the porous silicon core is 2.8%.

[0074] The mass percentage of silver in the carbon coating layer is 8.9%, and the mass percentage of gallium in the carbon coating layer is 10%.

[0075] 2. Preparation of the positive electrode sheet

[0076] The positive electrode active material (NCM811), polyvinylidene fluoride (PVDF), and conductive carbon black (SP) are mixed and stirred uniformly at a mass ratio of 94:3:3 to obtain a positive electrode slurry, and then the positive electrode slurry is coated on an aluminum foil through a coating process, and the positive electrode sheet is obtained after drying and cold pressing.

[0077] 3. Preparation of the negative electrode sheet

[0078] The silicon-carbon composite material, conductive agent SP, conductive agent SWCNT, and binder PAA are dissolved in a solvent at a mass percentage of 80:9:1:10, the solid content is controlled at 30%, and the negative electrode sheet is obtained by coating on a copper foil current collector and vacuum drying.

[0079] 4. Preparation of the lithium battery

[0080] The soft package battery is assembled by using the above-mentioned negative electrode sheet, the above-mentioned positive electrode sheet, 1 mol / L LiPF6 / EC+DMC+EMC (volume ratio of EC:DMC:EMC=1:1:1) electrolyte, PE+aluminum oxide separator, and a conventional production process for the shell.

[0081] Example 4

[0082] The difference between this embodiment and embodiment 1 is that the mass of silver in the porous silicon core is recorded as a, the mass of silver in the carbon-coated layer is recorded as b, a / b satisfies 2.6:1; the mass of boron in the porous silicon core is recorded as c, the mass of boron in the carbon-coated layer is recorded as d, c / d satisfies 3.2:1; other steps and parameter settings remain consistent with embodiment 1.

[0083] Embodiment 5

[0084] The difference between this embodiment and embodiment 1 is that the mass of silver in the porous silicon core is recorded as a, the mass of silver in the carbon-coated layer is recorded as b, a / b satisfies 8.8:1; the mass of boron in the porous silicon core is recorded as c, the mass of boron in the carbon-coated layer is recorded as d, c / d satisfies 7.5:1; other steps and parameter settings remain consistent with embodiment 1.

[0085] Embodiment 6

[0086] The difference between this embodiment and embodiment 1 is that the particle size of the porous silicon core is 3 μm, and the thickness of the carbon-coated layer is 35 nm; other steps and parameter settings remain consistent with embodiment 1.

[0087] Embodiment 7

[0088] The difference between this embodiment and embodiment 1 is that the particle size of the porous silicon core is 15 μm, and the thickness of the carbon-coated layer is 2 nm; other steps and parameter settings remain consistent with embodiment 1.

[0089] Embodiment 8

[0090] The difference between this embodiment and embodiment 1 is that the porous silicon powder is prepared by the following steps:

[0091] A platinum sheet is used as the cathode, a silicon sheet is used as the anode, and a hydrofluoric acid-ethanol solution (the volume ratio of hydrofluoric acid to ethanol is 1:10) is used as the electrolyte. The electrochemical etching is carried out at a current density of 500 mA·cm -2 for 9 min, and the porous silicon powder is prepared after ultrasonic peeling and water washing; other steps and parameter settings remain consistent with embodiment 1.

[0092] Comparative Example 1

[0093] The difference between this comparative example and embodiment 1 is that the lithium battery in this comparative example is a commercial fast-charging lithium battery. Specifically, in the commercial fast-charging lithium battery in this comparative example, the positive active material is NCM811, and the negative active material is graphite.

[0094] Comparative Example 2

[0095] The difference between the present comparative example and Example 1 is that the porous silicon core and the carbon coating layer of the silicon-carbon composite material do not contain the first and second doping elements; other steps and parameter settings are consistent with Example 1.

[0096] Comparative Example 3

[0097] The difference between the present comparative example and Example 1 is that the porous silicon core of the silicon-carbon composite material contains the first and second doping elements, but the carbon coating layer does not contain the first and second doping elements; other steps and parameter settings are consistent with Example 1.

[0098] Comparative Example 4

[0099] The difference between the present comparative example and Example 1 is that the porous silicon core of the silicon-carbon composite material does not contain the first and second doping elements, but the carbon coating layer contains the first and second doping elements; other steps and parameter settings are consistent with Example 1.

[0100] Comparative Example 5

[0101] The difference between the present comparative example and Example 1 is that the porous silicon core of the silicon-carbon composite material only contains the first doping element (silver), and the carbon coating layer only contains the second doping element (boron); other steps and parameter settings are consistent with Example 1.

[0102] Comparative Example 6

[0103] The difference between the present comparative example and Example 1 is that the porous silicon core of the silicon-carbon composite material only contains the second doping element (boron), and the carbon coating layer only contains the first doping element (silver); other steps and parameter settings are consistent with Example 1.

[0104] Comparative Example 7

[0105] The difference between the present comparative example and Example 1 is that an equal weight of silicon powder is used instead of porous silicon powder; other steps and parameter settings are consistent with Example 1.

[0106] Test method

[0107] I. Rate performance test

[0108] The rate performance of the lithium batteries of the above examples and comparative examples was tested, and the specific test method was as follows: at 25℃, 1C constant current discharge to 2.5V, stand for 10min, 6C constant current constant voltage charging to 4.2V, 0.05C cut-off, record the constant current charging capacity, constant current constant voltage charging total capacity and the highest temperature in the fast charging process, constant current charging ratio = constant current charging capacity / constant current constant voltage charging total capacity x 100%; the test results are recorded in Table 1.

[0109] II. Cycle performance test

[0110] The lithium batteries of the above examples and comparative examples were subjected to cycle performance test, and the specific test method was as follows: 2C constant current and constant voltage charging to 4.2V at 25℃, 0.05C cut-off, standing for 10min, 2C constant current discharging to 2.5V, standing for 10min, and cycling for 500 cycles, and the capacity retention rate of the lithium battery after 2C / 2C cycling for 500 cycles was recorded. The test results are shown in Table 1.

[0111] III. Element content test

[0112] The silicon element content, fluorine element content and phosphorus element content in the silicon-carbon composite materials prepared in the above examples and comparative examples were tested, and the test method was as follows:

[0113] The ion grinder can perform plane grinding and cross-section cutting on the sample by using argon ions; by using the ion beam grinding (CP) test principle, the argon ions are used to cut the section, and then X-ray energy spectrum analysis (EDS) is performed, so as to calculate the mass ratio of the first doping element in the porous silicon core and the first doping element in the coated carbon layer, and the mass ratio of the second doping element in the porous silicon core and the second doping element in the coated carbon layer.

[0114] IV. Porous silicon core particle size and coated carbon layer thickness test

[0115] TEM was selected for the test, and the test method was as follows:

[0116] (1) The porous silicon core sample prepared in the above examples and comparative examples was dispersed by ultrasonic vibration, the soft agglomeration was removed, and then the sample suspension was prepared by dispersing in water or other solvents;

[0117] (2) The copper mesh covered with carbon film or other polymer film was taken from the prepared sample suspension, and the required amount was taken out or sucked by a dropper, and then dropped on the copper mesh, and then dried by filter paper or air-dried, and then placed in the sample table for testing;

[0118] (3) Photograph in a representative and size step narrow place, and the size d of the porous silicon core was taken by arbitrarily selecting the morphology complete porous silicon core; after the first coated layer coated the porous silicon core sample, the coated carbon layer thickness h was taken by TEM test.

[0119] V. Fast charging time test

[0120] The lithium batteries in the above example 1 and comparative example 1 were subjected to fast charging time test, and the specific test method was as follows: 1C constant current discharging to 2.5V at 25℃, standing for 10min, 1C constant current charging to 10% SOC, standing for 10min, and the time of 6C constant current charging to 80% SOC was the fast charging time.

[0121] The fast charging time of the lithium battery in Example 1 is 7 min, and the fast charging time of the lithium battery in Comparative Example 1 is 35 min.

[0122] VI. Energy density test of lithium battery

[0123] The energy density of the lithium battery in Example 1 and Comparative Example 1 is tested. The specific test method is as follows: the weight of the test battery is weighed and recorded as m; the battery is placed in a clamp, a force of 3000 N is applied, the single battery is charged at a constant current of 0.33 C to 4.2 V, and then rested for 30 min, discharged at a constant current of 0.33 C to 2.5 V, and then rested for 30 min, and the cycle was repeated for 3 times; the discharge capacity (in Ah) and the energy E (average value of three cycles) are calculated, and the discharge energy density is E / m (in Wh / kg).

[0124] The energy density of the lithium battery in Example 1 is 350 Wh.kg -1 The energy density of the lithium battery in Comparative Example 1 is 288.1 Wh.kg -1 .

[0125] Table 1

[0126]

[0127] In combination with Examples 1-3, Comparative Examples 1-7 and Table 1, it can be seen that the fast charging lithium battery prepared by using the above negative electrode active material through the preparation of the porous silicon core and the carbon coating layer included on the surface of the porous silicon core has excellent rate performance and cycle capacity retention. Moreover, compared with the commercial fast charging lithium battery (Comparative Example 1) with fast charging time ≥ 30 min and energy density generally < 300 Wh.kg -1 The fast charging time of the lithium battery in Example 1 is only 7 min (charging interval is 10% SOC-80% SOC), and the energy density can reach 350 Wh.kg -1 .

[0128] This is because, in the present application, by doping the first and second doping elements in the porous silicon core and the carbon coating layer, on the one hand, the first and second doping elements can improve the electrical conductivity and thermal conductivity of the porous silicon core, improve the electrical conductivity and thermal conductivity of the negative electrode active material, and help to improve the structural stability and electrochemical performance of the negative electrode active material; and the doping of the second doping element can generate vacancies in the porous silicon core, which can help to reduce the resistivity of the negative electrode active material, thereby improving the rate performance of the negative electrode active material. On the other hand, the combination of the first and second doping elements can form a "buffer skeleton" in the porous silicon core that can buffer the volume expansion of the porous silicon core, and the combination of the first and second doping elements can also improve the structural stability of the carbon coating layer and the structural stability of the negative electrode active material, thereby improving the fast charging cycle stability of the lithium battery.

[0129] In combination with Examples 1-3, Comparative Example 6 and Table 1, it can be seen that using porous silicon powder to prepare the porous silicon core can provide more buffer space for the volume expansion caused by lithium ion deintercalation, improve the structural stability of the negative electrode active material, and improve the fast charging cycle stability of the lithium battery; since the porous silicon powder has a large specific surface area, it can improve the thermal conductivity of the porous silicon core under the action of the first and second doping elements, greatly relieve the heat generated during the fast charging cycle, and improve the safety of the lithium battery fast charging cycle.

[0130] In combination with Examples 1, Examples 4-5 and Table 1, it can be seen that by controlling the ratio of a / b and c / d to meet the above range, the distribution of the first and second doping elements in the porous silicon core and the carbon coating layer can be adjusted, and a porous silicon core with good rate performance and small volume expansion effect during the fast charging cycle can be obtained. In combination with the carbon coating layer, the electrical conductivity and thermal conductivity of the negative electrode active material are improved, and the cycle performance of the lithium battery is improved.

[0131] In combination with Examples 1, Examples 6-7 and Table 1, it can be seen that by controlling the particle size of the porous silicon core and the thickness of the carbon coating layer, the carbon coating layer can relieve the expansion of the porous silicon core to the greatest extent while ensuring that the negative electrode active material has high lithium ion transmission characteristics, thereby significantly improving the rate performance and cycle stability of the lithium battery.

[0132] In combination with Examples 1, Example 8 and Table 1, it can be seen that the porous silicon powder preparation process of the present application helps to obtain a porous silicon powder with a suitable pore structure that can minimize the damage to the structure of the lithium battery during the fast charging cycle, thereby improving the cycle stability of the lithium battery.

Claims

A silicon-carbon composite material, comprising a porous silicon core and a carbon coating layer covering the porous silicon core; The porous silicon core and the carbon coating layer both comprise a first doping element and a second doping element; wherein The first doping element comprises at least one of silver and copper; and the second doping element comprises at least one of boron, aluminum and gallium; The mass percentage of the first doping element in the porous silicon core is 0.1%-4%, and the mass percentage of the second doping element in the porous silicon core is 0.4%-3%; The mass percentage of the first doping element in the carbon coating layer is 2%-9%, and the mass percentage of the second doping element in the carbon coating layer is 4%-10%. The silicon-carbon composite material according to claim 1, wherein: The mass of the first doping element in the porous silicon core is denoted as a, the mass of the first doping element in the carbon coating layer is denoted as b, and a / b satisfies 2-9:

1. The silicon-carbon composite material according to claim 1, wherein: The mass of the second doping element in the porous silicon core is denoted as c, the mass of the second doping element in the carbon coating layer is denoted as d, and c / d satisfies 3-8:

1. The silicon-carbon composite material according to any one of claims 1-3, wherein: The particle size of the porous silicon core is 6-12 μm, and / or the thickness of the carbon coating layer is 2-30 nm. A preparation method of the silicon-carbon composite material according to any one of claims 1-4, comprising the following steps: S1, providing a silicon material, and etching the silicon material to obtain a porous silicon powder; S2, mixing the porous silicon powder, a first doping element source and a second doping element source, and performing ball milling to obtain the precursor; S3, placing the precursor in an inert atmosphere, heating and introducing a carbon source gas, and then cooling to obtain the silicon-carbon composite material. The method of claim 5, wherein: During the ball milling in S2, the ball milling speed is 200-400 r / min, and the ball milling time is 2-10 h; In S3, the temperature is raised to 550-1200 ℃, and the holding time is 0.3-12 h. The method of claim 5, wherein: The preparation process of the porous silicon powder comprises the following steps: Using a platinum sheet as a cathode and a silicon sheet as an anode, electrochemical etching is performed in an etching solution, and after cleaning, a porous silicon powder is obtained; The etching solution comprises a hydrofluoric acid-ethanol solution, and the volume ratio of hydrofluoric acid to ethanol in the hydrofluoric acid-ethanol solution is 1:1-8. The method of claim 7, wherein: The electrochemical etching process comprises the following steps: etching at a first current density of 550-800 mA.cm -2 etching for 2-5 min; continue etching at a second current density of 300-500 mA.cm -2 etching for 1-8 min. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer comprises the silicon-carbon composite material according to any one of claims 1-4. A lithium ion battery, comprising a positive electrode, a separator, an electrolyte and the negative electrode sheet according to claim 9.

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