Silicon-carbon composite material, preparation method and application thereof

Abstract

The application specifically discloses a silicon-carbon composite material and a preparation method and application thereof. The silicon-carbon composite material comprises 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; the mass proportion of the first doping element in the silicon-carbon composite material is 0.2%-5%; and the mass proportion of the second doping element in the silicon-carbon composite material is 0.5%-4%. The application has the advantages of reducing the internal resistance of a lithium battery, improving the rate performance of the lithium battery and the fast-charging cycle stability.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to a silicon-carbon composite material, its preparation method, and its application. Background Technology

[0002] In the development of new energy vehicles, fast charging technology has become one of the main directions for the industry to alleviate the problem of long charging times. Against this backdrop, using high-power charging for vehicle batteries has become a market trend. Among these trends, high-rate charging can further reduce charging time, making the research of high-rate, high-performance power batteries a key factor driving industry development.

[0003] Comparing various industries, research shows that in the consumer sector, small power batteries often achieve charging rates of over 10C, while in the automotive sector, power batteries primarily operate at 1C charging rates. Furthermore, current commercial fast charging technologies mainly focus on graphite systems, but directly applying graphite anodes to fast-charging lithium-ion batteries still faces some bottlenecks. Due to the layered structure of graphite, lithium ions diffuse over a relatively long distance between graphite layers, which to some extent affects the rate performance of lithium batteries. Additionally, under fast-charging conditions, the lithium intercalation potential in the graphite layers approaches the lithium deposition potential, significantly reducing the fast-charging cycle performance of lithium batteries.

[0004] Therefore, the current fast-charging performance of lithium-ion batteries is not good, which is not conducive to the industrial application of lithium batteries. Summary of the Invention

[0005] To improve the rate performance and fast-charging cycle stability of lithium batteries, this application provides a silicon-carbon composite material, its preparation method, and its application.

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

[0007] A silicon-carbon composite material includes a porous silicon core and a carbon coating layer covering the porous silicon core; both the porous silicon core and the carbon coating layer include a first dopant element and a second dopant element; wherein the first dopant element includes at least one of silver and copper; and the second dopant element includes at least one of boron, aluminum, and gallium.

[0008] The first dopant element accounts for 0.1%-4% of the mass of the porous silicon core, and the second dopant element accounts for 0.4%-3% of the mass of the porous silicon core.

[0009] The first dopant element accounts for 2%-9% of the mass of the coated carbon layer, and the second dopant element accounts for 4%-10% of the mass of the coated carbon layer.

[0010] This application employs a structure where a porous silicon core is coated with a carbon layer as the negative electrode active material, and controls the mass ratio of the first dopant and the second dopant in the porous silicon core and the coated carbon layer to be within a suitable range: First, since the silicon negative electrode material itself has low electronic conductivity, the introduction of the first dopant and the second dopant can improve the 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 heat generation during fast charging and discharging; In addition, since the second dopant lacks one electron when it forms a covalent bond with the four silicon atoms around it, a vacancy will appear at that location. The energy state of this vacancy belongs to the impurity energy level, which is usually located below the band gap and close to the valence band, which will greatly reduce the resistivity of the negative electrode active material, thereby helping to further improve the rate performance of the negative electrode active material. Secondly, the carbon coating layer covering the porous silicon core can isolate the electrolyte from contact with the porous silicon core, reducing the formation of unstable SEI films, thereby improving the structural and electrochemical stability of the negative electrode active material. Furthermore, on the one hand, the combination of the first and second dopant elements can not only generate a "buffer framework" with buffer expansion in the porous silicon core, improving the volume stability and cycle stability of the negative electrode active material, but also reduce the internal resistance of the porous silicon core, which helps to improve the fast-charging performance of the negative electrode active material. On the other hand, the combination of the first and second dopant elements can not only improve the expansion buffering effect of the carbon coating layer on the porous silicon core, further improving the cycle stability of the negative electrode active material, but also enable the carbon coating layer to have better conductivity than ordinary carbon coating layers, which helps to further improve the conductivity and rate performance of the negative electrode active material.

[0011] Preferably, the mass of the first dopant element in the porous silicon core is denoted as a, and the mass of the first dopant element in the coated carbon layer is denoted as b, where a / b satisfies 2-9:1.

[0012] Preferably, the mass of the second dopant element in the porous silicon core is denoted as c, and the mass of the second dopant element in the coated carbon layer is denoted as d, where c / d satisfies 3-8:1.

[0013] By controlling the values ​​of a / b and c / d, the distribution of the first and second doping elements in the porous silicon core and the coated carbon layer can be further adjusted. This helps to obtain a porous silicon core with good rate performance and small volume expansion effect during fast charging cycles. Furthermore, combined with a coated carbon layer with better conductivity, it can not only further improve the conductivity of the negative electrode active material, but also improve the thermal conductivity of the negative electrode active material, which helps to improve the fast charging cycle stability and safety of lithium batteries.

[0014] Preferably, the porous silicon core has a particle size of 6-12 μm, and / or the carbon coating layer has a thickness of 2-30 nm.

[0015] By controlling the particle size of the porous silicon core and the thickness of the carbon coating layer, the two work together to ensure high lithium-ion transport characteristics while also taking into account the buffering characteristics of the carbon coating layer on the volume expansion of the porous silicon core. This helps to improve the structural stability and electrochemical performance of the negative electrode active material, thereby improving the fast charging performance and fast charging cycle stability of the lithium battery.

[0016] Secondly, this application provides a method for preparing silicon-carbon composite materials, employing the following technical solution:

[0017] A method for preparing a silicon-carbon composite material includes the following steps:

[0018] S1, providing silicon material, and etching the silicon material to obtain porous silicon powder;

[0019] S2, the porous silicon powder, the first doping element source, and the second doping element source are mixed and ball-milled to obtain the precursor;

[0020] S3, the precursor is placed in an inert atmosphere, heated and a carbon source gas is introduced, and after being kept at the temperature, it is cooled to obtain the silicon-carbon composite material.

[0021] Preferably, in 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.

[0022] By controlling the ball milling process and the heating process, it is helpful to promote the diffusion distribution of the first dopant element and the second dopant element in the porous silicon core and the coated carbon layer, forming a silicon-carbon composite material with excellent fast charging performance.

[0023] Preferably, the first doping element source includes at least one of elemental silver and elemental copper; the second doping element source includes at least one of elemental boron, elemental aluminum, and elemental gallium.

[0024] Preferably, the inert atmosphere includes at least one of nitrogen, helium, and argon; the carbon source gas includes at least one of methane, acetylene, ethylene, and ethane.

[0025] Preferably, the preparation process of the porous silicon powder includes the following steps:

[0026] Using a platinum sheet as the cathode and a silicon sheet as the anode, electrochemical etching is performed in an etching solution, and porous silicon powder is obtained after cleaning.

[0027] The etching solution includes a hydrofluoric acid-ethanol solution, wherein the volume ratio of hydrofluoric acid to ethanol in the hydrofluoric acid-ethanol solution is 1:1-8.

[0028] Preferably, the electrochemical etching process includes the following steps: at a first current density of 550-800 mA.cm -2 Etching continues for 2-5 minutes at a second current density of 300-500 mA / cm². -2 Etch for 1-8 minutes.

[0029] The porous silicon powder obtained by etching using this method has a uniform and dense tubular porous structure, which allows the volume expansion generated during charging and discharging to mainly occur radially within the tubular structure. This can minimize the structural damage to the porous silicon powder caused by the volume expansion during fast charging and discharging.

[0030] Thirdly, this application provides a negative electrode sheet, which adopts the following technical solution:

[0031] A negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer comprises a silicon-carbon composite material as described above.

[0032] Fourthly, this application provides a lithium-ion battery, which adopts the following technical solution:

[0033] A lithium-ion battery includes a positive electrode, a separator, an electrolyte, and a negative electrode sheet as described above. Detailed Implementation

[0034] To better understand and implement this application, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some of the embodiments of this application, and not all of them.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0036] Unless otherwise stated, all numerical values ​​for the amounts of expressed components, reaction conditions, etc., used in the specification and claims are to be understood as being modified by the term "about". Therefore, unless otherwise indicated, the numerical parameters set forth herein are approximate values ​​that can be varied to obtain the desired performance.

[0037] The word “and / or” as used in this article refers to one or all of the elements mentioned.

[0038] The terms "include" and "contain" as used in this article cover both cases where only the mentioned elements exist and cases where other unmentioned elements exist in addition to the mentioned elements.

[0039] All percentages in this application are weight percentages unless otherwise stated.

[0040] Unless otherwise stated, the terms “a,” “an,” “an,” and “the” as used in this specification are intended to include “at least one” or “one or more.” For example, “a component” refers to one or more components, and therefore more than one component may be considered and may be employed or used in the implementation of the described embodiments.

[0041] Example 1

[0042] 1. Preparation of silicon-carbon composite materials

[0043] The specific steps for preparing porous silicon powder in S1 are as follows:

[0044] A platinum sheet was used as the cathode, a silicon wafer as the anode, and a hydrofluoric acid-ethanol solution (hydrofluoric acid to ethanol volume ratio of 1:4) was used as the etching solution at 700 mA / cm². -2 Electrochemical etching was performed for 4 minutes at a current density of 400 mA.cm. -2 After 5 minutes of electrochemical etching, ultrasonic stripping, and water washing, porous silicon powder was obtained.

[0045] S2, the above porous silicon powder, elemental silver and elemental boron are mixed and ball-milled at 300 r / min for 6 h to obtain porous silicon cores;

[0046] S3. The porous silicon cores were placed in nitrogen, heated to 800°C and methane was introduced. After holding at this temperature for 6 hours, the temperature was lowered to 25±2°C to obtain the silicon-carbon composite material.

[0047] The silicon-carbon composite material includes a 9μm porous silicon core and a 16nm thick coated carbon layer;

[0048] Silver accounts for 2% of the mass of the porous silicon core, and boron accounts for 1.5% of the mass of the porous silicon core;

[0049] Silver accounts for 5% of the mass of the carbon coating, and boron accounts for 6% of the mass of the carbon coating.

[0050] 2. Preparation of positive electrode sheet

[0051] The ternary cathode active material (NCM811), polyvinylidene fluoride (PVDF), and conductive carbon black (SP) are mixed and stirred evenly in a mass ratio of 94:3:3 to obtain a cathode slurry. The cathode slurry is then coated onto aluminum foil through a coating process, and after drying and cold pressing, a cathode sheet is obtained.

[0052] 3. Preparation of negative electrode sheet

[0053] The above-mentioned silicon-carbon composite material, conductive agent SP, conductive agent SWCNT and binder PAA were dissolved in a solvent at a mass percentage of 80:9:1:10 and mixed. The solid content was controlled at 30%. The mixture was coated on a copper foil current collector and vacuum dried to obtain the negative electrode sheet.

[0054] 4. Lithium-ion battery manufacturing

[0055] The soft-pack battery is assembled using the above-mentioned negative electrode, the above-mentioned positive electrode, 1 mol / L LiPF6 / EC+DMC+EMC (EC:DMC:EMC volume ratio = 1:1:1) electrolyte, PE+alumina separator, and shell using conventional manufacturing processes.

[0056] Example 2

[0057] 1. Preparation of silicon-carbon composite materials

[0058] The specific steps for preparing porous silicon powder in S1 are as follows:

[0059] A platinum sheet was used as the cathode, a silicon sheet as the anode, and a hydrofluoric acid-ethanol solution (hydrofluoric acid to ethanol volume ratio of 1:1) as the electrolyte. The electrolyte was applied at 550 mA / cm². -2 Electrochemical etching was performed for 5 minutes at a current density of 450 mA, followed by etching at a second current density of 450 mA. -2 Porous silicon powder was prepared by electrochemical etching for 8 minutes, followed by ultrasonic stripping and water washing.

[0060] S2, the above porous silicon powder, copper and aluminum are mixed and ball-milled at 200 r / min for 10 h to obtain porous silicon cores;

[0061] S3. The porous silicon cores were placed in helium, heated to 550°C and acetylene was introduced. After holding at this temperature for 12 hours, the temperature was lowered to 25±2°C to obtain a silicon-carbon composite material.

[0062] The silicon-carbon composite material includes an 11μm porous silicon core and a 30nm thick coated carbon layer;

[0063] Copper accounts for 0.5% of the mass of the porous silicon core, and aluminum accounts for 0.8% of the mass of the porous silicon core.

[0064] Copper accounts for 2.5% of the mass of the carbon coating, and aluminum accounts for 4.8% of the mass of the carbon coating.

[0065] 2. Preparation of positive electrode sheet

[0066] The ternary cathode active material (NCM811), polyvinylidene fluoride (PVDF), and conductive carbon black (SP) are mixed and stirred evenly in a mass ratio of 94:3:3 to obtain a cathode slurry. The cathode slurry is then coated onto aluminum foil through a coating process, and after drying and cold pressing, a cathode sheet is obtained.

[0067] 3. Preparation of negative electrode sheet

[0068] The above-mentioned silicon-carbon composite material, conductive agent SP, conductive agent SWCNT and binder PAA were dissolved in a solvent at a mass percentage of 80:9:1:10 and mixed. The solid content was controlled at 30%. The mixture was coated on a copper foil current collector and vacuum dried to obtain the negative electrode sheet.

[0069] 4. Lithium-ion battery manufacturing

[0070] The soft-pack battery is assembled using the above-mentioned negative electrode, the above-mentioned positive electrode, 1 mol / L LiPF6 / EC+DMC+EMC (EC:DMC:EMC volume ratio = 1:1:1) electrolyte, PE+alumina separator, and shell using conventional manufacturing processes.

[0071] Example 3

[0072] 1. Preparation of silicon-carbon composite materials

[0073] The specific steps for preparing porous silicon powder in S1 are as follows:

[0074] A platinum sheet was used as the cathode, a silicon sheet as the anode, and a hydrofluoric acid-ethanol solution (hydrofluoric acid to ethanol volume ratio of 1:8) as the electrolyte. The electrolyte was applied at 800 mA / cm². -2 Electrochemical etching was performed for 2 minutes at a current density of 400 mA, followed by etching at a second current density of 400 mA. -2 Porous silicon powder was prepared by electrochemical etching for 2 minutes, followed by ultrasonic stripping and water washing.

[0075] S2, the above porous silicon powder, elemental silver and elemental gallium are mixed and ball-milled at 400 r / min for 2 h to obtain porous silicon core;

[0076] S3. The porous silicon cores were placed in argon gas, heated to 1200℃ and ethylene and ethane were introduced. After holding at this temperature for 1 hour, the temperature was lowered to 25±2℃ to obtain a silicon-carbon composite material.

[0077] The silicon-carbon composite material includes a porous silicon core with a particle size of 7 μm and a coated carbon layer with a thickness of 2 nm;

[0078] Silver accounts for 4% of the mass of the porous silicon core, and gallium accounts for 2.8% of the mass of the porous silicon core;

[0079] Silver accounts for 8.9% of the mass of the carbon coating, and gallium accounts for 10% of the mass of the carbon coating.

[0080] 2. Preparation of positive electrode sheet

[0081] The ternary cathode active material (NCM811), polyvinylidene fluoride (PVDF), and conductive carbon black (SP) are mixed and stirred evenly in a mass ratio of 94:3:3 to obtain a cathode slurry. The cathode slurry is then coated onto aluminum foil through a coating process, and after drying and cold pressing, a cathode sheet is obtained.

[0082] 3. Preparation of negative electrode sheet

[0083] The above-mentioned silicon-carbon composite material, conductive agent SP, conductive agent SWCNT and binder PAA were dissolved in a solvent at a mass percentage of 80:9:1:10 and mixed. The solid content was controlled at 30%. The mixture was coated on a copper foil current collector and vacuum dried to obtain the negative electrode sheet.

[0084] 4. Lithium-ion battery manufacturing

[0085] The soft-pack battery is assembled using the above-mentioned negative electrode, the above-mentioned positive electrode, 1 mol / L LiPF6 / EC+DMC+EMC (EC:DMC:EMC volume ratio = 1:1:1) electrolyte, PE+alumina separator, and shell using conventional manufacturing processes.

[0086] Example 4

[0087] The difference between this embodiment and Embodiment 1 is that the mass of silver in the porous silicon core is denoted as a, the mass of silver in the carbon coating layer is denoted as b, and a / b satisfies 2.6:1; the mass of boron in the porous silicon core is denoted as c, the mass of boron in the carbon coating layer is denoted as d, and c / d satisfies 3.2:1; other steps and parameter settings are consistent with Embodiment 1.

[0088] Example 5

[0089] The difference between this embodiment and Embodiment 1 is that the mass of silver in the porous silicon core is denoted as a, the mass of silver in the carbon coating layer is denoted as b, and a / b satisfies 8.8:1; the mass of boron in the porous silicon core is denoted as c, the mass of boron in the carbon coating layer is denoted as d, and c / d satisfies 7.5:1; other steps and parameter settings are consistent with Embodiment 1.

[0090] Example 6

[0091] The difference between this embodiment and Embodiment 1 is that the particle size of the porous silicon core is 3μm and the thickness of the carbon coating layer is 35nm; the other steps and parameter settings are consistent with Embodiment 1.

[0092] Example 7

[0093] 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 coating layer is 2 nm; the other steps and parameter settings are consistent with Embodiment 1.

[0094] Example 8

[0095] The difference between this embodiment and Embodiment 1 is that the porous silicon powder is prepared using the following steps:

[0096] A platinum sheet was used as the cathode, a silicon sheet as the anode, and a hydrofluoric acid-ethanol solution (hydrofluoric acid to ethanol volume ratio of 1:10) as the electrolyte, at 500 mA / cm². -2 Electrochemical etching was performed at a current density of 1000 kJ / min for 9 min, followed by ultrasonic stripping and water washing to produce porous silicon powder; other steps and parameter settings were consistent with those in Example 1.

[0097] Comparative Example 1

[0098] The difference between this comparative example and Example 1 is that the lithium battery in this comparative example is a commercially available fast-charging lithium battery. Specifically, in the commercially available fast-charging lithium battery in this comparative example, the positive electrode active material is NCM811 and the negative electrode active material is graphite.

[0099] Comparative Example 2

[0100] The difference between this comparative example and Example 1 is that neither the porous silicon core nor the coated carbon layer of the silicon-carbon composite material contains the first dopant element or the second dopant element; the other steps and parameter settings are consistent with those of Example 1.

[0101] Comparative Example 3

[0102] The difference between this comparative example and Example 1 is that in the silicon-carbon composite material package, only the porous silicon core contains the first dopant element and the second dopant element, while the coating carbon layer does not contain the first dopant element and the second dopant element; other steps and parameter settings are consistent with Example 1.

[0103] Comparative Example 4

[0104] The difference between this comparative example and Example 1 is that the porous silicon core in the silicon-carbon composite material does not contain the first doping element and the second doping element, while the coated carbon layer contains the first doping element and the second doping element; other steps and parameter settings are consistent with Example 1.

[0105] Comparative Example 5

[0106] The difference between this comparative example and Example 1 is that in the silicon-carbon composite material, the porous silicon core is doped with only the first doping element (silver), and the carbon coating layer is doped with only the second doping element (boron); the other steps and parameter settings are consistent with Example 1.

[0107] Comparative Example 6

[0108] The difference between this comparative example and Example 1 is that in the silicon-carbon composite material, the porous silicon core is doped with only the second doping element (boron), and the carbon coating layer is doped with only the second doping element (silver); the other steps and parameter settings are consistent with Example 1.

[0109] Comparative Example 7

[0110] The difference between this comparative example and Example 1 is that an equal weight of silicon powder is used instead of porous silicon powder; all other steps and parameter settings are the same as in Example 1.

[0111] Test methods

[0112] I. Ratio Performance Test

[0113] The lithium batteries of the above embodiments and comparative examples were subjected to rate performance tests. The specific test method was as follows: at 25°C, 1C constant current discharge to 2.5V, stand for 10 minutes, 6C constant current and constant voltage charging to 4.2V, and 0.05C cutoff. The constant current charging capacity, the total constant current and constant voltage charging capacity, and the highest temperature during fast charging were recorded. The constant current charging ratio = constant current charging capacity / total constant current and constant voltage charging capacity × 100%. The test results are recorded in Table 1.

[0114] II. Cyclic Performance Testing

[0115] The lithium batteries of the above embodiments and comparative examples were subjected to cycle performance tests. The specific test method was as follows: at 25°C, 2C constant current and constant voltage charging was performed to 4.2V, 0.05C was cut off, and the batteries were left to stand for 10 minutes. Then, 2C constant current discharging was performed to 2.5V, and the batteries were left to stand for 10 minutes. The batteries were cycled for 500 cycles. The capacity retention rate of the lithium batteries after 2C / 2C cycles was recorded. The test results are recorded in Table 1.

[0116] III. Element Content Test

[0117] The silicon, fluorine, and phosphorus contents in the silicon-carbon composite materials prepared in the above embodiments and comparative examples were tested using the following methods:

[0118] The ion polishing instrument can perform both planar polishing and cross-sectional cutting of samples using argon ions. Utilizing the ion beam polishing (CP) testing principle, after cross-sectional cutting with argon ions, X-ray energy dispersive spectroscopy (EDS) is performed to calculate the mass ratio of the first dopant element in the porous silicon core to the first dopant element in the coated carbon layer, and the mass ratio of the second dopant element in the porous silicon core to the second dopant element in the coated carbon layer.

[0119] IV. Testing of Porous Silicon Core Size and Coated Carbon Layer Thickness

[0120] The TEM test was selected, and the test method is as follows:

[0121] (1) The porous silicon core samples prepared in the above examples and comparative examples are dispersed by ultrasonic vibration to remove soft agglomerates, and then dispersed in water or other solvents to obtain a sample suspension.

[0122] (2) Take a copper mesh covered with a carbon film or other polymer film, scoop it from the prepared sample suspension or use a dropper to obtain the required amount, drop it onto the copper mesh, blot it dry with filter paper or air dry it, and then place it on the sample stage for testing.

[0123] (3) Take photos in representative areas with narrow size distribution and good dispersion, and arbitrarily select porous silicon cores with complete morphology to measure size d; after the first coating layer coats the porous silicon cores to prepare the sample, measure the thickness h of the coating carbon layer by TEM test.

[0124] V. Fast Charging Time Test

[0125] The fast charging time of the lithium batteries in Example 1 and Comparative Example 1 above was tested. The specific test method was as follows: at 25°C, the batteries were discharged at 1C constant current to 2.5V, left to stand for 10 minutes, charged at 1C constant current to 10% SOC, left to stand for 10 minutes, and charged at 6C constant current to 80% SOC. This time was the fast charging time.

[0126] The test results showed that the fast charging time of the lithium battery in Example 1 was 7 minutes, and the fast charging time of the lithium battery in Comparative Example 1 was 35 minutes.

[0127] VI. Lithium-ion battery energy density test

[0128] The energy density of the lithium batteries in Example 1 and Comparative Example 1 was tested. The specific test method was as follows: the weight of the test battery was weighed and recorded as m; the battery was placed in a fixture and a force of 3000N was applied. The single battery was charged to 4.2V with a constant current of 0.33C, rested for 30min, discharged to 2.5V with a constant current of 0.33C, and rested for 30min. This cycle was repeated 3 times. The discharge capacity (in Ah) and energy E (average of three cycles) were calculated. The discharge energy density = E / m (in Wh / kg).

[0129] The energy density of the lithium battery in Example 1 was found to be 350 Wh·kg. -1 The energy density of the lithium battery in Comparative Example 1 is 288.1 Wh·kg. -1 .

[0130] Table 1

[0131]

[0132] Based on Examples 1-3, Comparative Examples 1-7, and Table 1, it can be seen that the fast-charging lithium battery prepared using the above-mentioned negative electrode active material through the energy-generating porous silicon core and the carbon coating layer included on the surface of the porous silicon core exhibits excellent rate performance and cycle capacity retention. Furthermore, compared to commercial fast-charging lithium batteries (Comparative Example 1) with fast-charging times ≥30 min and energy densities generally <300 Wh·kg, the fast-charging performance is significantly better. -1 In Example 1 of this application, the lithium battery has a fast charging time of only 7 minutes (charging range of 10% SOC-80% SOC) and an energy density of up to 350Wh.kg. -1 .

[0133] This is because, in this application, by doping both the porous silicon core and the coated carbon layer with a first dopant element and a second dopant element, the combination of the first and second dopant elements can improve the electrical and thermal conductivity of the porous silicon core. This improves both the electrical and thermal conductivity of the negative electrode active material, thus enhancing its structural stability and electrochemical performance. Furthermore, the doping of the second dopant element creates vacancies in the porous silicon core, which helps reduce the resistivity of the negative electrode active material, thereby improving its rate performance. On the other hand, the combination of the first and second dopant elements can form a "buffer framework" in the porous silicon core that can buffer the volume expansion of the porous silicon core. The combination of the first and second dopant elements can also improve the structural stability of the coated carbon layer, further enhancing the structural stability of the negative electrode active material, thereby improving the fast-charging cycle stability of the lithium battery.

[0134] Based on Examples 1-3, Comparative Example 6, and Table 1, it can be seen that using porous silicon powder to prepare porous silicon cores can provide more buffer space for the volume expansion caused by lithium-ion insertion / extraction, further improving the structural stability of the negative electrode active material and enhancing the fast-charging cycle stability of lithium batteries. Because porous silicon powder has a large specific surface area, the thermal conductivity of the porous silicon core can be further improved under the action of the first and second doping elements, greatly alleviating the heat generated during fast-charging cycles and improving the safety of lithium battery fast-charging cycles.

[0135] Combining Examples 1, Examples 4-5 and Table 1, it can be seen that by controlling the ratios of a / b and c / d to meet the above range, it helps to adjust the distribution of the first and second doping elements in the porous silicon core and the coated carbon layer, resulting in a porous silicon core with good rate performance and small volume expansion effect during fast charging cycles. Combined with the coated carbon layer, this further improves the conductivity and thermal conductivity of the negative electrode active material, thereby improving the cycle performance of the lithium battery.

[0136] Based on Examples 1, 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, while ensuring that the negative electrode active material has high lithium-ion transport characteristics, the carbon coating layer can alleviate the expansion of the porous silicon core to the greatest extent, and significantly improve the rate performance and cycle stability of the lithium battery.

[0137] Combined with Examples 1 and 8 and Table 1, it can be seen that the porous silicon powder preparation process of this application helps to obtain porous silicon powder with a suitable pore structure. The pore structure in the porous silicon powder can minimize the destructive effect of volume expansion on the structure during the fast charging cycle of lithium battery, and help improve the cycle stability of lithium battery.

[0138] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application, but such modifications or substitutions are all within the scope of protection of this application.

Claims

1. A silicon-carbon composite material, characterized in that: It includes a porous silicon core and a carbon coating layer covering the porous silicon core; Both the porous silicon core and the coated carbon layer include a first doping element and a second doping element; Wherein, the first doping element includes at least one of silver and copper; the second doping element includes at least one of boron, aluminum, and gallium; The first dopant element accounts for 0.1%-4% of the mass of the porous silicon core, and the second dopant element accounts for 0.4%-3% of the mass of the porous silicon core. The first dopant element accounts for 2%-9% of the mass of the coated carbon layer, and the second dopant element accounts for 4%-10% of the mass of the coated carbon layer. The mass of the first dopant element in the porous silicon core is denoted as a, and the mass of the first dopant element in the coated carbon layer is denoted as b, where a / b satisfies 2-9:1; The mass of the second dopant element in the porous silicon core is denoted as c, and the mass of the second dopant element in the coated carbon layer is denoted as d, where c / d satisfies 3-8:

1.

2. The silicon-carbon composite material according to claim 1, characterized in that: The porous silicon core has a particle size of 6-12 μm, and / or the carbon coating layer has a thickness of 2-30 nm.

3. The method for preparing the silicon-carbon composite material according to any one of claims 1-2, characterized in that: Includes the following steps: S1, providing silicon material, and etching the silicon material to obtain porous silicon powder; S2, the porous silicon powder, the first doping element source, and the second doping element source are mixed and ball-milled to obtain the precursor; S3, the precursor is placed in an inert atmosphere, heated and a carbon source gas is introduced, and after being kept at the temperature, it is cooled to obtain the silicon-carbon composite material.

4. The method for preparing the silicon-carbon composite material according to claim 3, characterized in that: In 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.

5. The method for preparing the silicon-carbon composite material according to claim 3, characterized in that: The preparation process of the porous silicon powder includes the following steps: Using a platinum sheet as the cathode and a silicon sheet as the anode, electrochemical etching is performed in an etching solution, and porous silicon powder is obtained after cleaning. The etching solution includes a hydrofluoric acid-ethanol solution, wherein the volume ratio of hydrofluoric acid to ethanol in the hydrofluoric acid-ethanol solution is 1:1-8.

6. The method for preparing silicon-carbon composite material according to claim 5, characterized in that: The electrochemical etching process includes the following steps: at a first current density of 550-800 mA / cm² -2 Etching continues for 2-5 minutes at a second current density of 300-500 mA / cm². -2 Etch for 1-8 minutes.

7. A negative electrode sheet, characterized in that: It includes a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer includes the silicon-carbon composite material as described in any one of claims 1-2.

8. A lithium-ion battery, characterized in that: It includes a positive electrode, a separator, an electrolyte, and a negative electrode sheet as described in claim 7.

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