Preparation method and application of hard carbon-silicon composite material

Abstract

The invention relates to a preparation method and application of a hard carbon-silicon composite material, and the preparation method comprises the following preparation steps: S1, soft carbon coating: dispersing silicon monoxide in a solution containing a soft carbon precursor, mixing, and drying; then under the protection of inert atmosphere, heating and carrying out first pyrolysis treatment to obtain soft carbon coated silicon monoxide; and S2, hard carbon coating: dispersing the soft carbon-coated silicon monoxide obtained in the step S1 in a solution containing a hard carbon precursor, mixing, drying, heating under the protection of inert gas again, and carrying out secondary pyrolysis treatment, so that the hard carbon precursor is converted into a hard carbon layer to cover the existing soft carbon layer, thereby forming the hard carbon-silicon composite material. According to the invention, silicon monoxide is mainly used as a silicon source, a soft carbon precursor is firstly pyrolyzed to form a flexible soft carbon coating layer on the surface of silicon monoxide, and then a hard carbon precursor is further pyrolyzed to form a hard carbon layer outside the soft carbon layer, so that the hard carbon-silicon composite material with the core-shell structure is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of anode materials for lithium-ion batteries, and more specifically, relates to a preparation method and application of a hard carbon-silicon composite material. Background Art

[0002] Lithium-ion batteries are rechargeable batteries widely used in modern electronic devices. Among them, the anode material of the battery is crucial for its performance. With the rapid development of fields such as portable electronic devices and electric vehicles, the demand for high-performance lithium-ion batteries is also increasing day by day, and exploring new anode materials has become a research hotspot. Among them, silicon-based materials are regarded as the core candidate materials for the next-generation lithium-ion battery anodes due to their high theoretical capacity (4200 mAh / g). However, during the charge and discharge process of silicon-based materials, the volume expansion rate is high (>300%), which easily leads to problems such as electrode structure damage, active material shedding, increased electrode internal resistance, poor cycle stability, electrode pulverization, repeated rupture of the SEI film, and short cycle life, severely limiting their practical applications.

[0003] In the prior art, the classification of silicon-based anode materials includes silicon-carbon composites, silicon oxides, etc., which improve the battery performance through different preparation methods and structural designs. Combining silicon with carbon materials is one of the effective ways to improve the cycle stability of silicon-based materials. Silicon-carbon composites are composites that combine silicon and carbon, aiming to utilize the high theoretical capacity of silicon while alleviating its volume expansion during the charge and discharge process. Among them, the main schemes for silicon-carbon composite include graphite doping, soft / hard carbon coating, etc. Carbon coating on silicon is the main means to alleviate volume expansion. However, the existing carbon-silicon composites still have deficiencies in structural design and performance optimization, and it is difficult to meet the requirements of practical applications. Therefore, it is of great significance to develop a preparation method of carbon-silicon composites that can effectively inhibit the volume expansion of silicon and improve the cycle performance of the battery. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method and application of a hard carbon-silicon composite material, which is particularly suitable for the anode material of high-performance lithium-ion batteries and can significantly improve the cycle stability and rate performance of the battery.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A preparation method of a hard carbon-silicon composite material includes the following preparation steps:

[0007] S1. Soft carbon coating: Dispersing silicon monoxide in a solution containing a soft carbon precursor, mixing and then drying; subsequently, under the protection of an inert atmosphere, heating for the first pyrolysis treatment to obtain soft carbon-coated silicon monoxide;

[0008] S2. Hard carbon coating: Disperse the silicon suboxide coated with soft carbon obtained in step S1 in a solution containing a hard carbon precursor, mix and dry, and then heat under the protection of an inert gas for a second pyrolysis treatment to convert the hard carbon precursor into a hard carbon layer covering the existing soft carbon layer, forming a hard carbon-silicon composite material.

[0009] As a preferred technical solution of the present invention, the mass ratio of the silicon suboxide to the soft carbon precursor is 1:1 - 3.

[0010] Specifically, the mass ratio of the silicon suboxide to the soft carbon precursor is 1:1, 1:1.2, 1:1.5, 1:2, 1:2.5, 1:2.8 or 1:3.

[0011] Preferably, the solution containing the soft carbon precursor / the solution containing the hard carbon precursor is an organic solvent such as ethanol or toluene.

[0012] Preferably, in step S1 / S2, the dispersion is ultrasonic dispersion.

[0013] Preferably, in step S1 / S2, the inert atmosphere is nitrogen or argon. By introducing an inert gas, silicon oxidation can be avoided.

[0014] Preferably, in step S1, the temperature of the first pyrolysis treatment is 700 - 900 °C, and the pyrolysis time is 1 - 3 hours; in step S2, the temperature of the second pyrolysis treatment is 1200 - 1500 °C, and the pyrolysis time is 3 - 6 hours.

[0015] Preferably, in step S2, the mass ratio of the silicon suboxide coated with soft carbon to the hard carbon precursor is 1:1 - 3.

[0016] Specifically, the mass ratio of the silicon suboxide coated with soft carbon to the hard carbon precursor is 1:1, 1:1.2, 1:1.5, 1:2, 1:2.5, 1:2.8 or 1:3.

[0017] It is easy to think of without creative effort that in step S2, after the second pyrolysis treatment, the hard carbon-silicon composite material can be purified from the liquid-phase components by operations such as cooling, washing, and drying.

[0018] As a suitable but non-limiting specific example, after the second pyrolysis treatment, hydrogen can also be introduced to reduce residual oxygen-containing functional groups and reduce interface defects.

[0019] Further, the washing step after the second pyrolysis treatment is: washing the product with an HF solution (5 - 10%) to remove surface impurities.

[0020] Further, the structure of the hard carbon-silicon composite material is a silicon core layer, a soft carbon intermediate layer, and a hard carbon outer layer, and the thickness ratio of each layer is 1:1 - 2:2 - 3.

[0021] The hard carbon-silicon composite material prepared by the present invention first pyrolyzes a soft carbon precursor to coat silicon monoxide, and then pyrolyzes a hard carbon precursor, forming a core-shell structure with silicon monoxide as the core, successively coated with a soft carbon layer and a hard carbon layer. Among them, the presence of silicon monoxide can, on the one hand, serve as part of the silicon source, endowing the composite material with a relatively high theoretical specific capacity; on the other hand, as an active substance, the oxidation state of silicon helps to stabilize the structure of silicon.

[0022] During the charge and discharge process of the silicon layer, the silicon element provided by silicon monoxide serves as an active substance. Due to the insertion and extraction behavior of lithium ions during the charge and discharge process, the silicon will undergo volume expansion and contraction. The flexibility of the soft carbon layer enables it to undergo elastic deformation like an elastic buffer pad when the silicon expands in volume, effectively buffering / absorbing the stress generated by the volume expansion of the silicon layer, reducing the volume strain of the silicon layer, reducing the generation of internal cracks in the material, improving the cycle performance, preventing the fragmentation of silicon particles and the destruction of the electrode structure caused by stress concentration, and thus improving the overall stability of the material.

[0023] As the hard carbon layer contained in the material of this application, it provides overall structural support and good electrical conductivity. On the one hand, the hard carbon layer can play the role of an "armor", providing a certain degree of support and protection for the internal structure, providing a stable framework structure for the entire composite material, restricting the excessive deformation of the soft carbon layer during long-term charge and discharge, preventing the excessive deformation and collapse of the soft carbon layer and the silicon layer during long-term cycling, and enhancing the stability of the entire composite material structure; on the other hand, the good electrical conductivity of hard carbon helps to improve the electron transfer efficiency, reduce the internal resistance of the battery, prevent problems such as pulverization and SEI side reactions of the composite material during battery assembly and use, thereby improving the charge and discharge performance of the battery and helping to maintain the integrity of the electrode.

[0024] This unique core-shell structure design makes the material not prone to structural collapse and active substance shedding during long-term cycling, ensuring the stable performance of the battery, extending the service life of the battery, realizing the double buffering and protection of silicon by soft carbon and hard carbon, enabling the composite material to maintain the integrity of the structure during the charge and discharge cycle, and thus significantly improving the cycle performance of the material. Therefore, the double carbon layers of the present invention cooperate to inhibit volume expansion, ensuring the ion transfer efficiency during the charge and discharge process of the battery, and improving the cycle stability and capacity retention rate of the silicon-based negative electrode.

[0025] Specifically, in step S1, the first pyrolysis causes the soft carbon precursor to decompose, polymerize, and carbonize on the surface of silicon suboxide, forming a soft carbon layer tightly bonded to the silicon suboxide, providing flexible buffering; the second pyrolysis causes the hard carbon precursor to pyrolyze on the outer surface of the soft carbon layer to form a hard carbon layer, enhancing electrical conductivity and structural stability. In the whole process, various substances interact with each other and cooperate to form a film, improving the comprehensive performance, and finally constructing a hard carbon-silicon composite material with a core-shell structure.

[0026] Preferably, the silicon suboxide is micron or nanoscale silicon powder. The silicon suboxide can be pre-controlled to have a micron or nanoscale particle size by ball milling or spray drying.

[0027] As a suitable but non-limiting specific example, the purity of the silicon suboxide > 99%.

[0028] Preferably, the silicon suboxide is pre-modified by pickling. The acid is preferably a HCl / HF solution with a concentration of 2 - 5%. By pickling, the surface oxide layer can be removed to reduce side reactions; surface defects can also be introduced to enhance the interfacial bonding with carbon.

[0029] Preferably, the soft carbon precursor and the hard carbon precursor are pre-oxidized or cross-linked to improve their thermal stability and void formation ability.

[0030] Preferably, the soft carbon precursor is at least one of pitch-derived carbon, coke, graphitized mesophase carbon microbeads (MCMB), carbon fiber, high-temperature coal tar pitch, phenolic resin, and petroleum coke, having good pyrolysis carbonization performance.

[0031] Among them, the soft carbon negative electrode material has the advantages of low and stable charge-discharge potential platforms, large charge-discharge capacity, high efficiency, and good cycle performance. After the heat treatment temperature of the soft carbon reaches the graphitization temperature, it has a high degree of graphitization. In the present invention, the addition of soft carbon mainly utilizes the buffering effect of the carbon material to relieve the stress caused by the volume change of silicon, enhance the electrical conductivity, reduce the direct contact between silicon and the electrolyte, improve the decomposition of the electrolyte caused by the dangling bonds on the silicon surface, and improve the cycle stability and electrochemical performance of the battery.

[0032] Preferably, the hard carbon precursor is at least one of resin carbon, anthracite, starch, coconut shell, cellulose, lignin, or polyacrylonitrile; these materials can form hard carbon with a unique pore structure and mechanical properties after pyrolysis.

[0033] Preferably, the resin carbon is phenolic resin-derived carbon, epoxy resin-derived carbon, or resorcinol-formaldehyde resin. This type of carbon can inhibit excessive graphitization and retain the amorphous carbon structure.

[0034] Hard carbon is a non-graphitizable carbon with a disordered microstructure, rich in defect sites and voids. Even when the heat treatment temperature of hard carbon reaches the graphitization temperature, the material still has a disordered structure. Its preparation often uses biomass (such as bagasse, corn waste) as a precursor, which is low-cost and environmentally friendly.

[0035] A hard carbon-silicon composite material, which is prepared by the preparation method as described above.

[0036] A negative electrode material for a lithium battery, comprising the hard carbon-silicon composite material as described above, or comprising the hard carbon-silicon composite material prepared by the preparation method as described above.

[0037] A lithium battery, comprising the hard carbon-silicon composite material as described above, or comprising the hard carbon-silicon composite material prepared by the preparation method as described above.

[0038] An application of the hard carbon-silicon composite material as described above in the preparation of a lithium battery, wherein the hard carbon-silicon composite material is used to prepare the negative electrode material of the lithium battery.

[0039] The hard carbon-silicon composite material of the present invention is mainly applied to the field of lithium ion batteries and can be used as the negative electrode material of lithium ion batteries. In lithium ion batteries, the silicon-carbon composite material is used as the negative electrode, and stores lithium ions through the alloying of silicon and lithium and the insertion mechanism of carbon, having a much higher specific capacity than graphite, potentially improving the energy density of the battery; the addition of carbon provides structural stability and conductivity, improving the cycle life. Its design balances high capacity and cycle stability and is suitable for next-generation high-energy density batteries.

[0040] The beneficial effects of the present invention:

[0041] (1) In the prior art, there are problems such as limited buffering effect and insufficient structural stability in a single carbon layer, making it difficult to balance stress buffering and conductivity optimization, difficult to balance flexibility and rigidity, and difficult to meet the requirements of high-energy density lithium ion batteries for long cycle life. For example, a single soft carbon coating has a relatively high specific surface area, good cycle stability and flexibility, but insufficient mechanical strength, making it difficult to suppress silicon expansion for a long time; while a single hard carbon direct coating has a relatively high energy density, mechanical strength and good conductivity, but insufficient flexibility, and is prone to cracking due to interfacial stress concentration. The hard carbon-silicon composite material of the present invention uses silicon monoxide as the silicon source, and forms a composite structure with silicon monoxide as the core and successively coated with a soft carbon layer and a hard carbon layer by stepwise pyrolysis of the soft carbon precursor and the hard carbon precursor. The high flexibility of the soft carbon layer can effectively buffer the volume expansion stress of silicon, and the hard carbon layer provides a stable conductive network and mechanical support, significantly improving the cycle performance and rate performance. The soft carbon layer and the hard carbon layer form a stable interface through chemical bonding, reducing the rupture of the SEI film, effectively solving the volume expansion problem of silicon-based materials in the application of lithium ion batteries, and improving the cycle performance of the battery.

[0042] (2) The preparation method of the present invention has low cost, controllable process, is compatible with the existing lithium-ion battery anode production line, and is suitable for large-scale production. Detailed implementation manners

[0043] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following combines examples to detail the specific implementation manners, structures, features, and their effects according to the present invention as follows.

[0044] Example 1

[0045] A preparation method of a hard carbon-silicon composite material includes the following preparation steps:

[0046] S1. Soft carbon coating: Mix silicon monoxide powder and a soft carbon precursor (high-temperature coal tar pitch) in a mass ratio of 1:2, ball-mill and disperse for 60 minutes to ensure uniform mixing, then add an ethanol solution and ultrasonically disperse for 60 minutes. Subsequently, transfer the mixed solution to a reaction kettle, and under nitrogen protection, heat it to 800 °C at a heating rate of 5 °C / min and hold for 2 hours for the first pyrolysis reaction. After the pyrolysis is completed, naturally cool it to room temperature to obtain silicon monoxide coated with soft carbon;

[0047] S2. Hard carbon coating: Uniformly mix the obtained silicon monoxide coated with soft carbon and a hard carbon precursor (polyacrylonitrile) in a mass ratio of 1:2, also add ethanol and ultrasonically disperse for 60 minutes. Subsequently, transfer the mixed solution to another reaction kettle, and under nitrogen protection, heat it to 1300 °C at a heating rate of 5 °C / min and hold for 4 hours for the pyrolysis reaction; after the pyrolysis is completed, naturally cool it to room temperature, wash it with a 5% HF solution to remove surface impurities, and dry it to obtain a hard carbon-silicon composite material.

[0048] Example 2

[0049] The difference between this example and Example 1 is that the soft carbon precursor in this example is petroleum coke.

[0050] Example 3

[0051] The difference between this example and Example 1 is that the hard carbon precursor in this example is lignin.

[0052] Example 4

[0053] The difference between this example and Example 1 is that the mass ratio of the silicon monoxide powder and the soft carbon precursor (high-temperature coal tar pitch) in this example is 1:1.

[0054] Example 5

[0055] The difference between this example and Example 1 is that the mass ratio of the silicon monoxide coated with soft carbon and the hard carbon precursor (polyacrylonitrile) in this example is 1:1.

[0056] Comparative Example 1

[0057] The difference between this comparative example and Example 1 is that the preparation method of the hard carbon-silicon composite material in this comparative example is as follows:

[0058] In step S1, hard carbon coating is carried out first, that is, silicon monoxide powder and a hard carbon precursor (polyacrylonitrile) are mixed and stirred evenly according to a mass ratio of 1:2, and then heated to 1300 °C for the first pyrolysis to obtain silicon monoxide coated with hard carbon; in step S2, the obtained silicon monoxide coated with hard carbon is mixed with a soft carbon precursor (high-temperature coal tar pitch) at a ratio of 1:2, and then the temperature is raised to 800 °C for the second pyrolysis to obtain a hard carbon-silicon composite material.

[0059] Comparative Example 2

[0060] The difference between this comparative example and Example 1 is that in this comparative example, silicon monoxide, a soft carbon precursor (high-temperature coal tar pitch) and a hard carbon precursor (polyacrylonitrile) are mixed according to a mass ratio of 1:2:2, and then ethanol is added for dispersion. Subsequently, it is heated to 1300 °C for pyrolysis reaction to obtain a hard carbon-silicon composite material. Other components, preparation steps and parameters are the same.

[0061] Comparative Example 3

[0062] The difference between this comparative example and Example 1 is that in this comparative example, silicon monoxide and a hard carbon precursor (polyacrylonitrile) are directly mixed according to a mass ratio of 1:2 for hard carbon coating to form a hard carbon-silicon composite material. That is, the silicon monoxide in this comparative example is not pre-coated with soft carbon.

[0063] Comparative Example 4

[0064] The difference between this comparative example and Example 1 is that in this comparative example, nitrogen is not introduced for protection in both step S1 and step S2.

[0065] Comparative Example 5

[0066] The difference between this comparative example and Example 1 is that in step S1 of this comparative example, the mass ratio of silicon monoxide powder to the soft carbon precursor (high-temperature coal tar pitch) is 2:1, and in step S2, the mass ratio of the soft carbon-coated silicon monoxide to the hard carbon precursor (polyacrylonitrile) is 2:1.

[0067] The hard carbon-silicon composite materials prepared in Examples 1-5 and Comparative Examples 1-5 are made into lithium batteries for electrochemical performance testing.

[0068] Electrode Preparation: The prepared hard carbon-silicon composite material, acetylene black conductive agent, and polyvinylidene fluoride binder are mixed at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone is added as a solvent, and the mixture is stirred evenly to form a slurry. The slurry is coated on a copper foil current collector and vacuum dried at 80 - 120 °C for 12 - 24 hours, and then punched into electrode sheets of appropriate size.

[0069] Battery Assembly: A lithium sheet is used as the counter electrode, a Celgard 2400 separator is used as the separator, and a mixed solution of ethylene carbonate (EC) / diethyl carbonate (DEC) (volume ratio 1:1) with 1 mol / L LiPF6 is used as the electrolyte. A lithium-ion battery is assembled in a glove box filled with argon.

[0070] Constant Current Charge and Discharge Performance Test:

[0071] The lithium-ion batteries prepared in the examples and comparative examples are charged at a set constant current of 0.2C to the upper limit voltage of 4.2V. During this period, the voltage-time curve and capacity data are recorded. After reaching the cut-off voltage, the charging is switched to constant voltage charging until the current drops to the set threshold of 0.05C.

[0072] After charging is completed, the lithium-ion batteries prepared in the examples and comparative examples are discharged at a constant current of 0.2C at the same rate until the discharge cut-off voltage of 2.2V is reached. The discharge capacity and voltage plateau are recorded. The above steps are repeated 5 times, and the Coulomb efficiency is recorded.

[0073] Among them, the Coulomb efficiency = (first discharge specific capacity / first charge specific capacity) × 100%

[0074] Cycling Performance Test:

[0075] The lithium-ion batteries prepared in the examples and comparative examples are subjected to charge and discharge cycling tests at a current density of 0.1C. After 100 cycles, the capacity retention rates of the electrodes in the examples and comparative examples are recorded.

[0076] Quantitative Test of Volume Expansion Rate:

[0077] An in-situ dilatometer is used to detect the change in electrode thickness after 100 cycles, and the volume expansion rate is calculated.

[0078] The test results are shown in Table 1.

[0079] Table 1

[0080]

[0081]

[0082] As can be seen from the test results in Table 1, the hard carbon-silicon composite materials of Examples 1-5 exhibit excellent cycle stability and higher initial Coulomb efficiency, demonstrating that the hard carbon-silicon composite materials of the present invention have stable electrochemical performance and structure compared with Comparative Examples 1-4, and have obvious advantages in suppressing the volume expansion of silicon and improving cycle stability.

[0083] In summary, the hard carbon-silicon composite materials obtained in the present invention exhibit excellent cycle performance and rate performance in the field of lithium-ion batteries, and have broad application prospects.

[0084] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the disclosed technical content within the scope of the technical solution of the present invention. However, as long as it does not depart from the technical solution content of the present invention, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a hard carbon-silicon composite material, characterized in that, It includes the following preparation steps: S1. Soft carbon coating: Dispersed silicon monoxide in a solution containing a soft carbon precursor, mixed and dried; then, under the protection of an inert atmosphere, heated for the first pyrolysis treatment to obtain silicon monoxide coated with soft carbon; S2. Hard carbon coating: Dispersed the silicon monoxide coated with soft carbon obtained in step S1 in a solution containing a hard carbon precursor, mixed and dried, and again under the protection of an inert gas, heated for the second pyrolysis treatment to convert the hard carbon precursor into a hard carbon layer covering the existing soft carbon layer, forming a hard carbon-silicon composite material.

2. The preparation method of the hard carbon-silicon composite material according to claim 1, characterized in that, The mass ratio of the silicon monoxide to the soft carbon precursor is 1:1-3.

3. The preparation method of the hard carbon-silicon composite material according to claim 1, characterized in that, The solution containing the soft carbon precursor is an ethanol or toluene organic solution; and / or, the solution containing the hard carbon precursor is an ethanol or toluene organic solution.

4. The preparation method of the hard carbon-silicon composite material according to claim 1, characterized in that In step S1, the temperature of the first pyrolysis treatment is 700-900°C, and the pyrolysis time is 1-3 hours; in step S2, the temperature of the second pyrolysis treatment is 1200-1500°C, and the pyrolysis time is 3-6 hours.

5. The preparation method of the hard carbon-silicon composite material according to claim 1, characterized in that The mass ratio of the silicon monoxide coated with soft carbon to the hard carbon precursor is 1:1-3.

6. The preparation method of the hard carbon-silicon composite material according to claim 1, characterized in that, The structure of the hard carbon-silicon composite material is a silicon core layer, a soft carbon intermediate layer, and a hard carbon outer layer, and the thickness ratio of each layer is 1:1-2:2-3.

7. The preparation method of the hard carbon-silicon composite material according to claim 1, wherein, The silicon monoxide is pre-modified by pickling.

8. The preparation method of the hard carbon-silicon composite material according to claim 1, wherein The soft carbon precursor is at least one of pitch-derived carbon, coke, graphitized mesophase carbon microspheres, carbon fiber, high-temperature coal tar pitch, phenolic resin, and petroleum coke; the hard carbon precursor is at least one of resin carbon, anthracite, starch, coconut shell, cellulose, lignin, or polyacrylonitrile.

9. A hard carbon-silicon composite material, characterized in that, The hard carbon-silicon composite material is prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the hard carbon-silicon composite material as described in claim 9 in the preparation of a lithium battery, characterized in that, The hard carbon-silicon composite material is used for preparing the negative electrode material of a lithium battery.

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