A multi-stage buffered silicon-carbon electrode active material and electrode for lithium batteries

Abstract

The application discloses a kind of multistage buffering silicon-carbon electrode active material and electrode for lithium battery, which comprises a substrate and a coating layer coated on the surface of the substrate, the coating layer has at least two layers, and the outer side of the hard carbon coating layer is wrapped with a soft carbon coating layer.The substrate comprises a hollow silicon-carbon material.The application adopts two aspects of improvement: first, the hollow structure in the hollow silicon-carbon material;Second, the hard carbon coating layer;The above two aspects work together to provide sufficient buffer space for the volume expansion of silicon, making the silicon-carbon electrode active material have good structural stability and can better overcome the structural collapse caused by silicon expansion.The silicon-carbon electrode active material is coated with soft carbon, which reduces the direct contact between silicon and electrolyte, reduces the occurrence of side reactions, and improves the electrical conductivity.

Description

Technical Field

[0001] This invention belongs to the field of silicon-carbon electrode active materials, specifically relating to a multi-level buffered silicon-carbon electrode active material, its preparation method, and an electrode for lithium batteries. Background Technology

[0002] Silicon-based materials are ideal anode materials for lithium-ion batteries due to their high theoretical specific capacity, suitable voltage platform, and low cost. However, problems such as volume expansion during charging and discharging leading to electrode structure damage, and excessive side reactions caused by direct contact between silicon and electrolyte resulting in capacity reduction, lead to poor cycle stability. Carbon materials can improve the electronic conductivity and structural stability of silicon-based materials. Therefore, silicon-carbon electrode active materials have become one of the important routes for the commercialization of silicon-based anodes.

[0003] In existing technical solutions for silicon-carbon electrode active materials, a common approach is to composite silicon and carbon materials to obtain a multi-level buffer structure in order to alleviate the volume expansion effect of silicon during electrocycling, improve the conductivity of the active material, and reduce the occurrence of side reactions between silicon and electrolyte during electrocycling. Patent application CN109167031A discloses a nano-silicon-carbon composite material with a multi-level structure, using silicon nanoparticles as the core, amorphous carbon as the intermediate coating layer, and fluorinated carbon as the outer shell. Amorphous carbon-coated nano-silicon is prepared by in-situ deposition of alloyed, dealloyed, and pyrolytic carbon, respectively, and then ball-milled and mixed with fluorinated carbon as the outermost coating. The prepared nano-silicon-carbon composite material has good electrochemical performance and silicon's barrier properties against electrolyte. However, the buffer space of the dealloyed nano-silicon is relatively limited, making it difficult to meet the requirements of long-term cycling. Furthermore, the technical difficulty of obtaining uniform coating through dealloying and in-situ deposition is relatively high, making it difficult to enter large-scale industrial applications.

[0004] Therefore, optimizing silicon-carbon electrode active materials, solving the structural cracking caused by the volume expansion of silicon materials in existing technologies, improving the conductivity of materials, and reducing the numerous side reactions caused by direct contact between silicon and electrolyte, so as to make them suitable for lithium battery electrodes and improve cycle performance, is an important problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a multi-level buffered silicon-carbon electrode active material, its preparation method, and an electrode for lithium batteries.

[0006] The present invention provides a multi-level buffered silicon-carbon electrode active material, the silicon-carbon electrode active material comprising a matrix and a coating layer covering the surface of the matrix, the coating layer having at least two layers, the coating layer being a hard carbon coating layer wrapped with a soft carbon coating layer on the outside, and the matrix comprising hollow silicon-carbon material.

[0007] To overcome the structural collapse caused by silicon expansion, the multi-stage buffered silicon-carbon electrode active material of this invention employs two improvements: first, the hollow structure within the hollow silicon-carbon material; second, a hard carbon coating layer. These two aspects work synergistically to provide sufficient buffer space for silicon volume expansion, giving the silicon-carbon electrode active material good structural stability and effectively overcoming structural collapse caused by silicon expansion. Furthermore, by coating the silicon-carbon electrode active material with soft carbon, direct contact between silicon and the electrolyte is reduced, minimizing side reactions and simultaneously improving conductivity.

[0008] Preferably, the hard carbon coating layer comprises a porous hard carbon coating layer, and the hollow silicon-carbon material comprises a hollow porous silicon-carbon material. The porous structure in the porous hard carbon coating layer and the porous structure in the hollow porous silicon-carbon material, together with the hollow structure in the hollow silicon-carbon material and the hard carbon coating layer, work synergistically to effectively reduce the volume change of the silicon-carbon electrode active material and improve structural stability.

[0009] Preferably, the mass ratio of silicon to carbon in the silicon-carbon electrode active material is 1:(0.5 to 1.5). Within this range, the silicon-carbon mass ratio effectively ensures a balance between the lithium storage capacity and the electrocycle stability of the silicon-carbon electrode active material, resulting in optimal economic benefits.

[0010] Preferably, the hollow inner diameter of the hollow silicon-carbon material is 300–1500 nm. Within this range, the hollow inner diameter of the hollow silicon-carbon material provides sufficient buffer space for silicon volume expansion while also possessing sufficient strength to improve the structural stability of the hollow porous silicon-carbon structure.

[0011] Preferably, the outer diameter of the hollow silicon-carbon material is 400–2000 nm.

[0012] Preferably, the thickness of the hard carbon coating layer is 60–400 nm. A thickness within this range is beneficial for conductivity, buffering, and protection, while also facilitating lithium-ion transport and increasing the capacity of the silicon-carbon electrode active material.

[0013] Preferably, the silicon-carbon electrode active material further includes heteroatom doping, wherein the heteroatoms include at least one of nitrogen atoms and sulfur atoms. Nitrogen atoms and sulfur atoms are electron donors, providing additional charge carriers and effectively reducing the band gap, thereby improving the overall conductivity of the carbon material. Furthermore, nitrogen-doped silicon-carbon electrode active materials have more active surfaces, providing more lithium storage sites, thus increasing the battery's energy storage capacity and resulting in higher energy density.

[0014] Preferably, the heteroatoms in the silicon-carbon electrode active material account for ≤1 wt% of the mass fraction of the silicon-carbon electrode active material.

[0015] Preferably, the preparation method of the multi-level buffered silicon-carbon electrode active material includes the following steps:

[0016] (1) Reduce silicon oxide to obtain porous silicon;

[0017] (2) Prepare an electrospinning core layer solution, an electrospinning intermediate layer solution, and an electrospinning outer layer solution. The electrospinning core layer solution includes a pyrolytic polyolefin, the electrospinning intermediate layer solution includes a pyrolytic polyester and the porous silicon, and the electrospinning outer layer solution includes a heteroatomic organic polymer. Perform coaxial electrospinning on the electrospinning core layer solution, the electrospinning intermediate layer solution, and the electrospinning outer layer solution to obtain spun fibers.

[0018] (3) The spun fibers are heat-treated to obtain a hollow porous silicon-carbon material coated with hard carbon.

[0019] (4) The hollow porous silicon material coated with hard carbon is coated with soft carbon to obtain silicon-carbon electrode active material.

[0020] The present invention discloses a method for preparing a multi-level buffered silicon-carbon electrode active material. The pyrolytic polyolefin in the electrospun core layer disappears during heat treatment. The pyrolytic polyester and silicon in the electrospun middle layer form a porous silicon-carbon structure under heat treatment conditions. The pyrolytic polyester has a certain degree of flexibility and viscosity, which can maintain the uniformity and stability of silicon dispersion. The heteroatom-containing organic polymer in the electrospun outer layer pyrolyzes into heteroatom-containing porous hard carbon. The porous hard carbon formed after the pyrolysis of the heteroatom-containing organic polymer has high strength, which is beneficial to improving the stability of the structure. The entire structure forms a hollow porous silicon-carbon nanofiber structure coated with hard carbon. The uniform distribution gives the fibers a certain structural strength and conductivity, improving the long-term cycling performance of the material.

[0021] Preferably, in step (1), the specific method for reducing silicon oxide to obtain porous silicon is as follows: biomass silicon oxide is reduced to porous silicon by aluminothermic reduction. The biomass silicon oxide comes from the crude product of rice husk and bamboo leaves after calcination and grinding. The crude product, aluminum powder (particle size not higher than 5μm), and aluminum chloride are heated to 250-350℃ and kept at the temperature for 5-10 hours in a mass ratio of 1:(1-2):(5-15). After cooling, the porous silicon is obtained by soaking in hydrochloric acid solution, etching with HF, washing with deionized water, and drying.

[0022] Preferably, in step (2), the pyrolytic polyolefin includes at least one of styrene-acrylonitrile copolymer (molecular weight Mw = 100,000 to 150,000), high-density polyethylene (molecular weight Mw = 200,000 to 350,000), and copolymer polypropylene (molecular weight Mw = 2,500,000 to 350,000).

[0023] Preferably, in step (2), the concentration of the pyrolytic polyolefin in the electrospun core layer solution is 15wt% to 35wt%.

[0024] Preferably, in step (2), the pyrolytic polyester includes at least one of polymethyl methacrylate (Mw = 500,000 to 1,500,000), polyethylene terephthalate (Mw = 30,000 to 60,000), and polyacrylamide (Mw = 15,000 to 40,000).

[0025] Preferably, in step (2), the concentration of the pyrolytic polyester in the electrospun intermediate layer solution is 10wt% to 20wt%.

[0026] Preferably, the solvent of the electrospun intermediate layer solution is a mixture of DMF and acetone, wherein the volume ratio of DMF to acetone is (0.5-2):1.

[0027] Preferably, in step (2), the heteroatom-containing organic polymer includes at least one of polyaniline and polyvinylpyrrolidone.

[0028] This scheme prepares a multi-level buffer structure silicon-carbon electrode active material composed of multi-layer carbon materials through coaxial electrospinning. Nitrogen is doped into the silicon-carbon material to form a good structural framework and electron transport channels. The large specific surface area and high surface area-to-volume ratio of the prepared nanofiber structure increase the number of active sites, which is conducive to the full wetting of the electrolyte and improves the energy storage efficiency. It effectively improves the structural stability of the material, reduces the volume change of the silicon-carbon material during the electrocycle, and enhances the cycling performance and rate performance of the material.

[0029] Preferably, the molecular weight of the polyaniline is 20,000 to 50,000; and the molecular weight of the polyvinylpyrrolidone is 50,000 to 100,000.

[0030] Preferably, in step (2), the concentration of polyaniline in the electrospinning outer layer solution is 5 wt% to 10 wt%, and the concentration of polyvinylpyrrolidone in the electrospinning outer layer solution is 1 wt% to 5 wt%.

[0031] Preferably, in step (2), the electrospinning voltage of the coaxial electrospinning is 0.5 to 1.2 kV / cm, and the flow rate is 1 to 3 mL / h.

[0032] Preferably, in step (3), the heat treatment specifically involves: heating to 250–400°C for heat preservation and shaping, followed by heat treatment at 800–1000°C under inert gas protection.

[0033] Preferably, the heat treatment is carried out at 250-400°C for 1 hour and at 800-1000°C for 3 hours.

[0034] Preferably, in step (4), the method of coating the hard carbon-coated hollow porous silicon with soft carbon specifically involves: by mass fraction, 0.5-5 parts of soft carbon precursor (softening point 200-250℃), 1 part of crude porous silicon-carbon material, 50-100 parts of solvent (tetrahydrofuran / water v / v = (1-5):1), and 0.1-1 parts of dispersant (at least one of polyvinylpyrrolidone, polyethyleneimine, and polyvinyl alcohol) are ball-milled and dispersed in a ball mill, followed by spray drying and granulation, and carbonization under an inert atmosphere (800-1000℃, 2-5 hours) to obtain silicon-carbon electrode active material.

[0035] Preferably, the soft carbon precursor is asphalt, and the softening point of the asphalt is 200-250°C.

[0036] The present invention also provides an electrode for lithium batteries, comprising the aforementioned multi-level buffered silicon-carbon electrode active material. Detailed Implementation

[0037] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0038] Example 1

[0039] In this embodiment, the silicon-carbon electrode active material with multi-level buffer has a silicon-carbon mass ratio of 1:0.9, a hollow inner diameter of 760 nm, an outer diameter of 1000 nm for the hollow porous silicon-carbon, a thickness of 120 nm for the porous hard carbon coating layer, and a nitrogen element content of 0.5 wt% in the silicon-carbon electrode active material.

[0040] The preparation method of the multi-level buffered silicon-carbon electrode active material in this embodiment is as follows:

[0041] (1) Preparation of porous silicon: 50 parts of crushed dry rice husks were washed with 0.1M hydrochloric acid by mass fraction, then washed with deionized water until neutral, dried at 90-100℃ for 12 hours, calcined at 500℃ for 5 hours under argon protection, then cooled, ground, and sieved to obtain crude silicon oxide; 1 part of crude silicon oxide, 2 parts of aluminum powder with a particle size of 1μm and 10 parts of aluminum chloride were mixed evenly, heated to 300℃ and kept at 7 hours, cooled to room temperature, washed with 1M hydrochloric acid solution, and the filter cake was etched by adding 1M HF solution and keeping at 40℃ for 1 hour, washed with deionized water until neutral, dried under vacuum at 80℃ for 12 hours, and ground to obtain porous silicon.

[0042] (2) Preparation of hard carbon coated hollow porous silicon carbon: A 20wt% DMF solution was prepared using styrene-acrylonitrile copolymer with a weight average molecular weight of 120,000 as the electrospinning core layer solution. DMF / acetone (1 / 1v / v) solutions with concentrations of 10wt% and 15wt% were prepared using porous silicon and polymethyl methacrylate with a weight average molecular weight of 1,000,000 as the electrospinning intermediate layer solutions. DMF solutions with concentrations of 7wt% and 3wt% were prepared using polyaniline with a weight average molecular weight of 35,000 and polyvinylpyrrolidone with a weight average molecular weight of 80,000 as the electrospinning outer layer solutions. Solution; set the electrospinning voltage to 16 kV, the distance between the spinning needle and the receiving plate to 20 cm, the spinning temperature to 25 °C, and the solution flow rates of the core layer, intermediate layer, and outer layer to 1.8 mL / h, 1.2 mL / h, and 2.2 mL / h, respectively, for coaxial electrospinning to obtain spun fibers; the spun fibers were heated to 300 °C in air and held for 1 hour for shaping, and then heated to 1000 °C in nitrogen atmosphere at a controlled heating rate of 10 °C / min and held for 3 hours, and then cooled to obtain hard carbon-coated hollow porous silicon carbon with an inner diameter of 760 nm, an outer diameter of 1000 nm, and a hard carbon shell thickness of 120 nm.

[0043] (3) Preparation of multi-stage buffered silicon-carbon electrode active material: by mass fraction, 1 part of hard carbon-coated hollow porous silicon-carbon, 2.5 parts of asphalt with a softening point of 220℃, 100 parts of a mixed solution of tetrahydrofuran / water (2 / 1v / v), 0.1 parts of polyvinylpyrrolidone, and 0.2 parts of polyvinyl alcohol were dispersed by ball milling in a ball mill under nitrogen protection. The ball mill speed was 50 rpm and the ball milling was carried out for 5 hours. Then, the material was spray-granulated at 220℃ and heated to 1000℃ in 5% hydrogen-argon atmosphere for 3 hours. After cooling, the silicon-carbon electrode active material was obtained. The proportion of nitrogen element in the silicon-carbon electrode active material was 0.5wt%, the aspect ratio was 20:1, and the specific surface area was 5.5m2 / g.

[0044] The silicon-carbon mass ratio, hollow inner diameter, hollow porous silicon-carbon outer diameter, porous hard carbon coating thickness, and nitrogen content (nitrogen mass fraction) of the silicon-carbon electrode active materials prepared in Examples 2-8 and Comparative Examples 1-3 (using the coaxial electrospinning method of the present invention) are shown in Table 1.

[0045] Table 1

[0046]

[0047] Comparative Example 3

[0048] The silicon-carbon electrode active material in this comparative example has no soft carbon coating layer (using the coaxial electrospinning method of the present invention), and the rest is the same as the structure and structural parameters of Example 1.

[0049] Performance testing

[0050] Electrode preparation: Silicon-carbon electrode active material, conductive agent, binder and solvent are prepared into electrode slurry. The electrode slurry is coated on the current collector. The coated electrode sheet is rolled and then formed into an electrode sheet.

[0051] 1. Participants

[0052] The electrode sheets prepared from the silicon-carbon electrode active materials of all examples and comparative examples were used as negative electrodes. A lithium sheet was used as the positive electrode, a microporous polypropylene membrane as the separator, and 1 mol / L LiPF6 (solvent consisting of equal volumes of dimethyl carbonate and dipropyl carbonate) as the electrolyte. These were assembled with the aforementioned negative electrode sheets in an argon-filled glove box to form a coin-type lithium-ion battery, which served as the test object for this test example. After the coin-type lithium-ion battery was left to stand for 24 hours, a charge-discharge test was conducted at 500 mA, with the charge-discharge range between 0.01 and 3.0 V.

[0053] 2. Test Content

[0054] (1) Electrochemical performance testing

[0055] After the assembled coin-type lithium-ion battery was left to stand for 24 hours, a charge-discharge test was conducted at a current of 500mA. The charge-discharge range was between 0.01 and 3.0V. The charge-discharge capacity of the first, second, and 300th cycles was tested. The capacity retention rate of the 300th cycle was calculated using the following formula: Capacity retention rate (%) = (Charging capacity of the 300th cycle / Charging capacity of the 2nd cycle) × 100%.

[0056] (2) Electrode Expansion Rate Test

[0057] After the assembled battery was charged and discharged for 300 cycles at a current of 500 mA / g, it was fully charged and then disassembled to test the thickness expansion rate of the prepared electrode sheet. The calculation formula is: thickness expansion rate = (electrode sheet thickness after 300 cycles / electrode sheet thickness in the second cycle - 1) × 100%.

[0058] 3. The test results are shown in Table 2 below:

[0059] Table 2

[0060]

[0061] As shown in Table 2, the electrode sheets prepared from the multi-level buffered silicon-carbon electrode active materials of Examples 1-8, when assembled into coin-type lithium-ion batteries as negative electrodes, exhibit a first-cycle coulombic efficiency ≥89% at 500mA current, a capacity retention rate ≥90% after 300 charge-discharge cycles, and an electrode thickness expansion rate ≤20%. In Comparative Example 1, the hollow inner diameter is 0, significantly reducing the buffer space for silicon volume expansion. After 300 cycles, the electrode expansion rate increased significantly from 10.2% in Example 1 to 242.6%, leading to structural instability of the silicon-carbon electrode active material and causing structural collapse. The first-cycle coulombic efficiency and the capacity retention rate after 300 cycles were also significantly lower than in Example 1. Comparative Example 2 lacks hard carbon coating, resulting in structural instability of the silicon-carbon electrode active material and causing structural collapse. This led to a significant decrease in the first-cycle coulombic efficiency and the capacity retention rate after 300 cycles compared to Example 1, while the electrode expansion rate after 300 cycles was significantly higher than in Example 1. Comparative Example 3 lacks a soft carbon coating layer, which is not conducive to reducing the side reactions between silicon and electrolyte. This results in a significant decrease in the initial coulombic efficiency and the capacity retention rate after 300 cycles compared to Example 1, while the electrode expansion rate after 300 cycles is significantly increased compared to Example 1.

[0062] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention 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 the present invention, but such modifications or substitutions are all within the scope of protection of the present invention.

Claims

1. A multi-level buffered silicon-carbon electrode active material, characterized in that, The silicon-carbon electrode active material includes a matrix and a coating layer covering the surface of the matrix; The coating layer has at least two layers, and the coating layer is a soft carbon coating layer wrapped around the outside of the hard carbon coating layer. The hard carbon coating layer includes a porous hard carbon coating layer, and the thickness of the hard carbon coating layer is 60-400 nm. The matrix includes hollow silicon-carbon material, which includes hollow porous silicon-carbon material, wherein the outer diameter of the hollow porous silicon-carbon material is 400-2000 nm and the hollow inner diameter is 300-1500 nm. The preparation method of the silicon-carbon electrode active material includes the following steps: (1) Reduce silicon oxide to obtain porous silicon; (2) Prepare an electrospinning core layer solution, an electrospinning intermediate layer solution, and an electrospinning outer layer solution. The electrospinning core layer solution includes a pyrolytic polyolefin, the electrospinning intermediate layer solution includes a pyrolytic polyester and the porous silicon, and the electrospinning outer layer solution includes a heteroatomic organic polymer. Perform coaxial electrospinning on the electrospinning core layer solution, the electrospinning intermediate layer solution, and the electrospinning outer layer solution to obtain spun fibers. (3) The spun fibers are heat-treated to obtain a hollow porous silicon-carbon material coated with hard carbon. (4) The hollow porous silicon-carbon material coated with hard carbon is coated with soft carbon material to obtain silicon-carbon electrode active material.

2. The multi-stage buffered silicon-carbon electrode active material according to claim 1, characterized in that, The mass ratio of silicon to carbon in the silicon-carbon electrode active material is 1:(0.5 to 1.5).

3. The silicon-carbon electrode active material with multi-level buffering according to claim 1, characterized in that, The silicon-carbon electrode active material further includes heteroatom doping, wherein the heteroatom includes at least one of nitrogen atoms and sulfur atoms.

4. The silicon-carbon electrode active material with multi-level buffering according to claim 1, characterized in that, The heat treatment in step (3) includes: heating to 250-400℃ and holding for shaping, followed by treatment at 800-1000℃.

5. An electrode for a lithium battery, characterized in that, The silicon-carbon electrode active material includes the multi-level buffer as described in any one of claims 1-4.

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