Silicon-carbon negative electrode material with egg yolk shell structure and preparation method of silicon-carbon negative electrode material

Abstract

The invention provides a yolk shell structure silicon-carbon negative electrode material and a preparation method thereof, the silicon-carbon negative electrode material is composed of micron-sized particles formed by silicon nanoparticles coated with a layer of uniform carbon shell, the surface of the micron-sized particles is coated with a carbon layer, a gap layer exists between the micron-sized particles and the carbon shell, and the carbon shell is coated with the carbon layer. A buffer space for silicon volume expansion is provided. The invention aims to solve the technical problems of grade piece cracking, internal resistance increase and battery cycle performance reduction caused by volume expansion of silicon.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon-carbon materials, and particularly relates to a yolk-shell structured silicon-carbon anode material and a preparation method thereof. Background Art

[0002] In recent years, renewable energy sources such as solar energy and wind energy have developed rapidly. However, due to the instability and discontinuity of these energy systems, large-scale energy storage devices are needed to make full use of them. Lithium-ion batteries, as rechargeable secondary batteries, play an important role, and the development of high-energy-density lithium-ion batteries is the key to meeting the increasingly demanding energy storage requirements. Silicon-based anode materials are considered ideal substitutes for graphite anodes due to their high theoretical capacity and natural abundance. However, problems such as short cycle life and low Coulomb efficiency seriously hinder their practical applications.

[0003] Carbon materials, due to their good electrical conductivity, low cost, and wide sources, can improve the electrical conductivity and surface stability of silicon-based materials, and are preferentially used as performance-improving additives for silicon-based anodes. Silicon-carbon materials have become the mainstream development direction of silicon-based anodes. Silicon-carbon anode materials have both excellent electrical conductivity and stable cycling performance of carbon materials, and also combine the higher lithium storage performance of silicon materials. They are the most likely commercialized lithium-ion battery anode materials after graphite anode materials. Carbon coating can improve the surface stability of silicon-based materials, but its ability to inhibit the volume expansion of silicon is general and cannot effectively solve the problem of silicon volume expansion. For example, the publicly disclosed patent 2020101339028 discloses a silicon anode material with a carbon nanotube carbon shell coating and a preparation method thereof. The required silicon anode material is formed by using carbon to coat the silicon-carbon material. However, there are no gaps between the carbon shell and the micron-sized particles during the preparation of the above preparation method, and the material is prone to pulverization and fragmentation and the electrode sheet is prone to cracking when inhibiting the silicon volume expansion. Therefore, it is particularly important to solve the problems that the currently carbon-shell-coated silicon anode materials are prone to material pulverization and fragmentation and electrode sheet cracking. Summary of the Invention

[0004] The purpose of the present invention is to provide a yolk-shell structured silicon-carbon anode material and a preparation method thereof, aiming to solve the technical problems of electrode sheet cracking, increased internal resistance, and decreased battery cycling performance caused by the volume expansion of silicon.

[0005] The present invention provides a yolk-shell structured silicon-carbon anode material, which is composed of micron-sized particles formed by silicon nanoparticles coated with a uniform carbon shell. The surface of the micron-sized particles is coated with a carbon layer, and there is a void layer between the micron-sized particles and the carbon shell for providing a buffer space for the volume expansion of silicon.

[0006] The present invention also provides a preparation method of a yolk-shell structured silicon-carbon anode material, and the preparation method includes the following steps:

[0007] Step 1: Uniformly coat a layer of carbon on the surface of nano-silicon particles;

[0008] Step 2: Disperse the carbon-coated nano-silicon material into an organic carbon solution with a low residual carbon rate, stir evenly and then spray dry to obtain micron-sized particles;

[0009] Step 3: Coat the micron-sized particles with a high residual carbon rate carbon source, and obtain a yolk-shell structured silicon-carbon anode material after carbonization.

[0010] A further improvement lies in that: the particle size of the nano-silicon particles in Step 1 is less than 100 nm.

[0011] A further improvement lies in that: the carbon source in Step 1 includes but is not limited to one or several of phenolic resin, asphalt, graphite, graphene, coal tar, polydopamine, glucose, and sucrose. A further improvement lies in that: the residual carbon rate of the organic carbon solution with a low residual carbon rate in Step 2 is less than 5%, including but not limited to materials such as PMMA and PVP.

[0012] A further improvement lies in that: the mass ratio of the low residual carbon rate organic carbon to the carbon-coated nano-silicon in Step 2 is between 1:0.1 - 10.

[0013] A further improvement lies in that: the solid content of the carbon-coated nano-silicon particles dispersed in the organic carbon solution in Step 2 is between 5 - 20%.

[0014] A further improvement lies in that: the micron-sized particles obtained by spray drying in Step 2 have a particle size between 1 - 20 μm.

[0015] A further improvement lies in that: the residual carbon rate of the high residual carbon rate carbon source in Step 3 is higher than 20%, including but not limited to materials such as phenolic resin, asphalt, polyurethane, and graphene.

[0016] A further improvement lies in that: when coating the micron-sized particles with the high residual carbon rate carbon source in Step 3, the structure of the micron-sized particles cannot be damaged.

[0017] The beneficial effects of the present invention are as follows: For the yolk-shell structured silicon-carbon anode material obtained by the preparation method, the carbon layer coated on the surface of the nano-silicon particles can improve the conductivity of silicon, inhibit the volume expansion of silicon, and avoid the formation of an unstable SEI film in contact with the electrolyte. Dispersing the nano-silicon particles coated with a carbon layer into an organic carbon solution with a low residual carbon rate, forming micron-sized particles after spray drying, and then coating another layer of high residual carbon rate organic carbon material on the outer layer. After high-temperature carbonization, the high residual carbon rate carbon source coated on the surface of the micron-sized particles pyrolyzes to form a carbon shell, and the low residual carbon rate organic carbon in the micron-sized particles completely pyrolyzes, forming a void between the carbon-coated nano-silicon particles and the outermost carbon shell, which can provide space for the volume expansion of silicon particles, avoid problems such as material pulverization, fragmentation, and electrode sheet cracking, and effectively improve the cycle stability of the battery. Brief Description of the Drawings

[0018] Figure 1 It is a schematic structural diagram of the yolk-shell structured silicon-carbon anode material prepared by the present invention.

[0019] Figure 2 It is a flow chart of the preparation method of the present invention. Detailed Embodiments

[0020] To deepen the understanding of the present invention, the present invention will be further described in detail below in conjunction with embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation to the protection scope of the present invention.

[0021] Embodiment 1

[0022] The yolk-shell structured silicon-carbon anode material is prepared by the following steps:

[0023] (1) Dispersing nano-silicon particles in a mixed solution of pure water and ethanol, and successively adding resorcinol, ammonia water (28 wt%), and formaldehyde (37 wt%), stirring for 6 h, standing overnight, washing repeatedly with pure water and ethanol, and collecting by centrifugation. The mass ratio of nano-silicon particles, resorcinol, ammonia water, and formaldehyde is 1:0.8:0.2:1.

[0024] (2) Carbonizing the nano-silicon particles coated with phenolic resin in argon at 800 °C for 2 h to obtain carbon-coated nano-silicon material.

[0025] (3) Dispersing the carbon-coated nano-silicon material into a toluene solution of PMMA, stirring evenly and then spray-drying to obtain micron-sized particles. The mass ratio of the carbon-coated nano-silicon material to PMMA is 1:1.

[0026] (4) Dispersing the micron-sized particles into an aqueous polyurethane solution, stirring evenly and then spray-drying to obtain a dry powder, and then carbonizing it in argon at 800 °C for 2 h to obtain the yolk-shell structured silicon-carbon anode material.

[0027] Embodiment 2

[0028] The yolk-shell structured silicon-carbon anode material is prepared by the following steps:

[0029] (1) Dispersing nano-silicon particles in a mixed solution of pure water and ethanol, and successively adding resorcinol, ammonia water (28 wt%), and formaldehyde (37 wt%), stirring for 6 h, standing overnight, washing repeatedly with pure water and ethanol, and collecting by centrifugation. The mass ratio of nano-silicon particles, resorcinol, ammonia water, and formaldehyde is 1:0.8:0.2:1.

[0030] (2) Carbonizing the nano-silicon particles coated with phenolic resin in argon at 800 °C for 2 h to obtain carbon-coated nano-silicon material.

[0031] (3) Disperse the carbon-coated nanosilicon material into the toluene solution of PMMA, stir evenly and then spray dry to obtain micron-sized particles. The mass ratio of the carbon-coated nanosilicon material to PMMA is 10:1.

[0032] (4) Disperse the micron-sized particles into the aqueous polyurethane solution, stir evenly and then spray dry to obtain a dry powder, and then place it in argon for carbonization at 800 °C for 2 h to obtain a yolk-shell structured silicon-carbon anode material.

[0033] Example 3

[0034] The yolk-shell structured silicon-carbon anode material is prepared by the following steps:

[0035] (1) Disperse the nanosilicon particles in a mixed solution of pure water and ethanol, sequentially add resorcinol, ammonia water (28 wt%), and formaldehyde (37 wt%), stir for 6 h, let stand overnight, wash repeatedly with pure water and ethanol, and collect by centrifugation. The mass ratio of the nanosilicon particles, resorcinol, ammonia water, and formaldehyde is 1:0.8:0.2:1.

[0036] (2) Carbonize the nanosilicon particles coated with phenolic resin in argon at 800 °C for 2 h to obtain a carbon-coated nanosilicon material.

[0037] (3) Disperse the carbon-coated nanosilicon material into the toluene solution of PMMA, stir evenly and then spray dry to obtain micron-sized particles. The mass ratio of the carbon-coated nanosilicon material to PMMA is 1:10.

[0038] (4) Disperse the micron-sized particles into the aqueous polyurethane solution, stir evenly and then spray dry to obtain a dry powder, and then place it in argon for carbonization at 800 °C for 2 h to obtain a yolk-shell structured silicon-carbon anode material.

[0039] Comparative Example 1

[0040] The difference between Comparative Example 1 and Example 1 is that in Comparative Example 1, a low residual carbon rate material is not used, and a yolk-shell structure is not formed.

[0041] (1) Disperse the nanosilicon particles in a mixed solution of pure water and ethanol, sequentially add resorcinol, ammonia water (28 wt%), and formaldehyde (37 wt%), stir for 6 h, let stand overnight, wash repeatedly with pure water and ethanol, and collect by centrifugation. The mass ratio of the nanosilicon particles, resorcinol, ammonia water, and formaldehyde is 1:0.8:0.2:1.

[0042] (2) Carbonize the nanosilicon particles coated with phenolic resin in argon at 800 °C for 2 h to obtain a carbon-coated nanosilicon material.

[0043] (3)Disperse micron-sized particles into an aqueous polyurethane solution, stir evenly and then spray dry to obtain a dry powder. Then place it in argon and carbonize at 800 °C for 2 h to obtain a yolk-shell structured silicon-carbon anode material.

[0044] Comparative Example 2

[0045] The difference between Comparative Example 2 and Example 1 is that there is no carbon layer coating on the surface of the core nano-silicon particles in Comparative Example 2.

[0046] (1)Disperse nano-silicon particles into a toluene solution of PMMA, stir evenly and then spray dry to obtain micron-sized particles. The mass ratio of nano-silicon particles to PMMA is 1:1. (2)Disperse the micron-sized particles into an aqueous polyurethane solution, stir evenly and then spray dry to obtain a dry powder. Then place it in argon and carbonize at 800 °C for 2 h to obtain a yolk-shell structured silicon-carbon anode material.

[0047] Performance test:

[0048] Assemble the materials prepared in Examples 1-3 and Comparative Examples 1-2 into button cells for electrochemical performance test and analysis. The specific scheme is as follows: Mix the prepared materials, conductive agent SP, and binder LA136 in a ratio of 8:1:1 and make a button cell of model 2032. The counter electrode is a lithium sheet, the separator is a Celgard 2400 microporous polypropylene membrane, the voltage range is 0.01-1.5 V, and constant current charge and discharge are carried out at a current density of 0.1 A g-1.

[0049] The test results are shown in Table 1.

[0050] Table 1

[0051]

[0052] Examples 1-3 are silicon-carbon anode materials with a yolk-shell structure, and the volume of the void layer is regulated by controlling the proportion of organic carbon with a low residual carbon rate. As can be seen from Table 1, by comparing the electrochemical performances of Examples 1-3, it can be seen that the volume of the void layer affects the cycle stability of the battery. When the mass ratio of the organic carbon with a low residual carbon rate to the micron-sized particles is 1:1, the volume of the obtained void layer is the best. In Example 2, the volume of the void layer is too small, providing insufficient space for the volume expansion of silicon. During the process of multiple expansions and contractions of silicon, the outer carbon shell is damaged, resulting in a decline in the cycle performance. In Example 3, the volume of the void layer is too large, the structure is unstable, and it is unable to effectively form a yolk-shell structure or the structure is easily damaged during the preparation of the electrode sheet, exposing the core carbon-coated nano-silicon particles, increasing the specific surface area, resulting in a decrease in the initial Coulomb efficiency and the cycle performance. Comparative Example 1 is a control for Example 1. In Comparative Example 1, a material with a low residual carbon rate is not used, and a yolk-shell structure is not formed, resulting in a significant decrease in the cycle performance of the battery. Comparative Example 2 is a control for Example 1. In Comparative Example 2, a carbon layer is not coated on the surface of the nano-silicon particles. The conductivity of the nano-silicon is relatively low, and the micron-sized particles of the core structure do not form a good conductive network, with a large internal resistance, resulting in a decline in the cycle performance of the battery.

[0053] The above are only the embodiments of the present invention, and thus do not limit the patent scope of the present invention. Any equivalent transformation made by using the content of the specification of the present invention, or directly or indirectly applied in the relevant technical field, shall be equally included in the patent protection scope of the present invention.

Claims

1. A silicon-carbon anode material with a yolk-shell structure, characterized in that: The silicon-carbon anode material is composed of micron-sized particles formed by silicon nanoparticles coated with a uniform carbon shell, and there is a void layer between the micron-sized particles and the carbon shell to provide a buffer space for the volume expansion of silicon.

2. A method for preparing a silicon-carbon anode material with a yolk-shell structure as described in claim 1, characterized in that: The preparation method includes the following steps: Step 1: Uniformly coat a carbon shell on the surface of the nano-silicon particles; Step 2: Disperse the carbon-coated nano-silicon material into an organic carbon solution with a low residual carbon rate, stir evenly and then spray-dry to obtain micron-sized particles; Step 3: Coat the micron-sized particles with a high residual carbon rate carbon source, and obtain a silicon-carbon anode material with a yolk-shell structure after carbonization.

3. The preparation method of a silicon-carbon anode material with a yolk-shell structure according to claim 2, wherein: In the said Step 1, the particle size of the nano-silicon particles is less than 100 nm.

4. The preparation method of a silicon-carbon anode material with a yolk-shell structure according to claim 2, characterized in that: In the said Step 2, the residual carbon rate of the organic carbon solution with a low residual carbon rate is less than 5%.

5. The preparation method of a silicon-carbon anode material with an egg yolk shell structure according to claim 2, wherein: In the said Step 2, the particle size of the micron-sized particles is between 1 - 20 μm.

6. The preparation method of a silicon-carbon anode material with a yolk-shell structure according to claim 2, characterized in that: In the said Step 3, the residual carbon rate of the high residual carbon rate carbon source is greater than 20%.

7. The preparation method of a silicon-carbon anode material with an egg yolk shell structure according to claim 2, characterized in that: When coating the micron-sized particles with the high residual carbon rate carbon source in the said Step 3, the structure of the micron-sized particles is not damaged.

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