Self-supporting silicon electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium ion battery negative electrode materials, and particularly relates to a self-supporting silicon electrode material and a preparation method and application thereof. The preparation method of the self-supporting silicon electrode material comprises the following steps: taking 2-methylimidazole as a ligand, and mixing the ligand with silicon powder in a first solvent system to obtain a ligand solution; the preparation method comprises the following steps: mixing a ligand solution and a cobalt salt solution under an ultraviolet irradiation condition, and standing in a dark place for reaction to form a ZIF-67 coated silicon core-shell structure, so as to obtain a composite material; and under a second solvent system, mixing the composite material with a polymer, performing freeze drying, and then performing calcination under a protective atmosphere to form a three-dimensional conductive support structure so as to obtain the self-supporting silicon electrode material. According to the invention, the surface of the silicon electrode and the ZIF-67 frame are chemically linked through ultraviolet light induction, so that the internal close combination of the silicon electrode is realized, the pulverization of the silicon electrode under high magnification and long circulation can be effectively buffered, and meanwhile, the cycle life of the battery is prolonged.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery negative electrode materials and relates to a self-supporting silicon electrode material and a preparation method and application thereof. Background Art

[0002] Silicon-based anode materials are recognized as the next generation anode materials with the greatest potential to improve the specific energy of lithium-ion batteries due to their theoretical specific capacity of up to 4200mAh / g, low working potential, rich element reserves and environmental friendliness.

[0003] However, the volume expansion of silicon-based negative electrode materials causes structural failure, which restricts the large-scale application of silicon-based negative electrode materials in the field of electrochemical energy storage. The volume change of silicon-based materials is due to the alloying and lithiation reaction mechanism. + Completely embedded to generate Li 22 When Si5 is used, the theoretical volume change reaches 320%. The cyclic stress generated in this process can cause the local structure of the material to become powdery and fall off, resulting in rapid capacity decay and poor cycle life.

[0004] Existing technologies often employ nanosizing, buffer structure construction, and in-situ coating strategies to mitigate the volume expansion-induced structural failure of silicon-based anode materials. When silicon particles are nanosized, volume expansion is significantly reduced, shortening the lithium-ion diffusion path, alleviating stress concentration, and reducing the expansion effect. Buffer structure construction involves introducing voids such as porous carbon and hollow polymer spheres to provide free expansion space and maintain structural stability. In-situ coating involves coating silicon particles with oxide or carbon layers to inhibit expansion.

[0005] Although these methods exhibit certain advantages under low-rate and short-cycle charge-discharge conditions, due to the slow lithium-ion insertion and extraction processes that have little impact on the material structure, under high-rate and long-cycle conditions, rapid ion migration exacerbates structural damage, and the lithium insertion and extraction processes further undermine structural stability. Repeated lithiation and delithiation cycles can lead to structural instability, causing severe electrode pulverization and significantly shortening the battery cycle life. Summary of the Invention

[0006] In order to solve the technical problems of severe pulverization and short cycle life of silicon electrodes under the above-mentioned high rate and long cycle conditions, the present invention provides a self-supporting silicon electrode material, a preparation method and application thereof.

[0007] The present invention uniformly coats ZIF-67 on the surface of nano-silicon to form a core-shell structure of ZIF-67 encapsulating silicon. At the same time, ultraviolet light is introduced into the ZIF-67 synthesis, and a chemical link between the surface of the silicon electrode and the ZIF-67 framework is induced by light to achieve a tight combination inside the silicon electrode. This can effectively buffer the pulverization of the electrode under high rate and long cycle conditions, while extending the cycle life of the battery. This solves the technical problems of the prior art in severe pulverization of the silicon electrode and short cycle life under high rate and long cycle conditions.

[0008] The present invention utilizes the organic components and Co nanoparticles in ZIF-67 to interlock and interconnect with the Si surface, forming a rigid interconnected structure of Co-N-Si chemical bonds. The Co atoms in ZIF-67 synergistically link the Si with the organic components, which can enhance the stability of the material and play a "rigidity" role. ZIF-67 can also enhance the conductivity of the electrode and accelerate ion / electron transport. At the same time, the polymer plays a flexible supporting role, accommodating the volume expansion of Si during the charge and discharge cycle, providing a "flexible" protective effect. In addition, the flexible polyacrylonitrile can adhere to the Si nanoparticles that pulverize under high-rate cycling, ensuring the structural integrity of the self-supporting Si negative electrode.

[0009] The first object of the present invention is to provide a method for preparing a self-supporting silicon electrode material, comprising the following steps: Using 2-methylimidazole as a ligand, the ligand and silicon powder are mixed in a first solvent system to obtain a ligand solution; under ultraviolet light conditions, the ligand solution and cobalt salt solution are mixed to form a ZIF-67 precursor. At the same time, photoexcitation induces a chemical link between the silicon surface and the ZIF-67 precursor to obtain a composite solution.

[0010] The composite solution is kept in the dark and allowed to react, so that ZIF-67 can grow directionally on the silicon surface to form a core-shell structure of ZIF-67 encapsulating silicon, thereby obtaining a composite material.

[0011] In a second solvent system, the composite material is mixed with a polymer and then freeze-dried to obtain a precursor; the precursor is calcined under a protective atmosphere so that the carbon network of the porous carbon skeleton generated by the pyrolysis of ZIF-67 is interwoven with each other to form a three-dimensional conductive support structure, thereby obtaining a self-supporting silicon electrode material; the polymer is polyacrylonitrile.

[0012] It should be noted that the present invention introduces ultraviolet light into the synthesis of ZIF-67. Under ultraviolet light conditions, the ligand solution and the cobalt salt solution are mixed to accelerate the coordination reaction between the cobalt ions in the cobalt salt solution and the imidazole ligands in the composite solution to form a ZIF-67 precursor. At the same time, light excitation promotes the chemical adsorption of the nano-Si surface and the ZIF-67 precursor to form a Co-N-Si rigid bonding structure, thereby achieving an interlaced interconnection between the organic ligands, Co atoms and the Si surface in ZIF-67. Preferably, the ultraviolet light conditions are: the power of the ultraviolet light source is 300W to 800W, the time is 30s to 50s, and the temperature is room temperature.

[0013] Preferably, the molar ratio of the ligand to the cobalt ions in the cobalt salt solution is 2-12:1; and the molar ratio of the ligand to the silicon powder is 1-6:10.

[0014] Preferably, the temperature for mixing the ligand and silicon powder is 30° C. to 55° C., and the time for mixing is 10 min to 30 min.

[0015] Preferably, the cobalt salt solution is obtained by mixing the cobalt salt and the first solvent at 30° C. to 55° C. for 10 min to 30 min.

[0016] Preferably, the first solvent is methanol.

[0017] Preferably, the particle size of the silicon powder is less than 100 nm.

[0018] Preferably, the temperature of the reaction is 70°C to 80°C and the time is 18 hours to 30 hours. In the present invention, the composite solution is allowed to react in the dark to inhibit excessive growth of ZIF-67 crystals, ensuring that the silicon core is completely coated, forming a core-shell structure of ZIF-67 encapsulating silicon.

[0019] Preferably, the mass ratio of the composite material to the polymer is 1:1.

[0020] Preferably, the molecular weight of the polymer is 2.5×10 4 ~8×10 4 Polyacrylonitrile, as the adhesive material of the self-supporting electrode, can form a sticky link with the Si surface, and will form a chemical bond during subsequent calcination and cyclization, strengthening the chemical link on the Si surface, making it less likely to pulverize during the charge and discharge process.

[0021] Preferably, the second solvent is a mixed solution of N,N-dimethylformamide and ethanol, with the volume ratio of N,N-dimethylformamide to ethanol being 1:1. The present invention uses ethanol as a volatile pore-forming agent, which easily maintains the structure and forms through-hole multi-level pores after drying. Furthermore, N,N-dimethylformamide strengthens the chemical bonds on the Si surface, effectively dissolves the polymer, and evaporates through freeze-drying.

[0022] Preferably, the freeze-drying time is 4 hours. The present invention maintains the original form of the composite material and fixes the three-dimensional network structure through freeze-drying, while drying the solvent in the composite material.

[0023] Preferably, the calcination temperature is 300°C and the calcination time is 20 to 24 hours. The precursor is calcined so that the porous carbon skeleton generated by the pyrolysis of ZIF-67 and the carbon network formed by the cyclization of the polymer are interwoven, forming a three-dimensional conductive support structure and maintaining structural stability. Since the polymer cyclizes at 300°C, the polymer will carbonize when the temperature is higher than 300°C. When the temperature is lower than 300°C, the cyclization is incomplete and requires a longer time.

[0024] The second object of the present invention is to provide a self-supporting silicon electrode material obtained by the above preparation method.

[0025] The third object of the present invention is to provide the use of the above-mentioned self-supporting silicon electrode material as a negative electrode material for lithium-ion batteries.

[0026] Compared with the prior art, the present invention has the following technical effects: 1. The present invention uniformly coats ZIF-67 on the surface of nano-silicon to form a core-shell structure of ZIF-67-wrapped silicon. At the same time, ultraviolet light is introduced into the ZIF-67 synthesis, and the chemical link between the silicon electrode surface and the ZIF-67 framework is induced by light to achieve a tight combination inside the silicon electrode. This can effectively buffer the pulverization of the electrode under high rate and long cycle conditions, while extending the cycle life of the battery. It solves the technical problems of the prior art in severe pulverization of the silicon electrode and short cycle life under high rate and long cycle conditions.

[0027] 2. The present invention utilizes a rigid interconnected structure formed by the staggered interconnection of ZIF-67 and the Si surface to improve the stability of the electrode material; at the same time, the cyclized polymer is used to play a flexible supporting role, which can accommodate the volume expansion of the Si electrode during the charge and discharge cycle. The present invention synergistically improves the performance of the self-supporting negative electrode material through the combination of rigidity and flexibility.

[0028] 3. The present invention uses self-supporting silicon electrode materials as the electrode materials for lithium-ion batteries. The structure is stable during the cycle, the internal part is not powdered, and the flexible cyclized polyacrylonitrile network layer limits the cracking of the material. At the same time, the Co atoms in ZIF-67 and the organic components synergistically link Si and provide stability. -1 At a current density of 820 mAh g -1 The discharge capacity is 960 mAh g after 100 cycles. -1 ; Lithium-ion batteries prepared using self-supporting silicon electrode materials have stable cycle capacity and strong structural stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a scanning electron microscope image of the self-supporting silicon electrode material prepared in Example 1.

[0030] Figure 2 The figure is a comparison chart of the capacities of the lithium-ion batteries prepared by Application Example 1 and Application Comparative Example 1.

[0031] Figure 3 This is the cycle curve of the lithium-ion battery prepared in Application Example 2.

[0032] Figure 4 The figures are rate curves of the lithium-ion battery prepared in Example 3 at different current densities. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand and implement the technical solution of the present invention, the present invention is further described below with reference to specific embodiments and drawings.

[0034] In the description of the present invention, unless otherwise specified, all reagents used are commercially available and all methods used are conventional techniques in the art.

[0035] Example 1 A method for preparing a self-supporting silicon electrode material comprises the following steps: Step 1: Prepare the composite solution: 10 mmol of cobalt nitrate was dissolved in 400 mL of methanol and stirred at 55° C. for 30 min to obtain a cobalt nitrate solution.

[0036] 60 mmol of 2-methylimidazole and 200 mmol of nano-silicon powder were dispersed in 500 mL of methanol and stirred at 55° C. for 30 minutes to obtain a ligand solution; wherein the particle size of the nano-silicon powder was less than 100 nm.

[0037] Under the condition of ultraviolet light irradiation with a power of 600 W, the cobalt nitrate solution and the ligand solution were mixed and stirred for 50 seconds to obtain a composite solution.

[0038] Step 2: Preparation of composite materials: The composite solution was allowed to stand at 80° C. for 24 h under light-proof conditions and filtered to obtain a composite material.

[0039] Step 3: Preparation of self-supporting silicon electrode material: After mixing 10 g of the composite material with 10 g of polyacrylonitrile, 30 mL of N,N-dimethylformamide and 30 mL of ethanol were added, and the mixture was stirred for 30 minutes, and then frozen at -30°C for 4 hours. The mixture was then allowed to stand and naturally dried before demolding to obtain a precursor.

[0040] Under a nitrogen protective atmosphere, the precursor was placed in a tube furnace and calcined at 300°C for 22 hours to obtain a self-supporting silicon electrode material.

[0041] Example 2 A method for preparing a self-supporting silicon electrode material comprises the following steps: Step 1: Prepare the composite solution: 8 mmol of cobalt nitrate was dissolved in 300 mL of methanol and stirred at 45° C. for 20 min to obtain a cobalt nitrate solution.

[0042] 30 mmol of 2-methylimidazole and 300 mmol of nano-silicon powder were dispersed in 300 mL of methanol and stirred at 45° C. for 20 minutes to obtain a ligand solution; wherein the particle size of the nano-silicon powder was less than 100 nm.

[0043] Under the condition of ultraviolet light irradiation with a power of 600 W, the cobalt nitrate solution and the ligand solution were mixed and stirred for 40 seconds to obtain a composite solution.

[0044] Step 2: Preparation of composite materials: The composite solution was allowed to stand at 75° C. for 24 h under light-proof conditions and filtered to obtain a composite material.

[0045] Step 3: Preparation of self-supporting silicon electrode material: After mixing 8 g of the composite material with 8 g of polyacrylonitrile, 20 mL of N,N-dimethylformamide and 20 mL of ethanol were added, and the mixture was stirred for 20 minutes, frozen at -30°C for 4 hours, and then allowed to stand and naturally dried before demolding to obtain a precursor.

[0046] Under a nitrogen protective atmosphere, the precursor was placed in a tube furnace and calcined at 300°C for 22 hours to obtain a self-supporting silicon electrode material.

[0047] Example 3 A method for preparing a self-supporting silicon electrode material comprises the following steps: Step 1: Prepare the composite solution: 5 mmol of cobalt nitrate was dissolved in 200 mL of methanol and stirred at 30° C. for 10 min to obtain a cobalt nitrate solution.

[0048] 20 mmol of 2-methylimidazole and 100 mmol of nano-silicon powder were dispersed in 200 mL of methanol and stirred at 30° C. for 10 minutes to obtain a ligand solution; wherein the particle size of the nano-silicon powder was less than 100 nm.

[0049] Under the condition of ultraviolet light irradiation with a power of 600 W, the cobalt nitrate solution and the ligand solution were mixed and stirred for 30 seconds to obtain a composite solution.

[0050] Step 2: Preparation of composite materials: The composite solution was allowed to stand at 70° C. for 24 h under light-proof conditions and filtered to obtain a composite material.

[0051] Step 3: Preparation of self-supporting silicon electrode material: After mixing 5 g of the composite material with 5 g of polyacrylonitrile, 10 mL of N,N-dimethylformamide and 10 mL of ethanol were added, and the mixture was stirred for 10 minutes, frozen at -30°C for 4 hours, and then allowed to stand, naturally dried, and demolded to obtain a precursor.

[0052] Under a nitrogen protective atmosphere, the precursor was placed in a tube furnace and heat treated at 300°C for 22 hours to obtain a self-supporting silicon electrode material.

[0053] Comparative Example 1 A method for preparing a self-supporting silicon electrode material comprises the following steps: Step 1: Prepare the composite solution: 10 mmol of cobalt nitrate was dissolved in 400 mL of methanol and stirred at 55° C. for 30 min to obtain a cobalt nitrate solution.

[0054] 60 mmol of 2-methylimidazole and 200 mmol of nano-silicon powder were dispersed in 500 mL of methanol and stirred at 55° C. for 30 minutes to obtain a ligand solution; wherein the particle size of the nano-silicon powder was less than 100 nm.

[0055] The cobalt nitrate solution and the ligand solution were mixed and stirred at room temperature for 50 seconds to obtain a composite solution.

[0056] Step 2: Preparation of composite materials: The composite solution was allowed to stand at 80° C. for 24 h under light-proof conditions and filtered to obtain a composite material.

[0057] Step 2: Preparation of self-supporting silicon electrode material: After mixing 10 g of the composite material with 10 g of polyacrylonitrile, 30 mL of N,N-dimethylformamide and 30 mL of ethanol were added, stirred for 30 minutes, frozen for 4 hours, and then allowed to stand and naturally dried before demolding to obtain a precursor.

[0058] Under a nitrogen protective atmosphere, the precursor was placed in a tube furnace and heat treated at 300°C for 22 hours to obtain a self-supporting silicon electrode material.

[0059] The difference from Example 1 is: The cobalt nitrate solution and the ligand solution were mixed without irradiation with ultraviolet light.

[0060] Application Example 1 A method for preparing a lithium ion battery comprises the following steps: In an argon-filled glove box, the self-supporting silicon electrode prepared in Example 1 was used as the active material, metallic lithium was used as the counter electrode and reference electrode, electrolyte model LB-002 was used as the electrolyte, and porous polypropylene was used as the diaphragm material to assemble CR2025 button cells to obtain a lithium-ion battery.

[0061] Application Example 2 A method for preparing a lithium ion battery comprises the following steps: In an argon-filled glove box, the self-supporting silicon electrode prepared in Example 2 was used as the active material, metallic lithium was used as the counter electrode and reference electrode, electrolyte model LB-002 was used as the electrolyte, and porous polypropylene was used as the diaphragm material to assemble CR2025 button cells to obtain a lithium-ion battery.

[0062] Application Example 3 A method for preparing a lithium ion battery comprises the following steps: In an argon-filled glove box, the self-supporting silicon electrode prepared in Example 3 was used as the active material, metallic lithium was used as the counter electrode and reference electrode, electrolyte model LB-002 was used as the electrolyte, and porous polypropylene was used as the diaphragm material to assemble CR2025 button cells to obtain a lithium-ion battery.

[0063] Comparative Application Example 1 A method for preparing a lithium ion battery comprises the following steps: In a glove box filled with argon, the self-supporting silicon electrode prepared in Comparative Example 1 was used as the active material, metallic lithium was used as the counter electrode and reference electrode, electrolyte model LB-002 was used as the electrolyte, and porous polypropylene was used as the diaphragm material to assemble CR2025 button cells to obtain a lithium-ion battery.

[0064] Experimental test: 1. Surface morphology test.

[0065] like Figure 1 As shown, ZIF-67 wraps the nano-Si particles, and ZIF-67 and the nano-Si particles are simultaneously wrapped by the outside of the cyclized PAN. During the cycle, Si can be confined inside the ZIF-67, keeping the active substance Si from falling off and maintaining structural stability.

[0066] 2. Electrical performance test.

[0067] like Figure 2As shown in the figures, the capacity retention rate of the lithium ion battery prepared by Example 1 is higher than that of the lithium ion battery prepared by Comparative Example 1, indicating that ultraviolet light irradiation plays an important role in the uniformity of the material. This is because under ultraviolet light, ZIF-67 slowly nucleates and precipitates chemical bond chains with the Si surface, achieving uniform interconnection of the internal rigid structure and synergistically with the external cyclized polyacrylonitrile, thereby jointly enhancing the structural stability of the material.

[0068] like Figure 3 As shown in the figure, the lithium ion battery prepared in Example 2 has outstanding cycle stability. After 100 cycles, the capacity is still 960 mAh g -1 This indicates that the self-supporting silicon electrode material has a stable structure during the cycle, and the interior is not powdered. The flexible cyclized polyacrylonitrile network layer limits the cracking of the material. At the same time, the Co atoms in ZIF-67 cooperate with the organic components to link Si and provide stability.

[0069] like Figure 4 As shown in the figure, the lithium ion battery prepared in Example 3 has good cycle stability. -1 At a current density of 820 mAh g -1 The discharge specific capacity shows that the self-supporting electrode material has stable cycle capacity and strong structural stability, while increasing the electrochemical performance of lithium-ion batteries.

[0070] Obviously, those skilled in the art may make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if such modifications and variations fall within the scope of the claims and their equivalents, the present invention is intended to encompass such modifications and variations. The above-described embodiments are merely preferred embodiments for the purpose of fully illustrating the present invention and are not intended to limit the scope of protection.

Claims

1. A method for preparing a self-supporting silicon electrode material, characterized in that: The following steps are involved: Using 2-methylimidazole as a ligand, the ligand and silicon powder are mixed in a first solvent system to obtain a ligand solution; Under ultraviolet light conditions, the ligand solution and the cobalt salt solution are mixed to form a ZIF-67 precursor. At the same time, light excitation induces a chemical link between the silicon surface and the ZIF-67 precursor to obtain a composite solution. The composite solution is kept in the dark and allowed to react so that ZIF-67 can grow directionally on the silicon surface to form a core-shell structure of ZIF-67 encapsulating silicon, thereby obtaining a composite material; In a second solvent system, the composite material is mixed with a polymer and freeze-dried to obtain a precursor; the precursor is calcined under a protective atmosphere so that the carbon network of the porous carbon skeleton generated by the pyrolysis of ZIF-67 is intertwined to form a three-dimensional conductive support structure, thereby obtaining a self-supporting silicon electrode material; The polymer is polyacrylonitrile.

2. The method for preparing a self-supporting silicon electrode material according to claim 1, wherein: The molar ratio of the ligand to the cobalt ions in the cobalt salt solution is 2 to 12:1; The molar ratio of the ligand to the silicon powder is 1~6:

10.

3. The method for preparing a self-supporting silicon electrode material according to claim 1, wherein: The particle size of silicon powder is less than 100nm; The molecular weight of the polymer is 2.5×10 4 ~8×10 4 .

4. The method for preparing a self-supporting silicon electrode material according to claim 1, wherein: The UV irradiation conditions are as follows: the power of the UV light source is 300W~800W, the time is 30s~50s, and the temperature is room temperature.

5. The method for preparing a self-supporting silicon electrode material according to claim 1, wherein: The mass ratio of composite material to polymer is 1:

1.

6. The method for preparing a self-supporting silicon electrode material according to claim 1, wherein: The cobalt salt solution is obtained by mixing the cobalt salt and the first solvent at 30°C to 55°C; the ligand and silicon powder are mixed at a temperature of 30°C to 55°C.

7. The method for preparing a self-supporting silicon electrode material according to claim 1, wherein: The reaction temperature should be 70℃~80℃ in the dark; The calcination temperature is 300°C.

8. The method for preparing a self-supporting silicon electrode material according to claim 1, wherein: The first solvent is methanol; The second solvent is a mixed solution of N,N-dimethylformamide and ethanol, and the volume ratio of N,N-dimethylformamide to ethanol is 1:

1.

9. A self-supporting silicon electrode material, characterized in that: The self-supporting silicon electrode material is prepared by the preparation method of the self-supporting silicon electrode material according to any one of claims 1 to 8.

10. Use of the self-supporting silicon electrode material according to claim 9 as a negative electrode material for a lithium-ion battery.

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