A negative electrode sheet, a method for manufacturing the same, and a battery

Abstract

This invention discloses a negative electrode sheet, its preparation method, and a battery. The preparation method includes mixing silicon nanoparticles with a graphene oxide dispersion to prepare a mixed dispersion, then adding a crosslinking agent and mixing evenly, and then coating it onto a substrate to obtain a wet film; then immersing the wet film in a poor solvent of graphene oxide, drying it to obtain a wrinkled composite film, and then reducing it to obtain the negative electrode sheet. The above method, through the addition of a crosslinking agent, enables interlayer crosslinking of the graphene oxide sheet, thereby improving the overall conductivity and structural stability of the electrode; by immersing the wet film in a poor solvent of graphene oxide to replace the graphene oxide dispersion solvent in the wet film, the graphene oxide can be transformed from its original stretched state to a contracted and wrinkled state, resulting in a negative electrode sheet containing a large amount of wrinkled graphene. This wrinkled graphene can effectively suppress silicon volume expansion, maintain the integrity of the electrode structure, and thus effectively improve the conductivity and cycle performance of the silicon-based negative electrode sheet.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a negative electrode sheet, its preparation method, and a battery. Background Technology

[0002] Lithium-ion batteries, as rechargeable secondary batteries, offer advantages over traditional lead-acid batteries, including high capacity, long cycle life, no pollution, and good safety. They are widely used in electronic devices, electric transportation, aerospace, military, and medical fields, playing a vital role in people's daily lives. Graphite, with its high conductivity and layered structure, is well-suited for lithium-ion intercalation and deintercalation. Currently, most commercial lithium-ion batteries use carbon-based anodes such as graphite, but its theoretical capacity of only 372 mAh / g is insufficient to meet the ever-increasing demand for high-capacity batteries. Silicon, as one of the anode materials for lithium-ion batteries, boasts a theoretical capacity of 4200 mAh / g, more than 10 times that of graphite, and offers advantages such as low lithium intercalation potential, lower cost, and environmental friendliness, making it a highly sought-after material. However, silicon's poor conductivity and significant volume expansion during charging and discharging easily lead to material pulverization, electrode breakage, and the continuous formation of new SEI films, resulting in rapid capacity decay and poor cycle stability, thus limiting the practical application of silicon as a lithium-ion battery anode.

[0003] To address the aforementioned issues, existing technologies primarily focus on silicon particle nanostructuring or combining silicon particles with carbon materials possessing excellent conductivity (such as graphite, graphene, carbon nanotubes, and carbon fibers) to improve the overall conductivity of silicon-based materials and simultaneously solve the problem of volume expansion during charge and discharge. Graphene, a two-dimensional monolayer material with a high specific surface area, effectively reduces the damage to electrode materials caused by silicon expansion and contraction during these processes. Furthermore, graphene's high conductivity helps mitigate the poor conductivity of silicon. However, the significant volume change of silicon during charge and discharge can damage conventional coating structures over long cycles. Therefore, effectively suppressing the silicon volume expansion effect is one of the key challenges in preparing high-capacity silicon-carbon anode materials. Summary of the Invention

[0004] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes a negative electrode sheet, a method for preparing the same, and a battery.

[0005] In a first aspect, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps:

[0006] S1. Mix silicon nanoparticles with graphene oxide dispersion to prepare a mixed dispersion;

[0007] S2. Add the crosslinking agent to the mixed dispersion and mix evenly, then coat it onto the substrate to obtain a wet film;

[0008] S3. The wet film is immersed in a poor solvent of graphene oxide and then dried to obtain a wrinkled composite film.

[0009] S4. The wrinkled composite film is subjected to reduction treatment to obtain the negative electrode sheet.

[0010] The method for preparing the negative electrode sheet according to embodiments of the present invention has at least the following beneficial effects: The method involves mixing silicon nanoparticles with a graphene oxide dispersion to uniformly disperse the silicon nanoparticles in the dispersion. A crosslinking agent is then added to create chemical bonds between the graphene oxide layers, enhancing the interlayer forces. This interlayer crosslinking better fixes the silicon nanoparticles between the layers, forming a more stable layered conductive network structure, thus improving the overall conductivity and structural stability of the electrode. The mixture formed by the crosslinking agent and the dispersion is then applied to a substrate to form a wet film. The wet film is then immersed in a poor solvent containing graphene oxide to displace the original solvent (referred to as a good solvent) from the wet film. The process involves using an agent to transform the environment of graphene oxide from a benign solvent to a poor solvent. This solvent change during immersion causes the graphene oxide sheets to change from an initially spread-out state in a benign solvent to a contracted and wrinkled state in a poor solvent, where they are difficult to disperse. After drying, the solvent evaporation causes the material to shrink further, resulting in a large number of wrinkled structures. Subsequent reduction treatment yields wrinkled graphene. This wrinkled graphene can be stretched when silicon expands, releasing the volume stress generated by silicon expansion, thereby effectively suppressing silicon volume expansion, maintaining the integrity of the electrode structure, and effectively improving the conductivity and cycle stability of silicon-based negative electrode sheets. Furthermore, this preparation method is simple, energy-efficient, and requires minimal equipment, making it suitable for laboratory operations or large-scale industrial production.

[0011] In some embodiments of the present invention, in step S1, the mass ratio of the silicon nanoparticles to the graphene oxide in the graphene oxide dispersion is 1:(0.5-1.0).

[0012] In some embodiments of the present invention, in step S1, the particle size of the silicon nanoparticles is less than 100 nm.

[0013] In some embodiments of the present invention, in step S1, the sheet size of graphene oxide in the graphene oxide dispersion is 5-50 μm. The concentration of the graphene oxide dispersion can be controlled at 10-30 g / L. Specifically, the graphene oxide dispersion can be prepared by dispersing graphene oxide in a solvent. In this application, the solvent capable of dispersing graphene oxide is referred to as a benign solvent for graphene oxide, including but not limited to acetone, tetrahydrofuran, N,N-dimethylformamide (DMF), methanol, ethanol, ethylene glycol, N-methylpyrrolidone (NMP), etc.; the graphene oxide is uniformly dispersed in the benign solvent, and the graphene oxide uniformly dispersed in the benign solvent exhibits a stretched state.

[0014] In some embodiments of the present invention, in step S2, the crosslinking agent has two or more active groups, which are groups that can react with oxygen-containing functional groups, such as amino, isocyanate, and carboxyl groups. Since graphene oxide has oxygen-containing functional groups, using a crosslinking agent with two or more active functional groups that can react with oxygen-containing functional groups allows the crosslinking agent to crosslink with different graphene oxide sheets, enabling the graphene oxide sheets to be connected by chemical bonds through the crosslinking agent.

[0015] In some embodiments of the present invention, the crosslinking agent is selected from at least one of diamine compounds and diisocyanate compounds.

[0016] In some embodiments of the present invention, in step S2, the mass ratio of the crosslinking agent to the graphene oxide in the mixed dispersion is 1:(10-100).

[0017] In step S2, the thickness of the crosslinking agent and the mixed dispersion coated on the substrate can be controlled between 0.5 and 3 mm. Specifically, the mixture of crosslinking agent and mixed dispersion can be coated on the substrate by scraping, with the scraper thickness being 0.5 to 3 mm and the scraping speed being 10 to 50 mm / s.

[0018] In step S3, the unsuitable solvent for graphene oxide is a solvent in which graphene oxide is difficult to disperse uniformly. To achieve the replacement of the solvent (i.e., the good solvent) of the original dispersed graphene oxide in the wet film by the unsuitable solvent of graphene oxide, the unsuitable solvent of graphene oxide is generally miscible with the solvent used in the graphene oxide dispersion. In some embodiments of the present invention, the unsuitable solvent of graphene oxide is selected from at least one of ethyl acetate, toluene, diethyl ether, and n-hexane. Furthermore, this step requires ensuring that the wet film is completely immersed in the unsuitable solvent of graphene oxide; the immersion time can be controlled between 2 and 8 hours.

[0019] In some embodiments of the present invention, in step S3, drying can be performed by heating drying, forced air drying, etc.; the temperature of heating drying can be controlled between 30 and 60°C.

[0020] In some embodiments of the present invention, in step S4, the reduction treatment is a chemical reduction. The chemical reduction can use a hydroiodic acid solution with a mass concentration of 10-30% as the reducing agent. Specifically, the wrinkled composite membrane can be immersed in the reducing agent, and the reduction time can be controlled within 1-6 hours.

[0021] In a second aspect, the present invention provides a negative electrode sheet prepared by any of the negative electrode sheet preparation methods proposed in the first aspect of the present invention. Specifically, the negative electrode sheet prepared by the above method comprises multilayer stacked graphene sheets and silicon nanoparticles distributed between the graphene sheets; the graphene sheets are wrinkled and connected by chemical bonds.

[0022] According to embodiments of the present invention, the negative electrode sheet has at least the following beneficial effects: In this negative electrode sheet, the graphene sheet can serve as a conductive framework to promote electron and ion migration, and can also serve as a template to suppress the aggregation and volume expansion of silicon nanoparticles; the silicon nanoparticles are distributed and fixed between the graphene sheets, providing high capacity while serving as a support to prevent the graphene sheets from re-stacking during lithium insertion / extraction, and there are strong bonding forces between the graphene sheets. The graphene sheets and silicon nanoparticles together constitute a layered conductive network structure, which can improve the overall conductivity and structural stability of the electrode; in addition, the graphene sheets have a rich wrinkled structure, which makes them stretchable and can effectively release the volume stress generated by silicon during cycling, thereby effectively suppressing silicon volume expansion, maintaining the integrity of the electrode structure, and effectively improving the conductivity and cycle stability of the silicon-based negative electrode sheet. When applied to a battery, it can improve the cycle stability of the battery.

[0023] In some embodiments of the present invention, the particle size of the silicon nanoparticles is less than 100 nm.

[0024] In some embodiments of the present invention, the graphene sheet has a sheet size of 5 to 50 μm.

[0025] In a third aspect, the present invention provides a battery comprising any of the negative electrode plates proposed in the second aspect of the present invention. Attached Figure Description

[0026] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein:

[0027] Figure 1 This is a schematic diagram of the structure of the negative electrode sheet prepared in Example 1;

[0028] Figure 2 A schematic diagram of the negative electrode sheet prepared in Comparative Example 1;

[0029] Figure 3The graph shows the cycle performance test results of button cell samples 1-2 prepared using the negative electrode sheets of Example 1 and Comparative Example 1, respectively. Detailed Implementation

[0030] The following will describe the concept and technical effects of the present invention clearly and completely with reference to the embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0031] Example 1

[0032] This embodiment prepares a negative electrode sheet, the preparation method of which includes the following steps:

[0033] S1. Graphene oxide with an average sheet size of 50 μm was dispersed in N,N-dimethylformamide (DMF), and after stirring and ultrasonic dispersion, a graphene oxide dispersion with a concentration of 20 g / L was obtained.

[0034] S2. With a mass ratio of silicon nanoparticles to graphene oxide of 1:0.5, silicon nanoparticles with a particle size of 50nm were added to the graphene oxide dispersion and ultrasonically stirred to obtain a uniformly dispersed mixed dispersion.

[0035] S3. Ethylenediamine is added to the mixed dispersion at a mass ratio of 1:100 to graphene oxide, stirred evenly, and then coated onto a glass substrate to obtain a wet film; wherein the thickness of the squeegee is 2 mm and the coating speed is 20 mm / s.

[0036] S4. Immerse the wet film in ethyl acetate for 4 hours to obtain a gelled composite film.

[0037] S5. The gelled composite film is heated and dried at 50°C to obtain a wrinkled composite film;

[0038] S6. Immerse the wrinkled composite membrane in a 10% hydroiodic acid solution for 2 hours, wash with ethanol, and dry to obtain the negative electrode sheet, the structural schematic diagram of which is shown below. Figure 1 As shown, it includes multi-layered stacked graphene sheets 11 and silicon nanoparticles 12 distributed and fixed between the graphene sheets 11, wherein the graphene sheets 11 are wrinkled and connected by chemical bonds.

[0039] Example 2

[0040] This embodiment prepares a negative electrode sheet. The difference between this embodiment and Example 1 is that isophorone diisocyanate is used instead of ethylenediamine used in step S3 of Example 1. Other operations are the same as in Example 1.

[0041] Example 3

[0042] This embodiment prepares a negative electrode sheet. The difference between this embodiment and Example 1 is that toluene is used instead of ethyl acetate in step S4 of Example 1 in step S4. Other operations are the same as in Example 1.

[0043] Comparative Example 1

[0044] This comparative example prepared a negative electrode sheet. The difference between this comparative example and Example 1 is that in this comparative example, the wet film was not immersed in ethyl acetate for treatment, but was directly heated and dried, followed by reduction treatment. The preparation method specifically includes the following steps:

[0045] S1. Graphene oxide with an average sheet size of 50 μm was dispersed in DMF and then stirred and ultrasonically dispersed to obtain a graphene oxide dispersion with a concentration of 20 g / L.

[0046] S2. With a mass ratio of silicon nanoparticles to graphene oxide of 1:0.5, silicon nanoparticles with a particle size of 50nm were added to the graphene oxide dispersion and ultrasonically stirred to obtain a uniformly dispersed mixed dispersion.

[0047] S3. Ethylenediamine is added to the mixed dispersion at a mass ratio of 1:100 to graphene oxide, stirred evenly, and then coated onto a glass substrate to obtain a wet film; wherein the thickness of the squeegee is 2 mm and the coating speed is 20 mm / s.

[0048] S4. The wet film is heated and dried at 50°C to obtain a composite film;

[0049] S5. Immerse the composite membrane in a 10% hydroiodic acid solution for 2 hours, wash with ethanol, and dry to obtain the negative electrode sheet, the structural diagram of which is shown below. Figure 2 As shown, it includes multilayer stacked graphene sheets 21 and silicon nanoparticles 22 distributed and fixed between the graphene sheets 21, wherein the graphene sheets 21 are connected by chemical bonds.

[0050] Comparative Example 2

[0051] This comparative example prepared a negative electrode sheet. The difference between this comparative example and Example 1 is that in step S2, the mass ratio of silicon nanoparticles to graphene oxide is 1:10; other operations are the same as in Example 1.

[0052] Comparative Example 3

[0053] This comparative example prepared a negative electrode sheet. The difference between this comparative example and Example 1 is that in step S4, ethanol is used instead of ethyl acetate, that is, the wet film is immersed in ethanol for soaking treatment. Other operations are the same as in Example 1.

[0054] Comparative Example 4

[0055] This comparative example prepared a negative electrode material. The difference between this comparative example and Comparative Example 1 is that after preparing the negative electrode sheet according to the preparation method of Comparative Example 1, the negative electrode sheet was then crushed into powdered negative electrode material with a particle size of 1-50 μm.

[0056] Comparative Example 5

[0057] This comparative example prepared a negative electrode material. The difference between this comparative example and Example 1 is that after the negative electrode sheet was prepared according to the preparation method of Example 1, the negative electrode sheet was then crushed into powdered negative electrode material with a particle size of 1-50 μm.

[0058] Comparative Example 6

[0059] This comparative example prepares an anode material. The difference between this comparative example and Example 1 is that in this comparative example, wrinkled graphene is prepared first, and then mixed with silicon nanoparticles to obtain the anode material. The specific preparation method includes the following steps:

[0060] S1. Graphene oxide with an average sheet size of 50 μm was dispersed in DMF and then stirred and ultrasonically dispersed to obtain a graphene oxide dispersion with a concentration of 20 g / L.

[0061] S2. Ethylenediamine is added to the graphene oxide dispersion at a mass ratio of 1:100, stirred evenly, and then coated onto a glass substrate to obtain a wet film; wherein the thickness of the squeegee is 2 mm and the coating speed is 20 mm / s.

[0062] S3. Immerse the wet film in ethyl acetate for 4 hours to obtain a gelled film.

[0063] S4. The gelled film is heated and dried at 50°C, and then broken to obtain wrinkled graphene oxide.

[0064] S5. The wrinkled graphene oxide was soaked in a 10% hydroiodic acid solution for 2 hours, washed with ethanol and dried to obtain wrinkled graphene.

[0065] S6. With a mass ratio of silicon nanoparticles to wrinkled graphene of 1:0.5, silicon nanoparticles with a particle size of 50nm are mixed evenly to obtain the negative electrode material.

[0066] Comparative Example 7

[0067] This comparative example prepared a negative electrode sheet. The difference between this comparative example and Example 1 is that in step S2, the mass ratio of silicon nanoparticles to graphene oxide is 1:0.1; other operations are the same as in Example 1.

[0068] Performance testing

[0069] The negative electrode sheets prepared in Examples 1-3, Comparative Examples 1-3, and 7 were used directly as self-supporting negative electrode sheets. A lithium metal sheet was used as the counter electrode, a Celgard 2400 microporous polypropylene membrane was used as the separator, and 1M lithium hexafluorophosphate was used as the electrolyte (the solvent was ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1) to assemble a button cell.

[0070] In addition, the negative electrode materials prepared in Comparative Examples 4 to 6 were mixed with conductive agent SP and binder LA136 in a mass ratio of 8:1:1 to prepare a negative electrode active slurry, which was then coated onto a copper foil current collector to prepare a negative electrode sheet. The thickness of the negative electrode active material layer formed on the current collector in the negative electrode sheet was 10 μm. Then, a lithium metal sheet was used as the counter electrode and a Celgard2400 microporous polypropylene membrane was used as the separator to assemble a button battery of model 2032.

[0071] The button cells prepared using the negative electrode sheets of Examples 1-3, Comparative Examples 1-3, Comparative Examples 4-6, and Comparative Example 7 are respectively designated as samples 1-10. The electrochemical performance of each button cell was tested and analyzed, specifically within the voltage range of 0.01-1.5V, using 0.1Ag... -1 Constant current charging and discharging were performed at a current density of [value missing], and the test results are shown in Table 1 and [value missing]. Figure 3 As shown.

[0072] Table 1

[0073]

[0074] As shown in Table 1 above, compared to the button cell samples 1-6 and 10 prepared using negative electrode sheets from Examples 1-3 and Comparative Examples 1-3 and 7 respectively, the button cell samples 7-9, which used negative electrode materials from Comparative Examples 4-6, exhibited lower initial specific capacity and initial coulombic efficiency. This is because the powdered negative electrode materials prepared in Comparative Examples 7-9 have a larger specific surface area, consuming more lithium ions during the formation of the SEI film in the first cycle, resulting in lower initial specific capacity and initial coulombic efficiency. The powdered negative electrode material also has a lower mass ratio of active material during electrode preparation. In contrast, button cell samples 1 and 4, using negative electrode sheets from Examples 1 and 1 respectively as self-supporting negative electrode sheets, do not require binders or conductive additives, achieving integrated electrode structure and better maintaining electrode structural integrity. The interconnected graphene sheets within the electrode sheets ensure rapid electron transport, resulting in superior cycle stability and rate performance. Furthermore, the button cell sample 9, using the negative electrode material prepared in Comparative Example 6, exhibited poor cycle stability. This is because the wrinkled graphene and silicon nanoparticles in the negative electrode material of Comparative Example 6 were solid-state mixed, failing to form a good coating structure, resulting in silicon being exposed in the electrolyte and the SEI film continuing to grow. While the wrinkled graphene provided some buffering for silicon expansion, its lack of a coating structure meant it was ineffective in suppressing silicon volume expansion. Button cell samples 1, 5, and 10 used negative electrode sheets prepared in Example 1, Comparative Example 2, and Comparative Example 7, respectively. Compared to the negative electrode sheet of Example 1, the negative electrode sheet of Comparative Example 2 had a lower silicon content, lower overall expansion rate, and higher cycle stability, resulting in higher capacity retention. However, its specific capacity was lower and did not meet the requirements. The negative electrode sheet of Comparative Example 7 had a higher silicon content, resulting in a higher specific capacity, but it could not maintain a good capacity retention rate. The negative electrode sheets of button battery samples 1 and 6 were prepared using the negative electrode sheets of Example 1 and Comparative Example 3, respectively. In the preparation process of the negative electrode sheet of Comparative Example 3, ethanol was used instead of ethyl acetate used in Example 1 to soak the wet film. This process could not produce wrinkled graphene, and thus could not effectively suppress the volume expansion of silicon. The cycle stability of the negative electrode sheet was not as good as that of battery sample 1 which used the negative electrode sheet of Example 1.

[0075] In addition, by comparing button battery samples 1, 4 and 5 in Table 1... Figure 1 As shown in the cycle performance test results of button battery samples 1 and 4, it can be seen that button battery samples 1 and 4 use the negative electrode sheets of Example 1 and Comparative Example 1, respectively. The initial specific capacity of the batteries is about 2500 mAh / g. However, after 100 cycles, the specific capacity of button battery sample 4 drops to 1531 mAh / g, while the specific capacity of button battery sample 1 is 1865 mAh / g. The cycle stability of the button battery assembled with the negative electrode sheet prepared in Example 1 is significantly improved. This is because the pleated structure of graphene alleviates the stress caused by the volume change of silicon during charging and discharging.

[0076] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a negative electrode sheet, characterized in that, Includes the following steps: S1. Silicon nanoparticles are mixed with graphene oxide dispersion to prepare a mixed dispersion; the graphene oxide dispersion is prepared by dispersing graphene oxide in a good solvent, and the good solvent is selected from at least one of methanol and ethanol. S2. Add the crosslinking agent to the mixed dispersion and mix evenly, then coat it onto the substrate to obtain a wet film; S3. The wet film is immersed in a poor solvent for graphene oxide and dried to obtain a wrinkled composite film; the poor solvent for graphene oxide is selected from at least one of toluene and diethyl ether. S4. The wrinkled composite film is subjected to reduction treatment to obtain the negative electrode sheet.

2. The method for preparing the negative electrode sheet according to claim 1, characterized in that, In step S2, the crosslinking agent has two or more active groups, which are groups that can react with oxygen-containing functional groups.

3. The method for preparing the negative electrode sheet according to claim 2, characterized in that, The crosslinking agent is selected from at least one of diamine compounds and diisocyanate compounds.

4. The method for preparing the negative electrode sheet according to claim 2, characterized in that, In step S2, the mass ratio of the crosslinking agent to the graphene oxide in the mixed dispersion is 1:(10~100).

5. The method for preparing the negative electrode sheet according to claim 1, characterized in that, In step S1, the mass ratio of the silicon nanoparticles to the graphene oxide in the graphene oxide dispersion is 1:(0.5~1.0).

6. The method for preparing the negative electrode sheet according to any one of claims 1 to 5, characterized in that, In step S1, the particle size of the silicon nanoparticles is less than 100 nm.

7. The method for preparing the negative electrode sheet according to any one of claims 1 to 5, characterized in that, In step S1, the sheet size of graphene oxide in the graphene oxide dispersion is 5~50μm.

8. A negative electrode sheet, characterized in that, It is prepared by the method of any one of claims 1 to 7.

9. A battery, characterized in that, Includes the negative electrode sheet as described in claim 8.

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