Preparation method of lithium battery negative electrode sheet and application thereof

Abstract

The present invention relates to a method for preparing a lithium battery negative electrode sheet and its application, belonging to the technical field of lithium-ion batteries. The method comprises the following steps: first, coating a layer of polydopamine on the surface of nano-silicon, then calcining at high temperature to obtain nitrogen-doped carbon layer-coated nano-silicon, then coating the surface of the nitrogen-doped carbon layer-coated nano-silicon with an ultrathin polydopamine layer to obtain polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, then mixing carbon fiber with the polydopamine and nitrogen-doped carbon layer double-coated nano-silicon to obtain a negative electrode active material, and finally, coating a mixture of the negative electrode active material, a conductive agent, a binder, and deionized water on at least one of the front and back surfaces of a negative electrode current collector to form a coating, and then drying and pressing to obtain a lithium battery negative electrode sheet. In the technical solution of the present invention, the obtained negative electrode sheet has extremely low volume expansion and excellent cycle stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a method for preparing a negative electrode sheet of a lithium battery and applications thereof. Background Art

[0002] In recent years, lithium-ion batteries have gradually become the main energy storage device for powering most consumer and portable electronic devices due to their advantages such as high energy density, long cycle life, environmental friendliness, and no memory effect. During the operation of lithium batteries, the negative electrode plays the role of storing ions and receiving electrons, and is one of the most critical materials in lithium-ion battery technology.

[0003] Graphite is a commercially viable anode material, but its specific capacity is low. Silicon, with its low cost, high theoretical specific capacity, and low operating voltage, is considered one of the most commercially promising high-performance anode materials for lithium-ion batteries. However, silicon's dramatic volume changes during charge and discharge, coupled with its low intrinsic conductivity, severely impact its electrochemical performance. Therefore, improving silicon-based materials and leveraging their energy density advantages to create silicon-based anode materials with long cycle life and high capacity is a key research priority. Summary of the Invention

[0004] The object of the present invention is to provide a preparation method and application of a lithium battery negative electrode sheet, which comprises the following steps: coating a layer of polydopamine on the surface of nano-silicon, calcining the nano-silicon at high temperature to obtain nitrogen-doped carbon layer-coated nano-silicon, then coating the surface of the nitrogen-doped carbon layer-coated nano-silicon with an ultra-thin polydopamine layer to obtain polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, mixing carbon fiber with polydopamine and nitrogen-doped carbon layer double-coated nano-silicon to obtain a negative electrode active material, and finally coating the negative electrode active material, a conductive agent, a binder and deionized water on at least one of the front and back sides of a negative electrode current collector to form a coating, and then drying and pressing the coating to obtain a lithium battery negative electrode sheet, which has extremely low volume expansion and excellent cycle stability.

[0005] The technical problem to be solved by this invention is that graphite is a suitable anode material for large-scale commercial applications, but its specific capacity is low. Silicon, with its low cost, high theoretical specific capacity, and low operating voltage, is considered one of the most commercially promising high-performance anode materials for lithium-ion power batteries. However, silicon's dramatic volume changes during charge and discharge, coupled with its low intrinsic conductivity, severely impact the electrochemical performance of silicon anodes. Therefore, improving silicon-based materials and leveraging their energy density advantages to prepare silicon-based anode materials with long cycle life and high capacity is a current research priority.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A method for preparing a lithium battery negative electrode sheet comprises the following steps:

[0008] A1. Coating a layer of polydopamine on the surface of nano-silicon, and then calcining it at high temperature to obtain nitrogen-doped carbon layer-coated nano-silicon;

[0009] The specific preparation process of step A1 is as follows: placing the nano-silicon in Tris buffer, ultrasonically dispersing for 1-3 hours, then adding dopamine hydrochloride, magnetically stirring for 25-30 hours, centrifuging, washing, and drying to obtain polydopamine-coated nano-silicon; and subjecting the polydopamine-coated nano-silicon to high-temperature calcination to obtain nitrogen-doped carbon layer-coated nano-silicon.

[0010] In the above preparation process, a layer of polydopamine is coated on the surface of nano-silicon by self-polymerization of dopamine, and then calcined at high temperature to carbonize the polydopamine layer into a nitrogen-doped carbon layer coating the nano-silicon. The coating of the nitrogen-doped carbon layer can improve the conductivity.

[0011] Furthermore, the particle size of the nano-silicon ranges from 20 to 500 nm.

[0012] Furthermore, the thickness of the polydopamine layer is 13-25 nm.

[0013] Furthermore, the usage ratio of nano-silicon, dopamine hydrochloride and Tris buffer is 1-2g:1g:350-500mL.

[0014] Furthermore, the high temperature calcination process is to increase the temperature from room temperature to 600-700°C at a heating rate of 5-10°C / min and keep the temperature for 4-8 hours.

[0015] A2. Coating an ultrathin polydopamine layer on the surface of the nitrogen-doped carbon layer-coated nano-silicon to obtain polydopamine and nitrogen-doped carbon layer double-coated nano-silicon;

[0016] The specific preparation process of step A2 is as follows: immersing the nitrogen-doped carbon layer-coated nano-silicon in Tris buffer, ultrasonically dispersing for 1-3 hours, then adding dopamine hydrochloride, magnetically stirring for 14-18 hours, centrifuging, washing with deionized water and ethanol alternately 2-3 times, and drying to obtain polydopamine and nitrogen-doped carbon layer double-coated nano-silicon;

[0017] During the preparation process, the ratio of nitrogen-doped carbon-coated nanosilicon to dopamine hydrochloride and the reaction time are controlled to produce an ultrathin polydopamine layer with a thickness of only 2-7 nm. Excessively thick polydopamine layers can hinder ion transport and reduce the conductivity of the nitrogen-doped carbon-coated nanosilicon. Furthermore, the dual coating of polydopamine and nitrogen-doped carbon effectively inhibits nanosilicon aggregation, increasing the specific surface area and, consequently, the specific capacity.

[0018] Furthermore, the thickness of the polydopamine layer is 2-7 nm.

[0019] Furthermore, the usage ratio of the nitrogen-doped carbon layer-coated nano-silicon, dopamine hydrochloride and Tris buffer is 3-4 g: 1 g: 750-900 mL.

[0020] Furthermore, the drying temperature is 100-130° C., and the drying time is 7-10 hours.

[0021] Furthermore, the pH of the Tris buffer is 8.5.

[0022] A3, mixing carbon fibers with polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, first magnetically stirring, then ultrasonically dispersing, to obtain a negative electrode active material;

[0023] During the preparation process, carbon fibers are mixed with nanosilicon double-coated with polydopamine and a nitrogen-doped carbon layer. The high adhesiveness of the polydopamine layer allows the double-coated nanosilicon to be evenly dispersed and firmly adhered to the carbon fiber surface, forming a three-dimensional network structure. This network structure provides abundant transmission channels for electron transport, thereby improving the conductivity of the negative electrode active material. Furthermore, the high elastic modulus of the polydopamine layer, the strength of the nitrogen-doped carbon layer, and the voids created by the three-dimensional network structure can all buffer the volume expansion of the nanosilicon. The voids created by the three-dimensional network structure also provide channels for ion transport, improving ionic conductivity.

[0024] When the negative electrode active material prepared above is mixed with a binder and a conductive agent to prepare a negative electrode sheet, it can bond with the binder and conductive agent, thereby improving the cohesive force of the negative electrode material. During the lithium battery's charge and discharge cycles, the negative electrode active material is less likely to fall off, thereby improving the cycle stability of the negative electrode sheet. The polydopamine layer on the surface of the negative electrode active material acts as a partial binder, not only improving the cohesive force of the negative electrode sheet, but also effectively solving the problem of reduced conductivity caused by excessive binder.

[0025] Furthermore, in step A3, the diameter of the carbon fiber is in the range of 70-200 nm.

[0026] Furthermore, in step A3, the mass ratio of the carbon fiber to the polydopamine and nitrogen-doped carbon layer double-coated nano-silicon is 1:3-6.

[0027] Furthermore, in step A3, the magnetic stirring time is 8-12 hours, and the ultrasonic dispersion time is 3-5 hours.

[0028] A4. Mix the negative electrode active material, conductive agent and binder to obtain a mixed dry material; add deionized water to the mixed dry material, stir and mix evenly to obtain a mixed slurry; apply the mixed slurry on at least one of the front and back sides of the negative electrode collector to form a coating, and then dry and press the coating to obtain a lithium battery negative electrode sheet.

[0029] Furthermore, in step A4, the mass ratio of the negative electrode active material, the conductive agent, and the binder is 85-99:0.5-10:0.5-5.

[0030] Furthermore, in step A4, the coating thickness is 10-350 μm and the surface density is 10-150 g / cm 2 .

[0031] Furthermore, in step A4, the viscosity of the mixed slurry is 2000-8000 mPa.s, and the solid content is 45-75%.

[0032] Furthermore, in step A4, the conductive agent is one or more of Ketjen black, conductive carbon black, acetylene black, graphite, graphene, graphene tubes, graphene fibers and carbon nanotubes.

[0033] Furthermore, in step A4, the binder is one or more of polyacrylic acid, lithium polyacrylate, polyacrylamide, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, sodium alginate, chitosan, ethylene glycol and guar gum.

[0034] Furthermore, in step A4, the negative electrode current collector is one or more of copper foil, zinc-nickel-plated copper foil, and carbon-coated copper foil.

[0035] The lithium battery negative electrode sheet prepared by the present invention is used in the preparation of lithium ion batteries.

[0036] Beneficial effects of the present invention:

[0037] (1) In the technical solution of the present invention, due to the double coating effect of the polydopamine layer and the nitrogen-doped carbon layer, the agglomeration of nano-silicon can be effectively suppressed, the specific surface area can be increased, and the specific capacity when used as a lithium-ion battery electrode can be increased.

[0038] (2) In the technical solution of the present invention, carbon fibers are mixed with polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, and the high adhesion of the polydopamine layer is utilized to make the polydopamine and nitrogen-doped carbon layer double-coated nano-silicon be evenly dispersed and firmly adhered to the surface of the carbon fibers, and form a three-dimensional network structure. The three-dimensional network structure provides abundant transmission channels for electron transmission and the high conductivity of the nitrogen-doped carbon layer, which synergistically improves the conductivity of the negative electrode active material.

[0039] (3) In the technical solution of the present invention, the polydopamine layer has a high elastic modulus, the nitrogen-doped carbon layer has a certain strength, and the voids generated by the three-dimensional network structure can buffer the volume expansion of nano-silicon. The voids generated by the three-dimensional network structure also provide channels for ion transmission, thereby improving the ionic conductivity, so that the prepared negative electrode sheet has excellent cycle stability.

[0040] (4) In the technical solution of the present invention, when the negative electrode active material is mixed with a binder and a conductive agent to prepare a negative electrode sheet, it can bond with the binder and the conductive agent, thereby improving the bonding strength of the negative electrode material. During the charge and discharge cycle of the lithium battery, the negative electrode active material is not easy to fall off, thereby improving the cycle stability of the negative electrode sheet; in addition, the polydopamine layer on the surface of the negative electrode active material acts as a partial binder, which not only improves the bonding strength of the negative electrode sheet, but also effectively solves the problem of decreased conductivity caused by excessive binder. DETAILED DESCRIPTION

[0041] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0042] Example 1

[0043] A method for preparing a lithium battery negative electrode sheet comprises the following steps:

[0044] A1. Coating the surface of nano-silicon with a layer of polydopamine, followed by high-temperature calcination to obtain nitrogen-doped carbon-layer-coated nano-silicon: 20 g of nano-silicon with a particle size of 20 nm was placed in 7 L of Tris buffer with a pH of 8.5, ultrasonically dispersed for 1 hour, then added with 20 g of dopamine hydrochloride, magnetically stirred for 25 hours, centrifuged, washed twice with deionized water and ethanol alternately, and then dried at 100°C for 10 hours to obtain polydopamine-coated nano-silicon; the polydopamine-coated nano-silicon was subjected to high-temperature calcination treatment, the temperature of which was increased from room temperature to 600°C at a heating rate of 5°C / min and maintained for 8 hours to obtain nitrogen-doped carbon-layer-coated nano-silicon;

[0045] A2. Coating an ultrathin polydopamine layer on the surface of nitrogen-doped carbon-coated nanosilicon to obtain polydopamine and nitrogen-doped carbon-coated nanosilicon: immerse 21 g of nitrogen-doped carbon-coated nanosilicon in 5.3 L of Tris buffer with a pH of 8.5, ultrasonically disperse for 1 hour, then add 7 g of dopamine hydrochloride, magnetically stir for 14 hours, centrifuge, wash twice with deionized water and ethanol alternately, and then dry at 100°C for 10 hours to obtain polydopamine and nitrogen-doped carbon-coated nanosilicon;

[0046] A3, 7g of carbon fiber with a diameter of 70nm was mixed with 21g of polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, first magnetically stirred for 8h, and then ultrasonically dispersed for 3h to obtain the negative electrode active material;

[0047] A4. The negative electrode active material, conductive carbon black, and polyacrylic acid were mixed in a ratio of 85:10:5 to obtain a mixed dry material; deionized water was added to the mixed dry material, and the mixture was stirred and mixed to obtain a mixed slurry having a viscosity of 2000 mPa.s and a solid content of 45%; the mixed slurry was applied to both sides of a copper foil to form a coating, dried at 50°C for 12 hours, and pressed into a sheet. The coating had a thickness of 10 μm and an area density of 10 g / cm 2 , and obtain the negative electrode sheet of the lithium battery.

[0048] Example 2

[0049] A method for preparing a lithium battery negative electrode sheet comprises the following steps:

[0050] A1. Coat the surface of nano-silicon with a layer of polydopamine, and then calcine it at high temperature to obtain nitrogen-doped carbon layer-coated nano-silicon: put 30g of nano-silicon with a particle size of 200nm into 8L of Tris buffer with a pH of 8.5, ultrasonically disperse it for 2h, then add 20g of dopamine hydrochloride, magnetically stir it for 27h, centrifuge it, wash it alternately with deionized water and ethanol three times, and then dry it at 110℃ for 9h to obtain polydopamine-coated nano-silicon; the polydopamine-coated nano-silicon is subjected to high-temperature calcination treatment, the process is heated from room temperature to 650℃ at a heating rate of 7℃ / min, and kept warm for 7h to obtain nitrogen-doped carbon layer-coated nano-silicon.

[0051] A2. Coating an ultrathin polydopamine layer on the surface of nitrogen-doped carbon-coated nanosilicon to obtain polydopamine and nitrogen-doped carbon-coated nanosilicon: immerse 24.5 g of nitrogen-doped carbon-coated nanosilicon in 5.6 L of Tris buffer with a pH of 8.5, ultrasonically disperse for 2 h, then add 7 g of dopamine hydrochloride, magnetically stir for 16 h, centrifuge, wash alternately with deionized water and ethanol three times, and then dry at 110°C for 9 h to obtain polydopamine and nitrogen-doped carbon-coated nanosilicon;

[0052] A3, 7g of carbon fiber with a diameter of 100nm was mixed with 28g of polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, first magnetically stirred for 10h, and then ultrasonically dispersed for 4h to obtain the negative electrode active material;

[0053] A4. The negative electrode active material, graphene, and carboxymethyl fiber are mixed in a ratio of 90:8:2 to obtain a mixed dry material; deionized water is added to the mixed dry material, and the mixture is stirred and mixed to obtain a mixed slurry having a viscosity of 4000 mPa.s and a solid content of 55%; the mixed slurry is coated on both sides of a zinc-nickel-plated copper foil to form a coating, which is dried at 50°C for 10 hours and pressed into a sheet. The coating has a thickness of 100 μm and an area density of 80 g / cm 2 , and obtain the negative electrode sheet of the lithium battery.

[0054] Example 3

[0055] A method for preparing a lithium battery negative electrode sheet comprises the following steps:

[0056] A1. Coat a layer of polydopamine on the surface of nano-silicon, and then calcine it at high temperature to obtain nitrogen-doped carbon layer-coated nano-silicon: put 40g of nano-silicon with a particle size of 350nm into 9L of Tris buffer with a pH of 8.5, ultrasonically disperse it for 3h, then add 20g of dopamine hydrochloride, magnetically stir it for 29h, centrifuge it, wash it alternately with deionized water and ethanol twice, and then dry it at 120℃ for 8h to obtain polydopamine-coated nano-silicon; the polydopamine-coated nano-silicon is subjected to high-temperature calcination treatment, the process is heated from room temperature to 700℃ at a heating rate of 9℃ / min, and kept warm for 6h to obtain nitrogen-doped carbon layer-coated nano-silicon.

[0057] A2. Coating an ultrathin polydopamine layer on the surface of nitrogen-doped carbon-coated nanosilicon to obtain polydopamine and nitrogen-doped carbon-coated nanosilicon: immerse 40 g of nitrogen-doped carbon-coated nanosilicon in 8.5 L of Tris buffer with a pH of 8.5, ultrasonically disperse for 3 h, then add 10 g of dopamine hydrochloride, magnetically stir for 17 h, centrifuge, wash alternately with deionized water and ethanol three times, and then dry at 120°C for 6 h to obtain polydopamine and nitrogen-doped carbon-coated nanosilicon;

[0058] A3, 7g of carbon fiber with a diameter of 150nm was mixed with 35g of polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, first magnetically stirred for 10h, and then ultrasonically dispersed for 5h to obtain the negative electrode active material;

[0059] A4. The negative electrode active material, carbon nanotubes, and sodium alginate were mixed in a ratio of 95:4:1 to obtain a mixed dry material; deionized water was added to the mixed dry material, and the mixture was stirred and mixed to obtain a mixed slurry having a viscosity of 6000 mPa.s and a solid content of 65%; the mixed slurry was applied to both sides of a copper foil to form a coating, dried at 50°C for 9 hours, and pressed into a sheet. The coating had a thickness of 300 μm and an area density of 120 g / cm 2 , and obtain the negative electrode sheet of the lithium battery.

[0060] Example 4

[0061] A method for preparing a lithium battery negative electrode sheet comprises the following steps:

[0062] A1. Coat the surface of nano-silicon with a layer of polydopamine, and then calcine it at high temperature to obtain nitrogen-doped carbon layer-coated nano-silicon: put 40g of nano-silicon with a particle size of 500nm into 10L of Tris buffer with a pH of 8.5, ultrasonically disperse it for 3h, then add 20g of dopamine hydrochloride, magnetically stir it for 30h, centrifuge it, wash it alternately with deionized water and ethanol twice, and then dry it at 130℃ for 7h to obtain polydopamine-coated nano-silicon; the polydopamine-coated nano-silicon is subjected to high-temperature calcination treatment, the process is heated from room temperature to 700℃ at a heating rate of 10℃ / min, and kept warm for 4h to obtain nitrogen-doped carbon layer-coated nano-silicon.

[0063] A2. Coating an ultrathin polydopamine layer on the surface of nitrogen-doped carbon-coated nanosilicon to obtain polydopamine and nitrogen-doped carbon-coated nanosilicon: immerse 40 g of nitrogen-doped carbon-coated nanosilicon in 9 L of Tris buffer with a pH of 8.5, ultrasonically disperse for 3 h, then add 10 g of dopamine hydrochloride, magnetically stir for 18 h, centrifuge, wash alternately with deionized water and ethanol three times, and then dry at 130°C for 7 h to obtain polydopamine and nitrogen-doped carbon-coated nanosilicon;

[0064] A3, 7g of carbon fiber with a diameter of 200nm was mixed with 42g of polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, first magnetically stirred for 12h, and then ultrasonically dispersed for 5h to obtain the negative electrode active material;

[0065] A4. The negative electrode active material, graphene fiber, and guar gum were mixed in a ratio of 99:0.5:0.5 to obtain a mixed dry material; deionized water was added to the mixed dry material, and the mixture was stirred and mixed to obtain a mixed slurry having a viscosity of 8000 mPa.s and a solid content of 75%; the mixed slurry was applied on both sides of the carbon-coated copper foil to form a coating, dried at 50°C for 8 hours, and pressed into a sheet. The coating had a thickness of 350 μm and an area density of 150 g / cm 2 , and obtain the negative electrode sheet of the lithium battery.

[0066] Comparative Example 1

[0067] Compared with Example 2, the thickness of the polydopamine layer coated on the outermost layer of nano-silicon in Comparative Example 1 is 15 nm, and the other steps and raw materials are the same as those in Example 2.

[0068] Comparative Example 2

[0069] Compared with Example 2, the outermost layer of the nano-silicon in Comparative Example 2 is not coated with a polydopamine layer, but only has a nitrogen-doped carbon layer. Other steps and raw materials are the same as those in Example 2.

[0070] Comparative Example 3

[0071] Compared with Example 2, in Comparative Example 3, the nano-silicon is not coated with a nitrogen-doped carbon layer, but only has a polydopamine layer. Other steps and raw materials are the same as those in Example 2.

[0072] Comparative Example 4

[0073] Compared with Example 2, no carbon fiber was added in Comparative Example 4, and other steps and raw materials were the same as those in Example 2.

[0074] Assembly of lithium-ion batteries: negative electrode, separator, positive electrode (containing 96.5% lithium nickel cobalt manganese oxide LiNi 0.8 Co 0.12 Mn 0.08 The battery cell is wound with O2 positive electrode active material, and the battery cell is placed in a battery shell. After vacuum drying, the electrolyte is injected into the battery shell. After packaging, standing, formation, and capacity separation, a lithium-ion battery is obtained.

[0075] Lithium-ion batteries were assembled using the negative electrode sheets prepared in Examples 1-4 and Comparative Examples 1-4, and the expansion rate and cycle stability of the electrode sheets were measured. The results are shown in Table 1.

[0076] test:

[0077] (1) Electrode expansion rate test: Measure the thickness of the negative electrode sheets after pressing in the embodiment and comparative example, as well as the thickness of the negative electrode sheets of the battery under full charge, and calculate the expansion rate of the negative electrode sheets.

[0078] (2) Cycling stability test: At room temperature of 25°C, the starting and ending voltages were 2.8 V and 4.25 V, respectively. The batteries prepared in the examples and comparative examples were charged from 1 C to 4.25 V, then charged at a constant voltage of 4.25 V until the current decreased to 0.05 C, and discharged at 0.2 C to 2.8 V. The capacity retention rates at the 50th, 200th, and 400th cycles were recorded.

[0079] Table 1

[0080]

[0081] It can be seen from the data in Table 1 that the electrode expansion rate of Examples 1-4 is low and has a high capacity retention rate, and the capacity retention rate can still reach more than 90% at the 400th cycle.

[0082] Specifically, by comparing the data of Example 2 and Comparative Example 1, it can be seen that the thickness of the polydopamine layer coated on the outermost layer of nano-silicon has a greater influence on the capacity retention rate, indicating that an increase in the thickness of the polydopamine layer will hinder the transmission of electrons and ions, resulting in a decrease in the capacity retention rate and a decrease in the battery cycle performance.

[0083] By comparing the data of Example 2 and Comparative Example 2, it can be seen that when the outermost layer of nanosilicon is not coated with polydopamine, the expansion rate and capacity retention rate of the negative electrode sheet are both deteriorated, indicating that when not coated with polydopamine, the nitrogen-doped carbon layer-coated nanosilicon cannot be evenly and firmly dispersed on the surface of the carbon fiber, but is simply blended with the nitrogen-doped carbon layer-coated nanosilicon, and cannot form a three-dimensional network structure of the electron transmission path. At the same time, the nitrogen-doped carbon layer-coated nanosilicon has poor adhesion to the binder and the conductive agent, which leads to a decrease in the cycle performance. Because the polydopamine coating with excellent elastic modulus is missing, the volume expansion inhibition effect of nanosilicon is reduced, so the expansion rate of the negative electrode sheet increases.

[0084] By comparing the data of Example 2 and Comparative Example 3, it can be seen that the electrode expansion rate is significantly increased when the nano-silicon is not coated with the nitrogen-doped carbon layer. In addition, the cycle stability is also significantly reduced when it is not coated with the nitrogen-doped carbon layer, indicating that the nitrogen-doped carbon layer has a significant effect on buffering the volume expansion of nano-silicon and can improve the cycle stability.

[0085] By comparing the data of Example 2 and Comparative Example 4, it can be seen that the electrode expansion rate and capacity retention rate are significantly reduced when carbon fiber is not added, indicating that when carbon fiber is added, a three-dimensional network structure and its gaps are formed between the carbon fibers adhered with polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, forming a complete electron and ion transmission path, thereby improving the capacity retention rate and reducing the electrode expansion rate.

[0086] Throughout the specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0087] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in similar ways. As long as they do not deviate from the invention or exceed the scope defined by the claims, they should all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a negative electrode sheet for a lithium battery, characterized in that: The following steps are involved: A1. Coating a layer of polydopamine on the surface of nano-silicon, and then calcining it at high temperature to obtain nitrogen-doped carbon layer-coated nano-silicon; A2. Coating an ultrathin polydopamine layer on the surface of the nitrogen-doped carbon layer-coated nano-silicon to obtain polydopamine and nitrogen-doped carbon layer double-coated nano-silicon; the thickness of the polydopamine layer is 2-7 nm; A3. Mixing carbon fibers with polydopamine and nitrogen-doped carbon layer double-coated nano-silicon, first magnetically stirring, and then ultrasonically dispersing to obtain a negative electrode active material; the mass ratio of carbon fibers to polydopamine and nitrogen-doped carbon layer double-coated nano-silicon is 1:3-6; A4. Mix the negative electrode active material, conductive agent and binder to obtain a mixed dry material; add deionized water to the mixed dry material, stir and mix evenly to obtain a mixed slurry; apply the mixed slurry on at least one of the front and back sides of the negative electrode collector to form a coating, and then dry and press the coating to obtain a lithium battery negative electrode sheet.

2. The method for preparing a lithium battery negative electrode sheet according to claim 1, wherein: The specific preparation process of step A1 is as follows: placing the nano-silicon in a Tris buffer solution, ultrasonically dispersing for 1-3 hours, then adding dopamine hydrochloride, magnetically stirring for 25-30 hours, centrifuging, washing, and drying to obtain polydopamine-coated nano-silicon; and subjecting the polydopamine-coated nano-silicon to high-temperature calcination to obtain nitrogen-doped carbon layer-coated nano-silicon; The particle size of the nano-silicon is in the range of 20-500 nm; the thickness of the polydopamine layer is 13-25 nm; the dosage ratio of the nano-silicon, dopamine hydrochloride and Tris buffer is 1-2 g:1 g:350-500 mL; the high-temperature calcination treatment process is to increase the temperature from room temperature to 600-700° C. at a heating rate of 5-10° C. / min and keep the temperature for 4-8 hours.

3. The method for preparing a lithium battery negative electrode sheet according to claim 1, characterized in that: The specific preparation process of step A2 is as follows: immersing the nitrogen-doped carbon layer-coated nano-silicon in Tris buffer, ultrasonically dispersing for 1-3 hours, then adding dopamine hydrochloride, magnetically stirring for 14-18 hours, centrifuging, washing with deionized water and ethanol alternately 2-3 times, and drying to obtain polydopamine and nitrogen-doped carbon layer-coated nano-silicon; The dosage ratio of the nitrogen-doped carbon layer-coated nano-silicon, dopamine hydrochloride and Tris buffer is 3-4 g: 1 g: 750-900 mL.

4. The method for preparing a lithium battery negative electrode sheet according to claim 2 or 3, characterized in that: The drying temperature in step A1 and step A2 is 100-130° C., and the drying time is 7-10 h; the pH of the Tris buffer is 8.

5.

5. The method for preparing a lithium battery negative electrode sheet according to claim 1, characterized in that: In the step A4, the mass ratio of the negative electrode active material, the conductive agent, and the binder is 85-99:0.5-10:0.5-5.

6. The method for preparing a lithium battery negative electrode sheet according to claim 1, characterized in that: In step A4, the conductive agent is one or more of Ketjen black, acetylene black, graphite, graphene tubes, graphene fibers and carbon nanotubes.

7. The method for preparing a lithium battery negative electrode sheet according to claim 1, characterized in that: In step A4, the binder is one or more of polyacrylic acid, lithium polyacrylate, polyacrylamide, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, sodium alginate, chitosan, ethylene glycol and guar gum.

8. The method for preparing a negative electrode sheet for a lithium battery according to claim 1, characterized in that: In step A4, the negative electrode current collector is one or more of copper foil, zinc-nickel-plated copper foil, and carbon-coated copper foil.

9. Use of a lithium battery negative electrode sheet prepared by the method according to any one of claims 1 to 8 in preparing a lithium ion battery.

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