Negative pole piece, preparation method thereof and battery

Abstract

The invention relates to the technical field of new energy, in particular to a negative pole piece, a preparation method thereof and a battery. The negative pole piece comprises a current collector and a negative active layer located on the current collector, the negative active layer comprises a negative active material, and the negative active material comprises an inner core, a buffer layer and a shell; the inner core comprises titanium dioxide; the shell covers the inner core, the shell comprises functionalized graphene, the functionalized graphene comprises graphene and a linking group, the linking group connects the graphene with the titanium dioxide, and the linking group comprises-C (O)-O-; the buffer layer is located between the titanium dioxide and the graphene, and the buffer layer comprises a polymer. According to the invention, the problems of agglomeration of graphene and interface bonding force with TiO2 can be improved, and the problem of relatively low capacity retention rate under high magnification caused by volume expansion and shrinkage of TiO2 is improved.

Description

Technical Field

[0001] This application relates to the field of new energy technology, and in particular to negative electrode sheets, their preparation methods, and batteries. Background Technology

[0002] Titanium dioxide (TiO2) is considered a promising next-generation lithium-ion battery anode material due to its high safety, excellent cycle stability, and moderate lithium-ion intercalation potential. However, TiO2's intrinsic electronic conductivity and ion diffusion rate are low, resulting in poor rate performance. Furthermore, significant volume changes occur during lithium-ion intercalation / deintercalation, leading to low capacity retention at high rates. These drawbacks limit the practical application of TiO2 as a lithium-ion battery anode material.

[0003] To improve conductivity, some studies have introduced graphene to construct three-dimensional conductive networks. However, graphene sheets are prone to aggregation due to van der Waals forces, which not only reduces the effective contact area but also leads to an increase in interfacial contact resistance and easy breakage of the conductive network during cycling. Furthermore, the addition of graphene has a limited buffering effect on the volume expansion of TiO2. Summary of the Invention

[0004] Based on this, the first aspect of this application provides a negative electrode sheet, the technical solution of which is as follows:

[0005] A negative electrode sheet includes a current collector and a negative electrode active layer located on the current collector, the negative electrode active layer including a negative electrode active material, the negative electrode active material including a core, a buffer layer and a shell;

[0006] The core comprises titanium dioxide;

[0007] The outer shell covers the core, and the outer shell includes functionalized graphene, which includes graphene and linking groups, the linking groups connecting the graphene to the titanium dioxide, and the linking groups include -C(O)-O-;

[0008] The buffer layer is located between the titanium dioxide and the graphene, and the buffer layer comprises a polymer.

[0009] The second aspect of this application provides a method for preparing a negative electrode sheet, the technical solution of which is as follows:

[0010] A method for preparing a negative electrode sheet includes the following steps:

[0011] A titanium dioxide precursor solution was prepared by mixing a titanium source and water.

[0012] Functionalized graphene dispersions were prepared by oxidizing graphene with an acidic substance and grafting carboxylic acid groups onto the graphene.

[0013] The titanium dioxide precursor solution is added to the functionalized graphene dispersion, so that the graphene is linked to the titanium dioxide precursor through the linking groups formed by the carboxylic acid groups. A polymer is added and stirred to distribute the polymer between the titanium dioxide precursor and the graphene to prepare a negative electrode active slurry.

[0014] The negative electrode active slurry is coated onto the current collector, dried, and then heat-treated to obtain the negative electrode active layer.

[0015] A third aspect of this application provides a battery comprising a negative electrode as described above.

[0016] Compared with traditional solutions, this application has the following advantages:

[0017] This application functionalizes graphene and uses it as an outer shell material. By linking graphene to titanium dioxide through functionalized graphene linking groups, the aggregation of graphene and the interfacial bonding force with TiO2 can be improved, thereby enhancing rate performance and cycle stability. Simultaneously, a polymer buffer layer is placed between the functionalized graphene shell and the titanium dioxide core, which, through elastic deformation, mitigates the low capacity retention at high rates caused by the volume expansion and contraction of TiO2. Detailed Implementation

[0018] The present application will be further described in detail below with reference to specific embodiments. The present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0020] Unless otherwise stated or in case of contradiction, the terms or phrases used herein shall have the following meanings:

[0021] In this application, the terms "multiple", "various", "multiple times", "multi-dimensional", etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or more than or equal to two.

[0022] In this application, "several" means at least one, such as one, two, etc., unless otherwise expressly and specifically defined.

[0023] In this application, the terms "optionally," "optionally," and "optional" refer to options that are optional, meaning they can be selected from either "with" or "without." If multiple "optional" options appear in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "optional" option is independent.

[0024] In this application, the terms "first aspect," "second aspect," "third aspect," and "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," and "fourth," etc., serve only a non-exhaustive enumeration purpose and should be understood not to constitute a closed limitation on quantity.

[0025] In this application, numerical intervals (i.e. numerical ranges) are involved. Unless otherwise specified, the selected numerical distributions within the above-mentioned numerical intervals are considered continuous, and include the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical range, as well as every value between the two numerical endpoints.

[0026] The first aspect of this application provides a negative electrode sheet. In one embodiment, the negative electrode sheet includes a current collector and a negative electrode active layer located on the current collector. The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a core, a buffer layer, and a shell.

[0027] The core comprises titanium dioxide;

[0028] The outer shell covers the core, and the outer shell includes functionalized graphene, which includes graphene and linking groups, the linking groups connecting the graphene to the titanium dioxide, and the linking groups include -C(O)-O-;

[0029] The buffer layer is located between the titanium dioxide and the graphene, and the buffer layer comprises a polymer.

[0030] This embodiment functionalizes graphene and uses it as the outer shell material. By linking graphene to titanium dioxide through the functionalized graphene's linking groups, the aggregation of graphene and its interfacial bonding with TiO2 can be improved, thereby enhancing rate performance and cycle stability. Simultaneously, a polymer buffer layer is placed between the functionalized graphene shell and the titanium dioxide core. This allows for elastic deformation, mitigating the low capacity retention at high rates caused by the volume expansion and contraction of TiO2.

[0031] In one embodiment, the buffer layer includes a first polymer layer, a second polymer layer, and a third polymer layer, wherein the first polymer layer is closer to the core than the second polymer layer, and the third polymer layer is closer to the graphene than the second polymer layer.

[0032] The first polymer layer comprises a first polymer, the second polymer layer comprises a second polymer, and the third polymer layer comprises a third polymer. The weight-average molecular weight of the first polymer is less than that of the second polymer, and the weight-average molecular weight of the second polymer is less than that of the third polymer.

[0033] The first polymer has a relatively low weight-average molecular weight and strong polarity. It forms strong hydrogen bonds with the titanium hydroxyl groups (Ti-OH) on the surface of the titanium dioxide core and easily diffuses into the mesoporous channels or surface gaps of titanium dioxide. The third polymer has a relatively high weight-average molecular weight and weak polarity. It interacts more strongly with the conjugated π bonds and functionalized groups (such as carboxyl and hydroxyl groups) on the graphene surface (van der Waals forces + weak hydrogen bonds), making it more easily adsorbed on the graphene surface. This forms a polymer buffer layer with a molecular weight gradient between titanium dioxide and graphene. The polymers of different molecular weights form a gradient distribution of "low viscosity ion-conducting inner layer - medium adhesion structure in the middle layer - high viscosity anti-expansion outer layer," which can achieve dynamic adaptation of "expansion-buffering," reduce the expansion rate, and match the expansion and contraction law of TiO2 throughout the charging cycle.

[0034] In one embodiment, the weight-average molecular weight of the first polymer is 2000-6000. The weight-average molecular weight of the second polymer is 10000-20000. The weight-average molecular weight of the third polymer is 40000-60000. The mass ratio of the first polymer, the second polymer, and the third polymer is (1.5-2.5):(2.5-3.5):1.

[0035] In one embodiment, the first polymer, the second polymer, and the third polymer are each independently selected from polyethylene glycol (PEG).

[0036] In one embodiment, the functionalized graphene further includes flame-retardant groups comprising a phosphazene ring skeleton.

[0037] In one embodiment, the core has a porous structure. The porosity of the core is 30% to 55%. The pore size of the core is 5 nm to 20 nm. The specific surface area of ​​the core is 60 m². 2 / g~150m 2 / g. In one embodiment, the core further includes lithium ions located within the porous structure of the core.

[0038] In one embodiment, the core has a particle size of 0.5 μm to 2 μm. The buffer layer has a thickness of 20 nm to 80 nm. The graphene has a thickness of 1 μm to 5 μm. The negative electrode active layer has a thickness of 40 μm to 60 μm.

[0039] In the aforementioned negative electrode, functional groups are grafted onto the outer graphene shell, allowing the -COOH groups of the functionalized graphene to form coordination bonds with TiO2, thus solving the interfacial delamination problem of traditional composite systems. Furthermore, the molecular weight gradient distribution of the polymer buffer layer, with its gradient molecular weight PEG, achieves dynamic "expansion-buffering" adaptability, reducing the expansion rate. The mesoporous structure of the titanium dioxide core further enhances capacity and ion transport. In summary, through interface control and structural design, a synergistic improvement in conductivity, anti-expansion properties, and safety is achieved. This overcomes the technical bottlenecks of traditional negative electrode sheets, such as weak interfacial interaction, poor structural adaptability, and limited functionality, exhibiting dynamic buffering capabilities, a highly efficient conductive network, and flame-retardant properties.

[0040] A second aspect of this application provides a method for preparing a negative electrode sheet. In one embodiment, the method for preparing the negative electrode sheet includes the following steps:

[0041] S10, a titanium source and water were mixed to prepare a titanium dioxide precursor solution.

[0042] In one embodiment, the titanium source is selected from tetrabutyl titanate.

[0043] In one embodiment, the titanium dioxide precursor solution further includes a template agent comprising citric acid, wherein the molar ratio of the titanium source to the citric acid is 1:(0.3~0.5). The template agent is removed by the heat treatment, giving the core a porous structure. In one embodiment, the temperature of the subsequent heat treatment is 200~250°C, and the duration of the subsequent heat treatment is 2~3 hours.

[0044] In one embodiment, the titanium dioxide precursor solution further includes a lithium source, wherein the lithium source comprises LiNO3. The molar ratio of the titanium source to the lithium source is 1:(0.05~0.2).

[0045] S20. Using an acidic substance to oxidize graphene, carboxylic acid groups are grafted onto the graphene to prepare a functionalized graphene dispersion.

[0046] In one embodiment, the acidic substance is a strong acid capable of oxidizing graphene and grafting carboxylic acid groups (-COOH) onto it. For example, the acidic substance is a mixture of sulfuric acid (H2SO4), nitric acid (HNO3), and water. The volume ratio of H2SO4 to HNO3 is (2~4):1.

[0047] In one embodiment, after grafting carboxylic acid groups onto the graphene, the method further includes the step of adding a flame retardant, the flame retardant having flame retardant groups, the flame retardant groups comprising a phosphazene ring skeleton. By adding the flame retardant, flame retardant groups are grafted onto the graphene. In one embodiment, the flame retardant is selected from hexachlorocyclotriphosphazene.

[0048] S30. The titanium dioxide precursor solution is added to the functionalized graphene dispersion, so that the graphene is connected to the titanium dioxide precursor through the linking group formed by the carboxylic acid group. A polymer is added and stirred to distribute the polymer between the titanium dioxide precursor and the graphene to prepare a negative electrode active slurry.

[0049] The titanium dioxide precursor solution is added to the functionalized graphene dispersion by dropwise addition. In one embodiment, the drop rate is 1-3 mL / min. During this process, the graphene is connected to the titanium dioxide precursor via -COO-.

[0050] In one embodiment, the polymer includes a first polymer, a second polymer, and a third polymer, wherein the weight-average molecular weight of the first polymer is less than the weight-average molecular weight of the second polymer, and the weight-average molecular weight of the second polymer is less than the weight-average molecular weight of the third polymer.

[0051] Stirring causes the polymer to distribute between the titanium dioxide precursor and the graphene, including the following steps:

[0052] Ultrasonic stirring is used to bring the first polymer closer to the titanium dioxide precursor relative to the second polymer to form a first polymer layer, and to bring the third polymer closer to the graphene relative to the second polymer to form a third polymer layer, wherein the second polymer layer formed by the second polymer is located between the first polymer layer and the third polymer layer.

[0053] Through ultrasonic stirring, the first polymer has a relatively low weight-average molecular weight and strong polarity. It forms strong hydrogen bonds with the titanium hydroxyl groups (Ti-OH) on the surface of the titanium dioxide core and can easily diffuse into the mesoporous channels or surface gaps of titanium dioxide. The third polymer has a relatively high weight-average molecular weight and weak polarity. It interacts more strongly with the conjugated π bonds and functionalized groups (such as carboxyl and hydroxyl groups) on the graphene surface (van der Waals forces + weak hydrogen bonds), and is more easily adsorbed on the graphene surface.

[0054] In one embodiment, the temperature of the ultrasonic stirring is 40~50°C. The power of the ultrasonic stirring is 300~500W. The duration of the ultrasonic stirring is 1~3 hours.

[0055] S40. Coat the negative electrode active slurry onto the current collector, dry it, and then heat treat it to obtain the negative electrode active layer.

[0056] In one embodiment, the current collector is copper foil. The coating thickness of the negative electrode active slurry is 60-90 μm. After coating, it is first vacuum dried at 80-90°C for 4-6 hours to remove free solvent, and then heat-treated at 200-250°C under an inert N2 atmosphere for 2-3 hours to solidify the gradient structure of the polymer buffer layer and remove citric acid, forming mesoporous structures. Finally, it is pressed into a tablet under a pressure of 15-25 MPa.

[0057] In the above preparation method, the polymer forms a buffer layer in the negative electrode active layer. The thickness of the buffer layer can be controlled by the ultrasonic stirring power; a higher ultrasonic power results in a thinner buffer layer, and a lower ultrasonic power results in a thicker buffer layer. Functionalized graphene serves as the outer shell of the negative electrode active layer, and the thickness of the graphene can be controlled by the ultrasonic power, time, concentration of acidic substances, and temperature. Specifically, reducing the ultrasonic power and time yields a thicker graphene layer. Reducing the concentration of acidic substances and the temperature also yields a thicker graphene layer. The concentration of acidic substances refers to the concentrations of H2SO4 and HNO3, and the temperature refers to the ultrasonic stirring temperature and the preparation temperature of the functionalized graphene dispersion.

[0058] The above preparation method simultaneously forms the negative electrode active material through in-situ preparation, and the preparation process is simple and easy to operate. Furthermore, in-situ preparation shortens the production cycle and ensures structural uniformity.

[0059] A third aspect of this application provides a battery, in one embodiment of which the battery includes a negative electrode as described above.

[0060] The following description is further illustrated with specific embodiments and comparative examples. Unless otherwise specified, the raw materials involved in the following specific embodiments and comparative examples are all commercially available. Unless otherwise specified, the instruments used are all commercially available. Unless otherwise specified, the processes involved are conventionally selected by those skilled in the art.

[0061] Example 1

[0062] This embodiment provides a negative electrode sheet and its preparation method, the steps of which are as follows:

[0063] Step 1: Preparation of titanium dioxide precursor solution

[0064] Tetrabutyl titanate and citric acid were mixed at a molar ratio of 1:0.3. Deionized water was added at a molar ratio of 1:10. The mixture was stirred at 40°C for 1 hour. LiNO3 was added at a molar ratio of 1:0.1 (tetrabutyl titanate to LiNO3). The mixture was stirred for 30 minutes to obtain a titanium dioxide precursor solution.

[0065] Step 2: Preparation of functionalized graphene dispersion

[0066] Graphene was added to an aqueous solution of H2SO4 / HNO3 (H2SO4 to HNO3 volume ratio 3:1) and ultrasonically oxidized for 3 hours at a power of 500W and a temperature of 55℃ to graft -COOH onto the graphene; then hexachlorocyclotriphosphazene was added and reacted at 85℃ for 5 hours to graft flame retardant groups, thus obtaining a functionalized graphene dispersion.

[0067] Step 3: Preparation of negative electrode active slurry

[0068] The above titanium dioxide precursor solution was added dropwise to the above functionalized graphene dispersion (dropping rate 1~3 mL / min), so that the graphene was connected to the titanium dioxide precursor through -COO-. At the same time, low, medium and high molecular weight PEG were added at a weight ratio of 2:3:1 (the weight average molecular weight of low molecular weight PEG was 4000, the weight average molecular weight of medium molecular weight PEG was 15000, and the weight average molecular weight of high molecular weight PEG was 50000). The mixture was ultrasonically stirred at 40°C (power 500W) for 2 hours to distribute the polymer between the titanium dioxide precursor and the graphene, thus preparing the negative electrode active slurry.

[0069] Step 4: Prepare the negative electrode sheet

[0070] The above-mentioned negative electrode active slurry was coated on copper foil with a coating thickness of 90 μm. It was first vacuum dried at 80 °C for 6 h to remove free solvent, and then heat-treated at 200 °C and under an inert N2 atmosphere for 3 h to solidify the PEG molecular weight gradient distribution structure and remove citric acid to form mesoporous structures. Finally, it was pressed under a pressure of 25 MPa to form a negative electrode active layer with a thickness of 50 μm, thus obtaining the negative electrode sheet.

[0071] Examples 2 to 3

[0072] The above embodiments provide a negative electrode sheet and its preparation method, which are basically the same as those in Embodiment 1. The main differences are shown in Table 1.

[0073] Table 1

[0074]

[0075] Comparative Example 1

[0076] This comparative example provides a negative electrode sheet and its preparation method, which is basically the same as that in Example 1, except that step 2 is different, specifically:

[0077] Step 2: Preparation of graphene dispersion

[0078] Graphene was added to deionized water; then hexachlorocyclotriphosphazene was added, and the mixture was reacted at 85°C for 5 hours to graft flame-retardant groups, thus obtaining a functionalized graphene dispersion.

[0079] Comparative Example 2

[0080] This comparative example provides a negative electrode sheet and its preparation method, which is basically the same as that in Example 1, except that step 3 is different, specifically:

[0081] Step 3: Preparation of negative electrode active slurry

[0082] The above titanium dioxide precursor solution was added dropwise to the above functionalized graphene dispersion (dropping rate 1~3 mL / min) so that the graphene was connected to the titanium dioxide precursor through -COO- to prepare a negative electrode active slurry.

[0083] The negative electrode sheets of the above embodiments and comparative examples were subjected to performance tests. The test items and methods are as follows:

[0084] Project 1: The morphology of the negative electrode active layer was examined using cryo-transmission electron microscopy. The core-shell outline of the negative electrode active layer was observed, and the thickness of the buffer layer and the graphene thickness were measured and are shown in Table 1.

[0085] Project 2: Oxidative Removal of Graphene Layer: A tube furnace was maintained at 450℃ for 3 minutes under an air atmosphere. Graphene was completely removed by oxidation to CO2 at this temperature, and the TiO2 mesoporous structure remained intact. Removal of the PEG buffer layer: The crucible was placed in a tube furnace, N2 was introduced, and the temperature was increased to 450℃ at a rate of 5℃ / min and maintained for 2 hours. PEG was completely decomposed under this atmosphere, and the temperature was much lower than the TiO2 mesoporous collapse temperature, without damaging the pore structure. The particle size, pore size, and porosity of the exposed TiO2 mesopores were measured using the BET method, and the results are shown in Table 1.

[0086] Project 3: After the graphene layer was removed by oxidation in Project 2, buffer layers at different positions with the core pointing towards the shell were taken, and the molecular weight of PEG was tested by gel permeation chromatography (GPC). The results showed that low molecular weight PEG was distributed closer to the core, while high molecular weight PEG was distributed closer to the shell.

[0087] Project 4: Assembly of electrode preparation: LFP, PVDF and conductive carbon black are mixed in a mass ratio of 8:1:1, and then mixed with water (solvent) to prepare positive electrode slurry. The slurry is coated onto aluminum foil with a scraper and dried under vacuum at 120°C to form a positive electrode active layer, thus obtaining the positive electrode sheet.

[0088] Solid electrolyte preparation: Lithium sulfide (Li₂S), phosphorus pentasulfide (P₂S₅), and LiCl powders with a purity higher than 99.9% were placed in an agate ball mill jar. High-energy ball milling was performed at 300–600 rpm for 10–30 hours under argon protection. The glassy powder was transferred to an alumina crucible and annealed in an argon-atmosphere tube furnace at 200–300 °C for 1–5 hours with a heating rate of 5 °C / min (slow heating to avoid powder agglomeration). After annealing, the furnace was cooled to room temperature to obtain the Li₆PS₅Cl sulfide electrolyte.

[0089] Negative electrode sheet: The negative electrode sheet prepared in each example and comparative example.

[0090] The above-mentioned positive electrode, solid electrolyte, and negative electrode were assembled into a battery. It was charged at a constant current (CC) of 1C to 3.65V, then switched to constant voltage (CV) charging until the current ≤0.05C was cut off. After standing for 10 minutes, it was discharged at 0.2C to 3.0V, and the discharge capacity (denoted as C0) was recorded. The battery was then discharged at constant currents of 0.5C, 1C, 2C, and 5C to 3.0V, and the discharge capacity (Cx) at each rate was recorded. The capacity retention rate was Cx / C0×100%. The test results are shown in Table 2.

[0091] Table 2

[0092]

[0093] As can be seen, the negative electrode prepared in the above embodiments has good rate performance, and still maintains a high capacity retention rate at high rates. Comparative Example 1 did not graft carboxyl groups onto graphene, resulting in problems with the interfacial bonding between graphene and TiO2, affecting rate performance. Comparative Example 2 did not form a buffer layer with a molecular weight gradient distribution between graphene and TiO2, leading to significant volume expansion and contraction of TiO2, affecting rate performance and resulting in a lower capacity retention rate at high rates.

[0094] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0095] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A negative electrode sheet, characterized in that, It includes a current collector and a negative electrode active layer located on the current collector, the negative electrode active layer including a negative electrode active material, the negative electrode active material including a core, a buffer layer and a shell; The core comprises titanium dioxide; The outer shell covers the core, and the outer shell includes functionalized graphene, which includes graphene and linking groups, the linking groups connecting the graphene to the titanium dioxide, and the linking groups include -C(O)-O-; The buffer layer is located between the titanium dioxide and the graphene, and the buffer layer comprises a polymer.

2. The negative electrode sheet according to claim 1, characterized in that, The buffer layer includes a first polymer layer, a second polymer layer, and a third polymer layer, wherein the first polymer layer is closer to the core than the second polymer layer, and the third polymer layer is closer to the graphene than the second polymer layer. The first polymer layer comprises a first polymer, the second polymer layer comprises a second polymer, and the third polymer layer comprises a third polymer. The weight-average molecular weight of the first polymer is less than that of the second polymer, and the weight-average molecular weight of the second polymer is less than that of the third polymer.

3. The negative electrode sheet according to claim 2, characterized in that, Includes at least one of the following features: (1) The weight-average molecular weight of the first polymer is 2000~6000; (2) The weight-average molecular weight of the second polymer is 10,000 to 20,000; (3) The weight-average molecular weight of the third polymer is 40,000 to 60,000; (4) The mass ratio of the first polymer, the second polymer and the third polymer is (1.5~2.5):(2.5~3.5):

1.

4. The negative electrode sheet according to claim 2, characterized in that, The first polymer, the second polymer, and the third polymer are each independently selected from polyethylene glycol.

5. The negative electrode sheet according to claim 1, characterized in that, The functionalized graphene also includes flame-retardant groups, which include a phosphazene ring skeleton.

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that, The kernel has a porous structure.

7. The negative electrode sheet according to claim 6, characterized in that, Includes at least one of the following features: (1) The porosity of the core is 30%~55%; (2) The aperture of the core is 5nm~20nm; (3) The specific surface area of ​​the core is 60m². 2 / g~150m 2 / g; (4) The core also includes lithium ions, which are located in the pore structure of the core.

8. The negative electrode sheet according to any one of claims 1 to 5 and 7, characterized in that, Includes at least one of the following features: (1) The particle size of the kernel is 0.5μm~2μm; (2) The thickness of the buffer layer is 20nm~80nm; (3) The thickness of the graphene is 1 μm to 5 μm; (4) The thickness of the negative electrode active layer is 40μm~60μm.

9. A method for preparing a negative electrode sheet, characterized in that, Includes the following steps: A titanium dioxide precursor solution was prepared by mixing a titanium source and water. Functionalized graphene dispersions were prepared by oxidizing graphene with an acidic substance and grafting carboxylic acid groups onto the graphene. The titanium dioxide precursor solution is added to the functionalized graphene dispersion, so that the graphene is linked to the titanium dioxide precursor through the linking groups formed by the carboxylic acid groups. A polymer is added and stirred to distribute the polymer between the titanium dioxide precursor and the graphene to prepare a negative electrode active slurry. The negative electrode active slurry is coated onto the current collector, dried, and then heat-treated to obtain the negative electrode active layer.

10. The method for preparing the negative electrode sheet according to claim 9, characterized in that, The polymer includes a first polymer, a second polymer, and a third polymer, wherein the weight-average molecular weight of the first polymer is less than that of the second polymer, and the weight-average molecular weight of the second polymer is less than that of the third polymer. Stirring causes the polymer to distribute between the titanium dioxide precursor and the graphene, including the following steps: Ultrasonic stirring is used to bring the first polymer closer to the titanium dioxide precursor relative to the second polymer to form a first polymer layer, and to bring the third polymer closer to the graphene relative to the second polymer to form a third polymer layer, wherein the second polymer layer formed by the second polymer is located between the first polymer layer and the third polymer layer.

11. The method for preparing the negative electrode sheet according to claim 10, characterized in that, Includes at least one of the following features: (1) The temperature of the ultrasonic stirring is 40~50℃; (2) The power of the ultrasonic stirring is 300~500W; (3) The ultrasonic stirring time is 1~3h.

12. The method for preparing the negative electrode sheet according to claim 9, characterized in that, After grafting carboxylic acid groups onto the graphene, the process further includes the following steps: A flame retardant is added, wherein the flame retardant has a flame retardant group, and the flame retardant group includes a phosphazene ring skeleton.

13. The method for preparing the negative electrode sheet according to claim 9, characterized in that, Includes at least one of the following features: (1) The titanium source is selected from tetrabutyl titanate; (2) The titanium dioxide precursor solution further includes a template agent, which includes citric acid. The molar ratio of the titanium source to the citric acid is 1:(0.3~0.5). The template agent is removed by the heat treatment, so that the core has a porous structure. (3) The titanium dioxide precursor solution further includes a lithium source, the lithium source includes LiNO3, and the molar ratio of the titanium source to the lithium source is 1: (0.05~0.2).

14. The method for preparing the negative electrode sheet according to any one of claims 9 to 13, characterized in that, The heat treatment satisfies at least one of the following conditions: (1) The heat treatment temperature is 200~250℃; (2) The heat treatment time is 2~3 hours.

15. A battery, characterized in that, Includes the negative electrode sheet according to any one of claims 1 to 8.

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