Secondary battery, electric device, preparation method for negative electrode active material, and preparation method for secondary battery

Abstract

The present disclosure provides a secondary battery, comprising a negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector. The negative electrode film layer comprises a negative electrode active material. The negative electrode active material comprises a core structure and a carbon coating layer arranged on at least part of the surface of the core structure. The core structure comprises a silicon-based material. The carbon coating layer contains an element iron. Carbon nanotubes are arranged on at least part of the surface of the carbon coating layer.

Description

Preparation methods of secondary batteries, electrical devices, and negative electrode active materials, and preparation methods of secondary batteries.

[0001] Cross-reference to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202510199694.4, filed on February 21, 2025, entitled "Secondary Battery, Electrical Device, Method for Preparing Negative Electrode Active Material and Method for Preparing Secondary Battery", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of lithium battery technology, and in particular to a secondary battery, an electrical device, a method for preparing a negative electrode active material, and a method for preparing a secondary battery. Background Technology

[0004] In recent years, with the increasingly wide application of rechargeable batteries, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and many other fields. Due to the significant development of rechargeable batteries, higher requirements have been placed on their rate performance, cycle stability, and initial coulombic efficiency. Summary of the Invention

[0005] This disclosure is made in view of the above-mentioned problems, and its object is to provide a secondary battery, an electrical device, a method for preparing a negative electrode active material, and a method for preparing a secondary battery. The secondary battery of the present invention has improved rate performance, good cycle stability, and initial coulombic efficiency.

[0006] This disclosure provides a secondary battery, including a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes a core structure and a carbon coating layer disposed on at least a portion of the surface of the core structure. The core structure includes a silicon-based material, and the carbon coating layer contains iron. Carbon nanotubes are disposed on at least a portion of the surface of the carbon coating layer. This provides an advantage for the secondary battery to have high energy density and improved rate performance, initial coulombic efficiency, and cycle performance.

[0007] In some implementations, the iron content in the negative electrode active material is 5%-20% by mass. This is beneficial for improving the rate performance and coulombic efficiency of the secondary battery, while also achieving high energy density and cycle performance.

[0008] In some implementations, the iron content in the negative electrode active material is 10%-20% by mass. This further improves the rate performance and coulombic efficiency of the secondary battery, while also achieving high energy density and cycle performance.

[0009] In some implementations, the iron element comprises 0-valent iron and / or +2-valent iron. This is beneficial for improving the rate performance, initial coulombic efficiency, and cycle performance of the secondary battery.

[0010] In some embodiments, the secondary battery satisfies one or more of the following characteristics: (1) the average aspect ratio of the carbon nanotubes is 200-1200; and / or, (2) the average length of the carbon nanotubes is 1 μm-6 μm; and / or (3) the average diameter of the carbon nanotubes is 5 nm-50 nm. This is beneficial for improving the rate performance and cycle performance of the secondary battery.

[0011] In some embodiments, the secondary battery satisfies one or more of the following characteristics: (1) the average aspect ratio of the carbon nanotubes is 900-1100; and / or, (2) the average length of the carbon nanotubes is 3 μm-5 μm; and / or (3) the average diameter of the carbon nanotubes is 5 nm-20 nm. This further improves the rate performance and cycle performance of the secondary battery.

[0012] In some embodiments, the mass percentage of carbon in the negative electrode active material is 5%-50%; and / or, the mass ratio of silicon to carbon in the negative electrode active material is 1:1-19:1. This is beneficial for the secondary battery to achieve both high energy density and good cycle performance.

[0013] In some embodiments, carbon nanotubes are deposited on at least a portion of the surface of the carbon coating layer through in-situ growth. This is beneficial for improving the rate performance and cycle performance of the secondary battery.

[0014] In some embodiments, the binder content in the negative electrode film layer is less than or equal to 0.5%. This is beneficial for improving the rate performance and initial coulombic efficiency of the secondary battery.

[0015] In some embodiments, the binder content in the negative electrode film layer is 0%-0.1%. This is beneficial for further improving the rate performance and initial coulombic efficiency of the secondary battery.

[0016] In some embodiments, the carbon coating also includes nitrogen. This is beneficial for improving the rate performance and initial coulombic efficiency of the secondary battery.

[0017] In some implementations, the mass percentage of nitrogen in the negative electrode active material is less than or equal to 0.1%. This is beneficial for improving the rate performance of the secondary battery.

[0018] In some embodiments, the silicon-based material includes one or more of elemental silicon, silicon-oxygen materials, silicon-carbon composite materials, and silicon alloy materials. This is beneficial for improving the energy density of secondary batteries.

[0019] In some embodiments, the volume distribution particle size Dv50 of the negative electrode active material is 40 nm to 200 nm; thereby, it is beneficial to improve the cycle performance of the secondary battery.

[0020] In some embodiments, the volume distribution particle size Dv90 of the negative electrode active material is 0.3 μm to 1 μm; this is beneficial to improving the cycle performance of the secondary battery.

[0021] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 7 to 10; thereby, it is beneficial to improve the cycle performance of the secondary battery.

[0022] In some embodiments, the specific surface area of ​​the negative electrode active material is 35 m². 2 / g~60m 2 / g. This is beneficial for improving the rate performance and cycle performance of secondary batteries.

[0023] In some embodiments, the negative electrode has a conductivity of 12.5 S / m to 67 S / m. This is beneficial for improving the rate performance and initial coulombic efficiency of the secondary battery.

[0024] In some embodiments, the secondary battery satisfies one or more of the following characteristics: (1) the thickness of the negative electrode film is 8 μm to 25 μm; and / or, (2) the coating weight of the negative electrode film is 1 mg / cm³. 2 ~5mg / cm 2 ; and / or, (3) the compaction density of the negative electrode film is 0.8 g / cm³. 3 ~1.2g / cm 3 This is beneficial for secondary batteries to have high energy density.

[0025] In some embodiments, the secondary battery includes an electrolyte, which includes at least one of a solid electrolyte, a semi-solid electrolyte, and a gel electrolyte.

[0026] In some embodiments, the electrolyte includes a sulfide electrolyte material.

[0027] In some embodiments, the secondary battery includes a positive electrode sheet, which includes a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, and modified compounds thereof.

[0028] The second aspect of this disclosure provides an electrical device that includes a secondary battery according to the first aspect of this disclosure, and thus has at least the advantages of the secondary battery of this disclosure.

[0029] This disclosure provides a third aspect of an anode active material, comprising a core structure and a carbon coating layer disposed on at least a portion of the surface of the core structure. The core structure comprises a silicon-based material, the carbon coating layer contains iron, and carbon nanotubes are disposed on at least a portion of the surface of the carbon coating layer. This is advantageous for secondary batteries to possess high energy density and improved rate performance, initial coulombic efficiency, and cycle performance.

[0030] In some implementations, the iron content in the negative electrode active material is 5%-20% by mass. This is beneficial for improving the rate performance and coulombic efficiency of the secondary battery, while also achieving high energy density and cycle performance.

[0031] In some implementations, the iron element comprises 0-valent iron and / or +2-valent iron. This is beneficial for improving the rate performance, initial coulombic efficiency, and cycle performance of the secondary battery.

[0032] In some embodiments, the secondary battery satisfies one or more of the following characteristics: (1) the average aspect ratio of the carbon nanotubes is 200-1200; and / or, (2) the average length of the carbon nanotubes is 1 μm-6 μm; and / or (3) the average diameter of the carbon nanotubes is 5 nm-50 nm. This is beneficial for improving the rate performance and cycle performance of the secondary battery.

[0033] In some embodiments, the mass percentage of carbon in the negative electrode active material is 5%-50%; and / or, the mass ratio of silicon to carbon in the negative electrode active material is 1:1-19:1. This is beneficial for the secondary battery to achieve both high energy density and good cycle performance.

[0034] In some embodiments, carbon nanotubes are deposited on at least a portion of the surface of the carbon coating layer through in-situ growth. This is beneficial for improving the rate performance and cycle performance of the secondary battery.

[0035] The fourth aspect of this disclosure provides a method for preparing a negative electrode active material, comprising the following steps:

[0036] A silicon core with a polymer film is mixed with an iron-based catalyst;

[0037] The temperature is increased to the first heat treatment temperature of 400℃-600℃ at a first heating rate of 5℃ / min-20℃ / min.

[0038] The material is held at a certain temperature for 5-50 hours initially, then heated to a second heat treatment temperature of 700-800℃ at a second heating rate of 5℃ / min-20℃ / min, and held for 1-5 hours to obtain the negative electrode active material. This process forms a carbon coating layer on the silicon core, containing iron, and carbon nanotubes are grown in situ on the carbon coating layer. This is beneficial for improving the rate performance, initial coulombic efficiency, and cycle performance of the secondary battery.

[0039] In some embodiments, the iron-based catalyst includes one or more of ferrocene, magnetite, ferric nitrate, and elemental iron. This facilitates the in-situ growth of carbon nanotubes and also improves the electronic conductivity of the negative electrode active material, thereby enhancing the rate performance and initial coulombic efficiency of the secondary battery.

[0040] In some embodiments, the mass ratio of the silicon core with polymer film to the iron-based catalyst is 1:0.075 to 1:0.70. This is beneficial for improving the rate performance, initial coulombic efficiency, and cycle performance of the secondary battery.

[0041] In some embodiments, the method for preparing a silicon core with a polymer film includes: adding the silicon core to a solution containing polymer monomers to carry out a polymerization reaction, thereby obtaining a silicon core with a polymer film. This is beneficial for improving the conductivity of the negative electrode active material.

[0042] In some embodiments, the mass ratio of the silicon core to the polymer in the solution containing polymer monomers is 1:0.75 to 1:1.6. This is beneficial for improving the coating efficiency of the carbon coating layer and also helps the secondary battery to achieve both energy density and cycle performance.

[0043] In some embodiments, the concentration of the polymer monomer in the solution is 0.05 mol / L to 0.1 mol / L. This is beneficial for forming a uniform and structurally stable carbon coating layer.

[0044] In some embodiments, the polymerization reaction includes reacting at 20°C to 150°C for 24 to 36 hours, thereby forming a polymer film.

[0045] In some embodiments, the polymer monomers include one or more of dopamine, resorcinol-formaldehyde, glucose, sucrose, vinylpyrrolidone, and tannic acid.

[0046] The fourth aspect of this disclosure provides a method for preparing a secondary battery, comprising the following steps to prepare a negative electrode sheet:

[0047] A negative electrode slurry is provided, comprising a negative electrode active material, a binder, and a solvent prepared by the method of the third aspect of this disclosure; the negative electrode slurry is coated onto a negative electrode current collector to form a negative electrode film layer. The resulting secondary battery exhibits improved rate performance, cycle performance, and initial coulombic efficiency.

[0048] In some embodiments, coating the negative electrode slurry onto the negative electrode current collector to form a negative electrode film layer includes: coating the negative electrode slurry onto the negative electrode current collector, and then holding it at 180°C to 250°C for 15 min to 30 min under an argon atmosphere to form a negative electrode film layer, wherein the binder content in the negative electrode film layer is less than or equal to 0.5 wt%. The resulting negative electrode sheet has good rate performance.

[0049] In some embodiments, the binder includes one or more of polypropylene carbonate, polylactic acid, and polyurethane. This helps to reduce the binder content in the negative electrode film layer.

[0050] In some embodiments, the solvent includes one or more of anisole, acetone, tetrahydrofuran, dimethylformamide, and toluene. Attached Figure Description

[0051] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of the present disclosure.

[0052] Figure 2 is an exploded view of a secondary battery according to an embodiment of the present disclosure shown in Figure 1.

[0053] Figure 3 is a schematic diagram of a battery module according to one embodiment of the present disclosure.

[0054] Figure 4 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0055] Figure 5 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 4.

[0056] Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0057] Figure 7 is a scanning electron microscope image of polydopamine-coated nanosilicon prepared in Example 1 of this disclosure.

[0058] Figure 8 is a scanning electron microscope image of the negative electrode active material prepared in Example 1 of this disclosure.

[0059] Figure 9 is an X-ray diffraction pattern of the negative electrode active material prepared in Example 1 of this disclosure.

[0060] Explanation of reference numerals in the attached figures:

[0061] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0062] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the secondary battery, power-consuming device, method for preparing negative electrode active material, and method for preparing the secondary battery. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for a full understanding of this disclosure by those skilled in the art and are not intended to limit the subject matter of the claims.

[0063] The "range" disclosed in this disclosure is defined by a lower limit and an upper limit, whereby a given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this disclosure, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0064] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0065] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0066] Unless otherwise specified, all steps of this disclosure may be performed sequentially or randomly, preferably sequentially. For example, if a method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if it is mentioned that the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0067] Sulfide-based all-solid-state batteries offer advantages such as high energy density and good safety performance. With the technological advancements in all-solid-state batteries, higher demands are being placed on their energy density, rate performance, and cycle performance. Silicon, due to its ultra-high specific capacity, can effectively improve the energy density of rechargeable batteries as a negative electrode active material. However, its low electronic conductivity and small lithium-ion diffusion coefficient result in low ion / electron transport efficiency in the negative electrode active material, limiting the rate performance of the rechargeable battery. Furthermore, the significant volume expansion of silicon during charge and discharge processes can easily damage the negative electrode sheet during rechargeable battery cycling. In existing technologies, to address the expansion problem of silicon materials, silicon-based active materials are typically used in combination with binders, or a buffer layer is incorporated into the negative electrode sheet to reduce the damage to the electrode structure caused by silicon expansion. However, commonly used binders, such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), are non-ionic / electronic active materials, which hinder ion / electron transport in the negative electrode. In all-solid-state batteries, due to the non-flowability of the solid electrolyte, the binder's obstruction of ion / electron transport is even more severe, thus limiting the improvement of the rate performance of the secondary battery. Buffer layers are usually made of ion / electron insulating materials, and the buffer layer increases the solid-solid interface, which is not conducive to ion / electron transport in the negative electrode.

[0068] In view of this, this disclosure provides a novel secondary battery, an electrical device, a method for preparing a negative electrode active material, and a method for preparing a secondary battery. The secondary battery provided by this disclosure has improved rate performance, cycle performance, and initial coulombic efficiency.

[0069] Secondary batteries

[0070] The term "secondary battery" as used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0071] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0072] In some implementations, the secondary battery includes a solid-state battery.

[0073] [negative electrode]

[0074] The secondary battery includes a negative electrode sheet, which includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material.

[0075] The negative electrode active material disclosed herein includes a core structure and a carbon coating layer disposed on at least a portion of the surface of the core structure. The core structure includes a silicon-based material, the carbon coating layer contains iron, and carbon nanotubes are disposed on at least a portion of the surface of the carbon coating layer.

[0076] In this disclosure, the negative electrode active material includes a core structure and a carbon coating layer disposed on at least a portion of the surface of the core structure. The core structure includes a silicon-based material, which, due to its high specific capacity, enables the secondary battery to have a high energy density. The carbon coating layer disposed on at least a portion of the surface of the core structure can mitigate the structural damage caused by volume expansion of the core structure during cycling, thereby improving the structural stability of the negative electrode film and thus enhancing the cycle stability of the secondary battery. Furthermore, the carbon coating layer also includes iron, which has excellent electron transport capabilities, beneficial for improving the electron transport efficiency in the negative electrode, thereby improving the rate performance and initial coulombic efficiency of the secondary battery. Additionally, carbon nanotubes are disposed on at least a portion of the surface of the carbon coating layer. As excellent electronic and ionic conductors, carbon nanotubes form an ion-electron conductive network in the negative electrode film, improving the ion and / or electron transport efficiency during cycling, thereby enhancing the rate performance and initial coulombic efficiency of the secondary battery.

[0077] In some embodiments, the mass percentage of iron in the negative electrode active material is 5%-20%. Maintaining this mass percentage within this range improves the electron transport efficiency in the negative electrode film while maintaining the specific capacity of the negative electrode active material. This, in turn, enhances the rate performance and coulombic efficiency of the secondary battery, while also achieving high energy density and good cycle performance. Exemplarily, the mass percentage of iron in the negative electrode active material can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, or any two of these values. In an optional embodiment, the mass percentage of iron in the negative electrode active material is 10%-20%, which further improves the rate performance and coulombic efficiency of the secondary battery, while also achieving high energy density and cycle performance.

[0078] In this disclosure, the elemental composition in the negative electrode film can be measured using methods conventional in the art. For example, inductively coupled plasma atomic emission spectrometry (ICP) can be used for testing.

[0079] In some embodiments, the iron element includes 0-valent iron and / or +2-valent iron. 0-valent iron has good electron transport efficiency, which is beneficial for improving the rate performance and initial coulombic efficiency of the secondary battery. During cycling, the 0-valent iron is partially lithium-ionized to form a Li-Si-Fe ternary alloy phase, in which the iron element is +2-valent. This helps reduce the volume expansion caused by lithium intercalation in the silicon material, thereby reducing the volume change of the negative electrode during charge and discharge, improving the structural stability of the negative electrode, and enhancing the cycle performance of the secondary battery.

[0080] In this disclosure, the valence state of elements in the negative electrode film can be measured using methods conventional in the art. For example, photoelectron spectroscopy (XPS) can be used for testing.

[0081] In some embodiments, the average aspect ratio of the carbon nanotubes is 200-1200. Exemplarily, the average aspect ratio of the carbon nanotubes can be 200, 300, 500, 800, 900, 1000, 1100, 1200, or a range from the above. By keeping the aspect ratio of the carbon nanotubes within the above range, the carbon nanotubes possess good elasticity and toughness, enabling the formation of an intertwined carbon nanotube network in the negative electrode active material. This provides a good long-range electron transport network, improving electron transport efficiency and thus enhancing the rate performance and initial coulombic efficiency of the secondary battery. Furthermore, the network structure formed by the carbon nanotubes helps suppress the expansion of the silicon material, improving the structural stability of the negative electrode film and thus enhancing the cycle performance of the secondary battery. In some optional embodiments, the aspect ratio of the carbon nanotubes is 900-1100.

[0082] In some embodiments, the average length of the carbon nanotubes is 1 μm-6 μm. Exemplarily, the length of the carbon nanotubes can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, or a range within the above range. By keeping the length of the carbon nanotubes within the above range, it is beneficial to form a carbon nanotube network, improve electron transport efficiency, and suppress silicon material expansion, thereby improving the rate performance and cycle performance of the secondary battery. In some alternative embodiments, the length of the carbon nanotubes is 3 μm-5 μm.

[0083] In some embodiments, the average diameter of the carbon nanotubes is 5 nm to 50 nm. Exemplarily, the diameter of the carbon nanotubes can be 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, or a range between these values. By keeping the diameter of the carbon nanotubes within these ranges, it is beneficial to improve the electron transport efficiency in the negative electrode film layer, thereby improving the rate performance of the secondary battery. In some optional embodiments, the diameter of the carbon nanotubes is 5 nm to 20 nm.

[0084] In this disclosure, the average aspect ratio of carbon nanotubes has a conventional meaning in the art, referring to the ratio of the longest diameter passing through the interior of the carbon nanotube to the diameter of the carbon nanotube, and can be measured using conventional methods in the art. Exemplarily, the longest diameter (length diameter) and the diameter of the carbon nanotube can be measured separately using the scale of a scanning electron microscope (SEM), and then the ratio between the longest diameter and the diameter is calculated; this is the aspect ratio. For example, a scanning electron microscope (SEM) image of the negative electrode sheet can be obtained using a scanning electron microscope (e.g., a ZEISS Sigma 300), referring to JY / T010-1996. As an example, the test can be conducted as follows: Randomly select a test sample with dimensions of 50mm x 100mm on the negative electrode. Randomly select multiple test areas (e.g., 5) within the test sample. At a certain magnification, read the length (i.e., the distance between the two furthest points on the carbon nanotube) and diameter of the carbon nanotubes in each test area, and calculate the aspect ratio. Calculate the aspect ratio values ​​for the carbon nanotubes in each test area, and take the arithmetic mean of the aspect ratios for all test areas. This is the average aspect ratio of the carbon nanotubes in the test sample. To ensure the accuracy of the test results, the above test can be repeated with multiple test samples (e.g., 10), and the average value of each test sample can be taken as the final test result.

[0085] In this disclosure, the average length and average diameter of carbon nanotubes have the common meaning in the art and can be measured using conventional methods in the art. For example, referring to the above-described aspect ratio test method, the measured length and diameter values ​​of the carbon nanotubes can be statistically analyzed, and the arithmetic mean can be taken as the average length and average diameter of the carbon nanotubes in the test sample. To ensure the accuracy of the test results, the above test can be repeated on multiple test samples (e.g., 10), and the average value of each test sample can be taken as the final test result.

[0086] In some embodiments, the mass percentage of carbon in the negative electrode active material is 5%-50%. Maintaining the carbon content within this range is beneficial for improving the electrical conductivity of the silicon-based material with its core structure, while also considering the energy density of the negative electrode active material. Furthermore, it helps to mitigate the volume expansion of the silicon-based material during cycling, improving the stability of the negative electrode active material and thus enhancing the cycle performance of the secondary battery. Exemplarily, the mass percentage of carbon in the negative electrode active material can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 30%, 35%, 40%, 45%, 50%, or a value within a range of any two of these values.

[0087] In some embodiments, the mass ratio of silicon to carbon in the negative electrode active material is between 1:1 and 19:1. Maintaining this mass ratio within this range is beneficial for the secondary battery to have a high energy density, and also helps to improve the structural stability of the negative electrode active material and enhance the cycle performance of the secondary battery. For example, the mass ratio of silicon to carbon in the negative electrode active material can be 1:1, 2:1, 4:1, 6:1, 8:1, 10:1, 12:1, 14:1, 15:1, 17:1, 19:1, or a value within a range of any two of these values.

[0088] In some embodiments, carbon nanotubes are disposed on at least a portion of the surface of the carbon coating layer through in-situ growth. This facilitates a tighter and more stable connection between the carbon nanotubes and the carbon coating layer, reduces physical contact failures in the conductive network caused by the expansion of the negative electrode active material, thereby improving the electronic conductivity and structural stability of the negative electrode active material, and enhancing the rate performance and cycle performance of the secondary battery.

[0089] In some embodiments, the binder content in the negative electrode film is less than or equal to 0.5%. This helps to reduce the obstruction of ion and / or electron transport by inactive materials in the negative electrode film, further improving the ion and / or electron transport efficiency in the negative electrode film, thereby improving the rate performance and initial coulombic efficiency of the secondary battery. Exemplarily, the binder content in the negative electrode film can be 0%, 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or a value within a range of any two of these values. In some optional embodiments, the binder content in the negative electrode film is 0%-0.1%. Further optionally, the binder content in the negative electrode film is 0%. This helps to further improve the ion and / or electron transport efficiency in the negative electrode film, thereby improving the rate performance and initial coulombic efficiency of the secondary battery.

[0090] In this disclosure, the binder content in the negative electrode film layer can be measured using methods conventional in the art. Since the binder material is a polymer with a low decomposition temperature, it decomposes and volatilizes during programmed temperature rise. Therefore, thermogravimetric analysis (TGA) can be used, exemplarily, to measure the binder content in the negative electrode film layer. As an example, the mass of the negative electrode film layer is measured as a function of temperature or time under programmed temperature control. By analyzing the thermogravimetric curve (TG curve) of the negative electrode film layer, the binder content in the negative electrode film layer can be determined.

[0091] In some embodiments, the carbon coating layer also includes nitrogen. Nitrogen has a slightly smaller atomic radius and greater electronegativity than carbon. Doping nitrogen into amorphous carbon and / or carbon nanotubes can improve the conductivity of amorphous carbon and carbon nanotubes, thereby improving the rate performance and first coulombic efficiency of the secondary battery.

[0092] In some implementations, the mass percentage of nitrogen in the negative electrode active material is less than or equal to 0.1%. This is beneficial for improving the conductivity of the negative electrode active material and enhancing the rate performance of the secondary battery.

[0093] In some embodiments, the silicon-based material includes one or more of elemental silicon, silicon-oxygen materials, silicon-carbon composite materials, and silicon alloy materials. Using the above-mentioned silicon materials as nano-silicon cores is beneficial for improving the energy density of secondary batteries.

[0094] In some embodiments, the volumetric particle size distribution (Dv50) of the negative electrode active material is 40 nm to 200 nm. By keeping the Dv50 of the negative electrode active material within this range, it is beneficial to reduce the volume expansion of the negative electrode active material during cycling, improve the structural stability of the negative electrode active material, and thus improve the cycle performance of the secondary battery. Exemplarily, the Dv50 of the negative electrode active material can be 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, or a value within a range of any two of these values. In some optional embodiments, the Dv50 of the negative electrode active material is 50 nm to 80 nm. This further improves the cycle performance of the secondary battery.

[0095] In some embodiments, the volume distribution particle size Dv90 of the negative electrode active material is 0.3 μm to 1 μm. By keeping the volume distribution particle size Dv90 of the negative electrode active material within this range, it is beneficial to reduce the problem of uneven stress caused by larger particles in the negative electrode active material, thereby improving the structural stability and cycle performance of the secondary battery. For example, the volume distribution particle size Dv90 of the negative electrode active material can be 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, or a value within a range of any two of these values. In some optional embodiments, the volume distribution particle size Dv90 of the negative electrode active material is 0.4 μm to 0.8 μm, and more preferably 0.5 μm.

[0096] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 7 to 10. By ensuring that the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is within this range, it reflects a uniform overall particle size distribution of the negative electrode active material. This helps reduce the problem of uneven stress caused by larger particles in the negative electrode active material, thereby improving the structural stability and cycle performance of the secondary battery. For example, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material can be 7, 7.5, 8, 8.5, 9, 9.5, 10, or a value within a range consisting of any two of these values.

[0097] In this disclosure, the volumetric distribution particle sizes Dv10, Dv50, and Dv90 represent the particle sizes corresponding to a cumulative volumetric distribution percentage of 10%, 50%, and 90%, respectively, and can be determined using instruments and methods known in the art. For example, they can be determined using a laser particle size analyzer, referring to GB / T19077-2016. The testing instrument can be the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK. An exemplary testing procedure is as follows: Take a clean beaker, weigh in an appropriate amount of the sample to be tested, add a surfactant, then add 20 ml of dispersant, and sonicate at 250 W / 3 min to ensure complete dispersion of the sample in the dispersant. After the dispersed sample is poured into the injection tower of the laser particle size analyzer, the particles circulate with the solution to the test optical path system. Under the irradiation of the laser beam (blocking degree: 25%-30%), the particle size distribution characteristics can be obtained by receiving and measuring the energy distribution of the scattered light.

[0098] In some embodiments, the specific surface area of ​​the negative electrode active material is 35 m². 2 / g~60m 2 / g. By ensuring the specific surface area of ​​the negative electrode active material is within the aforementioned range, it is beneficial for the negative electrode active material to possess an appropriate amount of pores, which can buffer the volume expansion of the negative electrode active material. On the one hand, this helps reduce the contact failure problem between the carbon coating layer and carbon nanotubes; on the other hand, it helps improve the structural stability of the negative electrode active material, thereby improving the rate performance and cycle performance of the secondary battery. For example, the specific surface area of ​​the negative electrode active material is 35m². 2 / g、40m 2 / g、45m 2 / g, 50m 2 / g、55m 2 / g、60m 2 / g or a value within a range of any two of these values. In some alternative embodiments, the specific surface area of ​​the negative electrode active material is 40m². 2 / g~50m 2 / g.

[0099] In this disclosure, the specific surface area of ​​the negative electrode active material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, Inc., USA.

[0100] In some embodiments, the electrode conductivity of the negative electrode is between 12.5 S / m and 67 S / m. By ensuring the powder conductivity of the negative electrode active material is within this range, it is beneficial to improve the electron transport efficiency of the negative electrode active material, thereby improving the rate performance and initial coulombic efficiency of the secondary battery. Specifically, the electrode conductivity of the negative electrode can be 12.5 S / m, 15 S / m, 17.5 S / m, 20 S / m, 21.2 S / m, 22 S / m, 22.2 S / m, 23.8 S / m, 25 S / m, 30 S / m, 35 S / m, 40 S / m, 45.5 S / m, 50 S / m, 55 S / m, 60 S / m, 66.7 S / m, or a value within a range of any two of these values.

[0101] In this disclosure, electrode conductivity refers to the electrode's ability to conduct current, and can be measured using conventional methods in the art. Exemplarily, it can be tested using a conductivity meter based on the four-probe testing principle, referring to standard GB / T1552-1995. The exemplary steps are as follows: Obtain the negative electrode to be tested (a sample can be taken during electrode preparation, or the negative electrode can be disassembled from a prepared secondary battery), compress its volume to a set pressure (e.g., 1 MPa) under hydraulic power at 25°C, measure the conductivity of the negative electrode online, and record the data.

[0102] In some embodiments, the thickness of the negative electrode film is 8 μm to 25 μm. Maintaining the thickness of the negative electrode film within this range is beneficial for achieving a high energy density in the negative electrode and also helps reduce lithium deposition and dendrite formation during secondary battery cycling. Exemplarily, the thickness of the negative electrode film can be 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 15 μm, 18 μm, 20 μm, 23 μm, 25 μm, or a value within a range of any two of these values.

[0103] In this disclosure, the thickness of the negative electrode film has a meaning known in the art and can be tested using methods known in the art. For example, after disassembling a secondary battery to obtain the negative electrode sheet, the thickness of the negative electrode film can be measured using a micrometer (e.g., a Mitutoyo 293-100 model with an accuracy of 0.1 μm). The thickness range given in this disclosure refers to the thickness range of the negative electrode film on one side of the negative electrode current collector.

[0104] In some embodiments, the coating weight of the negative electrode film is 1 mg / cm³. 2 ~5mg / cm 2 By keeping the coating weight of the negative electrode film within the aforementioned range, it is beneficial for the negative electrode sheet to have a high energy density. For example, the coating weight of the negative electrode film can be 1 mg / cm³. 2 2mg / cm 2 3mg / cm2 4mg / cm 2 5mg / cm 2 Or the value between any two of them within a range.

[0105] In some embodiments, the compaction density of the negative electrode film is 0.8 g / cm³. 3 ~1.2g / cm 3 By maintaining the compaction density of the negative electrode film within the aforementioned range, it facilitates better contact between the particles of the negative electrode active layer material, reducing porosity. This, in turn, improves both the conductivity and energy density of the negative electrode film. For example, the compaction density of the negative electrode film can be 0.8 g / cm³. 3 0.9g / cm 3 1.0g / cm 3 1.1g / cm 3 1.2g / cm 3 Or the value between any two of them within a range.

[0106] In this disclosure, the compaction density of the negative electrode film has a meaning known in the art and can be tested using methods known in the art. For example, after disassembling the battery as described above and obtaining the areal density and thickness of the negative electrode film, the compaction density of the negative electrode film is calculated using the following formula: Compaction density of the negative electrode film = Areal density of the negative electrode film / Thickness of the negative electrode film. Wherein, the areal density of the negative electrode film refers to the mass of the negative electrode film per unit area. It can be determined by the following method: After disassembling the secondary battery to obtain the negative electrode sheet, a certain area S (unit: cm²) is punched out. 2 Fifteen negative electrode sheets and 15 current collectors (from the same production batch as the negative electrode sheets) were weighed, and their average mass was calculated. The average mass of the negative electrode sheets is M1 (mg), and the average mass of the current collectors is M2 (mg). When the active material layer is only on one side of the current collector, the areal density is (M1-M2) / S. When the active material layer is on both sides of the current collector, the areal density is (M1-M2) / 2S.

[0107] Preparation method of negative electrode active material

[0108] This disclosure also provides a method for preparing a negative electrode active material, comprising the following steps:

[0109] A silicon core with a polymer film is mixed with an iron-based catalyst;

[0110] The temperature is increased to the first heat treatment temperature of 400℃-600℃ at a first heating rate of 5℃ / min-20℃ / min.

[0111] The material is kept at a temperature for 5-50 hours in the first heat treatment period, and then heated to a second heat treatment temperature of 700-800℃ at a second heating rate of 5℃ / min-20℃ / min, and kept at a temperature for 1-5 hours in the second heat treatment period to obtain the negative electrode active material.

[0112] This disclosure involves mixing the silicon core of a polymer film with an iron-based catalyst and then subjecting it to a first-stage heat treatment. This process causes the polymer film to gradually carbonize, forming a carbon coating layer, while simultaneously decomposing the iron-based catalyst to produce catalytically effective substances. A second-stage heat treatment at a higher temperature is then performed, allowing carbon nanotubes to grow in situ from the carbon coating layer formed by polymer carbonization under the influence of the catalytically effective substances. Simultaneously, the iron element from the iron-based catalyst is retained on the carbon coating layer.

[0113] In this disclosure, the first heating rate is 5°C / min to 20°C / min. This promotes a more uniform temperature within the reactants, reduces localized thermal stress concentration caused by excessively high local temperatures, and thus results in a more complete carbon coating structure formed by the carbonization of the polymer film, thereby improving the structural stability of the negative electrode active material. Exemplarily, the first heating rate can be 5°C / min, 8°C / min, 10°C / min, 12°C / min, 15°C / min, 18°C / min, 20°C / min, or any two of these values. The first heat treatment temperature is 400°C to 600°C. By keeping the first heat treatment temperature within this range, it is beneficial to achieve the carbonization reaction of the polymer film and the decomposition reaction of the iron-based catalyst. Exemplarily, the first heat treatment temperature can be 400°C, 450°C, 500°C, 550°C, 600°C, or any two of these values. The first time period is 5h to 50h. By keeping the first time period within the above range, it is beneficial to ensure a more complete reaction between the polymer film and the iron-based catalyst. For example, the first time period can be a value between 5h, 10h, 15h, 20h, 25h, 30h, 32h, 35h, 40h, 45h, 50h, or any two of these values.

[0114] The second-stage heat treatment is performed after the first-stage heat treatment. For example, the second heating rate is 5℃ / min-20℃ / min. This promotes a more uniform temperature within the reactants, reduces localized thermal stress concentration caused by excessively high local temperatures, and thus improves the structural stability of the negative electrode active material. On the other hand, it also improves the structural stability and uniformity of the in-situ grown carbon nanotubes. For example, the second heating rate can be 5℃ / min, 8℃ / min, 10℃ / min, 12℃ / min, 15℃ / min, 18℃ / min, 20℃ / min, or any two of these values. The second heat treatment temperature is 700℃-800℃. By keeping the second heat treatment temperature within the above range, carbon nanotubes are grown in situ from the carbon source in the carbon coating layer under the action of the catalyst. For example, the second heat treatment temperature can be 700℃, 720℃, 740℃, 760℃, 780℃, 800℃, or any two of these values. The second time period is 1h-5h, which helps to improve the structural stability and length of carbon nanotubes. For example, the second time period can be 1h, 1.5h, 2h, 2.8h, 3h, 3.5h, 4h, 4.3h, 4.5h, 5h, or any two of these values.

[0115] In some embodiments, the iron-based catalyst includes one or more of ferrocene, iron(III) oxide (Fe3O4), ferric nitrate, and elemental iron. These catalysts are beneficial for the in-situ growth of carbon nanotubes. In an optional embodiment, the iron-based catalyst includes ferrocene. Ferrocene decomposes at the first heat treatment temperature to produce carbon, metallic iron, and hydrogen. The combined effect of metallic iron and hydrogen helps improve the structural stability and uniformity of the in-situ grown carbon nanotubes. Furthermore, the metallic iron retained in the carbon coating layer improves the electronic conductivity of the negative electrode active material. Additionally, during lithium intercalation, the metallic iron and silicon form a silicon-iron alloy phase, which helps reduce the volume expansion caused by lithium intercalation in the silicon material. This, in turn, improves the cycle performance, rate performance, and initial coulombic efficiency of the secondary battery.

[0116] In some embodiments, the mass ratio of the silicon core with polymer film to the iron-based catalyst is 1:0.075 to 1:0.70. Maintaining this mass ratio within this range is beneficial for catalyzing the growth of carbon nanotubes on the carbon coating layer during subsequent heat treatment, and also for ensuring a suitable iron content in the prepared negative electrode active material. This improves the conductivity, rate performance, initial coulombic efficiency, and cycle performance of the secondary battery. Exemplarily, the mass ratio of the silicon core with polymer film to the iron-based catalyst can be 1:0.075, 1:0.14, 1:0.23, 1:0.29, 1:0.47, 1:0.67, 1:0.7, or a value within a range of any two of these values.

[0117] In some embodiments, the method for preparing a silicon core with a polymer film includes: adding the silicon core to a solution containing polymer monomers to carry out a polymerization reaction, thereby obtaining a silicon core with a polymer film. Through the interaction force between the polymer monomers and the surface of the silicon core, the formed polymer film is tightly bonded to the surface of the silicon core and has a high surface coverage. This improves the integrity of the subsequently prepared carbon coating layer, reduces the gap between the carbon coating layer and the silicon core, and enhances the conductivity of the negative electrode active material.

[0118] In some embodiments, the mass ratio of the silicon core to the polymer in the solution containing polymer monomers is 1:0.75 to 1:1.6. By maintaining this mass ratio within the aforementioned range, it is beneficial, on the one hand, to form a carbon coating layer encapsulating the silicon core during subsequent heat treatment, thereby increasing the coating efficiency of the silicon core. On the other hand, it is beneficial for the prepared negative electrode active material to have a suitable silicon-to-carbon ratio, thus contributing to a balance between energy density and cycle performance in the secondary battery. Exemplarily, the mass ratio of the silicon core to the polymer in the solution containing polymer monomers can be 1:0.75, 1:1.0, 1:1.25, 1:1.5, 1:1.6, or a value within a range of any two of these values.

[0119] In some embodiments, the concentration of the polymer monomer in the solution is 0.05 mol / L to 0.1 mol / L. Maintaining the polymer monomer concentration within this range facilitates uniform polymer dispersion, thereby promoting the formation of a uniform and structurally stable carbon coating layer on the silicon core surface. Exemplarily, the concentration of the polymer monomer in the solution can be 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.08 mol / L, 0.09 mol / L, 0.10 mol / L, or a value within a range of any two of these values.

[0120] In some embodiments, the polymerization reaction includes reacting at 20°C to 150°C for 24 to 36 hours. Under these conditions, it is beneficial for the polymer monomers to polymerize and form a polymer film.

[0121] In some embodiments, the polymer monomer includes one or more of dopamine, resorcinol-formaldehyde, glucose, sucrose, vinylpyrrolidone, and tannic acid. These polymer monomers can polymerize on the silicon core surface to form a polymer film and maintain close contact with the silicon core surface. In some alternative embodiments, the polymer monomer includes dopamine.

[0122] [positive electrode]

[0123] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode active material of the first aspect of the present disclosure.

[0124] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0125] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0126] In some embodiments, when the secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this disclosure is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of the following: O2), lithium nickel cobalt manganese aluminum oxide and its modified compounds. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0127] In some embodiments, the positive electrode active material includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, and modified compounds thereof.

[0128] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0129] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0130] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0131] [Electrolytes]

[0132] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This disclosure does not specify any particular type of electrolyte; it can be selected according to requirements.

[0133] In some embodiments, the electrolyte is a solid electrolyte, which includes an electrolyte material.

[0134] In some embodiments, the electrolyte material includes a sulfide electrolyte material. In some embodiments, the sulfide electrolyte material includes one or more of the following: lithium phosphorus-containing sulfide electrolyte material, lithium chloride-containing sulfide electrolyte material, lithium bromide-containing sulfide electrolyte material, lithium fluorine-containing sulfide electrolyte material, lithium boron-containing sulfide electrolyte material, lithium silicon-containing sulfide electrolyte material, lithium germanium-containing sulfide electrolyte material, lithium gallium-containing sulfide electrolyte material, lithium zinc-containing sulfide electrolyte material, lithium indium-containing sulfide electrolyte material, lithium aluminum-containing sulfide electrolyte material, and lithium tin-containing sulfide electrolyte material. Exemplarily, the above-mentioned sulfide electrolyte material may include one or more of lithium phosphorus sulfide, lithium germanium phosphorus sulfide, lithium tin phosphorus sulfide, lithium phosphorus sulfide chloride, lithium phosphorus sulfide bromide, and lithium silicon phosphorus sulfide chloride. Using the above-mentioned electrolyte material can improve the conductivity of the solid electrolyte film, thereby benefiting the initial coulombic efficiency and cycle performance of the secondary battery.

[0135] In some embodiments, the solid electrolyte further includes a binder. The binder includes one or more of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), or polyethylene (PE).

[0136] In some embodiments, the electrolyte may optionally include additives. For example, additives may include those that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0137] [Isolation membrane]

[0138] In some embodiments, the secondary battery also includes a separator. This disclosure does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0139] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0140] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0141] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0142] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0143] This disclosure does not impose any particular limitation on the shape of the secondary battery, which can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square secondary battery 5 as an example.

[0144] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0145] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0146] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 5 can be fixed in place using fasteners.

[0147] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0148] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0149] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0150] Electrical appliances

[0151] In addition, this disclosure also provides an electrical device, which includes a secondary battery provided by this disclosure. The secondary battery can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0152] As an electrical device, a rechargeable battery can be selected based on its usage requirements.

[0153] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0154] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0155] Methods for preparing secondary batteries

[0156] This disclosure also provides a negative electrode slurry comprising the aforementioned negative electrode active material or a negative electrode active material prepared according to the aforementioned method for preparing the negative electrode active material, a binder, and a solvent; the negative electrode slurry is coated onto a negative electrode current collector to form a negative electrode film layer. Thus, the prepared secondary battery exhibits improved rate performance, initial coulombic efficiency, and cycle performance.

[0157] In some embodiments, coating the negative electrode slurry onto the negative electrode current collector to form a negative electrode film layer includes: coating the negative electrode slurry onto the negative electrode current collector, and then holding it at 180°C to 250°C for 15 min to 30 min under an argon atmosphere to form a negative electrode film layer, wherein the binder content in the negative electrode film layer is less than or equal to 0.5 wt%.

[0158] A negative electrode slurry is prepared by adding a binder and solvent to the negative electrode active material. This slurry is then coated onto a negative electrode current collector to form a wet film, allowing the negative electrode active material to adhere and solidify on the current collector. The negative electrode sheet coated with the wet film is then heat-treated to evaporate the solvent and decompose the binder into volatile small molecules, which also evaporate during heating. The solvent and binder decomposition products are removed by purging under an argon atmosphere, thus preparing a negative electrode sheet that is virtually binder-free. The resulting negative electrode sheet exhibits excellent rate performance and initial coulombic efficiency.

[0159] In some embodiments, the binder includes one or more of polypropylene carbonate and polylactic acid. The binder decomposes into volatile small molecule products, such as carbon dioxide, carbon monoxide, and methanol, which are removed by purging under an argon atmosphere. This helps to reduce the binder content in the negative electrode film layer, thereby improving the rate performance and initial coulombic efficiency of the secondary battery. In some alternative embodiments, the binder includes polypropylene carbonate.

[0160] In some embodiments, the solvent includes one or more of anisole, acetone, tetrahydrofuran, dimethylformamide, and toluene. These solvents are volatile and easily removed.

[0161] Example

[0162] The following describes embodiments of this disclosure. The embodiments described below are exemplary and are only used to explain this disclosure, and should not be construed as limiting this disclosure. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0163] Example 1

[0164] Preparation of negative electrode active materials:

[0165] Step S1: 1g of 50nm nano-silicon nanoparticles are ultrasonically dispersed in 50mL of ethanol aqueous solution (ethanol to water volume ratio of 97:3), and 100mL of ammonia aqueous solution containing dopamine (PDA) (ammonia concentration of 0.5mol / L; PDA concentration of 0.1mol / L) is added. The mixture is stirred in an oil bath at 52℃ for 24 hours. After centrifugation, washing, and drying, polydopamine-coated nano-silicon sub-silicon is formed.

[0166] In step S2, the above-mentioned polydopamine-coated nano-silicon and 0.75g of ferrocene (mass ratio of approximately 2.53:0.75) are mixed evenly, heated to 550℃ at a rate of 5℃ / min and held for 30h, and then heated to 700℃ at a rate of 5℃ / min and held for 3h for carbonization to obtain the negative electrode active material.

[0167] Preparation of negative electrode sheet

[0168] The above-mentioned negative electrode active material, binder polypropylene carbonate (PPC), and solvent anisole were mixed by ball milling at a mass ratio of 1:0.05:5 for 1 hour at a ball milling rate of 350 rpm to obtain a negative electrode slurry. The negative electrode slurry was coated onto copper foil and then placed in a vacuum drying oven at 225°C under an argon atmosphere for 15 minutes to obtain the negative electrode sheet. The coating weight of the negative electrode sheet was 1 mg / cm³. 2 The thickness of the negative electrode film is 20 μm.

[0169] Parameter testing of negative electrode active materials

[0170] (1) Appearance

[0171] The morphology of the negative electrode active material was tested using scanning electron microscopy. The scanning electron micrographs of the polydopamine-coated silicon nanoparticles prepared in Example 1 and the final prepared negative electrode active material are shown in Figures 7 and 8, respectively. As can be seen from the figures, the silicon nanoparticles are encapsulated by a carbon coating layer, and carbon nanotubes are grown in situ on the surface of the carbon coating layer.

[0172] (2) XRD test

[0173] Phase analysis of the negative electrode active material was performed using X-ray diffraction (XRD). The XRD pattern of the negative electrode active material prepared in Example 1 is shown in Figure 9. As can be seen from Figure 9, silicon nuclei and iron elements form a silicon-iron alloy phase in the negative electrode active material.

[0174] (3) Aspect ratio test of carbon nanotubes in negative electrode active materials

[0175] Using a scanning electron microscope (e.g., ZEISS Sigma 300), refer to JY / T010-1996 to obtain scanning electron microscope (SEM) images of the negative electrode sheet. Specifically, arbitrarily select a test sample with a length × width of 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5) in the test sample, and at a certain magnification, read the length (i.e., the distance between the two farthest points on the carbon nanotube) and diameter in each test area, calculate the aspect ratio, and count the aspect ratio values ​​of the carbon nanotubes in each test area. Take the arithmetic mean of the aspect ratios of the carbon nanotubes in each test area, which is the average aspect ratio of the carbon nanotubes in the test sample.

[0176] (4) Mass percentage of iron in negative electrode active material

[0177] The mass percentage of iron in the negative electrode active material was determined using inductively coupled plasma atomic emission spectrometry (ICP).

[0178] Parameter testing of negative electrode sheet

[0179] (1) Electrode conductivity

[0180] Based on the four-probe testing principle and referring to standard GB / T1552-1995, the conductivity of the negative electrode sheet was tested using a powder conductivity meter (model FT-8100). The prepared negative electrode sheet was compressed to a set pressure value or intensity under hydraulic power, and the resistance, resistivity, and conductivity of the solid powder of the negative electrode active material were measured online and the data were recorded.

[0181] (2) The content of binder in the negative electrode film layer

[0182] Thermogravimetric analysis (TGA) was used to measure the binder content in the negative electrode film.

[0183] Preparation of positive electrode sheet

[0184] The positive electrode active material (NMC811), conductive agent (conductive carbon fiber VGCF), solid electrolyte (sulfide Li6PS5Cl), and binder (polytetrafluoroethylene, PTFE) were mixed in a mass ratio of 70:20:5:5, and the positive electrode sheet was prepared by continuous rolling. The rolling temperature was 80℃, and the thickness of the positive electrode sheet was 50μm.

[0185] Preparation of solid electrolyte layer

[0186] The electrolyte (Li6PS5Cl) and binder (polytetrafluoroethylene) were mixed at a mass ratio of 99:1, ground and dispersed evenly, and pressed at 10MPa for 60 seconds to obtain a solid electrolyte layer with a thickness of about 50μm.

[0187] Battery pack assembly

[0188] The cells are assembled in the order of positive electrode-solid electrolyte layer-negative electrode stacking, and after high-temperature densification treatment, a soft-pack battery is obtained.

[0189] Examples 2-5

[0190] The negative electrode active material was prepared using a method similar to that in Example 1. The difference was that the amount of ferrocene added was adjusted so that the iron content in the negative electrode active material had the values ​​shown in Table 1. The prepared negative electrode active material was then used to prepare a secondary battery using the same method as in Example 1.

[0191] Example 6

[0192] The negative electrode active material was prepared using the same method as in Example 1. The negative electrode active material, binder PVDF, and solvent anisole were mixed by ball milling at a mass ratio of 1:0.05:5 for 1 hour at a ball milling rate of 350 rpm to obtain a negative electrode slurry. The negative electrode slurry was coated onto copper foil, and after cold pressing and drying, a negative electrode sheet was obtained. The coating weight of the negative electrode sheet was 1 mg / cm³. 2 The thickness of the negative electrode film is 20 μm.

[0193] Comparative Example 1

[0194] The negative electrode active material was prepared using a method similar to that in Example 1, except that ferrocene was not added in step S2 of the preparation of the negative electrode active material.

[0195] The prepared negative electrode active material was used to prepare a secondary battery in the same manner as in Example 1.

[0196] Comparative Example 2

[0197] The negative electrode active material was prepared in a similar manner to that in Example 1, except that in step S2 of preparing the negative electrode active material, the temperature was only increased to 550°C at a rate of 5°C / min and held for 30 hours, and the second stage of heat treatment was not performed.

[0198] The prepared negative electrode active material was used to prepare a secondary battery in the same manner as in Example 1.

[0199] Performance testing of secondary batteries

[0200] (1) Initial Coulomb efficiency

[0201] The following test steps were performed on the secondary battery in Example 1 at 25°C:

[0202] ① Let stand for 5 minutes;

[0203] ② Charge at a constant current of 0.1C to 3.68V, then charge at a constant voltage of 3.68V to a current of 0.05C, and record the charging capacity C1 of the first cycle;

[0204] ③ Let stand for 5 minutes;

[0205] ④ Discharge at a constant current of 0.33C to 1.98V, and record the discharge capacity D1 of the first cycle;

[0206] First Coulomb efficiency (%) = first discharge capacity D1 / first charge capacity C1 * 100%.

[0207] (2) Ratio performance

[0208] ① Let stand for 5 minutes;

[0209] ② Charge at a constant current of 0.1C to 3.68V, then charge at a constant voltage of 3.68V to a current of 0.05C;

[0210] ③ Let stand for 5 minutes;

[0211] ④ Discharge at a constant current of 2C to 1.98V and record the discharge capacity.

[0212] (3) Capacity retention rate

[0213] The following test steps were performed on the secondary battery in Example 1 at 25°C:

[0214] ① Let stand for 5 minutes;

[0215] ② Charge at a constant current of 0.1C to 3.68V, then charge at a constant voltage of 3.68V to a current of 0.05C;

[0216] ③ Let stand for 5 minutes;

[0217] ④ Discharge at a constant current of 0.33C to 1.98V, and record the discharge capacity D1 of the first cycle;

[0218] ⑤ Repeat steps ① to ④ above 100 times, and record the discharge capacity Dn on the 100th cycle;

[0219] Capacity retention rate after 100 cycles (%) = Discharge capacity Dn of the 100th cycle / Discharge capacity D1 of the first cycle * 100%. The test results are shown in Table 2.

[0220] Table 1: Parameter test results of negative electrode active materials and negative electrode films in Examples 1-6 and Comparative Examples 1-3

[0221] Table 2: Performance test results of secondary batteries in Examples 1-6 and Comparative Examples 1-2

[0222] Based on the above results, compared to Comparative Example 1, Examples 1-6 improved the initial coulombic efficiency, rate performance, and cycle performance of the secondary battery by keeping the iron content in the negative electrode active material within the range of 5%-20%. Compared to Comparative Example 2, Examples 1-6 improved the initial coulombic efficiency, rate performance, and cycle performance of the secondary battery by enabling in-situ growth of carbon nanotubes in the negative electrode active material through a second-stage heat treatment. Thermogravimetric analysis showed that the mass percentage of binder in the negative electrode film layer of Examples 1-5 was less than 0.5%, while in Example 6, since no heat treatment was performed during the preparation of the negative electrode sheet, the binder was essentially retained in the negative electrode film layer. Compared to Example 6, Examples 1-5 further improved the conductivity of the electrode sheet by keeping the binder percentage in the negative electrode film layer to less than 0.5%, thereby further improving the initial coulombic efficiency, rate performance, and cycle performance of the secondary battery.

[0223] Examples 7-11

[0224] Examples 7-11 prepared negative electrode active materials using a method similar to that of Example 1, and used the prepared negative electrode active materials to prepare secondary batteries using the same method as in Example 1. The difference was that the method for preparing the negative electrode active materials was adjusted according to the parameters shown in Table 3 below.

[0225] Table 3: Preparation parameters and test results of negative electrode active material and negative electrode film in Examples 7-11

[0226] The secondary batteries prepared in Examples 7-11 were tested according to the battery performance test methods described above, and the results are shown in Table 4.

[0227] Table 4: Performance test results of the secondary batteries in Examples 7-11

[0228] Based on the above results, it can be seen that Examples 7-11 improved the initial coulombic efficiency, rate performance, and cycle performance of the secondary battery by making the aspect ratio of carbon nanotubes in the negative electrode active material range from 200 to 1200.

[0229] Examples 12-14

[0230] Examples 12-14 prepared negative electrode active materials using a method similar to that of Example 1, and used the prepared negative electrode active materials to prepare secondary batteries using the same method as in Example 1. The difference was that the method for preparing the negative electrode active materials was adjusted according to the parameters shown in Table 5 below.

[0231] Table 5

[0232] The secondary batteries prepared in Examples 12-14 were tested according to the battery performance test methods described above, and the results are shown in Table 6.

[0233] Table 6: Performance test results of the secondary batteries in Examples 12-14

[0234] As can be seen from the above results, the negative electrode active materials prepared by the polymer and iron-based catalysts disclosed in Examples 12-14 have good initial coulombic efficiency, rate performance and cycle performance.

[0235] It should be noted that this disclosure is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same essential structure and achieving the same effect as the technical concept within the scope of this disclosure are included in the technical scope of this disclosure. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, are also included in the scope of this disclosure without departing from the spirit of this disclosure.

Claims

1. A secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, The negative electrode film layer includes a negative electrode active material, the negative electrode active material includes a core structure and a carbon coating layer disposed on at least a portion of the surface of the core structure, the core structure includes a silicon-based material, the carbon coating layer contains iron, and carbon nanotubes are disposed on at least a portion of the surface of the carbon coating layer.

2. The secondary battery according to claim 1, wherein, The iron element accounts for 5%-20% of the mass of the negative electrode active material.

3. The secondary battery according to claim 2, wherein, The iron element accounts for 10%-20% of the mass of the negative electrode active material.

4. The secondary battery according to any one of claims 1 to 3, wherein, The iron element includes 0-valent iron and / or +2-valent iron.

5. The secondary battery according to any one of claims 1 to 4, wherein, It meets one or more of the following characteristics: (1) The average aspect ratio of the carbon nanotubes is 200-1200; and / or, (2) The average length of the carbon nanotubes is 1 μm-6 μm; and / or (3) The average diameter of the carbon nanotubes is 5nm-50nm.

6. The secondary battery according to claim 5, wherein, It meets one or more of the following characteristics: (1) The average aspect ratio of the carbon nanotubes is 900-1100; and / or, (2) The average length of the carbon nanotubes is 3μm-5μm; and / or (3) The average diameter of the carbon nanotubes is 5nm-20nm.

7. The secondary battery according to any one of claims 1 to 6, wherein, The mass percentage of carbon in the negative electrode active material is 5%-50%; and / or, In the negative electrode active material, the mass ratio of silicon to carbon is 1:1 to 19:

1.

8. The secondary battery according to any one of claims 1 to 7, wherein, The carbon nanotubes are disposed on at least a portion of the surface of the carbon coating layer by in-situ growth.

9. The secondary battery according to any one of claims 1 to 8, wherein, The binder content in the negative electrode film layer is less than or equal to 0.5%.

10. The secondary battery according to claim 9, wherein, The binder content in the negative electrode film layer is 0%-0.1%.

11. The secondary battery according to any one of claims 1 to 10, wherein, The carbon coating also includes nitrogen.

12. The secondary battery according to claim 11, wherein, The nitrogen element accounts for less than or equal to 0.1% of the mass of the negative electrode active material.

13. The secondary battery according to any one of claims 1 to 12, wherein, The silicon-based materials include one or more of elemental silicon, silicon-oxygen materials, silicon-carbon composite materials, and silicon alloy materials.

14. The secondary battery according to any one of claims 1 to 13, wherein, The negative electrode active material also satisfies one or more of the following characteristics: (1) The volume distribution particle size Dv50 of the negative electrode active material is 40nm to 200nm; (2) The volume distribution particle size Dv90 of the negative electrode active material is 0.3 μm to 1 μm; (3) The particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material is 7-10; (4) The specific surface area of ​​the negative electrode active material is 35m². 2 / g~60m 2 / g.

15. The secondary battery according to any one of claims 1 to 14, wherein, The negative electrode has a conductivity of 12.5 S / m to 67 S / m.

16. The secondary battery according to any one of claims 1 to 15, wherein, It meets one or more of the following characteristics: (1) The thickness of the negative electrode film is 8 μm to 25 μm; and / or, (2) The coating weight of the negative electrode film is 1 mg / cm³. 2 ~5mg / cm 2 ; and / or, (3) The compaction density of the negative electrode film is 0.8 g / cm³. 3 ~1.2g / cm 3 .

17. The secondary battery according to any one of claims 1 to 16, wherein, The secondary battery includes an electrolyte, which includes at least one of a solid electrolyte, a semi-solid electrolyte, and a gel electrolyte.

18. The secondary battery according to claim 17, wherein, The electrolyte includes sulfide electrolyte materials.

19. The secondary battery according to any one of claims 1 to 18, wherein, The secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer includes a positive electrode active material, the positive electrode active material includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide and their modified compounds.

20. An electrical device comprising a secondary battery as described in any one of claims 1 to 19.

21. A negative electrode active material, the negative electrode active material comprising a core structure and a carbon coating layer disposed on at least a portion of the surface of the core structure, the core structure comprising a silicon-based material, the carbon coating layer containing iron, and carbon nanotubes disposed on at least a portion of the surface of the carbon coating layer.

22. The negative electrode active material according to claim 21, wherein, It meets one or more of the following characteristics: (1) The iron element accounts for 5%-20% of the mass of the negative electrode active material; and / or (2) The iron element includes 0-valent iron and / or +2-valent iron.

23. The negative electrode active material according to claim 21 or 22, wherein, It meets one or more of the following characteristics: (1) The average aspect ratio of the carbon nanotubes is 200-1200; and / or (2) The average length of the carbon nanotubes is 1 μm-6 μm; and / or (3) The average diameter of the carbon nanotubes is 5nm-50nm.

24. The negative electrode active material according to any one of claims 21 to 23, wherein, The mass percentage of carbon in the negative electrode active material is 5%-50%; and / or, In the negative electrode active material, the mass ratio of silicon to carbon is 1:1 to 19:

1.

25. The negative electrode active material according to any one of claims 21 to 24, wherein the carbon nanotubes are disposed on at least a portion of the surface of the carbon coating layer by in-situ growth.

26. A method for preparing a negative electrode active material, comprising the following steps: A silicon core with a polymer film is mixed with an iron-based catalyst; The temperature is increased to the first heat treatment temperature of 400℃-600℃ at a first heating rate of 5℃ / min-20℃ / min. The material is kept at a temperature for 5-50 hours in the first heat treatment period, and then heated to a second heat treatment temperature of 700-800℃ at a second heating rate of 5℃ / min-20℃ / min, and kept at a temperature for 1-5 hours in the second heat treatment period to obtain the negative electrode active material.

27. The preparation method according to claim 26, wherein, The iron-based catalyst includes one or more of ferrocene, iron(II) oxide, ferric nitrate, and elemental iron.

28. The preparation method according to claim 26 or 27, wherein, The mass ratio of the silicon core with polymer film to the iron-based catalyst is 1:0.075 to 1:0.

70.

29. The preparation method according to any one of claims 26 to 28, wherein, The method for preparing the silicon core with a polymer film includes: adding the silicon core to a solution containing polymer monomers to carry out a polymerization reaction, thereby obtaining the silicon core with a polymer film.

30. The preparation method according to claim 29, wherein, The mass ratio of the silicon core to the polymer in the solution containing polymer monomers is 1:0.75 to 1:1.

6.

31. The preparation method according to claim 29 or 30, wherein, The concentration of the polymer monomer in the solution is 0.05 mol / L to 0.1 mol / L.

32. The preparation method according to any one of claims 29 to 31, wherein, The polymerization reaction includes a reaction at 20℃ to 150℃ for 24h to 36h.

33. The preparation method according to any one of claims 29 to 32, wherein, The polymer monomers include one or more of dopamine, resorcinol-formaldehyde, glucose, sucrose, vinylpyrrolidone, and tannic acid.

34. A method for preparing a secondary battery, comprising the following steps to prepare a negative electrode sheet: A negative electrode slurry is provided, the negative electrode slurry comprising a negative electrode active material, a binder, and a solvent prepared by the method of any one of claims 26 to 33; The negative electrode slurry is coated onto the negative electrode current collector to form a negative electrode film layer.

35. The preparation method according to claim 34, wherein, The step of coating the negative electrode slurry onto the negative electrode current collector to form a negative electrode film includes: The negative electrode slurry is coated onto the negative electrode current collector, and then held at 180℃~250℃ for 15min~30min under an argon atmosphere to form a negative electrode film layer, wherein the binder content in the negative electrode film layer is less than or equal to 0.5wt%.

36. The preparation method according to claim 34 or 35, wherein, The adhesive includes one or more of polypropylene carbonate and polylactic acid.

37. The preparation method according to claims 34 to 36, wherein, The solvent includes one or more of anisole, acetone, tetrahydrofuran, dimethylformamide, and toluene.

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