Secondary battery, method for manufacturing the same, energy storage system, and electric device

Abstract

The embodiment of the present application relates to the field of energy storage, and provides a secondary battery and a preparation method thereof, an energy storage system and an electric equipment. The negative electrode active material comprises a silicon-based material, a honeycomb-shaped porous graphite and a polypyrrole layer, the polypyrrole layer is coated on the surface of the honeycomb-shaped porous graphite, the silicon-based material is embedded in the pores of the honeycomb-shaped porous graphite, the silicon-based material comprises silicon particles, a poly-N-isopropyl acrylamide layer and a disulfide bond-containing polyaniline layer, the poly-N-isopropyl acrylamide layer and the disulfide bond-containing polyaniline layer are coated on the surface of the silicon particles, and the poly-N-isopropyl acrylamide layer is located between the silicon particles and the disulfide bond-containing polyaniline layer, and the honeycomb-shaped porous graphite comprises graphite sheets and a carbon layer with a honeycomb-shaped pore structure on the surface of the graphite sheets. The secondary battery formed by assembling the negative electrode active material has high cycle stability, high-temperature resistance, rate performance, specific capacity and safety.

Description

Technical Field

[0001] This application relates to the field of energy storage, and in particular to a secondary battery and its preparation method, energy storage system and electrical equipment. Background Technology

[0002] Currently, the cycle life, specific capacity of graphite materials, and electrode expansion properties of batteries can meet the requirements of conventional batteries. However, under certain extreme environments or when there are higher requirements for cycle life and capacity, the manufacturing process of batteries can have a certain impact on these functions.

[0003] The key factors for battery cycle life, cell capacity, and electrode expansion are the manufacturing process of the electrodes and their composition and structure. Summary of the Invention

[0004] The main objective of this invention is to provide a secondary battery, its preparation method, energy storage system, and electrical equipment, in order to solve the problems of low cycle life and specific capacity of batteries in the prior art.

[0005] To achieve the above objectives, according to one aspect of the present invention, a secondary battery is provided, comprising a positive electrode, a separator, a negative electrode, and an electrolyte. The negative electrode includes a current collector and a negative electrode active layer. The material of the negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent. The negative electrode active material includes a silicon-based material, honeycomb porous graphite, and a polypyrrole layer. The polypyrrole layer is coated on the surface of the honeycomb porous graphite. The silicon-based material is embedded in the pores of the honeycomb porous graphite. The silicon-based material includes silicon particles, a poly(N-isopropylacrylamide) layer, and a polyaniline layer containing disulfide bonds. The poly(N-isopropylacrylamide) layer and the polyaniline layer containing disulfide bonds are coated on the surface of the silicon particles, and the poly(N-isopropylacrylamide) layer is located between the silicon particles and the polyaniline layer containing disulfide bonds. The honeycomb porous graphite includes a graphite sheet and a carbon layer with a honeycomb porous structure located on the surface of the graphite sheet.

[0006] Furthermore, the average particle size of the silicon particles is 50–100 nm; and / or the average thickness of the poly-N-isopropylacrylamide layer is 45–55 nm; and / or the average thickness of the disulfide-bonded polyaniline layer is 27–33 nm.

[0007] Furthermore, the number average molecular weight of the polyN-isopropylacrylamide in the above-mentioned polyN-isopropylacrylamide layer is 8000 to 20000 Da; and / or, the number average molecular weight of the disulfide-bonded polyaniline in the disulfide-bonded polyaniline layer is 10000 to 30000 Da.

[0008] Furthermore, the disulfide bond density in the aforementioned disulfide-bonded polyaniline layer is 2 to 3 disulfide bonds per molecular chain; and / or, the porosity of the honeycomb porous graphite is 50 to 65%; and / or, the average pore size of the honeycomb porous graphite is 150 to 300 nm; and / or, the average thickness of the carbon layer is 1 to 5 μm; and / or, the average thickness of the graphite sheet is 100 to 200 μm, and the average sheet diameter is 10 to 50 μm.

[0009] Furthermore, the mass ratio of the aforementioned silicon-based material to honeycomb porous graphite is (94–95):(2–2.5); and / or, the average thickness of the polypyrrole layer is 50–150 nm; and / or, the number-average molecular weight of the polypyrrole in the polypyrrole layer is 15,000–35,000 Da; and / or, the mass ratio of the negative electrode active material, binder, and conductive agent is (96–97.5):(0.8–1.2):(2.4–3.2).

[0010] According to another aspect of the present invention, a method for preparing a secondary battery is provided, comprising: sequentially mixing, homogenizing, coating, and drying a negative electrode active material, a conductive agent, a binder, and a solvent to obtain a positive electrode sheet; fabricating a bare battery cell from the positive electrode sheet, a separator, and a negative electrode sheet; assembling the bare battery cell with a casing and a top cover; and injecting an electrolyte to obtain a secondary battery; the preparation steps of the negative electrode active material include: S1, mixing silicon particles, N-isopropylacrylamide, a first crosslinking agent, a first initiator, and a first solvent and performing a first polymerization reaction to obtain a poly(N-isopropylacrylamide) layer coated with silicon particles; S2, coating the silicon particles, aniline, a second crosslinking agent containing disulfide bonds, a second initiator, and a third... The two solvents are mixed and a second polymerization reaction is carried out to form a polyaniline layer containing disulfide bonds coated on the surface of the poly-N-isopropylacrylamide layer, thereby obtaining a silicon-based material; S3, graphite powder is pressed into graphite sheets, polystyrene particles are sprayed onto the surface of the graphite sheets and then calcined to obtain honeycomb porous graphite; S4, the silicon-based material is dispersed in a third solvent to obtain a suspension, and the honeycomb porous graphite is immersed in the suspension to obtain honeycomb porous graphite with silicon-based material embedded in the pores; S5, the honeycomb porous graphite with silicon-based material embedded in the pores is placed in a pyrrole solution for an electrochemical polymerization reaction to form a polypyrrole layer coated on the surface of the honeycomb porous graphite, thereby obtaining a negative electrode active material.

[0011] Further, the mass ratio of the aforementioned silicon particles, N-isopropylacrylamide, first crosslinking agent, and first initiator is (5–10):(1–2):(0.02–0.05):(0.05–0.1); and / or, the mass ratio of the poly(N-isopropylacrylamide) layer coating silicon particles, aniline, second crosslinking agent, and second initiator is (2–5):(1–1.2):(0.15–0.2):(0.4–0.5); and / or, the average thickness of the graphite sheet is 100–200 μm, and the average sheet diameter is 10–50 μm; and / or, the sprayed surface density of the polystyrene particles is 0.1–0.3 g / cm³. 2 ; and / or, the mass ratio of silicon-based material to the volume of the third solvent is (1-2 g):(10-50 mL); and / or, the mass ratio of honeycomb porous graphite to the volume of the suspension is (1.5-2.5 g):(5-20 mL); and / or, the mass ratio of honeycomb porous graphite with silicon-based material embedded in the pores to the volume of the pyrrole solution is (2.5-4.5 g):(100-150 mL).

[0012] Further, the temperature of the first polymerization reaction is 70–90°C; and / or the time of the first polymerization reaction is 6–12 h; and / or the temperature of the second polymerization reaction is 0–5°C; and / or the time of the second polymerization reaction is 12–24 h; and / or the temperature of the calcination treatment is 400–600°C; and / or the time of the calcination treatment is 1–3 h; and / or the impregnation treatment is carried out under vacuum conditions; and / or the temperature of the impregnation treatment is 25–60°C; and / or the time of the impregnation treatment is 30–120 min; and / or the current density of the electrochemical polymerization reaction is 0.1–1 mA / cm². 2 ; and / or, the temperature of the electrochemical polymerization reaction is 25–40 °C; and / or, the time of the electrochemical polymerization reaction is 1–5 h.

[0013] Further, the average particle size of the silicon particles is 50–100 nm; and / or, the average particle size of the polystyrene particles is 150–300 nm; and / or, the number-average molecular weight of the polystyrene particles is 50,000–60,000 Da; and / or, the first crosslinking agent is N,N'-methylenebisacrylamide and / or polyethylene glycol diacrylate; and / or, the first initiator is ammonium persulfate and / or azobisisobutyramidine hydrochloride; and / or, the first solvent is selected as water and / or ethyl acetate. Alcohol; and / or, the second crosslinking agent is dibenzoic acid disulfide; and / or, the second initiator is ammonium persulfate and / or ferric chloride; and / or, the second solvent is an aqueous HCl solution; and / or, the third solvent is N-methylpyrrolidone and / or dimethyl sulfoxide; and / or, the components of the pyrrole solution include pyrrole, water, oxidant and dopant, and the mass ratio of pyrrole, water, oxidant and dopant is (1-3):(10-12):(0.5-0.7):(0.2-0.4).

[0014] Further, the oxidant is selected from any one or more of FeCl3, ammonium persulfate and sodium persulfate; and / or, the dopant is selected from any one or more of sodium p-toluenesulfonate, sodium dodecylbenzenesulfonate and naphthalenesulfonic acid; and / or, the mass ratio of the negative electrode active material, binder and conductive agent is (96-97.5):(0.8-1.2):(2.4-3.2).

[0015] According to another aspect of the present invention, an energy storage system is provided, comprising a plurality of unit cells, wherein the unit cells are the aforementioned secondary cells or secondary cells prepared by the aforementioned method for preparing secondary cells.

[0016] According to another aspect of the present invention, an electrical device is provided, including the aforementioned energy storage system, the energy storage system being used to provide power to the electrical device.

[0017] The technical solution provided in this application has at least the following advantages:

[0018] In this application, a polypyrrole layer is coated on the surface of honeycomb porous graphite, which helps to suppress silicon particle expansion and improve the conductivity of the negative electrode active material. The silicon-based material is embedded in the pores of the honeycomb porous graphite. The presence of these pores helps to disperse stress through pore wall deformation during charging and discharging, reducing cracking of the graphite layer due to silicon expansion. The swelling degree of the polyN-isopropylacrylamide layer coating the silicon particle surface varies with temperature. At high temperatures, the polyN-isopropylacrylamide layer shrinks, helping to suppress the volume expansion of silicon particles; at low temperatures, the polyN-isopropylacrylamide layer swells, helping to enhance lithium-ion diffusion. The disulfide-bonded polyaniline layer achieves self-repair of cracks during charging and discharging through disulfide bond breaking / recombining, helping to maintain the integrity of the conductive network. This application combines silicon and graphite, which helps to improve the specific capacity of the negative electrode active material. Therefore, the negative electrode sheet prepared using the negative electrode active material of this application has high adhesion, high temperature stability, and low expansion rate. The secondary battery assembled with it has high cycle stability, high temperature resistance, rate performance, specific capacity, and safety. Attached Figure Description

[0019] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Unless otherwise stated, the drawings in the accompanying drawings do not constitute a limitation on scale. In order to more clearly illustrate the technical solutions in the embodiments of this application or in the conventional art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A schematic diagram of the honeycomb porous graphite structure in which silicon-based material is embedded in the pores is shown in this application.

[0021] Figure 2 A process flow diagram for preparing secondary batteries according to this application is shown.

[0022] The above figures include the following reference numerals:

[0023] 1. Silicon-based material; 2. Graphite sheet; 3. Carbon layer. Detailed Implementation

[0024] As analyzed in the background section of this application, the existing technology has problems with low cycle life and specific capacity of batteries. In order to solve this problem, this application provides a secondary battery and its preparation method, energy storage system and electrical device.

[0025] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0026] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0027] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists, A and B exist simultaneously, and B exists. In addition, the character " / " in this document generally indicates that the related objects before and after it have an "or" relationship.

[0028] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0029] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0030] In the description of the embodiments of this application, unless otherwise expressly specified and limited, the technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0031] In the accompanying drawings corresponding to the embodiments of this application, the thickness and area of ​​the layers are enlarged for better understanding and ease of description. When describing a component (such as a layer, film, region, or substrate) on or on the surface of another component, the component may be "directly" located on the surface of the other component, or there may be a third component between the two components. Conversely, when describing a component on the surface of another component, or when another component is formed or disposed on the surface of a component, it indicates that there is no third component between the two components. Furthermore, when describing a component as being "generally" formed on another component, it means that the component is not formed on the entire surface (or front surface) of the other component, nor is it formed on a portion of the edge of the entire surface.

[0032] In the description of the embodiments of this application, when a component "includes" another component, other components are not excluded unless otherwise stated, and other components may be further included. Furthermore, when a component such as a layer, film, region, or plate is referred to as being "on / located" on another component, it can be "directly on" the other component (i.e., located on the surface of the other component with no other components between them), or it can have another component present in between. Moreover, when a component such as a layer, film, region, or plate is "directly located" on another component, or when a component such as a layer, film, region, or plate is located on the surface of another component, it indicates that no other components are located in between.

[0033] The terminology used in the description of the various embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various embodiments and the appended claims, the term "part" is also intended to include the plural form unless the context clearly indicates otherwise. Components include layers, films, regions, or plates, etc.

[0034] The embodiments of this application will now be described in detail with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details have been provided in the embodiments of this application to facilitate a better understanding of the application. However, the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0035] In a typical embodiment of this application, a secondary battery is provided, including a positive electrode, a separator, a negative electrode, and an electrolyte. The negative electrode includes a current collector and a negative electrode active layer. The material of the negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent. The negative electrode active material includes a silicon-based material, honeycomb porous graphite, and a polypyrrole layer. The polypyrrole layer coats the surface of the honeycomb porous graphite, and the silicon-based material is embedded in the pores of the honeycomb porous graphite. The silicon-based material includes silicon particles, a poly(N-isopropylacrylamide) layer, and a disulfide-bonded polyaniline layer. The poly(N-isopropylacrylamide) layer and the disulfide-bonded polyaniline layer coat the surface of the silicon particles, and the poly(N-isopropylacrylamide) layer is located between the silicon particles and the disulfide-bonded polyaniline layer. The honeycomb porous graphite includes graphite sheets and a carbon layer with a honeycomb pore structure on the surface of the graphite sheets, such as... Figure 1 As shown, silicon-based material 1 is embedded in the honeycomb-like pores of carbon layer 3 located on the surface of graphite sheet 2.

[0036] In this application, the polypyrrole layer is coated on the surface of the honeycomb porous graphite (here, "surface" refers to the outer surface of the honeycomb porous graphite). Figure 1 (The polypyrrole layer is not shown in the image). This composite material helps to suppress silicon particle expansion and improves the conductivity of the negative electrode active material. The silicon-based material is embedded in the pores of the honeycomb porous graphite. The presence of these pores helps to disperse stress through pore wall deformation during charging and discharging, reducing cracking of the graphite layer due to silicon expansion. The swelling degree of the poly(N-isopropylacrylamide) layer coating the silicon particle surface varies with temperature. At high temperatures, the poly(N-isopropylacrylamide) layer shrinks, helping to suppress the expansion of the silicon particle volume; at low temperatures, the poly(N-isopropylacrylamide) layer swells, helping to enhance lithium-ion diffusion. The disulfide-bonded polyaniline layer achieves self-repair of cracks during charging and discharging through disulfide bond breaking / recombining, helping to maintain the integrity of the conductive network. This application combines silicon and graphite, which helps to improve the specific capacity of the negative electrode active material. Therefore, the negative electrode sheet prepared using the negative electrode active material of this application has high adhesion, high-temperature stability, and low expansion rate. The assembled secondary battery has high cycle stability, high-temperature resistance, rate performance, specific capacity, and safety.

[0037] It should be noted that the positive electrode, separator, and electrolyte in this application can all be obtained by purchasing or by preparing them using existing technologies.

[0038] Including but not limited to, the conductive agent is selected from any one or more of super conductive carbon black (Super-P), carbon nanotubes, graphene and carbon fiber; the binder is selected from any one or more of polyvinylidene fluoride, polyvinylidene fluoride, styrene-butadiene rubber, carboxymethyl cellulose and polyacrylic acid.

[0039] In one embodiment of this application, the average particle size of the silicon particles is 50-100 nm; and / or, the average thickness of the poly-N-isopropylacrylamide layer is 45-55 nm, which can be selected from 45 nm, 46 nm, 47 nm, 48 nm, 49 nm, 50 nm, 51 nm, 52 nm, 53 nm, 54 nm, 55 nm, and any two values; and / or, the average thickness of the disulfide bond-containing polyaniline layer is 27-33 nm, which can be selected from 27 nm, 28 nm, 29 nm, 30 nm, 31 nm, 32 nm, 33 nm, and any two values.

[0040] Controlling the average particle size of silicon particles within the aforementioned range helps reduce silicon particle agglomeration while ensuring a higher specific surface area, providing more interfaces for lithium-ion intercalation and deintercalation, thereby improving the battery's specific capacity. Controlling the average thickness of the poly(N-isopropylacrylamide) layer within the aforementioned range helps the poly(N-isopropylacrylamide) layer shrink at high temperatures, adhering tightly to the silicon particle surface and effectively limiting silicon particle expansion, without affecting lithium-ion transport efficiency. At low temperatures, the poly(N-isopropylacrylamide) layer swells, increasing lithium-ion diffusion channels, accelerating lithium-ion transport, and improving the battery's rate performance and high-temperature resistance. Controlling the average thickness of the disulfide-bonded polyaniline layer within the aforementioned range helps the disulfide-bonded polyaniline layer repair cracks promptly during charge and discharge processes without affecting lithium-ion transport efficiency, improving the structural stability of the negative electrode active material during cycling, thereby contributing to improved battery cycle performance and safety.

[0041] In one embodiment of this application, the number average molecular weight of the polyN-isopropylacrylamide in the polyN-isopropylacrylamide layer is 8000 to 20000 Da, and can be selected from 8000 Da, 10000 Da, 12000 Da, 14000 Da, 16000 Da, 18000 Da, 20000 Da, and any two values; and / or, the number average molecular weight of the disulfide-bonded polyaniline in the disulfide-bonded polyaniline layer is 10000 to 30000 Da, and can be selected from 10000 Da, 15000 Da, 20000 Da, 25000 Da, 30000 Da, and any two values.

[0042] Controlling the number-average molecular weight of poly(N-isopropylacrylamide) in the poly(N-isopropylacrylamide) layer within the aforementioned range helps ensure that the poly(N-isopropylacrylamide) layer possesses sufficient elasticity to buffer the volume changes of silicon particles while maintaining good thermal responsiveness and mechanical strength. This helps suppress silicon particle expansion, maintain electrode stability, and consequently improve the battery's cycle performance, heat resistance, and safety. Controlling the number-average molecular weight of disulfide-bonded polyaniline in the disulfide-bonded polyaniline layer within the aforementioned range helps to balance the self-healing and conductivity properties of the disulfide-bonded polyaniline layer, thereby helping the negative electrode active material maintain good electrochemical activity and mechanical stability during cycling.

[0043] In one embodiment of this application, the disulfide bond density in the disulfide bond-containing polyaniline layer is 2 to 3 disulfide bonds in the molecular chain; and / or, the porosity of the honeycomb porous graphite is 50 to 65%; and / or, the average pore size of the honeycomb porous graphite is 150 to 300 nm; and / or, the average thickness of the carbon layer is 1 to 5 μm; and / or, the average thickness of the graphite sheet is 100 to 200 μm, and the average sheet diameter is 10 to 50 μm.

[0044] The presence of disulfide bonds endows the polyaniline layer with a unique self-healing ability. Controlling the density of disulfide bonds in the polyaniline layer within the aforementioned range helps to improve its self-healing performance while maintaining its high flexibility, thereby enhancing the structural stability of the negative electrode active material and consequently improving the cycle stability of the battery. Controlling the porosity of the honeycomb porous graphite within the aforementioned range helps to provide sufficient space for the embedding of silicon-based materials while maintaining the stability of the negative electrode active material, thus contributing to an increase in the specific capacity of the secondary battery. Controlling the average pore size of the honeycomb porous graphite within the aforementioned range helps to optimize the embedding efficiency of the silicon-based material and promotes rapid lithium-ion transport, contributing to improved rate performance of the secondary battery. Controlling the average thickness of the carbon layer, the average thickness of the graphite sheets, and the average sheet diameter within the aforementioned ranges helps to improve the stability of the silicon-based material within the honeycomb pores while simultaneously increasing the specific capacity of the negative electrode active material.

[0045] In order to further increase the amount of silicon-based material embedded, thereby increasing the specific capacity of the secondary battery, in one embodiment of this application, the average pore size of the honeycomb porous graphite is 230-300 nm.

[0046] In one embodiment of this application, the mass ratio of the silicon-based material to the honeycomb porous graphite is (94-95):(2-2.5); and / or, the average thickness of the polypyrrole layer is 50-150 nm; and / or, the number average molecular weight of the polypyrrole in the polypyrrole layer is 15000-35000 Da; and / or, the mass ratio of the negative electrode active material, binder, and conductive agent is (96-97.5):(0.8-1.2):(2.4-3.2).

[0047] Controlling the mass ratio of silicon-based material to honeycomb porous graphite within the aforementioned range helps to increase the specific capacity of the negative electrode active material while reducing its expansion rate during cycling. Controlling the average thickness of the polypyrrole layer and the number-average molecular weight of polypyrrole within the aforementioned range helps to improve the structural stability and conductivity of the negative electrode active material while maintaining its high lithium-ion transport efficiency. Controlling the mass ratio of the negative electrode active material, binder, and conductive agent within the aforementioned range helps to improve both the specific capacity and cycle stability of the negative electrode sheet.

[0048] In another typical embodiment of this application, a method for preparing a secondary battery is provided, comprising: sequentially mixing, homogenizing, coating, and drying a negative electrode active material, a conductive agent, a binder, and a solvent to obtain a positive electrode sheet; fabricating a bare battery cell from the positive electrode sheet, a separator, and a negative electrode sheet; assembling the bare battery cell with a casing and a top cover; and injecting an electrolyte to obtain a secondary battery; the preparation steps of the negative electrode active material include: S1, mixing silicon particles, N-isopropylacrylamide, a first crosslinking agent, a first initiator, and a first solvent, and then performing a first polymerization reaction to obtain a poly(N-isopropylacrylamide) layer coated on silicon particles; S2, coating the silicon particles with the poly(N-isopropylacrylamide) layer... A second polymerization reaction is carried out after mixing silicon-coated particles, aniline, a second crosslinking agent containing disulfide bonds, a second initiator, and a second solvent to form a polyaniline layer containing disulfide bonds coated on the surface of a poly-N-isopropylacrylamide layer, thereby obtaining a silicon-based material; S3, graphite powder is pressed into graphite sheets, polystyrene particles are sprayed onto the surface of the graphite sheets, and then calcined to obtain honeycomb porous graphite; S4, the silicon-based material is dispersed in a third solvent to obtain a suspension, and the honeycomb porous graphite is immersed in the suspension for impregnation treatment to obtain honeycomb porous graphite with silicon-based material embedded in the pores. A schematic diagram of the structure of the honeycomb porous graphite with silicon-based material embedded in the pores is shown below. Figure 1 As shown in Figure S5, honeycomb porous graphite with silicon-based materials embedded in its pores is placed in a pyrrole solution for electrochemical polymerization to form a polypyrrole layer coating the surface of the honeycomb porous graphite, thereby obtaining a negative electrode active material.

[0049] In step S1, silicon particles, N-isopropylacrylamide, a first crosslinking agent, a first initiator, and a first solvent are mixed and subjected to a first polymerization reaction. N-isopropylacrylamide and the first crosslinking agent polymerize under the action of the first initiator, forming a poly(N-isopropylacrylamide) layer on the surface of the silicon particles. The swelling degree of the poly(N-isopropylacrylamide) layer varies with temperature. At high temperatures, the poly(N-isopropylacrylamide) layer shrinks, which helps suppress the expansion of the silicon particle volume; at low temperatures, the poly(N-isopropylacrylamide) layer swells, which helps enhance lithium-ion diffusion. In step S2, silicon particles coated with a poly(N-isopropylacrylamide) layer, aniline, a second crosslinking agent containing disulfide bonds, a second initiator, and a second solvent are mixed and subjected to a second polymerization reaction. Aniline and the second crosslinking agent containing disulfide bonds polymerize under the action of the second initiator, forming a poly(N-isopropylacrylamide) layer containing disulfide bonds. During charging and discharging, the poly(N-isopropylacrylamide) layer achieves self-repair of cracks through disulfide bond breaking / recombining, which helps... To maintain the integrity of the conductive network, in step S3, graphite powder is pressed into graphite sheets. Using the graphite sheets as a substrate, polystyrene particles are sprayed onto the surface of the graphite sheets. The polystyrene particles self-assemble to form a tightly packed hexagonal close-packed structure on the surface of the graphite sheets. After calcination, a carbon layer with a honeycomb-like porous structure is formed on the surface of the graphite sheets. In step S4, silicon-based materials are dispersed in a third solvent to obtain a suspension. The honeycomb-like porous graphite is then immersed in the suspension. The silicon-based materials are adsorbed into the pores of the honeycomb-like porous graphite. The presence of the honeycomb-like pores helps to disperse stress through pore wall deformation during charging and discharging, preventing silicon expansion from causing the graphite layer to crack. In step S5, the honeycomb-like porous graphite with silicon-based materials embedded in the pores is placed in a pyrrole solution for electrochemical polymerization. The pyrrole solution forms a polypyrrole layer that coats the surface of the honeycomb-like porous graphite through electrochemical polymerization. The presence of the polypyrrole layer helps to inhibit silicon particle expansion and improves the conductivity of the negative electrode active material. This application combines silicon and graphite, which helps to improve the specific capacity of the negative electrode active material. Therefore, the negative electrode sheet prepared by the method of this application has high adhesion, high temperature stability, and low expansion rate, and the assembled secondary battery has high cycle stability, specific capacity, and safety.

[0050] Including but not limited to, the solvents mentioned above are selected from any one or more of water, N-methylpyrrolidone, and dimethylformamide.

[0051] In one embodiment of this application, S1 further includes ultrasonic treatment of silicon particles with anhydrous ethanol to remove surface oxides.

[0052] In one embodiment of this application, the mass ratio of the silicon particles, N-isopropylacrylamide, first crosslinking agent, and first initiator is (5-10):(1-2):(0.02-0.05):(0.05-0.1); and / or, the mass ratio of the poly(N-isopropylacrylamide) layer coating silicon particles, aniline, second crosslinking agent, and second initiator is (2-5):(1-1.2):(0.15-0.2):(0.4-0.5); and / or, the average thickness of the graphite sheet is 100-200 μm, and the average sheet diameter is 10-50 μm; and / or, the sprayed surface density of the polystyrene particles is 0.1-0.3 g / cm³. 2 ; and / or, the mass ratio of silicon-based material to the volume of the third solvent is (1-2 g):(10-50 mL); and / or, the mass ratio of honeycomb porous graphite to the volume of the suspension is (1.5-2.5 g):(5-20 mL); and / or, the mass ratio of honeycomb porous graphite with silicon-based material embedded in the pores to the volume of the pyrrole solution is (2.5-4.5 g):(100-150 mL).

[0053] Controlling the mass ratio of silicon particles, N-isopropylacrylamide, the first crosslinking agent, and the first initiator within the aforementioned range helps to form a poly(N-isopropylacrylamide) layer of suitable thickness. This allows the poly(N-isopropylacrylamide) layer to shrink and adhere tightly to the surface of the silicon particles at high temperatures without affecting lithium-ion transport efficiency, effectively limiting the expansion of the silicon particles. At low temperatures, the poly(N-isopropylacrylamide) layer swells, increasing the diffusion channels for lithium ions, accelerating lithium-ion transport, and improving the rate performance and high-temperature resistance of the battery. Controlling the mass ratio of the poly(N-isopropylacrylamide) layer coating silicon particles, aniline, the second crosslinking agent, and the second initiator within the aforementioned range helps to form a polyaniline layer containing disulfide bonds of suitable thickness. This allows the polyaniline layer containing disulfide bonds to repair cracks in a timely manner during charge and discharge without affecting lithium-ion transport efficiency, improving the structural stability of the negative electrode active material during cycling, and thus contributing to improved battery cycle performance and safety. Controlling the average thickness, average sheet diameter, and spray areal density of the polystyrene particles within the aforementioned ranges helps to form a carbon layer of suitable thickness with a honeycomb-like porous structure on the surface of the graphite sheets. This helps to improve the stability of the silicon-based material within the honeycomb pores while simultaneously increasing the specific capacity of the negative electrode active material. Controlling the mass ratio of the silicon-based material to the volume of the third solvent, and the mass ratio of the honeycomb porous graphite to the volume of the suspension, within the aforementioned ranges helps to control the mass ratio of the silicon-based material to the honeycomb porous graphite within a suitable range. This helps to reduce the expansion rate of the negative electrode active material during cycling while simultaneously increasing its specific capacity. Controlling the mass ratio of the honeycomb porous graphite with silicon-based material embedded in the pores to the volume of the pyrrole solution within the aforementioned ranges helps to form a polypyrrole layer of suitable thickness. This helps to maintain a high lithium-ion transport efficiency in the negative electrode active material while improving its structural stability and conductivity.

[0054] In one embodiment of this application, a powder press is used to press graphite powder into graphite sheets. The mold containing the graphite sheets is moved to a high-pressure airless sprayer. Polystyrene particles are first sprayed onto one side of the graphite sheets, and then the mold is flipped over to spray polystyrene particles onto the other side of the graphite sheets.

[0055] In one embodiment of this application, the temperature of the first polymerization reaction is 70–90°C; and / or, the time of the first polymerization reaction is 6–12 h; and / or, the temperature of the second polymerization reaction is 0–5°C; and / or, the time of the second polymerization reaction is 12–24 h; and / or, the temperature of the calcination treatment is 400–600°C; and / or, the time of the calcination treatment is 1–3 h; and / or, the impregnation treatment is carried out under vacuum conditions; and / or, the temperature of the impregnation treatment is 25–60°C; and / or, the time of the impregnation treatment is 30–120 min; and / or, the current density of the electrochemical polymerization reaction is 0.1–1 mA / cm². 2 ; and / or, the temperature of the electrochemical polymerization reaction is 25–40 °C; and / or, the time of the electrochemical polymerization reaction is 1–5 h.

[0056] Controlling the temperature and time of the first polymerization reaction within the aforementioned range helps to control the number-average molecular weight of poly(N-isopropylacrylamide) within a suitable range. This helps ensure that the poly(N-isopropylacrylamide) layer has sufficient elasticity to buffer the volume changes of silicon particles while maintaining good thermal responsiveness and mechanical strength, thereby helping to suppress silicon particle expansion and maintain electrode stability. Controlling the temperature and time of the second polymerization reaction within the aforementioned range helps to control the number-average molecular weight of disulfide-bonded polyaniline within a suitable range. This helps to balance the self-healing and conductivity properties of the disulfide-bonded polyaniline layer, thereby helping to maintain good electrochemical activity and mechanical stability of the negative electrode active material during cycling. Controlling the temperature and time of the calcination treatment within the aforementioned range helps to form honeycomb porous graphite with suitable porosity and average pore size. This helps to provide sufficient space for embedding silicon-based materials while maintaining the stability of the negative electrode active material, optimizing the embedding efficiency of silicon-based materials, and promoting rapid lithium-ion transport, thereby helping to improve the specific capacity and rate performance of the secondary battery. Controlling the temperature and time of the impregnation treatment within the aforementioned range helps to improve the embedding efficiency of silicon-based materials. Controlling the current density, temperature, and time of the electrochemical polymerization reaction within the above-mentioned range helps to control the number-average molecular weight of polypyrrole within a suitable range, thereby helping to improve the structural stability and conductivity of the negative electrode active material while maintaining its high lithium-ion transport efficiency.

[0057] In one embodiment of this application, the average particle size of the silicon particles is 50-100 nm; and / or, the average particle size of the polystyrene particles is 150-300 nm; and / or, the number-average molecular weight of the polystyrene particles is 50,000-60,000 Da; and / or, the first crosslinking agent is N,N'-methylenebisacrylamide and / or polyethylene glycol diacrylate; and / or, the first initiator is ammonium persulfate and / or azobisisobutyramidine hydrochloride; and / or, the first solvent is water and... / or ethanol; and / or, the second crosslinking agent is dibenzoic acid disulfide; and / or, the second initiator is ammonium persulfate and / or ferric chloride; and / or, the second solvent is an aqueous HCl solution; and / or, the third solvent is N-methylpyrrolidone and / or dimethyl sulfoxide; and / or, the components of the pyrrole solution include pyrrole, water, oxidant and dopant, and the mass ratio of pyrrole, water, oxidant and dopant is (1-3):(10-12):(0.5-0.7):(0.2-0.4).

[0058] Controlling the average particle size of silicon particles within the aforementioned range helps reduce silicon particle agglomeration while ensuring a high specific surface area, providing more interfaces for lithium-ion intercalation and deintercalation, thereby improving the specific capacity of the battery. Controlling the average particle size and number-average molecular weight of polystyrene particles within the aforementioned range helps control the porosity and average pore size of honeycomb porous graphite within appropriate ranges, thus providing sufficient space for silicon-based material intercalation while maintaining the stability of the negative electrode active material, optimizing the intercalation efficiency of the silicon-based material, and promoting rapid lithium-ion transport, thereby improving the specific capacity and rate performance of the secondary battery. Controlling the types of the first crosslinking agent, first initiator, first solvent, second crosslinking agent, second initiator, second solvent, and third solvent within the aforementioned range helps improve the interaction between the components, thereby improving the efficiency and purity of the negative electrode active material preparation. Controlling the mass ratio of pyrrole, water, oxidant, and dopant within the aforementioned range helps improve the formation efficiency of polypyrrole.

[0059] In order to form a carbon layer with a more suitable pore size so that more carbon-based materials can be embedded in the pores, in one embodiment of this application, the average particle size of the polystyrene particles is 230-300 nm.

[0060] In one embodiment of this application, the oxidant is selected from any one or more of FeCl3, ammonium persulfate and sodium persulfate; and / or, the dopant is selected from any one or more of sodium p-toluenesulfonate, sodium dodecylbenzenesulfonate and naphthalenesulfonic acid; and / or, the mass ratio of the negative electrode active material, binder and conductive agent is (96-97.5):(0.8-1.2):(2.4-3.2).

[0061] Controlling the types of oxidants and dopants within the above-mentioned ranges helps to further promote the formation of polypyrrole. Controlling the mass ratio of the negative electrode active material, binder, and conductive agent within the above-mentioned ranges helps to improve the specific capacity of the negative electrode while also improving its cycle stability.

[0062] In another typical embodiment of this application, an energy storage system is provided, including multiple unit batteries, which are the aforementioned secondary batteries or secondary batteries prepared by the aforementioned secondary battery preparation method.

[0063] Because the above-mentioned energy storage system contains the secondary battery of this application, the energy storage system has high specific capacity, rate performance, cycle stability and high temperature resistance.

[0064] In another typical embodiment of this application, an electrical device is provided, including the aforementioned energy storage system, which is used to provide power to the electrical device.

[0065] Since the energy storage system of the aforementioned electrical equipment contains the secondary battery of this application, the electrical equipment has high specific capacity, rate performance, cycle stability, high temperature resistance and safety.

[0066] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0067] Example 1

[0068] Adopting such Figure 2The process flow shown is as follows for preparing a secondary battery. Specifically, silicon particles with an average particle size of 100 nm are dispersed in anhydrous ethanol, ultrasonically treated for 30 minutes, centrifuged at 8000 rpm for 10 minutes, and then vacuum dried at 60°C for 12 hours to obtain cleaned silicon particles. N-isopropylacrylamide, N,N'-methylenebisacrylamide, and water are mixed to obtain a first mixed solution. The cleaned silicon particles are dispersed in the first mixed solution, and nitrogen gas is purged for 30 minutes to remove oxygen. The temperature is raised to 80°C, and ammonium persulfate is slowly added dropwise. The mass ratio of silicon particles, N-isopropylacrylamide, N,N'-methylenebisacrylamide, and ammonium persulfate is 7:1.5:0.03:0.07. A first polymerization reaction is carried out for 9 hours. After centrifugation and washing (using deionized water three times), the product is vacuum dried at 60°C to obtain poly(N-isopropylacrylamide). Poly(N-isopropylacrylamide) coated silicon particles were prepared by mixing aniline, dibenzoic acid disulfide, and 1M HCl aqueous solution to obtain a second mixed solution. The poly(N-isopropylacrylamide) coated silicon particles were dispersed in the second mixed solution, cooled to 2°C, and ammonium persulfate was slowly added dropwise. The mass ratio of poly(N-isopropylacrylamide) coated silicon particles, aniline, dibenzoic acid disulfide, and ammonium persulfate was 3:1.1:0.17:0.45. A second polymerization reaction was carried out for 18 hours. The mixture was then centrifuged and washed (with ethanol three times) and vacuum dried to obtain the silicon-based material.

[0069] Graphite powder was pressed into graphite sheets using a powder press. The average thickness of the graphite sheets was 150 μm, and the average sheet diameter was 30 μm. The mold containing the graphite sheets was then transferred to a high-pressure airless sprayer. Polystyrene particles were first sprayed onto one side of the graphite sheets. The mold was then flipped over, and polystyrene particles were sprayed onto the other side of the graphite sheets. The average particle size of the polystyrene particles was 280 nm, the number average molecular weight was 55,000 Da, and the surface density of the sprayed polystyrene particles was 0.2 g / cm³. 2 After spraying, the material is calcined at 500℃ for 2 hours to obtain honeycomb porous graphite.

[0070] The aforementioned silicon-based material was dispersed in N-methylpyrrolidone to form a suspension, wherein the mass ratio of the silicon-based material to the volume of N-methylpyrrolidone was 2 g: 50 mL. The aforementioned honeycomb porous graphite was immersed in the suspension at 40 °C and kept under vacuum for 60 minutes, wherein the mass ratio of the honeycomb porous graphite to the volume of the suspension was 2 g: 10 mL. The impregnated graphite matrix was dried at 80 °C for 12 hours to remove the solvent, thereby obtaining honeycomb porous graphite with silicon-based material embedded in the pores.

[0071] Pyrrole, water, FeCl3, and sodium p-toluenesulfonate were mixed in a mass ratio of 2:11:0.6:0.3 to obtain a pyrrole solution. A honeycomb porous graphite with silicon-based material embedded in its pores was used as the working electrode to carry out electrochemical polymerization in the pyrrole solution. The mass ratio of the honeycomb porous graphite with silicon-based material embedded in its pores to the volume of the pyrrole solution was 3.5 g:120 mL, and the current density of the electrochemical polymerization reaction was 0.5 mA / cm². 2 The electropolymerization reaction was carried out at 25°C for 3 hours. Afterward, the product was dried at 60°C for 6 hours to obtain the negative electrode active material. This material comprises a silicon-based material, honeycomb porous graphite, and a polypyrrole layer. The polypyrrole layer coats the surface of the honeycomb porous graphite, and the silicon-based material is embedded in the pores of the graphite. The silicon-based material includes silicon particles, a poly(N-isopropylacrylamide) layer, and a polyaniline layer containing disulfide bonds. The poly(N-isopropylacrylamide) layer and the polyaniline layer containing disulfide bonds coat the surface of the silicon particles, with the poly(N-isopropylacrylamide) layer located between the silicon particles and the polyaniline layer containing disulfide bonds. The honeycomb porous graphite includes graphite sheets and a carbon layer with a honeycomb pore structure on the surface of the graphite sheets. The average thickness of the acrylamide layer is 50 nm, the number average molecular weight of poly-N-isopropylacrylamide is 15000 Da, the average thickness of the disulfide bond-containing polyaniline layer is 30 nm, the number average molecular weight of the disulfide bond-containing polyaniline is 20000 Da, the disulfide bond density in the disulfide bond-containing polyaniline layer is 3 disulfide bonds in the molecular chain, the porosity of the honeycomb porous graphite is 60%, the average pore size of the honeycomb porous graphite is 280 nm, the average thickness of the carbon layer is 3 μm, the average thickness of the graphite sheet is 150 μm, the average sheet diameter is 30 μm, the mass ratio of silicon-based material to honeycomb porous graphite is 94:2.3, the average thickness of the polypyrrole layer is 100 nm, and the number average molecular weight of the polypyrrole is 25000 Da.

[0072] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0073] Example 2

[0074] The difference from Example 1 is that the average particle size of the silicon particles is 50 nm, resulting in a negative electrode active material.

[0075] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0076] Example 3

[0077] The difference from Example 1 is that the average particle size of the silicon particles is 20 nm, and the final negative electrode active material is obtained.

[0078] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0079] Example 4

[0080] The difference from Example 1 is that the mass ratio of silicon particles, N-isopropylacrylamide, N,N'-methylenebisacrylamide and ammonium persulfate is 10:1:0.02:0.05, the temperature of the first polymerization reaction is 70°C and the time is 6 hours, and finally the negative electrode active material is obtained, wherein the average thickness of the poly(N-isopropylacrylamide) layer is 45 nm and the number average molecular weight of poly(N-isopropylacrylamide) is 8000 Da.

[0081] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0082] Example 5

[0083] The difference from Example 1 is that the mass ratio of silicon particles, N-isopropylacrylamide, N,N'-methylenebisacrylamide and ammonium persulfate is 5:2:0.05:0.1, the temperature of the first polymerization reaction is 90°C and the time is 12 hours, and the negative electrode active material is finally obtained, wherein the average thickness of the poly(N-isopropylacrylamide) layer is 55 nm and the number average molecular weight of poly(N-isopropylacrylamide) is 20000 Da.

[0084] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0085] Example 6

[0086] The difference from Example 1 is that the mass ratio of silicon particles, N-isopropylacrylamide, N,N'-methylenebisacrylamide and ammonium persulfate is 15:1:0.02:0.05, the temperature of the first polymerization reaction is 60°C and the time is 4 hours, and finally a negative electrode active material is obtained, wherein the average thickness of the poly(N-isopropylacrylamide) layer is 30 nm and the number average molecular weight of poly(N-isopropylacrylamide) is 6000 Da.

[0087] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0088] Example 7

[0089] The difference from Example 1 is that the mass ratio of poly(N-isopropylacrylamide) coating silicon particles, aniline, dibenzoic acid disulfide and ammonium persulfate is 5:1:0.15:0.4, the temperature of the second polymerization reaction is 0°C and the time is 12h, and finally the negative electrode active material is obtained, wherein the average thickness of the polyaniline layer containing disulfide bonds is 27nm and the number average molecular weight of the polyaniline containing disulfide bonds in the polyaniline layer is 10000Da.

[0090] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0091] Example 8

[0092] The difference from Example 1 is that the mass ratio of poly(N-isopropylacrylamide) coating silicon particles, aniline, dibenzoic acid disulfide and ammonium persulfate is 2:1.2:0.2:0.5, the temperature of the second polymerization reaction is 5°C and the time is 24h, and finally the negative electrode active material is obtained, wherein the average thickness of the polyaniline layer containing disulfide bonds is 33nm and the number average molecular weight of the polyaniline containing disulfide bonds in the polyaniline layer is 30000Da.

[0093] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0094] Example 9

[0095] The difference from Example 1 is that the mass ratio of poly(N-isopropylacrylamide) coating silicon particles, aniline, dibenzoic acid disulfide and ammonium persulfate is 8:1:0.15:0.4, the temperature of the second polymerization reaction is 0°C and the time is 4 hours, and finally the negative electrode active material is obtained, wherein the average thickness of the polyaniline layer containing disulfide bonds is 20 nm, and the number average molecular weight of the polyaniline containing disulfide bonds in the polyaniline layer is 8000 Da.

[0096] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0097] Example 10

[0098] The difference from Example 1 is that a powder press was used to press graphite powder into graphite sheets. The average thickness of the graphite sheets was 200 μm, and the average sheet diameter was 10 μm. The mold containing the graphite sheets was transferred to a high-pressure airless sprayer. Polystyrene particles were first sprayed onto one side of the graphite sheets. Then, the mold was flipped over, and polystyrene particles were sprayed onto the other side of the graphite sheets. The average particle size of the polystyrene particles was 260 nm, the number average molecular weight of the polystyrene particles was 60,000 Da, and the sprayed surface density of the polystyrene particles was 0.3 g / cm³. 2 After spraying, the material is calcined at a temperature of 600℃ for 1 hour to obtain the negative electrode active material. The honeycomb porous graphite has a porosity of 50%, an average pore size of 260nm, and an average carbon layer thickness of 5μm.

[0099] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0100] Example 11

[0101] The difference from Example 1 is that a powder press was used to press graphite powder into graphite sheets. The average thickness of the graphite sheets was 100 μm, and the average sheet diameter was 50 μm. The mold containing the graphite sheets was transferred to a high-pressure airless sprayer. Polystyrene particles were first sprayed onto one side of the graphite sheets. Then, the mold was flipped over, and polystyrene particles were sprayed onto the other side of the graphite sheets. The average particle size of the polystyrene particles was 300 nm, the number average molecular weight of the polystyrene particles was 50,000 Da, and the sprayed surface density of the polystyrene particles was 0.1 g / cm³. 2 After spraying, the material is calcined at a temperature of 400℃ for 3 hours to obtain the negative electrode active material. The honeycomb porous graphite has a porosity of 65%, an average pore size of 300nm, and an average carbon layer thickness of 1μm.

[0102] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0103] Example 12

[0104] The difference from Example 1 is that a powder press was used to press graphite powder into graphite sheets. The average thickness of the graphite sheets was 100 μm, and the average sheet diameter was 50 μm. The mold containing the graphite sheets was moved to a high-pressure airless sprayer. Polystyrene particles were first sprayed onto one side of the graphite sheets. Then, the mold was flipped over, and polystyrene particles were sprayed onto the other side of the graphite sheets. The average particle size of the polystyrene particles was 300 nm, the number average molecular weight of the polystyrene particles was 50,000 Da, and the sprayed surface density of the polystyrene particles was 0.5 g / cm³. 2 After spraying, the material is calcined at a temperature of 700℃ for 4 hours to obtain the negative electrode active material. The honeycomb porous graphite has a porosity of 40%, an average pore size of 300nm, and an average carbon layer thickness of 8μm.

[0105] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0106] Example 13

[0107] The difference from Example 1 is that the mass ratio of honeycomb porous graphite to the volume of the suspension is 2.5g:5mL, the impregnation time is 120min, and the negative electrode active material is finally obtained, wherein the mass ratio of silicon-based material to honeycomb porous graphite is 95:2.

[0108] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0109] Example 14

[0110] The difference from Example 1 is that the mass ratio of honeycomb porous graphite to the volume of suspension is 1.5g:20mL, the impregnation time is 30min, and the negative electrode active material is finally obtained, wherein the mass ratio of silicon-based material to honeycomb porous graphite is 94:2.5.

[0111] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0112] Example 15

[0113] The difference from Example 1 is that the mass ratio of honeycomb porous graphite to the volume of suspension is 1.5g:20mL, the impregnation time is 20min, and the negative electrode active material is finally obtained, wherein the mass ratio of silicon-based material to honeycomb porous graphite is 90:2.5.

[0114] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0115] Example 16

[0116] The difference from Example 1 is that pyrrole, water, FeCl3, and sodium p-toluenesulfonate were mixed in a mass ratio of 3:12:0.7:0.4 to obtain a pyrrole solution. The honeycomb porous graphite with silicon-based material embedded in its pores was used as the working electrode, and electrochemical polymerization was carried out in the pyrrole solution. The mass ratio of the honeycomb porous graphite with silicon-based material embedded in its pores to the volume ratio of the pyrrole solution was 2.5 g:150 mL, and the current density of the electrochemical polymerization reaction was 1 mA / cm². 2 The temperature was 40℃ and the time was 5h to obtain the negative electrode active material, in which the average thickness of the polypyrrole layer was 150nm and the number average molecular weight of the polypyrrole was 35000Da.

[0117] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0118] Example 17

[0119] The difference from Example 1 is that pyrrole, water, FeCl3, and sodium p-toluenesulfonate were mixed in a mass ratio of 1:10:0.5:0.2 to obtain a pyrrole solution. The honeycomb porous graphite with silicon-based material embedded in its pores was used as the working electrode, and electrochemical polymerization was carried out in the pyrrole solution. The mass ratio of the honeycomb porous graphite with silicon-based material embedded in its pores to the volume ratio of the pyrrole solution was 4.5 g:100 mL, and the current density of the electrochemical polymerization reaction was 0.1 mA / cm². 2 The temperature was 25℃ and the time was 1h to obtain the negative electrode active material, in which the average thickness of the polypyrrole layer was 50nm and the number average molecular weight of the polypyrrole was 15000Da.

[0120] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0121] Example 18

[0122] The difference from Example 1 is that pyrrole, water, FeCl3, and sodium p-toluenesulfonate were mixed in a mass ratio of 1:10:0.5:0.2 to obtain a pyrrole solution. The honeycomb porous graphite with silicon-based material embedded in its pores was used as the working electrode, and electrochemical polymerization was carried out in the pyrrole solution. The mass ratio of the honeycomb porous graphite with silicon-based material embedded in its pores to the volume ratio of the pyrrole solution was 6.5 g:100 mL, and the current density of the electrochemical polymerization reaction was 0.08 mA / cm². 2 The temperature was 25℃ and the time was 0.5h to obtain the negative electrode active material, in which the average thickness of the polypyrrole layer was 40nm and the number average molecular weight of the polypyrrole was 10000Da.

[0123] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0124] Comparative Example 1

[0125] The difference from Example 1 is that the addition of silicon particles was omitted, resulting in a negative electrode active material.

[0126] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0127] Comparative Example 2

[0128] The difference from Example 1 is that the addition of N-isopropylacrylamide and N,N'-methylenebisacrylamide was omitted, and the negative electrode active material was finally obtained.

[0129] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0130] Comparative Example 3

[0131] The difference from Example 1 is that the addition of aniline was omitted, resulting in a negative electrode active material.

[0132] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0133] Comparative Example 4

[0134] The difference from Example 1 is that dibenzoic acid is used instead of dibenzoic acid disulfide to obtain the negative electrode active material.

[0135] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0136] Comparative Example 5

[0137] The difference from Example 1 is that the polystyrene particle spraying is omitted, and the final negative electrode active material is obtained.

[0138] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0139] Comparative Example 6

[0140] The difference from Example 1 is that the addition of pyrrole was omitted, resulting in a negative electrode active material.

[0141] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0142] Comparative Example 7

[0143] The difference from Example 1 is that the obtained silicon-based material is directly placed in a pyrrole solution for electrochemical polymerization to finally obtain the negative electrode active material.

[0144] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0145] Comparative Example 8

[0146] The difference from Example 1 is that silicon particles are directly dispersed in N-methylpyrrolidone to form a suspension, and finally the negative electrode active material is obtained.

[0147] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above were mixed and homogenized into a slurry with a mass ratio of 97:1:3. After coating, rolling, slitting, tab forming and cutting, a negative electrode sheet was obtained. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator were stacked into a bare cell. The bare cell was then subjected to tab welding, encapsulation, electrolyte injection, formation and degassing and edge sealing in sequence to obtain a secondary battery.

[0148] The secondary batteries prepared in the examples and comparative examples were tested for their initial discharge specific capacity and capacity retention after 1000 cycles at an ambient temperature of 25°C and 0.5C, and at an ambient temperature of 45°C and 0.5C. The batteries that had been cycled at 45°C for 1000 cycles were also disassembled, and the increase in the thickness of the negative electrode sheet compared with that before cycling was measured. The test results are shown in Table 1.

[0149] Table 1

[0150]

[0151]

[0152] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0153] As shown in Table 1, the difference between Examples 1-3 lies in the average particle size of the silicon particles. When the average particle size of the silicon particles is in the range of 50-100 nm, the stability and activity of the negative electrode active material are stronger, which helps to make the secondary battery have higher initial discharge specific capacity and cycle stability, and the expansion rate of the negative electrode sheet is lower. The difference between Examples 1, 4-6 lies in the average thickness of the poly(N-isopropylacrylamide) layer and the number-average molecular weight of poly(N-isopropylacrylamide). When the average thickness of the poly(N-isopropylacrylamide) layer is 45-55 nm and the number-average molecular weight of poly(N-isopropylacrylamide) is 8000 Da-2000 Da, the difference is more pronounced. Within the 0 Da range, it helps to further suppress the expansion of silicon particles, thereby helping to reduce the expansion rate of the negative electrode. The difference between Examples 1 and 7-9 lies in the average thickness of the disulfide-bonded polyaniline layer and the number-average molecular weight of the disulfide-bonded polyaniline in the disulfide-bonded polyaniline layer. When the average thickness of the disulfide-bonded polyaniline layer is in the range of 27-33 nm and the number-average molecular weight of the disulfide-bonded polyaniline in the disulfide-bonded polyaniline layer is in the range of 10000-30000 Da, it helps to further maintain the integrity of the conductive network, thereby giving the secondary battery higher cycle stability. The difference between Examples 1 and 10-12 lies in the pore size of the honeycomb porous graphite. The porosity, average pore size, and average carbon layer thickness of the honeycomb porous graphite vary. When the porosity of the honeycomb porous graphite is 50-65%, the average pore size is 260-300 nm, and the average carbon layer thickness is 1-5 μm, more silicon-based material can be embedded within the honeycomb porous graphite, resulting in a higher initial discharge capacity of the secondary battery. The structure of the honeycomb porous graphite is also more stable, further suppressing the expansion of silicon particles, thus helping to reduce the expansion rate of the negative electrode. The difference between Examples 1 and 13-15 lies in the different mass ratios of silicon-based material to honeycomb porous graphite. When the mass ratio of porous graphite is in the range of (94-95):(2-2.5), silicon-based materials and honeycomb porous graphite have good compatibility, which can further improve the cycle stability of the negative electrode and reduce the expansion rate of the negative electrode. The difference between Examples 1 and 16-18 lies in the average thickness of the polypyrrole layer and the number-average molecular weight of polypyrrole. When the average thickness of the polypyrrole layer is in the range of 50-150 nm and the number-average molecular weight of polypyrrole is in the range of 15000 Da-35000 Da, the negative electrode active material has higher conductivity and stability, thereby enabling the secondary battery to have higher initial discharge specific capacity and cycle capacity retention.

[0154] In Comparative Example 1, while removing the silicon particles reduced the expansion of the negative electrode, the corresponding initial discharge specific capacity decreased significantly. In Comparative Example 2, the absence of a poly-N-isopropylacrylamide layer in the negative electrode active material hindered the suppression of silicon particle expansion, especially at high temperatures, leading to a significant increase in the expansion rate of the negative electrode after cycling. In Comparative Example 3, the lack of a disulfide-bonded polyaniline layer in the negative electrode active material resulted in insufficient self-repair capability during cycling, leading to poor cycle stability of the secondary battery. In Comparative Example 4, although a polyaniline layer was formed in the negative electrode active material, it lacked disulfide bonds, resulting in no self-repair capability during cycling and thus poor cycle stability of the secondary battery. In Comparative Example 5, the negative electrode active material lacked a disulfide bond layer. The expansion of silicon particles caused by the expansion of the carbon layer in the honeycomb porous structure leads to a significant increase in the expansion rate of the negative electrode sheet after cycling. In Comparative Example 6, the absence of a polypyrrole layer in the negative electrode active material results in excessively low conductivity and reduced structural stability, leading to lower initial discharge capacity and cycle stability in the secondary battery. In Comparative Example 7, the absence of honeycomb porous graphite in the negative electrode active material makes it difficult to effectively suppress the expansion of silicon particles, resulting in a high expansion rate of the negative electrode sheet. In Comparative Example 8, the absence of honeycomb porous graphite, polypyrrole layer, polyN-isopropylacrylamide layer, and disulfide-bonded polyaniline layer in the negative electrode active material results in poor stability of silicon particles during cycling, causing a severe decrease in the cycle capacity retention rate of the secondary battery and severe expansion of the negative electrode sheet.

[0155] In this application, a polypyrrole layer is coated on the surface of honeycomb porous graphite, which helps to suppress silicon particle expansion and improve the conductivity of the negative electrode active material. The silicon-based material is embedded in the pores of the honeycomb porous graphite. The presence of these pores helps to disperse stress through pore wall deformation during charging and discharging, reducing cracking of the graphite layer due to silicon expansion. The swelling degree of the polyN-isopropylacrylamide layer coating the silicon particle surface varies with temperature. At high temperatures, the polyN-isopropylacrylamide layer shrinks, helping to suppress the volume expansion of silicon particles; at low temperatures, the polyN-isopropylacrylamide layer swells, helping to enhance lithium-ion diffusion. The disulfide-bonded polyaniline layer achieves self-repair of cracks during charging and discharging through disulfide bond breaking / recombining, helping to maintain the integrity of the conductive network. This application combines silicon and graphite, which helps to improve the specific capacity of the negative electrode active material. Therefore, the negative electrode sheet prepared using the negative electrode active material of this application has high adhesion, high temperature stability, and low expansion rate. The secondary battery assembled with it has high cycle stability, high temperature resistance, rate performance, specific capacity, and safety.

[0156] Those skilled in the art will understand that the above embodiments are specific examples of implementing this application, and in practical applications, various changes in form and detail can be made without departing from the spirit and scope of this application. Any person skilled in the art can make various alterations and modifications without departing from the spirit and scope of this application; therefore, the scope of protection of this application should be determined by the scope defined in the claims.

Claims

1. A secondary battery, comprising a positive electrode, a separator, a negative electrode, and an electrolyte, wherein the negative electrode comprises a current collector and a negative electrode active layer, and the material of the negative electrode active layer comprises a negative electrode active material, a binder, and a conductive agent, characterized in that, The negative electrode active material includes a silicon-based material, a honeycomb porous graphite, and a polypyrrole layer. The polypyrrole layer coats the surface of the honeycomb porous graphite, and the silicon-based material is embedded in the pores of the honeycomb porous graphite. The silicon-based material includes silicon particles, a poly(N-isopropylacrylamide) layer, and a polyaniline layer containing disulfide bonds. The poly(N-isopropylacrylamide) layer and the polyaniline layer containing disulfide bonds coat the surface of the silicon particles, and the poly(N-isopropylacrylamide) layer is located between the silicon particles and the polyaniline layer containing disulfide bonds. The honeycomb porous graphite includes graphite sheets and a carbon layer with a honeycomb porous structure on the surface of the graphite sheets. The average thickness of the poly(N-isopropylacrylamide) layer is 45-55 nm; the average thickness of the polyaniline layer containing disulfide bonds is 27-33 nm; the average thickness of the carbon layer is 1-5 μm; and the average thickness of the polypyrrole layer is 50-150 nm.

2. The secondary battery according to claim 1, characterized in that, The average particle size of the silicon particles is 50~100nm.

3. The secondary battery according to claim 1, characterized in that, The number-average molecular weight of the poly-N-isopropylacrylamide in the poly-N-isopropylacrylamide layer is 8000~20000 Da; and / or, the number-average molecular weight of the disulfide-bonded polyaniline in the disulfide-bonded polyaniline layer is 10000~30000 Da.

4. The secondary battery according to claim 1, characterized in that, The disulfide bond density in the disulfide-bonded polyaniline layer is 2-3 disulfide bonds per molecular chain; and / or, the porosity of the honeycomb porous graphite is 50-65%; and / or, the average pore size of the honeycomb porous graphite is 150-300 nm; and / or, the average thickness of the graphite sheet is 100-200 μm, and the average sheet diameter is 10-50 μm.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The number average molecular weight of the polypyrrole in the polypyrrole layer is 15,000 to 35,000 Da; and / or, the mass ratio of the negative electrode active material, the binder and the conductive agent is (96 to 97.5): (0.8 to 1.2): (2.4 to 3.2).

6. A method for preparing a secondary battery, comprising: sequentially mixing, homogenizing, coating, and drying a negative electrode active material, a conductive agent, a binder, and a solvent to obtain a negative electrode sheet; fabricating a bare cell using a positive electrode sheet, a separator, and the negative electrode sheet; assembling the bare cell with a casing and a top cover; and injecting an electrolyte to obtain the secondary battery; characterized in that... The preparation steps of the negative electrode active material include: S1, silicon particles, N-isopropylacrylamide, a first crosslinking agent, a first initiator and a first solvent are mixed and subjected to a first polymerization reaction to obtain silicon particles coated with a poly-N-isopropylacrylamide layer; S2, the poly(N-isopropylacrylamide) layer coated with silicon particles, aniline, a second crosslinking agent containing disulfide bonds, a second initiator, and a second solvent are mixed and subjected to a second polymerization reaction to form a polyaniline layer containing disulfide bonds coated on the surface of the poly(N-isopropylacrylamide) layer, thereby obtaining a silicon-based material; S3, graphite powder is pressed into graphite sheets, polystyrene particles are sprayed onto the surface of the graphite sheets and then calcined to obtain honeycomb porous graphite. S4, the silicon-based material is dispersed in a third solvent to obtain a suspension, and the honeycomb porous graphite is placed in the suspension for impregnation treatment to obtain honeycomb porous graphite with silicon-based material embedded in the pores; S5, the honeycomb porous graphite with silicon-based material embedded in the pores is placed in a pyrrole solution for electrochemical polymerization reaction to form a polypyrrole layer coating the surface of the honeycomb porous graphite, thereby obtaining the negative electrode active material; The mass ratio of the silicon particles, the N-isopropylacrylamide, the first crosslinking agent, and the first initiator is (5~10):(1~2):(0.02~0.05):(0.05~0.1). The mass ratio of the poly(N-isopropylacrylamide) layer coated with silicon particles, the aniline, the second crosslinking agent, and the second initiator is (2~5):(1~1.2):(0.15~0.2):(0.4~0.5). The surface density of the polystyrene particles being sprayed is 0.1~0.3 g / cm³. 2 ; The mass ratio of the silicon-based material to the volume of the third solvent is (1~2g):(10~50mL). The mass ratio of the honeycomb porous graphite to the volume of the suspension is (1.5~2.5g):(5~20mL). The ratio of the mass of the honeycomb porous graphite with silicon-based material embedded in the pores to the volume of the pyrrole solution is (2.5~4.5g):(100~150mL).

7. The method for preparing a secondary battery according to claim 6, characterized in that, The graphite sheets have an average thickness of 100-200 μm and an average diameter of 10-50 μm.

8. The method for preparing a secondary battery according to claim 6, characterized in that, The temperature of the first polymerization reaction is 70~90℃; and / or the time of the first polymerization reaction is 6~12h; And / or, the temperature of the second polymerization reaction is 0~5℃; and / or, the time of the second polymerization reaction is 12~24h; And / or, the calcination temperature is 400~600℃; and / or, the calcination time is 1~3h; And / or, the impregnation treatment is carried out under vacuum conditions; and / or, the temperature of the impregnation treatment is 25~60°C; and / or, the time of the impregnation treatment is 30~120 min; And / or, the current density of the electrochemical polymerization reaction is 0.1~1 mA / cm². 2 ; and / or, the temperature of the electrochemical polymerization reaction is 25~40℃; and / or, the time of the electrochemical polymerization reaction is 1~5h.

9. The method for preparing a secondary battery according to claim 6, characterized in that, The average particle size of the silicon particles is 50-100 nm; and / or the average particle size of the polystyrene particles is 150-300 nm; and / or the number-average molecular weight of the polystyrene particles is 50,000-60,000 Da. And / or, the first crosslinking agent is N,N'-methylenebisacrylamide and / or polyethylene glycol diacrylate; And / or, the first initiator is ammonium persulfate and / or azobisisobutyramidine hydrochloride; And / or, the first solvent is selected as water and / or ethanol; And / or, the second crosslinking agent is dibenzoic acid disulfide; And / or, the second initiator is ammonium persulfate and / or ferric chloride; And / or, the second solvent is an aqueous solution of HCl; And / or, the third solvent is N-methylpyrrolidone and / or dimethyl sulfoxide; And / or, the pyrrole solution comprises pyrrole, water, oxidant and dopant, wherein the mass ratio of the pyrrole, the water, the oxidant and the dopant is (1~3):(10~12):(0.5~0.7):(0.2~0.4).

10. The method for preparing a secondary battery according to claim 9, characterized in that, The oxidant is selected from one or more of FeCl3, ammonium persulfate, and sodium persulfate; and / or the dopant is selected from one or more of sodium p-toluenesulfonate, sodium dodecylbenzenesulfonate, and naphthalenesulfonic acid. And / or, the mass ratio of the negative electrode active material, the binder and the conductive agent is (96~97.5):(0.8~1.2):(2.4~3.2).

11. An energy storage system comprising multiple cell units, characterized in that, The unit cell is a secondary battery as described in any one of claims 1 to 5 or a secondary battery prepared by the method for preparing a secondary battery as described in any one of claims 6 to 10.

12. An electrical appliance, characterized in that, The energy storage system includes the energy storage system of claim 11, which is used to provide power to the electrical equipment.

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