Secondary battery and preparation method thereof, energy storage system and electric equipment

Abstract

The embodiment of the invention relates to the field of energy storage, and provides a secondary battery and a preparation method thereof, an energy storage system and electric equipment. The negative electrode active material comprises a silicon-based material, honeycomb porous graphite and a polypyrrole layer, the surface of the honeycomb porous graphite is coated with the polypyrrole layer, the silicon-based material is embedded in pores of the honeycomb porous graphite, and the silicon-based material comprises silicon particles, a poly N-isopropylacrylamide layer and a disulfide bond-containing polyaniline layer. The poly (N-isopropylacrylamide) layer and the disulfide bond-containing polyaniline layer are coated on the surfaces of the silicon particles, the poly (N-isopropylacrylamide) layer is positioned between the silicon particles and the disulfide bond-containing polyaniline layer, and the honeycomb porous graphite comprises a graphite flake and a carbon layer which is positioned on the surface of the graphite flake and is of a honeycomb pore structure. The secondary battery formed by assembling the negative electrode active material disclosed by the invention has relatively high cycling stability, high temperature resistance, rate capability, gram volume and safety.

Description

Technical Field

[0001] The present application relates to the field of energy storage, and in particular to a secondary battery and a preparation method thereof, an energy storage system and electrical equipment. Background Art

[0002] At present, the battery cycle life, graphite material gram capacity, and electrode expansion can meet the requirements of conventional batteries. However, in certain extreme environments or when there are higher requirements for cycle life and capacity, the production of the battery will have a certain impact on these functions.

[0003] The key to battery cycle life, cell capacity and electrode expansion lies in the production of the electrode as well as its composition and structure. Summary of the Invention

[0004] The main purpose of the present invention is to provide a secondary battery and a preparation method thereof, an energy storage system and an electrical device to solve the problems of low cycle life and gram capacity of batteries in the prior art.

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

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

[0007] Furthermore, the number average molecular weight of the poly N-isopropylacrylamide in the poly N-isopropylacrylamide layer is 8000 to 20000 Da; and / or the number average molecular weight of the disulfide bond-containing polyaniline in the disulfide bond-containing polyaniline layer is 10000 to 30000 Da.

[0008] Furthermore, the density of disulfide bonds in the above-mentioned 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 diameter 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 above-mentioned 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 polypyrrole in the polypyrrole layer is 15,000-35,000 Da; 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).

[0010] According to another aspect of the present invention, a method for preparing a secondary battery is provided, wherein a negative electrode active material, a conductive agent, a binder and a solvent are sequentially mixed and homogenized, coated and dried to obtain a positive electrode sheet; the positive electrode sheet, the separator and the negative electrode sheet are made into a bare battery cell, the bare battery cell is assembled with a shell and a top cover, and an electrolyte is injected to obtain a secondary battery; 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 poly-N-isopropylacrylamide layer-coated silicon particles; S2, poly-N-isopropylacrylamide layer-coated silicon particles, aniline, a second crosslinking agent containing disulfide bonds, a second initiator and a first solvent are mixed and subjected to a first polymerization reaction to obtain poly-N-isopropylacrylamide layer-coated silicon particles; After the two solvents are mixed, a second polymerization reaction is carried out to form a disulfide bond-containing polyaniline layer coated on the surface of the poly N-isopropylacrylamide layer, thereby obtaining a silicon-based material; S3, the graphite powder is pressed into a graphite sheet, polystyrene particles are sprayed on the surface of the graphite sheet, and then calcined to obtain a honeycomb porous graphite; S4, the silicon-based material is dispersed in a third solvent to obtain a suspension, the honeycomb porous graphite is placed in the suspension for immersion treatment, and the honeycomb porous graphite with the silicon-based material embedded in the pores is obtained; S5, the honeycomb porous graphite with the silicon-based material embedded in the pores is placed in a pyrrole solution for 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] Furthermore, the mass ratio of the silicon particles, N-isopropylacrylamide, the first crosslinking agent and the 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-coated silicon particles, aniline, the second crosslinking agent and the 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 spraying surface density of the polystyrene particles is 0.1-0.3 g / cm 2 ; and / or, the ratio of the mass of the silicon-based material to the volume of the third solvent is (1-2 g):(10-50 mL); and / or, the ratio of the mass of the honeycomb porous graphite to the volume of the suspension is (1.5-2.5 g):(5-20 mL); and / or, the ratio of the mass of the honeycomb porous graphite with the silicon-based material embedded in the pores to the volume of the pyrrole solution is (2.5-4.5 g):(100-150 mL).

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

[0013] Furthermore, the average particle size of the silicon particles is 50 to 100 nm; and / or the average particle size of the polystyrene particles is 150 to 300 nm; and / or the number average molecular weight of the polystyrene particles is 50,000 to 60,000 Da; and / or the first cross-linking 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 ethyl acetate. alcohol; and / or, the second cross-linking 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, an oxidant and a 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] Furthermore, the above-mentioned 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, the binder and the conductive agent is (96-97.5):(0.8-1.2):(2.4-3.2).

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

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

[0017] The technical solution provided by the embodiments of the present application has at least the following advantages:

[0018] In the present application, the polypyrrole layer is coated on the surface of the honeycomb porous graphite, which helps to suppress the expansion of silicon particles on the one hand and helps to improve the conductivity of the negative electrode active material on the other hand. The silicon-based material is embedded in the pores of the honeycomb porous graphite. The presence of the honeycomb pores helps to disperse the stress through the deformation of the pore wall during charging and discharging, reducing the cracking of the graphite layer due to silicon expansion. The swelling degree of the poly N-isopropylacrylamide layer coated on the surface of the silicon particles can change with temperature. At high temperatures, the poly N-isopropylacrylamide layer shrinks, which helps to suppress the expansion of the volume of the silicon particles; at low temperatures, the poly N-isopropylacrylamide layer swells, which helps to enhance the diffusion of lithium ions. The disulfide bond-containing polyaniline layer achieves self-repair of cracks by breaking / recombining disulfide bonds during the charge and discharge process, which helps to maintain the integrity of the conductive network. The present application compounds silicon and graphite, which helps to improve the gram capacity of the negative electrode active material. Therefore, the negative electrode sheet prepared using the negative electrode active material of the present application has high adhesion, high temperature stability, and low expansion rate, and the assembled secondary battery has high cycle stability, high temperature resistance, rate performance, gram capacity and safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] One or more embodiments are exemplified by the figures in the corresponding drawings. These exemplified descriptions do not constitute a limitation on the embodiments. Unless otherwise stated, the figures in the drawings do not constitute a scale limitation. In order to more clearly illustrate the embodiments of the present application or the technical solutions in the traditional technology, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 A schematic diagram of the structure of honeycomb porous graphite with silicon-based materials embedded in the pores of the present application is shown;

[0021] Figure 2 The process flow chart of preparing secondary batteries in this application is shown.

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

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

[0024] As analyzed in the background technology of this application, the battery cycle life and gram capacity in the prior art are relatively low. In order to solve this problem, this application provides a secondary battery and a preparation method thereof, an energy storage system and an electrical device.

[0025] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

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

[0027] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists, A and B exist at the same time, and B exists. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0028] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0029] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0030] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0031] In the accompanying drawings corresponding to the embodiments of the present application, the thickness and area of ​​the layers are exaggerated for better understanding and ease of description. When describing a component (such as a layer, film, region or substrate) on another component or on the surface of another component, the component can be "directly" located on the surface of the other component, or there can be a third component between the two components. On the contrary, when describing a component on the surface of another component or when another component is formed or provided on the surface of a component, it means that there is no third component between the two components. In addition, when describing a component as "approximately" 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 partial edge of the entire surface.

[0032] In the description of the embodiments of the present application, when a component "includes" another component, unless otherwise specified, other components are not excluded, and other components may be further included. In addition, 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 another component may be present therebetween. In addition, 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 means that no other components are located therebetween.

[0033] The terms used herein in the description of the various embodiments are intended only to describe the specific embodiments and are not intended to be limiting. As used in the description of the various embodiments and the appended claims, "part" is intended to include the plural form unless the context clearly indicates otherwise. A component includes a layer, film, region, or plate.

[0034] The following detailed description of the various embodiments of the present application is provided in conjunction with the accompanying drawings. However, those skilled in the art will appreciate that many technical details are provided in the various embodiments of the present application to facilitate a better understanding of the present application. However, even without these technical details and the various variations and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0035] In a typical embodiment of the present application, a secondary battery is provided, comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, the negative electrode sheet comprising a current collector and a negative electrode active layer, the material of the negative electrode active layer comprising a negative electrode active material, a binder and a conductive agent, the negative electrode active material comprising a silicon-based material, a honeycomb porous graphite and a polypyrrole layer, the polypyrrole layer being coated on the surface of the honeycomb porous graphite, the silicon-based material being embedded in the pores of the honeycomb porous graphite, the silicon-based material comprising silicon particles, a poly (N-isopropylacrylamide) layer and a disulfide bond-containing polyaniline layer, the poly (N-isopropylacrylamide) layer and the disulfide bond-containing polyaniline layer being coated on the surface of the silicon particles, and the poly (N-isopropylacrylamide) layer being located between the silicon particles and the disulfide bond-containing polyaniline layer, the honeycomb porous graphite comprising a graphite sheet and a carbon layer presenting a honeycomb pore structure located on the surface of the graphite sheet, such as Figure 1 As shown, the silicon-based material 1 is embedded in the honeycomb pores of the carbon layer 3 located on the surface of the graphite sheet 2.

[0036] In the present application, the polypyrrole layer is coated on the surface of the honeycomb porous graphite (the surface here refers to the outer surface of the honeycomb porous graphite, Figure 1 (Polypyrrole layer not shown in the figure), on the one hand, helps to suppress the expansion of silicon particles, and on the other hand helps to 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 the honeycomb pores helps to disperse the stress through the deformation of the pore wall during charge and discharge, reducing the cracking of the graphite layer due to silicon expansion. The swelling degree of the poly N-isopropylacrylamide layer coated on the surface of the silicon particles can change with temperature. At high temperatures, the poly N-isopropylacrylamide layer shrinks, which helps to suppress the expansion of the volume of the silicon particles; at low temperatures, the poly N-isopropylacrylamide layer swells, which helps to enhance the diffusion of lithium ions. The disulfide bond-containing polyaniline layer achieves crack self-repair by disulfide bond rupture / recombination during the charge and discharge process, which helps to maintain the integrity of the conductive network. The present application compounds silicon and graphite, which helps to improve the gram capacity of the negative electrode active material. Therefore, the negative electrode sheet prepared using the negative electrode active material of the present application has high adhesion, high temperature stability, and low expansion rate, and the secondary battery assembled has high cycle stability, high temperature resistance, rate performance, gram capacity and safety.

[0037] It should be noted that the positive electrode sheet, separator and electrolyte of the present application can all be purchased or prepared using existing technologies.

[0038] Including but not limited to, the above-mentioned conductive agent is selected from any one or more of super conductive carbon black (Super-P), carbon nanotubes, graphene and carbon fiber; the above-mentioned 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 the present application, the average particle size of the above-mentioned silicon particles is 50 to 100 nm; and / or the average thickness of the poly (N-isopropylacrylamide) layer is 45 to 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 a range between any two values; and / or the average thickness of the disulfide bond-containing polyaniline layer is 27 to 33 nm, which can be selected from 27 nm, 28 nm, 29 nm, 30 nm, 31 nm, 32 nm, 33 nm, and a range between any two values.

[0040] Controlling the average particle size of silicon particles within the above range helps to reduce silicon particle agglomeration while allowing silicon particles to have a higher specific surface area, providing more interfaces for lithium ion embedding and de-embedding, thereby helping to increase the battery's gram capacity. Controlling the average thickness of the poly (N-isopropylacrylamide) layer within the above range helps to allow the poly (N-isopropylacrylamide) layer to shrink in a high-temperature environment without affecting the lithium ion transmission efficiency, clinging to the surface of the silicon particles and 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 the transmission of lithium ions, and improving the battery's rate performance and high-temperature resistance. Controlling the average thickness of the disulfide bond-containing polyaniline layer within the above range helps to allow the disulfide bond-containing polyaniline layer to repair cracks in a timely manner during the charge and discharge process without affecting the lithium ion transmission efficiency, thereby improving the structural stability of the negative electrode active material during the cycle process, thereby helping to improve the battery's cycle performance and safety.

[0041] In one embodiment of the present application, the number average molecular weight of the poly N-isopropylacrylamide in the poly N-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 a range between any two values; and / or the number average molecular weight of the disulfide bond-containing polyaniline in the disulfide bond-containing polyaniline layer is 10000 to 30000 Da, and can be selected from 10000 Da, 15000 Da, 20000 Da, 25000 Da, 30000 Da, and a range between any two values.

[0042] Controlling the number average molecular weight of poly(N-isopropylacrylamide) in the poly(N-isopropylacrylamide) layer within the above range 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 inhibit silicon particle expansion and maintain electrode stability, thereby improving the battery's cycling performance, heat resistance, and safety. Controlling the number average molecular weight of the disulfide-containing polyaniline in the disulfide-containing polyaniline layer within the above range helps to balance the self-healing and conductive properties of the disulfide-containing polyaniline layer, thereby helping the negative electrode active material maintain good electrochemical activity and mechanical stability during cycling.

[0043] In one embodiment of the present application, the density of disulfide bonds in the above-mentioned 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 gives the polyaniline layer a unique self-healing ability. Controlling the density of disulfide bonds in the disulfide bond-containing polyaniline layer within the above range helps to improve the self-healing performance of the disulfide bond-containing polyaniline layer while maintaining the high flexibility of the disulfide bond-containing polyaniline layer, thereby helping to improve the structural stability of the negative electrode active material, and further helping to improve the cycle stability of the battery. Controlling the porosity of the honeycomb porous graphite within the above range helps to provide sufficient space for embedding silicon-based materials while maintaining the stability of the negative electrode active material, thereby helping to improve the gram capacity of the secondary battery. Controlling the average pore size of the honeycomb porous graphite within the above range helps to optimize the embedding efficiency of the silicon-based material, while promoting the rapid transmission of lithium ions, and helping to improve the rate performance of the secondary battery. Controlling the average thickness of the carbon layer, the average thickness of the graphite sheet and the average sheet diameter within the above range helps to improve the gram capacity of the negative electrode active material while improving the stability of the silicon-based material in the honeycomb pores.

[0045] In order to further increase the embedding amount of the silicon-based material and thus increase the gram capacity of the secondary battery, in one embodiment of the present application, the average pore diameter of the honeycomb porous graphite is 230 to 300 nm.

[0046] In one embodiment of the present application, the mass ratio of the above-mentioned 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 polypyrrole in the polypyrrole layer is 15,000-35,000 Da; 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).

[0047] Controlling the mass ratio of the silicon-based material to the honeycomb porous graphite within the above range helps to reduce the expansion rate of the negative electrode active material during cycling while increasing the gram capacity of the negative electrode active material. Controlling the average thickness of the polypyrrole layer and the number average molecular weight of the polypyrrole within the above range helps to maintain a high lithium ion transmission efficiency of the negative electrode active material while improving the structural stability and conductivity of the negative electrode active material. Controlling the mass ratio of the negative electrode active material, the binder, and the conductive agent within the above range helps to increase the gram capacity of the negative electrode sheet while improving the cycling stability of the negative electrode sheet.

[0048] In another typical embodiment of the present application, a method for preparing a secondary battery is provided, wherein a negative electrode active material, a conductive agent, a binder and a solvent are sequentially mixed and homogenized, coated and dried to obtain a positive electrode sheet; the positive electrode sheet, the separator and the negative electrode sheet are made into a bare battery cell, the bare battery cell is assembled with a shell and a top cover, and then an electrolyte is injected to obtain a secondary battery; the preparation steps of the negative electrode active material include: S1, mixing silicon particles, N-isopropylacrylamide, a first cross-linking agent, a first initiator and a first solvent, and then performing a first polymerization reaction to obtain poly N-isopropylacrylamide layer-coated silicon particles; S2, coating the poly N-isopropylacrylamide layer with silicon particles; The silicon-coated particles, aniline, a second cross-linking agent containing disulfide bonds, a second initiator, and a second solvent are mixed and subjected to a second polymerization reaction to form a disulfide bond-containing polyaniline layer coated on the surface of the poly N-isopropylacrylamide layer, thereby obtaining a silicon-based material; S3, the graphite powder is pressed into a graphite sheet, polystyrene particles are sprayed on the surface of the graphite sheet and then calcined to obtain a honeycomb porous graphite; S4, the silicon-based material is dispersed in a third solvent to obtain a suspension, the honeycomb porous graphite is placed in the suspension and immersed in the suspension to obtain a honeycomb porous graphite with the silicon-based material embedded in the pores. The structural schematic diagram of the honeycomb porous graphite with the silicon-based material embedded in the pores is shown in FIG. Figure 1 As shown; S5, placing the honeycomb porous graphite with silicon-based materials embedded in the pores in a pyrrole solution for electrochemical polymerization to form a polypyrrole layer coated on the surface of the honeycomb porous graphite, thereby obtaining a negative electrode active material.

[0049] In step S1, silicon particles, N-isopropylacrylamide, a first cross-linking agent, a first initiator and a first solvent are mixed and then subjected to a first polymerization reaction. N-isopropylacrylamide and the first cross-linking agent undergo a polymerization reaction under the action of the first initiator to form a poly N-isopropylacrylamide layer on the surface of the silicon particles. The swelling degree of the poly N-isopropylacrylamide layer can change with temperature. At high temperature, the poly N-isopropylacrylamide layer shrinks, which helps to inhibit the expansion of the volume of the silicon particles; at low temperature, the poly N-isopropylacrylamide layer swells, which helps to enhance the diffusion of lithium ions. In step S2, silicon particles coated with the poly N-isopropylacrylamide layer, aniline, a second cross-linking agent containing a disulfide bond, a second initiator and a second solvent are mixed and then subjected to a second polymerization reaction. Aniline and the second cross-linking agent containing a disulfide bond undergo a polymerization reaction under the action of the second initiator. A disulfide bond-containing polyaniline layer is formed on the poly N-isopropylacrylamide layer. The disulfide bond-containing polyaniline layer achieves crack self-repair by breaking / recombining disulfide bonds during the charge and discharge process, which helps Maintain the integrity of the conductive network; in step S3, the graphite powder is pressed into a graphite sheet, the graphite sheet is used as a substrate, and polystyrene particles are sprayed on the surface of the graphite sheet. The polystyrene particles self-assemble on the surface of the graphite sheet to form a tightly arranged hexagonal close-packed structure, which is then calcined to form a carbon layer with a honeycomb pore structure on the surface of the graphite sheet; in step 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 immersion treatment. The silicon-based material is adsorbed into the pores of the honeycomb porous graphite. The presence of the honeycomb pores helps to disperse stress through pore wall deformation during charging and discharging, thereby avoiding cracking of the graphite layer caused by silicon expansion; in step S5, the honeycomb porous graphite with the silicon-based material embedded in the pores is placed in a pyrrole solution for electrochemical polymerization reaction. The pyrrole solution forms a polypyrrole layer coated on the surface of the honeycomb porous graphite through the electrochemical polymerization reaction. The presence of the polypyrrole layer helps to inhibit the expansion of silicon particles on the one hand, and on the other hand helps to improve the conductivity of the negative electrode active material. The present invention combines silicon and graphite to improve the specific capacity of the negative electrode active material. Consequently, the negative electrode sheet produced using the present invention exhibits high adhesion, high-temperature stability, and low expansion rate, and the assembled secondary battery exhibits high cycle stability, specific capacity, and safety.

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

[0051] In one embodiment of the present application, the above S1 further includes ultrasonically treating the silicon particles with anhydrous ethanol to remove oxides on the surface.

[0052] In one embodiment of the present application, the mass ratio of the silicon particles, N-isopropylacrylamide, the first cross-linking agent and the 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-coated silicon particles, aniline, the second cross-linking agent and the 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 spraying surface density of the polystyrene particles is 0.1-0.3 g / cm 2 ; and / or, the ratio of the mass of the silicon-based material to the volume of the third solvent is (1-2 g):(10-50 mL); and / or, the ratio of the mass of the honeycomb porous graphite to the volume of the suspension is (1.5-2.5 g):(5-20 mL); and / or, the ratio of the mass of the honeycomb porous graphite with the 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 crosslinker, and the first initiator within the aforementioned range helps form a poly(N-isopropylacrylamide) layer of suitable thickness. This allows the poly(N-isopropylacrylamide) layer to shrink at high temperatures, clinging to the silicon particle surface and effectively limiting its expansion, while not affecting lithium ion transmission efficiency. At low temperatures, the poly(N-isopropylacrylamide) layer swells, increasing lithium ion diffusion channels and accelerating lithium ion transmission, thereby improving the battery's rate performance and high-temperature resistance. Controlling the mass ratio of the poly(N-isopropylacrylamide) layer coating the silicon particles, aniline, the second crosslinker, and the second initiator within the aforementioned range helps form a disulfide-bonded polyaniline layer of suitable thickness. This allows the disulfide-bonded polyaniline layer to repair cracks in a timely manner during charge and discharge without affecting lithium ion transmission efficiency, thereby improving the structural stability of the negative electrode active material during cycling and ultimately enhancing the battery's cycling performance and safety. Controlling the average thickness, average sheet diameter, and spray density of the polystyrene particles of the graphite flakes within the aforementioned ranges helps form a carbon layer of appropriately thick, honeycomb-like pore structure on the surface of the graphite flakes, thereby improving the stability of the silicon-based material within the honeycomb pores while also increasing the gram capacity of the negative electrode active material. Controlling the ratio of the mass of the silicon-based material to the volume of the third solvent and the mass of the honeycomb porous graphite to the volume of the suspension within the aforementioned ranges helps control the mass ratio of the silicon-based material to the honeycomb porous graphite within an appropriate range, thereby helping to reduce the expansion rate of the negative electrode active material during cycling while also increasing the gram capacity of the negative electrode active material. Controlling the ratio of the mass of the honeycomb porous graphite with the silicon-based material embedded in the pores to the volume of the pyrrole solution within the aforementioned range helps form a polypyrrole layer of appropriately thick, thereby helping to improve the structural stability and conductivity of the negative electrode active material while maintaining a high lithium ion transport efficiency.

[0054] In one embodiment of the present application, graphite powder is pressed into graphite sheets using a powder tablet press, and the mold containing the graphite sheets is moved to a high-pressure airless sprayer. Polystyrene particles are first sprayed on one side of the graphite sheet, and then the mold is turned over and polystyrene particles are sprayed on the other side of the graphite sheet.

[0055] In one embodiment of the present 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 / cm 2 ; and / or, the temperature of the electrochemical polymerization reaction is 25 to 40° C.; and / or, the time of the electrochemical polymerization reaction is 1 to 5 hours.

[0056] Controlling the temperature and time of the first polymerization reaction within the above range helps to control the number average molecular weight of poly N-isopropylacrylamide within a suitable range, thereby helping to make the poly N-isopropylacrylamide layer have sufficient elasticity to buffer the volume change of silicon particles, while maintaining good thermal responsiveness and mechanical strength, thereby helping to inhibit the expansion of silicon particles and maintain the stability of the electrode. Controlling the temperature and time of the second polymerization reaction within the above range helps to control the number average molecular weight of disulfide bond polyaniline within a suitable range, thereby helping to take into account the self-healing properties and conductive properties of the disulfide bond polyaniline layer, thereby helping to make the negative electrode active material maintain good electrochemical activity and mechanical stability during the cycle. Controlling the temperature and time of the calcination treatment within the above range helps to form honeycomb porous graphite with appropriate porosity and average pore size, thereby helping 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 the rapid transmission of lithium ions, thereby helping to improve the gram capacity and rate performance of the secondary battery. Controlling the temperature and time of the impregnation treatment within the above 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 ranges helps to control the number average molecular weight of polypyrrole within an appropriate range, thereby helping to improve the structural stability and conductivity of the negative electrode active material while maintaining a high lithium ion transport efficiency of the negative electrode active material.

[0057] In one embodiment of the present application, the average particle size of the silicon particles is 50 to 100 nm; and / or, the average particle size of the polystyrene particles is 150 to 300 nm; and / or, the number average molecular weight of the polystyrene particles is 50,000 to 60,000 Da; and / or, the first cross-linking 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 from water and / or ethanol; and / or, the second cross-linking 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, an oxidant and a 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 above range helps to reduce the agglomeration of silicon particles while making the silicon particles have a higher specific surface area, which can provide more interfaces for the embedding and de-embedding of lithium ions, thereby helping to improve the gram capacity of the battery. Controlling the average particle size and number average molecular weight of polystyrene particles within the above range helps to control the porosity and average pore size of the honeycomb porous graphite within a suitable range, thereby helping to provide sufficient space for embedding silicon-based materials while maintaining the stability of the negative electrode active material, optimizing the embedding efficiency of the silicon-based material, and promoting the rapid transmission of lithium ions, thereby helping to improve the gram capacity and rate performance of the secondary battery. Controlling the types of the first cross-linking agent, the first initiator, the first solvent, the second cross-linking agent, the second initiator, the second solvent and the third solvent within the above range helps to improve the interaction between the components, thereby helping to improve the efficiency and purity of the preparation of the negative electrode active material. Controlling the mass ratio of pyrrole, water, oxidant and dopant within the above range helps to improve the generation efficiency of polypyrrole.

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

[0060] In one embodiment of the present application, the above-mentioned 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, the binder and the conductive agent is (96-97.5):(0.8-1.2):(2.4-3.2).

[0061] Controlling the types of oxidant and dopant within the above ranges helps further promote the formation of polypyrrole. Controlling the mass ratio of the negative electrode active material, binder, and conductive agent within the above ranges helps improve the negative electrode sheet's cyclic stability while increasing its gram capacity.

[0062] In another typical embodiment of the present application, an energy storage system is provided, comprising a plurality of unit cells, wherein the unit cells are the aforementioned secondary batteries or secondary batteries prepared by the aforementioned secondary battery preparation method.

[0063] Since the above energy storage system contains the secondary battery of the present application, the energy storage system has high gram capacity, rate performance, cycle stability and high temperature resistance.

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

[0065] Since the energy storage system of the above-mentioned electrical equipment contains the secondary battery of the present application, the electrical equipment has high gram capacity, rate performance, cycle stability, high temperature resistance and safety.

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

[0067] Example 1

[0068] Use Figure 2The process flow shown is used to prepare 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, deoxygenated with nitrogen for 30 minutes, heated 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, reacted for 9 hours, centrifuged and washed (deionized water × 3 times), and vacuum dried at 60°C to obtain poly (N-isopropylacrylamide). A poly(N-isopropylacrylamide) layer-coated silicon particles was prepared; aniline, dibenzoic acid disulfide, and a 1M HCl aqueous solution were mixed to obtain a second mixed solution, the poly(N-isopropylacrylamide) layer-coated silicon particles were dispersed in the second mixed solution, the mixture was cooled to 2°C, and ammonium persulfate was slowly added dropwise, wherein the mass ratio of the poly(N-isopropylacrylamide) layer-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, the reaction was carried out for 18 hours, the mixture was centrifuged and washed (ethanol × 3 times), and vacuum dried to obtain a silicon-based material.

[0069] Graphite powder was pressed into graphite sheets using a powder tablet 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 moved to a high-pressure airless sprayer. Polystyrene particles were first sprayed on one side of the graphite sheet. Then the mold was turned over and polystyrene particles were sprayed on the other side of the graphite sheet. The average particle size of the polystyrene particles was 280 nm, the number average molecular weight of the polystyrene particles was 55,000 Da, and the spraying surface density of the polystyrene particles was 0.2 g / cm 2 After spraying, calcination treatment is carried out at a temperature of 500° C. and a calcination time of 2 h to obtain honeycomb porous graphite.

[0070] The silicon-based material is dispersed in N-methylpyrrolidone to form a suspension, wherein the ratio of the mass of the silicon-based material to the volume of N-methylpyrrolidone is 2g:50mL. The honeycomb porous graphite is placed in the suspension for impregnation treatment at 40°C and maintained under vacuum conditions for 60 minutes. The ratio of the mass of the honeycomb porous graphite to the volume of the suspension is 2g:10mL. The impregnated graphite matrix is ​​dried at 80°C for 12 hours to remove the solvent to obtain a 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. The honeycomb porous graphite with silicon-based materials embedded in its pores was used as a working electrode, and electrochemical polymerization was carried out in the pyrrole solution. The mass ratio of the honeycomb porous graphite with silicon-based materials 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 / cm 2 , the temperature is 25 ° C, the time is 3 hours, and after the electropolymerization reaction, the product is dried at 60 ° C for 6 hours to obtain a negative electrode active material, wherein the negative electrode active material includes a silicon-based material, a 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 disulfide bond-containing polyaniline layer, the poly N-isopropylacrylamide layer and the disulfide bond-containing polyaniline layer are coated on the surface of the silicon particles, and the poly N-isopropylacrylamide layer is located between the silicon particles and the disulfide bond-containing polyaniline layer, the honeycomb porous graphite includes a graphite sheet and a carbon layer with a honeycomb pore structure located on the surface of the graphite sheet, the poly N-isopropylacrylamide layer and the disulfide bond-containing polyaniline layer are coated on the surface of the silicon particles, and the poly N-isopropylacrylamide layer is located between the silicon particles and the disulfide bond-containing polyaniline layer, the honeycomb porous graphite includes a graphite sheet and a carbon layer with a honeycomb pore structure located on the surface of the graphite sheet, and the poly N-isopropylacrylamide layer is located between the silicon particles and the disulfide bond-containing polyaniline layer. The average thickness of the acrylic acid amide layer is 50 nm, the number average molecular weight of poly N-isopropylacrylamide is 15,000 Da, the average thickness of the disulfide bond polyaniline layer is 30 nm, the number average molecular weight of the disulfide bond polyaniline is 20,000 Da, the density of disulfide bonds in the disulfide bond polyaniline layer is 3 disulfide bonds in the molecular chain, the porosity of the honeycomb porous graphite is 60%, the average pore diameter 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 the silicon-based material to the 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 25,000 Da.

[0072] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing 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, and the negative electrode active material is finally obtained.

[0075] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing 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 negative electrode active material is finally obtained.

[0078] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing 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. Finally, a 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 the poly N-isopropylacrylamide is 8000 Da.

[0081] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing 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. Finally, a negative electrode active material is obtained, wherein the average thickness of the poly N-isopropylacrylamide layer is 55 nm, and the number average molecular weight of the poly N-isopropylacrylamide is 20,000 Da.

[0084] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing 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. 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 the poly N-isopropylacrylamide is 6000 Da.

[0087] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0088] Example 7

[0089] The difference from Example 1 is that the mass ratio of the poly (N-isopropylacrylamide) layer-coated 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 12 hours. Finally, a negative electrode active material is obtained, wherein the average thickness of the disulfide bond-containing polyaniline layer is 27 nm, and the number average molecular weight of the disulfide bond-containing polyaniline in the disulfide bond-containing polyaniline layer is 10,000 Da.

[0090] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0091] Example 8

[0092] The difference from Example 1 is that the mass ratio of the poly (N-isopropylacrylamide) layer-coated 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 24 hours. Finally, a negative electrode active material is obtained, wherein the average thickness of the disulfide bond polyaniline layer is 33 nm, and the number average molecular weight of the disulfide bond polyaniline in the disulfide bond polyaniline layer is 30,000 Da.

[0093] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0094] Example 9

[0095] The difference from Example 1 is that the mass ratio of the poly (N-isopropylacrylamide) layer-coated 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. Finally, a negative electrode active material is obtained, wherein the average thickness of the disulfide bond polyaniline layer is 20 nm, and the number average molecular weight of the disulfide bond polyaniline in the disulfide bond polyaniline layer is 8000 Da.

[0096] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0097] Example 10

[0098] The difference from Example 1 is that the graphite powder is pressed into graphite sheets using a powder tablet press. The average thickness of the graphite sheets is 200 μm and the average sheet diameter is 10 μm. The mold containing the graphite sheets is moved to a high-pressure airless sprayer. Polystyrene particles are first sprayed on one side of the graphite sheet. Then, the mold is turned over and polystyrene particles are sprayed on the other side of the graphite sheet. The average particle size of the polystyrene particles is 260 nm, the number average molecular weight of the polystyrene particles is 60,000 Da, and the spraying surface density of the polystyrene particles is 0.3 g / cm 2 After spraying, calcination treatment is carried out at a temperature of 600°C and a calcination time of 1 hour to finally obtain a negative electrode active material, wherein the porosity of the honeycomb porous graphite is 50%, the average pore size of the honeycomb porous graphite is 260nm, and the average thickness of the carbon layer is 5μm.

[0099] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0100] Example 11

[0101] The difference from Example 1 is that the graphite powder is pressed into graphite sheets using a powder tablet press. The average thickness of the graphite sheets is 100 μm and the average sheet diameter is 50 μm. The mold containing the graphite sheets is moved to a high-pressure airless sprayer. Polystyrene particles are first sprayed on one side of the graphite sheet. Then, the mold is turned over and polystyrene particles are sprayed on the other side of the graphite sheet. The average particle size of the polystyrene particles is 300 nm, the number average molecular weight of the polystyrene particles is 50,000 Da, and the spraying surface density of the polystyrene particles is 0.1 g / cm 2 After spraying, calcination treatment is carried out at a temperature of 400°C and a calcination time of 3 hours to finally obtain a negative electrode active material, wherein the porosity of the honeycomb porous graphite is 65%, the average pore size of the honeycomb porous graphite is 300nm, and the average thickness of the carbon layer is 1μm.

[0102] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0103] Example 12

[0104] The difference from Example 1 is that the graphite powder is pressed into graphite sheets using a powder tablet press. The average thickness of the graphite sheets is 100 μm and the average sheet diameter is 50 μm. The mold containing the graphite sheets is moved to a high-pressure airless sprayer. Polystyrene particles are first sprayed on one side of the graphite sheet. Then, the mold is turned over and polystyrene particles are sprayed on the other side of the graphite sheet. The average particle size of the polystyrene particles is 300 nm, the number average molecular weight of the polystyrene particles is 50,000 Da, and the spraying surface density of the polystyrene particles is 0.5 g / cm 2 After spraying, calcination treatment is carried out at a temperature of 700°C and a calcination time of 4 hours to finally obtain a negative electrode active material, wherein the porosity of the honeycomb porous graphite is 40%, the average pore size of the honeycomb porous graphite is 300nm, and the average thickness of the carbon layer is 8μm.

[0105] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0106] Example 13

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

[0108] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0109] Example 14

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

[0111] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0112] Example 15

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

[0114] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0115] Example 16

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

[0117] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0118] Example 17

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

[0120] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0121] Example 18

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

[0123] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0124] Comparative Example 1

[0125] The difference from Example 1 is that the addition of silicon particles is eliminated, and the negative electrode active material is finally obtained.

[0126] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing 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 is eliminated, and the negative electrode active material is finally obtained.

[0129] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0130] Comparative Example 3

[0131] The difference from Example 1 is that the addition of aniline is eliminated, and the negative electrode active material is finally obtained.

[0132] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0133] Comparative Example 4

[0134] The difference from Example 1 is that dibenzoic acid is used to replace dibenzoic acid disulfide, and the negative electrode active material is finally obtained.

[0135] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0136] Comparative Example 5

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

[0138] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

[0139] Comparative Example 6

[0140] The difference from Example 1 is that the addition of pyrrole is eliminated, and the negative electrode active material is finally obtained.

[0141] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing 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 to undergo an electrochemical polymerization reaction, thereby finally obtaining a negative electrode active material.

[0144] The negative electrode active material, Super-P, polyvinylidene fluoride and N-methylpyrrolidone prepared above are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing 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 are mixed and homogenized, and the mass ratio of the negative electrode active material, polyvinylidene fluoride and Super-P is 97:1:3. Then, the negative electrode sheet is obtained by coating, rolling, slitting, tab forming and cutting. The lithium iron phosphate positive electrode sheet, negative electrode sheet and polyethylene separator are stacked into a bare battery cell, and the bare battery cell is sequentially subjected to tab welding, packaging, liquid injection, formation and vacuum sealing to obtain a secondary battery.

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

[0149] Table 1

[0150]

[0151]

[0152] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0153] From the data in Table 1, it can be seen that the difference between Examples 1 to 3 is that the average particle size of the silicon particles is different. When the average particle size of the silicon particles is in the range of 50 to 100 nm, the stability and activity of the negative electrode active material are stronger, which helps to make the secondary battery have a higher first discharge gram capacity and cycle stability, and the expansion rate of the negative electrode sheet is lower; the difference between Examples 1 and 4 to 6 is that the average thickness of the poly N-isopropylacrylamide layer and the number average molecular weight of the poly N-isopropylacrylamide are different. When the average thickness of the poly N-isopropylacrylamide layer is 45 to 55 nm and the number average molecular weight of the poly N-isopropylacrylamide is 8000 Da to 2000 Da, the negative electrode active material has a higher stability and activity, which helps to make the secondary battery have a higher first discharge gram capacity and cycle stability, and the negative electrode sheet has a lower expansion rate; when the average thickness of the poly N-isopropylacrylamide layer is 45 to 55 nm and the number average molecular weight of the poly N-isopropylacrylamide is 8000 Da to 2000 Da, the negative electrode sheet has a lower expansion rate. When the average thickness of the disulfide bond polyaniline layer is between 27 and 33 nm, and the number average molecular weight of the disulfide bond polyaniline in the disulfide bond polyaniline layer is between 10,000 and 30,000 Da, it helps to further maintain the integrity of the conductive network, thereby making the secondary battery have higher cycle stability; the difference between Examples 1 and 10 to 12 is that the pores of the honeycomb porous graphite The porosity, average pore diameter of the honeycomb porous graphite, and average thickness of the carbon layer are different. When the porosity of the honeycomb porous graphite is 50-65%, the average pore diameter of the honeycomb porous graphite is 260-300 nm, and the average thickness of the carbon layer is 1-5 μm, more silicon-based materials can be embedded in the honeycomb porous graphite, so that the secondary battery has a higher first discharge gram capacity, and the structure of the honeycomb porous graphite is more stable, which can further inhibit the expansion of silicon particles, thereby helping to reduce the expansion rate of the negative electrode sheet; the difference between Examples 1 and 13-15 is that the mass ratio of the silicon-based material to the honeycomb porous graphite is different. When the mass ratio of the silicon-based material to the honeycomb porous graphite is 50-65%, the average pore diameter of the honeycomb porous graphite is 260-300 nm, and the average thickness of the carbon layer is 1-5 μm, more silicon-based materials can be embedded in the honeycomb porous graphite, so that the secondary battery has a higher first discharge gram capacity, and the structure of the honeycomb porous graphite is more stable, which can further inhibit the expansion of silicon particles, thereby helping to reduce the expansion rate of the negative electrode sheet. When the mass ratio of the porous graphite is in the range of (94-95):(2-2.5), the silicon-based material and the honeycomb porous graphite have good adaptability, which can further improve the cycle stability of the negative electrode sheet and reduce the expansion rate of the negative electrode sheet; the difference between Examples 1 and 16-18 is that the average thickness of the polypyrrole layer and the number average molecular weight of the polypyrrole are different. When the average thickness of the polypyrrole layer is 50-150 nm and the number average molecular weight of the polypyrrole is in the range of 15000 Da-35000 Da, the negative electrode active material has higher conductivity and stability, thereby making the secondary battery have higher first discharge gram capacity and cycle capacity retention rate.

[0154] Although the omission of silicon particles in Comparative Example 1 can reduce the expansion of the negative electrode sheet, the corresponding first discharge gram capacity is significantly reduced; the negative electrode active material of Comparative Example 2 does not form a poly (N-isopropylacrylamide) layer, which is not conducive to inhibiting the expansion of silicon particles, especially the expansion at high temperature, resulting in a significant increase in the expansion rate of the negative electrode sheet after cycling; the negative electrode active material of Comparative Example 3 does not form a polyaniline layer containing disulfide bonds, and the negative electrode active material has insufficient self-repairing ability during the cycle, resulting in poor cycle stability of the secondary battery; although a polyaniline layer is formed in the negative electrode active material of Comparative Example 4, it does not contain disulfide bonds, and the negative electrode active material has no self-repairing ability during the cycle, so the secondary battery has poor cycle stability; the negative electrode active material of Comparative Example 5 does not inhibit The carbon layer with a honeycomb pore structure for the expansion of silicon particles causes a significant increase in the expansion rate of the negative electrode sheet after cycling; no polypyrrole layer is formed in the negative electrode active material of Comparative Example 6, resulting in too low electrical conductivity and reduced structural stability of the negative electrode active material, so that the secondary battery has a lower first discharge capacity and cycle stability; the negative electrode active material of Comparative Example 7 does not contain honeycomb porous graphite, and the expansion of silicon particles is difficult to be effectively suppressed, resulting in a higher expansion rate of the negative electrode sheet; the negative electrode active material of Comparative Example 8 does not contain honeycomb porous graphite, polypyrrole layer, poly N-isopropylacrylamide layer and disulfide bond-containing polyaniline layer, and the silicon particles have poor stability during the cycle, which seriously reduces the cycle capacity retention rate of the secondary battery and causes serious expansion of the negative electrode sheet.

[0155] In the present application, the polypyrrole layer is coated on the surface of the honeycomb porous graphite, which helps to suppress the expansion of silicon particles on the one hand and helps to improve the conductivity of the negative electrode active material on the other hand. The silicon-based material is embedded in the pores of the honeycomb porous graphite. The presence of the honeycomb pores helps to disperse the stress through the deformation of the pore wall during charging and discharging, reducing the cracking of the graphite layer due to silicon expansion. The swelling degree of the poly N-isopropylacrylamide layer coated on the surface of the silicon particles can change with temperature. At high temperatures, the poly N-isopropylacrylamide layer shrinks, which helps to suppress the expansion of the volume of the silicon particles; at low temperatures, the poly N-isopropylacrylamide layer swells, which helps to enhance the diffusion of lithium ions. The disulfide bond-containing polyaniline layer achieves self-repair of cracks by breaking / recombining disulfide bonds during the charge and discharge process, which helps to maintain the integrity of the conductive network. The present application compounds silicon and graphite, which helps to improve the gram capacity of the negative electrode active material. Therefore, the negative electrode sheet prepared using the negative electrode active material of the present application has high adhesion, high temperature stability, and low expansion rate, and the assembled secondary battery has high cycle stability, high temperature resistance, rate performance, gram capacity and safety.

[0156] Those skilled in the art will appreciate that the above-described embodiments are specific examples for implementing the present application, and that in actual applications, various changes in form and detail may be made thereto without departing from the spirit and scope of the present application. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be based on the scope defined in the claims.

Claims

1. A secondary battery comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a current collector and a negative electrode active layer, wherein 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, wherein the polypyrrole layer is coated on 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, wherein the poly (N-isopropylacrylamide) layer and the disulfide-bonded polyaniline layer are coated on 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 a graphite sheet and a carbon layer presenting a honeycomb pore structure located on the surface of the graphite sheet.

2. The secondary battery according to claim 1, wherein The average particle size of the silicon particles is 50 to 100 nm; and / or the average thickness of the poly (N-isopropylacrylamide) layer is 45 to 55 nm; and / or the average thickness of the disulfide bond-containing polyaniline layer is 27 to 33 nm.

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

4. The secondary battery according to claim 1, wherein The density of disulfide bonds 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 graphite sheet has an average thickness of 100 to 200 μm and an average sheet diameter of 10 to 50 μm.

5. The secondary battery according to any one of claims 1 to 4, characterized in that 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 polypyrrole in the polypyrrole layer is 15,000-35,000 Da; 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).

6. A method for preparing a secondary battery, comprising: mixing a negative electrode active material, a conductive agent, a binder, and a solvent, applying and drying them in sequence to obtain a positive electrode sheet; forming the positive electrode sheet, separator, and negative electrode sheet into a bare cell; assembling the bare cell with a housing and a top cover; and injecting an electrolyte to obtain the secondary battery; characterized in that: The steps of preparing 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 silicon particles coated with a poly (N-isopropylacrylamide) layer; S2, mixing the poly (N-isopropylacrylamide) layer-coated silicon particles, aniline, a second cross-linking agent containing disulfide bonds, a second initiator, and a second solvent, and performing a second polymerization reaction to form a disulfide bond-containing polyaniline layer coated on the surface of the poly (N-isopropylacrylamide) layer, thereby obtaining a silicon-based material; S3, pressing the graphite powder into graphite sheets, spraying polystyrene particles on the surface of the graphite sheets, and then calcining them to obtain honeycomb porous graphite; S4, dispersing the silicon-based material in a third solvent to obtain a suspension, placing the honeycomb porous graphite in the suspension for immersion treatment, to obtain a honeycomb porous graphite with the silicon-based material embedded in its pores; S5, placing the honeycomb porous graphite with silicon-based materials embedded in the pores in a pyrrole solution for electrochemical polymerization to form a polypyrrole layer covering the surface of the honeycomb porous graphite, thereby obtaining the negative electrode active material.

7. The method for preparing a secondary battery according to claim 6, wherein: 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)； and / or, the mass ratio of the poly (N-isopropylacrylamide) layer-coated silicon particles, the aniline, the second cross-linking agent, and the 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 spraying surface density of the polystyrene particles is 0.1-0.3 g / cm 2 ; and / or, the ratio of the mass of the silicon-based material to the volume of the third solvent is (1-2 g):(10-50 mL); And / or, the ratio of the mass of the honeycomb porous graphite to the volume of the suspension is (1.5-2.5 g): (5-20 mL); And / or, the ratio of the mass of the honeycomb porous graphite with the silicon-based material embedded in the pores to the volume of the pyrrole solution is (2.5-4.5 g):(100-150 mL).

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

9. The method for preparing a secondary battery according to claim 6, wherein: The average particle size of the silicon particles is 50 to 100 nm; and / or the average particle size of the polystyrene particles is 150 to 300 nm; and / or the number average molecular weight of the polystyrene particles is 50,000 to 60,000 Da; and / or, the first cross-linking agent is N,N'-methylenebisacrylamide and / or polyethylene glycol diacrylate; and / or, the first initiator is ammonium persulfate and / or azobisisobutylamidine hydrochloride; And / or, the first solvent is selected from water and / or ethanol; and / or, the second cross-linking 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, an oxidant and a dopant, and 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, wherein: 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, the binder and the conductive agent is (96-97.5): (0.8～1.2):(2.4～3.2)。 11. An energy storage system comprising a plurality of unit batteries, characterized in that: The unit cell is the secondary battery according to any one of claims 1 to 5 or a secondary battery prepared by the method for preparing a secondary battery according to any one of claims 6 to 10.

12. An electrical device, characterized in that: Including the energy storage system according to claim 11, the energy storage system is used to provide power for the electrical equipment.

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