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

Abstract

The invention relates to the field of energy storage, and provides a secondary battery and a preparation method thereof, an energy storage battery pack and electric equipment, and the preparation method comprises the following steps: forming a first active layer provided with a first hole and a second active layer provided with a second hole; the preparation method for forming the second active layer comprises the following steps: obtaining a camphor precursor based on second camphor particles and a pre-carbonization solution, and coating the surface of the camphor precursor with a carbonization material; carrying out heat treatment on the camphor precursor to obtain carbon-coated camphor particles; uniformly stirring the carbon-coated camphor particles, a second negative electrode material, a second conductive agent, a second adhesive and the second dispersion solution to obtain second coating slurry; the current collector with the first active layer is covered with second coating slurry, the second coating slurry can flow into part of the first holes, and a second active layer with carbon-coated second holes is formed after drying; and winding or laminating the positive plate, the negative plate and the diaphragm to obtain the secondary battery.

Description

Technical Field

[0001] The present application relates to the field of energy storage, and particularly to a secondary battery, a preparation method thereof, an energy storage battery pack, and an electrical device. Background Art

[0002] With the rapid development of the energy storage market, energy storage battery cells are accelerating their renewal and iteration. Currently, battery cell products are developing towards large capacity and high energy density. In addition to developing cathode and anode materials with higher specific capacities, in the design of battery cells, the energy density of the battery cells is often increased by increasing the coating surface density of the electrode sheets and the compaction density. However, this will significantly increase the electrochemical impedance of the electrode sheets. Especially for the anode electrode sheets, the increase in the electrochemical impedance of the electrode sheets will increase the risk of lithium plating during the charging process of the battery cells, resulting in the attenuation of the battery cell capacity.

[0003] Currently, the methods or preparation processes for improving the electrochemical impedance and enhancing the energy density of battery cells are relatively complex, and will affect the porosity of the anode electrode sheets and reduce the tortuosity of the electrode sheets, thus causing new problems. Summary of the Invention

[0004] Embodiments of the present application provide a secondary battery, a preparation method thereof, an energy storage battery pack, and an electrical device, which are at least beneficial to improving the porosity of the anode sheet, the electrochemical performance of the anode sheet, and enhancing the rate performance of the electrode sheet.

[0005] According to some embodiments of the present application, on the one hand, a preparation method of a secondary battery is provided, including: forming an anode sheet, the anode sheet including: a negative current collector, a first active layer, and a second active layer stacked in sequence, the first active layer having first pores, the second active layer having second pores, and the second porosity of the second active layer being greater than the first porosity of the first active layer; the preparation step of forming the second active layer includes: preparing carbon-coated camphor particles; preparing a second coating slurry, the second coating slurry including the carbon-coated camphor particles; covering the second coating slurry on the negative current collector having the first active layer, wherein the second coating slurry flows into at least part of the first pores and is dried to form the second active layer; during the drying process, the camphor particles in the carbon-coated camphor particles sublime to form the second pores; providing a cathode sheet and a separator, stacking the anode sheet, the separator, and the cathode sheet in sequence, obtaining a bare battery cell by winding or laminating, placing the bare battery cell into a battery case, injecting an electrolyte into the battery case, and then encapsulating to obtain a secondary battery.

[0006] In some embodiments, the second coating slurry further comprises: a second negative electrode material, a second conductive agent, and a second binder and a second dispersion solution; the mass fraction ratio of the carbon-coated camphor particles, the second negative electrode material, the second conductive agent, the second binder, and the second dispersion solution is (0.4~0.6):(96~97.2):(0.4~0.6):(2.0~2.8):(160~200).

[0007] In some embodiments, the second binder comprises styrene-butadiene rubber, carboxymethyl cellulose, and polyacrylic acid, and the mass fraction ratio of styrene-butadiene rubber, carboxymethyl cellulose, and polyacrylic acid is (0.8~1.2):(0.6~0.8):(0.6~0.8).

[0008] In some embodiments, in the process steps of obtaining the second coating slurry, the carboxymethyl cellulose, the polyacrylic acid, and the styrene-butadiene rubber are added in sequence, and at the same time, the rotation speed of stirring is controlled to show a trend of increasing first and then decreasing.

[0009] In some embodiments, the preparation steps of the carbon-coated camphor particles include: dispersing second camphor particles in a pre-carbonization solution, the pre-carbonization solution contains a carbonization material, drying to obtain a camphor precursor, and the surface of the camphor precursor is coated with the carbonization material; performing heat treatment on the camphor precursor to convert the camphor precursor into the carbon-coated camphor particles; the pre-carbonization solution comprises a sucrose aqueous solution, and the mass fraction of the sucrose aqueous solution in the pre-carbonization solution is 3 parts to 7 parts; the mass ratio of the second camphor particles in the sucrose aqueous solution is 55wt% to 70wt%.

[0010] In some embodiments, the process steps of the heat treatment include: placing the camphor precursor in a heat treatment device, introducing an inert gas, heating to 160°C to 200°C, and carbonizing for 5 min to 15 min, and obtaining the carbon-coated camphor particles after cooling; the thickness of the carbon shell in the carbon-coated camphor particles is 15 nm to 20 nm.

[0011] In some embodiments, the preparation steps of forming the first active layer include: uniformly stirring first camphor particles, a first negative electrode material, a first conductive agent, and a first binder and a first dispersion solution to obtain a first coating slurry; covering the first coating slurry on the negative electrode current collector, and the coating density is 100g / m 2 ~110g / m 2 , and obtaining the first active layer after drying, wherein, the first camphor particles sublime after drying, and the position occupied by the first camphor particles is converted into the first pores.

[0012] In some embodiments, after forming the first active layer, the following steps are further included: providing a carbon source gas and a carrier gas, and forming a layer of carbon nanotube layer on the surface of the first active layer by using a plasma chemical vapor deposition process at a temperature of 700 °C to 900 °C; the thickness of the carbon nanotube layer is 200 μm to 500 μm.

[0013] In some embodiments, the second particle size of the second camphor particles is smaller than the first particle size of the first camphor particles; the first particle size is 15 μm to 20 μm; the second particle size is 5 μm to 10 μm.

[0014] In some embodiments, the mass fraction ratio of the first camphor particles, the first negative electrode material, the first conductive agent, the first binder, and the first dispersion solution is (0.2 to 0.4):(96 to 97.2):(0.8 to 1.2):(2.0 to 2.8):(160 to 200).

[0015] In some embodiments, the first conductive agent includes carbon nanotubes and conductive carbon black, and the mass fraction ratio of the carbon nanotubes to the conductive carbon black is (0.4 to 0.6):(0.4 to 0.6).

[0016] According to some embodiments of the present application, on the other hand, an embodiment of the present application provides a secondary battery prepared by using the preparation method of the secondary battery described above, including: a battery case, which has a cavity inside, and an electrolyte is provided inside the cavity; a bare battery cell, which is located inside the cavity and immersed in the electrolyte; the bare battery cell includes a stacked positive electrode sheet, a separator, and a negative electrode sheet, wherein the negative electrode sheet includes: a negative electrode current collector, a first active layer, and a second active layer, the first active layer is located on the surface of the current collector, the second active layer is located on the surface of the first active layer, the first active layer has a first pore, the second active layer has a second pore wrapped by a carbon shell, and the second porosity of the second active layer is greater than the first porosity of the first active layer; the electrolyte is also located inside the first pore and the second pore.

[0017] In some embodiments, the negative electrode sheet further includes: a carbon nanotube layer, which is located between the first active layer and the second active layer.

[0018] According to some embodiments of the present application, on another aspect, an embodiment of the present application provides an energy storage battery pack, including: a secondary battery prepared by using the preparation method of the secondary battery according to any one of the above embodiments or the secondary battery according to the above embodiments.

[0019] According to some embodiments of the present application, on the other hand, an electrical device is provided, including: a secondary battery prepared by the method for preparing a secondary battery according to any one of the above embodiments, the secondary battery according to the above embodiments, or the energy storage battery pack according to the above embodiments.

[0020] The technical solutions provided by the embodiments of the present application have at least the following advantages: In the method for preparing a secondary battery provided by the embodiments of the present application, a first active layer and a second active layer are provided on the negative electrode current collector of the negative electrode sheet, and the second porosity of the second active layer is greater than the first porosity of the first active layer. In this way, the electrolyte can penetrate into the first active layer through the second holes of the second active layer, thereby reducing the difficulty of electrolyte infiltration and improving the electronic conductivity. Secondly, the second camphor particles coated with a carbon shell are used as the carrier of the second holes. After the second camphor particles sublime, the carbon shell can support the void structure, effectively solving the problem of the coating structure collapse caused by creating voids, and at the same time improving the electronic conductivity of the second active layer. Secondly, the first active layer has first holes. When forming the second active layer, the slurry can partially flow into the first holes to play a pinning effect, improving the bonding force between the first active layer and the second active layer. Description of the Drawings

[0021] One or more embodiments are exemplarily illustrated by the figures in the corresponding drawings. These exemplary illustrations do not limit the embodiments unless otherwise stated. The figures in the drawings do not constitute a proportional limitation; in order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required to be used in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0022] Figure 1 It is a flowchart of a method for preparing a secondary battery provided by an embodiment of the present application; Figure 2 It is a schematic structural diagram of a negative electrode sheet in a secondary battery provided by an embodiment of the present application. Detailed Embodiments

[0023] As can be seen from the background art, the porosity, electrochemical performance, and rate performance of the current negative electrode sheet need to be further improved.

[0024] Analysis reveals that one of the reasons for the porosity, electrochemical performance, and rate performance of the negative electrode sheet is that currently, to overcome the increased impedance caused by high areal density and high compaction, the positive and negative electrode formulations are usually adjusted. For example, the proportion of conductive particles is increased to increase the electronic conductivity of the electrode sheet. To improve ionic conductivity, a reasonable particle size ratio is usually used to increase the porosity of the electrode sheet and reduce the tortuosity of the electrode sheet. However, these measures have limited effects on solving the problem of increasing electrode impedance. Using a reasonable active material particle size ratio cannot significantly improve the ionic conductivity of the electrode sheet, and the porosity distribution of the electrode sheet is still unreasonable. Blindly increasing the proportion of conductive particles can reduce the electronic resistance of the electrode sheet within a certain range, but the ionic impedance, as the shortcoming of the electrode sheet, has a negligible effect on improving the electrochemical impedance of the entire electrode sheet. Therefore, simultaneously improving the electronic conductivity and ionic conductivity of the negative electrode sheet can effectively improve the electrochemical performance of the electrode sheet and enhance the rate performance of the electrode sheet. In addition, pore-forming agents are also used to increase porosity. Although these methods can increase the ionic conductivity of the electrode sheet, they are also prone to causing the collapse of the electrode sheet structure.

[0025] The embodiment of the present application provides a secondary battery, a preparation method thereof, an energy storage battery pack, and an electrical device. By sublimating camphor particles to form pores and coating the camphor particles with carbon to ensure the stability of the pores and prevent the pores from collapsing, and improving the conductivity based on the first active layer, the second active layer, and the carbon shell, the electrochemical performance and rate performance of the negative electrode sheet are improved.

[0026] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity, specific order, or primary-secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality" is more than two, unless otherwise specifically and clearly defined.

[0027] Referring to "embodiments" herein means that the specific features, structures, or characteristics described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0028] In the description of the embodiments of the present application, the term " / or" is only a description of the association relationship of the associated objects, indicating that there can be three relationships. For example, A and / or B can represent: the existence of A, the simultaneous existence of A and B, and the existence of B. In addition, the character " / " in this article generally represents an "or" relationship between the associated objects before and after.

[0029] In the description of the embodiments of the present application, the term "plurality" 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).

[0030] In the description of the embodiments of the present application, the orientation or positional relationship indicated by technical terms such as "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the embodiments of the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the embodiments of the present application.

[0031] In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installation", "connection", "connection", "fixation", etc. should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to specific circumstances.

[0032] In the drawings corresponding to the embodiments of the present application, for better understanding and convenience of description, the thickness and area of the layer are enlarged. When describing a component (such as a layer, film, region, or substrate) on or on the surface of another component, the component can be "directly" 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 "substantially" formed on another component, it means that the component is not formed on the entire surface (or front surface) of the other component, nor on a partial edge of the entire surface.

[0033] In the description of the embodiments of the present application, when a certain component "includes" another component, unless otherwise specified, other components are not excluded, and other components may further be 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 may be "directly on" another component (i.e., located on the surface of another component with no other components therebetween), or there may be other components therebetween. In addition, when a component such as a layer, film, region, plate, etc. is "directly located on" another component, or when a component such as a layer, film, region, plate, etc. is located on the surface of another component, it means that no other components are located therebetween.

[0034] The terms used in the description of the various embodiments herein are only for describing specific embodiments and are not intended to be limiting. As used in the description of the various embodiments and the appended claims, "the part" is also intended to include the plural form unless the context clearly indicates otherwise. Among them, the components include components such as layers, films, regions, or plates.

[0035] The embodiments of the present application will be described in detail below in conjunction with the accompanying drawings. However, those of ordinary skill in the art can understand that in the embodiments of the present application, many technical details are provided to help readers better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0036] Figure 1 A flowchart of a method for preparing a secondary battery provided in an embodiment of the present application; Figure 2 A schematic structural diagram of a negative electrode sheet in a secondary battery provided in an embodiment of the present application.

[0037] According to some embodiments of the present application, on the one hand, an embodiment of the present application provides a method for preparing a secondary battery. Referring to Figure 1 , the preparation method includes: forming a negative electrode sheet, the negative electrode sheet including: a negative electrode current collector, a first active layer, and a second active layer stacked in sequence, the first active layer having first pores, the second active layer having second pores, and the second porosity of the second active layer being greater than the first porosity of the first active layer; the preparation steps for forming the second active layer include: preparing carbon-coated camphor particles; preparing a second coating slurry, the second coating slurry including the carbon-coated camphor particles; covering the negative electrode current collector having the first active layer with the second coating slurry, wherein the second coating slurry flows into at least part of the first pores and is dried to form the second active layer; during the drying process, the camphor particles in the carbon-coated camphor particles sublime to form the second pores; providing a positive electrode sheet and a separator, stacking the negative electrode sheet, the separator, and the positive electrode sheet in sequence, and obtaining a bare battery cell by winding or laminating, and placing the bare battery cell into a battery case, injecting an electrolyte into the battery case and then encapsulating to obtain a secondary battery.

[0038] A negative electrode sheet is formed. The negative electrode sheet includes: a negative electrode current collector, a first active layer, and a second active layer. The first active layer is located on the surface of the negative electrode current collector, and the second active layer is located on the surface of the first active layer away from the first active layer. The first active layer has first pores, and the second active layer has second pores. The second porosity of the second active layer is greater than the first porosity of the first active layer. The preparation steps for forming the second active layer include: dispersing second camphor particles in a pre-carbonization solution, where the pre-carbonization solution contains a carbonization material, and drying to obtain a camphor precursor, with the surface of the camphor precursor coated with the carbonization material; performing heat treatment on the camphor precursor to convert the camphor precursor into carbon-coated camphor particles; stirring the carbon-coated camphor particles, a second negative electrode material, a second conductive agent, and a second binder evenly with a second dispersion solution to obtain a second coating slurry; covering the second coating slurry on the negative electrode current collector having the first active layer, where the second coating slurry flows into at least part of the first pores, and after drying, the second active layer is formed; where the camphor particles in the carbon-coated camphor particles sublime after drying, and the carbon shell of the carbon-coated camphor particles remains, and the positions occupied by the camphor particles in the carbon-coated camphor particles are converted into second pores with carbon coating.

[0039] In the preparation method of the secondary battery provided by the embodiments of the present application, the negative electrode current collector of the negative electrode sheet is provided with a first active layer and a second active layer, and the second porosity of the second active layer is greater than the first porosity of the first active layer. In this way, the electrolyte can penetrate into the first active layer through the second pores of the second active layer, thereby reducing the difficulty of electrolyte infiltration and improving the electronic conductivity. Secondly, using the second camphor particles coated with a carbon shell as the carrier of the second pores, after the second camphor particles sublime, the carbon shell can support the void structure, effectively solving the problem of the coating structure collapsing due to void formation, and at the same time improving the electronic conductivity of the second active layer. Secondly, the first active layer has first pores. When forming the second active layer, the slurry can partially flow into the first pores to play a pinning effect, improving the bonding force between the first active layer and the second active layer.

[0040] The above-provided preparation method will be described in detail below.

[0041] Classified by appearance, the prepared secondary battery can be divided into square cells, round cells or pouch cells. Classified by capacity, the secondary battery can be divided into models such as 50Ah, 100Ah, 150Ah, 200Ah, 280Ah, 306Ah, 314Ah, 500+Ah, 800+Ah and 1000+Ah. Classified by the chemical composition and working principle of the bare cell of the secondary battery, the secondary battery can be a lithium-ion battery, a lead-acid battery, a sodium-ion battery or a nickel-metal hydride battery. In the embodiments of the present application, the preparation method of the lithium-ion battery is taken as an example. Those skilled in the art can replace the lithium ions in the positive electrode sheet, negative electrode sheet and electrolyte with corresponding metal ions according to actual needs. For example, for a sodium-ion battery, the lithium transition metal oxide of the positive active material in the following is replaced with any one of the corresponding layered metal oxides (such as NaFeO2), polyanion compounds (NaFePO4) and Prussian blue compound systems (such as NaMnFe(CN)6-zH2O), and the electrolyte is replaced with any one of organic liquid electrolytes, solid composite electrolytes or solid electrolytes.

[0042] Reference Figure 2 , the negative current collector 100 can be a copper foil. The copper foil has low conductivity, can have high electron transport ability, and has weak lithium intercalation ability, capturing fewer lithium ions, thereby effectively reducing the loss of lithium ions. In some other embodiments, the negative current collector can also be a foam copper current collector, a copper mesh current collector and a three-dimensional nano-copper array current collector.

[0043] In some embodiments, the negative current collector 100 can be a composite current collector, including a polymer material layer and a metal coating. The metal coating is located on the upper and lower sides of the polymer material layer. The polymer material layer is PET (polyester), PP (polypropylene), PI (polyimide), etc., and the metal coating is a copper layer.

[0044] In some embodiments, the negative current collector 100 can also be a carbon-based current collector, that is, there is a layer of conductive carbon layer on the copper foil. The conductive carbon layer can serve as a protective layer to effectively protect the current collector to prevent corrosion of the metal current collector, thereby improving the life of the current collector; secondly, the conductive carbon layer itself has a low resistivity, so that excessive electrical losses will not be generated. Among them, the material of the conductive carbon layer can be flake graphite, spherical graphite, carbon nanotubes, graphene, etc.

[0045] It should be noted that the carbon-based current collector can be manifested as both the upper and lower surfaces of the copper foil being coated with a conductive carbon layer, or as the surface of a part of the copper foil being covered with a conductive carbon layer. Among them, the surface of a part of the copper foil being covered with a conductive carbon layer can include that the copper foil corresponding to the first active layer (and there is also a part of the tab area that does not cover the first active layer) has a conductive carbon layer; or the tab area has a conductive carbon layer; or, the first active layer and a part of the tab area have a conductive carbon layer, etc.

[0046] In some embodiments, the active material particles in the first active layer 110 and the second active layer 130 are the carriers for the oxidation reaction of the battery cell, such as the negative electrode material in the first active layer and the negative electrode material in the second active layer.

[0047] In some embodiments, the negative active materials can be divided into two major categories: carbon materials and non-carbon materials: the carbon-based materials include two types, graphite materials (natural graphite, artificial graphite, and mesophase carbon microspheres) and other carbon-based materials (hard carbon, soft carbon, and graphene); the non-carbon-based materials can be further divided into titanium-based materials, silicon-based materials, tin-based materials, nitrides, and metallic lithium, etc.

[0048] In some embodiments, the first active layer 110 and the second active layer 130 are provided on the negative current collector 100. By setting the two active material layers, firstly, since the first active layer 110 is closer to the negative current collector 100, the stability and conductivity of the lower surface where the negative current collector 100 is connected to the active coating are better, thereby improving the electrochemical performance and cycling performance of the negative electrode. Secondly, for the second active layer 130 that is far from the negative current collector 100 and is in contact with the separator, lithium deposition is likely to occur on the corresponding surface on the negative electrode side. Therefore, the second active layer 130 in contact with the separator can focus on stability and conductivity, which is conducive to the smooth insertion and extraction of lithium ions inside the positive and negative electrode materials.

[0049] Thus, the second active layer 130 is provided with second holes 131, the first active layer 110 is provided with first holes 111, and the second porosity of the second active layer 130 is greater than the first porosity of the first active layer 110. The second holes 131 can increase the porosity of the negative electrode sheet, which is conducive to the storage of the electrolyte, effectively improves the liquid retention ability of the negative electrode sheet, reduces the transmission distance of lithium ions during charge and discharge, creates voids between the active material particles of the negative electrode sheet, provides a buffer space for the cyclic expansion inside the active material particles, and further improves the charging ability of the negative electrode sheet.

[0050] In some embodiments, the preparation steps for forming the first active layer 110 include: uniformly stirring the first camphor particles, the first negative electrode material, the first conductive agent, and the first binder with the first dispersion solution to obtain the first coating slurry.

[0051] In some embodiments, the first camphor particles, named 1,7,7-trimethylbicyclo[2.2.1]heptan-2-one, have the chemical formula C 10 H 16 O. The average particle size of the first camphor particles is 15 μm to 20 μm. The particle size refers to the diameter of the particles, and the average particle size refers to the average of the diameters of most particles or the lengths of irregular particles.

[0052] In some embodiments, in the first coating slurry, the mass fraction ratio of the first camphor particles, the first negative electrode material, the first conductive agent, the first binder, and the first dispersion solution is (0.2~0.4):(96~97.2):(0.8~1.2):(2.0~2.8):(160~200). Thus, by increasing the proportion of the first conductive agent to 0.8~1.2, compared with the conventional conductive agent content of 0.4~0.6 (such as the mass fraction of the second conductive agent), the proportion of conductive particles in the negative electrode active material can be increased, thereby improving the electron conductivity of the first active layer 110, and further improving the electrochemical performance and cycling performance of the negative electrode sheet.

[0053] In some embodiments, the first negative electrode material may include graphite. Among them, the graphite can be graphite of secondary particles. In this way, the volume of the graphite of secondary particles is smaller, which is more conducive to ion diffusion. The graphite of secondary particles can provide lower impedance performance, and thus has higher conductivity. Herein, the secondary particles refer to the particle size of the aggregated particles, and the primary particles in the following refer to the particle size of a single fine crystal.

[0054] In some embodiments, the first conductive agent includes carbon nanotubes (abbreviated as CNT) and conductive carbon black (abbreviated as SP), and the mass fraction ratio of carbon nanotubes and conductive carbon black is (0.4~0.6):(0.4~0.6).

[0055] In some embodiments, the first binder includes styrene-butadiene rubber (abbreviated as SBR), carboxymethyl cellulose (abbreviated as CMC), and polyacrylic acid (abbreviated as PAA), and the mass fraction ratio of styrene-butadiene rubber, carboxymethyl cellulose, and polyacrylic acid is (0.8~1.2):(0.6~0.8):(0.6~0.8). Among them, an aqueous solution of styrene-butadiene rubber is used as the binder to reduce the phenomena of film peeling and carbon removal of the electrode sheet during the processes of coating, cutting, slitting, and winding of the electrode sheet production, optimize the processing performance of the electrode sheet, and the obtained electrode sheet has good flexibility; carboxymethyl cellulose mainly plays a role in dispersion and thickening, thereby reducing the amount of the binder; polyacrylic acid is a water-soluble polymer formed by polymerization of acrylic acid monomers. There are a large number of carboxylic acid groups on polyacrylic acid, which can form strong interactions with the active substances, so it has good bonding performance, and during the cycling process of the battery cell, it improves the cycling stability of the battery cell.

[0056] In some embodiments, the first dispersion solution may be deionized water.

[0057] In summary, the process method for forming the first coating slurry provided by the embodiments of the present application may be: the mass fraction ratio of each substance is graphite (the first negative electrode material): 96 parts to 97.2 parts, PAA (the first binder): 0.6 parts to 0.8 parts, SBR (the first binder): 0.8 parts to 1.2 parts, CMC (the first binder): 0.6 parts to 0.8 parts, SP (the first conductive agent): 0.4 parts to 0.6 parts, CNT (the first conductive agent): 0.4 parts to 0.6 parts, the first camphor particles: 0.2 parts to 0.4 parts, deionized water: 160 parts to 200 parts. Put the above amounts of graphite powder, SP, CMC and the first camphor particles into a stirring tank for dry mixing for 30 minutes, with the stirring revolution speed being 15 rpm and the dispersion speed being 500 rpm. Then add a small part (for example, 80 parts by mass) of deionized water for kneading and stirring for 60 min, with the stirring revolution speed being 20 rpm and the dispersion speed being 300 rpm. Then add the remaining deionized water (for example, 105 parts by mass) for high-speed dispersion, with the stirring revolution speed being 22 rpm and the dispersion speed being 1000 rpm. Then add CNT and PAA for dispersion for 30 min, with the stirring revolution speed being 22 rpm and the dispersion speed being 600 rpm. Finally, add SBR and continue stirring for 30 min, with the stirring revolution speed being 22 rpm and the dispersion speed being 500 rpm to obtain the first coating slurry.

[0058] Continue to refer to Figure 1 , the preparation steps for forming the first active layer 110 include: covering the first coating slurry on the negative electrode current collector 100, and the coating density is 100 g / m 2 ~110 g / m 2 , after drying, the first active layer 110 is obtained, the first camphor particles sublime, and the positions occupied by the first camphor particles are transformed into the first holes 111.

[0059] In some embodiments, after covering the first coating slurry on the negative electrode current collector, it is baked at 80 °C for 12 h. During the baking process, the first camphor particles will sublime and then break away from the coating, thus leaving holes at the original positions, that is, the first holes 111, which facilitates the subsequent inflow of the second coating slurry and forms the second active layer 130. In this way, the second active layer 130 is not only located on the surface of the first active layer 110 but also inside the first active layer 110, thereby being able to play a pinning effect and effectively improving the bonding force between the first active layer 110 and the second active layer 130.

[0060] In some other embodiments, an electrolytic graphite pore-forming process may be adopted, that is, the raw materials except the first camphor particles are mixed, coated, and dried to obtain a first active layer, and then the first active layer is placed in an electrolytic cell, and the negative current collector is connected to a power source. When an electric current passes through the electrolytic cell, the ions in the electrolyte will be electrolyzed, and then a chemical reaction will occur. This chemical reaction will cause etching on the surface of the graphite to form pores, that is, the first pores.

[0061] In some embodiments, referring to Figure 2 , after forming the first active layer 110, it further includes: providing a carbon source gas and a carrier gas, and using a plasma chemical vapor deposition process to form a carbon nanotube layer 120 on the surface of the first active layer 110 at a temperature of 700°C to 900°C. The carbon nanotube layer 120 effectively reduces the impedance caused by the coating interface and improves the lateral electron conductivity of the coating.

[0062] In some embodiments, the thickness of the carbon nanotube layer 120 is 200μm to 500μm. The thickness of the carbon nanotube layer 120 can be 200μm, 230μm, 250μm, 282μm, 303μm, 348μm, 369μm, 391μm, 403μm, 431μm, 466μm, 482μm or 500μm. The thickness of the carbon nanotube layer 120 can not only enable the subsequent formed second coating slurry to flow into the first pores 111 through the pores of the carbon nanotube layer 120 and the electrolyte can also flow into the first active layer 110 through these pores, but also the thickness of the carbon nanotube layer 120 can provide good conductivity to reduce the impedance between the first active layer 110 and the second active layer 130.

[0063] In some embodiments, the carbon source gas can be methane or ethanol. The carrier can be an inert gas such as argon.

[0064] Continuing to refer to Figure 1 , the steps of forming the second active layer 130 can be divided into: forming a camphor precursor, coating the surface of the camphor precursor with a carbonized material; forming carbon-coated camphor particles; forming a second coating slurry; forming the second active layer 130.

[0065] The process steps of forming the camphor precursor include: dissolving camphor particles in ethanol to form a saturated solution, obtaining micron-sized second camphor particles by spray drying, dispersing the second camphor particles in a pre-carbonization solution, stirring and evaporating the solvent to uniformly coat the second camphor particles with the carbonized material, and drying to obtain the camphor precursor with the surface coated with the carbonized material.

[0066] In some embodiments, the pre-carbonized solution includes an aqueous sucrose solution, and the mass fraction of the aqueous sucrose solution is 3 to 7 parts; the mass ratio of camphor particles in the aqueous sucrose solution is 55wt% to 70wt%.

[0067] The second camphor particles may be made of the same material as the first camphor particles, except that the average particle size of the first camphor particles is smaller than that of the second camphor particles. The average particle size of the first camphor particles is 15μm to 20μm; the average particle size of the second camphor particles is 5μm to 10μm.

[0068] The average particle size of the second camphor particles may specifically be 5μm, 6μm, 7μm, 8μm, 9μm or 10μm.

[0069] In some embodiments, the process steps of the heat treatment include: placing the camphor precursor in a heat treatment device, introducing an inert gas, heating to 160°C to 200°C, and carbonizing for 5 min to 15 min, and obtaining carbon-coated camphor particles after cooling. The carbonization process controls the carbonization temperature and time to prevent the second camphor particles from sublimating excessively, and carbon-coated camphor particles are obtained after cooling. The carbon shell retained in the carbon-coated camphor particles can effectively support the structure, prevent the collapse of the electrode structure, and at the same time improve the electronic conductivity of the second active layer 130.

[0070] In some embodiments, the thickness of the carbon shell 132 in the carbon-coated camphor particles is 15nm to 20nm. For example, the thickness of the carbon shell 132 can be 15nm, 16nm, 17nm, 18nm, 19nm or 20nm.

[0071] In some embodiments, the mass fraction ratio of the carbon-coated camphor particles, the second negative electrode material, the second conductive agent, the second binder, and the second dispersion solution is (0.4 to 0.6):(96 to 97.2):(0.4 to 0.6):(2.0 to 2.8):(160 to 200).

[0072] In some embodiments, the second binder includes styrene-butadiene rubber, carboxymethyl cellulose, and polyacrylic acid, and the mass fraction ratio of styrene-butadiene rubber, carboxymethyl cellulose, and polyacrylic acid is (0.8 to 1.2):(0.6 to 0.8):(0.6 to 0.8).

[0073] It should be noted that the second conductive agent in the second coating slurry is SP. The second negative electrode material is primary particle graphite. For other components, the second binder and the second dispersion liquid are the same as the first binder and the first dispersion liquid in the first coating slurry.

[0074] In some embodiments, in the process steps of obtaining the second coating slurry, carboxymethyl cellulose, polyacrylic acid, and styrene-butadiene rubber are added in sequence, and at the same time, the rotation speed of the stirring is controlled to show a trend of increasing first and then decreasing.

[0075] Specifically, the mass fraction ratios of various substances are as follows: graphite (graphite of primary particles): 96 parts to 97.2 parts, PAA: 0.6 parts to 0.8 parts, SBR: 0.8 parts to 1.2 parts, CMC: 0.6 parts to 0.8 parts, SP: 0.4 parts to 0.6 parts, carbon-coated camphor particles: 0.4 parts to 0.6 parts, deionized water: 160 parts to 200 parts. Put the above-mentioned amounts of graphite powder, SP, CMC, and carbon-coated camphor particles into a stirring tank for dry mixing for 30 minutes, with the stirring revolution speed being 15 rpm and the dispersion speed being 500 rpm. Then add a small part (80 parts by mass) of deionized water for kneading and stirring for 60 min, with the stirring revolution speed being 20 rpm and the dispersion speed being 300 rpm. Then add the remaining deionized water (105 parts by mass) for high-speed dispersion, with the stirring revolution speed being 22 rpm and the dispersion speed being 1000 rpm. Then add PAA for dispersion for 30 min, with the stirring revolution speed being 22 rpm and the dispersion speed being 600 rpm. Finally, add SBR and continue stirring for 30 min, with the stirring revolution speed being 22 rpm and the dispersion speed being 500 rpm to obtain the second coating slurry.

[0076] The process steps for forming the second active layer 130 include: coating the second coating slurry on the surface-treated first active layer 110, with the coating areal density being 100 g / m 2 ~110 g / m 2 , during the coating process, the coating slurry will flow into some of the first holes 111. Roll-press the double-layer negative electrode sheet obtained by the above coating, with the compaction density being between 1.5 g / cm 3 ~1.6 g / cm 3 . Bake the roll-pressed electrode sheet at 60 °C for 24 h. During the baking process, the camphor particles in the carbon-coated camphor particles will sublime and detach from the coating, thereby effectively increasing the porosity of the second active layer 130 and enhancing its ionic conductivity. At the same time, the remaining carbon-coated layer can enhance the electronic conductivity of this layer. Combining the excellent electronic conductivity of the first active layer 110 and the carbon nanotube layer 120 coating, it also makes up for the problems of large ionic impedance near the current collector and large electronic impedance far from the current collector in the thick negative electrode sheet, and the negative electrode sheet exhibits excellent rate performance.

[0077] It should be noted that the coating areal density ratios of the first coating slurry and the second coating slurry can be determined according to the actual situation. Part of the second coating slurry will fill the first holes 111, but will not completely fill the first holes 111, so that some of the first holes 111 can still be used to fill the electrolyte, and the first holes filled with the second coating slurry are part of the second active layer.

[0078] In summary, for the first active layer 110, measures such as using secondary particle type graphite for the first active layer 110 close to the negative current collector, increasing the proportion of conductive particles, introducing the carbon nanotube layer 120 to increase the conductivity ability, etc., ensure the improvement of the electron conductivity of this layer to the greatest extent, so as to ensure the electron conductivity of the second active layer 130 far from the negative current collector. At the same time, in order to ensure compliance with the connection in the coating and take into account the improvement of the ionic conductivity of the first active layer 110, pore-forming is carried out on the first active layer 110 using camphor particles. At the same time, a carbon nanotube layer 120 is deposited on the surface of the first active layer 110 by chemical vapor deposition, effectively reducing the impedance caused by the coating interface and improving the lateral electron conductivity of the coating.

[0079] For the second active layer 130, the template method is used to improve its porosity and thus its ionic conductivity. The template method is to add solid particles such as 1,7,7-trimethylbicycloheptan-2-one (camphor) with carbon-coated surfaces that are easily sublimated by heat in its coating slurry. These particles will sublimate by heat during the subsequent baking of the electrode sheet, thereby effectively improving the porosity of this layer of coating. At the same time, the second active layer 130 is the surface coating, and the application of the template method for the surface coating can ensure to the greatest extent that the template particles volatilize by heat without affecting the electrical performance of the subsequent electrode sheet. The improvement of the porosity of the second active layer 130 can ensure the ionic conductivity of the first active layer 110 to the greatest extent. At the same time, when camphor sublimates and volatilizes, the remaining carbon shell can effectively support the structure and prevent the collapse of the electrode sheet structure, while improving the electron conductivity of the second active layer 130. The above designs of the first active layer 110 and the second active layer 130 effectively ensure the rate performance of the high-coated coating.

[0080] After forming the negative electrode sheet, it further includes: die-cutting part of the negative current collector according to preset parameters so that the end of the negative current collector forms a negative electrode tab; cutting the negative current collector and forming multiple identical negative electrode sheets.

[0081] The positive electrode sheet includes a positive current collector and a positive active material layer, where the positive current collector can be aluminum foil.

[0082] The separator can be located between the positive electrode sheet and the negative electrode sheet to prevent short-circuit problems caused by physical contact between the positive electrode sheet and the negative electrode sheet. At the same time, it allows ions to be conducted through the electrolyte and hinders electron transmission, so that ions and electrons form a loop during the charge and discharge process of the battery.

[0083] The separator can be any one of a microporous membrane, a modified microporous membrane, a non-woven fabric separator, and a composite separator. The separator can be coated with polyvinylidene fluoride (PVDF) material to make the separator have excellent adhesiveness and flexibility.

[0084] Reference Figure 1, winding or laminating the positive electrode sheet, negative electrode sheet, and separator.

[0085] In some embodiments, the winding process is to form a single core by winding the separator, positive electrode sheet, and negative electrode sheet with a winding machine. The order is to wrap the positive electrode sheet with the negative electrode sheet, and then isolate the positive electrode sheet and the negative electrode sheet through the separator. The lamination process is to stack the separator, positive electrode sheet, and negative electrode sheet in sequence, and form a single battery cell through rolling and cutting.

[0086] The preparation method further includes: welding the electrode tabs, welding the electrode tabs to the adapter plate, and connecting the other end of the adapter plate to the electrode post; snapping the top cover onto the battery case. Among them, the adapter plate is located in the chamber, and the electrode post passes through the top cover.

[0087] Among them, the adapter plate includes at least a first adapter plate and a second adapter plate, the electrode post includes a positive electrode post and a negative electrode post, the first adapter plate is electrically connected to the positive electrode tab of the positive electrode sheet and the positive electrode post respectively, and the second adapter plate is electrically connected to the negative electrode tab of the negative electrode sheet and the negative electrode post respectively.

[0088] In the preparation method of the secondary battery provided by the embodiment of the present application, a first active layer 110 and a second active layer 130 are provided on the negative current collector of the negative electrode sheet, and the second porosity of the second active layer 130 is greater than the first porosity of the first active layer 110. In this way, the electrolyte can penetrate into the first active layer 110 through the second holes 131 of the second active layer, thereby reducing the difficulty of electrolyte infiltration and improving the electronic conductivity. Secondly, the second camphor particles coated with a carbon shell are used as the carrier of the second holes 131. After the second camphor particles sublime, the carbon shell 132 can support the void structure, effectively solving the problem of the coating structure collapse caused by creating voids, and at the same time improving the electronic conductivity of the second active layer 130. Secondly, the first active layer 110 has first holes 111. When forming the second active layer 130, the slurry can partially flow into the first holes 111 to play a pinning effect, improving the bonding force between the first active layer 110 and the second active layer 130.

[0089] Correspondingly, according to some embodiments of the present application, on the other hand, the embodiment of the present application further provides a secondary battery, which can be prepared by using the preparation method provided in the above embodiments. The same or corresponding technical features as those in the above embodiments will not be elaborated here.

[0090] According to some embodiments of the present application, on the other hand, an embodiment of the present application provides a secondary battery, including: a battery case having a cavity therein, and an electrolyte in the cavity; a bare electric core located in the cavity and immersed in the electrolyte; the bare electric core includes a stacked positive electrode sheet, a separator, and a negative electrode sheet, wherein the negative electrode sheet includes: a negative electrode current collector 100, a first active layer 110, and a second active layer 130, the first active layer 110 is located on the surface of the current collector, the second active layer 130 is located on the surface of the first active layer 110, the first active layer 110 has first holes 111 therein, the second active layer 130 has second holes 131 coated with a carbon shell 132 therein, and the second porosity of the second active layer 130 is greater than the first porosity of the first active layer 110; the electrolyte is also located in the first holes 111 and the second holes 131.

[0091] In some embodiments, the negative electrode sheet further includes: a carbon nanotube layer 120 located between the first active layer 110 and the second active layer 130.

[0092] According to some embodiments of the present application, on another aspect, an embodiment of the present application provides an energy storage battery pack, including: a secondary battery prepared by the preparation method of the secondary battery according to any one of the above embodiments or the secondary battery according to the above embodiments.

[0093] According to some embodiments of the present application, on yet another aspect, an embodiment of the present application provides an electrical device, including: a secondary battery prepared by the preparation method of the secondary battery according to any one of the above embodiments, the secondary battery according to the above embodiments, or the energy storage battery pack according to the above embodiments.

[0094] Hereinafter, the beneficial effects of the embodiments of the present application will be further described in conjunction with embodiments and comparative examples.

[0095] Example 1: (1)Preparation of the first coating slurry: The mass parts of each substance are as follows: graphite (raw material is secondary particles): 96.4 parts, PAA: 0.6 parts, SBR: 1.2 parts, CMC: 0.6 parts, SP: 0.4 parts, CNT: 0.4 parts, the first camphor particles: 0.4 parts, deionized water: 185 parts. Put the above amounts of graphite powder, SP, CMC and the first camphor particles into a stirring tank for dry mixing for 30 minutes, with the stirring revolution speed of 15 rpm and the dispersion speed of 500 rpm. Then add 80 parts of deionized water for kneading and stirring for 60 min, with the stirring revolution speed of 20 rpm and the dispersion speed of 300 rpm. Then add the remaining deionized water (105 parts) for high-speed dispersion, with the stirring revolution speed of 22 rpm and the dispersion speed of 1000 rpm. Then add CNT and PAA for dispersion for 30 min, with the stirring revolution speed of 22 rpm and the dispersion speed of 600 rpm. Finally, add SBR and continue stirring for 30 min, with the stirring revolution speed of 22 rpm and the dispersion speed of 500 rpm to obtain the first coating slurry.

[0096] (2)Preparation of the first active layer and preparation of the carbon nanotube layer: Coat the first coating slurry on the copper foil, with the coating surface density of 105 g / m 2 , bake the coated electrode at 80 °C for 12 h. During the baking process, the first camphor particles will sublime and detach from the coating, leaving the first holes in the original positions and forming the first active layer, which is convenient for the subsequent coating slurry to flow in and can play a pinning effect. Grow the carbon nanotube layer on the surface of the baked electrode. Introduce the carbon source gas (methane or ethanol) and the carrier gas (argon) into the reaction furnace in a certain proportion. At high temperature (700 °C) and high pressure, the carbon source gas decomposes. After the decomposed carbon source gas is gradually cooled, it bombards the first active layer. After a period of gas bombardment, a carbon nanotube layer will be formed on its surface. The thickness of the carbon nanotube layer is 300 μm.

[0097] (3)Preparation of carbon-coated camphor particles: Take a certain amount of camphor and dissolve it in ethanol to form a saturated solution. Obtain micron-sized second camphor particles by spray drying. Disperse the second camphor particles in a sucrose aqueous solution (7 parts by mass (the mass ratio of the second camphor particles in the solution is 55 wt%)), stir and evaporate the solvent to uniformly coat the second camphor particles with sugars, and dry to obtain sugar-camphor composite particles, that is, camphor precursors. Place the coated camphor precursors in a tube furnace, quickly heat up to 180 °C through an inert gas, and keep it for 15 minutes to carbonize the precursors, and cool to obtain carbon-coated camphor particles, and the carbon shell thickness is 20 nm.

[0098] (4)Preparation of the second coating slurry: The mass fractions of each substance are as follows: graphite (with primary particles as raw material): 97 parts, PAA: 0.6 parts, SBR: 0.8 parts, CMC: 0.6 parts, SP: 0.4 parts, carbon-coated camphor particles: 0.6 parts, deionized water: 185 parts. Put the above amounts of graphite powder, SP, CMC, and carbon-coated camphor particles into a stirring tank and dry mix for 30 minutes. The stirring revolution speed is 15 rpm, and the dispersion speed is 500 rpm. Then add 80 parts of deionized water and knead and stir for 60 min. The stirring revolution speed is 20 rpm, and the dispersion speed is 300 rpm. Then add the remaining deionized water (105 parts) for high-speed dispersion. The stirring revolution speed is 22 rpm, and the dispersion speed is 1000 rpm. Then add PAA and disperse for 30 min. The stirring revolution speed is 22 rpm, and the dispersion speed is 600 rpm. Finally, add SBR and continue to stir for 30 min. The stirring revolution speed is 22 rpm, and the dispersion speed is 500 rpm to obtain the second coating slurry.

[0099] (5) Preparation of the second active layer: Coating the second coating slurry on the surface-treated first active layer, and the coating surface density is 105 g / m 2 , during the coating process, part of the second coating slurry will flow into the first holes. Roll press the double-layer negative electrode sheet obtained by the above coating, and the compaction density is between 1.5 g / cm 3 ~ 1.6 g / cm 3 . Bake the roll-pressed electrode sheet at 60 °C for 24 h. During the baking process, the camphor in the carbon-coated camphor will sublime and separate from the coating, thereby forming the second holes and the second active layer.

[0100] Example 2: The difference from Example 1 is that the mass fraction of the first camphor particles in the first active layer is 0.2 parts.

[0101] Example 3: The difference from Example 1 is that the mass fraction of the carbon-coated camphor particles in the second active layer is 0.4 parts.

[0102] Example 4: The difference from Example 1 is that the temperature of the preparation process of the carbon nanotube layer is changed, that is, at 900 °C under high temperature and high pressure, the thickness of the carbon nanotube layer is 500 μm.

[0103] Example 5: The difference from Example 1 is that the preparation process of the carbon-coated camphor particles is changed. Control the mass fraction of the sucrose solution to be 3 parts, and the temperature in the tube furnace is 200 °C. The carbon shell thickness of the obtained carbon-coated camphor particles is 15 nm.

[0104] Comparative Example 1: The mass parts of each substance are as follows: graphite (the raw material is secondary particles): 96.8 parts, PAA: 0.6 parts, SBR: 1.2 parts, CMC: 0.6 parts, SP: 0.4 parts, CNT: 0.4 parts, deionized water: 185 parts. Put the above amounts of graphite powder, SP and CMC into a stirring tank and dry mix for 30 minutes. The stirring revolution speed is 15 rpm and the dispersion speed is 500 rpm. Then add 80 parts of deionized water and knead and stir for 60 min. The stirring revolution speed is 20 rpm and the dispersion speed is 300 rpm. Then add the remaining deionized water (105 parts) for high-speed dispersion. The stirring revolution speed is 22 rpm and the dispersion speed is 1000 rpm. Then add CNT and PAA and disperse for 30 min. The stirring revolution speed is 22 rpm and the dispersion speed is 600 rpm. Finally, add SBR and continue to stir for 30 min. The stirring revolution speed is 22 rpm and the dispersion speed is 500 rpm to obtain the first coating slurry.

[0105] Coat the first coating slurry on the copper foil, and the coating areal density is 210 g / m 2 , and then roll press the coated electrode sheet, and the compaction density is between 1.5 g / cm 3 .

[0106] Comparative Example 2: The difference from Example 1 is that it does not include the preparation of the carbon nanotube layer in step (2).

[0107] Comparative Example 3: The difference from Example 1 is that it does not include the preparation of the carbon-coated camphor particles in step (3). In the preparation of the second coating slurry in (4), the mass parts of each substance are as follows: graphite (the raw material is primary particles): 97 parts, PAA: 0.6 parts, SBR: 0.8 parts, CMC: 0.6 parts, SP: 0.4 parts, the second camphor particles: 0.6 parts, deionized water: 185 parts. Among them, the second camphor particles are still the second camphor particles provided in step (3), but the outer shell is not coated with a carbon shell.

[0108] Successively conduct electrode sheet wettability tests, electrode sheet conductivity tests on the above-mentioned examples and comparative examples, and prepare the negative electrode sheets provided in the above-mentioned examples and comparative examples into secondary batteries and conduct electrochemical performance tests, and summarize and record the test results in Table 1.

[0109] Wettability test of electrode sheets: The wettability of the electrode is manifested in the absorption capacity of the electrolyte and the absorption amount per unit time. Usually, the wettability test method is to compare the absorption rates of the same amount of electrolyte by different electrode sheets of the same weight, and the one that dries up first has a stronger absorption capacity; compare the absorption storage amounts of different electrode sheets of the same weight immersed in the electrolyte, and the one with a heavier weight has a stronger absorption amount. The negative electrode sheets prepared in the examples and comparative examples were taken with the same weight and tested for the electrolyte absorption rate and absorption amount respectively.

[0110] Conductivity test of electrode sheets: The conductivity test is mainly the test of the film resistance of the electrode sheets used in the battery. The test method is to measure the resistance value of the film under unit pressure and unit area. The film resistances of the negative electrode sheets prepared in the test examples and comparative examples were measured.

[0111] Electrochemical performance test: The test temperature is 25±2°C. Charge at a constant power of 0.5C until 3.65±0.01V; set aside for 10 minutes, then discharge at 0.5P (discharge rate), and the cut-off voltage is 2.5V. Record this capacity and use it as the initial capacity in the rate test; then charge at a constant current of 1C until 3.65±0.01V; set aside for 10 minutes; then discharge at 1P, and the cut-off voltage is 2.5V. Record this capacity and use it as the rate capacity at 1P. Among them, C-rate (abbreviated as C) is a value relative to the rated capacity of the battery. For example, if the rated capacity of a battery is 200Ah, then 1C is equivalent to a charging or discharging rate of 200A. According to this definition, 0.5C means that the battery can be fully charged in 2 hours, and 1C means that the battery can be fully charged in 1 hour. Similarly, 0.5P means that the battery can be discharged in 2 hours, and 1P means that the battery can be discharged in 1 hour.

[0112] Table 1

[0113] As can be seen from Table 1, when comparing Examples 1 to 5 with Comparative Example 1, adding the first camphor particles in the preparation process of the first active layer, forming the carbon nanotube layer after forming the first active layer, and adding the second camphor particles in the preparation process of the second active layer, and the outer layer of the second camphor particles is also coated with a carbon shell, can improve the rate of the negative electrode sheet absorbing the electrolyte, and can reduce the resistance of the negative electrode sheet to obtain better rate performance.

[0114] From the experimental data of Example 1 and Comparative Example 2, it can be seen that the carbon nanotube layer can reduce the resistance of the negative electrode sheet, but at the same time it will affect the absorption of the electrolyte. Therefore, combined with the data of Example 4, it can be known that controlling the thickness of the carbon nanotube layer can ensure that while having good absorption of the electrolyte, the resistance of the negative electrode sheet can be reduced.

[0115] From the experimental data of Example 1 and Comparative Example 3, it can be seen that the formation of the first camphor particles in the first active layer, that is, the formation of the first pores, but the second pores are not formed in the second active layer, resulting in a lower absorption rate of the electrolyte and a higher resistance.

[0116] Those of ordinary skill in the art can understand that the above embodiments are specific embodiments for implementing the present application. In actual applications, various changes can be made in form and details without departing from the spirit and scope of the present application. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present application. Therefore, the protection scope of the present application should be subject to the scope defined by the claims.

Claims

1. A method for preparing a secondary battery, characterized in that, Comprising: Forming a negative electrode sheet, the negative electrode sheet comprising: a negative electrode current collector, a first active layer, and a second active layer stacked in sequence, the first active layer having first pores, the second active layer having second pores, and the second porosity of the second active layer being greater than the first porosity of the first active layer; the preparation steps for forming the second active layer include: preparing carbon-coated camphor particles; Preparing a second coating slurry, the second coating slurry comprising the carbon-coated camphor particles; Covering the second coating slurry on the negative electrode current collector having the first active layer, wherein the second coating slurry flows into at least a part of the first pores and is dried to form the second active layer; wherein, during the drying process, the camphor particles in the carbon-coated camphor particles sublime to form the second pores; Providing a positive electrode sheet and a separator, stacking the negative electrode sheet, the separator, and the positive electrode sheet in sequence, obtaining a bare battery cell by winding or laminating, placing the bare battery cell into a battery case, injecting an electrolyte into the battery case, and then encapsulating to obtain a secondary battery.

2. The manufacturing method of the secondary battery according to claim 1, characterized in that, The second coating slurry further comprises: a second negative electrode material, a second conductive agent, a second binder, and a second dispersion solution; the mass ratio of the carbon-coated camphor particles, the second negative electrode material, the second conductive agent, the second binder, and the second dispersion solution is (0.4 - 0.6):(96 - 97.2):(0.4 - 0.6):(2.0 - 2.8):(160 - 200).

3. The manufacturing method of the secondary battery according to claim 2, wherein The second binder comprises styrene-butadiene rubber, carboxymethyl cellulose, and polyacrylic acid, and the mass ratio of styrene-butadiene rubber, carboxymethyl cellulose, and polyacrylic acid is (0.8 - 1.2):(0.6 - 0.8):(0.6 - 0.8).

4. The method for preparing a secondary battery according to claim 3, wherein In the process steps for obtaining the second coating slurry, the carboxymethyl cellulose, the polyacrylic acid, and the styrene-butadiene rubber are added in sequence, and at the same time, the rotation speed of stirring is controlled to show a trend of increasing first and then decreasing.

5. The manufacturing method of the secondary battery according to claim 1, characterized in that, The preparation steps for the carbon-coated camphor particles include: dispersing second camphor particles in a pre-carbonization solution, the pre-carbonization solution containing a carbonization material, drying to obtain a camphor precursor, and the surface of the camphor precursor being coated with the carbonization material; performing heat treatment on the camphor precursor to convert the camphor precursor into the carbon-coated camphor particles; the pre-carbonization solution comprises a sucrose aqueous solution, and the mass fraction of the sucrose aqueous solution in the pre-carbonization solution is 3 parts to 7 parts; the mass ratio of the second camphor particles in the sucrose aqueous solution is 55wt% - 70wt%.

6. The method for preparing a secondary battery according to claim 5, characterized in that, The process steps for the heat treatment include: placing the camphor precursor in a heat treatment device, introducing an inert gas, heating to 160°C - 200°C, and carbonizing for 5 min - 15 min, and cooling to obtain the carbon-coated camphor particles; the thickness of the carbon shell in the carbon-coated camphor particles is 15nm - 20nm.

7. The method for preparing a secondary battery according to claim 1, wherein The preparation steps for forming the first active layer include: Stirring first camphor particles, a first negative electrode material, a first conductive agent, a first binder, and a first dispersion solution evenly to obtain a first coating slurry; Cover the first coating slurry on the negative current collector, and the coating density is 100 g / m 2 ~110 g / m 2 , and the first active layer is obtained after drying. Among them, the first camphor particles sublime after drying, and the positions occupied by the first camphor particles are transformed into the first pores.

8. The manufacturing method of the secondary battery according to claim 7, characterized in that, After forming the first active layer, it further includes: providing a carbon source gas and a carrier gas, and forming a layer of carbon nanotube layer on the surface of the first active layer by using a plasma chemical vapor deposition process at a temperature of 700°C to 900°C; the thickness of the carbon nanotube layer is 200μm to 500μm.

9. The manufacturing method of the secondary battery according to claim 7, characterized in that, The second particle size of the second camphor particle is smaller than the first particle size of the first camphor particle; the first particle size is 15μm to 20μm; the second particle size is 5μm to 10μm.

10. The method for preparing a secondary battery according to claim 7, wherein The mass fraction ratio of the first camphor particle, the first negative electrode material, the first conductive agent, the first binder, and the first dispersion solution is (0.2 to 0.4):(96 to 97.2):(0.8 to 1.2):(2.0 to 2.8):(160 to 200).

11. The method for preparing a secondary battery according to claim 10, wherein The first conductive agent includes carbon nanotubes and conductive carbon black, and the mass fraction ratio of the carbon nanotubes and the conductive carbon black is (0.4 to 0.6):(0.4 to 0.6).

12. A secondary battery prepared by the method for preparing a secondary battery according to any one of claims 1 to 11, characterized in that, It includes: A battery case, which has a cavity inside, and an electrolyte is provided inside the cavity; A bare battery cell, which is located inside the cavity and immersed in the electrolyte; The bare battery cell includes a positive electrode sheet, a separator, and a negative electrode sheet stacked together. Among them, the negative electrode sheet includes: a negative electrode current collector, a first active layer, and a second active layer. The first active layer is located on the surface of the current collector, and the second active layer is located on the surface of the first active layer. The first active layer has first holes, and the second active layer has second holes wrapped by a carbon shell. The second porosity of the second active layer is greater than the first porosity of the first active layer; the electrolyte is also located inside the first holes and the second holes.

13. The secondary battery according to claim 12, characterized in that, The negative electrode sheet further includes: a carbon nanotube layer, which is located between the first active layer and the second active layer.

14. A energy storage battery pack, characterized in that, It includes: A secondary battery prepared by the method for preparing a secondary battery according to any one of claims 1 to 11, or a secondary battery according to claim 12 or 13.

15. An electrical device, characterized in that, It includes: A secondary battery prepared by the method for preparing a secondary battery according to any one of claims 1 to 11, a secondary battery according to claim 12 or 13, or an energy storage battery pack according to claim 14.

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