WO2020177623A1

WO2020177623A1 - Negative pole piece, secondary battery and apparatus thereof

Info

Publication number: WO2020177623A1
Application number: PCT/CN2020/077137
Authority: WO
Inventors: 梁成都; 曾毓群; 闫传苗; 柳金华
Original assignee: 宁德时代新能源科技股份有限公司
Priority date: 2019-03-01
Filing date: 2020-02-28
Publication date: 2020-09-10
Also published as: CN115602789A; CN111640940A

Abstract

Provided in the present application are a negative pole piece, a secondary battery and an apparatus thereof. The negative pole piece comprises a negative current collector and a negative membrane disposed on the negative current collector; the negative membrane comprises a first coating and a second coating; the first coating is disposed on the negative current collector and comprises a first negative active material and a first conductive agent; the second coating is disposed on the surface of the first coating away from the negative current collector and comprises a second negative active material and a second conductive agent. The first negative active material and the second negative active material are carbon materials having different graphitization degrees, and the graphitization degree of the first negative active material is greater than the graphitization degree of the second negative active material. The mass percentage content of the first conductive agent in the first coating is less than the mass percentage content of the second conductive agent in the second coating. The negative pole piece of the present application can enable the secondary battery to have high specific energy and excellent dynamic performance.

Description

Negative sheet, secondary battery and its device

Technical field

This application relates to the field of batteries, in particular to a negative electrode sheet, a secondary battery and a device thereof.

Background technique

With the increasing depletion of traditional fossil energy sources and the increasingly serious environmental pollution problems, the development of new energy sources is imperative, and secondary batteries, as green energy sources, have received widespread attention worldwide. At present, secondary batteries represented by lithium-ion batteries have been widely used in portable digital devices such as mobile phones, electric buses, electric vehicles and other fields. However, with the rapid development of power battery technology, companies are pursuing high battery specific energy while also Higher requirements are put forward on the dynamic performance of the battery (such as fast charging performance and power performance).

How to make the battery have good dynamic performance under the premise of ensuring high energy density is one of the urgent problems in the industry.

Summary of the invention

In view of the problems in the background art, the purpose of the present application is to provide a negative electrode sheet, a secondary battery and a device thereof, the secondary battery having both high specific energy and excellent dynamic performance.

In order to achieve the above objective, in the first aspect of the present application, the present application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode membrane arranged on the negative electrode current collector, the negative electrode membrane comprising a first coating Layer and a second coating layer, the first coating layer is disposed on the negative electrode current collector and includes a first negative electrode active material and a first conductive agent, and the second coating layer is disposed on the first coating layer away from the The negative electrode current collector is on the surface and includes a second negative electrode active material and a second conductive agent. The first negative electrode active material and the second negative electrode active material are carbon materials with different graphitization degrees, and the graphitization degree of the first negative electrode active material is greater than the graphitization degree of the second negative electrode active material. The mass percentage of the first conductive agent in the first coating layer is less than the mass percentage of the second conductive agent in the second coating layer.

In the second aspect of the present application, the present application provides a secondary battery including the negative electrode sheet described in the first aspect of the present application.

In the third aspect of this application, this application relates to a device including the secondary battery described in the second aspect of this application.

This application includes at least the following beneficial effects: This application can reasonably match the types and contents of the negative electrode active material and the conductive agent in the first coating and the second coating of the negative film during the design of the negative electrode. The secondary battery has both high specific energy and excellent dynamic performance. The device of the present application includes the secondary battery described in the second aspect of the present application, and thus has at least the same advantages as the secondary battery.

Description of the drawings

FIG. 1 is a schematic diagram of an embodiment of the secondary battery of this application;

FIG. 2 is a schematic diagram of an embodiment of the battery module of this application;

FIG. 3 is a schematic diagram of an embodiment of the battery pack of this application;

Figure 4 is an exploded view of Figure 3;

FIG. 5 is a schematic diagram of an embodiment of a device using a secondary battery as a power source of this application;

among them:

1- Battery pack;

2-Upper box;

3- lower box;

4- battery module;

5- Secondary battery.

detailed description

The negative electrode sheet, the secondary battery and the device thereof according to the present application will be described in detail below.

First, the negative electrode sheet according to the first aspect of the present application will be explained.

The negative electrode sheet according to the first aspect of the present application includes a negative electrode current collector and a negative electrode membrane provided on the negative electrode current collector. The negative electrode membrane comprises a first coating and a second coating, and the first coating is provided On the negative current collector and including a first negative active material and a first conductive agent, the second coating is disposed on the surface of the first coating away from the negative current collector and includes a second negative active material And the second conductive agent.

In the negative electrode sheet according to the first aspect of the present application, the first negative electrode active material and the second negative electrode active material are carbon materials with different graphitization degrees, and the graphitization degree of the first negative electrode active material Greater than the graphitization degree of the second negative electrode active material.

The graphitization degree of the first negative electrode active material and the second negative electrode active material can be calculated according to the following formula:

In the formula, g is the degree of graphitization; d ₀₀₂ is the interlayer spacing of the (002) crystal plane of the carbon material, and the unit of measurement is nm.

When a secondary battery is charged, ions are extracted from the positive electrode active material and inserted into the negative electrode active material through the electrolyte, while electrons are transferred to the negative electrode through an external circuit to maintain charge balance; the opposite is true during discharge. Taking carbon materials as an example, one of the mechanisms of ion intercalation in carbon materials is the formation of interlayer compounds in the layered graphite microcrystalline structure of carbon materials. Therefore, the degree of graphitization and interlayer spacing of carbon materials are closely related to the amount of ion intercalation of carbon materials. Related. Generally speaking, the graphitization degree of carbon materials is high (corresponding to the small interlayer spacing d ₀₀₂ ), and the ion insertion amount of carbon materials is high. This is because the high graphitization degree of carbon materials is easy to form low-potential interlayer compounds. The ion insertion amount is close to the theoretical value.

However, the surface anisotropy of carbon materials with a high degree of graphitization is usually greater. During the first charge of the secondary battery, the unevenness of the reduction and decomposition reaction of the electrolyte on the surface of the carbon material increases, resulting in the formation of SEI on the surface of the negative electrode. The membrane is loose and porous and cannot effectively block the co-intercalation of solvated ions, which may lead to the collapse of the carbon material structure. In addition, since the diffusion speed of ions along the ab-axis plane in the highly graphitized carbon material is greater than that along the c-axis direction, the insertion of ions is carried out at the boundary of the carbon material. The boundary area of the highly graphitized carbon material is small and the particles There is a large mutual barrier effect between them, which will cause a great kinetic hindrance to the diffusion of ions in the highly graphitized carbon material, and cannot be charged and discharged at a faster rate. The secondary battery has poor kinetic performance. problem.

If in the design of the secondary battery, the negative electrode membrane is only a single-layer structure and the carbon material has a high degree of graphitization, although it is beneficial to ion intercalation of the carbon material, it is beneficial to increase the specific energy of the secondary battery, but the layer spacing of the carbon material d _{002 is} small, which may hinder the diffusion of ions in the carbon material, so that the secondary battery cannot be charged and discharged at a faster rate, and the dynamic performance of the secondary battery is poor.

If in the design of the secondary battery, the negative electrode membrane is only a single-layer structure and the degree of graphitization of the carbon material is low, although it is beneficial to the diffusion of ions in the carbon material, the secondary battery can be charged and discharged at a faster rate. However, correspondingly, using a carbon material with a low degree of graphitization as a negative electrode active material may not be conducive to the specific energy of the secondary battery.

Further, although the addition of a conductive agent to the single-layer structure of the negative electrode film can achieve the purpose of improving the dynamic performance of the secondary battery, since the conductive agent is usually an inactive material, the increase in the content of the inactive material in the negative electrode film means The decrease in the content of the active material will therefore impair the specific energy of the secondary battery.

The negative electrode membrane of the present application has a multilayer structure, and the first negative electrode active material in the first coating layer provided on the negative electrode current collector has a high degree of graphitization, which enables the secondary battery to have the characteristics of high specific energy; The second negative electrode active material in the second coating layer on the first coating layer has a low graphitization degree, which can compensate for the problem of poor dynamic performance of the secondary battery caused by the high graphitization degree of the first negative electrode active material. Therefore, the secondary battery using the negative electrode film with the multilayer structure of the present application can have both high specific energy and excellent dynamic performance.

Preferably, the graphitization degree of the first negative electrode active material is 90% to 99.5%.

Preferably, the graphitization degree of the second negative electrode active material is 80%-98.5%.

Preferably, the first negative electrode active material is selected from one or more of artificial graphite, natural graphite, soft carbon, hard carbon, and mesocarbon microspheres.

Preferably, the second negative electrode active material is selected from one or more of artificial graphite, natural graphite, soft carbon, hard carbon, and mesocarbon microspheres.

In the negative electrode sheet according to the first aspect of the present application, the mass percentage of the first conductive agent in the first coating layer is less than the mass percentage of the second conductive agent in the second coating layer. The addition of the first conductive agent and the second conductive agent can improve the dynamic performance of the secondary battery, especially when the second coating contains a larger amount of the second conductive agent, the second coating as a high-kinetic transition layer can further Improve the dynamic performance of the secondary battery.

Under the same other conditions, when the content of the first conductive agent and the second conductive agent existing as inactive materials is small, it is not conducive to improving the dynamic performance of the secondary battery; and the first conductive agent existing as inactive materials The content of the conductive agent and the second conductive agent is continuously increasing, and the content of the first negative electrode active material and the second negative electrode active material is correspondingly reduced, which is not conducive to increasing the specific energy of the secondary battery.

Preferably, the mass percentage of the first conductive agent in the first coating is 0.5% to 3%. Further preferably, the mass percentage of the first conductive agent in the first coating is 1% to 2%.

Preferably, the mass percentage of the second conductive agent in the second coating is 1% to 6%. Further preferably, the mass percentage of the second conductive agent in the second coating is 2% to 5%.

In the negative electrode sheet according to the first aspect of the present application, preferably, the mass percentage of the first conductive agent in the first coating layer and the second conductive agent in the second coating layer The ratio of the mass percentage content is 1:(1.2～6). When the mass percentage ratio of the first conductive agent in the first coating to the second conductive agent in the second coating is within the above-mentioned range, the secondary can be balanced while reducing the amount of conductive agent as much as possible The specific energy and dynamic performance of the battery enable the secondary battery to take into account both high specific energy and excellent dynamic performance. Further preferably, the ratio of the mass percentage of the first conductive agent in the first coating to the mass percentage of the second conductive agent in the second coating is 1:(1.5 ~3).

In the negative electrode sheet according to the first aspect of the present application, preferably, the conductivity of the first conductive agent is lower than the conductivity of the second conductive agent. The negative electrode film of the present application has a multi-layer structure. When the conductivity of the second conductive agent in the second coating layer is greater than that of the first conductive agent in the first coating layer, it can be more conducive to ions in the negative electrode. The extraction and embedding in the diaphragm ensure that the secondary battery has excellent dynamic performance.

Preferably, the conductivity of the first conductive agent is 10 S/cm to 100 S/cm. Further preferably, the electrical conductivity of the first conductive agent is 10 S/cm to 50 S/cm.

Preferably, the conductivity of the second conductive agent is 30 S/cm to 200 S/cm. Further preferably, the conductivity of the second conductive agent is 40 S/cm to 150 S/cm.

Preferably, the first conductive agent is selected from one or more of conductive carbon black, carbon nanotubes, carbon nanofibers, and graphene.

Preferably, the second conductive agent is selected from one or more of conductive carbon black, carbon nanotubes, carbon nanofibers, and graphene.

In the negative electrode sheet described in the first aspect of the present application, in order to ensure that the secondary battery has a high specific energy without compromising the kinetic performance of the secondary battery, the second coating layer as a high-kinetic transition layer should not be too thick . Preferably, the ratio of the thickness of the first coating to the thickness of the second coating is (1-10):1. Further preferably, the ratio of the thickness of the first coating to the thickness of the second coating is (1-5):1.

In the negative electrode sheet according to the first aspect of the present application, the first coating layer further includes a first binder and a first dispersant, and the types of the first binder and the first dispersant are not specifically limited , Can be selected according to actual needs. Preferably, the first binder may be selected from one of polyacrylic acid, sodium polyacrylate, sodium alginate, polyacrylonitrile, polyethylene glycol, carboxymethyl chitosan, and styrene butadiene rubber (SBR) Or several; preferably, the first dispersant may be selected from sodium carboxymethyl cellulose (CMC).

In the negative electrode sheet according to the first aspect of the present application, the second coating layer further includes a second binder and a second dispersant, and the types of the second binder and the second dispersant are not specifically limited , Can be selected according to actual needs. Preferably, the second binder may be selected from one of polyacrylic acid, sodium polyacrylate, sodium alginate, polyacrylonitrile, polyethylene glycol, carboxymethyl chitosan, and styrene butadiene rubber (SBR) Or several; preferably, the second dispersant may be selected from sodium carboxymethyl cellulose (CMC).

It should be noted that the types of the first adhesive and the second adhesive can be the same or different, and can be selected according to actual needs.

In the negative electrode sheet described in the first aspect of the present application, the type of the negative electrode current collector is not specifically limited, and can be selected according to actual needs. For example, the negative electrode current collector can be copper foil or stainless steel foil, preferably , The negative electrode current collector is copper foil.

In the negative electrode sheet according to the first aspect of the present application, the preparation method of the negative electrode sheet may include the steps:

(1) Disperse the first negative electrode active material, the first binder, the first conductive agent, and the first dispersant in deionized water according to a certain mass ratio, and stir uniformly to obtain the first negative electrode slurry;

(2) Disperse the second negative electrode active material, the second binder, the second conductive agent, and the second dispersant in deionized water according to a certain mass ratio, and stir uniformly to obtain the second negative electrode slurry;

(3) Coating the first negative electrode slurry on the negative electrode current collector to form a first coating, and then coating the second negative electrode slurry on the first coating to form a second coating, which is obtained by cold pressing and slitting Negative plate.

Next, the secondary battery according to the second aspect of the present application will be explained.

The secondary battery according to the second aspect of the present application includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, wherein the negative electrode sheet is the negative electrode sheet according to the first aspect of the present application.

In the secondary battery described in the second aspect of the present application, the type of the separator is not specifically limited, and can be selected according to actual needs. For example, the isolation film may be polyethylene, polypropylene, polyvinylidene fluoride and their multilayer composite film, but is not limited to these.

In the secondary battery described in the second aspect of the present application, the type of the electrolyte is not specifically limited, and can be selected according to actual needs.

It should be noted that the secondary battery according to the second aspect of the present application may be a lithium ion battery, a sodium ion battery, or any other secondary battery using the negative electrode sheet according to the first aspect of the present application. Preferably, the secondary battery according to the second aspect of the present application is a lithium ion battery.

When the secondary battery is a lithium ion battery, the positive electrode active material in the positive electrode sheet may be selected from lithium transition metal composite oxides. Specifically, the positive electrode active material can be selected from lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate. One or more of these materials, but this application is not limited to these materials.

In some embodiments, the secondary battery may include an outer package for packaging the positive pole piece, the negative pole piece, and the electrolyte. As an example, the positive pole piece, the negative pole piece and the separator can be laminated or wound to form a laminated structure electrode assembly or a wound structure electrode assembly, the electrode assembly is packaged in an outer package; the electrolyte can be an electrolyte, which is infiltrated In the electrode assembly. The number of electrode assemblies in the secondary battery can be one or several, which can be adjusted according to requirements.

In some embodiments, the outer packaging of the secondary battery may be a soft bag, such as a pouch type soft bag. The material of the soft bag can be plastic, for example, it can include one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), and the like. The outer packaging of the secondary battery may also be a hard case, such as an aluminum case.

The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or other arbitrary shapes. Fig. 1 shows an electrochemical device 5 with a square structure as an example.

In some embodiments, the secondary battery can be assembled into a battery module, and the number of electrochemical devices contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

Fig. 2 is a battery module 4 as an example. 2, in the battery module 4, a plurality of secondary batteries 5 may be arranged in order along the length direction of the battery module 4. Of course, it can also be arranged in any other manner. Furthermore, the plurality of secondary batteries 5 can be fixed by fasteners.

Optionally, the battery module 4 may further include a housing having an accommodation space, and a plurality of secondary batteries 5 are accommodated in the accommodation space.

In some embodiments, the above-mentioned battery modules can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

Figures 3 and 4 show the battery pack 1 as an example. 3 and 4, the battery pack 1 may include a battery box and a plurality of battery modules 4 provided in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can be covered on the lower box body 3 and forms a sealed space for accommodating the battery module 4. Multiple battery modules 4 can be arranged in the battery box in any manner.

Finally, the device according to the third aspect of the present application is described, which includes the secondary battery described in the second aspect of the present application. The secondary battery provides power to the device. The device can be, but is not limited to, mobile devices (such as mobile phones, laptop computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf Vehicles, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

The device can select an electrochemical device, a battery module or a battery pack according to its usage requirements.

Figure 5 is a device as an example. The device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the requirements of the device for high power and high energy density of electrochemical devices, battery packs or battery modules can be used.

As another example, the device may be a mobile phone, a tablet computer, a notebook computer, etc. The device is generally required to be thin and light, and a secondary battery can be used as a power source.

In the following, a lithium ion battery is taken as an example, and the application is further described in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the application and not to limit the scope of the application.

The lithium ion batteries of Examples 1-4 and Comparative Examples 1-5 were prepared according to the following methods.

(1) Preparation of positive electrode sheet

The positive electrode active material LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , the conductive agent acetylene black, and the binder PVDF were mixed in a mass ratio of 96:2:2, and the solvent NMP was added, and the system was stirred under the action of a vacuum mixer until the system was uniform. Positive electrode slurry; uniformly coat the positive electrode slurry on the positive electrode current collector aluminum foil, dry at room temperature and transfer to an oven to continue drying, and then undergo cold pressing and slitting to obtain a positive electrode sheet.

(2) Preparation of negative electrode sheet

Disperse the first negative electrode active material, the first conductive agent, the first binder, and the first dispersant shown in Table 1 in deionized water in proportion, and stir until the system is uniform to obtain the first negative electrode slurry; The second negative electrode active material, the second conductive agent, the second binder, and the second dispersant shown in Table 2 are dispersed in deionized water in proportion, and stirred until the system is uniform to obtain the second negative electrode slurry; The extrusion coating equipment coats the first negative electrode slurry on the copper foil of the negative electrode current collector to form a first coating. After drying, coats the second negative electrode slurry on the first coating to form a second coating. After drying, transfer to an oven to continue drying, and then undergo cold pressing and slitting to obtain a negative electrode sheet.

(3) Preparation of electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF _{6 is} dissolved in the mixed The latter organic solvent is formulated into an electrolyte with a concentration of 1 mol/L.

(4) Preparation of isolation membrane

A polyethylene film is selected as the isolation film.

(5) Preparation of lithium ion battery

Lay the above positive electrode sheet, separator film, and negative electrode sheet in order, so that the separator film is located between the positive and negative electrode sheets for isolation, and then wind to obtain the electrode assembly; place the electrode assembly in the outer package and dry it Inject the electrolyte, go through the processes of vacuum packaging, standing, forming, and shaping to obtain a lithium ion battery.

Table 1 Parameters of the first coating of Examples 1-4 and Comparative Examples 1-5

Table 2 Parameters of the second coating of Examples 1-4 and Comparative Examples 1-5

Next, the test procedure of the lithium ion battery is explained.

(1) Specific energy test of lithium ion battery

At room temperature, the lithium ion battery is charged to the upper limit voltage at a rate of 1/3C, and then discharged to the lower limit voltage at a rate of 1/3C to obtain the energy during the discharge process of the lithium ion battery.

The specific energy of the lithium ion battery (Wh/Kg) = the energy during the discharge process of the lithium ion battery/the mass of the lithium ion battery.

(2) Rate performance test of lithium ion battery

At room temperature, charge the lithium-ion battery to the upper limit voltage at a rate of 1/3C, and then discharge to the lower limit voltage at 0.33C and 2C respectively, and use the discharge capacity of 0.33C as the reference group to calculate the lithium-ion battery to discharge at the rate of 2C The capacity retention rate.

(3) DC internal resistance test of lithium ion battery

At room temperature, charge the lithium ion battery to the upper limit voltage at a rate of 1/3C, which is 100% SOC, and then discharge the lithium ion battery to 50% SOC. After standing for 5 minutes, discharge at a rate of 4C for 10 seconds to obtain room temperature The direct current internal resistance (DCR) of the lower lithium-ion battery at 50% SOC.

Table 3 Performance test results of Examples 1-4 and Comparative Examples 1-5

From the analysis of the test results in Table 3, it can be seen that Examples 1-4 reasonably matched the types and contents of the negative active material and the conductive agent in the first coating and the second coating in the negative film of the lithium ion battery during the design of the negative film. , Can make the lithium ion battery have both high specific energy and excellent dynamic performance.

In Comparative Example 1, the negative electrode membrane has a single-layer structure, and the negative electrode active material is artificial graphite with a high degree of graphitization. Although it is beneficial to the insertion of lithium ions and is beneficial to increase the specific energy of the lithium ion battery, Comparative Example 1 The artificial graphite used has a small interlayer spacing d ₀₀₂ , which may hinder the diffusion of lithium ions and cannot improve the dynamic performance of the lithium ion battery.

In Comparative Example 2, the negative electrode film has a multilayer structure. The first coating uses artificial graphite with a lower graphitization degree, and the second coating uses soft carbon with a higher graphitization degree. The content of the conductive agent is higher than the content of the second conductive agent in the second coating. At this time, the specific energy of the lithium ion battery is lower, and the dynamic performance of the lithium ion battery is also poor.

In Comparative Example 3, the negative electrode membrane has a multi-layer structure, the first coating is made of natural graphite with a low degree of graphitization, the second coating is made of hard carbon with a high degree of graphitization, and the first coating is the first The content of the conductive agent is higher than the content of the second conductive agent in the second coating. At this time, the specific energy of the lithium ion battery is lower, and the dynamic performance of the lithium ion battery is also poor.

In Comparative Example 4, the negative electrode membrane has a multilayer structure, the first coating uses natural graphite with a low degree of graphitization, the second coating uses mesocarbon microspheres with a high degree of graphitization, and the second coating Relative to the first coating layer is set too thick, the specific energy of the lithium ion battery is lower at this time, and the dynamic performance of the lithium ion battery is also poor.

In Comparative Example 5, the negative electrode membrane has a multilayer structure. The first coating uses natural graphite with a low degree of graphitization, and the second coating uses artificial graphite with a high degree of graphitization. The first coating is relatively If the coating is too thick, the specific energy of the lithium-ion battery is low, and the dynamic performance of the lithium-ion battery is also poor.

Claims

A negative electrode sheet includes a negative electrode current collector and a negative electrode membrane arranged on the negative electrode current collector, wherein:

The negative electrode membrane includes a first coating and a second coating. The first coating is disposed on the negative current collector and includes a first negative active material and a first conductive agent. The second coating is disposed On the surface of the first coating away from the negative electrode current collector and comprising a second negative electrode active material and a second conductive agent;

among them,

The first negative electrode active material and the second negative electrode active material are carbon materials with different graphitization degrees, and the graphitization degree of the first negative electrode active material is greater than the graphitization degree of the second negative electrode active material;

The mass percentage of the first conductive agent in the first coating layer is less than the mass percentage of the second conductive agent in the second coating layer.
The negative electrode sheet according to claim 1, wherein:

The graphitization degree of the first negative electrode active material is 90% to 99.5%; and/or,

The graphitization degree of the second negative electrode active material is 80%-98.5%.
The negative electrode sheet according to claim 1, wherein:

The mass percentage of the first conductive agent in the first coating layer is 0.5% to 3%, preferably 1% to 2%; and/or,

The mass percentage of the second conductive agent in the second coating is 1% to 6%, preferably 2% to 5%.
The negative electrode sheet according to claim 1 or 2, wherein:

The first negative electrode active material is selected from one or more of artificial graphite, natural graphite, soft carbon, hard carbon, and mesocarbon microspheres; and/or,

The second negative electrode active material is selected from one or more of artificial graphite, natural graphite, soft carbon, hard carbon, and mesocarbon microspheres.
The negative electrode sheet according to claim 1 or 3, wherein the conductivity of the first conductive agent is less than the conductivity of the second conductive agent.
The negative electrode sheet according to claim 5, wherein:

The electrical conductivity of the first conductive agent is 10S/cm to 100S/cm, preferably 10S/cm to 50S/cm; and/or,

The conductivity of the second conductive agent is 30 S/cm to 200 S/cm, preferably 40 S/cm to 150 S/cm.
The negative electrode sheet according to any one of claims 1 to 6, wherein:

The first conductive agent is selected from one or more of conductive carbon black, carbon nanotubes, carbon nanofibers, and graphene; and/or,

The second conductive agent is selected from one or more of conductive carbon black, carbon nanotube, carbon nanofiber, and graphene.
The negative electrode sheet according to any one of claims 1 to 7, wherein the mass percentage of the first conductive agent in the first coating layer is equal to that of the second conductive agent in the second coating layer. The ratio of the mass percentage content in it is 1:(1.2-6), preferably 1:(1.5-3).
The negative electrode sheet according to claim 1, wherein the ratio of the thickness of the first coating layer to the thickness of the second coating layer is (1-10):1, preferably (1-5):1.
A secondary battery, comprising the negative electrode sheet according to any one of claims 1 to 9.
A device comprising the secondary battery of claim 10.