WO2025251663A1 - Negative electrode sheet and preparation method therefor, and battery and electric device - Google Patents

Abstract

A negative electrode sheet (100) and a preparation method therefor, and a battery and an electric device. The negative electrode sheet (100) comprises a current collector (10) and n active layers (20), wherein the current collector (10) has a first surface (11); the n active layers (20) are sequentially stacked on the first surface (11) in a first direction (X), the first direction (X) being perpendicular to the first surface (11); each active layer (20) comprises a first hard carbon material and/or a second hard carbon material; the content of the first hard carbon material in an (m+1)th active layer (20) is less than the content of the first hard carbon material in an mth active layer (20), the content of the second hard carbon material in the (m+1)th active layer (20) is greater than the content of the second hard carbon material in the mth active layer (20), the mth active layer (20) is closer to the first surface (11) than the (m+1)th active layer (20), and 1≤m≤n-1 is satisfied, both m and n being positive integers; and the proportion of the sloping-region capacity of the first hard carbon material is b1, and the proportion of the sloping-region capacity of the second hard carbon material is b2, satisfying b1<b2. By means of the negative electrode sheet (100), a battery achieves both a high energy density and a good charging capability.

Description

Negative electrode sheet and its preparation method, battery and electrical equipment

This application claims priority to Chinese Patent Application No. 202410711813.5, filed on June 3, 2024, entitled “Negative Electrode Sheet and Method for Preparation Thereof, Battery and Electrical Device”, the entire contents of which are incorporated herein by reference.

Technical Field

This application relates to the field of battery technology, specifically to a negative electrode sheet and its preparation method, a battery, and an electrical device.

Background Technology

Sodium-ion batteries are cheaper than lithium-ion batteries, but the energy density of existing sodium-ion battery systems is much lower than that of lithium-ion batteries, which limits the application scenarios of sodium-ion batteries.

Among them, sodium-ion battery anodes have significant room for improvement in specific capacity and compaction. However, improving the specific capacity of the anode requires improving its plateau region around 0V, which would increase the capacity proportion of the plateau region. But this would lead to a decrease in the capacity proportion of the ramp region, i.e., a decrease in power performance. It is difficult to achieve both simultaneously.

Summary of the Invention

The purpose of this application is to provide a negative electrode sheet and its preparation method, a battery, and an electrical device to solve the problem of the difficulty in balancing the power performance and battery capacity of sodium-ion batteries.

To achieve the objectives of this application, the following technical solution is provided:

In a first aspect, this application provides a negative electrode sheet, comprising:

A current collector having a first surface;

n active layers are sequentially stacked on the first surface along a first direction, wherein the first direction is perpendicular to the first surface;

Each of the active layers includes a first hard carbon material and/or a second hard carbon material; the content of the first hard carbon material in the (m+1)th active layer is less than the content of the first hard carbon material in the mth active layer, the content of the second hard carbon material in the (m+1)th active layer is greater than the content of the second hard carbon material in the mth active layer, the mth active layer is closer to the first surface than the (m+1)th active layer, and satisfies 1≤m≤n-1, where m and n are both positive integers;

The slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1 < b2;

Furthermore, the capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process at 0.1V vs. Na ⁺ /Na.

In one implementation, the following condition is also satisfied: 2≤n≤4.

In one embodiment, the reversible specific capacity of the first hard carbon material is a1, which satisfies: a1≥290mAh/g.

In one implementation, the following condition is also satisfied: 20% ≤ b1 ≤ 40%.

In one implementation, the following condition is also satisfied: 40% ≤ b2 ≤ 70%.

In one embodiment, the thickness of the active layer is h, which satisfies: h≥20μm.

In one implementation, the following condition is also satisfied: 30μm≤h≤60μm.

In one embodiment, the platform region capacity ratio of the first hard carbon material is e1, satisfying: 60% ≤ e1 ≤ 80%; the platform region capacity ratio of the second hard carbon material is e2, satisfying: 30% ≤ e2 ≤ 60%, and the capacity of the platform regions of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process from 0V vs. Na ⁺ /Na to 0.1V vs. Na ⁺ /Na.

Secondly, this application provides a method for preparing a negative electrode sheet, comprising the following steps:

A current collector is provided, the current collector having a first surface;

n active layers are sequentially stacked on the first surface along a first direction, wherein the first direction is perpendicular to the first surface;

Each of the active layers includes a first hard carbon material and/or a second hard carbon material; the content of the first hard carbon material in the (m+1)th active layer is less than the content of the first hard carbon material in the mth active layer, the content of the second hard carbon material in the (m+1)th active layer is greater than the content of the second hard carbon material in the mth active layer, the mth active layer is closer to the first surface than the (m+1)th active layer, and satisfies 1≤m≤n-1, where m and n are both positive integers;

The slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1 < b2;

Wherein, the capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process above 0.1V vs. Na ⁺ /Na.

Thirdly, this application provides a battery including a positive electrode, a separator, and a negative electrode as described in any of the first aspects, wherein the separator is disposed between the positive electrode and the negative electrode.

Fourthly, this application provides an electrical device, including an electrical appliance and a battery as described in the third aspect, wherein the battery supplies power to the electrical appliance.

The negative electrode sheet of this application has multiple active layers. The active layers contain a first hard carbon material to improve battery capacity and a second hard carbon material to improve battery rate. The first hard carbon material decreases in gradient along the direction away from the current collector in the active layers, while the second hard carbon material increases in gradient along the direction away from the current collector, i.e., closer to the electrode. The gradient of the first and second hard carbon materials in the multi-layer active layers improves the ion exchange rate near the electrode and the ion capacity near the current collector. The corresponding sodium-ion battery can achieve both improved power performance and battery capacity, resulting in improved battery capacity while having better rate performance and shorter fast charging time.

Attached Figure Description

To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

Figure 1 is a side view of the negative electrode and separator according to an embodiment;

Figure 2 is a flowchart of a method for preparing a negative electrode sheet according to an embodiment.

Explanation of reference numerals in the attached figures:
100 - Negative electrode, 10 - Current collector, 11 - First surface, 20 - Active layer, 200 - Separator, X - First direction.

Detailed Implementation

The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

It should be noted that when a component is said to be "fixed" to another component, it can be directly on the other component or it can be in a middle component. When a component is said to be "connected" to another component, it can be directly connected to the other component or it may be in a middle component.

Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and/or" as used in this application includes any and all combinations of one or more of the associated listed items.

The following detailed description of some embodiments of this application is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

Referring to Figure 1, this application provides a negative electrode 100, including a current collector 10 and n active layers 20. The current collector 10 has a first surface 11. The n active layers 20 are sequentially stacked on the first surface 11 along a first direction X, which is perpendicular to the first surface 11. Each active layer 20 includes a first hard carbon material and/or a second hard carbon material. The content of the first hard carbon material in the (m+1)th active layer 20 is less than the content of the first hard carbon material in the mth active layer 20, and the content of the second hard carbon material in the (m+1)th active layer 20 is greater than the content of the second hard carbon material in the mth active layer 20. The mth active layer 20 is closer to the first surface 11 than the (m+1)th active layer 20, and satisfies 1≤m≤n-1, where m and n are both positive integers.

The first hard carbon material and the second hard carbon material have specific special properties, namely: the slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1 < b2.

Among them, the capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process above 0.1V vs. Na ⁺ /Na.

When the electrode is a negative electrode 100, the corresponding current collector 10 can be any one of copper foil, composite copper foil or carbon-coated copper foil, or any one of aluminum foil, composite aluminum foil or carbon-coated aluminum foil.

The first and second hard carbon materials can be resin carbon (such as phenolic resin, epoxy resin, polyfurfuryl alcohol resin, etc.), organic polymer carbon (such as polyvinyl alcohol, polyvinyl chloride, polyvinylidene fluoride, polyacrylonitrile, etc.), carbon black (acetylene black prepared by chemical vapor deposition, etc.), biomass carbon (such as plant residues and shells, etc.), without limitation. Hard carbon materials, as a negative electrode material for sodium-ion batteries, are suitable for sodium-ion intercalation and deintercalation due to their large interlayer spacing and irregular structure.

In one embodiment, the reversible specific capacity of the first hard carbon material is a1, satisfying: a1≥290mAh/g. a1 can be 290mAh/g, 300mAh/g, 350mAh/g, 400mAh/g, etc., and is not limited.

In one implementation, the following condition is also satisfied: 20% ≤ b1 ≤ 40%. b1 can be 20%, 25%, 30%, 35%, or 40%, without limitation. Optionally, b1 also satisfies 30% ≤ b1 ≤ 40%.

The first hard carbon material has a1≥290mAh/g and the slope region capacity ratio b1 satisfies 20%≤b1≤40%, making the first hard carbon material a capacity-type hard carbon material. This results in a higher energy density for the battery and the more electricity it can store.

In one embodiment, the following condition is also satisfied: 40% ≤ b2 ≤ 70%. b2 can be 40%, 50%, 60%, or 70%, without limitation. Optionally, 40% ≤ b2 ≤ 60%. And when b1 is 40%, then b2 cannot be 40%. The second hard carbon material is a rate-controlled hard carbon material, enabling the battery to release or absorb energy more quickly, resulting in faster charging and discharging speeds and improved device efficiency.

The total mass of the first hard carbon material and the second hard carbon material in each active layer 20 may or may not be the same.

The number of active layers 20, n, satisfies 1 ≤ m ≤ n-1. Optionally, it also satisfies 2 ≤ n ≤ 4; specifically, n can be 2, 3, or 4. Multiple active layers 20 are stacked on the current collector 10, and as the active layers move away from the current collector 10 along the first direction X, the content gradient of the first hard carbon material in the active layers 20 decreases, while the content gradient of the second hard carbon material in the active layers 20 increases. Referring to Figure 1, the active layer 20 directly stacked on the first surface 11 is the first active layer 20, and the active layer 20 furthest from the first surface 11 is the nth active layer 20. The nth active layer 20 is in close contact with the diaphragm 200. Optionally, all the hard carbon material in the first active layer 20 is the first hard carbon material, and all the hard carbon material in the nth active layer 20 is the second hard carbon material.

The negative electrode 100 of this application is provided with multiple active layers 20. The active layers 20 are provided with a first hard carbon material to improve battery capacity and a second hard carbon material to improve battery rate. The first hard carbon material in the active layers 20 decreases in a gradient away from the current collector 10, and the second hard carbon material in the active layers 20 increases in a gradient away from the current collector 10, i.e., closer to the electrode. The multi-layer active layers 20 are provided with gradient-changing first and second hard carbon materials, which improves the ion exchange rate in the part near the electrode and the ion capacity in the part near the current collector. The corresponding sodium-ion battery can improve battery capacity while having better rate performance and shorter fast charging time.

In one embodiment, the thickness of the active layer 20 is h, satisfying: h ≥ 20 μm. Specifically, 30 μm ≤ h ≤ 60 μm. h can be 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, and is not limited.

When h≥20μm, the active layer 20 can provide more electrochemical reaction area, thereby improving the battery capacity and energy density.

When h > 60 μm, an excessively thick active layer 20 may increase the transport distance of electrons and ions, thereby increasing the battery's internal resistance and charging/discharging time. An excessively thick active layer 20 is also susceptible to the effects of lithium dendrites, leading to a decrease in battery performance. When 30 μm ≤ h ≤ 60 μm, the active layer 20 can balance battery capacity, energy density, and charging/discharging speed, and can also reduce battery weight, thereby improving battery energy density and portability.

In one embodiment, the platform region capacity ratio of the first hard carbon material is e1, satisfying: 60% ≤ e1 ≤ 80%; the platform region capacity ratio of the second hard carbon material is e2, satisfying: 30% ≤ e2 ≤ 60%, and the capacity of the platform regions of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process from 0V vs. ^Na+ /Na to 0.1V vs. Na ⁺ /Na.

e1 can be 60%, 65%, 70%, 75%, 80%, etc., without restriction. e2 can be 30%, 40%, 50%, 60%, etc., without restriction. Specifically, the sum of e1 and b1 is 100%, and the sum of e2 and b2 is 100%.

The first hard carbon material has a large plateau region capacity, which enables it to have a high specific capacity; the second hard carbon material has a large plateau region capacity, which enables it to have better rate performance.

Referring to Figures 1 and 2, this application also provides a method for preparing a negative electrode 100, comprising the following steps:

Step S10: Provide a current collector 10, the current collector 10 having a first surface 11;

Step S20: n active layers 20 are sequentially stacked on the first surface 11 along the first direction X, where the first direction X is perpendicular to the first surface 11.

Each active layer 20 includes a first hard carbon material and/or a second hard carbon material; the content of the first hard carbon material in the (m+1)th active layer 20 is less than the content of the first hard carbon material in the mth active layer 20, the content of the second hard carbon material in the (m+1)th active layer 20 is greater than the content of the second hard carbon material in the mth active layer 20, the mth active layer 20 is closer to the first surface 11 than the (m+1)th active layer, and satisfies 1≤m≤n-1, where m and n are both positive integers;

The capacity ratio of the slope region of the first hard carbon material is b1, and the capacity ratio of the slope region of the second hard carbon material is b2, satisfying: b1 < b2; wherein, the capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process above 0.1V vs. Na+/Na.

In step S20, n active layers 20 are sequentially stacked on the first surface 11 along the first direction X, which can be achieved by a multilayer coating method.

This application provides a battery including a positive electrode, a separator 200 and a negative electrode 100 as described in any of the preceding embodiments, wherein the separator 200 is disposed between the positive electrode and the negative electrode 100.

The battery can be a prismatic battery, a cylindrical battery, or other types such as a prismatic battery, without any restrictions.

The battery also includes a casing with a receiving cavity in which the positive electrode plate, separator 200, and negative electrode plate 100 are housed. The casing is made of a material with high structural strength, specifically metal, high-strength plastic, ceramic, etc. Metal materials include aluminum, aluminum alloy, magnesium alloy, iron, and iron alloys. The casing includes a base plate and side plates. The casing can be a one-piece structure, meaning the base plate and side plates are manufactured using a single molding process, such as stamping or casting, without limitation. The casing can also be a separate structure, with the side plates and base plate connected and fixed by welding, bonding, snap-fitting, screwing, etc. The wall thickness of the casing can be approximately uniform throughout, meaning the side plates can have a roughly uniform thickness, and the base plate and side plates can also have roughly the same thickness.

The positive electrode material of the battery can be a sodium-ion battery positive electrode active material. In specific embodiments, it includes one or more of sodium transition metal oxides, Prussian blue/white compounds, polyanionic compounds, mixed metal oxides, and layered oxides.

The separator 200 can be any type of woven membrane, nonwoven membrane (non-woven fabric), microporous membrane, composite membrane, rolled membrane, etc., without limitation. Several layers of separator 200 are used to separate several layers of positive and negative electrode sheets 100.

The positive electrode material, conductive agent, and binder are uniformly mixed and dispersed in NMP (N-methylpyrrolidone), coated onto a foil, and baked to obtain the positive electrode sheet. The conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60 (carbon 60), and carbon nanotubes; the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives, without limitation. This application does not specifically limit these materials; suitable materials can be selected according to actual application requirements.

This application provides an electrical device, including an electrical device and a battery as described in the foregoing embodiments, wherein the battery supplies power to the electrical device.

The battery fabrication process of one embodiment of this application is as follows:

The first hard carbon material and the second hard carbon material are mixed with conductive agent, binder and other materials in deionized water in a certain proportion, and then uniformly coated on the surface of current collector 10. The resulting negative electrode sheet 100 is baked and rolled.

The positive electrode, separator 200 and negative electrode 100 are wound sequentially and orderly to obtain the electrode core.

After the electrode core is encased, electrolyte is injected, and after aging, formation, and capacity testing, a complete secondary sodium-ion battery is obtained.

The technical solution of this application will be described in detail below through specific embodiments.

Example 1

This embodiment provides a negative electrode 100, which includes a current collector 10 and two active layers 20.

The first active layer 20 of the two active layers 20 is disposed on the first surface 11 of the current collector 10, and the second active layer 20 is disposed on the surface of the first active layer 20 away from the current collector 10. The mass ratio of the first hard carbon material and the second hard carbon material of the negative electrode 100 is 1:1. The first hard carbon material is resin carbon, and the second hard carbon material is biomass carbon. The thickness h of both the first active layer 20 and the second active layer 20 is 40 μm ± 2 μm.

The a1 of the first hard carbon material is 315.6 mAh/g, the reversible specific capacity of the second hard carbon material is a2, a2 is 280.4 mAh/g, the b1 of the first hard carbon material is 34.3%, and the b2 of the second hard carbon material is 43.4%.

The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 6:4.

The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 4:6.

Example 2

This embodiment provides a negative electrode 100, which is basically the same as that in Embodiment 1, except that:

The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 8:2.

In the second active layer 20, the mass ratio of the first hard carbon material to the second hard carbon material is 2:8.

Example 3

This embodiment provides a negative electrode 100, which is basically the same as that in Embodiment 1, except that:

The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 10:0.

The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 0:10.

Example 4

This embodiment provides a negative electrode 100, which is basically the same as that in embodiment 3, except that:

The second hard carbon material is resin carbon;

The a2 of the second hard carbon material is 262.6 mAh/g, and the b2 of the second hard carbon material is 60%.

Example 5

This embodiment provides a negative electrode 100, which is basically the same as that in embodiment 4, except that:

The a2 of the second hard carbon material is 248.8 mAh/g, and the b2 of the second hard carbon material is 70%.

Example 6

This embodiment provides a negative electrode 100, which is basically the same as that in embodiment 3, except that:

The a1 of the first hard carbon material is 300 mAh/g, and the b1 of the first hard carbon material is 36%.

Example 7

This embodiment provides a negative electrode 100, which is basically the same as that in embodiment 3, except that:

The a1 of the first hard carbon material is 290 mAh/g, and the b1 of the first hard carbon material is 39%.

Comparative Example 1

This comparative example provides a negative electrode 100, which is basically the same as that in Example 1, except that:

It has only one active layer 20, and the thickness of the active layer 20 in Comparative Example 1 is the same as the total thickness of the two active layers 20 in Example 1. The mass ratio of the first hard carbon material to the second hard carbon material in the active layer 20 is 5:5.

Comparative Example 2

This comparative example provides a negative electrode 100, which is basically the same as that in Example 1, except that:

The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 4:6.

In the second active layer 20, the mass ratio of the first hard carbon material to the second hard carbon material is 6:4.

Comparative Example 3

This comparative example provides a negative electrode 100, which is basically the same as that in Example 2, except that:

The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 2:8.

The mass ratio of the first hard carbon material to the second hard carbon material in the second active layer 20 is 8:2.

Comparative Example 4

This comparative example provides a negative electrode 100, which is basically the same as that in Example 3, except that:

The mass ratio of the first hard carbon material to the second hard carbon material in the first active layer 20 is 0:10.

In the second active layer 20, the mass ratio of the first hard carbon material to the second hard carbon material is 10:0.

Comparative Example 5

This comparative example provides a negative electrode 100, which is basically the same as that in Example 4, except that:

It has only one active layer 20, and the thickness of the active layer 20 in Comparative Example 5 is the same as the total thickness of the two active layers 20 in Example 4.

Comparative Example 6

This comparative example provides a negative electrode sheet that is basically the same as that in Example 5, except that:

It has only one active layer 20, and the thickness of the active layer 20 in Comparative Example 6 is the same as the total thickness of the two active layers 20 in Example 5.

Comparative Example 7

This comparative example provides a negative electrode sheet that is basically the same as that in Example 6, except that:

It has only one active layer 20, and the thickness of the active layer 20 in Comparative Example 7 is the same as the total thickness of the two active layers 20 in Example 6.

Comparative Example 8

This comparative example provides a negative electrode sheet that is basically the same as that in Example 7, except that:

It has only one active layer 20, and the thickness of the active layer 20 in Comparative Example 8 is the same as the total thickness of the two active layers 20 in Example 7.

Table 1 shows the distribution of the mass ratio of the first hard carbon material and the second hard carbon material in each layer of the current collector 10 in the negative electrode sheets of Examples 1-7 and Comparative Examples 1-8. Table 2 shows the data on the reversible specific capacity and the proportion of the ramp region of the first hard carbon material and the second hard carbon material in Examples 1-7 and Comparative Examples 1-8. Table 3 compares the energy density and charging capacity of the batteries corresponding to Examples 1-7 and Comparative Examples 1-8.

Table 1

Table 2

Table 3

Referring to Tables 1-3, and comparing Examples 1-3, it can be seen that as the gradient change of the first hard carbon material and the second hard carbon material increases, the specific capacity of the battery with the double active layer 20 increases and the fast charging time decreases.

Comparing Example 1 with Comparative Example 2, Example 2 with Comparative Example 3, and Example 3 with Comparative Example 4, it can be seen that when the mass ratio of the first hard carbon material and the second hard carbon material in the corresponding active layer 20 is reversed, the fast charging time of the battery increases.

Comparing Examples 1-3 and Comparative Example 1, it can be seen that when the specific capacity of the batteries is similar, the fast charging time of the battery with only the first active layer 20 is greater than that of the battery with two active layers 20.

Comparing Examples 4 and 5, it can be seen that the larger the proportion b2 of the slope region of the second hard carbon material, the smaller the reversible specific capacity a2 of the second hard carbon material. Comparing Examples 6 and 7, it can be seen that the larger the proportion b1 of the slope region of the first hard carbon material, the smaller the reversible specific capacity a1 of the first hard carbon material.

Comparing Examples 1-3 and Comparative Examples 2-4, it can be seen that the specific capacity of the batteries in Examples 1-3 is similar to that of the batteries in Comparative Examples 2-4. However, the fast charging time of the batteries in Examples 1-3 is shorter than that of the batteries in Comparative Examples 2-4. The largest difference in fast charging time is between Examples 3 and Comparative Examples 4, reaching a maximum of 9.2 minutes. Therefore, the battery of this application improves the charging capability while maintaining the specific capacity, that is, the battery of this application can balance battery capacity and power performance.

Comparing Examples 4 and 5, 5 and 6, 6 and 7, and 7 and 8, it can be seen that when the reversible specific capacity and the proportion of the slope region of the first hard carbon material and the second hard carbon material remain unchanged, changing the distribution of the first hard carbon material and the second hard carbon material has little effect on the specific capacity of the battery. However, the fast charging time of the battery using the negative electrode sheet of this application (a negative electrode sheet with double coating and the first hard carbon material closer to the current collector) is significantly reduced. Therefore, it can be considered that the batteries of Examples 4-7 reduce the fast charging time while ensuring the battery capacity. That is, the battery of this application can balance battery capacity and power performance.

Comparing Examples 3-5, it can be seen that changing the proportion b2 of the slope region of the second hard carbon material reduces the specific capacity of the battery, and the corresponding fast charging time is improved. However, when the specific capacity of the battery decreases by 5.25%, the fast charging time can be shortened by 26.2%, indicating that the battery of this application can balance battery capacity and power performance. Comparing Examples 1-5, it can be seen that the battery with a higher proportion of the first hard carbon material has a higher specific capacity but a longer fast charging time; the battery with a higher proportion of the second hard carbon material has a shorter fast charging time but a higher specific capacity; adding both the first and second hard carbon materials can balance the specific capacity and fast charging performance of the battery; further gradient design of the first and second hard carbon materials can further improve the fast charging performance of the battery.

In the description of the embodiments of this application, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art will understand that all or part of the processes for implementing the above embodiments and equivalent variations made in accordance with the claims of this application are still within the scope of this application.

Claims (11)

A negative electrode (100), characterized in that it comprises: A current collector (10) has a first surface (11); n active layers (20) are sequentially stacked on the first surface (11) along the first direction (X), and the first direction (X) is perpendicular to the first surface (11); Each active layer (20) includes a first hard carbon material and/or a second hard carbon material; the content of the first hard carbon material in the (m+1)th active layer (20) is less than the content of the first hard carbon material in the mth active layer (20), the content of the second hard carbon material in the (m+1)th active layer (20) is greater than the content of the second hard carbon material in the mth active layer (20), the mth active layer (20) is closer to the first surface (11) than the (m+1)th active layer (20), and satisfies 1≤m≤n-1, where m and n are both positive integers; The slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1 < b2; Wherein, the capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process above 0.1V vs. Na ⁺ /Na. The negative electrode (100) according to claim 1 is characterized in that it also satisfies: 2≤n≤4. According to claim 1, the negative electrode (100) is characterized in that the reversible specific capacity of the first hard carbon material is a1, which satisfies: a1≥290mAh/g. The negative electrode (100) according to claim 1 is characterized in that it further satisfies: 20% ≤ b1 ≤ 40%. The negative electrode (100) according to claim 1 is characterized in that it also satisfies: 40% ≤ b2 ≤ 70%. The negative electrode (100) according to claim 1 is characterized in that the thickness of the active layer (20) is h, which satisfies: h≥20μm. The negative electrode (100) according to claim 6 is characterized in that it also satisfies: 30μm≤h≤60μm. According to claim 1, the negative electrode (100) is characterized in that the capacity ratio of the plateau region of the first hard carbon material is e1, satisfying: 60% ≤ e1 ≤ 80%; the capacity ratio of the plateau region of the second hard carbon material is e2, satisfying: 30% ≤ e2 ≤ 60%, and the capacity of the plateau regions of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process from 0V vs. Na ⁺ /Na to 0.1V vs. Na ⁺ /Na. A method for preparing a negative electrode sheet (100), characterized in that it includes: A current collector (10) is provided, the current collector (10) having a first surface (11); n active layers (20) are sequentially stacked on the first surface (11) along a first direction (X), wherein the first direction (X) is perpendicular to the first surface (11); Each active layer (20) includes a first hard carbon material and/or a second hard carbon material; the content of the first hard carbon material in the (m+1)th active layer (20) is less than the content of the first hard carbon material in the mth active layer (20), the content of the second hard carbon material in the (m+1)th active layer (20) is greater than the content of the second hard carbon material in the mth active layer (20), the mth active layer (20) is closer to the first surface (11) than the (m+1)th active layer (20), and satisfies 1≤m≤n-1, where m and n are both positive integers; The slope region capacity ratio of the first hard carbon material is b1, and the slope region capacity ratio of the second hard carbon material is b2, satisfying: b1 < b2; Wherein, the capacity of the slope region of the first hard carbon material and the second hard carbon material is the capacity of the battery sodium intercalation process above 0.1V vs. Na ⁺ /Na. A battery, characterized in that it comprises a positive electrode, a separator (200), and a negative electrode (100) as described in any one of claims 1 to 8, wherein the separator (200) is disposed between the positive electrode and the negative electrode (100). An electrical device, characterized in that it includes an electrical device and a battery as described in claim 10, wherein the battery supplies power to the electrical device.