Negative electrode sheet, method for manufacturing the same, battery, and power storage device

Abstract

This application discloses a negative electrode sheet, its preparation method, a battery, and an electrical device. The negative electrode sheet includes a current collector and a negative electrode active material layer. The negative electrode active material layer is disposed on at least a portion of the surface of at least one side of the current collector. The negative electrode active material layer includes multiple sub-coating layers arranged sequentially along its thickness direction. Negative electrode active material is distributed in each of the multiple sub-coating layers, and nano-additives are distributed in at least the outermost sub-coating layer and the next outermost sub-coating layer. The volume expansion rate of the nano-additives after lithium intercalation is greater than that of the negative electrode active material after lithium intercalation, and the content of the nano-additives in the multiple sub-coating layers increases sequentially in the direction away from the current collector. After the first charge and discharge, this negative electrode sheet can form a gradient pore structure with progressively increasing porosity in the multiple sub-coating layers in the direction away from the current collector, which can effectively improve ion liquid phase transport and enhance the fast-charging performance and cycle performance of the thick-coated negative electrode sheet.

Description

Technical Field

[0001] This application belongs to the field of batteries, specifically relating to negative electrode sheets and their preparation methods, batteries, and electrical devices. Background Technology

[0002] Lithium-ion batteries are characterized by their green, environmentally friendly, high-energy, and low-carbon properties. They are not only used in energy storage systems for hydropower, thermal power, wind power, and solar power plants, but also widely applied in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. With current societal development, the demands on lithium-ion batteries are increasing, requiring high energy density, good fast-charging performance, and excellent cycle life. Currently, high-energy-density batteries typically feature thicker electrode plates. However, increased coating thickness increases the tortuosity of the negative electrode plate, hindering lithium-ion transport in the porous structure, accelerating lithium-ion diffusion polarization, and leading to deterioration in cycle performance. Summary of the Invention

[0003] In view of the technical problems existing in the background art, this application provides a negative electrode sheet, which aims to improve the fast charging performance and cycle performance of the thick-coated negative electrode sheet.

[0004] To achieve the above objectives, a first aspect of this application provides a negative electrode sheet, comprising: a current collector; and a negative electrode active material layer, the negative electrode active material layer being disposed on at least a portion of the surface of at least one side of the current collector, the negative electrode active material layer comprising multiple sub-coating layers arranged sequentially along its thickness direction, each of the multiple sub-coating layers containing a negative electrode active material, and at least the outermost sub-coating layer and the next outermost sub-coating layer containing nano-additives, the volume expansion rate of the nano-additives after lithium intercalation being greater than the volume expansion rate of the negative electrode active material after lithium intercalation, and the content of the nano-additives in the multiple sub-coating layers increasing sequentially in the direction away from the current collector.

[0005] Compared to existing technologies, the negative electrode sheet of this application has at least the following beneficial effects: By setting multiple sub-coatings and increasing the content of nano-additives in the sub-coatings sequentially away from the current collector, in-situ pore formation can be achieved by utilizing the volume expansion of the nano-additives after lithium intercalation. The irreversible volume expansion of the nano-additives allows the negative electrode sheet to form a gradient pore structure with progressively increasing porosity in the direction away from the current collector after the first charge and discharge, thereby effectively improving ion liquid phase transport and enhancing the fast-charging and cycle performance of the thick-coated negative electrode sheet. Furthermore, this negative electrode sheet can be flexibly adjusted by changing the content, particle size, and material of the nano-additives in different sub-coatings. By flexibly adjusting the porosity and pore size of different sub-coatings after the first charge and discharge, for example, based on the formation of a gradient structure in which the porosity of the negative electrode active material layer increases from the inside to the outside, the pore size of different sub-coatings after the first charge and discharge can be further adjusted to form a gradient pore structure in which both the porosity and pore size increase from the inside to the outside. This can further improve the liquid phase transport of ions, reduce the diffusion resistance of ions in the three-dimensional porous electrode, improve the utilization rate of the inner negative electrode active material, reduce lithium plating, and improve the fast charging performance and cycle performance of the thick-coated negative electrode. This enables the battery to achieve better fast charging capability and cycle performance while having a high energy density.

[0006] In some embodiments of this application, the negative electrode active material includes carbon materials.

[0007] In some embodiments of this application, the volume expansion rate of the nano-additive after lithium intercalation is not less than 100%. When the volume expansion rate of the nano-additive after lithium intercalation is within the given range, the fast charging performance and cycle performance of the thick-coated negative electrode can be further improved with a lower amount of nano-additive.

[0008] In some embodiments of this application, the negative electrode active material includes at least one of graphite material, hard carbon material, and soft carbon material.

[0009] In some embodiments of this application, the nano-additive includes at least one element selected from silicon, germanium, tin, antimony, bismuth, and phosphorus, and / or an oxide of at least one element selected from silicon, germanium, tin, antimony, bismuth, and phosphorus, and / or a compound formed from any of the elements selected from silicon, germanium, tin, antimony, bismuth, and phosphorus. When the nano-additive is selected within the given range, the expansion rate after lithium intercalation is relatively high, which can further improve the fast-charging performance and cycle performance of the thick-coated negative electrode with a lower amount of nano-additive.

[0010] In some embodiments of this application, the Dv50 particle size of the negative electrode active material is 10 μm to 20 μm. Controlling the Dv50 particle size of carbon materials, especially negative electrode active materials including graphite, within the given range is more beneficial for achieving both high initial discharge capacity and first-time efficiency.

[0011] In some embodiments of this application, the Dv50 particle size of the nano-additive is 20 nm to 150 nm. When the particle size of the nano-additive is within the given range, it is more conducive to forming a pore structure with appropriate and uniform pore size distribution and interconnection in the negative electrode active material layer, which can further improve the liquid phase transport of ions and improve the charging and cycling performance of the thick-coated negative electrode.

[0012] In some embodiments of this application, the porosity of the multilayer sub-coating increases sequentially after the first charge and discharge in the direction away from the current collector. Satisfying this condition can further improve ion liquid-phase transport and enhance the charging and cycling performance of the thick-coated negative electrode.

[0013] In some embodiments of this application, the average pore size of the pores formed after the first charge and discharge of the multilayer sub-coating increases sequentially in the direction away from the current collector. Satisfying this condition can further improve ion liquid-phase transport and enhance the charging and cycling performance of the thick-coated negative electrode.

[0014] In some embodiments of this application, the negative electrode active material layer includes at least three sub-coating layers arranged sequentially along its thickness direction, each of the sub-coating layers containing the nano-additive, or the nano-additive is distributed in all of the multiple sub-coating layers except the innermost layer.

[0015] In some embodiments of this application, the difference in the content of the nano-additives in two adjacent sub-coating layers gradually increases in the direction away from the current collector. Satisfying this condition can further improve ion transport in the liquid phase and enhance the charging and cycling performance of the thick-coated negative electrode.

[0016] In some embodiments of this application, the difference in porosity between adjacent sub-coating layers gradually increases after the first charge and discharge in the direction away from the current collector. Meeting this condition can further improve ion transport in the liquid phase and enhance the charging and cycling performance of the thick-coated negative electrode.

[0017] In some embodiments of this application, the multilayer sub-coatings have the same thickness, or the multilayer sub-coatings have progressively increasing thickness in the direction away from the current collector.

[0018] In some embodiments of this application, the negative electrode active material layer comprises three sub-coating layers arranged sequentially along its thickness direction. The content of the nano-additive in the outermost sub-coating layer is 0.7wt% to 1.5wt% of the mass of the sub-coating layer, optionally 1wt% to 1.1wt%. The content of the nano-additive in the next outermost sub-coating layer is 0.3wt% to 0.7wt% of the mass of the sub-coating layer, optionally 0.4wt% to 0.5wt%. The content of the nano-additive in the innermost sub-coating layer is 0 to 0.3wt% of the mass of the sub-coating layer, optionally 0.1wt% to 0.2wt%. When the amount of nano-additive in the three sub-coating layers is within the range given above, it is more beneficial to form a gradient pore structure with gradually increasing porosity from the inside to the outside and a suitable porosity range after the first charge and discharge. This can significantly improve fast charging performance while maintaining high energy density, without having a significant negative impact on cycle performance.

[0019] In some embodiments of this application, the difference between the content of the nano-additive in the innermost sub-coating layer and the content of the nano-additive in the next outermost sub-coating layer is 0.2 wt% to 0.4 wt%, and the difference between the content of the nano-additive in the outermost sub-coating layer and the content of the nano-additive in the next outermost sub-coating layer is 0.5 wt% to 0.7 wt%. When the difference in the content of the nano-additive in the three sub-coating layers is within the range given above, it can effectively improve the liquid phase transport of ions (such as lithium ions), significantly improve fast charging performance while maintaining high energy density, and at the same time, it will not have a significant negative impact on cycle performance.

[0020] In some embodiments of this application, based on the total mass of the negative electrode active material layer, the total content of the nano-additives is no more than 2.5 wt%, optionally 0.5 wt% to 1.8 wt%, and further optionally 0.6 wt% to 1.5 wt%. When the total mass concentration of the nano-additives in the negative electrode active material layer is within the given range, it will not significantly reduce the energy density, but will significantly improve the fast charging performance, while not having a significant negative impact on the cycle performance of the thick-coated negative electrode.

[0021] In some embodiments of this application, the negative electrode active material layer comprises three sub-coating layers arranged sequentially along its thickness direction. After the first charge and discharge, the porosity of the outermost sub-coating layer is 40%–43%, the porosity of the next outermost sub-coating layer is 36%–37%, and the porosity of the innermost sub-coating layer is 33%–35%. When the porosity of the three sub-coating layers is within the given range, the fast-charging performance and cycle performance of the thick-coated negative electrode can be further improved.

[0022] In some embodiments of this application, after the first charge and discharge, the difference in porosity between the innermost sub-coating layer and the next outermost sub-coating layer is 2% to 4%, and the difference in porosity between the outermost sub-coating layer and the next outermost sub-coating layer is 4% to 6%. When the porosity difference of the three sub-coating layers is within the given range, the fast-charging performance and cycle performance of the thick-coated negative electrode can be further improved.

[0023] In some embodiments of this application, the three sub-coating layers have the same thickness; or, the ratio of the thickness of the innermost sub-coating layer, the thickness of the next outermost sub-coating layer, and the thickness of the outermost sub-coating layer is (2±0.5):(3±0.5):(5±0.5).

[0024] In some embodiments of this application, the current collector is coated on one side or both sides, and the thickness of the negative electrode active material layer on one side of the current collector is 70 μm to 100 μm.

[0025] The second aspect of this application provides a method for preparing the negative electrode sheet of the first aspect of this application, comprising: (1) preparing a variety of negative electrode slurries, and containing nano-additives in at least two of the negative electrode slurries; (2) sequentially coating the variety of negative electrode slurries along the thickness direction of the current collector onto at least a portion of the surface of at least one side of the current collector to form a negative electrode active material layer including multiple sub-coatings, and increasing the content of nano-additives in the multiple sub-coatings in a direction away from the current collector, wherein: the variety of negative electrode slurries all contain negative electrode active materials, and the volume expansion rate of the nano-additives after lithium intercalation is greater than the volume expansion rate of the negative electrode active materials after lithium intercalation.

[0026] Compared with existing technologies, the method for preparing negative electrode sheets in this application has at least the following beneficial effects: not only is the process simple and easy to operate, but it also helps to control the porosity and pore size of each sub-coating after the first charge and discharge based on the type, content and particle size of nano-additives in each negative electrode slurry. This allows the active material layer of the negative electrode sheet to form a gradient structure with gradually increasing porosity in the direction away from the current collector after the first charge and discharge, thereby effectively improving ion liquid phase transport, enhancing the fast charging performance and cycle performance of the thick-coated negative electrode sheet, and enabling the battery to achieve better fast charging capability and cycle performance while having a high energy density.

[0027] A third aspect of this application provides a battery comprising: a negative electrode sheet according to the first aspect of this application, and / or a negative electrode sheet prepared by the method of preparing a negative electrode sheet according to the second aspect of this application.

[0028] In some embodiments of this application, the battery further includes: a positive electrode sheet, the positive electrode sheet comprising a lithium-rich manganese-based positive electrode material, the lithium-rich manganese-based positive electrode material comprising nLi₂MnO₃·(1-n)LiNi x Mn (1-x-y) M y O2, wherein 0.05≤n≤0.4, 0.3≤x<1, 0<y≤0.1, and M includes one or more elements selected from Na, Mg, Al, Ca, Ba, V, Zn, Ti, Fe, Co, Cr, Nb, W, Mo, Zr, Ta, and Hf.

[0029] The fourth aspect of this application provides an electrical device comprising: a negative electrode sheet of the first aspect of this application, and / or a negative electrode sheet prepared by the method of preparing a negative electrode sheet of the second aspect of this application, and / or a battery of the third aspect of this application.

[0030] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0031] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0032] Figure 1 This is a schematic diagram of the structure of a single-sided three-layer coated negative electrode sheet according to an embodiment of this application.

[0033] Figure 2 This is a schematic diagram showing the formation of a gradient pore structure in the sub-coating of a single-layer coated negative electrode sheet after its first charge and discharge, according to an embodiment of this application.

[0034] Figure 3 This is a schematic diagram of the structure of a negative electrode sheet with a double-sided three-layer coating according to an embodiment of this application.

[0035] Figure 4 This is a schematic diagram of one embodiment of an electrical device used as a power source according to an embodiment of this application.

[0036] Figure label:

[0037] 10 - Current collector; 20 - Negative electrode active material layer; 21 - Sub-coating layer; 21a - Outermost sub-coating layer; 21b - Sub-coating layer of the second outermost layer; 21c - Innermost sub-coating layer; d1 - Thickness of the outermost sub-coating layer; d2 - Thickness of the sub-coating layer of the second outermost layer; d3 - Thickness of the innermost sub-coating layer; d4 - Thickness of the active material layer located on one side of the current collector. Detailed Implementation

[0038] The present application will be further described below with reference to specific embodiments. It should be understood that these specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0039] In this application, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this application can be combined with other embodiments.

[0040] The "range" disclosed in this application is defined in the form of a lower limit and / or an upper limit. A given range is defined by selecting a lower limit and / or an upper limit, which defines the boundary of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form an undefined range, and any lower limit can be combined with other lower limits to form an undefined range, just as any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an undefined range. For example, if a range of no more than 20 is listed for a specific parameter, it is expected that ranges such as 2–18 and 5–15 satisfying no more than 20 are also to be understood. Furthermore, if the minimum range values ​​are listed as 1 and 5, and if the maximum range values ​​are listed as 10 and 30, then the following ranges are all to be expected: 1–10, 1–30, 5–10, and 5–30. In this application, unless otherwise stated, a numerical range such as "30 to 100" represents an abbreviation of any combination of real numbers between 30 and 100, where 30 and 100 are real numbers. For example, a numerical range of "1 to 30" means that all real numbers between "1 and 30" have been listed in this document, and "1 to 30" is simply an abbreviation of these numerical combinations.

[0041] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0042] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0043] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps S1 and S2, indicating that the method may include steps S1 and S2 performed sequentially, or it may include steps S2 and S1 performed sequentially. For example, the method may also include step S3, indicating that step S3 may be added to the method in any order. For example, the method may include steps S1, S2, and S3, or it may include steps S1, S3, and S2, or it may include steps S3, S1, and S2, etc.

[0044] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" may mean that other components not listed may also be included, or that only the listed components may be included. Additionally, in this application, the terms "a plurality of" or "multiple" refer to two or more types.

[0045] Unless otherwise specified, in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0046] 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 this application is for the purpose of describing particular embodiments only and is not intended to limit this application; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing description of the drawings, are intended to cover non-exclusive inclusion. Unless otherwise stated, the terms used in this application have their commonly understood meanings as understood by one of ordinary skill in the art. Unless otherwise stated, the numerical values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0047] Currently, high-energy-density battery electrode designs typically involve thicker coatings. For example, the negative electrode of a lithium-ion battery generally has a large coating thickness. However, during charging and discharging, the increased coating thickness increases the tortuosity of the negative electrode, making it difficult for lithium ions to transport in the liquid phase within the porous structure, accelerating lithium ion diffusion polarization, and leading to deterioration in cycle performance. To improve battery performance, many studies have been conducted in existing technologies. For example, there are double-coated negative electrode sheets, on which two active material layers are formed. The first active material layer is in close contact with the current collector and contains silicon particles and graphite. The second negative electrode active material layer is coated on the first negative electrode layer and contains only graphite. The first negative electrode active material layer ensures energy density, while the second negative electrode active material layer takes into account cycle performance. However, for thick-coated negative electrode sheets, since no silicon is added to the second negative electrode active material layer, the porosity of the upper negative electrode is lower than that of the lower negative electrode after the first charge. This is not conducive to the transport of lithium ions in the three-dimensional porous electrode, increases lithium ion diffusion polarization, and deteriorates cycle performance. For example, there is a scheme that forms a negative electrode active material layer on the negative electrode current collector and uses graphite and silicon-based materials, and amorphous carbon as the negative electrode active material, so that the porosity on the outer side of the negative electrode active material layer is greater than the porosity on the side closer to the current collector. By utilizing the porosity of the amorphous carbon itself, the porosity distribution in the negative electrode active material layer can be controlled to improve the cycle performance and fast charging performance of the battery. However, when the amorphous carbon content in the negative electrode active material layer is high, since the amorphous carbon has not been graphitized, it has a high oxygen content. These oxygen elements will undergo side reactions with lithium ions during charging and discharging, resulting in a high irreversible capacity.

[0048] In view of this, the first aspect of this application provides a negative electrode sheet, with reference to Figures 1-2 The negative electrode sheet includes: a current collector 10; and a negative electrode active material layer 20. The negative electrode active material layer 20 is disposed on at least a portion of the surface of at least one side of the current collector 10. The negative electrode active material layer 20 includes multiple sub-coating layers 21 arranged sequentially along its thickness direction. Negative electrode active material is distributed in each of the multiple sub-coating layers 21, and nano-additives are distributed in at least the outermost sub-coating layer 21a and the next outermost sub-coating layer 21b. The volume expansion rate of the nano-additives after lithium intercalation is greater than the volume expansion rate of the negative electrode active material after lithium intercalation, and the content of nano-additives in the multiple sub-coating layers 21 increases sequentially in the direction away from the current collector 10. It is understood that the negative electrode active material layer 20 includes at least two sub-coating layers 21, specifically including two, three, four, or more sub-coating layers. In this application, the content of nano-additives in a single sub-coating layer can be understood as the mass percentage or volume percentage of the nano-additives in that sub-coating layer, for example, it can be selected as the mass percentage. Additionally, it should be noted that the volume expansion rate after lithium intercalation in this application is calculated as (volume after lithium intercalation - original volume before lithium intercalation) / original volume before lithium intercalation × 100%.

[0049] The inventors discovered that for thick-coated negative electrode sheets, in-situ pore creation can be achieved by selecting appropriate additives and utilizing the volume expansion of lithium intercalation in nano-additives. The irreversible volume expansion of the nano-additives allows the negative electrode sheet to have a suitable porosity distribution after the first charge-discharge cycle, thereby improving the fast-charging and cycle performance of the thick-coated negative electrode sheet. Specifically, nano-additives with a large lithium intercalation volume expansion rate after the first charge-discharge cycle can enlarge the pores of the negative electrode active material (such as graphite). Figures 1-2 Understood, this application, by setting multiple sub-coatings and increasing the content of nano-additives in the sub-coatings sequentially in the direction away from the current collector, can form a gradient pore structure with progressively increasing porosity in the direction away from the current collector after the first charge and discharge (see reference). Figure 2 (Understanding) This can effectively improve ion liquid-phase transport and enhance the fast-charging and cycle performance of thick-coated negative electrode sheets. Furthermore, the porosity and pore size of different sub-coatings after the first charge-discharge can be flexibly adjusted by changing the content, particle size, and material of nano-additives in different sub-coatings. For example, based on forming a gradient structure where the porosity of the negative electrode active material layer increases sequentially from the inside to the outside, the pore size of different sub-coatings after the first charge-discharge can be further flexibly adjusted, resulting in a gradient pore structure where both porosity and pore size increase sequentially from the inside to the outside. This further improves ion liquid-phase transport, reduces the diffusion resistance of ions in the three-dimensional porous electrode, increases the utilization rate of the inner negative electrode active material, reduces lithium plating, and improves the fast-charging and cycle performance of the thick-coated negative electrode. This allows the battery to achieve good fast-charging capability and cycle performance while maintaining a high energy density.

[0050] In addition, in this application, for nano-additives and negative electrode active materials, in-situ XRD testing can be used to measure the volume before and after lithium intercalation. For example, the nano-additives or negative electrode active materials can be first made into coin cells, and then the half-cells can be placed in an in-situ XRD tester. While performing discharge testing, XRD testing can be performed at the same time to obtain the XRD patterns of the nano-additives or negative electrode active materials before and after lithium intercalation. Then, the cell parameters of the material before and after lithium intercalation can be extracted based on the patterns, and finally, the volume of the material before and after lithium intercalation can be calculated based on the cell parameters.

[0051] Furthermore, through in-depth research, the inventors discovered that, in addition to meeting the above conditions, the negative electrode sheet of this application can be further improved by controlling the selection, particle size, pore size distribution structure, amount of nano-additives, and porosity after the first charge and discharge, thereby further enhancing the fast-charging and cycle performance of the thick-coated negative electrode sheet. That is, in addition to meeting the above conditions, one or more of the following conditions may also be optionally met.

[0052] In some embodiments of this application, the negative electrode active material may include carbon materials, for example, it may be partially or entirely carbon materials. The negative electrode active material may simultaneously include carbon materials and other negative electrode active materials (such as silicon-based materials), or it may be entirely carbon materials. As a specific example, the negative electrode active material may include at least one of graphite materials, hard carbon materials, and soft carbon materials. Optionally, the negative electrode active material may include at least graphite materials. The selection of graphite materials, hard carbon materials, and soft carbon materials is not particularly limited, and those skilled in the art can flexibly choose according to actual needs. For example, the graphite material may be artificial graphite and / or natural graphite. Furthermore, the graphite material may be single-particle graphite or secondary granulated graphite, etc.

[0053] In some embodiments of this application, the volume expansion rate after lithium intercalation of the nano-additive can be no less than 100%, for example, no less than 120%, no less than 150%, no less than 175%, no less than 200%, no less than 250%, no less than 300%, no less than 400%, or 100% to 350%, or any range of the above values. The inventors have found that if the volume expansion rate after lithium intercalation of the nano-additive is too low, the pore size and porosity formed during the first charge-discharge process of the negative electrode will also be low. To achieve the expected porosity range, a larger amount of nano-additive is required, which not only affects the improvement of the fast-charging and cycle performance of the negative electrode but also affects the energy density of the negative electrode. When the volume expansion rate after lithium intercalation of the nano-additive is within the given range, it is more beneficial to achieve a suitable or expected porosity effect in the sub-coating with a lower amount of nano-additive, thereby better balancing the high energy density of the negative electrode with good fast-charging and cycle performance.

[0054] In some embodiments of this application, the nano-additive may include at least one element of silicon, germanium, tin, antimony, bismuth and phosphorus, and / or an oxide of at least one of silicon, germanium, tin, antimony, bismuth and phosphorus, and / or a compound (such as an alloy) formed by any of the elements of silicon, germanium, tin, antimony, bismuth and phosphorus. As some specific examples, the nano-additive may include at least one of metallic antimony, metallic tin, tin oxide, tin suboxide, elemental silicon, silicon oxide, silicon suboxide and so on. The volume shrinkage and expansion rate of nano-additives is affected by their material composition. The nano-additives within the given range have relatively high volume shrinkage and expansion rates. Taking silicon and tin as examples, their volume expansion rates after lithium intercalation can reach about 300%. Germanium, antimony, and bismuth also have volume expansion rates of over 100% after lithium intercalation, making it easier to meet the requirement that the volume expansion rate after lithium intercalation is greater than that of the negative electrode active material. For example, the volume expansion rates of silicon, germanium, tin, antimony, bismuth, and phosphorus after lithium intercalation are all greater than those of artificial graphite. When nano-additives are selected within the given range, they can further achieve a higher expansion rate after lithium intercalation, which is more conducive to obtaining a suitable or expected porosity effect in the sub-coating with a lower amount of nano-additives. This allows for a better balance between the high energy density of the negative electrode and good fast-charging and cycle performance.

[0055] In some embodiments of this application, the Dv50 particle size of the negative electrode active material can be 10μm to 20μm, for example, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, etc., or can be any range of the above values; optionally, the average particle size of the negative electrode active material can be 10μm to 20μm. The inventors have found that when using carbon-based negative electrode active materials, such as graphite, a smaller particle size results in a larger initial charge capacity, but also a larger irreversible capacity. Conversely, a larger particle size results in a higher initial efficiency but a smaller initial charge-discharge capacity. When the particle size of the negative electrode active material is within the given range, it is more beneficial to improve the utilization rate of the negative electrode active material, enabling the battery to have both a higher initial discharge capacity and a higher initial efficiency.

[0056] In some embodiments of this application, the Dv50 particle size of the nano-additive can be 20nm to 150nm, for example, 25nm, 35nm, 45nm, 55nm, 65nm, 75nm, 85nm, 95nm, 105nm, 115nm, 125nm, 135nm, 145nm, etc., or can be any range of the above values. The inventors have found that if the particle size of the nano-additive is too small, it will not only increase the processing cost, but also result in a smaller volume expansion after lithium intercalation. The pore size and porosity formed in the coating will be relatively small, making it difficult to effectively improve the liquid phase transport of ions (such as lithium ions) and improve the charging and cycling performance of the thick-coated negative electrode. On the other hand, if the particle size of the nano-additive is too large, the number of nano-additive particles will be significantly smaller at the same dosage. This will affect the uniformity of the distribution of the nano-additive in the negative electrode active material layer, and will also lead to a smaller number of pores and a significantly larger pore size in the negative electrode active material layer, which will also affect the improvement effect on ion liquid phase transport. When the particle size of the nano-additive is within the given range, it is more conducive to forming a pore structure with appropriate and uniform pore size distribution and interconnection in the negative electrode active material layer, which has a more significant effect on improving the liquid phase transport of ions and improving the charging and cycling performance of the thick-coated negative electrode.

[0057] In this application, the Dv50 particle size of the negative electrode active material and nano-additives can be obtained by laser particle size analyzer. Specifically, the powder particles can be dispersed in an appropriate amount of solvent to form a dispersion, and then the powder dispersion can be placed in a laser particle size analyzer to measure the particle size. In addition, in this application, the average pore size of the pores formed after the first charge and discharge of the coating is defined as the average value of the two-dimensional pore size of the coating cross-section. The specific test method for the average pore size of the pores formed after the first charge and discharge of the negative electrode coating is as follows: first, the cross-section of the negative electrode coating is obtained by argon ion beam profiler, and then several cross-sectional images of the negative electrode coating at the same magnification are obtained by scanning electron microscope. The average pore size of the sub-coating is obtained by statistically analyzing the two-dimensional pore size values ​​in the images.

[0058] In some embodiments of this application, reference is made to Figures 1-2 It is understood that the porosity of the multilayer sub-coating 21 increases sequentially after the first charge and discharge in the direction away from the current collector 10. Furthermore, the average pore size of the pores formed after the first charge and discharge of the multilayer sub-coating 21 in the direction away from the current collector 10 can also increase sequentially. Specifically, this can be achieved by controlling at least one of the following methods: the content, particle size, and material of the nano-additives in different sub-coatings. Selecting the above-mentioned gradient pore structure is more conducive to improving the liquid phase transport of ions and enhancing the charging and cycling performance of the thick-coated negative electrode.

[0059] In some embodiments of this application, reference is made to Figure 1It is understood that the negative electrode active material layer 20 may include at least three sub-coating layers 21 arranged sequentially along its thickness direction. Each sub-coating layer 21 contains nano-additives, or all sub-coating layers except the innermost layer contain nano-additives. Specifically, this can be achieved by controlling whether the coating slurry used to form the innermost sub-coating layer contains nano-additives. Both methods can achieve the effect of progressively increasing porosity of the multi-layer sub-coating layers after the first charge and discharge in the direction away from the current collector. Optionally, the presence of nano-additives in each sub-coating layer 21 further improves ion transport in the liquid phase and enhances the charging and cycling performance of the thick-coated negative electrode.

[0060] In some embodiments of this application, reference is made to Figure 1 Understanding is that, in the direction away from the current collector 10, the difference in the content of nano-additives in two adjacent sub-coating layers 21 can gradually increase (e.g., this can be achieved by controlling the mass concentration / volume concentration of nano-additives in the coating slurry used to form each sub-coating, based on the solid content of the coating slurry), in which case the negative electrode active material layer 20 includes at least three sub-coating layers 21. As a specific example, refer to... Figure 2 It is understood that when the negative electrode active material layer includes three sub-coatings arranged sequentially along its thickness direction, the content of nano-additives in the outermost sub-coating 21a can be w1, the content of nano-additives in the next outermost sub-coating 21b can be w2, and the content of nano-additives in the innermost sub-coating 21c can be w3, where w1-w2>w2-w3. During the research process, the inventors discovered that, in the direction away from the current collector, compared to making the content of nano-additives in the multi-layer sub-coatings change in an arithmetic progression (w1-w2=w2-w3) or the content increment gradually decrease (w1-w2<w2-w3), making the content of nano-additives in the multi-layer sub-coatings increase in a progression away from the current collector with a gradually increasing increment is more conducive to forming a porous structure in the negative electrode active material layer with a gradient increase in pore size and porosity from the inside to the outside. Furthermore, it is more effective in improving ion liquid-phase transport and enhancing the charging and cycling performance of the thick-coated negative electrode.

[0061] In some embodiments of this application, reference is made to Figure 1 Understanding is that, in the direction away from the current collector, the difference in porosity between two adjacent sub-coating layers 21 after their first charge-discharge can gradually increase (e.g., this can be achieved by controlling the mass / volume concentration of nano-additives in the coating slurry used to form each sub-coating, based on the solid content of the coating slurry), in which case the negative electrode active material layer 20 comprises at least three sub-coating layers 21. As a specific example, refer to... Figure 2It is understood that when the negative electrode active material layer comprises three sub-coating layers arranged sequentially along its thickness direction, after the first charge and discharge, the porosity of the outermost sub-coating 21a can be k1, the porosity of the next outermost sub-coating 21b can be k2, and the porosity of the innermost sub-coating 21c can be k3, where k1-k2>k2-k3. During the exploration process, the inventors discovered that, in the direction away from the current collector, compared to making the porosity of the multi-layer sub-coatings change in an arithmetic sequence (k1-k2=k2-k3) or the porosity increment gradually decrease (k1-k2<k2-k3), making the porosity of the multi-layer sub-coatings increase in the direction away from the current collector with a gradually increasing increment is more beneficial for improving ion liquid-phase transport, enhancing the charging and cycling performance of the thick-coated negative electrode, and thus making the battery possess both higher energy density and better fast-charging and cycling performance.

[0062] In some embodiments of this application, the thickness of the multilayer sub-coatings 21 can be either uniform or progressively increase in thickness away from the current collector 10. This can be achieved by controlling the solid content and coating thickness of the coating slurry used to form each sub-coating. Optionally, the multilayer sub-coatings 21 may progressively increase in thickness away from the current collector 10. The inventors have found that, compared to a uniform thickness distribution, progressively increasing the thickness of the multilayer sub-coatings away from the current collector is more beneficial for improving ion transport in the liquid phase and enhancing the charging and cycling performance of the thick-coated negative electrode. This, in turn, is more conducive to enabling the battery to possess both higher energy density and better fast-charging and cycling performance.

[0063] In some embodiments of this application, reference is made to Figures 1-3It is understood that the negative electrode active material layer 20 may include three sub-coating layers 21 arranged sequentially along its thickness direction. Considering that the thickness of the single-sided active material layer of the existing thick-coated negative electrode is usually no more than 150 μm, if there are more sub-coating layers, the thickness of a single sub-coating layer needs to be controlled even smaller, which will increase the coating difficulty. Moreover, compared with the structure of two sub-coating layers, the gradient pore structure formed by three sub-coating layers is more effective in improving ion liquid phase transport, charging and cycling performance. Therefore, the number of sub-coating layers of the negative electrode active material layer can be limited to three layers, thereby taking into account both the fast charging and cycling performance of the thick-coated negative electrode and reducing the operation difficulty. Furthermore, in the three sub-coating layers, the content of nano-additives in the outermost sub-coating 21a can be 0.7wt% to 1.5wt% of the mass of the sub-coating 21a, for example, 0.8wt%, 0.9wt%, 1wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, etc., or can be any range of the above values, such as 1wt% to 1.1wt%; the content of nano-additives in the next outermost sub-coating 21b can be 0.3wt% to 0.7wt% of the mass of the sub-coating 21b, for example, 0.35wt%. The content of nano-additives in the innermost sub-coating 21c can be 0 to 0.3 wt% of the mass of the sub-coating 21c, for example, 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, etc., or can be any of the above values, such as 0.1 wt% to 0.2 wt%. The mass content of nano-additives in different sub-coatings can be controlled by adjusting the mass concentration of nano-additives in the coating slurry that forms different sub-coatings or by adjusting the mass ratio of nano-additives with other raw materials. When testing the mass content of nano-additives in different sub-coatings of the prepared negative electrode sheet, inductively coupled plasma atomic emission spectrometry can be used. For example, the negative electrode sheet of each sub-coating can be digested by first passing a mixed solution of concentrated nitric acid and hydrofluoric acid, and then the resulting solution can be passed into an inductively coupled plasma atomic emission spectrometer to measure the mass content of nano-additives in different sub-coatings.

[0064] The inventors discovered that if the mass percentage of nano-additives in a single-layer sub-coating is too low, the porosity of the sub-coating after the first charge-discharge cycle will also be low, affecting ion transport in the liquid phase and fast-charging performance. Appropriately increasing the mass percentage of nano-additives in the single-layer sub-coating can improve the porosity of the sub-coating after the first charge-discharge cycle and improve the ion transport effect in the liquid phase. However, if the amount added is too high, it is equivalent to reducing the amount of negative electrode active material, which will not only reduce the energy density, but also cause excessive volume expansion after lithium intercalation, making it prone to breakage during cycling, consuming electrolyte, deteriorating cycle performance, and reducing discharge capacity retention. For the negative electrode active material layer structure of a three-layer sub-coating, controlling the amount of nano-additives in the three-layer sub-coating within the given range is more conducive to forming a gradient pore structure with progressively increasing porosity from the inside to the outside and a suitable porosity range after the first charge-discharge cycle. This can significantly improve fast-charging performance while maintaining high energy density, without significantly negatively impacting cycle performance.

[0065] In some embodiments of this application, reference is made to Figures 1-3 It is understood that when the negative electrode active material layer 20 includes three sub-coating layers 21 arranged sequentially along its thickness direction, the difference between the mass content of nano-additives in the innermost sub-coating layer 21c and the mass content of nano-additives in the next outermost sub-coating layer 21b (which can be understood as w2-w3) can be 0.2wt% to 0.4wt%, for example, 0.25wt%, 0.3wt%, 0.35wt%, etc., or any range of the above values; the difference between the mass content of nano-additives in the outermost sub-coating layer 21a and the mass content of nano-additives in the next outermost sub-coating layer 21b (which can be understood as w1-w2) can be 0.5wt% to 0.7wt%, for example, 0.55wt%, 0.6wt%, 0.65wt%, etc., or any range of the above values. When the given conditions are met, the content of nano-additives in the three-layer sub-coating can increase sequentially in the direction away from the current collector, and the increment gradually increases. This is more conducive to the formation of a porous structure in the negative electrode active material layer with a gradient increase in pore size and porosity from the inside to the outside. Furthermore, by ensuring that the incremental increment of nano-additives meets the above range, a suitable porosity difference and pore size difference can be obtained between adjacent sub-coating layers. This can effectively improve the liquid phase transport of ions (such as lithium ions), significantly improve fast charging performance while maintaining high energy density, and at the same time, it will not have a significant negative impact on cycle performance.

[0066] In some embodiments of this application, based on the total mass of the negative electrode active material layer 20, the total content of the nano-additives is no more than 2.5 wt%, for example, it can be 0.5 wt%, 0.6 wt%, 0.65 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, etc., or it can be a range of any of the above values, such as 0.5 wt% to 1.8 wt%, or even 0.6 wt% to 1.5 wt%. By controlling the total mass concentration of nano-additives in the negative electrode active material layer within a given range, the energy density will not be significantly reduced. Furthermore, the irreversible volume expansion of the nano-additives after lithium intercalation can create a certain porosity in the negative electrode active material layer. This can significantly improve fast charging performance while avoiding the significant deterioration of cycle performance due to excessive addition. It will not have a significant negative impact on the energy density and cycle performance of the thick-coated negative electrode.

[0067] In some embodiments of this application, reference is made to Figures 1-3It is understood that when the negative electrode active material layer 20 includes three sub-coating layers 21 arranged sequentially along its thickness direction, after the first charge and discharge, the porosity of the outermost sub-coating layer 21a can be 40% to 43%, for example, 41%, 42%, 43%, etc.; the porosity of the next outermost sub-coating layer 21b can be 36% to 37%, for example, 36.5%, etc.; and the porosity of the innermost sub-coating layer 21c can be 33% to 35%, for example, 34%, etc. Specifically, this can be achieved by controlling the material selection, addition amount, and particle size of the nano-additives in each sub-coating layer. The inventors discovered that if the porosity of each sub-coating is too low, it is difficult to effectively improve ion transport in the liquid phase and fast-charging performance. However, by controlling the porosity of the three sub-coatings to gradually increase within a given range in the direction away from the current collector, it is possible not only to significantly improve ion transport in the liquid phase, enhance fast-charging performance, and improve the utilization rate of the inner negative electrode active material, but also to reduce the probability of breakage during cycling when the volume expansion of the sub-coatings after lithium intercalation of nano-additives is too large. This effectively reduces or avoids the deteriorating effect on cycling performance, and better balances the fast-charging performance and cycling performance of the thick-coated negative electrode. Furthermore, the difference in porosity between the innermost sub-coating 21c and the next outermost sub-coating 21b can be 2% to 4%, for example, 2.5%, 3%, 3.5%, etc.; the difference in porosity between the outermost sub-coating 21a and the next outermost sub-coating 21b can be 4% to 6%, for example, 4.5%, 5%, 5.5%, etc. When this condition is met, the porosity of the three-layer sub-coating can increase sequentially in the direction away from the current collector, and the increment gradually increases. This can better improve the liquid phase transport of ions (such as lithium ions), improve fast charging performance, and at the same time, it will not have a significant negative impact on cycle performance. The porosity of the sub-coating after its first charge-discharge can be tested using the true density method, as per national standard GB / T24586-2009. For example, in a dry chamber, a certain number of negative electrode discs of equal area are first placed into a sample cup, and the apparent volume V2 is calculated. Then, the sample cup containing the sample is placed in a closed testing system of a true density analyzer, and helium gas is introduced according to the procedure. By detecting the gas pressure in the sample chamber and expansion chamber, the true volume V1 is calculated according to Bohr's law (PV = nRT), thus obtaining the porosity of the sample. The porosity calculation formula is as follows: Porosity P = (V2 - V1) / V2 × 100%, Apparent volume V2 = S × H × A, where: P - porosity, %; S – electrode area, cm². 2 H – Electrode thickness, cm; A – Number of electrodes, sheets; V1 – Actual sample volume, cm³ 3 V2 - Apparent volume of the sample, cm³ 3 .

[0068] In some embodiments of this application, reference is made to Figure 3It is understood that when the negative electrode active material layer 20 includes three sub-coating layers 21 arranged sequentially along its thickness direction, the thickness of the three sub-coating layers 21 can be the same, or it can increase layer by layer in the direction away from the current collector. As a specific example, the ratio of the thickness d3 of the innermost sub-coating layer 21c, the thickness d2 of the next outermost sub-coating layer 21b, and the thickness d1 of the outermost sub-coating layer 21a can be (2±0.5):(3±0.5):(5±0.5), for example, 1.5 / 2.5 / 4.5, 2 / 3 / 5, 2.5 / 3.5 / 5.5, 1 0.5 / 3.5 / 4.5, 2.5 / 3.5 / 4.5, etc., optionally, the difference between the thickness d1 of the outermost sub-coating 21a and the thickness d2 of the next outermost sub-coating 21b can be greater than the difference between the thickness d2 of the next outermost sub-coating 21b and the thickness d3 of the innermost sub-coating 21c. Even if d1-d2>d2-d3, the uniform thickness coating operation is convenient, while increasing the thickness of the three sub-coatings sequentially in the direction away from the current collector is more conducive to improving the liquid phase transport of ions (such as lithium ions) and improving the charging and cycling performance of the thick-coated negative electrode.

[0069] In some embodiments of this application, combined with Figure 1 and Figure 3 It is understood that a negative electrode active material layer 20 (such as...) can be coated on one side of the current collector 10. Figure 1 (As shown), a negative electrode active material layer 20 can also be evenly coated on both sides of the current collector 10 (such as...). Figure 3 As shown), it can be single-sided or double-sided coating, wherein, combined with Figure 3 It is understood that the thickness d4 of the active material layer 20 on one side of the current collector can be 70μm to 100μm, for example, 75μm, 80μm, 85μm, 90μm, 95μm, etc., or any range of the above values. The thickness of the negative electrode active material layer also affects the fast charging performance. In this application, by controlling the thickness of the negative electrode active material layer on one side of the current collector within the given range, it is beneficial to obtain a higher energy density while also taking into account good fast charging performance. The thickness of the negative electrode active material layer can be measured with a micrometer, specifically by measuring the electrode thickness at multiple points and statistically obtaining the average value of the electrode thickness.

[0070] The second aspect of this application provides a method for preparing the negative electrode sheet of the first aspect of this application, comprising: (1) preparing a variety of negative electrode slurries, and containing nano-additives in at least two of the negative electrode slurries; (2) sequentially coating the variety of negative electrode slurries along the thickness direction of the current collector onto at least a portion of the surface of at least one side of the current collector to form a negative electrode active material layer including a multilayer sub-coating, and increasing the content of nano-additives in the multilayer sub-coating in a direction away from the current collector, wherein: the variety of negative electrode slurries all contain negative electrode active materials, and the volume expansion rate of the nano-additives after lithium intercalation is greater than the volume expansion rate of the negative electrode active materials after lithium intercalation.

[0071] It should be noted that the method for preparing the negative electrode sheet in the second aspect of this application is based on the negative electrode sheet in the first aspect of this application. The features and effects described for the negative electrode sheet in the first aspect of this application are also applicable to the method for preparing the negative electrode sheet in the second aspect of this application, and will not be repeated here. Compared with the prior art, the method for preparing the negative electrode sheet in this application has at least the following beneficial effects: not only is the process simple and easy to operate, but it also facilitates the control of the porosity and pore size of each sub-coating after the first charge and discharge based on the type, content and particle size of the nano-additives in each negative electrode slurry. This allows the active material layer of the negative electrode sheet to form a gradient structure with gradually increasing porosity in the direction away from the current collector after the first charge and discharge, thereby effectively improving ion liquid phase transport, enhancing the fast charging performance and cycle performance of the thick-coated negative electrode sheet, and enabling the battery to achieve better fast charging capability and cycle performance while having a high energy density.

[0072] In some embodiments of this application, each negative electrode slurry may include a negative electrode active material, nano-additives, binders, and conductive agents. The selection of negative electrode active materials and nano-additives has been described in detail in the first aspect of this application and will not be repeated here. The selection of binders and conductive agents is not particularly limited, and those skilled in the art can flexibly select them according to actual needs. For example, the negative electrode active material may be a graphite-based material, the nano-additives may be silicon, silicon oxide, silicon suboxide, tin, tin oxide, stannous oxide, etc., the binder may be styrene-butadiene rubber and / or sodium carboxymethyl cellulose, etc., and the conductive agent may be acetylene black, superconducting carbon black, etc.

[0073] In some embodiments of this application, the raw material composition in each negative electrode slurry may be the same or different. Optionally, the raw material composition in each negative electrode slurry may be the same. Alternatively, the difference between each negative electrode slurry may be only in the mass ratio of negative electrode active material and nano-additive. Choosing this method is more conducive to flexibly controlling the content of nano-additive in each sub-coating and the porosity of each sub-coating after the first charge and discharge.

[0074] As a specific example, three types of negative electrode slurries can be configured, with the same raw material source for all three slurries. The only difference is the mass ratio of the negative electrode active material to the nano-additive. The volume expansion rate of the nano-additive after lithium intercalation is greater than that of the negative electrode active material after lithium intercalation. For example, the mass concentration of the nano-additive in the first negative electrode slurry can be no more than 0.3 wt%, the mass concentration of the nano-additive in the second negative electrode slurry can be 0.3–0.7 wt%, and the mass concentration of the nano-additive in the third negative electrode slurry can be 0.7–1.5 wt%. At the same time, the mass concentration of the nano-additive in the first, second, and third negative electrode slurries can be increased sequentially. Along the thickness direction of the current collector, a first type of negative electrode slurry, a second type of negative electrode slurry, and a third type of negative electrode slurry are sequentially coated on one side of the negative electrode current collector (i.e., single-sided coating), or along the thickness direction of the current collector, a first type of negative electrode slurry, a second type of negative electrode slurry, and a third type of negative electrode slurry are sequentially coated on opposite sides of the negative electrode current collector (i.e., double-sided coating), forming a negative electrode active material layer including multiple sub-coatings. At this time, in the direction away from the current collector, the porosity of the multiple sub-coatings increases layer by layer after the first charge and discharge.

[0075] A third aspect of this application provides a battery comprising: a negative electrode sheet according to the first aspect of this application, and / or a negative electrode sheet prepared using the method for preparing a negative electrode sheet according to the second aspect of this application. Optionally, the battery may be a secondary battery. Further optionally, the battery may be a lithium-ion battery.

[0076] Compared to conventional batteries, the negative electrode sheet prepared using the method of the first aspect of this application or the method of the second aspect of this application is more conducive to improving the fast-charging performance of high-energy-density batteries while also taking into account cycle performance. Furthermore, it is understood that the features and effects described regarding the negative electrode sheet of the first aspect of this application and the method for preparing the negative electrode sheet of the second aspect of this application also apply to the battery of the third aspect of this application, and will not be repeated here.

[0077] In some embodiments of this application, the battery further includes a positive electrode sheet. When the battery is a lithium-ion battery, the positive electrode sheet may include a lithium-rich manganese-based positive electrode material. Lithium-ion batteries are currently the mainstream power batteries, and their positive electrode materials mainly use lithium iron phosphate, ternary materials, lithium cobalt oxide, lithium manganese oxide, etc. However, traditional positive electrode materials have limited specific capacity, and it is difficult to improve the driving range by simply improving the process. Compared with common lithium-ion positive electrode materials, lithium-rich manganese-based positive electrode materials have higher specific capacity, which is more conducive to meeting the requirements of high energy density and improving driving range. Furthermore, the lithium-rich manganese-based positive electrode material may include nLi2MnO3·(1-n)LiNi x Mn (1-x-y) M yO2, wherein 0.05≤n≤0.4, 0.3≤x<1, 0<y≤0.1, and M includes one or more elements selected from Na, Mg, Al, Ca, Ba, V, Zn, Ti, Fe, Co, Cr, Nb, W, Mo, Zr, Ta, and Hf. This lithium-rich manganese-based cathode material has advantages such as high discharge specific capacity, high discharge voltage, high energy density, low cost, high safety, and long cycle life, and is more conducive to balancing the high energy density, fast charging performance, and cycle performance of the battery.

[0078] In addition to the positive and negative electrode plates, the battery typically includes a casing assembly, a separator, and an electrolyte. The positive and negative electrode plates, separator, and electrolyte are located within the cavity formed by the casing assembly. Depending on the type of battery, the casing assembly can be an aluminum-plastic film or a metal casing assembly, etc. The metal casing assembly can include square casing assemblies and cylindrical casing assemblies. When the casing assembly is a metal casing assembly, it typically includes a casing with an opening on at least one side and a cover for sealing the opening. Regardless of the type of battery, as long as its negative electrode plate includes the negative electrode plate of the first aspect of this application and / or a negative electrode plate prepared using the method of preparing a negative electrode plate according to the second aspect of this application, it can be understood as being included within the technical scope of the battery of the third aspect of this application. The characteristics of the negative and positive electrode plates have been described in the foregoing sections and will not be repeated here; the specific material or type of the separator can be selected using common choices in the art, and the composition of the electrolyte can also use common compositions in the art. Those skilled in the art can flexibly choose according to actual needs.

[0079] In some embodiments of this application, the battery can be either a stacked battery or a wound battery, and the wound battery can be either a prismatic battery or a cylindrical battery. Depending on the battery type, the positive electrode, negative electrode, and separator can be stacked to form a stacked unit, or they can be stacked and then wound to form a wound body.

[0080] In some embodiments of this application, the battery can be a single cell or a battery module assembled from cells. The number of cells included in a battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Furthermore, the battery module may also include a packaging assembly with a receiving space, which may include a base plate, side plates, and a cover plate.

[0081] In some embodiments of this application, the battery modules described above 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.

[0082] In addition, this application also provides an electrical device, which includes: a negative electrode sheet of the first aspect of this application, and / or a negative electrode sheet prepared by the method of preparing a negative electrode sheet of the second aspect of this application, and / or a battery of the third aspect of this application.

[0083] The battery, such as a cell, module, or pack, can serve as both a power source and an energy storage unit for the electrical device. This electrical device can include, but is not limited to, mobile devices (e.g., mobile phones, laptops), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, and energy storage systems. (Reference) Figure 4 To illustrate, as a specific example, the electrical device could be a vehicle.

[0084] Electrical devices can choose the specific type of battery according to their usage needs, such as battery cells, battery modules, or battery packs.

[0085] As an example, the electrical device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the battery, a battery pack or battery module can be used.

[0086] As another example, the device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use battery cells as their power source.

[0087] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0088] Example 1

[0089] (1) Preparation of positive electrode sheet

[0090] The lithium-rich cathode material 0.4Li2MnO3·0.6LiNi 0.5 Mn 0.5 O2, conductive agent acetylene black, and binder PVDF (polyvinylidene fluoride) are mixed in a weight ratio of 94:4:2. N-methylpyrrolidone solvent is added, and the mixture is thoroughly stirred to obtain a positive electrode slurry. This slurry is then coated onto both surfaces of the positive electrode current collector aluminum foil. After drying and cold pressing, a positive electrode sheet is obtained. The thickness of the positive electrode active material layer on one side of the positive electrode sheet is 66 μm, and the compaction density is 3 g / cm³. 3 .

[0091] (2) Preparation of negative electrode sheet

[0092] First, three negative electrode slurries with different contents of nano-additives were homogenized. The nano-additive was nano-silicon with a Dv50 particle size of 100nm, and the main negative electrode material was artificial graphite, resulting in three negative electrode slurries. The slurry preparation process is as follows: artificial graphite, nano-additives, conductive agent acetylene black, binder SBR (styrene-butadiene rubber), and binder CMC (sodium carboxymethyl cellulose) were mixed in a certain mass ratio, and deionized water was added as a solvent. The mixture was stirred thoroughly to obtain a homogeneous negative electrode slurry. Then, the three negative electrode slurries were coated in three layers onto the two surfaces of the copper foil of the negative electrode current collector. After drying and cold pressing, a negative electrode sheet with three sub-coatings was obtained (see the specific structure). Figure 3 (Understanding) The thickness of the active material layer on one side of the negative electrode sheet is 87 μm, and the compaction density is 1.65 g / cm³. 3 .

[0093] The innermost sub-coating is the first coating, the next outermost sub-coating is the second coating, and the outermost sub-coating is the third coating. The mass ratios of artificial graphite, nano-additives, acetylene black, SBR, and CMC in the first, second, and third coatings are 94.9:0.1:1.5:3.1:0.4, 94.5:0.5:1.5:3.1:0.4, and 94:1:1.5:3.1:0.4, respectively. The thickness ratio of the first, second, and third coatings is 2:3:5.

[0094] (3) Preparation of electrolyte

[0095] In an argon-atmosphere glove box with a water content of <10ppm, EC (ethylene carbonate), PC (polycarbonate), and DMC (dimethyl carbonate) were mixed in a weight ratio of EC:PC:DMC = 3:3:3. Then, LiPF6, VC (ethylene carbonate), DTD (ethylene sulfate), and PS (propylene sulfite) were added and stirred until homogeneous to obtain the electrolyte. The concentration of LiPF6 in the lithium-ion battery electrolyte was 1 mol / L, and the mass percentages of VC, DTD, and PS were 3%, 1%, and 1%, respectively.

[0096] (4) Preparation of the separating membrane

[0097] Polyethylene porous membrane is used as the separation membrane.

[0098] (5) Preparation of lithium-ion secondary batteries

[0099] The positive electrode sheet obtained in step (1), the separator in step (4), and the negative electrode sheet obtained in step (2) are stacked in sequence, so that the separator is in the middle of the positive and negative electrodes to play a role in isolation, and a bare cell is obtained; the bare cell is placed in an outer packaging, injected with the electrolyte prepared in step (3), and sealed for formation to obtain a lithium-ion secondary battery.

[0100] Example 2

[0101] The difference from Example 1 is that in step (2), the mass ratios of artificial graphite, nano-additives, acetylene black, SBR, and CMC in the first, second, and third coatings of the negative electrode sheet are 95:0:1.5:3.1:0.4, 94.7:0.3:1.5:3.1:0.4, and 94.3:0.7:1.5:3.1:0.4, respectively.

[0102] Example 3

[0103] The difference from Example 1 is that in step (2), the mass ratios of artificial graphite, nano-additives, acetylene black, SBR, and CMC in the first, second, and third coatings of the negative electrode sheet are 94.7:0.3:1.5:3.1:0.4, 94.3:0.7:1.5:3.1:0.4, and 93.5:1.5:1.5:3.1:0.4, respectively.

[0104] Comparative Example 1

[0105] The difference from Example 1 is that in step (2), no nano-additives are added to the negative electrode sheet. It can be prepared by one homogenization and one coating. The mass ratio of artificial graphite, acetylene black, SBR and CMC in the negative electrode slurry is 95:1.5:3.1:0.4.

[0106] Comparative Example 2

[0107] The difference from Example 1 is that in step (2), the preparation can be carried out by homogenization and coating once. The total mass of nano-additives in the negative electrode slurry is the same as the total mass of nano-additives in the negative electrode active material layer in Example 1. The mass ratio of artificial graphite, acetylene black, SBR and CMC is 94.33:0.67:3.1:0.4.

[0108] Example 4

[0109] The difference from Example 1 is that in step (2), the mass ratios of artificial graphite, nano-additives, acetylene black, SBR, and CMC in the first, second, and third coatings of the negative electrode sheet are 94.6:0.4:1.5:3.1:0.4, 94.1:0.9:1.5:3.1:0.4, and 93.9:1.1:1.5:3.1:0.4, respectively.

[0110] Example 5

[0111] The difference from Example 1 is that in step (2), the mass ratios of artificial graphite, nano-additives, acetylene black, SBR, and CMC in the first, second, and third coatings of the negative electrode sheet are 94.8:0.2:1.5:3.1:0.4, 94.7:0.3:1.5:3.1:0.4, and 94.5:0.5:1.5:3.1:0.4, respectively.

[0112] Example 6

[0113] The difference from Example 1 is that in step (2), the mass ratios of artificial graphite, nano-additives, acetylene black, SBR, and CMC in the first, second, and third coatings of the negative electrode sheet are 94.9:0.1:1.5:3.1:0.4, 94.35:0.65:1.5:3.1:0.4, and 94.15:0.85:1.5:3.1:0.4, respectively.

[0114] Example 7

[0115] The difference from Example 1 is that in step (2), the mass ratios of artificial graphite, nano-additives, acetylene black, SBR, and CMC in the first, second, and third coatings of the negative electrode sheet are 94.85:0.15:1.5:3.1:0.4, 94.45:0.55:1.5:3.1:0.4, and 94.05:0.95:1.5:3.1:0.4, respectively.

[0116] Example 8

[0117] The difference from Example 1 is that in step (2), the thickness of the active material layer on one side of the negative electrode sheet is 60 μm.

[0118] Example 9

[0119] The difference from Example 1 is that in step (2), the thickness of the active material layer on one side of the negative electrode sheet is 110 μm.

[0120] Example 10

[0121] The difference from Example 1 is that in step (2), the negative electrode sheet with two sub-coatings is prepared by two homogenizations and two coatings. The inner sub-coating is the first coating and the outer sub-coating is the second coating. The mass ratios of artificial graphite, nano-additives, acetylene black, SBR and CMC in the first coating and the second coating are 94.6:0.4:1.5:3.1:0.4 and 93.9:1.1:1.5:3.1:0.4, respectively. The thickness ratio of the first coating and the second coating is 3:7.

[0122] Comparative Example 3

[0123] The difference from Example 1 is that in step (2), the negative electrode sheet with two sub-coatings is prepared by two homogenizations and two coatings. The inner sub-coating is the first coating and the outer sub-coating is the second coating. The mass ratios of artificial graphite, nano-additives, acetylene black, SBR and CMC in the first coating and the second coating are 95:0:1.5:3.1:0.4 and 93.9:1.1:1.5:3.1:0.4, respectively. The thickness ratio of the first coating and the second coating is 3:7.

[0124] Example 11

[0125] The difference from Example 1 is that in step (2), the nano-additive is aluminum.

[0126] Example 12

[0127] The difference from Example 1 is that in step (2), the nano-additive is tin.

[0128] Example 13

[0129] The difference from Example 1 is that in step (2), the nano-additive is antimony.

[0130] Example 14

[0131] The difference from Example 1 is that in step (2), the nano-additive is bismuth.

[0132] Example 15

[0133] The difference from Example 1 is that in step (2), the nano-additive is germanium.

[0134] Characterization tests:

[0135] The secondary batteries prepared in the above embodiments and comparative examples were subjected to rate charging performance tests and cycle performance tests. The test results are shown in Table 1, and the test methods are as follows:

[0136] (1) Rate charging performance test

[0137] Under a constant temperature environment of 25℃, the secondary batteries of each embodiment and comparative example were discharged to 2.5V at 1 / 3C. After resting for 5 minutes, they were charged to 4.6V at 1 / 3C, and then charged at 4.6V under constant voltage until the current ≤0.05C. After resting for 5 minutes, the charging capacity at this time was recorded as C0. The batteries were then discharged to 2.5V at 1 / 3C, rested for 5 minutes, and then charged to 4.6V at 3C. After resting for 5 minutes, the charging capacity at this time was recorded as C1. The constant current ratio for 3C charging is C1 / C0 × 100%.

[0138] (2) Cyclic performance test

[0139] Under a constant temperature environment of 25℃, the capacitor is charged to 4.6V with a constant current of 1C, then charged to 0.05C with a constant voltage of 4.6V until the current drops to 0.05C, and then discharged to 2.5V with a constant current of 1C. The discharge specific capacity of the first cycle is obtained and denoted as Cd1. This charging and discharging process is repeated until the 500th cycle, and the discharge specific capacity after 500 cycles is denoted as Cd. n .

[0140] Capacity retention = Discharge specific capacity after 500 cycles (Cd) n ) / First-cycle discharge specific capacity (Cd1).

[0141] Table 1. Differences and test results between Examples 1-15 and Comparative Examples 1-3

[0142]

[0143]

[0144] Results and conclusions:

[0145] Comparative examples 1-15 and Comparative Example 1 show that the design of the negative electrode sheet of this application can effectively improve the fast-charging performance and cycle performance of the secondary battery. The main reason is that this application utilizes multi-layer sub-coatings to form a negative electrode active material layer on the negative electrode sheet, and the content of nano-additives in the multi-layer sub-coatings increases sequentially in the direction away from the current collector. Furthermore, nano-additives are distributed in at least the outermost and second-outermost sub-coatings. The irreversible expansion of the nano-additives can create pores in situ within the sub-coatings. By adjusting the content of nano-additives in different sub-coatings, a gradient pore structure with progressively increasing porosity can be formed in the direction away from the current collector after the first charge and discharge of the multi-layer sub-coatings. This significantly improves the transport of lithium ions in the liquid phase, thereby enhancing the fast-charging performance and cycle performance of the thick-coated negative electrode sheet. Specifically:

[0146] Comparing Example 1 and Comparative Examples 1-2, it can be seen that, compared to uniformly distributing nano-additives into the negative electrode active material layer, dividing the negative electrode active material layer into multiple sub-coatings with the content of nano-additives increasing sequentially in the direction away from the current collector is more conducive to improving the transport of lithium ions in the liquid phase and enhancing the fast-charging and cycle performance of the thick-coated negative electrode sheet. The main reason is that, although Comparative Example 2 also improved the porosity of the negative electrode active material layer after the first charge and discharge, which can improve the charging and cycle performance to a certain extent, its negative electrode active material layer did not form a gradient pore structure. In contrast, in Example 1, the multiple sub-coatings can form a gradient pore structure with progressively increasing porosity in the direction away from the current collector after the first charge and discharge, which is more conducive to improving the transport of lithium ions in the liquid phase and enhancing the utilization rate of the inner negative electrode active material.

[0147] Comparing Examples 1 and 2, it is evident that appropriately increasing the amount of nano-additives in each sub-coating of the negative electrode active material layer helps to further improve fast-charging and cycle performance. This is mainly because increasing the amount of nano-additives in each sub-coating helps to further increase the porosity of each sub-coating after the first charge-discharge cycle, which can better improve lithium-ion transport in the liquid phase and enhance the fast-charging and cycle performance of the thick-coated negative electrode sheet. Furthermore, the 3C rate charging current ratio and cycle performance of the secondary battery in Example 3 are relatively lower than those in Example 1 but significantly higher than those in Comparative Examples 1 and 2. This is mainly because the content of nano-additives in its outermost sub-coating is relatively high. Due to the excessive volume expansion of the nano-additives during lithium intercalation, they may break down during cycling, leading to a decrease in cycle performance. This demonstrates that an appropriate proportion of nano-additives helps to improve the charging capacity of the negative electrode and improve the cycle performance of the graphite negative electrode; however, excessive addition can lead to a decrease in cycle performance. Optionally, the content of nano-additives in the third coating can be 0.7wt% to 1.5wt% of the mass of the coating layer, and can be further selected as 1wt% to 1.1wt% considering cost and overall effect; the content of nano-additives in the second coating can be 0.3wt% to 0.7wt% of the mass of the coating layer; and the content of nano-additives in the first coating can be 0 to 0.3wt% of the mass of the coating layer, and can be further selected as 0.1 to 0.2wt%.

[0148] Comparing Examples 1-7 and Comparative Examples 1-2, it can be seen that although the content of nano-additives in the three sub-coatings of the negative electrode sheet in Example 4 is greater than that in Example 1, the 3C rate charge current ratio and cycle performance of its secondary battery are lower than those of Example 1 but higher than those of Comparative Examples 1-2. The main reason is that in Example 4, the difference in the content of nano-additives in adjacent coatings in the direction away from the current collector tends to decrease. The gradient pore structure formed after the first charge and discharge of the negative electrode active material layer is not as effective for lithium-ion liquid phase transport as in Example 1. Optionally, the content of nano-additives in the multi-layer sub-coatings can be increased sequentially in the direction away from the current collector. In addition, the 3C rate charge current ratio and cycle performance of the secondary batteries in Examples 6 and 7 are lower than those of Example 1 but higher than those of Comparative Examples 1-2. The main reason is that in Example 6, the difference in the content of nano-additives in the first and second coatings is greater than that between the second and third coatings. The difference in the content of nano-additives in the coating, specifically the difference in the content of nano-additives between adjacent coating layers in the direction away from the current collector in Example 6, also tends to decrease. In Example 7, the difference in the content of nano-additives between the first and second coating layers is equal to the difference in the content of nano-additives between the second and third coating layers. In Example 1, the difference in the content of nano-additives between the first and second coating layers is smaller than the difference in the content of nano-additives between the second and third coating layers. Furthermore, the total content of nano-additives in the negative electrode active material layer in Examples 6 and 7 is almost the same as that in Example 1. This further illustrates that, under the same total content of nano-additives, the difference in the content of nano-additives between adjacent coating layers in the direction away from the current collector tends to increase. The gradient pore structure formed in the negative electrode active material layer after the first charge and discharge is more conducive to improving the transport effect of lithium ions in the liquid phase, resulting in better improvement in fast charging performance and cycle performance. Optionally, the difference in the content of nano-additives between the first and second coating layers can be 0.2wt% to 0.4wt%, and the difference in the content of nano-additives between the third and second coating layers can be 0.5wt% to 0.7wt%.

[0149] Furthermore, the 3C rate charging current ratio and cycle performance of Example 5 are lower than those of Comparative Example 2 but higher than those of Comparative Example 1. The main reason for this is that the negative electrode active material layer in Example 5 can form a pore structure with a larger porosity gradient from the inside to the outside, which can improve the liquid phase transport of lithium ions to a certain extent. However, the content of nano-additives in each sub-coating is lower than that in Comparative Example 2, and the porosity formed after the first charge and discharge of each sub-coating is lower than that in Comparative Example 2. Therefore, the improvement effect on the liquid phase transport of lithium ions is not as good as that in Comparative Example 2. This also indirectly shows that adding nano-additives to the negative electrode active material layer to create pores in situ can form a certain porosity to improve the transport of ions in the liquid phase, thereby improving the charging performance and cycle performance. Moreover, the amount of nano-additives also affects the transport effect of ions in the liquid phase. Appropriately increasing the amount of nano-additives in the negative electrode active material layer helps to further improve the fast charging performance and cycle performance.

[0150] Comparing Examples 1, 8-9 and Comparative Examples 1-2, it can be seen that the total thickness of the negative electrode active material layer also affects fast charging performance and cycle performance. Increasing the coating thickness will reduce charging capacity and cycle performance. The number of sub-coating layers in the negative electrode active material layer and the content of nano-additives in different sub-coating layers can be adjusted according to different coating thicknesses. Optionally, when the negative electrode active material layer includes three sub-coating layers, the coating thickness of the negative electrode active material layer on one side of the current collector can be 70-100 μm.

[0151] Comparing Examples 1-7, 10 and Comparative Examples 1, 3, it is evident that including two sub-coating layers (inner and outer) in the negative electrode active material layer, and adding nano-additives to both sub-coating layers, can improve lithium-ion transport in the liquid phase, significantly enhancing fast-charging capability and cycle performance. Further comparison between Examples 1 and 10 shows that the total content of nano-additives in the negative electrode active material layer of Example 10 is greater than that of Example 1. This indicates that, with the same negative electrode active material layer thickness, compared to the gradient pore structure formed by two sub-coating layers, the gradient pore structure formed by three sub-coating layers is more conducive to achieving better lithium-ion transport in the liquid phase with a lower nano-additive content, resulting in a greater improvement in fast-charging capability and cycle performance.

[0152] Comparing Examples 1, 11-15, and Comparative Example 1, it can be seen that while aluminum as a nano-additive can improve lithium-ion transport in the liquid phase to some extent, its effect on improving fast-charging and cycle performance is not ideal, far lower than the effect of using nano-silicon, nano-tin, nano-antimony, nano-bismuth, and nano-germanium as nano-additives. This is mainly because the volume expansion rate of nano-aluminum after lithium intercalation is relatively low, only 90%. Although this is greater than the volume expansion rate of artificial graphite after lithium intercalation under the same conditions, the porosity formed in each sub-coating after the first charge and discharge is low, limiting the improvement effect on lithium-ion transport in the liquid phase. This also indicates that the low volume expansion rate of nano-additives after lithium intercalation has limited ability to improve the fast-charging capability and cycle performance of thick-coated negative electrodes. Optionally, the volume expansion rate of nano-additives after lithium intercalation can be made not less than 100%.

[0153] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A negative electrode sheet, characterized in that, include: current collector; as well as A negative electrode active material layer is disposed on at least a portion of the surface of at least one side of the current collector. The negative electrode active material layer includes multiple sub-coating layers arranged sequentially along its thickness direction. The negative electrode active material is distributed in each of the multiple sub-coating layers. At least the outermost sub-coating layer and the next outermost sub-coating layer contain nano-additives. The volume expansion rate of the nano-additives after lithium intercalation is greater than the volume expansion rate of the negative electrode active material after lithium intercalation. The content of the nano-additives in the multiple sub-coating layers increases sequentially in the direction away from the current collector. The nano-additives include at least one element of silicon, germanium, tin, antimony, bismuth and phosphorus, and / or an oxide of at least one of silicon, germanium, tin, antimony, bismuth and phosphorus, and / or a compound formed by any of silicon, germanium, tin, antimony, bismuth and phosphorus.

2. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material includes carbon materials; and / or, the volume expansion rate of the nano-additive after lithium intercalation is not less than 100%.

3. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material includes at least one of graphite materials, hard carbon materials, and soft carbon materials.

4. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material has a Dv50 particle size of 10μm to 20μm; and / or, the nano-additive has a Dv50 particle size of 20nm to 150nm.

5. The negative electrode sheet according to claim 1, characterized in that, The porosity of the multilayer sub-coating increases sequentially after the first charge and discharge in the direction away from the current collector.

6. The negative electrode sheet according to claim 1, characterized in that, The average pore size of the pores formed after the first charge and discharge of the multilayer sub-coating increases sequentially in the direction away from the current collector.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that, The negative electrode active material layer includes at least three sub-coating layers arranged sequentially along its thickness direction, each of which contains the nano-additive, or the nano-additive is distributed in all of the multiple sub-coating layers except the innermost layer.

8. The negative electrode sheet according to claim 7, characterized in that, At least one of the following conditions must be met: In the direction away from the current collector, the difference in the content of the nano-additive in two adjacent sub-coating layers gradually increases; In the direction away from the current collector, the difference in porosity between two adjacent sub-coating layers after the first charge and discharge gradually increases; The thickness of the multilayer sub-coatings is the same, or the thickness of the multilayer sub-coatings increases sequentially in the direction away from the current collector.

9. The negative electrode sheet according to claim 1 or 8, characterized in that, The negative electrode active material layer comprises three sub-coating layers arranged sequentially along its thickness direction. The content of the nano-additive in the outermost sub-coating layer is 0.7wt%~1.5wt% of the mass of the sub-coating layer; The content of the nano-additive in the sub-coating of the outermost layer is 0.3wt% to 0.7wt% of the mass of the sub-coating layer; The content of the nano-additive in the innermost sub-coating layer is 0~0.3wt% of the mass of the sub-coating layer.

10. The negative electrode sheet according to claim 9, characterized in that, The content of the nano-additive in the outermost sub-coating layer is 1 wt% to 1.1 wt% of the mass of the sub-coating layer; The content of the nano-additive in the sub-coating of the outermost layer is 0.4wt%~0.5wt% of the mass of the sub-coating layer; The content of the nano-additive in the innermost sub-coating layer is 0.1wt% to 0.2wt% of the mass of the sub-coating layer.

11. The negative electrode sheet according to claim 9, characterized in that, The difference in the content of the nano-additive in the innermost sub-coating layer and the content of the nano-additive in the next outermost sub-coating layer is 0.2wt%~0.4wt%, and the difference in the content of the nano-additive in the outermost sub-coating layer and the content of the nano-additive in the next outermost sub-coating layer is 0.5wt%~0.7wt%; and / or, Based on the total mass of the negative electrode active material layer, the total content of the nano-additives is no more than 2.5 wt%.

12. The negative electrode sheet according to claim 11, characterized in that, The total content of the nano-additives is no more than 0.5wt%~1.8wt%.

13. The negative electrode sheet according to claim 12, characterized in that, The total content of the nano-additives is no more than 0.6wt%~1.5wt%.

14. The negative electrode sheet according to claim 1 or 11, characterized in that, The negative electrode active material layer includes three sub-coating layers arranged sequentially along its thickness direction. After the first charge and discharge, the porosity of the outermost sub-coating layer is 40%~43%, the porosity of the second outermost sub-coating layer is 36%~37%, and the porosity of the innermost sub-coating layer is 33%~35%.

15. The negative electrode sheet according to claim 14, characterized in that, After the first charge and discharge, the difference between the porosity of the innermost sub-coating layer and the porosity of the next outermost sub-coating layer is 2% to 4%, and the difference between the porosity of the outermost sub-coating layer and the next outermost sub-coating layer is 4% to 6%.

16. The negative electrode sheet according to claim 15, characterized in that, The three sub-coating layers have the same thickness; or, the ratio of the thickness of the innermost sub-coating layer, the thickness of the next outermost sub-coating layer, and the thickness of the outermost sub-coating layer is (2±0.5):(3±0.5):(5±0.5).

17. The negative electrode sheet according to claim 1 or 12, characterized in that, The current collector is coated on one side or both sides, and the thickness of the negative electrode active material layer on one side of the current collector is 70μm~100μm.

18. A method for preparing a negative electrode sheet according to any one of claims 1 to 17, characterized in that, include: (1) Prepare multiple negative electrode slurries, and make at least two of the negative electrode slurries contain nano-additives; (2) The various negative electrode slurries are sequentially coated onto at least a portion of the surface of at least one side of the current collector along the thickness direction to form a negative electrode active material layer including multiple sub-coatings, and the content of nano-additives in the multiple sub-coatings increases sequentially in the direction away from the current collector. Among them, various negative electrode slurries include negative electrode active materials, and the volume expansion rate of the nano-additive after lithium intercalation is greater than the volume expansion rate of the negative electrode active material after lithium intercalation.

19. A battery, characterized in that, Includes the negative electrode sheet according to any one of claims 1 to 17, and / or the negative electrode sheet prepared by the method according to claim 18.

20. The battery according to claim 19, characterized in that, Also includes: The positive electrode sheet comprises a lithium-rich manganese-based positive electrode material, wherein the lithium-rich manganese-based positive electrode material comprises nLi₂MnO₃·(1-n)LiNi x Mn (1-x-y) M y O2, wherein 0.05≤n≤0.4, 0.3≤x<1, 0<y≤0.1, and M includes one or more elements selected from Na, Mg, Al, Ca, Ba, V, Zn, Ti, Fe, Co, Cr, Nb, W, Mo, Zr, Ta, and Hf.

21. An electrical appliance, wherein, The battery includes the negative electrode sheet according to any one of claims 1 to 17, and / or the negative electrode sheet prepared by the method according to claim 18, and / or the battery according to any one of claims 19 to 20.

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