Negative electrode sheet, method for manufacturing the same, and battery

Abstract

The application relates to the field of new energy batteries, in particular to a negative pole piece, a preparation method thereof and a battery. The negative pole piece comprises a current collector, a first negative pole active material layer arranged on the surface of the current collector, and a second negative pole active material layer arranged on the surface of the first negative pole active material layer away from the current collector; the first negative pole active material layer comprises a first silicon material, and the second negative pole active material layer comprises a second silicon material; wherein the sphericity S1 of the first silicon material is greater than or equal to 0.80; the sphericity S2 of the second silicon material is in the range of 0.30-0.65; the first silicon material and the second silicon material satisfy the following relationship: M1 >= M2, wherein M1 represents the mass percentage of the first silicon material in the first negative pole active material layer, and M2 represents the mass percentage of the second silicon material in the second negative pole active material layer. The battery cell prepared by using the negative pole piece has the advantages of significantly improved kinetic performance and long cycle performance, and high energy density of the battery cell.

Description

Technical Field

[0001] This invention relates to the field of new energy batteries, specifically to a negative electrode sheet, its preparation method, and a battery. Background Technology

[0002] With the rapid development of science and technology, continuously improving the energy density of lithium-ion batteries and developing high-energy-density lithium-ion batteries has become an inevitable trend in lithium-ion battery development. In terms of improving energy density, increasing the theoretical specific capacity of the anode active material in lithium-ion batteries is one of the most effective ways. Silicon materials have a theoretical specific capacity as high as 3579 mAh / g at room temperature, more than 10 times that of graphite-based anodes in currently commercial lithium-ion batteries. In the short term, it is considered the most promising anode material for commercial application to improve the energy density of lithium-ion batteries. However, silicon materials undergo a volume change of up to 300% during charge and discharge. This huge volume expansion can cause cracks in silicon particles, generating new interfaces, leading to the formation of a new SEI film that consumes electrolyte and affects battery performance. Furthermore, under this expansion, the conductive network of the anode is easily damaged, some particles lose electrical contact, battery capacity decays faster, and kinetic performance is lost. Therefore, how to suppress the volume expansion of silicon materials during charge and discharge and improve the stability and kinetic performance of silicon-containing anode sheets is a problem that urgently needs to be solved in the application of silicon materials as anodes in lithium-ion batteries. Summary of the Invention

[0003] In view of this, the present invention aims to provide a negative electrode sheet, its preparation method and battery, to solve the problem that silicon negative electrode materials in the prior art suffer from excessive volume expansion and insufficient kinetic performance during actual use, resulting in the inability to achieve both high energy density and fast charging performance.

[0004] To solve the above-mentioned technical problems, this application is implemented as follows:

[0005] A first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a current collector, a first negative electrode active material layer disposed on the surface of the current collector, and a second negative electrode active material layer disposed on the surface of the first negative electrode active material layer away from the current collector;

[0006] The first negative electrode active material layer includes a first silicon material, and the second negative electrode active material layer includes a second silicon material;

[0007] Wherein, the sphericity S1 of the first silicon material is ≥ 0.80; the sphericity S2 of the second silicon material is in the range of 0.30 to 0.65;

[0008] The first silicon material and the second silicon material satisfy the following relationship: M1≥M2, where M1 represents the mass percentage of the first silicon material in the first negative electrode active material layer, and M2 represents the mass percentage of the second silicon material in the second negative electrode active material layer.

[0009] Optionally, the sphericity S1 of the first silicon material is in the range of 0.80 to 0.95; and the sphericity S2 of the second silicon material is in the range of 0.40 to 0.60.

[0010] Optionally, the first silicon material and the second silicon material satisfy the following relationship: 1% ≤ M1 - M2 ≤ 5%.

[0011] Optionally, the mass percentage M1 of the first silicon material in the first negative electrode active material layer ranges from 5% to 30%; and / or, the mass percentage M2 of the second silicon material in the second negative electrode active material layer ranges from 0 to 25%.

[0012] Optionally, the mass ratio of the first negative electrode active material layer to the second negative electrode active material layer is 1:0.42 to 2.34.

[0013] Optionally, the first silicon material and the second silicon material are each independently selected from SiO2. x (0 < x < 2), at least one of pre-lithiated silicon oxide, pre-magnesiated silicon oxide, silicon alloy, milled silicon carbide, and vapor-deposited silicon carbide; the 5T powder compaction density of the first silicon material is 1.11 to 1.25 g / cc; the 5T powder compaction density of the second silicon material is 1.00 to 1.10 g / cc.

[0014] Optionally, the first negative electrode active material layer further includes a first active material, and the second negative electrode active material layer further includes a second active material; optionally, the first active material is a first graphite material, and the second active material is a second graphite material; optionally, the first graphite material and the second graphite material are each independently selected from at least one of artificial graphite, natural graphite, and mesophase carbon microspheres.

[0015] A second aspect of the present invention provides a method for preparing the above-mentioned negative electrode sheet, the method comprising the following steps:

[0016] S1. Prepare a first negative electrode active material layer slurry containing a first silicon material; coat the first negative electrode active material layer slurry onto the surface of the current collector and perform a first drying treatment to obtain a current collector with a first negative electrode active material layer on its surface.

[0017] S2. Prepare a second negative electrode active material layer slurry containing a second silicon material, coat the second negative electrode active material layer slurry onto the surface of the first negative electrode active material layer and perform a second drying treatment.

[0018] Optionally, the first negative electrode active material layer slurry further comprises a first active material, a first conductive agent, a first binder, a first thickener, and a first negative electrode solvent; the second negative electrode active material layer slurry further comprises a second active material, a second conductive agent, a second binder, a second thickener, and a second negative electrode solvent; the first conductive agent and the second conductive agent are each independently selected from at least one of conductive carbon black, graphene, and carbon nanotubes; the first binder and the second binder are each independently selected from at least one of styrene-butadiene rubber, polyacrylic acid, polyvinylpyrrolidone, polyaniline, polyimide, polysiloxane, polystyrene-butadiene rubber, epoxy resin, polyester resin, and polyurethane resin; the first thickener and the second thickener are each independently selected from sodium carboxymethyl cellulose and / or lithium carboxymethyl cellulose; the first negative electrode solvent is water; the second negative electrode solvent is water.

[0019] A third aspect of the present invention provides a battery comprising a negative electrode sheet, the negative electrode sheet comprising the negative electrode sheet described above and / or a negative electrode sheet prepared according to the preparation method described above.

[0020] The beneficial technical effects of the present invention through the above technical solution are as follows:

[0021] (1) The first negative electrode active material layer in the negative electrode sheet of the present invention adopts a first silicon material with high sphericity (S1≥0.80). The spherical structure design of this silicon material has strong compressive strength and high compaction density. When placed at the bottom of the electrode sheet, it can improve the overall compaction density of the negative electrode sheet and improve the energy density. Moreover, the high sphericity of the silicon material, when placed at the bottom, results in a large contact angle with the current collector, avoiding the silicon material from embedding into the current collector under high pressure roller pressure, which would cause the current collector to break. The high sphericity of the particles during the lithium insertion and extraction expansion process results in more uniform volume expansion stress and a low overall expansion rate of the material, which also effectively reduces the problem of current collector breakage caused by silicon expansion during cycling. The second negative electrode active material layer adopts a second silicon material with lower sphericity (0.30≤S2≤0.65). The low sphericity of this silicon material results in more surface active sites, which is conducive to the rapid transport and diffusion of lithium ions. Moreover, the low sphericity results in a low compaction density of the material. When placed on the upper layer of the electrode sheet, it is conducive to creating more pores after mixing with graphite, which is conducive to the rapid transport of electrolyte and lithium ions and improves the overall dynamic performance of the electrode sheet.

[0022] (2) The silicon content M1 of the first negative electrode active material layer and the silicon content M2 of the second negative electrode active material layer of the present invention are designed with a gradient, wherein the content of the first silicon material and the content of the second silicon material satisfy the following relationship: M1≥M2. The silicon material of the first negative electrode active material layer has a high compaction density and high structural strength, and its content at the bottom layer of the electrode is relatively high, which is beneficial to improving the overall energy density of the negative electrode. The silicon material of the second negative electrode active material layer has a lower compaction density, and a small amount of addition can effectively improve the kinetic performance of the upper electrode layer of the negative electrode. At the same time, it avoids the large volume expansion and contraction of silicon material during cycling, which would cause a large amount of silicon material to float to the surface of the active material layer, and silicon particles to puncture the separator and cause safety problems. This gradient design makes the compaction density of the upper and lower layers of the electrode more similar, avoiding the material layer peeling caused by a large difference in compaction density between the upper and lower layers.

[0023] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0024] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the following detailed description to explain the invention, but do not constitute a limitation thereof.

[0025] Figure 1 The diagram shown is a schematic of the negative electrode sheet of the present invention.

[0026] Explanation of reference numerals in the attached figures

[0027] 1. Current collector

[0028] 2. First negative electrode active material layer

[0029] 3. Second negative electrode active material layer

[0030] 4. First silicon material

[0031] 5. Second silicon material

[0032] 6. First active material

[0033] 7. Second active material Detailed Implementation

[0034] This invention discloses a negative electrode sheet, its preparation method, and a battery. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It is particularly important to note that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments. Those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0035] In the description of this invention, the list of items connected by the term "at least one of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0036] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0037] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0038] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0039] 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" can mean that other components not listed may also be included, or that only the listed components may be included.

[0040] To address the problem that silicon materials in existing technologies suffer from excessive volume expansion and insufficient kinetic performance during practical use, resulting in an inability to simultaneously achieve high energy density and fast charging performance, this invention adopts the following technical solution:

[0041] The first aspect of the present invention provides a negative electrode sheet, such as Figure 1 As shown, the negative electrode sheet includes a current collector, a first negative electrode active material layer disposed on the surface of the current collector, and a second negative electrode active material layer disposed on the surface of the first negative electrode active material layer away from the current collector;

[0042] The first negative electrode active material layer includes a first silicon material, and the second negative electrode active material layer includes a second silicon material;

[0043] Wherein, the sphericity S1 of the first silicon material is ≥ 0.80; the sphericity S2 of the second silicon material is in the range of 0.30 to 0.65;

[0044] The first silicon material and the second silicon material satisfy the following relationship: M1≥M2, where M1 represents the mass percentage of the first silicon material in the first negative electrode active material layer, and M2 represents the mass percentage of the second silicon material in the second negative electrode active material layer.

[0045] The negative electrode sheet of this invention includes a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer contains silicon material with high sphericity, which is beneficial for improving the overall structural stability of the negative electrode sheet and enhancing the cell's cycle performance and expansion performance. The second negative electrode active material layer contains silicon material with a sphericity in the range of 0.30 to 0.65, which helps to control the side reactions between the silicon material and the electrolyte while ensuring the cell's energy density, thus improving the cell's cycle and fast-charging performance. Furthermore, the content of the first silicon material and the content of the second silicon material in this invention satisfy the following relationship: M1 ≥ M2, which allows the compaction density of the upper and lower layers of the electrode sheet to be closer, avoiding material layer delamination caused by large differences in compaction density between the upper and lower layers.

[0046] The term "sphericity" used in this invention is a parameter characterizing particle morphology. The closer a particle's morphology is to a sphere, the closer its sphericity is to 1. In this invention, sphericity is obtained by direct measurement using equipment, such as a Morphologi-4 fully automated particle size and shape analyzer (purchased from Malvern Panaco, UK). The testing method can be dry dispersion and / or static imaging.

[0047] In an exemplary embodiment of the present invention, the sphericity S1 of the first silicon material is in the range of 0.80 to 0.95; the sphericity S2 of the second silicon material is in the range of 0.40 to 0.60. In this embodiment, the fast charging performance of the battery cell is less affected, the structural stability of the silicon material is improved, the overall structural stability of the negative electrode sheet is improved, and the cycle performance and expansion performance of the battery cell are improved.

[0048] According to the present invention, the gradient difference M1-M2 between the percentage content M1 of the first silicon material in the first negative electrode active material layer and the percentage content M2 of the second silicon material in the second negative electrode active material layer is controlled within a reasonable range. This results in a silicon negative electrode sheet and a battery cell containing the negative electrode sheet exhibiting relatively balanced energy density, fast-charging performance, cycle performance, and expansion performance. If the difference between M1 and M2 is too small, the content of the second silicon material in the second negative electrode active material layer is high. Due to the low compaction density of the second silicon material, the electrode sheet cannot be rolled to the target compaction density, causing the battery cell's energy density to decrease instead of increase. Furthermore, the increased content of the second silicon negative electrode material leads to a deterioration in the battery cell's fast-charging performance, cycle performance, and expansion performance. Simultaneously, the mismatch in volume expansion between the upper and lower electrode layers causes the electrode sheet to detach from the mold. If the difference between M1 and M2 is too large, the content of the second silicon material in the second negative electrode active material layer decreases, leading to a decrease in the battery cell's energy density. Moreover, the lower silicon content reduces the number of lithium intercalation sites, resulting in less pore space and a deterioration in fast-charging performance, but improves cycle and expansion performance. For example, the first silicon material and the second silicon material can satisfy the following relationship: 1% ≤ M1 - M2 ≤ 5%.

[0049] In an exemplary embodiment of the present invention, the mass percentage M1 of the first silicon material in the first negative electrode active material layer can be in the range of 5% ≤ M1 ≤ 30%; and / or, the mass percentage M2 of the second silicon material in the second negative electrode active material layer can be in the range of 0 < M2 ≤ 25%.

[0050] According to the present invention, a reasonable mass ratio of the first negative electrode active material layer and the second negative electrode active material layer can enable the silicon negative electrode sheet and the battery cell containing the negative electrode sheet to have a relatively balanced energy density, fast charging performance, cycle performance, and expansion performance. If the proportion of the first negative electrode active material layer is too small, that is, the amount of lower active material providing high compaction is reduced, the electrode sheet cannot be rolled to the target compaction density, the electrode sheet thickness increases, the battery cell energy density decreases, the fast charging performance deteriorates, and the overall proportion of the second silicon material in the electrode sheet increases, leading to an increase in side reactions with the electrolyte, and a deterioration in cycle performance and battery cell expansion. If the proportion of the first negative electrode active material layer is too large, the overall silicon content of the electrode sheet increases, the battery cell energy density increases, but the overall proportion of the second silicon material in the electrode sheet decreases, the fast charging performance deteriorates, and a slight lithium plating phenomenon occurs. In the present invention, the mass ratio of the first negative electrode active material layer to the second negative electrode active material layer can be 1:0.42 to 2.34. For example, the mass ratio of the first negative electrode active material layer to the second negative electrode active material layer can be any value among 1:0.42, 1:0.5, 1:0.75, 1:1, 1:1.25, 1:1.5, 1:1.75, 1:2, 1:2.25 and 1:2.34 or any value within the range of any two of the above values.

[0051] According to the present invention, the 5T powder compaction density of the first silicon anode can be 1.11 to 1.25 g / cc; the 5T powder compaction density of the second silicon anode can be 1.00 to 1.10 g / cc.

[0052] For example, the first silicon material and the second silicon material can each be independently selected from SiO2. x (0 < x < 2), at least one of pre-lithiated silicon oxide, pre-magnesiated silicon oxide, silicon alloy, sand-milled silicon carbide and vapor-deposited silicon carbide.

[0053] According to the present invention, the first negative electrode active material layer may further include a first active material, and the second negative electrode active material layer may further include a second active material; optionally, the first active material is a first graphite material, and the second active material is a second graphite material; optionally, the first graphite material and the second graphite material are each independently selected from at least one of artificial graphite, natural graphite, and mesophase carbon microspheres.

[0054] A second aspect of the present invention provides a method for preparing the above-mentioned negative electrode sheet, the method comprising the following steps:

[0055] S1. Prepare a first negative electrode active material layer slurry containing a first silicon material; coat the first negative electrode active material layer slurry onto the surface of the current collector and perform a first drying treatment to obtain a current collector with a first negative electrode active material layer on its surface.

[0056] S2. Prepare a second negative electrode active material layer slurry containing a second silicon material, coat the second negative electrode active material layer slurry onto the surface of the first negative electrode active material layer and perform a second drying treatment.

[0057] The method for preparing the negative electrode sheet of the present invention is simple and low in cost. It only requires controlling the sphericity and addition amount of silicon material added to the upper and lower layers during the negative electrode slurry preparation process. When the negative electrode sheet obtained by this preparation method is applied to the battery cell, it can significantly improve the dynamic performance, long cycle performance and energy density of the battery cell.

[0058] According to the present invention, the first negative electrode active material layer slurry may further contain a first active material, a first conductive agent, a first binder, a first thickener, and a first negative electrode solvent; the second negative electrode active material layer slurry may further contain a second active material, a second conductive agent, a second binder, a second thickener, and a second negative electrode solvent; the first conductive agent and the second conductive agent may each be independently selected from at least one of conductive carbon black, graphene, and carbon nanotubes; the first binder and the second binder may each be independently selected from at least one of styrene-butadiene rubber, polyacrylic acid, polyvinylpyrrolidone, polyaniline, polyimide, polysiloxane, polystyrene-butadiene rubber, epoxy resin, polyester resin, and polyurethane resin; the first thickener and the second thickener may each be independently selected from sodium carboxymethyl cellulose and / or lithium carboxymethyl cellulose; the first negative electrode solvent may be water; the second negative electrode solvent may be water.

[0059] In this invention, the mixing ratio of each component in the first negative electrode active material layer slurry and the second negative electrode active material layer slurry can be any of the mixing ratios commonly used in the art, and no further limitations are imposed here.

[0060] A third aspect of the present invention provides a battery comprising a negative electrode sheet, the negative electrode sheet comprising the negative electrode sheet described above and / or a negative electrode sheet prepared according to the preparation method described above.

[0061] The present invention will be further described in detail below through examples. All raw materials used in the examples are commercially available. The current collector used in the examples and comparative examples is copper foil.

[0062] Example 1

[0063] (1) A first silicon material, a first graphite material, a first conductive agent, a first binder, and a first thickener are mixed in water to obtain a first negative electrode active material layer slurry; the first negative electrode active material layer slurry is coated on the surface of a current collector and subjected to a first drying treatment to obtain a current collector with a first negative electrode active material layer on its surface. The first silicon material is vapor-deposited silicon-carbon with a sphericity S1 of 0.82 and a compaction density PD1 of 1.15 g / cc. The mass percentage M1 of the first silicon material in the first negative electrode active material layer is 10%. The first graphite material is artificial graphite, the first conductive agent is single-walled carbon nanotubes, the first binder is lithium polyacrylate (PAA-Li), and the first thickener is lithium carboxymethyl cellulose (CMC-Li). The mass ratio of the first silicon material, the first graphite material, the first conductive agent, the first binder, and the first thickener is 9.67%:87.03%:0.1%:2.8%:0.4%.

[0064] (2) The second silicon material, the first graphite material, the second conductive agent, the second binder, and the second thickener are mixed in water to obtain a second negative electrode active material layer slurry. The second negative electrode active material layer slurry is coated on the surface of the first negative electrode active material layer and subjected to a second drying treatment to obtain the negative electrode sheet of this embodiment. The second silicon material is vapor-deposited silicon-carbon with a sphericity S2 of 0.45 and a compaction density PD2 of 1.05 g / cc. The mass percentage M2 of the second silicon material in the second negative electrode active material layer is 7%. The second graphite material is artificial graphite, the second conductive agent is single-walled carbon nanotubes, the second binder is PAA-Li, and the second thickener is CMC-Li. The mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder, and the second thickener is 6.769%:89.931%:0.1%:2.8%:0.4%.

[0065] In this embodiment, the mass ratio of the first negative electrode active material layer and the second negative electrode active material layer is 1:1.

[0066] Example 2

[0067] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that the sphericity S2 of the second silicon material is 0.30 and the compaction density PD2 is 1.03 g / cc.

[0068] Example 3

[0069] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that the sphericity S2 of the second silicon material is 0.63 and the compaction density PD2 is 1.08 g / cc.

[0070] Example 4

[0071] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that: the mass percentage M2 of the second silicon material in the second negative electrode active material layer is 5%, and the mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder and the second thickener is 4.845%: 92.055%: 0.1%: 2.6%: 0.4%.

[0072] Example 5

[0073] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that: the mass percentage M2 of the second silicon material in the second negative electrode active material layer is 9%, and the mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder and the second thickener is 8.703%: 87.997%: 0.1%: 2.8%: 0.4%.

[0074] Example 6

[0075] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that the mass ratio of the first negative electrode active material layer and the second negative electrode active material layer is 1:2.34.

[0076] Example 7

[0077] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that the mass ratio of the first negative electrode active material layer and the second negative electrode active material layer is 1:0.42.

[0078] Example 8

[0079] The preparation method of the negative electrode sheet in this embodiment is generally the same as that in Embodiment 1, except that: the mass percentage M1 of the first silicon material in the first negative electrode active material layer is 5%, and the mass ratio of the first silicon material, the first graphite material, the first conductive agent, the first binder, and the first thickener is 4.845%:92.055%:0.1%:2.6%:0.4%; the mass percentage M2 of the second silicon material in the second negative electrode active material layer is 3%, and the mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder, and the second thickener is 2.907%:93.993%:0.1%:2.6%:0.4%.

[0080] Example 9

[0081] The preparation method of the negative electrode sheet in this embodiment is generally the same as that in Embodiment 1, except that: the mass percentage M1 of the first silicon material in the first negative electrode active material layer is 30%, and the mass ratio of the first silicon material, the first graphite material, the first conductive agent, the first binder, and the first thickener is 28.86%:67.34%:0.2%:3.2%:0.4%; the mass percentage M2 of the second silicon material in the second negative electrode active material layer is 25%, and the mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder, and the second thickener is 72.15%:24.05%:0.2%:3.2%:0.4%.

[0082] Example 10

[0083] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that the sphericity S1 of the first silicon material is 0.93.

[0084] Example 11

[0085] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that: the mass percentage M2 of the second silicon material in the second negative electrode active material layer is 3%, and the mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder and the second thickener is 2.907%: 93.993%: 0.1%: 2.6%: 0.4%.

[0086] Example 12

[0087] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that the mass ratio of the first negative electrode active material layer and the second negative electrode active material layer is 1:4.

[0088] Example 13

[0089] The preparation method of the negative electrode sheet in this embodiment is the same as that in Embodiment 1, except that the mass ratio of the first negative electrode active material layer and the second negative electrode active material layer is 1:0.25.

[0090] Example 14

[0091] The preparation method of the negative electrode sheet in this embodiment is generally the same as that in Embodiment 1, except that: the mass percentage M1 of the first silicon material in the first negative electrode active material layer is 3%, and the mass ratio of the first silicon material, the first graphite material, the first conductive agent, the first binder, and the first thickener is 2.907%:93.993%:0.1%:2.6%:0.4%; the mass percentage M2 of the second silicon material in the second negative electrode active material layer is 2%, and the mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder, and the second thickener is 1.938%:94.962%:0.1%:2.6%:0.4%.

[0092] Example 15

[0093] The preparation method of the negative electrode sheet in this embodiment is generally the same as that in Embodiment 1, except that: the mass percentage M1 of the first silicon material in the first negative electrode active material layer is 35%, and the mass ratio of the first silicon material, the first graphite material, the first conductive agent, the first binder, and the first thickener is 33.67%:62.53%:0.2%:3.2%:0.4%; the mass percentage M2 of the second silicon material in the second negative electrode active material layer is 30%, and the mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder, and the second thickener is 28.86%:67.34%:0.2%:3.2%:0.4%.

[0094] Comparative Example 1

[0095] The preparation method of the negative electrode sheet in this comparative example is the same as that in Example 1, except that the sphericity S2 of the second silicon material is 0.25 and the compaction density PD2 is 0.97 g / cc.

[0096] Comparative Example 2

[0097] The preparation method of the negative electrode sheet in this comparative example is the same as that in Example 1, except that the sphericity S2 of the second silicon material is 0.75 and the compaction density PD2 is 1.1 g / cc.

[0098] Comparative Example 3

[0099] The preparation method of the negative electrode sheet in this comparative example is generally the same as that in Example 1, except that: the mass percentage M1 of the first silicon material in the first negative electrode active material layer is 10%, and the mass ratio of the first silicon material, the first graphite material, the first conductive agent, the first binder, and the first thickener is 9.67%:87.03%:0.1%:2.8%:0.4%; the mass percentage M2 of the second silicon material in the second negative electrode active material layer is 11%, and the mass ratio of the second silicon material, the second graphite material, the second conductive agent, the second binder, and the second thickener is 10.637%:86.063%:0.1%:2.8%:0.4%.

[0100] Comparative Example 4

[0101] The preparation method of the negative electrode sheet in this comparative example is the same as that in Example 1, except that the sphericity S1 of the first silicon material is 0.75.

[0102] Comparative Example 5

[0103] A first silicon material, a second silicon material, a graphite material, a conductive agent, a binder, and a thickener were mixed in water to obtain a negative electrode active material layer slurry. This slurry was then coated onto the surface of a current collector and dried to obtain the negative electrode sheet of this comparative example. The first silicon material was vapor-deposited silicon-carbon with a sphericity S1 of 0.82, a compaction density PD1 of 1.15 g / cc, and a mass percentage M1 of 5% in the negative electrode active material layer. The second silicon material was also vapor-deposited silicon-carbon with a sphericity S2 of 0.45, a compaction density PD2 of 1.05 g / cc, and a mass percentage M2 of 3.5% in the negative electrode active material layer. The graphite material is artificial graphite, the conductive agent is single-walled carbon nanotubes, the binder is PAA-Li, and the thickener is CMC-Li. The mass ratio of the first silicon material, the second silicon material, the graphite material, the conductive agent, the binder, and the thickener is 4.845% : 3.3915% : 88.6635% : 0.1% : 2.6% : 0.4%.

[0104] Test Example 1

[0105] The negative electrode sheets prepared in Examples 1-11 and Comparative Examples 1-5, together with the positive electrode sheet and the separator, were used to prepare a battery cell. The method for preparing the positive electrode sheet included: using LiNi... 0.8 Co 0.1 Mn 0.1Using O2 material as the positive electrode material, a positive electrode material, conductive carbon black, multi-walled carbon nanotubes, and PVDF binder are mixed in a mass ratio of 97.5%:1.0%:0.5%:1.0%. An appropriate amount of NMP is added to adjust the viscosity of the slurry to 3000-10000 Pa·s, and the solid content to 50-60%, thus preparing the positive electrode slurry. The prepared positive electrode slurry is coated onto a positive electrode current collector aluminum foil, dried, and cold-pressed to obtain the positive electrode sheet. The cell preparation method includes: stacking the positive electrode, separator, and negative electrode in sequence, with the separator positioned between the positive and negative electrodes for isolation. Stacking yields a bare cell. The bare cell is placed in an outer packaging, injected with electrolyte, and sealed. After formation, degassing, and edge trimming processes, a lithium-ion battery is obtained.

[0106] The prepared battery cells were subjected to performance tests, and the test results are shown in Table 1.

[0107] (1) Energy density test:

[0108] The energy density of the lithium-ion battery was measured by performing 1C / 1C constant current charge and discharge tests within a voltage range of 2.8V-4.25V.

[0109] (2) 3C charging rate test:

[0110] At 25℃, discharge at 0.2C to 2.8V, let stand for 5 minutes, charge at 3C to 4.25V, charge at constant voltage to 0.05C and let stand for 5 minutes. The ratio of the constant current charging capacity to the total capacity is the charging rate.

[0111] (3) Lithium plating and delamination tests:

[0112] After charging the lithium-ion battery to 3C within a voltage range of 2.8V-4.25V, the electrode plates were disassembled to observe the lithium plating and delamination of the negative electrode plate.

[0113] (4) High-temperature cycling performance test:

[0114] The test temperature was 45℃. The battery was charged at a constant current of 1C to 4.25V, then charged at a constant voltage of 0.025C. After resting for 5 minutes, it was discharged at 1C to 2.8V. The capacity obtained in this step was used as the initial capacity. A 1C charge / discharge cycle test was performed, and the capacity decay curve was obtained by comparing the capacity at each step with the initial capacity. The capacity retention rate after 500 cycles at 45℃ was recorded to compare the high-temperature cycle performance of the battery.

[0115] (5) Electrode assembly full-charge expansion rate test:

[0116] The thickness of the fresh electrode assembly during half-charge is measured using a micrometer. When the cycle reaches 500cls, the electrode assembly is in a fully charged state. The thickness of the electrode assembly at this time is measured again using a micrometer. By comparing the thickness of the fresh electrode assembly during the initial half-charge, the expansion rate of the fully charged electrode assembly can be obtained.

[0117] Table 1

[0118]

[0119] As can be seen from the data in Table 1, the battery cell prepared by the negative electrode sheet in the application example has significantly improved energy density, kinetic performance and cycle performance.

[0120] Specifically, comparing Examples 1-3 with Comparative Examples 1 and 2, when the sphericity S2 of the second silicon material is <0.30, the surface active sites of the silicon material are significantly increased, the porosity of the upper layer increases after the electrode is rolled, and the fast charging performance is improved. However, the excessive active sites cause the side reactions between the silicon material and the electrolyte to intensify, leading to a deterioration in cycle performance and cell expansion. Moreover, the decrease in sphericity leads to a decrease in the compaction density of the silicon material, and the electrode cannot be pressed to the target compaction density, resulting in a decrease in the energy density of the cell. When the sphericity S2 of the second silicon material is >0.65, the surface active sites of the silicon material are reduced, the porosity of the upper layer decreases after the electrode is rolled, the fast charging performance deteriorates, and slight lithium plating occurs during 3C charging. The reduction in active sites reduces the side reactions between the silicon material and the electrolyte, and the cycle performance and cell expansion are improved.

[0121] Examples 1, 4, 5, 11 and Comparative Example 3 show that the gradient difference M1-M2 between the percentage of the first silicon material M1 in the first negative electrode active material layer and the percentage of the second silicon material M2 in the second negative electrode active material layer is controlled within a reasonable range. The resulting silicon negative electrode sheet and the battery cell containing the negative electrode sheet have relatively balanced energy density, fast charging performance, cycle performance and expansion performance.

[0122] Examples 1, 6, 7, 12, and 13 illustrate that a reasonable design of the first and second active layer mass ratios results in silicon anode sheets and cells containing these anode sheets with relatively balanced energy density, fast-charging performance, cycle performance, and expansion performance. When the first active layer mass ratio is <30%, the amount of compacted lower active material provided decreases. In this case, the electrode sheet cannot be rolled to the target compaction density, leading to increased electrode sheet thickness, decreased cell energy density, and deteriorated fast-charging performance. Furthermore, the overall proportion of the second silicon anode material in the electrode sheet increases, resulting in increased side reactions with the electrolyte, further worsening cycle performance and cell expansion. When the first active layer mass ratio is >70%, the overall silicon content of the electrode sheet increases, improving cell energy density. However, the overall proportion of the second silicon anode material in the electrode sheet decreases, leading to deteriorated fast-charging performance and slight lithium plating. This is because the increased proportion of the first silicon anode material in the electrode sheet improves the cell's cycle performance and expansion performance.

[0123] Examples 1, 8, 9, 14, and 15 illustrate that as the ratio of the first silicon material mass ratio M1 in the first active layer to the second silicon material mass ratio M2 in the second active layer increases, the energy density of the battery cell increases. However, the increased silicon content leads to a deterioration in the overall cycle performance and expansion performance of the battery cell. Therefore, the first silicon material mass ratio and the second silicon material mass ratio need to be rationally designed according to the actual battery cell design goals.

[0124] The comparison of Examples 1, 10 and Comparative Example 4 shows that as the sphericity S1 of the first silicon in the first active layer increases, the fast charging performance of the battery cell is less affected, but the overall structural stability of the negative electrode sheet is improved due to the improved structural stability of the silicon material, and the cycle performance and expansion performance of the battery cell are improved.

[0125] Comparative Example 5 shows that the first silicon material and the second silicon material are not coated in upper and lower layers, but are directly mixed with graphite and then coated in a single layer. The comparison between Example 1 and Comparative Example 5 illustrates that by designing the first silicon material and the second silicon material in upper and lower layers, the energy density, fast charging performance, cycle performance and expansion performance of the battery cell are significantly improved.

[0126] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized by, The negative electrode sheet comprises a current collector, a first negative electrode active material layer arranged on the surface of the current collector, and a second negative electrode active material layer arranged on the surface of the first negative electrode active material layer away from the current collector; The first negative electrode active material layer comprises a first silicon material, and the second negative electrode active material layer comprises a second silicon material; The sphericity S1 of the first silicon material is greater than or equal to 0.80, and the sphericity S2 of the second silicon material is in the range of 0.30-0.65; The first silicon material and the second silicon material satisfy the following relationship: M1≥M2, wherein M1 represents the mass percentage of the first silicon material in the first negative electrode active material layer, and M2 represents the mass percentage of the second silicon material in the second negative electrode active material layer; The first silicon material and the second silicon material satisfy the following relationship: 1%≤M1-M2≤3%; The mass percentage M1 of the first silicon material in the first negative electrode active material layer is in the range of 5%≤M1≤30%; The mass percentage M2 of the second silicon material in the second negative electrode active material layer is in the range of 0 The mass ratio of the first negative electrode active material layer to the second negative electrode active material layer is 1:0.42-2.

34.

2. The negative electrode sheet according to claim 1, characterized by The sphericity S1 of the first silicon material is in the range of 0.80-0.95, and the sphericity S2 of the second silicon material is in the range of 0.40-0.

60.

3. The negative electrode sheet according to claim 1, wherein The first silicon material and the second silicon material are each independently selected from at least one of SiO x , prelithiated silicon oxide, premagnesiated silicon oxide, silicon alloy, sand milled silicon carbon, and vapor deposited silicon carbon, wherein 0 < x < 2. The 5T powder compaction density of the first silicon material is in the range of 1.11-1.25 g / cc. The 5T powder compaction density of the second silicon material is in the range of 1.00-1.10 g / cc.

4. The negative electrode sheet according to claim 1, wherein The first negative electrode active material layer further comprises a first active material, and the second negative electrode active material layer further comprises a second active material.

5. The negative electrode sheet according to claim 4, characterized by The first active material is a first graphite material, and the second active material is a second graphite material.

6. The negative electrode sheet according to claim 5, characterized by The first graphite material and the second graphite material are each independently selected from artificial graphite and / or natural graphite.

7. A method for producing the negative electrode sheet according to any one of claims 1 to 6, characterized by The preparation method comprises the following steps: S1, preparing a first negative electrode active material layer slurry containing a first silicon material, coating the first negative electrode active material layer slurry on the surface of a current collector, and performing a first drying treatment to obtain a current collector with a first negative electrode active material layer arranged on the surface thereof; S2, preparing a second negative electrode active material layer slurry containing a second silicon material, coating the second negative electrode active material layer slurry on the surface of the first negative electrode active material layer, and performing a second drying treatment.

8. The preparation method according to claim 7, wherein The first negative electrode active material layer slurry further contains a first active material, a first conductive agent, a first binder, a first thickening agent, and a first negative electrode solvent, and the second negative electrode active material layer slurry further contains a second active material, a second conductive agent, a second binder, a second thickening agent, and a second negative electrode solvent. The first and second conductive agents are each independently selected from at least one of conductive carbon black, graphene, and carbon nanotube; The first and second binders are each independently selected from at least one of styrene butadiene rubber, polyacrylic acid, polyvinyl pyrrolidone, polyaniline, polyimide, polysiloxane, epoxy resin, polyester resin, and polyurethane resin; The first and second thickening agents are each independently selected from sodium carboxymethyl cellulose and / or lithium carboxymethyl cellulose; The first negative electrode solvent is water; The second negative electrode solvent is water.

9. A battery, characterized by The battery comprises a negative electrode sheet, and the negative electrode sheet comprises the negative electrode sheet according to any one of claims 1-6 and / or the negative electrode sheet prepared according to the preparation method of claim 7 or 8.

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