Multilayer structure negative electrode sheet and process for improving the electrochemical performance of wound structure batteries

Abstract

A multilayer negative electrode sheet and process for improving the electrochemical performance of a wound battery, comprising: a multilayer negative electrode sheet for improving the electrochemical performance of a wound battery, including a current collector, a first negative electrode layer near the negative electrode tab, a second negative electrode layer away from the negative electrode tab, and a third negative electrode layer on the second negative electrode layer; the conductivity of the first negative electrode layer is 2.56-2.85 S / cm, the conductivity of the second negative electrode layer is 0.75-1.32 S / cm, and the conductivity of the third negative electrode layer is 1.42-2.45 S / cm; the first negative electrode layer has a first... The negative electrode active material includes first graphite or a composite of first graphite and first silicon carbide; the second negative electrode active material of the second negative electrode layer includes second graphite or a composite of second graphite and second silicon carbide; the third negative electrode active material of the third negative electrode layer includes third graphite or a composite of third graphite and third silicon carbide; the compacted density of the first graphite 5T powder is 1.75-1.85, the compacted density of the second graphite 5T powder is 1.98-2.05, and the compacted density of the third graphite 5T powder is 1.88-1.96; a process for a multilayer structure negative electrode sheet.

Description

Technical Field

[0001] This invention relates to the technical field of negative electrode sheets and their preparation processes, and in particular to a multilayer structure negative electrode sheet and its process for improving the electrochemical performance of wound structure batteries. Background Technology

[0002] With the widespread application of lithium-ion batteries in mobile phones, watches and electric vehicles, the requirements for their energy density are also increasing. Existing battery systems can no longer meet people's growing demand for high energy density lithium-ion batteries.

[0003] Silicon-doped anodes are an effective means of improving battery energy density due to their high energy density, potential low cost, and broad application prospects.

[0004] However, conventional wound silicon-doped high-energy-density lithium-ion batteries encounter the following problems in practical applications:

[0005] 1. During charging, the current density near the negative electrode tab is relatively high, which can easily lead to adverse phenomena such as lithium plating and temperature rise. The deposited lithium can form dendrites, and the lithium dendrites and heat generation can easily damage the separator, causing the battery to short circuit, resulting in smoke, fire or even explosion, which poses a serious safety hazard. Therefore, it is necessary to suppress lithium plating on the negative electrode to ensure the safety of the battery.

[0006] Second, under high areal density and high compaction density, the electrolyte inside the electrode is not sufficiently and uniformly wetted, resulting in the active material of the negative electrode not being able to fully exert its potential. This easily leads to lithium plating and cycle failure during charge and discharge. Therefore, there is an urgent need for a new negative electrode technology that can simultaneously improve the energy density of conventional wound battery structures without reducing rate performance and cycle performance. Summary of the Invention

[0007] The purpose of this invention is to overcome the aforementioned deficiencies in the prior art and provide a multilayer negative electrode sheet and process for improving the electrochemical performance of wound battery structures, using a lower 5T powder compaction density range (1.75-1.85). The first graphite of the first negative electrode active material and the first negative electrode layer with a high conductivity range (2.56-2.85 S / cm) can greatly improve the kinetics of the negative electrode sheet near the tab and reduce lithium plating caused by conventional winding structures. A high 5T powder compaction density range (1.98-2.05) is selected. The use of graphite as the second negative electrode active material and a second negative electrode layer with a low conductivity range (0.75-1.32 S / cm) improves energy density and cycle performance. A medium-density 5T powder with a compaction density range of 1.88-1.96... The third graphite of the third negative electrode active material and the third negative electrode layer with a medium conductivity range (1.42-2.45 S / cm) enhance the kinetics of the negative electrode sheet, while ensuring high energy density and good cycle performance.

[0008] To achieve the above objectives, the present invention is accomplished through the following two aspects:

[0009] In a first aspect, the present invention provides a multilayer negative electrode sheet for improving the electrochemical performance of a wound structure battery, comprising a current collector, a first negative electrode layer coated on the current collector near the negative electrode tab, a second negative electrode layer coated on the current collector away from the negative electrode tab, and a third negative electrode layer coated on the second negative electrode layer, wherein the first negative electrode layer comprises a first negative electrode active material, the second negative electrode layer comprises a second negative electrode active material, and the third negative electrode layer comprises a third negative electrode active material;

[0010] The conductivity values ​​of the first negative electrode layer range from 2.56 to 2.85 S / cm, the conductivity values ​​of the second negative electrode layer range from 0.75 to 1.32 S / cm, and the conductivity values ​​of the third negative electrode layer range from 1.42 to 2.45 S / cm.

[0011] The first negative electrode active material includes first graphite or a composite of first graphite and first silicon carbide; the second negative electrode active material includes second graphite or a composite of second graphite and second silicon carbide; and the third negative electrode active material includes third graphite or a composite of third graphite and third silicon carbide.

[0012] The compacted density of the first graphite powder (5T) ranges from 1.75 to 1.85. The compacted density of the second graphite powder (5T) ranges from 1.98 to 2.05. The compacted density of the third graphite powder (5T) ranges from 1.88 to 1.96. ;

[0013] Preferably, the first negative electrode layer is close to the negative electrode tab and coated on both sides of the current collector, the second negative electrode layer is located at one end of the first negative electrode layer and away from the negative electrode tab and coated on both sides of the current collector, and the number of the third negative electrode layer is set to two, with the two third negative electrode layers corresponding one-to-one with the two second negative electrode layers.

[0014] On one side of the current collector, the distance between the first negative electrode layer and the negative electrode tab is set to L1, the coating length of the first negative electrode layer is set to L2, and the coating lengths of the second and third negative electrode layers are both set to L3.

[0015] On the other side of the current collector, the distance between the first negative electrode layer and the negative electrode tab is set to L4, the coating length of the first negative electrode layer is set to L5, and the coating lengths of the second and third negative electrode layers are both set to L6.

[0016] Preferably, on one side of the current collector, the coating length L2 of the first negative electrode layer and the coating length L3 of the second and third negative electrode layers satisfy the following ratio range: 5%≤L2 / (L2+L3)≤10%.

[0017] Preferably, on the other side of the current collector, the coating length L5 of the first negative electrode layer and the coating length L6 of the second and third negative electrode layers satisfy the following ratio range: 80%≤(L5+L6) / (L2+L3)≤95%.

[0018] Preferably, the first graphite, the second graphite, and the third graphite are all one or a mixture of two of artificial graphite and natural graphite.

[0019] Preferably, the volume median particle size (Dv50) of the first graphite is in the range of 4-8 μm; the volume median particle size (Dv50) of the second graphite is in the range of 13-17 μm; and the volume median particle size (Dv50) of the third graphite is in the range of 8.5-12.5 μm.

[0020] The volumetric median particle size (Dv50) of the first silicon carbon ranges from 3 to 6.5 μm; the volumetric median particle size (Dv50) of the second silicon carbon ranges from 9 to 11.5 μm; and the volumetric median particle size (Dv50) of the third silicon carbon ranges from 7 to 8.5 μm.

[0021] The specific surface area of ​​the first silicon-carbon alloy ranges from 4 to 5.5. The specific surface area of ​​the second silicon-carbon compound ranges from 1.0 to 2.5. The specific surface area of ​​the third silicon-carbon alloy ranges from 2.6 to 3.8. .

[0022] Preferably, the first negative electrode layer, the second negative electrode layer, and the third negative electrode layer all further include a conductive agent, a thickener, and a binder.

[0023] Preferably, the conductive agent of the first negative electrode layer, the conductive agent of the second negative electrode layer, and the conductive agent of the third negative electrode layer are all one or a mixture of two or more of conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0024] The thickeners of the first negative electrode layer, the second negative electrode layer, and the third negative electrode layer are all one or a mixture of two of CMC-Na (carboxymethyl cellulose-sodium) and CMC-Li (carboxymethyl cellulose-lithium);

[0025] The adhesives for the first negative electrode layer, the second negative electrode layer, and the third negative electrode layer are all one or a mixture of two or more of the following: styrene-butadiene rubber, styrene-acrylic rubber, polyurethane, and polyacrylic acid.

[0026] Preferably, when the first negative electrode active material of the first negative electrode layer is a composite of first graphite and first silicon carbon, the mass percentage of each material in the first negative electrode layer is as follows:

[0027] The proportion of graphite in the first negative electrode active material ranges from 57% to 99%.

[0028] The proportion of silicon-carbon in the first negative electrode active material ranges from 0% to 25%.

[0029] The proportion of conductive agent is 0-4%;

[0030] Thickener ratio range: 0-4%;

[0031] Adhesive ratio range: 1-10%;

[0032] When the second negative electrode active material of the second negative electrode layer is a composite of second graphite and second silicon carbon, the mass percentage of each substance in the second negative electrode layer is as follows:

[0033] The proportion of second graphite in the second negative electrode active material ranges from 0% to 99%.

[0034] The proportion of silicon-carbon in the second negative electrode active material ranges from 0% to 99%.

[0035] The proportion of conductive agent is 0-4%;

[0036] Thickener ratio range: 0-4%;

[0037] Adhesive ratio range: 1-10%;

[0038] When the third negative electrode active material of the third negative electrode layer is a composite of third graphite and third silicon carbon, the mass percentage of each substance in the third negative electrode layer is as follows:

[0039] The proportion of third graphite in the third negative electrode active material ranges from 50% to 99%.

[0040] The proportion of silicon-carbon in the third anode active material ranges from 0% to 50%.

[0041] The proportion of conductive agent is 0-4%;

[0042] Thickener ratio range: 0-4%;

[0043] Adhesive ratio range: 1-10%.

[0044] Secondly, the present invention provides a process for improving the electrochemical performance of a multilayer negative electrode sheet for a wound-structured battery, which is used to prepare a multilayer negative electrode sheet for improving the electrochemical performance of a wound-structured battery as described in the first aspect, and the preparation steps are as follows:

[0045] Step S1: Preparation of the first negative electrode slurry: The first graphite of the first negative electrode active material, the first silicon carbide of the first negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the first negative electrode slurry;

[0046] The proportions of the above substances are as follows:

[0047] The proportion of graphite in the first negative electrode active material ranges from 57% to 99%.

[0048] The proportion of silicon-carbon in the first negative electrode active material ranges from 0% to 25%.

[0049] The proportion of conductive agent is 0-4%;

[0050] Thickener ratio range: 0-4%;

[0051] Adhesive ratio range: 1-10%;

[0052] Step S2: Preparation of the second negative electrode slurry: The second graphite of the second negative electrode active material, the second silicon carbide of the second negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the second negative electrode slurry;

[0053] The proportions of the above substances are as follows:

[0054] The proportion of second graphite in the second negative electrode active material ranges from 0% to 99%.

[0055] The proportion of silicon-carbon in the second negative electrode active material ranges from 0% to 99%.

[0056] The proportion of conductive agent is 0-4%;

[0057] Thickener ratio range: 0-4%;

[0058] Adhesive ratio range: 1-10%;

[0059] Step S3: Preparation of the third negative electrode slurry: The third graphite of the third negative electrode active material, the third silicon carbide of the third negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the third negative electrode slurry;

[0060] The proportions of the above substances are as follows:

[0061] The proportion of third graphite in the third negative electrode active material ranges from 50% to 99%.

[0062] The proportion of silicon-carbon in the third anode active material ranges from 0% to 50%.

[0063] The proportion of conductive agent is 0-4%;

[0064] Thickener ratio range: 0-4%;

[0065] Adhesive ratio range: 1-10%;

[0066] Step S4: Coating is performed on both sides of the current collector using a three-layer extrusion coating die. Specifically, the coating areas are as follows: the first negative electrode slurry is coated on region 1 near the negative electrode tab; the second negative electrode slurry is coated on region 2 away from the negative electrode tab; and the third negative electrode slurry is coated on region 3 of the second negative electrode slurry. After drying in an oven, the other side of the current collector is coated again: the first negative electrode slurry is coated on region 4 near the negative electrode tab; the second negative electrode slurry is coated on region 5 away from the negative electrode tab; and the third negative electrode slurry is coated on region 5 of the second negative electrode slurry. In region 6 of the negative electrode slurry, the specific coating lengths are as follows: on the side of the negative electrode tab along the length of the current collector, the length of the empty foil region is L1, the length of region 1 is L2, and the lengths of regions 2 and 3 are L3; on the other side of the negative electrode tab along the length of the current collector, the length of the empty foil region is L4, the length of region 4 is L5, and the lengths of regions 5 and 6 are L6, wherein 5%≤L2 / (L2+L3)≤10%, and 80%≤(L5+L6) / (L2+L3)≤95%;

[0067] Step S5: After rolling and slitting processes, a multi-layer negative electrode sheet is obtained.

[0068] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0069] 1. The present invention is provided with a current collector, a first negative electrode layer is coated on the current collector and close to the negative electrode tab, a second negative electrode layer is coated on the current collector and away from the negative electrode tab, and a third negative electrode layer is coated on the second negative electrode layer.

[0070] Meanwhile, the conductivity of the first negative electrode layer > the conductivity of the third negative electrode layer > the conductivity of the second negative electrode layer, which makes the first negative electrode layer the highest in conductivity ranking, the third negative electrode layer in the middle, and the second negative electrode layer the lowest.

[0071] The 5T powder compaction density value of the first graphite in the first negative electrode layer is less than the 5T powder compaction density value of the third graphite in the third negative electrode layer, which is less than the 5T powder compaction density value of the second graphite in the second negative electrode layer. This results in the ranking of 5T powder compaction density values: the first graphite has the lowest compaction density, the third graphite is in the middle, and the second graphite has the highest compaction density.

[0072] Therefore, a lower 5T powder compaction density range (1.75-1.85) is selected near the negative electrode tab. The first graphite of the first negative electrode active material and the first negative electrode layer with a high conductivity range (2.56-2.85 S / cm) can greatly improve the kinetics of the negative electrode sheet near the tab and reduce lithium plating caused by conventional winding structures. A high 5T powder compaction density range (1.98-2.05) is selected. The use of graphite as the second negative electrode active material and a second negative electrode layer with a low conductivity range (0.75-1.32 S / cm) improves energy density and cycle performance. A medium-density 5T powder with a compaction density range of 1.88-1.96... The third graphite of the third negative electrode active material and the third negative electrode layer with a medium conductivity range (1.42-2.45 S / cm) enhance the kinetics of the negative electrode sheet, while ensuring high energy density and good cycle performance.

[0073] 2. In summary, the present invention provides a multilayer structure negative electrode sheet for improving the electrochemical performance of wound structure batteries. This avoids the lithium deposition at the negative electrode tab caused by conventional wound structures, and at the same time enables the negative electrode sheet to achieve both improved rate performance and cycle capacity of lithium-ion batteries under high energy density system design. Attached Figure Description

[0074] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0075] Figure 1 This is a schematic diagram of step S4 provided in Embodiment 2 of the present invention: coating is performed on both sides of the current collector by a three-layer extrusion coating die;

[0076] Figure 2 This is a schematic diagram of the structure of a multilayer negative electrode sheet for improving the electrochemical performance of a wound battery, provided in Embodiment 1 of the present invention;

[0077] Figure 3 This is a graph showing the test results of battery cycle performance testing for Experimental Example 1 and Comparative Example 1 provided in Embodiment 3 of the present invention;

[0078] Figure 4 This is a test result diagram of the surface of region 3 after the lithium plating test of the battery provided in Experiment Example 1 of Embodiment 3 of the present invention;

[0079] Figure 5 This is a test result diagram of the surface of region 3 after the lithium plating test of Comparative Example 2 provided in Embodiment 3 of the present invention. Detailed Implementation

[0080] The technical solution of this embodiment of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiment is one embodiment of the present invention, and not all embodiments thereof. Based on this embodiment of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0081] Example 1

[0082] Please see Figure 2 Embodiment 1 of the present invention provides a multilayer structure negative electrode sheet for improving the electrochemical performance of a wound structure battery, including a current collector, a first negative electrode layer coated on the current collector near the negative electrode tab, a second negative electrode layer coated on the current collector away from the negative electrode tab, and a third negative electrode layer coated on the second negative electrode layer. The first negative electrode layer includes a first negative electrode active material, the second negative electrode layer includes a second negative electrode active material, and the third negative electrode layer includes a third negative electrode active material.

[0083] The first negative electrode active material includes a composite of first graphite and first silicon carbide; the second negative electrode active material includes second graphite or a composite of second graphite and second silicon carbide; and the third negative electrode active material includes third graphite or a composite of third graphite and third silicon carbide.

[0084] The conductivity values ​​of the first negative electrode layer range from 2.56 to 2.85 S / cm, the conductivity values ​​of the second negative electrode layer range from 0.75 to 1.32 S / cm, and the conductivity values ​​of the third negative electrode layer range from 1.42 to 2.45 S / cm.

[0085] Therefore, the conductivity of the first negative electrode layer > the conductivity of the third negative electrode layer > the conductivity of the second negative electrode layer. In the ranking of conductivity values, the second negative electrode layer is the lowest, the third negative electrode layer is in the middle, and the first negative electrode layer is the highest.

[0086] The compacted density of the first graphite powder (5T) ranges from 1.75 to 1.85. The compacted density of the third graphite powder (5T) ranges from 1.88 to 1.96. The compacted density of the second graphite powder (5T) ranges from 1.98 to 2.05. Therefore, the compaction density of the first graphite in the first negative electrode layer is less than that of the third graphite in the third negative electrode layer, which is less than that of the second graphite in the second negative electrode layer. In the ranking of compaction density values ​​of 5T powder, the first graphite is the lowest, the third graphite is in the middle, and the second graphite is the highest.

[0087] The first graphite, the second graphite, and the third graphite are all one or a mixture of two types of artificial graphite and natural graphite.

[0088] The volumetric median particle size (Dv50) of the first graphite is 4-8 μm; the volumetric median particle size (Dv50) of the second graphite is 13-17 μm; and the volumetric median particle size (Dv50) of the third graphite is 8.5-12.5 μm.

[0089] The volumetric median particle size (Dv50) of the first silicon carbon ranges from 3 to 6.5 μm; the volumetric median particle size (Dv50) of the second silicon carbon ranges from 9 to 11.5 μm; and the volumetric median particle size (Dv50) of the third silicon carbon ranges from 7 to 8.5 μm.

[0090] The specific surface area of ​​the first silicon-carbon alloy ranges from 4 to 5.5. The specific surface area of ​​the second silicon-carbon compound ranges from 1.0 to 2.5. The specific surface area of ​​the third silicon-carbon alloy ranges from 2.6 to 3.8. .

[0091] The first negative electrode layer, the second negative electrode layer, and the third negative electrode layer all further include a conductive agent, a thickener, and a binder.

[0092] The conductive agent of the first negative electrode layer, the conductive agent of the second negative electrode layer, and the conductive agent of the third negative electrode layer are all one or a mixture of two or more of conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes;

[0093] The thickeners of the first negative electrode layer, the second negative electrode layer, and the third negative electrode layer are all one or a mixture of two of CMC-Na (carboxymethyl cellulose-sodium) and CMC-Li (carboxymethyl cellulose-lithium);

[0094] The adhesives for the first negative electrode layer, the second negative electrode layer, and the third negative electrode layer are all one or a mixture of two or more of the following: styrene-butadiene rubber, styrene-acrylic rubber, polyurethane, and polyacrylic acid.

[0095] When the first negative electrode active material of the first negative electrode layer is a composite of first graphite and first silicon carbon, the mass percentage of each material in the first negative electrode layer is as follows:

[0096] The proportion of graphite in the first negative electrode active material ranges from 57% to 99%.

[0097] The proportion of silicon-carbon in the first negative electrode active material ranges from 0% to 25%.

[0098] The proportion of conductive agent is 0-4%;

[0099] Thickener ratio range: 0-4%;

[0100] Adhesive ratio range: 1-10%;

[0101] When the second negative electrode active material of the second negative electrode layer is a composite of second graphite and second silicon carbon, the mass percentage of each substance in the second negative electrode layer is as follows:

[0102] The proportion of second graphite in the second negative electrode active material ranges from 0% to 99%.

[0103] The proportion of silicon-carbon in the second negative electrode active material ranges from 0% to 99%.

[0104] The proportion of conductive agent is 0-4%;

[0105] Thickener ratio range: 0-4%;

[0106] Adhesive ratio range: 1-10%;

[0107] When the third negative electrode active material of the third negative electrode layer is a composite of third graphite and third silicon carbon, the mass percentage of each substance in the third negative electrode layer is as follows:

[0108] The proportion of third graphite in the third negative electrode active material ranges from 50% to 99%.

[0109] The proportion of silicon-carbon in the third anode active material ranges from 0% to 50%.

[0110] The proportion of conductive agent is 0-4%;

[0111] Thickener ratio range: 0-4%;

[0112] Adhesive ratio range: 1-10%.

[0113] like Figure 2 As shown, the first negative electrode layer is close to the negative electrode tab and is coated on both sides of the current collector. The second negative electrode layer is located at one end of the first negative electrode layer and away from the negative electrode tab and is coated on both sides of the current collector. The number of the third negative electrode layers is set to two, and the two third negative electrode layers correspond one-to-one with the two second negative electrode layers.

[0114] On one side of the current collector, the distance between the first negative electrode layer and the negative electrode tab is set to L1, the coating length of the first negative electrode layer is set to L2, and the coating lengths of the second and third negative electrode layers are both set to L3.

[0115] On the other side of the current collector, the distance between the first negative electrode layer and the negative electrode tab is set to L4, the coating length of the first negative electrode layer is set to L5, and the coating lengths of the second and third negative electrode layers are both set to L6.

[0116] Wherein, on one side of the current collector, the coating length L2 of the first negative electrode layer, the coating length L3 of the second negative electrode layer and the third negative electrode layer satisfy the following ratio range: 5%≤L2 / (L2+L3)≤10%.

[0117] On the other side of the current collector, the coating length L5 of the first negative electrode layer and the coating length L6 of the second and third negative electrode layers satisfy the following ratio range: 80%≤(L5+L6) / (L2+L3)≤95%.

[0118] The first embodiment of the present invention provides a multilayer negative electrode sheet for improving the electrochemical performance of a wound battery. Its advantages include: the first negative electrode layer near the negative electrode tab has the lowest compaction density (1.75-1.85) due to the use of 5T powder. The first graphite layer has a high conductivity (2.56-2.85 S / cm) and combines the optimal coating length ratio of 5%≤L2 / (L2+L3)≤10%, which can greatly improve the dynamics of the negative electrode near the tab and greatly improve the speed of lithium ion insertion and extraction, thereby avoiding lithium plating at the negative tab caused by conventional winding structure.

[0119] The second and third negative electrode layers, located further away from the negative electrode tab, exhibit the following conductivity along the current collector -> second negative electrode layer -> third negative electrode layer direction: the second negative electrode layer has the lowest conductivity (0.75-1.32 S / cm), and due to the use of 5T powder compaction, it has the highest density (1.98-2.05). The second graphite layer ensures improved energy density and cycle stability of the lithium-ion battery. The third anode layer has moderate conductivity (1.42-2.45 S / cm), and due to the use of 5T compaction, its density is moderate (1.88-1.96 S / cm). The third graphite in the negative electrode improves the kinetics of the negative electrode, reduces lithium plating, and at the same time ensures high energy density and good cycle performance.

[0120] Example 2

[0121] Embodiment 2 of the present invention provides a process for improving the electrochemical performance of a multilayer negative electrode sheet for a wound-structured battery. This process is used to prepare a multilayer negative electrode sheet for improving the electrochemical performance of a wound-structured battery as described in Embodiment 1. The preparation steps are as follows:

[0122] Step S1: Preparation of the first negative electrode slurry: The first graphite of the first negative electrode active material, the first silicon carbide of the first negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the first negative electrode slurry;

[0123] The proportions of the above substances are as follows:

[0124] The proportion of graphite in the first negative electrode active material ranges from 57% to 99%.

[0125] The proportion of silicon-carbon in the first negative electrode active material ranges from 0% to 25%.

[0126] The proportion of conductive agent is 0-4%;

[0127] Thickener ratio range: 0-4%;

[0128] Adhesive ratio range: 1-10%;

[0129] Step S2: Preparation of the second negative electrode slurry: The second graphite of the second negative electrode active material, the second silicon carbide of the second negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the second negative electrode slurry;

[0130] The proportions of the above substances are as follows:

[0131] The proportion of second graphite in the second negative electrode active material ranges from 0% to 99%.

[0132] The proportion of silicon-carbon in the second negative electrode active material ranges from 0% to 99%.

[0133] The proportion of conductive agent is 0-4%;

[0134] Thickener ratio range: 0-4%;

[0135] Adhesive ratio range: 1-10%;

[0136] Step S3: Preparation of the third negative electrode slurry: The third graphite of the third negative electrode active material, the third silicon carbide of the third negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the third negative electrode slurry;

[0137] The proportions of the above substances are as follows:

[0138] The proportion of third graphite in the third negative electrode active material ranges from 50% to 99%.

[0139] The proportion of silicon-carbon in the third anode active material ranges from 0% to 50%.

[0140] The proportion of conductive agent is 0-4%;

[0141] Thickener ratio range: 0-4%;

[0142] Adhesive ratio range: 1-10%;

[0143] Step S4: Apply coating through a three-layer extrusion coating die (e.g., Figure 1 Coating is applied to both sides of the current collector, with the coating area and length as shown below. Figure 2 As shown. The specific coating areas are as follows: the first negative electrode slurry is coated in region 1 near the negative electrode tab, the second negative electrode slurry is coated in region 2 away from the negative electrode tab, and the third negative electrode slurry is coated in region 3 on the second negative electrode slurry. After drying in an oven, the other side of the current collector is coated again: the first negative electrode slurry is coated in region 4 near the negative electrode tab, the second negative electrode slurry is coated in region 5 away from the negative electrode tab, and the third negative electrode slurry is coated in region 6 on the second negative electrode slurry. The specific coating length is: along the length of the current collector on the side with the edge where the negative electrode tab is located, the empty foil... The length of region 1 is L1, the length of region 2 is L2, the length of region 3 is L3, and the length of empty foil region is L4, the length of region 4 is L5, and the length of region 5 and region 6 is L6 along the length of the current collector on the other side of the negative electrode tab. Specifically, the first negative electrode layer is close to the negative electrode tab and coated on both sides of the current collector, the second negative electrode layer is located at one end of the first negative electrode layer and away from the negative electrode tab and coated on both sides of the current collector, and the number of third negative electrode layers is set to two, with the two third negative electrode layers corresponding one-to-one with the two second negative electrode layers.

[0144] On one side of the current collector, the distance between the first negative electrode layer and the negative electrode tab is set to L1, the coating length of the first negative electrode layer is set to L2, and the coating lengths of the second and third negative electrode layers are both set to L3.

[0145] On the other side of the current collector, the distance between the first negative electrode layer and the negative electrode tab is set to L4, the coating length of the first negative electrode layer is set to L5, and the coating lengths of the second and third negative electrode layers are both set to L6.

[0146] On one side of the current collector, the coating length L2 of the first negative electrode layer and the coating length L3 of the second and third negative electrode layers satisfy the following ratio range: 5%≤L2 / (L2+L3)≤10%;

[0147] On the other side of the current collector, the coating length L5 of the first negative electrode layer and the coating length L6 of the second and third negative electrode layers satisfy the following ratio range: 80%≤(L5+L6) / (L2+L3)≤95%;

[0148] Step S5: After rolling and slitting processes, a multi-layer negative electrode sheet is obtained.

[0149] Specifically, in the multilayer negative electrode sheet prepared in step S5 above, the conductivity of the first negative electrode layer ranges from 2.56 to 2.85 S / cm, the conductivity of the second negative electrode layer ranges from 0.75 to 1.32 S / cm, the conductivity of the third negative electrode layer ranges from 1.42 to 2.45 S / cm, and the compacted density of the first graphite powder (5T) ranges from 1.75 to 1.85. The compacted density of the second graphite powder (5T) ranges from 1.98 to 2.05. The compacted density of the third graphite powder (5T) ranges from 1.88 to 1.96. .

[0150] Example 3

[0151] Embodiment 3 of the present invention provides experimental data for battery lithium plating test, galvanostatic intermittent titration (GITT) test and battery cycle performance test, respectively.

[0152] The experimental data in this third embodiment used 15 experimental cases and 15 comparative examples of lithium-ion batteries, among which:

[0153] (1). Regarding Experimental Example 1-15:

[0154] The negative electrode sheets in these 15 experimental examples were prepared using a multi-layer negative electrode sheet process described in Example 2 to improve the electrochemical performance of wound-structured batteries. The lithium-ion batteries were then prepared using existing techniques involving positive electrode sheet preparation -> negative electrode sheet -> separator -> positive electrode sheet winding -> liquid injection -> encapsulation -> formation -> capacity testing. All 15 experimental examples used the multi-layer negative electrode sheet structure described in Example 1 to improve the electrochemical performance of wound-structured batteries. The differences between these 15 examples are as follows: Based on Example 1, following the single-variable method, the conductivity values ​​or 5T powder compaction density values ​​of the first, second, or third negative electrode layers in Examples 2 to 15 are different from those in Example 1; or, on one side of the current collector, the specific ratio L2 / (L2+L3) of the coating length L2 of the first negative electrode layer and the coating length L3 of the second and third negative electrode layers in Examples 2 to 15 are different from those in Example 1.

[0155] (For specific differences, please see Table 1 mentioned below);

[0156] This document only discloses a more specific process for improving the electrochemical performance of a multilayer negative electrode sheet used in Experiment 1 (the preparation processes for Experiments 2-15 are the same as those for Experiment 1, except that the conductivity values ​​of the first, second, or third negative electrode layers or the 5T powder compaction density value of the active material graphite are different from those in Experiment 1, or the specific ratio value L2 / (L2+L3) of the coating length L2 of the first negative electrode layer and the coating length L3 of the second and third negative electrode layers on one side of the current collector is different from that in Experiment 1, which will not be repeated here):

[0157] Step S1: Preparation of the first negative electrode slurry: The first graphite of the first negative electrode active material, the first silicon carbide of the first negative electrode active material, the conductive agent, the thickener, and the binder are mixed together, a solvent (deionized water) is added, and the mixture is stirred with a vacuum mixer until it is uniformly mixed to prepare the first negative electrode slurry.

[0158] Step S2: Preparation of the second negative electrode slurry: The second graphite of the second negative electrode active material, the second silicon carbide of the second negative electrode active material, the conductive agent, the thickener, and the binder are mixed together, a solvent (deionized water) is added, and the mixture is stirred with a vacuum mixer until it is uniformly mixed to prepare the second negative electrode slurry.

[0159] The compacted density of the second graphite powder (5T) is 2.01. ;

[0160] Step S3: Preparation of the third negative electrode slurry: The third graphite of the third negative electrode active material, the third silicon carbide of the third negative electrode active material, the conductive agent, the thickener, and the binder are mixed together, a solvent (deionized water) is added, and the mixture is stirred with a vacuum mixer until it is uniformly mixed to prepare the third negative electrode slurry.

[0161] The compacted density of the 5T powder of the third graphite is 1.88. ;

[0162] In steps S1, S2 and S3 above, the 5T powder compaction density value of the first graphite is less than the 5T powder compaction density value of the third graphite and the 5T powder compaction density value of the second graphite.

[0163] Step S4: Apply coating through a three-layer extrusion coating die (e.g., Figure 1 Coating is applied to both sides of the current collector, with the coating area and length as shown below. Figure 2As shown. The specific coating areas are as follows: the first negative electrode slurry is coated in region 1 near the negative electrode tab, the second negative electrode slurry is coated in region 2 away from the negative electrode tab, and the third negative electrode slurry is coated in region 3 on the second negative electrode slurry. After drying in an oven, the other side of the current collector is coated again. The first negative electrode slurry is coated in region 4 near the negative electrode tab, the second negative electrode slurry is coated in region 5 away from the negative electrode tab, and the third negative electrode slurry is coated in region 6 on the second negative electrode slurry. The specific coating lengths are as follows: on the side with the edge of the negative electrode tab along the length of the current collector, the length of the empty foil area is L1, the length of region 1 is L2, and the length of regions 2 and 3 is L3. On the other side with the edge of the negative electrode tab along the length of the current collector, the length of the empty foil area is L4, the length of region 4 is L5, and the length of regions 5 and 6 is L6. Wherein, L2 / (L2+L3)%=7%;

[0164] Step S5: After cold pressing and slitting, a multi-layer negative electrode sheet is obtained.

[0165] In the multilayer negative electrode sheet prepared in step S5 above, the conductivity of the first negative electrode layer is 2.72 S / cm, the conductivity of the second negative electrode layer is 1.25 S / cm, and the conductivity of the third negative electrode layer is 2.16 S / cm.

[0166] (2). Regarding Comparative Examples 1-15:

[0167] The negative electrode sheets of Comparative Examples 1 to 15 were prepared using a process similar to that of Example 2 for improving the electrochemical performance of a wound-structure battery with a multilayer negative electrode sheet. Lithium-ion batteries were then fabricated using existing technology. The preparation processes of the negative electrode sheets of Comparative Examples 1 to 15 were identical to those of Example 2, except that the preparation processes of Comparative Examples 1 to 15 did not follow the order of conductivity of the first negative electrode layer > conductivity of the third negative electrode layer > conductivity of the second negative electrode layer in the process of Example 2. The following conditions must be met: the conductivity of the negative electrode layer is not within the range of 2.56-2.85 S / cm; the conductivity of the second negative electrode layer is not within the range of 0.75-1.32 S / cm; the conductivity of the third negative electrode layer is not within the range of 1.42-2.45 S / cm; the 5T powder compaction density of the first graphite layer is less than that of the third graphite layer, which is less than that of the second graphite layer; or the 5T powder compaction density of the first graphite layer is not between 1.75-1.85 S / cm. The value range is specified, or the compacted density value of the third graphite 5T powder is not between 1.88 and 1.96. The value range is specified, or the compacted density value of the second graphite powder (5T) is not between 1.98 and 2.05. Within the range of values ​​or on one side of the current collector, the coating length L2 of the first negative electrode layer, and the coating length L3 of the second and third negative electrode layers do not meet the following ratio range: 5%≤L2 / (L2+L3)≤10%;

[0168] Furthermore, the 5T powder compaction density value of the third graphite in Comparative Example 1 is lower than the above-mentioned range, the 5T powder compaction density value of the third graphite in Comparative Example 2 is higher than the above-mentioned range, the 5T powder compaction density value of the second graphite in Comparative Example 3 is lower than the above-mentioned range, the 5T powder compaction density value of the second graphite in Comparative Example 4 is higher than the above-mentioned range, the 5T powder compaction density value of the first graphite in Comparative Example 5 is lower than the above-mentioned range, and the 5T powder compaction density value of the first graphite in Comparative Example 6 is higher than the above-mentioned range.

[0169] Furthermore, the conductivity of the third negative electrode layer in Comparative Example 7 is lower than the above-mentioned range, the conductivity of the third negative electrode layer in Comparative Example 8 is higher than the above-mentioned range, the conductivity of the second negative electrode layer in Comparative Example 9 is lower than the above-mentioned range, the conductivity of the second negative electrode layer in Comparative Example 10 is higher than the above-mentioned range, the conductivity of the first negative electrode layer in Comparative Example 11 is lower than the above-mentioned range, and the conductivity of the first negative electrode layer in Comparative Example 12 is higher than the above-mentioned range.

[0170] Ranking of compaction density values ​​of 5T powder of the first graphite in Comparative Example 13 = 5T powder compaction density value of the third graphite = 5T powder compaction density value of the second graphite.

[0171] Comparative Example 14: L2 / (L2+L3) = 2% ≤ 5%;

[0172] Comparative Example 15: L2 / (L2+L3) = 13% ≥ 10%.

[0173] (Please see Table 1 below for specific differences)

[0174] (3). The conductivity of the first, second, and third negative electrode layers in Experimental Examples 1-15 and Comparative Examples 1-15, the specific parameters of the first graphite of the first negative electrode active material, the second graphite of the second negative electrode active material, and the third graphite of the third negative electrode active material, and the proportion of L2 to the total length of L2 and L3 are shown in Table 1 below:

[0175] Table 1

[0176]

[0177] The specific test methods for the battery lithium plating test, the galvanostatic intermittent titration (GITT) test, and the battery cycle performance test in this embodiment are as follows:

[0178] (1) Battery lithium plating test:

[0179] At 25°C, the lithium-ion batteries prepared in Experimental Examples 1-15 and Comparative Examples 1-15 were charged at a rate of 2C and discharged at a rate of 1C 20 times. Then, the fully charged batteries were disassembled to observe the lithium deposition on the negative electrode.

[0180] (2) Intermittent titration with constant current (GITT) test:

[0181] At 0°C, the lithium-ion batteries prepared in the experimental and comparative examples were charged at a constant current rate of 2C. The charging time was set so that the battery was paused for 3 seconds after each 1% state of charge (SOC) was reached, repeating this process until the battery reached the set cutoff voltage. The impedance was calculated using the voltage and charging current after the 3-second pause at each 1% SOC, and the impedance was plotted against the state of charge to obtain the impedance-state-of-charge curve. If the battery's impedance-state-of-charge curve shows a decreasing inflection point at the end of charging, it indicates that lithium plating has begun, and the state of charge corresponding to the inflection point is the SOC value for lithium plating.

[0182] (3) Battery cycle performance test

[0183] At 25°C, the lithium-ion batteries prepared in the experimental example and the comparative example were charged at a rate of 1.5C and discharged at a rate of 1C for 500 cycles, and the capacity retention rate of the lithium-ion batteries was recorded.

[0184] The test results of battery lithium plating test, galvanostatic intermittent titration (GITT) test, and battery cycle performance test for Experimental Examples 1-15 and Comparative Examples 1-15 in this Example 3 are shown in Table 2 below:

[0185] Table 2

[0186]

[0187] The results of the battery lithium plating test, galvanostatic intermittent titration (GITT) test, and battery cycle performance test conducted on Experimental Examples 1-15 and Comparative Examples 1-15 in this third embodiment are as follows:

[0188] When the compacted density of the third graphite powder of the third negative electrode active material is less than 1.88... (As in Comparative Example 1) or the compacted density of the second graphite powder of the second negative electrode active material is less than 1.98. (As in Comparative Example 3) or the compacted density of the first graphite powder of the first negative electrode active material is less than 1.75. When the conductivity of the third negative electrode layer is greater than 2.45 S / cm (as in Comparative Example 8), or the conductivity of the second negative electrode layer is greater than 1.32 S / cm (as in Comparative Example 10), or the conductivity of the first negative electrode layer is greater than 2.85 S / cm (as in Comparative Example 12), although lithium plating will not occur on the surfaces of regions 1 / 4 and 3 / 6, and the SOC value of the battery will not decrease, the capacity retention rate after 500 cycles at room temperature will be lower (e.g., ...). Figure 3 ).

[0189] When the compacted density of the third graphite powder of the third negative electrode active material is greater than 1.96... (As in Comparative Example 2) or the compacted density of the second graphite powder of the second negative electrode active material is greater than 2.05. (As in Comparative Example 4) or when the conductivity of the third negative electrode layer is less than 1.42 S / cm (as in Comparative Example 7) or the conductivity of the second negative electrode layer is less than 0.75 S / cm (as in Comparative Example 9), lithium plating will occur on the surface of region 3 / region 6 of the negative electrode sheet (e.g. Figure 5 (The darker areas in the image represent lithium plating). Lithium plating in the battery leads to a lower SOC value and lower capacity retention after 500 cycles at room temperature. If the compacted density of the first graphite powder of the first negative electrode active material is greater than 1.85... When the conductivity of the first negative electrode layer is less than 2.56 S / cm (as in Comparative Example 6), lithium plating will occur on the surface of region 1 / region 4 of the negative electrode sheet, that is, lithium plating will occur near the negative electrode tab, which will affect the SOC value and cycle performance of the lithium-ion battery.

[0190] As can be seen from Experimental Examples 1-13 and Comparative Examples 1-13, only when the 5T powder compaction density of the first active material (first graphite), the 5T powder compaction density of the second active material (second graphite), and the 5T powder compaction density of the third active material (third graphite) are respectively between 1.75 and 1.85... 1.98-2.05 1.88-1.96 For a battery to simultaneously possess the property of not plating lithium (e.g., within the specified range of values, and with the conductivity of the first negative electrode layer, the second negative electrode layer, and the third negative electrode layer respectively falling within the ranges of 2.56-2.85 S / cm, 0.75-1.32 S / cm, and 1.42-2.45 S / cm), it must also fall within the specified range of values. Figure 4 It exhibits excellent electrochemical performance, including a high lithium plating SOC value and high cycle capacity retention.

[0191] To reduce lithium plating near the tab caused by conventional winding structures, the first negative electrode layer uses a lower 5T powder compaction density range (1.75-1.85) compared to the second and third negative electrode layers. The first layer of graphite is the graphite. This is because the lower the 5T powder compaction density of graphite, the harder it is, and the more pressure resistant it is. This means that there are more pores inside the negative electrode sheet, which can enhance the absorption capacity of electrolyte and increase the number of lithium-ion channels. This can improve the speed of lithium-ion insertion or extraction and reduce lithium plating at the tab. Although a lower 5T powder compaction density of graphite can improve kinetics and reduce lithium plating at the negative electrode tab, if the 5T powder compaction density of graphite in the first, second, and third negative electrode layers is relatively low (as in Comparative Example 13), it will lead to poor contact between active materials or between active materials and conductive agents over a large area in the later stages of cycling. This will increase resistance, reduce charge and discharge efficiency, and result in a larger capacity loss after 500 cycles. Moreover, a lower 5T powder compaction density will reduce the overall compaction density of the negative electrode sheet, which is not conducive to improving the energy density of lithium-ion batteries. Therefore, graphite with a lower 5T powder compaction density is not suitable for the second and third negative electrode layers.

[0192] To improve the battery's energy density and cycle performance, the second negative electrode layer uses second graphite (1.98-2.05 g / L) with a higher powder compaction density range than the first and third negative electrode layers (5T). The higher the 5T powder compaction density of the active material graphite, the lower the hardness of graphite, which can increase the compaction density of the electrode, thereby increasing the energy density. Moreover, it can ensure close contact between active materials or between active materials and conductive agents during cycling, thus improving the cycle performance of lithium-ion batteries.

[0193] To reduce lithium plating on the surfaces of regions 3 and 6 without significantly lowering energy density, while maintaining good cycle performance and other electrochemical properties, the third anode layer is selected from graphite with a powder compaction density in the middle range of 5T (1.88-1.96 ppm) compared to the first and second anode layers. ).

[0194] In graphite-doped silicon negative electrode sheets, because graphite itself has relatively high conductivity, the influence of different types and contents of graphite on the conductivity of the electrode sheet is relatively small. Therefore, negative electrode sheets with different conductivity values ​​are generally prepared by selecting different contents of silicon-carbon, conductive agents, and binders. In Experiment 1-15 and Comparative Example 1-15 of this experiment, negative electrode sheets with different conductivity values ​​were prepared by selecting different contents of silicon-carbon, conductive agents, and binders. In order to improve the kinetics of lithium-ion batteries and reduce lithium deposition near the tab caused by conventional winding structures, the first negative electrode layer has a higher conductivity range (2.56-2.85 S / cm) compared to the second and third negative electrode layers. This is because the higher the conductivity, the smoother the electron transport in the electrode sheet, allowing the battery to exchange charges more efficiently during charging and discharging, thereby increasing the charging and discharging speed, improving the kinetics of the electrode sheet, and effectively reducing lithium deposition near the tab caused by conventional winding structures. Higher conductivity means better electrode dynamics, but also a lower proportion of active material, which is not conducive to improving energy density.

[0195] To improve the energy density and cycle performance of lithium-ion batteries, the conductivity range (0.75-1.32 S / cm) of the second anode layer is relatively low compared to the first and third anode layers. Lower conductivity means a higher proportion of active material, which is beneficial for increasing the battery's energy density. Moreover, lower conductivity results in a relatively slower electron transport speed, which can, to some extent, slow down the rate of side reactions within the battery, thus contributing to improved cycle stability and lifespan.

[0196] In order to reduce lithium plating, improve the kinetics of the negative electrode, and at the same time ensure high energy density and good cycle performance, the conductivity of the third negative electrode layer (1.42-2.45 S / cm) is in the middle range of the first and second negative electrode layers.

[0197] To further reduce lithium plating near the tab caused by conventional winding structures, it is crucial that the coating length of Region 1 be within a suitable range. As shown in Experimental Examples 1, 14-15, and Comparative Examples 14-15, the ratio of L2 (the coating length of Region 1) to the sum of L2 and L3 (the coating lengths of Region 2 or Region 3) should be within the range of 5%-10% for the battery to possess good overall electrochemical performance. If the ratio of L2 / (L2+L3) is less than 5% (as in Comparative Example 14), this will lead to insufficient kinetics in Region 1, making it prone to lithium plating during charge and discharge, thus reducing the battery's cycle performance. If the ratio of L2 / (L2+L3) is greater than 10% (as in Comparative Example 15), due to the larger coating length, this will not only reduce the energy density of the lithium-ion battery, but also cause poor electrical contact between active materials or between the active material and the conductive agent due to the low 5T powder compaction of graphite, resulting in a decrease in the battery's cycle capacity retention.

[0198] Therefore, in summary, the above test data demonstrates that the multilayer negative electrode sheet of this embodiment, which improves the electrochemical performance of a wound battery, utilizes a relatively low 5T powder compaction density range (1.75-1.85). The use of graphite as the first negative electrode active material, along with a high conductivity range (2.56-2.85 S / cm) and an optimal coating length (5% ≤ L2 / (L2+L3) ≤ 10%) for the first negative electrode layer, can significantly improve the kinetics of the negative electrode near the tab and reduce lithium plating caused by conventional winding structures. A higher 5T powder compaction density range (1.98-2.05) is also selected. The use of graphite as the second negative electrode active material and a second negative electrode layer with a low conductivity range (0.75-1.32 S / cm) improves energy density and cycle performance. A medium-density 5T powder with a compaction density range of 1.88-1.96... The third graphite of the third negative electrode active material and the third negative electrode layer with a medium conductivity range (1.42-2.45 S / cm) enhance the kinetics of the negative electrode sheet, while ensuring high energy density and good cycle performance.

[0199] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A multilayer negative electrode sheet for improving the electrochemical performance of wound-structured batteries, characterized in that, It includes a current collector, a first negative electrode layer coated on the current collector near the negative electrode tab, a second negative electrode layer coated on the current collector away from the negative electrode tab, and a third negative electrode layer coated on the second negative electrode layer. The first negative electrode layer includes a first negative electrode active material, the second negative electrode layer includes a second negative electrode active material, and the third negative electrode layer includes a third negative electrode active material. The conductivity values ​​of the first negative electrode layer range from 2.56 to 2.85 S / cm, the conductivity values ​​of the second negative electrode layer range from 0.75 to 1.32 S / cm, and the conductivity values ​​of the third negative electrode layer range from 1.42 to 2.45 S / cm. The first negative electrode active material includes first graphite or a composite of first graphite and first silicon carbide; the second negative electrode active material includes second graphite or a composite of second graphite and second silicon carbide; and the third negative electrode active material includes third graphite or a composite of third graphite and third silicon carbide. The compacted density of the first graphite powder (5T) ranges from 1.75 to 1.

85. The compacted density of the second graphite powder (5T) ranges from 1.98 to 2.

05. The compacted density of the third graphite powder (5T) ranges from 1.88 to 1.

96. .

2. The multilayer negative electrode sheet for improving the electrochemical performance of a wound battery according to claim 1, characterized in that, The first negative electrode layer is close to the negative electrode tab and is coated on both sides of the current collector. The second negative electrode layer is located at one end of the first negative electrode layer and away from the negative electrode tab and is coated on both sides of the current collector. The number of the third negative electrode layers is set to two, and the two third negative electrode layers correspond one-to-one with the two second negative electrode layers. On one side of the current collector, the distance between the first negative electrode layer and the negative electrode tab is set to L1, the coating length of the first negative electrode layer is set to L2, and the coating lengths of the second and third negative electrode layers are both set to L3. On the other side of the current collector, the distance between the first negative electrode layer and the negative electrode tab is set to L4, the coating length of the first negative electrode layer is set to L5, and the coating lengths of the second and third negative electrode layers are both set to L6.

3. The multilayer negative electrode sheet for improving the electrochemical performance of a wound battery according to claim 2, characterized in that, On one side of the current collector, the coating length L2 of the first negative electrode layer, the coating length L3 of the second negative electrode layer and the third negative electrode layer satisfy the following ratio range: 5%≤L2 / (L2+L3)≤10%.

4. The multilayer negative electrode sheet for improving the electrochemical performance of a wound battery according to claim 3, characterized in that, On the other side of the current collector, the coating length L5 of the first negative electrode layer and the coating length L6 of the second and third negative electrode layers satisfy the following ratio range: 80%≤(L5+L6) / (L2+L3)≤95%.

5. The multilayer negative electrode sheet for improving the electrochemical performance of a wound battery according to claim 1, characterized in that, The first graphite, the second graphite, and the third graphite are all one or a mixture of two types of artificial graphite and natural graphite.

6. The multilayer negative electrode sheet for improving the electrochemical performance of a wound battery according to claim 1, characterized in that, The volumetric median particle size (Dv50) of the first graphite ranges from 4 to 8 μm; the volumetric median particle size (Dv50) of the second graphite ranges from 13 to 17 μm; and the volumetric median particle size (Dv50) of the third graphite ranges from 8.5 to 12.5 μm. The volumetric median particle size (Dv50) of the first silicon carbon ranges from 3 to 6.5 μm; the volumetric median particle size (Dv50) of the second silicon carbon ranges from 9 to 11.5 μm; and the volumetric median particle size (Dv50) of the third silicon carbon ranges from 7 to 8.5 μm. The specific surface area of ​​the first silicon-carbon alloy ranges from 4 to 5.

5. The specific surface area of ​​the second silicon-carbon compound ranges from 1.0 to 2.

5. The specific surface area of ​​the third silicon-carbon alloy ranges from 2.6 to 3.

8. .

7. The multilayer negative electrode sheet for improving the electrochemical performance of a wound battery according to claim 1, characterized in that, The first negative electrode layer, the second negative electrode layer, and the third negative electrode layer all further include conductive agents, thickeners, and binders.

8. The multilayer negative electrode sheet for improving the electrochemical performance of a wound battery according to claim 7, characterized in that, The conductive agent of the first negative electrode layer, the conductive agent of the second negative electrode layer, and the conductive agent of the third negative electrode layer are all one or a mixture of two or more of conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes; The thickeners of the first negative electrode layer, the second negative electrode layer, and the third negative electrode layer are all one or a mixture of two of CMC-Na (carboxymethyl cellulose-sodium) and CMC-Li (carboxymethyl cellulose-lithium); The adhesives for the first negative electrode layer, the second negative electrode layer, and the third negative electrode layer are all one or a mixture of two or more of the following: styrene-butadiene rubber, styrene-acrylic rubber, polyurethane, and polyacrylic acid.

9. A multilayer negative electrode sheet for improving the electrochemical performance of a wound battery according to claim 7, characterized in that, When the first negative electrode active material of the first negative electrode layer is a composite of first graphite and first silicon carbon, the mass percentage of each material in the first negative electrode layer is as follows: The proportion of graphite in the first negative electrode active material ranges from 57% to 99%. The proportion of silicon-carbon in the first negative electrode active material ranges from 0% to 25%. The proportion of conductive agent is 0-4%; Thickener ratio range: 0-4%; Adhesive ratio range: 1-10%; When the second negative electrode active material of the second negative electrode layer is a composite of second graphite and second silicon carbon, the mass percentage of each substance in the second negative electrode layer is as follows: The proportion of second graphite in the second negative electrode active material ranges from 0% to 99%. The proportion of silicon-carbon in the second negative electrode active material ranges from 0% to 99%. The proportion of conductive agent is 0-4%; Thickener ratio range: 0-4%; Adhesive ratio range: 1-10%; When the third negative electrode active material of the third negative electrode layer is a composite of third graphite and third silicon carbon, the mass percentage of each substance in the third negative electrode layer is as follows: The proportion of third graphite in the third negative electrode active material ranges from 50% to 99%. The proportion of silicon-carbon in the third anode active material ranges from 0% to 50%. The proportion of conductive agent is 0-4%; Thickener ratio range: 0-4%; Adhesive ratio range: 1-10%.

10. A process for improving the electrochemical performance of a multilayer negative electrode sheet in a wound-structure battery, characterized in that, It is used to prepare a multilayer negative electrode sheet for improving the electrochemical performance of a wound-structure battery as described in any one of claims 1-9, and the preparation steps are as follows: Step S1: Preparation of the first negative electrode slurry: The first graphite of the first negative electrode active material, the first silicon carbide of the first negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the first negative electrode slurry; The proportions of the above substances are as follows: The proportion of graphite in the first negative electrode active material ranges from 57% to 99%. The proportion of silicon-carbon in the first negative electrode active material ranges from 0% to 25%. The proportion of conductive agent is 0-4%; Thickener ratio range: 0-4%; Adhesive ratio range: 1-10%; Step S2: Preparation of the second negative electrode slurry: The second graphite of the second negative electrode active material, the second silicon carbide of the second negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the second negative electrode slurry; The proportions of the above substances are as follows: The proportion of second graphite in the second negative electrode active material ranges from 0% to 99%. The proportion of silicon-carbon in the second negative electrode active material ranges from 0% to 99%. The proportion of conductive agent is 0-4%; Thickener ratio range: 0-4%; Adhesive ratio range: 1-10%; Step S3: Preparation of the third negative electrode slurry: The third graphite of the third negative electrode active material, the third silicon carbide of the third negative electrode active material, the conductive agent, the thickener, and the binder are mixed to prepare the third negative electrode slurry; The proportions of the above substances are as follows: The proportion of third graphite in the third negative electrode active material ranges from 50% to 99%. The proportion of silicon-carbon in the third anode active material ranges from 0% to 50%. The proportion of conductive agent is 0-4%; Thickener ratio range: 0-4%; Adhesive ratio range: 1-10%; Step S4: Coating is performed on both sides of the current collector using a three-layer extrusion coating die. Specifically, the coating areas are as follows: the first negative electrode slurry is coated on region 1 near the negative electrode tab; the second negative electrode slurry is coated on region 2 away from the negative electrode tab; and the third negative electrode slurry is coated on region 3 of the second negative electrode slurry. After drying in an oven, the other side of the current collector is coated again: the first negative electrode slurry is coated on region 4 near the negative electrode tab; the second negative electrode slurry is coated on region 5 away from the negative electrode tab; and the third negative electrode slurry is coated on region 5 of the second negative electrode slurry. In region 6 of the negative electrode slurry, the specific coating lengths are as follows: on the side of the negative electrode tab along the length of the current collector, the length of the empty foil region is L1, the length of region 1 is L2, and the lengths of regions 2 and 3 are L3; on the other side of the negative electrode tab along the length of the current collector, the length of the empty foil region is L4, the length of region 4 is L5, and the lengths of regions 5 and 6 are L6, wherein 5%≤L2 / (L2+L3)≤10%, and 80%≤(L5+L6) / (L2+L3)≤95%; Step S5: After rolling and slitting processes, a multi-layer negative electrode sheet is obtained.

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