Multilayer structure negative pole piece and process for improving electrochemical performance of winding structure battery

Abstract

The invention relates to a multilayer structure negative pole piece and process for improving the electrochemical performance of a winding structure battery, and the multilayer structure negative pole piece for improving the electrochemical performance of the winding structure battery comprises a current collector, a first negative pole layer close to a negative pole lug, a second negative pole layer far away from the negative pole lug, and a third negative pole layer on the second negative pole 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; a first negative electrode active substance of the first negative electrode layer comprises first graphite or a compound of the first graphite and first silicon carbon, and a second negative electrode active substance of the second negative electrode layer comprises second graphite or a compound of the second graphite and second silicon carbon; a third negative electrode active substance of the third negative electrode layer comprises third graphite or a compound of the third graphite and third silicon carbon; the compaction density of the first graphite 5T powder is 1.75 to 1.85, the compaction density of the second graphite 5T powder is 1.98 to 2.05, and the compaction density of the third graphite 5T powder is 1.88 to 1.96; the invention discloses a process of a multi-layer structure negative pole piece.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of negative electrode sheet and its preparation process, in particular to a multi-layer structure negative electrode sheet for improving the electrochemical performance of a winding structure battery and a process. BACKGROUND

[0002] With the wide application of lithium ion batteries in the fields of mobile phones, watches and electric vehicles, the requirement for the energy density of the lithium ion batteries is also increasing, and the existing battery system has been unable to meet the growing demand of people for high energy density of lithium ion batteries.

[0003] The silicon-doped negative electrode is an effective means to improve the energy density of the battery due to its high energy density, potential low cost and wide application prospect.

[0004] However, the silicon-doped negative electrode high energy density lithium ion battery with a conventional winding structure may have the following problems in actual application:

[0005] 1. During charging, the current density near the negative electrode tab of the negative electrode sheet is large, which is prone to cause lithium precipitation and temperature rise and other adverse phenomena, and the precipitated lithium forms dendrites, which can easily damage the separator and cause battery short circuit, resulting in battery smoke, fire and even explosion, which has serious safety hazards, so it is necessary to inhibit the lithium precipitation of the negative electrode and ensure the safety of the battery.

[0006] 2. Under high surface density and high compaction density, the electrolyte in the negative electrode sheet is not fully and uniformly infiltrated, which causes the active material of the negative electrode to not fully play a role, and during the charging and discharging process, lithium precipitation and cycle failure are prone to occur. Therefore, there is an urgent need for a new negative electrode sheet technology that can improve the energy density of the conventional winding structure battery while not reducing the rate performance and cycle performance. SUMMARY

[0007] The purpose of the present application is to overcome the above-mentioned defects in the prior art, and to provide a multi-layer structure negative electrode sheet and process for improving the electrochemical performance of a winding structure battery, which selects a first graphite of a first negative electrode active material with a lower 5T powder compaction density range value (1.75-1.85 The first negative electrode layer with a higher electrical conductivity range value (2.56-2.85 S / cm) can greatly improve the kinetics of the negative electrode sheet near the tab and reduce the lithium precipitation phenomenon caused by the conventional winding structure. The second graphite of a second negative electrode active material with a higher 5T powder compaction density range value (1.98-2.05 The second negative electrode layer with a lower electrical conductivity range value (0.75-1.32 S / cm) improves the energy density and cycle performance. The second graphite of a second negative electrode active material with a higher 5T powder compaction density range value (1.98-2.05 The third graphite of the third negative active material and the third negative electrode layer with the third median conductivity range value (1.42-2.45 S / cm) improve the kinetics of the negative electrode sheet, while ensuring high energy density and good cycle performance.

[0008] To achieve the above object, the present application is realized by the following two aspects:

[0009] In a first aspect, the present application provides a multi-layer structure negative electrode sheet for improving the electrochemical performance of a winding 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 active material, the second negative electrode layer comprises a second negative active material, and the third negative electrode layer comprises a third negative active material.

[0010] The conductivity value range of the first negative electrode layer is 2.56-2.85 S / cm, the conductivity value range of the second negative electrode layer is 0.75-1.32 S / cm, and the conductivity value range of the third negative electrode layer is 1.42-2.45 S / cm.

[0011] The first negative active material comprises a first graphite or a composite of the first graphite and a first silicon-carbon, the second negative active material comprises a second graphite or a composite of the second graphite and a second silicon-carbon, and the third negative active material comprises a third graphite or a composite of the third graphite and a third silicon-carbon.

[0012] The 5T powder compaction density value range of the first graphite is 1.75-1.85 The 5T powder compaction density value range of the second graphite is 1.98-2.05 The 5T powder compaction density value range of the third graphite is 1.88-1.96 ;

[0013] Preferably, the first negative electrode layer is near 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 is away from the negative electrode tab and is coated on both sides of the current collector, and the number of the third negative electrode layer is two, and each of the two third negative electrode layers corresponds to one of the two second negative electrode layers.

[0014] On one side of the current collector, the distance of the first negative electrode layer from the negative electrode tab is L1, the coating length of the first negative electrode layer is L2, and the coating length of the second negative electrode layer and the third negative electrode layer is L3.

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

[0016] Preferably, on one side of the current collector, the coating length L2 of the first negative electrode layer and the coating lengths L3 of the second negative electrode layer and the third negative electrode layer 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 lengths L6 of the second negative electrode layer and the third negative electrode layer satisfy the following ratio range: 80%≤(L5+L6) / (L2+L3)≤95%.

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

[0019] Preferably, the volume median particle size Dv50 of the first graphite ranges from 4 to 8 μm, the volume median particle size Dv50 of the second graphite ranges from 13 to 17 μm, and the volume median particle size Dv50 of the third graphite ranges from 8.5 to 12.5 μm.

[0020] The volume median particle size Dv50 of the first silicon-carbon ranges from 3 to 6.5 μm, the volume median particle size Dv50 of the second silicon-carbon ranges from 9 to 11.5 μm, and the volume 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 ranges from 4 to 5.5 The specific surface area of the second silicon-carbon ranges from 1.0 to 2.5 The specific surface area of the third silicon-carbon 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 comprise a conductive agent, a thickening agent 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 one or a mixture of two or more of conductive carbon black, single-walled carbon nanotubes and multi-walled carbon nanotubes.

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

[0025] The binder of the first negative electrode layer, the binder of the second negative electrode layer, and the binder of the third negative electrode layer are one of or a mixture of two or more of a styrene-butadiene rubber-based, a styrene-acryl rubber-based, a polyurethane-based, and a polyacrylic acid-based.

[0026] As preferred, when the first negative electrode active material of the first negative electrode layer is a composite of a first graphite and a first silicon-carbon, the mass ratio of each substance in the first negative electrode layer is as follows:

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

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

[0029] The proportion of the conductive agent ranges from 0% to 4%;

[0030] The proportion of the thickening agent ranges from 0% to 4%;

[0031] The proportion of the binder ranges from 1% to 10%;

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

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

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

[0035] The proportion of the conductive agent ranges from 0% to 4%;

[0036] The proportion of the thickening agent ranges from 0% to 4%;

[0037] The proportion of the binder ranges from 1% to 10%;

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

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

[0040] The proportion of the third silicon-carbon of the third negative electrode active material ranges from 0% to 50%;

[0041] The proportion of the conductive agent ranges from 0% to 4%;

[0042] The proportion of the thickening agent ranges from 0% to 4%;

[0043] The proportion of the binder ranges from 1-10%.

[0044] In a second aspect, the present application provides a process for preparing a multi-layer structure negative electrode sheet for improving the electrochemical performance of a wound structure battery, which is used to prepare a multi-layer structure negative electrode sheet for improving the electrochemical performance of a wound structure 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-carbon of the first negative electrode active material, the conductive agent, the thickening agent, and the binder are mixed to prepare the first negative electrode slurry;

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

[0047] The proportion of the first graphite of the first negative electrode active material ranges from 57-99%;

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

[0049] The proportion of the conductive agent ranges from 0-4%;

[0050] The proportion of the thickening agent ranges from 0-4%;

[0051] The proportion of the binder ranges from 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-carbon of the second negative electrode active material, the conductive agent, the thickening agent, and the binder are mixed to prepare the second negative electrode slurry;

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

[0054] The proportion of the second graphite of the second negative electrode active material ranges from 0-99%;

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

[0056] The proportion of the conductive agent ranges from 0-4%;

[0057] The proportion of the thickening agent ranges from 0-4%;

[0058] The proportion of the binder ranges from 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-carbon of the third negative electrode active material, the conductive agent, the thickening agent, and the binder are mixed to prepare the third negative electrode slurry;

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

[0061] The proportion range of the third graphite of the third negative electrode active material: 50-99%;

[0062] The proportion range of the third silicon-carbon of the third negative electrode active material: 0-50%;

[0063] The proportion range of the conductive agent: 0-4%;

[0064] The proportion range of the thickening agent: 0-4%;

[0065] The proportion range of the binder: 1-10%;

[0066] Step S4: coating on both sides of the current collector through a three-layer extrusion coating die, and the specific coating areas are as follows: the first negative electrode slurry is coated in area 1 close to the negative electrode tab, the second negative electrode slurry is coated on area 2 away from the negative electrode tab, and the third negative electrode slurry is coated in area 3 on the second negative electrode slurry; after drying in an oven, the other side of the current collector is coated, the first negative electrode slurry is coated in area 4 close to the negative electrode tab, the second negative electrode slurry is coated on area 5 away from the negative electrode tab, and the third negative electrode slurry is coated in area 6 on the second negative electrode slurry; the specific coating length is as follows: on the side of the edge where the negative electrode tab is arranged, the length of the empty foil area is L1, the length of area 1 is L2, and the lengths of area 2 and area 3 are L3; on the other side of the edge where the negative electrode tab is arranged, the length of the empty foil area is L4, the length of area 4 is L5, and the lengths of area 5 and area 6 are L6, wherein 5%≤L2 / (L2+L3)≤10%, 80%≤(L5+L6) / (L2+L3)≤95%;

[0067] Step S5: obtaining a multi-layer structure negative electrode tab after the rolling and slitting processes.

[0068] Compared with the prior art, the present application has the following advantages:

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

[0070] At the same time, the electrical conductivity of the first negative electrode layer > the electrical conductivity of the third negative electrode layer > the electrical conductivity of the second negative electrode layer, so that in the ranking of electrical conductivity, the first negative electrode layer is the highest, the third negative electrode layer is in the middle, and the second negative electrode layer is the lowest;

[0071] The 5T powder compaction density value of the first graphite in the first negative electrode layer < the 5T powder compaction density value of the third graphite in the third negative electrode layer < the 5T powder compaction density value of the second graphite in the second negative electrode layer, so that in the ranking of 5T powder compaction density values, the first graphite is the lowest, the third graphite is in the middle, and the second graphite is the highest;

[0072] Therefore, near the negative tab, the first negative active material of the first graphite with a lower 5T powder compaction density range value (1.75-1.85 ) and the first negative electrode layer with a higher electrical conductivity range value (2.56-2.85 S / cm) can greatly improve the kinetics of the negative electrode sheet near the tab and reduce the lithium precipitation phenomenon caused by the conventional winding structure. The second negative active material of the second graphite with a higher 5T powder compaction density range value (1.98-2.05 ) and the second negative electrode layer with a lower electrical conductivity range value (0.75-1.32 S / cm) improve the energy density and cycle performance. The third negative active material of the third graphite with a middle 5T powder compaction density range value (1.88-1.96 ) and the third negative electrode layer with a middle electrical conductivity range value (1.42-2.45 S / cm) improve the kinetics of the negative electrode sheet while ensuring higher energy density and good cycle performance.

[0073] 2、In summary, the multi-layer structure negative electrode sheet for improving the electrochemical performance of the winding structure battery provided by the application avoids the lithium precipitation at the negative tab caused by the conventional winding structure, and at the same time, the negative electrode sheet can improve the rate performance and cycle capacity of the lithium ion battery under the design of high energy density system. BRIEF DESCRIPTION OF DRAWINGS

[0074] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0075] Figure 1 is a structure schematic diagram of step S4: coating on both sides of the current collector by a three-layer extrusion coating die provided by embodiment two of the application;

[0076] Figure 2 is a structure schematic diagram of a multi-layer structure negative electrode sheet for improving the electrochemical performance of the winding structure battery provided by embodiment one of the application;

[0077] Figure 3 is a test result diagram of the battery cycle performance test of experimental example 1 and comparative example 1 provided by embodiment three of the application;

[0078] Figure 4 is a test effect diagram of the surface of region 3 of experimental example 1 after the battery lithium precipitation test provided by embodiment three of the application;

[0079] Figure 5 is the test effect diagram of the surface of area 3 of Comparative Example 2 provided by Embodiment Three of the present application after the battery lithium precipitation test. DETAILED DESCRIPTION

[0080] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are one of the embodiments of the present application, but not all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0081] Embodiment One

[0082] Please refer to Figure 2 , Embodiment One of the present application provides a multi-layer structure negative electrode sheet for improving the electrochemical performance of a winding structure battery, comprising a current collector, a first negative electrode layer coated on the current collector close to the negative electrode lug, a second negative electrode layer coated on the current collector away from the negative electrode lug, 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.

[0083] The first negative electrode active material comprises a composite of first graphite and first silicon-carbon, the second negative electrode active material comprises second graphite or a composite of second graphite and second silicon-carbon, and the third negative electrode active material comprises third graphite or a composite of third graphite and third silicon-carbon.

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

[0085] Therefore, the electrical conductivity of the first negative electrode layer > the electrical conductivity of the third negative electrode layer > the electrical conductivity of the second negative electrode layer, and in the ranking of the electrical 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 5T powder compaction density value of the first graphite ranges from 1.75 to 1.85 , the 5T powder compaction density value of the third graphite ranges from 1.88 to 1.96 , and the 5T powder compaction density value of the second graphite ranges from 1.98 to 2.05 ; therefore, the first graphite in the first negative electrode layer has the lowest compaction density value, the third graphite has a middle compaction density value, and the second graphite has the highest compaction density value in the ranking of 5T powder compaction density values.

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

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

[0089] The volume median particle size Dv50 of the first silicon-carbon is in the range of 3-6.5 μm, the volume median particle size Dv50 of the second silicon-carbon is in the range of 9-11.5 μm, and the volume median particle size Dv50 of the third silicon-carbon is in the range of 7-8.5 μm.

[0090] The specific surface area of the first silicon-carbon is in the range of 4-5.5 The specific surface area of the second silicon-carbon is in the range of 1.0-2.5 The specific surface area of the third silicon-carbon is in the range of 2.6-3.8 .

[0091] The first negative electrode layer, the second negative electrode layer, and the third negative electrode layer all further comprise a conductive agent, a thickening agent, 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 one of conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes or a mixture of two or more thereof.

[0093] The thickening agent of the first negative electrode layer, the thickening agent of the second negative electrode layer, and the thickening agent of the third negative electrode layer are one of CMC-Na (carboxymethyl cellulose-sodium) and CMC-Li (carboxymethyl cellulose-lithium) or a mixture of both.

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

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

[0096] The proportion of the first graphite of the first negative electrode active material is in the range of 57-99%.

[0097] The ratio range of the first silicon-carbon of the first negative electrode active material is 0-25%;

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

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

[0100] Binder 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 proportions of the various materials in the second negative electrode layer are as follows:

[0102] The proportion range of the second graphite of the second negative electrode active material is 0-99%;

[0103] The ratio range of the second silicon-carbon of the second negative electrode active material is 0-99%;

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

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

[0106] Binder 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 proportions of the various materials in the third negative electrode layer are as follows:

[0108] The proportion range of the third graphite of the third negative electrode active material is: 50-99%;

[0109] The ratio range of the third silicon-carbon of the third negative electrode active material is 0-50%;

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

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

[0112] The proportion range of binder: 1-10%.

[0113] like Figure 2 As shown, the first negative electrode layer is close to the negative electrode ear 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 is away from the negative electrode ear and is coated on both sides of the current collector, and the number of the third negative electrode layers is set to two, and the two third negative electrode layers correspond to the two second negative electrode layers respectively;

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

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

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

[0117] Wherein, at 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 negative electrode layer and the third negative electrode layer satisfy the following proportion range: 80%≤(L5+L6) / (L2+L3)≤95%.

[0118] The multi-layer structure negative electrode sheet of the embodiment one of the application has the advantages that: the first negative electrode layer close to the negative electrode tab has the lowest 5T powder compaction density (1.75-1.85 ), high first negative electrode layer conductivity (2.56-2.85 S / cm), and the best coating length proportion 5%≤L2 / (L2+L3)≤10%, which can greatly improve the kinetics of the negative electrode sheet close to the tab, greatly improve the speed of lithium ion insertion and extraction, and avoid lithium precipitation at the negative electrode tab caused by the conventional winding structure.

[0119] The second negative electrode layer and the third negative electrode layer far from the negative electrode tab have the second graphite with the highest 5T powder compaction density (1.98-2.05 ) in the current collector->second negative electrode layer->third negative electrode layer direction, which can ensure the improvement of the energy density and cycle stability of the lithium ion battery, the third negative electrode layer has the third graphite with the medium 5T compaction density (1.88-1.96 ), which improves the kinetics of the negative electrode sheet, reduces lithium precipitation, and ensures high energy density and good cycle performance.

[0120] Embodiment two

[0121] Embodiment two of the present application provides a process for preparing a multi-layer structure negative electrode sheet for improving the electrochemical performance of a wound structure battery, which is used for preparing a multi-layer structure negative electrode sheet for improving the electrochemical performance of a wound structure battery as described in embodiment one, and 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 carbon of the first negative electrode active material, the conductive agent, the thickening agent, and the binder are mixed to prepare the first negative electrode slurry;

[0123] The proportion of each of the above-mentioned substances is as follows:

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

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

[0126] The proportion of the conductive agent ranges from 0 to 4%;

[0127] The proportion of the thickening agent ranges from 0 to 4%;

[0128] The proportion of the binder ranges from 1 to 10%;

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

[0130] The proportion of each of the above-mentioned substances is as follows:

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

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

[0133] The proportion of the conductive agent ranges from 0 to 4%;

[0134] The proportion of the thickening agent ranges from 0 to 4%;

[0135] The proportion of the binder ranges from 1 to 10%;

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

[0137] The proportion of each of the above-mentioned substances is as follows:

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

[0139] The proportion of the third silicon-carbon of the third negative active material ranges from 0 to 50%;

[0140] The proportion of the conductive agent ranges from 0 to 4%;

[0141] The proportion of the thickening agent ranges from 0 to 4%;

[0142] The proportion of the binder ranges from 1 to 10%;

[0143] Step S4: coating on both sides of the current collector by a three-layer extrusion coating die (such as Figure 1 ), the coating area and length are shown in Figure 2 . The specific coating area is: the first negative electrode slurry is coated in area 1 close to the negative electrode tab, the second negative electrode slurry is coated in area 2 away from the negative electrode tab, and the third negative electrode slurry is coated in area 3 on the second negative electrode slurry. After drying in an oven, the other side of the current collector is coated, the first negative electrode slurry is coated in area 4 close to the negative electrode tab, the second negative electrode slurry is coated in area 5 away from the negative electrode tab, and the third negative electrode slurry is coated in area 6 on the second negative electrode slurry. The specific coating length is: on the side of the edge where the negative electrode tab is arranged, the length of the empty foil area is L1, the length of area 1 is L2, and the length of areas 2 and 3 is L3. On the other side of the edge where the negative electrode tab is arranged, the length of the empty foil area is L4, the length of area 4 is L5, and the length of areas 5 and 6 is L6. Specifically, 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 is away from the negative electrode tab and is coated on both sides of the current collector, and the number of the third negative electrode layer is two, with one-to-one correspondence between the two third negative electrode layers and the two second negative electrode layers.

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

[0145] On the other side of the current collector, the distance of the first negative electrode layer from the negative electrode tab is L4, the coating length of the first negative electrode layer is L5, and the coating length of the second and third negative electrode layers is 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 proportion 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, the coating length L6 of the second negative electrode layer and the third negative electrode layer satisfy the following ratio range: 80%≤(L5+L6) / (L2+L3)≤95%;

[0148] Step S5: obtaining the multi-layer structure negative electrode sheet through rolling and slitting processes.

[0149] The conductivity value of the first negative electrode layer in the multi-layer structure negative electrode sheet prepared in step S5 is in the range of 2.56-2.85 S / cm, the conductivity value of the second negative electrode layer is in the range of 0.75-1.32 S / cm, the conductivity value of the third negative electrode layer is in the range of 1.42-2.45 S / cm, the 5T powder compaction density value of the first graphite is in the range of 1.75-1.85 The 5T powder compaction density value of the second graphite is in the range of 1.98-2.05 The 5T powder compaction density value of the third graphite is in the range of 1.88-1.96 .

[0150] Example Three

[0151] Example Three of the present application provides experimental data respectively about battery lithium precipitation test, constant current intermittent titration method (GITT) test and battery cycle performance test.

[0152] The experimental data of this example three uses 15 experimental examples and 15 lithium ion batteries of comparative examples, wherein:

[0153] (1). About experimental examples 1-15:

[0154] The negative electrode sheets in the 15 experimental examples are prepared by the process of the multi-layer structure negative electrode sheet for improving the electrochemical performance of the winding structure battery in example two, and then the lithium ion batteries are prepared by the existing technology of preparing positive electrode sheet -> negative electrode sheet -> separator -> winding -> liquid injection -> packaging -> formation -> capacity distribution. The negative electrode sheets of the 15 experimental examples all adopt the structure of the multi-layer structure negative electrode sheet for improving the electrochemical performance of the winding structure battery in example one. The difference between the 15 experimental examples is that, based on experimental example 1, following the principle of single variable method, the conductivity value or the 5T powder compaction density value of the first negative electrode layer or the second negative electrode layer or the third negative electrode layer of the negative electrode sheet of experimental examples 2 to 15 is not the same as that of experimental example 1, or on one side of the current collector, the specific ratio value L2 / (L2+L3) of 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 of experimental examples 2 to 15 is not the same as that of experimental example 1

[0155] (Specific differences please see Table I mentioned below);

[0156] A more specific process for improving the electrochemical performance of a multi-layer structure negative electrode sheet of a wound structure battery used only for Experimental Example 1 (Experimental Examples 2-15 have the same preparation process as Experimental Example 1, only the electrical conductivity value of the first negative electrode layer or the second negative electrode layer or the third negative electrode layer or the 5T powder compaction density value of the active material graphite is different from Experimental Example 1, or on one side of the current collector, the specific ratio value L2 / (L2+L3) of 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 of Experimental Examples 2 to Experimental Examples 15 is different from Experimental Example 1, which will not be described here):

[0157] Step S1: Preparation of the first negative electrode slurry: mix the first graphite of the first negative electrode active material, the first silicon carbon of the first negative electrode active material, the conductive agent, the thickening agent, and the binder, add the solvent (deionized water), and stir with a vacuum stirrer to prepare the first negative electrode slurry;

[0158] Step S2: Preparation of the second negative electrode slurry: mix the second graphite of the second negative electrode active material, the second silicon carbon of the second negative electrode active material, the conductive agent, the thickening agent, and the binder, add the solvent (deionized water), and stir with a vacuum stirrer to prepare the second negative electrode slurry;

[0159] The 5T powder compaction density value of the second graphite is: 2.01 ;

[0160] Step S3: Preparation of the third negative electrode slurry: mix the third graphite of the third negative electrode active material, the third silicon carbon of the third negative electrode active material, the conductive agent, the thickening agent, and the binder, add the solvent (deionized water), and stir with a vacuum stirrer to prepare the third negative electrode slurry;

[0161] The 5T powder compaction density value of the third graphite is: 1.88 ;

[0162] In the above steps S1, S2 and S3, the 5T powder compaction density value of the first graphite < the 5T powder compaction density value of the third graphite < the 5T powder compaction density value of the second graphite;

[0163] Step S4: coating on both sides of the current collector by a three-layer extrusion coating die (such as Figure 1 ); Figure 2The specific coating area is: the first negative electrode slurry is coated in area 1 close to the negative electrode tab, the second negative electrode slurry is coated on area 2 away from the negative electrode tab, the third negative electrode slurry is coated in area 3 on the second negative electrode slurry, and after drying in the oven, the other side of the current collector is coated, the first negative electrode slurry is coated in area 4 close to the negative electrode tab, the second negative electrode slurry is coated on area 5 away from the negative electrode tab, and the third negative electrode slurry is coated in area 6 on the second negative electrode slurry. The specific coating length is: on the side of the edge where the negative electrode tab is arranged, the length of the empty foil area is L1, the length of area 1 is L2, and the length of areas 2 and 3 is L3; on the other side of the edge where the negative electrode tab is arranged, the length of the empty foil area is L4, the length of area 4 is L5, and the length of areas 5 and 6 is L6, wherein L2 / (L2+L3)%=7%;

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

[0165] In the multi-layer structure negative electrode sheet prepared in step S5, the electrical conductivity of the first negative electrode layer is 2.72 S / cm, the electrical conductivity of the second negative electrode layer is 1.25 S / cm, and the electrical 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 1-15 are prepared by a process similar to the multi-layer structure negative electrode sheet of Example 2 for improving the electrochemical performance of a wound structure battery. The lithium ion battery is prepared by the prior art. The preparation process of the negative electrode sheets of Comparative Examples 1-15 is the same as that of Example 2, and the only difference between them is that the preparation process of Comparative Examples 1-15 does not follow the ranking of the electrical conductivity of the first negative electrode layer > the electrical conductivity of the third negative electrode layer > the electrical conductivity of the second negative electrode layer or the value of the electrical conductivity of the first negative electrode layer is not within the value range of 2.56-2.85 S / cm or the value of the electrical conductivity of the second negative electrode layer is not within the value range of 0.75-1.32 S / cm or the value of the electrical conductivity of the third negative electrode layer is not within the value range of 1.42-2.45 S / cm or the ranking of the 5T powder compaction density values of the first graphite < the 5T powder compaction density values of the third graphite < the 5T powder compaction density values of the second graphite or the value of the 5T powder compaction density values of the first graphite is not within the value range of 1.75-1.85 or the value of the 5T powder compaction density values of the third graphite is not within the value range of 1.88-1.96 or the value of the 5T powder compaction density values of the second graphite is not within the value range of 1.98-2.05 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 do not satisfy the following ratio range: 5%≤L2 / (L2+L3)≤10% at the range of the value of the first negative electrode layer or at one side of the current collector;

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

[0169] Further, the electrical conductivity of the third negative electrode layer of Comparative Example 7 is lower than the above value range, the electrical conductivity of the third negative electrode layer of Comparative Example 8 is higher than the above value range, the electrical conductivity of the second negative electrode layer of Comparative Example 9 is lower than the above value range, the electrical conductivity of the second negative electrode layer of Comparative Example 10 is higher than the above value range, the electrical conductivity of the first negative electrode layer of Comparative Example 11 is lower than the above value range, and the electrical conductivity of the first negative electrode layer of Comparative Example 12 is higher than the above value range;

[0170] The compaction density value ranking of the 5T powder compaction density value of the first graphite of Comparative Example 13 = the 5T powder compaction density value of the third graphite = the 5T powder compaction density value of the second graphite;

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

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

[0173] (Specific differences please see Table 1 mentioned below)

[0174] (3). The specific parameters of the electrical conductivity of the first negative electrode layer, the second negative electrode layer and the third negative electrode layer of Experimental Examples 1-15 and Comparative Examples 1-15 and the first graphite of the first negative electrode active material, the second graphite of the second negative electrode active material, the third graphite of the third negative electrode active material, and the ratio of L2 to the sum of L2 and L3 are as follows:

[0175] Table 1

[0176]

[0177] The specific test methods of the battery lithium precipitation test, the constant current intermittent titration method (GITT) test and the battery cycle performance test of this embodiment three are as follows:

[0178] (1) Battery lithium precipitation test:

[0179] After the lithium ion battery prepared by the experimental example 1-15 and the comparative example 1-15 was charged at a rate of 2C and discharged at a rate of 1C repeatedly for 20 times at 25℃, the fully charged battery was disassembled, and the lithium precipitation of the negative electrode sheet was observed.

[0180] (2) Constant current intermittent titration method (GITT) test:

[0181] At 0℃, the lithium ion battery prepared by the experimental example and the comparative example was charged at a rate of 2C, and the charging time was set so that the battery was paused for 3s every 1% state of charge (SOC) after charging, and the process was repeated until the battery was charged to the set cut-off voltage. The impedance was calculated based on the voltage and charging current of the battery after 3s of sleep at the end of each 1% SOC charging, and the impedance-charge state curve was plotted. If the battery impedance-charge state curve shows a downward inflection point at the end of charging, it means that the battery has started to precipitate lithium, and the charge state corresponding to the inflection point is the SOC value of the battery lithium precipitation.

[0182] (3) Battery cycle performance test

[0183] After the lithium ion battery prepared by the experimental example and the comparative example was charged at a rate of 1.5C and discharged at a rate of 1C for 500 cycles at 25℃, the capacity retention rate of the lithium ion battery was recorded.

[0184] The test results of the battery lithium precipitation test, the constant current intermittent titration method (GITT) test and the battery cycle performance test of the experimental example 1-15 and the comparative example 1-15 in this example three are as follows in Table Two:

[0185] Table Two

[0186]

[0187] The test conclusions of the battery lithium precipitation test, the constant current intermittent titration method (GITT) test and the battery cycle performance test of the experimental example 1-15 and the comparative example 1-15 in this example three are as follows:

[0188] When the 5T powder compaction density of the third graphite of the third negative electrode active material is less than 1.88 (such as comparative example 1) or the 5T powder compaction density of the second graphite of the second negative electrode active material is less than 1.98 (such as comparative example 3) or the 5T powder compaction density of the first graphite of the first negative electrode active material is less than 1.75 (when the electrical conductivity of the third negative electrode layer is greater than 2.45 S / cm (such as Comparative Example 8) or the electrical conductivity of the second negative electrode layer is greater than 1.32 S / cm (such as Comparative Example 10) or the electrical conductivity of the first negative electrode layer is greater than 2.85 S / cm (such as Comparative Example 12), although lithium precipitation does not exist on the surface of the region 1 / region 4 and the region 3 / region 6 and the SOC value of the battery does not decrease, the capacity retention rate after 500 cycles at room temperature is low (such as Figure 3 ).

[0189] When the 5T powder compaction density of the third graphite of the third negative electrode active material is greater than 1.96 (when the 5T powder compaction density of the second graphite of the second negative electrode active material is greater than 2.05 (when the electrical conductivity of the third negative electrode layer is less than 1.42 S / cm (such as Comparative Example 7) or the electrical conductivity of the second negative electrode layer is less than 0.75 S / cm (such as Comparative Example 9), lithium precipitation exists on the surface of the region 3 / region 6 of the negative electrode sheet (such as Figure 5 , the dark part in the figure is the lithium precipitation position), the SOC value of the battery decreases, and the capacity retention rate after 500 cycles at room temperature is low. If the 5T powder compaction density of the first graphite of the first negative electrode active material is greater than 1.85 (when the electrical conductivity of the first negative electrode layer is less than 2.56 S / cm (such as Comparative Example 11), lithium precipitation exists on the surface of the region 1 / region 4 of the negative electrode sheet, i.e., lithium precipitation exists near the negative electrode tab, which further affects the SOC value of the lithium ion battery and the cycle performance.

[0190] From Experimental Examples 1-13 and Comparative Examples 1-13, it can be seen that only when the 5T powder compaction density of the first graphite of the first active material, the 5T powder compaction density of the second graphite of the second active material, and the 5T powder compaction density of the third graphite of the third active material are respectively within the value range of 1.75-1.85 , 1.98-2.05 , 1.88-1.96 and the electrical conductivity of the first negative electrode layer, the electrical conductivity of the second negative electrode layer, and the electrical conductivity of the third negative electrode layer are respectively within the value range of 2.56-2.85 S / cm, 0.75-1.32 S / cm, and 1.42-2.45 S / cm, the battery has good electrochemical performance such as no lithium precipitation (such as Figure 4 ), a high lithium precipitation SOC value, and a high cycle capacity retention rate.

[0191] In order to reduce the lithium precipitation phenomenon near the tab caused by the conventional winding structure, the first negative electrode layer is selected to have a lower 5T powder compaction density range (1.75-1.85 ) of the first graphite. This is because the lower the 5T tap density of the graphite, the relatively harder the hardness, the more resistant to pressure, which means that there are more pores in the interior of the negative electrode tab, the absorption capacity of the electrolyte can become stronger, and the number of lithium ion channels increases, so that the speed of lithium ion intercalation or deintercalation can be improved, and the generation of lithium precipitation at the tab is reduced. Although the smaller 5T tap density of the graphite can improve the kinetics and reduce the lithium precipitation at the negative electrode tab, if the 5T tap density of the graphite in the first negative electrode layer, the second negative electrode layer and the third negative electrode layer is relatively small (such as Comparative Example 13), this will lead to poor contact between the active material and the active material or the active material and the conductive agent in a large area at the later stage of the cycle, thereby increasing the resistance, reducing the charge and discharge efficiency, and causing the capacity loss to be large after 500 cycles. Moreover, the smaller 5T tap density will reduce the tap density of the entire negative electrode tab, which is not conducive to the improvement of the energy density of the lithium ion battery, so the second negative electrode layer and the third negative electrode layer are not suitable for using the graphite with the lower 5T tap density.

[0192] In order to improve the energy density and cycle performance of the battery, the second negative electrode layer uses the second graphite with a relatively higher 5T tap density range (1.98-2.05 ) than the first negative electrode layer and the third negative electrode layer. The greater the 5T tap density of the active material graphite, the smaller the hardness of the graphite, which can improve the tap density of the tab, thereby improving the energy density, and can ensure the close contact between the active material and the active material or the active material and the conductive agent during the cycle, thereby improving the cycle performance of the lithium ion battery.

[0193] In order to reduce the lithium precipitation on the surface of region 3 / region 6, without reducing the energy density too much, and also to ensure good cycle performance and other electrochemical properties, the third negative electrode layer uses the graphite with a relatively moderate 5T tap density range (1.88-1.96 ).

[0194] In the graphite silicon-doped negative electrode sheet, because the electrical conductivity of graphite itself is relatively large, the selection of different types and different contents of graphite has little effect on the electrical conductivity of the electrode sheet. Therefore, generally, negative electrode sheets with different electrical conductivities are prepared by selecting different contents of silicon-carbon, conductive agents and binders. In this experiment, examples 1-15 and comparative examples 1-15, negative electrode sheets with different electrical conductivities are prepared by selecting different contents of silicon-carbon, conductive agents and binders. In order to improve the kinetics of lithium ion batteries and reduce the lithium precipitation phenomenon near the tab caused by the conventional winding structure, the first negative electrode layer has a higher range value of electrical conductivity than the second and third negative electrode layers. (2.56-2.85 S / cm) This is because the greater the electrical conductivity, the more smooth the electron transmission in the electrode sheet, which allows the battery to exchange charges more efficiently during charging and discharging, thereby improving the charging and discharging speed, enhancing the kinetics of the electrode sheet, and effectively reducing the lithium precipitation phenomenon near the tab caused by the conventional winding structure. Higher electrical conductivity means better kinetics of the electrode sheet, and the proportion of active material is relatively small, which is not conducive to the improvement of energy density.

[0195] In order to improve the energy density and cycle performance of lithium ion batteries, the electrical conductivity of the second negative electrode layer (0.75-1.32 S / cm) is lower than that of the first and third negative electrode layers. Lower electrical conductivity means a higher proportion of active material, which is conducive to improving the energy density of the battery, and the slower electron transmission speed caused by lower electrical conductivity can slow down the speed of the battery's internal side reactions to some extent, which helps to improve the cycle stability and service life of the battery.

[0196] In order to reduce lithium precipitation, improve the kinetics of the negative electrode sheet, and at the same time ensure high energy density and good cycle performance, the electrical 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 the lithium precipitation phenomenon near the tab caused by the conventional winding structure, the coating length of region 1 is very important in a suitable range. As can be seen from experimental example 1, experimental examples 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 length of region 2 or region 3) should be in the range of 5%-10% for the battery to have good overall electrochemical performance. If the ratio of L2 / (L2+L3) is less than 5% (such as comparative example 14), this will lead to insufficient kinetics in region 1, which is prone to lithium precipitation during charging and discharging, thereby reducing the cycle performance of the battery. If the ratio of L2 / (L2+L3) is greater than 10% (such as comparative example 15), due to the larger coating length, not only will the energy density of the lithium ion battery be reduced, but also the low 5T powder compaction graphite will cause poor electrical contact between the active material and the active material or the active material and the conductive agent, thereby leading to a decrease in the cycle capacity retention rate of the battery.

[0198] Therefore, as described above, the test data described above shows that the multi-layer structure negative electrode tab of the first embodiment described above improves the electrochemical performance of the winding structure battery, the first graphite of the first negative electrode active material with a lower 5T powder compaction density range value (1.75-1.85 ) and the first negative electrode layer with a higher electrical conductivity range value (2.56-2.85 S / cm) and the best coating length (5%≤L2 / (L2+L3)≤10%) can greatly improve the kinetics of the negative electrode tab near the tab and reduce the lithium precipitation phenomenon caused by the conventional winding structure. The second graphite of the second negative electrode active material with a higher 5T powder compaction density range value (1.98-2.05 ) and the second negative electrode layer with a lower electrical conductivity range value (0.75-1.32 S / cm) improve the energy density and cycle performance. The third graphite of the third negative electrode active material with a middle 5T powder compaction density range value (1.88-1.96 ) and the third negative electrode layer with a middle electrical conductivity range value (1.42-2.45 S / cm) improve the kinetics of the negative electrode tab while ensuring higher energy density and good cycle performance.

[0199] The above is only a preferred embodiment of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure battery, characterized in that: The invention comprises a current collector, a first negative electrode layer coated on the current collector near the negative electrode ear, a second negative electrode layer coated on the current collector away from the negative electrode ear, 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; The conductivity value range of the first negative electrode layer is: 2.56-2.85 S / cm, the conductivity value range of the second negative electrode layer is: 0.75-1.32 S / cm, and the conductivity value range of the third negative electrode layer is: 1.42-2.45 S / cm; The first negative electrode active material includes a first graphite or a composite of the first graphite and a first silicon-carbon, the second negative electrode active material includes a second graphite or a composite of the second graphite and a second silicon-carbon, and the third negative electrode active material includes a third graphite or a composite of the third graphite and a third silicon-carbon; The 5T powder compaction density of the first graphite is in the range of 1.75-1.85 The 5T powder compaction density of the second graphite ranges from 1.98 to 2.

05. The 5T powder compaction density of the third graphite is in the range of 1.88-1.96 .

2. The multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure 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, and the number of the third negative electrode layers is set to two, and the two third negative electrode layers correspond to the two second negative electrode layers respectively; On one side of the current collector, the distance between the first negative electrode layer and the negative electrode ear is set to L1, the coating length of the first negative electrode layer is set to L2, and the coating lengths of the second negative electrode layer and the third negative electrode layer 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 ear is set to L4, the coating length of the first negative electrode layer is set to L5, and the coating lengths of the second negative electrode layer and the third negative electrode layer are both set to L6.

3. The multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure 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 coating length L3 of the third negative electrode layer satisfy the following ratio range: 5%≤L2 / (L2+L3)≤10%.

4. The multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure 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, the coating length L6 of the second negative electrode layer and the third negative electrode layer satisfy the following ratio range: 80%≤(L5+L6) / (L2+L3)≤95%.

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

6. The multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure battery according to claim 1, characterized in that: 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; the volume median particle size Dv50 of the third graphite is in the range of 8.5-12.5 μm; The volume median particle size Dv50 of the first silicon carbon is in the range of 3-6.5 μm; the volume median particle size Dv50 of the second silicon carbon is in the range of 9-11.5 μm; the volume median particle size Dv50 of the third silicon carbon is in the range of 7-8.5 μm; The specific surface area of ​​the first silicon carbon is in the range of 4-5.5 ; The specific surface area of ​​the second silicon carbon range is: 1.0-2.5 ; The specific surface area of ​​the third silicon carbon is in the range of: 2.6-3.8 .

7. The multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure battery according to claim 1, characterized in that: The first negative electrode layer, the second negative electrode layer and the third negative electrode layer also include a conductive agent, a thickener and a binder.

8. The multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure 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 thickener of the first negative electrode layer, the thickener of the second negative electrode layer and the thickener of the third negative electrode layer are all CMC-Na (carboxymethyl cellulose-sodium) and CMC-Li (carboxymethyl cellulose-lithium) or a mixture of the two; The binder of the first negative electrode layer, the binder of the second negative electrode layer and the binder of the third negative electrode layer are all one of styrene-butadiene rubber, styrene-propylene rubber, polyurethane and polyacrylic acid, or a mixture of two or more thereof.

9. The multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure 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 proportions of the various materials in the first negative electrode layer are as follows: The proportion range of the first graphite of the first negative electrode active material is 57-99%; The ratio range of the first silicon-carbon of the first negative electrode active material is 0-25%; The proportion range of conductive agent: 0-4%; Thickener ratio range: 0-4%; Binder 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 proportions of the various materials in the second negative electrode layer are as follows: The proportion range of the second graphite of the second negative electrode active material is 0-99%; The ratio range of the second silicon-carbon of the second negative electrode active material is 0-99%; The proportion range of conductive agent: 0-4%; Thickener ratio range: 0-4%; Binder 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 proportions of the various materials in the third negative electrode layer are as follows: The proportion range of the third graphite of the third negative electrode active material is: 50-99%; The ratio range of the third silicon-carbon of the third negative electrode active material is 0-50%; The proportion range of conductive agent: 0-4%; Thickener ratio range: 0-4%; The proportion range of binder: 1-10%.

10. A process for producing a multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure battery, characterized in that: It is used to prepare a multi-layer negative electrode sheet for improving the electrochemical performance of a wound structure battery as described in any one of claims 1 to 9, and the preparation steps are as follows: Step S1: preparing a first negative electrode slurry: mixing a first graphite of a first negative electrode active material, a first silicon carbon of a first negative electrode active material, a conductive agent, a thickener, and a binder to prepare a first negative electrode slurry; Among them, the proportions of the above substances are as follows: The proportion range of the first graphite of the first negative electrode active material is 57-99%; The ratio range of the first silicon-carbon of the first negative electrode active material is 0-25%; The proportion range of conductive agent: 0-4%; Thickener ratio range: 0-4%; Binder ratio range: 1-10%; Step S2: preparing a second negative electrode slurry: mixing the second graphite of the second negative electrode active material, the second silicon carbon of the second negative electrode active material, a conductive agent, a thickener, and a binder to prepare a second negative electrode slurry; Among them, the proportions of the above substances are as follows: The proportion range of the second graphite of the second negative electrode active material is 0-99%; The ratio range of the second silicon-carbon of the second negative electrode active material is 0-99%; The proportion range of conductive agent: 0-4%; Thickener ratio range: 0-4%; Binder ratio range: 1-10%; Step S3: preparing a third negative electrode slurry: mixing a third graphite as a third negative electrode active material, a third silicon carbon as a third negative electrode active material, a conductive agent, a thickener, and a binder to prepare a third negative electrode slurry; Among them, the proportions of the above substances are as follows: The proportion range of the third graphite of the third negative electrode active material is: 50-99%; The ratio range of the third silicon-carbon of the third negative electrode active material is 0-50%; The proportion range of conductive agent: 0-4%; Thickener ratio range: 0-4%; Binder ratio range: 1-10%; Step S4: Coating is performed on both sides of the current collector by a three-layer extrusion coating die head. The specific coating areas are: coating the first negative electrode slurry in area 1 close to the negative electrode ear, coating the second negative electrode slurry in area 2 away from the negative electrode ear, and coating the third negative electrode slurry in area 3 on the second negative electrode slurry. After oven drying, coating is performed on the other side of the current collector. Coating the first negative electrode slurry in area 4 close to the negative electrode ear, coating the second negative electrode slurry in area 5 away from the negative electrode ear, and coating the third negative electrode slurry in area 3 on the second negative electrode slurry. In area 6 on the negative electrode slurry, the specific coating length is: on the side of the negative electrode ear edge along the length direction of the current collector, the length of the empty foil area is L1, the length of area 1 is L2, and the lengths of areas 2 and 3 are L3; on the other side of the negative electrode ear edge along the length direction of the current collector, the length of the empty foil area is L4, the length of area 4 is L5, and the lengths of areas 5 and 6 are L6, among which 5% ≤ L2 / (L2+L3) ≤ 10%, 80% ≤ (L5+L6) / (L2+L3) ≤ 95%; Step S5: After rolling and slitting, a multi-layer structured negative electrode sheet is obtained.

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