Double-layer negative pole piece and preparation process thereof

Abstract

The double-layer negative electrode plate comprises a negative electrode current collector, a tab, a first negative electrode layer and a second negative electrode layer, the thickness of the first negative electrode layer is increased in the direction away from the tab, and the thickness of the second negative electrode layer is reduced in the direction away from the tab, the first negative electrode layer comprises a first negative electrode active material and a first binder, and the second negative electrode layer comprises a second negative electrode active material and a second binder; the oil absorption value of the first negative electrode active material is 25ml / 100g to 35ml / 100g, the swelling ratio of the first binder is 30 percent to 50 percent, the oil absorption value of the second negative electrode active material is 40ml / 100g to 50ml / 100g, and the swelling ratio of the second binder is 60 percent to 90 percent; the thickness of the first negative electrode layer close to the tab accounts for 10%-40% of the total thickness of the first negative electrode layer and the second negative electrode layer, and the thickness of the first negative electrode layer far away from the tab accounts for 60%-90% of the total thickness of the first negative electrode layer and the second negative electrode layer; 2, a preparation process of the double-layer negative pole piece; the invention has the advantages of ensuring high energy density and improving rate capability and cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of negative electrode pole piece structures and negative electrode pole piece preparation processes, and in particular to a double-layer negative electrode pole piece and a preparation process thereof. Background Art

[0002] Lithium-ion batteries offer advantages such as light weight, high energy density, high operating voltage, long cycle life, and zero pollution. They are widely used in a range of products, including mobile phones, watches, and electric vehicles. In recent years, with the increasing demand in the consumer and power battery markets, the demand for lithium-ion battery energy density has also increased.

[0003] Lithium-ion batteries can improve energy density by increasing the coating density and compaction density of the negative electrode, but this will also bring a series of problems:

[0004] First, during charging, the current density near the tab of the negative electrode is high, which makes lithium deposition easy;

[0005] Second, the coating density of the negative electrode is too high, which can easily lead to powder loss and poor rate performance during processing.

[0006] Third, increasing the compaction density of the negative electrode sheet will lead to a lower porosity of the negative electrode sheet, insufficient and uneven electrolyte infiltration, resulting in the inability to fully utilize the active material capacity of the negative electrode sheet, and easily causing lithium deposition and cycle failure during the charge and discharge process;

[0007] Therefore, there is an urgent need for a new battery technology that can improve battery energy density without reducing rate performance and battery cycle performance. Summary of the Invention

[0008] The present invention aims to overcome the above-mentioned defects in the prior art and provides a double-layer negative electrode sheet and a preparation process thereof. The present invention provides a double-layer negative electrode layer on the negative electrode current collector, wherein the oil absorption value of the negative electrode active material and the swelling rate of the binder in the first negative electrode layer are both smaller than those in the second negative electrode layer. In addition, combined with the structural distribution design, the thickness of the first negative electrode layer gradually increases in the direction away from the tab, and the thickness of the second negative electrode layer gradually decreases in the direction away from the tab, thereby achieving:

[0009] 1. The oil absorption value of the negative electrode active material and the swelling rate of the binder in the second negative electrode layer are relatively large, and the thickness of the second negative electrode layer gradually decreases in the direction away from the tab, which improves the degree of electrolyte infiltration, increases the speed of lithium ion insertion and extraction, is not easy to deposit lithium, and maintains good rate performance;

[0010] 2. The oil absorption value and swelling rate of the negative electrode active material in the first negative electrode layer are small, and the thickness of the first negative electrode layer gradually increases in the direction away from the tab, which helps to improve the bonding force between the negative electrode layer and the current collector, and is not easy to fall off during processing, which helps to improve the cycle performance of the battery;

[0011] 3. There is a boundary interface between the first negative electrode layer and the second negative electrode layer at both ends close to the pole ear and away from the pole ear, that is, near the pole ear, the thickness of the first negative electrode layer accounts for 10%-40% of the total thickness of the first negative electrode and the second negative electrode layer, and away from the pole ear, the thickness of the first negative electrode layer accounts for 60% to 90% of the total thickness of the first negative electrode and the second negative electrode layer, which prevents the edge of the electrode from being easily peeled off, improves the overall cohesion of the negative electrode plate at both ends, and helps to improve the cycle performance of the battery.

[0012] The combination of the above three can ensure the design of a high energy density system while effectively improving the battery's rate performance and cycle performance.

[0013] To achieve the above objectives, the present invention is implemented through the following three aspects:

[0014] In a first aspect, the present invention provides a double-layer negative electrode sheet, comprising a negative electrode current collector, a tab disposed on the negative electrode current collector, and a first negative electrode layer coated on the negative electrode current collector, wherein the thickness of the first negative electrode layer gradually increases in a direction away from the tab, a second negative electrode layer is coated on the first negative electrode layer, and the thickness of the second negative electrode layer gradually decreases in a direction away from the tab, the first negative electrode layer comprising a first negative electrode active material and a first binder, and the second negative electrode layer comprising a second negative electrode active material and a second binder;

[0015] The oil absorption value of the first negative electrode active material is in the range of 25 ml / 100 g to 35 ml / 100 g, and the swelling rate of the first binder is in the range of 30% to 50%. The oil absorption value of the second negative electrode active material is in the range of 40 ml / 100 g to 50 ml / 100 g, and the swelling rate of the second binder is in the range of 60% to 90%.

[0016] The thickness of one end surface of the first negative electrode layer close to the tab is set to H1, and the thickness of one end surface of the second negative electrode layer close to the tab is set to H3. The thickness of H1 accounts for 10%-40% of the total thickness of H1 and H3.

[0017] The thickness of the end surface of the first negative electrode layer away from the electrode tab is set to H2, and the thickness of the end surface of the second negative electrode layer away from the electrode tab is set to H4. The thickness of H2 accounts for 60%-90% of the total thickness of H2 and H4.

[0018] Preferably, the thickness of the first negative electrode layer gradually increases with a certain slope in a direction away from the electrode tab, and the thickness of the second negative electrode layer gradually decreases with a certain slope in a direction away from the electrode tab.

[0019] Preferably, the thickness of the first negative electrode layer gradually increases in a step-like manner along a direction away from the electrode tab, and the thickness of the second negative electrode layer gradually decreases in a step-like manner along a direction away from the electrode tab.

[0020] Preferably, the thickness of the first negative electrode layer gradually increases in a certain arc along the direction away from the electrode tab, and the thickness of the second negative electrode layer gradually decreases in a certain arc along the direction away from the electrode tab.

[0021] Preferably, the first negative electrode active material and the second negative electrode active material are graphite, silicon carbon or hard carbon respectively;

[0022] The first binder and the second binder are respectively one or a mixture of styrene-butadiene, styrene-acrylic, styrene-butadiene-acrylic or polyacrylate.

[0023] Preferably, the first negative electrode layer further includes a conductive agent and a thickener.

[0024] The mass proportions of the various substances in the first negative electrode layer are as follows:

[0025] The proportion range of the first negative electrode active material: 88-98%;

[0026] The proportion range of conductive agent: 0-3%;

[0027] Thickener ratio range: 0-3%;

[0028] The proportion range of the first binder: 0.5-10%;

[0029] The second negative electrode layer further includes a conductive agent and a thickener,

[0030] The mass proportions of the various substances in the second negative electrode layer are as follows:

[0031] The proportion range of the second negative electrode active material: 88-98%;

[0032] The proportion range of conductive agent: 0-3%;

[0033] Thickener ratio range: 0-3%;

[0034] The proportion range of the second binder is: 0.5-10%.

[0035] In a second aspect, the present invention provides a process for preparing a double-layer negative electrode sheet, which is used to prepare a double-layer negative electrode sheet as described in the first aspect, and the preparation steps include the following:

[0036] Step S1: mixing a first negative electrode active material, a conductive agent, a thickener, and a first binder according to a certain mass ratio, adding a solvent, and stirring with a stirrer to prepare a first negative electrode slurry;

[0037] Step S2: mixing the second negative electrode active material, the conductive agent, the thickener, and the second binder according to a certain mass ratio, adding a solvent, and stirring with a stirrer to prepare a second negative electrode slurry;

[0038] Step S3: Coating is performed on both sides of the negative electrode current collector using a double-layer extrusion coating die. The specific coating area and coating length are:

[0039] First, coat one side of the negative electrode current collector with the first negative electrode slurry in region 1 with a coating length of L1, and then coat the second negative electrode slurry in region 2 on the first negative electrode slurry with a coating length of L1;

[0040] After drying in an oven, the other side of the current collector is coated. The first negative electrode slurry is applied to area 3 with a coating length of L2. Then, the second negative electrode slurry is applied to area 4 on the first negative electrode slurry with a coating length of L2.

[0041] Among them, the thickness of the first negative electrode slurry located in area 1 and area 3 gradually increases in the direction away from the electrode tab, and the thickness of the second negative electrode slurry located in area 2 and area 4 gradually decreases in the direction away from the electrode tab. The negative electrode sheet is obtained through the processes of oven drying, roller pressing, slitting and welding the electrode tab to the negative electrode current collector.

[0042] In a third aspect, the present invention provides a process for preparing a double-layer negative electrode sheet, which is used to prepare a double-layer negative electrode sheet as described in the first aspect, and the preparation steps include the following:

[0043] Step S1: mixing a first negative electrode active material, a conductive agent, a thickener, and a first binder according to a certain mass ratio, adding a solvent, and stirring with a stirrer to prepare a first negative electrode slurry;

[0044] Step S2: mixing the second negative electrode active material, the conductive agent, the thickener, and the second binder according to a certain mass ratio, adding a solvent, and stirring with a stirrer to prepare a second negative electrode slurry;

[0045] Step S3: Coating is performed on both sides of the negative electrode current collector using a double-layer extrusion coating die. The specific coating area and coating length are:

[0046] First, coat one side of the current collector with the first negative electrode slurry, applying the coating lengths L4 and L5 to regions 5 and 6, respectively. Then, coat the second negative electrode slurry to regions 7 and 8, respectively, applying the coating lengths L3 and L4.

[0047] After drying in an oven, the other side of the negative electrode current collector is coated. The first negative electrode slurry is applied to regions 9 and 10, with coating lengths of L7 and L8, respectively. Then, the second negative electrode slurry is applied to regions 11 and 12, with coating lengths of L6 and L7, respectively.

[0048] Among them, the thickness of the first negative electrode slurry located in regions 5 and 9 gradually increases in the direction away from the tab, and the thickness of the second negative electrode slurry located in regions 8 and 12 gradually decreases in the direction away from the tab. Among them, the coating thickness of regions 6, 7, 10 and 11 are consistent, the coating thickness of region 6 is equal to the sum of the coating thicknesses of regions 5 and 8, and the coating thickness of region 10 is equal to the sum of the coating thicknesses of regions 9 and 12. The negative electrode sheet is obtained through the processes of oven drying, rolling, slitting and welding the tab to the negative electrode collector.

[0049] Preferably, the length L3 satisfies the following relationship: 0≤L3 / (L3+L4+L5)≤10%, the length L5 satisfies the following relationship: 0≤L5 / (L3+L4+L5)≤10%, the length L6 satisfies the following relationship: 0≤L6 / (L6+L7+L8)≤10%, and the length L8 satisfies the following relationship: 0≤L8 / (L6+L7+L8)≤10%.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1. A double-layer negative electrode sheet and a preparation process thereof of the present invention comprises a negative electrode current collector, a tab provided on the negative electrode current collector, and a first negative electrode layer coated on the negative electrode current collector, wherein the thickness of the first negative electrode layer gradually increases in a direction away from the tab, and a second negative electrode layer is coated on the first negative electrode layer, and the thickness of the second negative electrode layer gradually decreases in a direction away from the tab. At the same time, the oil absorption value of the first negative electrode active material of the first negative electrode layer is lower than the oil absorption value of the second negative electrode active material of the second negative electrode layer, the swelling rate of the first binder of the first negative electrode layer is lower than the swelling rate of the second binder of the second negative electrode layer, and near the tab, the thickness of the first negative electrode layer accounts for 10% to 40% of the total thickness of the first negative electrode layer and the second negative electrode layer, and away from the tab, the thickness of the first negative electrode layer accounts for 60% to 90% of the total thickness of the first negative electrode layer and the second negative electrode layer;

[0052] therefore:

[0053] Near the tab, the thickness of the first negative electrode layer accounts for 10%-40% of the total thickness of the first and second negative electrode layers. Far from the tab, the thickness of the first negative electrode layer accounts for 60%-90% of the total thickness of the first and second negative electrode layers, which improves the cohesion of the negative electrode sheet at both ends and helps improve the cycle performance of the battery.

[0054] Furthermore, the first negative electrode active material of the first negative electrode layer has a low oil absorption value, which can ensure a good cycle capacity retention rate of the battery;

[0055] Furthermore, the swelling rate of the first binder of the first negative electrode layer is relatively low, which can ensure the bonding strength between the first negative electrode layer and the current collector, and can effectively improve the powder loss between the two during processing, thereby improving the cycle performance of the battery;

[0056] Furthermore, the second negative electrode active material of the second negative electrode layer has a large oil absorption value, which can improve the ion transmission rate between the negative electrode plate and the electrolyte interface, and also reduce the resistance of ion migration, thereby ensuring good rate performance of the battery;

[0057] Furthermore, the second binder of the second negative electrode layer has a large swelling rate, which increases the pore structure inside the negative electrode plate. The second negative electrode active material can provide more lithium insertion sites, and the electrolyte absorption capacity is enhanced, which greatly increases the speed of lithium ion insertion and extraction, thereby effectively improving the battery charging performance. It makes the negative electrode plate less likely to release lithium under high current density conditions, ensuring good rate performance of the battery.

[0058] Therefore, the above-mentioned double-layer negative electrode sheet and its preparation process can effectively improve the rate performance and cycle performance of the battery while ensuring the design of a high energy density system. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0060] Figure 1 This is a cross-sectional view of a double-layer negative electrode sheet provided by Example 1 and Example 2 of the present invention, cut along the thickness direction (with the tab welded to the head or tail of the negative electrode current collector, and the thickness of the first negative electrode layer gradually increases at a certain slope in the direction away from the tab, and the thickness of the second negative electrode layer gradually decreases at a certain slope in the direction away from the tab);

[0061] Figure 2 It's about Figure 1 A top view in the direction of surface A;

[0062] Figure 3 It's about Figure 1 A top view in the direction of surface B;

[0063] Figure 4 This is a cross-sectional view of a double-layer negative electrode sheet provided by Example 1 of the present invention, cut along the thickness direction (with the tab welded to the middle of the negative electrode current collector, and the thickness of the first negative electrode layer gradually increases at a certain slope in the direction away from the tab, and the thickness of the second negative electrode layer gradually decreases at a certain slope in the direction away from the tab);

[0064] Figure 5 It's about Figure 4 A top view in the C-plane direction;

[0065] Figure 6 It's about Figure 4 A top plan view in the D-plane direction;

[0066] Figure 7 This is a cross-sectional view of a double-layer negative electrode sheet provided by Example 1 of the present invention, cut along the thickness direction (where the thickness of the first negative electrode layer gradually increases in a certain gradient away from the tab, and the thickness of the second negative electrode layer gradually decreases in a certain gradient away from the tab);

[0067] Figure 8 This is a cross-sectional view of a double-layer negative electrode sheet provided by Example 1 of the present invention, cut along the thickness direction (where the thickness of the first negative electrode layer gradually increases in a certain arc in the direction away from the electrode tab, and the thickness of the second negative electrode layer gradually decreases in a certain arc in the direction away from the electrode tab);

[0068] Figure 9 is a cross-sectional view of a double-layer negative electrode sheet provided by the first and third embodiments of the present invention, cut along the thickness direction;

[0069] Figure 10 This is a cross-sectional view of a double-layer negative electrode sheet provided by the prior art, cut along the thickness direction (the structure of the double-layer negative electrode sheet in this cross-sectional view corresponds to Comparative Example 13 mentioned in Example 4). DETAILED DESCRIPTION

[0070] The technical solution in this embodiment of the present invention will be clearly and completely described below with reference to the accompanying drawings in this embodiment of the present invention.

[0071] Example 1:

[0072] like Figure 1-9As shown, embodiment 1 of the present invention provides a double-layer negative electrode sheet, comprising a negative electrode current collector, a tab provided on the negative electrode current collector, and a first negative electrode layer coated on the negative electrode current collector, wherein the thickness of the first negative electrode layer gradually increases in a direction away from the tab, a second negative electrode layer is coated on the first negative electrode layer, and the thickness of the second negative electrode layer gradually decreases in a direction away from the tab, the first negative electrode layer comprises a first negative electrode active material and a first binder, and the second negative electrode layer comprises a second negative electrode active material and a second binder;

[0073] The oil absorption value of the first negative electrode active material is in the range of 25 ml / 100 g to 35 ml / 100 g, and the swelling rate of the first binder is in the range of 30% to 50%. The oil absorption value of the second negative electrode active material is in the range of 40 ml / 100 g to 50 ml / 100 g, and the swelling rate of the second binder is in the range of 60% to 90%.

[0074] The thickness of one end surface of the first negative electrode layer close to the tab is set to H1, and the thickness of one end surface of the second negative electrode layer close to the tab is set to H3. The thickness of H1 accounts for 10%-40% of the total thickness of H1 and H3.

[0075] The thickness of the end face of the first negative electrode layer away from the electrode tab is set to H2, and the thickness of the end face of the second negative electrode layer away from the electrode tab is set to H4. The thickness of H2 accounts for 60%-90% of the total thickness of H2 and H4.

[0076] There are two implementation methods for welding the tabs to the negative electrode current collector, including:

[0077] The first implementation method: Figure 1-3 As shown, the tab is welded to the head or tail of the negative electrode current collector;

[0078] The second implementation method: Figure 4-6 As shown, the tab is welded to the middle of the negative electrode current collector;

[0079] The thickness of the first negative electrode layer gradually increases in the direction away from the tab, and the second negative electrode layer is coated on the first negative electrode layer. The thickness of the second negative electrode layer gradually decreases in the direction away from the tab. There are three implementation methods, including:

[0080] The first implementation method: Figure 1-6 As shown in , 9 , the thickness of the first negative electrode layer gradually increases with a certain slope in the direction away from the tab, and the thickness of the second negative electrode layer gradually decreases with a certain slope in the direction away from the tab.

[0081] The second implementation method: Figure 7As shown, the thickness of the first negative electrode layer gradually increases in a step-by-step manner along the direction away from the tab, and the thickness of the second negative electrode layer gradually decreases in a step-by-step manner along the direction away from the tab.

[0082] The third implementation method: Figure 8 As shown, the thickness of the first negative electrode layer gradually increases in a certain arc along the direction away from the electrode tab, and the thickness of the second negative electrode layer gradually decreases in a certain arc along the direction away from the electrode tab.

[0083] The first negative electrode layer further includes a conductive agent and a thickener, and the second negative electrode layer further includes a conductive agent and a thickener.

[0084] The mass proportions of each substance in the first negative electrode layer are as follows:

[0085] The proportion range of the first negative electrode active material: 88-98%;

[0086] The proportion range of conductive agent: 0-3%;

[0087] Thickener ratio range: 0-3%;

[0088] The proportion range of the first binder is: 0.5-10%.

[0089] The mass proportions of each substance in the second negative electrode layer are as follows:

[0090] The proportion range of the second negative electrode active material: 88-98%;

[0091] The proportion range of conductive agent: 0-3%;

[0092] Thickener ratio range: 0-3%;

[0093] The proportion range of the second binder is: 0.5-10%.

[0094] The first negative electrode active material and the second negative electrode active material are respectively graphite, silicon carbon or hard carbon or other negative electrode active materials.

[0095] The first binder and the second binder are respectively one or a mixture of styrene-butadiene, styrene-acrylic, styrene-butadiene-acrylic or polyacrylate.

[0096] The conductive agent of the first negative electrode layer and the conductive agent of the second negative electrode layer are respectively one of conductive carbon black, multi-walled carbon nanotubes or single-walled carbon nanotubes or a mixture of several thereof.

[0097] The thickener of the first negative electrode layer and the thickener of the second negative electrode layer are respectively CMC-Na (carboxymethyl cellulose-sodium) or CMC-Li (carboxymethyl cellulose-lithium) or a mixture thereof.

[0098] A double-layer negative electrode sheet of embodiment 1 of the present invention has a first negative electrode layer whose thickness gradually increases in a direction away from the tab, a second negative electrode layer is coated on the first negative electrode layer, and the thickness of the second negative electrode layer gradually decreases in a direction away from the tab, the first negative electrode layer includes a first negative electrode active material and a first binder, and the second negative electrode layer includes a second negative electrode active material and a second binder; the oil absorption value of the first negative electrode active material is in the range of 25ml / 100g to 35ml / 100g, the swelling rate of the first binder is in the range of 30% to 50%, the oil absorption value of the second negative electrode active material is in the range of 40ml / 100g to 50ml / 100g, and the swelling rate of the second binder is in the range of 60% to 90%, so that the first negative electrode layer The oil absorption value of the first negative electrode active material is small, the oil absorption value of the second negative electrode active material of the second negative electrode layer is large, the swelling rate of the first binder of the first negative electrode layer is small, and the swelling rate of the second binder of the second negative electrode layer is large; at the same time, the thickness of the end surface of the first negative electrode layer close to the pole ear direction is set to H1, the thickness of the end surface of the second negative electrode layer close to the pole ear direction is set to H3, and the thickness of the thickness H1 accounts for 10%-40% of the total thickness of H1 and H3, the thickness of the end surface of the first negative electrode layer away from the pole ear direction is set to H2, the thickness of the end surface of the second negative electrode layer away from the pole ear direction is set to H4, and the thickness of the thickness H2 accounts for 60%-90% of the total thickness of H2 and H4; the advantages of this arrangement are:

[0099] 1. The oil absorption value of the first negative electrode active material in the first negative electrode layer ranges from 25ml / 100g to 35ml / 100g. Using a first negative electrode active material with a low oil absorption value ensures good cycle capacity retention. This is because the oil absorption value of a negative electrode active material is primarily related to the material's specific surface area, surface properties, particle shape, and particle size distribution. The oil absorption value of a negative electrode active material reflects its dispersibility in the negative electrode slurry. A first negative electrode active material with a low oil absorption value exhibits excellent dispersibility in the first negative electrode slurry. The first negative electrode slurry exhibits excellent anti-settling and dispersibility, making it easier to control the coating weight during the negative electrode current collector coating process. This allows the production of negative electrode sheets with uniform first negative electrode active material distribution and thickness, reducing the likelihood of localized current density unevenness during charge and discharge, improving battery cycle performance, and extending both the service life and storage life. However, the oil absorption value of graphite should not be too small, because too small an oil absorption value has high requirements on the particle regularity or particle size distribution of graphite, and the cost is relatively high, which is not conducive to the large-scale industrial production of lithium-ion batteries. Therefore, the oil absorption value range of the first negative electrode active material is set within a certain smaller range.

[0100] 2. The swelling rate of the first binder in the first negative electrode layer ranges from 30% to 50%. Using a first binder with a low swelling rate for the first negative electrode layer ensures a low battery cycle expansion rate. The swelling rate of the binder is measured by the amount of electrolyte absorbed after the binder is formed into a film, soaked in electrolyte, and then baked at high temperature. Under the same conditions, the more electrolyte the binder absorbs, the greater the swelling rate. A low swelling rate of the first binder helps improve the bonding between the first negative electrode layer and the negative electrode current collector, effectively reducing the delamination and powdering of the negative electrode sheet, thereby improving the battery's cycle performance.

[0101] 3. The oil absorption value of the second negative electrode active material in the second negative electrode layer ranges from 40ml / 100g to 50ml / 100g. Using a second negative electrode active material with a higher oil absorption value in the second negative electrode layer ensures good rate performance of the battery. Because of its larger specific surface area and rich pore structure, negative electrode active materials with high oil absorption values ​​can form a denser conductive network with the conductive agent in the negative electrode sheet, thereby reducing the negative electrode sheet resistance and improving electron transfer efficiency. This helps improve the battery's charge and discharge efficiency. Furthermore, high oil absorption means it can absorb more electrolyte, increasing the contact area between the negative electrode sheet and the electrolyte, allowing the electrolyte to penetrate more quickly and evenly into the interior of the negative electrode sheet. This not only increases the ion transfer rate at the interface between the negative electrode sheet and the electrolyte, but also reduces the resistance to ion migration, thereby improving the battery's rate performance. However, the oil absorption value of the second negative electrode active material should not be too high. This is because it will cause the second negative electrode active material to absorb too much of the second binder during the homogenization process, which is not conducive to effective contact and filling with the conductive agent particles, thereby reducing the cycle capacity retention rate. Therefore, the oil absorption value range of the second negative electrode active material is set to a relatively large range.

[0102] 4. The swelling rate of the second binder in the second negative electrode layer is 60% to 90%. Using a second binder with a higher swelling rate for the second negative electrode layer ensures good rate performance. A higher swelling rate increases the pore structure within the negative electrode sheet, allowing the second negative electrode active material to provide more lithium insertion sites, ensuring that more active lithium ions can be inserted. A higher swelling rate also enhances the electrolyte absorption capacity, allowing lithium ions to be quickly inserted into the second negative electrode active material, effectively improving the battery's fast charging performance and preventing lithium deposition at high current densities. However, the swelling rate should not be too high, as it increases the thickness of the negative electrode sheet, which in turn affects the battery's cycle capacity retention and cycle expansion rate. This is because the negative electrode active material expands during cycling, which can weaken the bond between the negative electrode active material and the negative electrode current collector, and may even cause debonding. Therefore, the swelling rate of the second binder is set within a relatively large range.

[0103] 5. The above 1-4 points are combined with the structural design that the thickness of the first negative electrode layer gradually increases in the direction away from the pole ear and the thickness of the second negative electrode layer gradually decreases in the direction away from the pole ear, and the structural design that the thickness of the first negative electrode layer accounts for 10%-40% of the combined thickness of the first negative electrode layer and the second negative electrode layer near the pole ear, and the thickness of the first negative electrode layer accounts for 60%-90% of the combined thickness of the first negative electrode layer and the second negative electrode layer away from the pole ear; in the first aspect, the thickness of the second negative electrode layer of the double-layer negative electrode sheet of the present invention gradually becomes thicker than the first negative electrode layer near the pole ear, and the oil absorption value of the second negative electrode active material of the second negative electrode layer is large and the second binder The swelling rate is large, which solves the technical problem of lithium plating caused by the high current density near the tab. Secondly, the thickness of the first negative electrode layer accounts for 10%-40% of the combined thickness of the first negative electrode layer and the second negative electrode layer. At a distance from the tab, the thickness of the first negative electrode layer accounts for 60%-90% of the combined thickness of the first negative electrode layer and the second negative electrode layer, which improves the overall cohesion of the negative electrode sheet at both ends and helps to improve the cycle performance of the battery. Thirdly, the oil absorption value, swelling rate and porosity of the negative electrode sheet are more reasonably distributed in the direction away from the tab, thereby effectively improving the rate performance and cycle performance of the battery while ensuring the design of a high energy density system.

[0104] Example 2:

[0105] like Figure 1-6 As shown, the second embodiment of the present invention provides a process for preparing a double-layer negative electrode sheet, which is used to prepare a double-layer negative electrode sheet as described in the first embodiment, and the preparation steps include the following:

[0106] Step S1: mixing a first negative electrode active material, a conductive agent, a thickener, and a first binder in a certain mass ratio, adding a solvent (the solvent may be deionized water), stirring with a blender, and mixing uniformly to prepare a first negative electrode slurry;

[0107] The oil absorption value of the first negative electrode active material is in the range of 25 ml / 100 g to 35 ml / 100 g, and the swelling rate of the first binder is in the range of 30% to 50%.

[0108] Among them, the above substances are as follows in proportion by mass:

[0109] The proportion range of the first negative electrode active material: 88-98%;

[0110] The proportion range of conductive agent: 0-3%;

[0111] Thickener ratio range: 0-3%;

[0112] The proportion range of the second binder: 0.5-10%;

[0113] Step S2: mixing the second negative electrode active material, the conductive agent, the thickener, and the second binder in a certain mass ratio, adding a solvent (the solvent may be deionized water), stirring with a blender, and mixing uniformly to prepare a second negative electrode slurry;

[0114] The oil absorption value of the second negative electrode active material is in the range of 40 ml / 100 g to 50 ml / 100 g, and the swelling rate of the second binder is in the range of 60% to 90%.

[0115] Among them, the above substances are as follows in proportion by mass:

[0116] The proportion range of the second negative electrode active material: 88-98%;

[0117] The proportion range of conductive agent: 0-3%;

[0118] Thickener ratio range: 0-3%;

[0119] The proportion range of the second binder: 0.5-10%;

[0120] Step S3: Coating is performed on both sides of the negative electrode current collector through a double-layer extrusion coating die, such as Figure 1 As shown, the specific coating area and coating length are:

[0121] First, coat one side of the negative electrode current collector with the first negative electrode slurry in region 1 with a coating length of L1, and then coat the second negative electrode slurry in region 2 on the first negative electrode slurry with a coating length of L1;

[0122] After drying in an oven, the other side of the negative electrode current collector is coated. The first negative electrode slurry is applied to area 3 with a coating length of L2. Then, the second negative electrode slurry is applied to area 4 on the first negative electrode slurry with a coating length of L2.

[0123] Among them, the thickness of the first negative electrode slurry located in area 1 and area 3 gradually increases along the direction away from the pole tab, and the thickness of the second negative electrode slurry located in area 2 and area 4 gradually decreases along the direction away from the pole tab. The thickness of the end face of the first negative electrode layer close to the pole tab is set to H1, and the thickness of the end face of the second negative electrode layer close to the pole tab is set to H3. The thickness of thickness H1 accounts for 10%-40% of the total thickness of H1 and H3; the thickness of the end face of the first negative electrode layer away from the pole tab is set to H2, and the thickness of the end face of the second negative electrode layer away from the pole tab is set to H4. The thickness of thickness H2 accounts for 60%-90% of the total thickness of H2 and H4. The negative electrode sheet is obtained through processes such as oven drying, rolling, slitting and welding the pole tab to the negative electrode collector.

[0124] Example 3:

[0125] like Figure 9 As shown, the third embodiment of the present invention provides a process for preparing a double-layer negative electrode sheet, which is used to prepare a double-layer negative electrode sheet as described in the first embodiment, and the preparation steps include the following:

[0126] Step S1: mixing a first negative electrode active material, a conductive agent, a thickener, and a first binder in a certain mass ratio, adding a solvent (the solvent may be deionized water), stirring with a blender, and mixing uniformly to prepare a first negative electrode slurry;

[0127] The oil absorption value of the first negative electrode active material is in the range of 25 ml / 100 g to 35 ml / 100 g, and the swelling rate of the first binder is in the range of 30% to 50%.

[0128] Among them, the above substances are as follows in proportion by mass:

[0129] The proportion range of the first negative electrode active material: 88-98%;

[0130] The proportion range of conductive agent: 0-3%;

[0131] Thickener ratio range: 0-3%;

[0132] The proportion range of the second binder: 0.5-10%;

[0133] Step S2: mixing the second negative electrode active material, the conductive agent, the thickener, and the second binder in a certain mass ratio, adding a solvent (the solvent may be deionized water), stirring with a blender, and mixing uniformly to prepare a second negative electrode slurry;

[0134] The oil absorption value of the second negative electrode active material is in the range of 40 ml / 100 g to 50 ml / 100 g, and the swelling rate of the second binder is in the range of 60% to 90%.

[0135] Among them, the above substances are as follows in proportion by mass:

[0136] The proportion range of the second negative electrode active material: 88-98%;

[0137] The proportion range of conductive agent: 0-3%;

[0138] Thickener ratio range: 0-3%;

[0139] The proportion range of the second binder: 0.5-10%;

[0140] Step S3: Coating is performed on both sides of the negative electrode current collector using a double-layer extrusion coating die. The specific coating area and coating length are:

[0141] First, coat one side of the current collector with the first negative electrode slurry, applying the coating lengths L4 and L5 to regions 5 and 6, respectively. Then, coat the second negative electrode slurry to regions 7 and 8, respectively, applying the coating lengths L3 and L4.

[0142] After drying in an oven, the other side of the negative electrode current collector is coated. The first negative electrode slurry is applied to regions 9 and 10, with coating lengths of L7 and L8, respectively. Then, the second negative electrode slurry is applied to regions 11 and 12, with coating lengths of L6 and L7, respectively.

[0143] The thickness of the first negative electrode slurry in regions 5 and 9 gradually increases in the direction away from the tab, and the thickness of the second negative electrode slurry in regions 8 and 12 gradually decreases in the direction away from the tab. The coating thicknesses of regions 6, 7, 10, and 11 are the same, the coating thickness of region 6 is equal to the sum of the coating thicknesses of regions 5 and 8, and the coating thickness of region 10 is equal to the sum of the coating thicknesses of regions 9 and 12.

[0144] Among them, in regions 5 and 9, the thickness of the end face of the first negative electrode layer close to the pole ear direction is set to H1, in regions 8 and 12, the thickness of the end face of the second negative electrode layer close to the pole ear direction is set to H3, and the thickness of thickness H1 accounts for 10%-40% of the total thickness of thicknesses H1 and H3; in regions 5 and 9, the thickness of the end face of the first negative electrode layer away from the pole ear direction is set to H2, in regions 8 and 12, the thickness of the end face of the second negative electrode layer away from the pole ear direction is set to H4, and the thickness of thickness H2 accounts for 60%-90% of the total thickness of thicknesses H2 and H4, in regions 7 and 11, the thickness of the second negative electrode layer is the sum of thicknesses H1 and H3, and in regions 6 and 10, the thickness of the first negative electrode layer is the sum of thicknesses H2 and H4.

[0145] Among them, the length L3 satisfies the following relationship: 0≤L3 / (L3+L4+L5)≤10%, the length L5 satisfies the following relationship: 0≤L5 / (L3+L4+L5)≤10%, the length L6 satisfies the following relationship: 0≤L6 / (L6+L7+L8)≤10%, and the length L8 satisfies the following relationship: 0≤L8 / (L6+L7+L8)≤10%.

[0146] The negative electrode sheet is obtained through processes such as oven drying, roller pressing, slitting and welding the tab to the negative electrode current collector.

[0147] Example 4:

[0148] The fourth embodiment of the present invention provides experimental data on a battery rate performance test and a battery cycle performance test.

[0149] First:

[0150] The experimental data of this fourth embodiment uses lithium-ion batteries from 7 experimental examples and 16 comparative examples, where:

[0151] A. The negative electrode sheets of these seven experimental examples were prepared using the double-layer negative electrode sheet preparation process of Example 2. Lithium-ion batteries were then prepared using the conventional process of preparing a positive electrode sheet, then winding the negative electrode sheet with a separator, then winding the positive electrode sheet, then injecting the electrolyte, then packaging, and finally forming the battery. The negative electrode sheets of these seven experimental examples all adopted the double-layer negative electrode sheet structure of Example 1. The differences between these seven experimental examples are as follows: based on Experimental Example 1 and following the principle of the single-variable method, the oil absorption value or swelling ratio of the first or second negative electrode layer of the negative electrode sheets of Experimental Examples 2 through 7 differed from that of Experimental Example 1. Alternatively, the H2 / (H2+H4)% or H1 / (H1+H3)% of Experimental Examples 2 through 7 differed from that of Experimental Example 1 (see Table 1 below for specific differences).

[0152] Only a more specific preparation process of a double-layer negative electrode sheet of Experimental Example 1 is disclosed here (the preparation processes of Experimental Examples 2-7 are the same as those of Experimental Example 1, except that the oil absorption value of the first negative electrode active material of the first negative electrode layer, the oil absorption value of the second negative electrode active material of the second negative electrode layer, the swelling rate of the first binder of the first negative electrode layer, or the swelling rate of the second binder of the second negative electrode layer are different from those of Experimental Example 1, or H2 / (H2+H4)% or H1 / (H1+H3)% are different from those of Experimental Example 1, which will not be repeated here):

[0153] Step S1: A first negative electrode active material, a conductive agent, a thickener, and a first binder are mixed in a mass ratio of 94.6:1.6:1.6:2.2, a solvent (the solvent can be deionized water) is added, and the mixture is stirred with a blender to prepare a first negative electrode slurry. The oil absorption value of the selected first negative electrode active material is 25 ml / 100 g, and the swelling rate of the first binder is 41%;

[0154] Step S2: The second negative electrode active material, the conductive agent, the thickener, and the second binder are mixed in a mass ratio of 94.6:1.6:1.6:2.2, a solvent (the solvent can be deionized water) is added, and the mixture is stirred with a blender to prepare a second negative electrode slurry. The oil absorption value of the selected second negative electrode active material is 42 ml / 100 g, and the swelling rate of the second binder is 70%;

[0155] Step S3: Coating is performed on both sides of the negative electrode current collector through a double-layer extrusion coating die, such as Figure 1As shown, the specific coating area and coating length are: first, the negative electrode current collector is coated on one side, and the first negative electrode slurry is coated in area 1, and the coating length is L1, and then the second negative electrode slurry is coated in area 2 on the first negative electrode slurry, and the coating length is L1; after oven drying, the other side of the negative electrode current collector is coated, and the first negative electrode slurry is coated in area 3, and the coating length is L2, and then the second negative electrode slurry is coated in area 4 on the first negative electrode slurry, and the coating length is L2; ​​wherein, the thickness of the first negative electrode slurry in area 1 and area 3 gradually increases along the direction away from the pole ear, and the thickness of the second negative electrode slurry in area 2 and area 4 gradually decreases along the direction away from the pole ear, and the negative electrode sheet is obtained through oven drying, rolling and slitting.

[0156] The thickness of the end surface of the first negative electrode layer close to the tab is set to H1, and the thickness of the end surface of the second negative electrode layer close to the tab is set to H3. The thickness of H1 accounts for 20% of the total thickness of H1 and H3.

[0157] The thickness of the end surface of the first negative electrode layer away from the electrode tab is set to H2, and the thickness of the end surface of the second negative electrode layer away from the electrode tab is set to H4. The thickness of H2 accounts for 80% of the total thickness of H2 and H4.

[0158] B. Among the 16 comparative examples, they are divided into comparative examples 1 to 12, comparative example 13, comparative example 14 to comparative example 16,

[0159] B1. More specifically, Comparative Examples 1 to 12 and Comparative Examples 14 to 16 were prepared by a double-layer negative electrode sheet preparation process similar to that of Example 2, and then lithium-ion batteries were prepared using existing technology. The only difference between the preparation process of Comparative Examples 1 to 12 and the preparation process of Example 2 is that the preparation process of Comparative Examples 1 to 12 does not follow the preparation process of Example 2.

[0160] The range of values ​​limited by the oil absorption value or swelling rate of the first negative electrode layer and / or the second negative electrode layer, and the difference between the preparation process of Comparative Examples 14 to Comparative Examples 16 and the preparation process of Example 2 is only that: the preparation process of Comparative Examples 14 to Comparative Examples 16 does not follow the thickness of the end face of the first negative electrode layer close to the pole ear direction is set to H1, the thickness of the end face of the second negative electrode layer close to the pole ear direction is set to H3, the thickness of the thickness H1 accounts for 10%-40% of the total thickness of H1 and H3, the thickness of the end face of the first negative electrode layer away from the pole ear direction is set to H2, and the thickness of the second negative electrode layer away from the pole ear direction is set to H3. The thickness of one end surface of the electrode is set to H4, and the thickness H2 accounts for a limited range of 60%-90% of the total thickness of H2 and H4. Meanwhile, Comparative Examples 1 to 5 differ only in the swelling rate of the second negative electrode layer, Comparative Examples 6 to 8 differ only in the oil absorption value of the second negative electrode layer, Comparative Examples 9 to 11 differ only in the swelling rate of the first negative electrode layer, Experimental Example 1 and Comparative Example 12 differ only in the oil absorption value of the first negative electrode layer, and Comparative Examples 14 to 16 differ only in H1 / (H1+H3)% and H2 / (H2+H4)% compared to Experimental Example 1. (For specific differences, please see Table 1 mentioned below.)

[0161] B2. Negative electrode structure of Comparative Example 13 (see Figure 10 ) Different from the double-layer negative electrode structure of Experimental Example 1, the preparation process of Comparative Example 13 is as follows:

[0162] Negative electrode preparation:

[0163] Step S1: A first negative electrode active material, a conductive agent, a thickener, and a first binder are mixed in a mass ratio of 94.6:1.6:1.6:2.2, a solvent (the solvent can be deionized water) is added, and the mixture is stirred with a blender to prepare a first negative electrode slurry. The oil absorption value of the selected first negative electrode active material is 25 ml / 100 g, and the swelling rate of the first binder is 41%;

[0164] Step S2: The second negative electrode active material, the conductive agent, the thickener, and the second binder are mixed in a mass ratio of 94.6:1.6:1.6:2.2, a solvent (the solvent can be deionized water) is added, and the mixture is stirred with a blender to prepare a second negative electrode slurry. The oil absorption value of the selected second negative electrode active material is 42 ml / 100 g, and the swelling rate of the second binder is 70%;

[0165] Step S3: Coating is performed on both sides of the negative electrode current collector through a double-layer extrusion coating die, such as Figure 10As shown, the specific coating area and coating length are: first, the negative electrode current collector is coated, and the first negative electrode slurry is coated in area 13, and the coating length is L9, and then the second negative electrode slurry is coated in area 14 on the first negative electrode slurry, and the coating length is L9; after oven drying, the other side of the negative electrode current collector is coated, and the first negative electrode slurry is coated in area 15, and the coating length is L10, and then the second negative electrode slurry is coated in area 16 on the first negative electrode slurry, and the coating length is L10; wherein, the thickness of the first negative electrode slurry in areas 13 and 15 and the thickness of the second negative electrode slurry in areas 14 and 16 remain unchanged along the direction away from the tab, and the negative electrode sheet is obtained through oven drying, rolling and slitting.

[0166] Among them, please see Figure 10 , the thickness H21 of the first negative electrode layer of Comparative Example 13 = the thickness H22 of the second negative electrode layer.

[0167] Preparation of a full battery: Place the separator between the positive and negative electrodes to isolate them, and wind them to obtain a bare cell. Place the bare cell in an aluminum-plastic film, inject the prepared electrolyte into the dried bare cell, and then go through vacuum packaging, standing, formation, capacity division and other processes to obtain a lithium-ion battery.

[0168] C. The specific parameters of the oil absorption value of the first negative electrode active material, the swelling ratio of the first binder, the oil absorption value of the second negative electrode active material, the swelling ratio of the second binder, H2 / (H2+H4)% and H1 / (H1+H3)% of the 7 experimental examples and 16 comparative examples are shown in Table 1 below:

[0169] (To simplify experimental comparison, H1=H4 and H2=H3 in the following 7 experimental examples and 16 comparative examples, so that H2 / (H2+H4)%+H1 / (H1+H3)%=100%, that is, H2 / (H2+H4)% and H1 / (H1+H3)% complement each other.)

[0170] Table 1

[0171]

[0172] D. The above-mentioned method for calculating the oil absorption value of the negative electrode active material is as follows:

[0173] Oil absorption value of negative electrode active material:

[0174] Oil absorption value, also known as oil absorption capacity, is a characteristic of the powder's ability to absorb oil. It is usually expressed by the volume of linseed oil added when the torque generated by the viscosity change of the powder and oil mixture reaches 70% of the maximum value. The detection and calculation method steps are as follows:

[0175] (ⅰ) Weigh the negative electrode active material, mass ≥ 20g;

[0176] (ii) Place the weighed sample into the mixing chamber of the oil absorption value tester, set the oil pump oil droplet speed U to 2.0mL / min and the rotor speed to 62.5r / min.

[0177] (iii) The oil absorption value D of the negative electrode active material is the volume of oil consumed per unit mass of sample, calculated according to the following formula:

[0178]

[0179] V-The volume of linseed oil consumed at 70% of the maximum torque, in cubic centimeters (cm3);

[0180] m--mass of powder sample, in grams (g);

[0181] The unit of oil absorption value D is ml / 100g.

[0182] E. The above-mentioned method for calculating the swelling rate of the binder is as follows:

[0183] (ⅰ) The adhesive was dried at room temperature and then transferred to an oven at 160°C for 6h to prepare a dry bubble-free film;

[0184] (ⅱ) After slicing, weigh the sample weight m1;

[0185] (iii) Place the sample in a test bottle and pour in a lithium-free electrolyte (EC:EMC:DEC = 3:2:5), with the electrolyte volume being no less than 15 times that of the test sample;

[0186] (ⅳ) The test bottle was placed in a 60 ℃ oven and heated for 72h;

[0187] (v). Then measure the weight of the sample again, m2;

[0188] (ⅵ) The swelling ratio W of the binder is calculated as follows:

[0189]

[0190] second:

[0191] The battery rate performance test and battery cycle performance test were performed on the above 7 experimental examples and 16 comparative examples of Example 4, respectively. The standards of these two test methods are as follows:

[0192] (1). Battery rate performance test

[0193] At 25°C, the lithium-ion batteries prepared in Experimental Examples 1-7 and Comparative Examples 1-16 were charged at a rate of 1-4C and discharged at a rate of 1C 20 times, and then the fully charged batteries were disassembled to observe the lithium deposition on the surface of the negative electrode.

[0194] (2). Battery cycle performance test

[0195] At 25° C., the lithium-ion batteries prepared in Experimental Examples 1-7 and Comparative Examples 1-16 were charged and discharged at a rate of 1C for 800 cycles. The capacity retention rate of the lithium-ion batteries and the battery expansion rate after full charge were recorded.

[0196] The above two test experiments were carried out, and the experimental data after the two tests are as follows in Table 2:

[0197] Table 2

[0198]

[0199] Therefore, from Table 2, we can see that

[0200] Experimental Examples 1 to 7 of the present application can ensure good battery rate performance and battery cycle performance;

[0201] Comparative Examples 1-5 show that when the swelling rate of the second binder gradually increases within the given range, rate performance improves with the increase in the swelling rate of the second binder. This is because the increased swelling rate enhances the electrolyte's wetting of the interior of the negative electrode sheet, reducing the diffusion resistance of lithium ions. However, simply changing the swelling rate of the second binder does not significantly improve rate performance, making it difficult to meet fast charging performance requirements. This is because relying solely on the second binder's liquid absorption capacity prevents the electrolyte from effectively wetting the interior of the negative electrode sheet. During the electrochemical reaction, lithium ions have difficulty embedding into the negative electrode sheet, resulting in lithium ion accumulation in the first negative electrode layer or on the surface of the second negative electrode layer, causing lithium plating. Furthermore, when the swelling rate of the second binder is too high (as in Comparative Example 5), the expansion rate after 800 cycles at room temperature is large, failing to meet customer usage requirements. Preferably, the swelling rate of the second binder in the second negative electrode layer is in the range of 60-90%.

[0202] When the oil absorption of the second negative electrode active material gradually increases within the given range (as in Comparative Examples 6-8), rate performance improves. This is because high-oil-absorption negative electrode active materials have a larger specific surface area and rich pore structure, allowing them to absorb more electrolyte, increasing the contact area between the negative electrode sheet and the electrolyte, allowing the electrolyte to penetrate more quickly and evenly into the negative electrode sheet. This not only improves the ion transport rate at the negative electrode sheet / electrolyte interface but also reduces the resistance to ion migration. When the oil absorption of the second negative electrode active material is less than 40 ml / 100 g (as in Comparative Example 6), rate performance is lower. When the oil absorption of the second negative electrode active material is greater than 50 ml / 100 g (as in Comparative Example 8), rate performance is good, but the capacity retention drops sharply after 800 cycles at room temperature. This is because the second negative electrode active material absorbs too much binder during the homogenization process, which is not conducive to effective contact and filling with the conductive agent particles. Only when the oil absorption value of the second negative electrode active material is in the range of 40-50ml / 100g can the lithium-ion battery take into account electrochemical properties such as rate performance and cycle capacity retention rate.

[0203] Further increasing the swelling ratio of the first binder based on Comparative Example 7 (as in Comparative Examples 9-11) enhances liquid absorption capacity and effectively improves rate performance. However, when the swelling ratio of the first binder exceeds 50%, the expansion ratio after 800 cycles at room temperature is high, resulting in excessive battery thickness and failure to meet requirements. Within an appropriate range of the swelling ratio of the first binder, the battery achieves both high rate performance and low cyclic expansion ratio. Preferably, the swelling ratio of the first binder in the first negative electrode layer is in the range of 30-50%.

[0204] Experimental Examples 1-5 and Comparative Example 12 show that the oil absorption of the first negative electrode active material exceeds 35 ml / 100 g, and the capacity retention rate after 800 cycles at room temperature is very low. Preferably, the oil absorption of the first negative electrode active material is in the range of 25-35 ml / 100 g, and the swelling rate of the first binder, the oil absorption of the second negative electrode active material, and the swelling rate of the second binder are all within the preferred numerical ranges. Only then can the battery simultaneously achieve high rate performance, high cycle capacity retention, and low expansion rate.

[0205] Experimental Example 1 and Comparative Example 13 differ in their coating methods. Comparative Example 13 employs a conventional double-layer coating, while the optimized coating method of Experimental Example 1 allows for more complete electrolyte infiltration of the second negative electrode layer near the tab, reducing lithium deposition near the tab and further improving the kinetic performance of the negative electrode sheet. Therefore, by optimizing the appropriate coating method, a negative electrode active material with a reasonable oil absorption value, and a binder with a reasonable swelling rate for each negative electrode layer, lithium deposition at the tab can be avoided, improving the battery's rate capability and cycle performance.

[0206] Experimental Examples 1, 6-7, and Comparative Examples 14-16 demonstrate that, away from the tabs, only when the H2 / (H2+H4)% ratio is between 60% and 90% can the battery simultaneously exhibit good rate performance, cycle capacity retention, and cycle expansion rate. If the H2 / (H2+H4)% ratio is less than 60% (as in Comparative Example 14) or greater than 90% (as in Comparative Example 15), the peeling force and cohesive force at both ends of the negative electrode sheet decrease, causing the negative electrode sheet to easily shed during charge and discharge, thereby affecting the battery's cycle capacity retention and cycle expansion rate. If the H2 / (H2+H4)% ratio is 100% (as in Comparative Example 16), meaning there is no interface between the first and second negative electrode layers at the ends away from the tabs, the battery's overall performance is compromised due to reduced rate performance and cycle capacity retention. Similarly, only when the thickness of H1 is within the range of 10%-40% (H1 / (H1+H3)%), the battery can have good rate performance, cycle capacity retention rate and cycle expansion rate at the same time.

[0207] Therefore, the above experimental data illustrate that the double-layer negative electrode plate of the first embodiment can effectively improve the rate performance and cycle performance of the battery while ensuring a high energy density system design.

[0208] The above are only 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 in the scope of protection of the present invention.

Claims

1. A double-layer negative electrode plate, characterized in that: The present invention comprises a negative electrode current collector, a tab provided on the negative electrode current collector, and a first negative electrode layer coated on the negative electrode current collector, wherein the thickness of the first negative electrode layer gradually increases in a direction away from the tab, a second negative electrode layer is coated on the first negative electrode layer, and the thickness of the second negative electrode layer gradually decreases in a direction away from the tab, the first negative electrode layer comprises a first negative electrode active material and a first binder, and the second negative electrode layer comprises a second negative electrode active material and a second binder; The oil absorption value of the first negative electrode active material is in the range of 25 ml / 100 g to 35 ml / 100 g, and the swelling rate of the first binder is in the range of 30% to 50%. The oil absorption value of the second negative electrode active material is in the range of 40 ml / 100 g to 50 ml / 100 g, and the swelling rate of the second binder is in the range of 60% to 90%. The thickness of one end surface of the first negative electrode layer close to the tab is set to H1, and the thickness of one end surface of the second negative electrode layer close to the tab is set to H3. The thickness of H1 accounts for 10%-40% of the total thickness of H1 and H3. The thickness of the end surface of the first negative electrode layer away from the electrode tab is set to H2, and the thickness of the end surface of the second negative electrode layer away from the electrode tab is set to H4. The thickness of H2 accounts for 60%-90% of the total thickness of H2 and H4.

2. A double-layer negative electrode sheet according to claim 1, characterized in that: The thickness of the first negative electrode layer gradually increases with a certain slope along the direction away from the electrode tab, and the thickness of the second negative electrode layer gradually decreases with a certain slope along the direction away from the electrode tab.

3. The double-layer negative electrode according to claim 1, characterized in that: The thickness of the first negative electrode layer gradually increases in a step-like manner along a direction away from the electrode tab, and the thickness of the second negative electrode layer gradually decreases in a step-like manner along a direction away from the electrode tab.

4. The double-layer negative electrode according to claim 1, characterized in that: The thickness of the first negative electrode layer gradually increases in a certain arc along the direction away from the electrode tab, and the thickness of the second negative electrode layer gradually decreases in a certain arc along the direction away from the electrode tab.

5. The double-layer negative electrode sheet according to claim 1, characterized in that: The first negative electrode active material and the second negative electrode active material are graphite, silicon carbon or hard carbon respectively; The first adhesive and the second adhesive are respectively one of styrene-butadiene rubber, styrene-propylene rubber, polyurethane or polyacrylic acid, or a mixture of several of them.

6. The double-layer negative electrode sheet according to claim 1, characterized in that: The first negative electrode layer further includes a conductive agent and a thickener, The mass proportions of the various substances in the first negative electrode layer are as follows: The proportion range of the first negative electrode active material: 88-98%; The proportion range of conductive agent: 0-3%; Thickener ratio range: 0-3%; The proportion range of the first binder: 0.5-10%; The second negative electrode layer further includes a conductive agent and a thickener, The mass proportions of the various substances in the second negative electrode layer are as follows: The proportion range of the second negative electrode active material: 88-98%; The proportion range of conductive agent: 0-3%; Thickener ratio range: 0-3%; The proportion range of the second binder is: 0.5-10%.

7. A process for preparing a double-layer negative electrode sheet, characterized in that: It is used to prepare a double-layer negative electrode sheet as claimed in any one of claims 1 to 6, and its preparation steps include the following: Step S1: mixing a first negative electrode active material, a conductive agent, a thickener, and a first binder according to a certain mass ratio, adding a solvent, and stirring with a stirrer to prepare a first negative electrode slurry; Step S2: mixing the second negative electrode active material, the conductive agent, the thickener, and the second binder according to a certain mass ratio, adding a solvent, and stirring with a stirrer to prepare a second negative electrode slurry; Step S3: Coating is performed on both sides of the negative electrode current collector using a double-layer extrusion coating die. The specific coating area and coating length are: First, coat one side of the negative electrode current collector with the first negative electrode slurry in region 1 with a coating length of L1, and then coat the second negative electrode slurry in region 2 on the first negative electrode slurry with a coating length of L1; After drying in an oven, the other side of the current collector is coated. The first negative electrode slurry is applied to area 3 with a coating length of L2. Then, the second negative electrode slurry is applied to area 4 on the first negative electrode slurry with a coating length of L2. Among them, the thickness of the first negative electrode slurry located in area 1 and area 3 gradually increases in the direction away from the electrode tab, and the thickness of the second negative electrode slurry located in area 2 and area 4 gradually decreases in the direction away from the electrode tab. The negative electrode sheet is obtained through the processes of oven drying, roller pressing, slitting and welding the electrode tab to the negative electrode current collector.

8. A process for preparing a double-layer negative electrode sheet, characterized in that: It is used to prepare a double-layer negative electrode sheet as claimed in any one of claims 1 to 6, and its preparation steps include the following: Step S1: mixing a first negative electrode active material, a conductive agent, a thickener, and a first binder according to a certain mass ratio, adding a solvent, and stirring with a stirrer to prepare a first negative electrode slurry; Step S2: mixing the second negative electrode active material, the conductive agent, the thickener, and the second binder according to a certain mass ratio, adding a solvent, and stirring with a stirrer to prepare a second negative electrode slurry; Step S3: Coating is performed on both sides of the negative electrode current collector using a double-layer extrusion coating die. The specific coating area and coating length are: First, coat one side of the current collector with the first negative electrode slurry, applying the coating lengths L4 and L5 to regions 5 and 6, respectively. Then, coat the second negative electrode slurry to regions 7 and 8, respectively, applying the coating lengths L3 and L4. After drying in an oven, the other side of the negative electrode current collector is coated. The first negative electrode slurry is applied to regions 9 and 10, with coating lengths of L7 and L8, respectively. Then, the second negative electrode slurry is applied to regions 11 and 12, with coating lengths of L6 and L7, respectively. Among them, the thickness of the first negative electrode slurry located in regions 5 and 9 gradually increases in the direction away from the tab, and the thickness of the second negative electrode slurry located in regions 8 and 12 gradually decreases in the direction away from the tab. Among them, the coating thickness of regions 6, 7, 10 and 11 are consistent, the coating thickness of region 6 is equal to the sum of the coating thicknesses of regions 5 and 8, and the coating thickness of region 10 is equal to the sum of the coating thicknesses of regions 9 and 12. The negative electrode sheet is obtained through the processes of oven drying, rolling, slitting and welding the tab to the negative electrode collector.

9. The process for preparing a double-layer negative electrode sheet according to claim 8, characterized in that: The length L3 satisfies the following relationship: 0≤L3 / (L3+L4+L5)≤10%, the length L5 satisfies the following relationship: 0≤L5 / (L3+L4+L5)≤10%, the length L6 satisfies the following relationship: 0≤L6 / (L6+L7+L8)≤10%, and the length L8 satisfies the following relationship: 0≤L8 / (L6+L7+L8)≤10%.

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