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

Abstract

The application provides a negative electrode sheet and a preparation method thereof, and a winding battery cell, and relates to the technical field of lithium ion batteries. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material, a conductive agent and a binder; in the length direction of the negative electrode sheet, the negative electrode active material layer comprises a starting end and a tail end arranged oppositely; the negative electrode active material layer is divided into n sub-active material layers in the direction from the starting end to the tail end; the mass content of the binder in the i-th sub-active material layer is greater than that in the (i-1)-th sub-active material layer; n is any integer in the range of 15-25; i is any integer in the range of 1-n. The application can solve the problem that the stress and expansion rate of the negative electrode sheet are different at different positions in the winding structure, which leads to uneven expansion, delamination or active material loss of the negative electrode sheet, and capacity retention rate reduction of the lithium ion battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a negative electrode sheet and its preparation method, and a wound battery cell. Background Technology

[0002] Currently, lithium-ion batteries possess advantages such as high voltage, high energy density, and long lifespan, and are widely used in energy storage and electric vehicles. The core is formed by winding positive and negative electrode sheets with a separator. During charging and discharging, the negative electrode sheet undergoes significant volume expansion due to lithium-ion insertion and extraction, especially in high-energy-density systems (such as silicon-carbon anodes). Therefore, the binder in the negative electrode sheet not only needs to provide adhesion and retention but also needs to restrain and buffer the volume expansion of the active material.

[0003] However, due to the geometric characteristics of the core structure, the expansion of the electrode near the winding center is limited during charging and discharging, while the expansion rate of the electrode far from the winding center is much greater than that of the electrode near the winding center, resulting in stress concentration and shedding of the active layer, which in turn affects the capacity retention rate of the lithium-ion battery. Summary of the Invention

[0004] In view of the technical problems existing in the background art, the main purpose of the present invention is to provide a negative electrode sheet and its preparation method, and a wound cell, so as to solve the problem that the existing negative electrode sheet has different stress and expansion rate at different positions in the winding structure, which leads to uneven expansion, delamination or loss of active material, and thus a decrease in the capacity retention rate of lithium-ion batteries.

[0005] To achieve the above objectives, according to one aspect of the present invention, a negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, a conductive agent, and a binder; in the length direction of the negative electrode sheet, the negative electrode active material layer includes a starting end and a tail end disposed opposite to each other; the negative electrode active material layer is divided into n segments of active material layer along the direction from the starting end to the tail end; the mass content of the binder in the i-th segment of active material layer is greater than the mass content of the binder in the (i-1)-th segment of active material layer; wherein, n is any integer from 15 to 25; and i is any integer from 1 to n.

[0006] Furthermore, when i=1, the mass content of the binder in the first active material layer is W1, where W1 is 0.6%~1.8%; when i=n, ​​the mass content of the binder in the nth active material layer is W... n W n The percentage is 2.2% to 4.5%.

[0007] More preferably, W1 is 0.8%~1.8%, W n It ranges from 2.2% to 3.5%.

[0008] Furthermore, along the direction from the starting end to the tail end, the length of the negative electrode active material layer is L, and the mass content of the binder in the i-th segment of the active material layer is W. i Satisfying the following relationship: W i =W1+(W n -W1)×[e k(i-1) / (n-1) -1] / (e k -1); where 4m≤L≤6m; 0.8≤k≤3.0; e is the natural constant, e=2.71828.

[0009] More preferably, n is any integer from 15 to 20; 1.0% ≤ W1 ≤ 1.5%; 2.5% ≤ W n ≤3.5%.

[0010] Further, in the first active material layer, the binder comprises, by total mass, 60%~90% styrene-butadiene rubber, 10%~40% sodium carboxymethyl cellulose, and 0%~5% polyacrylic acid; and / or, in the nth active material layer, the binder comprises, by total mass, 10%~40% styrene-butadiene rubber, 0%~5% sodium carboxymethyl cellulose, and 60%~90% polyacrylic acid.

[0011] Furthermore, the negative electrode active material includes artificial graphite and silicon-based materials; wherein, the silicon-based material includes at least one of silicon-carbon composite material, nano-silicon, and silicon suboxide; based on the total mass of the negative electrode active material, the mass content of artificial graphite is 80% to 97%, and the mass content of silicon-based materials is 3% to 20%.

[0012] Furthermore, based on the total mass of the negative electrode active material layer, the mass content of the negative electrode active material is 94%~98%, the mass content of the binder is 0.6%~4.5%, and the mass content of the conductive agent is 1.4%~5.4%.

[0013] Furthermore, the thickness of the negative electrode active material layer is 100μm~200μm.

[0014] Furthermore, the compaction density of the negative electrode is 1.5 g / cm³. 3 ~1.7g / cm 3 .

[0015] According to a second aspect of the present invention, a method for preparing a negative electrode sheet is provided, comprising the following steps: S1, mixing a first negative electrode active material, a first conductive agent, and a first binder to obtain a first negative electrode slurry; mixing a second negative electrode active material, a second conductive agent, and a second binder to obtain a second negative electrode slurry; wherein, the mass content of the first binder in the first negative electrode slurry is less than the mass content of the second binder in the second negative electrode slurry; S2, mixing the first negative electrode slurry and the second negative electrode slurry to obtain 1 to nth mixed negative electrode slurries respectively; wherein, the first negative electrode slurry in the i-th mixed negative electrode slurry... The mass content of the first negative electrode slurry in the (i-1)th mixed negative electrode slurry is less than the mass content of the first negative electrode slurry in the (i-1)th mixed negative electrode slurry, and the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry is greater than the mass content of the second negative electrode slurry in the (i-1)th mixed negative electrode slurry; n is any integer from 15 to 25; i is any integer from 1 to n; S3, along the length direction of the negative electrode current collector, from the beginning end to the end end of the length direction of the negative electrode current collector, the first to nth mixed negative electrode slurries are sequentially coated on the surface of the negative electrode current collector, and after drying and rolling treatment, n segments of active material layer are formed to obtain the negative electrode sheet.

[0016] Further, the mass content of the first binder in the first negative electrode slurry is W1, where W1 is 0.6%~1.8%; and / or, the mass content of the second binder in the second negative electrode slurry is W... n W n The percentage is 2.2% to 4.5%.

[0017] Furthermore, when i=1, based on the total mass of the first mixed negative electrode slurry, the mass content of the first negative electrode slurry is 95%~100%, and the mass content of the second negative electrode slurry is 0~5%; and / or, when i=n, ​​based on the total mass of the nth mixed negative electrode slurry, the mass content of the first negative electrode slurry is 0~15%, and the mass content of the second negative electrode slurry is 90%~100%.

[0018] Furthermore, the difference between the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry and the mass content of the second negative electrode slurry in the (i-1)-th mixed negative electrode slurry is 0.5%~16%.

[0019] Furthermore, the solid content of the first negative electrode slurry is 45%~50%; the solid content of the second negative electrode slurry is 45%~50%.

[0020] Furthermore, the viscosity of the first negative electrode slurry at 25°C is 2000 mPa·s to 5000 mPa·s; the viscosity of the second negative electrode slurry at 25°C is 2000 mPa·s to 5000 mPa·s.

[0021] Furthermore, the coating speed is 8 m / min to 12 m / min.

[0022] According to a third aspect of the present invention, a wound battery cell is provided, the wound battery cell being formed by sequentially stacking and winding a positive electrode sheet, a separator, and a negative electrode sheet; the starting end of the negative electrode sheet is located on the side of the wound battery cell near the winding center, and the tail end of the negative electrode sheet is located on the side of the wound battery cell away from the winding center; wherein, the negative electrode sheet includes the aforementioned negative electrode sheet, or a negative electrode sheet prepared by the aforementioned method for preparing a negative electrode sheet.

[0023] By applying the technical solution of this invention, the binder mass content in the negative electrode active material layer is gradually increased segment by segment from the starting end to the ending end. Specifically, the region near the starting end, close to the winding center and where expansion is restricted, has a lower binder mass content, which helps reduce the degree of constraint on the expansion of the negative electrode active material, thereby appropriately releasing expansion stress. Along the ending end, the external constraints on the negative electrode gradually weaken, and the degree of freedom of expansion gradually increases. Therefore, by gradually increasing the binder mass content segment by segment, the adhesion retention and expansion constraint capabilities of the negative electrode active material layer can be gradually enhanced, suppressing excessive expansion in regions far from the winding center. This helps to make the expansion behavior of the negative electrode at different winding positions more balanced, reducing the difference in expansion rate after cycling, and reducing pulverization, delamination, or loss of active material, thereby improving the capacity retention rate of the lithium-ion battery. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of the negative electrode sheet prepared in Example 1 of the present invention;

[0025] Figure 2 This is a graph showing the relationship between the binder mass content of the active material layers in segments 1 to 20 of the present invention and the segment number.

[0026] Figure 3 This is a schematic diagram showing the division of different test areas of the negative electrode sheets prepared in Examples 1-8 and Comparative Examples 1-3 of the present invention;

[0027] Figure 4 This is a schematic diagram of the cycle performance test curves of Embodiments 1-3 and Comparative Example 1 of the present invention;

[0028] Explanation of reference numerals in the attached figures:

[0029] 1-Negative electrode current collector; 2-Negative electrode active material layer; 21-First segment active material layer; 22-Second segment active material layer; 23-Third segment active material layer; 2n-Nth segment active material layer; A-Starting end of negative electrode active material layer 2 along the length of the negative electrode sheet; B-Tailing end of negative electrode active material layer 2 along the length of the negative electrode sheet; X-Direction from starting end A to tail end B;

[0030] 3-Negative electrode current collector; 4-Negative electrode active material layer; 31-Head region; 32-Middle region; 33-Tail region; A-The starting end of the negative electrode active material layer 4 along the length of the negative electrode sheet; B-The tail end of the negative electrode active material layer 4 along the length of the negative electrode sheet; C indicates the position of the negative electrode active material layer at 1 / 2L along the length of the negative electrode sheet; X indicates the direction from the starting end A to the tail end B. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0032] As described in the background section of this invention, existing technologies suffer from differences in stress and expansion rate at different positions of the negative electrode sheet in the winding structure, leading to uneven expansion, delamination, or loss of active material, which in turn causes a decrease in the capacity retention rate of the lithium-ion battery. To address these issues, in a typical embodiment of this invention, a negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a conductive agent, and a binder. Along the length of the negative electrode sheet, the negative electrode active material layer includes a starting end and a tail end disposed opposite to each other. Along the direction from the starting end to the tail end, the negative electrode active material layer is divided into n sub-segments of active material layer. The mass content of the binder in the i-th sub-segment of active material layer is greater than the mass content of the binder in the (i-1)-th sub-segment of active material layer. Wherein, n is any integer from 15 to 25; and i is any integer from 1 to n.

[0033] Figure 1 This is a schematic diagram of the structure of a negative electrode sheet provided in Embodiment 1 of the present invention. Figure 1 As shown, the negative electrode sheet includes a negative electrode current collector 1 and a negative electrode active material layer 2. The negative electrode active material layer 2 includes a first active material layer 21, a second active material layer 22, a third active material layer 23, ..., an nth active material layer 2n; A represents the starting end of the negative electrode active material layer 2 along the length of the negative electrode sheet, B represents the tail end of the negative electrode active material layer 2 along the length of the negative electrode sheet, and X represents the direction from the starting end A to the tail end B.

[0034] Specifically, the negative electrode sheet provided by the present invention includes a negative electrode current collector 1 and a negative electrode active material layer 2 disposed on at least one side of the negative electrode current collector 1. The negative electrode active material layer 2 may be disposed on only one surface of the negative electrode current collector 1, or it may be disposed on both surfaces of the negative electrode current collector 1. The negative electrode current collector 1 provides an electron conduction path and structural support for the negative electrode active material layer 2, which participates in the electrochemical reaction and forms the main lithium storage region of the negative electrode sheet.

[0035] The negative electrode active material layer 2 includes a negative electrode active material, a conductive agent, and a binder. The negative electrode active material is used to enable lithium ion insertion and extraction, the conductive agent improves the electronic conductivity of the negative electrode active material layer 2, and the binder maintains the structural stability of the negative electrode active material layer 2 and improves the bonding stability between the negative electrode active material layer 2 and the negative electrode current collector 1. The negative electrode active material is prone to volume expansion during charging and discharging; therefore, the content distribution of the binder affects the expansion constraint ability, anti-pulverization ability, and anti-delamination ability of the negative electrode active material layer 2.

[0036] Along the length of the negative electrode sheet, the negative electrode active material layer 2 includes a starting end A and a tail end B positioned opposite each other. Along the X direction, that is, along the direction from the starting end A to the tail end B, the negative electrode active material layer 2 is divided into n sub-segments of active material layer, namely, the first sub-segment active material layer 21, the second sub-segment active material layer 22, the third sub-segment active material layer 23, ..., the nth sub-segment active material layer 2n. This division refers to the formation of multiple regions with different binder mass contents along the length of the negative electrode active material layer 2; it does not require that adjacent active material layers have a clear physical gap or independent interface.

[0037] The binder mass content in the i-th active material layer is greater than that in the (i-1)-th active material layer, where i is any integer from 1 to n. In other words, along the X direction, the binder mass content in the 1st active material layer 21, the 2nd active material layer 22, the 3rd active material layer 23, ..., the nth active material layer 2n increases segment by segment. If the binder mass content in the 1st to the nth active material layers is denoted as W1, W2, W3, ..., W... n Then it satisfies W1 < W2 < W3 < ... < W n .

[0038] Meanwhile, n can be any integer from 15 to 25, meaning n can be 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25. By dividing the negative electrode active material layer 2 into 15 to 25 segments and gradually increasing the binder content in each segment along the X direction, it helps to ensure that the binder content in the negative electrode active material layer 2 increases gradually from the starting end A to the ending end B. This helps to reduce abrupt changes in composition between adjacent regions and also reduces the difficulty of slurry supply and coating control.

[0039] Regarding the value of i, since there is no 0th active material layer before the 1st active material layer 21, when i=2, it means that the binder mass content in the 2nd active material layer 22 is greater than the binder mass content in the 1st active material layer 21; when i=3, it means that the binder mass content in the 3rd active material layer 23 is greater than the binder mass content in the 2nd active material layer 22; and so on, when i=n, ​​it means that the binder mass content in the nth active material layer 2n is greater than the binder mass content in the (n-1)th active material layer. Thus, the negative electrode active material layer 2 forms a distribution with progressively increasing binder mass content along the direction from the starting end A to the tail end B.

[0040] Through the above configuration, a layer 2 of negative electrode active material with progressively increasing binder mass content is formed along the length of the negative electrode sheet. After the negative electrode sheet is wound to form a wound cell, the starting end A corresponds to the region near the winding center. This region is squeezed and constrained by the outer electrode sheet and other layered materials, resulting in a small expansion space. During charge-discharge cycles, it is prone to expansion restriction and stress concentration. The tail end B corresponds to the region far from the winding center. This region is subject to weaker external constraints, resulting in a larger expansion space. During charge-discharge cycles, it is more prone to significant thickness expansion. If the binder content is the same at all locations, it is difficult to compensate for the differences in stress and expansion space at different locations in the wound cell. This can easily lead to non-uniform expansion of the negative electrode sheet, causing localized pulverization, delamination, or loss of active material.

[0041] Therefore, this invention gradually increases the binder content in the negative electrode active material layer 2 from the starting end A to the ending end B. Specifically, the region near the starting end A, which is close to the winding center and has limited expansion, has a lower binder content, which helps to reduce the degree of constraint on the expansion of the negative electrode active material, thereby appropriately releasing expansion stress. Along the ending end B, the external constraint on the negative electrode sheet gradually weakens and the degree of expansion freedom gradually increases. Therefore, by gradually increasing the binder content, the adhesion retention and expansion constraint capabilities of the negative electrode active material layer 2 can be gradually enhanced, suppressing excessive expansion in regions far from the winding center. This helps to make the expansion behavior of the negative electrode sheet more balanced at different winding positions, reduce the difference in expansion rate after cycling, and reduce pulverization, delamination, or loss of active material, thereby improving the capacity retention rate of the lithium-ion battery.

[0042] in addition, Figure 1 The diagram illustrates the gradual increase in binder mass content in each segment of the active material layer by showing the dot-like filling density from the starting end A to the tail end B. This filling method is only used to indicate the trend of binder content change and is not used to limit the actual microstructure, pore structure or particle distribution of each segment of the active material layer.

[0043] In some embodiments, when i=1, the mass content of the binder in the first active material layer is W1, where W1 is 0.6%~1.8%; when i=n, ​​the mass content of the binder in the nth active material layer is W... n W n The binder content is 2.2%~4.5%. By controlling the binder mass content W1 in the first active material layer, it is helpful to ensure that the starting region meets the basic adhesion and structural maintenance requirements while avoiding excessive binder content. If the binder content is too high, it will correspondingly reduce the proportion of negative electrode active material in the negative electrode active material layer, reducing the active components that can participate in the lithium storage reaction per unit mass or unit area of ​​the negative electrode sheet, which is not conducive to improving the capacity of the negative electrode sheet and the energy density of the cell. If the binder content is too low, the structural stability of the negative electrode active material layer in the starting region is insufficient, and it is easy to experience powder shedding, cracking, or delamination during coating, rolling, winding, or cycling. By controlling the binder mass content W1 in the nth active material layer... n This helps improve the binding capacity and structural retention capacity of the tail region for the volume expansion of the negative electrode active material, thereby reducing the risk of pulverization, delamination, or loss of active material in this region. If W n If W is too low, the expansion constraint capacity of the tail region will be insufficient; if W n Excessively high levels of W1 will significantly reduce the proportion of the negative electrode active material and may affect ion transport and the overall energy density of the electrode. Specifically, W1 can be a range of 0.6%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, or any combination thereof; Wn It can be a range of 2.2%, 2.5%, 2.8%, 3.0%, 3.5%, 4.0%, 4.5%, or any combination thereof.

[0044] In some embodiments, W1 is 0.8%~1.8%, W n The content is 2.2%~3.5%. Further control is achieved by adjusting the binder mass content W1 in the first active material layer and the binder mass content W in the nth active material layer. n The range of W1 helps to further improve the binding capacity and structure retention capacity of the tail region to the volume expansion of the negative electrode active material while meeting the basic adhesion and structure retention requirements of the starting region. Specifically, W1 can be in the range of 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, or any combination thereof; n It can be a range of 2.2%, 2.5%, 2.8%, 3.0%, 3.5%, or any two of them.

[0045] In some embodiments, the length of the negative electrode active material layer along the direction from the starting end to the tail end is L, and the mass content of the binder in the i-th segment of the active material layer is W. i Satisfying the following relationship: W i =W1+(W n -W1)×[e k(i-1) / (n-1) -1] / (e k -1); where 4m≤L≤6m; 0.8≤k≤3.0; e is the natural constant, e=2.71828. By controlling the binder mass content Wi in the i-th segment of the active material layer to satisfy an exponential increasing relationship, the increasing law of binder content in each segment is further controlled. The exponential increasing relationship helps maintain a relatively low binder content in several segments of the negative electrode active material layer near the beginning, while making the increase in binder content in several segments of the negative electrode active material layer near the end more significant. This setting can reduce the proportion of inactive components in the beginning region while improving the expansion binding capacity and anti-pulverization capacity in the end region, thus better adapting to the differences in stress and expansion states at different locations in the winding structure.

[0046] In the formula, (i-1) / (n-1) represents the relative segment position of the i-th sub-active material layer among segments 1 to n. When i=1, (i-1) / (n-1) is 0, W i It equals W1; when i=n, ​​(i-1) / (n-1) is 1, W i equals W n Therefore, the adhesive content of the first and nth segments corresponds to the starting and ending ends, respectively, while the content of each intermediate segment increases progressively according to a preset rule.

[0047] By controlling k to be between 0.8 and 3.0, the steepness of the binder content increase curve can be adjusted. When k is small, the increase in binder content in each segment is relatively gradual; when k is large, the increase in binder content near the tail end is more significant. Controlling k between 0.8 and 3.0 helps to balance the lower binder content at the beginning, the higher expansion binding capacity at the tail end, and the smooth transition in the middle region, avoiding insufficient tail-end constraint due to excessively slow increase, and also avoiding abrupt changes in local composition due to excessively rapid increase. By controlling L to be between 4m and 6m, the length of the negative electrode active material layer can be made suitable for the electrode length range of common cylindrical wound cells, and it also helps to ensure good process matching between the number of segments, the coating length of each segment, and the material supply switching control.

[0048] Furthermore, e is a natural constant, e = 2.718. The natural constant e is introduced to achieve an exponentially increasing relationship, ensuring that the binder content in each segment increases exponentially from W1 to W. n Specifically, k can be a range of 0.8, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0 or any two of them; L can be a range of 4.0m, 4.2m, 4.5m, 5.0m, 5.5m, 5.8m, 6.0m or any two of them.

[0049] In some preferred embodiments, n is any integer from 15 to 20; 1.0% ≤ W1 ≤ 1.5%; 2.5% ≤ W n ≤3.5%. Further controlling the range of n helps ensure a smooth transition of binder content across different sections while reducing the difficulty of slurry switching and coating control. Simultaneously, further controlling the binder mass content range of the negative electrode active material layer from the beginning to the end helps further reduce the difference in expansion rate at different winding positions, further reducing the risk of localized pulverization, delamination, or loss of active material. Specifically, n can be any one of 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25; W1 can be a range of 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or any combination thereof; W n It can be a range of 2.5%, 2.9%, 3.0%, 3.1%, 3.2%, 3.5%, or any two of them.

[0050] In some embodiments, in the first active material layer, the binder comprises, by weight of the total binder, 60%~90% styrene-butadiene rubber, 10%~40% sodium carboxymethyl cellulose, and 0~5% polyacrylic acid; and / or, in the nth active material layer, the binder comprises, by weight of the total binder, 10%~40% styrene-butadiene rubber, 0%~5% sodium carboxymethyl cellulose, and 60%~90% polyacrylic acid. By controlling the specific composition of the binder in the first and nth active material layers, specifically, since the first active material layer is close to the starting end of the negative electrode active material layer, after the negative electrode sheet is wound to form a wound cell, this area, corresponding to the position near the winding center, is subjected to compression and constraint by the outer electrode sheet and other layered materials, resulting in a relatively small expansion space. This makes it more prone to expansion restriction and stress concentration during charge-discharge cycles. Therefore, the first active material layer employs a binder system primarily composed of styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC). The CMC molecular chain contains polar groups such as carboxymethyl groups, which helps improve the dispersion stability of the negative electrode slurry and enhances the adhesion stability between the negative electrode active material layer and the negative electrode current collector. SBR possesses a certain degree of flexibility, which helps improve the film-forming stability and winding resistance of the negative electrode active material layer. By controlling the mass content of SBR to 60%–90% and the mass content of CMC to 10%–40%, the first active material layer can maintain good current collector adhesion and film-forming stability while keeping the total binder content low. By controlling the mass content of polyacrylic acid to 0–5%, the risk of increased slurry viscosity or increased film rigidity due to excessive introduction of highly polar polyacrylic acid into the first active material layer can be reduced, thereby helping to reduce the risk of stress concentration near the winding center.

[0051] The nth active material layer is located near the tail end of the negative electrode active material layer. After the negative electrode sheet is wound to form a wound cell, this region, corresponding to a position far from the winding center, experiences relatively weaker external constraints and has a relatively larger expansion space, making it more prone to significant thickness expansion during charge-discharge cycles. Therefore, the nth active material layer employs a binder system primarily composed of polyacrylic acid and styrene-butadiene rubber. Polyacrylic acid contains numerous polar groups such as carboxyl groups, which can form strong interactions with oxygen-containing groups or polar sites on the surface of the silicon-based negative electrode material. This helps enhance the adhesion and binding effect on the negative electrode active material particles, thereby improving the ability of this region to restrict the volume expansion of the active material. Styrene-butadiene rubber has a certain degree of flexibility, which can work with polyacrylic acid to buffer the volume changes during cycling, reducing the risk of film cracking caused by excessive rigid constraints. By controlling the mass content of polyacrylic acid to 60%~90% and the mass content of styrene-butadiene rubber to 10%~40%, the nth active material layer can achieve both strong expansion binding ability and a certain degree of flexible buffering ability. By controlling the sodium carboxymethyl cellulose content to 0-5%, the binder system in the tail region can focus more on constraining and buffering the expansion of the silicon-based active material, reducing the risk of excessive expansion, pulverization, delamination, or loss of active material in this region.

[0052] Therefore, the first and nth active material layers differ not only in the total binder content but also in their binder composition. The first active material layer focuses on improving the adhesion of the current collector, film stability, and winding stability near the winding center; the nth active material layer focuses on improving the binding and buffering capacity of the active material volume expansion in areas far from the winding center. This differentiated binder system, combined with the design of gradually increasing binder mass content from the beginning to the end, helps to balance the expansion behavior of different winding positions of the negative electrode, reduce the difference in expansion rate after cycling, and reduce pulverization, delamination, or active material loss, thereby improving the capacity retention of the lithium-ion battery. Specifically, in the first active material layer, the mass content of styrene-butadiene rubber can be 60%, 65%, 70%, 75%, 80%, 85%, 90%, or any combination thereof; the mass content of sodium carboxymethyl cellulose can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, or any combination thereof; and the mass content of polyacrylic acid can be 0%, 1%, 2%, 3%, 4%, 5%, or any combination thereof. In the nth active material layer, the mass content of polyacrylic acid can be 60%, 65%, 70%, 75%, 80%, 85%, 90%, or any combination thereof; the mass content of styrene-butadiene rubber can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, or any combination thereof; and the mass content of sodium carboxymethyl cellulose can be 0%, 1%, 2%, 3%, 4%, 5%, or any combination thereof.

[0053] In some preferred embodiments, in the first active material layer, the binder, by weight of the total binder, comprises: 75%~85% styrene-butadiene rubber, 15%~25% sodium carboxymethyl cellulose, and 0~3% polyacrylic acid; and / or, in the nth active material layer, the binder, by weight of the total binder, comprises: 15%~25% styrene-butadiene rubber, 0~3% sodium carboxymethyl cellulose, and 75%~85% polyacrylic acid. Specifically, in the first active material layer, the mass content of styrene-butadiene rubber can be 75%, 76%, 77%, 78%, 80%, 85%, or any combination thereof; the mass content of sodium carboxymethyl cellulose can be 15%, 16%, 18%, 20%, 22%, 25%, or any combination thereof; and the mass content of polyacrylic acid can be 0%, 1%, 2%, 3%, or any combination thereof. In the nth active material layer, the mass content of polyacrylic acid can be 75%, 76%, 77%, 78%, 80%, 85%, or any combination thereof; the mass content of styrene-butadiene rubber can be 15%, 16%, 18%, 20%, 22%, 25%, or any combination thereof; and the mass content of sodium carboxymethyl cellulose can be 0%, 1%, 2%, 3%, or any combination thereof.

[0054] In some embodiments, the negative electrode active material includes artificial graphite and silicon-based materials; wherein the silicon-based material includes at least one of silicon-carbon composite materials, nano-silicon, and silicon suboxide; based on the total mass of the negative electrode active material, the mass content of artificial graphite is 80%~97%, and the mass content of silicon-based materials is 3%~20%. By controlling the inclusion of graphite and silicon-based materials in the negative electrode active material, it is helpful to utilize silicon-based materials to increase the capacity of the negative electrode sheet, while utilizing the better cycle stability and processing adaptability of graphite materials to mitigate the adverse effects of volume expansion of silicon-based materials. Furthermore, by controlling the specific type of silicon-based material, it is helpful to select appropriate silicon-based active components according to the battery capacity and cycle stability requirements, thereby balancing high capacity and structural retention during cycling. Specifically, based on the total mass of the negative electrode active material, the mass content of graphite can be within the range of 80%, 82%, 84%, 88%, 92%, 97%, or any combination thereof, and the mass content of silicon-based materials can be within the range of 3%, 5%, 10%, 15%, 20%, or any combination thereof.

[0055] In some embodiments, based on the total mass of the negative electrode active material layer, the mass content of the negative electrode active material is 94% to 98%, the mass content of the binder is 0.6% to 4.5%, and the mass content of the conductive agent is 1.4% to 5.4%. By controlling the mass content of each component in the negative electrode active material layer, it is helpful to balance the capacity, conductivity, and cycle stability of the negative electrode sheet, thereby improving the capacity retention rate of the lithium-ion battery. Specifically, the mass content of the negative electrode active material can be in the range of 94%, 95%, 96%, 97%, 98%, or any combination thereof; the mass content of the binder can be in the range of 0.6%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, or any combination thereof; and the mass content of the conductive agent can be in the range of 1.4%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.4%, or any combination thereof.

[0056] In some embodiments, the thickness of the negative electrode active material layer is 100 μm to 200 μm. Controlling the thickness of the negative electrode active material layer helps to ensure that it has suitable load capacity and mechanical stability. If the thickness is too small, the capacity per unit area of ​​the negative electrode sheet is low, which is not conducive to improving the energy density of the cell; if the thickness is too large, the ion transport and electron conduction paths inside the negative electrode active material layer become longer, and uneven internal and external stress, cracks, and pulverization are more likely to occur during cyclic expansion. Specifically, the thickness of the negative electrode active material layer can be 100 μm, 120 μm, 140 μm, 150 μm, 160 μm, 180 μm, 200 μm, or any combination thereof.

[0057] In some embodiments, the compaction density of the negative electrode is 1.5 g / cm³. 3 ~1.7g / cm 3 Controlling the compaction density of the negative electrode helps improve the particle contact stability and volumetric energy density of the negative electrode active material layer. If the compaction density is too low, the electrode porosity will be too large, resulting in insufficient particle contact, which may affect electron conduction and electrode strength. If the compaction density is too high, the porosity will be insufficient, potentially affecting electrolyte wetting and ion transport, and stress concentration will be more likely to occur during the cyclic expansion of silicon-based materials. Specifically, the compaction density of the negative electrode can be 1.5 g / cm³. 3 1.55 g / cm³, 1.6 g / cm³ 3 1.65g / cm 3 1.7g / cm 3 Or a range consisting of any two of them.

[0058] In two aspects, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps: S1, mixing a first negative electrode active material, a first conductive agent, and a first binder to obtain a first negative electrode slurry; mixing a second negative electrode active material, a second conductive agent, and a second binder to obtain a second negative electrode slurry; wherein, the mass content of the first binder in the first negative electrode slurry is less than the mass content of the second binder in the second negative electrode slurry; S2, mixing the first negative electrode slurry and the second negative electrode slurry to obtain first to nth mixed negative electrode slurries respectively; wherein, the mass content of the first negative electrode slurry in the i-th mixed negative electrode slurry is less than the mass content of the second binder in the i-th mixed negative electrode slurry. The mass content is less than the mass content of the first negative electrode slurry in the (i-1)th mixed negative electrode slurry, and the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry is greater than the mass content of the second negative electrode slurry in the (i-1)th mixed negative electrode slurry; n is any integer from 15 to 25; i is any integer from 1 to n; S3, along the length direction of the negative electrode current collector, from the beginning end to the end end of the length direction of the negative electrode current collector, the first to nth mixed negative electrode slurries are sequentially coated on the surface of the negative electrode current collector, and after drying and rolling treatment, n segments of active material layer are formed to obtain the negative electrode sheet.

[0059] In step S1, two base slurries with different binder contents are provided, wherein the mass content of the first binder in the first negative electrode slurry is less than the mass content of the second binder in the second negative electrode slurry. The first negative electrode slurry, as a low binder content slurry, is suitable for forming the sub-active material layer near the starting end; the second negative electrode slurry, as a high binder content slurry, is suitable for forming the sub-active material layer near the tail end. By preparing the two slurries separately, it avoids the need for individual slurry preparation for each stage, thereby simplifying the preparation process and helping to improve the stability and controllability of different batches of slurry.

[0060] In step S2, the first negative electrode slurry and the second negative electrode slurry are mixed to obtain the first to nth mixed negative electrode slurries. By adjusting the mixing ratio of the first and second negative electrode slurries, n mixed negative electrode slurries with progressively increasing binder content are formed. Simultaneously, the mass content of the first negative electrode slurry in the i-th mixed negative electrode slurry is less than that in the (i-1)-th mixed negative electrode slurry, and the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry is greater than that in the (i-1)-th mixed negative electrode slurry. Since the mass content of the first binder in the first negative electrode slurry is less than that of the second binder in the second negative electrode slurry, the binder mass content of the i-th mixed negative electrode slurry increases accordingly as the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry increases. Therefore, by adjusting the mixing ratio of the two base slurries, the binder content of the first to nth mixed negative electrode slurries can be gradually increased, which helps to reduce the preparation complexity and improve the process stability, thus providing a basis for the subsequent formation of a negative electrode active material layer with progressively increasing binder content.

[0061] In step S3, along the length of the negative electrode current collector, from the starting end to the tail end, the first to nth mixed negative electrode slurries are sequentially coated onto the surface of the negative electrode current collector. After drying and rolling, n segments of active material layers are formed, resulting in a negative electrode sheet. Since the binder content in the first to nth mixed negative electrode slurries gradually increases, the binder mass content in the first to nth segments of active material layers formed after coating, drying, and rolling also gradually increases. The drying process removes the solvent from the slurry, allowing the negative electrode active material, conductive agent, and binder to form a stable negative electrode active material layer. The rolling process improves the compaction density, particle contact stability, and electrode sheet thickness uniformity of the negative electrode active material layer. By sequentially coating the first to nth mixed negative electrode slurries and combining this with drying and rolling processes, a negative electrode sheet with a segmented increasing binder content along its length can be obtained.

[0062] Therefore, the method for preparing the negative electrode sheet of the present invention helps to prepare the target negative electrode sheet in a simpler and more controllable manner, thereby achieving a gradual increase in the binder content in the negative electrode active material layer along the length direction, and improving the adaptability of different positions of the negative electrode sheet to differences in stress and expansion.

[0063] In some embodiments, the mass content of the first binder in the first negative electrode slurry is W1, where W1 is 0.6% to 1.8%; and / or, the mass content of the second binder in the second negative electrode slurry is W... n W n The content is 2.2%~4.5%. This is achieved by controlling the mass content of the first binder in the first negative electrode slurry to be W1, and the mass content of the second binder in the second negative electrode slurry to be W... n This helps to ensure that the first and second negative electrode slurries correspond to the low binder content end and the high binder content end, respectively, thereby helping the first to nth mixed negative electrode slurries formed by mixing the two to cover the target range from low binder content to high binder content. Specifically, W1 can be a range of 0.6%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, or any combination thereof; W n It can be a range of 2.2%, 2.5%, 2.8%, 3.0%, 3.5%, 4.0%, 4.5%, or any combination thereof.

[0064] In some embodiments, when i=1, the mass content of the first negative electrode slurry is 95%~100% and the mass content of the second negative electrode slurry is 0~5% based on the total mass of the first mixed negative electrode slurry; and / or, when i=n, ​​the mass content of the first negative electrode slurry is 0~15% and the mass content of the second negative electrode slurry is 90%~100% based on the total mass of the nth mixed negative electrode slurry. By controlling the mass content of the first and second negative electrode slurries in the first mixed negative electrode slurry, it is helpful to bring the first mixed negative electrode slurry closer to the first negative electrode slurry, thereby forming a first segment active material layer with a lower binder content. Since the first segment is close to the starting end, this arrangement helps to reduce unnecessary binder usage in this region and increase the proportion of active material. Specifically, the mass content of the first negative electrode slurry in the first mixed negative electrode slurry can be 95%, 96%, 97%, 98%, 99%, 100%, or any combination thereof; the mass content of the second negative electrode slurry in the first mixed negative electrode slurry can be 0%, 1%, 2%, 3%, 4%, 5%, or any combination thereof.

[0065] By controlling the mass content of the first and second negative electrode slurries in the nth mixed negative electrode slurry, it is helpful to make the nth mixed negative electrode slurry similar to the second negative electrode slurry, thereby forming a nth segment active material layer with a higher binder content. This setting helps to improve the expansion binding capacity and anti-pulverization capacity of the tail-end region and reduce the risk of structural damage during cycling. Specifically, the mass content of the first negative electrode slurry in the nth mixed negative electrode slurry can be 0%, 3%, 5%, 8%, 10%, 12%, 15%, or any combination thereof; the mass content of the second negative electrode slurry in the nth mixed negative electrode slurry can be 90%, 92%, 95%, 96%, 98%, 100%, or any combination thereof.

[0066] In some embodiments, the difference between the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry and the mass content of the second negative electrode slurry in the (i-1)-th mixed negative electrode slurry is 0.5% to 16%. Controlling the difference between the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry and the mass content of the second negative electrode slurry in the (i-1)-th mixed negative electrode slurry helps to control the compositional variation between adjacent mixed negative electrode slurries. If the difference is too small, the binder content variation between the first and nth segments is not significant; if the difference is too large, the compositional abrupt change between adjacent sub-active material layers is large, which may lead to uneven local stress. Specifically, the difference between the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry and the mass content of the second negative electrode slurry in the (i-1)-th mixed negative electrode slurry can be a range of 0.5%, 4%, 8%, 10%, 14%, 16%, or any combination thereof.

[0067] In some embodiments, the solid content of the first negative electrode slurry is 45% to 50%; the solid content of the second negative electrode slurry is 45% to 50%. By controlling the solid content of the first and second negative electrode slurries, it is helpful to balance the slurry coatability, drying efficiency, and coating consistency. When the solid content is too low, the drying burden after coating increases, and the drying shrinkage is more obvious, which may affect the coating uniformity; when the solid content is too high, the slurry fluidity decreases, and the coating stability and mixing uniformity may deteriorate. Specifically, the solid content of the first negative electrode slurry can be a range of 45%, 46%, 47%, 48%, 49%, 50%, or any combination thereof; the solid content of the second negative electrode slurry can be a range of 45%, 46%, 47%, 48%, 49%, 50%, or any combination thereof.

[0068] In some embodiments, the viscosity of the first negative electrode slurry at 25°C is 2000 mPa·s to 5000 mPa·s; the viscosity of the second negative electrode slurry at 25°C is also 2000 mPa·s to 5000 mPa·s. Controlling the viscosity of the first and second negative electrode slurries at 25°C helps improve the coating stability and online mixing stability of the slurries. If the viscosity is too low, the slurry is prone to sagging or unstable coating edges; if the viscosity is too high, the slurry delivery, online mixing, and coating uniformity deteriorate. Controlling the viscosity within the above range helps ensure that the first and second negative electrode slurries maintain good coating compatibility even at different mixing ratios. Specifically, the viscosity of the first negative electrode slurry at 25°C can be within the range of 2000 mPa·s, 2500 mPa·s, 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, 4500 mPa·s, 5000 mPa·s, or any combination thereof; the viscosity of the second negative electrode slurry at 25°C can be within the range of 2000 mPa·s, 2500 mPa·s, 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, 4500 mPa·s, 5000 mPa·s, or any combination thereof.

[0069] In some embodiments of the present invention, in order to facilitate coating, it is necessary to ensure that the solid content and viscosity of the two slurries are similar, with the difference in solid content between the first negative electrode slurry and the second negative electrode slurry not exceeding 5% and the difference in viscosity not exceeding 1000 Pa. s.

[0070] In some embodiments, the coating speed is 8 m / min to 12 m / min. Controlling the coating speed helps to ensure that the first to nth mixed negative electrode slurries are stably and sequentially coated along the length of the negative electrode current collector, and helps to ensure good consistency in the length, thickness, and composition of the active material layers in each segment. A stable coating speed also allows for better controllability of the switching positions between segments when combined with segmented feeding or segmented mixing processes. Specifically, the coating speed can be a range of 8 m / min, 9 m / min, 10 m / min, 11 m / min, 12 m / min, or any combination thereof.

[0071] In three aspects, the present invention provides a wound battery cell, which is formed by sequentially stacking and winding a positive electrode sheet, a separator, and a negative electrode sheet; the starting end of the negative electrode sheet is located on the side of the wound battery cell close to the winding center, and the tail end of the negative electrode sheet is located on the side of the wound battery cell away from the winding center; wherein, the negative electrode sheet includes the above-mentioned negative electrode sheet, or a negative electrode sheet prepared by the above-mentioned method for preparing a negative electrode sheet.

[0072] Because this wound cell is prepared using the aforementioned high-performance negative electrode sheet, the binder content of the negative electrode sheet in the wound cell gradually increases from the side closer to the winding center to the side farther away from the winding center. Therefore, the negative electrode sheet at different positions in the wound cell can obtain different degrees of adhesion retention and expansion restraint capabilities, which helps to match the stress and expansion states at different locations in the winding structure. Specifically, the binder content is relatively low on the side closer to the winding center, which helps to reduce the proportion of unnecessary inactive components; the binder content is relatively high on the side farther away from the winding center, which helps to improve the restraint and buffering capacity of that position for the volume expansion of the negative electrode active material. Thus, this wound cell helps to reduce the risk of uneven expansion, pulverization, delamination, or loss of active material of the negative electrode sheet during cycling, thereby improving the capacity retention rate of the lithium-ion battery prepared in this way.

[0073] The present invention will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention.

[0074] Example 1

[0075] I. The negative electrode preparation method of this embodiment includes the following steps:

[0076] S1, Preparation of the first negative electrode slurry: The first negative electrode active material, the first conductive agent, and the first binder are added to a solvent and mixed to obtain the first negative electrode slurry. The first negative electrode active material includes a graphite and silicon-carbon composite material, wherein the silicon content in the silicon-carbon composite material is 15% by mass; the first conductive agent is conductive carbon black; and the first binder includes styrene-butadiene rubber and sodium carboxymethyl cellulose.

[0077] Based on the total mass of the solid components of the first negative electrode slurry, the mass content of the first negative electrode active material is 96%, the mass content of the first conductive agent is 2.8%, and the mass content of the first binder is 1.2%. Based on the total mass of the first negative electrode active material, the mass content of graphite is 90%, and the mass content of silicon-carbon composite material is 10%. Based on the total mass of the first binder, the mass content of styrene-butadiene rubber is 80%, and the mass content of sodium carboxymethyl cellulose is 20%.

[0078] Preparation of the second negative electrode slurry: The second negative electrode active material, the second conductive agent, and the second binder are added to a solvent and mixed to obtain the second negative electrode slurry. The second negative electrode active material includes a graphite and silicon-carbon composite material, the second conductive agent is conductive carbon black, and the second binder includes styrene-butadiene rubber and polyacrylic acid.

[0079] Based on the total mass of the second negative electrode active material, the mass content of graphite is 90%, and the mass content of silicon-carbon composite material is 10%. Based on the total mass of the solid components of the second negative electrode slurry, the mass content of the second negative electrode active material is 96%, the mass content of the second conductive agent is 1.0%, and the mass content of the second binder is 3.0%. Based on the total mass of the second binder, the mass content of polyacrylic acid is 80%, and the mass content of styrene-butadiene rubber is 20%.

[0080] The solid content of both the first and second negative electrode slurries is 48%. The viscosity of the first negative electrode slurry at 25°C is 3200 mPa·s, and the viscosity of the second negative electrode slurry at 25°C is 3300 mPa·s.

[0081] S2, the first negative electrode slurry and the second negative electrode slurry are mixed according to the proportions shown in Table 1 to obtain the first to 20th mixed negative electrode slurries respectively.

[0082] S3, along the length of the negative electrode current collector, the first to 20 mixed negative electrode slurries are sequentially coated onto the surface of the negative electrode current collector.

[0083] Wherein, along the direction from the starting end to the tail end, the mass content of the binder in the active material layer of the i-th segment is greater than the mass content of the binder in the active material layer of the (i-1)-th segment, where i is 1~20.

[0084] S4. The coated negative electrode current collector is dried and rolled to obtain the negative electrode sheet. After drying, the thickness of the negative electrode active material layer is 150 μm; after rolling, the compaction density of the negative electrode sheet is 1.6 g / cm³. 3 .

[0085] II. In this embodiment, along the length of the negative electrode sheet, the length L of the negative electrode active material layer is 5m. The negative electrode active material layer is divided into 20 sub-segments from the start end to the end end, i.e., n=20. The first sub-segment is closer to the start end, and the 20th sub-segment is closer to the end end. k=1.5, the binder mass content W1 in the first sub-segment is 1.2%, and the binder mass content W in the 20th sub-segment is... 20 It is 3.0%.

[0086] The mass content of the binder in the i-th sub-active material layer is calculated by taking the mass content of the binder in the first negative electrode slurry (1.2%), the mass content of the binder in the second negative electrode slurry (3.0%), and the mass content of the first and second negative electrode slurries in the i-th mixed negative electrode slurry. Taking the second mixed negative electrode slurry as an example: the mass content of the binder in the second sub-active material layer = the mass content of the binder in the first negative electrode slurry × the mass content of the first negative electrode slurry + the mass content of the binder in the second negative electrode slurry × the mass content of the second negative electrode slurry = 0.012 × 0.976 + 0.03 × 0.024 = 1.2432%. Similarly, the mass content of the binder in the first to 20th sub-active material layers is calculated, and then... Figure 2 The curves showing the relationship between the binder mass content of the active material layers from segment 1 to segment 20 and the segment number are shown. The fitted data satisfies the formula: W i =W1+(W n -W1)×[e k(i-1) / (n-1) -1] / (e k -1).

[0087] Table 1

[0088]

[0089] Example 2

[0090] The difference from Example 1 is that in this example, k=3.0; the binder mass content W1 in the first active material layer is 1.2%, and the binder mass content W in the 20th active material layer is... 20 3%;

[0091] Based on the total mass of the solid components of the first negative electrode slurry, the mass content of the negative electrode active material is 96%, the mass content of the conductive agent is 3.0%, and the mass content of the binder is 1.2%.

[0092] Based on the total mass of the solid components of the second negative electrode slurry, the mass content of the negative electrode active material is 96%, the mass content of the conductive agent is 1%, and the mass content of the binder is 3.0%. Specific results are shown in Table 2.

[0093] Table 2

[0094]

[0095] Example 3

[0096] The difference from Example 1 is that in this example, k=1.5, the binder mass content W1 in the first active material layer is 1.5%, and the binder mass content W in the 20th active material layer is... 20 It is 2.5%;

[0097] Based on the total mass of the solid components of the first negative electrode slurry, the mass content of the negative electrode active material is 96%, the mass content of the conductive agent is 2.5%, and the mass content of the binder is 1.5%.

[0098] Based on the total mass of the solid components of the second negative electrode slurry, the mass content of the negative electrode active material is 96%, the mass content of the conductive agent is 1.5%, and the mass content of the binder is 2.5%. Specific results are shown in Table 3.

[0099] Table 3

[0100]

[0101] Example 4

[0102] The difference from Example 1 is that in this example, the length L of the negative electrode active material layer along the length direction of the negative electrode sheet is 5m, and the negative electrode active material layer is divided into 15 segments of active material layer from the start end to the end end, i.e., n=15. The binder mass content W1 in the first segment of the active material layer is 1.2%, and the binder mass content W in the 15th segment of the active material layer is... 15 The figure is 3.0%. See Table 4 for the specific results.

[0103] Table 4

[0104]

[0105] Example 5

[0106] The difference from Example 1 is that, based on the total mass of the first adhesive, the mass content of styrene-butadiene rubber is 70%, the mass content of sodium carboxymethyl cellulose is 25%, and the mass content of polyacrylic acid is 5%.

[0107] Based on the total mass of the second adhesive, the mass content of polyacrylic acid is 70%, the mass content of styrene-butadiene rubber is 25%, and the mass content of sodium carboxymethyl cellulose is 5%.

[0108] Example 6

[0109] The difference from Example 1 is that in this example, k=0.5, the binder mass content W1 in the first active material layer is 2.0%, and the binder mass content W in the 20th active material layer is... 20 The figure is 3.0%. See Table 5 for the specific results.

[0110] Table 5

[0111]

[0112] Example 7

[0113] The difference from Example 1 is that, based on the total mass of the first adhesive, the mass content of styrene-butadiene rubber is 20%, the mass content of sodium carboxymethyl cellulose is 10%, and the mass content of polyacrylic acid is 70%.

[0114] Based on the total mass of the second adhesive, the mass content of polyacrylic acid is 20%, the mass content of styrene-butadiene rubber is 70%, and the mass content of sodium carboxymethyl cellulose is 10%.

[0115] Example 8

[0116] The difference from Example 1 is that, based on the total mass of the first adhesive, the mass content of styrene-butadiene rubber is 80% and the mass content of sodium carboxymethyl cellulose is 20%.

[0117] Based on the total mass of the second adhesive, the mass content of styrene-butadiene rubber is 80%, and the mass content of sodium carboxymethyl cellulose is 20%.

[0118] Comparative Example 1

[0119] The comparative example of the negative electrode preparation method includes:

[0120] S1, Preparation of negative electrode slurry: The negative electrode active material, conductive agent, and binder are mixed in a solvent to obtain the negative electrode slurry. The negative electrode active material includes graphite and silicon-carbon composite material, in which the silicon element content is 15% by mass; the conductive agent is conductive carbon black; and the binder includes styrene-butadiene rubber, sodium carboxymethyl cellulose, and polyacrylic acid.

[0121] Based on the total mass of the solid components in the negative electrode slurry, the mass content of the negative electrode active material is 96%, the mass content of the conductive agent is 2%, and the mass content of the binder is 2%. Based on the total mass of the negative electrode active material, the mass content of graphite is 90%, and the mass content of silicon-carbon composite material is 10%. Based on the total mass of the binder, the mass content of styrene-butadiene rubber is 30%, the mass content of sodium carboxymethyl cellulose is 20%, and the mass content of polyacrylic acid is 50%. The solid content of the negative electrode slurry is 48%. The viscosity of the negative electrode slurry at 25°C is 3200 mPa·s.

[0122] S2, along the length of the negative electrode current collector, the negative electrode slurry is coated onto the surface of the negative electrode current collector; the coated negative electrode current collector is then dried and rolled to obtain the negative electrode sheet. After drying, the thickness of the negative electrode active material layer is 150 μm; after rolling, the compaction density of the negative electrode sheet is 1.6 g / cm³. 3 .

[0123] Comparative Example 2

[0124] The difference between this comparative example and Example 1 is that...

[0125] The method for preparing the negative electrode in this comparative example includes the following steps:

[0126] S1, Preparation of the first negative electrode slurry: The first negative electrode active material, the first conductive agent, and the first binder are added to a solvent and mixed to obtain the first negative electrode slurry. The first negative electrode active material includes a graphite and silicon-carbon composite material, wherein the silicon content in the silicon-carbon composite material is 15% by mass; the first conductive agent is conductive carbon black; and the first binder includes styrene-butadiene rubber and sodium carboxymethyl cellulose.

[0127] Based on the total mass of the solid components of the first negative electrode slurry, the mass content of the first negative electrode active material is 96%, the mass content of the first conductive agent is 2.8%, and the mass content of the first binder is 1.2%. Based on the total mass of the first negative electrode active material, the mass content of graphite is 90%, and the mass content of silicon-carbon composite material is 10%. Based on the total mass of the first binder, the mass content of styrene-butadiene rubber is 80%, and the mass content of sodium carboxymethyl cellulose is 20%.

[0128] Preparation of the second negative electrode slurry: The second negative electrode active material, the second conductive agent, and the second binder are added to a solvent and mixed to obtain the second negative electrode slurry. The second negative electrode active material includes a graphite and silicon-carbon composite material, the second conductive agent is conductive carbon black, and the second binder includes styrene-butadiene rubber and polyacrylic acid.

[0129] Based on the total mass of the second negative electrode active material, the mass content of graphite is 90%, and the mass content of silicon-carbon composite material is 10%. Based on the total mass of the solid components of the second negative electrode slurry, the mass content of the second negative electrode active material is 96%, the mass content of the second conductive agent is 1.0%, and the mass content of the second binder is 3.0%. Based on the total mass of the second binder, the mass content of polyacrylic acid is 80%, and the mass content of styrene-butadiene rubber is 20%.

[0130] The solid content of both the first and second negative electrode slurries is 48%. The viscosity of the first negative electrode slurry at 25°C is 3200 mPa·s, and the viscosity of the second negative electrode slurry at 25°C is 3300 mPa·s.

[0131] S2, the first negative electrode slurry and the second negative electrode slurry are sequentially coated on the surface of the negative electrode current collector.

[0132] S3. The coated negative electrode current collector is dried and rolled to obtain the negative electrode sheet. After drying, the thickness of the negative electrode active material layer is 150 μm; after rolling, the compaction density of the negative electrode sheet is 1.6 g / cm³. 3 .

[0133] In this comparative example, along the length of the negative electrode sheet, the negative electrode active material layer is divided into two sub-active material layers from the start end to the end end, i.e., n=2. The first sub-active material layer is closer to the start end, and the second sub-active material layer is closer to the end end. The binder mass content W1 in the first sub-active material layer is 1.2%, and the binder mass content W2 in the second sub-active material layer is 3.0%.

[0134] Comparative Example 3

[0135] The difference between this comparative example and Example 1 is that, in this comparative example, the length L of the negative electrode active material layer along the length direction of the negative electrode sheet is 5m, and the negative electrode active material layer is divided into 5 sub-active material layers along the direction from the start end to the end end, i.e., n=5. The binder mass content W1 in the first sub-active material layer is 1.2%, and the binder mass content W5 in the fifth sub-active material layer is 3.0%.

[0136] Test methods

[0137] 1. Preparation of lithium-ion batteries

[0138] 1) Preparation of the positive electrode: The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), polyvinylidene fluoride and hexafluoropropylene (with a solid content of 3% by mass), and conductive carbon black (Super P, purchased from Shenzhen Kejing) were mixed in a mass ratio of 8:1:1. After being mixed evenly, the mixture was coated on aluminum foil and dried in a vacuum drying oven at 110°C for 14 hours. After rolling, NCM811 positive electrode sheet was obtained, wherein the thickness of the positive electrode active material layer after rolling was 60 μm.

[0139] 2) The negative electrode sheets prepared in the examples and comparative examples were stacked with the above-mentioned NCM811 positive electrode sheet and a 10μm thick PE separator, and then wound to obtain a wound battery cell. The starting end of the negative electrode sheet was located on the side of the wound battery cell closer to the winding center, and the tail end of the negative electrode sheet was located on the side of the wound battery cell farther from the winding center. The wound battery cell was installed in a battery casing, and an electrolyte was injected. The mass ratio of the electrolyte components EC:DMC:FEC = 18.9%:63.2%:17.9%, the lithium salt was LiPF6, and the lithium salt concentration was 1 mol / L. After encapsulation, settling, formation, and capacity testing, a 4695 cylindrical lithium-ion battery was obtained.

[0140] 2. Capacity retention test

[0141] The prepared lithium-ion battery was subjected to cycle testing in a constant temperature environment of 25℃. It was charged to 4.25V using a constant current and constant voltage of 0.5C, with a cutoff current of 0.05C, allowed to stand for 30 minutes, and then discharged to 2.5V using a constant current of 0.5C. The initial charging capacity was recorded as Q1, the initial discharging capacity as Q2, and the discharging capacity at cycle n as Q3. The capacity retention rate of the battery after cycling at room temperature at 0.5C for cycle n was calculated using the following formula: Capacity retention rate (%) = (Q3 / Q2) × 100%. When the cell reached 80% capacity retention (80% SOH), it was removed from the battery and the number of cycles at this point was recorded.

[0142] 3. Negative electrode expansion rate test

[0143] Batteries that have been cycled to 80% capacity retention are discharged to 2.5V at a low rate of 0.05C. Then, the batteries are disassembled in a controlled, dry room (dew point ≤ -50℃) to test the thickness of different locations on the negative electrode. The distinction between different locations on the electrode is shown in [link to documentation]. Figure 3 , Figure 3 This is a schematic diagram illustrating the division of different test areas of the negative electrode sheet provided in an embodiment of the present invention. In the diagram, 3 represents the negative electrode current collector, 4 represents the negative electrode active material layer, 31 represents the beginning region, 32 represents the middle region, 33 represents the end region, A represents the starting end of the negative electrode active material layer along the length of the negative electrode sheet, B represents the ending end of the negative electrode active material layer along the length of the negative electrode sheet, C represents the position at 1 / 2L along the length of the negative electrode sheet, and X represents the direction from the starting end A to the ending end B. The beginning region 31 is the region near the starting end A, the middle region 32 is the region located 15cm on each side of the position at 1 / 2L from the starting end, and the end region 33 is the region near the ending end B. Specifically, the length of the beginning region 31 along the X direction is 40cm, the length of the middle region 32 along the X direction is 30cm, and the length of the end region 33 along the X direction is 40cm. Figure 3The intervals between the beginning region 31, the middle region 32, and the end region 33 are only used to indicate the positional intervals between different test areas. They do not indicate that the negative electrode sheet is disconnected at the corresponding position, nor do they indicate that there is no negative electrode current collector or negative electrode active material layer at the corresponding position.

[0144] The formula for calculating electrode expansion rate is: Electrode expansion rate (%) = (Electrode thickness after cycling - Electrode thickness before cycling) / Electrode thickness before cycling × 100%, where the electrode thickness before cycling is measured by disassembling the battery at a low rate of 0.05C to 2.5V and then testing the thickness of the negative electrode at different locations in a controlled dry room (dew point ≤ -50℃). The closer the electrode expansion rates are in different regions, the better the electrode consistency across different regions.

[0145] 4. Test of capacity retention of negative electrode active material

[0146] Batteries that have been cycled to 80% capacity retention are discharged at a low rate of 0.05C to 2.5V. Then, the batteries are disassembled in a controlled drying room (dew point ≤ -50℃). The disassembled negative electrode is cleaned and soaked in DMC for 10 minutes, then air-dried. The negative electrode is then treated with deionized water to form a single-sided negative electrode sheet, and baked in an oven at 90℃ for 12 hours. The electrode sheet is then punched into a Φ14mm circular sheet, the separator is Φ19mm, and the lithium metal is Φ18mm. The casing uses a CR2032 standard kit. The coin cells are prepared in the following order: positive electrode casing, negative electrode sheet, separator, lithium metal, and negative electrode casing. They are then sealed using a cold-pressing machine at 5~7MPa and allowed to stand for 12 hours for activation. The preparation method for the negative electrode coin cells of uncycled lithium-ion batteries is the same.

[0147] The prepared lithium-ion battery and the uncycled lithium-ion battery were placed in a constant temperature environment of 25℃ for cycle testing. They were discharged at a constant current and constant voltage of 0.1C to 0.005V, with a cutoff current of 0.01C, allowed to stand for 30 minutes, and then charged at a constant current of 0.1C to 1.5V. The charging capacity of the lithium-ion battery was recorded as Q4, and the charging capacity of the uncycled lithium-ion battery as Q5. The capacity retention rate of the negative electrode active material was calculated using the following formula: Capacity retention rate of negative electrode active material (%) = (Q4 / Q5) × 100%.

[0148] The test results are shown in Table 6.

[0149] Table 6

[0150]

[0151] Figure 2 This is a graph showing the relationship between the binder mass content of the active material layers in segments 1 to 20 of Embodiment 1 of the present invention and the segment number. Figure 2 As shown, the horizontal axis represents the segment number of the sub-active material layer, and the vertical axis represents the binder mass content in the corresponding segment of the active material layer. Figure 2 It can be seen that the binder mass content in the active material layer from the 1st to the 20th segment in Example 1 increases segment by segment along the direction of increasing segment number, indicating that the negative electrode active material layer in Example 1 forms a segmented gradient structure with progressively increasing binder mass content along the direction from the start end to the end end.

[0152] Figure 4 The graphs show the cycle performance test curves of the lithium-ion batteries prepared in Examples 1-3 and Comparative Example 1 of this invention. Figure 4 As shown, the horizontal axis represents the number of cycles, and the vertical axis represents the capacity retention rate. Figure 4 It can be seen that, compared with Comparative Example 1, the lithium-ion batteries prepared in Examples 1-3 exhibit a slower decrease in capacity retention during cycling. This indicates that by forming a segmented structure with increasing binder mass content along the length of the negative electrode in Examples 1-3, the structural stability of the negative electrode during cycling is improved, thereby enhancing the cycling performance of the lithium-ion battery.

[0153] As shown in Table 6, compared with Comparative Examples 1-3, Examples 1-8, by making the binder mass content in the negative electrode active material layer gradually increase or differentiated along the direction from the start end to the end end, help to obtain different degrees of expansion release capacity and expansion binding capacity in the area near the winding center and the area far from the winding center. As a result, the overall difference in the negative electrode expansion rate of the beginning, middle and end of the winding in Examples 1-8 is reduced, and the overall capacity retention rate of the negative electrode active material at the end of the winding is improved. This helps to increase the number of cycles of the lithium-ion battery at 0.5C to 80% SOH to ≥1138 cycles.

[0154] In Comparative Example 1, a layer of negative electrode active material with uniform binder content was used. This failed to compensate for the differences in stress state and expansion space at different locations within the wound cell, resulting in a significant increase in expansion at the tail region. The expansion rates at the beginning, middle, and end of the winding varied considerably, and the capacity retention rate of the negative electrode active material at the tail was low at 72%. The number of cycles to reach 80% capacity retention at 0.5C was only 1060. Although Comparative Example 2 included two regions with different binder contents, it only formed a two-segment distribution with a lower starting point and a higher ending point. It lacked a multi-segment, gradual increase in the transition from the starting point to the ending point, which easily led to discontinuous changes in constraint capacity between different regions. Therefore, its expansion consistency and negative electrode active material capacity retention rate were still inferior to Examples 1-8, and the number of cycles to reach 80% capacity retention at 0.5C also decreased. Comparative Example 3 has fewer segments, consisting of only 5 active material layers. The binder content varies greatly between adjacent regions, making it difficult to form a sufficiently gentle binder content gradient. Therefore, its tail expansion rate is still relatively high at 20.1%, and the tail negative electrode active material capacity retention rate is still lower than that of Examples 1-8.

[0155] Furthermore, compared to Example 6, Examples 1-5 further control the gradient coefficient k within the range of 0.8-3.0 and control the binder mass content W1 in the first active material layer within the range of 0.6%-1.8%, making the increase in binder content in the negative electrode active material layer from the start end to the end more appropriate, which helps to balance the release of expansion stress near the winding center region and the expansion constraint far from the winding center region. In Example 6, due to the smaller k and higher W1, the binder content near the start end is higher and the gradient increase trend is slower, resulting in insufficient expansion release capacity near the winding center region. At the same time, the difference in expansion constraint between the end region and the start region is not sufficient, thus increasing the negative electrode expansion rate in the middle and end of the winding and decreasing the expansion consistency of different positions of the negative electrode sheet.

[0156] Compared to Example 7, Examples 1-5 further control the binder in the first active material layer to be mainly styrene-butadiene rubber and sodium carboxymethyl cellulose, and control the binder in the nth active material layer to be mainly polyacrylic acid and styrene-butadiene rubber. This makes the area near the winding center more focused on current collector adhesion, film stability, and winding resistance, while the area away from the winding center focuses more on active material expansion restraint and buffering capacity. In Example 7, because the binder system settings of the first and nth segments are inconsistent with the above matching relationship, the adhesion retention capacity, expansion release capacity, and expansion restraint capacity near and away from the winding center are difficult to match with the stress state of the corresponding areas, thereby increasing the expansion rate of the negative electrode at the winding tail and decreasing the capacity retention rate of the active material at the winding tail.

[0157] Compared to Example 8, Examples 1-5 further control the binder in the nth active material layer to include a higher content of polyacrylic acid, so that the tail region can enhance the adhesion and binding effect on the silicon-based anode material through the polar groups in polyacrylic acid, thereby improving the ability to limit the volume expansion of the active material in the region far from the winding center. In Example 8, both the first and second binders mainly adopt the styrene-butadiene rubber and sodium carboxymethyl cellulose system. The tail region lacks the strong expansion binding effect provided by polyacrylic acid, resulting in insufficient ability to suppress excessive expansion in the region far from the winding center. Consequently, the expansion rate of the tail anode is high, the capacity retention rate of the tail anode active material decreases, and the number of cycles at 0.5C to 80% capacity retention rate is reduced to only 1138 cycles.

[0158] In summary, Examples 1-8, by setting a negative electrode active material layer with progressively increasing binder mass content along the length of the negative electrode sheet, and further by using differentiated binder compositions, allow the expansion release capability and expansion restraint capability at different winding positions of the negative electrode sheet to better match the stress state and expansion space at the corresponding positions. This helps reduce the difference in expansion rate between the beginning, middle, and end of the winding, reducing pulverization, delamination, or loss of active material, and improving the cycle stability of the lithium-ion battery.

[0159] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a conductive agent, and a binder. Along the length of the negative electrode sheet, the negative electrode active material layer includes a starting end and a tail end disposed opposite to each other; along the direction from the starting end to the tail end, the negative electrode active material layer is divided into n sub-active material layers; The mass content of the adhesive in the i-th active material layer is greater than the mass content of the adhesive in the (i-1)-th active material layer; where n is any integer from 15 to 25, and i is any integer from 1 to n.

2. The negative electrode sheet according to claim 1, characterized in that, When i=1, the mass content of the binder in the first active material layer is W1, where W1 is 0.6%~1.8%; When i=n, ​​the mass content of the binder in the nth active material layer is W. n The W n The percentage is 2.2% to 4.5%; Preferably, W1 is 0.8%~1.8%, and W n It ranges from 2.2% to 3.5%.

3. The negative electrode sheet according to claim 2, characterized in that, Along the direction from the starting end to the tail end, the length of the negative electrode active material layer is L, and the mass content of the binder in the i-th segment of the active material layer is W. i The following relationship must be satisfied: W i =W1+(W n -W1)×[e k(i-1) / (n-1) -1] / (ie k -1); Where 4m≤L≤6m; 0.8≤k≤3.0; e is the natural constant, e=2.71828; The n is any integer from 15 to 20; the W1 is 1.0% to 1.5%; the W n It ranges from 2.5% to 3.5%.

4. The negative electrode sheet according to claim 2, characterized in that, In the first active material layer, the adhesive comprises, by weight of the total adhesive, 60%–90% styrene-butadiene rubber, 10%–40% sodium carboxymethyl cellulose, and 0%–5% polyacrylic acid; and / or, in the nth active material layer, the adhesive comprises, by weight of the total adhesive, 10%–40% styrene-butadiene rubber, 0%–5% sodium carboxymethyl cellulose, and 60%–90% polyacrylic acid.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that, The negative electrode active material includes artificial graphite and silicon-based materials; wherein, the silicon-based material includes at least one of silicon-carbon composite materials, nano-silicon, and silicon suboxide; based on the total mass of the negative electrode active material, the mass content of the artificial graphite is 80%~97%, and the mass content of the silicon-based material is 3%~20%; and / or, Based on the total mass of the negative electrode active material layer, the mass content of the negative electrode active material is 94%~98%, the mass content of the binder is 0.6%~4.5%, and the mass content of the conductive agent is 1.4%~5.4%.

6. The negative electrode sheet according to claim 5, characterized in that, The thickness of the negative electrode active material layer is 100μm~200μm; and / or, The compaction density of the negative electrode sheet is 1.5 g / cm³. 3 ~1.7g / cm 3 .

7. A method for preparing a negative electrode sheet according to any one of claims 1 to 6, characterized in that, Includes the following steps: S1, mix the first negative electrode active material, the first conductive agent and the first binder to obtain the first negative electrode slurry; mix the second negative electrode active material, the second conductive agent and the second binder to obtain the second negative electrode slurry; wherein, the mass content of the first binder in the first negative electrode slurry is less than the mass content of the second binder in the second negative electrode slurry; S2, the first negative electrode slurry and the second negative electrode slurry are mixed in proportion to obtain the first to nth mixed negative electrode slurries respectively; wherein, the mass content of the first negative electrode slurry in the i-th mixed negative electrode slurry is less than the mass content of the first negative electrode slurry in the (i-1)-th mixed negative electrode slurry, and the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry is greater than the mass content of the second negative electrode slurry in the (i-1)-th mixed negative electrode slurry; n is any integer from 15 to 25; i is any integer from 1 to n; S3, along the length direction of the negative electrode current collector, from the beginning to the end of the negative electrode current collector, the first to nth mixed negative electrode slurries are sequentially coated on at least one side of the negative electrode current collector. After drying and rolling, n segments of active material layer are formed to obtain the negative electrode sheet.

8. The method for preparing the negative electrode sheet according to claim 7, characterized in that, The mass content of the first binder in the first negative electrode slurry is W1, where W1 is 0.6% to 1.8%; and / or, the mass content of the second binder in the second negative electrode slurry is W... n The W n The percentage is 2.2% to 4.5%; and / or, When i=1, based on the total mass of the first mixed negative electrode slurry, the mass content of the first negative electrode slurry is 95%~100%, and the mass content of the second negative electrode slurry is 0~5%; and / or, when i=n, ​​based on the total mass of the nth mixed negative electrode slurry, the mass content of the first negative electrode slurry is 0~15%, and the mass content of the second negative electrode slurry is 90%~100%; and / or, The difference between the mass content of the second negative electrode slurry in the i-th mixed negative electrode slurry and the mass content of the second negative electrode slurry in the (i-1)-th mixed negative electrode slurry is 0.5% to 16%.

9. The method for preparing the negative electrode sheet according to claim 8, characterized in that, The first negative electrode slurry has a solid content of 45%~50%; the second negative electrode slurry has a solid content of 45%~50%; and / or, The viscosity of the first negative electrode slurry at 25°C is 2000 mPa·s to 5000 mPa·s; the viscosity of the second negative electrode slurry at 25°C is 2000 mPa·s to 5000 mPa·s; and / or, The coating speed is 8m / min to 12m / min.

10. A wound battery cell, characterized in that, The wound cell is formed by sequentially stacking and winding a positive electrode sheet, a separator, and a negative electrode sheet; the starting end of the negative electrode sheet is located on the side of the wound cell closer to the winding center, and the tail end of the negative electrode sheet is located on the side of the wound cell away from the winding center; wherein, the negative electrode sheet includes the negative electrode sheet according to any one of claims 1 to 6, or the negative electrode sheet prepared by the method of preparing the negative electrode sheet according to any one of claims 7 to 9.

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