Negative plate and lithium ion battery

Abstract

The invention provides a negative electrode plate and a lithium ion battery, the negative electrode plate comprises a negative electrode current collector, a first negative electrode active layer and a second negative electrode active layer, the negative electrode current collector comprises a first region and a second region, the first region is arranged on the negative electrode current collector, the second region is located between the edge of the negative electrode current collector and the edge of the first region, the first negative electrode active layer is arranged in the first region, and the second negative electrode active layer is arranged in the second region; the Young's modulus of the second negative electrode active layer is greater than the Young's modulus of the first negative electrode active layer. According to the negative electrode plate provided by the invention, the second negative electrode active layer is arranged in the second region of the negative electrode current collector, the Young modulus of the second negative electrode active layer is greater than that of the first negative electrode active layer, and the relatively large Young modulus can inhibit volume expansion generated by the edge of the negative electrode plate in the charging and discharging process, so that the extension stress in the width direction of the negative electrode plate is reduced; the condition of corner damage caused by excessive extension of the negative plate is improved, so that the safety performance of the lithium ion battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative plate and a lithium ion battery. BACKGROUND

[0002] With the increasing demand for lithium ion battery endurance of terminal electronic products, in order to pursue better energy density (ED) in the manufacturing process of lithium ion battery, silicon-carbon negative electrode material is introduced into the negative electrode of lithium ion battery. The silicon-carbon negative electrode material can bring higher ED in unit volume, but the silicon-carbon material has poor ductility. The large expansion coefficient and low ductility of the silicon-carbon negative electrode material cannot meet the requirement of the negative plate end on ductility in the cycle process of lithium ion battery, and the stress generated by the expansion of the negative plate can easily cause damage to the edge and corner of the battery cell, affecting the normal use of the lithium ion battery and further affecting the safety of its use. SUMMARY

[0003] In view of the problem of damage to the edge and corner of the battery cell caused by expansion stress in the use process of the silicon-carbon negative lithium ion battery in the prior art, a negative plate and a lithium ion battery are provided.

[0004] The technical solution adopted by the present application to solve the above technical problems is as follows: On the one hand, the present application provides a negative plate, which comprises a negative current collector, a first negative active layer and a second negative active layer. The negative current collector comprises a first region and a second region. The first region is arranged on the negative current collector, and the second region is located between the edge of the negative current collector and the edge of the first region. The first negative active layer is arranged in the first region, and the second negative active layer is arranged in the second region. The Young's modulus of the second negative active layer is greater than the Young's modulus of the first negative active layer.

[0005] Optionally, the ionic conductivity of the first negative active layer is greater than the ionic conductivity of the second negative active layer.

[0006] Optionally, the tensile strength of the second negative active layer is greater than the tensile strength of the first negative active layer.

[0007] Optionally, the first negative active layer comprises a first negative active material and a first binder, and the first binder is selected from styrene-butadiene rubber. The second negative active layer comprises a second negative active material and a second binder, and the second binder comprises styrene-butadiene rubber and polyacrylic acid.

[0008] Optionally, in the first negative electrode active layer, the mass percentage of the first binder is 0.8%-2.5%; in the second negative electrode active layer, the mass percentage of the second binder is 0.4%-1.5%, and in the second binder, the mass ratio of styrene-butadiene rubber to polyacrylic acid is (3-7):(3-7).

[0009] Optionally, the negative electrode current collector includes two second regions, which are respectively located on both sides of the first region in the width direction of the negative electrode current collector; the dimension R1 of a single second region in the width direction of the negative electrode current collector and the dimension R2 of the first region in the width direction of the negative electrode current collector satisfy the following condition: 5%≤R1 / R2≤10%.

[0010] Optionally, it also includes a flexible buffer layer disposed between the second region and the second negative electrode active layer; The flexible buffer layer includes a flexible buffer material, which includes one or more of graphene, carbon nanotubes, carbon nanofibers, and conductive polymers. The conductive polymer includes one or more of polythiophene, zirconia polymers containing chalcogens, and conductive polymers containing HOS. The thickness of the flexible buffer layer is X, where 0.5μm≤X≤2μm.

[0011] Optionally, the thickness of the second negative electrode active layer is Y, where 30μm≤Y≤70μm.

[0012] Optionally, the negative electrode sheet satisfies the following conditions: 1%Y≤X≤3%Y.

[0013] Optionally, the total thickness of the second negative electrode active layer and the flexible buffer layer is Z, where 30.5 μm ≤ Z ≤ 71.5 μm.

[0014] Optionally, the negative electrode sheet satisfies the following relationship: 0.99%Z≤X≤2.91%Z.

[0015] Optionally, the thickness of the first negative electrode active layer is 30.5 μm - 71.5 μm.

[0016] Optionally, both the first negative electrode active material and the second negative electrode active material comprise silicon-carbon materials; The silicon-carbon material accounts for 95%-98% of the mass in the first negative electrode active layer and 95%-98% of the mass in the second negative electrode active layer.

[0017] Optionally, the first negative electrode active layer further includes a first conductive agent, and the second negative electrode active layer further includes a second conductive agent. The first conductive agent and the second conductive agent each independently include one or more of carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, and graphene. The mass percentage of the first conductive agent in the first negative electrode active layer is 0%-0.5%, and the mass percentage of the second conductive agent in the second negative electrode active layer is 0.1%-0.5%.

[0018] On the other hand, the present invention provides a lithium-ion battery including the aforementioned negative electrode.

[0019] The beneficial effects of this application are as follows: The negative electrode sheet provided in this application includes a negative electrode current collector having a first region and a second region. The first region is disposed on the negative electrode current collector, and the second region is located between the edge of the negative electrode current collector and the edge of the first region. A second negative electrode active layer is disposed in the second region of the negative electrode current collector. The Young's modulus of the second negative electrode active layer is greater than that of the first negative electrode active layer. A negative electrode active layer with a larger Young's modulus has stronger resistance to deformation. Mechanical stress limits the excessive extension of the negative electrode sheet edge. Furthermore, the larger Young's modulus of the second negative electrode active layer can effectively suppress the volume expansion of the active material at the edge of the negative electrode sheet during charging and discharging, thereby reducing the extension stress in the width direction of the negative electrode sheet. This improves the situation where excessive extension of the negative electrode sheet causes damage to the cell's corners, further solving the problem of internal short circuits in lithium-ion batteries caused by damage to the cell's corners, thus improving the safety performance of lithium-ion batteries. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the negative electrode structure provided by the present invention.

[0021] The reference numerals in the accompanying drawings are as follows: 1. Negative electrode current collector; 2. First negative electrode active layer; 3. Second negative electrode active layer; 4. Flexible buffer layer. Detailed Implementation

[0022] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0023] In the description of this invention, it should be understood that the terms "longitudinal," "radial," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0024] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0025] Reference Figure 1 The present invention provides a negative electrode sheet, comprising a negative electrode current collector, a first negative electrode active layer and a second negative electrode active layer. The negative electrode current collector includes a first region and a second region. The first region is disposed on the negative electrode current collector, and the second region is located between the edge of the negative electrode current collector and the edge of the first region. The first negative electrode active layer is disposed in the first region, and the second negative electrode active layer is disposed in the second region. The Young's modulus of the second negative electrode active layer is greater than that of the first negative electrode active layer.

[0026] Specifically, the negative electrode sheet provided in this application includes a negative electrode current collector having a first region and a second region. The first region is disposed on the negative electrode current collector, and the second region is located between the edge of the negative electrode current collector and the edge of the first region. A second negative electrode active layer is disposed in the second region of the negative electrode current collector. The Young's modulus of the second negative electrode active layer is greater than that of the first negative electrode active layer. A negative electrode active layer with a larger Young's modulus has stronger resistance to deformation. Mechanical stress limits the excessive extension of the negative electrode sheet edge. Furthermore, the larger Young's modulus of the second negative electrode active layer effectively suppresses the volume expansion of the active material at the edge of the negative electrode sheet during charging and discharging, thereby reducing the extension stress in the width direction of the negative electrode sheet. This improves the situation where excessive extension of the negative electrode sheet causes damage to the cell's corners, further solving the problem of internal short circuits in lithium-ion batteries caused by damage to the cell's corners, thus improving the safety performance of lithium-ion batteries.

[0027] Specifically, the Young's modulus of the second negative electrode active layer is 2-5 GPa, and the Young's modulus of the first negative electrode active layer is 0.5-2 GPa; further, the Young's modulus of the second negative electrode active layer can be 2 GPa, 2.2 GPa, 2.35 GPa, 2.4 GPa, 2.5 GPa, 3 GPa, 3.2 GPa, 3.4 GPa, 3.41 GPa, 4 GPa, 4.5 GPa, or 5 GPa; the Young's modulus of the first negative electrode active layer can be 0.5 GPa, 0.8 GPa, 1 GPa, 1.05 GPa, 1.5 GPa, 1.7 GPa, 1.73 GPa, or 2 GPa.

[0028] In some embodiments, the ionic conductivity of the first negative electrode active layer is greater than that of the second negative electrode active layer.

[0029] Specifically, the first negative electrode active layer possesses excellent kinetic performance and high ionic conductivity, which can significantly improve the transport rate of lithium ions in the electrode, enhance the charge and discharge efficiency and power density of the battery, and thus help improve the energy density of the lithium-ion battery. The second negative electrode active layer, with its large Young's modulus, utilizes its strong resistance to deformation to form a stable structure at the edge of the negative electrode sheet, effectively suppressing the volume expansion of the active material and reducing the problems of cell corner structure damage and active material shedding caused by expansion. That is, by utilizing the synergy of the first and second negative electrode active layers, the advantages of the first negative electrode active layer in energy storage of lithium-ion batteries are fully utilized, while the mechanical protection characteristics of the second negative electrode active layer improve the adverse effects caused by expansion, so that the lithium-ion battery can maintain high energy density while suppressing the volume expansion generated at the edge of the negative electrode sheet.

[0030] In some embodiments, the tensile strength of the second negative electrode active layer is greater than the tensile strength of the first negative electrode active layer.

[0031] Specifically, the tensile strength of the second negative electrode active layer is greater than that of the first negative electrode active layer. The higher tensile strength allows the second negative electrode active layer to better resist the tensile stress generated by the volume expansion of the active material during charging and discharging in the edge region of the negative electrode current collector. This effectively prevents tearing, breakage, and detachment of the active material from the edge of the negative electrode sheet. The first negative electrode active layer maintains a lower tensile strength, which helps to maintain good flexibility in the corresponding first region on the negative electrode sheet, maintain efficient lithium-ion transport and electrode structural integrity. The synergistic effect of the first negative electrode active layer in the first region and the second negative electrode active layer in the second region enhances the mechanical toughness of the lithium-ion battery cell edge and ensures the overall electrochemical performance. This, in turn, helps to improve the structural stability and cycle life of the cell, while reducing the safety risks caused by the failure of the negative electrode edge.

[0032] Specifically, the tensile strength of the second negative electrode active layer is 3-5 MPa, and the tensile strength of the first negative electrode active layer is 0.6-2 MPa; further, the tensile strength of the second negative electrode active layer can be 3 MPa, 3.2 MPa, 3.23 MPa, 3.5 MPa, 3.7 MPa, 3.9 MPa, 3.94 MPa, 4 MPa, 4.5 MPa, or 5 MPa; and the tensile strength of the first negative electrode active layer can be 0.6 MPa, 1 MPa, 1.11 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa, 1.5 MPa, 1.6 MPa, 1.74 MPa, 1.8 MPa, or 2 MPa.

[0033] In some embodiments, the first negative electrode active layer includes a first negative electrode active material and a first binder, wherein the first binder is selected from styrene-butadiene rubber, and the second negative electrode active layer includes a second negative electrode active material and a second binder, wherein the second binder includes styrene-butadiene rubber and polyacrylic acid.

[0034] Specifically, the first negative electrode active layer uses styrene-butadiene rubber (SBR) as the first binder. SBR has good adhesion and flexibility, which can ensure the stable bonding between the first negative electrode active layer and the negative electrode current collector in the first region of the negative electrode sheet, while meeting the requirements of efficient lithium-ion transport inside the electrode. The second negative electrode active layer adopts a composite binder system of styrene-butadiene rubber and polyacrylic acid (PAA). SBR provides basic adhesion for the second negative electrode active layer. The molecular chain of PAA has abundant carboxyl groups, which can form strong chemical bonds with the surface of the negative electrode active material in the second negative electrode active layer. Combined with the fact that PAA has a large Young's modulus and resistance to deformation, it can effectively suppress the volume expansion of the negative electrode edge during the charging and discharging process, limit the excessive extension of the negative electrode edge, and enhance the structural stability of the negative electrode edge region.

[0035] In some embodiments, the first adhesive in the first negative electrode active layer has a mass percentage content of 0.8%-2.5%; the second adhesive in the second negative electrode active layer has a mass percentage content of 0.4%-1.5%, and the mass ratio of styrene-butadiene rubber to polyacrylic acid in the second adhesive is (3-7):(3-7).

[0036] Specifically, in the first negative electrode active layer, when the mass percentage of the first binder is in the range of 1.0%-1.6%, it is beneficial for the active material in the first negative electrode active layer to firmly bond with the negative current collector, maintain the stability of the electrode structure, and at the same time improve the loading of active material and energy density. In the second negative electrode active layer, the mass percentage of the second binder (styrene-butadiene rubber and polyacrylic acid composite system) has been verified. When the mass percentage of the second binder in the second negative electrode active layer is 0.8%-1.3%, it can give full play to the basic bonding advantages of styrene-butadiene rubber and effectively suppress the edge expansion of the negative electrode sheet by taking advantage of the high modulus characteristics of polyacrylic acid. The mass ratio of styrene-butadiene rubber to polyacrylic acid (3-7):(3-7) allows the two to synergistically enhance the edge mechanical strength and reduce stress concentration, while also balancing flexibility and rigidity. This avoids the electrode hardness becoming too brittle due to the high proportion of polyacrylic acid, or the expansion suppression effect being affected by the excessive proportion of styrene-butadiene rubber.

[0037] Specifically, the mass ratio of the styrene-butadiene rubber to polyacrylic acid can be 3:3, 3:4, 3:5, 3:6, 3:7, 4:3, 4:4, 4:5, 4:6, 4:7, 5:3, 5:4, 5:5, 5:6, 5:7, 6:3, 6:4, 6:5, 6:6, 6:7, 7:3, 7:4, 7:5, 7:6 or 7:7, and a more preferred mass ratio is (5-6):(4-5).

[0038] In some embodiments, the negative electrode current collector includes two second regions, which are respectively located on both sides of the first region in the width direction of the negative electrode current collector; The dimension R1 of the second region in the width direction of the negative electrode current collector and the dimension R2 of the first region in the width direction of the negative electrode current collector satisfy the following condition: 5%≤R1 / R2≤10%.

[0039] Specifically, the negative electrode current collector has a second region set on both sides in the width direction, forming a symmetrical region in the width direction of the negative electrode sheet that restricts the expansion and extension of the negative electrode sheet edge. By setting a second negative electrode active layer with a large Young's modulus on both sides of the first region, the extension stress on both sides of the negative electrode sheet edge caused by the volume change of the active material during charging and discharging can be precisely suppressed, effectively avoiding the problem of excessive extension of the negative electrode sheet edge causing damage to the cell corners. In addition, setting two second regions can evenly disperse the expansion stress in the width direction of the negative electrode sheet, prevent the negative electrode sheet from twisting and deforming due to uneven force on one side, optimize the uniform distribution of lithium-ion batteries in the width direction of the cell, and reduce the risk of local lithium plating.

[0040] In addition, the negative electrode sheet limits the size ratio of a single second region to the first region in the width direction to 5%≤R1 / R2≤10%. This ensures that the second region (high modulus active layer) covers the stress concentration area at the edge of the negative electrode sheet with a reasonable width, thereby improving the edge expansion and damage of the negative electrode sheet during charging and discharging. It also prevents the second region from excessively occupying the position of the negative electrode sheet and affecting the load of the first region on the first negative electrode active material. Thus, it can suppress the volume expansion generated at the edge of the negative electrode sheet while ensuring the high energy density of the lithium-ion battery.

[0041] In some embodiments, a flexible buffer layer is further included, the flexible buffer layer being disposed between the second region and the second negative electrode active layer; The flexible buffer layer includes a flexible buffer material, which includes one or more of graphene, carbon nanotubes, carbon nanofibers, and conductive polymers. The conductive polymer includes one or more of polythiophene, zirconia polymers containing chalcogens, and conductive polymers containing HOS. The thickness of the flexible buffer layer is X, where 0.5μm≤X≤2μm.

[0042] Specifically, the negative electrode sheet incorporates a flexible buffer layer between the second region and the second negative electrode active layer. This flexible buffer layer is made of flexible conductive materials such as graphene, carbon nanotubes, carbon nanofibers, or conductive polymers. On one hand, the high elasticity and flexibility of the flexible buffer material can effectively alleviate stress concentration between the second negative electrode active layer (high Young's modulus) and the negative electrode current collector, further suppressing the extension of the negative electrode sheet in its width direction, while avoiding interface cracking caused by excessive rigidity differences. On the other hand, the excellent conductivity of materials such as graphene and carbon nanotubes can enhance the electron transport efficiency between the current collector and the active layer, reducing contact resistance. Through the composite of the first negative electrode active layer, the second negative electrode active layer, and the flexible buffer layer, this negative electrode sheet coordinates and improves the mechanical stability and electrochemical performance of the lithium-ion battery cell.

[0043] Specifically, the thickness X of the flexible buffer layer is 0.5μm, 1μm, 1.5μm or 2μm.

[0044] In some embodiments, the thickness of the second negative electrode active layer is Y, where 30 μm ≤ Y ≤ 70 μm.

[0045] Specifically, the thickness of the second negative electrode active layer is 30μm≤Y≤70μm. Previous verification has shown that this thickness range ensures that the second active layer, with its large Young's modulus, provides sufficient mechanical support for the expansion and extension of the electrode edge, effectively suppressing volume expansion during charging and discharging and preventing damage to the cell's corners. However, when the thickness of the second negative electrode active layer is less than 30μm, it is difficult to effectively resist the expansion stress at the edge of the negative electrode. Conversely, when the thickness exceeds 70μm, it increases the overall internal resistance of the electrode and may lead to a decrease in coating uniformity due to excessive material accumulation. Therefore, when the thickness of the second negative electrode active layer is 30μm≤Y≤70μm, it ensures both effective constraint on the negative electrode edge and maintains the electrode's energy density.

[0046] Specifically, the thickness of the second negative electrode active layer can be 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 67μm or 70μm.

[0047] In some embodiments, the negative electrode sheet satisfies the following conditions: 1%Y≤X≤3%Y.

[0048] Similarly, verification revealed that when the thickness X of the flexible buffer layer is controlled at 1% to 3% of the thickness Y of the second active layer, it is more conducive to improving the mechanical stability and electrochemical performance of the electrode. If X < 1% Y, the buffer layer is too thin, which is difficult to effectively alleviate the rigidity difference between the second active layer with a large Young's modulus and the negative electrode current collector, and is prone to interface cracking. If X > 3% Y, the buffer layer is too thick, which affects the supporting stress of the second region of the negative electrode current collector in the second active layer, and will also increase the proportion of inactive material, affecting the energy density of the cell.

[0049] In some embodiments, the total thickness of the second negative electrode active layer and the flexible buffer layer is Z, where 30.5 μm ≤ Z ≤ 71.5 μm.

[0050] Specifically, the total thickness of the second negative electrode active layer and the flexible buffer layer is 30.5μm≤Z≤71.5μm. This thickness range is defined by the thickness of the second active layer being 30μm≤Y≤70μm and the thickness of the superimposed flexible buffer layer being 0.5μm≤X≤2μm. This ensures that the second active layer, which has a large Young's modulus, restricts the expansion and extension of the negative electrode edge, while the flexible buffer layer alleviates the stress concentration at the interface. In other words, while suppressing the expansion of the negative electrode edge and improving the interface compatibility, the energy density of the lithium-ion battery can also be guaranteed.

[0051] Specifically, the total thickness of the second negative electrode active layer and the flexible buffer layer can be 30.5μm, 32μm, 35μm, 37μm, 40.5μm, 41μm, 42μm, 46.5μm, 51μm, 57μm, 61μm, 65.5μm, 68μm, 71μm or 71.5μm.

[0052] In some embodiments, the negative electrode sheet satisfies the following relationship: 0.99%Z≤X≤2.91%Z.

[0053] Specifically, 0.99%Z≤X≤2.91%Z (where Z is the total thickness of the second negative electrode active layer and the flexible buffer layer, and X is the thickness of the flexible buffer layer). This ratio defines the thickness proportion of the flexible buffer layer in the negative electrode sheet. When X is controlled within 0.99%-2.91% of Z, it can ensure that the flexible buffer layer plays the best synergistic role in the composite structure of the negative electrode sheet. The thickness of the flexible buffer layer X ≥ 0.99% Z, ensuring that the flexible buffer layer has sufficient thickness to alleviate the interfacial stress between the second active layer and the negative electrode current collector, and to avoid coating cracking due to rigidity differences. The thickness of the flexible buffer layer X ≤ 2.91% Z, to prevent the flexible buffer layer from being too thick, which would be detrimental to the second active layer exerting a good mechanical constraint.

[0054] In some embodiments, the thickness of the first negative electrode active layer is 30.5 μm ≤ Z ≤ 71.5 μm.

[0055] Specifically, the second negative electrode active layer described in this application is mainly used to suppress the expansion of the negative electrode edge. The thickness of the first negative electrode active layer can be set to 40μm-50μm. This range can ensure the content of the first negative electrode active material in the first region of the negative electrode to improve the battery energy density.

[0056] In some embodiments, both the first negative electrode active material and the second negative electrode active material comprise silicon-carbon materials; The silicon-carbon material accounts for 95%-98% of the mass in the first negative electrode active layer and 95%-98% of the mass in the second negative electrode active layer.

[0057] Specifically, the silicon-carbon material includes graphite and silicon-carbon, and the mass percentage of the silicon-carbon material in the first negative electrode active layer can be 95%, 96%, 97% or 98%, and the mass percentage of the silicon-carbon material in the second negative electrode active layer can be 95%, 96%, 97% or 98%.

[0058] In some embodiments, the first negative electrode active layer further includes a first conductive agent, and the second negative electrode active layer further includes a second conductive agent. The first conductive agent and the second conductive agent each independently include one or more of carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotube, and graphene. The first conductive agent has a mass percentage of 0%-0.5% in the first negative electrode active layer, and the second conductive agent has a mass percentage of 0.1%-0.5% in the second negative electrode active layer.

[0059] Another embodiment of the present invention provides a lithium-ion battery including the aforementioned negative electrode sheet. Specifically, the lithium-ion battery of this application includes the aforementioned negative electrode sheet. The negative electrode sheet provided in this application includes a negative electrode current collector having a first region and a second region. The first region is disposed on the negative electrode current collector, and the second region is located between the edge of the negative electrode current collector and the edge of the first region. A reference is provided in the second region of the negative electrode current collector. Figure 1 The present invention provides a negative electrode sheet, including a negative electrode current collector 1, a first negative electrode active layer 2 and a second negative electrode active layer 3. The negative electrode current collector 1 includes a first region and a second region. The first region is disposed on the negative electrode current collector 1, and the second region is located between the edge of the negative electrode current collector 1 and the edge of the first region. The first negative electrode active layer 2 is disposed in the first region, and the second negative electrode active layer 3 is disposed in the second region. The Young's modulus of the second negative electrode active layer 3 is greater than that of the first negative electrode active layer 2.

[0060] Specifically, the negative electrode sheet provided in this application includes a negative electrode current collector 1 having a first region and a second region. The first region is disposed on the negative electrode current collector 1, and the second region is located between the edge of the negative electrode current collector 1 and the edge of the first region. At the same time, a second negative electrode active layer 3 is disposed in the second region of the negative electrode current collector 1. The Young's modulus of the second negative electrode active layer 3 is greater than that of the first negative electrode active layer 2. The negative electrode active layer with a larger Young's modulus has stronger resistance to deformation. By limiting the excessive extension of the edge of the negative electrode sheet through mechanical stress, the large Young's modulus of the second negative electrode active layer 3 can effectively suppress the expansion of the active material at the edge of the negative electrode sheet during charging and discharging, thereby reducing the extension stress in the width direction of the negative electrode sheet, improving the situation of cell corner damage caused by excessive extension of the negative electrode sheet, and further solving the problem of internal short circuit of lithium-ion battery caused by cell corner damage, thereby improving the safety performance of lithium-ion battery.

[0061] Specifically, the Young's modulus of the second negative electrode active layer 3 is 2-5 GPa, and the Young's modulus of the first negative electrode active layer 2 is 0.5-2 GPa; further, the Young's modulus of the second negative electrode active layer 3 can be 2 GPa, 2.2 GPa, 2.35 GPa, 2.4 GPa, 2.5 GPa, 3 GPa, 3.2 GPa, 3.4 GPa, 3.41 GPa, 4 GPa, 4.5 GPa, or 5 GPa; the Young's modulus of the first negative electrode active layer 2 can be 0.5 GPa, 0.8 GPa, 1 GPa, 1.05 GPa, 1.5 GPa, 1.7 GPa, 1.73 GPa, or 2 GPa.

[0062] In some embodiments, the ionic conductivity of the first negative electrode active layer 2 is greater than the ionic conductivity of the second negative electrode active layer 3.

[0063] Specifically, the first negative electrode active layer 2 possesses excellent kinetic performance and high ionic conductivity, which can significantly improve the transport rate of lithium ions in the electrode, enhance the charge and discharge efficiency and power density of the battery, and thus help improve the energy density of the lithium-ion battery. The second negative electrode active layer 3, with its large Young's modulus, utilizes its strong resistance to deformation to form a stable structure at the edge of the negative electrode sheet, effectively suppressing the volume expansion of the active material and reducing the problems of cell corner structure damage and active material shedding caused by expansion. That is, by utilizing the synergy of the first negative electrode active layer 2 and the second negative electrode active layer 3, the advantages of the first negative electrode active layer 2 in lithium-ion battery energy storage are fully utilized, and the mechanical protection characteristics of the second negative electrode active layer 3 are used to improve the adverse effects caused by expansion, so that the lithium-ion battery can maintain high energy density while suppressing the volume expansion generated at the edge of the negative electrode sheet.

[0064] In some embodiments, the tensile strength of the second negative electrode active layer 3 is greater than the tensile strength of the first negative electrode active layer 2.

[0065] Specifically, the tensile strength of the second negative electrode active layer 3 is greater than that of the first negative electrode active layer 2. The higher tensile strength allows the second negative electrode active layer 3 to better resist the tensile stress generated by the volume expansion of the active material during charging and discharging in the edge region of the negative electrode current collector 1, effectively preventing tearing, breakage, and loss of active material at the edge of the negative electrode sheet. The first negative electrode active layer 2 maintains a lower tensile strength, which helps to maintain good flexibility in the corresponding first region on the negative electrode sheet, maintain efficient lithium-ion transport and electrode structure integrity. The synergistic effect of the first negative electrode active layer 2 in the first region and the second negative electrode active layer 3 in the second region not only enhances the mechanical toughness of the edge of the lithium-ion battery cell but also ensures the overall electrochemical performance, thereby improving the structural stability and cycle life of the cell and reducing the safety risks caused by the failure of the negative electrode edge.

[0066] Specifically, the tensile strength of the second negative electrode active layer 3 is 3-5 MPa, and the tensile strength of the first negative electrode active layer 2 is 0.6-2 MPa; further, the tensile strength of the second negative electrode active layer 3 can be 3 MPa, 3.2 MPa, 3.23 MPa, 3.5 MPa, 3.7 MPa, 3.9 MPa, 3.94 MPa, 4 MPa, 4.5 MPa, or 5 MPa; and the tensile strength of the first negative electrode active layer 2 can be 0.6 MPa, 1 MPa, 1.11 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa, 1.5 MPa, 1.6 MPa, 1.74 MPa, 1.8 MPa, or 2 MPa.

[0067] In some embodiments, the first negative electrode active layer 2 includes a first negative electrode active material and a first binder, wherein the first binder is selected from styrene-butadiene rubber, and the second negative electrode active layer 3 includes a second negative electrode active material and a second binder, wherein the second binder includes styrene-butadiene rubber and polyacrylic acid.

[0068] Specifically, the first negative electrode active layer 2 uses styrene-butadiene rubber (SBR) as the first binder. SBR has good adhesion and flexibility, which can ensure the stable bonding between the first negative electrode active layer 2 and the negative electrode current collector 1 in the first region of the negative electrode sheet, while meeting the requirements of efficient lithium ion transport inside the electrode. The second negative electrode active layer 3 adopts a composite binder system of styrene-butadiene rubber and polyacrylic acid (PAA). SBR provides basic adhesion for the second negative electrode active layer. The molecular chain of PAA has abundant carboxyl groups, which can form strong chemical bonds with the surface of the negative electrode active material in the second negative electrode active layer 3. Combined with the fact that PAA has a large Young's modulus and resistance to deformation, it can effectively suppress the volume expansion of the negative electrode edge during the charging and discharging process, limit the excessive extension of the negative electrode edge, and enhance the structural stability of the negative electrode edge region.

[0069] In some embodiments, in the first negative electrode active layer 2, the mass percentage of the first adhesive is 0.8%-2.5%; in the second negative electrode active layer 3, the mass percentage of the second adhesive is 0.4%-1.5%, and in the second adhesive, the mass ratio of styrene-butadiene rubber to polyacrylic acid is (3-7):(3-7).

[0070] Specifically, in the first negative electrode active layer 2, when the mass percentage of the first binder is in the range of 1.0%-1.6%, it is beneficial for the active material in the first negative electrode active layer 2 to firmly bond with the negative current collector, maintain the stability of the electrode structure, and at the same time improve the loading and energy density of the active material. In the second negative electrode active layer 3, the mass percentage of the second binder (styrene-butadiene rubber and polyacrylic acid composite system) has been verified. When the mass percentage of the second binder in the second negative electrode active layer 3 is 0.8%-1.3%, it can give full play to the basic bonding advantage of styrene-butadiene rubber and effectively suppress the edge expansion of the negative electrode sheet by taking advantage of the high modulus characteristics of polyacrylic acid. The mass ratio of styrene-butadiene rubber to polyacrylic acid (3-7):(3-7) allows the two to synergistically enhance the edge mechanical strength and reduce stress concentration, while also balancing flexibility and rigidity. This avoids the electrode hardness becoming too brittle due to the high proportion of polyacrylic acid, or the expansion suppression effect being affected by the excessive proportion of styrene-butadiene rubber.

[0071] Specifically, the mass ratio of the styrene-butadiene rubber to polyacrylic acid can be 3:3, 3:4, 3:5, 3:6, 3:7, 4:3, 4:4, 4:5, 4:6, 4:7, 5:3, 5:4, 5:5, 5:6, 5:7, 6:3, 6:4, 6:5, 6:6, 6:7, 7:3, 7:4, 7:5, 7:6 or 7:7, and a more preferred mass ratio is (5-6):(4-5).

[0072] In some embodiments, the negative electrode current collector 1 includes two second regions, which are respectively located on both sides of the first region in the width direction of the negative electrode current collector 1; The dimension R1 of the second region in the width direction of the negative electrode current collector 1 and the dimension R2 of the first region in the width direction of the negative electrode current collector 1 satisfy the following condition: 5%≤R1 / R2≤10%.

[0073] Specifically, the negative electrode current collector 1 has a second region on both sides in the width direction, forming a symmetrical region in the width direction of the negative electrode sheet that restricts the expansion and extension of the negative electrode sheet edge. By setting a second negative electrode active layer 3 with a large Young's modulus on both sides of the first region, the extension stress on both sides of the negative electrode sheet edge caused by the volume change of the active material during charging and discharging can be precisely suppressed, effectively avoiding the problem of excessive extension of the negative electrode sheet edge causing damage to the cell corners. In addition, setting two second regions can evenly disperse the expansion stress in the width direction of the negative electrode sheet, prevent the negative electrode sheet from twisting and deforming due to uneven force on one side, optimize the uniform distribution of lithium-ion batteries in the width direction of the cell, and reduce the risk of local lithium plating.

[0074] In addition, the negative electrode sheet limits the size ratio of a single second region to the first region in the width direction to 5%≤R1 / R2≤10%. This ensures that the second region (high modulus active layer) covers the stress concentration area at the edge of the negative electrode sheet with a reasonable width, thereby improving the edge expansion and damage of the negative electrode sheet during charging and discharging. It also prevents the second region from excessively occupying the position of the negative electrode sheet and affecting the load of the first region on the first negative electrode active material. Thus, it can suppress the volume expansion generated at the edge of the negative electrode sheet while ensuring the high energy density of the lithium-ion battery.

[0075] In some embodiments, a flexible buffer layer 4 is further included, which is disposed between the second region and the second negative electrode active layer 3; The flexible buffer layer 4 includes a flexible buffer material, which includes one or more of graphene, carbon nanotubes, carbon nanofibers, and conductive polymers. The conductive polymer includes one or more of polythiophene, zirconia polymers containing chalcogens, and conductive polymers containing HOS. The thickness of the flexible buffer layer 4 is X, where 0.5μm≤X≤2μm.

[0076] Specifically, the negative electrode sheet incorporates a flexible buffer layer 4 between the second region and the second negative electrode active layer 3. This flexible buffer layer 4 is made of flexible conductive materials such as graphene, carbon nanotubes, carbon nanofibers, or conductive polymers. On one hand, the high elasticity and flexibility of the flexible buffer material can effectively alleviate stress concentration between the second negative electrode active layer (high Young's modulus) and the negative electrode current collector 1, further suppressing the extension of the negative electrode sheet in its width direction, while avoiding interface cracking caused by excessive rigidity differences. On the other hand, the excellent conductivity of materials such as graphene and carbon nanotubes can enhance the electron transport efficiency between the current collector and the active layer, reducing contact resistance. Through the composite of the first negative electrode active layer 2, the second negative electrode active layer 3, and the flexible buffer layer 4, the mechanical stability and electrochemical performance of the lithium-ion battery cell are coordinated and improved.

[0077] Specifically, the thickness X of the flexible buffer layer 4 is 0.5μm, 1μm, 1.5μm or 2μm.

[0078] In some embodiments, the thickness of the second negative electrode active layer 3 is Y, where 30 μm ≤ Y ≤ 70 μm.

[0079] Specifically, the thickness of the second negative electrode active layer 3 is 30μm≤Y≤70μm. Previous verification has shown that this thickness range ensures that the second active layer, with its large Young's modulus, provides sufficient mechanical support for the expansion and extension of the electrode edge, effectively suppressing volume expansion during charging and discharging and preventing damage to the cell's corners. However, when the thickness of the second negative electrode active layer 3 is less than 30μm, it is difficult to effectively resist the expansion stress at the edge of the negative electrode. Conversely, when the thickness exceeds 70μm, it increases the overall internal resistance of the electrode and may lead to a decrease in coating uniformity due to excessive material accumulation. Therefore, when the thickness of the second negative electrode active layer 3 is 30μm≤Y≤70μm, it ensures both effective constraint on the negative electrode edge and maintains the electrode's energy density.

[0080] Specifically, the thickness of the second negative electrode active layer 3 can be 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 67μm or 70μm.

[0081] In some embodiments, the negative electrode sheet satisfies the following conditions: 1%Y≤X≤3%Y.

[0082] Similarly, verification revealed that when the thickness X of the flexible buffer layer 4 is controlled at 1% to 3% of the thickness Y of the second active layer, it is more conducive to improving the mechanical stability and electrochemical performance of the electrode. If X < 1% Y, the buffer layer is too thin, which is difficult to effectively alleviate the rigidity difference between the second active layer with a large Young's modulus and the negative electrode current collector 1, and is prone to interface cracking. If X > 3% Y, the buffer layer is too thick, which affects the supporting stress of the second region of the negative electrode current collector 1 of the second active layer, and will also increase the proportion of inactive material, affecting the energy density of the cell.

[0083] In some embodiments, the total thickness of the second negative electrode active layer 3 and the flexible buffer layer 4 is Z, where 30.5 μm ≤ Z ≤ 71.5 μm.

[0084] Specifically, the total thickness of the second negative electrode active layer 3 and the flexible buffer layer 4 is 30.5μm≤Z≤71.5μm. This thickness range is defined by the thickness of the second active layer being 30μm≤Y≤70μm and the thickness of the flexible buffer layer 4 being 0.5μm≤X≤2μm. This ensures that the second active layer, which has a large Young's modulus, restricts the expansion and extension of the negative electrode edge, while the flexible buffer layer 4 can alleviate the stress concentration at the interface. In other words, while suppressing the expansion of the negative electrode edge and improving the interface compatibility, the energy density of the lithium-ion battery can also be guaranteed.

[0085] Specifically, the total thickness of the second negative electrode active layer 3 and the flexible buffer layer 4 can be 30.5μm, 32μm, 35μm, 37μm, 40.5μm, 41μm, 42μm, 46.5μm, 51μm, 57μm, 61μm, 65.5μm, 68μm, 71μm or 71.5μm.

[0086] In some embodiments, the negative electrode sheet satisfies the following relationship: 0.99%Z≤X≤2.91%Z.

[0087] Specifically, 0.99%Z≤X≤2.91%Z (where Z is the total thickness of the second negative electrode active layer 3 and the flexible buffer layer 4, and X is the thickness of the flexible buffer layer 4). This ratio defines the thickness proportion of the flexible buffer layer 4 in the negative electrode sheet. When X is controlled within 0.99%-2.91% of Z, it can be ensured that the flexible buffer layer 4 plays the best synergistic role in the composite structure of the negative electrode sheet. The thickness of the flexible buffer layer 4 is X≥0.99%Z, ensuring that the flexible buffer layer 4 has sufficient thickness to alleviate the interfacial stress between the second active layer and the negative electrode current collector 1, and to avoid coating cracking due to rigidity differences. The thickness of the flexible buffer layer 4 is X≤2.91%Z, to prevent the flexible buffer layer 4 from being too thick, which would be detrimental to the second active layer exerting a good mechanical constraint force.

[0088] In some embodiments, the thickness of the first negative electrode active layer 2 is 30.5 μm ≤ Z ≤ 71.5 μm.

[0089] Specifically, the second negative electrode active layer 3 described in this application is mainly used to suppress the expansion of the negative electrode edge. The thickness of the first negative electrode active layer 2 can be set to 40μm-50μm. This range can ensure the content of the first negative electrode active material in the first region of the negative electrode to improve the battery energy density.

[0090] In some embodiments, both the first negative electrode active material and the second negative electrode active material comprise silicon-carbon materials; The silicon-carbon material accounts for 95%-98% of the mass of the first negative electrode active layer 2, and the silicon-carbon material accounts for 95%-98% of the mass of the second negative electrode active layer 3.

[0091] Specifically, the silicon-carbon material includes graphite and silicon-carbon. The mass percentage of the silicon-carbon material in the first negative electrode active layer 2 can be 95%, 96%, 97%, or 98%, and the mass percentage of the silicon-carbon material in the second negative electrode active layer 3 can be 95%, 96%, 97%, or 98%.

[0092] In some embodiments, the first negative electrode active layer 2 further includes a first conductive agent, and the second negative electrode active layer 3 further includes a second conductive agent. The first conductive agent and the second conductive agent each independently include one or more of carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, and graphene. The first conductive agent accounts for 0%-0.5% of the mass of the first negative electrode active layer 2, and the second conductive agent accounts for 0.1%-0.5% of the mass of the second negative electrode active layer 3.

[0093] Another embodiment of the present invention provides a lithium-ion battery including the aforementioned negative electrode sheet. Specifically, the lithium-ion battery of this application includes the aforementioned negative electrode sheet. The negative electrode sheet provided in this application includes a negative electrode current collector 1 having a first region and a second region. The first region is disposed on the negative electrode current collector 1, and the second region is located between the edge of the negative electrode current collector 1 and the edge of the first region. A second negative electrode active layer 3 is disposed in the second region of the negative electrode current collector 1. The Young's modulus of the second negative electrode active layer 3 is greater than that of the first negative electrode active layer 2. A negative electrode active layer with a larger Young's modulus has stronger resistance to deformation. Mechanical stress limits the excessive extension of the negative electrode sheet edge. Furthermore, the larger Young's modulus of the second negative electrode active layer 3 can effectively suppress the volume expansion of the active material at the edge of the negative electrode sheet during charging and discharging, thereby reducing the extension stress in the width direction of the negative electrode sheet. This improves the situation where excessive extension of the negative electrode sheet causes damage to the cell corners, further solving the problem of internal short circuits in lithium-ion batteries caused by cell corner damage, thereby improving the safety performance of the lithium-ion battery. A second negative electrode active layer is provided, and the Young's modulus of the second negative electrode active layer is greater than that of the first negative electrode active layer. The negative electrode active layer with a larger Young's modulus has stronger resistance to deformation. Through mechanical stress, the excessive extension of the edge of the negative electrode sheet is limited. In addition, the larger Young's modulus of the second negative electrode active layer can effectively suppress the volume expansion of the active material at the edge of the negative electrode sheet during charging and discharging, thereby reducing the extension stress in the width direction of the negative electrode sheet. This improves the situation of cell corner damage caused by excessive extension of the negative electrode sheet, and further solves the problem of internal short circuit in lithium-ion batteries caused by cell corner damage, thereby improving the safety performance of lithium-ion batteries.

[0094] The present invention will be further illustrated by the following examples.

[0095] Table 1 Table 2 Example 1 This embodiment illustrates a negative electrode sheet and a lithium-ion battery disclosed in this invention, including the following operational steps: Preparation of negative electrode: Take 91% graphite, 5% silicon carbide, 2.5% SBR, 1% carboxymethyl cellulose (CMC), and 0.5% conductive carbon black (SP), add deionized water and mix evenly to obtain the first negative electrode active layer slurry; Take 91% graphite, 5% silicon carbide, 1.6% SBR, 0.9% PAA, 1.0% carboxymethyl cellulose (CMC), and 0.5% conductive carbon black (SP), add deionized water and mix evenly to obtain the second negative electrode active layer slurry; Take 75% carbon nanotubes (CNT), 18% sodium hydroxymethyl cellulose (CMC-Na), and 7% SBR, add deionized water and mix evenly to obtain a flexible buffer layer slurry; The flexible buffer layer slurry is coated on the second region, the first negative electrode active layer slurry is coated on the first region corresponding to the negative electrode current collector, and then the second negative electrode active layer slurry is coated on the surface of the flexible buffer layer slurry to obtain the negative electrode sheet. In this negative electrode sheet, the thickness of the flexible buffer layer is 1 μm, the thickness of the first negative electrode active layer is 40 μm, and the thickness of the second negative electrode active layer is 50 μm. The Young's modulus of the first negative electrode active layer is 1.05 GPa, and the tensile strength of the first negative electrode active layer is 1.11 MPa. The Young's modulus of the second negative electrode active layer is 2.35 GPa, and the tensile strength of the second negative electrode active layer is 3.23 MPa. Preparation of negative electrode: A slurry of positive electrode active material is coated onto a positive electrode current collector, and after drying, a positive electrode sheet is obtained. Preparation of lithium-ion batteries: After the positive electrode, separator, and negative electrode are arranged in order, the battery cell is prepared. The battery cell is placed in the casing, and electrolyte is injected. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0096] Example 2-17 Examples 2-17 illustrate a negative electrode and lithium-ion battery disclosed in this invention, including most of the operating steps in Example 1, with the following differences: The following parameters were used: the mass percentage of the first adhesive, the Young's modulus of the first negative electrode active layer, the tensile strength of the first negative electrode active layer, the mass percentage of the second adhesive, the mass ratio of the second adhesive styrene-butadiene rubber to the second adhesive polyacrylic acid, the Young's modulus of the second negative electrode active layer, the tensile strength of the second negative electrode active layer, and the thickness of the first negative electrode active layer, the thickness of the second negative electrode active layer, the thickness of the flexible buffer layer, the thickness of the flexible buffer layer, and the total thickness of the second negative electrode active layer and the flexible buffer layer, as shown in Examples 2-17 of Table 2.

[0097] Comparative Example 1 This comparative example is used to illustrate the negative electrode sheet and lithium-ion battery disclosed in this invention, including most of the operating steps in Example 1, with the following differences: The first and second regions of the negative electrode current collector are coated with a second negative electrode active layer.

[0098] Comparative Example 2 This comparative example is used to illustrate the negative electrode sheet and lithium-ion battery disclosed in this invention, including most of the operating steps in Example 1, with the following differences: The first negative electrode active layer is coated on both the first and second regions of the negative electrode current collector.

[0099] Comparative Example 3 This comparative example is used to illustrate the negative electrode sheet and lithium-ion battery disclosed in this invention, including most of the operating steps in Example 1, with the following differences: The positions of the first negative electrode active layer and the second negative electrode active layer are interchanged.

[0100] Performance testing The following performance tests were performed on Examples 1-17 and Comparative Examples 1-3 prepared above: 1. Thickness expansion rate test: 1) Place the battery in a dormant state at 25℃ for 5 minutes and test the initial thickness of the battery; 2) Charge at a constant current of 0.5C to 4.51V, then charge at a constant voltage of 4.51V to 0.05C; 3) Rest for 5 minutes; 4) Discharge at a constant current of 0.5C to 3.0V; 5) Repeat steps 2)-4) 100 times; The thickness of the battery was measured after 100 cycles when the cell was fully charged.

[0101] Expansion rate (%) = (Cell thickness after 100 cycles - Initial thickness) / Initial thickness × 100%.

[0102] 2. Width expansion rate test: 1) Record the cutting width of the negative electrode sheet of the battery cell; place the battery in a dormant state at a temperature of 25℃ for 5 minutes; 2) Charge at a constant current of 0.5C to 4.51V, then charge at a constant voltage of 4.51V to 0.05C; 3) Rest for 5 minutes; 4) Discharge at a constant current of 0.5C to 3.0V; 5) Repeat steps 2)-4) 100 times; The width of the negative electrode sheet in the fully charged state of the cell was determined by CT testing after 100 cycles.

[0103] Width expansion rate (%) = (Negative electrode width of the cell in full charge state after 100 cycles - Initial slitting width) / Initial slitting width × 100%; 3. Corner breakage rate test: 1) Record the cutting width of the negative electrode sheet of the battery cell; place the battery in a dormant state at a temperature of 25℃ for 5 minutes; 2) Charge at a constant current of 0.5C to 4.51V, then charge at a constant voltage of 4.51V to 0.05C; 3) Rest for 5 minutes; 4) Discharge at a constant current of 0.5C to 3.0V; 5) Repeat steps 2)-4) 100 times; The number of visible damages to the battery cells was visually inspected after 100 weeks.

[0104] Corner breakage rate (%) = Number of cells broken after 100 cycles / Total number of cells tested × 100% The test results are entered into Table 3.

[0105] Table 3 As can be seen from the test results in Table 3, in Example 1, the expansion rate in the electrode width direction is only 0.63%, the expansion rate in the cell thickness is 6.39%, and the corner breakage rate is as low as 0.2%. This is because the Young's modulus (2.35 GPa) and tensile strength (3.23 MPa) of the second negative electrode active layer in this example are significantly higher than those of the first negative electrode active layer (1.05 GPa, 1.11 MPa). In addition, with the size of the second region and the thickness of the buffer layer, the edge stress is effectively suppressed, while the first region maintains good ion transport capability. In Examples 2-6, different contents of the second binder were used. The test results showed that the electrode width expansion rate of Example 2 was 0.89% and the breakage rate was 3.6%; the electrode width expansion rate of Example 5 was 1.03% and the breakage rate was 6.6%. In Example 2, the content of the second binder was too high (2.5%), and the rigidity of the second active layer was too strong, resulting in an increase in the overall thickness expansion rate (9.56%). In Example 5, the content of the second binder was too low (0.6%), and the deformation resistance of the second active layer was insufficient (Young's modulus 1.65 GPa), the edge constraint failed, and the expansion rate and breakage rate deteriorated. The corner breakage rate of Example 8 was 1.3%, and the breakage rate of Example 9 (first active layer 20.5 μm) was 9.2%. It is speculated that the reason is that when the first adhesive is in the range of 0.8%-2.5%, the flexibility and ionic conductivity of the first active layer can be well guaranteed, reducing the overall expansion. The thickness of the first active layer in Example 9 is too thin, and the active material load is unstable, which can easily lead to structural damage. The breakage rate of Example 11 was 1.5%, and the breakage rate of Example 13 was 5.4%. The reason is that the thickness of the buffer layer is in the range of 0.5-2μm (as in Example 11), which can effectively alleviate the stress concentration between the second active layer and the current collector. When the material is too thin (0.2μm), the buffering effect is insufficient; when it is too thick (2.2μm), it occupies too much volume, both of which lead to an increase in the expansion rate.

[0106] In Examples 15-17, Examples 15 and 16 lack a second negative electrode active layer, and Example 7 lacks a flexible buffer layer. As can be seen from the test results, the corner breakage rate of all three is relatively high (23.6%-53.7%). This indirectly shows that setting a high Young's modulus active layer at the edge of the negative electrode sheet, and further coordinating with a flexible buffer layer, is beneficial to further suppress the expansion stress at the edge of the silicon-carbon negative electrode and improve the occurrence of corner breakage. Although the electrode width expansion rate of Comparative Example 1 is relatively low, the thickness expansion rate and breakage rate are extremely high. Since the first and second regions of Comparative Example 1 are coated with the second negative electrode active layer, the overall flexibility is sacrificed due to the excessive emphasis on deformation resistance. The breakage rates of Comparative Example 2 (using only the first active layer) and Comparative Example 3 (interchanging active layers) were much higher than those of Example 1, further illustrating that a single active layer (only the first active layer or only the second active layer) cannot simultaneously meet the requirements of high ionic conductivity and anti-expansion at the edge of the negative electrode.

[0107] Based on the parameter settings in Example 1, the expansion rate of lithium-ion batteries prepared using different types of flexible buffer materials was further tested: The test data of cell thickness expansion rate for Example 1 and Examples 18-20 are shown in Table 4. Table 4 As can be seen from the test results in Table 4, the cell thickness expansion rates of Example 1 (carbon nanofiber), Example 18 (carbon nanotube and carbon nanofiber), Example 19 (carbon nanotube and graphene), and Example 20 (carbon nanotube and conductive polymer) are all lower than those of Example 17 (14.37%) without a buffer layer, proving that the flexible buffer layer can effectively disperse stress. The expansion rates of Examples 19 and 20 (6.8% and 6.7%, respectively) were slightly higher than those of Example 18. This may be because graphene has relatively high rigidity, which weakens the flexible buffering effect to some extent. The conductivity of high-molecular conductive polymers (such as polythiophene) is slightly lower than that of carbon nanomaterials, which may result in a slightly lower local stress dispersion efficiency. However, the expansion rates of Examples 19 and 20 are still significantly better than those of the negative electrode without a buffer layer.

[0108] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector, a first negative electrode active layer and a second negative electrode active layer. The negative electrode current collector includes a first region and a second region. The first region is disposed on the negative electrode current collector, and the second region is located between the edge of the negative electrode current collector and the edge of the first region. The first negative electrode active layer is disposed in the first region, and the second negative electrode active layer is disposed in the second region. The Young's modulus of the second negative electrode active layer is greater than that of the first negative electrode active layer.

2. The negative electrode sheet according to claim 1, characterized in that, The ionic conductivity of the first negative electrode active layer is greater than that of the second negative electrode active layer.

3. The negative electrode sheet according to claim 1, characterized in that, The tensile strength of the second negative electrode active layer is greater than that of the first negative electrode active layer.

4. The negative electrode sheet according to claim 1, characterized in that, The first negative electrode active layer includes a first negative electrode active material and a first binder, wherein the first binder is selected from styrene-butadiene rubber. The second negative electrode active layer includes a second negative electrode active material and a second binder, wherein the second binder includes styrene-butadiene rubber and polyacrylic acid.

5. The negative electrode sheet according to claim 4, characterized in that, In the first negative electrode active layer, the mass percentage of the first binder is 0.8%-2.5%; in the second negative electrode active layer, the mass percentage of the second binder is 0.4%-1.5%, and in the second binder, the mass ratio of styrene-butadiene rubber to polyacrylic acid is (3-7):(3-7).

6. The negative electrode sheet according to claim 1, characterized in that, The negative electrode current collector includes two second regions, which are respectively located on both sides of the first region in the width direction of the negative electrode current collector; The dimension R1 of the second region in the width direction of the negative electrode current collector and the dimension R2 of the first region in the width direction of the negative electrode current collector satisfy the following condition: 5%≤R1 / R2≤10%.

7. The negative electrode sheet according to claim 1, characterized in that, It also includes a flexible buffer layer, which is disposed between the second region and the second negative electrode active layer; The flexible buffer layer includes a flexible buffer material, which includes one or more of graphene, carbon nanotubes, carbon nanofibers, and conductive polymers. The conductive polymer includes one or more of polythiophene, zirconia polymers containing chalcogens, and conductive polymers containing HOS. The thickness of the flexible buffer layer is X, where 0.5μm≤X≤2μm.

8. The negative electrode sheet according to claim 7, characterized in that, The thickness of the second negative electrode active layer is Y, where 30μm≤Y≤70μm.

9. The negative electrode sheet according to claim 7, characterized in that, The negative electrode sheet satisfies the following conditions: 1%Y≤X≤3%Y.

10. The negative electrode sheet according to claim 6, characterized in that, The total thickness of the second negative electrode active layer and the flexible buffer layer is Z, 30.5μm≤Z≤71.5μm.

11. The negative electrode sheet according to claim 10, characterized in that, The negative electrode sheet satisfies the following relationship: 0.99%Z≤X≤2.91%Z.

12. The negative electrode sheet according to claim 1, characterized in that, The thickness of the first negative electrode active layer is 30.5μm-71.5μm.

13. The negative electrode sheet according to claim 4, characterized in that, Both the first negative electrode active material and the second negative electrode active material include silicon-carbon materials; The silicon-carbon material accounts for 95%-98% of the mass in the first negative electrode active layer and 95%-98% of the mass in the second negative electrode active layer.

14. The negative electrode sheet according to claim 1, characterized in that, The first negative electrode active layer further includes a first conductive agent, and the second negative electrode active layer further includes a second conductive agent. The first conductive agent and the second conductive agent each independently include one or more of carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotube, and graphene. The mass percentage of the first conductive agent in the first negative electrode active layer is 0%-0.5%, and the mass percentage of the second conductive agent in the second negative electrode active layer is 0.1%-0.5%.

15. A lithium-ion battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1-14.

Images