Negative electrode sheet, method for producing negative electrode sheet, and secondary battery

Abstract

This invention relates to the field of secondary battery technology, specifically to a negative electrode sheet, a method for preparing the negative electrode sheet, and a secondary battery. The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one side of the negative current collector. The four corners of the negative electrode sheet have notched corner structures. A composite coating layer is disposed on the edge region of the negative electrode sheet, comprising a buffer layer and a conductive layer. The buffer layer is disposed on the inner side, and the conductive layer is disposed on the outer side of the buffer layer. The thickness of the composite coating layer gradually decreases from the edge of the negative electrode sheet towards the center. Through the combination of the notched corner structures and the composite coating layer, the electrode sheet maintains good interfacial contact and current distribution during charge-discharge cycles, while achieving puncture resistance, low impedance, and suppression of swelling, without sacrificing the battery's energy density and rate performance.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, specifically to a negative electrode sheet, a method for preparing the negative electrode sheet, and a secondary battery. Background Technology

[0002] With the development of new energy vehicles and the energy storage industry, stacked pouch batteries have become an important technological direction due to their advantages such as high energy density and low internal resistance. Currently, the negative electrode of stacked batteries usually adopts a right-angle punching design or a simple rounded transition treatment at the edge.

[0003] In practical applications, the corners and sides of the electrode exhibit a dual concentration of electric field and stress, resulting in a significantly higher current density at the edges compared to the central region. Simultaneously, burrs at the electrode edges continuously compress the separator during charging and discharging. These issues can lead to lithium plating at the negative electrode edges, separator puncture, and cyclic swelling due to poor interface contact, severely impacting the battery's cycle life and safety performance. Summary of the Invention

[0004] This invention provides a negative electrode sheet, a method for preparing the negative electrode sheet, and a secondary battery. For example, when the secondary battery is a lithium-ion secondary battery, it solves the problems of lithium deposition at the negative electrode edge, puncture of the separator, and poor interface contact leading to cycle swelling, which seriously affect the cycle life and safety performance of the battery.

[0005] To address the aforementioned problems, the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the four corners of the negative electrode sheet are provided with a notched corner structure; a composite coating layer is disposed on the edge region of the negative electrode sheet, the composite coating layer comprising a buffer layer and a conductive layer, the buffer layer being disposed on the inner side, the conductive layer being disposed on the outer side of the buffer layer, and the thickness of the composite coating layer gradually decreasing from the edge of the negative electrode sheet towards the center.

[0006] Optionally, the notched structure includes a C-corner and an R-corner, wherein the C-corner is a straight hypotenuse disposed at the corner of the negative electrode sheet, and the R-corner is disposed at the connection between the two ends of the C-corner and the side of the negative electrode sheet.

[0007] Optionally, the hypotenuse length of the C-angle is 0.3mm-1.2mm, the chamfer angle of the C-angle is 30°-60°, the radius of the R-angle is 0.2mm-1.0mm, and the ratio of the radius of the R-angle to the hypotenuse length of the C-angle is 0.5-0.8.

[0008] Optionally, the four straight sides of the negative electrode sheet and the hypotenuse of the C-angle are respectively provided with rounded corners, and the radius of the rounded corners is 0.05mm-0.2mm.

[0009] Optionally, the buffer layer is a composite layer of ceramic and adhesive, the conductive layer is a composite layer of conductive agent and elastic adhesive, and the coverage width of the conductive layer is smaller than the coverage width of the buffer layer.

[0010] Optionally, the thickness of the buffer layer and the conductive layer decreases linearly from the edge of the negative electrode sheet towards the center, and the cross-sectional profiles of the two at the edge of the electrode sheet together define a wedge-shaped cross-section with the opening facing the edge.

[0011] Optionally, the negative electrode and the matching positive electrode satisfy at least one of the following dimensional relationships: the difference between the main body length of the negative electrode and the main body length of the matching positive electrode is 0.4mm-1.0mm; the difference between the main body width of the negative electrode and the main body width of the matching positive electrode is 0.3mm-0.8mm; and the difference between the projected length of the C-angle hypotenuse of the negative electrode and the projected length of the chamfered hypotenuse of the matching positive electrode is 0.2mm-0.5mm. The ratio of the thickness of the single-sided active material layer of the negative electrode to the thickness of the single-sided active material layer of the matching positive electrode is 1.05:1 to 1.15:1.

[0012] The present invention also provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector. The corner area of ​​the negative electrode sheet is provided with a notched structure, the notched structure forming a non-right-angle boundary. A composite coating layer is disposed at the notched structure. The composite coating layer includes a buffer layer and a conductive layer. The conductive layer is disposed outside the buffer layer, and the thickness of the composite coating layer gradually decreases from the edge of the negative electrode sheet towards the center.

[0013] This invention also provides a method for preparing a negative electrode sheet, comprising the following steps: coating a negative electrode slurry on both sides of a negative electrode current collector and performing step drying to obtain a negative electrode sheet semi-finished product; performing a single punching process on the negative electrode sheet semi-finished product to simultaneously form the main body size of the electrode sheet, the corner notch structure, and the side micro-arcs, wherein the corner notch structure includes C-corners and R-corners; sequentially coating a buffer layer and a conductive layer on the edge region of the punched electrode sheet, wherein the thickness of the buffer layer and the conductive layer gradually decreases from the edge of the electrode sheet towards the center.

[0014] Optionally, the one-time punching forming adopts a precision punching die, the blade roughness of the precision punching die Ra≤0.02μm, the punching gap is controlled at 3%-5% of the thickness of the negative electrode semi-finished product, and the punching speed is 30-60 times / minute.

[0015] Optionally, the buffer layer and the conductive layer are coated using slot extrusion coating or piezoelectric inkjet printing coating processes.

[0016] Optionally, after coating the buffer layer and the conductive layer, they are respectively subjected to vacuum drying.

[0017] Optionally, it further includes: hot rolling the coated electrode sheet, wherein the hot rolling pressure is 8MPa-15MPa, the rolling temperature is 45℃-60℃, and the rolling speed is 5m / min-15m / min.

[0018] Optionally, when the negative electrode is a graphite system, the compaction density is 1.5 g / cm³-1.7 g / cm³; when the negative electrode is a silicon-carbon system, the compaction density is 1.3 g / cm³-1.5 g / cm³.

[0019] The present invention also provides a secondary battery, comprising a positive electrode and a negative electrode as described in any of the above.

[0020] Beneficial effects: The negative electrode sheet provided by this invention has a negative electrode active material layer disposed on at least one side of the negative electrode current collector, and corner-cut structures disposed at the four corners of the negative electrode sheet. The corner-cut structures can alter the electric field distribution in the corner regions of the electrode sheet. The current density originally concentrated at the right-angle tip is dispersed to the longer boundary formed by the corner, thereby reducing local current peaks. For example, when the secondary battery is a lithium-ion secondary battery, it can prevent lithium ions from depositing at the negative electrode edge due to insufficient insertion rate. Simultaneously, the corner-cut structures reduce abrupt transitions in the corner material, alleviating mechanical stress concentration caused by volume changes during charging and discharging, and preventing warping and deformation of the electrode corners due to repeated expansion and contraction.

[0021] Furthermore, a composite coating layer is formed at the edge region of the negative electrode sheet. This composite coating layer includes an inner buffer layer and an outer conductive layer. The buffer layer directly covers the burrs generated at the electrode sheet's cut surface, physically isolating them to prevent them from piercing the separator when the electrode sheet expands. Simultaneously, the buffer layer possesses a certain degree of elastic deformation capability, absorbing volume changes at the electrode sheet edge. The conductive layer provides a conductive path in the edge region, homogenizing the potentially high edge current density, reducing interfacial impedance, and preventing lithium plating caused by local overpotential. The thickness of the composite coating layer gradually decreases from the edge of the negative electrode sheet towards the center, ensuring that the coating layer is thickest at the edge and gradually thins towards the center until it disappears. This ensures that the burrs are completely encapsulated while avoiding unnecessary coverage or thickening of the active material coating in the main electrode sheet area.

[0022] By combining the notched structure with the composite coating, the electrode maintains good interfacial contact and current distribution during charge and discharge cycles, while achieving puncture resistance, low impedance and suppression of swelling, without sacrificing the battery's energy density and rate performance. Attached Figure Description

[0023] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0024] Figure 1 This is a cross-sectional schematic diagram of the corner of the negative electrode sheet in an embodiment of the present invention; Figure 2 This is a cross-sectional schematic diagram of the composite coating layer according to an embodiment of the present invention; Figure 3 This is a schematic flowchart illustrating a method for preparing a negative electrode sheet according to an embodiment of the present invention.

[0025] Explanation of reference numerals in the attached figures: 1. Negative electrode sheet; 11. Negative electrode current collector; 12. Negative electrode active material layer; 2. Corner-missing structure; 21. C-corner; 22. R-corner; 3. Composite coating layer; 31. Buffer layer; 32. Conductive layer. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] With the rapid development of new energy vehicles and the energy storage industry, the energy density, cycle life, and safety performance of secondary batteries have received increasing attention. Stacked pouch batteries, due to their advantages such as flexible design, high energy density, low internal resistance, and good safety performance, have become one of the main technological routes for high-end power batteries and energy storage batteries.

[0028] However, in practical industrial applications, there are still some technical problems to be solved for stacked pouch cells.

[0029] First, the battery cell is the basic structure of a battery, mainly composed of a positive electrode, a separator, a negative electrode, and a separator arranged in sequence. Conventional stacked batteries often use a right-angle punching design for the positive and negative electrodes, which can easily lead to electric field concentration at the corners and sides of the electrodes, with the current density in the edge region being significantly higher than that in the center region.

[0030] Meanwhile, stress concentration at the edges of the electrodes during charging and discharging can cause electrode warping, leading to misalignment between the positive and negative electrodes. When the active area of ​​the positive electrode exceeds the coverage of the negative electrode, lithium ions cannot be effectively embedded in the negative electrode and instead precipitate on the surface of the negative electrode, forming irreversible lithium dendrites. This not only shortens the cycle life of the battery but may also cause thermal runaway in severe cases.

[0031] Secondly, the edges of the electrode sheets formed by right-angle punching are prone to burrs. During the charge and discharge cycle, the electrode sheets expand and contract in volume, and the edge burrs will continuously squeeze the diaphragm, increasing the risk of the diaphragm being punctured, which may lead to internal short circuits or even fire and explosion.

[0032] Furthermore, stress concentration and uneven expansion at the electrode edges can lead to poor interfacial contact between the electrode and the separator, exacerbating electrolyte side reactions and increasing gas production. Particularly for high-expansion systems such as silicon-carbon anodes, the expansion rate of the edge region can be more than twice that of the central region, easily causing overall cell bulging and resulting in rapid capacity decay.

[0033] Therefore, the present invention provides a negative electrode sheet, a method for preparing the negative electrode sheet, and a secondary battery comprising the negative electrode sheet. Specifically, the secondary battery is a rechargeable battery commonly used in electric devices, such as a lithium-ion battery or a sodium-ion battery.

[0034] Reference Figure 1 and Figure 2 As shown, the first aspect of the present invention provides a negative electrode sheet 1, including a negative electrode current collector 11 and a negative electrode active material layer 12 disposed on at least one side surface of the negative electrode current collector 11.

[0035] Among them, the four corners of the negative electrode sheet 1 are provided with corner-cut structures 2.

[0036] A composite coating layer 3 is provided on the edge region of the negative electrode sheet 1. The composite coating layer 3 includes a buffer layer 31 and a conductive layer 32. The buffer layer 31 is disposed on the inner side, and the conductive layer 32 is disposed on the outer side of the buffer layer 31. The thickness of the composite coating layer 3 gradually decreases from the edge of the negative electrode sheet 1 towards the center.

[0037] The negative electrode 1 provided by this invention has a negative electrode active material layer 12 disposed on at least one side surface of the negative electrode current collector 11, and corner-cut structures 2 disposed at the four corners of the negative electrode 1. The corner-cut structures 2 can change the electric field distribution in the corner region of the electrode. The current density originally concentrated at the right-angle tip is dispersed to the longer boundary formed by the corner, thereby reducing the local current peak and preventing lithium ions from being deposited at the edge of the negative electrode due to insufficient insertion rate. At the same time, the corner-cut structures 2 reduce the sudden turning of the corner material, alleviate the mechanical stress concentration generated by the volume change of the electrode during charging and discharging, and prevent the corners of the electrode from warping and deforming due to repeated expansion and contraction.

[0038] Furthermore, a composite coating layer 3 is provided at the edge region of the negative electrode 1. This composite coating layer 3 includes an inner buffer layer 31 and an outer conductive layer 32. The buffer layer 31 directly covers the burrs generated at the electrode cutting surface, preventing the burrs from piercing the separator when the electrode expands through physical isolation. At the same time, the buffer layer 31 has a certain elastic deformation capability, which can absorb the volume change at the edge of the electrode. The conductive layer 32 provides a conductive path in the edge region, homogenizes the edge current density which is prone to being too high, reduces the interface impedance, and avoids lithium plating caused by local overpotential. The thickness of the composite coating layer 3 gradually decreases from the edge of the negative electrode 1 towards the center, making the coating layer thickest at the edge and gradually thinning towards the center until it disappears. This ensures that the burrs are completely wrapped, while avoiding unnecessary coverage or thickening of the active material coating in the main body of the electrode.

[0039] By combining the notched corner structure 2 with the composite coating layer 3, the electrode maintains good interfacial contact and current distribution during charge and discharge cycles, while achieving the effects of puncture resistance, low impedance and suppression of swelling, without sacrificing the energy density and rate performance of the battery.

[0040] Specifically, the four corners of the negative electrode sheet 1 are provided with corner-cut structures 2. A corner-cut structure 2 refers to a non-right-angle boundary formed by cutting away a portion of material from a corner that was originally a right angle. Its shape can be a straight bevel, a rounded edge, or a combination of a straight line and a rounded edge. In other words, corner-cut structures 2 include beveled corners, rounded corners, or other types of corner cuts. Corner-cut structures 2 can be bevels cut from straight edges, rounded corners cut from rounded edges, or irregular corner cuts. When corner-cut structures 2 are irregular, their cutting trajectory can be an irregular arc, a combination of multiple straight lines, a combination of a straight line and (regular or irregular) arcs, etc. Therefore, any cutting of the corners of the negative electrode sheet 1 to obtain a smoother structure falls under the meaning of corner-cut structures 2 in this embodiment.

[0041] The missing corner structure 2 can change the electric field distribution in the corner region, dispersing the current density originally concentrated at the right-angle tip to a longer oblique boundary, thereby reducing the local current peak. At the same time, the missing corner structure 2 reduces the abrupt turning of the corner material, alleviating the mechanical stress concentration caused by the change in electrode volume during charging and discharging.

[0042] Furthermore, a composite coating layer 3 is provided on the edge region of the negative electrode sheet 1.

[0043] Reference Figure 2 As shown, the composite coating layer 3 includes a buffer layer 31 and a conductive layer 32. The buffer layer 31 is disposed on the inner side, that is, on the side close to the negative electrode active material layer 12, and the conductive layer 32 is disposed on the outer side of the buffer layer 31, that is, on the side away from the negative electrode active material layer 12.

[0044] Both the buffer layer 31 and the conductive layer 32 cover the cut surface (i.e., the edge formed by punching) of the negative electrode sheet 1 and the surface of the active material layer near the edge. The thickness of the composite coating layer 3 gradually decreases from the edge of the negative electrode sheet 1 towards the center. That is, the composite coating layer 3 is thickest at the cut surface at the edge of the electrode sheet; along the direction from the edge to the center of the electrode sheet, the thickness of the composite coating layer 3 gradually decreases until it reaches zero at a certain position.

[0045] This gradient thickness design ensures that burrs at the edges are fully concealed, while avoiding unnecessary coverage of the active material coating on the main body of the electrode. The buffer layer 31 and the conductive layer 32 work together to maintain good interfacial contact and current distribution of the electrode during charge-discharge cycles.

[0046] In one alternative implementation, the missing corner structure 2 specifically includes a C-corner 21 and an R-corner 22.

[0047] C-angle 21 refers to the straight hypotenuse set at the corner of the negative electrode plate 1, that is, the corner of the electrode plate that was originally a right angle is cut off by a straight line to form a slanted straight edge. R-angle 22 is set at the connection between the two ends of C-angle 21 and the side of the negative electrode plate 1, that is, the two endpoints of C-angle 21 and the two sides of the electrode plate are respectively connected by arcs.

[0048] This composite notched structure 2, combining C-angle 21 and R-angle 22, utilizes the oblique cutting feature of C-angle 21 to alter the electric field distribution in the corner region, dispersing the current density originally concentrated at the right-angle tip to a longer oblique side region, thereby reducing local current peaks and preventing lithium ions from depositing at the negative electrode edge due to insufficient insertion rate. Simultaneously, C-angle 21 itself reduces abrupt changes in the corner material, alleviating mechanical stress concentration caused by volume changes during charging and discharging. Meanwhile, R-angle 22 fills the transition region between C-angle 21 and the side, eliminating new stress concentration points that might arise at the ends of a traditional oblique cut, allowing stress to be smoothly transmitted along the arc, preventing warping and deformation of the electrode corner due to repeated expansion and contraction.

[0049] In actual processing, C-angle 21 and R-angle 22 can be formed simultaneously in a single punching process without additional steps. The hypotenuse of C-angle 21 can be a straight line or slightly curved, but a straight line is preferred for easier mold design and punching accuracy control. R-angle 22 is a standard circular arc, with its center located on the angle bisector of the area formed by the end and side of C-angle 21.

[0050] The positions of C-corner 21 and R-corner 22 can cover the four corners of the negative electrode plate 1, and each corner contains one C-corner 21 and two R-corners 22 (located at both ends of C-corner 21 respectively).

[0051] Optionally, the angle between the hypotenuse of C-angle 21 and the side of the electrode can be adjusted according to actual needs. The length and angle of the hypotenuse of C-angle 21, as well as the radius of the arc of R-angle 22, can be selected based on the electrode size, the active material system, and the corner structure of the matching positive electrode. For example, for larger power battery electrodes, relatively larger C-angle 21 and R-angle 22 can be set; for smaller consumer electronics battery electrodes, relatively smaller C-angle 21 and R-angle 22 can be set.

[0052] Specifically, the hypotenuse length of C-angle 21 can be 0.3mm-1.2mm, and the chamfer angle of C-angle 21 can be 30°-60°. The radius of the arc of R-angle 22 can be 0.2mm-1.0mm. Furthermore, the ratio of the radius of R-angle 22 to the hypotenuse length of C-angle 21 is 0.5-0.8. These parameter ranges were obtained through extensive experimental optimization.

[0053] When the bevel is less than 0.3 mm, the bevel is too short, and the effect on current density dispersion is not obvious. The improvement of corner electric field concentration is limited, and the risk of edge lithium plating is still high. When the bevel is greater than 1.2 mm, the bevel is too long, which will occupy too much of the active material area at the corner of the electrode, resulting in a reduction in the effective area of ​​the electrode and a decrease in energy density.

[0054] When the missing corner angle is less than 30°, the hypotenuse is too gentle and the angle between it and the side is small, so the effect on changing the electric field is limited. In addition, there is still a large stress concentration at the connection between the end of C-corner 21 and the side, which is prone to warping during the cycle. When the missing corner angle is greater than 60°, the hypotenuse is too steep and the amount of material removed from the corner is small. It is still close to a right-angle structure, so the improvement of stress concentration is not obvious and the problem of electric field concentration at the corner still exists.

[0055] When R is less than 0.2mm, the radius of the arc transition is too small, which cannot effectively eliminate the stress concentration at the end of C-corner 21 and is prone to new sharp points, which increases the risk of diaphragm puncture. When R is greater than 1.0mm, the arc is too large and will interfere with C-corner 21, causing the actual hypotenuse length of C-corner 21 to be shortened, affecting the design effect of C-corner 21. At the same time, the excessively large arc will occupy too much side area, which may affect the overall structural strength of the electrode.

[0056] Maintaining the ratio between 0.5 and 0.8 ensures that the dimensions of C-angle 21 and R-angle 22 are coordinated, forming a smooth composite chamfer profile. If the ratio is less than 0.5, R-angle 22 is too small relative to C-angle 21, resulting in an insufficiently smooth transition and noticeable geometric abrupt changes at the end of C-angle 21. If the ratio is greater than 0.8, R-angle 22 is too large relative to C-angle 21, causing the arc to excessively encroach on the C-angle 21 region, weakening the bevel characteristics of C-angle 21 and affecting the electric field dispersion effect.

[0057] For example, the length of the hypotenuse of C-angle 21 can be 0.6mm-1.0mm, the chamfer angle can be 40°-50°, the radius of R-angle 22 can be 0.4mm-0.7mm, and the ratio can be 0.6-0.75.

[0058] For example, for a graphite system negative electrode, the length of the hypotenuse of the C-corner can be 0.8 mm, the chamfer angle can be 45°, the radius of the R-corner can be 0.5 mm, and the ratio of the radius of the R-corner to the length of the hypotenuse of the C-corner can be 0.625.

[0059] For silicon-carbon anode systems, due to the high expansion rate of silicon-carbon materials, stress concentration at the corners is more significant. Therefore, the size of the C-corner (C-corner 21) can be appropriately increased to better disperse stress and electric field. For example, the length of the C-corner hypotenuse can be 1.0 mm, the chamfer angle 50°, and the radius of the R-corner 0.7 mm, with a ratio of R-corner radius to C-corner hypotenuse length of 0.7. For high-voltage systems (such as charging upper limit voltage 4.4V), since the positive electrode expansion is also significant, the length of the C-corner hypotenuse can be 0.6 mm, the chamfer angle 40°, and the radius of the R-corner 0.4 mm, with a ratio of R-corner radius to C-corner hypotenuse length of 0.667.

[0060] These parameters can be selected and adjusted based on the specific battery capacity, electrode size, and negative electrode material system.

[0061] In practical applications, different values ​​within the above parameter range can all improve the corner electric field distribution and alleviate stress concentration. However, through orthogonal experimental verification, the above exemplary parameters can minimize the risk of edge lithium plating and improve cycle stability while ensuring energy density.

[0062] Optionally, the four straight sides of the negative electrode 1 and the hypotenuse of corner C 21 are respectively provided with rounded corners. The radius of the rounded corner is 0.05mm-0.2mm.

[0063] During the punching process, tiny burrs and stress concentration points inevitably form on the sides and beveled edges of the electrode sheet. Adding small rounded corners can eliminate these burrs and simultaneously make the stress distribution on the sides more uniform.

[0064] When the fillet radius is too small (less than 0.05 mm), burrs and stress concentration cannot be effectively eliminated; when the fillet radius is too large (greater than 0.2 mm), the effective width of the electrode will be reduced, affecting the energy density. Preferably, the fillet radius can be 0.08 mm to 0.15 mm, for example, 0.1 mm. These micro-rounded corners can be formed together with the notched corner structure 2 in a single punching process, without the need for additional steps.

[0065] In one alternative embodiment, the buffer layer 31 is a composite layer of ceramic and adhesive. The ceramic material provides insulation and mechanical strength, while the adhesive provides flexibility and adhesion. The ceramic material may be selected from one or more of boehmite, alumina, and zirconium oxide. The adhesive may be selected from one or more of polyvinylidene fluoride, styrene-butadiene rubber, and polyimide. The main function of the buffer layer 31 is to isolate burrs at the edge of the electrode sheet, prevent diaphragm puncture, and buffer the volume expansion at the edge of the electrode sheet.

[0066] For example, conductive layer 32 is a composite layer of a conductive agent and an elastic binder. The conductive agent provides a conductive path and reduces the edge interface impedance; the elastic binder provides flexibility, allowing conductive layer 32 to adapt to volume changes during electrode charging and discharging, preventing the coating layer from peeling off. The conductive agent can be selected from one or more of carbon nanotubes, graphene, and superconducting carbon black. The elastic binder can be selected from one or more of styrene-ethylene-butene-styrene block copolymers, hydrogenated nitrile rubber, and polyethylene oxide.

[0067] For example, conductive layer 32 can also be a binder-free conductive coating. Specifically, the conductive coating can be formed through physical / chemical film-forming processes.

[0068] The coverage width of the conductive layer 32 is smaller than that of the buffer layer 31. That is, in the direction from the edge of the electrode towards the center, the buffer layer 31 covers a wider area, while the conductive layer 32 covers a narrower area. Specifically, the total coverage width of the buffer layer 31 can be 0.5mm-2.0mm, and the total coverage width of the conductive layer 32 can be 0.2mm-0.5mm smaller than that of the buffer layer 31. This arrangement ensures that the buffer layer 31 fully covers the edge burr area, while the conductive layer 32 only covers the edge area most requiring current homogenization, avoiding over-coverage that could lead to increased impedance or obstruction of the active material.

[0069] Specifically, the thicknesses of the buffer layer 31 and the conductive layer 32 decrease linearly from the edge of the negative electrode 1 towards the center.

[0070] Linear reduction means that the amount of thickness reduction per unit distance (e.g., per 100 μm) is constant. This linear reduction can be precisely controlled through the coating process. For example, the thickness of the buffer layer 31 at the electrode edge is 2 μm-6 μm, and the thickness decreases by 0.5 μm-1 μm every 100 μm towards the center of the electrode until the thickness is 0. The thickness of the conductive layer 32 at the electrode edge is 1 μm-3 μm, and the thickness decreases by 0.3 μm-0.8 μm every 100 μm until the thickness is 0.

[0071] The cross-sectional profiles of the buffer layer 31 and the conductive layer 32 at the edge of the electrode jointly define a wedge-shaped cross-section with the opening facing the edge. This wedge-shaped cross-section refers to the fact that, on a cross-section perpendicular to the edge of the electrode, the thickness of the buffer layer 31 and the conductive layer 32 gradually decreases from the edge to the center, forming a wedge-like (triangular) shape, with the tip of the wedge pointing towards the center of the electrode and the wide side of the wedge located at the edge of the electrode.

[0072] This wedge-shaped cross-section design makes the coating layer thickest at the edge, which can fully wrap the burrs and provide sufficient buffering and conductivity; the closer to the center of the electrode, the thinner the coating layer, the less impact it has on the active material layer, and eventually it disappears naturally at a certain position without forming an abrupt step.

[0073] For example, the buffer layer 31 can be a composite slurry of boehmite and PVDF, which is applied to the edge of the electrode by slit extrusion. The coating width is 1.2 mm, the thickness at the edge is 4 μm, and it decreases by 0.5 μm every 100 μm.

[0074] The conductive layer 32 can be made of a composite slurry of carbon nanotubes and SEBS, with a coating width of 1.0 mm and a thickness of 2 μm at the edge, decreasing by 0.4 μm every 100 μm. After drying, the buffer layer 31 and the conductive layer 32 form a wedge-shaped cross-section with the thickness gradually decreasing from the edge to the center.

[0075] In some implementations, the negative electrode 1 and the matching positive electrode 1 satisfy the following dimensional relationship.

[0076] First, the difference between the main body length of the negative electrode 1 and the main body length of the matching positive electrode 1 is 0.4mm-1.0mm. The difference between the main body width of the negative electrode 1 and the main body width of the matching positive electrode 1 is 0.3mm-0.8mm.

[0077] The body length refers to the dimension of the electrode along its length, corresponding to the straight line segment between the two chamfered corners, excluding the chamfered corner area. The body width refers to the dimension of the electrode along its width, corresponding to the straight line segment between the two chamfered corners, excluding the chamfered corner area.

[0078] These differences ensure that, even with certain alignment tolerances, the negative electrode 1 can completely cover the main area of ​​the positive electrode during the stacking process, preventing the positive active area from exceeding the negative electrode range and causing lithium deposition. Preferably, the length difference can be 0.6mm-0.8mm, and the width difference can be 0.4mm-0.6mm.

[0079] Second, the difference between the projected length of the hypotenuse of the negative electrode 1 at angle C21 and the projected length of the chamfered hypotenuse of the matching positive electrode is 0.2mm-0.5mm.

[0080] The projection length of the hypotenuse of C-angle 21 refers to the projection length of the hypotenuse of C-angle 21 onto the electrode plane. The corner chamfer of the positive electrode can be either C-angle 21 or R-angle 22. When C-angle 21 is used, the projection length of the hypotenuse is directly compared; when R-angle 22 is used, it can be approximated as an arc, with its projection length being the chord length. Ensuring that the projection length of the hypotenuse of the negative electrode's C-angle 21 is greater than the projection length of the positive electrode's corner chamfer allows the negative electrode corner to completely cover the positive electrode corner, filling the corner matching blind spot. Preferably, this difference can be 0.25mm-0.4mm.

[0081] Third, the ratio of the thickness of the single-sided active material layer of the negative electrode 1 to the thickness of the single-sided active material layer of the matching positive electrode is 1.05:1 to 1.15:1.

[0082] Because the negative electrode active material layer 12 expands in volume after lithium ions are inserted into the negative electrode, while the volume change of the positive electrode is relatively small, setting the thickness of the negative electrode to be slightly larger than that of the positive electrode can accommodate the volume expansion after lithium ions are inserted into the negative electrode and prevent the edge of the positive electrode in the thickness direction from exceeding the range of the negative electrode. Preferably, this ratio can be from 1.06:1 to 1.12:1.

[0083] For example, for a 50Ah stacked pouch cell, the negative electrode 1 can have a main body length of 120mm and a main body width of 80mm, while the positive electrode 1 can have a main body length of 119.2mm and a main body width of 79.4mm. The hypotenuse length of the negative electrode C-angle 21 can be 0.8mm, with a projected length of approximately 0.8mm × cos45° = 0.566mm; the hypotenuse length of the positive electrode C-angle 21 can be 0.5mm, with a projected length of approximately 0.353mm, a difference of approximately 0.213mm. The single-sided active material layer thickness of the negative electrode can be 75μm, and the single-sided active material layer thickness of the positive electrode can be 70μm, with a ratio of 1.07:1.

[0084] In practical applications, those skilled in the art can select one of the above four size matching relationships based on the specific cell model design requirements, the fluctuation tolerance in the electrode preparation process, and the actual size parameters of the matching positive electrode. Alternatively, they can adaptively combine and configure any of the above relationships according to the matching requirements of the electrode edge alignment and the capacity of the active material to meet the requirements of negative electrode coating positive electrode under different performance and safety test standards.

[0085] In some embodiments of the present invention, the corner area of ​​the negative electrode sheet 1 is provided with a notched corner structure 2, which forms a non-right-angle boundary. The non-right-angle boundary means that the corner edge of the negative electrode sheet 1 is not a traditional 90° right angle, but an edge contour formed by punching or cutting, such as a bevel, a rounded edge, or a combination of a bevel and a rounded edge.

[0086] Specifically, right-angled edges tend to concentrate the electric field during charging and discharging, with edge current density typically higher than the central region. Lithium ions then deposit at the negative electrode edge due to insufficient insertion rate. Non-right-angled boundaries disperse the current concentrated at the right-angled tip across a longer boundary, reducing the peak value of the local current. Furthermore, right-angled punched corners experience mechanical stress concentration during volume changes during charging and discharging, leading to electrode warping. Non-right-angled boundaries reduce abrupt material transitions at the corners, allowing stress to be transmitted along smoother boundaries and preventing deformation of the electrode corners due to repeated expansion and contraction.

[0087] Furthermore, a composite coating layer 3 is provided at the corner structure 2. The composite coating layer 3 includes a buffer layer 31 and a conductive layer 32. The conductive layer 32 is disposed outside the buffer layer 31, and the thickness of the composite coating layer 3 gradually decreases from the edge of the negative electrode sheet 1 towards the center.

[0088] The buffer layer 31 covers the burrs generated on the cut surface of the electrode sheet, preventing the burrs from piercing the separator. The conductive layer 32 provides a conductive path in the edge area, homogenizes the edge current density, and reduces the interface impedance. The notched corner structure 2 and the composite coating layer 3 work together to ensure that the electrode sheet maintains good interface contact and current distribution during charge and discharge cycles, achieving the effects of puncture prevention, low impedance, and suppression of bulging.

[0089] For details regarding the specific shape and size parameters of the missing corner structure 2, as well as the material composition and coating method of the composite coating layer 3, please refer to the relevant descriptions above; they will not be repeated here.

[0090] Reference Figure 3 As shown, a second aspect of the present invention also provides a method for preparing a negative electrode sheet 1, comprising the following steps: Step S1: Coat both sides of the negative electrode current collector 11 with negative electrode slurry and perform step drying to obtain the negative electrode sheet 1 semi-finished product. Step S2: The negative electrode sheet 1 semi-finished product is punched and formed in one step to simultaneously form the main body size of the electrode sheet, the corner missing structure 2 and the side micro-circular arc, wherein the corner missing structure 2 includes C corner 21 and R corner 22. Step S3: A buffer layer 31 and a conductive layer 32 are sequentially coated on the edge area of ​​the punched electrode, and the thickness of the buffer layer 31 and the conductive layer 32 gradually decreases from the edge of the electrode towards the center.

[0091] The above preparation method uses a one-time punching process to simultaneously form the main body size of the electrode sheet, the corner missing structure 2, and the side micro-arc, avoiding burrs and positioning errors that may occur during secondary processing.

[0092] By precision punching after coating and drying, and then sequentially coating a buffer layer 31 and a conductive layer 32 with varying thicknesses, a complete protective structure can be formed at the edge of the electrode. The steps of this method are interconnected, enabling the stable fabrication of a negative electrode 1 with corner defects and an edge composite coating layer 3, thereby improving the current distribution at the electrode edge, reducing the risk of diaphragm puncture, and suppressing electrode bulging during cycling.

[0093] Meanwhile, this method is compatible with existing stacked battery production lines and can be implemented simply by changing the punching dies and adding a coating station, without requiring large-scale modifications to the production line.

[0094] In one optional embodiment, step S1 includes: first, preparing a negative electrode slurry by adding the negative electrode active material, conductive agent, binder, and dispersant to deionized water or an organic solvent in a certain proportion, and dispersing them by high-speed stirring to obtain the negative electrode slurry. Then, the negative electrode slurry is uniformly coated on both sides of the negative electrode current collector 11.

[0095] The negative electrode current collector 11 can be made of electrolytic copper foil with a thickness of 6 μm to 12 μm, such as 8 μm or 10 μm. The coating method can be transfer coating or extrusion coating, and the coating speed is determined according to the drying capacity and slurry properties, typically from 5 m / min to 20 m / min.

[0096] After coating, a step drying process is performed.

[0097] Step drying refers to dividing the drying area into multiple temperature segments, such as using a temperature rise and fall step of 80℃→100℃→120℃→100℃, with the residence time of each temperature segment being 1min-3min.

[0098] This drying method avoids coating cracking, binder floating, or uneven distribution of active material caused by excessively fast drying rates. After drying, a semi-finished negative electrode sheet 1 is obtained, in which the negative electrode active material layer 12 has been solidified on the surface of the current collector, but has not yet undergone punching and edge treatment.

[0099] Optionally, the solid content of the negative electrode slurry can be adjusted according to the coating process. For extrusion coating, the solid content can be appropriately increased to 48% to 50%; for transfer coating, the solid content can be controlled at 42% to 46%.

[0100] The maximum drying temperature for stepped drying should be determined based on the temperature resistance of the binder. For example, the maximum drying temperature for styrene-butadiene rubber binders should not exceed 120°C, while polyacrylic acid binders can withstand higher temperatures. The dried negative electrode sheet (semi-finished product 1) should undergo thickness testing to ensure that the single-sided dry film thickness is within the target range, such as 70μm to 85μm.

[0101] Specifically, a precision punching die is used for one-time forming. The cutting edge roughness Ra of the precision punching die is ≤0.02μm, meaning the cutting edge is mirror-polished. The punching gap is controlled at 3%-5% of the thickness of the semi-finished negative electrode sheet 1, and the punching speed is 30-60 times per minute. For example, the punching speed can be set to 40 times / minute.

[0102] In one specific embodiment, the buffer layer 31 and the conductive layer 32 are coated using slot extrusion coating or piezoelectric inkjet printing coating processes.

[0103] Slit extrusion coating involves uniformly extruding slurry onto the surface of a substrate through a slit die. The coating thickness and width are controlled by adjusting the gap between the die and the substrate, as well as the slurry supply pressure.

[0104] Piezoelectric inkjet printing coating utilizes piezoelectric elements to spray paste in the form of microdroplets onto the edge of the electrode. By controlling the spraying frequency and droplet size, a gradient coating thickness can be achieved. Both processes can achieve a gradual decrease in the thickness of the buffer layer 31 and the conductive layer 32 from the edge to the center.

[0105] For example, a piezoelectric inkjet printing coating process can be used to coat the inner insulating buffer layer 31 on the side and corner chamfered areas of the electrode sheet.

[0106] In some embodiments, after coating the buffer layer 31 and the conductive layer 32, they are respectively subjected to vacuum drying.

[0107] Vacuum drying refers to drying under reduced pressure. The drying temperature of the buffer layer 31 can be 100℃-120℃, and the drying temperature of the conductive layer 32 can be 90℃-110℃. Separate drying means first coating the buffer layer 31 and drying it, then coating the conductive layer 32 and drying it, to avoid the two layers of slurry from mixing.

[0108] For example, after the buffer layer 31 is coated, it is vacuum dried at 110°C for 3 min, and after the conductive layer 32 is coated, it is vacuum dried at 100°C for 2 min.

[0109] In an alternative embodiment, the method further includes hot rolling the coated electrode.

[0110] The pressure of hot rolling can be 8MPa-15MPa, the rolling temperature can be 45℃-60℃, and the rolling speed can be 5m / min-15m / min. Hot rolling can make the active material layer more compact, increase the compaction density, and at the same time, allow the buffer layer 31 and the conductive layer 32 to better bond with the active material layer.

[0111] For example, for a graphite-based negative electrode, a rolling temperature of 50°C, a pressure of 12MPa, and a speed of 8m / min can be used.

[0112] Specifically, the compaction density is controlled within different ranges depending on the active material system of the negative electrode 1. When the negative electrode 1 is a graphite system, the compaction density is 1.5 g / cm³ to 1.7 g / cm³. When the negative electrode 1 is a silicon-carbon system, the compaction density is 1.3 g / cm³ to 1.5 g / cm³. For example, the compaction density of the graphite system negative electrode 1 can be controlled at 1.65 g / cm³, and the compaction density of the silicon-carbon system negative electrode 1 can be controlled at 1.45 g / cm³.

[0113] A third aspect of the present invention also provides a secondary battery.

[0114] The secondary battery includes a positive electrode, a negative electrode 1, a separator, an electrolyte, and an aluminum-plastic film encapsulation shell. The negative electrode 1 is the negative electrode 1 described in any of the above embodiments.

[0115] The positive electrode includes a positive current collector and a layer of positive active material disposed on at least one side of the positive current collector. The positive current collector is aluminum foil, and the positive active material is selected from one or more of nickel-cobalt-manganese ternary materials, lithium iron phosphate, and lithium manganese oxide. The separator is a polyolefin separator, and the surface of the separator may be coated with a ceramic layer. The electrolyte is a mixture of lithium salt electrolyte and carbonate solvent, containing film-forming additives.

[0116] This secondary battery can be assembled using a stacking process, alternately stacking the negative electrode 1, separator, positive electrode 1, and separator to form a stacked cell. After stacking, processes such as hot pressing, tab welding, encapsulation, electrolyte injection, formation, and venting are performed to produce the finished battery.

[0117] The following three examples are used for comparative testing.

[0118] Example 1 This embodiment provides a 50Ah stacked soft-pack battery for power applications based on a graphite system.

[0119] The preparation process of negative electrode 1 is as follows.

[0120] Add 96.5 parts of artificial graphite, 1 part of superconducting carbon black, 1.5 parts of styrene-butadiene rubber binder, and 1 part of sodium carboxymethyl cellulose dispersant to deionized water, and disperse at high speed for 2 hours to obtain a negative electrode slurry with a solid content of 48% and a viscosity of 3500 mPa·s-4500 mPa·s.

[0121] The negative electrode slurry was uniformly coated on both sides of an 8μm thick electrolytic copper foil. A stepped drying process at 80℃, 100℃, 120℃, and 100℃ was used to remove the solvent, resulting in a semi-finished negative electrode sheet 1 with a single-sided dry film thickness of 75μm. A mirror-polished precision die was used to punch the semi-finished product, yielding a negative electrode sheet 1 with a main body size of 120mm in length and 80mm in width. Simultaneously, the C-angle 21 and R-angle 22 at the corners and the micro-rounded arc on the side were formed in one step. Specifically, the C-angle 21 has a bevel length of 0.8mm and a chamfer angle of 45°; the R-angle 22 has a radius of 0.5mm, and the ratio of the radius of R-angle 22 to the bevel length of C-angle 21 is 0.625; the radius of the micro-rounded arc on the side is 0.1mm. The punching gap was 4μm, and the punching speed was 40 times / minute.

[0122] A piezoelectric inkjet printing coating process is used to sequentially coat a buffer layer 31 and a conductive layer 32 on the side and corner chamfered areas of the electrode sheet.

[0123] The buffer layer 31 is composed of 80 parts boehmite and 20 parts polyvinylidene fluoride, with NMP as the solvent. The total width of the coating is 1.2 mm, the thickness at the edge is 4 μm, and the thickness decreases by 0.5 μm for every 100 μm extending towards the center. After coating, it is vacuum dried at 110 °C for 3 minutes.

[0124] The conductive layer 32 is composed of 30 parts of carbon nanotubes and 70 parts of SEBS. The total coating width is 1.0 mm, the thickness at the edge is 2 μm, and the thickness decreases by 0.4 μm for every 100 μm extending towards the center. After coating, it is vacuum dried at 100 °C for 2 minutes.

[0125] After drying, the material is hot-rolled at 50°C with a pressure of 12MPa and a rolling speed of 8m / min. The compaction density is controlled at 1.65g / cm³. After slitting and ultrasonic deburring, the finished negative electrode sheet 1 is obtained.

[0126] The preparation of the matching positive electrode is as follows.

[0127] 97 parts of NCM811 ternary material, 1 part of superconducting carbon black, and 2 parts of polyvinylidene fluoride were added to NMP and stirred to disperse, resulting in a positive electrode slurry with a solid content of 72%. This slurry was coated onto both sides of a 12 μm thick aluminum foil, with a single-sided dry film thickness of 70 μm and an areal density of 18 mg / cm². After drying and punching, a positive electrode sheet was obtained. The main dimensions of the positive electrode sheet were 119.2 mm in length, 79.4 mm in width, with a C-angle 21 bevel length of 0.5 mm, and a single-sided active material layer thickness of 70 μm.

[0128] The battery assembly and fabrication are as follows.

[0129] A 9μm thick double-sided ceramic-coated polyethylene diaphragm was used. The electrolyte was 1mol / L LiPF6, and the solvent was a mixture of EC, EMC, and DMC in a 1:1:1 volume ratio. Additives included 2% VC and 5% FEC. A Z-type stacking process was employed, with 32 stacked layers and a tab misalignment of no more than 0.1mm. A 113μm thick aluminum-plastic film was used for top and side sealing, and the membrane was allowed to stand for 24 hours after electrolyte injection.

[0130] The gradient formation process is as follows: constant current charging at 0.05C to 3.0V, resting for 30 minutes; charging at 0.1C to 3.5V, resting for 30 minutes; charging at 0.2C to 4.2V, resting for 30 minutes; 0.5C charge-discharge cycle for 2 weeks, followed by vacuum degassing and secondary packaging to obtain a stacked soft-pack battery with a rated capacity of 50.2Ah.

[0131] Example 2 This embodiment provides a 52Ah high-energy-density stacked soft-pack battery based on a silicon-carbon system.

[0132] The preparation process of negative electrode 1 is as follows.

[0133] 90 parts of silicon-carbon composite material (silicon content 10%), 5 parts of artificial graphite, 1.5 parts of carbon nanotubes, 2 parts of styrene-butadiene rubber binder, and 1.5 parts of sodium carboxymethyl cellulose dispersant were added to deionized water and stirred to disperse, thus obtaining a negative electrode slurry with a solid content of 45%.

[0134] The negative electrode slurry was coated on both sides of an electrolytic copper foil with a thickness of 10 μm, and the same step drying process as in Example 1 was used to obtain a semi-finished product with a single-sided dry film thickness of 80 μm.

[0135] The negative electrode sheet 1 with a main body size of 120mm in length and 80mm in width is obtained by punching, and the C-angle 21 and R-angle 22 at the corners and the micro-rounded arc on the side are formed simultaneously. Among them, the C-angle 21 has a hypotenuse length of 1.0mm and a chamfer angle of 45°, the R-angle 22 has a radius of 0.7mm and a ratio of 0.7, and the radius of the micro-rounded arc on the side is 0.15mm.

[0136] The buffer layer 31 is composed of 75 parts Al2O3 and 25 parts polyimide, with a coating width of 1.5 mm and an edge thickness of 5 μm, decreasing by 0.5 μm for every 100 μm. It is vacuum dried at 120 °C for 3 minutes.

[0137] The conductive layer 32 is composed of 25 parts graphene and 75 parts hydrogenated nitrile rubber, with a coating width of 1.2 mm and an edge thickness of 2.5 μm, decreasing by 0.4 μm for every 100 μm. It is vacuum dried at 110 °C for 2 minutes.

[0138] Then, the material is hot-rolled at 55℃ with a pressure of 10MPa and a compaction density controlled at 1.45g / cm³. After slitting and deburring, the finished negative electrode sheet 1 is obtained.

[0139] The matching positive electrode sheet uses NCM811 material with a single-sided dry film thickness of 72μm. The main body dimensions are 119.3mm in length and 79.3mm in width, with a corner C21 hypotenuse length of 0.6mm. 10% FEC is added to the electrolyte, and the remaining stacking, packaging, and formation processes are the same as in Example 1, resulting in a stacked soft-pack battery with a rated capacity of 52.1Ah.

[0140] Example 3 This embodiment provides a 48Ah stacked soft-pack battery with a high voltage of 4.4V.

[0141] The preparation process of negative electrode 1 is as follows.

[0142] A negative electrode slurry with a solid content of 47% was prepared by using 96 parts of artificial graphite, 1.2 parts of carbon nanotubes, 1.8 parts of styrene-butadiene rubber binder, and 1.0 parts of sodium carboxymethyl cellulose dispersant.

[0143] The coating is applied to both sides of an 8μm copper foil, and after step drying, a semi-finished product with a single-sided dry film thickness of 72μm is obtained.

[0144] The negative electrode sheet 1 with a main body size of 120mm in length and 80mm in width is obtained by punching, and the C-angle 21 and R-angle 22 at the corners and the micro-rounded arc on the side are formed simultaneously. Among them, the C-angle 21 has a hypotenuse length of 0.6mm and a chamfer angle of 40°, the R-angle 22 has a radius of 0.4mm and a ratio of 0.667, and the radius of the micro-rounded arc on the side is 0.08mm.

[0145] The buffer layer 31 consists of 82 parts ZrO2 and 18 parts polyvinylidene fluoride, with a coating width of 1.0 mm and an edge thickness of 3 μm, decreasing by 0.5 μm every 100 μm. It is vacuum dried at 110 °C for 3 minutes. The conductive layer 32 consists of 35 parts superconducting carbon black and 65 parts SEBS, with a coating width of 0.8 mm and an edge thickness of 1.5 μm, decreasing by 0.3 μm every 100 μm. It is vacuum dried at 100 °C for 2 minutes.

[0146] Then, the material is hot-rolled at 50°C with a pressure of 13 MPa and a compaction density controlled at 1.68 g / cm³ to obtain the finished negative electrode sheet 1.

[0147] The matching positive electrode uses 4.4V high-voltage NCM811 material, with a single-sided dry film thickness of 68μm. The main body dimensions are 119.4mm in length, 79.5mm in width, and the hypotenuse length of the C21 corner is 0.35mm. 1% LiPO2F2 and 3% FEC are added to the electrolyte, with a charging upper limit voltage of 4.4V. The remaining processes are the same as in Example 1, resulting in a stacked soft-pack battery with a rated capacity of 48.3Ah.

[0148] Comparative Example 1 This comparative example provides a conventional right-angle negative electrode 1 battery.

[0149] The same materials, formulation and battery manufacturing process as in Example 1 are used, with the only difference being that: the negative electrode sheet 1 is conventionally right-angle punched without C-angle 21 and R-angle 22, and without edge gradient composite coating layer 3. The positive and negative electrode size matching is conventional, that is, the length and width of the negative electrode are both 1.0 mm larger than the positive electrode, and there is no chamfered area matching with the thickness direction.

[0150] Comparative Example 2 This comparative example provides a single R-angle 22 chamfered negative electrode 1 battery.

[0151] The same materials, formulation and battery manufacturing process as in Example 1 are used, the only difference being that: the negative electrode sheet 1 is only provided with a single R-angle 22 chamfer, the radius of R-angle 22 is 0.5mm, there is no C-angle 21 design, there is no edge gradient composite coating layer 3, only the main body size is matched, and there is no matching of the chamfer area and the thickness direction.

[0152] Performance testing The batteries of Example 1, Example 2, Example 3, Comparative Example 1, and Comparative Example 2 prepared above were subjected to the following performance tests.

[0153] First charge and discharge performance test: At 25℃, charge at 0.2C constant current and constant voltage to their respective upper limit voltage, and then discharge at 0.2C constant current to 2.8V, and record the first charge and discharge efficiency.

[0154] Cyclic performance test: At 25℃, charge at 1C constant current and constant voltage to the upper limit voltage, and then discharge at 1C constant current to 2.8V to perform cycle life test. Record the capacity retention rate and thickness expansion rate after the corresponding number of cycles.

[0155] Low temperature performance test: After the battery is fully charged at -20℃, it is discharged at a constant current of 0.5C to 2.8V, and the discharge capacity retention rate is recorded.

[0156] Safety performance testing: Conduct needle penetration test, compression test and hot box test respectively, and record the test results.

[0157] Disassembly and analysis: After the cycle test, the battery was disassembled in the drying room to observe the lithium plating on the edge of the negative electrode 1 and whether the separator was punctured.

[0158] The test results are shown in Table 1.

[0159] Table 1 Performance test results of Examples 1-3 and Comparative Examples 1-2

[0160] As can be seen from Table 1, the batteries of Examples 1, 2 and 3 are significantly better than those of Comparative Examples 1 and 2 in terms of cycle life, capacity retention, thickness expansion control and low temperature performance. Moreover, no edge lithium plating or separator puncture marks were found after disassembly, and they did not catch fire or explode during needle penetration and extrusion tests.

[0161] The conventional right-angle electrode battery in Comparative Example 1 had poor cycle performance, severe edge lithium plating and separator puncture, and failed the safety test.

[0162] The performance of the single R-angle 22 electrode battery in Comparative Example 2 is improved compared to Comparative Example 1, but it still has problems such as slight lithium plating at the corner and micro-puncture of the separator. Its cycle performance and expansion control are not as good as the three embodiments of the present invention.

[0163] The above results show that the corner-cut structure 2 and the composite coating layer 3 of the present invention work together to effectively suppress edge lithium plating, prevent membrane puncture, reduce cyclic swelling, and maintain good low-temperature discharge capability and safety performance.

[0164] Obviously, the above embodiments are merely examples for clear illustration and are not intended to limit the embodiments. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all embodiments here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector (11) and a negative electrode active material layer (12) disposed on at least one side surface of the negative electrode current collector (11), wherein, The negative electrode sheet (1) has a corner-cut structure (2) at its four corners. The negative electrode sheet (1) has a composite coating layer (3) on its edge region. The composite coating layer (3) includes a buffer layer (31) and a conductive layer (32). The buffer layer (31) is disposed on the inner side, and the conductive layer (32) is disposed on the outer side of the buffer layer (31). The thickness of the composite coating layer (3) gradually decreases from the edge of the negative electrode sheet (1) towards the center.

2. The negative electrode sheet according to claim 1, characterized in that, The notched structure (2) includes a C-corner (21) and an R-corner (22). The C-corner (21) is a straight hypotenuse set at the corner of the negative electrode plate (1). The R-corner (22) is set at the connection between the two ends of the C-corner (21) and the side of the negative electrode plate (1).

3. The negative electrode sheet according to claim 2, characterized in that, The hypotenuse length of the C-angle (21) is 0.3mm-1.2mm, and the chamfer angle of the C-angle (21) is 30°-60°; the radius of the arc of the R-angle (22) is 0.2mm-1.0mm; and the ratio of the radius of the R-angle (22) to the hypotenuse length of the C-angle (21) is 0.5-0.

8.

4. The negative electrode sheet according to claim 2, characterized in that, The four straight sides of the negative electrode (1) and the hypotenuse of the C-angle (21) are respectively provided with rounded corners, and the radius of the rounded corners is 0.05mm-0.2mm.

5. The negative electrode sheet according to claim 1, characterized in that, The buffer layer (31) is a composite layer of ceramic and adhesive, the conductive layer (32) is a composite layer of conductive agent and elastic adhesive, and the coverage width of the conductive layer (32) is smaller than the coverage width of the buffer layer (31).

6. The negative electrode sheet according to claim 5, characterized in that, The thicknesses of the buffer layer (31) and the conductive layer (32) decrease linearly from the edge of the negative electrode (1) toward the center, and the cross-sectional profiles of the two at the edge of the electrode together define a wedge-shaped cross-section with the opening facing the edge.

7. The negative electrode sheet according to claim 2, characterized in that, The negative electrode (1) and the matching positive electrode satisfy at least one of the following dimensional relationships: The difference between the main body length of the negative electrode (1) and the main body length of the matching positive electrode is 0.4mm-1.0mm; The difference between the main body width of the negative electrode sheet (1) and the main body width of the matching positive electrode sheet is 0.3mm-0.8mm; The difference between the projected length of the hypotenuse of the C-angle (21) of the negative electrode (1) and the projected length of the hypotenuse of the chamfered corner of the matching positive electrode is 0.2mm-0.5mm; The ratio of the thickness of the single-sided active material layer of the negative electrode (1) to the thickness of the single-sided active material layer of the matching positive electrode is 1.05:1 to 1.15:

1.

8. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, characterized in that, The negative electrode plate (1) has a corner-cut structure (2) in the corner area, and the corner-cut structure (2) forms a non-right-angle boundary; A composite coating layer (3) is provided at the corner-cut structure (2). The composite coating layer (3) includes a buffer layer (31) and a conductive layer (32). The conductive layer (32) is disposed outside the buffer layer (31), and the thickness of the composite coating layer (3) gradually decreases from the edge of the negative electrode sheet (1) towards the center.

9. A method for preparing a negative electrode sheet, characterized in that, Includes the following steps: A negative electrode slurry is coated on both sides of the negative electrode current collector (11) and dried in a stepwise manner to obtain a negative electrode sheet (1) semi-finished product. The negative electrode sheet (1) semi-finished product is punched and formed in one step to simultaneously form the main body size of the electrode sheet, the corner missing structure (2) and the side micro-arc, wherein the corner missing structure (2) includes C corner (21) and R corner (22). A buffer layer (31) and a conductive layer (32) are sequentially coated on the edge area of ​​the punched electrode, and the thickness of the buffer layer (31) and the conductive layer (32) gradually decreases from the edge of the electrode towards the center.

10. The method according to claim 9, characterized in that, The one-time punching forming adopts a precision punching die, the blade roughness Ra≤0.02μm of the precision punching die, the punching gap is controlled at 3%-5% of the thickness of the negative electrode sheet (1) semi-finished product, and the punching speed is 30-60 times / minute.

11. The method according to claim 9, characterized in that, The buffer layer (31) and the conductive layer (32) are coated using slot extrusion coating or piezoelectric inkjet printing coating processes.

12. The method according to claim 9, characterized in that, After coating the buffer layer (31) and the conductive layer (32), they are vacuum dried respectively.

13. The method according to claim 12, characterized in that, Also includes: The coated electrode sheet is subjected to hot rolling, with a pressure of 8MPa-15MPa, a rolling temperature of 45℃-60℃, and a rolling speed of 5m / min-15m / min.

14. The method according to claim 13, characterized in that, When the negative electrode sheet (1) is a graphite system, the compaction density is 1.5 g / cm³-1.7 g / cm³; When the negative electrode sheet (1) is a silicon-carbon system, the compaction density is 1.3 g / cm³-1.5 g / cm³.

15. A secondary battery, characterized in that, It includes a positive electrode sheet and a negative electrode sheet (1) according to any one of claims 1-8.

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