Negative plate, preparation method of negative plate, laminated pole core and vehicle

Abstract

The invention relates to a negative plate, a preparation method of the negative plate, a laminated pole core and a vehicle, the negative plate comprises a current collector layer and an active material layer which are sequentially laminated along a first direction, two side edges of the current collector layer which are oppositely arranged at intervals along a second direction are defined as first side edges, and the second direction is crossed with the first direction; one of the two first side edges is provided with a tab, the active material layer comprises two edge layers arranged at intervals along the length direction of the first side edges and a middle layer located between the two edge layers, and the two edge layers are connected with the middle layer through transition layers; the porosity of the edge layer is greater than the porosity of the transition layer disposed adjacent to the edge layer, and the porosity of the intermediate layer is greater than the porosity of the transition layer. Through the design, the problems of non-uniform electrolyte distribution and lithium precipitation in the middle layer and transition layer areas are effectively improved, and the cycle and storage life of the battery cell is prolonged.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a negative electrode sheet, a method for preparing a negative electrode sheet, a laminated electrode core and a vehicle. Background Art

[0002] At present, lithium-ion batteries have become the core components of the development of electric vehicles. With the improvement of battery cell energy density, electrodes are moving towards high coating weight. However, the increase in electrode coating weight will lead to increased polarization of the electrode during the charging and discharging process, thereby reducing the dynamics of the battery cell. In addition, the increase in electrode coating weight will increase the water diffusion resistance in the electrode during the baking process of the battery cell. After baking, the excessive water content (or alcohol substances) in the electrode will cause the electrolyte to decompose and produce HF, further deteriorating the performance of the battery cell.

[0003] The existing short blade battery cells have significantly larger electrode pieces than the common square aluminum battery cells. During the fast charging process of the blade-shaped battery cells, the dynamic differences in each area of ​​the electrode piece are large. On the one hand, due to the uneven distribution of current density and temperature field, the potential difference is large, which in turn leads to lithium deposition problems at the edge of the battery cell electrode piece, which greatly affects the safe use of the battery cell. On the other hand, due to the large differences in the distribution of the electrolyte in each area of ​​the electrode piece, such as the less solvent and film-forming agent in the middle area, the aging rate of the negative electrode active material in the middle area is faster, which in turn deteriorates the battery cell cycle and storage performance. Summary of the invention

[0004] Based on this, it is necessary to provide a negative electrode sheet that can improve the edge power performance by reducing the degree of lithium deposition at the edge of the negative electrode sheet.

[0005] The present application provides a negative electrode sheet, comprising a current collector layer and an active material layer stacked in sequence along a first direction, wherein both sides of the current collector layer arranged relatively spaced apart along a second direction are defined as first sides, the second direction intersects with the first direction, one of the two first sides is provided with a pole ear, the active material layer comprises two edge layers spaced apart along the length direction of the first side and an intermediate layer between the two edge layers, the two edge layers and the intermediate layer are connected via a transition layer, the porosity of the edge layer is greater than the porosity of the transition layer arranged adjacent to the edge layer, and the porosity of the intermediate layer is greater than the porosity of the transition layer.

[0006] In one embodiment, the middle layer includes a first part and a second part arranged in sequence along the length direction of the first side edge, the two transition layers are respectively a first transition layer and a second transition layer, the two edge layers are respectively a first edge layer and a second edge layer, the first transition layer is located between the first edge layer and the first part, the second transition layer is located between the second edge layer and the second part, the porosity of the first part is greater than the porosity of the first transition layer, and the porosity of the second part is greater than the porosity of the second transition layer.

[0007] In one embodiment, the porosity of the middle layer, the porosity of the transition layer and the porosity of the edge layer are all 10% to 60%.

[0008] In one embodiment, the porosities of the two edge layers are equal, and the porosities of the two transition layers are equal.

[0009] In one embodiment, the widths of the middle layer and the edge layer along the length direction of the first side edge are smaller than the width of the transition layer along the length direction of the first side edge.

[0010] In one embodiment, the negative electrode sheet is in a rectangular shape, the first side is the short side of the current collector layer, the edge layer, the transition layer and the middle layer all extend along the long side of the negative electrode sheet, and the middle layer is located in the middle of the active material layer.

[0011] In one embodiment, the porosity of the edge layer is greater than or equal to the porosity of the middle layer; or, the porosity of the edge layer is less than the porosity of the middle layer.

[0012] The present application also discloses a method for preparing a negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet described in any one of the above embodiments, and the preparation method comprises the following steps in sequence:

[0013] Step S1, preparing an edge layer slurry of an edge layer, a transition layer slurry of a transition layer, and an intermediate layer slurry of an intermediate layer, wherein the edge layer slurry, the intermediate layer slurry, and the transition layer slurry are all composed of a negative electrode active material, a binder, a dispersant, and a conductive agent, wherein the negative electrode active material is composed of one or more of graphite, a silicon-based material, a hard carbon material, and natural graphite, the binder is composed of one or more of styrene-butadiene rubber, carboxymethyl chitosan, polyacrylonitrile, polyvinyl alcohol, and polyacrylic acid, the dispersant is composed of one or two of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose, and the conductive agent is composed of one or more of conductive carbon black, carbon nanotubes, and graphene;

[0014] Step S2, coating the edge layer slurry, transition layer slurry, middle layer slurry, transition layer slurry and edge layer slurry in sequence on the current collector layer along the length direction of the first side, and after coating, drying and rolling the coated negative electrode sheet to prepare a negative electrode sheet.

[0015] The present application also provides a laminated electrode core, comprising a negative electrode sheet, a separator and a positive electrode sheet stacked in sequence, wherein the negative electrode sheet is the negative electrode sheet described in any one of the above embodiments, and the collector of the negative electrode sheet is connected to the positive electrode sheet through the separator.

[0016] In one embodiment, the positive electrode sheet includes an active layer, and the charging capacity C1 of the active material layer of the negative electrode sheet and the charging capacity C2 of the active layer of the positive electrode sheet satisfy: C1 / C2=1.0-1.2.

[0017] The present application also provides a vehicle, comprising a battery having a laminated pole core, wherein the laminated pole core is the laminated pole core described in any one of the above embodiments.

[0018] Compared with the prior art, in the negative electrode sheet provided by the present application, the porosity of the middle layer is greater than the porosity of the transition layer, that is, by increasing the porosity of the middle layer, since the electrolyte is present in the pores, lithium ions are transmitted in the pores through the electrolyte, which increases the liquid retention of the solvent and film-forming agent at the location of the middle layer, reduces the aging rate of the negative electrode sheet at the location of the middle layer, and makes the electrolyte distribution difference between the middle layer and the transition layer not much, that is, effectively improves the problem of uneven electrolyte distribution in the area where the middle layer and the transition layer of the negative electrode sheet are located, and improves the cycle and storage life of the battery cell. By making the porosity of the edge layer greater than the porosity of the adjacent transition layer, the current density distribution in the area where the edge layer of the negative electrode sheet and the area where the transition layer are located can be uniform, reducing the potential difference, thereby reducing the probability of lithium precipitation problems at the edge. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the conventional technology, the drawings required for use in the embodiments or the conventional technology descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 It is a three-dimensional diagram of a negative electrode sheet according to an embodiment of the present application.

[0021] Figure numerals: 1, current collector layer; 11, first side; 2, active material layer; 21, first edge layer; 22, first transition layer; 23, intermediate layer; 231, first part; 232, second part; 24, second transition layer; 25, second edge layer; 3, pole ear. DETAILED DESCRIPTION

[0022] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present application, so the present application is not limited by the specific embodiments disclosed below.

[0023] It should be noted that when a component is referred to as being "fixed to" or "disposed on" another component, it may be directly on the other component or there may also be a centered component. When a component is considered to be "connected to" another component, it may be directly connected to the other component or there may be a centered component at the same time. The terms "vertical", "horizontal", "upper", "lower", "left", "right", "side", "top", "bottom" and similar expressions used in the specification of this application are only used to describe the various example structural parts and elements of the present application, but these terms are used here only for the purpose of convenience of explanation, and are determined based on the example orientations shown in the accompanying drawings, and do not represent the only implementation method. Since the embodiments disclosed in the application can be set in different directions, these terms indicating directions are only for illustration and should not be regarded as limitations. For example, "upper" and "lower" are not necessarily limited to directions opposite to or consistent with the direction of gravity.

[0024] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of this application, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0025] In the present application, unless otherwise clearly specified and limited, a first feature being “above” or “below” a second feature may mean that the first feature is directly in contact with the second feature, or the first feature and the second feature are indirectly in contact through an intermediate medium. Moreover, a first feature being “above”, “above” or “above” a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being “below”, “below” or “below” a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0026] It should be noted that "axial arrangement" means that the overall arrangement direction is along the axial direction, including but not limited to axial extension, and may form an angle with the axial direction.

[0027] Unless otherwise defined, all technical and scientific terms used in the specification of this application have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" used in the specification of this application includes any and all combinations of one or more related listed items.

[0028] like Figure 1 As shown, the present invention provides a negative electrode sheet. The negative electrode sheet includes a current collector layer 1 and an active material layer 2 which are sequentially stacked along a first direction y. Then, the first direction y is the stacking direction and also the thickness direction of the negative electrode sheet. It is defined that both sides of the current collector layer 1 which are relatively spaced apart along a second direction x are first sides 11. The second direction x intersects with the first direction y, and one side of the two first sides 11 is provided with a pole ear 3. The active material layer 2 includes two edge layers spaced apart along the length direction of the first side 11 and an intermediate layer 23 located between the two edge layers. The two edge layers and the intermediate layer 23 are connected by a transition layer. The porosity of the edge layer is greater than the porosity of the transition layer arranged adjacent to the edge layer, and the porosity of the intermediate layer 23 is greater than the porosity of the transition layer.

[0029] It can be understood that the porosity of the middle layer 23 is greater than the porosity of the transition layer, that is, by increasing the porosity of the middle layer 23, since the electrolyte exists in the pores, lithium ions are transmitted in the pores through the electrolyte, which increases the liquid retention of the solvent and film-forming agent at the location of the middle layer 23, reduces the aging rate of the negative electrode sheet at the location of the middle layer 23, and makes the electrolyte distribution difference between the middle layer 23 and the transition layer not much, that is, effectively improves the problem of uneven electrolyte distribution of the negative electrode sheet in the area where the middle layer 23 is located and the area where the transition layer is located, and improves the cycle and storage life of the battery cell. By making the porosity of the edge layer greater than the porosity of the adjacent transition layer, the current density distribution in the edge area of ​​the negative electrode sheet and the area where the transition layer is located can be uniform, reducing the potential difference, thereby reducing the probability of lithium precipitation problems at the edge.

[0030] The porosity of the edge layer and the middle layer 23 is greater than the porosity of the transition layer, which improves the uniformity of the motor reaction and activation, and effectively increases the life of the formed battery cell. In addition, since the pore channel inside the negative electrode is curved, the actual diffusion transmission path is longer than the linear distance in the thickness direction of the negative electrode sheet. By designing the gradient porosity of the edge layer, transition layer and middle layer 23, the electrode tortuosity can be optimized.

[0031] The porosity of the intermediate layer 23, the porosity of the transition layer and the porosity of the edge layer are all 10% to 60%. That is, the porosity of the intermediate layer 23, the porosity of the transition layer and the porosity of the edge layer are within the range of 10% to 60%.

[0032] It is understandable that when the porosity is greater than 60%, the conductive network on the negative electrode sheet will deteriorate, reducing the reaction rate, thereby affecting the subsequent charge and discharge efficiency of the battery. In addition, it will cause the bonding between the edge layer, transition layer and intermediate layer 23 to loosen easily, thereby increasing the risk of short circuit in the battery. When the porosity is less than 10%, it will hinder the flow of electrolyte and reduce the reaction rate; in addition, it will increase the accumulation of heat inside the battery, thereby causing the battery temperature to rise and causing thermal runaway problems. Designing the porosity to be 10% to 60% can ensure the bonding ability between the edge layer, transition layer and intermediate layer 23 while ensuring the flow rate of the electrolyte and reducing the accumulation of heat inside the battery, thereby improving the performance of the battery.

[0033] The porosity of the transition layer can be 10% or any value less than 60%, while the porosity of the middle layer and the edge layer can be 60% or any value between 10% and 60%.

[0034] The first direction y is perpendicular to the second direction x. In the present embodiment, the negative electrode sheet is in the shape of a cuboid, the first side 11 is the short side of the current collector layer 1, and the edge layer, transition layer and intermediate layer 23 are all extended along the long side of the negative electrode sheet. The intermediate layer 23 is located in the middle of the active material layer 2, that is, in the central area of ​​the negative electrode sheet. In this way, the porosity of the middle area of ​​the negative electrode sheet can be improved, the wetting problem of the middle area of ​​the battery cell can be improved, the electrolyte retention amount in the middle of the negative electrode sheet can be ensured, the uneven distribution of the electrolyte can be improved, and the storage and cycle life of the battery cell can be improved. In addition, the moisture baking time of the negative electrode sheet can be shortened later.

[0035] In one embodiment, the intermediate layer 23 is a complete active layer. In this embodiment, the intermediate layer 23 includes two parts arranged in sequence along the length direction of the first side 11, and the two parts are respectively a first part 231 and a second part 232. The porosity of the first part 231 is greater than the porosity of the transition layer arranged adjacent to the first part 231, and the porosity of the second part 232 is greater than the porosity of the transition layer arranged adjacent to the second part 232, that is, the porosity of the intermediate layer 23 is greater than the porosity of the adjacent transition layer.

[0036] The two edge layers are respectively the first edge layer 21 and the second edge layer 25, the first portion 231 is arranged adjacent to the first edge layer 21, and the second portion 232 is arranged adjacent to the second edge layer 25. The transition layer between the first portion 231 and the first edge layer 21 is defined as the first transition layer 22, then the porosity of the first portion 231 is greater than the porosity of the first transition layer 22. The transition layer between the second portion 232 and the second edge layer 25 is defined as the second transition layer 24, then the porosity of the second portion 232 is greater than the porosity of the second transition layer 24. In other words, the two transition layers are respectively the first transition layer 22 and the second transition layer 24.

[0037] The widths of the intermediate layer 23 and the edge layer along the length direction of the first side 11 are both smaller than the width of the transition layer along the length direction of the first side 11. It can be understood that the electrolyte distribution of the negative electrode sheet in the preset direction z can be further ensured to be uniform, reducing the probability of lithium deposition in the middle and edge. It should be noted that the preset direction z is the length direction of the first side.

[0038] In one embodiment, the porosity of the first edge layer 21 and the second edge layer 25 are not equal, and the porosity of the first transition layer 22 and the second transition layer 24 are also not equal. In another embodiment, the porosity of the two edge layers is equal, and the porosity of the two transition layers is equal, that is, the porosity of the first edge layer 21 and the second edge layer 25 is equal, and the porosity of the first transition layer 22 and the second transition layer 24 is equal.

[0039] In one embodiment, the porosity of the edge layer is greater than or equal to the porosity of the middle layer 23. In another embodiment, the porosity of the edge layer is less than the porosity of the middle layer 23.

[0040] The present invention also provides a laminated electrode core, which comprises a negative electrode sheet, a separator and a positive electrode sheet which are sequentially stacked, and a current collector of the negative electrode sheet is connected to the positive electrode sheet through the separator.

[0041] The positive electrode sheet includes an active layer, and the charging capacity C1 of the active material layer 2 of the negative electrode sheet and the charging capacity C2 of the active layer of the positive electrode sheet satisfy: C1 / C2=1.0-1.2. It can be understood that the aforementioned charging capacity ratio range is adopted to consider the performance of the battery under different conditions, to ensure that it can work normally under various working conditions, and to ensure that the battery cell has an open working window and does not deposit lithium during the entire life cycle.

[0042] The method for preparing the negative electrode sheet comprises the following steps in sequence:

[0043] Step S1, preparing an edge layer slurry of the edge layer, a transition layer slurry of the transition layer, and an intermediate layer slurry of the intermediate layer 23, wherein the edge layer slurry, the intermediate layer slurry, and the transition layer slurry are all composed of a negative electrode active material, a binder, a dispersant, and a conductive agent, wherein the negative electrode active material is composed of one or more of graphite, silicon-based materials, hard carbon materials, and natural graphite, the binder is composed of one or more of styrene-butadiene rubber, carboxymethyl chitosan, polyacrylonitrile, polyvinyl alcohol, and polyacrylic acid, the dispersant is composed of one or two of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose, and the conductive agent is composed of one or more of conductive carbon black, carbon nanotubes, and graphene;

[0044] Step S2, coating the edge layer slurry, transition layer slurry, middle layer slurry, transition layer slurry and edge layer slurry on the current collector layer in sequence along the length direction of the first side 11, and drying and rolling the coated negative electrode sheet after coating, so that the first edge layer 21, the first transition layer 22, the middle layer 23, the second transition layer 24 and the second edge layer 25 are sequentially arranged on the current collector layer along the length direction of the first side 11, that is, the above-mentioned negative electrode sheet is prepared. The above-mentioned current collector layer is a negative electrode copper foil.

[0045] It can be understood that since the internal pore channels of the electrode are curved, the electrode tortuosity can be optimized by adjusting the porosity of the edge layer, the middle layer 23 and the transition layer, so that the moisture and other impurities in the negative electrode sheet can be more easily volatilized in the subsequent vacuum baking process, thereby improving the baking efficiency of the negative electrode sheet and effectively improving the performance of the negative electrode sheet.

[0046] In the coating process of the above step S2, the coating amount in the preset direction z is changed by adjusting the mixed gram volume in the preset direction z, thereby achieving the adjustment and optimization of the porosity.

[0047] The present invention also provides a vehicle, which includes a battery, and the battery includes the above-mentioned laminated pole core.

[0048] The following specific examples will further illustrate the preparation method of the active material layer 2 on the negative electrode sheet and the positive electrode sheet. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention. If no specific conditions are specified in the examples, the conventional conditions or the conditions recommended by the manufacturer are followed. The reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased commercially.

[0049] Embodiment 1:

[0050] In step S1, the edge layer slurry is prepared by mixing graphite, silicon carbon, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 94.8:2:0.8:1.4:1 and dispersing at high speed. The transition layer slurry is prepared by mixing graphite, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 96.8:0.8:1.4:1 and dispersing at high speed. The intermediate layer slurry is prepared by mixing graphite, silicon carbon, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 94.0:2.8:0.8:1.4:1 and dispersing at high speed.

[0051] The positive electrode sheet preparation method is as follows: lithium iron phosphate, conductive carbon black, carbon nanotubes and polyvinylidene fluoride are mixed evenly in a mass ratio of 96.5:1:0.2:2.3, N-methyl-2-pyrrolidone solvent is added, and positive electrode slurry is prepared by high-speed dispersion, and then the positive electrode slurry is coated on the positive electrode current collector aluminum foil. After coating, the coated positive electrode sheet is dried and rolled to prepare the positive electrode sheet.

[0052] The positive electrode sheet and the negative electrode sheet prepared above are die-cut, and then stacked with the separator in a Z-shaped stacked core, and then packaged, baked, injected, formed, aged, sealed, and capacity divided to prepare a cell monomer, and finally, the cell is selected, assembled and packaged into a battery. The parameters and specific processes used in the specific packaging of the above battery are the same as those in the prior art, and will not be described in detail in this embodiment.

[0053] Embodiment 2:

[0054] In step S1, the edge layer slurry is prepared by mixing graphite, silicon carbon, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 95.8:1:0.8:1.4:1 and dispersing at high speed. The transition layer slurry is prepared by mixing graphite, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 96.8:0.8:1.4:1 and dispersing at high speed. The intermediate layer slurry is prepared by mixing graphite, silicon carbon, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 93.8:3:0.8:1.4:1 and dispersing at high speed.

[0055] The preparation method of the positive electrode sheet of this embodiment 2 is the same as that of the above-mentioned embodiment 1.

[0056] Embodiment 3:

[0057] In step S1, the edge layer slurry is prepared by mixing graphite, silicon carbon, conductive carbon black, styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 91.8:5:0.8:1.4:1 and then dispersing at a high speed.

[0058] The transition layer slurry is prepared by mixing graphite, conductive carbon black, styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 96.8:0.8:1.4:1 and then dispersing at a high speed.

[0059] The middle layer slurry is prepared by mixing graphite, silicon carbon, conductive carbon black, styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 93.8:3:0.8:1.4:1 and then dispersing at a high speed.

[0060] The preparation method of the positive electrode sheet of this embodiment 3 is the same as that of the above-mentioned embodiment 1.

[0061] Embodiment 4:

[0062] In step S1, the edge layer slurry is prepared by mixing graphite, silica, conductive carbon black, styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 91.8:5:0.8:1.4:1 and dispersing at high speed. The transition layer slurry is prepared by mixing graphite, conductive carbon black, styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 96.8:0.8:1.4:1 and dispersing at high speed.

[0063] The middle layer slurry is graphite, silica, conductive carbon black, styrene-butadiene rubber and sodium carboxymethyl cellulose, which are uniformly mixed in a mass ratio of 93.8:3:0.8:1.4:1 and prepared by high-speed dispersion.

[0064] The preparation method of the positive electrode sheet of this embodiment is: after uniformly mixing nickel-cobalt-manganese ternary positive electrode material (NCM622), conductive carbon black, carbon nanotubes and polyvinylidene fluoride in a mass ratio of 97:1:0.2:1.8, adding N-methyl-2-pyrrolidone solvent, preparing positive electrode slurry by high-speed dispersion, and then coating the positive electrode slurry on the positive electrode current collector aluminum foil. After coating, the coated positive electrode sheet is dried and rolled to prepare the positive electrode sheet.

[0065] Embodiment 5:

[0066] In step S1, the edge layer slurry is graphite, silica, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose, which are mixed evenly in a mass ratio of 94.3:2.5:0.8:1.4:1 and prepared by high-speed dispersion. The transition layer slurry is graphite, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose, which are mixed evenly in a mass ratio of 96.8:0.8:1.4:1 and prepared by high-speed dispersion. The intermediate layer slurry is graphite, silica, conductive carbon black, styrene butadiene rubber and sodium carboxymethyl cellulose, which are mixed evenly in a mass ratio of 93.8:3:0.8:1.4:1 and prepared by high-speed dispersion.

[0067] The method for preparing the positive electrode sheet of this embodiment is the same as the method for preparing the positive electrode sheet of the above-mentioned embodiment 4.

[0068] Comparative Example 1:

[0069] In step S1, the edge layer slurry, the transition layer slurry and the intermediate layer slurry are prepared by uniformly mixing graphite, conductive carbon black, styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 96.8:0.8:1.4:1 and then dispersing at a high speed.

[0070] This comparative example uses the same positive electrode sheet as in Example 1, namely, a lithium iron phosphate positive electrode sheet. The preparation method of the positive electrode sheet in this comparative example is the same as the preparation method of the positive electrode sheet in Example 1.

[0071] Comparative Example 2:

[0072] In step S1, the edge layer slurry, the transition layer slurry and the intermediate layer slurry are prepared by uniformly mixing graphite, conductive carbon black, styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 96.8:0.8:1.4:1 and then dispersing at a high speed.

[0073] The positive electrode sheet of this comparative example 2 is the same as the positive electrode sheet of Example 4, which is a nickel-cobalt-manganese ternary positive electrode sheet. The preparation method of the positive electrode sheet of this comparative example is the same as the preparation method of the positive electrode sheet in the above Example 4.

[0074] The porosity of the negative electrode sheets prepared in Examples 1 to 5 and the comparative example was characterized by mercury intrusion porosimetry. The results are shown in Table 1:

[0075]

[0076] The batteries prepared in the above Examples 1 to 3 and Comparative Example 1 were subjected to a cycle test, and the method was as follows: at 25°C, charged to 3.65V with a 3C constant rate, constant current and constant voltage, cut off with a 0.05C rate, left to stand for 1 hour, and then discharged to 2.0V with a 1C rate, left to stand for 1 hour, and the above charge and discharge process was cycled 200 times to obtain the capacity retention rate. The battery was then disassembled to observe the lithium deposition state in different areas of the negative electrode sheet, as shown in Table 2 below.

[0077] The batteries prepared in Example 4, Example 5 and Comparative Example 2 were subjected to a cycle test, and the method was as follows: at 35°C, a step-charging process strategy was adopted, and the battery was cycled for 600 cls in the range of 3-97%. The above charge and discharge process was cycled for 800 times to obtain the capacity retention rate, and then the battery was disassembled to observe the lithium deposition status in different areas of the negative electrode sheet, as shown in Table 2 below.

[0078]

[0079] The degree of lithium precipitation at the negative electrode full charge interface is defined as follows:

[0080] 0: Lithium deposition cannot be observed with the naked eye;

[0081] 1: Trace lithium deposition (lithium deposition area is less than 10% of the active material layer area in the preset direction z);

[0082] 2: Small area lithium deposition (lithium deposition area is equivalent to 10-20% of the area of ​​the active material layer in the preset direction z);

[0083] 3: Large-area lithium deposition (lithium deposition area > 30% of the active material layer area in the preset direction z).

[0084] The capacity retention rate of the above-mentioned embodiment 1 is 95.1%, the capacity retention rate of the embodiment 2 is 93.0%, the capacity retention rate of the embodiment 3 is 96.6%, and the capacity retention rate of the comparative example 1 is 90%. Compared with the comparative example 1, the above-mentioned embodiments 1 to 3 can significantly improve the capacity retention rate of the battery.

[0085] The capacity retention rate of Example 4 is 88.5%, the capacity retention rate of Example 5 is 92.5%, and the capacity retention rate of Comparative Example 2 is 85.2%. It can be seen that compared with Comparative Example 2, both Example 4 and Example 5 can significantly improve the capacity retention rate of the battery.

[0086] It can be seen from Comparative Examples 1 and 2 in Tables 1 and 2 above that when the first edge layer 21, the first transition layer 22, the first part 231, the second part 232, the second transition layer 24 and the second edge layer 25 of the negative electrode sheet all adopt equal porosity, due to the higher current density in the edge layer area of ​​the electrode sheet, the polarization of the charging process is worse, and a large area of ​​lithium deposition will appear in the area corresponding to the two edge layers of the negative electrode. The porosity of the middle layer is the porosity of the first part or the second part. In the area of ​​the middle layer of the negative electrode, it is more difficult for the electrolyte to infiltrate, which will also lead to the deterioration of the dynamics of the negative electrode middle layer, and trace lithium deposition will appear in the area of ​​the middle layer of the negative electrode sheet.

[0087] It can be seen from Examples 1 to 5 of Tables 1 and 2 above that: when the porosity of the edge layer and the porosity of the middle layer 23 differ from the porosity of the transition layer, that is, when the porosity of the first edge layer 21 and the first part 231 differ from the porosity of the first transition layer 22 by ≥5, no lithium deposition occurs in the first edge layer 21 and the first part 231, and when the porosity of the second edge layer 25 and the second part 232 differ from the porosity of the second transition layer 24 by ≥5, no lithium deposition occurs in the second edge layer 25 and the second part 232, that is, lithium deposition can be prevented from occurring in the entire area of ​​the negative electrode sheet, thereby ensuring the uniformity of the electrolyte in the negative electrode sheet, see Examples 3 to 5.

[0088] It can be seen from Examples 3 and 4 that when the porosity of the first edge layer 21 is greater than the porosity of the first part 231 which is greater than the porosity of the first transition layer 22, and the porosity of the second edge layer 25 is greater than the porosity of the second part 232 which is greater than the porosity of the second transition layer 24 (that is, when the porosity of the edge layer is greater than the porosity of the middle layer which is greater than the porosity of the transition layer), no lithium deposition occurs in any area of ​​the negative electrode sheet.

[0089] It can be seen from Examples 1, 2 and 5 that when the porosity of the first part 231 is greater than the porosity of the first edge layer 21 which is greater than the porosity of the first transition layer 22, and the porosity of the second part 232 is greater than the porosity of the second edge layer 25 which is greater than the porosity of the second transition layer 24, the greater the difference between the porosity of the edge layer and the porosity of the adjacent transition layer, the less likely it is for lithium deposition to occur in the area of ​​the negative electrode corresponding to the edge layer.

[0090] In summary, the above-mentioned embodiments 1 to 5 significantly improve the lithium deposition in the middle and edge areas of the negative electrode sheet while ensuring a high capacity retention rate, thereby effectively improving the performance of the battery.

[0091] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0092] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the scope of patent protection of the present application shall be subject to the attached claims.

Claims

1. A negative electrode sheet, characterized in that: The invention comprises a current collector layer (1) and an active material layer (2) which are sequentially stacked along a first direction, wherein both sides of the current collector layer (1) which are arranged relatively spaced apart along a second direction are first sides (11), the second direction intersects with the first direction, one of the two first sides (11) is provided with a pole ear (3), the active material layer (2) comprises two edge layers which are spaced apart along the length direction of the first side (11) and an intermediate layer (23) located between the two edge layers, the two edge layers and the intermediate layer (23) are connected via a transition layer, the porosity of the edge layer is greater than the porosity of the transition layer arranged adjacent to the edge layer, and the porosity of the intermediate layer (23) is greater than the porosity of the transition layer.

2. The negative electrode sheet according to claim 1, characterized in that: The intermediate layer (23) comprises a first part (231) and a second part (232) which are arranged in sequence along the length direction of the first side edge (11); the two transition layers are respectively a first transition layer (22) and a second transition layer (24); the two edge layers are respectively a first edge layer (21) and a second edge layer (25); the first transition layer (22) is located between the first edge layer (21) and the first part (231); the second transition layer (24) is located between the second edge layer (25) and the second part (232); the porosity of the first part (231) is greater than the porosity of the first transition layer (22); and the porosity of the second part (232) is greater than the porosity of the second transition layer (24).

3. The negative electrode sheet according to claim 1, characterized in that: The porosity of the intermediate layer (23), the porosity of the transition layer and the porosity of the edge layer are all 10% to 60%.

4. The negative electrode sheet according to claim 3, characterized in that: The porosities of the two edge layers are equal, and the porosities of the two transition layers are equal.

5. The negative electrode sheet according to claim 1, characterized in that: The negative electrode sheet is in the shape of a rectangular parallelepiped, the first side (11) is the short side of the current collector layer (1), the edge layer, the transition layer and the intermediate layer (23) all extend along the long side of the negative electrode sheet, and the intermediate layer (23) is located in the middle of the active material layer (2).

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that: The porosity of the edge layer is greater than or equal to the porosity of the middle layer (23); or, the porosity of the edge layer is less than the porosity of the middle layer (23).

7. A method for preparing a negative electrode sheet, characterized in that: The negative electrode sheet is the negative electrode sheet according to any one of claims 1 to 6, and the preparation method comprises the following steps in sequence: Step S1, preparing an edge layer slurry of the edge layer, a transition layer slurry of the transition layer, and an intermediate layer slurry of the intermediate layer (23), wherein the edge layer slurry, the intermediate layer slurry, and the transition layer slurry are all composed of a negative electrode active material, a binder, a dispersant, and a conductive agent, wherein the negative electrode active material is composed of one or more of graphite, silicon-based materials, hard carbon materials, and natural graphite, the binder is composed of one or more of styrene-butadiene rubber, carboxymethyl chitosan, polyacrylonitrile, polyvinyl alcohol, and polyacrylic acid, the dispersant is composed of one or two of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose, and the conductive agent is composed of one or more of conductive carbon black, carbon nanotubes, and graphene; Step S2, coating the edge layer slurry, transition layer slurry, middle layer slurry, transition layer slurry and edge layer slurry in sequence along the length direction of the first side edge (11) on the current collector layer (1), and after coating, drying and rolling the coated negative electrode sheet to prepare the negative electrode sheet.

8. A laminated electrode core, comprising a negative electrode sheet, a separator and a positive electrode sheet stacked in sequence, characterized in that: The negative electrode sheet is the negative electrode sheet according to any one of claims 1 to 6, and the current collector of the negative electrode sheet is connected to the positive electrode sheet through the separator.

9. The laminated pole core according to claim 8, characterized in that: The positive electrode sheet comprises an active layer, and the charging capacity C1 of the active material layer (2) of the negative electrode sheet and the charging capacity C2 of the active layer of the positive electrode sheet satisfy: C1 / C2=1.0-1.

2.

10. A vehicle comprising a battery having a laminated core, characterized in that: The laminated pole core is the laminated pole core described in claim 8 or 9.

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