Electrode assembly, method of manufacturing the same, and battery

Abstract

The application relates to an electrode assembly, a preparation method thereof and a battery. In the electrode assembly, a first active material layer of a first electrode sheet comprises a first main body part and a thinned part; the thickness of the thinned part gradually decreases from the first main body part to the thinned part; a second active material layer of a second electrode sheet comprises a second main body part and a mixed part; the mixed part is a multi-layer structure of a first sub-layer and a second sub-layer, the thickness of the first sub-layer gradually increases and the thickness of the second sub-layer gradually decreases from the second main body part to the mixed part; the limit compaction density of a first active material in the first sub-layer is smaller than the limit compaction density of a second active material in the second sub-layer and a third active material in the second main body part; the thickness of the multi-layer structure gradually increases from the second main body part to the mixed part, and the thickness of a convex part of the second main body part is greater than the thickness of the second main body part; the convex part is arranged opposite to the thinned part, and a part of a diaphragm in contact with the convex part is bent to the thinned part. Thus, lithium precipitation of a negative electrode can be avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to an electrode assembly, a preparation method thereof and a battery. BACKGROUND

[0002] A power battery is the main energy source of a new energy vehicle, and its performance determines the vehicle's endurance mileage, service life and safety. The basic structure in the battery is an electrode assembly, which is formed by winding or stacking a positive electrode sheet, a negative electrode sheet and a separator. Research shows that the distance between the positive and negative electrode sheets inside the electrode assembly is a key parameter affecting the active ion transmission path and the electrode potential distribution, which has an important influence on the overall performance of the battery.

[0003] In the manufacturing process of the electrode sheet, a slurry is generally applied to the current collector, and after drying and rolling, an active material layer is obtained. In order to reduce the risk of deformation of the electrode sheet during drying and rolling, the amount of slurry applied to the edge of the electrode sheet is usually reduced in the slurry coating process in actual production, so that the thickness of the edge area of the active material layer is thinned (referred to as a thinning area or a reduced area in the industry), and the thickness of the part of the active material layer located in the thinning area gradually decreases from the middle to the edge of the active material layer. However, due to the existence of the thinning area, when the electrode sheet is prepared into an electrode assembly by winding or stacking, the thinning areas in the positive and negative electrode sheets are oppositely arranged, and after heat pressing, there is a large gap between the thinning area and the separator, which can cause the negative reference potential of the thinning area to be lower than that of the normal area during charging, increase the transmission distance of active ions (lithium ions, sodium ions, etc.), and easily cause lithium precipitation at the corresponding position of the negative electrode sheet in the later stage of battery cycling, thereby affecting the cycle life of the battery and even threatening the safety of the battery. SUMMARY

[0004] Therefore, the present application provides an electrode assembly, a preparation method thereof and a battery to solve at least one problem in the background art.

[0005] In a first aspect, the present application provides an electrode assembly, comprising a first electrode sheet, a second electrode sheet and a separator arranged between the first electrode sheet and the second electrode sheet; the polarities of the first electrode sheet and the second electrode sheet are opposite;

[0006] The first electrode sheet comprises a first current collector and a first active material layer located on at least one surface of the first current collector in a first direction, the first active material layer comprises a first main body part and a thinning part, the thinning part is located on at least one side of the first main body part in a second direction; the thickness of the thinning part is less than the thickness of the first main body part, and the thickness of the thinning part gradually decreases from the first main body part to the thinning part;

[0007] The second tab includes a second current collector and a second active material layer located on at least one surface of the second current collector in the first direction, the second active material layer including a second main body portion and a mixed portion, the mixed portion being located on at least one side of the second main body portion in the second direction; the mixed portion being a multi-layer structure stacked by a first sub-layer and a second sub-layer in the first direction, the thickness of the first sub-layer gradually increasing and the thickness of the second sub-layer gradually decreasing in a direction from the second main body portion to the mixed portion; the first sub-layer including a first active material, the second sub-layer including a second active material, and the second main body portion including a third active material, the limit compaction density of the first active material being less than the limit compaction density of the second active material and the limit compaction density of the third active material; the thickness of at least part of the multi-layer structure being greater than the thickness of the second main body portion, the part of the multi-layer structure whose thickness is greater than the thickness of the second main body portion being defined as a protruding portion, the thickness of the protruding portion gradually increasing in the direction from the second main body portion to the mixed portion;

[0008] In the first direction, the protruding portion is arranged opposite to the thinned portion, and the part of the diaphragm in contact with the protruding portion is bent towards the thinned portion;

[0009] The first direction is the thickness direction of the first current collector, and the second direction is the width direction of the first current collector.

[0010] In combination with the first aspect of the present application, in an optional implementation, the electrode assembly satisfies at least one of the following features:

[0011] (1) In the first direction, the sum of the size of the gap between the mixed portion and the diaphragm and the size of the gap between the thinned portion and the diaphragm is less than or equal to 10 μm;

[0012] (2) In the first direction, the projection of the protruding portion is located within the projection of the thinned portion;

[0013] (3) The mixed portion further includes a flush portion, the flush portion being located between the protruding portion and the second main body portion, and the thickness of the flush portion being the same as the thickness of the second main body portion;

[0014] (4) The size of the mixed portion in the second direction is greater than the size of the protruding portion in the first direction;

[0015] (5) The difference between the size of the protruding portion and the size of the second main body portion in the first direction is less than the size of the protruding portion in the second direction;

[0016] (6) the maximum difference between the size of the first main body part and the size of the thinning part in the first direction is greater than or equal to the maximum difference between the size of the convex part and the size of the second main body part in the first direction;

[0017] (7) in the first direction, the sum of the thicknesses of the corresponding thinning part and the convex part is equal to the sum of the thicknesses of the first main body part and the second main body part;

[0018] (8) the first sub-layer is located between the second current collector and the second sub-layer;

[0019] (9) the second active material and the third active material are the same.

[0020] In combination with the first aspect of the present application, in an optional implementation, the second pole piece is a positive pole piece; and the electrode assembly satisfies at least one of the following characteristics:

[0021] (1) the first active material is lithium iron phosphate and / or lithium manganese iron phosphate; the limit compaction density of the first active material is 2.3 g / cm 3 ~2.8 g / cm 3 ;

[0022] (2) the second active material is a ternary material; the limit compaction density of the second active material is 3.3 g / cm 3 ~3.6 g / cm 3 ;

[0023] (3) the third active material is a ternary material; the limit compaction density of the third active material is 3.3 g / cm 3 ~3.6 g / cm 3 ;

[0024] (4) the gram capacity of the first active material is less than the gram capacity of the third active material, and the gram capacity of the second active material is less than or equal to the gram capacity of the third active material;

[0025] (5) the size of the mixed part in the second direction is 3 mm~20 mm;

[0026] (6) the size of the thinning part in the second direction is 0.5 mm~10 mm; and the maximum difference between the size of the second main body part and the size of the thinning part in the first direction is 0.5 μm~20 μm.

[0027] In combination with the first aspect of the present application, in an optional implementation, the second pole piece is a negative pole piece; and the electrode assembly satisfies at least one of the following characteristics:

[0028] (1) the first active material is a first graphite; the first active material has an ultimate compaction density of 1.55 g / cm 3 1.7 g / cm 3 ;

[0029] (2) the second active material is a second graphite; the second active material has an ultimate compaction density of 1.6 g / cm 3 1.7 g / cm 3 ;

[0030] (3) the third active material is a third graphite; the third active material has an ultimate compaction density of 1.6 g / cm 3 1.7 g / cm 3 ;

[0031] (4) the gram capacity of the first active material is greater than that of the third active material, and the gram capacity of the second active material is greater than or equal to that of the third active material;

[0032] (5) the size of the mixing part in the second direction is 0.5 mm to 10 mm;

[0033] (6) the size of the thinned part in the second direction is 3 mm to 10 mm; and the maximum difference between the sizes of the second main part and the thinned part in the first direction is 5 μm to 30 μm.

[0034] In a second aspect, the embodiments of the present application provide a method for manufacturing an electrode assembly, the method comprising:

[0035] forming a first active material layer on at least one surface of the first current collector along the first direction by a coating process to obtain a first electrode tab; the first active material layer comprises a first main part and a thinned part, the thinned part is located on at least one side of the first main part along the second direction; the thickness of the thinned part is less than that of the first main part, and the thickness of the thinned part gradually decreases from the first main part to the thinned part;

[0036] The second active material layer is formed on at least one surface of the second current collector in the first direction by a coating process to obtain a second pole piece; the second active material layer comprises a second main part and a mixed part, the mixed part is located on at least one side of the second main part in the second direction; the mixed part is a multi-layer structure formed by stacking a first sub-layer and a second sub-layer in the first direction, the thickness of the first sub-layer gradually increases and the thickness of the second sub-layer gradually decreases in the direction from the second main part to the mixed part; the first sub-layer comprises a first active material, the second sub-layer comprises a second active material, and the second main part comprises a third active material; the limit compaction density of the first active material is less than the limit compaction density of the second active material and the limit compaction density of the third active material; the thickness of at least part of the multi-layer structure is greater than the thickness of the second main part, and the part of the multi-layer structure whose thickness is greater than the thickness of the second main part is defined as a protruding part, and the thickness of the protruding part gradually increases in the direction from the second main part to the mixed part;

[0037] The diaphragm is arranged between the first pole piece and the second pole piece, the first active material layer and the second active material layer are arranged towards the diaphragm, and after lamination or winding, a pressing process is performed to obtain the electrode assembly; in the first direction, the protruding part is arranged opposite to the thinned part, and the part of the diaphragm in contact with the protruding part is bent towards the thinned part;

[0038] The polarities of the first pole piece and the second pole piece are opposite; the first direction is the thickness direction of the first current collector, and the second direction is the width direction of the first current collector.

[0039] In combination with the second aspect of the present application, in an optional implementation, the coating process for forming the second active material layer comprises:

[0040] The first slurry, the second slurry and the third slurry are prepared, the first active material is included in the first slurry, the second active material is included in the second slurry, and the third active material is included in the third slurry;

[0041] The first slurry and the second slurry are laminatedly coated on a mixing area on the surface of the second current collector, the mixing area is located on at least one side of the surface of the second current collector in the second direction, in the second direction, from the edge of the second current collector to the inside, the thickness of the mixed slurry composed of the first slurry and the second slurry gradually increases, and the thickness of the first slurry gradually decreases, and the thickness of the second slurry gradually increases; the third slurry is coated on other areas adjacent to the mixing area on the surface of the second current collector, the coating thickness of the third slurry is greater than or equal to the maximum thickness of the mixed slurry, after drying, rolling is performed to form the first sub-layer, the second sub-layer and the second main part.

[0042] In combination with the second aspect of the present application, in an optional implementation, the rolling is arc-shaped rolling; and / or, the temperature of the drying process is 80-150°C.

[0043] In combination with the second aspect of the present application, in an optional implementation, the electrode assembly satisfies at least one of the following features:

[0044] (1) In the first direction, the sum of the size of the gap between the mixing part and the separator and the size of the gap between the thinned part and the separator is less than or equal to 10μm;

[0045] (2) In the first direction, the projection of the protruding part is located within the projection of the thinned part;

[0046] (3) The mixing part further comprises a flush part, the flush part is located between the protruding part and the first main part, and the thickness of the flush part is the same as the thickness of the first main part;

[0047] (4) The size of the mixing part in the second direction is greater than the size of the protruding part in the first direction;

[0048] (5) The difference between the size of the protruding part and the first main part in the first direction is less than the size of the protruding part in the second direction;

[0049] (6) The maximum difference between the size of the first main part and the thinned part in the first direction is greater than or equal to the maximum difference between the size of the protruding part and the second main part in the first direction;

[0050] (7) In the first direction, the sum of the thickness of the corresponding thinned part and the protruding part is equal to the sum of the thickness of the first main part and the second main part;

[0051] (8) The first sub-layer is located between the first current collector and the second sub-layer;

[0052] (9) the second active material and the third active material are the same.

[0053] In combination with the second aspect of the present application, in an optional embodiment,

[0054] the second electrode tab is a positive electrode tab; the electrode assembly satisfies at least one of the following characteristics: (1) the first active material is lithium iron phosphate and / or lithium manganese iron phosphate; the limit compaction density of the first active material is 2.3 g / cm 3 ~2.8 g / cm 3 ; (2) the second active material is a ternary material; the limit compaction density of the second active material is 3.3 g / cm 3 ~3.6 g / cm 3 ; (3) the third active material is a ternary material; the limit compaction density of the third active material is 3.3 g / cm 3 ~3.6 g / cm 3 ; (4) the gram capacity of the first active material is less than that of the third active material, and the gram capacity of the second active material is less than or equal to that of the third active material; (5) the dimension of the mixed part in the second direction is 3 mm~20 mm; (6) the dimension of the thinned part in the second direction is 0.5 mm~10 mm; the maximum difference between the dimensions of the second main part and the thinned part in the first direction is 0.5 μm~20 μm.

[0055] Alternatively, the second electrode tab is a negative electrode tab; the electrode assembly satisfies at least one of the following characteristics: (1) the first active material is a first graphite; the limit compaction density of the first active material is 1.55 g / cm 3 ~1.7 g / cm 3 ; (2) the second active material is a second graphite; the limit compaction density of the second active material is 1.6 g / cm 3 ~1.7 g / cm 3 ; (3) the third active material is a third graphite; the limit compaction density of the third active material is 1.6 g / cm 3 ~1.7 g / cm 3 ; (4) the gram capacity of the first active material is greater than that of the third active material, and the gram capacity of the second active material is greater than or equal to that of the third active material; (5) the dimension of the mixed part in the second direction is 0.5 mm~10 mm; (6) the dimension of the thinned part in the second direction is 3 mm~10 mm; the maximum difference between the dimensions of the second main part and the thinned part in the first direction is 5 μm~30 μm.

[0056] In a third aspect, the embodiments of the present application provide a battery comprising the electrode assembly of any one of the first aspect or the electrode assembly prepared by the preparation method of any one of the second aspect.

[0057] Compared with the prior art, the embodiments of the present application have the following beneficial effects:

[0058] The electrode assembly, the preparation method thereof and the battery provided by the embodiments of the present application have the following beneficial effects: the electrode assembly comprises a first pole piece, a second pole piece and a separator arranged between the first pole piece and the second pole piece; in the first pole piece, the first active material layer comprises a first main body part and a thinned part, the thickness of the thinned part is less than the thickness of the first main body part, and the thickness of the thinned part gradually decreases in the direction from the first main body part to the thinned part; in the second pole piece, the second active material layer comprises a second main body part and a mixed part, the mixed part is a multi-layer structure formed by stacking a first sub-layer and a second sub-layer in a first direction, the thickness of at least part of the multi-layer structure is greater than the thickness of the second main body part, the part of the multi-layer structure whose thickness is greater than the thickness of the second main body part forms a protruding part, and the thickness of the protruding part gradually increases in the direction from the second main body part to the mixed part; in this way, the protruding part in the second pole piece is arranged opposite to the thinned part in the first pole piece, and after being wound or laminated and rolled, the part of the separator in contact with the protruding part is bent towards the thinned part under the action of the protruding part, and because the shape of the protruding part is adapted to the shape of the thinned part, the gap between the thinned part and the separator is significantly reduced, the active ion transmission distance is shortened, the problem that lithium is easily precipitated in the negative electrode due to the gap between the thinned part and the separator is effectively improved, and thus the cycle life and safety performance of the battery can be improved. In addition, in the actual preparation process of the second pole piece, the slurry corresponding to the second main body part can be coated at a normal coating thickness, the slurry corresponding to the mixed part can be coated in a way of being thinned relative to the normal coating thickness, and in the direction from the second main body part to the mixed part, the coating thickness of the slurry corresponding to the first sub-layer can be controlled to gradually increase, and the coating thickness of the slurry corresponding to the second sub-layer can be controlled to gradually decrease; in this way, because the slurry corresponding to the mixed part is coated in a way of being thinned, the risk of deformation of the pole piece can be reduced in the drying and rolling process of the second pole piece; at the same time, because the limit compaction density of the first active material is less than the limit compaction density of the second active material and the limit compaction density of the third active material, after being rolled, the thickness of the first sub-layer gradually increases and the thickness of the second sub-layer gradually decreases in the direction from the second main body part to the mixed part of the second pole piece, and at least part of the thickness of the multi-layer structure is greater than the thickness of the second main body part, forming the protruding part. That is, the present application can not only guarantee to reduce the risk of deformation of the pole piece by reducing the amount of slurry coated on the edge of the pole piece, but also can avoid the problem that lithium is easily precipitated in the negative electrode due to the large gap between the thinned part and the separator.

[0059] Additional aspects and advantages of the present application will be made apparent from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0060] The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments of the present application and together with the description serve to explain the present application. In the drawings:

[0061] Figures 1 to 5 A sectional view of an electrode assembly in a manufacturing process in the related art;

[0062] Figure 6 A structural schematic diagram of an electrode assembly according to an embodiment of the present application;

[0063] Figure 7 A structural schematic diagram of another electrode assembly according to an embodiment of the present application;

[0064] Figure 8 A flowchart of a manufacturing method of an electrode assembly according to an embodiment of the present application. DETAILED DESCRIPTION

[0065] In order to make the technical scheme and the beneficial effects of the present application more apparent and understandable, the following will be described in detail by combining the drawings and specific examples. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods not specified in the following examples are usually performed according to the conventional experimental conditions. The reagents and raw materials used in the present application are commercially available unless otherwise specified.

[0066] In the following description, numerous specific details are given to provide a thorough understanding of the application. However, it will be apparent to one of ordinary skill in the art that the application can be practiced without one or more of these specific details. In other instances, well-known features are not described in detail to avoid obscuring this application. In addition, it will be apparent to one of ordinary skill in the art that the present application, as described below, can be practiced in the absence of any element or step not specifically disclosed. The term "comprising" encompasses the terms "including", "having" and the like, as well as the term "consisting of".

[0067] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising", when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. As used herein the term "and / or" includes any and all combinations of associated items.

[0068] For a thorough understanding of the application, detailed descriptions will be made in the following description with specific steps and detailed structures, so as to illustrate the technical solutions of the application. The preferred embodiments of the application are described in detail as follows, however, the application can have other implementation manners in addition to these detailed descriptions.

[0069] Unless otherwise defined, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0070] Unless otherwise defined, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0071] For related art, please refer to Figure 5 The electrode assembly includes a first electrode sheet 100, a second electrode sheet 200, and a separator 300 disposed between the first electrode sheet 100 and the second electrode sheet 200; the polarity of the first electrode sheet 100 is opposite to that of the second electrode sheet 200. In the preparation process of the electrode assembly, first, please refer to Figure 1 A first slurry 101 is coated on the surface of the first current collector 110, one edge of the first slurry 101 is thinned, and after drying and rolling, please refer to Figure 2 A first active material layer 120 is formed on the surface of the first current collector 110, the first active material layer 120 includes a first main body part 121 and a first thinning part 1200, and a first electrode sheet 100 is obtained; next, please refer to Figure 3 A second slurry 102 is coated on the surface of the second current collector 210, one edge of the second slurry 102 is thinned, and after drying and rolling, please refer to Figure 4 A second active material layer 220 is formed on the surface of the second current collector 210, the second active material layer 220 includes a second main body part 221 and a second thinning part 2200, and a second electrode sheet 200 is obtained; then, please refer to Figure 5 The first electrode sheet 100, the separator 300, and the second electrode sheet 200 are assembled to obtain an electrode assembly. ByFigure 5 It can be seen that, since the first thinning portion 1200 and the second thinning portion 2200 are both arranged opposite to the separator 300, there is a large gap between the first thinning portion 1200 and the second thining portion 2200 and the separator 300, which affects the transmission of active ions and the potential of the pole piece, causes the negative reference potential of the thinning portion to be lower than that of the normal area during the charging process, increases the transmission distance of active ions, and easily causes lithium precipitation at the corresponding position of the negative pole piece in the later stage of battery cycle. That is, in the related art, in order to reduce the deformation risk of the pole piece during drying and rolling, the amount of slurry coating on the edge of the pole piece is reduced in the slurry coating process, so that the thickness of the edge area of the active material layer is thinned, but this inevitably increases the risk of lithium precipitation of the battery negative electrode, thereby affecting the cycle life of the battery, and even threatening the safety of the battery. Of course, Figure 2 In the related art, the first thinning portion 1200 is located on one side of the first main portion 121 along the width direction of the first current collector 110, and Figure 4 In the related art, the first thinning portion 1200 can be located on both sides of the first main portion 121 along the width direction of the first current collector 110, and the second thinning portion 2200 is also located on both sides of the second main portion 221 along the width direction of the first current collector 110. The first thinning portion 1200 and the second thinning portion 2200 located on both sides have a large gap between them and the separator 300.

[0072] Based on this, the embodiments of the present application provide an electrode assembly, such as Figure 6 and Figure 7As shown, the electrode assembly includes a first electrode tab 100, a second electrode tab 200, and a separator 300 disposed between the first electrode tab 100 and the second electrode tab 200; the first electrode tab 100 and the second electrode tab 200 are opposite in polarity; the first electrode tab 100 includes a first current collector 110 and a first active material layer 120 located on at least one surface of the first current collector 110 in a first direction, the first active material layer 120 includes a first main body part 121 and a thinned part 122, the thinned part 122 is located on at least one side of the first main body part 121 in a second direction; the thickness of the thinned part 122 is less than the thickness of the first main body part 121, and the thickness of the thinned part 122 gradually decreases in the direction from the first main body part 121 to the thinned part 122; the second electrode tab 200 includes a second current collector 210 and a second active material layer 220 located on at least one surface of the second current collector 210 in the first direction, the second active material layer 220 includes a second main body part 221 and a mixed part 222, the mixed part 222 is located on at least one side of the second main body part 221 in the second direction; the mixed part 222 is a multi-layer structure stacked by a first sub-layer 2221 and a second sub-layer 2222 in the first direction, the thickness of the first sub-layer 2221 gradually increases and the thickness of the second sub-layer 2222 gradually decreases in the direction from the second main body part 221 to the mixed part 222; the first sub-layer 2221 includes a first active material, the second sub-layer 2222 includes a second active material, the second main body part 221 includes a third active material, the limit compaction density of the first active material is less than the limit compaction density of the second active material and the limit compaction density of the third active material; the thickness of at least part of the multi-layer structure is greater than the thickness of the second main body part 221, the part of the multi-layer structure whose thickness is greater than the thickness of the second main body part 221 is defined as a protruding part 2201, the thickness of the protruding part 2201 gradually increases in the direction from the second main body part 221 to the mixed part 222; in the first direction, the protruding part 2201 is oppositely disposed with the thinned part 122, and the part of the separator 300 in contact with the protruding part 2201 is bent towards the thinned part 122; the first direction is the thickness direction of the first current collector 110, and the second direction is the width direction of the first current collector 110.

[0073] It should be noted that in the embodiments of the present application, the protruding part 2201 and the thinned part 122 are oppositely disposed, which can be understood as that the protruding part 2201 and the thinned part 122 are both disposed towards the separator 300, and the projections of the protruding part 2201 and the thinned part 122 in the first direction overlap. In addition, Figure 6 and Figure 7 In the above, the shape of the protruding part 2201 is only a schematic, and the way of gradually increasing the thickness of the protruding part 2201 in the direction from the second main body part 221 to the mixed part 222 can be linear increase or non-linear increase, such as curve increase, etc.; as long as it is suitable for the shape of the thinned part 122 in the first electrode tab 100.

[0074] It should also be noted that, Figure 6 and Figure 7 The thinning portion 122 located on one side of the first main body 121 along the second direction, and the mixing portion 222 located on one side of the second main body 221 along the second direction, are merely examples. In some other embodiments of this application, the thinning portion 122 may be located on both sides of the first main body 121 along the second direction, and the mixing portion 222 may also be located on both sides of the second main body 221 along the second direction. Of course, when the thinning portion 122 is located on one side of the first main body 121 along the second direction, and the mixing portion 222 is located on one side of the second main body 221 along the second direction, the thinning portion 122 and the mixing portion 222 need to be disposed on the same side of the electrode assembly in the second direction. In addition, the first active material layer 120 may be located on one or both surfaces of the first current collector 110 along the first direction. The second active material layer 220 may be located on one or both surfaces of the second current collector 210 along the first direction.

[0075] In the embodiments of the present application, one of the first and second pole pieces 100 and 200 is a positive pole piece, and the other is a negative pole piece. In the first pole piece 100, the first active material layer 120 includes a first main body part 121 and a thinned part 122, the thickness of the thinned part 122 is less than the thickness of the first main body part 121, and the thickness of the thinned part 122 gradually decreases in the direction from the first main body part 121 to the thinned part 122; in the second pole piece 200, the second active material layer 220 includes a second main body part 221 and a mixed part 222, the mixed part 222 is a multi-layer structure formed by stacking a first sub-layer 2221 and a second sub-layer 2222 in a first direction, the thickness of at least part of the multi-layer structure is greater than the thickness of the second main body part 221, the part of the multi-layer structure whose thickness is greater than the thickness of the second main body part 221 forms a protruding part 2201, and the thickness of the protruding part 2201 gradually increases in the direction from the second main body part 221 to the mixed part 222; in this way, the protruding part 2201 in the second pole piece 200 is arranged opposite to the thinned part 122 in the first pole piece 100, and after being wound or laminated and rolled in the assembly process of the electrode assembly, the part of the separator 300 in contact with the protruding part 2201 is bent towards the thinned part 122 under the action of the protruding part 2201, and since the shape of the protruding part 2201 is adapted to the shape of the thinned part 122, the gap between the thinned part 122 and the separator 300 is significantly reduced, the active ion transmission distance can be shortened, the problem that lithium is easily precipitated in the negative electrode due to the gap between the thinned part 122 and the separator 300 is effectively improved, and thus the cycle life and safety performance of the battery can be improved. In addition, in the actual preparation process of the second pole piece 200, the slurry corresponding to the second main body part 221 can be coated at a normal coating thickness, the slurry corresponding to the mixed part 222 can be coated in a way of being thinned relative to the normal coating thickness, and in the direction from the second main body part 221 to the mixed part 222, the coating thickness of the slurry corresponding to the first sub-layer 2221 can be controlled to gradually increase, and the coating thickness of the slurry corresponding to the second sub-layer 2222 can be controlled to gradually decrease; in this way, since the slurry corresponding to the mixed area is coated in a way of being thinned, the risk of deformation of the pole piece can be reduced in the drying and rolling process of the preparation of the second pole piece 200; at the same time, since the limit compaction density of the first active material is less than the limit compaction densities of the second and third active materials, after being rolled, the thickness of the first sub-layer 2221 gradually increases, and the thickness of the second sub-layer 2222 gradually decreases in the direction from the second main body part 221 to the mixed part 222 in the second pole piece 200, and at least part of the multi-layer structure can have a thickness greater than the thickness of the second main body part 221, forming the protruding part 2201. That is, the present application can not only reduce the risk of deformation of the pole piece by reducing the amount of slurry coated on the edge of the pole piece, but also avoid the problem that lithium is easily precipitated in the negative electrode due to the large gap between the thinned part 122 and the separator 300.

[0076] In some embodiments, in the first direction, the sum of the size of the gap between the mixing portion 222 and the separator 300 and the size of the gap between the thinning portion 122 and the separator 300 is less than or equal to 10 μm.

[0077] In the electrode assembly of the related art, the active material layer in the positive electrode tab and the negative electrode tab each has a thinning region, hereinafter referred to as a positive electrode thinning region and a negative electrode thinning region, respectively. In the first direction, the sum of the size of the gap between the positive electrode thinning region and the separator and the size of the gap between the negative electrode thinning region and the separator is usually greater than 10 μm, and can be as high as 70 μm. This results in a long lithium ion transport distance, and the negative electrode thinning region is prone to lithium precipitation after long-term cycling of the battery, thereby affecting the cycle life and safety of the battery. In the embodiments of the present application, by oppositely arranging the protruding portion 2201 in the second electrode tab 200 and the thinning portion 122 in the first electrode tab 100 in the first direction, the portion of the separator 300 in contact with the protruding portion 2201 is bent towards the thinning portion 122 under the action of the protruding portion 2201, and the protruding portion 2201 is adapted to the shape of the thinning portion 122, so that the sum of the size of the gap between the mixing portion 222 and the separator 300 and the size of the gap between the thinning portion 122 and the separator 300 is reduced to less than or equal to 10 μm. In this way, the active ion transport distance is significantly shortened, and the problem of lithium precipitation in the negative electrode caused by the gap between the thinning portion 122 and the separator 300 is better improved.

[0078] In some embodiments, in the first direction, the projection of the protruding portion 2201 can be located within the projection of the thinning portion 122. In this way, the effect of reducing the gap between the thinning portion 122 and the separator 300 by using the protruding portion 2201 can be fully played, thereby better improving the problem of lithium precipitation in the negative electrode caused by the gap between the thinning portion 122 and the separator 300.

[0079] Further, in the first direction, the edge of the protruding portion 2201 close to the second main body portion 221 and the edge of the thinning portion 122 close to the first main body portion 121 can coincide. In this way, the protruding portion 2201 and the thinning portion 122 can be better matched, thereby facilitating further reduction of the gap between the thinning portion 122 and the separator 300.

[0080] In some embodiments, please refer to Figure 6 and Figure 7 The mixing portion 222 further includes a flush portion 2202 located between the protruding portion 2201 and the second main body portion 221, and the thickness of the flush portion 2202 is the same as the thickness of the second main body portion 221.

[0081] In the actual preparation process of the second tab 200, the mixed part 222 can be coated in a way of thinning relative to the normal coating thickness in the embodiment of the present application, so as to reduce the risk of tab deformation in the drying and rolling process of preparing the second tab 200. At the same time, through the difference in the limit compaction density of the first active material and the second and third active materials, the thickness of the second main part 221 can be less than or equal to the thickness of the mixed part 222 after rolling, and the flush part 2202 is located between the protruding part 2201 and the second main part 221, so that the second tab 200 and the separator 300 can be better fitted, and the gap between the second tab 200 and the separator 300 caused by the thickness of the mixed area being less than the thickness of the second main part 221 can be avoided.

[0082] In some embodiments, the size of the mixed part 222 in the second direction is greater than the size of the protruding part 2201 in the first direction. In this way, it is beneficial to ensure that the mixed area realizes a smooth protruding transition compared to the second main part 221, and it is beneficial to make the shape of the protruding part 2201 better match the shape of the thinned part 122.

[0083] In some specific embodiments, the size of the mixed part 222 in the second direction can be 3mm-20mm, for example, it can be 3mm, 5mm, 7mm, 9mm, 10mm, 12mm, 14mm, 16mm, 18mm, 20mm or any value within any two of the above value ranges. In this way, it is beneficial to thin the coating of the mixed part 222 corresponding slurry, while avoiding the problem of capacity reduction of the tab caused by the mixed part 222 being too wide, that is, the ease of operation of the actual preparation process and the energy density of the electrode assembly can be considered.

[0084] When the size of the protruding part 2201 in the second direction is too large, the thickness of the part of the second active material layer 220 corresponding to the first main part 121 can be too large, which is not conducive to the close fitting between the tab and the separator 300; when the size of the protruding part 2201 in the second direction is too small, the protruding angle of the protruding part 2201 is too steep, which can not adapt to the shape of the thinned part 122 well, so as to not reduce the gap between the thinned part 122 and the separator 300 well. Therefore, in some specific embodiments, the size difference between the protruding part 2201 and the second main part 221 in the first direction is less than the size value of the protruding part 2201 in the second direction. In this way, it is beneficial to make the shape of the protruding part 2201 better adapt to the thinned part 122, so as to better reduce the gap between the thinned part 122 and the separator 300, and thus better improve the problem of negative electrode lithium precipitation.

[0085] In some embodiments, the maximum difference between the sizes of the first body portion 121 and the thinned portion 122 in the first direction is greater than or equal to the maximum difference between the sizes of the protruding portion 2201 and the second body portion 221 in the first direction. In this way, the matching degree of the protruding portion 2201 and the thinned portion 122 can be ensured, and the problem that the close fit between the positive and negative electrode sheets and the separator 300 is affected due to the excessively large size difference between the protruding portion 2201 and the second body portion 221 in the first direction can be avoided.

[0086] In some embodiments, in the first direction, the sum of the thicknesses of the corresponding thinned portion 122 and the protruding portion 2201 is equal to the sum of the thicknesses of the first body portion 121 and the second body portion 221. In this way, the gradually increasing thickness of the protruding portion 2201 can be matched with the gradually decreasing thickness of the thinned portion 122, so that the gap between the thinned portion 122 and the separator 300 can be better reduced, and the good fit effect between the positive and negative electrode sheets and the separator 300 can be ensured.

[0087] It can be understood that when the size of the thinned portion 122 in the second direction is too large or the thinning amount is too large, the capacity of the first electrode sheet 100 will be reduced; and when the size of the thinned portion 122 in the second direction is too small or the thinning amount is too small, the process implementation will be difficult. Therefore, in some embodiments, the size of the thinned portion 122 in the second direction can be 0.5 mm to 10 mm, for example, it can be 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, or any value within any two of the above value ranges, and the maximum difference between the sizes of the first body portion 121 and the thinned portion 122 in the first direction can be 0.5 μm to 20 μm, for example, it can be 0.5 μm, 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, or any value within any two of the above value ranges. The qualified thinning amount can be selected within the above range according to the design of the electrode sheet.

[0088] In some embodiments, referring to Figure 6 and Figure 7 the first sub-layer 2221 can be located between the second current collector 210 and the second sub-layer 2222.

[0089] In the actual preparation process of the second tab 200, the lower layer material is subjected to a greater pressure during the rolling process. In the embodiment of the present application, the first sub-layer 2221 is arranged below the second sub-layer 2222. Since the first active material in the first sub-layer 2221 has a smaller ultimate compaction density than the second active material in the second sub-layer 2222, the thickness of the active material layer corresponding to the mixing portion 222 is more easily controlled to decrease by a smaller amount than the thickness of the active material layer corresponding to the second main portion 221 during compaction, so as to form the protruding portion 2201.

[0090] In some embodiments, the second active material and the third active material can be the same. In this way, on the one hand, in the actual preparation process, the second sub-layer 2222 and the second main portion 221 can adopt the same slurry and can be coated in one step, simplifying the process; on the other hand, the second sub-layer 2222 and the second main portion 221 are integrated structures, which is beneficial to improve the electrochemical performance of the second active material layer 220.

[0091] In some embodiments, please refer to Figure 6 , the first tab 100 is a negative tab, and the second tab 200 is a positive tab.

[0092] Exemplarily, the first active material can be lithium iron phosphate and / or lithium manganese iron phosphate; the ultimate compaction density of the first active material can be 2.3 g / cm 3 ~2.8 g / cm 3 , for example, can be 2.3 g / cm 3 , 2.4 g / cm 3 , 2.5 g / cm 3 , 2.6 g / cm 3 , 2.7 g / cm 3 , 2.8 g / cm 3 , or any value between any two of the above value ranges. Generally, the lithium iron phosphate material will be subjected to nanocrystallization treatment, carbon coating treatment, etc., and therefore, the ultimate compaction density of the lithium iron phosphate material is generally between 2.3 g / cm 3 ~2.8 g / cm 3 ; the ultimate compaction density of the lithium manganese iron phosphate is generally between 2.3 g / cm 3 ~2.6 g / cm 3 .

[0093] Exemplarily, the second active material can be a ternary material; the ultimate compaction density of the second active material can be 3.3 g / cm 3 ~3.6 g / cm 3 , for example, can be 3.3 g / cm 3 , 3.4 g / cm3 3.5g / cm 3 3.6g / cm 3 Or any value between any two of the above ranges.

[0094] For example, the third active material can be a ternary material; the ultimate compaction density of the third active material can be 3.3 g / cm³. 3 ~3.6g / cm 3 For example, it can be 3.3 g / cm³. 3 3.4g / cm 3 3.5g / cm 3 3.6g / cm 3 Or any value between any two of the above ranges.

[0095] The true density of ternary materials is higher than that of lithium iron phosphate and lithium manganese iron phosphate. Therefore, the ultimate compaction density of ternary materials can be higher than that of lithium iron phosphate and lithium manganese iron phosphate. In this way, by utilizing the difference in ultimate compaction density between ternary materials and lithium iron phosphate and lithium manganese iron phosphate, the thickness difference distribution between the second main body 221 and the mixing zone after roll pressing can be achieved. Furthermore, by using the gradient design of the thickness of the slurry coating corresponding to the first sub-layer 2221 and the second sub-layer 2222, a protrusion 2201 with gradually changing thickness can be obtained.

[0096] In some specific embodiments, the ternary materials in the second and third active materials can be the same. In this way, on the one hand, in the actual preparation process, the second sublayer 2222 and the second main body 221 can use the same slurry and can be coated in one step, simplifying the process; on the other hand, the formed second sublayer 2222 and the second main body 221 are an integral structure, which is beneficial to improving the electrochemical performance of the second active material layer 220.

[0097] In some specific embodiments, the first sublayer 2221 may be located between the second current collector 210 and the second sublayer 2222. This facilitates, on the one hand, controlling the thickness reduction of the active material layer corresponding to the mixing portion 222 during compaction to be less than the thickness reduction of the active material layer corresponding to the second main body portion 221 during compaction, thus forming the protrusion 2201; on the other hand, the second sublayer 2222 containing ternary material located on the electrode surface can enhance the electrochemical kinetics at the thinned portion 122, thereby better preventing lithium plating at the negative electrode thinned portion 122.

[0098] In some specific embodiments, the gram capacity of the first active material is less than the gram capacity of the third active material, and the gram capacity of the second active material is less than or equal to the gram capacity of the third active material. In this way, the CB value (the ratio of the unit area negative electrode capacity to the unit area positive electrode capacity) at the thinned portion 122 is increased, thereby further avoiding lithium precipitation at the negative electrode thinned portion 122, ensuring the later cycle of the battery, and improving the cycle life and safety of the battery.

[0099] When the first tab 100 is a negative electrode tab and the second tab 200 is a positive electrode tab, the size of the mixed portion 222 in the second direction can be 3 mm to 20 mm, for example, 3 mm, 5 mm, 7 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, 20 mm, or any value within a range between any two of the above values; the size of the thinned portion 122 in the second direction can be 0.5 mm to 10 mm, for example, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, or any value within a range between any two of the above values; and the maximum difference in the size of the second main portion 221 and the thinned portion 122 in the first direction can be 0.5 μm to 20 μm, for example, 0.5 μm, 5 μm, 10 μm, 15 μm, 20 μm, or any value within a range between any two of the above values.

[0100] When the second tab 200 is a positive electrode tab, please refer to Figure 6 The second tab 200 can further include an insulating layer 400 on one side of the mixed portion 222 in the second direction. The insulating layer 400 can provide better insulation and prevent burrs, for example, to prevent short circuits between the positive and negative electrodes caused by burrs on the positive or negative electrode during winding.

[0101] In some embodiments, please refer to Figure 7 The first tab 100 is a positive electrode tab, and the second tab 200 is a negative electrode tab.

[0102] For example, the first active material can be a first graphite; the limit compaction density of the first active material can be 1.55 g / cm 3 ~1.7 g / cm 3 , for example, 1.55 g / cm 3 , 1.6 g / cm 3 , 1.65 g / cm 3 , 1.7 g / cm 3 , or any value within a range between any two of the above values.

[0103] The second active material can be a second graphite; the limit compaction density of the second active material can be 1.6 g / cm 3 ~1.7 g / cm3 For example, it can be 1.6 g / cm 3 1.65 g / cm 3 1.7 g / cm 3 or any value between any two of the above value ranges.

[0104] The third active material can be a third graphite; the limit compaction density of the third active material can be 1.6 g / cm 3 1.7 g / cm 3 For example, it can be 1.6 g / cm 3 1.65 g / cm 3 1.7 g / cm 3 or any value between any two of the above value ranges.

[0105] The limit compaction density of the first graphite is less than the limit compaction density of the second graphite and the limit compaction density of the third graphite, so that, by using the difference between the limit compaction densities of the first graphite and the second graphite, the thickness difference distribution of the second main body part 221 and the mixing area after rolling can be achieved, and by the gradual design of the corresponding slurry coating thicknesses of the first sub-layer 2221 and the second sub-layer 2222, the convex part 2201 with gradually changing thickness can be obtained.

[0106] In some specific embodiments, the second graphite and the third graphite can be the same. In this way, on the one hand, in the actual preparation process, the same slurry can be used for the second sub-layer 2222 and the second main body part 221, and the coating can be performed in one step, simplifying the process; on the other hand, the second sub-layer 2222 and the second main body part 221 are integrated structures, which is beneficial to improve the electrochemical performance of the second active material layer 220.

[0107] In some specific embodiments, the first sub-layer 2221 can be located between the second current collector 210 and the second sub-layer 2222. In this way, on the one hand, it is convenient to control the thickness reduction of the corresponding active material layer of the mixing part 222 in compaction to be less than the thickness reduction of the corresponding active material layer of the second main body part 221 in compaction, so as to form the convex part 2201; on the other hand, the second sub-layer 2222 containing the second graphite is located at the electrochemical kinetics of the cut-thin part 122 on the surface layer of the pole piece, thereby better avoiding lithium precipitation of the negative electrode.

[0108] In some specific embodiments, the gram capacity of the first active material is greater than the gram capacity of the third active material, and the gram capacity of the second active material is greater than or equal to the gram capacity of the third active material. In this way, it is beneficial to increase the CB value at the cut-thin part 122, thereby further avoiding lithium precipitation of the negative electrode, ensuring the later cycle of the battery, and improving the cycle life and safety of the battery.

[0109] When the first tab 100 is a positive tab and the second tab 200 is a negative tab, the size of the mixing portion 222 in the second direction can be 0.5mm-10mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm or any value between any two of the above ranges; the size of the thinning portion 122 in the second direction can be 3mm-10mm, for example, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm or any value between any two of the above ranges; the maximum difference in the size of the second main portion 221 and the thinning portion 122 in the first direction can be 5pm-30pm, for example, 5pm, 10pm, 15pm, 20pm, 25pm, 30pm or any value between any two of the above ranges.

[0110] When the first tab 100 is a positive tab, please refer to Figure 7 The first tab 100 can also include an insulating layer 400 on one side of the thinning portion 122 in the second direction. By providing the insulating layer 400, better insulation and burr prevention can be achieved, for example, to prevent short circuit caused by burrs on the positive or negative tab during winding.

[0111] The present application also provides a method for preparing an electrode assembly, please refer to Figure 8 The method for preparing an electrode assembly provided by the present application comprises the following steps:

[0112] S1: using a coating process to form a first active material layer on at least one surface of the first current collector in the first direction, to obtain a first tab; the first active material layer includes a first main portion and a thinning portion, the thinning portion is located on at least one side of the first main portion in the second direction; the thickness of the thinning portion is less than the thickness of the first main portion, and the thickness of the thinning portion gradually decreases from the first main portion to the thinning portion;

[0113] S2: forming a second active material layer on at least one surface of the second current collector in the first direction by a coating process to obtain a second pole piece; the second active material layer comprises a second main body part and a mixed part, the mixed part is located on at least one side of the second main body part in the second direction; the mixed part is a multi-layer structure stacked by the first sub-layer and the second sub-layer in the first direction, the thickness of the first sub-layer gradually increases and the thickness of the second sub-layer gradually decreases in the direction from the second main body part to the mixed part; the first sub-layer comprises a first active material, the second sub-layer comprises a second active material, the second main body part comprises a third active material, the limit compaction density of the first active material is less than the limit compaction density of the second active material and the limit compaction density of the third active material; the thickness of at least part of the multi-layer structure is greater than the thickness of the second main body part, the part of the multi-layer structure whose thickness is greater than the thickness of the second main body part is defined as a protruding part, the thickness of the protruding part gradually increases in the direction from the second main body part to the mixed part;

[0114] S3: placing a separator between the first pole piece and the second pole piece, the first active material layer and the second active material layer are arranged towards the separator, after stacking or winding, performing a pressing treatment to obtain an electrode assembly; in the first direction, the protruding part is arranged opposite to the thinned part, the part of the separator in contact with the protruding part is bent towards the thinned part; the polarity of the first pole piece and the second pole piece is opposite; the first direction is the thickness direction of the first current collector, and the second direction is the width direction of the first current collector.

[0115] In the embodiments of the present application, in the prepared first pole piece, the first active material layer comprises a first main body part and a thinned part, the thickness of the thinned part is less than the thickness of the first main body part, and the thickness of the thinned part gradually decreases in the direction from the first main body part to the thinned part; in the prepared second pole piece, the second active material layer comprises a second main body part and a mixed part, the mixed part is a multi-layer structure stacked by the first sub-layer and the second sub-layer in the first direction, the thickness of at least part of the multi-layer structure is greater than the thickness of the second main body part, the part of the multi-layer structure whose thickness is greater than the thickness of the second main body part forms a protruding part, and the thickness of the protruding part gradually increases in the direction from the second main body part to the mixed part; in this way, the protruding part in the second pole piece is arranged opposite to the thinned part in the first pole piece, and in the electrode assembly prepared after winding or stacking and rolling, the part of the separator in contact with the protruding part is bent towards the thinned part under the action of the protruding part, and since the shape of the protruding part is adapted to the shape of the thinned part, the gap between the thinned part and the separator is significantly reduced, the active ion transmission distance can be shortened, the problem that lithium is easily precipitated in the negative electrode due to the gap between the thinned part and the separator is effectively improved, and thus the cycle life and safety performance of the battery can be improved.

[0116] It should be understood that although each step in the above flowchart is shown in sequence according to the direction of the arrow, these steps are not necessarily executed in the order indicated by the arrow. Moreover, at least part of the steps in the above flowchart can include multiple steps or multiple stages, which are not necessarily executed at the same time or in sequence.

[0117] In step S1, a coating process is used to form a first active material layer on at least one surface of the first current collector in the first direction, thereby obtaining a first electrode sheet.

[0118] In an actual preparation process, the thickness of the slurry coated on the edge region of the first current collector can be thinner than the thickness of the slurry coated on the middle region of the first current collector. After drying (e.g., baking at 70-140°C) and rolling, the first electrode sheet is obtained, and the edge is thinned by coating, which can reduce the risk of deformation of the electrode sheet in the baking and rolling process of the electrode sheet preparation. Accordingly, the first active material layer formed includes a first main body portion and a thinned portion located on at least one side of the first main body portion in the second direction; the thickness of the thinned portion is less than the thickness of the first main body portion, and the thickness of the thinned portion gradually decreases in the direction from the first main body portion to the thinned portion.

[0119] In step S2, a coating process is used to form a second active material layer on at least one surface of the second current collector in the first direction, thereby obtaining a second electrode sheet. Specifically, the coating process for forming the second active material layer includes the following steps:

[0120] S21: preparing a first slurry, a second slurry, and a third slurry, the first slurry including a first active material, the second slurry including a second active material, and the third slurry including a third active material;

[0121] S22: laminating and coating the first slurry and the second slurry on a mixing region on the surface of the second current collector, the mixing region being located on at least one side of the surface of the second current collector in the second direction, in the second direction, from the edge of the second current collector to the inside, the thickness of the mixed slurry composed of the first slurry and the second slurry gradually increases, and the thickness of the first slurry gradually decreases, and the thickness of the second slurry gradually increases; coating the third slurry on other regions adjacent to the mixing region on the surface of the second current collector, the coating thickness of the third slurry being greater than or equal to the maximum thickness of the mixed slurry, after drying, rolling is performed to form a first sub-layer, a second sub-layer, and a second main body portion.

[0122] In the actual preparation process of the second pole piece, the third slurry is coated at a normal coating thickness, and the first slurry and the second slurry can be coated in a manner of being thinned relative to the normal coating thickness, and in the second direction, from the edge of the second current collector to the inside, the thickness of the mixed slurry composed of the first slurry and the second slurry gradually increases, and the thickness of the first slurry gradually decreases, and the thickness of the second slurry gradually increases. In this way, since the mixed slurry is coated in a manner of being thinned, the risk of deformation of the pole piece can be reduced in the drying and rolling process of the preparation of the second pole piece. At the same time, since the limit compaction density of the first active material is less than the limit compaction density of the second active material and the limit compaction density of the third active material, after rolling, in the second direction, from the edge of the second current collector to the inside, the thickness of the first sub-layer gradually decreases, the thickness of the second sub-layer gradually increases, and the thickness of at least part of the mixed part is greater than the thickness of the second main part, forming the protruding part. By oppositely arranging the protruding part and the thinned part, the part of the separator in contact with the protruding part is bent towards the thinned part under the action of the protruding part. Since the shapes of the protruding part and the thinned part are adapted to each other, the gap between the thinned part and the separator is significantly reduced, the active ion transmission distance is shortened, the problem of lithium precipitation in the negative electrode caused by the gap between the thinned part and the separator is effectively improved, and the cycle life and safety performance of the battery are improved. The application adopts the combination of active materials with different limit compaction densities to prepare an electrode assembly with a thinned part that can also be well attached to the separator, and the gap between the thinned part and the separator is significantly reduced or even the same as the normal coating area. That is, the application can not only reduce the risk of pole piece deformation by reducing the amount of slurry coated at the edge of the pole piece, but also avoid the problem of lithium precipitation in the negative electrode caused by the large gap between the thinned part and the separator.

[0123] In step S21, the preparation of the first slurry, the second slurry and the third slurry can all adopt processes known to those skilled in the art, which will not be described here. In addition, the first slurry, the second slurry and the third slurry usually also include a conductive agent and a binder. The types of conductive agent and binder, and the ratio of conductive agent, binder and active material can be set according to actual needs, which are not limited here.

[0124] In step S22, the first slurry, the second slurry and the third slurry can be coated and dried respectively, or they can be coated simultaneously using a multi-head die, and then dried together.

[0125] In step S22, the drying temperature can be 80-150°C, for example, it can be 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C or any value within any two of the above value ranges.

[0126] In step S22, the rolling can be arc-shaped rolling. In this way, the mixed slurry coating area and the third slurry coating area can be rolled in a region-by-region manner, and the limit compaction density differences of the first active material, the second active material and the third active material are utilized to ensure that the materials in each region reach the preset compaction density, form the second main body part and the mixed part, and make at least part of the thickness of the mixed part greater than the thickness of the second main body part. Specifically, the arc height of the arc-shaped rolling can be, for example, 0.02mm-0.05mm, because if the arc height is too high, the thinned coating area can not easily reach the designed compaction density, and if the arc height is too small, the designed compaction density of the pole piece can be easily exceeded, and overpressure can cause the active material to be difficult to delithiate. The rolling pressure of the arc-shaped rolling can be, for example, 30t-200t, which can be adjusted according to the size and performance of the equipment. Of course, the application also does not exclude the use of flat rolling. When flat rolling is used, region-by-region rolling can be performed, and different regions are provided with appropriate rolling parameters, thereby forming the above-mentioned second main body part and mixed part.

[0127] The pressing treatment in step S3 can be hot pressing treatment or cold pressing treatment. The structure of the electrode assembly prepared in step S3 can refer to Figure 6 and Figure 7 .

[0128] In some embodiments, in the first direction, the sum of the size of the gap between the mixed part 222 and the separator 300 and the size of the gap between the thinned part 122 and the separator 300 is less than or equal to 10μm. In this way, the active ion transmission distance is significantly shortened, and the problem that lithium precipitation easily occurs in the negative electrode due to the gap between the thinned part 122 and the separator 300 is better improved.

[0129] In some embodiments, in the first direction, the projection of the convex part 2201 can be located within the projection of the thinned part 122. In this way, the effect of reducing the gap between the thinned part 122 and the separator 300 by using the convex part 2201 can be fully played, thereby better improving the problem that lithium precipitation easily occurs in the negative electrode due to the gap between the thinned part 122 and the separator 300.

[0130] In some embodiments, please refer to Figure 6 and Figure 7 , the mixed part 222 further comprises a flush part 2202 located between the convex part 2201 and the second main body part 221, and the thickness of the flush part 2202 is the same as the thickness of the second main body part 221. In this way, the fit of the second pole piece 200 and the separator 300 can be better, and the situation that a gap occurs between the second pole piece 200 and the separator 300 due to the fact that the thickness of the mixed area is less than the thickness of the second main body part 221 can be avoided.

[0131] In some embodiments, the size of the mixing portion 222 in the second direction is greater than the size of the protruding portion 2201 in the first direction. In this way, it is beneficial to ensure that the mixing area achieves a smooth protruding transition compared to the second main body portion 221, and it is beneficial to make the shape of the protruding portion 2201 better match the shape of the thinned portion 122.

[0132] In some embodiments, the difference between the size of the protruding portion 2201 and the second main body portion 221 in the first direction is less than the size of the protruding portion 2201 in the second direction. In this way, it is beneficial to make the shape of the protruding portion 2201 better adapt to the thinned portion 122, thereby better reducing the gap between the thinned portion 122 and the separator 300, and thereby better improving the problem of lithium precipitation in the negative electrode.

[0133] In some embodiments, the maximum difference between the size of the first main body portion 121 and the thinned portion 122 in the first direction is greater than or equal to the maximum difference between the size of the protruding portion 2201 and the second main body portion 221 in the first direction. In this way, it is beneficial to ensure the matching degree of the protruding portion 2201 and the thinned portion 122, and to avoid the problem of affecting the close fit between the positive and negative electrode sheets and the separator 300 due to the size difference between the protruding portion 2201 and the second main body portion 221 in the first direction being too large.

[0134] In some embodiments, in the first direction, the sum of the thicknesses of the corresponding thinned portion 122 and the protruding portion 2201 is equal to the sum of the thicknesses of the first main body portion 121 and the second main body portion 221. In this way, the gradually increasing thickness of the protruding portion 2201 can be matched with the gradually decreasing thickness of the thinned portion 122, thereby better reducing the gap between the thinned portion 122 and the separator 300, and ensuring good fit between the positive and negative electrode sheets and the separator 300.

[0135] In some embodiments, please refer to Figure 6 and Figure 7 The first sub-layer 2221 can be located between the second current collector 210 and the second sub-layer 2222. Since the limit compaction density of the first active material in the first sub-layer 2221 is smaller than the limit compaction density of the second active material in the second sub-layer 2222, it is more convenient to control the thickness reduction of the active material layer corresponding to the mixing portion 222 in compaction to be less than the thickness reduction of the active material layer corresponding to the second main body portion 221 in compaction, so as to form the protruding portion 2201.

[0136] In some embodiments, the second active material and the third active material can be the same. In this way, on the one hand, in the actual preparation process, the second slurry 102 and the third slurry can adopt the same slurry, and can be coated in one step process, simplifying the process; on the other hand, the second sub-layer 2222 and the second main part 221 are integrated structures, which is beneficial to improve the electrochemical performance of the second active material layer 220.

[0137] In some embodiments, please refer to Figure 6 , the first electrode sheet 100 is a negative electrode sheet, and the second electrode sheet 200 is a positive electrode sheet.

[0138] Exemplarily, the first active material can be lithium iron phosphate and / or lithium iron manganese phosphate; the limit compaction density of the first active material can be 2.3g / cm 3 ~2.8g / cm 3 , for example, can be 2.3g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 , 2.8g / cm 3 or any value between any two of the above numerical ranges.

[0139] Exemplarily, the second active material can be a ternary material; the limit compaction density of the second active material can be 3.3g / cm 3 ~3.6g / cm 3 , for example, can be 3.3g / cm 3 , 3.4g / cm 3 , 3.5g / cm 3 , 3.6g / cm 3 or any value between any two of the above numerical ranges.

[0140] Exemplarily, the third active material can be a ternary material; the limit compaction density of the third active material can be 3.3g / cm 3 ~3.6g / cm 3 , for example, can be 3.3g / cm 3 , 3.4g / cm 3 , 3.5g / cm 3 , 3.6g / cm 3 or any value between any two of the above numerical ranges.

[0141] In the embodiments of the present application, the difference in the limit compaction density of ternary materials, lithium iron phosphate and lithium manganese iron phosphate can realize the thickness difference distribution of the second main body part 221 and the mixing area after rolling. The gradual design of the corresponding slurry coating thickness of the first sub-layer 2221 and the second sub-layer 2222 can obtain the convex part 2201 with gradually changing thickness. Moreover, ternary materials, lithium iron phosphate and lithium manganese iron phosphate are all commonly used positive active materials, without the need to increase special materials, which is convenient for mass production.

[0142] In some specific embodiments, the ternary material in the second active material and the third active material can be the same. In this way, on the one hand, in the actual preparation process, the second slurry 102 and the third slurry can use the same slurry, and can be coated in one step process, simplifying the process; on the other hand, the second sub-layer 2222 and the second main body part 221 are integrated structures, which is beneficial to improve the electrochemical performance of the second active material layer 220.

[0143] In some specific embodiments, the first sub-layer 2221 can be located between the second current collector 210 and the second sub-layer 2222. In this way, on the one hand, it is convenient to control the thickness reduction of the active material layer corresponding to the mixing part 222 in compaction to be less than the thickness reduction of the active material layer corresponding to the second main body part 221 in compaction, so as to form the convex part 2201; on the other hand, the second sub-layer 2222 containing ternary materials is located at the surface layer of the pole piece, which can improve the electrochemical kinetics at the thinning part 122, thereby better avoiding lithium precipitation at the negative thinning part 122.

[0144] In some specific embodiments, the gram capacity of the first active material is less than the gram capacity of the third active material, and the gram capacity of the second active material is less than or equal to the gram capacity of the third active material. In this way, it is beneficial to increase the CB value at the thinning part 122, thereby further avoiding lithium precipitation at the negative thinning part 122, ensuring the later cycle of the battery, and improving the cycle life and safety of the battery.

[0145] In the case that the first tab 100 is a negative tab and the second tab 200 is a positive tab, the size of the mixed portion 222 in the second direction can be 3mm-20mm, for example, 3mm, 5mm, 7mm, 10mm, 12mm, 14mm, 16mm, 18mm, 20mm or any value between any two of the above-mentioned values; the size of the thinned portion 122 in the second direction can be 0.5mm-10mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm or any value between any two of the above-mentioned values; the maximum difference between the sizes of the second main portion 221 and the thinned portion 122 in the first direction can be 0.5μm-20μm, for example, 0.5μm, 5μm, 10μm, 15μm, 20μm or any value between any two of the above-mentioned values.

[0146] In some embodiments, please refer to Figure 7 , the first tab 100 is a positive tab and the second tab 200 is a negative tab.

[0147] For example, the first active material can be a first graphite; the limit compaction density of the first active material can be 1.55g / cm 3 -1.7g / cm 3 , for example, 1.55g / cm 3 , 1.6g / cm 3 , 1.65g / cm 3 , 1.7g / cm 3 or any value between any two of the above-mentioned values.

[0148] The second active material can be a second graphite; the limit compaction density of the second active material can be 1.6g / cm 3 -1.7g / cm 3 , for example, 1.6g / cm 3 , 1.65g / cm 3 , 1.7g / cm 3 or any value between any two of the above-mentioned values.

[0149] The third active material can be a third graphite; the limit compaction density of the third active material can be 1.6g / cm 3 -1.7g / cm 3 , for example, 1.6g / cm 3 , 1.65g / cm 3 , 1.7g / cm 3 or any value between any two of the above-mentioned values.

[0150] The limit compaction density of the first graphite is less than the limit compaction density of the second graphite and the limit compaction density of the third graphite, so that the thickness difference distribution of the second main body part 221 and the mixing area after rolling can be realized by using the limit compaction density difference of the first graphite and the second graphite, and the convex part 2201 with gradually changing thickness can be obtained by the gradual design of the corresponding slurry coating thickness of the first sub-layer 2221 and the second sub-layer 2222. Moreover, graphite is a commonly used positive active material, and no special material needs to be added, which is convenient for mass production.

[0151] In some specific embodiments, the second graphite and the third graphite can be the same. In this way, on the one hand, in the actual preparation process, the second slurry 102 and the third slurry can use the same slurry, and can be coated in one step, simplifying the process; on the other hand, the second sub-layer 2222 and the second main body part 221 are integrated structures, which is beneficial to improve the electrochemical performance of the second active material layer 220.

[0152] In some specific embodiments, the first sub-layer 2221 can be located between the second current collector 210 and the second sub-layer 2222. In this way, on the one hand, it is convenient to control the thickness reduction of the corresponding active material layer of the mixing part 222 in compaction to be less than the thickness reduction of the corresponding active material layer of the second main body part 221 in compaction, so as to form the convex part 2201; on the other hand, the second sub-layer 2222 containing the second graphite is located at the surface layer of the pole piece, which can improve the electrochemical kinetics at the thinning part 122, thereby better avoiding lithium precipitation of the negative electrode.

[0153] In some specific embodiments, the gram capacity of the first active material is greater than the gram capacity of the third active material, and the gram capacity of the second active material is greater than or equal to the gram capacity of the third active material. In this way, it is beneficial to increase the CB value at the thinning part 122, thereby further avoiding lithium precipitation of the negative electrode, ensuring the later cycle of the battery, and improving the cycle life and safety of the battery.

[0154] When the first pole piece 100 is a positive pole piece and the second pole piece 200 is a negative pole piece, the size of the mixing part 222 in the second direction can be 0.5mm-10mm, for example, it can be 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm or any value within the range between any two of the above values; the size of the thinning part 122 in the second direction can be 3mm-10mm, for example, it can be 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm or any value within the range between any two of the above values; the maximum difference of the size of the second main body part 221 and the thinning part 122 in the first direction can be 5μm-30μm, for example, it can be 5μm, 10μm, 15μm, 20μm, 25μm, 30μm or any value within the range between any two of the above values.

[0155] The application also provides a battery comprising the electrode assembly of any one of the preceding embodiments or the electrode assembly prepared by the preparation method of any one of the preceding embodiments.

[0156] The battery in the embodiments of the application can be a winding type battery or a laminated type battery. It can be understood that the beneficial effects of the electrode assembly of any one of the preceding embodiments are applicable to the battery.

[0157] The technical solutions of the application will be further described below in combination with multiple embodiments and comparative examples.

[0158] Embodiment 1

[0159] The preparation method of the electrode assembly in this embodiment comprises the following steps:

[0160] Step S101: The positive active material 1 (corresponding to the first active material) adopts lithium iron phosphate (LiFePO4) material, and the ultimate compaction density is 2.5 g / cm 3 . According to the mass ratio of the positive active material 1, the conductive agent and the binder being 97.5:1.5:1, the homogenate of the positive active material 1 is completed, and the first slurry is obtained. The positive active material 2 (corresponding to the second active material and the third active material) adopts ternary material LiNi 0.6 Co 0.1 Mn 0.3 O2, and the ultimate compaction density is 3.4 g / cm 3According to the mass ratio of positive electrode active material 2, conductive agent, and binder of 97.5:1.5:1, the positive electrode active material 2 is homogenized to obtain the second slurry (i.e., the third slurry). The first slurry is coated on the mixing region on the surface of the positive electrode current collector (corresponding to the second current collector) (the mixing region is located on one side of the surface of the positive electrode current collector along the second direction). The second slurry is coated on other areas of the surface of the positive electrode current collector adjacent to the mixing region and on the first slurry. From the edge of the second current collector inward, the thickness of the mixed slurry composed of the first and second slurries gradually increases, and the thickness of the first slurry gradually decreases while the thickness of the second slurry gradually increases. The coating thickness of the second slurry on other areas of the surface of the positive electrode current collector adjacent to the mixing region is greater than or equal to the maximum thickness of the mixed slurry. After drying, an arc roller (arc height of 0.03 mm) is used. Roll pressing is performed to ensure that different coating areas achieve the designed compaction density, forming a second active material layer to obtain the positive electrode sheet (corresponding to the second electrode sheet). The second active material layer includes a second main body and a mixing section, with the mixing section located on one side of the second main body along a second direction. The mixing section is a multilayer structure formed by stacking a first sublayer and a second sublayer along a first direction. Along the direction from the second main body to the mixing section, the thickness of the first sublayer gradually increases, and the thickness of the second sublayer gradually decreases. At least a portion of the multilayer structure has a thickness greater than the thickness of the second main body, and this portion forms a protrusion. Along the direction from the second main body to the mixing section, the thickness of the protrusion gradually increases. Please refer to [reference needed]. Figure 6 In the rolled second electrode 200 (i.e., the positive electrode), the centerline of the second electrode 200 in the second direction (the width direction of the first current collector) is called the first centerline (e.g., ...). Figure 6 (As shown by the dashed line), the distance a = 95 mm between the protrusion 2201 and the first center line, the distance c = 90 mm between the mixing part 222 and the first center line, the width d of the mixing part 222 is 11 mm, the maximum thickness e = 0.05 mm, the width f of the protrusion 2201 is 6 mm, and the maximum thickness difference g between the protrusion 2201 and the second main body 221 is 10 μm. All distances and widths mentioned above refer to dimensions in the second direction.

[0161] Step S102: The negative active material graphite, conductive carbon (Super P), and binder are uniformly mixed in a mass ratio of 96.5:1:2.5 to obtain a slurry of the negative active material, and then the slurry is coated on the negative current collector (corresponding to the first current collector). The slurry is coated on one edge region of the negative current collector with a thickness less than that of other regions. After drying and rolling, a first active material layer is formed on the negative current collector to obtain a negative electrode sheet (corresponding to the first electrode sheet). The first active material layer includes a first main body part and a thinned part, and the thinned part is located on one side of the first main body part in the second direction. The thickness of the thinned part is less than that of the first main body part, and the thickness of the thinned part gradually decreases from the first main body part to the thinned part. Figure 6 In the rolled negative electrode sheet (i.e., the first electrode sheet 100), the size k of the thinned part 122 in the second direction is 8 mm, and the maximum thickness difference between the first main body part 121 and the thinned part 122 is 10 μm.

[0162] Step S103: The separator (polyethylene ceramic composite separator) is placed between the positive electrode sheet prepared in step S101 and the negative electrode sheet prepared in step S102, and the first active material layer and the second active material layer are arranged towards the separator. After winding, a pressing treatment is performed to obtain an electrode assembly; please refer to Figure 6 In the first direction (the first current collector thickness direction), the protruding part 2201 is arranged opposite to the thinned part 122, and the part of the separator 300 in contact with the protruding part 2201 is bent towards the thinned part 122; in the second direction, the distance b between the thinned part 122 in the negative electrode sheet (i.e., the first electrode sheet 100) and the first center line is 95 mm, i.e., a=b.

[0163] Example 2

[0164] The preparation method of the electrode assembly in this embodiment is basically the same as that in Example 1, and the difference lies in that:

[0165] The value of d is adjusted to 3 mm, the value of c is adjusted to 98 mm, the value of a is adjusted to 99 mm, the value of f is adjusted to 2 mm, and the value of b is adjusted to 95 mm, i.e., a>b.

[0166] Example 3

[0167] The preparation method of the electrode assembly in this embodiment is basically the same as that in Example 1, and the difference lies in that:

[0168] 1) The method for preparing the positive electrode sheet in step S101 is adjusted to prepare the negative electrode sheet, i.e., the negative electrode sheet is used as the second electrode sheet, and the components of the first slurry are adjusted as follows: negative active material 1 (first graphite, the limit compaction density is 1.6 g / cm 396.5:1:2.5; the components of the second slurry are adjusted as follows: negative active material 2 (second graphite, limit compaction density 1.65 g / cm 3 96.5:1:2.5. The arc height of the arc-shaped roller is adjusted to 0.04 mm;

[0169] 2) The method for preparing the negative electrode sheet in step S102 is adjusted to prepare a positive electrode sheet, i.e., the positive electrode sheet is the first electrode sheet, and correspondingly, the components of the slurry are adjusted as follows: positive active material LiNi 0.6 Co 0.1 Mn 0.3 O2, conductive carbon (Super P), and binder in a mass ratio of 97.5:1.5:1;

[0170] 3) a and b are adjusted to 92 mm, d is adjusted to 5 mm, e is adjusted to 0.06 mm, f is adjusted to 3 mm, g is adjusted to 15 μm, k is adjusted to 3 mm, and the maximum value of the thickness difference between the first main body part and the thinned part is adjusted to 10 μm.

[0171] Example 4

[0172] The method for preparing the electrode assembly in this example is basically the same as that in Example 3, except that:

[0173] b is adjusted to 90 mm, c is adjusted to 85 mm, d is adjusted to 10 mm, and k is adjusted to 5 mm.

[0174] Comparative Example 1

[0175] The method for preparing the electrode assembly in this comparative example is basically the same as that in Example 1, except that:

[0176] In step S101, the first slurry is replaced with the second slurry for coating, and the arc-shaped roller pressing is replaced with flat roller pressing. That is, the positive active material in the second active material layer is a single ternary material.

[0177] Comparative Example 2

[0178] The method for preparing the electrode assembly in this comparative example is basically the same as that in Example 1, except that:

[0179] In step S101, the positive active material 1, the positive active material 2, the conductive agent, and the binder are mixed uniformly in a mass ratio of 48.75:48.75:1.5:1 to obtain a positive mixed slurry, and the first slurry and the second slurry are both replaced with the positive mixed slurry for coating. That is, the positive active material in the second active material layer is a mixed material of the ternary material and lithium iron phosphate.

[0180] Comparative Example 3

[0181] The preparation method of the electrode assembly in the present comparative example is basically the same as that in Embodiment 1, except that:

[0182] In step S101, the first slurry is replaced by the second slurry for coating, that is, the positive active material in the second active material layer is all ternary material. At the same time, the slurry coating thickness on the mixed area on the surface of the positive current collector is greater than that on other areas, and correspondingly, after rolling, the thickness of the mixed part is greater than that of the second main part.

[0183] Comparative Example 4

[0184] The preparation method of the electrode assembly in the present comparative example is basically the same as that in Embodiment 3, except that:

[0185] In step S101, the first slurry is replaced by the second slurry for coating, and the arc rolling is replaced by the plane rolling. That is, the negative active material in the second active material layer is a single second graphite.

[0186] Comparative Example 5

[0187] The preparation method of the electrode assembly in the present comparative example is basically the same as that in Embodiment 3, except that:

[0188] In step S101, the negative active material 1, the negative active material 2, the conductive agent, and the binder are uniformly mixed according to a mass ratio of 48.25:48.25:1:2.5 to obtain a negative mixed slurry, and the first slurry and the second slurry are both replaced by the negative mixed slurry for coating. That is, the negative active material in the second active material layer is all a mixed material of the first graphite and the second graphite.

[0189] Comparative Example 6

[0190] The preparation method of the electrode assembly in the present comparative example is basically the same as that in Embodiment 3, except that:

[0191] In step S101, the first slurry is replaced by the second slurry for coating, that is, the negative active material in the second active material layer is all the second graphite. At the same time, the slurry coating thickness on the mixed area on the surface of the negative current collector is greater than that on other areas, and correspondingly, after rolling, the thickness of the mixed part is greater than that of the second main part.

[0192] The electrode assemblies prepared in the above examples and comparative examples were assembled into batteries, and the performance of the batteries was tested. The assembly steps of the batteries were as follows: the electrode assemblies were placed into an aluminum plastic film shell, top side sealing was performed, electrolyte was injected into the shell through the injection hole, the electrolyte was a solution of 1.2M LiPF6 in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (EMC) (the volume ratio of EC to EMC was 1:1), and after the electrolyte was injected, the final battery was obtained after formation and capacity distribution. The batteries were tested as follows:

[0193] (1) Lithium precipitation detection: to verify the lithium precipitation problem of the battery at the thinned part under fast charging, the battery was subjected to charge-discharge cycling in the range of 2.5V-4.25V in a step charging and 1C rate discharging manner. Specifically, the battery was subjected to constant current charging at a current of 1.5C, and the charging limit was 50% SOC (State of Charge) of the design capacity; constant current charging at a current of 0.5C, and the charging limit was 90% SOC of the design capacity; constant current charging at a current of 0.33C, and the charging limit was 99% SOC of the design capacity; constant current charging at a current of 0.1C, and the charging was stopped at the cut-off voltage 4.25V; standing for 30 min, and constant current discharging at a current of 1C to 2.5V. After 50 cycles according to the above steps, the battery was disassembled, and the lithium precipitation at the thinned part of the electrode sheet was observed.

[0194] (2) Capacity retention rate test: at 25°C, the battery was subjected to charge-discharge cycling in the range of 2.5V-4.25V in a step charging and 1C rate discharging manner. Specifically, the battery was subjected to constant current charging at a current of 1.5C, and the charging limit was 50% SOC of the design capacity; constant current charging at a current of 0.5C, and the charging limit was 90% SOC of the design capacity; constant current charging at a current of 0.33C, and the charging limit was 99% SOC of the design capacity; constant current charging at a current of 0.1C, and the charging was stopped at the cut-off voltage 4.25V; standing for 30 min, and constant current discharging at a current of 1C to 2.5V. The discharge capacity of the first cycle was recorded, and the discharge capacity of the battery was recorded when cycled to the 1000th cycle according to the above steps. The capacity retention rate of the 1000th cycle was (discharge capacity of the 1000th cycle / discharge capacity of the first cycle) x 100%.

[0195] The test results are shown in Table 1.

[0196] Table 1

[0197]

[0198] As can be seen from the data in Table 1, in Comparative Example 1, the positive electrode sheet is coated with a single slurry, i.e., the positive electrode active material is a single ternary material, so the difference in the limit compaction density of different active materials cannot be utilized, so that after rolling, the originally thinned coating area forms a protruding part with a thickness greater than that of the normal coating area, so that in the assembled electrode assembly, there is a large gap between the thinned part of the electrode sheet and the separator, causing lithium precipitation at the thinned part of the negative electrode sheet, reducing the cycle life of the battery. In Comparative Example 2, the positive electrode sheet is still coated with a single slurry, except that the ternary material is replaced by a mixture of ternary material and lithium iron phosphate in Comparative Example 1, but the limit compaction density of the active material at different coating areas has no difference, so that after rolling, the originally thinned coating area cannot form a protruding part with a thickness greater than that of the normal coating area, so that in the assembled electrode assembly, there is a large gap between the thinned part of the electrode sheet and the separator, causing lithium precipitation at the thinned part of the negative electrode sheet, reducing the cycle life of the battery. In Comparative Example 3, the positive electrode sheet is coated with a single slurry, but thick coating is performed at the edge region of the positive electrode current collector, i.e., contrary to the usual thinning coating, so that after rolling, the thickness of the active material layer of the thick coating area at the edge of the positive electrode sheet is greater than that of the active material layer in other regions, in order to reduce the gap between the thinned part of the negative electrode sheet and the separator in the assembled electrode assembly. However, due to the excessive thickness of the active material layer at the edge of the positive electrode sheet, the positive electrode active material is excessive, which reduces the CB value and also causes lithium precipitation at the thinned part of the negative electrode sheet, and the cycle life of the battery is greatly reduced.

[0199] In Comparative Examples 1 to 3, the negative electrode sheet corresponds to the first electrode sheet in the present application, and the positive electrode sheet corresponds to the second electrode sheet in the present application. In Comparative Examples 4 to 6, the positive electrode sheet corresponds to the first electrode sheet in the present application, and the negative electrode sheet corresponds to the second electrode sheet in the present application, i.e., only the positive electrode sheet and the negative electrode sheet are compared respectively. As can be seen from the data in Table 1, since Comparative Examples 4 and 5 have similar problems as Comparative Examples 1 and 2, the limit compaction density of the active material at different coating areas has no difference, so that after rolling, the originally thinned coating area cannot form a protruding part with a thickness greater than that of the normal coating area, so that in the assembled electrode assembly, there is a large gap between the thinned part of the electrode sheet and the separator, causing lithium precipitation at the thinned part of the negative electrode sheet, reducing the cycle life of the battery. In Comparative Example 6, the negative electrode sheet is coated with a single slurry, but thick coating is performed at the edge region of the negative electrode current collector, i.e., contrary to the usual thinning coating, so that after rolling, the thickness of the active material layer of the thick coating area at the edge of the negative electrode sheet is greater than that of the active material layer in other regions, in order to reduce the gap between the thinned part of the positive electrode sheet and the separator in the assembled electrode assembly. However, due to the excessive thickness of the active material layer at the edge of the negative electrode sheet, and the compaction density is consistent with that of the middle part of the negative electrode sheet, the current density is not uniform, which also causes lithium precipitation at the edge of the negative electrode sheet, and the cycle life of the battery is greatly reduced.

[0200] In Embodiments 1 and 2, the negative electrode sheet corresponds to the first electrode sheet in the present application, and the positive electrode sheet corresponds to the second electrode sheet in the present application. In Embodiments 3 and 4, the positive electrode sheet corresponds to the first electrode sheet in the present application, and the negative electrode sheet corresponds to the second electrode sheet in the present application. As can be seen from the data in Table 1, in the electrode assemblies of Embodiments 1 to 4, there is no lithium precipitation at the thinned portion of the negative electrode sheet, and the capacity retention rate of the 1000th cycle of the battery is maintained at a high level, that is, the cycle life of the battery is improved. It is thus shown that in the present application, whether the second electrode sheet is a positive electrode sheet or a negative electrode sheet, the difference in the limit compaction density of the active material at different coating regions and the design of the coating thickness can be utilized to form a protruding portion with a thickness greater than that of the normal coating region at the thinned coating region after rolling. In the electrode assembly assembled with the protruding portion of the second electrode sheet and the thinned portion of the first electrode sheet arranged opposite to each other, the part of the separator in contact with the protruding portion is bent towards the thinned portion under the action of the protruding portion after winding or stacking and rolling. Since the protruding portion and the thinned portion are in shape adaptation, the gap between the thinned portion and the separator is significantly reduced, which can shorten the transmission distance of active ions and effectively improve the problem of lithium precipitation in the negative electrode caused by the gap between the thinned portion and the separator, thereby improving the cycle life and safety performance of the battery.

[0201] It should be noted that the electrode assembly embodiments, the preparation method of the electrode assembly embodiments and the battery embodiments provided in the present application belong to the same concept. The technical features in the technical solutions recorded in each embodiment can be combined arbitrarily without conflict.

[0202] It should be understood that the above embodiments are exemplary and are not intended to include all possible implementations. Various modifications and changes can also be made to the above embodiments without departing from the scope of the present disclosure. Similarly, each technical feature of the above embodiments can be combined arbitrarily to form additional embodiments of the present application that have not been explicitly described. Therefore, the above embodiments only express several implementations of the present application and do not limit the protection scope of the present application.

Claims

1. An electrode assembly, characterized by, The electrode assembly includes a first electrode tab, a second electrode tab, and a separator disposed between the first electrode tab and the second electrode tab; the first electrode tab and the second electrode tab have opposite polarities; The first electrode tab includes a first current collector and a first active material layer located on at least one surface of the first current collector in a first direction, the first active material layer includes a first main body portion and a thinned portion, the thinned portion is located on at least one side of the first main body portion in a second direction; the thickness of the thinned portion is less than the thickness of the first main body portion, and gradually decreases from the first main body portion to the thinned portion; The second electrode tab includes a second current collector and a second active material layer located on at least one surface of the second current collector in the first direction, the second active material layer includes a second main body portion and a mixed portion, the mixed portion is located on at least one side of the second main body portion in the second direction; the mixed portion is a multi-layer structure stacked by a first sub-layer and a second sub-layer in the first direction, the first sub-layer is located between the second current collector and the second sub-layer, and the thickness of the first sub-layer gradually increases and the thickness of the second sub-layer gradually decreases from the second main body portion to the mixed portion; the first sub-layer includes a first active material, the second sub-layer includes a second active material, the second main body portion includes a third active material, the limit compaction density of the first active material is less than the limit compaction density of the second active material and the limit compaction density of the third active material; the thickness of at least part of the multi-layer structure is greater than the thickness of the second main body portion, the part of the multi-layer structure whose thickness is greater than the thickness of the second main body portion is defined as a protruding portion, and the thickness of the protruding portion gradually increases from the second main body portion to the mixed portion; In the first direction, the protruding portion is oppositely disposed with the thinned portion, and the part of the separator in contact with the protruding portion is bent towards the thinned portion; the first direction is the thickness direction of the first current collector, and the second direction is the width direction of the first current collector.

2. The electrode assembly of claim 1, wherein, The electrode assembly satisfies at least one of the following characteristics: (1) In the first direction, the sum of the size of the gap between the mixed portion and the separator and the size of the gap between the thinned portion and the separator is less than or equal to 10 μm; (2) In the first direction, the projection of the protruding portion is located within the projection of the thinned portion; (3) The mixed portion further includes a flush portion, the flush portion is located between the protruding portion and the second main body portion, and the thickness of the flush portion is the same as the thickness of the second main body portion; (4) The size of the mixed portion in the second direction is greater than the size of the protruding portion in the first direction; (5) The difference between the size of the protruding portion and the size of the second main body portion in the first direction is less than the size of the protruding portion in the second direction; (6) The maximum difference between the size of the first main body portion and the size of the thinned portion in the first direction is greater than or equal to the maximum difference between the size of the protruding portion and the size of the second main body portion in the first direction. (7) In the first direction, the sum of the thicknesses of the thinning portion and the protruding portion corresponding to the position is equal to the sum of the thicknesses of the first main portion and the second main portion; (8) The second active material and the third active material are the same.

3. The electrode assembly of claim 1, wherein, The second tab is a positive tab; the electrode assembly satisfies at least one of the following characteristics: (1) the first active material is lithium iron phosphate and / or lithium manganese iron phosphate; the first active material has an ultimate compaction density of 2.3 g / cm 3 2.8 g / cm 3 ; (2) the second active material is a ternary material; the ultimate compaction density of the second active material is 3.3 g / cm 3 3.6 g / cm 3 ; (3) the third active material is a ternary material; the ultimate compaction density of the third active material is 3.3 g / cm 3 3.6 g / cm 3 ; (4) The gram capacity of the first active material is less than the gram capacity of the third active material, and the gram capacity of the second active material is less than or equal to the gram capacity of the third active material; (5) The dimension of the mixed portion in the second direction is 3mm-20mm; (6) The dimension of the thinning portion in the second direction is 0.5mm-10mm; the maximum difference between the dimensions of the second main portion and the thinning portion in the first direction is 0.5um-20um.

4. The electrode assembly of claim 1, wherein, The second tab is a negative tab; the electrode assembly satisfies at least one of the following characteristics: (1) the first active material is first graphite; the first active material has an ultimate compaction density of 1.55 g / cm 3 1.7 g / cm 3 ; (2) the second active material is a second graphite; the second active material has an ultimate compaction density of 1.6 g / cm 3 1.7 g / cm 3 ; (3) the third active material is a third graphite; the third active material has an ultimate compaction density of 1.6 g / cm 3 1.7 g / cm 3 ; (4) The gram capacity of the first active material is greater than the gram capacity of the third active material, and the gram capacity of the second active material is greater than or equal to the gram capacity of the third active material; (5) The dimension of the mixed portion in the second direction is 0.5mm-10mm; (6) The dimension of the thinning portion in the second direction is 3mm-10mm; the maximum difference between the dimensions of the second main portion and the thinning portion in the first direction is 5um-30um.

5. A method of making an electrode assembly, characterized by, The method comprises: using a coating process to form a first active material layer on at least one surface of a first current collector in a first direction, to obtain a first tab; the first active material layer comprises a first main portion and a thinning portion, the thinning portion is located on at least one side of the first main portion in a second direction; the thickness of the thinning portion is less than the thickness of the first main portion, and the thickness of the thinning portion gradually decreases in the direction from the first main portion to the thinning portion; using a coating process to form a second active material layer on at least one surface of a second current collector in the first direction, to obtain a second tab; the second active material layer comprises a second main portion and a mixed portion, the mixed portion is located on at least one side of the second main portion in the second direction; the mixed portion is a multi-layer structure formed by stacking a first sub-layer and a second sub-layer in the first direction, the first sub-layer is located between the second current collector and the second sub-layer, in the direction from the second main portion to the mixed portion, the thickness of the first sub-layer gradually increases, and the thickness of the second sub-layer gradually decreases; the first sub-layer comprises a first active material, the second sub-layer comprises a second active material, the second main portion comprises a third active material, the limit compaction density of the first active material is less than the limit compaction density of the second active material and the limit compaction density of the third active material; the thickness of at least part of the multi-layer structure is greater than the thickness of the second main portion, the part of the multi-layer structure whose thickness is greater than the thickness of the second main portion is defined as a protruding portion, and the thickness of the protruding portion gradually increases in the direction from the second main portion to the mixed portion; The separator is arranged between the first and second electrode tabs, the first and second active material layers are arranged towards the separator, and after the lamination or winding, a pressing treatment is performed to obtain the electrode assembly; in the first direction, the protruding portion is arranged opposite to the thinned portion, and the portion of the separator in contact with the protruding portion is bent towards the thinned portion; The first and second electrode tabs have opposite polarities; the first direction is the thickness direction of the first current collector, and the second direction is the width direction of the first current collector.

6. The method of claim 5, wherein the electrode assembly is prepared by the steps of: The forming of the second active material layer by the coating process comprises: The first, second and third slurries are prepared, the first slurry comprises the first active material, the second slurry comprises the second active material, and the third slurry comprises the third active material; The first and second slurries are coated on the mixed area on the surface of the second current collector, the mixed area is located on at least one side of the surface of the second current collector along the second direction, and from the edge of the second current collector to the inside along the second direction, the thickness of the mixed slurry composed of the first and second slurries gradually increases, the thickness of the first slurry gradually decreases, and the thickness of the second slurry gradually increases; the third slurry is coated on the other area adjacent to the mixed area on the surface of the second current collector, the coating thickness of the third slurry is greater than or equal to the maximum thickness of the mixed slurry, and after drying, a rolling is performed to form the first sub-layer, the second sub-layer and the second main part.

7. The method of claim 6, wherein the electrode assembly is prepared by the steps of: The rolling is arc-shaped rolling; and / or the temperature of the drying treatment is 80-150°C.

8. The method of claim 5, wherein the electrode assembly is prepared by the steps of: The electrode assembly satisfies at least one of the following characteristics: (1) In the first direction, the sum of the size of the gap between the mixed part and the separator and the size of the gap between the thinned portion and the separator is less than or equal to 10μm; (2) In the first direction, the projection of the protruding portion is located within the projection of the thinned portion; (3) The mixed part further comprises a flush part, the flush part is located between the protruding portion and the first main part, and the thickness of the flush part is the same as the thickness of the first main part; (4) The size of the mixed part in the second direction is greater than the size of the protruding portion in the first direction; (5) The difference between the size of the protruding portion and the first main part in the first direction is less than the size of the protruding portion in the second direction; (6) The maximum difference between the size of the first main part and the thinned portion in the first direction is greater than or equal to the maximum difference between the size of the protruding portion and the second main part in the first direction; (7) In the first direction, the sum of the thicknesses of the corresponding thinned portion and protruding portion is equal to the sum of the thicknesses of the first and second main parts; (8) The second and third active materials are the same.

9. The method for preparing the electrode assembly according to claim 5, wherein The second electrode tab is a positive electrode tab; the electrode assembly satisfies at least one of the following characteristics: (1) the first active material is lithium iron phosphate and / or lithium iron manganese phosphate; the first active material has an ultimate compaction density of 2.3 g / cm 3 ~2.8 g / cm 3 ; (2) the second active material is a ternary material; the second active material has an ultimate compaction density of 3.3 g / cm 3 ~3.6 g / cm 3 ; (3) the third active material is a ternary material; the third active material has an ultimate compaction density of 3.3 g / cm 3 ~3.6 g / cm 3 ; (4) the gram capacity of the first active material is less than the gram capacity of the third active material, and the gram capacity of the second active material is less than or equal to the gram capacity of the third active material. (5) the size of the mixing portion in the second direction is 3 mm to 20 mm; (6) the size of the thinning portion in the second direction is 0.5 mm to 10 mm; and the maximum difference between the sizes of the second main portion and the thinning portion in the first direction is 0.5 μm to 20 μm; Alternatively, the second tab is a negative tab; the electrode assembly satisfies at least one of the following features: (1) the first active material is a first graphite; the first active material has an ultimate compaction density of 1.55 g / cm 3 ~1.7 g / cm 3 ; (2) the second active material is a second graphite; the second active material has an ultimate compaction density of 1.6 g / cm 3 ~1.7 g / cm 3 ; (3) the third active material is a third graphite; the third active material has an ultimate compaction density of 1.6 g / cm 3 ~1.7 g / cm 3 ; (4) the gram capacity of the first active material is greater than the gram capacity of the third active material, and the gram capacity of the second active material is greater than or equal to the gram capacity of the third active material. (5) the size of the mixing portion in the second direction is 0.5 mm to 10 mm; (6) the size of the thinning portion in the second direction is 3 mm to 10 mm; and the maximum difference between the sizes of the second main portion and the thinning portion in the first direction is 5 μm to 30 μm.

10. A battery, characterized by An electrode assembly prepared by the production method of the electrode assembly according to any one of claims 1 to 4 or the electrode assembly according to any one of claims 5 to 9.

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