Positive electrode sheet and lithium ion battery

Abstract

The application relates to a positive electrode sheet and a lithium ion battery. The positive electrode sheet comprises a current collector and a composite active material layer arranged on the current collector, the composite active material layer comprises a first active material layer, a second active material layer and a third active material layer which are sequentially stacked on one side of the current collector, the first active material layer comprises a first active material, the second active material layer comprises a second active material, and the third active material layer comprises a third active material, the expansion coefficient of the first active material > the expansion coefficient of the second active material > the expansion coefficient of the third active material; the first active material comprises LiNi x1 Co y1 M z1 O2, M is Mn or Al, x1 > 0, y1 > 0, z1 > 0, and the third active material comprises lithium iron phosphate. The positive electrode sheet can be stable in structure and is not easy to deform while the ternary material and lithium iron phosphate are combined.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a positive electrode and a lithium-ion battery. Background Technology

[0002] With the increasing demands for energy density, cycle life, and safety in lithium-ion batteries from fields such as electric vehicles, traditional ternary materials (generally NCM, "nickel-cobalt-manganese" ternary or NCA, "nickel-cobalt-aluminum" ternary) and lithium iron phosphate, used alone or in simple combinations, are no longer sufficient to meet the requirements. Further improvements in battery performance are needed through structural and material optimization. Ternary materials offer high energy density but relatively poor cycle stability and safety; lithium iron phosphate, on the other hand, boasts good cycle stability and safety but lower energy density. Related technologies often combine ternary materials and lithium iron phosphate in the cathode, aiming to complement each other's advantages. However, in practical applications (during charge and discharge), this cathode structure is unstable, prone to deformation and even cracking, affecting the performance and lifespan of the lithium-ion battery. Summary of the Invention

[0003] Therefore, it is necessary to provide a positive electrode that can combine ternary materials and lithium iron phosphate while maintaining structural stability and being resistant to deformation.

[0004] This invention provides a positive electrode sheet, comprising:

[0005] Current collector; and

[0006] A composite active material layer is disposed on at least one side surface of the current collector along the thickness direction;

[0007] The composite active material layer includes a first active material layer, a second active material layer, and a third active material layer. The first active material layer is disposed on one side surface of the current collector, the second active material layer is disposed on the side surface of the first active material layer away from the current collector, and the third active material layer is disposed on the side surface of the second active material layer away from the first active material layer.

[0008] The first active material layer includes a first active material, the second active material layer includes a second active material, and the third active material layer includes a third active material. The coefficient of thermal expansion of the first active material is greater than the coefficient of thermal expansion of the second active material, which is greater than the coefficient of thermal expansion of the third active material.

[0009] The first active material includes LiNi x1 Co y1 M z1 O2, M is Mn or Al, x1>0, y1>0, z1>0, and the third active material includes lithium iron phosphate.

[0010] In one embodiment, the second active material includes LiNi. x2 Co y2 M z2 Fe u O2, M is Mn or Al, x2>0, y2>0, z2>0, u>0.

[0011] In one embodiment, LiNi x2 Co y2 M z2 Fe u The molar content of Fe in O2 is 0.075-0.15; and / or

[0012] The Dv50 of the second active material is less than that of the first active material.

[0013] In one embodiment, the thickness of the second active material layer increases from the middle to both ends in the width direction of the positive electrode.

[0014] In one embodiment, the maximum thickness of the second active material layer is greater than or equal to one-fifth of the maximum thickness of the first active material layer.

[0015] In one embodiment, the maximum thickness of the second active material layer is 50-70 μm; and / or

[0016] The maximum thickness of the first active material layer is 120-180 μm; and / or

[0017] The maximum thickness of the third active material layer is 60-120 μm; and / or

[0018] The thickness of the composite active material layer is 210-270 μm.

[0019] In one embodiment, in the width direction of the positive electrode, the thickness of the first active material layer decreases from the middle to both ends; and / or

[0020] In the width direction of the positive electrode sheet, the thickness of the third active material layer decreases from the middle to both ends.

[0021] In one embodiment, the second active material layer further includes a solid electrolyte.

[0022] In one embodiment, the solid electrolyte is at least one selected from PVDF-LiClO4, PEO-LiTFSI, and LLZO; and / or

[0023] In the second active material layer, the content of the solid electrolyte is ≤ 20% of the content of the second active material.

[0024] The present invention also provides a lithium-ion battery, including the above-described positive electrode.

[0025] In the aforementioned positive electrode sheet, the first active material includes ternary materials, and the third active material includes lithium iron phosphate. That is, the aforementioned positive electrode sheet combines ternary materials and lithium iron phosphate, which allows the ternary materials and lithium iron phosphate to complement each other's advantages. As a result, the battery including the aforementioned positive electrode sheet has both high energy density and good cycle stability and safety.

[0026] Meanwhile, in the aforementioned positive electrode sheet, the first active material layer includes a first active material, the second active material layer includes a second active material, and the third active material layer includes a third active material. The expansion coefficient of the first active material is greater than that of the second active material, which is greater than that of the third active material. In other words, a second active material with an expansion coefficient between that of the ternary material and the lithium iron phosphate is placed between the first active material (ternary material) and the third active material (lithium iron phosphate). This alleviates the lattice mismatch problem between the ternary material and the lithium iron phosphate. Thus, during charging and discharging, the expansion of the first active material layer (ternary material layer), the second active material layer, and the third active material layer (lithium iron phosphate layer) after lithium intercalation can exhibit a gradient change. This reduces the expansion difference between layers, decreases the interaction force between layers, reduces internal stress, and avoids stress concentration at the interfaces of the active material layers, which could lead to deformation or even cracking at the interfaces. This reduces the probability of deformation or cracking of the aforementioned positive electrode sheet, improves its stability, and enhances the performance of the battery including the aforementioned positive electrode sheet, resulting in improved cell cycle performance, breakthroughs in rate performance, and increased lifespan. Attached Figure Description

[0027] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0028] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet in one embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of the positive electrode sheet in another embodiment of the present invention;

[0030] Figure 3 This is a schematic diagram of the positive electrode sheet in another embodiment of the present invention;

[0031] Figure 4 This is a schematic diagram of the positive electrode sheet in another embodiment of the present invention;

[0032] Figure 5 This is a schematic diagram of the positive electrode sheet in another embodiment of the present invention;

[0033] Figure 6 This is a schematic diagram of the positive electrode sheet in another embodiment of the present invention;

[0034] Figure 7 This is a schematic diagram of the positive electrode sheet in another embodiment of the present invention;

[0035] Figure 8 This is a schematic diagram of the positive electrode sheet in another embodiment of the present invention;

[0036] Figure 9 This is a schematic diagram of the positive electrode sheet of the comparative example of the present invention. Detailed Implementation

[0037] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0038] In the description of this application, it should be understood that, where they appear, the terms “center,” “longitudinal,” “lateral,” “length,” “width,” “thickness,” “upper,” “lower,” “front,” “rear,” “left,” “right,” “vertical,” “horizontal,” “top,” “bottom,” “inner,” “outer,” “clockwise,” “counterclockwise,” “axial,” “radial,” and “circumferential” indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0039] Furthermore, where applicable, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0040] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," "fixing," etc., shall be interpreted broadly. For example, they may refer to a fixed connection, a detachable connection, or an integral part; they may refer to a mechanical connection or an electrical connection; they may refer to a direct connection or an indirect connection through an intermediate medium; they may refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0041] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0042] It should be noted that, if an element is described as "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is described as "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0043] like Figure 1 As shown, an embodiment of the present invention provides a positive electrode 10. The positive electrode 10 includes a current collector 200 and a composite active material layer 300. The composite active material layer 300 is disposed on at least one surface of the current collector 200 along its thickness direction. Specifically, in this embodiment, the composite active material layer 300 is disposed on both sides of the current collector 200 along its thickness direction. This is beneficial for improving the energy density of the battery. It can be understood that in other embodiments, the composite active material layer 300 may only be disposed on one surface of the current collector 200 along its thickness direction.

[0044] In this embodiment, the composite active material layer 300 includes a first active material layer 310, a second active material layer 320, and a third active material layer 330. The first active material layer 310, the second active material layer 320, and the third active material layer 330 are sequentially stacked on one side surface of the current collector 200. That is, in the thickness direction of the current collector 200, the first active material layer 310 is disposed on one side surface of the current collector 200, the second active material layer 320 is disposed on the side surface of the first active material layer 310 away from the current collector 200, and the third active material layer 330 is disposed on the side surface of the second active material layer 320 away from the first active material layer 310. It can be understood that in other embodiments, in the thickness direction of the current collector 200, both sides of the current collector 200 are sequentially stacked with the first active material layer 310, the second active material layer 320, and the third active material layer 330.

[0045] The first active material layer 310 includes a first active material. The second active material layer 320 includes a second active material. The third active material layer 330 includes a third active material. The coefficient of thermal expansion of the first active material is greater than that of the second active material, which in turn is greater than that of the third active material.

[0046] The first active material includes LiNi x1 Co y1 M z1 O2, M is Mn or Al, x1>0, y1>0, z1>0, meaning the first active material includes ternary materials. The third active material includes lithium iron phosphate.

[0047] In the aforementioned positive electrode 10, the first active material includes a ternary material, and the third active material includes lithium iron phosphate. That is, the aforementioned positive electrode 10 combines ternary materials and lithium iron phosphate, which allows the ternary materials and lithium iron phosphate to complement each other's advantages. As a result, the battery including the aforementioned positive electrode 10 has both high energy density and good cycle stability and safety.

[0048] Meanwhile, in the aforementioned positive electrode 10, the first active material layer 310 includes a first active material, the second active material layer 320 includes a second active material, and the third active material layer 330 includes a third active material. The expansion coefficient of the first active material is greater than that of the second active material, which is greater than that of the third active material. In other words, a second active material with an expansion coefficient between the first active material (ternary material) and the third active material (lithium iron phosphate) is set between them, thereby alleviating the lattice mismatch problem between the ternary material and the lithium iron phosphate. Thus, during the charging and discharging process, the expansion of the first active material layer 310 (ternary material layer), the second active material layer 320, and the third active material layer 330 (lithium iron phosphate layer) after lithium insertion can exhibit a gradient change. This can reduce the expansion difference between layers, reduce the interaction force between layers, reduce internal stress, and avoid the occurrence of interface deformation or even cracking due to stress concentration at the interface of the active material layer. In turn, it can reduce the probability of deformation or even cracking of the positive electrode 10, improve the stability of the positive electrode 10, and enhance the performance of the battery including the positive electrode 10, thereby improving the cell cycle performance, achieving breakthroughs in rate performance, and increasing lifespan.

[0049] In this embodiment, the second active material includes LiNi. x2 Co y2 M z2 Fe u O2, M is Mn or Al, x2 > 0, y2 > 0, z2 > 0, u > 0. The second active material is LiNi. x2 Co y2 M z2 Fe u In O2, M is either Mn or Al, and the first active material is LiNi. x1 Co y1 M z1 In O2, M is either Mn or Al, meaning the range of M for the second active material is the same as the range of M for the first active material. In some embodiments, M for the second active material and M for the first active material can be the same, for example, both can be Mn or both can be Al. In some embodiments, M for the second active material and M for the first active material can also be different, for example, M for the second active material can be Mn, while M for the first active material can be Al.

[0050] In the above scheme, the chemical formula of the second active material is similar to that of the first active material. The biggest difference between the second and first active materials is that the second active material introduces Fe element into the ternary material. This not only allows the expansion coefficient of the second active material to be between that of the first and third active materials, but also allows the second active material to have a similar layered crystal structure to the first active material, facilitating lithium ion transport. Moreover, the introduction of Fe element into the second active material can not only bridge the gap between the first and third active materials and change the lattice distribution, but also improve the stability and safety of the second active material layer 320.

[0051] In this embodiment, LiNi x2 Co y2 M z2 Fe u The molar content of Fe in O2 is 0.075-0.15. That is, u / (1+X²+Y²+Z²+2) is 0.075-0.15. Specifically, LiNi x2 Co y2 M z2 Fe u The molar content of Fe in O2 can be 0.075, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, or any value within any two of the above ranges. Too high a molar content of Fe will lead to problems with LiNi. x2 Co y2 M z2 Fe u The structure of O2 changes, which is unfavorable for lithium-ion transport. The low molar content of Fe leads to a significant difference in the expansion coefficient between the second and first active materials, resulting in insufficient improvement in stress reduction caused by expansion. Considering these factors, LiNi is used... x2 Co y2 M z2 Fe u The molar content of Fe in O2 is 0.075-0.15.

[0052] In this embodiment, the Dv50 (median particle size) of the second active material is smaller than that of the first active material. The second active material is based on a ternary material with the addition of Fe. Considering that the addition of Fe reduces the ion transport capacity of the ternary material, the Dv50 of the second active material is set to be less than that of the first active material to improve the ion transport capacity in the second active material. Furthermore, the fact that the Dv50 of the second active material is less than that of the first active material also facilitates balanced ion transport in the middle and edge regions of the positive electrode 10.

[0053] In this embodiment, as Figures 2-8 As shown, in the width direction (left-right direction in the diagram) of the positive electrode 10, the thickness of the second active material layer 320 increases from the middle to both ends. In the width direction of the positive electrode 10, the edge regions are less constrained by the surrounding material compared to the middle regions, making them prone to stress concentration and cracking. To address this issue, in this embodiment, the thickness of the second active material layer 320 increases from the middle to both ends in the width direction of the positive electrode 10. This balances the constraining effect of the surrounding material on the edge and middle regions of the second active material layer 320 in the width direction of the positive electrode 10.

[0054] It should be noted that in the field of electrode sheet manufacturing, since electrode sheets are usually shipped in rolls, the length and width directions of the electrode sheets are defined. The direction in which a roll of electrode sheets is pulled out is the extension direction of the electrode sheet, which is the length direction. The other direction is the width direction. Therefore, in this embodiment, the width direction of the positive electrode sheet 10 does not specifically refer to the short side of the rectangle; the width direction of the positive electrode sheet 10 can also be the long side of the rectangle.

[0055] It should be noted that the edge regions in this application refer to the two end regions in the width direction of the positive electrode 10. There are two edge regions in the width direction of the positive electrode 10, and the middle region is the middle region in the width direction of the positive electrode 10, located between the two edge regions in the width direction. Generally, the length of a single edge region in the width direction of the positive electrode 10 is 3-8 mm. Specifically, the length of a single edge region in the width direction of the positive electrode 10 can be 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, or any value between any two of the above ranges.

[0056] It should be noted that the second active material layer 320, with its thickness increasing from the middle to both ends, can be prepared using existing coating processes. Therefore, the preparation method of the second active material layer 320 is not described in detail here.

[0057] In some embodiments, such as Figures 2-6 As shown, the minimum thickness of the middle region of the second active material layer 320 is 0 in the width direction of the positive electrode 10. In some embodiments, such as Figure 7 and Figure 8 As shown, in the width direction of the positive electrode 10, the minimum thickness of the middle region of the second active material layer 320 is not 0.

[0058] In some embodiments, such as Figure 2 , Figure 5 , Figure 6 and Figure 8 As shown, in the width direction of the positive electrode 10, the thickness of the second active material layer 320 increases from the middle to both ends, and in a cross-section of the positive electrode 10 parallel to the width direction, the middle region of the second active material layer 320 includes a point with the smallest thickness, rather than a segment. In some embodiments, such as Figure 3 , Figure 4 and Figure 7 In the width direction of the positive electrode 10, the thickness of the second active material layer 320 increases from the middle to both ends, and in the cross section of the positive electrode 10 parallel to the width direction, the middle region of the second active material layer 320 includes a segment with the smallest thickness, rather than a point.

[0059] Since the thickness of the second active material layer 320 increases from the middle to both ends in the width direction of the positive electrode 10, in order to ensure the normal transport of lithium ions, it is necessary to reduce the gap between interfaces. Therefore, in the width direction of the positive electrode 10, the thickness variation of the first active material layer 310 and / or the thickness variation of the third active material layer 330 must be adapted to the thickness variation of the second active material layer 320.

[0060] In some embodiments, such as Figures 2-4 and Figures 6-8 As shown, in the width direction of the positive electrode 10, the thickness of the first active material layer 310 decreases from the middle to both ends. This allows the thickness variation of the first active material layer 310 to match the thickness variation of the second active material layer 320, reducing the gap between the interfaces of the first and second active material layers 310 and ensuring normal lithium-ion transport. Furthermore, in the width direction of the positive electrode 10, the thickness of the second active material layer 320 increases from the middle to both ends, while the thickness of the first active material layer 310 decreases from the middle to both ends. Therefore, the second active material layer 320 can provide stronger constraint on the first active material layer 310, suppressing its expansion and reducing expansion differences. Furthermore, since the constraint of the edge region of the first active material layer 310 is weaker than that of the middle region, the expansion is more obvious. In the width direction of the positive electrode 10, the thickness of the first active material layer 310 decreases from the middle to both ends. This makes the content of the first active material in the edge region of the first active material layer 310 less than that in the middle region. As a result, the expansion of the edge region of the first active material layer 310 is less than that in the middle region, thereby balancing the expansion of the middle region and the edge region of the first active material layer 310 to a certain extent.

[0061] In some embodiments, such as Figure 5As shown, in the width direction of the positive electrode 10, the thickness of the third active material layer 330 decreases from the middle to both ends. In this way, the thickness variation of the third active material layer 330 can be adapted to the thickness variation of the second active material layer 320, so as to reduce the gap between the interface of the third active material layer 330 and the interface of the second active material layer 320, and ensure the normal transport of lithium ions.

[0062] In some embodiments, such as Figure 6 As shown, in the width direction of the positive electrode 10, the thickness of the first active material layer 310 decreases from the middle to both ends, and the thickness of the third active material layer 330 also decreases from the middle to both ends. This allows the thickness variation of the first active material layer 310 to match the thickness variation of the second active material layer 320, reducing the gap between the interfaces of the first and second active material layers 310 and ensuring normal lithium-ion transport. Similarly, it allows the thickness variation of the third active material layer 330 to match the thickness variation of the second active material layer 320, reducing the gap between the interfaces of the third and second active material layers 330 and ensuring normal lithium-ion transport.

[0063] It should be noted that in the cases where the thickness of the first active material layer 310 decreases from the middle to both ends, the thickness of the second active material layer 320 increases from the middle to both ends, and the thickness of the third active material layer 330 decreases from the middle to both ends, the increase and decrease in thickness can be continuous or stepwise. Specifically, in continuous increases and decreases, the side surface of the active material layer in the thickness direction can be a slope, while in stepwise increases and decreases, the side surface of the active material layer in the thickness direction can be a stepped surface.

[0064] It should be noted that this application mainly improves the thickness of the active material layer (first active material layer 310 and / or second active material layer 320 and / or third active material layer 330) in the width direction of the positive electrode 10. Unless otherwise specified, the thickness of the active material layer in the length direction of the positive electrode 10 is not improved; that is, the shape of the cross-section parallel to the width direction of the positive electrode 10 at different positions along the length direction of the positive electrode 10 is the same. In some embodiments, such as... Figures 2-4 , Figure 7 and Figure 8 As shown, the surface of the second active material layer 320 is flush with the surface of the first active material layer 310. This allows the thickness of the third active material layer 330 to be uniform throughout, making it easier to fabricate.

[0065] In some embodiments, such as Figure 5 and Figure 6As shown, the surface of the second active material layer 320 away from the first active material layer 310 is not flush. The surface of the second active material layer 320 away from the first active material layer 310 is filled by the third active material layer 330, making the surface of the third active material layer 330 flush with the surface of the second active material layer 320.

[0066] In this embodiment, the maximum thickness of the second active material layer 320 is greater than or equal to one-fifth of the maximum thickness of the first active material layer 310. This ensures that the second active material layer 320 has sufficient thickness, thereby providing adequate transition properties and facilitating the reduction of expansion differences between layers, decreasing interlayer forces, and lowering internal stress.

[0067] In this embodiment, the maximum thickness of the second active material layer 320 is 50-70 μm. Specifically, the maximum thickness of the second active material layer 320 can be 50 μm, 60 μm, 70 μm, or any value between any two of the above ranges. This ensures that the second active material layer 320 has sufficient transition effect.

[0068] In this embodiment, the maximum thickness of the first active material layer 310 is 120-180 μm. Specifically, the maximum thickness of the first active material layer 310 can be 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, or any value between any two of the above ranges. This ensures that the composite active material layer 300 of the positive electrode 10 has sufficient thickness.

[0069] In this embodiment, the maximum thickness of the third active material layer 330 is 60-120 μm. Specifically, the maximum thickness of the third active material layer 330 can be 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, or any value between any two of the above ranges. This ensures that the composite active material layer 300 of the positive electrode 10 has sufficient thickness.

[0070] In this embodiment, the thickness of the composite active material layer 300 is 210-270 μm. Specifically, the thickness of the composite active material layer 300 can be 210 μm, 220 μm, 230 μm, 240 μm, 250 μm, 260 μm, 270 μm, or any value between any two of the above ranges. This ensures that the composite active material layer 300 of the positive electrode 10 has sufficient thickness.

[0071] In this embodiment, the second active material layer 320 also includes a solid electrolyte. This effectively improves the ion transport capability of the second active material layer 320, enabling balanced ion transport in the middle and edge regions of the second active material layer 320.

[0072] In this embodiment, the solid electrolyte is at least one of PVDF-LiClO4 (polyvinylidene fluoride-lithium perchlorate composite solid electrolyte), PEO-LiTFSI (polyoxyethylene-lithium bis(trifluoromethanesulfonyl)imide composite solid electrolyte), and LLZO (lithium lanthanum zirconium oxide solid electrolyte). This avoids the solid electrolyte affecting the activity of the first, second, and third active materials. Furthermore, LLZO is preferred because it is heat-resistant, has no side reactions with the electrolyte, exhibits strong thermal stability and compatibility, and is highly feasible to manufacture.

[0073] In this embodiment, the content (mass content) of the solid electrolyte in the second active material layer 320 is ≤ 20% of the content (mass content) of the second active material. Specifically, the content (mass content) of the solid electrolyte can be 1%, 5%, 10%, 15%, 20% of the content (mass content) of the second active material, or any value between any two of the above ranges. If the content of the solid electrolyte is too high, the content of the second active material will be too low, resulting in insufficient expansion control. Therefore, the content (mass content) of the solid electrolyte in the second active material layer 320 is set to ≤ 20% of the content (mass content) of the second active material.

[0074] In this embodiment, the first active material layer 310 further includes a first binder and a first conductive agent. The mass ratio of the first active material, the first binder, and the first conductive agent is (96-98):(1-3):(1-2). This facilitates the fabrication of the first active material layer 310. Specifically, in this embodiment, the first active material is LiNi. x1 Co y1 M z1 O2, specifically LiNi 0.6 Co 0.3 Mn 0.1 O2, the first binder is PVDF (polyvinylidene fluoride), and the first conductive agent is SP (conductive carbon black, SuperP). It can be understood that in other embodiments, the first active material (LiNi) x1 Co y1 M z1 The chemical formula of O2 is not limited to LiNi 0.6 Co 0.3 Mn 0.1O2 can be used to obtain other chemical formulas of the first active material by changing x1, y1, and z1. It is understood that in other embodiments, the first binder is not limited to PVDF, and may also be polytetrafluoroethylene or acrylic acid. It is understood that in other embodiments, the first conductive agent is not limited to SP, and may also be LITX carbon black or KS-6 conductive graphite.

[0075] In this embodiment, the second active material layer 320 further includes a solid electrolyte, a second binder, and a second conductive agent. The mass ratio of the second active material, the solid electrolyte, the second binder, and the second conductive agent is (80-90):(8-15):(1-3):(1-2). This facilitates the fabrication of the second active material layer 320. Specifically, in this embodiment, the second active material is LiNi. 0.6 Co 0.3 Mn 0.1 Fe 0.45 O2, LLZO as the solid electrolyte, PVDF as the second binder, and SP as the second conductive agent. It can be understood that in other embodiments, the second active material (LiNi) x2 Co y2 M z2 Fe u The chemical formula of O2 is not limited to LiNi 0.6 Co 0.3 Mn 0.1 Fe 0.45 O2 can be used to obtain second active materials with other chemical formulas by changing x2, y2, z2, and u. It is understood that in other embodiments, the second binder is not limited to PVDF, and may also be polytetrafluoroethylene or acrylic acid. It is understood that in other embodiments, the second conductive agent is not limited to SP, and may also be LITX carbon black or KS-6 conductive graphite.

[0076] In this embodiment, the third active material layer 330 further includes a third binder and a third conductive agent. The mass ratio of the third active material, the third binder, and the third conductive agent is (96-98):(1-3):(1-2). This facilitates the fabrication of the third active material layer 330. Specifically, in this embodiment, the third active material is lithium iron phosphate, the third binder is PVDF, and the third conductive agent is SP. It is understood that in other embodiments, the third binder is not limited to PVDF; it can also be polytetrafluoroethylene or acrylic acid. It is understood that in other embodiments, the third conductive agent is not limited to SP; it can also be LITX carbon black or KS-6 conductive graphite.

[0077] The present invention also provides a lithium-ion battery. The positive electrode 10 of this lithium-ion battery is as described above.

[0078] This invention also provides an electrical device comprising the aforementioned lithium-ion battery. The aforementioned electrical device can be a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, power tool, energy storage device, amusement equipment, elevator, and lifting equipment, etc. Vehicles can be gasoline-powered vehicles, natural gas-powered vehicles, or new energy vehicles; new energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc.; spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc.; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, or electric airplane toys, etc.; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc.; energy storage devices can be energy storage walls, base station energy storage, container energy storage, etc.; amusement equipment can be a carousel, a drop tower, etc. This embodiment does not impose any special limitations on the aforementioned electrical device.

[0079] The present invention will be further described below with reference to embodiments.

[0080] I. Examples and Comparative Examples

[0081] Example 1 (with appendix) Figure 2 correspond)

[0082] 1. Preparation of positive electrode sheet

[0083] (1) Provide aluminum foil (current collector).

[0084] (2) Preparation of the first active material layer: The slurry of the first active material layer is prepared and coated on aluminum foil, dried, and the first active material layer is obtained. The first active material is LiNi. 0.6 Co 0.3 Mn 0.1 The active material is O2, the binder is PVDF, the conductive agent is SP, and the mass ratio of the first active material to the binder to the conductive agent is 97.3:1.7:1. The Dv50 of the first active material is 8 μm, the maximum thickness of the middle region of the first active material layer is 150 μm, the minimum thickness of the edge region of the first active material layer is 90 μm, and the width of the positive electrode is 100 mm. Along the width direction of the positive electrode, the thickness of the first active material layer decreases from the middle to both ends, and on a cross-section of the positive electrode parallel to the width direction, the maximum thickness of the middle region of the first active material layer is a single point, not a segment.

[0085] (3) Preparation of the second active material layer: Prepare a slurry for the second active material layer and coat it onto the first active material layer. Dry the slurry to obtain the second active material layer. The second active material is LiNi. 0.6 Co 0.3Mn 0.1 Fe 0.45 The molar content of O2 and Fe is 0.1125. The solid electrolyte is LLZO, the binder is PVDF, and the conductive agent is SP. The content ratio of the second active material to solid electrolyte to binder to conductive agent is 85:12.3:1.7:1. The Dv50 of the second active material is 4 μm. The minimum thickness of the middle region of the second active material layer is 0 μm, and the maximum thickness of the edge region is 60 μm. Along the width of the electrode sheet, the thickness of the second active material layer increases from the middle to both ends, and the surface of the second active material layer is flush with the surface away from the first active material layer. Furthermore, on a cross-section of the positive electrode sheet parallel to the width direction, the minimum thickness of the middle region of the second active material layer is a point, not a segment.

[0086] (4) Preparation of the third active material layer: A slurry for preparing the third active material layer is coated onto the third active material layer and dried to obtain the third active material layer, which is the positive electrode sheet. The third active material is lithium iron phosphate, the binder is PVDF, and the conductive agent is SP. The content ratio of the third active material to the binder to the conductive agent is 96.9:2:1.1. The thickness of the third active material layer is 60 μm and is the same everywhere. The sum of the thicknesses of the first active material layer, the second active material layer, and the third active material layer is 210 μm, that is, the thickness of the composite active material layer is 210 μm.

[0087] 2. Preparation of negative electrode sheet

[0088] The negative electrode active material graphite, the binder styrene-butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC), and the conductive agent conductive carbon black (SP) are mixed in a mass ratio of 96.5:1.8:0.9:0.8 and dispersed in the solvent deionized water to prepare a negative electrode slurry. The negative electrode slurry is coated on copper foil (current collector), and then the negative electrode sheet is obtained by baking and stamping.

[0089] 3. Selection of diaphragm

[0090] The diaphragm is made of commercial PE membrane.

[0091] 4. Battery manufacturing

[0092] The positive electrode, negative electrode, and separator are assembled into a battery.

[0093] Example 2 (with appendix) Figure 5 correspond)

[0094] The thickness of the first active material layer is 150 μm and is the same everywhere.

[0095] The minimum thickness of the middle region of the second active material layer is 0 μm, and the maximum thickness of the edge region is 60 μm. In the width direction of the electrode sheet, the thickness of the second active material layer increases from the middle to both ends, and in the cross-section of the positive electrode sheet parallel to the width direction, the minimum thickness of the middle region of the second active material layer is a point, not a segment.

[0096] The maximum thickness of the third active material layer in the middle region is 120 μm, and the minimum thickness in the edge region is 60 μm. Along the width of the electrode sheet, the thickness of the third active material layer decreases from the middle to both ends. Furthermore, on a cross-section of the positive electrode sheet parallel to the width direction, the maximum thickness of the third active material layer in the middle region is a single point, not a segment.

[0097] The sum of the thicknesses of the first active material layer, the second active material layer, and the third active material layer is 270 μm, which means the thickness of the composite active material layer is 270 μm.

[0098] Everything else is the same as in Example 1.

[0099] Example 3 (with appendix) Figure 3 correspond)

[0100] The maximum thickness of the middle region of the first active material layer is 150 μm, and the minimum thickness of the edge region is 90 μm. In the width direction of the electrode sheet, the thickness of the first active material layer decreases from the middle to both ends. In the cross-section of the positive electrode sheet parallel to the width direction, the maximum thickness of the middle region of the first active material layer is a segment, not a point. The length of the maximum thickness region of the middle region of the first active material layer in the width direction of the positive electrode sheet is 50 mm.

[0101] The minimum thickness of the middle region of the second active material layer is 0 μm, and the maximum thickness of the edge region is 60 μm. In the width direction of the electrode sheet, the thickness of the second active material layer increases from the middle to both ends. On the cross-section of the positive electrode sheet parallel to the width direction, the minimum thickness of the middle region of the second active material layer is a segment, not a point. The length of the minimum thickness region of the middle region of the second active material layer in the width direction of the positive electrode sheet is 50 mm.

[0102] The thickness of the third active material layer is 60 μm and is the same everywhere.

[0103] The sum of the thicknesses of the first active material layer, the second active material layer, and the third active material layer is 210 μm, which means the thickness of the composite active material layer is 210 μm.

[0104] Everything else is the same as in Example 1.

[0105] Example 4 (with appendix) Figure 7 correspond)

[0106] The maximum thickness of the middle region of the first active material layer is 150 μm, and the minimum thickness of the edge region is 90 μm. In the width direction of the electrode sheet, the thickness of the first active material layer decreases from the middle to both ends. In the cross-section of the positive electrode sheet parallel to the width direction, the maximum thickness of the middle region of the first active material layer is a segment, not a point. The length of the maximum thickness region of the middle region of the first active material layer in the width direction of the positive electrode sheet is 50 mm.

[0107] The minimum thickness of the middle region of the second active material layer is 10 μm, and the maximum thickness of the edge region is 60 μm. Along the width of the electrode sheet, the thickness of the second active material layer increases from the middle to both ends, and the surface of the second active material layer away from the first active material layer is flush. Furthermore, on a cross-section of the positive electrode sheet parallel to the width direction, the minimum thickness of the middle region of the second active material layer is a segment, not a single point; the length of the minimum thickness region in the middle of the second active material layer along the width direction of the positive electrode sheet is 50 mm.

[0108] The thickness of the third active material layer is 60 μm and is the same everywhere.

[0109] The sum of the thicknesses of the first active material layer, the second active material layer, and the third active material layer is 220 μm, which means the thickness of the composite active material layer is 220 μm.

[0110] Everything else is the same as in Example 1.

[0111] Example 5:

[0112] The second active material layer contains no solid electrolyte.

[0113] Everything else is the same as in Example 1.

[0114] Example 6:

[0115] The second active material layer contains no solid electrolyte.

[0116] The Dv50 of the second active material is the same as that of the first active material, both being 8 μm.

[0117] Everything else is the same as in Example 1.

[0118] Example 7:

[0119] The maximum thickness of the edge region of the second active material layer is 25 μm, which is less than (1 / 5) the maximum thickness of the first active material layer of 30 μm (1 / 5 * 150 μm).

[0120] Everything else is the same as in Example 1.

[0121] Example 8:

[0122] The second active material is LiNi.0.5 Co 0.3 Mn 0.2 Fe 0.3 O2, with an Fe content of 0.075.

[0123] Everything else is the same as in Example 1.

[0124] Example 9:

[0125] The second active material is LiNi. 0.5 Co 0.3 Mn 0.2 Fe 0.6 O2, with a Fe content of 0.15%.

[0126] Everything else is the same as in Example 1.

[0127] Example 10:

[0128] The second active material is lithium cobalt oxide (LCO), and the rest is the same as in Example 1.

[0129] Example 11 (with appendix) Figure 1 correspond):

[0130] The thickness of the first active material layer is 100 μm and is the same everywhere.

[0131] The second active material layer has a thickness of 80 μm and is the same everywhere.

[0132] The thickness of the third active material layer is 60 μm, and it is the same everywhere.

[0133] The sum of the thicknesses of the first active material layer, the second active material layer, and the third active material layer is 240 μm, which means the thickness of the composite active material layer is 240 μm.

[0134] Comparative Example 1 (and Appendix) Figure 9 correspond):

[0135] There is no second active material layer.

[0136] Everything else is the same as in Example 1.

[0137] II. Testing of the above embodiments and comparative examples

[0138] (1) Test of the coefficient of thermal expansion

[0139] The active material powder was pressed into a thin sheet to serve as the working electrode; a half-cell was assembled and fixed on an in-situ XRD sample stage, and a charge-discharge program was set. During the charge-discharge process, XRD patterns were acquired at regular voltage / time intervals, and the cell volume V was calculated using Jade software; the volume change rate under different lithium intercalation states was calculated.

[0140] (2) Capacity retention test and cracking test

[0141] The assembled battery was charged to 3.65V at a constant temperature of 25℃ using a constant current and constant voltage of 0.1C, with a cutoff current of 0.05C. It was then discharged to 2.5V at 0.33C to obtain the discharge capacity of the first cycle. This charge-discharge cycle was continued for N cycles (1000 cycles) to obtain the discharge capacity of the Nth cycle (1000 cycles). The capacity retention rate was calculated as the discharge capacity of the Nth cycle divided by the discharge capacity of the first cycle. After 1000 cycles, the cracking of the positive electrode was observed.

[0142] (3) Resistance test

[0143] Place the assembled battery in a constant temperature environment of 25℃ and perform a short pulse charge / discharge at a current of 1C for 10s. Record the voltage change ΔU and current change ΔI before and after the pulse. Calculate the DC resistance according to Ohm's law DCR=ΔU / ΔI.

[0144] The test results are shown in Table 1 below.

[0145] Table 1 Test results of Examples 1-11 and Comparative Example 1

[0146]

[0147] As shown in Table 1, the coefficient of thermal expansion of the first active material in Examples 1-11 and Comparative Example 1 is the same, which is 5%. This is because the first active material in Examples 1-11 and Comparative Example 1 is LiNi. 0.6 Co 0.3 Mn 0.1 O2. The coefficient of thermal expansion of the third active material in Examples 1-11 and Comparative Example 1 is the same, all being 1%, because the third active material in Examples 1-11 and Comparative Example 1 is lithium iron phosphate. The coefficient of thermal expansion of the second active material in Examples 1-7 and Example 11 is the same, all being 2%, while the coefficient of thermal expansion of the second active material in Examples 8-10 is different, because the second active material in Examples 1-7 and Example 11 is LiNi. 0.6 Co 0.3 Mn 0.1 Fe 0.45 O2, and the molar content of Fe is 0.1125, while the second active material in Example 8 is LiNi. 0.5 Co 0.3 Mn 0.2 Fe 0.3 O2, and its Fe content is 0.075; the second active material in Example 9 is LiNi. 0.5 Co 0.3 Mn 0.2 Fe 0.6O2, with an Fe content of 0.15%, and lithium cobalt oxide as the second active material in Example 10. Therefore, as shown in Table 1 above, the coefficient of thermal expansion of the active material is related to the material itself and is basically unrelated to the thickness and shape of the active material layer.

[0148] As shown in Table 1, the cracking situation in Examples 1-11 is better than that in Comparative Example 1. Furthermore, the discharge capacity, resistance, and capacity retention rate of Examples 1-11 are all better than those in Comparative Example 1. This indicates that setting a second active material with an expansion coefficient between that of the ternary material and the lithium iron phosphate between the first active material (ternary material) and the third active material (lithium iron phosphate) can indeed make the expansion of the first active material layer (ternary material layer), the second active material layer, and the third active material layer (lithium iron phosphate layer) after lithium intercalation exhibit a gradient change. This can reduce the expansion difference between layers, reduce the interaction force between layers, reduce internal stress, and avoid the occurrence of interface deformation or even cracking due to stress concentration at the interface of the active material layer. In this way, the probability of deformation or even cracking of the positive electrode sheet can be reduced, the stability of the positive electrode sheet can be improved, and the performance of the battery can be enhanced.

[0149] As shown in Table 1, the cracking performance of Examples 1-9 and Example 11 is better than that of Example 10, indicating that the second active material is LiNi. x2 Co y2 M z2 Fe u O2, compared to lithium cobalt oxide as the second active material, is more conducive to improving cracking.

[0150] As shown in Table 1, the cracking conditions in Examples 1-7 and Example 11 are better than those in Examples 8 and 9. This indicates that when the second active material is LiNi... x2 Co y2 M z2 Fe u The content of O2 and Fe affects the improvement of cracking. Therefore, LiNi can be set. x2 Co y2 M z2 Fe u The molar content of Fe in O2 is 0.075-0.15.

[0151] As shown in Table 1, the capacity retention rates of Examples 1-4 are better than those of Example 5. This indicates that adding a solid electrolyte to the second active material layer is beneficial to improving the capacity retention rate.

[0152] As shown in Table 1, the capacity retention rate of Example 5 is better than that of Example 6. This indicates that the Dv50 of the second active material is smaller than that of the first active material, which is beneficial to improving the capacity retention rate.

[0153] As shown in Table 1, the capacity retention rates of Examples 1-7 are better than those of Example 11. This indicates that the thickness of the second active material layer increases from the middle to both ends in the width direction of the positive electrode sheet, while the thickness of the first active material layer decreases from the middle to both ends and / or the thickness of the third active material layer decreases from the middle to both ends in the width direction of the positive electrode sheet, which is beneficial to improving the capacity retention rate.

[0154] As shown in Table 1, the capacity retention rate of Examples 1-6 is better than that of Example 7. This indicates that the maximum thickness of the second active material layer is greater than or equal to one-fifth of the maximum thickness of the first active material layer, which is beneficial to improving the capacity retention rate.

[0155] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0156] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A positive electrode plate, characterized in that, include: current collector; as well as A composite active material layer is disposed on at least one side surface of the current collector along the thickness direction; The composite active material layer includes a first active material layer, a second active material layer, and a third active material layer. The first active material layer is disposed on one side surface of the current collector, the second active material layer is disposed on the side surface of the first active material layer away from the current collector, and the third active material layer is disposed on the side surface of the second active material layer away from the first active material layer. The first active material layer includes a first active material, the second active material layer includes a second active material, and the third active material layer includes a third active material. The coefficient of thermal expansion of the first active material is greater than the coefficient of thermal expansion of the second active material, which is greater than the coefficient of thermal expansion of the third active material. The first active material includes LiNi x1 Co y1 M z1 O2, M is Mn or Al, x1>0, y1>0, z1>0, and the third active material includes lithium iron phosphate.

2. The positive electrode sheet as described in claim 1, characterized in that, The second active material includes LiNi x2 Co y2 M z2 Fe u O2, M is Mn or Al, x2>0, y2>0, z2>0, u>0.

3. The positive electrode sheet as described in claim 2, characterized in that, LiNi x2 Co y2 M z2 Fe u The molar content of Fe in O2 is 0.075-0.15; and / or The Dv50 of the second active material is less than that of the first active material.

4. The positive electrode sheet as described in claim 1, characterized in that, In the width direction of the positive electrode, the thickness of the second active material layer increases from the middle to both ends.

5. The positive electrode sheet as described in claim 4, characterized in that, The maximum thickness of the second active material layer is greater than or equal to one-fifth of the maximum thickness of the first active material layer.

6. The positive electrode sheet as described in claim 5, characterized in that, The maximum thickness of the second active material layer is 50-70 μm; and / or The maximum thickness of the first active material layer is 120-180 μm; and / or The maximum thickness of the third active material layer is 60-120 μm; and / or The thickness of the composite active material layer is 210-270 μm.

7. The positive electrode sheet as described in claim 4, characterized in that, In the width direction of the positive electrode, the thickness of the first active material layer decreases from the middle to both ends; and / or In the width direction of the positive electrode sheet, the thickness of the third active material layer decreases from the middle to both ends.

8. The positive electrode sheet as described in claim 1, characterized in that, The second active material layer also includes a solid electrolyte.

9. The positive electrode sheet as described in claim 8, characterized in that, The solid electrolyte is at least one of PVDF-LiClO4, PEO-LiTFSI, and LLZO; and / or In the second active material layer, the content of the solid electrolyte is ≤ 20% of the content of the second active material.

10. A lithium-ion battery, characterized in that, Includes the positive electrode sheet as described in any one of claims 1-9.

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