Positive electrode sheet, method for manufacturing the same, lithium ion battery, and electric device

Abstract

The application discloses a positive electrode sheet, a preparation method thereof, a lithium ion battery and an electric device. The positive electrode sheet has a middle region and at least one outer region located at the periphery of the middle region. The polarization of the middle region is greater than that of the outer region. By making the polarization of the middle region of the positive electrode sheet greater than that of the outer region, the phenomenon that the middle temperature of the electrode sheet is high, the expansion force is large, the polarization is small, the head and bottom temperatures of the electrode sheet are low, the expansion force is small, and the polarization is large during the cycle of the battery cell is offset. The uniformity of the polarization of the middle region and the outer region of the electrode sheet is improved, the uniformity of lithium intercalation from the positive electrode to the negative electrode is improved, the problem that the lithium intercalation amount of the outer region of the electrode sheet is lower than that of the middle region is alleviated, and the problem of lithium precipitation in the middle of the battery cell is improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a positive electrode sheet and its preparation method, a lithium-ion battery, and an electrical device. Background Technology

[0002] During cell cycling, uneven temperature and expansion force lead to different polarization along the height of the electrode. Due to heat dissipation, the temperature in the middle of the electrode is high, the expansion force is large, and the polarization is small; the temperature at the head and bottom of the electrode is low, the expansion force is small, and the polarization is large. Under the influence of uneven polarization distribution, lithium ions tend to diffuse and intercalate towards the middle of the electrode, resulting in a lower lithium intercalation amount in the outer region of the electrode than in the middle region, thus causing lithium plating in the middle of the cell. Summary of the Invention

[0003] This application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode that reduces lithium plating in the middle of the battery cell.

[0004] To achieve the above objectives, embodiments of this application provide a positive electrode sheet and its preparation method, a lithium-ion battery, and an electrical device.

[0005] In a first aspect, embodiments of this application propose a positive electrode sheet having a central region and at least one outer region located around the central region, wherein the polarizability of the central region is greater than that of the outer region.

[0006] Therefore, in the technical solution of this application embodiment, the polarization of the central region of the positive electrode sheet is greater than that of the outer region, which offsets the phenomenon that the central region of the electrode sheet has a high temperature, large expansion force, and small polarization due to heat dissipation during the cell cycle, while the head and bottom of the electrode sheet have a low temperature, small expansion force, and large polarization. This improves the uniformity of polarization in the central and outer regions of the electrode sheet, improves the uniformity of lithium intercalation from the positive electrode to the negative electrode, alleviates the problem that the amount of lithium intercalated in the outer region of the electrode sheet is lower than that in the central region, and improves the problem of lithium plating in the central region of the cell.

[0007] In any embodiment, the resistivity of the active material in the central region is greater than that in the outer region. By setting the resistivity of the active material in the central region of the positive electrode to be greater than that in the outer region, the polarization performance of the central region of the positive electrode is improved. Under the influence of temperature and expansion force, the consistency of polarization performance between the central and outer regions is improved, the uniformity of lithium intercalation from the positive to the negative electrode is improved, the problem of lower lithium intercalation in the outer region of the electrode compared to the central region is alleviated, and the problem of lithium plating in the middle of the cell is improved; and / or,

[0008] The carbon coating content of the active material in the central region is less than that in the outer region. By setting the carbon coating content of the active material in the central region of the positive electrode to be less than that in the outer region, the polarization performance of the central region of the positive electrode is improved. Under the influence of temperature and expansion force, the consistency of polarization performance between the central and outer regions is improved, the uniformity of lithium intercalation from the positive to the negative electrode is improved, the problem of lower lithium intercalation in the outer region of the electrode than in the central region is alleviated, and the problem of lithium plating in the middle of the cell is improved; and / or,

[0009] The median particle size of the active material in the central region is smaller than that in the outer region. By setting the median particle size of the active material in the central region of the positive electrode to be smaller than that in the outer region, the polarization performance of the central region of the positive electrode is improved. Under the influence of temperature and expansion force, the consistency of polarization performance between the central and outer regions is improved, the uniformity of lithium intercalation from the positive electrode to the negative electrode is improved, the problem of lithium intercalation in the outer region of the electrode being lower than that in the central region is alleviated, and the problem of lithium plating in the middle of the cell is improved.

[0010] In any embodiment, the central region has a dimension of a1 in the width direction of the positive electrode sheet, and a2 in the width direction of the positive electrode sheet, where 0 < a1 / a2 < 1. By setting a central region in the width direction of the positive electrode sheet, the polarization performance of the central region is higher than that of the outer region, thereby improving the uniformity of polarization in the central and outer regions of the electrode sheet, improving the uniformity of lithium insertion from the positive electrode to the negative electrode, alleviating the problem that the lithium insertion amount in the outer region of the electrode sheet is lower than that in the central region, and improving the problem of lithium plating in the middle of the cell. When a1 / a2 is less than 1 / 10, the central region is too narrow, and there is still local lithium plating. When a1 / a2 is greater than 3 / 4, the central region is too wide, and there is also local lithium plating. Optionally, 1 / 4 ≤ a1 / a2 ≤ 3 / 4.

[0011] In any embodiment, the resistivity of the active material in the central region is greater than that in the outer region. The resistivity of the active material in the central region is R1, and the resistivity of the active material in the outer region is R2, wherein 0 Ωcm < R1 - R2 < 55 Ωcm. By controlling the resistivity of the active material in the central region to be greater than that in the outer region, the polarizability of the central region is greater than that of the outer region. Simultaneously, controlling the resistivity difference within a suitable range is beneficial for improving the consistency of polarization performance between the central and outer regions. However, when R1 - R2 ≥ 55 Ωcm, the resistivity difference between the active materials in the central and outer regions is too large, which increases the probability of lithium plating in the outer region. Optionally, 5 Ωcm < R1 - R2 < 25 Ωcm.

[0012] In any embodiment, the resistivity of the active material in the central region is greater than the resistivity of the active material in the outer region. The resistivity of the active material in the central region is R1, and the resistivity of the active material in the outer region is R2, wherein: 0 Ωcm < R1 < 60 Ωcm; and / or, 0 Ωcm < R2 < 60 Ωcm. By controlling the appropriate resistivity of the active materials in the central and outer regions, the electron transport capability can be improved. When R1 ≥ 60 Ωcm, the resistivity of the active material in the central region is too high, affecting the electron transport capability. When R2 ≥ 60 Ωcm, the resistivity of the outer region itself is too high, affecting the electron transport capability.

[0013] In any embodiment, the carbon coating content of the active material in the central region is greater than that in the outer region. The carbon coating content of the active material in the central region is W1, and the carbon coating content of the active material in the outer region is W2, wherein: 0% < W2 - W1 < 3%. By controlling an appropriate difference in carbon coating content, the polarizability of the central region is greater than that of the outer region, which is beneficial to improving the consistency of polarization performance between the central and outer regions. When W2 - W1 ≥ 3%, the difference in carbon coating content between the active materials in the central and outer regions is too large, which will increase the probability of lithium plating in the outer region. Optionally, 0.3% < W2 - W1 < 1.2%.

[0014] In any embodiment, the carbon coating content of the active material in the central region is greater than the carbon coating content of the active material in the outer region. The carbon coating content of the active material in the central region is W1, and the carbon coating content of the active material in the outer region is W2, wherein: 0% < W1 < 3%; and / or, 0% < W2 < 3%. By controlling the appropriate carbon coating content of the active materials in the central and outer regions, electron transport capability can be improved. When W1 ≥ 3%, the carbon coating content of the active material in the central region is too high, reducing the cell energy density and affecting electron transport capability. When W2 ≥ 3%, the carbon coating content of the active material in the outer region is too high, reducing the cell energy density and affecting electron transport capability.

[0015] In any embodiment, the median particle size of the active material in the central region is greater than that in the outer region. The median particle size of the active material in the central region is d1, and the median particle size of the active material in the outer region is d2, wherein 0 μm < d2 - d1 < 3 μm. By controlling an appropriate difference in median particle size, the polarizability of the central region is greater than that of the outer region, which is beneficial to improving the consistency of polarization performance between the central and outer regions. When d2 - d1 ≥ 3 μm, the difference in median particle size between the active materials in the central and outer regions is too large, which will increase the probability of lithium plating in the outer region. Optionally, 0.2 μm < d2 - d1 < 1 μm.

[0016] In any embodiment, the median particle size of the active material in the central region is greater than the median particle size of the active material in the outer region. The median particle size of the active material in the central region is d1, and the median particle size of the active material in the outer region is d2, wherein: 0 μm < d1 < 3 μm; and / or, 0 μm < d2 < 3 μm. By controlling the appropriate median particle size of the active materials in the central and outer regions, the electron and ion transport capabilities of the central cathode can be regulated. When d1 ≥ 3 μm, the median particle size of the central region itself is too large, affecting the electron and ion transport capabilities. When d2 ≥ 3 μm, the median particle size of the outer region itself is too large, affecting the electron and ion transport capabilities.

[0017] In any embodiment, the positive electrode sheet includes at least two outer regions, which are located on either side of the central region in the width direction of the positive electrode sheet. By providing outer regions on both sides of the central region in the width direction, the polarization performance at the top and bottom ends of the electrode sheet is less than that at the middle position after winding, thus improving the effect of lithium plating in the middle of the cell.

[0018] In any embodiment, the positive electrode includes a current collector, a central coating layer is disposed on the central region of the current collector, and an outer coating layer is disposed on the peripheral region of the current collector located on at least one side of the central region of the current collector. The polarizability of the central coating layer is greater than that of the outer coating layer. By forming a central region and a peripheral region on the current collector to form corresponding central and outer regions, the polarizability of the central region can be made greater than that of the outer region by changing the polarizability of the coating layer.

[0019] In any embodiment, the material of the central coating layer includes: lithium iron phosphate with a first carbon coating content, wherein the mass percentage of the carbon coating layer in the lithium iron phosphate with the first carbon coating content is 0.01 to 0.5 wt.%.

[0020] The material of the outer coating layer includes: lithium iron phosphate with a second carbon coating content, wherein the mass percentage of the carbon coating layer in the lithium iron phosphate with the second carbon coating content is 0.01 to 1.5 wt.%.

[0021] The mass percentage of carbon coating in lithium iron phosphate with the first carbon coating content is less than the mass percentage of carbon coating in lithium iron phosphate with the second carbon coating content.

[0022] The polarization performance in the central region is higher than that in the outer region by using lithium iron phosphate with a first carbon coating content that has a lower mass percentage of carbon coating layer than the lithium iron phosphate with a second carbon coating content.

[0023] Secondly, embodiments of this application propose a method for preparing a positive electrode sheet, comprising the following steps:

[0024] Provide current collectors;

[0025] A central coating layer is provided in the middle region of at least one side of the current collector, and the middle region of the current collector has at least one peripheral region, the peripheral region being provided with an outer coating layer. The polarizability of the central coating layer is greater than that of the outer coating layer, thus obtaining a positive electrode.

[0026] By providing a central coating layer in the middle region of at least one side of the current collector, and having at least one peripheral region around the central region of the current collector, and providing an outer coating layer in the peripheral region, a positive electrode sheet can be obtained in which the polarization of the central coating layer is greater than that of the outer coating layer.

[0027] Thirdly, embodiments of this application provide a lithium-ion battery, including the positive electrode sheet of the first aspect of this application.

[0028] In any embodiment, the lithium-ion battery includes a primary lithium-ion battery and a secondary lithium-ion battery.

[0029] Fourthly, embodiments of this application provide an electrical device including a lithium-ion battery according to the third aspect of this application. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of the structure of a positive electrode sheet in one or more embodiments;

[0032] Figure 2 This is a schematic diagram of the structure of a positive electrode sheet in one or more embodiments;

[0033] Figure 3 This is a schematic diagram of the dimensions and structure of the central region of the positive electrode sheet in one or more embodiments.

[0034] Explanation of reference numerals in the accompanying drawings of the embodiments of this application:

[0035] label name label name 100 Positive electrode film <![CDATA[a1]]> Dimensions of the central region in the width direction of the positive electrode plate 1 Central region <![CDATA[a2]]> Dimensions of the positive electrode in the width direction 2 outer region

[0036] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0037] The following details embodiments of the positive electrode sheet, the method for preparing the positive electrode sheet, the solar power generation device, and the power consumption device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0038] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0040] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0041] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0042] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0043] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0044] During cell cycling, uneven temperature and expansion force lead to different polarization along the height of the electrode. Due to heat dissipation, the temperature in the middle of the electrode is high, the expansion force is large, and the polarization is small; the temperature at the head and bottom of the electrode is low, the expansion force is small, and the polarization is large. Under the influence of uneven polarization distribution, lithium ions tend to diffuse and intercalate to the middle of the electrode, resulting in a lower lithium intercalation amount at the head and bottom of the electrode than in the middle, thus causing lithium plating in the middle of the cell.

[0045] Therefore, numerous technologies have emerged to improve lithium plating in the middle of battery cells. For example, one negative electrode sheet has a higher negative electrode capacity per unit area in the edge layer than in the middle layer. When the negative electrode capacity per unit area in the middle layer equals the designed negative electrode capacity per unit area, it is equivalent to increasing the negative electrode capacity of the edge layer. Thus, even if the thickness of the outer edge of the edge layer gradually decreases due to slurry flow, the negative electrode capacity of the edge layer can be ensured not to be lower than the designed negative electrode capacity. This avoids insufficient negative electrode capacity caused by flow or edge effects at the edge of the coating layer, reducing the risk of lithium plating in this negative electrode sheet. However, because graphite with high specific capacity at the edges often has a smaller interlayer spacing and higher graphitization degree compared to graphite with low specific capacity in the middle, its limiting charge capability is lower. Increasing the specific capacity of graphite can improve lithium plating caused by insufficient CB (cumulative charge), but the insufficient charging capability of the graphite material itself may further lead to lithium plating.

[0046] Surprisingly, by setting the polarization performance of the central region of the positive electrode to be higher than that of at least one outer region, the difference in polarization performance between the central and outer regions of the positive electrode caused by inconsistent temperature and expansion force can be mitigated. This improves the uniformity of polarization in the central and outer regions of the electrode, enhances the uniformity of lithium intercalation from the positive electrode to the negative electrode, alleviates the problem that the amount of lithium intercalated in the outer region of the electrode is lower than that in the central region, and improves the problem of lithium plating in the middle of the cell.

[0047] Based on this, embodiments of this application provide a positive electrode sheet and its preparation method, a lithium-ion battery, and an electrical device.

[0048] Please see Figure 1 and Figure 2 In a first aspect, embodiments of this application provide a positive electrode 100 having a central region 1 and at least one outer region 2 located around the central region 1, wherein the polarizability of the central region 1 is greater than the polarizability of the outer region 2.

[0049] Therefore, in the technical solution of this application embodiment, by having a greater polarizability in the middle region 1 of the positive electrode 100 than in the outer region 2, the phenomenon that the middle part of the electrode has a high temperature, large expansion force, and small polarization due to heat dissipation during the cell cycle is offset, while the head and bottom of the electrode have a low temperature, small expansion force, and large polarization. This improves the uniformity of polarizability in the middle and outer regions 2 of the electrode, improves the uniformity of lithium intercalation from the positive electrode to the negative electrode, alleviates the problem that the amount of lithium intercalated in the outer region of the electrode is lower than that in the middle region, and improves the problem of lithium plating in the middle of the cell.

[0050] In any embodiment, the resistivity of the central region 1 is greater than that of the outer region 2. By setting the resistivity of the central region 1 of the positive electrode 100 to be greater than that of the outer region 2, the polarization performance of the central region 1 of the positive electrode 100 is improved. Under the influence of temperature and expansion force, the consistency of polarization performance between the central region and the outer region 2 is improved, the uniformity of lithium intercalation from the positive electrode to the negative electrode is improved, the problem of lower lithium intercalation in the outer region of the electrode than in the central region is alleviated, and the problem of lithium plating in the middle of the cell is improved; and / or,

[0051] The carbon coating content in the middle region 1 is less than that in the outer region 2. By setting the carbon coating content in the middle region 1 of the positive electrode 100 to be less than that in the outer region 2, the polarization performance of the middle region 1 of the positive electrode 100 is improved. Under the influence of temperature and expansion force, the consistency of polarization performance between the middle region and the outer region 2 is improved, the uniformity of lithium intercalation from the positive electrode to the negative electrode is improved, the problem of lithium intercalation in the outer region of the electrode being lower than that in the middle region is alleviated, and the problem of lithium plating in the middle of the cell is improved; and / or,

[0052] The median particle size of the central region 1 is smaller than that of the outer region 2. By setting the median particle size of the central region 1 of the positive electrode 100 to be smaller than that of the outer region 2, the polarization performance of the central region 1 of the positive electrode 100 is improved. Under the influence of temperature and expansion force, the consistency of polarization performance between the central region and the outer region 2 is improved, the uniformity of lithium intercalation from the positive electrode to the negative electrode is improved, the problem of lithium intercalation in the outer region of the electrode being lower than that in the central region is alleviated, and the problem of lithium plating in the middle of the cell is improved.

[0053] Please see Figure 3 In any embodiment, the central region 1 has a dimension a1 in the width direction of the positive electrode 100, and the positive electrode 100 has a dimension a2 in the width direction, wherein: 0 < a1 / a2 < 1, a1 / a2 can be 1 / 20, 1 / 15, 1 / 10, 1 / 5, 1 / 3, 1 / 2, 3 / 4, 4 / 5; and / or, 1 / 4 ≤ a1 / a2 ≤ 3 / 4. By setting a central region 1 of a certain size in the width direction of the positive electrode 100, the polarization performance of the central region 1 is higher than that of the outer region 2, thereby improving the uniformity of polarization of the central region and the outer region 2 of the electrode, improving the uniformity of lithium insertion from the positive electrode to the negative electrode, alleviating the problem that the lithium insertion amount in the outer region of the electrode is lower than that in the central region, and improving the problem of lithium plating in the middle of the cell.

[0054] In any embodiment, the resistivity of the active material in the central region is greater than that in the outer region. The resistivity of the active material in the central region is R1, and the resistivity of the active material in the outer region is R2, wherein: 0 Ωcm < R1 - R2 < 55 Ωcm; R1 - R2 can be 2 Ωcm, 5 Ωcm, 10 Ωcm, 15 Ωcm, 20 Ωcm, 25 Ωcm, 30 Ωcm, 35 Ωcm, 40 Ωcm, 45 Ωcm, 50 Ωcm, or 54 Ωcm. By controlling the resistivity of the active material in the central region to be greater than that in the outer region, the polarizability of the central region 1 is greater than that of the outer region 2. Simultaneously, the resistivity difference is controlled within a suitable range, which is beneficial for improving the consistency of polarization performance between the central region and the outer region 2. However, when R1 - R2 ≥ 55 Ωcm, the resistivity difference between the active materials in the central and outer regions is too large, which increases the probability of lithium plating in the outer region. Optionally, 5 Ωcm < R1 - R2 < 25 Ωcm.

[0055] In any embodiment, the resistivity of the active material in the central region is greater than the resistivity of the active material in the outer region. The resistivity of the active material in the central region is R1, and the resistivity of the active material in the outer region is R2, wherein: 0 Ωcm < R1 < 60 Ωcm; and / or, 0 Ωcm < R2 < 60 Ωcm. By controlling the appropriate resistivity of the central region 1 and the outer region 2, the electron transport capability can be improved. When R1 ≥ 60 Ωcm, the resistivity of the central region 1 itself is too high, affecting the electron transport capability. When R2 ≥ 60 Ωcm, the resistivity of the outer region 2 itself is too high, affecting the electron transport capability.

[0056] In any embodiment, the carbon coating content of the active material in the central region is greater than that in the outer region. The carbon coating content of the active material in the central region is W1, and the carbon coating content of the active material in the outer region is W2, wherein 0% < W2 - W1 < 3%. By controlling the appropriate difference in carbon coating content, the polarizability of the central region 1 is greater than that of the outer region 2, which is beneficial to improving the consistency of polarization performance between the central region and the outer region 2. When W2 - W1 ≥ 3%, the difference in carbon coating content between the active materials in the central region and the outer region is too large, which will increase the probability of lithium plating in the outer region. W2 - W1 can be 0.1%, 0.2%, 0.3%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 2.0%, 2.5%, or 2.9%. Optionally, 0.3% < W2 - W1 < 1.2%.

[0057] In any embodiment, the carbon coating content of the active material in the central region is greater than the carbon coating content of the active material in the outer region. The carbon coating content of the active material in the central region is W1, and the carbon coating content of the active material in the outer region is W2, wherein: 0% < W1 < 3%; and / or, 0% < W2 < 3%. By controlling the appropriate carbon coating content of the central region 1 and the outer region 2, the electron transport capability can be improved. When W1 ≥ 3%, the carbon coating content of the active material in the central region is too high, reducing the cell energy density and affecting the electron transport capability. When W2 ≥ 3%, the carbon coating content of the outer region is too high, reducing the cell energy density and affecting the electron transport capability.

[0058] In any embodiment, the median particle size of the active material in the central region is greater than that in the outer region. The median particle size of the active material in the central region is d1, and the median particle size of the active material in the outer region is d2, wherein: 0 μm < d2 - d1 < 3 μm; d2 - d1 can be 0.1 μm, 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.5 μm, 2.0 μm, 2.5 μm, or 2.9 μm. By controlling a suitable difference in median particle size, the polarizability of the central region 1 is greater than that of the outer region 2, which is beneficial to improving the consistency of polarizability performance between the central region and the outer region 2. When d2 - d1 ≥ 3 μm, the difference in median particle size between the active materials in the central region and the outer region is too large, which will increase the probability of lithium plating in the outer region. Optionally, 0.2 μm < d2 - d1 < 1 μm.

[0059] In any embodiment, the median particle size of the active material in the central region is greater than the median particle size of the active material in the outer region. The median particle size of the active material in the central region is d1, and the median particle size of the active material in the outer region is d2, wherein: 0 μm < d1 < 3 μm; and / or, 0 μm < d2 < 3 μm. By controlling the appropriate median particle size of the active materials in the central and outer regions, the electron and ion transport capabilities of the central cathode can be regulated. When d1 ≥ 3 μm, the median particle size of the central region itself is too large, affecting the electron and ion transport capabilities. When d2 ≥ 3 μm, the median particle size of the outer region itself is too large, affecting the electron and ion transport capabilities.

[0060] In any embodiment, the positive electrode 100 includes at least two outer regions 2, which are located on both sides of the central region 1 in the width direction of the positive electrode 100. By providing outer regions 2 on both sides of the central region 1 in the width direction, the polarization performance at the top and bottom ends of the electrode after winding is less than that at the middle position, thus improving the effect of lithium plating in the middle of the cell.

[0061] In any embodiment, the positive electrode 100 includes a current collector, a central coating layer is disposed on the central region of the current collector, and an outer coating layer is disposed on the peripheral region of the current collector located on at least one side of the central region of the current collector. The polarizability of the central coating layer is greater than that of the outer coating layer. By forming a central region and a peripheral region on the current collector to form a corresponding central region 1 and an outer region 2, the polarizability of the central region can be made greater than that of the outer region by changing the polarizability of the coating layer.

[0062] In any embodiment, the material of the central coating layer includes: lithium iron phosphate with a first carbon coating content, wherein the mass percentage of the carbon coating layer in the lithium iron phosphate with the first carbon coating content is 0.01 to 0.5 wt.%.

[0063] The material of the outer coating layer includes: lithium iron phosphate with a second carbon coating content, wherein the mass percentage of the carbon coating layer in the lithium iron phosphate with the second carbon coating content is 0.01 to 1.5 wt.%.

[0064] The mass percentage of carbon coating in lithium iron phosphate with the first carbon coating content is less than the mass percentage of carbon coating in lithium iron phosphate with the second carbon coating content.

[0065] The polarization performance in the central region is higher than that in the outer region by using lithium iron phosphate with a first carbon coating content that has a lower mass percentage of carbon coating layer than the lithium iron phosphate with a second carbon coating content.

[0066] In any embodiment, the material of the central coating layer includes, but is not limited to, at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium nickel cobalt aluminum oxide. The polarization of the central region can be higher than that of the outer region by changing the type of active material in the central coating layer, changing the particle size or carbon coating content of the active material in the central coating layer.

[0067] Secondly, embodiments of this application provide a method for preparing a positive electrode sheet, which includes the following steps:

[0068] Provide current collectors;

[0069] A central coating layer is provided in the middle region of at least one side of the current collector, and the middle region of the current collector has at least one peripheral region, the peripheral region being provided with an outer coating layer. The polarizability of the central coating layer is greater than that of the outer coating layer, thus obtaining a positive electrode.

[0070] By providing a central coating layer in the middle region of at least one side of the current collector, and having at least one peripheral region around the central region of the current collector, and providing an outer coating layer in the peripheral region, a positive electrode sheet can be obtained in which the polarization of the central coating layer is greater than that of the outer coating layer.

[0071] Thirdly, embodiments of this application provide a lithium-ion battery, including the positive electrode sheet of the first aspect of this application.

[0072] In any embodiment, the lithium-ion battery includes a primary lithium-ion battery and a secondary lithium-ion battery.

[0073] Fourthly, embodiments of this application provide an electrical device including a lithium-ion battery according to the third aspect of this application.

[0074] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0075] Example 1

[0076] A positive electrode plate in which the polarizability of the central region is greater than that of the outer region.

[0077] Example 2

[0078] A positive electrode plate in which the polarizability of the central region is greater than that of an outer region.

[0079] The parameters of the positive electrode sheets of Examples 3 to 35 and Comparative Examples 1 to 3 of this application are as shown in Table 1.

[0080] Table 1. Parameters of the cathodes in Examples 3 to 26 and Comparative Examples 1 to 6.

[0081]

[0082]

[0083] Example 27

[0084] A method for preparing a positive electrode sheet includes the following steps:

[0085] Provide current collectors;

[0086] A central coating layer is provided in the middle region of at least one side of the current collector, and the middle region of the current collector has at least one peripheral region, the peripheral region being provided with an outer coating layer. The polarizability of the central coating layer is greater than that of the outer coating layer, thus obtaining a positive electrode.

[0087] The positive electrode sheets of Examples 1 to 26 and Comparative Examples 1 to 6 were used to prepare lithium-ion secondary batteries. The negative electrode sheet, electrolyte, separator, and lithium-ion secondary battery can be prepared using the following methods:

[0088] Preparation of negative electrode:

[0089] Artificial graphite (specific capacity 340 mAh / g), conductive agent acetylene black, and binder SBR+CMC were mixed in a weight ratio of 95:1.5:3.1:0.4. Deionized water was added as solvent, and the mixture was stirred thoroughly to obtain a negative electrode slurry. This slurry was then coated onto both surfaces of the copper foil used as the negative electrode current collector. The coating weight of the negative electrode slurry was 0.108 g / 1540.25 mm. 2(Based on weight excluding solvent), the negative electrode sheet is obtained after drying and cold pressing.

[0090] Electrolyte preparation:

[0091] In an argon atmosphere glove box with a water content of <10ppm, EC, PC, and DMC were mixed in a weight ratio of EC:PC:DMC = 3:3:3. Then, LiPF6, VC, DTD, and PS were added to the mixed organic solvent and stirred until homogeneous to obtain the electrolyte. The concentration of LiPF6 in the lithium-ion battery electrolyte was 1 mol / L, and the mass percentages of VC, DTD, and PS were 3%, 1%, and 1%, respectively.

[0092] Preparation of the separating membrane:

[0093] Polyethylene porous membrane is used as the separation membrane.

[0094] Preparation of lithium-ion secondary batteries:

[0095] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. These are then wound to obtain a bare cell. The bare cell is placed in an outer package, infused with prepared electrolyte, and sealed for formation to obtain a lithium-ion secondary battery.

[0096] The following tests were conducted:

[0097] Room temperature cycle performance test of lithium-ion secondary batteries

[0098] At 25℃, the lithium-ion secondary battery was first charged at a constant current of 1C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour) to a voltage of 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C. After resting for 5 minutes, the lithium-ion secondary battery was discharged at a constant current of 1C to a voltage of 2.5V. This constitutes one charge-discharge cycle, and the discharge capacity of this cycle is the discharge capacity of the first cycle. The lithium-ion secondary battery was subjected to multiple charge-discharge cycles using the above method until the discharge capacity of the lithium-ion secondary battery decreased to 80%, and the number of cycles was recorded.

[0099] High-temperature cycle performance test of lithium-ion secondary batteries

[0100] At 60℃, the lithium-ion secondary battery was first charged at a constant current of 1C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour) to a voltage of 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C. After resting for 5 minutes, the lithium-ion secondary battery was discharged at a constant current of 1C to a voltage of 2.5V. This constitutes one charge-discharge cycle, and the discharge capacity of this cycle is the discharge capacity of the first cycle. The lithium-ion secondary battery was subjected to multiple charge-discharge cycles using the above method, and the discharge capacity of the 1000th cycle was measured.

[0101] The capacity retention rate of a lithium-ion secondary battery after 500 cycles at 60℃ = (discharge capacity of the 500th cycle / discharge capacity of the first cycle) × 100%.

[0102] The results of the statistical experiment are shown in Table 2.

[0103] Table 2 Performance determination of the positive electrode sheets in Examples 1 to 26 and Comparative Examples 1 to 6

[0104]

[0105]

[0106] As can be seen from Table 2, through Examples 1 to 2 and Comparative Examples 1 to 6, setting the polarization of the central position in the positive electrode sheet to be greater than that of the outer region can reduce the probability of lithium plating in the center of the cell.

[0107] As shown in Examples 3 to 7, by setting a central region in the width direction of the positive electrode sheet, the polarization performance of the central region is higher than that of at least one outer region, thereby improving the uniformity of polarization in the central and outer regions of the electrode sheet, improving the uniformity of lithium intercalation from the positive electrode to the negative electrode, alleviating the problem that the lithium intercalation amount in the outer region of the electrode sheet is lower than that in the central region, and improving the problem of lithium plating in the middle of the cell. The ratio of the central region in the width direction of the positive electrode sheet to the width dimension of the positive electrode sheet affects the effect of improving lithium plating in the middle. When a1 / a2 is less than 1 / 10, the central region is too narrow, and there is still local lithium plating. When a1 / a2 is greater than 3 / 4, the central region is too wide, and there is still local lithium plating.

[0108] As shown in Examples 8 to 13, by controlling the resistivity of the active material in the central region to be greater than that in the outer region, the polarizability of the central region is greater than that of the outer region. Simultaneously, controlling the resistivity difference within a suitable range is beneficial for improving the consistency of polarization performance between the central and outer regions. However, when R1-R2 ≥ 55 Ωcm, an excessively large resistivity difference between the active materials in the central and outer regions increases the probability of lithium plating in the outer region.

[0109] As can be seen from Examples 14 to 19, by controlling the appropriate difference in carbon coating content, the polarizability of the central region is made greater than that of the outer region, which is beneficial to improving the consistency of polarization performance between the central and outer regions. When W2-W1≥3%, the difference in carbon coating content between the active materials in the central and outer regions is too large, which will increase the probability of lithium plating in the outer region.

[0110] As shown in Examples 20 to 25, by controlling a suitable difference in median particle size, the polarizability of the central region is made greater than that of the outer region, which is beneficial to improving the consistency of polarization performance between the central and outer regions. When d2-d1≥3μm, the difference in median particle size between the active materials in the central and outer regions is too large, which will increase the probability of lithium plating in the outer region.

[0111] In summary, the positive electrode proposed in this application achieves a higher polarization in the central region than in the outer region. This counteracts the heat dissipation effect during cell cycling, which results in a higher temperature, greater expansion force, and lower polarization in the central region of the electrode, compared to a lower temperature, less expansion force, and higher polarization at the head and bottom of the electrode. This improves the uniformity of polarization in both the central and outer regions of the electrode, enhances the uniformity of lithium intercalation from the positive to the negative electrode, alleviates the problem of lower lithium intercalation in the outer region compared to the central region, and improves the problem of lithium plating in the central part of the cell.

[0112] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the patent protection scope of the present invention.

Claims

1. A positive electrode sheet characterized by comprising: The middle region has a polarization greater than that of the at least one outer region; The carbon-coated content of the active material in the middle region is less than that of the active material in the outer region, the carbon-coated content of the active material in the middle region being W1, and the carbon-coated content of the active material in the outer region being W2, wherein 0.3%≤W2-W1≤2%.

2. The positive electrode sheet according to claim 1, wherein The resistivity of the active material in the middle region is greater than that of the active material in the outer region; and / or The median particle size of the active material in the middle region is less than that of the active material in the outer region.

3. The positive electrode sheet according to claim 1, wherein The dimension of the middle region in the width direction of the positive electrode sheet is a1, and the dimension of the positive electrode sheet in the width direction is a2, wherein: 0 4. The positive electrode sheet according to claim 3, wherein 1 / 4≤a1 / a2≤3 / 4.

5. The positive electrode sheet according to claim 1, wherein The resistivity of the active material in the middle region is greater than that of the active material in the outer region, the resistivity of the active material in the middle region being R1, and the resistivity of the active material in the outer region being R2, wherein: 0Ω cm 6. The positive electrode sheet according to claim 5, wherein 5Ω cm 7. The positive electrode sheet according to claim 1, wherein The resistivity of the active material in the middle region is greater than that of the active material in the outer region, the resistivity of the active material in the middle region being R1, and the resistivity of the active material in the outer region being R2, wherein: 0Ω cm 0Ω cm 8. The positive electrode sheet according to claim 1, wherein 0.3%≤W2-W1≤1.2%.

9. The positive electrode sheet according to claim 1, wherein The carbon-coated content of the active material in the middle region is less than that of the active material in the outer region, the carbon-coated content of the active material in the middle region being W1, and the carbon-coated content of the active material in the outer region being W2, wherein: 0%≤W1≤3%; and / or 0%＜W2＜3%。 10. The positive electrode sheet according to claim 1, wherein The median particle size of the active material in the middle region is less than that of the active material in the outer region, the median particle size of the active material in the middle region being d1, and the median particle size of the active material in the outer region being d2, wherein: 0μm 11. The positive electrode sheet according to claim 10, wherein 0.2μm 12. The positive electrode sheet according to claim 1, wherein The median particle size of the active material in the middle region is less than that of the active material in the outer region, the median particle size of the active material in the middle region being d1, and the median particle size of the active material in the outer region being d2, wherein: 0μm 0μm 13. The positive electrode sheet according to claim 1, wherein The positive electrode sheet comprises at least two outer regions, which are located on both sides of the middle region in the width direction of the positive electrode sheet.

14. The positive electrode sheet according to claim 1, wherein The positive electrode sheet comprises a current collector, the middle region on the current collector is provided with a middle coating layer, and the outer region on the current collector and located on at least one side of the middle region of the current collector is provided with an outer coating layer, the polarization of the middle coating layer being greater than that of the outer coating layer.

15. The positive electrode sheet according to claim 14, wherein The material of the middle coating layer comprises: a first carbon-coated lithium iron phosphate, wherein the mass percentage of the carbon coating layer in the first carbon-coated lithium iron phosphate is 0.01-0.5wt.%. The material of the outer coating layer comprises: a second carbon-coated lithium iron phosphate, wherein the mass percentage of the carbon coating layer in the second carbon-coated lithium iron phosphate is 0.01-1.5wt.%. The mass percentage of the carbon coating layer in the first carbon-coated lithium iron phosphate is less than the mass percentage of the carbon coating layer in the second carbon-coated lithium iron phosphate.

16. A method for producing a positive electrode sheet, characterized by The method comprises the following steps: Providing a current collector; A middle coating layer is arranged on at least one side of the middle region of the current collector, and the periphery of the middle region of the current collector has at least one outer region, and the outer region is provided with an outer coating layer, the polarization of the middle coating layer is greater than the polarization of the outer coating layer, and a positive electrode sheet is obtained. The carbon-coated content of the active material in the middle region is less than the carbon-coated content of the active material in the outer region, the carbon-coated content of the active material in the middle region is W1, and the carbon-coated content of the active material in the outer region is W2, wherein 0.3%≤W2-W1≤2%.

17. A lithium-ion battery, characterized by, The positive electrode sheet comprises any one of claims 1-15.

18. The lithium-ion battery of claim 17, wherein, The lithium ion battery comprises a lithium ion primary battery and a lithium ion secondary battery.

19. An electrical device, comprising: The lithium ion battery comprises any one of claims 17 or 18. The lithium ion battery comprises any one of claims 17 or 18.

Images