Positive plate and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a positive plate which comprises a positive current collector and a positive coating coated on at least one side surface of the positive current collector, the contact impedance R of the positive electrode coating and the positive electrode current collector ranges from 0.0012 ohm cm < 2 > to 0.0225 ohm cm < 2 >; wherein the positive electrode coating comprises a positive electrode active material, the positive electrode active material comprises a first active material and a second active material, and the positive electrode current collector, the first active material and the second active material meet corresponding conditions. Therefore, by optimizing the structures and parameters of the positive electrode current collector and the positive electrode coating in the positive plate, the bonding quality of the positive electrode current collector and the positive electrode coating is effectively improved, so that the energy density, the rate capability and the cycling stability of the lithium ion battery are improved. The invention also discloses a lithium ion battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a positive electrode sheet and a lithium-ion battery. Background Art

[0002] With the widespread application of lithium-ion batteries in new energy vehicles, energy storage systems, and portable electronic devices, the requirements for their energy density, rate performance, and cycle life continue to increase. Among them, the performance of the positive electrode directly affects the overall performance of the lithium-ion battery.

[0003] Positive electrode sheets in related art generally consist of aluminum foil and a positive electrode active material layer coated on the surface of the aluminum foil. The bonding between the aluminum foil and the positive electrode active material layer directly affects the energy density, rate capability, and cycle life of lithium-ion batteries. To this end, related art provides a positive electrode sheet comprising a positive electrode active material, the positive electrode active material comprising a first positive electrode active material and a second positive electrode active material; the first positive electrode active material comprises single crystal particles, and the second positive electrode active material comprises polycrystalline particles.

[0004] During the implementation of the embodiments of the present disclosure, it was found that at least the following problems exist in the related art:

[0005] By adjusting the structure of the positive electrode active material, the bonding relationship between the aluminum foil and the positive electrode active material layer can be improved. However, when lithium-ion batteries are frequently charged and discharged, especially under high-rate charge and discharge and long-cycle conditions, the positive electrode active material particles are very easy to fall off from the surface of the aluminum foil, which not only affects its energy density, but also affects the cycle stability and service life.

[0006] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to ordinary technicians in this field. Summary of the Invention

[0007] In order to provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. The summary is not an extensive review, nor is it intended to identify key / critical elements or delineate the scope of protection of these embodiments, but rather serves as a prelude to the detailed description that follows.

[0008] The embodiments of the present disclosure provide a positive electrode sheet and a lithium-ion battery, which effectively improve the bonding quality between the positive electrode collector and the positive electrode coating by optimizing the structure and parameters of the positive electrode sheet, thereby improving the energy density, rate performance and cycle stability of the lithium-ion battery.

[0009] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode coating coated on at least one side of the positive electrode current collector; the contact resistance R between the positive electrode coating and the positive electrode current collector is in the range of 0.0012Ωcm2 ~0.0225Ωcm 2 ; wherein the positive electrode coating includes a positive electrode active material, the positive electrode active material includes a first active material and a second active material, and the positive electrode current collector, the first active material and the second active material meet the following conditions:

[0010]

[0011] Among them, D is the compaction density of the positive electrode sheet, V is the volume fraction of the first active material, C is the reduction rate of the thickness of the positive electrode collector, which ranges from 0.10 to 0.4, and G is the particle size ratio of the particles of the first active material to the particles of the second active material, which ranges from 1.33 to 5.6.

[0012] In some embodiments, the particle size D of the first active material is 50 ≥8μm, the particle size D of the second active substance 50 ≤6μm; wherein the volume fraction V of the first active material ranges from 5% to 25%.

[0013] In some embodiments, the reduction rate of the thickness of the positive electrode current collector is calculated as follows:

[0014]

[0015] Wherein, T1 is the thickness of the compressed positive electrode current collector, and T0 is the thickness of the uncompressed positive electrode current collector.

[0016] In some embodiments, the positive electrode current collector, the first active material, and the second active material further satisfy the following conditions:

[0017] When the particle size ratio R of the first type of active particulate matter to the second type of active particulate matter is less than 2.0, the reduction rate C of the thickness of the positive electrode current collector is greater than or equal to 0.15; and / or

[0018] When the particle size ratio R of the first type of active particulate matter to the second type of active particulate matter is greater than 4.0, the reduction rate C of the thickness of the positive electrode current collector is less than or equal to 0.35.

[0019] In some embodiments, the positive electrode active material of the positive electrode coating includes a ternary material or a lithium cobalt oxide material, and the first active material and the second active material are the same type of active material.

[0020] In some embodiments, the elements of the positive electrode active material include Ni, Co, Mn and Al; and the elements of the positive electrode active material further include one or more of Zr, Ti, Sr, Mg, Na, Mo, Ca, Ba, La and Y.

[0021] In some embodiments, the first active material comprises polycrystalline particles, and the second active material comprises single crystal particles or polycrystalline particles.

[0022] In some embodiments, the surface capacity of the positive electrode is in the range of 2.55 mAh / cm 2 ~3.55mAh / cm 2 .

[0023] In some embodiments, the positive electrode coating further includes a positive electrode conductor and a positive electrode binder; wherein the mass ratio of the positive electrode conductor, the positive electrode binder, and the positive electrode active material is in the range of 1:1:98 to 10:10:80.

[0024] In some embodiments, the positive electrode conductive agent includes carbon black or carbon nanotubes; and the positive electrode binder includes polyvinylidene fluoride.

[0025] In some embodiments, the compaction density D of the positive electrode sheet is in the range of 3.2 g / cm 3 ~3.8g / cm 3 .

[0026] In some embodiments, the lithium-ion battery includes a negative electrode sheet and a positive electrode sheet as described in the above embodiments.

[0027] The positive electrode sheet and lithium-ion battery provided by the embodiments of the present disclosure can achieve the following technical effects:

[0028] The positive electrode coating includes a positive electrode active material, and the positive electrode active material includes a first active material and a second active material; wherein, by optimizing the parameter relationship between the compaction density of the positive electrode sheet, the volume fraction of the first active material, the reduction rate of the thickness of the positive electrode collector, and the particle size ratio between the particles of the first active material and the particles of the second active material, the active material particles are in close contact with the positive electrode collector, thereby constructing an efficient electron and ion transmission channel, thereby improving the electron transmission efficiency.

[0029] At the same time, by optimizing the above parameters, the microstructure of the positive electrode sheet is also changed, thereby improving the bonding quality of the positive electrode current collector and the positive electrode coating, and further improving the energy density, rate performance and cycle stability of the lithium-ion battery.

[0030] The above general description and the following description are exemplary and explanatory only and are not intended to limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] One or more embodiments are exemplarily described by corresponding drawings. These exemplary descriptions and drawings do not limit the embodiments. Elements with the same reference numerals in the drawings are shown as similar elements. The drawings do not constitute a scale limitation. In addition,

[0032] Figure 1 Schematic diagram of a cathode fluid provided in an embodiment of the present disclosure. DETAILED DESCRIPTION

[0033] In order to be able to understand the features and technical content of the embodiments of the present disclosure in more detail, the implementation of the embodiments of the present disclosure is described in detail below in conjunction with the accompanying drawings. The accompanying drawings are for reference only and are not used to limit the embodiments of the present disclosure. In the following technical description, for the sake of convenience of explanation, a full understanding of the disclosed embodiments is provided through multiple details. However, one or more embodiments can still be implemented without these details. In other cases, to simplify the drawings, well-known structures and devices can be simplified for display.

[0034] In the description and claims of the embodiments of the present disclosure, as well as in the accompanying drawings, the terms "first," "second," and the like are used to distinguish similar items and are not necessarily used to describe a particular order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate to describe the embodiments of the present disclosure herein. In addition, the terms "including," "having," and any variations thereof are intended to cover non-exclusive inclusions.

[0035] In the embodiments of the present disclosure, the terms "upper", "lower", "inside", "middle", "outside", "front", "back" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings. These terms are mainly intended to better describe the embodiments of the present disclosure and their embodiments, and are not intended to limit the indicated devices, elements or components to having a specific direction, or to be constructed and operated in a specific direction. Moreover, in addition to being used to indicate directions or positional relationships, some of the above terms may also be used to indicate other meanings. For example, the term "upper" may also be used to indicate a certain dependency or connection relationship in certain circumstances. For those of ordinary skill in the art, the specific meanings of these terms in the embodiments of the present disclosure can be understood according to specific circumstances.

[0036] Furthermore, the terms "disposed," "connected," and "fixed" should be interpreted broadly. For example, "connected" can mean a fixed connection, a removable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection, an indirect connection through an intermediary, or an internal connection between two devices, elements, or components. Those skilled in the art will understand the specific meanings of these terms in the embodiments of this disclosure based on the specific circumstances.

[0037] Unless otherwise stated, the term "plurality" means two or more.

[0038] In the embodiment of the present disclosure, the character " / " indicates that the preceding and following objects are in an "or" relationship. For example, A / B means: A or B.

[0039] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0040] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present disclosure can be combined with each other.

[0041] In related technologies, the bonding relationship between the aluminum foil and the positive electrode active material layer can be improved by adjusting the structure of the positive electrode active material. However, when lithium-ion batteries are frequently charged and discharged, especially under high-rate charge and discharge and long-cycle conditions, the positive electrode active material particles are very easy to fall off from the surface of the aluminum foil, which not only affects its energy density, but also affects the cycle stability and service life.

[0042] Furthermore, the limited contact area between the positive electrode active material particles and the aluminum foil significantly hinders efficient electron transfer between the two. This results in lower electron transfer efficiency, exacerbating battery polarization, increasing the voltage drop during high-rate discharge, significantly reducing discharge capacity, and severely weakening the battery's rate performance. Furthermore, poor electron transfer conditions accelerate the destruction of the active material structure during cycling, further deteriorating the battery's cycling stability.

[0043] The present disclosure provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode coating coated on at least one side of the positive electrode current collector; the contact resistance R between the positive electrode coating and the positive electrode current collector is in the range of 0.0012 Ωcm 2 ~0.0225Ωcm 2 ;

[0044] The positive electrode coating includes a positive electrode active material, the positive electrode active material includes a first active material and a second active material, and the positive electrode current collector, the first active material and the second active material meet the following conditions:

[0045]

[0046] Among them, D is the compaction density of the positive electrode sheet, V is the volume fraction of the first active material, C is the reduction rate of the thickness of the positive electrode collector, which ranges from 0.10 to 0.4, and G is the particle size ratio of the particles of the first active material to the particles of the second active material, which ranges from 1.33 to 5.6.

[0047] The positive electrode sheet provided by the embodiment of the present disclosure has a positive electrode coating including a positive electrode active material, and the positive electrode active material includes a first active material and a second active material; wherein, by optimizing the parameter relationship between the compaction density of the positive electrode sheet, the volume fraction of the first active material, the reduction rate of the thickness of the positive electrode collector, and the particle size ratio of the particles of the first active material to the particles of the second active material, the active material particles are in close contact with the positive electrode collector, thereby constructing an efficient electron and ion transmission channel, thereby increasing the electron transmission efficiency.

[0048] At the same time, by optimizing the above parameters, the microstructure of the positive electrode sheet is also changed, thereby improving the bonding quality of the positive electrode current collector and the positive electrode coating, and further improving the energy density, rate performance and cycle stability of the lithium-ion battery.

[0049] In some specific embodiments, the positive electrode current collector, the first active material and the second active material meet the following conditions:

[0050]

[0051] In some embodiments, the particle size D of the first active material is 50 ≥8μm, particle size D of the second active substance 50 ≤6μm; wherein the volume fraction V of the first active material ranges from 5% to 25%.

[0052] In this embodiment, D 50 Refers to the median diameter or median particle size.

[0053] Combine Figure 1 As shown, in some embodiments, the reduction rate of the thickness of the positive electrode current collector is calculated as follows:

[0054]

[0055] Wherein, T1 is the thickness of the compressed positive electrode current collector, and T0 is the thickness of the uncompressed positive electrode current collector.

[0056] In some embodiments, the positive electrode current collector, the first active material, and the second active material further satisfy the following conditions:

[0057] When the particle size ratio R of the first type of active particulate matter to the second type of active particulate matter is less than 2.0, the reduction rate C of the thickness of the positive electrode current collector is greater than or equal to 0.15; and / or

[0058] When the particle size ratio R of the first type of active particulate matter to the second type of active particulate matter is greater than 4.0, the reduction rate C of the thickness of the positive electrode current collector is less than or equal to 0.35.

[0059] In some embodiments, the positive electrode active material of the positive electrode coating includes a ternary material or a lithium cobalt oxide material, and the first active material and the second active material are the same type of active material.

[0060] In an embodiment of the present disclosure, the ternary material includes nickel cobalt manganese oxide (NCM) or nickel cobalt aluminum oxide (NCA).

[0061] In the embodiment of the present disclosure, the first active material and the second active material are the same type of active materials, and the difference between them is that the average particle size of the first active material is larger than the average particle size of the second active material.

[0062] In some embodiments, the elements of the positive electrode active material include Ni, Co, Mn and Al; and the elements of the positive electrode active material further include one or more of Zr, Ti, Sr, Mg, Na, Mo, Ca, Ba, La and Y.

[0063] In some embodiments, the first active material includes polycrystalline particles, and the second active material includes single crystal particles or polycrystalline particles.

[0064] In some embodiments, the surface capacity of the positive electrode is in the range of 2.55 mAh / cm 2 ~3.55mAh / cm 2 .

[0065] In some embodiments, the positive electrode coating further includes a positive electrode conductor and a positive electrode binder; wherein the mass ratio of the positive electrode conductor, the positive electrode binder and the positive electrode active material ranges from 1:1:98 to 10:10:80.

[0066] In some embodiments, the positive electrode conductive agent includes carbon black or carbon nanotubes (CNTs); and the positive electrode binder includes polyvinylidene fluoride.

[0067] In some embodiments, the compaction density D of the positive electrode sheet is in the range of 3.2 g / cm 3 ~3.8g / cm 3 .

[0068] The present disclosure also provides a method for preparing a positive electrode sheet, comprising the following steps:

[0069] S101, mixing the first active substance and the second active substance according to a preset volume ratio;

[0070] S102, the mixed material particles, carbon black and polyvinylidene fluoride are thoroughly stirred and mixed in an N-methylpyrrolidone solvent system according to a preset mass ratio by a homogenizer to form a positive electrode slurry with a solid content of 60% to 75%;

[0071] S103 , coating the positive electrode slurry on at least one surface of the positive electrode current collector, and obtaining a positive electrode sheet after baking and rolling.

[0072] The embodiments of the present disclosure further provide a lithium-ion battery, including a lithium-ion battery comprising a negative electrode sheet and a positive electrode sheet as described in the aforementioned embodiments.

[0073] The specific structure of the positive electrode sheet refers to the above embodiments. Since the lithium-ion battery adopts all the technical solutions of all the above embodiments, it has at least all the technical effects brought by the technical solutions of the above embodiments, which will not be described one by one here.

[0074] In the disclosed embodiment, the lithium-ion battery further comprises a shell, in which the positive electrode sheet and the negative electrode sheet are encapsulated. The lithium-ion battery specifically comprises a cylindrical shell, the interior of which is used to accommodate a battery cell, wherein the top is a positive terminal, the bottom is a negative terminal, and a positive electrode column is provided on the positive terminal. Specifically, the positive electrode sheet, the negative electrode sheet and the diaphragm are stacked and then wound to form a cylindrical battery cell, the beginning of the winding is the end of the electrode sheet at the axis of the cylinder, and the end of the winding is the end of the electrode sheet on the outer surface of the cylinder. Among them, the positive electrode sheet comprises a strip-shaped positive electrode foil and a positive electrode coating and a first empty foil area coated on the surface of the strip-shaped positive electrode foil, and the negative electrode sheet comprises a strip-shaped negative electrode foil and a negative electrode coating and a second empty foil area coated on the surface of the strip-shaped negative electrode foil, the first empty foil area and the second empty foil area are perpendicular to the winding direction, and are formed into the top end face or bottom end face of the lithium-ion battery by flattening or cutting and stacking.

[0075] On this basis, an embodiment of the present disclosure provides a method for preparing a lithium-ion battery, comprising:

[0076] Preparation of positive electrode sheet: Mix the positive electrode coating materials, apply them on both sides of the aluminum foil, dry and cold press to obtain the positive electrode sheet;

[0077] Preparation of negative electrode sheet: Mix the negative electrode coating material and apply it on both sides of the aluminum foil. Then, a depression of a certain depth is opened in the negative electrode coating. After drying and cold pressing, the negative electrode sheet is obtained.

[0078] Preparation of battery cells: The positive and negative electrode sheets are rolled and slit, and then wound together with the separator to obtain battery cells;

[0079] Assembling lithium-ion batteries: Welding the tabs of the lithium-ion battery to the electrical connectors, placing them into the battery case, and performing the electrolyte injection, sealing, and formation processes to obtain a lithium-ion battery.

[0080] In addition, an embodiment of the present disclosure provides an electrical device, comprising a lithium-ion battery for providing power as described in the present application.

[0081] The present invention is further explained below with reference to the following examples.

[0082] Example 1

[0083] This embodiment 1 provides a method for preparing a lithium ion battery as follows:

[0084] Preparation of positive electrode sheet: The first active material and the second active material are mixed in a volume ratio of 7:3, wherein the particle size of the first active material is D 50 The particle size D of the second active material is 12 μm. 50 The mixed material particles, carbon black and polyvinylidene fluoride were mixed in an N-methylpyrrolidone solvent system in a mass ratio of 1.5:1.5:97 by a homogenizer, and the solid content was 65% to form a stable positive electrode slurry; the positive electrode slurry was applied to at least one side of the aluminum foil, wherein the thickness reduction rate of the aluminum foil after compression was 0.18; after baking and rolling, the compacted density was 3.2g / cm 3 The positive electrode.

[0085] Preparation of negative electrode sheet: Select negative electrode active material graphite, carbon nanotubes, sodium carboxymethyl cellulose and binder styrene butadiene rubber and stir them in deionized water at a mass ratio of 96:1.5:1:1.5 to form a negative electrode slurry, and then apply the negative electrode slurry to at least one side of the copper foil. After baking and rolling, a negative electrode sheet is obtained.

[0086] Preparation of electrolyte: lithium salt lithium hexafluorophosphate (LiPF6), organic solvent ethylene carbonate (EC), dimethyl carbonate (DMC), first type additive fluoroethylene carbonate (FEC), second additive vinyl sulfate (DTD) and third type additive vinylene carbonate (VC) are mixed in a mass ratio of 10:20:55:2:8:5 to obtain an electrolyte.

[0087] Preparation of the diaphragm: A high-porosity diaphragm is selected, in which the thickness of the base film polyethylene (PE) is 9 μm, the thickness of the ceramic coating on both sides of the base film is 1 μm, and the thickness of the polyvinylidene fluoride (PVDF) coating is 1 μm.

[0088] Lithium-ion battery assembly: The positive and negative electrode sheets are rolled, slit, and then wound together with the separator to form a cylindrical battery core. The core is then welded to the electrical connector and assembled into the battery casing. After completing the injection, sealing, and formation processes, the lithium-ion battery of Example 1 is obtained, which has a cylindrical casing.

[0089] Example 2

[0090] Example 2 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.4 g / cm 3, the reduction rate of the aluminum foil thickness is 0.22. Others are the same as in Example 1.

[0091] Example 3

[0092] Example 3 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.6 g / cm 3 , the reduction rate of the aluminum foil thickness is 0.26. Others are the same as in Example 1.

[0093] Example 4

[0094] Example 4 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.8 g / cm 3 , the reduction rate of the aluminum foil thickness is 0.30. Others are the same as in Example 1.

[0095] Example 5

[0096] Example 5 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode is 3.5 g / cm 3 The first active material and the second active material were mixed at a volume ratio of 5:5, and the reduction rate of the aluminum foil thickness was 0.16. Other conditions were the same as those in Example 1.

[0097] Example 6

[0098] Example 6 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 The first active material and the second active material were mixed at a volume ratio of 6:4, and the reduction rate of the aluminum foil thickness was 0.19. Other conditions were the same as those in Example 1.

[0099] Example 7

[0100] Example 7 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 , the reduction rate of the aluminum foil thickness is 0.22. Others are the same as in Example 1.

[0101] Example 8

[0102] Example 8 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 The first active material and the second active material are mixed at a volume ratio of 8:2, and the reduction rate of the aluminum foil thickness is 0.25. Other conditions are the same as those in Example 1.

[0103] Example 9

[0104] Example 9 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 The first active material and the second active material were mixed at a volume ratio of 9:1, and the reduction rate of the aluminum foil thickness was 0.28. Other conditions were the same as those in Example 1.

[0105] Example 10

[0106] Example 10 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 The particle size ratio of the first active material particles to the second active material particles was 1.33, and the reduction rate of the aluminum foil thickness was 0.15. Other conditions were the same as those in Example 1.

[0107] Example 11

[0108] Example 11 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 The particle size ratio of the first active material particles to the second active material particles was 3.0, and the reduction rate of the aluminum foil thickness was 0.23. Other conditions were the same as those in Example 1.

[0109] Example 12

[0110] Example 12 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 The particle size ratio of the first active material particles to the second active material particles was 4.0, and the reduction rate of the aluminum foil thickness was 0.27. Other conditions were the same as those in Example 1.

[0111] Example 13

[0112] Example 13 provides a lithium-ion battery. The difference between this example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 The particle size ratio of the first active material particles to the second active material particles was 5.6, and the reduction rate of the aluminum foil thickness was 0.32. Other conditions were the same as those in Example 1.

[0113] Comparative Example 1

[0114] Comparative Example 1 provides a lithium-ion battery. The difference between this comparative example and Example 1 is that the compaction density of the positive electrode sheet is 3.0 g / cm 3 , the reduction rate of the aluminum foil thickness is 0.08. Others are the same as in Example 1.

[0115] Comparative Example 2

[0116] Comparative Example 2 provides a lithium-ion battery. The difference between this comparative example and Example 1 is that the compaction density of the positive electrode sheet is 3.5 g / cm 3 The first active material and the second active material are mixed at a volume ratio of 4:6, and the reduction rate of the aluminum foil thickness is 0.10. Other conditions are the same as those in Example 1.

[0117] Here, the lithium ion batteries of Examples 1 to 13, and Comparative Examples 1 and 2 are subjected to corresponding tests to obtain actual parameters of the lithium ion batteries.

[0118] This embodiment provides a method for testing the rate performance of a lithium-ion battery (the positive electrode material is a ternary material, and the voltage window is 2.5V to 4.2V. The voltage window needs to be adjusted accordingly for different positive electrode materials). The lithium-ion battery is placed in a constant temperature box at 25°C for 4 hours and tested according to the following steps:

[0119] S201, at a discharge rate of 0.1C, discharge at a constant current until the voltage reaches 2.5V and then let it stand for 10 minutes;

[0120] S202, at a charge rate of 0.1C, charge at a constant current to 4.2V, and charge at a constant voltage to 0.01C, and let it rest for 10 minutes;

[0121] S203, at a discharge rate of 0.1C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 10 minutes. Read the capacity value Q1 at this time.

[0122] S204, at a charge rate of 0.1C, charge to 4.2V with constant current, and charge to a cutoff of 0.01C with constant voltage, and let stand for 10 minutes;

[0123] S205, at a discharge rate of 0.5C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 10 minutes;

[0124] S206, at a charge rate of 0.1C, charge to 4.2V with constant current, and charge to a cutoff of 0.01C with constant voltage, and let stand for 10 minutes;

[0125] S207, at a discharge rate of 1C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 10 minutes;

[0126] S208, at a charge rate of 0.1C, charge to 4.2V with constant current, and charge to a cutoff of 0.01C with constant voltage, and let stand for 10 minutes;

[0127] S209, at a discharge rate of 2C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 10 minutes;

[0128] S210, at a charge rate of 0.1C, charge to 4.2V with constant current, and charge to a cutoff of 0.01C with constant voltage, and let stand for 10 minutes;

[0129] S211, at a discharge rate of 3C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 10 minutes;

[0130] S212, at a charge rate of 0.1C, charge to 4.2V with constant current, and charge to a cutoff of 0.01C with constant voltage, and let stand for 10 minutes;

[0131] S213, at a discharge rate of 4C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 10 minutes;

[0132] S214, at a charge rate of 0.1C, charge to 4.2V with constant current, and charge to a cutoff of 0.01C with constant voltage, and let stand for 10 minutes;

[0133] S215, at a discharge rate of 5C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 10 minutes;

[0134] S216, at a charge rate of 0.1C, charge to 4.2V with constant current, and charge to a cutoff of 0.01C with constant voltage, and let stand for 10 minutes;

[0135] S217, at a discharge rate of 6C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 10 minutes;

[0136] S218, at a charge rate of 0.1C, charge at constant current to 4.2V, and charge at constant voltage to a cutoff of 0.01C, and let stand for 10 minutes;

[0137] S219. Under the condition of 8C discharge rate, discharge at constant current until 2.5V is cut off, and let it stand for 10 minutes.

[0138] The capacitance Q of each discharge rate N The ratio of Q1 to the first discharge capacity Q2 is used to obtain the rate performance of the lithium-ion battery, that is, the discharge capacity retention rate.

[0139] This embodiment also provides a method for testing the cycling performance of a lithium-ion battery. The lithium-ion battery is placed in a constant temperature box at 25° C. for 4 hours and tested according to the following steps:

[0140] S301, at a discharge rate of 0.1C, discharge at a constant current until the voltage reaches 2.5V and then let it stand for 5 minutes;

[0141] S302, at a charge rate of 0.2C, charge at a constant current to 4.2V, and charge at a constant voltage to 0.05C, and let it rest for 5 minutes;

[0142] S303, at a discharge rate of 0.2C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 5 minutes. Read the capacity value q0 at this time.

[0143] S304, at a charge rate of 1.0C, charge to 4.2V with constant current, and charge to a cutoff of 0.05C with constant voltage, and let stand for 5 minutes;

[0144] S305, at a discharge rate of 2.0C, discharge at a constant current until the voltage reaches 2.5V, and then let it stand for 5 minutes;

[0145] S306: Repeat steps S104 and S105 600 times, and record the 600th discharge capacity q 600 .

[0146] After the 600th discharge capacity q 600 The ratio of the discharge capacity q1 to the first discharge capacity q1 is used to obtain the cycle performance of the lithium-ion battery, that is, the cycle capacity retention rate.

[0147] This embodiment also provides a method for calculating the reduction rate of the thickness of the positive electrode current collector, which specifically includes the following steps:

[0148] Cut the positive electrode sheet into discs with a diameter of 0.7 cm;

[0149] The wafer was observed by cross section polisher-scanning electron microscope (CP-SEM) at a magnification of 1000 times;

[0150] Nano Measure software is used to measure the thickness of X aluminum foils at the compressed locations, and the five smallest thickness values ​​are selected and a first average value is calculated. The first average value is the thickness T1 of the aluminum foil after compression.

[0151] Then, use Nano Measure software to measure the thickness of X uncompressed portions of the aluminum foil, select the five largest thickness values, and calculate a second average value, which is the uncompressed thickness T0 of the aluminum foil.

[0152] pass Calculate the reduction rate of the positive electrode current collector thickness.

[0153] This embodiment also provides a method for measuring particles of an active substance, the specific steps of which include:

[0154] Cut the positive electrode sheet into discs with a diameter of 0.7 cm;

[0155] The discs were observed by cross-section polishing-scanning electron microscopy at a magnification of 1000 times;

[0156] Nano Measure software was used to measure the diameters of 100 large particles, and the average value was taken as the particle diameter. 50 The particle size of the first active material is denoted as D 50a The particle size of the second active material is recorded as D 50b .

[0157] This embodiment also provides a method for measuring the particle volume ratio of an active substance, the specific steps of which include:

[0158] Cut the positive electrode sheet into discs with a diameter of 0.7 cm;

[0159] The discs were observed by cross-section polishing-scanning electron microscopy at a magnification of 1000 times;

[0160] Counting the number of particles Y1 and the number of particles Y2 of the first active material in an area with a length of 50 μm and a width of 30 μm;

[0161] The volume of the particles of the first active substance is calculated by the following formula:

[0162]

[0163] The volume of the particles of the second active substance is calculated by the following formula:

[0164]

[0165] The volume ratio E of the particles of the first active substance to the total particles is:

[0166]

[0167] This embodiment also provides a method for testing the compaction density of a positive electrode sheet, the specific steps of which include:

[0168] S401, after rinsing with dimethyl carbonate 2-5 times, placing in a vacuum oven for drying to fully remove dimethyl carbonate;

[0169] S402, cutting off the coated area of ​​the positive electrode sheet, wherein the cut-off area is a disc of S;

[0170] S403, weighing the disc to obtain the sum of the mass of the foil and the coating as m1, and using a screw micrometer to measure the sum of the thickness of the foil and the coating as d1;

[0171] S404, scraping off all the powder on the surface of the wafer, wiping the wafer surface with NMP to fully remove the powder, and drying;

[0172] S405, weigh the disc again to obtain the mass of the foil as m2, and use a screw micrometer to measure the thickness of the foil as d2;

[0173] By formula Calculate the compacted density of the positive electrode sheet.

[0174] After Examples 1 to 12, and Comparative Examples 1 and 2 were subjected to the above tests, corresponding data were obtained.

[0175] Among them, the parameters measured according to Examples 1 to 12, the compaction density D, the volume fraction of the first active material (volume fraction V), the particle size ratio of the particles of the first active material to the particles of the second active material (particle size ratio G), the reduction rate of the thickness of the positive electrode current collector (reduction rate C), and the corresponding discharge capacity retention rate and cycle capacity retention rate are shown in the following table:

[0176] Compacted density D Volume ratio V Particle size ratio G Reduction rate C Discharge capacity retention rate Cycle capacity retention rate <![CDATA[g / cm 3 ]]> / / / ％ ％ Example 1 3.2 0.7 2.0 0.18 73.3 82.5 Example 2 3.4 0.7 2.0 0.22 77.1 85.3 Example 3 3.6 0.7 2.0 0.26 81.8 88.7 Example 4 3.8 0.7 2.0 0.30 85.6 92.3 Example 5 3.5 0.5 2.0 0.16 70.2 80.1 Example 6 3.5 0.6 2.0 0.19 74.4 83.2 Example 7 3.5 0.7 2.0 0.22 77.1 85.3 Example 8 3.5 0.8 2.0 0.25 80.8 87.6 Example 9 3.5 0.9 2.0 0.28 83.3 89.5 Example 10 3.5 0.7 1.33 0.15 72.4 81.3 Example 11 3.5 0.7 3.0 0.23 78.6 86.4 Example 12 3.5 0.7 4.0 0.27 80.2 87.8 Example 13 3.5 0.7 5.6 0.32 77.1 85.1 Comparative Example 1 3.0 0.7 2.0 0.08 62.3 75.4 Comparative Example 2 3.5 04 2.0 0.10 65.5 74.2

[0177] Table 1

[0178] In Table 1, by comparing Examples 1 to 4, it can be seen that the compacted density increases from 3.2 g / cm 3 Increased to 3.8g / cm 3 , the reduction rate C gradually increases from 0.18 to 0.30. Here, a higher compaction density requires a greater rolling pressure, which causes a greater degree of plastic deformation of the aluminum foil.

[0179] In terms of performance, both discharge capacity retention and cycle capacity retention are showing an upward trend. This is primarily due to the higher packing density, which allows for closer contact between the active material particles and the aluminum foil, increasing the electron transfer area and facilitating high-rate discharge performance. Furthermore, this close contact reduces active material shedding during cycling, enhancing the electrode structure and improving cycle stability. However, excessively high packing density can lead to low porosity and insufficient electrolyte wetting, thus compromising performance.

[0180] In Table 1, by comparing Examples 5 to 9, we can see that when the volume fraction V increases from 0.5 to 0.9, the reduction rate C increases from 0.16 to 0.28. Here, the increase in the volume fraction of the first active material makes the internal structure of the positive electrode sheet more compact, requiring greater pressure during roller pressing to achieve the compaction effect, resulting in an increase in the reduction rate C.

[0181] In terms of performance, the discharge capacity retention rate and cycle capacity retention rate have gradually improved. This is mainly due to the skeleton structure formed by the particles of the first active material, which enhances the conductivity and mechanical strength of the positive electrode sheet, facilitates the transmission of electrons and ions at high rates, and can better withstand volume changes during the cycle, reducing the pulverization and shedding of the active material and improving cycle performance.

[0182] In Table 1, a comparison of Examples 10 to 13 shows that as the particle size ratio G increases from 1.33 to 5.6, the reduction rate C increases from 0.15 to 0.27. Specifically, when the particle size ratio G is 5.6, the reduction rate C is 0.32. The increase in the particle size ratio indicates a larger difference in particle size between the first active material and the second active material, which improves the ability of small particles to fill gaps between larger particles within a certain range. This, in turn, requires greater pressure to compact the electrode, increasing the reduction rate C.

[0183] In terms of the performance of discharge capacity retention rate, the particle size ratio G shows an upward trend in the range of 1.33 to 4.0. The main reason is that the appropriate particle size ratio helps to form a good pore structure, promote electrolyte infiltration and lithium ion diffusion, and improve high-rate performance; however, when the particle size ratio G is 5.6, the particle size difference is too large, resulting in uneven distribution of small particles around large particles, and the pores in some areas are too large, which affects lithium ion transmission and causes the discharge capacity retention rate to decrease.

[0184] In terms of the performance of cycle capacity retention, the overall change is relatively small, but when the particle size ratio G is 5.6, it also decreases slightly. The reason is that the excessively large particle size ratio leads to uneven stress distribution between particles during the cycle, which accelerates the structural destruction of the active material.

[0185] In Table 1, it can be seen from Comparative Example 1 that when the compacted density is 3.0 g / cm 3 At this point, the porosity of the positive electrode coating is too high, increasing the contact impedance between the particles of the positive active material. At the same time, when the reduction rate C is 0.08, the actual total contact area between the positive current collector and the positive electrode coating is reduced, thereby reducing the electron transmission area and causing severe polarization at high rates. At the same time, when the compaction density is too low, the positive electrode coating is relatively loose and easily falls off, further accelerating capacity decay.

[0186] In Table 1, as shown in Comparative Example 2, when the volume fraction V is 0.4, which is relatively low, the large particles of the first active material are incomplete. This reduces the conductivity and mechanical strength of the positive electrode sheet. Furthermore, when the proportion of small particles of the first active material is too high, the specific surface area is increased, electrolyte side reactions are aggravated, and the cycle life is shortened.

[0187] In summary, this application optimizes the parameter relationship between the compaction density of the positive electrode sheet, the volume fraction of the first active material, the reduction rate of the positive electrode current collector thickness, and the particle size ratio between the particles of the first active material and the particles of the second active material, so that the active material particles are in close contact with the positive electrode current collector, thereby constructing an efficient electron and ion transmission channel, thereby increasing the electron transmission efficiency. At the same time, by optimizing the above parameters, the microstructure of the positive electrode sheet is also changed, thereby improving the bonding quality between the positive electrode current collector and the positive electrode coating, thereby improving the energy density, rate performance and cycle stability of the lithium-ion battery.

[0188] The above description and the accompanying drawings sufficiently illustrate the embodiments of the present disclosure to enable those skilled in the art to practice them. Other embodiments may include structural and other changes. The embodiments represent only possible variations. Unless expressly required, individual components and functions are optional, and the order of operations may vary. Portions and features of some embodiments may be included in or replace portions and features of other embodiments. The embodiments of the present disclosure are not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes may be made without departing from the scope thereof. The scope of the present disclosure is limited only by the appended claims.

Claims

1. A positive electrode sheet, characterized in that: The invention comprises a positive electrode current collector and a positive electrode coating coated on at least one side of the positive electrode current collector; the contact resistance R between the positive electrode coating and the positive electrode current collector is in the range of 0.0012Ωcm 2 ~0.0225Ωcm 2 ; The positive electrode coating includes a positive electrode active material, the positive electrode active material includes a first active material and a second active material, and the positive electrode current collector, the first active material and the second active material meet the following conditions: Among them, D is the compaction density of the positive electrode sheet, V is the volume fraction of the first active material, C is the reduction rate of the thickness of the positive electrode collector, which ranges from 0.10 to 0.4, and G is the particle size ratio of the particles of the first active material to the particles of the second active material, which ranges from 1.33 to 5.

6.

2. The positive electrode sheet according to claim 1, characterized in that The particle size D of the first active material 50 ≥8μm, the particle size D of the second active substance 50 ≤6μm; wherein the volume fraction V of the first active material ranges from 5% to 25%.

3. The positive electrode sheet according to claim 1, characterized in that The reduction rate of the thickness of the positive electrode current collector is calculated as follows: Wherein, T1 is the thickness of the compressed positive electrode current collector, and T0 is the thickness of the uncompressed positive electrode current collector.

4. The positive electrode sheet according to claim 1, characterized in that The positive electrode current collector, the first active material and the second active material also meet the following conditions: When the particle size ratio R of the first type of active particulate matter to the second type of active particulate matter is less than 2.0, the reduction rate C of the thickness of the positive electrode current collector is greater than or equal to 0.15; and / or When the particle size ratio R of the first type of active particulate matter to the second type of active particulate matter is greater than 4.0, the reduction rate C of the thickness of the positive electrode current collector is less than or equal to 0.

35.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: The positive electrode active material of the positive electrode coating includes a ternary material or a lithium cobalt oxide material, and the first active material and the second active material are the same type of active materials.

6. The positive electrode sheet according to claim 5, characterized in that: The elements of the positive electrode active material include Ni, Co, Mn and Al; and the elements of the positive electrode active material further include one or more of Zr, Ti, Sr, Mg, Na, Mo, Ca, Ba, La and Y.

7. The positive electrode sheet according to claim 6, characterized in that: The first active material includes polycrystalline particles, and the second active material includes single crystal particles or polycrystalline particles.

8. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: The surface capacity of the positive electrode sheet is in the range of 2.55 mAh / cm 2 ~3.55mAh / cm 2 The compaction density D of the positive electrode sheet is in the range of 3.2 g / cm 3 ~3.8g / cm 3 .

9. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: The positive electrode coating further includes a positive electrode conductor and a positive electrode binder; wherein the mass ratio of the positive electrode conductor, the positive electrode binder and the positive electrode active material is in the range of 1:1:98 to 10:10:

80.

10. A lithium ion battery, characterized in that: The invention comprises a negative electrode sheet and a positive electrode sheet as claimed in any one of claims 1 to 9.

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