Positive electrode sheet, secondary battery, battery module, battery pack, and electric device

Abstract

The application relates to a positive electrode sheet, a secondary battery, a battery module, a battery pack and a power utilization device. The positive electrode sheet comprises a positive electrode current collector; a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising a first active material layer and a second active material layer arranged in sequence in the direction away from the surface, the first active material layer comprising first composite particles, the first composite particles comprising first lithium iron phosphate particles and a first carbon layer coated on the surface of the first lithium iron phosphate particles, the second active material layer comprising second composite particles, the second composite particles comprising second lithium iron phosphate particles and a second carbon layer coated on the surface of the second lithium iron phosphate particles, wherein the graphitization degree of the first composite particles is greater than that of the second composite particles. The secondary battery adopting the positive electrode sheet can have higher rate performance and energy density.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a positive electrode sheet, a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] Lithium-ion rechargeable batteries are widely used in electric vehicles and consumer electronics due to their advantages such as high energy density, high output power, long cycle life, and low environmental pollution. The positive electrode is one of the core components of a lithium-ion rechargeable battery, and its performance has a significant impact on the battery's electrochemical performance.

[0003] With the widespread application of lithium-ion secondary batteries, increasingly higher requirements are being placed on the electrochemical performance of lithium-ion secondary batteries, and the performance requirements for the positive electrode are also gradually increasing. Summary of the Invention

[0004] This application provides a positive electrode sheet, a secondary battery, a battery module, a battery pack, and an electrical device, wherein the secondary battery can achieve both high rate performance and energy density.

[0005] To achieve the above objectives, a first aspect of this application provides a positive electrode sheet, comprising: a positive current collector; and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a first active material layer and a second active material layer sequentially stacked in a direction away from the surface, the first active material layer comprising a first composite particle, the first composite particle comprising a first lithium iron phosphate particle and a first carbon layer coated on the surface of the first lithium iron phosphate particle, the second active material layer comprising a second composite particle, the second composite particle comprising a second lithium iron phosphate particle and a second carbon layer coated on the surface of the second lithium iron phosphate particle, wherein the graphitization degree of the first composite particle is greater than that of the second composite particle.

[0006] Therefore, the positive electrode sheet of this application embodiment includes, on the one hand, two active material layers, each of which includes lithium iron phosphate particles coated with a carbon layer. The carbon layer has high conductivity, which can significantly improve the conductivity of the active material layer and significantly improve the contact resistance between the positive active material layer and the positive current collector, thereby improving the conductivity of the positive electrode sheet and thus improving the rate performance of the secondary battery using this positive electrode sheet. On the other hand, the two active material layers are a first active material layer and a second active material layer. The first active material layer is closer to the positive current collector, has a relatively higher graphitization, and the solid content of the slurry used to prepare the first active material layer is relatively low. The second active material layer is away from the positive current collector, has a relatively lower graphitization, and the solid content of the slurry used to prepare the second active material layer is relatively high. The difference in solid content between the slurries used to prepare the first and second active material layers results in a gradient difference in solvent concentration. The combined use of the two is beneficial for the thick coating of the positive active material layer, thereby improving the energy density of the secondary battery using this positive electrode sheet.

[0007] In any embodiment, the degree of graphitization of the first composite particles is 0.3 to 0.6, and optionally, the degree of graphitization of the first composite particles is 0.3 to 0.5. The relatively high degree of graphitization of the first composite particles can further improve the conductivity of the first active material layer, and the solid content of the slurry containing the first composite particles is relatively low.

[0008] In any embodiment, the degree of graphitization of the second composite particles is 0.05 to 0.3, and optionally, the degree of graphitization of the second composite particles is 0.1 to 0.2. The relatively low degree of graphitization of the second composite particles, while improving the conductivity of the second active material layer, also increases the solid content in the slurry used to prepare the second active material layer. The combination of low-solid-content and high-solid-content slurries further reduces the difficulty of thick coating and increases the thickness of the positive electrode active material layer.

[0009] In any embodiment, the first composite particle comprises primary particles, and the second composite particle comprises secondary particles formed by the aggregation of multiple primary particles. The average particle size D50 of the first composite particle is smaller than the average particle size D50 of the second composite particle. The difference in particle size between the second composite particle and the first composite particle enables the formation of a porous structure, thereby improving the kinetic performance of the secondary battery.

[0010] In any embodiment, the average particle size D50 of the first composite particles is 1 μm to 4 μm; optionally, it is 1 μm to 3 μm. Using the first composite particles within the above particle size range can increase the contact area with the electrolyte, thereby improving the rate performance of the secondary battery; and can also increase the compaction density during the formation of the first active material layer, thereby increasing the energy density of the secondary battery.

[0011] In any embodiment, the average particle size D50 of the second composite particles is 6 μm to 12 μm, and can be selected as 8 μm to 10 μm. Using second composite particles within the above particle size range, agglomeration is less likely to occur in the slurry when the second active material layer is formed; and the relatively large average particle size D50 of the second composite particles can significantly improve the rate performance of the secondary battery.

[0012] In any embodiment, the secondary particles have a spherical or near-spherical morphology. This morphology has a relatively large specific surface area, which can increase the contact area between the second composite particles and the electrolyte, thus improving the wetting performance of the electrolyte on the second composite particles, thereby improving the lithium-ion transport performance and enhancing the rate performance of the secondary battery.

[0013] In any embodiment, the specific surface area of ​​the first composite particle is 10 m². 2 / g~15m 2 / g; 12m can be selected. 2 / g~14m 2 / g. The first composite particles have a relatively large specific surface area and are uniformly dispersed, which can improve the wettability between the first composite particles and the electrolyte.

[0014] In any embodiment, the specific surface area of ​​the second composite particle is 5m². 2 / g~10m 2 / g; optional 6m 2 / g~8m 2 / g. The second composite particle has a relatively small specific surface area and a relatively large particle size, which allows it to form more pores with the first composite particle, which is beneficial for lithium ion transport.

[0015] In any embodiment, the ratio of the coating weight CW1 of the first active material layer to the coating weight CW2 of the second active material layer is 0.8 to 1.2; optionally, the ratio is 1. The first active material layer with a higher degree of graphitization and the second active material layer with a lower degree of graphitization work synergistically to improve the coating characteristics of the positive electrode active material layer, which is particularly beneficial for thick coating processing, thereby increasing the overall thickness of the positive electrode active material layer.

[0016] In any embodiment, the coating weight CW1 of the first active material layer is 0.115 mg / cm³. 2 ≤CW1≤0.195mg / cm 2 .

[0017] In any embodiment, the coating weight CW2 of the second active material layer is 0.115 mg / cm³. 2≤CW2≤0.195mg / cm 2 .

[0018] In any embodiment, the first active material layer includes a first conductive agent, with the mass content of the first conductive agent being A, calculated as 100% of the total weight of the first active material layer; the second active material layer includes a second conductive agent, with the mass content of the second conductive agent being B, calculated as 100% of the total weight of the second active material layer; wherein A < B; optionally, 1 wt% ≤ B A ≤ 3 wt%. Compared to the graphitization degree of the second active material layer, the graphitization degree of the first active material layer is relatively high. While ensuring the conductivity of the first active material layer, the amount of conductive agent used in the first active material layer can be reduced, and the mass proportion of the first composite particles can be increased, thereby improving the energy density of the secondary battery.

[0019] In any embodiment, the powder compaction density of the first active material layer at a pressure of 600 MPa is 2.4 g / cc to 2.65 g / cc, optionally 2.5 g / cc to 2.6 g / cc. Using the above-mentioned powder compaction density range can significantly improve the energy density of the secondary battery.

[0020] In any embodiment, the compacted density of the second active material layer powder at 600 MPa pressure is 2.2 g / cc to 2.45 g / cc, optionally 2.35 g / cc to 2.45 g / cc. The particles in the slurry for preparing the second active material layer have excellent flowability, dispersibility, and processing performance. The particles in the second active material layer have excellent flowability and are uniformly dispersed. Combined with the slurry for preparing the first active material layer, this facilitates the fabrication of the positive electrode active material layer and improves the quality of the positive electrode sheet.

[0021] The second aspect of this application provides a secondary battery, including a positive electrode, a separator, and a negative electrode, wherein the positive electrode is the positive electrode as described in any embodiment of the first aspect of this application.

[0022] A third aspect of this application provides a battery module, including a secondary battery as described in the second aspect of this application.

[0023] The fourth aspect of this application provides a battery pack, including a secondary battery as described in the second aspect of this application or a battery module as described in the third aspect of this application.

[0024] The fifth aspect of this application provides an electrical device, including a secondary battery as described in the second aspect of this application, a battery module as described in the third aspect of this application, or a battery pack as described in the fourth aspect of this application. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet provided in some embodiments of this application;

[0027] Figure 2 yes Figure 1 The cross-sectional view of the positive electrode plate shown along line AA;

[0028] Figure 3 This is a schematic diagram of the structure of the electrode assembly of a secondary battery provided in some embodiments of this application;

[0029] Figure 4 This is an exploded view of a secondary battery provided in some embodiments of this application;

[0030] Figure 5 These are schematic diagrams of the battery module provided in some embodiments of this application;

[0031] Figure 6 These are schematic diagrams of the battery pack structure provided in some embodiments of this application;

[0032] Figure 7 These are schematic diagrams of the electrical devices provided in some embodiments of this application;

[0033] Figure 8 These are TEM images of the first active material layer of the positive electrode sheet provided in some embodiments of this application;

[0034] Figure 9 These are TEM images of the second active material layer of the positive electrode sheet provided in some embodiments of this application;

[0035] Figure 10 These are partial SEM images of the positive electrode sheet provided in some embodiments of this application.

[0036] The reference numerals in the attached figures are explained as follows:

[0037] Y, thickness direction; 1, electrical device; 10, battery pack; 11, lower casing; 12, upper casing; 20, battery module;

[0038] 30. Secondary battery; 31. Top cover assembly; 32. Housing;

[0039] 40. Electrode assembly;

[0040] 50. Positive electrode plate; 51. Positive current collector; 511. Surface;

[0041] 52. Positive electrode active material layer; 521. First active material layer; 522. Second active material layer;

[0042] 60. Negative electrode sheet; 70. Separating membrane. Detailed Implementation

[0043] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack, and electrical device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0044] 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.

[0045] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0046] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0047] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means 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.

[0048] 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.

[0049] 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).

[0050] [Positive electrode plate]

[0051] In one aspect, embodiments of this application propose a positive electrode sheet.

[0052] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet provided in some embodiments of this application. Figure 2 yes Figure 1 The cross-sectional view of the positive electrode plate shown is taken along line AA. Figure 1 and Figure 2 As shown, the positive electrode 50 of this application embodiment includes a positive current collector 51 and a positive active material layer 52 disposed on at least one surface 511 of the positive current collector 51.

[0053] The positive electrode current collector 51 has two surfaces 511 that are opposite to each other in its own thickness direction X. The positive electrode active material layer 52 is disposed on either of the two surfaces 511 of the positive electrode current collector 51, or it can be disposed on both surfaces 511 respectively. Figure 1 The X direction shown indicates the thickness direction of the positive current collector 51.

[0054] The positive electrode current collector 51 can be a metal or a composite current collector. As a metal, exemplaryly, it can be a metal foil or a porous metal plate, such as aluminum or an aluminum alloy; specifically, the positive electrode current collector 51 is made of aluminum foil. As a composite current collector, exemplaryly, it can include a polymeric material substrate and a metal layer formed on at least one surface 511 of the polymeric material substrate. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy, etc.) on a polymeric material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0055] The positive electrode active material layer 52 includes a positive electrode active material, which is a lithium iron phosphate active material. During the charge and discharge cycle, it can perform reversible lithium ion insertion / extraction and electron migration. The positive electrode current collector 51 collects and outputs the current.

[0056] Generally, in lithium iron phosphate (LFP) particles, ferrous ions are located at the 4c ​​position of the oxygen octahedron, and lithium ions are located at the 4a position. The polyanionic phosphate structure exhibits good thermal stability. However, the octahedral FeO6 structure is separated by oxygen atoms in the tetrahedral phosphate ions, failing to form a continuous FeO6 network, resulting in poor electronic conductivity of LFP particles. Furthermore, the lithium-ion diffusion coefficient in LFP particles is low. These low conductivity and low lithium-ion diffusion coefficient lead to poor rate performance when using this type of LFP particle.

[0057] Carbon layers possess advantages such as good conductivity and large specific surface area. The inventors considered coating the surface of lithium iron phosphate particles with a carbon layer to form graphitized lithium iron phosphate particles, which could construct a good conductive network on the surface of the lithium iron phosphate particles, thereby improving the conductivity of the positive electrode 50. However, the inventors found that in order to improve the conductivity of the positive electrode, the degree of graphitization of lithium iron phosphate particles is relatively high. However, a higher degree of graphitization of lithium iron phosphate particles increases the difficulty of coating to form the positive electrode active material layer, which is not conducive to thick coating processing.

[0058] Based on the above problems, in this embodiment, the positive electrode active material layer 52 is divided into two layers. In other words, the positive electrode active material layer 52 includes a first active material layer 521 and a second active material layer 522 sequentially stacked in a direction away from the surface 511 of the positive electrode current collector 51. Both active material layers are made of graphitized lithium iron phosphate particles. Specifically, the first active material layer 521 is disposed close to the surface 511 of the positive electrode current collector 51, and the second active material layer 522 is disposed away from the surface 511 of the positive electrode current collector 51. The first active material layer 521 includes a first composite particle, which includes a first lithium iron phosphate particle and a first carbon layer coated on the surface 511 of the first lithium iron phosphate particle. The second active material layer 522 includes a second composite particle, which includes a second lithium iron phosphate particle and a second carbon layer coated on the surface 511 of the second lithium iron phosphate particle. Each layer can improve conductivity and significantly improve the contact resistance between the positive electrode active material layer 52 as a whole and the positive electrode current collector 51, thereby improving conductivity and thus improving the rate performance of the secondary battery using the positive electrode active material layer 52.

[0059] In this paper, the degree of graphitization refers to the extent to which carbon atoms form a close-packed hexagonal graphite crystal structure. The closer the lattice size is to the lattice parameters of ideal graphite, the higher the degree of graphitization. Here, it is used to characterize the carbon layer coated on lithium iron phosphate.

[0060] The two active material layers have different degrees of graphitization. The graphitization degree of the first composite particles in the first active material layer 521 is greater than that of the second composite particles in the second active material layer 522. In other words, the graphitization degree of the first composite particles is relatively higher, resulting in poorer compatibility with solvents in the slurry, such as N-methylpyrrolidone (NMP), making them more difficult to disperse. Consequently, the solid content in the slurry used to prepare the first active material layer 521 is relatively lower. Furthermore, when the graphitization degree of lithium iron phosphate particles is higher, the particles are more prone to slippage, which enhances the flexibility of the positive electrode 50 and reduces the risk of corner cracking during winding of the electrode assembly. The graphitization degree of the second composite particles is relatively lower, resulting in better compatibility with solvents such as N-methylpyrrolidone (NMP), easier dispersion, and a relatively higher solid content in the slurry used to prepare the second active material layer 522. Due to the difference in solid content, a solvent concentration gradient can be formed between the slurry for preparing the first active material layer 521 and the slurry for preparing the second active material layer 522, which is beneficial for the thick coating of the positive electrode active material layer 52, thereby improving the energy density of the secondary battery.

[0061] According to the embodiment of this application, the positive electrode 50 includes two active material layers, each of which includes lithium iron phosphate particles coated with a carbon layer. The carbon layer has high conductivity, which can significantly improve the conductivity of the active material layer and significantly improve the contact resistance between the positive active material layer 52 and the positive current collector 51, thereby improving the conductivity of the positive electrode 50 and thus improving the rate performance of the secondary battery using the positive electrode 50. On the other hand, the two active material layers are a first active material layer 521 and a second active material layer 522. The first active material layer 521 is located closer to the positive electrode current collector 51, and its graphitization is relatively higher, resulting in a relatively lower solid content in the slurry used to prepare the first active material layer 521. The second active material layer 522 is located away from the positive electrode current collector 51, and its graphitization is relatively lower, resulting in a relatively higher solid content in the slurry used to prepare the second active material layer 522. The difference in solid content between the slurries used to prepare the first active material layer 521 and the second active material layer 522 leads to a gradient difference in solvent concentration. The combined use of the two layers is beneficial for the thick coating of the positive electrode active material layer 52, thereby improving the energy density of the secondary battery using this positive electrode sheet 50.

[0062] Therefore, by using the positive electrode 50 of the present application embodiment, the secondary battery can simultaneously achieve high rate performance, energy density, and good overall electrochemical performance.

[0063] In some embodiments, the degree of graphitization of the first composite particles is 0.3 to 0.6. The lower limit of the degree of graphitization of the first composite particles can be 0.3, 0.4, or 0.5, and the upper limit can be 0.6, 0.5, or 0.4. The range of the degree of graphitization of the first composite particles can be a reasonable combination of any of the above upper and lower limits. Optionally, the degree of graphitization of the first composite particles is 0.3 to 0.5. A relatively high degree of graphitization of the first composite particles can further improve the conductivity of the first active material layer 521, and the solid content of the slurry containing the first composite particles is relatively low.

[0064] In some embodiments, the graphitization degree of the second composite particles is 0.05 to 0.3, with the lower limit being 0.05, 0.1, 0.15, 0.2, or 0.25, etc. The upper limit is 0.15, 0.20, 0.25, or 0.30, etc. The range of graphitization degree of the second composite particles can be a reasonable combination of any of the above upper and lower limits; optionally, the graphitization degree of the second composite particles is 0.1 to 0.2. The relatively low graphitization degree of the second composite particles, while improving the conductivity of the second active material layer 522, can also increase the solid content in the slurry used to prepare the second active material layer 522. The combination of low-solid-content and high-solid-content slurries can further reduce the difficulty of thick coating and increase the thickness of the positive electrode active material layer 52.

[0065] If the average particle size of the composite particles is too small or the specific surface area is too high, the contact area between the composite particles and the electrolyte will be too large. Under high voltage or strong oxidizing conditions, the electrolyte is prone to side reactions on the surface of the composite particles, which will worsen the gas generation problem, increase heat generation, and deteriorate the safety and cycle performance of the secondary battery. If the average particle size of the composite particles is too large or the specific surface area is too low, the path for lithium ions to insert and extract within the composite particles during charging and discharging will be too long, affecting the kinetic performance of the secondary battery.

[0066] To improve the performance of secondary batteries, this application achieves this goal by controlling the average particle size D50 and / or specific surface area of ​​the composite particles. D50 refers to the particle size corresponding to a cumulative volume percentage of 50% for the composite particles, i.e., the median particle size distribution. D50 can be measured, for example, using a laser diffraction particle size distribution measuring instrument (e.g., Malvern Mastersizer 3000).

[0067] In some embodiments, the first composite particle comprises primary particles, and the second composite particle comprises secondary particles formed by the aggregation of multiple primary particles. The average particle size D50 of the first composite particle is smaller than the average particle size D50 of the second composite particle. The difference in particle size between the second and first composite particles enables the formation of a porous structure, thereby improving the kinetic performance of the secondary battery.

[0068] As examples, the average particle size D50 of the first composite particles is 1 μm to 4 μm. The lower limit of the average particle size D50 of the first composite particles can be 1 μm, 2 μm, or 3 μm, etc. The upper limit of the average particle size D50 of the first composite particles can be 2 μm, 3 μm, or 4 μm, etc. The range of the average particle size D50 of the first composite particles can be a reasonable combination of any of the above upper and lower limits, and can be selected as 1 μm to 3 μm. Using first composite particles within the above particle size range can increase the contact area with the electrolyte, improve the rate performance of the secondary battery; and increase the compaction density during the formation of the first active material layer 521, thereby improving the energy density of the secondary battery.

[0069] As examples, the average particle size D50 of the second composite particles is 6 μm to 12 μm. The lower limit of the average particle size D50 of the second composite particles can be 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or 11 μm, and the upper limit of the average particle size D50 of the second composite particles can be 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, or 12 μm. The range of the average D50 of the second composite particles can be a reasonable combination of any of the above upper and lower limits, and can be selected as 8 μm to 10 μm. Using second composite particles within the above particle size range, the slurry is less prone to agglomeration when forming the second active material layer 522; and the relatively large average particle size D50 of the second composite particles can significantly improve the rate performance of the secondary battery.

[0070] Optionally, the second composite particles are spherical or near-spherical in shape. This type of shape has a relatively large specific surface area, which can increase the contact area between the second composite particles and the electrolyte. This is beneficial to the wetting performance of the electrolyte on the second composite particles, thereby improving the lithium-ion transport performance and thus enhancing the rate performance of the secondary battery.

[0071] In some embodiments, the specific surface area of ​​the first composite particle is 10 m². 2 / g~15m 2 / g. The lower limit of the specific surface area of ​​the first composite particle is 10m². 2 / g、11m 2 / g、12m 2 / g or 13m 2 / g, the upper limit of the specific surface area of ​​the first composite particle is 11m². 2 / g、12m 2 / g、13m 2 / g、14m 2 / g or 15m 2 / g, etc., the specific surface area of ​​the first composite particle can be any reasonable combination of the above upper and lower limits, and can be selected as 12m. 2 / g~14m 2 / g. The first composite particles have a relatively large specific surface area and are uniformly dispersed, which can improve the wettability between the first composite particles and the electrolyte.

[0072] In some embodiments, the specific surface area of ​​the second composite particle is 5 m². 2 / g~10m 2 / g. The lower limit of the specific surface area of ​​the second composite particles is 5m². 2 / g、6m 2 / g or 7m 2 / g, the upper limit of the specific surface area of ​​the second composite particles is 7m². 2 / g、8m 2 / g、9m 2 / g or 10m 2 / g, etc., the specific surface area of ​​the second composite particles can be any reasonable combination of the upper and lower limits mentioned above, and can be selected as 6m. 2 / g~8m 2 / g. The second composite particles have a relatively small specific surface area and a relatively large particle size, resulting in more voids when they are packed together. When the slurry containing the second composite particles is coated onto the first active material layer 521 to form the second active material layer 522, a void gradient difference can be formed between the first active material layer 521 and the second active material layer 522, which is beneficial to the transport of lithium ions and improves the kinetic performance.

[0073] In some embodiments, the ratio of the coating weight CW1 of the first active material layer 521 to the coating weight CW2 of the second active material layer 522 is 0.8 to 1.2. The lower limit of this ratio can be 0.8, 0.9, 1.0, or 1.1; the upper limit of this ratio can be 0.9, 1.0, 1.1, or 1.2. The range of this ratio can be a reasonable combination of any of the above upper and lower limits. Optionally, the ratio of the coating weight CW1 of the first active material layer 521 to the coating weight CW2 of the second active material layer 522 is 1. The first active material layer 521 with a higher degree of graphitization and the second active material layer 522 with a lower degree of graphitization work synergistically to improve the coating characteristics of the positive electrode active material layer 52, which is particularly beneficial for thick coating processes, thereby increasing the overall thickness of the positive electrode active material layer 52.

[0074] As some examples, the coating weight CW1 of the first active material layer 521 is 0.115 mg / cm³. 2 ≤CW1≤0.195mg / cm 2 The lower limit of CW1 can be 0.115 mg / cm³. 2 0.120 mg / cm 2 0.125 mg / cm 2 0.130 mg / cm 2 0.135 mg / cm2 0.140 mg / cm 2 0.145 mg / cm 2 0.150 mg / cm 2 0.155 mg / cm 2 0.160 mg / cm 2 0.165 mg / cm 2 0.170 mg / cm 2 0.175 mg / cm 2 Or 0.180 mg / cm 2 The upper limit of CW1 can be 0.125 mg / cm³. 2 0.130 mg / cm 2 0.135 mg / cm 2 0.140 mg / cm 2 0.145 mg / cm 2 0.150 mg / cm 2 0.155 mg / cm 2 0.160 mg / cm 2 0.165 mg / cm 2 0.170 mg / cm 2 0.175 mg / cm 2 0.180 mg / cm 2 0.185 mg / cm 2 0.190 mg / cm 2 Or 0.195 mg / cm 2 The range of CW1 can be any reasonable combination of any of the above upper and lower limits.

[0075] In some examples, the coating weight (CW2) of the second active material layer 522 is 0.115 mg / cm². 2 ≤CW2≤0.195mg / cm 2 The lower limit of CW2 can be 0.115 mg / cm³. 2 0.120 mg / cm 2 0.125 mg / cm 2 0.130 mg / cm 2 0.135 mg / cm 2 0.140 mg / cm 2 0.145 mg / cm 2 0.150 mg / cm 2 0.155 mg / cm 2 0.160 mg / cm 2 0.165 mg / cm 20.170 mg / cm 2 0.175 mg / cm 2 Or 0.180 mg / cm 2 The upper limit for CW2 is 0.125 mg / cm³. 2 0.130 mg / cm 2 0.135 mg / cm 2 0.140 mg / cm 2 0.145 mg / cm 2 0.150 mg / cm 2 0.155 mg / cm 2 0.160 mg / cm 2 0.165 mg / cm 2 0.170 mg / cm 2 0.175 mg / cm 2 0.180 mg / cm 2 0.185 mg / cm 2 0.190 mg / cm 2 Or 0.195 mg / cm 2 The range of CW2 can be any reasonable combination of any of the above upper and lower limits.

[0076] The first active material layer 521 and the second active material layer 522 work together to increase the overall coating weight of the positive electrode active material.

[0077] In some embodiments, the powder compaction density of the first active material layer 521 at 600 MPa pressure is 2.4 g / cc to 2.65 g / cc. The lower limit of the powder compaction density of the first active material layer 521 at 600 MPa pressure can be 2.4 g / cc, 2.45 g / cc, 2.50 g / cc, or 2.55 g / cc, and the upper limit of the powder compaction density of the first active material layer 521 at 600 MPa pressure can be 2.45 g / cc, 2.50 g / cc, 2.55 g / cc, 2.60 g / cc, or 2.65 g / cc. The range can be a reasonable combination of any of the above upper and lower limits, and can be selected as 2.5 g / cc to 2.6 g / cc. The first active material layer 521 using the above-mentioned powder compaction density range has a relatively high compaction density, which helps to improve the energy density of the secondary battery.

[0078] In some embodiments, the powder compaction density of the second active material layer 522 at 600 MPa pressure is 2.2 g / cc to 2.45 g / cc. The lower limit of the powder compaction density of the second active material layer 522 at 600 MPa pressure can be 2.2 g / cc, 2.25 g / cc, 2.30 g / cc, or 2.35 g / cc. The upper limit of the powder compaction density of the second active material layer 522 at 600 MPa pressure can be 2.30 g / cc, 2.35 g / cc, 2.40 g / cc, or 2.45 g / cc. The range can be a reasonable combination of any of the above upper and lower limits, and can be selected as 2.35 g / cc to 2.45 g / cc. The second active material layer 522, which uses the above-mentioned powder compaction density range, has a relatively low compaction density. The particles in the slurry used to prepare the second active material layer 522 have excellent flowability, dispersibility, and processing performance. The particles in the second active material layer 522 have excellent flowability and are evenly dispersed. When combined with the slurry used to prepare the first active material layer 521, it is beneficial to produce the positive electrode active material layer 52 and improve the quality of the positive electrode sheet.

[0079] The powder compaction density of the first active material layer 521 and the second active material layer 522 works together to increase the overall compaction density of the positive electrode active material layer 52, thereby increasing the energy density of the secondary battery.

[0080] The positive electrode active material layer 52 may optionally include a conductive agent. There are no particular limitations on the conductive agent; it can be any conductive agent known in the art. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The conductive agent can reduce the internal resistance of the positive electrode active material layer 52 and increase the conductivity between the positive electrode active material layer 52 and the positive electrode current collector 51. In the embodiments of this application, the use of carbon-coated lithium iron phosphate particles can improve the conductivity of the positive electrode active material layer 52 to a certain extent, thereby reducing the amount of conductive agent used to a certain extent, thus relatively increasing the amount of lithium iron phosphate particles used, improving the utilization rate of the positive electrode active material, which is beneficial for obtaining low electrode resistance and improving the energy density of the secondary battery.

[0081] In some embodiments, the first active material layer 521 includes a first conductive agent, with a mass content of A based on 100% of the total weight of the first active material layer 521. The second active material layer 522 includes a second conductive agent, with a mass content of B based on 100% of the total weight of the second active material layer 522; wherein A < B; optionally, 1 wt% ≤ B ≤ 3 wt%. Compared to the graphitization degree of the second active material layer 522, the graphitization degree of the first active material layer 521 is relatively high. While ensuring the conductivity of the first active material layer 521, the amount of conductive agent used in the first active material layer 521 can be reduced, and the mass proportion of the first composite particles can be increased, thereby improving the energy density of the secondary battery.

[0082] The positive electrode active material layer 52 may optionally include a binder. There are no particular limitations on the binder; it may be a binder known in the art. As an example, the binder may include one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), and polyvinyl alcohol (PVA).

[0083] The binder is used to bond the composite particles to the conductive agent, ensuring that the internal conductive network of the positive electrode 50 has good structural stability. Since the binder itself has poor conductivity, a relatively small amount of binder can be used to obtain low electrode resistance.

[0084] [Preparation method of positive electrode sheet]

[0085] Secondly, embodiments of this application also provide a method for preparing a positive electrode sheet.

[0086] The positive electrode sheet of this application embodiment can be prepared by coating. For example, a first positive electrode slurry is coated on at least one surface of the positive electrode current collector to obtain a first active material layer; a second positive electrode slurry is coated on the surface of the first active material layer opposite to the positive electrode current collector to obtain a second active material layer; then the first and second active material layers are subjected to processes such as drying and cold pressing to form a positive electrode active material layer on the positive electrode current collector, thereby obtaining the positive electrode sheet.

[0087] As examples, the preparation method of the positive electrode sheet includes the following steps:

[0088] The first composite particles, the first conductive agent, the binder, and any other components are dispersed in a solvent (e.g., N-methylpyrrolidone NMP) to form a first positive electrode slurry; the first positive electrode slurry is coated onto a positive electrode current collector.

[0089] The second composite particles, the second conductive agent, the binder, and any other components are dispersed in a solvent (e.g., N-methylpyrrolidone NMP) to form a second positive electrode slurry; the second positive electrode slurry is coated onto the first positive electrode slurry.

[0090] After the first positive electrode slurry and the second positive electrode slurry are dried and cold-pressed, the first active material layer and the second active material layer can be obtained respectively, thus obtaining the positive electrode sheet.

[0091] [Rechargeable Battery]

[0092] Thirdly, embodiments of this application also provide a secondary battery.

[0093] Figure 3 This is a schematic diagram of the structure of the electrode assembly of a secondary battery provided in some embodiments of this application; Figure 4 This is an exploded view of a secondary battery provided in some embodiments of this application.

[0094] like Figure 3 and Figure 4 As shown, the secondary battery in this embodiment of the application includes a positive electrode 50, a negative electrode 60, a separator 70, and an electrolyte.

[0095] The positive electrode 50 adopts the positive electrode of the first aspect embodiment of this application, or the positive electrode obtained by the preparation method of the second aspect embodiment of this application. Because the positive electrode of the first aspect embodiment of this application or the positive electrode obtained by the preparation method of the second aspect embodiment of this application is used, the secondary battery of this application simultaneously achieves high rate performance and high energy density.

[0096] The negative electrode 60 can be a lithium metal sheet, or it can be a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material. The negative electrode current collector is a copper foil.

[0097] In some embodiments, the negative electrode active material in the negative electrode active material layer may be a negative electrode active material known in the art for use in secondary batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0098] In some embodiments, the negative electrode active material layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0099] In some embodiments, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0100] In some embodiments, the negative electrode active material layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0101] In some embodiments, the negative electrode 60 can be prepared by dispersing the components used to prepare the negative electrode 60, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then drying, cold pressing and other processes to obtain the negative electrode 60.

[0102] This application does not impose any particular limitation on the type of separator 70; any known porous structure separator 70 with good chemical and mechanical stability can be selected. In some embodiments, the material of the separator 70 can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator 70 can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator 70 is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0103] In some embodiments, the positive electrode 50, the negative electrode 60, and the separator 70 can be fabricated into the electrode assembly 40 by a winding process or a stacking process. Figure 3 The electrode assembly 40 shown is a wound electrode assembly.

[0104] The electrolyte acts as a conductor of ions between the positive electrode 50 and the negative electrode 60. This application does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. The electrolyte can be liquid, gel, or entirely solid.

[0105] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0106] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0107] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0108] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0109] Please continue reading. Figure 3 and Figure 4 The secondary battery 30 includes a top cover assembly 31 and a housing 32, as well as an electrode assembly 40 and an electrolyte housed within the housing 32. The electrode assembly 40 includes a positive electrode 50, a negative electrode 60, and a separator 70. The positive electrode 50 or the negative electrode 60 includes tabs. During the charging and discharging process of the secondary battery 30, active ions repeatedly insert and extract between the positive electrode 50 and the negative electrode 60. The electrolyte acts as a conductor between the positive electrode 50 and the negative electrode 60. The separator 70 is disposed between the positive electrode 50 and the negative electrode 60, primarily preventing short circuits between the positive and negative electrodes while allowing ions to pass through. Specifically, the secondary battery 30 can be a wound or stacked battery, such as a lithium-ion secondary battery, a lithium primary battery, a sodium-ion battery, or a magnesium-ion battery, but is not limited to these types.

[0110] In some embodiments, the casing 32 of the secondary battery can be a rigid casing, such as a hard plastic casing, an aluminum casing, or a steel casing. The casing 32 of the secondary battery can also be a pouch, such as a pouch-type pouch. The material of the pouch can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0111] In some implementations, the secondary batteries can be assembled into a battery module. The battery module may contain one or more secondary batteries, the specific number of which can be selected by those skilled in the art based on the application and capacity of the battery module.

[0112] Figure 5 These are schematic diagrams of the battery module structure provided in some embodiments of this application, such as... Figure 5 As shown, in the battery module 20, multiple secondary batteries 30 can be arranged sequentially along the length of the battery module 20. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 30 can be fixed in place using fasteners.

[0113] Optionally, the battery module 20 may also include a housing with a receiving space in which a plurality of secondary batteries 30 are received.

[0114] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0115] Figure 6 These are schematic diagrams of the battery pack structure provided in some embodiments of this application, such as... Figure 6 As shown, the battery pack 10 may include a battery box and multiple battery modules 20 disposed within the battery box. The battery box includes an upper box 12 and a lower box 11, with the upper box 12 covering the lower box 11 to form a closed space for accommodating the battery modules 20. The multiple battery modules 20 can be arranged in any manner within the battery box.

[0116] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0117] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0118] Figure 7 This is a schematic diagram of the structure of an electrical device provided in some embodiments of this application; the electrical device 1 includes a secondary battery 30. The electrical device 1 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the secondary battery 30 of the electrical device 1, a battery pack or battery module can be used.

[0119] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0120] Example

[0121] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0122] The secondary batteries of Examples 1 to 35 and Comparative Examples 1 to 4 were all prepared according to the following method.

[0123] 1. Preparation of positive electrode current collector

[0124] Aluminum foil with a thickness of 13μm is used.

[0125] 2. Preparation of the positive electrode sheet

[0126] 2-1 Preparation of the first positive electrode slurry

[0127] The first composite particles, conductive carbon black, polyvinylidene fluoride binder, and dispersant shown in Table 1 were thoroughly mixed according to a preset mass ratio. Then, N-methylpyrrolidone was added and stirred to disperse the mixture, forming the first positive electrode slurry with a viscosity of 8000 mPa·s to 20000 mPa·s. The mass ratio of the total mass of the first composite particles and conductive carbon black, to the mass of the binder and dispersant was 97:2.5:0.5. The first composite particles were graphitized olivine-type lithium iron phosphate.

[0128] 2-2 Preparation of the second positive electrode slurry

[0129] The second composite particles, conductive carbon black, polyvinylidene fluoride binder, and dispersant shown in Table 2 were thoroughly mixed according to a preset mass ratio. Then, N-methylpyrrolidone was added and stirred to disperse the mixture, forming a second positive electrode slurry with a viscosity of 8000 mPa·s to 20000 mPa·s. The mass ratio of the total mass of the second composite particles and conductive carbon black, to the mass of the binder and dispersant was 97:2.5:0.5. The first composite particles were graphitized olivine-type lithium iron phosphate.

[0130] 2-3. Preparation of the positive electrode sheet

[0131] A double-sided, double-cavity coating device is used to coat the first positive electrode slurry and the second positive electrode slurry onto an aluminum foil. The coating is then dried, cold-pressed, slit, and prepared to obtain a positive electrode sheet. During the coating process, the first positive electrode slurry is located between the second positive electrode slurry and the aluminum foil.

[0132] 3. Preparation of negative electrode current collector

[0133] A copper foil with a thickness of 6μm is used.

[0134] 4. Preparation of conventional negative electrode sheets

[0135] The negative electrode active material graphite, conductive carbon black, binder styrene-butadiene rubber emulsion (SBR), and thickener sodium carboxymethyl cellulose (CMC) are thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 95:1.0:2.0:2 to form a uniform negative electrode slurry. The negative electrode slurry is coated onto the negative electrode current collector, and after drying and other processes, the negative electrode sheet is obtained.

[0136] 5. Separating membrane

[0137] PP film is used.

[0138] 6. Preparation of electrolyte

[0139] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 were mixed evenly to obtain an organic solvent. Then, 1 mol / L of LiPF6 was uniformly dissolved in the organic solvent.

[0140] 7. Preparation of lithium-ion secondary batteries

[0141] The positive electrode, separator, and negative electrode are stacked in sequence, then wound into a cell and packaged in a casing. The electrolyte is injected into the cell, and then the cell undergoes sealing, settling, hot and cold pressing, and formation processes to obtain a lithium-ion secondary battery.

[0142] Test section

[0143] Testing of composite particles

[0144] (1) Test of average particle size D50 of composite particles

[0145] The test was conducted using a Malvern 2000 (MasterSizer 2000) laser particle size analyzer according to the standard GB / T19077-2016 / ISO13320:2009. The test procedure was as follows: Pretreatment: Take a clean beaker, add an appropriate amount of the sample to be tested, add dispersant, and sonicate at 120W / 5min to ensure that the sample is completely dispersed in the dispersant.

[0146] Test: After the sample is poured into the injection tower, it circulates with the solution to the test optical path system. Under the irradiation of the laser beam, the particle size distribution characteristics of the particles can be obtained by receiving and measuring the energy distribution of the scattered light (shading degree: 8-12%). Specifically, the sample can be measured according to the standard GB / T19077-2016 / ISO 13320:2009.

[0147] (2) Graphitization degree test of composite particles

[0148] The composite particles were laid flat on a glass slide and placed under a laser Raman spectrometer. The Raman peaks on the surface were observed. The ratio of the intensities of the two peaks in the Raman spectrum, I1360 / I1580, was used to measure 3-5 points and take the average value to test the graphitization degree of the composite particles.

[0149] (3) Morphology test of composite particles

[0150] The samples were tested using a ZEISS Sigma 300 scanning electron microscope, and the morphology of the samples was observed in accordance with the standard JY / T010-1996.

[0151] The steps are as follows:

[0152] 1. Sample preparation: Cut a 5mm*5mm sample (including the abnormal area) with scissors and paste it on the sample stage with conductive adhesive. Use tweezers to make holes around the abnormal area to mark it.

[0153] 2. Parameter settings: Mode: In-lens, Voltage: 10KV, Aperture: 30um, Working distance: 4.5mm.

[0154] 3. Testing procedure: Move the sample at approximately 50x magnification to identify abnormal locations. Focus on the abnormal locations at 30K, 10K, 5K, 3K, 1K, 500, 200, and 50 (the minimum magnification should ideally cover the entire abnormal area). Focus on the normal areas at 30K, 10K, 5K, 3K, 1K, and 500 for imaging.

[0155] Performance testing of secondary batteries

[0156] (1) Rate performance test of lithium-ion secondary batteries

[0157] At 25°C, a fresh lithium-ion secondary battery is left to stand for 30 minutes, then discharged at a constant current rate of 0.33C to 2.5V, then discharged at a constant current rate of 0.33C to 2.0V, left to stand for 60 minutes, and then charged at a constant current rate of 0.33C to 3.65V. The constant voltage charging cutoff current is 0.05C.

[0158] At 25℃, after being left to stand for 30 minutes, the battery was discharged at a constant current rate of 0.33C to 2.5V, then discharged at a constant current rate of 0.33C to 2.0V, and then left to stand for 30 minutes. The discharge capacity of the lithium-ion secondary battery at a 0.33 rate was then measured.

[0159] At 25℃, after being left to stand for 30 minutes, the lithium-ion secondary battery was discharged at a constant current rate of 1C to 2.5V, then discharged at a constant current rate of 1C to 2.0V, and then left to stand for 30 minutes. The 1C discharge capacity of the lithium-ion secondary battery was then measured.

[0160] The 1C rate discharge capacity retention rate (%) of a lithium-ion secondary battery = 1C rate discharge capacity / 0.33C rate discharge capacity × 100%.

[0161] (2) DC internal resistance (DCR) test

[0162] At 25℃, the lithium-ion secondary battery was left to stand for 30 minutes, then charged at a constant current of 0.33C to 3.65V with a constant voltage charging cutoff current of 0.05C, left to stand for 5 minutes, then discharged at 0.33C with a cutoff current of 0.5C, left to stand for 60 minutes; then discharged at 5C for 30 seconds, and the voltage V1 at the end was recorded, left to stand for 40 seconds, then charged at a constant current of 3.75C for 30 seconds, left to stand for 60 minutes, then charged at a constant current of 0.33C to 3.65V with a constant voltage charging cutoff current of 0.05C, left to stand for 5 minutes, then discharged at 0.33C with a cutoff current of 0.9C, left to stand for 60 minutes, then discharged at 5C for 30 seconds, and the voltage V2 at the end was recorded, left to stand for 40 seconds; finally, charged at a constant current of 3.75C for 30 seconds, left to stand for 5 minutes.

[0163] DCR calculation formula: R=(V2-V1) / I, unit is mΩ.

[0164] (3) An exemplary method for testing the gravimetric energy density of a secondary battery is as follows:

[0165] Weigh the battery using an electronic scale (W, unit: kg); charge the battery at 25℃ with a constant current of 1 / 3C to its highest rated voltage, then charge it at a constant voltage until the current drops to 0.05C, let it stand for 15 minutes, then discharge it with a constant current of 1 / 3C to its lowest rated voltage, and let it stand for 5 minutes; obtain the battery's discharge energy; repeat the test 3 times and take the average value to obtain the battery's average discharge energy E (unit: Wh); the battery's weight energy density = E / W.

[0166] The parameters of Examples 1-27 and Comparative Examples 1-3 are shown in Tables 1 and 2.

[0167] Table 1

[0168]

[0169]

[0170] Table 2

[0171]

[0172]

[0173] The test results of Examples 1-27 and Comparative Examples 1-4 are shown in Table 3.

[0174] Table 3

[0175]

[0176]

[0177] As shown in Tables 1 to 3, Comparative Example 1, which uses a single layer of graphitized positive electrode active material, exhibits relatively superior rate performance. Compared to Comparative Example 1, the energy density of Examples 1 to 27 is significantly improved. Therefore, the secondary batteries of Examples 1 to 27 can simultaneously achieve both high rate performance and high energy density.

[0178] The first and second active material layers in Examples 1 to 6 have a high degree of graphitization, resulting in better rate performance and lower DC resistance in the secondary batteries. Furthermore, the rate performance improves with increasing graphitization. Example 3, in particular, exhibits relatively high rate performance and energy density.

[0179] In Examples 7 to 14, the secondary battery contains a high content of graphitized first active material layer, resulting in relatively good rate performance and low DC resistance; and due to the relatively high compaction density of the first active material layer, the energy density of the secondary battery is improved.

[0180] Examples 12 to 15 show that, by using a first active material layer with a relatively high degree of graphitization, the mass content of the conductive agent in the first active material layer is reduced, and the rate performance of the secondary battery does not deteriorate significantly, while the energy density is improved.

[0181] In Examples 16 to 21, within a reasonable compaction density range, as the compaction density of the first active material layer increases, the energy density of the secondary battery increases, and the rate performance also improves.

[0182] In Examples 22 to 27, within a reasonable compaction density range, as the compaction density of the second active material layer increases, the energy density of the secondary battery increases, and the rate performance also improves.

[0183] Figure 8 This is a TEM (Transmission Electron Microscope) image of the first composite particle in Example 1. Figure 9 This is a TEM image of the second composite particle in Example 1. Figure 10 This is a partial SEM (Scanning Electron Microscope) image of the positive electrode sheet in Example 1. Figures 8 to 10As shown, in Example 1, the average particle size D50 of the first active material layer is relatively small and the particles are densely distributed; the average particle size D50 of the second active material layer is relatively large, and a porous structure can be formed between the first and second active material layers.

[0184] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict.

Claims

1. A positive electrode sheet, comprising: Positive current collector; A positive electrode active material layer is disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a first active material layer and a second active material layer sequentially stacked away from the surface. The first active material layer includes a first composite particle, which includes a first lithium iron phosphate particle and a first carbon layer coating the surface of the first lithium iron phosphate particle. The second active material layer includes a second composite particle, which includes a second lithium iron phosphate particle and a second carbon layer coating the surface of the second lithium iron phosphate particle. The graphitization degree of the first composite particle is greater than that of the second composite particle. The degree of graphitization of the first composite particle is 0.3~0.6; The degree of graphitization of the second composite particles is 0.05~0.

3.

2. The positive electrode sheet according to claim 1, wherein, The degree of graphitization of the first composite particles is 0.3~0.5; and / or The degree of graphitization of the second composite particles is 0.1~0.

2.

3. The positive electrode sheet according to claim 1 or 2, wherein, The first composite particle includes primary particles, and the second composite particle includes secondary particles formed by the aggregation of multiple primary particles. The average particle size D50 of the first composite particle is smaller than the average particle size D50 of the second composite particle.

4. The positive electrode sheet according to claim 3, wherein, The average particle size D50 of the first composite particles is 1 μm to 4 μm, and / or The average particle size D50 of the second composite particles is 6μm~12μm.

5. The positive electrode sheet according to claim 4, wherein, The average particle size D50 of the first composite particles is 1 μm to 3 μm, and / or The average particle size D50 of the second composite particles is 8μm~10μm.

6. The positive electrode sheet according to claim 3, wherein, The secondary particles have a spherical or near-spherical morphology.

7. The positive electrode sheet according to claim 1, wherein, The specific surface area of ​​the first composite particle is 10 m². 2 / g ~15m 2 / g; and / or The specific surface area of ​​the second composite particle is 5m². 2 / g ~10m 2 / g.

8. The positive electrode sheet according to claim 7, wherein, The specific surface area of ​​the first composite particle is 12 m². 2 / g ~14m 2 / g, and / or The specific surface area of ​​the second composite particle is 6m². 2 / g ~8m 2 / g.

9. The positive electrode sheet according to claim 1, wherein, The ratio of the coating weight CW1 of the first active material layer to the coating weight CW2 of the second active material layer is 0.8 to 1.

2.

10. The positive electrode sheet according to claim 9, wherein, The ratio of the coating weight CW1 of the first active material layer to the coating weight CW2 of the second active material layer is 1.

11. The positive electrode sheet according to claim 9, wherein, The coating weight CW1 of the first active material layer is 0.115 mg / cm³. 2 ≤CW1≤0.195mg / cm 2 ; and / or The coating weight (CW2) of the second active material layer is 0.115 mg / cm². 2 ≤CW2≤0.195mg / cm 2 .

12. The positive electrode sheet according to claim 1, wherein, The first active material layer includes a first conductive agent, and the mass content of the first conductive agent is A, based on the total weight of the first active material layer being 100%. The second active material layer includes a second conductive agent, and the mass content of the second conductive agent is B, based on the total weight of the second active material layer being 100%. Where A < B.

13. The positive electrode sheet according to claim 12, wherein, 1wt%≤BA≤3wt%.

14. The positive electrode sheet according to claim 1, wherein, The first active material layer has a powder compaction density of 2.4 g / cc to 2.65 g / cc under a pressure of 600 MPa; and / or The compacted density of the second active material layer at a pressure of 600 MPa is 2.2 g / cc to 2.45 g / cc.

15. The positive electrode sheet according to claim 14, wherein, The first active material layer has a powder compaction density of 2.5 g / cc to 2.6 g / cc under a pressure of 600 MPa; and / or The compacted density of the second active material layer at 600 MPa pressure is 2.35 g / cc ~ 2.45 g / cc.

16. A secondary battery, comprising a positive electrode, a separator, and a negative electrode, wherein the positive electrode is the positive electrode as described in any one of claims 1 to 15.

17. A battery module comprising the secondary battery as described in claim 16.

18. A battery pack comprising a secondary battery as claimed in claim 16 or a battery module as claimed in claim 17.

19. An electrical device comprising a secondary battery as claimed in claim 16, a battery module as claimed in claim 17, or a battery pack as claimed in claim 18.

Images