Positive electrode piece, secondary battery, and electric device

Abstract

A positive electrode piece, comprising a current collector and a positive electrode active layer, the positive electrode active layer being arranged on at least one surface of the current collector; the positive electrode active layer comprises a first active layer directly coated on the current collector and a second active layer coated on the surface of the side of the first active layer away from the current collector; the first active layer comprises first lithium iron phosphate salt particles and / or first manganese iron phosphate salt particles, the second active layer comprises second lithium iron phosphate salt particles, the primary average particle diameter of the first lithium iron phosphate salt particles and / or the first manganese iron phosphate salt particles is 150 nm to 480 nm, and the primary average particle diameter of the second lithium iron phosphate particles is 500-3000 nm.

Description

Positive electrode sheet, secondary battery and electrical device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202410027211.8, filed on January 8, 2024, entitled “Positive Electrode Sheet, Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to a positive electrode sheet, a secondary battery, and an electrical device. Background Art

[0004] As a lithium-ion battery cathode material, lithium iron phosphate (LiFePO4) has rapidly become a global research hotspot due to its abundant resources, low price, environmental friendliness, and stable voltage in a two-phase reaction. Existing LiFePO4 cathode active materials used in cathode plates, such as nano-sized LiFePO4 and LiMnFePO4, have small particle sizes and high specific surface areas. These materials absorb water strongly when in contact with air, making them susceptible to side reactions at the solid-liquid interface. Furthermore, during cold pressing, they are prone to sticking to the rollers, stretching, and subsequently powder loss. Consequently, the actual coating requires a high binder content, which significantly impacts battery capacity.

[0005] Summary of the Invention

[0006] The present application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode sheet, a secondary battery and an electrical device, wherein the positive electrode sheet has high cohesion, low water absorption and can maintain a high energy density.

[0007] The first aspect of the present application provides a positive electrode plate, including a current collector and a positive electrode active layer, the positive electrode active layer is arranged on at least one side of the current collector, the positive electrode active layer includes a first active layer directly coated on the current collector, and a second active layer coated on the surface of the first active layer away from the current collector. The first active layer contains first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles, and the second active layer contains second lithium iron phosphate salt particles. The primary average particle size of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is 150nm-480nm, and the primary average particle size of the second lithium iron phosphate salt particles is 500nm-3000nm.

[0008] As described above, the positive electrode plate of the present application is provided with a positive electrode active layer comprising a first active layer and a second active layer, and the first active layer comprises lithium iron phosphate salt particles and / or lithium manganese phosphate salt particles (also referred to as "lithium manganese phosphate (manganese) iron salt particles") with a smaller primary average particle size close to the current collector.

[0009] Among them, by making the primary average particle size of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles and the second lithium iron phosphate salt particles of the embodiment of the present application within the above-mentioned range, the particles of the first active layer and the second active layer can be kept at a suitable micron-level size, thereby avoiding the problems of interfacial side reactions and processing difficulties caused by nano-sizing the particles, and preventing the particles from being limited by an excessively large size and causing a reduction in dynamic performance.

[0010] In addition, the second active layer contains lithium iron phosphate salt particles with a larger primary average particle size away from the current collector. By forming this structure, the positive electrode plate of the embodiment of the present application can enhance the cohesion of the surface layer due to the large-particle lithium iron phosphate salt particles provided on the surface. In addition, due to the relatively large particle size and small specific surface area of ​​the large-particle lithium iron phosphate salt particles, the water absorption of the material can be reduced, thereby maintaining the high energy density of the positive electrode plate. In addition, by arranging the large-particle lithium iron phosphate salt particles as the second active layer on the upper layer (the outer surface of the plate), it is possible to ensure that the large-particle lithium iron phosphate salt particles as the second active layer achieve better bonding, improve the compaction density of the positive electrode plate, and at the same time, the secondary battery having the positive electrode plate has excellent dynamic performance.

[0011] In any embodiment, the cohesive force of the positive electrode sheet is ≥32 N / m.

[0012] The cohesive force of the positive electrode sheet of the application is within the above range as measured by the measurement method described in the embodiments of the present application, thereby avoiding powder loss during the processing of the positive electrode sheet and improving the bonding force between the positive electrode surface and the isolation membrane.

[0013] In any embodiment, the first active layer has a thickness ranging from 80 μm to 140 μm, in some embodiments from 90 μm to 120 μm, and the second active layer has a thickness ranging from 8 μm to 50 μm, in some embodiments from 20 μm to 35 μm.

[0014] By setting the thickness of each group of the first active layer and the second active layer within the above range, the above technical effects of the present application can be further preferably achieved.

[0015] In any embodiment, the BET specific surface area of ​​the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is 10 m 2 / g-18m 2 / g, and in some embodiments the BET specific surface area is 12 m 2 / g-16m 2 / g, and the BET specific surface area of ​​the second lithium iron phosphate particles is 3m 2 / g-8m 2 / g, and in some embodiments the BET specific surface area is 4m 2 / g-7m 2 / g.

[0016] By ensuring that the BET specific surface areas of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles and / or the second lithium manganese iron phosphate salt particles of the embodiment of the present application are respectively within the above-mentioned ranges, it is possible to facilitate the stirring of the slurry containing the lithium iron phosphate salt particles and increase the solid content, thereby improving the processing problems of the battery cell and further increasing the volume energy density of the battery.

[0017] In any embodiment, the sphericity of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is ≤1.3, the sphericity of the second lithium iron phosphate salt particles is ≥1.5, and the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles are closer to spherical than the second lithium iron phosphate salt particles.

[0018] In this article, the term "sphericity" can be measured by the aspect ratio of the particles. The smaller the aspect ratio value, the better the sphericity. The aspect ratio is the ratio of the diameter of the major axis of a primary particle to the diameter of the minor axis. Let the diameter in the major axis be a and the diameter in the minor axis be b, and the aspect ratio = a / b. This parameter is often used to describe the morphology of particles and can be used to measure their sphericity. The closer the value is to 1, the higher the sphericity.

[0019] The sphericity can be measured using the method described in the examples of this application.

[0020] By ensuring that the sphericity of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles and the second lithium iron phosphate salt particles are each within the above range, the above technical effects of the present application can be further preferably achieved.

[0021] In any embodiment, the molecular formula of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is Li x’ M1 y’ Mn m’ Fe z’ M2 k’ PO4, wherein M1 includes at least one of Na, K, Ca, Al, and Mg, or a combination of two or more thereof, and M2 includes one of Ni, Cu, Zn, Ti, Nb, V, Cr, and Co, or a combination of two or more thereof, 0.95≤x'≤1.15, 0≤y'≤0.1, 0.1≤m'≤0.9, 0.1≤z'≤1, 0≤k'≤0.1, and the content of Mn is ≤1000ppm.

[0022] In the present application, there is no particular limitation on the composition of the first lithium iron phosphate salt particles and / or the first lithium iron manganese phosphate salt particles, and any existing small-particle lithium iron (manganese) phosphate salt particles can be used. For example, in an embodiment of the present application, by using the first lithium iron phosphate salt particles and / or the first lithium iron manganese phosphate salt particles having the above composition, and forming a double-layer structure with the second lithium iron phosphate salt particles in the embodiment of the present application, the above-mentioned effects of the present application can be better achieved.

[0023] In any embodiment, the molecular formula of the second lithium iron phosphate salt particle is Li m Fe x P y O j Q q , wherein Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, S, F, Cl, and Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0024] By making the second lithium iron phosphate salt of the embodiment of the present application have the above molecular formula and modifying it by adding one or more of the above elements, a high-load bulk modification of the second lithium iron phosphate salt particles is achieved. This helps to improve the bulk ion transport capacity of the lithium iron phosphate salt particles and can effectively solve the problem of poor kinetic performance inherent in large particles. In addition, by performing the above modification, the second lithium iron phosphate salt particles can exhibit good kinetic performance when used as a positive electrode material. In this application, the modification can specifically be manifested as doping and / or coating.

[0025] In any embodiment, Q includes at least one of Ti, V, Mg, and Nb, and in some embodiments is Ti. Calculated based on the total weight of the second lithium iron phosphate salt particles, the content of Q is 1000ppm-10000ppm, and in some embodiments the content is 2500ppm-6000ppm.

[0026] By doping the lithium iron phosphate salt of the embodiment of the present application with one or more of the above elements and making the content within the above range, the metal bulk modification of the second lithium iron phosphate salt particles can be better achieved while ensuring the primary average particle size of the lithium iron phosphate salt particles, further improving the bulk ion transport capacity of the lithium iron phosphate salt particles, and better solving the problem of poor kinetic performance inherent in large particles.

[0027] In any embodiment, the second lithium iron phosphate salt particles contain carbon, and the carbon content is 1.0 wt % to 2.0 wt % based on the total weight of the second lithium iron phosphate salt particles.

[0028] By setting the carbon content of the second lithium iron phosphate salt particles to 0.8 wt%-2.0 wt%, carbon can be coated on the surface of the second lithium iron phosphate salt particles to form a lithium iron phosphate material with a uniform and dense carbon coating layer, greatly improving the surface conductivity of the particles.

[0029] In any embodiment, the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles, and the second lithium iron phosphate salt particles are each independently single crystal particles and / or polycrystalline particles. In some embodiments, based on the total number of particles of the second lithium iron phosphate salt particles, the number of single crystal particles accounts for more than 90%.

[0030] By using single crystal particles of lithium iron phosphate in each active layer of the positive electrode plate of the present application, the phenomenon of polycrystalline particles being crushed and peeling off during the cold pressing process can be effectively avoided, thereby improving the bonding force between the second active layer and the first active layer or the isolation membrane, thereby improving the dynamic performance and energy density of the positive electrode plate.

[0031] In any embodiment, the capacity proportion of the second lithium iron phosphate salt particles η ≥ 88%, η is defined as: the battery containing the second lithium iron phosphate salt particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted as C1, and the capacity value at the discharge voltage of 2.0V is extracted as C2, η = C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50uA.

[0032] The capacity ratio η of the second lithium iron phosphate salt particles in the embodiment of the present application is ≥88%, which means that the second lithium iron phosphate salt particles of the present application can enable the secondary battery to exhibit good kinetic performance when used as a positive electrode material.

[0033] In any embodiment, the manufacturing process of the second lithium iron phosphate salt particles includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings, wherein the temperature of the first sintering is 500°C-760°C, and in some embodiments it is 550°C-720°C, and the carbon content of the material after the first sintering is 0.01 weight%-0.79 weight%, and in some embodiments it is 0.05 weight%-0.4 weight%; the temperature of the second sintering is 700°C-800°C, and in some embodiments it is 720°C-780°C, and the carbon content of the material after the second sintering is 0.8 weight%-2.0 weight%, and in some embodiments it is 1.0 weight%-1.6 weight%.

[0034] In the preparation process of the second lithium iron phosphate salt particles of the embodiment of the present application, two sintering processes are performed. By controlling the temperature of the first sintering process within the aforementioned range and ensuring that the carbon content of the sintered intermediate is within the aforementioned range, the lithium iron phosphate precursor obtained after the first sintering process can have a larger primary particle size, directly improving the powder compaction and electrode compaction density of the final product. Furthermore, the carbon source added before the first sintering process can effectively reduce the trivalent iron in the raw material, thereby improving the purity and stability of the product. By controlling the temperature of the second sintering process within the aforementioned range and ensuring that the carbon content of the sintered material is between 0.8% and 2.0% by weight, carbon can be coated on the surface of the second lithium iron phosphate salt particles, forming a lithium iron phosphate material with a uniform and dense carbon coating layer, significantly improving the surface conductivity of the particles.

[0035] In any embodiment, the first pulverization is performed after the first sintering, and the second pulverization is performed after the second sintering, wherein the Dv50 of the product after the first pulverization is 300nm-1200nm, and in some embodiments, it is 400nm-1100nm; the Dv50 of the product after the second pulverization is 500nm-5000nm, and in some embodiments, it is 700nm-2500nm.

[0036] In the preparation method of the second lithium iron phosphate salt particles of the embodiment of the present application, two post-sintering crushing processes are implemented. By performing the first crushing after the first sintering, the product has an average primary particle size of 300nm-1200nm, which can avoid the growth barrier of the carbon material and the modifying element to the crystal, and obtain a micron-sized lithium iron phosphate precursor. By performing the second crushing after the second sintering, the product has an average primary particle size of 500nm-3000nm, and lithium iron phosphate salt particles of the desired particle size can be obtained, resulting in a uniform and dense carbon coating layer, which significantly improves the surface conductivity of the particles.

[0037] In any embodiment, the first active layer and the second active layer may further arbitrarily contain a conductive agent, a binder and a dispersant, wherein, in the first active layer, the weight ratio of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles: binder: conductive agent: dispersant is 96-99: 0.5-3: 0.5-3: 0.5-3, and in some embodiments, it is 96: 1.0: 2.5: 0.5; in the second active layer, the weight ratio of the second lithium iron phosphate salt particles: binder: conductive agent: dispersant is 96-99: 0.5-3: 0.5-3: 0.5-3, and in some embodiments, it is 96.5: 0.5: 2.5: 0.5; the weight ratio of the binder in the first active layer is less than or equal to the weight ratio of the binder in the second active layer.

[0038] In any embodiment, the conductive agent includes any one or a combination of at least two of natural graphite, artificial graphite, conductive carbon black, carbon fiber, carbon nanotubes, graphene, and conductive polymers or metal powders, and in some embodiments, it is conductive carbon black. The binder includes any one or a combination of at least two of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, or polytetrafluoroethylene, and in some embodiments, it is polyvinylidene fluoride. The dispersant is not particularly limited as long as it is a dispersant commonly used in the art, including PVP (polyvinyl pyrrolidone) and the like.

[0039] By using the above-mentioned specific conductive agent, binder and dispersant, and making the weight ratio of the first lithium iron phosphate salt particles: binder: conductive agent: dispersant within the above-mentioned range, the positive electrode active layer of the present application can better achieve the above-mentioned effects of the present application.

[0040] The second aspect of the present application provides a secondary battery comprising the positive electrode sheet of the first aspect of the present application.

[0041] A third aspect of the present application provides an electrical device comprising the secondary battery according to the second aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG1 is a schematic structural diagram of a positive electrode sheet according to an embodiment of the present application.

[0043] FIG2 is an electron microscope image of a positive electrode sheet according to an embodiment of the present application.

[0044] FIG. 3 is a schematic diagram exemplarily showing the particle size of primary particles of the present application.

[0045] FIG. 4 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0046] FIG. 5 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 4 .

[0047] FIG6 is a schematic diagram of a battery module according to an embodiment of the present application.

[0048] FIG7 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0049] FIG8 is an exploded view of the battery pack shown in FIG7 according to an embodiment of the present application.

[0050] FIG. 9 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0051] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0052] The following describes in detail the embodiments of the positive electrode sheet, secondary battery, and electrical device of the present application. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0053] The "ranges" disclosed herein are defined in terms of lower and upper limits, where a given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise indicated, the numerical range "a to b" is a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0054] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0055] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0056] Unless otherwise specified, all steps of the present application may be performed sequentially, randomly, or optionally sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0057] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0058] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: 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).

[0059] The lithium iron phosphate active materials used in existing positive electrode plates, such as nano-sized lithium iron phosphate and lithium iron manganese phosphate, have small particle sizes, high specific surface areas, and strong water absorption. A layer of active material, typically coated with secondary particles, is used to enhance the surface cohesion of the plates. However, these secondary particles can crack and break during cold pressing.

[0060] Based on this, the present application proposes a positive electrode plate, in which a layer of primary particle material with a larger particle size is designed to be coated on the surface of the plate to improve the surface cohesion of the plate, thereby avoiding the problem of secondary particles being compressed and crushed, thereby improving the dynamic performance and energy density.

[0061] [Positive electrode]

[0062] The positive electrode plate of an embodiment of the present application includes a current collector and a positive electrode active layer, wherein the positive electrode active layer is arranged on at least one side of the current collector, and the positive electrode active layer includes a first active layer directly coated on the current collector, and a second active layer coated on the surface of the first active layer away from the current collector. The first active layer contains first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles, and the second active layer contains second lithium iron phosphate salt particles. The primary average particle size of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is 150 nanometers (nm)-480nm, and the primary average particle size of the second lithium iron phosphate salt particles is 500nm-3000nm.

[0063] In some embodiments, the primary average particle size of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles can be selected as 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, 210nm, 220nm, 230nm, 240nm, 250nm, 260nm, 270nm, 280nm, 290nm, 300nm, 310nm, 320nm, 330nm, 340nm, 350nm, 360nm, 370nm, 380nm, 390nm, 400nm, 410nm, 420nm, 430nm, 440nm, 450nm, 460nm, 470nm or 480nm, or a range between any two of the above values. In some embodiments, the primary average particle size of the second lithium iron phosphate particles may be 500 nm, 600 nm, 650 nm, 700 nm, 790 nm, 800 nm, 870 nm, 900 nm, 920 nm, 1000 nm, 1100 nm, 1200 nm, 1250 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, 2100 nm, 2200 nm, 2300 nm, 2400 nm, 2500 nm, 2600 nm, 2700 nm, 2800 nm, 2900 nm or 3000 nm, or a range between any two of the above values.

[0064] In this article, the term "primary average particle size" refers to the particle size value obtained by statistically analyzing the particle size of the particles using the long diameter statistical method under the field of view of a scanning electron microscope. Among them, in the particle size statistical process, particles with a primary average particle size less than or equal to 80nm are not included in the statistical range. The primary average particle size of this application is shown in Figure 3. The primary average particle size refers to the average particle size of the primary particles. In this article, "primary particles" refer to particles that do not have obvious agglomeration interfaces in the particle scanning electron microscope image, but may have tiny pores and point or line defects, which are different from powder particles that are the smallest units without structures such as stacking and flocculation. The determination method of the primary average particle size, the carbon content and the BET specific surface area described later can be carried out using the determination method described in the examples.

[0065] As described above, the positive electrode plate of the present application is provided with a positive electrode active layer comprising a first active layer and a second active layer, wherein the first active layer comprises lithium iron phosphate salt particles and / or lithium manganese phosphate salt particles (also referred to as "lithium manganese phosphate (manganese) iron salt particles") with a smaller primary average particle size close to the current collector.

[0066] Among them, by making the primary average particle size of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles and the second lithium iron phosphate salt particles of the embodiment of the present application within the above-mentioned range, the particles of the first active layer and the second active layer can be kept at a suitable micron-level size, thereby avoiding the problems of interfacial side reactions and processing difficulties caused by nano-sizing the particles, and preventing the particles from being limited by an excessively large size and causing a reduction in dynamic performance.

[0067] In addition, the second active layer contains lithium iron phosphate salt particles with a larger primary average particle size away from the current collector. By forming this structure, the positive electrode plate of the embodiment of the present application can enhance the cohesion of the surface layer due to the large-particle lithium iron phosphate salt particles provided on the surface. In addition, due to the relatively large particle size and small specific surface area of ​​the large-particle lithium iron phosphate salt particles, the water absorption of the material can be reduced, thereby maintaining the high energy density of the positive electrode plate. In addition, by providing the large-particle lithium iron phosphate salt particles as the second active layer on the upper layer (the outer surface of the plate), it is possible to ensure that the large-particle lithium iron phosphate salt particles as the second active layer achieve better bonding, improve the compaction density of the positive electrode plate, and at the same time, the secondary battery having the positive electrode plate has excellent dynamic performance.

[0068] In any embodiment, the cohesive force of the positive electrode sheet is ≥32 Newtons / meter (N / m).

[0069] The cohesive force of the positive electrode sheet of the application is within the above range as measured by the measurement method described in the embodiments of the present application, thereby avoiding powder loss during the processing of the positive electrode sheet and improving the bonding force between the positive electrode surface and the isolation membrane.

[0070] In any embodiment, the first active layer has a thickness ranging from 80 micrometers (μm) to 140 μm, in some embodiments from 90 μm to 120 μm, and the second active layer has a thickness ranging from 8 μm to 50 μm, in some embodiments from 20 μm to 35 μm.

[0071] In some embodiments, the thickness of the first active layer may be 80 μm, 81 μm, 84 μm, 87 μm, 90 μm, 93 μm, 96 μm, 99 μm, 100 μm, 103 μm, 106 μm, 109 μm, 110 μm, 113 μm, 116 μm, 119 μm, 120 μm, 123 μm, 126 μm, 129 μm, 130 μm, 133 μm, 136 μm, 139 μm or 140 μm, or any range therebetween. In some embodiments, the thickness of the first active layer can be 8 μm, 11 μm, 14 μm, 17 μm, 19 μm, 21 μm, 24 μm, 27 μm, 30 μm, 34 μm, 37 μm, 40 μm, 43 μm, 46 μm, 49 μm or 50 μm, or a range between any two of the above values.

[0072] By setting the thickness of each group of the first active layer and the second active layer within the above range, the above technical effects of the present application can be further preferably achieved.

[0073] In any embodiment, the BET specific surface area of ​​the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is 10 square meters / gram (m 2 / g)-18m 2 / g, and in some embodiments the BET specific surface area is 12 m 2 / g-16m 2 / g, the BET specific surface area of ​​the second lithium iron phosphate particles is 3m 2 / g-8m 2 / g, and in some embodiments the BET specific surface area is 4m 2 / g-7m 2 / g.

[0074] In some embodiments, the BET specific surface area of ​​the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles can be 10 m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g、15m 2 / g、16m 2 / g、17m 2 / g or 8m 2 / g, or a range between any two of the above values. In some embodiments, the BET specific surface area of ​​the second lithium iron phosphate particles can be 3m 2 / g、4m 2 / g、5m 2 / g、6m2 / g、7m 2 / g or 8m 2 The BET specific surface area can be measured using the method described in the examples.

[0075] By ensuring that the BET specific surface areas of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles and / or the second lithium manganese iron phosphate salt particles of the embodiment of the present application are respectively within the above-mentioned ranges, it is possible to facilitate the stirring of the slurry containing the lithium iron phosphate salt particles and increase the solid content, thereby improving the processing problems of the battery cell and further increasing the volume energy density of the battery.

[0076] In any embodiment, the sphericity of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is ≤1.3, and that of the second lithium iron phosphate salt particles is ≥1.5, and the sphericity of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is higher than the sphericity of the second lithium iron phosphate salt particles.

[0077] In this article, the term "sphericity" can be measured by the aspect ratio of the particles. The smaller the aspect ratio, the better the sphericity. The aspect ratio is the ratio of the diameter of the major axis of a primary particle to the diameter of the minor axis. Let the diameter in the major axis be a and the diameter in the minor axis be b, and the aspect ratio = a / b. This parameter is often used to describe the morphology of particles and can be used to measure their sphericity.

[0078] The sphericity can be measured using the method described in the examples of this application.

[0079] By ensuring that the sphericity of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles and the second lithium iron phosphate salt particles are each within the above range, the above technical effects of the present application can be further preferably achieved.

[0080] [First lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles]

[0081] In any embodiment, the molecular formula of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is Li x’ M1 y’ Mn m’ Fe z’ M2 k’PO4, wherein the M1 includes at least one of Na, K, Ca, Al, and Mg, or a combination of two or more thereof, the M2 includes one of Ni, Cu, Zn, Ti, Nb, V, Cr, and Co, or a combination of two or more thereof, 0.95≤x'≤1.15, 0≤y'≤0.1, 0.1≤m'≤0.9, 0.1≤z'≤1, 0≤k'≤0.1, and the Mn content is ≤1000 parts per million (1000 ppm).

[0082] In the present application, there is no particular limitation on the composition of the first lithium iron phosphate salt particles and / or the first lithium iron manganese phosphate salt particles, and any existing small-particle lithium iron (manganese) phosphate salt particles can be used. For example, in an embodiment of the present application, by using the first lithium iron phosphate salt particles and / or the first lithium iron manganese phosphate salt particles having the above composition, and forming a double-layer structure with the second lithium iron phosphate salt particles in the embodiment of the present application, the above-mentioned effects of the present application can be better achieved.

[0083] [Second lithium iron phosphate particles]

[0084] The primary average particle size of the second lithium iron phosphate particles of the embodiment of the present application is 500 nm to 3000 nm, and in some embodiments, 650 nm to 2500 nm, and the BET specific surface area is 3 m 2 / g~8m 2 / g, in some embodiments 4m 2 / g~7m 2 / g, calculated based on the total weight of the second lithium iron phosphate salt particles, the carbon content of the second lithium iron phosphate salt particles is Cx weight%, wherein 0.8≤Cx≤2.0, in some embodiments 1.0≤Cx≤1.6, and the ratio z of the BET specific surface area to Cx satisfies 1.5≤z≤8.5, and in some embodiments 3≤z≤6.

[0085] By making the primary average particle size of the second lithium iron phosphate particles of the embodiment of the present application 500nm to 3000nm and the BET specific surface area 3m 2 / g~8m 2 / g, the carbon content calculated based on the total weight of the second lithium iron phosphate salt particles is Cx weight % (0.8≤Cx≤2.0), and at the same time, the ratio z of the BET specific surface area of ​​the second lithium iron phosphate salt particles to Cx (i.e., BET specific surface area / Cx) satisfies the range of 1.5≤z≤8.5, which can improve the dynamic properties of the second lithium iron phosphate salt particles with an average particle size within the above range; the particles are kept at a suitable micron size, thereby avoiding the interface side reactions and processing difficulties caused by nano-sizing the particles, and the particles will not be limited to an excessively large size and thus suffer a reduction in dynamic performance; in addition, it is beneficial to the stirring of the slurry containing the second lithium iron phosphate salt particles and the increase of the solid content, thereby improving the processing problem of the battery cell, and thereby increasing the volume energy density of the battery; in addition, it can avoid the normal deintercalation and extraction of lithium ions due to the excessively high density of the carbon coating, thereby affecting the capacity of the battery cell.

[0086] Compared with the prior art, the second lithium iron phosphate salt particles in the embodiment of the present application have a smaller specific surface area when containing the same carbon content, which means that the particles contain less floating carbon, and can make the carbon contained in the second lithium iron phosphate salt particles more uniform and dense and each particle is coated, thereby improving the surface conductivity of the particles and improving their kinetic performance as a positive electrode material.

[0087] In any embodiment, the second lithium iron phosphate salt has a molecular formula of Li m Fe x P y O j Q q , wherein Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, Si, N, S, F, Cl, and Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0088] By making the second lithium iron phosphate salt of the embodiment of the present application have the above-mentioned molecular formula and modifying it by adding one or more of the above-mentioned elements, a high-load bulk modification of the second lithium iron phosphate salt particles is achieved. This helps to improve the bulk ion transport capacity of the second lithium iron phosphate salt particles and can effectively solve the problem of poor kinetic performance inherent in large particles. In addition, by performing the above-mentioned modification, the second lithium iron phosphate salt particles can exhibit good kinetic performance when used as a positive electrode material. In this application, the modification can specifically be manifested as doping and / or coating.

[0089] In any embodiment, the Q includes at least one of Ti, V, Mg, and Nb, and in some embodiments is Ti. Calculated based on the total weight of the second lithium iron phosphate salt particles, the content of Q is 1000ppm-10000ppm, and in some embodiments is 2500ppm-6000ppm. In some embodiments, calculated based on the total weight of the second lithium iron phosphate salt particles, the content of Ti, V, Mg and / or Nb can be 2500ppm, 2600ppm, 2700ppm, 2800ppm, 2900ppm, 3000ppm, 3100ppm, 3200ppm, 3300ppm, 3400ppm, 3500ppm, 3600ppm, 3700ppm, 3800ppm, 3900ppm, 4000ppm. m, 4100ppm, 4200ppm, 4300ppm, 4400ppm, 4500ppm, 4600ppm, 4700ppm, 4800ppm, 4900ppm, 5000ppm, 5100ppm, 5200ppm, 5300ppm, 5400ppm, 5500ppm, 5600ppm, 5700ppm, 5800ppm, 5900ppm or 6000ppm, or a range between any two of the above values.

[0090] By adding one or more of the above elements to the second lithium iron phosphate salt of the embodiment of the present application and making the content within the above range, the metal bulk modification of the second lithium iron phosphate salt particles can be better achieved while ensuring the primary average particle size of the second lithium iron phosphate salt particles, further improving the bulk ion transport capacity of the second lithium iron phosphate salt particles, and better solving the problem of poor kinetic performance inherent in large particles.

[0091] In any embodiment, the second lithium iron phosphate salt particles contain carbon, and the carbon content is 1.0 wt % to 2.0 wt % based on the total weight of the second lithium iron phosphate salt particles.

[0092] In the second lithium iron phosphate salt particles of the embodiment of the present application, the carbon may be present in a state of being mixed with the second lithium iron phosphate salt particles or coated on the second lithium iron phosphate salt particles. Alternatively, the carbon in the second lithium iron phosphate salt particles may be coated on the second lithium iron phosphate salt particles, thereby enabling the carbon to form a uniform and dense carbon coating layer on the surface of the second lithium iron phosphate salt particles, thereby improving the surface conductivity of the particles.

[0093] In any embodiment, the capacity proportion η of the second lithium iron phosphate salt particles is ≥88%, and the η is defined as: a battery containing the second lithium iron phosphate salt particles as a positive electrode material is charged and discharged twice at a constant current rate of 0.1 coulomb (C) in the voltage range of 2.0 volts (V) to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C1, and the capacity value at the discharge voltage to 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50 microamperes (uA).

[0094] η represents the platform retention performance of the material, and this value is strongly correlated with the discharge power performance of the battery. When this value is large, the battery can still maintain good power performance when discharged to a low SOC (battery state of charge), that is, the voltage drop of the battery is small when the battery is discharged with a large current at low power. The capacity proportion η of the second lithium iron phosphate salt particles in the embodiment of the present application is ≥88%, indicating that the second lithium iron phosphate salt particles of the present application can enable the secondary battery to exhibit good kinetic performance when used as a positive electrode material.

[0095] In any embodiment, the second lithium iron phosphate salt particles meet at least one of the following a)-f):

[0096] a) Dv10 of the second lithium iron phosphate particles is ≥ 0.2 μm;

[0097] b) the Dv50 of the second lithium iron phosphate particles is 0.5-5 μm;

[0098] c) Dv90 of the second lithium iron phosphate particles is ≤8 μm;

[0099] d) Dv99 of the second lithium iron phosphate particles is ≤ 10 μm;

[0100] e) The powder compaction density of the second lithium iron phosphate salt at a pressure of 3T is ≥ 2.25 g / cm3 (g / cm 3 );

[0101] f) The powder resistivity of the second lithium iron phosphate salt is less than 60 Ω·cm.

[0102] In this application, Dv10, Dv90, and Dv99 refer to the particle sizes corresponding to the 10%, 90%, and 99% cumulative particle size distribution percentages, respectively. The test method is the same as that for Dv50, and can be determined using the standards in the examples of this application.

[0103] It should be noted that the Dv10 is a large index, so the upper limit cannot be given, and the Dv90 is a small index, so the lower limit cannot be given.

[0104] By making the second lithium iron phosphate salt particles satisfy at least one of a) to f), the second lithium iron phosphate salt particles can better achieve the above technical effects.

[0105] In this application, the test method for the powder resistivity is: referring to the national standard GB / T33822-2017, using a powder resistivity meter (Suzhou Jinglattice, ST2722 model), weighing 1 gram (g) of sample (error within ±0.005g) and adding it to the feeding chamber, applying a pressure of 8 MPa, respectively testing the forward resistivity and reverse resistivity of the sample, and taking the average of the two as the powder resistivity of the sample.

[0106] In this application, the powder compaction density is defined as: during the external compression process, as the powder moves and deforms, larger gaps are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fit between particles is enhanced, thus forming a compact with a certain density and strength. The unit is g / cm 3 .

[0107] The compaction density test method follows the national standard GB / T 24533-2009. A certain amount of powder is placed on a compaction mold with a hollow center and two metal discs above and below. The powder is placed between the discs and a metal cylinder is placed on top. The mold is then placed on a compaction density instrument. Different pressures are set. The instrument reads the powder thickness at different pressures and calculates the compaction density using the equation ρ = m / v.

[0108] According to the following powder compaction density calculation results, ρc=m / V=m / (S×H); where ρc is the powder compaction density (g / cm 3 ), m is the mass of the material (g), S is the bottom area of ​​the mold (1.327 square centimeters (cm 2 )), H is the height of the sample after compaction (centimeter (cm)).

[0109] In some embodiments, the carbon content of the second lithium iron phosphate salt particles may be 0.8 wt%, 0.9 wt%, 1 wt%, 1.1%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2.0 wt%, or a range between any two of the above values.

[0110] By setting the carbon content of the second lithium iron phosphate salt particles to 0.8 wt%-2.0 wt%, carbon can be coated on the surface of the second lithium iron phosphate salt particles to form a lithium iron phosphate material with a uniform and dense carbon coating layer, greatly improving the surface conductivity of the particles.

[0111] In the lithium iron phosphate salt particles of the embodiments of the present application, the carbon may be present in a state of being mixed with the second lithium iron phosphate salt particles or in a state of being coated on the second lithium iron phosphate salt particles. In some embodiments, the carbon in the second lithium iron phosphate salt particles is coated on the second lithium iron phosphate salt particles, thereby enabling the carbon to form a uniform and dense carbon coating layer on the surface of the second lithium iron phosphate salt particles, thereby improving the surface conductivity of the particles.

[0112] In any embodiment, the second iron-lithium salt particles are single crystal particles and / or polycrystalline particles. In some embodiments, based on the total number of the second iron-lithium salt particles, the number of single crystal particles accounts for more than 90%.

[0113] By using the above-mentioned proportion of single crystal particles of lithium iron phosphate salt in each active layer of the positive electrode plate of the present application, the phenomenon of polycrystalline particles being crushed and peeling off during the cold pressing process can be effectively avoided, thereby improving the bonding force between the second active layer and the first active layer or the isolation membrane, thereby improving the dynamic performance and energy density of the positive electrode plate.

[0114] [Preparation Method of Second Lithium Iron Phosphate Particles]

[0115] In any embodiment, the manufacturing process of the second lithium iron phosphate salt particles includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings, wherein the temperature of the first sintering is 500 degrees Celsius (℃)-760℃, in some embodiments, 550℃-720℃, and the carbon content of the material after the first sintering is 0.01 weight%-0.79 weight%, in some embodiments, 0.05 weight%-0.4 weight%; the temperature of the second sintering is 700℃-800℃, in some embodiments, 720℃-780℃, and the carbon content of the material after the second sintering is 0.8 weight%-2.0 weight%, in some embodiments, 1.0 weight%-1.6 weight%.

[0116] In some embodiments, the temperature of the first sintering may be 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C or 760°C, or a range between any two of the above values. The temperature of the second sintering may be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C or 800°C, or a range between any two of the above values. The carbon content of the material after the first sintering can be selected to be 0.01 weight%, 0.05 weight%, 0.10 weight%, 0.15 weight%, 0.2 weight%, 0.25 weight%, 0.3 weight%, 0.35 weight%, 0.4 weight%, 0.45 weight%, 0.5 weight%, 0.55 weight%, 0.6 weight%, 0.65 weight%, 0.7 weight%, 0.75 weight%, 0.79 weight%; the carbon content of the material after the second sintering is 0.8 weight%-2.0 weight%, which can be selected to be 0.8 weight%, 0.9 weight%, 1 weight%, 1.1%, 1.2 weight%, 1.3 weight%, 1.4 weight%, 1.5 weight%, 1.6 weight%, 1.7 weight%, 1.8 weight%, 1.9 weight% or 2.0 weight%, or a range between any two of the above values.

[0117] In the preparation process of the second lithium iron phosphate salt particles of the embodiment of the present application, two sintering processes are implemented. Among them, by controlling the temperature of the first sintering within the above range and making the carbon content of the sintered intermediate within the above range, the lithium iron phosphate precursor obtained after the first sintering can have a larger particle size, which directly improves the powder compaction of the final product and the electrode compaction density. In addition, by adding a lower content of carbon source during the first sintering, the barrier effect of the carbon layer on the growth of lithium iron phosphate particles is greatly reduced, which is conducive to the crystallization growth of particles at a lower temperature, and is also conducive to the solid-phase diffusion reaction between the modifier and the lithium iron phosphate material, thereby achieving a higher concentration of metal ion doping. Compared with the traditional method of using high temperature to achieve particle growth, the above-mentioned preparation method of the present application can first synthesize large particles at a lower temperature, which can improve the phenomenon of cracking of the carbon layer on the surface of the particles at high temperature and improve the density of the surface carbon; at the same time, during the first sintering process, the carbon source can effectively reduce the trivalent iron in the raw material, thereby improving the purity and stability of the product; by controlling the temperature of the second sintering within the above range and making the carbon content of the sintered material 0.8 wt%-2.0 wt%, carbon can be coated on the surface of the second iron lithium salt particles to form a lithium iron phosphate material with a uniform and dense carbon coating layer, thereby greatly improving the surface conductivity of the particles.

[0118] In any embodiment, a first crushing is performed after the first sintering, and a second crushing is performed after the second sintering, wherein the Dv50 of the product after the first crushing is 300nm-1200nm, and in some embodiments, 400nm-1100nm; the Dv50 of the product after the second crushing is 500nm-5000nm, and in some embodiments, 700nm-2500nm.

[0119] In some embodiments, the Dv50 of the product after the first pulverization is 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1100 nm, or 1200 nm, or a range between any two of the above values. In some embodiments, the Dv50 of the product after the second crushing is 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1600nm, 1700nm, 1800nm, 1900nm, 2000nm, 2300nm, 2500nm, 2700nm, 3000nm, 3300nm, 3500nm, 3700nm, 4000nm, 4300nm, 4500nm, 4700nm, 5000nm, or a range between any two of the above values.

[0120] In this article, the term "Dv50" refers to the particle size corresponding to when the volume cumulative particle size distribution percentage in the particles reaches 50%. The determination of Dv50 can be carried out using the determination method described in the examples.

[0121] In the preparation method of the second lithium iron salt particles of the embodiment of the present application, two post-sintering crushing steps are implemented. By performing the first crushing after the first sintering, the product has an average primary particle size of 300nm-1200nm, which can avoid the growth barrier of the carbon material and the modifying element to the crystal, and obtain a micron-sized lithium iron phosphate precursor. By performing the second crushing after the second sintering, the product has an average primary particle size of 500nm-5000nm, and lithium iron phosphate salt particles of the desired particle size can be obtained, resulting in a uniform and dense carbon coating layer, which significantly improves the surface conductivity of the particles.

[0122] In any embodiment, the first active layer and the second active layer may further arbitrarily contain a conductive agent, a binder and a dispersant, wherein, in the first active layer, the weight ratio of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles: binder: conductive agent: dispersant is 96-99: 0.5-3: 0.5-3: 0.5-3, and in some embodiments, it is 96: 1.0: 2.5: 0.5; in the second active layer, the weight ratio of the second lithium iron phosphate salt particles: binder: conductive agent: dispersant is 96-99: 0.5-3: 0.5-3: 0.5-3, and in some embodiments, it is 96.5: 0.5: 2.5: 0.5; the weight ratio of the binder in the first active layer is less than or equal to the weight ratio of the binder in the second active layer.

[0123] In any embodiment, the conductive agent includes any one or a combination of at least two of natural graphite, artificial graphite, conductive carbon black, carbon fiber, carbon nanotubes, graphene, and conductive polymers or metal powders, and in some embodiments, it is conductive carbon black. The binder includes any one or a combination of at least two of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, or polytetrafluoroethylene, and in some embodiments, it is polyvinylidene fluoride.

[0124] By using the above-mentioned specific conductive agent, binder and dispersant, and making the weight ratio of the first lithium iron phosphate salt particles: binder: conductive agent: dispersant within the above-mentioned range, the positive electrode active layer of the present application can better achieve the above-mentioned effects of the present application.

[0125] In any embodiment, the preparation method of the second iron lithium salt particles includes: providing at least a lithium source, an iron source, a phosphorus source, a carbon source, a modifier, and a carbon film-forming agent as raw materials, wherein the mixing ratio of the lithium source, the iron source, and the phosphorus source, calculated on the basis of the atomic moles of each element, satisfies Fe:P=0.96-0.985, Li:Fe=1.0-1.1:0.95-1.1; the carbon source and the carbon film-forming agent, calculated on a weight ratio, satisfy the following: carbon source:carbon film-forming agent=9:1-2:8.

[0126] In some embodiments, the mixing ratio of the iron source to the phosphorus source, calculated on the basis of the atomic moles of each element, satisfies Fe:P=0.96:1, Fe:P=0.965:1, Fe:P=0.97:1, Fe:P=0.975:1, Fe:P=0.98:1 or Fe:P=0.985:1.

[0127] In some embodiments, the mixing ratio of the lithium source to the iron source, calculated on the basis of the atomic moles of each element, satisfies Li:Fe=1.0:1.1, Li:Fe=0.99:1.1, Li:Fe=0.98:1.1, Li:Fe=0.97:1.1, Li:Fe=0.96:1.1 or Li:Fe=0.95:1.1.

[0128] In some embodiments, the weight ratio of the carbon source to the carbon film-forming agent may be 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7 or 2:8.

[0129] By using raw materials containing a lithium source, an iron source, a phosphorus source, a carbon source, a modifier, and a carbon film-forming agent in the above-mentioned proportions, the primary average particle size, carbon content, BET specific surface area, and z value of the second lithium iron phosphate salt particles of the first aspect of this application can be well formed. Specifically, as mentioned above, by making the primary average particle size of the second lithium iron phosphate salt particles of the embodiment of the present application 500nm to 3000nm, the particles can be kept at a suitable micron-level size, thereby avoiding the problems of interface side reactions and processing difficulties caused by nano-sizing the particles, and the particles will not be limited to an excessively large size and thus reduce the kinetic performance. In addition, by making the BET specific surface area of ​​the second lithium iron phosphate salt particles of the embodiment of the present application 3m 2 / g~8m 2 / g, which can facilitate the stirring of the slurry containing the lithium iron phosphate salt particles and increase the solid content, thereby improving the processing of the battery cell and further increasing the volumetric energy density of the battery. In addition, by ensuring that the ratio z of the BET specific surface area of ​​the second lithium iron phosphate salt particles to Cx satisfies the range of 1.5≤z≤8.5, the carbon contained in the second lithium iron phosphate salt particles can be made more uniform and dense, thereby improving the surface conductivity of the particles.

[0130] In any embodiment, in the preparation method of the second iron lithium salt particles, the lithium source is a lithium compound, including one or more of lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide and lithium acetate, and in some embodiments, it is lithium carbonate; the iron source is an iron compound, including at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, ferric oxide and ferric oxyhydroxide, and in some embodiments, it is ferric oxide; the phosphorus source is a phosphate compound, including phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate. The present invention relates to a novel carbon-containing agent comprising one or more of ammonium, in some embodiments, phosphoric acid; the modifier comprises at least one of titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethanol, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate, in some embodiments, titanium dioxide; the carbon source comprises at least one of citric acid, glucose, sucrose, starch, fructose, and lactose, in some embodiments, glucose; the carbon film-forming agent comprises one or more of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinyl pyrrolidone, and polyvinyl alcohol, in some embodiments, polyaniline.

[0131] By selecting the above substances as the lithium source, iron source, phosphorus source, modifier, carbon source, and carbon film-forming agent used in the preparation method of the second lithium iron phosphate salt particles according to the embodiment of the present application, the primary average particle size, carbon content, BET specific surface area, and z-value of the second lithium iron phosphate salt particles according to the embodiment of the present application can be well formed. This can better achieve the large particle size of the second lithium iron phosphate salt particles according to the embodiment of the present application, while also achieving excellent kinetic performance when used as a positive electrode material.

[0132] In any embodiment, the heating rates in the first sintering and the second sintering are independently 2 degrees Celsius / minute (°C / min)-20°C / min, the first sintering constant temperature time is 1 hour (h)-6 hours; the second sintering constant temperature time is 2 hours-12 hours.

[0133] In some embodiments, the heating rates in the first sintering and the second sintering are each independently 2°C / min, 5°C / min, 7°C / min, 10°C / min, 13°C / min, 15°C / min, 17°C / min or 20°C / min.

[0134] In some embodiments, the constant temperature sintering time of the first sintering is 1 hour to 6 hours. In some embodiments, the constant temperature sintering time of the first sintering can be selected from 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours.

[0135] In some embodiments, the constant temperature sintering time of the second sintering is 2 hours to 12 hours. In some embodiments, the constant temperature sintering time of the second sintering can be selected from 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, 10.5 hours, 11 hours, 11.5 hours or 12 hours.

[0136] By controlling the heating rate and holding time during the first and second sintering processes within the aforementioned ranges, it is possible to prevent excessive side reactions during the sintering process caused by excessively rapid heating, thereby affecting the primary average particle size, carbon content, BET specific surface area, and z-value of the resulting second lithium iron phosphate salt particles. This allows for a larger particle size of the second lithium iron phosphate salt particles according to the embodiments of the present application to be achieved, while also achieving excellent kinetic performance when used as a positive electrode material.

[0137] In any embodiment, after the first sintering and before the second sintering, the product after the first sintering is subjected to carbon coating, and the amount of the carbon coating is 0.01-1.99%, in some embodiments, 0.2-1.5%, in some embodiments, 0.2 wt%-1.6 wt%, and in some embodiments, specifically 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt% or 1.6 wt%, or a range between any two of the above values.

[0138] By coating the product after the first sintering with carbon within the above-mentioned content range after the first sintering and before the second sintering, a uniform and dense carbon coating layer can be formed on the surface of the lithium iron phosphate particles during the second sintering process.

[0139] In any embodiment, the carbon coating is performed by vapor deposition during sintering, or by carbonizing and coating the carbon source at high temperature.

[0140] In the preparation method of the second lithium iron phosphate salt particles of the embodiment of the present invention, the embodiment of the carbon coating is not particularly limited. In some embodiments, the second lithium iron phosphate salt particles are carbon-coated by a vapor deposition method, thereby forming a uniform and dense carbon coating layer on the surface of the second lithium iron phosphate salt particles. In any embodiment, after the first sintering and the second sintering, the sintered product is crushed, and the crushing can be selected from one or more of mechanical crushing, sand grinding, and air flow crushing.

[0141] In the method for preparing the second lithium iron phosphate salt particles according to the embodiment of the present invention, the pulverization method is not particularly limited. By adopting the above-mentioned specific pulverization method, it is advantageous to obtain a desired primary average particle size.

[0142] In the method for preparing the second lithium iron phosphate salt particles according to the embodiment of the present invention, the pulverization method is not particularly limited. By adopting the above-mentioned specific pulverization method, it is advantageous to obtain a desired primary average particle size.

[0143] [current collector]

[0144] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming aluminum foil on a polymer material substrate (such as a substrate made of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0145] [Negative electrode]

[0146] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0147] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0148] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0149] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. 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, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional 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.

[0150] In some embodiments, the negative electrode film layer may further 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).

[0151] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0152] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0153] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0154] [Electrolytes]

[0155] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0156] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0157] 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 difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0158] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0159] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0160] [Isolation film]

[0161] In some embodiments, the secondary battery includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0162] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0163] [Secondary battery]

[0164] In one embodiment of the present application, a secondary battery is provided, which includes the positive electrode sheet.

[0165] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0166] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0167] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0168] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0169] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG4 shows a secondary battery 5 having a square structure as an example.

[0170] In some embodiments, referring to Figure 5, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0171] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0172] Figure 6 shows an example battery module 4. Referring to Figure 6 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured using fasteners.

[0173] In some embodiments, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 may be received in the receiving space.

[0174] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0175] Figures 7 and 8 illustrate an example battery pack 1. Referring to Figures 7 and 8 , the battery pack 1 may include a battery box and multiple battery modules 6 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0176] [Electrical devices]

[0177] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as 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., but is not limited thereto.

[0178] As an electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0179] Figure 9 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0180] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0181] Example

[0182] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0183] Example 1

[0184] 1. Preparation of lithium iron phosphate particles

[0185] 1) Preparation of the Second Lithium Iron Phosphate Particles

[0186] Lithium carbonate, ferric oxide, phosphoric acid, glucose, titanium dioxide (based on the total weight of the lithium iron phosphate particles, the amount of titanium dioxide added is sufficient to ensure a titanium content of 5000 ppm in the prepared lithium iron phosphate particles), and polyaniline are weighed separately. The weight ratios of Li, Fe, and P satisfy the following: Fe:P = 0.968:1, Li:Fe = 1:0.98, and the weight ratio of glucose to polyaniline satisfies the following: glucose:polyaniline = 1:2. The amount of glucose added is sufficient to ensure that the carbon content of the lithium iron phosphate precursor after the first sintering is completed is 0.15% by weight. Water is added to the above substances to obtain a mixture slurry.

[0187] The mixture was mixed in a ball mill and ground in a sand mill to a slurry with a solids content of 38% and a Dv50 of 0.40 μm. The mixture was then spray-dried (the negative pressure of the high-speed spray dryer was -650 to -200 Pa, the inlet temperature was 300°C to 360°C, and the outlet temperature was 100°C to 140°C). The dried reactants were placed in a sintering furnace for the first sintering step. The heating rate was controlled at 5°C / min, the holding temperature was controlled at 650°C, and the holding time was 4 hours. After cooling, the material was pulverized using a mechanical mill to obtain a powder.

[0188] Glucose as a carbon source and polyaniline as a carbon film-forming agent were added to the resulting powder, which was then mixed with water to produce a material with a solid content of 40%. The amounts of glucose and polyaniline added were such that the carbon content of the product after the second sintering was 1.2% (based on the total weight of the lithium iron phosphate particles), and the weight ratio of glucose to polyaniline was 1:2. The material was processed using a ball mill and a sand mill to produce a slurry with a Dv50 value of insoluble matter of 550 nm. The slurry was then spray-dried (using a high-speed spray dryer with a negative pressure of -650 to -200 Pa, an inlet temperature of 300°C to 360°C, and an outlet temperature of 100°C to 140°C). The dried reactants were then placed in a sintering furnace for a second low-temperature sintering (heating rate controlled at 5°C / min, sintering temperature of 750°C, and sintering time of 4 hours). After the material is cooled, it is crushed for the second time to an average particle size of 870 nm. After demagnetization, lithium iron phosphate salt particles are obtained, with a carbon content of 1.2% and a Ti content of 5000 ppm in the lithium iron phosphate salt particles.

[0189] 2) Preparation of the first lithium iron phosphate particles

[0190] The first lithium iron phosphate salt particles used in this application are commercially available products purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the product number BP-LFP-101; the second lithium manganese iron phosphate salt particles used in this application were purchased from Guangdong Brunp Recycling Technology Co., Ltd. with the product number BP-115.

[0191] 2. Preparation of active layer slurry

[0192] 1) Preparation of the first active layer slurry

[0193] First lithium iron phosphate salt particles, conductive carbon black as a conductive agent, polyvinylidene fluoride as a binder, and PVP (polyvinyl pyrrolidone) as a dispersant were mixed in a mass ratio of 96.0:2.5:1.0:0.5, and a certain amount of N-methylpyrrolidone was added, mixed evenly, stirred, and dispersed to prepare a first active layer positive electrode slurry. The BET specific surface area of ​​the first lithium iron phosphate salt particles was greater than the BET specific surface area of ​​the second lithium iron phosphate salt particles. The first lithium iron phosphate salt particles had a primary average particle size of 360 nm and a Dv50 of 1.0 μm.

[0194] 2) Preparation of the second active layer slurry

[0195] The second lithium iron phosphate salt particles, conductive carbon black as a conductive agent, polyvinylidene fluoride as a binder, and PVP (polyvinyl pyrrolidone) as a dispersant are mixed in a mass ratio of 96.5:2.5:0.5:0.5, wherein the second lithium iron phosphate salt particles are olivine structure single crystal lithium iron phosphate with a BET specific surface area of ​​7m 2 / g, Dv50 is 2.0μm, and the primary average particle size is 500nm. After mixing the above materials containing the positive electrode active material, a certain amount of N-methylpyrrolidone is added and mixed, and the mixture is evenly dispersed to form the first active layer slurry.

[0196] 3. Preparation of positive electrode sheet

[0197] After the slurry prepared above is stirred evenly, the viscosity of the slurry is adjusted to 8000-20000 mPa.s to prevent the slurry from stratification. Then, the slurry is coated on the substrate aluminum foil through a double-sided double-cavity coating device. The coating weight of the first active layer slurry is 300 mg / 1540.25 cm 2 , and the second positive electrode slurry coating weight is 100mg / 1540.25cm 2 After the first active layer slurry and the second positive electrode slurry are coated, they are dried, cold pressed, cut, and sliced ​​in sequence to finally obtain the positive electrode sheet.

[0198] 4. Preparation of negative electrode sheet

[0199] Artificial graphite as a negative electrode active material, conductive carbon black as a conductive agent, styrene-butadiene rubber (SBR) as a binder, and sodium carboxymethyl cellulose (CMC) as a thickener are mixed in a mass ratio of 95.0:1.0:2.0:2.0, added with deionized water and stirred, and dispersed evenly to form a negative electrode slurry. The negative electrode slurry is then coated on both sides of the substrate Cu foil by a double-sided coater. After both sides are completed, the negative electrode sheet is prepared by drying, cold pressing, slitting, and sheeting.

[0200] 5. Preparation of electrolyte

[0201] In an argon atmosphere glove box (H2O < 0.1ppm, O2 < 0.1ppm), organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed uniformly in a volume ratio of 3:7, and LiPF6 lithium salt was dissolved in the organic solvent to prepare a solution with a weight content of 12.5% ​​to obtain an electrolyte.

[0202] 6. Isolation film

[0203] Polypropylene film is used as the isolation film.

[0204] 7. Preparation of batteries

[0205] The positive electrode sheet, separator, and negative electrode sheet of Example 1 were stacked in order, with the separator positioned between the positive and negative electrode sheets to provide isolation. The cells were then wound to obtain a bare cell. The tabs were welded to the bare cell and placed in an aluminum casing. The cells were then baked at 80°C to remove moisture. The electrolyte was then injected and sealed to obtain an uncharged battery. The uncharged battery then underwent a series of steps, including resting, hot and cold pressing, formation, shaping, and capacity testing, to obtain the lithium-ion battery product of Example 1.

[0206] The lithium ion battery obtained in Example 1 was subjected to the performance test described below, and the results are shown in Figure 1.

[0207] In addition, the parameters of the first and second lithium iron phosphate salt particles in the positive electrode sheets were changed as shown in Table 1. In Example 11 and Comparative Example 2, first lithium manganese iron phosphate salt particles (purchased from Guangdong Brunp Recycling Technology Co., Ltd., item number BP-115) were used. The same procedures as in the examples were followed to produce Examples 2-14 and Comparative Examples 1-4. The cohesive force, compacted density, and dynamic performance of the resulting positive electrode sheets were measured.

[0208] 1. Parameter determination of battery components

[0209] 1. Primary average particle size

[0210] The electrode is cut open perpendicularly to the large surface of the electrode using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The longest diameter of the lithium iron phosphate (manganese) salt particles is statistically analyzed using the length diameter statistical method. Among them, the "primary average particle size" refers to the average value of the primary particle size of all particles, which is numerically equal to the total particle size value divided by the total number of particles. Among them, the primary particle size in the cross-section diagram refers to the longest distance connecting two points along the edge. Specifically, the total number of lithium iron phosphate (manganese) salt particles with a primary particle size greater than 50nm and the sum of the primary particle sizes of lithium iron phosphate (manganese) salt particles with a primary particle size greater than 50nm can be counted in the electron microscope scanning photo. The primary average particle size of the lithium iron phosphate (manganese) salt particles = the primary particle size of the total lithium iron phosphate (manganese) salt particles / the total number of lithium iron phosphate (manganese) salt particles. Among them, in the above-mentioned particle size statistical process, particles with a primary average particle size less than or equal to 50nm are not included in the statistical range.

[0211] 2. BET specific surface area

[0212] The specific surface area was tested by gas adsorption method according to the GB / T19587-2017 test standard, as follows: lithium iron phosphate (manganese) granular salt was taken as a sample, the sample tube was immersed in liquid nitrogen at -196°C, and the adsorption amount of nitrogen on the solid surface at different pressures was measured at a relative pressure of 0.05-0.30. The single molecular layer adsorption amount of the sample was obtained based on the BET multilayer adsorption theory and its formula, thereby calculating the specific surface area of ​​the material.

[0213] 3. Dv50

[0214] With reference to GB / T19077.1-2016, the Dv50 value of the lithium iron (manganese) phosphate salt particles was measured using a laser particle size analyzer (Malvern Master Size 3000). In addition, the Dv10, Dv90, and Dv99 values ​​of the present application were also measured in the same manner.

[0215] 4. Carbon content

[0216] After the lithium iron (manganese) phosphate salt particles are burned in a high-frequency induction furnace, the carbon content is tested using the infrared absorption method. The specific testing process is based on the standard GB / T 20123-2006 / ISO 15350:2000.

[0217] 5. Sphericity

[0218] After cold pressing, the electrode is cut perpendicularly to its broad surface using an Ar particle beam, exposing the end face. A scanning electron microscope image is then obtained. The SEM image of the electrode section is then analyzed using imaging software to measure the particle's major diameter (a) and minor diameter (b), and their ratio is calculated, representing the particle's aspect ratio.

[0219] 6. Cohesion

[0220] Select a copper plate with a smooth appearance, wipe the surface of the copper plate with dust-free paper and alcohol, and stick double-sided tape (model: nitto tape) on the copper plate. The tape width is 20 mm (mm) × length is 150 mm. The tape is parallel to the edge of the steel plate, and the distance between the tape and the two edges of the steel plate is equal. Take the electrode to be tested, use a blade to cut a sample with a width of 25 mm × length of 160 mm, and stick the cut electrode sample on the double-sided tape. Align the edge of the electrode with the edge of the copper plate to ensure that the electrode and the double-sided tape are flat. Use a copper glue with a length of about 30 mm to ensure that the copper glue is parallel to the electrode to be tested, and then use 2 kg (kg )'s pressure roller rolls back and forth 3-4 times; fold the copper glue upward and fix the end of the copper plate without the electrode with the lower clamp, and clamp the upper clamp on the paper tape and parallel to the copper plate; test on a computer connected to a tensile testing machine (model: INSTRON), first pre-stretch 3mm, set the rate to 10 millimeters / minute (mm / min), and the displacement distance to 15mm. After obtaining the data, intercept 8-15mm of data. Pre-stretch 3mm for each test, repeat the test four times, and take the average value as the cohesive force.

[0221] 7. Pole compaction density

[0222] The compacted density of the film layer on one side of the electrode = m / (V1-V2), where m represents the weight of the film layer, V1 represents the volume of the electrode, and V2 represents the volume of the current collector. m can be obtained by subtracting the weight of the current collector from the weight of the electrode. The product of the surface area of ​​the electrode and the thickness of the electrode is the volume V1 of the electrode, and the product of the surface area of ​​the electrode and the thickness of the current collector is V2. The thickness of the current collector and the thickness of the electrode are obtained by measuring the thickness of the empty foil in the tab area with a micrometer.

[0223] 2. Determination of battery performance

[0224] 1. Battery capacity ratio when discharged at 1C to 3.2V (η value)

[0225] The battery preparation and testing process is as follows: 2.0000g of sample was mixed with 0.1111g of conductive carbon black and 0.1111g of PVDF (at a mass ratio of 0.9:0.05:0.05), followed by the addition of 2.5g of the organic solvent NMP (N-methylpyrrolidone). After thorough mixing, the mixture was coated onto aluminum foil to form a 140μm-thick film. The film was then dried under vacuum at 120°C for 2 hours. A hole punch was used to punch the film into 13mm-diameter discs, which were then pressed using a tablet press at 10 MPa and kept at 120°C for 12 hours. The positive electrode was weighed, and the active material loading was 11mg-12mg. Coin-type cells were assembled in an argon-protected glove box with a lithium metal sheet as the negative electrode, an electrolyte consisting of a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate), LiPF6 electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator. The assembled batteries were tested on a blue battery tester. Within the 2.0V to 3.75V voltage range, the specific capacity was measured by charging / discharging at a constant current of 0.1C for two weeks, followed by a constant current of 1C for two weeks. The charging process was performed with a constant voltage of 3.75V and a cutoff current of 50uA.

[0226] A battery containing the second iron lithium salt particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C1, and the capacity value at the discharge voltage to 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50uA.

[0227] 2. Determination of kinetic performance (10% SOC DCR)

[0228] In this application, the dynamic performance of the battery is specifically reflected in the measurement of the DC internal resistance (power performance).

[0229] 25℃ power performance test:

[0230] Capacity calibration: The lithium-ion batteries prepared in each embodiment and comparative example were kept at 25°C for 2 hours, then charged at a constant current of 0.33C to 3.65V, and then charged at a constant voltage of 0.05C at 3.65V. After charging, the tested batteries were left at rest at 25°C for 2 hours, and then discharged at a DC current of 0.33C to 2.5V. The discharge capacity at room temperature was recorded as C0.

[0231] Adjust the SOC (battery state of charge): After keeping the calibrated lithium-ion battery at 25°C for 2 hours, discharge it at a 1 / 3C0 discharge rate for 144 minutes to adjust the lithium-ion battery capacity to 10% SOC;

[0232] Power test: After a 10% SOC lithium-ion battery is left at 25°C for 2 hours, it is discharged at a pulse current I of 3C0 for 30 seconds. The voltage before 3C0 discharge is recorded as V1, and the voltage at the end of the 30-second discharge is recorded as V2. The value of (V1-V2) / I is calculated, and this data can be used to characterize the battery's power performance. The lower the value of the above kinetic performance, the better the battery's kinetic performance.

[0233] 3. Parameters and Performance in Examples and Comparative Examples

[0234] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured, as shown in Table 1.

[0235] Table 1

[0236] According to Table 1, the positive electrode plates of Examples 1-14 of the present application include a first active layer and a second active layer, the first active layer includes first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles, and the second active layer includes second lithium iron phosphate salt particles, the primary average particle size of the second lithium iron phosphate salt particles is greater than the primary average particle size of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles and the two are within a specific range, and the resulting secondary battery has excellent kinetic performance and excellent battery performance.

[0237] In addition, according to Table 1, among Comparative Examples 1-4 of the present application, Comparative Example 1-2 does not have one of the first active layer or the second active layer, and although Comparative Examples 3-4 have the first active layer and the second active layer, the primary average particle size of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is too large or too small, and the kinetic performance of the secondary battery finally obtained is significantly lower than that of the embodiments of the present application, and cannot meet the performance requirements of the present application.

[0238] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included within the technical scope of the present application. In addition, without departing from the scope of the present application, any other modifications that can be imagined by those skilled in the art to the embodiments, or any other methods constructed by combining some of the constituent elements in the embodiments are also included within the scope of the present application.

Claims

1. A positive electrode plate, comprising a current collector and a positive electrode active layer, wherein the positive electrode active layer is disposed on at least one surface of the current collector. The positive electrode active layer includes a first active layer directly coated on the current collector, and a second active layer coated on the surface of the first active layer away from the current collector. The first active layer contains first lithium iron phosphate particles and / or first lithium manganese iron phosphate particles, and the second active layer contains second lithium iron phosphate particles. The primary average particle size of the first lithium iron phosphate particles and / or the first lithium manganese iron phosphate particles is 150 nm - 480 nm, and the primary average particle size of the second lithium iron phosphate particles is 500 nm - 3000 nm.

2. The positive electrode sheet according to claim 1, wherein The cohesive force of the positive electrode plate is ≥ 32 N / m.

3. The positive electrode sheet according to claim 1 or 2, characterized in that, The thickness range of the first active layer is 80 μm - 140 μm, and the thickness range of the second active layer is 8 μm - 50 μm.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The BET specific surface area of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is 10 m 2 / g - 18 m 2 / g, The BET specific surface area of the second lithium iron phosphate salt particles is 3 m 2 / g - 8 m 2 / g.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that, The sphericity of the first lithium iron phosphate particles and / or the first lithium manganese iron phosphate particles is ≤ 1.3, and the sphericity of the second lithium iron phosphate particles is ≥ 1.

5.

6. The positive electrode sheet according to any one of claims 1 to 5, characterized in that, The molecular formula of the first lithium iron phosphate particles and / or the first lithium manganese iron phosphate particles is Li x’ M1 y’ Mn m’ Fe z’ M2 k’ PO4, wherein M1 includes at least one or a combination of two or more of Na, K, Ca, Al, and Mg, and M2 includes one or a combination of two or more of Ni, Cu, Zn, Ti, Nb, V, Cr, and Co. 0.95 ≤ x' ≤ 1.15, 0 ≤ y' ≤ 0.1, 0.1 ≤ m' ≤ 0.9, 0.1 ≤ z' ≤ 1, 0 ≤ k' ≤ 0.1, and the content of Mn is ≤ 1000 ppm.

7. The positive electrode sheet according to any one of claims 1 to 6, characterized in that The molecular formula of the second lithium iron phosphate particle is Li m Fe x P y O j Q q , where Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, S, F, Cl, Br, 0.95 ≤ m ≤ 1.15, 0.9 ≤ x ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 < q ≤ 0.

1.

8. The positive electrode plate according to claim 7, wherein the Q includes at least one of Ti, V, Mg, and Nb. Based on the total weight of the second lithium iron phosphate particles, the content of Q is 1000 ppm - 10000 ppm.

9. The positive electrode sheet according to any one of claims 1 to 8, characterized in that, The second lithium iron phosphate particles contain carbon, and based on the total weight of the second lithium iron phosphate particles, the carbon content is 1.0 wt% - 2.0 wt%.

10. The positive electrode sheet according to any one of claims 1 to 9, characterized in that, The first lithium iron phosphate particles and / or the first lithium manganese iron phosphate particles, and the second lithium iron phosphate particles are each independently single crystal particles and / or polycrystalline particles.

11. The positive electrode plate according to any one of claims 1 to 10, characterized in that The capacity ratio η of the second lithium iron phosphate particles is ≥ 88%, and η is defined as: a battery containing the second lithium iron phosphate particles as the positive electrode material is charged and discharged at a constant current twice at a rate of 0.1C in the voltage range of 2.0V - 3.75V Subsequently, it is charged and discharged at a constant current once at a rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value when the discharge voltage is 3.2V is extracted and denoted as C1, and the capacity value when the discharge voltage reaches 2.0V is C2, η = C1 / C2, where the charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cut-off current of 50 μA.

12. The positive electrode sheet according to any one of claims 1 to 11, characterized in that, The manufacturing process of the second lithium iron phosphate particles includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings, wherein The temperature of the first sintering is 500°C - 760°C, and the carbon content of the material after the first sintering is 0.01 wt% - 0.79 wt%. The temperature of the second sintering is 700°C - 800°C, and the carbon content of the material after the second sintering is 0.8 wt% - 2.0 wt%.

13. The positive electrode sheet according to claim 12, wherein During the manufacturing process of the second lithium iron phosphate salt particles, the first pulverization is carried out after the first sintering, and the second pulverization is carried out after the second sintering, wherein, The Dv50 of the product after the first pulverization is 300 nm - 1200 nm; The Dv50 of the product after the second pulverization is 500 nm - 5000 nm.

14. The positive electrode sheet according to any one of claims 1 to 13, characterized in that, The first active layer and the second active layer further comprise a conductive agent and a binder, wherein, In the first active layer, the weight ratio of the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles : binder : conductive agent : dispersant is 96 - 99 : 0.5 - 3 : 0.5 - 3 : 0.5 - 3; In the second active layer, the weight ratio of the second lithium iron phosphate salt particles : binder : conductive agent : dispersant is 96 - 99 : 0.5 - 3 : 0.5 - 3 : 0.5 - 3; The weight ratio of the binder in the first active layer is less than or equal to the weight ratio of the binder in the second active layer.

15. The positive electrode sheet according to claim 14, characterized in that, The conductive agent includes any one or a combination of at least two of natural graphite, artificial graphite, conductive carbon black, carbon fiber, carbon nanotube, graphene, and conductive polymer or metal powder, The binder includes any one or a combination of at least two of polyvinylidene fluoride, copolymer of vinylidene fluoride and hexafluoropropylene, and polytetrafluoroethylene.

16. A secondary battery, characterized in that, Having the positive electrode tab according to any one of claims 1 - 15.

17. An electrical device, characterized in that, Having the secondary battery according to claim 16.

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