Positive electrode sheet, secondary battery, and electric device

Abstract

The application provides a positive electrode sheet, comprising a current collector and a positive electrode active layer arranged on at least one side of the current collector, wherein 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 lithium manganese iron phosphate salt particles, the second active layer comprises 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 nm-480 nm, and the primary average particle size of the second lithium iron phosphate salt particles is 500 nm-3000 nm.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a positive electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] As a positive electrode material of a lithium ion battery, lithium iron phosphate has rapidly become a global research hotspot due to its abundant resources, low price, environmental friendliness and stable voltage of two-phase reaction. The lithium iron phosphate positive electrode active material used in the existing positive electrode sheet, such as nanoscale lithium iron phosphate, lithium manganese iron phosphate and the like, has a small particle size and a high specific surface area, and has a strong water absorption when in contact with air, so that a side reaction easily occurs on the solid-liquid surface, and a phenomenon of sticking to a roller, extension and powder falling easily occurs during cold pressing. Therefore, a high content of binder is required for actual coating, and a high amount of binder actually affects the capacity of the battery. SUMMARY

[0003] The present application is made in view of the above-described problems, and aims to provide a positive electrode sheet, a secondary battery and an electric device, which have high cohesion, low water absorption and can maintain a high energy density.

[0004] A first aspect of the present application provides a positive electrode sheet, comprising a current collector and a positive electrode active layer, the positive electrode active layer being provided on at least one side of the current collector, the positive electrode active layer comprising a first active layer directly coated on the current collector, and a second active layer coated on a surface of the first active layer away from the current collector, the first active layer comprising first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles, and the second active layer comprising second lithium iron phosphate salt particles, the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles having a primary average particle size of 150 nm to 480 nm, and the second lithium iron phosphate salt particles having a primary average particle size of 500 nm to 3000 nm.

[0005] As described above, the positive electrode sheet of the present application is provided with a positive electrode active layer comprising a first active layer and a second active layer, the first active layer comprising lithium iron phosphate salt particles and / or lithium manganese iron phosphate salt particles (also referred to as "lithium (manganese) iron phosphate salt particles") having a small primary average particle size close to the current collector.

[0006] In the present application, by setting the primary average particle sizes 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 within the above-described range, the particles of the first active layer and the second active layer can be maintained at a suitable micron level, thereby avoiding the problems of interface side reactions and processing difficulties when the particles are nanosized, and the particles are not limited to a size that reduces the kinetic performance.

[0007] In addition, the second active layer contains large primary average particle size lithium iron phosphate salt particles away from the current collector, by forming this structure, the positive electrode sheet of the embodiments of the present application can improve the surface layer cohesion due to the large particle size of the large particle lithium iron phosphate salt particles on the surface, in addition, due to the relatively large particle size of the large particle lithium iron phosphate salt particles, the specific surface area is small, which can reduce the water absorption of the material, thereby maintaining the high energy density of the positive electrode sheet. 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 electrode sheet), the large particle lithium iron phosphate salt particles as the second active layer can achieve better adhesion, the compaction density of the positive electrode sheet is improved, and the kinetic performance of the secondary battery with the positive electrode sheet is excellent.

[0008] In any embodiment, the cohesion of the positive electrode sheet is ≥32 N / m.

[0009] The cohesion of the positive electrode sheet measured by the determination method described in the embodiments of the present application is in the above range, thereby avoiding the phenomenon of powder falling during the processing of the positive electrode sheet and improving the adhesion of the positive electrode surface and the separator film.

[0010] In any embodiment, the thickness of the first active layer is in the range of 80 μm-140 μm, and optionally 90 μm-120 μm, and the thickness of the second active layer is in the range of 8 μm-50 μm, and optionally 20 μm-35 μm.

[0011] By setting the thickness of each group of the first active layer and the second active layer in the above range, the above technical effects of the present application can be further preferably achieved.

[0012] 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-18 m 2 / g, and optionally 12 m 2 / g-16 m 2 / g, and the BET specific surface area of the second lithium iron phosphate salt particles is 3 m 2 / g-8 m 2 / g, and optionally 4 m 2 / g-7 m 2 / g.

[0013] By setting the BET specific surface area 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 embodiments of the present application in the above range, respectively, the stirring of the slurry containing the lithium iron phosphate salt particles and the improvement of the solid content can be facilitated, thereby improving the processing problem of the battery cell and further improving the volume energy density of the battery.

[0014] In any embodiment, the sphericity of the first lithium iron phosphate salt particles and / or 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 first lithium manganese iron phosphate salt particles are closer to spherical than the second lithium iron phosphate salt particles.

[0015] In the present document, 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 in the long axis direction to the diameter in the short axis direction of the primary particles, assuming that the diameter in the long axis direction is a and the diameter in the short axis direction is b, the aspect ratio = a / b; this parameter is often used to describe the morphology of particles and can be used to measure the sphericity, the closer the value is to 1, the higher the sphericity.

[0016] The determination of the sphericity can be carried out using the method described in the examples of the present application.

[0017] By making the sphericity of the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles and the second lithium iron phosphate salt particles each within the above range, the above technical effects of the present application can be further preferably achieved.

[0018] In any embodiment, the molecular formula of the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles is Li x’ M1 y’ Mn m’ Fe z’ M2 k’ PO4, wherein the M1 comprises at least one or a combination of two or more of Na, K, Ca, Al, Mg, the M2 comprises 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 ≤1000ppm.

[0019] The composition of the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles in the present application is not particularly limited, and any existing small particle lithium (manganese) iron phosphate salt particles can be used. Exemplarily, in the embodiments of the present application, by using the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles having the above composition, by forming a double-layer structure with the second lithium iron phosphate salt particles in the embodiments of the present application, the above effects of the present application can be better achieved.

[0020] In any embodiment, the molecular formula of the second lithium iron phosphate salt particles is Li m Fe x P y Oj Q q wherein Q comprises 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.

[0021] By modifying the second lithium iron phosphate salt of the embodiments of the present application to the above formula, and adding one or more of the above elements to it, high loading amount bulk phase modification of the second lithium iron phosphate salt particles is achieved, which helps to improve the bulk phase ion transport capacity of the lithium iron phosphate salt particles, and effectively solves the problem of poor inherent kinetic performance of 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 the present application, the modification can specifically be represented by doping and / or coating.

[0022] In any embodiment, Q comprises at least one of Ti, V, Mg, Nb, and can optionally be Ti. The content of Q is 1000 ppm-10000 ppm, and can optionally be 2500 ppm-6000 ppm, based on the total weight of the second lithium iron phosphate salt particles.

[0023] By doping one or more of the above elements in the lithium iron phosphate salt of the embodiments of the present application and ensuring the content is within the above range, metal bulk phase 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 phase ion transport capacity of the lithium iron phosphate salt particles, and better solving the problem of poor inherent kinetic performance of large particles.

[0024] In any embodiment, the second lithium iron phosphate salt particles contain carbon, and the content of carbon is 1.0 wt% - 2.0 wt%, based on the total weight of the second lithium iron phosphate salt particles.

[0025] 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, forming a lithium iron phosphate material with a uniform and dense carbon coating layer, and greatly improving the electrical conductivity of the particle surface.

[0026] 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, and the number of single crystal particles can account for 90% or more, based on the total number of second lithium iron phosphate salt particles.

[0027] By using single crystal particles of lithium iron phosphate salt in each active layer of the positive electrode tab of the present application, the phenomenon of crushing of polycrystal particles during cold pressing and film peeling can be effectively avoided, thereby improving the adhesion between the second active layer and the first active layer or the isolation film, and further improving the kinetic performance and energy density of the positive electrode tab.

[0028] In any embodiment, the capacity ratio η of the second lithium iron phosphate salt particles is ≥ 88%, wherein 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 rate of 0.1C in a voltage range of 2.0V-3.75V, and then charged and discharged once at a rate of 1C, in the charge and discharge test at a rate of 1C, the capacity value at a discharge voltage of 3.2V is denoted as C1, and the capacity value at a discharge voltage of 2.0V is denoted as C2, η = C1 / C2, wherein the charging process includes constant voltage charging, constant voltage 3.75V, and constant voltage cutoff current 50uA.

[0029] The capacity ratio η of the second lithium iron phosphate salt particles of the embodiments of the present application is ≥ 88%, which means that the second lithium iron phosphate salt particles of the present application can make the secondary battery show good kinetic performance when used as a positive electrode material.

[0030] 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 sintering processes, wherein the temperature of the first sintering is 500-760°C, and optionally 550-720°C, the carbon content of the material after the first sintering is 0.01-0.79wt%, and optionally 0.05-0.4wt%; the temperature of the second sintering is 700-800°C, and optionally 720-780°C, the carbon content of the material after the second sintering is 0.8-2.0wt%, and optionally 1.0-1.6wt%.

[0031] In the preparation process of the second lithium iron phosphate salt particles in the embodiments of the present application, twice sintering is implemented. Among them, by controlling the temperature of the first sintering in the above range and making the carbon content of the intermediate after sintering in the above range, the lithium iron phosphate precursor obtained after the first sintering can have a larger primary particle size, directly improving the powder compaction and the electrode sheet compaction density of the final product; at the same time, the carbon source added before the first sintering can effectively reduce the trivalent iron in the raw material, improve the product purity and stability; by controlling the temperature of the second sintering in the above range and making the carbon content of the material after sintering 0.8wt%-2.0wt%, the 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, greatly improving the surface conductivity of the particles.

[0032] In any embodiment, after the first sintering, first crushing is performed, and after the second sintering, second crushing is performed, wherein the Dv50 of the product after the first crushing is 300nm-1200nm, which can be selected as 400nm-1100nm; the Dv50 of the product after the second crushing is 500nm-5000nm, which can be selected as 700nm-2500nm.

[0033] In the preparation method of the second lithium iron phosphate salt particles in the embodiments of the present application, twice sintering and crushing are implemented. By performing the first crushing after the first sintering, the primary average particle size of the product is 300nm-1200nm, which can avoid the blockage of carbon materials and modified elements to the growth of crystals, and micron-sized lithium iron phosphate precursor is obtained; by performing the second crushing after the second sintering, the primary average particle size of the product is 500nm-3000nm, which can obtain lithium iron phosphate salt particles with desired particle size and obtain a uniform and dense carbon coating layer, greatly improving the surface conductivity of the particles.

[0034] In any embodiment, the first active layer and the second active layer can also arbitrarily contain a conductive agent, a binder and a dispersing agent, 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: the binder: the conductive agent: the dispersing agent is 96-99: 0.5-3: 0.5-3: 0.5-3, which can be selected as 96: 1.0: 2.5: 0.5; in the second active layer, the weight ratio of the second lithium iron phosphate salt particles: the binder: the conductive agent: the dispersing agent is 96-99: 0.5-3: 0.5-3: 0.5-3, which can be selected as 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.

[0035] 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 nanotube, graphene, and conductive polymer or metal powder, and is optionally 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 is optionally polyvinylidene fluoride; and the dispersant is not particularly limited as long as it is a dispersant commonly used in the art, and includes PVP (polyvinylpyrrolidone) and the like.

[0036] By using the above specific conductive agent, binder, and dispersant, and by setting the weight ratio of the first lithium iron phosphate salt particles : binder : conductive agent : dispersant within the above range, the positive electrode active layer of the present application can better achieve the above effects of the present application.

[0037] The second aspect of the present application provides a secondary battery including the positive electrode sheet of the first aspect of the present application.

[0038] The third aspect of the present application provides an electric device including the secondary battery of the second aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is a structural schematic diagram of a positive electrode sheet of an embodiment of the present application.

[0040] Figure 2 is a TEM image of a positive electrode sheet of an embodiment of the present application.

[0041] Figure 3 is a schematic diagram exemplarily showing the particle diameter of a primary particle of the present application.

[0042] Figure 4 is a schematic diagram of a secondary battery of an embodiment of the present application.

[0043] Figure 5 is Figure 4 a secondary battery of an embodiment of the present application shown in FIG. 6.

[0044] Figure 6 is a schematic diagram of a battery module of an embodiment of the present application.

[0045] Figure 7 is a schematic diagram of a battery pack of an embodiment of the present application.

[0046] Figure 8 is Figure 7 a battery pack of an embodiment of the present application shown in FIG. 8.

[0047] Figure 9 is a schematic diagram of an electric device using a secondary battery of an embodiment of the present application as a power source.

[0048] BRIEF DESCRIPTION OF DRAWINGS

[0049] 1: battery pack; 2: upper case; 3: lower case; 4: battery module; 5: secondary battery; 51: case; 52: electrode assembly; 53: cover plate. DETAILED DESCRIPTION

[0050] Hereinafter, embodiments of the positive electrode sheet, the secondary battery, and the electric device of the present application will be described in detail. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well, repeated description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0051] The "ranges" disclosed in the present application are defined in the form of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. In addition, if a minimum range value of 1 and 2 is listed, and if a maximum range value of 3, 4, and 5 is listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise specified, a numerical range "a-b" indicates a shorthand for any real combination of integers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is just a shorthand for these numerical combinations. In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0052] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0053] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0054] If not specifically stated, all steps of the present application can be performed in sequence or randomly, optionally in sequence. For example, the method comprises steps (a) and (b) indicates that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c) indicates that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0055] If not specifically stated, the present application refers to "including" and "comprising" as open-ended, and can also be closed. For example, "including" and "comprising" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0056] If not specifically stated, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any one of the following conditions satisfies the condition "A or B": 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 A and B are both true (or exist).

[0057] The existing positive electrode sheet uses a lithium iron phosphate positive electrode active material, such as a nano-sized lithium iron phosphate, a lithium manganese iron phosphate, and the like. The particle size of the material is small, has a high specific surface area, and has strong water absorption. Generally, an active material layer of secondary particles is coated on the surface to improve the surface cohesion of the electrode sheet, but the secondary particles will crack and break during cold pressing.

[0058] Based on this, the present application proposes a positive electrode sheet, which is coated with a layer of primary particle material with a larger particle size on the surface of the electrode sheet to improve the surface cohesion of the electrode sheet, which can avoid the problem of cracking and breaking of secondary particles under pressure, thereby improving the kinetic performance and energy density.

[0059] [Positive electrode sheet]

[0060] The positive electrode tab of the embodiments of the present application comprises a current collector and a positive electrode active layer disposed on at least one side of the current collector, the positive electrode active layer comprising a first active layer coated directly 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 comprising first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles, the second active layer comprising second lithium iron phosphate salt particles, the primary average particle size of the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles being 150-480 nm, and the primary average particle size of the second lithium iron phosphate salt particles being 500-3000 nm.

[0061] In some embodiments, the primary average particle size of the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles can be selected from 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, 210 nm, 220 nm, 230 nm, 240 nm, 250 nm, 260 nm, 270 nm, 280 nm, 290 nm, 300 nm, 310 nm, 320 nm, 330 nm, 340 nm, 350 nm, 360 nm, 370 nm, 380 nm, 390 nm, 400 nm, 410 nm, 420 nm, 430 nm, 440 nm, 450 nm, 460 nm, 470 nm, or 480 nm, or a range between any two of the above values. In some embodiments, the primary average particle size of the second lithium iron phosphate salt particles can be selected from 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.

[0062] In this context, the term "primary average particle size" refers to the particle size value obtained by statistically analyzing the particle size of the particles in the field of view of a scanning electron microscope using the long diameter statistical method. In the particle size statistical process, particles with a primary average particle size of less than or equal to 80 nm are not included in the statistical range. The primary average particle size of the present application is as follows: Figure 3The primary average particle diameter refers to the average particle diameter of primary particles. In the present context, "primary particles" refer to particles that have no apparent agglomeration interface in a scanning electron microscope image of the particles, but can have minute pores and point or line defects, as distinguished from powder particles that are the smallest unit without structures such as accumulation and flocculation. The primary average particle diameter, the carbon content described later, and the BET specific surface area can be measured using the measurement methods described in the examples.

[0063] As described above, the positive electrode tab of the present application is provided with a positive electrode active layer including a first active layer and a second active layer, the first active layer includes lithium iron phosphate salt particles and / or lithium manganese iron phosphate salt particles (also referred to simply as "lithium (manganese) iron phosphate salt particles") with a smaller primary average particle diameter close to the current collector.

[0064] Wherein, by making the primary average particle diameter 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 embodiments of the present application within the above 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 interface side reactions and processing difficulties when the particles are nano-sized, and without limiting the particles to a size that reduces the kinetic performance.

[0065] In addition, the second active layer includes lithium iron phosphate salt particles with a larger primary average particle diameter away from the current collector. By forming this structure, the positive electrode tab of the embodiments of the present application can improve the cohesion of the surface layer due to the presence of a layer of large particle lithium iron phosphate salt particles on the surface, and in addition, due to the relatively large particle size of the large particle lithium iron phosphate salt particles, the specific surface area is small, which can reduce the water absorption of the material, thereby maintaining the high energy density of the positive electrode tab. 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 tab), the large particle lithium iron phosphate salt particles as the second active layer can achieve better adhesion, the compaction density of the positive electrode tab is improved, and at the same time, the kinetic performance of the secondary battery with the positive electrode tab is excellent.

[0066] In any embodiment, the cohesion of the positive electrode tab is ≥ 32 N / m.

[0067] The cohesion of the positive electrode tab measured by the measurement method described in the examples of the present application is within the above range, thereby avoiding the phenomenon of powder falling during the processing of the positive electrode tab and improving the adhesion of the positive electrode surface and the separator film.

[0068] In any embodiment, the thickness of the first active layer is in the range of 80 μm-140 μm, and optionally 90 μm-120 μm, and the thickness of the second active layer is in the range of 8 μm-50 μm, and optionally 20 μm-35 μm.

[0069] In some embodiments, the thickness of the first active layer can be selected from 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 m or 140 μm, or a range between any of the above. In some embodiments, the thickness of the first active layer can be selected from 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.

[0070] 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.

[0071] In any of the embodiments, the BET specific surface area of the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles can be selected from 10 m 2 / g to 18 m 2 / g, optionally 12 m 2 / g to 16 m 2 / g, and the BET specific surface area of the second lithium iron phosphate salt particles can be selected from 3 m 2 / g to 8 m 2 / g, optionally 4 m 2 / g to 7 m 2 / g.

[0072] In some embodiments, the BET specific surface area of the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles can be selected from 10 m 2 / g, 11 m 2 / g, 12 m 2 / g, 13 m 2 / g, 14 m 2 / g, 15 m 2 / g, 16 m 2 / g, 17 m 2 / g or 8 m 2 / g, or a range between any two of the above. In some embodiments, the BET specific surface area of the second lithium iron phosphate salt particles can be selected from 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g or 8 m 2 / g, or a range between any two of the above values. The BET specific surface area can be measured using the method described in the examples.

[0073] By making the BET specific surface area 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 each within the above range, the stirring of the slurry containing the lithium iron phosphate salt particles and the solid content can be improved, thereby improving the processing problem of the battery cell and further improving the volumetric energy density of the battery.

[0074] 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 the sphericity 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.

[0075] In this context, the term "sphericity" can be measured by the aspect ratio of the particles, and the smaller the aspect ratio value, the better the sphericity; the aspect ratio is the ratio of the diameter in the long axis direction to the diameter in the short axis direction of the primary particles, assuming that the diameter in the long axis direction is a and the diameter in the short axis direction is b, the aspect ratio = a / b; this parameter is often used to describe the morphology of particles and can be used to measure the sphericity thereof.

[0076] The sphericity can be measured using the method described in the examples.

[0077] By making 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 each within the above range, the above technical effects of the present application can be further preferably achieved.

[0078] [First lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles]

[0079] 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 or a combination of two or more of Na, K, Ca, Al, Mg, the 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.

[0080] The composition of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles is not particularly limited in the present application, and any existing small particle lithium (manganese) iron phosphate salt particles can be used. Exemplarily, in the embodiments of the present application, by using the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles having the above composition, the above effects of the present application can be better achieved by forming a double-layer structure with the second lithium iron phosphate salt particles in the embodiments of the present application.

[0081] [Second lithium iron phosphate salt particles]

[0082] The primary average particle size of the second lithium iron phosphate salt particles in the embodiments of the present application is 500 nm to 3000 nm, and optionally 650 nm to 2500 nm, the BET specific surface area is 3 m 2 / g to 8 m 2 / g, and optionally 4 m 2 / g to 7 m 2 / g, the carbon content of the second lithium iron phosphate salt particles is Cx wt.%, wherein 0.8≤Cx≤2.0, and optionally 1.0≤Cx≤1.6, and the ratio z of the BET specific surface area to Cx satisfies 1.5≤z≤8.5, and optionally 3≤z≤6.

[0083] By making the primary average particle size of the second lithium iron phosphate salt particles in the embodiments of the present application be 500 nm to 3000 nm, the BET specific surface area be 3 m 2 / g to 8 m 2 / g, and the carbon content of the second lithium iron phosphate salt particles be Cx wt.%(0.8≤Cx≤2.0), while making the ratio z of the BET specific surface area of the second lithium iron phosphate salt particles to Cx (i.e. the BET specific surface area / Cx) satisfy the range of 1.5≤z≤8.5, the kinetic performance of the second lithium iron phosphate salt particles with the primary average particle size in the above range can be improved; the particles can maintain a suitable micron-level size, thereby avoiding the problems of interface side reactions and processing difficulties when the particles are nanometerized, and the kinetic performance will not be reduced due to the limitation of the size of the particles being too large; in addition, it is beneficial to the stirring of the slurry containing the second lithium iron phosphate salt particles and the improvement of the solid content, thereby improving the processing problems of the battery cell and further improving the volume energy density of the battery; in addition, the normal deintercalation of lithium ions can be avoided due to the high density of carbon coating, thereby affecting the capacity of the battery cell.

[0084] Compared with the prior art, the second lithium iron phosphate salt particles in the embodiments of the present application have a smaller specific surface area with the same carbon content, meaning that the particles contain less floating carbon, 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 the kinetic performance of the particles as a positive electrode material.

[0085] 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 comprises 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, Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0

[0086] By making the second lithium iron phosphate salt of the embodiments of the present application have the above-mentioned molecular formula, and adding one or more of the above-mentioned elements to modify it, the high load phase modification of the second lithium iron phosphate salt particles is realized, which helps to improve the 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 the present application, the modification can specifically be represented as doping and / or coating.

[0087] In any embodiment, the Q comprises at least one of Ti, V, Mg, Nb, and optionally Ti, and the content of the Q is 1000 ppm to 10000 ppm, and optionally 2500 ppm to 6000 ppm, based on the total weight of the second lithium iron phosphate salt particles. In some embodiments, the content of the Ti, V, Mg, and / or Nb is optionally 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, 3000 ppm, 3100 ppm, 3200 ppm, 3300 ppm, 3400 ppm, 3500 ppm, 3600 ppm, 3700 ppm, 3800 ppm, 3900 ppm, 4000 ppm, 4100 ppm, 4200 ppm, 4300 ppm, 4400 ppm, 4500 ppm, 4600 ppm, 4700 ppm, 4800 ppm, 4900 ppm, 5000 ppm, 5100 ppm, 5200 ppm, 5300 ppm, 5400 ppm, 5500 ppm, 5600 ppm, 5700 ppm, 5800 ppm, 5900 ppm, or 6000 ppm, or a range between any two of the above values, based on the total weight of the second lithium iron phosphate salt particles.

[0088] By adding one or more of the above elements to the second lithium iron phosphate salt of the embodiments of the present application and allowing the content to be within the above range, the metal body phase 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 body phase ion transport capacity of the second lithium iron phosphate salt particles, and better solving the problem of poor inherent kinetic performance of large particles.

[0089] In any embodiment, the second lithium iron phosphate salt particles comprise carbon, and the content of the carbon is 1.0 wt% to 2.0 wt%, based on the total weight of the second lithium iron phosphate salt particles.

[0090] In the second lithium iron phosphate salt particles of the embodiments of the present application, the carbon can be in a mixed state with the second lithium iron phosphate salt particles or coated on the second lithium iron phosphate salt particles. Optionally, the carbon in the second lithium iron phosphate salt particles is in a state of being coated on the second lithium iron phosphate salt particles, so that a uniform and dense carbon coating layer can be formed on the surface of the second lithium iron phosphate salt particles, thereby improving the surface conductivity of the particles.

[0091] In any embodiment, the capacity ratio η of the second lithium iron phosphate salt particles is ≥ 88%, wherein η is defined as follows: a battery comprising the second lithium iron phosphate salt particles as a positive electrode material is subjected to constant current charge and discharge twice in a voltage range of 2.0 V to 3.75 V at a rate of 0.1 C, and then subjected to constant current charge and discharge once at a rate of 1 C, wherein the capacity value at a discharge voltage of 3.2 V in the charge and discharge test at a rate of 1 C is denoted as C1, the capacity value at a discharge voltage of 2.0 V is denoted as C2, and η = C1 / C2, wherein the charging process includes constant voltage charging, the constant voltage is 3.75 V, and the constant voltage cutoff current is 50 uA.

[0092] η represents the platform retention performance of the material, which is strongly related to the discharge power performance of the battery. When the value is large, the battery can still maintain good power performance when discharged to a low SOC (state of charge of the battery), that is, the voltage drop of the battery is small when the battery is discharged at a large current at a low capacity. The capacity ratio η of the second lithium iron phosphate salt particles in the embodiments of the present application is ≥ 88%, which means that the second lithium iron phosphate salt particles in the present application can make the secondary battery exhibit good kinetic performance when used as a positive electrode material.

[0093] In any embodiment, the second lithium iron phosphate salt particles satisfy at least one of the following a) to f):

[0094] a) the Dv10 of the second lithium iron phosphate salt particles is ≥ 0.2 μm;

[0095] b) the Dv50 of the second lithium iron phosphate salt particles is 0.5-5 μm;

[0096] c) the Dv90 of the second lithium iron phosphate salt particles is ≤ 8 μm;

[0097] d) the Dv99 of the second lithium iron phosphate salt particles is ≤ 10 μm;

[0098] e) the powder compaction density of the second lithium iron phosphate salt under a pressure of 3T is ≥ 2.25 g / cm 3 ;

[0099] f) the powder resistivity of the second lithium iron phosphate salt is less than 60 Ω·cm.

[0100] In the present application, the above-mentioned Dv10, Dv90 and Dv99 refer to the particle size corresponding to the cumulative volume percentage of 10%, 90% and 99% in the particles, respectively. The test method is the same as the above-mentioned Dv50, which can be determined by using the standard in the embodiments of the present application.

[0101] 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.

[0102] By causing the second lithium iron phosphate salt particles to satisfy at least one of a)-f), the second lithium iron phosphate salt particles can better achieve the technical effects described above.

[0103] In the present application, the test method of the powder resistivity is as follows: referring to the national standard GB / T 33822-2017, using a powder resistivity meter (Suzhou Crystal, ST2722 type), 1 g of sample (with an error within ±0.005 g) is weighed and added to the charging cavity, an 8 MPa pressure is applied, and the forward resistivity and reverse resistivity of the sample are tested respectively, and the average of the two is taken as the powder resistivity of the sample.

[0104] In the present application, the definition of the powder compaction density is as follows: during the compression process of external force, as the powder moves and deforms, larger voids are filled, the contact area between particles increases, the interatomic attractive force is generated, and the mechanical bonding effect between particles is enhanced, thereby forming a compaction with a certain density and strength, and the unit is g / cm 3 .

[0105] The test method of the compaction density is as follows: referring to the national standard GB / T 24533-2009, a certain amount of powder is placed on the compaction mold, and a metal disc is placed on the upper and lower hollow parts of the mold. The powder is placed between the metal discs, and a metal cylinder is placed on the top. The mold is placed on the compaction density instrument, different pressures are set, and the thickness of the powder under different pressures can be read on the equipment. The compaction density is calculated by ρ = m / v.

[0106] According to the following powder compaction density calculation result, ρc = m / V = m / (S x H); in the formula, ρc is the powder compaction density (g / cm 3 ), m is the mass of the weighed material (g), S is the bottom area of the mold (1.327 cm 2 ), and H is the height of the sample after compaction (cm).

[0107] In some embodiments, the carbon content of the second lithium iron phosphate salt particles can be selected as 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.

[0108] By setting the carbon content of the second lithium iron phosphate salt particles to 0.8 wt%-2.0 wt%, the 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, and greatly improving the electrical conductivity of the particle surface.

[0109] In the lithium iron phosphate salt particle of the embodiments of the present application, the carbon can be in a state of being mixed with the second lithium iron phosphate salt particle or in a state of being coated on the second lithium iron phosphate salt particle. Optionally, the carbon in the second lithium iron phosphate salt particle is in a state of being coated on the second lithium iron phosphate salt particle, so that a uniform and dense carbon coating layer can be formed on the surface of the second lithium iron phosphate salt particle, thereby improving the surface conductivity of the particle.

[0110] In any embodiment, the second lithium iron salt particle is a single crystal particle and / or a polycrystal particle. Optionally, the number of the single crystal particles accounts for 90% or more based on the total number of the second lithium iron salt particles.

[0111] By using the single crystal particles of the lithium iron phosphate salt in the above-mentioned proportion in each active layer of the positive electrode tab of the present application, the phenomenon of the polycrystal particles being crushed during cold pressing and being detached from the film can be effectively avoided, so that the adhesion between the second active layer and the first active layer or the separator film can be improved, and the kinetic performance and the energy density of the positive electrode tab can be further improved.

[0112] [Preparation method of the second lithium iron phosphate salt particle]

[0113] In any embodiment, the manufacturing process of the second lithium iron phosphate salt particle 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 times of sintering, wherein the temperature of the first sintering is 500-760°C, optionally 550-720°C, the carbon content of the material after the first sintering is 0.01-0.79 wt%, optionally 0.05-0.4 wt%; the temperature of the second sintering is 700-800°C, optionally 720-780°C, the carbon content of the material after the second sintering is 0.8-2.0 wt%, optionally 1.0-1.6 wt%.

[0114] In some embodiments, the temperature of the first sintering can be selected to 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 these values. The temperature of the second sintering can be selected to 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 these values. The carbon content of the material after the first sintering can be selected to be 0.01 wt%, 0.05 wt%, 0.10 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, 0.5 wt%, 0.55 wt%, 0.6 wt%, 0.65 wt%, 0.7 wt%, 0.75 wt%, 0.79 wt%, and the carbon content of the material after the second sintering can be selected to be 0.8 wt% to 2.0 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 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 these values.

[0115] In the preparation of the second lithium iron phosphate salt particles in the embodiments of the present application, two sintering processes are implemented. By controlling the temperature of the first sintering within the above range and making the carbon content of the intermediate after sintering within the above range, the lithium iron phosphate precursor obtained after the first sintering can have a larger particle size, directly improving the powder compaction and the electrode compaction density of the final product. 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 during the crystallization growth process is greatly reduced, which is conducive to the crystallization growth of the particles at a lower temperature, and is conducive to the solid-phase diffusion reaction between the modifier and the lithium iron phosphate material, thereby realizing a higher concentration of metal ion doping. Compared with the traditional method of using high temperature to realize particle growth, the above preparation method of the present application can synthesize large particles at a lower temperature, can improve the phenomenon of cracking of the carbon layer on the surface of the particles at high temperature, and can 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 0.8wt%-2.0wt%, the carbon can be coated on the surface of the second lithium iron salt particles, forming a lithium iron phosphate material with a uniform and dense carbon coating layer, and greatly improving the surface conductivity of the particles.

[0116] In any embodiment, after the first sintering, a first crushing is performed, and after the second sintering, a second crushing is performed, wherein the Dv50 of the product after the first crushing is 300nm-1200nm, which can be 400nm-1100nm; the Dv50 of the product after the second crushing is 500nm-5000nm, which can be 700nm-2500nm.

[0117] In some embodiments, the Dv50 of the product after the first crushing is 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, or 1200nm, 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.

[0118] In the present specification, the term "Dv50" means the particle size at which the cumulative volume percentage of particles reaches 50%. The Dv50 can be measured using the measurement method described in the examples.

[0119] In the preparation method of the second lithium iron salt particles in the embodiments of the present application, the crushing after the second sintering is performed. By performing the first crushing after the first sintering, the primary average particle size of the product is 300 nm-1200 nm, which can avoid the growth blocking of the crystal by the carbon material and the modified element, and obtain a micron-level lithium iron phosphate precursor; by performing the second crushing after the second sintering, the primary average particle size of the product is 500 nm-5000 nm, which can obtain the lithium iron phosphate salt particles with the desired particle size, and obtain a uniform and dense carbon coating layer, greatly improving the surface conductivity of the particles.

[0120] In any of the embodiments, the first active layer and the second active layer can also optionally contain a conductive agent, a binder and a dispersing agent, 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: the binder: the conductive agent: the dispersing agent is 96-99: 0.5-3: 0.5-3: 0.5-3, which can be 96: 1.0: 2.5: 0.5; in the second active layer, the weight ratio of the second lithium iron phosphate salt particles: the binder: the conductive agent: the dispersing agent is 96-99: 0.5-3: 0.5-3: 0.5-3, which can be 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.

[0121] In any of the embodiments, 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, which can be conductive carbon black, and the binder includes any one or a combination of at least two of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene or polytetrafluoroethylene, which can be polyvinylidene fluoride.

[0122] By using the above specific conductive agent, binder and dispersing agent, and making the weight ratio of the first lithium iron phosphate salt particles: the binder: the conductive agent: the dispersing agent within the above range, the positive electrode active layer of the present application can better achieve the above effects of the present application.

[0123] In any of the embodiments, the method for preparing the second iron lithium salt particles includes: providing raw materials including at least a lithium source, an iron source, a phosphorus source, a carbon source, a modifier, and a carbon film forming agent, wherein the mixing ratio of the lithium source, the iron source, and the phosphorus source, in terms of atomic moles of each element, satisfies Fe:P=0.96-0.985, Li:Fe=1.0-1.1:0.95-1.1; and the carbon source and the carbon film forming agent satisfy carbon source:carbon film forming agent=9:1-2:8 in terms of weight ratio.

[0124] In some embodiments, the mixing ratio of the iron source and the phosphorus source, in terms of 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.

[0125] In some embodiments, the mixing ratio of the lithium source and the iron source, in terms of 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.

[0126] In some embodiments, the weight ratio of the carbon source and the carbon film forming agent can be 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, or 2:8.

[0127] By using the raw materials containing the lithium source, the iron source, the phosphorus source, the carbon source, the modifier, and the carbon film forming agent in the above ratios, the primary average particle size, the carbon content, the BET specific surface area, and the z value of the second lithium iron phosphate salt particles of the first aspect of the application can be well formed. Specifically, as described above, by setting the primary average particle size of the second lithium iron phosphate salt particles of the embodiments of the application to 500 nm-3000 nm, the particles can be kept at a suitable micron-level size, thereby avoiding the problems of interface side reactions and processing difficulties when the particles are nanometerized, and without limiting the particles to a size that reduces the kinetic performance. In addition, by setting the BET specific surface area of the second lithium iron phosphate salt particles of the embodiments of the application to 3 m 2 2 / g-8 m 2 / g, the stirring of the slurry containing the lithium iron phosphate salt particles and the improvement of the solid content can be facilitated, thereby improving the processing problems of the battery cell and further improving the volumetric energy density of the battery. In addition, by setting the ratio z of the BET specific surface area of the second lithium iron phosphate salt particles of the embodiments of the application to Cx to satisfy the range of 1.5≤z≤8.5, the carbon contained in the second iron lithium salt particles can be more uniform and dense, thereby improving the surface conductivity of the particles.

[0128] In any embodiment, in the preparation method of the second lithium iron phosphate salt particles, the lithium source is a compound of lithium, including one or more of lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide, and lithium acetate, and is optionally lithium carbonate; the iron source is a compound of iron, including at least one of iron hydroxide, ferrous chloride, diiron trioxide, iron phosphate, iron pyrophosphate, ferrous oxalate, iron powder, iron nitrate, magnetite, and iron hydroxide, and is optionally diiron trioxide; the phosphorus source is a phosphoric acid compound, including one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, and is optionally phosphoric acid; the modifier includes at least one of titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate, and is optionally titanium dioxide; the carbon source includes at least one of citric acid, glucose, sucrose, starch, fructose, and lactose, and is optionally glucose; and the carbon film forming agent includes one or a combination of polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol, and is optionally polyaniline.

[0129] By selecting the above-mentioned substances as the lithium source, the iron source, the phosphorus source, the modifier, the carbon source, and the carbon film forming agent used in the preparation method of the second lithium iron phosphate salt particles of the embodiments of the present application, the first average particle size, the carbon content, the BET specific surface area, and the z value of the second lithium iron phosphate salt particles of the embodiments of the present application can be well formed. Thus, the large particle size of the second lithium iron phosphate salt particles of the embodiments of the present application is better achieved, and excellent kinetic performance is achieved when the second lithium iron phosphate salt particles are used as a positive electrode material.

[0130] In any embodiment, the heating rate in the first sintering and the second sintering is independently 2°C / min-20°C / min, the first sintering constant temperature time is 1h-6h, and the second sintering constant temperature time is 2h-12h.

[0131] In some embodiments, the heating rate in the first sintering and the second sintering is 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.

[0132] In some embodiments, the constant temperature sintering time of the first sintering is 1h-6h. In some embodiments, the constant temperature sintering time of the first sintering is optionally 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, or 6h.

[0133] In some embodiments, the isothermal sintering time of the second sintering is 2h-12h. In some embodiments, the isothermal sintering time of the second sintering can be selected from 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h or 12h.

[0134] By controlling the temperature rising rate and the isothermal time in the first sintering and the second sintering within the above ranges, the excessive side reactions during sintering caused by too fast temperature rising can be prevented, thereby affecting the primary average particle size, the carbon content, the BET specific surface area and the z value of the obtained second lithium iron phosphate salt particles. Thus, the large particle size of the second lithium iron phosphate salt particles of the embodiments of the present application is better achieved, while excellent kinetic performance is achieved when it is used as a positive electrode material.

[0135] In any of the embodiments, 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%, which can be selected from 0.2-1.5%. Specifically, it can be selected from 0.2wt%-1.6wt%, and specifically, it can be selected from 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt% or 1.6wt%, or a range between any two of the above values.

[0136] By subjecting the product after the first sintering to carbon coating within the above content range before the second sintering, a uniform and dense carbon coating layer can be formed on the surface of the lithium iron phosphate salt particles during the second sintering.

[0137] In any of the embodiments, the carbon coating is performed by gas phase deposition during sintering or by carbonization of a carbon source at high temperature.

[0138] In the preparation method of the second lithium iron phosphate salt particles of the embodiments of the present application, the embodiment of the carbon coating is not particularly limited, and the second lithium iron phosphate salt particles can be subjected to carbon coating by gas phase deposition, thereby forming a uniform and dense carbon coating layer on the surface of the second lithium iron phosphate salt particles. In any of the embodiments, after the first sintering and after the second sintering, the sintered product is subjected to crushing, which can be selected from one or more of mechanical crushing, sand milling and air flow crushing.

[0139] In the production method of the second lithium iron phosphate salt particles according to the embodiment of the present application, the pulverization method is not particularly limited, and by using the specific pulverization method described above, it is possible to favorably obtain the desired primary average particle diameter.

[0140] In the production method of the second lithium iron phosphate salt particles according to the embodiment of the present application, the pulverization method is not particularly limited, and by using the specific pulverization method described above, it is possible to favorably obtain the desired primary average particle diameter.

[0141] [Current collector]

[0142] In some embodiments, the positive electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be employed. The composite current collector can 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 can be formed by forming an aluminum foil on a polymer material base material such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), or the like.

[0143] [Negative electrode tab]

[0144] The negative electrode tab includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0145] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0146] In some embodiments, the negative electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be employed. The composite current collector can 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 can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), or the like.

[0147] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more.

[0148] In some embodiments, the negative film layer can further optionally include a binder. The binder can 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).

[0149] In some embodiments, the negative film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0150] In some embodiments, the negative film layer can further optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0151] In some embodiments, the negative electrode sheet can be prepared by dispersing the components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry, coating the negative electrode slurry on a negative current collector, and then drying, cold-pressing, and the like to obtain the negative electrode sheet.

[0152] [Electrolyte]

[0153] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of the electrolyte is not particularly limited in the present application and can be selected as needed. For example, the electrolyte can be in a liquid state, a gel state, or a full solid state.

[0154] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0155] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorobisoxalate phosphate, and lithium tetrafluorobisoxalate phosphate.

[0156] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 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, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0157] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0158] [Separator]

[0159] In some embodiments, the secondary battery includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0160] 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, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0161] [Secondary battery]

[0162] In one embodiment of the present application, a secondary battery is provided, which includes the positive electrode sheet.

[0163] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During charging and discharging of the battery, active ions are repeatedly intercalated and deintercalated between the positive electrode sheet and the negative electrode sheet. The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, and mainly functions to prevent short circuiting of the positive and negative electrodes, while allowing ions to pass through.

[0164] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly through a roll-pressing process or a stacking process.

[0165] In some embodiments, the secondary battery can include an outer package. The outer package can be used to enclose the electrode assembly and the electrolyte.

[0166] In some embodiments, the outer package of the secondary battery can be a hard case, such as a hard plastic case, an aluminum case, a steel case, or the like. The outer package of the secondary battery can also be a soft pack, such as a pouch-type soft pack. The soft pack can be made of plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0167] The shape of the secondary battery according to the present application is not particularly limited, and can be cylindrical, square, or any other shape. For example, Figure 4 is a square structure as an example of a secondary battery 5.

[0168] In some embodiments, referring to Figure 5 , the outer package can include a case 51 and a cover plate 53. The case 51 can 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 case 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is enclosed in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be selected by those skilled in the art according to the specific actual needs.

[0169] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0170] Figure 6 is a battery module 4 as an example. Referring to Figure 6 , in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other way. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0171] Optionally, the battery module 4 can further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0172] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0173] Figure 7 and Figure 8is a battery pack 1 as an example. Referring to Figure 7 and Figure 8 In the battery pack 1, a battery case and a plurality of battery modules 6 disposed in the battery case can be included. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 can be disposed on the lower case 3 to form an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0174] [Electric device]

[0175] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, the battery module, or the battery pack provided by the present application. The secondary battery, the battery module, or the battery pack can be used as a power supply of the electric device, or can be used as an energy storage unit of the electric device. The electric device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0176] As the electric device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0177] Figure 9 is an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of high power and high energy density of the secondary battery for the electric device, the battery pack or the battery module can be used.

[0178] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the secondary battery can be used as a power supply.

[0179] Embodiment

[0180] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and cannot be understood as a limitation of the present application. If the specific technology or condition is not indicated in the embodiments, the technology or condition described in the literature in the art or according to the product instruction is used. If the reagent or instrument used is not indicated by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0181] Embodiment 1

[0182] 1. Preparation of lithium iron phosphate salt particles

[0183] 1) Preparation of second lithium iron phosphate salt particles

[0184] Lithium carbonate, iron trioxide, phosphoric acid, glucose, titanium dioxide (the amount of titanium dioxide added to meet the content of titanium in the prepared lithium iron phosphate salt particles of 5000 ppm based on the total weight of the lithium iron phosphate salt particles), and polyaniline were weighed respectively, wherein the weight ratio of Li element, Fe element and P element met Fe:P=0.968:1 and Li:Fe=1:0.98, and the weight ratio of glucose and polyaniline met glucose:polyaniline=1:2. The amount of glucose added met the content of carbon in the lithium iron phosphate precursor after the first sintering was 0.15% based on the weight of the lithium iron phosphate precursor. Water was added to the above-mentioned substances to obtain a mixture slurry.

[0185] The mixture slurry was mixed uniformly using a ball mill and was ground using a sand mill to obtain a slurry with a solid content of 38% and a Dv50 of 0.40 μm. The dried reactants were packed into a sintering furnace for the first sintering, the heating rate was controlled to be 5°C / min, the holding temperature was controlled to be 650°C, and the holding time was 4 h. After the material was cooled, the powder was obtained by mechanical grinding.

[0186] Glucose as a carbon source and polyaniline as a carbon film forming agent were added to the obtained powder, and then mixed with water to obtain a material with a solid content of 40%. The amount of glucose and polyaniline added met the carbon content of 1.2% in the product after the second sintering (based on the total weight of the lithium iron phosphate salt particles), and the weight ratio of glucose and polyaniline met glucose:polyaniline=1:2. The above-mentioned material was treated using a ball mill and a sand mill to obtain a slurry, and the Dv50 of the insoluble matter in the slurry was 550 nm. The slurry was 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 packed into a sintering furnace for the second low-temperature sintering (the heating rate was controlled to be 5°C / min, the sintering temperature was 750°C, and the sintering time was 4 h). After the material was cooled, it was secondly ground to an average particle size of 870 nm, and then the lithium iron phosphate salt particles were obtained after magnetic separation, the carbon content was 1.2%, and the content of Ti element in the lithium iron phosphate salt particles was 5000 ppm.

[0187] 2) Preparation of the first lithium iron phosphate salt particles

[0188] The first lithium iron phosphate salt particles used in the present application were commercially available and purchased from Guangdong Bangu Recycle Technology Co., Ltd. with a product code of BP-LFP-101; the second lithium manganese iron phosphate salt particles used in the present application were purchased from Guangdong Bangu Recycle Technology Co., Ltd. with a product code of BP-115.

[0189] 2, Preparation of the active layer slurry

[0190] 1) Preparation of the first active layer slurry

[0191] The first lithium iron phosphate salt particles, conductive carbon black as the conductive agent, polyvinylidene fluoride as the binder, and PVP (polyvinylpyrrolidone) as the dispersant are mixed in a mass ratio of 96.0:2.5:1.0:0.5, a certain amount of N-methylpyrrolidone is added, mixed and stirred, and dispersed to prepare the first active layer positive electrode slurry. The BET specific surface area of the first lithium iron phosphate salt particles is greater than that of the second lithium iron phosphate salt particles, the primary average particle size of the first lithium iron phosphate salt particles is 360 nm, and the Dv50 is 1.0 μm.

[0192] 2) Preparation of the second active layer slurry

[0193] The second lithium iron phosphate salt particles, conductive carbon black as the conductive agent, polyvinylidene fluoride as the binder, and PVP (polyvinylpyrrolidone) as the 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, the BET specific surface area is 7 m 2 / g, the Dv50 is 2.0 μm, and the primary average particle size is 500 nm. The above material containing the positive electrode active material is mixed, a certain amount of N-methylpyrrolidone is added, mixed and stirred, and uniformly dispersed to prepare the first active layer slurry.

[0194] 3) Preparation of the positive electrode sheet

[0195] After the above prepared slurry is uniformly stirred, the viscosity of the slurry is adjusted to 8000-20000 mPa.s, so that the prepared slurry does not delaminate, and 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 coating weight of the second positive electrode slurry is 100 mg / 1540.25 cm 2 . After the above first active layer slurry and second positive electrode slurry are coated, they are sequentially subjected to drying, cold pressing, slitting, and sheeting to finally obtain the positive electrode sheet.

[0196] 4) Preparation of the negative electrode sheet

[0197] The artificial graphite as the negative active material, the conductive carbon black as the conductive agent, the styrene butadiene rubber (SBR) as the binder, and the sodium carboxymethyl cellulose (CMC) as the thickening agent are mixed in a mass ratio of 95.0:1.0:2.0:2.0, deionized water is added for stirring, and the negative electrode slurry is prepared after being uniformly dispersed. Then, the negative electrode slurry is coated on both sides of the substrate Cu foil through a double-sided coating machine. After the coating on both sides is completed, the negative electrode sheet is prepared through drying, cold pressing, slitting, and sheeting.

[0198] 5. Preparation of electrolyte

[0199] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the organic solvents ethylene carbonate (EC) and methyl ethyl carbonate (EMC) are mixed uniformly in a volume ratio of 3:7, and LiPF6 lithium salt is dissolved in the organic solvents to prepare a solution with a weight content of 12.5%, thereby obtaining the electrolyte.

[0200] 6. Separation film

[0201] The polypropylene film is used as the separation film.

[0202] 7. Preparation of battery

[0203] The positive electrode sheet of Example 1, the separation film, and the negative electrode sheet are sequentially stacked with the separation film between the positive electrode sheet and the negative electrode sheet to play a role of separation, and then wound to obtain a bare cell. The bare cell is welded with tabs, and is loaded into an aluminum shell and baked at 80°C to remove water. Then, the electrolyte is injected and sealed to obtain a non-charged battery. The non-charged battery is sequentially subjected to processes of standing, hot and cold pressing, formation, shaping, capacity testing, and the like, thereby obtaining the lithium ion battery product of Example 1.

[0204] The lithium ion battery obtained in Example 1 is subjected to the performance tests described below, and the results are shown in Table 1.

[0205] In addition, the parameters of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles in the positive electrode sheet are changed as shown in Table 1, wherein the first lithium manganese iron phosphate salt particles (purchased from Guangdong Bangu Cycle Technology Co., Ltd., with a product number of BP-115) are used in Example 11 and Comparative Example 2, and the same processes as in the examples are performed except for the above, thereby obtaining Examples 2-14 and Comparative Examples 1-4. The cohesion, the sheet compaction density, and the kinetic performance of the obtained positive electrode sheet are measured.

[0206] I. Measurement of parameters of battery components

[0207] 1. Primary average particle size

[0208] The argon ion beam is perpendicular to the large surface of the pole piece to cut the pole piece and expose the cross section. The cross section is photographed by a scanning electron microscope. The longest diameter of the lithium iron phosphate (manganese) salt particles is statistically analyzed by a length-diameter statistical method. The "primary average particle size" refers to the average value of the primary particle size of all particles, which is equal to the total particle size value divided by the total number of particles. The primary particle size in the cross-sectional view refers to the longest distance between two points connected by an edge. Specifically, the total number of lithium iron phosphate (manganese) salt particles with a primary particle size greater than 50 nm and the sum of the primary particle sizes of lithium iron phosphate (manganese) salt particles with a primary particle size greater than 50 nm can be counted in the scanning electron microscope image. The primary average particle size of lithium iron phosphate (manganese) salt particles = total primary particle size of lithium iron phosphate (manganese) salt particles / total number of lithium iron phosphate (manganese) salt particles. In the above particle size statistical process, particles with a primary average particle size less than or equal to 50 nm are not included in the statistical range.

[0209] 2. BET specific surface area

[0210] The specific surface area is tested by gas adsorption method according to the test standard GB / T 19587-2017. Specifically, lithium iron phosphate (manganese) salt particles are taken as samples. The sample tube is immersed in liquid nitrogen at -196°C. The adsorption amount of nitrogen on the solid surface under different pressures is measured under 0.05-0.30 relative pressure. Based on the BET multilayer adsorption theory and its formula, the monolayer adsorption amount of the sample is obtained, and thus the specific surface area of the material is calculated.

[0211] 3. Dv50

[0212] According to GB / T 19077.1-2016, the Dv50 value of lithium iron phosphate (manganese) salt particles is measured using a laser particle size analyzer (Malvern Master Size 3000). In addition, the Dv10, Dv90 and Dv99 of the present application are also measured.

[0213] 4. Carbon content

[0214] After the lithium iron phosphate (manganese) salt particles are burned using a high-frequency induction furnace, the carbon content is tested by infrared absorption method. The specific testing process is based on the standard GB / T 20123-2006 / ISO 15350:2000.

[0215] 5. Sphericity

[0216] The cold-pressed pole piece is cut open by an Ar particle beam perpendicular to the large surface of the pole piece to expose the end surface. The picture is obtained by a scanning electron microscope. Then the scanning electron microscope picture of the pole piece cross section is analyzed by image software to measure the length a and the short diameter b of the particles, and obtain the ratio, i.e. the length-diameter ratio of the particles.

[0217] 6. Cohesion

[0218] Select a flat copper plate, wipe the surface of the copper plate with a dust-free paper and alcohol, then paste double-sided tape (model: nittotape) on the copper plate, the width of the tape is 20mm and the length is 150mm, the tape is parallel to the edge of the steel plate, and the distance from the tape to the edge of the steel plate is equal; take the test tab, cut a sample with a width of 25mm and a length of 160mm with a blade, and paste the cut tab sample on the double-sided tape, align the edge of the tab with the edge of the copper plate, and ensure that the tab is flat with the double-sided tape; use a copper glue with a length of about 30mm to ensure that the copper glue is parallel to the test tab, and then use a 2kg roller to roll back and forth 3-4 times; fold the copper glue upwards to the end of the copper plate where the tab is not pasted, fix it with the lower clamp, and clamp the paper tape and copper plate parallel with the upper clamp; test on the computer connected to the tensile testing machine (model: INSTRON), first pre-tension 3mm, set the speed to 10mm / min, and the displacement distance to 15mm, then obtain the data, cut off 8-15mm of data, pre-tension 3mm for each test, repeat the test four times, and take the average value as the cohesion.

[0219] 7. Tab compaction density

[0220] The compaction density of the film layer on one side of the tab = m / (V1-V2), m represents the weight of the film layer, V1 represents the volume of the tab, 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 tab, the product of the surface area of the tab and the thickness of the tab is the volume V1 of the tab, and the product of the surface area of the tab and the thickness of the current collector is V2. The thickness of the tab and the thickness of the current collector are measured by a micrometer.

[0221] II. Measurement of battery performance

[0222] 1. 1C discharge capacity ratio of battery to 3.2V (η value)

[0223] The preparation and testing process of the battery is as follows: 2.0000 g of the sample is mixed with 0.1111 g of conductive carbon black and 0.1111 g of PVDF (in a mass ratio of 0.9:0.05:0.05), 2.5 g of organic solvent NMP (N-methyl pyrrolidone) is then added, and after being mixed uniformly, a thin film with a thickness of 140 μm is coated on an aluminum foil, vacuum drying at 120°C for 2 h, using a puncher to punch a circular piece with a diameter of 13 mm, using a tablet press to press the tablet at 10 MPa, vacuum incubation at 120°C for 12 h, and weighing the weight of the positive electrode sheet, wherein the active material loading is 11 mg-12 mg. In an argon-protected glove box, a button cell is assembled, lithium metal is used as the negative electrode, the electrolyte is a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1, the electrolyte is LiPF6, and the separator is a Celgard 2400 microporous polyethylene film. The assembled battery is tested for electrical performance on a blue power tester. In the voltage range of 2.0 V-3.75 V, 0.1 C constant current charging / discharging is carried out for two weeks, followed by 1 C constant current charging / discharging for two weeks to test the specific capacity. There is a constant voltage process during the charging process, the constant voltage is 3.75 V, and the constant voltage cutoff current is 50 uA.

[0224] The battery comprising the second lithium-iron salt particles as the positive electrode material is subjected to constant current charging / discharging at a rate of 0.1 C for two times and then at a rate of 1 C for one time in the voltage range of 2.0 V-3.75 V, in the charging / discharging test at the rate of 1 C, the capacity value at a discharge voltage of 3.2 V is extracted and recorded as C1, the capacity value at a discharge voltage of 2.0 V is extracted and recorded as C2, and η = C1 / C2, wherein the charging process includes constant voltage charging, the constant voltage is 3.75 V, and the constant voltage cutoff current is 50 uA.

[0225] 2. Measurement of kinetic performance (10% SOC DCR)

[0226] The kinetic performance of the battery in the present application is specifically represented by the measurement of direct current resistance (power performance).

[0227] 25°C power performance test:

[0228] Capacity calibration: after the lithium ion batteries prepared from each example and the comparative example are incubated at 25°C for 2 h, 0.33 C constant current charging is carried out until 3.65 V, constant voltage charging is carried out at 3.65 V until 0.05 C, after the charging is completed, the measured battery is left at rest at 25°C for 2 h, and then 0.33 C direct current discharging is carried out until 2.5 V, and the normal temperature discharge capacity is recorded as C0;

[0229] Adjustment of SOC (state of charge of the battery): after the lithium ion battery with calibrated capacity is incubated at 25°C for 2 h, discharging is carried out at a discharging rate of 1 / 3 C0 for 144 min, and the capacity of the lithium ion battery is adjusted to 10% SOC;

[0230] Power test: after the lithium ion battery with 10% SOC is placed at 25℃ for 2h, it is discharged at a discharge rate of 3C0 under pulse current I for 30s, the voltage before 3C0 discharge is recorded as V1, and the voltage at the end of 30s discharge is recorded as V2; the value of (V1-V2) / I is calculated, and the data can represent the power performance of the battery. The lower the value of the above kinetic performance is, the better the kinetic performance of the battery is.

[0231] III. Parameters and performances in examples and comparative examples

[0232] The batteries of each example and comparative example were prepared according to the above method, and each performance parameter was measured, as shown in Table 1.

[0233] Table 1

[0234]

[0235] According to Table 1, the positive electrode sheet of the examples 1-14 of the application comprises a first active layer and a second active layer, the first active layer comprises first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles, and the second active layer comprises second lithium iron phosphate salt particles, the primary average particle size of the second lithium iron phosphate salt particles is greater than that of the first lithium iron phosphate salt particles and / or the first lithium manganese iron phosphate salt particles, and both are within a certain range, and the final obtained secondary battery has excellent kinetic performance and excellent battery performance.

[0236] In addition, according to Table 1, in the comparative examples 1-4, the comparative example 1-2 does not set one of the first active layer or the second active layer, and the comparative examples 3-4 set the first active layer and the second active layer, but 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 final obtained secondary battery is significantly reduced compared with the examples of the application, which cannot meet the performance requirements of the application.

[0237] It should be noted that the application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solution of the application are also included in the technical scope of the application. In addition, within the scope of the main idea of the application, various modifications of the embodiments can be made by those skilled in the art, and other ways can be constructed by combining part of the components in the embodiments.

Claims

1. A positive electrode tab, comprising a current collector and a positive electrode active layer, the positive electrode active layer being provided on at least one side of the current collector, the positive electrode active layer comprising a first active layer coated directly 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 comprising first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles, and the second active layer comprising second lithium iron phosphate salt particles, the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles having a primary average particle size of 150 nm to 480 nm, and the second lithium iron phosphate salt particles having a primary average particle size of 500 nm to 3000 nm; the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles having a sphericity of less than or equal to 1.3, and the second lithium iron phosphate salt particles having a sphericity of greater than or equal to 1.5, the sphericity being measured by the ratio of the length of the long axis to the length of the short axis of the particles, wherein the length of the long axis is denoted as a, the length of the short axis is denoted as b, and the ratio of the length of the long axis to the length of the short axis is a / b.

2. The cathode electrode of claim 1, wherein, the positive electrode tab having a cohesion of greater than or equal to 32 N / m.

3. The positive electrode plate of claim 1 or 2, wherein, the first active layer having a thickness in the range of 80 μm to 140 μm, and the second active layer having a thickness in the range of 8 μm to 50 μm.

4. The cathode electrode of claim 1, wherein The BET specific surface area of the first lithium iron phosphate salt particles and / or 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 cathode sheet of claim 1, wherein, 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, Mg or a combination of two or more thereof, and 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.

6. The cathode sheet of claim 1, wherein, the second lithium iron phosphate salt particles The molecular formula of the particle is Li m Fe x P y O j Q q wherein Q comprises 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 7.The positive electrode tab of claim 6, wherein the Q comprises at least one of Ti, V, Mg, and Nb, and the content of the Q is 1000 ppm to 10000 ppm, based on the total weight of the second lithium iron phosphate salt particles.

8. The cathode electrode of claim 1, wherein, the second lithium iron phosphate salt particles comprising carbon, and the content of the carbon is 1.0 wt% to 2.0 wt%, based on the total weight of the second lithium iron phosphate salt particles.

9. The positive electrode plate of claim 1, wherein, the first lithium iron phosphate salt particles and / or first lithium manganese iron phosphate salt particles, and the second lithium iron phosphate salt particles are each independently single crystal particles and / or polycrystal particles. 10.The positive electrode tab of claim 1, wherein the capacity ratio η of the second lithium iron phosphate salt particles is greater than or equal to 88%, wherein the η is defined as follows: a battery comprising the second lithium iron phosphate salt particles as a positive electrode material is subjected to constant current charge and discharge twice at a rate of 0.1 C in a voltage range of 2.0 V to 3.75 V, and then subjected to constant current charge and discharge once at a rate of 1 C, in the charge and discharge test at the rate of 1 C, the capacity value at a discharge voltage of 3.2 V is denoted as C1, and the capacity value at a discharge voltage of 2.0 V is denoted as C2, and η = C1 / C2, wherein the charging process includes constant voltage charging, the constant voltage is 3.75 V, and the constant voltage cutoff current is 50 uA.

11. The cathode electrode of claim 1, wherein, the manufacturing process of the second lithium iron phosphate salt particles comprises: 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 sintering processes, wherein The first sintering temperature is 500-760°C, and the carbon content of the material after the first sintering is 0.01-0.79 wt%; The second sintering temperature is 700-800°C, and the carbon content of the material after the second sintering is 0.8-2.0 wt%.

12. The cathode electrode of claim 11, wherein, In the manufacturing process of the second lithium iron phosphate salt particles, 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 300-1200 nm; The Dv50 of the product after the second crushing is 500-5000 nm.

13. The cathode sheet of claim 1, wherein, 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 the first lithium manganese iron phosphate salt particles: the binder: the conductive agent: the 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: the binder: the conductive agent: the 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.

14. The cathode electrode of claim 13, wherein, The conductive agent comprises 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 comprises any one or a combination of at least two of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, or polytetrafluoroethylene.

15. A secondary battery characterized by comprising: The positive electrode tab of any one of claims 1-14.

16. An electrical device, comprising: The secondary battery of claim 15.

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