Lithium iron phosphate cathode sheets and related secondary batteries, battery modules, battery packs, and electrical devices.

Abstract

The present application provides a lithium iron phosphate positive electrode sheet including a positive electrode current collector and a positive electrode active material located on the positive electrode current collector, the positive electrode active material having a volume average particle diameter D50 of 60-300 nm, preferably 60-200 nm, and a specific surface area of ​​15 m 2 / g, preferably 15-25m 2 / g, and the first lithium iron phosphate particles have a volume average particle diameter D50 of more than 800 nm, preferably 1000-1500 nm, and a specific surface area of ​​10 m 2 / g, preferably 5-10m 2 and second lithium iron phosphate particles having a particle size of 100 / g. A secondary battery manufactured using the lithium iron phosphate positive electrode sheet of the present invention has excellent kinetic performance and cycle performance.

Description

[Technical Field]

[0001] This application relates to the field of lithium battery technology, and more particularly to lithium iron phosphate cathode sheets and related secondary batteries, battery modules, battery packs, and electrical devices. [Background technology]

[0002] In recent years, the range of applications for lithium-ion batteries has expanded significantly. They are widely used in energy storage systems such as hydroelectric, thermal, wind, and solar power plants, as well as in various fields including power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. Among the many battery types, lithium iron phosphate batteries have attracted attention due to their high capacity and superior safety performance. However, as the use of secondary batteries becomes more widespread, so do people's demands for the dynamic and cycle performance of lithium iron phosphate batteries. Developing lithium iron phosphate batteries with superior dynamic and cycle performance remains an urgent challenge that engineers must address. [Overview of the Initiative] [Problems that the invention aims to solve]

[0003] This application was made in view of the above-mentioned problems, and aims to provide a lithium iron phosphate cathode sheet and related secondary batteries, battery modules, battery packs, and electrical devices, wherein a secondary battery to which the cathode sheet is applied has excellent kinetic performance and cycle performance. [Means for solving the problem]

[0004] A first aspect of the present application provides a lithium iron phosphate positive electrode sheet comprising a positive electrode current collector and a positive electrode active material located on the positive electrode current collector, the positive electrode active material being The volume-average particle size D50 is 60-300 nm, preferably 60-200 nm, and the specific surface area is 15 m². 2 Greater than / g, preferably 15-25m 2 A first lithium iron phosphate particle at / g, The volume-average particle size D50 is greater than 800 nm, preferably between 1000 and 1500 nm, and the specific surface area is 10 m². 2 Less than / g, preferably 5-10m 2 It contains a second lithium iron phosphate particle in a quantity of / g.

[0005] The positive electrode sheet described in the first aspect of the present application contains lithium iron phosphate particles of two different particle sizes and specific surface areas, fully utilizing the advantages of each, and can improve the dynamic performance and cycle performance of a secondary battery to which the positive electrode sheet is applied.

[0006] In any embodiment, the content of the first lithium iron phosphate particles is preferably 1-97%, preferably 18-68%, and more preferably 36-68%, based on the total weight of the positive electrode film layer in the lithium iron phosphate positive electrode sheet. Based on the total weight of the positive electrode film layer in the lithium iron phosphate positive electrode sheet, the content of the second lithium iron phosphate particles is 3-99%, preferably 32-82%, and more preferably 32-64%.

[0007] When the amounts of both types of lithium iron phosphate particles used are within the above range, the secondary battery manufactured using the positive electrode sheet described in the first aspect of this application has excellent kinetic and cycle performance.

[0008] In any embodiment, preferably, in the active material layer of the lithium iron phosphate positive electrode sheet, the weight ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is 1:0.1-9, preferably 1:0.4-4, and more preferably 1:0.4-1.5.

[0009] By adjusting the ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles used, the kinetic and cycle performance of the corresponding battery can be further improved.

[0010] In any embodiment, preferably, based on the total weight of the first lithium iron phosphate particles, the carbon content in the first lithium iron phosphate particles is 1.2 - 2.7%, preferably 1.4 - 2.2%, more preferably 1.4 - 2.0%. Based on the total weight of the second lithium iron phosphate particles, the carbon content in the second lithium iron phosphate particles is 0.7 - 1.3%, preferably 1 - 1.2%, more preferably 1.1 - 1.2%.

[0011] When the carbon content of both types of lithium iron phosphate particles is within the above range, the performance of the corresponding secondary battery can be further improved.

[0012] In any embodiment, preferably, the consolidation density of the positive electrode film layer in the lithium iron phosphate positive electrode sheet is 2 - 2.45 g / cm 3 and preferably 2.1 - 2.3 g / cm 3 and more preferably 2.15 - 2.25 g / cm 3 is.

[0013] In any embodiment, preferably, in the active material layer of the lithium iron phosphate positive electrode sheet, the first lithium iron phosphate particles and the second lithium iron phosphate particles are distributed in a layered manner.

[0014] In any embodiment, preferably, the second lithium iron phosphate particle layer is located on the surface of the positive electrode current collector, and the first lithium iron phosphate particle layer is located on the surface of the first lithium iron phosphate particle layer facing the positive electrode current collector.

[0015] By adjusting the distribution of the first lithium iron phosphate particle layer and the second lithium iron phosphate layer, especially when the second lithium iron phosphate particle layer is located on the surface of the current collector and the first lithium iron phosphate particle layer is located above the second lithium iron phosphate particle layer, the kinetic performance and cycle performance of the corresponding secondary battery can be further improved.

[0016] In any embodiment, preferably, the thickness of the first lithium iron phosphate particle layer is 65 - 250 μm, preferably 80 - 140 μm, more preferably 100 - 140 μm. The thickness of the second lithium iron phosphate particle layer is 60 - 250 μm, preferably 60 - 120 μm, more preferably 60 - 100 μm.

[0017] When the thickness of each lithium iron phosphate particle layer is within the above range, it helps to improve the cycle performance of the secondary battery, while also avoiding the loss of the kinetic performance of the secondary battery due to the excessive thickness of each layer.

[0018] In any embodiment, preferably, the thickness ratio of the first lithium iron phosphate particle layer to the second lithium iron phosphate particle layer is 1:0.1 - 9, preferably 1:0.4 - 1.5, more preferably 1:0.4 - 1.

[0019] When the thickness ratio of the first lithium iron phosphate particle layer to the second lithium iron phosphate particle layer is within the above range, it helps to further improve the kinetic performance and cycle performance of the corresponding secondary battery.

[0020] In any embodiment, preferably, the surface density of the first lithium iron phosphate particle layer is 0.1 - 0.3 g / 1540.25 mm 2 and preferably 0.15 - 0.25 g / 1540.25 mm 2 and more preferably 0.15 - 0.2 g / 1540.25 mm 2 and the surface density of the second lithium iron phosphate particle layer is 0.1 - 0.3 g / 1540.25 mm 2 and preferably 0.15 - 0.25 g / 1540.25 mm 2 and more preferably 0.15 - 0.2 g / 1540.25 mm 2 and.

[0021] A second aspect of the present application provides a secondary battery comprising a lithium iron phosphate cathode sheet according to the first aspect of the present application. The secondary battery can be manufactured by methods commonly used in the art.

[0022] A third aspect of the present application provides a battery module including a secondary battery according to a second aspect of the present application. The battery module can be manufactured by a method commonly used in the art.

[0023] A fourth aspect of the present application provides a battery pack including a battery module according to a third aspect of the present application. The battery pack can be manufactured by a method commonly used in the art.

[0024] A fifth aspect of the present application provides an electrical device comprising at least one selected from a secondary battery according to the second aspect of the present application, a battery module according to the third aspect of the present application, or a battery pack according to the fourth aspect of the present application. The electrical device can be manufactured by methods commonly used in the art. [Effects of the Invention]

[0025] In the lithium iron phosphate cathode sheet of this invention, the volume-average particle size D50 is 60-300 nm, and the specific surface area is 15 m². 2 The first lithium iron phosphate particles are larger than / g, and the volume-average particle size D50 is greater than 800nm, with a specific surface area of ​​10m². 2 The invention also includes second lithium iron phosphate particles smaller than / g, which helps to fully utilize the advantages of both the first and second lithium iron phosphate particles, thereby improving the dynamic and cycle performance of the secondary battery including the positive electrode sheet. Furthermore, by distinguishing the lithium iron phosphate particles by specific surface area, the invention avoids the situation where the particle size parameter becomes ineffective when lithium iron phosphate particles are distinguished solely by particle size, as the first lithium iron phosphate particles tend to aggregate.

[0026] The battery module, battery pack, and electrical device of the present application include a secondary battery provided by the present application and therefore have at least the same advantages as the aforementioned secondary battery. [Brief explanation of the drawing]

[0027] [Figure 1] This is a scanning electron microscope image of the first lithium iron phosphate particles of the present invention. As can be seen from Figure 1, the single particle size of the first lithium iron phosphate particles is small, and a large number of single particles are aggregated. [Figure 2] This is a scanning electron microscope image of the second lithium iron phosphate particles of the present invention. As can be seen from Figure 2, the particle size of the second lithium iron phosphate particles is large and does not aggregate easily. [Figure 3] The DC internal resistance (DCR) of the secondary battery corresponding to Example 1 of this application is shown at different temperatures. As can be seen from Figure 3, the DCR of the secondary battery corresponding to Example 1 is 18.26 mΩ at -25°C, 10.45 mΩ at -10°C, and 1.72 mΩ at 25°C. [Figure 4] These are the DC internal resistance (DCR) of the secondary battery corresponding to Comparative Example 1 of the present invention at different temperatures. As can be seen from Figure 4, the DCR of the secondary battery corresponding to Comparative Example 1 is 24.81 mΩ at -25°C, 13.91 mΩ at -10°C, and 3.14 mΩ at 25°C. [Figure 5] These are the DC internal resistance (DCR) of a secondary battery corresponding to Comparative Example 2 of this application at different temperatures. [Figure 6] This is a schematic diagram showing a secondary battery according to one embodiment of the present invention. [Figure 7] Figure 6 is an exploded view showing a secondary battery according to one embodiment of the present invention. [Figure 8] This is a schematic diagram showing a battery module according to one embodiment of the present invention. [Figure 9] This is a schematic diagram showing a battery pack according to one embodiment of the present invention. [Figure 10] Figure 9 is an exploded view showing a battery pack according to one embodiment of the present invention. [Figure 11]This is a schematic diagram showing an electrical device that uses a secondary battery as a power source according to one embodiment of the present invention. [Modes for carrying out the invention]

[0028] The following describes in detail embodiments of the lithium iron phosphate cathode sheet and its manufacturing method, secondary battery, battery module, battery pack, and electrical device of this application, with appropriate reference to the drawings. However, unnecessary details may be omitted. For example, detailed descriptions of already well-known matters and redundant descriptions of the same actual structure may be omitted. This is to avoid making the following description unnecessarily long and to facilitate understanding for those skilled in the art. The drawings and the following description are provided to enable those skilled in the art to fully understand this application and do not limit the subject matter described in the claims.

[0029] The “range” disclosed in this application is limited in the form of a lower and upper limit, and a given range is limited by selecting one lower limit and one upper limit, and the boundaries of the special range are limited by the selected lower and upper limits. The range thus limited may include or exclude boundary values ​​and can be any combination, that is, any lower limit can be combined with any upper limit to form a range. For example, if the ranges 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges 60-110 and 80-120 are also expected. Furthermore, if the minimum range values ​​1 and 2 and the maximum range values ​​3, 4 and 5 are listed, then the following ranges, 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5, are all expected. In this application, unless otherwise stated, the numerical range “ab” is an abbreviated representation of any combination of real numbers from a to b, where both a and b are real numbers. For example, the numerical range "0-5" refers to all real numbers between "0-5" listed herein, and "0-5" is simply an abbreviated representation of combinations of these numbers. Also, when a parameter indicates an integer ≥ 2, it is equivalent to disclosing that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0030] Unless otherwise specified, all embodiments and preferred embodiments of the present application can be combined to form new technical solutions.

[0031] Unless otherwise specified, all technical features and preferred technical features of this application can be combined to form new technical solutions.

[0032] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but it is preferable to perform them sequentially. For example, "The method includes steps (a) and (b)" means that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, "The method described above may include step (c)" means that step (c) can be added to the method in any order, for example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b).

[0033] Unless otherwise specified, the terms “equipment” and “include” as used in this application may be non-limiting or limiting. For example, “equipment” and “include” may further equip or include other components not listed, or they may equip or include only the listed components.

[0034] Unless otherwise specified, the terms "above" and "below" used in this application include the numbers themselves.

[0035] Unless otherwise specified, the term “or” in this application is inclusive. For example, the phrase “A or B” means “A, B, or both A and B.” More specifically, any of the following conditions satisfy the “A or B” condition: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) but B is true (or exists); or both A and B are true (or exist).

[0036] Through practical work, the inventors discovered that by combining lithium iron phosphate particles of different particle sizes, the kinetic and cycle performance of the corresponding secondary battery can be effectively improved. Furthermore, they unexpectedly discovered that by distinguishing lithium iron phosphate particles by particle size and specific surface area, they could avoid the problem where the particle size parameter becomes ineffective when distinguishing them solely by particle size, as the first lithium iron phosphate particle tends to aggregate.

[0037] The inventors found that, through further research, the kinetic and cycle performance of the corresponding secondary battery can be further improved by changing the carbon content of the first lithium iron phosphate particles and the second lithium iron phosphate particles, adjusting the usage ratio of each lithium iron phosphate particle, and layering different lithium iron phosphate particles and adjusting the thickness of each layer.

[0038] [Positive electrode sheet] A first aspect of the present application provides a lithium iron phosphate positive electrode sheet comprising a positive electrode current collector and a positive electrode active material located on the positive electrode current collector, wherein the positive electrode active material is The volume-average particle size D50 is 60-300 nm, preferably 60-200 nm, and the specific surface area is 15 m². 2 Greater than / g, preferably 15-25m 2 A first lithium iron phosphate particle at / g, The volume-average particle size D50 is greater than 800 nm, preferably between 1000 and 1500 nm, and the specific surface area is 10 m². 2 Less than / g, preferably 5-10m 2It contains a second lithium iron phosphate particle in a quantity of / g.

[0039] The positive electrode sheet according to this invention contains first lithium iron phosphate particles and second lithium iron phosphate particles. The first lithium iron phosphate particles have low DC internal resistance and excellent dynamic performance, and the second lithium iron phosphate particles have high compaction density and are less prone to aggregation. These advantages are fully utilized, and the dynamic performance and cycle performance of the corresponding secondary battery can be effectively improved. Furthermore, by distinguishing lithium iron phosphate particles by specific surface area, this invention avoids the situation where the particle size parameter becomes ineffective when lithium iron phosphate particles are distinguished solely by particle size, as the first lithium iron phosphate particles tend to aggregate.

[0040] As an example, the positive electrode sheet of the present application includes a positive electrode current collector and a positive electrode active material, the positive electrode current collector having two opposing surfaces in the thickness direction of itself, and the positive electrode active material being provided on one or both of the two opposing surfaces of the positive electrode current collector.

[0041] In this application, the volume-average particle size D50 and specific surface area can be measured by methods commonly used in this art, for example, the D50 particle size is measured according to GB / T 19077-2016 / ISO 13320:2009, and the specific surface area is measured according to GB / T 19587-2004 "Measurement of Specific Surface Area of ​​Solids by Gas Adsorption BET Method".

[0042] In some embodiments, the content of the first lithium iron phosphate particles is preferably 1-97%, preferably 18-68%, and more preferably 36-68%, based on the total weight of the positive electrode film layer in the lithium iron phosphate positive electrode sheet. Based on the total weight of the positive electrode film layer in the lithium iron phosphate positive electrode sheet, the content of the second lithium iron phosphate particles is 3-99%, preferably 32-82%, and more preferably 32-64%.

[0043] For example, the amount of first lithium iron phosphate particles used may be 19.4%, 38.8%, 48.5%, 58.2%, 64.7%, or 67.9%, and the amount of second lithium iron phosphate particles used may be 29.1%, 32.3%, 38.8%, 48.5%, 58.2%, or 77.6%.

[0044] Using too much of the first type of lithium iron phosphate particle leads to aggregation of many of these particles, which is detrimental to mass production and improving energy density. Similarly, too much of the second type of lithium iron phosphate particle may worsen the battery's dynamic performance and shorten its lifespan. When the content of both types of lithium iron phosphate particles is within the above range, the corresponding secondary battery will have low DC internal resistance and a long lifespan.

[0045] In some embodiments, preferably, in the active material layer of the lithium iron phosphate positive electrode sheet, the weight ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles is 1:0.1-9, preferably 1:0.4-4, and more preferably 1:0.4-1.5. For example, the weight ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles may be 1:4, 2:3, 1:1, 3:2, 2:1, or 7:3.

[0046] By adjusting the ratio of the first lithium iron phosphate particles to the second lithium iron phosphate particles used, the kinetic and cycle performance of the corresponding battery can be further improved.

[0047] In some embodiments, based on the total weight of the first lithium iron phosphate particles, the carbon content in the first lithium iron phosphate particles is preferably 1.2-2.7%, preferably 1.4-2.2%, and more preferably 1.4-2.0%. Based on the total weight of the second lithium iron phosphate particles, the carbon content in the second lithium iron phosphate particles is 0.7-1.3%, preferably 1-1.2%, and more preferably 1.1-1.2%.

[0048] If the carbon content of both types of lithium iron phosphate particles falls within the above range, the performance of the corresponding secondary battery can be further improved.

[0049] In this invention, the carbon content in lithium iron phosphate particles can be measured by a method commonly used in this field, for example, by infrared absorption spectroscopy. Specifically, the sample to be measured is burned in an oxygen stream to generate CO2. Under constant pressure, the energy of infrared radiation absorbed by CO2 is proportional to its concentration, so the carbon content can be calculated from the energy change before and after the measured CO2 gas passes through an infrared absorber.

[0050] In some embodiments, preferably, the tap density of the first lithium iron phosphate particle powder is 1-1.4 g / cm³. -3 Preferably, 1.1-1.3 g / cm³ -3 More preferably, 1.15-1.25 g / cm³ -3 That is the case.

[0051] In some embodiments, preferably, the tap density of the second lithium iron phosphate particle powder is 1.2–1.6 g / cm³. -3 Preferably, 1.3-1.5 g / cm³ -3 More preferably, 1.35-1.45 g / cm³ -3 That is the case.

[0052] The tap density of lithium iron phosphate particulate powder can be measured by methods commonly used by those skilled in the art, for example, according to GB / T 5162-2006 / ISO3953:1993.

[0053] In some embodiments, preferably, the compaction density of the first lithium iron phosphate particle powder is 2-2.2 g / cm³. 3 That is the case.

[0054] In some embodiments, preferably, the compaction density of the second lithium iron phosphate particle powder is 2-2.45 g / cm³. 3 That is the case.

[0055] Similarly, the compaction density can be measured by methods commonly used by those skilled in the art. For example, the compaction density can be measured by the following method: a certain amount of powder is placed in a mold specifically for compacting powder, the mold is then placed on a compaction density instrument, different pressures are set, and the thickness of the powder at different pressures (thickness after pressure relief) can be read from the instrument. The compaction density can then be calculated using the formula ρ = m / v.

[0056] In some embodiments, preferably, the compaction density of the positive electrode film layer in the lithium iron phosphate positive electrode sheet is 2-2.45 g / cm³. 3 Preferably, 2.1-2.3 g / cm³ 3 More preferably, 2.15-2.25 g / cm³ 3 The compaction density of the positive electrode film layer can be measured by a method commonly used in this field. For example, first, a positive electrode sheet is taken per unit area, its mass m1 is measured, then the mass m2 of the positive electrode foil material per unit area is measured, and the mass of the positive electrode film layer is obtained by subtracting m2 from m1. The compaction density of the positive electrode film layer can then be obtained by dividing this mass by the thickness of the positive electrode film layer (sheet thickness minus foil material thickness).

[0057] In some embodiments, preferably, the first lithium iron phosphate particles and the second lithium iron phosphate particles are distributed in a layered manner within the active material layer of the lithium iron phosphate cathode sheet.

[0058] In some embodiments, preferably, the second lithium iron phosphate particle layer is located on the surface of the positive electrode current collector, and the first lithium iron phosphate particle layer is located on the surface of the second lithium iron phosphate particle layer facing the positive electrode current collector.

[0059] Those skilled in the art will understand that the spatial structure design of the electrode sheet layer, particularly the design of an electrode sheet containing two or more active materials, has a significant impact on the performance of a lithium secondary battery. The performance of the battery can be further improved by designing the spatial structure of the electrode sheet. Specifically, in this application, when two types of lithium iron phosphate particles are coated in layers, and in particular the second lithium iron phosphate particle is coated on the surface of the positive electrode current collector and the first lithium iron phosphate particle is coated on the surface of the second lithium iron phosphate particle layer to form a two-layer distribution structure, the advantages of each of the two types of lithium iron phosphate particles can be more effectively utilized, improving the dynamic performance and cycle performance of the secondary battery.

[0060] In some embodiments, the thickness of the first lithium iron phosphate particle layer is preferably 65-250 μm, preferably 80-140 μm, and more preferably 100-140 μm. The thickness of the second lithium iron phosphate particle layer is 60-250 μm, preferably 60-120 μm, and more preferably 60-100 μm.

[0061] For example, the thickness of the first lithium iron phosphate particle layer may be, for example, 80 nm, 100 nm, 120 nm, or 140 nm, and the thickness of the second lithium iron phosphate particle layer may be, for example, 60 nm, 80 nm, 100 nm, or 120 nm. The thicknesses of the first lithium iron phosphate particle layer and the second lithium iron phosphate particle layer can be measured by methods commonly used in this art. For example, the thickness of the positive electrode film layer is measured first, and then the thickness is determined by the ratio of the amount of first lithium iron phosphate particles to the second lithium iron phosphate particles used.

[0062] If the lithium iron phosphate particle layer is too thick, there is a concern that it will degrade the battery's dynamic performance. Conversely, if the lithium iron phosphate particle layer is too thin, there is a concern that the advantages of the two-layer coating will not be fully realized, and further improvements in battery performance will not be possible.

[0063] In some embodiments, the thickness ratio of the first lithium iron phosphate particle layer to the second lithium iron phosphate particle layer is preferably 1:0.1–9, preferably 1:0.4–1.5, and more preferably 1:0.4–1. For example, the thickness ratio of the first lithium iron phosphate particle layer to the second lithium iron phosphate particle layer is 2:3, 1:1, 3:2, or 7:3.

[0064] When the thickness ratio of the first lithium iron phosphate particle layer to the second lithium iron phosphate particle layer is within the above range, it helps to further improve the kinetic and cycle performance of the corresponding secondary battery.

[0065] In some embodiments, preferably, the surface density of the first lithium iron phosphate particle layer is 0.1-0.3 g / 1540.25 mm 2 Preferably, 0.15-0.25g / 1540.25mm 2 More preferably, 0.15-0.2g / 1540.25mm 2 And, The surface density of the second lithium iron phosphate particle layer is 0.1-0.3 g / 1540.25 mm 2 Preferably, 0.15-0.25g / 1540.25mm 2 More preferably, 0.15-0.2g / 1540.25mm 2 That is the case.

[0066] Furthermore, this application does not impose any special restrictions on the coating method, and commonly used coating methods in this field, such as scraper coating, roller coating, and slit extrusion coating, can be employed.

[0067] In some embodiments, the positive electrode current collector can be a metal foil sheet or a composite current collector. For example, aluminum foil can be used as the metal foil sheet. The composite current collector includes a polymer material substrate layer and a metal layer formed on at least one surface of the polymer material substrate layer. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy) on a polymer material substrate (for example, a substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0068] In some embodiments, preferably, the lithium iron phosphate cathode sheet further comprises a binder. For example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resins.

[0069] In some embodiments, the binder preferably accounts for 0.1-3.5% of the total weight of the positive electrode film layer, and more preferably 0.5-2.5%.

[0070] In some embodiments, preferably, the lithium iron phosphate cathode sheet further comprises a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, cochin black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0071] In some embodiments, the conductive agent preferably accounts for 0.05-2.0% of the total weight of the positive electrode film layer, and more preferably 0.1-1.5%.

[0072] In some embodiments, preferably, the lithium iron phosphate cathode sheet further comprises other additives such as surfactants, wetting agents, and rheological modifiers. For example, the additives can be selected from one or more of the following: higher fatty acid salts, higher alkyl sulfonates, alkylaryl sulfonates, alkali metal salts such as lithium salts of lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, perfluoroalkanesulfonylimide salts, methyl octanoate, n-butyl pivalate, or lauryl acetate.

[0073] In some embodiments, the other additives preferably account for 0.05–2.0% of the total weight of the positive electrode film layer, and more preferably 0.1–1.5%.

[0074] In some embodiments, a positive electrode sheet can be manufactured as follows: components for manufacturing the positive electrode sheet, such as first lithium iron phosphate particles and second lithium iron phosphate particles, a conductive agent, a binder, and any other components, such as a surfactant, are dispersed in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is applied to a positive electrode current collector; and after processes such as drying and cold pressing, a positive electrode sheet can be obtained.

[0075] In some embodiments, preferably, a positive electrode slurry of first lithium iron phosphate particles and a positive electrode slurry of second lithium iron phosphate particles can be prepared, the positive electrode slurry of the second lithium iron phosphate particles can be applied to the side closer to the current collector, and the positive electrode slurry of the first lithium iron phosphate particles can be applied to the side of the surface of the second lithium iron phosphate particle layer that is farther from the current collector.

[0076] In some embodiments, preferably when producing a positive electrode slurry, the mixture is stirred until its viscosity reaches 7000-15000 mPa·s and immediately shipped.

[0077] [Secondary battery] A second aspect of the present application provides a secondary battery comprising a lithium iron phosphate positive electrode sheet as described in the first aspect of the present application. The secondary battery can be manufactured by methods commonly used in the art. For example, the positive electrode sheet, negative electrode sheet and separator can be wound together as an electrode assembly according to a certain process, and then an electrolyte can be injected into the resulting electrode assembly, followed by sealing and other processes to manufacture the secondary battery according to the present application.

[0078] Typically, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the battery charging and discharging process, active ions are intercepted and deintercepted between the positive and negative electrode sheets. The electrolyte is located between the positive and negative electrode sheets and serves to conduct ions. The separator is placed between the positive and negative electrode sheets and primarily serves to prevent short circuits between the positive and negative electrodes, while simultaneously allowing ions to pass through.

[0079] Other components of secondary batteries, such as the negative electrode sheet, electrolyte, and separator, will be described later.

[0080] [Negative electrode sheet] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, the negative electrode film layer containing a negative electrode active material.

[0081] For example, the negative electrode current collector has two opposing surfaces in the thickness direction of itself, and the negative electrode film layer is provided on one or both of the two opposing surfaces of the negative electrode current collector.

[0082] In some embodiments, the negative electrode current collector can be a metal foil sheet or a composite current collector. For example, copper foil can be used as the metal foil sheet. The composite current collector may include a polymer material substrate layer and a metal layer formed on at least one surface of the polymer material substrate layer. The composite current collector can be formed by forming a metal material (such as copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys) on a polymer material substrate (for example, a substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0083] In some embodiments, the negative electrode active material is a battery negative electrode active material well known in the art. For example, the negative electrode active material may include at least one of materials such as artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, stinate compounds, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as battery negative electrode active materials may be used. These negative electrode active materials may be used alone or in combination of two or more.

[0084] In some embodiments, the negative electrode film layer preferably further contains 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).

[0085] In some embodiments, the negative electrode film layer preferably further contains a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, cochin black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0086] In some embodiments, the negative electrode film layer preferably further contains other auxiliary agents such as a thickening agent (e.g., sodium carboxymethylcellulose (CMC-Na)).

[0087] In some embodiments, a negative electrode sheet can be manufactured as follows: the above-mentioned components for manufacturing the negative electrode sheet, such as a negative electrode active material, a conductive agent, a binder, and any other components, are dispersed in a solvent (e.g., deionized water) to form a negative electrode slurry; the negative electrode slurry is coated onto a negative electrode current collector; and the negative electrode sheet is obtained after going through processes such as drying and cold pressing.

[0088] [Electrolyte] The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. This application is not specifically limited to the type of electrolyte, which can be selected as needed. For example, the electrolyte can be a liquid, a gel, or an all-solid.

[0089] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution contains an electrolyte salt and a solvent.

[0090] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluoride phosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoride arsenate, lithium bisfluorosulfonylimide, lithium bistrifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium bis(oxalate)borate, lithium difluorobis(oxalate)phosphate, and lithium tetrafluorooxalate phosphate.

[0091] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, 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, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0092] In some embodiments, the electrolyte preferably further contains additives. For example, the additives may include negative electrode film forming additives, positive electrode film forming additives, and additives that can improve certain performance characteristics of the battery, such as additives that improve the overcharge performance of the battery and additives that improve the high-temperature or low-temperature performance of the battery.

[0093] [Separator] In some embodiments, the secondary battery further includes a separator. The present application is not particularly limited to the type of separator, and any known porous separator having good chemical and mechanical stability can be selected.

[0094] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator is not particularly limited and may be a single-layer thin film or a multilayer composite thin film. If the separator is a multilayer composite thin film, the materials of each layer are not particularly limited and may be the same or different.

[0095] A third aspect of the present application provides a battery module including the secondary battery described in the second aspect of the present application.

[0096] A fourth aspect of the present application provides a battery pack including the battery module described in the third aspect of the present application.

[0097] A fifth aspect of the present application provides an electrical device comprising at least one of the secondary battery described in the second aspect of the present application, the battery module described in the third aspect, or the battery pack described in the fourth aspect. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device and can also be used as an energy storage unit for the electrical device. The electrical device includes, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), trains, ships and satellites, and energy storage systems.

[0098] As the electrical device, a secondary battery, battery module, or battery pack can be selected depending on the requirements of use.

[0099] Furthermore, the secondary battery, battery module, battery pack, and electrical device of this application will be described below with due reference to the drawings.

[0100] In some embodiments, the secondary battery may include an enclosure. This enclosure can be used to seal the electrode assembly and electrolyte.

[0101] In some embodiments, the casing of the secondary battery may be a hard shell, such as a rigid plastic shell, an aluminum shell, or a steel shell. The casing of the secondary battery may also be a soft bag, such as a pouch soft bag. The material of the soft bag may be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0102] This invention does not particularly limit the shape of the secondary battery, and it may be cylindrical, rectangular, or any other shape. For example, Figure 6 is an example of a rectangular secondary battery 5.

[0103] In some embodiments, referring to Figure 7, the exterior may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and side plates may enclose a storage chamber. The housing 51 has an opening that communicates with the storage chamber, and the cover plate 53 may cover the opening to close the storage chamber. The positive electrode sheet, negative electrode sheet, and separator may form an electrode assembly 52 by a winding or laminating process. The electrode assembly 52 is sealed inside the storage chamber. The electrolyte permeates into the electrode assembly 52. ​​The number of electrode assemblies 52 included in the secondary battery 5 may be one or more, and can be selected by those skilled in the art according to their specific practical needs.

[0104] In some embodiments, the secondary battery can be assembled as a battery module, and the number of secondary batteries included in the battery module may be one or more, and the specific number can be selected by those skilled in the art depending on the application and capacity of the battery module.

[0105] Figure 8 shows an example of a battery module 4. Referring to Figure 8, in the battery module 4, multiple secondary batteries 5 are arranged sequentially along the length of the battery module 4. Of course, they may be distributed in any other manner. Furthermore, the multiple secondary batteries 5 can be secured with fasteners.

[0106] Preferably, the battery module 4 may further include an external case having storage space, in which a plurality of secondary batteries 5 are housed.

[0107] In some embodiments, the battery modules may be further assembled as a battery pack, and the number of battery modules included in the battery pack may be one or more, and a person skilled in the art can select the specific number depending on the application and capacity of the battery pack.

[0108] Figures 9 and 10 show an example of a battery pack 1. Referring to Figures 9 and 10, the battery pack 1 may include a battery box and a plurality of battery modules 4 provided inside the battery box. The battery box includes an upper box 2 and a lower box 3, and the upper box 2 can be covered by the lower box 3, forming a closed space for housing the battery modules 4. The plurality of battery modules 4 can be distributed inside the battery box in any manner.

[0109] Figure 11 shows an example of an electrical device. This electrical device may be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the demand for high power output and high energy density of the secondary battery in this electrical device, a battery pack or battery module can be used.

[0110] Other examples of the device may include mobile phones, tablets, and laptop computers. Such devices are typically required to be thin and can use rechargeable batteries as a power source. [Examples]

[0111] Examples of the present application are described below. The examples described below are illustrative and are used solely for interpreting the present application and should not be understood as limiting the present application. Where no specific technique or conditions are shown in the examples, the application should be carried out in accordance with the technique or conditions described in the literature of the art or in accordance with the product specification. Where the manufacturer of the reagents or equipment used is not indicated, they are conventional products available on the market.

[0112] The positive electrode active material for the embodiment of this application is shown in the table below. TIFF0007850742000001.tif100168

[0113] Example 1

[0114] Manufacturing of positive electrode sheets The first lithium iron phosphate particles (D50 is 300 nm, BET is 15.8 nm) 2 / g, carbon content 1.4%, second lithium iron phosphate particles (D50 is 1000nm, BET is 10nm) 2 Mix polyvinylidene fluoride (PVDF) and acetylene black (a conductive agent) in a weight ratio of 48.5%:48.5%:2.2:0.8 in a stirring tank. Stir for 15 minutes at a rotational speed of 25 rpm and a spinning speed of 800 rpm. Next, add N-methylpyrrolidone (NMP), a solvent with a solid content of 60%, to the above dry mix. Then, stir for 15 minutes at a rotational speed of 25 rpm and a spinning speed of 300 rpm. After that, adjust the rotational speed to 25 rpm and the spinning speed to 1200 rpm and stir for 220 minutes to uniformly disperse the mixture. Next, continue stirring at a rotational speed of 25 rpm and a spinning speed of 500 rpm to adjust the viscosity of the mixture to 10000 mPa·s, and immediately discharge to obtain a mixed slurry. Dispense 0.3 g / 1540.25 mm of the above mixed slurry. 2 The film is coated onto an aluminum foil with a thickness of 13 μm at a surface density, and both sides are coated. Then, the solvent is removed by drying, and the film is cooled to achieve a compaction density of 2.2 g / cm³ for the positive electrode film layer. 3 This is done. After cutting, the positive electrode sheet of Example 1 is obtained.

[0115] Manufacturing of negative electrode sheets The negative electrode slurry is prepared by dissolving artificial graphite (a negative electrode active material), acetylene black (a conductive agent), styrene-butadiene rubber (SBR) (a binder), and sodium carboxymethylcellulose (CMC-Na) (a thickener) in a weight ratio of 96:1:2:1 in deionized water (a solvent), stirring to homogeneously mix the mixture, and then applying a concentration of 9.7 mg / cm³ to the negative electrode slurry. 2 The negative electrode current collector copper foil is uniformly coated with the specified coating density, and after drying, cold pressing, and cutting, a negative electrode sheet is obtained.

[0116] electrolyte In an argon gas-atmosphereed glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) / ethyl methyl carbonate (EMC), which are organic solvents, are uniformly mixed in a volume ratio of 3 / 7. 12.5% ​​by weight (by weight of the ethylene carbonate / ethyl methyl carbonate solvent) of LiPF6 is dissolved in the above organic solvent and uniformly stirred to obtain an electrolyte.

[0117] Separator A commercially available PP-PE copolymer microporous film with a thickness of 16 μm and an average pore size of 80 nm (from Zhuoga Electronics Technology Co., Ltd., model number 20) will be used.

[0118] secondary battery A positive electrode sheet, a separator, and a negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrode sheets to act as a separator. By winding the stack, a cylindrical bare battery cell is obtained with a height of 146 mm and a diameter of 44.8 mm at the top and bottom surfaces. The distance between the positive electrode sheet and the negative electrode sheet is 16 μm, and the distance between the separator and the positive and negative electrode sheets is approximately 0 μm. The bare battery cell is placed in an enclosure, 100 g of the electrolyte is injected, and it is sealed to obtain a secondary battery.

[0119] Example 2-11 Aside from differences in particle size, specific surface area, carbon content, and the amount of lithium iron phosphate particles used between the first and second lithium iron phosphate particles, all other conditions were the same as in Example 1. Details of the specific conditions are shown in Table 1.

[0120] Example 12 Example 12 was similar to Example 1 in other conditions, in which the first lithium iron phosphate particles were added to a stirring tank in a weight ratio of first lithium iron phosphate particles: polyvinylidene fluoride (PVDF) and acetylene black as a conductive agent of 97:2.2:0.8 to produce a slurry, after which 0.3g / 1540.25mm 2A first layer of active material is obtained by coating an aluminum foil with a surface density of 13 μm thickness with a surface density of , and then adding a second layer of lithium iron phosphate particles to a stirring tank in a weight ratio of 97:2.2:0.8 with polyvinylidene fluoride (PVDF) and acetylene black as a conductive agent, and after producing a slurry, 0.3 g / 1540.25 mm 2 The first layer of active material is applied to the upper surface of the active material layer with a surface density of [value missing].

[0121] The amounts of the first lithium iron phosphate particles and the second lithium iron phosphate particles used were the same as in Example 1, i.e., the total amount of the upper and lower layers was the same as in Example 1. The thickness of the first lithium iron phosphate particle layer in the obtained positive electrode sheet was 100 μm, and the thickness of the second lithium iron phosphate particle layer was 100 μm.

[0122] Example 13 Except for applying the second lithium iron phosphate particles as the first active material layer to the surface of the aluminum foil and applying the first lithium iron phosphate particle layer on top of the second lithium iron phosphate particle layer, all other conditions in Example 13 are the same as in Example 12.

[0123] Examples 14-16 Except for changing the amount of lithium iron phosphate particles used so that the thickness of the first lithium iron phosphate particle layer and the second lithium iron phosphate particle layer are 80 μm / 120 μm, 120 μm / 80 μm, and 140 μm / 60 μm, respectively, the other conditions for Examples 14-16 are the same as for Example 13, and the details of the specific conditions are shown in Table 1.

[0124] Comparative Examples 1-3 Aside from differences in particle size, specific surface area, carbon content, and the amount of lithium iron phosphate used for the first and second lithium iron phosphate particles, all other conditions for Comparative Examples 1-3 were the same as in Example 1. Details of the specific conditions are shown in Table 1.

[0125] Comparative Example 4 Aside from setting the thicknesses of the first lithium iron phosphate particle layer and the second lithium iron phosphate particle layer to 16 μm and 184 μm, respectively, all other conditions for Comparative Example 4 are the same as those for Example 13.

[0126] Testing methods for related parameters

[0127] 1. Testing with a scanning electron microscope A suitable amount of lithium iron phosphate particles, the target of measurement, is prepared as a sample, and the sample morphology is observed using a ZEISS Sigma 300 scanning electron microscope with reference to standard JY / T010-1996.

[0128] 2. Testing of volume-average particle size D50 Refer to the standard GB / T 19077-2016 / ISO 13320:2009 Particle Size Distribution Laser Diffraction Method. Test using a laser particle size analyzer (Malvern 3000, MasterSizer 3000), with a helium-neon red light source as the primary light source. Take a clean beaker, add 1 g of the sample to be measured, add 20 ml of deionized water (ensure the sample concentration has a light shielding rate of 8-12%), add one drop of surfactant to lower the surface tension of the water and aid in particle penetration, and vibrate with ultrasound at 53 kHz / 120 W for 5 min to ensure complete dispersion of the sample. Open the laser particle size analyzer, clean the optical path system, and then automatically test the background. Stir the sample solution after ultrasonic vibration to ensure uniform dispersion, and if necessary, place it in the sample pool and begin particle size measurement. The measurement results can be read from the instrument.

[0129] 3. BET Test For the test method, refer to standard GB / T19587-2004, "Measurement of Specific Surface Area of ​​Solids by Gas Adsorption BET Method."

[0130] 4. DCR test Remove the battery and test its charge and discharge. First, fully charge the battery at 25°C and let it stand for 30 minutes after full charge. After standing, discharge it at a discharge rate of 1C for 30 minutes to adjust to 50% SOC and let it stand for 10 minutes. Adjust the temperature of the insulated box to -10°C and let it stand for 120 minutes, then measure the initial discharge voltage. Next, discharge it at a discharge rate of 3C for 10 seconds, record the lowest voltage during discharge, let it stand for 10 minutes, and obtain the DCR by dividing the difference between the initial 3C discharge voltage and the lowest voltage during 3C discharge by the current value (discharge rate is 3C, current is 78A).

[0131] 5. Cycle performance test At 25°C, the secondary batteries manufactured in the examples and comparative examples are charged with a constant current of 1C (i.e., completely discharged to the theoretical capacity current value within 1 hour) to 3.65V. Then, they are charged with a constant voltage at 3.65V until the current drops to 0.05C, left to stand for 5 minutes, and then discharged with a constant current of 1C to 2.5V, left to stand for 30 minutes. This constitutes one charge-discharge cycle, and the battery capacity C0 at this time is recorded. Following this method, the battery is subjected to n charge-discharge cycles, and the battery capacity after n cycles is recorded as C1. The cycle capacity retention rate of the battery at 25°C = C1 / C0 × 100%. If the measured cycle capacity retention rate of the battery is 80%, the corresponding number of cycles n is recorded.

[0132] [Table 1]

[0133] As can be seen from the above results, the secondary battery manufactured in Example 1-16 of this application has a lower DCR and better cycle performance than the battery in Comparative Example 1-4. Furthermore, the kinetic performance and cycle performance of the secondary battery can be further improved by adjusting the amount and spatial distribution of the first lithium iron phosphate particles and the second lithium iron phosphate particles.

[0134] Furthermore, this application is not limited to the embodiments described above. The embodiments described above are merely examples, and any embodiment that has substantially the same configuration as the technical idea and produces similar effects within the scope of the technical solutions of this application is included within the scope of this application. Furthermore, within the scope of this application, various modifications of the embodiments that a person skilled in the art could conceive of, and other forms constructed by combining some of the components of the embodiments are also included within the scope of this application, as long as they do not deviate from the spirit of this application. [Explanation of Symbols]

[0135] 1 Battery pack 2 Upper box 3 Lower box 4 Battery Modules 5 Secondary battery 51 Housing 52 Electrode Assembly 53 Top cover assembly

Claims

1. A lithium iron phosphate positive electrode sheet comprising a positive electrode current collector and a positive electrode active material located on the positive electrode current collector, wherein the positive electrode active material is The volume-average particle size D50 is 60-300 nm, and the specific surface area is 15 m². ２ A first lithium iron phosphate particle larger than / g, The volume-average particle size D50 is greater than 800 nm, and the specific surface area is 10 m². ２ It contains a second lithium iron phosphate particle that is less than or equal to / g, Based on the total weight of the first lithium iron phosphate particles, the carbon content in the first lithium iron phosphate particles is 1.2–2.7%. Based on the total weight of the second lithium iron phosphate particles, the carbon content in the second lithium iron phosphate particles is 0.7–1.3%. Based on the total weight of the positive electrode film layer in the lithium iron phosphate positive electrode sheet, the content of the first lithium iron phosphate particles is 36-68%. Based on the total weight of the positive electrode film layer in the lithium iron phosphate positive electrode sheet, the content of the second lithium iron phosphate particles is 32-64%. A lithium iron phosphate positive electrode sheet, wherein the second lithium iron phosphate particle layer containing the second lithium iron phosphate particles is located on the surface of the positive electrode current collector, and the first lithium iron phosphate particle layer containing the first lithium iron phosphate particles is located on the surface of the second lithium iron phosphate particle layer facing the positive electrode current collector.

2. The compaction density of the positive electrode film layer in the lithium iron phosphate positive electrode sheet is 2–2.45 g / cm³. ３ The lithium iron phosphate cathode sheet according to claim 1.

3. The lithium iron phosphate cathode sheet according to claim 1, wherein the active material layer in the lithium iron phosphate cathode sheet contains the first lithium iron phosphate particles and the second lithium iron phosphate particles, which are distributed in a separated layer.

4. The thickness of the first lithium iron phosphate particle layer is 65-250 μm. The lithium iron phosphate cathode sheet according to claim 3, wherein the thickness of the second lithium iron phosphate particle layer is 60-250 μm.

5. The lithium iron phosphate cathode sheet according to claim 3, wherein the thickness ratio of the first lithium iron phosphate particle layer to the second lithium iron phosphate particle layer is 1:0.1-9.

6. The surface density of the first lithium iron phosphate particle layer is 0.1–0.3 g / 1540.25 mm ２ And, The surface density of the second lithium iron phosphate particle layer is 0.1–0.3 g / 1540.25 mm ２ A lithium iron phosphate cathode sheet according to any one of claims 3 to 5.

7. A secondary battery comprising a lithium iron phosphate cathode sheet as described in claim 1.

8. A battery module including the secondary battery described in claim 7.

9. A battery pack comprising the battery module described in claim 8.

10. An electrical device comprising at least one selected from the secondary battery described in claim 7, the battery module described in claim 8, or the battery pack described in claim 9.

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