Positive electrode material, positive electrode sheet, secondary battery, and electrical apparatus

Abstract

A positive electrode material, comprising first lithium iron phosphate particles and second lithium iron phosphate particles. The primary average particle size of the first lithium iron phosphate particles is 500 nm to 3000 nm. Calculated based on the total weight of the second lithium iron phosphate particles, the carbon content of the second lithium iron phosphate particles is 1.5 wt% to 2.0 wt%, and the mass of the second lithium iron phosphate particles is greater than or equal to the mass of the first lithium iron phosphate particles. A battery prepared using said positive electrode material can have both good energy density and good cycle performance.

Description

Positive electrode material, positive electrode sheet, secondary battery and electrical device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202410177919.1, filed on February 8, 2024, entitled “Positive Electrode Material, Positive Electrode Sheet, Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

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

[0004] In recent years, the application of lithium-ion batteries has become increasingly widespread. They are widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. As lithium-ion batteries have achieved significant development, higher requirements have been placed on their energy density, cycle performance, and safety performance.

[0005] As a lithium-ion battery cathode material, lithium iron phosphate (LiFePO4) has rapidly become a global research hotspot due to its abundant resources, low price, environmental friendliness, and stable voltage in a two-phase reaction. Because LiFePO4's specific capacity is lower than that of ternary materials, recent research has focused on improving its capacity. However, focusing solely on improving LiFePO4's capacity inevitably compromises other battery properties, such as cycling performance.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode material, the battery prepared by the positive electrode material can have both good energy density and cycle performance.

[0008] In order to achieve the above-mentioned purpose, the first aspect of the present application provides a positive electrode material, comprising first lithium iron phosphate salt particles and second lithium iron phosphate salt particles, the primary average particle size of the first lithium iron phosphate salt particles is 500nm~3000nm, and the carbon content of the second lithium iron phosphate salt particles is 1.5wt%-2.0wt% calculated based on the total weight of the second lithium iron phosphate salt particles, and the mass of the second lithium iron phosphate salt particles is greater than or equal to the mass of the first lithium iron phosphate salt particles.

[0009] The cathode material provided in this application utilizes a combination of large-particle lithium iron phosphate (first lithium iron phosphate salt particles) and small-particle lithium iron phosphate (second lithium iron phosphate salt particles), effectively increasing the electrode compaction density and, in turn, the battery cell energy density. Furthermore, large-particle lithium iron phosphate offers excellent kinetics, high specific capacity, and good crystallinity, resulting in excellent cycling performance. Small-particle lithium iron phosphate also exhibits excellent cycling performance. Therefore, the combination of large-particle lithium iron phosphate and small-particle lithium iron phosphate allows the battery cell to achieve both high energy density and a longer cycle life.

[0010] In the positive electrode material provided in this application, the small-particle lithium iron phosphate used maintains a high carbon content, which can not only improve the electronic conductivity of the material, but also help to hide part of the initial capacity of the positive electrode material, thereby enabling the battery to have a longer cycle life.

[0011] In the positive electrode material provided in the present application, the mass of the small-particle lithium iron phosphate is kept greater than or equal to the mass of the large-particle lithium iron phosphate. That is, by mixing a small amount or an equal amount of large-particle lithium iron phosphate into the long-cycle small-particle lithium iron phosphate, the technical problem that energy density and long cycle cannot be taken into account at the same time can be effectively improved, and the long cycle performance of the battery can be maintained while improving its energy density.

[0012] In any embodiment, the mass ratio of the second lithium iron phosphate salt particles to the first lithium iron phosphate salt particles is 1:1 to 8:1.

[0013] When the mass ratio of large-particle lithium iron phosphate to small-particle lithium iron phosphate is controlled within the range of 1:1 to 8:1, the compaction density of the positive electrode sheet can be further improved, thereby further improving the energy density and cycle performance of the battery.

[0014] In any embodiment, the primary average particle size of the second lithium iron phosphate salt particles is 50 nm to 400 nm.

[0015] In any embodiment, the primary average particle size of the first lithium iron phosphate salt particles is 650 nm to 920 nm.

[0016] In any embodiment, the carbon content of the first lithium iron phosphate salt particles is 0.8 wt % to 1.5 wt %, calculated based on the total weight of the first lithium iron phosphate salt particles.

[0017] When the carbon content of large-particle lithium iron phosphate is controlled to be lower than that of small-particle lithium iron phosphate, a lower specific surface area and higher capacity can be maintained, thereby further improving the energy density of the battery; at the same time, large-particle lithium iron phosphate can also hide part of the initial capacity of the positive electrode active material, thereby further improving the long cycle life of the battery.

[0018] In any embodiment, the second lithium iron phosphate salt particle includes a core and a carbon coating layer on at least a portion of a surface of the core.

[0019] In any embodiment, the carbon coating layer includes a first carbon coating layer and a second carbon coating layer, and the first carbon coating layer is located between the inner core and the second carbon coating layer.

[0020] After the surface of lithium iron phosphate is coated with a high amount of carbon, the conductivity of the material can be further improved, thereby further enhancing the dynamic performance.

[0021] In any embodiment, the BET specific surface area of ​​the first lithium iron phosphate particles is 3 m 2 / g~8m 2 / g; and / or, the BET specific surface area of ​​the second lithium iron phosphate particles is less than or equal to 18.5 m 2 / g.

[0022] When the BET specific surface area of ​​large-particle lithium iron phosphate and small-particle lithium iron phosphate is controlled within the above range, it is beneficial to increase the contact area between the positive electrode material and the electrolyte, so that the electrochemical activity of the positive electrode material is further enhanced.

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

[0024] In any embodiment, Q includes at least one of Ti, V, Mg, and Nb, and the content of Q in the first lithium iron phosphate salt particles is 4000ppm-10000ppm calculated based on the total weight of the first lithium iron phosphate salt particles; and / or, the content of Q in the second lithium iron phosphate salt particles is 1000ppm-3000ppm calculated based on the total weight of the second lithium iron phosphate salt particles.

[0025] When the content of the Q element in the large-particle lithium iron phosphate and the small-particle lithium iron phosphate is controlled within the above range, the gram capacity of the positive electrode material can be further increased, so that the prepared battery has a better energy density.

[0026] In any embodiment, the positive electrode material is a single crystal particle and / or a polycrystalline particle.

[0027] In any embodiment, the capacity proportion η of the first lithium iron phosphate salt particles is ≥88%, and η is defined as: a battery containing the first lithium iron phosphate salt particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted as C1, and the capacity value at the discharge voltage of 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50μA.

[0028] When the capacity ratio of the first lithium iron phosphate salt particles has the above characteristics, the energy density and cycle performance of the battery can be further improved.

[0029] The second aspect of the present application provides a positive electrode plate, characterized in that it includes a positive electrode collector and a positive electrode material arranged on at least one surface of the positive electrode collector, wherein the positive electrode material is the positive electrode material of the first aspect of the present application.

[0030] In any embodiment, the surface density of the positive electrode active material on one side of the positive electrode sheet is 20.0 mg / cm 2 -35.0mg / cm 2 .

[0031] By coating the positive electrode active material thicker on the surface of the positive electrode sheet, the battery cell can achieve a higher volume energy density; at the same time, when the surface density of the positive electrode active material on the positive electrode sheet is controlled within a certain range, the battery energy density and cycle performance can be improved.

[0032] In any embodiment, the compaction density of the positive electrode active material layer on the positive electrode sheet is 2.30 g / cm 3 -2.70g / cm 3 .

[0033] When the electrode compaction density is controlled within a certain range, the battery can maintain a higher energy density.

[0034] The third aspect of the present application provides a secondary battery, characterized by comprising the positive electrode sheet of the third aspect of the present application.

[0035] A fourth aspect of the present application provides an electrical device comprising the secondary battery of the fourth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0037] FIG. 2 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 1 .

[0038] FIG3 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0039] Description of reference numerals: 5 secondary battery; 51 housing; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0040] Below, embodiments of the positive electrode material, the positive electrode sheet containing the positive electrode material, the secondary battery, the electric device and the manufacturing method thereof of the present application are described in detail. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0041] The "ranges" disclosed herein are defined in terms of lower and upper limits, where a given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise indicated, the numerical range "a to b" is a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0042] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0043] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0044] Unless otherwise specified, all steps of the present application may be performed sequentially, randomly, or optionally sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0045] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0046] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0047] Due to its structural characteristics, lithium iron phosphate (LiFePO4) has low electronic and ionic conductivity in its bulk phase, necessitating carbon coating and particle nanocrystallization to improve both electrical and ionic conductivity. However, nanocrystallization and surface carbon coating result in a low compaction density of the LiFePO4 material, which in turn affects its energy density. As market demands increase the range and cost of LiFePO4 vehicles, the demand for high-energy-density, long-cycle power batteries is growing.

[0048] [Cathode material]

[0049] In order to achieve the above-mentioned objectives, the present application provides a positive electrode material comprising first lithium iron phosphate salt particles and second lithium iron phosphate salt particles.

[0050] In some embodiments, the first lithium iron phosphate salt particles have an average primary particle size of 500 nanometers (nm) to 3000 nm.

[0051] In some embodiments, the primary average particle size of the first lithium iron phosphate salt particles is 650 nm to 920 nm, such as 650 nm, 870 nm, 920 nm, etc., or other unlisted values ​​within the range of 650 nm to 920 nm.

[0052] In some embodiments, the primary average particle size of the second lithium iron phosphate salt particles is 50 nm to 400 nm.

[0053] In some embodiments, the primary average particle size of the second lithium iron phosphate salt particles is 150 nm to 250 nm, such as 150 nm, 250 nm, etc., or other unlisted values ​​within the range of 150 nm to 250 nm.

[0054] The cathode material provided herein utilizes a combination of large-particle lithium iron phosphate (first lithium iron phosphate salt particles) and small-particle lithium iron phosphate (second lithium iron phosphate salt particles), effectively increasing the electrode compaction density and, consequently, the battery cell energy density. Furthermore, large-particle lithium iron phosphate exhibits excellent kinetics, high specific capacity, and good crystallinity, resulting in superior cycling performance. Small-particle lithium iron phosphate also exhibits excellent cycling performance. Therefore, the combination of large-particle lithium iron phosphate and small-particle lithium iron phosphate enables the battery cell to achieve both high energy density and a longer cycle life.

[0055] In some embodiments, the mass of the second lithium iron phosphate salt particles is greater than or equal to the mass of the first lithium iron phosphate salt particles.

[0056] In the positive electrode material provided in the present application, the mass of the small-particle lithium iron phosphate is kept greater than or equal to the mass of the large-particle lithium iron phosphate. That is, by mixing a small amount or an equal amount of large-particle lithium iron phosphate into the long-cycle small-particle lithium iron phosphate, the technical problem that energy density and long cycle cannot be taken into account at the same time can be effectively improved, and the long cycle performance of the battery can be maintained while improving its energy density.

[0057] In some embodiments, the mass ratio of the second lithium iron phosphate salt particles to the first lithium iron phosphate salt particles is 1:1 to 8:1, for example, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, 8:1, etc., or other values ​​not listed in the range of 1:1 to 8:1. In some embodiments, the mass ratio of the second lithium iron phosphate salt particles to the first lithium iron phosphate salt particles is 1:1, 3:1, 4:1, 7:1, 8:1, or other values ​​not listed in the range of 1:1 to 8:1.

[0058] When the mass ratio of large-particle lithium iron phosphate to small-particle lithium iron phosphate is controlled within the range of 1:1 to 8:1, the compaction density of the positive electrode sheet can be further improved, thereby further improving the energy density and cycle performance of the battery.

[0059] In some embodiments, the carbon content of the second lithium iron phosphate salt particles is 1.5 wt % to 2.0 wt %, calculated based on the total weight of the second lithium iron phosphate salt particles.

[0060] In some embodiments, the carbon content of the second lithium iron phosphate salt particles is 1.55 wt%-1.95 wt% calculated based on the total weight of the second lithium iron phosphate salt particles, and in some embodiments, it is 1.55 wt%, 1.70 wt%, 1.85 wt%, 1.95 wt%, etc., or other unlisted values ​​within the range of 1.55 wt%-1.95 wt%.

[0061] In the positive electrode material provided in this application, the small-particle lithium iron phosphate used maintains a high carbon content, which can not only improve the electronic conductivity of the material, but also help to hide part of the initial capacity of the positive electrode material, thereby enabling the battery to have a longer cycle life.

[0062] In some embodiments, the carbon content of the first lithium iron phosphate salt particles is 0.8 wt%-1.5 wt% calculated based on the total weight of the first lithium iron phosphate salt particles, and in some embodiments, it is 0.8 wt%, 1.2 wt%, 1.35 wt%, 1.5 wt%, etc., or other unlisted values ​​within the range of 0.8 wt%-1.5 wt%.

[0063] When the carbon content of large-particle lithium iron phosphate is controlled to be lower than that of small-particle lithium iron phosphate, a lower specific surface area and higher capacity can be maintained, thereby further improving the energy density of the battery; at the same time, large-particle lithium iron phosphate can also hide part of the initial capacity of the positive electrode active material, thereby further improving the long cycle life of the battery.

[0064] As used in this article, “carbon content” is determined using a carbon-sulfur analyzer in accordance with GB / T 20123-2006 / ISO 15350:2000.

[0065] As used herein, "carbon content" refers to the presence of carbon in the lithium iron phosphate particles, which may be mixed with the lithium iron phosphate particles or coated on the lithium iron phosphate particles. In some embodiments, "carbon content" refers to the presence of carbon in the lithium iron phosphate particles coated on the lithium iron phosphate particles, thereby enabling the carbon to form a uniform and dense carbon coating on the surface of the lithium iron phosphate particles, thereby improving the surface conductivity of the particles.

[0066] In some embodiments, the second lithium iron phosphate salt particle includes a core and a carbon coating layer on at least a portion of a surface of the core.

[0067] In some embodiments, the carbon coating layer includes a first carbon coating layer and a second carbon coating layer, wherein the first carbon coating layer is located between the inner core and the second carbon coating layer.

[0068] As used herein, "carbon coating layer" refers to the portion coated on the lithium iron phosphate particles, which may but does not necessarily completely coat the lithium iron phosphate particles. The use of "carbon coating layer" is only for ease of description and is not intended to limit this application.

[0069] After the surface of lithium iron phosphate is coated with a high amount of carbon, the conductivity of the material can be further improved, thereby further enhancing the dynamic performance.

[0070] In some embodiments, the BET specific surface area of ​​the first lithium iron phosphate particles is 3 m2 / g (m 2 / g)~8m 2 / g.

[0071] In some embodiments, the BET specific surface area of ​​the first lithium iron phosphate particles is 3 m 2 / g~7.5m 2 / g, in some embodiments 3m 2 / g、6m 2 / g、7m 2 / g, 7.5m 2 / g, etc., or 3m 2 / g~7.5m 2 / Other values ​​not listed in the g range.

[0072] In some embodiments, the BET specific surface area of ​​the second lithium iron phosphate particles is less than or equal to 18.5 m 2 / g.

[0073] In some embodiments, the BET specific surface area of ​​the second lithium iron phosphate particles is less than or equal to 13-18 m 2 / g, in some embodiments 13m 2 / g、15m 2 / g、16m 2 / g、18m 2 / g, etc., or 13-18m 2 / Other values ​​not listed in the g range.

[0074] As used herein, "specific surface area" or "BET" refers to the total area per unit mass of a material.

[0075] When the BET specific surface area of ​​large-particle lithium iron phosphate and small-particle lithium iron phosphate is controlled within the above range, it is beneficial to increase the contact area between the positive electrode material and the electrolyte, so that the electrochemical activity of the positive electrode material is further enhanced.

[0076] In some embodiments, the molecular formulas of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles are independently Li m Fex P y O j Q q , wherein Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0077] In this document, when a substance has a "molecular formula," the substance is not limited to the substance represented by the molecular formula but also includes other substances formed by further doping, modification, and / or coating based on the molecular formula, without limitation. The use of "molecular formula" is for ease of description only and is not intended to limit this application.

[0078] In some embodiments, the Q includes at least one of Ti, V, Mg, and Nb.

[0079] In some embodiments, Q comprises Ti.

[0080] In some embodiments, the content of Q in the first lithium iron phosphate salt particles is 4000 parts per million (ppm) to 10000 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles.

[0081] In some embodiments, the content of Q in the first lithium iron phosphate salt particles is 5000ppm-6000ppm, calculated based on the total weight of the first lithium iron phosphate salt particles, and in some embodiments is 5000ppm, 6000ppm, or other unlisted values ​​within the range of 5000ppm-6000ppm.

[0082] In some embodiments, the content of Q in the second lithium iron phosphate salt particles is 1000 ppm-3000 ppm, calculated based on the total weight of the second lithium iron phosphate salt particles.

[0083] In some embodiments, the content of Q in the second lithium iron phosphate salt particles is 1500ppm-2100ppm, calculated based on the total weight of the second lithium iron phosphate salt particles, and in some embodiments is 1500ppm, 2100ppm, or other unlisted values ​​within the range of 1500ppm-2100ppm.

[0084] When the content of the Q element in the large-particle lithium iron phosphate and the small-particle lithium iron phosphate is controlled within the above range, the gram capacity of the positive electrode material can be further increased, so that the prepared battery has a better energy density.

[0085] In some embodiments, the positive electrode material is single crystal particles and polycrystalline particles.

[0086] In some embodiments, the cathode material is a single crystal particle or a polycrystalline particle. In some embodiments, the cathode material is a single crystal particle. In some embodiments, the cathode material is a polycrystalline particle.

[0087] In some embodiments, the capacity ratio η of the first lithium iron phosphate salt particles is ≥88%.

[0088] In some embodiments, the capacity proportion η of the first lithium iron phosphate salt particles is ≥89.2%, and in some embodiments, it is 89.2%, 91.3%, 92.1%, 92.2%, etc., or other unlisted values ​​within the range of ≥89.2%.

[0089] The η is defined as: a battery containing the first lithium iron phosphate salt particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1 coulomb (C) in the voltage range of 2.0 volts (V) to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C1, and the capacity value at the discharge voltage to 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cutoff current of 50 microamperes (μA).

[0090] In some embodiments, the X-ray diffraction pattern of the second lithium iron phosphate salt particles tested in a fully charged state satisfies: a (311) crystal plane peak in the range of 35 degrees (°)-36°, a (011) crystal plane peak in the range of 20°-21°, and a ratio of the peak intensity I311 of the (311) crystal plane peak to the peak intensity I011 of the (011) crystal plane peak satisfies I311 / I011≥0.008.

[0091] When the capacity ratio of large-particle lithium iron phosphate has the above characteristics, or when the X-ray diffraction spectrum of small-particle lithium iron phosphate has the above characteristics, the energy density and cycle performance of the battery can be further improved.

[0092] The present application also provides a method for preparing a positive electrode material, characterized in that the positive electrode material contains first lithium iron phosphate salt particles, and the manufacturing process of the first 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 operations.

[0093] [Positive electrode]

[0094] The present application also provides a positive electrode plate, characterized in that it includes a positive electrode current collector and a positive electrode material arranged on at least one surface of the positive electrode current collector, wherein the positive electrode active material is the positive electrode material described in the present application, or the positive electrode material obtained according to the preparation method described in the present application.

[0095] In some embodiments, the surface density of the positive active material on one side of the positive electrode sheet is 20.0 mg / cm2. 2 )-35.0mg / cm 2 , in some embodiments 22.0 mg / cm 2 , 25.0mg / cm 2 , 27.0mg / cm 2 、35.0mg / cm 2 etc., or 20.0 mg / cm 2 -35.0mg / cm 2 Other values ​​not listed within the range.

[0096] By coating the positive electrode active material thicker on the surface of the positive electrode sheet, the battery cell can achieve a higher volume energy density; at the same time, when the surface density of the positive electrode active material on the positive electrode sheet is controlled within a certain range, the battery energy density and cycle performance can be improved.

[0097] In some embodiments, the compaction density of the positive electrode active material layer on the positive electrode sheet is 2.30 g / cm 3 -2.70g / cm 3 .

[0098] In some embodiments, the compaction density of the positive electrode active material layer on the positive electrode sheet is 2.30 g / cm 3 -2.68g / cm 3 , in some embodiments 2.30 g / cm 3 , 2.38g / cm 3 , 2.41g / cm 3 , 2.44g / cm 3 , 2.50g / cm 3 , 2.51g / cm 3 , 2.53g / cm 3 , 2.56g / cm 3 , 2.58g / cm 3 , 2.61g / cm 3 , 2.68g / cm 3 etc., or 2.30g / cm 3 -2.68g / cm 3 Other values ​​not listed within the range.

[0099] As used herein, "compacted density" is defined as: compacted density = surface density / (thickness of the electrode after rolling - thickness of the current collector). The determination method can refer to GB / T24533-2009.

[0100] When the electrode compaction density is controlled within a certain range, the battery can maintain a higher energy density.

[0101] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0102] In some embodiments, the positive electrode plate may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0103] In some embodiments, the positive electrode plate may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0104] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0105] [Negative electrode]

[0106] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0107] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0108] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0109] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0110] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0111] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0112] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0113] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0114] [Electrolytes]

[0115] The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. This application has no specific restrictions on the type of electrolyte, and it can be selected according to needs.

[0116] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0117] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0118] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0119] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0120] [Isolation film]

[0121] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0122] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0123] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0124] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0125] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0126] [Lithium secondary battery]

[0127] The present application also provides a lithium secondary battery, characterized in that it includes the positive electrode sheet described in the present application.

[0128] The present application has no particular limitation on the shape of the lithium secondary battery, which may be cylindrical, square, or any other shape. In some embodiments, FIG1 shows a secondary battery 5 with a square structure as an example.

[0129] In some embodiments, referring to Figure 2, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0130] [Electrical devices]

[0131] In addition, the present application also provides an electrical device, which includes the lithium secondary battery provided in the present application. The lithium secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0132] As the power-consuming device, a lithium secondary battery can be selected according to its usage requirements.

[0133] Figure 3 shows an example of an electric device. This device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of lithium secondary batteries, a battery pack or battery module can be used.

[0134] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a lithium secondary battery as a power source.

[0135] Example

[0136] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0137] 1. Preparation method

[0138] Example 1

[0139] (1) Preparation of positive electrode active material:

[0140] a) Preparation of the first lithium phosphate particles:

[0141] Lithium carbonate, ferric oxide, phosphoric acid, glucose, titanium dioxide (the amount of titanium dioxide added, based on the total weight of the lithium iron phosphate particles, is sufficient to ensure a titanium content of 5000 ppm in the prepared lithium iron phosphate particles), and polyaniline are weighed separately. The weight ratios of Li, Fe, and P satisfy the following: Fe:P = 0.968:1; Li:Fe = 1:0.98. The weight ratio of glucose to polyaniline satisfies the following: glucose:polyaniline = 1:2. The amount of glucose and polyaniline added is sufficient to ensure that the carbon content of the lithium iron phosphate precursor after the first sintering step is 0.15%. Water is added to the above substances to form a mixture slurry.

[0142] The mixture was mixed in a ball mill and ground in a sand mill to a slurry with a solids content of 38% and a Dv50 of 400 nm. The mixture was then spray-dried (the negative pressure of the high-speed spray dryer was -650 to -200 Pa, the inlet temperature was 300 to 360°C, and the outlet temperature was 100 to 140°C). The dried reactants were placed in a sintering furnace for the first sintering step. The heating rate was controlled at 5°C / min, the holding temperature was controlled at 650°C, and the holding time was 4 hours. After cooling, the material was pulverized using a mechanical mill to obtain a powder.

[0143] The resulting powder, glucose, polyaniline, and water were mixed (based on the total weight of the lithium iron phosphate particles, the amount of glucose and polyaniline added was such that the carbon content of the product after the second sintering was 1.2%, and the weight ratio of glucose to polyaniline was 1:2) to achieve a solids content of 40%. After uniform mixing, the mixture was processed using a ball mill and a sand mill to obtain a slurry with a Dv50 value of insoluble matter of 550 nm. The slurry was then spray-dried (using a high-speed spray dryer with a negative pressure of -650 to -200 Pa, an inlet temperature of 300°C to 360°C, and an outlet temperature of 100°C to 140°C). The dried reactants were then placed in a sintering furnace for a second low-temperature sintering (heating rate controlled at 5°C / min, sintering temperature of 750°C, and sintering time of 4 hours). After the material is cooled, it is crushed for the second time to an average particle size of 870 nm. After demagnetization, lithium iron phosphate salt particles are obtained, with a carbon content of 1.2% and a Ti content of 5000 ppm in the lithium iron phosphate salt particles.

[0144] b) Preparation of the second lithium phosphate particles:

[0145] FePO4, Li2CO3, the first carbon source glucose and titanium dioxide are weighed respectively (based on the total weight of the lithium iron phosphate particles, the amount of titanium dioxide added satisfies the titanium content in the prepared lithium iron phosphate particles to be 1500ppm), wherein the weight ratios of Li element, Fe element and P element satisfy: Fe:P=0.965:1; Li:Fe=1:0.98, the amount of the first carbon source added is based on the total weight of the prepared positive electrode active material, the weight content of the residual carbon of the first carbon source (i.e., the weight content w1% of the first carbon coating layer) is 1.55%, and mixed evenly, a small amount of water is added thereto, and the mixture is ground to a volume distribution particle size Dv50 of 360nm and then spray-dried; the spray-dried powder is placed in a sintering furnace, heated to 770℃ at 5℃ / min under a nitrogen atmosphere and kept at this temperature for 10h, and then cooled to obtain a lithium phosphate coated with the first carbon coating layer.

[0146] Take 8 kilograms (kg) of lithium-containing phosphate coated with the first carbon coating layer (denoted as m) and place it in a rotary kiln, introduce nitrogen at a flow rate of 4 liters (L) / minute and throughout the heating process, insulation process and cooling process. Heat to 750°C at 5°C / minute and keep warm for 25 minutes. During the insulation process, acetylene gas (deposition gas) is introduced with a gas flow rate V of 0.40L / minute. After the insulation is completed, stop introducing acetylene gas, then cool naturally, and crush by air flow to obtain LiFePO4 coated with two layers of carbon coating. Based on the total weight of the prepared positive electrode active material, the weight content w2% of the second carbon coating layer is 0.15%. Finally, second lithium phosphate particles with an average particle size of 250nm are obtained.

[0147] (2) Preparation of positive electrode slurry:

[0148] The first lithium phosphate salt particles and the second lithium phosphate salt particles are mixed in a mass ratio of 3:1 to obtain a positive electrode active material. The mixed positive electrode active material, conductive agent conductive carbon black, and binder polyvinylidene fluoride are mixed in a mass percentage of 96.0:2.0:2.0 and an appropriate amount of N-methylpyrrolidone is added. The mixture is fully mixed, stirred, and dispersed to form a positive electrode slurry.

[0149] (3) Preparation of positive electrode sheet:

[0150] The viscosity of the mixed and stirred slurry is adjusted to 8000-20000 mPa.s until the slurry does not delaminate. The slurry is coated on the surface of the substrate Al foil through a double-sided double-control coating device, and then dried, cold pressed, slit, and sheeted to finally obtain the positive electrode sheet.

[0151] (4) Preparation of negative electrode sheet:

[0152] Artificial graphite, conductive carbon black, styrene-butadiene rubber (SBR), and carboxymethyl cellulose sodium (CMC) are mixed uniformly in a mass percentage of 95:1.0:2.0:2.0, and a certain amount of deionized water is added. After stirring and dispersion, a negative electrode slurry is obtained. The negative electrode slurry is coated on a Cu foil substrate, and after drying, cold pressing, slitting, and sheeting, a negative electrode sheet is obtained.

[0153] (4) Preparation of batteries:

[0154] The positive electrode sheet, isolation film, and negative electrode sheet are stacked in order. The isolation film must be able to isolate the anode and cathode. The bare battery cell is obtained by winding, and the bare battery cell is placed in the outer packaging. The electrolyte is injected and the lithium-ion battery is finally obtained after the packaging, formation, exhaust and other processes.

[0155] Examples 2 to 12

[0156] The secondary batteries of Examples 2 to 12 are prepared in a similar manner to that of Example 1, except that the carbon content, Ti content, primary average particle size, molecular formula, BET specific surface area, and capacity ratio η of the first lithium iron phosphate salt particles, or the carbon content, Ti content, primary average particle size, molecular formula, and BET specific surface area of ​​the second lithium iron phosphate particles are adjusted, or the mass ratio of the first lithium iron phosphate salt particles to the second lithium iron phosphate salt particles is adjusted, or the surface density and compaction density of the positive electrode active material on the positive electrode sheet are adjusted. The different preparation parameters are detailed in Tables 1, 2, and 3.

[0157] Comparative Examples 1 to 3

[0158] The secondary batteries of Comparative Examples 1 to 3 are prepared in a similar manner to that of Example 1, except that the primary average particle size of the first lithium iron phosphate salt particles is adjusted, or the carbon content of the second lithium iron phosphate salt particles is adjusted, or the mass ratio of the second lithium iron phosphate salt particles to the first lithium iron phosphate salt particles is adjusted. The different preparation parameters are detailed in Tables 1, 2, and 3.

[0159] 2. Performance Testing

[0160] 1. Test methods for parameters related to positive electrode active materials

[0161] (1) Primary average particle size test method

[0162] The electrode is cut open perpendicularly to the large surface of the electrode using an argon ion beam to expose the cross section, which is photographed using a scanning electron microscope. The longest diameter of the lithium iron phosphate particles is statistically analyzed using the length diameter statistical method. The "primary average particle size" refers to the average value of the primary particle size of all particles, which is numerically equal to the total particle size value divided by the total number of particles. The primary particle size in the cross-section diagram refers to the longest distance between two points along the edge. Specifically, the total number of lithium iron phosphate particles with a primary particle size greater than 50 nm and the sum of the primary particle sizes of lithium iron phosphate particles with a primary particle size greater than 50 nm can be counted in the electron microscope scanning photograph. The primary average particle size of the lithium iron phosphate particles = the primary particle size of the total lithium iron phosphate particles / the total number of lithium iron phosphate 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.

[0163] (2) Carbon content test method

[0164] Lithium iron phosphate is burned in a high-frequency induction furnace and then the carbon content is tested using the infrared absorption method. The specific testing process is based on the standard GB / T 20123-2006 / ISO 15350:2000.

[0165] (3) Specific surface area test method

[0166] The specific surface area was tested by gas adsorption method according to the GB / T19587-2017 test standard, as follows: lithium iron phosphate granular salt was taken as a sample, the sample tube was immersed in liquid nitrogen at -196°C, and the adsorption amount of nitrogen on the solid surface at different pressures was measured at a relative pressure of 0.05-0.30. The single-molecule adsorption amount of the sample was obtained based on the BET multilayer adsorption theory and its formula, thereby calculating the specific surface area of ​​the material.

[0167] (4) Capacity ratio of the first lithium iron phosphate salt particle η

[0168] The battery preparation and testing process is as follows: 2.0000g of sample was mixed with 0.1111g of conductive carbon black and 0.1111g of PVDF (at a mass ratio of 0.9:0.05:0.05), followed by the addition of 2.5g of the organic solvent NMP (N-methylpyrrolidone). After thorough mixing, the mixture was coated onto aluminum foil to form a 140-micron-thick film. The film was then dried under vacuum at 120°C for 2 hours. The film was punched into 13mm discs using a hole punch. The discs were pressed using a tablet press at 10 MPa and kept at 120°C for 12 hours. The positive electrode was weighed, and the active material loading was 11-12mg. Coin-type cells were assembled in an argon-protected glove box. A lithium metal sheet served as the negative electrode, and the electrolyte consisted of a 1:1 (volume) mixture of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate), containing LiPF6 as the electrolyte. The separator was a Celgard 2400 microporous polyethylene membrane. The assembled batteries were tested on a blue battery tester. Within the 2.0V to 3.75V voltage range, the specific capacity was measured by charging / discharging at a constant current of 0.1C for two weeks, followed by a constant current of 1C for two weeks. The charging process was performed with a constant voltage of 3.75V and a cutoff current of 50uA.

[0169] A battery containing the lithium iron phosphate salt particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C1, and the capacity value at the discharge voltage to 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50uA.

[0170] 2. Test method for parameters related to positive electrode

[0171] (1) Test method for compaction density

[0172] The compacted density of the film layer on one side of the electrode = m / (V1-V2), where m represents the weight of the film layer, V1 represents the volume of the electrode, and V2 represents the volume of the current collector. m is obtained by subtracting the weight of the current collector from the weight of the electrode. The product of the surface area of ​​the electrode and the thickness of the electrode is the volume V1 of the electrode, and the product of the surface area of ​​the electrode and the thickness of the current collector is V2. The thickness of the current collector and the thickness of the electrode are obtained by measuring the thickness of the empty foil in the tab area with a micrometer.

[0173] (2) Surface density test method

[0174] The formula for calculating the surface density is (electrode weight - electrode aluminum foil weight) / electrode area. Specifically, you can first weigh the mass and area of ​​the aluminum foil, then weigh the mass and area of ​​the electrode. The calculated surface density of the electrode minus the surface density of the foil is the surface density of the dressing.

[0175] 3. Battery performance test method

[0176] (1) Cycle capacity retention rate

[0177] At 25±3℃, the battery cells were charged and discharged at constant power (0.5P / 0.5P, 2.5-3.65V), and the discharge capacity D at the 20th week was taken. 20 and 1000-cycle discharge capacity D 1000 The capacity retention rate can be obtained by comparing it with the discharge capacity D1 in the first week, that is: 20-week capacity retention rate = D 20 / D1*100%, 1000-cycle capacity retention rate = D 1000 / D1*100%.

[0178] (2) Energy density

[0179] At 25±3℃, the 0.5P discharge capacity (3.65-2.5V) of the battery cell was tested. The energy density (Wh / L) of the battery cell can be obtained according to the formula: Energy density = capacity * average voltage / battery cell volume.

[0180] 3. Analysis of test results of various embodiments and comparative examples

[0181] Batteries of various embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 3 below.

[0182] Table 1 Preparation parameters

[0183] Table 2 Preparation parameters

[0184] Table 3 Preparation parameters and performance parameters

[0185] The positive electrode materials of Examples 1 to 12 all use first lithium iron phosphate salt particles and second lithium iron phosphate salt particles, wherein the primary average particle size of the first lithium iron phosphate salt particles is 500nm to 3000nm, the carbon content of the second lithium iron phosphate salt particles is 1.5 weight% to 2.0 weight%, and the mass of the second lithium iron phosphate salt particles is greater than or equal to the mass of the first lithium iron phosphate salt particles. Therefore, the secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0186] In the positive electrode material provided in the present application, first lithium iron phosphate salt particles and second lithium iron phosphate salt particles are used at the same time, which is beneficial to hiding part of the initial capacity of the positive electrode material. Therefore, its capacity retention rate after 20 cycles at 25°C can reach more than 100%, thereby making the battery have a longer cycle life.

[0187] The mass of the second lithium iron phosphate salt particles of Comparative Example 1 is less than that of the first lithium iron phosphate salt particles.

[0188] From the comparison between Comparative Example 1 and Examples 1 to 12, it can be seen that the mass of the second lithium iron phosphate salt particles in the positive electrode material needs to be greater than or equal to the mass of the first lithium iron phosphate salt particles in order to effectively improve the energy density and cycle performance of the battery; and when the mass of the second lithium iron phosphate salt particles is less than the mass of the first lithium iron phosphate salt particles, it is not possible to effectively take into account both the improvement of the energy density and the cycle performance of the secondary battery.

[0189] In Comparative Example 2, the carbon content of the second lithium iron phosphate salt particles is the same as that of the first lithium iron phosphate salt particles, that is, the carbon content of the second lithium iron phosphate salt particles exceeds the range of 1.5 wt % to 2.0 wt %.

[0190] From the comparison between Comparative Example 2 and Examples 1 to 12, it can be seen that the carbon content of the second lithium iron phosphate salt particles in the positive electrode material needs to be controlled within the range of 1.5 wt%-2.0 wt% in order to effectively improve the energy density and cycle performance of the battery; and when the carbon content of the second lithium iron phosphate salt particles exceeds the range of 1.5 wt%-2.0 wt%, it is not possible to effectively take into account both the improvement of the energy density and the cycle performance of the secondary battery.

[0191] The primary average particle size of the first lithium iron phosphate particles of Comparative Example 3 exceeds the range of 500 nm to 3000 nm.

[0192] From the comparison between Comparative Example 3 and Examples 1 to 12, it can be seen that the primary average particle size of the first lithium iron phosphate salt particles in the positive electrode material needs to be controlled within the range of 500nm to 3000nm in order to effectively improve the energy density and cycle performance of the battery; and when the primary average particle size of the first lithium iron phosphate salt particles exceeds the range of 500nm to 3000nm, it is not possible to effectively take into account both the improvement of the energy density and the cycle performance of the secondary battery.

[0193] The mass ratio of the second lithium iron phosphate salt particles to the first lithium iron phosphate salt particles used in the positive electrode materials of Examples 1 to 12 is 1:1 to 8:1, and the secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0194] The primary average particle size of the second lithium iron phosphate salt particles used in the positive electrode materials of Examples 1 to 12 is 50 nm to 400 nm, and the secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0195] The manufacturing process of the first lithium iron phosphate salt particles in the positive electrode materials of Examples 1 to 12 includes: providing raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a carbon film-forming agent, and a modifier, and performing at least two sinterings. The secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0196] When the carbon content of the first lithium iron phosphate salt particles used in the positive electrode materials of Examples 1 to 12 is within the range of 0.8 wt % to 1.5 wt %, the secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0197] In the positive electrode materials of Examples 1 to 12, the second lithium iron phosphate particles used include a core and a carbon coating layer on at least a portion of the surface of the core. The secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0198] The BET specific surface area of ​​the first lithium iron phosphate particles used in the positive electrode materials of Examples 1 to 12 is 3 m 2 / g~8m 2 / g, or the BET specific surface area of ​​the second lithium iron phosphate particles is less than or equal to 18.5m 2 / g, the secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0199] The molecular formula of the first lithium iron phosphate salt particles or the second lithium iron phosphate salt particles of Examples 1 to 12 is Li m Fe x P y O j Q q In the embodiment, when Q is one or more of the elements Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, and in some embodiments, when it is Mn or Ti, the secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0200] When the Q content in the first lithium iron phosphate salt particles used in the positive electrode materials of Examples 1 to 12 is 4000ppm-10000ppm, or when the Q content in the second lithium iron phosphate salt particles is 1000ppm-3000ppm, the secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0201] When the capacity proportion η of the first lithium iron phosphate salt particles in the positive electrode materials of Examples 1 to 12 is ≥ 88%, the secondary batteries prepared therefrom all have excellent energy density and cycle performance.

[0202] The surface density of the positive active material on one side of the positive electrode sheets of Examples 1 to 12 is 20.0 mg / cm 2 -35.0mg / cm 2 The secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0203] The compacted density of the positive active material layer on the positive electrode sheets of Examples 1 to 12 is 2.35 g / cm3 -2.70g / cm 3 The secondary batteries prepared therefrom have both excellent energy density and cycle performance.

[0204] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included within the technical scope of the present application. In addition, without departing from the scope of the present application, any other modifications that can be imagined by those skilled in the art to the embodiments, or any other methods constructed by combining some of the constituent elements in the embodiments are also included within the scope of the present application.

Claims

1. A positive electrode material, characterized in that comprising first lithium iron phosphate salt particles and second lithium iron phosphate salt particles, The primary average particle size of the first lithium iron phosphate particles is 500 nm to 3000 nm. Based on the total weight of the second lithium iron phosphate salt particles, the carbon content of the second lithium iron phosphate salt particles is 1.5 wt % to 2.0 wt %. The mass of the second lithium iron phosphate salt particles is greater than or equal to the mass of the first lithium iron phosphate salt particles.

2. The positive electrode material according to claim 1, characterized in that The mass ratio of the second lithium iron phosphate salt particles to the first lithium iron phosphate salt particles is 1:1 to 8:

1.

3. The positive electrode material according to claim 1 or 2, characterized in that The primary average particle size of the second lithium iron phosphate particles is 50 nm to 400 nm.

4. The positive electrode material according to any one of claims 1 to 3, characterized in that The primary average particle size of the first lithium iron phosphate particles is 650 nm to 920 nm.

5. The positive electrode material according to any one of claims 1 to 4, characterized in that Calculated based on the total weight of the first lithium iron phosphate salt particles, the carbon content of the first lithium iron phosphate salt particles is 0.8 wt % to 1.5 wt %.

6. The positive electrode material according to any one of claims 1 to 5, characterized in that The second lithium iron phosphate salt particles include a core and a carbon coating layer on at least a portion of the surface of the core.

7. The positive electrode material according to claim 6, characterized in that The carbon coating layer includes a first carbon coating layer and a second carbon coating layer, wherein the first carbon coating layer is located between the core and the second carbon coating layer.

8. The positive electrode material according to any one of claims 1 to 7, characterized in that The BET specific surface area of the first lithium iron phosphate particles is 3 m 2 / g~8m 2 / g; and / or, the BET specific surface area of the second lithium iron phosphate particles is less than or equal to 18.5 m 2 / g.

9. The positive electrode material according to any one of claims 1 to 8, characterized in that The molecular formulas of the first lithium iron phosphate salt particles and the second lithium iron phosphate salt particles are independently Li m Fe x P y O j Q q , wherein Q includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.

1.

10. The positive electrode material according to claim 9, characterized in that wherein Q comprises at least one of Ti, V, Mg, and Nb, Calculated based on the total weight of the first lithium iron phosphate salt particles, the content of Q in the first lithium iron phosphate salt particles is 4000ppm-10000ppm; and / or, Calculated based on the total weight of the second lithium iron phosphate salt particles, the content of Q in the second lithium iron phosphate salt particles is 1000 ppm-3000 ppm.

11. The positive electrode material according to any one of claims 1 to 10, characterized in that The positive electrode material is single crystal particles and / or polycrystalline particles.

12. The positive electrode material according to any one of claims 1 to 11, characterized in that The capacity proportion η of the first lithium iron phosphate salt particles is ≥88%, where η is defined as: A battery containing the first lithium iron phosphate salt particles as the positive electrode material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V to 3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value at the discharge voltage of 3.2V is extracted and recorded as C1, and the capacity value at the discharge voltage to 2.0V is extracted as C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cutoff current of 50μA.

13. A positive electrode plate, characterized in that: The positive electrode material comprises the positive electrode material according to any one of claims 1 to 12.

14. The positive electrode sheet according to claim 13, characterized in that: The surface density of the positive active material on one side of the positive electrode sheet is 20.0 mg / cm 2 -35.0mg / cm 2 .

15. The positive electrode sheet according to claims 13 to 14, characterized in that: The compaction density of the positive electrode active material layer on the positive electrode sheet is 2.30 g / cm 3 -2.70g / cm 3 .

16. A secondary battery, characterized in that: A positive electrode sheet comprising the positive electrode sheet according to any one of claims 13 to 15.

17. An electrical device, characterized in that: A secondary battery according to claim 16 is included.

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