Secondary battery, preparation method thereof and electric device

Abstract

The invention provides a secondary battery, a preparation method thereof and an electric device. Particularly, the secondary battery comprises a positive pole piece, a negative pole piece, an isolating membrane and an electrolyte, the positive pole piece comprises a positive active material, the positive active material comprises lithium iron manganese phosphate particles and first lithium iron phosphate particles, the primary average particle size of the lithium iron manganese phosphate particles is 120-600nm, the primary average particle size of the first lithium iron phosphate particles is 120-600nm, and the secondary average particle size of the second lithium iron phosphate particles is 120-600nm. Based on the total area of the positive electrode active material in the cross section of the positive electrode plate, the area ratio of the first lithium iron phosphate particles is 2%-30%, and the area ratio of the lithium manganese iron phosphate particles is 70%-98%.

Description

[0001] This application is a divisional application based on the invention with application number 202410027059.3, application date January 8, 2024, applicant CATL, and invention title "Secondary Battery, Method for its Preparation and Electrical Device". Technical Field

[0002] This application relates to the field of secondary battery technology, and in particular to a secondary battery, its preparation method and power supply device. Background Technology

[0003] As a positive electrode active material for lithium-ion batteries, lithium manganese iron phosphate (MFP) has a higher voltage plateau, theoretical energy density, theoretical specific capacity, and lower cost compared to lithium iron phosphate (LFP). However, MFP suffers from poorer high-temperature (60°C) storage performance compared to LFP.

[0004] Therefore, there is a need to provide a positive electrode active material containing lithium manganese iron phosphate salt, which has good high-temperature storage performance. Summary of the Invention

[0005] This application is made in view of the above-mentioned issues, and its purpose is to provide a secondary battery with good high-temperature storage performance, its preparation method and power supply device.

[0006] The inventors have discovered that by adopting the technical solution of this application, the above-mentioned objectives can be achieved.

[0007] The first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive active material, which includes lithium manganese iron phosphate particles with a primary average particle size of 120-600 nm. The manganese leaching amount of the secondary battery is ≤50 ppm.

[0008] The secondary battery of this application exhibits good high-temperature storage performance. Furthermore, the secondary battery of this application also possesses good high-temperature cycle performance and a high solid content in the positive electrode slurry.

[0009] In any embodiment, the positive electrode active material further includes first lithium iron phosphate particles, wherein the primary average particle size s1 of the first lithium iron phosphate particles satisfies 500nm≤s1≤3000nm.

[0010] When the positive electrode active material also includes first lithium iron phosphate particles, and the primary average particle size s1 of the first lithium iron phosphate particles satisfies 500nm≤s1≤3000nm, the secondary battery of this application has a lower manganese dissolution, better high-temperature storage performance, better high-temperature cycle performance, and higher solid content of the positive electrode slurry.

[0011] In any embodiment, based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet, the area ratio of the first lithium iron phosphate particles is 2%-30%, and the area ratio of the first lithium iron manganese phosphate particles is 70%-98%.

[0012] In any embodiment, the positive electrode active material further includes second lithium iron phosphate particles, wherein the primary average particle size s2 of the second lithium iron phosphate particles satisfies 30nm≤s2≤200nm.

[0013] When the positive electrode active material also includes second lithium iron phosphate particles, and the primary average particle size s2 of the second lithium iron phosphate particles satisfies 30nm≤s2≤200nm, the secondary battery of this application has a lower manganese leaching amount, better high-temperature storage performance, and better high-temperature cycling performance.

[0014] In any embodiment, based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet, the area ratio of the second lithium iron phosphate particles is 1-10%; the area ratio of the first lithium iron phosphate particles is 2-25%; and the area ratio of the lithium manganese iron phosphate particles is 70%-97%.

[0015] In any embodiment, lithium manganese iron phosphate particles have the molecular formula Li m Fe x Mn r P y O j Q q Where Q includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95 ≤ m ≤ 1.15, x > 0, r > 0, 0.9 ≤ x + r ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 ≤ q ≤ 0.1, and / or The first lithium iron phosphate particles have the molecular formula Li m1 Fe x1 P y1 O j1 Q1 q1 Q1 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, with 0.95 ≤ m1 ≤ 1.15, 0.9 ≤ x1 ≤ 1, 0.95 ≤ y1 ≤ 1, 3.5 ≤ j1 ≤ 4, 0 < q1 ≤ 0.1, and / or The second lithium iron phosphate particles have the molecular formula Li m2 Fe x2 P y2 O j2 Q2q2 Q2 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, with 0.95≤m2≤1.15, 0.9≤x2≤1, 0.95≤y2≤1, 3.5≤j2≤4, and 0≤q2≤0.1.

[0016] In any embodiment, Q1 in the first lithium iron phosphate particles includes at least one of Ti, V, Mg, and Nb, and the content of Ti, V, Mg and / or Nb is 1000-10000 ppm based on the total weight of the first lithium iron phosphate particles.

[0017] In any embodiment, the specific surface area (BET) of the first lithium iron phosphate salt particles is 3 m². 2 / g-8 m 2 / g.

[0018] When the specific surface area (BET) of the first lithium iron phosphate particles is 3 m² 2 / g-8 m 2 At / g, the secondary battery of this application has a lower manganese leaching amount, a higher solid content in the positive electrode slurry, and a higher compaction density of the positive electrode sheet.

[0019] In any embodiment, the carbon content of the first lithium iron phosphate salt particles is calculated based on the total weight of the first lithium iron phosphate salt particles as Cx1% by weight, wherein 0.8 ≤ Cx1 ≤ 2.0.

[0020] In any embodiment, the carbon content of the first lithium iron phosphate particles is calculated based on the total weight of the first lithium iron phosphate particles as Cx1% by weight, and the ratio z1 of the specific surface area of ​​the first lithium iron phosphate particles to Cx1 satisfies 1.5≤z1≤8.5.

[0021] In any embodiment, the primary average particle size of the lithium iron phosphate particles is 150-210 nm, the primary average particle size s1 of the first lithium iron phosphate particles satisfies 870 nm ≤ s1 ≤ 3000 nm, and / or the primary average particle size s2 of the second lithium iron phosphate particles satisfies 60 nm ≤ s2 ≤ 200 nm.

[0022] In any embodiment, the capacity percentage η of the first lithium iron phosphate salt particles is ≥ 88%, where η is defined as: The battery using the first lithium iron phosphate salt particles as the positive electrode active material was charged and discharged twice at a constant current rate of 0.1C within a 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 1C rate, the capacity value extracted at the discharge voltage of 3.2V was recorded as C1, and the capacity value extracted at the discharge voltage of 2.0V was recorded as C2, and η=C1 / C2. The charging process included constant voltage charging, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0023] In any embodiment, the first lithium iron phosphate salt particles satisfy at least one of (a)-(f): (a) The Dv10 of the first lithium iron phosphate salt particles is ≥0.2 μm; (b) The Dv50 of the first lithium iron phosphate salt particles is 0.5-5 μm; (c) The Dv90 of the first lithium iron phosphate salt particles is ≤10μm; (d) The Dv99 of the first lithium iron phosphate salt particles is ≤12μm; (e) The compacted density of the first lithium iron phosphate powder under a pressure of 3 tons is ≥2.25 g / cm³. 3 ; (f) The resistivity of the first lithium iron phosphate powder is less than 60 Ω·cm.

[0024] In any embodiment, the aspect ratio of the second lithium iron phosphate salt particles is ≥1.3.

[0025] When the aspect ratio of the second lithium iron phosphate particles is ≥1.3, the secondary battery of this application has good high-temperature storage performance, low manganese leaching, good high-temperature cycling performance, and high specific capacity.

[0026] In any embodiment, the ratio of the intensity of the (020) crystal plane diffraction peak to the intensity of the (211) crystal plane diffraction peak of the second lithium iron phosphate salt particle is W≥1.03.

[0027] When the ratio of the intensity of the (020) crystal plane diffraction peak to the intensity of the (211) crystal plane diffraction peak of the second lithium iron phosphate salt particles is W≥1.03, the secondary battery of this application has good high-temperature storage performance, low manganese dissolution, good high-temperature cycling performance, and high specific capacity.

[0028] In any embodiment, the first lithium iron phosphate salt particles are mainly obtained by the following preparation method: The raw materials provided contain at least a lithium source, an iron source, a phosphorus source, an optional carbon film-forming agent, an optional carbon source, and an optional modifier, and are subjected to at least two sintering processes, wherein... The temperature for the first sintering is 500℃-760℃, and can be selected as 550℃-720℃; The temperature for the second sintering is 700℃-800℃, and can be selected as 720℃-780℃.

[0029] In any embodiment, the first lithium iron phosphate salt particles are mainly obtained by the following preparation method: The raw materials provided contain at least lithium source, iron source, phosphorus source, carbon source, carbon film-forming agent, and modifier, and are subjected to at least two sintering processes. The carbon content of the material after the first sintering is 0.01%-0.79% by weight, and can be selected as 0.05%-0.4% by weight. After the second sintering, the carbon content of the material is 0.8%-2.0% by weight, and can be selected as 1.0%-1.6% by weight.

[0030] In any embodiment, the method for preparing the first lithium iron phosphate salt particles includes the following steps: After the first sintering, a first pulverization is performed; after the second sintering, a second pulverization is performed. The Dv50 of the product after the first pulverization is 300nm-1200nm, and can be selected as 400nm-1100nm; The Dv50 of the product after the second pulverization is 500nm-5000nm, and can be selected as 700nm-2500nm.

[0031] A second aspect of this application provides an electrical device comprising a secondary battery as described in the first aspect of this application. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application; Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of this application is shown. Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application; Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application; Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown; Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application; Figure 7 This is a scanning electron microscope image of a first lithium iron phosphate salt particle with its primary particle size marked, according to an embodiment of this application.

[0033] Explanation of reference numerals in the attached figures: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the secondary battery, its manufacturing method, and electrical device of this application. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0034] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0035] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0036] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0037] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0038] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0039] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0040] As a positive electrode active material for lithium-ion batteries, lithium manganese iron phosphate (LFP) exhibits a higher voltage plateau, theoretical energy density, theoretical specific capacity, and lower cost compared to lithium iron phosphate (LFP). However, LFP suffers from poorer high-temperature (60°C) storage performance compared to LFP. Therefore, there is a need for a positive electrode active material containing LFP that possesses superior high-temperature storage performance.

[0041] Based on this, this application proposes a technical solution to solve the above-mentioned technical problems.

[0042] The first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive active material, which includes lithium manganese iron phosphate particles with a primary average particle size of 120-600 nm. The manganese leaching amount of the secondary battery is ≤50 ppm.

[0043] Lithium manganese iron phosphate (LFP) exhibits poor ionic and electronic conductivity, which is significantly affected by particle size. When the material contains large-sized LFP particles, the solid-phase transport of lithium ions is severely restricted, and the electron transport path during the electrochemical reaction is long, leading to a significant decrease in the material's specific capacity and kinetic performance. Studies have found that LFP materials exhibit strong surface oxidizability during the charged state, and due to the Jan Taylor effect, surface oxygen atoms readily react with protons in the electrolyte, resulting in manganese dissolution and electrolyte loss. Therefore, the particle size of LFP should not be designed to contain too many small particles; the micron content needs to be controlled at a low level. Experiments have shown that small particles with a diameter less than 120 nm exhibit significantly increased surface activity. This application utilizes LFP with low manganese dissolution and optimizes particle size selection, resulting in a secondary battery with good high-temperature storage performance, good high-temperature cycling performance, and high solid content in the positive electrode slurry, while also exhibiting good kinetic performance and high specific capacity.

[0044] In some embodiments, the primary average particle size of lithium manganese iron phosphate salt particles can be 140-500 nm. In some embodiments, the primary average particle size of lithium manganese iron phosphate salt particles can be 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, 210 nm, 220 nm, 230 nm, 240 nm, 250 nm, 260 nm, 270 nm, 280 nm, 290 nm, 300 nm, 350 nm, 400 nm, 450 nm, 480 nm, 490 nm, 500 nm, 550 nm, 600 nm, or a range of any two of the above primary average particle sizes or a value within that range.

[0045] In some embodiments, the manganese leaching amount of the secondary battery is ≤48 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤45 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤40 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤35 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤30 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤28 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤25 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤24 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤20 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤18 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤17 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤16 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤15 ppm. In some embodiments, the manganese leaching amount of the secondary battery is ≤14 ppm. In some implementations, the amount of manganese leached from the secondary battery is ≤12ppm.

[0046] In this application, the term "primary average particle size" refers to the average of the primary particle sizes of all particles, wherein the primary particle size is the longest distance connecting two points on the edge in a cross-sectional view.

[0047] The primary average particle size of lithium manganese iron phosphate (LFP) salt particles can be measured using methods and equipment known in the art. For example, it can be tested using scanning electron microscopy (SEM) and aortometric analysis. As an example, an argon ion beam is used to cut the electrode perpendicularly to its large surface, exposing the cross-section. The cross-section is photographed using an SEM, clearly showing the boundary between the first and second positive electrode active layers. Aortometric analysis is then used to statistically analyze the particle size of the positive electrode active layer containing LFP salt particles. Specifically, the total number of LFP salt particles with a primary diameter greater than 10 nm and the primary diameter of these particles can be counted from the SEM images. The primary average particle size of LFP salt particles is calculated as: (Primary diameter of total LFP salt particles / Total number of LFP salt particles). During the primary diameter analysis, particles with a primary diameter between 0 and 10 nm are excluded from the statistical scope.

[0048] Manganese leaching can be measured using methods and equipment known in the art. For example, the ICP testing procedure can be followed: retrieve the fully charged cell after 100 cycles, disassemble it in a glove box, remove the anode plate in a drying room, and gently scrape 1g of material from the center of the plate using a ceramic knife, ensuring the entire active material layer is scraped off to avoid inconsistencies in manganese content at different thicknesses. Seal the bag in a resealable bag and send it to the laboratory. Prepare an electronic scale, heating plate, 150ml quartz beaker, watch glass, funnel, 100ml volumetric flask, pliers (heat-resistant gloves), weighing spoon, and lint-free paper; confirm that the electronic scale's calibration date is within the validity period; expired scales cannot be used and must be recalibrated; connect the power supply, and the heating plate should heat up to 250℃ normally; weigh 0.2000 ±0.005g. The powder was placed in a beaker; acidification treatment: a dilute sulfuric acid solution with a volume ratio of 1:4 (concentrated sulfuric acid: ultrapure water = 1:4) was prepared; 20 ml of the prepared dilute sulfuric acid solution was added to the beaker; the heating plate was preheated to 250℃, and the sample was placed in the beaker for digestion for 30 min, with a quartz cap placed on the mouth of the beaker to reduce acid evaporation; after the sample digestion for 30 min was completed, it was removed and cooled to room temperature; the digested sample was transferred to a 100 ml glass volumetric flask and diluted to volume; then the ICP standard test procedure was performed: the test equipment temperature was 22±2℃, and the humidity was <60%; the trace element method was selected; the calibration standard was run: a standard curve was determined using the prepared standard solution; the element selected was Mn; finally, the amount of Mn dissolved was obtained through testing.

[0049] In some embodiments, the positive electrode active material further includes first lithium iron phosphate particles, wherein the primary average particle size s1 of the first lithium iron phosphate particles satisfies 500nm≤s1≤3000nm.

[0050] In specific implementation methods, as shown in the appendix Figure 7 The scanning electron microscope image of the first lithium iron phosphate salt particles is shown. The line segment marked by the double arrow in the particles is the primary particle size as defined in this application.

[0051] In some embodiments, the first lithium iron phosphate salt particles are primary particles.

[0052] In this paper, "primary particles" refers to particles that do not have obvious agglomeration interfaces in particle scanning electron micrographs, but may have tiny pores and point or line defects, which are different from the smallest unit of powder particles that do not have structures such as stacking and flocculation.

[0053] By setting the primary average particle size of the lithium iron phosphate particles to 500nm-3000nm, the particles can maintain a suitable micron-level size, thus avoiding interfacial side reactions and processing difficulties caused by nano-sizing of the particles, and preventing the reduction in kinetic performance due to excessively large particle size. In addition, it facilitates the stirring of the slurry containing the lithium iron phosphate particles and increases the solid content, thereby improving the cell processing and increasing the volumetric energy density of the battery. Furthermore, it avoids the impact of excessive carbon coating density on the normal insertion and extraction of lithium ions, which would otherwise affect the cell capacity.

[0054] When the positive electrode active material also includes first lithium iron phosphate particles, and the primary average particle size s1 of the first lithium iron phosphate particles satisfies 500nm≤s1≤3000nm, close packing can be achieved by mixing some large first lithium iron phosphate particles into the manganese iron phosphate material. At the same time, since the large particle component is lithium iron phosphate, the material has better kinetic performance compared with large manganese iron phosphate particles. Furthermore, after the large first lithium iron phosphate particles crack during cycling, since the particles do not contain manganese, the problems of electrolyte oxidation caused by the oxidation of the material and damage to the negative electrode SEI film caused by manganese dissolution are alleviated. This results in the secondary battery of this application having a lower manganese dissolution, better high-temperature storage performance, better high-temperature cycling performance, and higher solid content of the positive electrode slurry.

[0055] In some embodiments, the primary average particle size of the first lithium iron phosphate salt particles can be 250nm, 300nm, 350nm, 400nm, 440nm, 500nm, 550nm, 600nm, 650nm, 700nm, 730nm, 750nm, 780nm, 800nm, 830nm, 850nm, 870nm, 900nm, 950nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 2000nm, 2100nm, 2200nm, 2300nm, 2400nm, 2500nm, 2600nm, 2700nm, 2800nm, 2900nm, 3000nm, 3500nm, 4000nm, or a range of any two of the above primary average particle sizes or a value within that range.

[0056] In some embodiments, the primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 500nm ≤ s1 ≤ 3500nm.

[0057] When the primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 500nm≤s1≤3500nm, the secondary battery of this application has a lower manganese dissolution, better high-temperature storage performance, better high-temperature cycle performance, and higher solid content of the positive electrode slurry.

[0058] In some embodiments, the primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 500nm ≤ s1 ≤ 4000nm.

[0059] When the primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 500nm≤s1≤4000nm, the secondary battery of this application has a lower manganese leaching amount, better high-temperature storage performance, and higher solid content of the positive electrode slurry.

[0060] In some embodiments, the primary average particle size s1 of the first lithium iron phosphate particles satisfies 300 nm ≤ s1 ≤ 3500 nm. In some embodiments, the primary average particle size s1 of the first lithium iron phosphate particles satisfies 500 nm ≤ s1 ≤ 3000 nm. In some embodiments, the primary average particle size s1 of the first lithium iron phosphate particles satisfies 650 nm ≤ s1 ≤ 2500 nm.

[0061] In some embodiments, the first lithium iron phosphate salt particles are monocrystalline particles and / or polycrystalline particles.

[0062] In this application, the term "single crystal" refers to a structurally complete crystal grown from a single crystal nucleus. The single crystal in this application appears as a single unit under a transmission electron microscope image, and there are no grain boundaries within the single crystal.

[0063] In some embodiments, the single crystal of this application may have minor defects, such as internal micropores, a small number of points and surfaces, or a small number of particles adhering to each other on the surface of a particle.

[0064] In this application, the term "polycrystalline" refers to a crystal formed by the random orientation of small single-crystal grains, and grain boundaries exist within the polycrystalline structure.

[0065] In some implementations, the proportion of monocrystalline particles is greater than or equal to 90% based on the total number of first lithium iron phosphate particles. Controlling the proportion of monocrystalline particles within the above range results in a higher proportion of monocrystalline particles compared to polycrystalline and secondary agglomerates, with less obstruction of lithium ions by grain boundaries. This is beneficial for further improving the lithium ion transport rate and enhancing the kinetic performance of the secondary battery.

[0066] The primary average particle size of the first lithium iron phosphate (LFP) salt particles can be measured using methods and equipment known in the art. For example, it can be tested using scanning electron microscopy (SEM) and aortometric analysis. As an example, the electrode is cut open perpendicular to the large surface of the positive electrode using an argon ion beam to expose the cross-section. The cross-section is photographed using an SEM, clearly showing the boundary between the first and second positive electrode active layers. Aortometric analysis is then used to statistically analyze the particle size of the positive electrode active layer containing the LFP salt particles. Specifically, the total number of LFP salt particles with a primary diameter greater than 80 nm and the primary diameter of these particles can be counted from the SEM images. The primary average particle size of the LFP salt particles is calculated as: (Primary diameter of total LFP salt particles / Total number of LFP salt particles). Particles with a primary diameter between 0 and 80 nm are excluded from the primary diameter statistics.

[0067] In some implementations, based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet, the area ratio of the first lithium iron phosphate particles is 2%-30%, and the area ratio of the first lithium manganese iron phosphate particles is 70%-98%.

[0068] In some embodiments, the area ratio of the first lithium iron phosphate particles is 2-30% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet. In some embodiments, the area ratio of the first lithium iron phosphate particles is 2%, 23%, 25%, 30%, or any range of two of the above area ratios or values ​​within that range.

[0069] In some embodiments, the area ratio of lithium manganese iron phosphate particles is 70%-98% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet. In some embodiments, the area ratio of lithium manganese iron phosphate particles is 65%, 70%, 72%, 74%, 75%, 97%, or 98%.

[0070] The areas of the first and second lithium iron phosphate particles can be obtained by statistical analysis of the obtained electrode cross-sectional scanning electron microscope images using Avizo 3D software. The area of ​​each particle is divided by the total area to obtain the corresponding area ratio.

[0071] In some embodiments, the positive electrode active material further includes second lithium iron phosphate particles, wherein the primary average particle size s2 of the second lithium iron phosphate particles satisfies 30nm≤s2≤200nm.

[0072] By adding a small amount of ultrafine particles to the positive electrode active material, space utilization can be further improved. These ultrafine particles can be positioned within the gaps between medium-sized and ultra-large lithium iron phosphate (LFP) particles, further increasing the volumetric energy density, i.e., further improving the electrode's compaction density. Compared to ultrafine LFP, ultrafine LFP suffers some voltage loss on the voltage plateau, but its specific capacity and surface stability are significantly improved, especially its surface stability. Ultrafine LFP materials are difficult to coat effectively, exhibiting strong oxidizing properties on the surface during charging. Furthermore, due to the Jan Taylor effect, the high specific surface energy of small particles exacerbates manganese dissolution, worsening battery cycle life. The rechargeable battery of this application, by controlling the LFP particles to have a moderate particle size and containing fewer ultrafine and ultra-large particles, exhibits good cycle life and specific capacity. In addition, the rechargeable battery of this application ensures good compaction density by adding both large and small LFP particles to the positive electrode active material. For positive electrode active materials, according to the idealized close-packing model, to achieve good compaction density, the material must contain some large particles and ultrafine powders to form a close packing, thereby achieving both good high-temperature cycling performance and high positive electrode compaction density. When the positive electrode active material also includes second lithium iron phosphate particles, and the primary average particle size s2 of the second lithium iron phosphate particles satisfies 30nm≤s2≤200nm, the secondary battery of this application has low manganese dissolution, good high-temperature storage performance, and good high-temperature cycling performance.

[0073] In some embodiments, the positive electrode active material further includes second lithium iron phosphate particles, wherein the primary average particle size s2 of the second lithium iron phosphate particles satisfies 30nm≤s2≤100nm.

[0074] In some embodiments, the positive electrode active material further includes second lithium iron phosphate particles, the primary average particle size of which can be 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, or any range of two of the above primary average particle sizes or a value within that range.

[0075] The primary average particle size of the second lithium iron phosphate (LFP) particles can be measured using methods and equipment known in the art. For example, it can be tested using scanning electron microscopy (SEM) and aortometric analysis. As an example, the electrode is cut open perpendicular to the large surface of the positive electrode using an argon ion beam to expose the cross-section. The cross-section is photographed using an SEM, clearly showing the boundary between the second positive electrode active layer and the second positive electrode active layer. Aortometric analysis is then used to statistically analyze the particle size of the positive electrode active layer containing LFP particles. Specifically, the total number of LFP particles with a primary diameter greater than 10 nm and the primary diameter of LFP particles with a primary diameter greater than 10 nm can be counted from the SEM images. The primary average particle size of the LFP particles is calculated as: primary diameter of total LFP particles / total number of LFP particles. During the primary diameter statistics, particles with a primary diameter of 0 < 10 nm are not included in the statistics.

[0076] In some embodiments, the primary average particle size of the lithium iron phosphate particles is 150-210 nm, the primary average particle size s1 of the first lithium iron phosphate particles satisfies 870 nm ≤ s1 ≤ 3000 nm, and / or the primary average particle size s2 of the second lithium iron phosphate particles satisfies 60 nm ≤ s2 ≤ 200 nm.

[0077] In some implementations, based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet, the area ratio of the second lithium iron phosphate particles is 1-10%; the area ratio of the first lithium iron phosphate particles is 2-25%; and the area ratio of the lithium manganese iron phosphate particles is 70%-97%.

[0078] In some embodiments, the area ratio of the second lithium iron phosphate particles is 1-10% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet. In some embodiments, the area ratio of the second lithium iron phosphate particles is 1%, 3%, 5%, 10%, or any range of two of the above area ratios or values ​​within that range, based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet.

[0079] In some embodiments, the area ratio of the first lithium iron phosphate particles is 2-25% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet. In some embodiments, the area ratio of the first lithium iron phosphate particles is 2%, 23%, 25%, 30% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet, or a range of any two of the above area ratios or a value within that range. In some embodiments, the area ratio of the first lithium iron phosphate particles is 23-25% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet.

[0080] In some embodiments, the area ratio of lithium manganese iron phosphate particles is 70%-97% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet. In some embodiments, the area ratio of lithium manganese iron phosphate particles is 65%, 70%, 72%, 74%, 75%, 97%, or 98% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet. In some embodiments, the area ratio of lithium manganese iron phosphate particles is 65%-74% based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet.

[0081] The areas of the lithium iron phosphate (LFP) salt particles, the first LFP salt particles, and the second LFP salt particles can be obtained by statistical analysis of the obtained cross-sectional scanning electron microscope images of the electrode using Avizo 3D software. The area of ​​each particle is divided by the total area to obtain the corresponding area ratio.

[0082] In some embodiments, the lithium manganese iron phosphate salt particles have the molecular formula Li m Fe x Mn r P y O j Q q Where Q includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95 ≤ m ≤ 1.15, x > 0, r > 0, 0.9 ≤ x + r ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 ≤ q ≤ 0.1, and / or The first lithium iron phosphate particles have the molecular formula Li m1 Fe x1 P y1 O j1 Q1 q1 Q1 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, with 0.95 ≤ m1 ≤ 1.15, 0.9 ≤ x1 ≤ 1, 0.95 ≤ y1 ≤ 1, 3.5 ≤ j1 ≤ 4, 0 < q1 ≤ 0.1, and / or The second lithium iron phosphate particles have the molecular formula Li m2 Fe x2 P y2 O j2 Q2 q2Q2 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, with 0.95≤m2≤1.15, 0.9≤x2≤1, 0.95≤y2≤1, 3.5≤j2≤4, and 0≤q2≤0.1.

[0083] In some embodiments, the lithium manganese iron phosphate salt particles have the molecular formula Li m Fe x Mn r P y O j Q q Where m can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15; x+r can be 0.9 or 1.0; y can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15; j can be 3.5, 3.6, 3.7, 3.8, 3.9, or 4; and q can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1.

[0084] In some embodiments, the first lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O j1 Q q1 m1 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15; x1 can be 0.9 or 1.0; y1 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15; j1 can be 3.5, 3.6, 3.7, 3.8, 3.9, or 4; and q1 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1.

[0085] In some embodiments, the second lithium iron phosphate salt particles have the molecular formula Li m2 Fe x2 P y2 O j2 Q q2m2 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15; x2 can be 0.9 or 1.0; y2 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, or 1.15; j2 can be 3.5, 3.6, 3.7, 3.8, 3.9, or 4; and q2 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1.

[0086] Modifying the first and / or second lithium iron phosphate particles with elements Q1 and / or Q2 helps improve the ion transport capability of the cathode active material. These elements can create vacancies or alter interatomic bond lengths in the particle lattice, facilitating the movement of lithium ions within the lattice and effectively improving the conductivity of the particles themselves, thereby enhancing the kinetic performance of the cathode active material. In this application, the modification can specifically manifest as doping and / or coating.

[0087] In some embodiments, Q1 in the first lithium iron phosphate particles includes at least one of Ti, V, Mg, and Nb, and the content of Ti, V, Mg and / or Nb is 1000-10000 ppm based on the total weight of the first lithium iron phosphate particles.

[0088] In some embodiments, the content of Q1 in the first lithium iron phosphate particles is calculated based on the total weight of the first lithium iron phosphate particles and is 1000ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm, 8500ppm, 9000ppm, 9500ppm, 10000ppm, or a range consisting of any two of the above-mentioned contents of Q1 or a value within that range.

[0089] The Q1 content in lithium iron phosphate particles, for example, the Ti content, can be measured using methods and equipment known in the art. For example, it can be tested according to GB / T 33822-2017.

[0090] Existing lithium iron phosphate (LFP) salt particles generally have low or no modified element content. Increasing the Q1 element content in LFP salt particles helps to further improve their bulk ion transport capacity and kinetic performance. However, with further increases in the Q1 element content, the bulk ion transport capacity may not continue to increase, and it may even occupy lithium ion positions, affecting the specific capacity. The range of Q1 elements specified in this application helps to achieve better kinetic performance and specific capacity.

[0091] In some embodiments, Q1 in the first lithium iron phosphate particles includes Ti, and the content of Ti is 1,000-10,000 ppm, optionally 2,500-6,000 ppm, calculated based on the total weight of the first lithium iron phosphate particles.

[0092] In some embodiments, Q1 in the first lithium iron phosphate particles includes V, and the content of V is 1,000-10,000 ppm, optionally 2,500-6,000 ppm, calculated based on the total weight of the first lithium iron phosphate particles.

[0093] In some embodiments, Q1 in the first lithium iron phosphate particles includes Nb, and the Nb content is 1,000-10,000 ppm, optionally 2,500-6,000 ppm, calculated based on the total weight of the first lithium iron phosphate particles.

[0094] Based on the total weight of the first lithium iron phosphate particles, when the Ti content in the first lithium iron phosphate particles is 2500-6000ppm, further appropriately increasing the Ti content in the first lithium iron phosphate particles can help to further improve the bulk ion transport capability of the first lithium iron phosphate particles, so that the secondary battery of this application has better high-temperature storage performance and higher specific capacity.

[0095] In some embodiments, the specific surface area of ​​the first lithium iron phosphate salt particles is 3 m². 2 / g-8 m 2 / g.

[0096] In some embodiments, the specific surface area (BET) of the first lithium iron phosphate salt particles can be 3 m². 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g, 4.6m 2 / g、5m 2 / g, 5.5m 2 / g、6m 2 / g, 6.5m 2 / g、7m 2 / g, 7.5m 2 / g、8m 2 / g, 8.5m 2 / g、9m 2 / g, 9.5m 2 / g, 10m 2 / g, 10.5m 2 / g、11m 2 / g, 11.5m 2 / g、12m 2 / g, or a range of BET specific surface areas of any two of the above-mentioned first lithium iron phosphate particles, or a value within that range.

[0097] In this application, the term "specific surface area" or "BET" refers to the total surface area per unit mass of particles. In this application, the BET of lithium iron phosphate particles is related to factors such as the primary average particle size, carbon content, density of carbon coating, degree of adhesion between carbon and particles, and porosity of particles.

[0098] The specific surface area (BET) of particles can be measured using methods and equipment known in the art. For example, it can be tested according to the gas adsorption method, referring to GB / T 19587-2017. As an example, lithium iron phosphate particles are placed in a sample tube, which is then immersed in liquid nitrogen at -196°C. The amount of nitrogen adsorbed on the solid surface at different pressures (0.05-0.30) is measured. Based on the BET multilayer adsorption theory and its formula, the monolayer adsorption amount of the sample is calculated, thus obtaining the specific surface area of ​​the sample.

[0099] Excessively high BET values ​​for lithium iron phosphate (LFP) particles will increase their water absorption capacity, affecting the processing performance of the mixed cathode slurry. However, excessively low BET values ​​will reduce their specific capacity. Maintaining the BET of the LFP particles at 3 m... 2 / g-8 m 2 Within / g, it is beneficial to further balance the processing performance of the positive electrode slurry and improve the solid content, thereby improving the processing problems of the cell and thus increasing the volumetric energy density of the battery.

[0100] When the specific surface area of ​​the first lithium iron phosphate particles is 3 m² 2 / g-8 m 2At a concentration of / g, by introducing larger particles with smaller BET values, the specific surface energy of the positive electrode active material can be reduced, slowing down the spontaneous agglomeration and polymerization of the material after dispersion, reducing the viscosity of the slurry, and achieving a higher solid content. Simultaneously, the reduction in the specific surface area of ​​the positive electrode active material corresponds to a reduction in the active reaction area, which can reduce side reactions of the electrode under high voltage. The lower specific surface area of ​​the lithium iron phosphate particles also reduces the overall water absorption of the material. This reduction in moisture content further reduces side reactions during battery charging and discharging, resulting in a secondary battery with lower manganese dissolution, higher solid content in the positive electrode slurry, and higher compaction density of the positive electrode sheet.

[0101] In some embodiments, the specific surface area of ​​the first lithium iron phosphate salt particles is 3 m². 2 / g-6m 2 / g.

[0102] When the specific surface area (BET) of the first lithium iron phosphate particles is 3 m² 2 / g-6 m 2 At / g, the secondary battery of this application has good high-temperature storage performance, good high-temperature cycle performance, and high positive electrode compaction density.

[0103] In some embodiments, the carbon content of the first lithium iron phosphate particles is calculated as Cx1% by weight, based on the total weight of the first lithium iron phosphate particles, where 0.8 ≤ Cx1 ≤ 2.0.

[0104] When 0.8 ≤ Cx1 ≤ 2.0, by controlling the carbon content of large-particle lithium iron phosphate salts within a certain range, the conductivity and coating integrity of the large-particle lithium iron phosphate salt materials can be adjusted. Higher carbon content results in better kinetic performance, better particle conductivity, improved coating integrity, and reduced surface side reactions. However, excessively high carbon content leads to a high specific surface area, deteriorating cycle performance. High carbon coating also results in some carbon redundancy, existing as floating carbon or ineffective coating, worsening the material's processing performance, particularly significantly affecting the solids content of the pulp. The carbon content range specified in this application helps to further obtain better kinetic and processing performance.

[0105] In some embodiments, the carbon content of the first lithium iron phosphate particles is calculated based on the total weight of the particles, and is 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.15 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, or a range of any two of the above carbon contents or a value within that range.

[0106] In some embodiments, the carbon content of the first lithium iron phosphate particles is calculated based on the total weight of the first lithium iron phosphate particles as Cx1% by weight, wherein 0.8 ≤ Cx1 ≤ 1.2.

[0107] Based on the total weight of the first lithium iron phosphate particles, the carbon content of the first lithium iron phosphate particles is Cx1% by weight. When 0.8 ≤ Cx1 ≤ 1.2, the secondary battery of this application has good high-temperature storage performance, low manganese leaching, high solid content of positive electrode slurry, good high-temperature cycle performance, and high positive electrode sheet compaction density.

[0108] In some embodiments, the carbon contained in the first lithium iron phosphate particles is coated on the particle surface. In some embodiments, the carbon contained in the first lithium iron phosphate particles is embedded within the particles. In some embodiments, the carbon contained in the first lithium iron phosphate particles is partially coated on the particle surface and partially embedded within the particles.

[0109] In some embodiments, the ratio z1 of the specific surface area of ​​the first lithium iron phosphate salt particle to Cx1 satisfies 1.5 ≤ z1 ≤ 8.5. In some embodiments, the ratio z1 of the specific surface area of ​​the first lithium iron phosphate salt particle to Cx1 satisfies 3 ≤ z1 ≤ 6.

[0110] In some implementations, z1 can be 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.2, 3.5, 3.75, 3.8, 4, 4.2, 4.3, 4.5, 4.58, 4.8, 4.98, 5, 5.3, 5.42, 5.5, 5.71, 5.8, 5.83, 6, 6.3, 6.5, 6.7, 6.8, 7, 7.3, 7.5, 7.8, 7.9, 8, 8.3, 8.5, 8.6, or a range of any two of the above z1 values ​​or a value within that range.

[0111] The ratio z1 of the specific surface area BET to Cx1 of the first lithium iron phosphate (LiFePO4) particles can characterize the uniformity and density of the carbon contained in the particles. When the primary average particle size and carbon content of the LiFePO4 particles remain constant, a lower z1 ratio indicates higher carbon coating utilization, less floating carbon, and more uniform and dense carbon content. Improving the uniformity and density of the contained carbon is beneficial for further improving the kinetic performance and specific capacity of the LiFePO4 particles. However, excessively high carbon density may affect lithium-ion insertion / extraction, thus impacting the kinetic performance and specific capacity of the secondary battery to some extent. The z1 range specified in this application is conducive to achieving a suitable uniformity and density of carbon contained in the LiFePO4 particles, thereby improving the conductivity of the particle surface and further enhancing the specific capacity and kinetic performance of the secondary battery.

[0112] In some embodiments, the ratio z1 of the specific surface area of ​​the first lithium iron phosphate salt particle to Cx1 satisfies 3.8≤z1≤5.

[0113] When the ratio z1 of the specific surface area of ​​the first lithium iron phosphate salt particles to Cx1 satisfies 3.8≤z1≤5, the secondary battery of this application has good high-temperature storage performance, low manganese dissolution, high solid content of positive electrode slurry, good high-temperature cycle performance, and high positive electrode sheet compaction density.

[0114] In some implementations, the capacity percentage η of the first lithium iron phosphate salt particles is ≥ 88%, where η is defined as: The battery using lithium iron phosphate particles as the positive electrode active material was charged and discharged twice at a constant current rate of 0.1C within a 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 1C rate, the capacity value extracted at a discharge voltage of 3.2V was recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V was recorded as C2, where η=C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0115] In some implementations, η can be 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1%, or a range of any two of the above η values ​​or a value within that range.

[0116] The η value of the first lithium iron phosphate particles can be measured using methods and equipment known in the art. As an example, a coin cell is first prepared. The specific coin cell preparation process is as follows: 2.0000g of first lithium iron phosphate particles are mixed with 0.1111g of conductive carbon black and 0.1111g of polyvinylidene fluoride, and then added to 2.5g of the organic solvent N-methylpyrrolidone. After thorough mixing, a slurry is formed. The slurry is coated onto aluminum foil to a thickness of 140 micrometers, vacuum dried at 120°C for 2 hours, and then punched into discs with a diameter of 13mm using a punch. The discs are then pressed using a tablet press at 10 MPa and vacuum-insulated at 120°C for 12 hours to obtain the positive electrode. The weight of the positive electrode is measured, and the loading of lithium iron phosphate particles is 11-12mg. A button cell battery is assembled in an argon-protected glove box, with a lithium metal sheet as the negative electrode, a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent as the electrolyte, LiPF6 as the electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator.

[0117] The prepared coin cells were tested for electrical performance using a blue-light tester. Specifically, the coin cells were charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by one charge and discharge cycle at a constant current rate of 1C. In the 1C charge and discharge test, the capacity value at a discharge voltage of 3.2V was recorded as C1, and the capacity value at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0118] The capacity ratio η of the first lithium iron phosphate (LFP) particles reflects its kinetic performance and plateau retention performance. It can be adjusted by modifying the primary average particle size, carbon content, carbon source and carbon film-forming agent ratio, and modifier and its content. In this application, the LFP particles have an η value ≥ 88%, exhibiting good kinetic performance. Furthermore, when the η value is high, the secondary battery can still maintain good power performance when discharged to a low state of charge (SOC). That is, a battery with a high η value experiences a smaller voltage drop during low-charge, high-current discharge.

[0119] In some embodiments, the aspect ratio of the second lithium iron phosphate particles is ≥1.1. In some embodiments, the aspect ratio of the second lithium iron phosphate particles is ≥1.3.

[0120] When the aspect ratio of the second lithium iron phosphate salt particles is ≥1.3, the material has good crystallinity, which makes the secondary battery of this application have good high-temperature storage performance, low manganese leaching, good high-temperature cycle performance, and high specific capacity.

[0121] In this application, the term "aspect ratio" refers to the ratio of the diameter along the major axis to the diameter along the minor axis of a primary particle. Let the diameter along the major axis be a and the diameter along the minor axis be b, then the aspect ratio = a / b. This parameter is often used to describe particle morphology and can be used to measure its sphericity.

[0122] The aspect ratio of lithium iron phosphate particles can be measured using methods and equipment known in the art. For example, it can be tested by taking a cold-pressed electrode, cutting it open perpendicular to the large surface area using an Ar particle beam to expose the end face, and obtaining an image using a scanning electron microscope. Subsequently, the scanning electron microscope image of the electrode cross-section is analyzed using image processing software to measure the major axis a and minor axis b of the particles, and their ratio is obtained, which is the aspect ratio of the particles.

[0123] In some embodiments, the ratio W of the diffraction peak intensity of the (020) crystal plane to that of the (211) crystal plane of the second lithium iron phosphate particle is ≥0.98. In some embodiments, the ratio W of the diffraction peak intensity of the (020) crystal plane to that of the (211) crystal plane of the second lithium iron phosphate particle is ≥1.03. In some embodiments, the ratio W of the diffraction peak intensity of the (020) crystal plane to that of the (211) crystal plane of the second lithium iron phosphate particle is ≥1.08.

[0124] When the ratio of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane of the second lithium iron phosphate salt particles is W≥1.03, the material has good crystallinity. In addition, a higher W value indicates that more of the (020) crystal plane is exposed, which is more conducive to the insertion and extraction of lithium ions. This results in the secondary battery of this application having better high-temperature storage performance, lower manganese dissolution, better high-temperature cycling performance, and higher specific capacity.

[0125] The ratio W of the diffraction peak intensity of the (020) crystal plane to that of the (211) crystal plane can be measured using methods and equipment known in the art. For example, it can be tested by the following methods: The intensity ratio of the diffraction peaks of the second lithium iron phosphate particles was tested by X-ray diffraction. The second lithium iron phosphate particles were placed on the X-ray diffraction test platform of the Shimadzu XRD-7000 using a copper target X-ray diffractometer. The starting angle of the scan was 10°, the ending angle was 90°, and the step size was 0.013. The test was then started to obtain the diffraction pattern of the second lithium iron phosphate particles in the diffraction angle range of 10° to 90°. The ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane was determined according to the diffraction pattern.

[0126] In some embodiments, the specific surface area of ​​the second lithium iron phosphate salt particles is 11 m². 2 / g-14 m 2 / g. In some embodiments, the specific surface area of ​​the second lithium iron phosphate particles is 11 m². 2 / g-12 m 2 / g. In some embodiments, the specific surface area of ​​the second lithium iron phosphate particles is 12 m². 2 / g-14 m 2 / g.

[0127] In some embodiments, the carbon content of the second lithium iron phosphate particles is calculated as Cx2% by weight, based on the total weight of the particles, and the ratio z2 of the specific surface area of ​​the second lithium iron phosphate particles to Cx2 satisfies 8 ≤ z2 ≤ 11. In some embodiments, 8.1 ≤ z2 ≤ 10.4. In some embodiments, the carbon content of the second lithium iron phosphate particles is calculated as Cx2% by weight, based on the total weight of the particles, and the ratio z2 of the specific surface area of ​​the second lithium iron phosphate particles to Cx2 is 8.1, 8.9, 10.4, or any range of two of the above z2 values ​​or a value within that range.

[0128] In some embodiments, the carbon contained in the second lithium iron phosphate particles is coated on the particle surface. In some embodiments, the carbon contained in the second lithium iron phosphate particles is embedded within the particles. In some embodiments, the carbon contained in the second lithium iron phosphate particles is partially coated on the particle surface and partially embedded within the particles.

[0129] In some embodiments, the specific surface area of ​​lithium manganese iron phosphate salt particles is 8-20 m². 2 / g. In some embodiments, the specific surface area of ​​lithium manganese iron phosphate particles is 10-20 m². 2 / g.

[0130] In some embodiments, the specific surface area of ​​lithium manganese iron phosphate salt particles is 8 m². 2 / g、9 m 2 / g、10 m 2 / g, 10.5 m 2 / g、11 m 2 / g、12 m 2 / g、13m 2 / g、14 m 2 / g, 15m 2 / g、16 m 2 / g、17 m 2 / g、18 m 2 / g、19 m 2 / g、20 m 2 / g, or the range of any two specific surface areas mentioned above, or the value within that range.

[0131] In some embodiments, the specific surface area of ​​lithium manganese iron phosphate salt particles is 10-18 m². 2 / g. In some embodiments, the specific surface area of ​​lithium manganese iron phosphate particles is 10-17 m². 2 / g.

[0132] When the specific surface area of ​​lithium manganese iron phosphate salt particles is 10 m² 2 / g-17 m2 At / g, the secondary battery of the present application embodiment has good high-temperature storage performance, low manganese leaching, good high-temperature cycling performance, and high positive electrode compaction density.

[0133] In some embodiments, the carbon content of the lithium manganese iron phosphate particles is calculated as Cx wt%, based on the total weight of the particles, where 1.2 ≤ Cx ≤ 1.8.

[0134] In some embodiments, the carbon content of the lithium manganese iron phosphate salt particles is calculated as Cx wt%, based on the total weight of the particles, where Cx is 1.2, 1.3, 1.4, 1.5, 1.6, 1.65, 1.7, 1.8, or any range of two of the above Cx or a value within that range.

[0135] In some embodiments, the carbon contained in the lithium manganese iron phosphate particles is coated on the particle surface. In some embodiments, the carbon contained in the lithium manganese iron phosphate particles is embedded within the particles. In some embodiments, the carbon contained in the lithium manganese iron phosphate particles is partially coated on the particle surface and partially embedded within the particles.

[0136] In some embodiments, the ratio z of the specific surface area of ​​lithium manganese iron phosphate particles to Cx satisfies 7 ≤ z ≤ 13. In some embodiments, the ratio z of the specific surface area of ​​lithium manganese iron phosphate particles to Cx satisfies 7.1 ≤ z ≤ 12.5. In some embodiments, the ratio z of the specific surface area of ​​lithium manganese iron phosphate particles to Cx is 7, 7.1, 7.3, 7.5, 7.6, 7.9, 8.3, 8.6, 8.8, 9.1, 9.3, 9.4, 12.5, 13, or any range of any two z values ​​mentioned above, or a value within that range.

[0137] In some embodiments, the first lithium iron phosphate salt particles satisfy at least one of (a)-(f): (a) The Dv10 of the first lithium iron phosphate salt particles is ≥0.2 μm; (b) The Dv50 of the first lithium iron phosphate salt particles is 0.5-5 μm; (c) The Dv90 of the first lithium iron phosphate salt particles is ≤10μm; (d) The Dv99 of the first lithium iron phosphate salt particles is ≤12μm; (e) The compacted density of the first lithium iron phosphate powder under a pressure of 3 tons is ≥2.25 g / cm³. 3 ; (f) The resistivity of the first lithium iron phosphate powder is less than 60 Ω·cm.

[0138] In this application, the term "Dv10" refers to the particle size at which the cumulative volumetric particle size distribution percentage in the particle reaches 10%.

[0139] In this application, the term "Dv90" refers to the particle size at which the cumulative volumetric particle size distribution percentage in the particle reaches 90%.

[0140] In this application, the term "Dv99" refers to the particle size at which the cumulative volumetric particle size distribution percentage in the particle reaches 99%.

[0141] In this application, the term "powder compaction density" refers to the density of a compacted compact with a certain density and strength, formed during the external force compression process. This density is measured in g / cm³, as the powder moves and deforms, larger voids are filled, the contact area between particles increases, resulting in attractive forces between atoms and enhanced mechanical cohesion between particles. 3 .

[0142] In some implementations, the first lithium iron phosphate salt particle has a Dv10 < Dv50.

[0143] In some implementations, the Dv90 of the first lithium iron phosphate salt particle is greater than the Dv50.

[0144] In some embodiments, the Dv50 of the first lithium iron phosphate salt particles can be 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1700nm, 1900nm, 2000nm, 2300nm, 2500nm, 2700nm, 2900nm, 3000nm, 3200nm, 3400nm, 3600nm, 3800nm, 4000nm, 4200nm, 4400nm, 4600nm, 4800nm, or 5000nm, or a range or a value within that range composed of the Dv50 of any two of the products after the second pulverization.

[0145] The Dv10, Dv50, Dv90, and Dv99 of lithium iron phosphate particles can be measured using methods and equipment known in the art. For example, they can be determined using a laser particle size analyzer (Malvern MasterSize 3000) with reference to GB / T19077.1-2016.

[0146] The compacted density of lithium iron phosphate particles under different pressures can be measured using methods and equipment known in the art. For example, it can be measured using a compaction density instrument, referring to GB / T 24533-2009. Specifically, a certain amount of lithium iron phosphate particles is placed on a compaction mold (the mold diameter is known). The mold is hollow in the middle and has a metal disc at the top and bottom. The lithium iron phosphate particles are placed between the metal discs, and a metal cylinder is placed on top. The mold is placed on a compaction density instrument, and different pressures (e.g., 3T) are set. The thickness of the lithium iron phosphate particles under different pressures can be read on the instrument. The compacted density of the lithium iron phosphate particles is ρ=m / v, where v=(S×H), m is the mass of the lithium iron phosphate particles, S is the bottom area of ​​the mold, and H is the thickness of the lithium iron phosphate particles after compaction.

[0147] The powder resistivity of the first lithium iron phosphate particles can be measured using methods and equipment known in the art. For example, it can be measured using a powder resistivity meter (Suzhou Jingge, ST2722 type) according to GB / T 33822-2017. Specifically, a certain amount of lithium iron phosphate particles (e.g., 1g) is weighed and added to the feeding chamber of the powder resistivity meter. A pressure of 8 MPa is applied, and the forward and reverse resistivity of the lithium iron phosphate particles are measured separately. The average value of the two is taken as the powder resistivity of the lithium iron phosphate particles.

[0148] By ensuring that the first lithium iron phosphate particles satisfy at least one of (a)-(f), the first lithium iron phosphate particles can achieve the technical effects of this application more effectively.

[0149] In some embodiments, the first lithium iron phosphate salt particles are mainly obtained by the following preparation methods: The raw materials provided contain at least a lithium source, an iron source, a phosphorus source, an optional carbon film-forming agent, an optional carbon source, and an optional modifier, and are subjected to at least two sintering processes, wherein... The temperature for the first sintering is 500℃-760℃, and can be selected as 550℃-720℃; The temperature for the second sintering is 700℃-800℃, and can be selected as 720℃-780℃.

[0150] In some embodiments, the temperature of the first sintering can be 500°C, 530°C, 550°C, 580°C, 600°C, 630°C, 650°C, 680°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, or 760°C, or a range or a value within that range consisting of any two of the aforementioned first sintering temperatures; the temperature of the second sintering can be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, or 800°C, or a range or a value within that range consisting of any two of the aforementioned second sintering temperatures.

[0151] In some embodiments, the heating rates for the first and second sintering are each independently 2°C / min to 20°C / min.

[0152] In some embodiments, the heating rates in the first and second sintering are each independently 2°C / min, 5°C / min, 7°C / min, 10°C / min, 13°C / min, 15°C / min, 17°C / min, or 20°C / min.

[0153] In some embodiments, the isothermal sintering time for the first sintering is 1-6 hours. In some embodiments, the isothermal sintering time for the first sintering can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, or 6 hours.

[0154] In some embodiments, the isothermal sintering time for the second sintering is 2-12 hours. In some embodiments, the isothermal sintering time for the second sintering can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, 10 hours, 10.5 hours, 11 hours, 11.5 hours, or 12 hours.

[0155] Compared to traditional preparation methods that use high temperatures to achieve particle growth, the first lithium iron phosphate particles in this application undergo two sintering processes. Controlling the temperatures of the two sintering processes is beneficial for obtaining first lithium iron phosphate particles with the primary average particle size and specific surface area of ​​this application. Furthermore, controlling the heating rate, sintering temperature, and isothermal sintering time of the first and / or second sintering processes helps reduce side reactions, thereby better preparing the first lithium iron phosphate particles of this application. Furthermore, in conventional high-temperature sintering processes, the carbon coating layer on the particle surface is prone to cracking, reducing the integrity of the carbon coating. This application synthesizes large particles at low temperatures, which is beneficial for improving the consistency and uniformity of the surface carbon coating.

[0156] In some embodiments, the first lithium iron phosphate salt particles are mainly obtained by the following preparation methods: The raw materials provided contain at least lithium source, iron source, phosphorus source, carbon source, carbon film-forming agent, and modifier, and are subjected to at least two sintering processes. The carbon content of the material after the first sintering is 0.01%-0.79% by weight, and can be selected as 0.05%-0.4% by weight. After the second sintering, the carbon content of the material is 0.8%-2.0% by weight, and can be selected as 1.0%-1.6% by weight.

[0157] In some embodiments, the carbon content of the material after the first sintering can be 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.20 wt%, 0.25 wt%, 0.30 wt%, 0.35 wt%, 0.40 wt%, 0.45 wt%, 0.50 wt%, 0.55 wt%, 0.60 wt%, 0.65 wt%, 0.70 wt%, 0.75 wt%, 0.79 wt%, or a range consisting of any two of the above carbon contents or values ​​within that range; the carbon content of the material after the second sintering can be 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, or a range consisting of any two of the above carbon contents or values ​​within that range.

[0158] In the preparation method of this application embodiment, adding a carbon source before the first sintering can effectively reduce the trivalent iron in the raw materials, improving the purity and stability of the product. Furthermore, by controlling the temperature of the first sintering and the carbon content of the intermediate after the first sintering within the aforementioned range, it helps to increase the primary particle size of the first lithium iron phosphate particle precursor obtained after the first sintering. Specifically, during the first sintering process, a lower carbon content helps to reduce the barrier effect of the carbon layer on the growth process of the first lithium iron phosphate particles, which is beneficial for the crystallization growth of the first lithium iron phosphate particle precursor at a lower temperature. Simultaneously, it also facilitates the solid-phase diffusion reaction between any added modifier and the first lithium iron phosphate material, thereby facilitating the achievement of a higher concentration of metal ion modification. By controlling the temperature of the second sintering and the carbon content of the sintered material within the aforementioned range, it helps to better coat the surface of the first lithium iron phosphate particles with carbon, forming a uniform and dense carbon coating layer, which is beneficial for improving the surface conductivity of the first lithium iron phosphate particles, and enhancing their kinetic performance and specific capacity.

[0159] In some embodiments, raw materials containing at least a lithium source, an iron source, a phosphorus source, a carbon source, a modifier, and a carbon film-forming agent are provided, and sintering is performed at least twice.

[0160] In some implementations, the mixing ratios of lithium, iron, and phosphorus sources, based on the atomic molar number of each element, satisfy the following: Fe:P = (0.96-0.985):1 and Li:Fe = (1.0-0.95):1.1.

[0161] In some implementations, the mixing ratio of the iron source and the phosphorus source, based on the atomic moles of each element, satisfies the following: Fe:P=0.96:1, Fe:P=0.965:1, Fe:P=0.97:1, Fe:P=0.975:1, Fe:P=0.98:1, or Fe:P=0.985:1.

[0162] In some embodiments, the mixing ratio of lithium source and iron source, based on the atomic molar number of each element, satisfies the following: Li:Fe=1.0:1.1, Li:Fe=0.99:1.1, Li:Fe=0.98:1.1, Li:Fe=0.97:1.1, Li:Fe=0.96:1.1 or Li:Fe=0.95:1.1.

[0163] In some embodiments, the weight ratio of carbon source to carbon film-forming agent is (9-0.25):1. In some embodiments, the weight ratio of carbon source to carbon film-forming agent can be 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 0.8:1, 0.5:1 or 0.25:1.

[0164] In some embodiments, the lithium source is a lithium-containing compound. In some embodiments, the lithium source includes at least one selected from lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide, and lithium acetate. In some embodiments, the lithium source includes lithium carbonate.

[0165] In some embodiments, the iron source is an iron-containing compound. In some embodiments, the iron source includes at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, magnetite, and ferric hydroxide. In some embodiments, the iron source includes ferric oxide.

[0166] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one selected from phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. In some embodiments, the phosphorus source includes phosphoric acid.

[0167] In some embodiments, the carbon source includes at least one selected from citric acid, glucose, sucrose, starch, fructose, and lactose. In some embodiments, the carbon source includes glucose.

[0168] In some embodiments, the carbon film-forming agent includes at least one selected from polyethylene glycol, polyaniline, polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol. In some embodiments, the carbon film-forming agent includes poly(aniline).

[0169] In some embodiments, the modifier includes at least one selected from titanium dioxide, vanadium pentoxide, n-butyl titanate, ammonium metavanadate, niobium ethoxide, niobium oxalate, niobium pentoxide, magnesium hydroxide, and magnesium nitrate. In some embodiments, the modifier includes titanium dioxide.

[0170] By using raw materials in the above proportions, it is beneficial to form the first lithium iron phosphate particles of this application.

[0171] In some embodiments, the method for preparing the first lithium iron phosphate salt particles includes the following steps: After the first sintering, a first pulverization is performed; after the second sintering, a second pulverization is performed. The Dv50 of the product after the first pulverization is 300nm-1200nm, and can be selected as 400nm-1100nm; The Dv50 of the product after the second pulverization is 500nm-5000nm, and can be selected as 700nm-2500nm.

[0172] In this application, the term "Dv50" refers to the particle size at which the cumulative volumetric particle size distribution percentage in the particle reaches 50%.

[0173] In some embodiments, the Dv50 of the product after the first pulverization can be 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, or a range consisting of any two of the above-mentioned Dv50 values ​​of the product after the first pulverization, or a value within that range.

[0174] In some embodiments, the Dv50 of the product after the second pulverization can be 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm, 1500nm, 1700nm, 1900nm, 2000nm, 2300nm, 2500nm, 2700nm, 2900nm, 3000nm, 3200nm, 3400nm, 3600nm, 3800nm, 4000nm, 4200nm, 4400nm, 4600nm, 4800nm, or 5000nm, or a range consisting of any two of the above-mentioned Dv50 values ​​of the product after the second pulverization, or a value within that range.

[0175] In some embodiments, the pulverization includes one or more of mechanical crushing, grinding, sand milling, and air jet milling.

[0176] The Dv50 of particles can be measured using methods and equipment commonly used in the art. As an example, GB / T19077.1-2016 can be referenced to determine it using a laser particle size analyzer (Malvern Master Size 3000).

[0177] Controlling the Dv50 of the product after the first pulverization within the aforementioned range helps reduce the growth barrier effect of the added carbon source and any potentially added modifying elements on the precursor crystals of the first lithium iron phosphate particles, thus facilitating the preparation of micron-sized lithium iron phosphate particle precursors. Controlling the Dv50 value of the product after the second pulverization within the aforementioned range helps to obtain first lithium iron phosphate particles with the average particle size of the first pulverization step described in this application.

[0178] A second aspect of this application provides an electrical device comprising a secondary battery as described in the first aspect of this application.

[0179] In addition, the secondary battery and power-consuming device of this application will be described below with appropriate reference to the accompanying drawings.

[0180] In one embodiment of this application, a secondary battery is provided.

[0181] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0182] [Positive electrode plate] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode active material of the first aspect of this application.

[0183] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0184] In some embodiments, the positive 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 substrate and a metal layer formed on at least one surface of the polymer substrate. 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0185] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0186] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0187] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0188] [Negative electrode plate] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0189] As an example, the negative electrode current collector has two surfaces opposite each other in its own 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.

[0190] 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 substrate and a metal layer formed on at least one surface of the polymer substrate. 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0191] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0192] In some embodiments, the negative electrode film layer may optionally include an adhesive. The adhesive 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).

[0193] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0194] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0195] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0196] [Electrolytes] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0197] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0198] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0199] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0200] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0201] [Isolation membrane] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0202] 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 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.

[0203] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0204] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0205] 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. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0206] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured secondary battery 5.

[0207] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 using a winding or stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0208] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0209] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0210] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0211] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0212] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0213] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0214] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0215] Figure 6 This is an example of an electrical device. The device could 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 the secondary battery for this device, a battery pack or battery module can be used.

[0216] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0217] Example The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0218] Example 1 (1) Preparation of positive electrode slurry: Preparation of lithium iron phosphate particles: Lithium carbonate, ferric oxide, phosphoric acid, glucose, titanium dioxide (based on the total weight of lithium iron phosphate particles, with the amount of titanium dioxide added ensuring a titanium content of 5000 ppm in the prepared lithium iron phosphate particles), and polyaniline were weighed separately. The weight ratios of Li, Fe, and P elements were: Fe∶P = 0.968∶1, Li∶Fe = 1∶0.98, and the weight ratio of glucose to polyaniline was: glucose∶polyaniline = 1∶2. The amount of glucose added was such that after the first sintering, the carbon content accounted for 0.15% of the weight of the lithium iron phosphate precursor. Water was added to the above substances to obtain a slurry mixture.

[0219] The mixture was homogenized using a ball mill and then ground using a sand mill to obtain a slurry with a solid content of 38% and a Dv50 of 0.40 μm. 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℃ to 360℃, and an outlet temperature of 100℃ to 140℃). The dried reactants were then loaded into a sintering furnace for the first sintering, with a heating rate controlled at 5℃ / min, a holding temperature of 650℃, and a holding time of 4 hours. After cooling, the material was mechanically ground to obtain powder.

[0220] Glucose as a carbon source and polyaniline as a carbon film-forming agent were added to the obtained powder, and then mixed with water to obtain a material with a solid content of 40%. The amounts of glucose and polyaniline added were such that the carbon content of the product after the second sintering was 1.2% (based on the total weight of lithium iron phosphate particles), and the weight ratio of glucose to polyaniline was 1:2. A slurry was obtained by ball milling and sand milling, and the Dv50 of the insoluble matter in the slurry was 550 nm. Spray drying was performed (high-speed spray dryer with a negative pressure of -650 to -200 Pa, an inlet temperature of 300℃ to 360℃, and an outlet temperature of 100℃ to 140℃). The dried reactants were then loaded into a sintering furnace for a second low-temperature sintering (heating rate controlled at 5℃ / min, sintering temperature at 750℃, and sintering time at 4 h). After the material cools, it is crushed a second time to an average particle size of 870 nm. After demagnetization, lithium iron phosphate particles are obtained with a carbon content of 1.2% and a Ti element content of 5000 ppm.

[0221] The positive electrode active material was obtained by mixing lithium iron phosphate particles and lithium manganese iron phosphate particles (purchased from Guangdong Bangpu Recycling Technology Co., Ltd., item number CPP-001-001) according to the area ratio listed in Table 1. The above-mentioned mixed positive electrode active material, conductive carbon black, binder polyvinylidene fluoride and dispersant PVP were mixed in a weight percentage of 93:4.0:2.5:0.5 and N-methylpyrrolidone was added. After thorough mixing, stirring and dispersion, a positive electrode slurry was prepared.

[0222] (2) Preparation of the positive electrode sheet: Adjust the viscosity of the thoroughly mixed slurry to 8000-20000 mPa·s until it no longer separates. Then, use a double-sided, double-control coating machine to apply the slurry at a rate of 420 mg / 1540 mm. 2 The coating is applied to the surface of the Al foil substrate, and then dried, cold-pressed, slit, and sheeted to finally obtain the positive electrode sheet.

[0223] (3) Preparation of negative electrode sheet: Artificial graphite, conductive carbon black (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC) (thickener) were mixed evenly in a weight percentage of 95:1.0:2.0:2.0, and deionized water was added. After stirring and dispersing, a negative electrode slurry was obtained. The negative electrode slurry was then mixed at a concentration of 211 mg / 1540 mm. 2 The negative electrode sheet is obtained by coating the substrate Cu foil, drying, cold pressing, slitting, and sheet forming. (4) Battery fabrication: The positive electrode, separator, and negative electrode are stacked in sequence. The separator must be able to isolate the anode and cathode. The bare cell is obtained by winding. The bare cell is placed in the outer packaging, electrolyte is injected, and after processes such as encapsulation, formation, and degassing, a lithium-ion battery is finally obtained.

[0224] Example 2 The difference between Example 2 and Example 1 is that the primary average particle size of lithium manganese iron phosphate salt particles is 210 nm, and the specific surface area is 12 m². 2 / g, with a carbon content of 1.4% by weight, these lithium iron phosphate granules were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., with product number CPP-001-002.

[0225] Example 3 The difference between Example 3 and Example 2 is that the primary average particle size of lithium manganese iron phosphate salt particles is 120 nm, and the specific surface area is 15 m². 2 / g, with a carbon content of 1.65% by weight, these lithium manganese iron phosphate granules were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., with the product number SH-LMFP-1.

[0226] Example 4 The difference between Example 4 and Example 2 is that the primary average particle size of lithium manganese iron phosphate salt particles is 600 nm, and the specific surface area is 10 m². 2 / g, the lithium manganese iron phosphate granules were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., with the product number SH-LMFP-2.

[0227] Example 5 The difference between Example 5 and Example 2 is that the first lithium iron phosphate salt particles have an average primary particle size of 250 nm and a specific surface area of ​​9.5 m². 2 / g.

[0228] Example 6 The difference between Example 6 and Example 2 is that the primary average particle size of the first lithium iron phosphate salt particles is 300 nm, and the specific surface area is 7.5 m². 2 / g.

[0229] Example 7 The difference between Example 7 and Example 2 is that the primary average particle size of the first lithium iron phosphate salt particles is 4000 nm, and the specific surface area is 4.5 m². 2 / g.

[0230] Example 8 The difference between Example 8 and Example 2 is that the primary average particle size of lithium manganese iron phosphate salt particles is 480 nm, and the specific surface area is 11 m². 2 / g, the lithium iron phosphate granules were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., with product number CPP-001-003.

[0231] Example 9 The difference between Example 9 and Example 2 is that the primary average particle size of the first lithium iron phosphate salt particles is 3000 nm, and the specific surface area is 5 m². 2 / g.

[0232] Example 10 The difference between Example 10 and Example 2 is that the primary average particle size of the first lithium iron phosphate salt particles is 3500 nm, and the specific surface area is 4.6 m². 2 / g.

[0233] Example 11 The difference between Example 11 and Example 2 is that the first lithium iron phosphate salt particles have an average primary particle size of 500 nm and a specific surface area of ​​7 m². 2 / g.

[0234] Example 12 The difference between Example 12 and Example 2 is that the specific surface area of ​​the lithium manganese iron phosphate salt particles is 17 m². 2 / g, with a carbon content of 1.8% by weight, these lithium iron phosphate granules were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., with product number CPP-001-005.

[0235] Example 13 The difference between Example 13 and Example 2 is that the specific surface area of ​​the lithium manganese iron phosphate salt particles is 10 m². 2 / g, with a carbon content of 1.2% by weight, these lithium iron phosphate granules were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., with product number CPP-001-009.

[0236] Example 14 The difference between Example 14 and Example 2 is that the specific surface area of ​​the first lithium iron phosphate salt particles is 5.5 m². 2 / g, with a carbon content of 1.15% by weight.

[0237] Example 15 The difference between Example 15 and Example 2 is that the specific surface area of ​​the first lithium iron phosphate salt particles is 5 m². 2 / g, with a carbon content of 1.2% by weight.

[0238] Example 16 The difference between Example 16 and Example 2 is that the titanium content of the first lithium iron phosphate salt particles is 2500 ppm.

[0239] Example 17 The difference between Example 17 and Example 2 is that the titanium content of the first lithium iron phosphate salt particles is 6000 ppm.

[0240] Example 18 The difference between Example 18 and Example 2 is that vanadium pentoxide is used instead of titanium dioxide, and the specific surface area of ​​the first lithium iron phosphate salt particles is 8 m². 2 / g.

[0241] Example 19 The difference between Example 19 and Example 2 is that niobium pentoxide is used instead of titanium dioxide.

[0242] Example 20 The difference between Example 20 and Example 2 is that the specific surface area of ​​the first lithium iron phosphate salt particles is 3 m². 2 / g, with a carbon content of 0.8% by weight.

[0243] Example 21 The difference between Example 21 and Example 2 is that the specific surface area of ​​the first lithium iron phosphate salt particles is 12 m². 2 / g, with a carbon content of 1.4% by weight.

[0244] Example 22 The difference between Example 22 and Example 2 is that the primary average particle size of lithium manganese iron phosphate salt particles is 500 nm, and the specific surface area is 10.5 m². 2 / g, the lithium iron phosphate granules were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., with product number CPP-001-013.

[0245] Example 23 The difference between Example 23 and Example 2 is that the primary average particle size of lithium manganese iron phosphate salt particles is 140 nm, and the specific surface area is 20 m². 2 / g, the lithium iron phosphate granules were purchased from Guangdong Bangpu Recycling Technology Co., Ltd., with product number CPP-001-026.

[0246] Example 24 The difference between Example 24 and Example 2 is that the area ratio of the lithium iron phosphate salt particles is 70%, while the area ratio of the first lithium iron phosphate salt particles is 30%.

[0247] Example 25 The difference between Example 25 and Example 2 is that the area ratio of lithium manganese iron phosphate particles is 98%, while the area ratio of first lithium iron phosphate particles is 2%.

[0248] Example 26 The difference between Example 26 and Example 2 is that the titanium content of the first lithium iron phosphate salt particles is 1000 ppm.

[0249] Example 27 The difference between Example 27 and Example 2 is that the titanium content of the first lithium iron phosphate salt particles is 10,000 ppm.

[0250] Example 28 (1) Preparation of positive electrode slurry: The source / preparation method of lithium manganese iron phosphate particles and lithium iron phosphate particles is exactly the same as in Example 2.

[0251] The second lithium iron phosphate granules were purchased from Xiamen Tungsten New Energy Technology Co., Ltd., with the product number CPF-014-001.

[0252] The positive electrode active material is obtained by mixing lithium iron phosphate salt particles, lithium iron phosphate salt particles, and lithium iron phosphate salt particles according to the area ratio listed in Table 1. The above-mentioned mixed positive electrode active material, conductive carbon black, binder polyvinylidene fluoride, and dispersant PVP are mixed in a weight percentage of 93:4.0:2.5:0.5 and N-methylpyrrolidone is added. After thorough mixing, stirring, and dispersion, a positive electrode slurry is prepared.

[0253] (2) Preparation of the positive electrode sheet: It is exactly the same as Example 2.

[0254] (3) Preparation of negative electrode sheet: It is exactly the same as Example 2.

[0255] (4) Battery fabrication: It is exactly the same as Example 2.

[0256] Example 29 The difference between Example 29 and Example 28 is that the area ratio of lithium iron phosphate particles is 70%, the area ratio of first lithium iron phosphate particles is 25%, and the area ratio of second lithium iron phosphate particles is 5%.

[0257] Example 30 The difference between Example 30 and Example 28 is that the area ratio of lithium manganese iron phosphate particles is 65%, the area ratio of first lithium iron phosphate particles is 25%, and the area ratio of second lithium iron phosphate particles is 10%.

[0258] Example 31 The difference between Example 31 and Example 28 is that the area ratio of lithium manganese iron phosphate particles is 74%, the area ratio of first lithium iron phosphate particles is 25%, and the area ratio of second lithium iron phosphate particles is 1%.

[0259] Example 32 The difference between Example 32 and Example 28 is that the primary average particle size of the second lithium iron phosphate salt particles is 30 nm, and the specific surface area is 14 m². 2 / g, the second lithium iron phosphate granules were purchased from Xiamen Tungsten New Energy Technology Co., Ltd., with the product number XW-LFP-1.

[0260] Example 33 The difference between Example 33 and Example 28 is that the primary average particle size of the second lithium iron phosphate salt particles is 200 nm, and the specific surface area is 11 m². 2 / g, the second lithium iron phosphate particles were purchased from Xiamen Tungsten New Energy Technology Co., Ltd., with the product number XW-LFP-2.

[0261] Example 34 The difference between Example 34 and Example 28 is that the aspect ratio of the second lithium iron phosphate particles is 1.3, the ratio of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane is W=1.03, and the second lithium iron phosphate particles were purchased from Xiamen Tungsten New Energy Technology Co., Ltd., with the product number XW-LFP-3.

[0262] Example 35 The difference between Example 35 and Example 28 is that the aspect ratio of the second lithium iron phosphate particle is 1.3, the ratio of the intensity of the (020) crystal plane diffraction peak to the intensity of the (211) crystal plane diffraction peak of the second lithium iron phosphate particle is W=1.08, and the second lithium iron phosphate particle was purchased from Xiamen Tungsten New Energy Technology Co., Ltd., with the product number XW-LFP-4.

[0263] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that Comparative Example 1 does not contain lithium iron phosphate particles.

[0264] Comparative Example 2 The difference between Comparative Example 2 and Example 28 is that Comparative Example 2 does not contain lithium iron phosphate particles.

[0265] II. Battery Performance Testing 1) Primary average particle size First lithium iron phosphate particles: The electrode was cut open perpendicular to its large surface using an argon ion beam, exposing the cross-section. Scanning electron microscopy (SEM) images were taken of the cross-section, and the longest diameter of the first lithium iron phosphate (LFP) particles was statistically analyzed using a length-diameter statistical method. The "first-order average particle size" refers to the average of the first-order particle sizes of all particles, numerically equal to the total particle size divided by the total number of particles. In the cross-sectional image, the first-order particle size is defined as the longest distance connecting two points along the edge. Specifically, the total number of LFP particles with a first-order particle size greater than 80 nm and the sum of the first-order particle sizes of these particles can be counted from the SEM images. The first-order average particle size of LFP particles = total first-order particle size of LFP particles / total number of LFP particles. Particles with a first-order average particle size less than or equal to 80 nm are not included in the statistical analysis.

[0266] Second lithium iron phosphate particles: The electrode was cut open perpendicular to its large surface using an argon ion beam, exposing the cross-section. Scanning electron microscopy (SEM) images were taken of the cross-section, and the longest diameter of the lithium iron phosphate (LFP) particles was statistically analyzed using a length-diameter statistical method. The "first-order average particle size" refers to the average of the first-order particle sizes of all particles, numerically equal to the total particle size divided by the total number of particles. In the cross-sectional image, the first-order particle size is defined as the longest distance connecting two points along the edge. Specifically, the total number of LFP particles with a first-order particle size greater than 10 nm and the sum of the first-order particle sizes of these particles can be counted from the SEM images. The first-order average particle size of LFP particles = total first-order particle size of LFP particles / total number of LFP particles. Particles with a first-order average particle size less than or equal to 10 nm are not included in the statistical analysis.

[0267] Lithium manganese iron phosphate granules: The electrode was cut open perpendicular to its large surface using an argon ion beam, exposing the cross-section. Scanning electron microscopy (SEM) images were taken of the cross-section, and the longest diameter of the lithium manganese iron phosphate (LFP) particles was statistically analyzed using a length-diameter statistical method. The "first-order average particle size" refers to the average of the first-order particle sizes of all particles, numerically equal to the total particle size divided by the total number of particles. In the cross-sectional image, the first-order particle size is defined as the longest distance connecting two points along the edge. Specifically, the total number of LFP particles with a first-order particle size greater than 10 nm and the sum of the first-order particle sizes of these particles were counted in the SEM images. The first-order average particle size of LFP particles = total first-order particle size of LFP particles / total number of LFP particles. Particles with a first-order average particle size less than or equal to 10 nm were not included in the statistical analysis.

[0268] 2) Dv50 Referring to GB / T19077.1-2016, the Dv50 value of the particles was determined using a laser particle size analyzer (Malvern Master Size 3000). Furthermore, the Dv10, Dv90, and Dv99 values ​​described in this application were also determined in the same manner.

[0269] 3) Specific surface area The specific surface area was tested using the gas adsorption method, according to the GB / T19587-2017 testing standard. Specifically, lithium iron phosphate particles / lithium manganese iron phosphate particles were used as samples. The sample tube was immersed in liquid nitrogen at -196℃. The amount of nitrogen adsorbed on the solid surface under different pressures of 0.05-0.30 was measured. Based on the BET multilayer adsorption theory and its formula, the monolayer adsorption amount of the sample was obtained, and the specific surface area of ​​the material was calculated.

[0270] 4) Carbon content The carbon content of the first lithium iron phosphate particles, the second lithium iron phosphate particles, and the manganese iron phosphate particles was tested by infrared absorption method after combustion in a high-frequency induction furnace. The specific testing procedure was in accordance with the standard GB / T 20123-2006 / ISO15350:2000.

[0271] 5) The content of Q element (e.g., Ti element) in lithium iron phosphate particles The content of Q element in the first lithium iron phosphate granules was tested in accordance with GB / T 33822-2017.

[0272] 6) Area percentage The areas of the lithium iron phosphate particles, the first lithium iron phosphate particles, and the second lithium iron phosphate particles were obtained by statistical analysis of the obtained cross-sectional scanning electron microscope images of the electrode using Avizo 3D software. The area of ​​each particle was divided by the total area to obtain the corresponding area ratio.

[0273] 7) The ratio of the diffraction peak intensity of the (020) crystal plane to that of the (211) crystal plane is W The intensity ratio of the diffraction peaks of the second lithium iron phosphate particles was tested by X-ray diffraction. The second lithium iron phosphate particles were placed on the X-ray diffraction test platform of the Shimadzu XRD-7000 using a copper target X-ray diffractometer. The starting angle of the scan was 10°, the ending angle was 90°, and the step size was 0.013. The test was then started to obtain the diffraction pattern of the second lithium iron phosphate particles in the diffraction angle range of 10° to 90°. The ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane was determined according to the diffraction pattern.

[0274] 8) Aspect Ratio The aspect ratio is the ratio of the diameter along the major axis to the diameter along the minor axis of a primary particle. Let the diameter along the major axis be a and the diameter along the minor axis be b, then the aspect ratio = a / b. This parameter is often used to describe the morphology of particles and can be used to measure their sphericity.

[0275] Test method: After cold pressing, the electrode sheet was cut open perpendicular to the large surface area of ​​the electrode sheet using an Ar particle beam to expose the end face. Images were obtained using a scanning electron microscope. Subsequently, the scanning electron microscope images of the electrode sheet cross-section were analyzed using image processing software to measure the major axis a and minor axis b of the particles and obtain their ratio, which is the aspect ratio of the particles.

[0276] 9) The capacity percentage η of the first lithium iron phosphate particles in the examples and comparative examples when discharged to 3.2V and 2.0V respectively. First, a coin cell was prepared. The specific preparation process was as follows: 2.0000g of lithium iron phosphate particles were mixed with 0.1111g of conductive carbon black and 0.1111g of polyvinylidene fluoride, and then added to 2.5g of the organic solvent N-methylpyrrolidone. After thorough mixing, a slurry was formed. The slurry was coated onto aluminum foil to a thickness of 140 micrometers, dried under vacuum at 120℃ for 2 hours, and then punched into discs with a diameter of 13mm. The discs were then pressed using a tablet press at 10MPa and vacuum-treated at 120℃ for 12 hours to obtain the positive electrode. The weight of the positive electrode was measured, and the loading of lithium iron phosphate particles was found to be 11-12mg. A button cell battery is assembled in an argon-protected glove box, with a lithium metal sheet as the negative electrode, a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent as the electrolyte, LiPF6 as the electrolyte, and a Celgard 2400 microporous polyethylene membrane as the separator.

[0277] The prepared coin cells were tested for electrical performance using a blue-light tester. Specifically, the coin cells were charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V, followed by one charge and discharge cycle at a constant current rate of 1C. In the 1C charge and discharge test, the capacity value at a discharge voltage of 3.2V was recorded as C1, and the capacity value at a discharge voltage of 2.0V was recorded as C2, with η = C1 / C2. The charging process included constant voltage charging at a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0278] 10) Manganese leaching amount After 100 cycles of fully charged cell charging, disassemble it inside the glove box. In the drying room, remove the anode electrode and gently scrape 1g of material from the center of the electrode using a ceramic knife, ensuring the entire active material layer is scraped off to avoid inconsistencies in manganese content at different thicknesses. Seal the bag and send it to the laboratory. Prepare an electronic scale, heating plate, 150ml quartz beaker, watch glass, funnel, 100ml volumetric flask, pliers (heat-resistant gloves), weighing spoon, and lint-free paper. Confirm the electronic scale's calibration date is valid; expired scales cannot be used and must be recalibrated. Connect the power; the heating plate should heat up to 250℃. Weigh 0.2000 ± 0.005g. The powder was placed in a beaker; acidification treatment: a dilute sulfuric acid solution with a volume ratio of 1:4 (concentrated sulfuric acid: ultrapure water = 1:4) was prepared; 20 ml of the prepared dilute sulfuric acid solution was added to the beaker; the heating plate was preheated to 250℃, and the sample was placed in the beaker for digestion for 30 min, with a quartz cap placed on the mouth of the beaker to reduce acid evaporation; after the sample digestion for 30 min was completed, it was removed and cooled to room temperature; the digested sample was transferred to a 100 ml glass volumetric flask and diluted to volume; then the ICP standard test procedure was performed: the test equipment temperature was 22±2℃, and the humidity was <60%; the trace element method was selected; the calibration standard was run: a standard curve was determined using the prepared standard solution; the element selected was Mn; finally, the amount of Mn dissolved was obtained through testing.

[0279] 11) Solid content of positive electrode slurry Prepare an electronic balance (accuracy 0.0001), an oven, and a glass drying tray. Take 8-10g of the positive electrode slurry sample and evenly spread it on the sample tray. Record the slurry mass before drying as A. Close the oven door and heat. As heating progresses, the oven temperature continuously increases, reaching 130℃ for 5 hours. After drying, remove the sample from the oven and record the mass of the dried slurry. Repeat the drying process multiple times until the sample reaches a constant weight, recording the mass after drying as B. The slurry solid content = (A / B) × 100%.

[0280] 12) Electrode compaction density The term "electrode compaction density" used in this application refers to the "ultimate compaction density" of the electrode, and its testing method is as follows: The double-coated electrode sheets were cold-pressed using a roller press, and the elongation of the cold-pressed electrode sheets was tested. At the same time, the flexibility of the cold-pressed electrode sheets was evaluated.

[0281] By increasing the pressure of the roller press, electrodes with different compaction densities can be obtained. As the pressure increases, the compaction density of the electrode increases, the elongation of the electrode increases, and the flexibility of the electrode decreases. Excessive elongation of the electrode can easily lead to warping, while insufficient flexibility can easily lead to brittle fracture. Therefore, the lower of the compaction density corresponding to an electrode elongation of 6‰ or an electrode flexibility folding number of 3 times is defined as the limiting compaction density.

[0282] The compaction density is calculated by dividing the weight of a single positive electrode film layer by the volume of the positive electrode film layer.

[0283] 13) Gram capacity The specific testing process is briefly described as follows: (1) Place the battery in a 40℃ oven environment and let it stand for 2 hours until the battery temperature is maintained at 40℃; (2) Discharge the battery at a constant current of 1 / 3C to 2.0V; (3) Pause for 5 minutes; (4) Charge the battery at a constant current of 1 / 3C to 4.1V, and then charge it at a constant voltage of 4.1V until the cutoff current is 0.05C; (5) Pause for 5 minutes; (6) Discharge the battery at a constant current of 1 / 3C to 2.0V. This step is the actual cell test capacity. Combined with the mass of the positive electrode active material, the specific capacity of the battery can be calculated. Specific capacity = capacity / mass of positive electrode active material.

[0284] 14) High-temperature storage performance High-temperature storage performance was characterized by storing the battery at 60°C under full charge (charged to 4.1V) and measuring the time it took for the battery capacity to decay to 80% of its initial value.

[0285] 1. Let stand for 10 minutes, then discharge at a constant current of 0.33Cn (Cn is the battery capacity at 40℃ and 1 / 3C) to 2V.

[0286] 2. Let stand for 10 minutes, charge at a constant current of 0.33Cn to 4.1V, then charge at a constant voltage, with a cutoff current of 0.02Cn.

[0287] 3. Let stand for 2 hours, store at 60℃ for 30 days, and then take out to test the reversible capacity of the battery cell.

[0288] The specific testing process is briefly described as follows: (1) Place the battery in a 40℃ oven environment and let it stand for 2 hours until the battery temperature is maintained at 40℃; (2) Discharge at 1 / 3C constant current to 2.0V; (3) Pause for 5 minutes; (4) Charge at 1 / 3C constant current to 4.1V, and then charge at 4.1V constant voltage until the cutoff current is 0.05C; (5) Pause for 5 minutes; (6) Discharge at 1 / 3C constant current to 2.0V. This step is for testing the actual cell capacity.

[0289] 4. Conduct tests regularly until the reversible capacity of the battery cell degrades to 80% of its initial value.

[0290] 15) High-temperature cycling performance High-temperature cycling performance is characterized by measuring the number of cycles required when the battery capacity decays to 80% of its initial value using a 1C / 1C cycle at 60°C.

[0291] The specific process is briefly described as follows: (1) Place the battery in a 45℃ oven and let it stand for 2 hours until the battery temperature is maintained at 25℃; (2) Charge the battery with a constant current of 1C to 3.65V, and continue charging with a constant voltage until the charging current is less than 0.05C and then stop; (3) Pause for 5 minutes; (4) Discharge the battery with a constant current of 1C to 2.5V; (5) Pause for 5 minutes. Steps (2) to (6) constitute one charge-discharge cycle of the battery. Repeat steps (2) to (6) continuously until the battery capacity decays to 80% of the initial value.

[0292] III. Analysis of Test Results for Each Embodiment and Comparative Example Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The parameters of the positive electrode active material are shown in Table 1, and the performance test results are shown in Table 2.

[0293]

[0294]

[0295] Table 2: Performance Test Results

[0296] Based on the above results, it can be seen that the secondary batteries in Examples 1-31 all include a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive active material, which includes lithium manganese iron phosphate particles with a primary average particle size of 120-600 nm. The manganese leaching amount in the secondary battery is ≤50 ppm. A comparison between Examples 1-31 and Comparative Examples 1-2 shows that the secondary batteries of this application have better high-temperature storage performance. Furthermore, the secondary batteries of this application also have better high-temperature cycling performance and a higher solid content in the positive electrode slurry.

[0297] As can be seen from the comparison between Examples 2, 9, and 11 and Examples 5-6, when the positive electrode active material also includes first lithium iron phosphate particles, and the primary average particle size s1 of the first lithium iron phosphate particles satisfies 500nm≤s1≤3000nm, the secondary battery of the present application embodiment has a lower manganese dissolution, better high-temperature storage performance, better high-temperature cycling performance, and higher solid content of the positive electrode slurry.

[0298] As can be seen from the comparison between Examples 2, 9, 10-11 and Examples 5-6, when the positive electrode active material also includes first lithium iron phosphate particles, and the primary average particle size s1 of the first lithium iron phosphate particles satisfies 500nm≤s1≤3500nm, the secondary battery of the present application embodiment has a lower manganese dissolution, better high-temperature storage performance, better high-temperature cycling performance, and higher solid content of the positive electrode slurry.

[0299] As can be seen from the comparison between Examples 2, 7, 9, 10-11 and Examples 5-6, when the positive electrode active material also includes first lithium iron phosphate particles, and the primary average particle size s1 of the first lithium iron phosphate particles satisfies 500nm≤s1≤4000nm, the secondary battery of the present application embodiment has a lower manganese leaching amount, better high-temperature storage performance, and higher solid content of the positive electrode slurry.

[0300] As can be seen from the comparison between Example 2 and Examples 28-35, when the positive electrode active material also includes second lithium iron phosphate particles, and the primary average particle size s2 of the second lithium iron phosphate particles satisfies 30nm≤s2≤200nm, the secondary battery of this application embodiment has a lower manganese leaching amount, better high-temperature storage performance, and better high-temperature cycling performance.

[0301] A comparison of Examples 2, 16-17 and Examples 26-27 shows that, based on the total weight of the first lithium iron phosphate particles, when the Ti content in the first lithium iron phosphate particles is 2500-6000 ppm, the secondary battery of this application embodiment has better high-temperature storage performance and higher specific capacity.

[0302] A comparison of Examples 2, 14-15, 18, and 20 with Example 21 shows that when the specific surface area of ​​the first lithium iron phosphate salt particles is 3 m², 2 / g-8 m 2 At / g, the secondary battery of the present application embodiment has a lower manganese leaching amount, a higher solid content of the positive electrode slurry, and a higher compaction density of the positive electrode sheet.

[0303] A comparison of Examples 2, 10-11, 16 and Example 17 shows that when the specific surface area of ​​the first lithium iron phosphate salt particles is 3 m², 2 / g-6 m 2 At / g, the secondary battery of this application embodiment has good high-temperature storage performance, good high-temperature cycling performance, and high positive electrode compaction density.

[0304] A comparison of Examples 2, 14-15, 20 and Example 21 shows that, based on the total weight of the first lithium iron phosphate particles, the carbon content of the first lithium iron phosphate particles is Cx1% by weight. When 0.8 ≤ Cx1 ≤ 1.2, the secondary battery of this application embodiment has better high-temperature storage performance, lower manganese leaching, higher solid content of the positive electrode slurry, better high-temperature cycling performance, and higher positive electrode sheet compaction density.

[0305] A comparison of Examples 2, 14-15, 20 and Example 21 shows that when the ratio z1 of the specific surface area of ​​the first lithium iron phosphate salt particles to Cx1 satisfies 3.8≤z1≤5, the secondary battery of this application embodiment has better high-temperature storage performance, lower manganese leaching, higher solid content of positive electrode slurry, better high-temperature cycling performance, and higher positive electrode sheet compaction density.

[0306] As can be seen from the comparison between Examples 34-35 and Example 28, when the aspect ratio of the second lithium iron phosphate salt particles is ≥1.3, the secondary battery of this application embodiment has better high-temperature storage performance, lower manganese leaching, better high-temperature cycling performance, and higher specific capacity.

[0307] As can be seen from the comparison between Examples 34-35 and Example 28, when the ratio W of the diffraction peak intensity of the (020) crystal plane to the diffraction peak intensity of the (211) crystal plane of the second lithium iron phosphate salt particles is ≥1.03, the secondary battery of this application embodiment has better high-temperature storage performance, lower manganese leaching, better high-temperature cycling performance, and higher specific capacity.

[0308] A comparison of Examples 2, 12-13 and Example 23 shows that when the specific surface area of ​​lithium manganese iron phosphate particles is 10 m², 2 / g-17 m 2 At / g, the secondary battery of the present application embodiment has good high-temperature storage performance, low manganese leaching, good high-temperature cycling performance, and high positive electrode compaction density.

[0309] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, It includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a positive electrode active material, which includes lithium manganese iron phosphate particles and first lithium iron phosphate particles. The primary average particle size of the lithium manganese iron phosphate particles is 120nm-600nm. Based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet, the area ratio of the first lithium iron phosphate particles is 2%-30%, and the area ratio of the first lithium manganese iron phosphate particles is 70%-98%.

2. The secondary battery according to claim 1, characterized in that, The primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 500nm≤s1≤3000nm.

3. The secondary battery according to claim 1 or 2, characterized in that, The positive electrode active material also includes second lithium iron phosphate particles, wherein the primary average particle size s2 of the second lithium iron phosphate particles satisfies 30nm≤s2≤200nm.

4. The secondary battery according to claim 3, characterized in that, Based on the total area of ​​the positive electrode active material in the cross-section of the positive electrode sheet, the area ratio of the second lithium iron phosphate particles is 1-10%; the area ratio of the first lithium iron phosphate particles is 2-25%; and the area ratio of the lithium manganese iron phosphate particles is 70%-97%.

5. The secondary battery according to any one of claims 1-4, characterized in that, The lithium manganese iron phosphate particles have the molecular formula Li m Fe x Mn r P y O j Q q Q includes at least one of Al, Na, K, Mg, Cu, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, and has the following properties: 0.95 ≤ m ≤ 1.15, x > 0, r > 0, 0.9 ≤ x + r ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 ≤ q ≤ 0.1, and / or The first lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O j1 Q1 q1 Q1 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 ≤ m1 ≤ 1.15, 0.9 ≤ x1 ≤ 1, 0.95 ≤ y1 ≤ 1, 3.5 ≤ j1 ≤ 4, 0 < q1 ≤ 0.1, and / or The second lithium iron phosphate salt particles have the molecular formula Li m2 Fe x2 P y2 O j2 Q2 q2 Q2 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, with 0.95≤m2≤1.15, 0.9≤x2≤1, 0.95≤y2≤1, 3.5≤j2≤4, and 0≤q2≤0.

1.

6. The secondary battery according to claim 5, characterized in that, In the first lithium iron phosphate particles, Q1 includes at least one of Ti, V, Mg, and Nb, and the content of Ti, V, Mg and / or Nb is 1000-10000 ppm based on the total weight of the first lithium iron phosphate particles.

7. The secondary battery according to any one of claims 1-6, characterized in that, The specific surface area of ​​the first lithium iron phosphate salt particles is 3 m². 2 / g-8 m 2 / g.

8. The secondary battery according to any one of claims 1-7, characterized in that, Based on the total weight of the first lithium iron phosphate particles, the carbon content of the first lithium iron phosphate particles is Cx1% by weight, where 0.8 ≤ Cx1 ≤ 2.

0.

9. The secondary battery according to any one of claims 3-8, characterized in that, The primary average particle size of the lithium iron phosphate salt particles is 150-210 nm, the primary average particle size s1 of the first lithium iron phosphate salt particles satisfies 870 nm ≤ s1 ≤ 3000 nm, and / or the primary average particle size s2 of the second lithium iron phosphate salt particles satisfies 60 nm ≤ s2 ≤ 200 nm.

10. The secondary battery according to any one of claims 1-9, characterized in that, The capacity percentage η of the first lithium iron phosphate salt particles is ≥ 88%, where η is defined as: The battery using the first lithium iron phosphate salt particles as the positive electrode active material was charged and discharged twice at a constant current rate of 0.1C within a 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 1C rate, the capacity value extracted at the discharge voltage of 3.2V was recorded as C1, and the capacity value extracted at the discharge voltage of 2.0V was recorded as C2, and η=C1 / C2. The charging process included constant voltage charging, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

11. The secondary battery according to any one of claims 1-10, characterized in that, The first lithium iron phosphate salt particles satisfy at least one of (a)-(f): (a) The Dv10 of the first lithium iron phosphate salt particles is ≥0.2 μm; (b) The Dv50 of the first lithium iron phosphate salt particles is 0.5-5 μm; (c) The Dv90 of the first lithium iron phosphate salt particles is ≤10μm; (d) The Dv99 of the first lithium iron phosphate salt particles is ≤12μm; (e) The compacted density of the first lithium iron phosphate powder under a pressure of 3 tons is ≥2.25 g / cm³. 3 ; (f) The resistivity of the first lithium iron phosphate powder is less than 60 Ω·cm.

12. The secondary battery according to any one of claims 3-11, characterized in that, The aspect ratio of the second lithium iron phosphate salt particles is ≥1.

3.

13. The secondary battery according to any one of claims 3-12, characterized in that, The ratio of the intensity of the (020) crystal plane diffraction peak to the intensity of the (211) crystal plane diffraction peak of the second lithium iron phosphate salt particle is W≥1.

03.

14. The secondary battery according to any one of claims 1-14, characterized in that, The manganese leaching amount of the secondary battery is ≤50ppm.

15. The secondary battery according to any one of claims 1-14, characterized in that, The first lithium iron phosphate salt particles are mainly obtained by the following preparation method: Provide raw materials containing at least lithium source, iron source, phosphorus source, carbon film-forming agent, carbon source, and modifier, and perform at least two sintering processes, wherein... The temperature for the first sintering is 500℃-760℃; The temperature for the second sintering is 700℃-800℃.

16. The secondary battery according to claim 15, characterized in that, The first lithium iron phosphate salt particles are mainly obtained by the following preparation method: The raw materials provided contain at least lithium source, iron source, phosphorus source, carbon source, carbon film-forming agent, and modifier, and are subjected to at least two sintering processes. The carbon content of the material after the first sintering is 0.01%-0.79% by weight. The carbon content of the material after the second sintering is 0.8%-2.0% by weight.

17. The secondary battery according to claim 15 or 16, characterized in that, The preparation method of the first lithium iron phosphate salt particles includes the following steps: After the first sintering, a first pulverization is performed; after the second sintering, a second pulverization is performed. The Dv50 of the product after the first pulverization is 300nm-1200nm; The Dv50 of the product after the second pulverization is 500nm-5000nm.

18. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1-17.

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