Positive electrode active material, positive electrode sheet, lithium-ion battery, and electric device

Abstract

A positive electrode active material, a positive electrode sheet, a lithium-ion battery, and an electric device. The positive electrode active material comprises lithium iron phosphate particles. The volume proportion of the lithium iron phosphate particles with a particle size less than 1 μm is x, the volume proportion of the lithium iron phosphate particles with a particle size greater than 5 μm is z, and the range of z / x is 0.5-2.8.

Description

Positive electrode active materials, positive electrode sheets, lithium-ion batteries and electrical equipment

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is based on the Chinese patent application with application number 202311058729X and application date August 22, 2023, and claims the priority of the Chinese patent application. The entire content of the Chinese patent application is hereby introduced into this application as a reference. Technical Field

[0003] The present application relates to the field of battery technology, and in particular to a positive electrode active material, a positive electrode sheet, a lithium-ion battery, and an electrical device. Background Art

[0004] As a device for energy storage and conversion, lithium-ion batteries offer numerous advantages over traditional nickel-metal hydride and lead-acid batteries, including high energy density, high operating voltage, high energy conversion efficiency, long cycle life, and no memory effect. The cathode material, the most critical core component of lithium-ion batteries, is a key factor in determining their performance, including cost, voltage, energy density, rate capability, cycle life, and safety. Among the many cathode materials used in lithium-ion batteries, lithium iron phosphate (LiFePO4) boasts low cost, abundant raw materials, safety, environmental protection, and excellent thermal stability.

[0005] However, when lithium iron phosphate is used as a positive electrode material in related technologies, it still suffers from problems such as high internal resistance and poor conductivity, which restricts further improvement of battery performance using lithium iron phosphate as a positive electrode material. Therefore, there is room for improvement.

[0006] Summary of the Invention

[0007] The present application aims to solve at least one of the technical problems existing in the prior art. To this end, one purpose of the present application is to provide a positive electrode active material having a high compaction density, which can reduce impedance, improve conductivity, and enhance the capacity of the positive electrode active material.

[0008] The present application also proposes a positive electrode sheet having the above-mentioned positive electrode active material.

[0009] The present application also proposes a lithium-ion battery having the above-mentioned positive electrode sheet.

[0010] The present application also proposes an electrical device having the above-mentioned lithium-ion battery.

[0011] According to the positive electrode active material of the embodiment of the first aspect of the present application, it includes: lithium iron phosphate particles, the volume proportion of the lithium iron phosphate particles with a particle size less than 1 μm is x, the volume proportion of the lithium iron phosphate particles with a particle size greater than 5 μm is z, and the range of z / x is 0.5 to 2.8.

[0012] According to the positive electrode active material of the embodiment of the present application, by controlling the ratio of the proportion of lithium iron phosphate particles with a particle size greater than 5um to the proportion of lithium iron phosphate particles with a particle size less than 1um within the range of 0.5 to 2.8, the positive electrode active material can have a higher compaction density, reduce impedance, improve conductivity, and enhance the capacity of the positive electrode active material.

[0013] According to the second embodiment of the present application, the positive electrode plate includes: a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector, wherein the positive electrode active material layer includes the positive electrode active material according to the first embodiment of the present application.

[0014] According to the positive electrode sheet of the embodiment of the present application, by setting the positive electrode active material layer to include the above-mentioned positive electrode active material, the impedance can be reduced, the conductivity can be improved, and the capacity of the positive electrode active material layer can be enhanced.

[0015] A lithium-ion battery according to an embodiment of the third aspect of the present application includes: a shell; an electrode assembly, disposed in the shell, the electrode assembly including a negative electrode sheet and a positive electrode sheet, and the positive electrode sheet is the positive electrode sheet according to the embodiment of the second aspect of the present application.

[0016] According to the lithium-ion battery of the embodiment of the present application, by setting the positive electrode plate to the above-mentioned positive electrode plate, the impedance can be reduced, the conductivity can be improved, and the capacity of the positive electrode can be enhanced.

[0017] The electrical equipment according to the fourth embodiment of the present application includes: a lithium-ion battery according to the third embodiment of the present application.

[0018] According to the electrical equipment of the embodiment of the present application, by setting the above-mentioned lithium-ion battery, the lithium-ion battery can be used as a power source for the electrical equipment. Since the overall performance of the lithium-ion battery is good, the overall performance of the electrical equipment can be improved.

[0019] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become obvious from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0021] FIG1 is a comparison of scanning electron micrographs of lithium iron phosphate particles in Example 6 and lithium iron phosphate particles in Comparative Example 1;

[0022] FIG2 is a comparison of the particle size distribution of the lithium iron phosphate particles in Example 6 and the lithium iron phosphate particles in Comparative Example 1;

[0023] FIG3 is a comparison of X-ray diffraction patterns of the lithium iron phosphate particles in Example 6 and the lithium iron phosphate particles in Comparative Example 1;

[0024] FIG4 is a comparison of the compaction density-pressure curves of the lithium iron phosphate particles in Example 6 and the lithium iron phosphate particles in Comparative Example 1;

[0025] 5 is a comparison of powder resistivity-pressure curves of the lithium iron phosphate particles in Example 6 and the lithium iron phosphate particles in Comparative Example 1;

[0026] FIG6 is a comparison of EIS impedance spectra of half-button cells corresponding to the lithium iron phosphate particles in Example 6 and the lithium iron phosphate particles in Comparative Example 1;

[0027] FIG7 is a comparison diagram of the gram capacity-voltage curves of the half-button battery corresponding to the lithium iron phosphate particles in Example 6 and the lithium iron phosphate particles in Comparative Example 1. DETAILED DESCRIPTION

[0028] The following describes in detail embodiments of the present application. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application and are not to be construed as limiting the present application.

[0029] The following describes the positive electrode active material according to an embodiment of the present application.

[0030] According to the positive electrode active material of the embodiment of the first aspect of the present application, it includes: lithium iron phosphate particles, the volume proportion of lithium iron phosphate particles with a particle size less than 1um is x, the volume proportion of lithium iron phosphate particles with a particle size greater than 5um is z, and the range of z / x is 0.5 to 2.8.

[0031] It should be noted that the volume proportion of lithium iron phosphate particles with a particle size less than 1 μm refers to the ratio of the total volume of all lithium iron phosphate particles with a particle size less than 1 μm in the positive electrode active material to the total volume of all lithium iron phosphate particles in the positive electrode active material; the volume proportion of lithium iron phosphate particles with a particle size greater than 5 μm refers to the ratio of the total volume of all lithium iron phosphate particles with a particle size greater than 5 μm in the positive electrode active material to the total volume of all lithium iron phosphate particles in the positive electrode active material.

[0032] When the value of z / x is less than 0.5, it indicates that there are more lithium iron phosphate particles of smaller size. Too many lithium iron phosphate particles with smaller particle size will lead to lower compaction density, smaller spacing and gaps between lithium iron phosphate particles, smaller space for absorbing electrolyte, and correspondingly smaller amount of electrolyte absorbed. The electrolyte cannot achieve good infiltration of lithium iron phosphate particles, which will lead to a relatively large diffusion impedance of lithium ions between lithium iron phosphate particles. However, for lithium iron phosphate particles with smaller particle size, it is easier for charges to migrate within a single lithium iron phosphate particle, the speed is faster, and the charge migration impedance is smaller. In addition, the more particles of material of the same mass, the more difficult it is to disperse under the same process conditions. The larger the number of small-sized lithium iron phosphate particles, the more likely it is to cause particle agglomeration problems.

[0033] When the value of z / x is greater than 2.8, it indicates that there are more large-sized lithium iron phosphate particles, and there are too many lithium iron phosphate particles with larger particle sizes. For lithium iron phosphate particles with larger particle sizes, the diffusion path of lithium ions inside a single lithium iron phosphate particle is longer, the diffusion impedance is larger, the charge transfer impedance is also larger, and the electrochemical reaction activity is reduced. However, when there are more lithium iron phosphate particles with larger particle sizes, the compaction density can be increased, the spacing and gaps between lithium iron phosphate particles are larger, the space for absorbing electrolyte is larger, and the corresponding amount of electrolyte absorbed is also larger. The electrolyte can better infiltrate the lithium iron phosphate particles, so that the diffusion impedance of lithium ions between lithium iron phosphate particles is relatively small. In addition, when there are more large-sized lithium iron phosphate particles, in the process of preparing the positive electrode sheet, the internal stress of the positive electrode material of the positive electrode sheet is longer, and the large particles are easy to break, resulting in the loss of active material, the formation of dead lithium, and the loss of capacity.

[0034] In short, lithium iron phosphate particles with smaller particle sizes have lower compaction density, and the diffusion resistance of lithium ions between lithium iron phosphate particles is larger. In addition, lithium iron phosphate particles with smaller particle sizes are prone to agglomeration.

[0035] Larger lithium iron phosphate particles have a higher compaction density, which reduces the diffusion impedance of lithium ions between the particles, but increases the charge transfer impedance. In addition, smaller lithium iron phosphate particles have the problem of large particles being easily broken, resulting in loss of active material.

[0036] The present application controls the ratio of larger lithium iron phosphate particles and smaller lithium iron phosphate particles within a certain range, and controls the ratio of lithium iron phosphate particles with a particle size greater than 5um to the ratio of lithium iron phosphate particles with a particle size less than 1um within the range of 0.5 to 2.8. In this way, the ratio of larger and smaller particles in the lithium iron phosphate particles can be more appropriate, the stacking between the particles can be more reasonable, the positive electrode active material as a whole can have a higher compaction density, the impedance can be reduced, the conductivity can be improved, and the capacity of the positive electrode active material can be enhanced.

[0037] The lithium iron phosphate particles of the embodiment of the present application can be prepared by a high-temperature solid-phase method or a liquid-phase method. For example, the size and size distribution of the prepared lithium iron phosphate particles can be controlled by controlling reaction parameters (such as reaction time, reaction temperature, etc.).

[0038] For example, according to some embodiments of the present application, a lithium source, a divalent iron source, a phosphorus source, and a carbon source in a certain ratio are mixed uniformly by mechanical ball milling or sand milling to prepare a precursor; then, the precursor is reacted at a temperature of 600-1200°C in an inert atmosphere for 2-10 hours to produce lithium iron phosphate particles. The ratio of the lithium source, divalent iron source, phosphorus source, and carbon source can be (1.0-1.5):1:1:(0.01-0.2), which refers to the molar ratio.

[0039] For another example, according to some embodiments of the present application, a lithium source, a divalent iron source, and a phosphorus source in a certain ratio are stirred in a mechanically stirred hydrothermal kettle (solid content 20% to 60%) for 10 to 120 minutes to prepare a precursor; then, the precursor is reacted at a temperature of 120 to 200°C in an inert atmosphere for 2 to 12 hours to prepare lithium iron phosphate particles. The ratio of the lithium source, divalent iron source, phosphorus source, and carbon source can be (2.3 to 3.5):1:1, which refers to the molar ratio.

[0040] The lithium source may include at least one of lithium hydroxide (LiOH), lithium carbonate (Li2CO3), lithium bicarbonate (LiHCO3), lithium sulfate (Li2SO4), lithium chloride (LiCl), lithium nitrate (LiNO3) and lithium oxalate (Li2C2O4) and hydrates thereof.

[0041] The iron source may include at least one of ferrous oxide (FeO), ferrous oxide (Fe3O4), ferrous sulfate (FeSO4), ferric sulfate (Fe2(SO4)3), ferric phosphate (FePO4), ferrous chloride (FeCl2), ferric chloride (FeCl3), ferrous nitrate (Fe(NO3)2), ferric nitrate (Fe(NO3)3) and hydrates thereof.

[0042] The phosphorus source may include phosphoric acid (H3PO4), or a phosphorus-lithium compound such as lithium phosphate (Li3PO4), lithium dihydrogen phosphate (LiH2PO4), or lithium monohydrogen phosphate (Li2HPO4).

[0043] Carbon sources may include glucose, polyethylene glycol, etc.

[0044] According to the positive electrode active material of the embodiment of the present application, by controlling the ratio of the proportion of lithium iron phosphate particles with a particle size greater than 5um to the proportion of lithium iron phosphate particles with a particle size less than 1um within the range of 0.5 to 2.8, the positive electrode active material can have a higher compaction density, reduce impedance, improve conductivity, and enhance the capacity of the positive electrode active material.

[0045] According to some embodiments of the present application, the range of z / x is 1.5 to 2.5. For example, the value of z / x can be 1.5, 1.8, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, etc. By further controlling the ratio of the proportion of lithium iron phosphate particles with a particle size greater than 5 μm to the proportion of lithium iron phosphate particles with a particle size less than 1 μm within the range of 1.5 to 2.5, the positive electrode active material can have a higher compaction density, which can better reduce impedance, better improve conductivity, and better enhance the capacity of the positive electrode active material.

[0046] According to some embodiments of the present application, the volume fraction of lithium iron phosphate particles having a particle size greater than 5 μm and less than 20 μm is z. By controlling the particle size of the larger lithium iron phosphate particles to be between 5 μm and 20 μm, the particle size of the larger lithium iron phosphate particles can be made more reasonable, avoiding poor particle size consistency caused by the larger lithium iron phosphate particles being too large, and further improving the overall performance of the positive electrode active material containing the lithium iron phosphate particles.

[0047] According to some embodiments of the present application, x ranges from 11% to 30%, and z ranges from 5% to 40%. For example, the value of x can be 11%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, etc.; the value of z can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, etc.

[0048] By controlling the proportion of smaller-sized lithium iron phosphate particles within 11% to 30%, and the proportion of larger-sized lithium iron phosphate particles within 5% to 40%, the proportion of smaller-sized lithium iron phosphate particles and the proportion of larger-sized lithium iron phosphate particles are controlled within a reasonable range, and the proportion of smaller-sized lithium iron phosphate particles and the proportion of larger-sized lithium iron phosphate particles are both controlled within a lower range, so that the proportion of larger-sized lithium iron phosphate particles and smaller-sized lithium iron phosphate particles will not be too high, thereby making the particle size distribution of the lithium iron phosphate particles in the entire positive electrode active material more reasonable, and better improving the overall performance of the positive electrode active material with the lithium iron phosphate particles.

[0049] According to some embodiments of the present application, the volume proportion of lithium iron phosphate particles with a particle size of not less than 1 μm and not more than 5 μm is y, and the range of y is 40% to 80%. By controlling the proportion of lithium iron phosphate particles with a particle size of not less than 1 μm and not more than 5 μm to 40% to 80%, lithium iron phosphate particles with a size of 1 μm to 5 μm can occupy a larger proportion of the entire positive electrode active material, making the distribution of lithium iron phosphate particles of different particle sizes in the entire positive electrode active material more reasonable, resulting in a higher compaction density of the positive electrode active material, lower overall impedance, and better conductivity.

[0050] It should be noted that the volume ratio of lithium iron phosphate particles with a particle size of not less than 1 μm and not more than 5 μm refers to the ratio of the total volume of all lithium iron phosphate particles with a particle size of not less than 1 μm and not more than 5 μm in the positive electrode active material to the total volume of all lithium iron phosphate particles in the positive electrode active material.

[0051] According to some embodiments of the present application, the unit cell volume of lithium iron phosphate particles is V, and the unit of V is The value range of V is 291.1 to 294.6. If the single cell volume V of the lithium iron phosphate particles is too small, lithium ions will be difficult to embed. If the single cell volume V of the lithium iron phosphate particles is too large, the release of lithium ions may cause the single cell skeleton to be unstable, resulting in structural collapse, loss of active materials, and loss of capacity. By controlling the single cell volume V of the lithium iron phosphate particles to 291.1 to 294.6, the structure of the single cell of the lithium iron phosphate particles can be made more stable, less prone to structural collapse, and more conducive to the embedding of lithium ions into the lattice of the lithium iron phosphate particles, thereby further improving the electrochemical performance of the positive electrode active material.

[0052] According to some embodiments of the present application, the unit cell volume of lithium iron phosphate particles is V, and the unit of V is The value range of (z / x)*(V / 300) is 0.49 to 2.74. By satisfying the relationship between the single unit cell volume V of the lithium iron phosphate particles and the above z / x: 0.49≤(z / x)*(V / 300)≤2.74, the particle size distribution of the lithium iron phosphate particles and the single unit cell volume V of the lithium iron phosphate can be taken into account. This not only makes the particle size distribution of the lithium iron phosphate particles in the positive electrode active material reasonable, but also forms a stable unit cell, so that the positive electrode active material has a high compaction density, high conductivity, high gram capacity and excellent structural stability, thereby improving the electrochemical performance of the positive electrode active material.

[0053] According to the second embodiment of the present application, the positive electrode plate includes: a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector, and the positive electrode active material layer includes the positive electrode active material according to the first embodiment of the present application.

[0054] According to the positive electrode sheet of the embodiment of the present application, by setting the positive electrode active material layer to include the above-mentioned positive electrode active material, the impedance can be reduced, the conductivity can be improved, and the capacity of the positive electrode active material layer can be enhanced.

[0055] According to the third embodiment of the present application, the lithium-ion battery includes: a shell and an electrode assembly, the electrode assembly is arranged in the shell, the electrode assembly includes a negative electrode sheet and a positive electrode sheet, a separator is arranged between the negative electrode sheet and the positive electrode sheet, and the positive electrode sheet is the positive electrode sheet according to the above-mentioned second embodiment of the present application.

[0056] According to the lithium-ion battery of the embodiment of the present application, by setting the positive electrode plate to the above-mentioned positive electrode plate, the impedance can be reduced, the conductivity can be improved, and the capacity of the positive electrode can be enhanced.

[0057] The electrical equipment according to the fourth embodiment of the present application includes: a lithium-ion battery according to the third embodiment of the present application.

[0058] Optionally, the electric device may be an electric vehicle.

[0059] According to the electrical equipment of the embodiment of the present application, by setting the above-mentioned lithium-ion battery, the lithium-ion battery can be used as a power source for the electrical equipment. Since the overall performance of the lithium-ion battery is good, the overall performance of the electrical equipment can be improved.

[0060] The positive electrode active materials of the embodiments of the present application are further described below with reference to some embodiments and comparative examples.

[0061] The lithium iron phosphate particles used in the positive electrode active materials of each embodiment and comparative example were prepared. The lithium iron phosphate particles of each embodiment and comparative example were prepared by a high-temperature solid-phase method.

[0062] Comparative Example 1: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 700°C in an inert atmosphere for 8 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 22:78:0, and V was

[0063] Comparative Example 2: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 720°C in an inert atmosphere for 8 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 24:70:6, and V was

[0064] Example 1: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly in a ratio of 1.05:1:0.1 by mechanical ball milling or sand milling, and then reacted at 780°C in an inert atmosphere for 9 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 24:64:12, and V was

[0065] Example 2: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 790°C in an inert atmosphere for 8.5 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 24:71:5, and V was

[0066] Example 3: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 800°C in an inert atmosphere for 8 hours to obtain lithium iron phosphate (LFP) particles with a distribution ratio of 0-1um:1-5um:5-20um and a ratio of 21:58:21, and V was

[0067] Example 4: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 810°C in an inert atmosphere for 8 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 13:67:20, and V was

[0068] Example 5: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 815°C in an inert atmosphere for 9.5 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 12:66:22, and V was

[0069] Example 6: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 820°C in an inert atmosphere for 10 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 14:55:31. V is

[0070] Example 7: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 850°C in an inert atmosphere for 12 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 14:51:35, and V was

[0071] Example 8: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly in a ratio of 1.05:1:0.1 by mechanical ball milling or sand milling, and then reacted at 850°C for 10 hours in an inert atmosphere to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 14:47:39, and V was

[0072] Comparative Example 3: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 780°C in an inert atmosphere for 15 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 14:44:42, and V was

[0073] Comparative Example 4: Lithium carbonate (Li2CO3) + iron phosphate (FePO4) + polyethylene glycol were mixed uniformly by mechanical ball milling or sand milling in a ratio of 1.05:1:0.1, and then reacted at 780°C in an inert atmosphere for 18 hours to obtain lithium iron phosphate (LFP) particles with a particle size distribution ratio of 0-1um:1-5um:5-20um and a ratio of 13:44:43, and V was

[0074] The lithium iron phosphate particles prepared in each of the examples and comparative examples were subjected to electron scanning electron microscopy, particle size distribution test, X-ray diffraction pattern, compaction density test, and powder resistivity test.

[0075] Scanning electron microscopy (SEM) testing was primarily used to qualitatively analyze the particle size distribution of the lithium iron phosphate particles in each embodiment and each comparative example. The SEM testing was also used to examine the morphology of the lithium iron phosphate particles in each embodiment and each comparative example. The SEM was a Zeiss Sigma 300 with magnifications of 1k, 5k, 1w, and 3w.

[0076] The particle size distribution test is used to quantitatively analyze the particle size distribution of the lithium iron phosphate particles in each embodiment and each comparative example; instrument model: Master Size 3000, measurement range: 0.02-2000 μm, detection angle: 0-135°; to obtain particle size distribution data. Import the particle size distribution data into origin for plotting, where the horizontal axis is the particle size (μm) and the vertical axis is the volume distribution (%), to obtain the particle size distribution curve; click "Analyze" - "Integral" in origin, select the range from 0μm to the maximum particle size, and obtain integral area 1, which is the total volume of all lithium iron phosphate particles; select the range below 1μm to obtain integral area 2, which is the total volume of all lithium iron phosphate particles with a particle size less than 1um; select the range above 5μm to obtain integral area 3, which is the total volume of all lithium iron phosphate particles with a particle size greater than 5um, and the volume proportion of the lithium iron phosphate particles with a particle size less than 1um is x = integral area 2 / integral area 1, and the volume proportion of the lithium iron phosphate particles with a particle size greater than 5um is z = integral area 3 / integral area 1.

[0077] The X-ray diffraction patterns were used to detect and calculate the unit cell volume of the lithium iron phosphate particles in each embodiment and each comparative example; instrument model and parameters: PANalytical X'Pert3, Cu target, 5-80°, 5° / min.

[0078] The compaction density test is used to test the compaction density of the lithium iron phosphate particles in each embodiment and each comparative example under different pressures; instrument model: electronic pressure testing machine (force test), sample weight: 1±0.05g, pressurization displacement control: 10mm / min, pressure holding 30s; pressure relief displacement control: 30mm / min, pressure holding 10s, test pressure: 1T, 2T, 3T.

[0079] The powder resistivity test is used to test the powder resistivity of the lithium iron phosphate particles in each embodiment and each comparative example at different pressures; instrument model: Yuanneng prcd3100, 0-200 MPa.

[0080] For the lithium iron phosphate particles in each of the embodiments and comparative examples prepared above, as the positive electrode active material, the positive electrode active material lithium iron phosphate, the conductive agent (conductive carbon black, SP), and the binder (polyvinylidene fluoride, PVDF) are dispersed in the solvent N-methylpyrrolidone (NMP) in a mass ratio of 85:10:5, and mixed evenly to obtain a positive electrode slurry, which is then coated on the positive electrode current collector aluminum foil and dried to obtain a positive electrode active material layer, which is then stamped to obtain a positive electrode sheet. Lithium metal is used as the negative electrode to prepare a button battery, and the button battery is subjected to electrochemical performance testing, including EIS impedance and gram capacity. The parameters obtained by the test are referred to Table 1 below. It should be noted that the compacted density and powder resistivity in Table 1 below are measured at a pressure of 30 MPa.

[0081] Table 1

[0082] It can be seen from Table 1 above that when the z / x ratio is between 0.5 and 2.8, the compaction density of the lithium iron phosphate particles is larger, the powder resistivity is lower, the EIS impedance is lower, and the gram capacity is larger, thereby reducing the internal resistance and improving the conductivity and capacity; when the z / x ratio is between 1.8 and 2.5, the overall internal resistance is lower, the conductivity is better, and the capacity is higher.

[0083] Among them, the photos of the lithium iron phosphate particles in Example 6 and Comparative Example 1 detected by electron scanning electron microscopy (SEM) are shown in Figure 1. It can be seen from Figure 1 (especially at a magnification of 3w) that the particle size of the lithium iron phosphate particles in Example 6 is larger than that of the lithium iron phosphate particles in Comparative Example 1.

[0084] The specific distribution of particle sizes of the lithium iron phosphate particles in Example 6 and Comparative Example 1 is shown in Table 2 and FIG2 .

[0085] Table 2

[0086] Compared to the lithium iron phosphate particles in Comparative Example 1, the lithium iron phosphate particles in Example 6 have a larger overall particle size, and a higher proportion of large-sized particles. The particle size range of the lithium iron phosphate particles in Comparative Example 1 is: 0-1 μm accounts for 22%, 1-5 μm accounts for 78%, and 5-20 μm accounts for 0%. The particle size range of the lithium iron phosphate particles in Example 6 is: 0-1 μm accounts for 14%, 1-5 μm accounts for 55%, and 5-20 μm accounts for 31%.

[0087] The XRD data of the lithium iron phosphate particles in Example 6 and Comparative Example 1 are shown in Table 3 and Figure 3: The single cell of the lithium iron phosphate particles in Comparative Example 1 is slightly shorter in the a, b, and c axes, and the volume of the single cell is also smaller. In contrast, the single cell of the lithium iron phosphate particles in Example 6 is slightly longer in the a, b, and c axes than in the lithium iron phosphate particles in Comparative Example 1, and the volume of the single cell is

[0088] Table 3

[0089] 4 , which shows the compaction density-pressure curves of the lithium iron phosphate particles in Example 6 and Comparative Example 1, at different pressures, the compaction density of the lithium iron phosphate particles in Example 6 is higher than the compaction density of the lithium iron phosphate particles in Comparative Example 1.

[0090] 5 , which shows the powder resistivity-pressure curves of the lithium iron phosphate particles in Example 6 and Comparative Example 1, at different pressures, the powder resistivity of the lithium iron phosphate particles in Example 6 is significantly higher than that of the lithium iron phosphate particles in Comparative Example 1.

[0091] Referring to Figure 6, Figure 6 shows a comparison of the EIS impedance spectra of the button batteries corresponding to the lithium iron phosphate particles in Example 6 and Comparative Example 1. It can be seen from the semicircular arc part of the curve in Figure 6 that the charge transfer impedance of the button battery corresponding to the lithium iron phosphate particles in Example 6 is smaller than the charge transfer impedance of the button battery corresponding to the lithium iron phosphate particles in Comparative Example 1, which proves that the battery corresponding to the lithium iron phosphate particles in Example 6 has smaller charge and discharge polarization, which is also the reason for the higher capacity performance during the cycle.

[0092] Referring to Figure 7, Figure 7 shows a comparison of the gram capacity-voltage curves of the button batteries of the lithium iron phosphate particles in Example 6 and Comparative Example 1, reflecting the charge and discharge capacity. The gram capacity of the button battery corresponding to the lithium iron phosphate particles in Example 6 is 158mAh / g, and the gram capacity of the button battery corresponding to the lithium iron phosphate particles in Comparative Example 1 is 156mAh / g. The gram capacity of the battery corresponding to the lithium iron phosphate particles in Example 6 is higher than the gram capacity of the battery corresponding to the lithium iron phosphate particles in Comparative Example 1.

[0093] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0094] Although the embodiments of the present application have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to the embodiments without departing from the principles and intent of the present application, and that the scope of the present application is defined by the claims and their equivalents.

Claims

1. A positive electrode active material, characterized in that: include: The volume proportion of lithium iron phosphate particles with a particle size less than 1 um is x, and the volume proportion of lithium iron phosphate particles with a particle size greater than 5 um is z, and the range of z / x is 0.5 to 2.

8.

2. The positive electrode active material according to claim 1, characterized in that The range of z / x is 1.5 to 2.

5.

3. The positive electrode active material according to claim 1 or 2, characterized in that: The volume proportion of the lithium iron phosphate particles with a particle size greater than 5 um and less than 20 um is z.

4. The positive electrode active material according to any one of claims 1 to 3, characterized in that The range of x is 11% to 30%, and the range of z is 5% to 40%.

5. The positive electrode active material according to claim 4, characterized in that The volume proportion of the lithium iron phosphate particles having a particle size not less than 1 um and not more than 5 um is y, and the range of y is 40% to 80%.

6. The positive electrode active material according to any one of claims 1 to 5, characterized in that The unit cell volume of the lithium iron phosphate particle is V, and the unit of V is The value range of V is 291.1~294.

6.

7. The positive electrode active material according to any one of claims 1 to 6, characterized in that The unit cell volume of the lithium iron phosphate particle is V, and the unit of V is The value range of (z / x)*(V / 300) is 0.49~2.

74.

8. A positive electrode sheet, characterized in that: include: A positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material according to any one of claims 1 to 7.

9. A lithium ion battery, characterized in that: include: case; An electrode assembly is disposed in the shell, and the electrode assembly includes a negative electrode sheet and a positive electrode sheet, and the positive electrode sheet is the positive electrode sheet according to claim 8.

10. An electrical device, characterized in that: include: The lithium ion battery according to claim 9.

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