A lithium iron phosphate cathode material, a preparation method thereof, and a lithium battery

Abstract

The present invention discloses a lithium iron phosphate cathode material, a preparation method thereof, and a lithium battery, relating to the technical field of lithium ion batteries. The present invention adopts a specific method to test the pressing index of the lithium iron phosphate cathode material, so that the pressing index meets a specific range, and at the same time, the average circularity of larger particles in the Dv50-Dv100 range meets an appropriate range. Such lithium iron phosphate cathode materials have better particle movement ability, and the particles can move and rearrange at a faster speed during pressing, which is conducive to obtaining a cathode pole piece with a high tap density and improving the charge and discharge capacity of the battery. At the same time, it is beneficial to avoid particle breakage when pressed, so that it has good compressive resistance. Using the lithium iron phosphate cathode material provided by the present invention can enable the lithium battery to have both a high energy density and a capacity retention rate.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and more particularly, to a lithium iron phosphate cathode material, a preparation method thereof, and a lithium battery. Background Art

[0002] Lithium iron phosphate is one of the most competitive cathode active materials for lithium-ion batteries on the market today. Compared with lithium cobaltate and ternary cathode materials, it has a long lifespan and good safety performance. In addition, lithium iron phosphate has a theoretical specific capacity of 170 mAh g -1 and a platform discharge voltage of 3.4 V, so it has a considerable energy density.

[0003] The tapped density and the compression density (collectively referred to as the bulk density) are important indicators for evaluating cathode materials. The tapped density refers to the bulk density of powder materials after being tapped, and the compression density refers to the bulk density obtained after the powder materials are compressed and stabilized. Therefore, both of them reflect the mass of the cathode material contained per unit volume, which will directly affect the compression density of the prepared electrode sheet, and thus affect the energy density of the battery. Due to its crystal structure, the lithium iron phosphate cathode material has a low ion diffusion rate and conductivity. Existing technologies usually adopt the method of grading large and small particles to improve the tapped density and compression density of the lithium iron phosphate cathode material, so as to balance good bulk density and Li + diffusion rate. However, the tapped density and compression density of cathode materials are not limited to particle size distribution, but are also affected by many other factors.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a lithium iron phosphate cathode material, a preparation method thereof, and a lithium battery, aiming to provide a lithium iron phosphate cathode material with high compression density and good cycle performance.

[0006] The present invention is implemented as follows:

[0007] In a first aspect, the present invention provides a lithium iron phosphate cathode material, including a lithium iron phosphate active material, and having a carbon coating layer on the surface of the lithium iron phosphate active material;

[0008] The average circularity of the particles in the Dv50-Dv100 range of the lithium iron phosphate cathode material is greater than or equal to 0.6;

[0009] The pressing index of the lithium iron phosphate cathode material is 0.4-5.0, wherein the pressing index is obtained by the following method:

[0010] Obtain the density-pressure relationship curve of the lithium iron phosphate cathode material at different pressing speeds, linearly fit the part of the density-pressure relationship curve above 100 MPa to obtain the corresponding slope, plot the relationship curve between the pressing speed and the slope, and linearly fit the relationship curve between the pressing speed and the slope. The slope of the fitted line is the pressing index.

[0011] In an alternative embodiment, the particle strength of the lithium iron phosphate cathode material is 40 MPa - 150 MPa.

[0012] In an alternative embodiment, the lithium iron phosphate cathode material satisfies at least one of the following characteristics:

[0013] Characteristic A1: The particle strength is 60 MPa - 120 MPa;

[0014] Characteristic A2: The average circularity of the particles in the Dv50 - Dv100 range is 0.65 - 0.80;

[0015] Characteristic A3: The tap density is 1.5 g / cm 3 -3.0 g / cm 3 ;

[0016] Characteristic A4: The mass ratio of the carbon coating layer in the lithium iron phosphate cathode material is 0.5% - 5%.

[0017] In an alternative embodiment, it is defined that: compressibility factor = ;

[0018] The lithium iron phosphate cathode material satisfies a compressibility factor of 120 MPa - 300 MPa.

[0019] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate cathode material according to any one of the foregoing embodiments, including: mixing iron phosphate and a polymer solution containing lithium ions, and then mixing with a curing agent to carry out a curing reaction to obtain a gel; wherein, the polymer solution containing lithium ions is obtained by dissolving a soluble lithium salt and a polymer in a solvent;

[0020] Fragmentize and dry the gel to obtain a precursor;

[0021] Calcine the precursor to prepare carbon-coated lithium iron phosphate;

[0022] Mix carbon-coated lithium iron phosphates with different particle sizes.

[0023] In an alternative embodiment, the process of preparing the gel has at least one of the following characteristics:

[0024] Feature B1: The polymer is a soluble polymer containing at least one of hydroxyl group, vinyl group and epoxy group; the curing agent is selected according to the active groups contained in the polymer. When the polymer contains hydroxyl group, the curing agent is glycidyl ether; when the polymer contains vinyl group, the curing agent is a soluble photoinitiator; when the polymer contains epoxy group, the curing agent is a polyol.

[0025] Feature B2: The soluble lithium salt is selected from at least one of lithium chloride, lithium hydroxide, lithium acetate and lithium oxalate.

[0026] Feature B3: The solvent is selected from at least one of water, ethanol, methanol and isopropyl alcohol.

[0027] Feature B4: The concentration of lithium ions in the polymer solution containing lithium ions is 0.2M - 0.8M, the mass percentage of the polymer in the polymer solution containing lithium ions is 1% - 3%, and the mass ratio of the addition amount of the curing agent to the polymer is (1 - 5):100.

[0028] Feature B5: After mixing anhydrous iron phosphate, a dopant and the polymer solution containing lithium ions, then mixing with the curing agent for curing reaction. The dopant is a soluble compound containing a doping element, and the doping element is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, Y, Mg, Na, N, S and F; control the total molar amount of iron element and doping element in the anhydrous iron phosphate and the molar ratio of lithium element to be 1:(1.01 - 1.10).

[0029] Feature B6: The curing reaction method is selected from at least one of heating stirring reaction, ultraviolet irradiation stirring reaction and microwave irradiation stirring reaction; when using the heating stirring reaction method for curing reaction, control the reaction temperature to be 50°C - 80°C; when using the ultraviolet irradiation stirring reaction method for curing reaction, control the light wavelength to be 200nm - 400nm; when using the microwave irradiation stirring reaction method for curing reaction, control the power of microwave irradiation to be 200W - 500W.

[0030] Feature B7: The solid content of the prepared gel is 30% - 50%.

[0031] In an alternative embodiment, the gel is fragmented and then fluidized and dried.

[0032] Among them, fluidized drying means placing the fragmented gel in a fluidized bed for drying, controlling the drying temperature to be 80°C - 120°C, and the air flow rate to be 50mL / min - 400mL / min.

[0033] In an alternative embodiment, the process of preparing carbon-coated lithium iron phosphate from the precursor has at least one of the following features:

[0034] Feature C1: The calcination temperature is 550°C - 650°C, and the calcination time is 2h - 6h;

[0035] Feature C2: Calcination is carried out in an inert atmosphere;

[0036] Feature C3: Before the precursor is calcined, it enters a flash dryer for preheating. The inlet air temperature of the flash dryer is controlled at 300°C - 500°C, the outlet air temperature is 100°C - 150°C, the rotation speed is 10Hz - 30Hz, and the feeding frequency is 10Hz - 30Hz;

[0037] Feature C4: After the precursor is calcined, it is crushed and demagnetized to obtain carbon-coated lithium iron phosphate.

[0038] In an optional embodiment, the method of proportioning carbon-coated lithium iron phosphate with different particle sizes is selected from the first method or the second method;

[0039] Among them, the process of proportioning using the first method includes: mixing two kinds of carbon-coated lithium iron phosphate with different particle size distributions in proportion. The Dv50 of the two kinds of carbon-coated lithium iron phosphate in different particle size ranges are Dv50 a , Dv50 b , where 15μm ≥ Dv50 a ≥ 2.4Dv50 b ≥ 1μm; The mass ratio of the two kinds of carbon-coated lithium iron phosphate with Dv50 a and Dv50 b is (20% - 35%):(65% - 80%);

[0040] Among them, the process of proportioning using the second method includes: mixing three kinds of carbon-coated lithium iron phosphate with different particle size distributions in proportion. The Dv50 of the three kinds of carbon-coated lithium iron phosphate in different particle size ranges are Dv50 a , Dv50 b , Dv50 c , where 15μm ≥ Dv50 a ≥ 2.4Dv50 b ≥ 2.4 2 Dv50 c ≥ 2μm; The mass ratio of the three kinds of carbon-coated lithium iron phosphate with Dv50 a , Dv50 b and Dv50 c is (2% - 20%):(40% - 60%):(30% - 40%).

[0041] Thirdly, the present invention provides a lithium battery, comprising the lithium iron phosphate cathode material in any of the foregoing embodiments or the lithium iron phosphate cathode material prepared by the preparation method in any of the foregoing embodiments.

[0042] The present invention has the following beneficial effects: The present invention adopts a specific method to test the compaction index of the lithium iron phosphate cathode material , so that the compaction index meets a specific range, and at the same time, the average circularity of larger particles in the Dv50 - Dv100 range meets an appropriate range. Such lithium iron phosphate cathode materials have better particle movement ability, and the particles can move and rearrange at a faster speed during compaction, which is conducive to obtaining a cathode electrode sheet with a high tap density and improving the charge and discharge capacity of the battery. At the same time, it is beneficial to avoid particle breakage during compression, making it have good compressive resistance. Using the lithium iron phosphate cathode material provided by the present invention can enable the lithium battery to have both a high energy density and a capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, so they should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0044] Figure 1 is a density - pressure relationship curve graph;

[0045] Figure 2 is Figure 1 a schematic diagram after curve fitting;

[0046] Figure 3 is a compaction speed - slope relationship curve graph;

[0047] Figure 4 is a TEM test graph of Comparative Example 1 and Example 3;

[0048] Figure 5 is an SEM graph of the lithium iron phosphate cathode material prepared in Example 1;

[0049] Figure 6 is an SEM graph of the lithium iron phosphate cathode material prepared in Example 3. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are followed. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0051] An embodiment of the present invention provides a lithium iron phosphate cathode material, including a lithium iron phosphate active material, with a carbon coating layer on the surface of the lithium iron phosphate active material, forming a carbon-coated lithium iron phosphate cathode material.

[0052] The average circularity of the particles in the Dv50 - Dv100 range of the lithium iron phosphate cathode material provided by the embodiment of the present invention is greater than or equal to 0.6. The particles in this range are relatively large particles and have a greater impact on the overall movement ability of the powder material. The average circularity of this part of the particles can be 0.60, 0.65, 0.70, 0.80, 0.90, 1.00, etc.

[0053] The embodiment of the present invention creatively defines the pressing index of the lithium iron phosphate cathode material and controls the pressing index to be 0.4 - 5.0, such as 0.40, 0.50, 0.80, 1.00, 1.50, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50, 5.00, etc. Among them, the pressing index is obtained through the following method: obtain the density - pressure relationship curve of the lithium iron phosphate cathode material at different pressing speeds, perform a linear fitting on the part of the density - pressure relationship curve above 100 MPa (the linear growth part is usually above a pressure of 100 MPa) (requiring the goodness of fit r 2 ≥0.99) and obtain the corresponding slope, make the relationship curve between the pressing speed and the slope, and perform a linear fitting on the relationship curve between the pressing speed and the slope (requiring the goodness of fit r 2 ≥0.98) to obtain the slope of the fitting line as the pressing index. The specific test method refers to GB / T 11106 - 2022 "Metallic powders - Method for determining the compressive strength of cylindrical compacts", and then use software such as Origin to fit the tangent line of the relationship curve. The pressing index reflects the growth rate of the pressure increase speed of the lithium iron phosphate cathode material at different pressing speeds, and reflects the activity ability of the particles of the lithium iron phosphate cathode material.

[0054] It should be noted that the pressure growth rate of the powder material is inconsistent under different pressing speeds. The faster the pressing speed, the faster the pressure growth rate. The reason is that when pressure is applied, the powder particles dissipate the pressure through movement and rearrangement. However, if the pressing speed is too fast, the stress is difficult to dissipate quickly, resulting in a rapid increase in pressure. Therefore, the movement ability of the powder particles affects their pressure dissipation ability. The movement ability of the particles, that is, the quality of the stress dissipation ability, will directly affect their tapped density or compaction density, thereby affecting the energy density of the battery. Moreover, it also directly affects the stability during the cycling process of the cathode material, thereby affecting the cycle life and safety. Through a large number of experiments, the present invention finds that the particle movement ability of the lithium iron phosphate cathode material is related to its particle size distribution, particle morphology, and particle surface properties. Specifically, when the lithium iron phosphate cathode material particles have a good particle size grading effect, the particles only need to complete the filling of pores through a short movement path to dissipate stress, so the movement ability is relatively high. When the lithium iron phosphate cathode material particles have a high roundness, the contact area between the particles is the lowest, and the relative movement ability is relatively high, so the movement ability is relatively high. When the carbon coating layer of the lithium iron phosphate cathode material particles is relatively uniform, the friction between the particles is small, so the relative movement ability is relatively high.

[0055] The lithium iron phosphate cathode material provided by the embodiment of the present invention has a pressing index of 0.4 to 5, which has relatively excellent particle movement ability. During pressing, the particles can move and rearrange at a relatively fast speed, which is beneficial to obtaining a cathode electrode sheet with a high compaction density, improving the charge and discharge capacity of the battery, and at the same time, it is beneficial to avoid particle breakage when being pressed. Generally speaking, the broken particles may not be able to maintain the original conductive network structure, making the electron conduction path tortuous, increasing the internal resistance, and affecting the high-current discharge performance and efficiency of the battery. Usually, the smaller the pressing index , the better the movement ability of the particles.

[0056] In some embodiments, the roundness of the lithium iron phosphate cathode material particles is 0.5 - 1.0, such as 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, etc.

[0057] In some embodiments, the particle strength of the lithium iron phosphate cathode material is 40 MPa - 150 MPa, such as 40 MPa, 50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 110 MPa, 120 MPa, 130 MPa, 140 MPa, 150 MPa, etc. When the lithium iron phosphate cathode material has a pressing index When it is 0.4 - 5 and the particle strength is 40 - 150 MPa, the lithium iron phosphate cathode material has good particle mobility and high particle mechanical strength, and is not easily broken under pressure. Therefore, the lithium iron phosphate cathode material has a high tap density. The positive electrode sheet containing this lithium iron phosphate cathode material has a high tap density and a low internal resistance. The battery containing this positive electrode sheet has good charge and discharge capacity and cycle stability.

[0058] In a preferred embodiment, the particle strength of the lithium iron phosphate cathode material is 60 MPa - 120 MPa, and the average roundness of the particles in the Dv50 - Dv100 range is 0.65 - 0.80. It is appropriate that the strength and average roundness of the lithium iron phosphate cathode material particles are within the above ranges, which is beneficial to further improve the movement ability of the material and the tap density of the material. The tap density (test condition is 3T) is 1.5 g / cm 3 - 3.0 g / cm 3 For example, it can be 1.5 g / cm 3 、2.0 g / cm 3 、2.5 g / cm 3 、3.0 g / cm 3 etc. The mass ratio of the carbon coating layer in the lithium iron phosphate cathode material is 0.5% - 5%, for example, it can be 0.5%, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, etc.

[0059] Furthermore, define: compressibility factor = ; The lithium iron phosphate cathode material satisfies that the compressibility factor is 120 MPa - 300 MPa, for example, it can be 120 MPa, 150 MPa, 180 MPa, 200 MPa, 230 MPa, 250 MPa, 300 MPa, etc. The larger the compressibility factor, the stronger the compressive ability of the lithium iron phosphate cathode material, the higher the tap density of the obtained positive electrode sheet, so the capacity of the battery is higher and the cycle stability is better.

[0060] The embodiment of the present invention also provides a preparation method of a lithium iron phosphate cathode material. By combining the sol - gel method and the carbothermal reduction method, carbon - coated lithium iron phosphate with a uniform carbon coating layer and high roundness is prepared, and further a lithium iron phosphate cathode material is prepared through particle size ratio. The steps are as follows:

[0061] S1. Prepare a gel body

[0062] Dissolve a soluble lithium salt and a polymer in the same solvent to obtain a polymer solution containing lithium ions. After mixing iron phosphate (such as anhydrous iron phosphate) and the polymer solution containing lithium ions, then mix with a curing agent for a curing reaction to obtain a gel body.

[0063] It should be noted that anhydrous iron phosphate is dispersed in a polymer solution containing lithium ions to achieve the mixing of anhydrous iron phosphate and lithium ions. Under the condition of stirring, the polymer is slowly cross-linked to form a gel, thereby fixing the amounts of iron phosphate, lithium ions, and polymer contained in a unit volume, which is beneficial to preparing lithium iron phosphate with uniform shape and similar surface properties, thus improving the uniformity of the product and reducing the resistance. At the same time, the polymer serves as a carbon source during subsequent calcination and is uniformly coated on the surface of the iron phosphate solid, which not only helps to form a uniform carbon coating layer but also helps to improve the roundness of the obtained lithium iron phosphate.

[0064] In some embodiments, the soluble lithium salt is selected from at least one of lithium chloride, lithium hydroxide, lithium acetate, and lithium oxalate, and the soluble lithium salt can be any one or several of the above. The solvent is selected from at least one of water, ethanol, methanol, and isopropanol, and the solvent can be any one or several of the above.

[0065] In some embodiments, the polymer is a soluble polymer containing at least one of hydroxyl group, vinyl group, and epoxy group, and the polymer contains any one or several of the above active groups. The curing agent is selected according to the active groups contained in the polymer. When the polymer contains a hydroxyl group, the curing agent is a glycidyl ether, such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, etc.; when the polymer contains a vinyl group, the curing agent is a soluble photoinitiator, such as photoinitiator 2959, but not limited thereto; when the polymer contains an epoxy group, the curing agent is a polyol (such as ethylene glycol, glycerol, etc.), and the gel is prepared by reacting the polyol with the epoxy group.

[0066] Specifically, the polymer can be hyaluronic acid, vinyl grafted carboxymethyl cellulose, agar, guar gum, alginic acid, aliphatic epoxy resin, etc.

[0067] Furthermore, the concentration of lithium ions in the polymer solution containing lithium ions is 0.2M - 0.8M, such as 0.2M, 0.3M, 0.4M, 0.5M, 0.6M, 0.7M, 0.8M, etc. The mass percentage of the polymer in the polymer solution containing lithium ions is 1% - 3%, such as 1%, 2%, 3%, etc. The dosage of the curing agent is determined according to the dosage of the polymer, and the mass ratio of the added amount of the curing agent to the polymer is controlled to be (1 - 5):100, such as 1:100, 2:100, 3:100, 4:100, 5:100, etc.

[0068] In some embodiments, the curing reaction mode is selected from at least one of heating and stirring reaction, ultraviolet irradiation and stirring reaction, and microwave irradiation and stirring reaction. The curing reaction mode can be any one or several of the above, and can be selected according to the type of polymer. When the curing reaction is carried out by the heating and stirring reaction mode, the reaction temperature is controlled to be 50°C - 80°C, such as 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, etc. When the curing reaction is carried out by the ultraviolet irradiation and stirring reaction mode, the light wavelength is controlled to be 200nm - 400nm, such as 200nm, 250nm, 300nm, 350nm, 400nm, etc.; when the curing reaction is carried out by the microwave irradiation and stirring reaction mode, the power of the microwave irradiation is controlled to be 200W - 500W, such as 200W, 300W, 400W, 500W, etc. By regulating the reaction time, the solid content of the prepared gel is 30% - 50%, such as 30%, 35%, 40%, 45%, 50%, etc.

[0069] In some embodiments, a doping element is introduced into the gel. Anhydrous iron phosphate, a dopant, and a polymer solution containing lithium ions can be mixed, stirred and dispersed evenly, and then a curing agent is added while stirring. After adding, the curing reaction is carried out. Among them, the dopant is a soluble compound containing a doping element, and the doping element is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, Y, Mg, Na, N, S, and F. The doping element can be any one or several of the above. The molar ratio of the total molar amount of iron element and doping element in anhydrous iron phosphate to the molar amount of lithium element is controlled to be 1:(1.01 - 1.10), such as 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09, 1:1.10, etc.

[0070] S2. Fragmentation, drying

[0071] The gel is fragmented and then dried to remove surface solvents and other impurities to obtain a precursor.

[0072] In some embodiments, the fragmentation method can be sieving, rolling, grinding, etc. to prevent particle aggregation.

[0073] In some embodiments, fluidized bed drying can be employed to avoid agglomeration. Fluidized bed drying refers to drying the fragmented gel in a fluidized bed, controlling the drying temperature at 80°C - 120°C and the air flow rate at 50 mL / min - 400 mL / min to obtain precursor particles with relatively uniform particle sizes. Specifically, the drying temperature can be 80°C, 90°C, 100°C, 110°C, 120°C, etc., and the air flow rate can be 50 mL / min, 100 mL / min, 200 mL / min, 300 mL / min, 400 mL / min, etc.

[0074] S3. Calcination

[0075] The precursor is calcined, and after the polymer is carbonized, lithium iron phosphate coated with carbon is prepared.

[0076] In some embodiments, the calcination temperature is controlled at 550°C - 650°C, such as 550°C, 580°C, 600°C, 630°C, 650°C, etc.; the calcination time is 2 h - 6 h, such as 2 h, 3 h, 4 h, 5 h, 6 h, etc. The calcination process can be carried out in an inert atmosphere, and the inert atmosphere can be nitrogen, argon, etc.

[0077] In some embodiments, before the precursor is calcined, it enters a flash dryer for preheating. The inlet air temperature of the flash dryer is controlled at 300°C - 500°C, the outlet air temperature is 100°C - 150°C, the rotation speed is 10 Hz - 30 Hz, and the feeding frequency is 10 Hz - 30 Hz. Using the flash dryer for pre-calcination is beneficial for the particles to rapidly shrink at high temperatures, improving the bonding strength inside the material particles and thus enhancing the mechanical strength characteristics of the material.

[0078] In some embodiments, after the precursor is calcined, it is crushed and demagnetized to obtain lithium iron phosphate coated with carbon. The calcined agglomerated particles are dispersed by crushing, and magnetic impurities are removed by demagnetization.

[0079] S4. Particle size grading

[0080] Lithium iron phosphate coated with carbon with different particle sizes is proportioned, and the grading effect of the lithium iron phosphate cathode material is improved through the grading method, which is beneficial for increasing its tap density and compaction density, and thus beneficial for increasing the capacity of the battery.

[0081] In some embodiments, the method of proportioning lithium iron phosphate coated with carbon with different particle sizes is selected from the first method or the second method. The first method is to perform grading using two kinds of lithium iron phosphate coated with carbon with different particle size distributions, and the second method is to perform grading using three kinds of lithium iron phosphate coated with carbon with different particle size distributions.

[0082] Further, the process of proportioning by the first method includes: mixing two kinds of lithium iron phosphate coated with carbon having different particle size distributions in proportion. The Dv50 of the two kinds of lithium iron phosphate coated with carbon in different particle size ranges are Dv50 a , Dv50 b , where 15μm ≥ Dv50 a ≥ 2.4Dv50 b ≥ 1μm. That is to say, the Dv50 a of the large particles is greater than or equal to 2.4 times the Dv50 b of the small particles, and the values of Dv50 a and 2.4 times Dv50 b are both in the range of 1μm - 15μm, such as 1μm, 3μm, 5μm, 8μm, 10μm, 13μm, 15μm, etc. The mass ratio of the two kinds of lithium iron phosphate coated with carbon with Dv50 a and Dv50 b is (20% - 35%):(65% - 80%). The mass ratio of the large particle lithium iron phosphate coated with carbon to the small particle lithium iron phosphate coated with carbon can be 20%:80%, 25%:75%, 30%:70%, 35%:65%, etc.

[0083] Among them, the process of proportioning by the second method includes: mixing three kinds of lithium iron phosphate coated with carbon having different particle size distributions in proportion. The Dv50 of the large particle, medium particle and small particle three kinds of lithium iron phosphate coated with carbon in different particle size ranges are Dv50 a , Dv50 b , Dv50 c , where 15μm ≥ Dv50 a ≥ 2.4Dv50 b ≥ 2.4 2 Dv50 c ≥ 2μm. That is to say, the Dv50 a of the large particles is greater than or equal to 2.4 times the Dv50 b of the medium particles, and also greater than or equal to 2.4 2 times the Dv50 c of the small particles. And the values of Dv50 a and 2.4 times Dv50 b , 2.4 2 times Dv50 c are both in the range of 2μm - 15μm, such as 2μm, 5μm, 8μm, 10μm, 13μm, 15μm, etc. The lithium iron phosphate coated with carbon with Dv50 a , Dv50 b and Dv50 cThe mass ratio of the three kinds of carbon-coated lithium iron phosphate with large particles, medium particles and small particles is (2% - 20%):(40% - 60%):(30% - 40%), such as 2%:60%:38%, 5%:55%:40%, 8%:57%:35%, 10%:53%:37%, 13%:57%:30%, 15%:45%:40%, 18%:43%:39%, 20%:40%:40%, etc.

[0084] The embodiment of the present invention also provides a positive electrode sheet of a lithium battery, including the lithium iron phosphate positive electrode material provided by the embodiment of the present invention, which can improve the tap density of the positive electrode sheet. The positive electrode sheet of the lithium battery may further include a positive electrode current collector, and a positive electrode active coating is formed on at least one surface of the positive electrode current collector, and the lithium iron phosphate positive electrode material exists in the positive electrode active coating as a positive electrode active material.

[0085] The embodiment of the present invention also provides a lithium battery, including the positive electrode sheet of the lithium battery provided by the embodiment of the present invention, which can improve the charge and discharge capacity of the battery. The lithium battery may further include a negative electrode sheet, an electrolyte, a separator, etc. to form a complete battery structure, and the specific types of the negative electrode sheet, the electrolyte, and the separator are not limited. During the charge and discharge process of the lithium battery, active ions are embedded and removed back and forth between the positive electrode sheet and the negative electrode sheet, and the electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet.

[0086] The embodiment of the present invention also provides a device, including the above-mentioned lithium battery (secondary battery), battery module or battery pack. The secondary battery, battery module or battery pack can be used as the power source of the device or as the energy storage unit of the device. The device can be, but is not limited to, mobile devices (such as mobile phones, laptop computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc.

[0087] The features and properties of the present invention will be further described in detail below in conjunction with embodiments.

[0088] The present invention provides a lithium iron phosphate positive electrode material as shown in Table 1. The positive electrode sheet and the battery are prepared by using the lithium iron phosphate positive electrode material, and the performance of the electrode sheet and the battery is tested, as shown in Table 2.

[0089] Performance test methods and conditions:

[0090] Electrode sheet resistance: Tested with the Yuaneng Technology BER2100 multi-functional electrode sheet resistance meter, testing 10 different positions and calculating the average value.

[0091] Tap density: Determined in accordance with GB / T30835-2014.

[0092] Apparent density of the cathode material: The apparent density of the cathode material is determined in accordance with GB / T 30835-2014.

[0093] Compression index: Load 1 ± 0.01 g of the lithium iron phosphate cathode material into a steel mold, and use a Sansi CMT5000 universal mechanical testing machine to continuously compress the powder in the steel mold. The moving speed of the indenter (compression speed) is selected from at least 3 speeds in the range of 0.2 - 15 mm / min, for example, set to 0.2 mm / min, 1 mm / min, 10 mm / min, and 15 mm / min respectively. The mechanical testing machine automatically records the pressure and the displacement of the crossbeam during the compression process, and calculates the powder density and pressure according to the specifications of the steel mold and the mass of the powder to obtain the relationship curve between the powder density and the pressure at different compression speeds. Perform a linear fitting (requiring a goodness of fit r2 ≥ 0.99) on the linearly increasing part of the density-pressure relationship curve (usually above a pressure of 100 MPa) and obtain the corresponding slope 1. Each sample is measured at least three times for each speed, and the average value of the slope 1 is obtained; then plot the relationship curve between the compression speed and the average value of the slope 1, and the slope 2 obtained by performing a linear fitting on the relationship curve between the compression speed and the average value of the slope 1 is the compression index, and the goodness of fit r 2 ≥ 0.98 is valid data. The test method refers to GB / T 11106-2022 "Metallic powders - Method for determining the compressibility of cylindrical compacts by compression". Use software such as Origin to perform a linear fitting on the relationship curve. Before performing a linear fitting on the density-pressure relationship curve, the curve can be appropriately smoothed. Exemplarily, use the smoothing tool of Origin, select the Lowess model, and the span parameter is 0 - 0.5. Taking Example 5 as an example, the test process of the compression index is provided, as Figure 1 、 Figure 2 and Figure 3 shown.

[0094] Percentage of carbon content: It is determined in accordance with GB / T 30835-2014.

[0095] Chemical composition analysis: Use a PE Avio200 inductively coupled plasma optical emission spectrometer (ICP-OES).

[0096] Particle size: Test the Dv50 and Dv100 of the lithium iron phosphate cathode material, as well as the Dv10, Dv50, Dv90 and broadening coefficient Span of the carbon-coated lithium iron phosphate, in accordance with GB / T 19077.1 using an MS3000 laser particle size analyzer, where Span = (Dv90 - Dv10) / Dv50.

[0097] Particle strength: An MCT series of micro compression testing machines is used. The indenter applies a test force with a constant increasing speed to a single particle and fixes it between the flat indenter and the lower platen. Then, the deformation of the specimen is automatically measured. When the pressure suddenly drops, it is judged that the particle has been crushed. The particle strength is calculated according to formula (1) using the maximum pressure F before the pressure suddenly drops. At least 10 particles are taken from each sample for testing and the average value is calculated.

[0098] Particle strength = 2.8×F / (πd 2 ) Formula (1)

[0099] F represents the maximum pressure value corresponding to before the pressure suddenly drops, with the unit of mN; d represents the particle diameter, with the unit of μm.

[0100] Compaction density of the electrode sheet: The mass ratio of lithium iron phosphate material, binder, and conductive carbon black is 95:3:2. First, PVDF is dissolved in NMP to obtain a 5wt% binder solution, and then lithium iron phosphate material and conductive carbon black are added and stirred to obtain a slurry. The slurry is coated on an aluminum foil with a thickness of 20μm using a 300μm coating applicator, dried at 100°C for 8h, and the single-sided areal density is controlled to be 400±1g / m 2 ; Then, the dried electrode sheet is pressed. The temperature of the pressing roller is 40°C, and the pressure applied by the pressing roller is 6MPa to obtain the positive electrode sheet. The thickness of the positive electrode sheet is measured using a micrometer with a precision of 0.5μm, and the compaction density of the positive electrode sheet PD = areal density of the positive electrode sheet ÷ (thickness of the positive electrode sheet - 20μm).

[0101] Battery energy density: The prepared positive electrode sheet is assembled into a laminated battery, and the volume energy density and mass energy density of the battery are tested. The preparation method of the negative electrode sheet is as follows: The mass ratio of graphite, conductive carbon black, CMC, and styrene-butadiene rubber is 95.5:1:1.5:2; First, CMC is dissolved in water, then graphite and conductive carbon black are added and stirred; Finally, styrene-butadiene rubber is added, and after vacuum stirring is completed, it is screened and discharged to obtain a negative electrode slurry with a solid content of 50%. The negative electrode slurry is coated on a copper foil, cold-pressed, and slit to obtain a negative electrode sheet. The compaction density of the negative electrode sheet is 1.55g / cm 3 , and the thickness of the active material layer is 110μm; The electrolyte is EC / DMC / EMC + LiPF6 + additive, the concentration of LiPF6 is 1M, the separator is a 20μm PE ion exchange membrane; An aluminum-plastic film is heat-sealed as the outer shell; A Lanqi charging cabinet is used to form the assembled battery, and the conditions are: constant current charging at 0.1C to 3.8V, discharging at 0.1C to 2.0V; The energy density of the formed battery is tested.

[0102] Cycling capacity retention rate: Cycle 1000 times at a rate of 1C. The method is to charge at a constant current of 1C to 3.8V and then discharge at a constant current of 1C to 2.0V. Capacity retention rate = discharge capacity in the 1000th cycle ÷ discharge capacity in the first cycle × 100%.

[0103] TEM: Use FEI Titan Cubed G2 60 - 300 transmission electron microscope.

[0104] SEM: Use Nova NanoSEM 450. Roundness test method: Analyze according to the obtained SEM images using ImageJ image analysis software. Use ImageJ to identify individual particles that can be analyzed, and then use ImageJ to calculate the projected area and roundness of each particle (C = (4×π×A) / G 2 , where A is the area of the projected surface, G is the perimeter of the projected surface, and C is the roundness of the particle). The roundness of the lithium iron phosphate cathode material in Examples 2 - 3, 6, 8 - 10 is 0.5 - 1.0; Use an MS3000 laser particle size analyzer to test Dv50 and Dv100 of the lithium iron phosphate cathode material according to GB / T19077.1. Screen out particles with a projected area greater than or equal to (Dv50 / 2) 2 as samples in the SEM image, and count the average roundness of the samples. The sample size is not less than 30, that is, obtain the average roundness of the particles in the Dv50 - Dv100 range.

[0105] Table 1 Parameter detection results of the products provided in each example

[0106]

[0107] Table 2 Test results of electrode sheets and battery performance

[0108]

[0109] It can be seen from analyzing Table 1 and Table 2 that the lithium iron phosphate cathode material provided by the present invention takes into account both a relatively high tap density and cycling stability, and the prepared battery has both a relatively high energy density and capacity retention rate.

[0110] Example 1

[0111] The present example provides a preparation method of a lithium iron phosphate cathode material, and the steps are as follows:

[0112] (1) Dissolve lithium chloride, titanium oxysulfate, and hyaluronic acid in deionized water to obtain a polymer solution containing lithium ions. Add anhydrous iron phosphate and stir to keep it evenly dispersed. Then add ethylene glycol diglycidyl ether and heat to 60 °C while continuing to stir. Stop the reaction when the viscosity becomes too high to stir, obtaining a gel with a solid content of 48.2%.

[0113] Among them, the concentration of lithium ions in the polymer solution containing lithium ions is 0.2 M, and the mass percentage of the polymer in the polymer solution containing lithium ions is 1%; the addition amount of the curing agent is 3% of the polymer (mass fraction, the same below); anhydrous iron phosphate and titanium oxysulfate are fed according to the molar ratio of the total molar amount of Fe and Ti elements to the molar amount of Li element of 1:1.05, and the molar ratio of Fe and Ti is 0.95:0.05.

[0114] (2) After the gel is fragmented by pressing through an 80-mesh sieve, it is then sent to a fluidized drying bed for drying to constant weight to obtain a precursor.

[0115] Among them, the temperature of the fluidized bed is 120 °C, and the air flow rate is 100 mL / min.

[0116] (3) Place the precursor in a flash dryer for pre-calcination to obtain a pre-calcined precursor.

[0117] Among them, the inlet air temperature of the flash dryer is 500 °C, the outlet air temperature is 100 °C, the rotation speed is 30 Hz, and the feeding frequency is 30 Hz.

[0118] (4) Calcinate the pre-calcined precursor, crush it, and demagnetize it to obtain carbon-coated lithium iron phosphate.

[0119] Among them, the gas atmosphere for calcination is an inert atmosphere (nitrogen, the same below), the temperature is 650 °C, and the time is 5 h.

[0120] (5) Mix and proportion the carbon-coated lithium iron phosphate prepared from anhydrous iron phosphate with different particle sizes to obtain a lithium iron phosphate cathode material.

[0121] Table 3 Gradation scheme of Example 1

[0122]

[0123] Example 2

[0124] This example provides a method for preparing a lithium iron phosphate cathode material, and the steps are as follows:

[0125] (1) Dissolve lithium hydroxide and vinyl-grafted hydroxymethyl cellulose in deionized water to obtain a polymer solution containing lithium ions. Add anhydrous iron phosphate and stir to keep it dispersed evenly. Then add photoinitiator 2959, turn on the ultraviolet radiation and continue stirring (ultraviolet light wavelength is 254 nm). Stop the reaction when the viscosity becomes too high to stir, and obtain a gel with a solid content of 30.5%.

[0126] Among them, the concentration of lithium ions in the polymer solution containing lithium ions is 0.8 M, and the mass percentage of the polymer in the polymer solution containing lithium ions is 3%; the addition amount of the curing agent is 3% of the polymer; anhydrous iron phosphate is fed according to the molar ratio of Fe element to Li element of 1:1.1.

[0127] (2) After the gel is fragmented by pressing through an 80-mesh sieve, it is then sent to a fluidized drying bed for drying to constant weight to obtain a precursor.

[0128] Among them, the fluidized bed temperature is 80 °C and the air flow rate is 400 mL / min.

[0129] (3) Calcinate the precursor, crush and demagnetize it to obtain carbon-coated lithium iron phosphate.

[0130] Among them, the gas atmosphere for calcination is an inert atmosphere, the temperature is 550 °C, and the time is 6 h.

[0131] (4) Mix and proportion the carbon-coated lithium iron phosphate prepared from anhydrous iron phosphate with different particle sizes to obtain a lithium iron phosphate cathode material.

[0132] Table 4 Gradation scheme of Example 2

[0133]

[0134] Example 3

[0135] This example provides a preparation method of a lithium iron phosphate cathode material, and the steps are as follows:

[0136] (1) Dissolve lithium acetate and agar in deionized water at 50 °C to obtain a polymer solution containing lithium ions. Add anhydrous iron phosphate and stir to keep it dispersed evenly. Then add polyethylene glycol diglycidyl ether (Mn 500), and heat to 80 °C and continue stirring. Stop the reaction when the viscosity becomes too high to stir, and obtain a gel with a solid content of 42.7%.

[0137] Among them, the concentration of lithium ions in the polymer solution containing lithium ions is 0.5 M, and the mass percentage of the polymer in the polymer solution containing lithium ions is 2%; the addition amount of the curing agent is 2.5% of the polymer; anhydrous iron phosphate is fed according to the molar ratio of Fe element to Li element of 1:1.01.

[0138] (2) After the gel is crushed by pressing through an 80-mesh sieve, it is then sent to a fluidized drying bed for drying to constant weight to obtain a precursor.

[0139] Among them, the temperature of the fluidized bed is 100 °C and the air flow rate is 200 mL / min.

[0140] (3) The precursor is placed in a flash dryer for pre-calcination to obtain a pre-calcined precursor.

[0141] Among them, the inlet air temperature of the flash dryer is 400 °C, the outlet air temperature is 150 °C, the rotation speed is 30 Hz, and the feeding frequency is 20 Hz.

[0142] (4) The pre-calcined precursor is calcined, crushed, and demagnetized to obtain carbon-coated lithium iron phosphate.

[0143] Among them, the gas atmosphere for calcination is an inert atmosphere, the temperature is 600 °C, and the time is 3 h.

[0144] (5) The carbon-coated lithium iron phosphate prepared from anhydrous iron phosphate with different particle sizes is mixed and proportioned to obtain a lithium iron phosphate cathode material.

[0145] Table 5 Gradation Scheme of Example 3

[0146]

[0147] Example 4

[0148] The difference from Example 3 is that the gel is dried by ordinary drying, that is, the gel is placed in a blast oven at 100 °C for drying to constant weight;

[0149] Table 6 Gradation Scheme of Example 4

[0150]

[0151] Example 5

[0152] The difference from Example 3 is that in step (1), the mass percentage of the polymer in the polymer solution containing lithium ions is 3%; the addition amount of the curing agent is 5% of the polymer.

[0153] Table 7 Gradation Scheme of Example 5

[0154]

[0155] Example 6

[0156] (1) Lithium oxalate, zirconium acetate, and guar gum were dissolved in deionized water at 50 °C to obtain a polymer solution containing lithium ions. Anhydrous iron phosphate was added and stirred to maintain uniform dispersion. Then, ethylene glycol diglycidyl ether was added, and the mixture was heated to 80 °C and continuously stirred. The reaction was stopped when the viscosity became too high to stir, and a gel with a solid content of 34.9% was obtained.

[0157] Among them, the concentration of lithium ions in the polymer solution containing lithium ions was 0.5 M, and the mass percentage of the polymer in the polymer solution containing lithium ions was 2%; the addition amount of the curing agent was 1% of the polymer; anhydrous iron phosphate and the dopant were fed according to the molar ratio of the total molar amount of Fe element and dopant metal element to Li element of 1:1.1, and the molar ratio of iron to zirconium was 0.98:0.02.

[0158] (2) After the gel was fragmented by pressing through an 80-mesh sieve, it was sent to a fluidized drying bed and dried to a constant weight to obtain a precursor.

[0159] Among them, the temperature of the fluidized bed was 100 °C, and the air flow rate was 200 mL / min.

[0160] (3) The precursor was placed in a flash dryer for pre-calcination to obtain a pre-calcined precursor.

[0161] Among them, the inlet air temperature of the flash dryer was 500 °C, the outlet air temperature was 150 °C, the rotation speed was 10 Hz, and the feeding frequency was 30 Hz.

[0162] (4) The pre-calcined precursor was calcined, crushed, and demagnetized to obtain carbon-coated lithium iron phosphate.

[0163] Among them, the gas atmosphere for calcination was an inert atmosphere, the temperature was 550 °C, and the time was 4 h.

[0164] (5) The carbon-coated lithium iron phosphate prepared from anhydrous iron phosphate with different particle sizes was mixed and proportioned to obtain a lithium iron phosphate cathode material.

[0165] Table 8 Gradation Scheme of Example 6

[0166]

[0167] Example 7

[0168] The difference from Example 3 was that in step (3), the inlet air temperature of the flash dryer was 300 °C and the outlet air temperature was 100 °C.

[0169] Table 9 Gradation Scheme of Example 7

[0170]

[0171] Example 8

[0172] The difference from Example 3 is that the addition amount of the curing agent in step (1) is 1% of the polymer.

[0173] Table 10 Gradation Scheme of Example 8

[0174]

[0175] Example 9

[0176] The difference from Example 3 is that the addition amount of the curing agent in step (1) is 5% of the polymer.

[0177] Table 11 Gradation Scheme of Example 9

[0178]

[0179] Example 10

[0180] The difference from Example 3 is that the conditions of the flash dryer in step (3) are as follows: the inlet air temperature is 500 °C, the outlet air temperature is 150 °C, the rotation speed is 30 Hz, and the feeding frequency is 10 Hz.

[0181] Table 12 Gradation Scheme of Example 10

[0182]

[0183] Comparative Example 1

[0184] The difference from Example 3 is that no curing agent is added in step (1). The steps are as follows:

[0185] Dissolve lithium acetate and agar in deionized water at 50 °C to obtain a polymer solution containing lithium ions, add anhydrous iron phosphate and stir to keep it evenly dispersed, and heat to 80 °C and continue to stir until the solvent completely evaporates to obtain precursor 1.

[0186] Among them, the concentration of lithium ions in the polymer solution containing lithium ions is 0.5 M, and the mass percentage of the polymer in the polymer solution containing lithium ions is 2%; anhydrous iron phosphate is fed according to the molar ratio of Fe element to Li element of 1:1.01.

[0187] (2) Feed precursor 1 into a fluidized drying bed and dry it to constant weight to obtain precursor 2.

[0188] Among them, the fluidized bed temperature is 100 °C, and the air flow rate is 200 mL / min.

[0189] (3) Place precursor 2 in a flash dryer for pre-calcination to obtain a pre-calcined precursor.

[0190] Among them, the inlet air temperature of the flash dryer is 400 °C, the outlet air temperature is 150 °C, the rotation speed is 30 Hz, and the feeding frequency is 20 Hz.

[0191] (4) Calcinate the pre-calcined precursor, crush and demagnetize it to obtain carbon-coated lithium iron phosphate.

[0192] Among them, the gas atmosphere for calcination is an inert atmosphere, the temperature is 600 °C, and the time is 3 h.

[0193] (5) Mix and proportion the carbon-coated lithium iron phosphate prepared from anhydrous iron phosphate with different particle sizes to obtain the lithium iron phosphate cathode material.

[0194] Table 13 Gradation scheme of Comparative Example 1

[0195]

[0196] Comparative Example 2

[0197] Mix anhydrous iron phosphate, lithium acetate, and agar powder by ball milling, place them in a rotary kiln and calcine at 600 °C for 6 h, crush and demagnetize to obtain carbon-coated lithium iron phosphate. Anhydrous iron phosphate and lithium acetate are mixed according to the molar ratio n(Fe):n(Li)=1:1.01. The mixing ratio of anhydrous iron phosphate, lithium acetate, and agar powder is the same as that in Example 3.

[0198] Table 14 Gradation scheme of Comparative Example 2

[0199]

[0200] Comparative Example 3

[0201] Carbon-coated lithium iron phosphate of No. 1 in Example 4.

[0202] Performance characterization:

[0203] The TEM images of the lithium iron phosphate cathode materials prepared in Comparative Example 1 and Example 3 are as Figure 4 shown. It can be seen that the uniformity of the lithium iron phosphate cathode material prepared in Example 3 is better.

[0204] The SEM image of the lithium iron phosphate cathode material prepared in Example 1 is as Figure 5 shown, and the SEM image of the lithium iron phosphate cathode material prepared in Example 3 is as Figure 6 shown. It can be seen that the lithium iron phosphate cathode material prepared contains large particles and small particles, and the average circularity of the large particles (above Dv50) in Example 3 is higher than that in Example 1.

[0205] Combined with Table 1, Table 2 and the preparation method, analyze each example and comparative example:

[0206] Specifically, the compaction index of Comparative Example 1 is too large, resulting in a low compaction density, a large resistance of the prepared electrode sheet, and thus a low energy density. The reason is that the lack of a curing agent leads to a significant decrease in the uniformity of the carbon coating layer (see the TEM attached drawing), resulting in a high compaction index.

[0207] Compaction index of Example 1 It satisfies 0.4 - 5.0, but the particle strength does not satisfy 40 MPa - 150 MPa. Example 1 has a high particle strength. Compared with Example 5, the two have similar compaction indexes. However, due to the excessive particle strength of Example 1, its compaction density is low. The reason is that: the particle strength of Example 5 is low. When measuring the compaction density, the particles will be crushed to release stress, thereby increasing the compaction density. However, the fragmentation of the particles will lead to a longer electron / ion transport path, resulting in a larger resistance of the electrode sheet. The fragmentation of the particles will also lead to a deterioration of the cycling performance.

[0208] Compaction index of Example 2 It satisfies 0.4 - 5.0, the particle strength satisfies 40 MPa - 150 MPa, but does not satisfy 60 MPa - 120 MPa, and the compressibility factor does not satisfy 120 MPa - 300 MPa. Since Example 2 has more lithium iron phosphate with small particle sizes, it has a lower compaction index. Therefore, it has a higher compaction density but a higher resistance of the electrode sheet. In addition, the particle strength of Example 2 is low, so the cycling capacity retention rate is low.

[0209] Examples 3 - 5 have similar particle strengths but different compaction indexes. From the carbon coating layer percentage content, average circularity, and grading scheme of the three, the difference in the compaction index comes from the carbon coating layer percentage content and average circularity. From the performance tests, as the compaction index increases, the compaction density of the powder decreases, and thus the compaction density of the electrode sheet decreases; the resistance of the electrode sheet slightly increases. The reason is that an increase in the carbon content to a certain extent is beneficial to improving conductivity, but a decrease in circularity will lead to an increase in the interfacial resistance of the particles. Therefore, the change in the resistance of the electrode sheet under the two effects is not significant.

[0210] Examples 3, 6, and 7 have similar compaction indexes but different particle strengths. The greater the particle strength, the smaller the compaction density of the powder and the electrode sheet, but the smaller the resistance of the electrode sheet and the higher the cycling capacity retention rate. Although the average circularity of Examples 6 and 7 is small, their grading effect is good, and the number of lithium iron phosphate with small particle sizes is more. Therefore, their compaction indexes are similar to that of Example 3, indicating that the powder movement ability of the lithium iron phosphate cathode material can be regulated by adjusting the process parameters of the preparation method of the present invention.

[0211] The compressibility factors of Examples 3, 6, 8, and 10 meet the range of 120 MPa-300 MPa. Therefore, compared with other examples, they have the best overall performance in terms of good compaction density, electrode conductivity, and capacity retention.

[0212] During the preparation of Examples 3, 8, and 9, varying amounts of curing agent were added. The results indicate that a greater amount of curing agent added resulted in a higher degree of gel crosslinking, a higher compression index for the resulting lithium iron phosphate cathode material, and slightly increased particle strength. Further analysis revealed similar circularity, carbon coating content, and grading schemes among the three. However, when no curing agent was added (Comparative Example 1), the compression index of the lithium iron phosphate material was actually higher than that of Example 9. This is because without the addition of a curing agent, a gel cannot be formed, and the amounts of iron phosphate, lithium ions, and polymer contained per unit volume cannot be fixed. Consequently, the resulting lithium iron phosphate material exhibits severe agglomeration, resulting in a higher Dv50 and a wider span. Furthermore, TEM images of Example 3 and Comparative Example 1 show that the carbon coating uniformity of Comparative Example 1, which lacks a curing agent, is poorer than that of Example 3, resulting in a higher compression index. This indicates that the addition of a curing agent and its amount significantly influence the uniformity of the carbon coating. Neither the addition of a curing agent nor an excessive amount of curing agent is conducive to forming a uniform coating, leading to an increased compression index.

[0213] Different flash evaporation conditions were used during the preparation of Examples 3, 7, and 10. Specifically, as the inlet air temperature during flash evaporation increased, the particle strength of the material increased and the roundness became better. However, since the grading scheme of Example 7 contained more small-particle lithium iron phosphate, Examples 3 and 7 had similar compression indices. Since the grading scheme of Example 10 was similar to that of Example 3, Example 10 had a lower compression index due to its higher average roundness. Although its hardness was higher, due to its lower compression index, the compacted density of the powder was similar to that of Example 3.

[0214] In Comparative Example 2, the polymer and lithium salt are mixed with anhydrous iron phosphate by a solid phase method. The compression index of the obtained lithium iron phosphate positive electrode material is large, so the compaction density of the powder and the electrode sheet is low, and the electrode sheet resistance is high.

[0215] Compared with Example 4, Comparative Example 3 was not graded, so the compression index was relatively large.

[0216] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that, It includes lithium iron phosphate active material with a carbon coating layer on the surface of the lithium iron phosphate active material; The average circularity of the particles in the Dv50 - Dv100 range of the lithium iron phosphate cathode material is greater than or equal to 0.6; The compaction index of the lithium iron phosphate cathode material is 0.4 - 5.0, wherein the compaction index is obtained by the following method: Obtain the density - pressure relationship curve of the lithium iron phosphate cathode material at different pressing speeds, linearly fit the part above 100 MPa of the density - pressure relationship curve to obtain the corresponding slope, make the relationship curve between the pressing speed and the slope, and linearly fit the relationship curve between the pressing speed and the slope. The slope of the fitted line is the pressing index; The particle strength of the lithium iron phosphate cathode material is 40 MPa - 150 MPa; Definition: Compressibility factor = ; The lithium iron phosphate cathode material satisfies that the compressibility factor is 120 MPa - 300 MPa.

2. The lithium iron phosphate cathode material according to claim 1, wherein The lithium iron phosphate cathode material satisfies at least one of the following characteristics: Characteristic A1: The particle strength is 60 MPa - 120 MPa; Characteristic A2: The average circularity of the particles in the Dv50 - Dv100 range is 0.65 - 0.80; Feature A3: The compaction density is 1.5 g / cm 3 - 3.0 g / cm 3 ; Characteristic A4: The mass percentage of the carbon coating layer in the lithium iron phosphate cathode material is 0.5% - 5%.

3. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1-2, characterized in that, It includes: Mix ferric phosphate and a lithium - ion - containing polymer solution, and then mix with a curing agent for a curing reaction to obtain a gel; wherein, the lithium - ion - containing polymer solution is obtained by dissolving a soluble lithium salt and a polymer in a solvent; Fragmentize the gel and dry it to obtain a precursor; Calcine the precursor to prepare carbon - coated lithium iron phosphate; Mix the carbon - coated lithium iron phosphate with different particle sizes.

4. The preparation method according to claim 3, characterized in that, The process of preparing the gel has at least one of the following characteristics: Characteristic B1: The polymer is a soluble polymer containing at least one of hydroxyl, vinyl, and epoxy groups; the curing agent is selected according to the active groups contained in the polymer. When the polymer contains hydroxyl, the curing agent is glycidyl ether; when the polymer contains vinyl, the curing agent is a soluble photo - initiator; when the polymer contains epoxy groups, the curing agent is a polyol; Characteristic B2: The soluble lithium salt is selected from at least one of lithium chloride, lithium hydroxide, lithium acetate, and lithium oxalate; Characteristic B3: The solvent is selected from at least one of water, ethanol, methanol, and isopropyl alcohol; Characteristic B4: The concentration of lithium ions in the lithium - ion - containing polymer solution is 0.2 M - 0.8 M, the mass percentage of the polymer in the lithium - ion - containing polymer solution is 1% - 3%, and the addition amount of the curing agent to the mass of the polymer is (1 - 5):100; Feature B5: After mixing anhydrous iron phosphate, a dopant, and the lithium-ion-containing polymer solution, mix with a curing agent for a curing reaction. The dopant is a soluble compound containing a doping element, and the doping element is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, Y, Mg, Na, N, S, and F; control the total molar amount of iron element and the doping element in the anhydrous iron phosphate to the molar ratio of lithium element to be 1:(1.01 - 1.10); Feature B6: The curing reaction method is selected from at least one of heating and stirring reaction, ultraviolet irradiation and stirring reaction, and microwave irradiation and stirring reaction; when using the heating and stirring reaction method for the curing reaction, control the reaction temperature to be 50°C - 80°C; when using the ultraviolet irradiation and stirring reaction method for the curing reaction, control the light wavelength to be 200nm - 400nm; when using the microwave irradiation and stirring reaction method for the curing reaction, control the power of microwave irradiation to be 200W - 500W; Feature B7: The solid content of the prepared gel is 30% - 50%.

5. The preparation method according to claim 3, characterized in that, Fragmentize the gel and then perform fluidized drying; Among them, the fluidized drying means placing the fragmented gel in a fluidized bed for drying, controlling the drying temperature to be 80°C - 120°C, and the air flow rate to be 50mL / min - 400mL / min.

6. The preparation method according to claim 3, characterized in that, The process of preparing the lithium iron phosphate coated with carbon from the precursor has at least one of the following features: Feature C1: The calcination temperature is 550°C - 650°C, and the calcination time is 2h - 6h; Feature C2: Calcination is carried out in an inert atmosphere; Feature C3: Before the precursor is calcined, it enters a flash dryer for preheating, controlling the inlet air temperature of the flash dryer to be 300°C - 500°C, the outlet air temperature to be 100°C - 150°C, the rotation speed to be 10Hz - 30Hz, and the feeding frequency to be 10Hz - 30Hz; Feature C4: After the precursor is calcined, it is crushed and demagnetized to obtain the lithium iron phosphate coated with carbon.

7. The preparation method according to claim 3, wherein The method of proportioning the lithium iron phosphate coated with carbon of different particle sizes is selected from the first method or the second method; Among them, the process of mixing according to the first method includes: mixing two kinds of carbon-coated lithium iron phosphate with different particle size distributions in proportion. The Dv50 of the two kinds of carbon-coated lithium iron phosphate with different particle size ranges are Dv50 a and Dv50 b , where 15μm ≥ Dv50 a ≥ 2.4Dv50 b ≥ 1μm; the mass ratio of the two kinds of carbon-coated lithium iron phosphate with Dv50 a and Dv50 b is (20% - 35%):(65% - 80%); Among them, the process of proportioning using the second method includes: mixing the three carbon-coated lithium iron phosphates with different particle size distributions in proportion, and the Dv50 of the three carbon-coated lithium iron phosphates with different particle size ranges are Dv50 a , Dv50 b , Dv50 c , where 15μm ≥ Dv50 a ≥ 2.4Dv50 b ≥ 2.4 2 Dv50 c ≥ 2μm; the mass ratio of the three carbon-coated lithium iron phosphates with Dv50 a , Dv50 b and Dv50 c is (2% - 20%):(40% - 60%):(30% - 40%).

8. A lithium battery, characterized in that, Including the lithium iron phosphate cathode material according to any one of claims 1 - 2 or the lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 3 - 7.

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