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

Abstract

The present invention discloses a lithium iron phosphate cathode material, a preparation method thereof, and a lithium ion battery, relating to the technical field of batteries. The lithium iron phosphate cathode material includes primary particles and secondary particles, and the secondary particles are formed by the aggregation of the primary particles. The lithium iron phosphate cathode material satisfies FSSS = 5 - 40 and ρ% = 50% - 90%, where FSSS refers to the Fisher Sub-Sieve Size of the lithium iron phosphate cathode material, D<subgt;BET< / subgt; refers to the specific surface area particle size of the lithium iron phosphate cathode material, and ρ% refers to the relative density of the secondary particles. The present invention provides a lithium iron phosphate cathode material that simultaneously satisfies FSSS = 5 - 40 and ρ% = 50% - 90%. The lithium iron phosphate cathode material has good dispersibility, low aggregation degree, and high tap density. The positive electrode sheet prepared therefrom has the characteristics of uniform surface density and high peel strength. Therefore, the battery prepared has high specific capacity and cycle stability.

Description

Technical Field

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

[0002] Lithium iron phosphate is one of the most competitive cathode active materials for lithium ion batteries on the market. Compared with lithium cobaltate and ternary cathode materials, it has a long service life 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 an impressive energy density.

[0003] However, due to its crystal structure, the lithium iron phosphate cathode material has a low ion diffusion rate and conductivity. In the prior art, means such as nanosizing, coating, and doping are usually used to improve the Li + diffusion rate and conductivity of the material. However, these means have caused or affected the agglomeration behavior of the lithium iron phosphate cathode material to a certain extent. For example, the smaller the particle size of the cathode material, the more deviated the shape from the standard spherical shape, and the wider the distribution, the easier the cathode material is to agglomerate; for another example, when the coating layer is carbon material, if the carbon material is distributed in an island shape on the surface of the active material, it will increase the surface roughness of the cathode material, thereby enhancing the interaction force between particles and facilitating particle agglomeration; for another example, the preparation process has an impact on the surface chemical properties of the obtained lithium iron phosphate cathode material, thereby affecting the interaction between particles. The appearance of agglomerates results in extremely obvious scratches and particles during the cathode coating process, and at the same time, it will also cause uneven surface density of the electrode and difficulty in maintaining the uniformity of the cathode sheet, thus affecting the consistency of the battery.

[0004] In view of this, the present invention is specifically 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 ion battery.

[0006] The present invention is implemented as follows:

[0007] In a first aspect, the present invention provides a lithium iron phosphate cathode material, which includes primary particles and secondary particles, and the secondary particles are formed by agglomeration of primary particles. The lithium iron phosphate cathode material satisfies = 5~40 and ρ% = 50%~90%, where the FSSS refers to the Fisher particle size of the lithium iron phosphate cathode material, with the unit of μm; the D BET refers to the specific surface area particle size of the lithium iron phosphate cathode material, with the unit of μm; ρ% = 1 - H%, and ρ% refers to the relative density of the secondary particles, and H% refers to the porosity of the secondary particles.

[0008] In an alternative embodiment, the D BET is the particle size of the lithium iron phosphate cathode material measured and calculated by the nitrogen adsorption-desorption method, and the , wherein, the BET refers to the specific surface area of the lithium iron phosphate cathode material, and the value range of the BET satisfies ≤ 20 m 2 / g, and the refers to the true density of the lithium iron phosphate cathode material, and the value range satisfies 3.3 to 3.6 g / cm³.

[0009] In an alternative embodiment, the FSSS is the average particle size of the lithium iron phosphate cathode material measured by the steady-flow air permeability method, and the value range of the FSSS satisfies 0.4 to 5 μm;

[0010] and / or, the degree of agglomeration N of the lithium iron phosphate cathode material is less than 25000, wherein the degree of agglomeration ;

[0011] and / or, the tap density of the lithium iron phosphate cathode material is 1.9 to 3.0 g / cm 3 .

[0012] In an alternative embodiment, the lithium iron phosphate cathode material satisfies = 10 to 30;

[0013] and / or, the secondary particles of the lithium iron phosphate cathode material satisfy ρ% = 50% to 70%;

[0014] and / or, the lithium iron phosphate cathode material satisfies that the degree of agglomeration N is 200 to 12000;

[0015] and / or, the lithium iron phosphate cathode material satisfies FSSS = 1 to 2.5 μm;

[0016] and / or, the lithium iron phosphate cathode material satisfies that the BET is 9 to 15 m 2 / g.

[0017] In an alternative embodiment, the lithium iron phosphate cathode material includes an active material matrix and carbon material, and the general formula of the active material matrix is Li 1-x A x Fe 1-y M y (PO 4-z )D z, wherein, A is selected from one or more of Na and Mg; M is selected from one or more of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from one or more of F, S, N and Cl; 0 ≤ x ≤ 0.1; 0 ≤ y ≤ 0.1 and 0 ≤ z ≤ 0.1; the mass of the carbon material is 1% - 5% of the mass of the lithium iron phosphate cathode material.

[0018] In a second aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material as described in any one of the foregoing embodiments, which includes:

[0019] Mixing iron phosphate and a lithium salt as a material, and mixing the material with a carbon source having different gasification temperatures to obtain a precursor;

[0020] Subjecting the precursor to segmented calcination in an inert atmosphere, and then crushing, sieving, and demagnetizing to obtain the lithium iron phosphate cathode material.

[0021] In an alternative embodiment, the step of mixing the iron phosphate and the lithium salt includes at least one of Feature I - Feature IV;

[0022] Feature I: Before mixing the iron phosphate and the lithium salt, first add the iron phosphate into a jet mill for comminution treatment;

[0023] Feature II: The lithium salt is at least one of lithium carbonate, lithium oxalate, and lithium acetate;

[0024] Feature III: When mixing the iron phosphate and the lithium salt, a dopant is further added, and the dopant is at least one of a salt, an oxide, and a hydroxide of a doping element;

[0025] Feature IV: The iron phosphate and the dopant are fed according to a stoichiometric ratio.

[0026] In an alternative embodiment, the carbon source having different gasification temperatures includes a first carbon source with a gasification temperature of 400 - 600 °C, a second carbon source with a gasification temperature of 200 - 400 °C, and a third carbon source with a gasification temperature of 100 - 200 °C;

[0027] The first carbon source includes at least one of polyimide, polyphenylene ether, and polyetherimide;

[0028] The second carbon source includes at least one of glucose, polyethylene glycol, starch, sucrose, maltose, cellulose, chitosan, polyacrylonitrile, benzyl alcohol, and glycerol;

[0029] The third carbon source includes at least one of phenol, ethylene glycol, propylene glycol, butylene glycol, isopropanol, sorbitol, erythritol, fructose, malic acid, citric acid, and salicylic acid.

[0030] In an alternative embodiment, the step of mixing the material with carbon sources having different gasification temperatures includes at least one of features V - IX;

[0031] Feature V: The material is mixed with the carbon sources having different gasification temperatures in a jet mill device, and a gas stream is introduced during mixing. The pressure of the gas stream is 0.5 - 1.5 MPa, and the temperature is 25 - 120°C; the total mixing time is 15 - 60 min;

[0032] Feature VI: Before being mixed with the material, the carbon sources having different gasification temperatures are first formulated into a carbon source solution with a mass percentage concentration of 10% - 30%, and then the carbon source solution is atomized and then mixed with the material. Among them, the atomization pressure is 0.1 - 1.5 MPa;

[0033] Feature VII: The material is first mixed with the first carbon source, then with the second carbon source, and finally with the third carbon source;

[0034] Feature VIII: The total mass of the carbon sources having different gasification temperatures is 15% - 25% of the mass of the material;

[0035] Feature IX: When the material is mixed with the carbon sources having different gasification temperatures, the mass ratio of the first carbon source, the second carbon source, and the third carbon source is 0 - 15:2.5 - 15:2.5 - 10.

[0036] In an alternative embodiment, the staged calcination includes a first stage, a second stage, and a third stage. The first stage is heated to 180 - 220°C at a rate of 1 - 2°C / min and held for 1 - 2 h; the second stage is heated to 380 - 420°C at a rate of 2 - 3°C / min and held for 2 - 4 h; the third stage is heated to 650 - 750°C at a rate of 4 - 6°C / min and held for 4 - 8 h.

[0037] In a third aspect, the present invention provides a lithium - ion battery, which includes the lithium iron phosphate cathode material as described in any one of the foregoing embodiments or the lithium iron phosphate cathode material obtained by the preparation method of the lithium iron phosphate cathode material as described in any one of the foregoing embodiments.

[0038] The present invention has the following beneficial effects:

[0039] (1) The present invention provides a method that simultaneously meets The lithium iron phosphate cathode material with =5 to 40 and ρ% = 50% to 90% has good dispersibility, low degree of aggregation, and high tap density; the cathode sheet prepared with it as the cathode active material has the characteristics of uniform areal density and high peel strength, so the prepared battery has high specific capacity and cycle stability.

[0040] (2)The preparation method of the lithium iron phosphate cathode material provided by the present invention uses carbon sources with different gasification temperatures to gasify and form pores at different calcination stages, making the binding between material particles loose, inhibiting the adhesion and aggregation between particles, so that the obtained cathode material has a lower degree of aggregation. Through the preparation method provided by the present invention, it is possible to obtain a lithium iron phosphate cathode material that simultaneously satisfies =5 to 40 and ρ% = 50% to 90%. This lithium iron phosphate cathode material has good dispersibility, low degree of aggregation, and high tap density. The cathode sheet prepared from it has the characteristics of uniform areal density and high peel strength. Therefore, the prepared battery has high specific capacity and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] 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 use in 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.

[0042] Figure 1 SEM diagram of the lithium iron phosphate cathode material provided in Example 1 of the present invention;

[0043] Figure 2 SEM diagram of the lithium iron phosphate cathode material provided in Example 8 of the present invention;

[0044] Figure 3 SEM diagram of the lithium iron phosphate cathode material provided in Comparative Example 4 of the present invention;

[0045] Figure 4 Raman spectrum diagram of the lithium iron phosphate cathode materials provided in Example 1 and Example 4 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] 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 conditions not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchases.

[0047] A lithium iron phosphate cathode material, which comprises primary particles and secondary particles formed by agglomeration of the primary particles, and the lithium iron phosphate cathode material satisfies = 5 to 40 and ρ% = 50% to 90%, where FSSS refers to the Fisher Sub-Sieve Sizes of the lithium iron phosphate cathode material, with the unit of μm; D BET refers to the specific surface area particle size of the lithium iron phosphate cathode material, with the unit of μm; ρ% = 1 - H%, where ρ% refers to the relative density of the secondary particles, and H% refers to the porosity of the secondary particles.

[0048] The present invention provides a lithium iron phosphate cathode material that simultaneously satisfies and ρ% within a specific range. It has good dispersibility, low agglomeration degree, and high tap density. The cathode sheet prepared with it as the cathode active material has the characteristics of uniform areal density and high peel strength. Therefore, the prepared battery has high specific capacity and cycle stability.

[0049] Specifically, reflects the ratio of the particle size of the secondary particles to the particle size of the primary particles. The better the dispersibility of the lithium iron phosphate cathode material and the lower the agglomeration degree, the smaller it is. The of the lithium iron phosphate cathode material provided by the present invention ranges from 5 to 40. Too high or too low will both lead to a decrease in the tap density of the cathode material, and further lead to a decrease in the specific capacity of the lithium iron phosphate cathode material battery. Too high indicates a high agglomeration degree of the lithium iron phosphate cathode material, and there are likely to be large pores between the agglomerated secondary particles, resulting in a low tap density; too low indicates that the lithium iron phosphate cathode material is mainly dispersed primary particles. Since the porosity of the secondary particles (aggregates) is usually lower than that of the primary particle powder in the tapped state, too low an agglomeration degree will also cause a decrease in the tap density.

[0050] In some embodiments, ranges from 10 to 30, and can be, for example, any one of 10, 12, 15, 18, 20, 22, 25, 28, 30 or the range value between any two of them.

[0051] In some embodiments, the value range of FSSS satisfies 0.4 to 5 μm, and can be, for example, any one of 0.4 μm, 0.5 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm or the range value between any two of them; in some other embodiments, the value range of FSSS satisfies 1 to 2.5 μm.

[0052] The Fisher Sub-Sieve Sizing (FSSS) is the average particle size of the lithium iron phosphate cathode material measured by the steady-flow air permeability method. Its mechanism is as follows: Using air with constant pressure and constant flow passing through the powder sample tube, the specific surface area and average particle size of the powder are calculated according to the formula by measuring the gas pressure difference. Since the air flow is of constant pressure and constant flow, the specific surface area measured is the external specific surface area of the powder. Therefore, the average particle size obtained can reflect the particle size of the aggregates.

[0053] The specific surface area particle size ( ) is the volume-area average diameter, defined as the specific surface area of particles with this diameter, equal to the average value of the specific surface areas of all particles; it is measured and calculated by the nitrogen adsorption-desorption method. Its mechanism is as follows: By measuring the nitrogen adsorption amount of the powder sample at different pressures, the specific surface area (BET) of the powder is calculated according to the formula. Since the air flow is a pressurized air flow and the collected data is the gas adsorption amount, the specific surface area measured is the total specific surface area of the powder. Therefore, the particle size calculated based on the BET specific surface area can reflect the particle size of the primary particles. The calculation formula is as follows: , where BET refers to the specific surface area of the lithium iron phosphate cathode material, with the unit of m 2 / g, refers to the true density of the lithium iron phosphate cathode material, with the unit of g / cm 3 . According to definition, , that is ; at the same time, ; therefore .

[0054] In some embodiments, the value range of BET satisfies ≤20 m 2 / g; in some embodiments, the value range of BET is 9 - 15 m 2 / g, for example, it can be any one of 9 m 2 / g, 10 m 2 / g, 11 m 2 / g, 12 m 2 / g, 13 m 2 / g, 14 m 2 / g, 15 m 2 / g or the range value between any two of them; in some embodiments, the value range of satisfies 3.3 - 3.6 g / cm³, for example, it can be any one of 3.3 g / cm³, 3.4 g / cm³, 3.5 g / cm³, 3.6 g / cm³ or the range value between any two of them.

[0055] The relative density of the lithium iron phosphate secondary particles refers to the ratio of its apparent density to the true density of lithium iron phosphate. The apparent density refers to the ratio of the mass of the material to the apparent volume, and the apparent volume is the sum of the actual volume and the pore volume, that is, the apparent volume is the sum of the solid volume of lithium iron phosphate and the pore volume. The true density refers to the actual mass of the solid substance per unit volume in the absolutely dense state of the material, that is, the density after removing the internal pores. The relative density of the secondary particles can be calculated from the porosity (H%) of the secondary particles, that is = l - H%, the higher the porosity of the secondary particles, the lower the relative density, indicating that the structure of the secondary particles is looser and easier to disperse. The higher the tap density, the more uniform the surface density of the positive electrode prepared further and the higher the peel strength, and thus the higher specific capacity and cycle stability. The value ranges from 50% to 90%. If it is too high, the secondary particles are difficult to disperse, and if it is too low, the tap density of the lithium iron phosphate positive electrode material will be reduced; in some embodiments, the value ranges from 50% to 70%.

[0056] In some embodiments, the degree of agglomeration N of the lithium iron phosphate positive electrode material is less than 25000, where the degree of agglomeration .

[0057] In some embodiments, the degree of agglomeration N of the lithium iron phosphate positive electrode material is 200 - 12000. The lithium iron phosphate positive electrode material that meets the value range of the degree of agglomeration has better tap density, and the surface density of the prepared electrode is more uniform, so it has higher specific capacity and cycle stability.

[0058] The degree of agglomeration N reflects the number of primary particles contained in the secondary particles of the lithium iron phosphate positive electrode material. Specifically, the degree of agglomeration N = = = ( ) 3 × ; the smaller the value of the degree of agglomeration N, the lower the degree of agglomeration and the better the uniformity of the electrode. However, if the degree of agglomeration N is too low, it indicates that the relative density of the lithium iron phosphate positive electrode material or is too small, both of which will result in too low tap density of the material and thus lower specific capacity.

[0059] In some embodiments, the tap density of the lithium iron phosphate positive electrode material is 1.9 - 3.0 g / cm 3 , and the tap density is measured under a pressure of 3T.

[0060] In some embodiments, the lithium iron phosphate positive electrode material includes an active material matrix and carbon materials. The general formula of the active material matrix is Li 1-x A x Fe 1-y M y(PO 4-z )D z , wherein, A is selected from one or more of Na and Mg; M is selected from one or more of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from one or more of F, S, N and Cl; 0 ≤ x ≤ 0.1; 0 ≤ y ≤ 0.1 and 0 ≤ z ≤ 0.1; the mass of the carbon material is 1% - 5% of the mass of the lithium iron phosphate cathode material.

[0061] In addition, the present invention provides a method for preparing a lithium iron phosphate cathode material, which comprises the following steps:

[0062] S1. Mix ferric phosphate and a lithium salt as materials.

[0063] Wherein, before mixing ferric phosphate and the lithium salt, ferric phosphate is first added to a jet mill for comminution treatment; through the comminution treatment, ferric phosphate can be refined to facilitate subsequent uniform mixing with the lithium salt.

[0064] In this embodiment, the lithium salt includes but is not limited to at least one of lithium carbonate, lithium oxalate and lithium acetate; when mixing ferric phosphate and the lithium salt, a dopant can also be added according to actual product requirements, and ferric phosphate and the dopant are fed in accordance with the stoichiometric ratio. The dopant includes but is not limited to at least one of salts, oxides and hydroxides of doping elements.

[0065] S2. Mix the materials with carbon sources having different gasification temperatures to obtain a precursor.

[0066] Before mixing the carbon sources having different gasification temperatures with the materials, they are first formulated into a carbon source solution with a mass percentage concentration of 10% - 30%, and then the carbon source solution is atomized and then mixed with the materials, wherein the atomization pressure is 0.1 - 1.5 MPa; the materials and the carbon sources having different gasification temperatures are mixed in a jet mill, and a gas stream is introduced during mixing, the pressure of the gas stream is 0.5 - 1.5 MPa, and the temperature is 25 - 120 °C; the total mixing time is 15 - 60 min.

[0067] In this embodiment, the carbon sources having different gasification temperatures include a first carbon source with a gasification temperature of 400 - 600 °C, a second carbon source with a gasification temperature of 200 - 400 °C, and a third carbon source with a gasification temperature of 100 - 200 °C; the materials are first mixed with the first carbon source, then with the second carbon source, and finally with the third carbon source; the total mass of the carbon sources having different gasification temperatures is 15% - 25% of the mass of the materials; when the materials are mixed with the carbon sources having different gasification temperatures, the mass ratio of the first carbon source, the second carbon source and the third carbon source is 0 - 15:2.5 - 15:2.5 - 10, wherein when the mass of the first carbon source is 0, it means that the carbon source only includes the second carbon source and the third carbon source.

[0068] Among them, the first carbon source includes but is not limited to at least one of polyimide, polyphenylene ether, and polyetherimide; the second carbon source includes but is not limited to at least one of glucose, polyethylene glycol, starch, sucrose, maltose, cellulose, chitosan, polyacrylonitrile, benzyl alcohol, and glycerol; the third carbon source includes but is not limited to at least one of phenol, ethylene glycol, propylene glycol, butylene glycol, isopropyl alcohol, sorbitol, erythritol, fructose, malic acid, citric acid, and salicylic acid.

[0069] The above-mentioned first carbon source has the characteristics of strong molecular chain rigidity, high conjugation degree, and large amount of solid residue after decomposition compared with the second or third carbon source. Therefore, its graphitization degree is relatively high, which is beneficial to improving the conductivity of the cathode material. On the one hand, because the first carbon source is more hydrophobic than the second or third carbon source, it can be better coated on the surface of the material particles when premixed with the material; on the other hand, the conductive carbon network formed by its calcination can be closely distributed on the surface of the lithium iron phosphate particles during premixing.

[0070] S3. Calcinate the precursor in an inert atmosphere in stages, and then obtain the lithium iron phosphate cathode material after crushing, sieving, and demagnetization.

[0071] The staged calcination includes a first stage, a second stage, and a third stage. In the first stage, the temperature is raised to 180 - 220 °C at a rate of 1 - 2 °C / min and held for 1 - 2 h; in the second stage, the temperature is raised to 380 - 420 °C at a rate of 2 - 3 °C / min and held for 2 - 4 h; in the third stage, the temperature is raised to 650 - 750 °C at a rate of 4 - 6 °C / min and held for 4 - 8 h.

[0072] The above-mentioned lithium iron phosphate cathode material can be widely used in the preparation of batteries, and the batteries prepared have excellent electrochemical performance. In this regard, the present invention also provides a lithium-ion battery, which includes the above-mentioned lithium iron phosphate cathode material.

[0073] The battery provided by the present invention generally may include a positive electrode sheet, a negative electrode sheet, a separator interposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte. The method for preparing the battery should be known to those skilled in the art. For example, the positive electrode sheet, the separator, and the negative electrode sheet can each be a laminate, so that they can be cut into a target size and stacked in sequence, or wound to a target size to form an electrode core, and can be further combined with the electrolyte to form a battery.

[0074] In the battery provided by the present invention, the negative electrode sheet generally includes a negative electrode current collector and a negative electrode active material layer located on the surface of the negative electrode current collector, and the negative electrode active material layer generally includes a negative electrode active material. The negative electrode active material can be various materials of negative electrode active materials applicable to lithium-ion batteries in the art. For example, it can include, but is not limited to, one or a combination of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, or other metals that can form alloys with lithium. Among them, the graphite can be selected from one or a combination of artificial graphite, natural graphite, and modified graphite; the silicon-based materials can be selected from one or a combination of elemental silicon, silicon oxides, silicon-carbon composites, and silicon alloys; the tin-based materials can be selected from one or a combination of elemental tin, tin oxides, and tin alloys. The negative electrode current collector is generally a structure or part for collecting current, and the negative electrode current collector can be various materials applicable to the negative electrode current collector of lithium-ion batteries in the art. For example, the negative electrode current collector can include, but is not limited to, metal foils, and more specifically, it can include, but is not limited to, copper foils, etc.

[0075] In the battery provided by the present invention, the separator can be various materials of separators applicable to lithium-ion batteries in the art. For example, it can include, but is not limited to, one or a combination of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0076] In the battery provided by the present invention, the electrolyte can be various electrolytes applicable to lithium-ion batteries in the art. For example, the electrolyte generally includes an electrolyte and a solvent. The electrolyte can generally include lithium salts, etc. More specifically, the lithium salts can be inorganic lithium salts and / or organic lithium salts, etc. The lithium salts can be selected from one or a combination of LiPF6, LiBF4, LiN(SO2F)2 (abbreviated as LiFSI), LiN(CF3SO2)2 (abbreviated as LiTFSI), LiClO4, LiAsF6, LiB(C2O4)2 (abbreviated as LiBOB), LiBF2C2O4 (abbreviated as LiDFOB). For another example, the concentration of the electrolyte can be between 0.8 mol / L and 1.5 mol / L. The solvent can be various solvents applicable to the electrolytes of lithium-ion batteries in the art. The solvent of the electrolyte is generally a non-aqueous solvent, preferably an organic solvent, and specifically can include, but is not limited to, one or a combination of ethylene carbonate, propylene carbonate, butylene carbonate, pentylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, etc. or their halogenated derivatives.

[0077] The features and properties of the present invention will be further described in detail below in conjunction with embodiments.

[0078] The present invention provides a lithium iron phosphate cathode material as shown in Table 1, Table 2 and Table 3, and conducts tests on it.

[0079] The assembly of the coin cell includes the following steps:

[0080] (1) Slurry preparation

[0081] Add the binder PVDF to a part of NMP and stir in a double planetary mixer. The revolution speed is 80 rpm, the rotation speed is 4500 rpm, and the stirring time is 120 min to obtain a glue solution with a solid content of 7%. Add acetylene black to the glue solution and stir. The revolution speed is 80 rpm, the rotation speed is 4500 rpm, and the stirring time is 60 min to obtain a conductive slurry. Add the lithium iron phosphate cathode material and the remaining NMP to the conductive slurry. The revolution speed is 80 rpm, the rotation speed is 6500 rpm, and the stirring time is 90 min. Then stir at a rotation speed of 15 rpm for 30 min to defoam and obtain a cathode slurry with a solid content of 65%.

[0082] The mass ratio of the lithium iron phosphate cathode material, acetylene black and PVDF is 97.2:0.8:2. The average particle size of acetylene black is 40 nm and the specific surface area is 65 m 2 / g.

[0083] (2) Preparation of the electrode sheet

[0084] Coat the cathode slurry on an aluminum foil substrate using a double-sided reciprocating coater and obtain a cathode electrode sheet after drying. In the rolling process, set the main roller temperature to 40 °C and control the main roller pressure so that the compaction density of the electrode sheet is 2.55 g / cm 3 .

[0085] (3) Assembly of the coin cell

[0086] The negative electrode sheet is a lithium sheet, the separator is a polypropylene porous membrane, and the electrolyte is prepared by dissolving 1 mol of LiPF6 in a mixed solvent of 1 L of EC and DMC (volume ratio 1:1). Assemble the cathode electrode sheet, negative electrode sheet, electrolyte, and separator into a battery in a glove box under argon protection.

[0087] The test methods include:

[0088] (1) Carbon material content: Tested using a high-frequency infrared carbon and sulfur analyzer in accordance with YS / T 1028.4-2015.

[0089] (2) True density (ρ LFP ): Measured according to the helium method of GB / T 24203-2024.

[0090] (3)Fischer Sub-Sieve Sizes (FSSS): Measured in accordance with GB / T 3249-2022.

[0091] (4)Specific surface area (BET): Determined by the BET method using a Micromeritics physical adsorption instrument in accordance with GB / T 19587-2017, and then calculated based on to obtain , and then further calculated to obtain .

[0092] (5)Relative density of secondary particles (ρ%): Separate all or part of the secondary particles from the sample to be tested, then measure their porosity (H%), and calculate the relative density of the secondary particles through ρ% = 1 - H%.

[0093] In the embodiments of the present invention, the following method is used to separate secondary particles: Vibrating and screening the sample to be tested (the difference in mesh numbers of adjacent sieves is 10 meshes) to obtain sub-samples in multiple particle size ranges, separately subjecting the sub-samples in different particle size ranges to gravity sedimentation (in an aqueous medium) for separation, taking the precipitate and drying it at room temperature to obtain the secondary particles of the sub-sample, and then combining the secondary particles of all sub-samples.

[0094] In the embodiments of the present invention, a scanning electron microscope (SEM) is used to observe and calculate the porosity of secondary particles: Adjust the magnification until there is only one complete cross-section of a secondary particle in the view and take an SEM photo, then perform image analysis on the SEM photo using Image-Pro Plus software and calculate the porosity. The porosity = (sum of the areas of all pores of the secondary particle in the cross-section / cross-sectional area of the secondary particle) × 100%.

[0095] (6)The calculation formula for the degree of agglomeration N is N = ( ) 3 × .

[0096] (7)Compacted density (CD): Tested in accordance with GB / T 24533-2009, and the pressure end point is 3T (600 MPa).

[0097] (8)Areal density test: The areal density of the electrode sheet is tested using X-ray fluorescence spectrometry (XRF). The electrode sheet to be tested is cut into 20 small sample pieces of the same size for testing to obtain the content of each element, and calculate the mass and areal density of the lithium iron phosphate cathode material, take the average value as the average areal density, and calculate the standard deviation of the areal density sample.

[0098] (9)Peel strength test: Tested using the 180° peel test method on an Instron 3365 tensile testing machine at a pulling speed of 60 mm / min.

[0099] (10)Pole piece resistance: It was tested using the Yuaneng Technology BER2100 multi-functional pole piece resistance meter. Ten different positions were tested, and the average value was calculated, as well as the sample standard deviation was calculated.

[0100] (11)Electrochemical performance: The LAND battery test system was used for the constant current charge and discharge and cycle performance tests of the battery. At 25 °C, the charge and discharge voltage range was 2.5 - 4.2 V.

[0101] The test results are shown in Tables 1, 2, and 3.

[0102] Table 1. Statistical table of the general formula and parameter detection of different samples

[0103]

[0104] Table 2. Statistical table of the physical property detection of different samples

[0105]

[0106] Table 3. Statistical table of the electrochemical performance detection of different samples

[0107]

[0108] As can be seen from the above Tables 1, 2, and 3, the lithium iron phosphate cathode material of the embodiment of the present invention has a higher tap density and better comprehensive performance compared with the comparative example. The being less than 5 or greater than 40, or being greater than 90% will result in too small a tap density, thus affecting the charge-discharge specific capacity. Specifically, reflects the particle size ratio of secondary particles to primary particles and reflects the degree of agglomeration; can reflect the tightness of the agglomeration of primary particles in secondary particles; too low a tightness of agglomeration makes the particles easy to be completely crushed and rearranged during the compaction process, or too low a degree of agglomeration is likely to result in too much pore volume between particles, which is not conducive to increasing the tap density; too high a degree of agglomeration, or too high a tightness, makes it difficult to crush the secondary particles during the compaction process, and the pore volume formed by the large particles overlapping each other is too large, which is not conducive to increasing the tap density. Therefore and need to be maintained within a reasonable range to enable the cathode material to have a higher tap density. The tap density will affect the specific capacity and rate performance of the material. Therefore, the comparative example has poorer electrochemical performance compared with Example 1.

[0109] The lithium iron phosphate cathode material of Example 2 conforms to =5-40 and ρ%=50%-90%, but does not meet the range of agglomeration degree N below 25000. In this case, compared with other examples, the compaction density of the lithium iron phosphate positive electrode material is lower, resulting in lower electrode peel strength. Due to excessive agglomeration, the sample standard deviation of the surface density and the sample standard deviation of the electrode sheet resistance are both large, resulting in a lower compaction density and specific surface area, which reduces the lithium insertion and extraction speed, resulting in lower rate performance and poor cycle stability. The positive electrode material agglomeration degree N of the remaining examples is lower than 25000, and the performance is better.

[0110] Compared to Example 6, Examples 1, 3, and 5 exhibit higher compaction density. Furthermore, they exhibit higher electrode peel strength, lower sample standard deviations for areal density, and lower sample standard deviations for electrode resistance, resulting in higher rate capability and capacity retention. This indicates that lithium iron phosphate cathode materials exhibit superior overall performance when their degree of agglomeration falls within the optimal range of 200-12,000.

[0111] Compared to Examples 1, 3, and 5, Example 4 has a lower compaction density. This is because the degree of agglomeration in Example 4 is below the preferred range of 200, indicating that the positive electrode material has a low degree of agglomeration. Due to the low degree of agglomeration, the number of primary particles is large, meaning the overall particle size is small, resulting in a low compaction density, which further affects the material's specific capacity. Due to the low degree of agglomeration of the positive electrode material, the sample standard deviations of both the areal density and the sheet resistance are low. Furthermore, the low degree of agglomeration results in a high number of primary particles, meaning the overall particle size is small and the specific surface area is large, thus improving the adhesion of the positive electrode material coating to the current collector and increasing the peel strength.

[0112] Example 1, Example 5, and Comparative Example 2 have similar chemical compositions and , but with different , comparing the performance data of the three, we can see that with The compaction density increases first and then decreases. The reason is that too large or too small agglomeration will lead to more pores between particles, thereby reducing the compaction density. In similar situations, with It shows that the number of primary particles in a single secondary particle increases, which increases the overall compaction density. , indicating that the larger the particle size of the secondary particles or the smaller the particle size of the primary particles that make up the secondary particles, both will break the gradation balance and cause the compaction density to decrease; the compaction density has a great influence on the specific capacity of the lithium iron phosphate positive electrode material. The larger the value, the larger the standard deviation of the electrode surface density sample, the smaller the peel strength, the larger the electrode resistance, and the larger the standard deviation of the electrode resistance sample. Finally, Example 1 has excellent electrochemical performance, indicating that the lithium iron phosphate positive electrode It should be maintained within a certain range so that its degree of agglomeration N is maintained within a certain range.

[0113] Examples 2, 6, 7 and Comparative Example 3 have similar chemical compositions and , but have different . By comparing the performance data of the three, it can be seen that as increases, the compaction density first increases and then decreases. The reason is that the secondary particles with a relatively high relative density can, on the one hand, increase the overall compaction density, and on the other hand, it is difficult for them to be crushed during the compaction process, while the pores formed by the large secondary particles overlapping each other increase, resulting in a decrease in the compaction density; in addition, as increases, the agglomeration degree N of the lithium iron phosphate cathode material becomes larger, resulting in a decrease in the peel strength; as increases, the agglomeration degree N increases, the resistance of the electrode sheet increases, and both the sample standard deviation of the electrode sheet surface density and the sample standard deviation of the electrode sheet resistance will increase; finally, Example 7 has excellent electrochemical performance, indicating that the lithium iron phosphate cathode material or the agglomeration degree N should be maintained within a certain range.

[0114] In addition, the present invention also correspondingly provides preparation methods for the above product examples and product comparative examples.

[0115] Method Example 1

[0116] A preparation method of a lithium iron phosphate cathode material, the product obtained by the preparation corresponding to the product of Example 1 above. The preparation method of this example includes the following steps:

[0117] (1) Introduce air into the air jet milling device, and perform milling treatment on anhydrous iron phosphate with a Dv50 of 8.52 μm to obtain anhydrous iron phosphate with a Dv50 of 0.4 - 0.5 μm;

[0118] (2) Mix the anhydrous iron phosphate obtained in step (1) with lithium carbonate as the material, and prepare the first carbon source solution, the second carbon source solution and the third carbon source solution; successively introduce the atomized first carbon source solution, the second carbon source solution and the third carbon source solution into the air jet milling device with air as the medium to perform milling coating on the material, and obtain the first precursor, the second precursor and the third precursor respectively.

[0119] The anhydrous iron phosphate and lithium carbonate are mixed according to n(Fe) : n(Li) = 1:1.05.

[0120] The first carbon source solution is a 10 wt% polyimide DMF solution; the second carbon source solution is a 20 wt% glucose aqueous solution; the third carbon source solution is a 10 wt% ethylene glycol ethanol solution.

[0121] The feeding ratio of the material, the first carbon source solution, the second carbon source solution, and the third carbon source solution is 10 g: 5 g: 2.5 g: 5 g.

[0122] The pressure of the air flow is 1 ± 0.1 MPa; when the material is mixed with the first carbon source solution, the air flow temperature is 120 °C and the mixing time is 20 min; when the first precursor is mixed with the second carbon source solution, the air flow temperature is 100 °C and the mixing time is 15 min; when the second precursor is mixed with the third carbon source solution, the air flow temperature is 80 °C and the mixing time is 15 min.

[0123] The atomization pressure is 1 MPa.

[0124] (3) Place the third precursor in a rotary kiln and perform three-stage calcination under a nitrogen atmosphere, and then obtain the lithium iron phosphate cathode material through mechanical crushing, sieving, and demagnetization.

[0125] The three-stage calcination includes: the first stage is heated to 200 °C at a rate of 1 °C / min and held for 1 h; the second stage is heated to 400 °C at a rate of 2 °C / min and held for 3 h; the third stage is heated to 700 °C at a rate of 5 °C / min and held for 6 h.

[0126] Method Example 2

[0127] A preparation method of a lithium iron phosphate cathode material, and the obtained product corresponds to the product of the above Example 2. The preparation method of this example includes the following steps:

[0128] (1) Dry-mix anhydrous iron phosphate with Dv50 of 8.52 μm, lithium acetate, titanium oxysulfate, the first carbon source, the second carbon source, and the third carbon source to obtain a material, and put the material into a ball milling device for grinding and crushing to obtain a precursor, and control the Dv50 of the precursor to be 0.4 - 0.5 μm.

[0129] Anhydrous iron phosphate is mixed with titanium oxysulfate and lithium acetate according to n(Fe): n(Ti): n(Li) = 0.986: 0.014: 1.05.

[0130] The first carbon source is polyimide; the second carbon source is glucose; the third carbon source is erythritol.

[0131] The total mass of anhydrous iron phosphate, lithium acetate, and titanium oxysulfate, the feeding ratio of the first carbon source, the second carbon source, and the third carbon source is 10 g: 0.7 g: 0.7 g: 0.7 g.

[0132] Ball milling conditions: the frequency is 30 Hz, 1 mm zirconia beads are used for ball milling, the dosage ratio to the volume of the material is 1: 1, the rotation speed is 600 rpm, the ball milling time is 3 h, and the temperature is room temperature.

[0133] (2) Place the precursor in a rotary kiln and perform three-stage calcination under a nitrogen atmosphere, and then obtain the lithium iron phosphate cathode material through mechanical crushing, sieving, and demagnetization.

[0134] The three-stage calcination includes: the first stage is heated to 200 °C at a rate of 1 °C / min and held for 1 h; the second stage is heated to 400 °C at a rate of 2 °C / min and held for 2 h; the third stage is heated to 700 °C at a rate of 5 °C / min and held for 8 h.

[0135] Method Example 3

[0136] A preparation method of a lithium iron phosphate cathode material, and the product obtained by the preparation corresponds to the product of the above Example 3. The preparation method of this example includes the following steps:

[0137] The difference from Method Example 1 is:

[0138] The medium of the air flow pulverization device is nitrogen;

[0139] In step (2), lithium oxalate is used as the lithium source; sodium fluoride is also added to the material, and anhydrous iron phosphate, sodium fluoride, and lithium oxalate are mixed according to n(Fe):n(Na):n(Li)=0.988:0.002:1.05.

[0140] The first carbon source solution is a 10 wt% toluene solution of polyphenylene ether; the second carbon source solution is a 20 wt% ethanol solution of polyethylene glycol-400; the third carbon source solution is a 10 wt% ethanol solution of fructose; the feeding ratio of the material, the first carbon source solution, the second carbon source solution, and the third carbon source solution is 10:2.5:2.5:7.5.

[0141] The pressure of the air flow is 0.8 ± 0.1 MPa; the air flow temperature is 120 °C and the mixing time is 15 min when the material is mixed with the first carbon source solution; the air flow temperature is 70 °C and the mixing time is 15 min when the first precursor is mixed with the second carbon source solution; the air flow temperature is 70 °C and the mixing time is 30 min when the second precursor is mixed with the third carbon source solution.

[0142] The atomization pressure is 0.8 MPa.

[0143] The three-stage calcination in step (3) includes: the first stage is heated to 200 °C at a rate of 1 °C / min and held for 2 h; the second stage is heated to 400 °C at a rate of 2 °C / min and held for 2 h; the third stage is heated to 650 °C at a rate of 5 °C / min and held for 5 h.

[0144] Method Example 4

[0145] A preparation method of a lithium iron phosphate cathode material, and the product obtained by the preparation corresponds to the product of the above Example 4. The preparation method of this example includes the following steps:

[0146] The difference from Method Example 1 is as follows:

[0147] In step (2), lithium oxalate is used as the lithium source; the first carbon source is not added, the second carbon source solution is an ethanol solution of 20 wt% polyethylene glycol - 400; the third carbon source solution is an ethanol solution of 10 wt% ethylene glycol; the feeding ratio of the material, the second carbon source solution, and the third carbon source solution is 10:5:8.

[0148] The pressure of the air flow is 1.5 ± 0.1 MPa; the air flow temperature is 80 °C and the mixing time is 30 min when the material is mixed with the second carbon source solution; the air flow temperature is 80 °C and the mixing time is 20 min when the second precursor is mixed with the third carbon source solution.

[0149] The atomization pressure is 1 MPa.

[0150] The three - stage calcination in step (3) includes: the first stage is heated to 200 °C at a rate of 1 °C / min and held for 2 h; the second stage is heated to 400 °C at a rate of 2 °C / min and held for 4 h; the third stage is heated to 650 °C at a rate of 5 °C / min and held for 5 h.

[0151] Method Example 5

[0152] A preparation method of a lithium iron phosphate cathode material, and the product obtained by the preparation corresponds to the product of the above - mentioned Example 5. The preparation method of this example includes the following steps:

[0153] The difference from Method Example 1 is as follows:

[0154] In step (2), the first carbon source solution is a DMF solution of 16 wt% polyimide; the second carbon source solution is an aqueous solution of 10 wt% amylose; the third carbon source solution is an ethanol solution of 20 wt% citric acid; the feeding ratio of the material, the first carbon source solution, the second carbon source solution, and the third carbon source solution is 10:6.25:2.5:1.25.

[0155] The atomization pressure is 0.5 MPa.

[0156] The three - stage calcination in step (3) includes: the first stage is heated to 200 °C at a rate of 2 °C / min and held for 2 h; the second stage is heated to 400 °C at a rate of 2 °C / min and held for 2 h; the third stage is heated to 750 °C at a rate of 5 °C / min and held for 5 h.

[0157] Method Example 6

[0158] A preparation method of a lithium iron phosphate cathode material, and the product obtained by the preparation corresponds to the product of the above - mentioned Example 6. The preparation method of this example includes the following steps:

[0159] (1) Mix anhydrous iron phosphate with a Dv50 of 8.52 μm, lithium carbonate, titanium oxysulfate, a second carbon source, and a third carbon source to obtain a material. Place the material in a ball milling device for grinding and crushing to obtain a precursor, and control the Dv50 of the precursor to be 0.4 - 0.5 μm.

[0160] The anhydrous iron phosphate is mixed with titanium oxysulfate and lithium carbonate in a ratio of n(Fe):n(Ti):n(Li) = 0.988:0.012:1.05.

[0161] The second carbon source is polyethylene glycol - 400; the third carbon source is ethylene glycol.

[0162] The feeding ratio of the total mass of anhydrous iron phosphate, lithium carbonate, and titanium oxysulfate, the second carbon source, and the third carbon source is 10:1.5:1.

[0163] Grinding conditions: frequency is 30 Hz, zirconia beads with a diameter of 1 mm are used for ball milling, the volume ratio of the beads to the material is 1:1, the rotation speed is 600 rpm, the ball milling time is 3 h, and the temperature is room temperature.

[0164] (2) Place the precursor in a rotary kiln and conduct three - stage calcination under a nitrogen atmosphere, and then obtain the lithium iron phosphate cathode material through mechanical crushing, sieving, and demagnetization.

[0165] The three - stage calcination includes: the first stage is heated to 200 °C at a rate of 1 °C / min and held for 2 h; the second stage is heated to 400 °C at a rate of 2 °C / min and held for 2 h; the third stage is heated to 750 °C at a rate of 5 °C / min and held for 6 h.

[0166] Method Example 7

[0167] A preparation method of a lithium iron phosphate cathode material, the product obtained by the preparation corresponds to the product of the above - mentioned Example 7. The preparation method of this example includes the following steps:

[0168] The difference from Method Example 5 is that the material in step (2) further includes titanium oxysulfate, and the anhydrous iron phosphate is mixed with titanium oxysulfate and lithium carbonate in a ratio of n(Fe):n(Ti):n(Li) = 0.988:0.012:1.05; the feeding ratio of the material, the first carbon source solution, the second carbon source solution, and the third carbon source solution is 10:9.375:5:2.5.

[0169] Method Comparative Example 1

[0170] A preparation method of a lithium iron phosphate cathode material, the product obtained by the preparation corresponds to the product of the above - mentioned Comparative Example 1. The preparation method of this comparative example includes the following steps:

[0171] (1) Mix ferrous sulfate heptahydrate, lithium hydroxide, and phosphoric acid in a ratio of 2 g : 40 mL of solute to solvent according to n(Fe) : n(Li) : n(P) = 3 : 1 : 1, and disperse them in a mixed solvent (the volume ratio of ethylene glycol to water is 1 : 1). After stirring evenly, keep the temperature at 180 °C for 6 h. After solid-liquid separation, wash and dry to obtain lithium iron phosphate powder.

[0172] (2) Mix the lithium iron phosphate powder with glucose, place it in a rotary kiln, and calcine it under a nitrogen atmosphere. Then, obtain the lithium iron phosphate cathode material through mechanical crushing, sieving, and demagnetization. The calcination conditions are to heat up to 650 °C at a rate of 5 °C / min and keep the temperature for 5 h. The mass of the glucose is 15% of the mass of the lithium iron phosphate powder.

[0173] Method Comparative Example 2

[0174] A method for preparing a lithium iron phosphate cathode material, and the product obtained by this preparation corresponds to the product of the above Comparative Example 2. The preparation method of this comparative example includes the following steps:

[0175] (1) Mix ferrous sulfate heptahydrate, lithium hydroxide, and phosphoric acid in a ratio of 2 g : 40 mL of solute to solvent according to n(Fe) : n(Li) : n(P) = 3 : 1 : 1, and disperse them in a mixed solvent (the volume ratio of ethylene glycol to water is 1 : 1). After stirring evenly, keep the temperature at 180 °C for 6 h. After solid-liquid separation, wash and dry to obtain lithium iron phosphate powder.

[0176] (2) Mix the lithium iron phosphate powder with a mixed carbon source and put it into a ball milling device for grinding and crushing to obtain a mixed material.

[0177] The mixed carbon source is a mixture of polyimide, glucose, and erythritol with a mass ratio of 1 : 1 : 1.

[0178] The mass of the mixed carbon source is 15% of the mass of the lithium iron phosphate powder.

[0179] Ball milling conditions: frequency is 30 Hz, use 1 mm zirconia beads for ball milling, the volume ratio of the dosage to the material is 1 : 1, rotation speed is 600 rpm, ball milling time is 3 h, and the temperature is at room temperature.

[0180] (3) Place the mixed material in a rotary kiln and conduct three-stage calcination under a nitrogen atmosphere. Then, obtain the lithium iron phosphate cathode material through mechanical crushing, sieving, and demagnetization. The three-stage calcination includes: the first stage is to heat up to 200 °C at a rate of 2 °C / min and keep the temperature for 1 h; the second stage is to heat up to 400 °C at a rate of 2 °C / min and keep the temperature for 2 h; the third stage is to heat up to 700 °C at a rate of 5 °C / min and keep the temperature for 8 h.

[0181] Method Comparative Example 3

[0182] A preparation method of a lithium iron phosphate cathode material, and the product obtained by the preparation corresponds to the product of Comparative Example 3 above. The preparation method of this comparative example includes the following steps:

[0183] The difference from Method Example 1 is that the material in step (2) further includes titanium oxysulfate, and anhydrous iron phosphate, titanium oxysulfate, and lithium carbonate are mixed according to n(Fe):n(Ti):n(Li)=0.986:0.014:1.05. Step (3) uses one-stage calcination, that is, it is heated to 700°C at a rate of 2°C / min and held for 8 hours to obtain the lithium iron phosphate cathode material.

[0184] Method Comparative Example 4

[0185] A preparation method of a lithium iron phosphate cathode material, and the product obtained by the preparation corresponds to the product of Comparative Example 4 above. The preparation method of this comparative example includes the following steps:

[0186] The difference from Method Example 1 is that only the second carbon source solution is added in step (2), and one-stage calcination is used in step (3), which specifically includes the following steps:

[0187] (1) The same as Method Example 1;

[0188] (2) Mix the anhydrous iron phosphate obtained in step (1) with lithium carbonate as the material, and introduce the atomized second carbon source solution into the air flow pulverizer as the medium to pulverize and coat the material to obtain the precursor.

[0189] Anhydrous iron phosphate and lithium carbonate are mixed according to n(Fe):n(Li)=1:1.05.

[0190] The second carbon source solution is a 20wt% aqueous glucose solution.

[0191] The feeding ratio of the material to the second carbon source solution is 10g:7.5g.

[0192] The pressure of the air flow is 1±0.1MPa; the air flow temperature is 100°C when the material is mixed with the second carbon source solution, and the mixing time is 15 minutes.

[0193] The atomization pressure is 1MPa.

[0194] (3) Place the precursor in a rotary kiln and calcine it under a nitrogen atmosphere, and then obtain the lithium iron phosphate cathode material after mechanical crushing, sieving, and demagnetization. Calcination conditions: heat to 700°C at a rate of 2°C / min and hold for 8 hours.

[0195] Combined with the products and preparation methods of the present invention, as well as the product performances in the above Tables 1 - 3, it can be seen that in Method Example 2, since the mixing method of anhydrous iron phosphate with lithium acetate, titanium oxysulfate, the first carbon source, the second carbon source, and the third carbon source is dry mixing, the agglomeration degree of the product obtained at this time is significantly higher than that of other method examples. As a result, its tap density and peel strength are lower, the sample standard deviation of the areal density and the sample standard deviation of the electrode resistance are larger, and the rate performance and cycle stability are poorer. While Method Example 3 changed the reaction parameters within the scope of the present invention and still obtained good results. In Method Example 4, since the first carbon source was not added, compared with Example 1, its is lower, the sample standard deviation of the areal density and the sample standard deviation of the electrode resistance are both lower, and the specific capacity of the material also decreases slightly, but it is still significantly better than Comparative Examples 1 - 4, fully proving that the carbon source of the present invention can also achieve better effects even when there are only two kinds. Method Example 5 adjusted the types and dosages of the carbon source and the reaction conditions, and it still could obtain good performances. In Method Example 6, all the raw materials were directly ball-milled and mixed, which would lead to a significant increase in the FSSS and of the product obtained by its preparation, resulting in an increase in the agglomeration degree. Method Example 7 prepared a chemical composition similar to that of Method Example 6 according to the method of Method Example 5, but due to the and agglomeration degree in Method Example 7 being better than those in Method Example 6, its comprehensive performance is better.

[0196] In Method Comparative Examples 1 and 2, lithium iron phosphate powder was prepared according to the conventional method, and then mixed with the carbon source and calcined. Whether it was one-stage calcination or three-stage calcination, the comprehensive performance of the prepared lithium iron phosphate cathode material was significantly worse than that of the above method examples. Compared with Method Example 1, in Method Comparative Example 3, one-stage calcination was used in step (3), and at this time its effect was significantly worse than that of Method Example 1; the effect of Method Comparative Example 4 was even worse than that of Method Comparative Example 3, because only one kind of carbon source was added, and at this time the FSSS and of the prepared lithium iron phosphate cathode material were significantly improved, and ρ % and the agglomeration degree exceeded the scope of the present application, resulting in a significant reduction in the electrical performance of the product.

[0197] In addition, the present invention also provides Method Examples 8 - 14 and Method Comparative Example 5, and Method Examples 8 - 14 and Method Comparative Example 5 perform single-factor variable on a certain parameter of Method Example 1.

[0198] Specifically, the specific steps of Method Examples 8 - 14 and Method Comparative Example 5 are as follows:

[0199] Method Example 8

[0200] The difference from Method Example 1 is that the carbon source and the material are dry-mixed in a ball milling device, including the following steps:

[0201] Mix anhydrous iron phosphate with a Dv50 of 8.52 μm with lithium carbonate, the first carbon source, the second carbon source, and the third carbon source to obtain a material. Put the material into a ball milling device for grinding and crushing to obtain a precursor, and control the Dv50 of the precursor to be 0.4 - 0.5 μm.

[0202] The anhydrous iron phosphate and lithium carbonate are mixed according to n(Fe) : n(Li) = 1:1.05.

[0203] The first carbon source is polyimide; the second carbon source is glucose; the third carbon source is ethylene glycol.

[0204] The total mass of anhydrous iron phosphate and lithium carbonate, and the feeding ratios of the first carbon source, the second carbon source, and the third carbon source are 10 g : 0.5 g : 0.5 g : 0.5 g.

[0205] Ball milling conditions: frequency is 30 Hz, 1 mm zirconia beads are used for ball milling, the volume ratio of the dosage to the material is 1:1, the rotation speed is 600 rpm, the ball milling time is 3 h, and the temperature is room temperature.

[0206] The precursor is calcined, and the method is the same as that in step (3) of Method Example 1.

[0207] Method Example 9

[0208] The difference from Method Example 1 is that the carbon source and the material are mixed together in a jet mill device, including the following steps:

[0209] (1) The same as Method Example 1

[0210] (2) Prepare a carbon source mixed solution. The mass percentages of polyimide, glucose, ethylene glycol, and the solvent DMF in the carbon source mixed solution are 10% : 10% : 10% : 70% respectively; Mix the anhydrous iron phosphate and lithium carbonate obtained in step (1) as the material, and introduce the atomized carbon source mixed solution into the jet mill device with air as the medium to crush and coat the material to obtain a precursor.

[0211] The anhydrous iron phosphate and lithium carbonate are mixed according to n(Fe) : n(Li) = 1:1.05.

[0212] The feeding ratio of the material to the carbon source mixed solution is 10 g : 5 g.

[0213] The pressure of the air flow is 1 ± 0.1 MPa; the air flow temperature when the material is mixed with the carbon source mixed solution is 120 °C, and the mixing time is 20 min.

[0214] The atomizing pressure is 1 MPa.

[0215] (3) is the same as Method Example 1.

[0216] Method Example 10

[0217] The difference from Method Example 1 is that the atomized third carbon source solution, second carbon source solution, and first carbon source solution are introduced into the air flow pulverization device in sequence to pulverize and coat the material, and the first precursor, second precursor, and third precursor are obtained respectively.

[0218] Method Example 11

[0219] The difference from Method Example 1 is that in step (2), the feeding ratio of the material, the first carbon source solution, the second carbon source solution, and the third carbon source solution is 10 g: 8 g: 2.5 g: 2 g.

[0220] Method Example 12

[0221] The difference from Method Example 1 is that in step (2), the feeding ratio of the material, the first carbon source solution, the second carbon source solution, and the third carbon source solution is 10 g: 2 g: 2.5 g: 8 g.

[0222] Method Example 13

[0223] The difference from Method Example 1 is that in step (2), the mixing of the material and the carbon source solution is at room temperature of 25 °C.

[0224] Method Example 14

[0225] The difference from Method Example 1 is that a ball milling device is used instead of an air flow pulverization device for wet mixing, and the specific steps are as follows:

[0226] (1) is the same as Method Example 1;

[0227] (2) Mix anhydrous iron phosphate and lithium carbonate obtained in step (1) as the material, and prepare the first carbon source solution, the second carbon source solution, and the third carbon source solution; spray the atomized first carbon source solution onto the material under stirring, stir evenly and then place it in a ball milling device for the first ball milling to obtain the first precursor; spray the atomized second carbon source solution onto the first precursor under stirring, stir evenly and then place it in a ball milling device for the second ball milling to obtain the second precursor; spray the atomized third carbon source solution onto the second precursor under stirring, stir evenly and then place it in a ball milling device for the third ball milling to obtain the third precursor;

[0228] Anhydrous iron phosphate and lithium carbonate are mixed according to n(Fe):n(Li)=1:1.05. The atomization pressure is 1 MPa.

[0229] The first carbon source solution is a polyimide DMF solution with a concentration of 10 wt%; the second carbon source solution is an aqueous glucose solution with a concentration of 20 wt%; the third carbon source solution is an ethylene glycol ethanol solution with a concentration of 10 wt%.

[0230] The feeding ratio of the material, the first carbon source solution, the second carbon source solution, and the third carbon source solution is 10 g: 5 g: 2.5 g: 5 g.

[0231] Ball milling conditions: the frequency is 30 Hz, 1 mm zirconia beads are used for ball milling, the volume ratio of the dosage to the material is 1:1, and the rotation speed is 600 rpm; the ball milling temperature of the first ball milling is 120 °C and the time is 1 h; the ball milling temperature of the second ball milling is 100 °C and the time is 1 h; the ball milling temperature of the third ball milling is 80 °C and the mixing time is 1 h.

[0232] (3) is the same as Method Example 1.

[0233] Method Comparative Example 5

[0234] The difference from Method Example 1 is that only the second carbon source solution is added in step (2).

[0235] The products of Method Examples 8-14 and Method Comparative Example 5 were tested, and the test results are shown in Table 4.

[0236] Table 4. Statistical table of parameter detection of different samples

[0237]

[0238] As can be seen from the above table, in Method Example 8, anhydrous iron phosphate is mixed and ball milled with lithium carbonate, the first carbon source, the second carbon source, and the third carbon source to obtain a precursor. At this time, its FSSS and increase significantly, resulting in an increase in the degree of agglomeration and exceeding the scope of the present invention. In Method Example 9, all the carbon sources are mixed together without being added sequentially, which will cause a slight increase in FSSS, but , ρ% and the degree of agglomeration N are still within the preferred range of this application. In Method Example 10, the addition order of the carbon sources is opposite to that of Method Example 1. At this time, it will cause a slight increase in FSSS, but , ρ% and the degree of agglomeration N are still within the preferred range of this application. In Method Examples 11-12, the amounts of the first carbon source, the second carbon source, and the third carbon source added are adjusted. It can be seen that the change in the addition amounts of the first carbon source and the third carbon source will also have a certain impact on the product performance parameters. Among them, when the amount of the first carbon source increases and the amount of the third carbon source decreases, it will cause an increase in FSSS, a decrease in BET, , ρ% and the degree of agglomeration N increase. When the amount of the first carbon source decreases and the amount of the third carbon source increases, it will cause a decrease in FSSS and an increase in BET, , and the degree of agglomeration N decreases. Therefore, the above parameters can be adjusted by controlling the addition amount of the carbon source to control it within a reasonable range. In Method Example 13, the mixing temperature of the material and the carbon source solution was changed, and the obtained increased significantly and exceeded the preferred range of the present invention. In Method Example 14, a ball milling device was used instead of a jet milling device for wet mixing. It can be seen that whether it is jet milling or ball milling, it is possible to prepare a lithium iron phosphate cathode material that simultaneously satisfies and within a specific range. However, using jet milling to mix the materials can make the dispersion of each component in the precursor better, and the carbon source is evenly dispersed on the surface of anhydrous iron phosphate or lithium source particles, which is conducive to subsequent uniform gasification and pore formation, and a lithium iron phosphate cathode material with a uniform distribution of carbon materials is obtained. Through the above method examples, it can be seen that within the parameter range of the present invention, adjustments can be made to obtain a lithium iron phosphate cathode material that simultaneously satisfies and within a specific range. This lithium iron phosphate cathode material has good dispersion, low agglomeration degree, and high tap density. The positive electrode sheet prepared therefrom has the characteristics of uniform surface density and high peel strength. Therefore, the prepared battery has a high specific capacity and cycle stability.

[0239] In Comparative Example 5 of the method, only the second carbon source was added, which would cause the FSSS to exceed the range of the present invention, increase significantly, and finally cause the agglomeration degree to exceed the range of the present invention. This fully proves that adding only one carbon source cannot obtain a lithium iron phosphate cathode material with an agglomeration degree lower than 25000.

[0240] The present invention also performed electron microscopy scans on the products of the above Examples 1, 8 and Comparative Example 4. Please refer to the SEM images in Figure 1 、 Figure 2 and Figure 3 .

[0241] From Figure 1 、 Figure 2 and Figure 3 comparisons, it can be seen that the dispersion: Example 1 > Example 8 > Comparative Example 4.

[0242] Furthermore, the present invention also performed Raman spectroscopy tests on the lithium iron phosphate cathode materials provided in Examples 1 and 4: The graphitization degree of the lithium iron phosphate cathode material was characterized using a German Burker RFS100 / S Raman spectrometer, with an excitation wavelength of 458 nm. After background subtraction, it was fitted with a Gaussian function. The D-band characteristic peak corresponding to 1300 - 1400 cm -1 represents the A of sp³ hybridized carbon atoms 1gVibration, which reflects the defects or disordered structure of the carbon material, and the corresponding peak intensity is I D ; 1500 - 1600 cm -1 The corresponding G-band characteristic peak represents sp 2 hybridized carbon atoms' E 2g vibration, which reflects the ordered structure of the carbon material, and the corresponding peak intensity is I G , to evaluate the graphitization degree of the carbon material in the lithium iron phosphate cathode material. The test results are referred to Figure 4 .

[0243] Since the higher the graphitization degree, the better the electrical conductivity of the carbon material. The Raman spectra of Example 1 and Example 4 are referred to Figure 4 , the I G / I D of Example 1 is 1.22, and the I G / I D of Example 4 is 1.09, indicating that adding the first carbon source is beneficial to improving the graphitization degree of the lithium iron phosphate cathode material. Moreover, the carbon source feeding amount in Method Example 4 is greater than that in Method Example 1, but the carbon material content in Example 1 is higher, indicating that adding the first carbon source is beneficial to increasing the solid residue amount of the carbon source, thereby increasing the conductive carbon.

[0244] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A lithium iron phosphate cathode material, characterized in that, It includes primary particles and secondary particles, and the secondary particles are formed by agglomeration of the primary particles. The lithium iron phosphate cathode material satisfies = 5 to 40 and ρ% = 50% to 90%, where the FSSS refers to the Fisher Sub-Sieve Size of the lithium iron phosphate cathode material, with the unit of μm; the D BET refers to the specific surface area particle size of the lithium iron phosphate cathode material, with the unit of μm; ρ% = 1 - H%, where ρ% refers to the relative density of the secondary particles, and H% refers to the porosity of the secondary particles. The FSSS is the average particle size of the lithium iron phosphate cathode material measured by the steady-flow air permeability method, and the value range of the FSSS satisfies 0.4 to 5 μm; The D BET is the particle size of the lithium iron phosphate cathode material measured and calculated by the nitrogen adsorption and desorption method. The , where BET refers to the specific surface area of the lithium iron phosphate cathode material, and the value range of BET satisfies ≤ 20 m 2 / g. The refers to the true density of the lithium iron phosphate cathode material. The value range satisfies 3.3 - 3.6 g / cm³. The agglomeration degree N of the lithium iron phosphate cathode material is lower than 25000, where the agglomeration degree .

2. The lithium iron phosphate cathode material according to claim 1, characterized in that The tap density of the lithium iron phosphate cathode material is 1.9~3.0 g / cm 3 .

3. The lithium iron phosphate cathode material according to claim 2, wherein The lithium iron phosphate cathode material satisfies = 10 to 30; And / or, the secondary particles of the lithium iron phosphate cathode material satisfy ρ% = 50% - 70%; And / or, the lithium iron phosphate cathode material satisfies that the degree of agglomeration N is 200 - 12000; And / or, the lithium iron phosphate cathode material satisfies FSSS = 1 - 2.5 μm; And / or, the lithium iron phosphate cathode material satisfies a BET of 9 to 15 m 2 / g.

4. The lithium iron phosphate cathode material according to any one of claims 1-3, characterized in that, The lithium iron phosphate cathode material includes an active material matrix and carbon material, and the general formula of the active material matrix is Li 1-x A x Fe 1-y M y (PO 4-z )D z , wherein A is selected from one or more of Na and Mg; M is selected from one or more of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from one or more of F, S, N and Cl; 0≤x≤0.1; 0≤y≤0.1 and 0≤z≤0.1; the mass of the carbon material is 1% to 5% of the mass of the lithium iron phosphate cathode material.

5. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1-4, characterized in that, It includes: Mixing iron phosphate and a lithium salt as a material, and mixing the material with carbon sources having different gasification temperatures to obtain a precursor; Subjecting the precursor to staged calcination in an inert atmosphere, and then crushing, sieving, and demagnetizing to obtain the lithium iron phosphate cathode material.

6. The preparation method of the lithium iron phosphate cathode material according to claim 5, wherein The step of mixing the iron phosphate and the lithium salt includes at least one of Feature I - Feature IV; Feature I: Before mixing the iron phosphate and the lithium salt, first add the iron phosphate into a jet mill for comminution treatment; Feature II: The lithium salt is at least one of lithium carbonate, lithium oxalate, and lithium acetate; Feature III: When mixing the iron phosphate and the lithium salt, a dopant is further added, and the dopant is at least one of a salt, an oxide, and a hydroxide of a doping element; Feature IV: The iron phosphate and the dopant are fed according to a stoichiometric ratio.

7. The preparation method of the lithium iron phosphate cathode material according to claim 5, characterized in that, The carbon sources having different gasification temperatures include a first carbon source with a gasification temperature of 400 - 600 °C, a second carbon source with a gasification temperature of 200 - 400 °C, and a third carbon source with a gasification temperature of 100 - 200 °C; The first carbon source includes at least one of polyimide, polyphenylene ether, and polyetherimide; The second carbon source includes at least one of glucose, polyethylene glycol, starch, sucrose, maltose, cellulose, chitosan, polyacrylonitrile, benzyl alcohol, and glycerol; The third carbon source includes at least one of phenol, ethylene glycol, propylene glycol, butylene glycol, isopropyl alcohol, erythritol, fructose, malic acid, citric acid, and salicylic acid.

8. The preparation method of the lithium iron phosphate cathode material according to claim 7, wherein The step of mixing the material with carbon sources having different gasification temperatures includes at least one of Feature V - Feature IX; Feature V: Mix the material with the carbon sources having different gasification temperatures in a jet mill, introduce a gas stream during mixing, the pressure of the gas stream is 0.5 - 1.5 MPa, and the temperature is 25 - 120 °C; the total mixing time is 15 - 60 min; Feature VI: Before mixing the carbon sources having different gasification temperatures with the material, first prepare a carbon source solution with a mass percentage concentration of 10% - 30%, and then atomize the carbon source solution and mix it with the material, wherein the atomization pressure is 0.1 - 1.5 MPa; Feature VII: First mix the material with the first carbon source, then mix it with the second carbon source, and finally mix it with the third carbon source; Feature VIII: The total mass of the carbon sources having different gasification temperatures is 15% - 25% of the mass of the material; Feature IX: When mixing the material with the carbon sources having different gasification temperatures, the mass ratio of the first carbon source, the second carbon source, and the third carbon source is 0 - 15:2.5 - 15:2.5 - 10.

9. The preparation method of the lithium iron phosphate cathode material according to claim 5, wherein The segmented calcination includes a first stage, a second stage and a third stage. In the first stage, the temperature is raised to 180 - 220°C at a rate of 1 - 2°C / min and held for 1 - 2 h; in the second stage, the temperature is raised to 380 - 420°C at a rate of 2 - 3°C / min and held for 2 - 4 h; in the third stage, the temperature is raised to 650 - 750°C at a rate of 4 - 6°C / min and held for 4 - 8 h.

10. A lithium-ion battery, characterized in that, It includes the lithium iron phosphate cathode material as described in any one of claims 1 - 4 or the lithium iron phosphate cathode material obtained by the preparation method of the lithium iron phosphate cathode material as described in any one of claims 5 - 9.

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