Lithium iron phosphate positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. According to the present invention, the lithium iron phosphate positive electrode material is provided, the lithium iron phosphate positive electrode material comprises small-particle lithium iron phosphate and large-particle lithium iron phosphate, the particle size distribution curve of the lithium iron phosphate positive electrode material presents a double-peak or double-peak-like peak type, the particle size corresponding to the highest point of the peak intensity of the left peak is 0.2 [mu] m < = Dp1 < = 0.6 [mu] m, and the particle size corresponding to the highest point of the peak intensity of the right peak is 0.2 [mu] m < = Dp2 < = 0.6 [mu] m; the volume ratio Vp1 corresponding to the highest peak intensity is greater than or equal to 6.0% and less than or equal to 8.0%; the particle size Dp2 corresponding to the highest peak intensity of the right peak is more than or equal to 1.0 mu m and less than or equal to 1.5 mu m, and the volume ratio Vp2 corresponding to the highest peak intensity of the right peak is more than or equal to 8.0% and less than or equal to 10.0%; through compounding of small-particle lithium iron phosphate and large-particle lithium iron phosphate, the overall compaction density and electrochemical performance of the lithium iron phosphate positive electrode material are improved, and wide industrial application is facilitated.

Description

Technical Field

[0001] This application belongs to the technical field of lithium - ion batteries, and particularly relates to a lithium iron phosphate cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] A lithium - ion battery is composed of a cathode material, an anode material, a separator, and an electrolyte, and has the advantages of a high working voltage, a large energy density, a low self - discharge rate, and a long cycle life; based on the above advantages, lithium - ion batteries are more widely used in various fields.

[0003] With the rapid development of the electric vehicle industry, the demand for high - safety, high - performance, low - cost, and environmentally friendly lithium - ion batteries is increasing day by day. The cathode material is an important part of a lithium - ion battery, and its cost proportion in the lithium - ion battery even exceeded 80% at one time. Therefore, it is crucial to develop a high - cost - performance lithium - ion cathode material.

[0004] Due to its low cost, high energy density, excellent cycle performance, and good safety performance, the market demand for lithium iron phosphate has been increasing in recent years. However, the energy density of lithium iron phosphate is lower than that of ternary materials, and the two core performance indicators affecting the energy density of lithium iron phosphate are the powder tap density and the discharge specific capacity. Therefore, developing a lithium iron phosphate cathode material with a high tap density and a high discharge specific capacity is a technical problem that needs to be focused on breaking through in the entire industry. Summary of the Invention

[0005] The purpose of this application is to provide a lithium iron phosphate cathode material, a preparation method thereof, and an application thereof, aiming to solve the problem in the prior art that it is impossible to simultaneously improve the tap density and the discharge specific capacity of the lithium iron phosphate cathode material.

[0006] To achieve the above - mentioned application purpose, the technical solution adopted in this application is as follows:

[0007] In the first aspect, this application provides a lithium iron phosphate cathode material. The lithium iron phosphate cathode material includes small - particle lithium iron phosphate and large - particle lithium iron phosphate, and the particle size distribution curve of the lithium iron phosphate cathode material presents a bimodal or quasi - bimodal peak shape, where the particle size D corresponding to the highest peak intensity of the left peak satisfies 0.2 μm ≤ D p1 ≤ 0.6 μm, and the volume ratio V corresponding to the highest peak intensity satisfies 6.0% ≤ V p1 ≤ 8.0%; the particle size D corresponding to the highest peak intensity of the right peak satisfies 1.0 μm ≤ D p2 ≤ 1.5 μm, and the volume ratio V corresponding to the highest peak intensity satisfies 8.0% ≤ V p2 ≤ 10.0%.

[0008] In some embodiments, the lithium iron phosphate cathode material includes a first structural formula and a second structural formula. The first structural formula is Li x Fey M1 z PO4@C, 1.005 ≤ x ≤ 1.05, 0.975 ≤ y ≤ 0.990, 8.36×10 -5 ≤ z ≤ 1.67×10 -4 ; The second structural formula is Li x’ Fe y’ M2 z’ PO4@C, 0.98 ≤ x’ ≤ 1.00, 0.950 ≤ y’ ≤ 0.965, 0 ≤ z’ ≤ 8.36×10 -5 ; wherein, M1 and M2 are doping elements and are independently selected from at least one of Ti, Mg, V, Nb, and Mn.

[0009] In some embodiments, the lithium iron phosphate cathode material includes small particle lithium iron phosphate and large particle lithium iron phosphate, wherein the mass ratio of small particle lithium iron phosphate to large particle lithium iron phosphate is (3 - 10):1.

[0010] In some embodiments, the lithium iron phosphate cathode material is spherical or quasi-spherical.

[0011] In some embodiments, the average particle size of the primary particles of the lithium iron phosphate cathode material is 200 - 350 nm, wherein the proportion of the number of primary particle sizes d ≤ 200 nm is 35.0% ≤ P1 ≤ 70.0%, and the proportion of the number of primary particle sizes d ≥ 600 nm is 2.5% ≤ P2 ≤ 10.0%.

[0012] In some embodiments, based on the total mass of the lithium iron phosphate cathode material being 100%, the total carbon content is 1.0 wt% - 1.5 wt%, and the total content of the doping element is 3000 - 10000 ppm.

[0013] In some embodiments, the powder tap density of the lithium iron phosphate cathode material ≥ 2.60 g / cm 3 .

[0014] In some embodiments, the discharge specific capacity of the lithium iron phosphate cathode material at 25°C at 1C ≥ 140 mAh / g.

[0015] In a second aspect, the present application provides a method for preparing a lithium iron phosphate cathode material, comprising the following steps:

[0016] After first-stage grinding of precursors A and B with different raw material ratios, they are respectively placed in the upper chamber and the lower chamber of a layered crucible for first sintering, and after sintering, intermediate C and intermediate D are obtained;

[0017] Mix and grind a dopant and intermediate C to obtain a first mixture, and mix and grind a carbon source and intermediate D to obtain a second mixture;

[0018] The first mixture and the second mixture are respectively placed in the upper chamber and the lower chamber of a layered crucible for secondary sintering to obtain lithium iron phosphate E and lithium iron phosphate F, and then the lithium iron phosphate cathode material is obtained after air-flow mixing and pulverization. Among them, the lithium iron phosphate cathode material includes small-particle lithium iron phosphate and large-particle lithium iron phosphate.

[0019] In some embodiments, in the raw materials of precursor A, the molar ratio of the lithium source to iron phosphate is (1.005 - 1.05):1, and among them, the atomic ratio of iron to phosphorus in the raw materials is 0.975 - 0.990:1.

[0020] In some embodiments, in the raw materials of precursor B, the molar ratio of the lithium source to iron phosphate is (0.980 - 1.000):1, and among them, the atomic ratio of iron to phosphorus in the raw materials is 0.950 - 0.965:1.

[0021] In some embodiments, the particle size D of the slurry of precursor A 50 is 0.2 - 0.4 μm, and the particle size D of the slurry of precursor B 50 is 0.6 - 0.8 μm.

[0022] In some embodiments, the mass ratio of precursor A to precursor B is (3 - 10):1.

[0023] In some embodiments, the mass ratio of the first mixture to the second mixture is (3 - 10):1.

[0024] In some embodiments, the material of the layered crucible is graphite, and the length and width dimensions of the upper chamber and the lower chamber are the same. The heights of the upper chamber and the lower chamber are h1 and h2 respectively, and h1:h2 = (3 - 10):1.

[0025] In some embodiments, in intermediate C, based on the total mass of intermediate C being 100%, the carbon content is 1.3 wt% - 1.8 wt%, and the content of the doped metal is 4000 - 8000 ppm.

[0026] In some embodiments, in the first mixture, the addition amount of the dopant is 0 - 2000 ppm; the particle size D of the slurry 50 is 0.2 - 0.4 μm.

[0027] In some embodiments, in lithium iron phosphate E, based on the total mass of lithium iron phosphate E being 100%, the carbon content is 1.3 wt% - 1.8 wt%, and the content of the doped metal is 4000 - 10000 ppm.

[0028] In some embodiments, in intermediate D, based on the total mass of intermediate D being 100%, the carbon content is 0.2 wt% - 0.8 wt%, and the content of the doped metal is 0 - 4000 ppm.

[0029] In some embodiments, in the second mixture, the addition amount of carbon is 0.3 wt% to 0.8 wt%; the slurry particle size D 50 is 0.8 to 1.2 μm.

[0030] In some embodiments, in lithium iron phosphate F, based on the total mass of lithium iron phosphate F being 100%, the carbon content is 1.0 wt% to 1.5 wt%, and the content of the doped metal is 0 to 4000 ppm.

[0031] In some embodiments, the sintering temperature of the first sintering and the second sintering is 700 to 800 °C, and the sintering time is 12 to 24 h.

[0032] In some embodiments, the parameters of the air flow mixing and pulverizing are: the classification frequency is 50 to 100 Hz, the pressure intensity is 0.4 to 0.6 MPa, and the feeding rate is 1 to 5 kg / h.

[0033] In a third aspect, the present application provides a lithium ion battery, including the above-mentioned lithium iron phosphate positive electrode material or a lithium iron phosphate positive electrode material prepared by the preparation method of the above-mentioned lithium iron phosphate positive electrode material.

[0034] The lithium iron phosphate positive electrode material provided in the first aspect of the present application is obtained by compounding small particle lithium iron phosphate and large particle lithium iron phosphate, and the particle size distribution curve of the lithium iron phosphate positive electrode material presents a bimodal or quasi-bimodal peak shape, wherein the particle size D corresponding to the highest peak intensity of the left peak is 0.2 μm ≤ D p1 ≤ 0.6 μm, and the volume ratio corresponding to the highest peak intensity is 6.0% ≤ V p1 ≤ 8.0%; the particle size D corresponding to the highest peak intensity of the right peak is 1.0 μm ≤ D p2 ≤ 1.5 μm, and the volume ratio corresponding to the highest peak intensity is 8.0% ≤ V p2 ≤ 10.0%; it can be seen that the provided lithium iron phosphate positive electrode material is obtained by compounding small particle lithium iron phosphate and large particle lithium iron phosphate, wherein the small particle size particles can greatly reduce the lithium ion transmission path, improve the lithium ion transmission efficiency, thereby improving the electrochemical performance, while the large particle size particles can effectively improve the tap density. The common improvement of these two core performances can effectively improve the energy density of the lithium iron phosphate material, and greatly broaden the application prospect of the lithium iron phosphate material in the power field.

[0035] The preparation method of the lithium iron phosphate cathode material provided in the second aspect of the embodiments of the present application. In this preparation method, precursors A and B with different raw material ratios are respectively provided, and it is creatively proposed to load precursors A and B into the upper and lower chambers of the sagger respectively. Due to the gaps between the precursor powders, the heat conduction effect between the powder particles is poor, which often causes the materials in the saggers placed at different positions to be at different temperatures. The lower chamber is close to the actual temperature in the furnace to form large-particle lithium iron phosphate, and the actual temperature of the upper chamber is lower than the actual temperature of the furnace to form small-particle lithium iron phosphate. At the same time, the cation ratio in precursor A is higher than that in precursor B. The increase in the cation ratio is beneficial to increasing the energy barrier for the growth of lithium iron phosphate particles, facilitating the preparation of small-particle lithium iron phosphate. This preparation method realizes the simultaneous preparation of large- and small-particle lithium iron phosphate, and the large-scale preparation of lithium iron phosphate with high tap density and high capacity. The process is simple, environmentally friendly, and low in cost, which is conducive to the industrial development of lithium iron phosphate cathode materials.

[0036] The lithium-ion battery provided in the third aspect of the present application. Since the provided lithium-ion battery includes the above-mentioned lithium iron phosphate cathode material or the lithium iron phosphate cathode material prepared by the preparation method of the above-mentioned lithium iron phosphate cathode material, due to the excellent properties of high tap density and high capacity of the obtained lithium iron phosphate cathode material, the obtained lithium-ion battery has a high cycle efficiency and a high charge-discharge specific capacity. Description of the Drawings

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the following drawings are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0038] Figure 1 It is the particle size distribution curve of the lithium iron phosphate finished product obtained in Example 1 of the present application;

[0039] Figure 2 It is the SEM image of the lithium iron phosphate finished product obtained in Example 1 of the present application;

[0040] Figure 3 It is the SEM image of the lithium iron phosphate finished product obtained in Comparative Example 2 of the present application;

[0041] Figure 4 It is the charge-discharge curve of the lithium iron phosphate finished product obtained in Example 1 of the present application. Detailed Embodiments

[0042] In order to make the technical problems to be solved, technical solutions and beneficial effects of this application more clear and understandable, the following further elaborates on this application in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not used to limit this application.

[0043] In this application, the term "and / or" describes the association relationship of associated objects and indicates that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Among them, A and B can be singular or plural. The character " / " generally represents an "or" relationship between the front and back associated objects.

[0044] In this application, "at least one" means one or more, and "a plurality" means two or more. "At least one of the following" or its similar expressions refer to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0045] It should be understood that in various embodiments of this application, the magnitudes of the serial numbers of the above processes do not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of this application.

[0046] The terms used in the embodiments of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The singular forms "a" and "the" used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0047] The weights of the relevant components mentioned in the specification of the embodiments of this application not only can refer to the specific contents of each component, but also can represent the proportional relationship of the weights between each component. Therefore, as long as the contents of the relevant components in the specification of the embodiments of this application are scaled up or down in proportion, they are within the scope disclosed in the specification of the embodiments of this application. Specifically, the mass in the specification of the embodiments of this application can be mass units well-known in the chemical industry such as μg, mg, g, kg, etc.

[0048] The terms "first" and "second" are used for descriptive purposes only to distinguish objects such as substances from each other, and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX may also be referred to as the second XX, and similarly, the second XX may also be referred to as the first XX. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features.

[0049] In the first aspect of the embodiments of the present application, a lithium iron phosphate cathode material is provided. The lithium iron phosphate cathode material includes small particle lithium iron phosphate and large particle lithium iron phosphate, and the particle size distribution curve of the lithium iron phosphate cathode material presents a bimodal or quasi-bimodal peak shape, wherein the particle size corresponding to the highest peak intensity of the left peak is 0.2 μm ≤ Dp1 ≤ 0.6 μm, and the volume ratio corresponding to the highest peak intensity is 6.0% ≤ Vp1 ≤ 8.0%; the particle size corresponding to the highest peak intensity of the right peak is 1.0 μm ≤ Dp2 ≤ 1.5 μm, and the volume ratio corresponding to the highest peak intensity is 8.0% ≤ Vp2 ≤ 10.0%.

[0050] The particle size distribution curve of the lithium iron phosphate cathode material provided in the first aspect of the embodiments of the present application presents a bimodal or quasi-bimodal peak shape, wherein the particle size corresponding to the highest peak intensity of the left peak is 0.2 μm ≤ D p1 ≤ 0.6 μm, and the volume ratio corresponding to the highest peak intensity is 6.0% ≤ V p1 ≤ 8.0%; the particle size corresponding to the highest peak intensity of the right peak is 1.0 μm ≤ D p2 ≤ 1.5 μm, and the volume ratio corresponding to the highest peak intensity is 8.0% ≤ V p2 ≤ 10.0%; it can be seen that the provided lithium iron phosphate cathode material is obtained by compounding small particle lithium iron phosphate and large particle lithium iron phosphate. Among them, the small particle size particles can greatly reduce the lithium ion transmission path, improve the lithium ion transmission efficiency, and thus improve the electrochemical performance, while the large particle size particles can effectively improve the tap density. The common improvement of these two core performances can effectively improve the energy density of the lithium iron phosphate material and greatly broaden the application prospect of the lithium iron phosphate material in the power field.

[0051] In the conventional lithium iron phosphate cathode material with a compound of large and small particle sizes, the raw material ratios of particles with different particle sizes are usually the same. mainly in the processing process, different grinding methods are used to obtain particles with different particle sizes for compounding.

[0052] In some embodiments, the lithium iron phosphate cathode material includes a first structural formula and a second structural formula. The first structural formula is Li x Fe y M1 z PO4@C, 1.005 ≤ x ≤ 1.05, 0.975 ≤ y ≤ 0.990, 8.36 × 10 -5≤z≤1.67×10 -4 On the one hand, the lithium iron phosphate of the first structural formula has a relatively high lithium content. When the lithium iron phosphate is used as the cathode material, the compounded lithium iron phosphate of the first structural formula means that during battery charging, more lithium ions can be embedded into the lattice of the lithium iron phosphate, and during discharging, more lithium ions can also be extracted to participate in the electrochemical reaction, thereby increasing the theoretical specific capacity of the battery. At the same time, it helps to optimize the crystal structure of the lithium iron phosphate, making the diffusion of lithium ions in the lattice smoother. During the charge and discharge process, lithium ions can be embedded and extracted more quickly, thereby improving the charge and discharge efficiency of the battery and enhancing its high-rate charge and discharge performance. On the other hand, the lithium iron phosphate of the first structural formula has a relatively high iron-to-phosphorus ratio. A relatively high iron-to-phosphorus ratio means an increase in the relative content of iron ions, which can further enhance the stability of this framework structure. During the charge and discharge process of the battery, it helps to resist the structural changes caused by the embedding and extraction of lithium ions, reducing the possibility of lattice distortion and structural collapse, thereby improving the cycle stability of the battery and extending its service life. At the same time, the relatively high iron-to-phosphorus ratio can change the electron cloud distribution and electron conduction characteristics of the material, which is beneficial to improving the electronic conductivity of the material. Better electronic conductivity can accelerate the transmission speed of electrons inside the material, enabling the electrochemical reaction to proceed more efficiently, thereby enhancing the charge and discharge performance and energy density of the battery. In addition, the relatively high lithium content and the relatively high iron-to-phosphorus ratio are both beneficial to increasing the proportion of cations. An increase in the proportion of cations is conducive to increasing the energy barrier for the growth of lithium iron phosphate particles.

[0053] In some embodiments, the second structural formula is Li x’ Fe y’ M2 z’ PO4@C, 0.98 ≤ x’ ≤ 1.00, 0.950 ≤ y’ ≤ 0.965, 0 ≤ z’ ≤ 8.36×10 -5 Compared with the lithium iron phosphate of the first structural formula, the lithium iron phosphate of the second structural formula has a relatively low lithium content and a relatively low iron-to-phosphorus ratio. By controlling the relatively low lithium content and the relatively low iron-to-phosphorus ratio, the proportion of cations is ensured to be relatively low. The relatively low proportion of cations reduces the energy barrier for the growth of lithium iron phosphate particles. The compounding of the obtained lithium iron phosphate of the first structural formula and the lithium iron phosphate of the second structural formula is beneficial to increasing the overall tap density of the material, improving the energy density of the battery, enhancing the cycle performance of the battery, and strengthening the safety of the battery.

[0054] In some embodiments, both M1 in the first structural formula and M2 in the second structural formula are doping elements, where M1 and M2 are independently selected from at least one of Ti, Mg, V, Nb, and Mn. The doping elements can improve the crystal structure, increase the diffusion rate of lithium ions, and thus enhance the battery capacity, making lithium ions more easily embedded and extracted during the charge and discharge process, thereby improving the charge and discharge efficiency and capacity of the battery.

[0055] In some embodiments, based on the total mass of the lithium iron phosphate cathode material being 100%, the total mass of the doping elements is 3000 - 10000 ppm. In some preferred embodiments, based on the total mass of the lithium iron phosphate cathode material being 100%, the total mass of the doping elements is 4000 - 8000 ppm.

[0056] In some specific embodiments, based on the total mass of the lithium iron phosphate cathode material being 100%, the total mass of the doping elements includes but is not limited to typical but non - restrictive values such as 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, 7000 ppm, 8000 ppm, 9000 ppm, 10000 ppm, etc.

[0057] In some embodiments, both small - particle lithium iron phosphate and large - particle lithium iron phosphate include a carbon coating layer, which is beneficial to protecting the lithium iron phosphate core material from the effects of water and carbon dioxide in the air, protecting the structural stability of the material, and improving the overall performance of the battery.

[0058] In some embodiments, based on the total mass of the lithium iron phosphate cathode material being 100%, the total carbon content is 1.0 wt% - 1.5 wt%. In some preferred embodiments, based on the total mass of the lithium iron phosphate cathode material being 100%, the total carbon content is 1.1 wt% - 1.4 wt%.

[0059] In some specific embodiments, based on the total mass of the lithium iron phosphate cathode material being 100%, the total carbon content includes but is not limited to typical but non - restrictive values such as 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, etc.

[0060] In some embodiments, the morphology of lithium iron phosphate is spherical or quasi - spherical.

[0061] In some embodiments, the average particle size of the primary particles of the lithium iron phosphate cathode material is 200 - 350 nm, wherein the proportion of the number of primary particle sizes d ≤ 200 nm is 35.0% ≤ P1 ≤ 70.0%, and the proportion of the number of primary particle sizes d ≥ 600 nm is 2.5% ≤ P2 ≤ 10.0%.

[0062] In some embodiments, the average particle size of the primary particles of the lithium iron phosphate cathode material includes but is not limited to typical but non - restrictive values such as 200 nm, 210 nm, 220 nm, 230 nm, 240 nm, 250 nm, 260 nm, 270 nm, 280 nm, 290 nm, 300 nm, 310 nm, 320 nm, 330 nm, 340 nm, 350 nm, etc.

[0063] In some embodiments, the mass ratio of small-particle lithium iron phosphate to large-particle lithium iron phosphate is (3 to 10):1. By controlling the mass ratio of small-particle lithium iron phosphate to large-particle lithium iron phosphate, the tap density and capacity of the overall lithium iron phosphate cathode material are maximized.

[0064] In some embodiments, the tap density of the lithium iron phosphate cathode material powder ≥ 2.60 g / cm 3 .

[0065] In some embodiments, the discharge specific capacity of the lithium iron phosphate cathode material at 1C at 25 °C ≥ 140 mAh / g.

[0066] In traditional methods, to prepare high-tap-density and high-capacity lithium iron phosphate particles using particle size grading, large-particle and small-particle lithium iron phosphate particles are often prepared separately and then mixed evenly. This occupies the resources of the production line, affects production efficiency, and reduces production capacity.

[0067] However, the second aspect of the embodiments of the present application discloses a method for preparing a lithium iron phosphate cathode material, including the following steps:

[0068] S01. After first-stage classification and grinding of precursor A and precursor B with different raw material ratios, they are respectively placed in the upper chamber and the lower chamber of a layered crucible for first-stage sintering, and intermediate C and intermediate D are obtained after sintering;

[0069] S02. Mix and grind a dopant and intermediate C to obtain a first mixture, and mix and grind a carbon source and intermediate D to obtain a second mixture;

[0070] S03. Place the first mixture and the second mixture in the upper chamber and the lower chamber of the layered crucible respectively for second-stage sintering to obtain lithium iron phosphate E and lithium iron phosphate F, and then obtain the lithium iron phosphate cathode material after air flow mixing and pulverization. Among them, the lithium iron phosphate cathode material includes small-particle lithium iron phosphate and large-particle lithium iron phosphate.

[0071] The method for preparing a lithium iron phosphate cathode material provided in the second aspect of the embodiments of the present application. In this preparation method, precursor A and precursor B with different raw material ratios are respectively provided, and it is creatively proposed to separately load precursor A and precursor B into the upper chamber and the lower chamber of the crucible. Due to the gaps between the precursor powders, the heat conduction effect between the powder particles is poor, and the materials in the crucibles placed in different positions are often subjected to different temperatures. The lower chamber is close to the actual temperature in the furnace to form large-particle lithium iron phosphate, and the actual temperature of the upper chamber is lower than the actual temperature of the furnace to form small-particle lithium iron phosphate. This preparation method realizes the simultaneous preparation of large-particle and small-particle lithium iron phosphate, the large-scale preparation of high-tap-density and high-capacity lithium iron phosphate, has a simple process, is environmentally friendly, has low costs, and is conducive to the industrial development of lithium iron phosphate cathode materials.

[0072] In step S01, after the precursors A and B with different raw material ratios are subjected to first-stage classification grinding, they are respectively placed in the upper and lower cassettes of a layered crucible for first sintering, and intermediates C and D are obtained after sintering.

[0073] In some embodiments, in the raw materials of precursor A, the molar ratio of the lithium source to iron phosphate is (1.005 - 1.05):1, and among them, the atomic ratio of iron to phosphorus in the raw materials is 0.975 - 0.990:1. By controlling precursor A to have a high lithium content and iron-phosphorus ratio, precursor A has a high cation occupancy ratio. The increase in cation occupancy ratio is beneficial to enhancing the energy barrier for the growth of lithium iron phosphate particles, facilitating the preparation of small-particle lithium iron phosphate.

[0074] In some embodiments, in the raw materials of precursor B, the molar ratio of the lithium source to iron phosphate is (0.980 - 1.000):1, and among them, the atomic ratio of iron to phosphorus in the raw materials is 0.950 - 0.965:1. Precursor B has a low lithium content and iron-phosphorus ratio, and the low cation occupancy ratio does not increase the energy barrier for the growth of lithium iron phosphate particles, facilitating the preparation of large-particle lithium iron phosphate.

[0075] In some embodiments, both precursor A and precursor B further include a dopant and a carbon material.

[0076] In some embodiments, the lithium source is lithium carbonate or lithium hydroxide.

[0077] In some embodiments, the dopant is one or a combination of several of titanium dioxide, magnesium nitrate, magnesium acetate, vanadium pentoxide, or niobium oxide.

[0078] In some embodiments, the carbon source is one or a combination of several of glucose, sucrose, polyethylene glycol, citric acid, ascorbic acid, starch, phenolic resin, commercial carbon powder, carbon nanotubes, graphene, acetylene black, and carbon aerogel; preferably, it is one or a combination of several of glucose, polyethylene glycol, sucrose, and starch.

[0079] In some embodiments, before step S01 is carried out, step S00 is further included: mixing an iron source and a phosphorus source to prepare iron phosphate. Among them, the iron source and phosphorus source used are common raw materials in the art, and the prepared iron phosphate can be added to the raw materials to prepare precursors A and B with a lithium source and the like.

[0080] Furthermore, the precursors A and B are respectively subjected to grinding treatment, and among them, the particle size D of the slurry of precursor A obtained 50 is 0.2 - 0.4 μm, and the particle size D of the slurry of precursor B 50is 0.6 to 0.8 μm; through grinding treatment, two precursor slurries with different particle sizes are obtained, which is beneficial to obtaining lithium iron phosphate particles with two different particle sizes for compounding subsequently.

[0081] In some embodiments, the mass ratio of precursor A to precursor B is (3 to 10):1. The purpose of controlling the mass ratio of precursor A and B is to control the proportion of large and small particles in the final lithium iron phosphate product. If the proportion of precursor A is too high, the proportion of small particles will be high, resulting in a low tap density. If the proportion of precursor A is too low, the proportion of large particles will be too high, and the capacity will deteriorate.

[0082] In some embodiments, the mass ratio of the first mixture to the second mixture is (3 to 10):1. The purpose of controlling the mass ratio of the first mixture and the second mixture is to control the proportion of large and small particles in the final lithium iron phosphate product. If the proportion of the first mixture is too high, the proportion of small particles will be high, resulting in a low tap density. If the proportion of the first mixture is too low, the proportion of large particles will be too high, and the capacity will deteriorate.

[0083] Due to the gaps between the precursor powders, the heat conduction effect between the powder particles is poor, often resulting in different temperatures for the materials in the crucibles placed at different positions. The lower crucible is close to the actual temperature in the furnace to form large-particle lithium iron phosphate, and the actual temperature of the upper crucible is lower than the actual temperature of the furnace to form small-particle lithium iron phosphate.

[0084] In some embodiments, the material of the layered crucible is graphite, and the length and width dimensions of the upper crucible and the lower crucible are the same. The heights of the upper crucible and the lower crucible are h1 and h2 respectively, and h1:h2 = (3 to 10):1.

[0085] In some embodiments, precursor A is placed in the upper crucible for the first sintering to obtain intermediate C, and precursor B is placed in the lower crucible for the first sintering to obtain intermediate D.

[0086] In some embodiments, in intermediate C, based on the total mass of intermediate C being 100%, the carbon content is 1.3 wt% to 1.8 wt%, and the content of the doped metal is 4000 to 8000 ppm. In intermediate D, based on the total mass of intermediate D being 100%, the carbon content is 0.2 wt% to 0.8 wt%, and the content of the doped metal is 0 to 4000 ppm. In intermediate C, a higher carbon content is beneficial for the carbon layer to coat the lithium iron phosphate particles more tightly, and a higher metal doping is beneficial for increasing the energy barrier for the growth of the lithium iron phosphate particle size, which is beneficial for inhibiting the growth of the particle size. On the contrary, intermediate D with a lower carbon content and metal doping amount is beneficial for promoting the growth of the lithium iron phosphate particle size.

[0087] In some embodiments, the first sintering is roller hearth kiln sintering, the sintering temperature is 700 - 800 °C, and the sintering time is 12 - 24 h. In some specific embodiments, the sintering temperature includes but is not limited to typical but non - restrictive values such as 700 °C, 710 °C, 720 °C, 730 °C, 740 °C, 750 °C, 760 °C, 770 °C, 780 °C, 790 °C, 800 °C, etc. In some specific embodiments, the sintering time includes but is not limited to typical but non - restrictive values such as 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, 21 h, 22 h, 23 h, 24 h, etc.

[0088] In step S02, the dopant and intermediate C are mixed and ground to obtain a first mixture, and the carbon source and intermediate D are mixed and ground to obtain a second mixture.

[0089] In some embodiments, the dopant and intermediate C are mixed and ground to obtain a first mixture, and a dopant with a doping metal amount accounting for 0 - 2000 ppm of the mass of intermediate C is added during the sanding process; the purpose here is to grind some large - sized particles during the first sintering process and break up particle agglomerates, which is beneficial to increasing the proportion of small - sized particles. If the dopant added to intermediate C is too high, it is easy to generate impurity phases, and if the addition amount is too low, it cannot achieve the effect of refining particles and improving capacity.

[0090] In some embodiments, the slurry particle size D of the first mixture 50 is 0.2 - 0.4 μm. To ensure that the resulting product is lithium iron phosphate with small particle size. If the particle size of the slurry of the first mixture is too large, there may be some agglomerates that cannot be broken up, and during the second sintering process, the agglomerates fuse to form large particles; if it is too small, it will increase the sanding duration and energy consumption, which is not environmentally friendly and economical.

[0091] In some embodiments, the carbon source and intermediate D are mixed and ground to obtain a second mixture. The provided carbon content accounts for 0.3 wt% - 0.8 wt% of the mass of intermediate D; the sanding here is rough grinding, only to break up the adhesion of particles, and at the same time, the purpose of supplementing part of the carbon source is to make the surface carbon layer coating of large - sized lithium iron phosphate particles more complete and ensure the electrical performance of large - sized lithium iron phosphate particles. If too little carbon is added, the overall carbon content of the final lithium iron phosphate H will be too low, affecting capacity performance. If too much carbon source is added, it is easy to form an overly dense carbon coating layer, inhibiting the formation of large particles and affecting the compaction density.

[0092] In some embodiments, the slurry particle size D of the second mixture 50 is 0.8 - 1.2 μm, to ensure that the resulting product is large - sized lithium iron phosphate with large particle size. If the particle size is too large, it will lead to agglomerates that cannot be broken up, affecting capacity, and if it is too small, there will be too many small particles, which is not conducive to the improvement of compaction density.

[0093] In step S03, the first mixture and the second mixture are respectively placed in the upper cartridge and the lower cartridge of a layered crucible for secondary sintering to obtain lithium iron phosphate E and lithium iron phosphate F, and then the lithium iron phosphate cathode material is obtained after airflow mixing and pulverization. The lithium iron phosphate cathode material includes small-particle lithium iron phosphate and large-particle lithium iron phosphate.

[0094] In some embodiments, based on the total mass of lithium iron phosphate E being 100%, the carbon content is 1.3 wt% - 1.8 wt%, and the content of doped metal is 4000 - 10000 ppm.

[0095] In some embodiments, based on the total mass of lithium iron phosphate F being 100%, the carbon content is 1.0 wt% - 1.5 wt%, and the content of doped metal is 0 - 4000 ppm.

[0096] In some embodiments, the secondary sintering is all carried out in a roller hearth kiln, the sintering temperature is 700 - 800 °C, and the sintering time is 12 - 24 h.

[0097] In some embodiments, the parameters of airflow mixing and pulverization are: the classification frequency is 50 - 100 Hz, the pressure intensity is 0.4 - 0.6 MPa, and the feeding rate is 1 - 5 kg / h.

[0098] In this preparation method, the differential preparation of large and small particles is achieved by using iron phosphate with high and low iron-to-phosphorus ratios, mixing high and low metal doping amounts and high and low carbon contents, and the differentiation is strengthened by controlling the particle size D of different sanding slurries. 50 More importantly, the use of a layered crucible is introduced. By controlling the different heating temperatures in different spaces inside the layered crucible, the effect of simultaneously preparing large and small particles in the same sintering is achieved. After mixing, they produce a grading effect. The prepared lithium iron phosphate product has both a high tap density and excellent electrochemical properties. At the same time, it is not necessary to transform the original production line equipment, and there is no need to add new fixed asset investment, which greatly improves the production efficiency, reduces the production cost, and is conducive to the industrial development of lithium iron phosphate cathode materials.

[0099] In some embodiments, a positive electrode sheet is provided, including the above-mentioned lithium iron phosphate cathode material or the lithium iron phosphate cathode material prepared by the preparation method of the above-mentioned lithium iron phosphate cathode material.

[0100] The provided positive electrode sheet includes the above-mentioned lithium iron phosphate cathode material or the lithium iron phosphate cathode material prepared by the preparation method of the above-mentioned lithium iron phosphate cathode material. Since the obtained lithium iron phosphate cathode material has excellent properties of high tap density and high capacity, the obtained positive electrode sheet has high rate performance and can be widely applied to various lithium-ion batteries.

[0101] In the third aspect of the embodiments of the present application, a lithium-ion battery is provided, including the above-mentioned lithium iron phosphate cathode material or the lithium iron phosphate cathode material prepared by the preparation method of the above-mentioned lithium iron phosphate cathode material.

[0102] For the lithium-ion battery provided in the third aspect of the embodiments of the present application, since the provided lithium-ion battery includes the above-mentioned lithium iron phosphate cathode material or the lithium iron phosphate cathode material prepared by the preparation method of the above-mentioned lithium iron phosphate cathode material, and since the obtained lithium iron phosphate cathode material has excellent properties of high tap density and high capacity, therefore, the obtained lithium-ion battery has a high cycle efficiency and a high charge-discharge specific capacity.

[0103] The following is illustrated with specific examples.

[0104] Example 1

[0105] A high-tap-density and high-capacity lithium iron phosphate cathode material and its preparation method

[0106] The specific operation steps are as follows:

[0107] (1) Weigh the raw material mixed precursors A and B respectively. Among them, the iron-to-phosphorus ratio of iron phosphate in precursor A is 0.975, the molar ratio of lithium carbonate to iron phosphate is 1.01:1, the mass ratio of the dopant TiO2 to iron phosphate is 1:95, the mass ratio of glucose to iron phosphate is 0.12:1, and control the particle size D of the sand-milled slurry 50 to be 0.25 μm; the iron-to-phosphorus ratio of iron phosphate in precursor B is 0.960, the molar ratio of lithium carbonate to iron phosphate is 0.990:1, the mass ratio of the dopant TiO2 to iron phosphate is 1:36, the mass ratio of glucose to iron phosphate is 0.05:1, and control the particle size D of the sand-milled slurry 50 to be 0.6 μm; load the spray-dried precursors A and B into the upper and lower compartments of a layered graphite crucible according to a mass ratio of 3:1. The length and width dimensions of the upper and lower compartments are the same, and the heights of the upper and lower compartments are h1 and h2 respectively, and h1:h2 = 3:1. Send the loaded graphite crucible into a roller hearth kiln for sintering. The sintering temperature is 780 °C and the sintering time is 20 h to obtain the intermediates C and D after the first sintering; after testing, the carbon content of intermediate C in this example is 1.43%, and the Ti content is 6051 ppm; the carbon content of intermediate D is 0.49%, and the Ti content is 1863 ppm;

[0108] (2) Grind the intermediate C obtained in step (1) and the dopant TiO2 together, where the mass ratio of intermediate C to TiO2 is 1:0.0033, and control the particle size D of the sand-milled slurry 50It is 0.23 μm, and the first mixture is obtained after spray drying; the intermediate D obtained in step (1) and glucose are ground together, where the mass ratio of the intermediate D to glucose is 1:0.05, and the particle size D of the grinding slurry is controlled 50 to be 0.80 μm, and the second mixture is obtained after spray drying;

[0109] (3) The first mixture and the second mixture in step (2) are respectively loaded into the same layered crucible as in step (1) according to a mass ratio of 3:1. Among them, the first mixture is correspondingly loaded into the upper crucible of the layered crucible, and the second mixture is correspondingly loaded into the lower crucible of the layered crucible; the loaded graphite crucible is sent to a roller hearth kiln for secondary sintering. The sintering temperature is 780 °C, and the sintering time is 24 h, and lithium iron phosphate E and F are respectively obtained; subsequently, lithium iron phosphate E and F are simultaneously sent to air jet milling, the classification frequency is 50 Hz, the pressure intensity is 0.5 MPa, and the feeding speed is 2 kg / h, and finally a high tap density and high capacity lithium iron phosphate finished product is obtained. After testing, the carbon content of lithium iron phosphate E in this example is 1.39%, and the total Ti doping amount is 7869 ppm; the carbon content of lithium iron phosphate F is 1.21%, and the total Ti doping amount is 1764 ppm.

[0110] Example 2

[0111] A high tap density and high capacity lithium iron phosphate cathode material and its preparation method

[0112] The specific operation steps are as follows:

[0113] This example is basically the same as the process of Example 1, except for the following differences:

[0114] a. In step (1), the precursors A and B are respectively loaded into the layered crucible according to a mass ratio of 7:1, and in step (3), the first mixture and the second mixture are respectively loaded into the layered crucible according to a mass ratio of 7:1;

[0115] b. In step (1), the primary sintering and in step (3), the upper crucible and the lower crucible of the layered crucible used in the secondary sintering have different heights. Among them, the ratio of the upper crucible h1 to the lower crucible h2 is 7:1, and other steps and parameters remain unchanged.

[0116] Example 3

[0117] A high tap density and high capacity lithium iron phosphate cathode material and its preparation method

[0118] The specific operation steps are as follows:

[0119] This example is basically the same as the process of Example 1, except that in step (2), the additive is changed from TiO2 to V2O5, and the mass ratio of the intermediate C to V2O5 is 1:0.0036, and other steps and parameters remain unchanged.

[0120] Example 4

[0121] A high-compaction and high-capacity lithium iron phosphate cathode material and its preparation method

[0122] The specific operation steps are as follows:

[0123] This example is basically the same as the process of Example 1, except that in step (2), the carbon source is changed from glucose to PEG-4000, and the mass ratio of intermediate D to PEG-4000 is 1:0.06, and other steps and parameters remain unchanged.

[0124] Comparative Example 1

[0125] A lithium iron phosphate cathode material and its preparation method

[0126] The specific operation steps are as follows:

[0127] (1) Grind the mixing ratio parameters of the raw material precursors A and B in Example 1 respectively, and then load the spray-dried precursors A and B into 2 ordinary graphite crucibles according to the mass ratio of 3:1 respectively. The heights of the 2 ordinary crucibles are the same, and the crucibles are placed side by side left and right; send the 2 loaded graphite crucibles into a roller hearth kiln for sintering, the sintering temperature is 780 °C, and the sintering time is 20 h to obtain the intermediate products C and D after the first sintering;

[0128] (2) It is exactly the same as step (2) of Example 1;

[0129] (3) Load the first mixture and the second mixture in step (2) into the same 2 ordinary graphite crucibles as in step (1) according to the mass ratio of 3:1 respectively, and then send the loaded graphite crucibles into a roller hearth kiln for secondary sintering. The sintering temperature is 780 °C, and the sintering time is 24 h to obtain lithium iron phosphates E and F respectively; then send lithium iron phosphates E and F into air flow crushing according to the mass ratio of 3:1, the classification frequency is 50 Hz, the pressure intensity is 0.5 MPa, and the feeding speed is 2 kg / h to finally obtain the high-compaction and high-capacity lithium iron phosphate finished product.

[0130] Comparative Example 2

[0131] A lithium iron phosphate cathode material and its preparation method

[0132] The specific operation steps are as follows:

[0133] (1) Grind according to the ratio parameters of the raw material mixed precursor A in Example 1. Divide the spray-dried precursor A into two parts according to a mass ratio of 3:1 and load them into the upper and lower compartments of the layered graphite crucible respectively. The sizes and placement methods of the upper and lower crucibles are the same as those in Example 1. Send the loaded graphite crucible into a roller hearth kiln for sintering at a sintering temperature of 780 °C and a sintering time of 20 h to obtain the intermediates C and D after the first sintering;

[0134] Steps (2) and (3) of this comparative example are exactly the same as those in Example 1.

[0135] Comparative Example 3

[0136] A lithium iron phosphate cathode material and its preparation method

[0137] The specific operation steps are as follows:

[0138] (1) Grind according to the ratio parameters of the raw material mixed precursor B in Example 1. Divide the spray-dried precursor B into two parts according to a mass ratio of 3:1 and load them into the upper and lower compartments of the layered graphite crucible respectively. The sizes and placement methods of the upper and lower crucibles are the same as those in Example 1. Send the loaded graphite crucible into a roller hearth kiln for sintering at a sintering temperature of 780 °C and a sintering time of 20 h to obtain the intermediates C and D after the first sintering;

[0139] Steps (2) and (3) of this comparative example are exactly the same as those in Example 1.

[0140] Comparative Example 4

[0141] A lithium iron phosphate cathode material and its preparation method

[0142] The specific operation steps are as follows:

[0143] This comparative example is exactly the same as step (1) in Example 1. After obtaining the intermediates C and D, directly perform air jet milling and pulverization mixing according to a mass ratio of 3:1 without using secondary grinding and secondary sintering.

[0144] Performance test and result analysis

[0145] (1) Characterize the performance of the lithium iron phosphate cathode material obtained in Example 1

[0146] The powder tap density of the high tap density and high capacity lithium iron phosphate cathode material prepared in Example 1 is 2.619 g / cm 3 , the total carbon content is 1.28%, and the total metal doping amount of Ti is 6101 ppm; the particle size distribution curve of the product is as Figure 1 shown, where the particle size of lithium iron phosphate D 50 = 1.06 μm, and its particle size distribution curve presents a bimodal peak shape. The particle size D corresponding to the highest peak intensity of the left peakp1 = 0.56 μm, the volume fraction V corresponding to the highest peak intensity of the left peak p1 = 6.75%; the particle size D corresponding to the highest peak intensity of the right peak p2 = 1.36 μm, the volume fraction V corresponding to the highest peak intensity of the right peak p2 = 8.79%.

[0147] The SEM image of the high-compaction and high-capacity lithium iron phosphate cathode material prepared in Example 1 is as Figure 2 shown. It can be seen that the lithium iron phosphate particles exhibit a spherical-like morphology. The proportion P1 of the number of small particles with a primary particle size d ≤ 200 nm is 36.4%, and the proportion P2 of the number of large particles with d ≥ 600 nm is 5.1%. The average size of the primary particle size is 227.3 nm. The SEM image of the high-compaction and high-capacity lithium iron phosphate cathode material prepared in Comparative Example 2 is as Figure 3 shown. It can be seen that the morphology of the material prepared in Comparative Example 2 is uniform and the particle size is small, and there is no particle gradation.

[0148] (2) Electrochemical performance test

[0149] The powder materials prepared in Examples 1-4 and Comparative Examples 1-4 were made into coin cells, and the specific steps are as follows:

[0150] (1) Preparation of the slurry. 2.33 kg of the cathode active material, 0.012 kg of superconducting carbon black (SP), and 0.048 kg of the binder polyvinylidene fluoride (PVDF) were simultaneously added to a 500 mL agate ball milling jar, and then 1.6 kg of the solvent N-methylpyrrolidone (NMP) was added. The slurry was prepared by ball milling at a rotation speed of 360 r / min for 4 h;

[0151] (2) Coating of the slurry. The scale of the doctor blade of the coater was adjusted, and the ball-milled slurry was evenly coated on the aluminum foil. The coated electrode was placed in a vacuum drying oven at a temperature of 130 °C and baked for 3 h;

[0152] (3) Rolling and punching. The aluminum foil coated with the slurry was placed flat in the middle of the rollers, and the electrode was rolled; the rolled electrode was placed with the front side closely against the punched hole, and punched successively; the compaction density of the electrode was controlled at 2.0-2.4 g / cm 3 , with a diameter of 14 mm and a thickness of 0.05-0.10 mm; the punched electrode was placed in a vacuum drying oven at a temperature of 130 °C and baked for 3 h;

[0153] (4) Assemble the coin cell. In the glove box, assemble it in the order of the negative electrode case, shrapnel, steel sheet, lithium sheet, separator, positive electrode sheet, and positive electrode case. During the process, inject 10 μL of electrolyte, and then use a sealing machine to seal the coin cell. Perform electrochemical performance tests on these four groups of coin cells.

[0154] The characterization results of the carbon content and metal doping amount of the intermediates C and D obtained after the first sintering in the examples and comparative examples of this application are shown in Table 1 below:

[0155] Table 1

[0156]

[0157] The characterization results of the carbon content and metal doping amount of the lithium iron phosphate E and F obtained after the second sintering in the examples and comparative examples of this application are shown in Table 2 below:

[0158] Table 2

[0159]

[0160]

[0161] The statistical results of the particle size distribution and primary particle size of the lithium iron phosphate finished products prepared in the examples and comparative examples of this application are shown in Table 3 below:

[0162] Table 3

[0163]

[0164] The carbon content, total metal doping amount, and powder compaction density results of the lithium iron phosphate finished products prepared in the examples and comparative examples of this application are shown in Table 4 below:

[0165] Table 4

[0166]

[0167] As can be seen from Table 4, the overall compaction density of the lithium iron phosphate finished products obtained in Examples 1 to 4 is ≥ 2.601 g / cm 3 , while the overall compaction density of the lithium iron phosphate finished products obtained in Comparative Examples 1 to 4 is ≤ 2.467 g / cm 3 .

[0168] The room-temperature electrochemical characterization results of the lithium iron phosphate finished products prepared in the examples and comparative examples of this application are shown in Table 5 below:

[0169] Table 5

[0170]

[0171]

[0172] As can be seen from Table 5 and Figure 4 it can be known that for the coin cells made of the high-voltage chamber and high-capacity lithium iron phosphate cathode materials prepared in Examples 1 to 4, the discharge specific capacity at 1C at room temperature is ≥140.0 mAh / g, the discharge specific capacity at 5C can reach up to 127.55 mAh / g at most, and the capacity retention rate is ≥98.15%, showing excellent rate performance; among them, the electrical properties of the high-voltage and high-capacity lithium iron phosphate cathode material prepared in Example 1 are as Figure 4 shown. Its discharge specific capacity at 0.1C at room temperature is 160.39 mAh / g, and the discharge specific capacity at 1C reaches 145.05 mAh / g. In Comparative Examples 1, 3, and 4, the discharge specific capacity at 1C of the coin cells at room temperature is ≤140.0 mAh / g, and the capacity retention rate is ≤96.07%, with poor electrochemical performance. Among them, the discharge specific capacity at 1C of the coin cell prepared from the lithium iron phosphate cathode material obtained in Comparative Example 2 is 142.76 mAh / g, and the discharge specific capacity at 5C also reaches 126.94 mAh / g. Although the material obtained in Comparative Example 2 has excellent electrochemical performance, combined with the content of Table 4, the tap density of the material obtained in Comparative Example 2 is 2.159 g / cm 3 , it can be seen that the material of Comparative Example 2 cannot have both high electrochemical performance and high tap density characteristics at the same time.

[0173] Obviously, the lithium iron phosphate cathode materials obtained in Examples 1 to 4 all have relatively excellent electrochemical performance and relatively high tap density.

[0174] In summary, the lithium iron phosphate cathode material provided in the embodiments of the present application is obtained by compounding small-particle lithium iron phosphate and large-particle lithium iron phosphate. Moreover, the particle size distribution curve of the lithium iron phosphate cathode material presents a bimodal or quasi-bimodal peak shape, where the particle size D corresponding to the highest peak intensity of the left peak satisfies 0.2 μm ≤ D p1 ≤ 0.6 μm, and the volume ratio V corresponding to the highest peak intensity satisfies 6.0% ≤ V p1 ≤ 8.0%; the particle size D corresponding to the highest peak intensity of the right peak satisfies 1.0 μm ≤ D p2 ≤ 1.5 μm, and the volume ratio V corresponding to the highest peak intensity satisfies 8.0% ≤ V p2 ≤ 10.0%; it can be seen that the provided lithium iron phosphate cathode material is obtained by compounding small-particle lithium iron phosphate and large-particle lithium iron phosphate. Among them, the small-particle size particles can greatly reduce the lithium ion transmission path, improve the lithium ion transmission efficiency, and thus improve the electrochemical performance, while the large-particle size particles can effectively improve the tap density. The simultaneous improvement of these two core performances can effectively improve the energy density of the lithium iron phosphate material and greatly broaden the application prospect of the lithium iron phosphate material in the power field.

[0175] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included within the protection scope of the present application.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material comprises small-particle lithium iron phosphate and large-particle lithium iron phosphate, and the particle size distribution curve of the lithium iron phosphate positive electrode material presents a double peak or a quasi-double peak type, wherein the particle size corresponding to the highest peak intensity of the left peak is 0.2 μm≤D p1 ≤0.6μm, the volume proportion corresponding to the highest peak intensity is 6.0%≤V p1 ≤8.0%; the particle size corresponding to the highest peak intensity of the right peak is 1.0μm≤D p2 ≤1.5μm, the volume proportion corresponding to the highest peak intensity is 8.0%≤V p2 ≤10.0%.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The lithium iron phosphate positive electrode material includes a first structural formula and a second structural formula. The first structural formula is Li x Fe y M1 z PO4@C, 1.005≤x≤1.05, 0.975≤y≤0.990, 8.36×10 -5 ≤z≤1.67×10 -4 ; The second structural formula is Li x’ Fe y’ M2 z’ PO4@C, 0.98≤x'≤1.00, 0.950≤y'≤0.965, 0≤z'≤8.36×10 -5 ; Wherein, M1 and M2 are doping elements and are independently selected from at least one of Ti, Mg, V, Nb and Mn.

3. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The mass ratio of the small-particle lithium iron phosphate to the large-particle lithium iron phosphate is (3-10):1; and / or, The particle morphology of the lithium iron phosphate positive electrode material is spherical or quasi-spherical; and / or, The average particle size of the primary particles of the lithium iron phosphate positive electrode material is 200-350 nm, wherein the number of primary particles with a diameter of d≤200 nm accounts for 35.0%≤P1≤70.0%, and the number of primary particles with a diameter of d≥600 nm accounts for 2.5%≤P2≤10.0%.

4. The lithium iron phosphate positive electrode material according to claim 2, characterized in that: Taking the total mass of the lithium iron phosphate positive electrode material as 100%, the total carbon content is 1.0wt% to 1.5wt%, and the total content of doping elements is 3000 to 10000ppm; and / or, The powder compaction density of the lithium iron phosphate positive electrode material is ≥2.60g / cm 3 and / or, The lithium iron phosphate positive electrode material has a 1C discharge capacity of ≥140 mAh / g at 25°C.

5. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The steps include: Precursors A and B with different raw material ratios are subjected to first classification grinding, and then placed in the upper and lower saggers of the layered sagger for first sintering, to obtain intermediates C and D after sintering; The dopant and the intermediate C are mixed and ground to obtain a first mixture, and the carbon source and the intermediate D are mixed and ground to obtain a second mixture; The first mixture and the second mixture are placed in the upper and lower boxes of the layered sagger respectively for a second sintering to obtain lithium iron phosphate E and lithium iron phosphate F, and then the lithium iron phosphate positive electrode material is obtained after airflow mixing and crushing, wherein the lithium iron phosphate positive electrode material includes small-particle lithium iron phosphate and large-particle lithium iron phosphate.

6. The method for preparing the lithium iron phosphate positive electrode material according to claim 5, characterized in that: In the raw material of the precursor A, the molar ratio of the lithium source to the iron phosphate is (1.005-1.05):1, wherein the atomic ratio of iron to phosphorus in the raw material is 0.975-0.990:1; and / or, In the raw material of the precursor B, the molar ratio of the lithium source to the iron phosphate is (0.980-1.000):1, wherein the atomic ratio of iron to phosphorus in the raw material is 0.950-0.965:1; and / or, The particle size D of the slurry of the precursor A 50 is 0.2 to 0.4 μm, and the particle size D of the slurry of the precursor B is 50 0.6 to 0.8 μm; and / or, The mass ratio of the precursor A to the precursor B is (3-10):1; and / or, The mass ratio of the first mixture to the second mixture is (3-10):

1.

7. The method for preparing the lithium iron phosphate positive electrode material according to claim 5, characterized in that: The layered sagger is made of graphite, and the length and width of the upper sagger are consistent with those of the lower sagger. The heights of the upper sagger and the lower sagger are h1 and h2 respectively, and h1:h2=(3-10):

1.

8. The method for preparing a lithium iron phosphate positive electrode material according to claim 5, characterized in that: In the intermediate C, the total mass of the intermediate C is 100%, the carbon content is 1.3wt% to 1.8wt%, and the content of the doped metal is 4000 to 8000ppm; and / or, In the first mixture, the dopant addition amount is 0 to 2000 ppm; the slurry particle size D 50 0.2 to 0.4 μm; and / or In the lithium iron phosphate E, based on the total mass of the lithium iron phosphate E being 100%, the carbon content is 1.3wt% to 1.8wt%, and the content of the doped metal is 4000 to 10000 ppm.

9. The method for preparing a lithium iron phosphate positive electrode material according to claim 5, characterized in that: In the intermediate D, the total mass of the intermediate D is 100%, the carbon content is 0.2wt% to 0.8wt%, and the content of the doped metal is 0 to 4000ppm; and / or, In the second mixture, the amount of carbon added is 0.3wt% to 0.8wt%; the slurry particle size D 50 0.8 to 1.2 μm; and / or In the lithium iron phosphate F, the total mass of the lithium iron phosphate F is 100%, the carbon content is 1.0wt% to 1.5wt%, and the content of the doped metal is 0 to 4000ppm.

10. The method for preparing a lithium iron phosphate positive electrode material according to claim 5, characterized in that: The sintering temperature of the first sintering and the second sintering is 700-800° C., and the sintering time is 12-24 hours; and / or, The parameters of the airflow mixing and pulverizing are: classification frequency of 50-100 Hz, pressure intensity of 0.4-0.6 MPa, and feed rate of 1-5 kg / h.

11. A lithium ion battery, characterized in that: The invention comprises the lithium iron phosphate positive electrode material according to any one of claims 1 to 4 or the lithium iron phosphate positive electrode material prepared by the preparation method of the lithium iron phosphate positive electrode material according to any one of claims 5 to 10.

Images