Lithium iron phosphate positive electrode material, preparation method therefor and use thereof

Abstract

A lithium iron phosphate positive electrode material, a preparation method therefor and a use thereof. A value K of the lithium iron phosphate positive electrode material is less than or equal to 1, and K is calculated by means of the grain size and the b-axis length of the lithium iron phosphate positive electrode material. The positive electrode material has excellent properties, including low resistivity, high conductivity, high compaction density and high capacity. During the research process, it was found that when the value K of the lithium iron phosphate positive electrode material is less than or equal to 1, the excellent properties such as high conductivity, low resistivity, high compaction density and high capacity can be ensured. The powder resistivity of the lithium iron phosphate positive electrode material is smaller than 10Ω·cm, the compaction density is greater than 2.2 g / cm3, the 0.1 C discharge capacity is greater than 160mAh / g, the 1C discharge capacity is greater than 150mAh / g, and the material has the advantage of high capacity.

Description

A lithium iron phosphate positive electrode material and its preparation method and application

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on November 28, 2023, with application number 202311615121.2 and invention name “A lithium iron phosphate positive electrode material, its preparation method and application”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application belongs to the technical field of electrode material preparation, and specifically relates to a lithium iron phosphate positive electrode material and its preparation method and application. Background Art

[0004] Lithium iron phosphate (LiFePO4) has become one of the most widely used cathode materials for lithium-ion batteries due to its low cost, stable charge-discharge platform, environmental friendliness, and high safety. Currently, domestic shipments of LiFePO4 have surpassed those of ternary materials. However, the material's conductivity remains a pressing issue within the industry, as it directly impacts the battery's electrical performance. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present application is to overcome the defects of the lithium iron phosphate positive electrode material in the prior art, such as poor conductivity, thereby providing a lithium iron phosphate positive electrode material and its preparation method and application.

[0006] To this end, this application provides the following technical solutions.

[0007] In a first aspect, the present application provides a lithium iron phosphate positive electrode material, wherein K of the lithium iron phosphate positive electrode material is less than or equal to 1, and K satisfies the relational formula 1: K=D+(b-6)×100 Relational formula 1

[0008] D is the grain size of the lithium iron phosphate cathode material, 0.05 μm ≤ D ≤ 0.15 μm;

[0009] b is the b-axis length of the lithium iron phosphate cathode material,

[0010] A second aspect of the present application provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0011] (1) mixing iron phosphate, a lithium source, a dopant, a carbon source, and a flux and grinding the mixture to obtain a slurry;

[0012] The iron-phosphorus ratio of the ferric phosphate is 0.975-0.99, and the specific surface area is 6-10m 2 / g; the particle size D50 of the slurry is 300-500nm; the doping element in the dopant includes at least two of vanadium, titanium, tungsten and magnesium; the mass content of the doping element in the lithium iron phosphate positive electrode material is 1000-5000ppm; the flux is at least one of lithium fluoride, lithium phosphate and lithium dihydrogen phosphate;

[0013] (2) drying and sintering the slurry to obtain a lithium iron phosphate positive electrode material with K≤1;

[0014] Where, K=D+(b-6)×100 Relationship 1

[0015] D is the grain size of the lithium iron phosphate cathode material, 0.05 μm ≤ D ≤ 0.15 μm;

[0016] b is the b-axis length of the lithium iron phosphate cathode material,

[0017] When using XRD to determine the grain size D and b-axis length of lithium iron phosphate cathode materials, it is necessary to ensure that R wp ≤10%, R wp is the residual variance factor of the weighted graph, which can ensure the accuracy of refinement.

[0018] The molar ratio of the iron phosphate to the lithium in the flux is 1:(0.02-0.06).

[0019] The dopant includes at least two of vanadium pentoxide, titanium dioxide, tungsten trioxide and magnesium oxide.

[0020] When the dopant includes at least two of vanadium pentoxide, titanium dioxide, tungsten trioxide and magnesium oxide, the mass ratio of different dopants is not specifically limited. When the dopant includes two of vanadium pentoxide, titanium dioxide, tungsten trioxide and magnesium oxide, the mass ratio can be but not limited to 1:1, 1:2, 1:3, 2:1, 2:5, etc.; when the dopant includes three of vanadium pentoxide, titanium dioxide, tungsten trioxide and magnesium oxide, the mass ratio can be but not limited to 3:2:1, 4:3:1, 1:1:1, 2:1:1, 2:5:3, 5:10:7, etc.; when the dopant includes four of vanadium pentoxide, titanium dioxide, tungsten trioxide and magnesium oxide, the mass ratio can be but not limited to any value such as 4:3:2:1, 7:7:6:4, 1:3:1:1, 1:1:1:1, 2:5:7:10, 5:10:15:6, etc.

[0021] The mass content of carbon element in the lithium iron phosphate positive electrode material is 1.3-1.5wt%;

[0022] Optionally, the carbon source is at least one of polyethylene glycol, glucose, carbon nanotubes, sucrose and starch.

[0023] When the carbon sources include two or more, the mass ratio of different carbon sources is not specifically limited. When the carbon sources include two, the mass ratio of the two carbon sources can be, but is not limited to, 2:1, 3:2, 1:1, 2:7, 8:3, etc.; when the carbon sources include three, the mass ratio of the three carbon sources can be, but is not limited to, 2:1:1, 3:2:7, 1:1:1, 2:7:6, 8:3:5, etc.; when the carbon sources include four, the mass ratio of the four carbon sources can be, but is not limited to, 2:1:1:6, 3:2:7:1, 1:1:1:9, 2:7:6:6, 8:3:5:2, etc.; when the carbon sources include five, the mass ratio of the five carbon sources can be, but is not limited to, 2:1:1:6:7, 3:2:7:1:4, 1:1:1:9:1, 2:7:6:6, 8:3:5:2:1, etc.

[0024] The preparation method satisfies at least one of (1) to (3):

[0025] (1) The lithium source is lithium carbonate and / or lithium hydroxide monohydrate;

[0026] (2) The molar ratio of the iron phosphate to the lithium in the lithium source is 1:(0.97-1.02);

[0027] (3) The mass content of water in the slurry is 50-70%.

[0028] The drying is spray drying;

[0029] Optionally, the spray drying parameters are: feed frequency is 10-30 Hz, atomizer frequency is 40-60 Hz;

[0030] Optionally, the inlet air temperature is 240-270°C;

[0031] Optionally, the outlet air temperature is 80-100°C.

[0032] The specific steps of sintering include: heating to 750-820° C. at a heating rate of 1.5-4° C. / min and then keeping the temperature for 8-12 hours.

[0033] In a third aspect, the present application provides a pole piece, comprising a current collector and a coating attached to the current collector; the raw material of the coating comprises the above-mentioned lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared by the above-mentioned preparation method.

[0034] A fourth aspect of the present application provides a battery, comprising the above-mentioned lithium iron phosphate positive electrode material, the lithium iron phosphate positive electrode material prepared by the above-mentioned preparation method, or the above-mentioned electrode sheet.

[0035] The technical solution of this application has the following advantages:

[0036] 1. The lithium iron phosphate positive electrode material provided in this application has a K value ≤ 1, where K is calculated based on the grain size and b-axis length of the lithium iron phosphate positive electrode material. The positive electrode material has excellent properties such as low resistivity, high conductivity, high compaction density, and high capacity. During the research process, the inventors found that a K value ≤ 1 for the lithium iron phosphate positive electrode material can ensure that it has excellent properties such as high conductivity, low resistivity, high compaction density, and high capacity. The powder resistivity of the lithium iron phosphate positive electrode material is less than 10Ω·cm, and the compaction density is greater than 2.2g / cm 3 , 0.1C discharge capacity is greater than 160mAh / g, 1C discharge capacity is greater than 150mAh / g, it has the advantage of high capacity.

[0037] 2. The present application provides a method for preparing a lithium iron phosphate cathode material, which can produce a lithium iron phosphate cathode material with good comprehensive properties such as low resistivity, high conductivity, high compaction density, and high capacity. The powder resistivity of the lithium iron phosphate cathode material produced in this application is less than 10Ω·cm, and the compaction density is greater than 2.2g / cm 3 , 0.1C discharge capacity is greater than 160mAh / g, 1C discharge capacity is greater than 150mAh / g, with the advantage of high capacity. This application produces a lithium iron phosphate positive electrode material with K≤1 by controlling the iron-phosphorus ratio and specific surface area of ​​the iron phosphate, as well as the slurry particle size, the amount of doping elements used, and the type of flux. The lithium iron phosphate positive electrode material with K≤1 has comprehensive properties such as low resistivity, high conductivity, high compaction density, and high capacity.

[0038] Lithium iron phosphate has a one-dimensional lithium ion transport channel, with lithium ions transported along the b-axis. The shorter the b-axis, the faster the lithium ion transport and the better the ionic conductivity. Furthermore, the smaller the grain size, the higher the capacity. During their research, the inventors discovered that a K value of ≤1 for lithium iron phosphate cathode materials ensures excellent properties such as high conductivity and low resistivity. Furthermore, the method of this application can achieve a K value of ≤1 for lithium iron phosphate cathode materials, thereby improving their conductivity and capacity.

[0039] 3. The preparation method of the lithium iron phosphate positive electrode material provided in this application can further optimize the conductivity by adjusting the amount of co-solvent when preparing the lithium iron phosphate positive electrode material.

[0040] This application further optimizes the type of carbon source to ensure the carbon content in the lithium iron phosphate cathode material product; by optimizing the spray drying parameters, the spherical size and moisture content of the dried material can be controlled. For example, if the feed frequency is too fast, the material particles are large and the moisture content is high; if the atomizer frequency is too low, the particles are large, which is not conducive to improving the compaction density and the subsequent process. This application optimizes the air inlet temperature and air outlet temperature to facilitate the discharge of moisture, and does not generate high energy consumption, cause carbon volatilization, and affect the carbon content of the product. When preparing the lithium iron phosphate cathode material, this application can further improve its electrical performance by controlling the sintering temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0042] FIG1 is a scanning electron microscope image of the lithium iron phosphate positive electrode material of Example 1 of the present application. DETAILED DESCRIPTION

[0043] The following examples are provided to further better understand the present application, but are not limited to the best implementation mode described herein, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the scope of protection of the present application.

[0044] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0045] In a specific embodiment, the molecular formula of ferric phosphate can be represented by Fe x PO4, wherein x ranges from 0.975 to 0.99, ensuring that the iron-phosphorus ratio of the iron phosphate is 0.975 to 0.99. In various embodiments, the molecular weight of the iron phosphate can be calculated by 55.85x+31+16×4.

[0046] Example 1

[0047] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0048] (1) With an iron-phosphorus ratio of 0.982 and a specific surface area of ​​8.1m 2 / g of anhydrous ferric phosphate was used as a raw material, which was mixed with lithium carbonate, titanium dioxide, vanadium pentoxide, polyethylene glycol, sucrose, lithium phosphate, and water in a mass ratio of 100:24.1:0.42:0.36:8:2:0.8:206.64, stirred for 30 minutes, and then ground to obtain a slurry with a particle size D50 of 356 nm.

[0049] (2) The slurry was spray-dried at a feed frequency of 26 Hz, an atomizer frequency of 48 Hz, an air inlet temperature of 260°C, and an air outlet temperature of 85°C. The sprayed dry material was then sintered by heating the temperature to 785°C at a heating rate of 3°C / min and then holding the temperature for 10 hours. After air flow pulverization, a lithium iron phosphate cathode material satisfying K≤1 was obtained. The K value of the lithium iron phosphate cathode material is calculated using equation 1.

[0050] Example 2

[0051] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The difference from Example 1 is that the iron-phosphorus ratio of anhydrous iron phosphate is changed. The iron-phosphorus ratio of anhydrous iron phosphate in this embodiment is 0.976. Other aspects are the same as Example 1.

[0052] Example 3

[0053] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The difference from Example 1 is that tungsten trioxide is used instead of vanadium pentoxide, and the mass ratio of iron phosphate, lithium carbonate, titanium dioxide, tungsten trioxide, polyethylene glycol, sucrose, lithium phosphate, and water is 100:24.1:0.42:0.26:8:2:0.8:206.64. Other factors are the same as Example 1.

[0054] Example 4

[0055] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material. The difference from Example 1 is that the sintering temperature is changed. The sintering temperature of this embodiment is 810° C., and the rest is the same as Example 1.

[0056] Example 5

[0057] This embodiment provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0058] (1) With an iron-phosphorus ratio of 0.979 and a specific surface area of ​​6.5m 2 / g of anhydrous ferric phosphate was used as a raw material, which was mixed with lithium hydroxide monohydrate, tungsten trioxide, magnesium oxide, carbon nanotubes, starch, lithium fluoride, and water in a mass ratio of 100:27.6:0.13:0.52:8:2:0.8:206.64, stirred for 25 minutes, and then ground to obtain a slurry with a particle size D50 of 472 nm.

[0059] (2) The slurry was spray-dried at a feed frequency of 15 Hz, an atomizer frequency of 55 Hz, an air inlet temperature of 260°C, and an air outlet temperature of 85°C. The sprayed dry material was then sintered by heating the temperature to 755°C at a heating rate of 2.5°C / min and then holding the temperature for 9 hours. After airflow pulverization, a lithium iron phosphate cathode material satisfying K≤1 was obtained. The K value of the lithium iron phosphate cathode material is calculated using equation 1.

[0060] Comparative Example 1

[0061] This comparative example provides a method for preparing a lithium iron phosphate positive electrode material. The difference from Example 1 is that the iron-phosphorus ratio of the anhydrous iron phosphate is changed. The iron-phosphorus ratio of the anhydrous iron phosphate in this comparative example is 0.965, and the rest is the same as Example 1.

[0062] Comparative Example 2

[0063] This comparative example provides a method for preparing a lithium iron phosphate positive electrode material, which differs from Example 1 in that the amount of titanium dioxide added is changed to 0.82, and the rest is the same as Example 1.

[0064] Comparative Example 3

[0065] This comparative example provides a method for preparing a lithium iron phosphate positive electrode material, which differs from Example 1 in that no flux is added (ie, no lithium phosphate is added), and the rest is the same as Example 1.

[0066] Test example

[0067] This test example provides performance tests of lithium iron phosphate cathode materials in various embodiments and comparative examples, specifically:

[0068] (1) The powder resistivity, compaction density, carbon content, and doping element content of the lithium iron phosphate cathode material were obtained by ICP testing. The results are shown in Table 1.

[0069] The K value of the lithium iron phosphate positive electrode material of each embodiment and comparative example is calculated by equation 1, as shown in Table 1;

[0070] Where, K=D+(b-6)×100 Relationship 1

[0071] D is the grain size of lithium iron phosphate cathode material, 0.05≤D≤0.15um;

[0072] b is the b-axis length of the lithium iron phosphate cathode material,

[0073] D and b are obtained by XRD test; when obtaining D and b values, it is necessary to ensure that R wp ≤10%, R wp is the residual variance factor of the weighted graph, which can ensure the accuracy of refinement.

[0074] (2) Preparation method of button half-cell: lithium iron phosphate positive electrode material (from each embodiment and comparative example), carbon black (SP), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 90:5:5, N-methylpyrrolidone (NMP) was added, mixed evenly, and coated on aluminum foil. The mixture was dried at 100°C for 4 hours, cut into positive electrode sheets with a diameter of 12 mm, and assembled to obtain button half-cells. The battery performance was tested after standing for 12 hours. The results are shown in Table 1.

[0075] Battery capacity test method: using Xinwei testing cabinet, 0.1C charging, cut-off voltage 3.75V, 0.1C discharging, cut-off voltage 2V, 0.5C charging, cut-off voltage 3.75V, 1C discharging, cut-off voltage 2V.

[0076] Table 1 Performance results of positive electrode materials in various embodiments and comparative examples

[0077] The above results show that the lithium iron phosphate cathode material with K≤1 in the present application has good conductivity and low resistivity. The specific surface area of ​​the cathode material in the present application is 11-14m 2 / g, carbon content is 1.3-1.5wt%, and compacted density is greater than 2.2g / cm 3 Furthermore, the positive electrode material also has a higher discharge capacity.

[0078] Furthermore, in Comparative Example 1, when the iron-phosphorus ratio of the iron phosphate was lower than 0.975, a lithium iron phosphate positive electrode material with a K value of ≤ 1 could not be obtained; in Comparative Example 2, the mass content of the doping element in the lithium iron phosphate positive electrode material was 7000 ppm, and a lithium iron phosphate positive electrode material with a K value of ≤ 1 could not be obtained; and in Comparative Example 3, when no flux was added, a lithium iron phosphate positive electrode material with a K value of ≤ 1 could not be obtained. The above results show that the present application can obtain a lithium iron phosphate positive electrode material with a K value of ≤ 1 by controlling parameters such as the iron-phosphorus ratio of the iron phosphate and the doping element content, thereby ensuring the conductivity and capacity of the positive electrode material, while also having a good compaction density.

[0079] (3) FIG1 is a scanning electron micrograph of the lithium iron phosphate cathode material prepared in Example 1 of the present application at 50,000 times magnification. As can be seen from the figure, the lithium iron phosphate cathode material has a small particle size, lithium ions are easily intercalated and deintercalated, and the ionic conductivity is good. The figure also shows that the conductive carbon layer adheres to the particle surface, further improving the electronic conductivity. This further illustrates the good conductivity of the lithium iron phosphate cathode material with a K value of ≤ 1 provided by the present application.

[0080] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: The K of the lithium iron phosphate positive electrode material is less than or equal to 1, and K satisfies the relational formula 1: K=D+(b-6)×100. D is the grain size of lithium iron phosphate positive electrode material, 0.05μm≤D≤0.15μm; b is the b-axis length of the lithium iron phosphate positive electrode material, 2. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: (1) mixing iron phosphate, a lithium source, a dopant, a carbon source and a flux and grinding the mixture to obtain a slurry; The iron-phosphorus ratio of the ferric phosphate is 0.975-0.99, and the specific surface area is 6-10m 2 / g; the particle size D50 of the slurry is 300-500nm; the doping elements in the dopant include at least two of vanadium, titanium, tungsten and magnesium; the mass content of the doping elements in the lithium iron phosphate positive electrode material is 1000-5000ppm; the flux is at least one of lithium fluoride, lithium phosphate and lithium dihydrogen phosphate; (2) drying and sintering the slurry to obtain a lithium iron phosphate positive electrode material with K≤1; Where, K = D + (b-6) × 100 Relation 1 D is the grain size of lithium iron phosphate positive electrode material, 0.05μm≤D≤0.15μm; b is the b-axis length of the lithium iron phosphate positive electrode material, 3. The preparation method according to claim 2, characterized in that: The molar ratio of the iron phosphate to the lithium in the flux is 1:(0.02-0.06).

4. The preparation method according to claim 2 or 3, characterized in that: The dopant includes at least two of vanadium pentoxide, titanium dioxide, tungsten trioxide and magnesium oxide.

5. The preparation method according to any one of claims 2 to 4, characterized in that: The mass content of carbon element in the lithium iron phosphate positive electrode material is 1.3-1.5wt%; Optionally, the carbon source is at least one of polyethylene glycol, glucose, carbon nanotubes, sucrose and starch.

6. The preparation method according to any one of claims 2 to 5, characterized in that: Satisfy at least one of (1)-(3): (1) The lithium source is lithium carbonate and / or lithium hydroxide monohydrate; (2) The molar ratio of the iron phosphate to the lithium in the lithium source is 1:(0.97-1.02); (3) The mass content of water in the slurry is 50-70%.

7. The preparation method according to any one of claims 2 to 6, characterized in that: The drying is spray drying; Optionally, the spray drying parameters are: feed frequency is 10-30 Hz, atomizer frequency is 40-60 Hz; Optionally, the temperature of the inlet air is 240-270°C; Optionally, the outlet air temperature is 80-100°C.

8. The preparation method according to any one of claims 2 to 7, characterized in that: The specific steps of sintering include: heating to 750-820° C. at a heating rate of 1.5-4° C. / min and then keeping the temperature for 8-12 hours.

9. A pole piece, characterized in that: It comprises a current collector and a coating attached to the current collector; the raw material of the coating comprises the lithium iron phosphate positive electrode material according to claim 1 or the lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 2-8.

10. A battery, characterized in that: It comprises the lithium iron phosphate positive electrode material according to claim 1, the lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 2 to 8, or the pole piece according to claim 9.

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